Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium (Studies in Surface Science and Catalysis)

Studies in Surface Science and Catalysis 142 - Part A IMPACT OF ZEOLITES AND OTHER POROUS MATERIALS ON THE NEW TECHNOLO...

Author: R. Aiello | G. Giordano | F. Testa

27 downloads 2315 Views 102MB Size Report

This content was uploaded by our users and we assume good faith they have the permission to share this book. If you own the copyright to this book and it is wrongfully on our website, we offer a simple DMCA procedure to remove your content from our site. Start by pressing the button below!
Report copyright / DMCA form

DOWNLOAD PDF

Studies in Surface Science and Catalysis 142 - Part A IMPACT OF ZEOLITES AND OTHER POROUS MATERIALS ON THE NEW TECHNOLOGIES AT THE BEGINNING OF THE NEW MILLENNIUM

This Page Intentionally Left Blank

S t u d i e s in S u r f a c e S c i e n c e a n d C a t a l y s i s Advisory Editors: B. Delmon and J.To Yates

Vol. 142

IMPACT OF ZEOLITES OUS MATERIALS ON THE NEW TECHNOLOGIES AT THE BEGINNING OF THE NEW MILLENNIUM PART A Proceedings of the 2 ~ International FEZA (Federation of the European Zeolite Associations) Conference Taormina, Italy, September 1-5, 2002

Organized by the ITALIAN ZEOLITE ASSOCIA TION under the auspices of the Federation of the European Zeolite Associations Edited by R. A i e l l o , G. G i o r d a n o a n d F. T e s t a

Dipartimento di Ingegnefia Chimica e dei Materiali, Universitd della Calabria Arcavacata di Rende, Italy

0

2002

ELSEVIER Amsterdam - Boston - London - New York- Oxford - Paris - San Diego San Francisco - Singapore - S y d n e y - Tokyo

ELSEVIER SCIENCE B.V. Sara Burgerhartstraat 25 P.O. Box 211, 1000 AE Amsterdam, The Netherlands

9 2002 Elsevier Science B.V, All rights reserved.

This work is protected under copyright by Elsevier Science, and the following terms and conditions ~ p l y to its use: Photocopying Single photocopies of single chapters may be made for personal use as allowed by national copyright laws. Permission of the Publisher and payment of a fee is required for all other photocopying, including multiple or systematic copying, copying for advertising or promotional purposes, resale, and all forms of document delivery. Special rates are available for e d ~ institutions tlmt wish to make photocopies for non-profit educational classroom use. Permissions may be sought directly from Elsevier Science via their homepage (http:/h~v.elsevier.eom) by selecting 'Customer support' and then 'Permissions'. Alternatively you earl send an e-mail to: [email protected], or fax to: (+44) 1865 853333. In the USA, users may clear permissions and make payments through the Copyright Clearance Center, Inc., 222 Rosewood Drive, Danvers, MA 01923, USA; phone: (+I) (978) 7508400, fax: (+1) (978) 7504744, and in the UK through the Copyright Licensing Agency Rapid Clearance Service (CLARCS), 90 Tottenham Court Road, London WIP 0LP, UK; phone: (+44) 207 63 i 5555; fax: (+44) 207 631 5500. Other countries may have a local reprographic rights agency for payments. Derivative Works Tables of contents may be reproduced for internal circulation, but permission of Elsevier Science is required for external resale or distribution of such material. Permission of the Publisher is required for all other derivative works, including compilations and translations. Electronic Storage or Usage Permission of the Publisher is required to store or use electronically any material contained in this work, including any chapter or part of a chapter. Except as outlined above, no part of this work may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without prior written permission of the Publisher. Address permissions requests to: Elsevier Science Global Rights Department, at the mail, fax and e-mail addresses noted above. Notice No responsibility is assumed by the Publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. Because of rapid advances in the medical sciences, in particular, independent verification of diagnoses and drug dosages should be made.

First edition 2002 Library of Congress Cataloging in Publication Data A catalog record from the Library of Congress has been applied for.

British Library Cataloguing in Publication Data A catalogue record from the British Library has been applied for.

ISBN: ISSN:

0-444-51174-1 0167-2991

The paper used in this publication meets the requirements of ANSI/NISO Z39.48-1992 (Permanence of Paper). Printed in The Netherlands.

PREFACE

It is a pleasure to present the Proceedings of the Second Conference of the Federation of the European Zeolite Associations where are collected the contributions of internationally renowned researchers in the field of the Science and Technology of micro and mesoporous materials. Aim of the Conference, organized by the Italian Zeolite Association, is to create an international forum where researchers from academia as well as from industry can bring and discuss ideas finalized to evaluate the impact of zeolites and other porous materials on the new technologies at the beginning of the new millennium. Among the others, in fact, the technologies for the production of chemicals, which will become always more important for maintaining our standard of life and our environment safe, will need substantial innovation and we hope that this book will be a source of new ideas for further fundamental and applied research work not only for the participants of the Conference but also for the whole scientific community. These proceedings report the oral and poster communications presented during the FEZA Conference, subdivided into 8 thematic sessions. The volume contains also the full text of the three plenary and two keynote lectures. The scientific contributions, coming from 35 countries both European and extra European, testify of the great vitality of the zeolite science in its various branches, from those always represented at the zeolite conferences (synthesis, catalysis, ion exchange and modification, natural zeolites... ) to the new emerging areas (mesoporous materials, environmental sciences, computational chemistry, advanced materials... ) and, at the same time, of the blend of multidisciplinary knowledge involved in this science in continuous evolution. The editors would like to acknowledge the dedication of the members of the Paper Selection Committee: A. Alberti, G. Centi, M. Derewinski, F. Fajula and J. B.Nagy, and express their gratitude to all the referees who contributed to the selection of the Conference papers. A special and grateful acknowledgment has to be addressed to Dr. A. Katovic (Treasurer) for her great involvement all along the Conference organization. Rosario Aiello Girolamo Giordano Flaviano Testa Editors


vii The Federation of the European Zeolite Associations (FEZA) was constituted in Szombathely (Hungary) on 12 July 1995 by the representatives of the National Zeolite Associations from France, Germany, Hungary, Italy, the Netherlands and UK, plus Bulgaria and Spain, which were going at that time to constitute the respective national associations. The Constitution of the FEZA was approved on 25 January 1996. At the same date and in successive meetings of the FEZA Committee, other national associations were accepted, i.e., the Romanian Zeolite Association, the Georgian Association of Zeolites, the Polish Zeolite Association, the Czech Zeolite Group, and finally, in the course of last meeting in Montpellier, on 9 July 2001, the admission of Portugal and Slovakia was decided. Among the objects of the Federation, there is the task to arrange Specialist Workshops, Euroconferences or Meetings of an educational character. Accordingly, in the three-year period from 1996 to 1998, a series of six Euroworkshops on Zeolites have been organized, with the financial support of the European Union, on synthesis; ordered mesoporous materials; sorption, diffusion and separation; natural zeolites; application in catalysis, and modification and characterization. In the same frame, the FEZA originated the proposal of a cycle of Euresco Conferences on Zeolite Molecular Sieves. The first Euroconference of this cycle has been held in Obernai (France) during the last March on the "Isomorphous Substitution by Transition Metals". The proposal to organize an International Thematic Conference trader the auspices of the FEZA was made by the leading members of the Hungarian Zeolite Association during the FEZA Committee Meeting, held in Budapest in 1998. Although this type of Conference was not expressly considered in the FEZA Constitution, the proposal was accepted with enthusiasm by the members of the Committee. The 1st International FEZA Conference was therefore held in Eger (Hungary) on 1-4 September 1999, on the theme "Porous Materials in Environmentally Friendly Processes". The Eger Conference was a very successful Conference and this encouraged the FEZA Committee to continue on the same way. Now I have the particular pleasure and pride to present the volume constituting the Proceedings of the 2nd International FEZA Conference, which will be held in Taormina (Italy) on 1-5 September 2002 on the theme "Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium". The reading of the contents and the information directly gathered from the organizers makes me convinced that this will be a very successful Conference either for the richness of themes or for the quality of the contributions. In addition, I am sure that these Proceedings will be prepared by the Editors and printed by the Publisher with the usual care and attention to the printing quality. One last information for the reader. The next FEZA Conference, the 3rd of the series, will be held in Prague in August-September 2005, under the auspices of the Czech Zeolite Group on the theme of "Molecular Sieves from Basic Research to Industrial Applications". Carmine Colella Chairman of the FEZA Committee


SUPPORT AND SPONSORING (as of May 30, 2002) The Organizing Committee wishes to thank various Institutions and Companies for their financial support to FEZA 2002.Their contributions allowed a reduced registration fee for students and a bursary program. INSTITUTIONS Universit~ della Calabria Dipartimento di Ingegneria Chimica e dei Materiali- Universith della Calabria Universit~ di Messina Universit~ di Catania Consorzio Interuniversitario Nazionale per la Scienza e la Tecnologia dei Materiali (INSTM) COMPANIES EniTecnologie Sasol Italy Philips Netzsch Jeol Micromeritics UOP M.S. COECO Pirossigeno


xi

ORGANIZING COMMITTEE

Chairman R. Aiello

University of Calabria, Italy

Vice-Chairman G. Giordano


Secretary E Testa


Treasurer A. Katovic


Members E Crea S. Crisafulli S. Galvagno A. Parmaliana

University University University University

of of of of

Calabria, Italy Catania, Italy Messina, Italy Messina, Italy

INTERNATIONAL SCIENTIFIC ADVISORY BOARD (Council of FEZA)

C. Colella (President) H. Van Bekkum (Former President) E Di Renzo (Secretary) M.W. Anderson P. Behrens J. Ceika P. Ciambelli E Hudec I. Kiricsi D.C. Koningsberger S. Kowalak C. Minchev J. Perez-Pariente E Ramoa Ribeiro R. Russu G. Tsitsishvili

Italy The Netherlands France United Kingdom Germany Czech Republic Italy Slovakia Hungary The Netherlands Poland Bulgaria Spain Portugal Romania Georgia

xii PAPER SELECTION COMMITTEE R. Aiello A. Alberti G. Centi M. Derewinski E Fajula J.B. Nagy

University of Calabria, Italy University of Ferrara, Italy University of Messina, Italy Polish Academy of Science, Krakov, Poland CNRS-ENSCM, Montpellier, France University of Namur, Belgium

xiii

CONTENTS

ZEOLITE SYNTHESIS AND CHARACTERIZATION Zeolite characterization with spectroscopic methods A. Zecchina, G. Spoto, G. Ricchiardi, S. Bordiga, E Bonino, C. Prestipino and C. Lamberti (PLENARY LECTURE) Synthesis of alumino, boro, and gallosilicate zeolites by steam-assisted conversion method and their characterization R. Bandyopadhyay, Y. Kubota and Y. Sugi Aluminium distribution in MCM-22. The effect of framework aluminium content and synthesis procedure J. D~dedek, J. Cejka, M. Oberlinger and S. Ernst Grafting of aluminium on dealuminated H-BEA using alkoxides A. Omegna, M. Haouas, G. Pirngruber and R. Prins Influence of various synthesis parameters on the morphology and crystal size of zeolite Zn-MFI A. Katovic, G. Giordano and S. Kowalak In situ dynamic light scattering and synchrotron X-Ray powder diffraction study of the early stages of zeolite growth G. Artioli, R. Grizzetti, L. Carotenuto, C. Piccolo, C. Colella, B. Liguori, R. Aiello and P Frontera Synthesis of MCM-22 zeolite by the vapor-phase transport method S. Inagaki, M. Hoshino, E. Kikuchi and M. Matsukata Defect-flee MEL-type zeolites synthesized in the presence of an azoniaspiro-compound R. Millini, D. Berti, D. Ghisletti, W.O. Parker, Jr., L.C. Carluccio and G. Bellussi Chemical and structural aspects of the transformation of MCM-22 precursor into ITQ-2 R. Schenkel, J.-O. Barth, J. Kornatowski and J.A. Lercher Nanocrystalline ZSM-5: a highly active catalyst for polyolefin feedstock recycling D.P Serrano, J. Aguado, J.M. Escola and J.M. Rodriguez Modeling superoxide dismutase: immobilizing a Cu-Zn complex in porous matrices and activity testing in H202 decomposition K. Hernadi, D. M~hn, I. Labddi, I. Pdlink6, E. Sitkei and I. Kiricsi Crystal growth of zeolite Y studied by computer modelling and atomic force microscopy JR. Agger and M. W. Anderson Interaction of small molecules with transition metal ions in zeolites: the effect of the local environment P Nachtigall, M. Davidovd, M. Silhan and D. Nachtigallovd

3

15

23 31

39

45

53 61 69 77

85 93

101

xiv Preparation and characterization of mesoporous TS-1 catalyst K. Johannsen, A. Boisen, M. Brorson, 1. Schmidt and C.J.H. Jacobsen Observations of layer growth in synthetic zeolites by field emission scanning electron microscopy S. Bazzana, S. Dumrul, J. Warzywoda, L. Hsiao, L. Klass, M. Knapp, J.A. Rains, E.M. Stein, M.J Sullivan, C.M. West, J Y. Woo and A. Sacco, Jr. XANES and XPS studies of titanium aluminophosphate molecular sieves M.H. Zahedi-Niaki, E Beland, L. Bonneviot and S. Kaliaguine An investigation of the intermediate gel phases of A1PO4-11 synthesis by solid state NMR spectroscopy Y. Huang, R. Richer and C. Kirby The benzene molecule as a probe for steric hindrance at proton sites in zeolites: an IR study B. Onida, B. Bonelli, L. Borello, S. Fiorilli, E Geobaldo and E. Garrone Structural characterization of Co- and Si-substituted A1PO-34 synthesized in the presence of morpholine A. MartuccL A. AlbertL G. CrucianL A. Frache and L. Marchese Chemical linking of MFI-type colloidal zeolite crystals P. Agren, S. Thomson, Y. Ilhan, B. Zibrowius, W. Schmidt and E Schfith Synthesis and characterization of MCM-22 zeolites for the N20 oxidation of benzene to phenol D. MelonL R. MonacL E. RombL C. Guimon, H. Martinez, 1. Fechete and E. Dumitriu Novel solid strong base derived from zeolite supported CaO X. W. Han, G. Xie, Y. Chun, X.. W. Yan, Y. Wang, J Xue and JH. Zhu ZSM-5 spheres prepared by resin templating L. Tosheva and J Sterte Novel Nanocomposite Material A. Carati, C. Rizzo, L. Dalloro, B. Stocchi, R. Millini and C. Perego Vibrational and optical spectroscopic studies on copper-exchanged ferrierite G. Turnes Palomino, S. Bordiga, C. Lamberti, A. Zecchina and C. Otero Aredm Variable temperature FTIR spectroscopy of carbon monoxide adsorbed on protonic and rubidium-exchanged ZSM-5 zeolites C. Otero Are6n, M. Pe~arroya Mentruit, M. Rodriguez Delgado, G. Turnes Palomino, O. V. Manoilova, A.A. Tsyganenko and E. Garrone Preparation and characterization of Zn-MFI zeolites using short chain alkylamines as mineralizing agents S. Valange, B. Onida, E Geobaldo, E. Garrone and Z. Gabelica

109

117

125

135

143

151 159

167 175 183 191 199

207

215

Crystal growth of nanosized LTA zeolite from precursor colloids S. Mintova, B. Fieres and T. Bein

223

Synthesis of hybrid zeolite disc from layered silicate Y. Kiyozumi, M. Salou and E Mizukami

231

XV

Effect of alkali metal ions on synthesis of zeolites and layered compounds by solid-state transformation T. Nishide, H. Nakajima, Y. Kiyozumi and E Mizukami (A1)-ZSM-12: syhnthesis and modification of acid sites J. Cejka, G. Ko~ovd, N. Zilkov6 and I. Hrub6 Formation of new microporous silica phase in protonated kanemite-TMAOHwater system E KoolL Y. Kiyozumi, M. Salou and E Mizukami Raman spectroscopic studies of the templated synthesis of zeolites P.P.H.JM. Knops-Gerrits and M. Cuypers Preparation, characterization and catalytic activity of non-hydrothermally synthesized saponite-like materials R. Prihod'ko, M. Sychev, E.J.M. Hensen, J.A.R. van Veen and R.A. van Santen Self-bonded A1, B-ZSM-5 pellets C. Perri, P. De Luca, D. Vuono, M. Bruno, J. B.Nagy and A. Nastro Syntheses and characterization of A1, B-LEV type zeolite from systems containing methyl-quinuclidinium ions D. Violante, P. De Luca, C.V. Tuoto, L. Catanzaro, M. Bruno, J. B.Nagy and A. Nastro Synthesis and ion exchange properties of the ETS-4 and ETS-10 microporous crystalline titanosilicates C.C. Pavel, D. Vuono, A. Nastro, J. B.Nagy and N. Bilba Quasiisothermal degradation kinetics of tetrapropylammonium cations in silicalite-1 matrices O. Pachtova, M. Kodigik, B. Bernauer and E Bauer Cationic silver clusters in zeolite rho and sodalite J. Michalik, J. Sadlo, M. Danilczuk, J. Perlinska and H. Yamada The first example of a small-pore framework hafnium silicate Z. Lin and J. Rocha Synthesis, characterization and catalytic activity of vanadium-containing ETS-10 P. Brand, o, A.A. Valente, J. Rocha and M. W. Anderson Infrared evidence for the reversible protonation of acetonitrile at high temperature in mordenite J. Czyzniewska, S. Chenevarin and E Thibault-Starzyk Spectroscopic and catalytic studies on Cu-MCM-22: effect of copper loading A.J.S. Mascarenhas, H.O. Pastore, H.M.C. Andrade, A. Frache, M. Cadoni and L. Marchese Preparation and properties of MFI zincosilicate S. Kowalak, E. Szymkowiak, M. Gierczyfiska and G. Giordano Influence of Cs loading and carbonates on TPR profiles of PtCsBEA L. Stievano, C. Caldeira, M.E Ribeiro and P. Massiani New evidences for the fluoride contribution in synthesis of gallium phosphates V.1. Pdrvulescu, C.M. Visinescu, M.H. Zahedi-Niaki and S. Kaliaguine

239 247

255 263

271 279

287

295

303 311 319 327

335 343

351 359 367

xvi High-field ESR spectroscopy of Cu(I)-NO complexes in zeolite CuZSM-5 A. Pb'ppl and M. Hartmann

375

Characterization of acid sites in dehydrated H-Beta zeolite by solid state NMR E'. Montouillout, S. Aiello, E Fayon and C. Fernandez

383

Characterization and quantification of aluminum species in zeolites using high-resolution 27A1 solid state NMR A.A. Quoineaud, E Montouillout, S. Gautier, S. Lacombe and C. Fernandez Control of AFI type crystal synthesis with additional gel components J Kornatowski, G. Zadrozna and JA. Lercher

391 399

Synthesis and characterization of mordenite (MOR) zeolite derived from a layered silicate, Na-magadiite T. Selvam and W. Schwieger

407

Hydrothermal synthesis and characterization of new phosphate-based materials prepared in the presence of 1,4-dimethylpiperazine L. Josien, A. Simon-Masseron, S. Fleith, E Gramlich and J Patarin

415

Modeling of crystal growth at early stages of analcime synthesis from clear solutions B. Suboti~, R. Aiello, J. Bronik and E Testa

423

Synthesis of zincosilicate molecular sieve VPI-7 using vapor phase transport J Dong, C.E Xue and G. Liu

431

Combined IR and catalytic studies of the role of Lewis acid sites in creating acid sites of enhanced catalytic activity in steamed HZSM-5 J Datka, B. Gil, P. Baran and B. Staudte Heterogeneity of Cu + in CuZSM-5, TPD-IR studies of CO desorption J Datka and P. Kozyra

439 445

Speciation and structure of cobalt carbonyl and nitrosyl adducts in ZSM-5 zeolite investigated by EPR, IR and DFT techniques P. Pietrzyk, Z. Sojka, B. Gil, J Datka and E. Broctawik

453

Spectroscopic and catalytic behaviour of [015-CsHs)Rh(TI4-1,5-CsH12)] in Mt56Y and Hs6Y (M ' = Li, Na, K, Rb and Cs) E.C. de Oliveira, R.G. da Rosa, H. 0. Pastore

461

Improved synthesis procedure for Fe-BEA zeolite D. Aloi, E Testa, L. Pasqua, R. Aiello and J B.Nagy One-step benzene oxidation to phenol. Part I: preparation and characterization of Fe-(A1)MFI type catalysts G. Giordano, A. Katovic, S. Perathoner, E Pino, G. Centi, J B.Nagy, K. Lazar and P. Fejes

469

477

xvii CATALYSIS

From micro to mesoporous molecular sieves: adapting composition and structure for catalysis 487 A. Corma and M.T. Navarro (PLENARY LECTURE) One step benzene oxidation to phenol. Part II: catalytic behavior of Fe-(A1)MFI zeolites 503 S. Perathoner, F. Pino, G. CentL G. Giordano, A. Katovic, J. B.Nagy, K. Lazar and P. Fejes Synthesis, structure, and reactivity of iron-sulfur species in zeolite ZSM-5 511 R. W. Joyner, M. Stockenhuber and O.P. Tkachenko Characterization of FeMCM-41 and FeZSM-5 catalysts to styrene production 517 J.R.C. Bispo, A.C. Oliveira, M.L.S. Corr~a, J.L.G. Fierro, S.G. Marchetti and M. C. Rangel Fischer-Tropsch synthesis. Influence of the presence of intermediate iron reduction species in Fe/Zeolite L catalysts 525 N.G. Gallegos, M.V. CagnolL J.E Bengoa, A.M. Aloarez, A.A. Yeramidm and S. G. Marchetti On the necessity of a basic revision of the redox properties of H-Zeolites 533 Z. Sobalik, P. Kubdmek, O. Bortnovsky, A. Vondrovdt, Z. Tva~Skovr, JE. Sponer and B. Wichterlovdt The role of zeotype catalyst support in the synthesis of carbon nanotubes by CCVD 541 K. Hernadi, Z. Krnya, A. Siska, J Kiss, A. Oszkr, J. B.Nagy and I. Kiricsi The influence of water on the activity of nitridated zeolites in base-catalyzed reactions 549 S. Ernst, M. Hartmann, T. Hecht, P. Cremades Ja~,n and S. Sauerbeck Selective catalytic reduction of N20 with light alkanes and N20 decomposition over Fe-BEA zeolite catalysts 557 T. Nobukawa, K. Kita, S. Tanaka, S. Ito, T. Miyadera, S. Kameoka, K. Tomishige and K. Kunimori Hydroxymethylation of 2-methoxyphenol catalyzed by H-mordenite: analysis of the reaction scheme 565 E Caoani, L. Dal Pozzo, L. Maselli and R. Mezzogori Unraveling the nature and location of the active sites for butene skeletal isomerization over aged H-Ferrierite 573 S. van Donk, E. Bus, A. Broersma, J.H. Bitter and K.P. de Jong Hydroconversion of aromatics over a Pt-Pd/USY catalyst 581 C. Petitto, G. Giordano, E Fajula and C. Moreau Hydrodearomatization, hydrodesulfurization and hydrodenitrogenation of gas oils in one step on Pt,Pd/H-USY 587 Z Varga, J. Hancsrk, G. Tolvaj, W.I. Horvrth and D. Kall6 Reformate upgrading to produce enriched BTX using noble metal promoted zeolite catalyst 595 S.H. Oh, K.H. Seong, Y.S. Kim, S. ChoL B.S. Lim, J.H. Lee, J. Woltermann and Y.E Chu

xviii Dehydroisomerization of n-butane to isobutene over Pd/SAPO-11. The effect of Si content of SAPO-11, catalyst preparation and reaction condition Y. Wei, G. Wang, Z Liu, P Xie and L. Xu Vapor phase propylene epoxidation over Au/Ti-MCM-41 catalyst: influence of Ti grafting A.K. Sinha, T. Akita, S. Tsubota and M. Haruta Intrinsic activity of titanium sites in TS-1 and Al-free Ti-Beta U Wilkenh6ner, D.W. Gammon and E. van Steen The effect of zeolite pore size and channel dimensionality on the selective acylation of naphtalene with acetic anhydride J (~ejka, P Prokegovd, L. Ceroenfi and K. Mikulcovd Alkylation of phenol with methanol over zeolite H-MCM-22 for the formation of p-cresol G. Moon, K.P M6ller, W. B6hringer and C.T. O'Connor Relative stability of alkoxides and carbocations in zeolites. QM/MM embedding and QM calculations applying periodic boundary conditions L.A. Clark, M. Sierka and J. Sauer H-Beta zeolite for acylation processes: optimization of the catalyst properties and reaction conditions P Botella, A. Corma, E Rey and S. Valencia Aniline methylation on modified zeolites with acidic, basic and redox properties I.L Ioanova, O.A. Ponomoreva, E.B. Pomakhina, E.E. Knyazeva, V.V. Yuschenko, M. Hunger and J. Weitkamp Aldol condensation catalyzed by acidic zeolites T. Komatsu, M.Mitsuhashi and T. Yashima Role of intracrystalline tunnels of sepiolite for catalytic activity Y. Kitayama, K. Shimizu, T. Kodama, S. Murai, T. Mizusima, M. Hayakawa and M. Muraoka Catalytic wet oxidation of reactive dyes with H202 over mixed (A1-Cu) pillared clays S.-C. Kim, D.-S. Kim, G.-S. Lee, J.-K. Kang, D.-K. Lee and Y.K. Yang Application of zeolites as supports for catalysts of the ethylene and propylene polymerization I.N. Meshkova, T.A. Ladygina, T.M. Ushakova, N. Yu. Kovaleva and L.A. Novokshonova Catalytic properties of beta zeolite exchanged with Pd and Fe for toluene total oxidation J. Jacquemin, S. Siffert, J.-E Lamonier, E. Zhilinskaya, A.Aboukai's Hydroisomerization of n-Butane over Pd/HZSM-5 and Pd/Hmordenite with and without binder P Ca~izares, E Dorado, P Shnchez and R. Romero Butane isomerization on several H-zeolite catalysts S. De RossL G. Moretti, G. Ferraris and D. Gazzoli Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene JL. Valverde, A. de Lucas, P Sdnchez, E Dorado and A. Romero

603

611 619

627

635

643

651 659

667 675

683

691 699

707 715 723

xix Influence of cocations on the activity of Co-MOR for NO/N20 SCR by propene I. Asencio, E Dorado, JL. Valverde, A. De Lucas and P. Sdnchez Catalytic performance of mesoporous silica SBA-15-supported noble metals for thiopene hydrodesulfurization M. Sugioka, T. Aizawa, Y. Kanda, T. Kurosaka, Y. Uemichi and S. Namba

731

739

Skeletal isomerization of 1-hexene to isohexenes over zeolite catalysts Z. Wu, Q. Wang, L. Xu and S. Xie

747

Preparation and catalytic characterisation of Al-grafted MCM-48 materials M. Rozwadowsla', M. Lezanska, J Wloch, K. Erdmann and J Kornatowski

755

Photoreduction of incorporated molecules in zeolite X: methylviologen K.T. Ranjit and L. Kevan

763

Effective utilization of residual type feedstock to middle distillates by hydrocracking technology S.K. Saha, G.K. Biswas and D. Biswas

771

Direct analysis of deactivated catalysts in 1-pentene isomerization by high-resolution fast atom bombardment mass spectrometry J.M. Campelo, E Lafont and J.M. Marinas

781

Selection of an active zeolite catalyst and kinetics of vapor phase esterification of acetic acid with ethyl alcohol A.M. Aliyev, E.E. Sarijanoo, O. Tun 9 Sava~gi, R.Z. Mikailov, T.N. Shakhtakhtinsky, A. Sario~lan, P.E Poladly and A.R. Kuliyeo Hydrodesulfurization of dibenzothiophene over Mo-based catalysts supported by siliceous MCM-41 A. Wang, Y. Wang, Y. Chen, X. Li, P. Yao and T. Kabe Acylation of 2-methoxynaphtalene over ion-exchanged ~-Zeolite ]. C. Kantarh, L. Artok, H. Bulut, S. Ydmaz and S. Olkii Development of new ZSM-5 catalyst-additives in the fluid catalytic cracking process for the maximization of gaseous alkenes yield A.A. Lappas, C.S. Triantafillidis, Z.A. Tsagrasouli, V.A. Tsiatouras, I.A. Vasalos and N.P. Evmiridis Characterization of H and Cu mordenites with varying SIO2/A1203 ratios, by optical spectroscopy, MAS NMR of 29Si, 27A1 and 1H, temperature programmed desorption and catalytic activity for nitrogen oxide reduction V. Petranovskii, R.E Marzke, G. Diaz, A. Gomez, N. Bogdanchikova, S. Fuentes, N. Katada, A. Pestryakov and V. Gurin A comparison of SAPO, GaPSO, MgAPO and GaPO's as DeNOx catalysts V.I. Pdrvulescu, M. Alifanti, M.H. Zahedi-Niaki, P. Grange and S. Kaliaguine The influence of textural properties of MFI type catalysts on deactivation phenomena during oligomerization of butenes G. Giordano, E Cavani and E Trifir6

787

795 799

807

815

823

831

XX

Dehydrogenation of propane over various chromium-modified MFI-type zeolite catalysts V.A. Tsiatouras, T.K. Katranas, C.S. Triantafillidis, A.G. Vlessidis, E.G. Paulidou and N.P Evmiridis Effect of Pd addition on the catalytic performance of H-ZSM-5 zeolite in chlorinated VOCs combustion R. Lrpez-Fonseca, S. Cibridm, J.1. Guti~rrez-Ortiz and J.R. Gonzdtlez-Velasco Influence of the amount and the type of Zn species in ZSM-5 on the aromatisation of n-hexane A. Smie~kovLt, E. Rojasovr, P Hudec, L. Sabo and Z. Zidek Simultaneous desulfurization and isomerization of sulfur containing n-pentane fractions over Pt/H-mordenite catalyst J. Hancsrk, A. Holl6, I. Valkai, Gy. Szauer and D. Kall6 Propylene polymerization using various metal-containing MCM-41 as cocatalyst T. Miyazaki, Y. Oumi, T. Uozumi, H. Nakajima, S. Hosoda and T. Sano Oxidation of cyclohexene catalyzed by manganese(III) complexes encapsulated in two faujasites M. Silva, R. Ferreira, C. Freire, B. de Castro and JL. Figueiredo Heavy aromatics upgrading using noble metal promoted zeolite catalyst S.H. Oh, S.I. Lee, K.H. Seong, Y.S. Kim, JH. Lee, J Woltermann, WE. Cormier and Y.E Chu Preparation of iron-doped titania-pillared clays and their application to selective catalytic reduction of NO with ammonia D.-K. Lee, S.-C Kim, S.-J. Kim, J.-K. Kang, D.-S. Kim and S.-S. Oh Sulfated Zr-pillared saponite: preparation, properties and thermal stability L. Bergaoui, A. Ghorbel and J.-E Lambert Isomerization and hydrocracking of n-decane over Pt-Pd/A1MCM-41 catalysts S.P. Elangovan, C Bischof and M. Hartmann Influence of nickel metal distribution in Ni/Y-zeolite on the reactivity toward CO hydrogenation D.-S. Kim, S.-C. Kim, S.-J Kim and D.-K. Lee Hydrodechlorination of chlorinated compounds on different zeolites B. Imre, Z. Krnya, I. Hannus, J. Halrsz, J B.Nagy and I. Kiricsi Ammoxidation of ethylene into acetonitrile over Co-zeolites catalysts M. Mhamdi, S. Khaddar-Zine and A. Ghorbel Physicochemical characterization of vanadium-containing K10 epoxidation catalyst I. Khedher, A. Ghorbel and A. Tuel Conversion of aromatic hydrocarbons over MCM-22 and MCM-36 catalysts E. Dumitriu, I. Fechete, P. Caullet, H. Kessler, V. Hulea, C. Chelaru, T. Hulea and X. Bourdon HE-DE exchange and migration of Ga in H-ZSM5 and H-MOR zeolites M. Garcia-Sanchez, P. Magusin, E.JM. Hensen and R.A. van Santen

839

847

855

863 871

879 887

895 903 911

919 927 935 943 951

959

xxi Catalytic conversion of trichloroethylene over HY-zeolite E. Finocchio, C. Pistarino, P. Comite, E. Mazzei Justin, M. Baldi and G. Busca FT-IR studies of internal, external and extraframework sites of FER, MFI, BEA and MOR type protonic zeolite materials G. Busca, M. Beoilacqua, T. Armaroli and M. Trombetta NO reduction with isobutane on Fe/ZSM-5 catalysts prepared by different procedures M.S. Batista and E.A. Urquieta-Gonzdlez

967

975 983

Catalytic and infrared spectroscopic study of NO+CO reaction over iron-containing pillared montmorillonite 991 E L6nyi, J. Valyon and I. Kiricsi A study on alkylation of naphtalene with long chain olefins over zeolite catalyst 999 H. Guo, Y. Liang, W. Qiao, G. Wang and Z. Li Synthesis of anthraquinone from Phthalic Anhydride with Benzene over Zeolite Catalyst 1007 Y. Wang, W.-R. Miao, Q. Liu, L.-B. Cheng and G.-R. Wang Simultaneous hydrogenation and ring opening of aromatics for diesel upgrading on Pt/zeolite catalysts. The influence of zeolite pore topology and reactant on catalyst performance M.A. Arribas, A. Martinez and G. Sastre Catalytic combustion of chlorobenzene over Pt/zeolite catalysts S. Scird, S. Minicd, C. CrisafullL G. Burgio and V. Giuffrida Ag and Co exchanged ferrierite in lean NOx abatement with CH4 P. Ciambelli, D. Sannino, M.C. Gaudino and M. Flytzani-Stephanopoulos The effect of sulfate ion on the synthesis and stability of mesoporous materials M.L. Guzmdn-Castillo, H. Armend6riz-Herrera, A. Tob6n-Ceroantes, D.R. Acosta, P. Salas-Castillo, A. Montoya de la F. and A. Vfzquez-Rodriguez Catalytic behavior of Cd-clinoptilolite prepared by introduction of cadmium metal onto cationic sites G. Onyestydtk and D. Kall6

1015 1023 1031 1039

1047

MESOPOROUS M O L E C U L A R SIEVES

Confinement at nanometer scale: why and how? E Di Renzo, A. Galarneau, P. Trens, N. Tanchoux and E Fajula (PLENARY LECTURE)

1057

Anchorage of dye molecules and organic moieties to the inner surface of Si-MCM-41 Y. Rohlfing, D. W6hrle, J. Rathousk~, A. Zukal and M. Wark Mesocellular aluminosilicate foams (MSU-S/F) and large pore hexagonal mesostructures (MSU-S/H) assembled from zeolite seeds: hydrothermal stability and properties as cumene cracking catalysts Y. Liu and T.J Pinnaoaia

1067

1075

xxii Fabrication of large secondary mesopores in MCM-41 particles assisted by aminoacids and hydrophobic functional groups 1083 I. Diaz and J P~rez-Pariente

Hexagonal and cubic thermally stable mesoporous Tin(IV) phosphates with acidic, basic, and catalytic properties

1091

C. Serre, A. Auroux, A. Geruasini, M. Hervieu, G. Ferey

Characterization of [Cu]-MCM-41 by XPS and CO or NO adsorption heat measurements

1101

M. Broyer, J.P Bellat, O. Heintz, C. Paulin, S. Valange and Z Gabelica

Synthesis and characterisation of iron-containing SBA-15 mesoporous silica

1109

E Martinez, Y.-J Han, G. Stueky, JL. Sotelo, G. Ouejero and JA. Melero

Synthesis and characterization of mesoscopically ordered surfactant/co-surfactant templated metal oxides

1117

T. Czuryszla'ewicz, J. Rosenholm, E Kleitz and M. Linden

Preparation of novel organic-inorganic hybrid micelle templated silicas. Comparison of different routes for materials preparation

1125

D.J. Maequarrie, D.B. Jackson, B.L. King and A. Watson

Structure and catalytic performance of cobalt Fischer Tropsch catalysts supported by periodic mesoporous silicas

1133

A. E Khodakou, R. Bechara and A. Gribooal-Constant

Highly dispersed VOx species on mesoporous supports: promising catalysts for the oxidative dehydrogenation (ODH) of propane

1141

A. Briickner, P Rybarczyk, H. Kosslick, G.-U. Wolf and M. Baerns

Modelling mesoporous materials

1149

M. W. Anderson, C.C. Egger, G.JT. Tiddy and J.L. Casei

Acidity and thermal stability of mesoporous aluminosilicates synthesized by cationic surfactant route

1157

M. Derewinski, M. Machowska and P Sarv

Mesoporous silicate as matrix for drug delivery systems of non-steroidal antinflammatory drugs

1165

R. Aiello, G. Cauallaro, G. Giammona, L. Pasqua, P Pierro and E Testa

Aluminum incorporation and interracial structures in A1SBA-15 mesoporous solids: double resonance and optically pumped hyperpolarized 129XeNMR Studies

1173

E. Haddad, J.-B. d'Espinose, A. Nossov, E Guenneau and A. G~d~on

Tailoring the pore size of hexagonally ordered mesoporous materials containing acid sulfonic groups

1181

R. van Grieken, J.A. Melero and G. Morales

Novel vesicular mesoporous material templated by catanionic surfactant self-assembly 1189 X. W. Yan and J.H. Zhu

Preparation and characterization of Co-Fe-Cu mixed oxides via hydrotalcite-like precursors for toluene catalytic oxidation J. Carpentier, J.-E Lamonier, S. Siffert, H. Laversin, and A. Aboukai's

1197

xxiii Catalytic oxidation over transition metal doped MCM-48 molecular sieves C. WeL Q. Cai, X. Yang, 14(.Pang, Y. Bi and K. Zhen Highly selective oxidation of aromatic hydrocarbons (styrene, benzene and toluene) with H202 over Ni, Ni-Cr and Ni-Ru modified MCM-41 catalysts V..Parvulescu, C. Anastasescu, C. Constantin and B.L. Su Mesoporous materials as supports for heteropolyacid based catalysts M. Gulbihska, M. wrjtowski and M. Laniecki Synthesis and characterization of A1-MCM-48 type materials using coal fly ash P. Kumar, N.K. Mal, Y. OumL T Sano and K. Yamana Synthesis of well-aligned carbon nanotubes on MCM-41 W. Chen, A.M. Zhang, X. Yan and D. Han Synthesis and characterization of CuO and Fe203 nanoparticles within mesoporous MCM-41/-48 silica C. Minchev, R. Krhn, T. Tsoncheva, M. Dimitrov, 1. Mitov, D. Paneva, H. Huwe and M. Frrba Study of the porosity of montmorillonite pillared with aluminum/cerium M.J. Hernando, C. Blanco, C. Pesquera and E Gonzdlez X-ray absorption fine structure investigation of MCM-41 materials containing Pt and PtSn nanoparticles prepared via direct hydrothermal synthesis C. Pak, N. Yao and G.L. Hailer Ordered assembling of precursors of colloidal faujasite mediated by a cationic surfactant J Agfindez, 1. Diaz, C. Mdrquez-Alvarez, E. Sastre and J P~rez-Pariente Synthesis, characterisation and catalytic activity of SO3H-phenyl-MCM-41 materials F. Mohino, I. Diaz, J. POrez-Pariente and E. Sastre Synthesis of ordered mesoporous and microporous aluminas: strategies for tailoring texture and aluminum coordination V. Gonzdlez-Pe~a, C. Mdrquez-Alvarez, E. Sastre and J P~rez-Pariente Characterization of a heteropolyacid supported on mesoporous silica and its application in the aromatization of a-pinene H. Jaramillo, L.A. Palacio and L. Sierra Catalytic activity, deactivation and re-use of A1-MCM-41 for N-methylation of aniline J.M. Campelo, R.M. Leon, D. Luna, J.M. Marinas and A.A. Romero Restructured V-MCM-41 with non-leaching vanadium and improved hydrothermal stability prepared by secondary synthesis N.K. Mal, P. Kumar, M. Fujiwara and K. Kuraoka Comparative study of MCM-41 acidity by using the integrated molar extinction coefficients for infrared absorption bands of adsorbed ammonia A. Taouli and W. Reschetilowski Confinement of nematic liquid crystals in SBA mesoporous materials L. Frunza, S. Frunza, A. Schrnhals, U. Bentrup, R. Fricke, 1. Pitsch and H. Kosslick Synthesis and characterization of bimetallic Ga,A1-MCM-41 and Fe,A1-MCM-41 R. Bfrjega, C. Nenu, R. Ganea, Gr. Pop, S. "-erban and T. Blasco

1205

1213 1221 1229 1237

1245

1253

1261 1267 1275

1283

1291 1299

1307

1315 1323 1331

xxiv Fischer-Tropsch synthesis on iron catalysts supported on MCM-41 and MCM-41 modified with Cs A.M. Alvarez, J E Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramidn and S. G. Marchetti Coordination and oxidation states of iron incorporated into MCM-41 K. Ldzdr, G. Pdl-Borb~ly, A. Szegedi and H.K. Beyer Synthesis and characterization of In-MCM-41 mesoporous molecular sieves with different Si/In ratios W. Brhlmann, O. Klepel, D. Michel and H. Papp The effect of niobium source used in the synthesis on the properties of NbMCM-41 materials 1. Nowak Inclusion of europium(III) ~-diketonates in mesoporous MCM-41 silica A. Fernandes, J. Dexpert-Ghys, C. Brouca-Cabarrecq, E. Philippot, A. Gleizes, A. Galarneau and D. Brunel Synthesis, characterization and catalytic properties of mesoporous titanostanno silicate, Ti-Sn-MCM-41 N.K. Mal, P. Kumar, M. Fujiwara and K. Kuraoka Alternative synthetic routes for NiA1 layered double hydroxides with alkyl and alkylbenzene sulfonates R. Trujillano, M.J. Holgado and V. Rives Spectroscopic studies on aminopropyl-containing micelle templated silicas. Comparison of grafted and co-condensation routes D. Brunel, A.C. Blanc, E. Garrone, B. Onida, M. Rocchia, JB.Nagy and D. J Macquarrie Preparation, characterization, stability and catalytic reactivity of the 3d transition metals incorporated MCM-41 molecular sieves V. Pdrvulescu and B.L. Su Amine-functionalized SiMCM-41 as carrier for heteropolyacid structures L. Pizzio, P. Vdzquez, A.Kikot and E.Basaldella Acidity of mesoporous aluminophosphates and silicas MCM-41. A combined FTIR and UV-Vis-NIR study E. GianottL V. Dellarocca, E.C. Oliveira, S. Coluccia, H.O. Pastore and L. Marchese Modification of silica walls of mesoporous silicate and alumino-silicate by reaction with benzoyl chloride L. Pasqua, E Testa, R. Aiello, G. Madeo and J. B.Nagy

1339

1347

1355

1363 1371

1379

1387

1395

1403 1411

1419

1427

ADVANCED MATERIALS AND APPLICATIONS Options for the design of structured molecular sieve materials J. Sterte , J. Hedlund and L. Tosheoa (KEYNOTE)

1437

XXV

Chromium containing zeolite beta macrostructures V. Naydenov, L. Tosheva and J Sterte Semiconductor nanoparticles in the channels of mesoporous silica and titania thin films M. Wark, H. Wellmann, J Rathousk~ and A. Zukal Spin-coating induced self-assembly of pure silica and Fe-containing mesoporous films N. Petkov, S. Mintooa and T. Bein Guanidine catalysts supported on micelle templated silicas. New basic catalysts for organic chemistry D.J. Macquarrie, K.A. Utting, D. Brunel, G. Renard and A. Blanc Attempts on generating basic sites on mesoporous materials X.W. Yan, X. W. Han, W.Y. Huang, J.H. Zhu and K. Min Application of zeolite in the health science: novel additive for cigarette to remove N-nitrosamines in smoke Z Xu, Y Wang, JH. Zhu, L.L. Ma, L. Liu and J Xue Direct synthesis of ZSM-5 crystals on gold modified by zirconiumphosphonate multilayers S. Dumrul, J Warzywoda and A. Sacco, Jr. Square root relationship in growth kinetics of silicalite-1 membranes P. Nov6k, L. Brabec, O. Solcov6, O. Bortnovsky, A. Zik[mov6 and M. Ko6i~ik Transport characteristics of zeolite membrane from dynamic experiments A. Zikfnov6, B. Bernauer, V. Fila, P. Hrab6nek, J Hradil, V. Krystl and M. Ko6ifik Incorporation of zeolites in polyimide matrices P. Sysel, M. Fry6ov6, R. Hobzov6, V. Krystl, P. Hrab6nek, B. Bernauer, L. Brabec and M. Ko6i~ik The formation mechanism of ZSM-5 zeolite membranes Y LL J. ShL J Wang and D. Yan Mesoporous molecular sieves for albumin A.Y. Eltekov and N.A. Eltekova Characterizing the novel porous superbase K+/ZrO2 by probe adsorption: a Raman study W.Y. Huang, Y Wang, Q. Wang and Q. Yu The synthesis and characterization of zeolite ZSM-5 and ZSM-35 films by self-transformation of glass J. Dong, W. Fan, G. Liu and J. Li Preparation of mesoporous materials as a support for the immobilization of lipase A. Macario, V. Calabr~, S. Curcio, M. De Paola, G. Giordano, G. lorio and A. Katovic

1449 1457 1465

1473 1481

1489

1497 1505 1513 1521

1529 1537 1545

1553 1561

ADSORPTION, DIFFUSION, SEPARATION AND PERMEATION Adsorption and diffusion of linear and dibranched C6 paraffins in a ZSM-5 zeolite E. Lemaire, A. Decrette, JP. Bellat, JM. Simon, A. M~thioier and E. Jolimaftre

1571

xxvi Adsorption of indole and benzothiophene over zeolites with faujasite structure J.L. Sotelo, M.A. Uguina and V.1. Agueda

1579

Determination of microporous structure of zeolites by t-plot method - State-of-the-art 1587 P. Hudec, A. Smiegkovd, Z. Zidek, P. Schneider and O. Solcov6 Binary mixture adsorption of water and ethanol on silicalite Y. OumL A. Miyajima, J. Miyamoto and T. Sano

1595

Influence of water adsorption on zeolite Beta C. Flego, G. Pazzuconi and C. Perego

1603

Diffusion and adsorption of hydrocarbons from automotive engine exhaust in zeolitic adsorbents D. Caputo, M. Eik and C. Colella

1611

Kinetic processes during sorption and diffusion of aromatic molecules on medium pore zeolites studied by time resolved IR-spectroscopy H. Tanaka, S. Zheng, A. Jentys and J.A. Lercher

1619

Calorimetric study of C2H4 adsorption on synthetic zeolites with Na § and Ca 2§ cations 1627 1. V. Karetina, G.Ju. Zemljanova and S.S. Khvoshchev A1-MCM-48: synthesis and adsorption properties for water, benzene, and nitrogen M. Rozwadowski, M. Lezanska, R. Golembiewski, K. Erdmann and J. Kornatowsla"

1631

A frequency-response study of the kinetics of ammonia sorption in zeolite particles Gy. Onyestydk, J. Valyon and L. V.C. Rees

1639

An attempt to correlate the non-isothermal desorption behavior of heterocyclic compounds on a NaY zeolite B. Hunger, 1.A. Beta, C. Engler, E. Geidel, O. Klepel and H. B6hlig

1647

Determination of diffusion coefficient for Cu(II) retention on chemically activated clinoptilolite R. Pode, T. Todinca, A. lovL R. Radovet and G. Burticd

1655

Dynamics of sorption columns in dewatering of bioethanol using zeolites M. Boldi~, K. Melzoch, J. Pokorny and M. Ko~i~ik

1663

Adsorption properties of MCM-41 materials for the VOCs abatement G. Calleja, D.P. Serrano, J.A. Botas and EJ. Guti~rrez

1671

Adsorption of linear and branched paraffms in silicalite: thermodynamic and kinetic study I. Gener, J. Rigoreau, G. Joly, A. Renaud and S. Mignard

1679

Location and transport properties of ammonia molecules in a series of faujasite zeolite structures as studied by FT-IR and 2H-NMR spectroscopies 1687 E Gilles, J.-L. Blin, H. Toufar and B.L. Su Characterization of mesoporous solids: pore condensation and sorption hysteresis phenomena in mesoporous molecular sieves M. Thommes, R. K6hn and M. Fr6ba

1695

xxvii NATURAL ZEOLITES Ion exchange selectivity of phillipsite A.E GualtierL E. Passaglia and E. Galli Sorption of ammonia from gas streams on clinoptilolite impregnated with inorganic acids K. Ciahotn~, L. Melenov6, H. Jirglov6, M. Boldi~ and M. Ko6iHk Microtopographic features and dissolution behavior of natural zeolite surfaces studied by Atomic Force Microscopy (AFM) M. VoltolinL G. Artioli and M. Moret Occurrence and crystal structure of magnesian chabazite E. Passaglia and O. Ferro Treatment of urban dump leachates with natural zeolite packed bed column T. Rodriguez F., E. Acevedo del Monte, G. Mori and B. Rafuzzi Phosphorous removal from wastewater by bioaugmented activated sludge with different amounts of natural zeolite addition J. Hrenovic and D. Tibljas Zeolitized tufts as pedogenic substrate for soil re-building. Early evolution of zeolite/organic matter proto-horizons A. Buondonno, E. Coppola, M. BuccL G. Battaglia, A. Colella, A. Langella and C. Co#ella Neapolitan yellow tuff for the recovery of soils polluted by potential toxic elements in illegal dumps of Campania Region E. Coppola, G. Battaglia, M. BuccL D. Ceglie, A. Colella, A. Langella, A. Buondonno and C. Colella Application of Jordanian faujasite-phillipsite tuff in ammonium removal K.M. Ibrahim Evidence of the relationship occurring between zeolitization and lithification in the yellow facies of Campanian Ignimbrite (southern Italy) A. Langella, P. De Simone, D. Calcaterra, P. Cappelletti and M. de' Gennaro

1705

1713

1721 1729 1737

1743

1751

1759

1767

1775

ION E X C H A N G E AND MODIFICATION

Characterisation of iron containing molecular sieves - the effect of T-element on Fe species 1785 P. Decyk, M. Trejda, M. Ziolek and A. Lewandowska Thermal decomposition of sodium azide in various microporous materials 1793 Gy. Onyesty6k Modifying the acidic properties of PtZSM-5 and PtY zeolites by appropriately varying reduction methods 1801 A. Tam6sL K. Niesz, 1. P6link6, L. Guczi and 1. Kiricsi

xxviii Vibrational studies of iron phthalocyanines in zeolites P.P.H.JM. Knops-Gerrits, E Thibault-Starzyk and R. Parton

1809

The effect of dealumination on the A1 distribution in pentasil ring zeolites J Dgdedek, V. Grbov6 and B. Wichterlov6

1817

Pb(II) ion exchange on zeolite-supported magnetite. Characterization of process by effective diffusivity coefficient V. Pode, T. Todinca, R. Pode, V. Dalea and E. Popovici

1825

Galliation of beta zeolite by the pH control method Y. Oumi, S. KikuchL S. Nawata, T. Fukushima and T. Sano

1833

Ion exchange behaviour of two synthetic phillipsite-like phases C. Colella, B. de' Gennaro, B. Liguori and E. Torracca

1841

Competitive exchange of lead(II) and cadmium(II) from aqueous solution on clinoptilolite S. Berber-Mendoza, R. Leyva-Ramos, J. Mendoza-Barron and R.M. GuerreroCoronado Competitive ion exchange of transition metals in low silica zeolites C. Weidenthaler, Y Mao and W. Schmidt

1849

1857

STRUCTURE ANALYSIS AND MODELLING Computational methods for the design of zeolitic materials M. Elanany, K. Sasata, T. Yokosuka, S. Takami, M. Kubo and A. Miyamoto

1867

(KEYNOTE)

A theoretical investigation on pressure-induced changes in the vibrational spectrum of 1877 zeolite bikitaite E. Fois, A. Gamba, G. TabacchL O. Ferro, S. Quartieri and G. Vezzalini Flexible aluminium coordination of zeolites as function of temperature and water content, an in-situ method to determine aluminium coordinations J.A. van Bokhoven, A.M.J. van der Eerden and D.C. Koningsberger

1885

Structure analysis of boron-silicalite and of a "defect-free" MFI-silicalite by synchrotron radiation single crystal X-ray diffraction M. Milanesio, D. Viterbo, L. Palin, G.L. Marra, C. Lamberti, R. Aiello and E Testa

1891

Density functional theory modelling EPR spectra of Cu(II) in Y zeolite D. Berthomieu, J.M. Duc~r~ and A. Goursot

1899

Molecular modeling: a complement to experiment for designing porous materials used 1907 in separation technologies by adsorption S. Girard, C. Mellot-Draznieks, G. FOrey and P. Pullumbi NMR-crystallographic studies of aluminophosphate A1PO4-40 C.M. Morais, C. Fernandez, V. Montouillout, E Taulelle and J. Rocha

1915

xxix Structural characterization of borosilicates synthesized in the presence of ethylenediamine S. Zanardi, A. Alberti, R. Millini, G. Bellussi and G. Perego Molecular dynamics simulations of water confined in zeolites P. Demontis, G. Stara and G.B. Suffritti EXAFS and optical spectroscopy characterisation of silver within zeolite matrices S.G. Fiddy, N.E. Bogdanchikova, V.P. Petranooskii, J.S. Ogden and M.Avalos-Borja Molecular dynamics simulations of static and dynamic properties of water adsorbed in chabazite S. Jost, S. Fritzsche and R. Haberlandt Correlations in anisotropic diffusion of guest molecules in silicalite-1 S. Fritzsche and J. Kiirger A combined anomalous XRPD, EXAFS, IR, UV-Vis and photoluminescence study on isolated and clustered silver species in Y zeolite C. Prestipino, C. Lamberti, A. Zecchina, S. Cresi, S. Bordiga, L. Palin, A.N. Fitch, P. Perlo and G.L. Marra DFT and IR studies on copper sites in CuZSM-5: structure-redox conditionsdenox activity relationship E. Broctawik, J. Datka, B. Gil and P. Kozyra Diffusion of water in silicalite by molecular dynamics simulations: ab initio based interactions C. Bussai, S. Hannongbua, S.Fritzsche and R. Haberlandt Comparison of small size alumino- and borosilicates optimised by periodic Hartree-Fock A. V. Larin and D.P. Vercauteren Monte Carlo simulation of the temperature dependence of adsorption of nitrogen and oxygen by LiLSX zeolite S.R. Jale, D. Shen, M. Biilow and ER. Fitch Density functional theory calculations of Henry's constant for N2, 02 and Ar molecules in Ca-A and Ca-LSX zeolites G. De Luca, P. Pullumbi and N. Russo Author index Subject index

1923 1931 1939

1947 1955

1963

1971

1979

1987

1995

2003

2011 2021


Z E O L I T E SYNTHESIS AND C H A R A C T E R I Z A T I O N


Studies in Surface Science and Catalysis 142 R. AieUo, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Zeolite characterization with spectroscopic m e t h o d s A. Zecchina 1'2, G. Spoto 1'2, G. Ricchiardi 1'2, S. Bordiga 1'2'3, F. Bonino 1, C. Prestipino 1'3 and C. Lamberti 1'2'3 1 Department of Inorganic, Physical and Material Chemistry, Via P. Giuria 7, 10125 Turin (I) 2 INSTM Unith di Torino 3 INFM Unit~ di Torino Universit~ (I)

Among the different spectroscopic techniques reported in the plenary lecture presented at the FEZA 2002 conference, in this work, we will focus on IR spectroscopy only, devoted to the characterization of the acid strength of the BrCnsted groups in zeolites In particular, in this brief review we will show how the systematic study of the spectroscopic manifestations observed by IR spectroscopy upon dosing to H-zeolites probe molecules with increasing proton affinity will allow to establish a spectroscopic acidity scale for the BrOnsted groups hosted in different zeolites. 1. INTRODUCTION It is universally recognized that the BrCnsted groups represent the most reactive species present in the channels and cavities and that most of the catalytic properties of the zeolites in the acidic form can be ascribed to these species [ 1-3]. The ability of the BrCnsted groups to interact with molecules entering the channels has been the subject of many investigations with physical techniques and, among them, vibrational spectroscopy has definitely played the major role in elucidating the interaction mechanisms and the structure of the formed species. In particular, one of the key questions associated with the activity of the BrCnsted groups present in the zeolite channels is related to the estimation of their acid strength and to the comparison with that of the most common mineral acids and superacids [4].To illustrate in a systematic way the results obtained by the application of vibrational spectroscopy, a useful approach is to describe the interaction of the BrCnsted site of zeolite 1~ (chosen as a prototype system) with molecules characterised by a proton affinity (PA) ranging in a very broad interval. To this end the following sequence of molecules (ordered following their PA) will be illustrated: N2 (PA = 118.2 kcal mol-1), CO (PA = 141.9 kcal mol-1), C2H4 (PA = 162.6 kcal mol-1), C3H6 (PA = 179.5 kcal mol-i), H20 (PA = 166.5 kcal mol-1), CH3CN (PA = 188.6 kcal mol-1), CH3OH (PA = 181.9 kcal mol-1), (CH3)20 (PA = 192.1 kcal mol-1), THF (PA = 196 kcal mol 1) and Py (PA = 204 kcal mo1-1) [5]. All these molecules have a basic character increasing along the sequence, and when appropriate temperature and pressure conditions are adopted, they can form hydrogen bonded adducts whit the BrCnsted groups, as shown in Scheme 1.

B!

i/i l

0,,.. /Ox, ..,,0

o':S

0 0

+B

0,,, /([Ix

o=S ,

OO

,,0 Scheme 1

The temperature conditions allowing the formation of the internal acid-base adducts change gradually in the above series from 77 K to 300 K. In fact the N2 and CO (which are very weak bases) adducts are stable only at very low T, while the adducts (or the salts) formed by interaction with the bases at the end of the series are stable at RT. The adducts formed by the acid-base reaction illustrated in scheme 1, have stretching and bending modes which differ from those of the original BrCnsted group because the hydrogen bonding perturbation is usually associated with profound modifications of the v(OH), 8(OH) and ),(OH) vibrational frequencies and minor changes of the internal modes of the bases B. In this brief review we want to show that the systematic study of these modifications form the basis of the so called spectroscopic method for the estimation of the acid strength of the BrCnsted groups in zeolites [6-15]. 2. D I S C U S S I O N

By starting from the molecules with lower PA, Figure la and lc report the modifications induced by the hydrogen bonding perturbation on the v(OH) stretching mode of the BrCnsted groups of 13 zeolite as function of the pressure of N2 and CO respectively. From the spectra it is clearly emerging that upon dosage of the base B the v(OH) mode of unperturbed groups (band at 3614 cm -1) is gradually consumed while that of the v(OH.-.B) vibration (shifted to lower frequency) simultaneously shows up. The clear isosbestic points observed in both spectra ensure that the 1:1 process illustrated in Scheme 1 is really occurring in a stoichiometric way. Other important observations are: (i) the negative shift A V(OH) increases on passing from N2 (AV = -126 c m -1) to C O (A~- = -319 cm-1), i.e. with the proton affinity of the base; (ii) the full width at half maximum (FWHM) of the v(OH) mode increases on passing from the unperturbed BrCnsted groups (FWHM -- 20 cm 1) to the N2 (FWHM -- 85 cm -1) and CO (FWHM -- 220 cm -1) adducts. It can be easily verified that the FWHM is roughly 90of the shift AV. The resuks illustrated in a) and b) are the typical ones expected for the presence of linear hydrogen bonds [16,17] and represent further and clear demonstration of the formation of 1:1 adducts. Following the immense literature on the IR spectroscopy of the hydrogen-bonded systems [ 16-18], the shift to lower frequency and the increase of the bandwidth are due to the decrease of the force constant induced by the polarisation of the O-H bond and by coupling of the v(OH) with the v(O...B) modes of the adducts, which consequently can be better expressed as v(OH.--B) + v (O.-.B). As briefly mentioned before, the formation of the hydrogen bonded adducts can be accompanied also by a perturbation of the internal modes of the base (in the present case the N-N or the C-O stretching modes).

(a)

(b) 0.02

!

3750 3 5 0 0 3250 W a v e n u m b e r (cm 4)

I

i Io. a.u. i 1

, (c)

2360

a.u.

23'40'23'20

Wavenumber (cm-1)

(d)

I'll

i

3750 3 5 0 0 3250 Wavenumber (crn4)

' 22'oo ' 2~'so ' 21'oo Wavenumber (crn4)

Fig.1. IR spectra of increasing equilibrium pressures of N2 and CO adsorbed at liquid nitrogen temperature on activated H-I3 zeolite, parts (a,b) and (c,d) respectively. (a), (c): O-H stretching region. (b): N-N stretching region. (d) C-O stretching region. In each part the dotted line spectrum is that recorded before gas dosage.

As far as carbon monoxide is concerned, the perturbation of the v(CO) mode upon formation of the OH...CO adduct is shown in Figure ld. It is clear that the stretching frequency undergoes a consistent blue shift (AV (CO) = +34 cm 1) with respect to the gas phase value. This is the result of the CO bond polarisation subsequent to the hydrogen bond formation. An analogous result has been obtained for the adducts of CO with Na +, K +, Cs +, Ag + cations in zeolites, a subject which will be extensively discussed in refs. [ 19-26]. At highest filling conditions, also the silanols located on the external surfaces of the microcrystals or at internal defects form hydrogen bonded adducts with CO. The shift induced on the v(OH) stretching frequency of the silanols is definitely smaller (AV = -90 cm -1) than that observed for the BrCnsted sites: this is simply the consequence of the fact the shift of the v(OH) mode of the acid centres caused by the interaction with a given base is related to the acid strength of the group itself. In other words this different response simply reflect the fact that the OH groups of the structural BrCnsted sites are much stronger acid than the OH groups of the silanols. We will demonstrate in the following that this observation can be supported by a large amount of experimental observations obtained with different bases and different zeolites, so proving its general validity. We anticipate that this general correlation, which is the extension to heterogeneous systems of the well known Bellamy-Hallam-Williams (BHW) relation extensively documented in homogeneous phase [27-28], will form the basis of the spectroscopic method for the acid strength evaluation of the BrCnsted groups of zeolitic systems. Finally, notice also the band at 2138 cm -1 in Figure ld, favored at the highest dosages, which is due to liquid-like CO physically adsorbed in the channels and only interacting with the hydroxyl free, homopolar part of the internal surface [19-21]. Although very weak, mention must be made also of the peaks at 2230 cm -1 of adsorbed CO (Figure ld), because it indicates that in the 13-zeolite treated under vacuum at 673 K a small fraction of sites with very large polarising character are present which can represent potential sites for acid catalysed reactions. These sites, whose concentration is strongly influenced by the thermal treatments and can vary from one sample to the other, are A13+ ions in trigonal coordination deriving from the thermally induced migration of framework Al atoms into partial framework position [19-21]. Coming back to adsorbed nitrogen (Figure lab), it is worth noticing that although the interaction of nitrogen with the structural BrCnsted groups is very weak, the induced polarisation of the N-N bond (only RAMAN active in the gas phase) is sufficient to make the v(NN) mode of the OH.-.NN adduct slightly IR active and hence to originate an appreciable absorption in the 2400-2300 cm -1 range. The v(NN) stretching frequencies of the adducts with structural BrCnsted groups (2330 cm 1) and with silanols (2325 cm -1) are upward shifted as expected [29]. On the basis of the literature concerning homogeneous systems the formation of hydrogen bonded adducts should be accompanied by an upward of the ~5 and ~, modes. Unfortunately, due the overshadowing effect of the skeletal vibrations, it was not possible to measure the effect of the formation of the hydrogen bonded species on the ~5and T modes. In this review article it not possible to continue in the same detailed way the description of the spectra obtained with molecules like ethene, propene, acethylene, etc. which come immediately after N2 and CO in the PA scale. We consequently move to acetonitrile (PA = 188.6 kcal mol-1). The reasons of this choice are twice: i) the acetonitrile probe (CH3CN and CD3CN) has been studied extensively over a great variety of zeolites [4,30-36]; ii) the acetonitrile-zeolite complex is characterised by a complex spectroscopy generated by Fermi resonance effects. As these effects are dominant in the spectra of the adducts of structural BrCnsted sites with bases of medium-strong PA, their detailed illustration for the acetonitrile

complexes can be useful for the comprehension of a great variety of experiments involving different and stronger bases. The spectra of increasing doses of deuterated acetonotrile adsorbed on [~-zeolite [36] are illustrated in Figure 2.

l'+

0.1 a.u.

II I 41

++

db +t, I, +.

41.

,l

9 .+

i

ii '%

.I ++41+

41111

i. 9 ,i+

,i,. ,i. l,

l,

,i.

+

::

3500

A ":. ..B " i

3000

2500

C

2000

1500

Wavenumber (crn Fig.2. IR spectra of increasing equilibrium pressures of CD3CN adsorbed on H-I~ zeolite. Solid line spectra 1-9 refer to CD3CN equilibrium pressures in the 0-10-1 Torr interval, while the dotted line one refers to a much higher pressure (30 Torr). Labels A, B and C denote the three components due to Fermi resonance effects (see text). As found before for N2 and CO we observe the progressive erosion of the structural BrCnsted groups because of the formation of hydrogen bonded adducts (full line spectra in Figure 2); at the highest filling conditions also the band due the silanol groups is eroded (dotted line spectrum). While upon interaction with the nitrile molecule the silanol band originates a broad peak shifted at lower frequency ( AV = -345 cm-1; FWHM = 260 cm-1), two absorptions with apparent maxima at 2856 and at 2452 cm -1 (hereafter named A and B respectively) originate from the structural BrCnsted peak (instead of the single one expected on the basis of the previous results). Other relevant features of the spectra illustrated in Figure 2 are: i) the v(CN) modes of the structural BrCnsted groups and of the weaker silanols are found at 2297 and 2275 cm 1 respectively (i.e. at frequencies higher than those of the free molecule); ii) a novel band at 1325 cm 1 shows up with coverage which can be ascribed to the 8 mode of the BrCnsted-acetonitrile group. The last result demonstrates that the interaction has become sufficiently strong to shift the 8 mode in a frequency range not dominated by the framework vibrations (a fact which makes it observable). The observation of the precise position of the 8 mode gives us the key for the explanation of the presence of A-B doublet. In fact as the minimum separating the A and B partners is observed at a frequency corresponding

to the twice of the 8 mode, it can be readily inferred that it corresponds to the Evans window generated by Fermi resonance effect between the v(OH...B) _+v(O-.-B) mode centred at 2680 cm "l (FWHM = 750 em "1) and the 28 overtone. This explanation finds justification in the abundant spectroscopic literature of hydrogen bonded systems [5,17,18,30] and on the observations concerning adduets with stronger bases (vide infra). In Figure 2 also a band at 1680 cm "1 (labeled with the symbol C) is clearly evident. A similar band is observed for aeetonitrile on H-ZSM-5 and H-MOR [30]. The assignment of this peak will be given in the following after a general introduction to the Fermi resonance effects in hydrogen bonded systems.

Fig.3. Qualitative representation of the IR spectra of weak (a-c), medium (d-f) and strong (g-h) A-H...B or A'---H-B§ H-bonded complexes. The grey areas correspond to regions obscured by the skeletal modes of the zeolite frameworks. For each spectrum the evolution of the proton potential as a function of the A-H distance is also schematically illustrated (fight).

To guide the understanding of the resuks obtained with other bases of larger proton affinity, we think that it is useful to represent schematically the dependence of the v, 8, )', 28, 2)' frequencies upon the O...B distance (Figure 3) used as measure of the strength of the acid base interaction. This dependence has been somewhat freely deduced from the literature data concerning homogeneous compounds and does not have fully quantitative meaning. Notwithstanding this fact, it can be successfully utilized to illustrate the IR spectroscopy of the hydrogen bonding interactions occurring in the zeolites. From this Figure 3 the following seven important points can be underlined: (i) The frequency of the v(OH.-.B) + v(O...B) mode decreases gradually following the well known curve established for homogenous compounds [15]; the shift AV is accompanied by a progressive broadening of the band (FWFM ___-90 AV); (ii) The frequencies of the ~5(OH-.B) and ),(OH...B) modes behaves in a opposite way and the same do the 28 and 2), overtones (the upward shifts are however definitely smaller); (iii) When the frequency of the 28 overtone falls within the stretching band a Fermi resonance occurs with formation of a Evans window and doubling of the peak (A and B bands) [37,38]; (iv) When both the frequencies of the 28 and 2), overtones fall within the v(OH...B) band, the broad v(OH-.-B) band is partitioned by the Fermi resonance into three peaks (named A, B and C) [38]. The relative intensity of the C band with respect to the A and B doublet increases as the strength of the hydrogen bond increases and becomes gradually dominant; (v) For the strongest hydrogen bonding interactions (i.e. for negative shifts of the stretching mode of the order of 2000 cm -1) the v, ~5 and y curves directly intersect. Under these circumstances, corresponding to a fiat potential well characterized by a single minimum [39], direct mixing is occurring and distinction between v, 8 and ), modes becomes impossible. Following the homogeneous literature [17,18] this condition corresponds to that of an hesitating proton; (vi) For strongest hydrogen bonds the downwards shifted v(OH) fall in the range typical of the internal modes of the base B: this fact can further complicate the assignment of the IR spectra; (vii) When the base B approaches proton affinity values near to 200 kcal mo1-1, proton tranfer occurs with formation of hydrogen bonded BH § This is for instance the case for NH3 [40] and Py [5,41] which lead to the formation of Z-.-.H-NH3 § and Z-...HPy+ adducts. As Z- is a weak base (the conjugated acid is strong) and NH4§ and PyH § are also weak acids, the acid-base hydrogen bonding interaction is weak and the shift A V of the v(NH) mode is consequently that typical of a weak interaction. As the PA of Z- is lower than that of NH3 and Py, it is quite conceivable that at the highest filling conditions also (H3N-H.-.NH3) § and (Py-H...Py) § dimers can be present in the zeolite channels. On the basis of these considerations we can now understand the essential features of the sequence of spectra reported in Figure 4, where the IR spectra of the interaction products of series of bases (ranging from N2 to Py) of increasing PA with zeolite H-13 are illustrated [5]. The gradual shift to lower frequency of the broad absorption associated with the perturbed OH group is well documented. The formation and the evolution of the A, B and C peaks and of their baricenter upon the change of the adsorbate and of the proton affinity is also clearly emerging. Notice how the greatest shift is occurring for THF (PA = 196 kcal mo1-1) and how the Py shifts is definitely smaller (because of protonation). Very similar resuks have been obtained on H-MOR, on HZSM-5 and on H-Y [30,41,42] so showing that the considerations illustrated before have general character. As for every adsorbate with proton affinity ranging from 118 to 196 kcal/mole we can determine the shift A V of the BrCnsted sites and of the silanols, we have the possibility to

10 plot them in a XY diagram (Figure 5) and to verify if the BHW relation, whose validity has been well established in solution [27,28], is also holding for the hydrogen bonding interactions occurring in the zeolite channels. 9

i

eta +

...

N2

CO H4

t J (D O t,-

[

CH,,CN _ /~

t~

L_

O r

OH

,
90% (from IH and 13C NMR). The bromide was then dissolved in deionized water, exchanged in O H form by electrodialysis, and roto-evaporated, after which the concentration was determined by acid titration. Zeolite syntheses were performed according to the following general recipe. The source of silica (Sylobloc 47) was added under vigorous stirring to an aqueous solution containing NaOH, EASU-OH, H2SO4 and, when required, A12(804)3" 16H20. The resulting homogeneous suspension was charged into a stainless steel oscillating autoclave and heated at 443 K for 7 14 days. The crystalline product was filtered, dried at 393 K and finally calcined at 823 K. 2.2. Characterization A1 content was determined by ICP-AES using a Thermo Jarrell Ash spectrometer; SiO2 was determined gravimetrically; quantitative C, H and N analyses were carded out with a Perkin Elmer 2400 analyzer. Sample morphology of the samples was examined by Scanning Electron Microscopy (SEM), using a Jeol LV5400 microscope operating at 25 keV accelerating voltage. The standard gold coating technique was used to avoid sample charging. Thermogravimetric (TG) analyses were carded out over the 293 - 1173 K interval with 10 K'min 1 heating rate and 300 ml-minl air flow using a Mettler TG50 thermobalance, controlled with a Mettler TC 11A microprocessor. Solid state NMR spectra (1H decoupled) were obtained with samples (200 mg) in 7 mm zirconia rotors, undergoing magic angle spinning (MAS) at 5 ld-tz on a Bruker ASX-300 spectrometer. 13C and 29Si shifts were referenced to adamantane (39.5, 29.5 ppm) and tetrakis(trimethylsilyl)silane (-9.8 and -135.2 ppm), respectively. X-Ray powder diffraction (XRD) data were recorded on a Philips X'PERT diffractometer over the 3 < 20 < 53 ~ angular region, with steps of 0.02 ~ 20 and 20 s/step accumulation time; the CuKo~ radiation (~, = 1.54178 A) was used. SXPD data were collected at room temperature on a calcined, pure-silica MEL sample loaded in a Lindemann capillary (0.3 mm i.d.), rotating at 1 Hz, at the GILDA beamline BM08 of the ESRF (Grenoble) during the experiment 08-02-174. Details about the setting of the beamline and of data collection and treatment are given elsewhere [ 16]. A wavelength of ~, = 0.82714(2) A was used and the data were collected on a Fuji Image Plate located 204.83 mm from the sample and perpendicular to the incident beam. The elaboration of the scanned digital image was carded out with the Fit2d software package [ 17] and the diffraction data finally converted to into a conventional 1/20 profile, covering the 3.5 < 20 < 55 ~ angular range

63 with fixed angular steps of 0.028452 ~ 20. Rietveld refinement of the SXPD pattern was carded out with the GSAS software package [18]. The structure model of the MEL framework was that reported in [ 13]. 2.3. C o m p u t a t i o n a l details

The search for the minimum energy location of the ASU and EASU cations within the MEL, MFI, MTW and MOR porous structures was performed with the Monte Carlo docking procedure proposed by Freeman et al. [19]. 100 SDA conformations were periodically extracted from a 0.2 ps long high temperature (1500 K) Molecular Dynamics (MD) trajectory of the isolated SDA molecule, and randomly docked within the zeolite model. Finally, energy minimization of the crudely docked structures gave a representation of the low energy sites and an estimation of the binding energy (B.E.). The search was successively refined using the recently developed Quench Dynamics protocol, which allows the complete inspection of the conformational space of the docked molecules to be performed [20]. For each Zeolite/SDA conformation, a 100 ps long MD simulation was run in the NVT ensemble at 3000 K; every 200 fs, the MD run was interrupted and the resulting conformation was minimized and stored in an archive file for successive elaboration. The simulation was run maintaining the purely siliceous MFI framework fixed. Monte Carlo docking calculations were performed with the MSI Catalysis 4.0.0 software package, employing the cflgl__czeo forcefield. Quench Dynamics calculations wee carded out with the MSI Cerius 2 program All the calculations were performed on a Silicon Graphics Octane workstation with the MSI Cerius 2 4.2 Materials Science software package, using the COMPASS forcefield [21 ].

Table 1 Description of the most representative syntheses. Sample 8iO2/A1203 Ri/SiO2 Na/SiO2 H20/SiO2 1 -0.2 0.1 45 2 -0.2 0.1 45 3 -0.2 0.1 45 4 100 0.2 0.1 45 5 100 0.2 0.1 45 6 100 0.2 0.1 45 7 50 0.2 0.1 45 8 25 0.2 0.1 45 9 ~ 0.2 0.1 20 10 ~ 0.2 0.1 30

pH 12.1 12.1 12.1 11.9 11.9 11.9 12.5 11.4 12.0 11.9

T (K) Time(d) Products 14 A M ii 428 443 7 AM + MEL 443 14 MEL 428 14 AM 443 7 AM + MEL 443 14 MEL 443 14 AM 443 14 AM 443 14 Quartz 443 14 Quartz 11 -0.2 0.1 60 11.7 443 14 AM + MEL 12 100 0.2 0.1 30 12.4 443 7 AM + MEL 13 100 0.2 0.1 30 12.4 443 14 MEL 14 ~ 0.2 -30 12.9 443 14 AM + MEL 15 -0.35 0.1 45 12.0 443 4 SSZ-31 + MEL 16 ~ 0.35 0.1 45 12.0 443 6 MEL + SSZ-31 17 ~ 0.35 0.1 45 12.0 443 7 MEL i 1-ethyl-6-azonia-spiro-[5,5]-undecane hydroxide; ii Amorphous

64

d

b

e~

C

k

H

g 3

i J

g

'

-'""- - i

.pp.,. e ' 0 " 6'o" " "4'0" " a ' 0 " " 6 Figure 1. ]3C CP-MAS NMR spectrmn of as-synthesized sample 3.

I

. . . .

I

. . . .

-105

I

. . . .

-110

I

"

-115

"'"

"

'/

"

-120

"""

"

I"

"

-125

" " ' ' "

i

zmm

Figure 2. 298i MAN NMR spectnnn of calcined sample 3.

3. RESULTS AND DISCUSSION 3.1. Synthesis Before describing the influence of the synthesis parameters, it must be pointed out that the crystallization of MEL is sensitive to the source of silica used, since it was preferentially obtained with Sylobloc 47 or TEOS. When Aerosil 200 or Ludox AS40 were used, amorphous and layered phases were mainly recovered, while with sodium silicate the crystallization of MOR was observed. All the representative syntheses described in Table 1 were performed using Sylobloc 47 as silica source. Pure MEL can be obtained both as purely siliceous and as aluminosilicate phase, provided that the molar SIO2/A1203ratio in the reaction mixture is kept _> 100 and the hydrothermal treatment is performed at 443 K for 14 days (Table 1). No crystalline phases were obtained from aluminum-reach reaction mixtures (runs 7 and 8) or by decreasing the temperature to 428 K (runs 1 and 4). Another critical parameter is the H20/SiO2 molar ratio, which should be close to 45. Quartz was in fact obtained from more concentrated aluminum-free reaction mixtures (runs 9 and 10) while partially crystallized MEL was recovered upon increasing the H20/SiO2 molar ratio to 60 (run 12). It is interesting to note that in the presence of aluminum the H20/SiO2 molar ratio can be decreased to 30 (run 13) without any influence, however, on the crystallization kinetics. Upon increasing the EASU/SiO2 molar ratio to 0.35 a different behavior was observed, in the sense that pure MEL is obtained only after 7 days of hydrothermal treatment and its formation occurs through the consumption of SSZ-31 which is formed first (runs 15-17). 3.2. Physico-chemical and structural analysis In the pure silica system, defect-free MEL crystallizes in the form of elongated tetragonal prisms terminating with tetragonal pyramids (Figure 1). When A1 is added to the synthesis gel, the shape of the crystals is maintained but the average dimensions decrease. TG analysis showed that -- 4 EASU molecules/unit cell are decomposed in an unique and sharp step in the range 6 0 0 - 800 K (peak temperature in the DTG pattern at 698 K) (Figure 2). The decomposition of the organic molecules occurs without any apparent loss of crystallinity.

65

100

i / ~

95

m

--0.002

,..1

g - -0.004

85 300

i 400

,

~'

~

500

600

700

"

Temp.

Figure. 3. SEM micrograph of pure silica MEL sample 3.

i

~

i

Ii

800

900

1000

1 00

-0.006

(K)

Figure 4. TG (--) and DTG ( ..... ) traces of pure silica MEL sample 3.

Solid state 13C NMR spectroscopy of the as-synthesized MEL catalyst confirmed that the SDA (EASU) remains intact within the pores of the crystalline product (Figure 3). The splitting of signal a in figure X is attributed to two different (chiral) orientations around this carbon. High-resolution 29Si MAS NMR studies of Fyfe and co-workers [9-11] revealed that between 283 and 333 K dealuminated ZSM-11 undergoes a displacive phase transition. This temperature dependence of the lattice structure gave six 29 Si peaks at 373 K and eight peaks at 263 K, implying a lowering of the unit cell symmetry with decreasing temperature from the I4m2 to the 1-4 space group. 29Si MAS NMR spectroscopy of the purely siliceous sample 3 revealed three composite peaks (centered at - 111.6, - 113.8 and - 116.3 ppm), broader than those reported previously [9-11 ]. The poor resolution of this spectrum (Figure 4), compared to Fyfe and co-workers, is presumably due to small deviations from perfect ordering not easily detected by XRD. Highly resolved spectra were obtained only aider steaming highly crystalline ZSM-11 at 1023 K [9-11]. According to them, at room temperature, the most suitable structural model would be the low-temperature one (space group 1-4). However, Terasaki et al. found that the SXPD pattern of a pure silica MEL sample can be nicely refined with both the I-4m2 and 1-4 models and that the refined structures display very similar

2-Theta

[~!

~.LL

A,

.........................

5

! .... 10

i .... 15

i,, 20

,i

....

i ....

2~

30

2-Theta

[o]

i ....

35

i ....

40

"__ "

7: . _'?-'_'~

i ....

45

50

$

10

15

20

25

30

2-The~

35

40

45

50

55

[o]

Figure 5. (A) XRD pattem of calcined MEL sample 3; the inset shows the sharp (110) reflection. (B) Experimental (+), calculated (--) and difference (lower) SXPD profiles for calcined MEL sample 3. Vertical bars indicate the positions of Bragg reflections.

66 features [ 13]. van Koninsgveld et al. in the single crystal X-ray structure refinement of assynthesized MEL reached the same conclusion [ 14]. Figure 5A shows the typical XRD pattern of a calcined MEL sample. High crystallinity permitted the presence of the weak 110 reflections at 20 = 6.2 ~ to be monitored. Since the intensity of this reflection rapidly decreases with increasing amounts of MFI intergrowth, its observation indicates that the samples possess a defect-free MEL type structure [12]. This conclusion was confirmed by the Rietveld refinement of the SXPD data, carded out in the topological I-4m2 tetragonal space group, starting from the structural model reported by Terasaki et al. [13]. This model proved to be suitable for reproducing the experimental SXPD pattern, as demonstrated by the final Rietveld plot reported in Figure 5B. The main crystallographic and geometric data are summarized in Table 2. The fractional atomic coordinates (available upon request) are very close to those reported in ref. 13. It must be pointed out that the Si-O bond distances are spread over a narrow range (1.594- 1.602 A) as a consequence of the soft constraint imposed on these distances during the refinement ( d s i - o = 1.60 + 0.01). On the contrary, no constraints were imposed on the O-O distances and, in spite of that, the O-T-O angles are close to the value expected for the tetrahedral coordination (Table 2). Table 2 Main crystallographic Space Group a (A) c (,~) V (.~3) Rp Rwp R(F 2) Red. X2 No. Reflections

and geometric data. I-4m2 (#119) 20.0777(3) 13.4154(2) 5407.9(2) 0.0181 0.0276 0.0703 1.213 1156

T-O distance Mean: min / max: O-T-O angle Mean: min / max: T-O-T angle Mean: min / max:

1.599(2) 1.594 - 1.602,4, 109.5 ~ 105.0 - 111.6 ~ 152.6 ~ 138.0 - 173.7 ~

3.3. Modeling studies In a previous paper, we have shown that, depending from the SIO2/A1203 molar ratio in the reaction mixtm'e, 6-azonia-spiro-[5,5]-undecane (ASU), the unsubstituted parent SDA of EASU, favors the formation of MTW, MOR and of ERS-10, a zeolite with a still unknown framework structure [22]. Here we demonstrate how the simple substitution of a hydrogen atom by an ethyl group in {x-position produces completely different results, in the sense that, apart from the partial crystallization of SSZ-31, defect-free MEL is selectively favored by the new SDA. In order to understand the reasons for this selectivity, we have carded out a molecular modeling study to determine minimum energy location and energetic of EASU in the different microporous structures. The framework structures considered were MEL, MOR, MTW and MFI, the latter was included because of the structural analogy with MEL. The nonbonding (van der Waals) energies (Ewdw) for the EASU cation in the different zeolites are listed in Tab. 3. According to these data, the EASU cation strongly stabilizes the MEL

67

MEL

MH

Figure 6. Lowest energy conformations of EASU cation in MEL and MFI.

structure by 3 1 . 9 - 34.1 kJ-moll relative to MOR and MTW and 46.0 kJ.mol l relative to MFI. It is worth noting that, among the zeolite frameworks considered, MFI is predicted to be the least stabilized by EASU and this is in agreement with the lack of i-type of stacking in these crystals (see above). An inspection of the lowest energy conformations of EASU in MEL and MFI revealed that this cation nicely fits the intersections between the straight 10MR channels of MEL (Figure 6). The same is not true in the case of MFI, in which the cation lies again at the channel intersection but with the two rings developing along the linear 10MR channels (Figure 6).

Table 3 Non-bonding (van der Waals) energy for the different zeolite/EASU and ASU systems. Zeolites experimentally obtained with the two SDA's are marked in bold. ZEOLITE EASU ASU Evdw(kJ'mol"1) AEvdwi(kJ.mol"1) Evdw(kJ-mol"1) AEvdwi(kJ.mo1-1) MEL -132.2 0 -109.0 0 MOR - 100.3 +31.9 -100.3 +8.7 MTW -98.1 +34.1 -103.6 +5.4 MFI -86.2 +46.0 -87.0 +22.0 i Non-bonding energy relative to MEL/EASU and MEL/ASU systems

According to the data reported in Table 3, ASU seems to stabilize MEL better than the experimentally obtained MOR and MTW by 8.7 and 5.4 kJ.mol l, respectively. This contradiction can be explained by considering that the latter two zeolites are further stabilized by the effective packing of ASU molecules in the straight 12MR channels [23]. This stabilization is expected to be larger than that occurring in MEL, where the ASU cations are preferentially located at the channel intersections.

68 4. CONCLUSIONS A new SDA, 1-ethyl-6-azonia-spiro-[5,5]-undecane (EASU), was used for synthesizing pure defect-free MEL-type zeolites. The absence of intergrowth phenomena with MFI-type zeolite was confirmed by Rietveld refinement of the SXPD data and by the presence of the weak and sharp (110) reflection in the XRD pattern. The high specificity of EASU towards the crystallization of MEL was confi17ned by molecular modeling studies.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8.

P. Chu, U. S. Patent 3 709 979 (1973) assigned to Mobil. L.D. Rollmann and E. W. Valyoesik, U. S. Patent 4 108 881 (1978) assigned to Mobil. J.P. MeWilliams and M. K. Rubin, U. S. Patent 4 894 212 (1989) assigned to Mobil. E.W. Valyoesik, U. S. Patent 4 914 963 (1990) assigned to Mobil. J.S. Beck and J. D. Schlenker, U. S. Patent 5 213 786 (1993) assigned to Mobil. G.T. Kokotailo, P. Chu and S. L. Lawton, W. M. Meier, Nature 275 (1978) 199. G. Perego, M. Cesari and G. Allegra, J. Appl. Cryst. 17 (1984) 403. G. Perego, G. Bellussi, A. Carati, R. Millini and V. Fattore, in: Zeolite Synthesis. ACS Symposium Series no. 398 (M. L. Occelli and H. E. Robson Eds.), American Chemical Society, Washington DC, 1989, p. 360. 9. B.H. Toby, M. M. Eddy, C. A. Fyfe, G. T. Kokotailo, H. Strobl and D. E. Cox, J. Mater. Res. 3 (1988) 563. 10. C. A. Fyfe, H. Gies, G. T. Kokotailo, C. Pasztor, H. Strobl and D. E. Cox, J. Am. Chem. Soc. 111 (1989) 2470. 11. C. A. Fyfe, Y. Feng, H. Grondey, G. T. Kokotailo and A. Mar, J. Phys. Chem. 95 (1991) 3747. 12. Y. Nakagawa, WO Patent 95/09812 (1995) assigned to Chevron. 13. O. Terasaki, T. Ahsuna, H. Sakuma, D. Watanabe, Y. Nakagawa and R. C. Medrud, Chem. Mater. 8 (1996) 463. 14. H. van Koningsveld, M. J. Den Exter, J. H. Koegler, C. D. Laman, S. L. Njo and H. Graafsma, in Proc. 12th Intern. Zeolite Conf. (M. M. J. Treacy, B. K. Markus, M. E. Bisher and J. B. Higgins Eds.), MRS, Warrandale (PA), 1999, p. 2419. 15. P. M. Piccione and M. Davis, Microporous Mesoporous Mater. 49 (2001) 163. 16. R. Millini, G. Perego, D. Berti, W.O. Parker, Jr., A. Carati and G. Bellussi, Microporous Mesoporous Mater. 35-36 (2000) 387. 17. A. Larson and R. B. Von Dreele, GSAS Manual, Los Alamos Report No. LAUR-86-748, Los Alamos National Laboratory, USA, (1986) 18. A. P. Hammerslay, S. O. Svensson, M. Hanfland, A. N. Fitsch and D. H~iusermann, High Pressure Res. 14 (1996) 235. 19. C. M. Freeman, C. R. A. Catlow, J. M. Thomas and S. Brode, Chem. Phys. Lett. 186 (1994) 231. 20. R. Millini, Stud. Surf. Sci. Catal. 135 (2001) 264. 21. H. Sun, J. Phys. Chem., 102 (1998) 7338. 22. R. Millini, L. C. Carluccio, F. Frigerio, W. O. Parker, Jr., and G. Bellussi, Microporous Mesoporous Mater. 24 (1998) 199.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

69

Chemical and structural aspects of the transformation of the M C M - 2 2 precursor into ITQ-2 R. Schenkel, J.-O. Barth, J. Kornatowski, and J.A. Lercher Institut ~ r Technische Chemic, Technische Universitat M0nchen, 85747 Garching, Germany MCM-22 materials were prepared from precursors with varying Si/A1 ratios. During transformation of swollen MCM-22 into ITQ-2 strong desilication and amorphization were observed, which increased in intensity with decreasing aluminum concentration of the precursor. The ease of delamination increased in the same order. Exfoliation of the MCM-22 precursor to ITQ-2 was best performed at Si/A1 ratios above 20.

1. I N T R O D U C T I O N

Layered materials have a high potential as catalysts and supports, since two-dimensional slit-pores provide an easier access to organic molecules exceeding the typical size of zeolitic micropores. These crystalline layered structures can be intercalated [1,2], yielding materials which show advantages in comparison to amorphous oxides, i.e., (i) a high internal order and homogeneous distributions of the slit-pores due to the crystallinity of the layers, and (ii) well defined micro-/mesoporosity. MCM-36 [ 1] is a typical example of such a molecular sieve. Materials, similar to MCM-36, but with a completely disordered structure of layers, have been reported by Corma et al. [3,4], who observed that the zeolitic layers of the MCM-22 precursor can be fully separated by means of ultrasonic treatment. This procedure yields crystalline monolayers. The resulting material has been named ITQ-2 [3,4]. It consists of 2.5 nm thick sheets with a 10-membered ring channel system inside the sheets. Its specific surface area has been found [3,4] to be high (600-700 m2/g) and structurally well defined. However, this type of transformation depends subtly on the composition of the materials and the chemical methods employed. We report here on the influence of the synthesis routes, their modification, and the importance of chemical composition (A1 concentration) of the MCM-22 precursors on the formation and properties of lTQ-2. 2. EXPERIMENTAL MCM-22 precursors were synthesized using the procedure described by He et al.[5] (reaction gels with Si/A1 = 8, 12, and 50, under static conditions). One part of the precursors was filtered, washed with water, dried at room temperature in air and finally calcined at 823 K under flowing N2 with 8% 02 for 48 h to produce crystalline MCM-22. The other part of the non-dried and non-calcined precursors was swollen 5 with aqueous solutions of hexadecyltrimethylammonium chloride (CTMAC1, 25%) and tetrapropylammonium hydroxide (TPAOH, 40%) at relative weight ratios MCM-22 (P) / CTMAC1 / TPAOH equal

70 to 1:4:1.2, and pH adjusted to 13.5 with an NaOH solution. The swollen materials were filtered, washed, dried at room temperature and, in form of ca. 5 wt.% water slurries, treated in an ultrasonic bath [3,4] (120W, 35 kHz) at pH=12.5 [4] for 36 h. Then, several drops of concentrated HC1 were added to obtain pH below 2, which allowed an easier recovering of the resulting ITQ-2 solids by centrifugation. After drying at ambient conditions, the organic material was removed by calcination in a procedure analogous to that for calcining MCM-22. The overall Si/A1 ratio was determined from elemental analysis with atomic absorption spectroscopy (AAS, UNICAM 939). Powder XRD patterns were recorded with a Philips XPERT PRO diffractometer using CuI~ radiation. XRD simulations were performed using "Cerius 2" (Version 4.6, MSI). Nitrogen adsorption (PMI automated BET-sorptometer) was measured at 77.4 K for samples activated in situ at 673 K for 20 h. Scanning electron microscopy (SEM, JEOL 500) and transmission electron microscopy (TEM, JEOL 2010) were used to characterize crystal habitus and structural aspects. The adsorption of pyridine and 2,6-di-tert-butyl-pyridine (DTBPy) were followed by in situ IR spectroscopy (Bruker IFS-88, resolution 4 crn-1) at 373 K under partial pressures of 10-3 - 10-2 mbar. For this, the zeolite samples were pressed into self-supporting wafers (ca. 5 mg) and activated in vacuum (p31p 2D CP, 6000 scans were acquired for each of the 64 tl slices. The 31p and 27A1 shifts were referenced to 85% H3PO4 and 1M AI(NO3)3 aqueous solution, respectively. For ~H --->31p CP experiments, the Hartman-Hahn condition was determined using (NI-h)H2PO4. The repetition time of 5 s was used and 160 scans were acquired for each spectrum. The powder XRD patterns of the gels were recorded on a Rigaku diffractometer using Co Ka radiation (a wavelength of 1.7902/~). 3. RESULTS and DISCUSSION

The powder XRD patterns of the intermediate gel phases are shown in Figure 1. The initial gel (without heating), the gel aged at 90 ~ for 24 hrs and the solid material produced by heating the aged gel at 200 ~ for 50 min. are all amorphous. The first evidence of A1PO4-11 crystals in the diffraction patterns was observed after heating the aged reaction mixture at 200 ~ for 60 min. Heating the aged gel at 200 ~ for more than 80 min. yields the pure crystalline AIPO4-11. We did not observe any metavariscite and variscite phases in the early stage of the reaction as reported by Tapp and co-workers [5]. The 31p and 27A1 MAS spectra (Figure 2) were obtained to probe the local chemical environments of P and AI atoms. The spectra of the gel heated at 200 *C for more than 80 minutes are identical to those of pure crystalline A1PO4-11, which is consistent with the powder XRD patterns. For the initial gel, the 27A1 MAS spectrum contains a very weak peak at around 45 ppm and a strong peak a t - 7 ppm with a prominent shoulder on the low-field side at about 4 ppm. Based on the shift value, the 45 ppm peak may be assigned to tetrahedral A1 sites in the aluminophosphate gel. As shown in Figure 2, the intensity of this peak increases with increasing the heating time. The position of this tetrahedral peak gradually shifted towards high-field side with increasing heating time, indicating that there is a slight change in the chemical environment. The assignments for the main peak at -7 ppm and its low-field shoulder at 4 ppm are ambiguous. The -7 ppm peak may be assigned to octahedral AI sites in the aluminophosphate gel. The shoulder at 4 ppm could be due to either the octahedral A1 in unreacted alumina or five-coordinated AI in an aluminophosphate gel. It is unclear if these two maximums at 4 and -7 ppm are due to two separate resonance signals or just one A1 site with asymmetric lineshape resulting from a large quadrupolar coupling constant. The 31p spectrum of the initial gel has a sharp peak at 3 ppm and a broad peak a t - 1 1 ppm. After aged at 90 ~ for 24 hrs, the sharp peak at 3 ppm disappeared and the spectrum only has a very broad resonance centered a t - 1 4 ppm. The assignment of the broad peak at around-14 ppm is also worth mentioning. In previous studies of A1PO synthesis, a broad peak in the range between-10 a n d - 2 0 ppm has also been observed in the gel samples

137

. . . . . . . .

_

_

,

. . +

,___ _~.~~~~i~.+~ , i~~i;:. 200 ~ for 60 min

200 ~ for 50 min

90 ~ for 24 hrs ~d':'-"--"-'~L;-.~-;-";-7~ :-';7~II-~Z--', ~" L :-.~:~ ;-,'~" ~'-'~'~"L~~-~::' .'~- -~-. -;.:~_t..~-:- :.t.-~..~:;_-_'-:.~--.:: ~-:;- +t_':"

Initial gel without heating 9

i

!

!

!

i

!

10

20

30

40

50

60

2 Theta Fig. 1 Powder XRD patterns of the gels

%- ~ ~- ;,j

138

27A1 MAS

31p MAS

36 ppm~,..~

-27 P I 2 ~ ~ 31 " _ . ~ - , ppm

AIPO4-11

200 ~ for ~

-___

200 ~ for

200 ~ fo

.____r 4 ppm 4 ppm r r45 ~ 7 ~ 100

50

0

-~ -7ppm 3ppm ~ -11 ppm Initialgelwith~ "~1 -50

ppm from 1.0 M AI(NO3)3 Fig. 2

- ! 00

40

20

0

-20

ppm from 85% H3PO4 31p and 27A1MAS spectraof the gels

-40

-60

139 obtained in the early stages of the crystallization [7]. This peak has been assumed to be the amorphous aluminophosphate species. However, there is no direct proof that this P peak is actually connected to the AI sites. Several other P species such as mono-, di- and polyphosphates, hydrogen monophosphates and dihydrogen phosphates can also appear in this region [8]. Heating the gel at 200 ~ for 60 min. resulted in the appearance of a very weak shoulder at about -26 ppm on the high-field side of t h e - 1 4 ppm main peak. This shoulder is more prominent in the spectrum of the sample heated at 200 ~ for 80 min. Since the position of the weak shoulder is in the region where two P peaks of pure A1PO4-11 (-27 and-31 ppm) also appear, we may assume that this broad shoulder is due to a small amount of A1PO4-11 crystallites, which is also seen in powder XRD pattern. As discussed above, the 31p and 27A1 MAS spectra only provide limited structural information and ambiguities do exist in spectral assignments. To further characterize the structure of the intermediate gel phases, we have carried out 27A1 ---->31p cross polarization experiments. The cross polarization is mediated by heteronuclear dipolar interactions [9]. Thus, this technique provides connectivity information through the distance dependence of the CP process. Previously, Fyfe and co-workers have shown that 27A1/31P CP can be used to characterize the crystal structure of AlPO-based molecular sieves [6]. This method has also been employed to study aluminophosphate glasses [10]. Very recently, we have shown, for the first time, that 27A1 ---->31p CP can also be used to extract the AI-O-P connectivity in the gel phases of AIPOs and SAPOs synthesis [2]. We first examined the initial gel without heating. A previous study suggested that mixing the aluminum oxide with phosphoric acid and organic amine at room temperature results in precipitation of aluminum oxide hydrate in a more reactive colloidal form that serves as a precursor for further reaction and that the subsequent pretreatment of the gel processor at about 90 ~ converts aluminum oxide hydrate to aluminophosphate complexes which eventually lead to microporous AIPO4-11 [11]. If this was the case, there should be no aluminophsphate species existing in the initial gel. Our 31p MAS spectrum shows two peaks at 3 and -11 ppm, respectively. Tapp et al. reported that amine phosphate might form in the initial gel [5]. Since some unprotonated monophosphates do have chemical shifts near 0 ppm [8], the sharp peak at 3 ppm may be assigned to amine phosphate. However, the chemical environment o f - 1 1 ppm peak is not clear at all. Figure 3 shows the twodimensional 27A1 ~ 31p CP spectrum of initial gel without heating, from which several interesting features are immediately apparent. The P projection only contains a broad CP 3~ signal at-14 ppm. The sharp resonance at 3 ppm seen in P MAS spectrum does not appear in the projection indicating that this P species is not linked to any of the AI sites observed in 27A1 MAS spectrum. This result confirms that the 3 ppm peak is likely due to amine phosphate which is not connected to any AI species. In AI projection, two peaks are observed at 41 and -7 ppm. The shoulder at 4 ppm appearing in the corresponding A1 MAS spectrum does not show up in the projection, suggesting that this resonance is due to the unreacted aluminum oxide and not five-coordinated AI in the aluminophsphate gel. The broad peak a t - 1 4 ppm in P projection is correlated to both 41 and -7 ppm peaks in A1 projection. Our results clearly indicate that mixing AI and P sources together with the structure directing agent at room temperature yield a solid mixture containing amine phosphate, unreacted aluminum oxide and amorphous aluminophosphate species. Both tetrahedral and octahedral A1 sites are present in the aluminophosphate gel, and both are

140 -11

' PMAs-JI L - - 31p project'

g

~

7

~:

.........................

!

[[

. ~

" --'"--"-'--"--------"--" "

~~_~_

31p slice via oct. A1

;

'

~

9 ~v] 31p shce

9

"

-I 9

-4~

.---

-4~

i i

I I

~

i

41 II

a

20

.... . .... ..... !0

0

-10

.... . .... . -20

-30

-40

f r o m 85% H3PO 4

Fig. 3 2D 27A1 to 31p CP spectrum of initial gel without heating

long contacttime

@

|._~

' '

' 1 .... 11

I ....

o

I ....

-11

20

~,o~

I ....

-io

0

h/

-20 ~

-40

co,aclti'mT?'~,

(

~

20

~ -711 9

'

"

0

-20

p p m ~ o m 85% H3PO 4

-40 "

from 85% H3PO 4

Fig. 4 2D 17AIto 3tp CP spectrum of the gel heated at 200~ min.

Fig. 5 ~H to 31p CP spectrum of the gel heated at 200~ min.

141 connected to the P. Careful inspection of 2D spectrum reveals that the broad peak a t - 1 4 ppm in P projection actually encompasses two components. The first component is a sharp P peak centered a t - 1 8 ppm, which is only connected to AI peak at 41 ppm. It represents a small amount of P atoms with only tetrahedral AI as the nearest neighbors. The second component is a broad peak a t - 1 4 ppm connected to both tetrahedral and octahedral A1 sites. These connectvities can be seen more clearly in the P slices taken through the tetrahedral and octahedral A1 sites (Figure 3). These two components are apparently due to the P sites with different degrees of condensation. We have further characterized the gel heated at 200 ~ for 60 minutes by twodimensional 27A1 --~ 31p CP (Figure 4). Two peaks were observed in the A1 projection corresponding to the tetrahedral (39 ppm) and octahedral (-7 ppm) A1 sites, both of which are connected to P sites. The 4 ppm peak seen in the AI MAS spectrum did not appear, implying that this A1 site is not part of the aluminophosphate gel and probably due to the unreacted A1 source. A broad P peak positioned a t - 1 6 ppm is connected to both tetrahedral and octahedral AI sites, indicating that this broad resonance represents P atoms, which are not fully condensed. The P projection also contains a well defined broad shoulder at about-27 ppm and this shoulder is connected to tetrahedral AI site only. The chemical shift of this shoulder is very close to the frequencies of the peaks in crystalline A1PO4-11. These results suggest that the chemical environment of this P site is P(-OA1)4. Additional evidence for this assignment is that compared to the same shoulder in the P MAS spectrum the intensity of the shoulder a t - 2 7 ppm was significantly enhanced relative to t h e - 1 6 ppm main peak. Fyfe and co-workers have carried out the 27A1 ~ 29Si CP on various zeolites and found the found that the relative enhancement of Si signals could be related to the number of AI atoms in neighboring T sites [ 12]. In our case, the enhancement of 31p at -27 ppm is consistent with our argument that -27 ppm P peak is due to fully condensed P(-OAI)4 environment, whereas -16 ppm is due to partially condensed P sites (where the number of AI atoms in the first coordination sphere is less than 4). However, care must be taken when interpreting CP intensity since cross polarization from a quadrupolar nucleus such as 27A1 (1=5/2) is usually very inefficient due to the difficulty in spin locking [13]. For this reason, we also conducted the IH ~ 31p CP on the same gel sample. Figure 5 shows that the short contact time (0.1 ms) favors only-16 ppm main peak, suggesting strong dipolar interactions between proton and P a t - 1 6 ppm. This result implies that the broad-16 ppm peak indeed represents P sites which are not fully polymerized and of mixed coordination, P(OH)x(OAI)n.x (where x is 1-3). The CP spectrum obtained with long contact time of 10 ms contains mainly a weak peak a t - 2 7 ppm. The fact that the very long contact time is needed to cross polarize the -27 ppm peak indicates indirectly that the this peak is due to fully condensed P site with P(-OA1)4 environment. In summary, we have examined the evolution of the gel phases as a function of crystallization time by solid-state NMR. We have also carefully characterized several gel phases obtained at several different stages of the reaction. In contrast to previous reports, we found that aluminophsphate species exists in the gel phases formed at room temperature. The different P and AI sites were identified and their connectivity mapped out unambiguously by A1/P CP experiments. The solid sample corresponding to the beginning of the nucleation (200 ~ min.) was also characterized. Valuable structural information

142 regarding the gel structure was obtained by CP method and this information is not readily available from simple AI and P MAS experiments.

Acknowledgements Y.H acknowledges the financial support from Natural Science and Engineering Research Council of Canada for a research grant and the Canada Foundation for Innovation for the award of a solid-state NMR spectrometer. R.R. thanks OGSST for a scholarship. REFERENCES 1. For reviewers see a) H. Gies, B. Marler, U. Werthmann, in Molecular Sieves: Science and Technology, Vol. 1, (Eds: H.G. Karge and J. Weitkamp), pp35-64, Springer, Berlin, 1998; b) M.E. Davis, R.F. Lobo, Chem. Mater., 4 (1992) 756; c) R. J. Francis and D. O'Hare, J. Chem. Soc. Dalton Trans., (1998) 3133; d) S. Oliver, A. Kuperman, G.A. Ozin, Angew. Chem. Int. Ed., 37 (1998) 46. 2. Y. Huang, D. Machado, Micropor. Mesopor. Mater., 47 (2001) 195. 3. S.T. Wilson, B.M. Lock, E.M. Flanigen, US Patent No. 4 310 440 (1982). 4. J.M. Bennett, J.W. Richardson Jr., J.J. Pluth, J.V. Smith, Zeolites, 7 (1987) 160. 5. N.J. Tapp, N.B. Milestone, D.M. Bibby, Zeolites, 8 (1988) 183. 6. C.A. Fyfe, K.T. Mueller, H. Grondey, K.C. Wong-Moon, J. Phys. Chem., 97 (1993) 13484. 7. a) M.E. Davis, C. Monte, P.E. Hathaway, J.M. Grace, in Zeolites: Facts, Figures, Future (Eds: P.A. Jacobs and R.A. van Santen), Elsevier, Amsterdam, 1989, pp 199-215. b) H. He, J. Klinowski, J. Phys. Chem., 98 (1994) 1192; c) E. Jahn, D. Mueller, J. Richter-Mendau in Synthesis of Microporous Materials, Vol. I (Eds: M.L. Occelli, H.E. Robson),Van Nostrand Reinhold, New York, 1992, pp249-265; d).S. Prasad, S.B. Liu, Chem. Mater., 6 (1994) 633. 8. a) I.L. Mudrakovskii, V.P. Shmachkova, N.S. Kotsarenko, V.M. Mastikhin, J. Phys. Chem. Solids, 47 (1986) 335; b) P. Hartmann, J. Vogel, B. Schnabel, J. Magn. Reson., 111 (1994) 110. 9. A. Pines, M.G. Gibby, J.S. Waugh, J. Chem. Phys., 59 (1973) 569. 10. R.M. Wenslow, K. Fiske, K.T. Mueller, in Sold-State NMR Spectroscopy of Inorganic Materials (Ed: J.J. Fitzgerald), ACS Symposium Series 717, 1999. 11. X. Ren, S. Komameni, D.M. Roy, Zeolites, 11 (1991) 142. 12. C.A. Fyfe, K.C. Woog-Moon, Y. Huang, H. Grondey, K.T. Mueller, J. Phys. Chem., 99 (1995) 8707. 13. a) A. J. Vega, Solid State NMR, 1 (1992) 17; b) A. J. Vega, J. Magn, Reson., 96 (1992) 50.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 9 2002 Elsevier Science B.V. All rights reserved.

143

The benzene molecule as a probe for steric hindrance at proton sites in zeolites: an IR study. B. Onida, B. Bonelli, L. Borello, S. FioriUi, F. Geobaldo and E. Garrone* Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24 - 10129 Torino, Italy. When a molecule, like benzene, is engaged in H-bonding with an acidic proton in zeolites, secondary interactions with the surrounding walls may occur, orienting the molecule and effecting the spectroscopic measure of the acidity. Information is gained on the geometry of the site from an otherwise disturbing phenomenon. 1. INTRODUCTION Probe molecules are extensively used to study H-bond formation with acidic hydroxyls in solids, the bathochromic shift of the stretching frequency (Avoi0 being assumed as measure of the acidic strength of the OH species [1]. In recent years Bellarny-Hallam-William (BHW) plots [2, 3], where shifts in frequency caused by H-bonding of two acidic groups to a set of mildly basic molecules are plotted one against the other, have been used: a straight line is observed, the slope of which is the measure sought. With microporous solids, such as zeolites, the probe molecule must h e small, so to be able to diffuse inside the pore system and reach the sites. A widely used candidate is CO [1], which, however, must be adsorbed at low temperature, because of the weakness of its interaction. Benzene may be used instead: adsorption occurs at room temperature and similar AVOH are observed [ 1, 4]. Though bulkier than CO, benzene can~still diffuse inside most of zeolitic channels, even if sometimes diffusion may be slow. When bulky molecules are used~ the possibility arises that steric hindrance be exerted by the surroundings of the acidic hydroxyl species, so hampering the interaction, and affecting the measure of acidity. This aspect is dealt with in the present paper by scrutinizing a set of data concerning different molecules and solids. Data reported concern the interaction of the isolated SiOH species in severely dehydrated Aerosil, and the Bronsted site Si(OH)A1 in a few zeolitic systems: ZSM-5, MCM-22, theta, SAPO-40. Data for silica and ZSM-5 are from literature [1, 5]. Also considered is the mesoporous silica MCM-41, exhibiting the same isolated hydroxyl species as Aerosil, in order to check whether any steric hindrance is introduced by mesoporosity. The set of weakly basic molecules employed are: N2, CO, ethylene, benzene, propene, toluene, 1,3,5- trimethylbenzene (TMB), the strength of which as bases is in the order listed

[5].

Ethene and propene can both engage in H-bonding and act as proton acceptors. At RT proton transfer to ethene is slow with all zeolites considered, so that the H-bonded ~*Corresponding author, E-mail: [email protected], FAX: +39-011-5644699

144 complex can be readily observed. Proton transfer to propene is faster, but only with ZSM-5 the process requires fast time-resolved experiments [6]. 2. EXPERIMENTAL

SiO2 and ZSM-5 (Si/A1 = 25) were from Degussa and Zeolist, respectively. SAPO-40 (Si/AI = 0.12) and MCM-22 (Si/AI = 14) samples were prepared according to the literature [7, 8]. MCM-41 was prepared according to [_9], treated in flowing air up to 823 K and maintained at the same temperature for six hours, in order to remove the template. Theta (Si/A1 = 20) was prepared at the University of Calabria [10]~ For FT-IR measurements, the powders were pressed into thin, self-supporting wafers; spectra were collected, at a resolution of 2 cm 1, in the 4000-500 cm "1 range, on a Bruker FTIR Equinox 55 spectrometer, equipped with a MCT cryodetector (128 scan). Pre-treatments were carried out using a standard vacuum frame, in a IR cell equipped with KBr windows. Wafers were outgassed at 773 K. Adsorption of CO and N2 wascarried outat the nominal temperature of 77 K. The sollware 'Moldraw' [ 11 ] has been used to define the bulkiness of the probes. 3. RESULTS AND DISCUSSION

Figure 1 reports some experiments for the zeolite MCM,22_ Curve a is the spectrum, in the OH stretching region, of the bare sample, after outgassing in vacuo at 773 K. The other curves refer to the contact of some of the probe molecules employed. H-bonding shills the O-H vibration to frequencies the lower, the more basic is the probe, i. e. the order of basicity is N2 < CO < C2H4 0.2 a.u.

----________&

t,--

-E

,-.,..

0 .(3

,

C2H4 "

200

o o

100

...i N2

0

~

9 N2

~

~ ~

"'

I

0

'

20

I

'

40

I

'

I

60

'

I

'

I

'

"

I

'

'

9

-1

~

."

"1-

o

I

'

I

'

I

'

I

'

,.o 9

o ~

~

,

~

o

,

o []

.."

400

~

"T, 9

C7H8 C6H 6

CO

E 300

C3H6

"9

." C2H 4 []

.. 9

O

o

."

~

"T"

.m

(/); > 200 (I) 115. "o o~

I

e

25-

i

0

9

i

i r i I

.m ,.i-,

I

,1~,'C6H6

30-

/

" 20

'

d

CzH8

,' A~

25

9

35

,& 06H6

I

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0 molecular hindrance (nm)

i

C

30--

l

CO

i C H

I

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 mnler.ulAr hindrance. (nm~ 35

I i

;

#

i

N2 9

I

9

I

'

I

.

0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 molecular hindrance (nm)

9

I

" '

,

I I I

9

I

9

I

9

I

9

I

9

I

9

0.0-0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 molecular hindrance (nm)

Figure 4. Correlation between extent of hindrance and largest transverse molecular diameter o f probe molecule for SAPO-40 (a), ZSM-5 (b), MCM-22 (c) and theta (d).

149 4. CONCLUSIONS When secondary interactions with the walls take place, the strength of H-bonding is under-evaluated. On the other hand, the study of H-bonding with relatively bulky molecules, like benzene, may give a measure of hindrance at the site, and information can be drawn from an otherwise disturbing phenomenon. Surroundings may have an orientating effect of molecules at the proton site [12], and influence the reactivity of the molecule at the active site. Indeed, with zeolites where significant hindrance at the Bronsted site is observed, like MCM-22 [ 13] and theta [ 14], proton transfer to light olefins at room temperature is slower than that observed for zeolites which show smaller hindrance at the proton site (ZSM-5, mordenite) [6, 15] in spite of the fact that the spectroscopically measured acidity is the same. ACKNOWLEDGMENT We thank Drr Girolamo Giordano for theta samples, Dr. Flaviano Testa and Prof Rosario Aiello for MCM-22 samples, Prof. Piero Ugliengo for Moldraw software and for fruitful discussions. REFERENCES

1. E. A. Paukshtis and E. NrYurchenko, React~ Kinek Catal. Lett., 16 (1981) 131_ ; A. Zecchina and C. Otero Aregn, Chem. Soc. Rev., (1996) 287 and references therein; J. A. Lercher, C. Gr0ndling and G_ Eder-Mirth, Cat_zL Today, 27 (1996) 353 and references therein; L. M. Kustov, V. B. Kasansky, S. Beran, L. Kubelkovh, and P. Jim, J. Phys. Chem., 91(20) (1987) 5247;_tL Knt~uzinger aM_S. Huher, 1. Chem. Sot., Faraday Trans, 94(15) (1998) 2047 and references therein; E. Garrone, A. Barbaglia, B. Onida, B. Civalleri and P. Ugliengo, Phys. Chem. Chem_ Phys., 1 (1999) 4649 and references therein. 2. G. C. Pimentel and A. L. McClellan, in The Hydrogen Bond, W. H. Freeman and Co., San Francisco, 1960. 3. H. Kn6zinger, in The H-bond: recent advances in theory and experiment, P~ Schuster, G. Zundel and C. Sandorfy (eds), North Holland, Amsterdam, 1976, p. 1269. 4. D. Barthomeuf, Zeolite, 1993, 13, 626; B. L. Su and D. Barthomeuf, J. Catal., 139 (1993) 81. 5. C. Paz~, S. Bordiga, C. Lamberti, M. Salvataggio andA. Zecchina, J. Phys. Chem. B, 101 (1997) 4740 6. G. Spoto, S. Bordiga, G. Ricchiardi, D. Scarano, A. Zecchina and E. Borello, J. Chem. Soc. Faraday Trans., 90 (1994) 2827. 7. N. Dumont, Z. Gabelica, E. (3. Derouane andL. B. McCusker, Microporous Materials, 1 (1993) 149. 8. Corma A., C. Corell and J. P&ez-Pariente, Zeolites 15 (1995) 2. 9. Q. Cai, W. Y. Lin, F. S. Xiao, W. Q. Pang, x. H. Chen and B. S. Zou, Microp. Mesop. Materials, 32 (1999) 1. 10. G. Giordano, A. Katovic, A. Fonseca and L B_ Nagy, Stud. Surf. Sci. Catal. 135 (2001) 175. 11. P. Ugliengo, D. Viterbo and G. Chiari, Z. Kristallogr. 9 (1993) 207. 12. E. Kemner, I. M. de Schepper and G. J.Kearley, Chem. Commun., (2001) 2466.

150 13. B. Onida, F. Geobaldo, F. Testa, R. Aiello and E. Garrone, J. Phys. Chem., 106 (2002) 16840 14. F. Geobaldo, S. Fiorilli, B. Onida, A. Katovic, G. Giordano and E. Garrone, in preparation. 15. Geobaldo, F., Spoto, G., Bordiga, S., Lamberti, C. and Zecchina, A., J. Chem. Soc. Faraday Trans., 93(6) (1997) 1243.


Structural characterization of Co- and synthesized in the presence of morpholine.

151

Si-substituted

AIPO-34

A. Martucci l*, A. Alberti l, G. Cruciani 1, A. Frache 2 and L. Marchese 2. 1Dipartimento di Scienze della Terra, Sezione di Mineralogia, Petrografia e Geofisica, Corso Ercole I d'Este, 32, I- 44100 Ferrara, Italy 2Dipartimento di Scienze e Tecnologie Avanzate, Universit/t del Piemonte Orientale "A. Avogadro", C.so Borsalino 54, I-15100 Alessandria, Italy.

SAPO-34, CoAPO-34 and CoAPSO-34 materials, synthesized using morpholine as structure directing agent, were structurally characterized by single-crystal and X-ray powder diffraction (XRD) data. HF was also used for the crystallization of CoAPO-34. The incorporation of Co and/or Si atoms in the framework sites was determined by different routes in the structure refinement. In CoAPO-34, at room temperature one of the A1 framework sites was sixfold-coordinated with four O and two F atoms. Fluorine bridged two A1 atoms. The loss of fluorine occurred at around 450~ and restored the tetrahedral coordination of the 6-coordinated A1 atom. In agreement with TGA analysis, the presence of two morpholine molecules in each cage of all these chabazite-type materials was assessed by structural studies. -NH2 vibrations in the IR spectra of assynthesized samples indicated that some morpholine molecules are protonated and act as counterbalancing charges of the negative framework. Structural modifications under heating, followed by in-situ high temperature XRD, showed that CoAPO-34 materials start to lose crystallinity at around 730~ Bridging hydroxyls [Co2+-O(H)-P] were found by FTIR, and the presence of tetrahedral Co 2+ ions was inferred by UV-Vis-NIR spectroscopy. 1 INTRODUCTION When cobalt ions are incorporated into the tetrahedral framework sites of microporous aluminophosphates (A1POs) or silicoaluminophosphates (SAPOs), selective heterogeneous catalysts are produced [1]. Co-substituted alumino- or silicoaluminophosphates are materials which may combine both redox and acid properties. SAPO-34, for instance, a material with chabazite-type framework, is a particularly good shape-selective catalyst for methanol to light-olefin conversion [2]. A1PO-34 type structures have recently been synthesized in the presence of morpholine as structure-directing agent; HF was also used for crystallization [3]. The aim of this work is to determine the structures of Co-substituted A1PO-34 and SAPO-34 in the *to whom correspondence should be addressed: [email protected]; [email protected]

152 presence of morpholine in order to have direct evidence of the metal substitution in the A1 framework sites, and to localize the organic species in the chabazite cages. CoAPO34, including F, was also studied via X-ray structure determination in order to understand its ability to coordinate A13+ [4], and to follow the structure modifications of F-CoAPO-34 under heating. FTIR and DR UV-Vis spectroscopies were also used to monitor the presence of cobalt ions in framework positions. 2. E X P E R I M E N T A L

SAPO-34 and CoAPSO-34 were synthesized by mixing the appropriate amounts of AI(OH)3, H3PO4 and distilled water. The mixture was stirred until a uniform gel was obtained. SiO2 and/or Co(CH3COO)2 were then added, followed by morpholine. The resulting gels (0.08Co:0.92Al:0.25Si:0.90P:50HzO:l.25Morph.) were crystallized in a Teflon-lined autoclave under autogenous pressure at 195~ for 10 days. CoAPO-34 was synthesized in a similar way, but without SiO2 and with the addition of HF (F-CoAPO34; 0.08Co:0.92Al:lP:0.35F:50H20 :l.25Morph.). TGA analysis was carried out on a TA instrument (SDT2960 mod). The weight loss percentages from 200 to 700~ of all as-synthesised materials was in the range 18-20%, this means that two molecules of morpholine could be present within the chabasite cages. Single crystal X-ray diffraction data were collected at room temperature on three crystals of SAPO-34 and two of CoAPSO-34 (with different Co/A1 ratios), using a Nonius KappaCCD diffractometer equipped with a CCD detector. The chemical compositions of one SAPO-34, and two CoAPSO-34 crystals were determined by an ARL-SEMQ electron microprobe in the wavelength dispersive mode, at 15 K V , with 20nA sample current and defocused beam (20~tm), on the same crystals used for the Xray data collection. Powder diffraction data of F-CoAPO-34 were collected at high resolution at the Swiss-Norwegian beamline (SNBL) at the X-ray synchrotron source of ESRF (Grenoble). A sample of F-CoAPO-34 was also packed in an open capillary and heated "in situ" using a hot air stream, at the GILDA beamline at ESRF. During the heating process powder diffraction patterns were collected on a translating image plate in the temperature range 50-850~ FTIR spectra were recorded on a Bruker IFS 88 spectrometer, and diffuse reflectance (DR) UV-Vis-Nir spectra on a Perking Elmer Lambda 19 equipped with a reflectance sphere attachment.

3. RESULTS AND DISCUSSION

Single-crystal structure refinements of SAPO-34 and CoAPSO-34 were carried out, starting from the crystallographic data reported for the CoAPSO-34 structure [5] in space group R-3. The position of the morpholine molecules was determined by Fourier and difference Fourier maps. Incorporation of Si and Co in tetrahedral sites was assessed by: a) analysis of the T-O bond distances. For Si-O we assumed the value of 1.603A found in the purely siliceous chabazite [6], for A1-O the value of 1.725 A found in the structurally-related SAPO-47 structure [7], and for Co-O and P-O the values 1.93 and 1.52 A respectively [8].

153 b) the Co and A1 occupancies in sites T1 and T2 obtained by scattering curve refinements. The refinements of SAPO-34 crystals showed slight differences in the S i - P substitutions and, in accordance with criterion a), the T2 site was occupied by about 78% P and 22% Si, in excellent agreement with the results of Pluth and Smith [7], whereas chemical analyses gave about 15% Si. The two crystals of CoAPSO-34 differed mainly in their Co and Si contents in the tetrahedral sites. One of these, in accordance with criteria a) and b), showed about 15% of Co in T1, but no Si in T2, whereas the other crystal was characterized by about 10% Co in T1 and 5% Si in T2. According to the chemical analyses, the A1 tetrahedron was occupied by 19% Co and the P tetrahedron by 5% Si in the first crystal, whereas in the second one the two tetrahedral were occupied by 15% Co and 9% Si, respectively. Structure refinements showed the presence of two morpholine molecules related by the inversion centre. Each of these occupied three different positions with 1/3 of occupancy rotated around the threefold axis and the oxygen atom placed, as a pivot, on the triad (Figure 1).

Figure 1. Projection along [111] of the chabazite cage, showing the morpholine molecules in SAPO-34 and CoAPSO-34.

154 The distances from the framework oxygens of all atoms of the organic molecule were always greater than 3.3A, thus indicating that morpholine was only weakly bonded, if at all, to the framework. This behaviour is quite unusual: however, the presence of water molecules near the center of the 8-rings suggested that different morpholine molecules could be connected by means of hydrogen bonds through water molecules to form morpholine-H20 chains (Figure 2). The structure refinement of F-CoAPO-34, obtained by high resolution X-ray diffraction data at the SNBL beamline of ESRF, strongly resembles that of A1PO-34 performed by Harding and Kariuki [9]; the symmetry is triclinic, with space group P-1. The framework in both cases in the asymmetric unit is composed of three tetrahedral P atoms, two tetrahedrally coordinated A1 atoms and one octahedrally coordinated A1 atom. This framework site is coordinated to four O and two F atoms. Therefore, the sixfold coordination of an A1 atom found in the F-A1PO-34 structure is maintained also when Co atoms are located in the framework (see spectroscopic results). We observed that fluorine bridged two A1 atoms, which were found closer together than other framework atoms. The presence of bridging fluorine led to a distortion of the trigonal symmetry of the CHA framework.

Figure 2. Projection along [100] of the chabazite cage, showing the chains of morpholine and water molecules in SAPO-34 and CoAPSO-34.

155 Two fully occupied sites of morpholine were found at the intersection of the six- and eight-rings (Figure 3).

Figure 3. Projection along [010] of the chabazite cage of CoAPO-34 containing two morpholine molecules. The N atom of the morpholine molecules is suitably located for hydrogen bonding (2.63A) with one framework oxygen. The "in situ" analysis of the powder diffraction data collected at the GILDA beamline at the X-ray synchrotron source of ESRF showed that, when CoAPO-34 was heated to around 450~ it lost both fluorine and morpholine molecules. The unit cell parameters became strongly rhombohedral with a ~ b ~ c 9.33A and ct ~ [~ ~ 7 ~ 94.2~ but the symmetry remained triclinic P-1. The original sixcoordinated A1 atoms became four-coordinated after removing the fluorine atoms, and this restored the three-dimensional topology of the chabazite-type structures. A memory of the coordination and distortion was nevertheless maintained after template and fluorine removal, and the threefold axis of the rhombohedral symmetry was not restored. The results of the X-ray structure refinements for Si- and Co- containing materials, as well as for A1PO-34 [9], indicated clearly that two morpholine molecules are always present in the chabazite cage of A1PO-34-type structures. This figure perfectly fits the TGA data reported in the experimental section. As a result, morpholine can be either protonated or unprotonated in these structures. IR and Raman measurements, however,

156 showed that protonated morpholine molecules are indeed present in both SAPO-34 and A1PO-34 [3]. More recent spectroscopic studies (see below) suggest that, also in Cocontaining materials, protonate morpholine molecules are present. 3.1 Spectroscopic results Fig. 4 shows the IR spectra (1800-1500 cm "l) of CoAPO-34 taken under vacuum at temperatures ranging from 200 to 550~ Absorption at 1604 cm 1, which progressively disappeared as the temperature increased, is assigned to a bending mode of +NH2 groups in protonated morpholine. FTIR studies (figure not reported for the sake of brevity) revealed that, like to structurally related CoAPO-18 materials, acid and redox centres are present in CoAPO-34 materials, confirming that Cobalt ions isomorphously substituted some A13+ ions in the A1PO-34 framework [10,11]. The acid sites were related to Bronsted hydroxyl groups bridged between Co 2+ and p5+ and associated with the presence of Co(II)/Co(III) redox couples. In fact, two vibrational absorption at ca. 3575 and 905 cm -1, assigned respectively to the stretching and bending modes of bridged OH groups, were present in the FTIR spectra. These hydroxyl groups disappear after calcinations in 02, and were restored by reducing in H2 at 400~ Finally, DR UVVis-NIR spectra (figure not reported) showed the presence of bands in the visible (20.000-15.000 cm -1, triplet) and in the near infrared (10000-4000 cml), attributable to 4Tl(P) --~ 4A2(F) and 4Tl(F) --+ 4A2(F) ligand field transition of tetrahedrally coordinated Co 2§ centres which should be located at framework sites.

30

0 t""

20

E r

2/

L,.

I-- 10

0

L

1800

,

I

1700

,

I

1600

1500

Wavenumbers [cm1]

Figure 4." FTIR spectra of CoAPO-34 recorded after evacuating the samples at increasing temperatures" a) 200~ b) 400~ c) 550~

157 REFERENCES

1. J.M. Thomas, Angew. Chem. Int. Ed. Engl. 33 (1994) 913. 2. Y. Xu, C.P. Grey, J.M. Thomas, A.K. Cheetham, Catal. Lett., 4 (1990) 251. 3. L. Marchese, A. Frache, E. Gianotti, G. Martra, M. Caus/t, S. Coluccia Micro. Meso. Mat. 30 (1999) 145. 4. P.F. Feng, X. Bu, T.E. Gier, G.D. Stucky, Micro. Meso. Mat. 23 (1998) 221. 5. G. Nardin, L. Randaccio, V. Kaucic, N. Raijic, Zeolites 11 (1991) 192. 6. M.J. Diaz_Cabanas, P.A. Barrett, M. A. Camblor, Chem. Commun. (1998) 1881. 7. J.J. Pluth, J.V. Smith, J. Phys. Chem. 93 (1989) 6516. 8. R.D. Shannon, Acta Crystallogr. A32 (1976) 751. 9. M.M. Harding, B.M. Kariuki, Acta Crystallogr. C50 (1994) 852. 10. L. Marchese, G. Martra, N. Damilano, S. Coluccia, J.M. Thomas, Stud. Surf. Sci. Catal., 101 (1996) 861 11. E. Gianotti, L. Marchese, G. Martra, S. Coluccia, Cat. Today, 54 (1999) 547



159

Chemical linking of MFl-type colloidal zeolite c r y s t a l s P. Agren*, S. Thomson t, Y. Ilhan, B. Zibrowius, W. Schmidt, F. Schiith MPI ftir Kohlenforschung, 45470 MOlheim, Germany Nanoparticles were synthesized using solutions of hydrolyzed TEOS and aluminate TPAOH at room temperature. These nanoparticles were crosslinked by reaction with 1,7dichlorooctamethyltetrasiloxane. The resulting solid after calcinations is X-ray amorphous, but they show NMR and IR signatures akin to MFI type zeolites. They have mieroporous properties like a zeolitic structure, but in addition provide a mesopore system resulting from the crosslinking. The acid site strength of aluminosilicate materials is lower than that of HZSM-5, as shown by pyridine desorption at different temperatures followed by IRspectroscopy. 1. INTRODUCTION Colloidal zeolite crystals have been intensively investigated over the last years (for instance [1,2]). However, the colloidal dimension is rather wide, ranging from the nanometer to the thousand nanometer scale. Conventionally, zeolite crystals are termed "colloidal", if they have sizes of several ten nanometers. It has also been known for quite some time, that even smaller entities of only several nanometers in size exist in zeolite synthesis solutions under specific conditions which are assumed to be the precursors of MFI type zeolites [3,4]. These entities which were primarily detected by small angle scattering, typically had sizes of 3-5 nm. At their first detection, their internal structure remained fairly unclear, although it was thought that they contain some structural elements of the zeolites which eventually are formed from them. More far reaching conclusions, based on NMR spectroscopy, IRspectroscopy, small angle X-ray scattering, electron microscopy, gel permeation chromatography and other techniques, with respect of the structures of such entities in clear solution systems were drawn by Martens and coworkers. The authors presented evidence for the presence of MFI "nano slab s" at intermediate stages of a clear solution synthesis [5] which upon heating assemble into macroscopic zeolite crystals [6]. For some reactions it has been proposed that they occur advantageously on the external crystallite surface of zeolites. This includes the Beckmann rearrangement of cyclohexanone oxime to e-caproclactame [7] in the gas phase, although this has recently been disputed for the liquid phase reaction [8] as well as for the gas phase reaction, where it was shown that both educt and product have access to the pore system of the MFI structure [9]. The higher activity of smaller crystallite size material was explained by an increased effectiveness factor of such small particle catalysts. In other systems, however, the reacting molecules are clearly too bulky to enter the pore system, and their conversion over zeolitic catalysts could be improved, if particles with a high fraction of external surface area were used [10]. It seems thus interesting to produce zeolitic materials with a maximum external surface area and investigate their properties in such reactions. Present address: Vaisala Oyj, P.O. Box 26, FIN-00421 Helsinki, Finland t Present address: Materials Division, Building 3, Australian Nuclear Science and TechnologyOrganisation (ANSTO), PMB 1, Menai, 2234, New South Wales, Australia

160 If the zeolite nanoslabs proposed by Martens and coworkers could be used in the form in which they are assumed to be present in clear solution, a maximum of external surface area could be achieved, since in such materials essentially the whole surface area would be external or could at least be accessible to reactant molecules. However, if these very small entities existing in pre-crystallizing solutions are removed by extraction or by solvent evaporation, or if these solutions are heated, agglomeration of the primary units takes place with a concomitant loss of external surface area. To maintain a high external surface area, we have attempted to capture the nanosized particles in a matrix which provides access to their external surface. For this purpose we have used silicones with reactive groups on both ends or aluminum Keggin ions. The following discussion will focus on samples which where crosslinked with the silicone 1,7-dichloro-octamethyltetrasiloxane. 2. EXPERIMENTAL 2.1. Materials

For the preparation of the nanosized zeolites clear solution syntheses were carried out according to published procedures, but without the aging step to keep the particles as small as possible. The molar composition of the solution was 12 TPAOH : 25 SiO2 : 1.67 A1203 : 480 1-120 : 100 EtOH. The alumosilicate solution was prepared by combining to precursor solutions. For solution 1, TEOS was hydrolyzed by stirring with two thirds of the TPAOH solution (40wt.%) for 24 h at room temperature. Solution 2 was prepared by dissolving freshly precipitated AI(OH)3 gel (prepared according to Schoeman et al. [ 11]) in one third of the TPAOH solution and the rest of the water. After combining the two solutions they were transferred to sealed polypropylene bottles and aged for different times at room temperature. After the desired aging time, 4 ml were removed and put in a polyprolylene beaker with a stirring bar. 1 ml of 1,7-dichlorooctamethyltetrasiloxane was added under vigorous stirring. A gel was formed after approximately 5 h. The gel was aged for another 24 h at room temperature, then dried at 363 K for 24 h. Calcination was carried out by heating to 823 K with a rate of 1 K/min and kept at 823 K for 5 h. A reference sample of unlinked nanoparticles was prepared by adding THF to the solution after the appropriate aging time, particles were salted out into the organic phase by NaCI addition, and the product was obtained by evaporation of the THF, according to the procedure described in [12]. In the following the discussion will focus on three sets of samples, non-crosslinked material SO (uncalcined) and S0C (calcined), crosslinked after 3.5 h of aging $3.5 and $3.5C and crosslinked after 24 h of aging $24 and $24 C. 2.2. Characterization

Samples were characterized by nitrogen adsorption with a Micromeritics ASAP 2010 at liquid nitrogen temperature. Prior to the measurements, samples were pretreated overnight under vacuum at 473 K. 29Si and 27A1 MAS NMR measurements were carried out using a Bruker Avance 500 spectrometer at a spinning rate of 15 kHz. XRD measurements were performed on a STOE STADI P transmission diffractometer with a position sensitive detector. Selected samples were analyzed by transmission electron microscopy on a Hitachi HF 2000 microscope equipped with a cold field emitter gun. IR spectra of adsorbed pyridine were recorded using a Nicolet spectrometer and a home built in situ cell. Prior to pyridine admission, samples were outgassed in vacuum at 673 K for two hours. Then samples were exposed to 4 mbar of pyridine at 413 K and evacuated, before spectra were taken. To assess acid site strength, samples were heated to temperatures of 473 K, 523 K, 598 K and 673 K and after each step cooled to 413 K to record the spectra.

161 3. RESULTS AND DISCUSSION 3.1. Coordination of silicon and aluminum centers MAS NMR spectroscopy provides information on the coordination state and the connectivities of silicon and aluminum in the samples. In Fig. 1, the 29SiNMR spectra of the samples SO and $24 are shown. The spectrum of SO shows three distinct signals at 91.5, 101.3 and 109.6 ppm, which can be assigned to Q2, Q3 and Q4 silicon atoms. The spectra are quite similar to those reported earlier for isolated "nanoslabs" by Ravishankar et al. [5]. In the crosslinked sample $24 the signal corresponding to Q2 silicon has vanished and the relative intensity of the Q3 signal is strongly decreased. Two new signals are visible at 18.9 and 22.4 ppm which are caused by silicon connected to the methyl groups in the silicone-linker. The reduction of the Q2 and Q3 signals in the crosslinked material can be attributed to the reaction of the Si-C1 group of the silicone with surface hydroxyl groups of the nanoparticles. The strong reduction of the NMR signals corresponding to the not fully connected silicon atoms shows, that the reaction with the silicone is quite efficient. After calcinations the signals at 18.9 and 22.4 ppm have disappeared, due to the removal of the Si-20 -40 .4]0 -80 -100 -120 -140 CH3 groups. The relative intensity of the Q4 signal has still increased slightly. This means that also the backbone of the silicone consists of almost fully Figure 1" 29SiMAS NMR spectra of connected silicon atoms, resulting from a ZSM-5 nanoparticles, SO, (top trace) condensation process during calcinations. and of crosslinked nanoparticles aged 27A1 MAS NMR spectra of all samples studied for 24 tl, $24, (bottom trace). typically showed two signals at 52 ppm (tetrahedral aluminum) and 1 ppm (octahedral aluminum) corresponding to about equal amounts of octahedral and tetrahedral aluminum. For low aluminum concentrations (Si/A1 = 50 and smaller), complete incorporation even in nanosized crystals has been proven [11]. However, the amount of aluminum offered in our syntheses was rather high (Si/A1 - 7.5) to create a high concentration of acid sites. It could therefore be expected that some of the aluminum was not incorporated in the zeolite framework but is present in the sample as extraframework alumina. The NMR data show that the reaction of the silicone with the surface silanols proceeds smoothly and goes to completion. The amount of silicone added (2.85 mmol) is approximately sufficient to react with all free silanol groups (7.3 mmol silicon in the aliquot taken). Since the medium in which the reaction is carried out, is aqueous, some hydrolysis and condensation of the silicone with other silicone molecules can occur as well. This would be detected in the NMR spectra as a high fraction of Q3 species left. However, since the hydrolysis and condensation are acid catalyzed and the surface silanols are moderately acidic, preferential reaction with these is expected and also detected by the reduction in Q3 intensity. ................................................................................

i .........

,..i,,.,,,.t,.,I.,,..,,.,,,,,,,,.,,.,...,,,,,,,

3.2. Textural properties Nitrogen physisorption isotherms of the three samples SOC, $3.5C and $24C are shown in Fig. 2. Strong differences can be detected between the three samples. SOC has a micorpore volume of 0.19 cm3/g in good agreement with data on nanocrystalline ZSM-5 and also bulk ZSM-5 [13]. However, different to bulk materials with MFI structure, there is still a slight increase of the volume adsorbed up to about p/p0 = 0.4. This is attributed to supermicropores between the individual agglomerated nanocrystals and to adsorption on the appreciable

162

450

-

400

"4

,._, 350 r

SO $3.5C $24C

300-

co 2 5 0 O3

E

2O0

.o. 1 5 0

~=:~=:~:=~

"o

" 100

50

9 ~

0010

~.oe

o.'2

'

' 0'8 o14 p/po

'

o18

'

l io

Figure 2: Nitrogen adsorption isotherms of the samples SOC, $3.5C and $24C. external surface area of the materials. In addition, the isotherm is not fully reversible at low pressures. In order to rule out the possibility of artifacts, we used samples from another batch, different equilibration times, another adsorptive (Ar) and another instrument. However, in all cases the non-reversibility was observed. The isotherm for $3.5C has a totally different character. There is no indication of microporosity, the isotherm increases smoothly, as for the adsorption on nonporous samples and then increases steeply beyond p/p0 = 0.8. This is indicative of a sample with porosity in the meso- to macropore range. As for SOC, non-reversibility of the isotherm was observed. The isotherm of $24C shows the presence of a microporous material with an additional mesopore system, as targeted for in the synthesis. The steep in crease at low relative pressures indicates microporosity, while the increase at higher relative pressures and the high pressure hysteresis proves the presence of mesopores with a rather wide size distribution. The BET equivalent surface area corresponds to 600 m2/g, although this value has to be interpreted with caution, since the BET algorithm can not directly be applied to samples with microporosity. Nevertheless, the appreciable slope in the range of relative pressures between 0.1 and 0.4 is indicative of a rather high surface area in mesopores and on the external surface. Other than for S0C and $3.5C, the isotherm of $24C was fully reversible. The differences in the isotherms of the samples $3.5C and $24C can be explained by the different degree of crystalline organization of the nanoparticles. Full organization of the silicate framework to a crystalline material is expected to take some time, as has been reported for the crystallization of colloidal zeolite A [2]. The nanoparticles which were formed in that study were not immediately crystalline, but started to crystallize from the middle towards the shell. A similar process is most likely occurring in the ZSM-5 system studied here as well. After aging at 3.5 h, the colloidal particles are unporous gel particles, which are linked with the silicone, creating the meso/macropore-system after calcinations. The sample aged for 24 h, in contrast, has been convened to the MFI structure, which creates the zeolite-like microporosity in addition to the meso/macropore system due to crosslinking. Further increases in the aging time from 4 to 14 days did not lead to further increase of the micropore system nor did it have an observable effect on the secondary pore structure, nor did aging at 368 K for 3 days before crosslinking. Similar observations were reported by Persson

163 et al. [ 11 ], who reported that the size of the colloidal zeolite crystal reached a maximum after 48 h. The lack of reversibility of the nitrogen sorption isotherms for samples S0C and $3.5C is not fully clear, yet. A tentative explanation takes into account the fact that in these samples relatively loose packed or not well organized particles are present, which might swell during nitrogen adsorption and thus lead to a non-closure of the isotherm. In contrast, $24C consists of strongly linked, well organized particles, for which no textural changes during nitrogen adsorption are expected to occur. 3.3. Structural Properties

All samples were completely amorphous according to X R analysis. Only the broad feature around 20-25 ~ (2| could be observed, which is characteristic for amorphous silica. TEM analysis of dry sols, however, revealed lattice fringes in some small particles of a few nanometers in size, indicative of crystalline material. This, however, is no contradiction to the XRD data, since very small crystalline domains will lead to so strongly broadened reflections, that they can not be distinguished from the background. Such lack of Bragg reflections has been observed before for similar colloidal zeolite crystals consisting of very small crystalline domains [ 12]. For MFI type materials with a lack of long range periodicity, the IR band around 550 to 580 cm4 assigned to a double five-ring vibration has been interpreted as the most conclusive evidence for the presence of MFI structural elements at least on a local scale [ 14]. This band is also observed for the crosslinked sample $24 (Fig. 3). The sharp bands at 806, 1263 and 2962 cm4 are assigned to vibrations of the silicone linkers. T h ~ were only observed in crosslinked, uncalcined samples, corroborating the results of the Si NMR analysis. Thus, although no Bragg reflections were detected for the samples, TEM and IR data reveal, that the materials consist of small ZSM-5 crystallites, which are linked by silicone moieties.

50-

u 9 40--

f

E ~ 30c

f

f

f

I--

N 2o10-

bO 0

35'oo

30'00

25'oo

20'00

Wavenumbers (cm-l)

15'oo

o'oo

Figure 3 FTIR spectrum of sample $24 in KBr. The band at 563 cm" is indicative for ZSM5 in the crosslinked material. The sharp bands at 806, 1263 and 2962 cm-1 are assigned to vibrations of the silicone linker

164 3.4. Acidity

The acidity of sample $24C was considerably different compared to a commercial HZSM5 sample (SM-27, calcined to give the H-form, Alsipenta). The spectra of the OH-region of $24C after water desorption at 673 K have their prominent features in the range above 3700 cm 4 (Fig. 4). No band at 3610 cm4 which is characteristic for Si-OH-AI bridges is observed, in contrast to normal HZSM-5 which could indicate that aluminum was not incorporated in the crystals in spite of the detection of tetrahedraUy coordinated aluminum in the w 29Si NMR spectra. The spectrum of ~ (a) { 4M' ,v{ I~ _~., I sample $24C shows three bands in the silanol region, an intense one at 3743 cm1' and two shoulders at 3733 and 3724 cmwhich become visible only after dehydration. A band at about 3680 cm-1 which vanishes upon heating, is attributed to vicinal silanols, which are easily dehydrated [15] The 3743 cm 1 band is (c) .i attributed to silanol groups on the external ai00 a~0 a~0 a~0 ~ surface of the particles, which explains the 39o0 ~0 Wavenumber(cm-1) high intensity of this band, since the colloidal particles have a high external Figure 4: IR spectra of sample $24C in the surface area. The other bands at lower OH-stretching region. (a) at 413 K (b) after wavenumbers are attributed to internal evacuation at 573 K (c) after evacuation at silanol groups [16] or tentatively to 673 K. Spectra are offset for clarity. asymmetric hydrogen bonded silanol on the external surface [ 15]. Upon heating to 673 K there is a substantial reduction in the intensity of the silanol bands, which indicates substantial dehydroxylation. Using adsorbed pyridine as probe molecule, Broensted as well as Lewis acidity could be detected in the samples (Fig. 5). The bands correspond to those typically observed for such types of acid sites, i.e. 1454 cm-1 (Lewis) 1491 cm"1 (Lewis and Broensted), 1543 cm 1 (Broensted), 1620 cm"1 (Lewis) and 1635 cm-1 03roensted). The intensity of the 1453 crn-1 .,... .4-

8

3-

.~ .2-

.1-

O1700

18'5o

16'oo

15'5o

I~'oo

~&0

14'00

Wavenumbers (cm- 1)

Figure 5: Spectra of pyridine adsorbed on $24C after different desorption temperature. From top: 413K, 473 K, 523 K, 593 K, 673 K. Spectra are offset for clarity.

~350

165 band is substantial and comparable to that observed for the commercial HZSM-5 reference sample, indicating that the external silanols have Broensted acidic character. Upon heating, most of the Broensted adsorbed pyridine is removed upon heating at 593 K, but a substantial reduction is observed already at appreciably lower temperature. In comparison, the commercial reference sample still had substantial amounts of pyridine adsorbed to Broensted acid sites after heating at 673 K and only at 723 K all pyridine was completely desorbed from the Broensted acid sites. Corresponding to the absence of an OH-vibration around 3610 crn1 the acid site strength detected by thermal desorption of adsorbed pyridine is thus substantially lower than for HZSM-5. The crosslinked colloidal MFI type material has thus a quite different acidity compared to HZSM-5 which should affect the catalytic properties. The altered acidity can be attributed to several factors. (i) the high external surface area and the correspondingly high concentration of terminal silanol groups will provide a high number of moderately acidic sites (ii) the apparent lack of aluminum incorporation to give rise to strong bridging hydroxyl groups and (iii) possibly Si-OH groups resulting from the silicone crosslinker which also might have differing acidity compared to regular silanol groups in zeolites or amorphous silica. 4. CONCLUSIONS Crosslinking of colloidal zeolite crystals with a silicone spacer provides an interesting possibility to create an additional pore system in such materials. With properly aged precursor solutions, in which colloidal particles with the MFI framework topology are present, a solid having both micro- and meso/macroporosity could be produced with the micropore volume corresponding to that expected for an MFI type zeolite. Such materials might be interesting for reactions, in which a high external surface area of zeolitic materials in combination with an additional pore system minimizing diffusion resistances is desired. However, several factors still need to be fine-tuned and better understood. (i) the question why no strong acid sites could be produced in these materials has to be addressed; (ii) the fraction of surface silanols which are used for crosslinking and those which are used for the catalytic reaction need to be optimized; (iii) the possibilities for tailoring the meso/macropore system by using silicone linkers with differently long polysiloxane backbone need to be explored; (iv) the reason for the non-complete reversibility of the sorption isotherm of samples aged for shorter times before crosslinking should be better understood. If these and related questions can be solved, the route could give access for materials with interesting properties. One could, for instance, use them even in the uncalcined form, if applications at low temperatures are envisaged. For a reaction to proceed on the external surface, template molecules in the pore system are not critical and need not be removed. In addition, the silicone molecules could provide a special environment, the properties of which could be tuned by different substituents on the silicon atoms. Catalytic tests of such and related materials are in progress, and results will be available at the time of the conference. ACKNOWLEDGEMENTS We would like to thank B. Spliethoff for recording the TEM. This work was partially supported by the EU (HPRN-CT- 1999-00025) which is gratefully acknowledged REFERENCES 1. B.J. Schoeman, J. Sterte, J.E. Otterstedt, J.Chem.Soc.Chem.Commun., (1993) 994. 2. S. Mintova, N.H. Olson, V. Valtchev, T. Bein, Science, 283 (1999) 958.

166 3. W.H. Dokter, H.F. van Garderen, T.P.M. Beelen, R.A. van Santen and W. Bras, Angew.Chem.Int.Ed.Engl.. 34 (1995) 73. 4. P.P.E.A. de Moor, T.P.M. Beelen, B.U. Komanschek, O. Diat and R.A. van Santen, J.Phys. Chem.B, 101 (1997) 11077. 5. R. Ravishankar, C.E.A. Kirschhock, P.P Knops-Gerrits, E.J.P. Feijen, P.J. Grobet, P. Vanoppen, F.C. De Schryver, G. Miehe, H. Fuess, B.J. Schoeman, P.A. Jacobs and J.A. Martens, J.Phys.Chem.B, 103 (1999) 4960. 6. C.E.A. Kirschhock, V. Buschmann, S. Kremer, R. Ravishankar, C.J.Y. Houssin, B.L. Mojet, R.A. van Santen, P.J. Grobet, P.A. Jacobs and J.A. Martens, Angew.Chem.Int.Ed.Engl., 40 (2001) 2637. 7. H. Ichihashi and H. Sato, Appl.Catal. A - General, 221 (2001) 359 with further references. 8. M.A. Camblor, A. Corma, H. Garcia, V. Semmer-Herledan and S. Valencia, J.Catal. 177 (1998) 267. 9. H. Kath, R. Gl~iser and J. Weitkamp, Chem.Eng.Technol., 24 (2001) 150. 10. For instance: A. Chica and A. Corma, J.Catal., 187 (1999) 167. 11. A.E. Persson, B.J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites, 15 (1995) 611. 12. R. Ravishankar, C. Kirschhock, B.J. Schoeman, D. De Vos, P.J. Grobet, P.A. Jacobs and J.A. Martens, in: M.M.J. Treaty, B.K. Marcus, M.E. Bisher, J.B. Higgins (eds.), Proceedings of the 12th International Zeolite Conference, MRS, Pennsylvania 1999, p. 1825. 13. R. Van Grieken, J.L. Sotelo, J.M. Menendez, and J.A. Melero, Micropor.Mesopor.Mater., 39 (2000) 135. 14. R.F. Howe, Stud.Surf.Sci.Catal., 102 (1996) 97. 15. G.P. Heitmann, G. Dahlhoff, and W.F. HOlderich, J.Catal. 186 (1999) 12. 16. M. Trombetta and G. Busca, J.Catal. 187 (1999) 521.


167

Synthesis and characterization of MCM-22 zeolites for the N20 oxidation of benzene to phenol D. Meloni a, R. Monaci a, E. Rombi a, C. Guimon b , H. Martinez b, I. Fechete ~ and E. Dumitriu ~ a Dipartimento di Scienze Chimiche, Universit~ di Cagliari, Complesso Universitario di Monserrato, S.S. 554 Bivio per Sestu, 09042 Cagliari, Italy Tel.: +39 070 6754422; Fax: +39 070 6754388 b Laboratoire de Physico-Chimie Moltculaire, UMR (CNRS) 5624, Htlioparc Pau-Pyrtntes, 2 Av. P.Angot, 64000 Pau, France Laboratory of Catalysis, Technical University oflasi, 71 D. Mangeron Ave., 6600-Iasi, Romania MCM-22 zeolites with various Si/A1 and/or Si/Fe ratios were synthesized and characterized by XRD, microcalorimetry and other techniques. The catalytic activity and selectivity of MCM22 zeolites were investigated in the gas-phase oxidation of benzene with N20 at 673 K. The oxidation of benzene produces phenol as main product. The influence of the nature of catalysts (i.e., the content of iron, the nature of active sites) on catalytic activity of MCM-22 was investigated. In addition, some results about the nature of coke, which causes the Fe-MCM-22 deactivation, are exposed.

1. INTRODUCT!QN The one step oxidation of benzene to phenol with N20 seems to be an interesting altemative to the current commercial processes for the synthesis of phenol. Various materials have been tested as catalysts (mixed metal oxides, zeolites), but the most promising results were obtained when zeolites of MFI structure, like Fe-silicalite or H-ZSM-5 [1,2], have been used (selectivities greater than 90%, almost 100%). Until now the mechanism of benzene hydroxylation has not been elucidated. Panov et al. [3] proposed that a particular type of extra-framework iron is responsible for the hydroxylation reaction, whereas others proposed that Brtnsted acid sites [4] or the extra-framework aluminum playing the role of Lewis acid sites [5] are involved in this reaction. Recently, Juttu et al. [6] reported the catalytic activity of MCM-zeolites for the gas phase hydroxylation of benzene by dinitrogen monoxide (15% benzene conversion at 603 K and nearly 100% selectivity towards phenol), and they found that the presence of Br6nsted acidity is critical for this reaction, but there is no correlation of the activity with the iron content The last assertion agrees with [4], but it is contradictory to the results reported previously by others authors. Therefore, the aim of this work is to study the oxidation of benzene to phenol with N20 at 673 K and atmospheric pressure, over different samples of Fe-MCM-22 zeolite and to investigate the influence of iron content on their catalytic activity, as well as the influence of the acidity of samples. In addition, some results about the nature of coke, which causes the Fe-MCM22 deactivation, are exposed.

168 2. EXPERIMENTAL

2.1. Catalyst preparation MCM-22 zeolites were hydrothermally synthesized according to a slightly modified procedure given by Corma et al. [7]. Aerosil 200 (Degussa) and sodium aluminate (56 % A1203, 37,5% Na20, Carlo Erba) were used as Si and A1 sources, respectively, and hexamethyleneimine (99% HMI, Aldrich) as a template, while sodium hydroxide was used for the adjustment of pH. A typical gel composition, expressed as molar ratio of the oxides, was 2.7Na20-1.0A1203-30SiO215HMI-1340H20. After aged with stirring for 60 min at room temperature the resulting gel was transferred into Teflon-lined autoclave. The gel was crystallized at 423 K for 7 days under stirring (60 rpm). The crystalline product was washed thoroughly with deionized water until pH 50 O.0

0.2 6 ' 0' 8 ' ' 0'4'0' Relative p~essure'(p/po )" '

1.0

0.0 10

i

I

i

i

i i i i

1 O0

. . . . . . . | i i | i i i

Pore d i a m e t e r (D)/A

1000

Figure 5. A typical nitrogen adsorption/desorption isotherm for the ZSM-5 spheres prepared (a) and the corresponding BJH pore size distribution (b). Open symbols, adsorption; solid symbols, desorption. the SIO2/A1203 ratio of the synthesis solution, the crystal growth rate, the particle concentration and the conversion of metal oxides to MFI are increased whereas the average particle size is decreased. The pore structure of the ZSM-5 spheres prepared comprised of the micropores of the ZSM-5 and mesopores emanating from the removal of the ion exchanger. A typical nitrogen adsorption/desorption isotherm (exemplified with the one for ZSM-5 spheres prepared with a SIO2/A1203 ratio of 100 for 9 h of treatment without aging at a synthesis solution to resin weight ratio of 10) is shown in Fig. 5a. The isotherm is of type IV, characteristic of mesoporous materials but with a relatively steep increase at low relative pressures indicating a substantial microporosity. The dual pore size system is evident from the BJH pore size distribution shown in Fig. 5b. Table 1. Influence of various parameters on the crystallinity (cryst, %) and the BET surface area (SA, m2g-1) of the ZSM-5 spheres prepared. Parameter Treatment time cryst/S A Aging cryst/S A SIO2/A1203 ratio cryst/SA SIO2/A1203 ratio cryst/SA Synthesis solution to resin weight ratio cryst/S A

Sample 7h 10/847

50, 8h 5/740 50, 24h 39/421 1:1, 12h 13/933

8h 24/642 8h 18/660 100, 8h 24/642 100, 24h 76/302 1:2, 12h 18/775

24h 9h 40/641 76/302 9h 22/693 noA1, 8h 66/351/ noA1, 24h 100/324 1:5, 12h 52/629

189 Data on the degree of crystallinity and the BET surface area of ZSM-5 samples is given in Table 1. Generally, the BET surface areas decreased with an increase in the sample crystallinity. From this table the influence of various synthesis parameters on the crystallinity and the BET surface areas can be seen. Thus, samples prepared with aging have lower crystallinity and higher surface areas compared to non-aged ones. Decreasing the SIO2/A1203 ratio and the synthesis solution to resin weight ratio also leads to a decrease of crystallinity and an increase of the BET surface area values.

4. CONCLUSIONS ZSM-5 spheres were prepared employing macroporous strongly basic resin beads as shapedirecting templates. Resin beads were mixed with ZSM-5 clear synthesis solutions and after a hydrothermal treatment at 170~ ZSM-5 was crystallized within the pore system of the ion exchanger. The resin was removed thereafter by calcination leaving behind ZSM-5 spherical particles. Crystallinity of the ZSM-5 spheres was dependent on the treatment time, aging of the synthesis solution, the SIO2/A1203 ratio of the synthesis solution and the synthesis solution to resin weight ratio used. The crystallinity increased with an increase in the time of treatment, the SIO2/A1203 ratio of the synthesis solution and the synthesis solution to resin weight ratio. Also, samples of inferior crystallinity were obtained after aging the synthesis solution - resin mixture for 24 h at room temperature prior to synthesis. The BET surface areas of the ZSM-5 spheres were decreasing with an increase in the sample crystallinity in the range 850 - 300 m 2 g-J. The pore structure of the samples prepared consisted of both microporous (ZSM-5) and mesopores (from the removal of the ion exchanger).

ACKNOWLEDGEMENTS The partial financial support from the Swedish Research Council for Engineering Sciences (VR) is gratefully acknowledged.

REFERENCES 1. K . H . Rhodes, S. A. Davis, F. Caruso, B. Zhang and S. Mann, Chem. Mater. 12 (2000) 2832. 2. X . D . Wang, W. L. Yang, Y. Tang, Y. J. Wang, S. K. Fu and Z. Gao, Chem. Commun. (2000) 2161. 3. Y.-J. Lee, S. Lee, Y. S. Park, K. B. Yoon, Adv. Mater. 13 (2001) 1259 4. B. Zhang, S. A. Davis, N. H. Mendelson and S. Mann, Chem. Commun. (2000) 781. 5. L. Huang, Z. Wang, J. Sun, L. Miao, Q. Li, Y. Yah and D. Zhao, J. Am. Chem. Soc. 122 (2000) 3530. 6. C . J . H . Jacobsen, C. Madsen, J. Houzvicka, I. Schmidt and A. Carlsson, J. Am. Chem. Soc. 122 (2000) 7116. 7. I. Schmidt, A. Krogh, K. Wienberg, A. Carlsson, M. Brorson and C. J. H. Jacobsen, Chem. Commun. (2000) 2157. 8. L. Tosheva, V. Valtchev and J. Sterte, J. Mater. Chem. 10 (2000) 2330.

190 9.

L. Tosheva, B. Mihailova, V. Valtchev and J. Sterte, Microporous Mesoporous Mater. 39 (2000) 91. 10. L. Tosheva, B. Mihailova, V. Valtchev and J. Sterte, Microporous Mesoporous Mater., 48 (2001) 31. 11. Y. Yah, S. R. Chaudhuri and A. Sarkar, Chem. Mater. 8 (1996) 473. 12. A. E. Persson, B. J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites 15 (1995) 611.

Studies in Surface Science and Catalysis 142 R. Aielio, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

191

N o v e l nanocomposite material A. Caratia, C. RIZZO a, L. Dalloro b, B. Stocchia, R. Millinia, C. Perego a a EniTecnologie, Via Maritano 26, 20097 San Donato Mil. (MI), Italy

b Polimeri Europa, Istituto Guido Donegani, Via Fauser 4, 28100 Novara, Italy A spherical nanocomposite S-1/SiO2 has been prepared, starting from a milky slurry of silicalite-1 (S-I) crystallites and a fluid siliceous gel, precursor of an amorphous silica, with a narrow pore size distribution in the mesopore region (MSA). The spherical particles show a core of nanocrystalline S-1 and an outer shell of nanoporous MSA, so the novel composite material is formed by two different nanostructured units, each preserving their individuality. The performances of nanocomposite material in the Beckmann rearrangement of cyclohexanone oxime are comparable with those of S-1, while the MSA binder behaves as an inert. 1.

INTRODUCTION

Nanomaterials, characterized by at least one dimension (i.e. length, width, thickness, pore diameter) in the nanometer size range, usually defined as 1 to 100 ran, are of high interest in catalysis. Indeed their very high surface-to-volume ratio plays a key role in catalysis, being related to accessibility and availability of active sites. The ability to tailor the materials at the nanostructure level opens new opportunities in the optimization of catalysts. For example, the performances of zeolite catalysts are heavily affected by their crystal size. Generally small crystals reduce diflhsion limitation, so increasing catalytic activity and stability. Several examples are reported in literature. For hydrocarbon transformations, the stability of ZSM-5 increases for crystal size of 5-100 nm [1]. For phenol hydroxylation, the catalytic activity of TS-1 decreases as crystal dimensions increase [2]. For Beckmann rearrangement, zeolitic metal-silicates having an external surface area > 5 m2/g and crystallite dimensions < 500 nm give high yields to ~caprolactam [3 ]. Therefore nanocrystalline zeolites (i.e. crystal size < 100 nm) are preferred for catalyst formulation, but tmfortunately, as crystal size decreases, separation of zeolite from its mother liquor becomes more difficult [4]. Another important role in industrial catalysts is played by the binder, indeed zeolites can not be used as made in catalytic reactor, but they have to be mixed with an inorganic binder in order to increase their particle dimension and to permit their use in industrial plant. Binder can modify the performance of the final catalyst, due to its own activity and/or causing variation in diffusion, for example due to pore blockage.

192 In this work the preparation of a novel nanocomposite material based on a zeolite (S-l) and a nanoporous binder (MSA) is described [5]. MSA is mainly mesoporous with a lower contribution of micropores, it shows a narrow pore size distribution that can be modulated in mesoporous region [6]. The catalytic activity of the nanocomposite material is compared with that of the individual components in the Beckmann rearrangement of cyclohexanone oxime.

2. EXPERIMENTAL

2.1. Synthesis

Synthesis ofS-1. S-1 was prepared by adding the solution of 417 g of Si(OC2H5)4 to 102 g of tetrapropyl ammonium hydroxide (TPAOH) and 152 g of water. The so obtained sol had the following molar composition: TPAOH/SiO2 = 0.25, H20/SiO2- 4. It was transferred to 1 liter stainless-steel autoclave and heated at 190 ~ under autogenous pressure and stirred for 2 h. After hydrothermal treatment a milky slurry was obtained. A known amount of slurry was centrifuged to give a white solid, which was dried overnight at 120 ~ and calcined at 550 ~ for 5 h. Synthesis of mesoporous siliceous MSA. 485 g of deionized water were charged in 1 liter stainless-steel autoclave. 92 g TPA-OH (40 wt % in aqueous solution free from alkali cations) and 417 g of Si(OC2H5)4 were added in sequence. The sol was heated at 60~ for lh, obtaining a homogeneous fluid gel with the following molar composition: TPAOH/SiO2 = 0.09, H20/SiO2 = 15. A small part of the gel was aged 15 h at room temperature, dried at 100 ~ and calcined 8 h in air at 550 ~ Synthesis of nanocomposite S-1/MSA. A mixture 30:70 wt/wt (calculated on SiO2) of siliceous MSA gel and milky S-1 slurry was prepared. 4 wt % of polyvinylalcohol (PVA) was added. The mixture was heated at 70~ for 3h and aged for 15 h at room temperature. Then it was pumped to a pressure nozzle of a spray dryer Niro-Mobile HI-TEC, at a rate of 5 1/min. The outlet temperature was about 100 ~ The sample is reported as "Spray A". For comparison a milky slurry of S-l, added with 4 wt % PVA, was sprayed in the same operative conditions. The sample is further reported as "Spray B". 2.2. Characterization X-ray powder diffraction (XRD) analyses were performed with a Philips X'PERT vertical diffractometer equipped with a proportional detector and a secondary monochromator. Data were collected stepwise in the 3 < 20 < 53 ~ angular region with 0.02 ~ 20 step and 20 s/step accumulation time; the CuKa (~ = 1.54178 A) radiation was used. Morphology of the samples was investigated with a Jeol JSM-5400LV Scanning Electron Microscope (SEM), using the Back Scattered Electrons (BSE) technique, and with a Philips EM420 Transmission Electron Microscope (TEM) operating at 120 kV accelerating voltage. For SEM-BSE analysis, a small amount of sample was embedded in epoxy resin and the surface ground with sandpaper until internal sections of the particles were obtained. The surface was finally polished with diamond past in water. To avoid sample charging, a thin layer of carbon was deposited by sputtering on the surface of the sample. For TEM observations, a small amount of powdered sample was embedded in epoxy resin and ultrathin sections ( 5 0 0 - 1000 A thick) were successively cut with a Reichert-Jung ultramicrotome equipped with a diamond knife.

193 Textural properties of all samples were determined by nitrogen isotherms at liquid N2 temperature, using a Micromeritics ASAP 2010 apparatus (static volumetric technique). Before determination of adsorption-desorption isotherms the samples (-~ 0.2 g) were outgassed for 16 h at 350 ~ under vacuum. The specific surface area was evaluated by 2parameters linear BET plot in the range p/pO 0.01-0.2. The total pore volume was evaluated by Gurvitsch rule.

2.3. Catalytic activity The catalysts were tested in a tubular fixed-bed glass microreactor (length: 200 mm, internal diameter: 11.5 mm, axial sheath for thermocouple with external diameter of 4 mm). The catalyst (size: 42+80 mesh, charge: 0.5 g) was diluted with granular quartz up to a volume of 2 cm 3 and positioned in the reactor between two layers of quartz. The cyclohexanone oxime was dissolved in methanol and toluene, preheated and introduced into the reactor together with a nitrogen stream. The reactant mixture was vaporized before coming into contact with the catalyst. Before the test the catalyst was dried in a nitrogen stream for 1 h at a temperature higher than 300 ~ and accustomed to the reaction by feeding only the solvents mixture for at least 30 minutes. Operating condition: Temperature = 350 ~ WHSV (referred to the cyclohexanone oxime feeding and to the catalyst active phase) = 4.5 h -1, methanol/toluene/N2/cyclohexanone oxime molar ratio = 10:l 0:8:1. The mixture of effluent vapors from the reactor was condensed and samples were collected for evaluation of catalytic performances by GC analysis.

3.

RESULTS

S-1 is obtained in quantitative yield with respect to SiO2 in form of a white powder. XRD analysis indicates the presence of a pure and well-crystallized MFI-type structure with the expected monoclinic symmetry. SEM analysis shows the typical morphology of the S-1 crystals, i.e. well formed elongated hexagonal prisms, with dimensions in the range (400-700) x (100-300) x (40-70) nm. Since one dimension of the crystals is lower than 100 nm, this S-1 sample can be classified as nanomaterial. As reported in the experimental section, the milky slurry obtained from the S-1 synthesis was mixed with a siliceous MSA gel and PVA to give the spray-dried sample Spray A. A second sample, Spray B, was also prepared without the addition of the MSA gel to the mixture. Representative SEM-BSE micrographs of the two samples are shown in Figures 1 and 2, respectively. Both samples are constituted by spherical particles with diameter in the range 50 - 120 ~tm, but with different textural properties. In Spray A the particles are constituted by a compact outer shell, 3 - 6 ~tm thick, which includes an inner low density phase (Figure 1). On the contrary, only this low density phase was observed in Spray B, which was obtained without the addition of the binder (Figure 2). A more detailed insight in the particle morphology was obtained by TEM analysis. Figure 3a shows a representative micrograph of sample Spray A, corresponding to the boundary zone between the outer shell and the low density inner phase. It is evident that the outer shell is constituted by the amorphous silica phase (MSA) while the S-1 crystals are mainly present in the core of the particles.

194 The presence of S-1 crystals is unambiguously confirmed both for their typical morphology (see inset in Figure 3a) and for the observation of the lattice fringes at highest magnifications (Figure 3b).

Figure 3 a, b. TEM micrographs of sample Spray A

195

250 - o - s-i -"

..o- MSA

200

Spray A

(b)

Spray B "~ 150 0 r~

100 E

50

0

~

1.E-06

1 .E-05

1 .E-04

1 .E-03

l .E-02

1 .E-01

1 .E+00

Relative Pressure (p/pO) Figure 4 a, b. N2 adsorption/desorption isotherms for the samples under investigation. The relative pressure is reported both in linear (a) and logaritmic (b) scale.

196 In Table 1 the textural properties of Spray A and B are summarized and compared with those of S-1 and siliceous MSA. The isotherms are reported in Figure 4 a. Table 1 Textural properties of S-l, MSA, Spray A, Spray B

Sample

Specific surface area

Total pore volume

(m2/g)

(mUg)

I

390

0.25

MSA

IV + (I)

460

0.34

Spray A

I +(IV)

540

0.37

Spray B

I + (IV)

390

0.31

S-1

Isotherm type

S-1 is characterized by a reversible Type Ia isotherm distinctive of primary micropore filling. Furthermore, the horizontal plateau over a wide range of high p/pO is indicative of a relatively small amount of multilayer adsorption on the open surface. The Spray B isotherm is very similar to S-1 as far as p/pO _ 0.85, indeed they are characterized by the same specific surface area, but Spray B shows a total pore volume higher than S-1. That is justified by the presence of the porosity arising from interparticle voids evidenced by the hysteresis loop at high relative pressure (p/pO > 0.9) and the asymptotic approach to the saturation vapor pressure. MSA shows a Type IV + (I) with a H2 hysteresis loop. So the material is mainly mesoporous with a low contribution of micropores responsible of the adsorption observed at very low relative pressure, p/pO < 0.1. The H2 hysteresis type is usually attributed to different size of pore mouth and pore body (this is the case of ink-bottle shaped pores) or to a different behavior in adsorption and desorption in near cylindrical through pores. Spray A isotherm is a combination of the isotherms of the other three samples. An irreversible Type I + (IV) isotherm with two hysteresis loop is observed for Spray A: the first one shows a lower area with respect to MSA sample, due to the low content of binder in the sample, the second one is quite similar of that observed for Spray B and it is related to the presence of the porosity arising from interparticle voids. In order to better compare the micropore region, the isotherms are also displayed as logarithmic plots (Figure 4 b). In agreement with their uniform micropore size both S-1 and Spray B show a clear inflection point at very low p/p o (10"6 - 10".5,). The inflection point at very low p/pO is not detectable in the MSA, due to the small contribution of micropores. The Spray A shows all the pore size contributions. In Table 2 the catalytic behaviors of S-l, MSA and Spray A are compared after 20 h of time on stream. Spray A and S-1 show the same catalytic performances: a very high conversion of cyclohexanone oxime (Cox) with a high selectivity to e-caprolactam (ScPL). By contrast, MSA shows a very low catalytic activity.

197 Table 2 Catalytic perfong,antes in Beckmann rearrangement. % Cox

o~ SCPL

(after 20h)

(after 20h)

S-1

99

96

MSA

5

70

Spray A

98

95

Sample

See experimental

4. DISCUSSION The proposed formulation process can be performed avoiding the separation of solid phase from mother liquor, that constitutes a problem for nanocrystalline zeolitic materials. Spherical particles of Spray A are formed by a low density core containing the S-1 crystals, while MSA binder forms the outer shell. According to this Spray B, obtained by spray drying the S-1 sltm~ without binder, shows only the low density phase. Nanocomposite Spray A sample exhibits a high surface-to-volume ratio, as described for nanostructured solids, related to both the tridimensional organization of internal surfaces (micro/mesoporosity of S-1 and siliceous MSA, respectively) and to the external exposed faces (nanometric S-1 particles). In the Beckmann rearrangement of cyclohexanone oxime, the siliceous MSA shows the required performances of a good binder: very low catalytic activity and quite good selectivity. In the nanocomposite material the performances of S-1 are not reduced by the presence of the binder. This behavior can be related to the peculiar morphology obtained that plays down the interactions between crystalline and amorphous phases. Amorphous silica can play its role of binder, without modifications of the catalytic behavior of zeolite. Thank to its controlled pore distribution in mesopore region no diffusion problems are evidenced. The described nanocomposite preparation is suitable also for other zeolite structures. These nanocomposites can be used as active phase for preparation of extrudate catalysts or as such as catalyst for fluid or transported bed reactor [6]. 5.

CONCLUSION

A spherical nanocomposite S-l/siliceous MSA can be prepared, starting from the S-1 mother liquor, avoiding the liquid-solid separation. The nanocomposite shows a very peculiar morphology, with a low density core of S-1 and an outer shell of nanoporous amorphous silica, so the interactions between zeolite and binder are minimized. The peculiarity of this nanocomposite is that both the components are nanomaterials characterized by a well defined morphology: S-1 particles exhibit one dimension lower than 100 nm and nanoporous silica has a narrow pore size distribution in the mesopore region. The nanocomposite described has been tested in Beckmann rearrangement, showing the same activity of S-l, no effects of binder is observed.

198

Acknowledgements The authors would like to acknowledge Carlo Barabino and Giuseppe Botti for their precious technical support. REFERENCES 1. E.J. Rosinski, A.B. Schwartz, C.J. Plank, US 3,926,782 (1975). 2. A.J.P.H. van der Pol, J.H.C. van Hooff, Appl. Catal. A, 92 (1992) 113. 3. H. Sato, K. Hirosa, M. Kitamura, Y. Umada, N. Ishii, H. Tojima, EP 242,960 (1987). 4. I. Schmidt, A. Krogh, K. Weinberg, A. Carlsson, M. Brorson, C.J.H. Jacobsen, Chem. Commun., (2000) 2157. 5. A. Carati, G. Botti, L. Dalloro, EP 1,106,576 (2001). 6. A. Carati, C. Rizzo, M. Tagliabue, C. Perego, Stud. Surf. Sci. Catal., 130B (2000) 1085.


199

V i b r a t i o n a l and optical spectroscopic studies on c o p p e r - e x c h a n g e d ferrierite G. Turnes Palomino, a S. Bordiga, b C. Lamberti, b A. Zecchina b and C. Otero Arefina Departamento de Qufmica, Universidad de las Islas Baleares, 07071 Palma de Mallorca, Spain Dipartimento di Chimica IFM, Universit~t di Torino, 10125 Torino, Italy CuLferrierite was prepared by reaction between the protonic form of the zeolite and CuC1 vapour. UV-Vis spectroscopy showed that copper was present in the zeolite mainly in the form of Cu ~ ions; no evidence was found for the presence of Cu I~ ions or copper species in microaggregates. FTIR spectroscopy showed that the zeolite adsorbs CO at liquid nitrogen temperature with formation of CuX(CO)n adducts, mainly mono- and dicarbonyls. After dosing with dioxygen at room temperature CuLferrierite was partially oxidized. This partially oxidized sample was found to be capable of oxidizing carbon monoxide. 1. INTRODUCTION Cu-exchanged ZSM-5 zeolites were found to display high catalytic activity towards the decomposition of nitrogen oxides, including direct decomposition of NO to N2 and O2 and the selective catalytic reduction of NO by hydrocarbons in the presence of excess oxygen [1-9]. Both of these processes are of considerable practical interest, since nitrogen oxides are regarded as being major air pollutants. Because of its potential use in the solution of air pollution problems, Cu-ZSM-5 and other copper-exchanged zeolites are currently under active investigation. The reason why these materials are active for the decomposition of nitrogen oxides is still a matter of discussion and active research, and characterization of the active copper species is a prime requirement for understanding their catalytic activity. In this sense, numerous studies have been carried out in order to elucidate the nature of copper sites in zeolites and to correlate structural data with redox chemistry [10-16] and catalytic activity [3,4,7,11,17,18]. Although most research has been focussed on Cu-ZSM-5, an MFI type zeolite, Cu-exchanged zeolites belonging to other structural types have also been investigated [4,8,16,18,19-25]. These studies have shown that the catalytic activity for the decomposition of nitrogen oxides differs depending on the zeolite structure type [16,25]. This different catalytic behaviour could probably be correlated with the different coordination environment and accessibility of the sites occupied by extraframework copper cations in the different zeolites, but more research is desirable in order to enlarge the available set of experimental results, which would hopefully help in understanding catalytic activity. As a contribution to this field, and following previous studies on Cu-ZSM-5 [17,26], Cu-MOR [27] and Cu-Y [28] zeolites, we report here on spectroscopic characterization of Cuferrierite (Cu-FER) obtained by reacting H-FER with gas phase CuC1. We have used IR and UV-Vis spectroscopies to obtain information on the oxidation state of copper ions, their coordination environment and their redox behaviour.

200 2. E X P E R I M E N T A L

The starting ferrierite zeolite used in this study was a commercial alkaline sample (Na,KFER) supplied by Engelhard Corporation (Iselin, NJ, USA); it had a Si/A1 ratio of 8. From this sample, the corresponding ammonium form was obtained by ion exchange with an ammonium nitrate solution. The NH4-FER sample thus obtained was thermolysed and outgassed at 473 K, to yield H-FER; this protonic form was then reacted with CuC1 vapour at 573 K and finally outgassed at 823 K to remove excess CuC1. More details on the experimental set-up and procedures used were reported (for CuI-ZSM-5) elsewhere [17]. To check ion exchange, Fourier transform IR spectroscopy was used. Nearly complete exchange was obtained, as shown in Figure 1, were IR spectra in the O-H stretching region (before and after ion exchange) are reported. After ion exchange, the band at 3601 cm l , which corresponds to bridged Si(OH)A1 hydroxyls, disappears, thus proving that all acidic protons were replaced by copper ions. Further IR spectroscopic characterization of Cu-FER 0.4was carried out by using adsorbed CO as a probe molecule. For these infrared measurements, a thin selfsupported wafer was prepared and introduced in an IR cell which allowed in situ thermal treatments, gas dosage and v low-temperature measurements o 0.2 (,to be made. The spectra were .,Q collected with a resolution of 2 0 cm ~ on a Bruker FTIR IFS66 ..Q ,< spectrometer equipped with an MCT cryodetector. Although for low temperature measurements the IR cell was permanently cooled with liquid 2 nitrogen, the actual sample temperature (under the IR 0.0 beam) was likely to be ca. IO03800 36;0 34;0 ll0K. Waven um be r/cm -1 UV-Vis diffuse reflectance spectra were obtained at room temperature on a Varian Cary5 spectrometer using a quartz Fig. 1. IR spectra in the O-H stretching region cell designed to allow in situ of ferrierite before (spectrum 1) and after high temperature treatments (spectrum 2) the exchange procedure. and gas dosage.

201 3.

RESULTS AND DISCUSSION

3.1. Outline of the ferrierite structure Ferrierite, structure type FER in the IUPAC nomenclature [29], is a zeolite with orthorombic structure. In the framework, five-membered ring building units are connected to form 10-membered ring channels running along the [001] direction (main channels). These channels have an elliptical cross section, 4.2x5.4 ,~ in diameter, and are intersected at right angles by 8-membered ring channels (3.5x4.8 A) running parallel to the [010] direction. Cation location in Na,Mg-FER has been studied crystallographically by Vaughan [30] and by Barrer and Marshall [31], among others, who found two occupied cation sites. Of these, only one, located in the main channel, is accessible to adsorbed molecules. Recent synchrotron X-ray diffraction [32], luminescence [33] and computational [34] studies on copper-exchanged ferrierite are substantially in agreement with the previous crystallographic (X-ray diffraction) work. According to the synchrotron radiation studies of Attfield et al. [32], the accessible copper site is located at the intersection of 10-membered ring and 8-membered ring channels, and the copper ion is coordinated to only two oxygen atoms of the zeolite framework. This low coordination could presumably lead to a high catalytic activity. Computational studies [34], however, seem to suggest that, besides the site at the channel intersection, Cu I ions (in CuLFER) could also be located on the wall of the main channel, where they would have a higher coordination number: 3 to 4. Although details are not yet completely settled, the situation does not seem to be much different from that found with Cu IZSM-5 for which both EXAFS measurements [26] and theoretical studies [35,36] have shown that there are two different (accessible) sites for extraframework Cu I ions; which can be coordinated to either two or three to four oxygen atoms of the zeolite framework. This structural analogy between CuLFER and CuLZSM-5 suggests that CuLFER can be active in the catalytic decomposition of nitrogen oxides. 3.2. UV-Vis spectroscopy Figure 2 shows the UV-Vis diffuse reflectance spectrum of CuI-FER outgassed at 823 K.

2.5

2.0

--~ -1

1.5

v .~

1.0

v

0.5

0.0 50000

'

i 40000

,

i 30000

'

Wavenumber/cm

I 20000

,

i 10000

-1

Fig. 2. Diffuse reflectance UV-Vis spectrum of Cu-FER outgassed at 823 K.

202 The intense bands observed in the 50000-30000 cm -1 region can be assigned, on the basis of the known spectra of copper complexes [37-39], to metal-to-ligand charge transfer transitions. The absence of any absorption in the 20000-5000 cm -~ range, where d--->d transitions of CUIIwould appear [37-39], proves that copper ions are present as Cu I. Possible formation of reduced species, as copper metal clusters, which would give rise an absorption edge at c a . 17000 cm -~ [40,41], can also be excluded. Therefore, these results from UV-Vis spectroscopy confirm that copper in the Cu-FER sample here described is present, at least mainly, in the monovalent oxidation state; no divalent Cu H species or zerovalent copper aggregates have been detected. 3.3. IR spectroscopy of adsorbed CO Figure 3 shows the IR spectra, in the C - O stretching region, of carbon monoxide adsorbed, at liquid nitrogen temperature and increasing dosage, on the CuI-FER sample previously outgassed at 823 K. At the lowest CO equilibrium pressure the spectrum shows a single infrared absorption band at 2157 cm -1. According to data reported in the literature for other CuLcontaining zeolites [17,19,21,26-28,42,43], the 2157-cm -1 band is assigned to monocarbonyl CuI...CO species. Bands at nearly the same frequency were reported for CO adsorbed on CuI-ZSM-5 (2157 cm -1) [17,26], CuI-MOR (2159 cm -1) [27], CuLY (2159 cm -1) [28] and CuI-~ (2157 cm -l) [44]; they were invariably assigned to monocarbonyl Cu[...CO adducts.

0.8 .

D

//11

0.15.

4

///,.~~p

////

0.6

(1) t-" t~

/

0.4

2170 cm.1 j 2178 cm 1

O

r/

\\\\

o.o5 ________________~______________~/ __,__

0

/

2175

< 0.2

0.0' 2200

' Wavenumber/cm

2100

'

2000

1

Fig. 3. FTIR spectra, at 77 K, of CO adsorbed on Cu-FER. Increasing equilibrium pressure from c a . 0.1 to 20 Torr (spectra 1 to 4). Inset shows an expanded view of the 2225-2150 cm -l region.

203 Upon increasing the CO equilibrium pressure the band at 2157 cm -I is gradually eroded and simultaneously two new bands develop at 2178 and at 2149 cm -~. These facts strongly suggest that copper monocarbonyl species add a second CO ligand to yield CuI(CO)2 dicarbonyl adduct. In agreement with previous reports [17,26-28], the bands at 2178 and 2149 cm -~ are assigned, respectively, to the symmetric and the asymmetric C - O stretching modes of the CuI(CO)2 adducts. At the highest CO equilibrium pressure two new, and weak, IR absorption bands appear at 2191 and 2170 cm -j. These weak bands should be assigned [17,26-28] to tricarbonyl CuI(CO)3 species. This result suggests that a small portion of copper (I) ions can accept a third CO ligand, although most of them remain, even at the highest CO equilibrium pressure used, as CuI(CO)2 adducts. Qualitatively, these results are similar to those found for the CO/CuI-ZSM-5 system [ 17,26], where successive formation of Cu I mono-, di- and tricarbonyl species was also observed upon increasing the CO equilibrium pressure. However, the proportion of tricarbonyl species (as compared to that of dicarbonyls) seems to be smaller in the case of CO adsorbed on CuI-FER, thus suggesting that ferrierite contains a smaller proportion of Cu I ion in a low coordination state. Note that Cu ~ ions which have only two nearby framework oxygens would be more likely to from tricarbonyls than those having three or four oxygen neighbours.

3.4. Characterization and properties of the oxidized copper ferrierite In order to investigate the redox behaviour of Cu I ions present in copper-exchanged ferrierite, a sample previously outgassed at 823 K was put in contact with 20 Torr of oxygen at room temperature. Immediately after dosing with oxygen the sample colour was seen to change from white to grey. This change is reflected in the corresponding UV-Vis spectrum, depicted in Figure 4, which shows the development of a new (and broad) absorption band at about 24000 cm -z. According to published data on copper oxides, this new absorption band can be assigned to optical (d-d) and charge transfer transitions involving Cu n ions [37-39,45]. These results prove that contact with oxygen at room temperature causes oxidation of part of the Cu ~ ions. Most of them, however, should remain in the monovalent state, since the UVVis spectrum (Figure 4) still displays strong features of the non oxidized material (bands in the 50000 to 30000 cm -1 range). Following contact with oxygen, CO was dosed on the sample (at room temperature) and an FTIR spectrum was run; this spectrum is shown in Figure 5. It displays the IR absorption bands corresponding to the CuI(CO)2 species (2149 and 2178 cm-l), and also a complex band which has a sharp peak at 2349 cm -l. The spectrum of adsorbed CO (Figure 5) gives further evidence that most copper ions remain in the monovalent oxidation state even after contact with oxygen. Note, however, that Cu I tricarbonyls are not formed at room temperature. A relevant feature of the full line spectrum in Figure 5 is the presence of a distinctive band which peaks at 2349 cm -~ and has a shoulder at about 2355 cm -~. The sharp peak at 2349 cm -~ corresponds to gas phase carbon dioxide (asymmetric stretching vibration mode ~)3), while the shoulder at about 2355 cm -I should be assigned [46] to the same ~)3 mode of CO2 slightly perturbed by extraframework cations in the zeolite sample. The point to remark is that the above IR measurements prove that the oxidized copper ferrierite sample is capable of oxidizing CO to CO2. This is in consonance with the known ability of Cu H oxides to act as oxidation catalysts [45-50], but it should be noted that CO2 was not produced after CO adsorption on a CuI-ZSM-5 sample previously dosed with oxygen [ 15]. This different behaviour of Cu~-ZSM-5 and CuLFER is worth of further investigation, since it could have relevant consequences in the potential use of these materials as catalysts for air

204 2.5

0.8

2.0 0.6 5

'~1.5

o9

"~ 1.0

0.4

o

0.2

0.5

. .................................................. i 0.0 50000

i

i

,

i

,

t

40000 30000 20000 10000 Wavenumber/cm 1

Fig. 4. Diffuse reflectance UV-Vis spectra of CuI-FER before (dotted line) and after (solid line) contact with oxygen at room temperature.

3.0 2400

23~00'

22~00'

i 21;0

Wavenumber/cm

' 2000

-1

Fig. 5. FTIR spectra of CO (ca. 50 Torr) adsorbed on Cu-FER before (dotted line) and after (solid line) contact with oxygen at room temperature.

pollution control (e.g. in automobile exhaust catalytic converters). Further experimental research is in progress in order to address this interesting point, as well as in an attempt to characterize the oxidized copper species; to this end, EPR spectroscopy is now being applied.

Acknowledgments This contribution has been supported by the Italian MURST: COFIN Area 03 bando 2000, and by the Spanish DGESIC" Project No. PB97-0147.

REFERENCES 1 2 3 4 5 6 7

M. Iwamoto, H. Furukawa, Y. Mine, F. Uemura, S. Mikuriya and S. Kagawa, J. Chem. Soc., Chem. Commun., (1986) 1272. Y. Li and W. K. Hall, J. Phys. Chem., 94 (1990) 6145. M. Iwamoto and H. Hamada, Catal. Today, 10 (1991) 57. J. Valyon and W. K. Hall, J. Phys. Chem., 97 (1993) 1204. B. Wichterlovfi, Z. SobalN and M. Skokenek, Appl. Catal. A., 103 (1993) 269. M. Iwamoto and H. Yahiro, Catal. Today, 22 (1994) 5. M. Shelef, Chem. Rev., 95 (1995) 209.

205 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33

34 35 36 37 38

A. Corma, V. Fornds and E. Palomares, Appl. Catal. B, 11 (1997) 233. A. Fritz and V. Pitchon, Appl. Catal. B, 13 (1997) 1. J. Sfirkfiny, J. L. D'Itri and W. M. H. Sachtler, Catal. Lett., 16 (1992) 241. D.J. Liu and H. Robota, Appl. Catal. B, 4 (1994) 155. B. Wichterlovfi and J. Dedecek, J. Phys. Chem., 99 (1995) 16327. T. Beutel, J. S~irkfiny, G. D. Lei, J. Y. Yan and W. M. H. Sachtler, J. Phys. Chem., 100 (1996) 845. B. Wichterlov~i, J. Dedecek, Z. Sobalfk, A. Vondrova and K. Klier, J. Catal., 169 (1997) 194. G. Turnes Palomino, P. Fisicaro, E. Giamello, S. Bordiga, C. Lamberti and A. Zecchina, J. Phys. Chem. B, 104 (2000) 4064. R. Bul~nek, B. Wichterlovfi, Z. Sobal~ and J. Tichs), Appl. Catal. B, 31 (2001) 13. G. Spoto, A. Zecchina, S. Bordiga, G. Ricchiardi, G. Martra, G. Leofanti and G. Petrini, Appl. Catal. B, 3 (1994) 151. A. Corma, A. Palomares and F. Mfirquez, J. Catal., 170 (1997) 132. Y. Huang, J. Am. Chem. Soc., 95 (1973) 6636. J. Dedecek, Z. Sobal~, Z. Tvaruzkova, D. Kaucky and B. Wichterlovfi, J. Phys. Chem., 99 (1995) 16327. Y. Kuroda, H. Maeda, Y. Yoshikawa, R. Kumashiro and M. Nagao, J. Phys. Chem. B, 101 (1997) 1312. H. Miessner, H. Landmesser and K. Richter, J. Chem. Soc., Faraday Trans., 93 (1997) 3417. Z. Sobah'k, J. Dedecek, I. Ikonnikov and B. Wichterlovfi, Microporous Mesoporous Mater., 21 (1998) 525. P.J. Carl and S. C. Larsen, J. Phys. Chem. B., 104 (2000) 6568. A.E. Palomares, F. Mfirquez, S. Valencia and A. Corma, J. Mol. Catal. A, 162 (2000) 175. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina, F. Geobaldo, G. Vlaic and M. Bellatreccia, J. Phys. Chem. B, 101 (1997) 344. C. Lamberti, S. Bordiga, A. Zecchina, M. Salvalaggio, F. Geobaldo and C. Otero Arefin, J. Chem. Soc., Faraday Trans., 94 (1998) 1519. G. Turnes Palomino, S. Bordiga, A. Zecchina, G. L. Marra and C. Lamberti, J. Phys. Chem. B, 104 (2000) 8641. Ch. Baerlocher, W. M. Meier and D. H. Olson, Atlas of Zeolite Framework Types, 5th Ed., Elsevier, Amsterdam (2001). P.A. Vaughan, Acta Cryst., 21 (1966) 983. R.M. Barrer and D. J. Marshall, J. Chem. Soc., 2296 (1964) 1964. M.P. Attfield, S. J. Weigel and A. K. Cheetham, J. Catal., 172 (1997) 274. B. Wichterlovfi, J. Dedecek and Z. Soball'k, "Proceedings 12th International Zeolite Conference", M. M. J. Treacy, B. K. Marcus, M. E. Bisher, E. Higgins, eds; Materials Research Society: Warrendale, PA, 1999, p. 941. P. Nachtigall, M. Davidovfi and D. Nachtigallovfi, J. Phys. Chem. B, 105 (2001) 3510. D. Nachtigallovfi, P. Nachtigall, M. Sierka and J. Sauer, Phys. Chem. Chem. Phys., 1 (1999) 2019. P. Nachtigall, D. Nachtigallovfi and J. Sauer, J. Phys. Chem. B, 104 (2000) 1738. B.N. Figgis, Introduction to Ligand Fields, Interscience, London, 1966. G. Ferrandi and S. Murlidharan, Coord. Chem. Rev., 36 (1981) 45.

206 39 B. J. Hathaway, in Comprehensive Coordination Chemistry (G. F. Wilkinson, R. D. Gillard and J. A. McCleverty, Eds.), Pergamon Press, Elmsford, N.Y., 1987, Vol. 5, p. 533. 40 J. Texter, D. H. Strome, R. G. Herman and K. Klier, J. Phys. Chem., 81 (1977) 333. 41 C. Lamberti, G. Spoto, D. Scarano, C. Paz6, M. Salvalaggio, S. Bordiga, A. Zecchina, G. Turnes Palomino and F. D'Acapito, Chem. Phys. Lett., 269 (1997) 500. 42 M. Iwamoto, H. Yahiro, K. Tanda, N. Mizumo, Y. Mine and S. Kagawa, J. Phys. Chem., 95 ( 1991) 3727. 43 J. Sfirkfiny, J. L. D'Itri and W. M. H. Sachtler, Catal. Lett., 16 (1992) 241. 44 G. Turnes Palomino, A. Zecchina, E. Giamello, P. Fisicaro, G. Berlier, C. Lamberti and S. Bordiga, Stud. Surf. Sci. Catal., 130 (2000) 2915. 45 L. Gang, J. Van Grondelle, B. G. Anderson and R. A. van Santen, J. Catal., 186 (1999) 100. 46 B. Bonelli, B. Civalleri, B. Fubini, P. Ugliengo, C. Otero Arefin and E. Garrone, J. Phys. Chem. B, 104 (2000) 10978. 47 N.N. Sazonova, A. V. Simakov and H. Veringa, React. Kinet. Catal. Lett., 57 (1996) 71. 48 F. Dannevang, US patent 5,587,134 (1996). 49 A. Wollner and F. Lange, Appl. Catal. A, 94 (1993) 181. 50 T. Curtin, F. O'Regan, C. Deconinck, N. Kntittle and B. K. Hochneff, Catal. Today, 55 (2000) 189.

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

207

Variable temperature FTIR spectroscopy of carbon monoxide adsorbed on protonic and rubidium-exchanged ZSM-5 zeolites C. Otero Arena, a M. Pefiarroya Mentruit, a M. Rodriguez Delgado, a G. Turnes Palomino, a O. V. Manoilova, b A. A. Tsyganenkob and E. Garrone c a Departamento de Quimica, Universidad de las Islas Baleares, 07071 Palma de MaUorca, Spain b Institute of Physics, St. Petersburg University, 198904 St. Petersburg, Russia c Dipartimento di Scienza dei Materiali ed Ingegneria Chimica, Politecnico di Torino, 10126 Torino, Italy Infrared spectroscopic studies have shown that adsorbed carbon monoxide interacts with Bronsted acid Si(OH)A1 groups of the zeolite H-ZSM-5 forming hydrogen bonded H...CO and H...OC species, which were characterized by C-O stretching IR absorption bands at 2174 and 2120 cm1, respectively. In the rubidium-exchanged zeolite, corresponding Rb+--.CO and Rb§ adducts gave characteristic bands at 2161 and 2119 cm~, respectively. Variabletemperature FTIR spectroscopy has shown that, in both cases, C- and O-bonded species are interrelated through a temperature dependent equilibrium in which the proportion of Obonded adducts increases with increasing temperature. By means of van't Hoff plots, the corresponding enthalpy change was found to be AH~ 4.6 kJ mol"l for the CO/H-ZSM-5 system and AH~ 1.8 kJ mol"~ for CO/Rb-ZSM-5. 1. INTRODUCTION Interaction of carbon monoxide with zeolites in their protonic (acid) form is well known [1-3] to give rise to hydrogen bonded OH.-.CO adducts which involve the bridging Si(OH)A1 hydroxy groups of the zeolite (Bronsted acid sites). Similarly, when CO is adsorbed on zeolites exchanged with alkali metal cations, formation of M+.'-CO (M = Li, Na, K, Rb, Cs) adducts occurs. Low-temperature IR spectra of adsorbed CO invariably show a main C--O stretching band which is upward shifted with respect to that of free CO (2143 cm~), and which corresponds to the above adducts [2,3]. For the series of alkali-metal exchanged M +ZSM-5 zeolites, this cation-specific high frequency (HF) band was observed at wave numbers which gradually decrease from 2195 cm"1 for Li§ [4] down to 2157 cm"~ for Cs+ [5]; while for CO adsorbed on H-ZSM-5 it was observed at about 2173 cm"~ [2,6]. Besides the HF band, a minor low-frequency (LF) band was also noted in many infrared spectra of CO adsorbed on cation exchanged zeolites [7-9]. This LF band appears below 2143 crn"1 and is also cation-specific. For the above series of M+-ZSM-5 zeolites it was observed at wave numbers gradually increasing from 2100 crn! for Li+ [4] up to 2122 cm! for Cs+ [5]. Quantum chemical calculations have shown [10-12] that this LF band corresponds to the fundamental C-O stretching mode of M+.--OC adducts, where the CO molecule interacts with the cation through the oxygen atom. It was also recently documented [13] that CO adsorbed on the protonic faujasite-type zeolite H-Y forms both OH...CO and OH...OC hydrogenbonded species upon interaction with the zeolite Bronsted acid sites; which is in consonance

208 with the known ability of CO to form hydrogen bonded OC...HF and CO---HF adducts with hydrogen fluoride [14]. Previous variable-temperature infrared spectroscopic studies [15-17] have shown that Cand O-bonded species are in a temperature-dependent equilibrium which can be described by equation (1), where Z stands for the zeolite framework and M + is an alkali-metal cation or a proton: ZM+...CO = ZM+...OC

(1)

The enthalpy change, AH ~ involved in the above linkage isomerization equilibrium was determined for a few systems. In kJ mol -~, it amounts to 3.8 for CO adsorbed on Na-ZSM-5 [15], 2.4 forCO on Na-Y [16], 3.2 forCO on K-ZSM-5 [18] and 4.3 forCO on H-Y [13]. It is clear that the value of AH ~ depends on the specific cation considered, on the zeolite structure type and (most likely) on the nature of the interaction between CO and the adsorbing centre: hydrogen-bonded or purely electrostatic. However, it seems clear that more experimental measurements are needed before general trends can be established. The aim of this paper is to analyse temperature-dependent infrared spectra of CO adsorbed on H-ZSM-5 and Rb-ZSM-5, so as to extend previous work and to advance towards a more detailed understanding of the factors involved in the interaction between carbon monoxide and zeolites. Some preliminary results on H-ZSM-5 were reported elsewhere [19]; the CO/Rb-ZSM-5 system is documented here for the first time. 2. EXPERIMENTAL The zeolite H-ZSM-5, structure type MFI [20], was synthesized following standard procedures [21]. It had a nominal Si/A1 ratio of 25. Powder X-ray diffraction showed good crystallinity, and absence of any diffraction lines not assignable to the corresponding structure type [22]. From H-ZSM-5, the rubidium-exchanged sample was obtained by ion exchange with an aqueous solution of rubidium nitrate. The process was monitored by disappearance of the original IR absorption band corresponding to bridged Si(OH)A1 hydroxy groups. After ion exchange, the Rb-ZSM-5 sample was also checked by powder X-ray diffraction. For infrared studies, a thin self-supported wafer of each sample was prepared and outgassed (activated) in a dynamic vacuum (residual pressure < 10 -4 Torr) for 2 h at 700 K inside an IR cell which allowed in situ high-temperature activation, gas dosage, and variabletemperature measurements to be carried out. Details on the design and performance of this home-made infrared cell were given elsewhere [23,24]. Liquid nitrogen was used for refrigeration and temperature was measured by means of a platinum resistance thermometer inserted close to the sample wafer. For better thermal contact between the zeolite wafer and the refrigerated cell body, about 0.3 Torr of helium was admitted into the sample compartment before running the background spectrum at liquid nitrogen temperature. Carbon monoxide was then dosed to an equilibrium pressure of about 1.2 Torr, the cell was closed and infrared spectra were recorded at 77 K and on gradual warming up of the infrared cell following removal of liquid nitrogen. A series of spectra was thus taken (for each sample) at about 10 K intervals in the range going from 77 K to room temperature. These transmission FTIR spectra were recorded at 3 cm -I resolution by means of a Bruker IFS66 FTIR instrument. The zeolite blank spectrum taken at 77 K before CO admission, was used as a background; all the spectra shown in this work are background subtracted.

209 3. RESULTS AND DISCUSSION In the O - H stretching region, the background infrared spectrum of the H-ZSM-5 sample showed the characteristic absorption bands at 3748 cm -t (silanols) and at 3615 cm 1 (bridging Si(OH)A1 hydroxy groups). The silanol band was not affected by adsorbed CO (1.2 Torr), while that corresponding to the BrOnsted acid OH group was shifted to 3300 cm l (and enlarged) as a consequence of hydrogen bonding [2,25]. Fig. 1 shows some selected variabletemperature IR spectra in the C - O stretching region. Interaction of CO with the zeolite BrOnsted acid groups gives rise to a major IR absorption band at 2174 cm -~, and also to a minor (and complex) band in the 2115-2130 cm l region.

1.0-

1

J

2

0.9 0.8 0.7 0.6 0 c.Q 0 o9 ,.Q

" c

6 4

.c_

2

10

15

20

25

30

35

40

45

50

55

60

2e/degrees Figure 2. X-ray diffraction patterns of kanemite (1), intercalated kanemite at 3h after the cation exchange (2), silicalite precursors heated at 130 ~ for 5 min. (3), 20 min. (4), 1 h 50 rain. (5), 2 h (6), 4 h (7), 16 h (8) and 46 h (9). To measure the amount of TPA + in the silicalite disc, TG-DTA measurement was done. The weight loss calculated from the exothermic peak due to the combustion decomposition of TPA + are summarized in Table 1 together with product. when the silicalite disc was calcined at 0 to 400~

The hybrid disc was not obtained

because TPA + still remained in the

silicalite discs (Table 1, Runs 1-3 : 4.1 - 9.8 wt %), and TPA + may penetrate from the silicalite side to MOR precursor side during the MOR synthesis process.

Therefore, the

calcination process of the silicalite disc is required to synthesize the hybrid disc.

In the case

of after the TPA + is almost perfectly removed from the silicalite disc (Table 1, Run 4 : 0.3 wt%), MOR was formed on the silicalite disc.

235 Table 1. Weight loss of silicalite disc and product Run No.

Calcination

Weight loss

Product

(~ )

( wt %)

1

no

9.8

( upper / down) MFI / MFI

2

300

8.3

MFI / MFI

3

400

4.1

MFI / MFI

4

550

0.3

MFI / MOR

Figure 3. Hybrid zeolite disc from kanemite (shaped disc with 20 mm in diameter) Hybrid zeolite disc from kanemite are shown in Figure 3. The shaped disc with 20 mm in diameter and observed

ca.

1 mm in thick was obtained.

during the transformation.

The reduction of the volume was not

In kanemite, water molecules are present at the

external surface, between the layers around the Na cations and within the hexagonal rings constituting the layers [7]. d-spacings were

ca.

From XRD data, kanemite and TPA intercalated kanemite

10.2 and 12.7A, respectively. This result suggests that the silicate sheets

of kanemite are intercalated by monolayer of TPA +. intercalated kanemite gave 1.1 of

29Si-MAS-NMR spectra of TPA

Q4/Q3ratio, then the intensity of Q4 peak is higher than that

of original kanemite and increases during the solid state transformation. These results show that silicate layers of kanemite start condensing during the intercalation of TPA + and the phenomenon continues during the solid state transformation, leading to the formation of a three-dimensional network.

236 Moreover, the disc showed good mechanical strength (compression strength) over 10 kg/cm 2. Figure 4 shows compression strengths [8] of hybrid disc and FCC catalyst ( Nikki. Co.:CK-300, including the binder ). In comparison with the commercial product, there is no inferiority on the mechanical strength. Physical properties of the hybrid zeolite disc are summarized in Table 2.

Large

differences in morphology and SiO2 / A1203 ratio between two sides of the disc obtained finally were observed.

The silicalite-side of the disc took an intergrowth structure of

prism-like crystals (ca. 1-2~tm), the other side (mordenite-side) was composed of scale-like crystals (ca. > 1lam). The SiO2 / A1203 ratios for the silicalite- and mordenite-sides of the disc were __12. The CO(NH2)2 concentration does not affect the chemical shift (6) and the width of the Al(tet)band in ZnSP. However, it clearly is important for MgSP (Fig. 1B): the narrowest Al(tet) signal is detected for the solid produced with an increased urea concentration of 0.40 mol/L. This also resulted in increased crystallinity for NiSP and CoSP. Therefore, the materials prepared in the presence of 0.40 mol/L CO(NH2)2 were subjected for the further investigations. 298i MAS NMR spectra of ZnSP and MgSP with Si/A1 = 12 (Fig. 1C) exhibit three resonances of decreasing intensities at about -96, -92 and -87 ppm (Q3 Si(0A1), Q3 Si(1A1) and Q3 Si(2A1), respectively) originating from Si coordinated with A13+ in the tetrahedral sheets [ 1]. The lack of the resonance at -102 ppm evidences the absence of amorphous silica. However, the band at -87 ppm cannot be assigned unambiguously, since Q2 Si(0A1) also shows a resonance at comparable value. The synthetic saponites, especially MgSP, are composed of small platelets [ 1] with consequently a high amount of Si4+ situated at their edges. These atoms might produce a signal in the region of Q2 Si. This surmise is strengthened by the observation that ZnSP composed of a larger particles [ 1] indeed exhibits the band at -86.7 ppm with a lower intensity. Acidic smectites are frequently used as a catalysts. Consequently, we paid attention to proper preparation procedures to convert the materials to their acidic form. Figure 2 shows the 27A1 MAS NMR spectra of H-form of MgSP obtained via acid treatment and calcination of its NH4+-form. After treatment of MgSP with 0.05 M HC1, a new resonance in 27A1 NMR appeared at 55-56 ppm (Fig. 2) due to the non-framework tetrahedral A1 caused by the leaching. The band intensity increased with treatment time. Simultaneously, the relative content of the Al(oct) remains practically invariable (Fig. 2), Hence, some structural 55.5 66.1: distortion occurred upon the acid treatment. Calcination of (NH4)MgSP at 673 K drastically ~" modified its 27A1 NMR spectrum, i.e., the band at 55 ;~ i 8.3 ,,, ppm becomes dominant, and an "-i \ :',.,/;, 3 additional band of the Al(oct) at ~= :: ",.,. ,,/5.1 "-,., around-5.2 ppm appears (Fig. 2), indicating complete structural collapse. Similar results were obtained for ZnSP. i The N2 adsorption isotherm of ZnSP after 24 h of (Fig. 3) is 100 50 0 -50 almost of Type II (typical of the mesoporous solids) [13], with ~5 [ppm] H2 hysteresis, being insignificantly altered with the synthesis time. Figure 2:27A1 MAS NMR spectra of MgSP treated with 0.05 M HC1 during (1) 3 h; (2) 18 h; (3) (NH4)MgSP calcined at 673 K.

:ii

..... ~ ................. . . : : : : : . . : : : : : i : : : :

..................

, : : : : : . . . . . . .::: .:::: :.::::!. ............... . . . . . . . . . . . , , ~ . . . . . . . . . . . . . . . . . . . . . . ......... ! . . . . . . . . . . . . . . . . . . . . . . :

:~ ........................

275 150

ZnSP

MgSP 450

1

x t_--7.~ --~'~/ an

100

--

300 "d

50

150 f

2

O

o

.

0 450

NiSP

ol

~

i

300

......

i

,.

I

.

CoSP

.i

e,y~~' 1 ,4,v

>

300

2

200

~,~".W

@v,,-

, i

0.0

0.2

0.4

0.6

0.8

!

I

i

0.0 0.2 0.4 1.0 Relative pressure [P/Po]

J,

I

0.6

I

, I

}

0.8

I

1.0

Figure 3. N2 adsorption-desorption isotherms of saponites after different synthesis times: (1") 12 h, (1) 24 h and (2) 36 h. (3) Mesoporous silica included as a reference. For the other saponites, the isotherm shape transforms from ahnost Type IIb to Type Ib for the solids obtained for 36 h. This points to the development of micropores. The hysteresis loop is also changed from Type H1 to almost Type H4, characteristic of the slit-like pores [13]. Table 1 Chemical composition and surface area (SBET), total pore volume ( V t o t a l ) , m i c r o p o r e mes0pore ( V m e s o ) volumes of samples after synthesis for 36 h and 0.40 mol/L of urea Sample Si A1 M 2+ aBET Vtotal Vmicro (m2/g) (cc/g) (cc/g) ZnSP 7.38 0.62 7.62 218 0.183 0.004 MgSP 7.43 0.57 5.24 592 0.323 0.303 NiSP 7.39 0.61 6.40 540 0.340 0.250 CoSP 7.39 0.6i 5.12 405 0.270 0.239

(Vmicro)

Vmeso

(cc/g) 0.179 0.019 0.090 0.031

and LL

276 The comparison of these isotherms with that of mesoporous silica (Type II + H1 hysteresis) (Fig. 3, sample 3), suggests that Mg-, Ni, and Co-saponites prepared during 24 hours contain some amorphous admixture. The Si/A1 ratios and the synthesis times (up to 72 h) do not notably affect the saponite textural properties. However, the saponite textural properties strongly depend on the nature of octahedral cations (Table 1). The chemical composition of saponite-like materials (XRF, in molar ratios) reflects the similarity of the Si/A1 ratios in starting gels and in the resulting materials. The amount of the octahedral cations slightly deviates from the ideal saponite composition (Table 1). Temperature programmed reduction of NiSP obtained after 24 h proceeds in two steps with maxima at 570 and 778 K (Fig. 4). The first small TPR peak agrees with the reduction of Ni(OH)2 impurity, while the second one might be attributed to the reduction of the Ni 2+ located in the lattice, since interlayer Ni 2+ cations were reduced at a lower temperature (728 K) as determined for Ni-exchanged MgSP (Fig. 4). The synthesis duration of 36 h lead to the first peak disappearance, and the TPR pattern contains a single reduction step centred at 800 K (Fig. 4) indicating complete Ni 2+ incorporation. The sharper reduction peak relates to increased crystallinity obtained by prolonged synthesis. While in the case of CoSP virtually no Co(OH)2 was detected, the reduction takes place at considerably higher temperatures. The sharper main reduction step comparing with the NiSP (Fig. 4) shows that Co 2+ cations are better distributed in the saponite lattice. All data collected clearly indicate that the nature of octahedral cation has a strong effect on the formation rate of non-hydrothermally synthesized saponite-like materials. Based on the crystallization model proposed by Kloprogge [9], one may assume that the saponite crystallization initiates from separated sheets. Since the main fraction of tetrahedral layers was arranged in the Si/A1 gel prior to the synthesis starting point, the formation of octahedral brucitetype sheets becomes the rate-determining factor of the structure formation. In this case, the ability of octahedral cation to form such sheets at appointed pH plays a decisive role. One such example, here, is the remarkable difference in crystallization rate for the Zn 2+ cation compared to Mg 2+, Ni 2+ and Co 2+. Additional study is necessary to fully understand the underlying mechanism.

f

&

400

600

800

1000

400

996 600

800

1000

Temperature [K] Figure 4. TPR patterns of NiSP and CoSP after different synthesis duration (1) 24 h, (2) 36 h; (3), (4) Ni- and Co- exchanged MgSP, respectively.

277 3.2. Catalysis The catalyst acid-base properties can be characterized by different methods and here we applied 2-methyl-3-butyne-2-ol (MBOH) and 2-propanol (iPrOH) decomposition [14]. The MBOH transformation over NiSP and CoSP containing H + and Na + charge-compensating cations undergoes the acid and redox catalyzed cleavage reaction giving 3-methyl-3-buten-l-in (Mbyne), acetylene and acetone. The same is also observed for iPrOH decomposition. These findings indicate that Ni 2+ and Co 2+ located in the octahedral positions of the saponite lattice, mainly exposed at the crystal can generate redox active sites. The alcohol decomposition over H-forms of ZnSP and MgSP proceeds only through the acid catalyzed pathway. The activity of these catalysts in the used reactions depends on the preparation procedure (Fig. 5), as could be expected on the basis of the NMR data. The acid-treated MgSP and ZnSP exhibit higher activity than that of the H-form prepared therlnal decomposition of the NH4§ form. This can be explained by the structural collapse of this material. The occurrence of an optimum in the catalyst activity with the acid treatment time may be attributed to the optimal ratio between the formed Bronsted and Lewis (non-framework A13+ cations) acid sites. Formation of the non-framework A1, which yields the Lewis sites, was clearly evidenced by 27A1 MAS NMR in the acid activated saponite. Since cumene dealkylation and n-heptane isomerisation require participation of the stronger acid sites compared with the alcohol decomposition, they show the presence of those sites in the H-form of Zn- and Mgcontaining saponites. The Si/A1 ratio influences the catalyst activity in following sequence: 12 > 26 > 8, and by analogy to zeolites, is most likely related to the Next Nearest Neighbour effect.

r---i

~

8

~

.i ~ iPrOH I

~"

2.0

~ - ~

f-'~

n-Heptane

0.36 ~'~ .~,m

6 •215 ,%1 ~ - - ,

E~ 4 ~

M)(."

*

c;f;c

,,,,' -'-" ..\x

~

:"" ~

,',,~ ~

......

=

0 . . . . ::: . . . . . . . " 1 2

,. -.,. x

(x x

il..l

,~

-"-:

' 3

Samples

4

1.0

0.35

0.5

0.0

0

1

2

3

Samples

4

0.34

Figure 5. Quasi steady-state activity (2 h run-time) of MgSP treated with 0.05 M HC1 during (1) 3 h, (2) 6 h, (3) 18 h, respectively, and (4) (NH4)MgSP calcined at 673 K in reaction under study. Left the activity in MBOH and iPrOH decomposition and right the activity in n-heptane isomerisation and cumene dealkylation.

278 4. C O N C L U S I O N S In the present study, we have shown that a wide variety of parameters influence the properties of Zn-, Mg-, Ni- and Co- containing saponite-like materials prepared under non-hydrothennal conditions. Most importantly, the nature of the divalent octahedral cation has a profound effect on the saponite crystallization. Especially, the use of Zn 2+ results in the efficient formation of crystallized materials within 24 h as opposed for Mg 2+, Ni 2+ and Co 2+ for which synthesis times of at least 36 h were required. The octahedral layer composition also leads to variations in the textural properties. Whereas MgSP is mainly microporous, NiSP and CoSP contain a small fraction of mesopores and ZnSP is mainly mesoporous. The materials with predominantly tetrahedrally coordinated A13+ can be obtained at Si/A1 >_ 12. The H-form of Zn and Mgsaponites can be prepared via their acid treatment with diluted HC1. The Ni- and Co-containing solids possess both acid and redox active sites. Furthermore, saponites with Zn and Mg in the octahedral sheets being ion-exchanged with protons display a high catalytic activity in alcohol decomposition, cumene cracking and n-heptane isomerisation. 5. A C K N O W L E D G E M E N T S The authors thank Ms. M.C. Mittelmeijer-Hazeleger (University of Amsterdam, The Netherlands) for the adsorption measurements and Dr. K. Erdmann (Nicholas Copernicus University, Torun, Poland) for some NMR experiments. These investigations were supported in part by Ukrainian Ministry of Education and Science and by a Spinoza grant (to R.A.v.S.) from the Dutch Science Foundation. REFERENCES

1. 2. 3 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

R..J.M.J. Vogels, M.J.H.V. Kerkhoffs, J.W. Geus, Stud. Surf. Sci. Catal. 91 (1995) p. 1153. M. Sychev, R. Prihod'ko, Stud. Surf. Sci. Catal.118 (1998) p. 967. M. Sychev, R. Prihod'ko, I. Astrelin, P.J. Stobbelaar, R.A. van Santen, Book Abstract, EUROCLAY'99 Conf., Krakow, Poland (1999) 135.. R. Prihod'ko, M. Sychev, I. Astrelin, K. Erdmann, E.J.M Hensen, R.A. van Santen, Rus. J. Appl. Chem. (2001), accepted for publication R.A. Sheldon and R.A. van Santen (eds.), Catalytic Oxidation, World Sci. Publ., Singapore, 1995. R.G. Leliveld, W.C.A. Huyben, A.J. van Dillen, J.W. Geus, D.C. Koningsberger, Stud. Surf. Sci. Catal.106 (1997) p. 137. D. Bassett, H.W. Habgood, J. Phys. Chem. 64 (1960) 769. E. Booij, J.T. Kloprogge, J.A.R. van Veen, Clays and Clay Minerals 44 (1996) 774. J.T. Kloprogge, Thesis, University of Utrecht, Utrecht, The Netherlands,1992. L.Li, X Liu, Y.Ge, R.Xu, J. Rocha, J. Klinowski, J. Phys. Chem. 97 (1993) 10389. A.P. Hagan, M.G. Lofthouse, F.S. Stone, M.A. Trevethan, Stud. Surf. Sci. Catal.3 (1979) p.417. J. Dedecek, B, Wichterlova, J.Phys. Chem. B. 103 (1999) 1462. F. Rouquerol, J. Rouquerol, K.Sing, Adsorption by Powder and Porous Solids. Principles, Methodology and Applications, Acad. Press, San Diego, 1999, pp.439-441. C. Lahousse, J. Bachelier, J.-C. Lavalley, H. Lauron-Pernot, A.-M. Le Govic, J. Mol. Catal. 87 (1994) 329.


279

Self-bonded AI, B-ZSM-5 pellets. C. Perri a, P. De Luca a, D. Vuono a, M. Bruno a, J. B.Nagyb and A. Nastroa aDipartimento di Pianificazione Territoriale, Universit/t della Calabria, Arcavacata di Rende, 87030 Rende (Cs) Italy. bLaboratoire de R.M.N, Facult6s Universitaires Notre Dame de la Paix, B-5000 Namur, Belgium. Synthesis of self-bonded ZSM-5 pellets is carried out using Li salts containing gels of composition: xLi20-(y+z)Na20-yAl203-zB203-30TPABr- 150 SIO2-480 H20 where z= 18,36; 5

(18) Based on this model the concentration cj, of component j in the intercrystalline volume can be determined:

S=hj," (uSr + S,h.i, ) cj, = S, hj ,am= (1- exp

40-

8

20-

D

0r 573

623

673

723

773

823

Temperature / K

Figure 6. Conversion-temperature plot for the decomposition of 0.5% N20 in helium (GHSV = 45000 hq): , , Cu(50)-MCM-22; 0 , Cu(100)-MCM-22; A, Cu(150)-MCM-22; 0 , Cu(200)MCM-22 and O, Cu(200)-ZSM-5. Figure 6 shows the catalytic performances in the nitrous oxide decomposition as a function of the temperature for all Cu-MCM-22 samples discussed in this work. The activity increased as a function of the copper content until an exchange level of 141%, that presented exactly the same catalytic behavior of Cu(200)-ZSM-5, reaching 100% of conversion (5,0 x 10 -7 mo1N20.gcat-l.s"l) at 773 K. Cu(200)-MCM-22 (cation exchange level of 183%) is slightly less active than Cu(150)-MCM-22 (141%) and reached the maximum activity only at 823 K. This difference can be accounted for by the presence of a larger amount of copperbased oxidic phases in the first one, as shown by TPR, F T I R - CO and F T I R - NO, and confirms that oligomeric species are the most active for the decomposition of nitrous oxide. Isolated copper species do not seem to be as active as the oligomeric cationic species, since the under-exchanged sample, Cu(50)-MCM-22, is nearly inactive in the range of investigated temperatures, reaching only 7% of conversion at 550~ It is known that the adsorption of NO on Cu-ZSM-5 originates bands related to monoand dinitrosilic complexes of Cu(I) that are partially oxidized to mononitrosilic complexes of Cu(II), while for Cu-MCM-22 the experiments show that the oxidation process is fast and complete. There is also formation of nitro, nitrito and nitrate complexes in Cu-ZSM-5, but in lower amounts than that observed for Cu(200)-MCM-22. These results show that, in spite of the different copper species formed in the MWW and MFI structures, it is possible to attain high activities in MCM-22 zeolite as well.

4. CONCLUSIONS Over-exchanged Cu-MCM-22 is a catalyst as active as Cu-ZSM-5 in the decomposition of nitrous oxide (N20). Its catalytic performances can be interpreted on the basis of the different copper species formed in the MWW structure. TPR experiments showed

350 the existence of isolated Cu2+ ions in ion-exchange sites, cationic oligomeric species, such as [ C u - O - Cu] z+, and copper oxide species deposited in the cavities and/or channels of the MCM-22. FTIR of adsorbed CO confirmed the existence of well dispersed oxidic phases. Upon NO adsorption it was observed the formation of mononitrosylic complexes of copper(I), that were oxidized to mononitrosylic copper(II) complexes. This process was complete and NO pressure dependent for the over-exchanged sample with 183% of cation exchange, where an extensive formation of bands in the region of nitro, nitrito and nitrate complexes, was also observed, and this attributed to the reactivity of copper oxidic phases towards NO. ACKNOWLEDGEMENTS

This work was supported by the Brasilian "Fundaq~o de Amparo ~t Pesquisa do Estado de Sao Paulo" (FAPESP, grant number 99/10391-8) and by the Italian "Ministero della Ricerca Scientifica e Tecnologica" (MURST, "Progetti di Rilevante Interesse Nazionale", Cofinanziamento 2000). M.C. thanks "Associazione per lo Sviluppo Scientifico e Tecnologico del Piemonte" for a grant-in-aid. REFERENCES

1. J. N. Armor, CataL Today 26 (1995) 99. 2. M. E. Leonowicz, J. A. Lawton, S. L. Lawton, M. K. Rubin, Science 266 (1994) 1910. 3. A. Corma, A. E. O. Palomares, V. Forn6s, Res. Chem. Intermed. 24 (1998) 613. 4. A. J. S. Mascarenhas, H. M. C. Andrade, H. O. Pastore, Stud. Surf Sci. Catal. 135 (2001) 322. 5. A. Fraehe, M. Cadoni, C. Bisio, L. Marchese, A. J. S. Mascarenhas, H. O. Pastore, J. Phys. Chem. Submitted. 6. H. Robson, Micropor. Mesopor. Mater. 22 (1998) 551. 7. A. L. S. Marques, J. L. F. Monteiro, H. O. Pastore, Micropor. Mesopor. Mater. 32 (1999) 131. 8. M. Iwamoto, H. Yahiro, Y. Torikai, T. Yoshoka, N. Mizuno. Chem. Lett. (1990) 1967. 9. F. Kapteijn; G. Marbhn; J. Rodriguez-Mirasol; A. J. Moulijn. J. Catal. 167 (1997) 271. 10. C. Lamberti, S. Bordiga, M. Salvalaggio, G. Spoto, A. Zecchina, F. Geobaldo,M. Vlaic, M. Belatreccia, J. Phys. Chem. B: 101 (1997) 344. 11. C. M~quez-Alvarez, G. S. McDougall, A. Guerrero-Ruiz, I. Rodriguez-Ramos, Appl. Surf Sci. 78 (1994) 477. 12. B. Wichterlovfi, J. Dedecek, Z. Sobalik, A. Vondrov~i, K. Klier, J. Catal. 169 (1997) 194. 13. G. D. Borgard, S. Molvik, P. Balaraman, T. W. Root, J. A. Dumesie, Langmuir 11 (1995) 2065. 14. V. M. Rakic, R. V. Hercigonja, V. T. Dondur, Micropor. Mesopor. Mater. 27 (1999) 27. 15. T. Wasowicz, A. M. Prakash, L. Kevan, Microporous Mater. 12 (1997)107. 16. A. Frache, A. Mascarenhas, M. Cadoni, H.O. Pastore, L. Marchese, in preparation. 17. H. J. Jang, W. K. Hall, J. L. d'Itri, J. Phys Chem. 100 (1996) 9416. 18. G. Spoto, S. Bordiga, D. Scarano, A. Zeeehina, Catal. Lett. 13 (1992) 39. 19. G. T. Palomino, S. Bordiga, A. Zecchina, G. L. Marra, C. Lamberti, J. Phys. Chem. B: 104 (2000) 8641. 20. M. V. Konduru, S. S. C. Chuang, J. Catal. 196 (2000) 271.


351

Preparation and properties of M F I zincosilicate Stanistaw Kowalak, Ewa Szymkowiak, Monika Gierczyflska, Girolamo Giordano a Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznafi, Poland, e-mail: skowalak@ main.amu.edu.pl a Department of Chemical Engineering and Materials, University of Calabria, via P. Bucci, 1-87030 Rende (CS), Italy The following study demonstrates a possibility to synthesize zincosilicate MFI by hydrothermal crystallization using sodium zincate as zinc source. It reminds the conventional method of hydrothermal zeolite synthesis. The earlier syntheses involved mostly complexed Zn cations. This way relatively high Zn loading (Si/Zn = 33) can be attained. The properties of the resulting samples depend on the origin of sodium zincate and on zinc content. The resulting products (particularly after ion-exchange modification) show remarkable catalytic activity for cumene cracking, 2-propanol decomposition and cyclohexane transformation towards benzene. 1. INTRODUCTION The syntheses of the crystalline metallosilicate molecular sieves often require protection of the metal cations to be introduced against the precipitation in highly alkaline initial mixture [1]. Usually anions such as oxalates, phosphates or amines [2] were admitted into the initial mixtures in order to coordinate the metal cations. We always [3-5] admitted a substantial amount of phosphoric acid to the initial gels containing Zn(NO3)2, silicon sources (such as Ludox, BDH silica gel, silicic and fluorosilicic acid, sodium silicate), sodium hydroxide and tetrapropylammonium bromide (TPAB). We have noticed a considerable influence of silicon source on properties of resulting materials. The samples obtained with contribution of BDH silica gel were active for cumene cracking [5], whereas the samples of similar Zn content prepared with silicic acid indicated only the presence of weak acid sites and hence enabled to catalyze 2-propanol decomposition [3]. The zinc containing ZSM-5 zeolites were reported as active catalysts for aromatization of light paraffins (Cyclar process) [7], regardless the localization of Zn atoms in the zeolite phase. The localization of zinc atoms in the MFI zincosilicates is still not quite clear [3-5]. Although the MFI zincosilicates indicate ionexchange properties, there is no linear correlation between ion-exchange capacity and zinc loading. The NMR data suggest a migration of Zn atoms from the framework position upon thermal treatment [5]. The following study comprises syntheses of series of MFI zincosilicates of various Zn content. The principal difference in preparation procedure compared to earlier [2-5] works consists in employing sodium zincate as a zinc source. The procedure [6] is analogous to the conventional synthesis of aluminosilicate zeolites that always commences with forming a gel by merging respective sodium silicate and aluminate.

352 2. EXPERIMENTAL

2.1 Synthesis The principal materials used for syntheses were as follows: water glass (SIO2=27.25%, Na20=13.64%, 1-120=59.11%) as silicon source, NaOH, TPABr (Aldrich) as template, zinc was added in form of sodium zincate. The latter was prepared either from Zn(NO3)26 1-120 (Aldrich) or ZnO (POCh, Poland) by dissolving in concentrated NaOH. In the case of Zn(NO3)2 6H20 two different procedures were applied. One comprised the alkalization of zinc nitrate solution with NHaOH and forming of Zn(OH)2, which was then carefully washed in order to remove the nitrates. Then the resulting Zn(OH)2 was dissolved with adequate amount of NaOH solution. In other case the zinc nitrates were treated with stoichiometric amount of NaOH without any nitrate removal. The Zn loading in prepared samples varied in range of Si/Zn 1000 - 33. The initial gel was formed by mixing of water glass and TPABr solutions and then adding the zinc and sodium sources. Some amount of phosphoric acid was used for adjusting pH of the gel to the value of 11. Other acids (e.g. HF, H2SO4) were applied in some experiments and they also resulted in forming of MFI structure. The mixtures were magnetically stirred for an hour at room temperature then placed in the Teflon lined autoclave and heated at 170~ The crystallization period was 23 hours for the samples of lowest Zn content and 46 hours for the others. The crystallization products were washed with distilled water and dried. Then they were calcined in air at 450~ for 16 h. The composition of the mixtures and synthesis conditions are listed in Table 1. Some of the resulting samples were modified by means of ion-exchange with various cations (NH4 +, Na § Ca2+ Cu2+, Zn2+, Ce3§ Al3+). The aqueous solutions (0.1M) of respective salts were applied for modification conducted at room temperature. The ammonium modifications were calcined at 350~ in air in order to obtain the hydrogen forms. 2.2. Sample characterization The resulting samples were characterized by means of XRD (TUR M-62), SEM (Philips SEM 515), FTIR (Bruker Vector 22), DTA, DTG were conducted in air atmosphere with the temperature increase of 5~ ( SETARAM SETSYS 12 ). Adsorption measurements involved nitrogen adsorption isotherm, BET surface area measurement (Quantachrome AUTOSORB-1). ICP-AES (ARL 3410) was used for elemental analysis, TPR with hydrogen (Asap ChemSorb 2705). The catalytic tests for cumene cracking and for 2-propanol decomposition were conducted in a pulse micro-reactor attached to the gas-chromatograph. The catalyst samples (powder 0.015g) were activated prior to reaction at 400~ for 30 min in helium stream. The cumene reaction was carried out at 350~ and 2-propanol decomposition at 230~ respectively. The pulses of 1ttl were injected. The reaction of cyclohexene was carried out in a flow system [8]. The catalyst was pretreated in nitrogen at 500~ and the reaction was conducted at the same temperature. The results of catalytic tests were compared with activity of aluminosilicate zeolites ZSM-5 (Si/Al=27, A1SiPenta) modified with H and Zn cations. 3. RESULTS AND DISCUSSION As shown in Table 1 the crystallization time of 46 hours was sufficient for attaining high crystallinity. The samples of lowest Zn loading attained high crystallinity already after

353 23 hours of heating. The products were easy to be recovered by filtration. The crystallization period longer than 46 hours usually resulted in very hard monolith, which was very difficult to be removed from the autoclave vessel. The XRD data indicate poor crystallinity of the samples of highest Zn loading. The influence of applied zinc source on structure of products is conspicuous. The samples prepared with Zn(NO3)2 treated with N a O H without nitrate anions removal showed lower crystallinity then those involving sodium zincates obtained from zinc oxide or pure hydroxide. Table 1 Gel composition and crystallization condition Gel molar ratio Si 9 Zn 9 T 9 Na 9 P 9 H20

Zn source

Crystallization time (h)

1 1 1 1

: : : :

0,001 0,01 0,02 0,03

: : : :

0,08 0,08 0,08 0,08

: : : :

0,12 0,24 0,24 0,48

: : : :

0,32: 0,32: 0,32: 0,32:

40 40 40 40

Zn(OH)2 Zn(OH)2 Zn(OH)2 Zn(OH)2

23 46 46 46

1 1 1 1

: : : :

0,001 0,01 0,02 0,03

: : : :

0,08 0,08 0,08 0,08

: : : :

0,12 0,24 0,24 0,24

: : : :

0,32: 0,32: 0,32: 0,32:

40 40 40 40

ZnO ZnO ZnO ZnO

23 46 46 46

1 1 1 1

: : : :

0,001 0,01 0,02 0,03

: : : :

0,08 0,08 0,08 0,08

: : : :

0,12 0,24 0,24 0,48

: : : :

0,32: 0,32: 0,32: 0,32:

40 40 40 40

Zn(NO3)2 Zn(NO3)2 Zn(NO3)2 Zn(NO3)2

23 46 46 46

T-template, P- phosphoric acid Typical X R

data are given in Fig. 1.

Source of zinc- ZnO Si/Zn

I....... ]

]1

Source of zinc- Zn(N03)2 ~

Si/Zn :~

33i 4 Fig. 1. X R

142 Th et 24

34

4

14 2 Th et 24

patterns for the as synthesized samples of different zinc origin.

34

354 The calcination at 450~ does not bring any noticeable changes in crystallinity of the samples. The influence of zinc source on the morphology of the resulting samples is even more noticeable in the electron microscopy pictures. The presented SEM photographs (Fig. 2) show that crystallites obtained from Zn(OH)2 are very fine (about l l.tm) and very uniform. The samples prepared in the presence of nitrates are also fine but not as uniform as in the earlier series. In the case of ZnO used for preparing sodium zincate the resulting samples form circular agglomerates of about 5 lam in diameter. The loading of zinc also affects the morphology of the products. The contribution of amorphous phase is seen for the samples of highest Zn content (Fig.3).

Fig.3. SEM photographs of series prepared with ZnO of indicated Zn loading. Thermal analyses (TG and DTA) conducted in air indicate some influence of zinc loading on the course of the curves. The weight loss in the temperature range up to 400~ is almost insignificant for the sample of low Zn content. It becomes more substantial for the samples of higher Zn loading. The weight loss in this range results from desorption of water, but it is not reflected in distinctive DTA peaks. The second step of weight loss starts at about 400~ and is attributed to thermal decomposition of template. The contribution of this step in total weight loss decreases with the Zn content. The decomposition of organic template comprises oxidation which is reflected in very distinctive exothermic effect at about 400~ The temperature of this peak is 400~ for the samples of lowest Zn content and it attains the temperature 414~ for the other samples. The increase in decomposition temperature might result from contribution of the ZnO42- tetrahedra in the framework, that provides stronger bonding of alkylammonium cations. The exothermic effect is proceeded with the endothermic

355 effect at -440~ (only for the sample of lowest Zn content the peak is at 420~ - ~ ~ The effect can be attributed to desorption of template oxidation products. The Ill DTA endothermic effect is overlapped by a J]440oc ~~,~.,,,,,-~ subsequent small exothermic peak at ~-- . . . . ~ . . . . . . 450~ which can reflect a further oxidation. There is no considerable 42~~ TG difference in profiles of DTA curves for the MFI zineosilieate samples of different zinc origin. Thermoprogrammed Si/Zn=100 reduction (TPR) in hydrogen stream (Fig. 5) indicates significant influence of zinc source on reduction ability of the resulting samples. The zinc atoms are the most susceptible to be reduced in hydrogen among all the atoms in sodium forms of the investigated samples. The sample obtained from ZnO does not show any distinctive sharp peak on the TPR curve. Only a very broad and insignificant signal reflecting a hydrogen consumption is noticeable in the range 500 - 850~ The sample prepared with Zn(OH)2 indicates two distinct peaks at 620 and 700~ In the case of the sample prepared with Zn(NO3)2 the profile of the TPR is even more complex. Besides the mentioned peaks at 620 and 700~ there are two additional signals at 650 and 730~ It is 445"C not easy to interpret the results of TPR in details, but it is unambiguous that 414 ~C Si/Zn=33 different behavior of the studied samples results from different zinc surrounding in each case. The nitrogen adsorption measurements DTA (Fig. 6) indicate that specific surface area decreases with increasing zinc content. The samples of lowest Zn loading show the surface area about 300m2/g (BET). , 438oC 465~ TG The decrease in surface area is most drastic for the samples prepared in the 0 200 400 600 800 1000 presence of nitrates. 4000c

Si/Zn=1000

i

Temperature

i

[~

Fig.4. DTA and TG curves (in air, temperature rise 5~ for indicated samples prepared with Zn(OH)2.

356 Si/Zn=lO0

zinc source: ~ 003)2

-

50

r

!

i

i

250

450

650

850

Temperature

[~

Fig. 5. TPR profiles for the sodium forms of indicated samples. The temperature rise was 10~ gas (Ar 90%, H2 10%) flow 100ml/min, sample weight 0.03g.

"

4OO

-.- z,,(om.2

t -i- ZnO

~. 3oo

. . . . .

i

0 1000

100

50

33

Si/Zn

Fig. 6. Correlation between zinc content and surface area (BET).

The catalytic tests for 2-propanol decomposition (Fig.7) indicate much lower activity of the samples (of Si/Zn = 100) obtained in the presence of nitrates. The activity depends very much on cation introduced by means of ion exchange. The samples containing sodium or calcium are practically inactive. The modification with A1 cations always results in highest activity of all series. It is possible that some of introduced A1 ions replace zinc from the framework and contribute in generation of relatively strong acid sites. The hydrogen and cerium modifications of the samples of different origination showed also high activity. It is peculiar

Fig. 7. Activity (expressed as conversion) of indicated samples for 2-propanol decomposition.

357 that modification with Zn cations of the sample prepared with Zn(OH)2 results in very low activity whereas the other two samples modified with Zn are among the most active ones. The decomposition of 2-propanol results mostly in generation of propene. Only in the case of modification with Cu and Ce cations the contribution of acetone in the products is noticeable. The correlation between the Zn loading and the catalytic activity is not very clear. The zincfree silicalite-1 does not show any activity for the reactions applied for the catalytic tests. The introduction of some zinc into the structure (Si/Zn = 1000) is reflected in remarkable activity. Further increase in Zn content (Si/Zn = 100) brings about enhanced activity, but then the activity declines with growing Zn loading (Si/Zn = 50). It is even more dramatic for the Zn richest samples (Si/Zn = 33). In the case of the latter it is likely that low activity can result from lower crystallinity and some structure defects. The obtained samples show some activity for cumene cracking (Fig. 8). The activity of the samples obtained from Zn(OH)2 is distinctively prevailing for the samples of low Zn content and it decreases for the sample of Si/Zn = 50. The activity of series prepared with ZnO increases with Zn loading up to Si/Zn = 50. The last sample is even more active than the analogous sample prepared with Zn(OH)2. All the samples of Si/Zn = 33 are inactive for this reaction. Similarly to the 2-propanol reaction the samples modified with Na and Ca are inactive. 10-

..............................................................................S~i~Zii=i~............. 9

,_8-

10

~'6

%'~ "~6-

~4-

"~4-

r~2-

~2

0

H

Zn

Cu

AI

Ce

Ca

Na form

0

J~................................................................................. AI form Zn(OrI~ ZnO

lOOO

,

lOO

~

50

,

,

33 Si/Zn

Fig. 8. Activity of the indicated samples for cumene cracking. The preliminary test reactions of cyclohexane transformation [8] were conducted over investigated zincosoilicate series modified either with Zn cations or with protons. The same cation modifications of zeolites ZSM-5 were used for comparison. The reaction led to benzene as a result of dehydrogenation as well as to a variety of hydrocarbons as a result of cracking and subsequent oligomerization of cracking products. The conversion of cyclohexene over the samples under study was usually much lower than that obtained over zeolite ZSM-5. The selectivity of zeolites towards benzene was-~7% for the H form and -74% for the Zn form. Similar selectivity to that of Zn-ZSM-5 was noticed for H-form of zincosilicates. The respective Zn-modifications exhibited higher selectivity towards benzene. The zincosilicate MFI samples of intermediate zinc loading (i.e. Si/Zn 100 and 50) showed the highest activity and highest selectivity. The series prepared with Zn(2qO3)2 indicated again much lower activity than the other ones. The results confirm the earlier observation [7] that the origin and localization of zinc inside the MFI structure is not decisive for the catalytic activity for dehydrogenation and Cyclar process.

358 4. CONCLUSIONS The presented series of samples indicate that zincosilicate MFI can be obtained not only with Zn cations introduced into the initial mixture in form of simple salt solution where the ZnZ+ions are complexed by phosphate anions [3-5] or amines [2]. We have demonstrated that sodium zincate can be used as effective source of zinc for the MFI zincosilicate synthesis and the resulting samples can attain a substantial content of Zn. Although some amount of n3PO4 was used for adjusting pH of the gel in the studied series, the high crystallinity could be also attained with other acids such as HF or H2SO4. It is very conspicuous that the origin of applied zincate affects significantly the results of syntheses. The samples that involved zincates obtained by dissolving of ZnO or freshly precipitated pure Zn(OH)2 in NaOH show always better crystallinity and much higher catalytic activity than the series prepared from Zn(NO3)2 treated with NaOH and containing residual nitrates. The presence of nitrates in the crystallization mixtures affects very much the properties of the resulting products. The attempts to introduce the amounts of zinc higher than Si/Zn = 33 usually led to amorphous products. The correlation between zinc loading and catalytic activity is not very clear. The samples of the highest loading are always least active. It is likely that the structure of these samples are partially defected what is reflected in XRD data and in low surface areas. The modification of zincosilicate MFI with various cation influences very much their catalytic properties. It is interesting that aluminum cations introduced by means of ion exchange result in relatively high activity for the reactions involving acid sites. It is likely that some Al ions can replace the framework zinc. REFERENCES:

1. R. Szostak, Molecular Sieves, Principles of Synthesis and Identification, Van Nostrand Reinhold Catalysis Series, 1988. 2. S. Valange, Z. Gabelica, B. Onida, E. Garrone, Proc. 12th Int. Zeolite Conf. Baltimore 1998 (Eds. M.M.J. Treaty et al.), MRS, Warrendale, Penn., IV, (1999) 2711. 3. S. Kowalak, E. Szymkowiak, A. Jankowska, G. Giordano, Proc. 9th Int. Symp. Heterogeneous Catal., Varna, Inst. of Catal., Bulg. Acad. of Sci., Sofia 2000 (p. 229). 4. S. Kowalak, E. Szymkowiak, I. Lehmann, G. Giordano, Stud. Surf. Sci. Catal., 135 (2001) 311. 5. A. Katovic, E. Szyrnkowiak, G. Giordano, S. Kowalak, A. Fonseca and J. B.Nagy, Stud. Surf. Sci. Catal., 135 (2001) 337. 6. S. Kowalak, E. Szymkowiak, M. Gierczyfiska, Pol. Pat. pending, WP 362 (2001). 7. I.E. Maxwell and WH.J. Stork, Stud. Surf. Sci. Catal., 85 (1991) 571. 8. F. Roessner, E. Szymkowiak, to be published.

Studies in SurfaceScienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa(Editors) 9 2002 ElsevierScienceB.V.All rightsreserved.

359

I n f l u e n c e of Cs l o a d i n g a n d c a r b o n a t e s on T P R profiles of P t C s B E A * L. Stievano a, C. Caldeira b, M. F. Ribeiro b and P. Massiani ~, aLaboratoire de R6activit6 de Surface (CNRS-UMR 7609), Universit6 Pierre et Marie Curie, 4 Place Jussieu, casier 178, 75252 Paris CEDEX 05, France, e-mail: [email protected] bDepartemento de Engenharia Quimica, Instituto Superior T6cnico, Av. Rovisco Pals, 1049-001 Lisboa, Portugal The study by Temperature Programmed Reduction (TPR) of calcined PtCsBEA containing increasing Cs loadings, and therefore possessing increasing basicities, shows that the amount of Cs strongly influences the reducibility. The simultaneous mass spectrometry analysis suggests that a special care should be taken in the interpretation of the TPR profiles, especially for basic supports on which carbonates may form upon simple exposure of the samples to the CO2 of the air. Their reduction during TPR produces additional peaks, which alter the patterns and the quantitative evaluation of the consumption of hydrogen. 1. I N T R O D U C T I O N Pt supported on zeolites is involved in numerous catalytic processes. Even though the zeolite support is mostly used in its acidic form [1], the use of basic zeolites is being developed with the aim of controlling the electronic and hence the catalytic properties of the Pt metal particles [2]. A well-known example is that of basic KL that led in the 80's to the development of the industrial PtKL catalyst for dehydrocyclisation of n - p a r ~ s [3]. Three main interpretations were proposed to explain the specific activity of Pt on KL, which were recently reviewed [2,4-5]. They are (i) a molecular die effect, that involves the geometric constraints imposed by the rigid network of L (linear zeolitic channel), (ii) an electron transfer from the negatively charged oxygen atoms of the basic zeolitic framework to the metal particles (metal-support interaction) and/or (iii) the presence of very small Pt particles which might as well influence the electronic structure of the unsaturated Pt atoms at the surface. The high selectivity to aromatisation for Pt supported on other basic oxides with varying pore systems, * Financial support from CNRS, ICCTI CERC3 and the Embassy of France in Portugal is gratefully acknowledged.

360 as in particular Cs-exchanged BEA (three-dimensional large-pore zeolitic framework) [6] and Mg(AI)O [7] suggests that the die effect is, in fact, not determining. On the contrary, the role played by the Pt nanoparticles is supported by the observation on basic PtBEA, that the higher the basicity of the zeolite, the higher the dispersion of Pt and the selectivity towards aromatics in the conversion of n-hexane [6]. The reason for the appearance of small Pt particles in the presence of a strongly basic support, however, is still uncertain. The high dispersion, in fact, depends upon the nature and the location of the Pt species formed along the different steps of the preparation. In the case of PdNaX, it was shown that the acido-base properties of the support influence the stabilisation of the metal cations during the calcination step prior to reduction [8]. This generates specific microenvironments at the internal surface of the zeolite that may affect the final size of the noble metal particles. Temperature Programmed Reduction (TPR) is one of the most commonly used techniques to i d e n t ~ the chemical nature and the location of the calcined Pt species in the porosity; it is often presented as a routine one. However, a survey of literature shows many discrepancies and/or uncertainties about peak assignments. For instance, peaks at 200-250~ in the TPR patterns of calcined PtKL were attributed to the reduction of either Pt09 [9] or Pt ~-+ions [10], and the attributions varied also in the case of PtBEA [11-12]. Moreover, phenomena such as He spillover were taken as responsible for additional H2 consumptions [13]. In the present paper, TPR is employed to study the nature of the Pt species supported on CsBEAs having increasing Cs loadings, and therefore increasing basicities. In view of the controversial literature data, and in order to improve the comprehension of the TPR profiles of Pt supported on basic zeolites [6], the reduction of the catalysts is followed simultaneously by TPR and mass spectrometry (MS). 2. EXPERIMENTAL

2.1. Catalyst preparation Two parent acidic HBEAs, with same Si/A1 ratios (13 • 1), were provided after template removal at 550~ in air (Enitecnologie and RIPP); they are henceforth abbreviated as H~EM and H~RwP. They were ion-exchanged in a 0.5 N aqueous solution of CsC1 (50~ under stirring for 2 hours), the procedure being repeated three times with the aim to fully exchange the protons by Cs cations (nominal Cs/A1 molar ratio of 1.0). The so-obtained CsBEAs (samples Cs-exch.~ENi and Cs-exch.~RiPP) were subsequently exchanged with appropriate amounts of a 2.5.10 .2 M [Pt(NH3)4]C12 aqueous solution (weight of Pt per weight of dry zeolite in the suspension of 1%). The basicity of the Pt-containing CsBEAs was further increased by wetness impregnation with appropriate solutions of CsOH giving samples with nominal Cs/A1 molar ratios of 1.2, 1.4 and 1.6. The samples were finally calcined at 300~ for 2 hours using a high air flow (800 ml/min) and a low heating rate (50~ in order to prevent the

361 autoreduction of the Pt precursor by the NH3 ligands, which would lead to the direct formation of large particles of metallic Pt with no interest un catalysis. They were subsequently stored in room atmosphere before characterisations. All calcined Pt-containing samples will be henceforth referred to as PtCSx~ENI and PtCsx~Ripp, where x is the Cs/A1 molar ratio determined by chemical analysis.

2.2. T e c h n i q u e s of c h a r a c t e r i s a t i o n The contents of Cs, A1, Si, and Pt were measured by atomic absorption (Central Analysis Service of the CNRS, France). The porosity was studied by N2 physisorption on a Micromeritics ASAP 2010 instrument after evacuation in vacuo at 300~ The micropore volumes were determined using the t-plot method. The quantitative TPR profiles were obtained on about 80 mg of sample by using a gas mixture of 5% Hz in At, a gas flow rate of 20 ml/min and a heating rate of 7.5~ Before TPR, the samples were dehydrated in situ in flowing Ar at 200~ for i h to remove most of the adsorbed water. The He consumption was measured by a differential thermal conductivity detector (TCD). In order to analyse the nature of the gases evolved during reduction, a mass spectrometer (Hiden Analytical) was coupled to the TPR apparatus. The mass spectrometer was connected at the exit of the reduction reactor in order to allow the analysis of all gases evolved (mass range 3-50). A desiccator was placed before the TCD in order to adsorb water and/or ammonia that would influence the TPR profiles. 3. R E S U L T S AND D I S C U S S I O N

3.1. I n f l u e n c e o f the s u p p o r t on Cs l o a d i n g For all Pt-containing samples, the Pt loading is between 1.0 to 1.1 wt %, showing that all the Pt(NH3)42+ ions present in solution were exchanged in the 2.0-

-

,i

"/

~= 1.5

.f-

i

i

O

1.0-

C

o

e 0.5-

0.0 0.0

0.4

0.8

1~2

1.6

2.0

Nominal Cs/AI molar ratio

Fig. 1. Experimental Cs loading as a function of the nominal one

362

E'

I-03

g

~)0

'

'

_ ,.~

4oo-

' ........

'

'

!

'

'

'

I

'

'

' 1

'

'

'

HPEN I

~" 300-

0

I

I

I

0.0

0.2

0.4

"

I

I

0.6

0.8

Relative Pressure [P/P~

I

1.0

Fig. 2. N~. physisorption isotherms of the two parent HBEAs zeolite, whatever the parent BEA. Moreover, the close Si/A1 ratios measured for all materials (from 12 to 14) suggest that none of the BEA supports was dealuminated during the exchange treatments. On the contrary, the parent BEA strongly influenced the Cs loadings, as shown by Fig. 1, on which the experimental Cs/A1 molar ratios are reported as a function of the nominal ones. For the PtCsx~RiPP series, the two values are close, indicating that the protons were fully exchanged by Cs § cations during exchange in solution, and confirming the presence of over-exchanged Cs after the subsequent impregnation treatments. At variance, only about haft of the starting protons were substituted by Cs § cations in the case of H~ENI, although the exchange procedure was the same as above. As a result, some Bronsted acidity is left in Cs-exchanged PtCso.4~ENI (nominal Cs/AI=I.0), in agreement with preliminary FT-IR results of the adsorption of NH3 (G. Martra et al., to be published.) The residual acidity is progressively removed by the increasing addition of Cs by impregnation, the experimental Cs/A1 molar ratio reaching a maximum value close to 1 (sample PtCs0.93~ENI, nominal Cs/A1 = 1.6). 3.2. I n f l u e n c e o f t h e p a r e n t B E A o n p o r o s i t y

The lower Cs-exchange capacity of H~ENIis most probably related to a lower crystallinity of this sample, as reflected by broader and less intense X-ray diffraction peaks (not shown) and illustrated by the different shapes of the N2 physisorption isotherms for the two parent HBEAs (Fig. 2). Indeed, although both zeolites have rather similar micropore volumes (0.20 and 0.24 cm3-g"1 for H~ENI and H~RIPP, respectively), the steeper slope of the isotherm for H~ENI reveals additional physisorption of nitrogen by meso- and macroporous surfaces, which is in line with the larger external surface area expected for the much smaller crystallites of H~ENIthan those of H~RIPP(sizes below 80 nm and between 200 to 500 nm, respectively).

363

{

0.3

,

.

.

.

,

.

.

.

,

.

.

.

,

O.2-

0.~

-

PtCsxp~

I

0

o.o

I

0.0

'

,

'

!

,

'

0.4 Experimental

,

I

,

0.8 Cs/AI

molar

,

,

I

t .2 ratio

Fig. 3. Micropore volumes for the two BEA series as a function of the Cs loading.

For the two BEA supports, the addition of Cs has the effect of decreasing progressively the micropore volume, which becomes nearly half of the original one for the highest Cs loadings (Fig. 3). The micropore volume thus reaches a value as low as 0.10 ml/g in the case of PtCs0.93~ENI (nominal Cs/AI molar ratio of 1.6) which puts a limit to a further addition of Cs since the active internal surface of the zeolite would become to small for catalytic applications. 3.3. I n f l u e n c e o f t h e Cs l o a d i n g on r e d u c i b i l i t y The TPR profiles of the two series of samples are shown in Fig. 4 and Fig. 5, together with the MS signals of masses 15 and 18 that are representative of the mare MS phenomena occurring during reduction. Masses 16 and 17 (not shown) were also observed, with profiles almost identical to those of masses 15 and 18, respectively. In addition, very weak peaks were detected in the 28-30 mass range, but they are not discussed due to their very low signal to noise ratio. In the case of the Cs-exch.~ENi support, the consumption of I-I2 is barely identified, as expected from the absence of Pt in this sample (Fig. 5). However, a broad TPR signal with low intensity is detected for Pt-free Cs-exch.BRmP, revealing a non-negligible H2 consumption in the temperature range 250-600~ (Fig. 4). Interestingly, this signal is still identified when Pt is added (PtCso.85~RmP), and an additional broad TPR peak appears at low temperature (100-250~ related to a reduction of the platinum species. The Pt-reduction peak is slightly shifted to higher temperatures for Cs-richer PtCs126~RIPP and PtCsl.42~RIPP (maximum at about 250~ the main effect of the higher Cs loading being a dramatic increase of the intensity. For the Cs over-exchanged samples, a H2 consumption still takes place at high temperatures (400-600~ but its contribution is secondary compared to the total reduction process. The influence of the Cs loading on the shape of the TPR patterns is also well shown by the behaviour of the PtCSx~ENI series (Fig. 5). PtCs0.42~ENI, which has the lowest Cs loading, exhibits a single broad peak centred at 380~ Its shape

364

and position are rather different from those observed for the previous samples, but they are similar to those reported by Lee et al. [14] for Pt supported on HBEA, in line with the residual Bronsted acidity left in PtCso.42~ENI. These authors attributed this peak to a reduction of Pt ~+ ions. However, the high H9 consumption in our case (molar ratio H/Pt = 4.1) would rather suggest the reduction of Pt 4§ On increasing the Cs loading (samples PtCs0.63~ENI to PtCs0.93~ENI), the peak at 400~ strongly decreases in intensity whereas a peak similar to that observed for PtCso.85~RiPP with comparable Cs loading (temperature range 100-250~ becomes dominant. Simultaneously, the He consumption first decreases and then increases again (total molar H/Pt = 1.9, 2.4 and 4.3, on going from PtCs0.63~ENI to PtCs0.93~ENI). Such total H/Pt values, which could agree with the reduction of Pt ~+ and/or Pt 4§ species, are significantly lower than those measured for Cs over-exchanged PtCsl.26~RIPP and PtCs1.42~RIPP (total molar H/Pt = 6.6 and 7.1, respectively). 3.4. N a t u r e o f the r e d u c i b l e s p e c i e s The abnormally high H/Pt molar ratios (higher than 4) for the Cs overexchanged samples cannot be explained by assuming the simple reduction of Pt 2§ or Pt 4§ Therefore, it must involve the contemporaneous reduction of non platinic species, as was proposed in a previous paper [6]. For the two samples, the deconvolution of the TPR profiles shows, on the one hand, that the dominant H2 consumption (intense peak centred at 200~ actually corresponds to a H/Pt ratio slightly larger than 4, in line with the observation of PtO2 by EXAFS (P. Massiani et al., to be published). On the other hand, the residual consumption

~mass

15

........... mass 18

20 A PtCSl.42~'pJpp

"~15 I o E

PtCsl.~l~R~op CO C

C 0

~0

r =

5

-r"

0

m

09 PtCs o ~13.~p 9 0

200

400

Temperature[~

600

800

I [-

0

l

200

l

400

l

600

. , 800

T e m p e r a t u r e [~

Fig. 4. TPR (left) and MS profiles (right) for the samples prepared with H~RIPP

365

occurring above 250~ (molar H/Pt close to 2) is analogous to that measured for Pt-free Cs-exch.~RiPP, suggesting reduction processes in which Pt is not involved. The hypothesis that species other than Pt are reduced during TPR is supported by the MS data. Indeed, an evolution of mass 15 (same for mass 16) is observed for all samples between 250 ~ and 600~ and both the intensity and the shape of the mass 15's signals parallel those of the TPR peaks in this temperature range, as especially well illustrated for C s - e x c h . ~ R i p p (broad signal between 250 ~ and 600~ Fig. 4) and PtCs0.42~ENI (strong TPR and MS signals centred at 400~ Fig. 5). On the contrary, the mass 15's patterns significantly differ from those of mass 18 and must correspond to different phenomena. The parallel observation of masses 15 and 16 could be due to the desorption of ammonia. However, the fact that they are not associated to a parallel evolution of mass 17 (same as mass 18) indicates that the formers must rather be assigned to an evolution of methane, which most probably results from the reductive decomposition of carbonates, possibly formed on the supports by exposition to air. This conclusion is stromgly supported by the He consumption found on Pt-free Cs-exch.~RiPP, Oil which ammonia cannot be present (initial calcination at 550~ no exchange by Pt(NH3)4~+). In this hypothesis, however, the good correlation between the TPR and mass 15 profiles as well as the high intensity of the peaks for partially acidic PtCs0.42~ENI is rather unexpected, since carbonates should better form on the most basic supports. However, other effects such as steric hindrance and size of the crystallites may also play a role in the formation of these compounds. Work is currently being performed to confirm our hypothesis. The association of masses 18 and 17 is typical of the evolution of water, resulting either from reduction phenomena or from the direct desorption of water. For the two Cs-exchanged supports, two peaks of mass 18 are observed. The first one, located below 200~ cannot correspond to a reduction, since no corresponding TPR signal is observed. Most probably, it reveals the presence of

20 el)

PtCso.~l~

0")

~15 -'r" ,..,, O

5

PtCso.721~E.t

E

,~ ~0

PtCso 631~

.m

O

03

09

PtCso.421~F_~

ffJ e'O

mass 15 Cs-exch. PEN~ .......... mass 18

fro

' ' 6 o " 80o Temperature [~

o""'2oo""4oo

6oo"" '" "8oo

....

Temperature ['C]

Fig. 5 TPR (left) and MS profiles (right) for the samples prepared with H~ENI

366 residual water in the Ar used for the pre-treatment, which could have been reabsorbed onto the support during the cooling prior to reduction. Such a reabsorption cannot be excluded for the other samples, for which a similar peak is always observed. The attribution of the second peak of mass 18 (250-600~ is more complex, since it can reveal reduction processes, but could also be partly related to the dehydroxilation of the zeolitic support. The latter effect is expected to occur especially at temperatures higher t h a n 400~ at which a weak but regular production of water is observed for all samples.

4. C O N C L U S I O N S The TPR profiles of calcined PtCsBEAs strongly depend on the Cs loading, which is itself influenced by the zeolite morphology. Moreover, these profiles are affected by the possible reductive decomposition of carbonate species, as suggested by the MS observation of mass 15, attributable to the evolution of CH4 during the reduction of both Pt-free and Pt-containing samples. As a consequence, the TPR data should not be quantitatively interpreted on the basis of the reduction of solely platinum species, but additional consumptions of H2 occurring in a large temperature range have to be taken into account.

REFERENCES

1. P.A. Jacobs, Stud. Surf. Sci. Catal., 12 (1982) 71. 2. D. Barthomeuf, Catal. Rev. Sci. Eng., 38 (1996) 521. 3. J . R . Bernard, in "Proc. 5th Int. Zeol. Conf., Naples" (L.V.C. Rees, Ed.), Heyden, London (1980) 686. 4. R.J. Davis, Heterog. Chem. Rev., 1 (1994) 41. 5. P. Meriaudeau, C. Naccache, Catal. Rev. Sci. Eng., 39 (1997) 5. 6. T. Becue, F. J. Maldonado, A. P. Antunes, J. M. Silva, M. F. Ribeiro, P. Massiani, M. Kermarec, J. Catal. 181 (1999) 244. R. J. Davis, and E. G. Derouane, J. Catal., 132 (1991) 269. 8. A. Sauvage, M. Oberson de Souza, M. J. Peltre, P. Massiani, D. Barthomeuf, , J. Chem. Soc., Chem. Commun. (1996) 1325. D. J. Ostgard, L. Kustov, K. R. Poeppelmeier, W. M. H. Sachtler, J. Catal. 133 (1992) 342. 10. J. Zheng, J. Dong, Q. Xu, C. Hu, Catal. Lett. 37 (1996) 25. 11. L. W. Ho, C. P. Hwang, J. F. Lee, I. Wang, C. T. Yeh, J. Mol. Catal. A: Chemical, 136 (1998) 293. 12. A. B. da Silva, E. Jordao, M. J. Mendes, P. Fouilloux, Appl. Catal. A: General, 148 (1997) 253. 13. G. Martra, S. Coluccia, P. Davit, E. Gianotti, L. Marchese, H. Tsuji, H. Hattori, Res. Chem. Intermed., 25 (1999) 77. 14. J. K. Lee, H. K., Rhee, Catal. Today, 19 (1997) 235. .

.


367

N e w evidences for the fluoride contribution in synthesis of gallium phosphates V. I. P~rvulescu a, C. M. Visinescu a, M. H. Zahedi-Niaki b and S. Kaliaguine b aUniversity of Bucharest, Faculty of Chemistry, Department of Chemical Technology and Catalysis, B-dul Regina Elisabeta 4-12, Bucharest 70346, Romania, E-mail: [email protected] bDepartment of Chemical Engineering Laval University, Quebec (Quebec), Canada, G1K 7P4 Gallium phosphates were prepared by hydrothermal synthesis under autogeneous pressure. Gel compositions with formulas Ga203. x P2Os.y HF.z Py(Pr3N).70 1-120 with x-- 0.5 till 1; y= 1 till 5, z-- 1.7 till 3.4 for Py and 1.5 till 3 for Pr3N were obtained under vigorous stirring by successive dissolution in a phosphoric acid/water solution of Ga203, hydrofluoric acid and template. These materials were characterized by XRD, N2 adsorption-desorption isotherms at 77 K, TG-DTA, F T I ~ NH3-DRWT, XPS, SEM, MAS-NMR. The increase of the F/Ga gel composition ratios was found to enhance the crystallinity of the samples and, in certain limits, to improve the thermal stability. 1. I N T R O D U C T I O N Open-framework materials such as aluminum- and aluminum/gallium phosphates raise an increasing interest because of their potential applications as catalysts and of their specific structural architectures. Since the discovery of microporous aluminophosphates synthesized by using organic amines or quaternary ammonium cations as templates [1] and of microporous gallophosphates [2-4], numerous phosphates-based crystalline materials have been reported [5]. The phosphates have provided new microporous frameworks, since the constituent metal polyhedra are not limited to tetrahedra like the aluminosilicate zeolites [6]. Guth et al. [7] were the first who developed the fluoride synthesis method. The use of this method in combination with the ability of gallium to adopt various coordinations, namely, 4, 5 or 6, allowed the synthesis of new structures like cloverite, ULM, ZON, M ~ , MU etc. [8-12]. The addition of fluoride ions in the synthesis medium was reported to favor mineralization, allowing rapid assimilation of other reagents into a soluble, highly reactive form [13]. Fluorine remains incorporated in the framework. The diversity of these new materials also results from the nature of the organic molecules used as ligands (1,3-diaminopropane, 1,4-diaminobutane, methylamine [13 ], 1-methylimidazole [14], cyclohexylamine [ 15], 1,6-diaminohexane [16], pyridine [ 17], N, N, N', N'-tetramethylethylenediamine [18] etc.). From the studies published to date about the synthesis of these gallium phosphates the complexity of these syntheses emerges. Several factors including temperature, reaction time, reagents and solvents percentage of vessel filling could exert a crucial effect on the product characteristics. Gallium phosphates exhibit however only a poor thermal stability. In spite of the

368 numerous examples of synthesized structures, very scarce information was reported about the effect of the gel fluorine content on the thermal stability of the products. The aim of this contribution is to provide additional information on the role of fluorine in the synthesis of gallium phosphates. For such a purpose, gallium phosphates were prepared using various gel compositions, and Pr3N and Py as templates. These were characterized by XRD, N2 adsorption-desorption isotherms at 77 K, TG-DTA, FTIR, NH3-DRIFT, XPS, MAS-NMR. 2. EXPERIMENTAL Gallium phosphates were prepared by hydrothermal synthesis under autogeneous pressure. Ga203 (Alfa AESAR, 99%+), phosphoric acid (85% H3PO4, BDH), hydrofluoric acid (40%, BDH), pyridine (Py) and tripropyl amine (Pr3N) both from Aldrich were used as reactants. Gel compositions with formulas Ga203. x P2Os.y HF.z Py(Pr3N).70 H20 with x-- 0.5 till 1; y= 1 till 5, z = 1.7 till 3.4 for Py and 1.5 till 3 for Pr3N were obtained under vigorous stirring by successive dissolution in a phosphoric acid/water solution of Ga203, hydrofluoric acid and template. Each dissolving step was controlled by measuring the pH value. After 24h stirring the gel was placed without stirring in a teflon lined autoclave, heated at 445 K for 24 h, and then cooled instantaneously to room temperature. After several washings, the material was dried and analyzed using several techniques: XRD, N2 adsorption-desorption isotherms at 77 K, TG-DTADSC, FTIR, NH3-DRIFT, XPS, MAS NMR. The same analyses were also made for catalysts calcined at 623 K. Crystallinity of the prepared samples was evidenced by X-ray diffraction (XRD) using a Siemens D5000 X-ray diffractometer with CuKa radiation source and scintillation counter. N2 adsorption-desorption isotherms were recorded at 77 K, in the relative pressure range 104 - 0.99, with a Micromeritics ASAP 2000 equipment, after the samples were degassed at 323 K for 12 h, at 10 -4 Pa. TG-DTA-DSC curves were collected using a SETARAM 92 16.18 equipment. In situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) spectra were collected using a Bruker IFS88 infrared spectrometer with KBr optics and a DTGS detector. Pure samples were placed inside a commercial environmental chamber (Spectra-Tech 0030-103) attached to a diffuse reflectance accessory (Spectra-Tech collector). To investigate the stability of the adsorbed ammonia species during temperature elevation, the spectra were recorded under helium (30ml/min) at room temperature, 373,523 and 573 K, after exposure to flowing ammonia for 30 min at room temperature. Infrared Fourier Transform spectra were recorded with the same apparatus. Spectra in the lattice vibrations range were recorded for wafers of sample mixed with KBr in a Pyrex IR cell, with KBr windows. The XPS spectra were obtained using a SSI X probe FISONS spectrometer (SSX -100/ 206) with monochromated Al-Kot radiation. The spectrometer energy scale was calibrated using the Au 4f7/2 peak (binding energy 84.0 eV). For the calculation ofthe binding energies, the Cls peak of the C-(C, H) component at 284.8 eV was used as an internal standard. The composite peaks were decomposed by a fitting routine included in the ESCA 8,3 D software. The superficial composition of the investigated samples was determined using the same software. The Ga, P, Fls and NI~ peaks were investigated.

369 Table 1. Textural characteristics of the investigated samples Sample Gel composition

VP14 VP16 VP17 VP 18 VP 19 VP20 VP 15

Surface area, m 2 g-1 as-synthesized, calcined degassed at 323 K 9 4 12 6 14 7 14 9 14 11 12 9 8 3

Ga203. P205.1.5 HF.1.7 Py.70 H20 Ga203. P205.2 HF.1.7 Py.70 H20 Ga203. P205.3 HF. 1.7 Py.70 H20 Ga203. P205.4 HF. 1.7 Py.70 1-t20 Ga203. P205.5 HF. 1.7 Py.70 H20 Ga203. P205.5 HF.3.4 Py.70 H20 Ga203. P205.1.5 HF. 1.5 Pr3N.70 H20

The 3Zp and ~3C NMR analyses were conducted at room temperature using a Bruker ASX-300 spectrometer. The chemical shifts were referenced to ml(NO3)3 solution in water and adamantane for 31p and 13C, respectively. Spinning rate was 10 KHz and 31p MAS NMR spectra were recorded with both 1H decoupling and CP MAS method. A 90 degree pulse with 4.8 ms duration was used for both 3Zp and ~3C NMR experiments. 3. RESULTS 3. 1. Textural characteristics

Table 1 compiles the surface areas of as-synthesized gallophosphates and calcined samples. All the as-synthesized samples after degassing at 323 K exhibit a very low surface area. The calcination at 623 K led to an important decrease of the surface area, which except for the samples prepared with a Ga203/HF ratio of 1:5, was about 50%. For the samples VP18-VP20, the loss in the surface area was less than 30%.

VP 16

sample

"'|"

15

20

25

211~a

3O

35

4O

45

5O

692

|

J

|

|

w

690

i

w

w

|

w

688

|

|

w

!

w

686

|

|

w

!

~

684

w

|

i

!

w

682

Binding energy, eV

Fig. 1. XRD pattems of as-synthesized samples Fig.2. XPS spectra in the region of Fls energies

w

|

|

680

370 3.2. XRD Fig. 1 shows the X R pattems of two as-synthesized gallophosphates VP-16 and VP-18 samples. The same peaks were detected atter calcining at 625K, but under these conditions their intensity was decreased. The increase of the F/Ga gel composition ratios was found to enhance the crystallinity of these samples and, in certain limits, to improve the thermal stability as was appreciated from the modification in the intensity of the main peaks. A similar XRD pattern was previously reported by Schott-Darie et al. [14] as a novel structure without giving any assignment. These authors also used Py as template agent. After calcining at 773 K, the peaks in the range 2theta 5-10 disappeared, the resulted material being a mixture of amorphous and non-porous crystallized phases. 3.3. XPS XPS analysis (Fig. 2) suggested a different superficial packing of the materials as a function of the gel composition and the thermal treatment (Table 1). This become very evident for the calcined samples where the chemical composition may suggest a surface enrichment in F, while the XPS spectra recorded in the region of Fls binding energies clearly show the disappearance of the species binded to the N containing organic molecule. The two bands in the region Of Fls binding energies may suggest two types of fluorine as Loiseau and Ferey [16] already proposed: one corresponding to the CJa framework, which is strongly bound, and another corresponding to fluorine bound to N species. Fig. 2 show that the species corresponding to the framework F remained unchanged. XPS analysis suggest a different location of fluorine comparing to data reported in the literature [16]. Higher C_ra/F and Ga/N ratios than chemical ones indicate that these elements are not surface species. On the contrary, for P, the chemical and XPS ratios are in a very good concordance. Analysis of the Ga2p3 species gave additional information about the transformations occurring during calcination. In non-calcined microporous phosphates gallium, as Ga2p3, was found to be located at 1116.5 eV. After calcining, a second species was appeared at around 1118.7 eV. The relative ratios of these bands depended on the calcination temperature, but also on the fluorine content in the gel composition. Table 2. Comparative chemical and XPS atomic ratios Sample Chemical analysis of fresh samples C_ra/P Cra/F C_rafN VP14 1.0 1.33 118 VP16 10 1.00 118 VP16 ~ 10 1.00 118 VP17 ~ 10 0.66 118 VP18 ~ 10 0.50 118 VP19 1.0 0.40 118 VP20 1.0 0.40 0.59 VP15 1.0 1.33 1.33 c: calcined

Ga/P 0.96 1.11 1.14 1.03 0.97 0.92 0.87 1.30

XPS analysis Gaff 4.26 3.95 15.68 21.51 35.46 3.95 3.84 13.08

Cra/N 0.57 0.43 0.43 0.41 0.43 0.51 0.56 0.52

371

3.4. TG-DTA-DSC analysis The thermograms exhibited several sequences: two exothermic below 553 K, attributed to water desorption and template degradation, and other three after 553 K up to 993 K due to oxidation and burning of the template and to structure rearrangements caused by the template elimination. 3.5. MAS NMR

31p MAS NMR spectra of both as-synthesized form of gallium phosphate VP16 and VP18 samples are presented in Fig. 3. Two separate peaks resulted at-10.7 and -4.5 ppm These two resonance peaks that are assigned to tetrahedral phosphorous in different crystallografic positions were observed in other gallium phosphates [15]. The calcined form of both these two samples has provided essentially the same 31p NMR spectra with a single broad peak at-10.7 ppm This suggests that organic template molecules are interacting in different manner with lattice phosphorous atoms compared to water molecules. The ~3C MAS NMR spectra of both assynthesized gallium phosphates samples are identical (Fig. 4). These show two resonance peaks at 145.7 and 127.9 ppm, which correspond to Py. The 13C MAS NMR results show that Py molecules are more likely protonated and immobile in GAPO's structures. These two peaks were completely vanished after removing of template by calcination. 3.6. NH3-DRIFT Fig 5 shows typical NH3-DRIFT spectra for gallium phosphates. DRIFT lattice vibration spectra of these samples provided evidence for the structure of the framework of these materials. The main peaks in the range 500-1300 cm -1 correspond to the vibrations ofthe gallium phosphate framework, to the bending of the TO 4 tetrahedra, and to the symmetric and asymmetric stretching vibrations of the T-O-T linkages, respectively. The bands in the range 1400-1600 cm 1 are assigned to various N species. These may correspond to the different bonds in which is involved pyridinic N A large band due to OH and NH species is also present in the range 29003600 cm -"

I

,)2 ] 2acs

(2)

where C~csis the variance of the chemical shift distribution around its average value <scs>. The problem of the distribution of quadrupolar parameters liQ (CQ, 11) has been already addressed by Czjzek et al. [15] in the case of solids isotropic on the average. With these assumptions, Czjzek et al. have shown that the corresponding distribution may be calculated by the following equation:

rlQ(CQ,n)-~-CQ-n-(1--6-)-exp-:-T-Q.(I+ C~Q

2C~Q

)

(3)

In this model, the function liQ (CQ,rl) only depends on one parameter ~Q, which represents the variance of each components of the EFG tensor. The total distribution function that appears in eq. 1 is finally the product of the two chemical shift and quadrupolar coupling distributions.

386

Al, v

I

I

I

I

I

120 lOG 80

!

I

I

60

I

40

I

I

20

I

I

0

I

I

-20

I

I

-40

I

I

-60

I

I

I

--80 (ppm)

Figure 1: Experimental and simulated 27A1MAS spectra (rot = 30 kHz) of hydrated H-Beta zeolite


3.1.27Ai MAS NMR experiments on hydrated H-Beta zeolite The 27A1 MAS spectrum of the hydrated H-Beta sample is shown in figure 1. It exhibits at least four types of aluminum sites: two narrow and well-resolved signals at 58 and 1 ppm both flanked by a broader shoulder on the fight side. The two groups of peaks can be directly attributed to tetrahedral and octahedral aluminum, respectively. The narrow peaks correspond to aluminum with relatively symmetric environment (referred to as Alw-1 and Alvi-1), and the broad shoulders are characteristic of more distorted and environment distributed sites (Alw-2 and Al~-2). The NMR parameters obtained from the simulation of these spectra, taking into account distribution of quadnJpolar coupling constant, are given in table 1. The narrow signal AIIv-I is unambiguously assigned to aluminum at tetrahedral sites in the regular zeolite framework. The second narrow signal Al,a-1 have been attributed to an octahedrally coordinated framework aluminum, consisting of an aluminum atom connected to four lattice oxygen atoms, the oxygen of a water molecule and the oxygen of a hydronium ion [2-5, 16, 17]. This species is induced by the presence of water and only observable in the protonated zeolite. It reverses back to tetrahedral aluminum after an ammonia treatment. The presence of the more distorted tetrahedraJ and octahedraJ coordination should be related to the strong hydrolysis treatment experienced by the zeolite. Indeed, the steaming at 773 K causes complete hydrolysis of a part of the tetrahedral framework aluminum, which is then extracted from the framework.

387 Table 1 NMR parameters determined from 27A1 MAS spectra: " 6cs in ppm, b CO in MHz, c relative intensity ,,

tetrahedral aluminum

sample

,,

,

,,,,

,

,,

octahedral aluminum

58 a (2.7) b

~ 58 (5.9)

2 (1.9)

~-3 (6.6)

[38 %1c

[33 %]

[7 %1

[2 3%]

dehydrated

42 (8.4)

59 (15.3)

53 (2.1)

H-Beta

[52 %]

[45 %]

[3 %]

H-Beta

,,,,,,

The aluminum species corresponding to AlIv-2 and Alvi-2 are then assigned to extraframework tetrahedral and octahedral aluminum. These distorted sites represent about 50% of the total intensity in the sample. From the present study of aluminum in the hydrated zeolite, and from literature results, it must be concluded that the calcination of the Beta zeolite leads to the formation of both tetrahedral and octahedral extra-framework species, corresponding to 50% of the observable aluminum atoms. Nevertheless, both octahedral aluminum species are also expected to transform back to a tetrahedral coordination upon dehydration that should have similar effect than ion-exchange [5]. 3.2. 27A! MAS NMR experiments on dehydrated H-Beta zeolite Dehydration causes dramatic distortion of aluminum coordination geometry.

(b)

'

'

I

(ppm) 400

'

'

'

I

200

'

'

'

I

0

'

'

'

'1

-200

'

'

'

I

-400

'

Figure 2:27A1NMR experiments in fully dehydrated H-Beta zeolite, (a) and (b) static and rotor-synchronized (rot: 10 kHz) whole-echo spectra (c) infinite rate spectrum obtained from the QPASS experiment (rot: 10 kHz).

388 The aluminum atoms are then subject to very large quadrupole interactions which broaden their signal so that many authors consider them as "NMR invisible" [6]. Figure 2(a-c) shows 27A1 whole-echo spectra of the fully dehydrated H-Beta recorded in static conditions and with a 10 kHz spinning speed as well as the infinite rate spectrum obtained from the QPASS experiment. As expected, the static spectrum is broad, -100 kHz, and poorly resolved. We do not distinguish any quadrupolar lineshape, indicating strongly distributed environments and no-averaged residual dipolar interactions or chemical shift anisotropy. With magic angle spinning, the signal splits in at least 7 spinning sidebands, without any characteristic lineshape. The sideband free QPASS spectrum is more resolved and consists in, at least, two components. The main site (maximum around -0 ppm) exhibits an asymmetric lineshape with a steep low-field edge and a trailing high-field edge, characteristic of a distribution of Electric Field Gradient (EFG). All the spectra were fitted to retrieve isotropic chemical shifts and population with MASAI. The spectra were considered simultaneously in order to get the best agreement. The finite spinning rates and pulses train efficiency were taken into account by the sottware. The proposed deconvolution is based on some hypotheses. According to TRAPDOR experiments reported elsewhere [ 10], we postulated the presence of two different aluminum sites with quadrupole coupling constant in the range of 10 and 16 MHz. As the lines are characteristic of NMR parameters distribution, a Czjzek model was used for the distribution of EFG [15]. Finally, even if no aluminum with lower CQ constant could be detected in TRAPDOR experiment, the 10 kHz echo spectrum clearly shows the presence of a third weaker component. This signal could be attributed to aluminum not connected to proton. Simulated and experimental spectra are shown in figure 2, and corresponding NMR parameters are given in table 1. According to this simulation, aluminum atoms are found at least in three different environments in the dehydrated H-Beta zeolite. All of them are tetracoordinated to oxygen atoms, which agrees with the hypothesis concerning the transformation of octahedral to tetrahedral aluminum upon dehydration. Each of the two main components represents approximately 50% of the total intensity.

J

:

I

9

j t'~

./

.....

//- 1000

Fel.IMFIht-S

hydrothermal synthesis

> 1000

-

Fel.IMFIht-A55


55

Fe2.2MFIht-A54


54

Fe2.3MFIht-A90


Fel.sMFIie-A13

ion exchange

Fe3.6MFIie-A13

Fe0.4MFIcvD-A13

AI/Fe

%Fe

(wt.)

Note

< 0.05

1.1

Commercial samples fi"omAlsi Penta (SN27) Pure Fe-fi,ee Hsilicalite-1 Pure Al-free Fesilicalite-1

1.48

1.1

(Fe-A1)ZSM5 sample

0.75

2.2

(Fe-A1)ZSM5 sample

90

0.91

2.2

(Fe-A1)ZSM5 sample

13

3.71

1.8

ion exchange

13

1.85

3.6

CVD*

13

16.7

0.4

200

....

Parent zeolite is commercial Alsi Penta (SN27)

FeC13 is mixed with the zeolite and heated in air * 9FeC13 sublimation. # C indicates commercial sample, S silicalite and A indicates the Si/A1 ratio.

Fe0.sMFIss~-A62

solid state reaction

62

1.97

0.8

2.2 Catalytic Tests

Before the catalytic tests the Fe-(A1)MFI samples were activated in-situ at a temperature ranging from 600 to 700~ in the presence or absence of steam. The catalytic tests were made in a fixed-bed reactor at a temperature typically of 400~ feeding a mixture containing (i) 3% benzene and 6% N 2 0 in h e l i u m o r (ii) 2 0 % benzene and 3% N 2 0 in helium.

506 Sometimes steam (up to about 3%) was also added to the feed. The total flow rate was 3 L/h and the amount of catalyst 0.Sg (contact time of 0.6 s g/ml). The feed was prepared using an already calibrated mixture of N20 in helium and adding benzene using an infusion pump and a vaporizer chamber. The feed could be sent either to the reactor or to a by-pass for its analysis. The feed coming out of the reactor or from the bypass could be sent to vent or to one of two parallel absorbers containing pure toluene as the solvent (plus calibrated amounts of tetrahydrofuran as the internal standard) cooled at about 15~ in order to condense all organic products. The line to the absorbers was heated at about 200~ in order to prevent condensation of the products. The vent, after condensation of the organic products, was sent to a sampling valve for analysis of the residual gas composition. The reactor outlet stream was sent alternatively to the two parallel absorbers for a given time (typically 3 or 5 rain), in order to monitor the change in the catalytic activity averaged over this time. N20, O2, N2 and total oxidation products (CO and CO2) were analyzed using TCD-Gas chromatography and a 60/80 Carboxen-1000 column, whereas benzene and phenol (as well as other minor aromatic by-products) were determined by FID-Gas chromatography using a ECONO-CAP SE-30 "wide bore" column or a Mass-GC equipped with a capillary Chrompack CP-Sil 5CB-MS Fused Silica column.


3.1 Role of catalyst composition Reported in Figure 1 is the catalytic behavior (productivity and selectivity to phenol in benzene hydroxylation with N20) as a function of the time-on-stream of a series of Fe(Al)MFI type catalysts prepared by hydrothermal synthesis. The characteristics and composition of these samples are reported in Table 1. The samples were selected in order to summarize the influence of the composition of the catalyst on the activity and rate of deactivation. A commercial H-ZSM5 sample (H-ZSMS-C) showed effectively good activity, in agreement with literature data. However, it deactivated very quickly and in about 1 h the productivity decreased by a factor of about 50. The selectivity to phenol progressively decreases from an initial value of around 65-70% to less than 10%. It should be noted that similarly to most of the commercial samples, H-ZSMS-C contains traces of iron (see Table 1) due to contamination during the industrial preparation. When a pure Fe-free Silicalite-1 catalyst (H-MFI-S) was tested, no formation of phenol at all could be observed (Figure 1). Silicalite-1 has the same MFI structure as ZSM-5, but is A1flee. The synthesis of Silicalite leads to the presence of structural defects (hydroxyl nests), as confirmed by the presence of silanol groups evidenced by FTIR spectroscopy. Pretreatment of the sample at 700~ in helium before the catalytic tests causes dehydroxylation of these hydroxyl nests forming oxygen vacancies which may activate N20, because they behave as F-centres. In agreement, the catalyst shows an initial activity in N20 decomposition (around 22% N20 conversion aiter 5-10 min of time-on-stream). This suggests that defects in the zeolite can activate N20, but are unable to either form active oxygen species in benzene hydroxylation and/or to activate the organic substrate (benzene) for this reaction.

507 1,6 ,-'r "7 ..c -~

1,4

+ --O-~ --v--

1,2

F: E 1,0 ...... or-. 9 0,8 o

+ Fel.IMFIht-S - - 0 - - Fe 2 aMFI~A90

0,6

._> "o

H-ZSM5-C Fez2MFI~-A54 H-MFI-S Fel.lMFI~-A55

0,4

~o

13. 0,2 0,0 0,0

0,5

1,0

1,5

2,0

2,5

Time, h

I

100

9

I

I

I

I

80 -.

~

0

60

o eeQ.

~

40

2o 0

0,o

.= ~v,

,

h

i 0,5

,

L~,

,

i

1,0

L

L

,

L

J

1,5

L

,

,

,

i

2,0

,

,

,

,v

i

J_

2,5

Time, h

Figure 1. Behavior of the catalysts as a function of the time-on-stream in the productivity and selectivity to phenol by benzene hydroxylation with N20. Catalysts pretreated at 700~ in helium for two hours. Reaction conditions: T=400~ 3% benzene, 6% N20, balance He. When an Al-free Fe-Silicalite (Fel.IMFIht-S) is used the productivity to phenol is low, but quite stable. No change in the productivity was observed up to about 20 hours of time-onstream. The selectivity to phenol is initially low and progressively increases up to final constant values of about 50%. The conversion of N20 is low (about 5%) and nearly constant with increasing time-on-stream. Therefore, in the absence of A1 the activity of the catalyst towards phenol synthesis is low, but remarkably stable, suggesting that A1 sites may play a role in the synthesis of phenol, but also in the side reactions leading to catalyst deactivation. When both Fe and A1 are present in the zeolite (Fel.lMFIht-A55 and Fe2.2MFIht-A54) the productivity to phenol increases by a factor of about 10 or 20, respectively and the selectivity

508 to phenol also markedly increases reaching values higher than 90% for Fel.lMFIht-A55. The productivity to phenol is approximately proportional to the amount of iron (compare phenol productivities for Fe~.tMFIht-A55 and Fe2.EMFIht-A54; the latter has twice the iron content and double the productivity of the former, while the Si/A1 content is the same). The selectivity to phenol, on the contrary, is higher in the case of the sample having the lower iron content. The trend with time-on-stream is quite similar in the two catalysts. The productivity to phenol decreases, although much less dramatically than in the case of HZSMS-C, while selectivity to phenol increases. The conversion of N20 is initially around 40% and then decreases to about 10-15% for Fe1.lMFIht-A55. When this result is compared with that of the sample having a comparable iron content, but no aluminium ions (Fel.lM~Iht-S), it is evident that the presence of A1 ions (or probably better sites related to A1, such as AI-OH Bronsted acid) together with iron ions is a condition necessary for efficient activation of N20. However, sites for the decomposition of N20 are also present which progressively become inactivated by the carbonaceous-type species formed on the catalyst, causing its deactivation. In fact, the rate of N20 conversion in Fel.lMFIht-A55 decreased by a factor of about 6-8 during the 2.5 hours of the experiments (Figure 1), while the productivity to phenol decreased by a factor of about 2. The side decomposition of N20 produces 02 (which is also detected in the reactor outlet) and probably 02 determines the total combustion of benzene, phenol or reaction intermediates and thus the formation of CO2 (CO is usually observed only in traces). This explains why in parallel to the decrease in N20 conversion, an increase in the selectivity to phenol is observed. With increasing iron content (Fe2.2MFIht-A54) the initial conversion of N20 is about 65% and then decreases to about 20-25% after 2.5h of time-on-stream. This indicates that the increase in iron determines an increase in the number of active sites for phenol synthesis (the productivity is about twice), but probably also increases the formation of a higher amount of a second type of iron species. Reasonably these species are small iron-oxide particles within the zeolite cavities which form in the process of partial migration of iron from framework to non-framework positions during the initial catalyst pretreatment. The increased amount of iron favours the process of aggregation of these iron species and thus a higher amount of aggregated iron oxide which is responsible for the decomposition of N20 to N2 + 02 instead of N2 + c~-O. As a consequence, the productivity of phenol in Fe2.2MFIht-A54 is higher than in Fel.lMFIht-A55, but the selectivity to phenol lower. Decreasing the amount of A1, while maintaining constant the amount of iron (compare Fe~.3MFIht-A90 with Fe2.2MFIht-A54) leads to an increase in both the productivity and selectivity, although the general trend remains again similar. This indicates that probably an A1/Fe ratio close to 1 is the optimal compromise to maximize both productivity and selectivity to phenol, suggesting that the active sites for phenol synthesis comprise both A1 and Fe. 3.2 Role of methodology in iron introduction in Fe-MFI catalysts In order to analyze in a concise way the effect of different methods of addition of iron in Fe-(A1)MFI samples on the catalytic performances (activity and deactivation), catalytic data have been summarized in Table 2. The following parameters are reported: (i) the maximum observed phenol productivity, which is generally obtained at the beginning of reaction or aiter about 20 minutes of time-on-stream (see Figure 1), (ii) the phenol productivity alter 2 hours, which gives an indication of the catalyst stability, and (iii) the selectivity in correspondence with the maximum phenol productivity and after 2 hours of time on stream.

509 Table 2 compares the behavior of some selected samples in which Fe was introduced postsynthesis by (i) an ion exchange method (Fel.sMFIie-A13 and Fe3.6MFIie-A13) using an aqueous solution of iron-ammonium-sulphate, (ii) CVD (contacting the anhydrated zeolite at 300~ with a flow of FeC13 in N2) and (iii) solid state reaction (FeC13 and the zeolite are mixed homogeneously and heated to 400~ For better comparison, data for two samples prepared by the hydrothermal method and containing both Fe and A1 (Fe1.1MFIht-A55 and Fe2.2MFIht-A54) as well as data for the parent zeolite used for ion-exchange and CVD (HZSM5-C) are also reported. Table 2. Phenol productivity and selectivity for Fe-(A1)MFI samples in which iron was introduced using different methods. Before benzene oxidation the catalysts were pretreated at 700~ in helium for two hours. Reaction conditions as in Figure 1. Selectivity3 Phenol productivity Sample Max1 2 h2 Max3 2h3 Fel.1MFIht-A54

0.38

0.18

81

92

Fe2.2MFIht-A55

0.81

0.62

56

69

H-ZSM5-C

0.44

0.02

61

11

Fel.sMFIie-A13

0.21

0.01

28

12

Fe3.6MFIie-A13

1.03

0#

78

17#

Fe0.aMFIcvo-A13

0.29

0.135

38

12

0.04 0# 7 1 Fe0.sMFIssr-A62 1 Maximumphenol productivity (mmolh"1g-l) at 400~ 2 Phenol productivity(mmol h~ g-l) at 400~ after 2 h of reaction 3 Phenol selectivity (%) at 400~ expressed on the total product basis in corrispondence to the maximum productivity and alter 2h of reaction. # The catalyst is completelydeactivatedjust after 1 h of reaction. The selectivitycorresponds therefore to a reaction time of 50 minutes $ The productivityto phenol does not further decrease in tests up to 20h of time-on-stream. The post-synthesis introduction of iron in the Fe-(A1)MFI catalysts generally leads to lower selectivities and productivities to phenol, although in some cases (e.g. Fe3.6MFIie-A13) good initial behavior is obtained. With respect to the behavior of the parent zeolite (HZSM5-C), the productivity to phenol increases more than twice and an increase in the selectivity to phenol could also be noted. However, similarly to H-ZSM5-C, after 1 h the productivity to phenol becomes negligible. When the iron content is lower, much poorer performances were observed, being higher the rate of side decomposition of N20. Therefore, the addition of iron by ion exchange could lead to catalysts having an initial activity better than the parent zeolite, but the effect of fast deactivation discussed before did not change. The addition of iron by CVD leads to not very selective catalysts due to a high rate of side N20 decomposition, but stable activity was noted after an initial decrease in phenol productivity. According to previous characterization, the CVD (sublimation) method leads to the formation of isolated or binuclear iron species [15], probably having characteristics similar to those of the active species responsible for the generation of the selective ~-O species during the interaction with N20. However, especially when the feed or the zeolite is

510 not fully anhydrated, nanoparticles of iron oxides also form [16], responsible for the decomposition of N20 and lowering of the selectivity. In agreement, the sample prepared by solid state reaction and in which the formation of the latter species is enhanced leads to very few selective and active catalysts in phenol formation. 4. CONCLUSIONS The Si/A1 and Si/Fe ratio in Fe-MFI, as well as also the method of addition of iron to the catalyst, have a marked influence on the catalyst activity and rate of deactivation during onestep oxidation of benzene to phenol. If deactivation is not taken into consideration, the comparison of different catalysts and the analysis of the structure-activity relationship may not be correct. Other factors not reported here due to space limitations determine the catalytic activity and stability, namely the temperature and atmosphere of pre-activation of the catalyst, the zeolite structure and the feed composition (N20/benzene ratio and their concentrations, and presence of H20 in the feed). These factors will be discussed in a subsequent manuscript. 5. REFERENCES

1. S. Niwa, M. Eswaramoorthy, J. Nair, A. Raj, N. Itoh, H. Shoji, T. Namba, F. Mizukami, SCIENCE, 295 (2002): 105. 2. G. Centi, S. Perathoner, F. Vazzana, CHEMTECH, 29(12) (1999) 48. 3. A.S. Kharitonov, G.I. Panov, K.G. Ione, V.N. Romannikov, G.A. Sheveleva, L.A. Vostrikova, V.I. Sobolev, US Patent 5 110995 (1992). 4. P.P. Nott6, Topics in Catal., 13 (2000) 387. 5. E.Suzuki, K. Nakashiro, Y. Ono, Chem. Lett. (1988) 953. 6. R. Burch, C. Howitt, Appl. Catal. 86 (1992) 135. 7. M. Gubelmann, P.-J. Tirel, US Patent 5 001 280 and E.P. 0 341 113, both assigned to Rhone-Poulenc. (1988) 8. V.I. Sobolev, K.A. Dubkov, E.A. Paukshits, L.V. Pirutko, M.A. Rodkin, A.S. Kharitonov, G.I. Panov, Appl. Catal. A 141 (1996) 185. 9. G.I. Panov, V.I. Sobolev, A.S. Kharitonov, J. Mol. Catal. 61 (1990) 85. 10. Zhobolobenko, Mendeleev Commun., 28 (1993). 11. J.L. Motz, H. Heiniche, W. HSlderich, Studies in Surf. Sci. Catal., 105 (1997) 1053. 12. L.V. Pirutko, V.S. Chemyavsky, A.K. Uriate, G.I. Panov, Appl. Catal. A, 227 (2002) 143 13. G. Giordano, A. Katovic, S. Perathoner, F. Pino, G. Centi, J. B.Nagy, K. Lazar, P. Fejes, Stud. Surf. Sci. Catal., part I, this issue. 14. G. Centi, S. Perathoner, G. Romeo, Stud. Surf Sci. Catal., 135 (2001) 181. 15. B. Wichterlova, J. Dedecek, Z. Sobalik, in Catalysis by Unique Metal Ion Structures in Solid Matrices, G. Centi, B. Wichterlova, A. Bell Eds., NATO Science Series II Vol. 13, Kluwer Acad. Pub.: Dordrecht (The Netherlands), Ch. 3, p. 31. 16. G. Centi, F. Vazzana, Catal. Today, 53 (1999) 683.

Studies in Surface Scienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa (Editors) 9 2002 Elsevier ScienceB.V. All rightsreserved.

511

S y n t h e s i s , s t r u c t u r e , a n d r e a c t i v i t y of i r o n - s u l f u r s p e c i e s in z e o l i t e ZSM-5 Richard W. Joyner a, Michael Stockenhuber" and Olga P. Tkachenko b aThe Catalysis Research Laboratory, Department of Chemistry and Physics, Nottingham Trent University, Clifton Lane, Nottingham NG11 8NS, UK. bPermanent address: Zelinsky Institute of Organic Chemistry, 47 Leninskii Prospect, Moscow, Russia. Iron-oxo species involving isolated iron atoms and nanoclusters can be formed within the pore structure of the zeolites ZSM-5 by a range of procedures, including aqueous exchange and chemical vapour deposition. We report that these can be sulfided by exposure to hydrogen sulfide at 623 K, forming analogous iron- sulfur entities. Characterisation by XPS and in particular EXAFS spectroscopy shows that there is almost complete replacement of oxygen by sulfur in the immediate iron coordination sphere, with an Fe - S distance of 2.26 A. The iron-sulfur materials are weakly active for the hydration of acrylonitrile to acrylamide, with the best materials achieving about 0.4 turnovers per iron species present. The catalyst appears to be subject to extensive hydrolysis during reaction. 1. INTRODUCTION A variety of oxo-species are produced when iron is introduced by different methods into the channels of microporous and mesoporous materials. The formation of oxo-bridged iron dimers is claimed to occur in ZSM-5 when a chemical vapour deposition route is used [1], while monomeric species predominate in MCM-41 [2]. When aqueous exchange is the method of introduction, we have reported extensive evidence that unusual oxo-nanoclusters are formed in ZSM-5 [3] and there is also some clustering when iron is introduced into zeolite beta by this method [4]. In ZSM-5 the average cluster contains four iron atoms and a similar number of oxygen atoms. The clusters are characterised by Fe - O distances in the range 1.93 - 2.10 A and also by unusually short iron iron distances of ca 2.55 .~. Because of these short iron - iron distances, we have drawn analogies with the structures of small sulfur-containing clusters that are know to occur in nature, and which can also be synthesised in the laboratory. Specifically we have drawn analogies with high-potential iron proteins (HIPIP) [5] and ferredoxins [6]. It is thus of interest to study whether any of'the iron-exchanged materials can be sulfided, and to examine what the structure and properties of the resultant materials might be. In particular we would like to mimic the enzyme

512

pseudomonas chlororaphis. This enzyme is thought to contain an iron-sulfur entity

as its active site, and it is able to catalyse hydration reactions, including the industrially significant hydration of acrylamide [7]. Our structural and reactivity results are the subject of this paper. 2. EXPERIMENTAL

The preparation and characterisation of the iron-oxo nanoclusters by aqueous exchange with H-ZSM-5 has been fully described [3]. Samples were also prepared by chemical vapour deposition, using the method first used by Chen and Sachtler [8]. About 500 mg of each sample was heated to 373 K in helium overnight and then exposed to hydrogen sulfide (Air Products, > 99.9%, 5% in helium) at 20 ml min -1 for 2 hours. The samples, which were originally yellowbrown in colour, were found to be black after sulfiding. The sulfided materials were characterised by X-ray photoelectron spectroscopy (XPS) using a VG ESCA-3 spectrometer, and by in situ iron K edge X-ray absorption spectroscopy (EXAFS) with fluorescence detection, at the Daresbury synchrotron radiation source. Standard experimental and data analysis methods were employed for both of these techniques, which have been fully described previously [3]. 3. RESULTS A N D DISCUSSION 3.1 Characterisation of sulfided materials

The sulfided materials were principally characterised by XPS and EXAFS, and some XPS results are listed in Table 1. Table 1: XPS results Catalyst 2.45% Fe-ZSM-51 iron (II) sulfide 2

Sulfur 2p binding energy/eV 163.6 and 170.6 162.5

Iron 2p3/2 binding energy/eV

Fe/S Ratio

710.8

1.2

710.0

1.2

1 Prepared by chemical vapour deposition followed by sulfiding. 2 BDH Technical Grade.

Figure 1 shows a typical iron K-edge EXAFS spectrum for the CVD sample after sulfidation, together with a spectrum calculated using the parameters listed in Table 2. The spectrum is dominated by an iron - sulfur interatomic distance of 2.26 + 0.02 A with a coordination number of 2.4 + 0.5. Before sulfiding, the main feature was an iron- oxygen distance with a mean interatomic value of ca 1.95 .~ and a co-ordination number of ca 4. There are, however three other shells of

513

i

i

|

i

A 1

i 4,

6

8

10

K:.Angstrom I J

9

.

.

,

lo

B

t

1="-4

It

o 1

3

5

7

p

Distance/A Figure 1. Experimental (solid line) and calculated iron K edge EXAFS for a catalyst prepared by chemical vapour deposi(ion and subsequently sulfide& A) Reciprocal space: B) Fourier transform. For details see the text, and for the parameters used for the calculated spectrum see Table 2.

514 neighbours that contribute to the calculated EXAFS in a statistically significant way. There is a residual oxygen shell at 1.93 A with a coordination number of about 0.4. The significance of the Fe - A1 and Fe - Fe shells of neighbours will be discussed elsewhere. Table 2 Parameters used to model the experimental EXAFS spectrum shown in Figure 1. Neighbour Oxygen Sulfur Iron Aluminium

Interatomic Distance/.A 1.93 2.26 2.74 3.20

Coordination Number 0.4 2.4 0.5 1.0

Debye-Waller Factor / A 2 0.008 0.015 0.020 0.014

The colour change on sulfidation and the XPS results show that sulfur has been introduced into these materials. The EXAFS results go further, in that they demonstrate almost complete displacement of oxygen by sulfur in the first iron coordination shell. This is indicated by the reported distance, 2.26 A, which is typical of the iron-sulfur interatomic separation in sulfides. The thermodynamic driving force for sulfidation is probably the formation of water, through the reaction: Fe- O + HzS .... > Fe- S + H20 For the bulk compounds, this reaction is thermodynamically favourable for iron (II) oxide (AHo29s = - 91 kJ tool-l), but not for Fe203. The sulfur 2p binding energy of ca 163 eV is consistent with the presence of sulfide. The higher binding energy observed is believed to be the result of atmospheric oxidation to sulfate during transfer to the spectrometer, indicating the lability of the sulfur species present. Similar behaviour has previously been observed in studies of sulfided chromia catalysts [9].

3.2 Reactivity The catalytic hydration of acrylonitrile to yield acrylamide is a reaction of industrial importance, which is usually catalysed by Raney copper. Since this Cu/A1 alloy catalyst undergoes slow decay and is subject to fouling by thermally polymerised product [10], there is interest in developing alternative catalysts. Several enzymes are used industrially in Japan [7], including the iron containing pseudomonas chlororaphis. We have therefore tested sulfided Fe-ZSM-5 materials for catalytic activity in the hydration of acrylonitrile. Reaction has been carried out in aqueous solution (1 ml acrylonitrile in 12.5 ml water) at temperatures in the range 338- 363 K, with product analysis by off-line gas chromatography. The results of a typical experiment with sulfided Fe-ZSM-5 prepared initially by the CVD route are shown in Figure 2. Some acrylamide is produced, but the activity is not high and we calculate that the maximum yield observed represents

515 only about 0.4 turnovers per iron atom. No other products were detected, so selectivity to acrylamide is 100%. We are therefore still some way away from mimicking the activity of the enzyme P. chlororaphis, which achieves its saturation yield of 45% acrylamide after a few hours at 20~ [7]. However other iron containing materials that we tested showed even lower activity than sulfided FeZSM-5. With the unsulfided iron containing zeolite the maximum yield found was < 0.2%, and no activity at all was detected with powdered iron sulfide. Our material gradually loses sulfur during reaction, as evidenced by the loss of its black colour. We believe that prolonged hydrolysis reverses the sulfidation reaction, since the black colour is also lost over several hours in boiling water. By contrast, the material is not reoxidised in air after many hours at > 100~ It is therefore possible that the formation of acrylamide is indeed catalytic (i.e. producing more than one turnover per active site), but that the catalyst is destroyed by a parallel hydrolysis reaction. We are currently investigating this.

0.7

0.6

0.5

e- 0.4 o (n L_

r 0 0

0.3

T increased to 95 C

0.2 T increased to 75~ 0.1

0

10

20

30

40

50

60

70

80

90

100

Time/h

Figure 2. Results of the hydration of acrylonitrile using sulfided Fe-ZSM-5 and the reaction conditions indicated in the text. The reaction was started at 65~ and the temperature was increased at the points indicated by the arrows. REFERENCES

1. 2.

P. Martura, L. Drozdova, A. Kogelbauer and R. Prins, J. Catal., 192 (2000) 236. M. Stockenhuber, M.J. Hudson and R.W. Joyner, J. Phys. Chem. B., 104 (2000) 3370.

516 3. R.W. Joyner and M. Stockenhuber, J. Phys. Chem. B., 103 (1999) 5963. 4. M. Stockenhuber, R.W. Joyner, G.S. Paine, unpublished results. 5. See e.g.S.P. Cramer in X-ray absorption, Principles, applications and techniques of EXAFS, Ed. D.C. Koningsberger and R. Prins, J. Wiley New York, 1988 and references therein. 6. C.R. Kissinger, E.T. Adman, L.C. Sieker and L.H. Jensen, J. Amer. Chem. Soc., 110 (1988) 8721. 7. See the review by H. Yamada and M. Kobayashi, Biosci. Biotech. Biochem., 60 (1996) 1391. 8. H.-Y. Chen and W.M.H. Sachtler, Catal. Today, 42 (1998) 73. 9. B.W.L. Southward, G.J. Hutchings, R.W. Joyner and R.A. Stewart, Catal. Lett., 68 (2000) 75. 10. M.S. Wainwright, Preparation and utilisation of Raney copper catalysts, in Catalysis of Organic Reactions, Ed. R.E. Malz Jr., pub Marcel Dekker, New York 1996, pp. 213.


Characterization of FeMCM-41 and FeZSM-5 Catalysts Production

517

to Styrene

J. R. C. Bispo a, A. C. Oliveiraa, M. L. S. Corr~a a , J. L. G. Fierro b, S. G. Marchetti ~ and M. C. Rangel a aInstituto de Quimica, Universidade Federal da Bahia. Campus Universithrio de Ondina, Federag~o. 40 170-280, Salvador, Bahia, Brazil, e-mail: [email protected] bInstituto de Catalisis y Petroleoquimica, CSIC, Campus UAM, Cantoblanco, 28049 Madrid, Spain ~ Facultad de Ciencias Exactas, Universidad National de La Plata, 1900, 47 y 115, La Plata, Argentina FeMCM-41 and FeZSM-5 catalysts have been prepared and tested in the dehydrogenation of ethylbenzene to produce styrene. These new catalytic systems have high specific areas and can stabilize the trivalent state of iron. It was found that the FeZSM-5 catalysts are more active and selective than the FeMCM-41 ones and this behavior is explained in terms of the higher amount of the active oxidation state (Fe 3+) on the catalyst surface. These catalysts also result more active and selective than t~Fe203 (hematite) and are less toxic than the chromium-containing commercial catalysts used for this purpose.

1. INTRODUCTION Styrene monomer is one of the most important high value chemical used extensively for the manufacture of plastics, including crystalline polystyrene and styrene-butadiene rubber (SBR) [1]. Direct catalytic dehydrogenation of ethylbenzene has been the dominant technology for styrene production since its first commercial application [2-4]. In this process ethylbenzene is dehydrogenated to styrene and hydrogen over a catalyst in the presence of steam; toluene and benzene are formed as by-products. The overall reaction is highly endothermic and thermodynamically limited and thus the conversion is increased when increasing the temperature. The feed is much diluted with steam for limiting the coking rate of the catalyst, decreasing the temperature fall due to the reaction and improving the conversion at the equilibrium [5]. The most widely used industrial catalysts comprise iron oxide and promoters such as K2CO3, Cr203, CeO2, MOO3, V205 and so on [2]. Other promoters like aluminum, cadmium, magnesium, manganese, nickel and uranium oxides as well as rare-earths

518 have been used [6]. However, potassium-promoted iron oxide is better than any other catalyst known for ethylbenzene dehydrogenation in the presence of steam [2,3]. It is generally believed that potassium acts as a chemical promoter in the catalyst, whereas chromium oxide is a textural promoter stabilizing the high surface area of the active phase [2, 3, 7]. Despite its high activity and selectivity, the commercial catalyst still has some disadvantages which need to be improved in order to minimize the manufacturing costs of styrene. On the one hand, the active oxidation state is unstable; hematite (~Fe203) is preferred for styrene production, but it tends to go into oxides with lower oxidation states and even to elemental iron, and they catalyze carbon formation and dealkylation [3]. On the other hand, the iron-based catalysts have low specific areas and deactivate with reaction time being susceptible to poisoning by halides and residual organic chlorine impurities [2]. The most serious deactivation is caused by the loss of potassium promoter, which migrates in two directions as the catalyst ages. Potassium chloride is found downstream in the water layer of the condensed product as well as in the center of the catalyst pellets [2,3]. Besides, the large amounts of steam used in commercial operations increase the operational costs. The catalyst has the additional disadvantage of being toxic causing damage to the humans and to the environment. Therefore, the investigation for new systems which have high specific area, can stabilize the trivalent state of iron and are potassium and chromium free is much needed. With this goal in mind, this work deals with the evaluation of FeMCM-41 and FeZSM-5 as catalysts to the styrene production. 2. EXPERIMENTAL The FeZSM-5 sample was prepared by mixing an aqueous solution of ferric sulfate (0.09 mol.L -1) with an aqueous solution of sodium metasilicate (2.5 mol.L 1) and a tetrapropylammonium bromide (template) solution (0.2 mol.L 1) under stirring. The resulting solution was kept in an autoclave at 170 ~ for 72 h. Then, the sample was rinsed with water, centrifuged and dried at 120 ~ for 2h. After this, the solid was calcined at 500 ~ under nitrogen flow (3 h) and under air flow (5 h). The FeMCM-41 sample was prepared from a gel with sodium metasilicate (2.5 mol.Ll), tetramethylcethylammonium bromide (0.02 mol.Ll), ferric sulfate (0.1 mol.L1) and tetramethylammonium hydroxide, which was aged for 4 h under stirring and then kept under hydrothermal treatment in autoclave. The pH was adjusted to 12 with ammonium hydroxide, aged for 4 h under stirring and kept in an autoclave under hydrostatic pressure in an oven at 140 ~ for 16 h. The sample was rinsed with water, centrifuged and dried at 90 ~ for 12 h. The material was calcined at 500 ~ under nitrogen flow (lh) and under air flow (6 h). An iron oxide sample (hematite) was also prepared to be used as a reference catalyst. This solid was prepared by adding, under stirring, an aqueous solution of iron nitrate (1.0 mol.L 1) and a concentrated (25% w/w) solution of ammonium hydroxide to a beaker with water. The sol produced was centrifuged, rinsed with water, dried in an oven at 120 ~ and calcined at 500 ~ under nitrogen flow (2 h).

519 The iron contents were determined by inductively coupled plasma atomic emission spectroscopy (ICP/AES) by using an Arl 3410 model equipment. The absence of the templates in the catalysts was confirmed by Fourier transform infrared spectroscopy in the range of 4000-400 cm-1 using a model Valor II Jasco spectrometer and KBr discs. The structure of the FeZSM-5 and of the FeMCM-41 was confirmed by X-ray diffractometry experiments performed at room temperature with a Shimadzu model XD3A instrument using CuKa radiation generated at 30 kV and 20 mA. The specific area was measured (BET method) in a Micromeritics model ASAP 2000C equipment on samples previously heated under nitrogen (150 ~ 2 h). The temperature programmed reduction (TPR) was performed in a Micromeritics model TPD/TPO 2900 equipment, using a 5% H2/N2 mixture. The M6ssbauer spectra were obtained in transmission geometry, with a 512- channel constant acceleration spectrometer at 25 ~ A source of 57Co in Rh matrix of nominally 50 mCi was used. Velocity calibration was performed against a 121am thick ct-Fe foil. All isomer shifts mentioned in this paper are referred to this standard at 25~ The spectra were evaluated by using a least-squares nonlinear computer fitting program with constraints. Lorentzian lines were considered with equals widths for each spectrum component. The spectra were folded to minimize the geometric effects. X ray photoelectron spectra were obtained with a VG ESCALAB 200R spectrometer equipped with a MgKet X-ray radiation source (hv = 1253.6 eV) and a hemispherical electron analyzer. The powder samples were pressed into small stainless steel cylinders and mounted onto a manipulator which allowed the transfer from the preparation chamber into the spectrometer. Before the analysis, they were outgassed (10 .9 mbar) or reduced in hydrogen at 500 ~ (1 h). The Si2p peak was chosen as an internal reference. This reference was in all cases in good agreement with the BE of the C ls peak, arising from contamination, at 284.9 eV. This reference gave an accuracy of + 0.1 eV. The catalyst performance was evaluated using 0.2 g of powder within 50 and 325 mesh size, and a fixed-bed microreactor, providing there is no diffusion effect. The experiments were carried out under isothermal condition (530 ~ and at atmospheric pressure, employing a steam to ethylbenzene molar ratio of 10. The reactor, containing the catalyst, was heated under nitrogen flow (60 ml.s 1) up to the reaction temperature. Then the feed was interrupted and the reaction mixture was introduced. The reaction mixture was obtained by passing a nitrogen stream through a saturator with ethylbenzene and then through a chamber where it was mixed with steam. The gaseous effluent was collected in a condenser and the organic phase was analyzed by gas chromatography, using a CG-35 instrument. In order to save the energy related to the steam consumption, the catalysts were also evaluated in the absence of steam.

3. RESULTS AND DISCUSSION Table 1 compiles the amount of iron in the catalysts, their specific area and their rates of ethylbenzene conversion and selectivity towards styrene in the steady state. It can be seen that the FeZSM-5 sample is active only in the presence of steam. This can

520 Tablel. Amount of iron (%Fe) and specific area of the catalysts (Sg) and their activity (a), activity per weight of iron (a/g) and selectivity (S) to styrene of the catalysts in the dehydrogenation of ethylbenzene Sample % Fe Sg a.lO 3 (mol.hl.g q) a/g (mol.hl.g Fe q) s (%) (m2.g1) With Without With Without steam steam steam steam FeZSM-5 1.32 425 3.5 0.0 4.7 100 0 FeMCM-41 1.04 1112 1.8 0.9 3.1 52 33 c~-Fe203 71.32 17 2.4 0.0 4.4. 10 -3 92 0

be related to the role of steam in reacting with the carbonaceous deposits according to the Boudart reaction. On the other hand, the FeMCM-41 sample was active both in the presence and in the absence of steam, although the activity and selectivity strongly decreased without it As FeMCM-41 has larger pores than FeZSM-5, one can suppose that it can afford a large amount of coke and then it can work even without steam. The other role of steam is to keep the trivalent state of iron highly selective to styrene [2,3]. As MCM-41 and ZSM-5 did not show any activity towards the reaction, the performance of the catalysts may be directly related to iron oxides. The FeZSM-5 catalyst was more active than the hematite-based sample. As hematite has larger amount of iron, and then larger number of active sites than the zeolite, one can conclude that iron is much more active in the zeolite structure, as confirmed by the values of activity per gram of iron. The FeZSM-5 catalyst was the most selective whereas the FeMCM-41 sample was the least one. Figure 1 shows the activity and the selectivity to styrene as a function of time. In all eases, the activity varied in the first hours of reaction and reached stables values after 4h. Concerning the selectivity, FeZSM-5 and hematite show stable values since the beginning of the reaction, while FeMCM-41 showed stable values only after 6h. ~,,

8

120

~.

6

,-~ 100 ~ 80

"~

4

",=, 60

2

40 20

9

0 ................. 0 2 4 Time (h)

(a)

6

8

0,

0

~ 1

~ 2

, , 3 4 5 Time (h)

6

,

7

(b)

Figure 1. (a) Activity and (b) selectivity to styrene of the catalysts in the dehydrogentaion ofethylbenzene. -4k- e~-Fe203; -" F e M C M - 4 1 ; - - I FeZSM-5

521 The TPR profile of pure hematite showed two peaks around 400 ~ and 740 ~ ascribed to the reduction of Fe 3+ and Fe 2+, respectively [8]. The TPR curve of FeMCM41 showed only a peak centered at 500 ~ whereas the curve of FeZSM-5 displayed a large peak beginning at the same temperature. As stated early [9] this peak is due to Fe 3+ reduction in zeolite structure. Therefore, it can conclude that the iron reduction is more difficult on both MCM-41 and ZSM-5 structures as compared to hematite. The M6ssbauer spectra of FeMCM-41 samples were poorly-defined with a low signal-to-noise ratio probably due to the low amount of iron. In the fresh sample a signal related to Fe 3+ species exchanged and/or superparamagnetic Fe203 was found. After the reaction, carried out with or without steam, a fraction of iron went into 7Fe203 and the sample was attracted by a magnet. This can be explained by considering that during the reaction very small crystallites of magnetite were produced which went into maghemite under the oxidizing conditions of the Mossbauer experiments. Table 2 shows the M6ssbauer parameters of the FeMCM-41 catalysts. Table 2. M6ssbauer hyperfine parameters catalyst tested with steam and WS Species Parameters 7-Fe203 H(T) 8(mm/s) 2e(mm/s) Fe 3+ exchanged 5(mm/s) and/or Fe203 sp A(mm/s)

at 25 ~ of FeMCM-41 catalysts. S represents the the catalyst tested without steam FeMCM-41 FeMCM-41 (S) FeMCM-4 I(WS) 48 + 1 47.5 + 0.4 0.4 + 0.1 0.41 + 0.06 0.03 (*) 0.1 + 0.1 0.32+0.03 0.33 (*) 0.33 + 0.02 0.78i0.04 0.9 + O.1 0.94 + 0.03

*parameter held fixed while fitting; sp: superparamagnetic; H: hyperfine field; 8: isomer shift; 2e: quadrupole shift; A: quadrupole splitting. The flesh FeZSM-5 catalyst shows the M6ssbauer parameters which fitted well to hematite.Using the magnetic excitation model [10], the average diameter of the crystallites can be estimated as 17.2 nm, which means that they are outside the channels of the zeolite, in accordance with previous work [ 11]. After the reaction, performed with steam, these crystallites grew up to 20.1 nm and no other phase, besides hematite, was noted. On the other hand, the reaction carried out without steam caused a phase change, producing maghemite detected by the strong reduction of the hyperfine filed (H) and the zero value of the quadrupole shift (2e). Again, this phase is related to the small crystallites of magnetite that were produced under the reaction conditions. In all spectra, there is a very weak central signal which may correspond to Fe 3§ species exchanged with the zeolite and/or superparamagnetic hematite related to a fraction of very small crystallites which could be located inside the channels. Table 3 shows the M6ssbauer parameters. These results show that magnetite is produced more easily in the FeMCM-41 structure than in the FeZSM-5 one, in the presence of steam. Therefore, MCM-41 is not able to avoid magnetite formation even in the presence of steam. On the

522 Table 3. M6ssbauer hyperfine parameters at 25 ~ of FeZSM-5 catalysts. S represents the catalyst tested with steam and WS the catalyst tested without steam. Species Parameters FeZSM-5 FeZSM-5(S) FeZSM-5(WS) H(T) 50.9 + 0.10 51.2 + 0.10 ~-Fe203 8(mm/s) 0.37 + 0.02 0.37 + 0.01 2e(mm/s) -0.19 + 0.03 -0.20 _+0.01 H(T) 49.5 + 0.10 y-Fe203 5(ram/s) 0.32 + 0.01 - 0.06 + 0.03 2e(mm/s) Fe 3+ exchanged 0.37(*) 8(mm/s) 0.37(*) 0.37(*) and/or Fe203 sp *Parameter held fixed while fitting; sp: superparamagnetic; H: hyperfine field; ~: isomer shift; 2e: quadrupole shift; A: quadrupole splitting. other hand, the FeZSM-5 catalysts showed only hematite after the reaction (with steam), showing that the zeolite structure is able to stabilize hematite. Without steam, hematite changed to magnetite which went into maghemite during the M6ssbauer experiments. Table 4 shows the binding energies (BE) of some characteristic core levels of Fe, O and Si in the FeMCM-41 samples as well as the surface amount of Fe 3+ species. The fresh catalyst showed a binding energy of 710.5 eV which is typical ofFer+species in hematite [ 11]. After the catalytic test, carried out with steam, the catalyst surface still showed only Fe 3+ species. When the reaction was performed without steam, however, a fraction of Fe 3+ was reduced to Fe 2+ (45%) as inferred by the BE of 709.7 eV [11]. After the reduction, carried out in the XPS equipment, some Fe 3+ is reduced to Fe 2+ in all samples, showing that the Fe 3§ state was stabilized during the reaction. The surface atomic ratio Fe/Si of the catalyst is also shown in Table 4. It can be seen that the reaction causes an enrichment of iron on the surface. The reduction performed in the XPS equipment increased the amount of iron on the catalyst surface even more. The binding energies (BE) of some characteristic core-levels of Fe, O, A1 and Si in the FeZSM-5 samples as well as the surface amount of Fe 3§ species are shown in Table 5. The flesh catalyst showed binding energies of 710.0 and 711.7 eV which are typical of Fe 2+ and Fe 3+ species respectively [11]. The presence of the reduced species can be explained by an effect of vacuum during the treatment of the samples, in the XPS equipment. The Fe 3§ reduction, produced by the outgassing, could be due to the dehydroxilation of the zeolite. Similar results have been found by other authors in Fe-zeolite L [12] and Fe-zeolite X [13] systems. After the catalytic tests, the amount of Fe 3§ species as well as the total amount of iron on the catalyst surface increased. After the reduction, carried out in the XPS equipment, a part of Fe 3+ is reduced to Fe 2+ in all samples and the amount of iron on the catalyst surface increased even more. By comparing the Fe/Si ratio of the catalysts, we see that the FeZSM-5 samples have much more iron on the surface, and then more Fe 3+ species, than the FeMCM-41 catalysts.

523 Table 4. Binding energies (eV), surface atomic ratios and surface amount of Fe 3§ of fresh and spent FeMCM-41 catalysts taken on samples previously treated under vacuum or under hydrogen at 500 ~ S represents the catalyst tested with steam and WS the catalyst tested without steam. Sample Cls Si2p Ols Fe2p3/2 Fe 3+ Fe/Si (%) (atom) FeMCM-41 (vac) 284.6 103.4 532.9 710.5 100 0.0054 (H2, 500~ 284.6 103.4 532.0 709.6 0.0069 711.5 40 FeMCM-41 (S) (vac) 284.6 103.4 532.9 709.7 0.0075 711.5 55 (H2, 500~ 284.6 103.3 532.0 709.5 0.0078 711.6 35 FeMCM-41 (WS)(vac) 284.6 103.4 532.9 711.3 100 0.0068 (H2, 500~ 284.6 103.4 532.9 709.7 0.0079 711.7 56 Table 5. Binding energies (eV), surface atomic ratios and surface amount of Fe 3+ of fresh and spent FeZSM-5 catalysts taken on samples previously treated under vacuum or under hydrogen at 500 ~ S represents the catalyst tested with steam and WS the catalyst tested without steam Sample Cls Si2p A12p Fe2p3/2 Fe 3+ Fe/Si (atom) FeZSM-5 (vac) 284.6 103.5 74.6 710.0 0.015 711.7 37 (U2, 500~ 709.4 284.6 103.3 74.5 711.2 31 0.017 FeZSM-5 (S) (vac) 284.6 103.4 74.4 709.7 0.019 711.5 43 (H2, 500~ , 709.6 103.3 74.5 711.4 33 0.022 284.6 FeZSM-5 (WS) (vac) 284.6 103.4 74.5 709.7 0.018 711.4 43 (H2, 500~ 709.7 284.6 103.4 74.4 711.6 32 0.020

From these results it can be concluded that the higher activity of FeZSM-5 catalyst, as compared to FeMCM-41 one, can be ascribed to its higher amount of iron on the surface surface However, the FeMCM-41 catalyst seems to be more resistant against deactivation by coke blockage of the pores, since it has larger pores than the other.

524 CONCLUSIONS FeMCM-41 and FeZSM-5 catalysts are both active towards ethylbenzene dehydrogenation in the presence of steam. Particularly, FeZSM-5 catalyst is more active and selective than the FeMCM-41 one. The better performance of FeZSM-5 catalyst is due to the higher amount of iron deposited on the external zeolite surface and their ability in stabilizing the active oxidation state (Fe3§ on the surface. Finally, since the FeZSM-5 catalyst is more active and selective than pure hematite and is chromium and potassium-free, it is a promising candidate to commercial applications. ACKOWLEDGEMENTS The authors thank the financial support from PADCT/FINEP. J. R. C. B. and A. C. O. acknowledge their undergraduate scholarship. REFERENCES 1. Kirk-Othmer, Encyclopedia of Chemical Technology, John Willey and Sons, New York, 1984, p. 770. 2. E.H. Lee, Catal. Rev., 8 (1973) 285. 3. B.D. Herzog and H.F. Raso, Ind. Eng. Chem. Prod. Res. Dev., 23 (1984) 187. 4. S. S. E. H. Elsanashaie, B. K. Abdallah, S. S. Elshishini, S. Olkowalter, M. B. Noureldeen and T. Aboudani, Catal. Today, 64 (2001) 151. 5. Ph. Courty and J.J. Le Page, in: B. Delmon, P. Grange, P. Jacobs and G. Poncelet (Editors), Preparation of Catalysts II (Studies in Surface Science and Catalysis), Elsevier, Amsterdam, 1979, p. 293. 6. G.H. Riesser, U.S. Patent 4,144,197 (1979). 7. A.K. Vijh, J. Chem. Phys., 72 (1975) 5. 8. J.C. Gonzalez, M.G. Gonzb]ez, M.A. Laborde and N. Moreno, Appl. Catal., 20 (1986) 3. 9. L.J. Lobree, I. Hwang, J.A,. Reimer and A.T. Bell, J. Catal., 186 (1999) 242. 10. P. Fejes, J. B. Nagy, K. L~izar and J. Halask, Appl. Catal., 190 (2000) 117. 11. C.D. Wagner, W.M. Riggs, L.E. Davis, J.F. Moulder and G.E. Muilenberg, Handbook of X-Ray Photoelectron Spectroscopy, Perkin-Elmer Coorporation, Eden Prairie, 1978, p. 76. 12. S. Morup and H. Topsoe, Appl. Phys. 11 (1976) 63. 13. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramian and R.C. Mercader, Hyperfine Interactions, C4 (1999) 61. 14. J.A. Morice and L.V.C. Rees, Trans. Faraday Soc. 64 (1968) 1388.


525

Fischer-Tropsch synthesis. Influence of the presence of intermediate iron reduction species in Fe/Zeolite L catalysts. N.G. GaUegos, M.V. Cagnoli, J.F. Bengoa, A.M. Alvarez, A.A. Yeramihn and S.G. Marchetti

CINDECA, Fac. Cs. Exactas, Fac. Ingenieria, U.N.L.P., CIC, CONICET. Calle 47 N ~ 257 (1900) La Plata, Argentina. Two catalyst to be used in the Fischer-Tropsch reaction, using zeolite-L in potassic form as support of iron species were prepared through to different methods of impregnation with iron salt. X-Ray Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) in controlled atmosphere, between room temperature (RT) and 15 K, H2 chemisorption and Volumetric Oxidation (VO) were used to characterise the solids. The impregnation of the zeolite L under inert gas allowed to obtain a fraction ofFe ~ in contact with Fe 2+ ions that enhanced the activity of the sites. The two catalysts presented similar selectivity towards hydrocarbons and low chain growth. 1. INTRODUCTION It is well known that in iron catalysts supported on different solids such as A1203, SiO2, MgO and zeolites, it is not possible to obtain complete reduction to Fe ~ when the iron concentration is low (approximately < 10%w/w) [1, 2]. Therefore, most of the iron catalysts used in the Fischer-Tropsch synthesis have, in addition to Fe ~ intermediate iron reduction species like Fe 2+. However, the role of Fe 2§ on the activity and selectivity in the CO hydrogenation has not been studied yet. In this paper, a commercial Zeolite L in potassic form (ZLK) was used as metal support for CO hydrogenation. The choice of this support was carried out since this reaction is sensitive to the structure. This means that the activity and selectivity of the catalyst depend on its metallic crystal size [3]. Catalysts with a narrow size distribution lead to good selectivity towards to a desirable product. Making use of the structure of channels and cages ofzeolites it is possible to reach this purpose. In order to determine the influence of the intermediate iron reduction species on the activity and selectivity of the Fischer-Tropsch reaction, two Fe/ZLK catalysts were prepared by two different methods.

2. EXPERIMENTAL SECTION Two precursors were prepared using the commercial form of the ZLK (Tosoh Corp.), with the ideal unit cell composition of dehydrated form of K9A195i27072 and 290 m~/g of specific surface area. One of them, was obtained by dry impregnation in air of the

526 zeolite with aqueous solution (pH=0.5) of a concentration to yield a solid with 5.84% w/w ofFe. Then it was calcined following the programme described in [4]. This sample was called p-Fe/ZLK(a). The other precursor was obtained outgassing the support at 773K and 0.05 torr for 1 h to eliminate the water present inside the channels and cages of the zeolite. After this time the system was filled with ultra high purity He up to 500 torr. Then, the Fe(NO3)3.9HzO aqueous volume solution equal to the pore volume of the zeolite was added to yield a solid with 4.56% w/w of iron that was calcined in the same way that p-Fe/ZLK(a).This solid was called p-Fe/ZLK(v). Both precursors were reduced in H2 stream (60 cm3/min) from 298 to 698 K at 2.66 ~ and were kept at 698 K during 26 h. The resulting solids were named c-Fe/ZLK(a) and c-Fe/ZLK(v), and characterised by XRay Diffraction (XRD), Specific Surface Area (BET), M6ssbauer Spectroscopy (MS) at 298 and 15 K, 1-12chemisorption and volumetric oxidation (VO). These last two techniques were performed in a conventional static volumetric equipment with grease-free vacuum valves. The Hz uptakes at the same initial pressure, but at different temperatures between RT and 673 K were measured to determine the temperature in which the adsorption capacity is maximum. Volumetric oxidation experiments are based on the conversion of all iron species in the sample to Fe203 when it was heated in an O~ atmosphere at temperatures higher than 620 K [5]. The M6ssbauer spectra were obtained in transmission geometry with a 512-channel constant acceleration spectrometer. A source of 57Co in Rh matrix of nominally 100mCi was used. Velocity calibration was performed against a 6-1am-thick ~-Fe foil. All isomer shifts (6) mentioned in this paper are referred to this standard. The temperature between 15 and 298 K was varied using a Displex DE-202 Closed Cycle Cryogenic System. All MSssbauer spectra of the catalyst were obtained in controlled atmosphere using a cell specially built for this purpose to be used inside the cryogen [6]. The spectra were evaluated using a least-squares nonlinear computer fitting program with constraint. Lorentzian lines were considered for each spectra components. The catalytic tests were carried out in a fixed bed reactor with a H~:CO ratio of 3:1, 543 K, 1 atm of total pressure, 20 cm3/min of total volumetric flow and a space rate of 0.25 s"l. The reaction products were analyzed by gas chromatography using FID and TCD as detectors, and a GS-Alumina capilar column and Chromosorb 102 packed column respectively. 3. RESULTS AND DISCUSSION

The preservation of the crystalline structure of the samples after impregnation and calcination process was checked analysing its X-Ray difraction patterns (not shown). The same peaks as those for ZLK were obtained, although the relative intensities of these varied slightly. These results may be due to a decrease of crystallinity after impregnation and calcination process and/or a lower crystallographic planes periodicity due to the presence of Fe oxides inside the zeolite channels. The specific surface area of the precursors is of 33 m2/g for p-Fe/ZLK(a) and 45m2/g for p-Fe/ZLK(v). The very important decrease in specific surface area in comparison with ZLK would indicate that a great fraction of iron species are located inside the zeolite channels reducing the pore mouth sizes. The M6ssbauer spectra ofp-Fe/ZLK(a) and p-Fe/ZLK(v) at 298 and 15 K are shown in Figure 1.

527 'I

'I '

I

"

i

I

"I

I

I

I

oO

't

,,

.

.

I

.

.

.

I

....... i

........ I

.

I

.......... I' "

o

r C

.o W l/} =,,,

tl

E

W

t~ t_

I--

p-

I

I

-12

-8

I ....

-4

I

0

I

I

4

Velocity (mm/s)

8

,1

12

1

I

I

-12

-8

-4

,

i

I

!

0

4

8

,I

12

Velocity (mm/s)

Figure 1" M6ssbauer spectra of the precursors at 298 and 15 K. At 298 K a paramagnetic doublet is observed in both precursors, while an additional magnetic sextet is also displayed by p-Fe/ZLK(a). When the temperature is lowered to 15 K, a second magnetic signal is observed in p-Fe/ZLK(v). Instead, in p-Fe/ZLK(a) only two resolved signals are still noticed, but the background is significantly curved. The spectrum at 298 K of p-Fe/ZLK(v) have hyperfme parameters (Table 1) that can be assigned to two Fe 3+ species: small particles of (x-Fe203 and/or Fe 3+ ions exchanged with the support. When the temperature decreased to 15 K, it was possible to determine the existence of two magnetic signals assignable to the "core" (sextet with higher magnetic field) and "shell" (sextet with smaller magnetic field) of (~-Fe203 "clusters" [7]. Assuming homogeneous semi-spherical particles and using the ratio of the areas of the two sextuplets, it was possible to estimate an average "cluster" diameter of 1.1 nm. Therefore, these "clusters" could be located inside the channels of the zeolite L. This result, analyzed in connection with the DRX andBET results, mentioned above, confima the existence of a great quantity of iron oxide microcrystals situated inside the zeolite structure.

528 Table 1" M/3ssbauer hyperfine parameters of the precursors. Temp. 298 K

Specie s c~-Fe203

Parameters .... p-Fe/ZLK(v) p-Fe/ZLK(a) H(T) 51.2 + 0.1 8(mm/s) 0.38 + 0.02 2e(mm/s) ~ -0.24 + 0.03 Fe 3+'' 8(mm/s) 0.34~:0.01 0'.'32 _+0.0i ....... A(mm/s). . . . . . . . . . 0.90-a:0.0! 0.87 + 0.01 15 K a-Fe203 H(T) 49.3+0.1 53.7 + 0.1 ~5(mm/s) 0.50• 0.45 + 0.02 2e(mm/s) . . . . -0.06• .... 0.36 + 0,,.03 ot-Fe203 H(T) 46.3+0.1 46.3" 5(mm/s) 0.46• 0.47* 2e (mm/s) ...... -0.03+0.02 -0.01 * Fe 3§ ~5(mm/s) 0.46• 014i~ 0.01 ................... A(mm/s) 1.03• !.0! + 0.01 *Constant used for the fit. The remaining doublet was assigned to Fe 3+ exchanged with the support, and/or superparamagnetie ct-Fe203 particles. The hyperfme parameters (Table 1) at 298 K of p-Fe/ZLK(a) can be assigned to the same iron species than in p-Fe/ZLK(v). When the temperature decreased to 15 K, the spectrum displays a curved background probably originated in a fraction of small particles undergoing an incomplete magnetic splitting. The fitting was simulated with one sextet, one doublet and a second sextet of very broad lines [3]. The relative area of the sharper sextet corresponding to ct-Fe203 (11+_2%) is the same (within experimental errors) at RT and at 15 K. To estimate roughly the average size for this fraction, we applied the Collective Magnetic Excitation Model (CMEM) [8]. A diameter of 20 nm is obtained. This value indicates that these particles must be located out of the channels of the zeolite. Although the fitting procedure is a rough approximation to the physical process actually taking place, the method yields an estimate of the fraction of the particles in the relaxing magnetic regime (55+7 %). Since these particles at 15 K have not reached the degree of magnetic order of the p-Fe/ZLK(v) particles, their size must be even smaller than 1.1 nm. Figure 2 shows the M6ssbauer spectra in controlled H2 atmosphere of both catalysts c-Fe/ZLK(v) and c-Fe/ZLK(a) at 298 and 15 K. At RT both display a magnetic sextet and several intense and highly overlapped central signals. The spectra were interpreted in terms of a superposition of one magnetic sextet, one paramagnetir doublet and one singlet. In addition to the above mentioned signals, other doublet appears in c-Fe/ZLK(v). When the temperature decreases to 15 K the spectra show the presence of two magnetic sextets, a paramagnetic doublet and a superparamagnetic singlet for both solids, and c-Fe/ZLK(v) displays an additional magnetic sextet. The values of the hyperfme parameters at 298 and 15 K are shown in Table 2. At 15 K, the values are characteristic of magnetic Fe ~ (Fe~ Fe304, Fe 2+ exchanged with the support and superparamagnetir Fe ~ (Fe~ [9, 10]. The additional sextet in c-Fe/ZLK(v) is assigned to Fe 2+ ions considering its isomer shift value. Since, we found the Fe 2+ signal magnetically splitted and its hyperfine magnetic field is very similar to the ct-Fe ~ value, we

529 think that this species is magnetically coupled with Fe~ Therefore, the Fe 2+ would be decorating the F e ~ [9]. The weak magnetic signal, assignable to Fe304, that can be seen in the MS at 15 K, can be attributed to the incomplete reduction of the oxides. Its quantity is too small to sort out one more interaction in the spectrum fitted at RT from the statistical noise. The Fe~ existence at so low temperature such as 15 K indicates the presence of very small particles of Fe*, at least smaller than ~2.9nm [11] in both catalysts. The presence of the Fe~ fraction aRer a reduction treatment suggests that these microcrystals are located inside the channels of the zeolite since this situation would avoid the sintering process. The same amount of Fe ~ inside the support (Table 3) is achieved in both catalysts, although in p-Fe/ZLK(v) all the iron oxide is inside the channels and in p-Fe/ZLK(a) there is a fraction of iron oxide out of the zeolite structure. Therefore, in the former sample a percentage of the iron crystallites have migrated to the external surface, during the reduction step.

'"i

'~

9. , : . '

I

.......

I

I

I

..': :~,. I

I

,%

=. (O cO u)

E r r

0~ L

p.

c-FelZLK(v) 298 K

c-FelZLK(a) 298 K

"""

~'~

~. v

.,,"

; ; z ", ~ -

~ -. . .

-

c-FelZLKla) 15K

c-FelZLK(v) 15K

II I

t

I

I

I

I

!

-12

-8

-4

0

4

8

Velocity (mmls)

I

.I

12 -12

!

I

!

L.

1

!

-8

-4

0

4

8

12

Velocity (mmls)

Figure 2: M6ssbauer spectra of the catalysts at 298 and 15 K.

530 Table 2: Hyperfine M6ssbauer parameters of c-Fe/ZLK(v) and c-Fe/ZLK(a) ~

Species Fe ~ (magnetic)

Parameters

...... c-Fe/ZLK(v) ....... c,Fe/ZLK(a ) 298 K 15 K 298 K 15 K H (T) 33.2-~0.1 34.2+0.1 33.0+0.1 34.1_+0.1 8(Fe) (mm/s) 0.00+0.01 0.12+0.01 0.01+0.01 0.11+0.01 2e ( ~ s ) -0.01+0.01 -0.01a:0.01 0.00" 0.00" H (T) 49.5+0.6 48.6+0.7 8(Fe) (mm/s) 0.58+0.08 0.77+0.09 .

FesO4 (magnetic)

(mnVs) Fe 2+ (m) (Coupled with Fe~ (m)) Fe 2+ (exchanged) Fe 2+

H (T) 8(Fe) (mm/s) 2e (mm/s) A (mm/s) 8(Fe) (mm/s) A (mm/s) 8(Fe) (mm/s) LVe0sp 8(Fe) (mm/s) *Constant used for the fit

o.oo*

1.59+0.03 1.18+0.01 0.'44+0.04 1.15+0.02 0.04+0.01

35.1~0.2 1.35+0.03 0.04+0.06 2.06+0.03 1.26+0.02 ....... 0112•

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

1.36~0.07 1.23+0.04

..... '0.03•

.

.

.

o.oo* .

.

1.82+0.08 1.39_+0.04

0.02+0.05

There is a good agreement between the experimental 02 uptake for the complete reoxidation of the reduced catalysts and the consumption of 02 calculated from the percentage of each species obtained fi'om the M6ssbauer spectra at 15 K (Table 4). The cross-checking of volumetric oxidation results with the complex MOssbauer spectra of these catalysts is the only reliable method that should be used in spectra of such complexity if one has not the capability to take "in situ" spectra of samples with an external magnetic field. Other choices for assignments of the iron species, different from those of Table 2, lead to unacceptable differences between both techniques. From these results, it can be deduced that although we obtained two precursors with the same iron oxide species, the catalysts have different iron species aRer reduction. The number of Fe ~ surface atoms, was determined by 1-12chemisorption at 673 K in 10-80 Torr pressure range. The high H2 consumption observed allowed us to verify the existence of very small metallic Fe crystals inside the channels [10]. Assuming that hydrogen atoms are chemisorbed only on Fe ~ surface atoms and considering that there are two fractions of Fe ~ crystals [12], it is possible to estimate their average diameter values. Assuming a semi-spherical shape, the fraction of microcrystals located inside the zeolite structure cannot exceed 2.6 nm diameter considering the dimension of the channels. This fraction is superparamagnetic (Fe~ in the M6ssbauer spectra of both Table 3" Percentages of iron species obtained by MSssbaucr spectroscopy at 15K

c-Fe/ZLK(v) c-Fe/ZLK(a)

Fe ~ FesO4 ,,(magnetic) (magnetic) 30-A:2 5q-1 45+3 13+5

Species (%) Fe 2+(Coupled with Fe~ 13+1 ---

Fe 2+ (exchanged) 38• 27+6

Fe~ 144-1 15+4

531 Table 4: Values of O2 uptake, H2 chernisorption and Fe~ crystal diameter of catalyts.

c-Fe/ZLK(v) e-Fe/ZLK(a)

Experimental 02 uptake (~tmol O4g) 496:525 521:556

Theoretical O~ uptake (txmol O2/g) 442:514 553:542

Experimental Fe~ Diameter H2 ehemisorption (nm)

(/.tmol H2/g) 31 62

>_14.3 6.0-13.0

samples, and their percentages were obtained from these spectra. The more important structural difference between both catalysts is the presence of Fe 2§ decorating the extemal Fe ~ surface crystals in c-Fe/ZLK(v). The existence of this fraction of Fe 2§ leads to a decrease of the Fe~ and Fe304 amounts. On the other hand, the quantity of Fe ~ internal crystals (that represents the 80% of the total active sites) is the same in both catalysts. The activity and selectivity results are shown in Table 5. The tumover frequencies to total hydrocarbons of both catalysts were obtained assuming one active site per Fe~ surface atom. In the pseudo-steady state, the activity per site is about three times higher in cFe/ZLK(v) than in c-Fe/ZLK(a), and the activity per gram is twice higher in e-Fe/ZLK(v) than in c-Fe/ZLK(a). Since the inner Fe~ fraction is equal in both catalysts, the activity difference between them cannot be assigned to this fraction. In consequence, the different behaviour of the catalysts may be attributed to the presence of Fe 2+ decorating the Fe~ surface of crystals located outside the channels in fresh c-Fe/ZLK(v). Theoretical models have demonstrated that the main effect of cations in contact with a metal is an electrostatic one [ 13], which is essentially of short range. However, a long range effect is possible as a result of a cumulative electrostatic field, generating zones of minimum potential energy at the surface. Consequently, the bond between the Fe ~ and the CO adsorbed becomes stronger, while at the same time, the intra-moleeular CO bond is weakened, increasing the catalyst activity. After 48hs of reaction the Fe~ is carburized in both catalyst and in c-Fe/ZLK(v) the sextet of Fe2+ appears magnetically coupled with z-FesC2 maintaining the promoter effect of this species. This behaviour was demonstrated by MS in controlled atmosphere (not shown spectra). The olefirgparaffm ratio is similar for both catalysts. These results can be justified taking into account that the conversion values and the support basicity are practically equal in both samples Finally, similar methane production and Schulz-Flory coefficients ((z) are observed in both catalysts. Therefore, it can be deduced that Fe 2+ ions magnetically coupled with Fe~ Table 5" Activity and selectivity results

Total hydrocarbon molecules/site.see xl 04 Total hydrocarbon moleeules/g.sec xl 0"~6 CO conversion (%) Olefin/paraffln ratio(without CI-I4) cn4 (%) Schulz-Flory coefficient (a)

c-Fe/ZLK(v) 8.90 3.32 1.7 1.40 38 0.22

c-Fe/ZLK(a) 2.60 1.94 1.5 2.34 42 0.30

532 do not influence on the catalysts selectivity and chain growing. Bearing in mind that the 80% of the total active sites correspond to Fe~~, the low ot coefficient values can be justified taking into account that on very small metallic particles, the chain propagation finishes at low molecular weight hydrocarbon (up to C4) [14]. The small metallic crystal size avoids the presence of enough CHx neighbour groups to produce the propagation chain, although steric impediments inside the pores of the zeolite do not be ruled out. 4. CONCLUSIONS Through two different impregnation methods we obtained two catalysts with only one structural difference: the Fe2§ ions magnetically coupled with Fe located outside the channels of the zeolite ZLK. These ions enhanced the activity of the Fe~ sites for the total hydrocarbon production by an electrostatic effect favouring the CO dissociation. Instead, the selectivity and chain growth is not modified by the presence of these ions. REFERENCES

1. G.B. Raupp and W.N. Delgass, J.Catal., 58 (1979) 337. 2. M.V. Cagnoli, S.G. Marchetti, N.G. Gallegos, A.M. Alvarez, R.C. Mercader and A.A. Yerami~, J. Catal, 123 (1990) 21. 3. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramihn and 1LC. Mercader, Hyperfme Interactions (C), 4 (1999) 61. 4. S.G. Marchetti, A.M. Alvarez, J.F. Bengoa, M.V. Cagnoli, N.G. Gallegos, A.A. Yeramiatt and M Schmal, Actas do XVII Simp6sio Ibero-americano do Cat~ilise (J.M.0rfiio, J.L. Faria, J.L. Figueiredo, Eds.), p.97, Porto, Portugal (2000). 5. M. Boudart, A. Delbouille, J.A. Dumesic, S. Khamrnouma and H. Topsoe, J.Catal. 37 (1975) 486. 6. S. G. Marchetti, J. F. Bengoa, M. V. Cagnoli, A. M. Alvarez, N. G. Gallegos, A. A. Yerami~n and R. C. Mercader, Meas. Sci. Tech. 7 (1996) 758. 7. M.Vasquez-Mansilla, R.D. Zysler, C. Arciprete, M.I. Dimitrijewits, C. Saragovi, J.M. Greneche, J. of Magnetism and Magnetic Materials, 204 (1999) 29. 8. S. Morup and H. Topsae, Appl. Phys. 11 (1976) 63. 9. M.V. Cagnoli, N.G. Gallegos, A.M. Alvarez, J.F. Bengoa, A.A. Yeramihn and S.G. Marchetti, Studies in Surface Science and Catalysis, 135 (2001) 272. 10. A.M. Alvarez, S.G. Marchetti, M.V. Cagnoli, J.F. Bengoa, R.C. Mercader and A.A. Yerami~a., Applied Surface Science, 165 (2000) 100. 11. F. Bodker, S. Morup, M.S. Pedersen, P. Svedlindh, G.T. Jonsson, J.L. Garcia-Palacios and F.J. Lazaro, J. Magn. Magn. Mater., 925 (1998) 177. 12. S.G. Marchetti, M.V. Cagnoli, A.M. Alvarez, J.F. Bengoa, R.C. Mercader and A.A. Y~eramihrg Appl. Surf. Sei., 165 (2000) 91. 13. J.W. Niemantsverdriet, "Spectroscopy in Catalysis" VCH, Weinheim, (1995), p. 219. 14. L. Guczi, in "New Trends in CO Activation", Studies in Surface Science and Catalysis, Ed. L. Guezi, Elsevier, 64 (1991) 350.


533

On the necessity of a basic revision of the redox properties of H-zeolites Z. Sobalik, P. Kubhnek, O. Bortnovsky, A. Vondrovh, Z. Tvarfi~kovh, J.E. Sponer, and B. Wichteflov/t J. Heyrovsl@ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, CZ-182 23 Prague 8, Czech Republic. Negligible activity in NO-NO2 equilibration and SCR of NOx by propane, as well as benzene hydroxylation and N20 decomposition reactions was obtained over laboratory synthesized H-MFI, H-FER and H-BEA zeolites containing Fe with concentration below 50 ppm. On the other hand significant activity was found over H-zeolite samples of the same structural type and similar Si/Al values, but with the content of iron impurities of several hundreds of ppm. The consequence of this evidenced role of such low levels of iron content, i.e. at the level usually present in the commercial samples mostly used for the catalytic studies, and not considered in the analysis of the redox function of these systems, is discussed. INTRODUCTION There is generally accepted assumption that the protons of H-zeolites display redox properties, as H-forms of zeolites have been shown to exhibit catalytic activity in typical redox reactions. The list of such reactions includes NO-NO~ equilibration [ 1-3], selective catalytic reduction of NOx by paraffins [4], N20 decomposition, and benzene hydroxylation to phenol using nitrous oxide [5] and hydrocarbons oxidations. Reaction mechanisms of these reactions over H-zeolites are still not fully understood, nevertheless in some of them the zeolitic proton redox activity is expected to take part. Most elaborated are arguments in support of the proton redox activity in the NO-NO2 equilibration, a reaction connected with the selective catalytic reduction of NOx (SCRNOx) by paraffins. It has been recognized that the NO2 formation constitutes the prerequisite for effective reduction of NO to nitrogen by propane over H-ZSM-5 [4,6]. As no other active function was assumed in the H-form of the zeolite used for this reaction, the redox activity of the proton of bridged OH was postulated [ 1,3,4] to provide for the fairly high activity of H-ZSM-5 in the NO/O~/C3H8 reaction (see Ref. [2]). The concept of the redox function of the bridged proton was further detailed by Lukyanov et al. [3] providing direct experimental evidence of a high activity of a commercial H-ZSM5 sample in NO-NO2 equilibration, which yielded similar activity as ion exchanged CoZSM-5. Although the concept of redox activity of protonic sites has been generally accepted, there was always some base for a criticism of such view. It was indicated by

534 several experiments; e.g. Shelef [7], found the H-ZSM-5 sample nearly inactive in the NO oxidation. Less straightforward is the evidence on the active site for N20 decomposition and related benzene hydroxylation by N20. There are several controversial suggestions in the literature on the nature and structure of active sites in zeolite catalysts for hydroxylation of benzene to phenol by N20. Ono et al. [8], and Burch and Howitt [9] suggested the Broensted acid sites in H-ZSM-5 to be the active sites, and radical mechanism has been proposed to take place with participation of zeolite protons [8]. Strong acidic A1-Lewis sites, created during high-temperature dehydroxylation or under steaming of H-ZSM-5 were suggested to be the active sites for stabilization of an atomic oxygen, formed by N20 decomposition, on unsaturated aluminium, and its transfer to benzene molecule [10,11 ]. It could be summarized that in accordance with the concepts of these authors, the H-zeolites contain all necessary active functions to run the reaction. However, most of these studies were carried out with parent commercial zeolite samples with unspecified content of iron impurities. On the other hand a mechanism has been proposed, which includes redox function of Fe-connected sites and role of protons in activation of hydrocarbon. Extra-framework dinuclear iron species in ZSM-5 were suggested by Panov et al. [12,13] to be the active sites transferring so called a oxygen formed by N20 decomposition to benzene. Nott6 [14] also assumed, that the presence of Broensted acid sites is essential for benzene to phenol hydroxylation together with the a-sites, primarily linked to the presence of iron. In the presented paper we attempt to evaluate whether the occurrence of iron traces, always present in the commercial zeolites up to the range of hundreds of ppm, could have basically influenced the conclusions on the redox activity of H-zeolites, as found in the literature. For this purpose a set of test reactions, i.e. NO equilibration, SCR of NOx by propane, N20 decomposition and benzene hydroxylation was used. 1. EXPERIMENTAL SECTION

1.1 Catalyst preparation Representative collection of the frequently used zeolite system for the mentioned reactions were chosen, i.e. ZSM-5 (MFI), ferrierite (FER), and beta (BEA), see Table 1. Table 1. Commercial zeolite samples Sample

Si/A1

Fe (ppm)

Producer

LOT

ZSM-5

39 14.1 12.5 8.5 12.7

410 180 200 170 250

PQ~) Slovnaft 2) Slovnaff TOSOH 3) PQ

ZH-5

FER BEA

010812B HB-8-25

pQ Corporation, U.S.A., 2)Research Institute of Petroleum and Hydrocarbon Gases, SR; 3) TOSOH Corporation, Japan.

1)

535 Table 2. Laboratory prepared pure zeolites Sample

Si/Al

CFe, ppm ~)

MFI MFI FER BEA

28 19.5 8.2 11.9

30 50 50 26

1) The data given are based on the average of three analyses. The error of the given values for Fe analysis was estimated to be +5 ppm. Laboratory synthesized zeolites MFI, FER and BEA (see Table 2) were prepared using materials with iron content below 1 ppm of Fe. If necessary the prepared zeolites were transformed into NH4-form by 4-fold ion exchange by 1M NH4NO3 071uka, Fe contents 1 ppm) in demineralized water. MFI samples were prepared using a solution of A1 nitrate (Fluka) in deionized water, tetraethyl orthosilica (Fluka) and ethanol (laboratory grade). The mixture was stirred for 90 min, and a solution of tetrapropylammonium hydroxide in water (20% - Fluka) was added; the resulting mixture was stirred for 90 min. The reaction gel crystallized for 72 hrs at 170 ~ FER was synthesized using fumed silica (Cab-o-Sil) and pure metallic A1 dissolved in solution of sodium hydroxide (Merck) and potassium hydroxide (Fluka). No organic template was used in this synthesis. BEA was synthesized with fumed silica (Cab-o-Sil), aluminium nitrate (Fluka) and TEAOH (35 wt.%, Aldrich). XRD analysis indicated highly crystalline materials for all the samples. The elemental analysis was done by X-ray fluorescence spectroscopy and inductively coupled plasma emission spectrometry. 1.2. Structural characterization

FTIR spectra of samples in the form of self-supported pellets (- 5 mg/ cm 2) were recorded at room temperature on a Magna-IR System 550 FTIR Nicolet with a lowtemperature MCT-B detector using heatable cell connected to a vacuum/gas manifold. Analysis of the transmission window region evidencing the local perturbation of the zeolite framework due to cation presence in the cationic positions was based on the data for Fe-FER, Fe-BEA [15] and Fe-MFI [ 16]. The concentration of Broensted and Lewis sites in zeolites was determined by quantitative analysis of the spectra of adsorbed d3acetonitrile using the extinction coefficients listed in ref. [ 17]. X-band ESR spectra of Fe(III) ions were monitored at RT on an ESR spectrometer ERS-220, Germany. As an internal standard Mn(II) ions in solid matrix were used. The samples were before the spectra measurements pretreated in a stream of dry oxygen at 480 ~ for 2 hrs, then cooled to RT and evacuated for 30 minutes at 10.2 Pa, and sealed. Model calculations of simplified model of Fe cationic site were performed employing DFT/Becke3LYP level of theory using the Gaussian98 suit of programs.

536

g2.3

~

~i~

~~f''~

Figure 1 X-band ESR spectra of MFI with 30 and 410 ppm of Fe dehydrated at 450 ~ the H-ZSM-4 with 1200 ppm Fe added by ion exchange taken from Ref. [21 ]

Spectra of

1.3. Catalytic tests

The catalytic activity tests were carded out in a flow catalytic setup using a fixed-bed through-flow mieroreactor (total flow of the gas mixture 100 ml/min) with 0.4 g (NO/NO2, SCR, N20 decomposition) or 0.5 g (benzene hydroxylation) equipped with an on-line chemical analysis of the inlet and outlet streams, combining a standard GC analysis (HP 5890II series, model G1540A) (all reactions), with NOx/NO ehemiluminiseenee analysis (Analyser Vamet 138, CR) (NO-NO2, SCR and N20 decomposition). Prior to the catalytic tests of NO-NO2 equilibration, SCR-NOx, or N20 decomposition the catalysts were heated for 4 hours in the stream of He, and then the system was stabilized in a reaction mixture at 350 ~ until a stable catalytic performance was reached (usually over 3 h). Then the activity was measured between 150 and 450 ~ A stable activity was usually reached after about 1 h and was stable for several hours. Composition of the inlet stream in NO-NO2 equilibration test consists of 1000 ppm NO and 2.5 % 02, balanced by He to a total flow of 100 ml/min. The catalytic tests of SCR of NOx by propane was carried using stream consisting of 1000 ppm of NO, 1000 ppm of propane, 4% of 02, total flow of 100 ml/min in He. The gas mixture of 1000 ppm N20 in He was used for tests for N20 decomposition. The reaction mixture for benzene hydroxylation contained 20 vol. % of N20, 20 vol. % of benzene, and nitrogen as a balance. Prior to the reaction the catalyst was activated in a stream of oxygen at temperature of 480 ~ for 1.5 hr and then in a nitrogen stream at the reaction temperature

537 for 30 rain. The reaction was carried out at 350 ~ It has been checked that the activity of the empty reactor (filled by glass balls) was negligible for any of the reactions under the experimental conditions used. 2. RESULTS AND DISCUSSION

2.1.Characterization of samples The iron content in the commercial samples was between 170 ppm, and 410 ppm. By using very pure materials for zeolites synthesis was possible to suppress the content of iron to or below of 50 ppm. The total acidity of the zeolites was consistent with the Si/A1 value. Brondsted acidity prevails on both the H-ZSM-5 and H-FER samples, while higher proportion of Lewis acidity was found in the H-BEA samples (see discussion in

Ref. [18]). Structural characterization of the iron species at the level of several hundreds ppm provided only a limited information. The FTIR method was based on the approach already presented in Ref. [15]. The ,,transmission window" region between 980 and 850 cmlwas previously shown to evidence bonding of divalent metal cation to the local framework structure of the cation position (see [19]. A band at 915 cm1 was identified for both dehydrated Fe-FER and Fe-BEA samples, with intensity proportional to the iron cation content [Montp.]. For FeZSM-5 samples this band was found at 928 cmq [ 16]. Actually a very weak band at 915 cmq was found in the commercial FER sample with 170 ppm of Fe. This band was absent in the laboratory prepared FER with 50 ppm of Fe. This evidences that at least a part of the iron present in the commercial FER is placed in the regular cationic position. No band in this region was identified in either BEA or ZSM-5 samples. However, no conclusion could be drawn from this finding, as the intensities of the band due to local perturbation of the BEA and ZSM-5 samples are generally weaker than in FER, and thus the concentration of Fe in cationic sites at the commercial samples is probably well below the detection limit. ESR spectra indicate that the H-MFI zeolites with 30 ppm and 50 ppm Fe contains iron predominantly in the form of oxide-like species (broad signal at g 2.3, AH 130 mT). In samples containing 180 and 410 ppm Fe a signal at g 4.3, AH 5 mT, of Td coordinated Fe at cationic sites appeared, in addition to a broad signal (see Fig. 1). At higher Fe concentration (1200 ppm) additional signals at g 6.0 and 5.6 have been reported [21] and related to the hydroxylation of benzene. These can be hardly seen at the low Fe concentrations. It could be summarized that a direct structural analysis of such low iron content could provide only a limited evidence suggesting the presence of iron in both the ion-exchange position and as the oxide-like species. With the absence of a reliable structural data providing for identification of the active species, (i.e. single cations, defective oxides, dinuclear species ), it could be speculated that the suggested model calculations of the proposed structures could provide some insight. Actually, both the Fe (II) and Fe (III) in the cationic position displayed according to the model calculation extreme charge transfer effects between the zeolite framework and the metal, thus actually bringing the iron cation reduction. As a result, the computed NBO charges of Fe(II) and Fe(III) were 0.91 and 1.8, respectively.

538 2.2. Catalytic results

NO-N02 equilibration. The NO-NO2 equilibration of the NO-O2 mixture (the results expressed as a fraction of the NO2 content at the equilibrium composition of the gas mixture; see Tab. 3) indicated high activity of the commercial zeolites and low (FER) or negligible for pure samples (MFI, BEA). Despite the expected difference in the state of Fe impurities and difference in the zeolite structure, the catalytic performance in this reaction was roughly proportional to the amount of iron present in the zeolite. SCR of NOx by propane.As typical for SCR-NOx by paraffins, the temperature profile is characterized by a bell shape curve with maximum at about 350 ~ (not shown). Typically the declining part of the curve is connected with a complete consumption of paraffin. To eliminate the role of this parameter the data used here for evaluation of the role of iron impurities presence are those taken at lower temperature, eliminating thus the role of changing NOx/paraffin ratio. The data on the commercial and pure analogs obtained at 300 ~ are summarized in Table 3. It should be stressed that these data were obtained working with the concentration of NO2 in the inlet of the reactor below 100 ppm. Previous results have shown negligible role of high iron content for the activity of H-zeolites in SCR of NO2 rich streams [22], indicating that the reaction of propane with NO2 run exclusively on the protonic sites of a zeolite. Accordingly, the change in the performance of the two classes of samples (pure vs. commercial) in the NO/O2/propane mixtures resides exclusively in the role of iron impurities in NO to NO2 oxidation. Table 3. Summary of the data on catalytic performance of the zeolites in the NO-NO2 equilibration, SCR of NOx by propane, N20 decomposition and benzene hydroxylation. zeolite

eft, ppm

MFI

30 50 180 410

FER BEA

Reaction N20 decomposition2), %

NO-NO2 equilibration 1), %

SCR-NOx 2), %

1.5

9

0

20

19

3

50 170

11

10

2

39

32

17

26 250

2 52

19 60

Benzene hydroxylation3), mmol/g.h 0.5 0.8 6.4 7.5

1)Ratio of the equilibrium NO2 concentration at the outlet; 2)NOx or N20 conversion, %; 3)Rate of Phenol formation, T-O-S 25 rain, mmol/g.h

539

N20 decomposition. Only a limited set of experiments is presented here. Nevertheless, it is documented that by increasing iron content to the level of the commercial FER sample would provide for appreciable N20 decomposition. Benzene hydroxylation. Nearly all literature data on benzene hydroxylation have been accumulated over H-MFI type of catalysts. Accordingly, the experiments were limited also here to this material. Laboratory prepared H-ZSM-5 zeolites with very low iron content (30 and 50 ppm Fe), exhibited very low activity in benzene hydroxylation, in contrast to the commercial samples containing 180 and 410 ppm Fe. As shown previously [21], the concentration of protons in the zeolite itself does not correlate with the activity for benzene hydroxylation. It could be summarized, that in all the reaction studied, i.e. NO-NO2 equilibration, SCR-NOx in the NO/O2/propane mixture, N20 decomposition and benzene hydroxylation, a decrease of the iron content below the level typical of the standard commercial products to about 50 ppm brings about a striking suppression of the catalytic activity of the H-zeolite structures studied. Based on the results it could be speculated, that in the two categories of reactions, i.e. connected to NO or N~O species, the simpler processes, i.e. NO or N20 activation, are directly and exclusively connected with the iron active sites, while in the more complex processes, including the propane or benzene activation, i.e. SCR-NOx in NO/O2/propane mixture or benzene hydroxylation, the iron active sites cooperate with the zeolitic protons. By assuming the iron present as active site for the individual reactions would provide a very high TOF values (between 20 - 90 h-l), a value highly above the TOF value of standard catalysts for such reaction systems. Moreover, the ESR findings indicate that only small fraction of iron present bear catalytic function, thus bringing the actual value of TOF to even higher values. Based on the presented data we can speculate that the list of reactions, where the role of the iron impurities in the parent H-zeolite was overlooked in the analysis of the reaction mechanism, would go farther. Actually, we would propose the well-known positive effect of the oxygen traces in cracking over H-zeolites as a next candidate for such analysis. CONCLUSSIONS Using a set of commercial zeolite samples and laboratory prepared pure zeolites a role of iron impurities in redox activity of H-zeolites was proven. Only a limited structural analysis on this iron concentration level could be provided, and even these are based on extrapolation of the tendencies from the studies carried out at higher concentration levels. The evidence has been given suggesting, that, using the standard commercial parent samples and omitting in the data analysis the presence and contribution of the iron impurities to the catalytic reactions, could influence understanding of the redox reactions over H-form of zeolites. All this brings us to the conclusion that there exists a necessity to make a basic revision of the data on the redox properties of the H-zeolites as presented in literature. Actually, we suggest that in most cases the already published results should be re-evaluated after replacing for the new analysis the parent "H-zeolite" by a correct HFe-zeolite.

540

Acknowledgments Financial support from the Grant Agency of the Czech Academy of Sciences (Grants A 4040007 and S 4040016) is gratefully acknowledged.

REFERENCES 1. I. Halasz, A. Brenner, K.Y.S. Ng, and Y. Hou, J. Catal. 161 (1996) 359. 2. A.Y. Stakheev, C.W. Lee, S.J. Park, and P.J. Chong, Catal. Lett. 38 (1996) 271. 3. D.B. Lukyanov, G. Still, J.L. d'Itri, and W.K. Hall, J. Catal. 153 (1995) 265. 4. M. Sasaki, H. Hamada, Y. Kintaichi, and T. Ito, Catal. Lett. 15 (1992) 297. 5. V. Zholobenko, I.N. Senchenya, L.M. Kustov, and V.B. Kazansky, Kinet. Catal. Eng. Ed. 32 (1991) 132. 6. H. Hamada, Y. Kintaichi, M. Sasaki.,T. Ito, and M. Tagata, Appl. Catal. (1990) L1 64. 7. M. Shelef, C.N. Montreuil, and H.W. Jen, Catal. Lett. 26 (1994) 277. 8. E. Suzuki, K. Makasiro, and Y Ono, Chem. Soc. Jap. Chem. Commun. (1988) 953 9. R. Butch and C.Howitt, Appl. Catal A: General 103 (1993) 135. 10. J.L. Motz, H. Heinichen, W.F. Hoelderich, J. Mol. Catal. A: Chemical 136 (1998) 175. 11 L.M. Kustov A.L Tarasov., V.I Bogdan., A.A Tyrlov, and J.W. Fulmer, Catal. Today, 61 (2000) 123. 12 G.I. Panov, G.A. Sheveleva, A.S. Kharitonov, V.N. Romannikov, L.A. Vostrikova, Appl. Catal., A:Oeneral, 82 (1992) 31. 13. O.I. Panov, V.I. Sobolev, K.A, Dubkov, A.S. Kharitonov, J. Hightower, W.N. Delgass, E. Iglesia, A.T. Bell, Proc. 11th Inter. Congr. Catal., Baltimore, 1996, Stud. Surf. Sci. Catal., 101 (1996) 493. 14. P. Notte, Topics Catal., 13 (2000) 387. 15. Z. Sobalik, J.E. Sponer, and B. Wichterlovh, in A. Corma, F.V. Melo, S. Mendioros, and J.L.G. Fierro (Eds.), Studies in Surface Science and Catalysis, Vol. 130B, Elsevier, Granada, (2000) 1463 16. H.Y. Chen, X. Wang, and W.M.H. Sachtler, Phys. Chem Chem. Phys., 2 (2000) 3083. 17. B. Wichterlov/t, Z. Tvarfi~kovfi, Z. Sobalik, and P. Sarv, Microporous Mesoporous Mater., 24 (1998) 223. 18. O. Bortnovsky, Z. Sobalik, and B. Wichterlov~, Microporous. Mesoporous. Mater., 46 (2001) 265. 19. J.E. Sponer, Z. Sobalik, J. Lesczynski, and B. Wichterlovh, J. Phys. Chem. B, 105 (2001) 8285. 20. Z. Sobalik, J. E. Sponer, Z. Tvarfi~kov~i, A. Vondrovh, S. Kuryiavar, and B. Wichterlov~i, in A. Galameau, F. Di Renzo, F. Fajula, and J. Vedrine (Eds.),Studies in Surface Science and Catalysis Vol. 135 Elsevier, Montpellier, (2001) 136. 21. P. Kubhnek, B. Wichterlovfi, and Z. Sobalik, J Catal., submitted. 22. Z. Sobalik, J.E. Sponer, A. Vondrovit, Z. Tvarfi~kovfi, and B. Wichterlovb., Catal. Today, in press.


541

The role of zeotype catalyst support in the synthesis of carbon nanotubes by CCVD K. Hernadi a*, Z. K6nya a, A. Siska a, J. Kiss b, A. Oszkd b, J. B.Nagy c and I. Kiricsi a aUniversity of Szeged, Applied and Environmental Chemistry Department, H-6720 Szeged, Rerrich Bela ter 1, Hungary bReaction Kinetics Research Group of the Hungarian Academy of Sciences, Department of Solid State and Radiochemistry, University of Szeged, P.O. Box 168, H-6701 Szeged, Hungary CLaboratoire RMN, Facult6s Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, B5000 Namur, Belgium

The effect of zeotype support on the selectivity of carbon nanotube formation in the catalytic decomposition of acetylene was investigated. Catalyst supports with various pore diameters were tested. Formation and the quality of carbon deposit were followed by transmission electron microscopy (TEM) and the state of supported catalyst particles was investigated by in situ X-ray photoelectron spectroscopy (XPS) measurements. It was found that only catalyst particles deposited on the external surfaces of porous support could efficiently take part in the catalytic carbon nanotube formation.

1. INTRUDU CTI O N The catalytic chemical vapor deposition (CCVD) method for the production of carbon nanotubes is of great interest among researchers since it gives large quantity, good quality single- (SWNT) and/or multi- (MWNT) wall carbon nanotubes. In this procedure simple hydrocarbons as methane, ethylene, acetylene, or benzene, toluene

Authors thanks to the European Commission (RTN Program, NANOCOMP network, RTN 11999-00013), to the Hungarian Ministry of Education (FKFP 0643/2000) and to the National Science Foundation of Hungary (OTKA T25246).

542 were used predominantly [1]. Transition metals, most frequently Fe, Ni or Co, supported on oxides or zeolites were the catalyst precursors [2]. When bimetallic catalyst w a s used, alloy phase was formed, which was supposed to be the active component of the catalyst. The relatively high yield and excellent quality of carbon nanotubes were explained by the peculiar behavior of this alloy phase [3]. Recently, several papers dealt with the mechanism of nanotube formation [4]. Particularly, the role of the catalyst support and the particle size of the metal have been discussed [5]. The most frequently used catalyst supports were silica, zeolites and alumina. Well-crystallized carbon nanotubes were formed on catalysts supported by these materials [6]. A part of these supports are molecular sieves having sharp pore diameter distribution in molecular dimensions (0.4-1 nm), pore diameter of the others is much larger. The role of pore structure of the support in the formation of nanotubes is one of the most intriguing problems to be answered. Structural and textural properties of pyrolitic carbon formed in the inner pores of zeolitic structures have been studied [7]. In this paper we present results on the role of zeotype catalyst support and the state of the metal in the CCVD production of MWNT.

2. EXPERIMENTAL

2.1. Preparation and characterization of catalysts Zeolites (NaA: Hungalu Co., KL: Union Carbide, NaY: Union Carbide, 13X: Union Carbide) and mesoporous zeolite-like materials (MCM-41, SiMCM-48, A1MCM-48: synthesized in our laboratory [8-9]) were loaded with metal ions using ion exchange, impregnation and isomorphous substitution. After evaporating the solvent, the catalyst sample was dried at 400 K overnight. Catalyst samples prepared by the impregnation method contained 2-5 wt% of Co. Since these materials have high ion-exchange capacity, and when they are in contact with cobalt ion containing solution, ion exchange immediately starts. Upon drying the solution onto the zeolite the ion concentration in the solution increases, consequently, the ion exchange in the zeolite goes to completion. After this point, extra ion incorporation takes place if the initial Co content of the solution used for the preparation of a given amount of zeolite is larger than the ion exchange capacity of the zeolite. This is the source of Co ions on the outer surface of the zeolite catalysts. Ion exchange of A1MCM-41 in aqueous solution of Co acetate (0.1 molYdm3) was the preparation procedure for CoA1MCM-41(ex) catalyst. Ion exchange was performed twice at 343 K for 12 h each time (0.5 mmol metal ion/g silicate). CoMCM-41(iso) sample was prepared by isomorphous substitution of Si for Co

543 following the description in [10]. Co/A1MCM-41, SiMCM-48 and A1MCM-48 were prepared by impregnation. Calculated amount of Co salt was dissolved in distilled water, which was evaporated slowly under gentle heating. The composition of the catalyst was checked by X-ray fluorescence (XRF) analysis. The transition metal content was determined by classical analytical methods. The zeolite samples and the MCM materials showed the characteristic X-ray diffraction (XRD) pattern. The BET surface areas of the samples were determined by N2 adsorption isotherms measured at 77 K using a volumetric apparatus. For the MCM samples, the pore size distribution was calculated by the Barett-Joyner-Halenda method [11] from the adsorption data. 2.2. Synthesis of carbon nanotubes The catalytic reaction was carried out in a fix-bed flow reactor in the temperature range of 900-1100 K. The catalyst samples were placed in a quartz boat that was put into a horizontal tube reactor. Before introducing the reactant mixture (10% acetylene 90% N2, with a flow rate of 300 ml/min) the catalysts were purged by nitrogen stream (300 ml/min) in order to remove water and pretreat the catalyst at 999 K. The reactions were carried out for reaction time of 30 min. In situ XPS measurements were carried out to clarify the state of cobalt on the supported catalyst and the reaction was conduated in the sample prelaaration chamber of XPS instrument. [see details in ref. 12] 2.3. Characterization of the product MWNTs Since the initial weight of catalysts introduced into the reactor was known, we measured the weight increase after the reaction. From these data the total carbon production was determined. For the charaterization of catalyst activity, carbon yield (ratio of carbon deposit and catalyst) calculated as following was used: Carbon yield = (mafterreaction-- mcatalyst) / 1Tlcatalyst (g/g) For TEM and HRTEM Philips CM20 and JEOL 200CX were used, respectively. For the preparation of sample holder grids, the glue technique was used described elsewhere in detail [2]. Nominal composition, surface area, pore diameter and activity data of the catalysts are given in Table 1.

544 Table 1 Characterization of catalyst samples Sample

Metal content Surface area Pore diameter (wt%) (m2g1) (nm)

Co/A Co/L Co/Y

Activity (g/g)

2 2 2

435 216 632

0.51 0.60 0.74

0.03 0.20 0.19

Co/13X Co/A1MCM-41 CoA1MCM-4 l(ex)

2 5 0.29

615 931 931

0.74 3.4 3.4

0.21 0.73 0.82

CoMCM-41 (iso)

0.01

931

3.4

0.96

5 5

1078 994

3.1 3.0

0.67 0.71

Co/SiMCM-48 Co/A1MCM-48

3. RESULTS 3.1. Formation of M W N T s

There are obvious differences between these catalysts concerning both the quantity and the quality of MWNT formed. Using impregnated zeolite-supported catalysts of pore diameter less than 1 nm, well-graphitized carbon ~anotubes could be grown almost independently of the type of the support. Neither surface area nor pore diameter affected significantly the quality and the quantity of carbon nanotubes. For illustration, Fig. 1 shows electron microscopy images of carbon nanotubes grown over various zeolite-supported cobalt catalysts.

m

.

Figure 1. Carbon nanotubes formed on the surface of a) Co/NaY; b) Co/13X catalysts; c) high resolution image of a carbon nanotube.

545 similar dimension would be able to regulate the inner or the outer diameter of the forming carbon nanotubes. Close scrutiny of our samples (Figs. 2 and 3a) revealed, however, that the pore size of the mesoporous supports and the diameter of carbon nanotubes showed no correlation.

Figure 2. Carbon nanotubes formed on the surface of a) Co/SiMCM-48 and b) Co/A1MCM-48 catalysts. No nanotube could be detected on CoA1MCM-41 (iso) (Fig. 3b) and only a slight indication of nanotube formation is seen on CoMCM-41(ex). Numerous, wellgraphitized nanotubes formed on Co/A1MCM-41 (Fig. 3a). It is worth to emphasize here that the samples proved to be inactive in the production of MWNTs were prepared by isomorphous substitution and ion exchange, not by impregnation. 3.1. In situ XPS characterization As far as the reducibility of cobalt ions is concerned detailed in situ X P S investigations were carried out. XPS spectra of the catalyst samples were taken under vacuum at both ambient and reaction temperatures, then, measurements were performed in acetylene atmosphere. Significant changes were observed after the sample was kept at 1000 K in acetylene atmosphere for 60 min. In such a strong reducting atmosphere we could detect reduction of cobalt ions.

546

Figure 3. Electron microscopy image of samples a) Co/MCM-41 and b) CoMCM41 (is0) after CCVD.

4. DISCUSSION

Zeolites have a relatively small pore size, typically a few A. Mesoporous molecular sieves have uniform hexagonal (MCM-41) and cubic (MCM-48) pore systems ranging from 10 A to more than 100 A. This significant difference inspired us to test these materials as catalyst support in the formation of carbon nanotubes having similar dimensions. As it is known from zeolite chemistry, the ion exchange positions of zeolites are situated in their pore system, which is of molecular dimensions. In our case, zeolite NaY has a pore opening, i.e., an entrance for the ions and/or molecules, around 0.7 nm. Its ion-exchange capacity depending on the Si/A1 ratio of the framework varies, but its upper limit is around 5 mmol/g dry zeolite. This 5 mmol/g Co 2+ ion is bound to particular positions in the cage system and is accessible only for molecules of kinetic diameter less than 0.7 nm. This is true for the reverse way as well. Though, only those molecules can leave the pores whose diameter is smaller than the pore exit that is identical to the entrance. From this follows that carbon nanotube formation takes place on those metallic particles, which are generated from ions sitting on the outer surface of zeolite crystals, since the outer diameter of the thinnest MWNT is much bigger than 0.7 nm, the pore size of the zeolite. Similar consideration can be done for the other zeolitic supports applied in our system. Their pore diameter is lower than or equal to that of NaY.

547 For the MCM structures the situation is similar. Here, MWNT formation was observed neither on CoA1MCM-41 (ex) nor on CoMCM-4 l(iso). The case of the former is identical to that mentioned above for the ion-exchanged zeolites. Here, the pore opening is bigger (-3 nm), however, the Co 2§ ions are in the channels, but the pores are too small to be the nests of MWNT generation. The case of an isomorphous substituted sample is even simpler. Presumably, all Co ions are chemically bound in the wall of MCM-41 in this sample. These Co ions are immobile, almost irreducible, therefore, there is no or a very small chance to form clusters on the outer surface of the material. Therefore, they cannot act as active sites in the MWNT generation. We proved that cobalt-containing samples prepared by impregnation are good catalysts for the generation of MWNT from acetylene via CCVD. The activity differences found for the various supports can be explained by the necessary localization of the catalytically active components on the outer surface, at those places of support where a MWNT can easily accommodate, i.e., in the big pores like a silica gel has [13]. Interaction between cobalt particles and catalyst support seems to be of significant importance. Catalyst activity may slightly vary with Si/A1 ratio. Since carbon yield determined after reaction is only an approximate measure of the synthesis (MWNT content of carbon deposit varies in wide range), quantitative considerations cannot be done. Concerning the characteristics of zeotype support materials listed in Table 1, no correlation was found between these data and catalytic activity in carbon nanotube formation. Increasing amount of deposited carbon with larger pore diameter is due to stuffing the pores with non-graphitic carbon. This activity is independent of the selectivity of carbon nanotube formation for which exclusively catalyst particles on the outer surface was found to be responsible. Consequently, using mesoporous material as catalyst support, instead of presumable controlling effect, the overwhelming part of carbon deposit is composed of amorphous carbon. No indication was found suggesting that formation of MWNT starts in the pores of MCM type catalysts. Actually, their pore diameter is much smaller than that of the MWNT. From this it follows that only those catalyst particles, which are deposited on the outer surfaces can have a role in the formation of carbon nanotubes. Our in situ ESCA experiments showed that cobalt ions are reduced by the reactant acetylene and we found no indication of any kind of cobalt oxide after treatment the sample at 1000 K.

548 REFERENCES

1 L.B. Avdeeva, D.I. Kochubey, Sh.K. Shaikhutdinov, Appl. Catal. A: General 176 (1999) 135; T.E. Muller, D.G. Reid, W.K. Hsu, J.P. Hare, H.W. Kroto, D.R.M. Walton, Carbon 35 (1997) 951; A.M. Benito, Y. Maniette, E. Munoz, M.T. Martinez, Carbon 36 (1998) 681. 2. A.L Balch, M.M. Olmstead, Chem. Rev. 98 (1998) 2123; K. Hernadi, A Fonseca, J. B.Nagy, A. Fudala, D. Bemaerts, A. Lucas, Zeolites 17 (1996) 416; A. Carlsson, T. Oku, J.O. Bovin, G. Karlsson, Y. Okamoto, N. Ohnishi, O. Terasaki, Chem. Eur. J. 5 (1999) 244. 3. Z. K6nya, J. Kiss, A. Oszk6, A. Siska, I. Kiricsi, Phys. Chem. Chem. Phys. 3 (2001) 155. 4. R. Sen, A. Govindaraj, C.N.R. Rao, Chem. Phys. Lett. 267 (1997) 276; A. Peigney, C. Laurent, O. Dumortier, A. Rousset, J. Eur. Ceram. Soc. 18 (1998) 1995. 5. M. Terrones, W.K. Hsu, H.W. Kroto, D.R.M. Walton, Topics in Current Chemistry, Springer Verlag, Berlin Heidelberg, Vol. 199 (1999) p: 189-234. 6. K. Hemadi, A Fonseca, J. B.Nagy, D. Bemaerts, Springer Series in Materials Science: Supercarbon, Springer Berlin Heidelberg (1998) p: 81; A. Kukovecz, I. Willems, Z. Konya, A. Siska and I. Kiricsi, Phys. Chem. Chem. Phys. 2 (2000) 3071. 7. J. Rodriguez-Mirasol, T. Cordero, L.R. Radovic, J.J. Rodriguez, Chem. Mater. 10 (1998) 550. 8. J.S. Beck, J.C. Vartuli, G.J. Kennedy, C.T. Kresge, W.J. Roth and S.E. Schramm, Chem. Mater., 6 (1994) 1816. 9. R. Schmidt, D. Akporiaye, M. St6cker and O.H. Ellestad, J. Cfiem. Soc., Chem. Commun., (1994) 1493. 10. J.M. Kim, J.H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem. 99 (1995) 16742. 11. E.P. Barrett, L.G. Joyner, P.P. Halenda, J. Am. Chem. Soc. 73 (1951) 373. 12. K. Hemadi, Z. K6nya, A. Siska, J. Kiss, A. Oszk6, J. B.Nagy and I. Kiricsi, Mater. Chem. Phys., in press 13. K. Hemadi, A Fonseca, P. Piedigrosso, J. B.Nagy, D. Bemaerts, J. Riga, A. Lucas, Catal. Lett. 48 (1997) 229.


549

The influence o f w a t e r on the activity o f nitridated zeolites in b a s e - c a t a l y z e d reactions S. Ernst, M. Hartmann, T. Hecht, P. Cremades Ja6n and S. Sauerbeck Department of Chemistry, Chemical Technology, University of Kaiserslautern, P.O. Box 3049, D-67653 Kaiserslautern, Germany

The ammonia treatment of zeolites with FAU-topology at 700 ~ leads to materials in which Si-NH2, Si-NH-Si and Si-NHz-A1 groups are present, i.e. NaXN, NaYN and DAYN (nsi/nAl = 30). These catalysts are active in base-catalyzed reactions, such as Knoevenagel condensations. It could be shown that the catalytic activity increases with increasing nsi/nAlratio. Furthermore, TG-MS experiments revealed that the uptake of water leads to slow hydrolysis with concomitant release of ammonia. The sensitivity to water increases with increasing hydrophilicity of the catalyst, viz. increasing aluminum content of the parent zeolite. It is, however, possible to preserve the catalytic activity by keeping the freshly nitridated material in a dry nitrogen atmosphere. 1. INTRODUCTION Recently, a novel method for the preparation of zeolites and zeolite-like microporous and mesoporous materials with basic properties has been described [1,2]. Essentially, it is based on a high-temperature (i.e., 700 ~ to 850 ~ ammonia treatment of the molecular sieves. The materials obtained by such a treatment are active catalysts in typical base-catalyzed reactions like Knoevenagel condensations. By analogy with nitridated amorphous oxynitrides (e.g. [3,4]), the active basic sites in the new materials are supposed to result from a replacement of a certain amount of framework oxygen atoms by nitrogen. By substitution of bridging oxygen atoms or of oxygen in silanol groups in zeolites, Si-NH-Si and Si-NH2 groups are formed. The occurrence of both species in nitridated zeolites has been detected by Fourier-Transform-IR spectroscopy [2]. However, it has also been found in the studies conducted so far that these nitrogen-containing sites are prone to hydrolysis in the presence of water vapor with a concomitant loss of active sites. Hence, the catalytic activity of the nitridated zeolites strongly depends on their "history", viz. contact with moist air etc. Therefore, it was the aim of the present study to investigate in detail the influence of water on the catalytic activity of the novel type of catalyst. For this purpose, nitridated zeolites with FAU topology were stored for selected periods under different conditions (i.e., under nitrogen, under a controlled moisture-containing atmosphere and at elevated temperatures in air) and subsequently tested for their catalytic activities in the Knoevenagel condensation of benzaldehyde with propanedinitrile.

550 2. E X P E R I M E N T A L SECTION The zeolites NaX, NaY and dealuminated Y (DAY, nsi/nAl = 30, prepared via SiC14treatment) were used as parent materials. The nitridations were performed in an ammonia flow ( V = 60 ml- min -~ ) at 700 ~ for a period of 42 h. Afterwards, the materials were cooled down in a flow ( V = 60 ml. min -1 ) of dry nitrogen. Aliquots were then transferred as quickly as possible into the prepared reaction mixture or stored for selected periods under nitrogen, in a desiccator over a saturated aqueous solution of ammonium chloride or at 100 ~ in air. Xray powder diffraction patterns were collected on a Siemens-axs D5005 diffractometer using CuK~ radiation. TG-MS experiments were performed using a SETARAM setsys-16/MS instrument with a heating rate of 13= 5 K. min -~ in a nitrogen flow. The DRIFT spectra were collected using a Nicolet Nexus spectrometer equipped with a high-temperature in-situ chamber. The pyrrole adsorption experiment was performed in-situ in the FT-IR spectrometer at 50 ~

using a pyrrole-loaded flow of nitrogen (X7 = 40 ml. min-1,

Ppyrrole =

1 kPa ). The

catalytic experiments were conducted in the liquid phase at 8 0 ~ using 4 m m o l benzaldehyde, 4 mmol propanedinitrile and 0.2 g of catalyst in 10 cm 3 toluene as solvent. 3.

RESULTS

3.1 Characterization XRD powder patterns of the nitridated zeolites NaXN, NaYN and DAYN are shown in Figure 1. In the sample notations, the suffix "N" indicates that the zeolite has been treated with ammonia. The thermal stability of the nitridated materials after different storage times in a desiccator was studied by thermogravimetry coupled with mass spectroscopy. The maximum of the observed weight loss is around 150 ~ and fragments with m/z = 16 (NH3) and m/z = 18 (H20) are detected in the mass spectra. In Figure 2, the relative intensities of the signals of water (rn/z = 18) and ammonia (m/z = 16) are plotted as a function of the storage time for zeolite NaYN. The increase of the signal for the desorbed water is correlated to the decrease of the signal for ammonia. The nitridated materials have also been characterized by infrared spectroscopy in the diffuse reflectance mode (DRIFT). As shown earlier, the DRIFT-spectrum of the ammonia treated zeolite DAYN exhibits four additional bands, viz. at 1622, 1553, 1536 and 1450 cm -1, which were tentatively assigned according to Table 1 [2]. In particular, the bands at 1553 and 1536 cm -1 are indicative of Si-NH2 and Si-NH2-A1 groups [5]. Furthermore, several spectral changes are observed above 3000 cm ~ The intensity of the bands at 3729 cm 1 and 3616 cm -1 decreases with ammonia treatment. These bands are typically assigned to Si-OH groups and Si-OH-A1 groups [5], respectively. Simultaneously, five new bands appear at 3441, 3380, 3289, 3135 and 3035 cm 1 (cf. Figure 3). It is well known that NH3 reacts at high temperatures with dispersed SiO2 forming Si-NH2 and Si-NH-Si groups [6,7]. The Si-NH2 group is characterized by IR vibrations at 1 5 5 0 - 1550 cm ~ (SNH2), 3 4 4 0 - 3455 cm -1 (vsNH2) and 3520-3540 cm 1 (vasNH2) [8,9]. An additional band at 3390 cm -1 has been assigned to Si-NHSi groups [6]. Fink and Datka have assigned the band at 3289 cm 1 to the vsNH2 vibration of Si-NHz-A1 groups [5]. Physically adsorbed ammonia produces the band at 3378 cm 1 and the band at 3035 cm ~ is indicative for the formation of ammonium ions [10]. The bands observed in DAYN and their assignments are summarized in Table 1.

551

'

'

'

I

'

'

'

'

I

'

'

'

'

I

'

'

'

'

I

'

'

'

'

I

'

'

'

'

i

I

r--

I... v

NaYN

....... ,

,

,

I

,

,

,

,

10

I

,

,

,

,

20

I

,

,

,

_NaxN

,

30

I

I

I

............ I

I

I

40

I

,

50

I

60

Angle 2 0 / ~ Fig. 1" X-ray powder patterns of the ammonia-treated zeolites NaXN, NaYN and DAYN. "

'

'

I

"

"

'

'

I

'

"

'

'

I

,

,

,

,

I

"

'

'

'

I

'

'

O

..,,.

l-

xi

"

'

I

'

'

"

"

O

. . . .mlz ,

L_

V

0

m/z-18(H20

)

c-

20

40

60

80

100

120

140

Storage Time / h Fig. 2: Intensities of the signals for m/z = 16 and 18 in the TG/MS-experiments for zeolite NaYN, which was stored for different times in a desiccator over a saturated aqueous solution of ammonium chloride.

552 Table 1. Assignment of IR bands of nitridated DAYN zeolites. Species

Si-NH-Si

Assignment 5NH4 + v sNH4+ vasNH4+ 8NH2 vsNH2 va~NH2 8NH2 vsNH2 vasNH2 5NH3 vsNH3 v~sNH3 vNH

Si-OH Si-OH-A1

vOH vOH

NH4+

Si-NH2

Si-NHa-A1

NH3 adsorbed

....

e-

I . . . . . . . . I

I I

I I

,,

I|

|

.

9

|

1

5 4

3 2

|

9

|

9

|

a

|

*

,

'

Band in Figure 3

1553 3441 n.obs. 1536 3289 3340 1622 n.obs. 3378 3390-3400 (shoulder) 3729 3616

I .... I

,

0o

Wave number / cm "l 1450 3035

S

DAYN

I

,d r O o

I I I

i

I I

I -

I I I I

er I

3500

i

i

I

i

I I I

I

3400

I

I

I

- - + - . . . . . ~ I I I I I I I I i

3300

,

,

i

I

,

3200

i

,

,

/

iDA Y

______J-

I I I I . . . .

3100

I

. . . .

3000

I

------

I I I I I I

. . . .

2900

2800

W a v e n u m b e r / cm -~ Fig. 3" DRIFT spectra of untreated DAY and of ammonia treated DAYN. The asterisk (*) marks bands produced by the adsorption of NH3 on the ZnSe windows of the DRIFT cell.

553

After adsorption of pyrrole, an intense band at 3467 cm l with a shoulder at 3403 c m "1 is observed in the IR spectrum. These bands are assigned to the NH stretching mode of pyrrole based on a comparison with the liquid phase and the gas phase spectrum of pyrrole [ 11].

3.2 Knoevenagel condensation 3.2.1. Influence of the nsi/nAl-ratio In Figure 4, the conversions of propanedinitrile in the Knoevenagel condensation of propanedinitrile with benzaldehyde over freshly nitridated NaXN, NaYN and DAYN catalysts are shown in dependence of reaction time. The activity of the fresh materials strongly depends on the nsi/nArratio of the catalyst. With decreasing aluminum content, an increase in catalytic activity for the base-catalyzed Knoevenagel condensation is found. After two hours of reaction, the conversion of propanedinitrile over DAYN amounts to 85 %, while over NaYN and NaXN conversions of 60 % and 25 %, respectively, are observed.

100 0--9. t.. -O ~) (.{3. LO

80 60

9

X

tO t/l t_

> to o

20it"

/

9 NaXN (nsi/nAt

0

5

10

=

1.3)

NaYN (nsi / nA, = 2.5) /

o

DAYN (nsi/nAi = 30)

v

0

:

15

20

25

30

Reaction Time / h Fig. 4" Conversion of propanedinitrile in the Knoevenagel condensation over nitridated materials with different nsi/nAl ratios (reaction conditions see experimental section). 3.2.2. Influence of the storage time The results of the storage tests with nitridated zeolite NaY are shown in Figure 5. As a measure for the catalytic activity, the conversion of propanedinitrile after a reaction time of 2 h (Xzh) was used. For the fresh materials (storage time = 0 h), the catalytic activity depends on the nsi/nAl-ratio of the catalyst (cf. Figure 4). The activity of DAYN decreases very slowly with increasing storage time (Figure 5). NaYN shows a decrease to 50 % of its origin activity after ca. 50 h of storage over a saturated ammonium chloride solution. NaXN has a very small initial activity which declines only slightly with storage time.

554 100,,.

~~

.~.. "

,

. . . .

,

. . . .

,

. . . . v

_

80

-

._ C

., o

x

60

13_

I

40

.o

v

>

NaXN NaYN DAYN

9

20

0

-w-

j

0

T0

~

50

100

150

2( )0

Storage Time / h Fig. 5" Conversion of propanedinitrile after 2 h of reaction (X2h) over NaXN, NaYN and DAYN after different storage times in a desiccator over a saturated ammonium chloride solution. 70

o~ K)

60

Y

m .n t_

.-

"O

50

T

r

9 9

t~

~s 40

&.

X t-

Nitrogen Desiccator Oven

30

0 r

~ >

20

o 0

10

t-

O

A W

0

50

100

150

200

250

300

350

Storage Time / h Fig. 6" Conversion of propanedinitrile after 2 h of reaction time for different storage times of NaYN. The storage conditions are explained in the text above.

555 3.2.3. Influence of the storage conditions To evaluate the influence of water on the catalytic activity of nitridated materials, aliquots of nitridated zeolite NaYN were stored under different conditions, viz. under dry nitrogen, in an oven at 100 ~ and in a desiccator over saturated ammonium chloride solution. The initial activity of ammonia treated NaY (NaYN) was maintained by storing the material under a dry nitrogen atmosphere (cf. Figure 6). The catalytic activity is almost constant even for storage times of more than 300 h. In contrast, the activity of the NaYN catalyst stored in a desiccator decreases with storage time. While the fresh catalyst exhibits a propanedinitrile conversion of 52 % after 2 h of reaction, the conversion decreases to ca. 20 % after keeping the catalyst for 100 h in the desiccator. For still longer storage times, the catalytic activity remains almost constant. The samples stored at 100 ~ show an increased deactivation rate, which can be attributed to accelerated hydrolysis at elevated temperatures.

4. DISCUSSION The nitridation of zeolites results in the formation of Si-NH2, Si-NH-Si and Si-NH2-A1 groups, which is evident from the IR bands at 3441, 3390 and 3289 cm l (cf. Figure 3), respectively. The TG-MS experiments reveal a weight loss with a maximum around 150 ~ which is due to the simultaneous release of water and ammonia. It is, therefore, evident that the Si-NH2, Si-NH-Si and Si-NHz-A1 groups formed by nitridation are prone to hydrolysis with concomitant release of ammonia. The ammonia formed is presumably initially adsorbed on the zeolite but later replaced via adsorption of water. Therefore, after a certain storage time mainly water is desorbed from the zeolite (cf. Figure 2). The extend of hydrolysis depends on the hydrophobicity of the catalyst, which increases with increasing nsi/nAl-ratio. The nitridation of mesoporous materials such as MCM-41 and MCM-48 with different nsi/nA1ratios indeed shows the same trend [12], which supports the validity of the above made assumption. The results of the catalytic experiments also show that the activity of the nitridated zeolites is strongly affected by hydrolysis. The catalytic activity of the fresh catalyst depends on the nsi/nAl-ratio of the parent material. The initial activity (as expressed by propanedinitril conversion at a reaction temperature of 80 ~ and a reaction time of 2 h) decreases with increasing aluminum content. These differences in (initial) activity are tentatively explained by a higher number of basic sites in DaYN as a consequence of a higher concentration of silanol groups prior to the nitridation treatment. It is furthermore interesting to note that full conversion (even after 24 h of reaction) is not reached over NaYN and NaXN. The maximum conversion is 90 % and 55 %, respectively. The reason for the catalyst deactivation is still unclear, however, one possible explanation is that the pores of the less active catalysts (which are more hydrophilic) are blocked by strong adsorption of the bulky and polar condensation product and/or polar reactants. The loss of activity of the nitridated catalyst stored over a saturated NHaC1 solution is small for DAYN and marked for NaYN and NaYN, which is in line with the increasing hydrophobicity from NaX to DAY. It is, therefore, inevitable to only compare catalysts which have been stored under controlled conditions. The catalytic activity, which decreases rapidly when the catalyst is stored in a controlled humid environment (cf. Figure 6), is maintained if the catalyst is kept under dry nitrogen. Keeping the catalyst in an oven at elevated temperature (100 ~ increases the hydrolysis rate and/or the desorption rate of ammonia and results in

556 faster deactivation. From our results, it follows that hydrolysis is the major reason for deactivation of the nitridated catalysts. CONCLUSIONS In the present contribution, zeolites NaX, NaY and DAY were treated with ammonia at elevated temperatures to create basic Si-NH2, Si-NH-Si and Si-NHz-A1 groups inside their structures. The existence of these groups was proven by DRIFT spectroscopy. The novel materials exhibit catalytic activity in the base-catalyzed Knoevenagel condensation of benzaldehyde with propanedinitrile. A decrease in catalytic activity is observed upon extended exposure to moisture due to hydrolysis of the nitrogen containing groups and, hence, release of ammonia. The rate of deactivation decreases with increasing hydrophobicity in the following order: NaXN > NaYN > DAYN ( n s i / n A l = 30). The results so far indicate that the uptake of water and subsequent hydrolysis are responsible for the deactivation of the catalysts. The storage of the freshly nitridated zeolites in a dry nitrogen atmosphere preserves their catalytic activity, while storage of the fresh catalysts in air at elevated temperature accelerates the hydrolysis reaction. ACKNOWLEDGEMENTS

Financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie und Max-Buchner-Forschungsstiftung is gratefully acknowledged. REFERENCES

1. S. Ernst, M. Hartmann, S. Sauerbeck and T. Bongers, Appl. Catal. A 200 (2000), 117. 2. S. Ernst, M. Hartmann and S. Sauerbeck, in: "Zeolites and Mesoporous Materials at the Dawn of the 21 st Century", A. Galarneau, F. Di Renzo, F. Fajula and J. Vddrine (Eds.), Studies in Surface Science and Catalysis, Vol. 135, Elsevier, Amsterdam (2001), 175. 3. P. Grange, P. Bastians, R. Conanec, R. Marchand and Y. Laurent, Appl. Catal. A 114 (1994), L91. 4. M.J. Climent, A. Corma, V. Forn6s, A. Frau, R. Guil-L6pez, S. Iborra and J. Primo, J. Catal 163 (1996), 392. 5. P. Fink and J. Datka, J. Chem. Soc., Faraday Trans. 1 85 (1989) 3079. 6. B.A. Morrow and I.A. Cody, J. Phys. Chem. 80 (1976) 1998. 7. P. Fink and I. Plotzki, Wiss. Z. Friedrich-Schiller-Universit~it Jena, Math.-Nat. R. 37 (1988), 691. 8. P. Fink, I. Plotzki, G. Rudakoff, Wiss. Z. Friedrich-Schiller-Universit~it Jena, Math.-Nat. R. 37 (1988), 911. 9. P. Fink, I. Plotzki, Wiss. Z. Friedrich-Schiller-Universitgt Jena, Math.-Nat. R. 25 (1976), 853. 10. G.T. Kerr and G. F. Shipman, J. Phys. Chem. 72 (1968) 3071. 11. H. F6rster, H. Fuess, E. Geidel, B. Hunger, H. Jobic, C. Kirschhock, O. Klepel and K. Krause, Phys. Chem. Chem. Phys. 1 (1999) 593. 12. S. Sauerbeck, M. Hartmann and S. Ernst, unpublished results.


Selective catalytic reduction of N20 with decomposition over Fe-BEA zeolite catalysts

557

light

alkanes

and

N20

T. Nobukawa a, K. Kita a, S. Tanaka a, S. Ito a, T. Miyadera b, S. Kameoka a , K. Tomishige a and K. Kunimori a alnstitute of Materials Science, University of Tsukuba, Tsukuba, Ibaraki 305-8573, Japan bResearch Institute of Energy Utilization, National Institute of Advanced Industrial Science and Technology, Tsukuba, Ibaraki 305-8569, Japan Fe ion-exchanged zeolite catalysts (Fe-BEA, Fe-MFI) were found to be active for selective catalytic reduction (SCR) of N20 with light alkanes (i.e., CH4 and C2H6) even in the presence of excess oxygen. In the NEO/CEH6/O2 system over Fe-BEA catalyst, N20 plays an important role in the oxidation of C2H6 (i.e., activation of C2H6 at an initial step). HE-TPR and catalytic measurements of Fe-BEA catalysts with different Fe contents revealed that the active sites for SCR of N20 with CH4 and N20 decomposition are Fe ion species, and Fe oxide aggregates are inactive. In O2-TPD studies, we observed new desorption peaks from Fe-BEA catalyst after N20 treatment. In the isotopic tracer study of N20 decomposition, the result shows that 02 formation on Fe-MFI catalyst proceeds via Eley-Rideal mechanism, which is different from the case of supported Rh catalysts. 1.

INTRODUCTION

Nitrous oxide (N20), which also contributes to catalytic stratospheric ozone destruction, is a strong greenhouse-effect gas with a global warming potential (GWP) per molecule of about 300 times that of carbon dioxide (CO2) [1]. From the point of view of environment, therefore, it is important to study removal of N20 in the emission gases. Catalytic decomposition of N20 [1] and SCR of N20 with reductants such as hydrocarbons [2-8] and ammonia [9] have been proposed as the effective methods of N20 abatement. Fe ion-exchanged zeolites have been investigated as interesting catalysts, which are active for SCR of N20 with C3H6 or C3H8 in the presence of excess O2, H20 and SO2 [3, 5]. Recently, we reported the simultaneous removal of N20 and CH4 as the strong greenhouse-effect gases over Fe-BEA by SCR of N20 with CH4 [6, 8]. The Fe-BEA catalyst was more active than the other Fe-zeolite catalysts (Fe-MFI etc.) [6]. In this work, the activities of SCR of N20 with CH4 and N20 decomposition have been investigated over Fe-BEA catalysts with different Fe contents, and the natures of active sites have been studied by means of HE-TPR and O2-TPD

558 techniques. Enhancement of C2H6 oxidation by the presence of N20 in SCR of N20 with C2H6 has been studied by a transient reaction technique etc.. Isotopic tracer studies using 1802 were also carried out over Fe-MFI (or Fe-BEA) to elucidate the mechanism of 02 formation for N20 decomposition. 2.

EXPERIMENTAL

Fe-zeolite catalysts have been prepared by two different ion-exchange methods: (i) wet ion-exchange (IE) with a dilute solution of FeSO4 at 50 ~ for 20 h under nitrogen atmosphere, followed by calcination in air at 500~ for 12 h [3, 6] and (ii) solid-state ion-exchange (SSI) with FeC12 and zeolite, which were mixed intensively in a ball mill under ambient conditions for 1 h, followed by calcination in air at 400 ~ for 5 h [10, 11]. The zeolite supports (H-BEA, SIO2/A1203=27.3 and Na-MFI, SIO2/A1203=23.8) were supplied by TOSOH Co. The loading weight of Fe was changed from 0 to 3.1wt% by SSI method in order to elucidate the effect of Fe contents on the catalytic activities. An Fe/MFI (IMP) catalyst (2.9wt% Fe) was also prepared by impregnation method using Fe(NO3)3 in order to compare catalytic activities and O2-TPD spectra. The reaction was carried out in a standard fixed-bed flow reactor, capable of rapid switching (ON-OFF) of the gases, using a mixture of N20 (0- 1300 ppm), HCs (CH4, C2H6:0 - 1000 ppm), 02 (0 - 10%) in He flow (SV=60,000 hl). In the TPR experiment, the sample was heated in 5% H2/Ar flow (30 ml/min) at a constant heating rate of 10 ~ and H2 consumption was monitored by TCD. The O2-TPD experiment was carried out in a microcatalytic pulse reactor [ 12]. The effluent was analyzed in an on-line gas chromatograph system equipped with a TCD and differentially pumped quadrupole mass spectrometer. The temperature was increased from room temperature to 800 ~ at a constant heating rate of 10 ~ and kept at 800 ~ for 30 min. In the isotopic study, N2160 was pulsed onto 1802-treated Fe-zeolite catalyst, and desorbed 02 molecules were monitored by means of mass spectrometry [ 12]. 3.

RESULTS AND DISCUSSION

3.1. The H2-TPR and O2-TPD studies and the activities of SCR with CH4 and decomposition of N20 Figure 1 shows TPR profiles of Fe-BEA catalysts (ion-exchanged by SSI or IE) with different Fe contents after the calcination treatment. A peak between 70 ~ and 140 ~ is an artifact, which is due to desorption of residual Ar [13]. As shown in Fig. 1 (A), the sample 1 exhibits three main reduction peaks at 470 ~ 530 ~ and 580 ~ together with the tail peaks at the lower temperatures. A peak at 530 ~ is due to the reduction of remaining C1 in the catalyst, because this peak was removed after washing by deionized water (not shown). Other two peaks were due to the reduction of Fe oxides (i.e., Fe203---~Fe0) [14, 15]. The samples 2 and 3 show one main reduction peak at 400 ~ The amount of the H2 consumption expressed

559 as the ratio H2/Fe for this peak was 0.37 and 0.43, respectively • (see also Fig. 1 (B)). In e 5 agreement with previous works 2 on TPR of Fe-BEA [14] and ._g Fe-MFI [15], this peak could correspond to the reduction of Fe 3§ to Fe2§ The lower ratio of 200 400 600 H2 consumption to Fe suggests 8 that a part of the Fe species was "1still reduced or was unreducible I in these conditions. The sample 3 200 400 600 also has a small reduction peak Temperature / ~ at 520 ~ which correspond to Fig. 1. H2-TPR of Fe-BEA catalysts:(1) 3.1wt% SSI, the reduction of Fe oxide [15]. (2) 1.5wt% SSI, (3) 0.77wt% SSI, (4) 0.30wt% SSI, The sample 4 shows very small (5) 1.6wt% IE. broad peak because of very small amount of Fe loading, but the peak may be attributed to the reduction of Fe 3+ to Fe 2+. As for the sample 1, the tail peaks at around 400 ~ may also be attributed to the reduction of Fe 3+ to Fe 2+, although the amount of the Fe ion species appears to be smaller than that of the sample 2. As shown in Fig. 1 (B), the sample 5, which was prepared by the wet ion-exchange (IE) method, exhibits almost the same TPR profile as that of the sample 2, although a small peak (530~ due to the reduction of Fe oxides was observed. These results indicate that Fe 3§ ion species are prepared by either SSI or IE method followed by the calcination treatment and that Fe oxide aggregates are formed with higher Fe loading. Figure 2 shows the results of SCR with CH4 and N20 decomposition on Fe-BEA catalysts with different Fe contents. The activities were increased with increasing the amount of Fe until 1.5wt%, beyond which the activity levelled off in both reaction systems. The results show that the active sites of these reactions are Fe ion species and that Fe oxide aggregates are inactive. The results of these catalytic activities are consistent with the results in Fig. 1. The activities of 3.1wt% Fe-BEA were a little lower than these of 1.5wt% Fe-BEA, which suggests that the amount of active Fe ions may be smaller and/or the Fe oxide aggregates may block the inlet pores of the zeolite. Figure 3 shows the activities of N20 decomposition over Fe catalysts, which were prepared by wet ion-exchange (IE) and impregnation (IMP) methods. We have already reported that Fe-BEA catalyst has higher activity in SCR of N20 with CH4 than that of Fe-MFI catalyst [6]. This tendency was also found for N20 decomposition. It is clear that Fe/MFI (IMP) has almost no activity even at 500 ~ which means that the active species for N20 decomposition are Fe ions and that Fe oxide aggregates are inactive, in agreement with the results in Fig. 2.

560 ~100 Z o

r

O

z +0

o~ 100 Z

80

O

40

9

20

y

O

..+o------_._. o

80

Z

60

N,...

O

,-- 40 O 9 tO

cO

q

(a)

,4.-J

0"

60

~

r

1

2

level / w t %

Loading

~

3

20 0

(c) 300

400 Temperature

Fig. 2. The activity of SCR with CH4 (Q" N20 950 ppm, CH4 500 ppm, 02 10% at 350 ~ and decomposition of N20 (O: N20 950 ppm at 480 ~ over of Fe-BEA (SSI) with the different ion-exchange levels.

500 / ~

Fig. 3. The activities of N20 decomposition (N20 950 ppm) over Fe catalysts; (a) 1.6wt% Fe-BEA (IE), (b) 2.9wt% Fe-MFI (IE), (c) 2.9wt% Fe/MFI (IMP).

Figure 4 shows O2-TPD spectra from the Fe catalysts after 02 or N20 pretreatment. As shown in Fig. 4 (a), Fe/MFI (IMP) does not desorb any significant amount of oxygen. For Fe-BEA (IE) after the 02 treatment, 02 starts to desorb above 520 ~ with a maximum occurring around 700 ~ (Fig. 4 (b)). After the N20 treatment, new peaks appeared at lower temperatures (300 ~ - 600 ~ The total amount of desorption was increased by the N20 treatment (see Table 1). There have been some reports that active oxygen species are formed by exposing Fe ion-exchanged zeolite catalysts to N20 [14, 16, 17], although detailed points are controversial in the 5 present stage. O

o a

'-+:9 .......... .+-.~ .......... ..,--....... I

200

,

I

400

*

I

600

Temperature

,

--I

............ (.a) -

800

"

~ . . . .

]

"*

........

[ . . . . . .

m]

800

/ ~

Fig. 4. O2-TPD profiles from Fe catalysts after 02 or N20 pretreatment: (a) 2.9wt% Fe/MFI

(IMP), N20 at 250 ~ for lh; (b) 1.6wt% Fe-BEA (IE), O2 at 500 ~ for lh; (c) 1.6wt% Fe-BEA (IE), N20 at 500 ~ for lh.

Table 1 Amount of desorbed oxygen from the O2-TPD experiments on the pretreated catalyst surface (see Fig. 4). Spectrum pretreatment 02 (lJmol)

O/Fe

(a)

N20

0.38

0.03

(b)

02

1.64

0.47

(C)

N20

2.49

0.72

561 Panov et al. [17] reported that O atom from N20 molecule (so called c~-oxygen), which cannot be produced by 02, readily reacted with benzene to produce phenol over Fe-MFI catalyst even at room temperature. However, such high reactivity with benzene was not observed on an N20-treated Fe-MFI catalyst at room temperature [16]. In addition, it should be noted that no desorption of NO was observed during O2-TPD experiments in our Fe-BEA catalyst system [ 14]. SCR of N20 with C2H6 Enhancement of C2H6 + 02 reaction by the presence of N20 Figure 5 shows N20 and C2H6 conversions in SCR of N20 with C2H6 over Fe-BEA catalyst in the presence of excess O2. For a comparison, the activities in SCR of N20 with CH4 are also shown in Fig. 5, which are taken from the Ref. [8]. The N20 conversion was increased significantly by adding reductant (CH4 and C2H6) even in the presence of excess 02 (i.e., SCR), and there was a plateau in CH4 conversion (at 300 ~ -~ 450 ~ after N20 conversion reached to 100% [8]. This is due to the fact that N20 in the mixture gas was completely consumed by the reaction with CH4, and no reaction of CH4 with 02 took place at these temperatures. On the other hand, the oxidation behavior of C2H6 in SCR of N20 was significantly different from that of CH4. No plateau in the C2H6 conversion was observed in the N20/OE/CEH6 system, but the C2H6 conversion reached to 100% at ca. 350 ~ Therefore, it was found that the oxidation of Call6 by coexistent 02 concomitantly occurred. As shown in Fig. 5, however, the oxidation of C2H6 by 02 required higher temperatures (> 350 ~ This result indicates that the oxidation of C2H6 by 02 in SCR of NaO is significantly enhanced by 3.2.

-

-

100

A

loo

V

! !

80

;

[ l i//

Z

o

O Z O

60

t~~

to 40 >

ot-- 20 O

,b q

-~0

/

O N20/O2/CH4 e N20/O2/C2H6 X N20/O 2

[-] CH,/O 2

02H6/O2

80

"~

t i l

._o ~ 40 > e0

'

0

I

esS

~r 60

o..-o ....... -6" sS s

/

r

20

:

;

/1

I

'

" 400

Temperature / ~

'

0

400

Temperature / ~

Fig. 5. Conversions of N20 and HC in various reaction systems over 1.6wt% Fe-BEA catalyst (IE).; N20 (950ppm), 02 (10%) and HC (CH4: 500ppm, C2H6: 300ppm).

562 C2Hs/O=iN20/C2Hs/O~ N=O/C2H6 the presence of N20, and that N20 plays ~--i-~ ~ 100 important roles at an initial step in the 100 ~, ~;~ oxidation of C2H6by 02 (i.e., abstraction of H o~ 80 80 atom etc.). -r"tD In order to examine in more detail the ~ 60 60 :~ oxidation behavior of C2H6 in SCR of N20, O O twe performed the transient reaction .9 40 40 ~(!) experiments in the N20/O2/C2H6 system. The O9 result is shown in Fig. 6. The C2H6 conversion => 20 0 0 was drastically increased by adding N20 to C2H6/O2 flow at 350 ~ while oxidation of 4~ 80 120 160 208 C2H6 by 02 did not occur at this temperature. T i m e / min After changing to N20/C2H6 flow on removal Fig. 6. Transient responses for C2H6 of 02 from the N20/O2/C2H6 flow at 350 ~ oxidation on addition of N20 to C2H6/O2 the C2H6 conversion decreased to ca. 18%. In flow and on removal of O2 from this case, the C2H6 conversion was small NEO/CEH6/O2 flow at 350 ~ over 1.6wt% simply due to the depletion of the oxidant (i.e., Fe-BEA catalyst (IE).; N20 (1300ppm), N20). These results directly demonstrate that C2H6:(1000ppm) and O2 (10%). the oxidation of C2H6 by 02 is significantly enhanced by the presence of N20. It should be noted that CO formation was observed only at the N20/O2/C2H6 flow. Therefore the result apparently shows that the formation of CO2 occurred in the reaction of C2H6 with N20 (in the N20/C2H6 flow) and that the formation of CO occurred in the reaction of C2H6 with 02 (in the N20/O2/C2H6 flow). The in situ DRIFT measurements [18] showed that absorption peaks of CxHy(a) and/or CxHyOz(a) species were observed in exposing the catalyst to the N20/C2H6 mixture. Therefore, we propose that N20 plays important roles in the formation of CxHy(a) and/or CxHyOz(a) species in the initial state of C2H6 oxidation, and these species can react with 02 to produce COx and H20. I -..,.

I i

3.3.

i

The isotopic study of N20 decomposition over Fe-MFI c a t a l y s t - Mechanism of 02 formation -

The reduced Fe-MFI catalyst (2.9wt% Fe, IE) was oxidized with 1802 (purity 96.5%) gas at 500 ~ and pulsed N20 decomposition was carried out at 420 ~ The result is shown in Table 2. An isotopic equilibrium constant, Ke = [180 160]2 / [1802] [1602], should be considered to judge incidental exchange reactions that would disguise the experimental results [12]. If the exchange reaction equilibrates, Ke should be close to 4 [19]. As shown in Table 2, Ke is 3.95, which suggests that the isotopic exchange of oxygen in CO2 equilibrates (Table 2, Expt. 1). Therefore, the isotopic fraction of 180 (1~ in the product CO2 should be equal to that of the surface oxygen, i.e., 0.23. After the pulsed CO2 experiment, an N2160 pulse was injected onto the catalyst at 420 ~ (Table 2, Expt. 2), and the 18f of the product 02 was 0.13, which is

563

almost the half value of that of the surface oxygen. In addition, the Ke value of oxygen produced from N20 decomposition was infinity (Table 2, Expt. 2), because 1802 was not detected. It should be noted that the exchange of oxygen in N20 with surface oxygen (eqn. (1)) could be neglected because of very low lSfvalue in the outlet N20 (Table 2, Expt. 2). As a separate experiment, we confirmed that the exchange of oxygen between the gas phase and the surface oxygen is almost negligible (Table 2, Expt. 3, 4). Therefore, we suggest that O2 formation proceeds via Eley-Rideal (ER) mechanism (eqn. (2)). N2160 + N2160 +

180(a) ~ 180(a) ~

N2180 + 160(a) N2 + 160180

(1) (2)

The ER mechanism has also been suggested for Fe-BEA, on which the l ~ o f the desorbed 02 from N20 decomposition had the half value of that of the surface oxygen. However, an isotopic mixing might have occurred, judging from the equilibrium constant (Ke = 3.4) probably because of higher activity of Fe-BEA. In addition, an isotopic study of SCR of N20 with CH4 was not possible, because produced CO2 made complete isotopic mixing (Ke - 4.0). The present result is in contrast with the mechanism of N20 decomposition over supported Rh catalysts [12], where Langmuir-Hinshelwood (LH) type desorption (20(a) 02) has been proposed. For some systems such as Fe ion-exchanged zeolite catalysts, where active sites are isolated, it may be reasonable that the ER mechanism prevails [20]. The active sites of N20 decomposition over Fe-MFI catalyst may be Fe ion species such as binuclear oxo species [13, 16, 21], which may also be supported by the fact that the O/Fe ratio from the O2-TPD experiment is 0.47 (Table 1, (b)). On the other hand, Delahay et al. [22] proposed that mononuclear Fe-oxo species are the most active sites for SCR of N20 with NH3. The isotopic study in this work clearly showed that the ER mechanism prevailed when N2160 was pulsed onto the 1802-treated Fe-zeolite catalyst. However, more detailed studies are needed to elucidate the mechanism of 02 formation during steady-state N20 decomposition reaction. The isotopic fraction of 180 (l~f) and the isotopic equilibrium constant (Ke) in the Table 2 product molecules from 1802, C1602 and N2160 pulses over Fe-MFI catalyst at 420 ~ ii

Product

18f91?~,

Kr

180

CO2

0.23

3.95

N2160

180

02

0.13

cr

2

N2160

180

N20

3

1802

160

02

0.95

4

1802

m

02

0.96 b)

Experiment No.

Pulse

1

C1602

2

Surface species

'a) The isotopic abundance of 180 is 0.002. b) The l~r the catalyst.

0.00 a)

0.18

the incident pulse measured without

564 4.

CONCLUSIONS

Studies of SCR of N20 with CH4 and N20 decomposition over Fe-BEA with different Fe contents prepared by means of SSI revealed that the most effective loading level was 1.5wt%. The active sites of N20 decomposition may be binuclear Fe-oxo species, which has also been suggested in SCR of NO with hydrocarbons [13, 16, 21]. In the N20/C2H6/O2 system over Fe-BEA catalyst, N20 plays an important role in the oxidation of C2H6 (i.e., activation of C2H6 at an initial step). In the isotopic study of N20 decomposition, 02 formation on the Fe-MFI catalyst proceeds via Eley-Rideal mechanism. REFERENCES

1. 2. 3. 4. 5. 6.

F. Kapteijn, J. Rodriguez-Mirasol and J. A. Moulijn, Appl. Catal. B, 9 (1996) 25. Y. Li and J.N. Armor, Appl. Catal. B, 3 (1993) 55. C. Pophal, T. Yogo, K. Yamada and K. Segawa, Appl. Catal. B, 16 (1998) 177. M. K6gel, R. M6nnig, W. Schwieger, A. Tissler and T. Turek, J. Catal., 182 (1999) 470. G. Centi and F. Vazzana, Catal. Today, 53 (1999) 683. S. Kameoka, T. Suzuki, K. Yuzaki, T. Takeda, S. Tanaka, S. Ito, T. Miyadera and K. Kunimori, Chem. Commun., (2000) 745. 7. S. Kameoka, K. Yuzaki, T. Takeda, S. Tanaka, S. Ito, T. Miyadera and K. Kunimori, Phys. Chem. Chem. Phys., 3 (2001) 256. 8. S. Kameoka, K. Kita, T. Takeda, S. Tanaka, S. Ito, K. Yuzaki, T. Miyadera and K. Kunimori, Catal. Lett., 69 (2000) 169. 9. M. Mauvezin, G. Delahay, F. Kil]lich, B. Coq and S. Kieger, Catal. Lett., 62 (1999) 41. 10. M. Rauscher, K. Kesore, R. M6nnig, W. Schwieger, A. Til31er and T. Turek, Appl. Catal. A: General 184 (1999) 249. 11. R. Giles, N. W. Cant, M. K6gel, T. Turek and D. L. Trimm, Appl. Catal. B: Environmental 25 (2000) 75. 12. S. Tanaka, K. Yuzaki, S. Ito, S. Kameoka and K. Kunimori, J. Catal., 200 (2001) 203. 13. H.-Y. Chen and W. M. H. Sachtler, Catal. Today, 42 (1998) 73. 14. B. Coq, M. Mauvezin, G. Delahay and S. Kieger, J. Catal., 195 (2000) 298. 15. R. Q. Long and R. T. Yang, J. Catal., 194 (2000) 80. 16. E1-M. E1-Malki, R. A. van Santen and W. M. H. Sachtler, J. Catal., 196 (2000) 212. 17. G.I. Panov, A.K. Uriarte, M.A. Rodkin and V.I. Sobolev, Catal. Today, 41 (1998) 365. 18. S. Kameoka, K. Kita, S. Tanaka, T. Nobukawa, S, Ito, K. Tomishige, T. Miyadera and K. Kunimori, Catal. Lett. in press. 19. Ozaki, A., "Isotopic Studies of Heterogeneous Catalysis." Kodansha, Tokyo, 1977. 20. A. U Yakovlev, G. M. Zhidomirov and R. A. van Santen, Catal. Lett., 75 (2001) 45. 21. P. Marturano, L. Drozdovfi, A. Kogelbauer and R. Prins, J. Catal., 192 (2000) 236. 22. G. Delahay, M. Mauvezin, B. Coq and S. Kieger, J. Catal., 202 (2001) 156.


565

Hydroxymethylation of 2-methoxyphenol catalyzed by H-mordenite: analysis of the reaction scheme F. Cavani, L. Dal Pozzo, L. Maselli, R. Mezzogori Dipartimento di Chimica Industriale e dei Materiali, Viale Risorgimento 4, 40136 Bologna, Italy. [email protected]. Tel & fax +39-0512093680 The reaction scheme for the hydroxyalkylation of 2-methoxyphenol (guaiacol) with aqueous solutions of formaldehyde (formalin) aimed at the synthesis of 3-methoxy-4hydroxybenzyl alcohol (p-vanillol), catalyzed by an H-mordenite zeolite was analyzed. Specific attention was focused on the nature of the by-products obtained in the presence and in the absence of methanol -a component present in formalin. In the presence of methanol the main products were the vanillol isomers, and the prevailing by-product was 3-methoxy-4hydroxybenzylmethylether, obtained by reaction between p-vanillol and methanol. In the absence of methanol the prevailing by-products were diarylmethanes. Even though both byproducts can be considered secondary ones from a chemical point of view, i.e., formed starting from vanillols, they were found to form even at low guaiacol conversion. Therefore vanillols, once formed in the zeolitic pores, generate therein the corresponding benzyl carbocations and undergo nucleophilic attack by either methanol, guaiacol or another vanillol. The reactivity of vanillic alcohol isomers was also directly checked, both in the presence and in the absence of formaldehyde and of methanol. The results show the different tendencies of the isomers to give consecutive transformations. 1. INTRODUCTION The hydroxyalkylation of activated arenes (containing functional groups such as the hydroxy or methoxy groups) with aldehydes and ketones is a reaction of interest for the production of drugs, polymers, and food additives [1-3]. For instance, the hydroxymethylation of 2-methoxyphenol (guaiacol) represents one-step in the multistep synthesis of 3-methoxy-4-hydroxybenzaldehyde (vanillin, VA), an environmentally friendly process for the production of this important food additive [2]. OH ~/OCH3

OH H~C~H

H2C~ OH

Hydroxyalkylations are catalyzed by Lewis-type acids, like A1C13, and mineral Br6nsted acids. Some papers and patents have appeared in recent years, where zeolitic materials are

566 described as catalysts for this reaction [1-6]. Solid acid materials are highly desirable catalysts, since the environmental impact of the process benefits from easier separation of the catalyst, the absence of liquid wastes containing inorganic salts, and less severe corrosion problems [ 1]. Usually, the condensation between arenes and aldehydes is carried out in the liquid phase, and large-pore zeolites are necessary in order to make the reaction occur at an acceptable rate in the condensed phase. When formaldehyde is the reactant, water is the solvent, since the aqueous solution of formaldehyde (formalin) is the simplest, cheapest and most available reactant from the commercial point of view. This implies the need for hydrophobic zeolites, in order to avoid preferential filling of the pores by more polar water molecules rather than by the aromatic substrate [7]. In previous studies, we have analyzed the main reaction parameters affecting catalytic performance with zeolitic catalysts, and the effect of the methanol concentration in the formalin solution [8,9]. The objective of the present study was to analyze the nature of the by-products which are formed under different reaction conditions, with specific attention being given to the consecutive products of transformations occurring on VA isomers. 2. EXPERIMENTAL Catalytic tests were carried out in a glass, batch reactor, loading 48 ml of an aqueous solution of formaldehyde, and 1 g of a commercial zeolite (H-mordenite HM-45 supplied by Engelhard) characterized by a Si-to-A1 atomic ratio equal to 23. The mixture was then heated to 80~ and 4 ml of guaiacol were added under stirring. The reaction mixture was left at 80~ under vigorous stirring (600 rpm) for varying reaction times up to 12h. For the tests on VAs, 0.4 g of each compound was loaded in the reactor, while all the other amounts and conditions remained the same as for the tests of guaiacol hydroxymethylation. The commercial aqueous solution of formaldehyde typically contains 37 wt.% formaldehyde and 10-15% methanol (the latter inhibits the formation of higher molecular weight polyoxymethylenes, which would precipitate and separate from the aqueous solution). Some tests were carried out using non-commercial aqueous solutions of formaldehyde, containing 29-30 wt.% formaldehyde and 1.5 wt.% methanol. The products were analyzed by HPLC (TSP Spectra Series), equipped with an Alltech Hypersil ODS column, and with a UV-Vis TSP UV 150 detector (~, 280 nm). Elution was done with a mixture of acetonitrile and water. Identification of products was made by GC-MS and by comparison with the retention time of standard components (when available).


3.1. The distribution of products in guaiacol hydroxymethylation Plotted in Figure 1 is the selectivity to products as a function of guaiacol conversion (the latter having been varied by varying the reaction time), for tests carried out using a commercial formalin solution containing approx. 15 wt. % methanol and 37% formaldehyde (Figure 1 left), and for tests carried out using a non-commercial formalin solution, containing a minimal concentration of methanol (1.5 wt.), and 29 % formaldehyde (Figure 1 right). The conversion of guaiacol had a considerable effect on the distribution of products. The main

567 products were vanillic alcohols (o-VA: 2-hydroxy-3-methoxybenzyl alcohol; m-VA: 3hydroxy-4-methoxybenzyl alcohol; p-VA: 3-methoxy-4-hydroxybenzyl alcohol), diaryl compounds (having mainly MW 260, with a minor amount of compounds having MW 290), and monoaryl compounds other than VAs. Amongst the latter, the predominant compounds were 2-methoxy-3-hydroxybenzylmethylether (MW 168), obtained by etherification between p-VA and methanol, and the compound obtained by etherification of p-VA with the hemiformal (MW 198). The main differences between the two sets of tests concern (i) the overall higher selectivity to VAs obtained in the presence of methanol, and (ii) the nature of the by-products, which were substantially different in the two cases. Summarized in Figure 2 are the details concerning the selectivities to the by-products, for the tests carried out under methanol-rich conditions and methanol-lean conditions, and in correspondence with two different guaiacol conversions. For tests carried out under methanol-rich conditions, and for low guaiacol conversion, the prevailing by-products were monoaryl compounds, while only traces of diaryl compounds (MW 260) were found. At higher conversion, the main by-products remained the monoaryl compounds (MW 168 and 198). In all cases the formation of di-hydroxyalkylated monoaryl by-products was negligible. Blank tests made in the absence of the zeolite demonstrated that a mild acidity is sufficient to protonate VAs, generate the corresponding benzyl cation and let it undergo nucleophilic attack by methanol or hemiformal. In the case of tests carried out under methanol-lean conditions, the main by-products at low guaiacol conversion were diaryl compounds having MW 260. The latter is obtained by reaction between one molecule of guaiacol and one of VA. The same compound can also be formed by reaction between two VA molecules (MW 290) followed by elimination of one formaldehyde molecule. Traces of triaryl by-products were detected. At high guaiacol conversion, diaryl and polyaryl compounds were the predominant by-products. The data indicate that the presence of methanol drastically modifies the distribution of byproducts. Methanol preferentially reacts with VAs, forming monoaryl ether, thus inhibiting the formation of diaryl by-products. This effect is observed even at low guaiacol conversion, and this indicates that the rates of the parallel reactions for the formation of the primary products are affected considerably by the presence of methanol, due to the fact that the latter is a better nucleophile than VAs.

.>

70

70

60

60

50

50

40

40

30

30

20

20

IIi1~_. i

0

20

m l

40

m

_.j

[

60

m

l

guaiacol conversion (%)

10 m i

80

0 100

0

20

40

60

guaiacol conversion (%)

80

100

Figure 1. Selectivity to the products as a function of guaiacol conversion. Left: tests carried out with 15 wt.% methanol in formalin. Right: tests carried out with 1.5 wt.% methanol in formalin. Symbols: selectivity to p-VA (A), o-VA ( . ) , m-VA (11), by-products (O).

568 20 .~ 16

._~ 12 ~

g

~

4

!iii!iiiiil !iiiiiiiiil

~ 30

ii!i!iiiiil

~

i!iiiiiiiil

iiiiiiiiiil 21% conv.

50 I

~i~i~ 49% conv.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

20 __~__

o 18% conv

67% conv

Figure 2. Selectivity to the by-products at different conversion levels. Left: tests carried out with 15 wt.% methanol in formalin. Right: tests carried out with 1.5 wt.% methanol in formalin. Dotted bars: 2-methoxy-3-hydroxybenzylmethylether (MW 168). Open bars: bisarylmethanes (MW 260). Full bars: other monoaryl by-products (mainly MW 198). Figure 1 (left) shows that in the presence of methanol the main consecutive reactions occurred on p-VA, the selectivity of which showed the steepest decrease with increasing guaiacol conversion, especially when the latter was higher than 60%. The selectivity to m-VA and o-VA initially decreased slightly, but then stabilized and finally even increased, possibly because of the non-negligible contribution of isomerization reactions for very long reaction times (up to 12 h reaction time was necessary to reach the highest guaiacol conversion). Also, it is possible that in the presence of extensive catalyst deactivation, the contribution of homogeneous acidity becomes important (the liquid bulk is always acid, due to the presence of formic acid), and finally a product distribution similar to the one achieved with homogeneous catalysis develops [8]. In the latter case, i.e., in the absence of shape-selectivity effects, the selectivity to p-VA was initially slightly higher than that to o-VA, but then for prolonged reaction times (i.e., with possible development of isomerization equilibria) the two isomers formed in comparable amounts, and both were approximately twice the amount of mVA, i.e., with a relative distribution of isomers similar to that observed in Figure 1 (left) for longer reaction times. Moreover, in heterogeneous tests and under conditions of catalyst deactivation (i.e., for long reaction times) the reaction of VA etherification with methanol is probably no longer reversible, and the selectivity to p-VA drops quickly. In the case of tests done in the absence of methanol (which were carried out for much shorter reaction times, since in the absence of methanol the conversion of guaiacol was much higher [8]), the highest contribution to consecutive reactions was again on p-VA, the selectivity of which exhibited a continuous decrease with increasing guaiacol conversion (Figure 1 right). The selectivity to o-VA and m-VA also exhibited a non-negligible decrease. In this case, the main contribution to the VAs disappearance was the irreversible formation of diarylmethanes (Figure 2 right), but also the monoaryl by-products formed in non-negligible amounts, despite the relatively low concentration of methanol. The higher reactivity of p-VA towards consecutive reactions is due to both a steric effect (p-VA and m-VA diffuse more quickly than o-VA into zeolitic cavities due to the lower steric hindrance), and to electronic effects. The carbocation which develops by protonation of the hydroxymethyl group and exit of water in p-VA is more stable (and therefore its formation is quicker) than the corresponding benzyl cations which form in o-VA and m-VA. This is due to the delocalization of the charge in those positions in the aromatic ring which feel more the

569 electron-donating mesomeric effect of the hydroxy group and less the electron-attracting inductive effect of the substituents. The benzyl cation formed in p-VA undergoes nucleophilic attack by (i) methanol or hemiformal (yielding the monoaryl by-products), (ii) guaiacol (yielding the three possible isomers of diarylmethane), and (iii) a second molecule of p-VA, o-VA or m-VA (again yielding the same three isomers of diarylmethane, after elimination of formaldehyde). The higher reactivity of p-VA as compared to the other isomers also explains why only three isomers of diarylmethane (MW 260) were found. A final consideration concerns the experimental evidence that the formation of all byproducts, at both low and high conversion, include VAs (a primary product) as the reactant, and thus correspond to compounds formed through consecutive reactions. On the contrary, Figure 1 clearly shows the important contribution of parallel reactions, with a selectivity to VAs which when extrapolated to nil conversion is not total. Therefore, even though VAs might be considered the exclusive primary products from a chemical and mechanistic point of view, they can not be considered as such from a kinetic point of view. This contradiction can be explained by making the hypothesis that a fraction of VAs generates a stable carbocation in the zeolitic pores, and is transformed into by-products before going into the bulk liquid phase. This effect should theoretically mainly involve o-VA, which is sterically more hindered than the other isomers, and the counterdiffusion of which therefore should be slower, with a corresponding higher time constant. This hypothesis might also explain why the presence of methanol had a considerable effect on the initial selectivity to o-VA (i.e., measured at low guaiacol conversion). Figure 1 shows that the overall selectivity to VAs was higher when the reaction was carried out in the presence of methanol, and the difference was mainly due to the higher selectivity to o-VA, which was systematically higher (approx. 10% more) at both low and high guaiacol conversion. Methanol may in part inhibit the interaction between o-VA and the active sites, and thus decrease the mean residence time of the molecule in the pores and accelerate its counterdiffusion, saving it from further transformations. This may have a positive effect on the initial selectivity to this compound. On the contrary, the effect of methanol on initial selectivity was practically nil for p-VA (compare Figure 1 left and right). On the other hand, p-VA in the bulk liquid phase can readily re-enter the zeolite and generate stable carbocations inside the cavities, thus undergoing consecutive transformations more quickly than the other isomers do.

3.2.The reactivity of vanUlic alcohols In order to better understand the observed phenomenology, reactivity tests were made by loading separately in the reactor each one of the three VA isomers, and by carrying out the reaction in the presence of the H-mordenite catalyst, under different reaction conditions (Figure 3): (i) in the presence of only water, (ii) with water and methanol (with an alcohol content simulating the amount which is present in the reaction medium for tests carried out with the commercial formalin solution), and (iii) with the commercial formalin solution (i.e., containing = 15 wt.% methanol). All the other reaction conditions (i.e., time, temperature and amount of catalyst) were kept the same as for the standard reactions of guaiacol hydroxymethylation. The following considerations can be drawn: 1) The tests done with VAs in water (Figure 3 top left) are indicative of the tendency of each VA isomer to react in the presence of the H-mordenite, to yield either intramolecular or

570 intermolecular transformations in acid media. The following reactivity scale was found: p-VA > m-VA > o-VA, which does not fully correspond to the trend expected on the basis of the relative stability scale of the benzyl cation generated by protonation of the hydroxymethyl group and exit of water (i.e., p-VA > o-VA > m-VA). It is useful to mention that under the same conditions guaiacol was completely unreactive. The high reactivity of p-VA derives from (i) the higher stability of the corresponding benzyl cation, which makes the latter a strong electrophilic agent, and (ii) the higher diffusivity of the molecule in the pores of the zeolite. The low reactivity of o-VA is likely due to the slower diffusion of the molecule into the zeolite, made more difficult by steric hindrance (and indeed this shape-selectivity effect is the reason why with H-mordenite a considerably higher selectivity to p-VA than to o-VA is obtained in guaiacol hydroxymethylation with respect to tests carried out in homogeneous acid media [8]). These data confirm that vanillic alcohols give consecutive reactions of transformations even in the absence of formaldehyde and methanol, and that under these conditions the most reactive VA is p-VA. The contributions to conversion were: (i) isomerization, (ii) formation of small amounts of diaryl compounds, and (iii) formation of products which were not analytically found, thus explaining the "C-unbalance" seen in Figure 3. Concerning the formation of diaryl compounds, since they are insoluble in water but soluble in water/methanol, it was possible to detect them by adding methanol to the batch after conclusion of the reaction. Surprisingly, the prevailing diaryl compounds detected were those having MW 260. This indicates that after condensation of two VA molecules, a formaldehyde molecule is soon eliminated. The low yield to diaryl compounds indicates that this reaction is not rapid under these conditions. The extent of isomerization of p-VA and o-VA was much less than that of m-VA. Under the hypothesis of kinetic control, this is explained by considering that in the case of m-VA the driving force for the intramolecular shift of the hydroxymethyl group from the meta position to the ortho or to the para position is the formation of an intermediate carbocation which is more stable than that originally formed. This reaction is analogous to the ipso-substitution in the isomerization of alkylaromatics. Since the formation of heavy, polyaryl compounds (which are not eluted in the column under our analytical conditions) was unlikely, the C-unbalance was due to the retention of VAs in the zeolite. The absolute amount of VA which was retained in the zeolite porosity, as inferred from the C-unbalance, follows the scale p-VA > m-VA ~ o-VA, that does not correspond to the stability scale of the corresponding benzyl cations. This hypothesis is also confirmed by the colour which developed in the mordenite when put in contact with VAs (absorption at 550 nm wavelength from p-VA, at 430 nm from o-VA and at 700 nm from mVA), indicating the presence of the corresponding cations adsorbed on the zeolite surface and stabilized by this interaction [3]. A strong interaction of the reactant with the catalyst has also been proposed to occur in the case of furfuryl alcohol hydroxymethylation, at low concentration of formaldehyde [ 10]. Due to the interaction between furfuryl alcohol and the zeolite, and to the competition between the two reactants, saturation of the catalyst surface occurred, with inhibition of the reaction rate. 2) In the case of tests carried out in the presence of water/methanol mixtures (Figure 3, top right), the o-VA and m-VA conversions were enhanced with respect to tests carried out in the presence of only water, while the conversion of p-VA was substantially unaffected by the presence of methanol. Also in this case, guaiacol was completely unreactive.

571 The formation of diaryl compounds was very low, and the main product obtained was that of etherification with methanol. Also in this case, however, the majority of converted VA indeed contributed to the C-unbalance, thus to the amount of vanillol retained in the catalyst pores. The absolute amount of missing VA followed the scale: p-VA > o-VA >> m-VA. The substantial absence of diaryl compounds suggests that the formation of the latter only occurs in the presence of formaldehyde and in the absence of methanol. In guaiacol hydroxymethylation and in the presence of methanol, methanol reacts quickly with p-VA, and the rate of condensation of two VA molecules becomes practically nil. An alternative explanation is a solvent effect, since the benzyl cation may be solvated by methanol molecules, thus hindering the attack on a second p-VA molecule. The effect of methanol on VAs conversion can be interpreted by considering that p-VA reacts through a SNl-type mechanism (as usually occurs for more stable carbocations), and therefore the rate of generation of the cation (the rate-determining step) is substantially unaffected by the presence of a nucleophile (while the distribution of the products obtained, instead, is obviously affected by the type of nucleophilic species present in the reaction medium). On the contrary, in the case of o-VA and m-VA, the corresponding benzyl cations are less stable, and the mechanism of transformation may reasonably involve a SN2-type mechanism, in which the concentration of methanol (the nucleophilic species) contributes positively to the reaction rate. In confirmation of this, the scale of VA reactivity for the formation of the products of etherification was m-VA > o-VA > p-VA, which corresponds to the reverse of the scale of stability for the corresponding benzyl cations. In other words, in the presence of the nucleophilic methanol, m-VA quickly reacts, and yields a large fraction of the arylmethylether (and correspondingly the fraction of vanillol retained in the zeolite is very low), while p-VA generates a stable cation, and behaves similarly in the absence and in the presence of methanol (except for the different type of product obtained), with a large fraction of the cation retained in the zeolite. Therefore, under these conditions any shape-selectivity effect of the zeolite is rendered nil by the presence of a strong chemical interaction with pVA. 3) When the reaction was carried out in the presence of the commercial formalin, further differences between VAs were observed. The conversion of all VA isomers was lower than in the previous cases; the scale of reactivity was m-VA = o-VA > p-VA. This is because formaldehyde (present in large excess) is preferentially protonated and competes with VAs for protonation on the active sites. Therefore, the nature of the rate-determining step for VA conversion is different from that occurring in the absence of formaldehyde. The products of VA transformation were mainly monoaryl compounds obtained by etherification with the hemiformal or with methanol; minor amounts of products obtained by hydroxymethylation of VAs were also found. The lower degree of interaction of VAs with the acid sites of the zeolite is also demonstrated by the decrease in the C-unbalance, which indicates a decrease in the absolute amount of species which are retained in the zeolite. This is more evident for p-VA (the corresponding C-unbalance was very low), which generates the most stable carbocation and reacts quicker than the other isomers. Furthermore, p-VA itself is a better nucleophilic agent, and reacts quicker with protonated formaldehyde than the other VA isomers do. These data also are in line with the higher contribution of consecutive reactions (i.e., etherification) occurring on p-VA rather than on other isomers in tests of guaiacol hydroxymethylation. The high yield to monoaryl compounds obtained from m-VA can be attributed to a contribution of the transformation of p-VA, the latter having been generated by isomerization of m-VA.

572 80

80 ~

60

......................

:!:! :.:.

"u "~' 40

iii ;-." !i:i

ou

~.'. !iii

20

--

*~ .~ 60

~ "~'40

1 ...~,

....

:::"- . . . . . . . . :::"

:::: ::::

i:!] iii.~ :::"

:i:i i'i" :i:i

o 20

:::: ":': ..

,

o-VA

50 40

,g~9 30 ~ 2o 8 10 0

,

m-VA

m

p-VA

iilnl

....

:;:-"

,

o-VA

~

::::

-

i:i: iiii ::::

[ -I

:':" ::::

-II

....

;';'

,

m-VA

":':

I

p-VA

Vanillols conversion Sel. to isomerization Sel. to monoaryl compounds Sel. to diaryl compounds C unbalance

iil i

o-VA

.....

i

m-VA

p-VA

Figure 3. Vanillol conversion and yield of products for tests done in only water (top left), in water/methanol (top right), and in commercial formalin (bottom). 4. REFERENCES 1. R.A. Sheldon, H. van Bekkum, in "Fine Chemicals through Heterogeneous Catalysis", R.A. Sheldon and H. van Bekkum (Eds.), Wiley-VCH, 2001, p. 1. 2. P. Metivier, in "Fine Chemicals through Heterogeneous Catalysis", R.A. Sheldon and H. van Bekkum (Eds.), Wiley-VCH, 2001, p. 173. 3. A. Corma, H. Garcia, J. Chem. Soc., Dalton Trans., (2000) 1381. 4. C. Moreau, F. Fajula, A. Finiels, S. Razigade, L. Gilbert, R. Jacquot, M. Spagnol, in "Catalysis of Organic Reactions", F.A. Herkes (Ed.), Marcel Dekker, New York, 1998, p. 51. 5. C. Moreau, S. Razigade-Trousselier, A. Finiels, F. Fazula, L. Gilbert, WO patent 96/37452 (1996), assigned to Rhone-Poulenc Chimie. 6. N. Barthel, A. Finiels, C. Moreau, R. Jacquot, M. Spagnol, J. Molec. Catal., A: Chemical, 169 (2001) 163 7. A. Finiels, P. Geneste, J. Lecomte, F. Marichez, C. Moreau, P. Moreau, J. Molec. Catal., A: Chemical, 148 (1999) 165. 8. F. Cavani, M. Corrado. R. Mezzogori, J. Molec. Catal., in press 9. F. Cavani, R. Mezzogori, Catal. Org. React., submitted 10. J. Lecomte, A. Finiels, P. Geneste, C. Moreau, J. Molec. Catal., A: Chemical, 133 (1998) 283.


573

Unraveling the Nature and Location of the Active Sites for Butene Skeletal Isomerization over Aged H-Ferrierite Sander van Donk, Eveline Bus, Alfred Broersma, Johannes H. Bitter and Krijn P. de Jong* Department of Inorganic Chemistry and Catalysis, Debye Institute, Utrecht University, P.O. Box 80083, 3508 TB Utrecht, The Netherlands *Corresponding author, fax: +31302511027; e-mail: [email protected]

The relation between the catalytic performance, the number and location of the accessible active sites and the nature of carbonaceous deposits was established for aged H-ferrierite during n-butene skeletal isomerization. In situ infrared spectroscopy reveals that the deposition of carbonaceous species significantly lowers the number of Bronsted sites. With short time-on-stream such deposits display large reactivity and induce by-product formation but also contribute to part of the isobutene production. With prolonged time-on-stream the deposits are converted into non-reactive carbon species and accordingly isobutene selectivity is enhanced. Probing with d3-acetonitrile does not reveal the presence of carbenium ions at this stage. Additionally, it is established that part of the Bronsted acid sites in the 10 membered-ring channels are still accessible and most likely catalyze the selective conversion of n-butene into isobutene.

1.

INTRODUCTION

Over the last decade a growing demand for isobutene has provided large industrial and scientific interest for the skeletal isomerization of linear butenes to isobutene. Isobutene is used in the petrochemical industry for the production of e.g. polyisobutene, methacrolein, synthetic rubber and MTBE. Zeolites containing 10-membered ring (MR) pores have proven to exhibit high selectivities for the butene skeletal isomerization [1-3]. Especially the twodimensional zeolite H-ferrierite in its acidic form (H-FER), for which the 10 MR main channels are interconnected by 8 MR side pores, exhibits an exceptionally high selectivity and stability [4,5]. Nevertheless initial isobutene selectivity is rather poor, but with longer timeon-stream (TOS) very high selectivities are reached which is often associated with a change in the prevailing reaction pathway [6-10]. Conclusive evidence for this was provided by Meriaudeau et al. [8], de Jong et al. [9] and Cejka et al. [10] using 13C-labelled butenes, demonstrating that over a fresh H-FER scrambling of the 13C-label occurred whereas with prolonged TOS hardly any scrambling was observed. This clearly indicates that initially nonselective dimerization-cracking reactions running over the H-FER Bronsted acid sites dominate the catalytic action. However, with prolonged TOS a highly selective reaction pathway prevails [8-10] and coincides with the presence of carbonaceous deposits that largely fill the H-FER internal pore-volume [3,7,9].

574 The nature of the selective reaction pathway over aged H-FER and the role of the carbonaceous deposits are still under debate. Several authors [8,11-14] suggested a monomolecular reaction pathway as the selective route for isobutene production. However, the direct conversion of n-butene into isobutene over a plain Bronsted acid site involves the formation of an energetically and thermodynamically highly unfavorable primary carbenium ion [15]. Guisnet et al. [6,16] therefore proposed an altemative pathway in which no primary carbenium intermediate is formed. This so-called pseudo-monomolecular reaction pathway should run over alkyl-aromatic tertiary carbenium ions prefixed in the coke instead of plain Bronsted acid sites. The aim of this study is to unravel the role of the carbonaceous deposits and the number, nature and location of the active sites during butene skeletal isomerization over aged H-FER. Therefore the following research strategy is employed: (1) The catalytic performance of H-FER as a function of the amount of carbonaceous species deposited is evaluated under differential conditions in a catalysis set-up including a tapered element oscillating microbalance (TEOM). (2) The nature of carbonaceous deposits with TOS is established by in situ infrared (IR) spectroscopy. (3) The locations of the vacant Bronsted acid sites with TOS are determined using in situ IR spectroscopy and subsequent deconvolution. Assignment of the differently located Bronsted-groups is established by taking into account previous studies [17,18] regarding the H-FER structure. (4) H-FER samples with different amounts of deposits, obtained after evaluation in the TEOM, were subsequently examined by IR spectroscopy establishing the nature and number of the accessible active sites by in situ probing with d3-acetonitrile (CD3CN). This enables discrimination between plain Bronsted acid sites and adsorbed carbenium ions [ 19,20]. Elucidation of the nature of the active sites contributes to the longstanding discussion whether a monomolecular [8,11-14] or a pseudo-monomolecular [6,16] reaction pathway dominates the catalytic action over aged H-FER. Moreover, based on the relation between the catalytic performance, the number and location of the active sites and the role of carbonaceous deposits, an overall reaction scheme for butene skeletal isomerization over aged H-FER as a function of TOS is introduced. 2.

EXPERIMENTAL

2.1. Ferrierite samples and catalysis measurements High silicon NH4-FER (Zeolyst Int. Si/A1 30) was activated in a dry N2 flOW at 823 K for 12 hours to obtain H-FER. The acid site density of H-FER is 0.53 mmol.g -1, determined by temperature programmed desorption-thermogravic analysis (TPD-TGA) of n-propylamine, and the micropore volume is 0.132 ml.g 1, established by N2 physisorption. Catalysis measurements were performed in a set-up including a tapered element oscillating microbalance (Rupprecht & Pataschnik TEOM 1500 PMA), which offers the possibility to quantitatively monitor the carbon deposition rate and amount in situ, see Hershkowitz and Madiara [21] and Chen et al. [22].

575 The TEOM is connected to a Shimadzu 17A gas chromatograph with a Chrompack PLOT capillary column (fused silica-A1203/KCL, 50m x 0.32mm) and a flame ionization detector, to analyze reaction products. For the differential catalysis measurements the TEOM reactor was loaded with 5-10 mg of H-FER particles (90-150 gin) with quartz wool on top and bottom of the bed to keep the particles firmly packed. The samples were dried in situ in N2 at 623 K and after switching to a pure n-butene gas-flow (Hoek Loos, 1-butene, > 99.5%), the catalytic performance and carbon uptake were monitored at 623 K, 1.3 bar. Conversion and selectivity were determined for H-FER samples with different amounts of deposits under differential conditions (conversion < 10 mole%) by adjusting the weight hourly space velocity (WHSV). Conversion is defined as the molar ratio of all products (4 n-butene) to all compounds detected. Selectivities are calculated as the molar ratio of a certain product to all products (;~ n-butene). All mass changes were corrected for temperature- and gas density differences by performing blank runs over inert samples. Immediately after the catalysis experiments the n-butene flow was switched off and the aged H-FER samples were studied by IR spectroscopy, probing with CD3CN (see also section 2.3.). It has been checked and confirmed for all aged H-FER samples that catalytic performance was not affected by cooling down to room temperature and contacting with air.

2.2. In situ infrared spectroscopy Spectra were recorded during the skeletal isomerization of n-butene at 623 K and 1.0 bar with an FT-IR spectrometer (Perkin Elmer Spectrum One) equipped with an in situ flow cell. The H-FER sample was pressed into a self-supporting wafer o f - 3 mg with 0.5 cm diameter, by applying a pressure of 200 MPa. The wafer was placed in a cylindrical oven and dried at 623 K under a He flow (10 ml.min-1). Next n-butene was added to the flow at 0.2 bar partial pressure. The first 4 hours of n-butene reaction, IR spectra were recorded at a time interval of 5 minutes using a MCT detector (20 scans in 22 seconds). After 4 hours spectra were recorded at a time interval of 5 hours using a MIR-TGS detector (100 scans in 9 minutes). The spectra were measured in transmission mode from 4000 to 1000 c m -1 (4 c m -1 resolution) and normalized to the overtone lattice vibration of the dried H-FER between 1880-1860 cm -1 (A = 0.3), which is a good measure of the thickness of the wafer. The v(OH) stretch region (4000-3000 cm -1) was deconvoluted using the Origin 6.1 software program, assuming Gaussian bandshapes. 2.3. Infrared spectroscopy: probing with d3-acetonitrile Probing experiments with CD3CN (Acros, 99% purity) were performed in the FT-IR instrument described above and sample preparation was identical. After drying the sample at 623 K under a He flow (10 ml.min-1), the sample was cooled to 448 K and the adsorption of CD3CN (partial pressure 5-10 mbar in 10 ml.min 1 He) was started. The applied conditions were chosen based on experimental checks revealing that 448 K is the highest temperature and 5 mbar the lowest partial pressure at which the maximum amount of Bronsted acid sites is probed. Spectra were measured in transmission mode from 4000 to 1000 cm -1 (4 cm -1 resolution) and normalized to the overtone lattice vibration of H-FER. In order to quantify the amount of CD3CN adsorbed on the sample, the v(CN) stretch region (2600-2100 cm -1) was deconvoluted using the Origin 6.1 software program.

576 Table 1. Conditions and catalytic performances of the aged H-FER samples at 623 K and 1.3 bar. Reaction conditions

WHSV (gc4=.gH.FER'I.h-1) TOS (h)

H-FER5.0C

H-FER6.6C

H-FER6.8C

169 4

142 20

21 300

5.0 9.5 35.5

6.6 8.9 38.0

6.8 8.5 91.2

16.1 5.7

12.6 4.8

1.8 1.6

T E O M - catalysis results

carbon uptake (guptake.gH_FER-1. 102) n-butene conversion (mole%) isobutene selectivity (mole%) Reaction rates

n-butene conversion (g.gH_FER'1.h"l) isobutene formation (g.gH.FER-1.h"l)

3.

RESULTS

3.1. On the catalytic performance The catalytic performance of H-FER as a function of the amount of carbonaceous deposits was evaluated in a catalysis set-up including a TEOM. Table 1 summarizes the applied conditions and the main results. Upon aging lower WHSV's are demanded to obtain similar n-butene conversions o f . . 9 mole% . This indicates that H-FER becomes less reactive with TOS, although in the end for H-FER6.8C n-butene is converted into isobutene with a selectivity o f - 91 mole% . By taking into account the applied W H S V ' s for the aged H-FER samples, reaction rates for the conversion of n-butenes and the formation of isobutene are calculated and also displayed in table 1. 3.2. On the nature of the carbonaceous deposits The in situ IR spectra for the region specific of carbonaceous species on H-FER are displayed in figure 1. The absorption at 1514 cm 1 corresponds to the C-C bond vibration of non-condensed aromatics [16,23]. This band is moderately present after 1 h TOS and increases with longer n-butene contact. Contributions around 1580 cm 1 and 1616 cm -1 reveal the formation of condensed aromatic species [16,23] and the signal around 1420 cm -1 indicates these species to be attached to cyclopentane tings. Moreover, the bending modes that are characteristic for alkyl-groups are present at 1352 c m 1 and 1438 cm -1 [16,23]. 3.3. On the location of the vacant Bronsted acid sites Figure 2 shows the decrease in intensity of the band characteristic for the Bronsted acid sites in H-FER as monitored by in situ I R spectroscopy during n-butene reaction. Initially the number of vacant Bronsted acid sites largely reduces upon n-butene contact. A f t e r - 17 hours TOS there is no further reduction and 5 % of the initial number of Bronsted sites is still vacant.

-201/ 2 :3

1.6

o

1.2

r == o

0.4 0.0

.-..100

40

0 h/ 5h 11 9

,

9

,

1600 1550 1500 14'50 14'00' 13'50 wavenumber

( c m "1)

Figure 1. In situ IR spectra of carbon deposited on H-FER during n-butene reaction at 623 K and 1.0 bar. The spectrum of the fresh H-FER is subtracted.

577

"o r

Figure 2. Intensity of the total band corresponding to Bronsted acid sites on H-FER with TOS at 623 K and 1.0 bar, normalized to the band at Oh TOS.

--&-- 10 MR

.Q "O

~ 6o c tli

-9 40

II1 ~

.

.

.

.

.

"g 20

c

A --

0

,

o

,

1o

~

,

,

20 30 TOS (hours)

,

,

40

.

Figure inset. Relative intensities of the peaks characteristic for Bronsted acid sites in the 10 MR channels (A) and in the 8 MR channels (ll) with short TOS. The values are normalised on their respective initial values at Oh TOS and obtained by deconvolution of the total Bronsted band.

50

The Bronsted band consists of a number of OH vibrations that are located at different positions in the H-FER framework. Zholobenko et al. [17] and Domokos et al. [18] demonstrated for fresh H-FER that it is possible to deconvolute this band into its separate contributions. In good agreement with the set of parameters proposed by Domokos et al. [ 18], we recently identified four types of Bronsted OH groups located in 10-, 8-, 6- and 5membered tings [24]. Moreover, it was demonstrated for the first time that it is possible to distinguish the differently located OH groups in the H-FER structure during the reaction of nbutene [24]. The inset in figure 2 displays the change in relative peak areas with short TOS of the sites in the 10 MR main channels and 8 MR side channels. 3.4. On the nature and number of the accessible active sites The H-FER samples displayed in table 1 were probed with CD3CN to establish the number and nature of the active sites. In figure 3 the difference spectra of the v(OH) and v(CN) stretch vibrations for fresh H-FER and for the aged H-FER samples are shown. Upon adsorption of CD3CN the difference band assigned to the Bronsted sites at 3580 cm -1 (left figure) is reduced in intensity for all aged H-FER samples as compared to the fresh H-FER. After CD3CN adsorption the Bronsted band does not disappear completely (not shown in figure), implying that not all sites are accessible for CD3CN. Additionally, the maximum of the remaining Brensted acid band is shifted towards slightly lower frequencies. 0.0

O c

i,., O

v(CN)

(,,)

0.2

~

E:

-0.1

V

H-FER

fresh

v(OH)

-0.2 '

36'oo ' 35'5o wavenumber (cm 4)

I,,. O ,~ .Q

H'iI~Rfresh

0.1

0.0

2300

2250

wavenumber (cm "1)

Figure 3. IR difference spectra of CD3CN (5-10 mbar in 10 ml.min -1 He-flow) adsorbed on fresh HFER, H-FERS.0C, H-FER6.6C and H-FER6.8C at 448 K. Left: v(OH)-region, negative peaks correspond to the disappearance of the OH stretch vibration. Right: v(CN)-region, positive peaks correspond to the appearance of the CN stretch vibration.

578

H-FER6.8C H-FER6.6C H-FER5.0C

0

500

1000

1500

TOF (mmol.mmol'l.h "1)

2000

Figure 4. Turn-over frequencies for the aged H-FER samples at 623 K and 1.3 bar; defined as mmol butene converted (I) or mmol isobutene formed (!"1) per mmol accessible Bronsted acid site (as probed by CD3CN) per hour.

The disappearance of the Bronsted peak correlates with the appearance of the peak at 2292 cm -1 (fight figure). This peak can be solely assigned to the stretch vibration of v(CN) coordinated to a Bronsted acid site, as has been reported in other studies [20,25-27]. The v(CN) stretching mode allows quantification of the accessible Bronsted acid sites present in the aged H-FER samples. The numbers are calculated by taking the peak areas relative to fresh H-FER, for which the number of Bronsted acid sites is 0.53 mmol.g 1 according to n-propylamine TPD-TGA. Considering the number of accessible Bronsted acid sites, the reaction rates presented in table 1 can be transformed into turn-over-frequencies (TOF). The TOF's are displayed in figure 4, giving the moles of n-butenes converted or moles of isobutenes formed per mole accessible Bronsted acid site, as probed by CD3CN, per hour. Bystrov [19] and Jolly et al. [20] reported that the adsorption of CD3CN on zeolites aged in hydrocarbon reactions may result in the appearance of a strong band between 2387 cm -1 and 2377 cm -1, characteristic for the vibration of v(CN) bound to a C + of a carbocation. However, we recently showed that no such band is present upon CD3CN probing of the aged H-FER samples as well as upon probing during butene skeletal isomerization [24]. 4.

DISCUSSION

From the results presented in table 1, figure 1 and by other groups [3,7,9], it is obvious that the formation of carbonaceous deposits accompanies the skeletal isomerization of nbutene over H-FER. Figures 2 and 3 reveal that the deposition of carbon reduces the overall number of accessible Bronsted acid sites. The inset in figure 2, obtained after deconvolution of the Bronsted band [24], indicates that already with short TOS Bronsted sites are lost in both the 8 MR side pores and 10 MR main channels. Moreover, the 8 MR-signal rapidly levels off implying that its entrances are blocked, leaving sites inaccessible for n-butene. Figure 3 proofs that with extensive aging of H-FER, Bronsted sites remain accessible for CD3CN. Given that the OH groups in the 8 MR's vibrate at lower wavenumber than those in the 10 MR's [17,18,24], the observed shift of the peak-maximum confirms that sites in the side-pores are inaccessible for CD3CN. Therefore the Bronsted sites that are still accessible will be predominantly located in the 10 MR channels; hence these sites are involved in the catalytic action. This result is in excellent agreement with Domokos et al. [ 18] who reported on the relation between acid site locations in sodium exchanged H-FER samples and their catalytic performance. In these studies a structure-activity relation was observed between the presence of Bronsted acid sites in the 10 MR channels and the selective formation of isobutene. We show that this structure-activity relation also exists under conditions where carbonaceous deposits largely fill the H-FER micropore volume [24].

579 prevailing reaction and location of the active sites

Non-selective bimolecular reactions throughout the crystals and carbon deposition Both non-selective reactions of deposits and selective isobutene formation over Bronsted sites in the 10 MR pore entrances Selective isobutene formation over Bronsted sites in the 10 MR pore entrances

N-BUTENE ~.__

$:

fast

ISOBUTENE, PROPENE, PENTENES,etc.

H-FER

fresh, without deposits

fast ; oligomerization, cyclization

L_

"~ p,

E

(~ I

H-FER

aged with aikyl-aromatic deposits

slow [H-transfer

N-BUTENE ~ I S O B U T E N E , PROPENE,

ks/~PENTENES, etc. "~ f-

H-FER

aged with condensed aromatic deposits

ISOBUTENE N-BUTENE

~l~w

ISOBUTENE

Figure 5. Schematic overview of the prevailing reactions, the nature of carbonaceous deposits and the locations of the active sites during n-butene skeletal isomerization over H-FER with TOS. Guisnet et al. [6,16] claimed that with prolonged TOS all Bronsted acid sites are inaccessible for reactants. A so-called pseudo-monomolecular reaction pathway running over alkyl-aromatic tertiary carbenium ions captured inside the pore-entrances of the zeolite should dominate the selective catalytic action. However, the CD3CN probing results [24] do not support the presence of carbenium ions and the occurrence of a pseudo-monomolecular pathway. This was further confirmed by Asensi et al. [13], who showed that on a high-silica H-FER (Si/A1 = 59, i.e. low number of Bronsted acid sites) high isobutene selectivities were reached without the substantial formation of deposits. At atmospheric or higher pressures the deposition of carbonaceous species during butene skeletal isomerization can not be avoided, see table 1 and figure 1. The role of these deposits is rather ambiguous. According to figures 2 and 3, the deposition of carbon lowers the amount of Bronsted sites, consequently suppressing non-selective bimolecular reactions. Table 1 indicates that the deposition of reactive species significantly contributes to the overall product formation, although the catalytic action is not at all selective since besides isobutene numerous by-products are formed. With extensive aging of H-FER, figure 1 reveals that the nature of deposits changes from hydrogen rich towards hydrogen-poor aromatics. These last species are not very reactive and accordingly isobutene selectivity is indirectly enhanced. Table 1 shows that for the aged and selective catalyst H-FER6.8C, these alterations result in a net decrease of both the n-butene conversion and the isobutene formation rate, indicating that indeed part of the isobutene was initially formed from these deposits. Figure 5 introduces an overview of the relations between the catalytic performance, the number, nature and location of the active sites and the role of carbonaceous deposits during butene skeletal isomerization over aged H-FER with TOS. 5.

CONCLUSIONS

In situ I R spectroscopy reveals that during butene skeletal isomerization the deposition of carbon significantly lowers the number of vacant Bronsted acid sites. Initially such deposits are involved in non-selective reactions. However, with prolonged TOS the deposits change into non-reactive species and accordingly isobutene selectivity is enhanced. Probing with CD3CN reveals that at this stage no carbenium ions are present, while Bronsted acid sites in

580 the 10 MR channels are still accessible. The latter sites most likely catalyze the selective conversion of n-butene into isobutene. ACKNOWLEDGEMENT

We thank Dr. T. Visser and Dr. F. de Groot for their contributions. This work was financially supported by the Netherlands Organization for Scientific Research (NWO/CW 700-97-019). REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

A.C. Butler and C.P. Nicolaides, Catal. Today 18 (1993) 443. P. M6riaudeau and C. Naccache, Adv. Catal. 44 (1999) 505. S. van Donk, J.H. Bitter and K.P. de Jong, Appl. Catal. A Gen. 212 (2001) 97. P. Grandvallet, K.P. de Jong, H.H. Mooiweer, A.G.T.G. Kortbeek and B. KraushaarCzarnetzki, European Patent No. 501 677 (1992), to Shell. H.H. Mooiweer, K.P. de Jong, B. Kraushaar-Czarnetzki, W.H.J. Stork and B.C.H. Krutzen, Stud. Surf. Sci. Catal. 84 (1994) 2327. M. Guisnet, P. Andy, N.S. Gnep, C. Travers and E. Benazzi, J. Chem. Soc. Chem. Commun. (1995) 1685. W.-Q. Xu, Y.-G. Yin, S.L. Suib and C-L. O'Young, J. Phys. Chem. 99 (1995) 758. P. M6riaudeau, R. Bacaud, L.N. Hung and T.A. Vu, J. Mol. Catal. A 110 (1996) L177. K.P de Jong, H.H. Mooiweer, J.G. Buglass and P.K. Maarsen, Stud. Surf. Sci. Catal. 111 (1997) 127. J. Cejka, B. Wichterlov~ and P. Sarv, Appl. Catal. A Gen. 179 (1999). G. Seo, H.S. Jeong, D.-L. Jang, D.L. Cho and S.B. Hong, Catal. Lett. 41 (1996) 189. J. Houzvicka and V. Ponec, Ind. Eng. Chem. Res. 36 (1997) 1424 M.A. Asensi and A. Martinez, Appl. Catal. A Gen. 183 (1999) 155. G. Seo, M.-Y. Kim and J.-H. Kim, Catal. Lett. 67 (2000) 207. D. Brouwer and J. Oelderik, Rec. Trav. Chim. Pays Bas 87 (1968) 1435. P. Andy, N.S. Gnep, M. Guisnet, E. Benazzi and C. Travers, J. Catal. 173 (1998) 322. V.L. Zholobenko, D.B. Lukyanov, J. Dwyer and W.J. Smith, J. Phys. Chem. B 102 (1998)2715. L. Domokos, L. Lefferts, K. Seshan and J.A. Lercher, J. Mol. Catal. A Chem. 162 (2000) 147. D.S. Bystrov, Zeolites 12 (1992) 328. S. Jolly, J. Saussey and J.C. Lavalley, Catal. Lett. 24 (1994) 141. F. Hershkowitz and P.D. Madiara, Ind. Eng. Chem. Res. 32 (1993) 2969. D. Chen, A. Gronvold, H.P. Rebo, K. Moljord and A. Holmen, Appl. Catal. A Gen. 137 (1996) L1. Z.R. Finelli, C.A. Querini, N.S. Figoli and R.A. Comelli, Appl. Catal. A Gen. 187 (1999) 115. S. van Donk, E. Bus, A. Broersma, J.H. Bitter and K.P. de Jong, submitted. G. Pelmenschikov, R.A. van Santen, J. J~inchen and E. Meijer, J. Phys. Chem. 97 (1993) 11071. J. Kotrla and L. Kubelkova, Stud. Surf. Sci. Catal. 94 (1995) 509. C. Paz6, A. Zecchina, S. Spera, G. Spano and F. Rivetti, Phys. Chem. Chem. Phys. 2 (2000) 5756.


581

H y d r o c o n v e r s i o n o f aromatics over a P t - P d ~ S Y catalyst C. Petitto a'b, G. Giordano b, F. Fajulaa and C. Moreau a aLaboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM- CNRS, Eeole Nationale Sup6rieure de Chimie de Montpellier, 8 Rue de l'Ecole Normale, 34296 MontpeUier Cedex 5, France bDipartimento di Ingegneria Chimica e dei Materiali, Universit/t della Calabria, Arcavacata di Rende (CS), Italia The hydroconversion of 1-methylnaphthalene was investigated over a 0.5 wt % Pt-0.3 wt % Pd/USY catalyst at 310 ~ and 5 MPa 1-12in the presence of different amounts of thiophene. Under those operating conditions, hydrogen transfer is not limiting and thermodynamics favours saturated products. The influence of sulphur present in the feed (200 to 1600 ppm S) was evaluated in the three major reactions involved in the hydroconversion process, i.e. hydrogenation, isomerisation and ring-opening reactions. Hydrogenation of 1methylnaphthalene into methyltetralines is always rapid whatever the sulphur amount. Hydrogenation of methyltetralines into the corresponding methyldecalines becomes lower with increased sulphur content, whereas isomerization of methyltetralines into alkylindanes as well as ring-opening of methyltetralines into alkylbenzenes becomes favoured. A mechanism involving hydrogen spillover, as recently proposed in the literature, would also account for the present results. 1. INTRODUCTION The aromatic content of diesel feedstock can vary broadly. Because of the increasing demand for cleaner distillates, de-aromatization of petroleum fractions is a basic process in the refinery. With respect to the new specifications for diesel fuel composition (decreased sulphur content, increased octane index, reduced particulate emissions, decreased aromatic content), highly active sulphur resistant catalysts were recently developed, mainly based on Pt, Pd, or mixtures of both, supported on acidic carriers [1-4]. In this paper, we present a study of the hydroconversion of 1-methylnaphthalene over a Pt-Pd/USY catalyst in the presence of thiophene. The influence of sulphur present in the feed is investigated in order to evaluate its effect on the course of the three major reactions involved, i.e. hydrogenation, isomerization and ring opening.

582 2. EXPERIMENTAL The P t - P d ~ S Y catalyst was prepared by impregnation procedures [ 1] in order to yield 0.5 wt % of Pd and 0.3 wt % of Pt. The catalyst was calcined in air for 2 h at 200 ~ and 4 h at 500 ~ and reduced in a 1-12flow for 3 h at 350 ~ Experiments were carried out in a 0.3 liter stirred autoclave working in the batch mode. 150 mg of the freshly reduced catalyst are added to 100 ml of a 0.1 M solution of 1-methylnaphthalene in cyclohexane to which is added thiophene (200 to 1600 S ppm). When the temperature reached 310 ~ hydrogen was introduced at the required pressure. Products were analysed by gas chromatography and identified by comparison with authentic samples and/or by GC/MS analyses. 3. RESULTS Preliminary experiments were performed to find experimental 1-12pressure and temperature conditions to ensure that i- hydrogen transfer is not diffusion limiting, ii- temperature is such that it is possible to measure accurately the partitioning of products resulting from the transformation of methyltetralines and iii- thermodynamics favours saturated products [5]. CH3

CH3

dimethylindanes

alkyltoluenes

l

A

4 -

i

C.H3~ Io. 3

i

5- and6-methyltetralines

1-methylnaphthalene

CH3

2_

~

--=CH3

cis + transmethyldecalines

1

1-and2-methyltetralines

CH3 ~ C H 3 dimethylindanes

3- ~ - - ~ 1 ' ~ R alkylbenzenes

Scheme 1: Simplified reaction scheme for the hydroconversion of 1-methylnaphthalene over a Pt-Pd/USY catalyst at 310 ~ and 5 MPa 1-12. In a typical run over the Pt-Pd/USY catalyst at 310 ~ in cyclohexane as the solvent, without thiophene added and 5 MPa 1-12 (Scheme 1), the main reaction intermediates in the

583 absence of sulphur are 5- and 6-methyltetralines and 1- and 2-methyltetralines (Fig. 1). 5methyltetraline results from hydrogenation of the benzene ring adjacent to the methyl group and 6-methyltetraline would result from isomerisation of 5-methyltetraline through formation of a bridged arenium ion [6]. 1-methyltetraline results from the hydrogenation of the ring bearing the methyl group and 2-methyltetraline would result from isomerisation of 1methyltetraline through formation of bridged cationic species as proposed for the cracking of lOO 8o

[]

-~ 60 "i

~o

o

2

40 20 o-

w

0

2

4

6

8

0

Time, h

2

4

6

8

Time, h

Figure l. Hydroconversion of 1-methylnaphthalene over Pt-Pd/USY catalyst, 310 ~ 5 MPa H2, cyclohexane as solvent.

Figure 2. Isomerisation and ring-opening percentages during 1-methylnaphthalene hydroconversion.

tetraline for example [7]. Hydrogenation over metal catalysts then features a behaviour similar to that observed for sulphided catalysts, i.e. one ring is less aromatic in character than the other one and, consequently, is more rapidly hydrogenated [8]. After 8 h of reaction, about 85 % of methyldecalines are formed with a mass balance close to 90-95 %. The percentage of C-C bond cleavage of methyltetralines into alkylbenzenes passes through a maximum at = 5 % alter 3 h and then decreases due to cracking of the lateral alkyl chain into C]0 and C9 hydrocarbons. In a similar manner, isomerisation of methyltetralines into alkylindanes also passes through a maximum at = 4 % after 2 h and then decreases relatively rapidly to yield both C5 ring-opening and cracking products. 100

100

8O

80

- 9

:~ ~

60

60

ppm

--D-200 --e- 400 -o--800 ~1600

ppm ppm

ppm

_

~

ppm

/

,

9 ------'

9

E 8

40

IlX L#/

0

~

- e - 400 ppm

-0-800

I!/

0

i,~

--~-200 ppm

2o I-//

~ 1

2

3

i,i,m

20

9

1600 ppm 4

Time, h

Figure 3. Influence of S content on the hydrogenation of 1-methylnaphthalene over Pt-Pd/USY catalyst, 310 ~ 5 MPa H~.

0

2

o

4

Time, h

6

8

Figure 4. Influence of S content on the formation of methyldecalines over PtPd/USY catalyst, 310 ~ 5 MPa H2.

584 In the presence of thiophene in the starting feed, significant effects are observed on the three major reactions involved for hydroconversion of 1-methylnaphthalene, i.e. hydrogenation, isomerisation and ring-opening reactions. Hydrogenation of 1-methylnaphthalene into 1- and 5- methyltetralines is always rapid whatever the amount of sulphur (Fig. 3) except at 1600 ppm S content. At the same time, hydrogenation of methyltetralines into the corresponding methyldecalines decreases with increased sulphur content (Fig. 4). Ring-opening of methyltetralines into alkylbenzenes (Fig. 5) as well as isomerisation of methyltetralines into alkylindanes (Fig. 6) then become favoured.

12

~R 10

- ' l - O ppm --g--200 ppm [ - - ~ 4 0 0 ppm / --0--800 ppm /

.,~

~ J ~ ~

~u ~ "I- -

~ /

~

~1600 ppm --o-800 ppm +400 ppm - ~ - 2 0 0 ppm

6 I

i~ ,3 4

~R 5

8 2

2

1

0

o

2

4 Time, h

6

8

Figure 5. Influence of S content on the ring-opening of methyltetralines into C11alkylbenzenes over Pt-Pd/USY catalyst, 310 ~ 5 MPa 1-/2.

0

2

4 Time, h

6

8

Figure 6. Influence of S content on the isomerisation of methyltetralines into C llalkylindanes over Pt-Pd/USY catalyst, 310 ~ 5 MPa H2.

In Table 1 are summarized the results obtained after 8 h of reaction and corresponding to a mass balance of 90-95 %. Table 1 Effect of sulphur amount on the main routes from 1-methylnaphthalene over Pt-Pd/USY at 310 ~ and PH2 = 5 MPa after 8 h of reaction time : concentrations in methyltetralines, methyldecalines, C11-alkylbenzenes and C11-alkylindanes. S content 0 ppm 200 ppm 400 ppm 800 ppm 1600 ppm

Methyltetralines (%) 0 10 40 53 61

Methyldecalines (~) 95 70 37 20 10

Cl~-alkylbenzenes (%) 2.5 8 12 14 12

C~l-alkylindanes (~) 1 1.5 3.5 6 6

A better manner to account for the importance of the effect of sulphur added in the feed is to take the partitioning of hydrogenation, ring-opening and isomerisation products from methyltetralines converted (Table 2). It can then be seen that ring-opening and isomerisation of methyltetralines represent more than 50 % of the reaction pathway at high sulphur content.

585 Table 2 Effect of sulphur amount on the partitioning of hydrogenation, ring-opening and isomerization products from methyltetralines converted on Pt-Pd/USY at 310 ~ and PH2 = 5 MPa after 8 h reaction time. S content 0 ppm 200 ppm 400 ppm 800 ppm 1600 ppm

Hydrogenation into methyldecalines (%) 96 87.5 71 50 36

Ring-opening into C11-alkylbenzenes (%) 2.5 10 23 33 43

Isomerisation into C11-alkylindanes (%) 1 2 6 17 23

A similar analysis can also be obtained from the experimental results reported at isoconversion of methyltetralines (Table 3). The influence of sulphur present in the feed is more important on C-C bond cleavage than on isomerisation reactions. Table 3 Effect of sulphur amount on ring-opening and isomerization products from methyltetralines converted at different methyltetralines conversions (10 to 30 %) over Pt-Pd/USY at 310 ~ and Pm = 5 MPa. S content 0 ppm 200 ppm 400 ppm 800 ppm 1600 ppm

C 11-alkylbenzenes %

C 11-alkylindanes %

10 %

20 %

30 %

10 %

20 %

30 %

1 3 5 9 14

3 7 6 14

4 9 12

2 3 2 3 6

3 4 3 6

3.5 3.5 3.5

4. DISCUSSION

Two additional experiments were also performed in order to have more information on the influence of hydrogen and acidity on the partitioning of reaction products and on the mechanism of hydroconversion : i- when hydrogen is removed from the autoclave after formation of methyltetralines, no further reaction occurs except partial dehydrogenation to 1methylnaphthalene. This clearly means that hydrogen must be present for hydrogenation reactions, of course, and also for cleavage of C-C bonds to yield alkylbenzenes, and ii- the addition of a stoichiometric amount of USY zeolite to the catalyst leads to an increase in the isomerisation steps rather than in hydrogenation or C-C bond cleavage reactions. Furthermore, the results obtained in the presence of Pt-Pd/USY catalysts closely parallel those obtained in the presence of conventional hydrotreatment catalysts, i.e. inhibiting effect of H2S on hydrogenation reactions and favourable effect on hydrocracking reactions through hydrogen spill-over were observed [9]. In the recent literature concerning aromatics hydroprocessing, it seems to be admitted that the modifications of the electronic properties of metal and acidic sites would be responsible for the enhancement of the thioresistance of Pt-Pd

586 catalysts supported on acidic carriers [ 1-3, 10]. Hydrogenation of aromatics would take place on acidic sites by hydrogen spilled-over from the metal sites, the participation of hydrogen spill-over to reactivity requiring close proximity of metal and acidic sites [3]. However, as already assumed by us from the comparison between hydrotreatment and metallic catalysts [8,11], protonic and hydride species resulting from the dissociation of hydrogen would be responsible for hydrogenation and hydrogenolysis reactions, respectively. In addition, the dissociation of H2S generated from the S-precursors leads to the formation of protonic and nucleophilic H S and/or S2 species [12] which should also be taken into account, as well as the possibility for tetralines and decalines intermediates to act as hydrogen donors. From the experimental results obtained, reduction of the hydrogenation route and corresponding increase of both isomerisation and C-C bond cleavage routes, we have then recently proposed that protonic species would be responsible for aromatics hydrogenation and that hydride species would be responsible for C-C bond cleavage [ 13], isomerisation reactions being more concerned by the acidity of the support as recently proposed in the literature [4].

REFERENCES

1. T. Fujikawa, K. Idei, T. Ebihara, H. Mizuguchi and K. Usui, Appl. Catal. A: Gen., 192

(2000) 253.

2. R.M. Navarro, B. Pawelec, J.M. Trejo, R. Mariscal and J.L.G. Fierro, J. Catal., 189 (2000) 184. 3. B. Pawelec, R. Mariscal, R.M. Navarro, S. van Bokhorst, S. Rojas and J.L.G. Fierro, Appl. Catal. A: Gen., 225 (2002) 223 and references therein. 4. M.A. Arribas and A. Martinez, Stud. Surf. Sci. Catal., 130 (2000) 2585. 5. B. Demirel and W.H. Wiser, Fuel Process. Technol., 55 (1998) 83. 6. Y.A. Borisov, N.I. Raevskii, E.S. Mortikov, V.A. Plakhornik and I.I. Lichehiner, Bull. Acad. Sci. USSR, Division Chemical Sciences, 3 (1992) 574. 7. A. Corma, V. Gonzalez-Alfaro and V. Orchill6s, J. Catal., 200 (2001) 34. 8. C. Moreau and P. Geneste, in "Theoretical Aspects of Heterogeneous Catalysis", J.B. Moffat, Editor, Van Nostrand Reinhold, N.Y., 1990, p. 256. 9. S. Giraldo de Le6n, P. Grange and B. Delmon, Appl. Catal. A: Gen., 107 (1993) 101, and references therein. 10. E. Guillon, J. Lynch, D. Uzio and B. Didillon, Catal. Today, 65 (2001) 201. 11. A. Finiels, P. Geneste and C. Moreau, J. Mol. Catal., A: Chem., 107 (1996), 385. 12. J. L6glise, L. Finot, J.N.M. van Gestel and J.C. Duchet, Stud. Surf. Sci. Catal., 127 (1999) 51. 13. C. Petitto, G. Cfiordano, F. Fajula and C. Moreau, Catal. Commun., 3 (2002) 15.


587

Hydrodearomatization, hydrodesulfurization and hydrodenitrogenation of gas oils in one step on Pt, Pd/H-USY Z. Varga a, J. Hancs6k ~, G. Tolvajb, W/thin6 I. Horvhthb, D. Kall6 c aDepartment of Hydrocarbon and Coal Processing, University of Veszpr6m, Veszpr6m, P.O. Box 158, H-8201, Hungary bDivision of Production and Trade, MOL - Hungarian Oil and Gas Co., Sz/tzhalombatta, P.O. Box 1, H-2443, Hungary r Research Center, Institute of Chemistry, Hungarian Academy Sciencies, Budapest, P.O. Box 17, H-1525, Hungary The investigation of hydrodearomatization, hydrodenitrogenation and hydrodesulfurization of gas oils (up to 188 ppm sulfur, 193 ppm nitrogen and 38.4% total aromatic content) over Pd, Pt catalysts supported on USY zeolite, whose Pd/Pt mass ratio was varied between 6:1 and 1:3, and total metal contents were between 0.90 and 0.93, are presented. The effect of change of Pd/Pt ratio on HDA, HDN and HDS activities are demonstrated. The advantageous process parameters for HDA, HDN and HDS of gas oils over a selected catalyst (e.g. Pd/Pt mass ratio 2:1) were determined. Under optimum process parameters the conversion of polyaromatics was higher than 80%, that of nitrogen compounds higher than 89% and sulfur content of the obtained products was lower than 50 ppm in case of every applied feed. 1. INTRODUCTION During the last 20 years the sulfur content of diesel fuels has been reduced stepwise from 1% to 350 ppm by January, 2000 and is to be reduced tO 50 ppm by January, 2005 in the European Union. The reason of the sulfur content reduction beside the environmental protection arises mainly because of proper working of exhaust gas affertreatment devices [ 1]. Reduction of the aromatic content of diesel fuels is relevant, too, since this contributes to increase of the cetane number and reduction of particulate and NOx emissions [2]. Hydrodearomatization (HDA) catalysts were developed recently providing hydrodesulfurization (HDS) function up to 500 ppm sulfur content beside saturation of aromatics [3-8]. Gas oil fractions having higher sulfur content are hydrodesulfurized previously on supported Co-Mo or Ni-Mo catalysts. These types of catalysts, mainly Co-Mo, do not provide, however, sufficient hydrogenating activity needed for the elimination of the most refractive sulfur and nitrogen compounds e.g. alkylated dibenzothiophenes and alkylated carbazoles, respectively [9-10]. Reduction of the nitrogen content of diesel fuels is similarly important, because NOx is favorably formed during the burning of the organonitrogen compounds resulting in air pollution (acid rains, ozone formation) and corrosion in the engine

588 and exhaust system, furthermore, they decrease the base content of the engine oil. Accordingly, search is required for catalysts having hydrodenitrogenation (HDN) activity as well as HDS and HDA activities. The objective of the authors was to find a catalyst suitable both for the saturation of aromatics of gas oils and reduction of their sulfur and nitrogen contents in one catalytic step. 2. EXPERIMENTAL HDA, HDS and HDN experiments were carried out with catalysts containing Pt and Pd in different ratios on H-USY support. The main properties of the support are the following: SIO2/A1203 ratio 33.5, total and mesopore surface areas 592.5 m2/g and 51 m2/g calculated using BET-plots and t-plots, respectively. The bimetallic Pt-Pd catalysts were prepared by incipient wetness impregnation using [Pt(NH3)4]C12 and [Pd(NH3)4]C12. The total amount of metals was 0.90-0.93% and the Pd/Pt ratio was varied between 6:1 and 1:3. Metal contents and ratios are summarized in Table 1. Pd, Pt, Si and A1 contents were measured by ICP apparatus (Jobin Yvon Ultima ICP-AES), and metal dispersion was determined from the amount of chemisorbed CO. After impregnation the catalysts were dried at 70~ calcined in oxygen stream at 210~ reduced in situ in 1-12before the catalytic tests at 400~ for 12 hours. The experiments were carried out in a high pressure flow apparatus. The 100 cm 3 tube reactor was working without back mixing. The feedstocks were gas oil fractions of different aromatic, sulfur and nitrogen contents. Their most important properties are summarized in Table 2. The catalysts were investigated between 260 and 340~ at total pressures of 30-40 bar, hydrogen to hydrocarbon ratios (in the following H2/HC) of 600-1000 Nm3/m3 and liquid hourly space velocity (in the following LHSV) of 1.0-2.0 h"1. The properties of the feeds and products were determined by test methods according to the standard EN 590:2000 regarding to commercial type diesel fuels, the sulfur content by pyro-fluorescent method (ASTM D 5453), the nitrogen content by pyro-chemiluminescent method (ASTM D 4629) and the aromatic content by high performance liquid chromatography (IP 391:1995). The percentile decrease of aromatic, sulfur and nitrogen contents were determined and defined as HDA, HDS and HDN activities, respectively. Table 1 The metal contents and Pd/Pt ratios of catalysts Catalyst Pd/Pt mass ratio Pd content, % Pt content, % Total metal content, % Dispersion

I

II

III

IV

V

6:1 0.80 0.13 0.93 055

4:1 0.72 0.18 0.90 0.51

2:1 0.60 0.31 0.91 0.48

1:1 0.45 0.46 0.91 0.43

1:3 0.23 0.69 0.92 0.41

589 Table 2 Properties of gas oil feedstocks Properties Density, 15~ kg/m 3 Sulfur, ppm Nitrogen, ppm Total aromatics, % Mono-ring aromatics, % 2+-rings aromatics, % Boiling point, ~ IBP 10% 50% 90% EP

A 832.4 118 77 25.7 21.4 4.3

B 839.2 128 85 28.9 22.3 6.6

Feed C 842.1 139 80 29.2 24.8 4.4

D 843.8 161 81 30.4 25.9 4.5

E 861.9 188 193 38.4 26.8 11.6

201 225 271 334 364

209 226 276 338 365

211 226 278 341 365

218 226 282 344 368

223 240 292 353 373


First the influence of Pd/Pt ratio on the HI)A, HDN and HDS activity was investigated. The results obtained with extreme feeds ("A" and "E") showed that both saturation of aromatics and elimination of nitrogen and sulfur took place, but to different extent. Fig. 1 shows the change of HDA activity as function of the metal ratio of catalysts in case of feed "E" at 280~ 40 bar, H2/HC 800 m3/m3, LHSV = 1.0 h"1. The conversion of total aromatics decreased from about 39.2% to 21.1% as the Pd/Pt ratio increased. Presumably the saturation of aromatic compounds, mainly that of mono aromatics, requires higher hydrogenation activity which is provided by the platinum metal. Fig. 2 shows the HDS and HDN activities as functions of the Pd/Pt ratio. As it can be seen the conversion of sulfur compounds monotonously increased from about 51.2% to 81.2% with increasing Pd/Pt ratio. While the conversion of nitrogen compounds first increased with the Pd/Pt ratio and reaching a maximum (86.1%) at Pd/Pt ratio 2:1 began to decrease. On the base of the experimental results it could be assessed that the maximum of the HDS and HDN activities did not coincide. The explanation of the different change of I-IDS and HDN activities requires further investigations, mainly determination of the types of sulfur and nitrogen compounds being present in the feed and products. Experiments carried out with the other extreme feed "A" provided similar results. According to the results of the preliminary experiments catalyst having Pd/Pt mass ratio of 2:1 is of best performance because the obtained products have sulfur content less than 50 ppm and polyaromatic content less than 2%, satisfying the requirements of the European Union for diesel fuels coming into force by 2005. In addition, this catalyst has the highest HDN activity. The results of the experiments using this catalyst will be discussed in detail. In the next step of investigation the advantageous process parameters of I D A , HDN and HDS with the previously selected catalyst (Pd/Pt ratio is 2:1) were determined.

590 40

35

-...4 r oa

30

9

20

O9

____------q~-------~

--e~

iC40

--A--

iC4 =

--V--

gO4

9

I

X--X~X~X~X-------------- X

0

'

I

50

'

I

100

'

I

150

'

' I

200

'

X

Reaction Time

104

=

--------v~v

--4v-- total C4 = --+-CI-C3

_______._&~&

--X~

05*

./

10

04.

I

250

'

X ~ X I

300

'

b I

350

~'

I

400

'

'1

450

(min)

Figure 2 The yield and selectivity of the products at different reaction time 4. CONCLUSION

The structure of Si species in SAPO-11 changed with Si content and an extreme value for bronsted acid amount can be found. This can be used to explain the extreme value of the catalytic properties. Increasing the impregnation time of the catalyst will promote bifunctional catalyzing reaction such as isomerization and cracking, but weaken the function of supported Pd particles. The acid function and metallic function

609 of the catalyst changed with time on stream in the opposite way, and the catalytic property changes indicate the same trends. All the effect talked above, the effect of the Si content of SAPO-11, preparation condition and reaction time, show the interaction of acid support and supported metal particles of the catalysts.

REFERENCES

1. R.Byggningsbacka, N. Kumar, L.-E. Lindfors, Catal. Lett., 55 (1988) 173 2. B. Didillion, C. Travers, J. P. Burzynski, U.S.Patent 5,866,746 (1999) 3. A. Vieira, M.A. Tovar, C. Pfaff, P. Betancourt, B. Mendez, C. M. Lopez, F. J. Machado, J. Goldwasser, M. M. Ramirez de Agudelo, M. Houalla, J. Molec. Catal. A: Chemical, 144 (1999) 101 4. B. M. Lok, C.A. Messina, R. L. Patton, R. T. Gajek, T. R. Canan, E. M. Flanigen, U.S. Patent 4,440,871



Vapor phase propylene epoxidation over Au/Ti-MCM-41 graRing

611

catalyst: influence o f Ti

A.K. Sinhaa, T. Akitaa, S. Tsubotaa and M. Harutab aEnvironmental Catalysis Research Group, Special Division of Green Life Technology, AIST, 1-831, Midorigaoka, Ikeda 563-8577, Japan bResearch Institute for Green Technology, AIST, Onogawa 16-1, Tsukuba 305-8569, Japan Vapor-phase epoxidation of propylene using H2 and 02 was carried out over gold-Ti-MCM41 catalysts. Gold nanoparticles were homogeneously dispersed on the titanium incorporated MCM41 type of supports by deposition-precipitation (DP) method. Ti was incorporated into the mesoporous MCM-41 framework by (a) one-step method - hydrothermally during synthesis or by post-synthesis grafting and by (b) two-step method - hydrothermal incorporation followed by postsynthesis grafting. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. The Ti-MCM-41 supports prepared by two step titanium incorporation led to higher catalytic activity (after Au deposition) than Ti-MCM-41 prepared by one-step Ti incorporation method, at similar propylene oxide selectivities and hydrogen efficiencies. Presence of more Ti sites isolated from each other is thought to be responsible for the enhanced activity of the catalysts comprising of titanium incorporated by two-step method. 1. INTRODUCTION Propylene oxide (PO) is an industrially important chemical for the manufacture of polyurethane, unsaturated resins, surfactants and other products. Industrially PO is produced using two processes: Chlorohydrin process and Halcon (hydroperoxide) process (1). The former process produces environmentally unfriendly chlorinated organic by-products as well as calcium chloride, while the latter process produces equimolar amounts of co-products and requires heavy capital investment. Extensive efforts are being made since long to develop alternative process for direct gasphase propylene epoxidation using oxygen [2, 3] because it is a promising method of tremendous industrial significance which can replace currently used environmentally disadvantageous chlorohydrin process and hydroperoxide processes. Recently Enichem utilized TS-1 (MFI) as a catalyst for the epoxidation of propylene in the liquid phase using hydrogen peroxide [4-6]. However, due to the very high production cost of H202 and its handling problems it would be highly desirable to produce H202 in-situ. Toso Co. Ltd. [7] has developed Pd/TS-1 catalyst for the in-situ generation of of H202 from H2 and 02. Recently H61derich and coworkers [8] modified the catalyst developed by Toso with Pt and reported that the improved yield of PO could be ascribed to the maintainance of Pd in its +2 oxidation state. Our research work on the catalysis by gold [9-11] has opened a new stage for the direct epoxidation of propylene using H2 and O2. In a series of papers we have reported the vapor phase epoxidation of propylene over highly dispersed nanosize Au particles supported on TiO2, TiO2/SiO2 [ 12, 13] and titanosilicates such as TS- 1, TS-2, Ti-13, Ti-MCM-41 and Ti-MCM-48 [ 15-17]. These findings are now being followed by few other researchers [18-20] and companies [21,22]. But the problems for industrial viability still exist due to low PO yields and low H2 efficiency. Efficiency of supported metal catalysts often depends on metal dispersion and metal-support interaction. The

612 presence of well dispersed tetrahedrally coordinated Ti sites and the Au nanoparticles on the support surface is thought to be necessary for epoxidation activity [15-17] It is possible to incorporate titanium into mesoporous MCM-41 supports during hydrothermal synthesis [23, 24] or by postsynthesis grafting using titanocene [25] or titanium isopropoxide. Titanium incorporation by a twostep method, (1) during hydrothermal synthesis followed by (2) post-synthesis grafting could be a very effective method to generate more number of titanium sites in the MCM-41 structure which are highly dispersed and more accessible to reactant molecules. The present work reports the improvement in PO yield and H2 efficiency of Au catalysts supported on Ti-MCM-41 prepared by the two-step titanium incorporation method. Earlier studies on the gas phase epoxidation of propylene over Au/Ti-MCM-41 (15) have shown low propylene conversions. 2. EXPERIMENTAL Ti-MCM-41 and MCM-41 supports were prepared according to literature procedures [20, 21]. Ti grafting on the MCM-41 and Ti-MCM-41 support surfaces (dehydrated in vaccum at 300~ was performed in a glove box in an inert atmosphere to avoid TiO2 precipitation according to the method of Maschemeyer et al. [22] using (a) titanocene dichloride (TiCC) and (b) titanium isopropoxide (TilPO) corresponding to desired amount of Ti. Titanocene dichloride was dissolved in chloroform and was allowed to penetrate into dried MCM-41 powder in an inert atmosphere. The sample thus treated was exposed in-situ to triethylamine to activate the surface silanol groups of MCM-41. The color of the suspension changed from red via orange to yellow, due to substitution of the chloride with siloxide ligands. After extensive washing with chloroform, the organic components of this material were removed by clacination at 540~ under dry oxygen. For titanium grafting using titanium isopropoxide, MCM-41 sample calcined and vacuum-dehydrated was mixed with a solution of titanium isopropoxide in anhydrous hexane corresponding to the desired atomic % Ti. The mixture stood for 1 h and was then filtered and washed with anhydrous hexane. The grafted material was calcined at 400~ for 4 h in oxygen to convert unreacted alkoxide ligands into Ti-OH groups, and to remove residual isopropyl alcohol and hexane. The MCM-41 and Ti-MCM-41 materials were characterized by XRD (Rigaku R i n t - 2400, Cu-KGt radiation, 40 kV, 40 mA), UV-Vis (Photal Otsuka Electronics, MC-2530 UV/VIS light source), and nitrogen adsorption/desorption, BET surface area measurements (Micromeritics ASAP 2010 apparatus. Gold nanoparticles were deposited on the supports by deposition precipitation method [13-15] using aqueous HAuCI4 solution (corresponding to 2 wt % Au) and NaOH as precipitant followed by calcination in air at 300 ~ The catalytic tests were carried out in a vertical fixed-bed U-shaped quartz reactor (i.d. 10 mm) using a feed containing 10 vol% each C3H6, H2 and 02 diluted with Ar passed over the catalyst (0.15 g) bed at a space velocity of 4000 h-Xcm3/g.cat. The temperature was controlled and measured using a glass tube covered Cr-A1 thermocouple located in the center of the catalyst bed. Prior to testing, the catalysts were first pretreated at 250~ for 30 min. in a stream of 10 vol% H2 in Ar, followed by 10 vol% 02 in Ar streams. The feeds and products are analyzed using on-line GCs equipped with TCD (Porapak Q column) and FID (HR-20M column) detectors and auto injector. 3. RESULTS AND DISCUSSIONS

Table 1 lists the various titanium containing MCM-41 supports, their titanium content, BET surface areas, pore sizes and pore volume. The BET surface areas of the supports with up to 3.0% Ti content was similar (900-1200 m2gl). The BET surface area is found to decrease with increasing amount of Ti grafted onto Ti-MCM-41 sample (samples (3) to (8)). The BJH average pore diameter is found to decrease with increasing amounts of grafted Ti. The post-synthetically grafted Ti is expected to react with the surface silanol groups of the walls in a random fashion in the most accessible sites near the pore mouth and wider pores. As a consequence there is clear decrease in pore size with increasing amount of grafted titanium.

613 Table 1 Surface properties of titanium containing MCM-41 samples. Catalyst [Ti/Si(H)+Ti/Si(G)] Surface area

Pore size

(m2g"1)

(~)

Pore volume

(cm3g-1)

(1) Ti-MCM-41 0.015(H) 1270.4 38.4 1.80 (2) Ti-MCM-41 0.03(H) 1016.6 29.2 0.87 (3) Ti/Ti-MCM-41 0.015(H)+0.015(G) 1192.3 35.0 1.56 (4) Ti/Ti-MCM-41 0.015(H)+0.03(G) 907.3 35.0 0.77 (5) Ti/Ti-MCM-41 0.015(H)+0.04(G) 882.9 29.6 0.72 1029.9 36.0 1.09 (6) Ti-+Ti-MCM-41 0.015(H)+0.0015(a) 903.1 29.8 0.71 (7) Ti-~Ti-MCM-41 0.015(H)+0.03(G) 864.9 27.6 0.68 (8) Ti-->Ti-MCM-41 0.015(H)+0.04(G) 1205 39.4 1.60 (10) Ti-->MCM-41 0.015(G) 906.8 27.2 0.52 (11) Ti--->MCM-41 0.03(G). Ti/Si(H), ratio of hydrothermally incorporated Ti; Ti/Si(G), ratio of Ti grafted. Ti incorporation method: Samples (1), (2) hydrothermally during synthesis; samples(3)-(5) grafting using titanocene dichloride on Ti-MCM-41; samples (6)-(8) Ti grafting using titanium isopropoxide on Ti-MCM-41; samples (9), (10) by Ti grafting on MCM-41.

v

.m

i

c-

~) r-

1

;

; 2

i theta

g

;

7"

!

8

(d e g .)

Figure 1. XRD patterns for the titanium incorporated MCM-41 samples (1), (2), (3), (4), (5), (6), (9) (see Table 1 for the details of titanium composition and incorporation mode).

614 XRD spectra for the various titanium containing MCM-41 samples are shown in Figure 1. Powder XRD analysis showed that both pure MCM-41 and pure Ti-MCM-41 maintain their structure and crystallinity after Yi grafting. The samples exhibited well defined (1 0 0) reflection. There is slight decrease in the intensity of the higher order peaks which could be due to slight decrease in long range order after Ti grafting. The samples after Ti grafting also show a slight shift in the XRD peak position to higher 2-theta values.No higher order reflections could be seen, indicating the absence of bulk (> 1000 A particle size) anatase. UV-Vis spectra of the titanium containing MCM-41 samples are shown in Figure 2. The UVVis analysis of these samples show a band near 220 nm range due to tetrahedrally coordinated Ti. With increasing Ti content in the samples the UV-vis spectra was found to become broader at higher wavelength region probably due to formation of Ti-O-Ti clusters. Generally a shoulder a t - 3 3 0 nm

O

4.5 %) into the MCM-41 supports may cause the formation of Ti-O-Ti type clusters as suggested by broadening of the UV-Vis spectra (Figure 2), resulting in lower PO selectivity (catalysts # (5) and (8)) and more CO2 formation. 4. CONCLUSIONS Two-step titanium incorporation (hydrothermally, during crystallization followed by postsynthesis grafting) into mesoporous MCM-41 results in a catalyst support with more isolated/dispersed Ti sites. Gold nanoparticles deposited on such Ti containing support, by DP method, show better propylene epoxidation activity, better hydrogen efficiency and less deactivation than Ti-MCM-41 supports prepared by one-step titanium incorporation method, either hydrothermally or by post-synthesis grafting. The improved activity could be attributed to the more number of isolated Ti sites which are partly used as sites for Au nanoparticle deposition and may stabilize hydroperoxo species (formed at Au sites) that react with the propylene adsorbed on the silica matrix of the catalyst support surface. ACKNOWLEDGEMENT A. K. Sinha gratefully acknowledges the financial support in the form of STA Fellowship from Science and Technology Agency of Japan. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

S.L. Ainsworth, Chem. Eng. News 9 (1992); M. Mccoy, Chem. Eng. News 19 (2001). K. Murata and Y. Koyozumi, Chem. Commun., (2001) 1356. H. Orzesek, R. P. Schulz, U. Dingerdisses, W. E. Maier, Chem. Eng. Technol., 22 (1999) 8. M.G. Clerici, G. Bellusi and U. Romano, J. Catal., 129 (1991) 159. B. Notari, Catal. Today, 18 (1993) 163. E. Occhiello, Chem. Ind., 761 (1997). A. Sato, T. Miyake and T. Saito, Shokubai (Catalysts), 34 (1992) 132. R. Meiers, U. Dingerdissen and W. F. H61derich, J. Catal., 176 (1998) 376 M. Haruta, Catalysis Surveys of Japan, 1 (199&) 61 and references therein. M. Haruta, Catal. Today, 36 (1997) 123 and references therein. M. Haruta, Stud. Surf. Sci. Catal., 110 (1997) 123 and references therein. T. Hayashi, K. Tanaka and M. Haruta, Shokubai, 37 (1995) 72. T. Hayashi, K. Tanaka and M. Haruta, J. Catal., 178 (1998) 566. Y.A. Kalvachev, T. Hayashi, K. Tanaka and M. Haruta, Stud. Surf. Sci. Catal.,

618 110 (1997) 965. 15. M. Haruta, B. S. Uphade, S. Tsubota and A. Miyamoto, Res. Chem. Intermed. 24 (1998)329. 16. B.S. Uphade, M. Okumura, S. Tsubota and M. Haruta, Appl. Catal. A: Gen., 190 (2000) 43. 17. B.S. Uphade, Y. Yamada, T. Nakamura and M. Haruta, Appl. Catal. A: Gen., 215 (2000) 137. 18. T.A. Nijhuis, H. Huizinga, M. Makkee and J. A. Moulijn, Ind. Eng. Chem. Res. 38 (1999) 884. 19. E.E. Stangland, K. B. Stavens, R. P. Andres and W. N. Delgass, J. Catal., 191 (2000) 332. 20. G. Mul, A. Zwijnenburg, B. van der Linden, M. Makkee and J. A. Moulijn, J. Catal. 201 (1) (2001) 128. 21. R. G. Bowman, H. W. Clark, J. J. Maj, G. E. Hartwell, PCT/US97/11414, PCT Pub. No. WO 98/00413 (1998). 22. T. Hayashi, M. Wada, M. Haruta and S. Tsubota, Jpn. Pat. Pub. No. H 10-244156, PCT Pub. No. WO97/00869, U.S. Patent 5,932, 750 (1999). 23. A. Corma, M. T. Navarro and J. Perez-Parieme, J. Chem. Soc., Chem. Commun. 147 (1994). 24. P.T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature 368 (1994) 321. 25. T. Maschmeyer, F. Ray, G. Sankar and J. M. Thomas, Nature 378 (1995) 159. 26. F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. Leofanti, G. Petrini, Catal. Lett. 16 (1992) 109. P. E. Sinclair, G. Sankar, C. Richard, A. Catlow, J. M. Thomas, T. Maschmeyer, J. Phys. Chem. B 101 (1997) 4232.


619

Intrinsic activity of titanium sites in TS-1 and Al-free Ti-Beta U. Wilkenh6ner 1, D.W. Gammon:, E. van Steen 1. Catalysis Research Unit, ~Dept. Chemical Engineering, 2Dept. of Chemistry, University of Cape Town, Private Bag, Rondebosch 7701, South Africa; E-mail: [email protected] Dedicated to Professor Jens Weitkamp on the occasion of his 60th birthday The intrinsic activity of Ti-sites in the pores of TS-1 and Al-free Ti-Beta in the phenol hydroxylation with water and methanol as a solvent is estimated using the Thiele-modulus approach. The observed kinetic data were modelled using 2na order rate expressions. The intra-crystalline diffusivity of phenol was determined using Zero Length Column chromatography. The intrinsic rate constant for phenol consumption with water as a solvent over TS-1 is smaller than the one over Al-free Ti-Beta. Due to strong selective adsorption of phenol with water as a solvent, definite conclusions on the intrinsic activity of Ti-sites in these two titano-silicates with water as a solvent cannot be made. Selective adsorption is not important with methanol as a solvent. Based on the intrinsic activity with methanol as a solvent, it can be concluded that titanium sites in TS-1 are more active than those in Ti-Beta.

1. INTRODUCTION Crystalline titanium substituted silicates, such as TS-1 and Ti-Beta, are a well-known class of materials for selective oxidation using peroxides as the oxidant. Although the Ti-O bond length in both TS-1 and Ti-Beta is identical, force field calculations showed that the bond angles differ [ 1]. This can potentially lead to a different intrinsic activity of the active Ti-site. It must, however, be kept in mind that the force field calculations were performed for a titanium site in perfect tetrahedral coordination. In freshly calcined, dehydrated TS-1 titanium is in a tetrahedral coordination [2]. Under reaction conditions [3] and in the presence of water or alcohols [4] the coordination of titanium in titano-silicates such as TS-1 increases. The active sites in phenol hydroxylation are titanium peroxide species, which are formed upon hydrolysis of the Ti-O-Si bonds [5]. It has been shown that TS-1 is more active for the epoxidation of linear olefins than Al-free Ti-Beta [6-8]. However, it was recognised that diffusional constraints may exist in the epoxidation of 1-octene over Ti-Beta [9]. Diffusional limitations are well known for phenol hydroxylation [10,11] and anisole hydroxylation [12] over both TS-1 and Al-free Ti-Beta. In this study the intrinsic activity of the titanium sites in TS-1 and Al-free Ti-Beta for phenol hydroxylation are estimated taking diffusional constraints into account.

620 2. EXPERIMENTAL SECTION In the experiments a number of TS-1 samples and Ti-Beta samples of different crystallite sizes were used, viz. TS-I: 0.1 l.tm, 3x10x45 ~tm (diffusional path length ca. 31xm), Ti-Beta: 0.9 ~tm, 2-5 ~m. Small crystallites of TS-1 (Si/Ti=33) were synthesised according to the method described by Thangaraj et al. [13]. Large crystals ofTS-1 (Si/Ti=33) were synthesised according to the procedure described by Milestone et al. [14]. Large crystals of Al-free TiBeta (Si/Ti=40) were synthesised using the procedure adopted by Blasco et al. [8]. The synthesis procedure for small Al-free Ti-Beta crystals is identical to the one for the large crystals except for the addition of nano-sized, dealuminated zeolite Beta seeds (obtained by acid washing of nano-sized zeolite Beta crystals at 358K for 24hrs). All materials were shown to be crystalline (XRD). Scanning electron micrographs of the resulting crystals are shown in Figure 1. The absence of amorphous TiO2 in these materials was shown using DR-UV/VIS.

TS-1 (dcrysta~ca. 0.1 ~m)

AI-free Ti-Beta (dcrystat ca. 0.9 p,m)

TS-1 (3x10x45 ~m)

AI-free Ti-Beta

(dcrystal--

2-5 l~m)

Figure 1" Scanning electron micrographs of the titano-silicates (TS-1 and Al-free Ti-Beta) used in this study (note different magnifications)

621 In order to estimate the activity in the pores of TS-1, the external surface of the TS-1 ( d ~ l ~ 0.1 ~m) was inertised by cyclic, low temperature deposition of tetra-ethoxysilane (20 cycles) [5]. It was shown that this technique leads to the inertization of the external surface of TS-1 without significantly affecting access to the pore mouth [11 ]. The external surface of the other titano-silicates is sufficient small in comparison to the total surface area so that their contribution to the overall activity can be neglected. The diffusivity of phenol was determined using zero length column chromatography (ZLC) [15]. The transient desorption of phenol was determined at temperatures between 273K and 353K with water and methanol as a solvent using the large crystals of TS-1 and Al-free TiBeta (70 mg). Flow rates were varied between 0.5 and 3 ml/min to ensure that the response curves were not governed by interstitial fluid hold-up or axial dispersion. The independence of the transient desorption of concentration was confirmed by variation of the phenol concentration. Although the adsorption constant can principally be obtained from ZLC-experiments, the adsorption was also obtained from partition chromatographic measurements [ 16]. Phenol hydroxylation (12.8 mmol phenol) was carried out in a small batch reactor at 333K using 30% H202 in water (0.6ml; 5.3 mmol H202) and 0.2g catalyst in 5 ml solvent (water or methanol). The concentration profile of phenol, hydroquinone, catechol and H202 were fitted with second order rate expressions, from which the observed rate constant for the phenol consumption can be determined [5,11 ].

3. RESULTS AND DISCUSSION 3.1 Intra-crystalline diffusivity and adsorption of phenol in TS-I and Al-free Ti-Beta The effective diffusivity of phenol with water and methanol as a solvent was determined using the large crystals of TS-1 and Al-free Ti-Beta. Figure 2 shows the temperature dependency of the intra-crystalline diffusivity of phenol with water as a solvent in TS-1. The intra-crystalline diffusion coefficients are about a magnitude of order larger than the value reported previously (210 "18 m2/s at 333K [10]). The literature value was determined from kinetic experiments with different crystal sizes of TS-1, i.e. under reaction conditions, in the presence of the solvent, 1-120, and H202. From the Arrhenius plot, the activation energy is estimated to be of 14.8 kJ/mol. This compares well with the reported activation energy 14.7 kJ/mol for the di~sion of phenol in NaX with water as a solvent [ 14]. The diffusivity of phenol in the pores of crystalline titano-silicates is dependent on both the solvent and the pore structure (see Table 1). The effective diffusivity of phenol with water as a solvent is 4-8 times slower than with methanol as a solvent. The effective diffusivity of phenol diffiasivity in the large pore titano-silicate Ti-Beta is 3-8 times larger than in the medium pore TS-1. The Henry coefficients extracted from ZLC experiments compare well with those determined using partition chromatography. Strong selective adsorption of phenol is observed with water as a solvent. The strong adsorption of phenol in TS-1 with water as a solvent is not surprising since the TS-1 framework is known to be hydrophobic and thus less polar molecules will be preferentially adsorbed. Al-free Ti-Beta is obviously less hydrophobic due to diminished wallsorbent interaction. With methanol as a solvent the concentration in the pores is almost identical to that in the bulk liquid.

622 1E-15

E ch I-c

":- 1E-16 0 e~ e-

a 1E-17

I

2.5

I

I

3

I

I

3.5

4

10001T, K -1 Figure 2:

Temperature dependency of intra-crystalline diffusivity of phenol in TS-1 with water as a solvent

Table 1"

Effective diffusivity as determined using ZLC and the relative strength of adsorption determined using partition chromatography in TS-1 and Al-free Ti-Beta

Dr phenol, m2/s Solvent T, K 5.2 10"17 H20 303 4.4 10"16 CHaOH 333 Al-free Ti-Beta H20 303 3.9 10-16 CHaOH 333 1.5 10"15 1 Henry's constant extracted from ZLC experiments 2 Henry's constant determined using partition chromatography Catalyst TS-1

K1 77.7 0.61

K2 84.3 0.66 17.2 1.31

The much slower diffusivity of phenol with water as a solvent as compared to the diffusivity with methanol as a solvent might be attributed to the difference in phenol-solvent interaction and the phenol-phenol interaction. The interaction between phenol and water in the pores of TS-1 and Al-ffee Ti-Beta is poor, since water adsorbs much less strong in the pores. Thus, the main interaction with water as a solvent will be the phenol-phenol interaction slowing down the diffusion of phenol out of the pores of the titano-silicate. With methanol as a solvent phenol in the pores of the titano-silicate will be surrounded by methanol molecules.

3.2 Phenol hydroxylation using TS-I and AI-free Ti-Beta It is well known that phenol hydroxylation using TS-1 [5, 10-12] and Ti-Beta [11,12] is strongly mass transfer limited. Figure 3 shows the effect of the crystal size of TS-1 and AIfree Ti-Beta on the phenol conversion with water as a solvent as a function of reaction time. It can be clearly seen that the small crystals of TS-1 are less active than the larger crystal of Ti-

623

Beta. This is only observed when the external surface of TS-1 is inertised and is not contributing to the overall consumption of phenol [5]. It can further be seen that phenol hydroxylation with water as a solvent over Al-free Ti-Beta is clearly mass transfer limited.

20

20

TS-

AI-free Ti-Beta (ca. 0.9 pm) I

O

0

Al-.free Ti-Beta (2-5 pro)

.,..

....

U; L

I/1 L

>~

~o~ ~10 _.o

m

o

C

[]

a. Jr

9

0

,

9

100

,

9

200

i

300

Reaction time, min

Figure 3:

i

400

100

200

3(X)

Phenol hydroxylation with water as a solvent over TS-1 (left; O: d~stal ca. 0.1 txm; O: 3x10x45 ktm) and Al-free Ti-Beta (fight; I1: d r ca. 0.9 lxm; [-i: 2-5 ~tm). Reaction conditions: T=333K; 5.6 ml I-/20; rn~t=0.12g; Cph=o~,0=2.5mol/;%, CH2o2,0= 0.83 mol/~,. Solid lines represent fit to second order rate expressions.

The kinetic data were fitted with second order rate expressions [ 11 ]: phenol + H202 --> hydroquinone/catechol/tar + H20 r1 = k 1 -Cphenol -CH202 products + H202

--> tar + H20

r2 = k 2 -Cproducts -CH202

2H202

->

r3 - k 3 - C 2

2H20+ 89

H202

Thus, the rate of consumption and the change in the phenol and H202 concentration can be expressed as: dCphenol

-

d ~

= k 1" C phenol" C H 2O2

_ d C H 2 0 2 = k 1 9Cphenol "CH202 + k 2 "Cproducts "CH202 + k 3 .C 2 dt

4OO

Reaction time, min

H202

The observed rate constants were determined by fitting these expressions to the measured concentrations of phenol, hydroquinone, catechol and H202 (tar formation was monitored over the mass balance). The observed rate constants for the conversion of phenol (kl) using either water or methanol as a solvent over TS-1 and Al-free Ti-Beta are given in Table 2. The observed activity of both TS-1 and Al-free Ti-Beta is higher with water as a solvent than with methanol as a solvent. Phenol adsorbs preferentially when water is present (see Table 1). This would lead to a high concentration of phenol in the pores and thus enhance the rate of phenol conversion, since phenol hydroxylation is mass transfer limited.

624 Table 2:

Observed rate constants for phenol conversion (kl in L/(molhr'g)) in phenol hydroxylation at 333K TS-1

Solvent 0.1 gtm~ 1-120 2.75 CH3OH 1.26 1 External surface inertised

Al-free Ti-Beta 0.9 l.tm ca. 3 l~m 2.46 1.29 0.45 0.40

3x10x45 l.tm 0.21 0.34

Using the experimentally determined diffusion coefficients (see Table 1), the effectiveness factor can be estimated (see Figure 4). For all titano-silicates a spherical geometry was chosen, except for the large crystals of TS-1, which were approximated as slab-shaped particles. Small crystallites of TS-1 (d~,~tal ~ 0.1 lxm) have an effectiveness factor close to one for the phenol hydroxylation with both solvents. The phenol hydroxylation with methanol as a solvent approaches an effectiveness factor of equal to one for Al-free Ti-Beta crystals with a crystal size of ca. 0.9 lam. A direct comparison of the observed rate constant for phenol consumption over small crystals of TS-1 and small crystals of Al-free Ti-Beta leads to the conclusion that TS-1 is more active per gram oftitano-silicate.

1.0

1.0

A

0.8 m M

"6

o.6

=

0.6

~ 0.4

~.0.2 w 0.0 0.1

~ 0.2

1

10

Thiele m o d u l u s ,

Figure 4:

100

solvent: CH30H

0.8

9 0.4

.>

m

0.0 0.01

i

t

.,,.

0.1

1

10

100

Thiele m o d u l u s ,

Effectiveness factor of titano-silicates in phenol hydroxylation with water (left) or methanol (fight) as a solvent as a function of Thiele modulus (@: TS-1, d ~ l ca. 0.1 l.tm; O: TS-1, 3x10x45 ~tm; m: Al-free Ti-Beta, d~y~l ca. 0.9 ~tm; VI: Al-free Ti-Beta 2-5 lam). Reaction conditions: T=333K; 5 ml solvent; rn~ = 0.12g; Cph~ol,0= 2.5mol/1, CH2o2,0= 0.83 mol/1 - H202 added as 30% H202 in 1-120.

Knowing the effective diffusivities (also at reaction temperature) and the observed rate constants the effectiveness factor for the phenol hydroxylation can be determined. From these data the intrinsic rate constants can be obtained. With water as a solvent, the intrinsic 2"a order rate constant is lower with TS- 1 (2.8 L/(molhrg) corresponding to 1.6 L/(molsmol Ti-sites)) than with Al-free Ti-Beta (3.6 L/(molhrg) corresponding to 2.5 L/(mol's'mol Ti-sites)).

625 The diffiasivity used to estimate the intrinsic activity of the titanium sites was based on ZLC-experiments. This yielded a diffusivity of phenol in water, which is ca. 40 times larger than the estimate by van der Pol et al. [10]. In our experiments the diffiasivity was estimated using TS-1 pre-equilibrated in H20/phenol mixture. The diffusivity estimated by van der Pol et al. [10] was based on kinetic experiments. The coordination sphere of the titanium sites may differ in the ZLC-experiments from those under reaction conditions [2-4]. A change in the coordination of titanium is expected to affect the diffusivity in TS-1 more severely than the dif~sivity of phenol in Al-free Ti-Beta. Taking the diffusivity of phenol in TS-1 as reported by van der Pol et al. [10], the intrinsic 2"~ order rate constant with TS-1 equals 4.9 E/(molhrg) (corresponding to 2.8 E/(molsmol Ti-sites)). It should however be kept in mind that the intrinsic 2nd order rate constants are calculated using bulk concentrations. Any selective adsorption effect will falsify the possible conclusion. Phenol is selectively adsorbed with water as a solvent. Furthermore, phenol is more strongly adsorbed in TS-1 than in M-free Ti-Beta. Phenol hydroxylation requires the access of two reagents to the active site, viz. phenol and H202. Due to the selective adsorption the concentrations of phenol and H202, within the pores will differ significantly from those in the bulk of the fluid. In order to estimate the intrinsic activity of the titanium sites in TS-1 and AIfree Ti-Beta for phenol hydroxylation with water as a solvent, the adsorption of H202 needs to be taken into account, which is unknown. With methanol as a solvent effectiveness factors of approximately 1 were obtained for phenol hydroxylation over small TS-1 crystals and relatively small Al-free Ti-Beta crystals. A direct comparison of the observed activity does show that titanium sites in TS-1 are more active for phenol hydroxylation with methanol as a solvent than those in Al-free Ti-Beta. Furthermore, selective adsorption does not seem to play a significant role with methanol as a solvent. Hence, the intrinsic activity of the titanium sites can be estimated from phenol hydroxylation with methanol as a solvent. Based on the intrinsic rate constants with methanol as a solvent (TS-I: 1.27 L/(molhrg) corresponding to 0.7 E/(molsmol Ti-sites); Ti-Beta: 0.45 E/(molhrg) corresponding to 0.3 M(molsmol Ti-sites)), it can be concluded that the titanium sites in TS-1 are intrinsically more active. The intrinsic activity of titanium sites in TS-1 can even be larger than the one reported here, if the coordination of the titanium sites under reaction conditions is larger than under ZLC conditions. This would lead to a reduced diffusivity of phenol in TS-1 under reaction conditions, whereas the diffiasivity of phenol in Al-free Ti-Beta would hardly be affected. 4. CONCLUSIONS The intrinsic activity of titanium sites in TS-1 and Al-free Ti-Beta for phenol hydroxylation was estimated using the diffiasivity of phenol with water and methanol as a solvent in TS-1 and Al-free Ti-Beta as determined by ZLC and kinetic experiments. The kinetic experiments were evaluated using 2"a order rate expressions. The intrinsic activity was then estimated using a Thiele-modulus approach. The intrinsic activity of the titanium sites in TS-1 are at least twice as active as the titanium sites in Al-free Ti-Beta for the phenol hydroxylation with methanol as a solvent. With water as a solvent this approach fails due to the strong selective adsorption of phenol in TS-1 and to a lesser extent in Al-free Ti-Beta.

626 ACKNOWLEDGEMENT

Financial support for this study from Sasol, AECI, NRF, DTI (via THRIP) and UCT is gratefully acknowledged.

REFERENCES:

1. G. Sastre and A. Corma, Chem. Phys. Lett. 302 (1999), 447-453. 2. S. Pei, G.W. Zajae, J.A. Kaduk and J. Faber, Catal. Lett. 21 (1993), 333-344. 3. V. Bolis, S. Bordiga, C. Lamberti, A. Zeeehina, A. Carati, F. Rivetti, G. Spano and G. Petrini, Mieroporous and Mesoporous Mat. 30 (1999), 67-76. 4. E. Astorino, J.B. Peri, R.J. Willey and G. Busea, J. Catal. 157 (1995), 482-500. 5. U. Wilkenh6ner, G. Langhendries, F. van Laar, G.V. Baron, D.W. Gammon, P.A. Jaeobs and E. van Steen, J. Catal. 203 (2001), 201-212. 6. C.B. Dartt and M.E. Davis, Appl. Catal. A 143 (1996), 53-73. 7. J.C. van der Waal, P.J. Kooyman and J.C. Jansen, Mieroporous and Mesoporous Mat. 25 (1998), 43-57. 8. T. Blaseo, M.A. Camblor, A. Corma, P. Esteve, J.M. Guil, A. Martinez, J.A. PerdigonMelon and S. Valencia, J. Phys. Chem. B 102 (1998), 75-88. 9. J.C. van der Waal, Phi) thesis, Technical University Deltt (1998). 10. A.J.H.P. van der Pol, A.J. Verduyn and J.H.C. van Hooff, Appl. Cat. A 92 (1992), 113130. 11. U. WilkenhOner, PhD thesis, University of Cape Town (2002). 12. U. WilkenhOner, D.W. Gammon and E. van Steen, Stud. Surf. Sei. and Cat. 135 (2001), 368. 13. A. Thangaraj, M.J. Eapen, S. Sivasanker and P. Ratnasamy, Zeolites 12 (1992), 943-950. 14. N.B. Milestone and N.S. Sahasrabudhe, Proe. 12th Int. Zeolite Conf. (M.M.J. Treaey, B.K. Marcus, M.E. Bisher, J.B. Higgins, Eds.), Vol. 3, p. 1901-1908, Materials Research Society, Warrendale (1990). 15~ D.M. Ruthven and P. Stapleton, Chem. Eng. Sei. 48 (1993), 89-98. 16. G. Langhendries, D.E. De Vos, G.V. Baron and P.A. Jaeobs, J. Catal. 186 (1999), 1-11. 17. F. Awum, S. Narayan and D. Ruthven, Ind. Eng. Chem. Res. 27 (1988), 1510-1515.


627

The effect of zeolite pore size and channel dimensionality on the selective acylation of naphthalene with acetic anhydride JiN 12ejka1, Pavla Proke~ovfi 1, Libor (;erven~ and Katefina Mikulcovfi 2 1j. Heyrovsk) Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-182 23 Prague 8, Czech Republic 2 Department of Organic Technology, Institute of Chemical Technology, Technick/t 5, CZ-166 28 Prague 6, Czech Republic Acylation of naphthalene was investigated over different large pore zeolites with the aim to synthesise selectively 2-acetylnaphthalene as the demanded product for various perfume and flavour compositions. Acetic anhydride was found to be the proper acylating agent while acetyl chloride and acetic acid exhibited very low acylation activity. The highest acetylnaphthalenes yield was achieved with zeolite Beta possessing medium concentration of acid sites and excess of naphthalene together with stepwise adding of acetic anhydride to the reaction mixture.

1. INTRODUCTION Acylation reactions represent the most important method in organic chemistry to synthesize aromatic ketones, which are at present important intermediates for the production of fine chemicals. The reaction usually proceeds via interaction of various aromatic hydrocarbons with chlorides or anhydrides of relevant carboxylic acids at the presence of proper catalyst. The conventional catalysts like Lewis acids (e.g. A1C13, BF3, ZnC12) or Broensted acids (HF, H3PO4) have been usually employed to catalyze these acylation reactions. However, the technical and environmental drawbacks connected to the utilization of these catalysts are evident. More than stoichiometric amount of Lewis acid is required to catalyze effectively acylation reactions, which leads to the formation of stable complex of product with Lewis acid. The decomposition of this complex leads to the formation of pure product, however, Lewis acid is destroyed without the possibility of the catalyst regeneration (1). This is the main reason while during the last decade a number of contributions appeared showing that various solid acids can be successfully employed to catalyze acylation reactions (2). Among them zeolites played a significant role as highly active and selective heterogeneous catalysts for acylation of different aromatic hydrocarbons. Particularly, acylation of anisole, toluene or 2-methoxynaphthalene were investigated with a significant effort over medium and large pore zeolites. Rohan et al. (3) acylated anisole to para-methoxyacetophenone and showed that the critical issue of this reaction

628 was the easy deactivation of the zeolite catalyst probably due to the formation of various bulky compounds inside the zeolite channel system. The shape-selectivity was successfully used in the case of 2-methoxynaphthalene acylation to desired 6-acetyl-2- methoxynaphthalene, which is of a particular interest for the production of the anti-inflammatory drug Naproxen. It was shown that 1-acetyl-2-methoxy naphthalene was mainly achieved in experiments carried out in the kinetic regime, while 2,6- and 2,8-isomers were favoured by thermodynamic factors (4). While the activity of zeolites in acylation of 2-methoxynaphthalene decreased in the following order Y > Beta > Mordenite > ZSM-12, zeolite Y preferred mainly the formation of 1-acetyl-2- methoxynaphthalene. The channel system of ZSM-12 was expected to be too small to accommodate acetyl-methoxy-naphthalenes (5). On the other hand zeolite Beta and mordenite exhibited high selectivity towards desired 6-acetyl isomer (6,7). Based on these numerous studies it is evident that not only the choice of the proper zeolite catalyst and reaction conditions plays very important role to achieve high conversion and selectivity but also the utilization of proper solvent and its concentration are key parameters for optimum zeolite behaviour. Recently, Rhodia company announced a new industrial process for acylation of anisole to para-acetylanisole employing zeolite catalyst (8). In this contribution the effect of zeolite pore size and dimensionality on the activity and selectivity of zeolites Y, Mordenite, ZSM-12, Beta and zeolite L is investigated in the acylation of naphthalene. Acetic anhydride, acetyl chloride and acetic acid were tested as acylating agents. In addition, the role of the type and concentration of individual acid sites on their catalytic behaviour is discussed. The interesting product, 2-acetylnaphthalene is widely used in perfume compositions, mainly in Neroli Orange Blossom, Magnolia, Honeysuckle, Jasmine and also in flavour compositions in imitation of Strawberry, Grape, various Citrus and berry-compositions (9). 2. EXPERIMENTAL S E C T I O N 2.1. Zeolites and methods used

Large pore zeolites Y (Si/A1 = 2.8), Mordenite (Si/A1 = 10.0), Beta (Si/A1 - 12.5-75), ZSM-12 (Si/A1 = 58) and zeolite L (Si/A1 = 3.2) were investigated in the naphthalene acylation with acetic anhydride. Zeolite Y was obtained from the Research Institute for Oil and Hydrocarbons, Bratislava (Slovak Republic), ultrastabilized zeolites Y, Mordenite and Beta were purchased from Zeolyst (USA). ZSM-12 and zeolite L were synthesised in our laboratory. The crystallinity and phase purity were checked by X-ray powder diffraction (Siemens D5005) with CuKa radiation in Bragg Brentano geometry and the size of the crystals by scanning electron microscopy (Jeol). The concentration and type of acid sites were determined by FTIR spectroscopy (Nicolet Protege 460) using self-supporting wafers d3-acetonitrile as probe molecule. The characteristics of zeolites used are depicted in Table 1.

629 2.2 Catalytic experiments Catalytic experiments were carried out in a 50 ml glass vessel equipped with a reflux condenser and magnetic stirrer. The reaction vessel was located in an oil bath with controlled temperature, all experiments performed in this study were carried out at 135 ~ In the typical experiments naphthalene (1.54 M), internal standard (tetradecane) and a solvent (mixture of cis- and trans-decaline) were mixed and heated in the reaction vessel to the reaction temperature. If not indicated elsewhere, the typical naphthalene to acetic anhydride molar ratio used was 2 : 1. After that an activated zeolite catalyst was added to the reaction mixture. Zeolite activation was performed at 450 ~ in a stream of air for 90 min followed by zeolite cooling to the ambient temperature in the dessicator. Finally, acetic anhydride was given to the reaction mixture and the experiment started. The tests with acetic acid or acetyl chloride were carried out in the same way. To follow the time-on-stream values of conversion and selectivity, small amounts of reaction mixture were taken away at preset time values.

Reaction products were analyzed using a gas chromatograph equipped with flame ionization and mass spectrometric detector (HP 6850 with an autosampler Agilent 7683) employing a high-resolution capillary column HP-1 (length 30 m, internal diameter 0.32 mm). In addition, reaction products were identified by gas chromatograph combined with mass spectrometer (HP 5890 Series I I - 5971A).

3. RESULTS AND DISCUSSION In our previous paper (10) we have shown that large pore zeolite catalysts can selectively catalyze naphthalene acylation with acetic anhydride to 2-acetylnaphthalene, which is a desired product for the preparation of various perfume and flavour compositions. The individual zeolite catalysts differed in acetylnaphthalenes yield (ratio of acetylnaphthalenes obtained related to the theoretical concentration of acetylnaphthalenes), selectivity and the resistance against deactivation. Therefore, now we tried to optimize the reaction conditions, type of acylating agent and the ratio between naphthalene and acylating agent with the aim to increase the long term stability of the zeolite and the selectivity to 2-acetylnaphthalene. The main characteristics of zeolites investigated, including dimensions of their pores, Si/A1 ratios and acetylnaphthalenes yields and selectivities to 2-acetylnaphthalene in naphthalene acylation with acetic anhydride are summarized in Table 1. 3.1. Effect of concentration of active sites The data in Table 1 clearly shows that there is a significant effect of the concentration of acid sites of zeolites in naphthalene acylation with acetic anhydride. It seems that this effect is reflected in the acetylnaphthalene yields despite the role of zeolite structure. The highest yields were obtained with zeolites possessing medium concentration of acid sites (e.g. for zeolite Beta/3 about 24-26 % under standard reaction conditions). The increase in the concentration of acid sites (zeolites Y and L, Mordenite, Beta/l) led to the significant decrease in the acetylnaphthalenes yield. It is seen from Table 1 that the maximum acetylnaphthalenes yield achieved with zeolites possessing the Si/A1 ratio lower than 10 was less than 3 % in maximum, in contrast to zeolites with Si/A1 ratio between 35-55. In the later case the acetylnaphthalenes yield reached more than 20 %

630 (Beta/3 and/4). On the other hand, further decrease in the concentration of acid sites for zeolite Beta/5 (Si/A1 = 75) resulted in a decrease in the acetylnaphthalenes yield to about 11-12 % (Fig. 1). Similar results were found with zeolite Y and its ultra-stabilized forms. While the acetylnaphthalenes yield for zeolite Y was about 1 % , USu and/2 exhibited significantly higher yields around 11-12 % (Fig. 2). On the other hand, the selectivity to 2-acetylnaphthalene was significantly lower in comparison to zeolite Beta (32-34 % and 72-80 %, respectively). To examine the effect of the type of acid sites on acetylnaphthalenes yield zeolites Beta/1 and/6 were compared. While Beta/1 possesses about 30 % of Broensted and 70 % of Lewis acid sites, Beta/6 (calcined in the stream of ammonia) exhibits about 70 % of Broensted and 30 % of Lewis acid sites. It is seen from Table 1 that slighly higher acetylnaphthalenes yield was observed for Beta/1 having more Lewis sites and lower total concentration of acid sites. It indicates very significant role of adsorption and desorption or even internal diffusion processes in this reaction, which overcome the possible effect of different type of acid sites. On the basis of these data it can be inferred that there exists some optimum concentration of acid sites to catalyze this acylation reaction with high acetylnaphthalenes yield. When high concentration of acid sites increases the rate of acetic anhydride decomposition and subsequent reactions, which led to the formation of various hydrocarbon deposits and in fact to the deactivation of the zeolite. The decomposition of acetic anhydride provided also the formation of acetic acid. It was

Table 1 Characteristics of zeolites used, acetylnaphthalene yields and selectivities to 2acetylnaphthalene in naphthalene acylation with acetic anhydride Zeolite Channel Channel Si/A1 ACN yield c (%) 2-ACN a system diameter (nm) ratio Selectivity (%) ZSM-12 1D 0.57 x 0.61 58 < 1.0 n.d. Zeolite L 1D 0.71 3.2 < 1.0 n.d. Zeolite Y 3D 0.74 2.8 < 1.0 n.d. USY/1 3D 0.74 15 11.4 34 USY/2 3D 0.74 40 11.5 32 Mordenite 1D a 0.67 x 0.70 10.0 2.7 73 H-beta/1 3D 13.4 7.3 79 H-beta/2 3D 17.5 9.0 77 H-beta/3 3D 0.76 x 0.64 37.5 24-26 75 H-beta/4 3D 0.55 x 0.55 55 23.3 73 H-beta/5 3D 75 11.7 74 H-beta/6 b 3D 13.3 3.8 71 a 8 membered ring of mordenite is too small to accommodate naphthalene molecules, b sample calcined in a stream of ammonia to increase the concentration of Broensted sites, c ACN yield - yield of acetylnaphthalenes to their theoretical amount, which can be formed based on naphthalene to acetic anhydride molar ratio at reaction time = 120 min, d 2-ACN selectivity- at reaction time = 120 rain (maximum acetylnaphthalenes yield).

631

30 25 "o" 20 ~,9 15

5 13,4

18,8

37,5

55,4

75

Molar ratio Si/AI

Fig. 1 Dependence of acetylnaphthalenes yield on Si/A1 ratio for zeolite Beta in acylation of naphthalene with acetic anhydride (Experimental conditions: 1.2 g of catalyst, naphthalene/acetic anhydride molar ratio=2, reaction time 120 min, temperature 135 ~

reported by Servotte et al. (11) that acid forms of zeolites catalyze easily the transformation of acetic acid into a large number of various hydrocarbons which causes the formation of bulky compounds, which cannot be easily desorbed from the zeolite channel system. This results in the fast deactivation of the respective zeolite catalyst. However, when only a small amount of acid sites is available acetylnaphthalenes yield again is rather low. In addition, it is necessary to emphasize the role of hydrophobicity-hydrophilicity of the zeolite used for adsorption/desorption and diffusion processes. We have shown (10) that the presence of more polar solvent (sulfolane) led to a lower conversion compared to decalin. This can affect also the concentration of available acid sites for the reaction. The higher is the concentration of acid sites, the slower is the desorption and transport of the reaction products. This is probably the second reason why some optimum concentration of acid sites is needed to achieve a good adsorption/desorption balance which favours high acetylnaphthalenes yield in this reaction. Fig. 3 depicts the effect of the ratio of naphthalene to acetic anhydride on the resulting acetylnaphthalenes yield and selectivity to 2-acetylnaphthalene. In agreement with results of Botella et al. (2) it is evident that this ratio influences significantly the yield of acetylnaphthalenes. This can be probably described to the inhibiting effect of acetylnaphthalenes formed during the reaction. Thus, the increase in the naphthalene to acetic anhydride molar ratio should result in the increase in acetylnaphthalenes yield. The excess of naphthalene probably facilitates easier desorption of acetylnaphthalene molecules. It is clearly seen in Fig. 3 that with increasing this ratio to 4 : 1, the acetylnaphthalene yield increases from about 6 to more than 35 %. On the other hand, no important changes in selectivities to 2-acetylnaphthalene were observed, the

632

selectivity to 2-acetylnaphthalene was about 70-80 % despite very significant differences in the reaction rate.

yield 4~ ImI~ ACN Selectivity 30 r

lff

20
o

10

t~

15

40

Molar ratio Si/AI 2 Acetylnaphthalenes yield and selectivity to 2-acetylnaphthalene over zeolites USY/1 and USY/2 in acylation of naphthalene with acetic anhydride (Experimental conditions: 1.2 g of catalyst, naphthalene/acetic anhydride molar ratio = 2, reaction time 120 min, temperature 135~ Fig.

8O

40

m ACN yield

35

-

7O

30

-

60 o

-E 25

-

50 ~

"~ 20 "~,

-

40 ~-

-

20

r

15-

~

10

Z

Selectivity

30 ._> o

10 0

1:1

2:1

4:1

Molar ratio (naphthalene" acetanhydride) Fig. 3 Dependence of acetylnaphthalenes yield on molar ratio of naphthalene to acetic anhydride for zeolite Beta (Si/A1 = 37.5); (Experimental conditions" 1.2 g of zeolite, reaction time 120 min, temperature 135 ~

633 3.2. Effect of the zeolite structure While the concentration of acid sites controls the activity of the zeolite catalysts, which is reflected in the acetylnaphthalenes yield (Fig. 1), the selectivity to individual acetylnaphthalene isomers is governed by the zeolite structure. From one side, shape selectivity influences significantly the ratio between 1- and 2-acetylnaphthalenes but also the formation of larger molecules, which are hardly to be desorbed from the zeolite channel system. In spite of the fact, that significantly different yields of acetylnaphthalenes were found over zeolite Beta possessing different concentration of acid sites (see Table 1), the selectivity to 2-acetylnaphthalene was always between 72-80 %. However, with USY/1 and USY/2 the selectivity to 2-acetylnaphthalene was only 32-34 %. It is evident that transition state selectivity strongly influences the ratio between the individual reaction products as the more open structure of USY zeolites enables the formation of sterically more demanded product (1-acetylnaphthalene). This is in contrast to zeolite Beta, the channel system of which is formed via threedimensional channels without larger cavities. In this channel system the formation of 1acetylnaphthalene is severely limited from sterical reasons and 2-acetylnaphthalene is preferentially formed. 3.3 Effect of acylating agent Three different types of acylating agents were used in this study, namely acetic anhydride, acetyl chloride and acetic acid, which in addition is also formed during the decomposition of acetic anhydride. While high yields of acetylnaphthalenes were achieved with acetic anhydride, almost no acylation activity was found for acetyl chloride and also acetic acid. During the decomposition of one molecule of acetic anhydride one molecule of acetic acid can be formed. This means that during the acylation reaction relatively high concentration of acetic acid is present in the reaction mixture. Despite this fact practically no acetylnaphthalenes were formed with acetic acid as acylating agent. Thus, it can be inferred that acetic acid probably does not represent the proper acylating agent for naphthalene acylation and naphthalene is acylated by acetyl group formed during the decomposition of acetic anhydride. In addition, as acetic acid is not the acylating agent, it means that contributes significantly to the deactivation of the catalysts due to its high reactivity on the acidic zeolites.

No higher acylated naphthalenes were obtained in the liquid phase during the reaction over any of zeolite catalysts used. This is probably due to the deactivating effect of acetyl group on aromatic hydrocarbons, which prevents the second acylation. In toluene acylation with acetic anhydride it was shown that further products (various aromatic ketones) can be formed inside of the channel system of zeolite Beta. These ketones can be removed via Soxhlet extraction with chloroform. They form only several percents of products, however, no di-acetyl products were found (2). Further experiments showed that to achieve the high acetylnaphthalenes yield over zeolite Beta/3, a combination of both the excess of naphthalene in the reaction mixture and the stepwise adding of acetic anhydride resulted in a significant improvement of the naphthalene conversion and the stability of the zeolite used (10).

634 4. CONCLUSIONS The highest acetylnaphthalenes yield in acylation of naphthalene with acetic anhydride was reached with zeolite Beta having the Si/A1 ratio around 35-40. Higher concentration of acid sites favoured rapid deactivation of the zeolite probably due to the subsequent reactions of acetic acid formed via acid catalyzed acetic anhydride decomposition and slow desorption of bulky products. Acetic anhydride was found to be the only active acylation agent while almost no acylation activity was observed with acetic acid or acetyl chloride. This indicates that acetic acid formed during the reaction can hardly be used as acylating agent. Very significant shape selective effect on the formation of individual acetylnaphthalene isomers was observed. While the selectivity to 2-acetylnaphthalene reached about 72-80 % over zeolite Beta, only 32-34 % were found with USY zeolites. It is evident that selectivity in this reaction is controlled by the restricted transitions state selectivity. Because of the higher reactivity of acetic anhydride compared to naphthalene the excess of the later one is recommended to decrease the rate of the deactivation of the catalyst. In a similar manner the stepwise addition of acetic anhydride into the reaction mixture led to higher naphthalene conversion.

REFERENCES

1.

P. Mrtivier, in Fine Chemicals though Heterogeneous Catalysis (Eds. R.A. Sheldon, H. van Bekkum), Wiley-VCH, Weinheim 2001, p. 161. 2. P. Botella, A. Corma, J.M. Lopez-Nieto, S. Valencia and R. Jacquot, J. Catal. 195 (2000) 161, and references therein. 3. D. Rohan, C. Canaff, E. Fromentin, M. Guisnet, J. Catal. 177 (1998) 296. 4. M.G. Clerici, Top. Catal. 13 (2000) 373. 5. G. Harvey, G. Maeder, Collect .Czech. Chem. Commun. 57 (1992) 863. 6. D. Das, S. Cheng, Appl. Catal. A 201 (2000) 159. 7. M. Casagrande, L. Storaro, M. Lenarda, R. Ganzerla, Appl. Catal. A 201 (2000) 263. 8. M. Spagnol, L. Gilbert, E. Benazzi and C. Marcilly, WO 96/35655 (1996). 9. S. Arctander, in Perfume and Flavour Chemicals (Aroma Chemicals), Publ. by the author, Montclair, N.J. (USA), 1969. 10. L. 12erven~, K. Mikulcovfi and J. Cejka, Appl. Catal. A, 223 (2002) 65. 11. Y. Servotte, J. Jacobs and P.A. Jacobs, Acta Phys. Chem. Szegediensis (1985) 611.


635

A l k y l a t i o n of P h e n o l with M e t h a n o l over Zeolite H - M C M - 2 2 for the F o r m a t i o n of p - C r e s o l G. Moon, K.P. M/311er, W. B~3hringer and C.T. O'Connor Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Private Bag, Rondebosch, Cape Town, 7701, South Africa The alkylation of phenol with methanol over H-MCM-22 was investigated and the performance of this catalyst compared with that of H-ZSM-5 and amorphous silica-alumina. The reaction was studied in the liquid phase (batch reactor) at temperatures between 200 250~ and pressures between 20 - 42 bar and in the gas phase (flow reactor) at temperatures between 250 - 400~ and feed partial pressures of 0.2 bar. The C (ring) : O (side chain)alkylation ratio was found to be more dependent on reaction conditions (i.e. gas/liquid phase, pressure and temperature) than catalyst type. The highest C : O alkylation ratios were obtained using gas phase reactions and were similar for all three catalysts. The highest p/ocresol ratios were obtained using H-MCM-22 in the liquid phase at 200~ Reasons are proposed to explain the different ring vs side chain alkylation and cresol isomer distributions for the different catalysts and the different reaction conditions. 1. I N T R O D U C T I O N

Para-cresol is an important intermediate in the formation of antioxidants and preservatives for plastics, motor oil and foods, p-Cresol can be formed in the alkylation of phenol with methanol. Typical catalysts used for the alkylation of phenols are H 2 8 0 4 , BF3, MgO, A1203 and SiO2-supported Fe/V [ 1,2]. The methyl group can alkylate in two ways on the phenol; firstly, on the oxygen, which is know as O-alkylation and secondly, on the benzene ring, known as C-alkylation, as shown in Figure 1. OH -!-

CH30H

OH

OCH 3 +

O-alkylation

or

C-alkylafion

Figure 1: Schematic diagram of the alkylation of phenol with methanol.

+ OH 3

636 The primary products of this reaction are anisole, o-cresol and p-cresol, m-Cresol is the thermodynamically favoured cresol isomer but is not kinetically favoured by electrophilic substitution as the hydroxyl group on the phenol is strongly ortho- and para-directing [3]. Possible secondary products are xylenols (dimethylphenols) and methylanisoles. It is possible to produce a high selectivity to o-cresol over basic catalysts or alumina [2, 4, 5]. Highly selective synthesis of m-cresol and p-cresol respectively is desirable due to the high cost of separating these isomers but no successful process has yet been developed [6]. A mixture of o- and p-cresol without m-cresol can easily be separated by distillation. Various zeolites have been studied for this reaction. These include H-Y [6, 7, 8, 9], H-ZSM-5 [6, 10, 11, 12], H-mordenite [5] and H-beta [13]. However, these zeolites have shown poor paraselectivity with p/o- cresol ratios of 0.7 for H-ZSM-5 [6], 0.3 for H-Y [5, 11] and no p-cresol was formed over H-mordenite [5]. This paper presents results in which H-MCM-22 and H-ZSM-5 were compared for the alkylation of phenol with methanol and in which silica-alumina was used as a benchmark for a non-porous acid catalyst. MCM-22 has a pore structure that consists of two independent channel systems [14]. The first, a three-dimensional channel system, consists of supercages defined by 12-membered rings having a diameter of 7.1 A and a length of 18.2/~. These supercages are interconnected through 10-membered ring openings with dimensions of 4.0 x 5.4 A. The second channel is a two-dimensional sinusoidal channel defined by 10-membered rings (4.0 x 5.9 /~). There is no direct access between these two channel systems. The external surface of MCM-22 has 12-membered ring pockets, which have a depth of 7.1 /~. ZSM-5 has a three-dimensional pore structure consisting of two interconnecting 10membered ring channels, one of which is sinusoidal (5.3 x 5.6/~) and the other straight (5.1

x 5.5 h) [15]. The focus of the study was on the selectivity of the various catalysts to the specific cresol isomers and to the C (ring) vs O (side chain) alkylation as a function of liquid phase batch and gas phase flow reactor systems. 2. EXPERIMENTAL Liquid phase experiments were carried out in a 600 ml stirred batch reactor at temperatures between 200 and 250~ The reactor was loaded with 200 g reactants (equimolar ratio of methanol and phenol) and 5 g catalyst. The reactions were run under autogenous pressure, e.g. approximately 23 bar at 200~ Gas phase experiments were carried out in an isothermal flow reactor at temperatures between 250 and 400~ The feed mixture of equimolar amounts of methanol and phenol were pumped via an evaporator and nitrogen was added as the diluent gas. The feed partial pressure was 0.2 bar, total pressure 1 bar, and a space velocity of approximately 14 h -1. The products were analysed using gas chromatography with a Chrompack CP Cresols capillary column. Toluene was used as an internal standard and a carbon balance of between 96 and 102% was obtained.

637 H-MCM-22 (Si/Al = 12, platelet morphology, average diameter of 0.5 ktm, thickness of 0.1 - 0 . 2 l.tm) was synthesised according to the method described by Raviskankar et al. [16]. Commercial samples of H-ZSM-5 (Si/A1 = 45, spherical morphology, average diameter 0.2 0.3 l.tm) and amorphous silica-alumina (SiOx-AlxO3, Si/Al = 8, < 106 microns) were obtained from Stid-Chemie and Akzo Nobel, respectively. 3. RESULTS AND DISCUSSION

In the gas phase experiments, the catalysts initially deactivated rapidly over the first two hours and then reached essentially a steady state with respect to phenol conversion. This initial deactivation period was accompanied by some change in product selectivity. Table 1 shows the phenol conversion, C : O-alkylation ratio and p/o-cresol ratio over H-MCM-22, HZSM-5 and silica-alumina, at 300~ In terms of phenol conversion, H-ZSM-5 was the most active catalyst studied. Although this zeolite had a low aluminium content it is unlikely that this difference in conversion was due to differences in strength of acid sites. The lower decline of conversion in the case of H-ZSM-5 was probably due to it being less susceptible to coking during the initial reaction period. All the catalysts showed similar product distributions with anisole and cresol isomers dominant. Small amounts (< 4 % steady state selectivity) of secondary products, viz. methylanisoles and xylenols, were present. There is an indication of enhanced selectivity to p-cresol over the zeolites compared to silica-alumina. H-MCM-22 and H-ZSM-5 produced similar p/o-cresol ratios of ca. 0.5 whereas for silica-alumina this was ca. 0.4. The p/o-cresol was essentially unchanged with time-on-stream and conversion, m-Cresol was also formed over all the catalysts at this temperature, but consisted of less than 5% of the cresol fraction. At 300~ the C : O-alkylation ratio initially declined but in the steady state levelled out at approximately 0.6 over all the catalysts studied. Thermodynamic equilibrium calculations at 300~ predict that the products should be mainly cresols. Moreover, at this temperature the distribution of cresol isomers was close to an o : p : m = 65 : 30 : 5 which is far removed from the thermodynamically predicted distribution of 32 : 8 : 60. These product distributions indicate that the reaction is kinetically and not thermodynamically controlled at the reaction conditions used. Table 1. Phenol methylation over H-ZSM-5, H-MCM-22 and silica-alumina (gas phase, 300~ 0.2 bar feed partial pressure, weight hourly space velocity of 14 h-l). Phenol conversion (%) C : O-alkylation ratio p/o-Cresol ratio Initial sample (15 minutes time-on-stream ) H-ZSM-5 19.2 0.98 0.55 H-MCM-22 9.3 0.71 0.56 SiO2 - A 1 2 0 3 5.3 0.55 0.38 Steady state (> 5 hours time-on-stream) H-ZSM-5 11.1 0.64 0.54 H-MCM-22 3.1 0.59 0.49 SiO2 - A 1 2 0 3 2.9 0.56 0.38

638 Table 2. Phenol methylation major products converted over H-MCM-22 (gas phase, 300~ 0.2 bar feed partial pressure, weight hourly space velocity of 14 h -1) Reactants Conversion Selectivity of products formed (%) (%) Anisole Cresols Methylanisoles Xylenols Phenol 19.2 37.8 2.1 40.9 Anisole 7 Anisole/phenol 4* 68.7 12.7 1.6 17.0 (1/1 molar ratio) 0.2 68.0 0.0 15.9 15.9 o-Cresol 4 0.7 88.2 0.0 5.7 5.4 p-Cresol 7 0.0 97.7 0.0 1.2 1.1 m-Cresol 4 * Conversion of anisole Table 3. Cresol distribution formed when converting the phenol methylation major products over H-MCM-22 (gas phase, 300~ 0.2 bar feed partial pressure, weight hourly space velocity of 14 h -1) Cresols distribution (%) Reactants o-Cresol p-Cresol m-Cresol Ansiole 69 11 20 Anisole/phenol (1/1 molar ratio) o-Cresol p-Cresol m-Cresol Thermodynamic equilibrium

53

28

19

9 26 32

15 74 8

85 91 60

In order to elucidate the reaction pathways a study was made of the reactions over HMCM-22 at 300~ in the gas phase, of each of the major products formed during phenol methylation, viz. ansiole, o-cresol, p-cresol and m-cresol. These results are shown in Tables 2 and 3. Anisole formed mainly methylanisoles and cresols. Cresols are mainly formed through monomolecular methyl shift. The formation of methylanisole clearly is due to a methyl transfer mechanism which initially forms phenol and methylanisole. Further methyl transfer then may occur from anisole to the phenol product to form more cresols. This is confirmed in the reaction where an equimolar amount of phenol was co-fed with the anisole. The conversion of anisole decreased as expected from its reduced partial pressure. Cresols were the major product. The decrease in the amount of o-cresol was almost quantitatively equivalent to the increase in p-cresol in the anisole and anisole/phenol reactions. This implies that in the absence of phenol monomolecular methyl transfer dominates, producing o-cresol. In the presence of reactant phenol, however, a bimolecular transfer from the anisole to the phenol para position is now possible resulting in much higher p-cresol selectivity (Table 3). Each cresol isomer was converted over H-MCM-22. In the case of o-cresol and p-cresol, mcresol was the major product as expected. Since in the methylation of phenol over all the catalysts studied, only a small amount of methylanisoles, xylenols (< 4 % selectivity) and m-cresol (< 2 % selectivity) were formed, it can be deduced that secondary reactions of anisole and the cresols occur only to a small

639 extent at 300~ At 200~ in the liquid phase the conversion of pure cresols, anisole and the anisole/phenol mixture over H-MCM-22 was insignificantly small [17]. This indicated that the anisole, o-cresol and p-cresol formed at those conditions were all primary products. It is important to note that at 200~ at conversions of less than 15 %, selectivities were always found to be independent of conversion over all the catalysts studied [17] thus allowing the comparison of the catalysts at different conversions. The effect of temperature on the product distribution from phenol methylation over HMCM-22 was investigated, in the gas phase, in the range 250 - 400~ These results are shown in Figure 2 which summarizes all the results in the liquid and gas phase in terms of C : O-alkylation ratio and p/o-cresol ratio, respectively. Decreasing the temperature decreased the C : O-alkylation ratio from 1.36 (400~ to 0.53 (250~ (Figure 2a) but caused the p/o-cresol ratio to increase from 0.41 (400~ to 0.54 (250~ (Figure 2b). Decreasing the temperature also decreases the formation of m-cresol from 16 to 4 mol % in the cresol fraction. This indicates that at higher temperatures either the primary formation of m-cresol is enhanced or alternatively that the isomerization of the cresols to the thermodynamically most stable isomer, viz. m-cresol, is enhanced. The reaction was also studied in the liquid phase at even lower temperatures, viz. 200 250~ At these conditions H-ZSM-5 was still more active than H-MCM-22 and silicaalumina in terms of phenol conversion (Table 4). Temperature had little effect on the p/oselectivity over H-ZSM-5 and silica-alumina. The most significant effects observed under these conditions were that H-ZSM-5 showed a 3 - 4 fold greater C : O-alkylation selectivity than the other two catalysts whereas H-MCM-22 had a similarly greater extent of p/o-cresol selectivity. The fact that H-MCM-22 had a similar C : O-alkylation selectivity to silica-alumina and that H-ZSM-5 had a similar p/o-selectivity to silica-alumina may be the result of the extent to which the reactions occur on the external surface of the zeolites as opposed to inside the pores. Consequently H-MCM-22 and H-ZSM-5 were modified by carrying out a selective Na/H exchange in order to remove the internal acid sites using a method described by Chester et al. [18]. These catalysts were tested at 200~ in the liquid phase. Over the internally poisoned H/Na-MCM-22 the p/o-cresol molar ratio was only 0.63 compared to 1.30 for the HMCM-22. In the case of H/Na-ZSM-5 the p/o-cresol ratio decreased to 0.26 compared to 0.38 for the H-ZSM-5 (Table 4). These results show clearly that for both zeolites, at these reaction conditions, phenol methylation occurs to a significant extent on the internal acid sites. Table 4. Comparison of the different catalyst for the lic,tuid phase batch alkylation (200~ 21 - 23 bar, modified weight hourly space velocity = 8 h -1) . Phenol conversion (%) C : O-alkylation ratio p/o-Cresol ratio H-ZSM-5 9.5 0.3 0.4 H-MCM-22 2.4 0.08 1.3 Sit2 - A 1 2 0 3 0.4 0.07 0.4 H/Na-ZSM-5 1.2 0.2 0.6 H/Na-MCM-22 2.0 0.06 0.3 * Note: The selectivities did not change with residence time or conversion respectivity [ 17]

640 1.5

o

O (a)

o

O

9 ~,=,i

~

1.0

6 o

0

0 0.5 T' V A

0 0.0 150

O0 V

0,0

9

200

250

300

350

400

450

Temperature (~ 1.5 9

9

o

O

(b)

.~,-4

1.0o

I

0.5-

9 v 0.0

9

200

150

O0 v 9

250

~ Q

O0

,

300

O0

9

350

,

400

450

Temperature (~ 9 O V V II

H-MCM-22 liquid phase (23 - 42 bar) H-MCM-22 gas phase (0.2 bar) H-ZSM-5 liquid phase (23 - 40 bar) H-ZSM-5 gas phase (0.2 bar) SiO2-A1203liquid phase (20 bar)

I"1

SiO2-Al203gas phase (0.2 bar)

9 ik

H/Na-MCM-22 liquid phase (21 bar) H/Na-ZSM-5 liquid phase (21 bar) Thermodynamic equilibrium

Figure 2. The gas and liquid phase (a) C 9O-alkylation ratio and (b) #o-cresol ratio over HMCM-22, H-ZSM-5 and amorphous silica-alumina.

641 Table 5. The relationship between spaciousness index [ 19] and C : O-alkylation over different catalysts (liquid phase, 200~ 21 - 23 bar, modified weight hourly space velocity = 8 h-l). Spaciousness index C : O-alkylation ratio p/o-Cresol ratio H-ZSM-5 1 0.3 0.4 H-mordenite [ 17] 6 0.2 0.5 H-MCM-22 8 0.08 1.3 H-beta [ 17] 16 0.08 0.5 H-USY [ 17] 22 0.08 0.6 SiO2 - A1203 22 0.07 0.4 As mentioned above, in the liquid phase at 200~ H-ZSM-5 showed a 3 - 4 fold higher selectivity to cresols than H-MCM-22 or silica-alumina. It has been reported [19] that HZSM-5 has a lower spaciousness index (1) than H-MCM-22 (8) and silica-alumina (22), this index being a measure of internal free space inside the pore system of a zeolite. Table 5 shows the spaciousness indices reported for various catalysts and the C : O-alkylation ratios found in this study. In the more spacious catalysts the C : O-alkylation ratio was 0.08 and the p/ocresol ratio was ca. 0.5. In the medium restricted zeolites (H-MCM-22) there is no change in the C : O-alkylation ratio but there is more shape-selectivity to the p-cresol. In the highly restricted zeolite (H-ZSM-5) the selectivity to anisole has decreased and the selectivity to ocresol has increased when comparing to the medium restricted zeolites. It can be speculated that the anisole converts on site into the more stable o-cresol as also found by Parton et al. [20]. This would explain the differences in the C : O-alkylation ratios for the two zeolites and the low p/o-cresol ratio in H-ZSM-5. In the gas phase reactions, which were carried out at temperatures between 250 - 400~ a comparative experiment at 300~ showed no difference in the C : O-alkylation ratio between the zeolites and silica-alumina. This could be due to the fact that the reaction occurs very rapidly at these temperatures and predominantly on the outer shell of the zeolite. The H-ZSM5 sample used has about 2.5 times greater external surface per gram than the sample of HMCM-22. This compares approximately to the activity of these catalysts at 300~ (Table 1). This is also supported by the small difference in the p/o-cresol ratio over H-MCM-22 and HZSM-5 compared to silica-alumina at these conditions. Lastly it should be noted that at 250~ there is a large difference between the liquid phase and gas phase results with respect to both the C : O-alkylation ratio and the p/o-cresol ratio (Figure 2). The difference in the reaction pressure (ca. 42 bar vs 0.2 bar) may give rise to this difference. However, further experiments are being carried out to investigate whether there may be a hysteresis phenomenon in this temperature range due to the existence of a metastable intermediate complex. 4. CONCLUSIONS In the liquid phase at temperatures in the range 200 - 250~ H-MCM-22 showed much greater p-cresol content in the cresol fraction from the methylation of phenol than H-ZSM-5 and silica alumina but H-ZSM-5 had a much greater selectivity to ring alkylation. The low p/o-cresol ratio (0.4) in H-ZSM-5 could be due to anisole converting in its relatively more restricted pore system to the more stable o-cresol. The reaction was shown to be occurring on

642 both the internal and external acid sites and also that shape-selective formation of p-cresol was occurring in the zeolites pores. Gas phase experiments in a flow reactor at 300~ showed almost no difference in the C : O-alkylation ratio between the catalysts studied and the zeolites demonstrated only slightly higher selectivity to p-cresol than the silica-alumina. The similarity between the performance of the three catalysts at this higher temperature is probably due to the higher reaction rates, effectively resulting in the reaction occurring on the outer shell of the catalyst. The large difference between the gas phase and liquid phase C : O-alkylation ratio and the p/o-cresol ratio may be due to the difference in reaction pressure. In the liquid phase experiments, the product spectrum consists of anisole, o-cresol and p-cresol implying that the product mixture formed can easily be separated by distillation to yield high purity p-cresol. REFERENCES 1. S. Subramanian, A. Mitra, C.V.V. Satyanarayana and D.K. Chakrabarty, Appl. Catal., 159 (1997) 229 2. J.S. Beck and W.O. Haag, Handbook of Heterogeneous Catalysis, Vol 5, G. Ertl, H. Kn6zinger and J. Weitkamp (eds.), Wiley-VCH: Weinheim (1997) 2131 3. P. Sykes, A Guidebook to Mechanism in Organic Chemistry, 6th ed., Longman Scientific and Technical: Essex (1986) 153 4. S.C. Lee, S.W. Lee, K.S. Kim, T.J. Lee, D.H. Kim and J.C. Kim, Catalysis Today, 44 (1998) 253 5. M. Marczewski, G. Perot and M. Guisnet, Stud. Surf. Sci. Catal., 41 (1988) 273 6. R.F. Patton, J.M. Jacobs, D.R. Huybrechts and P.A. Jacobs, Stud. Surf. Sci. Catal., 46 (1989) 163 7. S. Namba, T. Yashima, Y. Itaba and N. Hara, Stud. Surf. Sci. Catal., 5 (1980) 105 8. L. Garcia, G. Giannetto, M.R. Goldwasser, M. Guisnet and P. Magnoux, Catalysis Lett., 37 (1996) 121 9. M. Marczewski, G. Perot and M. Guisnet, React. Kinet. Catal. Lett., 57 (1996) 21 10. M. Renaud, P.D. Chantal and S. Kaliaguine, Can. J. Chem. Eng., 64 (1986) 787 11. S. Balsama, P. Beltrame, P.L. Beltrame, P. Carniti, L. Forni and G. Zuretti, Appl. Catal., 13 (1984) 161 12. R. Pierantozzi and A.F. Nordquist, Appl. Catal., 21 (1986) 263 13. J. Xu, A-Z. Yan and Q-H. Xu, React. Kinet. Catal. Lett., 62 (1997) 71 14. S.L. Lawton, M.E. Leonowicz, R.D. Partridge, P. Chu and M.K. Rubin, Microporous Mater., 23 (1998) 109 15. Ch. Baerlocher, W.M. Meier and D.H. Olson, Atlas of Zeolite Framework Types, 5th ed., Elsevier: Amsterdam (2001) 16. R. Raviskankar, D. Bhattacharya, N.E. Jacob and S. Sivasanker, Microporous Mater., 4 (1995) 83 17. G. Moon, K.P. M/Jller, W. BOhringer and C.T. O'Connor, Stud. Surf. Sci. Catal., 135 (2001) 310 18. A.W. Chester, A.S. Fung, C.T. Kresge and W.J. Roth, US Patent 5,779,882 (1998) 19. J. Weitkamp, S. Ernst and L. Puppe, Catalysis and Zeolites. Fundamentals and Applications, J. Weitkamp and L. Puppe (eds.), Springer: Berlin (1999) 327 20. R.F. Parton, J.M. Jacobs, H. Van Ooteghem and P.A. Jacobs, Stud. Surf. Sci. Catal., 46 (1989) 211


643

Relative Stability of Alkoxides and Carbocations in Z e o l i t e s . Q M / M M Embedding and QM Calculations Applying Periodic Boundary Conditions Louis A. Clark, Marek Sierka t and Joachim Sauer Humboldt-Universit/it zu Berlin, Institut ffir Chemie, Unter den Linden 6, D- 10099 Berlin, Germany 1. I N T R O D U C T I O N The role of carbocations in hydrocarbon conversions over acidic zeolite catalysts is much debated. These carbocations generally take two forms. Carbenium ions have three-coordinated carbon centers and are typically formed by protolytic dehydrogenation of saturated hydrocarbons or by protonation of an unsaturated hydrocarbon. In comparison, carbonium ions are also cations, but have a five-coordinated carbon center. They are often formed by adding a proton to a saturated hydrocarbon. It was first assumed that carbenium and carbonium ions were as common in zeolites as they are in homogeneous reactions [ 1]. Later, magic angle spinning (MAS) NMR studies failed to find these intermediate species in zeolites [2, 3]. Around the same time, quantum chemistry calculations employing small cluster models also came to the conclusion that these ionic species were only present as extremely short-lived transition states [4, 5]. More recently, evidence for persistent carbocation intermediates has begun to appear. Experimental studies have succeeded in identifying bulky carbocation species in zeolites [6-8] and theoretical predictions have been made concerning when they might be stable in zeolites based on their proton affinities [9]. Our recent theoretical studies, which we outline here, have succeeded in finding stable carbocation intermediates. First, the briefest description of the methodology and contrast to other methods is given. We then illustrate the performance of the method using a case-study of isobutene conversion. Finally, evidence for carbocation intermediates during the m-xylene disproportionation reaction is presented. 2. C O M P U T A T I O N A L

METHODOLOGY

In our computational studies, we have begun to incorporate what we believe is a more realistic model of the active site environment into our calculations. This is done using a simple, but effective technique often termed mechanical embedding that enables us to handle much larger systems than is possible with pure quantum mechanical (QM) methods. In this technique, the energies, gradients and second derivatives from QM and molecular mechanical (MM) calculations are combined to yield a consistent composite description of the potential energy tPresent address: Institut ~r Physikalische Chemie, Universit~tKarlsruhe (TH), Kaiserstr. 12, D-76128 Karlsruhe, Germany

644 surface [ 10, 11 ]. (1)

E(S)QM-Pot = E(S)Pot -E(I)Pot +E(I)QM

E(S)QM-Potis the composite QM/MM energy of the system and E(S)Pot is the MM energy of the whole system. The higher accuracy description of the active site from the QM method is incorporated by subtracting out the MM results (E (I)Pot) in a small region on and around the active site and replacing them by QM results in the same region (E (I) QM). QM/MM embedding techniques in varying degrees of sophistication are gaining popularity [ 12-14] The advantage of this embedding method over full periodic QM calculations is that it is considerably faster. If the potential energy function is a good approximation to the QM method used, the QM-Pot energy converges quicklytowards the full periodic QM result when increasing the size of the QM cluster. Unfortunately, current DFT fails to give reasonable interaction energies for van der Waals systems [ 15, 16]. It seems that different functionals behave differently, PW91 as used in most plane wave codes for solid state applications such as CPMD seems to perform better than B3LYP or BLYP [17]. For studies in zeolites, this phenomenon manifests itself most strikingly by giving unreasonable adsorption energies, even in calculations where the description of the active site itself is probably reasonable [ 18, 19]. The incorporation of the MM calculations, which are capable of describing adsorption correctly [20-24], into the QM-Pot calculations allows for more realistic description of the system. The QM part (DFT) is limited to a small cluster model of the active site of the zeolite. Figure 1 gives an example of the QM/MM partitioning. The QM part consists of 3 tetrahedra only, but provides a good description of the bond breaking- bond making portion.

Here

'.

,

A

~

B

Figure 1: QM/MM system partitioning for the isobutene in FAU calculations. A) MM portion B) QM portion. The partitioning for m-xylene in FAU is identical, but a larger fraction of the QM portion is hydrocarbon. The MM interatomic potential functions describe the van der Waals interactions of the hydrocarbon molecule with the wall of the zeolite cavity and the relaxation of the framework structure. We combine the cvff force field for hydrocarbons [25] with the proven ion-pair shellmodel potential for acidic zeolites [26, 27]. Charges on the hydrocarbons are critical for the success of the simulations. Among the charge options tested is a combination of bond increments as used by existing force fields and potential derived charges for the extra-charge on carbocations. For isobutene adsorption two different models of distributing the extra charge over the hydrocarbon part of the alkoxide are used which give very similar results. Assigning

645 the full extra charge to the C atom directly bound to the zeolite framework is not recommended. Final results for m-xylene adsorption and conversion employ charges fit to electrostatics from the cluster model. The QM portion of the calculations were done with TURBOMOLE [28, 29] using the B3LYP functional [30, 31], TZP basis sets on the oxygen atoms and DZP basis sets on all other atoms. For comparison periodic DFT studies are made on m-xylene in H-FAU (cell size: 17.43A x 17.50A x 17.49A) and its reaction products using plane wave basis sets (cut-off 70 Rydberg), norm-conserving Trouiller-Martins pseudopotentials and the PBE functional [32]. The CPMD code [33] was employed. 3. R E S U L T S

3.1. Isobutene and m-xylene adsorption When unsaturated hydrocarbons such as isobutene and m-xylene interact with zeolitic hydroxyl groups an adsorption complex is formed. It is expected that the energy of the initial adsorption step suffers most from the problem of insufficient description of van der Waals interactions by DFT. We will show that our QM-Pot approach which limits the DFT description to the site of specific interactions while using forcefields for the van der Waals interactions is well suited to tackle the adsorption step and the bond breaking- bond forming steps. Our embedded cluster QM-Pot approach yields for isobutene an adsorption energy of 55 kJ/mol - a reasonable value for a C4-hydrocarbon. The DFT contribution to this value obtained for a 3T cluster model (cf. Figure 1) is 22 kJ/mol while the potential function contribution is 33 kJ/mol. This is in excellent agreement with results obtained by Sinclair et al. [34]. The embedded cluster is 55 kJ/mol and the constrained cluster result (which is comparable to our QM contribution) is 22 kJ/mol. Note that the QM method used by Sinclair et al. is MP2 for a slightly different cluster and the forcefield used is also somewhat different. The constrained cluster result is also DFT and the same functional is used. The adsorption energy for m-xylene calculated by comparing the bare acidic zeolite and isolated m-xylene species to the adsorbed state using the embedded cluster approach is approximately 62 kJ/mol, the DFT part of this result is 12 kJ/mol only. This is consistent with results of about 20 kJ/mol obtained for adsorption of toluene on small cluster models [35]. The QM contribution of the embedded cluster result is expected to be smaller than the result for a free cluster model because the QM/MM geometry optimizations may move the structure away from the pure QM minimum to a region where the MM energy is also low. Applying DFT to the periodic zeolite structure using periodic boundary conditions (CPMD) we find an adsorption energy of approximately 28 kJ/mol. Experimental values are between 60 to 85 kJ/mol on NaY and KY zeolites [36-38]. We see again that the failure of DFT to give reasonable long-range dispersion interactions results in unreasonable predictions for the adsorption energy.

3.2. Formation of t-butyl alkoxide The study of isobutene reaction with the acidic site of the faujasite zeolites illustrates our methodology. The complex formed between the hydrocarbon and the acidic hydroxyl group (~-complex), in principle, can rearrange into an alkoxide (Figure 2) or into a carbocation attached to the anionic zeolite surface. Quantum chemical calculations can provide information about the existence of alkoxide and carbocation intermediates and their stabilities relative to

646 the rt-complex. The reliability of quantum chemical calculations depends on both the choice of a reliable method and a realistic zeolite model. Small or medium size cluster models suffer from missing steric repulsion. For example, in chabazite the stability of alkoxides obtained by reaction with ethene, propene and iso-butene relative to the r~-complex is found to decrease in this sequence due to steric interactions with the zeolite surface around the active site [34]. This behavior cannot be correctly described by small cluster models. Our embedded cluster method shows that the adsorption energy for the t-butoxide structure (~-complex) is 95+5 kJ/mol, i.e. the 6-complex is by 40+5 kJ/mol more stable than the rt-complex. Boronat et al. report the alkoxide more stable by 31 kJ/mol [39]. The embedded cluster approach used by Sinclair et al. predicts the ~-complex more stable by 9 kJ/mol (MP2), while the constrained cluster result (DFT) is 18 kJ/mol. 9~ , , , ~

~

Figure 2: Isobutene in the faujasite zeolite structure. Shown here are the r~-complex (left) and the o-complex (alkoxide) structures (right).

3.3. Xylene disproportionation results In an effort to exemplify, understand and provide strategies for control of the phenomenon known as Transition State Shape Selectivity, we have studied a classic zeolite-based reaction computationally. The methyl transfer (disproportionation) reaction between two m-xylene molecules has long been assumed to be one where environmental confinement shifts reaction selectivity by directly influencing the formation of reaction transition states. As part of this study, we have located carbenium and carbonium ions that contribute to the critical shape-selectivity step in the reaction. The classical mechanism, thought to dominate in large-pore zeolites proceeds through a diphenylmethane carbonium species [35, 40--42]. After adsorption of the first m-xylene molecule (AE = -62 kJ/mol, cf. section 3.1 .) dehydrogenation at one of the methyl groups yields a surface alkoxide on the zeolite pore wall, ZOH + m-xylene (ads) --+ ZO-CH2-C6Hn-CH3 + H2

AE = + 58 kJ/mol

(2)

There is also a less stable carbenium ion intermediate shown to be a local minimum on the potential energy surface, ZO-CH2-C6H4-CH3 ~ ZO- + CH3-C6Hn-CH2+

AE = + 53 kJ/mol

(3)

A second m-xylene molecule entering the same cavity can interact with the alkoxide (figure 3A) or with the carbenium ion (figure 3B). We also consider the carbonium ion (figure 3C) obtained from the former, ZO-CH2-C6Hn-CH3 + m-xylene (ads) --+ ZO- + CH3-C6Hn-CH2 -[C6Hn(CH3)2] +

(4a,b)

647 ZO- + CH3-C6H4-CH + + m-xylene (ads) --+ ZO- + CH3-C6H4-CH2 -[C6H4(CH3)2] + which is one of the possible intermediates in the xylene disproportionation reaction. .

'

,

'

.

B

~

C

,.-

Figure 3: View of the alkoxide (A), carbenium (B) and carbonium (C) structures. This is comparable to typical energy barriers in related reactions and means that the ionic species are stable. We find that both the carbenium ion and the carbonium ion species are stationary points on the potential energy surface. This is in contrast to previous cluster-based results [35, 42] and shows that the inclusion of the pore environment stabilizes these ionic intermediates. Full periodic calculations using the PBE functional in the CPMD code also indicate that the carbenium and carbonium ions are stationary points. Preliminary calculations indicate that there is an energy barrier of approximately 45 kJ/mol between the carbenium and the alkoxide species. Note that the barrier could be different for other zeolites and if more acid sites that our in 1 A1/supercage model were included. Since this barrier is comparable to other barriers in the reaction, it seems likely that these species play an large role in the overall reaction mechanism. 5. C O N C L U S I O N S Our recent work applying the QM-Pot mechanical embedding methodology to acid catalyzed zeolite reactions has produced results that elucidate basic mechanisms. A note is also made of the synergy between the QM and MM methods. Taken together, they provide a reasonable description of bond breaking and forming from the QM as well as long-range dispersion interactions from the faster MM methodology. We also find that some aromatic carbenium and carbonium ions are stationary points on the potential energy surface and may be stable enough to observe experimentally. 6. A C K N O W L E D G E M E N T S This work has been supported by the 'Fonds der Chemischen Industrie' and the 'Deutsche Forschungsgemeinschaft'. Computer time on the T3E at Zentrum fuer Informationstechnik Berlin is acknowledged. LAC acknowledges the support of the Alexander von Humboldt Foundation.

648 REFERENCES 1. G. A. Olah, A. Molnar, Hydrocarbon Chemistry, Wiley, New York, N.Y., 1995. 2. T. Xu, J. H. Zhang, E. J. Munson, J. F. Haw, Chem. Commun. 23 (1994) 2733-2735. 3. J. F. Haw, J. B. Nicholas, T. Xu, L. W. Beck, D. B. Ferguson, Acc. Chem. Res. 29 (1996) 259-267. 4. V. B. Kazansky, M. V. Frash, R. A. van Santen, Catal. Letters 28 (1994) 211-222. 5. V. B. Kazansky, M. V. Frash, R. A. van Santen, Appl. Catalysis A 146 (1996) 225-247. 6. L. Femandez, V. Marti, H. Garcia, Phys. Chem. Chem. Phys. 1 (1999) 3689-3695. 7. W. Adam, I. Casades, V. Fornes, H. Garcia, O. Weichold, J. Org. Chem. 65 (2000) 39473951. , ,i , 8. W. G. Song, J. B. Nicholas, J. F. Haw, L Phys: Chem. B 105 (2001) 4317-4323. 9. J. B. Nicholas, J. F. Haw, J. Am. Chem. Soc. 120(1998) 11804-11805. 10. U. Eichler, C. M. Kolmel, J. Sauer, J. Comput. Chem. 18 (1997) 463-477. 11. M. Sierka, J. Sauer, J. Chem. Phys. 112 (2000) 6983-6996. 12. J. Gao, M. A. Thompson (Eds.), Combined Quantum Mechanical and Molecular Mechanical Methods, Vol. 712 of ACS symposium series, ACS, Washington, DC, 1998. 13. T. Z. Mordasini, W. Thiel, Chimia 52 (1998) 288-291. 14. J. Sauer, M. Sierka, J. Comput. Chem. 21 (2000) 1470-1493. 15. Y. K. Zhang, W. Pan, W. T. Yang, J. Chem. Phys. 107 (1997) 7921-7925. 16. T. A. Wesolowski, O. Parisel, Y. Ellinger, J. Weber, J. Phys. Chem. A 101 (1997) 78187825. 17. S. Tsuzuki, H. P. Luthi, J. Chem. Phys. 114 (2001) 3949-3957. 18. A. M. Vos, X. Rozanska, R. A. Schoonheydt, R. A. V. Santen, F. Hutschka, J. Hafner, J. Am. Chem. Soc. 123 (2001) 2799-2809. 19. T. Demuth, L. Benco, J. Hafner, H. Toulhoat, F. Hutschka, J. Chem. Phys. 114 (2001) 3703-3712. 20. R. Q. Snurr, A. T. Bell, D. N. Theod0rou, J. Phys. Chem. 97 (1993) 13742-13752. 21. T. J. H. Vlugt, R. Krishna, B. Smit, J. Phys. Chem. B 103 (1999) 1102-1118. 22. A. Gupta, L. A. Clark, R. Q. Snurr, Langmuir 16 (2000) 3910-3919. 23. M. D. Macedonia, D. D. Moore, E. J. Maginn, Langrnuir 16 (2000) 3823-3834. 24. A. H. Fuchs, A. K. Cheetham, J. Phys. Chem. B 105 (2001) 7375-7383. 25. A. T. Hagler, S. Lifson, P. Dauber, J. Am. Chem. Soc. 101 (1979) 5122, (as implemented in the Discover software of Accelrys Inc.). 26. K. P. Schr6der, J. Sauer, J. Phys. Chem. 100 (1996) 11043-11049. 27. M. Sierka, J. Sauer, Faraday Discussions 106 (1997) 41-62. 28. R. Ahlrichs, M. B~, M. H~iser, H. Horn, C. M. K61mel, Chem. Phys. Lett. 162 (1989) 165. 29. O. Treutler, R. Ahlrichs, J. Chem. Phys. 102 (1995) 346-354. 30. A. D. Becke, J. Chem. Phys. 98 (1993) 5648-5652. 31. C. Lee, W. Yang, R. G. Parr, Phys. Rev. B 37 (1988) 785-789. 32. J. P. Perdew, K. Burke, M. Emzerhof, Phys. Rev. Lett. 77 (1996) 3865-3868. 33. J. Hutter, A. Alavi, T. Deutsch, M. Bemasconi, S. Goedecker, D. Marx, M. Tuckerman, M. Parrinello, CPMD 3.4.1, MPI ftir Festk6rperforschung and IBM Zurich Research Laboratory (1995-1999). 34. P. E. Sinclair, A. D. Vries, P. Sherwood, C. R. A. Catlow, R. A. V. Santen, J. Chem. Soc. Faraday Trans. 94 (1998) 3401-3408. 35. X. Rozanska, X. Saintigny, R. A. V. Santen, F. Hutschka, J. Catal. 202 (2001) 141-155. 36. E. Santacesaria, D. Gelosa, P. Danise, S. Carrh, Ind. Eng. Chem. Process. Des. Dev. 24 (1984) 78-83.

649 37. 38. 39. 40. 41.

D. M. Ruthven, M. Goddard, Zeolites 6 (1986) 275-282. J. Bellat, M. Simonot-Grange, Zeolites 15 (1995) 219-227. M. Boronat, P. Viruela, A. Corma, Phys. Chem. Chem. Phys. 3 (2001) 3235-3239. M. A. Lanewala, A. P. Bolton, J. Org. Chem. 34 (1969) 3107-3112. M. Guisnet, N. S. Gnep, S. Morin, Microporous and Mesoporous Mater. 35-36 (2000) 4759. 42. S. R. Blaszkowski, R. A. van Santen, in: K. Morokuma, D. G. Truhlar (Eds.), Transition State Modeling for Catalysis, ACS Symp. Series 721, ACS, Washington DC, 1999, pp. 307-320.



651

H-Beta zeolite for acylation processes: optimization of the catalyst properties and reaction conditions P. Botella, A. Corma, F. Rey and S. Valencia Instituto de Tecnologfa Qufrnica, Av. de los Naranjos s/n, 46022 Valencia, Spain Beta zeolite has been optimized for acylation processes. In the acylation of toluene and mxylene with acetic anhydride Beta samples of low Si/A1 framework ratio and nanocrystals show improved activity and slower deactivation. In the acylation of 2-methoxynaphthalene with acetic anhydride the selective dealumination of the external surface of a nanocrystalline Beta enhances the shape selectivity of the zeolite, increasing the selectivity to the less sterically hindered 2-acetyl-6-methoxinaphthalene. Finally, the control of the process conditions is vital in the case of the acylation with carboxylic acids. Water from the liquid phase should be removed to avoid side reactions that decrease the quality of the final product. 1. INTRODUCTION The Friedel-Crafts acylation and the related Fries rearrangement are widely used in the industry for the production of arylketones as intermediates in the synthesis of fine chemicals and pharmaceuticals (1). It is known that the main drawback of these processes is the use of stoichiometric amounts of Lewis acids (i.e. A1C13, BF3, etc.) as catalysts (2), which are destroyed at the end of the reaction producing undesirable wastes. Heterogeneous catalysis with solid acids have recently introduced an alternative to leave the classical homogeneous process using less hazardous acylating agents, like carboxylic acids and their anhydrides (3). Many different solid acids have been tested in the last decade for this purpose (4-5), but zeolites have found application for the industrial production of aromatic ketones. H-Beta zeolite is one of the most widely used zeolites for acylation of arylethers (i.e.: anisole, 2-methoxynaphthalene, etc. (6-7) and substituted aromatic rings (i.e., phenol, toluene, etc.) (8-9). Moreover, recently, RHODIA has reported the first industrial application of this catalyst for the anisole acylation (10). A drawback of these catalysts is their deactivation due to the strong adsorption of the arylketone (11-12). It should be possible to improve the performance of H-Beta zeolite as an acylation catalyst through a better catalyst design and optimization of the reaction conditions. Accordingly, we have firstly studied the catalytic behavior of several Beta samples with different Si/A1 molar ratio, crystallite size and external surface acidity in the acylation of toluene (TOL), m-xylene (XYL) and 2-methoxynaphthalene (2-MN) with acetic anhydride (AA). Secondly, a particular case of acylation with carboxylic acids with application in the fragrances industry, the acylation of anisole (ANI) with propionic acid (PA) to produce the precursor of anetole, has also been studied. It will be presented that the reaction conditions and especially the presence of water strongly influences the performance of the catalyst and the quality of the f'mal product.

652

2. EXPERIMENTAL Several Beta zeolites in protonic form have been tested in the present work. A commercial Beta was provided by P.Q. Industries (CP811) and the rest were prepared in our laboratory. Samples N7.5 and N16 are nanocrystalline zeolites synthesized in an alkali-free basic medium (13) while sample L8 has been prepared by a new synthesis procedure (14). All samples were calcined at 580 ~ before using. A surface dealuminated nanocrystalline Beta, NH54, was prepared by treating sample N16 with a 1 M HC1 solution (90 ~ 5 h) in order to remove both framework and extra-framework Al from the external surface. For this purpose the zeolite was acid-treated before removing the template and then it was calcined at 580 ~ The A1 content in the samples was determined by atomic absorption spectrophotometry (Varian spectrAA-10 Plus). Crystallinity was measured by powder X-ray diffraction, using a Phillips PW1710 diffractometer with CuKa radiation and compared with a standard sample. Acidity was measured by the pyridine adsorption-desorption method, while surface area was calculated from the N2 adsorption isotherms (77 K) in a Micromeritics ASAP 2000 instrument. The crystal size was determined from the SEM images obtained in a JEOL 6300 scanning electron microscope. The most relevant physicochemical properties of these zeolites are summarized in Table 1. The acylation of the different substrates with AA was carried out in batch conditions at autogeneous pressure. Acylation of TOL was carried out in a stainless steel stirred 150-ml autoclave (Autoclave Engineerings) under N2 atmosphere, while the acylation of XYL and 2MN were carried out in a 25-ml three-neck round-bottom flask, connected to a reflux cooler system, under argon and with magnetic stirring. All reagents were supplied by Aldrich. In the TOL acylation, 1.00 g of catalyst activated in situ reacted at 150 ~ with a mixture of 400 mmol of TOL and 20 mmol of AA, and the reaction was performed for 4 h. Acylation of XYL was carried out at 110 ~ with 1.00 g of Beta activated in situ and a mixture of 100 mmol of XYL and 10 mmol of AA, and the reaction was performed for 3 h. For the acylation of 2-MN 0.20 g of catalyst were activated in situ. Then, a mixture of 4.0 mmol of 2-MN, 2.0 mmol of AA and 3.0 ml of chlorobenzene was introduced and the reaction was carried out at 132 ~ for 24 h. The propionylation of ANI with PA was carried out at 154 ~ and autogeneous pressure in a 25-ml three-neck round-bottom flask, connected to a reflux cooler system, under argon atmosphere and with magnetic stirring. 0.50 g of catalyst were activated in situ. Then a mixture of 100 mmol of ANI and 20 mmol of PA was added, and the reaction was carried out for 48 h. When necessary water was removed from the reactant mixture by a Dean-Stark or by a molecular sieve column (3/~) installed "on line". In all cases, the reaction products were analyzed by GC in a Varian 3350 Series instrument equipped with a HP-5 column and a FID detector, using nitrobenzene as internal standard. Products were also identified by mass spectrometry in a Varian Saturn II GC-MS model working with a Varian Star 3400 GC and using reference samples.

3. RESULTS AND DISCUSSION 3.1. Acylation of toluene and m-xylene with acetic anhydride Friedel-Crafis acylation of TOL and XYL with AA is a very selective process for the production of the para isomers 4-methoxyacetophenone (MAP) and 2,4-dimethylacetophe none (DMAP), respectively (Scheme 1). These reactions involve the production of one mole-

653 Table 1 Physicochemical characteristics of H-Beta zeolites. Acidity (l.tmolpy)(3) Br6nsted Sample Si/A10) AreaBET ExternalArea Crystal 250~ 350~ 400~ (m2 g-l) (m 2 g-l) (~tm) kz(CuSO4)> ke(Cu(HCOO)2)>> kz(Cu(CH3COO)2).

Stokes radii of anion species are r(CH3COO)=0.22nm, r(HCOO-)=0.16nm, r(Cl)=0.12nm and r(SO42)=0.11nm. From the reaction kinetics data in the exchange reaction with various Cu(II)-salts, the exchange rate of Mg z§ with Cu z§ depended on the Stokes radii of anion species of the Cu(II) salts. This indicates that the diffusion of Cu e+ in to the intracrystalline tunnels of sepiolite contributes to the reaction rate. Table 1 Kinetic parameter of the substitution of Mg2§ in sepiolite lattice with Cu2§ rate constant

Activation

solution

k2

energy / kJ tool"1

CuC12

0.613

23.5

CuSO4

0.541

21.1

Cu(HCOO)2 Cu(CI-I~COO)2

0.535 0.085

19.2 19.4

328K

'~ 0.6

"~0.4

! 0.2

0

0

?

~t

fi

Figure 2 Exchange reaction between sepiolite and Cu(HCOO)s.solution. Sep: lOOmg; Cu(HCO0)2:50mL(lmmol/I~

Sepiolite:200mg; concentration of Cu(II)-salts: 0.1-10 mmol/L (500mL); reaction temp.: 298K. Activation energy was calculated from the data obtained in the temperature range from 288K to 318K

3.1.2 Results of ESR m e a s u r e m e n t s To determine the substituted site of Cu 2+ in sepiolite, the well orientated fiber shape bundle Cu2§

sepiolite sample was

HI/O0"

used for angular dependency measurements of ESR spectra. As shown in Fig.3, clear unisotoropic

ESR

spectra

due

to

Co)

poler

Cu 2§

(gx=2.390, gr=2.115, gz=2.045; Ax=8.5, Ay=3.0, Az=6.0 roT) were observed. These values strongly suggest t h a t the coordination

Figure 3 ESR spectra of well oriented fibrous sepiolite

679 of Cu 2+ is a distorted octahedral[5]. Further, the dissolved Mg e+ was found in the aqueous solution after the reaction. From the results, we concluded t h a t the Mg e§ along tunnel wall of sepiolite are substituted by Cu e+. 3.2 Catalytic activiW of sepiolite

3.2.1 Shape selective catalytic reaction

100%

To investigate the role of tunnels of sepiolite,

catalytic

cyclodehydration

80%

of

DEG was studied by using sepiolite alone.

60%

1,4-Dioxane

40%

Ethylene

was

glycol,

the

main

ethanol,

product.

acetic

acid,

20%

acetoaldehyde and crotonaldehyde were observed in the product. At an early stage

of

the

reaction,

the

yield

of

1,4-dioxane was 100%, the decrease of 1,4-dioxane formation and an increase of

0% 1.5

3

7.8

13.9

slmerticial velcNzitv ( 1O0 m/min~

II acetoaldehyde

[] ethanol [] acetic acid El ethylene glycol

@ 2-methyl-l,3-dioxolane I erotonaldehyde [] dioxane

by'products formation were observed with an increase of the TOS. This means t h a t conversions of 1,4-dioxane occurred on the

Figure 4 Effect of superficial Velocity on the distribution of products reaction temp: 573K;TOS:30min;WHSV:0.2h1

catalyst. To clarify the reaction path, reactions were carried out by using flow reactors of various diameters packed with the constant weight of the sepiolite at the constant WHSV. Since in each reaction, no DEG has been detected at the initial stage of the reaction, the distributions of the product at 30 min of TOS are shown in Fig.4. The superficial gas velocity is defined by vs=(flow rate of D EG)/(cross section of the reaction tube). The existence of the optimum superficial velocity for 1,4-dioxane formation indicates that a diffusion of DEG into the inner surface of

lOO 9O =

80

___- "~ ~.~.,.

~~ 7o_ 4? 60

.~ so 2 4o

-

k

_

.

o. .~ 30

"

17 3 0 m ~

,t~ 2o

"

0

~

60mh

,~.

A 90mh

lO

I

0

0.1

~

I

0.2 WHSV/h

R eactbn I m p .

,

I

0.3

~

I

0.4

,

0.5

-1 533K, s e p b l i ~ Ig

Figure 5 Relation between WHSV and selectivity for 1.3-butadiene formation.

the tunnels of sepiolite and desorption of 1,4-dioxane are important process. The selectivity of 1,4-dioxane formation depended on WHSV of the reactant. When DEG was supplied less t h a n adsorption capacity of DEG on sepiolite, the selectivity of 1,4-dioxane formation was high. On

680 the other hand, supplying an excess of DEG over the adsorption capacity of the catalyst, the selectivity for the 1,4-dioxane formation decreased as shown in Fig.5. The coordination of DEG to the Mg 2§ was observed in situ XRD measurements of the sepiolite. The results show that a shape selective 1,4-dioxane formation occurs the inner surface of sepiolite tunnels, while by-products formation occurs on the outer surface of sepiolite tunnels.

3.2.2 Acid-base bihmctional catalyst Metal oxide such as Mn, Zn, Ni and Cu supported on sepiolite (MOx-sepiolite) showed a high activity and selectivity for the catalytic conversion of ethanol to 1,3-butadiene, although ethylene and diethylether produced from ethanol by using sepiolite alone. At the early stage of the reaction acetaldehyde produced and decreased with an increase of 1,3-butadiene in the progress of the reaction. The yields of C4 on MOx-sepiolite catalysts were 50% -85% as shown in Table 2. Since 1,3-butadiene was produced via following process [6]" Aldohl Condensation 2CH3CHO -CH3CH(OH)CH2CHO I A ~ A ~ dehydration hydrogen transf~'+2C2HsOH CH3CH=CHCH2OH+CH3CHO ~ CH3CH=CHCHO A I dehydration A+B .2H 2 2C2HsOH ~ B

T

CH2=CH-CH=CH2

I A:acidir site I B:basie site

both acidic and basic sites are necessary. There are only acidic sites ((pKa=6.8 " "1.5) on sepiolite alone. An existence of basic sites on the ZnO-sepiolite catalyst (pK,=9.8) was confirmed by Benesi's titration method and the substituted Zn 2+ ions with Mg 2+ along the tunnel wall behaved as basic sites for the catalytic conversion of ethanol [7]. To clarify roles of acidic and basic sites for 1,3-butadiene formation, the catalysts were poisoned by acidic and basic materials. The results are shown in Table 3. Acidic materials such as CO2 and phenol inhibited the formation of the 1,3-butadiene formation and enhanced the ethylene formation. A small amount of NH3 and pyridine enhanced the 1,3-butadiene formation and inhibited the ethylene formation. However, when the catalyst treated with a large amount of NH3, both the 1,3-butadiene and ethylene formations were inhibited. This means that both acidic and basic sites, on the catalyst play important roles for the 1,3"butadiene formation. Especially, the basic site on the catalyst is more important than acidic site, ie. the basic site forms by the exchange of Mg 2+ along the tunnel wall with divalent transition metal ions and it plays as an important role for acetaldehyde formation step as shown in scheme.

681 Table 2 M a x i m u m yield of 1,3-butadiene on various m e t a l oxide supported on sepiohte Catalyst sepiolite Mn/sepiolite Ni/sepiolite Co/sepiolite V/sepiolite Zrgsepiolite Cu/sepiolite

1,3-butadiene

yield/% butene isomer

C4 total

0 31 33 55 43 67 58

0 20 36 6 9 18 10

0 51 69 62 52 85 68

Table 3 Effect of poisoning by acidic and basic materials on yields of 1.3-butadiene from ethanol on manganese supported on sepiolite

poison no

EtOH conversion conversion1 initial rate wt% mmol min~ 92.5 0.102

yield wt% 54.5

1.3-butadiene initial rate mmol min~ 0.023

SB2 60

ethylene yield SEa wt% 15.4 13

phenol

82.3

0.074

46.0

0.010

46

21.9

20

CO2

97.9

0.099

45.0

0.023

52

19.8

21

pyridine 89.5 0.085 62.5 0.024 65 3.4 4 NH3 (5.3x102pa) 95.2 0.213 71.0 0.037 66 6.8 5 NH3 (1.4xl04pa) 92.6 0.048 48.0 0.011 53 10.4 8 Catalyst:0.4g; Mn content is 7 mmol per lg ofsepiolite; reaction temp.:553K; 1 reaction time:24h. 21,3-butadiene formation selectivity SB=( C4H6 yield)/(EtOH conversion). 3ethylene formation selectivity SE-(C2H4 yield)/( EtOH conversion). 3.3.3 Catalytic activity for S u z u k i reaction The

paUadium-catalysed

cross'coupling

reaction

of

aryl

ha]ides

and

phenylboronic acid (Suzuki reaction) is one of the most i m p o r t a n t methods for forming sp2-sp 2 carbon-carbon bonds both in m o d e r n synthetic c h e m i s t r y and h a s great potential for i n d u s t r i a l application [8]. _~~._ ~~_ Pd'sepi~ K2CO3 ~ R X+ B(OH)2 ~R DMF As shown in Table 4, the Pd-sepiolite catalysts showed higher activities t h a n a commercial Pd supported on carbon (Pd/C) for the Suzuki reaction of various aryl h a h d derivatives a n d phenylboronic acid. The reactivity of aryl bromide is generally lower t h a n t h a t of aryl iodide. In our study, no difference in the reactivity between aryl bromide and aryl iodide could be observed. Pd-sepiolite

682 gave slightly higher yield of the substrate involving electron withdrawing substituents than electron donating substituents. Mubofu e t al. reported that electron withdrawing groups have relatively little effect on the reaction rate and selectivity in the reactions over Pd supported on modified silica catalysts [9]. The average particle size and dispersion of Pd in the Pd-sepiolite were 1.6 nm and 67% respectively. The high activity of Pd-sepiolite for Suzuki reaction is attributed to the high dispersion and micro size Pd particles in the catalyst. Table 4 Suzuki reactions of various aryl halide derivatives catalyzed by Pd-sepiolite substrate 'catalyst Aryl h a l i d e Pd/mmol C6HsI 0.02 CBHsBr 0.02 4-NHeC6HsBr 0.02 4"CH30CBHsBr 0.02 4-CHaCOC6HsBr 0.02 4-NO2C~HsBr 0.02 C6H~Br 0.001

Temp. Time Yield /~ /h /% 100 20 80 100 20 76 100 20 77 100 20 75 100 20 81 100 8 96 130 24 61

TON 40(}0 3800 3850 3750 4050 4800 61000

4-CH30C6HsBr

0.001

130

24

48

48000

4-CHaCOC6HsBr

0.001

130

20

92

92000

4-MeOC6H6Br

0.2(Pd/C)

130

24

63

315

aryl halide:2.5 mmol; phenylboronic acid :7.5 mmol; potassium acetate :7.5 retool; the catalyst; DMF: 5 mL

References 1. A. Preisinger, "Clays & Clay Minerals" Earth Sci. Series, vol.12(1963)pp365, Pergamon Oxford. 2. H. Nagata, S. Shimoda and T. Sudo, Clays Clay Miner., 22(1974) 285. 3. Y. Kitayama, H. Katoh, T. Kodama and J. Abe, Appl. Surf.Sci., 121/122(1997) 331 4. Y. Kitayama and A. Abe, J. Chem. Soc. Jpn., 1989,1824. 5. D. E. Billing, B. J. Hathway and P. Nicholis, J. Chem. Soc. (A),1969, 316. 6. H. Niiyama, S. Morii and E. Echigoya, Bull. Chem. Soc., 45(1972)655. 7. H. A. Benesi, J. Am Chem., 78(1956)5490. 8. N. Miyaura, T. Yanagi andA. Suzuki, Synth. Commun. 11(1981)513 9. E. B. Mubofu, J. H. Clark and D. J. Macquarrie, Green Chem.,3(2001)23.


683

Catalytic wet oxidation o f reactive dyes with H 2 0 2 over m i x e d (A1-Cu) pillared clays Sung-Chul Kim, Dul-Sun Kim, Geun-Seon Lee, Ju-Ki Kang, Dong-Keun Lee a and Young Kuk yangb aDepartment of Chemical Engineering/Environmental Protection, Environment and Regional Development Institute, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyungnam 660-701, Korea bKukbo Ind. Co. Ltd, Sangpyung-dong 157-5, Chinju, Kyungnam 660-340, Korea AI-Cu pillared clays were prepared by direct introduction of A1-Cu pillaring solution into the dilute bentonite suspension. AI-Cu pillared clays had d001 spacing of about 18A and had surface area of about 140m2/g or higher. A1-Cu pillared clays showed excellent activity toward the catalytic wet peroxide oxidation of reactive dyes. Complete removal of reactive dyes could be achieved within 20min at atmospheric pressure and 80 ~ which are extremely milder operation condition when compared with the conventional catalytic wet oxidation process. The pillared clays were also stable against the leaching out of Cu. I. INTRODUCTION Catalytic wet oxidation has recently been the subject of numerous investigations to reduce the amount of organic pollutants in wastewaters[1-6]. Catalytic wet oxidation(CWO) is a liquid phase oxidation of organic materials in water with oxygen(the main source of oxygen is generally air) on the surface of catalyst. Although the use of catalyst makes the reaction conditions milder than uncatalyzed wet oxidation, the catalytic wet oxidation process still requires high temperature(higher than 150~ and high pressure(1-5MPa). This severe operating conditions can lead to high installation costs, and thereby practical applications of this process are limited. The catalytic wet peroxide oxidation(CWPO) is a liquid phase catalytic oxidation with hydrogen peroxide. The catalytic wet peroxide oxidation could be a more efficient process than the catalytic wet oxidation since the oxidizing properties of hydrogen peroxide are stronger than those of molecular oxygen. Moreover, the reaction conditions when hydrogen peroxide is used as oxidant can be lowered upto 80 ~ and l atm[7], allowing the possibility to treat a large amount of polluted wastewater without a too large energy consumption. In our previous paper copper was proved to act as a catalyst to accelerate the decomposition of H202 into hydroxyl radicals(HO.)[7]. Homogeneous copper nitrate, copper chloride and copper This research was supported by Korea Ministry of Industry and Resource.

684 sulfate catalysts were extremely active for the complete oxidation of organic pollutants in water into CO2 and H20. However, additional separation process was required for the removal of copper cation before the treated water was discharged. Accordingly a new heterogeneous catalyst containing copper cation needs to be developed for the practical application of the catalytic peroxide oxidation. Pillared clays are thermally stable microporous solids which are promising catalysts in numerous areas[8,9]. Copper-containing pillared clay might be a promising catalyst for the successful wet peroxide oxidation of organic pollutants. Dyehouse effluents from the textile industry impose serious environmental problems because of their color and their high chemical oxygen demand (COD). Discharge of highly colored waste is not only aesthetically displeasing, but it also interferes with the transmission of light and upsets the biological processes which may then cause the direct destruction of aquatic communities present in the receiving stream. Of all the dyestuffs used the reactive dyes present major problems because they get hydrolyzed to the extent of 20% while dyeing textile substrates and therefore are discharged into the effluents in unrecoverable form. They possess high tinctorial power and, therefore, always exist in textile dyeing effluents, though in trace amounts. The removal of color and COD from dyehouse wastewater to meet the discharge standards is currently a major problem in the textile industry. In this study catalytic wet peroxide oxidation of reactive dyes was carded out in a batch reactor using mixed (A1-Cu) pillared clays as catalysts, and the catalytic performance of the clays was investigated. 2. EXPERIMENTAL 2.1. Materials

High purity reactive black 5 and blue 19 from Aldrich Co. were employed in this study, and the chemical structure of these dyes are shown in Figure 1. Pure bentonite(DongYang Bentonite Co.) was used as the starting clay without any further purification procedure. A1 and Cu chloride(Aldrich Co.) were used as the precursors for pillaring solution.

NaO3SOCHzCHa--~~ N=N SO~4a C) ~k~/ HO 'k/-~ H2N O tt NaO3SOCH2CI'lz~ N-N/ ~O--~~ Reactive black 5

\ SO3Na

~" I

~" O

H2

SO3Na

NH---~ \~/

~/

\ SO2CHaCHaOSO3 Reactive blue 19

Figure 1. Chemical structures of reactive black 5 and reactive blue 19.

685

2.2. Catalyst preparation A1-Cu pillared clays were prepared by direct introduction of AI-Cu pillaring solution into the dilute bentonite suspension. The pillaring solutions were prepared by dissolving 0.1M A1 and Cu chlorides in 0.2M NaOH solution. The hydrolysis molar ratio OH/(AI+Cu) was kept to be 2. Solution volumes were adjusted to have a Cu/(AI+Cu) ratio between 0% and 20%. The pH of the solution was about 3.8 and the solutions remained clear. The dilute bentonite suspension(l%w/w) was prepared by adding the purified bentonite powder into the corresponding distilled deionized water. The pillaring reaction was carried out under continuous vigorous stirring at 40 ~ by adding the pillaring solution drop by drop into the bentonite suspension. The prepared samples were filtered and washed with deionized water until Cl-free samples were obtained, and then dried at 120 ~ for 6h. The dried samples were finally calcined at 400 ~ for 6h.

2.3. Reaction procedures and analysis The oxidation of two reactive dye aqueous solution was performed in a glass reactor of 1L capacity equipped with a condenser, stirrer and air flow controller. The reaction were conducted at atmospheric pressure and 80 ~ Air was bubbled into the solution during the reaction, and the flow rate of air was kept to be 200mL/min. Liquid sample were immediately filtered and analyzed for total organic carbon(TOC), hydroxyl radical(HO.), color unit and residual materials in water. TOC was measured with a Shimadzu 5000A TOC analyzer. Electron paramagnetic resonance(EPR) spin trapping of the HO" formed during the catalytic wet oxidation was performed using a Varian E-4 spectrometer. 5,5-Dimethyl-l-pyrroline N-oxide(DMPO, purchased from Aldrich Co.) was used as a trapping agent because it efficiently scavenges HO" through the following reaction to produce the DMPO/HO- adduct, which has a characteristic EPR spectrum[7,10].

MeMe~-~H + I

O-

140.

=

Me H Ma~,.N[-N--~OH t

O-

Color unit of the samples was measured by following ADMI(American Dye Manufacture Institute) tristimulus filter method[l 1], and H202 concentration was measured by a colorimetric method using a UV/Visible DMS 90 Varian spectrophotometer[12]. X-ray diffraction(XRD) patterns of the A1-Cu pillared clay were recorded at a scanning rate of 1~ 20/min with a Simens D-5000 diffractometer(CuKtx radiation). Surface area was determined by using nitrogen as the sorbate at 77K in a static volumetric apparatus(Micromeritics ASAP2010). 3. RESULTS AND DISCUSSION

3.1. Characterization of the catalyst X-ray diffraction patterns of the starting bentonite and A1-Cu pillared clays are shown in Figure 2. The prepared pillared clays will be abbreviated by the symbols of A1-PILC and Al(mole% in the pillaring solution)-Cu(mole% in the pillaring solution)-PILC. A1-PILC

686 denotes the pillared clay with alumina. AI(90)-Cu(10)-PILC is, for example, the AI-Cu pillared clay prepared by the initial pillaring (e) solution having the 90mole% AI and 10mole% eu, respectively. The 20 angle of the (001) (d) reflection of the pure bentonite was 7.5 ~ which corresponded to a d(c) spacing of 11.77 A. The corresponding 20 angles of the (hk) two-dimensional peaks were at 19.6 ~ (b) and 35.3 ~. The diffraction at 20 of 19.6 ~ was the summation of hk indices of (02) and (01), and the diffraction of 35.3 ~ was the summation of hk indices of (13) and I , I ~ I ~ I , , I (20). The peak at 20 of 27.9 ~ was a 50 0 I0 20 30 40 2O reflection of the quartz impurity[9]. Upon pillaring with AI and Cu the Figure 2. XRD patterns of bentonite(a), A1d001 peak shifted lower 20 values of PILe(b), Al(95)-Cu(5)-PILC(c), Al(90)-eu(10)PILC(d),AI(80)-Cu(20)-PILC(e). about 4.9 ~ corresponding to the increase in the d0o~ spacing, while the rest of the structure was not clearly affected. The d00~ spacing of the pillared clays was about 18.0A. In Table 1 are listed the summarized properties of the AI-Cu pillared clays. BET surface area increased significantly after intercalation.

3.2. Catalytic wet peroxide oxidation of reactive dyes To assess the extent of uncatalyzed oxidation of reactive black 5 solution, wet oxidation was performed without any catalyst and H202 at atmospheric pressure and 80"C. The initial concentration of reactive black 5 solution was 1,000mg/L. As can be seen in Figure 3, no detectable extent of the uncatalyzed oxidation of the reactive black 5 solution could be achieved. Even in the presence of 10g Ai(90)-Cu(10)-PILe the reaction did not proceed at all. The addition of H202(20mL, 0.5N), however, enhanced the efficiency of the oxidation remarkably. Most of reactive black 5 could successfully be oxidized within 20min. Table 1. Summarized prc ~erties of A1-Cu pillared clays d001(A) surface area(m2/g) Clay Bentonite AI-PILC AI(95)-Cu(5)-PILC AI(90)-Cu(10)-PILe Al(85)-Cu(15)-PILe AI(g0)-Cu(20)-PILC

11.77 17.02 18.1 18.0 18.0 18.0

9

33.2 142.3 164.5 149.4 146.8 142.9

Cu(%) 0.80 1.12 1.71 2.50

687 100

100 .=.-

80 A

(.,) 0 I..-

lOO

8o

8o

20

20

60

- i

40 20 =

5

10

15

20

25

_~ 30

Time(min)

Figure 3. Changes in TOC during the oxidation of reactive black 5 solution (O:uncatalyzed oxidation(without catalyst and H202), A:catalytic wet oxidation(with 10g AI(90)-Cu(10)-PILC in the absence of H202), ==:catalytic wet peroxide oxidation (with 10g AI(90)-Cu(10)-PILC and H202).

I

0

0

5

10

15

20

25

,0 30

Time(rain)

Figure 4. Correlation between TOC removal(O), H20~ consumption(A) and HO. formation(ll) during the catalytic wet peroxide oxidation of reactive black 5 with 10g AI(90)-Cu(10)-PILC.

The addition of H202 to wet oxidation systems has been known to enhance the reaction rate leading to high conversion in short time[13]. The fast reaction rate of the catalytic wet oxidation with H202 as opposed to the uncatalyzed oxidation and the catalytic wet oxidation with air is due to the decomposition of H202 to give two hydroxyl radicals which react with reactive black 5 in water. In Figure 4 are shown the removal of TOC together with the concentration of H202 consumed and HO. produced during the reaction with reactive black 5 in the presence of 10g AI(90)-Cu(10)-PILC. The removal of TOC was shown to be strongly related to the consumption of H202 which will be decomposed into HO.. A separate experiment of H202 decomposition in the absence of any reactive black 5 was carried out at the same reaction condition. The concentration of H202 was the same as that in the experiment of Figure 4. The measured changes in the concentration of H202 and HO. are plotted in Figure 5. As seen, in accordance with the consumption of H202 the formation of HO- occurs during the reaction. The rates of both the H202 consumption and HO. production increased greatly by the action of AI(90)-Cu(10)-PILC which must have played an important role on the activation of H202 decomposition and the subsequent HO. formation. The subtracted amount of HO., corresponding to the difference between HO- formed in Figure 5 and HO. remained in Figure 4 must have participated in the oxidation of reactive black 5 in water. As discussed from the result in Figure 5, AI(90)-Cu(10)-PILC catalyst could increase the production rate of HO" greatly. This indicates that the use of a catalyst will further enhance the rate of oxidation of reactive black 5 solution. Figure 6 shows a comparison between the results of wet peroxide oxidation with A1-Cu pillared clays having different amount of Cu.

688 lOO -

2 ~

_m

80

ttl

80

=~

60

-,-.m

0 e~ 40 "I" 2O 00

lOO

n I

-

C

m

5

10

15

20

Time(rain)

25

30

Figure 5. Time dependence of H 2 0 2 conversion and HO" formation during H 2 0 2 decomposition in t h e absence o f the catalyst(O) and in the presence of 10g Al(90)-Cu(10)-PILC(A).

2O 0

0

5

10

15

20

25

30

Time(rain)

Figure 6. Effects of Cu content in the AICu pillared clays on the removal of TOC(O:A1-PILC, A:AI(95)-Cu(5)-PILC, I:Al(90)-Cu(10)-PILC, V: AI(85)-Cu(15)PILC, . : Al(80)-Cu(20)-PILC).

There was a considerable increase in the reaction rate by using A1-Cu pillared clays instead of A1-PILC. About 14% removal of TOC was achieved in 30min with the A1-PILC, while in the presence of the A1-Cu pillared clays it took only about 20min for the complete removal of TOC. In addition the removal efficiency of TOC increased with increasing amount of Cu in the AI-Cu PILCs. Catalytic wet oxidation of reactive blue 19 was also conducted in the presence of 10g AICu pillared clay catalysts, 20mL 0.5N H202 solution. The initial concentration of reactive blue 19 solution was 1,000mg/L. As shown in 100 Figure 7, reactive blue 19 could successfully be removed by using AI-Cu pillared clay as 80 catalysts. Figure 8 represents the results of TOC and =~ 60 color unit change during catalytic wet 0 oxidation of reactive black 5 and reactive blue o k40 19 at five different initial H202 concentration in the presence of 10g AI(90)-Cu(10)-PILC. 20 When the H202 dosage was 5mL, the final TOC values were not significantly different 0 from the initial values, while the color units 0 5 10 i5 20 25 30 have dropped by about 60%. This means that Time (min) the consumption of H 2 0 2 does not Figure 7. Effects of Cu content in the A1- immediately mineralize the organics in the Cu pillared clays on the removal of reactive dye solution. Most of the organic TOC((O:A1-PILC, A:Al(95)-Cu(5)-PILC, carbons remain in the solution, but the newly ll:Al(90)-Cu(10)-PILC, V:AI(85)-Cu(15) formed organics have lower color unit per molecule. In addition both the removal of -PILC, O:AI(80)-Cu(20)-PILC). TOC and color unit was completed within A

689 100

=

=

=

._.

=

~100

100

100 It.

80

80

80

"e ";&'It

~

60

60

40

40

60

I-.

40

_o

0

0 I-

A

=~

00

"-

9 ".. 9

20

0

5

10

15

20

Time (min)

25

30

0

"'o.. " "~

: $..

20

"'.

20

""...

"-

" " " "O.

.

s

" " * .....

40

.... " . . . . .

20

-... .&.

":~

0

60

""0.

1~

i% 2~

v

2s

_o o

r

,,~0

3o

Time (min)

(A) (B) Figure 8. Effects of concentration on the removal of TOC(--) and color(---) during the catalytic wet oxidation of reactive black 5(A) and reactive blue 19(B) with 5mL(O), 10mL(A), 15mL(n), 20mL(V) and 30mL(~) of 0.5N 1-1202solution. 20min when the dosage of H202 w a s more than 20mL. These behaviors suggest that the oxidation proceeded in more than one step. The first step involves the breakdown of the large dye molecules into smaller molecules of intermediate organics. The next step will be the degradation of the smaller molecules into carbon dioxide and water. The residual organics remaining in the solution during the reaction were analyzed and identified with a HPLC-Mass spectrometer. The main organics were lower molecular weight carboxylic acids such as acetic acid, maleic acid, fumaric acid, oxalic acid and formic acid. Acetic acid and maleic acid are known to be highly refractory materials, and the oxidation of these carboxylic acids was proved to be the rate-controlling steps in the conventional catalytic wet oxidation with air[6,14,15].

3.3 Catalytic wet oxidation of a real dyehouse effluent A real effluent, produced from the washing process of a certain dyeing industry, was employed for the catalytic wet oxidation with 10g AI(80)-Cu(20)-PILC and 20mL 0.5N H202 solution. In order for dyeing textile substrates the industry had used the aqueous solutions of reactive black 5, reactive blue 19 and reactive red 198. In addition small amount of some penetrating agent together with NaOH were contained in the effluent. The dark black reddish effluent had TOC value of 6,900mg/L and its color unit was 5,200. Figure 9 shows the time dependence of the removal of TOC and color. Most of the TOC and color unit were removed after 30min reaction. The visual appearance of the effluent changed greatly during the reaction. The dark black reddish color began to disappear, and was remained strong red color which became weaker and weaker. After 30min reaction the weak red color was completely discolored. 3.4 Stability of the catalyst During the catalytic wet peroxide oxidation the active component Cu might be leached out from the A1-Cu pillared clays. To investigate the stability of the AI-Cu pillared clays with respect to metal leaching, the concentrations of dissolved Cu and A1 in the solution were analyzed using ICP. NO detectable amount of dissolved Cu and A1 could be measured.

690 7000 5OOO

60O0

4000

50OO

Accordingly at the reaction conditions employed in this research no leaching of Cu and A1 can be said to occur. 4. CONCLUSIONS

2000

Q

2000

IOO0

1000 0

0

5

10

15

20

Time/mint

25

30

0

Figure 9. Removal of TOC(--) and color(---) during the catalytic wet oxidation of a real dyehouse effluent with 10g AI(80)-Cu(20)PILC.

A1-Cu pillared clays were prepared by direct introduction of A1-Cu pillaring solution into the dilute bentonite suspension, and the prepared AI-Cu pillared clays were employed as catalysts for the wet peroxide oxidation of reactive dyes at atmospheric pressure and 80 ~ The pillaring reaction for the preparation of A1-Cu pillared clays was performed at 40~ with OH/(AI+Cu) molar ratio of 2 and at Cu/(AI+Cu) ratio

of 0, 5, 10 and 20%, respectively. Upon intercalation the d00~ spacing increased from 11.77A to 18.0A. In addition the surface area of the pillared samples also increased from 33.2m2/g of the pure bentonite upto 164.5m2/g. AI-Cu pillared clays were proved to act as excellent catalysts for the wet peroxide oxidation of reactive dyes. The initial 1,000mg/L reactive dyes could be completely removed in just 20min with 10g AI(90)-Cu(10)-PILC at atmospheric pressure and 80 ~ The catalysts were also extremely stable against the leaching out of active Cu component into the aqueous solution. REFERENCES

1. E Luck, Catal. Today, 27 (1996) 195. 2. J. Levee and A. Pintar, Catal. Today, 24 (1995) 51. 3. A. Pintar and J. Levee, Catal., 135 (1992) 345. 4. P. Gallezot, N. Laurin and P. Isnard, Appl. Catal. B, 9 (1996) L 11. 5. D. Duprez, F. Delanoe, Jr. J. Barbier and P. Isnard, G. Blanchard, Catal. Today, 29 (1996) 317. 6. D.-K. Lee and D.-S. Kim, Catal. Today, 63 (2000) 249. 7. D.-K. Lee, D.-S. Kim and S.-C. Kim, Stud. Surf. Sci. Catal., 133 (2001) 297. 8. A. Vaccari, Catal. Today, 41 (1998) 53. 9. R.T. Yang, N. Tharappiwattananon and R.Q. Long, Appl. Catal. B, 19 (1998) 289. 10. E.P. Sargent and E.M. Grady, Can. J. Chem., 54 (1976) 275. 11. W. Allen, W.B. Prescott, R.E. Derby, C.E. Garland, J.M. Peret and M. Saltzman, Proc. 28 ~ Ind. Waste. Conf., 142 (1973) 661. 12. G.M. Eisenberg, Ind. Eng. Chem., 15 (1943) 327. 13. S.H. Lin and Y.F. Wu, Environ. Technol., 17 (1996) 175. 14. H.R. Devlin and I. Harris, J. Ind. Eng. Chem. Fundam., 23 (1984) 387. 15. L. Li, P. Chem and E.F. Gloyna, AICHE J., 37 (1991) 1687.


691

Application of zeolites as supports for catalysts of the ethylene and propylene polymerization I.N.Meshkova, T.A.Ladygina, T.M.Ushakova, N.Yu.Kovaleva, L.A.Novokshonova Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygin St.4, 119 991 Moscow, [email protected]

The Na-form of zeolite ZSM-5 (SIO2/A1203=24) was studied as a support for organometallic catalysts of olefin polymerization. Zeolite-fixed aluminoxanes were prepared by partial hydr~ ,ys~s of trimethylaluminium with the zeolite's internal water.It was shown that aluminoxanes synthesized on the zeolite surface form heterogenized complexes with CpzZrC12 and Et[Ind]zZrC12 which are active for a long time in ethylene and propylene polymerization without addition of another aluminiumorganic cocatalyst. Yields were up to 1200kgPE/molZr.bar.h. and 4000kgPP/molZr.bar.h.The activation energy of ethylene and propylene polymerization in the presence of Na-ZSM-5(HzO)/A1Me3-Et[Ind]2 ZrCI2 is equal to 32 and 48,5 kJ/mol, respectively. Molecular weight and melting point of polyethylene obtained with such zeolite supported Zr-cene catalysts are higher than those of polyethylene formed with the corresponding homogeneous metallocene systems.

1. INTRODUCTION Immobilization of organometallic catalysts for olefin polymerization arose as a challenge immediately after they have been discovered and continues to be of particular significance. Most publicati, ,s in this field are focused on catalysts supported on the inorganic carrierssilica and alumina. The first works involving zeolites as supports appeared as recently as the mid-1990s [1-4]. All methods used for heterogenization of soluble metallocene catalysts on zeolite require the introduction of the external methylaluminoxane (MAO) or AIR3 to the supported catalyst for its activation in olefin polymerization [ 1,2,4]. In the present paper we report the application of zeolites as supports for metallocene catalysts for ethylene and propylene polymerization. Earlier we developed the method of metalorganic catalysts preparation on the surface of hydrated supports [5,6]. By this method fixed alkylaluminoxanes were formed on the surface of undehydrated Na-ZSM-5 zeolite by partial hydrolysis of trimethylaluminium (TMA) with intemal zeolite's water and were used for the heterogenization of Zr-cene (CpzZrC12, Et[Ind]2ZrC12) complexes. The kinetics of ethylene and propylene polymerization on Na-ZSM-5(HzO)/TMA-Zr-cenes systems were investigated and the properties of PE and PP obtained with these zeolite supported Zr-cene catalysts and appropriate homogeneous systems were compared.

692 2. EXPERIMENTAL SECTION

2.1. Materials The Na-form "f ZSM-5 had the following characteristics: SiO2/A1203 =24, total volume of pores was equal to 0,287 cm 3 / g, surface area determined with the use of benzene to 40 m 2/g, the inside water content - to 8,9wt.%. Components of catalysts: Cp2ZrC12 and Et[Ind]2 ZrC12 were used as received from Aldrich Co. TMA (from GNIIHTEOS, Moscow) and MAO (10 wt% in toluene solution, from Aldrich Co) were used without further purification. TMA contained 37,2wt% of A1 and 60,6wt% of Me. Toluene ( from Aldrich Co) was used as a solvent. Ethylene and propylene were of polymerization-grade purity. 2.2. Preparation of supported catalysts The preparation of zeolite supported catalysts involved two steps : the preparation of the zeolite-fixed methylaluminoxanes and the formation of heterogenized complexes from the zirconocene and fixed alkylaluminoxanes. The synthesis of fixed methylaluminoxanes and the preparation of zeolite supported Zr-cene catalysts were realized in a 0,4 L glass reactor equipped with stirrer and water jacket for thermostating. The reactor was evacuated and then filled with argo'-. Zeolite (1,3 g) containing 8,9 wt% of internal water was introduced into the reactor. After the removal of argon from the reactor the zeolite was suspended in 30 cm 3 of toluene. TMA (43 wt% solution in toluene) was added dropwise to the uncalcinated zeolite over one hour. The reaction of TMA with the zeolite water was carried out at 22 ~ with intensive stirring of components. The end of partial hydrolysis of TMA by the zeolite water was determined by the completion of the gaseous product (methane) evolution. After removal of methane from reaction zone 30 cm 3 of a toluene solution of the zirconocene (Cp2ZrC12 or of Et[Ind]2 ZrCI2) was added. The Zr content in catalyst was varied in the range of 7.10 .6 - 3.10 -3g/g zeolite. Molar ratio of A1/Zr was changed from 140:1 to 5000:1. 2.3. Polymerization of ethylene and propylene Slurry ethylene and propylene polymerization was carried out in the same reactor where the catalytic complex had been prepared. The ethylene was introduced into the system immediately after Zr-cene addition.The monomer concentration, temperature and rate of stirring were kept constant during a polymerization run. 2.4. Characteristics of PE and PP The molecular-weight characteristics of PE samples were measured by GPC (Waters,150C) in ortho-dichlorobenzene at 140 ~ C. The melting point (m.p.) and crystallinity(~) of polymer products were determined on a Dupont differential scanning calorimeter (DSC 910), with a heating rate of 10 K. min l from295 to 425 K. The PP isotacticity was determined by the Luongo method [8].

3. RESULTS and DISCUSSION A peculiarity of zeolites is a high mobility of the inside zeolite water. In contrast to the structure water of the other highly hydrated supports, for example, aluminium hydroxide and

693 Table 1. The consumption of AOC in dehydration of supports and the yield of gaseous products

H20 in

jg_o

3 4

Supw'rt

support, wt.%

Uncalcinated zeolite Zeolite heated at 300~ Uncalcinated kaolin Uncalcinated AI(OH)3

AOC

AOC consumption, mmol/g

RH

Yield of RH, mmol/g

8,9

A1Me3

5,4

CH4

5,8

5,4

AIMe3

2,2

CH4

2,25

13,8

A1Et2C1

0,1

C2H6

0,1

36,4

A1Et2C1

0,08

C2H6

0,08

kaolin, the zeolite water is completely removed by the heating up to 800- 1000~ without the zeolite structure destruction and can be readsorbed [7]. It was shown that in the case of zeolite aluminiumalkyls react both with water adsorbed on the external surface of zeolite and with water in its channels. As is seen ~rom Table 1 data the consumption of aluminiumorganic compound (AOC) in the dehydration process of zeolites as well as the gas (RH) evolution accompaining this process are markedly higher (Table 1, runs 1,2) than in chemical dehydration of other inorganic supports by AOC (Table 1, runs 3,4). We compared the sorption properties of zeolite ( Na-ZSM - 5, previously heated in vacuo at 300 ~ with respect to TMA and water. According to the isotherms of adsorption (Fig.l, curve l) the gaseous TMA does not penetrate into the micropores of the zeolite contrary to water (Fig. 1, curves 2,3). On the base of these sorption data we propose that the fixed methylaluminoxanes from TMA and zeolite's water and the Zr-cene complexes with fixed methylaluminoxanes can not be formed inside of the zeolite structure.In reaction with TMA the zeolite's water migrates from channels of zeolite and the formation of fixed aluminoxanes and then of complexes with Zr-cene takes place on the external zeolite surface.

o~,mmo! '~, zv

10 8 ~2 6 24~_y;_ L__~-L- ~ - ....} 2 --1 0

0

0,2

0,4

0,6 P/Ps

0,8

,

o

9

1

Fig. 1. Isotherms of adsorption at 22 ~ C of TMA (1), adsorption (2) and desorption (3) of H20 for Na-ZSM-5 zeolite heated in vacuo at 300~ C.

694

,400

............................. a;

Rp, k g P E / m o l Z r bar h

............

1000

i

6OO

-

---~--"'

--

I

200

,

---

Fig.2. Rate-time profiles for ethylene polymerization with zeolite supported Zrcene catalysts on the base of Cp2ZrC12 (a) and Et[Ind]2ZrC12 (b).

....

1

_--~__._....~

...................................................

-3- .........

0 0

20

40

60

time, min

Rp, k g P E / m o l Z r

bar h

1200

800

b) .

.

.

.

1

400 ,,

0

.

.

,

100 time,

3~~

~

~

50

b) Na-ZSM-5(H20)/TMA/Et[Ind]2ZrC12; toluene; 0,23 bar; 0,05 wt.% Zr; [A1]/[Zr]=4300; Ypo~.,~ 34 (1), 50 (2), 70(3)

-:

t 0

~o_o

a) Na-ZSM-5(H20)/TMA/Cp2ZrC12 (1,2,3); homogeneous Cp2ZrC12-MAO catalyst (4); toluene; 0,46 bar; Tpol. = 34~ Zr, wt.%: 0,024 (1), 0,028 (2), 0,16 (3); [Zr] x 105 , mol/l: 14 (4); [AOC] x 103 , mol/l" 68(1), 60 (2), 90 (3). 70 (4).

150

,

'

--~ 200

min

The high content of inside zeolite water (about 9 wt.%) and its mobility permit to form aluminoxanes on the zeolite surface in the amount enough for activation of Zr-cene component of catalyst. The systems Na-ZSM-5(HzO)/TMA/CpzZrCI2 and Na-ZSM5(HzO)/TMA-Et[Ind]2 ZrCI2 are active in the ethylene polymerization without addition of other cocatalysts for a long time (more than 100 hours). Yields were up to 1200 kgPE/molZr bar h. and 4000 kgPP/mol Zr.bar.h. (Fig.2a,b and Table 2). By this means the zeolite supported Zr-cc;,e catalysts put forward by us are significantly different from known catalysts [1,2,4,9] obtained by adsorption of zirconium compound on zeolite calcined and treated by MAO. These zeolite/MAO/Cp2ZrCl2 catalysts [2] are inactive in ethylene polymerization.They are activated only by addition of free MAO into the reaction zone. The kinetic .regularities of ethylene and propylene polymerization by our zeolite supported Zr-cene catalysts (the polymerization rate dependences of Zr-cene and monomer concentration and of temperature) are the same as for other immobilized catalysts for olefin polymerization.

Table2. Ethylene and propylene polymerization with the zeolite supported and homogeneous Zr-cene catalysts.

Run

Catalyst

i

molar

Mw

Aa

Mw/Mn

m.p.2

OC

mol/l

hb, %

Iiso c,

%

-

111

3.007 4600 1650 320 3.024 C2H4 470 3.160 0.140*

0.058 0.058 0.056 0,056

34 34 34 34

82 475 500 1500

202750 273800 52900 133700

22 10 18 4.8

133.5 135 123.5 127.5

85 85 76 75

5.

IV

0.018

0.025

75

1200

135700

30

127

81

6. 7. 8. 9. 10.

IV IV IV IV V

0.030 0.015 0.016 0.018 0.026*

30 40 62 75 75

230 48500 4200 43000 1155 3600** 3160 1500** 6600 2O0Ob*

18.6 8.8

135 123 waxes waxes waxes

63 50

1. 2. 3. 4.

I I I1

4300

I 840 4350 4100 5000 1725

C3H6

0.327 0.260 0.119 0.125 0.125

695

Clatalyst: I- ZSM-S(H20)/TMA/Cp2ZrC12 ; 11- ZSM-S(H20)/TMA/Cp2ZrClz + MAO; 111- CpzZrClz -MAO; IV- ZSM-5(H20)/TMA/Et[Ind]2ZrClz ;V- Et[Ind]zZrCl2 +MAO. 1- Activity of catalyst, kg polymer/ mol Zr.bar.hour; 2- The polymer crystallinity; c- The PP isotacticity [lo]. li- [Zr], molil; **- Mn.

-

88 82 40 10 15

696 Log Rp/IM]

Fig.3. Log(Rp / [M]) vs.1/T with Na- ZSM-5 (H20)/TMA/Et[Ind]2ZrC12 catalyst. Monomer: ethylene (1), propylene (2).

4.00 3.50 3.00 2.50 2.00 2.80

-

I

-

3.00

3.20

3.40

I/T103, K -1

The ethylene and propylene polymerization rates (Rp) with this zeolite supported catalyst increase with increasing temperature in the range of 30-75~ (Fig.2b,3). The energy of activation of ethylene polymerization is equal to 32 kJ/mol, in the case of propylene polymerization - 48,5 kJ/mol. At the same time it is known that the rate of ethylene polymerization in the presence of homogeneous Zr-cene systems reaches a maximum value in the range of 45 - 50 o C and decreases with a further increase of temperature [ 10]. The specific rate of ethylene polymerization increases with the enhancement of the Zrcene content in Na-ZSM-5(H20)/TMA/Cp2ZrC12 catalyst. It peaks at the surface Zr-cene conzentration enue i to about 3.10 .6 mol Zr/g zeolite and then Rp decreases. The complex dependences of Rp of ethylene and propylene polymerization on monomer concentration .were observed. The olefin concentration dependences of Rp have the fractional order between 1 and 2. That is two olefin molecules are involved in the insertion transition state [ 11 ]. The PE and PP properties obtained with Na-ZSM-5(HzO)/TMA/Zr-cene catalysts are presented in the Table 2. It is seen that the molecular weight, molecular weight distribution (Mw/Mn ratio); the melting point and crystallinity of polyethylene formed with zeolite supported catalyst are higher than those of polyethylene obtained with homogeneous CpzZrCI2 - MAO catalyst (Table 2, runs 1,2 and 4). The tendency for an increase of molecular weight and the melting point of PE in the case of catalyst immobilization on the support is a typical for metallocomplex catalysis.The PP properties obtained with zeolite supported and homogeneous Zr-cene catalysts are closely related (Table 2, run 9 and 10). The external MAO addition to Na-ZSM-5(H20)/TMA/Cp2ZrC12 catalyst increases its activity but decreases the molecular weight and melting point of polyethylene formed (Table2, run3). It is possible that part of active centers of the zeolite supported catalyst transfer to solution from the surface upon introduction of a free MAO into the reaction zone.

4. CONCLUSION The use of heterogenized Zr-cene catalysts based on fixed aluminoxanes as a product of partial hydrolysis of TMA with internal zeolite's water, allows to replace unstable cocatalyst

697 MAO by TMA and avoids the addition of free MAO for formation of the zeolite supported Zr-cene complexes. We suppose that the synthesis of MAO directly on the zeolite support and the absence ot free MAO may be one of the ways to the reduction of supported Zr-cene catalyst leaching. In this case the appearance of homogeneous active centres in the reaction zone is less possible. The positive temperature coefficiency of polymerization rate as well as the increase of molecular weight and melting point of PE obtained with the zeolite supported Zr-cene catalyst developed in this work, compared to PE produced by the homogeneous Zr-cene system confirms this view.

REFERENCES

1. Ciardelli F., Altomare A. Conti G.,//Macromol. Symp. 1994. 8__00.P.29. 2. Woo S.I,Ko Y.S., Han T.K//Macromol. Rapid. Commun. 1995.16. P.489. 3. MeshkovaI.N.,UshakovaT.M.,GuruliN.T.,GultsevaN.M., Novokshonova L.A.// Intern.Symp. "ZEOLITE-95"Sofia,Bulgaria. 1995.P.238 4. Marques M.F.v.,Coutinho F.M.B.//36 Intern.Syrup. Macromolecules.Seoul, Korea. 1996. 5. USSR Invei;tor's Cetificate 1066193//(1982).Byul.Izobr. 1985. P. 199. 6. Meshkova I.N.,Ushakova T.M., Dyachkovskii F.S.//31 IUPAC Macromolecular Symp. Merseburg. GDR. 1987.P. 100. 7. Dahl J.M., Jens K.J.//Catal. Today. 1992.13. P.345 8. Luongo J.P.//J. Appl. Polym. Sci. 1960.3. P.302. 9. Michelotti M., Arribas G., Bronco S., Altomare A.,//J.Mol. Catal. A: Chemical. 2000.152. P.167. 10. Giannnetti E., Nicolletti G.M. and Mazzochi R. J.Polym.Sci. Part A" Polym. Chem. 1985.23.P.2117 11. Ystenes M.//J. Catal. 1991. 129. P.383.


Studies in Surface Science and Catalysis 142 R. Aiello, G. GiordanoandF. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

699

Catalytic properties of Beta zeolite exchanged with Pd and Fe for toluene total oxidation J. Jacquemin, S. Siffert*, J.-F. Lamonier, E. Zhilinskaya and A. Abouka'fs Laboratoire de Catalyse et Environnement, E.A. 2598, Universit6 du Littoral - C6te d'Opale, MREID, 145, avenue M. Schumann, 59140 Dunkerque, France

Dealuminated beta zeolites exchanged with Pd and Fe were prepared to investigate the influence of iron and dealumination on the activity and selectivity of Pd/BEA zeolite for toluene total oxidation. The specific areas determined by BET method and EPR studies allowed to know that the palladium would be more easily agglomerated on the BEA than on the DBEA. Moreover, a quantification of the palladium saturation on the BEA zeolite was deduced by EPR. Effects of dynamic and static oxidation and weak and strong reduction treatments were studied by EPR. Several isolated and interacted Pd + species and hole centers were detected. The Pd was much reduced after the catalytic test in dealuminated and Fe doped samples. This result could be directly correlated to the catalytic deactivation. The deactivation could be also explain by the type of coke deposed on the catalyst and by the hydroscopic behavior of the samples. Addition of Fe or dealumination could prevent the deactivation and then lead to better catalysts for VOCs oxidation. 1. INTRODUCTION Volatile Organic Compounds (VOCs) in industrial gas represent a serious environmental problem. An effective way of removal is complete catalytic oxidation to harmless products such as H20 and CO2. In order to make the reaction economically attractive, highly active catalysts at low temperatures are required. Supported precious metals such as Pt and Pd are well established as efficient catalysts for VOC combustion [1 ] and palladium is cheaper and often more active for oxidation than platinum [2]. Moreover, iron is often used for catalytic oxidation of VOCs [3] and could imply special properties of the catalyst for deactivation [4]. However, the support is also very important for the efficiency of the catalyst. Zeolites have a good potential for VOC adsorption but the formation of water during the combustion could be a deactivation agent [5]. Therefore, in this paper, we prepared dealuminated beta zeolites exchanged with Pd and Fe for a good activity and low deactivation. Toluene, which is often found in industrial exhausts, has been chosen as probe molecule for the catalytic oxidation test. The objective of this work is to investigate the influence of iron and dealumination on the activity and selectivity of 0.5wt%Pd/BEA zeolite for toluene total oxidation. The palladium valence was also studied for this reaction.

700 2. EXPERIMENTAL 2.1. Catalyst preparation Na-Beta zeolite (Si/AI=10, from P.Q. Corporation, called BEA) was dealuminated by HC1 (0.2M, 3h, 80~ and exchanged with NaC1 (1M, 2h, 60~ 7 times) to obtain the solid called DBEA (Si/Al=66). The samples were exchanged by palladium (18h, 60~ to have 0.Swt%Pd/DBEA. The exchange by palladium was carried with two different complexes: Pd(NO3)2 for all the catalysts and Pd(NH3)4C12 only for the preparation of the solid called 0.5wt%Pd(2)/BEA to compare the influence of the both solids for the VOC oxidation. A second exchange was done by iron (Fe(NO3)2, 24h, 25~ to have 0.2wt%Fe,0.5wt%Pd/DBEA. [3-zeolite was also exchanged in the same ways without previous dealumination (samples called .../BEA). 2.2. Catalyst characterization The XRD patterns were obtained at room temperature with a Siemens D5000 diffractometer using Cu-K~ radiation. Chemical elementary analysis of the samples were obtained by the research centre of Vernaison (France, CNRS). Thermal analysis measurements were performed using a Netzsch STA 409 equipped with a microbalance. The sample (40 mg) was treated under a flow of air (75 mL.min -1) and the temperature was raised at a rate of 5~ min ~ up to 1000~ The specific areas of solids are determined by the BET method using a Quantasorb Junior apparatus, and the gas adsorbed at -196~ is pure nitrogen. The electron paramagnetic resonance (EPR) measurements are performed at 20~ on a EMX Bruker spectrometer. A cavity operating with a frequency of 9.3 GHz (X-band) is used. The magnetic field is modulated at 100 kHz. Precise g values are determined from precise frequency and magnetic field values. Reduction studies of the catalysts followed by EPR were carried on a vacuum ramp with 5 vol%H2 in N2 or pure Ha. For the FTIR studies, self-supported wafers were pressed from 25 mg of each sample at a pressure of about 1 ton/cm 2 and then placed in a sample holder inside a Pyrex cell with NaC1 windows, which allowed the pre-treatment of the samples (300~ in vaccum < 10-6 Torr for 2 h), the introduction of toluene (1 gL (9.4.10 .6 mol) to 30 gL (28.2.10 -s tool)) and the recording of the spectra. The IR spectra were recorded after adsorption-desorption equilibrium of toluene using a Fourier Transform Perkin-Elmer Spectrometer Spectrum 2000 at room temperature. 2.3. Activity tests The samples were tested in the total oxidation of toluene for 24h on stream. Before the catalytic test, the solid (200 mg) was calcined under a flow of air (4 L.h l) at 500~ (2~ 1) during 4 hours. Toluene oxidation was carried out in a flow microreactor and studied at about 250~ Temperature was controlled at the intemal oven surface and in the catalytic bed. Toluene was mixed with a flash-injector in a flow of air (99 mL/min) adjusted by a mass flow controller. The space velocity was about 50000 h 1. The analysis of combustion products was performed evaluating the toluene conversion and the CO/(CO + CO2) molar ratio from a Perkin Elmer autosystem chromatograph equipped with TCD and FID.

701 Table 1 Loss of BET specific areas after Pd and Fe exchanges on BEA and DBEA (mE/g) 0.0 lwt%Pd 0.05wt%Pd 0.Swt%Pd 0.5wt%Pd + 0.2wt%Fe BEA DBEA

52 8

64 22

112 29

128


XRD patterns of the pure BEA and the dealuminated DBEA are similar. The cristallinity of the ]3-zeolite is then not altered by dealumination. The thermal behaviour and the decomposition of the palladium complexes (Pal(NO3)2 and Pd(NH3)4C12) of the samples 0.Swt%Pd/BEA, 0.Swt%Pd/DBEA and 0.5wt%Pd2/BEA are followed by thermal analysis and the TGA-DTA curves displayed two endothermic transitions for the all these samples. The first one corresponds to the loss of water at about 100~ The samples 0.5wt%Pd/BEA and 0.Swt%Pd(2)/BEA present a water loss of about 8 % whereas the sample 0.Swt%Pd/DBEA showed only 6 % water loss. This difference is due to dealumination which implies a lower hydrophilic character and should led to less deactivation of the catalyst during oxidation of VOCs [5]. The second endothermic transition around 230~ and 320~ for 0.5wt%Pd/BEA and 0.5wt%Pd(2)/BEA is due to the decomposition of the Pd complexes, respectively, Pd(NO3)2 and Pd(NH3)4CI2. The samples BEA and DBEA present important specific areas of, respectively, about 595 and 653 m2/g. The losses of BET specific areas after Pd and Fe exchanges are displayed in Table 1. The decrease of surface area after Pd exchange is always higher for BEA than for DBEA. More agglomeration of Pd on BEA than on DBEA could explain this tendency. This result will be correlated to EPR measurements. Fig. 1 shows the EPR spectra of 0.5%Pd/BEA recorded at T=293K for different treatment conditions : calcined at 773K under air (a); evacuated 293K (b); reduced at 523K (c) and reduced at 773K (d). All the spectra are composed of three signals A, B and S. The A signal centred at g = 4.30 could be attributed to isolated Fe 3§ ions presented as impurity in the solids [7]. The B signal has a g = 2.09+0.01 and a 8H = 1300G and is most remarkable in calcined and evacuated at 293K samples. Intensity of this signal (i) increases slightly with decreases of measurement temperature (exchange interaction with antiferromagnetic component); (ii) increases with increase the % of Pd exchanged up to 0.05w% and further stays still up to 0.5w% of Pd (saturation effect in quantity of paramagnetic Pd agglomerates); (iii) increases with increase of the reduction treatment (Fig. 2). On the base of these facts, we can suppose that some of the Pd ions are present in an oxidation state r 2 (may be in 3 or certainly 1) and are under the influence of the strong exchange interaction with ferrimagnetic or antiferromagnetic component. The interaction in Pd ions system should be antiferromagnetic because the decrease of the measurement temperature from 293K to 77K leads to a slight increase of the signal intensity (i). The same type of magnetic interaction has been observed in small Pd particles [8]. The absence of the structure peculiarities for B signal does not allow to determine unambiguously the valence state of Pd ions as it has been made in other works [9]. However, the number of this type of palladium species (B) was calculated as a function of the %Pd exchange on BEA and the saturation of this Pd species is found between 0.01 and 0.05 wt%Pd exchange (ii). Signal S with the parameters g=2,0045 and 8H = 15G and Lorentian line shape is observed in EPR spectra of all the samples after the reduction treatment at T=523K and higher. The intensity of this signal increases with increase of the exchange ratio

702 of Pd and the reduction treatment temperature (Fig.2). It should be noted that identification of the narrow symmetric line with g-=ge is difficult, but Lorentian shape of signal indicates the exchange interaction between the corresponding paramagnetic species. In our case taking into account the condition of reduction treatment and composition of studied solids, it should be the oxygen located hole for corresponding defect centres [10]. It must be noted that the increase of number of S centres is in inverse proportion to quantity of agglomerated B centres (iii), i.e. the reduction of agglomerated Pd ions from paramagnetic state to diamagnetic one leads to appearance of the defects of S type. The dealuminated sample 0.5%Pd/DBEA shows the same types of EPR spectra as the 0.5%Pd/BEA ones under vacuum and reduction treatment. The difference in quantity of signal B and signal S for these treatments are shown in fig. 2. It may be noted that the dealumination of the samples leads to a decrease of quantity of agglomerated Pd ions under all the conditions of treatment (in 1.6 time on average) and their easier reduction just up zero at 773K, an increase of S type of paramagnetic species under the same conditions (in 1.67 time on average). This result is correlated to the above BET study. After dynamic oxidation of 0.5%Pd(2)/BEA at 773K under flow of air and vacuum treatment at T=773K, only weak signal A is present. After the vacuum treatment at 673 and 77K with or without previous calcination, several EPR signals of paramagnetic defect species appears. Signal S' with g=2.0036, 5H = 15G and Lorentian line shape as the signal S may be also attributed to a oxygen located hole centre. This defect is more intense in the previously calcined samples. Under static oxidation (02, 2h, 293K), the signal S' completely disappears even without vacuum treatment (Fig.3). The static oxidation does not lead to creation of new paramagnetic species, on the contrary it eliminates the existed above defect species. Fig.3 shows the spectra EPR at T=77K of statically oxide Pd(2)/BEA under the weak (3%H2/Ar, p=200torr, T=293K) reduction conditions for different times of reduction. The large variety of Pd + centres are manifested themselves in these spectra. The impurity of Fe 3+ (signal A) and agglomerated Pd (signal B) are also present in whole EPR spectra (not shown), but the signals of defect S and S' are absent. A

60-

B a

i

50

b

2,5 2

~" 40 & 30 1

-

1

X 10

2o

d

x 0,09

1000

"

10 |

|

i

|

2000

3000

4000

5000

6000

H (Gauss)

Figure 1: EPR spectra at 293K of Pd/BEA zeolites (a) calcined at 500~ under air ; (b) treated under vacuum at 293K; reduced under H2 at (c) 523K, (d) 773K.

..................

without vaccum treatment 500~

H2, 20'

250~

0,5 0

H2, 20'

500~

Figure.2 Dependence of the quantity of agglomerated paramagnetic Pd species (NsPd) and of S species (NsS) on treatment condition for A: DBEA and [2" BEA.

703 The observed EPR signals with parameters: signals C (gl 1=2-628; g• ), signal D (g[ t=2.496), signal E (gl I=2.434) ; signal F (gl I=2-356) may be attributed to isolated Pd + [9] species in different local symmetry environment. It must be noted that the first appearance (perpendicular component of C signal) of Pd + is observed even after 5 minutes of reduction under the weak reduction condition. Further treatment during 50min at T=293K leads to increase of intensity of Pd + EPR spectra (C-F), which partially disappear (species E and F) during the longer reduction (5 h). The further increase of reduction time up to 15 h results in complete disappearance of all above observed Pd+ species and appearance of a new radical G signal (gl l=1.988; gx=2-0505) which is similar to signal observed earlier in [9] but under different (oxidation)treatment conditions and is attributed to Pd2+-Q radical. Fig.4 shows EPR spectra of the Pd(2)/BEA reduced under the strong reduction conditions (H2, 100torr) at T=293 and 393K for different times. A first 5 h reduction leads to appearance of a new Pd + centre (signal H: gl 1=2,444; gx=2.0944) or a transformation of one of precedent (C, D or E) ones and increase of intensity of earlier observed F centres. A continuous increase of reduction time up to 15 h eliminates the H signal. At the same time the beginning of the interaction between the Pd+ ions is manifested itself in EPR spectra by appearance of a signal I with isotropic giso=2.128. The increase of the reduction temperature up to T=393K leads to consecutive increase of quantity of Pd + ions which are under an influence of others (Fig.4c, d). The appearance and increase of I species proves that under the noted reduction conditions, we can continue the progressive reduction of palladium species, i.e. there is an important reserve of Pd 2+ not yet reduced. The reduction effect on Pd after the catalytic test will be discuss later. ~i H ~t F

g

x 0 25

'

g•

F

b

L'2

\ ; 2 .... glt~5 2400

I 2900

I 3400

H(gauss)

Figure 3. EPR spectra of statically oxided Pd(2)/BEA under the weak reduction during (a) 5 min ; (b) 50 min ; (c) 5 h ; (d) 15 h.

ff

2420

I

I

i

t

~

t

2620

2820

3020

3220

3420

3620

~em=SS)

Figure 4. EPR spectra at 77K under the strong reduction of sample fig.3d at (a) T=293K, 5 h; (b) T=293K, 15 h; (c) T=393K, 1 h; (d) T=393K, 3 h.

704 The IR spectra of adsorbed toluene on the samples displayed two characteristic bands at 1650 cm -1 and 1490 cm -1 corresponding to C-C ring vibrations [6] but also C-H stretching vibrations bands of the aromatic ring between 3050 and 3020 cm -~. The characteristic bands of adsorbed toluene are not shifted from one to another studied sample. The study of the intensity of toluene sorbed peaks with increasing amounts of toluene in the cell shows different behaviours of the solids. Adsorbed toluene is visible when 1gL (9.4.10 .5 moles) is injected in the cell for 0.5wt%Pd/DBEA whereas 5gL and 10gL of toluene are the minimum amounts respectively for 0.5wt%Pd/BEA and 0.2wt%Fe,0.5wt%Pd/BEA. The toluene is then more easily adsorbed on 0.5wt%Pd/DBEA and that adsorption is more difficult on the Fe exchanged catalyst. Nevertheless, the sample 0.5wt%Pd/BEA was saturated with adsorbed toluene before 0.5wt%Pd/DBEA and 0.2wt%Fe,0.5wt%Pd/BEA. The toluene total oxidation was studied by the conversion of toluene at 250~ versus time on stream for the calcined catalysts 0.5wt%Pd/BEA, 0.5wt%Pd2/BEA, 0.5wt%Pd/DBEA and 0.2wt%Fe,0.5wt%Pd/BEA (Fig.5). Although the oxidation of the toluene on these catalysts was producing only water and CO2, all the toluene was not converted at the beginning of the experiment. After 3 minutes under flow, 96% of the toluene was oxidized on 0.5wt%Pd(2)/BEA, 94% on 0.5wt%Pd/BEA, 90% on 0.5wt%Pd/DBEA and only 80% on 0.2wt%Fe,0.5wt%Pd/BEA. However, this conversion increased to 100% for all the catalysts after some time on stream : after 5 hours for 0.5wt%Pd/BEA and 0.5wt%Pd/2BEA but only after 10 h for 0.5wt%Pd/DBEA and 0.2wt%,0.5wt%Pd/BEA. The total conversion is nevertheless not stable during our experiment for the both more active catalysts at the beginning of the experiment (0.5wt%Pd/BEA and 0.5wt%Pd(2)/BEA). The deactivation could be partially explained by the DTA curves of the sample after the catalytic test (Table 2). Two exothermic transitions at about 250 and 500~ and only one at 500~ are present for, respectively, the dealuminated 0.5wt%Pd/DBEA sample and the non-dealuminated samples (0.5wt%Pd/BEA and 0.2wt%Fe,0.5wt%Pd/BEA). These transitions should correspond to the combustion of some deposed cokes on the catalysts. 100 a

95

90

"".

85I d~ / 0

8

Time (h)

16

24

Figure 5 : Toluene conversion at 250~ versus time on stream a : 0.5%Pd(2)/BEA, b : 0.5%Pd/BEA, c : 0.5%Pd/DBEA, d:O.5%Pd,O.2%Fe/BEA.

705 The losses of cokes of, respectively, 1.7% + 2.3% = 4% and 3.7% for 0.5wt%Pd/DBEA and 0.2wt%Fe,0.5wt%Pd/BEA samples are similar whereas a higher quantity is found on 0.5wt%Pd/BEA. This difference could explain the deactivation observed. But also because the first type of coke present on 0.5wt%Pd/DBEA is already decomposed at 250~ and that temperature is the oxidation temperature of toluene in the catalytic test. The second type of coke decomposed at 500~ is then more stable. Another explanation of the deactivation could be also due to the formation of water during the toluene oxidation. Non-dealuminated samples after the catalytic test present higher water losses (corresponding to an endothermic transition at about 80~ The water formed during the toluene oxidation is more sorbed on the more hydroscopic samples and then could decrease the activity of the catalysts. Moreover, the EPR spectra of these samples after the catalytic test still present another result. In Table 3, there are the number of B (NsPd) and S centres (NsS) species of the samples after the toluene catalytic test. The toluene oxidation reaction produces a reduction effect for all the studied catalysts. In Fig.2, the quantity of paramagnetic species B' (g=2.25-2.34, 6H=1100-1200 G) which have been attributed in [9] to palladium clusters, and S species after the test are presented. The catalytic effect is then similar to the reduction treatment at -573K. The value of S centres (Ns) increases after test in order successive : Pd(2)/BEA to Pd/BEA to PdFe/BEA to Pd/DBEA. For the quantity of B centres the order is inverse. So the agglomerated Pd ions are in order more reduced from Pd(2)/BEA to Pd/BEA to PdFe/BEA and to dealuminated Pd/DBEA sample. This result could be directly correlated to the catalytic deactivation observed for Pd(2)/BEA and Pd/BEA after 24 h on stream (Fig. 5). In fact, the more the palladium is reduced, the more its activity for oxidation reaction [ 11 ]. Therefore, the addition of 0.2wt% of Fe or the zeolite dealumination could prevent the deactivation and then lead to better catalysts. Table 2 : DTA results on used catalyst Water loss (80~ Pd/BEA PdFe/BEA Pd/DBEA

8.9% 8.0% 5.8%

Coke 1 loss (250~

Coke 2 loss (500~

0% 0% 1.7%

4.7% 3.7% 2.3%

Table 3 : Number of B (NsPd) and S (NsS) centres after the toluene catalytic test

Pd(2)/BEA Pd/BEA PdFe/BEA Pd/DBEA

g factor (S)

NsS (r.u.)

g factor (Pd +)

NsPd (r.u.)

2.0046 2.0045 2.0046 2.0030

2.15 1.98 2.34 3.16

2.34 2.25 2.28 2.30

0.327 0.352 0.289 0.233

706 4. CONCLUSION This study has shown that an addition of iron or the dealumination of the zeolite could prevent the deactivation of Pd/BEA for toluene total oxidation. Palladium would be more easily agglomerated on the BEA than on the dealuminated BEA (BET and EPR measurements). Moreover, the palladium saturation in function of the wt% of exchanged Pd on the BEA zeolite was deduced by EPR. Several isolated and interacted Pd + species and hole centers were also detected by EPR under dynamic and static oxidations and weak and strong reduction treatments. Moreover, the Pd was much reduced after the catalytic test in dealuminated and Fe doped samples. This result could be directly correlated to the catalytic deactivation of the other samples. The deactivation could be also explain by the type of coke deposed on the catalyst and by the hydroscopic behavior of the samples. REFERENCES

1. C.-H. Lee and Y.-W. Chen, Appl. Catal. B 17 (1998) 279 2. J.R. Gonzhlez-Velasco, A. Aranzabal, J.I. Guti6rrez, R. Lopez-Fonseca, M.A. Guti6rrezOrtiz, Appl. Catal. B 19 (1998) 189. 3. P.O. Larsson, A. Andersson, B. Svensson and L.R. Wallenberg, in 'Environmental Catalysis', G. Centi et al. (Eds), Rome, (1995) 547 4. M. Ogura, S. Kage, M. Hayashi, M. Matsukata and E. Kikuchi, Appl. Catal. B 27 (2000) L213 5. C.K.W. Meininghaus, R. Prins, Microporous. Mater. 35 (2000) 349 6. A.E. Palomares, G. Eder-Mirth and J.A. Lercher, J. Catal. 168 (1997) 442 7. J. Matta, J.-F. Lamonier, E. Abi-Aad, E.A. Zhilinskaya, A. Abouka'is, Phys. Chem. Chem. Phys. 1 (1999) 4975 8. S. Sako and K. Ohshima, J. Phys. Soc. Jap. 65(12) (1996) 4062 9. A.M. Prakash, T. Wasowicz, and L. Kevan, J.Phys.Chem. 101 (1997) 1985 10. L.D. Bogomolova, V.A. Jachkin, S.A. Prushinsky, S.A. Dmitriev, S.V. Stefanovsky, Y.G. Teplyakov, F. Caccavale, E. Cattaruzza, R. Bertoncello, F. Trivillin, J.Non-Cryst.Solides, 210 (1997) 101 11. E.M. Cordi, J.L. Falconer, J. Catal. 162 (1996) 104


707

H y d r o i s o m e r i z a t i o n o f n - B u t a n e over P d / H Z S M - 5 and P d / H m o r d e n i t e with and w i t h o u t binder. P. Cafiizares, F. Dorado*, P. Sfinchez and R. Romero Departamento de Ingenieria Quimica, Facultad de Ciencias Quimicas, Universidad de Castilla - La Mancha, Campus Universitario s/n, 13004 - Ciudad Real, Spain.

The influence of a clay binder (bentonite) on the acid properties and performance of Pd/HZSM-5 and Pd/HM catalysts with different Si/A1 ratios for the hydroisomerization of nbutane has been studied. Temperature-programmed desorption of ammonia, atomic absorption spectroscopy, chemisorption and surface area measurements were used to characterize the catalysts. After agglomeration, some zeolite protons are neutralized by clay sodium and, consequently, a lower n-butane conversion is obtained. However, the decrease in conversion will be compensated by a much higher isobutane selectivity. The product selectivity is also strongly influenced by the binder due to the fact that zeolite hydrogen transfer activity, metal/acid site balance, and diffusion of products are modified. The isomerization activity for Pd/HZSM-5 samples were higher than for Pd/HM catalysts due to mordenite is expected to be more sensitive to the increased in the length of the effective diffusional pathway due to its one-dimensional pore structure.

1. INTRODUCTION Isomerization of n-butane to isobutane is an important commercial process. Isobutane is utilized in alkylation and production of isobutene for the synthesis of methyl-tert-butylether (MTBE), tert-butylalcohol (TBA), polyisobutene and other products. One possible route for isomerization of n-butane is the use of zeolites as catalysts, so that it is of great interest to study the influence of binder on the catalytic performance in this reaction. Most industrial zeolite catalysts require the zeolite to be pelletized with a binder to obtain larger and more resistant particles and to avoid an extremely high pressure drop in fixed bed reactors. Although binders are not active as catalysts, they can influence the catalytic performance [1 ]. This is very important for the development of industrial zeolite catalysts. The presence of binder can affect the acidic properties of a zeolite as a result of changes in the proton exchange efficiency and/or blocking of zeolite channels during the pelletization process.

*To whom correspondence should be addressed. E-mail.'[email protected] Phone: +34-926-295300. Fax: +34-926-295318

708 Bentonite is a clay mineral with wet binding properties. Sodium bentonite contains exchangeable sodium cations. When dispersed in water it breaks down into small plate-like particles negatively charged on the surface, positively charged on the edges. This unique ion exchange is responsible for the binding action. Several catalysts have been successfully used for the skeletal isomerization of n-butane, e.g. HMOR, HZSM-5 and Hbeta [2]. The use of mordenite zeolites have received much attention, mainly due to the specific pore geometry combined with the strong acidic properties. The unidirectional mordenite structure has a cross section with so-called side pockets allowing (small) molecules to pass each other. However, this zeolite is highly sensitive to coke deactivation [3]. n-Butane isomerization has been investigated over HMOR and Pt/HMOR catalysts [4]. According to most of the authors, this reaction occurs via bimolecular pathway, as the monomolecular isomerization mode would involve a primary (and hence very unstable) carbenium-ion intermediate. Metal in mordenite has also been suggested to contribute to the isomerization of n-butane via a parallel bifunctional route involving dehydrogenation of the alkane on the metal site [5]. It is interesting to rationalize the steric requirements for the bimolecular mechanism in the one-dimensional pore of mordenite. The kinetics diameters of branched alkanes (e.g. isobutane) approach the pore size of mordenite, implying that the individual molecules cannot pass each other within one channel. This type of motion is referred to as single-file diffusion [6]. Generally, concentration gradients inside the pores are the driving force for the directed motion of the molecules. However, in single-file diffusion, a displaced molecules is more likely to return to its original position than to proceed further, since the latter would stipulate a further concentration of the molecules ahead, which seems to be highly constrained because of the space limitations in one-dimensional pores. Another possibility is to use HZSM-5 zeolite, because of its acidity, shape selectivity, and temperature resistence. It can be used as catalyst to isomerize light paraffins once promoted with a metal [7]. In this work, several acid catalysts based on ZSM-5 and mordenite zeolites with different Si/A1 ratios and agglomerated with bentonite were prepared. The aim of this work is to study the influence that the pelletization process can induce on the catalyst.

2. E X P E R I M E N T A L

ZSM-5 (Si/A1 = 15 and 40) and Mordenite (Si/A1 = 10 and 45) were supplied by Zeolyst International. Bentonite was supplied by Aldrich Chemical Co. Characterization data of the raw materials (clay and zeolites) are summarized in Table 1. The method of preparation consisted of three steps: agglomeration of zeolite, incorporation of functions (acid and metallic), and activation of the metal part. The ammonium forms of zeolite were calcined at 550 ~ for 15 h to obtain the protonic form. For the binding process, the zeolite and the clay were mixed together and suspended in water at 60 ~ for 2 h, dried at 120~ overnight and finally crushed to 0.50-1.00 mm. The dried material was finally calcined in static air at 550 ~ for 15 h. In order to avoid a possible activity decrease due to partial exchange of the strongest protonic zeolite acid sites with alkaline cations from the binder during the preparation [8], the agglomerated catalysts were ion exchanged with 35 mL/g of 0.6 N HC1. Metal incorporation was carried out by an impregnation technique using an aqueous Pd(NO3)2 solution. The solvent was then removed by evaporation under vacuum. The metal concentration of the

709 impregnating solution was calculated to always yield a final Pd content of 0.82 wt%. After metal incorporation, the catalysts were air-calcined at 400 ~ for 4 h and reduced in situ under a hydrogen flow of 190 mL/(min g) at 450 ~ for 4 h. Catalysts are named as follows: firstly, the symbol of the metal is shown (Pd), followed by a character representing the zeolite (Z for ZSM-5 or M for mordenite). The subsequent number indicates the zeolite Si/A1 ratio. For non-agglomerated samples, further characters are not included. For bound catalysts, however, there is another character related to the binder name (B for bentonite), and finally a number that represents the amount of binder in the catalyst (65 wt% for all the agglomerates samples). Temperature-programmed desorption of ammonia (TPDA), surface area measurements and pore size distributions, atomic absorption spectroscopy (AA), and chemisorption measurements have been used to characterize the catalysts, as described in a previous paper [9]. The crystal size of zeolite particles was determined using a Mastersizer 2000 analyzer. Its technology is based on both Fraunhofer and Mie theories about light scattering. Prior to measurement, each sample was dispersed in water for two minutes in ultrasonic baths. The measurements of zeolite crystal sizes have an error of + 1%. The experiments were carried out in a flow-type apparatus designed for continuous operation. This apparatus consisted of a gas feed system for each component hydrogen and nbutane (+99.95% purity, significant impurities being isobutane [max. 400 ppm] and propane [max. 100 ppm]), with individual control by mass flow meters, a fixed-bed downflow reactor, and an exit gas flow meter. Experimental conditions were: weight of catalysts 5 g, reaction temperature 350-430 ~ total pressure 1.013 bar, W/Fn-c4 = 1 gzeolite/(g/h), and H2/n-C4 molar ratio 8.3:1. Reaction gas products were analyzed with a Hewlett-Packard gas chromatograph using a fused silica PLOT AlzOJKC1 column and an FID detector. Results from a reproduced experiment showed that conversion and isobutane selectivity have an error of +5%. 3. Results and discussion

Acid properties of the raw materials are given in Table 1. As it can be seen, for both zeolites (HZSM-5 and HMOR), the presence of the metal does not significantly change the zeolite acid properties. Table 1 Characterization data for the raw materials Catalyst Zeolite Sodium Acidity data Si/AI ratio content Total Acidity (wt%) (mmol NH3/g) Bentonite 0.48 0.038 HZSM-5 15 0.04 0.707 HZSM-5 40 0.04 0.467 PdZ15 15 0.04 0.712 PdZ40 40 0.04 0.469 HMOR 10 0.05 0.775 HMOR 45 0.01 0.409 PdM10 10 0.05 0.777 PdM45 45 0.01 0.411

Weak Acidity (mmol NH3/g) 0.038 0.031 0.018 0.030 0.017 0.074 0.036 0.072 0.035

Yd (~ 274 300 300 290 289 305 306 303 306

Strong Acidity (retool NH3/g) 0 0.676 0.449 0.682 0.452 0.701 0.373 0.705 0.376

Yd (~ 419 403 400 402 505 492 506 490

710 Table 2 summarizes the acidity data for the agglomerated catalysts. The experimentally measured acidity properties for all PdZ15B65, PdZ40B65, PdM10B65, and PdM45B65 catalysts show a derivation from the predicted values calculated from the contribution of the raw materials: the weak acidity of the agglomerated catalysts is higher than the predicted values, whereas the opposite effect is observed for strong acidity. First, it should be considered whether these changes could be due to total blocking of zeolite channels. To verify this possibility, the surface area of the samples was measured. Table 3 resumes these measurements together with theoretical predicted values calculated from the contribution of nonagglomerated zeolite and binder. The results show that blocking can be excluded since values of surface area are in line with those predicted. The decrease in the number of expected strong sites can be attributed to solid-state ion exchange between zeolite protons and clay sodium during the calcination that follows the ion-exchange step with HC1 [1,10]. Besides, Na + cations are also weak acid sites [ 11 ], so that the increase in weak acid sites density is due to this cation. In summary, experimental acidity values do not match the theoretically calculated values due to solid-state ion exchange and some interference from Na +. As the binder influences the strong acid site density of the zeolite, the catalyst performance will depend on the binder. This can be seen in Figure 1, where n-butane conversion for these samples is shown. Taking into account that we use a W/Fn-c4 parameter based on grams of zeolite, we should first of all refer strong acidity to grams of zeolite to relate conversion to strong acid site density (Table 3). As expected, the higher the strong acid site density per gram of zeolite, the higher the conversion.

Table 2 Characterization data for Pd/HZSM-5/bentonite and Pd/HMOR/bentonite catalysts Mechanical Dm Acidity data Catalyst Weak Aciditya Strong Acidity b resistance (%) Total Acidity (%fines) (mmol NH3/gcat) (mmol NH3/gcat) (mmol NH3/gcat) 24.7 0.273 c 0.036 c 0.237 c PdZ15B65 0.7 0.267 a 0.057 a 0.210 a +60.3 e -11.2 e 20.1 0.188 c 0.031 c 0.157 c PdZ40B65 0.7 0.185 d 0.038 a 0.147 d +22.6 e -6.5 e 22.3 0.296 ~ 0.049 c 0.247 c PdM10B65 0.7 0.295 d 0.060 d 0.235 d +22.4 e -4.8 e 25.3 0.168 c 0.037 c 0.132 ~ PdM45B65 0.7 0.165 d 0.040 d 0.125 d +8.11 e -4.6 e a Desorption temperature of ammonia ~ 290 ~ b Desorption temperature of ammonia ~ 390 ~ for ZSM-5 based catalyst, and ~ 440 ~ for mordenite-based catalysts c Predicted value proportionally calculated from the contribution of the non-agglomerated zeolite and binder. a Experimental value e Deviation from the predicted value (%)

711 Table 3 Strong acid site density per gram o f zeolite, surface area, and pore v o l u m e m e a s u r e m e n t s Pore Volume (~tL/gcat) Strong Acidity a

Surface area a Deviation from the (mmol NH3/gzeolite) (m2/g) predicted value (%)

Sample Bentonite HZSM-5 (Si/Al=l 5) HZSM-5 (Si/AI=40) PdZ15 PdZ40 HMOR (Si/AI=I 0) HMOR (Si/Al=45) PdM10 PdM45 PdZ 15B65 PdZ40B65 PdM 10B65 PdM45B65 a Experimental value

--0.676 0.449 0.682 0.452 0.701 0.373 0.705 0.376 0.600 0.420 0.671 0.357

37 441 441 440 438 535 530 537 535 181 182 221 220

------------------2.3 2.8 4.2 4.1

Micropores 4.3 140.1 140.1 149.2 137.7 195.6 196.1 190.4 188.0 47.1 46.4 78.4 73.5

Meso- and macropores 88.5 37.0 37.0 28.0 30.1 41.0 39.5 30.8 30.6 70.5 69.8 140.6 148.9

Figures 2 and 3 s h o w isobutane selectivity versus n-butane conversion for these catalysts. As it can be seen, agglomerated catalysts showed a higher selectivity to isobutane than their respective parent zeolite. This result is in good agreement with the bimolecular m e c h a n i s m proposed for n-butane isomerization [12].

45

45

(a)

40

,~,35

~35

-~ 30

30

~E2 5 • -~

(b)

40

g25

20

..~

r~ 1,1

~a15

20

~15

10

10 5

5

0

0 PdZl5

PdZ15B65 PdZ40

Catalyst

PdZ40B65

F-] PdM10

PdM10B65 PdM45

PdM45B65

Catalyst

Figure 1. Influence o f binder on n-butane conversion (Reaction Temperature = 370 °C, P = 1.013 bar, W/Fn-c4 = 1 gzeolite/(g h) and H2/n-C4 = 8.3) for (a) P d / H Z S M - 5 catalysts and (b) P d / H M O R catalysts.

712 100 90 80 ~,70 60 "~ 50 m 40 3O 2O 10 0

--m~--.PdZ 151365 9 PdZ40 - • .... PdZ40B65 20

40 X (mol %)

",

60

80

Figure 2. Isobutane selectivity as a function of n-butane conversion for Pd/HZSM-5 and Pd/HZSM-5/Bentonite.

100 90 80 ~, 70 60 50 E 40 '~' c~ 30 20 10 0

- ' ! ~ PdM10B65 --iv- PdM45 PdM45B65 20

40 X (tool %)

6O

Figure 3. Isobutane selectivity as a function of n-butane conversion for Pd/HM and Pd/HM/Bentonite.

The first step is the formation of a butylcarbenium ion through (i) protonation of butane by a Br6nsted acid site and subsequent abstraction of H2, (ii) hydride abstraction by a Lewis acid site, or (iii) protonation of trace olefins formed by thermal cracking [13]. Then, the C8+ ions can be formed via reaction of the butylcarbenium ion with an olefin forming an octylcarbenium ion or via reaction with an alkane forming an octylcarbonium ion. The C8 carbenium ions, after isomerization and p-scission, will yield n-butane and isobutane. The C8 carbonium ions, after disproportionation, will yield propane and pentane. So, the relative concentration of carbenium and carbonium ions determines the selectivity of the catalysts. A higher strong acid site density involves a higher hydrogen transfer activity, which leads to a shorter lifetime of carbenium ions and to a higher concentration of carbonium ions in the zeolite pores. Therefore, according to this mechanism, a lower isomerization and a higher disproportionation activity will be obtained if the strong acid site density is increased. Tables 4 and 5 show the hydrocarbon selectivity, at approximately 25 mol % conversion. As it can be seen easily, the highest formation of propane and pentanes (disproportionation activity) was observed for samples without binder, i.e. the catalysts with the highest zeolite strong acid site density, as one would expect. On the other hand, Tran et al. considered that a high disproportionation selectivity could be also due to a long diffusion pathway for organic molecules [14]. Thus, while the reactant molecules diffuse through the channels, they could undergo many successive intermolecular reactions with consequently a preferential formation of propane (butanes and pentanes are very reactive compared to propane and can be transformed into this product). First of all, it was measured the zeolite crystal size for these samples, as the length of the intracrystalline diffusion depends on it. However, very similar results were obtained for all samples (about 4 tam). Secondly, it was considered a partial blockage of the zeolite micropore mouths, which would lead to an increase in the length of the effective diffusional pathway. A metal dispersion of 20-25% was found for all the parent zeolites. The average diameter of the metal

713 particles, calculated from a theorical expression [9], would be then about 46.3-57.9 A, too big to think that the palladium particles could be located into the zeolite main channels. Thus, partial blocking of the zeolite micropore mouths by the metal is expected for the pure zeolites. However, for the bindered catalysts, the big metal particles are likely located not on the zeolite surface, but into the meso- and macropores provided by the binder (Table 3). Zeolite mouth pore partial blockage is then avoid, with consequently a lower diffusional constraint. Thus the diffusion pathway for the non-agglomerated samples would be clearly much longer than for the agglomerated catalysts, which strongly favors propane formation, as observed (Tables 4 and 5).

Table 4 Hydrocarbon selectivity at approx. 25 mol% conversion over Pd/HZSM-5/bentonite catalysts. Catalyst PdZl5 PdZ15B65 PdZ40 PdZ40B65 Selectivity (%) CH4

2.9

0.7

1.3

2.3

C2H6

2.6

1.1

2.1

1.3

C2H4

0

0

0

0

C3H8

11.8

9.2

11.2

4.7

C3H6

0

0

0

0

i-C4H1o

73.3

80.0

76.6

86.3

C4H8a

0.0

0.3

1.6

2.2

n-CsHlo + i-C5Hlo b

9.5

8.7

7.2

3.3

a All isomers; b Hexanes or higher were not obtained

Table 5 Hydrocarbon selectivity at approx. 25 mol% conversion over Pd/HMOR/bentonite catalysts. Catalyst PdM10 PdM10B65 PdM45 PdM45B65 Selectivity (%) CH4

0.4

0.4

0.7

0.9

C2H6

1.6

1.1

1.6

1.2

C2H4

0

0

0

0

C3H8

18.2

10.3

13.7

9.4

C3H6

0

0

0

0

i-C4Hlo

65.2

77.8

71.3

78.2

C4H8a

0.1

0.5

3.6

3.6

n-CsH10 + i-C5H10b

14.5

9.9

9.1

6.7

a

All isomers; b Hexanes or higher were not obtained

714 In agreement with this assumption, it was observed that isomerization activity for Pd/HZSM-5 catalysts was higher than for Pd/HMOR samples. Differences in isomerization selectivity for both type of samples could be explained again on the bases of differences in acidity and pore size. Mordenite is expected to be more sensitive to the increase in the length of the effective diffusional pathway due to its one-dimensional pore structure. Thus, effective diffusional pathway for Pd/HMOR samples would be clearly much longer than that for PdHZSM-5 catalysts, and consequently, a higher formation of propane and pentane will be obtained over the former, as it can be seen in Tables 4 and 5. Finally, it is interesting to compare the catalysts PdZ40 and PdM45. Strong acid site densities per gram of zeolite was higher for the former than for the latter. However, the selectivity to isobutane was higher for PdZ40, which confirms that the long diffusional pathway for Pd/MOR samples strongly favors propane formation.

CONCLUSIONS The presence of a clay binder affects the acidic properties of zeolite based catalysts, and consequently their performance in the hydroisomerization of n-butane. There is a solid ion exchange between zeolite protons and clay sodium cations. The neutralization of some zeolite protons by the clay leads to a lower n-butane conversion. However, this negative effect can be compensated by a higher selectivity to isobutane, which is due to both an easier product diffusion and a lower disproportionation activity. Catalysts based on Pd/HZSM-5 had a higher isomerization activity than those based on Pd/HMOR, as mordenite, with one-dimensional pore structure, is quite more sensitive to diffusional restrictions. REFERENCES

1. V.R. Choudhary, P. Devadas, A.K. Kinage, M Guisnet. Appl. Catal., 162 (1997) 223. 2. E. Babfirek, J. Novfikovfi. Appl. Catal. A., 185 (1999) 123. 3. M. Guisnet, P. Marnoux. Catalyst Deactivation 1994, Stu. Surf. Sci. Catal., in: B.Delmon, G. F. Froment (Eds.). 88 (1994) 53. 4. C. Bearez, F. Avendano, F. Chevalier, M. Guisnet, Bull.Soc. Chim. Fr., 3 (1985) 346. 5. R.A. Asuquo, G. Eder-Mirth, J.A.Z. Pieterse, K. Seshan, J.A. Lercher. J. Catal., 168 (1997) 292. 6. J. K/~rger, M. Petzold, H. Pfeifer, S. Ernst, J. Weitkamp. J. Catal., 136 (1992) 283. 7. W.O. Haag, R.M. Lago. U.S. Patent 4,374,296 (1983). 8. J.M. Fougerit, N.S. Gnep, M. Guisnet, P. Amigues, J.L. Duplan, F. Hughes. Stud. Surf.Sci. Catal., 84 (1994) 1723. 9. P. Cafiizares, A. de Lucas, F. Dorado, A. Dur~n, I. Asencio. Appl. Catal., 169 (1998) 137. 10. M.D. Romero, J.A. Calles, A. Rodriguez, A. de Lucas. Microporous and Mesoporous Materials, 9 (1997) 221. 11. V.R. Choudhary, V.S. Nayak. Zeolites, 5 (1985) 15. 12. R.A. Asuquo, G. Eder-Mirth, J.A. Lercher. J. Catal., 155 (1995) 376. 13. R. Shigeishi, A. Garforth, I. Harris, J. Dweyer. J. Catal., 130 (1991) 423. 14. M.-T. Tran, N.S. Gnep, G. Szabo, M. Guisnet. J. Catal., 174 (1998) 185.


715

Butane i s o m e r i z a t i o n on several H-zeolite catalysts Sergio De Rossi*, Giuliano Moretti, Giovanni Ferraris and Delia Gazzoli Centro CNR SACSO c/o Dipartimento di Chimica, Universit/l "La Sapienza", Piazzale A. Moro 5, 00185 Roma, Italy. (e-mail: sergio.derossi@uniroma 1.it)

H-ZSM-5, H-Beta, H-Mordenite and H-Y zeolites are active catalysts for n-butane isomerization. The level of activity and resistance to poisoning are a function of the concentration of the Bronsted acid sites and framework structure. H-L and H-Ferrierite zeolites have a very low activity. Silicalite-1 (S-l) and mesoporous silica-alumina (MSA) are not active. The isobutane yield on ultrastable H-Y (Si/AI=3.15) catalyst is lower in comparison to tungsta supported on zirconia (WOx/ZrO2), H-Mordenite, H-Beta and H-ZSM-5 catalysts, however, H-Y catalysts are more resistent to poisoning and highly selective towards isobutane. In fact, no deactivation of H-Y catalysts was observed after 5 h of time on stream. On the contrary, WOx/ZrO2, H-Mordenite and H-Beta catalysts, under the same experimental conditions, are deactivated in less than 1 h of time on stream. It is suggested that the stability of the H-Y catalysts in comparison to the other acidic zeolites catalysts may be due to the 3-dimensional structure of H-Y zeolites, made of large supercages interconnected by apertures of 12 oxygen atoms. H-ZSM-5 catalysts are stable with time on stream but their selectivity to isobutane is very low. Active acidic molecular sieves with monodimensional structure (H-Mordenite, H-Beta) may favour the formation of polyenyl unsaturated chains, the precursors of the coke responsible for the catalysts deactivation. 1. INTRODUCTION The isomerization of n-butane has a specific interest because isobutane can be alkylated with butenes to produce high octane number isooctanes and is a starting material for producing isobutene via dehydrogenation. Isobutene can be reacted with methanol or ethanol to produce methyl-tert-butyl (MTBE) or ethyl-tert-butyl (ETBE) ethers, which are employed as additive of gasoline, to increase the antiknock power and improve the combustion [ 1,2]. The use of halogen-containing catalysts, to reach the acid strength necessary for skeletal isomerization, is becoming problematic for environmental reasons. Hence the search for strongly acidic catalysts which avoid the use of halogens is an important goal of the refining industry.

This paper is dedicated to Prof. Alessandro Cimino on the occasion of his 75 th birthday. Financial support from Italian Murst (Programmi di Ricerca di Rilevante Interesse Nazionale) is gratefully acknowledged.

716 Zirconia based catalysts, especially sulfated zirconia [Refs. 3,4 and references therein] and tungsta supported on zirconia [Ref. 5 and references therein], have high activity and selectivity for n-butane isomerization. On the sulfated zirconia catalysts the catalytic activity was correlated with the number of Bronsted acid sites. In particular the strongest sites, which are able to retain pyridine against evacuation at 150 ~ were suggested to be the active sites for butane isomerization [3,4]. The main problem with these catalysts is their fast deactivation due to coke deposition. On sulfated zirconia catalysts it was found that less than 0.1 wt % of carbonaceous deposits were sufficient to reduce of 90 % the initial activity in less than 1 h of time on stream [4]. H-Mordenites were tested by Asuquo et al. [2] as an interesting alternative to conventional n-butane isomerization catalysts, although also for these catalysts the main limitation was their fast deactivation due to coke deposition. The initial activity and selectivity to isobutane (n-butane 2% in He) were similar to that found on zirconia based catalysts at 250 ~ however, at higher temperature the selectivity decreases due to the increased formation of propane and pentanes. Later, the same group found that the deactivation can be minimized by using H-Mordenite containing 0.25 wt % Pt in the presence of hydrogen (Hz/n-butane ratios in the range 0-10, n-butane 20% in He) [6]. The isomerization of n-butane (10% in N2) over H-Mordenite samples with framework Si/A1 ratios from 6.6 to 80 was studied at 250 ~ by Trung Tran et al. [7]. According to these authors, the positive effect of the acid-site density on the activity is a demonstration that the reaction mechanism is bimolecular (dimerization-isomerization-cracking), as previously suggested by Asuquo et al. [2]. Babfirek and Novfikov~i investigated the role of Bronsted and Lewis acid sites in the isomerization of n-butane (Hz/n-butane ratio = 120) over HMordenite, H-ZSM-5 and H-Beta zeolites [8]. They found that the increased number of Lewis acid sites, obtained by vacuum treatment of the samples at 650 ~ results in a substantial suppression of both n-butane conversion and selectivity to isobutane. The same authors also investigated the effect of the addition of Pt to the acidic zeolites. It was found that Pt particles, with low dispersion, increase the n-butane conversion as well as the yield of isobutane [9]. The present contribution reports a study of H-zeolites as catalysts for n-butane isomerization, extending previous literature data to other framework types. The results obtained on reference zirconia based catalysts will be compared with those obtained on Hzeolites under the same experimental conditions. The catalytic properties can be related to the different framework types and acid strength. In a previous paper dealuminated H-Y zeolites with framework Si/A1 atomic ratios in the range 3-11, obtained by leaching with diluted HC1 solutions, were found very interesting catalysts for n-butane isomerization in comparison to WOx/ZrO2 catalysts [10]. 2. EXPERIMENTAL

2.1. Catalysts preparation The H-zeolites and the other catalysts studied in this work are reported in Table 1. The commercial zeolites, initially in the sodium or potassium form (L zeolite), were transformed in the ammonium form by three consecutive ion exchanges with 3 M ammonium nitrate solutions at 90~ for 4 h (1 g of zeolite per 100 mL), and finally transformed in the acid form by treatment at 500~ for 5 h in air.

717 Table 1. Chemical and textural properties of the investigated catalysts. Sample Si/A1 [A1] (a) [H +] (a) (Manufacturer and label) at./unit cell at.g-lxl0 2~ H-ZSM-5 Sfid Chemie 12.5 7.1 7.4 Eka Nobel EZ 472 16 5.7 5.9 PQ CBV 5020 25 3.7 3.9 Stid Chemie 50 1.9 2.0 Eka Nobel EZ 112 84 1.1 1.2 Sfid Chemie 120 0.79 0.82 This laboratory (S- 1) 0.00 0.00 H-Y UOP LZY 54 2.5 55 29 Tosoh US HSZ-330 HUA 3.15 46 24 H-Ferrierite Engelhard EZ 500 8.4 3.8 11 H-L Engelhard EZ 200 3.05 8.9 20 H-Beta This laboratory 12 4.9 7.7 H-Mordenite Engelhard EZ 321 8.1 5.3 11 Stid Chemie 25 1.8 3.9

MSA This laboratory

90

11

SBET

m2g-1

Sexternal m2g-1

V~t

mLg -1

400 439 443 331 346

74 29 96 54 8

309 729

9 43

0.132 0.272

378

14

0.144

414

22

0.156

0.143 0.182 0.162 0.140 0.166

0.219

555 476 554

29 35

0.175 0.208

697

522

0.080

w wt.-% [w] WOx/Zr02 at. g-1x 10.20 This laboratory 17 5.6 50 50 0.000 (a) Calculated on a dry basis and assuming only the presence of framework aluminum. The analytical content (by atomic absorption spectroscopy) of Na was < 0.2 wt.-% for all H-zeolite samples, the residual K content of the H-L zeolite was 2.7 wt.-%. Silicalite-1 (S-I) and amorphous mesoporous silica-alumina (MSA) with Si/A1 = 90 were prepared according to procedures developed by EniTecnologie SpA [Ref. 11 and references therein]. H-Beta zeolite was prepared according to the procedure reported by Kiricsi et al. [12]. The tungsta/zirconia sample was prepared by equilibrium adsorption starting from hydrous zirconia, and an ammonium metatungstate solution [5]. The final calcination temperature was 800 ~ for 5 h.

2.2. Catalysts characterization and catalytic tests The H-zeolites catalysts were characterized by X-ray diffraction (XRD) using CuK~ (Nifiltered) radiation (Philips automated PW1729 diffractometer) and by textural analysis (BET

718 specific surface area, external surface area and micropore volume by t-test using the Harkins & Jura reference isotherm equation [13]) by N2 adsorption-desorption at -196 ~ (Micromeritics ASAP 2010 analyzer). Before adsorption, the solids were preheated under vacuum in three steps: 1 h at 150 ~ 1 h at 250 ~ and finally 4h at 350 ~ The results are reported in Table 1. Catalytic isomerization of n-butane was carried out in a flow apparatus, including: (i) a feeding section equipped with independent mass flow controllers; (ii) a down flow silica reactor, containing ca. 1 g of catalyst supported on a fritted disk, and vertically positioned in an electrical heater thermoregulated to within +1 ~ (iii) a gaschromatograph equipped with a flame ionization detector connected to an integrator for peak area evaluation. Before each catalytic run the catalyst was heated in flowing oxygen at 500 ~ for 0.5 h. The reaction was run with pure n-butane at atmospheric pressure, the weight hourly space velocity (WHSV) was 0.8 h -1 and the reaction temperature 300 ~ 3. R E S U L T S AND DISCUSSION 3.1. Activity and selectivity As shown in Figure 1 H-Mordenite and H-Beta zeolites are the most active catalysts for n-butane isomerization to isobutane at 300 ~ but their activity substantially decrease with

20

~

A

1

- 12

H-Mordenite Si/A1 = N 50

. ~~~~MM-5

Si/A12 16_

.

_

H-Y Si/A1 = 3.15

5

@

~.

r

'H-

0

o

sb

16o

1go

:,6o

:,go

aoo

1.0 .._, >~

~

H-L Si/A1 = 3.05

0.5

1 MSA Si/A1 = 90 o.o

0

=

=,=

50

=

=

100

-_,

= , =

150

.

200

,

250

300

time on stream / min Figure 1. Isobutane yield for the n-butane isomerization at 300 ~ of the H-zeolites, tungsta/zirconia and MSA catalysts as a function of time on stream (WHSV=0.8 h-l).

719 time on stream. H-L and H-Ferrierite zeolites have a very low activity. S-1 and MSA are completely inactive. H-ZSM-5 and ultrastable H-Y (Si/A1 = 3.15) zeolites have a level of activity comparable to the WOx/ZrO2 catalyst and, at variance with this catalyst, are remarkably stable with time on stream. For all the active catalysts reported in Figure 1, the selectivity to isobutane as a function of time on stream is reported in Figure 2. The selectivity of H-ZSM-5 catalyst is only 20 %, the main reaction product being propane. The selectivities of H-Mordenite and H-Beta is 40-60 %. (H-Mordenite with Si/A1 = 8.1, not shown in Figures 1 and 2, underwent faster deactivation and the selectivity decreased with time on stream.) H-Y with Si/A1 = 2.5 showed very low activity (less than 2 %), whereas ultrastable H-Y with Si/A1 = 3.15 was substantially more active (about 6 %), stable with time on stream and fairly selective to isobutane (more than 80 %). Higher isobutane yields (ca. 12 %) were obtained on dealuminated H-Y zeolites with Si/A1 atomic ratios in the range 4.85 - 10.2 [ 10]. Tungsta/zirconia reference catalyst underwent fast deactivation with time on stream. The activity decreased by a factor of 5 in less than 3 hours of time on stream. The selectivity to isobutane was, however, close to 90% and constant with time on stream. 3.2. Effect of the Bronsted

acid sites concentration

and zeolite framework

The Bronsted acid sites are the active sites for the isomerization of n-butane and their strength and concentration are factors of paramount importance [2,6-10]. The effect of the concentration of the Bronsted acid sites in the MFI structure ( S-1 and H-ZSM-5 zeolites ) on the n-butane isomerization activity, measured after 2 h on stream, was studied in detail. As shown in Figure 3 H-ZSM-5 catalysts with a concentration of Bronsted acid sites less than ca. 1 x l020 sites/g, which correspond to less than 1 framework A1 atom per unit cell (uc), are not active.

100

WOx/ZrO2 = ~D

o

~9 0

80

v

w

1.

w

_

_

v

w

H-Y Si/A1 = 3.15

H-Mordenite Si/A1 = 25

60

H-Y Si/A1 = 2.5 w

~

.

-

.# .~9

40

~D

~

H-ZSM-5 Si/A1 = 16

20 0

0

5'0

160

z60

time on stream / min

300

Figure 2. Selectivity to isobutane at 300 ~ as a function of time on stream (WHSV=0.8 h-l).

720 As shown in Figure 1, amorphous MSA catalysts with ca.1.1 x l020 Bronsted acid sites/g is inactive. On the other hand Figure 3 shows that H-ZSM-5 with a similar concentration of Bronsted acid sites is active confirming that the Brensted acid sites in H-ZSM-5 are much stronger than the Si(OH)A1 groups in MSA [see Ref. 11 and references therein]. Figure 3 shows that in the range between ca. 1 and 6 x l 020 Bronsted acid sites/g the turnover frequency is constant. Between 6 and 7 xl020 Bronsted acid sites/g the activity of H-ZSM-5 levels off. Similar results were reported for H-Mordenite catalysts by Trung Tran et al. [7]: the rate of n-butane transformations is very low and not proportional to the concentration of Bronsted acid sites when they are less than ca. 1 per uc. A levelling off of the activity occurs at ca. 4.5 Bronsted acid sites/uc. The higher activity of ultrastable H-Y catalyst with Si/A1 = 3.15, in comparison to the H-Y catalyst with Si/A1 = 2.5, may be related to its higher surface area and micropore volume (see Table 1) which can contribute to an easier access to the Bronsted acid sites. We also recall that enhanced acidity of dealuminated H-Y zeolites compared to nondealuminated was observed by spectroscopic and catalytic experiment by Kotrel et al. [14]. In a previous work we found that dealuminated H-Y catalysts with framework Si/A1 atomic ratios in the range 3-11, and containing also same extra-framework aluminum species, are highly active and selective for n-butane isomerization [ 10]. Figures 4(a) and 4(b) show that in H-ZSM-5 the increase of the reaction rate, by either increasing the concentration of the Brensted acid sites or increasing the reaction temperature, leads to a decrease of isobutane yield and to an increase of the propane yield. Due to the fact that the diffusion coefficient of isobutane is much lower than that of propane in the H-ZSM-5 channels [ 15], we may tentatively suggest that at low rate the reaction is under chemical

25 o

_~ 20 "7

r.~

2 m 0

5

0

0

1

2

3

4

5

6

7

[H+] / ions g l x 10-20 Figure 3. n-butane isomerization on various H-ZSM-5 zeolites (WHSV-0.8 h-l). Correlation between isobutane production rate and the concentration of Bronsted acid sites.

721 control (main product isobutane), whereas at high rate the reaction is under diffusion control (main product propane). 3.3. Reasons for the slow deactivation of H-ZSM-5 and ultrastable H-Y catalysts H-ZSM-5 and ultrastable H-Y zeolites have a much higher stability with time on stream in comparison to both WOx/ZrO2, an acid catalyst extensively investigated for alkane isomerization [5], and the more active H-Mordenite and H-Beta zeolites. The deactivation of these catalysts is due to coke produced by entrapped unsaturated chains that cyclize to form mono- and poly-cyclic aromatics [2,6-9]. As previously reported [2,6-9], over H-Mordenite, H-Beta and H-ZSM-5 catalysts nbutane is converted to isobutene via a bimolecular mechanism at low temperature. In the first step, n-butane forms a butyl-carbenium ion by protonation at the strong Bronsted acid sites and subsequent dehydrogenation. A similar mechanism may be suggested in the case of H-Y catalysts. As shown in Figure 1, H-Ferrierite and H-L zeolites have a very low activity for n-butane isomerization, probably because their Bronsted acid sites are not strong enough to form the butyl-carbenium ion. The very weak Bronsted acid sites in MSA (A1-OH-Si) and S-1 ( internal silanols nests) [Ref. 11 and references therein] is the rationale to justify their inactivity. On the other hand, the activity for the 1-butene isomerization on H-Ferrierite is explained by Paz~ et al. [16] with a bimolecular mechanism, which would favour the formation of coke over zeolitic structures with monodimensional channels and pore apertures of molecular dimensions, like H-Mordenite, H-Ferrierite and H-Beta [17]. Moreover, note that "one coke molecule" trapped in a channel can block, in a monodimensional zeolite, the access of the reactant to all the acid sites of this channel.

80

i

i

i

(a)

i

i

i

9

80

60

60

40

40

20

20

0

0

20

40

60 80 Si/A1

100 120 140

0

'

300

(b)

350

T/~

'

400

Figure 4. (a) Conversion and selectivities for n-butane isomerization on H-ZSM-5 catalysts as a function of the Si/A1 atomic ratio at 300 ~ (WHSV=0.8 h-l). (b) Conversion and selectivities for n-butane isomerization on H-ZSM-5 catalyst with Si/Al=84 as a function of the reaction temperature. Symbols: 9 --- total conversion; 9 = selectivity to isobutane; A= selectivity to propane.

722 We suggest that the high activity and the stability with time on stream of ultrastable H-Y zeolites, in comparison to the other zeolites investigated in this work, may be tentatively explained by the 3-dimensional structure of the H-Y zeolite that favours the diffusion of reactant and product decreasing the residence time and the ensuing degradation to coke. 4. CONCLUSIONS The main result reported in this study is the remarkable activity and stability with time on stream of ultrastable H-Y catalysts for the isomerization of n-butane at 300 ~ The activity of these catalysts is lower than that reported for zirconia based catalysts and for HMordenite, and H-Beta catalysts. These more active catalysts, however, suffer a severe loss of activity with time on stream due to the poisoning of the acid sites by coke. The selectivity on H-Y catalysts is similar to that obtained on the most active tungsta/zirconia and sulfated zirconia catalysts. It seems that on ultrastable H-Y catalysts the formation of coke deposits is avoided due to the presence of strong Bronsted acid sites in the large supercages which favour the diffusion of the isomerized products as they are formed and hinder the formation of coke. Considering that in commercial practice high selectivity is usually more valuable than high activity, the relative comparison between ultrastable H-Y and H-ZSM-5, HMordenite and H-Beta catalysts is strongly in favour of the ultrastable H-Y zeolites. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

P. M~riaudeau and C. Naccache, Adv. Catal. 44 (2000) 505. R.A. Asuquo, G. Eder-Mirth and J. A. Lercher, J. Catal. 155 (1995) 376. K. Shimizu, N. Kounami, H. Wada, T. Shishido and H. Hattori, Catal. Lett. 54 (1998) 153. B. Li and R. D. Gonzalez, Catal. Lett. 54 (1998) 5. S. De Rossi, G. Ferraris, M. Valigi and D. Gazzoli, submitted for publication. R.A. Asuquo, G. Eder-Mirth, K. Seshan, J.A.Z. Pieterse and J. A. Lercher, J. Catal. 168 (1997) 292. M.- Trung Tran, N.S. Gnep, G. Szabo and M. Guisnet, Appl. Catal. A 170 (1998) 49. E. Babfirek and J. Novfikovfi, Appl. Catal. A 185 (1999) 123. E. Babfirek and J. Novfikovfi, Appl. Catal. A 190 (2000) 241. S. De Rossi, G. Moretti, G. Ferraris and D. Gazzoli, Catal. Lett., in the press. G. Moretti, C.Dossi, A. Fusi, S. Recchia, R. Psaro, Appl. Catal. B 20 (1999) 67. I. Kiricsi, C. Flego, G. Pazzuconi, W. O. Parker, Jr., R. Millini, C. Perego and G. Bellussi, J. Phys. Chem. 98 (1994) 4627. W.D. Harkins and G. Jura, J. Am. Chem. Soc. 66 (1944) 1366. S. Kotrel, J.H. Lunsford and H. Kn6zinger, J. Phys. Chem B, 105 (2001) 3917. B. Millot, A. M6thivier, H. Jobic, H. Moueddeb and M. B6e, J. Phys. Chem. B, 103 (1999) 1096. C. Pazb, B. Sazak, A. Zecchina and J. Dwyer, J. Phys. Chem B, 103 (1999) 9978. W.M. Meier, D.H. Olson and Ch.Baerlocher, Atlas of Zeolite Framework Types, Elsevier, Fifth Revised Edition 2001. World Wide Web under: http://www.izastructure.org/databases/.


723

Metal loaded Ti-pillared clays for selective catalytic reduction of NO by propylene +. J. L. Valverde*, A. de Lucas, P. S~inchez, F. Dorado and A. Romero Department of Chemical Engineering, Faculty of Chemistry. Castilla-La Mancha University. Campus Universitario s/n. 13071-Ciudad Real. Spain.

Metal loaded Ti-PILCs have been used as catalysts for the selective reduction of NO by propylene. Cu, Ni, and Fe ion exchanged Ti-PILCs were prepared. The influence of the metal loading was studied. When the metal loading increases, the catalytic activity also increases reaching a maximum of NO conversion and then decreased. Cu-TiPILs exhibited the highest NO conversion. Cu-PILCs prepared by impregnation were compared with those prepared by ion exchange. In general the ion exchange method resulted to be more adequate for the preparation of the catalyst. The presence of Cu 2+ species in the ion-exchanged samples could be the responsible of this behaviour.

1. INTRODUCTION. Pillared clays constitute one of the most studied families among the new groups of microporous materials developed by molecular engineering. These materials, also known as cross-linked clays or pillared inter-layered clays (PILCs), are synthesised by exchanging the inter-layered cations of the clay with inorganic polyoxocations, followed by calcination. The polyoxocations are then converted to metal oxide clusters by dehydration and dehydroxylation, leading to a permanently opening of the clay layers [ 1]. Properties as acidity, surface area, and pore size distribution of PILCs offer new shape selective catalysts similar to the zeolites. Nevertheless, thermal stability, lower than zeolites, limits their use as catalysts to specific reactions at relatively low temperatures. Emphasize in special titanium pillared clays (Ti-PILCs) by its catalytic activity in the selective reduction (SCR) of NOx of great importance from the environmental point of view. +Financial support from European Commission (ContractERK5-CT-1999-00001) and DGICYT (Direcci6n General de Investigaci6nCientificay T6cnica, Project 1FD97-1791, Ministryof Education, Spain) is gratefully acknowledged. *Corresponding author: Fax (+34) 926 29 53 18; e-mail:[email protected]

724 These materials showed an excellent thermal stability, high surface area and acidity, and its activity is almost unchanged in the presence of the poisons SO2 and H20, which are present in NOx containing streams [2]. Potential applications of PILCs in catalytic processes of a redox nature would require the clay structure to accommodate transition metal ions that are known to easily change their oxidation state [3]. A large number of catalyst, such as V 2 0 5 - W O 3 (or MoO3)/TiO2, other transition metals oxides (e.g., Fe, Cr, Co, Ni, Cu, Nb, etc.), and doped catalyst, as well as zeolite-type catalyst (e.g., H-ZSM-5, Fe-Y, Cu-ZSM-5), have been found active in this reaction. Despite the high activity of vanadium-based catalysts [4], major disadvantages remain, such as their toxicity and high activity for the oxidation of SO2 to SO3. In this work Ni, Fe and Cu have been used as metals for the preparation of active catalysts (metal-Ti-PILCs) in the SCR NOx reaction. The influence of the metal loading method for the Cu-Ti-PILCs catalyst preparation is also described.

2. EXPERIMENTAL.

2.1. Catalyst preparation. The starting clay was a purified grade bentonite (Fisher Company), with a particle size c 0

;L

9

4030-

,..,.,

mr

f

20-

# --

0 z

10-

Ni-TiPILC

-m--

Fe-TiPILC

- - -A- - - C u - T i P I L C

0 0

I

I

2

4

I

I

I

6 8 10 Ni, Fe, Cu ( w t % )

I

I

12

14

16

Figure 2. Influence of the metal loading on the catalytic activity of SCR NO.

60 50-

A

04

Z o

40-

c

.s >

=

o r

O z

30 20-

r - - ~

Ion

exchange

- -impregnation

10-

0 z

I

I

I

I

I

5

6

7

8

9

10

Cu ( w t % )

Figure 3. Influence of the preparation procedure of Cu-TiPILC samples on the SCR NO catalytic activity.

728 Table 1. Metal loading, acidity, and textural properties of samples. SAMPLE Bentonite TiPILC Fe-Ti-PILC Ion Exchange

Ni-Ti-PILC Ion Exchange

Cu-Ti-PILC Ion Exchange

Cu-Ti-PILC Impregnation

Metal Acidity Surface Area Micropore Area (wt %) (mmol NH3/g) (mZ/g) (m2/g) 0.0 0.0 5.8 8.0 12.6 15.5 1.6 2.9 3.4 3.6 4.6 7.4 9.0 9.5 4.6 8.0 8.6 9.7

0.132 0.529 0.441 0.469 0.608 0.668 0.467 0.468 0.470 0.472 0.620 0.731 0.894 0.766 0.500 0.502 0.508 0.515

35.2 273.2 244.3 217.5 201.3 197.4 260.3 247.3 236.3 222.7 241.6 234.3 201.8 198.8 259.3 228.0 226.6 225.0

15.1 224.5 195.2 154.8 143.1 149.8 195.7 180.2 173.5 160.4 202.2 189.1 153.7 137.5 227.1 202.9 198.7 197.4

Pore Volume (cm3/g) 0.069 0.266 0.234 0.234 0.241 0.200 0.269 0.278 0.265 0.257 0.226 0.236 0.219 0.233 0.229 0.207 0.198 0.180

Copper ion exchanged Ti-PILC showed the highest acidity due probably to the intrinsic acidity of the Cu 2+ species as demonstrated below by TPR analysis.

3.2. Catalytic Activity. Figure 2 shows the catalytic results achieved in the NO-SCR reaction by using Ti-PILCs ion exchanged with different amounts of Ni, Fe and Cu. Conversion of NO increased with the metal content until a maximum. On the other hand, Ni and Fe Ti-PILCs presented low conversions (under 35 %) as compared with Cu-Ti-PILC. Both the high acidity and the adequate redox characteristics of the copper species formed should explain this behaviour. It is important to note that the most effective temperature defined as the temperature of the maximum NO conversion was around 250~ over Cu ion exchanged samples, whereas Fe an Ni presented higher temperatures, 325 ~ and 425 ~ respectively. Since the metal providing the best results was the Cu, a thoroughly study comparing two ways to introduce this metal (ion exchange and impregnation) was carried out (Figure 3). When the Cu content is low the catalyst prepared by impregnation presents higher conversion than that obtained by ion exchange. Nevertheless, when the Cu content is high, similar conversion values are obtained. Sample prepared by ion exchange with 7.4 wt % of Cu presents the highest catalytic activity.

729 These results could be explained because both the preparation method and the Cu content influences the nature and the positions of the metal on the clay. TPR can be used to identify and quantify the metal species in samples. Figure 4 shows the Hz-TPR profiles of ion exchanged and impregnated Cu samples. The peak at the lowest temperature would be related with the presence of CuO aggregates [6]. The other two reduction peaks suggests a two-step reduction process of isolated Cu 2+ species [7]. The peak at the lower temperature would indicate that the Cu 2+ to Cu + reduction process occurred. The other peak at the highest temperature suggests that the produced Cu + was further reduced to Cu ~ As can be seen on Figure 4, the only peak that clearly appears on impregnated samples was the one at the lowest temperature with a small shoulder that could be related with the second reduction peak (Cu 2+ to Cu+), whereas the other two peaks are absent. On the contrary, Cu ion exchanged exhibited the three above-mentioned peaks. This result seem indicates that the increase on the catalytic activity observed in Cu ion exchanged samples as compared with the impregnated ones, is due to the presence of Cu 2+ species.

o

~

Impregnation

r/l

~ 250~ Ion Exchange

J 0

'

I

200

'

I

400

'

I

600

Temperature (~ Figure 4. TPR profiles of Cu ion exchanged and impregnated samples.

730 Cu introduced by ion exchange firstly occupies very stable positions of the clay structure, practically inaccessible to the reagents or any molecule test. Once these positions are filled, the copper place in less stable but more accessible positions, which seem to be more catalytically active. The decrease on activity observed at high Cu content could indicate that the metal is deposited mainly as Cu oxide aggregates [6]. The impregnation method favours the deposition of Cu as Cu oxide on the surface, although it is accompanied of a simultaneous ion exchange process that lead the metal to accessible positions. 4. CONCLUSIONS. Metal loaded titanium pillared clays are active as catalysts in the SCR NO reaction. Cu loaded samples showed the highest activity as compared with Fe and Ni catalyst. The high acidity and mainly the redox nature of the Cu species are the responsible of this behaviour. The NO conversion increases with increasing the metal loading of samples, reaching a maximum and then, a decrease was observed. The preparation procedure of Cu loaded samples influences the catalytic activity. Cu ion exchanged samples showed the best results. This fact could be attributed to the presence of accessible Cu2§ species on the ion-exchanged samples. TPR result is in agreement with the higher acidity of theses samples.

REFERENCES

1. Gil, A.; Gandia, L.M., Catal. Rev-SCI. ENG, 42, (2000) 145. 2. A. Bahamonde, F. Mohino, M. Rd~oUar, M. Yates, P. Avila, S. Mendioroz, Catal. Today 69 (2001) 233. 3. K. Bahranowski, M. Gasior, A. Kielski, J. Podobinski, E.M. Serwicka, L.A. Vartikian, K. Wodnicka, Clays Clay Miner. 46 (1998) 98. 4. N.Y. Topsoe, H.Topsoe, J.A. Dumesic, J. Catal. 151 (1995) 241. 5. R.Q. Long, R.T. Yang, J. Catal. 196 (2000) 73. 6. R.T. Yang, N. Tharappiwattananon, R.Q. Long, App. Catal. B 19 0998) 289. 7. R. BulAnek, B. Wichtedovfi, Z. Sobalik, J. Tich:, Appl. Catal. B 31 (2001) 13.


Influence + propene.

o f cocations

on the activity o f C o - M O R

731

for N O / N 2 0

S C R by

I.Asencio*, F. Dorado, J.L. Valverde, A. De Lucas and P. Shnchez. Department of Chemical Engineering, Faculty of Chemistry. UCLM. Campus Universitario s/n. 13004-Ciudad Real. Espafia (Spain).

Zeolite based catalysts are known to be active for the selective reduction of NO by hydrocarbons. However, the activity strongly decreases with the presence of water in the feed. In this work, the effect of silver on the performance of Co-exchanged mordenite catalysts for NO and N20 reduction with propene in presence of poisons (1-120, SO2) has been investigated. Silver was chosen because it is active itself and it possesses a relativity weak af~nity for water [ 1]. Zeolite form (H vs. Na) is known to have influence on the catalytic performance as far as SCR by hydrocarbons is concerned [2], so that both acid and sodium zeolite forms were also studied.

I. I N T R O D U C T I O N

Selective catalytic reduction (SCR) of NOx by hydrocarbons is under investigation worldwide as a very interesting technique for NOx reduction to N2 in exhaust gas streams from both mobile and stationary sources. Since the 1992 disclosure by Li and Armor [3] of SCR of NOx with CH4 over cobalt zeolites, a large number of studies using cobalt and other metals and zeolite hosts have been reported. Several metals have been found to possess high selectivity for the reduction of NOx to nitrogen with hydrocarbons in the presence of excess oxygen. However, these catalysts are limited by a narrow operating temperature window and low hydrothermal stability. Most of the research was focussed on transition-metal-containing zeolites such as Cu/ZSM5 [4,5] and Co/ZSM5 [6,7,8]. The main problem of this kind of catalyst is the decline of activity in presence of poisons such water or sulphur oxide. In this work, we have studied the effect of cocations (Ag, Na) on the performance of Coexchanged mordenite catalyst, on the reduction of NOx. Nitric oxide, dinitrogen monoxide and the mixing, were used. The change in the catalytic activity upon the addition of water and + Financial support from European Commission (Contract ERK5-CT-1999-00001) and DGICYT (Direcci6n General de Investigaci6n Cientifica y Trcnica, Project 1FD97-1791,Ministry of Education, Spain) is gratefully acknowledged. * Corresponding author: Fax (+34) 926 29 53 18.

732 SO2 was also examined. Silver was chosen because it is active itself and it possesses a relativity weak ~ t y for water [ 1]. Zeolite form (H vs. Na) is known to have influence on the catalytic performance as far as SCR by hydrocarbons is concerned [2], so that both acid and sodium zeolite forms were also studied. 2. EXPERIMENTAL.

2.1. Catalyst preparation. Mordenite was supplied in the sodium form by PQ Corp., with an atomic ratio of Si/AI=7.5, crystallinity = 100% and 7.8 ~tm average particle size. The acid form of the zeolite was obtained by exchanging the Na + with 25 mL/g of 0.6 N HC1 under agitation at room temperature for 14 h. Metal incorporation was carded out by the ion exchange technique. The zeolite was added to Ag(NO3) and/or Co(CH3-COO)2 solutions (25 mL of 0.1 N solution/g of catalyst). The mixture was kept under agitation at 30 ~ for 14 h. Next, the suspension was filtered and thoroughly washed with deionised water in order to completely remove the occluded salt. The resulting solid was drying at 120 ~ for 14 h. After the last metal incorporation, the catalysts were air calcined at 550 ~ for 4 h. These catalysts were referred to as a function of the metal loading. For instance, Na-Ag(1.2%)Co(2.3%)-MOR corresponds to a sodium mordenite catalyst with a silver content of 1.2% and cobalt content of 2.3%, both by weight.

2.2. Catalyst test. Activity tests of the catalysts were carried out in a fixed bed reactor. 1000 ppm NO or 500 ppm N20 or 500 ppm NO+250 ppm N20, 1000 ppm C3I-I6, 5% 02 were used as flue gas component. 5% H20 and 50 ppm SO2 were used in some experiments. He was used as a balance gas at a total flow-rate of 125 mL/min. The flow rates were controlled by calibrated Brooks flowmeters. H20 was introduced in the reactor with a pump (SAGE Instruments, 341B). The space velocity of the feed was 15000 hq (GHSV). The reaction was studied in the 200-500 ~ temperature interval. The outlet gases were analyzed using a gas chromatograph equipped with a TCD detector and a 1010 Carboxen column (Supelco) for the separation of 02, N2, N20, C3I-I6, CO2 and CO2 and a chemiluminiscence NOx analyzer (Eco Physics CLD 700 EL ht) for NO and NO2. NO2 presence was not observed at any experiment.

2.3. Catalyst characterization. X-ray diffraction (XRD) patterns were measured with a Philips model PW 1710 di~actometer using Ni-filtered CuKa radiation. Total acid-site density of the samples was measured by a temperature programmed desorption (TPD) of ammonia, by using a Micromeritics TPD/TPR analyzer. Samples were housed in a quartz tubular reactor and pretreated in flowing helium (99.999%) while heating at 15 ~ min1 up to 500 ~ After 0.5 h at 500 ~ the samples were cooled to 180 ~ and saturated for 0.25 h in an ammonia (99.999%) stream. The sample was then allowed to equilibrate in a helium flow at 180 ~ for 1 h. Finally; ammonia was desorbed using a linear heating rate of 15 ~ minq. Temperature and detector signals were simultaneously recorded. The average relative error in the acidity determination was lower than 3%. Temperature programmed reduction (TPR) measurements were carried out with the same apparatus previously described. After loading, the sample was outgassed by heating at 20 ~

733 min "1 in an argon flow to 500 ~

This temperature was kept constant for 30 min. Next, it was cooled to 25 ~ and stabilized under an argon/hydrogen (99.999%, 83/17 volumetric ratio) flow. The temperature and detector signals were continuously recorded while heating at 20 ~ min"1. A cooling trap placed between the sample and the detector retained the liquids formed during the reduction process. TPR profiles were reproducible with an average relative error in the determination of the reduction maximum temperatures lower than 2%. Details of TPD and TPR measurements were published in an earlier work [9]. The metallic content (wt %) was determined by atomic absorption measurements by using a SpectrAA 220 FS analyzer. In all cases, calibrations from the corresponding patron solutions were performed. Chemical compositions are listed in Table 1. 3. RESULTS AND DISCUSSION.

3.1. XRD analysis. X-ray measurements were carried out using Ni-filtered CuKcz radiation. Figure 1 shows the results of XRD measurements of all catalysts. For all the samples, only diffraction peaks of the mordenite are observed. Additional peaks indicating the presence of Co304 are not detected. The relative intensity of the strongest mordenite lines is lower in the modified catalysts than in the parent zeolite. Moreover, background of the Ag loaded samples is higher than Na-MOR and Na-Co (3.0%)-MOR ones. This fact seems to indicate that crystallinity of the zeolite was lower for the ion-exchanged samples. The relative intensity of the strongest peak for each sample is listed in Table 1. A peak at 20 = 32.8 ~ appearing in silver containing catalysts should indicates the presence of Ag20 [ 10].

Table 1. Composition and XRD analysis of mordenite catalysts. -iV . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~

.....................................

Na-MOR

6.]

Na-Co(3.0%)-MOR

0.4

H - A g ( 1 . 4 % ) - C o ( 2 . 6 % ) - M O R ~ 0.1 ii

i .

.

.

.

.

.

.

.

.

.

i .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

~. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

.

.

.

.

.

.

.

-

-

100

-

3.0

74

1.2

2.3

56

2.8

Na-Ag(1.2%)-Co(2.2%)-MOR

ii

........................

-

Relative intensity of Na Ag Co the strongest peaks content ~content c o n t e n t of M O R measured (wt.%) (wt.%) (wt.%) , by X R D

Catalyst

_

7-T

.

.

.

.

.

.

.

.

.

! .

~ 1.4

2.6

51

ii .

.

j .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

i

i :

734

I

I

I

I

I

f

I

I

t

I

5

10

15

20

25

30

35

40

45

50

2 | Degree Figure 1. XRD patterns of." a) Na-MOR; b) Na-Co(3.0%)-MOR; c) Na-Ag(1.2%)-Co(2.3%)MOR; d) H-Ag(1.4%)-Co(2.6%)-MOR.

3.2. TPD analysis. Table 2 lists for all the catalysts the weak and strong acid site density and the metal content. In the same table the Co ion-exchange levels, determined taking as reference the number of aluminium atoms in the structure, are also summarized. It can be observed that the catalysts loaded with silver present more acidity than the sample without silver. This effect could be explain taking into account that silver can show strong Lewis acidity. Thus, an increase of silver amount would lead to an increase of strong acidity. For the H-Ag(1.4%)-Co(2.6%)-MOR sample an increase of strong acidity is obtained, due to the presence of protons in the zeolite.

735 Table 2. Weak and strong acid sites density ofmordenite catalysts. .

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

: ................................

. ..............

=,- . . . . .

~ ............................................

,. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~,-

.......

7 .....

...-,---~-

=-:=

Weak acid sites, Strong acid Co Ionsites density ~content exchange density (mmol NH3/g) (a) (mmol NH3/g) tb)~ (wt.%) level (%)

Catalyst

Na-Co(3.0%)-MOR

3.0

55

2.3

42

0.786

0.300

0.600

1.492

=

- .........................................................................................

Na-Ag(1.2%)-Co(2.3%)-MOR i

.........................................................................

? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

~ H-Ag(1.4%)-Co(2.6%)-MOR i ....

2.6

'

48

_ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

:

.......................................................................................

:

0.387

1.763

_'. ........................................

9_ .............................................

-:__- _ - -

(a) Desorption temperature = 300 ~ (b) Desorption temperature= 480 ~

3.3. TPR analysis. TPR of the catalysts provided useful information about reducibility of the cobalt and silver components of the catalysts. Na-Co(3.0%)-MOR sample shows only one reduction peak at 705 ~ (Figure 2).

/!

Na-Co(3.0%)-M OR

'

,

!

I

I

....

Na-Ag(1.2%)-eo(2.3%)-M OR

~ .......

H-Ag(1.4%)-C~(2.6%)-M OR

,

I

!

I

'

a; I

/

: 0

p ~

I

:

'~_/..-'" ~

~

",

_..~

T

]

200

400

-__

Temperature, ~ Figure 2. TPR profiles.

/N

~

_/

-7---

_.,:Pd/SBA- 15 >Rh/SBA- 15 >Ru/SBA- 15.

100 ,

.

.

.

.

.

.

.

.

.

.

80

.

.

.

.

Presulfided CoMo/AI,03

60 I~ O

40

5wt%Pd/S

~ 2O t

5

%Pt/SBA- 15

5wt%Rh/SBA- 15

5wt%Ru/SBA- 15

/'~'7.~

0

~

~:z~

1

~

.

~ -

....

!

| ....

I

2 3 4 Time. on stream(hour)

W/F = 87.9 g. hr/mol,

.

I

5

I-I=/Thlophene = 30

Figure 1. Hydrodesulfurization of thiophene over noble metal/SBA-15 catalysts at 350~ The activity of Pt/SBA-15 was the highest among noble metals/SBA-15 catalysts and this activity was almost the same as that of commercial CoMo/AIzO3 catalyst. The reaction products in the HDS of thiophene over Pt/SBA-15 were mainly C4 hydrocarbons (butane 90%, butenes 9%) and small amount of CI-C 3 (1%) hydrocarbons. These results indicate that Pt/SBA-15 catalyst has high hydrogenating ability for unsaturated C 4 hydrocarbons and low hydrocracking activity for hydrocarbons in the presence of hydrogen sulfide. As the activity of Pt/SBA-15 catalyst was almost the same as that of CoMo/A1203 catalyst, we examined the catalytic activities of noble metals supported on AISBA-15 for the HDS of thiophene at 350~

in order to develop much more highly active SBA-15 based HDS catalysts.

It was found that the catalytic activities of noble metals/A1SBA-15 were higher than those of noble metals/SBA-15. Pt/A1SBA-15 showed the highest activity among noble metals/AlSBA-15 catalysts as shown in Figure 2. The Pt/A1SBA-15 catalyst showed high and stable activity and this activity was higher than that of CoMo/AI203 catalysts.

742

100 AISBA-15

80 P, 9 60 40

O

-

5wt%Pd/Ak~A-15 '1,,..~_=_..

-_ ~_ -_ _~ _= " _ - - _ ! '

\

20

5

II

m

m

_an

|

.,.i

. .....

_n

_-

V A-15

5wI~FIu/NSBA-15 -,

0

,.-,-~

:.. "

1

~.=

+

--~

2

"

I. . . .

3

4

I.

.

!

5

Time on stream(hour)

Figure 2. Hydrodesulfurization of thiophene over noble metal/AlSBA- 15 ~talysts at 350"C.

3.2.

Properties of Pt/AISBA-15 catalyst

As the Pt/A1SBA-15 catalyst showed high and stable activity for the HDS of thiophene, we studied in detail the catalytic properties of Pt/A1SBA-15 for the HDS of thiophene in order to clarify the cause of high activity of Pt/AISBA-15 for the HDS of thiophene. The effect of introduction of hydrogen sulfide on the catalytic activity of Pt/A1SBA-15 was examined in order to learn more about the origin of high and stable activity of Pt/A1SBA-15 catalyst. The introduction of hydrogen sulfide (3ml/min) was performed using a microfeeder with a glass syringe; the concentration of hydrogen sulfide in the hydrogen stream was ca.5 vol%. The catalytic activity of Pt/A1SBA-15 was remarkably decreased by the introduction of hydrogen sulfide in the course of HDS reaction. However, the decreased activity was almost restored after cutting off the introduction of hydrogen sulfide as shown in Figure 3. This shows that hydrogen sulfide is reversibly adsorbed on Pt/AISBA-15 and Pt/AISBA-15 catalyst has high sulfur-tolerant properties for the HDS of thiophene as well as Pt/FSM-16 and Pt/MCM-41 catalysts described in the previous paper [8, 9]. By this reason, Pt/AISBA-15 shows high and stable activity for the HDS of thiophene. We also examined the effect of introduction of ammonia on the catalytic activity of Pt/A1SBA-15 in the HDS of thiophene in order to clarify the role of acidic properties of Pt/A1SBA-15 in the HDS of thiophene. The introduction of ammonia (3ml/min) was carried out using microfeeder with glass syringe as well as that of hydrogen sulfide. It was revealed that

743 the catalytic activity of Pt/A1SBA-15 was decreased by the introduction of ammonia (ca.5 vol%) in the course of HDS reaction and the decreased activity was completely regenerated after cutting off the introduction of ammonia as shown in Figure 3. This result indicates that the acid site of Pt/A1SBA- 15 catalyst play an important role for the HDS of thiophene. 100

90 - ~ 80

NH~ Introduction (3 ml/min)

Introduction ~

70

v I::

o

60

(

= 50

>

)

oo 40 30

20 10 0

2

4

6

"lime on stream (hour)

8

10

Figure 3. Effect of introduction of hydrogen sulfide and ammonia on the catalytic acdvity of Pt/AISBA-15 in the hydrodesulfurization of thiophene at 350~

3.3. XRD analysis of Pt/SBA-15 and Pt/AISBA-15 catalysts Figure 4 shows the XRD analysis of A1SBA-15 and noble metal/AlSBA-15 catalysts before reduction. Almost the same XRD patterns as that of A1SBA-15 were obtained before and after loading of noble metals. This indicates that the structure of A1SBA-15 was maintained after loading of noble metals. In the case of SBA-15 before and after loading of noble metals, the situation was quite similar to that of AISBA- 15. Furthermore, almost the same XRD patterns of noble metals were observed in noble metals supported on SBA-15 and A1SBA-15 except platinum. No peaks of Pt were observed in the XRD analysis of Pt/A1SBA-15 but the sharp peaks of Pt were observed in the XRD analysis of Pt/SBA-15 as shown in Figure 4. These results indicate that Pt particles in Pt/A1SBA-15 are loaded on A1SBA-15 with high dispersion but Pt in Pt/SBA-15 is loaded on SBA-15 with relatively large particle size. Since we have reported that the acid site of HZSM-5 zeolite enhances the dispersion of Pt on HZSM-5 in our previous paper [5], high dispersion of Pt on AISBA-15 may be due to high acidity of A1SBA-15. By these results, it can be assumed that Pt/AISBA-15 has higher ability of activation of hydrogen, that is, the formation of spillover hydrogen, on highly dispersed Pt particles than Pt/SBA-15.

744

l i~ I~

a)AISEIA-- 15 b)RIVAISBA--- 15 c) Pd/AISBA-- 15

-" ~

;-;~:~

d) Ru/AISBA--- 15 e) Pt/AISBA-- 15 f)Pt/SBA-- 15

;:,,~='C'~~

9

_==

O)

o

c)

I 5

10 15 20 25 30 35 40 45 50 5 5 ' 6 0 2 0/degree

65 70 75 80 85 90

Figure 4. XRD patterns for noble metallAlSBA-15eatalystsbefore reduction. 3.4.

Mechanism of HDS of thiophene on Pt/AISBA-15 Catalyst

Pt/A1SBA-15 catalyst showed higher activity for the HDS of thiophene than Pt/SBA-15 and commercial CoMo/A1203 catalysts. We also studied the active sites and reaction mechanism in the HDS of thiophene over Pt/AISBA-15 catalyst. 100 90 8O vI:: 70 o

El 2-Propanol dehydration I= Cumene cracking

"~ 60 o

> g: 50 oo 40 30

20 10

Low activity SBA--15

AISBA-- 15

Figure 5. Catalytic activities of SBA-15 and AISBA-15 for the dehydration of 2-propanol (200~

and cracking of cumeme (400"C).

745 We evaluated the acidic properties of SBA-15 and A1SBA-15 by the reactions of 2-propanol dehydration (200~

and cumene cracking (400~

using pulse reactor. It was revealed that

SBA-15 showed very low activity for both reactions but AISBA-15 showed remarkably high activity for these reactions as shown in Figure 5. These results indicate that AISBA-15 has high acidity and there exists the Broensted acid sites on A1SBA-15. We also confirmed the existence of the Broensted acid sites at 1547 cm -1 on A1SBA-15 by the observation of b-T-IR spectra of pyridine adsorbed on AISBA-15 as shown in Figure 6. Thus, we supposed that the Broensted acid site of A1SBA- 15 acts as active site for the activation of thiophene in the HDS of thiophene.

ID

o r-co

b)AISBA-15

x~ 0

1700

1600

1500

1400

Wavenumber (cm -I)

1300

SBA- 15and AISBA- 15 were evacuated at 500~ for 2 hrs. Pyridine was

adsorbed at 150~ followed by evacuation at 150,"C for 0.5 hr. Figure 6. Infrared spectra of pyridine adsorbed on SBA-15 and A1SBA-15.

On the basis of these results, we propose a possible mechanism for the HDS of thiophene over Pt/A1SBA-15 as shown in Scheme 1. In the proposed mechanism, the Broensted acid site in the Pt/AISBA-15 acts as active site for the activation of thiophene and Pt acts as active site for the activation of hydrogen to form spillover hydrogen. The spillover hydrogen formed on Pt attacks the activated thiophene on the Broensted acid site on A1SBA-15.

746

HzS + C4Hydrocarbon

H~ H

H-~Hydmg~_

f~[

Activated thiophene ] , 4 - - - -

Pt

.

~

.

[~

.

.

.

.

H+ Br~nsted ,

0 9

.

"' '

~ ~ d , e ~ AISBA-15

. . . . . . . .

|

acid site 9

Acid" sltef " '

............

Scheme 1. The possible mechanism of hydrodesulfurization of thiophene over

Pt/AISBA-15 catalyst. 4. C O N C L U S I O N It was revealed that the Pt/A1SBA-15 catalyst showed high and stable activity for the HDS of thiophene and this activity was higher than that of commercial CoMo/AI203 HDS catalyst. Therefore, it is concluded that there is a possibility for using Pt/AISBA-15 as highly active new HDS catalyst for bulky organic sulfur compounds in the petroleum feedstocks. ACKNOWLEDGEMENT

This work was partly supported by KAWASAKI STEF.I~21 Century Foundation, Japan and Petroleum Energy Center of Japan. REFERENCES

1. M. Laniecki and W. Zmierczak, Zeolites, 11(1991)18. 2. Y. Okamoto, Catal. Today, 39(1997)45. 3. M. Sugioka, Erdol & Kohle, Erdgas, Petrochemie, 48(1995)128. 4. M. Sugioka, F. Sado, T. Kurosaka and X. Wang, Catal. Today, 45(1998)327. 5. T. Kurosaka, M. Sugioka and H. Matsuhashi, Bull. Chem. Soc. Jpn, 74(2001)747. 6. K. M. Reddy, B. Wei and C. Song, Catal. Today, 43(1998)261. 7. A. Wang, Y. Wang, T. Kabe, Y. Chen, A. Ishihara and W. Qian, J. Catal., 199(2001)19. 8. M. Sugioka, L. Andalaluna, S. Morishita and T. Kurosaka, Catal. Today, 39(1997)61. 9. M. Sugioka, S. Morishita, T. Kurosaka, A. Seino, M. Nakagawa and S. Namba, Stud. Surf. Sci. Catal., 125(1999)531.


747

Skeletal Isomerization of 1-Hexene to Isohexenes over Zeolite Catalysts Zhihua Wu, Qingxia Wang, Longya Xu and Sujuan Xie Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, E R. China Several zeolite catalysts such as SAPO-11, ZSM-11, ZSM-12, etc. were selected to convert 1-hexene to branched hexenes in this work. Pore size of the zeolite catalyst plays an important role on the yield and the distribution of branched isohexenes. And the zeolite catalysts with the pore size of 0.6nm are optimum to produce dimethylbutenes (DMB). SAPO-11 zeolite is a suitable skeletal isomerization catalyst, especially in the production of methyl pentenes. Under the following reaction conditions: WHSV=I.0 h~, HJhexene=8, T=250 ~ P=0.2 MPa, the yield of skeletal isohexenes remains above 80% at the prolonged time-on stream of 80 h, accompanying low C5., C7+ products and low carbon deposition on the catalyst. 1. INTRODUCTION The catalytic reactions for converting unbranched olefins into branched olefins, such as the skeletal isomerization of n-butenes to isobutene, are important processes for the large-scale production of raw materials for chemical industry. To guide the screening of catalysts for the desired processes, tremendous of work has also been devoted to the mechanistic studies of these processes. To date, there are at least two proposed models for the skeletal isomerization of olefins, monomer model and dimerization model. Guisnete [1] reported that there were three steps from n-butenes to isobutene: (i) dimerization of n-butenes, (ii) skeletal isomerization of dimers, and (iii) cracking of the octene isomers. In contrast, Houzvickn [2] proposed that the dominating process for the skeletal n-butene isomerization was monomolecular and the bimolecular mechanism was mainly responsible for the formation of byproducts, such as propene and pentenes. Also, the results of Mooiweer [3] favored the mechanism of skeletal n-pentenes isomerization to isopentenes to be monomolecular. Isomerization reactions of olefins are affected by various factors. Asensi reported that the selectivity of n-butene to isobutene was greatly improved with the increased Si/A1 ratio in MCM-22. Further characterization of these catalysts revealed that the increased Si/A1 ratio led to a lower acid site density. Since these acid sites were proposed to the sites for the bimolecular side-reaction, a decreased acid site density in those catalysts was attributed to the increased isobutene selectivity [4]. Besides the acid site density, the pore size of the zeolites also affects the selectivity of the isomerization reaction. The results of Feng [5] indicated that the outcome of 2-methyl-2-pentene isomerization reaction was also greatly influenced by the pore sizes of zeolites. This was supported by the feeding experiment with several octenes over open-surface and microporous materials and it was found that the 10-membered ring (10-MR) channels were hardly accessible to double-branched hydrocarbons and the diffusion through the 10-MR by triple-branched were denied [2].

748 In the present paper, the catalytic performance of zeolites for the isomerization of 1-hexene to branched hexenes was investigated in a continuous-flow fixed bed reactor. Reported herein are the preliminary skeletal isomerization results. 2. EXPERIMENTAL

2.1. Catalyst preparation ZSM-11 (Si/AI=700), ZSM-35 (Si/AI=15) and ZSM-12 (Si/AI=50) zeolites were synthesized in our laboratory. SAPO-11 and Y-type zeolites were produced by another laboratory in our Institute of Chemical Physics. Si-ZSM11 and Si-SAPO11 were prepared by binding the zeolite and silica sol according to a definite weight ratio together, while the catalyst, A1-SAPOll, was prepared by binding A1203 and SAPO-11. The solids were calcined in air at 550 ~ for 3 h before reaction. Si-ZSM35 was prepared by binding silica sol and ZSM-35 zeolite, then was calcined in air at 550 ~ for 3 h. The catalyst was exchanged with 0.8 M ammonium nitrate solution two times (for 2 h each time), then impregnated with magnesium nitrate aqueous solution, calcined at 500 ~ for 2 h. The catalyst was about 8 wt % Mg loading. Si-ZSM12 was prepared as the catalyst Si- ZSM35, and the catalyst was about 1 wt % Mg loading. The catalyst, Si-Y, was prepared from NaY by exchanging with 0.8 M ammonium nitrate solution only one time.

2.2. Reaction performance 1-Hexene of 96.92% purity obtained from Acros Organics was used. The major impurities were 3-methyl-1-pentene (0.66%), 2- and 3-hexenes (2.41%). Olefin isomerization reaction was carried out in a microreactor (9 mm I. D., 39mm O.D.), with 3.5g catalyst (20-40 mesh). The reactor was heated from room temperature to 400 ~ at a rate of 200 ~ in a flow of hydrogen then maintained at 400 ~ for an hour. After that, it was cooled to the reaction temperature. As the desired reaction temperature was reached, the mixture feed of 1-hexene and hydrogen (1:8 molar ratio) was passed through the reactor instead of hydrogen. The tail gas was analyzed by an on-line gas chromatography equipped with a 9-m squalane column and TCD, while the liquid product was analyzed by a Varian 3800 gas chromatography with a 100-m Pond capillary column and FID. Yields to the different reaction products are calculated according to the following equation: % Yield (i) =100 •

weight of product i formed Weight of 1-hexene fed

2.3. Catalyst characterization 2.3.1. NH3 temperature programmed desorption (NH3-TPD) A catalyst sample of 140mg was first heated from room temperature to 600 ~ at a ramping rate of 25 ~ and then held at 600 ~ for 30 min under a flow of 30ml/min pure helium. The system was then cooled to 150 ~ in a He stream. At 150 ~ the adsorption of the catalyst was carried out in a He stream containing ammonia until it was saturated. Then, the sample was swept with helium. When the baseline of gas chromatography was stable, the NH3 desorption profile of the catalyst was performed from 150 ~ to 600 ~ at a heating rate of 20 ~ The amount of desorption NH3 was monitored by a thermal conductivity detector and quantified by the pulse method.

749 Table 1 Influence of temperature on the performance of Si-ZSM11 (H2/1-hexene=8, P=0.2 MPa, SV=I.0h "l) Temp. Yield of product (wt%) ~ Cs. 1-hexene hexene(-2,-3) branched hexenes 350 0.00 96.27 3.13 0.61 400 0.00 52.98 40.06 6.96 500 0.00 17.12 67.20 15.57

C7+ 0.00 0.00 0.11

2.3.2. Thermogravimetric Thermogravimetric (TG) data was acquired on a Perkin Elmer Pyrisl TGA apparatus. The used catalyst of about 10 mg was heated to 150 ~ and held at 150 ~ for 30 min under a flow of 20ml/min N2. Then N2 was switched to air and the catalyst was heated from 150 ~ to 800 ~ at a rate of 10 ~ and the weight of catalyst was monitored by the thermo-balance and recorded. 3. RESULTS AND DISCUSSION

3.1. Reaction performance of Si-ZSMll for skeletal isomerization of 1-hexene The effect of temperature on the performance of skeletal isomerization of 1-hexene to branched hexenes (BH) over Si-ZSM11 catalyst was studied. The results are shown in Table 1. The skeletal isomerizaion reaction does not occur until the reaction temperature rises up to 400 ~ And the amount of branched hexenes increases from 6.96% to 15.57% when the temperature increases from 400 ~ to 500 ~ The C7+ products appear at 500 ~ due to the polymerization of hexenes. Si/A1 ratio in ZSM-11 zeolite is 700, and the average distance of an A13§ ion in zeolite to the closest one is 4.23 nm, while the length of a 1-hexene molecule is 1.03nm. This means that the closest distance between A13+is 2 times greater than the size of a 1-hexene molecule. This excludes the possibilities of the interaction of 1-hexene absorbed on different A1> sites. Thus, the branched isohexenes in the product without C5. and Cv+ at 400 ~ might come from monomolecular hexenes adsorbed on the catalyst. In a word, the skeletal isomerization of 1-hexene to branched hexenes is monomolecular. However the farmation of C7+ at 500 ~ might come from the direct reaction between the hexene absorbed on the acid site of catalyst surface and the 1-hexene existed in the gas phase. The formation of C7+ is agreement with that of Eley-Riedeal mechanism. Thus, it can be inferred from the above results that the skeletal isomerization of 1-hexene to isohexenes over Si-ZSM-11 zeolite catalyst is monomolecular mechanism. 3.2. Reaction performance over difference zeolites Here, we investigated the relation between skeletal isomerization of 1-hexene to BH and the acid density of catalysts with similar acid strength, and table 2 shows reaction performance. The results from Figure 1 show that the acid densities of catalysts decrease as the following: Si-Y >> Si-ZSM35>Si-ZSM12>>Si-SAPOll, while the values of isohexenes over the catalysts from Table 2 are: Si-ZSM35>Si-SAPOll> Si-ZSM12 >Si-Y. The results show that the yield of branched BH over Si-ZSM35 is highest and that of Si-Y is the lowest. Since the acid site density of Si-Y is the highest among the catalysts used. The above results indicated that the acid density of a catalyst is not the sole factor directly related with the

750 Table 2 Reaction results of 1-hexene isomerization to isohexenes over catalysts (H2/1-hexene=8, P=0.2 MPa, SV=I.0h l, T=270 ~ Yield of product (wt %) Catalyst Pore diameter Acidity* (nm) (mmol/g) C5. Hexene (-1, BH (DMB)

-2,-3)

Si-Y 0.80-0.90 0.439 Si-ZSM12 0.57x0.61 0.244 Si-ZSM35 0.42x0.54 0.290 Si-SAPO11 0.39x0.63 0.035 * Values calculated from NH3-TPD

0.31 2.20 0.58 0.00

93.99 35.22 25.96 46.68

5.09 (0.13) 48.20 (3.34) 69.11 (3.95) 53.32 (1.07)

C7+ 0.61 14.38 4.35 0.00

skeletal isomerization of 1-hexene to BH if it plays an important role. Microporous materials produce less dimerization reaction than open-surface materials [2]. This implies that the pore size of the catalyst might also be crucial for the catalytic production of the desired products. And this led to our further studies on the effect of catalyst pore diameter on the selectivity of the catalyst. The results from Table 2 indicate that the catalyst pore diameter between about 0.4 nm to 0.6 nm was optical for the 1-hexene skeletal isomerization. When the catalyst pore diameter is above 0.8nm, the value of branched hexenes is very low. Our data clearly indicate that, besides the acid site density, the micropore size of the zeolite is responsible for the highest selectivity shown in the case of Si-SAPO 11. The percentage of each branched isohexene in the product mixture may be affected by the catalyst pore diameter. Figure 2 shows the ratio of the dimethylbutenes (DMB) percentage in the branched isohexene mixture over catalysts tested in our experiment to that of calculated equilibrium value (Equilibrium value refers to ref. [6]). Although the yield of branched isohexenes over Si-SAPOll is more than that over Si-ZSM12, the yield of DMB over SiSAPO 11 is less than that over Si-ZSM12. The differences in the product distribution over the catalysts used might be attributed to the pore size differences. Microporous materials such as SAPO-11 do not allow free diffusion of tribranched or even dibranched hydrocarbon [2]. It is highly possible that the wall of 0.39 nm wide pore 0.5 0.4

0.3 0.2

0. i 150

250

350

450

550

Desorption Temperature(~

Fig. 1 NH3-TPD profiles of different zeolites

Si-Y

Si-ZSMI2 Si-ZSM35 Si-SAP011

Fig.2 Ratio of DMB percentage in BH in this test to that of equilibrium.

751

,

'

i

,

i

9

1.6

--m-- G Product

50-

1.4 e 40-

"~ i

---o-- Br C7+anehed ProduetIsohexenes --a~

1.2

~9 30>., 1.0 Space Velocity (h -~)

Fig.3 Effect of space velocity

2~

24o

2~

~

a~o

uo

Reaction T e l n p e r a t u r e (~

Fig.4 Effect of reaction temperature

suppresses DMB production. Or even if the DMB were produced in the pore of SAPO-11/Si with high percentage, the very slow diffusion rate of DMB would make DMB stay in the pore. All of these might explain the low percentage of DMB in the product mixture. It is conceivable that catalyst with pore size about 0.6 nm might generate a product mixture with higher DMB percentage. 3.3 Reaction performance of Si-SAPOll under different reaction conditions Based on the discussion above, it can been seen that Si-SAPOll is a good catalyst for skeletal isomerization of 1-hexene. The effects of space velocity and reaction temperature on the performance of skeletal isomerization of 1-hexene to isohexenes over Si-SAPO 11 catalyst were investigated. The results are shown in Figure 3 and 4. The higher the space velocity is, the lower the yield of skeletal isohexenes is. The DMB percentage in the branched isohexene mixture decreases with increasing space velocity. Table 3 exhibits the production rate of methyl pentenes (MP) and DMB at various space velocities. The result shows that the production rate of MP is larger than that of DMB. If every acid site can convert 1-hexene to skeletal isohexenes, at the utmost about 180 MP and 2.3 DMB can be produced one hour at one site over Si-SAPO 11 catalyst at 250 ~ Since DMB come from the skeletal isomerazation of monomethyl pentenes, which in turn come from the skeletal isomerizatiion of 1-hexene [7]. Thus, two consecutive isomerization steps are required to produce DMB from 1-hexene. As an intermediate for the production of DMB, it is reasonable that monomethyl pentene production is higher than that of DMB.

Table 3 Influence of space velocity on the produce rate (H2/1,hexene=8, P=0.2 MPa, T=250 ~ WHSV( h 4) ............. 0,5 ....... 1.0 MP Produce rate(mmol, h 1. g-l) 3.10 5.27 DMB produce rate(mmol, h "1. g4) 0.051 0.080

1.5 6.35 0.074

2.0 6.15 0.063

752 Table 4 Influence of binder on the reaction performance (H2/1-hexene=8, WHSV=I h "l, P=0.2 MPa, T=250 ~ Catalyst Yield of produce (%) DMB/BH C 5. BH C7+ (%) Si'SAPO 11 0.00 44.97 0.00 1.5 A1-SAPO11 0.37 79.19 7.84 7.1

Acidity (~tmol/g) 350 ~ 450 ~ 33.0 1.8 62.5 16.5

600 ~ 2.2

When the reaction temperature rises from 250 ~ to 310 ~ the yield of skeletal isohexenes increases and achieves the highest value at 310 ~ The results also reveal that the C1 and C2 product are not observed in the temperature range between 250 ~ and 340 ~ The yield of Cs is larger than that of C4 in the temperature range between 280 ~ and 340 ~ The yields of C5. and C7+ products, especially that of propane, rise quickly with increased temperature. The yield of propane increases from zero at 280 ~ to 4.36% at 340 ~ while that of Cs only increases from 0.06% to 0.72%. At 340 ~ the ratio of C9/C3 is only 0.036, while the ratios of C7/Cs and C8/C4 are 2.014 and 6.653 respectively. The wide production ratio distribution indicates that there might be multiple mechanisms employed in this catalytic process. The relative low C9/C3 ratio compared to these of C7/C5 and C8/C4 implies that the production mechanism of C3 might be different from that of C4 and C5. The dimerizationcracking process produces C4 and Cs product, however maybe 13-scission of polymers produces C3 product [7]. The high reaction temperature is more benefit for 13-scission than dimerizationcracking. 3.4. Influence of binder on the reaction performance on SAPO-11

Table 4 shows the effect of the binder on the reaction performance of SAPO-11. And Figure 5 shows the NH3-TPD of A1-SAPO 11 and Si-SAPO 11. Compared to Si-SAPO 11, A1SAPOll is more acidity. And the yields of both DMB and by-product are higher. The alumina, which is impregnated in the form of an acidic aluminium nitrate, has enough acidity to efficiently convert 1-pentene to skeletal isomers [8]. In the process of binding SAPO-11 and A1203, nitric acid was added and aluminium nitrate is produced and then impregnated onto A1203,. This process is equivalent to the direct impregnation of aluminium nitrate on A1203. This is why the acidity of A1-SAPO 11 is higher than that of A1-SAPO 11. Since these acidic sites are responsible for the isomerization of 1-hexenes to BH, it is expected that the yield of BH over A1-SAPO11 is higher than that over Si-SAPO 11. The surface of alumina is opensurface, and open-surface favors the dimerization of olefins and cracking ' ' AI-'SAP() 1 1' l, Si-SAPO 1 1 ] dimerization [2]. Thus, the higher yields of both C5. and C7+ products over A1r SAPO 11 catalyst, as indicated in Table 4, Eare expected and consistent with the ...::::::::::l- .... properties of the corresponding catalyst 2;o3;o 4;o s;o 600 used. Desorl)tion Temperature(~ Neither Cl, C2, nor C3 product is observed in the product mixture in the case of A1-SAPO 11 at 250 ~ This is Fig.5 NH3-TPD profiles of different catalysts

ill

~1-

~i111,11.

753 90

L~

80

i

/_~_mm __| ~ n__m ~__m.lm__| __m~__n

9 --~"~'-m~mm~m

70

9

60 10

--o---A--

Dim ethylbutenes C~+ P r o d u e t

9

9

| , . J

\.:-J'~:~,~:-o-:>~:-~o~,~.o_.~.,__~~ 9

2'0

40 Tim

e

oll

9

i

60 St.r

u

80 earn

9

i

100

9

120

(h)

Fig.6 Infuence of time-on-stream over A1-SAPO 11 (H2/1-hexene=8, WHSV=I.0h 1, P=0.2 MPa, T=250 ~

different to that over Si-SAPO 11 at 340 ~ Since the production of C 3 products is proposed to be the result of the [3-scission mechanism, the higher yield of C3 product at higher temperature indicates that the [3-scission mechanism is favored at higher temperature. 3.5. Stability test of AI-SAPOll catalyst Figure 6 shows the stability of A1-SAPOll catalyst at WHSV = 1.0 h 1, HJhexene = 8, T-250 ~ P=0.2MPa. The yield of branched hexenes is usually 80% or above when the reaction time is less than 78 hours. After 126 hours the yield of skeletal isohexenes is still higher than 60% at the same reaction condition. The test shows that the catalyst is robust and has a relatively stable performance over long time. The C~, C2 and C3 products have not been observed from the start to the end of the stability test, while a few of C4 and Cs products are observed. The yield of Cs. is always less than 0.4%. The sum of C5 is larger than that of C4. It is also shown that the yield of C7+ is always larger than that of C5.. When the carbon number of product is larger than 6, the yield of the corresponding product decreases with the increasing chain length. Also, both C5. and C7+ products decrease with increasing time on stream. The DMB yield always decreases with increasing time on stream, even when the yield of BH keeps above 80%. The yield of DMB decreases from 5.60% at the start to 95 2.21% at 78 h, and to 1.37% at 126 h. Accordifigly, the ratio of DMB/BH ,.c:: decreases from 7.07% to 2.78%, and to 93 2.27%. These results imply that the active sites responsible for the production of the DMB lost activity at a rate faster than that 2;0 360 460 560 660 760 of MR Figure 7 shows TG of used A1-SAPO11 Temper ature (~ catalyst. The coke is about 4.3 w. %. The Fig.7 TG of used A1-SAPO11 abruptly temperature point of weight lost is "~

~JO

94'

754 about 470 ~ active sites.

This shows they might be responsible for the lost of activity of the catalytic

4. CONCLUSIONS Our data presented in this paper favors the monomolecular skeletal isomerization of 1hexene to branched isohexenes over Si-ZSMll zeolite catalyst. Also, the skeletal isomerization of 1-hexene to isohexenes is not only influenced by the acid strength and acid site density, but also by the zeolite catalyst pore size. It has also been found that the A1-SAPO 11 catalyst is an excellent catalyst for the skeletal isomerazation of 1-hexene. High yield of skeletal isohexenes with monomethyl pentenes as the major product, and high catalyst stability are obtained. Furthermore, the low yields of the side products, such as C5 and C7+ products, and low carbon deposition on the catalyst over long time make this catalyst attractive for future target for further optimization. ACKNOWLEDGEMENT We thank Dr. L. Xu for providing SAPO-11 zeolite used in this paper. REFERRENCES 1. M. Guisnet, E Andy, N. S. Gnep, E. Benazzi and C. Travers, J. Catal., 158 (1996) 551. 2. J. Houzvicka and V. Ponec, Ind. Eng. Chem. Res., 36 (1997) 1424. 3. H. H. Mooiweer, K.P. de Jong, B.Kraushaar-Czametzki, W.H.J. Stork and B.C.H. Krutzen, Stud. Surf. Sci. Catal., 84 (1994) 2327. 4. M. A. Asensi, A. Corma, and A. Martinez, J. Catal., 158 (1996) 561. 5. X. Feng, J. S. Lee, J. W. Lee, J. Y. Lee, D. Wei and G. L. Haller, Chem. Eng. J., 64 (1996) 255 6. J. E. Kilpatrick, E. J. Prosen, K. S. Pitzer and E D. Rossini, J. Res. Nati. Bur. Standarts., 36 (1946) 559. 7. W. A. Groten and B. W. Wojciechowski, J. Catal., 122 (1990) 362. 8. C. Lin, H. Yang, C. Lai, C. Chang, L. L. K. Kuo and K. Yung, Skeletal Isomerization of Olefins with an Alumina Based Catalyst, US Patent No. 5 321 193 (1991)


755

Preparation and catalytic characterisation o f Al-grafted M C M - 4 8 materials M. Rozwadowski, *aM. Lezanska, a J. W l o c h , a K. Erdmarln, a and J. Komatowskib aFaculty of Chemistry, Nicholas Copernicus University, Gagarina 7, 87-100 Torun, Poland bLehrstuhl II fttr Technische Chemie, Technische UniversiN't Mttnchen, Lichtenbergstr. 4, 85747 Garching bei Mttnchen, Germany

Samples of A1-MCM-48 were prepared by grafting A1 onto the pure siliceous material and used as catalysts for cumene cracking and conversion of 2-propanol. The former reaction yielded mainly benzene and propene, which indicated the presence of strong Bronsted acid sites in the catalysts. The conversion of 2-propanol resulted mainly in dehydration of the substrate, yielding propene and diisopropyl ether. The catalytic activity of A1-MCM-48 grew with both the A1 content and reaction temperature. The concentrations of the Bronsted and Lewis acid sites increased with the A1 content of the material as well. 1. INTRODUCTION Mesoporous molecular sieves of the M41S family [ 1,2] have extensively been studied with respect to their unique properties [3-6]. Many efforts have been focused on silica- and alumina-based materials as potential catalysts for the reactions involving large organic molecules [7-10]. Purely siliceous M41S does not show significant catalytic activity because of its electrically neutral skeleton with no ion-exchange capability. However, substitution of silicon with various metals generates acidity in these materials and modifies their surface properties. This is a promising way to synthesise materials applicable in catalysis [11,12]; for example, introduction of boron [7,13], titanium [14-16], vanadium [17], and gallium [8] has been reported. Incorporation of aluminium is also interesting in relation to catalytic applications and has been discussed in numerous papers, especially in the case of the MCM-41 materials. Reports, although not so many, on the introduction of A1 into MCM-48, another member of the M41S family, have been published as well [18-20]. Such a modification of the M41S structure seems to be of particular importance as it can give rise to the Bronsted acid sites. These centres should primarily be responsible for the catalytic activity of the mentioned materials. Generally, the Al-containing molecular sieves can be obtained by a hydrothermal (i.e., direct) synthesis or by post-synthesis methods of impregnation or grafting. Jun and Ryoo [21 ] investigated the catalytic activity of mesoporous molecular sieves of different channel systems (MCM-41, MCM-48, and KIT-I; Si/A1 = 19 and 38) in the Friedel-Crafts alkylation reaction. They demonstrated that the materials prepared with the post-synthesis procedures were superior to those synthesised directly with respect to the structural order and

756 accessibility of the A1 centres to reactants. The authors suggest that the latter is caused by the fact that, in the case of the hydrothermal synthesis, a part of A1 becomes located inside the pore walls, especially when the A1 content is relatively low. Cheng et al. [22] showed that A1grafted MCM-41 exhibited a considerably higher acidity as compared to that of A1-MCM-41 obtained hydrothermally (both materials with Si/A1 = 20). This was reflected in the results of cumene cracking. However, when Si/A1 was in the range of 1-6 [23], the materials synthesised directly exhibited a higher acidity but their structure was not typical of MCM-41. On the other hand, the Al-grafted samples retained the MCM-41 structure. Corma et al. [24] found that the acid strength of A1-MCM-41 synthesised hydrothermally was lower than that of zeolite USY and higher than that of amorphous aluminosilicates. The aim of this work was to study the catalytic reactions of cumene cracking and conversion of 2-propanol over the Al-grafted MCM-48 samples. It was expected that the content and/or distribution of A1 might affect the strength of the Bronsted acid centres similarly as in zeolites. Therefore, we attempted to correlate the postulated reaction mechanisms with the acidic strength of these sites.

2. EXPERIMENTAL

2.1. Samples The MCM-48 material was synthesised from a mixture containing suspension of SiO2 (Ultrasil, Degussa) in water and both tetramethylammonium hydroxide and cetyltrimethylammonium chloride as templates [25]. Four different A1-MCM-48 samples were prepared by grafting aluminium onto the purely siliceous MCM-48 parent material. Aluminium isopropoxide dissolved in n-hexane was chosen as the source of aluminium for the grafting process. The resulting materials were calcined at 803 K under air for 4 h. The samples are referred to as A1-MCM-48(n) where n denotes the Si/A1 molar ratios in the reaction mixtures, equal to 32, 15, 5, and 2. The Si/A1 ratios of the calcined A1-MCM-48 samples were determined with the atomic absorption spectroscopy (AAS) (see Table 1). More details on the sample preparation can be found elsewhere [26]. 2.2. Catalysis The catalytic tests were carried out with a pulsed method using a vertical flow microreactor connected to a Shimadzu GC-14B gas chromatograph equipped with a flame ionization detector. The catalyst samples (5 mg) were placed in the reactor and treated thermally at 723 K under helium for 1 h. Cumene was injected at 25-min intervals (eight injections, 1-~tl portions) and the reaction was run at 623,673, and 723 K. The chromatographic column was packed with Carbowax 4000 and the carrier gas (helium) was flowing at a rate of 30 ml/min. In the case of 2-propanol, four injections (1-~tl portions) were applied in 15-min intervals, the reaction temperatures were 523 and 573 K, and the column was packed with Porapak N.

2.3. Acid sites For the analysis of the Bronsted and Lewis acid sites present in the studied A1-MCM-48 materials, the IR spectra were recorded with a Bruker 48 PC spectrometer equipped with a MCT detector. The samples in the form of wafers were activated in situ in the IR cell at 633 K for 1 h. Then, pyridine (POCh, Poland, dried over KOH) taken in excess of the amount necessary to neutralise all the acid sites was adsorbed at 430 K. Subsequently, the physisorbed

757 pyridine was removed under 30-min evacuation at the same temperature and then the IR spectra were recorded. Concentrations of both the Bronsted and Lewis acid sites were calculated from intensities of the IR bands assigned to pyridinium ions (HPy +) and to pyridine molecules bonded to Lewis sites (PyL) at 1545 and 1455 cm-1, respectively. The extinction coefficients used for the calculations were determined for pyridine adsorbed on both the zeolite HY containing only the Bronsted acid sites and the dehydroxylated zeolite HY containing only the Lewis acid sites. They were equal to 0.070 and 0.100 cm 2 gmo1-1 for HPy + and PyL, respectively.


The low-angle XRD powder patterns of the studied samples demonstrated a set of peaks (including the 211 and 220 reflections), indicating a typical system of uniform cubic pores [26]. These pores are considered as the primary mesopores while void space between adjoining crystallites and large mesopores in the particles that do not form any ordered structures are referred to as the secondary mesopores [26]. The combined volume of both the primary and secondary mesopores is defined as a total pore volume. Table 1 shows some structural parameters of the studied samples. Although the values of SBET and Vt somewhat decreased with the increase in the content of A1, they were relatively high. This suggested that the materials might exhibit noteworthy catalytic properties. The reaction of the catalytic cracking of cumene results in a series of compounds with different numbers of carbon atoms in a molecule, propene and benzene being the main products [27,28]. Comparison of the level of the cumene conversion performed over different samples at a given temperature allows one to arrange these samples with respect to their acidity [29]. Here, it was found that the cumene conversion increased with the content of aluminium in the A1-MCM-48 materials (Fig. 1). For a given sample, the cumene conversion Table 1 Structural parameters of the studied MCM-48 materials [26] Parameter

Sample parent A1-MCMA1-MCMA1-MCMA1-MCMMCM-48 48(32) 48(15) 48(5) 48(2) Si/A1 (AAS) n.a. 34.5 12.7 3.8 3.5 d211 [nm] 3.71 3.40 n.d. 3.32 n.d. ao [nm] 9.10 8.33 n.d. 8.14 n.d. SBET [m2 g-~] 1315 1245 1188 1051 1030 2 -1 St [m g ] 1294 1213 1164 1010 995 Next [m2 g-l] 284 259 136 154 209 Sp [m2 g-l] 1010 954 1028 856 786 Vp [cm3g-l] 0.718 0.662 0.673 0.538 0.513 Vt [cm 3 g-l] 0.958 0.884 0.794 0.713 0.711 d211 is the (211) interplanar spacing, a 0 - unit cell parameter, aBET- the BET specific surface area, St - total surface area, Sr - external surface area, Sp - surface area of primary mesopores, Vp - volume of primary mesopores, Vt - total pore volume, n.a. - not applicable, and n.d. - not determined.

758 increased also with the reaction temperature, as seen for A1-MCM-48(5) (Fig. 2). A similar picture was observed for A1-MCM-48(2)while for A1-MCM-48(15)and A1-MCM-48(32)the increase in the conversion with the temperature was clearly lower. In general, the conversion decreased slightly with the number of injections (Figs. 1 and 2). The rate of this decrease was more pronounced for the samples with higher contents of A1 and practically independent of the reaction temperature. These observations suggest a more efficient coking of the catalysts with the higher A1 contents. 50

50

4O

4O

O []

,-.- 30

._o

A

v~

[]

[]

[]

O O

> 20

(~

o

O (.)

O

[]

=~ 20

,~

A

8

O

[]

[]

30

O

[]

10

A

[]

A

O

10

[]

O

0

0 o

o I

I

I'

I

i

I

I

I

2

3

4

5

6

7

"

0

l

8

I

I

I

I

I

I

I"

I

1

2

3

4

5

6

7

8

injection no.

injection no.

Figure 1. Cumene conversion at 723 K over A1-MCM-48 with different Si/A1 ratios: 3.5 (+), 3.8 (O), 12.7 (A), and 34.5 ( 9

Figure 2. Cumene conversion at 623 ( 9 673 (A), and 723 K (F]) over A1-MCM-48 with Si/A1 = 3.8.

60

60 b

b

b

P

_

O

40

I

1

._>

b 1~

b 14_

Np 9

~ ' 4O

~b 4

4_

-

14

>,

I I

O Q) Q)

20

20

1 I -I

"~

1 3.5

m

[ 12.7

3.8

34.5

Si/AI Figure 3. Selectivity (after the injection no. 1) of cumene conversion at 723 K over A1MCM-48 with the Si/A1 ratios as indicated; b, benzene, p, propene, and m, a-methylstyrene.

623

m

",

673

T [K]

,r 723

Figure 4. Selectivity of cumene conversion at temperatures indicated over A1-MCM-48 with Si/A1 = 3.8; b, benzene, p, propene, m, a-methylstyrene; 1 and 4 denote selectivities after the injections no. 1 and 4, respectively.

759 In accordance with the literature findings, benzene and propene were the main products of the cumene cracking over A1-MCM-48 (Figs. 3 and 4). a-Methylstyrene was another product found in significant amounts. Some not determined compounds were also observed though they were present in trace amounts only. These products were neglected while calculating selectivity. At 723 K, the relative yield of benzene and propene slightly increased and that of a-methylstyrene slightly decreased with the growth of the aluminium content of the catalysts (Fig. 3). On the other hand, the relative yield of benzene and a-methylstyrene slightly decreased whereas that of propene slightly increased with the rising reaction temperature, as observed for A1-MCM-48(5) (Fig. 4). The selectivities of all the products did not change much with the injection number (Fig. 4). As known [27,29], benzene and propene are formed on strong Bronsted acid sites while a-methylstyrene forms at electron-acceptor centres. Thus, the presented observations (Fig. 3) suggest that the number of the electron-acceptor centres decreased while that of the Bronsted acid sites slightly increased with the A1 content of the catalysts. According to stoichiometry of the reaction, the cracking of cumene should yield equal amounts of benzene and propene. The observed lower amounts of propene (Figs. 3 and 4) result most probably from the fact that propene undergoes to a greater extent the conversion to carbonaceous deposits, especially at lower temperatures. In the case of conversion of 2-propanol, two reactions were assumed to occur: (i) dehydration, which leads to formation of propene and diisopropyl ether and (ii) dehydrogenation, which yields acetone [30,31 ]. As found here, the conversion at 523 K increased from c a . 50 to 100% with the Si/A1 of the A1-MCM-48 samples decreasing from 34.5 to 3.5 (Fig. 5). Thus, the catalytic activity of the studied materials grew clearly with the A1 content of the catalysts. At 573 K, the conversion over all the catalysts was approximately 100%. Interestingly, the conversion did not depend on the number of injections. Propene was the main 100

0

'0

0

9

A

A

A

O

O

O

I

I

8O E 0

cO

o

60

0

40

I

1

2 3 injection no.

I

4

Figure 5. 2-Propanol conversion at 523 K over A1-MCM-48 with different Si/A1 ratios: 3.5 (+), 3.8 ([]), 12.7 (zx), and 34.5 ( 9

760 product of this reaction. At 523 K, selectivity toward propene was c a . 98.5% for A1-MCM48(32) and it increased up to c a . 100% with the content of A1 (Fig. 6). Diisopropyl ether was the other important product while acetone was detected in trace amounts only. The selectivity toward propene decreased slightly with the number of injections of 2-propanol. At 573 K, however, the contribution of propene for all the samples was practically 100% and did not decrease with the injection number. The relation between the level of conversion of the examined compounds and the A1 content was confirmed by the IR analysis of the acid centres. As found, the studied A1-MCM48 catalysts differ in the concentrations of the Bronsted and Lewis acid centres that determine the course of the conversion of cumene and 2-propanol. The calculated concentrations of the sites in the parent MCM-48 material and selected A1-MCM-48 samples are listed in Table 2. Some amount of the Lewis sites detected in the parent material is presumably due to traces of A1 present in the reagents used for the synthesis. As seen from the table, the Bronsted acidity of A1-MCM-48(5) is only c a . 3.5 times higher than that of A1-MCM-48(32) although the A1 content is ca. 10 times higher. This implies that the Al-rich sample contains likely a relatively high amount of aluminium that is not incorporated into the structure of the material and does not give rise to the Bronsted acidity. Another reason for the observed catalytic behaviour of the studied samples may be connected with a different acid strength of the catalyst centres. The acid strength can decrease with the rising concentration of the centres that control the examined reactions. Such a dependence, although not very clear, has been found by us for the MCM-41 materials [ 10]. The analysis of the acid strength of the centres of the A1-MCM-48 samples is in progress. These results and comparison of the catalytic activity between Al-grafted MCM-48 and other molecular sieves (zeolites, amorphous aluminosilicates) are planned to be included in a next paper. 100

98

96 .__>

~6

"~

94

92

90

3.5

3.8

12.7

34.5

Si/AI

Figure 6. Selectivity toward propene in conversion of 2-propanol at 523 K over A1-MCM-48 with the Si/A1 ratios as indicated; 1 and 4 denote selectivities after the injections no. 1 and 4, respectively.

761 Table 2 Concentrations of Bronsted and Lewis acid sites Acid sites [~tmol g-l] Bronsted Lewis

MCM-48 0 32

Sample A1-MCM-48(32) 32 150

A1-MCM-48(5) 110

430

4. CONCLUSIONS The examined A1-MCM-48 materials differ in the concentrations of the Bronsted and Lewis acid sites, which increase with the A1 content. High conversion of the reaction of cumene cracking over A1-MCM-48 indicates the presence of strongly acidic Bronsted sites. The conversions of cumene and 2-propanol grow with both the A1 content and reaction temperature. Benzene and propene are the main products of the cumene cracking, a-methylstyrene being another product found in considerable amounts. In the case of the 2-propanol conversion, dehydration is the principal reaction. It leads to formation of propene in predominating amounts and of diisopropyl ether. The concurrent reaction of dehydrogenation yields acetone in trace amounts only. In spite of large differences in the A1 contents of the catalyst, the results of the catalytic reactions do not indicate significant differences in their Bronsted acidity. An increase in the concentration of the Bronsted sites may cause some decrease in their acidic strength. Further investigations are in progress.

ACKNOWLEDGEMENT Thanks are due to Prof. J. Datka (Krakow, Poland) for the IR analysis of acid centres. The work was supported in part by the State Committee for Scientific Research (KBN).

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S. Beck, Nature, 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, and J.W. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Monnier, F. Schtith, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke, and B.F. Chmelka, Science, 261 (1993) 1299. 4. S. Biz and M.L. Occelli, Cat. Rev. Sci. Eng., 40 (1998) 329. 5. A. Corma, Chem. Rev., 97 (1997) 2373. 6. W.J. Roth and J.C. Vartuli, Stud. Surf. Sci. Catal., 135 (2001). 7. A. Sayari, C. Danumah, and I.L. Moudrakovski, Chem. Mater., 7 (1995) 813. 8. C.-F. Cheng, H. He, W. Zhou, J. Klinowski, J.A.S. Goncalves, and L.F. Gladden, J. Phys. Chem., 100 (1996) 390.

762 9. M. Rozwadowski, M. Lezanska, J. Wloch, K. Erdmann, G. Zadrozna, and J. Komatowski, Stud. Surf. Sci. Catal., 135 (2001). 10. M. Rozwadowski, J. Datka, M. Lezanska, J. Wloch, K. Erdmann, and J. Kornatowski, Phys. Chem. Chem. Phys., 3 (2001) 5082. 11. A. Sayari, Chem. Mater., 8 (1996) 1840. 12. A. Sayari, in "Recent Advances and New Horizons in Zeolite Science and Technology", H. Chon, S.I. Woo, and S.-E. Park (eds.), Elsevier, Amsterdam, 1996, Ch. 1. 13. A. Sayari, I.L. Moudrakovski, C. Danumah, C.I. Ratcliffe, J.A. Ripmeester, and K.F. Preston, J. Phys. Chem., 99 (1995) 16373. 14. A. Corma, M.T. Navarro, and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1994) 147. 15. P.T. Tanev, M. Chibwe, and T.J. Pinnavaia, Nature, 368 (1994) 321. 16. A. Sayari, K.M. Reddy, and I.L. Moudrakovski, Stud. Surf. Sci. Catal., 98 (1995) 19. 17. K.M. Reddy, I.L. Moudrakovski, and A. Sayari, J. Chem. Soc., Chem. Commun., (1994) 1059. 18. R. Schmidt, H. Junggreen, and M. St6cker, Chem. Commun., (1996) 875. 19. R. Ryoo, S. Jun, J.M. Kim, and M.J. Kim, Chem. Commun., (1997) 2225. 20. M. Hartmann and C. Bischof, Stud. Surf. Sci. Catal., 117 (1998) 249. 21. S. Jun and R. Ryoo, J. Catal., 195 (2000) 237. 22. L.Y. Chen, Z. Ping, G.K. Chuah, S. Jaenicke, and G. Simon, Microporous Mesoporous Mater., 27 (1999) 231. 23. R. Mokaya and W. Jones, J. Chem. Soc., Chem. Commun., (1997), 2185. 24. A. Corma, A. Martinez, V. Martinez-Soria, and J.B. Monton, J. Catal., 153 (1995) 25. 25. C.T. Kresge, M.E. Leonowicz, W.J. Roth, and J.C. Vartuli, US Patent No. 5 098 684, (1992). 26. M. Rozwadowski, M. Lezanska, R. Golembiewski, K. Erdmann, and J. Kornatowski, Stud. Surf. Sci. Catal. (submitted). 27. D.B. Dadyburjor and A. Bellare, J. Catal., 126 (1990) 261. 28. A. Corma, V. Fornes, M.T. Navarro, and J. Perez-Pariente, J. Catal., 148 (1994) 569. 29. D. Best and B.W. Wojciechowski, J. Catal., 47 (1977) 11. 30. Y. Ukisu, S. Sato, G. Muramatsu, and K. Yoshida, Catal. Lett., 11 (1991) 177. 31. R. Burch, P.J. Milligton, and A.P. Walker, Appl. Catal. B, 4 (1994) 65.


Photoreduction o f Methylviologen

Incorporated Molecules

763

in Zeolite X:

Koodali T. Ranjit and Larry Kevan* Department of Chemistry, University of Houston, Houston, Texas, 77204-5003 The photoreduction of methylviologen (MV 2+) was examined in zeolite X. A series of alkali metal ion-exchanged zeolite X materials with ion-exchanged methylviologen was photoionized with 320 nm light at room temperature in the absence of any reducing counteranion. Photoreduction of methylviologen containing alkali metal ion-exchanged zeolite X results in the formation of methylviologen cation radicals (MV+'). The radicals were identified by electron spin resonance (ESR). Upon irradiation at room temperature the samples turn light blue in color and a single line ESR spectrum characteristic of the methylviologen radical cation is observed. The photoyield depends on the nature of the alkali metal ion-exchanged into the zeolite framework. The photoyield increases in the

order

Li-X/MV 2+ < Na-X/MV 2+ < K-X/MV 2+ < Rb-X/MV 2+

Rb-X/MV 2+ > K-X/MV 2+ > Na-X/MV 2+ > Li-X/MV 2+ > H-X/MV 2+. Thus the photoreduction efficiency can be controlled by the nature of the metal ion in the ionexchange sites. TGA results clearly show that the amount of methylviologen incorporated in different M-X zeolites is similar, suggesting that the difference in the photoyield is due to other factors such as the electron donor strength of the zeolite framework. The cations in the zeolites, in addition to compensating the negative charges in the framework, also govern the electron donor strength of the zeolite framework. The electron donor strength or basicity of the framework has been demonstrated to increase upon increasing the electropositivity of the cation[18,30,31,43]. Thus the electron donor strength of zeolites increases in the order Li-X< Na-X < K-X < Rb-X < Cs-X. In order to directly calculate the electron donor strength of the zeolite framework, it is necessary to have the effective ionization potential of the zeolite framework Ip(Z). However, values of Ip(Z) are not known for solid zeolite X although Ip(Z) has been estimated to be 11.4 eV for Na-ZSM-5153].

768 Sanderson's electronegativity equalization principle has served as a theoretical basis to correlate the experimentally observed electron donor strength of the framework and the partial charge of the framework oxygens[48]. We use Sanderson's partial charges of the framework oxygen atoms as a measure for the framework electron donor strength since they have been shown to be linearly correlated with the experimentally observed framework electron donor strengths. Sanderson's partial charges of the framework oxygen atoms for the MV 2+ doped X zeolites were calculated on the basis of the chemical compositions.The values of Sanderson's electronegativity for each element Si, A1, O, Li, Na, K, Rb and Cs were taken from the literature[54]. The relationship between the photoyield and the calculated Sanderson's partial charge of the framework oxygens is quite linear and leads us to conclude that the yield of the photoproduced MV +" increases with an increase in the negative charge density of the framework oxygens, that is, upon increasing the framework electron donor strength or basicity. The fact that the photoyield and the stability of photoproduced MV +" decrease in the order Cs-X/MV 2+ > Rb-X/MV 2+ > K-X/MV 2+ > Na-X/MV2+ > Li-X/MV 2+ clearly indicate that not only the single electron transfer step to form MV +" but also the MV +" stability or decay dynamics are dependent on the electron donor nature of the cation. A possible explanation is that the major pathway for the decay of photoproduced MV +" cation radical is back electron transfer from MV +" to a radical center in the zeolite to regenerate MV 2+ in its ground state and an oxygen lone pair in the framework.

5.

CONCLUSIONS Microporous alkali metal ion containing zeolite X

show stable photoinduced

charge separation of methylviologen molecules. The MV +" cation radical photoyield depends on the electron donor strength or basicity of the zeolite as determined by its metal cation. The photoyields for a series of alkali metal containing zeolite X materials can be linearly correlated with Sanderson's partial charges on the framework oxygens. The electron donor sites are believed to be oxygen sites in the framework. The MV +" photoyield is dependent on the basicity and the pore size of the zeolite. The results clearly indicate that Cs-X zeolites provide the most appropriate steric and electrostatic environment to retard back electron transfer and increase the lifetime of photogenerated radical ions from methylviologen for many days at room temperature.

769 6.

ACKNOWLEDGMENT

This research was supported by the Chemical Sciences, Geosciences and Biosciences Division, Office of the Basic Energy Sciences, U.S. Department of Energy, the Texas Advanced Research Program and the Environmental Institute of Houston.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10.

11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27.

D. Gust, T.A. Moore and A.L. Moore, Acc. Chem. Res., 34 (2001)40. J.C. Scaiano and H. Garcia, Acc. Chem. Res., 32 (1999) 783. T.J. Meyer, Acc. Chem. Res., 26 (1989) 198. P.P. Infelta, M. Gr/itzel and J.H. Fendler, J. Am. Chem. Soc., 102 (1980) 1479. J.K. Hurst, L.Y.C. Lee, L.Y.C. and M. J. Gr/itzel, M., J. Am. Chem. Soc., 105 (1983) 7048. M.P. Lanot and L. Kevan, J. Phys. Chem., 95 (1991) 10178. B. Xiang and L. Kevan, Colloid Surf. A, 72 (1993) 11. B. Xiang and L. Kevan, Langmuir, 11 (1995) 860. B. Xiang and L. Kevan, J. Phys. Chem., 98 (1994) 5120. H. M. Sung-Suh and L. Kevan, J. Chem. Soc. Faraday Trans., 94 (1998) 1417. J.S. Krueger, J.E. Mayer and T.E. Mallouk, J. Am. Chem. Soc., 110 (1988) 8232. L. A. Vermeulen and M. E. Thompson, Nature, 358 (1992) 656. A. Slama-Schwok, D. Avnir and M. Ottolenghi, Nature, 355 (1992) 240. L. Persaud, A.J. Bard, A. Campion, M.A. Fox, T. E. Mallouk, S.E. Webber and J.M. White, J. Am. Chem. Soc., 109 (1987) 7309. M. Wolszczak and Cz. Stradowski, Radiat. Phys. Chem., 3 (1989) 355. T. M. Bockman and J. K. Kochi, J. Org. Chem., 55 (1990) 4127. T. Watanabe and K. Honda, J. Phys. Chem., 86 (1982) 2617. J. Wheeler and J.K. Thomas, J. Phys. Chem., 86 (1982) 4540. H.J.D. McManus and L. Kevan, J. Phys. Chem., 95 (1991) 5996. P.K. Dutta and W. Turbeville, J. Phys. Chem., 96 (1992) 9410. S. Lukac and J.R. Harbour, J. Am. Chem. Soc., 93 (1983) 4248. M.J. Colaneri, L. Kevan and R. Schmehl, J. Phys. Chem., 93 (1989)397. M.J. Colaneri, L. Kevan, D. H. P. Thompson and J.K. Hurst, J. Phys. Chem., 91 (1987) 4072. K.B. Yoon and J.K. Kochi, J. Am. Chem. Soc., 110 (1988) 6586. K.B. Yoon and J.K. Kochi,. J. Am. Chem. Soc., 111 (1989) 1128. S. Sankararaman, K.B.Yoon, T. Yabe and J.K. Kochi, J. Am. Chem. Soc., 113 (1991) 1419. R. M. Barrer, .Zeolites and Clay Minerals as Sorbents and Molecular Sieves, Academic Press, London, 1978.

770 28.

H. van Bekkum, E. M. Flanigen and J.C. Jansen, J.C. (eds.), Introduction to Zeolite Science and Practice, Elsevier, Amsterdam, 1991. 29. J.A. Rabo, C. L. Angell, P.H. Kasai and V. Schomaker, Discuss. Faraday Soc., 41 (1966) 328. 30. W.J. Mortier, J. Catal., 55 (1978)138. 31. T.L. Barr and M.A. Lishka, J. Am. Chem. Soc., 108 (1986) 3178. 32. Y. Ono, Stud. Surf. Sci. Catal., 5 (1980) 19. 33. I. Mochida and Y. Yoneda, . J. Org. Chem., 33 (1968) 2161. 34. H. Hattori, Chem. Rev., 95 (1995) 537. 35. V.K. Kaushik, S. G.T. Bhat and D.R. Corbin, Zeolites, 13 (1993) 671. 36. W.J. Mortier and R.A. Schoonheydt, Prog. Solid State Chem., 16 (1985) 1. 37. M. Huang, A. Adnot and S. Kaliaguine, . J. Am. Chem. Soc., 114 (1992) 10005. 38. S.Y. Choi, Y.S. Park, S.B. Hong, and K. B. Yoon,. J. Am. Chem. Soc., 118 (1996) 9377. 39. K. Tanabe, M. Misono, Y. Ono and H. Hattori, Stud. Surf. Sci. Catal., 51 (1989) 1. 40. Y. Okamoto, M. Ogawa, A. Maezawa and T. Imanaka, J. Catal., 112 (1988) 427. 41. T. Yashima, K. Sato, T. Hayasaka and N. Hara, J. Catal., 26 (1972)303. 42. D. Barthomeuf and B.-H. Ha, J. Chem. Soc., Faraday Trans., 69 (1973) 2173. 43. L. Uytterhoeven, D. Dompas and W.J. Mortier, J. Chem. Soc., Faraday Trans., 88 (1992) 2753. 44. T.L. Barr, Zeolites, 10 (1990) 760. 45. C. Mirodatos, P. Pichat and D. Barthomeuf, J. Phys. Chem, 80 (1976) 1335. 46. A. Corma, V. Fornes, H. Garcia and M.A. Marti, Chem. Mater., 7 (1995) 2136. 47. F.R. Chen and J.J. Fripiat, J. Phys. Chem., 96 (1992) 819. 48. C.J. Rhodes, J. Chem. Soc., Faraday Trans., 87 (1991) 3179. 49. H.J.D. McManus, C. Finel and L. Kevan, Radiat. Phys. Chem., 45 (1995) 761. 50. M. Alvaro, H. Garcia, S. Garcia, F. Marquez and J.C. Scaiano, J. Phys. Chem. B, 101 (1997) 3043. 51. Y.S. Park, S.Y. Um and K.B. Yoon,.J. Am. Chem. Soc., 121 (1999)3193. 52. M.F. Baldovi, F.L. Cozens, V. Fornes, H. Garcia and J.C. Scaiano, Chem. Mater., 8 (1996) 152. 53. D.W. Werst, E.E. Tartakovsky, E. A. Piocos and A.D. Trifunac, J. Phys. Chem. B, 98 (1994) 10249. 54. J.E. Huheey, E.a. Keiter, R. L. Keiter, Inorganic Chemistry, 4 th ed., Harper Collins College Publications, New York, 1993, p 187.

Studies in Surface Science and Catalysis 142 R. AieUo, G. Giordano and F. Testa (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

771

Effective utilization of residual type feedstock to middle distillates by hydrocracking technology and D. Biswas b. S.K. Saha ~*, G.K.Blswas, " aChemical Engineering Department, Jadavpur University, Calcutta-700032, India bChemical Technology Department, Calcutta University, Calcutta-70009, India Hydrocracking is an attractive technique among th~secondary conversion processes. The processing problem, however, goes up markedly as the crude oil .quality decreases such like ~

gravity while on the other hand increases the conradson carbon, sulfur and

metal contents are due to excessive consumption of petroleum products. Hydrocracking is the most flexible in respect to change in feed quality that handles poor quality feeds easily to produce lighter products. We studied a case using 60:40 combination of reduced crude oil and cycle oil containing 50% aromatics with 1.15% S, and 0.1% N having pour point +24~

Temperature, pressure, and residence time were studied as a process

parameters. Catalytic parameters were also studied. The maximum yield of middle distillates was found to be 49.51% under the following condition: temperature = 623 K, pressure = 7.0 MPa, initial hydrogen partial pressure = 6.0 MPa, residence time = 900 see, feed = 250 g, and catalyst = 10 g 20:80 ratio of A:Z (A- amorphous silica-alumina, Zmolecular sieve 13X). Palladium metal was chosen for hydrogenation site. 1. INTRODUCTION In the modem refinery, catalytic hydrocracking is an attractive among the secondary conversion processes to get more valuable products as well as clean atmosphere from heavier petroleum fraction. The versatility of this process makes it easy to equilibrate the supply and demand of fuels such as gasoline, diesel, and jet fuel. The main goal of hydrocraeking conversion is the reduction of the average carbon number, and the production of branched isomerization of linear paraffins is desirable to improve *Correspondence should be addressed to: S.K Saha Department of Chemistry, Faculty of Engineering, Gifu University, C_hfia501-1193, Japan. E-mail: ksshyama168 @hotmail.com

772 the quality of the different petroleum fractions. Demand patterns of petroleum products have been changed from gasoline to middle distillates and the change continues at present, all over the world [1]. In this perspective, hydrocracking is considered to be the best economic way of converting heavy ends to quality fuels, particularly to middle distillates. A recent report suggests that hydrocracking of polyaromatic compounds proceeds via initial hydrogenation of peripheral ring to naphthenic ring, [2] cleavage into aliphatic substitutes and isomerise to a branched naphthenic compound and finally undergoes into dealkylafion. Another report studied on the role of dispersed phase Mo catalyst in hydrocracking of Guado H [3] revels that cracking reaction occurs essentially through the normal cracking pathway, and that Mo catalyst can considerably inhibit coke formation and enhance desulfurisation. Evidence complemented~y the works on hydrocracking of vacuum gas oil assembled in studies with using highly dispersed metals such as W, Mo, Co and Ni [4,5,6] explored that higher temperature favours more coke whereas lower pressure gives rise to middle distillate with mild acidity. Refractory cycle oil feed could be easily hydrocracked over SiO2-A1203-Ce exchanged Y containing Ni and Mo to jet fuels [7]. Ultrastable Y zeolite catalyst has been found more active to increase middle distillates compared to commercial LZY-82 catalyst [8]. Omega zeolite containing catalyst [9] has also been reported to afford high conversion and selectivity to middle distillates. Studies conducted by Saha et al. [10] on refinery waste to middle distillates reports hydrogen partial pressure plays a vital role for the hydrocracking of refinery waste mainly refractory type of compounds. Various catalyst types viz. zeolites, amorphous SiO:z-A1203, ZrO2-SiO2, USY-zeolite, ZSM-5 etc. were tried as cracking site while Ni, Mo, W, Pt and Pd etc. were studied as hydrogenation site by a number of researchers [ 11]. Still, better catalyst is in search for economic process technology as well as quality products. In this work large pore molecular sieve 13X and SIO2-A1203 amorphous supports were chosen for cracking site and palladium metal for hydrogenation site. 13X molecular sieve adsorb critically larger diameter molecules, such as aromatics and branched chain hydrocarbon and offer very good mass transfer rate in parallel, palladium metal has higher hydrogenation capacity. Our present work designed with the mixed feed is so far the first report on catalyst support variation for middle distillate yield. The present paper deals with hydrocraeking of residual feed (mixed feed) with catalyst support variation from amorphous SiO2-A1203 to zeolite 13X and their combination at different proportion. Various parameters were also studied for maximum middle distillate yield.

773 2. EXPERIMENTAL

2.1 Feed and catalyst preparation Reduced crude oil (RCO) blended with cycle oil in the proportion of 60:40 ratio, having characteristics listed in Table 1. Feed was characterized using standard method. F.or the catalyst preparation, molecular sieve 13X support was procured from the market while the amorphous silica-alumina support was made in the laboratory. Silica-alumina ratio was maintained as to 70:30 for both catalysts. The ammonium form of molecular sieve 13X as prepared by ion-exchange of sodium form, then dried and calcined to give the protonated H-form by a treatment with a m m o n i u ~ i t r a t e solution. 0.5% palladium metal was loaded as palladous chloride in both supports by impregnation method. The detail method of preparation of the catalyst have been described elsewhere [12]. The stability of catalyst was checked by DT-TGA. The characteristics of the catalysts have been shown in Table 2.

2.2 Reactor set-up Experiments were carried out in a rocking type batch reactor of laboratory scale (1 dm3 capacity). Details of the reactor and the assembly of other parts were described elsewhere [5]. The reactor was charged with requisite amount of feed and catalyst, and closed. Purging was done with nitrogen gas to ensure an oxygen free environment inside the reactor. Initially, desired pressure was maintained with hydrogen or nitrogen or both. Purity of hydrogen and nitrogen used here was 99.6% and 99.5% respectively. The total pressure was maintained by only nitrogen. The pressure reading was obtained from the pressure gauge, and the valve was properly closed and checked with soap solution for any leakage. Heating was applied and the temperature was regulated by variac. After attaining desired temperature, rocking of the reactor was started and continued for a definite residence time. At the end of residence time, gas and vapor originated inside the reactor was allowed to pass through an ice-cooled spiral condenser. The liquid product was condensed while non-condensable gaseous product was allowed to pass through the scrubbing system for H2S absorption. The scrubber contained 10% NaOH solution. AKer H2S absorption rest of the gas was passed through a wet gas meter and escape to the atmosphere. The liquid product was analyzed by standard methods for petroleum products (IS/ASTM).

774 Table 1 Properties of feedstock at 60:40 combination of reduced crude oil and cycle oil Parameters

Values

Specific gravity, 60~176

0.8874

Viscosity at 100~ cSt

7.70

Sulfur wt., %

1.15

Nitrogen wt., %

0.10

Ramsbottom carbon residue wt., %

0.911

Carbon to hydrogen ratio

7.55

Pour point, ~

+24 240-576

Boiling range, ~

,,,

,

Table 2 Catalyst properties of palladium loaded molecular sieve 13X and amorphous silica-alumina _

|

,

Items

,

i

,,

Molecular sieve 13X

Amorphous silica-alumina

With palladium

With Palladium

Surface area (m2/g)

336.30

133.60

Total pore volume (cc/g)

0.3973

0.088

0.327 0.0870

0.3302 0.0840

81.64

16.02

Acidity (retool/g) Brrnsted acidity Lewis acidity Pore size distribution (%) > 1000A < 1000 A

18.56

,,,,,

83.98 . . . .

,

, .

.

.

.


The process parameters studied were the temperature (573 to 683 K), partial pressure of hydrogen (2.0 to 6.0 MPa) and residence time (420 to 1800 sec). During process parameter study 250 g feed and 25 g catalyst of palladium metal loaded with at a combination of 80:20 A:Z were used. Table 3 reveals that the percentage of conversion at 573 K was only 58.60%, which increased to 93.53% at 663 K beyond which percentage conversion slowly decreased to 83.76% at 683 K. However, the yield of middle distillates was the highest, which was 33.80%, at temperature 623 K within the temperature range

775 studied. The decrease of percentage of middle distillates at higher temperature might be due to secondary cracking reactions occurred beyond temperature of 623 K, thereby augmenting yield of light distillate and gaseous product. It is, therefore, expected that the endothermic cracking reaction predominated over exothermic hydrogenation reaction, and the fact was supported by the increasing tendency of % aromatics at higher temperature. Partial pressure of hydrogen was studied with predetermined total pressure, which was 7.0 MPa at 623 K. The effect of hydrogen partial pressure has been shown in Table 4. It has been observed that effect of hydrogen partial pressure plays a significant role during hydrocracking reaction. The experimental data reveal that there is an increase in the production of middle distillates with corresponding increase of light distillate, and an increase in hydrogen partial pressure up to 6.0 M P ~ i t h correspondingly decreases in heavy distillate. It clearly indicates that initially hydrogenation of higher hydrocarbons makes cracking easier for yielding lighter products. The maximum middle distillate was found at 6.0 MPa hydrogen partial pressure. At higher hydrogen partial pressure, product quality was better and also coke deposition was minimum. Smoke point and octane index of the middle distillate cuts were higher. Table 5 shows the effect of residence time. To investigate the influence of residence time on hydro cracking of residual type feedstock, increasing reaction time from 420 see to 1800 sec resulted in conversion from 65.95 to 78.42%. However, it has been observed that percentage yield of middle distillate is increased with increment of residence time up to 900 sec reaching maximum value of 41.55% which was the summation of MDL-middle distillate light (150-250~ 17.95% and MDH- middle distillate heavy (250-320~

cut of

cut of 23.6% after which the

percentage yield of middle distillates falls. These results indicate that longer reaction time like 1800 see is not beneficial to hydrocrack, rather 900 see might be better choice. This is probably due to the fact residence time less than 900 see is not sufficient to complete the reaction while a longer residence time results in undesirable side reactions, such as partial polymerization and condensation, thus decreasing middle distillate. Catalytic parameters were studied at predetermined process condition and optimum feed to catalyst ratio. For the study of catalyst cracking site variation, 10 g of catalyst was used. Effect of cracking site variation has been shown in Table 6. The study was conducted with catalyst support varying from amorphous silica-alumina to zeolite 13X and their combination at different proportion viz. 80:20, 50:50 and 20:80. Palladium was the metallic support in all the cases for hydrogenation site. It was revealed from the study that neither amorphous silica-alumina nor zeolitel3X was suitable as cracking site when used individually for hydrocracking of residual feed to lighter products especially middle distillate. Their combination, however, was more effective for this purpose resulting in high conversion

776 and more yields of middle distillates of good quality. Again, zeolite rich A:Z of 20:80 combination was far better than amorphous rich combination. The result showed more middle distillate production having lower aromatic content, thus an improved burning characteristics (higher smoke point) and better engine performance (higher Cetane Index). Coke deposition was also minimal. 100% amorphous or zeolite-based catalyst alone was not effective. This implies that there must be some synergistic effect when amorphous-zeolite combination was used. This may be due to the fact that in one hand, amorphous catalyst has good stability against sulfur compounds present in the feed and high selectivity for middle distillate. On the other hand, zeolitic catalyst may have difficulty in converting some of the larger and higher boiling component to lighter product. Hence, presence of certain percentage of a~orphous catalyst in the zeolite matrix would be beneficial in hydrocracking of residual type feedstock. Table 3 Effect of temperature on hydrocracking of mixed feed oil (total pressure: 4.5 MPa, hydrogen partial pressure: 4.5 MPa, residence time: 900 see, feed: 250 g, catalyst: 25 g, A:Z = 80:20) ,,

Items

,

..

,,i

,.,,

i

,

,

Temperature (K) 573

623

663

683

Percentage conversion

58.60

92.44

93.53

83.76

Gas Light distillate (IBP-150~

20.00 11.50

38.75 18.55

39.68 21.43

60.32 7.14

MDL (150-250~

10.20

15.25

15.00

5.25

MDH (250-320~ Heavy distillate (320~

14.90 41.40

18.18 7.55

15.01 6.46

8.24 16.24

Coke

2.00

2.00

2.40

2.80

% Aromatics in MDL (Vol.)

35.00

25.00

26.00

27.00

Smoke point ofMDL, mm

14.00

18.00

18.00

17.00

% Aromatics in MDH (Vol.)

32.00

22.00

24.00

26.00

Cetane Index of MDH

33.00

48.00

46.00

46.00

777 Table 4 Effect of hydrogen partial pressure (temperature: 623 K, total pressure: 7.0 MPa, residence time: 900 see, feed: 250 g, catalyst: 25 g, A:Z = 80:20) ,

,,,

--

Items

=

,

,,

, , ,

,

,,

,,,

Hydrogen partial pressure (MPa) 2.0

4.5

6.0


57.20

73.20

76.61

Gas

14.03

19.64

24.00

Light distillate (mP-150~

4.00

6.82

8.26

MDL (150-250~

14.85

18.50

17.95

MDH (250-320~

19.10

22.24

23.60

Heavy distillate (320~

42.80

26.80

23.39

Coke

5.20

6.00

2.80


28.00

26.00

22.00


18.00

18.00

20.00

% Aromatics in MDH (Voi.)

25.00

25.00

20.00

Cetane Index of MDH

44.00

47.00

,,

48.00 ,,,,,

.

.

.

.

.

.

.

.

Table 5 Effect of residence time (temperature: 623 K, total pressure: 7.0 MPa, hydrogen partial pressure: 6.0 MPa, feed: 250 g, catalyst: 25 g, A:Z = 80:20) .

.

.

.

,,,

,,

,

,,

,

,

,

.

.

.

.

Items

Residence time (see) 420

900

1800


65.95

76.61

78.42

Gas

11.05

24.00

26.00

Light distillate (IBP-150~

9.36

8.26

6.60

MDL (150-250~

17.75

17.95

18.52

MDH (250-320~

23.00

23.60

21.30

Heavy distillate (320~

34.04

23.39

21.58

Coke

4.80

2.80

6.00


26.00

22.00

26.00


19.00

20.00

20.00


25.00

20.00

25.00

Cetane Index of MDH

47.00

48.00

46.00

778 Table 6 Effect of catalyst cracking site variation (temperature = 623 K; total pressure = 7.0 MPa; hydrogen partial pressure = 6.0 MPa; feed = 250 g; catalyst = 10 g, all catalysts are loaded with palladium metal) ,,,,,,

,,

,,

i

J

,

Items

,

|

i,,l|l

,,

,, i

i

,

,

A

A'Z

A'Z

A'Z

Z

(100%)

(80:20)

(50:50)

20:80

(100%)


79.21

83.91

85.48

85.78

61.43

Gas

30.27

29.60

29.18

21.13

18.51

Light distillate (IBP-150 ~

4.26

7.41

10.08

12.34

2.75

MDL (150-250 ~

19.01

20.46

22.12

19.68

17.85

MDH (250-320 ~

23.67

24.04

22.74

29.83

20.31

Heavy distillate (320 ~ +)

20.79

16.10

1.4.52

14.22

38.58

Coke

2.00

2.40

2.00

2.80

2.00


26.00

25.00

25.00

20.00

22.00


18.00

20.00

19.00

22.00

20.00


20.00

20.00

21.00

16.00

16.00

47.00

47.00

50.00

Cetane index of MDH

45.00 ,

,

,

,

,

,,

49.00 ,

,

i

i

m,,

4. CONCLUSION Higher catalytic activity was observed with larger external surface area, due to the greater number of pore opening. Greater surface area, high pore volume and presence of majority of pores in the macro pore regions were the positive result for hydro cracking of residual type feedstock with zeolytic rich catalyst. The above result showed that hydrocracking reaction was not suitable at higher temperature and higher residence time but higher hydrogen partial pressure was favorable for middle distillate yield. Palladium metal based catalyst showed lower stability in presence of high sulfur containing feeds though properties of middle distillate was better.

The maximum yield of middle

distillates was found 49.51% under the following reaction condition: temperature = 623 K, Pressure - 7.0 MPa (hydrogen partial pressure 6.0 MPa), residence time = 900 sec and feed to catalyst ratio = 25:1. ACKNOWLEDGEMENTS We are greatly indebted to Prof. Y. Sugi, Department of Chemistry, Faculty of Engineering, Gifu University, Japan for helpful discussion.

779 REFERENCES

1. K.P. De Jong, Catalysis Today, 29 (1996) 171-178. 2. N. Masakatsu, A. Kenji, S. Murats, H. Matsui, Catalysis Today, 29 (1996) 235-240. 3. L. Chenguang, Q. Guohe, L. Wenjie, Z. Yajie, Shiyou Xuebao Shiyo Jiagong, 10 (2) (1994)29-37:C.A.-121 (1994) 259259y. 4. W. Kotowski, B. Heinz, B. Karsten, E Wolfgang, Chem.-Ing. -Tech., 69 (1/2) (1997) 103-107 :C.A.- 126 (1997) 279922r. 5. C.R. Lahiri and D. Biswas, Physica, 139&I40B (1986) 725-728. 6. A.Corma, & Martinez, V. Martinerz-soda and J.B. Monton, J. Catal, 153 (1995) 25-31~ 7. R.J. White, US 3,983,029 (1976) : C.A.-86 (1977) 109024n. 8. K. Nitta, S. Nakai, Japan Pat. 62,297,389 (1987) : C.A.-108 (1988) 115616w. 9. F. Raatz, C. Marcilly, E Dufresue, Fr. Pat. 214,042 (1985) : C.A.-106 (1987) 216856. 10.C.R. Lahiri, S.K. Saha, D. Biswas and G.K. Biswas, Selection of Refinery configuration by linear programming modeling in petroleum refining and petrochemical based industries in Eastern India (Eds) R.K. Saha, S. Ray, B.R. Maity. S. Ganguly, D. Bhattacharya, S.L. Chakraborty, Allied Publishers Ltd. New Delhi (2000) 99-101. l l.J.S. Bawa, N. Ray, R.E Dabral and M. Lal, Hydrocracking-A literature Review, Hydrocarbon Technology, (1991) 149-152. 12.S.K. Saha, Studies on Hydrocracking Characteristics for Middle Distillate, Ph.D (Engg.) thesis, Jadavpur University, India (2000).


Studies in SurfaceScienceand Catalysis 142 R. Aiello,G. Giordanoand F. Testa(Editors) 9 2002 ElsevierScienceB.V. All rightsreserved.

781

D i r e c t a n a l y s i s of d e a c t i v a t e d c a t a l y s t s i n 1 - p e n t e n e i s o m e r i z a t i o n b y High-Resolution Fast Atom Bombardment Mass Spectrometry J.M. Campelo-, F. Lafont b and J.M. Marinas a a Organic Chemistry Department, University of Cordoba, Campus de R a b a n a l e s Edificio Marie Curie (C3), E-14014 C6rdoba, Spain b Mass Spectrometry Lab-SCAI, University of Cordoba, Old Sciences Faculty, Av. San Alberto Magno s/n, E-14004 C6rdoba, Spain High-Resolution Fast-Atom Bombardment Mass Spectrometry (HR-FAB-MS) was used to study the nature of the coke formed in the isomerization of 1-pentene over silicoaluminophosphate catalysts with different pore structure (SAPO-5, SAPO-11 and SAPO-34). The formation of polyaromatic hydrocarbons was obtained in those catalysts with large pore sizes (SAPO-5) whereas in those with medium-small pore sizes (SAPO-34), polyunsaturated long chain hydrocarbons were formed. 1. INTRODUCTION In zeolite catalysts Mass Spectrometry has been applied to temperatureprogrammed desorption (TPD) of adsorbed species, such as ammonia and pyridine [1] as well as in conjunction with thermal gravimetric (TG) analysis for the study of gases from pyrolysis of tetrapropylammonium template molecules from MFI-type zeolites [2] and of occluded templates in the ALPO-11 [3], VPI-5 [3], SAPO-5, SAPO-11 and SAPO-34 [4] molecular sieves. These applications always involve EI-MS ionization technique. Fast-atom bombardment (FAB-MS) has been applied to analyse a wide variety of compounds, particularly those nonvolatile or thermally labile, and to directly analyse spots in TLC [5]. In FAB-MS, the "analyte" dispersed in a viscous liquid (called the matrix) is bombarded with neutral gas ions (typically Cs) that are accelerated through a large potential difference. The interaction of these "fast atoms", and the analyte/matrix dispersion, results in the ejection of ions and neutral molecules from the matrix surface into the vapour phase [6]. FAB-MS is classified as a soft ionization technique in that fragmentation of analyte molecules is extremely small. The resulting spectrum displays a prominent ion indicative of the sample molecular weight. Due to these characteristics FAB-MS seems to be an ideal technique to obtain structural and

782 molecular weight information from coke depositions with a multi-component nature. This work studies the nature of coke formed during 1-pentene isomerization in several silicoaluminophosphate molecular sieves by means of HR-MS employing FAB as the ionisation technique, indicating that this is a suitable and powerful technique for these purposes. 2. EXPERIMENTAL Hydrothermal synthesis of SAPO-5, SAPO-11 and SAPO-34 catalysts has been previously described [7]. Their textural and acid properties are collected in Table 1. Table 1 Textural and acid properties of SAPO-5, SAPO- 11 and SAPO-34 catalysts

SBET

S~P

dp

PY~73 a

PY67~b

NH3 c

(m2/g)

(m2/g)

(A)

(lamol/g)

(~tmol/g)

(lamol/g)

SAPO-5

183

128

7.3

259

210

686

SAPO- 11

110

65

6.3x3.9

107

83

503

SAPO-34

120

117

3.8

50

50

870

Catalyst

Pyridine adsorption at 573 K; b Pyridine adsorption at 673 K; c TPD-NH3 from 350 to 723K.

a

SAPO-5, SAPO" 11 and SAPO-34 catalysts were studied by FAB-MS after their utilization in 1-pentene isomerization during 10 h at 400~ (time after which their catalytic performance is practically negligible due to coke deactivation). The amount of coke deposited was quantified by thermogravimetric-mass spectrometry (TG/MS) experiments according to a previously described method [8], using electron impact (EI, 70 eV)) technique. Thus, portions of deactivated catalyst, were placed into a deep quartz sample capillary tube, which was inserted into the electron impact ionization source of an AutoSpec-EBE Mass Spectrometer (Micromass, Manchester, UK), and ramped from 100~ to 600~ under high vacuum (10 .6 mbar). Coke molecules are completely desorbed and this was monitored by the total ion current from scan mass experiments.

783 The same spectrometer was used for coke analysis by FAB technique. Thus, portions of 1 mg of deactivated catalysts were ultrasonically dispersed into 0.1 ml of FAB matrix (thioglycerol, glycerol, NBA and NPOE were tested; however, the best results were obtained with thioglycerol), and directly deposited on the FAB probe tip of the mass spectrometer. The optimised MS conditions were as follows: cesium was used as the bombardment ions operating at an accelerating voltage of 30 kV; spectra were acquired with a scan rate of 4 s per decade over a mass range of 150-1200 Daltons (positive ion mode) after calibrating with ICs; accurate mass measurements were obtained by scanning at 10000 resolution (10% valley definition) using an internal mass reference mixture (PEG 300, 600 and 1000 average molecular weight) added to the matrix (1 wt%) for correcting masses to an accuracy of 10 ppm. 3. RESULTS AND DISCUSSION Electron impact mass spectra (EI-MS) experiments shows that the amount of coke deposited was more important for SAPO-5 (16.3 wt%) and SAPO-34 (12.3 wt%), the most active catalysts, than for SAPO-11 (8.1 wt%) catalyst. Deactivated catalysts were directly introduced into the FAB ion source of the mass spectrometer (dispersed in thioglycerol) for characterisation of coke compounds. The high energy of cesium ions, which were accelerated with 30 kV, allows identifying not only the coke located in the pore mouths but also those molecules placed inside the pores. Figures 1, 2 and 3 show the HR-FAB mass spectra obtained from deactivated solids under vacuum (106mbar). Mass spectra of solids have always many peaks, some of those from matrix and the main corresponds to coke molecules and indicating the presence of a complex mixture of hydrocarbons. In the spectra we can observe mainly protonated molecular ions [M+H] § typically of FAB, with the absence of breakdown peaks, which facilitates their interpretation. Identification of compounds were carried out by their molecular weight as well as by their elemental composition obtained from the highresolution data. FAB mass spectra of SAPO-11 (Figure 1) and SAPO-34 (Figure 2) indicate the presence of large polyunsaturated hydrocarbon chains from approximately 200 to 400 umas, whose elemental composition (according to high-resolution mass spectra) indicates greater unsaturated levels in the molecules desorbed. These results have the same tendency observed with the same catalysts on methanol conversion where coke was studied by NMR, FT-IR and HR-EI-MS [9]. SAPO-5 (Figure 3) showed the presence of a great variety of peaks, where we can find peaks at m/z: 228, 254, 276 and 278 that correspond to C18H12, C2oH14, C22H12 and C22H14 (confirmed by high resolution) and can be attributed to polyaromatic hydrocarbon.

784

95 90

"

69

97

85

80

7

64

!II

II0 123

2b 15 i.~

222 180 I

137

h,152 l

i ~ 2 3 5 150

200

3

z|

Figure 1. HR-FAB mass spectra from deactivated SAPO- 11 catalyst.

L

I00~ 95/: 9O85: 8O-

l

252

757o_=

65 60:= 554 5o4 454

40-" 35~

170 151

3o_.=" 25~ 2o_.=

15_~ 102

8E 276

'

h 11293 331

- -

)

2

25o

3

I o

~/

Figure 2. HR-FAB mass spectra from deactivated SAPO-34 catalyst.

785

141 15

16

14

13

109

I

2

J-

219252

-,

[

153

E 76

9

,

93

331 365 3

~-' :-

4 7 485

.405

505

555

!~

150

Figure 3. HR-FAB mass spectra from deactivated SAPO-5 catalyst.

In the SAPO-5 case, the higher unsaturation level mainly corresponds to polyaromatic hydrocarbons from 200 to 600 umas with a molecular composition of CnHm (always m "'0

Therefore the catalyst, 13 zeolite has more catalytic activity in reaction of esterification of acetic acid with ethyl alcohol. Below is given a development of the theoretical grounded kinetic model of the reaction of esterification of acetic acid with ethyl alcohol on the catalyst, 13 zeolite. On the basis of analysis of the literature material on a study of mechanism of liquid phase reactions of esterification of acetic acid with alcohols and the results of investigation of kinetic laws of the reaction have been suggested two hypothesizes of probable stage mechanism of proceeding of vapor phase reaction of esterification of acetic acid with ethyl alcohol. Hypothesis 1. Ethyl acetate is formed on acidic sites of the catalyst at interaction of strongly adsorbed molecules of acetic acid with weak adsorbed molecules of ethyl alcohol on scheme: C2HsOH + Z
ZCH3COOH

ZC2HsOH + ZCH3COOH

k*

O II > 2Z + CH3-C-O-C2H5 + 1420

According to this hypothesis, the equation of rate of the reaction r =

k * KlK:plp 2 kK 2plp: -(l+KlP, +K2P2) 2 (l+KlP, +K2P2) 2

(3)

where k*Kl=k since K2>>K1, then r=

kK2PlP2 (l+KEP2) 2

(4)

Hypothesis 2. Ethyl acetate is formed on acidic sites of the catalyst at interaction of adsorbed molecules of acetic acid with molecules of ethyl alcohol from vapor phase on scheme: CH3COOH + Z
ZCH3COOH

ZCH3COOH +C2HsOH

O I! > Z + CH3-C-O-C2H5 + 1-I20

793 According to this hypothesis, the equation of rate of the reaction r = kK2PlP-----------Lz l+K2p2

(5)

Q OeRT K 2 -K 2

Both of these models have been subjected to statistical analysis on the basis of the data of Table 2. The objective function was F(X)= ~--'

(6)

Where X signifies the set of parameters of the considered model. The kinetic parameters obtained for the two models (equations 4, 5) are reported in Table 4. According to these results, the equation (4) satisfactorily describes the experimental data. Table 4. Kinetic parameter of equations (4) and (5) for the 13zeolite catalyst Equation (4) 0.49.10 9 ko [mole/h.g-atm] 15.00 E [kcal/mole] 0.6-103 K~ [atm1] Q [kcal/mole] Mean of error for the runs (% on the yield of ethyl acetate)

4.00 6.48

Equation (5,,) 0.86-108 16.47 0.51-108 8.82 14.79

Thus in the studied conditions 13 zeolite with ~=25 shows relatively high catalytic activity and selectivity in the reaction of vapor phase esterification of acetic acid with ethyl alcohol. Reaction rate studies allow to be made concerning the following probable stage mechanism for the above reaction; ethyl acetate is formed on acidic sites of this catalyst at interaction of strongly adsorbed molecules of acetic acid with weak adsorbed molecules of ethyl alcohol. - mole ratio, C2HsOH:CH3COOH:He; X - conversion of ethyl alcohol; S - selectivity on ethyl acetate; T - temperature of the reaction; r - rate of the reaction; Z - silicate modulus, SiOz/A1203; p - bulk density of the catalyst; A 1 - yield of ethyl acetate, %; A 2 - yield of ethyl ether, %; A 3 - yield of ethylene, %; n i0 _ initial mole flow rate of i component; ni current mole flow rate of i component; p i - partial pressure of i component; i - component, 1 alcohol, 2 - acetic acid, 4 - water, 5 - ethyl acetate; E - activation energy ; v - space velocity on the liquid mixture of reagents; Gear - weight of catalyst; Veat- volume of the catalyst; P total pressure; k0 - preexponential factor in kinetic law; Ki - adsorption constant of i component; K~ -preexponential factor for adsorption of i component; Q - heat of adsorption. ACKNOWLEDGMENTS This research is sponsored by NATO's Scientific Affairs Division in the framework of the Science for Peace.

794 REFERENCES

1. A.M.Aliyev, A.Sh.Melikova, R.M.Babayev, A.R.Kuliyev. Azerbaijan oil economy, 1996, ~o12. 2. Russian Patent ~r 2059606 ~ 4 C07C 69/12, C07C 67/08. 3. A.s. 1549945 USSR. B.I. 1990, ~r


795

Hydrodesulfurization of dibenzothiophene over Mo-based catalysts supported by siliceous MCM-41 Anjie Wang a, Yao Wanga, Yongying Chen a, Xiang Li, PinNing

Yao a

and Toshiaki Kabe b

a State Key Laboratory of Fine Chemicals, Dalian University of Technology, Dalian 116012, China b Department of Applied Chemistry, Faculty of Technology, Tokyo University of Agriculture and Technology, 2-24-16 Nakacho, Koganei, Tokyo 184, Japan

Deep hydrodesulfurization (HDS) catalysts were prepared by depositing Co-Mo or Ni-Mo species over siliceous MCM-41. The extremely high surface area of MCM-41 favors the dispersion of the active species, resulting in a very high reactivity in desulfurizing dibenzothiophenes. A maximum HDS activity was observed at Co/Mo or Ni/Mo molar ratio of 0.75 for the supported catalysts, higher than that of the conventional ]t-A1203 supported catalysts. A 35S isotope tracer technique was used to investigate the HDS reaction mechanism. It was revealed that sulfur atoms retained on the surface could be removed only by the introduction of a sulfur-containing compound, indicating that sulfur atom exchange between sulfur-containing compounds and the active sites is involved in the HDS reaction. Accordingly, a reaction mechanism for HDS is proposed. 1. INTRODUCTION Hydrodesulfurization (HDS) is a key process in producing clean engine fuels. Since cobalt or nickel promoted molybdenum/tungsten sulfides are basically established as the active species, the proper selection of a better support becomes an effective approach in developing high-performance HDS catalysts. Mesoporous MCM-41 has been the focus of much research interest since its discovery because it offers a uniform pore sizes of 15 to 100 and a very high surface area(> 1000 m 2/g). The MCM-41 supported catalysts have been developed for a variety of reactions[ 1], including hydrodesulfurization of petroleum fractions[2]. However, It is worth noting that all the researches on MCM-41 type supports in HDS catalyst preparation have focused on A1-MCM-41, probably hoping that the acidity of the support may help to crack the polyaromatic sulfur-containing compounds so as to improve the HDS activity. But no great improvement in the HDS activity has been The research was partly supported by the Natural Science Foundation of China (20003002) and by the Young Promising Teachers' Funds from the Education Ministry of China.

796 achieved. We have reported the high-performance HDS catalysts by depositing Co or Ni promoted Mo and W sulfides over siliceous MCM-41 [3,4]. 2. EXPERIMENTAL All the reagents in synthesizing siliceous MCM-41 and in preparing the catalysts were of chemical grade. Siliceous MCM-41 was synthesized according to the procedure in previous paper.[5] The catalysts were prepared by the wet impregnation method. The catalysts were presulfided prior to HDS reaction of DBT. A model fuel containing 1wt% DBT in decalin was used to investigate the HDS activities of the prepared catalysts. To elucidate the reaction mechanism of hydrodesulfurization, an isotope tracer technique was used. [3] 3. RESULTS AND DISCUSSION Figure 1 and 2 show the conversion of DBT as a function of temperature over Co-Mo/MCM-41 and Ni-Mo/MCM-41 respectively, compared with a commercial deep hydrodesulfurization catalyst (DHDS) on Japanese market. It is shown that both series of catalysts showed very high activity for converting DBT into hydrocarbons, yielding almost complete conversion of DBT at temperatures over 320 ~ A maximum HDS activity was observed at Co/No or Ni/Mo atomic ratio of 0.75 for the MCM-41 supported catalysts, different from that of conventional '/-A1203 supported catalysts. The selectivity results show that HDS over Co-Mo/MCM-41 mainly takes the route of hydrogenolysis while hydrogenation plays a more important part in HDS of DBT over Ni-Mo/MCM-41.

100

! 00

80

,

,

~

80

o= 60

~

60

40

~

40

20 0 200

,

20

240

280

320

360

400

Temperature (~

Figure 1 DBT conversion as a function of HDS reaction temperature. ( O ) MOO3, ([--]) CoO, ( ~ ) Co-No (0.25), (A)Co-Mo (0.50), (~)Co-Mo (0.75), ( I )Co-No (1.00), ( O ) Commercial DHDS catalyst

0 200

r 240

A ~ 280

320

I 360

, 400

Temperature f'C)

Figure 2 DBT conversion as a function of HDS reaction temperature. (O) MOO3, (O) Ni, ( ~ ) Ni-No (0.25), (A)Ni- Mo (0.50), (/~)Ni-Mo(0.75), ( I ) N i - M o (1.00), (f-l) Ni-Mo(1.25), (O) Commercial DHDS catalyst

797 The dynamic behavior of sulfur atom during HDS of DBT was investigated by the 35S isotope tracer technique. The HDS of DBT was conducted over sulfided Co-Mo/MCM-41 catalyst at 280 ~ and 5.0 MPa. A typical profile of radioactivity with reaction time is illustrated in Figure 3. A decalin solution of 1 wt% [32S]DBT was pumped into the reactor to start the HDS reaction. After the conversion of [32S]DBT became constant, a flow of 1 wt% [35S]DBT in decalin was introduced into the reactor to replace the [32S]DBT solution. The radioactivity of the unreacted [35S]DBT in the liquid product increased and approached a steady state immediately after the introduction of [35S]DBT, whereas there was a delay for the radioactivity of the released [35S]H2S to reach a steady state. After both the radioactivity of the unreacted [35S]DBT and that of the released [358]H28 became constant, a flow of decalin switched to replace the [35S]DBT solution. Both the radioactivity of unreacted [35S]DBT and that of released [35S]H2S decreased sharply. Little [35S]DBT was detected during the period of purging with decalin and hydrogen, suggesting that there is no physically absorbed HzS on the surface of the catalyst. After a long period of purging, a flow of 1 wt% [32S]DBT solution was introduced again to replace decalin. No noticeable radioactivity change was detected in the liquid product, but a peak of 35S radioactivity in the gas phase appeared in the form of [35S]H2S. It should be noted that a similar profile was obtained for Ni-Mo/MCM-41 catalyst at the same reaction conditions. The 35S isotope tracer investigation for both Ni-Mo/MCM-41 and Co-Mo/MCM-41 revealed that the sulfur atoms removed from DBT molecules are not released directly into gas phase as HzS but retained on the surface of the catalyst. Since purging with decalin in the presence of H2 could not remove the retained sulfur atoms on the catalyst, it is unlikely that the retained 35S exits in the form of adsorbed HzS on the surface. On the other hand, these sulfur atoms could be released by the introduction of new DBT molecules, indicating that sulfur atom exchange between DBT molecules and sulfur species on the surface of the catalyst may be involved during HDS of DBT and the sulfur atoms on the surface may serve as active sites. Accordingly, a catalytic circle 20 . . . . ' during HDS over sulfided Ni-Mo ~ [3'SIDBT Decalin [32SlDBT catalysts is proposed, as shown in ~ I ~ Scheme 1. The mechanism briefly ~ 25 o~#~o o c,,.,o Lr-~o~ o9 describes the possible reaction steps ~,.~ l0 involved on the local site. It is ~o~. ~ 9 assumed that gaseous hydrogen adsorbs .~. dissociatively on the surface of catalysts and the hydrogen species consumed in the reaction are supplemented by means 0 ___~. , , "h,-,,-~,~'~--~ . . . . ? of spillover on the surface.[6] 0 100 200 300 400 500 600 When sulfur-containing molecule Reaction time (min) absorbs on the sulfide cluster in the 111 Figure 3 Change in radioactivities of mode through hydrogen bonding, [35S]DBT(O) and [35S]H2S ( O ) with cluster reconstruction occurs to reaction time accommodate the incoming molecule,

798 "~ "H

H I

SH ~

H

/

H I

\U ,

H

/

SH

H

H

'

%)/\""..o.7 \ / / / W S ....

: ~.,,.~S-----S-7

i

H - "l " s

H

H

H

/---=d,..------E.,.,s---S-7

~,/

's

\//

7

s..~NM

'-----,- H2S

H

I

/'N~"

H.

HS

\

SH H

/

Mo4'

1 -

H

-

7-7 /

H- -'~-,S/ S

"

H

H

H

H

1~--I

t___,

N~ /

1

.-H

. ',.,v,-~--7s--- 7

/

Hydrocarbon

"(propionyl>acetyl. Hence, the formation of more bulky isomers, such as 1-acyl isomer, should be disfavoured owing to sterical constraints placed by the geometry of straight channels within the zeolite, which has 7.6 x 6.4 A opening. With propionic anhydride, the overall selectivity to the 1- and 6- position acylation was lower (85-87%), this being due to the formation of a diacylated ether product (3-4%) and increased selectivity to an acylated isomer with unknown structure (6-7%). Analogous to those with acetic anhydride, somewhat higher conversion was achieved over In13, yet product distribution was comparable. The conversion was lower and 6-position acylation was more selective over Lal3. The conversion of 2-MN with benzoic anhydride over In13 was lower compared to that achieved with HI), and the selectivity trend altered in favour of 1-benzoyl-2methoxynaphthalene product with the catalyst. In the case of Lal3, the conversion was only 450 ~ (22). The presence of vinyl chloride indicated that the abstraction of HC1 was the first step in the reaction scheme. The formation of this intermediate was appreciably suppressed when using Pd/H-ZSM-5. The maximum concentration of vinyl chloride obtained was 735 ppm with H-ZSM-5, but when adding Pd to the zeolite the peak amount was reduced to 100 ppm. As regards TCE, trace amounts of perchloroethylene were observed as a by-product. This compound was generated by chlorination of the TCE and was partially destroyed at higher temperature (23). Pd/H-ZSM-5 zeolite led to larger quantities of this undesired by-product t h a n H-ZSM-5, since this concentration increased from 120 ppm to 355 ppm. This increase is due to the known activity of noble metals in chlorination reactions (24).

852 T a b l e 1. Selectivities towards desired by-products. DCE

H-ZSM-5 Pd/H-ZSM-5

TCE

CO2,%

HCI,%

CO2, % HC1, %

54 100

96.6 91.3

63 73

57.0 42.5

As far as CO and CO2 formation was concerned, the formation of CO2 was relatively favoured as temperature increased with both catalysts, but Pd/H-ZSM5 zeolite was more selective towards CO2 formation instead of CO, which was the major product formed over H-ZSM-5. The high activity of Pd for CO oxidation is the cause of this beneficial effect (25). When decomposing DCE, the selectivity to CO2 obtained with the metal loaded zeolite at 550 ~ was 100% but this improvement in CO2 selectivity was less noticeable with TCE, as only 73% selectivity was achieved (Table 1). The H-form zeolite showed a lower selectivity to C12 generation. On the contrary, the metal loaded zeolite considerably promoted the formation of this toxic by-product by the Deacon reaction (2HCl+ 89 (6,26). The selectivity towards C12 was higher for the case of TCE rather t h a n for DCE due to the H:C1 ratio greater t h a n 1. Table 1 sumarises the selectivities obtained to the desired oxidation products (CO2 and HC1) in the decomposition of the chlorinated compounds with both catalysts. It must be pointed out that the combustion of DCE over H-ZSM-5 and Pd/H-ZSM-5 was accompanied by the formation of coke due to the polymerisation of vinyl chloride (27). By contrast, no coke deposition was observed during TCE combustion with any of the catalysts. Carbon balances closed above 95-100% when decomposing TCE, but they were found to be higher than 100% at elevated temperatures in the DCE reaction due to the combustion of the coke formed during the reaction. Chlorine balance was in the range 65-85% in the oxidation of both chlorinated compounds. It is known that AI-O bonds in the zeolite framework can be easily attacked by the HC1 formed during reaction leading to the formation of volatile A1C13 which causes the partial collapse of the framework and the blockage of the porous structure (28). Furthermore, a change in colour of the Pd/H-ZSM-5 catalyst from grey to orange yellowish was observed at the end of the activity test with both compounds. This colour fitted quite well with that of metal chloride complexes (PdC12), thus the interaction of chlorine with the metal in presence of halocarbons could also explain the unfitted chlorine balance (6).

853 4. CONCLUSIONS Both catalysts, H-ZSM-5 and Pd/H-ZSM-5, studied in this work showed a high activity in the chlorinated VOCs decomposition. Pd/H-ZSM-5 zeolite was found to be most active in the oxidation of TCE, while no noticiable difference between both catalysts was noticed for DCE combustion. The main oxidation products were CO, CO2, HC1 and C12. Additionally, vinyl chloride was detected in DCE reaction, indicating that the first step in the mechanism is the dehydrochlorination of the feed. On the other hand, perchloroethylene was also observed in TCE conversion as a result of the chlorination of the feed molecule. The addition of the metal to the zeolite improved both the activity and the selectivity towards CO2, but this also involved a significant increase in undesired by-products formation such as chlorine and highly chlorinated hydrocarbons. ACKNOWLEDGEMENTS The authors whis to thank Universidad del Pals Vasco/EHU (9/UPV 0069.310-13517/2001) and Ministerio de Ciencia y Tecnologla (PPQ2001-1364) for the financial support. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.

H. Muller, K. Deller, B. Despeyroux, E. Peldszus, P. Kammerhofer, W. Kuhn, R. Spielmannleitner and M. Stoger, Catal. Today 17 (1993) 383. A.R. Gavaskar, B.C. Kim, S.H. Rosansky, S.K. Ong and E.G. Marchand, Environ. Prot. 14 (1995) 33. S. Chatterje and H.L. Greene, J. Catal. 130 (1991) 76. S. Chatterjee, H.L. Greene and Y.J. Park, J. Catal. 138 (1992) 179. C. Gerard-Gomez, M. Dufaux, J. Morel, C. Naccache and Y.B.Taarit, Appl. Catal. A 165 (1997) 371. J.R. Gonz~lez-Velasco, A. Aranzabal, J.I. Guti6rrez-Ortiz and R. L6pezFonseca, Appl. Catal. B 19 (1998) 189. P. Cafiizares, A. de Lucas, F. Dorado and J. Aguirre, Microporous Mesoporous Mater. 42 (2001) 245. W.M.H. Sachtler, Catal. Today 15 (1992) 419. V.Z. Radkevich, M.F. Savchits and Y.G. Egiazarov, Russian J. Appl. Chem. 7 (1997) 759. S.T. Homeyer and W.M.H. Sachtler, J. Catal. 117 (1989) 91. M. Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno and H. Ohuchi, Appl. Catal. B 9 (1996) 167. B. Ramachandran, H.L. Greene and S. Chatterjee, Appl. Catal. B 8 (1996) 157. R. L6pez-Fonseca, A. Aranzabal, J.I. Guti6rrez-Ortiz, J.I..s and J.R. Gonz~lez-Velasco, Appl. Catal B 30 (2001) 303. G.C. Bond and N. Sadeghi, J. Appl. Chem. Biotechnol. 25 (1975) 241.

854 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28.

P.S. Chintawar and H.L. Greene, Appl. Catal. B 13 (1997) 81. H. Windawi and M. Wyatt, Platinum Metals Rev. 37 (1993) 186. H. Windawi and Z.C. Zhang, Catal. Today 30 (1996) 99. S. Karmakar and H.L. Greene, J. Catal. 138 (1992) 364. M. Tajima, M. Niwa, Y. Fujii, Y. Koinuma, R. Aizawa, S. Kushiyama, S. Kobayashi, K. Mizuno and H. Ohuchi, Appl. Catal. B 9 (1996) 167. S. Chatterjee, H.L. Greene and Y.J. Park, J. Catal. 138 (1992) 179. R. L6pez-Fonseca, A. Aranzabal, P. Steltenpohl, J.I. Guti~rrez-Ortiz and J.R. Gonz~lez-Velasco, Catal. Today 62 (2000) 367. G. Sinquin, J.P. Hinderman, C. Petit and A. Kiennemann, Catal. Today 54 (1999) 107. H. Shaw, Y. Wang, T.C. Yu and A.E. Cerkanowicz, ACS Syrup. Ser. 495 (1992) 358. J.R. Gonz~lez-Velasco, A. Aranzabal, R. L6pez-Fonseca, R. Ferret and J.A. Gonz~lez-Marcos, Appl. Catal. B 24 (2000) 33. S. Chatterjee and H. Greene, Appl. Catal. A 98 (1993) 139. J.C. Lou and S.S. Lee, Appl. Catal. B 12 (1997) 111. S. Imamura, H. Tarumoto and S. Ishida, Ind. Eng. Chem. Res. 28 (1989) 1449. Z. Konya, I. Hannus and I. Kiricsi, Appl. Catal. B 8 (1996) 391.


855

Influence of the amount and the type of Zn species in ZSM-5 on the aromatisation of n-hexane A.Smiegkov~t, E.Rojasov~t, P.Hudec, L.~;abo, and Z.2;idek Department of Petroleum Technology and Petrochemistry, Faculty of Chemical and Food Technology, Slovak University of Technology, Radlinsk6ho 9, 812 37 Bratislava, Slovakia The aromatisation ofn-hexane over a series of zinc modified (0.03 5-0.50 mmolZn/g)acid ZSM-5 zeolite (1.09 mmolAl/g) prepared by i) ion exchange or by ii) mechanical admixing of ZnO was investigated. The reaction of aromatisation was carried out at 420 ~ at atmospheric pressure. At this temperature thermal cracking ofn-hexane doesn't contribute to the conversion. Over ion exchanged samples with increasing amount of zinc in the zeolite the conversion of n-hexane comparing with acid ZSM-5 at first continuously decreases goes through the minimum and than begins to rise, while the selectivity to aromatics continuously raises. As was shown the differences in the acidity of samples are very low so from the results we conclude that the role of the Zn in cationic positions in n-hexane conversion depends on the zinc concentration. Zinc species at low concentration probably due to their localization are not active in n-hexane activation but only in dehydrogenation of the oligomerized products. Mechanical mixtmes of ZnO + acid ZSM-5 in contrary to the ionexchanged samples had constant activity/selectivity independently on the amount of ZnO. Their activity and selectivity are roughly on the level of ion-exchanged sample containing 0.085 mmol Zn/g. From the results follows that at 420 ~ ZnO species in mechanical mixtures are not active in n- hexane conversion and the catalytic performance of the mixtures is probably the consequence of a solid state ion exchange of Zn into zeolite from ZnO species. From the constant activity/selectivity of the mechanical mixtures containing different amount of ZnO follows that the extent of assumed solid state ion exchange is limited and is controlled probably by the surface characteristics of the zeolite and not by the amount of ZnO. 1. INTRODUCTION Effective conversion of light paraffmie hydrocarbons contained in natural gas and associated gases and of butane and light naphtha component such as n-CsH~2 and n-C6Ht4 derived from petroleum refining processes to more valuable chemicals is of the great importance. One of the important targets of the conversion would be the synthesis of important aromatics-benzene, toluene and xylenes for the petrochemical industry. It has been reported by several authors that ZSM-5 zeolites modified with gallium[ 1,2,3,4,5,6],

856 indium [6], Zn [7,8,9,10,11,12], Ni [12], Pt [7], or with the mixture of Ni and Zn[13] belong to the most convenient catalyst for conversion of this light alkanes to the aromatics. Polyfunctional Zn and Ga metalosilicates of ZSM-5 structure modified with Pt [ 14,15] were also found as selective catalysts for aromatisation of light alkanes. As is known ZSM-5 zeolites modified with zinc are a very active catalyst for this purpose. A number of works has been published from the studies of light alkanes aromatisation carded out on Zn ZSM-5 catalysts prepared by ion exchange [7,8] wet impregnation [10,13,16,17] or by mechanical mixing of ZnO and ZSM-5. Enhanced yields of aromatics obtained over these catalysts are generally attributed to the participation of the Zn in activation of alkanes and in conversion of naphtenes to aromatics. However, the influence of the type of Zn species on the activity/selectivity in aromatisation of light alkanes is still a point of discussion. The literature of Zn containing zeolites also shows that the activation of the catalysts and the experiments has been carded out at very different temperatures (420-600 ~ and that very different alkanes as model feedstocks are used - from C2- up to C7 alkanes or their mixtures. Thus, this fact can lead to the observed controversial conclusions conceming the role of Zn species in aromatics formation. Depending on the type of alkane used at high reaction temperatures thermal cracking can contribute to the alkane conversion. Moreover, at high testing or activation temperatures the loss of zinc by sublimation is pretended [ 10] from both, cationic or non-framework positions. Thus, the aim of this work was to investigate the catalytic behaviour of Zn containing ZSM-5 in dependence of Zn concentration where zinc species were incorporated by different methods, n-Hexane was used as a model feed. The reaction temperature was 420 ~ and catalysts were activated at 480 ~ Our previous experiments showed that at these temperatures neither thermal cracking of n-hexane nor zinc loss from the catalysts during activation occur. 2. EXPERIMENTAL The NaZSM-5 zeolite (Si/AI=14, 1.09 mmol A1/g) was supplied by Slovnaft Research Institute of Petroleum and Hydrocarbon Gases, Inc. Bratislava. NHa-ZSM-5 and NH4ZnZSM-5 samples (0.035-0.35 mmol Zn/g) marked ZnZSM-5 were prepared by exchanging procedures described in [ 18]. Mechanical mixtures ofZnO (p.a) and NH4ZSM-5 (0.085-0.50 mmol Zn/g) marked ZnO+ZSM-5 were prepared by thorough homogenisation of the mixture in a mill. The acidity of the catalysts was determined by temperature programmed desorption of ammonia (TPDA). Adsorption of ammonia was carried out at 220 ~ after activation of samples at 480 ~ for 1 h in a stream of helium. The quantity of ammonia desorbed in the temperature range from 220 to 550 ~ was detected by titration of the excess of 0.1 M HaSO4. Conversion of n-hexane was performed in vapour phase, in a continuous glass flow microreactor contained 100 mg of catalyst at atmospheric pressure in a stream of nitrogen (20 ml/min) saturated with n-hexane at 0~ The microreactor was on-lined with a gas chromatograph. Before the catalytic tests were performed, the catalysts were in situ activated in a stream of dry air (50 ml/min) at 480 ~ for 1 hour.

857 In the case of mechanical mixture before TPDA measurements and catalytic tests the samples were pre-activated in a stream of dry air at 480 ~ for three hours. Conversion of n-hexane was carried out at 420 ~ The reaction products were analysed each 20 minutes within 180 minutes existing run by gas chromatography (Hewlett Packard 5890, SERIE II) using a HP-1 capillary column (15 m x 0.530 mm) and FID detector. 3.RESULTS AND DISCUSSION The characteristics of the samples are show in the Table 1. Table 1 Characteristics of the investigated samples Zn content, Sample (mmol/g) NH~SM-5* 0.07ZnZSM-5 0.17ZnZSM-5 0.18ZnZSM-5 0.25ZnZSM-5 0.30ZnZSM-5 0.4ZnZSM-5 0.46ZnZSM-5 0.54ZnZSM-5 0.7 ZnZSM-5 Mechanical mixtures 0.17ZnO+ZSM-5 0.40ZnO+ZSM-5 0.46ZnO+ZSM-5 0.54ZnO+ZSM-5 0.7ZnO+ZSM-5 1.0ZnO+ZSM-5 * NHaZSM-1.09 mmol A1/g

0 0.035 0.085 0.09 0.125 0.15 0.2 0.23 0.275 0.35

Acidity, TPDA (mmol NH3 des.)/g total > 450 ~ 0.91 0.21 0.89 0.90 0.27 0.88 0.84 0.85 0.28 0.90 0.29 0.87 0.29 0.80 0.39 0.85 0.42

0.085 0.2 0.23 0.275 0.35 0.5

0.97 0.85 0.85 0.79 0.85 0.89

0.28 0.26 0.27 0.27 0.26 0.28

In our previous works [18,19,20] was reported that Zn in cationic positions in zeolite represents Lewis acid sites and causes acid sites strength redistribution in the zeolite. This conclusion came from the fact that with increasing amount of Zn in cationic position (see Tab.l). slightly increases the amount of ammonia desorbing over 450 ~ .In contrary to the ion-exchanged samples in mechanical mixtures the amount of ZnO practically has no effect on the amount of ammonia desorbing over 450~ From the data given in Table 1 can be also seen that in spite of the considerable differences in the content of Zn in samples there are very slight differences in their acidity. The conversion of n-hexane in dependence of the amount of Zn over ion exchanged samples and over the mechanical mixtures is graphically presented of Fig.1.

858 60-~

~

50-

e1! t~ 4 0 X

0 J~ I e,,

~:

,,.. 3 0 -

t

0 tO

x 9

x x 9

x

"~ 2 0 0 r,, 0

o

x ZnZSM-5 9ZnO+ZSM-5

10-

0

[

T

[

1

0,1

0,2

0,3 Zn, (mmollg)

0,4

T. . . . . . . . . . . . . . . . . . . . .

0,5

:

0,6

Fig. 1 Conversion of n-hexane over ion exchanged ZnZSM-5 and mechanical mixtures ZnO+ZSM-5, {T= 420 ~ p=0.1 MPa, WHSV = 2.6 h1 } During testing, the conversion of n-hexane in dependence on the time on both series of samples was practically constant. So the value of arithmetic average of conversion was used for evaluation of the catalysts. In our previous work [18] was shown that the conversion of n hexane at 420 ~ on sodium form of ZSM-5 and also on a pure A1203 was zero indicating that thermal cracking at this temperature doesn't contribute to the n-hexane conversion. On Fig. 1 can be seen that the conversion of n-hexane is relatively high on pure acid ZSM-5. This means that Broensted sites of NH4ZSM-5 are active in carbcation formation fi'om n-hexane via hydride abstraction. After modification of the zeolite with Zn by ion exchange with increasing amount of Zn the conversion at first decreases passes the minimum at about 0.125 mmol Zn/g zeolite and then begins to rise. At concentration 0.35 mmol Zn/g zeolite the conversion of n-hexane reaches the level of that obtained on pure NH4ZSM-5 sample. We can also see that at concentration lower than 0.27 mmol Zn/g zeolite, there are samples with different content of Zn having the similar activity in n-hexane conversion. Different situation can be observed on mechanical mixtures. Before testing mechanical mixtures were pre- activated at 480 ~ for tree hours. This procedure was chosen on the basis of our previous results published in [20]. In this work was investigated the influence of the temperature (450,500,550 ~ and the time ofpre-treatment (1,3,6, and 10.5 hours) on the activity/selectivity of the mechanical mixture ZnO+ZSM-5 containing 0.35mmol Z n / g in aromatisation of n-hexane. Results showed that at temperatures 450 and 500 ~ the activity/selectivity of the catalyst slightly changed with the time of activation but remained constant aJter pretreatment exceeding 3 hours. As can be seen on Fig. 1 mechanical mixtures

859 containing different content of ZnO have practically the constant catalytic activity. We have previously observed that on mechanical mixtures of ZnO with non-acidic carder as alumina [18] or NaZSM-5 [21] the conversion of n-hexane at 420 ~ was practically zero. This proved that ZnO as a separate phase at this reaction temperature is not active in n-hexane conversion. The constant activity of our mechanical mixtures with different content of ZnO confirms that ZnO cannot be an active component in these catalysts. On Fig.1 can be also observed that the level of n hexane conversion on our mechanical mixtures is lower comparing with that on pure acid NH4ZSM-5 and is comparable with the level of conversion reached on two ion-exchanged samples containing 0.085 or 0.123 mmolZn/g. The decrease ofn-hexane conversion over the mechanical mixtures comparing with pure acid zeolite on the level reached on ion-exchanged samples with low content of Zn indicates that the catalytic performance of mechanical mixtures is a result of some interaction between ZnO and the zeolites. Selectivity to aromatics in conversion of n-hexane in dependence of the amount of Zn in samples are presented in Fig.2 Results demonstrated on Fig.2 show that over ion exchanged samples in contrary to the conversion with increasing amount of Zn the selectivity to aromatics continuously raises and 60-

5O v

W

.2 40 t~

x x

E

x ZnZSM-5

x

9ZnO+ZSM-5

0L .

m 30 0

._> 20 X

0

9

9

9

9

,,=,=

10

0

0,1

T

1

0,2

0,3

1- ...................

0,4

T. . . . . . . . . . . . . . . . . . . . . . . .

0,5

0,6

Zn, (mmollg) Fig.2 Selectivity to aromatics in n-hexane conversion over ion exchanged ZnZSM-5 and mechanical mixtures ZnO+ZSM-5 {T= 420~ p= 1 MPa, WHSV=2.6 h ~} on Zn containing samples is evidently higher of that on pure acid ZSM-5. The selectivity to aromatics over mechanical mixtures analogous to the conversion is constant independently

860 on the content of ZnO in the mixture and reached the level of ion exchanged sample containing 0.085 mmolZn/g. The composition of aromatic fraction of the products are given in Table 2.As can be seen the prevailing aromatics are benzene toluene and xylenes. As conceming the BTX fraction composition we can see that at low Zn concentration in the zeolite in contrary to the acid ZSM-5 toluene has the highest concentration in BTX fraction. Its concentration in BTX fraction continuously decreases with increasing amount of Zn in the zeolite and simultaneously raises the concentration ofxylenes. On samples containing >0.2 mmol Zn/g the order of the concentration of the components in BTX fraction changed and is xylene> toluene > benzene and the composition of BTX fraction is very close to the thermodynamic equilibrium[22]. Table 2 C omp0sition of aromatics conv. arom. BTX*/arom. Sample .... % .... NH4ZSM-5 2.06 0.84 0.07ZnZSM-5 3.28 0.87 0.17ZnZSM-5 3.40 0.88 0.18ZnZSM-5 4.2 0.87 0.25 ZnZS M-5 4.36 0.86 0.30ZnZSM-5 6.95 0.88 0.4ZnZSM-5 10.41 0.83 0.46ZnZSM-5 12.93 0.82 0.54ZnZSM-5 17.26 0.81 0.7 ZnZSM-5 23.92 0.76 ZnO+ZSM-5 3.14 0.85 * BTX = (B)enzene, (To)luene, (X)ylenes

B/BTX

To/BTX

X/BTX

0.12 0.10 0.15 0.18 0.21 0.18 0.18 0.18 0.19 0.18 0.13

0.39 0.52 0.49 0.48 0.45 0.43 0.38 0.38 0.37 0.36 0.43

0.49 0.38 0.36 0.36 0.37 0.37 0.44 0.43 0.44 0.46 0.44

For the atypical activity/selectivity relationship in n-hexane aromatisation on ion-exchanged samples as follows from Fig.1 and Fig.2 we have not an unambiguous explanation. More detailed study can be made for elucidation of this observation. But one of the possible reasons could be a different position of the zinc cations in the zeolite framework in dependence on the concentration of zinc. At low concentration Zn cations probably don't contribute to the n-hexane activation and the conversion decreases with decreasing amount of Broensted acid sites in samples. The continuos increase of selectivity to aromatics with the increase of content of Zn in ionexchanged samples indicate that Zn at low concentration supports only the reactions of dehydrogenation of oligomerized products. Zn species probably due to their localization begin to activate n- hexane only at higher concentration and the conversion increases. Whether the activation occurred via dehydrogenation of n-hexane or hydride abstraction is still the subject of discussion. As concerning the mechanical mixture, at our reaction conditions ZnO seems to be inactive in n-hexane aromatisation. The activity/selectivity of the mechanical mixtures is comparable to the activity/selectivity of the ion exchanged sample containing 0.085 mmol Zn/g. We assume that the catalytic performance of the mechanical mixture is probably a

861 result of solid-state ion exchange of Zn from ZnO into zeolite. The fact that the conversion is constant and doesn't depend on the content of ZnO indicates that the degree of this exchange is limited. From the similar activity/selectivity of mechanical mixtures to the activity /selectivity ofionexchanged sample with low content of Zn can be deduced that the proposed SSIE proceeds to a relatively low degree Our preliminary hypothesis is that the extent of interaction of Zn species is probably not controlled by the concentration of ZnO in the mixture but by the concentration of cationic sites accessible for the migrating Zn species. They can be Zn2+(need two sites in suitable distance, ZnOH + at high temperatures hardly probable) or some clusters of the types as (ZnOZn) 2+ forming from ZnO at higher temperatures. These clusters can be too large to migrate inside the zeolite channels and accessible sites for them are only the sites on the extemal surface of the zeolite. So in this case the distribution of A1 in zeolite crystal and the crystal sizes can play an important role on the extent of expected SSIE. 4. CONCLUSIONS The presented results showed that activity/selectivity of ZnZSM samples prepared by ion exchange in n-hexane aromatisation significantly depend on the amount of Zn. While the selectivity to aromatics in comparison with pure acid form with increasing amount of zinc continuously raises the conversion at first decreases, passes the minimum at concentration roughly 0.125mmol of Zn/g and than begin to rise. This phenomenon we suppose is connected with the localization of the zinc in the zeolite. At low concentration Zn species have low activity for n -hexane activation but support reaction of dehydrogenation of oligomerized products. . The activity/selectivity of mechanical mixtures with a different content of ZnO were constant and were on the level of ion-exchanged sample containing 0.085 mmolZn/g. So we suggest that ZnO is an inactive component and in mechanical mixtures SSIE of Zn from ZnO proceeds during thermal pre-treatmen and Zn in cationic positions are responsible for the catalytic performance of mechanical mixtures. From the results followed that the extent of proposed SSIE is rather low and is not controlled by the content of ZnO in mechanical mixtures but probably by the zeolite crystals and surface characteristics. Results demonstrated that the roll of zinc in cationic position of ZSM-5 zeolite in n-hexane aromatisation at 420 ~ depends on the concentration of Zn in the zeolite and also showed that ZnO species at 420 ~ are inactive in n-hexane aromatisation.

REFERENCES

1.J.Kanai, Succesfull Design of Catalysis, Elsevier Science Publishers, Amsterdam, 1988 2. M.Guisnet, N.S.Gnep, Applied Catalysis A:General, 146 1 (1996) 33 3.K.M.Dooley, G.L.Price, V.I. Kanarizev, V.I.Hart, Catalysis Today, 31 3-4 (1996) 305 4. R.LevanMao, Y.Jianhua, L.A.Dufrense, R.Carli, Catalysis Today, 31 3-4 (1996) 293 5. M.Guisnet, N.S.Gnep, Catalysis Today, 31,3-4, (1996) 275 6. J.Hal/~sz, Z.Konya, A.Fudala, A B6res, I.Kiricsi, Catalysis Today, 31 3-4, (1996) 293 7. P.Meriandeau, G. Sapaly, C.Naccache, Proceed. ot the Inter. Symp. on Chemistry and

862 Microporous Crystals, Tokyo, June 26.-29. (1990) 267 8. J.Kanai, N.Kawata, Journal of Catalysis 114 (1988) 284 9. A,.Hagen, F. Roesner, Zeolites and Microporous Crystals, (1994) 313 10. F.Roessner, A.Hagen, M.Mrocsek, H.G.Karge, K-H Steiberg, Procc. 10thInt. Cong.on Catal. 12.-24 July, (1992), Budapest, 1707 11. J.Heemsoth, E Tegeler, F.Roessner, A.Hagen. Microporous and Mesoporous Materials,46(2001) 185 12. L.Wei, Z, Gui, H.S.Din~, X.T.Zhang, H.Y. Li, L.Song, Z.L.Sun, L.V.C. Rees, Stud.Surf.Sci. Catal.13u' Intemational Zeolite Conference, Montpellier, July 18-13, (2001)24-P-31 13. J.Z.Gui, H.S. Ding, N.N. Liu, Y.R.Gao, Z.L.Cheng, X.T.Zhang, B.Ma, L.Song, Z.L.Sun, L.V.S.Rees Stud.Surf.Sci. Catal.13th International Zeolite Conference, Montpellier, July 18-13, (2001) 28-0-02 14. A.Matsuoka, J.B.Kim, T.Inui, Microporous and Mesoporous Material, 35-36 (2000) 89 15. J.Kanai N. Kawata, Applied Catalysis 55 (1989) 115 16. N.Wiswanadhan, A.R.Pradhan, N.Ray, S.C.Vishnoi, U.Shanker, T.S.R.Prasada Rao, Applied Catalysis A: General 137(1996) 225 17.Y. Ono, H. Nakatani, H.Kitagawa, E.Suzuki, Successful Design of Catalyst. (1988) Elsewier Science Publishers, B.V.Amsterdam, 279 18. E.Rojasov~i, A.Smiegkov~i, P.Hudec,Collect. Czech.Chem.Commun 64 (1999) 168 19.E.Rojasov~, A.Smie~kov~i,P.Hudec, Zidek, React.Kinet.Catal.Lett. 66 (1999) 91 20. E.Rojasov~i, A.Smiegkov~i, P.Hudec, ~.idek Collect. Czech.Chem.Commun. 65 (2000) 1506 21. E.Rojasov~i, A.Smie~kov~, P.Hudec, ~idek, Stud. Surf. Scie. Catal.125 (1999) 441 22. Alberty R.A. Ind.Eng.Chem.Fundam. 28 (1988) 211


863

Simultaneous desulfurization and isomerization of sulfur containing n-pentane fractions over Pt/H-mordenite catalyst J. Hancs6k a, A. Hol16 b, I. Valkai b, Gy. Szauer b, D. Kall6 c

a Department of Hydrocarbon and Coal Processing, University ofVeszpr6m, Veszpr6m, P.O.Box 158, H-8201, Hungary b Division of Production and Trade, M O L - Hungarian Oil and Gas Co., Sz~tzhalombatta, P.O.Box 1, H-2443, Hungary c Chemical Research Center, Institute of Chemistry, Hungarian Academy of Sciences, Budapest, P.O.Box 17, H-1525, Hungary A new field of the catalytic application of Pt/H-mordenite catalysts is presented for production of gasoline blending components having high octane number and reduced sulfur content (A1MCM-41>> TiMCM-41. The use of the parent siliceous MCM-41 in place of the Metal-MCM-41 gave no polymer. Therefore, the difference in the cocatalytic activity of Metal-MCM-41 seems to be attributable to the difference in the acidity of Metal-MCM-41, probably the acidic strength. The characteristics of polypropylenes produced were hardly dependent on the kind of Metal-MCM-41 and its Si/Metal ratio. However, there were slight differences in Tm, Mw and Mw/Mn between polypropylenes produced using the Metal-MCM-41 and MAO. No difference in isotacticity [mmmm] pentad was observed. These results suggest that ZnMCM-41 and GaMCM-41 as well as A1MCM-41 can activate the metallocene compound and that the stereospecificity of rac-Et(Ind)2ZrC12 hardly changed by activation with Lewis acid sites on the Metal-MCM-41. No change in the stereospecificity of rac-Et(Ind)2ZrC12 was also confirmed from 13C NMR spectra of polypropylenes produced. As shown in Fig. 3, no difference in the microstructure of polypropylene was observed. 3.3. Relationship between cocatalytic activity and Lewis acidity To get further information concerning activation of metallocene compound by the Metal-MCM-41, the relationship between the cocatalytic activity and the Lewis acidity was investigated. Fig. 4 shows the relationship between the polymer yield and the amount of Lewis acid. The amount of Lewis acid sites is normalized based on the peak intensity at 1454 cm -1 in the FTIR spectrum of pyridine adsorbed on the A1MCM-41 (Si/Al-13.4). A clear tendency was not observed. It is recognized that the acid strength of solid acid catalysts such as zeolite and silica-alumina is not uniform, namely there exists a distribution of acid strength. Therefore, the acid strength of the Metal-MCM-41 was roughly evaluated by changing the desorption temperature of pyridine. Namely, pyridine vapor was adsorbed onto the Metal-MCM-41 at 150~ for 1 h and the FTIR spectra was recorded at ,~ 6 10 e various stages of pyridine desorption, which was e7 continued by evaporation at ~ .3 o 4 progressively highly temperatures (150-400~ As shown in Fig. 5, the intensity ~ 2 .11 o of the peak at ca. 1451 4 9 e8 cm-]due to Lewis bound CD 1 5 pyridine decreased gradually 0 ~ e,2 3 4 5 with an increase in the Normalized amount of Lewis acid sites of Metal-MCM-41 evacuation temperature. However, the degree of Fig. 4 Relationship between the polymer yield and the reduction in the peak intensity normalized amount of Lewis acid sites. was markedly dependent on Arabic numbers denote Sample No. in Table 1. the kind of metal incorporated

877

(A) 9

.

BL

I

I,

I

B B+L.~__

(D) B L

BBLj

9

k..__ (d) '~_ (c)

0

0.375Pt-0.068Pd _A 0.25Pt- 0.135Pd I

"O

50

~

~

~ 40

~' 60

"1o

Xc.. .o_

,I

' .........

o

20 10

10 o~

200

250

300

350

.....

o ,'o'g~

400

~or176

200

Reaction T e m p e r a t u r e / ~

' . . . . . . . . . .

250

300

~1

350

400

Reaction Temperature / ~

Fig. 1" Conversion of n-decane (left) and isomer yields (right) as a function of the reaction temperature over different Pt-Pd/A1MCM-41 catalysts.

100 9O end ....

o-e.

8O

PH2

kPa = 1M

--10

~(("

~= 70 8

~

3o

0

20

50

'"

.

.

.

,

.

.

.

.

.

.

|

.

.

.

.

|

.

.

.

.

|

.

.

.

.

|

i 0.25Pt- 0.135Pd 0.125Pt - 0.068Pd

"o

~ 3o

x~ 5o 4O

~.

~o 40

60

"~

60

~ 2o ]1~'? _,~

I-~-- 0"5Pt- 0"27Pd_ _ I~ 0.25Pt- 0.135Pd ]

10

10 O~ 200

250 3o0 350 Reaction Temperature / ~

400

0~ j 200

.

.

250

300

I

350

400

Reaction Temperature / ~

Fig. 2: Conversion of n-decane (left) and isomer yields (right) over Pt-Pd/A1MCM-41 catalysts with different metal content. In Figure 1, the results of n-decane conversion over bimetallic Pt-Pd catalysts with different npt/npd ratios are depicted in comparison to the monometallic catalysts. Conversion of n-decane over the monometallic catalysts 0.27 Pd/HA1MCM-41 and 0.5 Pt/A1MCM-41 starts at reaction temperatures around 250 ~ (Figure 1 (left)). For conversions up to 30 %,

914 isomerization is the sole reaction. With increasing conversion, hydrocracking of C~0 isomers occurs as a consecutive reaction. The yield of feed isomers passes through a maximum around 55 % and then declines again (Figure 1(right)). These results are in complete agreement with the well known reaction mechanism for isomerization and hydrocracking of alkanes. It is noteworthy that due to the high reaction temperature needed to achieve full conversion over the monometallic clusters containing catalysts aromatization also occurs to a significant extent. The yield for substituted benzene isomers, viz. 1-ethyl-3,5-dimethyl benzene, 1-ethyl2,4-dimethyl benzene, 1,2,3,4-tetramethyl benzene and 1,2,4,5 tetramethyl benzene, at a reaction temperature of 410 ~ amounts to 8.3 % over 0.27 Pd/HA1MCM-41 (Xn-De = 91%) and to 14 % over 0.5 Pt/HA1MCM-41 (Xn-De = 99 %). Aromatization (on the metal function) is favored at high reaction temperatures and competes with hydrocracking (on the acid sites). The bimetallic systems are significantly more active compared to the monometallic catalysts, n-Decane conversion already starts at 200 ~ and the maximum yield of feed isomers is already reached at reaction temperatures around 300 ~ which is 50 to 80 ~ below the maximum for the monometallic systems. However, the maximum isomer yield reaches only 45 % for the 0.375Pt-0.068Pd/A1MCM-41 catalyst and 38 % for 0.25Pt0.135Pd/A1MCM-41 (Figure l(right)). In Figure 2, variation of the n-decane conversion (left) and the isomer yield (right) with the reaction temperature as function of the metal content for the bimetallic Pt-Pd systems is depicted. The activity and the isomer yield increases in the order 0.125 Pt-0.068Pd/A1MCM-41 < 0.25Pd-0.135Pt/A1MCM-41 < 0.5 Pt-0.27Pd/A1MCM41. Due to the lower reaction temperature, no aromatization products are detected over these catalysts. In Table 1, the isomerization, hydrocracking and aromatization yields at the temperature of maximum isomer yield are compared. A lower temperature at maximum isomerization indicates higher activity of the catalyst. All catalysts containing bimetallic clusters exhibit a higher catalytic activity as compared to the monometallic catalyst. In particular, the catalyst 0.25Pt-0.135Pd/A1MCM-41 exhibits superior activity, while the isomerization selectivity is still high. The higher activity of the bimetallic systems is tentatively attributed to a higher hydrogenation/dehydrogenation activity of the bimetallic Pt-Pd clusters and, hence, a higher concentration of alkenes, which are the intermediates in the hydroconversion of alkanes [3]. Table 1 Yields of isomers (Yiso.), cracked products (Ycr.) and aromatization products (Yarom.) at maximum isomer yield. Metal compound Tmax / ~ Xn-decane/ % Yiso./% Ycr./% Yarom./% S iso./% 0.5 wt.-% Pt

355

81.0

53.5

24.6

2.9

66.0

0.27 wt.-% Pd

380

74.3

54.5

17.1

1.4

73.4

0.375 Pt-0.068 Pd

320

77.4

44.0

33.4

0.0

56.8

0.25 Pt- 0.135 Pd

300

58.6

38.2

20.4

0.0

65.2

0.125 Pt- 0.203 Pd

310

69.6

37.3

32.3

0.0

53.6

0.125Pt- 0.068 Pd

330

82.4

32.4

49.9

0.1

39.3

0.5 Pt - 0.27 Pd

310

73.3

48.9

24.4

0.0

66.7

915 The variations of the monobranched and dibranched isomers with the conversion are the same irrespective of the sample considered and are in close agreement with our data on the monometallic catalysts [11 ]. This illustrates the consecutive nature of the transformation of the monobranched into dibranched and tribranched isomers which is favored at high temperature. The formation of tribranched isomers is detected only at high temperature and to a small extent. Moreover cracking of dibranched and tribranched isomers is fast compared to cracking of monobranched isomers. Therefore, the selectivity for monobranched isomers is high over the whole conversion range. The distribution of the monobranched isomers changes with reaction temperature and approaches thermodynamic equilibrium at high conversion levels. Detailed analysis of the cracked products (Table 2) shows that methane and ethane formation is significantly reduced for all catalysts containing bimetallic Pt-Pd clusters compared to 0.5Pt/A1MCM-41. Therefore, hydrogenolysis (cracking on the metal function) does contribute to a smaller extend to the yield of the cracked products, which is probably a consequence of the increased activity at lower reaction temperatures. 0.25Pt0.135Pd/A1MCM-41 and 0.5Pt-027Pd/A1MCM-41 exhibit product distributions which are characteristic for ideal hydrocracking, which is further supported by the high selectivity for the iso-isomers in the C4 and C5 fraction, respectively. It is assumed that these catalysts possess a better balance between the two catalytic functions. Furthermore aromatization is almost completely suppressed, which is a consequence of the reduced reaction temperature and, hence, the improved balance between the acid and metal sites. Table 2. Distribution of the cracked products at a cracking yield YCr. of 15 %. Metal

0.5 Pt

0.27 Pd

0.375 Pt-

0.25 P t -

0.125 Pt-

0.125Pt-

0.5 P t -

0.068 Pd

0.135 Pd

0.203 Pd

0.068 Pd

0.27 Pd

Number of moles/100 moles cracked C1

37

7

10

1

9

10

5

C2

9

2

8

0.5

8

10

5

C3+C7

45

37

50

26

69

63

43

C4+C6

80

105

95

103

80

73

96

C5

43

60

48

71

45

39

53

C8

6

0

5

0

6

8

3

C9

7

0

4

3

4

5

1

227

211

220

204.5

221

208

206

C4

42.3

55.5

40.0

68.9

24.0

12.6

55.0

C5

48.5

57.3

45.0

70.5

30.3

15.8

60.0

Total % iso in

916

100 (a) C~ fraction

90 L_

80

E

70

0 o ~

60

-o r

o

50

.Q

40

c t~ L_ 0

c

30

--i--0---0---[3--

0

20

o L_

w

10 I

10

20

,

I

30

,

I

40

,

I

50

Conversion

,

0.25Pt - O. 135Pd 0.375Pt- O.068Pd O. 125Pt - 0.203Pd 0.27 Pd O.5Pt

I

60

,

70

I

,

80

I

90

100

X n _ d e c a n e ] 0~

100 o-?,

90

(t) L_

80

E

70

0 (/)

, m

-o

60

o

50

_Q

40

o

30

o

20

(I) (-. c

E 0

LL

(b) C 4 fraction

,r

--0---I-

10 0

,

0

I

10

,

I

20

,

I

30

,

I

40

,

I

50

Conversion

,

I

60

,

0.125Pt- 0.203Pd 0.375Pt - O.068Pd 0.25Pt - O. 135Pd --i0.27 Pd - - - 0 - O.5Pt , , , , , ,

70

80

90

X n . d e c a n e / 0~

Fig. 3" Hydrocracking of n-decane over Pt-Pd/A1MCM-41 catalysts, percentage of branched isomers in the (a) C5 and (b) C4 fraction.

I

I II

I

I

100

917 In Figure 3a and Figure 3b, the amount of branched isomers in the C5 and C4 fraction, respectively, are plotted as a function of n-decane conversion. For 0.27Pd/A1MCM-41 and 0.25Pt-0.135Pd/A1MCM-41, the C4 and C5 products are mainly branched and their relative amount changes only slightly with the degree of conversion. In contrast, over all other catalysts mainly n-alkanes are formed at low conversion level, while the formation of branched isomers dominates at high conversion. The iso-selectivity for the 0.25Pt-0.135Pd catalyst is 68 % and 70 % in the C4 fraction and the C5 fraction, respectively. Comparable results are obtained for a 0.5Pt/CaY zeolite, where the selectivities for branched C4 and C5 isomers amount to 66 % and 73 %, respectively [3]. The observed high selectivities for linear cracking products (in particular at low conversion levels) can only be partially explained by the occurrence of hydrogenolysis (cracking at the metal function), which would result in the formation of n-isomers. The spectrum of cracked products (Table 2) reveals that hydrogenolysis indeed occurs over 0.5Pt/A1MCM-41, 0.375Pt-0.068Pd/A1MCM-41 and 0.125Pt-0.203Pd/A1MCM-41, but not to a sufficient extend to explain the high selectivities for n-butane and n-pentane at low conversion. Consequently, the product spectrum is presumably also influenced by several types of [3-scission [12]. Cracking of monobranched alkenes proceeds through a type C mechanism (which involves two secondary carbenium ion intermediates) and results in the formation of two linear alkanes. Dibranched alkenes undergo cracking through type B 1 or B2 mechanisms (one secondary plus one tertiary carbenium ion) and equimolar amounts of branched and linear products are formed. Finally, tribranched alkenes are cracked through type A [~-scission (two tertiary carbenium ions) and result in the formation of branched isomers. Type D ~-scissions via primary carbenium ions are typically not observed. Over zeolite catalyst, type A cracking is much faster than type B which is again faster than type C cracking. Therefore, over zeolite catalysts ca. 70 % of the cracked products are formed via type A [~-scission, which accounts for the high selectivities for branched isomers [13]. To explain the lower selectivities for branched products over our catalysts, the occurrence of type B or type C hydrocracking has also to be considered. The relative high reaction temperatures (as compared to zeolite-based catalysts) might result in a sufficient reaction rate for these mechanisms to proceed. The substitution of part of the platinum by palladium results in bimetallic catalysts, which exhibit an increased catalytic activity in n-decane conversion. We have found that the bimetallic catalyst exhibits distinctly different properties compared to the monometallic catalysts. Similar results were obtained for Pt-Pd clusters supported on LaNaY [ 14] and on HBeta [4]. In the latter case, it was found that the dispersion of bimetallic clusters is significantly higher compared to the monometallic systems, which results in a better proximity of the metal and the acid sites. It is generally accepted that a good "balance" between the acid sites and the metal sites is important for a high activity of the catalyst and a high isomerization yield. The observed higher catalytic activity and decreased hydrogenolysis activity for our Pt-Pd/A1MCM-41 catalysts, in particular for 0.25Pt-0.135Pd/A1MCM-41, is therefore presumably a consequence of a higher dispersion of both platinum and palladium. Hydrogen adsorption and spectroscopic studies aiming at the clarification of this point are currently underway.

918 4. C O N C L U S I O N S In comparison to monometallic Pd/A1MCM-41 and Pt/A1MCM-41 catalysts, bimetallic PtPd/A1MCM-41 materials are superior bifunctional catalysts for n-decane isomerization. The use of bimetallic Pt-Pd clusters results in higher catalytic activity and a higher yield of C10 isomers at a substantially lower reaction temperature. This improvement is due to a better balance between the two catalytic functions. Compared to palladium, platinum is a more active hydrogenation/dehydrogenation function, but the realization of sufficient dispersions with current catalysts preparation techniques is difficult. Large platinum clusters catalyze ndecane hydrogenolysis, which is significantly reduced for the bimetallic Pt-Pd/A1MCM41(23) catalysts. It is therefore assumed that the dispersion of the Pt-Pd clusters is enhanced in comparison to the monometallic Pd and Pt clusters, respectively.

ACKNOWLEDGEMENTS Financial support by Deutsche Forschungsgemeinschaft (DFG) and Fonds der Chemischen Industrie is gratefully acknowledged.

REFERENCES 1. 2. 3. 4. 5.

M.L. Coonradt and W.E. Garwood, Ind. Eng. Chem. Prod. Res. Dev. 3 (1964) 38. R. Mokaya, W. Jones, S. Moreno and G. Poncelet, Catal. Lett. 49 (1997) 87. J. Weitkamp, Ind. Eng. Chem. Prod. Res. Dev. 21 (1982) 550. E. Blomsma, J.A. Martens and P.A. Jacobs, J. Catal. 165 (1997) 241. P.Meriaudeau, Vu.A. Tuan, G. Sapaly, Vu.T. Nghiem and C. Naccache, Catal. Today 49 (1999) 285. 6. J.S. Beck, J.C. Vartuli, W. J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. 7. K.J. Del Rossi, G.H. Hatzikos, A. Huss, US Patent 5,256.277 assigned to Mobil Oil Corp. (1993). 8. K. Chaudhari, T.K. Das, A.J. Chandwadkar and S. Sivasanker, J. Catal. 186 (1999) 81. 9. A. Klemt and W. Reschetilowski, Chem. Ing. Tech. 73 (2001) 872. 10. M. Hartmann, S. Racouchot and C. Bischof, Microporous and Mesoporous Mater. 27 (1999) 309. 11. C. Bischof and M. Hartmann, in "Zeolites and Mesoporous Materials at the Dawn of the 21 st Century", A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (eds.), Studies in Surface Science and Catalysis, Vol. 135, 26-P-18, Elsevier: Amsterdam (2001). 12. J. Weitkamp, P.A. Jacobs and J.A. Martens, Appl. Catal. 8 (1983) 123. 13. J.A. Martens, P.A. Jacobs and J. Weitkamp, Appl. Catal. 20 (1986) 283. 14. S.P. Elangovan and M. Hartmann, unpublished results.


919

Influence o f nickel m e t a l distribution in N i / Y - z e o l i t e on the reactivity t o w a r d CO hydrogenation Dul-Sun Kim, Sung-Chul Kim, Seong-Ji Kim and Dong-Keun Lee Department of Chemical Engineering/Environmental Protection, Environment and Regional Development Institute, Gyeongsang National University, Kajwa-dong 900, Chinju, Kyungnam 660-701, Korea The influence of nickel metal distribution on the properties of Ni/Y-zeolite catalysts for CO hydrogenation was investigated. The studies using TPR/TPO, TEM, XRD and FMR have shown the existence of a bidispersion of nickel metal particles; i.e., small particles were restricted inside the zeolite pores, and large particles were formed outside the zeolite crystal. CO hydrogenation reaction was performed in a fixed bed reactor operated at 1.5Mpa, 270 ~ and H2/CO ratio of 3. Nickel metal distribution affected the activity and hydrocarbon product distribution significantly. With increasing fraction of nickel metal inside the zeolite pores the activity continued to decrease, but the production of the long chain and olefinic hydrocarbons was enhanced remarkably. Especially when all the nickel metals were restricted within the pores of zeolite, a bimodal product distribution of CI and C4 was observed. This distribution was proved to be due to the rapid dimerization of the primarily formed ethylene to butene probably on the Ni + ion within the zeolite pores. 1. INTRODUTION Recent researches in CO hydrogenation aim mainly at improving the hydrocarbon product selectivity, and zeolites came to appear to be a promising support because they can be utilized to prepare catalysts containing highly dispersed metals, to show molecular seiving effect and to induce polyfunctional activity[ 1-5]. Metals of ruthenium[6-9], iron[7,8,10] and cobalt[2-5, 7,8,10], which have been shown to be acitve for CO hydrogenation, were employed for the preparation of metal-zeolite catalysts. Chain-length limitation in hydrocarbon product distribution occurred on the small metallic aggregates restricted into the zeolite pores[6-8], and anomalously high selectivities for certain hydrocarbons were also observed for iron- [ 10] and cobalt-zeolite[ 10,11 ] catalysts. Nickel metal is also known to be active for CO hydrogenation[ 12,13], but its main product is methane. Accordingly nickel metal has generally been excluded from the candidates of controlling the hydrocarbon product distribution. Our investigations on the reduced nickel-zeolite catalysts have shown the existence of a bidispersion of nickel particles; i.e., small particles were restricted inside the zeolite pores, and large particles were formed outside the zeolite crystal. Although methane is believed to be produced as a main product on the large nickel particles, a different pattern of hydrocarbon product distribution can be inferred to occur on small nickel particles restricted into pores of

920 zeolite. In this research Ni/Y-zeolite catalysts having different nickel metal distribution were prepared, and the influence of nickel metal distribution on the properties of Ni/Y-zeolite catalyst toward CO hydrogenation was investigated.

2. EXPERIMENTAL 2.1. Materials and catalysts NaY zeolite with a Si/A1 molar ratio of 2.5 was provided from Strem Chemicals. Ni(NO3)2-6H20, supplied from Alpha Division, was used as a precursor for the catalyst preparation. Hydrogen(Matheson, 99.999%) was further purified by passing it through an oxy- and a molecular sieve trap. Carbon monoxide(Takachiho, 99.95%) was passed through a molecular sieve trap. Oxygen(Matheson, 99.99%) was used without further purification procedure. The catalyst samples were prepared by a conventional ion exchange method. After suspending 10g zeolite in 1000mL of 0.04N aqueous solution of Ni(NO3)2"6H20, the mixture were stirred continuously at 85~ for 48h. After being ion exchanged, the samples were filtered and washed sufficiently to remove remaining nickel salt solution which was not yet ion exchanged. By repeating the ion exchange four times Ni/Y-zeolite catalyst having about 10wt% nickel loading could be obtained. The prepared Ni/Y-zeolite was dried in air at 80 ~ for 12h, and was then reduced in hydrogen stream to the desired extent of reduction. All the catalysts used in the present study will be abbreviated by the symbols of (reduction temperature, ~ time, h) in the rest of the text. The catalyst of 500-8, for example, denotes the 10wt% Ni/Y-zeolite catalyst reduced at 500~ for 8h. All the catalysts were heated to the reduction temperature at a linear rate of 2 ~ 2.2. Characterization Temperature programmed reduction(TPR) and oxidation(TPO) experiments were performed in a closed system with gas circulating unit to determine the amount of nickel metal inside and outside the zeolite by following the procedure of Jacobs et al.[ 14,15]. Ferromagnetic resonance(FMR) spectra of nickel metal were recorded at X-band frequencies on a Varian E-4 spectrometer working at a microwave power of 13mW and field modulation of 100KHz. DPPH was used as a standard to determine the g-factors. The quartz sample tube was designed for the in situ operation. X-ray diffraction line broadening was measured on a diffractometer(JEOL, JDX-1193A) using CuKot radiation. Nickel particle sizes were calculated from the line broadening data for Ni(111) using Scherrer equation. Transmission electron microscopy was done with a JEOL 200 CX microscope using 160KeV electrons. About 40 measured points were used to calculate the average value of nickel crystallite size. The adsorption isotherms of H2 were measured at 25~ in a conventional Pyrex glass volumetric adsorption apparatus. The gas uptakes were obtained by extrapolating the straight portion of the isotherms to zero pressure, and the irreversible uptakes were determined from the difference between the total and reversible ones. The dispersion of nickel particles were calculated from the irreversible uptakes of H2[ 16].

921 2.8

,_, x

2.3. C O h y d r o g e n a t i o n

2.1

[

U ~

. \r

~

/

E

!!

0

t

'

i

I \

"

/'",

"

"

iii

",,,

CO hydrogenation reaction was performed in a fixed bed reactor operating at 1.5Mpa, 270~ and HJCO ratio of 3. The steady state rates were measured after 1Oh from the introduction of the reactants to the catalysts.

3. RESULTS AND DISCUSSION 200

300

400

TEMPERATURE

500

600

(~

3.1. C h a r a c t e r i z a t i o n

A typical result of TPR/TPO consecutive experiments for a 10wt% Ni/Y-zeolite catalyst is shown in Figure 1. Curve A denotes the rate of nickel ion reduction to nickel metal which may be formed inside or outside the zeolite. The degree of nickel ion reduction can be calculated from the ratio of the amount of the hydrogen consumed and the total amount of the nickel ion in zeolite by considering the reaction of Ni 2++ H2 ~ Ni ~ + 2H § Upon oxidation each metal phase inside or outside the zeolite may be oxidized at different routes as suggested by Jacobs et al.[15]. Curve B represents the oxidation rate, but no discrimination could be achieved. During a second reductive treatment of this oxidized sample (curve C) a new low temperature maximum together with a new high temperature maximum in the reduction rate is observed. At the end of the first and second reduction nearly the same amounts of hydrogen were consumed. From the peaks of the low- and hightemperature maxima the amounts of nickel metal phase inside and outside the zeolite can be deduced, respectively. In Table 1 are listed the degree of reduction and the fraction of the amounts of nickel metal inside and outside the zeolite. Both the degree of reduction and the fraction of nickel metal outside the zeolite are known to increase with increasing reduction temperature and time. Figure 1. Typical curves of TPR/TPO experiments.

Table 1. Degree of reduction and nickel metal distribution in the Ni/Y-zeolite catalysts reduced at different conditions Catalyst

Degree of reduction(%)

500-12 500-4 500-1 400-12 400-4 450-1 400-1 300-1

60.0 59.8 59.7 44.3 40.1 37.5 35.0 8.5

Nickel metal distribution(%) Inside Outside 64.0 36.0 60.0 40.0 58.5 41.5 32.2 67.8 28.4 71.6 26.1 73.9 25.0 75.0 0.0 100.0

922

Figure 2. Four representative TEM photographs of Ni/Y-zeolite catalysts (a:500-8, b: 400-8, c:350-8, d:300-1). Figure 2 shows the four representative photographs obtained by TEM on the 10wt% Ni/Yzeolite catalysts reduced at different conditions. Photographs a, b and c are representative for the samples of 500-8, 400-8 and 350-8, respectively. The size of nickel crystallites existing at the exterior surface of the zeolite is shown to increase with increasing reduction temperature indicating that more and more metal migrates out of the zeolite pores and agglomerates as bulky crystallites. When the sample was reduced at 300 ~ for l h(photograph d), no nickel crystallites of detectable size are found. In this case most of the nickel crystallites are believed to exist within the pores of the zeolite and the size will be less than that of faujasite supercage(_=_l.3nm) FMR spectra for the 10wt% Ni/Y-zeolite catalysts do change significantly with reduction conditions as shown in Figure 3. When the catalyst was reduced at 300~ for l h, nearly symmetric and narrow peak(line width=ll00G) appears at the g-value of 2.22. As the reduction temperature increases, the peak becomes asymmetric and its line width broadens to have 2300 gauss for the 600-1 catalyst. Following the suggestions by Jacobs et al.[17], the symmetric and narrow peak is representative of the small nickel metal within the pores of Yzeolite, while the highly asymmetric broad peaks are mainly due to the large nickel metal at the exterior surface of the zeolite. Aforementioned results of TPR/TPO, TEM and FMR show the existence of the bidispersion of nickel metal inside and outside the zeolite.

923 The sizes of nickel crystallites obtained from X-ray line broadening, TEM and H2 chemisorption are listed 600-1 in Table 2. Between the sizeg from XX 1/25 ray line broadening and those from H2 chemisorption incompatible results Z are obtained. The size from X-ray line broadening increases with increasing reduction temperature, while the size calculated from H2 chemisorption E3 does not show a distinct tendency and rr is always larger than that from X-ray X 1/4 line broadening. In particular the size of 45.0nm for the 300-1 catalyst from u3 H2 chemisorption is unreasonably too z iii large when compared with the TEM FZ X1 00-1 photograph(Figure 2-d) and X-ray line broadening which showed no nickel crystallites of detectable size. 1000 3000 5000 Kubo et al.[18] reported that H (gauss) atomically dispersed platinum was inactive to H2 chemisorption. Therefore, if the zeolite includes a Figure 3. FMR spectra of Ni/Y-zeolite catalysts number of nickel species inactive to (detection temperature = 20 ~ H2 chemisorption, the size from H2 chemisorption must have been overestimated. The size of nickel crystallites was calculated again from H2 chemisorption by assuming that the nickel metal phase within the zeolite pores is inactive to H2 chemisorption. The sizes from the modified hydrogen chemisorption(listed in the fifth column in Table 2) agree well with those from X-ray line broadening. Therefore H2 chemisorption is believed to be suppressed on the small nickel metal particles within the zeolite pores.

E

Table 2. Nickel crystallite size in the Ni/Y-zeolite catalysts Nickel crystallite size (nm) Catalyst XRD TEM H2 Chem.

H2 Chem.*

500-12 20.4 31.0 30.6 19.6 500-8 21.8 30.9 30.4 18.8 500-1 16.3 29.9 17.5 450-1 15.2 43.0 11.2 400-8 13.6 28.9 36.0 10.9 400-1 12.4 30.4 7.6 350-8 12.2 21.7 58.1 7.0 300-1 + + 45.0 * Nickel crystallite size was calculated by following the assumption that the nickel metals inside the zeolite pores are inactive to H2 chemisorption. + No nickel crystallites of detectable size were observed.

924 Table 3. Activity and hydrocarbon product distribution Catalyst

Turnover frequency Product distribution (wt%) Nco x 103 (see-1) C1 C2 C3 C4 C5 C6 500-12 231.1 80.3 10.9 5.9 1.8 0.7 0.4 500-8 295.4 88.1 7.8 3.6 0.5 500-1 140.9 75.4 12.6 9.3 1.5 1.2 400-12 86.4 81.0 10.5 5.0 2.1 1.4 450-1 50.2 55.6 16.7 20.6 6.0 1.1 400-1 26.1 60.4 13.6 16.3 7.9 2.0 350-1 20.3 56.0 13.0 9.3 11.0 0.7 300-1 1.8" 40.4 11.7 7.0 40.9 * Turnover frequency of the 300-1 catalyst was calculated under the assumption that the size of nickel crystallites is the same as that of faujasite supercage(=1.3nm).

3.2. CO hydrogenation In Table 3 are listed the summarized data for CO hydrogenation. The activity was expressed in terms of turnover frequency for CO conversion, i.e., the number of CO molecules converted per catalytic site per second. The hydrocarbon products are represented as the number of carbon atoms per molecule, and their respective concentrations are listed as weight percentage. The activity continues to decrease with increasing fraction of nickel metal within the pores of the zeolite (or decreasing size of nickel crystallites). Van Hardeveld and Hartog[19] have shown that the decrease in nickel crystallite size from 21 to 4nm favored the formation of a more strongly bound CO species. As CO adsorbs less strongly on the metal, hydrogen competes more easily with CO for the adsorption sites thereby enhancing the rate of hydrogenation. This is consistent with the suggestion of Vannice[20] that the most active metal surface is that which adsorbs CO the least strongly. The increasing adsorption strength of CO with decreasing size of nickel crystallites seems also to be concerned with the hydrocarbon product distribution in Table 3. Fu and Bartholomew[21 ] showed that longer chain hydrocarbons were produced on the surface with the more strongly bound CO owing to the longer residence time of carbon containing reaction intermediates on the surface. Therefore the production of long chain hydrocarbons seems to be favored with increasing fraction of nickel crystallites inside the zeolite. Especially when all the nickel metals are restricted within the pores of the zeolite(in this case 300-1 catalyst), a bimodal product distribution of C~ and C4 was observed. Several papers concerning the dimerization of ethylene over transition metal exchanged zeolite catalysts have been published. Riekert[22] found that NiY was an active catalyst for ethylene dimerization. Ni + in zeolite framework was proved to be active site for ethylene dimerization[23,24]. The bimodal distribution in the 300-1 catalyst might be resulted from the dimerization of the primarily formed ethylene to butene on the active species of Ni + ion which may be present within the zeolite. To verify this ethylene was fed in a closed system where 0.5g sample of 300-1 catalyst was loaded at 270 ~ The dimerization of ethylene to butene did occur at very fast rate. Moreover the addition of small amount of ethylene to the H2/CO feed(CO:H2:C2H4 = 1:3:0.15 in molar ratio) resulted in the significant increase in the formation of butene. These two results confirm the dimerization of the primarily formed ethylene to butene on the 300-1

925

u+

1.0

1.0

0.8

0.8

0.6

0.6

0.4

0.4

o 0.2 0.0 300

~

o

+

,~--

o

o~

0.2

350

' 400

' 450

catalyst. Figure 4 shows the changes in the formation of olefinic hydrocarbons with reduction temperature. With increasing reduction temperature(or decreasing fraction of nickel metal inside the zeolite pores) more olefins are formed. This seems to be due to the suppression of hydrogen chemisorption on the small nickel metal restricted within the pores of the zeolite as discussed previously.

0.0 500

Reduction temp. ( ~

4. C O N C L U S I O N S

Figure 4. Changes in the fraction of olefinic hydrocarbons over the Ni/Y-zeolite catalysts with reduction temperature,

TPR/TPO, TEM and FMR investigations on the reduced Ni/Yzeolite catalysts have shown the existence of a bidispersion of nickel metal particles. Small nickel metal particles were restricted inside the zeolite, while large particles were formed outside the zeolite crystal. CO hydrogenation reactivities of the reduced Ni/Y-zeolite catalysts were significantly affected by the distribution of nickel metals in the catalyst. With increasing fraction of nickel metal inside the zeolite the activity continued to decrease, but the production of the long chain and olefinic hydrocarbons was enhanced significantly. Especially when all the nickel metals were restricted within the zeolite, a bimodal hydrocarbon product distribution of C~ and C4 were observed. This was ascribed to the dimerization of the primarily formed ethylene to butene probably on the active species of Ni § ion. REFERENCES 1. K.M. Minachev and Y.I. Isakov, Zeolite Chemistry and Catalysis(J.A. Rabo, Eds.), American Chemical Society, Washington, 1976. 2. D.-K. Lee and S.-K. Ihm, Appl. Catal., 32 (1987) 85. 3. D.-K. Lee and S.-K. Ihm, J. Catal., 106 (1987) 386. 4. J.H. Lee, S.-K. Ihm and D.-K. Lee, React. Kinet. Catal. Letts., 57 (1996) 301. 5. J.H. Lee, D.-K. Lee and S.-K. Ihm, Stud. Surf. Sci. Catal., 83 (1994) 355. 6. H.H. Nijs, P.A. Jacobs and J.V. Uytterhoeven, J. Chem. Soc. Chem. Comm., 180 (1979) 1095. 7. D.B. Tratchenko and I. Tratchenko, J. Mol. Catal., 13 (1981) 1. 8. D.B. Tratchenko, G. Coudurier and N.D. Chau, Stud. Surf. Sci. Catal., 12 (1982) 123. 9. Y.W. Chew, H.T. Wag and J.G. Goodwin Jr., J. Catal., 83 (1983) 415. 10. L.F. Nazar, G.A. Ozin, F. Hugues and J. Godber, J. Mol. Catal., 21 (1983) 313. 11. D. Fraenkel and B.C. Gates, J. Amer. Chem. Soc., 102 (1980) 2478. 12. M.A. Vannice, J. Catal., 37 (1975) 449.

926 13. M. A. Vannice, J. Catal., 44 (1976) 152. 14. P.A. Jacobs, M. Tielen, J.P. Linart, J.B. Uytterhoeven and H.K. Beyer, J. Chem. Soc. Faraday I, 72 (1976) 2793. 15. P.A. Jaeobs, J.P. Linart, H. Nijs and J.B. Uytterhoeven, J. Chem. Soc. Faraday I, 73 (1977) 1745. 16. C.H. Bartholomew and R.B. Pannell, J. Catal., 65 (1980) 390. 17. P.A. Jacobs, H. Nijs, J. Verdonck, E.G. Derouane, J.P. Gilson and A. Simoens, J. Chem. Soe. Faraday Trans. I, 75 (1979) 1196. 18. T. Kubo, H. Arai, H. Tominaga and T. Kunugi, Bull. Chem. Soc. Japan, 45 (1972) 607. 19. R. Van Hardeveld and F. Hartog, Advances in the Catalysis, Vol. 22, p. 75, Academic Press, New York, 1972. 20. M.A. Vannice, J. Catal., 44 (1976) 152. 21. L. Fu and C.H. Bartholomew, J. Catal., 92 (1985) 376. 22. L. Liekert, J. Catal., 19 (1970) 8. 23. I.V. Elev, B.N. Shelimov and V.B. Kazansky, J. Catal., 89 (1984) 470. 24. L. Zheng, G. Wang and X. Bai, Proe. 7th Intern. Cone On Zeolite, Tokyo, 965 (1986).


927

Hydrodechlorination of chlorinated compounds on different zeolites B. Imre a, Z. K6nya a, I. Hannus a*, J. Halfisz a, J. B.Nagyband I. Kiricsi a aDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich t6r 1, H-6720 Szeged, Hungary bLaboratoire de RMN, Facult6s Universitaires Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium

Adsorption and transformation of chlorinated hydrocarbons on zeolite Y-FAU modified by different metals such as Co, Pt or both were studied. The combined instrumental methods implied revealed that trichloro-ethylene reacts immediately after adsorption giving ethane and HC1 as main products appearing in the gas phase, tetrachloroethane gives similar product distribution. CHC1F2 transforms on the surface resulting in adsorbed CO and complex surface intermediates involving the acidic OH groups of the zeolite.

1. INTRODUCTION Chlorinated compounds emitted into the atmosphere are responsible for diminishing the ozone layer in the stratosphere. Therefore, their environmentally safe and economically valuable transformation is very important and a big challenge as well. Two main potential ways have been suggested: their (oxidative) destruction to form environmentally safe products or their transformation to potentially valuable chemical compounds by hydrodechlorination [ 1]. It has been assumed that zeolites are appropriate catalysts both in the oxidative decomposition [2,3] and in the hydrodechlorination of chlorinated hydrocarbons [4]. It was shown that different transition metals (for example Co), nobel metals (Pt), and their bimetallic form on zeolite support are promising catalysts for hydrodechlorination [5,6]. We report here on the preparation of Co-, Pt- and Co,Pt ion-exchanged and impregnated Y-type zeolites, their characterization and investigation in reductive hydrodechlorination reactions of different chlorinated compounds.

2. EXPERIMENTAL NaY-FAU (Union Carbide product) was the parent zeolite with Na58A158Si1340384 This work was performed with the help of grants FKFP 400/2000 and OTKA T 025248, Hungary, and with financial help from CGRI, Belgium.

928 unit cell composition. NaY zeolite was impregnated with Pt(NH3)4C12 solution. After drying the Pt content of this P t ~ a Y sample was 1.2 %. Co,NaY- and Pt,NaY-FAU samples were obtained after ion exchange of NaY-FAU in C0(NO3)2 and Pt(NH3)4C12 solution followed by filtering, washing and drying. Unit cell composition of ion-exchanged samples are Co14Na30A158Si1340384 and Pt21Na16A158Si1340384,respectively. Pt/Co,NaY-FAU sample was obtained from Co,NaY-FAU impregnated by 1.2 % Pt. The used reactants (trichloro-ethylene, TCE, 1,1,2,2 tetrachloro-ethane, TCA, and CHC1F2, HCFC-22) were commercial products. Self-supporting wafer technique was employed for acidity and adsorption measurements. The wafers (10 mg/cm2) were prepared from the powdered zeolites and placed into the sample holder of the in situ IR cell. The pretreatment of the samples was as follows: the temperature of the wafer was slowly increased to 723 K under continuous evacuation of the cell and it was held at 723 K for 2 h. Hydrogen was used for the reduction, 26.6 kPa (200 Torr) Ha was introduced in to the cell and reacted with the wafer at 573 K, for 1 h, followed by degassing at the same temperature for an additional hour. After this treatment the sample was cooled to room temperature and the background spectrum of the zeolite was recorded. 16 scans were accumulated for a spectrum. For the acidity measurements 1.33 kPa (10 Torr) of pyridine was introduced into the cell containing the pretreated, reduced self-supporting wafer and heated to 473 K. After 1 h adsorption, the cell was evacuated for 1 h at the same temperature. After cooling the sample to room temperature spectra of the adsorbed pyridine were taken. The peak areas of the bands around 1450 and 1540 cm ~, characteristic of pyridine bonded to Lewis and Brtinsted acid sites [7], were integrated and were divided by the mass of the wafer. These values (summarised in Table 1) were used to characterize the acidity of the samples.

Table 1 Data on surface acidity measured by pyridine adsorption Zeolites NaY-FAU Pt~aY-FAU Co,NaY-FAU Pt/CoY-FAU Pt,NaY-FAU HY-FAU

Surface acidity (cml/mg) * Br6nsted Lewis 0.00 Traces 0.05 0.59 0.11 0.67 0.14 0.47 0.34 0.03 0.78 0.09

*Integrated absorbances from the IR spectra of adsorbed pyridine divided by the mass of the self-supported wafer For in situ IR spectroscopic adsorption and reaction measurements the self-supported wafers were outgassed and reduced with hydrogen. After 1.33 kPa (10 Torr) vapour of the reactant and 26.6 kPa (200 Torr) of H2 were introduced into the cell and heated up to preselected temperatures. The spectra were recorded at beam temperature in each case. Structural changes occurring in the zeolite framework upon reaction with chlorinated compounds were investigated by KBr pellet technique of IR spectroscopy.

929 Spectra were run on a Mattson Genesis 1 FTIR spectrometer (lithium tanthalate detector), with a resolution of 2 cm "x. In situ MAS 13C NMR measurements were performed on an MSL-400 BRUKER spectrometer operating at 100.6 MHz (4.0 ~ts (| pulse). The samples were packed into the NMR tubes and evacuated at 723 K for 2 h. The activated zeolite samples were loaded with the reactant, then the tubes were carefully sealed to achieve proper balance and a high spinning rate (3.8 kHz). Spectra were recorded after heating the tube at various preselected temperatures. The catalysts placed in a glass reactor were pretreated in vacuo and ,finally, at 723 K in 100 cm3/min H2 flow. After adjusting the desired reaction temperature the reactant was fed using hydrogen flow with a flow rate around 50 cm3/min. Hydrodechlorination reactions of different reactants were carried out in a flow system applying GC product analysis (Shimadzu apparatus with capillar column). 3. RESULTS AND DISCUSSION 3.1. Acidity measurements Table 1 summarizes the data of BrOnsted and Lewis acidity of our metal-containing zeolites and that of a neutral NaY-FAU and a mostly BrOnsted acidic HY-FAU for comparison. (The latter sample was obtained by ion exchange of NaY-FAU in NH4C1 solution followed by filtering, washing, drying and deammonization at 723 K in vacuum.) Figure 1 shows the OH-region of the IR spectra of the catalysts used. The Pt/NaY-FAU, impregnated sample has no acidic OH groups in the 3500-3700 cm l range (spectrum a). In good agreement with this, it has practically no Br6nsted acidity : -,, i : .... determined by pyridine adsorption (see ! C / "+"-,, Table 1). It means that during the impregnation only negligible extent of ion-exchange took place, and b ,^ "/'~"..................... therefore,Br6nsted acidity was not p+~.+ formed during the reduction of Pt 2+ ions. fail ....... ..+,~.~ The Co,NaY-FAU zeolite has both Br6nsted and Lewis acidity. It is ........................... well-known that the reduction of Co 2+i ions is difficult in exchanged position 3800 3600 3400 3200 3000 of zeolites. This, and the low ionWevenumberO/can) exchange degree are responsible for the relatively low Br6nsted acidity. Figure 1. Infrared spectra of the OH region of The reduction of PtZ+-ions our catalysts; (a) Pt~aY, (b) Co,NaY, (c) takes place completely under the Pt/Co,NaY, (d) Pt,NaY. .............................................................................................................

930 experimental conditions applied. In this process acidic OH groups are formed as spectrum d in Figure 1 shows. Three absorptions were obtained at 3740, 3644 and 3548 cm "l. These are assigned, as a non acidic terminal SiOH, and two acidic, bridging OH groups sited in superand sodalite cages, respectively. The acidity of the Pt/Co,NaY-FAU bimetallic zeolite sample is similar to the Co,NaY-FAU (see data in Table 1), because the impregnated Pt-salt did not create too many new acidic OH-group.

3.2. Infrared spectroscopy

Trichloro-ethylene, (TCE) The gas phase spectra of TCE reacted over Pt~aY-FAU (impregnated) are depicted in Figure 2. The characteristic absorptions of the starting material are as follows: different C-C1 vibrations below 1000 cm 1, C-H deformation at 1250, C-H stretching at 3080 cm 1, and the band due to the C=C bond appeared at 1580 cm ]. It is clearly seen that hydrodechlorination reaction takes place with increasing temperature. In the spectrum recorded after heat treatment at 473 K characteristic vibration of HC1 centered at 2880 cm 1 appeared. At higher temperature the intensity of different C-C1 vibrations, C-H stretching and deformation vibrations, C=C vibration of TCE decreased and simultaneously, the intensity of a broad band at 3000 cm 1 and a smaller at 1500 cm 1 due to the C-H stretching and deformation vibration of ethane [8] increased. Formation of HC1 could also be followed.

I

,

i

'

'

i

L

i

/ h/

i!;

'

c

,,

~!/!,,~,...........

!,!.i ~j ........

b i . . . . . . . . r , " .........

ii

'

t

i ,J ~,

, ! ,I

h !i f~,,j i'

#

1

3600

3400

3200

3000

~umberO,~rn)

2800

2600

I t

........ "Ls ........:............ :..j:......... :, 1600

1400

1200

10O0

[

'

,~.

,:,.t,..': ;L..L-4 800

600

WIl~l:)l~l/c~nn)

Figure 2. Infrared gas phase spectra of trichloro-ethylene reacted over Pt/NaY-FAU; A) C-H stretching region, B) C=C, C-H deformation and C-C1 vibration region; (a) starting material, (b) after reaction at 473 K, (c) at 573 K and (d) at 673 K.

931 The increasing signal of HC1 on the high frequency side of the ethane band reveals the enhanced formation of HC1 with increasing temperature.

Tetrachloro-ethane, (TCA) The gas phase spectrum of TCA is similar to the spectum of TCE, however, the absorption of C=C bond is lacking. The hydrodechlorination takes place with increasing temperature over Pt~aY-FAU catalyst and the main products are ethane and HC1 as well. Figure 3 shows the spectra of TCA adsorbed and reacted on the self-supported wafer of Pt~aY-FAU. A broad band developed in the OH region probably as a result of the reaction of HC1 with the Na+-ion of the zeolite. A doublet with broad bands centered at around 2700 and 2400 cm l became clearly visible with increasing temperature. This is characteristic of the H-bonded interaction between the HC1 and acidic OH group on zeolite. There are several explanations for this phenomenon. According to Ozin et al. the band at 2700 cm l belongs to the X)OHstretching and the band at 2400 cm "l to the ~Ci-H stretching vibration in the O-H'"(C1-H)n structure [9]. By another explanation the reason of this phenomenon is a special Fermi resonance between a combination vibration (~oH + ngOH--El ) and an overtone (26on) [ 10,11 ].

~.~

: / :

/ i /'

i /

"-.,

",..,. "'~

W 0.2 s. With high-concentration feeds we can evaluate (at lower temperatures when conversion is small and the assumption for a differential reactor can be applied) activation energy of the order of 55 kJ/mol, i.e. definitely smaller than that measured for low-concentration feeds. This shows that, with these concentrated feeds, the pore diffusion step is kinetically not negligible at low temperature and can become determinant at higher temperatures. As a result of this, the best conversion results were in fact obtained in the 0.05-0.2 s contact time range with high concentration feed. We further investigated the contact time range above 0.2 s with high concentration TCE stream (10000 ppm), stoichiometric water vapour (1:2) and in the absence of oxygen (Fig. 4). Also in this case we observe a decrease of the conversion by increasing contact times in agreement with a dit~sion-limited regime, but we observe later a plateau in the conversion versus contact time curve. . L

v\

3 =

2

"-

1

o

A TCE 0.1%, tau

-,%

=1.2s X TCE1%,tau= 0.02 s

m

0 0.00

0.0012

0.0014

1/T r l i K 1

0.0016

Figure 3. Arrhenius plot of activation energies, TCE conversion over HY zeolite (stoichiometric water, excess oxygen). On the other hand, the comparison of Fig. 2 and 4 indicates that, in the range near x -~ 0.2 s, the presence of oxygen has a positive effect on the conversion, measured after 30 min on stream, i.e. when apparent steady state conditions are reached. Actually, in the absence of oxygen and 1% TCE concentration the reaction rate (conversion) declines with time, so that the points reported in Fig. 4 are to be taken with precaution. Experiments of conversion of 10000 ppm TCE with stoichiometric water but in the absence of oxygen and x = 0.13 s show that after 3 h the conversion declined down to 5 % but it stays later constant for 20 h. Accordingly, after even short runs in the absence of oxygen the catalyst is completely black while after runs in the presence of oxygen the catalyst is white or pale grey. This again suggests that coking is significant in particular in the absence of oxygen.

971 0 ........................................................................................................................................................................................................................... ~ 50 [] HY (TCE, noO2)

40

o FER (TeE + 02) xFER (TCE, noO2)

0

~

20

[]

[]

10

0.1

I

I

I

I

0.15

0.2

0.25

0.3

-clsl

0.35

Figure 4. TCE conversion over HY and FER zeolite at 823 K (stoichiometric water). Chlorinated by-products (i.e. tetrachloroethylene) have also been found in trace amounts at temperatures where conversion is not complete. Selectivity to CO is above 90%, the rest being CO2, and this could indicate that the water gas shift or the CO oxidation reactions occur at a low extent. To have a confirmation of the likely role of pore-diffusion in the behaviour of zeolite HY, we also performed some runs with the smaller pore zeolite H-FER. Note that TCE should not enter the cavities of the FER structure, so that we can suppose that in this case the pore diffusion should not affect conversion. In fact, we can see that in the same contact time range and in the same conditions, the conversion on H-FER increases by increasing contact time. So, at x ~ 0.13 s H-FER is much less active than HY, just because the external sites of the zeolite H-FER crystals are weaker as acids than the typical internal zeolite sites [7] and the available catalytic area is much lower if the internal zeolite surface cannot work. On the other hand, when the HY zeolite catalyst enters in the diffusion-limited region, the H-FER catalyst becomes more active than HY. So this behaviour strongly supports the conclusion that, with > 0.2 s and 10000 ppm TCE, pore-diffusion limited conditions are reached on the HY catalyst. 3.2. F T - I R studies.

FT-IR spectroscopy has been applied to have some characterisation of the catalyst and information on the reaction mechanism. In Fig. 5 the skeletal IR spectra of the fresh catalyst, of the catalyst alter catalytic runs with 10000 ppm of TCE in the reactor in excess oxygen and after the prolonged experiments performed in the absence of oxygen are reported. The spectrum of the fresh catalyst fully agrees with that of HY zeolite [8], and is dominated by the Si-O-Si asymmetric stretching modes (1056 cm1 and 1170 cm1, shoulder), the Si-O-Si symmetric stretching / bending modes (818 cm-1) and the modes due to Si-O-Si rocking (596, 513 and 456 cm-1). After runs in oxygen the spectrum presents only a slight modification with the appearance of a more pronounced band near 925 cm1. In the case of zeolites a band in this region can be assigned to Si-(OH) stretching, but, on the other hand, an assignment to C-CI stretching mode of transformation products of TCE with sp3 carbon atoms is also possible.

972 On the contrary, the spectrum recorded after prolonged runs performed in the absence of oxygen is deeply modified, in particular in the region of Si-O-Si stretchings where a sharp component at 1102 cm"1 appears together with a new shoulder near 1220 cm1. These new features can be due to Al-free silica-like particles that are in fact characterized by higher Si-OSi stretchings than Al-containing silicas or zeolites [8]. This is likely an evident effect of dealumination. It is consequently concluded that the deactivation at concentrated feeds is not only due to coking but also by simultaneous dealumination.

1.0i ro

0-8i

o co

< 0.4 1000 Wavenumbers (cm-t) Fig. 5. FT-IR spectra of flesh catalyst (a), after catalytic runs with 10000 ppm of TCE in the reactor in excess oxygen (b) and after the prolonged experiments performed in the absence of oxygen (c), all in KBr pressed disks. The FT-IR spectra of the pure catalyst powder after high temperature activation and TCE interaction at room temperature are reported in figure 6. After activation at 673 K, a main band is evident in the spectrum at 3741 cm1, due to the terminal silanol groups while at lower frequencies a complex absorption shows three maxima at 3688, 3624 and 3550 cm-1. The band at 3688 cm1 is generally assigned to OH stretching of extraframework groups, while the 3624 and 3550 cmq are due to the high frequency (HF) and low frequency (LF) bridging SiOH-A1 groups typical of the HY structure. This spectrum is typical for a high-Al content HY zeolite with some extraframework alumina. After trichloroethylene adsorption at room temperature bands due to physisorbed TCE are detected at 3085 cm1 (stretching CH) and 1583 cm1 (stretching C=C). The perturbation of these bands with respect to gas-phase IR bands (detected at 3098 cm~ and 1580 cm1, respectively) is not very significant and this indicates that the molecular interaction at room temperature is medium-weak. Outgassing at room temperature allows a progressive weakening of IR peaks, which disappear after 30 min in vacuum, thus indicating the disappearance of the molecularly adsorbed TCE. The interaction at room temperature is reversible and involves the OH groups, which are restored after evacuation at room temperature. All the OH's groups are involved, but those due to HF OH's and silanols are involved to a bigger extent, as evidenced in the subtraction spectra. The data show that a

973 medium strong interaction occurs with the OH stretching modes at the main HF zeolite cavity but also on the external crystal surface where terminal silanols are located.

0 25-

9 ~.. f .~,~k~

o.=. T J

"

Evacuatedat r.t.

~

~

1.01

Evacuatedat r.t.

oo,

o 00

4173O

ActivatedHY

~

~~-~

~--/~ ~)x -

-0.05 2OOO VVavenumbers (cm-1)

Fig. 5. FT-IR spectra of activated HY, and after adsorption of TCE and successive evacuation, in the enlargement: FT-IR subtraction spectra [surface + TCE] - [activated surface], OH stretching region After heating the catalyst in the presence of TCE gas at increasing temperatures new broad bands are detected at 1580 cm1 (with a shoulder at 1635 crn1) and at 1470 cm~. These bands are characteristic of carboxylate species formation. A likely assignment is to dichloroacetate species. By analysis the gas phase we can detect the typical sharp rotovibrational features of HC1 starting from 773 K. In these conditions the above cited carboxylate bands raise their maximum intensity, and decrease at immediately higher temperatures. 0.6 o tO ($)

4

v

> 40

100

8O

3~

PtBEA75(C) ~ : : ~ :-" PtBEA300(C) / / - - O - -PtBEA75(Y) / /

- - "A" - -

.9

200

~

400

0 200

250 300 350 Temperature (~

400

Fig. 2. PhC1 conversions (C) and PhClx yields (Y) vs. reaction temperatures over zeolite supported Pt samples.

1027

0.8 v

x r ,13_

~X~

0.6

.>_ oo

PtFAU5 PtBEA75

O -t-,

PtMFI30

0.4

PtFER20 0.2 0 60

70

80

90

100

Conversion (%)

Fig. 3. PhClx selectivity vs. PhC1 conversion levels over zeolite supported Pt samples (reaction temperature: 350~ It is important to underline that stability tests carried out in the 300-340~ range showed that under the experimental conditions used, all samples presented no measurable decrease in the activity during 10 hours of operations. It must be also reminded that on all supports alone the oxidation of PhC1 occurs at much higher temperatures (light-offs higher than 400~ than on the corresponding supported Pt sample.

3.2. Characterization of acid sites FT-IR of adsorbed pyridine and TPD of ammonia experiments were performed in order to study the acidic properties (nature and strength of acid sites) of zeolite supported Pt samples. Pyridine is widely used as a probe molecule for Lewis or Bronsted acidities [9]. The ring stretching vibrations are the most sensitive modes of pyridine with regard to the nature and strength of the adsorptive interaction. Bands at ca. 1640 cm1 and 1540 cm-1 are assumed to be characteristics for pyridinium ions (adsorption of pyridine on Bronsted sites), whereas bands in the region 1600-1630 cm-1 and 1440-1455 cm-1 are attributed to coordinatively adsorbed pyridine on Lewis sites. These latter bands increase in wavenumber as the strength of interaction increases and therefore gives an indication of the strength of Lewis acid sites [9]. Fig. 4 shows the FT-IR spectra in the region 1700-1400 crn-~ of zeolite supported Pt samples after admission of pyridine and subsequent evacuation at 250~ to eliminate the physisorbed pyridine. All samples exhibited bands characteristics of both Bronsted and Lewis acidity. However it appears clearly that H-ferrierite supported samples contain predominantly Bronsted acid sites and rather few Lewis acid sites. The ratio of Lewis to Bronsted sites increases moving from Pt/H-ZSM5 to Pt/H-beta to Pt/H-Y catalysts. From the figure it can be also noted that, according to the literature [10] on each type of zeolite the number of both Bronsted and Lewis sites strongly decreases on increasing the SiO2/Al203 ratio of the zeolite.

1028

PtFER20

_~.~L,.~

._....__..F~.~ _

,,-:,.

PtMFI30

v

O t-

PtMFI280

t-

,-, - PtMFI280

t/) (/) r t~

0 .Q

. 200

100

0 ~ 0

0.2

0.4

0.6

0.8

1

P/P0

Figure 1. Adsorption-desorption isotherms of N2 at 77K on MCM-41 (left), trimethylbenzeneswelled MCM-41 (middle), and SBA-15 (fight). Lowest closure point of the hysteresis loop at p/po 0.42.

1059

The passage from a dense phase to a low-density gas in conditions in which the liquid-gas interface is unstable is strongly reminiscent of the definition of supercritical phenomena. The shift of the critical point in capillary conditions was calculated in the early stages of the density functional theory (DFT) methods [18, 19]. Adsorption phenomena on MCM-41 have been recently investigated to ascertain at which extent the lowest limit of the hysteresis loop can be identified with the capillary critical point [20-22]. The lowest closure points of the hysteresis loop in adsorbents of different mesopore size at several temperature levels can be plotted by using reduced coordinates Pr = P/Pc and Tr = T/To. In a log Pr VS. 1/Tr graph, the points for each adsorbate are aligned on straight lines passing through the critical point T = Tc and P = Pc. This behaviour indicates that the lowest limit of the hysteresis loop can be described by a law of corresponding states [ 16].

13.. O

1.0

1.5

2.0

2.5

1/Tr Figure 2. Position of the lowest closure point of the hysteresis loop in reduced coordinates for several adsorbates and pores of various size (Data from [ 17]). From a practical point of view, these linear correlations allow to predict the lowest closure point of hysteresis for a given adsorbate at any temperature once a single lowest closure point has been determined. The lowest limit of hysteresis seems not to depend on the nature of the adsorbent, the data of Figure 2 having been obtained on adsorbents as different as silica, titania, and carbon. Very likely, the presence of one or more adsorbed layers between the surface of the adsorbent and the condensing phase smooths down the potential differences between the different solids. 3. METHODS OF ASSESSMENT OF THE POROSITY

Adsorption methods are the main tools to characterize the porosity of any kind of adsorbents. In the case of the ordered mesoporous adsorbents prepared by micelle-templated synthesis, other techniques can usefully integrate the evidences from adsorption methods. High-resolution transmission electron microscopy (TEM) has provided some spectacular breakthroughs in the knowledge of micelle-templated materials. For instance, the section of the pores of MCM-41 has been recognized to be hexagonal by TEM [23], the structure of SBA- 1 and SBA-6 have been solved by TEM and microdiffraction [24], and the connections between the mesopores of SBA-15 have been identified by TEM of platinum replicas [25].

1060 Notwithstanding these outstanding results on carefully selected samples, TEM is not a userfriendly technique to provide routine quantitative data. Beyond the inevitable problems of homogeneity of the sample, it has been shown that any evaluation of wall thickness by TEM methods can be severely affected by the focusing procedure [26]. X-ray diffraction can provide relevant information on the periodical properties of micelletemplated materials. In the case of well-ordered solids, several diffraction lines can be observed and allow to identify the symmetry group. Once the symmetry (usually hexagonal or cubic) is known, the cell parameter a can be easily determined from the diffraction angles. The cell parameter is correlated to the pore size. In the case of MCM-41, a = d + t, where d is the diameter of the hexagonal pore and t is the thickness of the wall between the pores. In the case of MCM-48, a = 2(d + t). If the diffraction angles provide useful information on the cell parameter, any quantitative exploitation of the intensity of the diffraction lines is much more difficult. The diffraction pattern of the ordered mesoporous materials do not allow to define the position and occupancy of individual atomic sites, but depends on the average contrast between empty and filled parts of the solid. The presence of residual template or any adsorbate inside the porosity decreases the contrast level and strongly affects the intensity of the diffraction lines [27]. The periodicity of micelle-templated silicas allow to use the results of X-ray diffraction to better understand the adsorption data. The main information on the pore size is provided by the pressure of the adsorption and desorption steps of the type IV isotherm. How to calculate a pore size from the experimental isotherm? Several models allow to calculate the adsorptiondesorption isotherm and X-ray diffraction data allow to independently evaluate the reliability of these methods. The plain geometry of the pores of MCM-41 allows to calculate the pore size from the cell parameter a and the mesopore volume Vp, independently on the pressure of the isotherm step [28]. These values of diameter Dvmes+x~ are reported in Figure 3 for several samples of MCM-41. 100

-

~ 80 ~< 60 .,,.~

o

,J,

/

_! / D B d B

#r-/

"x

DDvr 3"

~ o 40

f

DBJH

J

20

I

/

S

s

9 D Vmes+XRD t:l 4V/S*

0(}.0. . . .0.2. . . . 0.4 . . . . 0.6 . . . . . 0.8 . .

p/p0

1.0

Figure 3. Comparison among several methods to evaluate the pore diameter of MCM-41 as a function of the relative pressure of the desorption step of N2 at 77 K [28]. The widely used BJH (Barrett, Joyner and Halenda) method, which applies the Kelvin equation to the calculation of the pore size, patently underestimates the pore diameter. Better results are obtained by the method of Broekhoff and de Boer [29], which modifies the Kelvin equation to

1061 take into account the interaction with the curved surface of the adsorbent. The DFF methods also provide a good evaluation of the pore diameter. It can be observed that the region of the smallest mesopores should be outside the limits of validity of the Kelvin equation: the instability of the meniscus for pressure below the lowest limit of the hysteresis seems to render meaningless any calculation based on the curvature of the meniscus. However, the method of Broekhoff and de Boer provides also in this pressure range a fair evaluation of the pore diameter at which desorption takes place. The usual Gurvitch method, in which the equivalent hydraulic diameter of the pore is calculated from the ratio between pore volume and surface area, severely underevaluates the pore diameter. The drawback of the method does not come from the geometry of the system but from the use of an inadequate value for the area corresponding to an adsorbed N2 molecule. The routine value of 16.2 ~2 for N2 molecule is calculated from the density of liquid nitrogen and has been shown to be valid for the adsorption on an hydrocarbon-lined surface. In the case of the adsorption on a silica surface, a more appropriate value is 13.5 ]k2 per molecule [30]. If this value is used in the calculation of a surface area S* from the volume of the BET monolayer, the formula D = 4V/S* provides a correct estimate of the pore diameter. 4. INFLUENCE OF THE PORE GEOMETRY ON THE PROPERTIES OF THE ADSORBENT

The model of the hexagonal honeycomb of MCM-41 can be used to study the correlations among geometry-dependent properties of the solid [31]. The surface area Sg (m2 g-l) of a perfect honeycomb is a function of the cell parameter a (nm) and the wall thickness t (nm): Sg = 4103 (a-t) / psit (2a-t) 2000 1800 [ 1600 1400 ~

1200

~ 1000 -% 800 600 400 200 0

2

4

6

8

10

a(nm)

Figure 4. Surface area of a hexagonal silica honeycomb as a function of cell parameter and wall thickness [31].

1062 In Figure 4 are reported the surface areas calculated from the geometry of the honeycomb taking into account the volumic mass of amorphous silica psi 2.2 g cm "3. For each value of wall thickness, a very low surface area is expected when the cell parameter is only slightly higher than the wall thickness. For more realistic honeycomb with a >> t, the surface area only depends on t and goes towards an asymptotic value Sg = 2103 / psit The pore volume Vf of the hexagonal honeycomb can be calculated by the same model as (a-t) 2 Vf-PsiXtx(2a-t ) 4.5 4 3.5 3 "7

2.5 o

x.j

2 1.5 1 0.5 0 2

4

6

8

10

a(nm) Figure 5. Pore volume of a hexagonal silica honeycomb as a function of cell parameter and wall thickness. The calculated volumes are reported in Figure 5 as a function of cell parameter a and wall thickness t. Starting from a = t, the pore volume increases about linearly with the cell parameter by a slope which increases when the wall thickness becomes smaller. These estimations of the properties of perfect silica honeycombs allow to evaluate at which point experimental solids correspond to the ideal geometry. Differences between calculated and experimental porosity have been at the basis of the characterization of the microporosity of SBA- 15 [32]. An abacus of the properties of MCM-41 as a function of the cell parameters can also orient the choice of the proper material for a given application. Such a choice is always the result of a compromise between conflicting properties. For instance, thinner walls allow to encapsulate a larger volume of fluid phase by using a smaller amount of confining solid. As a consequence, it should be desirable to use an adsorbent with the thinnest possible walls. A lowest limit to wall thickness is imposed by the stability of the solid.

1063 As an example, mechanical stability can be calculated for cellular solids. The crushing strength ar for the brittle failure of a hexagonal honeycomb upon in-plane uniaxial loading is ar162 = 4/9 (t/L) 2 where ae is the crushing strength of the bulk material (7.2 GPa for vitreous silica), t is the wall thickness and L is the side of the hexagonal cell [33]. This correlation, which becomes a_~=~x t2~ a c ~ (a-t) ~ if the cell parameter a is used instead of the cell side L, has been compared with experimental data on the stress-strain relationship of MCM-41, and has be found to slowly underestimate the actual strength [34]. The difference between experiment and calculation was probably due to a significant component of axial loading in the powder bed used for testing. It seems reasonable to assume that the calculated strength of the honeycomb provides a conservative evaluation of the strength of ordered MCM-41.

2.5

0nm

2 -~ 1.5 1 0.5

t-05 nm-.___ 0

2

4

-----___ 6

8

10

a (nm) Figure 6. Crushing strength of a hexagonal silica honeycomb upon in-plane uniaxial loading as a function of cell parameter and wall thickness. In Figure 6, the calculated strength of hexagonal silica honeycombs are reported. The crushing strength decreases when the unit cell becomes larger, as faster as the walls are thinner. It is clear that, for a given unit cell, any decrease of the wall thickness brings about a decrease of mechanical stability. Thermal stability has also been shown to be significantly worsened by a decrease of wall thickness [34]. The tailoring of the properties of the solid and their relevance to the catalytic behaviour have been recently reviewed [35, 36].

1064 5. T E M P L A T E EFFECT IN THE SYNTHESIS

The template effect in zeolite synthesis has often been described as the organisation of silica units around an organic molecule to form a structure which can retain the memory of the shape of the template after its extraction. The generality of this model, developed on a limited number of successful examples, has been questioned by the synthesis of very large-pore zeolites, VPI-5 and cloverite, with no direct correlation between the size and shape of the small template molecules and the large voids of the pore system. The need for a simultaneous organisation of the template and the silicate units was already clear in the formation of inorganic-templated zeolites, in which silicate units enter the coordination sphere of hydrated cations. It seems more and more clear that only in a limited number of instances templated synthesis corresponds to the condensation of network-forming units around a pre-existing template. In most cases, inorganic units assemble together with the template molecules in aggregates quite different from the state of the template alone. This effect of self-assembly was early shown for materials at the borderline between micropores and mesopores, formed by aggregation of small organic molecules [37], and confirmed by the formation of ordered silica-alkyltrimethylammonium mesophases well below the concentration threshold for the formation of surfactant mesophases in the absence of silica [38]. The mechanism of self-assembly of ordered mesoporous materials has been largely elucidated by the charge matching effect in the case of cationic surfactants [4, 7]. When nonionic surfactants are used as templates, the formation of ordered materials at low charge concentration is more difficult to model. A good example is provided by the complex synthesis of SBA-15, a well-ordered hexagonal mesoporous silica formed in the presence of polyethylene oxide (PEO)-polypropylene oxide (PPO) triblock copolymers [8]. The micelles of nonionic surfactant in solution are surrounded by a corona of hydrated PEO chains protruding from the micelle surface. The repulsion between these organic brushes keeps the micelles at a minimum distance of 3-4 nm [39]. PPO

Figure 7. Schematic representation of micelles of polyethylene oxide (PEO)-polypropylene oxide (PPO) triblock copolymers. A rise in temperature decreases the hydration of the PEO chains and the repulsion forces between micelles. SBA-15 is formed by a long low-temperature reaction of the surfactant with silica issued from the hydrolysis of tetraethyl orthosilicate. The solid formed presents a well-ordered mesoporosity between thick walls containing a disordered microporosity [32]. When this solid

1065 is treated at higher temperature, the mesopore size increases and the microporosity disappears. This behaviour is only partially comparable with the behaviour of the surfactant in the absence of silica. At high temperature, protruding PEO chains are indeed replied upon micelles also in the absence of silica, and the decreased repulsion between micelles is at the basis of the cloud point phenomena, but the increase of pore size with temperature seems much larger than the micelle swelling observed in the absence of silica [40]. It seems that, in the synthesis of mesoporous materials by using nonionic surfactants as templates, a step of impregnation by silica of a preexisting mesophase can compete with cooperative changes of the silica-surfactant system when temperature conditions are changed. The complexity of the formation mechanism hinders the prediction of the properties of the final material. ACKNOWLEDGMENT

The authors gladly acknowledge Edoardo Garrone for useful discussions on the physical chemistry of adsorbed phases. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147. F. Schiith, Chem. Mater. 13 (2001) 3184. A. Monnier, F. Schtith, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Krishnamurty, P.Petroff, A. Firouzi, M. Janicke, B.F. Chmelka, Science 261 (1993) 1299. A. Firouzi, D. Kumar, L.M. Bull, T. Besier, P. Sieger, Q. Huo, S.A. Walker, J.A. Zasadzinski, C. Glinka, J. Nicol, D. Margolese, G.D. Stucky, B.F. Chmelka, Science 267 (1995) 1138. A. Galarneau, F. Di Renzo, F. Fajula, L. Mollo, B. Fubini, M.F. Ottaviani, J. Colloid Interface Sci. 201 (1998) 105. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schiith, G.D. Stucky, Nature 368 (1994) 317. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, G.D. Stucky, J. Am. Chem. Soc. 120 (1998) 6024. G.A. Ozin, A. Kuperman, A. Stein, Angew. Chem. Int. Ed. Engl. 28 (1989) 359. L. Chen, P.J. Klar, W. Heimbrodt, F. Brieler, M. Fr/3ba, H.A. Krug von Nidda, A. Loidl, Physica E 10 (2001) 368. C.G. Wu, T. Bein, Science 264 (1994) 1757. F. Marlow, M.D. McGehee, D. Zhao, B.F. Chmelka, G.D. Stucky, Adv. Mater. 11 (1999) 632. J. Rathousky, A. Zukal, O. Franke and G. Schulz-Ekloff, J. Chem. Soc. Faraday Trans. 91 (1995) 937. J. Janchen, H. Stach, M. Busio, J.H.M.C. van Wolput, Thermochim. Acta 312 (1998) 33. P.L. Llewellyn, F. Schiith, Y. Grillet, F. Rouquerol, J. Rouquerol and K.K. Unger, Langmuir 11 (1995) 574. F. Di Renzo, E. Garrone, A. Galarneau, P. Trens, N. Tanchoux, D. Brunel, B. Fubini, F. Fajula, submitted.

1066 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

C.G.V. Burgess, D.H. Everett, J. Colloid Interface Sci. 33 (1970) 611. H. Nakanishi, M.E. Fisher, J. Chem. Phys. 78 (1983) 3279. R. Evans, U. Marini Bettolo Marconi and P. Tarazona, J. Chem. Phys. 84 (1986) 2376. P.I. Ravikovitch, S.C. O' Domhnaill, A.V. Neimark, F. Schiith, K.K. Unger, Langmuir 11 (1995) 4765. K. Morishige, M. Shikimi, J. Chem. Phys. 108 (1998) 7821. S. Gross, G.H. Findenegg, Ber. Bunsenges. Phys. Chem. 101 (1997) 1726. V. Alfredsson, M. Keung, A. Monnier, G.D. Stucky, K.K. Unger, F. Schtith, J. Chem. Soc. Chem. CommurL 1994, 921. Y. Sakamoto, M. Kaneda, O. Terasaki, D.Y. Zhao, J.M. Kim, G. Stucky, H.J. Shin, R. Ryoo, Nature 408 (2000) 449. Z. Liu, O. Terasaki, T. Ohsuna, K. Hiraga, H.J. Shin, R. Ryoo, ChemPhysChem (2001) 229. S. Schacht, M. Janicke, F. Schtith, Microporous Mesoporous Mater. 22 (1998) 485. B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Microporous Materials 6 (1996) 375. A. Galarneau, D. Desplantier, R. Dutartre, F. Di Renzo, Microporous Mesoporous Mater. 27 (1999) 297. J.C.P. Broekhoff, J.H. de Boer, J. Catal. 10 (1968) 377. L. Jelinek, E. s. Kov~its, Langmuir 10 (1994) 4225. F. Di Renzo, D. Desplantier, A. Galarneau, F. Fajula, Catal. Today 66 (2001) 75. A. Galarneau, H. Cambon, F. Di Renzo, F. Fajula, Langmuir 17 (2001) 8328. L.J. Gibson, M.F. Ashby, Cellular Solids: Structure and Properties, 2nd ed., Cambridge 1997. A. Galarneau, D. Desplantier-Giscard, F. Di Renzo, F. Fajula, Catal. Today 68 (2001) 191. F. Di Renzo, A. Galarneau, P. Trens, F. Fajula, in Handbook of Porous Materials, F. Schiith, K. Sing, J. Weitkamp (Eds.), Wiley-VCH, 2002, 1311. D. Trong On, D. Desplantier-Giscard, C. Danumah, S. Kaliaguine, Appl. Catal. A 222 (2001) 299. G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde-Massara, G. Perego, Stud. Surface Science Catal. 84 (1994) 85. C.F. Cheng, Z. Luan, J. Klinowski, Langmuir 11 (1995) 2815 J.N. Israelachvili, H. Wennerstrtim, J. Phys. Chem. 96 (1992) 520 C. Booth, D. Attwood, Macromol. Rapid Commun. 21 (2000) 511.


1067

Anchorage of dye molecules and organic moieties to the inner surface of SiMCM-41 Yven Rohlfing a, Dieter W0hrle a, Ji[i Rathousl@ b, Arno~t Zukalb and Michael Wark c a Institute of Organic and Macromolecular Chemistry, University Bremen, D-28334 Bremen, Germany. b j. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, CZ-182 23 Prague 8, Czech Republic. c Institute &Physical and Electrochemistry, Universitfit Hannover, D-30167 Hannover, Germany. Based on siliceous MCM-41 material obtained according to a pH dependent method of homogeneous precipitation host-guest-systems with covalently gra~ed organic moieties, especially chromophores, were prepared. The highly ordered mesoporous material does not suffer from the multi-step procedure including passivation of surface silanol groups, functionalization with 3-aminopropyltriethoxysilane and subsequent anchorage of dye molecules and anhydrides via peptide, sulfonamide and anhydride imide bonds. The present work gives a survey on the incorporation of anhydrides and chromophores of four different types and proves their uniform distribution in the host pore system as well as their diffusion stability by different optical spectroscopy methods and physisorption studies. 1. INTRODUCTION Incorporation of organic moieties into porous inorganic solids results frequently in strong changes of the physicochemical properties of both, host and guest material. The embedding of larger organic molecules into intact zeolite structures by diffusion is restricted by the small pore diameters of the host; crystallization inclusion of guest species may conduct to lattice defects [ 1]. Since BECK et al. closed the gap between microporous and macroporous materials by introducing the mesoporous molecular sieves of the M41S family in 1992 [2], these limitations in the accessibility could be overcome. The wide pore opening and narrow pore size distribution of the channels open the way for manifold inclusion chemistry [3], e.g. covalent grafting of silane precursors followed by covalent or ionic anchorage of organic moieties. The tunable pore size and the opportunities for modification of interior channel walls of the highly ordered Si-MCM-41 lead to tailor-made materials with higher mechanical stability and hydrophobicity for catalysis and adsorption [4]. In the present communication a multi-step procedure for covalent anchorage of dye molecules is described. The definitive covalent graffing was lead out via peptide, sulfonamide or anhydride imide bonding.

1068 The properties of structure and surface of the organically modified materials were investigated by different analytic methods like N2 adsorption measurements or UV/VIS and IR spectroscopy in diffuse reflectance. The homogeneous distribution of the chromophore molecules was also proved by confocal fluorescence microscopy.

2. EXPERIMENTAL SECTION 2.1 Synthesis of siliceous MCM-41 and pre-silylation of its external surface The Si-MCM-41 host was synthesized by the homogeneous precipitation method using sodium metasilicate as silica source and cetyltrimethylammonium bromide as structuredirecting agent [5]. All the silylation reactions, described in the following, were carried out with freshly dried Si-MCM-41 material under nitrogen atmosphere. The solvents were distilled over desiccants under inert gas. In order to passivate the external surface 1 g of Si-MCM-41 was given in a flask and evacuated for 2 h at 10.3 mbar. Subsequently, the flask was filled with nitrogen. The sample was suspended in 30 mL dry tetrahydrofurane (THF) and 300 ~L of diphenyldichlorosilane (Ph2SiC12) were added under stirring. After 45 min the solid was filtered and extensively washed with THF and dichloromethane. Afterwards it was dried in a heating box and evacuated (10 .3 mbar) for 2 h. 2.2 Functionalization of the inner surface of Si-MCM-41 The functionalization was performed by anchoring varying amounts of 3-aminopropyltriethoxysilane (APTES) at the silanol groups of the inner pore walls. As solvent either dichloromethane or toluene were used depending on the desired reaction temperature. A detailed description of the reaction conditions has been given previously [6]. 2.3 Covalent bonding of dyes and anhydride functions The dyes 4'-dimethylaminoazobenzene-4-carbonic acid 1, Zn phthalocyanine tetrasulfonylchloride acid 3a and Si phthalocyanine tetrasulfonylchloride 3b (ZnPcTSCI) were prepared in our laboratory. RhodamineB sulfonylchloride 2 (FLU~) and 1-ethyl-l'-[hexanoicacid N-succinimide ester] indodicarbocyanine 6,6'-disulfonic acid 4 (AMERSHAM PHARMACL~) are commercial available. The inserted amounts of dyes are related to the amounts of parent Si-MCM-41 used. For the anchoring the dyes 1 - 3b the procedure was as follows. A distinct amount of 3-aminopropylsilyl-Si-MCM-41 (usually 0 . 5 - 0 . 7 g) was mixed in a flask with a desired quantity of the dye (0.001 - 1 mmol), dried under N2 atmosphere and suspended in 30 mL dichloromethane. The suspensions were pre-cooled with an ice/NaC1 mixture and stirred for 1.5 h. For the activation of the dyes either dicyclohexylcarbodiimide (DCC), dissolved in dichloromethane, or pyridine were added as reaction promoters. The reaction mixture was allowed to reach ambient temperature. After 20 h of stirring the recovered solid was extensively washed, subjected to a Soxhlet treatment for 3 d and dried in a heating box (see chapter 2.1).

1069 In case of 1-ethyl-l'-[hexanoic acid N-succinimide ester] indodicarbocyanine 6,6'-disulfonic acid (Cy5) 4 series of Si-MCM-41 samples containing extremely low dye concentrations were synthesized in order to perform single molecule detection (see below). To 1 g Si-MCM-41, silylated with 5 mmol of APTES, alcoholic solutions containing 4.7-10 -3- 4.7.10 -9 mmol of 4 were added. Then the reaction mixtures were stirred for 20 h in the absence of light. The obtained solids were vigorously washed with ethanol and subsequently treated in a Soxhlet apparatus. For the anchoring of the maleic anhydride 5 and phthalic anhydride 6, aiming the application of the anhydride group as potential coupling function for the attachment of diarylethene dyes, each 5 mmol were added to suspensions of aminopropylsilyl-functionalized Si-MCM-41 in ethanol. The reactions were stopped after heating overnight under reflux and the obtained functionalized solids were manipulated as described before. 2.4 Characterization Measurements of nitrogen isotherms at -196 ~ C on reference silica gels and on Si-MCM-41 samples were performed on a MICROMERITICS ASAP 2010 volumetric adsorption instrument. Before the measurements purely siliceous samples were degassed at 300~ for 24 h; the chemically modified samples were degassed at 100 ~ C for 48 h. XRD patterns were recorded on a PHILn,S X'pert Alpha 1 diffractometer. Diffuse reflectance UV/VIS spectra were obtained with a PERKIN-ELMER Lambda 9 spectrometer. The samples were also examined by DRIFT (diffuse reflectance infrared fourier transform) spectroscopy carried out with an BIO-RAD FTS-60A instrument equipped with a praying mantis (HARmCK) and a sealed sample holder with a vacuum system giving a base pressure of 10-8 bar. Single molecule detections via confocal fluorescence micrographs were carried out with a CARL ZEISS UMSP 80 microscope spectral photometer.

3. RESULTS AND DISCUSSION The surface silanol groups of the Si-MCM-41 parent material were functionalized with 3-aminopropyltriethoxysilane precursors. For grafting dyes and organic moieties to the amino groups peptide (a), sulfonamide (b) or anhydride imide bonding (c) were employed (Scheme 1). Activation is necessary in case of carbonic acid groups, e.g. by addition of dicyclohexylcarbodiimide (DCC) or by preparation of the anhydride, respectively. Sulfonic acid groups were activated by chlorination and further reaction in presence of pyridine. The outer surface of Si-MCM-41 particles and the most reactive silanol sites were blocked in a pre-silylation procedure with diphenyldichlorosilane under mild conditions in absence of promotors [7] to ensure the exclusive functionalization of the inner surface in the third step.

o -~- ~ O ~ s i ~

o ~N/x~r~

----~0 ~--O~si~Ss~ -~/

b

O~k//O ~

k"-~

--

0

~ ~ ~ 0/

o c o/~~

Scheme 1. Different methods for covalent grafting on amino-functionalized silica supports.

1070 Recent investigations designate that post-synthetic grafting also allows the embedding of single molecules in homogeneous distribution in lowest concentrations without any preliminary treatment (see chapter 3.2). It could be demonstrated that every synthesis step can be detected by DRIFT spectroscopy and XRD patterns [6]. The parent silica and pre-silylated materials are showing the characteristic reflections (100, 110, 200, 210) in X-ray diffraction patterns. The peak intensities of the diphenyldichlorosilane treated samples are slightly decreased. Due to the modification of the inner pore walls the intensity of reflections, especially of the 110-, 200- and 210-peaks, considerably decreases and the relative intensity of the 200-peak increases. The negligible changed d spacings confirm the intact structure. The chemical modification of silica surfaces results in several changes in the DRIFT spectra. The sharp absorption band at 3745 cm-1 and the broad band down to 2500 crn1 are attributed to the free silanol groups and hydrogen-bonded silanol groups, respectively [8]. The pre-silylation results in a slight increased intensity of the band of free silanol groups. Effected by further functionalization of the pore walls the band at 3745 crn-~ disappeared while the broad band ascribed to residual silanol groups shiRed to lower wavenumbers. The occurrence of characteristic peaks is indicating the organic modification, e.g. 3365 cm1, 3305 cm-~ (N-H), 2975 cm~, 2935 cm1, 2895 cm1 (C-H) for aminopropylsilane moieties [9]. Peaks in the fingerprint region are signifying the presence of amine, amide and imide functions.

i

CH3--....~

S02CI

/

ClO2S

\ ~

-

.~3

o

~

,~SO3

~=

1

0

4

!

k~'CH3

Scheme 2. Multi-step procedure for anchoring dye molecules: pre-silylation, functionalization of inner surface, grafting of 4'-dimethylaminoazobenzene-4-carbonic acid 1, rhodamine B sulfonylchloride 2, Si/Zn phthalocyanine tetrasulfonylchloride 3 and 1-ethyl-l'[hexanoic acid N-succinimide ester] indodicarbocyanine 6,6'-disulfonic acid (Cy5) 4.

1071 Scheme 2 shows the incorporated dyes coupled with the amino-functionalized interior surface of Si-MCM-41. Resulting from host-guest interactions and confining pores the inserted dyes exhibit a higher organization of molecular dipoles (see chapter 3.2) and alteration of UV/VIS spectra in comparison to measurements in solution.

3.1 Anchorage of azo 1_, rhodamine dye 2 and phthalocyanine dyes 3

~l~' " ii

!

I a

I -" I '~ I o" .,/,,"

,

I

, :

I t

~ . " ",

t

I

I

1

"

:'

/

500

',b ~

600

700

800

Wavelength /nm Figure 1. Diffuse reflectance UV/VIS spectra of covalently grafted SiPcPTSC1 (a, solid), covalently grafted ZnPTSC1 (b, dashed) and ionically anchored ZnPTS (c, dotted), 0.005 mmol dye offered/g.

The incorporation of dye molecules into the channel system of Si-MCM-41 conducts to a shift of the absorption bands in UV/VIS reflectance spectroscopy. The main absorption bands of azo dye 1 and rhodamine dye 2 are blue-shifted from 468 nm, measured in transmittance in aqueous solution (pH 7), to 441nm and from 565nm to 553 nm, respectively. It could be shown previously [6] that this strongly depends on the changed chemical environment in the modified pores containing residual basic amino-functions and silanol groups. Reference experiments with adsorbed azo dyes in modified and parent material indicate a strong red-shift in case of the pure Si-MCM-41 and only negligible differences of absorbanee for amino-modified materials. For providing strong fluorescence the presence of individual chromophore molecules is necessary. In case of anchored rhodamine dye the fluorescence maximum was found at 0.0075 mmol 2/g Si-MCM-41. It is claimed out that the aggregation of dyes is mostly inhibited at this low concentration.

Figure 1 represents spectra of anchored phthalocyanines in the Q-band region. The main absorption bands at higher wavelengths are attributed to monomers whereas the shoulder peaks at ~ ~ 600 nm are caused by excitation of dimers. In opposite to SiPc, with silicone coordinated by two OH-groups, ZnPc derivates tend to aggregate. Thus, spectra of SiPcTSC1 3b (a, main absorption ~ ~ 664 nm) indicate a mostly monomeric incorporation and a blue-shift in comparison to spectra from organic solution in DMSO (~ ~ 676 nm). Covalent anchoring of ZnPcTSC1 3a enhances the aggregation, confirmed by a broadening of the band at ~, ~ 677 nm (b). Recently, it was demonstrated that the increased aggregation in the constrained space of mesopores can be avoided by ionic anchorage of ZnPTS in the pores of Si-MCM-41 functionalized with trimethylammonium silane precursors (c, ~ ~ 667 nm) [ 10].

1072

3.2 Anchorage of dicarbocyanine dye 4_ Dicarbocyanine dyes are mainly used as slow-response potentiometric fluorescence dyes and fluorescent labels in life science. Cy5 dye 4 exhibits a maximum of absorption at ~, = 650 nm while the fluorescence maximum is found at ~, = 670 nm. Due to its 5-carbon alkyl chain between the indolenine units it is highly fluorescent and shows a satisfying photostability. Thus the dye is applicable for the detection in confocal fluorescence microscopy at low excitation energy. For linkage onto amino-functionalized supports 4 is activated by a N-succinimide ester. Figure 2. x-y-Scan of Cy5 covalently grafted to Si-MCM-41. (a) 4.7.10 -6 mol, (b) 9.4-10 .8 mol and (c) 4.7-10 -1~mol Cy5 4 offered per 1 g siliceous parent material. Si-MCM-41 particles fixed by embedding in poly(methyl methacrylate) (Figure 2) were scanned with laser light (~,ox= 633 nm, 1.2 ~tW). The excitation laser beam reflected by a dichroic mirror was focused by a lens (microscope objective) to a diffraction-limited spot in the sample. The emitted fluorescence light passes through the same optical dements and a pinhole [ 11 ]. The detected volume element has a height of 0 . 6 - 0.8 ~tm and 0.3 ~tm diameter waist. Figure 2 shows x-y-scans of particles with different dye concentrations. The necessary high dilution of chromophores can be obtained by photobleaching of particles with higher dye concentrations (a, b) or direct offer of very small amounts of activated dye (c). Thus, the method proves the graining of single chromophores onto the internal pore walls of the host material. Almost every detected single molecule followed a preferential orientation along the z-axes of the Si-MCM-41 particles indicating a strict homogeneity of the incorporation and hexagonal texture [ 12].

3.3 Anchorage of maleic anhydride 5 and phthalic anhydride 6 Aiming the prospective anchoring of ethene moieties of optical switchable diarylethene dyes also the reactions of maleic anhydride and phthalic anhydride with amino-functionalized Si-MCM-41 were carried out. Figure 3 presents the nitrogen adsorption isotherms on samples obtained by two-step grafting procedures. The isotherms on modified samples functionalized with APTES and subsequently gratted with anhydrides manifest the influence of the chemical modification causing decreases in the surface areas of mesopores and mesopore volumes (Table 1). The bigger organic moiety (phthalic imide) conducts to a stronger decrease of the pore diameter. This signifies that a high degree of the anhydride is anchored by imide bonding. The isotherms on all samples were processed by the method of comparison plots up to relative pressure p/po = 0.8 (Figure 4) [ 13]. As reference data the isotherms on unmodified and APTES-modified macroporous silica DAVISIL were used. All the comparison plots are

1073 30

30

25 :0

25 etl) ~z 20

.~~ 15

.~~ 15

10

10

5

5

,.e~ i

"~

0 0,0

012

014

016

0

0,8

p/po Figure 3. N2 adsorption isotherms on Si-MCM-41 (o), APTES-functionalized ([], +2), maleic anhydride-grat'ted (A, +4) and phthalic anhydride-grained (V, +6) Si-MCM-41.

0

5

10

1'5

20

25

are f / p,m O1 m-2 Figure 4. Comparison plot for Si-MCM-41 (o), APTES-functionalized (D, +2), maleic anhydride-gra~ed (A, +4) and phthalic anhydride-gra~ed (V, +6) Si-MCM-41.

characterized by two linear parts. The first one corresponds to the formation of a monolayer and the beginning of multilayer adsorption. The linear fit goes through the origin and its slope gives the total surface area Stot. The subsequent steep increase is caused by the capillary condensation of nitrogen in the mesopores. The decrease of external surface area S~t of chemically modified samples, determined from the second linear part (plateau), is probably caused by a loss of the smallest particles in the course of preparation (Table 1). The geometric diameter of mesopores calculated by the equation Dme = 4 Vme)/Smemarkedly decreases with increasing amount of organic species on the mesopore surface. It is obvious that the coveting of the mesopore surface with organic compounds causes a certain narrowing of the pores. However, the calculated values of Dmo of samples with imide bonding seem to be unrealistically small. Since the decrease in mesopore volume is more pronounced than the decrease in mesopore surface, additional phenomena must be taken into account. Organic species can cause an enhanced roughness of the host surface, which manifests itself in some increase in the mesopore surface and in a decrease of geometric diameter Dm,, consequently. Table 1. Texture parameters from comparison plots of adsorption isotherms, i.e. total (Stot), external (S~) and mesopore surface area (Sine), mesopore volume (Vmo)and diameter (Dine). Stot [me8"1] Soxt[m2g"1] Si-MCM-41, parent APTES-functionalized Maleic imide Phthalic imide

1082 819 770 716

228 111 94 92

Smo [m~g -1]

Vine[cm38"1]

Dme [nm]

854 708 676 624

0.682 0.451 0.357 0.310

3.19 2.55 2.11 1.99

1074 4. CONCLUSIONS Mesoporous Si-MCM-41 was utilized as a host for monomeric grafted chromophores. Covalent anchorage of the organic guests was obtained in a multi-step post-synthetic treatment of the silica. It was proved, especially by XRD measurements and the construction of comparison plots of N2 adsorption isotherms, that there is no damage of the pore texture due to steps of surface modification. Optical methods for solid-state measurements like DR/FT spectroscopy, diffuse reflectance UV/VIS and fluorescence spectroscopy signify the strong interaction of grafted dye and the host material. Especially confocal fluorescence microscopy is a powerful tool not only for detecting homogenous distribution and orientation of chromophores but also proving the high order of the molecular sieve.

5. A C K N O W L E D G E M E N T

The authors gratefully acknowledge funding from Deutsche Forschungsgemeinschaft (DFG) (WO 237/16-3). We thank C. Br~iuchle, E. Kneuper and C. Seebacher (Department of Chemistry and Pharmacy, LMU Munich) for the confocal fluorescence microscope measurements.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

D. Wrhrle, G. Schulz-Ekloff, Adv. Mater. 6 (1994) 875. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. K. Mrller, T. Bein, Chem. Mater. 10 (1998) 2950. X.S. Zhao, G.Q. Lu, X. Hu, Micropor. Mesopor. Mater. 41 (2000) 37. (a) G. Schulz-Ekloff, J. Rathousk~, A. Zukal, Microporous Mesoporous Mater. 27 (1999) 273; (b) J. Rathousk~, M. Zukalova, A. Zukal, J. Had, Collect. Czech. Chem. Commun. 63 (1998) 1893. Y. Rohlfing, D. Wrhrle, M. Wark, G. Schulz-Ekloff, J. Rathousl~, A. Zukal, Stud. Surf. Sci. Catal. 129 (2000) 295. D.S. Shephard, W. Zhou, T. Mashmeyer, J.M. Matters, C. L. Roper, S. Parsons, B.F.G. Johnson, M. J. Duer, Angew. Chem. 110 (1998) 2847. X. S. Zhao, G. Q. Lu, A. K. Whittaker, G. J. Millar, H. Y. Zhu, J. Phys. Chem. B 101 (1997) 6525. X. S. Zhao, G. Q. Lu, J. Phys. Chem. B 102 (1998) 1556. Y. Rohlfing, O. Barrels, D. WOhrle, M. Wark, 14. Deutsche Zeolith-Tagung, Book of Abstracts, 2002, Frankfiart. W.P. Ambrose, P.M. Goodwin, J.H. Jett, A. Van Orden, J.H. Werner, R.A. Keller, Chem. Rev. 10 (1999) 2947. C. Br~iuchle, Y. Rohlfing, C. Seebacher, D. Wrhrle, paper in preperation. M. Jaroniec, M. Kruk, J.P. Olivier, Langmuir 15 (1999) 5410.


Mesocellular

aluminosilicate

foams

1075

(MSU-S/F)

and

large

pore

hexagonal

m e s o s t r u c t u r e s ( M S U - S / H ) a s s e m b l e d from zeolite seeds: h y d r o t h e r m a l stability and properties as c u m e n e cracking catalysts Yu Liu and Thomas J. Pinnavaia* Departmem of Chemistry, Michigan State University, East Lansing, MI 48824-1322

Mesostructured aluminosilicate foams (pore size > 20 nm) and very large pore hexagonal SBA-15 analogs (pore size >8 nm), denoted MSU-S/F and -S/H, respectively, were assembled from aluminosilicate nanoclusters that seed the crystallization of zeolites Y, ZSM-5 and Beta. Although the zeolite seeds are nucleated under basic pH conditions and the mesostuctures are assemble at acidic pH, the results of hydrothermal stability tests in steam at 800~ and in boiling water for 250 h reveal that MSU-S/F and MSU-S/H aluminosilicates are much more hydrolytically stable than their A1-MCF and A1-SBA-15 analogues. 27A1 MAS NMR spectra indicate that calcined MSU-S/F and MSU-S/H mesostructures retained between 80 and 95% of the aluminum centers in tetrahedral framework sites at an overall Si/A1 ratio of 50. Both mesostructures showed high activity for acid - catalyzed cumene cracking at 300 ~ 1. INTRODUCTION Significant advances have been made in improving the hydrothermal and steaming stability as well as acidity of A1-MCM-41 by using "zeolite seeds" or "aluminosilicate nanoclusters" as precursors [ 1-4]. These zeolite seeds promote zeolite nucleation by adopting A104 and SiO4 connectivities and consequentially resemble the primary and secondary building units of crystalline zeolite on the wall of the final mesostructured materials. The first demonstration of this approach to stable aluminosilicate mesostructures utilized faujasitic zeolite (FAU) seeds to construct the walls of a hexagonal MCM-41 structure [ 1]. Shortly after, we [1,2] and others [3,4] have used ZSM-5 (MFI) and zeolite Beta (BEA) seeds to assemble steam-stable A1-MCM-41 derivatives containing the 5-ring subunits of these pentasil zeolites. Unlike MCM-41 mesostructures, which are assembled under basic pH conditions compatible with protozeolitic seeds formation, mesostructured cellular foams (MCF) [5,6] and Corresponding author, email: [email protected]. This research is supported by the National Science Foundation through CRG grant 99-03706.

1076 distributions were calculated from the N2 adsorption branch using the BJH model. TEM images were taken on a JEOL 100CX with a CeB6 gun that was operated at an acceleration voltage of 100 kV. 27A1 MAS NMR spectrum were recorded on a Varian VXR-400S spectrometer with 7 mm zirconia rotor, a spinning frequency of 4 kHz. External Al(H20)63+ with a chemical shift of 0 ppm was used as a reference. To test the hydrothermal stability ofmesoporous MSU-S/F aluminosilicate materials, 0.15 g calcined samples were put into 20 ml H20 and boiling for 250 h. The steam stability was tested by exposure 0.2 g samples in 20 ml/min N2 flow saturated with 20% water vapor at 800~ for 2 h. The nitrogen stream was bubbled through a water bath at a controlled temperature to achieve the desired partial pressure of water vapor. Cumene cracking experiments at 300 ~ were performed according to methods described in our previous report [ 1,2]. 3. RESULTS AND DISCUSSIONS 3.1 Mesocellular MSU-S/F Aluminosilicate Foams The pure silica nanoclusters that nucleate a pentasil MFI structure have been extensively studied by Martens et al. [9-11 ]. The procedure used for the extraction and subsequent stable in acid media and suitable for assembling MCF and SBA-15 analogs at under strong acid conditions. In agreement with our previous results, the 27A1MAS NMR spectra showed only one peak

300(

After steaming

Before steaming 250( "0

200(

o

A

A

150( O>

100(

50C Z

o'.~ o'.~ o'.~ 0'.~ ,

0.2 0.4 0.6 0.8

1

P/Po

Figure 1. N2 isotherms for me so structured aluminosilicate foams before and after exposure to 20% steam in nitrogen at 800~ for 2 h: (A) MSU-S/FFAu, (B) MSU-S/FMFI, (C) MSU-SFBEA, (D) MCE Each isotherm is offset by 500 cm3g-1.

200

i

I

I

l

I

150

100

50

0

-50

-100

ppm

Figure 2. 27A1 MAS NMR spectra of A: MSU-SFFAu, B: MSU-SFMFI, C: MSU-SFBEA and D: A1-MCF

1077 very large pore hexagonal (SBA-15) [5,7] mesostructures require acidic reaction conditions for assembly. Such conditions may not be favorable for the incorporation of zeolitic subunits into the framework walls. As we show here, however, zeolite seeds can indeed be used under the acid conditions needed to assemble mesostructures that are structurally analogous to MCF and SBA-15. In addition to describing the physical properties of these mesostructures, we show that the resulting mesostructures are effective acid catalysts for cumene cracking.

2. EXPERIMENTAL 2.1 Synthesis Faujasitic zeolite Y (FAU), zeolite ZSM-5 (MFI), and zeolite Beta (BEA) seeds (Si/A1 = 50) were prepared using procedures analogous to those described previously [8-13]. FAU seeds (Si/A1- 5.6) were prepared by reacting sodium silicate and sodium aluminate at 100 ~ for 12 h, diluting the mixture with sodium silicate solution to obtain a Si/A1 ratio of 50, and then digesting the mixture an additional 12 h at 100 ~ before use. MFI and BEA seeds were prepared using aluminum sec-butoxide and tetraethylorthosilicate as precursors and tetrapropyl- and tetraethyl ammonium ions as structure directors, respectively. These solutions were digested at 100 ~ for 3-6 h before use in constructing mesostructures. A mesostructured aluminosilicate cellular foam (Si/A1 = 50) was prepared from FAU seeds by adding the seeds to a microemulsion containing Pluronic 123 surfactant, (EO)20(PO)70(EO)20, and 1,3,5-trimethylbenzene (TMB) as a co-surfactant to provide a mixture with the molar composition 1.00 SiO2:0.010 A1203:0.013 P123:0.51 TMB: 70.0 H20. The pH of the mixture was adjusted to 4.5-6.5 by the addition of 1.7 M H2SO4, aged at 25-60~ for 20-40 h and finally heated at 100~ under static conditions for 24 h. Analogous foam compositions were prepared from MFI and BEA seeds by adding the seeds to an acidic emulsion of P123, TMB and HC1 to obtain a reaction mixture with the composition 1.00 SiO2:0.010 A1203:0.017 P123:0.79 TMB: 4.95 HCI: 158 H20. The strongly acidic mixture (pH < 2) was stirred at 35 ~ for 20 h, and then the mixture was allowed to digest under static conditions at 100 ~ for 24 h. The as-made products were washed, air-dried, and calcined at 600~ for 4 h to remove the surfactant. Extending the use of zeolite seeds to the assembly of large pore hexagonal structures, we prepared SBA-15 analogs using the same FAU, MFI, and BEA seeds precursors, reaction conditions, and procedures that were used to prepare the above MSU-S/F mesostructures, except that the TMB co-surfactant was eliminated from the reaction mixtures. Two comparison samples of 2 mol% Al-mesostructured cellular foam (denoted as A1-MCF) and 2 mol% A1-SBA-15 (denoted as A1-SBA-15) were prepared exactly as above one formed by MFI and BEA seeds except that a same stoichiometric ratios of Al(i-BuO)3 and TEOS were added to the microemulsion and surfactant solution simultaneously. 2.2 Characterization Powder X-ray diffraction patterns were measured using Cu-Ka radiation (~=1.542 A) and a Rigaku Rotaflex. N2 adsorption and desorption isotherms were obtained at-196~ on a Micromeritics ASAP 2010 Sorptometer using static adsorption procedures. Pore size

1078 Table 1 Textural properties of calcined mesostructured aluminosilicate foams (Si/A1 = 50) before and after hydrothermal stability tests. Window size Cell size Surface area Pore vol. Cumene conv. Sample (%) (rim) (rim) (m2/g) (cc/g)

MSU-SF~ Before steaming After steaming Boiling H20 MSU-SFMFI Before steaming After steaming Boiling H20 MSU-SFBEA Before steaming After steaming Boiling H20 MCF Before steaming After steaming Boiling H20

132 126 118

208 204 196

570 462 273

1.79 1.46 0.78

33

102 95 90

201 195 187

888 748 463

1.95 1.68 0.96

35

128 124 117

220 215 210

861 737 647

2.18 1.86 1.87

36

110 105 -

228 220 -

715 147 103

1.79 0.44

450~ the weight loss due to the isolated methyl groups is added to the desorption of propylthiol fragments and the water resulting from the condensation of silanol groups. Figure 3 depicts 298i MAS NMR spectra of the samples functionalised with methyl groups and with a mixture of methyl and methylproylthiol groups. Resonances at -101 ppm and -111 ppm correspond to the Q3 ((SiO)3SiOH) and Q4 ((SiO)4Si) silicon species, respectively. The signal centred at-65 ppm has been assigned to silicon atoms attached to the methyl groups in T 3 configuration, CH3 Si(OSi)3, whereas the shoulder a t - 5 6 ppm is attributed to T 2 centres, i.e., Si atoms attached to one residual OH group, CH3 (SiO)2SiOH. The new signal observed at-19.6 ppm in the spectrum of the sample 2 has been assigned to =Si(CH3)(CHzCHzCHzSH) organic moieties. It can be observed in Table 2 that the total functionalisation degree of the two samples is practically the same, - 30%, according to the equal content of functional groups of the two gels. Table 2

29Si M_AS M R data for extracted samples functionalised with methyl-propilthiol (sample 2) and methyl groups (sample 3). Normalised peak area is in brackets.

Q4

Q3

T~

T2

7,12

Si-(OSi)4

-Si-OH

=Si-CH3

=Si(OH)CH3

=Si-[CH3,SH]

2

-110,7 (50%)

-101,1 (19%)

-65,1 (14%)

-56,3 (3%)

-19,7 (14%)

3

-110,2 (53%)

-101,1 (16%)

-65,2 (27%)

-55,1 (3%)

-

Sample

1087 The N2 isotherm of the extracted sample 1 obtained in the absence of 400 MTMS (Figure 4) shows a smooth adsorption of nitrogen at low p/p0 2 values, characteristic of the pore E ~ 300 filling in the mesopore region [15]. E A very small hysteresis loop is observed, which is usually .~ 200 associated to the interparticular ,.D porosity in MCM-41 [ 16, 17]. ~ 100 However, if MTMS is added to the synthesis gel, the N2 isotherm ' I ' i ' I ' of the resulting extracted solid 0,0 0,2 0,4 0,6 0,8 shows the presence of a clear P/Po hysteresis loop (sample 2 in Figure Figure 4. N2 isotherms of the samples. 4), which is abruptly closed at p/p0 0.47. At the same time, a strong increase of the pore volume from 0.07 cm3/g to 0.53 cm3/g is found. It is remarkable that this effect is not observed if pure amines are used as co-surfactants [ 13]. This result suggests that the presence of methyl groups decorating the pore walls of MCM-41 would induce the formation of such a large secondary mesopores in the presence of leucine. Therefore, a sample that contains only methyl groups but no sulphur was prepared. The N2 isotherm of this sample shows a strong enhancement of the secondary mesoporosity at roughly the same relative pressures as in the sulphur-containing sample (sample 3 in Figure 4). According to the shape of the isotherm, such porosity would be associated to pores with "ink bottle" morphology having a very large inner diameter and an homogeneous size of the pore mouth, as suggested by the sharpness of the desorption branch. The pore size distribution obtained from the adsorption branch of the isotherm indicates the presence of pores having 13~ average diameter in the pure thiol-containing material, corresponding to the conventional functionalised structured mesopores. The maximum of the distribution shifts to 15 A in the sample that do not contain thiol groups. Indeed, the low pore size is similar to that previously found for thiol or methyl/thiol-containing samples prepared from CTA + or mixtures of this cation with dodecylamine [ 13, 18-19]. The good pore ordering of these functionalised MCM-41 type structures is confirmed by transmission electron microscopy (TEM). The strong interaction between the electrons and the sample results in high symmetry order selected area electron diffraction (SAED) patterns along both parallel and perpendicular to the c axis (Figure 5). There is a good agreement in the cell parameter, 37 and 39 A, calculated by Figure 5. ED patterns of sample 2 along directions perpendicular (A) and parallel (B) electron diffraction (ED) and XRD to the channels axis. respectively. In the TEM images it can be also clearly seen that the high ordered pore 3

1088

arrangement of the MCM-41 is randomly interrupted by regions with low contrast (Figure 6), which would correspond to the large mesopores detected by nitrogen adsorption in samples 2 and 3. Indeed, these regions are much more abundant in the pure methyl sample (sample 3, Figure 6C), which possesses higher secondary mesoporosity. Nevertheless, these mesopores seem to extend several unit cells across the particles, permeating their entire bulk. Regarding the cause that produces the formation of such a large volume of secondary mesopores, it has to be considered first that they are not observed if pure amines are used as co-surfactants in the synthesis gels [ 13]. Therefore, it seems likely that the acid group present in the leucine molecule should be involve in the process leading to such large cavities. Some other interesting features can also be observed by TEM in these materials. It is remarkable the peculiar morphology of the particles (Figure 7), having in general "peanut" shape, which has been already observed when an amine is used as co-surfactant [13]. Second, what it seems to be a thick amorphous layer surrounding the particles is clearly observed in the samples having secondary mesopores (Figure 6C). This layer is not present in the pure thiol-sample, where the hexagonal packing of the pores is observed even at the edge of the particles (Figure 6A), and it could be related eventually with the mechanism of the selfassembly process leading to the mesoestructured material. Indeed, particles with a distinct morphology are detected in samples 2 and 3 (Figure 8). They resemble vesicles surrounded by a thick edge that reminds the thick amorphous surface of the "peanut" particles previously mentioned. This fact, together with the similar thickness of the amorphous layer around the particles, could lead us to think that they are first stages in the growing process of the material in such a way that the mesoporous structure should grow up inside these particles. These vesicles have not been observed in other functionalised materials. The interest of these functionalised materials having high porosity relies upon the possibility to control the population and size of the big mesopores. The presence of these large cavities might have catalytic implications, as they would probably affect the diffusion properties of the material, and hence the final catalytic performance. This aspect is being actually explored. '# ~"~'" " "

"

~7~~~

"

"~k~!"4t

"4

C 20 nm

Figure 6. TEM images along the channel direction of A) sample 1 with MCM-41 type structure, B) some weak contrasts in sample 2, and C) high presence of secondary mesoporosity in sample 3.

1089

.ks

,. ,', ",, f,," .i~ ~?"

100 nm Figure 7. Image of a typical "peanut" shape particle (sample 3). Elongation takes place following the direction perpendicular to the c axe.

:(i!

Figure 8. Particles with vesicle like morphology present in sample 3.

1090 Acknowledgement The authors acknowledge the CICYT (Spain) for financial support within the Project MAT2000-1167-C02-02 and O. Terasaki for the TEM facilities and his helpful discussion. The help of T. Blasco and C. Marquez in collecting and analyzing the 298i MAS NMR is greatly appreciated. I. Diaz acknowledges the Spanish Ministry of Education for a Ph.D. grant.

4. REFERENCES 1. 2. 3. 4.

S.L. Burkett, S. D. Sims, S. Mann, Chem. Commun. (1996) 1367. C.E. Fowler, S. L. Burkett, S. Mann, Chem. Commun. (1997) 1769. M.H. Lim, C. F. Blanford, A. Stein, J. Am. Chem. Soc. 119 (1997) 4090. W.M. Van Rhijn, D. E. De Vos, B. F. Sels, W. D.; Bossaert, P. A. Jacobs, Chem. Commun. (1998)317. 5. M.H. Lim, C. F. Blanford, A. Stein, Chem. Mater. 10 (1998) 467. 6. C.E. Fowler, B. Lebeau, S. Mann, Chem. Commun. (1998) 1825. 7. W.M. Van Rhijn, D. E. De Vos, W. Bossaert, J. Bullen, B. Wouters, P. J. Grobet, P. A. Jacobs, Stud. Surf. Sci. Catal. 117 (1998) 183. 8. M.H. Lira, A. Stein, Chem. Mater. 11 (1999) 3285. 9. F. Babonneau, L. Leite, S. J. Fontlupt, Mater. Chem. 9 (1999) 175. 10. R. J. P. Corriu, C. Hoarau, A. Mehdi, C. Reye, Chem. Commun. (2000) 71. 11. V. Goletto, M. Imperor, F. Babonneau, Stud. Surf. Sci. Catal. 129 (2000) 287 (several papers about co-condensation in that issue) 12. I. Diaz, C. Mfirquez-Alvarez, F. Mohino, J. P6rez-Pariente, E. Sastre, J. Catal. 193 (2000) 295. 13. I. Diaz, C. Mfirquez-Alvarez, F. Mohino, J. P6rez-Pariente, E. Sastre, Micropor. Mesopor. Mater. 44-45 (2001) 203. 14. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992) 710. 15. A. Sayari, M. Kruk,. M. Jaroniec, Cat. Lett. 49 (1997) 147. 16. M. Kruk, M. Jaroniec, R. Ryoo, J. M. Kim, Chem. Mater. 11 (1999) 2568. 17. M. Kruk, M. Jaroniec, Y. Sakamoto, O. Terasaki, R. Ryoo, C. H. Ko, J. Phys. Chem. B, 104 (2000) 292. 18. I. Diaz, C. Mfirquez-Alvarez, F. Mohino, J. P6rez-Pariente, E. Sastre, J. Catal. 193 (2000) 283. 19. I. Diaz, F. Mohino, J. P6rez-Pariente, E. Sastre, Appl. Catal. A: Gen. 19 (2001) 205.


1091

Hexagonal and Cubic Thermally Stable Mesoporous Tin(IV) Phosphates with Acidic, Basic and Catalytic Properties (b) Christian SERRE (a)*, Aline AUROUx , Antonella GERVAS1NI (c), Maryvonne HERVIEU (d) and Gdrard FEREY (a)

(a)Institut Lavoisier, UMR CNRS 8637, Universitd de Versailles St-Quentin en Yvelines, 45 Avenue des Etats-Unis, 78035 Versailles Cedex, France; Fax:(33)l 39 25 43 58; e-mail: [email protected] (b)Institut de Recherches sur la Catalyse, CNRS, 2 avenue Einstein, 69626 VILLEURBANNE Cedex, France, Fax:(33)-(0)472-44-53-99; e-mail: [email protected] (c)Dipartimento di Chimica Fisica de Electrochimica, Universit~ degli Studi di Milano, via C. Golgi 19; 1-20133 Milano Italy; fax. 0039 02 70638129; e-mail: [email protected] (d)CRISMAT, ISMRA, UMR CNRS, 6 Boulevard du Marrchal Juin, 14050 Caen, France; Fax:(33) 2 31 95 16 00; Email: [email protected] Thermally stable hexagonal and cubic mesoporous tin(IV) phosphates have been synthesised via a fluoride route using alkyl di- or tri-methylammonium bromide surfactants. X-ray powder diffraction and HREM show that the hexagonal phase exhibits a MCM-41 organisation while the structure of cubic phase is similar to those of the micellar cubic solid SBA-1. Removal of the surfactant by calcination under air atmosphere of both phases does not destabilise the framework and leads to mesoporous solids exhibiting surface area within the 200-630 m2/g range and pore sizes between 12 and 25 A. Finally, microcalorimetric studies of the adsorption of NH3 and SO2, indicated that both solids exhibit both a strong acidic and a weak basic character. First de-NOx catalytic tests have also shown positive results for both solids, which indicates that these porous solids represent an interesting family of materials with good catalytic properties. 1. INTRODUCTION. Since the initial work of Mobil researchers on the synthesis of mesoporous silica,[ 1] extending the composition of mesoporous materials to metal oxides other than silica for applications in acid, redox catalysis or photocatalytic processes, is a great challenge.[ 2-4]

*author for correspondence

1092 Up to now, numerous non-silica based mesostructured solids based on metal oxides of A1, Ti, , Nb, V, W,[ 5-9] or metal phosphates of A1, Zr, or V,[ 10-12] have been reported. However, cubic mesoporous solids are still scarce despite results concerning oxides of Nb,[13] Sb,[14] Ti,[6b] and Zr[15] reported previously. The first mesotextured metallophosphate with a cubic structure was described recently by Mizuno et aL[ 16] However, its framework collapses after removal of the surfactant. Tin oxide is widely used as a semi-conductor or as a catalyst for oxidation of organic compounds. Synthesis of tin-based mesoporous solids is therefore of a great interest. To date, several mesotextured tin oxides have been reported.[ 17] However, they exhibit either a low thermal stability, a lack of long-range order or low surface areas. We recently reported new hexagonal or lamellar titanium(IV) fluorophosphates synthesised via the fluoride route.[ 18] By extending this pathway to the tin(IV) system, we report here the first hexagonal and cubic porous tin(IV) fluorophosphates which exhibit both a high thermal stability, a long-range order and high surface areas.[ 19] For catalysis application, the determination of the acid-base or redox properties of the samples is of great importance. The acidic and basic character determination as well as first de-NOx catalytic tests are reported here.

2. EXPERIMENTAL SECTION. These solids are first prepared by mixing SRF4 with a aqueous solution of phosphoric acid with a final P/Sn ratio between 4 and 16. Then, the surfactant (Cetyl or tetradecyl trimethylammoniumbromide (CTAB or TTAB)) solution is poured into the tin solution under stirring (S/Sn ratio: 1:2). The final tin concentration is 0.1 Mol/1. The suspensions are aged at 90~ ovemight in a Teflon-lined PARR bomb and cooled down to room temperature. The solids, filtered, washed and dried at room temperature, are finally calcined under air 8 hours at 400~ with a 2~ heating slope. X-ray powder diffraction patterns were conducted on a conventional high resolution (0-20) Siemens DS000 Diffractometer using ~,Cu Ka in steps of 0.02 ~ for 6 s per step with 1/01 mm slits. Tin, phosphorus, fluorine, carbon, nitrogen and hydrogen contents were determined at the C.N.R.S. Central Laboratory of Analysis of Vemaison (69, France). Ratios of P/Sn, F/Sn and S/Sn (S=Surfactant) equal respectively to 1.35, 1 and 0.65 for the TTAB-hexagonal and 1.55, 0.65 and 0.45 for the TTAB-cubic as-synthesised solids were measured. Only traces of fluorine were reported on the calcined solids. The BET surface area measurements were measured with a Micromeretics ASAP 2010 apparatus using nitrogen (N2) as the adsorbed gas. The High Resolution Electron Microscopy (HREM) was performed with a TOPCON 002B microscope (point resolution of 1.8A) equipped with an Energy Dispersive Spectroscopy (EDS) analyser. Samples were prepared by dispersing the powder in alcohol without grinding. Ammonia and sulfur dioxide were used as probe molecules to probe the acidity and basicity of the samples respectively.

1093 The microcalorimetric studies of ammonia and SO2 adsorption were performed at 353K in a heat flow calorimeter of the Tian-Calvet type ( C80 from Setaram) linked to a conventional volumetric line. Before each experiment the samples were outgassed overnight at 673K. The differential heats of adsorption were measured as a function of coverage by repeated addition of amounts of gas until an equilibrium pressure of about 66pa was reached. Then, the samples were evacuated for 1 hour at the adsorption temperature and a second adsorption was done in order to allow the determination of chemisorption uptakes [a]. The catalytic tests were carded out with samples of mass ~ 0.15 g contained in a quartz tubular microreactor (5 mm ID). The reactant stream was provided from a set of mass flow controllers (Bronkhorst, Hi-Tec) supplying 1000 ppmV NO and 1000 ppmV C2H4, and 20,000 ppmV 02 in helium at a total flow rate of 50 cm3(STP)/min, with the reactor at close to atmospheric pressure. Contact time was maintained constant at 0.168 kg*s*l -~. The interval from 200 up to 750~ of reaction temperature was investigated. The exit gas stream from the reactor flowed through a gas cell (pathlength 2.4 m multiple reflection gas cell) in the beam of an FTIR spectrometer (Bio-Rad with DTGS detector). The spectrometer gave analyses for NO, N20, and NO2 for N-containing species, and C2H4, CO and CO2 for C-containing species. The measurements were carried out at 0.50 cm -~ resolution with an accuracy of + 10 ppmV for NO, and 4 for N20 and NO2 using lines at 1876, 2225, and 1619, respectively, respectively. The samples were contained in the reactor between plugs of quartz wool and initially pretreated in flowing 20% O2/He while raising the temperature in stages up to 350~ and maintained it for 4 h. The tests were repeated three times using fresh portions of catalyst and working in low (200-500~ high (450-750~ and medium (350-600~ zones of reaction temperature, respectively.

3. SYNTHESIS AND CHARACTERISATION. The existence of either hexagonal or cubic phases is strongly dependent on syntheses parameters such as concentration, P/Sn, S/Sn ratios and the alkyl chain lengths (S for surfactant). Using alkyl tdmethylammonium surfactants, hexagonal solids are obtained for long alkyl chains surfactants (n>14) while cubic phases appear for small chain surfactants (n,

/

t-" (D .i-,

.=_ .o_ .i-, t./) 0 0 0

jrx,

: Calcined

Fe-SBh-

_ ~

,6

958" cm 1 i

, ~

d

0 r 13..

980 cm 1 35100 2800

I

::

1400 1200 10'00 W avenumbers (cm 1)

i i I

800

6(;0

Figure 4. PAS-FTIR spectra of Fe-SBA-15 materials

'

400

F

.

/ .

.

I

.

. I

I

'

I

'

200 250 300 350 400 450 5 C)0 550 600 Wavelength (nm)

Figure 5. DRUV-VIS spectra of calcined FeSBA- 15 materials

1115

Diffuse reflectance in UV-VIS. DR UV-VIS spectra of calcined Fe-SBA-15 materials are depicted in Figure 5. Spectrum of Fe-SBA-15 material, denoted as DS-2, is dominated by a strong absorption in the range of 200-300 nm, due to metal-oxygen charge transfer. The bands centred at 218 and 250 nm, present in all the Fe-SBA-15 materials, indicate the presence of iron in tetrahedral coordination at framework positions [14]. Unlike Fe-SBA-15 materials prepared through direct synthesis (DS samples) and post synthetically routes (IE samples), materials synthesised with iron chloride as metal source (IC samples) showed a clear modification of UV-VIS spectra: (i) broadening of charge transfer bands, (ii) a shift towards higher wavelengths and (iii) an increase in absorbance in the high-wavelength tail. These features observed for IC samples evidence the presence of Fe in extra-framework positions as iron oxide clusters located onto the walls of mesostructured materials. Finally, it must be remarkable that the disappearance of 958 cm -1 IR band and the subsequent appearance of 980 cm -1 band upon calcination for DS-2 and IE-1 samples seem to be related with the low presence of extra-framework iron species confirmed by DR UV-VIS spectroscopic results. EPR spectroscopy. EPR spectra of calcined Fe containing SBA-15 materials measured at 77K are shown in Figure 6. Three different signals appeared at geff values = 4.3, 2.4 and 2.0, which are usually observed in iron containing silica matrix [15]. The assignments of these signals are ambiguous and different works are described in literature [9,11,14,15]. According to these studies, transitions at 4.3 and 2.0 seem to be attributed to trivalent (paramagnetic) iron in the distorted and symmetrical tetrahedral framework sites whereas the signal centred at 2.4, which resulted especially visible at 77K, might be assigned to nanosized (superparamagnetic) clusters located within the mesopores of SBA-15. From these results, it is inferred that during the process of loading and subsequent calcination treatment, trivalent iron is partially substituted in the silicate framework and part is present as extraframework nanoparticles located in the channels. However, the proportion of both iron species is clearly -4.3 -2.0 Samples I IC-1~ IC-2 lIE-1 IDS-; dependent on the strategy of synthesis. -2.4 g ~ 72"~++ 4.3) EPR results correlates fairly well with those obtained by PAS-FTIR and DR IC-1 UV-VIS. IC samples showed a low geff [(2+4.3)/2.4] ratio suggesting a higher presence of extraframework Fe species and confirming DR UV-VIS results. On the other hand, post-synthetic route using Fe ethoxide in anhydrous conditions (sample IE-1) as well as the direct synthesis procedure (sample DS-2) led to a higher tetrahedral substitution of Fe species in the silica framework after calcination treatment. Likewise, iron-containing materials are usually unstable upon calcination, 0 2000 4000 6000 8000 10000 showing a high amount of Magnetic field (G) extraframework iron species after Figure 6. EPR spectra at 77K of calcined Fe-SBA-15 thermal treatments. In this work, it must materials

1116 be noteworthy that calcined DS and IE SBA-15 materials present a high thermal stability of framework iron species as it is evidenced by EPR and DRUV-VIS spectra. 4. CONCLUSIONS Iron-containing ordered SBA-15 materials have been synthesised by grafting routes (IC and IE samples) and through direct synthesis procedures (DS samples). Iron contents around 1 wt. % were achieved for IC and DS samples, whereas values up to 3.3 wt % have been obtained for IE samples using iron ethoxide in absolute anhydrous conditions. Spectroscopic techniques demonstrate that Fe species are partially isomorphously substituted or grafted in the silica framework accompanied by the presence of extraframewok iron oxide clusters located within the channels. The degree of iron loadings as well as the environment of these metallic species in the silica network is strongly dependent on the synthesis strategy. Further efforts are currently carried out in order to minimize the presence of unstable iron species, which are responsible of leaching effects in heterogeneous catalytic systems. These Femodified mesostructured materials display high surface areas (650-800 A0, defined pore size (55-60 A~) and remarkable hydrothermal stability which are appealing features for oxidation catalytic processes.

ACKNOWLEDGEMENTS We wish to thank Dr. J.Soria et al. from Instituto de Catfilisis y Petroqufmica (CSIC) for their help with EPR measurements.

REFERENCES 1. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky. J. Am. Chem. Soc., 120 (1998) 6024. 2. J.Y. Ying, C.P. Mehnert and M.S. Wong. Angew. Chem. Int. Ed., 38 (1999) 56. 3. A. Stein, B.J. Melde and R.C. Schroden. Adv. Mater., 12(19) (2000) 1403. 4. K. Moller and T. Bein, Chem. Mater., 10 (1998) 2950. 5. Z. Luan, E. M. Maes, P. A.W. van der Heide, D. Zhao, R.S. Czernuszewicz and L. Kevan, Chem. Mater., 11 (1999) 3680. 6. Y. Yue, A. Gdd6on, J. Bonardet, N. Melosh, J. D'Espinose and J. Fraissard, Chem. Commun., (1999) 1967. 7. Z. Luan, M. Hartmann, D. Zhao, W. Zhou and L. Kevan, Chem. Mater., 11 (1999) 1621. 8. V. Antochshuk and M. Jaroniec, Chem. Commun., (1999) 2373. 9. A.B. Bourlinos, M.A. Karakasides and D. Petridis, J. Phys. Chem., 104 (2000) 4375. 10. W. Zhang, B. Glomski, T.R. Pauly and T.J. Pinnavaia, Chem. Commun., (1999) 1803. 11. P. Selvam, S.E. Dapurkar, S.K. Badamali, M Murugasan and H. Kuwano, Catal. Today, 68 (2001) 69. 12. M.S. Morey, S. O'Brien, S. Schwarz and G.D. Stucky, Chem. Mater., 12 (2000) 898. 13. M.S. Morey, S. O'Brien, S. Schwarz and G.D. Stucky, Chem. Mater., 12 (2000) 3435. 14. S.K. Badamali, A. Sakthivel and P. Selvam, Catal. Lett., 65 (2000) 153. 15. A. Tuel and S. Gontier, Chem Mater., 8 (1996) 114.


1117

Synthesis and characterization of mesoscopically ordered surfactant/cosurfactant templated metal oxides T. Czuryszkiewicz a, J. Rosenholm a, F. Kleitz b, M. Lindena'* aDepartment of Physical Chemistry, Abo Akademi University, Porthansgatan 3-5, FIN-20500 Turku, Finland bDepartment of Heterogeneous Catalysis, Max-Planck-lnstitut fer Kohlenforschung, Kaiser-Wilhelm-Platz 1,45470 MQIheim, Germany * To whom correspondence should be addressed. The objective of this study was to synthesize mesoscopically ordered surfactant/cosurfactant templated metal oxides (silica, titania and zirconia). Hexadecyltrimethylammonium bromide was used as structure directing agent. 1-octanol was used as co-surfactant and trimethylbenzene was added to achieve a more swollen structure. Hexagonall~/ordered titanium- and zirconium- oxo phosphates with dl00-spacings of around 80 A were successfully derived. The silica materials reached dlo0-spacings of 50 A. The surfactant removal of the silica and titanium oxo phosphate materials succeeded with preservation of the hexagonal structure upon calcination, while the removal of the surfactants from zirconium oxo phosphate by thermal treatment was unsuccessful, and resulted in a collapsed structure. The materials were characterized by XRD and N2-sorption. 1. I N T R O D U C T I O N

The synthesis of porous silica [1-3], titania [4], and zirconia [5-8], with a twodimensional hexagonal arrangement of pores have been reported by several authors. Usually quaternary alkylammonium-based surfactants have been used as structure directing agents. The inorganic-surfactant interaction, in the titania and zirconia synthesis referred to, is electrostatic in nature and occurs between negatively charged sulfate groups of the transition metal poly-ions and the positively charged surfactant. The synthesis pH is very low due to the strongly acidic nature of the transition metal ions. Although the dl0o-spacing of these materials is about 40 - 45 ~,, microporous materials are often obtained after removal of the surfactant. However, truly mesoporous transition metal oxides can also be prepared using block-copolymers as structure directing agents [9,10], due mainly to the larger size of the block-co-polymer supramolecular aggregates compared to those of standard surfactants. This report deals with another approach to enhance the swelling of the

1118

composite mesophase by the addition of substituted, aromatic swelling agent with or without the use of a non-ionic co-surfactant under acidic conditions. This approach has proved very useful in the synthesis of large pore silica materials under basic conditions [11]. The experimental procedure follows those previously described in the literature for mesoscopic titania [4], zirconia [12], and silica [13], respectively, with the exception that 1-octanol has been used as a co-surfactant and 1,2,3trimethylbenzene as a swelling agent. The as-synthesized and calcined materials have been characterized by SAXS/XRD and N2-sorption, respectively. 2. EXPERIMENTAL 2.1. Chemicals Hexadecyltrimethylammonium bromide, CTAB, (Aldrich); 1,2,3-trimethylbenzene, TMB, (Merck); 1-octanol (Fluka); titanium isopropoxide (Merck); zirconium sulfate (Alfa); tetraethylorthosilicate, TEOS, (Aldrich), ethanol 99.9% (Primalco), hydrochloric acid fuming 37% (Merck), phosphoric acid 85% (Merck) and sulfuric acid 96% (J.T. Baker) were used as received without further purification. The water was purified by distillation and de-ionization. The syntheses of the transition metal oxides and the silica were performed at 30~ and room temperature, respectively, with a stirring rate of 500 rpm. 2.2. Titanium-Oxo Phosphate The synthesis was performed according to the procedure described by Blanchard et al. [4], with the addition of an organic swelling agent and a co-surfactant. A total of 6.6xl 0 .3 mol CTAB was used in the following molar ratios, CTAB/H20/H2SO4/Ti(iOPr)4=0.3/399.625/1.59/1. TMB and 1-octanol were used as swelling agent and co-surfactant, respectively, and they were added to the CTAB solution in molar ratios of TMB to CTAB=I, 2, 3 and 4 and 1-octanol to CTAB=0.048, 0.096 and 0.144. The samples were typically calcined 2h at 250~ 350~ and 450~ respectively, with a heating rate of 1 K/2min. 2.3. Zirconium-Oxo Phosphate The swollen zirconium-oxo phosphates were synthesized according to the report of Ciesla et al. [12], modified with the addition of TMB and 1-octanol. A total of 6.87x10 .3 mol CTAB, was used in the following molar ratios, CTAB/H20/Zr(SO4)2.4H20=0.54/476.9/1. TMB and 1-octanol were added to the CTAB solution in molar ratios of TMB to CTAB=I, 2, 3, and 4 and 1-octanol to CTAB=0.046, 0.092, and 0.138, respectively. The samples were typically calcined 2h at 250~ 350~ and 450~ respectively, with a heating rate of 1 K/2min. 2.4. Silica The synthesis of SBA-3 type mesoporous silica, with a synthesis composition of CTAB/HCI/H20/Si=O.12/9.2/130/1 in molar ratios, proposed by Babonneu et al. [13] was modified by the addition of TMB and 1-octanol. The molar ratio of TMB to CTAB was 1, 2.5, 4 and 5, and the molar ratio of 1-octanol to CTAB was 0.1, 0.2 and 0.3, respectively. The reactant mixture was stirred for 3 h, filtered, rinsed and dried at

1119

90~ The dried samples were calcined for 4 h at 600~ K/min.

with a heating rate of 1

2.5. Analysis The Small-Angle X-ray Scattering (SAXS) measurements were performed on a Kratky compact small-angle system. A Seifert ID-3003 X-ray generator operating at a maximum intensity of 50 kV and 40 mA, provided the Cu Ks radiation of wavelength 1.542 ,&,. A Ni filter was used to remove the KI3 radiation, and a W filter was used to protect the detector from the primary beam. The system was equipped with a position-sensitive detector consisting of 1024 channels of 55.4 I~m each. The sample to-detector-distance was 277 mm. In order to minimize the background scattering from air, the camera volume was kept under vacuum during the measurements. The measurements were performed on wet and calcined samples. The N2-sorption isotherms were determined at 77 K using an ASAP 2010 sorptometer (Micromeritics). The calcined samples were outgassed at 423 K prior to the measurements. 3. RESULTS

The hexagonal structure of the as-synthesized titanium- and zirconium-oxo phosphates showed to be very well ordered, as seen from the XRD-diffractograms presented in Figures l a and lb. The (100) reflections are very narrow and the (110) and (200) reflections are also clearly visible, even though more markedly for the titanium oxide materials. When TMB was added to the systems there was a distinct increase of the dlo0-spacing.

5

n(TMB)/n(CTAB)=4

>.,

(1)

~,J ~

n(TMB)/n(CTAB)=2 ~

v

~J n(TMB)/n(CTAB)=4

>,, 03 c" (1)

n(TMB)/n(CTAB)=~

-4-'-'

n(TMB)/n(CTAB)=2

n(TMB)/n(CTAB)=~

n(TMB)/n(CTAB)=0

~B)/n(CTAB)=0 !

1

1

2

I

3

I

I

4 5 2O (deg)

I

6

I

7

Figure la. XRD pattern of titanium oxo phosphates, with n(TMB)/n(CTAB) = 0, 1,2, and 4.

1

I

2

I

3

I

I

4 5 20 (deg)

I

6

I

7

8

Figure lb. XRD pattern of zirconium oxo phosphates with n(TMB)/n(CTAB) = 0, 2, 3, and 4.

1120

The very well ordered hexagonal structure of the titanium oxo phosphates was maintained with the TMB addition, while the zirconium oxo phosphate showed a slight broadening of the (100) reflection, indicating some loss of order. The addition of TMB using the titanium oxo-phosphate synthesis resulted in a material with a maximum dloo-spacing of 68 ,&,, which should be compared to the d~oo-spacing of 42 A of the material containing nor co-surfactant neither swelling agent. Figure 2 presents the dloo-spacing versus added amount TMB for the three systems investigated. The zirconium oxo phosphates reached dloo-spacings of 79 A, also hexagonal in structure, at a TMB to CTAB molar ratio of 4. The d~o0-spacing was enlarged almost twice in this case, compared to the original material. Addition of TMB to the silica synthesis resulted in an increase of the d~oo-spacing from 38 ,&, to 47 ,~,, at a TMB to CTAB molar ratio of 5.

Zirconia Titania

80 o~" 70

Silica

O3

~9 60

A

,~0 5O

9

0

9

40 I

0

a

I

1

,

I

m

I

,

I

2 3 4 n(TU B)/n(CTAB)

,

I

5

Figure 2. dloo-spacing versus added amount TMB in molar ratio to CTAB, for the zirconia, titania and silica systems. At 1-octanol to CTAB molar ratios up to 0.144 a hexagonally ordered titanium oxo phopshate phase was formed at TMB/CTAB=3 and 4. A larger amount of 1-octanol at TMB/CTAB=4 resulted in the formation of a lamellar phase. With the addition of 1octanol to the zirconium oxide synthesis, a mixture of a hexagonal and a lamellar phase were always formed, except at a very high TMB amount and a low 1-octanol addition (1-octanol/TMB/CTAB=0.048/4/1). Figure 3 clearly demonstrates the linear increase in dloo-spacing with increasing 1-octanol to CTAB molar ratio, for the titanium oxide and silica systems. The titania oxo phosphate reached dlo0-spacings of 78 A when 1-octanol and TMB were added in the molar amounts of 0.144 and 4 to CTAB. As shown in Figure 3 the dl0o-spacing of silica was increased by 13 A, when TMB and 1-octanol were added. Larger ratios of 1-octanol to CTAB resulted in a disordered silica structure.

1121

80

o
.,

~9

>,, .

E (1)

_

as-synthesized

E calcined at 600~

I

2

I

3

1

4

I

5

1

6

I

7

2e (deg) Figure 4a. XRD pattern of SBA-3 and swollen SBA-3.

I

1

.

L

2

1

3

I

4

!

5

I

6

1

7

1

8

9

2e (deg) Figure 4b. XRD pattern of swollen SBA3, as-synthesized and calcined at 600~

1122

The as synthesized swollen silica material, possessed a hexagonal order with dlo0spacings reaching 50 ,& when TMB was added. Two intense low-angle reflections were observed upon addition of 1-octanol, as shown in Figure 4a. The reflection at lower d-spacing clearly corresponds to a hexagonal phase, while the reflection at higher d-spacing probably originates from a distorted hexagonal structure, as previously observed for S+I materials synthesized in presence of hexane [15]. Figure 4b demonstrates the maintained structure of the mesoscopically ordered swollen silica upon calcination. The calcined silica synthesized with addition of TMB and 1-octanol had BET surface areas around 1000 m2g1 and pores in the mesoporous range of 3-6 nm. Figure 5a shows nitrogen sorption isotherms for a silica material containing 1-octanol and TMB in the molar ratios of 0.2 and 4 to CTAB, and a corresponding material with no added swelling agent or co-surfactant. The BET surface area of the material with no added organics was 1400 m2g1 and the nitrogen sorption isotherm indicated a mesoporous structure with pore sizes in the range of 2-3 nm. The nitrogen sorption isotherm of the swollen SBA-3 showed a hysteresis with a clear indication of a bimodal pore structure in the adsorption branch, in agreement with the XRD results. The TMB containing titanium oxo phosphates with n(TMB)/n(CTAB)=3 showed a typical type IV isotherm indicative of mesoporosity, see Figure 5b.

80O o 600

,~.

rim--nm-nmn----umn-mm-~

-~ 500

~ ~

300

0

200~!

- - . - - Ti n(TMB)/n(CTAB)=3 Ti: n(TMB)/n(CTAB)=2 - - " - - Ti: no TMB

90

m 60

"0

100 - - o - - n(1-octanol)/n(TMS)/n(CTAB)=0.2/4/1 0 - - ' - - ~BA-3 . . . . . . 0.0 0.2 0.4 0.6 0.8 1.0 Relative pressure (p/po)

Figure 5a.

150

~ 120

.~ 400

~

180

N2-sorption

isotherm

of

SBA-3 and swollen SBA-3, calcined at 600~

-0 >

30 0 --~

0.0

"nnl---tdll~

0.2 0.4 0.6 0.8 Relative pressure (p/p0)

1.0

Figure 5b. N2-sorption isotherms of titanium oxo phopsphate materials with n(TMB)/n(CTAB) = 0, 2 and 3, calcined stepwise to 450~

The stability of the material upon heating was dramatically increased when TMB was used. A substantial contraction of about 1.5 nm was observed during calcination, but the mesoporous structure was still retained. The BET-surface areas and the total pore volumes decreased with increasing calcination temperature.

1123

4. DISCUSSION

The d~00-spacing of titania and zirconia composite mesostructures could controllably be increased up to 70 ,&, and 80 ~,, respectively, (TMB to CTAB molar ratio of 4) without the loss of the long-range order of the hexagonal mesophase. The swelling of the hexagonal mesophase of titania could further be increased to 78 ,& through the use of 1-octanol as a co-surfactant, the 1-octanol to TMB molar ratio being the key parameter for determining the transition from a hexagonal to lamellar phase in the material. For zirconia, the addition of 1-octanol generally led to the formation of a mixed hexagonal-lamellar mesophase. Surfactant removal by calcination was successful for titania. However, loss of order for zirconia was usually observed upon removal of the template. In contrast, the d~oo-spacing of SBA-3 type 2-D hexagonally ordered silica could controllably be increased to 50 ,&, using the same approach, clearly indicating the potential of the use of mixed surfactant templates together with a swelling agent for the synthesis of large pore mesoscopic materials. Replacing the 1-octanol by an equal molar amount of CTAB did not provide any further swelling of the titania or silica materials, why it is clear that 1-octanol facilitates the formation of a supra-molecular assembly with a lower interfacial curvature needed for an enhanced solubilization of TMB. The formation of the inorganic-surfactant mesophase occurs immediately upon mixing of the reagents for both titania and zirconia due to the presence of large transition metal poly-ions in solution. The large size of the inorganic poly-ions decreases the interfacial flexibility of the composite mesophase, limiting the solubilization capacity of the composite mesophase [15]. Therefore the locus of solubilization of the swelling agent in the initially formed surfactant micelles is crucial for the performance of the swelling agent. Here, kinetic stabilization of the swollen micelles is of utmost importance. The addition of a co-surfactant aids in stabilizing surfactant aggregates with a lower interfacial curvature. Largely swollen, ordered inorganic-surfactant mesophases can therefore be easily synthesized by the right choice of swelling agent and co-surfactant. 5. CONCLUSIONS

The synthesis of large pore mesoscopic titania, zirconia and silica materials has successfully been performed. The dloo-spacings of the hexagonal structures increased with the addition of TMB and 1-octanol, used as swelling agent and cosurfactant, respectively. The dloo-spacings of titania and zirconia were almost doubled in size (40 ,&, - 80 ,&,) with the addition of organics, while the d~0o-spacing of silica increased with 13 ,&,. The calcined silica and titanium oxo phosphate materials showed a hexagonal structure while the thermal instability of swollen zirconia oxo phosphate still remains a challenge for future work.

1124

REFERENCES 1. J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. TW. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, and J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2. J. Frasch, B. Lebeau, M. Soulard, J. Patarin, Langmuir, 16 (2000) 9049. 3. R. Richer, L. Mercier, Chem. Mater., 13 (2001) 2999. 4. J. Blanchard, F. Schath, P. Trens, M. Hudson, Microporous Mesoporous Mater., 39 (2000) 163. 5. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, F. Schtith, Angew. Chem. Int. Ed., 35 (1996) 541. 6. J. Reddy, A. Sayari, Catal. Lett., 38 (1996) 219. 7. D.J. McIntosh, R.A. Kydd, Microporous Mesoporous Mater., 37 (2000) 281. 8. M. Linddn, J. Blanchard, S. Schacht, S.A. Schunk, F. Schtith, Chem. Mater., 11 (1999) 3002. 9. Q. Huo, D.I. Margolese, U. Ciesla, D. Demuth, P. Feng, T. E. Gier, P. Sieger, A. Firouzi, B. F. Chmelka, F. Schtith, G. D. Stucky, Chem. Mater., 6 (1994) 1176. 10. P. Yang, D. Xhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater., 11 (1999) 2813. 11. A. Lind, B. Spliethoff, M. Lind6n, submitted. 12. U. Ciesla, M. Fr6ba, G. Stucky, F. Scht~th, Chem. Mater. 11 (1999) 227. 13. F. Babonneau, L. Leite, S. Fontlupt, J. Mater. Chem., 9 (1999) 175. 14. Kleitz, W. Schmidt, F. Schtith, Microporous Mesoporous Mater., 44-45 (2001) 95. 15. M. Linddn, P. Agren, S. Karlsson, P. Bussian, H. Amenitsch, Langmuir, 16 (2000) 5831.


1125

Preparation o f novel organic-inorganic hybrid Micelle Comparison of different routes for materials preparation

Templated

Silicas.

Duncan J Macquarrie*, Dominic B Jackson, Bethan L King and Andrea Watson Department of Chemistry, University of York, Heslington, YORK, YO 10 5DD, England A series of different routes to organic-inorganic hybrids, based on the (co) condensation of silica (and a functional silane) around an amine template, followed by different template removal methodologies, is described. High loadings and bifunctional materials can be readily achieved with the direct template displacement method, while different alcohols can be grafted onto the surface during conventional template removal techniques. 1. INTRODUCTION The development of efficient routes to organic-inorganic hybrid materials, such as organically modified silicas has led to a range of important materials, of use in catalysis, adsorbency and a range of other applications. Of the many routes available for their synthesis, one of the most promising involves the direct co-condensation of two (or more) silanes in the presence of non-

~

___.~

(RO)4Si~

reTo~

RS, R +(RO)45i

_ i~~--..~

Scheme 1. Different routes to organically modified Micelle Templated Silicas ionic interactions[I,2]. While these routes lead to materials with slightly lower long range order than the ionically templated routes[3,4], they allow easy extraction of the template, meaning that a wide range of organic functionality can be incorporated directly in the sol-gel process, and that the template can be readily recovered and reused.[5] The latter is a major

1126

advantage both in terms of cost and environmental impact, both of which are important in the acceptance of such new materials in areas such as the development of cleaner chemical processes. In order to fully exploit the products of this technology, it is critical to investigate the synthetic methodology in order to understand the processes occurring. Here we present results of investigations into the extraction of neutral amine template using a variety of methods. The results indicate that the standard extraction methodology (involving ethanol) is a non-passive method which results in surface functionalisation. We also present results demonstrating that direct removal of template with a functional silane is possible in non-ionic systems, as well as in the ionic-template materials, as reported by Jaroniec and Antochshuk[6-8]. We also show that the latter process can be extended to remove template from functional materials, leading to higher loaded materials and bifunctional materials. 2. EXPERIMENTAL All chemicals were purchased from Lab suppliers and were used without further purification. Distilled water was used throughout. Preparation of MTS materials was carried out using literature procedures[ 1,3] Porosity and surface area measurements were carried out using dinitrogen on a Micromeritics ASAP2100 instrument. Thermal analysis was carried out on a Stanton-Redcroft 625 instrument; elemental analysis was carried out at the University of Manchester. Extractions in solvents were carried out at the reflux temperature of the solvent, unless stated otherwise. 2.1. Removal of OEt groups by HCI treatment This was accomplished by stirring the materials (lg) in 1.2M HC1 (50ml) containing a few drops ethanol as "wetting agent" at room temperature for 24 hours, or at 55~ for 6 hours. Filtration and washing with water and ethanol was followed by drying at 100~ Elemental analysis indicated that no chlorine was detected. 2.2. Grafting of functional silanes to extracted materials (acid treated or not) The material (lg) to be grafted was suspended in the solvent (120ml), and the silane (15retool) was added. The suspension was heated at the desired temperature (reflux unless stated otherwise) for 24 hours. 2.3. Knoevenagel reaction The catalyst under test (0.5g) was suspended in cyclohexane (25ml) and 0.25g n-dodecane internal standard was added. The suspension was brought to reflux and 20mmol of ethyl cyanoacetate and 20mmol of cyclohexanone were added. The reaction was monitored by GC. 2.4. Reactive extractions The as-synthesised material (i.e as filtered from the reaction mixture) was dried at 100~ for 16h before reactive extractions were carried out. l g of the material to be extracted was suspended in the appropriate solvent (ethanol or toluene, 25ml), 5.1mmol silane was added, and the reaction mixture stirred at the appropriate temperature for 24h. The solid was then filtered and washed with the solvent (3x25ml) and dried. In the case of neat extractions, 50mmol, of silane was used and the mixture treated at 55~ for 24h.

1127 3. RESULTS AND DISCUSSION The preparation of a range of organic-inorganic hybrid materials, using aqueous ethanol solvent and n-dodecylamine has already been described.[1,9] Template extraction from these materials is achieved through the use of ethanol, either at reflux or using continuous extraction such as a Soxhlet apparatus. The resultant materials are template free (as evidenced by Thermal Analysis, NMR, IR and elemental analysis) but contain residual OEt groups, purported to be from incomplete hydrolysis of the tetraethoxysilane precursor. However, it is possible that some of these groups are formed during the ethanol extraction stage. It has been suggested that a high concentration of such groups may impede attachment of functional silanes,[10] and thus it is important to investigate this step of the process to understand the source of these groups, and optimise the incorporation of functional silanes. 3.1. Template extraction and reaction of the surface with alcohols Template extraction from an as-synthesised all silica MTS was thus attempted with a range of solvents (dichloroethane, ethyl acetate, THF, toluene, methanol, ethanol i-propanol), after drying of the as-synthesised material at 110~ for 24h. It was found that only alcohols and refluxing toluene could remove template. While the alcohols would remove template even from wet material, toluene required the material to be dried. Extraction with ethanol caused the template to be completely removed and resulted in a material with 4.2mmol g-Z of OEt groups. Toluene extraction gave a similar material, but with only 3.2mmol g-10Et loading. In order to investigate the role that OEt groups play in the grafting of aminopropyl silane, we have undertaken a series of grafting experiments onto MTS materials which have been pretreated in different ways. The results are summarised in Table 1.

Table 1 Summary of results from grafting of ar ainopropyl groups under various conditions Extractio HC1 treatment Loading AMP Loading OEt before Loading OEt after before grafting (mmol g-l) grafting (mmol grafting (mmol g-l) EtOH No 0.45 4.2 2.3 Toluene No 1.37 3.3 0.85 EtOH Yes 2.64 1.71 0.08 Toluene Yes 1.41 1.39 0.52

g-l)

.......

Thus it can be seen that, in ethanol, the high loading of OEt groups substantially reduces the amount of aminopropyl groups which are attached to the MTS, whereas in toluene, very little influence is seen. Remarkably, the loss of OEt groups is almost complete in the ethanolgrafted material which has undergone acid treatment. Thus, while acid treatment removes a significant amount of OEt groups, those remaining are almost completely removed by the grafting process. This may indicate that amine catalysis is more effective than acid catalysis, or that there are two subsets of OEt groups whose immediate environment requires different conditions for hydrolysis (it should be noted that amines are generally considered as catalysts for hydrolysis / condensation reactions. Losses of OEt during the toluene experiments is thought to be due to hydrolysis from residual water on the surface of the materials. In no case is there evidence that the losses of OEt groups results exclusively from a concerted

1128 grafting/displacement of OEt, although such a pathway cannot be ruled out entirely. Thus the grafting and loss of OEt processes are thought to be predominantly unrelated. The reactivity of alcohols towards the surface of these materials can also be exploited to attach larger alcohols, which may serve as surface modifiers, reducing build-up of polar impurities during catalytic reactions. We have thus treated extracted materials with 1-hexanol and 2-phenylethanol in order to examine this possibility. The Cg-alkoxy materials were then grafted with aminopropyl trimethoxysilane to give basic catalysts, active in the Knoevenagel reaction [ 11 ]. 80 7o 60 5040 30

Pheny lethy Ioxy @0.86m mol/g -" [email protected]/g - - x - untreated

10 0

0

i

1

r

q

10

20

30

4O

time (minutes)

Figure 1. Reaction rates in the reaction of ethyl cyanoacetate and cyclohexanone using three differently alkoxy-functyionalised aminopropyl MTS materials. Rates are normalised for amine content. These catalysts are known to be poisoned by the adsorption of a polar impurity, and the incorporation of such non-polar groups onto the surface was thought to be a route to reducing their deactivation. Figure 1 indicates that this was successful, with the most highly functionalised material having the longest lifetime. 3.2. Reactive extraction with organosilanes. Recently, Antochshuk and Jaroniec reported on the surprising simplicity with which simple silanes such as trimethylchlorosilane and RSi(OMe)3[6-8] could enter the as-synthesised MCM material, and displace the template. Similarly, Onida et al. have also published work indicating the diffusion through the template-filled material is simple, even for large dye molecules.[12] The potential to remove template from the as-synthesised neutrally templated materials has not been explored, but would be of value as an alternative route to functional materials using the neutral amine method. Furthermore, the extension to extraction of organically modified materials to increase the loading of organic functionality, as well as to extract with a second organosilane to give bifunctional materials would be extremely interesting, and would provide access to a range of materials with interesting properties.

1129 3.2.1. Extraction with trimethyl chlorosilane. An as-synthesised material prepared from the condensation of TEOS in the presence of ndodecylamine was used to investigate the removal of template in the initial phase of the investigation. Initially, the template was removed under conventional conditions (extraction with ethanol in a Soxhlet extractor. The resultant material was compared to two other materials prepared by subsequent grafting of trimethylchlorosilane at room temperature either neat or as a solution in toluene. The results of this preliminary work are summarised in Table 2. As can be seen, the functionalisation of the materials with TMS gives TMS loadings of 2.3mmol g-1 and 1.95mmol g-1 for the neat and toluene samples respectively. This probably reflects the larger quantities of silane used in the neat experiments. Surface areas, pore volumes and pore diameters all drop upon functionalisation, by similar amounts in each of the two experiments, as would be expected for the incorporation of the silane groups in the pores of the material. Direct reaction of trimethylchlorosilane with the as-synthesised material (after drying at 100~ for 16h) was found to successfully remove the template whether the silane was used neat or as a toluene solution. After reaction, the material was filtered and washed with toluene. Elemental analysis and porosimetry indicated that the template had been successfully removed in both cases, and that incorporation of silane had taken place. It should be noted that no removal of template occurs at room temperature with toluene under the conditions of the experiment. The quantities of silane incorporated were 2.5mmol g-1 and 2.3mmol g-1 for the neat and the toluene experiment respectively. These figures indicate that the grafting is approximately as effective whether the template is removed or not, and the overall loading is similar to the loadings of silanes onto similar mesoporous systems.J13] Perhaps surprisingly, the surface areas are only slightly lower than the parent material, although pore volume and pore diameter are reduced somewhat more than in the two-step process described earlier.

Table 2 Grafting of trimethylsilyl roups onto Micelle Tem 91ated Silica Loading SSA Pore volume Sample (cm 3 g-l) (mmol gq) (m2g "l) 1084 0.96 Control (EtOH extraction only) Extracted then grafted neat 2.3 517 0.63 Extracted then grafted in PhMe 2.0 580 0.69 Direct extraction neat 2.5 906 0.54 Direct extraction in PhMe 2.3 963 0.56

Pore diameter

(rim) 2.4 2.1 2.2 1.8 1.8

3.2.2. Extraction with functional silanes. Having established that trimethylchlorosilane can extract template under mild conditions, and concomitantly functionalise the surface of the material to a significant extent, we then attempted to remove the template in this way, but with functional silanes. Again, this was successful, with a range of silanes being successfully used to replace the template in a mild and simple process. As was found with the direct template replacement with trimethylchlorosilane, the pore diameter is relatively small, with reductions observed in the surface area and pore volume as compared to the control experiment (as shown in Table 3).

1130 Table 3. Template qtisplacement with functional silanes

Silane

Conditions

Loading (mmol g")

SSA (m2g-1)

ore volume Pore diameter

(cm3g"1) (nm) Control 1.6 759 Aminopropyl 0.31 1.6 Aminopropyl Ethanol, reflux 2.3 759 0.63 2.2 Aminopropyl Ethanol 20~ 2.3 618 0.51 2.3 Aminopropyl Toluene, 20~ 1.9 561 0.19 1.8 Control 1.0 722 0.57 1.9 Ch!oropropyl Chloropropyl Ethanol, reflux 1.3 861 0.39 1.8 Chloropropyl Ethanol, 20~ 1.5 905 0.46 1.7 Chloropropyl Toluene, reflux 1.9 593 0.39 1.7 ercaptopropyl Control 1.9 816 0.45 1.9 ercaptopropyl Ethanol, reflux 1.2 868 0.51 18 ercaptopropyl Ethanol 20~ 1.4 840 0.47 1.9 ercaptopropy! Toluene, reflux 2.3 391 0.27 2.0 Iodopropyl Control 0.3 643 0.76 2.2 Iodopropyl Ethanol, reflux 0.8 527 0.23 1.9 Iodopropyl Toluene, reflux 1.2 14 0]02 6.2 Allyl Toluene, reflux (**) 937 0.66 1.8 Cyanoethyl Toluene, reflux 2.0 810 0'32 117 (*) Phenyl Toluene, reflux 788 0.46 1.7 vinyl Toluene, reflux ....(*) ... 777 0.39 1.8 Loadings determined by elemental analysis, based on heteroatom. For those materials(*) with hydrocarbon chains, loadings have not been determined, but presence confirmed by FTIR spectroscopy. .

.

.

.

.

.

.

.

.

.

.

.

The loadings are again good, and the quantity loaded is independent of whether triethoxy or trimethoxy silanes are used. It can be seen from the above that all the silanes shown above can be used to effect direct template removal and grafting. Only in the case of the iodopropyl material under conditions of refluxing toluene was structural collapse evident. For all others, loadings were comparable to, or often higher than those achieved by conventional solvent extraction and subsequent grafting. It should be noted that some template removal would be expected with ethanol, either hot or cold. However, in comparison with the control experiments, the loadings and final physical parameters of the materials indicate that the process here is not a simple extraction followed by grafting. While there is no obvious pattem regarding the relative effects of the different reaction conditions, grafting in toluene, whether at reflux or at room temperature is often the most effective in terms of achieving a high loading, although this often gives a lower surface area and pore volume, partly at least due to the increased amount of organics in the pore. Thus it can be seen that the method is general, and usually gives materials of equivalent or superior physical properties to the conventional route.

3.2.3. Template displacement with functional silanes in functional silicas A further extension of this work demonstrates that it is equally straightforward to displace template from an as-synthesised material which has organofunctional silanes present in the

1131 preparation step. Here, co-condensation of TEOS with MeO)3SiR (or Me3SiC1) was carried out, and the template removed by displacement with a second silane. The second silane can be the same or different to the original, leading to increased loadings, or to bifunctional materials. Examples of this approach are given in Table 4, using aminopropyl modified assynthesised material, although the approach appears to be more general: Table 4. Templatt displacement in aminopropylsilicas Loading AMP Loading 2 nd SSA Second Pore volume Pore diameter cm 3 g-1 (nm) (mmol g-l). Silane (mmol g-l) (m 2 g-l) silane Chloropropyl 1.2 1.8 304 0.19 2.0 1.1 1.9 229 Mercaptopropyl 0.14 1.9 Aminopropyl 2.25 359 0.23 1.9 Yrimethylsilyl'~ 1.2 1.6 562 0.30 1.8 * Loading theoretically identical for all four batches. Therefore the amount of aminopropyl grafted in the third example is expected to be ca. 1. l mmol g~. t Amine is protonated by the HCI released Here again, it seems that the procedure succeeds in attaching functional groups and simultaneously removing template. The loading of the second silane is typically higher than the first, and the overall amount attached reached 3mmol g-1 in two cases, very high values for such materials. Initial results indicate that the procedure can be extended to other silanes, both in the sol-gel stage, and in the template exchange step. Such a method holds promise for extending the loadings achievable in such material, without the structural collapse which is evident at or below 2.4mmol g~ in the direct co-condensation route[9] or the similar levels which can be directly grafted.[13] 4. CONCLUSIONS Organically functionalised micelle templated silicas can be prepared in several ways in order to control the degree of surface coverage with OR groups. Such groups can be important modifiers of the activity of catalysts, and additional control can be exercised by choice of solvent, giving further flexibility to the method. Additionally, reactive displacement of template, by the use of silanes, has been shown to be a general and valuable method for the preparation of high loading materials and bifunctional materials in a simple and direct route. ACKNOWLEDGEMENTS DJM thanks the Royal Society for a University Research Fellowship, DBJ and AW thank the EPSRC for funding, BLK the University of York. REFERENCES 1. 2. 3. 4. 5.

D J Macquarrie, Chem. Commun (1996) 1961 R Richer and L Mercier, Chem. Commun (1998) 1775 P T Tanev and T J Pinnavaia, Science, (1995) 267 865 S A Bagshaw; E Prouzet, T J Pinnavaia, Science (1995) 267 1242 D J Macquarrie, Green Chem (1999) 1 195

1132 6. V Antochshuk, M Jaroniec, Chem. Mater., (2000) 12 2496 7. V Antochshuk, M Jaroniec., Stud. Surf. Sci., Catal., (2000) 129 265 8. V Antochshuk, M Jaroniec., Chem. Commun., (1999) 2373 9. D J Macquarrie, D B Jackson, J E G Mdoe, J H Clark, New J Chem., (1999) 23 539 10. K Cassier, P van der Voort, EF Vansant, Chem. Commun., (2000) 2489 11. D J Macquarrie, D B Jackson, Chem. Commun., (1997) 1781 12. B Onida, B Bonelli, L Flora F Geobaldo, CO Arean, E Garrone, Chem. Commun., (2001) 2216 13. D Brunel, Microp. Mesop. Mater., (1999) 27 329


Structure and catalytic p e r f o r m a n c e of cobalt Fischer T r o p s c h supported by periodic m e s o p o r o u s silicas

1133

catalysts

A.Y. Khodakov, R. Bechara and A. Griboval-Constant Laboratoire de Catalyse de Lille, USTL, Cit6 Scientifique, Brit. C3, 59655 Villeneuve d'Ascq, France The structure of cobalt catalysts supported by periodic mesoporous silicas at different stages of preparation was characterized by XRD, N2 adsorption, XPS, in situ Xray absorption and TGA. It was shown that the size and reducibility of supported cobalt particles were strongly affected by porous structure; larger and more easily reducible particles being detected in wider pore silicas. Cobalt dispersion was found to be controlled by silica pore sizes even at high cobalt contents (up to 30 wt.%) It was shown that catalytic behavior of cobalt supported mesoporous silicas in Fischer Tropsch synthesis strongly depended on cobalt dispersion and catalyst porous structure. Wide pore SBA-15 supported Co catalysts were found to be much (about 5-10 times) more active than narrow pore MCM-41 supported catalysts with the same cobalt content. Product distribution was found to be a function of cobalt particle sizes and cobalt reducibility. Fischer Tropsch reaction rates increased monotonically with increase in cobalt content up to 30 wt %, whereas product distributions for completely reduced wide pore catalysts were nearly the same at high and low cobalt loadings. 1. INTRODUCTION Concerns about environment and rational management of natural resources are major reasons for renewed interest in Fischer-Tropsch (FT) synthesis [1]. FT synthesis produces valuable hydrocarbons from relatively cheap synthesis gas. Synthetic liquid fuels prepared using FT technology contain negligible concentrations of sulfur and heavy metals relative to crude oil. Availability of large reserves of natural gas in different countries is another motivation for further development of FT technology. The efficiency of FT technology can be improved by using catalysts with higher hydrocarbon productivities and lower methane selectivities [2]. The reaction proceeds on supported metal particles. The catalytic performance is a function of metal dispersion, reducibility and volumetric density of active metal sites. Cobalt catalysts have been found to be most suitable for synthesis of higher hydrocarbons [3]. Most of FT publications have addressed optimization of metal function in catalysts supported by amorphous oxides. Our approach suggests that porosity of the oxide support could play a significant, often decisive role in the performance of FT supported catalysts. Broad pore size distributions in common catalytic supports make it difficult to draw unambiguous conclusions about influence of catalyst pore sizes on FT reaction rates and selectivities. Tailored pore size distribution in recently discovered periodic mesoporous silicas [4, 5] makes these materials model catalytic supports for FT catalysts. In the present work the effects of porosity on the

1134

structure of supported cobalt species, on FT reaction rates and selectivities were studied over wide ranges of silica mesopore diameters and cobalt contents. MCM-41 and SBA-15 periodic mesoporous silicas were used as catalyst supports. A commercial mesoporous silica was also studied for comparison. 2.

EXPERIMENTAL

MCM-41 and SBA-15 type periodic mesoporous silicas (PMS) were ~;ynthesized using halide cetyltrimethyl and dodecyl ammonium compounds and polyethylene glycols as templates. BET surface areas, total pore volumes, pore diameters calculated from nitrogen isotherms and details of synthesis procedure are presented in Table 1 and references therein. Low angle XRD patterns (~,=1.668 /k) showed an intense peak at 20-2.3 degrees and low intensity peaks at 3-6 degrees for MCM-41 and an intense peak at 20-0.9 degrees and low intensity peaks at 1.6-1.9 degrees for SBA-15 materials. The observed XRD patterns were characteristic of the hexagonal structure [4, 5]. A commercial silica (Cab-o-sil M-5) was agglomerated by wetting and dried in an oven at 393 K overnight. Cobalt was introduced by incipient wetness impregnation using solutions of cobalt nitrate. The samples were dried overnight at 373 K and calcined at 773 K for 5 h. Cobalt contents were varied between 5 and 30 wt %. The nomenclature for the catalysts (xPMSn) consists of two parts; the first part (x) indicates calculated cobalt weight content; the second part (PMSn) refers to a periodic mesoporous silica used as a support. The catalysts were characterized by XRD, XPS, adsorption techniques, in situ X-ray absorption and TGA. In situ X- ray absorption and low angle XRD measurements were Table 1. Adsorption properties and synthesis of mesoporous silicas. Silica Type Adsorption properties Synthesis .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

=_

_

SBET, TPV, Pore Template mZ/g cm3/g diameter, A PMS1 MCM-41 1481 0.75 =20 dodecyltrimethylammonium bromide PMS2 MCM-41 742 0.59 =20 cetyltrimethyl ammonium chloride PMS3 SBA-15 679 0.78 42 poly (ethylene glycol)block- poly (propylene glycol)- block- poly (ethylene glycol) PMS4 SBA-15 887 1.91 91 poly (ethylene glycol)block- poly (propylene glycol)- block- poly (ethylene glycol) SiO2 213 0.84 330 (Cab)

,,,,

,,

Source Ref. of SiO2 Fumed [21] silica Fumed silica TEOS

[22]

TEOS

[26]

-

Cabo-sil M5

[25]

1135 2

1600

O.O6

1400

Boo

o

004

d

0.03

3

/ /2

0.02

~ i

4O0

001

2OO 0

1

0,05

0

02

0.4 0.6 Relative Pressure, P/P.

0

08

10

4 '

loo

,,,I lOOO

Pore Diameter, A

Figure 1. Isotherms of nitrogen adsorptiondesorption on narrow MCM-41 and wide pore SBA-15 periodic mesoporous silicas 9 1- PMS1 (flpore=20 A) and 2- PMS4 (dpore-91 A). The isotherm of PMS4 is offset for clarity.

Figure 2 Pore size distribution curves calculated from nitrogen desorption isotherms: 1- PMS1, 2- PMS2, 3-PMS3, 4-PMS4, 5 - SiO2(Cab).

carried out in L.U.R.E., Orsay (France) using synchrotron radiation from the DCI storage ring. The extent of overall cobalt reduction was calculated from TGA weight loss in the atmosphere of hydrogen at 753 K assuming stoichiometric reduction of Co304 to metallic cobalt. The data were corrected by subtracting weight losses of silica supports treated under the same conditions. Co content in the samples was measured by atomic absorption. Prior to catalytic measurements, the catalysts were reduced at 753 K for 5 h in a flow of hydrogen. FT catalytic rates and selectivities were measured at atmospheric pressure in a fixed bed flow reactor (H2/CO=2) with on-line analysis of products by gas chromatography. The reaction rates and hydrocarbon selectivities were measured at quasi-state 2 conditions generally attained after 7 hours on-stream. FT reaction 1.5 rates were normalized by the o ~ 400 number of cobalt atoms. The hydrocarbon 0.5 selectivities were calculated on carbon basis. The chain growth probabilities, a, were calculated Cobalt content, from the slope of the curve ln(Sn/n) Figure 3. Surface areas and total pore volume versus n, where n is the carbon periodic silica as a function of cobalt content in number of the hydrocaroon and S, PMS4 silica. the selectivity to corresponding hydrocarbon. 1000

2.5

800

600

v

0

9

.

9

5

10

15

0

20

wt.%

25

30

1136 4

b 0.5

!2: t-

2

0.4 0.3, 0.2.

0.5

2

0.1,

~

'

,

r ,,

77

Photon Energy, keV

,81

0 0

1

2

3

4

5

6

7

R,A

Figure 4. XANES spectra (a) and moduli of Fourier transform of EXAFS (b) for oxidized catalysts: 1- 5CoPMS2, 2- 5CoPMS3, 3- 5CoPMS4, 4- 5CoSiO2(Cab) and 5bulk C o 3 0 4 . 3. RESULTS AND DISCUSSION 3.1. Structure of mesoporous silicas and supported cobalt species Figure 1 shows isotherms of nitrogen adsorption-desorption on mesoporous silicas" MCM-41 and SBA-15. Isotherms without hysteresis loop are observed for MCM-41 type silicas (Figure 1, curve 1). This type of isotherms is typical for mesoporous silicas with pore diameters smaller than 30 A [4]. Nitrogen adsorption isotherms of SBA-15 silicas (Figure 1, curve 2) belong to type IV in accordance with classification of Brunauer [6]. All SBA-15 isotherms show a reversible part and a type A hysteresis loop at higher pressures. The P/Po position of the inflection points is related to a diameter of mesopores. The pore sizes in SBA-15 silicas varied in the range from 42 to 91 A as a function of synthesis procedure (Figure 2). Pore size distributions calculated from the desorption branches of the isotherms using BJH method [7], were much more narrow in periodic mesoporous silicas than in the commercial SiO2(Cab). Cobalt impregnation led to a decrease in specific surface areas and total pore volumes for both MCM-41 and SBA-15 type silicas (Figure 3), but did not affect the shape of pore distribution curves. The surface areas however, remain high even at higher cobalt loadings (30 wt. %). Small C O 3 0 4 crystallites in oxidized catalysts were observed by XRD, X ray 6OO

a

b

5OO

5O0

5

z;~o

3

3

2 1

100

lOO

1

0

0 55

6O

65 2 " l ' h ~ , degree

70

s5

60

65

70

2 Theta

Figure 5. XRD patterns of oxidized catalysts: a- 5 wt% Co catalysts supported by PMS1 (1), PMS2 (2), PMS3 (3), PMS4 (4), SiO2(Cab) (5) silicas; b- catalysts supported by PMS4 silica containing 5 wt% (1), 10 wt%(2), 20 wt%(3) and 30 wt% (4) Co.

1137 Table 2. Characterization of Co catalysts Co catalyst Co Pore C0304'"'crystallite content diameter, diameter from, A wt % ,& XRD XPS

TGA extent of overall reduction at753 K, %

5CoPMS1 5CoPMS2 5CoPMS3 5CoPMS4 30CoPMS4 5CoSiO2(Cab) 30CoSiO2 (Cab)

43.2 62.9 72.8 94.8 94.4 -

5.47 5.67 6.95 5.39 27.3 4.75 26.0

20 20 43 75 75 200-300 200-300

57 43 92 121 125 230 286

8 16 70 67 74 76 103

absorption and XPS. The X-ray absorption near edge structure (XANES) and the moduli of Fourier transforms of EXAFS of 5 wt. %Co/SiO2 oxidized catalysts and bulk Co304 are presented in Figure 4. The XANES spectra of oxidized Co supported silicas were practically identical; they resemble the spectrum of bulk Co304. Similarity of both XANES spectra and of moduli of Fourier transform shows similar local structure of Co species in bulk Co304 and oxidized Co supported catalysts. XRD patterns of Co supported catalysts are presented in Figure 5. Only Co304 crystalline phase was detected. Very broad Co304 X-ray diffraction patterns were observed for supports with pore sizes smaller than 30 ,~, (PMS1 and PMS2). Table 2 shows that the size of Co304 crystallites estimated from the width of XRD profiles using Scherrer equation depends on the pore diameters in mesoporous silicas; larger Co304 crystallites are found in large pore periodic (PMS3, PMS4) and commercial (Cab-o-sil) silicas. An increase in cobalt loading up to 30-wt% in periodic mesoporous silicas did not results in any noticeable modifications of the width of XRD patterns and therefore Co304 particle sizes (Figure 5b, Table 2). This suggests that most of cobalt particles introduced by 3.5 impregnation are located in 3 catalyst pores where their size is g 2.5 limited by diameters of silica ~ 2 pores even at high cobalt contents. z 1 XPS measurements of 0.5 particle sizes using Kerkhof and Moulijn model [8] showed 0 7.69 7.71 7.73 7.75 7.77 7.79 7.81 similar dependence of CoaO4 Photon Energy, keV particle sizes on silica pore Figure 6. XANES spectra of CoO (1), 5CoPMS2 (2), diameter (Table 2). The 5CoPMS3 (3), 5CoPMS4 (4) and 5CoSiO2 (Cab) (5) differences in absolute values of catalysts reduced in situ at 773 K and Co foil (6). Co304 particle sizes evaluated The spectra were measured in hydrogen at room from XRD and XPS data seem to temperature. be related to the limitations of these techniques. The sizes of

1138 Co304 crystallites measured by XRD were found slightly larger 120 than silica pore diameters. It is 200 known that the limitations and 100 approximations of analysis of E 150 _~ XRD profiles using Scherrer mm 100 equation could overestimate the actual particle sizes [9]. It should 5O be also noted that particles supported on periodic 0 10 20 30 40 50 60 70 80 90 1 O0 mesoporous silicas co, rid adopt a Pore Diameter, A slightly elongated shape in the mesopores. Kerkhof and Moulijn Figure 7. Diameters of C0304 crystallites measured model assumes uniform by XRD and extent of overall cobalt reduction as distribution of the supported functions of silica pore sizes (5 wt.%Co/SiO2 phase between the bulk and outer catalysts). surface of catalyst grains. Previous works [10] however, showed that impregnation of silicas followed by calcinations could lead to enrichment of CO304 particles on the external surface of the SiOe grains. Higher concentration of CO304 near the outer surface of catalyst grains could lead to higher intensity of Co2p XPS signal and therefore, to underestimating CO304 particle sizes. In agreement with previous reports [11, 12] in situ X-ray absorption and TGA showed that reduction of catalysts proceeds as C0304--->C00-->C0. The reduced catalysts were characterized by in situ X-ray absorption. The XANES spectra and moduli of Fourier transform of EXAFS of the catalysts and of Co foil are shown in Figure 6. Comparison of X ray absorption data of the reduced Co catalysts with those of Co foil indicates the presence of Co metal species in all samples. At the same time, the resemblance of the XANES spectrum of 5CoPMS2 and CoO suggests considerable concentrations of unreduced CoO phase in that sample. As the diameter of pores in supported Co catalysts increases from 5CoPMS2 to 5CoPMS4 , the near-edge spectrum of cobalt shifts from one resembling that of CoO to one resembling Co foil. XANES also showed high extent of cobalt reduction in commercial wide pore 5CoSiOe(Cab). The extent of reduction measured by TGA and X-ray absorption was found to depend on pore sizes (Table 2, Figure 6). Larger cobalt particle in wide pore SBA-15 silicas were significantly easier to reduce than small particles in narrow pore catalysts (Figure 7). The effect of particle sizes on the reducibility of Co species was attributed to the interaction between metal and support. As shown previously [12], in smaller particles this interaction could be much stronger than in larger ones and this interaction was likely to stabilize small oxidized particles and clusters in silica. 8o

60

o~

'~ -o

40

20

.

.

.

.

.

.

.

.

,

0

3.2. Catalytic behavior of cobalt supported mesoporous silicas. Catalytic results are presented in Table 3 and Figure 8. Wide pore SBA-15 supported Co catalysts were found to be much (about 5-10 times) more active than MCM-41 supported Co catalysts with the same cobalt content (Table 3). Higher methane (25%) and lower C5+ selectivities were observed on narrow pore silicas, whereas the chain growth probabilities (c~) were in the range of 0.66-0.78 for both narrow and wide pore catalysts.

1139 Table 3. Catalytic performance of mesoporous silicas in FT synthesis (H2/CO=2, P= 1 bar, Tr=463 K, 7 h on-stream, conversion < 5%). Co catalyst Reaction rates, 10-~ CH4 selectivity, C- C5+ selectivity, Ci

5%CoPMS1 5%CoPMS2 5 %CoPMS3 5 %CoPMS4 30%CoPMS4 5%CoSiO2(Cab) 30%CoSiO2(Cab)

S-1

%

O-~

0.5 0.1 1.55 1.38 2.55 2.68 1.88

24.9 22.7 15.2 15.3 15.2 16.9 16.5

50.1 57.0 65.0 68.4 68.0 60.7 62.4

0.66 0.78 0.74 0.77 0.77 0.70 0.70

Characterization results show that narrow pore silica supported catalysts contain smaller cobalt particles. These smaller particles were found more difficult to reduce than larger one. It can be suggested therefore, that FT activity could be a function of cobalt particle size and their reducibility. This suggestion is consistent with previous reports. Reuel and Bartholomew found [13] that activity of Co supported catalysts prepared by impregnation increased with decreasing metal dispersion. This effect was assigned to changes in surface structure with decreasing particle size and to electronic modifications due to interaction of small crystallites with the support. Higher methane selectivity was observed on partially reduced cobalt species [13]. Iglesia et al [2, 14] showed that FT is structure-insensitive reaction by definition of Boudart; lower activity of small cobalt particles observed in previous reports was explained in terms of lower reducibility of smaller cobalt particles and their possible reoxidation by water or by other reaction products at FT reaction condition. In line with these observations, our results also display lower activity of smaller cobalt particles in FT synthesis. Lower reducibility of smaller particles of cobalt oxide in narrow pores of mesoporous silicas (Table 2) seems to be one of the major reasons responsible for their lower activity and higher methane selectivity. FT reaction rates increase with increase in cobalt content in periodic mesoporous silicas (Figure 8). This finding is consistent with relatively high cobalt dispersion observed by both XRD and ,40 XPS in periodic silicas with high ,35~ ~2 cobalt loadings. For commercial ~o ~ silicas with wide pore size distribution r" =o curves the activity per cobalt atom decreases at higher cobalt loadings in ,t;agreement with a decrease in cobalt dispersion (Table 2, 3). Narrow pore 0 ,0 0 5 10 15 20 25 30 size distributions and high specific Co content, % surface areas (>700-1000 mZ/g) of Figure 8. FT reaction rate and methane periodic mesoporous silicas are likely selectivity as function of Co content in MSS4 to facilitate stabilization of relatively peridic mesoporous silicas. small cobalt particles (~100 ~,) and to prevent them from sintering even at higher cobalt loadings. Therefore, in m

2o

lO

1140 periodic mesoporous silicas with higher metal contents, high local concentrations of cobalt sites could be suggested. High volumetric densities of cobalt sites in periodic mesoporous silica with high cobalt loadings seems to be one of the reasons [14] responsible for their enhanced activity. 4. CONLUSIONS The results showed that the structure of supported cobalt species, their reducibility and catalytic behavior in FT synthesis were strongly affected by pore sizes and porous structure of periodic mesoporous silicas. The sizes of cobalt particles were found to depend on pore sizes in silicas even at high cobalt contents (30 wt.%); smaller and hardly reducible particles being observed in narrow pore supports. Catalytic measurements reveled strong impact of pore sizes on catalytic behavior: FT reaction rates increased more than 5-10 times as pore sizes increased from 20 to 100 A. A larger diameter of catalyst pores also led to significantly lower methane selectivities. Relatively high cobalt dispersion stabilized by porous structure of periodic mesoporous silicas in catalysts with high cobalt contents led to high activity of these materials in FT synthesis. ACKNOLEDGEMENTS

The authors thank C. Guelton for TGA measurements and Dr. V.L. Zholobenko for providing PMS1 silica. The authors are grateful to F. Villain (D42) and D. Durand (D43) for help during synchrotron experiments. The Laboratoire pour l'Utilisation du Rayonnement Electromagndtique (L.U.R.E.), Orsay, France is acknowledged for the use of beamline. REFERENCES

1. 2. 3. 4.

A.M. Thayer, Chem. & Eng. News, March 13 (2000) 20. E. Iglesia, S.C. Reyes, R.J. Madon, and S.L. Soled, Adv. Catal., 39(1993) 221. P. Chaumette, Revue IFP 51(1996) 711. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, CT. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc., 114(1992) 10834. 5. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka, G.D. Stucky, Science, 279 (1998) 548. 6. S. Brunauer, L.S. Deming, W.S. Deming and E., Teller, J. Am. Chem. Soc. 62(1940), 1723. 7. E.P. Barrett, L.G. Joyner, and P.P. Halenda, J.Am.Chem.Soc. 73(1951) 373. 8. F.P.J. Kerkhof and J.A. Moulijn, J. Phys. Chem., 83(1979) 1612. 9. P. Ganesan, H.K. Kuo, A. Saaverda, and R.J. DeAngelis, J.Catal. 52(1978) 319. 10. D.G. Castner, P.R.Watson and I.Y. Chan, J.Phys. Chem. 93(1989) 3188 11. D.G. Castner, P.R.Watson and I.Y. Chan, J.Phys.Chem., 94(1990) 819. 12. A.Y. Khodakov, J. Lynch, D. Bazin, B. Rebours, N. Zanier, B. Moisson, and P. Chaumette, J.Catal., 168(1997) 16. 13. R.C. Reuel, and C.H. Bartholomew, J.Catal., 85(1984) 78. 14. E.Ig.'_esia, Applied Catalysis A:General, 161 (1997) 59.


1141

Highly dispersed VOx species on mesoporous supports: Promising catalysts for the oxidative dehydrogenation (ODH) of propane A. Brtickner, P. Rybarczyk, H. Kosslick, G.-U. Wolf and M. Baerns Institute for Applied Chemistry Berlin-Adlershof Richard-Willstiitter-Str. 12, D-12489 Berlin, Germany

Dedicated to Prof. Jens Weitkamp on the occasion of his 60 th birthday

VOx species supported on mesoporous A1203, SBA-15 and MCM-48 materials have been studied by simultaneous in situ-EPR/UV-vis/on line-GC measurements and tested as catalysts for the ODH of propane. Highest propene yields can be obtained with catalysts of very high surface area in which active V sites have a mean valence state close to +4, are highly dispersed and preferably tetrahedrally coordinated by oxygen. Coke deposits formed at high reaction temperature and propane concentrations do not deactivate the catalysts but enhance the propene selectivity by covering acidic sites of the support.

1. INTRODUCTION Vanadia-based catalysts are widely used in a number of industrial oxidation processes. Particularly, oxidative dehydrogenation (ODH) of light alkanes to the corresponding alkenes would be an attractive subject since cheap and environmentally friendly starting materials can be converted to valuable olef'ms used as feedstock for other processes. The ODH of propane has been extensively studied since the 1970's and discussed in several review papers [1-3]. Unfortunately, maximum propene yields obtained so far hardly exceed 20 % since total combustion of both propane and propene leads to low selectivities, in particular at higher degrees of conversion. Due to these limitations, the ODH of propane is still far from being attractive for industrial application and a major goal of research is to develop highly selective catalysts for this process. Recemly we have shown by comparative studies of a variety of vanadia-comaining mixed metal oxides that selectivity and activity strongly depend on the valence state, coordination and dispersion of the V ions [4]. Highest selectivities could be obtained with catalysts exposing preferably isolated and/or low oligomeric tetrahedral VOx species with a mean surface V valence close to +4 although their intrinsic activity was lower than that of V sites in octahedral symmetry and/or within VOx clusters. Furthermore we have seen that, under reaction conditions, coordination and valence state of the V species equilibrate and differ from those of flesh and used catalysts in ambiem atmosphere. This clearly indicates the need of using in situ-techrfiques for deriving reliable structure-reactivity relationships.

1142 In this work, we tried to achieve the desired high dispersion of vanadium sites in preferably tetrahedral coordination by depositing VOx species on mesoporous support materials (A1203 and SiO2). This implies low surface concentration of active sites which in turn might lower the overall activity of the catalysts. To compensate for this undesired consequence, supports with very high surface area were used. Changes of the structure and valence state of the active V sites under reaction conditions have been studied by a novel simultaneous coupling of in situ-EPR/on line-GC/UV-vis-DRS [5].

2. EXPERIMENTAL Catalysts with 2.8 and 5.8 wt.-% V were prepared by wet impregnation of mesoporous A1203 [6], MCM-48 [7], and SBA-15 [8] materials with aqueous solutions of NHaVO3. The suspensions were treated at 70 ~ and ambient pressure for 1 h in a rotary evaporator d h reactor t u b e = =~ 1 before water was removed at the same temperature in vacuum. The obtained powders were dried at 150 ~ in vacuum and finally calcined quartz wool at 600 ~ for 6 h in air. Structure, redox behaviour and valence state of the VOx sites as well as surface ~products acidity have been studied by TPR, - ~ conr'~-'tion to FTIR, potentiometric titration, 51Von t i n e - G C NMR, EPR and UV-vis-DRS. ~ p # e ....................... =i UV-vis-DRS spectra were recorded by a Cary 400 UV-vis Figure 1. Experimental set-up for simultaneous in situspectrometer (Varian) equipped EPR/on line-GC/UV-vis coupling with a diffuse reflectance accessory including a heatable reaction chamber (praying mantis, Harrick). To reduce light absorption, the catalysts were diluted with ~-A1203 (calcined at 1473 K for 4h). Spectra deconvolution was performed by the program GRAMS32 (Galactic). EPR spectra were recorded by the c.w. spectrometer ELEXSYS 500-10/12 (Bruker) in Xband. For in situ-studies a homemade flow reactor equipped with a temperature programmer and connected to a gas dosing apparatus was used [9, 10]. For on-line product analysis the reactor outlet was connected to a GC 17AAF capillary gas chromatograph (Shimadzu) equipped with a 30 m x 0.32 mm Silicaplot column (Chrompack) and a FID. For simultaneous EPR/on line-GC/UV-vis coupling, a fibre optic quartz sensor (Optran WF, 200 x 1.5 mm) was directly implemented in the EPR flow reactor through a Teflon gasket (Fig. 1) [5]. The sensor is connected to an AVS-PC-2000 plug-in spectrometer (Avantes) by fibre optic cables (2000 x 0.4 mm). In a typical run, 100 mg catalyst particles (0.3 - 0.6 mm) were treated with

1143 mixtures of C3H8 and 02 in a molar ratio of 2 diluted by different amounts of N2 to adjust W/F values between 0.6 and 2.7 g h mo1-1. Acidic surface sites were determined by pyridine adsorption using a FTIR spectrometer (Bruker IFS 66) equipped with a heatable adsorption cell. Self-supporting wafers were pretreated in vacuum at 400 ~ FTIR spectra were recorded after pyridine adsorption at room temperature and subsequent evacuation. BET surface areas and mean pore diameters were determined by N2 adsorption a t - 1 9 6 ~ using a Gemini III 2375 surface area analyzer (Micromeritics). The mean vanadium valence state was determined by potentiometric titration using a variant of the method developed by Niwa and Murakami [ 11 ]. Catalytic tests were performed in a fixed-bed U-type quartz reactor at 500 ~ (educt mixture: 40% C3H8, 20% 02, balance N2; W/F = 0.6-0.9 goat h mol'lprooane).


2. 1. Structural properties of the catalysts Pentavalent vanadium sites in supported VOx catalysts give rise to intense charge-transfer (CT) bands in the respective UV-vis-DRS spectra (Fig. 2). From the position of the lowenergy CT band conclusions on the coordination number and the degree of V site agglomeration can be derived. In the spectra of hydrated VOx/MCM-48 and VOx/SBA-15 samples 0,5

o,2oJ

o.~5~f~

A

0.5

o,4

A

0,3

0.3

0,2

0.2

0,1

0.1

0,0

0.0

LL

0,080,04 0,00

0.8

0,4

032-

360

d o - 5bo - 6bo - 7bo-

nm

0.4

0,3

0.6

0,2

0.4

0,1

0.2

O,Oi

936o

4oo

~~bo?6osoo

nm

0.0

3oo 46o 560 6ob ~o6 8~

nm

Figure 2. Room temperature UV/VIS-DRS spectra of samples 2.8 % V/A1203(left), 2.8 % V/SBA15-200 (middle) and 2.8 % V/MCM-48 before (A) and after dehydration in air at 773 K (B) which are very similar these bands occur above 450 nm suggesting octahedral coordination (Fig. 2A [12]). This is also supported by a SlV-NMR signal at -270 ppm for V s§ in octahedral symmetry [12]. After heating in air to 500 ~ these signals disappear since the octahedral V

1144 sites loose coordinated water ligands and become tetrahedral. Accordingly, the most intense low-energy CT band occurs at 320 nm being characteristic of mainly isolated tetrahedral VO4 sites while the small band around 380 nm indicates the presence of some VO4 sites connected via V-O-V bridges, too (Fig. 2B [13]). The latter band is not visible in VOx/MCM-48, probably due to its very high surface area (Table 1) which facilitates high V dispersion. In agreement with the UV-vis-DRS results, a single line at -575 ppm is observed in the 5~V-NMR spectra a~er dehydration which is assigned to tetrahedrally coordinated V 5+ [12]. These spectral changes are completely reversible by rehydrating the samples in ambient atmosphere suggesting that all V sites are exposed on the surface and accessible to water ligands and, thus, also to potential reactant molecules. Thus, it is justified to calculate VOx surface densities and turnover frequencies (TOF) assuming that all V sites are exposed (Table 1). In contrast to VO• and VOx/MCM-48, the majority of V sites in as-synthesized VOx/ml203, is in tetrahedral coordination which is confirmed, too, by an intense 5 1 V - N M R signal at -560 ppm [12]. The V coordination number does almost not change upon heating (Fig. 2A and B). However, a weak band at 455 nm points to the presence of a small amount of octahedrally coordinated, rather oligomeric vanadium sites which do not become tetrahedral upon heating (Fig. 2B). They are evidenced, too, by a very weak 5~V-NMR peak at -330 ppm. Table 1. Structural properties and catalytic results at 500 ~ of catalysts with 2.8 wt.-% of V Sample

Surface densitya

[V/nm~] VOx/A1203 VOx/SBA50 VOx/SBA200 VOx/MCM48

1.0 0.43 0.7 0.37

SBET Mean pore [ m 2 / g ] diameter

[A]

273 645 421 889

48.2 52.6 190.5 26.2

Mean V valence 4.81 4.81 4.83 4.86

TOF a, b S(C3H6)b Ymax(C3H6) [s"l] [%] [%] 0.44 0.18 0.26 0.21

73.3 83.3 82.3 80.1

12.3 14.5 12.4 18.0

aapparent values, calculated assuming exposure of all V sites, b for Xpropane--2.5 - 3.9 %

Table 2. Acidic surface sites determined by FTIR of adsorbed pyridine Relative band area b Sample a

VOx/A1203 VOx/SBA50 VOx/SBA200 VOx/MCM48

1445 crn-~

1540 c m -1

2.3 1.3 2.0 1.6

0.2 0.3 0.1

a V content 2.8 wt.-%, b normalized on the specific surface area

As shown in Table 1, all samples contain a small amount of tetravalent V species. They are also detected by a characteristic EPR signal with hyperfine structure arising from VO 2+ sites in hydrated sampies. This line is, however, not observed in dehydrated VOx/SBA-15 and VOx/MCM48 samples due to the change to tetrahedral V coordination which shortens the relaxation times. Acidic surface sites in the catalysts were determined by FTIR spectroscopy of adsorbed pyridine using the band area at 1445 cm"1 (Lewis sites) and 1540 cm 1 (Bronsted sites) (Table 2). All catalysts

1145 contain Lewis sites, their concentration being highest for the alumina-supported sample. Bronsted sites are almost negligible in silica-supported catalysts and are not detectable at all in VOx/A1203.

2.2. Behaviour of V sites under reaction conditions In situ-EPR/on line-GC/UV-vis experiments have been performed with all samples during heating under ODH conditions. For example, the results obtained with VOx/A1203 are shown in Fig. 3. UV-vis spectra of the fresh catalyst at 20 ~ are dominated by CT bands of V 5+ as observed accordingly also in Fig. 2A. By raising the temperature stepwise to 400 ~ light absorption increases gradually above 500 nm due to partial reduction of V 5+ to V 4+, the d-d transitions of which fall in the higher wavelength range of the spectrum [ 14] (Fig. 3A, left). It is interesting that this reduction, which is also confirmed by the growing EPR signal of interacting and isolated VO 2§ species (Fig. 3A, middle), starts already at temperatures well below the onset ofpropene formation.

7 6 ~,

60 % C3H8/30 %02 / N2 W/F = 1.25 g h mol -~

T! ~

5TF~.,.

\

vz2o,

700

;00

5~o - - - - - - = ' ~ l ~ [ ~ f - - - - - -

--.

1

,,

300

;00

5-

lOOmT

X/nm

B

0

(~ '-3

e o~, 2 ,,,

1

o

600mT

\%,.,. / \ & ~ ; / 3 o o o c \ ~ < 2 o o o 0 , .

300

,

~

~

,

-

500 700 ;k,/nm

,

/z 500~

4oo~ - - - - - - " ...........

20~ ~

900

I'

-

100 mT

-~

b

20 4.0 60 ~}0 "100 X, S / %

'1

_,--2~ ~

............

I'

600 mT

Figure 3. A) In situ-EPR/UV-vis/on line-GC measurement of sample VOx/A1203 (5.8 wt.-% V)

during ODH of propane and B) in situ-EPR/UV-vis measurement of the same sample during heating in H2 flow. Further heating above 400 ~ gives rise to a strong increase of absorbance in the whole visible range of the spectrum (Fig. 3A, left). This is caused by the formation of carbon depos-

1146 its which have been detected, too, by FTIR spectroscopy. The contributions of carbon deposits and reduced V species to the overall absorbance can be distinguished by comparing the in situ-UV-vis spectra of Fig. 3A with those of Fig. 3B obtained at similar temperatures in 1-12 flow in which only reduction of V s+ but no carbon deposition is possible. It is interesting to note that the catalyst is not deactivated by carbon deposits (Fig. 3A, right). Obviously, those carbonaceous residues are mainly deposited on the support material while the active VOx species remain flee. This could reduce the surface acidity of the support under reaction conditions and, thus, be the reason for the strongly increasing propene selectivity in the initial period of the reaction (Fig. 3A, right). By using a VOx/ml203 catalyst with a lower V loading (2.8 wt.-%) and under less severe reaction conditions coke formation can be at least partly suppressed (Fig. 4, left). In this case, only a slight increase of propene selectivity occurs in the initial reaction period and the final value after 45 min time on stream is less than 40 % although the degree of conversion is markedly lower in comparison to the experiment shown in Fig. 3A. g'~ 2 55-

T/*C 4

50-

/30"

45-

//14

35-

//,

3025-

//~ "

~

1s "----'-"y~-,..,-'~

.,,

40-

5

7

o ----"/3/111,~.,'"~

4 5 o -----~M;illlP.~

- ..................... "

"

i

,,,',

=

/ / , /,,,, /. . 4 o o - ~ / - ~ l t l ~ - - - ' - ' / / //," , , - 3 5 0 --~-'i,,.-.,. I ~ _ ~ - - . - - - " / /-

20-

'

==

,

! ~

X

is

propene pro

ne

151005

4dO

6(:;0 8d0

v/nm

20-

-

"-'-

Ig

1'0 20 '30 4'0 x.s/%

Figure 4. In situ-EPR/UV-vis/on line-GC measurement of sample VOx/A1203(2.8 wt.-% V) during heating in a flow of 28 % C3H8, 14 % O2/N2 (W/F = 2.7 g h mo14).

The in situ-UV-vis spectra of VOx/MCM-48 and VO• catalysts do also indicate that V 5+ is partly reduced under ODH conditions. However, in contrast to VOx/A1203, the intensity of the VO 2+ signal observed in the in situ-EPR spectra of VO,/MCM-48 and VOx/SBA-15 is negligible. As shown by UV-vis measurements, the VOx species in these materials are essentially in tetrahedral coordination (Fig. 2B). When this coordination symmetry persists during reduction, the respective V 4+ species remain EPR-silent at ambient and elevated temperatures. However, when a flow of wet nitrogen is passed through the catalyst bed after cooling to room temperature, the typical EPR signal of VO 2+ species in octahedral and/or square-pyramidal coordination appears since the tetrahedral V 4+ species formed under reaction conditions adsorb additional water ligands. In agreement with the in situ-UV-vis spectra and the mean V valence state of the used catalysts determined by potentiometric titration this indicates clearly, that VOx species on silica supports are also reduced to a certain degree un-

1147 der ODH conditions. As for VOx/A1203, coke formation does also take place on these catalysts depending on the reaction conditions.

2.3. Catalytic tests Catalytic tests of samples with the same V content under similar reaction conditions revealed that the intrinsic activity of the VOx sites reflected by TOF values as well as the propene selectivities do not differ much for the three silica-supported VOx catalysts (Table 1). This agrees well with the fact that their local structure and valence state under reaction conditions is also very similar. Moreover, the different pore diameters seem to be of minor influence (Table 1). Due to the much higher surface area of sample VOx/MCM-48, the maximum propene yield achieved with this catalyst is higher in comparison to the VOx/SBA-15 samples. The intrinsic activity of VOx/A1203 (TOF values, Table 1) is higher in comparison to that of VOx/SBA-15 and VOx/MCM-48. As shown by the in situ-studies described above, a certain amount of octahedral vanadium sites is present under reaction conditions in VOx/A1203. These species and the higher number of V-O-V bonds might be the reason for the higher intrinsic activity of the VOx sites in comparison to VOx/SBA-15 and VOx/MCM-48. However, propene selectivities over VOx/A1203 are lower than over the silica-supported samples. This might be due to the higher concentration and strength of Lewis acidic sites that have been detected on VOx/A1203 by FTIR spectroscopy of pyridine adsorption. 3. CONCLUSIONS By comparing the properties of VOx species deposited on mesoporous supports with their catalytic performance in the ODH of propane some general relationships can be derived: 9 Vanadium catalyzes this reaction in both oxidation states +5 and +4. However, under reaction conditions, initial V 5§ is partly reduced whereby a mean equilibrium valence state well below +5 is established depending on the feed composition. This process lowers the vanadium redox potential and could be one reason for enhanced propene selectivities. 9 High specific surface areas and low surface acidity of the support material (especially valid for VOx/MCM-48) favour the formation of isolated, completely accessible V sites being beneficial for high propene selectivities. Their low intrinsic activity (TOF values) can be compensated for by high specific surface areas of the support, thus, leading to maximum propene yields. 9 V sites tetrahedrally coordinated by oxygen seem to be less active but more selective than higher coordinated ones. This is evident by comparing the results of VOx/AI203 containing besides VO4 also VO6 units with those of VOx/MCM-48 in which vanadium is essentially tetrahedrally coordinated under reaction conditions. 9 Feed compositions containing propane in excess can give rise to coke deposits. However, these species do not deactivate the V sites but cover preferably acidic sites of the support which in tuna enhances the propene selectivity. These results suggest that good catalysts should contain highly dispersed, preferably tetrahedrally coordinated VOx species on non- or low-acidic support surfaces. The catalytic data obtained in this work with VOx supported on mesoporous A1203 and SiO2 promise that further

1148 improvement could still be achieved by optimizing these materials on the basis of the knowledge described above. ACKNOWLEDGEMENT

The authors thank Dr. U. Bentrup, Dr. D. MtiHer and Mrs. R. Jentzsch for experimental support and the German Federal Ministry of Education and Research for financial support (grant no. 03C0280). REFERENCES

1. 2. 3. 4. 5. 6.

7. 8. 9. 10. 11. 12. 13. 14.

T. Blasco and J. M. L6pez-Nieto, Applied Catalysis A: General, 157 (1997) 117. M.A. Banares, Catalysis Today, 51 (1999) 319. E.A. Mamedov and V. Cort6s Cober~_n, Applied Catalysis A: General, 127 (1995) 1. P. Rybarczyk, H. Berndt, J. Radnik, M.-M. Pohl, O. Buyevskaya, M. Baems and A. Brtickner, J. Catal., 202 (2001) 45. A. Briickner, Chem. Commun., (2001) 2122. H. Kosslick, R. Eckelt, D. Mtiller, M.-M. Pohl, M. Richter, R. Fricke, Proccedings of the International Conference on Advanced Materials ,,Materials Week", Mttnchen, September 25th-28th, 2000. V. Alfredsson and M. W. Andersson, Chem. Mater., 8 (1996) 1141. Z. Luan, M. Hartmann, D. Zhao, W. Zhou and L. Kevan, Chem. Mater., 11 (1999) 1621. H.G. Karge, J.-P. Lange, A. Gutsze and M. Laniecki, J. Catal., 114 (1988) 144. A. Brtickner, B. Kubias and B. Lticke, Catal. Today, 32 (1996) 215. M. Niwa, and Y. Murakami, J. Catal., 76 (1982) 9. G. Centi, S. Perathoner, F. Trifir6, A. Aboukais, C. F. Aissi and M. Guelton, J. Phys. Chem., 96 (1992) 2617. X. Gao and I. E. Wachs,, J. Phys. Chem., 104 (2000) 1261. C.J. Ballhausen and H. B. Gray, Inorg. Chem., 1 (1962) 111.


1149

M o d e l l i n g M e s o p o r o u s Materials M.W. Anderson a, C.C. Egger a, G.J.T. Tiddy b, J.L. Casci c* aUMIST Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, UK bDepartment of Chemical Engineering, UMIST, P.O. Box 88, Manchester M60 1QD, UK CSynetix, P.O. Box 1, Billingham. Cleveland TS23 1LB UK

1. I N T R O D U C T I O N Mesoporous materials of the MCM or SBA variety are by their nature organised amorphous material. Consequently, in order to describe their structure, it is necessary to utilise a model which is able to accommodate both the organisation and the disorder. Such models are useful for a number of reasons. First, as a method of characterisation, if a model can be generated then a variety of experimental data can be simulated e.g. x-ray diffraction, electron microscopy, gas adsorption etc. Second, a structural model allows further properties of a phase to be anticipated. Third, a model allows a visualistion of a structure which aids our understanding of these novel complex materials. Fourth, the details of the structure yield clues to the synthetic mechanism thereby aiding strategies to design and control new structures. Recently it has been shown [1 ] that an electron density map of a mesoporous structure can be directly determined from electron crystallography. That is, a very well ordered sample was examined by electron microscopy, both electron diffraction patterns and images were collected providing, after indexing, both intensity and phase information resulting in a low resolution electron density map (low resoltuion means that only the wall structure is located, as precise atomic coordinates are random for an amorphous structure). This is the ultimate method to directly determine the structure (wall curvature and thickness) of such a material. However, the technique is laborious and is not suitable for screening materials. Furthermore, subtle structural features are not readily extracted from the resulting three-dimensional electron density map. Our approach is to build structures using a certain amount of previous knowledge in a manner which is then easily manipulated to reflect different synthetic conditions and qualities of material. In this paper we concentrate on the structure of SBA-1 [2] a material first synthesised by the Santa Barbara group, Stucky et al. 2. BUILDING S T R U C T U R E S BY HAND SBA-1 is a hydroxylated silica mesoporous material which is synthesised using a suffactant template cetyl-triethylammonium bromide in a highly acidic silica solution. Under the conditions of synthesis the surfactant forms globular micelles which pack together to give a cubic unit cell. The space group of the resulting material appears to be Pm3n and is related to the suffactant mesophase known as the I1 phase[3]. Working on the basis that the resulting This work was funded by Synetix.

1150 silica structure will in some manner wrap around the globular micellar water structure, in order to describe the silica walls in an analytical mathematical form a type of mathematics is required which will easily describe surfaces wrapped around spheres and distorted spheres. Such a mathematics exists based upon the Gauss distribution function: e

_x 2

--C

and is described in detail by Jacob and Andersson[4]. For our purposes we will require spheres for which the x, y, and z coordinates are given by the equation:

e -(x~§247 = C The radius of the sphere is determined by the constant C and the centre of the sphere can be moved to any coordinate h, k, I by the following transformation: e-[(x-hi +(y-kr162 ] = C The sphere can be elongated or squashed in any dimension to produce for instance an oblate ellipsoid by the following transformation: e-[a (~:-hr +b~O,-er +b3(z-l)Z].__C Finally, an object with a different radius can be formed not only by changing C but also by adding a constant, a, within the exponential thus: e "-[~ (~-h)~+b~('v-e)~+b~(z- t)~] = C This provides the tools to build a mesoporous material synthesised from globular micelles by now adding these functions in the exponential scale. Figure 1 shows what happens when two exponential functions are added together, one representing a sphere, and a second oblate ellipsoid displaced to a different coordinate. When the objects are far apart they form perfect spheres or ellipsoids. However, as they approach the surface begins to form a continuous wrapping. Such a construction should be ideal for the description of mesoporous materials based on globular micelles as the silica surface should indeed wrap around the micellar body forming a continuous surface. Pores

G

a

C

b d

Figure 1. a) and b) are oblique and top views of a small sphere and oblate ellipsoid generated with Gauss distribution functions. c) and d) show how the surfaces wrap as the object become larger and approach one another.

1151 will be generated by the excluded zone between the objects. The arrangement of micelles in the Pm3n, 11, structure is given by adding 21 objects together according to the following equation:

s {O'-h)~+Cv-k)~+~:z-')}+ ~ s h,k,t h',~',I' Table 1. h,k,l fractional coordinates of 9 spheres

Table 2. h ' k' /' coordinates for 12 ellipsoids with values of bl, b2 and b3 in terms o f f r~ = radius o f sphere

rz

f = radius o f long axis o f ellipsoid radius o f short axis o f ellipsoid

h 0

k 0

1

0 0

= radius o f short axis o f ellipsoid

C = e -~

a = r~ + In(C)

1 0

h' 1

k' 0.25

1' 0.5

bl 1/f2

be 1

0

0

1

0.75

0.5

1/f2

1

1 0

0 1

1

1

0

1

0

1

0

1

1

1

1

1

0.5

0.5

0.5

0 0 0.5 0.5 0.5 0.5 0.25

0.25 0.75 1 1 0 0 0.5

0.5 0.5 0.25 0.75 0.25 0.75 1

1/f2 1/ f 2 l/ f 2 1/ f 2 1/f2 1If 2 1

1 1 l/ f 2 l/ f 2 1/f2 l/ f 2 1/f2

b3 1/f2 1 / f2 1/f2 1/f2 1 1 1 1 1/f2

0.75 0.25 0.75

0.5 0.5 0.5

1 0 0

1 1 1

1/ f 2 1/f2 1 / f2

1/ f 2 1/f2 1 / f2

Figure 2: Pm3n arrangement of 9 spherical micelles, one marked $5, (on a body centre) and 12 oblate ellipsoids, marked El-E4, two on each face o f t h e unit cell. Based on a unit cell of 85A the sphere radius and the radius of the short axis of the ellipsoid are both 10A resulting in the constant, a, equal zero.

1152 The first summation represents 9 spheres with a body centred arrangement and the second summation 12 oblate ellipsoids. The coordinates for these objects are given in tables 1 and 2 as well as the derivation of the constants. The result is the arrangement of micelles shown in figure 2. This figure is just a schematic representation of the relative positions of the spherical and oblate micelles and the sizes are scaled in order to aid the reader to understand this arrangement. In this figure the extension of the oblate ellipsoid is given by the factor f, which in this case has been chosen as 1.3. Although the final structure can be calculated for any unit cell in order for the mathematics to remain robust it is important that the constant C does not become excessively large or small. In order to prevent this from occurring all surface calculations have been based on a unit cell of 6A which is then scaled accordingly. '~

:':~,i ~ , ~ . , r

................ : : , ; G ' ~ : . r

~.!-',~_.!~,:'... " ",,,: :t-.tl ~ . ~ g , _~ "- 'r 9" p " r .

....t

' ~~ ~ ~t ~"

~~

t

~

,

~

~

9 . . , t l - ;o .. -.

"...

.~:~: ':

: ....:.,,,-t.~.'~..

".

t -' ~ ; ; ~ " . . ~". ~ : - ,,t ' t

, ~,

.r . ~ . -

..

.

~'~*~m~I~["~3.:--.-"" ~7",':'" " r,'.-v;...,'-. " "~,. ,,'." ".'_,,.2 II'

~ J ~ ' ~ m ~ i I I ~ l ~ '

r ~ ~ , , ~ M ~ m m ~ I

""

9

9 ,-

~

~

~

~ :

~"

. .. ,~ = e,,~.. ~ 9 t- ~

.~,r . ! -

9:.~ r

" .,~".'t . . . . ~'..~

" - : --I',

.m

~.--

9 " 9 ".

Figure 3: left picture shows the surface generated with an 85A unit cell, radii rl and r2 equal 19.5A and an oblateness given by f=l.18A; right picture shows the atomistic model of SBA-1 by placing a random army of silicon atoms on the outer side of the surface. The structure ofSBA-1 is then built by increasing the size of the spheres and ellipsoids until they merge and the surface becomes continuously wrapped. However, as the surface will wrap into adjacent unit cells it is important to include in the calculation a further 12 virtual ellipsoids. The coordinates of these twelve ellipsoids can be derived from those in Table 2 by replacing the coordinate 0.25 by -0.25 and 0.75 by 1.25. Consequently a total of 33 objects are required to describe the whole structure ofSBA-1 (9 spheres, 12 ellipsoids and 12 virtual ellipsoids). When this is done the result is the surface shown in figure 3. One unit cell is shown and windows can be seen which are generated as the surface wraps from one object to the next. This surface will represent the periphery of the edge of the wall of SBA-1 which will presumably be in contact with a water sheath around the micelles. In order to generate an atomistic model of SBA-1 it is the necessary to fill the space on the outer side of the surface, not occupied by the surfactant molecules and water layer with a random array of silicon atoms. This method was successfully used before in an atomistic description of

1153 MCM-4815]. The density of silicon atoms is maintained by keeping an average Si...Si separation of 3.5/!t and the atomistic model so formed is shown in the lower part of figure 3. In order to optimise the parameters, radius of the sphere rs, radius of the short axis of the ellipsoid re and degree of oblatenessfa large number of structures were generated in this manner from which the x-ray diffraction pattern and electron micrographs could be generated for comparison between the our model and experimental data. The results of some of these calculations are shown in figure 4.

'

!i

.............

~"

t

. . . . .

t

~

t

.

123456123456123456 "20

.

.

.

.

.

.

234567

. ............. .

.

.J ' , . . . . .

.~,,s,~

.....

; _ : ~ _ _ ~

Figure 4: top 16 x-ray diffraction patterns calculated with sphere radius rl equal to the radius of the short axis of the oblate ellipsoid r2. The value off, the measure of oblateness is given inset in each figure. The 16 atomistic models generated are layed out below in the same order. The x-ray diffractions pattems agree very well with those reported in the literature and with those that we measure. The best agreement between is found for rs=rz=l 9 to 19.5/!t and f=l.18. As soon as rs deviated from r2 extra reflections appeared, most significantly the [ 110] reflection, which rapidly became very strong and is not observed experimentally. The fact that the radius of the sphere and that of the short axis of the oblate ellipsoid are similar is not surprising as both will be governed by the length of the surfactant chain. The models which best fit the x-ray diffractions patterns also show a strong correspondence between the projected electron potential maps (not shown) and the electron micrographs described in the literature[ 1]. As the ellipsoids tend to spheres (when f tends to 1) the size of the two types of micelles has to be significantly different (e.g. 23A and 19A radii to be able to explain the electron micrographs. This was the description for the structure given previously[1 ], however, the x-ray diffraction pattems of such a structure are vastly incorrect, including a large [110] reflection (see figure 5).

1154

I. -,',:L:e~,-,.

...i,..

.......... -:~-~s_-7..r

: 1 show low extent of the dealumination and hence reasonable thermal stability. The pyridine adsorption showed, that both the Br6nsted and Lewis acid centres are present in the mesoporous materials. The Lewis acidity is predominating in all the samples under study. The number of the Br6nsted sites depends on the Si content and is the highest for sample SA(5). It has been also shown that nature of the Lewis sites present depends on the mesoporous aluminosilicate composition. For the A1 rich samples, the Lewis centres are mainly the extraframework species containing octahedral A1. In the case of more stable, silicon rich preparations (Si/A1 > 1), trigonally coordinated A1 atoms i.e. AI(OSi)3 formed as a result of dehydroxylation of the neighbouring Br6nsted sites account for a high number of Lewis acid sites. 5. ACKNOWLEDGEMENTS This work was supported by the NATO Science for Peace Programme under project NATO SfP 974217. REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature (London) 359 (1992) 710. 2. Z. Luan, C-F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 3. Z. Luan, C-F. Cheng, H. He, J. Klinowski, J. Phys. Chem., 99 (1995) 10590. 4. A. Corma, V. Fornes, M.T. Navarro, J. Perez-Pariente, J. Catal., 148 (1994) 569. 5. R.B. Borade, A. Clearfield, Catal. Lett., 31 (1995) 267. 6. R. Mokaya, W. Jones, Chem. Commun., (1998) 1839. 7. S. Cabrera, J. E1 Haskouri, S. Mendioroz, C. Guillem, J. Latorre, A. Beltr/m-Porter, D. Beltr~in-Porter, M.D. Marcos, E Amor6s, Chem. Commun., (1999) 1679. 8. S. Cabrera, J. E1 Haskouri, C. Guillem, J. Latorre, A. Beltr/m-Porter, D. Beltr~in -Porter, M.D. Marcos, P. Amor6s, Solid State Sci., 2 (2000) 405. 9. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltr~in, D. Beltr/m, S. Mendioroz, M.D. Marcos, P. Amor6s, Adv. Mater., 11 (1999) 379. 10. G. Engelhardt, D. Michel, "High-Resolution Solid State NMR of Silicates and Zeolites ", John Viley & Sons, Chichester, 1987. 11. J. Weglarski, J. Datka, H. He, J. Klinowski, J. Chem. Soc, Faraday Trans., 92 (24) (1996) 5161. 12. R.B. Borade, A. Clearfield, Synthesis of Porous Materials: Zeolites, Clays and Nanostructures, M.L. Occelli, H. Kessler, eds. Marcel Dekker, Inc., New York, 1997. 13. C.A. Emeis, J. Catal., 141 (1993) 347. 14. F. Di Renzo, B. Chiche, E Fajula, S. Viale, E. Garrone, Stud. Surf. Sci. Catal., 101 (1996) 851.


M e s o p o r o u s silicate as matrix for drug delivery a n t i n f l a m m a t o r y drugs

1165

s y s t e m s of non-steroidal

R. Aiello ~ G. Cavallaro*, G. Giammona*, L. Pasqua ~ P. Pierro*, F. Testa ~ ~ di Ingegneria Chimica e dei Materiali, Universit~ della Calabria, Via Pietro Bucci 87030 Rende (CS) ITALIA *Dipartimento di Chimica e Tecnologie Farmaceutiche, Universit~ di Palermo, Via Archirafi 32 90123 Palermo ITALIA 1. I N T R O D U C T I O N

During the last two decades significant advances have been made in the area of controlled release technology in the attempt to overcome drawbacks due to the use of the conventional dosage forms such as frequent administrations, toxic and side effects, high doses; to this aim Drug Delivery Systems (DDSs) seem to have many interesting properties being able to modify the absorption and/or the distribution and/or the elimination of a drug and, so, to modify its pharmacokinetic profile [1,2]. The advantages due to the use of a DDSs are considerable; in fact, among other things, these systems are potentially able to maintain therapeutic drug levels for large periods of time preventing the onset of potentially toxic peaks in drug concentration, to reduce the amount of drug and the number of administrations and to protect the drug against chemical and enzymatic degradations. Therefore with a DDS is possible not only to improve the therapeutic efficacy of a known drug but also to allow the administration of drugs characterised by a short plasma half-life and/or by remarkable collateral and toxic effects. Furthermore, another advantage in the development of a known drug as DDS is in relatively low cost compared to that of the discovery of a new drug. Both natural and synthetic materials have been tested and proposed as component of DDSs [3] and many efforts have been doing in many fields to synthesise materials with the "ad hoc" biological, technological and mechanical properties for each application in drug delivery. The aim of this investigation has been to test the possibility of employing a particular inorganic mesoporous matrix based on siliceous material as drug controlled release device. The introduction of the M41S family of mesoporous materials by Mobil Corporation scientists in 1992 created a new field in research on advanced materials. MCM-41 is the member of the M41S family that has been more extensively studied [4]. It shows hexagonal arrays of cylindrical mesopores. Pore size ranges between 15 and 100 ]k depending basically on the surfactant employed for the synthesis process. Pore walls present free silanol groups that could be reactive toward appropriate guest molecules. These properties allow MCM-41 to work as a matrix for adsorption and release of suitable organic molecules such as drugs without any chemical modification of pore walls. Recently disks of MCM-41 charged with Ibuprofen, an anti-inflammatory drug, were prepared and characterised [5].

1166 In this contest, we have evaluated the capacity of a mesoporous silicate matrix to entrap drugs and, subsequently, to release them under different experimental conditions mimicking some biological compartments. Non-steroidal anti-inflammatory agents such as Diflunisal (Dr), Naproxen (Np), Ibuprofen (Ib) and his sodium salt (IbNa) have been employed in this study. The four drug impregnated matrices have been characterised with regard to drug content and swelling measurements in aqueous media which simulate some biological fluids such as gastric and intestinal liquids. Moreover the capacity of the mesoporous matrix to act as delivery system by carrying out in vitro studies under experimental conditions mimicking gastrointestinal fluids has been evaluated.

~H3

OOH

~H3

CHCOOE

OH

(CH3)2CHC2 h H

CH30~ DIFLUNISAL

NAPROXEN

HCOOH

IBUPROFEN

Fig. 1. Chemical Structure of Diflunisal, Naproxen and Ibuprofen 2. EXPERIMENTAL SECTON

2.1. MCM-41 synthesis MCM-41 was obtained from gel with the following molar composition: 1SiO2-0.2NaOH0.04AI(OH)3-0.2CTABr-40H20. The initial gel was prepared according to the following procedure: 24.3 g of cetyltrimethylammonium bromide (Aldrich), 1.04 g of Ai(OH)3 (Pfaltz & Bauer), 2.7 g of NaOH (Carlo Erba) and 20 g of fumed SiO2 (Sigma) were added in this order in 240 g of bi-distilled water. The resulting gel was aged for 2 hours at room temperature and then transferred to a Teflon-lined autoclave in a thermostated oven at 150 ~ for 24 hours. The synthesis product was filtered, washed with distilled water and then dried at 80 ~ The synthesized sample was calcined in air at the temperature of 550 ~ for 8 hours with a heating rate of 1 ~ The N2 adsorption-desorption volumetric isotherm was measured on a Micromeritics Asap 2010 instrument. Sample was pre-treated under vacuum at 300~ to a residual pressure of 2 ktmHg. Surface area was obtained by BET linearization in the pressure range 0.05 to 0.2 P/Po. Lattice pore volume was obtained from the amount of nitrogen gas adsorbed at the top of the rising section of the of type IV isotherm. The pore size distribution was calculated on the basis of desorption data by employing the Barrett-JoynerHalenda (BJH) method [6].

2.2. Drug loading by soaking procedure As an example, drug loading into mesoporous matrix was achieved by impregnation, soaking for three days at room temperature, under continuos stirring, 600 mg of the matrix in a concentrated solution (200 mg of Diflunisal in 10 ml of methanol) acting as a swelling agent. Methanol was also used for Np and Ib, while water was used for IbNa. The solvent was then removed by filtration and the samples dried under vacuum.

1167

2.2.1 Determination of drug amount entrapped in the matrix In order to determine the amounts of loaded drug into the mesoporous matrix, different methods have been employed depending on the drug: 1. sample impregnated with Df was dispersed in methanol at room temperature and kept under constant stirring for 4 days; 2. samples impregnated with Np and Ib were extensively extracted with methanol at 60~ for 4 hours; 3. sample impregnated with Ib-Na was dispersed in water at 60~ and kept under constant stirring for 3 days. The solvents after extraction were collected by filtration under vacuum and assayed by HPLC analysis for the quantitative determination of the drug loaded in the matrices. HPLC analyses were carried out on a system consisting of a Varian 9012 Liquid Chromatography equipped with a Rhedyne Injector 7125 (fitted with a 10 B1 loop), a Kontron HPLC Detector 432 on line with a computerized HP workstation. In the method HPLC, a reversed phase C18 column (BBondapack; 10 Bm of 250 mm x 4.6 mm i.d., obtained from Waters) equipped with a direct-connect guard column C18 (Waters) was used; mobile phase (flow 1 mL/min) was methanol:phosphoric acid (0.1% v/v) in the ratio 70:30 (v/v) for Df analyses and acetonitrile:acetic acid (5 g/l) in the ratio 50:50 (v/v) for Np, Ib and IbNa. Eluate was monitored at 254 nm for all drugs.

2.2.2. Swelling studies 25 mg of the impregnated matrices were kept in contact with 20 ml of double-distilled water, HC1 0.1N at pH 1 or 20 ml of a phosphate buffer (NaC1, Na2HPO4, KH2PO4) at pH 6.8 until the swelling equilibrium was reached, then each swollen sample was filtered, blotted with paper and weighed. Water content (WC %) was calculated as follows: WC (%) -- (Ws-Wd)/Ws X 100 where Ws and Wd are the weights of the swollen and dry matrix, respectively.

2.2.3. Drug release studies In vitro release studies of drug from mesoporous silicate matrix were carried out by keeping

25 mg of each sample of matrix loaded with drug in 20 ml of a 0.1N HC1 solution (pH 1.0 simulated gastric juice), at 37_+0.1~ in a water bath with magnetic stirring (100 r.p.m.), for 2 hours. As, in all cases, the releases were not complete after this period of incubation time, a mixture of Na3PO4 0.2M and NaOH 0.1N was added in order to adjust the pH to 6.8 (simulated intestinal fluid). Sink conditions were maintained throughout the experiment. Then, at suitable time intervals, samples were filtered and the aqueous solutions analysed by HPLC according to the conditions previously reported.

3. RESULTS AND DISCUSSION The reaction of cetyltrimethylammonium bromide, aluminium hydroxide, sodium hydroxide and fumed silica in water at 150 ~ for 24 hours in autoclave gave rise to a solid material that after proper filtration and calcination gave mesoporous material belonging to MCM-41 family. The obtained sample was characterized by elemental analysis, X-ray diffraction, thermogravimetric analysis and N2 adsorption- desorption isotherm at 77K.

1168 The amount of aluminium in 100 g of MCM-41 sample, calculated by elemental analysis, is equal to 1.04 g, confirming the complete incorporation of aluminium into the mesoporous material. X-ray powder diffraction pattern of calcined MCM-41 shows a single reflection (Fig. 2) that indicates the small size of the crystals [7]. The unit cell parameter a~ (distance from centres of two adjacent pores) is 49.2 A. The N2 adsorption-desorption isotherm at 77K (Fig. 3) shows the main nitrogen uptake typical of mesopore filling in the range 0.3-0.4 P/Po. Pore volume at P/Po=0.8 is 0.92 cm3/g while specific surface area calculated according BET method is 1124 m2/g. Average pore diameter (4V/A by BET) is 43]k. The BJH pore size distribution obtained from desorption isotherm of mesoporous sample is shown in Fig. 4. Two main regions of nitrogen consumption can be noted on the logaritmic plot of pore volume versus pore diameter. The sharper one is assigned to pores whose diameters ranges between 26 and 32 A, the regular micelles-produced porosity typical of MCM-41 materials. BJH method was shown to underestimate the size of pores [8-10] and therefore it can be used just for a qualitative evaluation of the actual pore size distribution. The broader one is assigned to pores whose diameter ranges between 300 and 1000 A. It derives from the amount of nitrogen desorbed in the high pressures region on the isotherm (relative pressures comprised between 0.9 and 1) and is assigned to very large mesopores or macropores generated from further silica condensation on the composite particles. Molecular size of active agents investigated (Diflunisal, 10.3x6.0/~ Ibuprofen 11.5x6.0/~ Naproxen 12.6x7.7 A) are widely compatible with calculated pore diameter so that the influence of different size on dynamic behaviour in mesoporous channels is negligible. The composite sample obtained from synthesis has been calcined at 550~ Mesoporous matrices used for drug loading have been stored at room temperature and atmospheric pressures so that it is reasonable to suppose that silica surface is in a maximum state of hydroxylation and hydration. Progressive heating of sample first removes mukiple layers of physically adsorbed water on silica surface. Dry silica surface is in a maximum state of hydroxylation and isolated singles, geminals and vicinals hydroxyl groups are present. At this stage hydration is totally reversible. Successively the concentration of isolated single OH groups increases and concentration of vicinal OH groups decreases. Water desorption is fast, rehydroxylation is reversible. Further heating completely removes vicinal OH groups while free single and free geminal OH groups still remain on the silica surface. Rehydroxylation is reversible for weakened and strained siloxane bridges (Si-O-Si). Overall amount of OH groups on the surface progressively decreases and concentration of siloxane bridges increases. Siloxane surface is hydrophobic and the reydroxylation rate is very low at room temperature. Hydroxyl population is now not reversibly decreased. This last phenomenon is probably not important for samples calcined at 550~ Thermogravimetric analysis of calcined sample (Figure 5, curve a) shows a weight loss starting at 300 ~ assigned to the condensation of two SiOH groups with loss of a molecule of water. Starting from this assumption a concentration of 1 SiOH group every ca. 4 atoms of Si can be calculated. The surface of silicoaluminate MCM-41 sample is characterized by hydroxyl groups with different acid strength: 1) SiOH groups showing increasing acidity with the increase of their hydrogen bonding interaction; these sites have weak acidity; 2) aluminium atoms form Br6nsted acid sites AI(OH)Si that are due to the presence of hydroxyl groups on aluminium oxide species and Lewis acid sites generated from an electron vacancy on a aluminium atom in a cluster of aluminium oxide. Figure 5 (curve b) shows thermogravimetric analysis of Df-loaded matrix. Three main regions are found: the first, below 200~ is assigned to removal of muki-

1169

600

~

~Adsorpt!on

l

500

~

400 "6 ;> 300

t"

_.=

"~ 200 r~

100

.f ,, ,

4

6 8 10 2 Theta Degrees

1'2

14

Fig. 2. X-ray pattern of calcined MCM- 41

0.0

o12 " 0 1 4 .... 0 ' . 6 " 0 ' . 8 Relative Pressure P/Po

1.0

Fig. 3. N2 adsorption-desorption isotherm at 77 K of calcined MCM-41

8 7-

6-

~32" 1 100 Pore diaraXer (Angstrom)

1000

Fig. 4. BJH desorption pore size distribution. Table 1. Drug 10adingvalues-

Diflunisal Naproxen ~6uprofen.... Ibuprofen sodium salt Drug Loading % (a) 8.7 7.3 6.4 6.9 (a) Amount of drug impregnated (%W/w) with respect to the impregnateci mesoporous matrix. and mono-layer of adsorbed water; a region, between 200 and 620~ assigned to the combined effect of condensation of vicinal silanols, degradation of drug molecules and removal of drug fragments adsorbed on the surface; the last region corresponding to temperatures higher than 620~ corresponds to the dehydroxylation on silica surfaces. Simple soaking procedures were used for the loading of each drug into the matrix using a proper solvent as swelling agent. Therefore the mechanism of the loading of the drug into the matrix involves the occurrence of physic interaction between drug molecule groups and matrix residues. In Table 1 drug loading values for Df, Np, Ib and IbNa are reported.

1170 In order to evaluate the affmity of prepared matrices towards aqueous medium, the value of water content percentage (WC%) was determined in aqueous media which simulate some biological fluids such as gastric (pH 1) and intestinal (pH 6.8) as well as in bidistilled water. In order to understand how the drug presence influences the swelling behaviour, the results obtained from matrices containing Df, Np, Ib and IbNa have been compared with those related to the matrix without any drug. The results of these experiments are reported in Table 2. Table 2. Percentage of water content of free and drug loaded MS matrices Experimental conditions

MS

H20 65.5 pH=l 77.9 pH=6.8 74.7 (") Each measurement is the

% WATER CONTENT (,o Df loaded-MS Ib loaded-MS IbNa loadedMS 59.7 73.7 66.8 68.9 73.9 70.8 64.0 75.4 68.1 average of three experiments.

100

Np loadedMS 74.4 73.0 76.7

(a)

98

(b)

96 94" 92 ~

~

90

88 86 84 82 80

0

100

200

300

400

500

Temperature(~

6()0

700

8000"

' 100 ' 200 ' 300

400 ' 500

600 ' 700

800

Temperature(~

Fig. 5. Thermogravimetric curves of" (a) mesoporous matrix and (b) Df-loaded matrix These results show that the mesoporous matrix has a high water affinity and this makes it potentially biocompatible; furthermore they show that the drugs do not affect the value of WC %. With reference to the release of the drug from the matrix, it is important to outline that many variables, such as physicochemical properties of the drug, hydrophilicity of the mesoporous silica and loading method of the drug, control release kinetics. Figures 6, 7, 8 and 9 show drug release rates expressed as percent of drug delivered, related to the drug loading value, as a function of time (see paragraph 2.2.3). In particular Figure 6 shows that drug release of Df is quite low at pH 1.0, whereas it increases quickly when pH jumps (after two hours) to a value near to the neutrality. On the contrary, drug release of Ib occurs already at pH 1.0 and it seems to be complete within 2 hours; for Np about 75% of the drug is released within 1 hour and the release becomes complete after pH change, while for IbNa drug release starts quickly at pH 1.0, but it is not complete also after pH change and until about 12 hours.

1171 100

i

i

I

t

6

1oo

75

75

so

},0

25

25

0

.

.

.

.

2

4

6

$

.

.

10

12

0 0

.

.

.

.

.

.

2

4

6

8

10

12

Time (hours)

Time (hours)

Fig. 6. Diflunisal release at pH 1 and 6.8 as a function of time 100"

Fig. 7. Ibuprofen release at pH 1 and 6.8 as a function of time

o'|

es

P

a-50.

'l

25"

0 0

.

.

.

.

2

4

6

8

1h~ 0m~)

.

10

.

12

0 0

,

,

,

,

2

4

6

8

,

10

,

12

lh~(lms)

Fig. 8. Naproxen release at pH 1 and 6.8 as Fig. 9. Sodium Ibuprofen release at pH 1 and a function of time 6.8 as a function of time Being the average pore size quite greater than the major diameter of all examined drug molecules, these quite different release behaviours seem not to be correlated to the drug ability to go across the pores of the matrix, but different phenomena have to be evoked. In the case of Ib and Np, the strong burst effect should be reasonable explained mainly considering the occurring of surface pore adsorption phenomena; besides this interaction seems not affected by pH changes. On the contrary, in the case of Df the change of pH value markedly increases the amount of dissociated drug molecules whose affinity for the solvent seems to be greater then for matrix groups. It seems reasonable to prevent the release of a drug not soluble in water or acid at the gastric level by means of an inorganic matrix and allowing it in the intestinal treat at neutral pH where the drug solubility increases. Electrostatic interaction between IbNa and matrix groups are probably the responsible for the incomplete release of this drug molecules. 4. CONCLUSIONS The suitability of the mesoporous silicate matrix as drug delivery systems has been evaluated by using different nonsteroid anti-inflammatory agents as model drugs. In particular, it has been shown that this type of matrix is able to trap the bioactive agents by a soaking procedure and, then, to release them in conditions mimicking the biological fluids. Besides the high affinity of these matrices for water make them potentially biocompatible.

1172 Release data suggest that the matrix impregnated with Diflunisal can offer a good potential as system for the controlled drug release. In fact, only 20% of drug is released at the gastric level allowing, in this way, the reduction of side effects related with oral administration of nonsteroidal anti-inflammatory agents and the release of the most part of drug in the intestinal duct. In our opinion that the activation of composite samples drastically influences the adsorption properties of the final porous matrix. In this regard, hydroxyl population, which is the main responsible for the interaction between drug and silicate surface, depends on the activation method. Chemical methods, other than calcination, can be, in fact, used for the removal of organic micelles without any thermal exposure of the sample. Further work is needed to verify if these methods can improve the DDSs properties of the MS samples. ACKNOWLEDGMENTS

The authors thank Prof. Nevio Picci (Department of Chemistry, University of Calabria) for his useful suggestions during the development of this research. REFERENCES

1. 2. 3. 4.

R. Langer; Nature,392 (1998) 5. J.R.B. Brouwers; J. Pharm. World Sci., 18 (1996) 158. E. Methiowitz, Enciclopedia of controlled drug delivery, Wiley, New York, 1999. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck; Nature, 359 (1992) 710. 5. M. Vallet-Regi, A. Ramila, R.P. del Real, J. Perez-Pariente; Chem. Mater.,13 (2001) 308. 6. E.P. Barrer, L. Joyner, P.P. Halenda, J.Am.Chem. Soc.,73 (1951) 373 7. A.Corma, Chem. Rev., 97 (1997) 2373 8. P.I. Ravikovitch, S.C.O. Domhnaill, A.V. Neimark, F.Scuth and K.K. Unger, Langmuir, 11 (1995) 4765 9. M.W. Maddox, K.E.Gubbins, Int. J. Thermophys.,15 (1994) 1115 10. C. Lastoskie, K.E. Gubbins and N.Quirke, J.Phys. Chem., 97 (1993) 4786


1173

Aluminum Incorporation and Interfacial Structures in AISBA-15 mesoporous solids- double resonance and Optically Pumped Hyperpolarized 129Xe N M R Studies. Elias Haddad a, Jean-Baptiste d'Espinose b, Andrei Nossov a, Flavien Guenneau a, Antoine G6d6ona* Laboratoire Syst6mes Interfaciaux h l'Echelle Nanom6trique (SIEN). CNRS-FRE 2312 aUniversit6 Pierre et Marie Curie, case courrier 196, 4 place Jussieu, 75252 Paris Cedex 05, France. Email : [email protected] b Physique Quantique, ESPCI, 10 rue Vauquelin, 75231 Paris Cedex 05, France Aluminum-incorporated SBA-15 mesoporous materials have been obtained by direct synthesis. The surfactant- aluminosilicate interaction during synthesis was studied by double resonance NMR and confronted with the structural properties of the materials obtained after calcinations. Continuous-flow laser-polarized ~29Xe NMR spectroscopy was applied for the first time to explore the porosity of the A1SBA-15 mesoporous molecular sieves. TRAPDOR experiments firmly established a strong interaction between segments of the PEO block of the surfactant with the silica-alumina framework. ~H Dipolar Dephasing revealed that the amount of segments rigidified by this interaction increased with the maturation time. The increased rigidity of the surfactant is to be linked with the increased mesoscopic ordering during maturation, resulting in the higher mesoporous surface obtained after calcinations. The invariability of the TRAPDOR effect proved that the strength of the interaction, that is the degree of interpenetration of the organic/inorganic phases remained the same irrespective of maturation time. Together with the dramatic decrease of the microporous volume with maturation time, this established that the origin of the microporosity of A1SBA-15 is to be found in the incomplete hydrolysis of the TEOS precursor itself rather than in the incomplete PEOaluminosilicate phase separation. 1. INTRODUCTION We have synthesized acid A1SBA-15 mesoporous solids with regular channels and very high thermal and hydrothermal stability [ 1]. Incorporation of A1 was established by HETCOR double resonance l H - 27A1 NMR [2]. A1SBA-15 materials retain the hexagonal order and physical properties of purely siliceous SBA-15. They present higher thermal stability and catalytic activity in cumene cracking reaction than A1MCM-41 solids. To better understand the origin of these improved properties, textural results from N2 porosity measurements are confronted with molecular scale double resonance MAS

1174 NMR results in order to discuss the incorporation of A1 and the interpenetration of the organic/inorganic phases during synthesis. Indeed, recent publications have evidenced the significant occurrence of a microporous "corona" around the internal surface of the mesopores in SBA-15 [3,4]. Considering that micropores result from the calcination of an incompletely hydrolyzed silicate precursor, it is of primary importance, if one wants to be able to control the extent of the microporosity, to understand at the molecular scale why the silicate network did not fully condense: Is it because of the interpenetration of the hydrophilic part of the surfactant with the forming inorganic phase [3]? Or is it because the organometallic TEOS precursor was not fully mineralized prior to calcination? To address this question, samples of different hydrolysis levels were prepared by varying the maturation time. It was then possible to investigate phase separation and ordering in the parent material by NMR double resonance between the organic protons and the aluminum of the solid, the results were then related to the structural properties of the calcined final mesoporous A1SBA-15. 2. EXPERIMENTAL 2.1. Materials and synthesis Al-containing SBA-15 mesoporous solids were synthesized by using tetraethyl orthosilicate (TEOS), aluminum tri-tert-butoxide, and triblock poly(ethylene oxide)poly(propylene oxide)-poly(ethylene oxide) (EOEoPO70EO20) Pluronic 123 copolymers. The synthesis conditions were described elsewhere [ 1]. After being stirred for 3 hours the gel solution was transferred into a Teflon bottle and heated at 100 ~ for different reaction or maturation times 0, 16 and 48 h. The solid products were filtered (parent composites) and finally calcined (calcined samples) in air flow (9 L h-1) at 823 K for 4 h with a heating rate of 24 K h ~. In what follows, the samples are denoted A1SBA-15. 2.2. Hyperpolarized 129XeNMR 129Xe NMR spectra were collected on a Bruker AMX 300 spectrometer operating at 83.03 MHz. Hyperpolarized (HP) xenon was produced in the optical pumping cell in the fringe field of the spectrometer magnet. The gas mixture containing 800 torr of He and 40 torr of Xe polarized to ca. 1% was delivered at 70 cc/min flow rate to the sample via plastic tubing. 256 FIDs were accumulated with 10~ts (n/2) pulses and 5s delays. 2.3. MAS-NMR Magic angle spinning nuclear magnetic resonance (MAS NMR) experiments were performed on a Bruker ASX500 spectrometer at 11.7 T. 27A1one-pulse experiments were performed at 14 kHz with a selective pulse (<x/6) duration of 0.5 ps, recycle time 1 s, and 5000 acquisitions. ~H one-pulse experiments were performed at 13 kHz with a n/2 pulse duration of 5.5 ~ts, recycle time 20 s, and 8 acquisitions. The 1H to 27A1 Cross-Polarization (CP) pulse scheme (Fig. la) used to establish 2D heteronuclear correlation (HETCOR or WISE) [5] was the usual one. The experimental conditions for CP to an abundant quadrupolar nuclei were determined by using the same considerations developed in our study of alumina surfaces by 1n to 27A1 CP [6]. The

1175 spectra were obtained with radio frequency magnetic field strengths (,Q/2rc) of approximately 44 kHz for 1H and 90 kHz for 27A1, and a spinning frequency (vr) of 4 kHz. The JH{a7A1} TRAPDOR pulse scheme is given in Fig. lb [7]. The dipolar dephased lH spin echo is recorded after evolution at different integer multiples of the rotation period (So) and compared to the spin echo obtained under the same conditions but with additional dephasing due to irradiation of Z7A1during half of the evolution period (S). The 27A1 TRAPDOR dephasing of the ~H signal (So-S)/So is then plotted as a function of the evolution time. The radio frequency fields were 45 kHz and 140 kHz for 1H and 27A1. n/2

1H

~1

n/2

rt

1H Aq

27A1

]

CP

I t~ V

27A1

Aq "-

Fig.la. Scheme of the HETCOR pulse Fig. lb. Scheme of the TRAPDOR sequence, pulse sequence.

2.4. Structural characterization N2 adsorption/desorption isotherms were measured on a Micromeritics ASAP-2000 instrument at liquid N2 temperature. Specific surface areas were calculated from the adsorption isotherms by the BET method, and pore size distributions from the desorption isotherms by the BJH method. To assess the presence of micropores, we used the modified c~s-plot analysis [8]. 3. RESULTS AND DISCUSSION

3.1. Structural properties The structural properties of A1SBA-15 samples have been analyzed in earlier studies by Synchrotron XRD and transmission electron microscopy [9,10]. The five wellresolved XRD peaks associated with 2D p6mm hexagonal symmetry and the well ordered hexagonal arrays of mesoporous channels from TEM images indicate the high structural quality of these materials. The nitrogen adsorption-desorption isotherms of the A1SBA-15 studied samples have been used to obtain information about their mesoporosity. These isotherms, illustrated in figure 2, are similar to those reported earlier [2]. As the maturation time decreased, the N2 adsorption isotherms preserved the same shape with a significant decrease of the hysteresis loop towards lower P/P0 values. This led to smaller surface area and mean pore diameter. The pore size distributions (PSD) for the studied samples are shown in figure 3. They indicated that the mesoporous channels were very regular with a narrow gaussian distribution centered at 4.5, 7.0, and 8.2 nm respectively for samples at 0, 16 and 48 h. of maturation time. The textural characteristics of the A1SBA-15 samples are listed in table1.

1176 The adsorption-desorption isotherms have been ft~her used to evidence an additional microporosity. The specific mesoporous volume and the micropore volume were calculated using modified as-plots. The micropore volume evolved inversely to the maturation time. 48 h

0,10

48 h

1000

0,08 800

16h ~

00

O >

0,06

3

9

rabon

400

"~

"E

o,o4

no maturation

G) O

i5

,Cu,Fel (HT) with x = 0, 0.5, 1, 1.5 and 2. Co2Fe~(OH) sample has also been prepared by coprecipitation of nitrate elements with NaOH. The final pH was 8 and the precipitate was immediately filtered, washed several times with deionized water and dried at 100~ for 24 hours. In order to study the catalytic behaviour of these samples, a calcination treatment has been performed under flow of air (4 L.h 1) at 500~ (2~ 1 and 4 hours at desired temperature) ; the solids obtained were named Co2.• (HT) and CozFel500 (OH). 2.2. Characterization The specific areas of solids were determined by BET method using a Quantasorb Junior apparatus and the gas adsorbed at -196~ is pure nitrogen. The structures of solids were analysed by X-ray diffraction (XRD) technique in a Siemens D5000 diffractometer equipped with a copper anode. The XRD patterns were recorded at room temperature. All the XRD patterns were assigned using the JCPDS data-base. Differential thermal analysis (DTA) (Netzsch STA 409 equipped with a microbalance (TG)) was conducted in flow air (75 mL.min 1) at a heating rate of 5~ 1 from room temperature to 1000~ with around 50 mg of sample. The IR spectrum of each sample was recorded by accumulating 16 scans at 2 cm ~ resolution using a Perkin Elmer System 2000 Fourier transform infrared spectrometer.

2.3. Catalytic tests Before the catalytic test, the solid (200 mg) was calcined under a flow of air (4 L.h ~) at 500~ during 4 hours. Toluene oxidation was carried out in a flow microreactor and studied up to the reaction starts (ignition temperature). Then, the catalyst deactivation was studied for 48 hours. The reactive flow was composed of 99 mL.min ~ of air and 2.6 mL.min t of gaseous toluene. A high quantity of toluene was used in order to observe the catalytic behaviour in severe conditions. The analysis of combustion products was performed using a Varian 3600 chromatography equipped with TCD and FID. 3. RESULTS AND DISCUSSION 3.1. XRD

3.1.1. Uncalcined samples Figure l a shows the XRD pattern of C02Fe~(OH) sample. The comparison of the XRD pattern with the JCPDS data base indicates the presence of Fe3Oa, Co304 or CoFe204 phases. Ifideed, the XRD lines of these three oxide phases are very closed. However, the formation of Fe304 phase can be eliminated since the presence of Fe 2+ ions in the solution is necessary to form the Fe(n)Fe~m)204 spinel. Moreover, the reduction of Fe 3+ into Fe 2+ by Co 2+ in the solution

1199

is not possible taking into consideration the oxydo-reduction potential values of Co~+/Co 2+ (1.9V) and Fe3+/Fe2+ (0.77V). Using Co 2+ and Fe 2+ nitrates, Christoskova et al. [7] showed that C02Fel freshly prepared and calcined at 200~ are amorphous. So, it seems that the oxidation of Co 2+ into Co 3" to lead to C0304 spinel oxide does not take place at low temperature. Moreover, the synthesis of MnFe204 ferrites by coprecipitation of Fe 3+ and Co 2+ ions is possible in the temperature range of 50-100~ [8]. Therefore, the XRD pattern of our sample can be attributed to the CoFe204 phase : the simultaneous presence of Fe 3§ and Co 2§ into the solution allows to form this oxide at low temperature, the excess of cobalt in the solid (Co/Fe = 2) is probably in amorphous phase.

d "2".

[]

C

,.a

d c

.,..,

e~

nzx

~D

[]

[]

ix

zx

b

[] []

'

5

o

a

25

45

65

20 (o) Figure 1 9XRD patterns of (a) Co2Fe~(OH), (b) Co2Fel(HT), (c) Co2FelS00(OH) and (d) C02Fel500(HT) samples.

5

[]

25

o

45

oo

a 65

20 (o) Figure 2 9 XRD patterns of uncalcined Co2_xCuxFel (HT) samples with (a) x = 0.5, (b) x = 1, (c) x = 1.5, (d) x = 2.

The XRD pattern of Co2FeI(HT) is shown in Figure lb. It revealed narrow, symmetric, strong lines at low 20 values and weaker and less symmetric lines at high 20 values (see D on Figure 1) characteristics of layered materials. From the position of the two strongest lines of crystallographic indices (003) and (006), the lattice distances, d003 and d006, were calculated and used to determine the lattice parameter " c' " representing the thickness of a hydroxide layer and an interlayer. The "c" parameter is equal to " 3c' ". This parameter depends on the anion size (component in the interlayer), the value of MU/M m ratio (M = Co 2+, Cu 2" and Fe 3+) and the degree of hydration. The lattice parameter " a " was determined from the (110) reflection line. This parameter depends on the nature of the cations (in the bmcite-like sheet) and the value of Mn/M m ratio [4]. The crystallographic parameters obtained for Co2Fe~(HT) sample (a = 3.1267 A and c = 22.8001 A) are closed to those found in the literature for a Co/Fe ratio of 3 [5]. The XRD patterns of Co2..~Cu.~Fe~ (HT) with x = 0.5, 1, 1.5 and 2 are shown in Figure 2. The hydrotalcite phase was formed for x = 0.5 (Figure 2a). The crystallographic parameters of Co~.sCu0.sFe~ (HT) sample (a = 3.1257 .~ and c = 22.7759 A) are lower than those of Co2Fel (HT) sample. This result can be explained by the substitution of Co 2+ by Cu 2., the ionic radius

1200 of Cu 2§ (0.69 ,a,) being lower than this of Co 2+ (0.745 A) [4]. For ColCulFel (HT) sample (Figure 2b), a low crystallised HT phase (see [] in Figure 2b) was observed but it was mixed with another non identified phase (see A in Figure 2b). With increasing copper content, an amorphous phase was obtained. Then, for the sample without cobalt (x = 2), the monoclinic malachite phase (Cu2(OH)2CO3) was observed. Therefore, when the Cu quantity is superior to 1, the HT phase is not pure or not formed. Even though Cu 2+ ions have a suitable ionic radius to form anionic clays, they give rise preferentially to the precipitation of malachite-like phases, because of the Jahn-Teller effect, which for d 9 ions favours the formation of distorted octahedral structures [9]. But associated cations (M 2+ = M g 2+, Co 2+ or Zn 2+) favour the entrance of Cu 2+ ions into aluminium (A13+) based hydrotalcite phases, which are obtained without side phases for Cu2+/m 2+ ratios < 1 [4]. According to this result, when copper is associated with Co 2+, iron (Fe 3+) based HT phase is also formed for Cu/C0=0.33 and Cu/Co=l. The surface area values of CoiFed(OH) and C02..~CuxFe~ (HT) are reported in Table 1. For C02Fel(OH) sample, the formation of amorphous phase with CoFe204 is confirmed by the high surface area value. When the HT phase is formed, the BET value is three times lower. The surface areas of others untreated samples increased with the Cu content. The rise of Cu quantity (up to x = 1.5) probably leads to an amorphous phase having a high specific area. The surface area of Cu2Fel(HT) is lower according to the malachite phase crystallisation. Table 1 Surface areas (m2.g 1) of CozFel(OH) and Co2.xCuxFel (HT) with x = 0, 0.5, 1, 1.5 and 2 Samples x = 0 (OH) x = 0 (HT) x = 0.5 x= 1 x = 1.5 x=2 Uncalcined 152 55 86 168 213 109 Calcined 66 74 78 70 58 26 After test 22 30 31 31 18 10

3.1.2. Calcined samples The XRD patterns of Co2.xCuxFel (HT) with x = 0.5, 1, 1.5 and 2 calcined at 500~ are presented in Figure 3. The X R profile of C02Fel500(OH) was similar to that of Co2Fe~(OH). However, the lines were more narrow, indicating a higher crystallisation of the solid. This result was confirmed by the ~ decrease of the surface area value. The XRD 2 pattern of C02Fel500(HT) sample revealed the destruction of the HT phase to form a similar phase to that of C02Fe~500(OH). But, for Co2Fe~500(HT), the diffraction peaks were broader according to the higher BET value of 5 25 45 65 C02Fel500(HT). A high surface area and a 20 (o) better metal dispersion were often observed when oxides were derived from HT phases [4]. Figure 3 9XRD patterns of Co>xCu.~Fe~(HT) The broadness of diffraction peaks can be also samples calcined at 500~ with (a) x = 0.5, explained by the presence of a mixture of two (b) x - 1, (c) x = 1.5, (d) x = 2. oxide phases 9CoFe204 and Co304.

1201 Indeed, the oxidation of Co 2+ into Co 3§ to form the Co304 spinel could occur during the calcination at 500~ For x = 0.5, the destruction of the hydrotalcite phase into spinels is also observed (Figure 3a). For x = 1 and 1.5 (Figure 3b and 3c), in addition to the oxide spinel phases, monoclinic CuO phase (tenorite) is produced (see * in Figure 3). The CuO quantity increases with the rise of copper content in the solid. For x = 2 (Figure 3d), the presence of CuO and ct-Fe203 (haematite) phases is detected. The haematite phase seems not to be formed when cobalt is present in the solid. Uzunova et al. [6] shown that for Co-Fe oxide derived from HT phase, ot-Fe203 phase could be formed if cobalt content in the solid is low (Fe/Co>l), the cobalt-rich samples leading to a cobalt-substituted Fe304 phase (CoxFe3.xO4). 3.2. T G - DTA The TG-DTA curves of CozFel(OH) and CozFel(HT) are shown in Figure 4. A broad endothermic peak in the DTA profile of Co2Fe~(OH), accompanied with a slow mass loss (TG) was observed. The shoulder of the DTA peak can be attributed to the presence of cobalt (II) and/or iron (III) hydroxides, the decomposition of such amorphous compounds taking place respectively at 266~ and 238~ (broad peak). On the contrary, for Co?Fel(HT) sample, a narrow endothermic signal registered at 170~ indicated the collapse of the HT structure, to form metal oxides as Co304 and/or CoFe204. This decomposition temperature value is in good agreement with data reported in the literature. In fact, Uzunova et al. [5] observed the destruction of Co3Fel(HT) at 170~ The endothermic signal was accompanied with a quick mass loss (28.01%) which corresponds to the removal in a single step of hydroxide groups, carbonates anions and interlayer water in Co?Fe~(OH)6(co3Z)vz, 1.25 HzO hydrotalcite, yielding Co304 and CoFezO4 spinels. In addition, CozFe~(OH) and CozFe~(HT) samples presented a second endothermic signal at respectively 970~ and 965~ which can be attributed to the thermal decomposition of cobalt and/or iron spinels. Indeed, starting from CoCo204, an endothermic DTA signal accompanied with a mass loss value of 5.49% was observed at 933~ The experimental mass loss value corresponds to the theoretical one obtained with the reaction : Co304 --+ 3 CoO + '/2 Oz. The DTA signals obtained during the calcination of Co2.xCu.~Fe~(HT) samples with x = 0.5, 1, 1.5 and 2 are presented in Figure 5. For x = 0.5, the narrow endothermic peak was always present indicating the destruction of the HT structure. For x _> 1, the HT phase has not been formed and the endothermic signal around 170~ disappeared. However, an exothermic signal (without weight mass loss) around 510-530~ appeared. The DTA signal intensity increased with copper content. It can be attributed to the crystallisation of CuO phase. Indeed, after calcination at 500~ for 4 hours, the presence of CuO phase (tenorite) has been detected by XRD analysis for x > 1, the quantity of CuO increasing with the copper content in the solid. The signal at around 900~ corresponding to the spinel destruction, was present in the samples containing cobalt and disappeared for CuzFe1(HT) sample. The DTA signal intensity decreased and the maximum was shifted to lower temperatures when cobalt content decreased. This result confirms that the endothermic peak corresponds to the decomposition of a cobalt based spinel. For the CuzFel (HT) sample, the DTA curve revealed a single endothermic peak at 333~ which corresponds to the destruction of malachite structure. Indeed, the experimental mass loss value (21.5%) is in agreement with the theoretical one (20.07%) and corresponds to the destruction of malachite into c~-Fe203 (haematite) and CuO, oxides detected by XRD analysis after calcination at 500~ during 4 hours.

1202

531~

::i ~

/

~

!28.01% d

.~o "E- |

~o 170~

~

//

25

9

I

225

]

3

3

3

~

1

I I

xq73~

--N'I

425 625 825 Temperature (~C)

Figure 4 : TG-DTA curves of (a) CozFel(OH) and (b) CozFel(HT) samples. 3.3.

~

970oct

13.85%[

I

_

b .,..~

D

b[~

./v_...~ I

_

1

20

220

I

!

420 620 Temperature (~

t

820

Figure 5 : DTA curves of Co2,~CuxFel(HT) (a) x = 0.5, (b) x = 1, (c) x = 1.5, (d) x = 2.

FTIR

The IR spectrum of Co2Fel(OH) sample in the range of 450-4450 cm 1 (not presented) showed two bands characteristics of the stretching vibrations of metal-oxygen bonds. The first band at 559 cm 1 could be associated with the BOB3 vibrations in the spinel lattice and the second band at 656 cm "1 could be attributed to the ABO3 vibrations (B = metal ions in an octahedral position and A - metal ions in a tetrahedral position). In comparison with C o 3 0 4 spinel, a slight difference in wavenumbers was however observed : 571 and 664 cm 1 for respectively the both bands characteristics of the stretching vibrations of Co-oxygen bonds [5, 10]. In fact, the lower wavenumbers associated with the decrease of the bond strength of cation-oxygen could be explained by the substitution of Co ions with Fe ions leading a bathochromic shift of the two bands. This IR result confirmed the formation at low temperature of CoFe204 spinel assumed from XRD analysis. FTIR spectra in the range of 1200-1600 cm l of Co2Fel(OH) and COz..~CuxFel(HT) samples with 0 < x _< 2 are presented in Figure 6. Figure 6a showed a prominent band at 1384 cm ~ which can be attributed to free nitrate vibrations (nitrates not eliminated during washing of the precipitate) [11]. In the other hand, Co2Fe~(HT) and Co~.sCu0.sFel(HT) samples (Figure 6b and 6c) showed a strong band at 1353 cm ~ which can be attributed to u(CO3) of interlayer carbonates [5] in accordance with the formation of the hydrotalcite phase. When the HT phase has not been formed (x >_ 1), this band disappeared and a double band detected at 1397-1530 cm ~ (Figure 6d and 6e) was getting more significant with increasing copper concentration in the sample. This doublet can be assigned to carbonates in interaction with Cu 2+ ions [12] in a bidentate complex [13]. For the CuzFe~(HT) sample (Figure 60, shoulders at 1423 cm ~ and 1498 cm ~ appeared and can be explained by the presence of carbonates in a monodentate complex [13]. The rise of the Cu content increases the amount of carbonate anions and in the case of the malachite structure leads to different types of carbonates which have different interactions with copper. Alejandre et al. [12] n o t i c e d t h e same trends studying Cu-AI hydrotalcite.

1203 The calcination at 500~ of Co2Fel(OH) sample leaded to the elimination of the nitrates anions since the strong band at 1384 c m "1 disappeared. No significant change (shift in wave numbers) was observed for the bands characteristics to the spinel lattice vibrations. IR analysis of Co2FeI500(HT) revealed the disappearance of the bands related to HT structure and the appearance of the bands at 545 and 640 cm -1 characteristics to the spinel structure vibrations. 1522 1498

II Ignition Temperature

1,397

[] Catalyst Temperature 450 400 e

1353

d

o

350 300 250 200

r

150 100 50 1600

1500 1400 1300 Wavenumbers (cm l )

1200

Figure 6 : IR profiles of (a) Co2Fel (OH) and Co2.xCuxFel (HT) samples with (b) x = 0, (c) x = 0.5, (d) x = 1, (e) x = 1.5 and (f) x = 2.

0

I

a

b

c

i

1

1

d

e

f

Figure 7 : Ignition and catalyst temperatures of (a) Co2Fe1500 (OH) and Co2,,Cu.,Fe1500 (HT) with (b) x = 0, (c) x = 0.5, (d) x = 1, (e) x = 1.5 and (f) x = 2.

3.4. C A T A L Y T I C A C T I V I T Y The catalytic activity of CO2_xCuxFe1500(HT) for the total oxidation of toluene has been evaluating by comparing the ignition temperature (Figure 7). In order to check the role of the catalyst preparation method, the same experiment with Co2Fe1500(OH) was performed. The result included in Figure 7, showed clearly that the use of the HT phase precursor of the oxide phase was beneficial since the ignition temperature was 25~ lower for Co2Fet500(HT). The exothermicity of the reaction could be estimated by comparing the ignition and the catalyst temperatures. Regarding the oxides derived from the HT phase, it is obvious to deduce that higher cobalt content induces higher exothermicity. The difference observed between Co2Fel500(OH) and Co2Fel500(HT) in terms of reactivity and exothermicity can be explained by different dispersion of" cobalt and iron coming from different preparation methods for precursors [4]. Besides, higher specific area was obtained for Co2Fe1500(HT) (Table 1). The copper addition in the solid produced a beneficial effect on the ignition temperature. But the rise of copper content in the sample increased the ignition temperature. The formation of well developed CuO crystallites (evidenced by XRD analysis for x ___ 1) probably lowers the number of available active sites at the surface. Bahranowski et al. [6] shown the role of interface boundaries between CuO and CuCr204 since the combustion of toluene over CuO + CuCr204 mixture was less effective than Cu-Cr mixed oxide including CuO. However the lower ignition temperature was obtained for COl.sCu0.sFe~500(HT) in which CuO phase has

1204 not been produced. So the best reactivity of this sample can be explained by the formation of another spinel phase including copper as CuyMzO4 (M = Fe or Co). The only carbonaceous product for the toluene oxidation was CO2 except for Co2Fe1500(OH) where benzene appeared in trace amounts. The formation of benzene can be explained by the mechanism of combustion in which the first step is the cracking of the C-C bonds before oxidation to CO2 and H20. For the toluene, the cracking of C-C bond of the exocyclic methyl is the easier rupture. The formed benzene is very stable and then much more difficult to crack. The presence of benzene can therefore be explained for the less active catalysts as CozFe~500(OH). No deactivation for Co containing samples was observed after 48 hours on stream despite of the strong decrease of specific area (Table 1). The lack of dependence between catalytic deactivation and BET values can be due to the complex phase composition of mixed oxides. 4. CONCLUSION This study reports an investigation in the total toluene oxidation on Co2.xCu.~Fel mixed oxides synthesized by hydrotalcite method. For x > 1, the HT structure was not formed because of a strong interaction of Cu 2+ and CO32. However a layered material was obtained for x < 1. For x < 2, the thermal decomposition at 500~ of the different hydroxides led to cobalt and/or ferrite spinels. These mixed oxides were more active than Fe203 obtained for x = 2. A catalytic synergetic effect was observed for copper containing samples. But the ignition temperature increased with the Cu content. The disappearance of the synergetic effect could be related to the presence of CuO, which probably lowers the number of available active sites at the surface. Better performance of Co~.sCu0.sFel derived from a HT precursor could be explained by CuyMzO4 (M - Fe or Co) oxide formation besides ferrite spinels. REFERENCES

[ 1] P. Papaeffhimiou, T. Ioannides, X.E. Verykios, Appl. Catal. B 13 (1997) 175 [2] P.O. Larsson, A. Andersson, B. Svensson, L.R. Wallenberg, in "Environmental Catalysis", G. Centi et al. (Eds), Rome, (1995) 547 [3] S. Scire, S. Minico, C. Crisafulli, S. Galvagnio, Catalysis Communications, 2 (2001) 229 [4] F. Cavani, F. Trifiro, A. Vaccari, Catal. Today, 11 (1991) 173 [5] E. Uzunova, D. Klissurski, I. Mitov, P. Stefanov, Chem. Mater., 5 (1993) 576 [6] K. Bahranowski, E. Bielanska, R. Janik, T. Machej, E.M. Serwicka, Clay Minerals, 34 (1999) 67 [7] St.G. Christoskova, M. Stoyanova, M. Georgieva, Appl. Catal. A, 208 (2001) 235 [8] R.M. Cornell, Clay Minerals, 23 (1988) 329 [9] A. Vaccari, Catalysis Today, 41 (1998) 53 [10] St.G. Christoskova, M. Stoyanova, M. Georgieva, D. Mehandjiev, Materials Chemistry and Physics, 60 (1999) 39 [11] J.T. Kloprogge, L. Hickey, R.L. Frost, Applied Clay Science, 18 (2001) 37 [12] A. Alejandre, F. Medina, X. Rodriguez, P. Salagre, J.E. Sueiras, J. Catal., 188 (1999) 311 [13] K. Nakamoto, Infrared spectra of inorganic and coordination compounds, second edition, Willey Interscience, New-York, 1970, p. 169


1205

Catalytic oxidation over transition metal doped MCM-48 molecular sieves Changping Wei a*, Qiang Cai b, Xuwei yange, Wenqin pangb, Yingli Bi ~ and Kaiji Zhen~ aDepartment of Chemistry Engineering, Jilin Institute of Technology, Changchun 130012, P.R.China bKey Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P.R.China CDepartment of Chemistry, Jilin University, Changchun 130023, P.R.China A series of MCM-48 mesoporous molecular sieves doped with Ti, Cr, Mo, Zr, and Cu were synthesized by hydrothermal crystallization and characterized by XRD, UV, HRTEM and N2 adsorption. Selectively catalytic oxidation of a-long chain eicosanol to the corresponding t~eicosanoic acid over this series of catalysts has been tested. The optimum reaction temperature and reaction time were given. Experimental results show that MCM-48 molecular sieves doped a suitable amount of transition metal may enhance the yield and the selectivity of a-eicosanoic acid. The sort and the amount of the doped transitional metal have important effect on the catalytic activity. 1. INTRODUCTION Much attention has been paid to a new family of mesoporous molecular sieves denoted as M41S [1-2]. Due to their regular pore arrangement and narrow pore size distribution, they extended the range of ordered microporous molecular sieves. Ti- and V-substituted MCM41 and Ti-substituted hexagonal mesoporous silica such as Ti-HMS have been synthesized [37]. The mesoporous M41S materials have expanded significantly the possibilities for processing bulky molecules for catalytic and adsorption purposes. But as a catalyst, MCM-48 characterized by a three-dimensional channel system has several advantages over MCM-41 which has a one-dimensional channel system. For instance, the three-dimensional pore system is more resistant to blockage by extraneous ions than the one-dimensional pore system. Thus, MCM-48 may be applied to chemical industry and biochemistry [8-11 ]. In this work, we synthesized MCM-48 mesoporous molecular sieves doped with Ti, Cr, Mo, Zr, and Cu by hydrothermal crystallization, and characterized them by XRD, UV, HRTEM and N2 adsorption. The catalytic performance of M (Ti, Cr, Mo, Zr, and Cu)-MCM-

*Corresponding author, E-mail: [email protected]; Fax: 86-0431-5952413.

1206 48 for the oxidation of a-eicosanol to a-eicosanoic acid has been tested. 2. E X P E R I M E N T A L

2.1. Synthesis of M-MCM-48 molecular sieves The M-MCM-48 (M = Ti, Cr, Mo, Zr, and Cu) molecular sieves were synthesized [12] hydrothermally using TEOS, transition metal salts, CTAB, NaOH and distilled water. The procedure is described below: NaOH was dissolved in distilled water, then transition metal salts and the CTAB were added. When the solution became homogeneous, TEOS was added and the resulting solution was transferred to an autoclave and heated at 373 K for three days. The products were washed with distilled water, dried at ambient temperature and calcined at 823 K for 4h. The obtained catalyst samples are as M-MCM-48, where M denotes the transition metal. 2.2. XRD measurement The X-ray diffraction patterns of the M-MCM-48 were recorded on a SCINAG XDS-2000 Diffractometer with Cu-I~ radiation.

2.3. UV and HRTEM measurement The UV diffusion reflection spectra were recorded on a UV-3100 (HITACHI). HRTEM profiles were obtained on a HIACHI-8100 transmission electron microscope operated at 200 KV within a thin section prepared by ultramicroscope. 2.4. Adsorption / desorption measurement Nitrogen adsorption and desorption isotherms at 77K were measured using a Micromeritics ASAP 2400 Instrument. The data were analyzed by the BJH (Barrett-Joyner-Halenda) method using the Halsey equation for multilayer thickness. The pore-size distribution was obtained from the analysis of adsorption branch of the isotherm. 2.5. Test of the catalytic oxidation The catalytic oxidation of a-eicosanol was carried out in a 4-neck flask equipped with a stirring rod, a thermometer, an oxygen inlet and a condenser. Reactions were carried out at 413 K for 5h. 0.1-0.2 g catalyst (100 mesh) was used. The a-eicosanol was purified before used. Conversion of a-eicosanol and the yield of a-eicosanoic acid were calculated according to a stearic acidity of the product which was determined as following: 1.0 g product was dissolved in 70 ml hot ethanol. To the solution 6 drops of phenol phthalein and excessive amount of 0.2 M KOH were added, followed by titration with 0.2 M HC1. The stearic acidity was calculated based upon the titer.


3.1. X-ray diffraction The X-ray diffraction pattems of M-MCM-48 (M = Ti, Cr, Mo, Zr, and Cu) (Figure 1) are

1207 in agreement with those of typical MCM-48 materials [13]. All as-synthesized samples exhibited a very strong diffraction peak at around 2.30 ~ two weak peaks at 2.70 ~ and 4.40 ~ corresponding to diffraction planes of (211), (220) and (332), respectively. The XRD patterns of calcined M-MCM-48 looked similar to those of the as-synthesized samples except that the diffraction peaks shifted slightly to the higher 20 angle. 3.2. U V spectra and H R T E M profiles

The existence of metal atoms in MCM-48 framework were confirmed by UV and HRTEM analysis. The UV spectra of the Si-MCM-48 and the Ti-MCM-48 are shown in Figure 2. The band at 210 nm was assigned to isolated framework titanium in tetrahedral coordination, and

The as-synthesized

The calcined

d

g~

Zr-MCM-48 Ti-MCM.-48

Si-MCM-48 1

3

5

7

9

2

4

6

8

10

20, ~

Figure 1. XRD patterns of Si-MCM-48 and 2% M-MCM-48 catalysts

1208 the band at 230 nm was assigned to framework titanium in octahedral coordination[4]. A band at ca. 270 nm was attributed to extraframe titanium [ 14]. HRTEM images of Si-MCM-48 and Ti-MCM-48 were shown in Figure 3 indicating HRTEM images along (111) direction. The data obtained from HRTEM are well consistent with those obtain from XRD [4]. Both UV and HRTEM results indicated that the titanium atoms exist in the MCM-48 framework. 3.3. Pore size distribution The pore-size distribution of the pure silicon and M-MCM-48 were measured. Si-MCM-48, Zr-MCM-48, and Cr-MCM-48 all have a narrow pore diameter distributions at around 2.6 nm. But the pore radius of Zr-MCM-48 is smaller, which is caused by residual of small amount of heteratom oxides inside the channels. Since the atomic radius of chromium is smaller than that of zirconium, the pore size of Cr-MCM-48 is larger than that of Zr-MCM48. 3.4. Reaction conditions We first carried out the gas phase (non-catalytic) oxidation of a-eicosanol as a blank test and the results indicated that the yield of t~-eicosanoic acid is low (14.6 %). There are brokenoff chain materials in products measured by GC-MS, which can not be separated easily. Over

0.6

I

d

O t o

-o- Ti-MCM-48

-o-

Si-MCM-48

0.4

r~

o.2

0

200

250

300 350 400 WAVELENGTHS, nm

450

Figure 2. UV spectra of Si-MCM-48 and Ti-MCM/-48 catalysts

1209

Figure 3a. HRTEM profiles of Si-MCM-48 catalyst

Figure 3b. HRTEM profiles of Ti-MCM-48 catalyst

other catalysts such as simple metal oxides, the highest yield of ~t-eicosanoic acid was 26 %. However, the selectivity of a-eicosanoic acid was greatly enhanced when M-MCM-48 were used. GC-MS measurements indicate no cracking and decarboxylation occurring during the catalytic reaction. The effect of temperature on catalytic activity over Ti-MCM-48 was studied and experimental results are given in Table 1. The optimum reaction temperature was 413K. In this work unless particular state, in most run reaction temperature was 413K. The same conclusion can be drawn for M (Cr, Mo, Zr, and Cu)-MCM-48. The product of the oxidation of a-eicosanol over Ti-MCM-48 at 413K was extracted from the reaction system for composition analysis. The results show that after 5h of reaction, the highest yield of a-eicosanoic acid was obtained (Table 1). Further increasing reaction time did not result in a higher yield. This probably is due to decarbonation of the acid caused by heating for a longer time.

Table 1 Effect of reaction temperature and reaction time on yield (CH3(CH/)IsCOOH) over 1% ( n (Ti) / n (Si) = 0.01) Ti-MCM-48 catalysts Reaction temperature (K)

Yieldaof a-eicosanic acid (%)

393 403 413 423 433

18.4 40.2 54.4 38.8 21.4

aReaction time: 5h. bReaction temperature: 413K.

Reaction time (h) 3.0 5.0 7.5 10.0

Yield bof a-eicosanic acid (%) 27.0 54.4 54.0 46.4

1210 Table 2 Effect of M content on catalytic oxidation activity over M-MCM-48 catalysts The catalyst Ma-MCM-48

Selectivityb of ct-eicosanic acid (%)

Yield b of a-eicosanic acid (%)

31.9 44.9

14.9 47.8 51.6 54.4 30.0 41.8

n (Ti) / n (Si) = 0 n n n n n

(Ti) / n (Ti) / n (Ti) / n (Cu)/n (Cu)/n

(Si) (Si) (Si) (Si) (Si)

= 0.001 = 0.005 = 0.01 -- 0.01 = 0.02

aM = Ti, Cr, Mo, Zr, and Cu. bReaction temperature: 413K; Reaction time: 5h.

3.5. Influence of M content on catalytic oxidation activity The effect of M content on catalytic activities was examined. As shown in table 2, the yield of the desired product, a-eicosanoic acid, increases gradually with increasing Ti content and reaches a maximum over 1% Ti-MCM-48. However further increase in the Ti content results in a decrease in the yield. The effect of Cu content on catalytic activities over Cu-MCM-48 was also shown in Table 2. Figure 4 gave the effect of M content on catalytic activities over Cr-MCM-48, Mo-MCM-48 and Zr-MCM-48. In Table 2 and Figure 4, the yield and the selectivity of t~-eicosanic acid both increase with increasing M content. Influence of different transition metals on the catalytic properties for this reaction is seen from Table 2 and Figure 4.

70

Yield

/

6s

< 65-

~

60-

60~

~

r~

O 55 ~509 r 45-

40

4s ~ 0

i

4

;

Cr DOPED AMOUNTS, %

....

10

40

"~

Figure 4a. Effect of Cr content on the Yield and selectivity of a-eicosanoic acid over x%CrMCM-48 catalysts

1211 65:

---,

~ 0,10 o,o

0

o~

0.4

0,6

o,8

1,o

0,05 0,00 5

|

,

10

15

. . . . . .

J.

20

.

.

.

.

T

i

,

25

30

35

40

Average pore radius [A]

All materials with well defined mesoporous structure shows on the respective adsorptiondesorption isotherms the characteristic step at p/po - 0.3 (see the example on Fig.2). On the other hand an appearance of large, irregular hysteresis loop at p/po-~0.7-0.9 indicated that regular mesoporosity for such samples does not exist any more. This was reflected in pore size distribution. Figure 1 exemplifies the typical pore size distribution for the samples containing boron, however, similar shapes were obtained for those with different content of AI. Increasing boron content gives less uniform porous structure, what is demonstrated by the shift of the average pore radius towards higher values. Impregnation of the mesoporous materials of well defined structure, as well as those with less uniform porosity, with phosphomolybdic heteroploacid (I~A) or its nickel (NiHPA) or cobalt (CoHPA) salts resulted both in the decrease of total volume of nitrogen adsorbed and in consequence decrease in BET surface area of all samples. After impregnation with solutions containing 20% of HPA, N H P A or CoHPA a pore volume in majority of samples decreased of about 50%. Curves presented on Fig.2 and Fig.3 shows that incorporation of heteropolyacid and its salts inside regular mesopores do not cause changes in the geometry of channels. The shape of nitrogen adsorption-desorption isotherms is well preserved indpendently of the

1225 700

,

600 I 500

- 0 - A14 .-o-9 A14(5)HPA A14(10)HPA ---O-- A14(20)HPA

-

E "o 4 0 0 t,. o w "1o ~n 300 o

>

200

el

100

0,0

I 9

i

0,2

i

0,4

1

,

0,6

"

' i

0,8

I

1,0

Relative pressure.[p/po]

Fig. 3. Nitrogen isotherms of the A14 samples loaded with different amount of HPA the template applied and concentration of molybdenum containing compound. In contrast, an application of ammonium heptamolybdate (AHM) as a source of Mo for impregnation resulted in almost complete clogging of the mesoporous structure. No maximum characteristic for mesopores after impregnation with AHM (Mo concentration the same as for HPA) was observed. Samples containing boron showed very similar effects upon incorporation of HPA, NiHPA or CoHPA inside the mesoporous structure. Infrared studies indicated that upon calcination of supported heteropolycompounds at 675 K the Keggin structure is still well preserved what was demonstrated by the presence in the infrared spectrum characteristic bands at 958 (M=O2) and 880 cm "1 (M-O3-M) [20]. The measurements of acidity with pyridine as a probe molecule, confirmed the absence of any acidity for siliceous materials. Presence of A1 or B atoms in the mesoporous materials generate relatively strong acidity, of the Lewis type (presence of bands at 1444 and 1600 cml). The strongest Lewis type acidity was observed for samples with Si/A1 or Si/B higher or equal 17. For such samples bands at 1444 and 1600 cmldisappeared only after desorption at 523 K. The appearance of the bands at 1540 and 1452 crn] for mesoporous materials upon impregnation with HPA and calcination at 675 K indicates that generation both Brrnsted and Lewis acidity inside the channels of MCM-41 occurs. These acidic centers are relatively strong because the characteristic bands disappears only after desorption at temperatures higher than 473 K. According to the works of Kozhevnikov [14] it is expected that proton a~er dehydration can be localized on the terminal oxygen atoms of Keggin units. An impregnation of MCM-41 materials with nickel or cobalt salts of HPA in oxidizing atmosphere results in complete elimination of the Brrnsted acidity detectable via infrared spectroscopy. The only acidity detected was of the Lewis type and proportional to the concentration of the impregnating salt. According to the suggestion of Misono [21 ] the source of proton acidity in heteropolysalts can be related with the reduction of cations as well. This reason prompted us

1226 400 350

....

300

- -

5 w t % HPA 1 0 w t % HPA 2 0 w t % HPA

-'2 250 200 ~Q

150 IO0 ..r,. . . . .

0

200

"'" . . . . . .

f

,

j

i

400

600

800

1000

"~"

1200

Temperature [ K I 250 9

9

----

9

5 wt.% Ni-HPA 10 w t % Ni-HPA 20 w t % Ni-HPA

7 ,.,

150

&

~

100

0 400

200

600

800

1000

Temperature [ K ] 250 9 9----

200

5 w t % Co-HPA 10 wt % Co-HPA 20 wt % Co-HPA

7 150

~1

100

200

400

600

800

1000

Temperature [ K ]

Fig. 4. TPR profiles of M C M - 41 impregnated with HPA, NiHPA and CoHPA.

1200

to perform a series of experiments with temperature programmed reduction (TPR) of the studied systems. Fig. 4. presents the TPR profiles of A14 mesoporous material supported with different amounts of phosphomolybdic hetropolycompounds. Similar shapes and intensities of the TPR profiles were always obtained for samples containing A1 or B (Si/A1 or Si/B close to 17), however, those maxima we re always shifted of about 50 K towards lower temperature. The assignment of the characteristic reduction maxima to the specific oxidation number using TPR technique is very difficult, especially that there are not too many literature data concerning this specific topic. Misono [20] proposed for the reduction of the solid H3[PMo12040] multistep mechanism involving reduction to the Mo 5+ ions without removal of oxygen, followed by formation of Mo 4+ species and water evolution. In our case with supported NiHPA and CoHPA mainly two reduction maxima were always observed. Both for low and high loadings samples containing NiHPA showed better reduction abilities (temperatures lower of about 50 K) than for those with CoHPA. Comparison of these results with pure supported HPA suggests that the first maximum (- 750-820 K) is related with the reduction of M06+ probably to Mo4+.Simultaneusly the reduction of nickel or cobalt ions can occur at the same range of temperatures. This can find a confirmation in the inten-

1227 Table 2 Hydrogenolysis o f tripropylamine over A16 and B 16 supports impregnated with HPA and NiHPA. T e m p . r e a c t i o n - 525 K, Mo c o n t e n t - 6 . 8 wt.%.

Conversion after 1 hour [%]

Si/AI or B/A1

H2-675K* HPA Ni-HPA

H2S/H2-675K* HPA Ni-HPA

NH3-875K* HPA Ni-HPA

A16 A16.2 A16.4 A16.6 A16.8

oo 17.5 8.7 5.7 4.3

12.4 35.3 19.4 10.0 13.8

18.2 53.0 21.0 18.4 20.0

46.7 28.2 23.3 22.4 20.8

66.5 43.2 41.8 40.0 35.3

60.4 93.2 30.6 18.3 9.2

B16 B16.1 B16.2 B16.4 B16.8

oo 35.0 17.1 9.3 4.0

4.2 7.1 13.1 17.8 13.7

-

8.4 10.2 24.2 28.2 13.9

-

4.8 12.0 21.0 20.3 14.9

Support

88.3 100.0 60.8 -

* pretreatment in different atmospheres Table 3 Activity and selectivity during hydrogenolysis of tripropylamine over MCM-41 materials loaded with molybdic heterpolycompounds. Temp. reaction- 575 K, Mo c o n t e n t - 6.8 wt.%

Catalyst

Reduction H 2 - 675 K Activity[%]

A14(5) H P A A14(10) H P A A14(20) H P A A14(5) N i H P A A14(10) N i H P A A14(20) N i H P A A14(5) C o H P A A14(10) C o H P A A14(20) C o H P A

43.7 75.4 80.3 70.1 95 2 100.0 59.4 778 86.0

Selectivity[%] 16.7 20.3 26.8 20.8 50.8 37.0 17.7 45.3 41.2

Sulfidation H2S/H2 - 675 K Activity[%] 69.2 84 3 100.0 100.0 100.0 100.0 88.6 93.3 90.0

Selectivity[%] 243 26 5 308 42.0 63.3 48.6 24.0 49.7 43.3

sities of the maxima at 750 and 770 K respectively for Ni 2§ and Co2+.These maxima shows much higher intensities than those related with supported HPA and same concentrations. Second maximum at 920 K can be linked with more deep reduction of molybdenum ions and at this point a total decomposition and reduction even to Mo o can not be excluded. Still remain unresolved the degree of reduction of nickel and cobalt ions in supported NiI-IPA or CoHPA. The detailed study concerning this topic is currently under way. The intention of TPR measurements was to find out any effects which can be related to the

1228 catalytic activity of the studied materials in the tripropylamine hydrogenolysis reaction. Table 2 and 3 presents part of the catalytic tests results. It was found that among the products mainly propane and small amounts of propylene were present. All catalysts deactivate in time, however, alter one hour of reaction significant stabilization in conversion was observed. In all studied cases the best catalytic performance was found for catalysts impregnated with NiHPA. These catalysts independently of the pretreatment conditions, temperature of reaction or concentration of the heteroplycompound showed the best catalytic activity. Moreover, also selectivity towards propane was the highest. High activity of the NiHPA based catalysts can be related to the low reduction temperature of nickel ions and consequently lower temperature of Ni-Mo-S or Ni-Mo-N species formation. The absence of strong BrOnsted acid sites for NiHPA or CoHPA based catalysts decrease the ability of carbocations formations and finally results in better selectivity. Proposed synthesis and test reaction allowed us to establish the first rules governing the preparation future good catalysts for HDN reaction based on MCM41 materials and reacting with large organic molecules.

Acknowledgement This work was supported by the Polish State Committee for Scientific Research (KBN) within the project : 7 T09B 027 21. REFERENCES 1. 2. 3. 4.

C.T. Kresge, M. Leonowicz. W. Roth, J.S. Beck, Nature, 359(1992)710 T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull.Chem.Soc.Japan, 63(1990)988 S. Bagshaw, E. Prouzet, T. Pinnavaia, Science 269(1995)1242 Q. Huo, D. I. Margolese, U. Ciesla, P.Feng, T. E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schuth, G. D. Stucky, Nature, 368(1994)317 5. J.Kim, C. Shin, R. Ryoo, Catalysis Today, 38(1997)221 6. M.T. Anderson, P. Newcomer, Mater. Res. Soc. Symp. Proc. 3 71 (1995) 117 7. G.S. Attard, C.G. Goltner, J.M. Corker, S. Henke, R.H. Templer, Angew. Chem. Intl. Ed. Engl. 36(1997) 1315 8. H. Kang, Y.-W. Jun, J.-I. Park, K.-B. Lee, J. Cheon, J. Chem. Mater. 12(2000)3530 9. S. Ernst, M. Selle, Microporous and Mesoporous Mater. 27 (1999) 355 10. E. Armengol, A. Corma, V. Fornes, H. Garcia, J. Primo, Appl. Catal.A., 181 (1999)305 11. J. F. Diaz, K.J. Balkus Jr., J. Mol. Catal. B. Enzymatic, 2 (1996) 115 12. S. Koner, K. Chaudhari, T. K. Das, S. Sivasanker, J. Mol. Catal.A. Chem. 150 (1999)295 13. I. V. Kozhevnikov, A. Sinnema, R.J.J. Jansen, K. Pamin, H. van Bekkum, Catal. Lett., 30 (1995) 241 14. I. V. Kozhevnikov, K. R. Kloestra, A. Sinnema, H.-W. Zandbergen, H. van Bekkum, J. Mol. Catal.A. Chem., 114 (1996) 287 15. K. Nowiflska, W. Kaleta, Appl. Catal. A., 203 (2000) 91 16. W. Kaleta, K. Nowifiska, Chem. Comm. 2001,535 17. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C. T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J.Am. Chem. Soc., 114 (1992) 10834. 18. R. Schmidt, D. Akporiaye, M.Stocker, O.H.Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61 19. G.A. Tsigdinos, Ind. Eng. Chem., Prod. Res. Develop., 13 (1974) 267 20. M. Misono, Catal.Rev.-Sci. Eng, 29 (1987) 269 21. T. Okuhara, N. Mizuro, M. Misono, Adv. Catal., 41 (1996) 113


1229

Synthesis and characterization of A 1 - M C M - 4 8 type materials using coal fly ash P. Kumar*, N.K. Mal, Y. Oumi 1, T. Sano I and K. Yamana Ceramic Section of Chemistry & Food Department, Industrial Research Institute of Ishikawa Kanazawa, Ishikawa 920-0223, Japan. 1School of Materials Science, Japan Advanced Institute of Science & Technology, Tatsunokuchi, Ishikawa 923-1292, Japan.

Supernatant of the coal fly ash solution was used to prepare aluminum containing MCM48 (A1-MCM-48). It was found that most of the Si and A1 components in the fly ash could be effectively transformed into MCM-48 when a surfactant mixture containing cationic cetyltetramethylammonium bromide, CTMABr and tetraoxyethylene dodecyl ether, C12(EO)4 were used as templates. Alkali fusion was found to be necessary as it improves the hydrothermal condition for synthesis of the mesoporous materials. High degree of aluminum incorporation into the tetrahedral positions was revealed when A1-MCM-48 was prepared under controlled pH condition. 1. INTRODUCTION MCM-48 (cubic, space group Ia3d) with its highly branched and interwoven threedimensional networks of the mesopore channels is one of the most interesting mesoporous materials among many mesoporous silica molecular sieves [1]. It is believed that MCM-48 is much more resistant to pore blockage than one-dimensional channel system with a hexagonal MCM-41 while being used as absorbents and catalyst supports [1-6]. Although the discovery of the MCM-48 materials was reported simultaneously with the hexagonal MCM-41 in 1992 [1-2], research reports until now on synthesis and application of these materials have been severely biased to MCM-41 [3-4]. The bias may be attributed largely to the fact that the synthesis of MCM-48 required very specific synthesis conditions [5]. Presently however, both the economic and environmental costs for large-scale manufacture of these materials are high due to the cost and toxicity of both templates and preferred silica source. A variety of silica sources are generally used to prepare these materials including fumed silica and silicon tetraethoxide. The industrial manufacture of mesoporous materials is likely to be economically prohibitive if silicon alkoxides and fumed silica in particular are selected. * Corresponding author. Tel: + 49-241/80-20115; Fax: + 49-241/8022-291 E-mail: pnt67 @hotmail.com Present address: Chemical Technology and Heterogeneous Catalysis University of Technology RWTH Aachen, Worringerweg 1, 52074 Aachen, Germany.

1230 Since the synthesis of MCM-48 requires some very specific condition, a variety of synthesis routes have been developed in order to overcome the synthesis shortcomings [7]. These synthesis results demonstrated that the crystallinity of the MCM-48 pass through an optimum as a function of time. The MCM-48 products were obtained as an intermediate between a hexagonal or disordered surfactant-silica mesophase and a more stable lamellar mesophase [8]. Similarly, one report suggested that the transformation of the MCM-48 mesophase to lamellar can be quenched by adjusting the pH of the reaction mixture [9]. Another report indicated that the mixed surfactant approach resulted into high quality MCM48 as an energetically favored mesophase [10]. Very recently, it was reported that the use of gemini surfactants induce the formation of cubic structure even using fumed silica as silicon source [11]. All these studies indicate that the formation of MCM-48 type materials is possible under certain synthesis conditions. Coal combustion, which accounts for about 37% of the world's electricity production generates, about 600 million Tons, coal fly ash per year as a by-product [12]. Current applications of this vast amount of coal fly ash (only 15%) is not enough and requires further attention to utilize this waste material [13-16]. Since fly ash contains mainly amorphous aluminosilicates (glassy phase) and some crystalline minerals (quartz, mullite, etc.), it can be used as a raw material for the synthesis of porous materials. Very recently we have reported our studies on the synthesis of aluminum containing MCM-41 (A1-MCM-41) and SBA-15 type of materials and their characterization as well as the catalytic properties [17-18]. To further extend this synthesis regime, we have carried out the studies on the preparation condition of A1-MCM-48 type materials using coal fly ash as the silicon and aluminum source [19]. In this report various characterization techniques such as 27A1MAS NMR, FF-IR, TEM, N2 adsorption and cumene cracking reaction are used to further evaluate the materials obtained.

2.

EXPERIMENTAL

2.1.

Materials Coal fly ash used in this study was obtained from Nanao-Ota power plant, Hokuriku and used as obtained. The chemical composition of fly ash revealed apart from the main constituents such as silica (67.5%) and alumina (18.7%), the other impurities such as Fe203, CaO, MgO, K20, TiO2, Cr203, P205 Na20, K20 and SO3 with 3.6%, 2.0%, 0.7%, 0.9%, 0.8%, 0.9%, 0.3%, 0.2%, 0.4%, 0.7%, respectively. The specific surface area (BET) and cation exchange capacity (CEC) of the coal fly ash were found to be 4.5 mE/g and 0.8 meq/100g, respectively. 2.2. Synthesis of AI-MCM-48 The supernatant obtained from fused fly ash powder was used as the silica and aluminum source [17]. The concentrations of Si, A1 and Na measured in supernatant were 11,000, 380 and 35,000 ppm, respectively. The detail synthesis procedure for MCM-41 was followed from our previous study [ 18]. Different samples of MCM-48 type materials with varying Si/A1 ratio were prepared using both single surfactant and a surfactant mixture of CTMABr and C12(EO)4 (Aldrich) [19]. In brief all batches were prepared using a synthesis gel with the following molar composition: CTMABr/C12(EO)4]I-I20/Si = 0.35-0.55/0.15-0.25/100/1. The Si/A1 ratio

1231 was also varied from 60 to 14. To remove the surfactant in the mesoporous materials, the assynthesized sample was calcined in air under static conditions at 813 K for 6 hours, with a linear temperature ramp of 0.5K / min and two plateaus of 60 minutes each at 423 and 623 K.

2.3. Analysis and characterization Powder X-ray diffraction (XRD) patterns obtained from CuK~ radiation were measured by using MAX18X. cE The chemical composition was analyzed by the LilEB404 method using the X-ray fluorescence (XRF) technique (Philips PW2400). BET specific surface area was determined from NE-adsorption at liquid nitrogen temperature (Belsorp 28SA). Transmission electron microscope (TEM) image was obtained by using JEOL 2010. FI'-IR spectra of the self supporting wafers were measured by JEOL JIR-7000. 27A1 MAS NMR spectra were obtained on a Varian VXP-400.

2.4.

Catalytic activity

The cumene cracking was performed in an atmospheric pressure flow system. The sample placed in the quartz tube reactor of a 10mm inner diameter was dehydrated at 673 K for 1 h in a nitrogen stream. The temperature was then brought into a reaction temperature (623 K). The reactant was fed into the catalyst bed with micro-feeder. Nitrogen was used as a carrier gas (40 ml/min), the contact time (W/F) was 0.20 h, and the partial pressure of the cumene was 7.9 kPa. On line product analysis was done on a Shimadzu GC-17A gas chromatograph (FID) with a GL-Science TC-1 capillary column (30 m).

3. RESULTS AND DISCUSSION 9

|

9

|

9

3.1. AI-MCM-48 prepared by direct thermal synthesis from supernatant Figure 1 shows the XRD patterns of different MCM phases of calcined samples prepared under different surfactant/silica ratio. It can be seen that the low concentration of surfactant (CTMABr) results into MCM-41 type materials as suggested from the XRD pattern (Fig. la) with four peaks that are consistent with indexing to a hexagonal cell, typical of an MCM-41 type product. The observation of three higher angle reflections other than the dl00 indicates that the product is likely to possess the symmetrical hexagonal pore structure of MCM41. A further increase in surfactant concentration resulted into mesophases, poor in hexagonal structural order as indicated from the gradual disappearance of diffraction peaks assigned to (110), (200) and (210) reflections (Figure lb, lc). By increasing the concentration of CTMABr in the synthesis gel, a phase transitions from hexagonal to lamellar passing

211

4

-_~" --=

110 2

a-~ 4

6

8

20/degree Figure 1. XRD profiles of the different calcined MCM type materials. CTMABr/SiO2 9a = 0.22, b = 0.35, c, d and e = 0.55; C12(EO)4/SIO2: d = 0.15 and e = 0.18

1232 Table 1 Physical properties of the raw material and the calcined mesoporous Sample

/SiO2

/SiO2

SBET/ m 2 g-1

Fly ash A1-MCM-41 (a)

0.20

-

4.5 761

Si/A1 Pore d 100 volume /nm / c m 3 g-1 2.9 . . . 14.0 0.57 4.24

A1-MCM-41 (b)

0.35

-

738

18.5

0.57

A1-MCM-41 (c)

0.55

-

731

65.0

A1-MCM-48 (d)

0.55

0.15

639

A1-MCM-48 (e)

0.55

0.18

848

A1-MCM-48 (f)

0.55

0.18

A1-MCM-48 (g)

0.55

0.18

1 2 3 4

Surf 1 Surf 2

d 211 /nm .

ao 3

/nm

Pore size4 /nm

. -

4.9

2.8

3.56

-

4.1

2.9

0.57

3.56

-

4.1

2.7

62.3

0.55

-

3.17

7.8

2.5

59.4

0.82

-

3.04

7.4

3.0

760

18.2

0.76

-

2.98

7.3

3.0

756

14.0

0.74

-

2.98

7.3

3.0

cetyltrimethyl ammonium bromide tetraoxyethylene dodecyl ether unit cell parameter, using 2d100/~/3 for MCM-41 and d211~/6 for MCM-48 Dollimore-Heal method

through an intermediate state of cubic structure is reported [ 1-4]. But using the supernatant as a silica source it was not observed, in other words MCM-48 formation was not facilitated under the synthesis condition using CTMABr alone. Figure l d and l e shows the XRD patterns of materials the surfactant-silica mesophase obtained from the starting mixtures of CTMABr/C12(EO)4 = 0.55/0.15 and 0.55/0.18, respectively. It can be seen that the presence of neutral surfactant has resulted into mesophase, identical to the cubic MCM-48. We observed that the optimum condition for MCM-48 using the supernatant as a silica source was CTMABr/ClE(EO)4 = 0.55/0.18 as it showed the sharpest XRD pattern. From the XRD pattern in Figure le, a highly ordered MCM-48, without any trace of lamellar phase peaks was obtained. The high ordered array of these materials could be inferred from the presence of a well defined set of diffraction peaks between 3 ~ and 6 ~ in the XRD patterns assigned to the (211), (220), (321), (420), (422) and (431). Two more samples A1-MCM-48 (f) and A1MCM-48 (g) (XRD not shown) with high aluminum concentration was then prepared using the similar composition. Table 1 summarizes characteristics of the calcined mesoporous materials obtained. The gel representing higher than 0.18 of C 1 2 ( E O ) 4 resulted either into unidentified mesophase or didn't show any XRD pattern. The (211) reflection is found at approximately 3.6 nm for all the as-synthesized samples. This correspond to a unit cell size of -- 8.7 nm. For the calcined samples the same reflection occurs at 3.1 nm, a unit cell length of --7.5 nm. This shrinkage of the unit cell (--13%) during calcinations probably is due to silanol condensation. This magnitude of unit cell shrinkage was in the range of values normally reported in the literature using other silicon source, approximately in the 5-15% range [14-17]. The same tendency is observed for the (220) reflection, suggesting that the supernatant of coal fly ash containing dissolved silica species could be used as the source materials for the preparation of such kind of materials.

1233 The N~ adsorption-desorption isotherms of 600 different samples (c, d and e) are shown in ,,.-:,. Figure 2. It belongs to a reversible type IV n 500 isotherm, characteristic for mesoporous o~~:)o c} materials. An inflection point is observed at g o relative pressures between 0.25 and 0.3. din400 (5) This corresponds to the filling of the mesopores and the sharp increase in the 300 ~=,~'~" [] " adsorbed volume indicates a uniform poresize distribution. It can be seen (Table 1) "~ 200 @ that the presence of neutral surfactant 0 M C M 4 8 (mixed,e) facilitates the formation of MCM-48. The ::3 [] MCM-48 (mi~Ex:l, d ) presence of a small hysteresis loop in sample c, indicates the formation of lamellar A M C M 4 8 (single. c ) phase which is very similar to the studies 0,,. I that has been reported at the high surfactant/ 0 0.5 1 silica ratio [20]. TEM image of microRelative pressure (P/Po) sectioned sample (Figure 3) also showed well developed pores arranged on the cubic Figure 2. N2 isotherms of different samples. plane (sample g), confirming that the materials possess the pore system symmetries that are inferred from XRD and N2 isotherms. Another factor that affected the formation of cubic phase was the pH of the supematant-surfactant mesostructure. Generally, a high pH condition is a major driving force for the transformation to lamellar [21]. In our case the pH adjustment to 10.2 during the synthesis arrested this transformation and also helped to improve the product yields. This is in agreement with the report where the pH adjustment was mentioned as a means for quenching the transformation of the MCM-48 mesophase to lamellar [ 10]. A mixed surfactant approach has been reported in the literature for the preparation of mesoporous materials [20,22]. In many cases, two different surfactants are completely miscible and form liquid crystalline misceller mesophase cooperatively. This phase behavior becomes more complicated when silica and alumina sources are present in the form of supernatant of coal fly ash. Supematant is a highly alkaline solution of silicate and aluminate (anions) and are strongly attracted by electrostatic interaction surrounding the head groups of the CTMABr, which may lead to the high concentration of the anions on the surface of the surfactant micelles. The neutral surfactant has no strong interaction with the ~;~:i? " 50nm , ,~,f~.i anions, and consequently its incorporation to the micelles will bring a dilution of the anions at the surface. This low surface Figure 3. TEM image of sample g. concentration may further lead to a certain

1234 contraction of the micelles surface, resulting in a phase transition from hexagonal to cubic. At this stage we are not advancing any explanation about the complexities of phase behavior of the supernatant-surfactant mesostructures in the aqueous solution, however we believe that C12(EO)4 acts more as a diluents and based on our observation facilitated the formation of MCM-48 structure.

3.2. Acidity of various AI-MCM-48 samples One of the most important features of our study using coal fly ash is the aluminum incorporation into the framework of the synthesized materials [17-18]. We found in the previous study on A1-MCM-41 that although there is no clear explanation for a large amount of tetrahedrally (Ta) coordinated framework aluminum in A1-MCM-41 derived from the supernatant, the supernatant is very effective for preparation of A1-MCM-41 without any Oh nonframework (0 ppm) aluminum. Very similar results we have also observed for the different MCM-48 samples. In Figure 4 the 27A1MAS NMR spectra of A1-MCM-48 (samples e, f and 100 50 0 -50 g) are presented. Chemical shift is referenced to 1 M Al(NO3)3 aqueous solution and the peak 27A1MAS-PPN~ spectra for Alintensity was normalized based on 1 g of Figure4. material. A single peak at ca 54 ppm, without MCM-48 prepared from supernatant of any evidence of any Oh aluminum can be seen fused fly ash powder. Si/A1 ratio; e=59.4, f=18.2, g=14.0 in all three samples, the intensity for which increased with low Si/A1 ratio. This is H H interesting and suggests the formation of acid sites in the mesoporous system. To further authenticate this, the samples were tested for pyridine adsorption using FT-IR. Aluminum in tetrahedral position creates ion exchange site associated with the charge compensating Na § ions. Figure 5 shows IR spectra of pyridine < adsorbed on the samples (Si/A1 = 59.4, 18.2 and 14.0 for e, f and g, respectively) after degassing at 423 K for 30 min. The samples did not show any acidity as expected, the weak bands at 1446 and 1598 cm -1 are probably due to pyridine 1600 1500 1400 adsorbed via H-bond interaction. When the samples were ion-exchanged twice with the Wave number (cm-1) NH4+ salt and calcined (protonation), a clear Figure 5. Fr-IR spectra of adsorbed pattern of acidity generated on the samples can be seen in Figure 6. Intense bands were pyridineonA1-MCM-48 samples before measured around 1456 cm 1 and 1623 cm 1 protonation. !

i .

.

.

.

1235 (Lewis acid sites), 1556 c m "1 (BrCnsted acid sites) and 1494 cm -1 (overlapping BrCnsted and Lewis L B+L acid sites). The intensity of these bands increases with the A1 content of the samples, showing a corresponding increase in the number of acidic sites. However, majority of acid sites generated on the samples were found to be Lewis acid sites (Figure 6) and the peaks arising from BrCnsted acid sites disappeared after evacuation at 523 K for 1 h, suggesting that the acidic strength of the BrCnsted acid sites in the A1-MCM-48 synthesized is very weak. Nevertheless, it is I I 1600 1500 1400 interesting to observe the acidity in the A1-MCM48 derived from coal fly ash, which confirms the Wave number (cm-1) aluminum incorporation suggested by the 27A1 MAS NMR measurement. Figure 6. FF-IR spectra of adsorbed pyridine on protonated A1-MCM-48 Catalytic activity of the A1-MCM-48 samples. prepared was further evaluated using the cumene cracking reaction at different time on stream and compared with the A1-MCM-48 prepared from 25 pure chemicals. The initial activity of coal fly ash derived materials (Si/AI= 18.2) was lower compared to the initial activity of A1-MCM-48 9 (Si/AI= 22.0) prepared from pure chemicals. Taking into account the fact that the cracking reaction require medium to strong BrCnsted acid r sites and the peaks derived from BrCnsted acid o [] a sites disappeared after evacuation at 523 K, it is suggested that the acidity of A1-MCM-48 prepared from the supernatant of coal fly ash is = "0" b o o o very weak. In other words, all the aluminums I I present in the A1-MCM-48 prepared from fly ash are catalytically not active. A number of reports 0 1 2 3 Time on stream (h) provides sufficient evidence for the partial inaccessibility of aluminum due to its Figure 7. Conversion profile of cumene incorporation in separate aluminum phases or on protonated A1-MCM-48 samples deeply imbedded in the porous walls if an prepared from (a) pure chemicals and (b) coal fly ash. aluminum source is added to the initial synthesis gel [26-27]. Our observation in this study provides further support for this point that the catalytic active sites are not connected with the total aluminum concentration but linked only to the amount of accessible aluminum, preferably on the surface.

12

4. CONCLUSIONS Supernatant of coal fly ash can be used as a raw material for the synthesis of aluminum containing MCM-48. The use of surfactant mixture has greatly facilitated the synthesis of

1236 MCM-48 performed under controlled pH condition. A high aluminum incorporation in tetrahedral position is revealed in the mesoporous materials which in turn generate ionexchange sites as well as acid sites when measured by pyridine adsorption using FF-IR. The experimental data produced here suggest that the coal fly ash could be a suitable source of silicon/aluminum with a low economy and environmentally friendly reagent for the preparation of well ordered mesoporous materials. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 680. 3. A. Corma, Chem. Rev., 97 (1997) 2373. 4. A. Sayari, Y. Yang, M. Kruk and M. Jaroniec, J. Phys. Chem. B, 103 (1999) 3651. 5. J..M. Kim, S. K. Kim and R. Ryoo, J. Chem. Soc., Chem. Commun., (1998) 259. 6. C.L. Landry, S. H. Tolbert, K. W. Gallis, A. M. Monnier, G, D. Stucky, P. Norby and J. C. Hanson, Chem. Mater., 12 (2001) 1600. 7. M.L. Pena, Q. Kan, A. Corma and F. Rey, Microporous Mesoporous Mater., 44-45 (2001) 267. 8. A. Corma, Q. Kan, and F. Rey, J. Chem. Soc., Chem. Commun., (1998) 579. 9. J. Xu, Z. Luan, H. He, W. Zhou and L. Kevan, Chem. Mater., 10 (1998) 3690. 10. R. Ryoo, S.H. Joo and J.M. Kim, J. Phys. Chem. B, 103 (1999) 7435. 11. P. Van Der Voort, M. Mathieu, F. Mees and E. F. Vansant, J. Phys. Chem. B, 102 (1998) 8847. 12. C. Zevenbergen, J.P. Bradley, L.P.V. Reeuwijk, A.K. Shyam, O. Hjelmar and R.N.J. Comans, Environ. Sci. Technol., 33 (1999) 3405. 13. G. Belardi, S. Massimilla and L. Piga, Resource, Conservation and Recycling, 24 (1998) 167. 14. A. Singer and V. Berkgaut, Environ. Sci. Technol., 29 (1995) 1748. 15. S. Rayalu, N. K. Labhasetwar and P. Khanna, U.S. Patent No. 6027708 (22 February 2000). 16. N. Shigemoto, S. Sugiyama, H. Hayashi and K. Miyaura, J. Mater. Sci., 30 (1995) 5777. 17. P. Kumar, Y.Oumi, K. Yamana and T. Sano, J. Ceram. Soc. Japan, 109 (2001) 968. 18. P. Kumar, N. K. Mal, Y.Oumi, K. Yamana and T. Sano, J. Mater. Chem., 11 (2001) 3279. 19. P. Kumar, Y. Oumi, K. Yamana and T. Sano, accepted to Nanoporous Materials III, June 12-15 th 2002, Canada. 20. G. Oye, J. Sjoblom and M. Stocker, Microporous Mesoporous Mater., 27 (1999) 171. 21. R. Ryoo and J.M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 22. J. L. Palous, M. Turmine and P. Letellier, J. Phys. Chem. B, 102 (1998) 5886. 23. K.R. Kloetstra, H.W. Zandergen and H. van Bekkum, Catal. Lett., 33 (1995) 157. 24. A. Jentys, K. Kleestofer and H. Vinek, Microporous Mesoporous Mater., 27 (1999) 321.


1237

Synthesis of w e l l - a l i g n e d carbon nanotubes on M C M - 4 1 Wei Chen, Ai Min Zhang*, Xuewu Yan, Dongcheng Han Department of chemistry, Nanjing University, Nanjing, 210093, P.R. China Fax: +86-25-3317761, E-mail: [email protected] Well-Aligned carbon nanotubes (CNTs) have been fabricated on mesoporous molecular sieves (MCM-41) embedded with iron oxide nanoparticles by chemical vapor deposition (CVD). Benzene with 1% thiophene was used as the carbon source. And large pore size MCM-41 was obtained by using 1,3,5-trimethyl benzene (TMB) as swelling agent. It has been found the mesoporous MCM-41 is an ideal substrate for growing well-aligned carbon nanotubes.

1. I N T R O D U C T I O N Since the discovery of carbon nanotubs, both theoretical models and experimental measurement have demonstrated their remarkable mechanical as well as novel electrical and magnetic properties. Growing Well-Aligned CNTs is important for obtaining functional devices for use as scanning probes [1] and sensors, as new field emitters in panel displays [2], and single-molecular transistors in microelectronics [3]. Aligned carbon nanotubes have been prepared either by postsynthesis fabrication [4] or by synthesis-induced alighment [5]. Recently, Jung Sang Suh [6] fabricated highly ordered two-dimensional CNTs on porous anodic alumina templates; Ren [7] used plasma-enhanced CVD and synthesized self-aligned CNTs on glass substrates. Previous studies show that the template plays an important role in the procedure of CNTs growth. Since the appearance of mesoporous molecular sieves [8,9], such as MCM-41, it has been found that mesoporous molecular sieve (MCM-41) is an ideal substrate for encapsulating catalyst [10,11]. Here we report the well-aligned CNTs have been obtained by using CVD over iron oxide nanoparticles embedded in MCM-41. It is known that the size of formed micelles determines the pore size of final mesoporous materials [12]. Some researchers have already used post-synthesis treatments [13,14], surfactants of different chain lengths [15] and polymers such as triblock-copolymers [16] as templates or incorporation of swelling agent to form large pore mesoporous materials. In the previous studies, 1,3,5-trimethylbenzene (TMB)[17,18] and decane [19] have been used as expanders, and materials with pore size superior to 80 A were obtained. In order to synthesize * Corresponding author.

1238 CNTs with uniform diameters through controlling the size distribution of active iron particles, we want to synthesis MCM-41 with large pore diameter. Here, we used TMB as swelling agent to expand the pore diameter of mesoporous materials. 2. E X P E R I M E N T A L 2.1. Preparation of catalyst Cetyltrimethylammonium bromide (CTAB) was first dissolved in water with stirring at room temperature to obtain a clean colloidal solution. 1,3,5-trimetyl benzene (TMB), tetraethyl orthosilicate (TEOS) and NaOH were then separately added drop by drop to the solution. After being stirred at room temperature for 1 hour, the homogenous gel with the molar composition of 1.0 cetyltrimethylammonium bromide (CTAB)" x TMB' 20.0 tetraethyl orthosilicate (TEOS): 10.0 NaOH" 1500.0 H20 (0~< x ~< 2.5) was sealed in Teflon autoclaves and statically heated at 373K for 72 hours. Resultant white product was filtered and washed several times with hot deionized water. After drying it was calcined at 773 K in air for 6 hours. The loading of iron oxides onto MCM-41 was carried out by the wet impregnation technique with a 1.6 M aqueous solution of Fe(NO3)3 9H20 for certain time. Then the resulting product was washed with deionized water and dried at room temperature under vacuum for several hours. Afterward the material was calcined at 673K under N2 atmosphere for 6 hours, which led to a transformation of iron nitrate to ferric oxide indicated by the disappearance of the IR band of the NO3-at 1380 cm -~. 2.2. Growth of carbon nanotubes CNTs were prepared in a conventional CVD equipment consisted of a horizontal tubular furnace and gas flow controlling units. A typical growth experiment, about 50 mg catalysts was put into ceramic boat inside a quartz tube. The catalysts were first actived at 500~

for

1.5 h in N2 with flow rate of 60 ml/min, and subsequently reduced by H2 (60 ml/min) at 500~ for l h, then rise to reaction temperature, at 8 0 0 - 9 5 0 ~ maintained for 1 h with the N2 flowing rate of 60 ml/min. Finally the benzene vapor with 1% thiophene was draw into the reaction system by hydrogen gas at certain flowing rate for 30 rain. Carbon nanotubes formed over catalyst were weighed at room temperature. 2.3. Characterization of the carbon nanotubes and catalysts The morphology and diameter of carbon nanotubes were observed by the JEM-200CX type transmission electron microscope (TEM). The crystallogram was determined with Japan

1239 D/max-Y RA X-ray diffractometer using CuK~, radiation (X = 1.54178 ). Pore diameter distribution and specific surface area were performed on an ASAP 2000 adsorption apparatus made by Micromeritics Corporation. The chemical compositions of catalysts were analysed with atom scan 2500 ICP emission spectrometer. 3. R E S U L T S A N D D I S C U S S I O N

3.1. Synthesis of carbon nanotubes The key result we reported in this research work is the synthesis of Well-Aligned CNTs using the new catalyst, MCM-41 embedded with iron nanoparticles. The TEM image of the as-synthesized material (see Figure 1) shows the well-aligned carbon nanotubes with diameter from 10 to 15 nanometers. For a typical 30 rain growth experiment at 900~

the average

weight increase percent using the catalyst contained 2.0 wt.% of ferric oxide is about 22 wt.%, which is relative to the total weight of the catalyst. It is obvious from Fig.1 that nanotubes self-assemble into aligned structures. We have predicted the possible aligned mechanism in our current work. As the nanotubes growth, their outmost walls interact with those of neighboring nanotubes via van der Waals force to form a large bundle with sufficient rigidity. This rigidity enables nanotubes to keep growing along the original direction. Even the outmost nanotubes are held by the inner nanotubes without branching away.

-~.,

,~

,

~..,.~ "

~,,

, ~,

]

!

Figure 1. Well-aligned carbon nanotubes on MCM-41 embedded with iron oxide nanoparticles We have found that the catalyst preparing process is a crucial step in obtaining the high performance carbon nanotubes. Impregnation with aqueous solution of ferric nitrate for more

1240 than one hour will cause the collapse of the mesoporous structures due to the poor hydrothermal stability of MCM-41 in acidic solutions (pH < 1.0 ), which was indicated by the disappearance of the typical XRD reflection peaks of MCM-41 after impregnation. This collapse would significantly reduce the total surface areas and pore volume. As a result, iron oxide nanoparticles, could not be well dispersed in such template. But, the well-dispersed nanoparticles are very essential to CNTs growth as indicated in other people's work [20-22]. So, in order to avoid this limitation, we adopted different ways: 1. the impregnation time was reduced; 2. impregnation carried out in methanol solution of ferric nitrate; and 3. ultrasonic disperse was adopted. In our experiment, all the typical hkl reflections of the MCM-41 XRD pattern were well maintained after impregnation or ultrasonic loading compared with the assynthesis materials, showing that the loading process in such condition has little influence on the mesoporous phase of MCM-41. The details of the XRD results are shown in figure 2 and figure 3 respectively. Besides the typical MCM-41 reflections, no additional peaks are observed, indicating that no crystalline iron oxide phase has been formed outside the pore structure.

Figure 2. XRD pattern after impregnation with ferric nitrate aqueous solution for l0 rain

Figure 3. XRD pattern atker impregnation with ferric nitrate methanol solution for 1 hour

1241 We also found that the pore diameter, surface area and pore volume of MCM-41 were changed little after impregnation, which was indicated by BET experiment. The results of BET experiment are shown in Table 1. So, with an average MCM-41 pore diameter of 2.9nm, the iron oxide nanoparticles should be dispersed well, which is the vital factor to synthesis Well-Aligned CNTs. Table 1 BET results of as synthesised materials and the materials after impregnation. BJH surface area (mZigi................i;0reV0iume .. ....................P0re::~ciiameter .. ................. (cc/g) (nm) As-synthesised materials

1313.35

0.83

3.40

After impregnation

1297.32

0.78

2.75

3.2. Synthesis of MCM-41 with large pore diameter by using TMB as the swelling agent The pore structure of the mesoporous MCM-41, as the substrate of catalyst, influences immediately on the states of loading iron nanoparticles. Beck et al. [8,9] have demonstrated that the pore size of MCM-41 can be varied as a function of the concentration of expander molecules such as TMB. According to the methodology introduced by Beck et al, we obtained enlarged pore size materials only at the molar ratio of 1.0 CTAB: 2.5 TMB. The experiment results also indicated that the quantity of smeller (TMB) is an important factor on the phase and pore diameter of final mesoporous materials. In our experiment, the molar composition of mixture may be described as: 1.0 CTAB: x TMB: 20.0 TEOS : 10.0 NaOH: 1500.0 H20 ( 0 ~ x ~ 2.5) As a result, the mixture of MCM-41 and MCM-50 or pure MCM-50 were obtained when the CTAB / TMB molar ratio is between the range of 1.0 to 2.0. The lamellar MCM-50 occurred when the molar ratio of TMB/CTAB reached 1.5. However. However when the molar ratio further increasing, the hexagonal MCM-41 was restored again and the pore diameter was enlarged. The chemical composition of mixture and products of synthesis materials for expanding procedure are presented in table 2. We predicted the possible phase transformation mechanism of MCM-41 pore size expanding procedure by using TMB as the swelling agent. As Kunieda et a1.[23] said in his paper, the penetrate tendency was very large for alcohol and aromatic hydrocarbons such as m-xylene. In this case, there will be no significant change in the micelle size by using 1,3,5trimethyl benzene (TMB) as the swelling agent at the lower TMB/CTAB ratio (less than 1.5). But this penetration would destroy the structure of hexagonal MCM-41, and result in the

1242 formation of lamellar MCM-50. While increasing the amount of TMB, TMB molecules would congregate to form "big oil particles", and " dissolve" in the organic hydrophobic tail of the surfactant (CTAB). The hydrophobic solvate interaction of the aromatic molecule with the hydrocarbon tails is analogous to the hydrophilic solvate interaction of water with the charged head groups of surfactants (CTAB). In this sense the inorganic/organic molecular ion pair species are organized with the organic TMB molecules as a co-solvent for the hydrophobic portion of the bi-phase synthesis mixture. As a result, the pore diameter of MCM-41 would be enlarged, which was checked by the increased dl00 value of MCM-41 reflection peaks. Table 2 Chemical compositions of mixture and products of synthesis mesoporous material CTAM

TMB

H20

0.05

0.0

75.0

0

MCM-41 (fine)

0.5

0.05

0.05

75.0

1.0

MCM-41 and MCM-50

0.5

0.05

0.075

75.0

1.5

Disordered MCM-50

1.0

0.5

0.05

0.100

75.0

2.0

MCM-50 and MCM-41

1.0

0.5

0.05

0.125

75.0

2.5

MCM-41

TEOS

NaOH

1.0

0.5

1.0 1.0

TMB :CTAB

Product

Above procedure was indicated by the XRD patterns (see figure 4-8). When the molar ratio of TMB/CTAB was 1.0, the reflection peaks of lamellar MCM-50 were occurred around 2 0 = 3.4 (figure 5), and the dl00Value (44.125 A ) of the 100 reflection peaks of MCM-41 phase was changed little compared with the pure MCM-41 (41.925 A) (figure 4). When the ratio reached 1.5, no reflection peaks of MCM-41 phase were detected and only MCM-50 reflection peaks could be observed (figure 6), which means that MCM-41 phase was completely transformed to MCM-50 phase. At the molar ratio of 2.0, the reflection peaks of MCM-41 phase occurred again, and the dl00 value was increased to 56.718 A (figure 7), indicated that the pore of MCM-41 was enlarged by the expander molecule (TMB). But the MCM-50 phase still existed at this condition. Finally, when the ratio reached 2.5, the reflection peaks of MCM-50 phase were disappeared in the XRD patterns, only the pure MCM-41 with enlarged pore diameter was found, and value of dl00 was 69.531 A (figure 8). But the 110 and 200 reflection peaks of hexagonal MCM-41 couldn't be observed due to the broadening the 100 reflection peak. We also found that the 100 reflection peak shifted toward smaller angel region when the pore was enlarged, which was as the same as the previous work [8,9,19].

1243

2-11o

~

t

20o

1

L,

I

I

-

Figure

4.

TMB

9C T A B

,~

Figure

= 0.0

~,

5.

r

o,

TMB

,=

r

9C T A B

e

,,,

o

= 1.0

\ I;

Figure

6.

TMB

9C T A B

Figure

= 1.5

Figure

8.

TMB

"CTAB

7.

TMB

' CTAB

= 2.0

= 2.5

4. C O N C L U S I O N We have synthesized Well-Aligned CNTS on MCM-41 with diameter from 10 to 15 nanometers. Our synthetic approach involves prepare of mesoporous molecular sieves (MCM-41), impregnation with ferric nitrate aqueous solution, and chemical vapor deposition. All of these allow the production of the Well-Aligned carbon nanotubes. And we have synthesized the MCM-41 with large pore diameter, which will be used to fabricate different

1244 carbon nanotubes grown from the pores of the template in our future work.

REFERENCE 1. J.H.Hanfer, C.L.Cheung, A.T.Woolley, C.M.Lieber, Progress in Biophysics & Molecular Biology, 77 (2001) 73. 2. De Heer, W.A. Bonard, J.M. Fauth, et al., Adv. Mater., 9 (1997 ) 87. 3. S.Frank, P.Poncharal, Z.L Wang, W.A.De Heer, Science, 280 (1998) 1744. 4. W.A. De Heer, W.S.Bacsa, C.A. Telain, T. Gerfin, R.Humphreybaker, L.Forro, D. Ugarte, Science, 268 (1995) 845. 5. S.Huang, L. Dai, A.W.H Mau, J. Mater. Chem, 9 (1999) 1221. 6. J.S. Sub and J.S. Lee, Applied Physics Letters, 75 (1999) 2047. 7. Z. F. Ren, Z. P Huang, et.al science, 282 (1998) 1105. 8. C.T. Kresge, M.E. Leonowicz, W.J.Roth, J.C.Vartuli, J.S.Beck, Nature, 359 (1992) 710. 9. J. S. Beck, et al., J. Am. Chem. Soc., 114 (1992) 10834. 10. F.Michael, et al., Chem. Mater., 274 (1999) 1701. 11. T.Abe, Y.Tachibana, T.Uemastsu, M.Iwamoto, J. Chem. Soc. Chem.Commun. (1995) 1617. 12. A.Corma, Chem. Rev., 97 (1997) 2373. 13. Q.Huo, D.I. Margolez, G.D. Stucky, Chem. Mater., 8 (1996) 1147. 14. A. Sayari, P. Liu, M.Kruk, M. Jaroniec, Chem. Mater., 9 (1997) 2499. 15. A. Sayari,V.R. Karra, R.J. Sudhakar, Presented at the Symposium on Synthesis of Zeolites, Layered compounds and other Microporous Solids, 209th National Meeting of the American Chemical Society, Anaheim, CA, 1995. 16. D. Zhao, J.Feng, Q. Huo, N.Melosh, G.H.Fredrickson, B.F.Chmelka, G.D.Stucky, Science 279 (1995) 548. 17. J. S. Beck, U.S. Patent, 5,057,57296 (1991). 18 P.J. Branton, J.Dougherty, G.Lockhart, J.W.White, Charact. Porous Solids IV(1997) 668. 19 J.L. Blin, C. Otjacques, G. Herrier, and Bao-Lian Su. Langmuir, 16 (2000) 4229. 20 W.W. Li, S.S. Xie, et al., Science, 274 (1996) 1701. 21 J. Kong, A. Cassell, H.Dai, Chem.Phys.Lett., 292 (1998) 4. 22 E. Flahaut et al., Chem.Phys.Lett., 300 (1999) 236. 23 H. Kunieda, K. Ozawa, K.L.Huang, J. Phys. Chem. B, 102 (1998) 831.


1245

Synthesis and characterization of CuO and Fe203 nanoparticles within mesoporous MCM-41/-48 silica C. Minchev~, R. K6hnb, T. Tsoncheva~, M. Dimitrov~, I. Mitov ~, D. Paneva r H. Huwe d and M. Fr6ba d* aInstitute of Organic Chemistry, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria bInstitute of Inorganic and Applied Chemistry, University of Hamburg, Germany ~Institute o f Catalysis' Bulgarian Academy o f Sciences, 1113 Sofia, Bulgaria d Institute of Inorganic and Analytical Chemistry, Justus-Liebig-University Giessen, Germany

Two simple methods for the synthesis of pure siliceous MCM-41 and MCM-48 silica materials, modified with CuO or Fe203 nanoparticles, located almost exclusively within the mesopores are presented. The modified samples were characterized by powder X-ray diffraction, nitrogen physisorption, temperature progranamed reduction, X-ray absorption spectroscopy (XANES/EXAFS) or M6ssbauer spectroscopy and methanol decomposition as a catalytic test reaction. The existence of small, slightly disordered metal oxide nanoparticles was proved. The redox and catalytic behavior of the modified samples depending on the metal oxide, the preparation method used and the type of the mesoporous support are studied and compared to the corresponding bulk oxide phases. 1. INTRODUCTION In the last years the mesoporous MCM-41 and MCM-48 molecular sieves have gained large interest as catalytic supports for metals, metal oxides and organometallic compounds [1-4]. In case of Cu and Fe containing mesoporous materials different methods of preparation: direct synthesis or post-synthetic modification have been described [5-10]. Depending on the preparation method used, the presence of metal ions and / or highly dispersed metal oxide species has been proved. In the present work the preparation of CuO and Fe203 modified MCM-41 and MCM-48 silica materials by means of different impregnation techniques in aqueous or organic media has been investigated. The aim of this study is to examine the influence of the different methods used on the dispersion and redox behavior of the generated CuO or Fe203 species and their catalytic activity in methanol decomposition as a test reaction.

2. EXPERIMENTAL The parent MCM-41 and MCM-48 silica materials with specific surface areas of 1000-1300 m2/g were synthesized by standard procedures described elsewhere [8,11 ]. After *Corresponding author: FAX: **49-641-9934109. E-mail: Michael'Fr~176

1246 drying, the as-synthesized silica samples were calcined in air for 6 h at 823 K. The loading of copper or iron oxide was carried out by two different impregnation techniques: Method A: The parent materials were stirred for 1 h first at room temperature and then for 1 h at 323 K with 0.023 M solution of Cu(II) or Fe(III) acetylacetonate in chloroform. After that the chloroform was evaporated. The sample was dried at room temperature. Method N: The parent materials were stirred at room temperature with 0.5 M aqueous solutions of Cu(II) or Fe(III) nitrates. The obtained product was dried at room temperature and then under vacuum for several hours. After drying all modified materials were calcined in air at 770 K (CuO/MCM-41/-48) or 670 K (Fe203/MCM-41/-48). Reference samples of bulk copper oxide (CuO) and iron oxide (Fe203) were prepared by mechanical mixing of amorphous silica and 4% wt CuO and 7% wt Fe203. The samples were characterized by powder X-ray diffraction (P-XRD), nitrogen physisorption, temperature programmed reduction combined with thermogravimetric analysis (TPR-TGA) and M6ssbauer spectroscopy (MS) as described elsewhere [ 11,12]. The methanol decomposition to H2, CO and/or methane was conducted at 450-700 K at atmospheric pressure in a flow type apparatus and Ar as carrier gas. Product analysis were carried out by online gas chromatography [ 13]. Before the catalytic experiments the samples were pretreated in situ in air at 773 K for 2 h. 3. R E S U L T S A N D D I S C U S S I O N 3.1. Textural characterization Representative examples of the nitrogen adsorption/desorption isotherms and pore diameter distributions of the samples are shown in Figure 1 and 2. The isotherms show type IV profiles (IUPAC classification) for the parent and modified materials as expected for mesoporous systems. The BET surface area varies in the range of 870-1280 m2/g (Tablel). After the modification reductions in the pore diameter, pore volume and BET surface area are observed. These changes are more pronounced in case of MCM-48 silica in comparison to MCM-41 samples (Figure 1a and b). The observed effects are substantially influenced by the preparation method used for the copper containing samples. However no essential differences have been observed in case of iron modified samples as shown in Figure 2a and b.

Table 1: Characteristics of copper and iron oxide modified samples. Sample Metal content Precursor: Host structure* (% wt) metal Cu-M 1-A 3.7 acetonate MCM-41 Cu-M8-A 3.7 acetonate MCM-48 Cu-M 1-N 3.3 nitrate MCM-41 Cu-M8-N 3.8 nitrate MCM-48 Fe-M 1-A 6.8 acetonate MCM-41 F e-M 8-A 6.8 acetonate MCM-48 Fe-M 1-N 6.8 nitrate MCM-41 Fe-M8-N 6.3 nitrate MCM-48

BET surface (mZ/g) 1277 1119 876 1075 873 1117 980 1018

* MCM-41 (1000-1300 m2/g) and MCM-48 (1200 m2/g) silica were used as parent materials.

1247 ads

600

des

pristine MCM-41 -

J ads des 600~ p~is.neMCM~8 -

~a.a/,

~J .

a4o0t=~f

,-.-, 500 E ~'o 400

300

E _= o 200-

. . . .

200

~:~5

des

pristine MCMJ,8 -

600

,...-, 500 E 400

ads

700

,,"

.

.

,

012 0.4

1 2 3 4 p.ore diameter [nm],

0.6

0'.8 ' 1.0

E 300 _= 0 > 200 100

~.. 0.0

0

'.2

p.ore 2iamet3r. [nm:

014 0.6

O'.8

1.0

relative pressure P/Po relative pressure P/Po Fig. 2:N2 physisorption isotherms (77 K) for pristine MCM-48 silica in comparison to iron oxide modified Fe-M8-N(a) and Fe-M8-A(b). Insets depict pore diameter distributions (BJH).

Some powder X-ray diffraction patterns of the investigated samples are presented in Figure 3 a and b. The modified molecular sieves still show all reflections typical of the corresponding parent material, but with lower intensity. A comparable decrease in the signal intensity due to the filling of the mesopores for the MCM-48 and MCM-41 silica systems is found for both modification methods A and N. So, according to the textural characterization by physisorption and P-XRD neither pore blocking nor structural collapse for both copper and iron modified mesoporous M41S phases is observed. Except for the sample Cu-M1-N (Figure 3 a, enlargement) no additional reflections typical of CuO (35.7 ~ and 38.55 ~ 2 0) or Fe203 (24.1 ~ 33.0 ~ and 35.6 ~ 2 0) are observed in the P-XRD patterns of the modified mesoporous materials as shown exemplary in the enlargement of Figure 3 b. This indicates that practically no crystalline metal oxide phase has been formed in all cases. The very high dispersion of iron oxide species is also confirmed by the M6ssbauer

1248

|

8000 7000 ,-, 6 0 0 0

"~ 5000

o 5000

.ca. 4000

>, 4 0 0 0

"~ 3 0 0 0 e,,, 2000

.,..,

e-

@

7000 6000

3000

"c 2 0 0 0

, m ,

._=

Cu-M1-N

1000

10

2'0

30 2 0 [ ~]

1000 0

40

10

20

30

40

20[3

Fig. 3" P-XRD for pristine MCM-41/-48 silica in comparison to CuO modified Cu-M1-N(a) and Cu-MS-N(b). Enlargement shows 2 0 region (300-45 ~ for the strongest CuO reflections. Table 2: M6ssbauer parameters of the iron oxide modified samples Sample Components IS QS Heff [mm/s] [mm/s] [kOe] Fe-M1-A Dbl - S P M - Fe3+octa 0.34 0.99 -

FWHM [mm/s] 0.55

G [%] 100

Fe-M8-A

Dbl - S P M - Fe3+octa Sxt - c~-Fe203

0.33 0.36

1.02 - 0.12

514

0.61 0.54

93 7

Fe-M1-N

Sxt- c~-Fe203 3+ Dbl - S P M - Fe octa

0.37 0.36

- 0.10 0.70

513 -

0.38 0.53

32 68

Fe-M8-N

Sxt - c~-Fe203 Dbl - SPM - F-3+ e octa

0.37 0.35

- 0.11 0.72

512 -

0.31 0.52

22 78

IS: isomer shift related to (x-iron; QS" quadrupole splitting; He~r: internal magnetic field; FWHM: line width; G: relative weight of the components; SPM: superparamagnetic. data (Table 2). The M6ssbauer spectrum of Fe-M1-A sample represents quadruple doublet, while those of Fe-MS-A, Fe-M1-N, and Fe-M8-N samples are a superposition of the lines of sextet (Sxt) and doublet (Dbl) components. The determined parameters for all spectral components show that they belong to high spin Fe3+-ions. The parameters of the sextet part are close to those for c~-Fe203 (IS = 0.37 mm/s, QS = - 0.11 mm/s, Herr = 513 kOe). The main component of the doublet part in the spectra belongs to Fe3+-ions in octahedral coordination (IS = 0.33-36 mm/s, QS = 0.7-1.1 mm/s). This component could be assigned to nanosized particles of iron oxide with superparamagnetic behavior (SPM). It should be noted that this component is smallest for the sample Fe-M1-N. Obviously the metal oxides are spread as very small species within the mesoporous host material. This is confirmed by the combined results obtained from the physisorption, P-XRD, M6ssbauer and X-ray absorption spectroscopy. The X-ray absorption spectroscopy (XAS) data [8,14] show that these metal oxides are slightly disordered and do not have the same structure as their corresponding bulk oxide phases. Consequently, both impregnation techniques (Table 1) allow the formation of very highly dispersed CuO or Fe203 nanoparticles located almost exclusively within the mesopores of the parent silica materials. More over, by method N haematite nanoparticles located within the

1249 mesopores even at very high Fe203 concentrations (up to 42.5 wt %) could be obtained [15]. At the same time the increase of the copper concentration above 6-7 wt% for CuO/M41S samples leads to the formation of large CuO particles on the outer surface of the support [ 16]. 3.2. TPR-TGA measurements TPR-TGA measurements on copper oxide modified MCM-41 and MCM-48 silica showed that the reduction to copper metal is achieved in a temperature range of 473-750 K [4,11]. Bulk copper oxide particles are reduced to the copper metal at ca. 633 K while the highly dispersed nanostructures within the mesoporous host structure (Cu-M1-N and Cu-M8-N) are reduced in a temperature range of 473-590 K with TPR peaks between 523-573 K [4,11]. More complicated is the case of the samples Cu-M1-A and Cu-MS-A where due to the reduction process two temperature ranges are found. A first reduction step is observed between 473-590 K ascribed to highly dispersed material. However, the main reduction takes part at relatively high temperatures 600-750 K which is assigned to the reduction of large copper oxide particles of various size [11 ] and isolated copper entities stabilized by the host structure. On the contrary the reduction of the iron oxide nanoparticles to stable products failed for iron contents below 20 %wt (5% H2 in Ar, 300-873 K). No differences in the reduction behavior due to the preparation method were found. Under these conditions the reduction of bulk haematite to magnetite is observed at 573 K and further reduction at temperatures above 673 K leads to metal iron. Even at higher H2 concentrations the reduction of the host/guest compounds leads only to a partial reduction of Fe(III) to Fe(II) as indicated by XAS data [15]. This mixed iron oxide exhibits a ferromagnetic behavior like magnetite. 3.3. Catalytic studies Low catalytic activity in methanol decomposition to CO and hydrogen, not exceeding 10 % even at 750 K, is observed on bulk CuO. Quite different is the case when CuO is supported on mesoporous silica materials (Figure 4 a and b). All catalysts show a good activity above 500 K (Figure 4 a). CO is the main product in all cases, but at lower temperatures methylformiate (up to 20 %) is also registered. The samples exhibit comparable catalytic properties in the range of 550-600 K, but a lower activity for Cu-M1-N above 600 K is observed. This effect seems to be due to the faster deactivation of Cu-M1-N sample, which is confirmed by the experiments under isothermal conditions at 650 K (Figure 5 a). The activity of the latter decreases more than 3 times for 2 hours, while it is preserved almost unchanged when acetylacetonate was used as a precursor. At the same time all copper modified MCM-48 silica materials exhibit stable catalytic activity despite of the used copper precursor. The iron analogues are less active in methanol decomposition and they show catalytic activity just above 600 K (Figure 6 a and b). Methane, as well as CO are the main products registered in the whole investigated temperature interval. Small amounts of CO2, dimethyl ether (DME), and C2-C4 hydrocarbons are also observed. Despite the close conversion of the samples under thermo-programmed regime, some differences in their products distribution are registered (Figure 6 b). Higher CO yield for Fe-M1-N is observed. It decreases essentially when acetylacetonate was used as a precursor. At the same time all samples obtained on MCM-48 silica show a good selectivity to methane independent of the preparation method

1250 applied. On contrary to the corresponding copper samples, all iron modified catalysts exhibit fast deactivation at 650 K (Figure 5 b). Their activity decreases about 2.5 times in the first 30 minutes time on stream and remains almost unchanged within the next two hours. The bulk Fe203 reference material exhibits also high catalytic activity above 600 K (not depicted). However in comparison with the M41S supported iron oxide catalysts, it is characterized with high selectivity to CO (about 100%) and low degree of deactivation. The parent MCM-41 and MCM-48 silica materials do not exhibit catalytic activity in the whole temperature interval (not depicted). So the observed effects could be considered as an

~oo~

o. C

i I

o

/,/~i~

S o--~>~, 9

0

5S0

9

o - - Fe-M1-N - - o - - Fe-M8-N

.

--*--

. . . . . 600 650

40

o..%.,6 t

@

ii:,.

-0

L

"'. d"..

q..5;~.

--o-- Fe-M1-N --o-- Fe-M8-N --.--

'.. '~.; ...... 'q "'i="/.. -.

40 N"

"'"'/"'"'O

F e-M8-A

~..

}-60 '~__ "'..

'..

20

20

9 '.

o~

...@

Fe-M8-A 700

750

0

550

o

~ 600

650

700

750

100

temperature [K] temperature [K] Figure 6. Total conversion (a) and yield (b) of CO and CH4 (dotted) vs. temperature in methanol decomposition on various iron oxide containing samples.

evidence for some differences in the state of the nanostructured metal/metal oxide species depending on the preparation method and the host structure. Rapid reduction of the active phase to metal copper and following agglomeration of the latter seems to be most probably the reason for the conversion decrease with time on stream for the Cu-M1-N sample (cf. Figure 5 a). This processes seems to be favored by the one-dimensional channel system of MCM-41 silica, which is more susceptible to pore blocking effect in comparison to the MCM-48 silica. However, the slower deactivation of all acetylacetonate samples could be an indication of different mode of active phase deposition. We assume that this is caused by a higher dispersion of copper precursor species homogeneously spread within the whole mesoporous system. This is due to the preparation method A with chloroform which allows an increased wetting of the whole hydrophobic channel system during impregnation in comparison to the water/nitrate system that does not achieve the inner parts of the long mesoporous channels of the MCM-41 host structure. In case of the three-dimensional mesoporous channel system of the MCM-48 silica this effect is not observed due to the better accessibility. Comparable effects were found for the iron oxide system with respect to the selectivity of sample Fe-M1-N. In this case the blocking effect of the active sites within the mesoporous host system leads probably to the reduced selectivity for methane (cf. Figure 6 b). For all iron containing samples the reduction changes the iron oxide species (see also part 3.2) and that could be the reason for the observed fast decrease in the catalytic activity (cf. Figure 5 b) as it was shown recently [ 12]. 4. CONCLUSIONS Two simple methods for the preparation of highly dispersed CuO or Fe203 nanoparticles loaded within mesoporous silica by impregnation with metal acetylacetonates or nitrates are presented. The results from the complex investigation as P-XRD, physisorption, TPR-TGA, MS and the catalytic methanol decomposition show that these particles are located almost

1252 exclusively within the mesopores while the host structure is preserved. Some unusual redox and catalytic properties of these materials have been registered. The catalytic properties differ for the hexagonal MCM-41 silica material depending on the preparation method. The threedimensional MCM-48 silica material shows no differences in the catalytic properties for both preparation methods. The advantage of the metal nitrate impregnation is the possibility to achieve much higher metal contents in one single impregnation/calcination procedure. ACKNOWLEDGEMENTS Financial support from the National Science Fund at the Ministry of Education and Science of Bulgaria, Bulgarian Academy of Sciences and the Deutsche Forschungsgemeinschaft (Fr1372/2-1,2-2) and the Fonds der Chemischen Industrie is gratefully acknowledged. Ch. M. also wishes to thank to the Deutscher Akademischer Austauschdienst for the permanent support. The very helpful assistance of Uta Sazama is gratefully acknowledged. REFERENCES [ 1] [2] [3] [4] [5] [6] [7]

A. Corma, Chem. Rev., 97 (1997) 2373. U. Ciesla and F. Scht~th, Microporous Mesoporous Mater., 27 (1999) 131. F. Scht~th, A. Wingen, J. Sauer, Microporous Mesoporous Mater., 44-45 (2001) 465. R. KOhn and M. FrOba, Catal. Today, 68 (2001) 227. M. Hartmann, Stud. Surf. Sci. Catal., 128 (2000) 215. M. Ziolek, I. Sobczak, P. Decyk, I. Nowak, Stud. Surf. Sci. Catal., 125 (1999) 633. A. Zecchina, D. Scarano, G. Spoto, S. Bordiga, C. Lamberti, G. Bellussi, Stud. Surf. Sci. Catal., 117 (1998) 343. [8] M. Fr6ba, R. K6hn, G. Bouffaud, O. Richard, G. v. Tendeloo, Chem.Mater., 11 (1999) 2858. [9] M. Stockenhuber, R. W. Joyner, J. M. Dixon, M. J. Hudson, G. Grubert, Microporous Mesoporous Mater., 44-45 (2001) 367. [ 10] A. Wingen, W. Schmidt, F. Scht~th, A. C. Wie, C. N. Liao, K. J. Chao, Stud. Surf. Sci. Catal., 135 (2001) 317. [11 ] C. Minchev, R. KOhn, T. Tsoncheva, M. Dimitrov, M. Fr6ba, Stud. Surf. Sci. Catal., 135 (2001) 235. [12]T. Tsoncheva, M. Dimitrtov, D. Paneva, I. Mitov, R. KOhn, M. Fr6ba, C. Minchev, React. Kinet. Catal. Lett., 74 (2001) 385. [13] T. Tsoncheva, R. Dimitrova, C. Minchev, Appl. Catal. A: General, 171 (1998) 241. [ 14] R. K6hn, C. Minchev, M. Fr6ba, Annual Report, Hamburger Synchrotronstrahlungslabor, (2001) 759. [15] R. K6hn, PhD Thesis, University of Hamburg, 2001. [ 16] R. K6hn, C. Minchev, and M. Fr/Sba, unpublished results.


1253

Study o f the p o r o s i t y o f m o n t m o r i l l o n i t e pillared with a l u m i n u m / c e r i u m M.J. Hernando, C. Blanco, C. Pesquera, F. Gonzfilez Inorganic Group. Department of Engineering Chemistry and Inorganic Chemistry. University of Cantabria. Avda de los Castros, s/n 39005-Santander. SPAIN This study compares the porosity created in montmorillonites pillared with aluminum/cerium pillars with that of samples pillared only with aluminum. Studies of the pore size distribution indicate that the A1Ce-pillared samples have a new porous system, with a pore size at the limit between microporosity and mesoporosity. The presence of larger pores in the A1Ce-pillared samples is due to the inorganic polyoxycations intercalated between the clay layers. These polyoxycationes are larger than the Keggin ion intercalated in the samples pillared only with aluminum. After successive thermal treatments the micropore volume is still high in the A1Ce-pillared samples, whereas the thermal stability of the micropore volume, developed after the pillared process, is lower in the Al-pillared samples, being reduced practically to zero after thermal treatment at the same temperature. Moreover, the two different types of pores generated in the A1Ce-pillared samples show different thermal evolution and this compares them with the thermal evolution of the one single type of pores generated in the Al-pillared samples.

1. INTRODUCTION Pillared clays are smectite clay minerals that have been modified through the introduction of large inorganic polyoxycations into their interlayer regions, followed by calcination. The intercalated polycations increase the basal spacing of the clay and, upon heating, the resultant materials contain metal oxide pillars capable of preventing the collapse of the interlayer spaces. As a result, an interlayer space of molecular dimensions, a two-dimensional porous network, is generated. The microporous structure created, the high specific surface area and the presence of acid centers both on the surface of the layers and on their pillars make these materials suitable for use as adsorbents and catalysts (1-2). To prevent the clay layers from sintering, the thermal resistance of the pillars must be increased. One well-tested way of achieving this is to use mixed pillars in the materials (3-4), and the most widely used methods have been doping of Al-pillaring solutions with lanthanide cations. Shabtai et al were the first to prepare pillared solids containing Ce and A1 (5). Later, Sterte (6) and McCauley (7) found that the incorporation of lanthanide elements in the preparation of the pillaring agent resulted in material whose basal spacing was greater than that in conventional materials. In this study, we have prepared and characterized montmorillonite pillared with A1 and Ce under different synthesis conditions. The textural and structural parameters of the materials were compared with those of montmorillonite pillared only with A1. We have applied low pressure nitrogen adsorption data to obtain a quantitative evaluation of the microporosity of the synthesized materials and their evolution with thermal treatment.

1254 2. EXPERIMENTAL SECTION

2.1. Starting material The starting material used in this work was a montmorillonite from Wyoming, supplied by Missouri University and denominated here as Wy. The fraction 0.1 This new step in the level of nitrogen adsorption is seen more clearly when the isotherms of nitrow adsorption are represented in semilogarithmic scale (left of figure 3). Between P/Po=5.10- and P/Po=2.10 -2, the A1Ce-Wy25-400 sample displays a notable increase in nitrogen adsorption. This step in nitrogen adsorption, in a certain range of relative pressures, must be due to the insertion of larger polyoxycations between the clay sheets during the pillaring process (greater basal spacing), thus this generated a greater space interlayer and, consequently, pores of larger diameter. This results in nitrogen adsorption up to high relative pressures. This adsorption is at the limit between microporous and mesoporous size, corresponding to a diameter of 20A in the classification of Dubinin (8). However, a single level of adsorption at low pressures is observed in the Al-pillared sample, in figure 2 (left) the step around P/Po=0.1 does not appear, which indicates a single type of size of micropores.

250 --*- AICe-Wy-25-400 4 - AICe-Wy-25-500 AICe-Wy-25-600 --e-AICe-Wy-25-700 AICe-Wy-25-800

AE~ ~o ~ -lo ,.Q '-- 150 o oo -lo

(D

E 100 o > 50 0 0.00001

o.oool

o.ool

o o~

o~

Relative Pressure (P/Po)

1

0

0.2

0.4

0.6

0.8

Relative Pressure (P/Po)

Figure 3. N2 adsorption isotherms at 77 K of the A1Ce-Wy-25 sample after thermal treatments, between 400 and 800~ Relative pressure axis: left, logarithmic and right, linear.

1257 Table 1 Surface area and pore volumes calculated by the BET equation and the t-plot Samples

SBET(m2/g)

A1Ce-Wy-25-400 A1Ce-Wy-50-400 A1Ce-Wy-75-400 A1-Wy-400 A1-Wy

Vad(cm3/g)

373 411 277 346 33

0.312 0.337 0.189 0.226 0.042

Vmp/Wad (%)

Vmp(Cm3/g) 0.196 0.223 0.100 0.131 0

62.8 66.1 52.9 57.9 0

In figure 3, it is possible to observe how the new step, which is present in the pillared samples with A1Ce, remains when materials are treated at temperatures of up to 700~ As temperature is increased, a decrease in intensity of the step is observed with inferior nitrogen adsorptions when the samples are treated at higher temperatures. This relationship between the level of nitrogen adsorption and the increase in thermal treatment, is also noted in the A1Wy sample (figure 2). In both samples the adsorption branches of the isotherms (figures 2 and 3), at high pressures P/Po>0.2, remain parallel to each other after the consecutive thermal treatments, which indicates that the mesoporosity of the materials is not modified, affecting the decrease in the adsorption of nitrogen at low pressure values and therefore to the microporous zone. In addition, the adsorption branches of the Al-pillared material (A1-Wy) are seen to display lower values that decrease more quickly with the thermal treatment. Table 1 shows the values for specific surface area, SBET and volume of N2 adsorbed at P/Po=0.98, Vad, of pillared samples and the raw material. All the pillared samples show a more developed porosity than the raw material (montmorillonite). The specific surface area and pore volume increase in both types of pillared samples, but the increase is greater in the A1Ce-pillared samples (A1Ce-Wy-25 and A1Ce-Wy-50) than in the Al-pillared samples (A1Ce-Wy-75 and A1-Wy). The increase in specific surface area was consistent with the expansion of the structure observed by XRD. The A1-Wy-400 sample had a SBETof 346 mZ/g while the A1Ce-Wy-50-400 had a value of 41 lmZ/g. 200 "~5o -o

~loo..p E 50 >o 0

0

i

2

3 Thic~nessS(A), H6arkins7and j8ura

9

Figure 4. t-plot of pillared samples: e, Al-Wy-400 and a, A1Ce-Wy-50-400.

1258 Figure 4 shows the t-plot for two of the pillared samples. From this, the micropore volume,

gmp, was calculated (Table 1). The micropore volume is seen to increase in the pillared samples, and again the increase is greater in the A1Ce-pillared samples. This increase in micropore volume, which is greater than that in specific surface area shows a two-fold increase in the A1Ce-Wy-50-400 sample over the A1-Wy-400 sample (0.223 and 0.131 cm3/g respectively). The former sample presents a second step in the adsorbed volume at greater thickness, t, than in the A1-Wy-400 sample, which presents only one step. This second step is related to relative pressure of adsorption around P/Po = 0.1, which corresponds to the abovementioned increase in the adsorption isotherm. The ratio of micropore volume to total pore volume, Vmp/gad, is given in Table 1. There is a higher ratio of micropore volume to total pore volume for the A1Ce-pillared samples than for the Al-pillared samples. This indicates greater development of microporosity and an increase in pore size in the former. The micropore size distribution and cumulative pore volume of the pillared samples was obtained by the DFT method, using the slit-like pore model, given the laminar structure of the pillared montmorillonite samples. Figure 5 show the micropore distribution and cumulative pore volume by the DFT method for samples A1-Wy-400 (Figure 5, left) and A1Ce-Wy-50400 (Figure 5, right). For the latter, the micropore volume is seen to accumulate approximately two type of pores. The cumulative pore volume curve shows two steps, each associated with one of the two pore distribution maxima. In contrast, for the A1-Wy-400 sample, the pore volume distribution presents a single type pore, which corresponds to the smallest diameter pore for the A1Ce-Wy-50-400 sample.

o.14

0.07 0.06~

o.1s

0.05~

m

>~

0.0.2

~o.0~

0.030
o.1

0.02 9

-5 E 0.050

0

001 ~-~10

15

20

25

30

Pore Width

35 (A)

40

45

0 50

Figure 5. Distribution and cumulative pore volume by the DFT method of the pillared samples: left, A1-Wy-400 and right, A1Ce-Wy-50-400.

1259 0.25

400 350

,0.2

300

~o.~5

"~

2~o~ 200 't?:

~0.1 o "-

150 o 100

~0.05

5O 0 400

500

600

0 800

700

Temperature (~

Figure 6. Thermal evolution of the specific surface area of the samples: , , A1-Wy and II, A1Ce-Wy-25 and the micropore volume of the samples: A, A1-Wy and O, A1Ce-Wy-25. The hydrothermal stability of the pillars was also studied by means of the N2 isotherms of the samples subjected to hydrothermal treatment. Figure 6 presents the thermal evolution of specific surface area, SBET, and micropore volume, Vmp, of the A1Ce-Wy-25 sample from 400~ until 800~ and the A1-Wy sample from 400~ to 700~ It can be seen that with the successive thermal treatments up to 700~ low values of specific surface area and micropore volume are reached in the A1-Wy sample. While, for the A1Ce-Wy-25 sample these textural parameters are still high at the same temperature. After treatment at 700~ the SBETof the A1Ce-Wy-25 and A1Ce-Wy-50 samples was 304 and 270 m2/g, respectively. In contrast, the A1Ce-Wy-75 and A1-Wy samples treated at 700~ had a SBETof 43 and 50 m2/g, indicating a total collapse of the pillars, with microporosity falling sharply to values similar to those of the initial sample. This can be attributed to the presence of cerium in the pillars which gives an increase in the basal spacing of these samples and seems to display greater thermal stability than in the samples that only incorporate A1 in this process. E~ E >o

g

0.25

0.045

0.2 E0.03 0.15

0.025

J~ ~1~

L / r~ /

Ill

/*, ~1

-l- AICe-Wv-25-5o0 -i- AICe-Wy-25-600 -e- AICe~Wy~2~700

~ 0.02

"-= 0.1

~

E

0.015

-- 0.01

0 0.05

0.005 10

20

30

Pore Width (/~)

40

50

0

0

10

20

30

40

50

Pore Width (,~)

Figure 7. Distribution (right) and cumulative pore volume (left) by the DFT method of the pillared sample A1Ce-Wy-25 after thermal treatments between 400 and 800~

1260 In order to study the evolution of the microporosity generated in these materials with thermal treatments, the micropore size distribution and cumulative pore volume of the pillared samples have been analyzed. Figure 7 shows the micropore distribution (right) and cumulative pore volume (left) for the A1Ce-Wy-25 sample at different temperatures. This figure shows that when the sample was subjected to thermal treatment, the behaviour of the two type of pores was totally different. As seen the smaller pores remained nearly constant, while the larger pores decreased. This may be because the larger pores protect the smaller pores from thermic damage in the A1Ce-Wy-25 sample. 4. CONCLUSIONS The montmorillonite pillared with aluminum and cerium has inorganic polyoxycations incorporated between the clay sheets. This modifies the textural characteristics of the raw material. The specific surface area is increased and the porous structure, particularly the micropore volume, with generation of pores at the limit between microporosity and mesoporosity. The A1Ce-pillared samples have larger pores than those generated in the A1pillared materials, whose pores clearly belong to the micropore region. In addition to the fact that the textural parameters show higher values, they are thermally more stable, maintaining high values of specific surface area and micropore volume up to 700~ Thanks to this greater thermal stability in the A1Ce-pillared samples, which show two types of pores, the largersized pores prevent the collapse of the smaller-sized pores, as the collapse of the larger-sized pores when subjected to thermal treatments simultaneously protects those of smaller diameter against deterioration. ACKNOWLEDGMENT: We acknowledgment to CICYT for financial support of this work under Project MAT 99/1093-CO2-O2. REFERENCES 1. J. T. Kloprogge, J. Porous Mater. 5 (1998) 5. 2. A. Gil, L.M. Gandia, M.A. Vicente, Catal. Rev.- Sci. Eng. 42 (2000) 145. 3. X. Tang, W. Q. Shu, Y.F. Shen, S.L. Suib, Chem. Mater. 7 (1995) 102. 4. M.J. Hernando, C. Pesquera, C. Blanco, I. Benito, F. Gonz~.lez, Chem. Mater. 8 (1995) 76. 5. J. Shabtai, M. Rossell, M. Tokarz, Clays Clay Miner. 32 (1984) 99. 6. J. Sterte, Clays Clay Miner. 39 (1991) 167. 7. J.R. McCauley, U.S. Patent No. 4,818,737 (1988). 8. S.J.Gregg, K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press, London, 1982. 9. J. H. de Boer, J. Catal. 3 (1964) 268. 10. W. D. Harkins, G.J. Jura, J. Chem. Phys. 11 (1943) 431. 11. J. P. Olivier, J. Porous Mater. 2 (1995) 9. 12. M.J. Hernando, Thesis, University of Cantabria, 2000. 13. S. M. Bradley, R.A. Kydd, J. Yamdagni, J. Chem. Soc., Dalton Trans. 7 (1990) 102. 14. C. Pesquera, F. GonzS.lez, I. Benito, S. Mendioroz, J.A. Pajares, Appl. Catal. 8 (1991) 587. 15. F. Gonz~lez, C. Pesquera, C. Blanco, I. Benito, S. Mendioroz, Inorg. Chem. 31 (1992) 727.


1261

X-ray absorption fine structure investigation o f M C M - 4 1 materials containing Pt and PtSn nanoparticles prepared via direct hydrothermal synthesis Chanho Pak a, Naisheng Yaob and Gary L. Hallerb aMaterials and Devices Laboratory, Samsung Advanced Institute of Technology, EO. Box 111, Suwon, 440-600, Korea bDepartment of Chemical Engineering, Yale University, EO. Box 208286, New Haven, CT 06520-8286, USA

Pt- and PtSn- containing MCM-41 materials were prepared by direct introduction of metal precursors with fumed silica and cetyltrimethylammonium surfactant. The precursor of Pt, with or without Sn precursor, was introduced in the initial synthesis mixture by dissolving both Pt(NHa)4(NO3)and K2SnO3. The highly ordered MCM-41 mesoporous structure of two samples was revealed by a four-peaked XRD patterns, sharp capillary condensation in nitrogen isotherms, and a high surface area. Alter reduction, the coordination number of Pt particles in the MCM-41 samples decreased from 8.7 to 6.4 with simultaneous incorporation of Sn. The nearest Pt-Pt distance is almost the same, 0.276 ran, in both samples. The whiteline intensity of X-ray absorption at the Pt LIn edge was decreased to that of metal foil during the reduction. It is suggested from the coordination number of the first shell that Pt particles are generated in the MCM-41 mesoporous molecular sieves by direct introduction and that simultaneous addition of Sn leads to a decrease of particle size. 1. INTRODUCTION Mesoporous materials such as the M41S family and SBA-n have stimulated interest as sorbents, catalysts and supports of high surface area and uniform pore size. Pt-containing mesoporous materials prepared by direct incorporation [1-3] or post impregnation [4-7] with several precursors have been studied for the conversion of carbon monoxide [1 ], aromatics [4], and hexane [5,6]. Recently, we investigated three preparation methods for the preparation of Pt particles in MCM-41 material and obtained highly dispersed Pt particles in MCM-41 [7]. In this study, the Pt- and PtSn-containing MCM-41 samples were prepared by direct introduction of the Pt precursor without or simultaneously with the Sn precursor. The structure of MCM-41 demonstrated by X-ray diffraction is slightly perturbed by introduction of metal atoms, but retains a highly ordered structure. The particle size estimated by the coordination number of Pt particle was decreased by simultaneous introduction of Sn.

1262 2. EXPERIMENTAL

It-containing MCM-41 (It-MCM-41) material was synthesized by a modified method used for pure MCM-41 [7,8]. Ten g of tetramethylammonium silicate (SACHEM, 10-wt% SiO2) was mixed with 50g of doubly deionized water (DW) and then 2.5g of HiSil-233 (Pittsburgh Plate Glass) was added. Pt(NH3)4NO3 (0.0968g, Aldrich) was added to the above solution. Then, cetyltrimethylammonium (CTA) hydroxide solution (29.0g), obtained from ion exchange of the 20 wt% of CTABr aqueous solution with Amberjet 4400 OH (Sigma) resin by batch mixing, was poured into the silica solution with vigorous stirring. After stirring for 10rain, the solution pH was corrected to 11 with dilute H2SO4 solution. The synthetic gel, having molar ratios of 3.67 SiO2:0.30 (TMA)20:0.02 It: 1 CTAOH: 255 H20, was transferred to a polypropylene (PP) bottle and placed in an autoclave at 383 K for 96 h. For the ItSn-containing MCM-41 (ItSn-MCM-41) was prepared by the same procedure as above except the simultaneous addition of 0.2989 g of K2SnO3 (STREM) with Pt(NH3)4NO3. The molar ratios in the mixture at this time was 3.67 SiO2:0.30 (TMA)20:0.02 It: 0.08 Sn: 1 CTAOH: 255 H20. The resulting product was recovered by filtration on a Btichner funnel, washed with DW, and dried in air at room temperature (RT). Calcination of this product was conducted by heating from RT to 823 K at 1K/min and holding for 6 h in flowing air. Reduction of the calcined sample was carried out with 5% H2/He flow at 623 K for 2 h. The XRD patterns were recorded on a SCINTAG X-ray diffractometer (Cu I~, k= 0.154 ran, 40 kV, 45mA). The scanning range of 20 was between 1.6 ~ and 7 ~ with a step increase of 0.02. The sample powder was pressed into a depressed square area of a plastic slide to obtain a smooth plane of sample with regard to the slide surface. Nitrogen adsorption-desorption isotherms were measured at 77 K with a static volumetric instrument (Autosorb-lC, Quantachrome) after outgassing the sample at 473 K to a vacuum of ca. 10"7 bar. A Baratron (10 .6 - 10.2 bar) pressure transducer was used for low-pressure measurement. At each isotherm point, the saturated vapor pressure of liquid nitrogen was measured in a reference cell. Thermal analysis was performed by simultaneous TG-DTA measurements in flowing air using the STA 449C of NETZSCH Co. Samples were heated in the temperature range 3 0 0 1073K at a heating rate 10 K/min. X-Ray absorption measurement was performed at the I t Lm edge (11564 eV), using Si(lll) as the monochromator crystal at station X18B in NSLS, 2.5 GeV storage ring, Brookhaven National Laboratory. Samples were pressed into self-supporting wafers and placed into an in-situ cell, equipped with Kapton windows, gas inlet, outlet and heating unit around the sample, which allowed gas treatment and measurement. In the XANES analysis the first inflection points of a metal foil reference and of all samples were adjusted and aligned to the edge energy of the metal foil. The X-ray absorption fine structure (XAFS) spectra were analyzed relative to the reference XAFS from the I t foil by using UWXAFS 2.01 from the Department of Physics at the University of Washington.

1263 3. RESULTS AND DISCUSSION The color of Pt and PtSn containing MCM-41 samples changed from light gray to dark gray after calcination, which is evidence of the presence of metal particles in these materials. The loading of each metal was analyzed by elemental analysis as listed in Table 1. The amount of Pt in the two samples is very similar at about 1.3 wt%, which indicates the loading of Pt is not effected by the simultaneous addition of Sn with this synthesis method. All samples displayed at least four well-resolved peaks corresponding for p6m symmetry in the XRD patterns below 2 0 - 7 ~ as shown in Fig. 1 [1,6-8]. The direct introduction of Pt and Sn precursors into the synthesis mixture for MCM-41 materials did not affect significantly the formation of the hexagonal mesoporous structure. A This result agreed with the 5 previous observation [1]. The intensities of the most intense peaks of Pt-MCM-41 and PtSnMCM-41 samples were decreased from that of pure MCM-41, which suggests that the hexagonal structure was perturbed a little by introduction of Pt and Sn metal atoms. The four peaks indexed as (1 00), (1 1 0), ( 2 0 0 ) and ( 2 2 0 ) 2 3 4 5 6 planes. The lattice constant of the 2 Theta (degree) structure was calculated from the Figure 1. Powder XRD patterns of (a) pure MCM-41, (b) PtSn-MCMequation a0 = 2d100A/3. The unit 41 and (c) Pt-MCM-41. cell parameters are given in Table 1. The lattice constant is increased by 0.3 nm for Pt-MCM-41 and is the same for PtSn-MCM-41 when compared with the pure MCM-41. This is additional evidence of a small perturbation of the hexagonal structure of MCM-41 by transition metal atoms.

Table 1. Physical properties ofPt-MCM-41 and PtSn-MCM-41 samples. Sample MCM-41 Pt-MCM-41

Matal loading

(wt%)

Surface area (m//g)

Lattice Constant

Wall thickness a

3.7 3.8

4.6 4.9

0.9 1.1

3.6

4.6

1.0

Pore size (nm)

1261 Pt: 1.3 780 Pt: 1.2 PtSn-MCM-41 893 Sn: 3.1 aobtained from the difference between lattice constant

and pore size.

(nm)

(nm)

1264

100

*-

b

80

k

,,m

o 6O I

400

600

800

t

I

1000

Temperature (K)

400

600

800

1000

Temperature (K)

Figure 2. (a) Weight change curves for (--) Pt-MCM-41 and ( - - - ) PtSnMCM-41 and (b) weight change derivatives Figure 2 shows the TGA plots of weight loss observed for the Pt- and PtSn-MCM-41 samples. Both samples exhibited typical patterns in the plots as reported in the literature for M41S materials [9-13]. The final residue is 64 wt% for Pt-MCM-41 and 60 wt% for PtSnMCM-41, respectively. In the case of PtSn-MCM-41 sample, the weight loss is larger than that of Pt-MCM-41. This is attributed to the additional weight loss of the Sn component. In Fig. 2 b, three regions of 600 weight loss were observed in both samples. The first weight loss between RT and 390 K is due to adsorbed water (-~5 wt%). Most of weight loss (20-23 A wt%) was observed in the second region b ( 3 9 0 - 563 K), which is caused by the 400 E decomposition of surfaetants [9-13]. A difference of both samples was obvious Q} in the third weight loss region (563-773 K). It appears that this region is related E to the decomposition of the metal 200 complexes. The increased portion of weight loss in this region supports this interpretation because the PtSn-MCM41 sample had additional SnO32 complex in the as-made sample. 0 N I I I I Fig. 3 shows the nitrogen adsorption0.0 0.2 0.4 0.6 0.8 1.0 desorption isotherms of Pt-MCM-41 Relative Pressure (pip.) and PtSn-MCM-41 attar calcination. A Figure 3. Nitrogen adsorption-desorption capillary condensation of a typical Type isotherms of (a) Pt-MCM-41 and VI isotherm was exhibited in the range (b) PtSn-MCM-41

~

I

1265 of 0.2-0.4 relative pressure (P/P0) without hysteresis for all samples. This is a characteristic of highly ordered mesoporous structures such as MCM-41 and MCM-48 [8,14,15]. The surface area of samples was decreased to 780 m2/g for Pt-MCM-41 and 893 m2/g for PtSn-MCM-41 from that (1261 m2/g) of MCM-41 material. Although the introduction of metal atoms caused a decrease of the surface area, the absolute value of the materials is still high so that the mesoporous structure is slightly disturbed. The pore size was calculated using the BJH method with the corrected form of the Kelvin equation and the statistical film thickness curve for porous silica, both derived from the Kruk, Jaroniec and Sayari (KJS) approach [16]. The pore size for both Pt-MCM-41 and PtSnMCM-41 samples is almost identical (3.7 + 0.1nm) to the pure MCM-41 materials. The suggests that the pore structure of MCM-41 is retained, even when transition metal atoms were directly introduced in the initial synthesis mixture, as was the case for Cr and V [8,17]. The X-ray absorption near edge structure (XANES) was recorded during the in situ reduction of the calcined sample from RT to 623K (not shown). The whiteline area at the Lin edge of Pt was gradually decreased with increased temperature, which indicates that the Pt was reduced to the zero valent state. The temperature of maximum reduction rate, estimated from the rate of change ofthe white line area [7], is 423K for Pt-MCM-41 and 388K for PtSnMCM-41, respectively. For XAFS analysis, after Fourier transforming, the spectra in R space of the isolated Pt-Pt coordination shell were obtained using UWXAFS 2.01 [18] as shown in Fig. 4. The height of the peaks in R space is indicative of the number of Pt atoms that are present at the corresponding distance between two atoms. The fitted spectra in R space are also illustrated in Fig. 4. The structural parameters are calculated from the fitting with Pt foil as reference and listed in Table 2. The average distance between Pt-Pt atoms of the first shell was 0.276 nm for Pt- MCM-41 and 0.277 nm for PtSn-MCM-41, respectively, which is almost identical to the value of the bulk Pt foil. The coordination number of the first shell is decreased from the 8.7 for Pt-MCM-41 to 6.4 for PtSn-MCM-41 by simultaneous introduction of Sn. For the

b

_= I

1 2 3 4 Distance (A)

5

0

1

2 3 4 Distance (A)

Figure 4. Fourier transfoms magnitudes of (a) Pt-MCM-41 and (b) PtSn-MCM-41 ( ( - - ) experimental and (ooo) fitted).

9 . . . .

1266 Table 2. XAFS fitting results for the Pt-MCM-41 and PtSn-MCM-41 samples. Sample Distance ( n m ) Coordination number Debye-Wallerfactor Pt-MCM-41 0.276 8.7 0.0038 PtSn-MCM-41 0.277 6.4 0.0031 determination of particle size, the high-angle XRD and transmission electron microscopy is now in progress. Highly ordered mesoporous Pt and PtSn containing MCM-41 materials were prepared by direct hydrothermal synthesis. The mesoporous structure of these materials is suggested from the XRD, TGA and nitrogen isotherms. It was hypothesized from the XAFS analysis that the simultaneous introduction of Sn with Pt precursor caused the decrease of the coordination number at the first shell of Pt particles. ACKNOWLEDGEMENTS

We acknowledge financial support from DOE, Office of Basic Energy Science. C. Pak also thanks the Korea Science and Engineering Foundation (KOSEF) for partial financial support for post-doctoral fellowship. REFERENCES

1. U. Junges, W. Jacobs, I. Voigt-Martin, B. Krutzsch, F. Schuth, Chem. Commurt, (1995) 2283. 2. M.A. Aramendia, V. Borau, C. Jimenez, J.M. Marinas, F.J. Romero, Chem. Commun., (1999) 873. 3. M. Chatterjee, T. Iwasaki, Y. Onodera, T. Nagase, Catal. Lett., 61 (1999) 199. 4. A. Corma, A. Martinez, V. Martinez-Soria, J. CataL, 169 (1997) 480. 5. T. Takeguchi, J.-B. Kim, M. Kang, T. Inui, W.-T. Cheuh, G.L. Haller, J. CataL, 175 (1998) 1. 6. K. Chaudhari, T.K. Das, A.J. Chandwadkar, S. Sivasanker, J. CataL, 186 (1999) 81. 7. N. Yao, C. Pinckey, S. Lim, C. Pak, G.L. Haller, Micro. Mesopo. Mater., 44-45 (2001) 377. 8. C. Pak, G.L. Haller, Micro. Mesopo. Mater., 48 (2001) 165. 9. Y. Cesteros, G.L. Haller, Micro. Mesopo. Mater., 43 (2001) 171. 10. M. Kn~, M. Jaroniec, R. Ryoo, S.H. Joo, Chem. Mater., 12 (2000) 1414. 11. A.A. Romero, M.D. Alba, J. Klinowski, J. Phys. Chem. B, 102 (1998) 123. 12. S. Kawi, M. Te, Catal. Today, 44 (1998) 101. 13. M. Busio, J. Janchen, J.H.C. van Hooff, Micro. Mater., 5 (1995) 211 14. M. Jaroniec, M. Kn~, H.J. Shin, R. Ryoo, Y. Sakamoto, O. Terasaki, Micro. Mesopo. Mater., 48 (2001) 127. 15. F. Rouquerol, J. Rouquerol, K. Sing, Adsorption by powders and porous solids: Principles, Methodology and Applications, Academic Press, San Diego, 1999. 16. M. Knfl(, M. Jaroniec, A. Sayari, Langmuir, 13 (1997) 6267. 17. S. Lim, G.L. HaUer, Appl. Catal. A: Gen., 188 (1999) 277. 18. M. Newville, P. Livins, Y. Yacoby, J.J. Rehr, E.A. Stern, Phys. Rev. B, 47 (1993) 14126.


1267

Ordered assembling of precursors of colloidal faujasite mediated by a cationic surfactant. J. Agfindez, I. Diaz, C. M/trquez-Alvarez, E. Sastre and J. P6rez Pariente*. Instituto de Cat/disis y Petroleoquimica, C.S.I.C. Campus Cantoblanco, 28049 Madrid (Spain)

The synthesis of ordered microporous materials have been carried out by adding a cationic surfactant, cetyltrimethylammonium bromide, to TMA-containing solutions precursors of colloidal faujasite. The influence of the synthesis temperature on the physicochemical properties and catalytic activity in m-xylene conversion of the calcined samples has been studied. Calcination of the as-made, MCM-41-1ike samples produces a strong unit cell contraction, and microporosity develops. No FAU structural elements are detected by IR spectroscopy. The reaction rate for m-xylene transformation is much higher than that of conventional A1-MCM-41, but lower than that of USY, whereas the isomerization/disproportionation ratio is close to 10.

1. INTRODUCTION The discovery of the Ordered Mesoporous Materials (OMM) opened the possibility to process organic molecules bulkier than those currently converted by conventional zeolites. The isomorphous substitution of part of the silicon atoms of the framework by aluminum imparts structural acidity, but it was soon realised that the acidity and stability of the aluminum-containing materials is below that required for most applications in the refining industry (1). The mild acidity has been attributed to the absence of structural ordering within the silica-alumina inorganic framework of the material. On the other hand, it is well known since long the strong acidity and good stability (generally speaking) of many zeolite structures, although obviously these properties vary from one structure to another, as well as a function of the A1 content. Therefore, it has been argued that the eventual ordering of the framework of mesoporous materials would increase their acidity and thermal stability. This goal, if reached, would have important consequences for the catalytic application of these materials in many chemical processes, especially those dealing with molecules excessively large to be activated by microporous structures. Fuelled by these potential benefits, a number of researches have been pursued, aiming to confer some structural order to the framework of OMM. Basically, three different approaches have been followed: A) on-site structuring of the framework during the self-assembling process leading to the mesostructured material. B) *Author for correspondence. Phone: 34 91 585 4784. Fax: 34 91 585 4760. E-mail: [email protected]. Http://www.icp.csic.es/gtm

1268 post-synthesis recrystallization of MCM-41 by treatment with a template-containing solution. C) the use of pre-ordered blocks synthesised e x s i t u and their further assembly by addition of surfactants. Following route A, mixtures of ZSM-5 and MCM-41 are obtained by using tetrapropylammonium (TPA) and a cationic surfactant (2-4). Composite materials containing zeolite and MCM-41 are obtained by using procedure B. For instance, mixtures of ZSM-5 and MCM-41 are obtained by hydrothermal treatment of MCM-41 exchanged with TPA (5) or by impregnation of amorphous mesoporous aluminium-containing silica (Si/A1 = 100) with a TPAOH solution (6). Aluminosilicate mesostructures stable under steaming have been obtained by using precursors of the zeolites faujasite (7), ZSM-5 (8) and Beta (8-10), i.e., route C. However, although the benefits of the assembling of the so-called nanoclustered aluminosilicate precursors on the steam stability of the OMM is well documented, the increase in stability is not accompanied by a simultaneous enhancement of the acidity. An exception to this behaviour has been reported recently by Zhang and col. (10), who claimed the synthesis of a mesoporous material that possesses an acid strength similar to that of zeolite H-Beta. However, if precursors of zeolite Y are used to built up the mesoporous material, only minor differences between this material and conventional A1-MCM-41 in the cracking of cumene are observed before steaming (7). In contrast to this, the use of ZSM-5 seeds leads to a three-fold increase of activity for the same reaction (8). The synthesis of the precursors of both ZSM-5 and Beta requires the use of an organic cation, TPA and TEA (tetraethylammonium), respectively, whereas the reported synthesis of the faujasite precursor made use of a pure inorganic seeding gel (7). Therefore, it would be possible that the presence of organic cations in the zeolite gel precursor be required to obtain materials possessing high acid strength. Accordingly, we have carried out the synthesis of mesoporous materials by using as building blocks precursors of zeolite faujasite obtained in the presence of tetramethylammonium (TMA) cations. The procedure reported in ref. (11), which allows the synthesis of colloidal crystals of faujasite, has been followed.

2. E X P E R I M E N T A L 2.1. Materials

Synthesis gels precursors of faujasite with a molar composition: A1203 : 1.53 (TMA)20 : 0.088 Na20 : 3.62 SiO2 : 246 H20 were prepared as follows: freshly precipitated AI(OH)3 was added under stirring to a solution containing TMAOH (25 wt%, Aldrich) and NaOH (Prolabo), until complete dissolution of the AI(OH)3. Then, Ludox SM30 (Aldrich, previously exchanged with Dowex HCR-S ion exchange resin) was added and the mixture stirred for 45 minutes. The resulting opalescent solution was poured into polypropylene bottles and kept at 100~ for 24 h. After this, a hexadecyltrimethylammonium (CTA) bromide aqueous solution (20 wt%) was added at room temperature, and the bottles were heated again at selected temperatures for 3 h. For T > 100~ 60ml, teflon-lined, stainless-steel autoclaves were used. The resulting solid was filtered, washed with deionized water and dried at 60~ overnight. The total yield of oxides averaged 60%. The samples were calcined at 550~ under continuous flow of N2 (130 cm3-min-1) for lh, followed by air (130 cm3.min-1) for 6 h. Reference A1-MCM-41 (Si/A1 = 15) was prepared following ref. (12). A commercial ultrastable Y zeolite (USY CBV-720,

1269 unit cell parameter a0 = 24.28 A), kindly provided by Zeolyst International, was also used as reference in the m-xylene isomerization experiments.

2.2. Characterization X-ray powder diffraction patterns were collected using CuKc~ radiation, on a Seiffert XRD 3000P diffractometer operating at low angle (1-10 ~ using a primary automatic divergence slit and a 0.2 mm detector slit. Analysis of the organic material present in the solid was carried out using a Perkin-Elmer 2400 CHN analyser. Thermogravimetric analyses (TGA) were performed using a Perkin-Elmer TGA7 instrument, from 30 to 900~ at a heating rate of 10~ -1 under air flow. Adsorption of nitrogen was carried out at-196~ using a Micromeritics ASAP 2000 apparatus. Specific surface areas were calculated following the BET procedure. The Si/A1 ratio of calcined samples, finely grounded and dispersed in water, was measured using a Rich & Seifert EXTRA-II total reflectance X-ray fluorescence (TXRF) spectrometer. For the transmission electron microscopy (TEM) experiments, the samples were crushed, dispersed in acetone and dropped on a holey carbon grid. Micrographs were recorded using a JEOL JEM 2000FX microscope operating at 200 kV equipped with an X-ray detector (XEDS) and a Philips 120 Biotwin microscope operating at 120 kV equipped with a Gatan CCD camera. Infrared (IR) spectra in the range 400-4000 cm -1 were recorded at 4 cm -1 resolution, in the transmission mode, using a Nicolet 5ZDX FTIR spectrometer. IR spectra of the solid samples diluted in KBr were recorded at room temperature. For acid sites -2 characterisation, the samples were pressed into self-supporting wafers of 6 to 12 mg.cm thickness, placed into a glass cell provided with CaF2 windows and greaseless stopcocks, and heated under vacuum (10 .3 Pa) at 350~ for 8 h; pyridine (8 Torr) was dosed at room temperature, the sample subsequently evacuated at selected temperatures for 1 h, and IR spectra recorded at room temperature. The amount of pyridine adsorbed on acid sites were estimated from the integrated absorbance of the IR bands centred at ca. 1547 (Br6nsted sites) and 1456 cm -1 (Lewis sites), assuming for the integrated molar extinction coefficients the values 1.67 and 2.22 cm.gmo1-1, respectively (13). 2.3. Catalytic Activity Isomerization of m-xylene was carried out in a fixed bed reactor at atmospheric pressure, working at a temperature of 400~ and a molar ratio N2/m-xylene - 4. The contact time was varied accordingly to obtain conversions below 10%. The reaction products were analysed by gas chromatography in a Hewlett-Packard GC 5710A provided with a thermal conductivity detector and equipped with a column filled with DC-200 methylsilicone (16%) and Bentone 34 (3%) on Chromosorb W (80-100 mesh). In order to compare the activity of the catalysts in the absence of deactivation, the initial activity (V0) of all products were calculated by extrapolating the conversion at time zero of reaction (14).

3. RESULTS AND DISCUSSION The X-ray diffraction patterns of the samples obtained from gels heated at different temperatures are given in Figure 1. Reflections corresponding to faujasite were not observed. Patterns characteristics of MCM-41 were obtained, the ordering of which increases with the synthesis temperature, whereas the unit cell dimensions follow the opposite trend (Table 1).

1270

[s2:il

[, $2-1 cal. ] 5 20 25 30 35 40

"-2',

d

s 2, A ~

r~

3

5

40

]s2-3ca,I J 5 10 15 20 25 30 35 40 I x 1.7 2

3

4

5

6

1

z0

2

3

4

5

6

Figure 1. XRD patterns of as-made and calcined samples. Upon calcination, only one intense peak at 20 - 3.7 ~ is observed in the XRD patterns (Figure 1), which intensity also increases with the synthesis temperature. The TEM image of sample S1-3 calcined, synthesised at 80~ shows that this is a heterogeneous material exhibiting some MCM-41 orientations (Figure 2A). On the other hand, the calcined sample synthesised at 150~ (sample $2-3) possesses large domains exhibiting well ordered hexagonal pore arrangement characteristic of MCM-41 (Figure 2B). From the TEM image, a unit cell dimension (a0) of 29 A is calculated for sample $2-3 by Fast Fourier Transform (FFT). This value compares well with the a0 value determined from XRD (27.7 A), assuming that the intense peak at 3.7 ~ corresponds to the [100] reflection of the hexagonal p6mm symmetry. For sample $2-3, the Si/A1 ratio obtained by TXRF is 2.7, close to the value of 3.0 determined by XEDS. Table 1. Synthesis conditions and properties.

Sample

S1-3 $2-1 $2-2 $2-3

as-made Synthesis d temperature spacing (~ (A) 80 100 120 150

. . 38.55 38.45 37.49

,

ao

d spacing (A)

(A) .

. . 44.6 44.4 43.3

.

. . 23.56 23.39 23.26

Calcined ao Surface area @) (m2.~-') 27.2 27.2 27.7

544 617 593 642

Micropore volume (cm3-~-~) 0 0.12 0.15 0.24

1271

~flnm_v_...

imilllmllmm

." ,,

B

~'.

Figure 2. TEM images of calcined samples S 1-3 (A) and $2-3 (B). It is noteworthy the large unit cell contraction brought about by calcination, --- 17 A, and particularly the fact that despite of this severe shrinkage, the hexagonal arrangement is nevertheless maintained in the samples synthesised at T > 100~ (Table 1). This remarkable cell contraction might be due to the presence of a large number of structural defects, i.e., SiOH groups. From the chemical analysis and the TG data, the total content of silanol groups has been estimated to be 0.95 mol per mol of SiO2, i.e., a very high population of structural defects. Indeed, the large amount of organic material contained in the solid, as evidenced by TG (Figure 3), points to a thin pore wall. The reference A1-MCM-41 material contains 35wt%

,

i

i

i

i

220 t 200-[" ~

a.

_

_

t "/ -

'a

300

!

"

~

0.6

b

~

0.5

a

~"

0.4

I ; i ,

o

e~

C

_.

. . . . . . .

o

b

I :)t : c el

200.

o

/

100 0 0.0

012

014

016

P/Po

018

1.0

0.0

I , ~ !

a^ , : ,

~tE 0.3 0.2

,,

let , ~

tq':

!

2

~"

.

,

9

|

,

.

t

o~ ,'~

4

',

".

! t

,

"

|

6 D (nm)

" ' '

|

8

10

Figure 1. Nitrogen adsorption-desorption isotherms at 77 K and BJH pore size distributions (desorption branch) of aluminas synthesised in 1,4-dioxane with surfactants Triton X-114 (a), Tergitol 15-S-9 (b) and Tergitol 15-S-15 (c), and aged at 25~ Samples calcined at 550~ prepared from sols obtained in 2-butanol [ 14], the most intense resonance is observed at ca. 5 ppm, which corresponds to hexacoordinated aluminum species (AlW). A second, less intense resonance is observed at ca. 70 ppm, attributed to tetracoordinated aluminum (AlW). Finally, the weak resonance at ca. 35 ppm indicates the presence of a small fraction of aluminum atoms in 5-fold coordination. In order to obtain more acidic aluminas, it would be necessary to increase the number of coordinatively unsaturated A1. For this reason, we have tested the effect of fluoride ions added to the synthesis gel as ammonium fluoride. Figure 3 shows the 27A1 MAS NMR spectrum of one selected sample synthesised in 2-butanol in the presence of ammonimn fluoride (spectrum d) and that of a sample prepared using the same synthesis conditions but without modifier (spectrum c), both calcined at 550~ It can be observed that the use of ammonium fluoride produces a significant increase in the intensity of the resonances assigned to tetra- and

50nm

30nm

Figure 2. TEM images of alumina samples synthesised in 1,4-dioxane, in the presence of Tergitol 15-S-15, using no modifiers (left) and using ethyl acetoacetate as modifier (right). The gels were aged at 95~ and the obtained sols, calcined at 550~

1287 Table 2 Pore structure of mesoporous aluminas prepared from sols obtained in 1,4-dioxane Calcined at 6000C

Calcined at 550~ Ageing temperature /oc

Surfactant (number of EO units)

25 25 25 95 95 95

X-114 15-S-9 15-S-15 X-114 15-S-9 15-S-15

BET surface area /m2.g-1

(8) (9) (15) (8) (9) (15)

Pore diameter /nm

550 500 445 490 525 510

3.5 4.8 5.2 3.5 4.0 4.5

Pore volume /cm3"g-1

0.44 0.75 0.78 0.51 0.71 0.71

BET surface area /m2.g-1

Pore diameter /nm

4.1 6.5 7.1 4.2 5.8 6.0

370 350 355 350 355 375

Pore volume /cm3-g-1

0.50 0.76 0.85 0.60 0.72 0.74

pentacoordinated aluminum, particularly the later one. The N2 adsorption-desorption isotherms of calcined aluminas synthesised in the presence of ammonium fluoride (Figure 4,a) are characteristic of mesoporous solids. These isotherms are similar to those of samples prepared using identical synthesis conditions but without ammonium fluoride. However, the hysteresis loop is shifted to higher relative pressure. The nitrogen adsorption data reported in Tables 1 and 3 indicate that the addition of ammonium fluoride to the synthesis gel leads to aluminas with higher pore size and lower BET surface area. The TEM images of aluminas synthesised in the presence of fluoride ions (not shown for brevity) reveal that this porosity is textural, as in the case of aluminas synthesised with no modifiers.

d

r/3 ~D

,

|

200

100

()

-100

Chemical shift (ppm)

-200

200

100

0

-100

-200

Chemical shift (ppm)

Figure 3.27A1 MAS NMR spectra of aluminas synthesised in 1,4-dioxane using Tergitol 15-S9 and aged at 95~ (left), and in 2-butanol using Triton X-114 and aged at 25~ (right). The modifiers added to the synthesis gel were: none (a,c), ethyl acetoacetate (b) or ammonium fluoride (d). All the samples were calcined at 550~

1288 400

b 100

300 [--, c,r

>

o,.......................................................... C

9

75

200 0

,,'~

~0 50

100

;> 25 f

0.0

012

0'.4

016

0'.8

1.0

0

0.0

012

P/P0

0'.4

016

0'.8

" ....

1.0

P/P0

Figure 4. Nitrogen adsorption-desorption isotherms of aluminas synthesised using modifiers: ammonium fluoride (a), ethyl acetoacetate (b,c) and triethanolamine (d). The surfactant used was Triton X-114 and the solvents, 2-butanol (a,b,d) and 1,4-dioxane (c). The sols were aged at 95~ (except sample a, aged at 25~ and calcined at 550~

3.3. Microporous aluminas A second strategy that we have explored is the chemical modification of the aluminum alkoxide precursor with chelating ligands, in order to retard the hydrolysis and condensation reaction rates [17]. As a result of such a modification, the number of coordinatively unsaturated aluminum atoms, and therefore the acidity of the alumina, is increased. For this purpose, we have carried out the hydrolysis of aluminum sec-butoxide modified with ethyl acetoacetate and triethanolamine. Figure 3 shows the 27A1 NMR spectrum of an alumina sample synthesised using ethyl acetoacetate as modifier and calcined at 550~ (spectrum b). A strong increase is observed in the intensity of the resonances assigned to tetra- and pentacoordinated aluminum as compared

Table 3 Pore structure of mesoporous aluminas prepared from sols obtained in 2-butanol, using ammonium fluoride as modifier. Calcined at 600~

Calcined at 550~ Ageing temperature /oC

Surfactant (number of EO units)

BET surface area

25 25 55

X-114 (8) 15-S-9 (9) 15-S-9 (9)

260 325 335

/m2.g -I

Pore diameter /nm

6.8 5.8 5.7

Pore volume Icm3.g -I

0.56 0.62 0.67

BET surface area

/m2.g -I

115 180 180

Pore diameter /nm

12.0 8.6 11.0

Pore volume /cm3"g-I

0.50 0.54 0.66

1289 Table 4 Pore structure of microporous aluminas prepared from aluminum sec-butoxide modified with chelating ligands, and calcined at 550~ Solvent

Modifier

2-butanol 2-butanol 2-butanol 2-butanol 1,4-dioxane 1,4-dioxane 1,4-dioxane

triethanolamine ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate ethyl acetoacetate

Ageing Surfactant temperat (number of ure EO units) /oC 25 95 95 95 95 95 95

X-114 (8) X-114 (8) 15-S-9 (9) 15-S- 15 (15) X-114 (8) 15-S-9 (9) 15-S- 15 (15)

BET surface area /m2.g -1 290 325 305 310 290 360 400

Micropore Total pore volume volume /cm3.g -1 /cm3"g-I

0.10 0.09 0.10 0.11 0.09 0.12 0.12

0.13 0.15 0.14 0.14 0.13 0.16 0.19

to a sample obtained from the non-modified alkoxide precursor (spectrum a). The actual modification of the aluminum alkoxide by ethyl acetoacetate and the preservation of the bidentate complex during hydrolysis and condensation of the chelated aluminate precursor were confirmed by infrared spectroscopy. The spectrum of the gel showed the characteristic carbonyl stretching bands at ca. 1636 and 1613 cm -I indicative of the bidentate complex [18], which is preserved until calcination at temperatures above 300~ The TEM image of a sample synthesised from ethyl acetoacetate-modified aluminum secbutoxide and calcined at 550~ (Figure 2, right) shows the presence of irregularly shaped pore channels, in marked contrast to samples synthesised with the non-modified alkoxlde. Therefore, besides the effect on aluminum coordination, the chemical modification of the precursor with ethyl acetoacetate also produces a dramatic change in the porous structure of the a!umina. Thic suggests teat the stabi!ity of the chelate with regard to hydrolysis and condensation reactions prevents the formation of the octahedral hydroxyaquo AI(III) complexes that account for the tendency to form the pseudoboehmite platelets in the absence of modifiers. The nitrogen adsorption-desorption isotherms of aluminas synthesised with ethyl acetoacetate- or triethanolamine-modified aluminum sec-butoxide and calcined at 550~ (Figure 4, right) show that these samples are microporous. The BET surface area, the micropore volume and the total pore volume of aluminas synthesised using different solvents and surfactants are reported in Table 4. The micropore volume has been calculated as the intercept of the straight line that fits the low relative pressure region of the adsorption isotherm in a Dubinin-Radushkevitch plot [19], and the density of tile adsorbed nitrogen has been taken as 0.81 cm3.g-~. As shown in Table 4, after calcination at 550~ aluminas with BET surface area in the range 300 to 400 m2.g -1, and micropore volume around 0.1 cm3.g -I are obtained. The effect of surfactant on porosity is much more pronounced for samples synthesised in 1,4-dioxane respect to those obtained in 2-butanol.

1290 4. CONCLUSION In the absence of modifiers, organised mesoporous aluminas are obtained by hydrolysis of aluminum tri-sec-butoxide, in the presence of polyethylene oxide surfactants, either in 2butanol or 1,4-dioxane solution. In 1,4-dioxane, the alumina pore size can be tuned by modifying the head volume of the surfactant, in contrast to 2-butanol. It is proposed that this effect be related to the surfactant aggregation into micelles in the former solvent. The addition of fluoride does not modify the mesoporous structure of the alumina but causes a decrease in surface area and an increase in the amount of tetra- and pentacoordinated aluminum. Microporous aluminas with high concentration of tetra- and pentacoordinated aluminum are obtained when the aluminum alkoxide precursor is modified by chelating agents such as ethyl acetoacetate or triethanolamine.

Acknowledgements The authors thank the CICYT (Spain) project MAT2000-1167-C02-02 for financial support, Dr. I. Diaz for collecting the TEM images, and Dr. T. Blasco for collecting the NMR spectra. V.G.P. acknowledges the Conacyt (M6xico) for a Ph.D. grant.

REFERENCES 1. C. Misra, Industrial Alumina Chemicals, ACS monograph 184, ACS, Washington, 1986 2. Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E, Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schfith and G.L. Stucky, Chem. Mater. 6 (1994) 1176 3. M. Yada, M. Machida and T. Kijima, Chem. Commun. (1996) 769 4. M. Yada, H. Hiyoshi, K. Ohe, M. Machida and T. Kijima, Inorg. Chem. 36 (1997) 5565 5. M. Yada, H. Kitamura, M. Machida and T. Kijima, Langmuir 13 (1997) 5252 6. A. Stein and B. Holland, J. Porous Mater. 3 (1996) 83 7. N.R.E. Radwan, A.A. Atia and A.M. Youssef, Ads. Sci. Technol. 17 (1999) 17 8. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltrfin, D. Beltrfin, S. Mendioroz, M.D. Marcos and P. Amords, Adv. Mater. 11 (1999) 379 9. S. Valange, J.-L. Guth, F. Kolenda, S. Lacombe and Z. Gabelica, Micropor. Mesopor. Mater. 35-36 (2000) 597 10. F. Vaudry, S. Khodabandeh and M.E. Davis, Chem. Mater. 8 (1996) 1451 11. S.A. Bagshaw, T.J. Pinnavaia, Angew. Chem. Int. Ed. Engl. 35 (1996) 1102 12. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature 396 (1998) 152 13. W. Zhang and T.J. Pinnavaia, Chem. Commun. (1998) 1185 14. V. Gonz/dez-Pefia, I. Diaz, C. Mfirquez-Alvarez, E. Sastre and J. P6rez-Pariente, Micropor. Mesopor. Mater. 44-45 (2001) 203 15. V. Gonzfilez-Pefia, C. Mfirquez-Alvarez, E. Sastre and J. P6rez-Pariente, Stud. Surf. Sci. Catal. 135 (2001) 1072 16. A. Ray, Nature, 231 (1971) 313 17. C.J. Brinker, G.W. Scherer, Sol-gel science, Academic Press, San Diego, 1990 18. F. Babonneau, L. Coury and J. Livage, J. Non-Cryst. Sol., 121 (1990) 153 19. S.J. Gregg, K.S.W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982


1291

Characterization of a heteropolyacid supported on mesoporous silica and its application in the aromatization of ~-pinene H. Jaramillo a, L. A. Palacio b and L. Sierra b aDepartment of Chemistry. bGroup of Material Science University of Antioquia, A. A. 1226 - Medellin, Colombia

H3PW12040.xH20 was loaded (33 wt%) by impregnation into a calcined mesoporous silica. The Keggin structure was characterized by XRD, TGA, N2 adsorption and 31p MAS NMR. The bi-functionality (acid/redox) of this catalyst was then studied using the transformation of tx-pinene between 40 and 160 ~ in a batch reactor. The substrate is transformed into camphene and terpinene by isomerization on the acid sites. The latter is transformed in a second step at higher temperature into p-cymene by a dehydrogenation on the acid and redox sites. 3-p-menthene and carvomenthene are produced, in low yield, together with p-cymene, by a disproportionation reaction of the terpinenes. 1. INTRODUCTION Catalytic properties of the heteropolyacids (HPAs) have attracted the attention in the two precedent decades due to the versatility of these compounds, since they possess acid and redox properties [1,2,3]. Mineral acids as catalysts cause a reduced life of the reactors and equipment used in many important processes. The HPAs could replace them giving good catalytic efficiency with less difficulties in their handling. To carry out heterogeneous catalysis in liquid phase, HPAs are usually supported on a carrier such as activated carbon, titania, alumina or silica [4,5]. The present work studies the characterization of the 12-Tungnstophosphoric acid: H3PW12040 (HPW) supported on a mesoporous silica and its use in the heterogeneous catalytic conversion of tx-pinene. A reasonable amount of research work has been done on apinene reactions for their applications in fine chemistry [5,6,7,8]. Among such reactions, the isomerization of ct-pinene by heterogeneous acid catalysis produces mono-, bi- and tricyclic terpenes. With catalysts offering also redox sites, the isomerization of tx-pinene can be followed by dehydrogenation to give aromatic compounds such as cymenes. 2. EXPERIMENTAL SECTION The Keggin form of HPW was prepared following the method of Grutther [9] and Misono [10], with a modification of the extraction and purification. 75 g of Na2WO4.2H20 and 37.5 g of Na2HPO4.12H20 were dissolved in 100 ml of hot water; then 90 ml of concentrated HC1 were added. The extraction was made with 120 ml of ether. In order to favor the formation of the Keggin anion, the part with the ether was subjected to acid hydrolysis by means of the addition of concentrated HC1 (20 ml). 40 ml of H20 and more ether were added to the acidic ether solution to form three layers from which the most dense layer containing the HPA species in ether was evaporated by air bubbling and the recovered solid was dried at 40~

1292 The mesoporous silica was prepared by using a sodium silicate solution and the neutral surfactant Triton X-100 as template, with a method developed in our laboratory in which the mechanism S~ ~ is partly replaced by the mechanism S~ [ 11 ]. After calcination to remove the surfactant, the mesoporous silica was impregnated with an aqueous solution of HPW in order to obtain 33% of load. Before using, the catalyst was activated at 130~ under vacuum. The as-synthesized mesoporous silica, and the supported HPW were characterized by XRD with a Phillips PW 1130 equipment, by nitrogen adsorption in a Micromeritics ASAP2010, by TGA in a TA Instruments Hi-Res TGA 2950, by FTIR in a Mattson 5000 FT-IR Spectrometer and by 31p MAS NMR in a Bruker MSL 300 instrument. A preliminary evaluation of the acidity of the supported HPW was carried out based upon the catalytic dehydration of 2-propanol in a continuous reactor at 100~ with a space velocity of 248 cm3/g.min (catalyst weight of 125 mg). For the catalytic transformation of ot-pinene a static batch reactor was used, in which the temperature was set between 40 and 160~ The reaction was followed by gas chromatography (Perkin Elmer Gas Chromatograph equipped with a capillar Carbowax 20 M column and Q-Mass 910 Mass Spectrometer).

3. RESULTS AND DISCUSSION 3. 1. Characterization 3.1.1. Characterization of the mesoporous silica The XRD pattem for this material shows a peak with a d spacing value o f - 3 . 8 nm corresponding to the d~00 reflexion for the mesoporous material. The N2 adsorption results afford a BET area of 783 m2/g, a pore volume of 0.76 cm3/g and an average pore diameter of 2.0 nm (BJH method), which are characteristic of mesoporous phases.

3.1.2. Characterization of HPW: as-synthesized and supported on the mesoporous silica. XRD analysis The XRD pattem for the as-synthesized HPW compound shown in figure 1, corresponds to that assigned to the Keggin species H3PW12Oa0.xH20 by Chen et al [12]. Since the crystallographic array varies with the number of hydration water molecules, it is possible from the information of the unit cell and space group to determine this number. By using the software Checkcell [ 13], it was determined that the HPW XRD pattem corresponds to a cubic unit cell with ao = 1.2113(3) nm and to the space group Pn3m, from which it can be concluded that HPW crystallizes with 6 water molecules [ 14]. r

0.7

~-

0.3

t

~=~

~"

"~"

0

;

I

(.q

~

1 l

r,~

I

i

o

oo

o, 0.0

10.0

20.0

30.0

40.0

2O

Figure 1. XRD pattern for the as-synthesized HPW (H3PWlzO4o.6H20)

1293 100

0.05

100

0.05

-TGA

. . . . DTGA 0.04

;r

0.03 ~

96.

o o~

94,

92

0.01

o

1~o

'

~;o Temperature,

~;o *C

,;o

'

0.00 500

0.04

98

0.03 .~

:~ 96

~ ~

0.02 94

92

0.01

0

.

. 100

. 200

.

300

Temperature,

*C

400

,

500

,

0.00 600

Figure 2. TGA thermographs. (a) as-synthesized HPW, (b) supported HPW In order to check if the crystalline structure of HPW still remains in the supported HPW, a XRD pattern was performed for HPW supported on the mesoporous silica (33 wt %). The lines of the crystalline HPW are not seen as it is the case when the HPW load on mesoporous silicas is lower than 50 wt % [15,16]. The strong peak observed at a d spacing value of 3.8 nm, corresponds to the dl00 reflexion of the mesoporous silica.

TGA analysis The TGA thermogram for the as-synthesized HPW (figure 2a) shows a first weight loss of 3.4 % until 140~ corresponding to non-coordinated water and a second weight loss of 3.1% between 140 and 320~ due to hydration water, which corresponds to 5.3 water molecules for each Keggin anion. This value is close to the one found by XRD. There is a third weight loss of 0.5% between 320 and 500 ~ which is produced by the condensation of hydroxyl groups formed by the bonding of terminal oxygen of the Keggin anion with the acidic protons. The TGA thermogram for the supported HPW (figure 2b) shows an important and steep weight loss between 25 and 140 ~ followed by a gradual loss between 140 and 500~ This means that the loss of hydration water in the supported HPW occurs at lower rate than for the as-synthesized HPW. This indicates that the removal of this kind of water is more difficult probably due to the inclusion of the hydrated HPW inside of the mesopores. This effect could stabilize the HPW on the support.

31p MAS NMR analysis

The 31p MAS NMR spectra for the as-synthesized HPW and for the supported HPW are shown in figure 3a and 3b respectively. The as-synthesized HPW exhibits a signal at -15.89 ppm, which can be assigned to the Keggin species with 6 hydration water molecules [8]. The signal at-13.47 ppm can correspond to lacunary species formed during the preparation of the HPW [11]. The resonance signals between-0.05 y - 5 . 1 6 ppm correspond to HxPO4(3"x)" species, where 0 < x

+

dipentene

_ 10 which show the typical characteristics as it is well known from systems mentioned above. 2. EXPERIMENTAL

2.1. Synthesis procedure a) Sodium form: Na-In-MCM-41 molecular sieves were synthesized starting from an aqueous solution of cetyltrimethylammoniumbromide (CTAB), Aerosil as silicon source and a suspension of sodium water glass in water as cationic agent. Then the indium source

1356 (In(NO3)3) was added and the gel was stirred for lh. Generally, the molar composition of the resulting gels were 0.004...0.06 In(NO3)3: SiO2:0.31 CTAB" 0.17 Na20:25.7 H20. The obtained starting gel was heated at 90~ for 96 h. The as-synthesized products were filtered and washed first with distilled water and in a second step with methanol to remove residual template molecules. After drying at 90~ for 6 h in air the solid product was calcined for 12 h at 540~ b) Hydrogenform: Pure silica MCM-41 was synthesized from aqueous silica solution in the system SiO2 (tetramethoxysilane as silicon source), ethylenediamine (EN), water, cetyltrimethylammoniumbromide as described by Oberhagemann et al. 7 Again In(NO3)3 as indium source was added to obtain the In-MCM-41. The gel was stirred for 1 h. Generally, the molar composition of the resulting gels were 0.008...0.2 In(NO3)3: SiO2:0.36 CTAB: 2.2 EN: 62 H~O. The obtained starting gel was heated at 95~ for 170 h. After the hydrothermal synthesis the same procedure was used as for the sodium form of the In-MCM-41. 2.2. Sample characterization

X-ray Diffraction. The XRD powder pattem were recorded using a Philips PW 1877 diffractometer with CuKa radiation, 0.05 ~ step size, and 2.5 s step time of each point.

Nitrogen adsorption. Nitrogen adsorption-desorption isotherms were measured at 77.4 K using a ASAP 2010 (Micromeritics) analyzer. The specific surface area and the pore size were determined following the BET procedure. Temperature Programmed Desorption. The TPD experiments were carried out in a home built flow apparatus with helium as carrier gas in a temperature range from 300 to 900 K. For evolved gas detection a mass spectrometer was used. 115In-MAS-NMR measurements. The solid state 115In-MAS-NMR spectra were performed at a resonance frequency of 65.768 MHz at a Bruker MSL 300 spectrometer, zirconia rotors 7 mm in diameter spun at rotation frequency of 4.5 kHz. A recycle delay of 100 ms, short pulses of 2.1 gs, and 100000 scans for each spectrum were applied. External In(NO3)3 (chemical shift of 0 ppm) was used as reference. 3. RESULTS AND DISCUSSIONS The In-MCM-41 molecular sieves were synthesized with Si/In ratios between 5...120. Both the as-synthesized and the calcined materials were investigated using XRD powder diffraction. The obtained XRD pattern of the In-MCM-41 (shown in Figure 1) with a Si/In ratio higher 10 posses the typical low-angle reflections that are characteristic of hexagonal ordered mesophases. At Si/In ratio of 10 the intensity of the reflections are decreased, indicating a transition from the MCM-41 structure to an amorphous material having a undefined structure. The collapse of the MCM-41 structure at lower Si/In ratios which is clearly demonstrated in Figure l a and l b can be attributed to the fact that the amount of indium molecules in the synthesis gel are to large to form the MCM-41 molecular sieves. Both the sodium and the hydrogen form of the molecular sieves show the same tendencies. Obviously, other processes are preferred in the hydrothermal synthesis if the amount of indium molecules increase in the gel leading to an amorphous product. The obtained products have a yellowish gleam which can be attributed to a mixture of indium oxide and indium silicate.

1357

t

Si/In a:

I

5

b: 10 c:20

]l

2

ratio:

I

4

I

I

6 8 2 Theta

I

10

12

Figure 1: Powder XRD pattern of calcined In-MCM-41 with different Si/In ratios prepared using ethylenediamine as cationic agent. The most reliable information about the mesopore structure of solids is obtained from low temperature nitrogen adsorption isotherms, which allow the calculation of specific surface area, pore volume, and mesopore size distribution. 12 The nitrogen adsorption-desorption isotherms are shown in Figure 2. Materials with a Si/In ratio higher than 10 possess the well known type IV isotherms. Considering the isotherm of the material with a Si/In ratio of 10 (compare Figure 2b) it is seen that the type IV character is weakly distinct in comparison with material of higher Si/In ratios. This should be caused by a transition from the MCM-41 structure to a more amorphous material as it can be observed by XRD measurements too. The isotherm of the sample with a Si/In ratio of 5 (Figure 2a) shows that irregular macropores are formed corresponding to a structural collapse of the material. From the adsorption data pore sizes between 38...45 A were calculated which are summarized in Table 1. Furthermore, the BET surface areas are estimated show relatively high values between 735...950 m2/g which are comparable with well known A1-MCM-41 material. 13 The material with Si/In ratio of 10 shows a BET surface area of 410 m2/g which is between the values of MCM-41 (735...950 mVg) and materials without MCM-41 structure (84 m2/g). As the relative pressure increase (p/po>0.3) the isotherms (Figure 2c, 2d) exhibit a sharp inflection characteristic of capillary condensation within the mesopores, and the P/Po position of the inflection point is related to

1358

Si/In ratio: a: 5 b: I0 c: 20 d: Si-MCM41

I

0,0

i

I

i

0,2

I

0,4

i

I

i

0,6

I

m

0,8

I

1,0

Figure 2: Nitrogen adsorption isotherms at 77.4 K of various In-MCM-41 samples (H-Form) the diameter of the mesopores. The sharpness in this step indicates uniform pores. The hysteresis loop at p/po>0.9 reflects that the larger pores are filled at high pressures. Contrary to this findings the isotherms in Figure 2b and 2a are quite different demonstrating that the amount of indium species strongly influence the formation of the structure. The values in Table 1 correspond with the results of the XRD and NMR measurements which indicates that at Si/In >_ 10 the MCM-41 is formed. With increasing amounts of indium in the synthesis gel an amorphous material is obtained indicating that indium is expelled from the structure. Table 1" Specific pore volumes, pore diameters, and BET surface areas estimated from nitrogen adsorption-desorption isotherms S i/In ratio (H-Form) 5* 10 20 volume 0.16 0.41 0.72

specific pore (cm3/g) pore diameter (in A) 78 BET surface area, (in m2/g) 84 *no MCM-41 structure is obtained.

40 414

39 735

52 0.80

Si/In ratio (Na-Form) 30 60 120 0.72 0.76 0.86

38 834

45 617

40 760

38 914

1359 The l15In MAS NMR spectra of the samples ( Figure 3a-c) show two broad signals which are assigned to four- (at about 780 ppm) and six-coordinated (at about -900 ppm) indium species. According of these results it can be established that more than 50% of the employed indium is incorporated into the framework. As it is expected the intensity increases with increasing indium content. Furthermore the spectral intensities of ll5In peaks in samples with Si/In > 10 are unaffected by calcination. It must be mentioned that the highly electric quadrupole interactions (indium: nucleus spin ofI - 9/2) strongly broadens the NMR signals in the solidstate powder spectra. Furthermore it is known 14 that satellite transitions in addition to the central transition take place which are become visible in the recorded spectra. This influences the quality and the signal noise ratio of the 115In MAS NMR spectra. Because of the molecule size of the indium in all spectra a high amount was detected as so called extra-framework species which is slightly dependent on the used Si/In ratio. The spectrum of the material with a Si/In ratio of 5 (Figure 3d) is quite different in comparison with spectra in Figure 3a-c. As it is clearly seen the intense lines are shiited to about 1100 ppm and-1000 ppm, respectively, as a result of a changed environment of the zlSIn nuclei. This is a further hint that no MCM-41 structure was obtained. The NMR results represent a further evidence that it is possible to synthesize In-MCM-41 with a high content of indium into the framework. Further information about the state of incorporation should be possible if the MQMAS (multiple quantum magic-angle spinning) NMR method is applied.

d

Jb F 2000

1500

1000

500

0

-500

-1000 -1500 -2000

ppm Figure 3: 115In-MAS-NMR spectra of ln-MCM-41 mesoporous molecular sieves with a Si/In ratio of 120 (a), 60 (b), 30 (c), and 5 (d). All spectra were recorded at 65.768 MHz with a rotation frequency of 4.5 kHz.

1360 It is well known, that incorporation of aluminum in siliceous materials like zeolites or mesoporous molecular sieves generates Bronsted and Lewis acid centers. Indium, as an element of the 3rd main group like aluminum, should be able to generate acidic centers too. However, one should be expected, that the strength of the In-generated centers is lower in comparison to aluminum. The temperature programmed desorption (TPD) of ammonia has been established as a powerful tool to investigate the acidic properties of solids. ~5 The aim of our investigations was to elucidate in which manner the introduction of indium influences the acidic properties of the materials. The obtained temperature programmed desorption files are shown in Figure 4. Considering the curves (Figure 4a-d) it is seen that all studied samples undergo interactions with the adsorbed ammonia. Except the In-free Si-MCM-41 material (Figure 4e), which did not show any desorption, all curves have a maximum at about 500 K. This maximum should be assigned to weak acid centers. The materials with typical MCM-41 structure (lower Incontent) have an additional shoulder at about 550 K. This could be an indication for the existence of different In-species in these materials.

i

Si a

/ In :

ratio"

5

b: 10 c: 20 d: 43 : "-

a

300

I

400

"

e

500

.

e

a

600

T

I

700

-41

I

I

800

a

I

900

/K

Figure 4: Temperature programmed desorption curves of different In-MCM-41 molecular sieves (a-d) in comparison with pure siliceous material (e).

1361

6,0x10 J

5,0x10 g~

N ~, Z

4,0x10 3,0xl 0 2,0x10

-9

M CM-structurc

-9

non MCM-structure

-9 I

0

I

10

a

I

20

In/% (Si+In=100%)

Figure 5: Comparison of the amount of desorbed ammonia in dependence of the amount of

indium in the MCM-41 material. The amount of desorbed ammonia correlates to the content of indium but not in a linear way. For a more detailed elucidation, we compared the amount of desorbed ammonia in relation to the amount of indium in the mesoporous molecular sieves. As it is shown in Figure 5 two different areas are visible representing the material with MCM-41 structure and non MCM-41 structure. Obviously, the material without MCM-41 structure possess a higher affinity against ammonia which can be described with a higher acidity. This clearly demonstrates that the acidic properties depends on the environment of the indium molecules. Generally, the results of the TPD studies demonstrate that the obtained materials can be applied in several procedures where acidic centers are involved in catalytic processes. 4. CONCLUSIONS A series of indiumsilicate MCM-41 materials prepared from gels with Si/In ratios between 5...120 have been synthesized using indium nitrate as source of indium. The products were characterized by different methods like XRD, NMR, N2 adsorption, and TPD measurements. The quality of the molecular sieves strongly depends on the amount of indium. The results of the XRD and the nitrogen adsorption-desorption isotherms confirm the MCM-41 structure. ~5In MAS NMR indicates that indium is tetrahedrally incorporated into the framework of the molecular sieves with amounts of more than 50% in the calcined samples. However, an increasing amount of indium leads to a collapse of the structure and forms macropores. Generally, it could be demonstrated that it is possible to synthesize In-MCM-41 materials in their sodium- and hydrogen form which have the typical properties of the M41S family and it is of potential interest as catalyst.

1362 A C K N O W L E D G M E N T : This work was supported by the DFG. Contract No: SFB 294/G8

and Contract No: PA194/4-4. REFERENCES

1

10 11 12 13 14 15

T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S.Beck, Nature, 359 (1992) 710. Z. Luan, C.-F. Cheng, W. Zhou and J. Klinowski, J. Phys. Chem., 99 (1995) 1018. C.-G. Wu and T. Bein, J. Chem. Soc., Chem. Commun., (1996) 925. W. B6hlmann and D. Michel, Stud. Surf. Sci. and Cat., 135 (2001) 202. C.-F. Cheng and J. Klinowski, J. Chem. Soc., Faraday Trans., 92 (1996) 289. A. Sayari, C. Danuman and I.L. Moudrakovski, Chem. Mater., 7 (1995) 813. U.Oberhagemann, M. Jeschke, and H. Papp, Microporous and Mesoporous Mat., 33 (1999) 165. D. Trong On, P.N. Joshi, and S. Kaliaguine, J. Phys. Chem. 100 (1996) 6743 C.-F. Cheng, H. He, W. Zhou, J. Klinowski, J.A.S. Goncalves, and L.F. Gladden, J. Phys. Chem., 100 (1996) 390 N.H. Heo, S.W.Jung, S.W. Park, M. Park, and W.T. Lim, J. Phys. Chem. B, 104 (2000) 8372 M. Chatterjee, D. Bhattcharya, H. Hayashi, T. Ebina, Y. Onodera, T. Nagase, S. Sivasanker, and T. Iwasaki, Microporous and Mesoporous Mater., 20 (1998) 87 P.T. Tanev and L.T. Vlaev, J. Colloid Interface Sci., 160 (1993) 110 P. Selvam, S.K. Bhatia, and C.G. Sonwane, Ind. Eng. Chem. Res. 40 (2001) 3237 D. Freude, R.A. Meyers (Ed.), Encyclopedia of Analytical Chemistry, John Wiley & Sons Ltd, Chichester, (2000), pp 12188-12224 B.M. Lok, B. K. Marcus, C. L. Angnell, Zeolites 6 (1986) 185


1363

The effect o f niobium source used in the synthesis on the properties o f N b M C M - 4 1 materials Izabela Nowak A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland; e-mail: nowakiza@amu, edu.pl Niobium-containing mesoporous molecular sieves (NbMCM-41) prepared by using different sources of niobia were characterized by means of N2 adsorption, XRD, TEM, SEM combined with microanalysis and IR spectroscopy in the skeletal region. The results showed that the nature of Nb-sources influences the crystallite diameter, volume, and wall thickness of the mesoporous material. Only NbMCM-41 prepared from Nb ammonium oxalate complex exhibits the uniform pore size distribution. 1. INTRODUCTION The relatively large interest of MCM-41-type mesoporous materials that has arisen since 1992 [ 1] lies in the fact that they can be tailored to form pores with narrow size distribution, from 2 to 10 nm. In silica MCM-41 material, silicate matrix surrounds hollow tubes which are arranged in a relatively regular two-dimensional hexagonal array. This opened up a field of fundamental and applied science, where narrow pore size distribution and high surface area could be required. The easy modification of MCM-41 by isomorphously substitution tetrahedral silicon atoms with other novel elements such as AI, Ti, V, B, Ga, etc makes MCM-41 a potentially useful catalyst. By structural ordering, MCM-41 has long-range order which is derived from a regular array of unisized cylindrical mesopores, and manifests itself in X-ray diffraction (XRD) at small angles (2-8 ~ 20), transmission electron microscopy (TEM), and adsorption analysis. On the other hand, scanning electron microscopy (SEM) images reveal the morphologies. The synthesis and properties of niobium and siliceous containing mesoporous sieves of MCM-41 type were first time described by our group [2-3] and simultaneously, Nb-doped mesoporous sieves were synthesized by Zhang and Ying [4]. Recently, it was found a very high activity of NbMCM-41 mesoporous molecular sieves in the oxidation of thioethers to sulfoxides with hydrogen peroxide [5,6] and in the direct conversion of cyclohexene into trans-2-alkoxycyclohexanol in the presence of H202 [7]. The first synthesis of NbMCM-41 mesoporous molecular sieves was carried out using niobium oxalate as a source of Nb [2]. The obtained materials were partially disordered especially when the content of niobium was high (Si/Nb=16). The aim of this study was to synthesize NbMCM-41 materials using the other sources of niobium, especially those available commercially, i.e., niobium chloride and ammonium-oxalate complex of niobium. The another goal was to compare their structure and properties with those found for NbMCM-41 prepared from niobium oxalate and for MCM-41 impregnated with niobium salts.

1364 2.

EXPERIMENTAL

2.1. Syntheses procedure In the previous report [2] it was shown that a mesoporous molecular sieve MCM-41 could be successfully synthesized with niobium oxalate as a source of niobium. The same procedure of the preparation of niobium-containing MCM-41 materials was applied in this study, i.e. niobium oxalate (Nb(HC204)5, CBMM, Brazil), niobium chloride (NbCIs, Aldrich) or ammonium-oxalate complex of niobium (NbO(C204)3(NH4)3, CBMM, Brazil) were added to an aqueous solution of the sodium silicate (27% SiO2 in 14% NaOH, Aldrich) mixed vigorously with surfactant (cetyltrimethylammonium chloride, 25 wt. % solution in water, Aldrich). In the case of niobium chloride, the niobium source was first dissolved in ethanol solution in a dry-box under dry nitrogen flow. The formed gel was stirred for about 0.5 h before 20 g of distilled water was added. The gel was loaded into a stoppered PP bottle and heated without stirring at 373 K for 48 h. The precipitated product was recovered by filtration, extensively washed with distilled water, and dried in air at ambient temperature. The product was finally calcined at 773 K for lh in helium flow and 6 h in air. NbMCM-41 obtained from different niobium sources, i.e. niobium oxalate, ammoniumoxalate complex of niobium or niobium chloride, will be designated in this paper as Nb(O)MCM-4 l-Y, Nb(Oc)MCM-4 l-Y, and Nb(CI)MCM-4 I-Y, where: O stands for oxalate, Oc - oxalate complex, CI - chloride, and Y - Si/Nb atomic ratio in the synthesis mixture, respectively. The chemical analyses showed that the obtained Si/Nb ratios for samples prepared with different sources of niobium were very close to the theoretical of 32 and are as follows: 36 for oxalate, 33 -chloride, and 32 for ammonium complex of niobium.

2.2. Characterization The physico-chemical properties were studied by means of X-ray diffraction (XRD), transmission electron microscopy (TEM), N2 adsorption-desorption analysis, scanning electron microscopy (SEM) and infrared spectroscopy (IR). Powder X-ray diffraction data were collected on a TUR-42 diffractometer using Cu Kc~ radiation (~=0.154 nm) with 0.02 ~ step size. For TEM measurements, all calcined materials were crushed in an agate mortar, dispersed in ethanol, and deposited on a microgrid. The transmission electron micrographs were taken on a JEOL-2000 operated at 80 keV. Scanning electron microscopy was performed on a JEOL JSM-5400 using an accelerating voltage of 20 keV with microanalysis. The N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus. The sample was outgassed at 623 K under vacuum prior to the adsorption. The data were analyzed by the BJH (Barret-Joyner-Halenda) method and the surface area and the pore size distribution curve were obtained from the analysis of desorption portion of the isotherm. However, the calculations from adsorption branch of isotherm were performed as well in the case of some pore blocking (not presented in this paper). Infrared (IR) spectra were recorded with a Vector 22 (BRUKER) FTIR spectrometer. The samples were measured by diluting them to 1 wt. % in KBr. 3. RESULTS AND DISCUSSION The Nb-containing mesoporous material prepared with various niobium sources showed the distinct textural and structural characteristics. The calcined NbMCM-41 materials with Si/Nb ratio of 16 (Fig. 1A) gave well-defined hexagonal XRD patterns with a main peak (indexed as [ 100] assuming a hexagonal unit cell) at 20 ~ 2 ~ and up to 3 signals in the region

1365 20=3-8 ~ These reflections are due to the ordered hexagonal array of parallel niobosilica tubes [5]. The materials with less niobium content have got a very sharp signal indexed as [ 100] and not well resolved signals in the higher angle region (Fig. 1B). It is well known that by the means of X-ray diffraction it is not possible to quantify the purity of the material. Corma [8] attributed the apparently "less-resolved" XRD pattern to the formation of smaller although no less ordered MCM-41 crystallites. MCM-41 is considered as crystalline on a macroscopic level because of the regular arrangement of the mesopores in honeycomb fashion; therefore, it is possible to estimate crystallite size perpendicular to the basal plane with the help of the Xray diffraction pattern using the Scherrer equation: d=0,9 9L/(B 9cos0), where B is the peak width at half-maximum, E - wavelength, 0 - Bragg angle [9]. The data in Table 1 suggest that, with higher silica to niobia ratio the smaller A B crystallites were obtained. To elucidate the pore structure of MCM-41 transmission electron 30 microscopy is usually r used. Fig. 2 shows a TEM image of the hexagonal "~: 20 arrangement of uniform, jb -~4 nm sized pores in the samples of Nb(O)MCMlO 41-32 and Nb(C1)MCM41-32. However, the unambiguous analysis of o 4 6 8 2 4 8 lo the pore size and 2| o 2e, o thickness of the pore walls is very difficult and Figure 1. Powder X-ray diffraction patterns of the NbMCM-41 possible without materials prepared with Si/Nb ratio of 16 (A) or 32 (B) by using not additional simulations niobium oxalate (a), ammonium-oxalate complex of niobium because of the focus (b), and niobium chloride (c).

,o

rA.

/,

c

!

!

Table 1. Estimates of d spacing, pore, crystallite, and particle size for various niobium-containing MCM-41 samples.

d spacing, nm

Average pore diameter, nma

Crystallite diameter, lam b

Particle diameter, lamc

Nb(O)MCM-41-16

3.68

3.90

0.035

2.0

Nb(Oc)MCM-41-16

3.74

3.24

0.028

-

Nb(O)MCM-41-32

3.81

4.56

0.025

2.0

Nb(Oc)MCM-41-32

3.56

4.62

0.018

6.0

Nb(CI)MCM-41-32

3.51

3.30

0.019

2.0

Sample

The data was estimated: a _ by using gas adsorption (BJH -des. ), b_ from line broadening of X-ray diffraction, and c_ from optical microscopy.

1366

.::.:." 9 .

~!.. . . . .

..::!::...~:: .

..:..i:: 9

. . . . . . .

~ ~ ' . ' - ' ~ . ~ . - ~ ~ : - ~ " - . r ~ : : 8 ~ . : : '

..

100

i ~ -.

0 0.0

0.2

0.4 0.6 relative pressure (P/Po)

0.8

Figure 1. N 2 adsorption-desorption isotherms; {Eu(thd)~} n~

1.0 as in Table 1.

Table 1 S t a r t i n g compositions" X ~.,(~d)3(mg) for 100mg of d e h y d r a t e d M C M - 4 1 , chemical analyses results,and t e x t u r a l characteristics" specific area (A), pore volume (V) and pore d i a m e t e r (D) of {Eu(thd)x}M samples ~.

Samples a n~ Process 1 2 3 4 5

wet wet wet wet Vapour

X 25 50 75 100 100

A n a l y s e s (% w e i g h t ) Si Eu C H O 34.5 32.1 30.5 29.4 24.5

4.5 8.2 11.1 13.3 11.0

2.5 0.5 2.8 0.7 4.0 0.8 5.7 1.2 15.7 2.5

58 56.2 53.6 50.1 46.3

N 2adsorption results A (m2/g) V (ml/g) D (rim) 774 708 646 625 338

0.55 0.48 0.44 0.38 0.13

3.6 3.45 3.3 3.2 2.3

The N 2 adsorption/desorption isotherms for {Eu(thd)x}M are displayed in Figure 1 and the t e x t u r a l characteristics are g a t h e r e d in Table 1 The mesoporous s t r u c t u r e is preserved after the inclusion process. However, the specific surface, pore volume and pore size are reduced with respect to the pure MCM-41 ( A~ T = 952 m2/g, pore volume = 0.74ml/g, pore diameter = 3.65nm), and decreased as a function of the complex loadings. The sample prepared according to the vapour process shows the highest pore reduction, in agreement with the largest a m o u n t of organic p a r t found by chemical analyses.

1374

3.2. Quantitative analyses by Energy Loss Spectroscopy (EELS) Results from EELS and chemical analyses are compared in Table 2. Quantitatively, the localized and bulk determinations are coherent. At the spatial resolution of the probe (150 nm), the samples are monophasic and the europium ions are homogeneously dispersed inside the silica matrix. Table 2. Atomic ratios [Eu]/[Si] measured by EELS* versus bulk analyses. ......

s

.pie

.....

........

2

3 .

.

.

.

4 ........

' [Eu]/[Si] EELS 3.3_+0.3% 5.7_+0.5% "7.4_+0.7% 8.1_+0.8% [Eu]/[Si] bulk 2.40_+0.06% 4.72_+0.09% 6.73_+0.10% 8.33_+0.10% 8.30_+0 10% * [Eu]/[Si] ratios calculated as ([Eu]/[O])*([O]/[Si]). [Eu]/[O] determined by comparison with experimental ratio (Eu M-edge / O K-edge) in the standard Eu203. [Si]/[O] by calculated Si and O's K-edge cross sections. .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

3.3. X-ray diffraction. Pure and loaded MCM-41 exhibit, in the low-diffraction angles range, diagrams characteristic of the hexagonal a r r a n g e m e n t of the channels (Figure 2). It has been established t h a t the peak intensities are all the more weakened with respect to pure MCM-41 silica as the electronic density from inserted m a t t e r increases in the tunnels (for instance in Ref [4]). This is the general trend observed here if one considers the Eu(thd) 3 / MCM-41 ratio in the initial mixtures (Table 1). The difference observed between samples n~ (wet) and n~ (vapour) could be linked to the larger amount of ligand measured after reaction for the second one. It must be noticed t h a t none of the diffractograms recorded at larger diffraction angles shows peaks indicative of the free europium complex. '

C"

.

'

'

I

~.a~,~" '

'

/ joure

'

I ......

'

'

'

I

'

'

"

3 4 L _

co rC ,

2

3

4

2 theta Figure 2. XRDiffractograms; {Eu(thd) X}n~

as in Table 1.

,

5

1375

2.3.

IR s p e c t r o s c o p y

pure M C M - 4 1

,m

pure4~ CM- 1

c.>,

.Q

3000

2900

2800

fl) o c .Q 0 .Q

3800

3700

3600 wavenumbercm

Figure 3.IR spectra ; {Eu(thd) x} n~

3500

3400

-1

as in Table 1.

The IR spectra, were normalized to the intensity of MCM-41 lattice vibrations . On Figure 3 are displayed the two more between 2100 and 1750 c m -1 characteristic w a v e n u m b e r s ranges. The sharp absorption at 3741 cm 1 appearing after removal of the adsorbed water in MCM-41 has been conclusively assigned to vo~ in isolated silanol groups on the walls of the tunnels ~ . g ;. ref. [5]). After impregnation of activated MCM41, the intensity of this-vibration diminishes with increasing content of the loaded complex, according to the elemental analyses. In sample n~ the isolated silanol signal completely disappears and it appears a band at 3705 cm -1 t h a t could be due to c o m p l e x - hydrogen bonded silanols. Intensities of vibrations characteristic of the ligands (vcm,c~) in the 3000 - 2800 cm 1 range increase simultaneously from sample n ~ to n~ These observations coherently evidence t h a t the metalorganic species are grafted on the mesoporous walls via the silanol groups.

3.4.

Visible fluorescence The Eu 3§ luminescence displayed on Figure 4 and 5 were recorded at room t e m p e r a t u r e and pressure. The narrow emission lines 5Dj-~7Fj, are characteristic of the europium i m m e d i a t e environment ; this is illustrated for instance by the differences observed between Eu(thd) 3 and Eu(dbm)~ emission spectra in the 550680 n m range (~Dj--~ 7F~3) : The excitation spectra exhibit simultaneously narrow intra-4ff lines and broader features due to the ligand part.

9s p u e q uo!~daosqe pue~.~[ oq~ u! 'uiu OL8 ao O~;g " sA~oaa~ ~;q u ~ o q s sq~uoIOAeA~ SUO!~m!OXO 'mu099-0~g 9uo~s.rm~t "tuu 119 ~uvo~!uotu 'uIu00g-0~ ~ 9uo.t~e~t.oxx "lA~{(uzqp)n~i} pug ~(tuqp)nx jo oouoosout.uzn~I "9 o2n~.t~i

OOZ

.

.

.

I

.

i

1 I

009 I

I

s I !

i

I

(tuu) q],BUOlOABAA OOg O0"lr '

I

'

"

'

'

I

'

'

'

'

00~ I

'

'

~v

9

!

-.

I

l~{(wqp)n3}

I

.........

~

,~.

,

i I I I

C

g .

..

,

I

..,~,

..

.

.

I

.

.

',

'

I,

.,l'

.."

.'

. .

(9~I~- ~'~ ~ uIU 068 zo ( ~

.

r~

I

.

.,,,

.

~

"

s u ~ o q s sq~uoIoAU~ uo.t~u].roxo 'tlIU099-0~g 9uo.tss.tm~t "uzu I I 9 ~u!Jo~!uouI 'uzu00g-0g~" uo.t]u~.tox~t "lAI{X(pq~)n~t} puu ~(pq])n~[ jo oouoosou.tmn~I "~ ozru~.~i

OOZ w

(WU) Li::I6UOIOAI~M

009

I

'

'

.....

I

.

009

'

'

'

'

I

,-, ; . . . . . . . . I

I

"-.

OOg

00"17.

'

'

-'",;

', :'.; I I I I

I I I I

'

'

I

'

'

,, I , , " , :',,

t/

I

'

"''

I

t

I I

'

'

I

'

'

"

',

~1.-"

I

" =

g.

Ii

_~.

g

g( ,Pq::l)n3 -

~i |,

I

.

.,

9

9L~!

1377 The characteristic features of the corresponding Eu(L) 3 precursor are not recovered in the emission spectra of the {Eu(L)~}M hybrids, which unambiguously proves the modification of the Eu surroundings. In the luminescence excitation spectra, the intra-4ff excitations 7Fo.1 --> ~D2 (468 nm) 7Fo.1 --~L 6 (390 nm) appear with different relative intensities. F e a t u r e s relative to the ligand, which give evidence for the , , a n t e n n a effect- also exhibit drastic variations before and after their immobilization on the silica substrate. F u r t h e r investigations are needed to explain accurately these observations. As they stand, the luminescence excitation data corroborate the emission part and prove the strong interaction between the mesoporous substrate and the metal complexes. This interaction affects both the Eu 3§ surroundings and the ligand to metal energy transfers.

4.

DISCUSSION

These investigations of two mixed systems denoted {Eu(L)x}M obtained by reacting europium ~-diketonates [Eu(L) 3] with the hexagonally structured, mesoporous silica MCM-41 (M) must be discussed under several aspects. Two ligands were considered: (2,2,6,6-tetramethyl-3,5-heptanedionate) = (thd), and (1,3-diphenyl-l,3-propanedionate) = (dbm). For {Eu(thd)x}M samples, two reaction routes were compared: the classical "wet" process (the impregnation of the matrix by a solution of the precursor), and the "vapour" process (the reaction of the m a t r i x with the sublimated complex). The two synthesis routes lead to the homogeneous immobilisation of Eu(L) x species within the channels of the mesoporous silica, as concluded from X-ray diffraction and localized analysis (EELS) results. Nitrogen adsorption / desorption experiments prove that the mesoporous structure is preserved after the inclusion process in b o ~ routes. A comparison of the wet and the vapour routes may be done by considering samples n ~ 4 and n~ (Table 1): a higher ligand to europium ratio, stronger reductions of the pore volume, of the hexagonal X-ray diffraction and of the (yon isolated silanols) vibration intensities are observed for the sample elaborated via the vapour process. The m a x i m u m insertion rate, expressed by the [Eu]/[Si] atomic ratio, is 8.5% for both synthesis routes, t h a t is about 2/3 of the n u m b e r of silanols per silicium measured on similar substrates (13.2 % in Ref. [6]). This fact, and the observed diminution of the voH vibration intensity simultaneously with the increase of Eu loading strongly suggest t h a t the immobilisation takes place via Si-O-Eu grafting. The characteristic Eu 3§ emissions of the metal-organic precursors are lost after grafting and the emission spectra are very similar for all the hybrid materials investigated, whatever the synthesis route and even the nature of the ligand (thd or dbm). Ligand to metal energy transfer still occurs after grafting on the silica surface, but t h e observed strong modifications of the Eu 3+ luminescence excitation spectra are not still understood. Similar observations

1378 have been reported in Ref. [7] when the complex Eu(dbm)3phen (phen = 1,10phenanthroline) is intercalated in unmodified- or modified- MCM 41s. 5. C O N C L U S I O N The inclusion of europium ~- diketonates in mesoporous MCM-41 silica has thus been realized by impregnation as well as by vapour reaction. The different characterization methods employed coherently suggest that the immobilisation occurs via the grafting of the Eu 3+ on the free silanol groups of the surface. The luminescence characteristics of the europium 13- diketonates precursors are strongly modified by the reaction, but still exhibit ligand to metal energy transfer (the so-called ,,antenna effect~). It is worthy of note that the conclusions we have reached are valid for samples having been submitted to severe post-synthesis outgassing. Further investigations are now in progress to study the different reaction steps in the synthesis routes.

REFERENCES 1. J. Dexpert-Ghys, C. Picard and A. Taurines. J. of Inclusion Phenomena and Macrocyclic Chemistry, 39:(2001) 261. 2. D. Desplantier-Giscard, O. Collart, A. Galarneau, P. Van der Voort, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal., 129 (2001) 665 3. K.J. Eisentraut and R.E. Sievers, J. Am. Chem. Soc., 87 (1965) 5254. 4. B. Marler, U. Oberhagemann, S. Vortmann, H.Gies Microporous Mater., 6 (1996) 375. 5. J. Chen, Q. Li, R. Xu and F. Xiao, Angew. Chem. Int.Ed. Engl. 34 (1995) 2694. 6. G. Gerstberger, C. Palm and R. Anwander, Chem. Eur. J. 5, 3 (1999) 997. 7. L. Fu, Q. Xu, H. Zhang, L. Li, Q. Meng and R. Xu, Mater. Sci. and Engineering B28 (2002) 68.


1379

Synthesis, characterization and catalytic properties of mesoporous titanostanno silicate, Ti-Sn-MCM-41 Nawal Kishor Mal,a'* Prashant Kumar,b Masahiro Fujiwaraa and Koji Kuraoka a aAIST

Kansai, 1-8-31 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, JAPAN

bCeramic Section, Industrial Research Institute of Ishikawa, Kanazawa 920-0223, JAPAN

Mesoporous titanostanno silicate, Ti-Sn-MCM-41 was prepared by direct hydrothermal synthesis. Samples were characterized using XRD, SEM, UV-visible, FT-IR, Sorption techniques (N2 and HzO), TG-DTA and 29Si MAS NMR. Sn-containing samples are more hydrophilic in nature and contain more hydroxyl groups in comparison to Si-MCM-41. The selectivity of cyclohexene to cyclohexene oxide in the presence of tert-butyl hydroperoxide over Sn-Ti-MCM-41 is higher than Ti-MCM-41.

1. I N T R O D U C T I O N

Microporous titanostanno silicate (MFI structure) is highly active and selective propylene epoxidation catalyst [ 1]. However, channel of the MFI structure less than 0.6 nm restricts the diffusion of relatively larger molecules such as cyclohexene. Following the discovery of M141S silica mesoporous molecular sieves by Beck et al. [2] in 1992, COrma et al. reported the synthesis of mesoporous organic (-ell3) containing Ti-MCM-41 [3]. Organic (-ell3) containing Ti-MCM-41 is highly active and selective for the epoxidation of cyclohexene [3]. However, regeneration of this catalyst at high temperature may be a serious problem due to presence of methyl group attached to framework silica. In this report, for the first time, we describe the synthesis of mesoporous Ti-Sn-MCM-41, their characterization by using X R , SEM, UV-visible, FT-IR, TG-DTA, sorption techniques (N2 and H20) and 29Si MAS NMR, and their catalytic activity in the epoxidation of cyclohexene in presence of tert-butyl hydroperoxide (TBHP) as oxidant. +NNM is grateful to AIST, Tokyo for STA fellowship.

1380 2. MATERIALS AND METHODS 2.1. Materials and synthesis

The reactant used in this study were silicon ethoxide (99.9%, Aldrich Chem.) (TEOS), cetyltrimethylammonium bromide (96%, Kanto Chem.) (CTMABr), tetramethylammonium hydroxide (25% aqueous, Aldrich Chem.) (TMAOH), titanium ethoxide (Aldrich Chem.) (TEOT), tin(IV) chloride (99%, Wako Chem.) (SnC14), 2-propanol (2-PrOH, Wako Chem.) and H2SO4 (96%, Wako Chem.). Ti-Sn-MCM-41 samples were synthesized in absence of sodium using following molar composition: 1.0 SiO2 : 0.48 CTMABr : 0.67 TMAOH : (0.0 0.033) TiO2 : (0.0- 0.010) SnO2 : (1.0-4.0) 2-PrOH : 0.20 H2504 : 80 H20 with pH ofthe gel was varied between 10.26 to 11.46. In a typical preparation, 24.43 g of TMAOH and 18.22 g of CTMABr were dissolved in 100 g of water by stirring at 308 K to give a clear solution. 20.85 g of TEOS was added and stirred for 30 min. 0.38 g of TEOT (Si/Ti - 60) in 10 g of 2-PrOH was then added followed by 0.26 g of SnC14 (Si/Sn = 100) in 10 g of 2-propanol under stirring for 3 h. Finally, 2.0 g of H2SO4 in 126 g of H20 was added and stirred for 2 h. The resultant gel (pH = 10.31) was aged at room temperature for 1 day then heated statically at 373 K for 4 days under autogenous pressure. The products were filtered, washed, dried at 378 K and calcined at 823 K for 6 h to yield Ti-Sn-MCM-41. Ti-MCM-41 was prepared using the same procedure without addition of SnCI4, where as Si-MCM-41 was prepared without addition of titanium and tin sources. 2.2. Characterization

Elemental analyses of the samples were carried out using ICP (Shimadzu ICPV-1017). Characterization of the samples was carried out using XRD (Shimadzu XRD-6000), N2 sorption at 77 K (Bellsorp 28 instrument), H:O sorption at 298 K and at fixed p/po ratio of 0.5 in electrobalance (Chan, USA), FT-IR (JASCO FT/IR-230, UV-visible (JASCO V-560), and thermogravimetric (TG) and differential thermal analysis (DTA) with heating rate of 10 K min1 (Seiko, SSC/5200). 29Si M_AS NM~ spectra were obtained on a Varion VXP-400. 2.3. Catalytic oxidation reactions

Liquid phase epoxidation reaction of cylcohexene was carried out batch wise in two-necked round bottom flask fitted with a condenser and placed in oil bath at 333 K for 5 h under the reaction conditions; 0.10 g catalyst, 12.2 mmol cyclohexene, 5 ml acetonitrile (solvent), 4.05 mmol TBHP (70% aqueous). The reaction products were analyzed in a capillary GC (HP 5880) using 50 m long silicon gum column and identified by known standards and GC-MS.

1381

2.4. Methods The BET surface area [4] was calculated in the relative pressure range between 0.04 and 0.2. The average pore diameter (APD) was calculated using adsorption branch of isotherms. Total pore volume was determined from the amount adsorbed at relative pressure of 0.99 [4]. The pore size distributions were calculated from adsorption branches of the nitrogen adsorption isotherms using Barrett-Joyner-Halenda (BJH) method [5].

3. RESULTS AND DISCUSSION 3.1. Synthesis, structure and sorption properties Physico-chemical characterization of various titanium tin containing MCM-41 samples are shown in Table 1. All the samples were prepared in absence of sodium because in the presence of sodium formation of SnO2 takes place (confirmed by UV-visible). The Si/Ti molar ratio in product is higher than in synthesis gel for all samples. Where as Si/Sn ratio in product is lower than in synthesis gel except for sample 3 (Table 1). In Fig. 1, XRD profiles of samples 1, 2, 3 and 6 are shown. Four peaks in the XRD patterns of all the samples are observed, which are characteristics of long range ordering of a typical MCM-41 material. Ti-MCM-41 and Ti-Sn-MCM-41 samples have higher interplanar, dl00 spacing than Table 1. Physico-chemical Characterization of Ti-Sn-MCM-41 a Mole ratio in gel Sample

pH

Si/Ti

1

11.46

2

11.15

60

3

10.26

30

.

Si/Sn .

in product

dloo/

ao/

aBET/ Vp/

APD

HeO b

Si/Ti

nm

nm

m2g-1 cm3g"1

/nm

/wt%

3.56

4.11

1 0 8 0 0.93

3.54

16.2

Si/Sn

.

.

oo

68

-

3.68

4.25

1 0 4 5 0.89

3.63

17.3

52

210

3.77

4.35

930

3.67

20.4

200

0.82

4

10.31

60

100

61

89

3.74

4.32

998

0.78

3.61

24.5

5

10.49

60

200

75

164

3.72

4.30

967

0.75

3.56

20.1

6

10.60

60

600

64

405

3.70

4.27

1 0 1 1 0.80

3.58

18.4

7

10.86

60

900

89

533

3.67

4.24

1 0 2 1 0.85

3.59

17.8

8

10.75

100

200

126

174

3.65

4.21

1 0 5 8 0.86

3.58

19.7

9c

-

91

-

2.84

3.28

881

oo

77

-

-

-

adl00: X-ray diffraction (100) interplanar spacing; ao: unit cell parameter = 2d100/3v2; Vp: primary meospore volueme; APD: Average pore diameter = 1.213d100((2.2Vp)/(l+2.2Vp))v2; bSorption capacity of water measured gravimetrically at 298 K and P/P0 = 0.5. CDatataken from ref. 7 for comparison.

1382 Si-MCM-41 (sample 1) due to substitution of Si4+ ions by relatively larger Ti4+ and ~, t~ v

d

Sn 4+

c

spacing is much higher than expected

ions. However, an increase in dl00

compared with Si-MCM-41 because the

e-

b

_.=

content of tin (IV) chloride increases in the synthesis gel the pH of the gel decreases.

a

4 i

2

,

6

1'o

20 (degree)

After lowering the pH of the synthesis gel an increase in dl00 spacing was reported by Wang et al. [6]. For all the samples pH of

Figure 1. XRD profiles of different samples:

the synthesis gel varies between 11.46 and

(a) sample 1, (b) sample 2, (c) sample 3, and

10.26. Difference in dl00 spacing also may

(d) sample 6.

cause due to large uncertainties on low diffraction angle values. Sample 9

(Ti-MCM-41), data taken from reference 7 for comparison of catalytic activity, has much smaller dl00 spacing (2.84) in comparison to our samples because characteristics ofMCM-41 strongly affected by the.synthesis conditions [8]. Another possible reason is that sample 9 shows less defined X R , much broader peak at dl00 plane and do not observe any peak at higher order by Blasco et al [7]. In Fig. 2, N2 adsorption-desorption isotherms and pore size distribution of samples 1, 2 and 6 are shown. All the samples show a typical type IV isotherm with narrow pore size distribution with peak pore diameter at 2.8 nm. However, pore size distributions of samples 2 and 6 are relatively broader compared with sample 1 (Si-MCM-41) probably due to presence of Ti and Sn. BET specific surface area and pore volume of Ti and Sn containing samples are marginally lower than for Si-MCM-41 (Table 1). As the content of tin increases in the samples the H20 sorption capacity of samples gradually increased from 16.2 to 24.5. Thus, tin containing samples are more hydrophilic than Si-MCM-41 (Table 1).

800

--;

600 {3)

>

~'2

A

:~

400

,oJ...I".,._

2.01

! .......

0.0

0.2

0.4

0.6

0:8

Relative pressure (P/Po)

110

B

....... : ..............

08]

200

2.8nm

..... / ~"~'*........................... b

oo! -.,.'!-...,........ -'-.... 2

4

6

Pore diameter (nm)

8

....

10

Figure 2. (A) N2 adsorption-desorption isotherms and (B) pore size distribution curve of (a) sample 1, Sample 2 and (b) sample 6.

1383

* CO 2

r

b

b

t 200

360 " Wavelength

460 (nm)

s6o

4o'oo

" 3o'oo

2o'oo

Wave n u m b e r ( c m "1)

~ooo

Figure 3. UV-visible spectra of (a) sample 2, Figure 4. FT-IR spectra of (a) sample 2 (b) sample 5 and (c) sample 6.

and (b) sample 6.

3.2. UV-visible and FT-IR UV-visible spectra of calcined samples 2, 5 and 6 are shown in Fig. 3. All these samples show single band near 220 nm, is taken as proof of Ti [9,10] and Sn [11,12] incorporation into the silica walls. The absence of band near 250-270 nm or 300-330 nm indicates that no extra framework species (partially polymerized hexacoordinate Ti and Sn species) or completely polymerized TiO2 and SnO2 phase, respectively, existed in these samples [9-12]. However, we observed the extra framework Ti species (band near 270 rim) in the case of sample 3, which contains low Si/Ti ratio (30) in gel. FT-IK spectra of samples 2 and 6 are shown in Fig. 4. Presence of vibration band at 1081 and 1079 cm 1 in the samples 2 and 6, respectively, which are lower than compared to pure Si-MCM-41 (1089 cml), may be considered as a substitution of Ti and Sn in the frame work of Ti and Sn containing samples [13]. Samples 2 and 6 show band at 960 and 962 cm 1, respectively, assigned to the framework vibration of Si-O-M (M = Ti, and/or Sn) bond [13,14]. However, in the case of pure Si-MCM-41, this vibration band is also exit due to presence of excess silanol groups in the calcined material. Therefore, vibration band near 960 cm -1 can not be taken as a proof for substitution of titanium and tin in the structure. 3.3. Thermogravimetric analysis (TGA) Thermogravimetric (TG) and differential thermal analysis (DTA) of the Si-MCM-41 (sample 1) and Ti-Sn-MCM-41 (sample 6) are given in Fig. 5. Thermal patterns of the both samples are qualitatively similar [7]. The total weight losses are 48.1 and 49% for sample 1 and 6, respectively. Total weight losses for other samples at 1073 K remain in the 47-52% range. Four distinct weight losses were observed in thermo diagram [15-17]. Weight loss below 409 K corresponds to desorption of physisorbed water (or ethanol) in the voids formed by crystals agglomeration. Three other weight losses can be distinguished; 409-547 K,

1384

1~ t

20

b

10

60. .

"

i

400

"

,

'

,

600

,

i

800

,

x.~~~0

-10

J

1000

Terrtaerature (K)

Terrl:erature (K)

160o

Figure 5. TG-DTA profiles in air of as-synthesized samples (a) Si-MCM-41 (sample 1) and (b) Ti-Sn-MCM-41 (Sample 6). 547-593 K, and above 593 K are related to breakage, decomposition and combustion of residual organics associated with three exothermic processes. Weight loss above 628 K is attributed to condensation of hydroxy groups.

3.4. 29SiMAS NMR 29Si MAS NMR spectra of calcined form of Ti-Sn-MCM-41 (sample 6) is shown in Fig. 6. Intensity of Si(-OSi)3(-OH) (i.e. Q3 at-99 ppm) is comparable to Si (-OSi)4 (i.e. Q4, at -109 nm) even after calcination at 823 K. It indicates that titanium tin containing samples contains much amount of hydroxy groups. Probably the substitution of titanium and tin in the sample generates many defect sites, which shows considerable contribution :from Q3 species. i

.....

I" 0

"

9

~

~"

9 - - 5

I

~ O

""'"

9

9 --110

"' 0

9

-

-'"

1

- - l S

' tD

9

"

'

9 2CO

Figure 6.29Si MAS NMR ofTi-Sn-MCM-41 (Sample 6).

0

~.

i

1385 Table 2. Catalytic activity in the epoxidation of cyclohexene Sample

Mole ratio in product

Cyclohexene

TBHP efficie-

Cyclohexene oxide

Si/Ti

conversion (%)

ncya (mole%)

selectivity (%)

Si/Sn

2

68

-

19.4

58.2

96

3

52

210

15.5

46.5

98

4

61

89

17.8

53.4

99

5

75

164

16.3

48.9

100

6

64

405

21.0

63.0

100

7

89

533

19.0

57.0

99

8

126

174

16.2

48.6

99

9b

77

-

14.1

-

93

aTBHP efficiency = mole% of TBHP consumed in the formation of cyclohexene oxide, cyclohexadiol, cyclohexene-2-ene-l-ol and cyclohexene-2-ene-l-one, bData taken from ref. 7 for comparison, reaction conditions: 0.3 g catalyst, 56.3 mmol of cyclohexene, 14.0 mmol TBHP, 333 K, 5 h reaction time.

3.5. Catalytic reactions Catalytic activity of titanium tin containing samples in the oxidation of cyclohexene in presence of TBHP as oxidant are given in Table 2. As the contents of titanium and tin increased the cylcohexene conversion increases except sample 3. In the case of sample 3 extra framework Ti species was detected by UV-visible, which is inactive in the reaction. Maximum cyclohexene conversion was obtained with sample 6 (21%). TBHP efficiency defind as the mole% of TBHP consumed in the formation of cyclohexene oxide, cyclohexadiol, cyclohexene-2-ene-l-ol and cyclohexene-2-ene-l-one. TBHP efficiency is 48.9 and 63% over sample 5 and 6, respectively. Cyclohexene oxide is major product over all titanium tin containing samples and other products are cyclohexadiol, cyclohexene-2-ene-1-ol and cyclohexene-2-ene-l-one. However, cyclohexene oxide selectivity is 100% over sample 5 and 6 and no other products were detected. It clearly indicates that in presence of tin cylcohexene oxide selectivity increased. Cycloohexene oxide selectivity over sample 9 is reported to be 93% by Blasco et al. [7].

CONCLUSION In conclusion, mesoporous Ti-Sn-MCM-41 was prepared for the first time in absence of sodium by direct hydrothermal synthesis. The resultant materials possess BET specific surface area of 967 to 1045 m2g~ range, pore volume of 0.75 to 0.89 crn3g~ range and

1386 average pore diameter of 3.56 to 3.67 nm range. UV-visible predicts the presence of tetrahedral coordination of titanium and tin in the samples. Tin containing samples are more hydrophilic in nature compared with pure Si-MCM-41. Cyclohexene conversion and cylcohexene oxide selectivity are 21% and 100%, respectively over Sn-Ti-MCM-41 (Si/Ti = 64, Si/Sn = 405).

REFERENCES

.

2.

L. Nemeth, G. J. Lewis and R. R. Rosin, US patent No. 5,780,654 (1998). C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 3 59 (1992) 710.

.

A. Corma, J. L. Jorda, M. T. Navarro and F. Rey, Chem. Commun., (1998) 1899.

4.

S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 60 (1938) 309.

5.

E. R Barrett, L. G. Joyer and P. R Halenda, J. Am. Chem. Soc., 73 (1951) 373.

6.

A. Wang and T. Kabe, J. Chem. Soc., Chem. Commun., (1999) 2067.

7.

T. Blasco, A. Corma, M. T. Navarro and J. P. Pariente, J. Catal., 156 (1995) 65.

8.

C.- Y. Chen, H.- X Li and M. E. Davis, Micropor. Mater., 2 (1993) 17.

9.

A. Zechinna, G. Spoto, S. Bordiga, A. Ferrero, G. Petrini, G. Leofanti and M. Padovan, Stud. Surf. Sci. Catal., 69 (1991) 251.

10. 11. 12.

T. Blasco, M. A. Camblor, A. Corma and J. P~rez-Pariente, J. Am. Chem. Soc., 115 (1993) 11806. N. K. Mal and A. V. Ramaswamy, J. Mol. Catal., 105 (1996) 149. N. K. Mal, V. Ramaswamy, S. Ganapathy and A. V. Ramaswamy, Appl. Catal. A 125 (1995) 233.

13.

N. K. Mal and A. V. Ramaswamy, J. Chem. Soc., Chem Commun., (1994) 1933.

14.

M. R. Boccuti, K. M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal., 48 (1989) 133.

15.

C.- Y. Chen, H.- X Li and M. E. Davis, Micropor. Mater., 2 (1993) 17.

16.

R. Schmidt, D. Akporiaye, M. Stocker and O. Ellestad, Stud. Surf. Sci. Catal., 84 (1994) 61.

17.

P. T. Tanev and T. J. Pinnavaia, Chem. Mater., 8 (1996) 2068.


1387

Alternative synthetic routes for NiAI layered double hydroxides with alkyl and alkylbenzene sulfonates Raquel Trujillano, Maria JesOs Holgado and Vicente Rives* Departamento de Quimica Inorg/mica, Universidad de Salamanca, Salamanca, Spain (e-mail [email protected]).

A study is presented on different synthetic routes to prepare layered double hydroxides (LDHs) containing organic sulfonates hosted in the interlayer space. The solids prepared have been characterized by powder X-ray diffraction, FT-IR spectroscopy, mass spectrometry, and thermal methods. Anionic exchange and precipitation in the presence of NaOH have been used, but urea hydrolysis represents the best route to obtain crystalline solids, with low specific surface area, although coprecipitation of amorphous AI(III) oxohydroxides is only avoided when a large excess of urea is used. 1. INTRODUCTION Synthesis of new materials involved in the preparation of composites is deserving much attention in recent years. Usually, swelling layered materials have been used for these purposes, mainly layered clays, both cationic clays, as well as anionic clays, also known as layered double hydroxides (LDHs) or hydrotalcite-like compounds. These LDHs are able to host both organic and inorganic anions (A) swelling their structure; they thereof constitute appropriate materials for the synthesis of nanocomposites. The intercalated anions are also able to be exchanged, depending on the strength of the bonds between the anions and the hydroxyl layers; if such a bond is of the type A-water-layer the anion is simply surrounded by water molecules, and the bond strength to the layers is rather week; otherwise, if the bond is A-layer the bond is strong, the anion becomes grafted to the layers and is not exchangeable

[1].

Several methods have been described in the literature to prepare this sort of materials. The importance of the synthesis procedure on stacking of the layers and location of the interlayer anion has been stressed in different papers. We here report on different methods for the synthesis of LDHs with Ni(II) and AI(III) in the layers, which contain organic surfactants differing in the length of the organic chain (eight or twelve methylene groups, with or without phenyl endings), in the interlayer, in order to ascertain the optimum synthesis conditions to prepare these materials as nanocomposite precursors.

1388 2. E X P E R I M E N T A L

2.1 Synthesis procedures The surfactants used were octanesulfonic acid sodium salt, dodecanesulfonic acid sodium salt, octylbenzenesulfonate acid sodium salt and dodecylbenzenesulfonic acid sodium salt (the corresponding anions will be labelled as OS, DS, OBS, and DBS, respectively). All of them were provided by Fluka (Switzerland) and were used without any further purification, as well as other chemicals. Gases were from L'Air Liquide (Spain). The synthesis methods used to prepare the NiA1 anionic clay with the surfactant in the interlayer spacing were, anionic exchange, and direct synthesis method by coprecipitation with NaOH or urea as precipitation agents.

2.1.1. Anionic exchange The starting material Ni-A1 anionic clay with a Ni/A1 molar ratio of 2 and chloride in the interlayer was prepared by the coprecipitation method described by Miyata [3]. 100 ml of a 0.4 M NiCI2 and 0.2 M A1C13 aqueous solution was dropwise added to 250 ml of decarbonated water; the pH was controlled with a 725 Dosimat coupled to a 691 pH-meter, both from Metrohm, and maintained constant at 8.3 by adding NaOH 1M. The synthesis was carried out under inert atmosphere of N2. The green solid suspension obtained was aged during 24 hours by stirring at room temperature, then washed with decarbonated water and dried under vacuum. The sulfonate salts of the surfactants used were dissolved in decarbonated water (0.4 M). A given amount of the starting material was suspended in 100 ml of decarbonated water and then an aqueous solution of the surfactant solution was added. The amount of the surfactant was twice the anion exchange capacity (AEC) of the clay, calculated from the AI(III) content. The mixture was stirred during three days under inert atmosphere at room temperature, then washed with decarbonated water, centrifuged and dried under vacuum. Samples were labelled as NiA1OS, NiA1DS, NiA1OBS and NiA1DBS. 2.1.2. Direct Synthesis with NaOH as precipitating agent A 200 ml solution of NiC12 (0.16 M) and A1C13 (0.08 M) in decarbonated water with a Ni/A1 molar ratio of 2 was dropwise added to 100 ml of a decarbonated water solution of the surfactant. The amount of the surfactant was twice the AEC of the clay. The pH was controlled with a 725 Dosimat coupled to a 691 pH-meter, both from Metrohm, and maintained at 8.3 with 1 M NaOH. The suspension obtained was magnetically stirred for three days under inert atmosphere at room temperature, and then it was washed with decarbonated water, centrifuged and dried in vacuo. A portion of the suspension was hydrothermally treated in a Teflon lined stainless steel bomb during 5 days at 105 o C, and the solid was then washed and dried in the same conditions above given. Solids obtained by this method were named as NiAIOSNa, NiA1DSNa, NiA1OBSNa and NiA1DBSNa. 2.1.3. Direct Synthesis by precipitation with urea A 200 ml solution of NiC12 (0.8 M) and A1C13 (0.4 M) in decarbonated water with a Ni/A1 molar ratio of 2 was added to a 300 ml solution of the surfactant which amount was twice that corresponding to the AEC of the clay. Urea was added to this mixture with a molar urea:total cations ratio of 10:3 or 20:3. The mixture was stirred during three days under inert atmosphere and reflux conditions at 95 ~ then washed with decarbonated water, centrifuged

1389 and dried under vacuum. Samples thus obtained were called NiA1OSU, NiA1DSU, NiA1OBSU and NiA1DBSU. 2.2. Characterisation Techniques Elemental chemical analysis for Ni, A1, S, and Na were carried out by conventional techniques in Servicio General de An/disis Quimico Aplicado (Universidad de Salamanca, Spain). Powder X-ray diffraction patterns (PXRD) were recorded with a Siemens D-500 instrument, using Cu Kc~ radiation ()~=1.54050 A) and equipped with AT Difract software. FTIR spectra were recorded using a Perkin Elmer FT1730 instrument, using KBr pellets; 100 spectra (recorded with a nominal resolution of 4 cm ~) were averaged to improve the signalto-noise ratio. Thermogravimetric (TG) and differential thermal analysis (DTA) were carried out using TG-7 and DTA-7 instruments from Perkin Elmer, in flowing oxygen (50 ml min ~) at a heating rate of 10 ~ min 1, and using alumina (from Merck) calcined at 1200 ~ as a reference for the DTA studies. Specific surface area assessment and pore size analysis were carried out using a Gemini instrument from Micromeritics. The sample (ca. 80-100 mg) was previously degassed in flowing nitrogen at 150 ~ for 2 h in order to remove physisorbed water in a FlowPrep 060 apparatus, also from Micromeritics, and the data were analysed using published software [4]. Mass spectra were recorded by the FAB (Fast Atom Bombardement) method in a VG-AutoSpec spectrometer; the Cs emission was 1 gA and acceleration voltage 3 5 kV. 3. RESULTS AND DISCUSSION 3.1. Elemental chemical analysis Elemental chemical analysis data are summarized in Table 1, together with the proposed formulae for some of the solids prepared. Table 1 Elemental chemical analysis data and sample Ni* AI* S* NiA1C1 31.11 7 . 3 6 - - NiA1OS 23.94 5 55 5.79 NiAIOSU 21.88 8.35 6.04 NiA1OSNa 25,27 7.32 4.46 NiA1DS 23.74 5.01 5.4 NiA1DSU 18.90 4.16 5.14 NiA1DSNa 18.84 4.41 5.14 NiA1OBS 21.14 4.38 4.69 NiA1OBSU n.m. n.m. n.m. NiA1OBSNa 19.8 4.86 5.82 NiA1DBS 14.92 3,57 3.6 NiAIDBSU 12.19 5.8 4.36 NiA1DBSNa 16.63 3.85 5.37 *weight percentage; tatomic ratio;

....

proposed formulae of the solids prepared Ni/Al~f A1/St Formula 1.94 .... [Nio.66oAlo.34o(OH)2](C1)o.34l.03H20 ....... 1.98 1.14 [Nio.665Alo.335(OH)2](S)o.2941.24H20 1.20 1.64 [Nil.xAlx(OH)2] (S)x nH20 1.59 1.95 [Nil.xAlx(OH)2] (S)x nH20 . 2.17 1.10 [Nio.685Alo.315(OH)2](S)o.2861.14H20 2.08 0.96 [Ni0.676Alo.324(OH)2](S)o.338" 2.08H20 1.96 1.02 [Ni0.662Alo.337(OH)2](S)o.330 1.24H20 2.21 1.11 [Ni0.689Alo.31o(OH)2](5)o.279 nH20 n.m. n.m. [Nil.xAlx(OH)2] (S)x nH20 1.87 0.99 [Nio.652A10.348(OH)2](S)0.351"l.12H20 1.92 1.17 [Ni0.658A10.342(OH)2](S)o.292 2.3H20 0.97 1.58 [Ni~.xAI• (S)x nH20 1.99 0.85 [Ni0.665AI0.335(OH)2](S)o.3941.18H20 n.m. = not measured.

1390 The amount of water has been calculated from the first weight loss in the TG curves. The Ni/A1 molar ratio is in almost all cases close to the expected value of 2. The molar AI/S should be close to 1, as each interlayer monovalent anion balances the positive charge in the layers because the introduction of an AI(III) cation. In some cases the A1/S markedly exceeds the expected value, suggesting precipitation of A1 hydroxides (probably as an amorphous material) outside the LDHs crystallites; so, as the amount of A1 in the LDH layers and of sulfonate in the interlayers cannot be determined, the formula of the corresponding solid has not been calculated. In this same samples, the Ni/A1 is markedly lower than the expected value of 2, which is the minimum value for a stable hydrotalcite. The amount of Na found in some cases is almost negligible, indicating that the surfactant anions are not as sodium salts, but probably inserted in the LDH interlayer. 3.2. Powder X-ray diffraction The PXRD diagram for the starting NiAIC1 material is typical of an anionic clay with interlayer chloride anions [5], the interlayer spacing d(003)(7.8 ~) coinciding with the value reported by Miyata [6]. The diagrams for representative surfactant-exchanged samples are given in Fig. 1. All diagrams correspond to layered materials, with basal spacings ranging from 21 to 30 A. These values are in the range reported by several authors for LDHs with interlayer anions containing hydrocarbon chains [7, 8].

2500

~"

~

0

7~...~NiAIDS

[

i

10

20

I

I

30 40 2 e - Cu Ko~ (o)

./~. x2

I

[

50

60

70

Figure 1. PXRD diagrams of parent NiA1CI and of sulfonate-containing LDHs prepared by anionic exchange. Such a spacing decreases as the length of the hydrocarbon chain does. With respect to the application of different synthesis routes to prepare the same compound, the PXRD diagrams

1391 for samples prepared with octane sulfonate are included in Fig. 2. Spacing decreases and crystallinity slightly increases when the sample has been prepared by hydrolysis with urea; probably this is a result of the smooth increase in pH during hydrolysis of urea, thus providing conditions for a better crystallisation of the solid. It should be stressed that diffraction maxima due to crystalline phases other than the expected LDH were not recorded, so indicating that, if such additional phases exist (as assumed for samples NiAIOSU, NiAIOSNa, and NiA1DBSU, from the elemental chemical analysis data), they should be in an amorphous state, and, probably, its concentration should not be very large.

~"

I 1000

d

g

/~ 0

....

I

I

10

20

I

~SNa.~,,.X 2] I

I

I

I

30 40 50 60 70 2 0 - CU Ka (~ Figure 2. PXRD diagrams of octanesulfonate-containing LDHs. 3.3. Mass spectrometry The PXRD diagrams of samples NiAIDBS and NiA1OBS are rather similar to those of the corresponding sodium sulfonate salts. In order to check the absence of sulfonate species adsorbed on the external surface of the LDH crystallites, the solid samples prepared, as well as the pure sulfonates and the parent NiAICl LDH, were analysed by mass spectrometry. The MS of the pure surfactants showed the signals expected for these molecules. The spectra of the solids prepared and of the original NiA1CI LDH did not show any detectable signal, or they were extremely weak. These results indicate that the LDHs are hardly volatilised in the MS chamber and that the surfactant anions hosted in the interlayer space are strongly held; on the contrary, if the surfactant molecule was adsorbed on the external surface of the crystallites, such a volatilisation would not be hindered. Consequently, these results demonstrate that there is no appreciable amount of "free" surfactant externally adsorbed on the surface of the LDHs prepared. 3.4. FT-IR spectroscopy The FT-IR spectrum of NiA1C1 (not shown) is typical of a hydrotalcite with intercalated chloride [9]. Representative FTIR spectra for some of the solids prepared are included in Fig. 3. The broad absorption in the 3600-3300 cm I is due to stretching mode of OH groups (both from the layers and from intercalated water molecules); it should be stressed that a shoulder

1392 around 3200 cm "~, reported in the literature to be originated by OH stretching mode of hydroxyl groups hydrogen-bonded to intercalated carbonate anions, is not present, thus confirming the absence of carbonate impurities. The strong absorption at 425 cm "1, recorded in the spectra of all samples prepared, is due to M-O vibrations in the layers, and is characteristic of this sort of layered solids [10]. The other bands recorded are mostly due to the interlayer species. Broadly speaking, the bands are in close positions to those of the pure surfactants, although in all cases, a shift towards lower wavenumbers is observed. Sulfonate anions strongly absorb at 1230-1120 cm ~, and also show weak bands in the 1080-1025 cm "~ range, due to the antisymmetric and symmetric modes, respectively, of the SO3 group. If the sulfonate anion contains also aromatic rings, four bands are also recorded close to 1230, 1990, 1130, and 1040 cm 1 (three SO and one S-phenyl vibrations interactions) [ 11 ]. These bands, except that at 1130 cm "a, are recorded also in the spectra of the non-aromatic sulfonates, as well as in the LDHs prepared from them. The bands at 1013-1011 c m l are due to centroantisymmetric vibrations, and are recorded in the spectra of DBS and OBS surfactants, and of their corresponding LDHs. The bands in the 850-600 cm ~ are due to in-plane quadrant bending, and those in 550-400 cm ~ to the benzene ring vibrations, which involve ring bending by quadrant. These bands shift towards lower wavenumbers and their intensities decrease when the surfactant is hosted in the LDHs. Finally, bands in the 2960-2800 and 1025-700 cm ~ ranges are due to different C-H and C-C vibrations of the benzene and alkyl chains.

..

I

v

T

~

DBS

O t-t~

E C" L_

4000

I

3000

I

2000

I

11~00

0

wavenumbers (cm Figure 3. Representative FT-IR spectra of LDHs samples containing the indicated sulfonate.

3.5. Thermal analyses The DTA curve of parent NiA1C1 hydrotalcite shows two endothermic effects close to 130 and 290 ~ due to removal of interlayer water molecules and through condensation of layer OH groups, respectively [12]. The curves for the sulfonate-containing LDHs are completely

1393 different, and the curves are dominated by exothermic effects due to combustion of the organic chains. The synthetic route followed does not have a strong effect on the shape of the curves nor positions of the maxima. The curves corresponding to the samples prepared by ionic exchange are given in Fig. 4. These four curves can be grouped in two subgroups: those corresponding to samples NiA1OS and NiA1DS show two exothermic effects between 214287 and 280-380 ~ with a weak shoulder at 400 ~ for those samples containing aromatic rings, however, two additional effects are recorded, a weak one at ca. 150 ~ and a rather sharp peak at 470 ~ probably, combustion of the aromatic ring should be in some sort of way related to the exothermic effect close to 470 ~

5~

exo (9 o

I

0

I

I

200

400 600 800 temperature (~ Figure 4. DTA diagrams of LDHs samples containing the indicated sulfonate. The TG curves show a first effect due to removal of interlayer water and then a strong weight loss due to combustion of the interlayer surfactant and evolution of water from the layer hydroxyl groups. 3.6. Surface texture

The N2 adsortion-desorption isotherms at-196 ~ correspond in all cases to type II in the IUPAC classification [ 13 ], indicating they are macroporous or non-porous, with unrestricted monolayer-multilayer adsorption. It has been previously reported [ 14] that during adsorption experiments, N2 molecules are unable to enter the interlayer space of LDHs containing simple anions, as chloride, nitrate or carbonate, while if large anions with large formal negative charge (e. g., hexacyanoferrates, polyoxometalates, Keggin-type anions) are hosted in the interlayer space such an access is possible, thus the solids behaving as microporous. In our case, despite the large interlayer space, which would made it accessible to N2 molecules, such a space should be highly populated and packed with monovalent surfactant anions, so leaving not too much room available for the N2 molecules; consequently, microporosity is not observed.

1394 A relationship between the synthesis route and specific surface area development seems to exist. So, samples prepared by anion exchange show a specific surface area similar to that of parent NiA1C1, suggesting anion exchange takes place without destruction of the layers nor the crystallites. On the contrary, samples prepared in NaOH or urea media show different specific surface area development; those prepared by precipitation with NaOH show SBET values in the range 15-80 m g , the specific value probably depending on the drying rate. However, the samples prepared by hydrolysis with urea show SBETvalues close or even lower than 8 m 2 g-1. These results are in agreement with PXRD results above commented, as the most crystalline samples (prepared in urea) show the lower specific surface area values. 4. CONCLUSIONS Among the three synthetic routes tested, urea hydrolysis leads in all cases to the most crystalline materials, although coprecipitation of amorphous compounds of the trivalent cation cannot be avoided; the pure LDH was obtained only when a large excess of urea was used. Changes in specific surface area are in agreement with the change observed in crystallinity, SBETincreasing when the amorphous co-product is formed.

Acknowledgments. Finantial support from MCyT (grant MAT2000-1148-C02-01) is acknowledged. REFERENCES 1. J. Inacio, C. Taviot-Gu6ho and J. P. Besse, Appl Clay Sci., 18 (2001) 255. 2. F. Leroux M. Adachi-Pagano, M. Intissar, S. Chauvi6re, C. Forano and J-P. Besse, J. Mater. Chem., 11 (2001) 105. 3. S. Miyata, Clays Clay Min., 26 (1978) 441. 4. V. Rives, Ads. Sci. Technol., 8 (1991) 95. 5. V. A. Drits and A. S. Bookin, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 37. 6. S. Miyata, Clays Clay Min., 31 (1989) 511. 7. M. Meyn, K. Beneke and G. Lagaly, Inorg. Chem., 29 (1990) 5201. 8. H.-P. Boehm, J. Steinle and C. Vieweger, Angew. Chem. Int. Ed. Engl., 16 (1977) 265. 9. J. T. Kloprogge and R. L. Frost, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 139. 10.M.J. Hern~indez-Moreno, M. A. Ulibarri, J. L. Rend6n and C. J. Serna, Phys. Chem. Solids, 12 (1985) 34. 11. N. B. Colthup, L. H. Daly and S. E. Wiberley (eds.), Introduction to Infrared and Raman Spectroscopy, Academic Press, San Diego, California, 1990. 12. V. Rives, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 115. 13. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 57 (1985)603. 14. V. Rives, in Layered Double Hydroxides: Present and Future, V. Rives (Ed.), Nova Sci. Pub., Inc., New York, 2001, p. 233.


1395

Spectroscopic Studies on Aminopropyl-containing Micelle Templated Silicas. Comparison of grafted and co-condensation routes D. Brunel a*, A C Blanc a, E Garrone b, B Onida b*, M Rocchia b, J B.Nagy c, D J Macquarrie d*

(a) Laboratoire des Mat6riaux Catalytiques et Catalyse en Chimie Organique, ENSCM, UMR-5618-CNRS, 8, rue de l'Ecole Normale, 34296 Montpellier, c6d6x 5 France (b) Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24, 1-10129 Turin, Italy (c) Laboratoire de R6sonance Magn6tique Nucl6aire, Facult6s Universitaires Notre-Dame de la Paix, 63 rue de Bruxelles, B-5000 - NAMUR, Belgium (d) Centre for Clean Technology, Department of Chemistry, University of York, Heslington, YORK, YO 10 5DD, England Spectroscopic studies on three aminopropyl-Micelle Templated Silicas indicate significant differences between the interactions of the amine groups and the surface. These differences correlate to the differences in catalytic and stoichiometric activity which have already been reported 1. INTRODUCTION The recent development of routes to novel highly structured and regular silicas such as the M41-S family, and others such as the neutral amine templated and polyether templated Micelle Templated Silicas (MTS) has opened up new areas of inorganic chemistry.[1] Such materials display structural regularity expressed in terms of regular pore size and shape (controllable via template length and synthesis conditions), and high surface areas, typically of the order 1000 m2g1. Such features make these materials eminently suitable candidates for many applications, including catalysis (where regular, controllable pores is of obvious importance in terms of selectivity), adsorption and many other materials applications such as molecular wires and sensors. In terms of catalysis, the MTS group of materials has been well investigated as a catalyst support, with a range of organic species being supported onto its surface, and many catalytic applications investigated.[2] These catalytic materials are accessible via grafting methods first developed for amorphous silicas, relying on the fact that the walls of the MTS materials are broadly similar to amorphous silica. A second methodology for the incorporation of organic groups has been

1396 developed, whereby the organic moiety is incorporated into the material during the synthesis of the MTS.[3] Thus, a silica precursor (tetraethoxysilane, TEOS) and an organosilane (RSi(OMe)3) are co-condensed in the presence of the template (a neutral amine) leading directly to a MTS containing the R group as an intrinsic part of the surface of the material. Removal of template by solvent extraction leaves the material ready for use. Analogous materials can also be prepared using quaternary ammonium[4] and polyether block copolymers.[5] These materials have been used as catalysts in similar reaction types to the grafted MTS and grafted silica analogues, and their physical properties, such as pore size distribution, surface areas and thermal behaviour have been studied. While their overall activity is generally broadly similar, there are significant differences in the detail of their behaviour which hints at important differences in surface chemistry. For example, in the materials where R - CH2 CH2CH2NH2, all three materials catalyse the Knoevenagel condensation of ethyl cyanoacetate with cyclohexanone with similar rates, but the condensation of ethyl cyanoacetate with benzaldehyde is fastest with grafted materials, and is considerably slower with the in-situ materials. Stoichiometric reactions with benzaldehyde to form the imine are likewise different, with very large differences in rate (orders of magnitude) even within the in-situ materials. Despite this striking behaviour, their spectroscopic properties have not yet been investigated in depth. We present here studies on aminopropyl-containing MTS materials prepared from the grafting route and from the direct co-condensation route. Two different co-condensed materials are investigated, as they display intriguing and large differences in the chemistry of the amine.

H20,t- EtOH ~ # ~ _ . , ~ . _ D-C12H25 NH2 @~t{~~l~

~ (RO)aSi

remove template

I RSi(OMe) + (RO)4Si remove._ template

Figure 1. Different routes to preparing the MTS-G and MTS-C catalysts

2. EXPERIMENTAL Grafted MTS materials (MTS-G) and materials prepared by co-condensation MTS-C) were prepared according to previously published methods.[2b,6] MTS-C50 refers to a material prepared by the co-condensation route using a 50vo1% water : 50vo1% ethanol solvent mixture; MTS-C70 to a material prepared by the co-condensation route using a 70vo1%

1397 water: 30vo1% ethanol solvent mixture. These two materials have been shown to have reaction rates with benzaldehyde differing by several orders of magnitude[6]. MTS-C70 reacts completely within 20 minutes, whereas MTS-C50 requires 6 days under the same conditions. MTS-G reacts completely within 2 minutes. Quantitative MAS NMR spectra were obtained using a Bruker MSL400 spectrometer operating at 100.6MHz (13C) and 79.5MHz (29Si) respectively. Infra-red spectra were obtained using self-supporting wafers in a home-made infra-red cell. Wafers were prepared at pressures of 100 mV; strong site, 0 < E < 100 mV; weak site, -100 < E < 0 mV and very weak site, E < -100 mV.

1417 o -50

1000 ,-~

-100

% \ %

750

-150

MS

500

0

-200 250

-250 -300

E~" -350 "-"

U.I 1250

0

MS-F I 0,0

I

i

0,5

,,

i

1,0

-250

I 1,5

2,0

O l OO -~ I:~ 750

--

500

--

r

1250

~

0,0

TPA/MS-F '

I

0,5

'

I

1,0

'

I

'

I

'

1,5

2,0

lOOO 750. 500

%

250 o ~ -250

MPA/MS-F '

0,0

I 0,5

'

I 1,0 meq/g

'

I 1,5

-25o

' 2,0

~MPA/MS-F-L

~ 0,0

TP/~S-F-L '

I

0,5

'

I

1,0

meq/g

'

1,5

Figure 5. Potentiometric titration of SiMCM-41 and functionalized SiMCM41 (a), supported MPA (TPA) (b, c) and the catalysts after leaching with ethanol/water (d). MS and MS-F were titrated in order to compare their acidities. In Figure 5a, it can be seen t h a t MS presented higher acidity t h a n MS-F. It is evident from this result that the grafting process interchanges some acidic sites of SiMCM-41 by amine groups of 3-aminopropyltriethoxysilane. Additionally, MPA and TPA presented similar acid strength values when they are supported on MS, 1017 mV for MPAfMS and 1062 mV for TPA/MS (Figure 5b and 5c). These results can be attributed to the presence of Keggin structures t h a t remain unaltered onto the MS surface[l]. It was supposed that at least one proton of H3PW(Mo)1204o will react with the OH of silanol leading to a SiOH2 § group, which should act as a counter ion for the polyanion. The acidity decreased considerably for TPA when is supported on MS-F (-80 mV, Figure 5b). For MPA/MS-F, the acid strength is similar to MPA supported on MS, but the plateau of potentiometric curve is very narrow. This behavior could be due to different interaction of MPA and TPA on the functionalized support

1418 with respect to MS as a consequence of the different superficial groups. HPA are linked to the MS-F through proton transfer from the acids to the amine group, resulting in an electrostatic bond between t h e - N H 3 + and the heteropolyanion. In addition, this difference in acidity could be assigned to a change of the HPA proton positions. The protons would be localized on the most highly charged oxygen atoms, they could migrate from bridged to terminal oxygens. It is interesting to point out that acidity do not change for the catalysts after the leaching with ethanol/water when they are supported on MS and MS-F (Figures 5b, 5c and 5.d). The catalytic activities of TPA/MS-F-L and MPA/MS-F-L in the esterification of acetic acid with isoamyl alcohol are being tested [7]. They display high activity and selectivity for the studied reaction. It was determined by atomic absorption spectrometry that no leaching of TPA or MPA occur during the reaction. These results show that HPA supported onto functionalized mesoporous silica seem to be promising solids to be used as heterogeneous catalysts in liquid phase reactions. References 1. P.Vfizquez, M.Blanco and C.Cficeres, Catal. Lett. 60, 205 (1999). 2. L.Pizzio, C.Chceres and M.Blanco, Appl. Catal. A: General 167, 283 (1998). 3. K.J.Edler, J.W.White, Chem.Mat. 1997, 1226 (1997). 4. M.Lasp6ras, T.Lloret, L.Chaves, I.Rodrlguez, A.Cauvel, D.Brunel, Stud.Surf.Sci.Catal. 108, 75 (1997). 5. W.Zhang, M.Froba, J.Wang, P.T.Tanev, J.Wong and T.J.Pinavaia ,

J. Chem.Am.Soc. 118, 9164 (1996). 6. Spectroscopic techniques for organic chemists, James W. Cooper, Ed. John Wiley & Sons (New York) (1980). 7. L. R. Pizzio, P.V. Vhzquez, C.V. Chceres, M.N. Blanco, unpublished results.


1419

Acidity of Mesoporous Aluminophosphates and Silicas MCM-41. A Combined FTIR and UV-Vis-NIR Study Enrica Gianotti a*, Valeria Dellarocca a, Erica C. Oliveirab, Salvatore Coluccia a, Heloise O. Pastoreb and Leonardo Marchese ~ aDipartimento di Chimica IFM, Universit/t di Torino, v. P. Giuria, 7, 10125, Torino - Italy blnstituto de Quimica, Universidade Estadual de Campinas, CP 6154, 13083-970, Campinas, SP, Brasil ~Dipartimento di Scienza e Tecnologie Avanzate, Universit/l del Piemonte Orientale, "A. Avogadro", C.so Borsalino, 54, 15100, Alessandria - Italy. The surface acidity of new mesoporous aluminophosphates, silicoaluminophosphates and [Si]-MCM-41 was studied by means of FTIR and DR UV-Vis-NIR spectroscopies using ammonia as probe molecule. Moreover, a novel Ti-gratted mesoporous ALPO was synthesised and studied by the use of DR-UV-Vis spectroscopy.

1. Introduction

Microporous aluminophosphates materials (A1PO-n) with various crystalline structures, are known as the first example of inorganic molecular sieves composed of a material other than silica [ 1]. In the search for new synthesis methods, that could afford channel systems with pores in the range of mesoporosity, phosphate-based molecular sieves, like cloverite [2] and VPI-5 [3], have been prepared and displayed ring systems larger than the usual 12 T atoms found in large pore zeolites. Despite the large channels and/or cavities, the actual openings in these solids are not larger than 1.2 nm, still limiting their use to small reactants. A further limitation to their application is that their thermal resistance is not significant. It was not until the advent of mesoporous silicates and aluminosilicates that the possibility of preparing aluminophosphates with pore apertures larger than the ones already known turned into a reality [4]. Recently, cetyltrimethylammonium bromide was used as a structure-directing agent to synthesise mesoporous ALPOs and Mg-ALPOs [5]. In this contribution we report the synthesis of mesoporous aluminophosphates (ALPO) and silicoaluminophosphate (SAPO) and their spectroscopic characterisation using FTIR and DR UV-Vis-NIR techniques and NH3 as molecular probe to monitor the surface acidity. MCM-41 was also studied for comparison. In the view of the great interests in Ti-based materials for selective oxidation reactions, mesoporous ALPO was functionalised grafting titanium complexes on the hydroxyl groups (P-OH or A1-OH) present on the ALPO surface. This new material was studied by the use of FTIR and DR UV-Vis spectroscopy for obtaining information on surface hydroxyls and coordination of Ti(IV) centres.

1420

2. E x p e r i m e n t a l

Section

Mesoporous ALPO and SAPO materials were synthesised using cetyltrimethylammonium bromide (CTMABr) as surfactant, aluminium sulphate and orthophosphoric acid; in the case of SAPO, tetraethylorthosilicate (TEOS) was used as source of silicon (Si/AI = 0.38, Si/P = 0.30). Titanium-mesoporous ALPO was synthesised following the procedure used by Maschmeyer et al. [6] to prepare Ti-grafted MCM-41 (Ti = 1.2 wt %). In order to eliminate the surfactant, the samples were outgassed at 500~ and then calcined in 100 torr O2 at the same temperature. MCM-41 was prepared according to literature methods [7]. FTIR spectra on pelletised samples were collected using a Bruker IFS88 spectrometer and UV-Vis-NIR Diffuse Reflectance experiments were performed on a Perkin Elmer (Lambda 19) spectrometer equipped with an integrating sphere attachment. 3. R e s u l t a n d D i s c u s s i o n

In these systems the mesopores are randomly oriented and do not present an hexagonal organisation, as indicated by the appearance of only the (100) peak in the X-ray diffractogram. The study of the thermal decomposition of the surfactant in mesoporous ALPO and SAPO was followed by in situ FTIR spectroscopy. Fig. 1 shows the FTIR spectra of the assynthesised mesoporous ALPO (section A) and SAPO (section B) after outgassing at increasing temperatures from 200~ to 500~ (curves a to e). After outgassing at 200~ water was desorbed and a broad band between 3800-3200 cmq due to H-bonded P-OH and AI-OH groups observed in the ALPO spectrum (Fig. 1A, curve a). At higher outgassing temperatures (curves b-e), dehydroxylation took place and oxygen-sharing-AIOa and-PO4, along with free AI-OH and P-OH groups, were formed (scheme 1). In fact, bands at 3670 cmq, assigned to the stretching of isolated P-OH groups, and bands at 3789 and 3720 cmq, due to the stretching of free AI-OH groups, were present even at 350~ (curve c) and increased in intensity at higher temperatures (curves d and e).

Oj

1

i'

H.

""O /

1

H_

"-O /

I

H..o/H -

-H20

I

u_ l AI 0~'\0 ~/ P~. uk 00~_-O 1 ~ 0 j ~ ' ' O _ -

_

0

_

"~

_

O/H

o

H

o/

0 ~ O \o/P,~n / t \ - . /

-

_

0"7_00 .

_

-

,

o

u

i

~_

Scheme 1 Bands in the range 3050-2800 crnq which decreased with increasing temperature up to 500~ (curve e), and disappeared after calcination at 550~ (curve f), are assigned to the C-H stretching vibrations of the -CH2 (2922, 2851 cmq) and -CH3 (2964 and 2876 cmq) groups of the hydrocarbon chain of the surfactant. The same behaviour is observed for two bands at 1458 and 1580 crnq. The former was assigned to the bending of-CH2 groups, whereas the latter is of unknown nature. The surfactant preserved its cationic form after synthesis as revealed by bands at around 3025 and 1482 crnq assigned to -CH3 stretching

1421 and bending vibrations in -N(CH3)3 + polar heads of the template. In the case of ALPO, the positive charges might be counterbalanced by either bromide ions, introduced during the synthesis, or PO" and A10" groups present at the ALPO/surfactant interface, while for MCM-41 and MCM-48 (spectra not reported) the anionic counterpart is represented by

sio- [81.

The spectrum of SAPO (Fig. 1B) outgassed at 200~ (curve a), showed broad bands at 3750-2500 cm"1 range and a band at 1660 crn"1 respectively due to stretching and bending modes of H-bonded water molecules. It is of note that water molecules are more tightly bonded on SAPO than on ALPO revealing that the surface acid/base properties of the two materials are different. The presence of micropores, where 1-120 molecules can be entrapped, within the inorganic walls of SAPO might also explain this behaviour. A weak band at 3692 cm"~, due to the presence of H-bonded hydroxyls was also found. All of these bands completely disappeared after outgassing at 400~ (curve d). The bands corresponding to the polar heads of the surfactant (3033 and 1475 cm~) decreased in intensity more rapidly than those of the hydrocarbon chains (bands at 2964 for -CH3 and 2925, 2855 and 1465 cml for -CH2) being nearly completely removed upon treatment at 400oc. The decomposition of the surfactant lead to the formation of the P-OH, Si-OH and A1-OH groups (bands at 3670, 3735 and 3789 cml ) and was complete only upon calcination at 550~ (curve f).

...................

l

~

.....

|

H

a

.... 3600 3200 2800 Wavenumber[era"~] 9

3600

3200

2800

1800

1500

..

t

,,

t

// 1800 1500 -

9

I .

Fig-1 - FTIR spectra of mesoporous ALPO (section A) and mesoporous SAPO (section B) recorded after outgassing the sample at: 200~ (a), 300~ (b), 350~ (c), 400~ (d), 500~ (0), after calcination in 100 tort 02 at 550~ (f)

I

1422 Fig.2 shows FTIR spectra in the OH stretching region of mesoporous ALPO, Ti-ALPO and MCM-41 upon calcination at 550~ in 02. In the ALPO spectrum (curve a), bands at 3789 and 3720 crnq are assigned to the stretching mode of free A1-OH groups and the band at 3675 crnq to the stretching mode of isolated P-OH groups. Similar bands were present in the spectrum of Ti-ALPO but with lower intensity (curve b), and this is a clear evidence of the fact that some OH groups were used to gratt titanium ions. MCM-41 (curve d) exhibited only a narrow band at 3745 cm"l due to the stretching vibration of isolated Si-OH groups.

3745 4

It !

C

.]

%

..

. . . . . . . . . . . .

3,2o 3900

!

3 8 0 0 3 7 0 0 3 6 0 0 3 5 0 0 3400 Wavenumber [cm"1]

Fig.2 - FTIR spectra of calcined ALPO (a), Ti-ALPO (b) and MCM-41 (c) NH3 was used as molecular probe to monitor the acidity of hydroxyl groups both in mesoporous ALPO (Fig. 3 section A) and MCM-41 (Fig. 3 section B). The adsorption of 10 mbar NH3 on ALPO (Fig. 3A, curve a) produced bands at 3380, 3280, 1620 and 1460 emq, while simultaneously, the bands due to the stretching modes of both AI-OH (3789, 3720 cm"l) and P-OH groups (3675 cmq) completely disappeared. The bands at 3380, 3280 and 1460 cmq are assigned to the asymmetric and synm~tdc stretching and to the asymmetric bending modes of NH4+ ions formed by proton transfer from the surface hydroxyl groups to NH3 molecules, whereas the band at 1620 cnaq is assigned to the asymmetric bending mode of NH3 adsorbed on Lewis acid sites likely due

1423 to AI ions located on the inner surface of the mesopores [9,10]. After outgassing the sample at room temperature for lh (curve b), the bands due to NH4 + ions decreased in intensity and the band due to P-OH stretching vibration (3675 cm "l) reappeared, although of smaller intensity than that observed before adsorbing NH3 (curve e). Also the band at 1620 cm"~ was still present atter outgassing the sample at room temperature. Only after outgassing at 350~ (curve d), the bands of NH4 § disappeared and the absorptions due to the stretching of A1-OH and P-OH groups were completely restored. NH3 on Lewis acid sites was also desorbed at 350~

//

A

1

B

i:

a r~

C

II

iI

, .. 9

'

,

,

,

,

.

,

..

3600 3300 3000 2700

/;

,

.

, " . " ' - I

1800 1600 1400

"

9

,

d - " - ] 7 . . - - - . ~ - - 7 - - - ' t e

3600 3300 3000 2 7 0 0

,

.

,

,

, l

"'180016001400

Wavenumber [em-~1 Fig.3 - FTIR spectra of NH3 adsorption on mesoporous ALPO (section A) and on MCM-41 (section B). Section A - curve a: 10 mbar NH3, curve b: outgassed sample at room temperature for lh, curve e: outgassed sample at 150~ curve d: outgassed sample at 350~ curve e: sample before NH3 adsorption. Section B - curve a: 10 mbar NH3, curve b: 1 mbar NHa, curve e: outgassed sample at room temperature for lh, curve d: sample before NH3 adsorption.

1424 The adsorption of 10 mbar of NH3 on [Si]-MCM-41 (Fig. 3B, curve a) produced, in the FTIR spectra, bands at 3405, 3330 and 3320 crn"1, overlapped to a broader band centred at 3030 crn"l. The band at 3330 cm "~, due to the asymmetric stretching mode of NH3 molecules in gas phase, disaplpeared by decreasing ammonia pressure (curves b). The broad band centred at 3030 cm" is assigned to the stretching O-H of the silanol groups Hbonded to NH3 molecules (Si-O-H .... NH3). The bands at 3405 and 3320 crn"l are respectively due to the asynm~tric and symmetric stretching vibrations of NH3 H-bonded to silanols. At lower frequency, bands at 1635 and 1625 cm"1 were also formed. The band at 1625 crn"l is due to the asymmetric bending mode of NH3 molecules in gas phase, whereas the band at 1635 crn"1 is assigned to the asymmetric bending mode of the ammonia molecules adsorbed on Si-OH groups. All these bands completely disappeared upon outgassing the sample at room temperature for lh (curve c), and the narrow band at 3745 cm"l, due to stretching mode of free Si-OH groups, was almost completely restored, giving a spectrum which was similar to the one recorded for the bare MCM-41 sample (curve d) [11,12]. These results show that AI-OH and P-OH hydroxyl groups in ALPO are sufficiently acidic to protonate ammonia molecules, and that NH4+ is decomposed at temperatures lower than in acid zeolites. This particular acidity sets this material as intermediate between Si-OH and zeolitic bridged OH acid groups, specific of silicate and/or aluminosilicates structures. These materials might be used with advantages in organic reactions where a mild acidity is necessary. NIR spectra of ALPO (curve a) and MCM-41 (curve b) calcined and outgassed at 550~ are shown in Fig.4. 0.3

-

:

.

.

/ /

.

.

.

.

.

.

.

.

4530

:,,

02

t |

',

7321 t

4610~

/

b,

~17190

~, . , . r

s

o.o

8000-7500

7000

6500"" 50004500-4-000 W avenum ber r "~

Fig.4 - NIR spectra of mesoporous ALPO (curve a) and MCM-41 (curve b)

1425 The ALPO spectrum showed broad bands at 7190 cm~ and at 4530 cm~ with a shoulder at 4610 cm"~, the former being due to the first overtone of the stretching mode and the latter to the combination of stretching and bending modes of isolated OH groups. Such bands are broad and asymmetric in that, different hydroxyl groups (AI-OH and P-OH), are present in ALPO material. The spectrum of MCM-41 showed a band at 7321 crn"~ due to the first overtone of the stretching mode of isolated Si-OH groups (2VOH), and a band at 4530 cml assigned to the combination of the deformation with the stretching OH modes (VoH + ~oH). These bands are narrow because only Si-OH groups are present in MCM-41. Surface OH groups, P-OH and A1-OH in ALPO and Si-OH in MCM-41, are the sites for anchoring metals ions, for example Ti ions. In this way it is possible to obtain a variety of Ti-functionalised mesoporous catalysts with tuned polarity (from MCM-41 to ALPOs) for reactions involving bulky hydrocarbons. In Ti-based materials the active sites for selective oxidations are isolated tetrahedral Ti(IV) centres. DR UV-Vis spectroscopy was used to achieve information about the coordination of Ti(IV) sites in Ti-ALPO mesoporous material (Fig. 5). The spectrum of the as-synthesised sample in air (curve a) showed a broad absorption centred at ca. 250 ran, which after calcination (curve b) became sharper and shifted to 230 nm. These features indicates that the titanium sites existed mainly in tetrahedral coordination, in fact, bands in the 210-230 nm range arise from oxygen to tetrahedral Ti(IV) ligand-to-metal charge transfer (LMCT) [11-14]. The broadness of the band in the spectrum of as-anchored sample was due to the presence of electron-reach pentadienyl ligands bonded to titanium centres.

3

2

0 200

~'\a

250

300

350

400

450

500

Wavelength [nm]

Fig. 5 - DR VU-Vis spectra of mesoporous TiALPO. Curve a: as-synthesised sample in air; curve b: calcined sample in vacuo

1426 In conclusion the synthesis of mesoporous ALPO and SAPO materials and their characterisation by spectroscopic means is reported. ALPO showed acidic properties intermediate between silicas and zeolites which can be useful for catalytic reactions where mild acidity is required. Measurements of the acidity of the meso-SAPO is in progress. These materials are also interesting as support for preparing metal-functionalised catalysts. An example of Ti-gratted material is reported and showed that tetrahedral Ti(IV) sites, the active centres for many selective oxidations, are present. ACKNOWLEDGEMENTS: The Italian MURST (Progetto di Rilevante Interesse Nazionale, Cofin. 2000) and the Brazilian FAPESP (Funda~.o de Amparo h Pesquisa no Estado de S~.oPaulo) are acknowledged.

REFERENCES

1. S.T.Wilson, B.M. Lok, C.A. Messina, T.R. Cannan, E.M. Flanigen, J.Am. Chem. Soc., 104 (1982) 1146 2. M. Estermann, L.B. McCuster, C. Baerlocher, A. Merrouche, H. Kessler, Nature 352 (1991)320 3. M.E.Davis, C. Saldarfiaga, C. Montes, J. Garces, C. Crowder, Nature 331 (1998) 698 4. T. Kimura, Y. Sugahara, K. Kuroda, Micropor. Mesopor. Mater. 22 (1998) 115 5. N.C. Masson, H.O. Pastore, Micropor. Mesopor. Mater., 44 (2001) 173 6. T. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature, 378 (1995) 159. 7. C.T. Kresge, M.E. Leonowicz, W.J. Vartuli, J.S. Beck, Nature 359 (1992) 710M. 8. L.Pefia V. Dellarocca, F. Rey, A. Corma, S. Cosuccia, L. Marchese, Micron. Mesop. Mater., 44-45 (2001) 345 9. L.H. Little, Infrared Spectra of Adsorbed Species, Academic press, New York, (1966) 10. R.L. Puurunen, A. Root, S. Haukka, E.I. Iiskola, M. Lindblad, A.O.I. Krause, J. Phys. Chem. B, 104 (2000) 6599] 11. L. Marchese, E. Gianotti, T. Maschmeyer, G. Martra, S. Coluccia and J.M. Thomas, I1 Nuovo Cimento, 19D (1997) 1707. 12. E. Gianotti, V. Dellarocca, L. Marchese, G. Martra, S. Coluccia, T. Maschmeyer in preparation 13. P.E. Sinclair, G. Sankar, C.R.A. Catlow, J.M. Thomas and T. Maschmeyer, J. Phys. Chem. B, 101 (1997) 4232. 14. L.Marchese, T. Maschmeyer, E. Gianotti, S. Coluccia and J.M. Thomas, J. Phys.Chem. B, 101 (1997) 8836.


1427

Modification of silica walls of mesoporous silicate and alumino-silicate by reaction with benzoyl chloride. L. Pasqua a, F. Testa b, R. miello b, G. Madeo b and J. B.Nagy~ aDipartimento di Ingegneria dei Materiali e della Produzione, Universit~t Federico II, Piazzale V. Tecchio 80, 80125 Napoli, Italy bDipartimento di Ingegneria Chimica e dei Materiali, Universit~t degli Studi della Calabria, Via Pietro Bucci, 87030 Rende, Italy. CLaboratoire de R. M. N., Facult6s Universitaires Notre-Dame de la Paix, 61 Rue de Bruxelles, B-5000 Namur, Belgium.

Preparation of mesoporous silicate and alumino-silicate by different synthesis methods and their surface modification are reported. Specific surface area of starting samples ranges between 400 and 1100 m2/g. Surface properties of pore walls of obtained samples were investigated by reaction with benzoyl chloride. FF-IR spectra of modified samples show a peak centred at 1705 cm -1 assigned to the stretching vibration of carbonyl group of benzoic ester. Modified samples show lower specific surface areas and pore volume with respect to the correspondent values for starting materials. The weight ratio between benzoyl and silica was also calculated by thermogravimetric analysis. Mesoporous materials and their reactivity open new perspectives in the possibility of designing specific catalysts. 1. I N T R O D U C T I O N

A new family of mesoporous molecular sieves, named M41S, was recently discovered by Mobil's researchers [ 1]. MCM-41 is the member of the M41S family characterized by a regular hexagonal array of uniformely-sized mesopores. Mesopore diameters can be varied from approximately 20 to ca. 100 A depending on the surfactant employed in the synthesis, the pore size increasing with the chain length of the surfactant. These materials have important applications in catalysis, metal ion extraction, optical applications, etc.. Catalytically active materials have been prepared by introduction of inorganic heteroatoms by grafting metallocene complexes on mesoporous silica creating well-defined active sites [2]. Hybrid inorganic-organic materials are produced when chemically active groups are covalently linked to the inorganic framework of mesoporous materials either by post-synthetic grafting or by simultaneous condensation of siloxane and organosiloxane precursors, the last one containing a non-hydrolysable Si-C bond [3-5]. This method was also applied for bulky chromophores [6]. MCM-41 was also studied as a drug delivery system. In particular, inclusion and delivery mechanism of Ibuprofen, an anti-inflammatory drug, were investigated [7]. Immobilization of enzymes in MCM-41 host was also studied [8].

1428 Molecular design of the active site is one of the elements of greatest importance in heterogeneous catalysis. High reactivity of acyl chlorides of every kind of molecule allows, in fact, fast modifications of pore walls of mesoporous materials representing a valid tool for modelling pores or giving the pores the appropriate chemical functionality, being the synthesis of a chemically-interactive or shape-selective (or both) composite inorganic-organic catalyst the ultimate goal. In this work the preparation of mesoporous silicate and alumino-silicate by different synthesis method is reported. The cationic surfactants cetyltrimethylammonium bromide, cetylpyridinium chloride and the neutral surfactant polyoxyethylene(10)isononylphenylether (Nonfix 10, Condea) were used as structure directing agents while tetraethylorthosilicate, fumed silica, and sodium silicate solution were used as silica sources. Synthesized and modified products were characterized by X-Ray powder diffraction, nitrogen adsorption-desorption, thermogravimetric analysis. Chemical ability of free silanols on surface of pore walls toward organic functional group of guest species were investigated by reaction with benzoyl chloride.

2. EXPERIMENTAL 2.1. Materials and methods The reagents utilized were: cetyltrimethylammonium bromide (CTABr, Aldrich), cetylpyridinium chloride (CPIC1, Aldrich), polyoxyethylene(10)isononylphenylether (Nonfix 10, Condea), benzoyl chloride (Sigma), ammonium fluoride (Carlo Erba), and NaOH (Prolabo). The silicon source was tetraethylorthosilicate (TEOS), fumed silica (SigmaAldrich), sodium silicate solution (Sigma). X-Ray powder diffraction patterns were measured on a Philips PW1710 diffractometer using Cu-Kct radiation (40 Kv, 20 mA) over the range 1~ ~ The N2 adsorptiondesorption volumetric isotherms at 77 K were measured on a Micromeritics Asap 2010 apparatus. Samples were pre-treated under vacuum at 300~ to a residual pressure of 2 l.tmHg. Benzoylated samples were treated at 230~ to the same residual pressure. Surface area of the samples was obtained by BET linearization in the pressure range 0.05 to 0.2 P/Po. Lattice pore volume was obtained from the amount of nitrogen gas adsorbed at the top of the rising section of the isotherms of type I or IV. FT-IR spectra of calcined and modified samples were recorded on a Bruker IFS-28 spectrometer in the absorbance mode with a resolution of 2 cm -1. For each spectrum 256 scans were acquired. 2.2. Synthesis Mesoporous materials submitted to chemical modifications of pore walls were obtained from different synthesis procedures at different temperatures and pH. Sample A and sample B were obtained starting from the gel with the following molar composition: SiO2-0.2NaOH-x AI(OH)a-0.2CTABr-40H20 where x is 0.04 and 0.02, respectively, for samples A and B. 10 grams of fumed silica were added to a solution consisting of AI(OH)3 0.52 g (for sample A) or 0.26 g (for sample B) and 1.35 g of NaOH in 120 grams of distilled water. The gel was aged for 2 hours at room temperature and then transferred to a Teflon-lined container in a thermostated oven at 140~ for 24 h. The synthesis product was then filtered and dried at 80~

1429 Samples C and D were prepared in fluoride medium [9] at 50~ The molar composition of the gel was: TEOS -0.21 CTABr-10 NH4F- x Al(NO3)3-146 H~O where x is 0.01 and 0, respectively, for samples C and D. 13.7 g of TEOS were added to a solution containing 4.98 g of CTABr and 10g of NHnF in 174 g of distilled water. In the synthesis C 0.248 g of Al(NO3)3 9H20 were added before TEOS. The as-synthesized solid was recovered by filtration, washed and dried at 70~ Syntheses of samples E and F were performed in the presence of cetylpyridinium chloride starting from a gel of the following molar composition: SiO2 -0.25 NaOH -0.25 CPC1-x AI(OH)3 -40 H20 where x is 0.02 and 0, respectively, for samples E and F. The resulting gel was stirred for 15 minutes, then transferred to a Nalgene polypropylene bottle and heated under static conditions at 80~ The recovered solid was filtered, washed and dried at 80~ The neutral surfactant polyoxyethylene(10)isononylphenylether (Nonfix 10, Condea) was used in the synthesis of sample G. The molar composition of the gel was: 1SIO2-0.6 NaOH-0.064 Nonfixl0-0.8 HC1-58.1 H20 14.6 g of sodium silicate were added to the surfactant solution (2.9 g of Nonfix 10 in 57.4 g of H20) after complete dissolution of Nonfix 10. Finally, 5.33 g of 37% HC1 were added and the resuking gel was aged for 24 hours at room temperature and heated in oven for 24 hours at 100~ [ 10]. The template was removed by calcination from all the samples at 550~ under flowing air with a heating rate of 1~ followed by a static step of 8 hours. 2.3. Chemical modification

Silica pore walls of calcined mesoporous materials were chemically modified by means of esterification of free silanols with benzoyl chloride. Typically, 0.5 g of calcined mesoporous materials were suspended in 10 ml of anhydrous tetrahydrofuran under nitrogen atmosphere; 1 ml of benzoyl chloride was then added and the suspension was kept under stirring for 20 hours. The modified product was filtered, washed with ethanol, dried in oven at 80~ and submitted to FF-IR analysis. A band at 1705 cm -1 appears in the spectrum after esterification. The same band disappears after the modified sample was hydrolysed at room temperature for 3 hours (buffer solution pH=l, Merck). Nitrogen adsorption-desorption at 77 K on resulting hydrolysed sample shows that the total chemical process of modification and hydrolysis does not affect the pore structure. 3. RESULTS AND DISCUSSIONS

As-synthesized samples have been characterized by X-Ray powder diffraction and thermogravimetric analysis. X-ray powder diffraction pattern of synthesized samples do not exhibit higher diffraction angle peaks but just a broad band in addition to the main reflection peak (hkl 100). The absence of higher angle peaks is typical of poorly ordered porous system on the long range. Calcined samples are characterized by X-ray powder diffraction and nitrogen adsorption-desorption at 77 K. Table l shows the physico-chemical properties of assynthesized materials before and after calcination. Specific surface area of starting samples ranged between 400 and 1100 mZ/g, while pore volume values are comprised between 0.37 and 1.02 cm3/g. Samples C and D, synthesized in fluoride medium at pH around neutrality, show

1430 relatively low pore volume and specific surface area values. At neutral pH hydrolysis rate is low and condensation is fast. In these conditions, the polymerization of silica accounts for increased wall thickness and lower incorporation of surfactant [ 11]. Tablel Unit cell parameters for as-synthesized (ao) and calcined materials (a~,~), specific surface area SBEX,pore volume and surfactant/silica mass ratio for calcined materials. Sample

ao (A)

aca~(A)

BET Surface Area (m2/g)

Pore volume (cm3/g)

CTA/SiOz mass ratio

A B C D E F G

47.3 50.3 49.2 56.0 54.9 49.4 64.2

44.3 49.4 49.2 56.0 52.9 49.6 64.2

777 933 496 400 1034 1150 992

0.56 0.74 0.41 0.37 0.72 1.02 0.80

0.91 0.60 0.26 0.20 0.66 0.96 0.39

Surface properties of pore walls of obtained samples were investigated by reaction with benzoyl chloride in tetrahydrofuran under nitrogen atmosphere at room temperature for 20 hours. Free silanols on external surface of particles and on the surface of pore walls of mesoporous materials are potential electron donor species, so they are able to react with the electrophilic carbon atom of benzoyl chloride. Esterification of free silanols on pore walls was determined by Fr-IR spectroscopy and thermogravimetrical analysis. Porosity of modified samples was evaluated by nitrogen adsorption-desorption at 77 K. Table 2 shows physico-chemical properties of modified materials. Lattice parameters of modified materials are very similar to values of calcined materials presented in Table 1. Table 2 Unit cell parameters (ao), specific surface area SBET, pore volume, FT-IR band of modified materials. Sample

ao (A)

A B C

47.3 46.9 49.2

BET Surface Pore volume Area (m2/g) . . . . cm3/g 759 803 488

0.44 0.63 0.4

FF-IR band Wavenumber (cm -1)

CO-~/SiO2

1705 1705 -

0.040 0.020 0.014

Modification procedure does not affect the texture of the solids. Modified samples show lower specific surface areas and pore volume compared to the correspondent values for starting materials shown in Table 1. The band at 1705 cm -1 in the FF-IR spectra (Figure 2 for sample A) of modified samples is assigned to the stretching vibration of carbonyl group of benzoic ester. This band, not present in the spectra of starting calcined materials (Figure 1 for sample

1431 A), disappears after hydrolysis of ester function. For samples C and D no FT-IR band at 1705 cm -1 was detected. Silanols groups are probably too diluted on the surface of mesoporous materials and the low amount of carbonyl group is not detected by FI'-IR technique. Amount of benzoylation was quantitatively estimated by thermogravimetric analysis for samples A and G. In order to quantify the weight loss that occur at high temperature due to further condensation of silicate walls the calcined sample was thermally treated at 150~ with a

1.3

1.0-

o

0.8

0.5'

3500

'

3000 '

2500

'

20b0

'

15'00'

1000

-1

Wavenumber cm Figure 1. FI'-IR spectrum of calcined sample A.

0.9

17 oo

0.8

0.7 "35'00'30'00'25'00'20'00"15'00' Wavenumber crn-1 Figure 2. FT-IR spectrum of modified sample A.

1000

1432 heating rate of l~ Then a 6 hours static step followed and finally the sample was heated at 850~ with a heating rate of 5~ The same procedure was successively adopted for modified sample. The molar ratios between benzoyl group and silica are shown in Table 2. Figure 3 shows nitrogen adsorption-desorption isotherms of calcined and modified sample A. The main nitrogen uptake for calcined sample A takes place around a relative pressure P/Po=0.3.

600

----~-- Adsorption ----- Desorption

o,'•~500 o

Sample A calcined

400 300

o

Sample A modified

< 200 100 0.0 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

Relative Pressure P/Pn Figure 3. Nitrogen adsorption-desorption isotherms of calcined and modified sample A.

1000

- - m - - Adsorption ~ - - Desorption

8oo

~ 4O0 200 0.0

0.I

l

0.2

0.3

I

0.4

0.5

i

06

0.7

I

0.8

0.9

1.0

Relative pressure P/Po Figure 4. Nitrogen adsorption-desorption isotherms of calcined and modified sample F.

1433

800

~ m ~ Adsorption - - o ~ Desorption

700 600-

Sample G calcined

500. 400Sample G modified

.

~

300-

100

.

0.0 0.1

I.

02

'

0.3

I

0.4

'

0.5

I

0.6

'

0.7

I

0.8

'

0.9

1.0

Relative pressure P/Po Figure 5. Nitrogen adsorption-desorption isotherms of calcined and modified sample G. The isotherm is a Type IV one, reversible without hysteresis loop. It can be noted that the pore filling step of the isotherm moves toward lower relative pressure and the pore volume decreases after chemical modification. Shift of the nitrogen uptake and decrease of pore volume suggest that benzoic ester occupies part of the available space in the starting calcined materials. The same behaviour is shown by samples F (Figure 4) and G (Figure 5) (both calcined and modified) and also by samples B and E (not reported). Pore volume and specific surface areas of samples decrease following esterification. Table 3 shows BET surface area and pore volume ratios between modified and starting calcined materials. Sample C and D show BET surface area and pore volume ratio at the border line with experimental error. Sample A and sample G show the most important deviations from unity for pore volume ratio. Table 3 S B E T and pore volume ratios between modified and starting calcined materials. Sample

A

B

C

D

E

F

G

SBE'r Ratio Mod/Calc PV Ratio Mod/Calc

0.97 0.78

0.86 0.85

0.98 0.98

0.96 0.97

0.92 0.96

0.97 0.86

0.82 0.79

The 13CN/~Rspectra unambiguously show the presence of benzoyl group grafted on the MCM-41 wall. The chemical shifts are quasi identical to those of benzoic methyl ester and different from those of benzoic esters. The highest difference is in the carbonyl chemical shift. The difference stems from the fact that O-SiR3 group is less electron donor than the CH3-O-

1434 group. Chemical properties of silica surface depend on population of free silanols and on their accessibility. Modification rates depend on reactivity of precursors, diffusion limitations and steric factors. The last two factors could be neglected in the case of mesopores and benzoyl chloride. Samples C and D, synthesised in fluoride media, do not reveal chemical modifications. At neutral pH condensation rate is fast, the gelling time of a silica sol is at its minimum [12] and materials with increased wall thickness and low amount of silanols on the pore walls are produced. 4. CONCLUSIONS The above reported results show that new possibilities are open in designing molecular host. Molecules that, on one side, approach silica walls and reacts with free silanols, as it occurs in the case of benzoyl chloride can carry on the opposite side, instead of phenyl group, every kind of organic functional groups. This makes possible to create appropriate active sites in terms of size, polarity, and presence of chelating agents, or metals so that the desired reaction can be catalysed in a tailored cavity as it naturally happens in enzymes. 5. ACKNOWLEDGMENTS

The authors thank the Belgian Programme PAI P5/10 on "Quantum size effects in nanostructured materials". REFERENCES

1. J.S. Beck, J.C. Vartuli, V.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. Mc Cullen, J.B. Higgins and J.L. Schlenker, J.Am. Chem. Soc., 114 (1992) 10834. 2. T. Maschmeyer, F. Rey, G. Sankar and J.M. Thomas, Nature 378 (1995) 159. 3. S.L. Burkett, S.D. Sims and S. Mann, Chem. Commun (1996) 1367. 4. M.H. Lim and A. Stein, Chem. Mater. 11 (1999) 3285. 5. D. Brunel., Microporous Mesoporous Mater., 27 (1999) 329. 6. C.E. Fowler, B. Lebeau and S. Mann, Chem. Commun, (1999) 201. 7. M. Vallet-Regi, A Ramila, R:P. Del Real and J. Perez Pariente, Chem. Mater., 13 (2001) 308. 8. J.F. Diez and K. J. Balkus Jr., J. Mol. Catal. B: Enzymatic 2 (1996) 115. 9. L. Pasqua, F. Testa, R. Aiello, Proceed."5 ~ Congresso Nazionale Scienza e Tecnologia delle Zeoliti" Ravello (Sa) Italy, l-5 ottobre 2000 p. 41. 10. F. Cavallaro, L. Pasqua, F.Testa and R. AieUo, Abstract 2nd Feza Conference Taormina 1-5 Sept.2002. 11. L. Pasqua, F. Testa, R. Aiello F. Di Renzo and F. Fajula, Microporous Mesoporous Mater., .44-45 (2001) 111. 12. F. Di Renzo, F. Testa, J.D. Chen, H. Cambon, A. Galarneau, D. Plee and F. Fajula, Microporous Mesoporous Mater.,28 (1999) 437.

ADVANCED MATERIALS AND APPLICATIONS



1437

Options for the Design of Structured Molecular Sieve Materials J. Sterte, J. Hedlund and L. Tosheva Division of Chemical Technology, Lule~t University of Technology, 971 87 Lulegt, Sweden Options for structuring of molecular sieve materials at the crystal, particle and reactor level are discussed in general and exemplified by work performed in our group. Zeolite crystal manipulation at Lule~ University of Technology (Ltu) includes the preparation of colloidal zeolites as well as zoned materials. Molecular sieve films have been prepared using the seeding approach developed by the group using a variety of zeolite-substrate combinations with a number of different applications intended. Of particular interest has been to develop high flux molecular sieve membranes. A method for the preparation of three-dimensional molecular sieve macrostructures of great interest in catalytic applications has been developed and modified.

1. I N T R O D U C T I O N Structured materials have attracted an increasing interest during the last few years. Although no strict definition has been given, structured materials are usually referred to as materials designed and prepared in a controlled manner to overcome limitations of existing materials prevailing in commercial use. The control could be effected on the crystal level, the particle level or the reactor level and the design involve both the physical and the compositional structure of the material. The borders are not definite but structures at the molecular level (e.g. crystalline structures, isomorphous substitutions etc.) are usually not included. In the book by Cybulsky and Moulijn [1] on "Structured Catalysts and Reactors", the editors choose not to define such catalysts but rather to set the limitations by giving examples. The examples given include (i) Monolith catalysts (honeycombs), (ii) Membrane catalysts, and (iii) Arranged catalysts, the last group including catalysts arranged in arrays and structural catalysts derived from structural packings for distillation and absorption columns. They have thus chosen not to include structures at or below the level of particles with the motivation that randomness in packing always will result in a lack of uniform structure at the reactor level. To limit the scope of structured molecular sieve materials to the three groups given by Cybulski and Moulijn would, however, be to narrow the field too much and would not correspond to the use of the term structured materials in the literature. Molecular sieves are widely used in catalysts as well as in adsorption applications. The reasons are their well known properties in terms of defined pore sizes (selectivity), surface area, ion-exchange capacity, and catalytic activity. Taken alone or in combination, these properties provide possibilities for the preparation of materials with unique properties for a huge number of applications.

1438 To increase the utility of molecular sieves as components in structured materials, further insights in possibilities of manipulating and control properties of molecular sieve crystals, molecular sieve films, self-bonded molecular sieve structures and composite materials comprising molecular sieves are needed. Such insights form the basis for the design of structured molecular sieve materials tailored for optimum performance in applications such as catalysis, separation technology and chemical sensors. This paper intends to give a brief overview of recently developed possibilities for the controlled preparation of molecular sieve materials with a more extensive treatment of such methods developed in our laboratories.

2. MODIFIED CRYSTALS

Tailoring of molecular sieve crystals may involve control of properties such as size (size distribution), shape or compositional gradients. It may also concern modifications aimed at e.g. controlling the properties of the external surface of the crystals. Since single crystals often are primary building blocks for structured molecular sieve materials, possibilities to prepare discrete crystals with the desired size and shape are of great interest. Some preparative procedures recently developed rely on the use of colloidal suspension of molecular sieves as seeds for further growth and in a number of applications the size of the crystals are important since it affects parameters such as diffusion length and ratio between external and internal surface area. Methods for the preparation of stable colloidal suspensions of a number of molecular sieve types have been developed during the last decade. Some relevant examples are given in Table 1.

Table 1 Molecular sieves prepared in the form of stable colloidal suspensions Molecular sieve Appr. Size range (nm) References Hydroxysodalite Zeolite A Silicalite-1

30-50 100-500 >40

Ti-silicalite- 1 (TS 1) ZSM-5 ZSM-2 Faujasite

80-250 100-250 100-500 40-150

Zeolite L Offretite Zeolite Beta

>30 >60 10-200

Schoeman et al. [6] Schoeman et al. [7] Persson et al. [8] Anthonis et al. [9] Li et al. [ 10] Zhang et al. [ 11-12] Persson et al. [ 13] Schoeman et al. [ 14] Schoeman et al. [7] Li et al. [ 15] Verduijn et al. [ 16] Verduijn, [ 17] Camblor et al. [18] Schoeman et al. [ 19]

1439 Most of these examples have been developed within our group using preparative methods involving crystallization from clear synthesis solutions. Although several of the most interesting molecular sieve types are present in this list, a number of interesting zeolite types has not yet been prepared in this form. For some applications it is desirable to grow extremely large single crystals of molecular sieves. During the last years significant progress in this area has been achieved by using nonaqueous growth media for crystallization. Strategies for the preparation of large single crystals have been reviewed by Qiu et al. [2] and some applications of large single crystals are discussed by Scandella et al. [3]. It may also be desirable to limit the surface acidity or to control the adsorption properties of the external surface area of molecular sieve crystals. It can e.g. be of interest to have an outer shell of silicalite-1 on an active core crystal consisting of ZSM-5 or Ti-Silicalite. This can be envisaged to be realized either by a truly zoned crystal, i.e. a single crystal with a compositional gradient from core to shell or by a (large) crystal covered with a polycrystalline film of silicalite-1. These options are further discussed below in connection with molecular sieve films. Another possible way to adapt the performance of molecular sieves in a given application is to modify the surface properties of the crystals [4]. This can be done with the purpose of introducing a diffusion constraint e.g. by chemical vapor deposition. Objectives can also be to deactivate existing catalytic sites at the surface or introducing an additional catalytic function selectively at the surface. Another possible purpose may be to change the hydrophobicity e.g. by application of methods described by Caro et al. [5].

3. Z E O L I T E FILMS Three main routes for the preparation of continuous zeolite films can be distinguished in the vast number of papers devoted to zeolite films and membranes. These routes are the vapor phase method, the method of direct crystallization and seeding methods. Focus will be on the last of these approaches and on work performed at Ltu. In the vapor phase method, first reported by Xu et al. [20], the support is first coated with a parent aluminosilicate gel. The amorphous gel is then hydrothermally treated with vapors of template molecules and converted into zeolite. The technique has been explored by several groups and is well described by Matsukata et al. [21]. This method is versatile but reproducibility appears to be an issue, at least when it is applied for membrane preparation. The method of direct crystallization is the most widely used approach. From a practical point of view, this approach benefits from being a single step method. The preparation is straightforward, the support is treated directly in a synthesis mixture whereupon zeolite crystals nucleate on the support and grow into a continuous film. The main disadvantage is that nucleation must occur in a synthesis mixture. For instance, a synthesis mixture that is capable of creating a sufficient concentration of nuclei on the surface may not exist. The nuclei concentration is dependent of the chemical properties of the support and the nuclei concentration may be low thus ruling out the possibility for preparation of thin and continuous films etc. Despite the limitations, the approach has been successfully demonstrated for the preparation of a large number of zeolites on various types of substrates [22]. In seeding methods, pre-synthesized seeds are somehow concentrated on the support surface and the seeded support is subsequently hydrothermally treated in order to grow the

1440 seeds into a dense film on the support. Advantages with this route are that nucleation no longer must occur on the surface of the support, and the chemistry of the support becomes less important. Instead, nucleation (and limited growth) is carried out in a separate step, during the crystallization of the seed crystals. In the second step, the seed crystals are concentrated and attached to the support. Several techniques are available to accomplish this, such as dip coating, spin coating or electrostatic adsorption, the latter technique facilitating monolayer adsorption. Another advantage with the method is thus that the concentration of nuclei/seed crystals on the surface may be high, provided that the seeding is effective. If the seed crystals are small and adsorbed in a monolayer, it is possible to grow very thin and dense films, which may be difficult/impossible with the alternative methods. In the last step, the seeded support is hydrothermally treated, the seeds grow and a dense film is formed. As opposed to the method of direct crystallization, a synthesis mixture with low nucleation rate can be used. In fact, it is even possible to use a synthesis mixture were no zeolite would form in the absence of seed crystals. For sensitive supports, a synthesis mixture with low alkalinity could be selected. An additional advantage is thus that a wider range of synthesis mixtures can be used for film growth, due to the presence of seed crystals. A disadvantage with the method is that it relies on the existence of small seed crystals, and the fact that it is multi step procedure makes it more labor intensive. The development at Ltu of a seeding technique for preparation of zeolite films was initiated in the mid-nineties, recognizing the facts that extremely thin and continuous films are desired in most applications. This resulted in a method employing colloidal zeolite crystals as seeds and electrostatic adsorption on a modified substrate surface as a means to facilitate a dense momolayer coating [23]. In a first test of the concept, silicalite-1 films were prepared on silicon wafers [24]. Colloidal silicalite-1 seed crystals were prepared according to Persson [8] and the seeds were adsorbed elctrostatically, employing a cationic polymer to reverse the charge of the silicon surface. Films with thickness ranging from about 100 to 800 nm, depending on the synthesis time, were prepared. Since then, the same basic concept has been used, modified in a number of ways to facilitate the growth of various zeolite types on a variety of different supports, see Table 2. SEM top view images of a few examples of zeolitesupport combinations are shown in Figure 1. Table 2 Examples of zeolite films prepared at Lulegt University of Technology References Molecular sieve Support Film thickness (nm) Silicalite- 1 Silicalite- 1 S ilicalite- 1 Silicalite- 1 ZSM-5 ZSM-5 (Si/AI= 10) Zoned MFI A FAU FAU Beta

Silicon wafer Carbon fiber Gold Steel Gold Quartz wafer Quartz wafer Alumina wafer Alumina wafer Steel Tantalum

100-800 -3200 100-800 200-800 200-250 200-4000 6300 200-800 150-2700 2000-6000 - 140

Hedlund et al. [24] Valtchev et al. [25] Engstr6m et al. [26] Wang et al. [27] Mintova et al. [28] Mintova et al. [29] Li et al. [30] Hedlund et al. [31 ] Lassinanti et al. [32] Wang et al. [33] Schoeman et al. [ 19]

1441

Figure 1. A 1000 nm thick silicalite-1 film on silicon wafer (a), a 1500 nm thick ZSM-5 (Si/AI=10) film on porous alumina (b), a 800 nm thick LTA film on alumina wafer (c) and a cracked 6 ~tm thick FAU film on a stainless steel support (d). Besides exploring the possibilities of preparing samples of various zeolite-support combinations, we have studied the preferred orientation of the crystals in the films and explored how to control the preferred orientation [34-36]. It has been found that the preferred orientation of the crystals making up the film is controlled mainly by the orientation of the seed crystals and by competitive growth during the film formation. Recently, compositionally zoned MFI films have been prepared [37]. Silicalite-1 films have been grown on ZSM-5 and vice versa. The aluminum content in the ZSM-5 part has been varied. It appears that truly zoned films, i.e. films with a continuously propagating channel system throughout the entire film, only form if the first layer is silicalite-1 or if the compositional difference between the ZSM-5 part of the film and the silicalite-1 film is small.

4. MACROSTRUCTURES Synthetic zeolites are normally produced as a crystalline powder. Prior to using the zeolites as e.g. catalysts and adsorbents, the powder is usually formed into agglomerates such as spheres, tablets and extrudates. Methods for forming the zeolite powder into agglomerates

1442 include the addition of non-zeolitic binders, generally various types of clays. The binders provide macro particles that conform to the process requirements of activity, pressure drop and attrition resistance. However, the binding additives may also affect the zeolite performance. For instance, since the binder is typically present in amounts of up to about 50 wt.% of zeolite, the binder reduces the adsorption capacity of the zeolite agglomerate. Also, during the preparative procedures, the binder can block the pores of the zeolite thus slowing the rate of mass transfer to and from the zeolite pores. Thus, the preparation of molecular sieve macrostructures (e.g., macroscopic structures with dimensions greater than 0.1 mm) is of great technological interest. Several approaches to prepare zeolite macrostructures have been reported in the scientific literature. Generally, molecular sieve macrostructures can be prepared by transforming shaped monoliths consisting of zeolite precursors into a zeolite using molecular structure-directing agents, e.g. ref. [37], or using other, non-zeolitic monoliths, that are removed after the synthesis. In the latter case, the monoliths are often composed of organic materials, e.g. 3D arrays of polystyrene spheres [38] or polyurethane foams [39], and the macrostructures obtained after the template removal are characterized by a controlled macroporosity. We developed a method for the preparation of self-bonded zeolite macrostructures, the resin templating method [40-42]. Conceptually, the method is based on the use of macroporous anion exchange resins as shape-directing macrotemplates. Anion exchange resins are supplied as spherical beads and are available in various particle sizes. The introduction of zeolite nutrients (e.g., negatively charged silica and alumina species that are present in the zeolite synthesis solutions) into these resins is facilitated by the ability of the macrotemplate to exchange anions. The permanent pore structure of the macroporous resins allows the zeolite crystallization to take place within the beads. In addition, being an organic material, the resin may easily be removed after the synthesis by calcination. Figure 2 represents a schematic illustration of the different stages of production of molecular sieve macrostructures by the resin templating method. Firstly, a mixture of resin beads and zeolite synthesis solution is hydrothermally treated. As a result of the treatment, zeolite is crystallized both in the bulk solution and within the resin. The resin-zeolite composite formed is separated from the zeolite crystallized in the bulk and as a final step the resin is removed by calcination leaving self-bonded zeolite spheres with a shape and size similar to the original resin beads.

9 9

Ion exchange resin

Hydrothermal synthesis

Calcination

~

Ion exchange resin microporous material

~Se!f-bonded body

Figure 2. Schematic illustration of the procedure for preparing self-bonded molecular macrostructures by the resin templating method.

1443 The resin templating method was firstly developed for the preparation of silicalite-1 macrostructures due to the relatively simple reaction system compared to the aluminum containing zeolites [40]. The method was then applied to synthesize zeolite macrostructures such as zeolite beta and ZSM-5 [41,42]. Figure 3a shows typical SEM images of molecular sieve-resin composite particles. These composite beads were similar in shape and size to the original resin beads. Generally, no shrinkage and change in appearance were observed upon removal of the ion exchanger (Fig. 3b). Depending on the synthesis conditions (e.g., synthesis time, synthesis solution, temperature, synthesis solution to resin weight ratio used) hollow spheres or spheres with a reduced size may be obtained as well [6]. The particles building up the sphere interiors always had a size of about 100 nm, which is comparable to the pore size of macroporous resins. Larger zeolite crystals were observed on the sphere surfaces. This is related to the fact that while the surface of the resin beads is open and exposed to the synthesis solution, the resin polymer chain restricts crystal growth within the resin to the size of the resin pores. Besides the self-bonded form achieved by the resin templating method, another advantage is the controlled dual pore structure of the spheres prepared by the method. The calcined macrostructures contain micropores emanating from the molecular sieve (or the amorphous silica that is present in the amorphous or semi-crystalline spheres) and mesopores resulting from the removal of the ion exchanger. The microporosity is dependent on the zeolite type and the ratio between crystalline and amorphous material present. The mesoporosity is predetermined from the resin polymer chains but is also dependent to a certain degree on the material formed within the resin beads. These pore structure features are exemplified in Fig. 4a showing three nitrogen adsorption isotherms, for silicalite-1, ZSM-5 and zeolite beta. The isotherms are all of type IV characteristic of mesoporous materials with a relatively steep increase at low pressures indicating a substantial microporosity. From the corresponding pore size distributions presented in Fig. 4b is seen that the ones for the MFI type materials are very similar (the two samples have similar degree of crystallinity), whereas the one for the zeolite beta is slightly different. Depending on the synthesis conditions and the amount of a high surface area amorphous material present, spheres with specific BET surface areas of up to 1000 mZg-1 were obtained.

O0 l.tm~l

Figure 3. SEM images of silicalite-l-resin composite beads (a) and the silicalite-1 spheres obtained after the removal of the resin (b).

1444 500

0.8

a

b A:

450 0.6

m~ 400 350 @ r,g3

0.4

300

~,!~7 ! iiii .:

250 @ 200 >.

150 -

~

_A_ZX_A_A-ZX-/X-ZXZXaxa-~"

.

0.0 I

0.0

i ,,

0.2

0.2

,

I

0.4

,

I

06.

,

Relative pressure

I

0.8

(p/po)

, , ,

'

,

1.0

10

, ,

,

i ,

, i

,,,l ,,1,

i

i

,

, ,

, , , , , , ,,,,

i i I

100

.

.

,

.

.

,

.

,

.

,,,

,

1000

Pore diameter (D)/A

Figure 4. Nitrogen adsorption isotherms of silicalite-1 (D), ZSM-5 (A) and zeolite beta (o) spheres (a); solid symbols, adsorption; open symbols; desorption; and the corresponding BJH pore size distributions (b). Employing gel type resins in the resin templating method results in the formation of a high surface area amorphous silica macrostructures. This is due to steric hindrance effects: the gel resins are characterized by an "apparent" porosity of no greater than 4 nm, which is not enough to allow zeolite crystal formation within those resins. Materials with very high surface areas, up to 1600 mZg-l,were prepared by the resin templating method using gel resins [43]. In a recent development of the method, a number of modified molecular sieve macrostructures were prepared. Firstly, molecular sieve-resin composite beads are prepared as previously described. In a second step, transition metal anions are introduced into the composite beads by ion exchange employing the residual ion exchange of the resin. Finally, the organic compounds are removed by combustion. The method allows the introduction of basically any transition metal providing that it is available in an anionic form. For instance, vanadium and tungsten containing silicalite-1 macrostructures of various crystallinity and transition metal content have been prepared and extensively characterized [44]. A major advantage of the method is that the transition metals are distributed throughout the molecular sieve spheres, a result of the presence of the three-dimensional resin chains responsible for the ion exchange. By adjusting the conditions for ion exchange (e.g., the pH of the transition metal solution, the solution concentration and solution to composite weight ratio as well as the properties of the molecular sieve material formed within the resin), materials of desirable metal loading and high surface areas may be obtained. In combination with the dual pore structure and controlled macro shape, this makes the modified molecular sieve macrostructures interesting materials for catalytic applications.

1445 5. A P P L I C A T I O N S The main application areas for structured molecular sieve materials are membranes, catalysts and sensors. In cooperation with other research groups, a few of our films have been tested in sensor applications [45,46]. Recently, a greater fraction of our work, briefly described below, has been devoted to zeolite membranes and catalysis applications. ZSM-5 films with high aluminum content have been explored in membrane applications. The zeolite was crystallized in the absence of template molecules and calcination was thus not necessary. Crack formation during calcination could thus be avoided [47]. However, we found that cracks still developed in the membranes at the elevated temperatures needed to dry the membrane [48]. By applying a support masking technique, and grow thin (500 nm) silicalite-1 films, very good membranes with extremely high flux, higher than previously reported, could be prepared with high reproducibility [49,50], see Table 3. Molecular sieves and zeolites are currently used in numerous catalytic applications taking advantage of the unique properties of these materials. Considering this, and the general interest in structured materials, it is not surprising that a significant effort currently is being devoted to the development of structured molecular sieve catalysts. A number of research groups in academia as well as industry around the world are presently active in the development of structured molecular sieve materials primarily for use as catalytic or gas separation membranes and other types of structured catalysts such as catalytic distillation packing materials but also for a number of other technologically sophisticated applications such as films on conventional packing materials and zoned zeolite crystals. There is a number of possible ways to utilize molecular sieves in novel applications such as structured catalysts as illustrated schematically in Figure 5. In cooperation with the Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, South Africa, we are presently studying ZSM-5 films in catalysis applications. Catalysts configured as in a) and c) in Figure 5 are explored. Test reactions are p-xylene isomerisation and triisopropylbenzene cracking. The first results show that defects such as cracks or grain boundaries in the zeolite film may play a major role in these materials. Defects may have a positive effect in a) since they increase the effective diffusivity in the film and thus increases the activity of the catalyst. Defects are not acceptable in c) if they provide additional pathways in the material and thus reduces or eliminates the effect of the outer coating. However, it seems as if the film is thin enough, defects are not formed.

Table 3. Permeation data from mixtures of butanes, hexanes or xylenes for silicalite-1 membranes. The permeance of the favored component is given in 10 -l~ m o l / ( m Z s 9Pa) for each system. System Permeance a Temperature/~ n-butane/iso-butane

9800

9.0

25

n-hexane/2,2-dimethyl-butane

5600

227

390

para-xylene/ortho-xylene

3000

16

390

1446 a

b

c Carrier Zeolite Catalyst

Figure 5. A carrier material (inert or catalyst) can be coated with an inert or catalytically active zeolite film (a), carrier material with catalytic coating (e.g. metal) can be coated with a inert or catalytically active zeolite film (b) or carrier material can be coated with a zoned zeolite film with varying composition i.e. framework aluminum content (c).

6. CONCLUSIONS Structured molecular sieve materials are of great interest in a number of application areas. The potential for eliminating limitations of materials presently used by tailored design of molecular sieve materials for specific applications is considerable. Often, conceptual possibilities are identified but the preparative methods needed for practical realization are lacking. Such methods are however currently being developed concurrently with an increasing understanding of the fundamental mechanisms directing the outcome of preparations of molecular sieve based materials. Thus, methods for the controlled preparation of molecular sieve crystals, films, macrostructures and composite materials with desired properties have been developed during the last decade. At Ltu we have focused our activities at this area of development and made some contributions to the recent progress. Efforts in academia as well as in industry within this area are increasing This is a fact that warrants a further development towards new and exiting materials in years to come.

REFERENCES 1. A. Cybulski and J.A. Moulijn (eds.), Structured catalysts and reactors, Marcel Dekker Inc., New York, 1998. 2. S. Qiu, J. Yu, G. Zhu, O. Teresaki, Y. Nozue, W. Pang and R. Xu, Microporous Mesoporous Mater., 21 (1998) 245. 3. L. Scandella, G. Binder, T. Mezzacasa, J. Gobrecht, R. Berger, H. P. Lang, C. Gerber, J. K. Gimzewski, J. H. Koegler and J. C. Jansen, Microporous Mesoporous Mater., 21 (1998) 403. 4. H. Manstein, K. P. M/511er, W. B6hringer and C. T. O'Connor, Microporous Mesoporous Mater., 51 (2002) 35. 5. J. Caro, M. Noack and P. K61sch, Microporous Mesoporous Mater., 22 (1998) 321. 6. B.J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites, 14 (1994) 208. 7. B.J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites, 14, (! 994) 110. 8. A.E. Persson, B. J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites 14 (1994) 557.

1447 9. M.H. Anthonis, A. J. Bons and J. P. Verduijn, PCT WO 97/25272 (1997). 10. Q. Li, D. Creaser and J. Sterte, Microporous Mesoporous Mater., 31 (1999) 141. 11. G. Zhang, J. Sterte, B. Schoeman, J. Chem. Soc. Chem. Commun, (1995) 2259. 12. G. Zhang, J. Sterte and B. Schoeman, Chem. Mater., 9 (1997) 210. 13. A. E. Persson, B. J. Schoeman, J. Sterte and J.-E. Otterstedt, Zeolites 15 (1995) 611. 14. B. J. Schoeman, J. Sterte and J.-E. Otterstedt, J. Colloid Interface Sci. 170 (1995) 449. 15. Q. Li, D. Creaser and J. Sterte, accepted for publication in Chem. Mater. 16. J. P. Verduijn, M. M. Mertens and M. H. Anthonis, PCT WO 97/03021 (1997) 17. J. P. Verduijn, PCT WO 97/03019 18. M. A. Camblor, A. Corma, A. Mifsud, J. P6rez-Pariente and S. Valencia, in: Chon et al. (Eds.) Progress in Zeolite and Microporous Materials, Studies in Surface Science and Catalysis, vol 105A, p.341. Elsevier Science, Amsterdam, 1997. 19. B. J. Schoeman, E. Babouchkina, S. Mintova, V. Valtchev and J. Sterte, J. Porous Materials, 8 (2001) 13. 20. W. Xu, J. Dong, J. Li, J. Li, F. Wu, J. Chem. Soc. Chem. Commun. (1990) 755. 21. M. Matsukata and E. Kikuchi, Bull. Chem. Soc. Jpn. 70 (1997) 2341. 22. J. Coronas and J. Santamaria, Catalysis Today 51 (1999) 377. 23. J. Sterte, J. Hedlund and B. J. Schoeman, US Patent No. 6 177 373 (2001). 24. J. Hedlund, B. J. Schoeman, J. Sterte, In: H. Chon, S.-K. Ihm., Y. S. Uh (eds.) Progress in Zeolites and Microporous Materials, Studies in Surface Science and Catalysis, vol 105, p. 2203, Elsevier Science, Amsterdam, 1997. 25. V. Valtchev, B. J. Schoeman, J. Hedlund, S. Mintova and J. Sterte, Zeolites 17 (1996) 408. 26. V. Engstr6m, B. Mihailova, J. Hedlund, A. Holmgren and J. Sterte, Microporous Mesoporous Mater. 38 (2000) 51. 27. Z. Wang, J. Hedlund and J. Sterte, Microporous Mesoporous Mater., in press. 28. S. Mintova, J. Hedlund, B. J. Schoeman, V. Valtchev and J. Sterte, Chem. Commun. 15 (1997). 29. S. Mintova, J. Hedlund, V. Valtchev, B. Schoeman and J. Sterte, J. Mater. Chem. 7 (1997) 2341. 30. Q. Li, J. Hedlund, D. Creaser and J. Sterte, Chem. Commun. 7 (2001) 527. 31. J. Hedlund, B. Schoeman and J. Sterte, Chem. Commun. (1997) 1193. 32. M. Lassinantti, J. Hedlund and J. Sterte, Microporous Mesoporous Mater. 38 (2000) 25. 33. Z. Wang, J. Hedlund and J. Sterte: Synthesis of FAU type films on steel supports using a seeding method, in: A. Galarneau, F. Di Renzo, F. Fajula and J. Vedrine (Eds.), Studies in Surface Science and Catalysis, Zeolites and mesoporous materials at the dawn of the 21st century, Elsevier Science, Amsterdam, in press. 34. J. Hedlund, S. Mintova and J. Sterte, Microporous Mesoporous Mater. 28 (1999) 185. 35. J. Hedlund, J. Porous Mater. 7 (2000) 455. 36. S. Mintova, J. Hedlund, V. Valtchev, B. Schoeman and J. Sterte, J. Mater. Chem. 10 (1998) 2217. 37. I. Kiricsi, S. Shimizu, Y. Kiyozumi, M. Toba, S. Niwa and F. Mizukami, Microporous Mesoporous Mater., 21 (1998) 453. 38. B. T. Holland, L. Abrams and A. Stein, J. Am. Chem. Soc., 121 (1999) 4308. 39. Y.-J. Lee, J. S. Lee, Y. S. Park and K. B. Yoon, Adv. Mater., 13 (2001) 1259. 40. L. Tosheva, V. Valtchev and J. Sterte, Microporous Mesoporous Mater., 35-36 (2000) 621.

1448 41. L. Tosheva, B. Mihailova, V. Valtchev and J. Sterte, Microporous Mesoporous Mater., 48 (2001) 31. 42. L. Tosheva and J. Sterte, proceedings of the 2n~ FEZA conference, Giardini Naxos, Italy, 1-5 September 2002. 43. L. Tosheva, V. Valtchev and J. Sterte, J. Mater. Chem, 10 (2000) 2330. 44. L. Tosheva and J. Sterte, Chem. Commun. 1112 (2001) 45. S. Mintova, B. Schoeman, V. Valtchev, J. Sterte, S. Mo and T. Bein, Adv. Mater. 9 (1997) 585. 46. R. Bjorklund, J. Hedlund, J. Sterte and H. Arwin, J. Phys. Chem. B 102 (1998) 2245. 47. J. Hedlund, M. Noack, P. K61sch, D. Creaser, J. Sterte and J. Caro, J. Memb. Sci. 159 (1999) 263. 48. M. Lassinantti, F. Jareman, J. Hedlund, D. Creaser and J. Sterte, Catalysis Today, 67 (2001) 109. 49. M. H. Anthonis, A. J. Bons, H. W. Deckman, J. Hedlund, W. F. Lai and J. A. J. Peters, International patent application WO 00/53297 (2000). 50. M. H. Anthonis, A. J. Bons, H. W. Deckman, J. Hedlund, W. F. Lai., J. A. J. Peters, International patent application WO 00/53298 (2000).


1449

C h r o m i u m c o n t a i n i n g zeolite beta m a c r o s t r u c t u r e s Valeri Naydenov, Lubomira Tosheva and Johan Sterte

Division of Chemical Technology, Lulegt University of Technology, S-97187 Lule&, Sweden luto @km. luth.se Chromium containing zeolite beta macrostructures were prepared by a multi-step procedure employing macroporous anion exchange resin beads as shape-directing macrotemplates. In a first step, zeolite beta was crystallized in the pores of the ion exchanger. Secondly, chromium oxoanions were introduced into the resin-zeolite beta composites thus obtained using the residual ion exchange capacity of the resin. Finally, the resin was removed by calcination leaving behind chromium containing zeolite beta spheres. Spheres of various chromium content were synthesized and characterized by AAS, XRD, EDS, UV-vis DRS spectroscopy and nitrogen adsorption measurements. The properties (chromium loading, crystallinity, pore structure) of the Cr-beta spheres prepared were dependent on the starting resin-zeolite composites as well as on the chromium solution to composite weight ratio used for the Cr ion exchange. 1. INTRODUCTION Chromium based catalysts are generally composed of finely divided chromia supported on a carrier such as alumina and silica. The nature of the chromium species in these catalysts is dependent on the support, pretreatment conditions and chromium loading. Chromia supported catalysts are active in ethene and propene polymerization, dehydrogenation reactions such as n-butane to butene, propane to propene, i-butane to i-butene as well as for selective catalytic reduction of NOx [1-13]. The catalysts used in paraffin dehydrogenation and ethylene polymerization are commonly prepared by the incipient impregnation method [1-7]. Molecular sieves are often used as catalyst supports due to their well-defined pore architectures, high surface areas and high thermal stability. Molecular sieve and mesoporous materials, in which chromium was directly incorporated into the support during the synthesis, have been prepared as oxidation reaction catalysts [8-13]. A major disadvantage of using molecular sieves as supports is that they are generally produced as powders, which are difficult to handle and further processing is necessary to produce macroparticles such as pellets and spheres, generally by mixing the zeolites with amorphous binders. The binding additives, however, dilute the adsorption properties of the zeolite and cause diffusion limitations. Recently, a method for producing molecular sieves in the form of spheres using an ion exchange resin as a shape-directing macrotemplate was reported [14,15]. In this method, zeolites are crystallized in the pores of ion exchange resin beads and upon removal of the resin, zeolite spheres with a shape and size similar to the original resin beads are obtained. In this contribution we report on the preparation of chromium containing zeolite beta spheres by

1450 the resin templating method. Chromium anionic species are introduced into the resin-zeolite beta composite spheres using the residual ion exchange capacity of the resin.

2. EXPERIMENTAL SECTION A batch of resin-zeolite beta composite spheres was prepared according to the previously described procedure [14] using a macroporous strongly basic resin (Dowex MSA-1, chloride form, bead size distribution 0.3-1.2 mm). A 0.010M Na2Cr2OTo2H20 (Merck, 99%) solution was employed to introduce anionic chromium species into the composite particles. Two series of experiments were performed utilizing as-synthesized composites (assuming that the resin in these composites is in the OH- form) and composites, in which the resin was converted into a C1- form by passing a 10 wt.% NaC1 (Riedel-de Hahn, >99.8%) solution through an ionexchange column loaded with the composites [16]. The dichromate solution was added to resin-zeolite beta composites dried at room temperature in a beaker at three different solution to composite weight ratios, namely 10, 30 and 50, and the corresponding samples were designated as BetaCrx(OH or C1), where x is equal to 10, 30 or 50. The beakers were placed on a shaker for 48 h. After the ion exchange, the chromium containing composites were separated, rinsed with distilled water, dried at room temperature and calcined at 600~ for 6 h, after heating to this temperature at a rate of 1~ min -~. The chromium content in the zeolite beta spheres was determined by flame atomic absorption spectrometry (AAS, Perkin-Elmer 3100) after fusing the samples with LiBO2 according to the procedure described in ref. [17]. X-ray diffraction (XRD) patterns were collected with a Siemens D5000 powder diffractometer using Cu K~ radiation. Relative crystallinity was evaluated from the area of the most intense zeolite beta reflection peak, which appears at 20 = 22.5 ~ The most crystalline sample (pure zeolite beta spheres obtained from resin-zeolite beta composites in an OH- form) was used as a standard sample for these calculations. UV-vis DRS diffuse reflectance spectra (UV-vis DRS) were obtained with a Perkin-Elmer Lambda 2 UV-vis spectrometer equipped with a Labsphere RSA-PE 20 Reflectance Spectroscopy Accessory and operating in a single beam mode. A white SRS-99 standard reference material was used for a background correction. Calcined spheres were ground into a powder prior to the XRD and UV-vis DRS spectroscopy study. A scanning electron microscope (SEM), Philips XL 30 equipped with a LaB6 emission source, was used to study the morphology of the samples. The distributions of chromium in the zeolite beta spheres was studied by line-scan energy dispersive spectroscopy (EDS) using a Link ISIS Ge energy dispersive X-ray detector. Flat and polished cross sections of the spheres were prepared prior to these measurements by embedding the chromium spheres in an epoxy resin (Epofix, Struers). Nitrogen adsorption desorption measurements were performed with a Micromeritics ASAP 2010 instrument after degassing calcined spheres at 300~ overnight. Specific surface areas were calculated with the BET equation and total pore volumes were obtained by converting the amount adsorbed at relative pressure of 0.995 to the volume of liquid adsorbate. Pore-size distributions were determined by the BJH method (desorption isotherm).

1451 Table 1. Properties of the calcined pure zeolite beta and chromium containing spheres. Sample

Cr content (wt%)

Degree of crystallinity

BET surface area, SBET

Total pore volume, Vp

Beta (OH) Beta (C1) BCrl0(OH) BCrl0(C1) BCr30(OH) BCr30(C1) BCr50(OH) BCr50(C1)

0.7 1.2 2.4 2.9 3.5 4.3

100 70 81 69 65 67 60 60

624 666 624 651 588 607 582 614

0.64 0.73 0.79 0.79 0.75 0.74 0.79 0.76

(%)

(m2~ -1)

(cm3g -l)

3. R E S U L T S A N D D I S C U S S I O N

Visually, the calcined chromium containing zeolite beta spheres were yellowish or greenish colored depending on the chromium content. SEM analysis showed no changes in the appearance of the spheres due to the introduction of chromium and the particles were similar in size and shape to the original resin beads (not shown, see ref. [ 15]). No changes in the morphology of the particles building up the sphere were observed by SEM. The chromium content in the samples increased with an increase in the chromium solution to the resin-zeolite composite ratio (Table 1). Higher chromium loadings were obtained using

C

rae3 r .4,,,a

20

40

60

20 degrees

80

Figure 1. XRD patterns of calcined BCrl0(OH) (a), BCr30(OH) (b) and BCr50(OH) (c) samples and a reference Cr203 XRD pattern (d).

1452

1.0 ,

,

,

,

, , ,

,

,

,

,

, , ,

: : : : :: :i~+: : ::

"

,;,'+i i: i!i

----o---BCrl0(OH) ----+---Beta(C1)

-,,-BC

lO(C1)

r162

~,0.5

e

-

.:~++_r,t,/ii:',::,~

~..

iz~:~,:,iiix

. ~ '

0.0

,

10

,

::::::

~o. ii

%

: :::::: : :iiiii .

.

.

.

.

.

.

\ct~

,

100 Pore Diameter

1000 (D)/A

Figure 2. BJH desorption pore size distributions of calcined zeolite Beta and Cr-Beta spheres. composites in a C1- form. XRD analysis showed that the crystallinity of the chromium samples decreased compared to pure zeolite beta spheres with the decrease being higher for the samples of higher chromium content (Table 1). An interesting observation is the inferior crystallinity of the Beta(C1) spheres compared to Beta(OH). Considering the fact that the conversion of the resin into a C1- form was performed in the presence of zeolite in the pores of the resin, there might be a certain degree of amorphization of the zeolite during the ion exchange. The higher surface area and pore volume for this sample were in accordance with this suggestion (Table 1). Fig. 1 shows XRD patterns of the three BetaCr(OH) samples. By increasing the chromium content the intensity of the reflection peaks of zeolite beta decreased and peaks corresponding to Cr203 appeared. The Cr203 peaks are somewhat difficult to distinguish because of their closeness to the zeolite beta peaks. Nevertheless, the above statement is best exemplified by the Cr203 peak at 20 = 36.2 ~ which can barely be seen in the XRD pattern of BCrl 0(OH) and which is of comparatively high intensity in the XRD pattern of BCr50(OH). The BET surface areas of the chromium containing zeolite beta spheres were lower compared to pure zeolite beta samples (Table 1). On the other hand, total pore volumes were higher. The nitrogen adsorption isotherms recorded for all spheres were of type IV with a substantial microporosity (not shown, see ref. [ 15]). The mesopores present are related to the removal of the ion exchanger whereas the micropores are due to the presence of zeolite beta and high surface area amorphous material. Differences were observed in the BJH pore size distributions of the zeolite spheres prepared. Firstly, by comparing the pure zeolite beta samples it can be noted that the broad mesopore size distribution centered at 400 * for Beta(OH) becomes narrower and shifts to the left (50-60*) for the Beta(C1) sample. The

1453

0

'

'2"0'0

'

'

400 '

'

length, gm

'

'6"0'0

Figure 3. Typical SEM image of a cross-sectioned Cr-Beta sphere (a) and typical EDS line scan analysis of chromium over it (b). changes in the pore structure of Beta(C1) are related to the conversion of the resin in the resinzeolite composites into a C1- form. However, the resin-zeolite composite is a very complex system and the exact reasons for the changes observed are not clear. Pore-size distributions for the chromium containing spheres were similar with a substantial part of the pore volume found in mesopores in the range 80-100 * (Fig. 2). The distribution curves were broader and shifted to the right compared to the one for Beta(C1) and narrower and left-shifted compared to the Beta(OH) one. Again, the reasons for these changes in the pore structure of the chromium containing samples are unclear. The changes are more likely related to changes in the resin polymer chains during the chromium ion exchange. Thus, the type of chromium species exchanged might differ when starting with resin-zeolite composites in an OH- and in a C1- form. It is well known that in Cr(VI) solutions there is an equilibrium between chromate and dichromate ions which is very sensitive to the pH value [1]. The equilibrium is shifted towards chromate ions with an increase of pH. Such an increase occurs when the ion exchange is performed with composites in the OH- form due to the release of OH- ions into the solution. However, a further discussion in this direction would be highly speculative. Although the material appeared homogeneous upon grinding, EDS line scan analysis was used to further investigate the chromium distribution within the zeolite spheres. Figure 3 shows a SEM micrograph of a cross-sectioned sphere (a) and the EDS line scan analysis of chromium over it (b). Chromium was relatively evenly distributed across the sphere. Similar results were obtained for all the chromium spheres.

1454

f

9

d

,

200

I

300

,

I

400

500

600

700

800

Wavelength/nm Figure 4. UV-vis DRS spectra of calcined Cr-beta samples: BCrl0(OH) (a), BCrl0(C1) (b), BCr30(OH) (c), BCr30(C1) (d), BCr50(OH) (e), BCr50(C1) (f).

Further, UV-vis spectroscopy was used to study the chromium species in the chromium containing zeolite beta spheres. Generally, chromate species have absorption bands at 275 and 375 nm, dichromate- at 275, 322 and 445 nm and pseudo-octahedral Cr 3+ - at 625 nm (the Cr 3+ bands at 295 and 465 nm are overlapping with the stronger chromate and dichromate bands) [1,18]. Figure 4 shows UV-vis DRS spectra of the calcined chromium containing zeolite beta spheres. In the spectrum of BCrl0(OH) (Fig 4a) bands at 265, 302, 355, 445 and a very weak band at 600 nm are present. The bands may be associated with the presence of grafted chromates (the bands at 265 and 355 nm) and dichromates (445 nm) and Cr203 (302, 600 nm). By increasing the chromium content the amount of surface dichromates and Cr203 is increased (the bands at 445 and 600 nm, respectively) whereas the surface chromates are decreased (265, 355 nm). The spectra of the chromium samples prepared with the OHcomposites and the Cl-composites were similar and no conclusion about the influence of the counter ion of the resin-zeolite composite can be drawn. 4. CONCLUSIONS Chromium containing zeolite beta spheres were prepared using macroporous anion exchange resin as a shape directing macrotemplate. Firstly, resin-zeolite beta composites were synthesized by crystallizing the zeolite into the resin pores. Chromium was then introduced into the composites as oxoanions using the residual ion exchange capacity of the resin. Finally, the resin was removed by calcination leaving behind chromium containing zeolite beta spheres. The chromium content in the spheres was dependent on the chromium solution to resin-zeolite composite weight ratio as well as on the resin form in the composites OH- or

1455 C1-. The crystallinity of chromium containing samples was inferior compared to pure zeolite beta spheres. The pore structure of the materials prepared consisted of both micropores (the zeolite) and mesopores (from the removal of the resin) with features dependent on the pretreatment conditions. A major advantage of the method reported is that the materials prepared may be directly used as catalysts in e.g. fixed-bed reactors and no further processing to form macroparticles is necessary thus avoiding deterioration effects due to the addition of binders in conventionally prepared zeolite supported catalysts. This makes the chromium containing zeolite beta spheres prepared according to the procedure potentially beneficial oxidation catalysts. ACKNOWLEGMENTS The partial financial support from the Swedish Research Council for Engineering Sciences (VR) is gratefully acknowledged. REFERENCES 1. 2. 3. 4.

B.M. Weckhuysen, I. E. Wachs, R. A. Schoonheydt, Chem. Rev. 96 (1996) 3327. B.M. Weckhuysen, R. A. Schoonheydt, Catal. Today 51 (1999) 223. A. Hakuli, M. E. Harlin, L. B. Backman, A. O. I. Krause, J. Catal. 184 (1999) 349. F. Cavani, M. Koutyrev, F. Trifiro, A. Bartolini, G. Ghisletti, R. Iezzi, A. Santucci, G. Del Piero, J. Catal. 158 (1996) 236. 5. B.M. Weckhuysen, D. Wang, M. P. Rosynek, J. H. Lunsford, J. Catal. 175 (1998) 338. 6. B.M. Weckhuysen, D. Wang, M. P. Rosynek, J. H. Lunsford, J. Catal. 175 (1998) 347. 7. A.V. Salker, W. Weisweiler, Appl. Catal. A: General 203 (2000) 221. 8. Z. Zhu, Z. Chang, L. Kevan, J. Phys. Chem. B 103 (1999) 2680. 9. D. Escalante, L. Giraldo, M. Pinto, C. Pfaff, V. Sazo, M. Matjushin, B. Mendez, C. M. Lopez, F. J. Machado, J. Goldwasser, M. M. Ramirez de Agudelo, J. Catal. 169 (1997) 176. 10. S. Yuvaraj, M. Palanichamy, V. Krishnasamy, Chem. Commun. (1996) 2707. 11. N. van der Puil, Widyawati, J. C. Jansen, H. van Bekkum, in: J. Weitkamp, H. Karge, H. Pfeifer, W. H/51derich (eds.), Zeolites and Related Mesoporous Materials: State of the Art, Amsterdam, Elsevier, 1994, p. 211. 12. T. Chapus, A. Tuel, Y. Ben Taarit, C. Naccache, Zeolites 14 (1994) 349. 13. M. Selvam, M. P. Vinod, Appl. Catal. A: General 134 (1996) L 197. 14. L. Tosheva, V. Valtchev, J. Sterte, Micropor. Mesopor. Mater. 35-36 (2000) 621. 15. L. Tosheva, B. Mihailova, V. Valtchev, J. Sterte, Micropor. Mesopor. Mater. 48 (2001) 31. 16. C. E. Harland, Ion Exchange: Theory and Practice, 2 ed., The Royal Society of Chemistry, Cambridge, 1994, pp. 63-75. 17. C. O. Ingamells, Talanta, 11 (1964) 665. 18. B. M. Weckhuysen, A. A. Verberckmoes, A. R. De Baets, R. A. Schoonheydt, J. Catal. 166 (1997) 160.



1457

Semiconductor nanoparticles in the channels of mesoporous silica and titania thin films Michael Wark l, Hartwig Wellmann 1, Jiri Rathousk~, 2 and Arno~t Zukal 2 1Institute of Applied and Physical Chemistry, University of Bremen, FB 2 - Chemistry, D-28334 Bremen, Germany, e-mail: [email protected]. 2 j. Heyrovsk~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, CZ-18223 Prague, Czech Republic, e-mail: [email protected]. A procedure for the preparation of porous silica thin films was developed, which is based on the use of block copolymers as structure directing agents and the application of the dipcoating technique. The prepared films exhibit an uniform thickness and a large roughness factor indicating the highly developed porosity. The addition of C d 2+ ions to the reaction gel for silica films prior to their formation, followed by the calcination of films at 623 K and treatment with HzS or HzSe is the preferred procedure for the creation of CdS or CdSe nanoparticles. The occurrence of the size-quantization effect confirms that the particles are located within the pores and that their size corresponds to the pore width of the host. The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. Well-organized film is formed only from a completely hydrolyzed precursor. The photocatalytic activity of such organized film is comparable with that of the most active commercial anatase powders. 1. INTRODUCTION The most important feature of ordered mesoporous materials is their ability to form thin films [1]. The motivation for synthesis of mesoporous thin films originates from the appreciation of their technological potential as membranes, sensors, surfaces for heterogeneous catalysis and insulating layers. Pore accessibility, which is of fundamental importance in almost all applications, is highly enhanced in structures which possess threedimensional symmetries, e.g. bicontinuous cubic. Such structures can be obtained using block copolymers as structure-directing agents [2]. The additional aspect of mesoporous films is the possibility to use them as a hosting matrix for nanoparticles, which opens a new field of application, relying on quantum size effects of the confined particles. First syntheses of nanoparticle loaded silica mesoporous films have been reported recently dealing with SiGe [3], Si [4] and Ag [5] nanoparticles. Although most of the published experimental research has focused on silica as the inorganic framework constituent, there is a major stimulus behind the research into the nonsilica mesoporous films, especially titanium dioxide, namely its potential for converting light to electrical energy or chemical energy by its solar-driven band gap excitation. The application of the reaction schemes originally developed for siliceous materials has been found much less successful for the synthesis of mesoporous transition metal oxides due to their facile crystallization and subsequent grain growth, which leads to the loss of the original

1458 mesoporous structure. However, recently a novel approach has been developed, enabling to obtain mesoporous titanium dioxide with highly interesting structural properties based on the usage of amphiphilic poly(alkylene oxide) block copolymers as structure directing agents in non-aqueous solution for organizing the network-forming titanium dioxide species [6]. Only very recently first successful preparations of mesoporous titania films have been reported [7,8]. In the present communication, the loading of silica mesoporous thin films with nanoparticles of CdS and CdSe is reported for the first time. Further the above mentioned synthetic approach for the preparation of titania films will be analyzed with respect to the effects of decisive processing parameters and it will be shown that thus prepared materials can be effectively used in the photocatalytic destruction of an important organic water pollutant, viz. 4-chlorophenol.

2. MATERIALS AND METHODS 2.1.

Materials

2.1.1. Preparation of silica mesoporous thin films loaded with CdS and CdSe A typical synthesis using the surfactant Brij 56 (Aldrich, C16H33(C2H40)10H) was performed as follows. 8.35 g of tetraethylorthosilicate (TEOS) were dissolved in 16 g ethanol, followed by 4 g of 0.1 M HC1 and the sol was heated under reflux for 1 hour. After the solution had cooled to ambient temperature, 3 g of Brij 56 dissolved in 16 g ethanol were added under vigorous stirring and the mixture was allowed to age at ambient temperature for 2 hours. Subsequently, the viscous mixture was used for dip-coating quartz glass or silicon wafers 3 cm • 3 cm in size, which had been previously cleaned with acetone, at a constant velocity of 1 m m s -1. After drying at ambient temperature for 1 hour, the films were calcined in air at 350~ for 2 hours (heating rate: 1~ rain-l). With surfactants PE 9400 (BASF, (C2H40)21(CH(CH3)CH20)47(C2H40)21H) and cetylpyridinium chloride (Aldrich) the compositions of the reaction mixtures were different. With PE 9400, 8.35 g of TEOS dissolved in 16 g ethanol was mixed with 7.2 g of 0.1 M HC1. After heating under reflux for 1 hour and cooling to ambient temperature, 4 g of PE 9400 in 24 g ethanol were added. For the preparation of films with cetylpyridinium chloride, 8.7 g of TEOS, 2.22 g of 0.1 M HC1 and 3 g of cetylpyridinium chloride were dissolved in 36 g ethanol and heated under reflux for 20 h. After cooling to ambient temperature, thus prepared reaction mixtures were used for dip-coating the wafers. In some cases, 0.56 g of cadmium acetate were added to the surfactant solutions in order to incorporate Cd 2+ ions. Alternatively, Cd 2+ ions were introduced by impregnating the silica films with a methanolic solution of cadmium acetate, whose concentration was adjusted according to the desired loading, which was between 1 and 10 wt.% Cd. Cd 2+ ions were transformed into CdS or CdSe nanoparticles by a treatment with H2S or H2Se, respectively. Prior to and after the treatment all the physisorbed species were removed by evacuation. 2.1.2. Preparation of mesoporous titanium dioxide using block copolymers First, 0.9 g of Pluronic P-123 (BASF) were dissolved in 11 mL of ethanol. To this solution, 1 mL of titanium tetrachloride was added under vigorous stirring. The mixture was maintained in an open beaker at 40~ for 5 days, the evaporated ethanol being filled up every 12 h. Thus prepared clear yellowish solution could be stored at room temperature for several

1459 weeks without apparent changes. Films of different thickness were prepared by spreading various amounts of the stock solution on the glass support. The liquid layer was subsequently gelled in air at 40~ for 7 days and calcined at 400~ for 5 h in air. Finally, the titania films were peeled off the support (samples I-A, I-B, I-C, I-D). 2.2.

Measurements The thickness of the films was determined by a Veeco DEKTAK 3030ST profilometer and a NT-MDT Smena B atomic force microscope. Scanning electron micrographs were obtained by a Hitachi S-900 apparatus. Powder X-ray diffraction data were collected with a Siemens D 5005 diffractometer in the Bragg-Brentano geometry using CuKc~ radiation. Adsorption isotherms of nitrogen and krypton were measured at -196~ with an ASAP 2010 instrument (Micromeritics). FT-IR, UV/Vis and X-ray photoelectron spectra were measured by Biorad FTS-60A, Varian Cary and Physical Electronics PHI 5600 spectrometers. Raman spectra were measured using a T64000 spectrometer (Instruments, SA, France) equipped with an Olympus BH2 microscope. Photocatalytic activity of the TiO2 samples was studied using 4-chlorophenol as model pollutant. Photodegradation of this compound was examined employing a tube photoreactor where TiO2 was dispersed in water. After illumination, 4-chlorophenol follows three separate reaction pathways: hydroxylation, substitution and direct charge-transfer oxidation forming 4-chlorocatechol, hydroquinone and non-aromatic compounds as primary intermediates, respectively. The reaction rate was calculated according to the first-order kinetics.

3. RESULTS AND DISCUSSION 3.1.

Mesoporous films with embedded CdS and CdSe Using all the three mentioned templates, i.e. block copolymers Brij 56 and PE 9400 as well as the ionic cetylpyridinium chloride, thin films on quartz glass or Si substrates were obtained by dip-coating. To ensure the homogeneity of the films, the presence of some amount of solvent (ethanol) was important, the thickness of the films being varied between 50 and 800 nm by evaporating different amounts of the solvent in a rotary evaporator. The thickness of the films was determined by profilometrv, which enabled to measure areas of about 1 cm 2. Consequently, the step in the height l~etween film and wafer was determined with a higher accuracy than by atomic force microscopy, allowing to scan only an area of about 10 lam2. The morphology of the films was determined with a high precision by AFM. Fig. 1 shows a typical AFM micrograph of a 300 nm thick film, which was synthesised with the template Brij 56 and was supported on a Si wafer. The roughness over the measured area of 2 ~tm2 was very small, achieving only + 7 nm. Scanning over more extensive parts (about 300 ~tm x 300 ~tm in size) ensured that the roughness obtained was typical of the whole film. Therefore, the formation of islands of particles was ruled out. The observed unevenness, however, also demonstrated that the films were not formed as a single entity but consisted of a large number of individual closely packed structures, whose diameter was between 100 and 200 nm. The characterization of the regularity of the pore structure of the films by X-ray diffraction was difficult due to the very small amount of the scattering material. Nevertheless, reflections with relatively sharp maxima at 20 = 1.4 and 1.6 ~ were found.

1460

nm 14

lO 8

0,01

6

1,0x,

,.-..-,f 1,8

0,13"".

~lWj~3f

0,6

pm

),8 ,

1,2

pm

1,4

1,6

4 2 o

0.4

o,o~;- ~

Figure 1. Atomic force micrograph of a 300 nm thick mesoporous SiO2 film prepared using Brij56 as a structure-directing agent.

Basic data about the porosity of films were obtained by the analysis of Kr adsorption isotherms. From the adsorbed amount of Kr and the geometrical area of the film of about 0.5 cm 2 a roughness factor of about 40 was calculated. Since AFM also for this films showed only a variation of + 10 nm in the surface topology, the high roughness factor demonstrates the presence of a porosity within the film. The N2 adsorption isotherm on the corresponding powdered sample exhibited a strong increase up to p/p0 ~ 0.25, which indicates the presence of a distinct amount of small mesopores with a diameter around 2 nm. The analysis of the isotherm gives a BET surface area of 915 cm2/g and a mesopore volume of 0.432 cm3/g. The measured values in combination with the results of X-ray diffraction clearly demonstrate the formation of silica materials with structured mesopores both in powders and films. Since no Coulomb interactions of cations with the matrix are possible in silica films, postsynthetic loading with Cd-species can only be achieved by impregnating with a suitable cadmium salt. These procedure, however, led to an inhomogeneous distribution of Cd 2§ ions as well as CdS or CdSe nanoparticles within the film. Fig. 2b shows a film obtained by dropping a solution of cadmium acetate in methanol on the film, drying and precipitation of CdSe by H2Se. The dark spots show CdSe particles, which are mainly concentrated at the edge of the film. If the film was impregnated by dip-coating, a more homogeneous distribution of the Cd-salt was achieved (Fig. 2c), but the concentration of the salt could hardly be controlled. The addition of Cd 2§ ions to the reaction gel prior to the formation of films enabled to obtain films, which very homogeneously tumed their color to orange after treatment with H2Se (Fig. 2d). Except for the intense color there are no deviations from the colorless films which were prepared without addition of cadmium acetate to the synthesis gel (Fig. 2a). With templates PE 9400 and cetylpyridinium chloride, the addition of cadmium salt led to the fast formation of a solid gel, from which film preparation by dip-coating was no longer possible.

1461

Figure 2. Photographs of parent and CdSe-loaded mesoporous SiO2 films; unloaded transparent 300 nm thick parent film (a), film with CdSe particles, introduction of Cd 2+ ions by impregnation with a droplet of a cadmium acetate solution (b), film with CdSe particles, introduction of the Cd 2+ ions by dip-coating from a cadmium acetate solution (c) and film with homogeneously distributed CdSe nanoparticles, introduction of the Cd 2§ ions directly to the synthesis gel prior to dip-coating (d). The latter exhibits a homogeneous bright orange color.

The composition of films treated at different temperatures was checked by XP spectroscopy. Calcination at 500~ led to a complete evaporation of highly dispersed Cd species. Thus, a diminution of the calcination temperature was necessary. Temperaturedependent in-situ IR studies in an O2/Ar gas flow showed that surfactant molecules were quantitatively removed from the films quantitatively at 350~ within 2 hours [9]. After calcination at 350~ a Cd content of 2.1 atom-% was detected for a Cd-containing film by XPS, a value which is only 0.3 atom-% less than that of a film dried at 80~ After treatment with H2Se, the Se content of the film calcined at 350~ was higher than that of the non-calcined film, since free pores allowed the penetration of H2Se. However, only about 50% of the Cd 2§ ions reacted to CdSe, probably due to the fact that a considerable portion of Cd 2+ ions was incorporated within the silica walls during the gelation process and was not accessible for the H2Se. In all the samples the binding energy of the Se3ds/2 electrons was between 54.7 and 55.0 eV, which is in good agreement with literature data for CdSe [10]. In the film calcined at 500~ containing almost no Cd, an orange-red color appeared after treatment with H2Se and a considerable amount of Se was found. However, the binding energy of the Se3ds/2 electrons was increased to 55.4 eV, which is typical for elemental selenium. This observation indicates the silica film tends to catalyze the decomposition of H2Se. All the CdS and CdSe loaded transparent mesoporous SiO2 films were homogeneously colored, pale yellow (CdS) and orange (CdSe), respectively, indicating the occurrence of sizequantization effects with respect to the bulk materials, which were yellow-orange (bulk-CdS, absorption edge around ) ~ - 520 nm) and grey-red (bulk-CdSe, absorption edge at )~ = 712 nm). In the prepared films the observed absorptions increased considerably at around )~- 500 nm (CdS) and ~ = 590 nm (CdSe), and the first excitonic shoulders could be deduced from the second derivations of the spectra at )~ = 483 nm (CdS) and 2~ = 557 nm (CdSe).

1462 According to the tight-binding model, introduced by Lippens and Lannoo for the estimation of diameters of semiconductor nanoparticles from the altered opto-electronic features [11 ], mean diameters of about 2.5 nm were calculated for both CdS and CdSe nanoparticles [9]. The flatness of the absorption edges, however, indicates that the size-distribution is rather broad, about 2.5 + 0.8 nm. Nevertheless, this particle size corresponds well to the widths of the host pores (around 2 nm), confirming that the CdS and CdSe particles were formed within the pores, where they were protected against further growth even due to heating at 373 K.

3.2. Mesoporous titanium dioxide prepared using block copolymers The stock solution for the film preparation contains an ethoxide-modified titanium chloride, formed by the reaction: TIC14 + x EtOH ~ TiC14_x(OEt)x + x HC1, where x ~ 2. The formed TiClx(OC2Hs)4_• species, which are rather stable against hydrolysis, associate preferentially with poly(ethylene oxide) moieties to produce a self-assembling complex. The necessary prerequisite for the formation of ordered material is the hydrolysis of titanium-containing species. Due to their stability, this process is strongly dependent on such parameters as the sufficient supply of water vapor and the length of the hydrolysis. Finally, calcination in air removes quantitatively the organic template. Chemical analysis by XPS has confirmed that the product does not contain any detectable amounts of elements other than titanium and oxygen, i.e. the removal of the organic component was complete. Because of the intended application in the continuous effluent decontamination and the aimed study into the effect of the completeness of the hydrolysis on the structure properties of mesoporous titania, the samples were prepared in the form of films of variable thickness. With thin films (samples I-A and I-B, density of 2 mg/cm 2 and 4 mg/cm 2, respectively), the full hydrolysis occurs due to a good accessibility for the air humidity during the aging. This ensures the creation of a highly uniform and regularly arranged porous structure with a narrow pore size distribution as has been proved by SEM and N2 adsorption (Fig. 3).

I

8

6

,

I-C

4

2

0 0,0

J 0,4

J 0,8

/ 1,2

I 16,

i 2,0

P/Po Figure 3. SEM image of the thinnest film I-A (left) and adsorption isotherms of N2 at-196~ (right). The start point of each isotherm is shifted by P/P0 = 0.4. The solid symbols denote desorption.

1463 Table 1. Structure parameters of films prepared using block copolymers Dpc 9 Sample da SBET~ (mg/cm 2) (m2/g) (nm) I-A 2 105 4.4 I-B 4 94 4.6 I-C 6 127 4.0, 5.2 I-D 8 104 6.2,16.0 a density of the film, b BET surface area, c mean pore size (two values correspond to a bimodal porous structure).

With medium (sample I-C, density of 6 mg/cm 2) and thick films (sample I-D, density of 8 mg/cm 2) the hydrolysis is far from being complete. Consequently, larger pores are formed in addition to smaller ones during the calcination of the non-hydrolyzed fraction. This leads to the formation of a bimodal porous structure (Fig. 3, right). The structure parameters of all the films studied are given in Table 1. X-ray diffractograms and the Raman spectra evidence that all the samples contain a pure anatase phase. The presence of an amorphous titania component is probable because X-ray diffractograms exhibit decreased intensity of reflections due to anatase in comparison with a pure reference material (Bayer). This component does not seem, however, to exhibit any characteristic Raman signal, which would distinguished it from anatase.

3.3. Photocatalysis It was recently demonstrated that mesoporous titania prepared using ligand assisted templating methods has low photocatalytic activity compared to the crystalline phase despite its high surface area [12]. This low activity is due to the incomplete extraction of the surfactant and the amorphous titania channel walls. The authors conclude that partially crystallized titania is essential for obtaining high photocatalytic activity. It this study we have found that by optimizing the synthesis condition a highly active photocatalysist can be synthesized using block copolymers, whose activity compares well even with the best commercial materials (such as PKP 09040, Bayer). There are, however, severe requirements, which should be met. The preparation of a highly active photocatalyst requires the complete hydrolysis of the precursor, as that is the case with samples I-A and I-B. Consequently such a photocatalyst is characterized by a regularly arranged porous structure with a narrow pore size distribution. Rate constants of the decomposition of 4-chlorophenol calculated according to the first-order kinetics are given in table 2. Table 2. Decomposition of 4-chlorophenol Sample Rate constant of the decomposition of 4-chlorophenol (10 4 S"l) I-A I-B Non-optimum films Bayer

3.49 3.42 1.4-2.4 3.37

1464 4. CONCLUSIONS Procedures for the formation of thin, nanostructured, transparent and crack-free films of silica up to a thickness of about 800 nm have been developed by use of block copolymers as structure-directing agents. The crack formation is avoided by lowering the calcination temperature to 350~ The films contain uniform mesopores, whose diameters are about 2 nm. The mesopores can be used to host highly dispersed CdS and CdSe nanoparticles, showing size-quantization effects. The porous structure of titania films prepared using a poly(alkylene) block copolymer in an ethanolic medium depends on their thickness because the completeness of the hydrolysis of the titania precursor is decided by its accessibility for air humidity. Well-organized film is formed only from a completely hydrolyzed precursor. The photocatalytic activity of such organized film is comparable with that of the most active commercial anatase powder. ACKNOWLEDGMENTS

This work was supported by the Grant Agency of the Academy of Sciences of the Czech Republic (contract No. A4040804). REFERENCES 1. S. Pevzner, O. Regev and R. Yerushalmi-Rozen, Curr. Opin. Colloid. Interface Sci., 4 (2000) 420. 2. D. Zhao, P. Yang, N. Melosh, J. Feng, B.F. Chmelka and G.D. Stucky, Adv. Mater. 10 (1998) 1380. 3. Y.S. Tang, S.J. Cai, G.L. Jin, K.L. Wang, H.M. Soyer and B.S. Dunn, Thin Solid Films, 321 (1998) 76. 4. O. Dag, G.A. Ozin, H. Yang, C. Reber and G. Bussiere, Adv. Mater. 11 (1999) 474. 5. Y. Pyuto, J.-M. Berquier, C. Jacquiod and C. Ricolleau, Chom. Commun. (1999) 1653. 6. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. 7. L. Kavan, J. Rathousky, M. Gr~itzel, V. Shklover and A. Zukal, J. Phys. Chem. B 104 (2000) 12012. 8. D. Grosso, G.J. de A.A. Soler-Illia, F. Babonneau, C. Sanchez, P.-A. Albouy, A.Brunet-Bruneau and A.R. Balkenende, Adv. Mater. 13 (2001) 1085. 9. H. Wellmann, J. Rathousky and M. Wark, Thin solid films, 2001, submitted. 10. G.E. Muilenberg (Ed.), Handbook of X-ray photoelectron spectroscopy, Perkin-Elmer Corporation, 1979. 11. P.E. Lippens and M. Lannoo, Phys. Rev. B 39 (1989) 10935. 12. V.F. Stone and R.J. Davis, Chem. Mater. 10 (1998) 1468.


1465

Spin-coating induced self-assembly of pure silica and Fe-containing mesoporous films N. Petkov, S. Mintova* and T. Bein* Department of Chemistry, University of Munich, Butenandtstr. 11-13 (E), 81 377 Munich, Germany Pure silica and Fe-containing mesoporous thin films have been prepared from preformed silica/triblock copolymer/ethanol precursor solutions by spin- coating. The Xray diffraction results show transformation of the mesophase structure from onedimensional hexagonal to three-dimensional cubic structure by increasing solvent evaporation rate or by thermal transformation during the calcination process. Thus cubic pure silica and Fe-containing mesoporous films have been obtained possessing accessible mesopore channel system suitable for variety of nanotechnolgical and catalytic applications. The films show very smooth and homogeneous morphology and can be prepared with controllable thickness by varying the conditions of the spin-coating procedure. The UV-Vis spectroscopy and thermal analysis results suggest that Fe species are preferentially introduced inside the mesoporous channel system. I. INTRODUCTION The ability to utilize the structural and functional characteristics of the mesoporous films provides an excellent avenue for numerous potential applications ranging from catalysis to the development of electronic and optical devices on a nanometer scale [ 1-4]. Although excellent progress has been made in the preparation of mesoporous films with desired mesophase structure and surface morphology there are still several drawbacks from the standpoint of nanotechnology. Many envisioned nanotechnological applications could benefit by the preparation of continuous mesoporous films that possess threedimensional accessible pore system and provides easy contact to the support which offers a host channel matrix for the growth of nanomaterials. Recently evaporation induced self-assembly route for the preparation of continuous mesoporous films showing smooth and homogeneous surface has been proposed [ 1, 2, 57]. This approach employs preformed silica/surfactant/solvent coating solutions with surfactant concentrations below the critical micellar concentration that are dip- or spincoated on different substrates [5-7]. The rapid solvent evaporation induces self-assembly of the silica/surfactant precursor species and leads to the formation of homogeneous and very smooth mesoporous film. Through the variation of the initial surfactant concentration [5, 7, 8] and the type of the surfactant [9], mesoporous films with different "Correspondingauthor: [email protected]

1466 mesophase structure including cubic and three-dimensional hexagonal structures have been prepared. Furthermore the evaporation induced self-assembly method gives an opportunity for incorporation of different functional groups in the micellar interiors by mixing the precursor silica/surfactant/solvent assemblies with the desired molecular functionality and subsequent coating [ 1, 2]. In this study we report on the preparation of mesostructured pure silica and Fe-loaded films with different mesophase pore systems on silicon wafers. Evaporation induced selfassembly approach performed via spin- coating is used to deposit thin mesoporous films from pure silica and Fe-containing silica/nonionic triblock copolymer/ethanol precursor solutions. 2. EXPERIMENTAL Clear precursor silica/surfactant assemblies were prepared by mixing acid hydrolyzed tetraetoxyisilane (TEOS 98%, Aldrich) at 60~ with ethanol solution of PEO-PPO-PEO triblock copolymer (Pluronic 123 - BASF). Iron was introduced during the mixing of the two solutions as Fe(NO3)3.9H20 and resulted in yellow transparent sol. The final molar ratio of the coating solution was 1TEOS: 0.017PEO-PPO-PEO: 0.06HCI: 60EtOH: 10H20:(0.1 or 0.2)Fe(NO3)3.9H20. The pure silica sample is designated as MSil, whereas the samples prepared with 0.1Fe(NO3)3.9H20 and 0.2Fe(NO3)3.9H20 are designated as MSilFel and MSilFe2, respectively. The coating solutions were aged for 2 h prior to preparation of the thin films. The mesoporous films were deposited on acetone cleaned silicon wafers (25 x 25 mm) by spin-coating (SCS P6700) with rotation speed of 4000 rpm at room temperature (RT). For comparison thick films were prepared by slow evaporation of the same coating solutions deposited drop-wise on precleaned Si-wafers at room temperature for 24 h. These samples are labeled as MSi2, MSi2Fel and MSi2Fe2 for the pure-silica and Fe-containing samples, respectively. Finally the mesoporous films were thermally treated in flowing N2 at 300~ for 12 h, followed by calcination in flowing air at 500~ for 6 h. The mesophase structure of the as-synthesized and calcined films was determined by X-ray diffraction in the 00-20~ scan mode (Scintag XDS 2000 cooled Ge detector, Cu K~ radiation). The surface morphology was evaluated from atomic force microscopy images (AFM) (Nanoscope N S E - Digital Instruments). The thickness of the mesoporous films was determined from the scratched parts of the films by scanning electron microscopy (SEM) (Philips XL 40). The degree of loading of Fe species into the mesoporous materials was determined by thermal analysis (TA) (DuPont Instruments - 951 TA) and UV-Vis spectroscopy (Hitachi U-3501 Spectrophotometer). 3. RESULTS AND DISCUSION 3.1. Structure of the mesoporous films

The structure of the mesoporous films deposited on Si wafers has been determined by X-ray diffraction. Figure 1 represents the X-ray diffraction patterns of the as-deposited pure-silica and Fe-containing mesoporous films. The pure-silica film shows several diffraction peaks that can be indexed as the (111), (200), (210), (220), and (440) reflections of the Pn3m cubic space group with high unit cell size a - 184 A (Figure 1a). The measured and calculated 20~ values for the primitive cubic Pn3m space group are presented as an inserted table in Figure 1. Similar results have been reported before for

1467 the primitive cubic mesophase structure identified as a bicontinuous Pn3m prepared by spin- coating of silica/cationic surfactant solutions [8]. However cubic mesophase structure showing Im3m symmetry has been observed for the dip-coated mesoporous films prepared with triblock copolymers with higher molar ratio of ethylene oxide to propylene oxide [9]. Our data for the films prepared by spin- coating of silica/triblock copolymer mixture suggest the existence of the bicontinuous cubic mesophase structure with Pn3m symmetry. After calcination the observed space group symmetry is preserved, but the reflections appear at higher 20 ~ values due to the contraction of the silica framework (Figure 2a). The unit cell size of the calcined pure-silica mesoporous film is a= 116 A and corresponds to 37 % contraction of the silica framework. The XRD patterns of the as-deposited MSilFel film shows three peaks with decreased intensity that can be indexed as (100), (200) and (300) reflections of the highly ordered one-dimensional hexagonal mesophase structure with channels oriented parallel to the substrate surface (Figure l b). The unit cell size of the corresponding as-deposited onedimensional hexagonal structure is a = 108/~. Upon calcination several reflections are observed, and they can be indexed in primitive cubic symmetry of the same bicontinuous Pn3m mesophase structure obtained already for the pure-silica sample (Figure 2b). The unit cell size of the calcined MSilFel film is a = 115 A, and it has similar value to that of the pure-silica sample. The X-ray results reveal that mesophase transformation has taken place during the calcination process leading to the formation of three-dimensional mesophase structure due to shrinkage of the silica framework. (100)

hkl 20 ~ 100 1.06 200 2.02

hkl 20 ~ 20cal 200 1.70 1.70 211 2.08 2.08

(C)

~. (200) . ~ 0 0 3.05

(19o) A

II i[

!1

9~= , ] ]

,, I,c)

. . . . . . hkl 20 ~ 100 0.94

%~ ;200] "3""" 200 1.87 vv) 300 2.71

(b) ,,,,,

xlt)t)

~" (111~? 00) ~i ~ \ .~~ o ( ~

)

hkl 111 200 211

20 ~ 1.33 1.53 2.03

20cal 1.33 1.53 1.89

hkl 111 200 211

20 ~ 20cal 1.31 1.32 1.52 1.52 1.98 1.86

(b)

(200) ,1,,,11 1, 1 l)gll v

i

0.7

~(21~ I(21~)~ k..----

(440)

111 0.83 0.83,'aX 200 0.93 0.96I, )

~440

(a)

2.62 2.63 X40

i

i

2.7

4.7

2 theta (degrees)

(

Figure 1. XRD patterns of the as-deposited (a) MSil, (b) MSilFel, and (c) MSilFe2 mesoporous films.

1

2 2 theta (~egrees)

4

Figure 2. XRD patterns of the calcined (a) MSil, (b) MSilFel, and (c) MSilFe2 mesoporous films.

1468 Similar mesophase transformation process has been observed before for the cubic mesoporous films prepared by dip- coating of silica/cationic surfactant solution [5]. The as-deposited MSilFe2 film shows one very intensive and two less intensive reflections that are consistent with one-dimensional hexagonal mesophase structure with unit cell size of a = 96 A (Figure lc). In comparison to sample MSilFel the mesophase order and the unit cell size is decreased probably due to the introduction of higher amount of Fe salt in the coating mixure. The calcined MSilFe2 film shows only two very broad and low intensive reflections similar to (200) and (211) reflections of the bicontinuous cubic Pn3m mesophase structure (Figure 2c). Apparently the introduction of the Fesource in the synthetic mixture provokes formation of one-dimensional mesophase structures (as-deposited samples) but enables the formation of three-dimensional cubic mesophase through thermal transformation during the calcination process. In order to elucidate the role of the solvent evaporation during the formation of mesoporous films, thicker films are prepared by slow evaporation of the same coating solutions deposited drop-wise on Si-wafers at room temperature. The XRD patterns of the as-deposited films are presented in Figure 3. The results show that preferentially onedimensional hexagonal mesophase structures with channels running parallel to the substrate surface are formed from the three coating solutions. The corresponding unit cell size of the one-dimensional hexagonal mesophase structures are: aMsi2-- 115 /~ aMSi2Fel= 106 ~ and aMSi2Fe2 = 103/~. (100) A i II ~ (200) l l L A

hkl 20 ~ 100 1.00 200 1.96 3002.95

(c)

(c)

, ,

(100) A ~i II .~-~ I!

(200) i

(300)

hkl 100 200 300

20 ~ 0.94 1"86 (b) 2.78

.~~

111 1.89 1.89 200 2.20 1.18

O

(100) (100)

hkl 20 ~ 100 0.88

(200)

,

~

(300)

JLJ i

0.7

hkl 20 ~ . . . . . 100 1.86 tzuu) _ 200 3.88

]~176 0 ~175~ (a)

i

2.7 2 theta (degrees)

i

4.7

Figure 3. XRD patterns of the asdeposited (a) MSi2, (b) MSi2Fel, and (c) MSi2Fe2 mesoporous films.

1

2

3

2 theta (degrees)

4

5

Figure 4. XRD patterns of the calcined (a) MSi2, (b) MSi2Fel, and (c) MSi2Fe2 mesoporous films.

1469 The observed decrease in the unit cell size from the pure-silica to the Fe-containing films is attributed to the introduction of Fe salt in the synthetic mixture. After calcination of film MSi2, the X-ray pattern exhibits two reflections that can be straightforwardly assigned to (100) and (200) reflections of one-dimensional highly ordered mesophase structure (Figure 4a). The unit cell size of the calcined sample is a = 48 A and corresponds to 58 % shrinkage of the silica framework. For the calcined MSi2Fel film two reflections that are indexed as (111) and (200) values are detected and assigned to the primitive cubic Pn3m symmetry (Figure 4b). The same mesophase transformation process has been observed for the thinner films prepared by spin- coating of solutions MSilFel and MSilFe2. The unit cell size of the thicker film (MSi2Fel) is a - 81 A, and it is much smaller than that of the MSilFel cubic film (a - 115 A) suggesting higher stability of the silica framework towards thermal contraction. One broad reflection shifted to higher 20 ~ values of about 2.4 20 ~ was recorded for the sample MSi2Fe2 alter calcination (Figure 4c). This pattern suggests decreased mesostructural order of the sample, and it is attributed to more disordered mesophase structure. Obviously the spin-coating process induces formation of three-dimensional mesophase structures due to the rapid evaporation of the solvent, which promotes the formation of spherical micellar interiors on the substrate surface and leading to self-assembling in a cubic mesostracture. In contrast, the slow evaporation of the solvent promotes slower silica condensation and enables formation of tubular micelles on the substrate surface, which are self-assembled into hexagonal arrangement of the mesoporous channels parallel to the surface. 3.2. Morphology and thickness of the mesoporous films The surface features and the thickness of the mesoporous films are evaluated by contact mode AFM images and by scanning electron micrographs of the scratched parts of the films. Figure 5 shows the AFM image of the calcined MSil mesoporous film, where a sausages-like type of morphology is seen on the film surface. At higher magnification, irregular objects with dimensions of less than 0.5 ~tm could also be seen. The film roughness calculated from the AFM image is estimated to be around 9 nm. The SEM image of the calcined MSil sample shows that very thin (- 90 nm) and continuous film along the whole Si-wafer was prepared with no cracks and corrugation (Figure 6). The Fe-loaded thin mesoporous film deposited using the same spin-coating conditions and calcination procedure is shown in Figure 7. The observed sausages-like type of surface morphology is slightly distorted in the MSilFel compared to the MSil film. The roughness of the Fe-loaded film is similar to that of the pure silica film deposited under the same spin-coating conditions. The thickness of the calcined MSilFel film is about 120 nm, which is slightly higher than the thickness of the pure-silica film (Figure 8). The Fe-loaded film is continuous and no surface inhomogenates or cracks were observed alter calcination. The as-deposited thicker films (data not shown) exhibit continuous but uneven surface morphology alter slow evaporation of the coating solutions at room temperature. Upon calcination the MSi2, MSi2Fel and MSi2Fe2 films show cracks and they partially delaminate from the substrate.

1470

Figure 5. AFM image of the calcined cubic MSil film.

Figure 6. SEM image of the calcined cubic MSi 1 film.

The thickness of these films determined from the SEM images is estimated to be in the micrometer range (~ 21am). The above results demonstrate that the surface morphology and the film thickness of pure silica and Fe-containing films deposited by spin- coating are analogous. In the both cases, very smooth and homogeneous films with nanometer thickness suitable for nanotechnological applications can be prepared. In addition, the surface morphology and the thickness of the films can be easily controlled by changing the solvent evaporation rate. The slower solvent evaporation results in thicker micrometer films showing uneven and bumpy surface. 3.3. Fe-loading of the mesoporous materials The UV-Vis spectra of the calcined pure-silica and Fe-containing samples obtained aider slow evaporation of the coating mixtures at room temperature are depicted in Figure 9.

Figure 7. AFM image of the calcined cubic MSilFel film.

Figure 8. SEM image of the calcined cubic MSilFel film.

1471

1.1

110

.•(c)

100

~ 90

~ so

O

70 -

O

~0.8

(c)

~ 6o ~-_

__

- .

__

:

b)

_

50 0.5

t

250

450 Wavelenght (nm)

I

650

Figure 9. UV-vis spectra of (a) MSi2 (b) MSi2Fel, and (c) MSi2Fe2 samples after delaminating from the substrate.

(a)

40 30

130

230

330

430T(C~

Figure 10. TG-analysis of (a) MSi2 (b) MSi2Fel, and (c) MSi2Fe2 samples alter delaminating from the substrate.

As expected, the pure-silica sample shows no absorbance in the range of 250-700 nm (Figure 9a). On the other hand, the Fe-containing samples show absorbance in the region 250-400 nm with maximum at 290 nm indicative for the ligand to metal Fe 3§ charge transfer (Figure 9 b, c). On the other hand the absorbance bands in the range of 450-600 nm related to the absorbance of bulky Fe203 or FeO(OH) particles are absent in our samples [10]. The XRD measurements taken at high 20 ~ angles and the microscopy confirm the absence of large bulky particles of Fe oxides. Our results suggest the encapsulation of Fe oxide nanoparticles and clusters inside the mesoporous channels upon calcination and are consistent with the previously published result [ 10]. The degree of Fe-loading and the stability of the mesoporous films were determined by TG-analysis. The TG curves of the pure-silica and Fe-containing mesoporous samples are shown in Figure 10. The MSi2 sample shows that the surfactant is removed in a single decomposition step in the temperature interval of 160-230 ~ and the weight loss is estimated to be around 52 wt. %. In the case of the Fe-loaded mesoporous sample the weight loss behavior is different compared to the pure-silica sample. The combustion of surfactant takes place at higher temperature interval (250-330 ~ and leads to lower weight loss of 46 wt. % and 37 wt. % for MSi2Fel and MSi2Fe2 samples, respectively (Figure 10 b, c). The difference of 8 wt. % and 15 wt. % for the MSi2Fel and MSi2Fe2 samples compared to the pure-silica sample is probably due to the incorporated Fe species into the mesoporous host. 4. CONCLUSION Evaporation induced self-assembly via spin- coating has been employed for the formation of mesoporous films from preformed pure-silica and Fe-containing silica/triblock copolymer/ethanol solutions. The X-ray diffraction results reveal the

1472 transformation from one-dimensional hexagonal to the three-dimensional cubic mesophase structure by increasing the solvent evaporation rate or by thermal transformation during the calcination process. The rapid solvent evaporation of the coating mixture induces formation of three-dimensional micellar interiors, which selfassembled in cubic mesophase structure on the silicon substrate. Varying the solvent evaporation rate and silica condensation process in the coating solutions can control the different morphology and thickness of the films. The results from UV-Vis spectroscopy and TG analysis suggest that the iron species are preferentially introduced inside the mesoporous channel system. The mesoporous films containing Fe-nanoparticles are suitable supports for variety of catalytic applications. The growth of carbon nanotubes by catalytic decomposition of hydrocarbons is currently being investigated. REFERENCES

1. H. Fan, Y. Lu, A. Stump, S. T. Scott, T. Baer, R. Schunk, V. Perez-Luna, G. P. Lopez and C. J. Brinker, Nature, 405 (2000) 56. 2. D. Doshi, N. Huesing, M. Lu, H. Fan, Y. Lu, K. Simmons-Potter, B. G. Pottr Jr., A. Hurd and C. J. Brinker, Nature, 290 (2000) 107. 3. G. Wirnsberger and G. D. Stucky, Chem. Mater., 12 (2000) 2525. 4. R. Hayward, P. Alberius-Henning, B. Chmelka and G. D. Stucky, Micropor. Mesopor. Mater., 44-45 (2001) 619. 5. Y. Lu, R. Ganguli, C. A. Drewien, M. T. Anderson, C. J. Brinker, W. Gong, Y. Guo and J. I. Zink, Nature, 389 (1997) 364. 6. C.J. Brinker, Y. Lu, A. Sellinger and H. Fan, Adv. Mater., 11 (1999) 579. 7. M. Ogawa and N. Masukawa, Micropor. Mesopor. Mater., 38 (2000) 35. 8. I. Honma, H. Zhou, H. Kundu and A. Endo, Adv. Mater., 12 (2000) 1529. 9. D. Zhao, P. Yang, N. Melosh, J. Feng, B. Chmelka and G. D. Stucky, Adv. Mater., 10 (1998) 1380. 10. S. E. Dapurkar, S. K. Badamali and P. Selvam, Catal. Today, 68 (2001) 63. 11. M. Iwamoto, T. Abe and Y. Tachibana, J. Mol. Catal., 155 (2000) 143.


Guanidine Catalysts Supported Catalysts for Organic Chemistry

on Micelle T e m p l a t e d

1473

Silicas. N e w

Basic

D. J. Macquarrie *a, K. A. Utting a, D. Brunel b, G. Renard b and A. Blanc b (a) Centre for Clean Technology, Department of Chemistry, University of York, Heslington, YORK, YO 10 5DD, England (b) Laboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM-CNRS, 8, Rue de l'Ecole Normale- 34296 Montpellier, France. The preparation of guanidines covalently linked to the framework of Micelle Templated Silicas is discussed. These materials can behave as efficient catalysts in base catalysed epoxidation. Control over surface chemistry is very beneficial in enhancing selectivity and efficiency with respect to utilisation of the oxidant hydrogen peroxide. Such systems contribute to the search for cleaner manufacturing technologies. The same catalysts are active in the Linstead variation of the Knoevenagel reaction. Initial work aimed at preparing chiral guanidines attached to Micelle Templated Silica is also presented. 1. INTRODUCTION The development of novel catalysts supported on inorganic solids such as silica has received a great deal of attention recently. With the drive towards cleaner chemical processes intensifying, the search for efficient catalytic routes to products and intermediates is increasingly important, and progress is being made in acid, base and oxidation catalysts, many of which are based on the new highly structured solids such as the MCMs and related materials[1 ]. Many important synthetic protocols exist which rely on soluble base catalysts, and we, and others, have in the past addressed some of these with simple amines attached to silica.[2-7] While these catalysts are very effective in a range of important reactions involving mild base catalysis, they fail in other reaction types where more powerful bases are required. The types of organic bases which are of interest are the guanidines, considerably more powerful than the simple amines commonly used so far. One such reaction type which is of importance in synthetic chemistry, and which is amenable to guanidine catalysis is the base catalysed epoxidation of electron-deficient alkenes (Scheme 1).[8,9] This reaction is particularly appealing, since it delivers functionality efficiently, is of importance in synthesis, and requires only the clean, cheap oxidant hydrogen peroxide, as terminal oxidant. There is also the as-yet untapped potential to develop chiral guanidines supported on structured inorganic matrices which would allow the enantioselective preparation of important building blocks for natural product synthesis, such as 1 in Scheme 1, a structural motif in a range of antiobiotics of the manumycin family.

1474 0

0

0

Scheme 1. Epoxidation of electron deficient alkenes. Previous work in the area of has been carried out by the groups of Jacobs,[ 10] Brunel[11 ] and Jaenicke. [ 12] Jacobs' approach was to attach the bicyclic guanidine triazabicyclo[4,4,0]undec-5-ene (TBD) to a silica support modified with the glycidyl group. Ring opening of this epoxide led to the attachment of the guanidine unit. This product was active in the epoxidation reaction mentioned earlier (with excellent selectivity towards the organic component, but poor utilisation of the hydrogen peroxide (ca. 20% selectivity), as well as being a good catalyst for both the Michael and Knoevenagel reactions. The catalyst described by Brunel was prepared by displacement of the chlorine of a chloropropyl-micelle templated silica with the same bicyclic guanidine, followed by liberation of the free base by treatment with a stronger guanidine base. The resultant catalyst was found to be efficient for the base catalysed ring-opening of epoxides to form monoglycerides. A similar approach was used by Jaenicke to provide a catalyst active for the same ring-opening reaction.[12] We now present our results on the development of supported guanidines and their application in model reaction types. The achiral guanidines are based on tetramethylguanidine, rather than TBD. Chiral guanidines are also prepared, based on the work of Isobe, who has developed routes to soluble chiral guanidines.[ 13-16] 2. PREPARATION OF CATALYSTS Our approach to achiral guanidines is based on the reaction of chloropropyl silanes, either attached to a silica surface or prior to grafting, with 1,1,3,3-tetramethylguanidine (TMG). Removal of HC1 liberates the free guanidine, and is accomplished using 1-methyl-TBD, a stronger base than TMG. Various routes were attempted, with these being summarised in Scheme 2. Catalysts were prepared by either grafting onto a pre-formed Micelle Templated Silica (MTS-G) of by direct sol- gel preparation of an organically modified MTS (MTS-D). Preliminary attempts to attach TMG via epoxide ring-opening were unsuccessful, probably due to steric hindrance from the methyl groups impeding attak of the imine N on the epoxide. Thus, emphasis was placed on the nucleophilic displacement of the chloride and subsequent liberation of the free guanidine. The guanidine silane 2 is known[17], and was prepared in a straightforward manner, the product being very moisture sensitive, as might be expected. This was grafted directly to a Micelle Templated Silica to give 5, but templated sol-gel preparations resulted in only amorphous material.

1475

(MeO)3Si~/~XCI

(a)

=" (MeO)3Si ~ (/

1

3

7

4

1

N--------~NMe2~ ) ~ NMe2 8

/~

NMe2 N~"~ 2 NMe2 " ~ (b)

NMe2 ~ ~ N _ _ . ~ N~---~NMe2

5

NMe2 NMe2

6

N----"~NMe2 NMe2

Scheme 2. Summary of synthetic routes to catalysts. (a) Displacement of chloride by TMG, followed by liberation of free base; (b) templated sol-gel synthesis of modified silica; (c) grafting onto preformed MTS This is likely to be due to the guanidine unit becoming protonated in the aqueous environment of the system, something which is known to hinder tempated synthesis.[18] Both templated sol-gel and grafting routes to the chloropropyl systems 3 and 4 proceeded in a straightforward manner, and displacement of the chloride was easily achieved. Silylation with N,O-bistrimethylsilylacetamide was carried out before attachment of guanidine. This improves the ease with which physisorbed guanidine can be removed, by substantially reducing the interaction of the guanidine with the surface of the silica, as well as reducing the interaction of the basic chemisorbed guanidines with the surface. Positive effects on hydrogen peroxide utilisation have also been noted in Ti-MTS systems in epoxidation reactions after surface passivation,[ 19,20] and it was hoped that these would also be found here. A summary of the physical parameters of the succesfully prepared materials is given in Table 1. Table 1 Physica! parameters of chosen catalysts Material Parent MTS 5 7 8

SSA (m 2g-1) 972 221 1281 807

Pore diameter (nm)

loading (mmol g-l)

3.0 1.9 2.4 2.5

1.7 1.4 0.4

1476 3. CATALYTIC RESULTS The above catalysts were investigated in the epoxidation of 2-cyclohexenone (Scheme 1), and in the Linstead-Knoevenagel condensation (Scheme 3).[21,22]

HO2C~"'+~CO2H

~ C H O

THF

-H20

H~ ,.,4,CHO CeH1/C~C'h"CsHll 10

06H13~~/COOH 9

\

COOH I -cO2 C5H1'~ 0H2002H 9

Scheme 3. Linstead-Knoevenagel condensation The epoxidation of 2-cyclohexenone proceeded smoothly at room temperature in methanol (Table 2). Best results were obtained when the hydrogen peroxide was added incrementally over the period of the reaction, rather than all at the start. Using this method, we were able to obtain good selectivities towards the enone, and towards hydrogen peroxide. The major byproduct consisted of small amounts (2-3%) of a product derived from the addition of methanol to the enone were always observed (even in blank reactions with no catalyst); this product could be suppressed by changing solvent to a bulkier alcohol such as i-propanol, although this reduced the rate and conversion significantly. Selectivities towards enone were excellent (up to 89%) and match those in the literature. Importantly, selectivity towards hydrogen peroxide were substantially improved, due to the reduction in the amount nonproductively decomposed. This is due to surface passivation, and has been noted in other TiMTS systems[19,20] In this way we were able to improve the hydrogen peroxide utilisation from the 21% reported[ 10] to over 50%. This corresponds to a substantial decrease in the final volume of the reaction mixture, and to a substantial decrease in the quantity of water present. The condensation of malonic acid with heptanal is a key step in the synthesis of coconut oil lactone, and is carried out by the Linstead variation of the Knoevenagel condensation, usually using tris(hydroxyethyl)amine as catalysts and solvent. Here, the solvent chosen was THF, largely because solubility problems with malonic acid in most simple organic solvents preclude effective heterogeneous catalysis. Catalytic quantities of the guanidine catalysts can be used in THF to bring about the Linstead condensation, as summarised in Table 3 and Scheme 3. Condensation of the two components to give product 9 is relatively simply achieved in the cases attempted, but two main difficulties were encountered:- firstly, the formation of condensation products (10) from two molecules of aldehyde dimerising were high (14%) in one catalyst (8), but in the slightly less active 5, these products were minimised. The major loss of selectivity came as a result of the incomplete decarboxylation of the

1477 intermediate diacid, which can be isolated (and converted to the product in a separate reaction step). This stems from the low temperature of reflux in THF, with higher boiling solvents not dissolving sufficient malonic acid to allow reaction to proceed. Table 3. Results of supporte J guanidine catal, rsis of the Linstead condensation. _atalyst Conversion Selectivity tumover 8 95% 58% 158 5 78% 40% 31

Time 48h 48h

4. CHIRAL SUPPORTED GUANIDINES The approach investigated to obtain chiral guanidines is represented in Scheme 4. The first variation, involving the condensation of the chiral cyclohexane-diamine (instead of 11) with urea in a high temperature step[ 13-16] was only partly successful, with very low yields being obtained. Subsequent steps have not yet been studied.

H2N Ph

NH2

/ '., Ph

H2N

11

~ ,, II 0

/NH 2

O ,,,,LL,. -- HN NH.,, ~

Ph

'

2xMel

[

12

Ph

\

MeN'~NMe+ [.,,

Ph 14

[MTS]---NH2

13

Hx

.HCI

oxalyl chloride

",, Ph

,,Ph

S / ' ~ L _ N___~~I---1"

~ MT

NMe [

Ph

CI

Ph

=Mej

O ,~

--L /N~ph

removal of HCI

"

15

final catalyst 16

Scheme 4 - Scheme for the preparation of chiral guanidine supported on MTS Instead, we have concentrated our initial efforts on the second route involving 1,2-diphenyl, 1,2-diaminoethane 11, Scheme 4. Here, we have successfully prepared the supported chiral guanidine on a Micelle Templated Silica, in the form of its HC1 salt. Efforts are ongoing to liberate the free base, and to evaluate the product. The first three steps up to 14 proceeded smoothly, and according to the literature. Coupling of 14 with a trimethylsilyl-passivated aminopropyl grafted micelle templated silica was carried out after drying of the aminopropylsilica under vacuum (lmbar) at 150~ overnight. The crude reaction mixture containing 14 was cooled to room temperature, and excess oxalyl chloride was removed under vacuum, leaving a suspension of 14 in chlorobenzene. To this

1478 was added the activated aminopropyl silica, and the mixture stirred for 48 hours. Triethylamine was added and stirring continued for 3 hours. Filtration and washing with methanol and DMF yielded the product 15. Elemental analysis indicated a 0.66mmol/g loading of guanidine (and 0.66mmol/g C1).

5. EXPERIMENTAL All solvents used were analytical grade, and were not purified further unless specified. Reagents were purchased from Aldrich, and were used as received. The MTS used was prepared following a published route[6], and was calcined to remove template shortly before use. Grafting was carried out following published methods.[6] The templated sol-gel method has been described previously.[23,24] Silane 2 was prepared following a literature method.[17] Porosity and surface area measurements were carried out using dinitrogen on a Micromeritics ASAP2100 instrument. NMR spectra were recorded on a Bruker Model AM 300 spectrometer operating at 75.470 MHz with Fourier transform. The instrument setting were the following : 90 ~ pulse of 4.80 ms; proton decoupling power : 30 G; contact time : 5.10 .3 s; delay time : 5 s; rotor spinning speed : 5 KHz

5.1 Passivation of chloropropyl supports Chloropropyl-MTS (2.00g) was refluxed in dry toluene (75ml). To the suspension was added N,O-bis trimethylsilyl acetamide (5.6ml, 27.5mmol) and the reaction mixture refluxed for 2.5h. After cooling, the solution was filtered and washed with toluene (25ml). The solid was subjected to Soxhlet extraction with methanol for 6 hours, and subsequently dried at 100~ 5.2 Grafting of guanidine onto chloropropyl-containing supports The material from the previous experiment (1.3g) was activated at 150~ for 2h before reaction. The activated solid was then suspended in dry toluene (30ml), and 1,1,3,3tetramethylguanidine (TMG, 1.00g, 8.7mmol) and 1-methyl-l,5,9-triazabicyclodecane (TBD, 0.3ml, 2.0mmol) were then added. The mixture was heated to reflux and held at this temperature for 7h. After cooling to room temperature, the mixture was filtered, and the solid washed thoroughly with toluene (3x 20ml), methanol (20ml) 1:1 v/v methanol / water (20ml) methanol (2x 20ml) and then extracted in a Soxhlet apparatus containing dichloromethane / diethyl ether (1:1) for 18h. This yielded 1.3g of product, the physical parameters of which are recorded in Table 1. 5.3 Grafting of guanidine silane 2 to MTS To activated (150~ vacuum, 18h) MTS (2.00g) was added dry toluene (50ml) and silane 2 (5mmol, 1.4g) The mixture was heated to reflux and held there for 15 hours. After cooling, the solid was filtered and washed thoroughly with methanol (3x50ml) and then extracted (Soxhlet, ether, dichloromethane 1:1). NMR (~3C, CPMAS) 10.5, SiCH2; 24.7, SiCH2CH2; 40.4 4x NCH3; 47.8 (CH30); 51.9 (CH2N=C); 162.2 (N=C(NMe2)2.

5.4 Epoxidation To a solution of 2-cyclohexenone (0.96g, 10mmol) in methanol (10ml) at 20~ the catalyst was added (0.10g) followed by hydrogen peroxide (30% v/v in water) dropwise. The reaction

1479 was followed by GC, using 2-phenyl-2-butanol in methanol as external standard, and products identified by GC-MS and by comparison with an authentic standard. Hydrogen peroxide was added until no more conversion was seen, and the total quantity added used to determine the selectivity towards oxidant. 5.5 Linstead-Knoevenagel condensation Heptanal (1.14 g, 10 mmol) and malonic acid (1.04 g, 10 mmol) are added to THF (30 ml). To this mixture was added the catalyst (150rag, activated at 100~ overnight under vacuum) and the mixture was heated to reflux under nitrogen atmosphere. Reaction is followed by GC, using dodecane in THF as external standard. 3-Nonenoic acid and 2-pentyl-2-nonenal were identified by GC/MS and by comparison with authentic compounds. Dicarboxylic acids are not detected by GC. After two days, the reaction is stopped and the catalyst filtered and washed with THF. The solvent is removed and the crude mixture recovered. Isolation of products was achieved by flash chromatography (SDS silica 60 AC.C/70-200 mesh, cyclohexane/diethyl ether/formic acid 90:10:1). 5.6 Attachment of chiral chloramidinium salt to AMPS (a) preparation of passivated aminopropyl-MTS. To 6.0g aminopropyl-MTS[6] (preheated at 150~ under vacuum) was added trimethylsilylimidazole (14g). The suspension was stirred under nitrogen for 8 hours at 60~ The solid was filtered and washed with toluene, DMF, methanol, acetone (each 2x35ml) before being extracted (Soxhlet, 50:50 ether : dichloromethane). The product was then dried at 60~ for 1 day. (b) Coupling reaction. The urea 14 (0.85g, 3mmol) was heated at 70~ in 20ml of chlorobenzene and oxalyl chloride (0.5g, 4mmol) added. Heating was continued for 14h. The flask was then cooled to 25~ and a vacuum applied to remove excess oxalyl chloride. After the removal of oxalyl chloride was complete, the passivated aminopropyl-MTS was added ((0.7g) and the reaction stirred for 48h. Triethylamine (0.2g) was then added and stirred for a further 3 hours. The solid was then filtered and washed with methanol and DMF. Soxhlet extraction with ether : dichloromethane (50:50) was then carried out overnight before drying at 50~ Loading from elemental analysis was 0.66mmol/g guanidine. 6. CONCLUSIONS The preparation of a range of guanidines supported on Micelle Templated Silicas is presented. It is possible to prepare these materials without significantly damaging the structure of the solid matrix, and control over surface properties is also achieved. Catalytic activity is good in two reaction types. Initial attempts to prepare chiral supported guanidines are underway, and supported salts have been prepared. Liberation of the free base is currently being undertaken 7. ACKNOWLEDGEMENTS DJM thanks the Royal Society for a University Research Fellowship, and for travel funds to carry out this work. KAU thanks the EPSRC for a Project Studentship. DJM and DB are grateful to the British Council / EGIDE Alliance scheme for travel funds. Authors are grateful to Dr Francois Fajula for consistent support, and Anne Derrien for useful preliminary work. They wish also to thank Dr Annie Finiels for NMR analysis and Dr Patrick Graffin for GCMS analysis.

1480 REFERENCES

1. "Fine Chemicals Through Heterogeneous Catalysis", ed. R A Sheldon, H Van Bekkum, Wiley-VCH (Weinheim) 2001 2. D J Macquarrie and D B Jackson, Chem. Commun., (1997) 1781 3. J E G Mdoe, J H Clark and D J Macquarrie, Synlett, (1998) 625 4. D J Macquarrie, Green Chem., (1999) 1 195 5. M Lasperas, T Llorett, L Chaves, I Rodriguez, A Cauvel and D Brunel, Stud. Sure Sci. Catal., (1997), 108 75 6. D Brunel, Microp. Mesop. Mater. (1999) 27 329 7. B M Choudary, M L Kantam, P Sreekanth, T Bandopadhyay, F Figueras, A Tuel, J. Mol. Cat., A, (1999) 142 361 8. T Genski, G Macdonald, X D Wei, N Lewis, R J K Taylor, Synlett, 795 (1999) 9. C.L. Dwyer, C. D. Gill, O. Ichihara and R. J. K. Taylor, Synlett, 2000, 704 10. Y V Subba Rao, D E de Vos, P A Jacobs, Angew. Chem. Int. Ed. Engl., (1997) 36 2661 11. A Derrien, G Renard, D Brunel, Stud. Sure Sci. Catal., (1998) 117 445 12. X Lin, G K Chuah, S Jaenicke, J. Mol. Cat., A, (1999) 150 287 13. T Isobe, K Fukuda, T Ishikawa, Tetrahedron Asymmetry, (1998) 9 1729 14. T Isobe, K Fukuda, T Ishikawa, J. Org. Chem., (2000) 65 7770 15. T Isobe, K Fukuda, T Ishikawa, J. Org. Chem., (2000) 65 7774 16. T Ishikawa, Y Araki, T Kumamoto, H Seki, K Fukuda, T Isobe, Chem. Commun (2000) 245 17. T Takago (to Shin-Etsu Chemical Industry Co. Ltd.), Ger. Often. 2 827 293 (1979) [US Patent, 4 248 992 (1981) 18. R J P Corriu, A Mehdi, C Rey6, C R Acad. Sci, Paris, t.2, S6rie IIc 35 (1999) 19. T Tatsumi, K A Koyano, N Igarishi, Chem. Commun., (1998) 325 20. M B D'Amore, S Schwarz, Chem. Commun., (1999) 121 21. S E Boxer and R P Linstead, J. Chem. Soc., (1931) 740 22. R P Linstead and E G Noble, J. Chem. Soc., (1933) 557 23. D J Macquarrie, Chem. Commun, (1996) 1961 24. D Macquarrie, D B Jackson, J E G Mdoe and J H Clark, New J. Chem, 1999 23 539


1481

A t t e m p t s on generating basic sites on m e s o p o r o u s materials* X.W. Yan, X.W. Han, W.Y. Huang, J.H. Zhu** and K. Min Department of Chemistry, Nanjing University, Nanjing 210093, China

The stability of A1-MCM-48 could be obviously enhanced by post-synthetic treatment. Basic mesoporous materials were thus prepared by introducing basic guest through microwave irradiation and impregnation along with ion exchange, exhibiting a considerable basicity in dehydrogenation of 2-propanol. 1. INTRODUCTION Basic selective catalysis is a developing field in the application of zeolite. Moreover, limited to their small pore size, zeolites are not efficient for large size molecule reactions such as pharmaceutical chemical reactions; therefore new basic material with uniform mesoporous size distribution is desirable. A family of mesoporous molecular sieves designated as M41S was developed on 1990's [1 ], and since then the research of novel mesoporous materials and their properties has attracted more and more attention [2-3]. Hexagonal mesoporous molecular sieve MCM-41, the representative of M41S, has been used to prepare basic catalyst either as ion-exchanged substrate for alkali-containing derivative [4], or as support for composite with basic organic group grafted [5]. Another important member of M41S series is the MCM-48 with cubic (Ia3d) mesophase that is expected to be a better candidate for preparation of basic catalyst because of its interwoven three-dimensional channel system [6]. However, the question on the preparation of satisfactory mesoporous supports and the generation of basic sites in an environmentally benign way still remains to be answered. For prepare basic zeolite, it is known that introducing basic guests is easier to create strong basic sites on zeolite than adjusting the chemical composition of host framework [7], and existence of aluminum in support is proven to be beneficial for preparing solid base [8]. Consulting these facts, we study the preparation of aluminum-containing MCM-48 (denoted as A1-MCM-48) and the modification with magnesia by the use of microwave radiation. *The key laboratory of chemical engineering and technology of Jiangsu province and Analysis Center of Nanjing University financially support this subject. ** Corresponding author, E-mail: [email protected], FAX: 0086-25-3317761.

1482 2. E X P E R I M E N T A L

A1-MCM-48 was prepared in basic condition, with the CTAB (Cationic surfactant cetyltrimethylammonium bromide) used as a template and TEOS (tetraethyl orthosilicate) as the silica source. The alumina source was fresh aluminum hydroxide gel, which was obtained from the recovery of the precipitation of aluminum sulfate solution and equivalent sodium hydroxide. In a typical procedure, a clear solution was obtained by dissolving 35 g CTAB in 115 g distilled water and 53 g of 1.5 mol/L sodium hydroxide solution was added dropwise into the emulsion formed by 33 g TEOS and 7.8 g distilled water. After hydrolyzing for about 3 rain, the TEOS emulsion was poured into the CTAB solution under vigorous stirring for 0.5 h. Then the aluminum hydroxide gel was added with vigorous stirring. The obtained mixture was sealed in a Teflon-lined stainless autoclave and kept at 373 K for 3 d, and the synthesis molar composition was TEOS:0.06AI(OH)3:0.6CTAB:0.4NaOH:60H20. The product was recovered by filtration, washed with distilled water and dried at ambient temperature. In a typical process of hydrothermal post-synthetic treatment, as-made product was mixed thoroughly with distilled water at 1 g/100 ml ratio [9], sealed again in the autoclave and kept at 373 K for 14 d, followed by recovery, washing and drying. The obtained material was calcined in N2 at 823 K for 1 h followed by in air for 5 h to remove the organic surfactant. Basic samples were prepared in the different ways as described below: (1). 2g MCM-48 was stirred in 200 ml solution of cesium nitrate and refluxed at 373 K for ion exchange, then filtrated, washed with adequate distilled water and dried at ambient temperature to obtain Cs+-A1-MCM-48, denoted as Cs+/ie sample. (2). 10 g NaX zeolite, supplied by Nanjing Inorganic Chemical Factory, was added to 100 ml NaC1 solution of 1 mol/L then vigorously stirred for 10 h under 353 K, followed by drying at 373K over night. This procedure was repeated for another 3 times. In succession, a 0.23 mol/L CsNO3 solution was used as the substitution of NaC1 solution, and ion exchanging was conducted for another 4 times as described above. The residue was washed, dried, and calcined in air at 823 K for 4 h. (3). 0.2 g A1-MCM-48 was added into 25 ml 0.02 mol/L magnesium acetate solution. After volatilization of the liquid, the residue was calcined in air at 773 K for 4 h to yield magnesium acetate impregnated A1-MCM-48, and denoted as MgO/imp sample. (4). MgO or CaO, dried at 473 K prior to use, was ground with A1-MCM-48 at a given ratio and named as MgO/mix or CaO/mix sample; some of them was then irradiated in a domestic microwave oven for 3-40 rain [10]. The finally obtained material was denoted as MgO/mw or CaO/mw sample. To characterize the resulting sample, XRD patterns were recorded on a Rigaku D/max-rA diffractometer employing CuKo~ radiation, from 0.5 ~ to 10 o for mesophase and 20 o to 60 o for alkaline-earth metal oxide detection. The X-ray tube was operated at 30 kV and 50 mA, 40 kV and 70 mA respectively. Nitrogen adsorption-desorption isotherms at 77 K were measured using a Micromeritics ASAP 2000 instrument, and the sample was activated at 573 K in vacuum line. The data were analyzed by Barrett-Joyner-Halenda (BJH) method using Hasley equation for multi-layer thickness. Pore size distribution curve of the sample came from the

1483 analysis of adsorption branch of isotherm, and the pore volume was taken at the P/Po=0.9869 signal point. The element molar ratios of components in the sample were calculated with the data investigated on a Shimadz VF-320 X-ray fluorescent spectrometer. All samples were melted in the flux agent before measurement. The X-ray tube was operated at 40 kV and 60 mA. To evaluate the basic catalysis function of sample, decomposition of 2-propanol was performed in a conventional flow-type reactor with a WHSV of 2.4/h, in the manner as described previously [11 ]. Although the reaction had been carried out for several hours, only the data of 0.5h was cited to compare the catalytic properties of the sample. 3. RESULTS AND DISCUSSION Figure 1 shows the XRD patterns of as-made A1-MCM-48, calcined and post-treated samples, and the patterns of as-made A1-MCM-48 can be indexed as a cubic mesophase belonging to Ia3d space group [6]. Since MCM-48 is less reliable to be impregnated in electrolyte aqua solution overnight, it was post-synthesis treated to improve its stability. As evident in the XRD patterns, sharp and more peaks emerged in the range of 20 between 3~ and 6~ along with the substantially greater intensity of peaks, indicating that the stability and long range mesoscopic ordering of the pores of MCM-48 has been promoted during the posttreatment. The enhanced stability has a significant influence, as demonstrated later, on the existence of mesoporous structure in the basic derivatives prepared under rigorous conditions. Figure 2A shows the N2 adsorb-desorption isotherms of post-treated A1-MCM-48 and Cs+/ie sample. They are type IV adsorption isotherms with two hysteresis loops. The sharp loop in the P/Po range of 0.2 to 0.4 corresponds to capillary condensation within uniform mesopores, and its steep slope reflects the uniform pore size of the sample. Another larger and longer loop

~-., ..',! r !li

9~_

i/ iio '."

9

'~04

bz"

i,,,

I'C Jl~i o40'"'o403 /

IV

/

2

li

I I :

v . . O ";, r

",-,,'I-'4.: ""

4

''r.... 03 "6

8

10

2e Figure 1. Powder X-ray diffraction patterns of as-made A1-MCM-48 (curve a), its calcined sample (curve b, dotted line), and its post-treated sample with subsequently calcination (curve c, dashed line), which has been indexed.

1484

i--

700

8

A

m 600 131

7

!4

E 500

,s

o

-o 400 r ..Q

'..0 300

3

t~ "13

< 200

o 2

13_

(1.)

E 100 o 0

>

5

012'014'016 018" Relative Pressure (P/Po)

0.0

1.0

1 o lO

100 1000 Pore Diameter (A)

Figure 2. (A) N2 adsorb-desorption isotherms of (a) the post-synthesis treated A1-MCM-48 and (b) Cs+/ie sample; (B) Pore size distribution curve of the post-synthesis treated A1-MCM-48. between P/Po of 0.5 and 1.0 can be probably attributed to a wide distribution range of the inter space among the sample particles. Influence of Cs + ion exchange on the structure of MCM-48 emerged on the isotherm of Cs+/ie sample whose slope in the first hysteresis loop of appeared from 250 to 350 cm3/g STP, lowered about 100 units than that of A1-MCM-48 (from 300 to 500 cm3/g STP). The total adsorption capacity and mesoporous volume of Cs+/ie sample seems to be ascended after ion-exchange procedure, but the mesoporous structure is well maintained proven by the clear first hysteresis loop. In the BJH plot of pore volume versus pore diameter (Fig.2B), a very narrow pore size distribution of A1-MCM-48 with a pore diameter of 3.4 nm was observed, and its fwhm of about 0.6 nm indicates a well-defined and uniform pore-size distribution. Besides, this sample has a BET surface area of 983 m2/g and a pore volume of 1.07 cm3/g (see Table 1). Figure 3 show the XRD patterns of three derivatives from A1-MCM-48, Cs+/ie, MgO/imp and MgO/mw samples along with the MgO/mix sample. Among them the Cs+/ie and MgO/mix

A

:..

,,

a

d

2

4

6 2--e

8

10

20

'

3'0

'

4'0

'

2e

Figure 3. XRD patterns of the sample derived from post-treated A1-MCM-48. a) MgO/mw; b) Cs+/ie, c) 10%MgO/imp and d) 10%MgO/mix sample.

5'0

'

60

1485 samples had a XRD patterns same as that of the parent material, indicating no obvious collapse formed even though the former was treated in hot water. However, impregnation in electrolyte solution for a long time left no distinct fine structure in the 20 range of 30-6~ on the XRD spectrum of MgO/imp sample, despite the cubic mesophase still preserved enough for further tests. This disadvantage results from the effect of ionic strength to accelerate hydrolysis, since if the sample was impregnated in a concentrated solution of 0.5 mol/L Mg(Oac)2 under the same condition, only a broad peak remained in the XRD patterns (not shown). For the sample of MgO/mw, only the intensity of XRD peaks was slightly lowered as seen in Fig 3A, though the dispersion of MgO by microwave radiation was conducted in an open system and the host was exposed to moisture during the process. The use of microwave radiation seems not only energy and time efficient to disperse basic guest on mesoporous materials, but also in favor of avoiding structural damage of the support. Figure 3B reveals the dispersion of MgO on A1-MCM-48. There was no MgO phase on the XRD patterns of impregnated sample, but a small one survived on that of MgO/mw and a large one in the MgO/mix sample. Through the comparison of MgO/mw sample and the mixture, it is clear that the majority of MgO has been dispersed on A1-MCM-48 by microwave radiation, similar to that reported on zeolite KL and NaY [ 10]. What is amazing about the survived XRD peak of MgO is whose intensity kept similar in MgO/mw sample, no matter how the radiation time was prolonged to 40 min or the mass ratio of MgO to the support was halved to 0.05/1. Abundance of silica in mesoporous support seems not beneficial for dispersion of MgO by microwave radiation, since the same amount of MgO cannot be dispersed on the porous silica either. However, on the zeolite ZSM-5 with a similar Si/A1 ratio as the MCM-48, a perfect dispersion of MgO has been observed [ 10], which indicates the more important role-played by geometric structure of the support than the chemical composition for dispersion of the basic guests. Intrinsic property of A1-MCM-48 host structure is another factor for the preservation of MgO particles. As mentioned below, some tiny fragments may be caused in the channel of A1MCM-48 by microwave radiation and wrapped the MgO particles, forming an obstacle for continuous dispersion of MgO. Table 1 lists the porosity characteristics of the basic derivatives, and among them the surface area and pore volume as well as the d value is smaller than that of A1-MCM-48. The pore size

Table 1. Structural properties of A1-MCM-48 and its basic derivatives Sample d" Surface area Pore size b (nm) (m2/g) (nm) A1-MCM-48 3.9 983 3.4 Cs+/ie 3.7 850 3.3 MgO/imp 3.6 814 3.4 5%MgO/mw 3.7 612 3.9 ! 0%MgO/mw 3.8 527 4.2 a) d (211) spacing, b) BJH adsorption average pore diameter.

Pore volume (cm3/g) 1.07 0.84 0.78 0.69 0.66

1486 Table 2. Decomposition of 2-propanol over the solid catalysts Reaction condition: in N2 (20 mmol/h), WHSV=2.4/h, 0.5h. Sample Temp. 1 Conv. 2 Selectivity Sample (K) (%) (%)3 A1-MCM-48 Cs+/ie

773 100 0 5%MgO/mix 673 100 2.60 723 100 9.66 773 100 15.13 5%MgO/mw MgO/imp 723 100 0 773 100 0.38 NaX 673 100 0.70 10%MgO/mw 723 99.56 0.11 773 100 2.58 10%CaO/mix CsX 673 69.37 20.93 723 87.50 1 3 . 4 1 10%CaO/mw 773 97.56 21.58 1Temp means the reaction temperature; 2 It represents the 3 The selectivity of acetone in the products of reaction.

Temp. 1 Conv. 2 (K) (%)

Selectivity (%)3

673 99.9 0.91 723 100 0.46 773 100 1.31 673 100 0.64 723 100 4.20 773 99.97 7.31 723 100 0.84 773 100 1.08 723 85.97 11.07 773 97.60 22.22 723 100 8.75 773 100 29.69 conversion of 2-propanol.

of Cs+/ie or MgO/imp sample was unchanged, but a rather enlargement was observed on the pore size of MgO/mw sample which seems to be accelerated by loading MgO: the more MgO loaded, the more obvious phenomena appeared. The different variation in the structure of A1MCM-48 result from different preparation methods. In aqueous solution, the host decomposed exteriorly with water immersing in it. For radiation, however, microwave penetrates through the wall of A1-MCM-48 and affects both outer and inner of the host at the same time, causing more remarkable crumbling in the inner pore especially in the presence of MgO. The role of MgO played in this procedure is not clear yet but seems very important. Mesoporous materials and zeolites can be microwave-assisted synthesized [12], so it is not unusual for them to be stable in microwave radiation though their surface became very hot [10,13]. The MgO guest adsorbed microwave energy and thus interacted with the host during the dispersion process to form tiny particles in which the MgO is wrapped, but a further study is desirable. Table 2 illustrates the performance of A1-MCM-48 and the basic derivatives in the reaction of 2-propanol decomposition. A1-MCM-48 was inactive for dehydrogenation of 2-propanol but active for dehydration due to the existence of acid sites. Acetone formed on Cs+/ie and MgO/mw sample at 673 K and its selectivity increased unequally with temperature, even higher than that on zeolite NaX, a typical basic zeolite (Table 2). For comparison the catalytic properties of MgO/mix were also listed in Table 2, but the acetone selectivity above 723 K was lower than MgO/mw. Although the catalysis function of MgO/mw sample for 2-propanol dehydrogenation rose totally from the introduced basic component, loading MgO of 10 wt.-% or more on the host did not enhance the selectivity of acetone in products. On the MgO/imp sample, however, no acetone was found up to 723K and only negligible acetone selectivity appeared at 773 K. Similar phenomena had been reported on the NaY sample impregnated

1487 Table 3. The component molar ratio of the basic mesoporous materials Sample Si/Cs Cs+/ie 36.65 MgO/imp 10%MgO/mw -

Si/A1 11.60 11.23 9.59

with Mg(NO3) 2 followed by calcination of 873K, on which the formed MgO was high dispersed on zeolite and lost its intrinsic catalytic properties [7]. In comparison with those zeolites loaded MgO [ 10], the resulting basicity on MCM-48 is relative weak. This difference, in our opinion, results from the geometric structure of the host. For instance, ion exchange with Cs + is a common way to enhance the basicity of molecular sieve, and the Cs +exchanged in MCM-48 presents a better catalytic performance than those derived from MgO loaded indeed. The reason is simple; every Cs § ion can act as the separate active site but the MgO disperses in the form of particle so the actual reactive area of MgO is much small according to its atomic amount. However, the basicity of Cs+/ie sample, represented by the selectivity of acetone in this probe reaction, was obviously weaker than that of zeolite CsX as demonstrated in Table 2. Microscopic structure of the surface assumedly takes charge of this difference. Basic zeolite CsX owns quasi-crystal framework, whereas mesoporous material is built with amorphous silica wall that is lack of both Br6nsted and Lewis site [ 14]. Additionally, the Si/A1 ratio of zeolite X is much lower than mesoporous MCM-48, which means the former can provide more acid sites than the latter for ion exchange. Since the concentrations of acid sites in hydrogenated mesoporous alumino-silicate are lower than the concentration of fourcoordinate aluminum [14], A1-MCM-48 host is unable to provide the anticipated number of active site for alkali matter though even its A1/Si ratio is low (ca circa 0.10). Moreover, as the component ratios in Table 3 reveals, the Si/A1 ratio of Cs+/ie and MgO/imp sample is actually higher than that of MgO/mw sample. This variation results from the especial preparation process; Unlike dispersion of MgO under microwave irradiation that occurs on the surface of solid in dry circumstance, ion exchange or impregnation is usually performed in aqueous solutions and therefore dealumination may take place in some extent. As the result, the ratio of Si/Cs is obviously higher than Si/A1 (Table 3), indicating a relatively low concentration of Cs § is exchanged on the parent MCM-48. This is not extraordinary since Cs § can only substitute the ion on the original Br6nsted site in host, so the number of Br6nsted site provided by the host limits the amount of exchanged Cs +ion. A further proof on this comes from the report on the sample of MCM-41 and zeolite NaY loaded KNO 3 [15] in which the latter exhibited a stronger basicity than the former. Clearly the lack of acid sites in mesoporous materials makes it inefficient to introduce basicity through ion-exchange method. For these intrinsic reasons, many methods, which can be used on zeolite to create strongly basic sites, do not work on the mesoporous molecular sieves. Consulting the performance of MgO/mw sample in 2-propanol dehydrogenation, microwave radiation is proven to be one optimistic way to get more resultant basic mesoporous catalyst.

1488 Without dependence on the amount of acid sites in support, microwave-assisted dispersion can introduce more basic guest on mesoporous host than alkali ion exchange; rather, the operation is much simple without any pollution or waste. Moreover, the obtained basicity in the sample can be adjusted by choosing suitable type and amount of guest material and modifying the surface of the host [10,13]. As the proof to pursue this point further, CaO was dispersed on A1MCM-48 by microwave radiation to replace MgO, and formed basic sites on the mesoporous material. As expected, these new basic sites exhibited a high catalytic activity for dehydrogenation of 2-propanol therefore more acetone formed at 773 K on CaO/mw than that on CsX zeolite, which would be discussed elsewhere in detail. From the results discussed above some conclusions can be made. Hydrothermal postsynthetic treatment significantly enhanced the stability of A1-MCM-48, so the host can keep its mesoporous structure in electrolyte aqua solution or microwave radiation. A considerable basicity can be introduced on the host of A1-MCM-48 by use of ion exchange, impregnation or microwave-assisted dispersion of basic guest such as MgO or CaO, possessing catalytic activity for dehydrogenation of 2-propanol. Among these methods, microwave radiation is proven to be a cost-effective and environmental benign process for preparing the basic mesoporous materials. ACKNOWLEDGEMENT We appreciate Professor Q.H. Xu and Y.Q. Liang (Nanjing University) for their helpful advice.

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359(1992) 710. 2. RT. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368(1994) 321. 3. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka, and G.D. Stucky, J. Am. Chem. Soc., 120(1998) 6024. 4. K.R. Kloeststra and H. Van Bekkum, J. Chem. Soc., Chem. Commun., (1995) 1005. 5. I. Rodriquez, S. Iborra, A. Corma, F. Rey and J.L. Jorda, Chem. Commun., (1999) 593. 6. V. Alfredsson and M.W. Anderson, Chem. Mater., 8(1996) 1141. 7. J.H. Zhu, Y. Chun, Y. Wang and Q.H. Xu, Chin. Sci. Bull., 44(1999) 1926. 8. J.H. Zhu, Y. Chun, Y. Qin and Q.H. Xu, Micropor. Mesopor. Mater., 24(1998) 19. 9. Q. Huo, D.I. Margolese and G.D. Stucky, Chem. Meter., 8(1996) 1147. 10. Y. Wang, J.H. Zhu, Y. Chun and Q.H. Xu, Micropor. Mesopor. Mater. 26(1998) 175. 11. J.L. Dong, J.H. Zhu and Q.H. Xu, Appl. Catal. A, 112(1994) 105. 12. D.S. Kim, J.S. Kim, J.S. Chang and S.E. Park, Stud. Surf. Sci. Catal. 135(2001) 333. 13. B.I. Whittington and N.B. Milestone, Zeolites, 12(1992) 815. 14. J. Weglarski, J. Datka, H.Y. He and J. Klinowski, J. C. S. Faraday Trans., 92(1996): 5161. 15. Y. Chun, J.H. Zhu, Y. Wang, D.K. Sun and Q. H. Xu, Proc.12th Intern. Zeol. Conf., (Eds: M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins), MRS 1999, Vol.2, 989-996.


1489

A p p l i c a t i o n o f zeolite in the health science" novel additive for cigarette to r e m o v e N - n i t r o s a m i n e s in smoke* Y. Xu, Y. Wang, J.H. Zhu**, L.L. Ma, L. Liu and J. Xue Chemistry Department, Nanjing University, Nanjing 210093, China

This paper reports the especial function of zeolite to remove N-nitrosamines along with the latest progress in new cigarette containing zeolite. N-nitrosamines in cigarette smoke can be strongly adsorbed and catalytic degraded by zeolite additive without any change in the taste of cigarette.

1. INTRODUCTION In the realm of health science, one of the new areas of study is the application of zeolites, because zeolite has the unique function of selective adsorption and catalysis. Among many efforts involved in slow release drugs [1], enzyme mimetic drugs [2], anti-tumor drugs [3] etc, a noteworthy example is the zeolite additive in cigarette to remove carcinogenic agents like Nnitrosamines [4-6]. Unlike the improved cigarette filter wrapped zeolites for adsorption of toxic chemical, zeolite additive in cigarette rod could perform catalysis when it approaches the hot zone in the burning cigarette, especially to decompose N-nitrosamines, the most active carcinogenic compounds in smoke [4-5]. Of the N-nitrosamines about half could be eliminated from the mainstream (MS, inhaled by smokers) or the sidestream (SS, the smoke formed in between puffs), and the taste of the cigarette was kept according to the results obtained in the laboratory [4]. However, many suspicions exist on the zeolite additive concerning how the Nnitrosamines are adsorbed in zeolite and if the addition of zeolite changes the properties of cigarette when the experiment is preformed in an industry scale. In this paper we try to explore the adsorption manner of N-nitrosamines in zeolite through analyzing the degradation products of N-nitrosodimethylamine (NDMA), N-nitrosopyrrolidine (NPYR) and Nnitrosohexamethyleneimine (NHMI) on different zeolites by use of GC-MS technique and * National Advanced Materials Committee of China and Analysis Center of Nanjing University financially support this subject. ** Corresponding author, E-mail:[email protected], FAX: 0086-25-3317761.

1490 TPSR (temperature programmed surface reaction) method. Moreover, the latest progress in new cigarette containing zeolite is also reported. 2. EXPERIMENTAL NaY, NaZSM-5, NaA and KA zeolites are commercially available in powder form. HZSM-5 zeolite was obtained by an ion exchange method from NaZSM-5 [7], and the surface basicity of NaY was enhanced by dispersion of magnesia with microwave irradiation [8]. NDMA, NPYR and NHMI had been purchased from Sigma and dissolved in methylene chloride [7] TPSR of N-nitrosamines adsorbed on zeolite was carried out in a flow reactor, an "on line" HP-5890 II GC with HP-5972 MSD was employed to explore how the carcinogenic compound was degraded on zeolite [6]. The test cigarette samples containing 3% of zeolite were prepared in two methods as reported by Meier et al. [4,9]. One was hand-rolled by mixing zeolite with finely cut tobacco until the mix looked perfectly uniform [4]; for another zeolite was added in solution and sprayed onto tobacco before cigarette manufacturing [9]. 4 or 30 cigarettes were smoked in the glass-made chamber designed by Caldwell [10] or Miyake [11 ], and the smoldering smoke or mainstream was collected to measure the observed content of N-nitrosamines. All of the smoke was pulled through 60-100 mL citrate-phosphate buffer at a rate of 1 L/min so that the N-nitrosamines could be absorbed. The buffer solution was extracted with methylene chloride then the combined organic fractions were dried over a bed of anhydrous sodium sulfate and concentrated to a final volume of 25mL. N-nitrosamines were chemically denitrosated and analyzed by use of a SP-830 spectrophotometer (ColeParmer)[7]. For the analysis of tobacco specific N-nitrosamines (TSNA) in cigarette smoke, Heinr Borgwaldt 20 port and 1-port smoking machines were used under the standard condition [12]. Thermal Energy Analyzer (Thermo Electron 502B) and HP 5890B GC equipped SPB-5 fused silica capillary column were employed and N-nitrosoamine-N-Propylamine (NDPA) were utilized as ISTD. 3. RESULTS AND DISCUSSION Figure 1 reveals the strong adsorption of N-nitrosamines on zeolites. When the zeolite NaY adsorbed NPYR was evacuated at 553 K, there was no characteristic Mass signal of NPYR with m/e of 100 appeared on the MS spectrum (Fig. 1A). This fact indicates the absence of NPYR desorption from the adsorbent though the temperature is close to the boiling point of the carcinogenic compound. Some fragments with m/e of 30, 41, 44 and 53 emerged on the spectrum along with the signal of water (m/e of 18) and N2 (m/e of 28), and they could be tentatively assigned to the degraded products of NPYR such as N20 or (CH3)2N§ (m/e = 44), 2methyl-l-H-pyrrole (m/e=53), and 1-nitroso-pyrrolidine (m/e of 30 and 41 ) [6]. A further confirmation came from Figure 1B in which zeolite NaY adsorbed the N-nitrosamines from tobacco and was evacuated at 514 K. No any characteristic Mass signal of NDMA (m/e =74) and NPYR (m/e = 100) [13] or TSNA such as NNN (N-nitrosonornicotine, with m/e of 178)

1491

18

160

28

,,,-->.

160

A

~" 120

=. 120

8 80 '8 ~
> Lxaz (Lae/z is the width of the mass transfer zone at time 0. The solution is subject to a radiation boundary condition at layer surface accounting for the respective rates of diffusing particles attachment and detachment [11]:

Yh = -hL exp(hZ Dt)erfc(h4r-Dt )- I + -~-77Th

(6)

where h = ko/D, ka is the rate constant of species attachment to the layer surface and D is the corresponding diffusion coefficient of a low molecular silicon containing species or coefficient Do characterizing the Brownian motion. The first term can be neglected for sufficiently large v/t. Thus, after an induction period, the function is reduced to a straight-line and the yield is Yh = V/(4D/~L 2) V/t - D/(kdL). Denoting the slope of the straight line Yh vs. V/t as k, one can estimate D as:

D - re. (kL/2) 2.

(7)

An important feature of this model is that slope of Yh VS. V/t is proportional to L s or when plotted Yh.L vs. V/t for various depths of liquid column above the support, the experimental points should fall into a narrow stripe along the straight-line.

1508 (ii) Assumption: The growth kinetics of crystal layer on the horizontally oriented support (yield Yh) is controlled exclusively by the sedimentation of colloidal particles of radius r which is invariant of synthesis time t. Thus, Yh (t) reads as:

(8)

Yh = (Vs. t)/L for synthesis time t _~ tm= = L/vs; Yh = 1 for t > tm=

For the sedimentation velocity of silicalite-1 colloidal particles in water at 170 ~ holds: vs = 2 7 1 1 ks. r with ks = 1.46 x 10 m .s-. A close value would correspond to compact colloidal silica particles. Table 1 gives an insight into the effect of sedimentation and Brownian motion. When plotting (Yh.L) vs. t, the experimental points should fall- similarly as in the case (i) into a narrow stripe along the corresponding dependence, however, the time dependence for both models is different. 3. EXPERIMENTAL The synthesis of silicalite-1 layers was carried out in 100 ml Teflon lined autoclaves. The starting molar composition was 10 SiO2" 1 TPABr 91 NaOH" 800 H20. Tosil (30 wt.% SiO2, pH = 9.0, supplied by Silchem Ltd., Czech Republic) served as the SiO2 source. SiO2 particles exhibit the size distribution plotted in Fig. 1 and measured by dynamic light scattering on a HORIBA LB-500 instrument. The bottom of the reaction vessel was covered either with mercury or with a Teflon disk. The reaction temperature was 170 or 180 ~ synthesis lasted 3 - 300 h. Membranes were characterized by SEM, helium density and XRD. Mass yields on horizontal and vertical areas were evaluated as Yh = m h / m t h e o r and Yv = mv/mtheor, respectively, where mtheor i s t h e mass of the total amount of SiO2 contained in the synthesis batch and mh, mv are masses of respective layers.

30 maximum: at 24 nm 2.5

standard deviation: 13 nm

2.0 0~ ._z,1,5

I

a 1.0 0.5 0.0

I

ex .,m~ I 0 20

40

t 60

80

100

120

140

nm

Fig. 1" Distribution of SiO2 particle size in Tosil (clear colloidal solution)

4. RESULTS AND DISCUSSION A typical membrane morphology is shown in Fig. 2. The quantity (Yh.L) was plotted vs. t m (Fig. 3). The linear form of Yh vs. t m is consistent with the growth kinetics controlled by diffusion of low molecular species or by a Brownian motion of colloidal particles. The respective diffusion model described above can be used for the growth kinetics in the region of low and medium yields (< 0.5). The full triangles in Fig. 3 belong to the synthesis on Teflon support. The importance of a diffusion or Brownian transport is also evident from Fig. 4. After synthesis at 180 ~ weighing of bottom as well as wall layers was performed and the yields Yh and Yv were plotted vs. t I/2 . The sedimentation can be excluded when considering the growth of the vertical layer, however, it cannot be neglected in the mass transport to the bottom.

1509

"

~'~.-"-.";Z',,~

-'9

" :~

-x.,~',X,'~,~- , ' -

~-+~-

s~

E .__..

| ,i i

x A,~"

.= 0 . 3 0

0,," /

"m

/

:-f 0.40

,,'"

,,"

.ai

8

Iti ill

/

/

| 0.20

E "

| / 8o ,i~," 6

'"F"o

/0

o

x

f

,

/

,," ,,,'0 ,/

//

4

t ~r2 [h~2]

Fig. 3" Yh.L for L= 9 mm (X), 16 mm (OHg, A-Teflon) and 31 mm (O) (at 170 ~

0

,/

O" 0 ,0 o

,,'"(3 I

'

I

'

2

t lr2

'

[h wx]

'

14

16

Fig. 4: Synthesis at 180 ~ yields from the bottom ( 0 ) and the wall ( 0 )

1510

4O00

o)

3000

smooth side

2000 1000

(o40)

1

10

'

I

'

20

:::it .c_

(060)

(0100)

I

30

I 10

20

10

20

'

I

40

'

1

50

rouo.e 30

40

50

2O00

1000

0

I

'

I

30

'

I

40

'

I

50

2e

Fig. 5: XRD of the membrane synthesized at 170 ~ h and that of a silicalite-1 powder The XRD pattern of the upper (rough) and bottom (smooth) side of the zeolite layer grown on mercury and that of a silicalite-1 powder sample are shown in Fig. 5 and indicate that the crystal layer consists of a

pure silicalite-1 phase. XRD pattern of the mercury side of the membrane clearly demonstrates the preferred orientation of crystals where the most of b-axes of the crystals are perpendicular to the mercury plane, i.e. the ac-faces of the crystals are parallel with the Hg surface. 0k0 diffraction lines (020, 040, 060, 080 and 0100) of the mercury side of the layer are more intensive in comparison with those of the solution side which are almost identical to these found in the XRD pattern of silicalite-1 powder. This indicates that the crystals of the solution side of the layer are not oriented. A diffusion coefficient was estimated fi'om the experimental slope of Yh vs. t I/2 according to the formula (7) as D - 1.5 x 10-6 cm2.s1. This value is practically the same for 170 and 180 ~ and it is at least one order of magnitude lower than that calculated for monomeric Si species at synthesis temperature. This estimation was made using diffusion coefficients of monomeric Si species measured by Jander and Jahr [12] at room temperature and extrapolating the data to 170 ~ using the activation energy Eo = 10 kJ/mol. Thus, rather a Brownian motion of nanoparticles is more probable to represent the determining step of crystal growth rate than a diffusion of low molecular species. Our value of D corresponds to the particle size of ca 25 nm (of. Table 1).

It appears that aider decreasing the reaction temperature to 160-140 ~ the activation energy of growth (calculated with the assumption that a diffusion process is still rate controlling) increases and reaches the value range (> 40 kJ/mol) given in the literature for a kind of a surface reaction. Such a behaviour suggests that in this temperature region a consecutive combination of two rate steps affect the layer growth. 5. CONCLUSION The silicalite-1 layer growth rate at 170-180 ~ on vertically oriented support is most likely controlled by Brownian motion of nanoparticles. The mass yield on horizontal area increases more rapidly with time due to an additional contribution of colloidal particles sedimentation. These horizontal yields are inversely proportional to the height of solution column. Mass yields on mercury and Teflon do not differ.

1511 ACKNOWLEDGEMENT

This study was supported by Grant Agency of the Czech Republic via grants No. 203/99/0522 and No. 104/01/0945. The authors thank to Mr. A. Gaba (Labimex, Prague) and Mr. H. Wachernig (Retsch, Haan) for arranging the measurement by dynamic fight scattering. REFERENCES

1. R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London, 1982, pp. 152-153. 2. B.J. Schoeman, J. Sterte, J.-E. Ottersted, Zeolites 14 (1994) 568. 3. V. Valtchev, S. Mintova, L. Konstantinov, Zeolites 15 (1995) 679. 4. J. Hedlund, S. Mintova, J. Sterte, Microporous Mesoporous Mater. 28 (1999) 185. 5. V. Valtchev, S. Mintova, Microporous Mesoporous Mater. 43 (2001) 41. 6. Y. Kyiozumi, F. Mizukami, K. Maeda, S. Niwa, Advanced Materials 8 (1996) 517. 7. G.H. Weiss in Advances in Chemical Physics vol XIII, I. Prigodine (ed.), J. Wiley, London, 1967. 8. R.J. Stokes, D. F. Evans, Fundamentals of Interracial Engineering, Wiley-VCH, New York, 1997, p. 98. 9. N.N. Tunicky, V.A. Kaminsky, S. F. Timashev, Metody fizikokhimicheskoi kinetiky, izd. Khimia, Moskva, 1972, pp. 14-23. 10. T.G.M. van de Ven, Colloidal Hydrodynamics, Academic Press, London, 1989, p. 70. 11. J. Crank, The Mathematics of Dii~sion, Clarendon Press, Oxford, 1956, pp. 34 - 37. 12. G. Jander, K.F. Jahr, Kolloid-Beihefie, W. Ostwald (ed.), vol. 41, T. Steinkopf, Dresden 1935.



1513

Transport characteristics o f zeolite m e m b r a n e from d y n a m i c e x p e r i m e n t s Arlette Zikfinovfi b, Bohumil Bernauer a*, Vlastimil Fila a, Pavel Hrabfinek a, Jifi Hradil r Vladislav Krystl a and Milan Ko6ifik b

a

Institute of Chemical Technology, Technickfi 5, 166 28 Praha 6, Czech Republic

b j. Heyrovsk2~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejgkova 3, 182 23 Prague 8, Czech Republic c Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovsk2~ Sq. 2, 162 06 Prague 6, Czech Republic The analysis of the Wicke-Kallenbach dynamic experiments in semi-open arrangement showed that for single component and binary mixture permeation the effective diffusivities can be estimated from responses to step changes in partial pressure of components at the cell open side. The various techniques were used to study single component permeation of nitrogen and binary nitrogen - methane mixture and to evaluate M a x w e l l - Stefan diffusion coefficients of the species. Comparison of the results from single component permeation and counter-current binary permeation is presented.

1. INTRODUCTION Prediction of the separation performance of molecular sieve materials requires knowledge on multicomponent diffusion and adsorption parameters. Measurements of transport related characteristics of microporous zeolitic matrices using uptake sorption techniques such as gravimetric or piezometric ones fail frequently due to very low overall time constants of transient processes and also to rate mechanisms superimposed on intracrystalline diffusion. Consequently there exist orders of magnitude differences between values of transport parameters measured with different techniques. These techniques can be divided in macroscopic (or transport) methods and microscopic (or spectroscopic) ones. The macroscopic methods provide the value of flux as a function of concentration gradient. Examples of such methods are uptake measurements in a static or flow system, frequency response techniques or various modifications of chromatographic methods. Examples of microscopic techniques to measure self-diffusion of components in the absence of a concentration gradient are the NMR methods, quasi-elastic neutron scattering or light scattering1.

Corresponding author. E-mail: [email protected]

1514 A large effort in a development of zeolite-based membranes offers new routes to investigation of zeolitic transport cf. e.g. 1-3. Permeation through a membrane composed of randomly oriented and intergrown non uniform zeolitic crystals is different from permeation in a single crystal and the diffusion in intercrystalline voids can interfere with true intracrystalline transport. For zeolites such as those of MFI type which contain after synthesis in their structure molecules of organic species there is a simple test of zeolite membrane integrity based on impermeability of as-synthesized zeolite layers. Thus, e.g. a long-time invariability of an air pressure difference with respect to atmospheric pressure in a vessel separated from the ambient atmosphere by an as-synthesized membrane would indicate that intercrystalline voids have no significant influence on the permeation experiment. Apart from diffusion in a perfect zeolite channel system there are grain boundaries within apparent single crystals of silicalite-1. The occurrence of grain boundaries is enhanced in polycrystalline randomly oriented zeolite layers. In isolated silicalite-1 crystals there was investigated the effect of grain boundaries in detail by light microscopy4-6. In references 4'5 a counter diffusion of iodine against large organic molecules was investigated and it was evidenced that at high crystal loading the boundaries represent a leakage for mass transport. By contrast, the grain boundaries represent in the same type of silicalite-1 crystals (90~ a barrier for mass transport of isobutane 6. The membrane technique seems to be an appropriate tool to study the mass transport in zeolitic materials. A benefit would be in particular when results are directed to check the extent of crystal coalescence in zeolite polycrystalline layers and that of the interfacial area of grain boundaries in them during the synthesis. The aim of the present work is to analyze the transient and steady state diffusion and permeation measuring techniques to estimate the transport related parameters of polycrystalline zeolite layers. In the present work we focus our attention on coarse grained layers of Silicalite-1 grown on c~-alumina disks. As model sorbing species to study the zeolitic transport we selected deliberately relatively weakly adsorbing components N2 and CH4. Phenomenological model to analyze the mass transport in zeolitic matrix will be that based on Maxwell-Stefan formulation 7. 2. EXPERIMENTAL 2.1. Materials

The membrane supports used in this study consist of an asymmetric a-alumina filtration elements in the form of disks Q 20 mm, thickness of 3 mm prepared in our laboratories in cooperation with Carborundum Electric company. The details of synthesis and characterization can be found elsewhere 8. The body of the disk consisted of two layers:(i) ~ 2.8 mm in depth with average pore diameter of 12 ~tm and (ii) ~ 0.2 mm in depth of the a-alumina grain size 3 ~tm and a maximum defect of the radius 0.8 gm (estimated by the bubble point method). The magnitude A of the area of the composite membrane exposed to the gas phase was A = 3.0x 10-4 m 2. Silicalite-1 layers were synthesized on the surface of the supports following the procedure by Giroir-Fendler et al. 9. The synthesis mixture was composed of silica (12g of Aerosil 380, Deggusa), 100 ml 1.0 M solution of TPAOH (Fluka). The synthesis took place in Teflon lined stainless steel autoclaves. Disks were fixed in Teflon holders to keep them in vertical position and favored the silicalite-1 growth on the side containing the fine pores. Synthesis and crystallization took place at 180~ for 72 hours.

1515 The calcination in air flow 300 ml/min was performed to remove the template using temperature program: sample was heated 0.5~ to 550~ kept at 550~ for 24 hours, cooled 2~ to 30~ 2.2. Measurement of permeation flows The experimental arrangement used in this study was as follows: 1) A standard Wicke-Kallenbach cell (W-K cell) operated in steady state is shown in Fig.

9

•(2) )

1 The measured quantities of the system for the open W-K cell are the input (P/o1),~ ;o

t P/~I)~-~

~

e/~2)I

I Figure 1. Schematic representation of Wicke- Kallenbach cell.

and output (~o),fi(2)) partial pressures of component i in the compartments (1) and (2) of the cell and the corresponding input (Fo('), Fo(2)) and output ( f ( 1 ) , f (2)) total molar flow rates. The global flux densities of the species (Ni) can be calculated from mass balances of the cell. 2) A semi-open W-K cell was constructed to observe transient processes exclusively. Its operation can be described using the schematic representation of the standard W-K cell depicted in

V(2)

Figure 1. The compartment (2) was closed i.e. Fo(2) = 0 . Volume of the compartment (2) was stirred with a magnetically driven propeller and was equipped with a septum to take samples for GC analysis 9The membrane was tightened with a Viton "O" ring. Sufficiently high molar flow Fo(1) ( Z Ni 60 ml/min. The semiopen W-K cell was equipped with a difference pressure manometer to indicate changes in total pressure difference between the compartment (1) and (2) during the transient process. A precise barometer measured the total ambient pressure Pa. A rough estimate of the thickness L of the composite layer was made using SEM. The effective membrane thickness L was also an adjustable parameter in the model referred to below (cf. Eqs. (2) to (5)). The simulation procedure provides an adequate value of L even for transient processes very close to quasi-stationary ones i.e. for low values of the parameter H defined by Eq. (1). 3) The non-stationary permeation of gases through the membranes was also measured using a semi-open permeation apparatus with a reservoir of the testing gas of the volume V = 2.0x 10 -3 m 3 and with the gas pressurized at experiment beginning to the pressure difference A Po ~ (0.5, 5 ) bar over Pa. AP(t) was monitored by a precise gauge. 3. RESULTS AND DISCUSSION Results of non-stationary diffusion measurements of binary mixtures N2 - CH4 performed at 298 K in the semi-open W-K cell under counter-diffusion conditions are

1516

1 -

A g 2 ) c n 4 (t) / A g l ) c n 4 (t) iil

i 00

.0.51

t 4000

t Is] 6000

-1

-1.5 -2

-

-2.5 -

9

-3-3.5

9

_

-4 -4.5

_

-5

80 00

exemplified in Figure 3 as the plots In {1 A P(2)CH4 (t) / A P(1)CH4 (t)}. The data are based on transient responses of the CH4 content in the volume U 2) to a step change in the composition in chamber (1) from the CH4 molar fraction XCH4 O - ) "- 0 ( f l O W of pure nitrogen) to XCH4(O+) -- 0.2 (flow of the corresponding mixture nitrogen + CH4). Overall change A Pz-(t) was found to

Figure 2 In {1 - A P(2)CH4 (t) / A pa) CH4 (t)} -- t plot for diffusion of C H 4 against N2 through silicalite-1 membrane. Results from two identical measurements are presented

(m,~).

be less than 0.05 x Ap(1)CH4(0+). Thus, we neglected at this stage the pressure change. It can be seen that the plots exhibit in a broad interval of the 1 - A P(e)ci-i4 (t) / A pr (t) values i.e. between 1x 10-2 and 5x 10 .2 a linear shape y = - a t. This is the consequence of a low value of the parameter H (cf. Eq. (1)) which represents the ratio of the amount of sorbing species accumulated at equilibrium in the membrane to that accumulated in the volume V(2) = 68 cm 3.

HCH4 = LA(K~

(2)/RT) = 1.13x 10.2 0 : (i) the sorption capacity of the membrane for the tested species is negligible with respect to the reservoir accumulation capacity of the volume V and at the same time (ii) the permeability of the membrane does not change during the transient process as a result of species immobilization. Figure 2 shows the results of permeation measure-ment of N2. For the total time constant (pJ)i of the transient permeation process in the region of a linear sorption isotherm we deduced the formula (1) consistent with formulas presented for macroporous bodies 1~ 9 (l-t0i = L2/2Div + L2/ (DivHi )

(6)

Here L denotes an effective thickness of the zeolite layer, D i v is the diffusion coefficient of the species i through the membrane layer in the Fickian approach and Hi is defined as:

Hi = LA (Ki)t /(V/RT)

(7)

1519 It represents the ratio of the amount of sorbing species accumulated at eqilibrium in the membrane (of the volume LA) to that accumulated in the volume V. (Ki)t is the slope of sorption isotherm at the partial pressure Pi - Pa of the permeating species. For sorption descibed by Langmuir type isotherm equation (7) makes possible to estimate the upper bound to Hi which corresponds to the condition Pa = 0 (evacuation of the line at the membrane outlet). Thus, the estimate of (KN2)t for N2 is (K~ = qsat,N2 bN2 >- (KN2)t and qsat,N2bN2 - - 3.8x 10- 3 mol/(Pa.m 3) 12 and the estimate of HN2 i s "

(8)

HN2 < 5 . 3 3 x 1 0 -5

This result implies that the first term in equation (6) which characterizes the nonstationary diffusion in the membrane is negligible as compared to the second one. The evaluation of the data from the experiment exemplified in Figure 3 is as follows: From the slope of the plot one obtains a = 1.614x 10 .4 s and then using equation (9) the quantity kN2/L. Here ku2 stands for the permeability of membrane for pure N2: kN2/L = ~. V/(ART) = 4.34x 10 -7 mol/(s.m2pa)

(9)

The corresponding value of the parameter kN2/L 2 c a n be estimated from (9) using the thickness L = 4.1 x 10 -5 m obtained from the above diffusion experiment: kN2/Z 2

=

(10)

1.059x 10 -2 mol/(s.m 3 Pa)

and the corresponding value of the quantity DN2v/L 2 c a n DN2v/L 2 = kN2/(L 2 Ks

) =

be estimated as"

3.03 s-1

(11)

Here Ks (Ks = 3.49• .3 mol/m3/pa) is the initial slope of the secant of the sorption isotherm between the pressures at the inlet membrane side PN2 = 2x 105 Pa and that at the outlet pressure side PN2 = 1• 105 Pa at the beginning of permeation process. The Maxwell-Stefan diffusion coefficient Div is related to the Fickian diffusion coefficient by the relation 7 : DiV = D i z /I~i = Div / ( 0 lnPi / 0 lnqi)

(12)

Using the Langmuir isotherm one obtains for 1--'N2the formula: 1-'N~ = ~

1

(13)

1--0N2

where

|

= -

qN 2 -

q sat,N2

Thus, at the membrane inlet (PN2 = 2x 10 5 Pa) 1"N2(0) "- 1.192 for t = 0 and at the experiment membrane outlet (for any t _>0 PN2 = lxlO 5 Pa) 1-'N2 = 1.087. Taking the mean value ofI-'N2 = 1.14 one obtains DN2v/L 2 = 2.66 s -1 .

1520 4. CONCLUSIONS The Maxwell-Stefan diffusion coefficient of N2 obtained from transient permeation experiments was found to be reproducibly lower as compared with that obtained from transient diffusion experiment with the mixture C H 4 - N2. This result was obtained at the conditions that a contingent change of the total pressure in the chamber (2) of the W-K cell were neglected. Thus, for the diffusion coefficient of N2 in silicalite-1 gives permeation technique of single component the value 4.47x 10.9 mZ/s whereas the diffusion measurements in binary mixture the value 2.62x10 8 mZ/s. For diffusion coefficient of CH4 one obtains from diffusion measurements in binary mixture the value 5.05x 10-8 mZ/s. ACKNOWLEDGEMENT

This research was supported by the Grant Agency of the Academy of Sciences of the Czech Republic as Grant No 4040901 and Grant Agency of Czech Republic, grant no. 104/01/0945. Professor W.E.Stewart (University of Wisconsin) and his coworkers are acknowledged for permission to use GREG package in our parameter estimation problem. REFERENCES

1. J. K~irger and D.M Ruthven., Diffusion in zeolites and others microporous solids, Wiley, New York, 1992. 2. J.M. van de Graaf, F. Kapteijn and J. A. Moulijn, AICHE J. 45 (1999) 497. 3. F.Kapteijn, J. A. Moulijn and R. Krishna, Chem. Eng. Sci. 55 (2000) 2923. 4. M. Ko~ifik, J. Komatowski, V. Masafik, P. Novfik, A. Zikfinovfi, J. Maixner, Microporous and Mesoporous Materials 23 (1998) 295. 5. V. Masafik, P. Novfik, A. Zikfinovfi, J. Komatowski, J. Maixner, M. Ko~ifik, Collect. Czech. Chem. Commun. 63 (1998) 321. 6. O. Geier, S. Vasenkov, E. Lehmann, J. K/irger, R.A. Rakoczy, J. Weitkamp, Stud. Surf. Sci. Catal 135 (2001) 154. 7. R. Krishna, Chem. Eng. Sci. 45 (1990) 1779. 8. J. Pavlfi, J. Kudovfi, A. Zikfinovfi, M. Ko6ifik, P. Uchytil, O. Solcovfi, J. Ro6ek, V. Fila, B. Bemauer, V. Krystl, P. Hrabfinek, Chemick6 listy (in Czech), submitted. 9. A. Giroir-Fendler, A. Julbe, J. Ramsay, J.-A. Dalmon, Patent WO 95/297/751. 10. A. Burghardt, J.M. Smith, Chem. Eng. Sci. 34 (1979) 267. 11. D. Amost, P. Schneider, Chem. Eng. Sci. 49 (1994) 393. 12. T.C. Golden and S.J. Sircar, J. Colloid. Interface. Sci. 162 (1994) 182. 13. A. Vignes, Ind. Engng. Chem. Fundam. 5 (1966) 189. 14. B. Bemauer, M. Kocirik, V. Fila, V. Krystl and J. Fulem, Computers&Chemical Engineering, submitted. 15. J. Villadsen, M. Michelsen, Solution of Differential Equations Models by Polynomial Approximation, Prentice Hall, New York 1978. 16. K. E. Brenan, S. L. Campbell, and L. R. Petzold, Numerical Solution of Initial-Value Problems in Differential-Algebraic Equations, Elsevier, New York, 1989. 17. P. N. Brown, A. C. Hindmarsh, and L. R. Petzold, SIAM J. Sci. Comp., 15, 1467(1994). 18. W.E. Stewart, M. Caracotsios, J. P. Sorensen, AICHE J. 38 (1992)641.


1521

Incorporation of zeolites in polyimide matrices P. Sysela, M. Fry(,ovfia, R. Hobzovfi~, V. Krystlb, P. Hrabfinekb, B. Bernauer b, L. Brabec c and M. K o ~ i ~ c aDepartment of Polymers, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic bDepartment of Inorganic Technology, Institute of Chemical Technology, 166 28 Prague 6, Czech Republic r Heyrovslc~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Prague 8, 182 23 Czech Republic 3-Aminopropyltriethoxysilane terminated polyimide precursors (polyamic acids) and Silicalite-1 were used to prepare zeolite-filled polyimide films. The accessibility of the pores of the zeolite built in both non-treated and treated polyimide matrices was studied using sorption of iodine from the vapour phase. Transport properties of the filled films were investigated using small probe molecules of hydrogen and methane. 1. INTRODUCTION Polymeric membranes have been successfully applied in numerous processes of gas separation. Principal requirements of the membrane technologies are on increasing permeability at sufficiently high selectivity. In this respect pure polymeric membranes reached their limit [ 1]. A promissing route to membranes of improved permeabilities consists in incorporation of microporous materials into polymers. Thus, the incorporation of zeolites into a rubbery polymeric membrane may result in an improvement of its properties both in separation of gases and pervaporation [2,3]. Lower selectivies have been achieved by incorporation of zeolites into glassy polymers. One reason of this result was generation of voids at the polymer zeolite interface [4-6]. This drawback may, however, be overcompensated in many respects with outstanding mechanical and chemical stability of some high-performance glassy polymers at elevated temperatures. Aromatic polyimides (PI) exhibit very good chemical, mechanical and dielectric stability at temperatures up to 200-250~ They are mostly used in (micro)electronics, aviation industry, aerospace investigation and as polymeric separation membranes [7]. Non-porous polyimide membranes show high separation factors for separation of

1522 mixtures of permanent gases but low permeability for both permanent gases and organic vapours [8]. Asymmetric membranes [9], hollow-fiber membranes [10] and non-porous membranes [ 11] based on polyimides, poly(ether-imide)s and poly(amideimide)s crosslinked with poly(ethylene adipate), respectively, were successfully employed in separation of organic vapours from permanent gases. The aim of the present work is to examine feasibility of Silicalite-1 - PI composites which would exhibit (i) a sufficient interracial adhesion of phases, (ii) accessibility of zeolitic phase for sorbing molecules, (iii) an enhanced flow of species at reasonable selectivities. 2. EXPERIMENTAL 2.1. Chemicals

Pyromellitic dianhydride (PMDA) and 4,4"-(hexafluoroisopropylidene)diphthalic anhydride (6FDA) were heated to 180~ overnight in a vacuum before use. 4,4"Oxydianiline (ODA) (all Aldrich, Czech Republic) and p-aminopropyltriethoxysilane (APTES) (ABCR, Germany) were used as received. N-Methyl-2-pyrrolidone (NMP) (Merck, Czech Republic) and N,N-dimethylformamide (DMF) (Aldrich) were distilled under vacuum over phosphorus pentoxide and stored in an inert atmosphere. 2.2. Procedures

Polyimide precursors, polyamic acids (PAA), were prepared in a 250 ml two-necked flask equipped with a magnetic stirrer and a nitrogen inlet/outlet. PAA based on a dianhydride and ODA with uncontrolled number average molecular weight (M.) were prepared by the reaction of equimolar amounts of the dianhydride and the diamine in NMP or DMF (solid content 15 wt.%) at room temperature for 24 h [7]. A typical example of the Pl-aminopropyltriethoxysilane terminated PAA (6FDAODA) with Mn = 10000 g mol is as follows: 6FDA was dissolved in NMP or DMF and the terminating agent (APTES) was added to the reaction mixture and allowed to react with 6FDA for 2 h. 6FDA was then added and the reaction (solid content 15 wt.%) was allowed to proceed at room temperature for 24 h. Zeolite crystals were prepared according to the protocol by Kornatowski [12]. They were S ilicalite-l-90~ with S i/A1 ratio cca 350 and the lenght Lc cca 190 ~tm. The most crystals are of prismatic form with a minimum crosses and rosettes. The template (tetrapropylammonium bromide) was removed in a single stage calcination process in the flow of air (60 ml min l ) using the heating programme applied to a shallow bed of crystals. ZSM-5 was supplied by PQ Zeolites (Conteka) and NaY by VURUP Bratislava. Membranes were prepared by dispersion of the zeolite in PAA solution in NMP or DMF (by stirring for 2 h) and subsequent casting of the mixture on a Teflon substrate.

1523 After solvent removal the films were heated in subsequent steps up to 230~ for 2 h. The content of zeolites in the films was 10 wt.%.

2.3.

Instrumental techniques

Sorption experiments using Iodine Indicator Technique (involving light microscopy) [13,14] were performed with crystals embedded in polyimide matrix. Fine iodine particles placed at the beginning of the experiment into the cell represented the source of iodine vapour. The sorption kinetics was monitored by taking coloured photographs of the filled films at different contact times. The photographs provided information on uniformity of colouring process and the time interval necessary for reaching a limiting intensity of crystal colouring. Optical observations of sorption by transmission light microscopy were done with a microscope Peraval Interphako (Carl Zeiss Jena) coupled with a digital camera Nikon Coolpix 950. A sorbent loaded with volatile species was examined in a closed glass cell with a total thickness of about 2.5 mm (including the glass) and an internal space of a cylindrical shape (15 mm in diameter and 1.2 mm in depth). All the observations were performed at room temperature under air. The permeability and selectivity of the composite membranes were investigated using Wicke-Kallenbach cell and small probe molecules as hydrogen and methane [151. 3. RESULTS AND DISCUSSION

3.1. System selection The characterization of the zeolites tested in this study is given in Table 1. Distribution of both ZSM-5 and NaY inclusions in polyimide films was strongly nonuniform (based on observations by light microscopy - crystal aglomerates form dark patterns) probably due to their hydrophilic character. The most uniform distribution of zeolitic inclusions was obtained with Silicalite-1 crystals (Figure 1). We were not successful in the preparation of self-standing PI-zeolite films with PI Table 1 Characterization of zeolites Crystal length

(~m)

Silicalite- 1 ZSM-5 NaY

190 1-3 1

Pore size

Si / A 1

0.5 0.5 0.8

350 25 2

(nm)

Hydrophilicity

very low middle high

1524 based on PMDA and ODA (both with uncontrolled or controlled Mn). The most probable reason of miscarriage was PI (PMDA-ODA) chain rigidity. On the contrary, PI-zeolite films based on 6FDA and ODA (both with uncontrolled or controlled M,) were self-standing up to 50 wt.% zeolite content. All composites based on PAA (6FDA-ODA) with uncontrolled Mn (see Figure 2 for its preparation) and zeolites summarized in Table 1 exhibited very high permeabilities and very low selectivities. One reason of this result was undoubtedly a generation of interfacial voids at the polymer-zeolite interface. This phenomenon is typical for glassy polymer- zeolite combination [4].

~,,

~

.

~, .I ~

"~.~,~;";~"!;:~i~i,::~:.; ~;

-:

,..~

,

~

,,

@

} ,,

9 I

II

~ ~'

O

,,

9

200gr~l '

- ,:~..~

g,~

_, ,~

?oo ,

"k a

!

'~o

!

b

,!,5

,20O~tm I

I

Figure 1. Light micrographs of PI (6FDA-ODA)-zeolite films based on p-aminopropyltriethoxysilane terminated PAA (6FDA-ODA) with theoretical M, = 10000 o g mol 1 and 10 wt,~ of a) Silicalite-1, b) ZSM-5 and c) NaY

1525

/7---~, / = k u

NMP

[

RT, N 2, 24 h.

-H20I ,~I

r-

/

k ....,~.~

]

%.~'-../~

I_o"

x,,~_~ !

In

Figure 2. Preparation of PI (6FDA-ODA) with uncontrolled Mn To improve the adhesion between the matrix and the zeolite we prepared PAA (6FDA-ODA) with controlled Mn terminated with APTES (see Figure 3 for its preparation). The theoretical Mn was 10000 g mo1-1. The imidization was accompanied with the formation of Si-O-Si bonds between the polymer matrix and the zeolite surface bearing hydroxyl groups (Figure 4). The silylation of the zeolite with APTES was studied as a tool to improve zeolite incorporation in the polyimide matrix [6].

~F3

o c--- ~

o ",,,.~- ~ c

o

/O--C2H5 H2N---( CH2)3--S~'O--C2H5 O--C2H5

NMP

RT, N.2, 24 h, OF3

[

I"~176 F ,_. o /

(H~C20)3Si-(CH2)s.

,

O

OII

,, j F - ~ o _ ~ ~

c-" ~-

_c. ~ . . c _

~r""

/CF3

~..coo.

~. 2.7 '0,

04 0

if) t--

I'~ 0

g I13

....,

g

r-

b

E

800

700

600

500

400

Wavenumber/cm-1 Figure 1. Raman spectra of ZrO 2 (a) dried and calcined at (b) 300~ (c) 500~ (d) 700~

300

200

800

700

600

500

400

300

200

Wavenumber/cm -~

Figure 2. Raman spectra of 20%KNO3/ZrO 2 (a) dried and calcined at (b) 300~ (c) 500~ (d) 700~

tetragonal form at 645 and 268 cm -~ were also presented. In addition, the band intensity of potassium nitrate at 716 cm -1 became weaker. It is noticeable that the metastable tetragonal crystalline is still maintained in large extent even when the sample was heated at 700~ (Fig.2d), while calcination at 700~ diminished the band at 716cm 1 completely, indicating the decomposition of KNO 3 on the surface of ZrO 2 as shown in Fig.2d. After calcination at 700~ the free ZrO 2 transferred completely to monoclinic form, but on the sample loaded K salt the stability of metastable tetragonal form seemed to be significantly enhanced because it still remains in a large extent. These Raman results are in good agreement with those XRD tests on the same sample as reported previously [3]. Obviously, such stabilization is related to the introduction of potassium salt, probably due to the interaction of K ions with ZrO 2. Duwez et al. [6] reported the stabilization of high-temperature tetragonal crystal after incorporation of a Ca 2+ into ZrO2, and those Ca 2+ ions were assumed to occupy the position of Zr 4+ and formed an O 2- vacancy. Therefore these K § ions were inferred to occupy the position of octa-coordinated Zr 4+ which is more stable than the hepta-coordinated Zr 4+, due to interaction of K salt with zirconia [3].

3.2 Formation of superbasic sites In order to investigate the basic site of 20%KNO3/ZrO2, this sample was heated at 600~ for 5h in air and consequently kept in air for 24h at room temperature. Formation of carbonate-like substance on the sample was observed by IR spectroscopy. As shown in Fig.3, new bands of carbonates-like species appeared at 1344,1260 and 1030 cm -1 (Fig.3b), meanwhile the 1380 cm I band of N-O asymmetric stretching vibration of NO3- disappeared. Moreover, the band at 1615 cm 4 became significantly broader to 1580-1540cm ~, the former was assigned tentatively to bicarbonate species due to the binding of CO2 to surface OHgroups, while the latter could be attributed to an asymmetric stretching vibration of surface

1548

b S"

,

18oo

i

16oo

,,

,

i

1380

1615 ,

1

120o 1400 Wavenumber/crn 1

i

,

1000

Figure 3. IR spectra of 2 0 % K N O 3 / Z r O 2 (a)dried (b) calcined at 600~

1800

,

I

1600

,

I

i

1400

1200

1000

Wavenumber/cm 1

~-Figure 4. IR spectra of ZrO2 (a) dried (b) calcined at 600~

monodentate and bidentate species [7]. As a comparison, no such new characterizing bands were observed on the spectra of free ZrO2 after calcination at 600~ in air as shown in Fig.4b. Fukuda and Tanabe examined the adsorption of CO2 on CaO and observed the phenomena that the monodentate carbonate complex changed into bidentate carbonate complex when the evacuation temperature was increased from room temperature to 300~ [8]. The bidentate carbonate species was proposed to be formed by CO2 binding not only to surface oxygen but also to surface metal ions, while monodentate carbonate species can be formed by the CO2 adsorbed on surface basic sites 0 2. even if at room temperature. On the surface of 20%KNO3/ZrO2 sample, the surface carbonate species presented not only in the form of monodentate but also in the form of bidentate, as demonstrated in Fig.3, providing some information on the basicity of the sample. Carbon dioxide is known to tentatively absorb on the electron-pair donor sites on the surface since it is a strong-pair acceptor. Besides, the surface lattice oxygen contributed to the producing monodentate carbonate species while bidentate carbonate species were formed by linkage to Mn§ site)[9-11 ]. In addition, different basic samples will give different adsorpti6r/::states of CO2. As Fukuda and Tanabe reported [8], monodentate carbonate species were the predominate of CO 2 adsorbed on the surface of MgO, while on the surface of CaO only monodentate species were formed at room temperature. Consulting these results it is reasonable to infer that calcination for the sample of 20KNOg%/ZrO2in air made KNO3 decomposed over the surface of support. As a result, basic sites formed on the sample and adsorbed CO2 from atmosphere to form the monodentate carbonate and bidentate carbonate species. Moreover, the monodentate carbonate can be assumed resulting from the adsorption of CO~ on the stronger EPD oxygen sites of surface and bidentate carbonate species is located in the oxygen which is adjacent to K § ions. For the sample of 20 O~KNO3/ZrO2, a high activity was observed in the isomerization of cis-2-butene at 0~ when this catalyst was evacuated at 500~ 12]. Formation of basic sites on the K§ sample should account for the high catalytic activity, no matter how they were generated

1549 a[1.5

120-

lOO'

-

100

0.5

95

0.0

~

90

~.5

-1.0 ~ %.

85

f~.~

.m 60-

0.0

40130

20.

334

4oo 6o0 Tern peratu re (~

2()0

9.

105

.

~

0

"

.

ca -

V

looo

800

5

.... 0.0 *'~

200

105~--

.

100

=~95

- 0 . 5 ~: ~5 ID

9o

.

"

460

.... 60b

Tern peratu re (~

.

.

~ 800

.

-1.5

1000

d 0.5 0.0 "~

y----.,._

8

o

-1.o gE

82

! -1.5

8oo

"

100

~

-1.0

tl.0

80.

e._~

105

. _

-1.o ~

[

-1.s ~

80 0

.

105 .

.

.

200

.

.

.

~oo~, ~ ~o

.

.

.

400

.

9

600 Tern peratu re (*C) .

.

,,

800

1000

.

e~~,0.5

~

~,~o.o ~

'

85 80

0....

266 ....... 460 ' 660 Temperature(~

F-

2.0

. ........

s60

-

Tern peratu re (*C) 105

..............

f

100.

1.0 0.5 ~"

95

o.o

.~.o ~

90-

-0.5

-1.5 E

85-

L-2 0

1000"

8Qo

-~-1.o ~

82

._::_

~

200

400 600 Temperature (*C)

800

100~ 's

Figure 5. TG-DTA profile of (a) KNO 3 (b) ZrO2 (c) 7.5%KNO3/ZrO2 (d) 14% KNO3/ZrO2 (e) 20% KNO3/ZrO 2 (f) 27% KNO3/ZrO 2 samples. below 500~ in vacuum or 600~ in air through the decomposition of KNO3. Pure KNO3 cannot decompose until the temperature rise above 600~ as shown in Fig.5a. However, the decomposition temperature of KNO 3 loaded on ZrO2 was become much lower than that of pure KNO3. As evident in the TG-DTA curves of K + modified samples shown in Fig.5, there is only one endothermic peak centered about 82~ resulting from the desorption of the physical adsorbed water in free zr02 (Fig.5b). On the TG-DTA curve of 7.5%KNO3/ZrO2 sample, two peaks centered about 82~ and 546~ and the second one corresponded to decomposition of the guest material (Fig.5c). When the loading amount of KNO3 increased to

1550 14 wt.-% and 27 wt.-%, three endothermic peaks emerged at 87~ 551~ and 830~ for the former (Fig.5d) and another three appeared near 82~ 556~ and 884~ for the latter (Fig.5e). The temperature of the final weight loss in the KVZrO2 samples that usually occurred above 800~ increased with the increase of loading amount of potassium salt, from 830~ (14%KNOJZrO2) to 884~ The reason, as what revealed on K+/A1203 [ 1], is the different interaction states of the loaded KNO 3 with the support. Among the supported of K + species, some of them was highly dispersed and interacted very strongly with the surface of ZrO 2 but another may be still the micro-crystal KNO 3 with the similar properties as the pure KNO 3 owing to lack of interaction with support. As expected, the KNO 3 interacted with support can decompose at the temperature much lower than that needed for pure KNO3, meanwhile those without interaction still need high temperature for decomposition. Therefore the contribution to the formation of superbasic sites only comes from those highly dispersed K + species, decomposed near 550~ and represented'~y the second peak in TG-DTA spectra, whereas those decomposed above 800~ is not associated with the generation of strong basic sites. 3.3 Adsorption of probe molecular on K+/ZrO2 Pyrrole can be used to detect strong basic sites because of its very weak acidity. Acting as an H-bonding donator, pyrrole interacts with surface basic site to form a strong colinear NH-O band complex that results in a strong characteristic bond in the ring-stretching region. As shown in Fig.6, the band at 3135 cm -t for pure liquid pyrrole (Fig.6a) can be attributed to CH stretching vibration [13], and the two weaker bands at 1468 and 1380 cm t may be assigned to C=C and C-C stretching vibration. The strong band at 1145 cm -1 is the NH bending vibration [14]. The spectra of free ZrO2 (Fig.6b) evacuated at 773 K gave no new band, since both the basic sites and acidic sites on the surface of ZrO 2 were too weak, so the adsorbed pyrrole was thus completely desorbed in the evacuation process even at room temperature. For the sample of 14%KNO3/ZrO 2, however, the spectra of pyrrole adsorption is very different (Fig.6c). A sharp band at 1136 cm 4 due to the NH band deformation was observed, while the NH bending q.~

.-~ u) oN

-

u

d

co

V

[ 3500

i 3000

.,

, 2500

i

.

2000

t 1500

,

, 1000

W o v e n u tuber/era -~

Figure 6. Raman spectrum of (a) liquid Pyrrole, (b)ZrO2, (c) 14%KNO3/ZrO2, (d) 20%KNO3/ZrO2

1551 e3 Lo o

~

e

oo 1o

Or) r-

__= ,d-"

L

3500

I ......

3000

,

I

2500

J

t

2000

L

I

1500

o~ oo

,,

!

1000

i

I

500

W a v e n u m b e f f c m -1 Figure 7. Raman spectrum of (a) liquid Pyrrolidine (b) ZrO2,(c ) 14%KNOJZrO2, (d)20%KNO3/ZrO 2, (f) 27%KNO3/ZrO2 (g) 34%KNO3/ZrO 2 vibration of pure pyrrole appeared near 1145 cm 4. This bathochromic shift may result from the formation of H-bond between surface basic site and NH band of chemical adsorbed pyrrole molecule, because the formation of H-bond lowered the strength of NH band so that the vibration of the latter shifted down to lower frequency. As the amount of potassium salt on ZrO2 increased to 20 wt.-%, the intensity of the peak at 1136 cm -1 was obviously enlarged (Fig.6d), mirroring the increase of basic sites. Moreover, a series of new bands appeared at 3095,1362 and 1321 cm "1, the former could be assigned to the v[NH-O] species [15] and the two latter probably represented the C=C and C-C:~mg stretching vibration [16]. Binet and coworkers [5] studied the dissorciate and undissociate adsorption of pyrrole on oxides and propose that the degree of ionicity/covolency of the 02. surface species may parallel their basicity. When the 02. species was very basic, the easily polarizable hydrogen in pyrrole will be bridge-linked to the 02. and result in [-NH-O] species that bond at 3100-2700 cm -~ range. Based on these facts, the weak band at 3095 cm ~ on 20%KNO3/ZrO ~ can be tentatively assigned to [-NH-O] species. Although pyrrolidine possesses a stronger basicity than pyrrole, it is still an acid probe molecule when it interacts with a superbase. After pyrrolidine adsorbed on the K+/ZrO2 samples, a shift of N-H bending vibration from 1454 to 1321cm ~ emerged on the spectrum as shown in Fig.7, accompanied with a small one for the C-N ring stretching from 1280 to 1257 cm -~. In contrary no obvious shift was observed in the C-H stretching band at 2963 cm -1. Moreover, the C-H deformation vibration at 899 cm 4 was split into two weak bands at 912 and 871cm -~ respectively, which can be tentatively attributed to the polarity

1552 change of C-H band and will be discussed elsewhere in detail. The use of N-methylpyrrole on the sample of K+/ZrO2 seems not successful, because no obvious adsorption was observed on the obtained spectrum of 20%KNO3/ZrO2 (Figure was not shown). Probably the replacement of the H atom connected to N atom by the group of-CH3 hinders the probe molecule to interact with the basic sites by the formation H-bond. Another possible reason is strength of adsorption. The adsorbed species may be physically eliminated during the evacuation at room temperature, and a further investigation is thus desirable. CONCLUSION (1). Strong interaction exists between KNO3 on and ZrO2 and results in the stabilization of metastabte tetragonal form of ZrO2 and lowers the decomposition temperature of K N O 3 o n Zr0s than that of crystalline KNO3. (2). Two states of KNO3 locate on the surface of ZrO2. One is the high dispersed K + species that decompose around 550~ and play a important role in the formation of superbase, another is the bulk KNO3 that decompose above 700~ but not participate to create superbasic sites. (3). The strong basic sites on the potassium modified ZrO2 sample can be characterized by the adsorption of pyrrole, and a strong coliner NH-O species is formed by H-bond with surface basic sites. N-methylpyrrole is not a convenient probe since it hardly chemically adsorbs on the surface of solid superbase. REFERENCES

1.Y. Wang, J.H. Zhu and W.Y. Huang, Phys. Chem. Chem. Phys., 3 (2001) 2537. 2.T. Baba, H. Handa and Y. Ono, J. Chem. Sot., Faraday Trans., 90 (1994) 187. 3.Y. Wang, W.Y. Huang, Y. Chun, J.R. Xia and J.H. Zhu, Chem. Mater. 13 (2001)670. 4. M. Huang and S. Kaliaguina. J. Chem. Soc. Faraday Trans., 88 (1992) 751. 5. C.Binet, A.Jady, J. Lamotte and J.C. Lavalley, J. Chem. Soc., Faraday Tran. 92(1996) 123. 6. P. Duwez and F. H. Brown, J. Am. Ceram. Soc., 35(!952) 109. 7. J. A. Lercher, C. Colombier and H. Noller, J. Chem_=Soc., Faraday Trans., 80(1984) 949. 8. Y. Fukuda and K. Tanabe, Bull. Chem. Soc. Jpn., 46(1973) 1616. 9. B. W. Krupay and Y. Amenomita, J. Catal., 67(1981)362. 10. Y. Morikawa and Y. Amenomiya, J. Catal. 48(1977) 120. 11. J. V. Evan and T. L.Whateley, J. Chem. Soc., Faraday, 63(1967)2769. 12.Y. Wang, W.Y. Huang, Z.Wu, Y.Chun and J.H.Zhu, Marterial letter, 40(2000)198. 13. T.D. Klots,R.D. Chirico and W.V. Steele, Spectrochimaca Acta.50A 4(1994)765 14. R.C.Lord, Jr and F. A. Miller. J. Chem.Phys. 10(1942)328 15. J. C. Lavalley, Catalysis Today, 27(1996)377 16. B.Tian and G.Zerbi. J. Chem. Phys., 92(1990)3886.


1553

The S y n t h e s i s and C h a r a c t e r i z a t i o n o f Zeolite Z S M - 5 and Z S M - 3 5 F i l m s b y Self-transformation o f Glass Jinxiang Dong, Wugang Fan, Guanghuan Liu, Jinping Li Research Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, Shanxi, P.R. China

Zeolite ZSM-5 and ZSM,35 films have been synthesized successfully by selftransformation of glass discs. The synthesis was performed by the vapor phase method. Ethylenediamine (EDA) was used as the organic template agent_ The synthesis conditions were studied. The phase of the zeolite film was dependent on the ratio of EDAJH20. The properties of the Zeolite ZSM-5 and ZSM-35 films were investigated by calcination, aqueous vapor treatments at 493K and were characterized by XRD, SEM. The films didn't peel off atter this series of treatments~ This showed that the films connected with the substrate firmly.

1. INTRODUCTION Zeolites have been widely used in many fields including catalysts, separation, ion exchange, sensing etc. In most cases, zeolites are preferred as powder composition whereas they should have suitable configurations for practical uses (powder zeolites are used as washing builders). Zeolites in membrane or film configuration have been an attractive subject for several years because their potential in separation and catalysis. A membrane is an intervening phase separating two phases and/or acting as an active or passive barrier to the transportation of matter between p ~ s e s adjacent to it under driving force. It can be gas, liquid or solid. Generally, it has greater lateral dimension than its thickness[1 ]. Zeolite films or membranes can be classified as asymmetric films (supported) or symmetric films (self-supported). The sencond type is constituted by a pure zeolite phase; the first type is a zeolite thin layer formed on different supports. Many materials can be used as supports, such as glass[2], quartz[3], stainless steel disc[4], silicon[5], alloy[6], polymeric[7], ceramic[8]~ alumina[9], mercury[10], filter paper[ll] etc. The combination of catalysis and distillation is one the new trend in chemical engineering to reduce the enormous space taken up by distillation towers. If the zeolitic films could be employed as elements, it would require the films to be of high thermal stability and of a certain mechanical strength. Symmetric films have good

1554 engineering properties (strength, hardness) and are more suitable for this technique, it can be prepared by direct synthesis, pressuring sol,gel method and sQ on, but this kind of film can't be connected to the support material very well and can be removed easily. Comparing with traditional commonly used hydrothermal synthesis systems, the vapor phase method[12], which avoids the direct contact of the solid phase and the liquid phase, can decrease the waste of liquids and enhances the productivity of zeolite. It has been employed for preparing zeolitic films, for example, Dong et al. formed ZSM-5 and ZSM-35 self-supported films by this way[ 13], Matsukata and Nishiyama using vapor phase synthesized a layer of mordenite and analcime on porous alumina [14]. Recently, Dong et al. prepared a ZSM-5 film on boron glass disc in the vapor phase of ethylamine (EA) and H20 by the same method[ 15]. In this work, some conditions of synthesis of self-transformation zeolite ZSM-5 and ZSM-35 film on glass discs were investigated. The influence of different factors in synthesis process was studie& The stability of the films and the joint situation of the films with the substrate after calcination and aqueous vapor treatments were investigated by XRD and SEM. The results showed that the film firmly connected with the substrate.

2. EXPERIMENTAL AND CHARACTERIZATION 2.1 Materials

Glass disc (30mm• for the convenience of XRD characterization) was used as the starting substrate. The prior treatment of the glass disc was a complete immersion into an erosion solution for 2-~3h at room temperature and then a cleaning with distilled water. EDA (A.R.>99%), triethylamine (Et3N)(A.R.>99%) and n-propylamine (PrNH2) (>98.5%), n-butylamine (ButNH2)(>98~5%) in reagent grade were used for the synthesis. The synthesis method (vapor phase method) was described in a previous paper [12]. The glass disc was mounted in a special autocalve. The liquid phase at the bottom of the autoclave is composed of template and H20 at different ratio. The calcination of the assynthesized glass film was conducted in a crucible oven_ The temperature was raised at a rate of 1K/min from room temperature to 823K; then kept constant for 6h in air, and then allowed to cool to room temperature. The aqueous vapor treatment of the zeolite films was carried out in the same autoclave of the synthesis. However, the liquid phase was distilled water. The heating treatment was carried out at 493K for 72h, at the same time, XRD and SEM inspected phase and morphology change of the zeolite film respectively. The glass disc should be carefully brushed and cleaned before XRD examination in order to get rid of the peeled off zeolite after the treatments.

1555 2.2 Characterizations The XRD identification was carried out with a Rigaku D/max 2500 X-ray powder diffractometer. Cu Ko~ radiation was used. The relative crystallization of zeolite ZSM-5 film was calculated by making the summation of intensities of the four peaks 20=7.92,23.10,23.27,23.98, and divided by highest crystallization. For ZSM-35, the eight peaks 20 = 9.23, 22.21, 22.30, 22A3~ 22_92, 23.47, 23 .76,24.07 were used as reference peaks. The morphology of a small piece of zeolite film detached from the integrated glass disc was observed with SEM (JEOL JSM-35). 3. RESULTS AND DISCUSSION 3.1 Synthesis conditions The synthesis of zeolite films was different from the traditional hydrothermal synthesis. The basic factors which influence film formation included liquid phase composition (template agent), the type of substrate, crystallization time and reaction temperature. Firstly, different kinds of glasses were used to discover the possibility of preparing this type of self-transformation film(Table 1). The glass discs without boron (common glass) and the boron glass discs (B203=12.8wt%) were used as starting substrate to synthesis. It was found that the common glass transformed with EDA as template agent in the steam of water and EDA. Whenthe EDA/H20=0.30(mol ratio), at 493K for 288 h, ZSM-5 films can form on the glass disc. However, the films had lower crystallization than that forming on boron glass surface and it peeled off after calcination treatment(823K,6h). So in the following experiments, boron glass discs were used as starting substrate. Secondly, different organic amines were tried as template to investigate the possibility of synthesis. In most experiments, EDA was used as an effective template, which will be discussed in details. When the Et3N was added to the aqueous solution of EDA, the ZSM-5 film could also be formed on the glass disc. Yet, it was also found the PrNH2 and ButNH2 were invalid_ as template for the glass disc transformation at 493K for 288h(Table2), although they can be used as template for synthesizing the powder of zeolite ZSM-5. The reason for the selectivity of template seemed to be the different dissociation constants in water. Table 1 The influence of glass category The category of glass Boron glass Size (mm) 30•215 Phase ZSM-5

Common 8 1 a s s 30•215 ZSM-5

Common glass 30• ZSM-5

1556

Table2 The comparison of different of template a~ents Run 1 2 Template EDA EDA/Et3N Mol ratio (template/H20) 0.30 1.37/0.83 Table3 Synthesis conditions of zeolite film Run t EDA/H20 (mot ratio) 6.5 Phase FER

3 PrNH2 0.36

on Boron-glass disc 2 3 2.4 1. t FER FER+MFI

4 ButNH2 0.30

4 0.60 FER+MFI

5 0.40 MFI

6 0.30 MFI

Thirdly, the influence of ratio of EDA and H20 on the zeolite film formation on glass was investigated at 493K for 288h. The results showed that when the molar ratio in 2.4-6.5, zeolite ZSM-35 film was obtained; when the ratio was changed from 0.30-0.40, zeolite ZSM-5 fdm was synthesize_& Between these ranges there was a film of mixed crystal grew on the glass disc (Table3). From the synthesis results, it can be seen that an appropriate ratio EDA/H20 was of great importance for the transformation of the glass disc.. Fourthly, the influences of crystallization temperature on the self-transformations of the glass disc were investigated at 453-493K for 240h. The results showed that in this temperature range the two films could be formed. Finally, the influence of crystalline time on the synthesis of zeolite film was investigated by choosing 144h, 19211, 240h, 288h, 384h and 400h respectively at 493K. For zeolite ZSM-5 film, it was found that the crystallization increased along with the crystallization time until the time was up to 384h(Fig.1). The growth of ZSM,35 film seemed to be slower than that of the ZSM-5 film. = 1.20 N

"~

0.8

--

----0-- ZSM-5 film

r~

~,o.6

-

L)

o0.4

~

0.2

~

o o

t

-~

I

I

I

I

I

I

I

I

I

50

100

150

200

250

300

350

400

450

Time (h)

Fig. 1 The growth of zeolite films with the heating time

ZSM-35film

1557 3.2 The influence of the a f t e r - t r e a t m e n t on the rdms

The organic amine molecules should be removed before zeolite can be used as catalyst and adsorbent by calcination~ According to the process showed in 2.2, the two kinds of zeolite films were calcined and then treated with aqueous vapor to investigate the properties of the films. The film grew on the common glass disc would peel off after calcinations. Fig.2 and Fig.3 were the XRD patterns of the zeolite ZSM-35 and ZSM-5 film that were formed by self-transformation of boron glass disc respectively.

tt'i

ttlt

(a)

~-""---"~ ~l

(b)

r

j .aJ

c)

5

10

15

20 20

25

30

Fig.2 XRD patterns of ZSM-3 5 film (a) After aqueous vapor treatment (b) After calcination (c) As-synthesized film (d) Starting substrate

5

10

15

20

2e

(a)

(b)

,

(c)

25

30

Fig.3 XRD patterns of ZSM-5 film (a) Film after calcination (b) After calcination (c) As-synthesized film (d) Starting substrate

From the X R patterns, it can be seen that (I) The glass disc was amorphous. (II) The zeolite film was synthesized. (III) After calcination at 823K for 6h, aqueous vapor treatment at 493K for 72h, the phase of the zeolite ZSM,5 and ZSM-35 didn't change. The crystallization even was improved after aqueous vapor treatment.

1558

(a) The as-synthesized ZSM-5film

(a) The as-synthesized ZSM-3 5film

(b) The ZSM.5film after calcination

(b) The ZSM-35film after calcination

(c)After aqueous vapor treatment Fig.4 SEM images of ZSM-5

(c) After aqueous vapor treatment Fig.5 SEM images of ZSM-3 5

1559

Fig.6 The surface of starting substrate From the series of SEM images (FigA and Fig.5), it can be seen that the two kinds of zeolite films not only maintain a good phases stability(XRD results) but also have good morphology after calcination and aqueous vapor treatment. It is proven that this kind of self-transformation film is effectively connected to the substrate and it is resistant to peeling off comparing with other films. Fig.6 showed that the surface of the starting substrate after chemical treatment. The surface of the glass disc was rough. It seemed suitable for the growth of zeolite film on it. This glass disc can be partly transformed into zeolite films in the vapor of EDA and H20. XRD and SEM showed that zeolite films grown on the glass disc was resistant to calcination and aqueous vapor treatments. 4.CONCLUSIONS We studied the synthesis conditions of zeolite ZSM-5 and ZSM-35 film formation on glass by the self-transformation with the vapor phase method. Zeolite films were obtained using EDA and H20 as liquid phase. When the EDA/H20 varied from 6.5-2.4, the FER zeolite phase was obtained; when the ratio was from 0.40-0.30, MFI was obtained. This kind of zeolite film connect with the substrate welt and can endure calcination and aqueous vapor treatments, so it has good quality that maybe could find future application.

ACKNOWLEDGEMENT

The authors gratefully thank the f'mancial assistance provided by Natural Science Foundation of Shanxi Province (20011012).

1560 REFERENCES

1.A. Tavolaro, Adv. Mater., l l , No~12(1999)975. 2.J.G.Tsikoyiannis, W.O. Haag. Zeolite, 12(1992) 126. 3.G.J.Myatt, P.M.Budd, C.Price, S.W.Carr, J.Mater.Chem.2(1992) 1103. 4.T.Sano, H.Yanagishita, Y.Kiyozumi, F.Mizukami, K.Haraya, J.Membr.Sci., 95(1994)221. 5.J.C.Jansen, W.Nugroho, H.van Bekkum. Proc.9th Int~ Zeolite Conf.,1(1992)247. 6.Y.Kiyozumi, K.Maeda, F.Mizukami, Stud.Surf.Sci.Catal.98(1994)278. 7.M.W.Anderson, K.S.Pachis. J.ShLS. W.Carr, L Mater. Chem.,(1992)255. 8.M.D.Jia, K.V.Peinemann, R.D.Behling. J.Membr. Sci., 82(1993) 15. 9.NANishiyama, K~Ueyama, M.Matsukata, Stu& Surf. SckCatal.,105(1997)2195. 10.Y.Kiyozumi, F.Mizukami, K.Maeda, M. Toba, S.I.Niwa, Adv.Mater.,8(1996)517. 11.T.Sano, Y.Kiyozumi, K.maeda, M.Toba, S~Nivca,F.Mizukami, Proc,_9th Int. Zeolite conf., 1(1992)239. 12.W.Y.Xu, J.X.Dong, J. Chem. Sock, Chem. Commun.,(1990)755. 13.J.X. Dong, T.Dou, X.Zhao, L.Gao, J. Chem. Sot., Chem. Commu.,(1992)1056. 14.M.Matusakata, N.Nishiyama, K~Ueyama,Microporous Mater.,7(1996)299. 15.W.Y. Dong, C. L. Long Chem.Commun.,(2000)1067.


1561

Preparation of mesoporous materials as a support for the immobilisation of iipase. A. Macario, V. Calabr6, S. Curcio, M. De Paola, G. Giordano, G. Iorio, A. Katovic Dipartimento di Ingegneria Chimica e dei Materiali, Universit~ della Calabria via P. Bucci, 187030 Rende (CS), Italy In this paper the immobilisation of the lipase enzyme on a M41S type mesoporous material support is presented. Different M41S type materials, with a pore diameter ranging from 37 A to 48 A, were synthesised starting from different silica sources, in order to immobilise, by physical adsorption, lipase with a spherical molecular diameter close to 41 A. The efficiency of the immobilisation carried out at an optimal pH=7, different temperatures and time, were tested by UV Spectrophotometry. The maximun amount of bound lipase resulted to be around 472 mg at 0~ compared to the 362 mg at 25~ and 119 mg at 40~ Indeed, the maximun immobilisation efficiency is around 47%. The activity of enzyme immobilised was tested by hydrolysis of triglycerides in olive oil into fatty acids and the lipase immobilised has 78% of the activity of the free enzyme. 1. INTRODUCTION The most important industrial applications of lipase regard the synthesis and the hydrolysis of glycerides. The principal glycerides inquiries are in the food, pharmaceutic and cosmetic industries [1]. In particulary, the mono and diglycerides are often used as emulsifier and some triglycerides are used as economic substitute of cocoa butter [2,3]. The glycerides are, today, produced by inorganic catalyst at high temperature (130~176 consequently, with high energy cost [4]. Recently, owing to the problems related to high reaction and separation costs, new methods for glycerides synthesis were proposed as possible alternatives. Particularly, an enzymatic process, in which a biocatalyst is used, allows to carry out the reactions under milder conditions. The advantages of using immobilised enzymes, as compared to their native state, pertain the utilization of processes that are easier to control and to operate, allowing the reutilization of catalyst that may be readily separated from the substrate. Enzyme molecules are, generally, immobilised by entrapment within a porous matrix, by physical adsorption or by chemical based methods and, usually, maintain most of their catalytic activity. In recent years, different enzyme immobilisation techniques were reported [5-9]. As far as lipase is concerned, no study regarding its immobilisation into the internal surface of a mesoporous material has been presented. M41S are hydrophobic materials that allow immobilising enzyme through a solvent containing both lipase and the support, under stirring. Physical adsorption is, therefore, attained.

1562 Aim of the present study is the preparation and the characterization of a novel biocatalyst with lipase immobilised in MCM-41 support. Many mesoporous materials, having pores diameter comparable to spherical molecular diameter of used lipase, were synthesized. The effect of the synthesis time, the different silica sources and the different organic cations on the pore size of the support were investigated. The amount of lipase immobilised in the synthesised supports, with a pore size comparable to the dimensions of lipase, has been quantitatively evaluated. Finally, the activity and stability of immobilised enzyme, as regards the hydrolysis of triglycerides, were tested.

2. EXPERIMENTAL 2.1 Materials The enzyme used is produced by NOVO and its trade name is Palatase: a purified 1,3specific lipase from Rhizomucor miehei produced by submerged fermentation of a genetically modified Aspergillus oryzae micro-organism. Optimun working conditions are reported as 40~ and pH = 7.5. Calculated MW is 29536 g/mol. The enzyme spherical molecular diameter has an influence on the support synthesis. The lipase spherical molecular diameter was estimated as 41 A, on the basis of experimental evidences [ 10]. The synthesized support was MCM-41. The following materials were used for purely siliceous MCM-41 synthesis: silica gel (BDH) with a specific surface of 550 m2/g; Z6osil silica with a specific surface of 175 m2/g; cetyltrimethylammonium bromide (CTABr 99%, Aldrich); tetraethylammonium hydroxide solution (TEAOH 40%, Fluka), tetramethylammonium hydroxide solution (TMAOH 25%, Fluka). 2.2 Synthesis and characterization of MCM-41 support Pure siliceous MCM-41 with different pores diameter were synthesized from the following molar composition, with the optimal CTA/Si molar ratio [ 11 ]:

0,07 X20- 1 SiO2- 0,12 CTABr- 30 H20 in which X represents the cations (TMA + or TEA +) added as hydroxides. The mixture was heated at 140~ in a Teflon-steel autoclave. The time of crystallization was varied from 1 to 12 days. The solid phases were recovered by filtration and washed with distilled water and, then, dried at 100~ for 24 h. Samples were calcined in air at 550~ for 10h in order to remove the template. The long-range structures of the MCM-41 materials were characterized by XRD patterns collected on a Phillips PW 1710 diffractometer with CuKcz radiation. The samples were scanned from 1 to 10~ (20) in steps of 0,02 ~ with a count time of 1 second at each point. The specific surface area, the mesopore volume and the pore diameter of MCM41, after calcination, were determined by N2 adsorption isotherms obtained at 77K using an ASAP 2010 Micromeritrics instrument.

2.3 Lipase immobilization 0,4 g of calcined MCM-41 were mixed with 50 ml of 0,5 M phosphate buffer pH 7 containing 1 g of lipase and was stirred for different times at three different temperatures (0~ 25~ and 40~ The mixture was, then, filtered and the MCM-41 containing lipase

1563 Table 1 Activities of lipase in its native state Amount of free Lipase Temperature of (mg) Immobilisation (~ 119,0 40 362,2 25 471,7 0

Activities (U/L) 8820 11760 15400

was washed twice with deionised water and dried at 25~ ovemight. The filtrate was collected in a small graduated cylinder to measure its exact volume. The amount of immobilized enzyme was calculated from the difference of the absorbance at 280 nm before and after addition of the support. The enzyme concentration of the initial solution and of the filtrate was calculated by calibration curve determined by Perkin-Elmer Spectrophotometer UV. A linear regression equation is applicable to obtained data: C o n c e n t r a t i o n = 1 3 , 8 8 3 * A b s o r b a n c e , with a correlation coefficient of 0,9971. The absorbance of the initial lipase solution was 1,636 corresponding to a concentration Co of 22,71 mg ml 1. The amount of enzyme adsorbed on the MCM-41 (Win, [mg]) was determined from the following lipase mass balance: WIL = CoV0 - CfVf, in which Co is the initial enzyme concentration (mg mll); V0 is the initial volume of lipase solution (ml); Cf is the enzyme concentration of the filtrate (mg mll); Vf is the volume of the filtrate (ml). 2.4 Activity of immobilised Lipase. The activity of immobilised lipase was tested and compared to the activity of the lipase in its native state by the Tietz and Fiereck method [12] that involves hydrolysis of triglycerides contained in olive oil to give fatty acids, diglycerides and, a lesser extent, monoglycerides, and glycerol. The activity assay procedure is the same as reported in the Tietz and Fiereck method, both for soluble and immobilised lipase [12]. Lipase activity in its native state, in the same amount that has been immobilised at the three tested temperatures, is reported in table 1.

3. RESULTS AND DISCUSSION 3.1 Structural characteristics of the MCM-41 support The dl00 spacing and the hexagonal lattice parameter (ao) of the samples synthesized with different silica sources and different organic cations at different reaction times are reported in the tables 2, 3 and 4. The organic cations strongly affect the pore diameter of as synthesised M41S materials. As matter of fact, in the presence of TMA + ions in the initial reaction mixture, the pore diameters increase when the reaction time is prolonged (see samples 1-7 table 2). On the contrary, independently of the silica source, the presence of TEA + ions in the synthesis hydrogel induce a reduction of the pore diameter value, for a longer reaction time (see samples 8-14, table 3, and 15-17 table 4). When the silica Z6osil was used, the decrease of pore diameter is detected after 6 days of reaction time, whereas, when silica gel from BDH was used, the larger pore diameter is showed after only 2 days of reaction at 140~ Probably, the different behaviour observed in the samples synthesised with different organic

1564 Table 2 XRD dloo spacing, hexagonal lattice parameter of uncalcined samples synthesized with Z6osil as silica source and TMA + organic cation, reacted at 140~ [ 14]. Sample Synthesis time [h] dloo [ A ] ao [ A ] Final phase 1 24 37,3 43,1 MCM-41 2 68 41,2 47,6 MCM-41 3 120 43,1 49,8 MCM-41 4 144 43,5 50,2 MCM-41 5 168 43,9 50,7 MCM-41 6 192 44,1 50,9 MCM-41 7 288 45,7 52,8 MCM-41 Table 3 XRD dloo spacing, hexagonal lattice parameter of uncalcined samples synthesized with Z6osil as silica source and TEA + as organic cation, reacted at 140~ [ 14]. Sample Synthesis time [h] dloo [ A ] ao [ A ] Final phase 8 24 41,2 47,6 MCM-41 9 68 42,1 48,6 MCM-41 10 120 42,6 49,2 MCM-41 11 144 47,2 54,5 MCM-41 12 168 42,7 49,3 MCM-41 13 192 42,6 49,2 MCM-41 14 288 40,9 47,2 MCM-41 Table 4 XRD dl00 spacing, hexagonal lattice parameter of uncalcined samples synthesized with BDH as silica source and TEA + as organic cation, reacted at 140~ [ 14]. Sample Synthesis time [h] dloo [ A ] ao [ A ] Final phase .... 15 48 47,8 55,2 MCM-41 16 120 44,1 50,9 MCM-41 17 144 42,7 49,3 MCM-41 cations and silica source are due to the different interaction of organic cation and silica [ 13] and to the different length of organic chain. In table 5 are reported the wall-thickness of calcined and uncalcined samples. The wallthickness of uncalcined samples was calculated by the geometric methods [14], where the volume of the N2 adsorbed was substituted with the surfactant volume calculated by thermal analysis, while the wall-thickness of calcined samples was calculated considering the difference between the unit cell and the pore diameter calculated by N2 adsorption. The contraction of the unit cell after calcination is observed only for the samples prepared with BDH as silica source (N~ and 17), on the contrary, the samples 7 and 11 show an unit cell opening and, then, a wall-thickness reduction. Since the contraction of unit cell after calcination is caused by condensation of the Si-OH unit in the channel wall, the reason of this difference, probably, is the high specific surface area of the BDH silica and its greater acidity. In any case, the pore walls thickness is comparable to the largest wall-thickness reported in

1565 Table 5 XRD dl00 spacing, hexagonal lattice parameter and wall-thickness of uncalcined and calcined samples. Sample before calcination after calcination dl00 [A] ao [ ,~ ] wall-thickness dloo [ ,h ] ao [ ,h ] wall-thickness

[X]

7 11 15 16 17

45,7 47,2 47,8 44,1 42,7

52,8 54,5 55,2 50,9 49,3

[A]

14,9 15,3 13,5 11,6 9,3

48,1 49,0 44,1 40,8 41,9

55,5 56,6 50,9 47,1 48,4

14,0 14,7 15,2 -

Table 6 Properties of the MCM-41 calcined samples, obtained from the BJH method. Sample Specific surface area Mesopore Volume a0 Pore diameter [m2 g-l] [cm3 g-l] [A] [A] 7 11 15

950 1028 1087

0,95 1,08 0,77

55,5 56,6 50,9

41,5 42,0 35,7

literature, from other authors, for the MCM-41 materials [15]. In table 6 are reported the results of N2 adsorption/desorption analysis. It can be observed that pore size of the samples increases when the ao parameter enlarge. When the Z6osil was used as silica source, large pore volume and lower specific surface area were measured. However, for all samples the large pore volume indicates that no structural collapse occurs during calcination. The sample 11 shows the highest pore diameter, while the sample synthesized with BDH as silica source (15) has the highest specific surface area but smaller pore diameter. Therefore, in order to immobilize the highest amount of lipase, the sample 11 was chosen. 3.2 I m m o b i l i s e d lipase on M C M - 4 1

In order to better understanding the role played by temperature, three different immobilisation experiments have been performed at the temperatures of 0~ 25~ and 40~ For these experiments two characteristic quantities, i.e. the ratio between the filtrate volume and the volume of initial lipase solution (Rv) and the ratio between the weight of dried MCM-41 after and before the immobilization (Rw), were considered and evaluated after 24 hours. Results are summarised in the following table 7 and figure 1. These results show that the adsorption is favoured at low temperatures and the higher amount of lipase immobilised is at the temperature of 0~ Figure 1 shows the dynamic evolution of adsorbed lipase on M41S materials at different temperature. The initial trend of the curve shows a very fast adsorption. When the time is prolonged, the amount of adsorbed enzyme increase slowly. This behaviour is observed for all temperatures tested; besides, the time necessary to achieve the plateau region is independent of the temperature and is around 1080 minutes (18 hours). In agreement with physical adsorption theory, the amount of lipase adsorbed decreases with

1566 Table 7 Rv and Rw of the immobilisations carried out at three Experiments T (~ 1 0 2 25 3 40

different temperatures for 24h. Rv Rw 0,90 3,050 0,92 1,325 0,96 1,275

5001

A

400

....................

!

300

.... ~

~ .............................

200

...............................................................

[]

1

i ............................

~

Wil (mg)

1

lOO o

0

500

1000

1500

Time (rain.)

Figure 1 Dynamic evolution of adsorbed lipase on MCM-41 at three different temperatures. increasing temperature. Since the rate of a chemisorption increases with temperature, we suppose that only a physical adsorption on M41S occurs. The maximum amount of immobilised lipase is equal of 472 mg. The maximum immobilisation efficiency, defined as the ratio between the amount of immobilised enzyme and its initial value, evaluated at 0~ is equal to 47,2%. Compared to lipase immobilisation on other supports, such as membrane [16-17] or zeolites [7], characterised by immobilisation efficiency of about 33%, the proposed process is seems suitable and promising. One of the most important characteristics of immobilised enzymes is the preservation of their catalytic activity throughout the immobilisation procedure. Immobilised and native lipase performances were tested, in exactly the same conditions, for the hydrolysis of triglycerides, contained in olive oil, to fatty acids. The activities of immobilised lipase on M41S type materials are shown in table 8. It can be observed that the highest activity of bounded lipase

1567 Table 8 Activity of lipase immobilised on MCM-41 Temperature of Activity (U/L) Immobilisation (~ 0 12040 25 5600 40 2800

With RESPECT to the activity of the free enzyme 78% 48% 32%

Table 9 Stability of immobilised lipase after repeated use Activity (U/L) Activity (U/L) Cycles Number Enzyme Immob. at 0~ Enzyme Immob. at 25~ 1 12040 5600 2 8400 5040 3 6720 4480 4 5320 4060

Activity (U/L) Enzyme Immob. at 40~ 2800 2100 1540 1484

Table 10 Activity related at first hydrolyses cycle of the lipase immobilised at 0~ Number of Relative Activity (%) cycles 0~ 25~ 40~ 1 100 46,5 23,3 2 69,8 41,9 17,4 3 55,8 37,2 12,8 4 44,2 33,7 12,3 is that corresponding to an immobilisation performed at 0~ its value is 78% of that exhibited by native enzyme. Although lipase lost about 22% of its original activity throughout the immobilisation, this result is satisfactory if compared to the activity loss observed during immobilisation on other types of supports [7,8,16,17], that might even reach 65%, with respect to the initial enzyme activity. In table 9 and 10 the stability and the relative activity of immobilised lipase on M41S materials, after four repeated hydrolysis cycles, are reported. The highest activity was measured at 0~ although this corresponds to a more significant decay after different process cycles. In any case, we observe a high activity for the samples prepared at 0~ Further studies are to be devoted to the comprehension of the deactivation phenomena that affect lipase under such particular immobilisation conditions.

4. CONCLUSIONS First of all, the silica source and the nature of organic cations strongly affect the properties of as synthesised materials. In fact, the samples synthesised from BDH silica gel and TEA + ions show a larger pore diameter after shorter reaction time. The use of Zdosil silica source and TEA + ions in the initial reaction mixture favours the formation of M41S

1568 materials with larger pore diameter, wall-thickness, mesoporous volume and good specific surface area. With different synthesis procedure, it is possible to modulate the characteristics of mesoporous materials in order to optimiser lipase immobilisation. Lower temperatures drastically influence the immobilisation of lipase and, then, the activity and stability of enzymatic catalyst with respect to triglycerides hydrolysis. Finally, the obtained results show that the immobilisation of lipase on mesoporous materials is a very promising alternative as compared to the enzyme immobilisation on other support. REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

Kim S.M., Rhee J.S., Production of medium-Chain Glycerides by immobilized Lipase in a Solvent-Free System, JACOS, Vol. 68, N ~ 7 (1991). Macrae A.R., Biocatalysts in organic Synthesis, Proceedings of an International Symposium, 14-17 april 1985. Katchalski-Katzir E., Immobilised enzymes - learning from past successes and failures, Tibtech Vol. 11 (1993). Mariani E., Chimica Industriale Applicata, UTET, (1983). Lie E. and Molin G.,J. Chem. Tech. Biotechnol. 50 (1991) 549-553. Goncalves A., Lopez J., Lemos F., Ribeiro F., Prazeras D. M. F., Cabral J., and Aireshbarros M. R., J. Mol. Catal. B-Enzymatic 1 (1996) 53-60. Knezevic Z., Mojovic L., and Adnadjevic B.; Enzyme Microb.Technol., 1998, vol. 22, March. Felipe Diaz J., Kenneth J. Balkus Jr.; J. Molec. Catal. B." Enzymatic 2 (1996) 115-126. Jing He, Xiaofen Li, D.G. Evans, Xue Duan, Chengyue Li; J. Molec. Catal. B: Enzymatic 11 (2000) 45-53. Perry' s Chemical Engineers' Handbook, sez. 17-20, table 17-10 Katovic A., Giordano G., Bonelli B., Onida B., Garrone E., Lentz P., Nagy J.B., Microporous and Mesoporous Materials; 44-45 (2001) 275-281. Tietz NW, Fiereck EA, A specific method for serum Lipase determination, Clin. Chim. Acta 13:352, 1966. R.M. Barrer, Zeolites, 1 (1981) 130; Galarneau A., Desplantier D., Dutartre R., Di Renzo F., Microporous and Mesoporous Materials; 24 (1999) 297-308. Coustel N., Di Renzo F., Fajula F., J. Chem. Soc., Chem. Commun., 1994,967. Bosley J.A., Clayton J.C., Peilow A.D., Immobilisation of Lipase for use in non-aqueous media, Uniliver research, 1995. Taylor F., Lipase Membrane Reactor for Continuos Hydrolisis of Tallow, in Engineering of~with Lipase, Ed. by F.X. Malcata, NATO ASI Series, Vol. 317; pp. 455-472.

ADSORPTION, DIFFUSION, SEPARATION AND PERMEATION



1571

Adsorption and diffusion of linear and dibranched C6 paraffins in a ZSM-5 zeolite E. Lemaire a, A. Decrette a, J.P. Bellat a*, J.M. Simon a, A. M6thivier b and E. JolimaRre b a LRRS, UMR 5613 CNRS Universit6 de Bourgogne, 9 av. A. Savary, BP 47870, 21078 Dijon Cedex, France b Institut Frangais du P6trole, 1-4 av. du Bois Pr6au, 92852 Rueil-Malmaison, France

The adsorption of n-hexane and 2,2-dimethylbutane on a commercial ZSM-5 zeolite is studied under isothermal and isobaric conditions. ZSM-5 exhibits two different behaviours with linear and dibranched alkanes. A substep at 4 molec.uc 1 is observed on the adsorption isotherm of n-hexane at 348 K. A singular adsorption-desorption process is evidenced on the adsorption isobar at 5.5 kPa with 2,2-dimethylbutane at a temperature close to 343 K when the sample is activated at 298 K under vacuum. The diffusivities have been determined by fitting directly the uptake curves with a numerical resolution of second Fick's law based on finite difference method. Microporous diffusion seems to be rate limiting but surface barrier could also be significant. The diffusivities are almost independent on the filling and are 100 times greater with the linear alkane than with the dibranched one.

1. I N T R O D U C T I O N Over the last years, refining and petrochemicals have become aware of environmental and human health protection. New legislative standards on pollutant emissions impose to reduce some dangerous compounds as aromatic hydrocarbons from the gasoline. However, this reduction would be detrimental to the octane number and a good way to maintain it is to increase the content of highly branched alkanes and to reduce that of linear and monobranched ones. In order to satisfy this octane requirement, C6-C~0 isomerization units will become more common. So it is necessary to be able to separate the different isomers of alkanes after isomerization. MFI-type zeolites may be used to perform such a separation. The adsorption of linear and branched paraffins on silicalite or ZSM-5 is subject to an extended research, often oriented to chemical engineering applications as for example the separation of gas by permeation through a zeolitic membrane 1' 2, 3. Indeed, in the last decade, a lot of experimental data on the diffusion of single paraffins and their mixtures in MFI type zeolites have been published 4' 5, 6. However, the data on adsorption equilibria are less numerous. Regarding the C6 paraffins the adsorption isotherm of n-hexane on silicalite has been largely studied 7, 8, 9, 10 but few results about the adsorption of its branched isomers like 2methylpentane or 2,2-dimethylbutane are quoted in the literature ~' 12. A relatively good agreement is observed among the data on adsorption equilibria given by the different authors. * To whom correspondence should be addressed. Email: [email protected]

1572 On the other hand, the diffusion data are much more scattered. They mainly depend on the technique used (macroscopic or microscopic approach). The adsorption experiments are usually performed on well crystallized samples, synthesized at the laboratory scale and few studies have been done on commercial adsorbents. However, the size and the morphology of zeolite particles, which depend on the synthesis procedure as well as the thermal treatment of the adsorbent before adsorption, may influence the adsorption properties of the material. Moreover, the adsorption of C6 paraffins on MFI zeolite is always studied at constant temperature. No adsorption data under isobaric conditions, which is a convenient way to study the effect of the temperature on the adsorption equilibria, are given in the literature, at our knowledge. This work is then devoted to the adsorption and diffusion of single C6 paraffin isomers in the gas phase as n-hexane and 2,2-dimethylbutane on a commercial ZSM-5 zeolite in order to use it in adsorption/separation process of linear and branched alkanes.

2. EXPERIMENTAL The adsorbent was supplied by the Institut Frangais du P6trole. It was a commercial template-free ZSM-5 without binder. The Si/A1 ratio was of 500. As shown in the scanning electron micrograph in Figure l, the powdered sample is composed of spherical aggregates of different sizes, with a diameter between 1 and 5 ~tm. Each aggregate is composed of interpenetrating crystals with the size in range between 0.2 and 1 gm. The microporous volume estimated by nitrogen adsorption at 77 K is of 0.184 cm3.g-1. This value is in a good agreement with the crystallographic porous volume of ZSM-5 zeolite (0.18 - 0.19 cm3.g -1)13. However, the porous volume determined on the adsorption isotherm at the relative pressure p/po = 0.95 is 0.204 cm3.g-1. This indicates the presence of a great external surface and secondary mesopores. The adsorptives n-hexane (HEX) and 2,2-dimethylbutane (22DMB) were provided by Prolabo with specified purities over 99% and were dried over a 4A zeolite before application. The adsorption isotherms of single HEX and 22DMB on ZSM-5 were measured at 348 and 443 K for pressure ranging from 10 -4 to 10 kPa by using a home made manometric device. The sample (m = 400 mg) was first activated in situ at 673 K under secondary vacuum (10 .5 kPa) for 12 hours and then, cooled to adsorption temperature. The adsorption isotherms were drawn step by step. The adsorbent was submitted to a first amount of pure alkane vapor introduced into the adsorption cell. Once a plateau of pressure was reached, a following equilibrium step was performed by varying the pressure. Kinetics of adsorption (uptake curves) was followed by recording the pressure as a function of time and by this way, the diffusion coefficients were calculated. The adsorption of alkanes under isobaric conditions was performed by means of a Mc Bain

Figure 1: Scanning electron micrograph of the commercial ZSM-5 zeolite. The particles are far from having the coffin shape usually observed on ZSM-5 zeolite.

1573

thermobalance. Prior each adsorption experiment, the zeolite (m = 15 mg) was activated in situ under secondary vacuum either at 673 K for 12 hours or at 298 K for 2 hours. The sample was then submitted to a constant hydrocarbon vapor pressure of 5.5 kPa at 298 K. This step corresponded to the first adsorption up to the quasi-saturation of the micropores. The desorption branch was then drawn by increasing step by step the temperature of the sample under constant pressure from 298 K to 673 K. Additionnal adsorption-desorption cycles were performed by increasing or decreasing the temperature.

2. RESULTS AND DISCUSSION 2.1. Adsorption isotherms

The adsorption isotherms are shown in Figure 2 and 3. They are reversible and exhibit a general type I shape of the IUPAC classification. Nevertheless the adsorption isotherm of n-hexane shows in the low pressure region a slight substep, which occurs at a filling close to 4 molec.uc 1. This substep is more noticeable on Figure 4 with a logarithmic scale for the pressure axis. This phenomenon has been observed by other authors 7' 15 before and with other adsorptives like p-xylene 14 and tetrachloroethene15. The substep is interpreted as the signature of an adsorption mechanism on two different energetic sites. According to Richards et al. 7 the first part of the isotherm would correspond to the adsorption of 4 molecules of n-hexane in the sinusoidal channels and the second one to the packing of the molecules in the straight channels, sinusoidal channels and intersections. However this adsorption model is questionable and the origin of the substep on the adsorption isotherm of molecule with a size close to that of the pore opening is still subject to discussion. Concerning the adsorption capacities, our data are self-consistent and are comparable with the data published for these systems (Table 1). The slight differences observed in the adsorption capacities are attributed to few defects present in our sample. It is worth pointing out on the adsorption isotherms of n-hexane at 348 K that the plateau is not exactly horizontal in the pressure range 2 - 10 kPa. This cannot be attributed to an adsorption on the external surface because the relative pressure is too much low (p/po < 0.08). It seems that the last two molecules of n-hexane encounter difficulties to be adsorbed in the channels. At high filling the steric hindrance becomes important in the micropores and the packing of the last molecules progressively occurs by force of the pressure. As expected, the adsorption capacity 8 -~

~ 6

HEX

4

9

3 O

"z2

--o

.....

o '

9

2

~

1 I

0

2

4 6 p/kPa

8

10

Figure 2" Adsorption isotherms of C6 alkanes on ZSM-5 at 348 K [open circles: experience; solid line: DSL model for HEX and SSL model for 22DMB.

0

2

I

I

4 6 p/kPa

I

8

10

Figure 3: Adsorption-desorption of C6 alkanes on ZSM-5 at 443 K [open circles: experience; solid line: SSL model].

1574

6 O

4

i

Figure 4: Adsorption isotherm of n-hexane on ZSM-5 at 348 K. A diffuse substep marc be noticed at the filling of 4 m o l e c . u c . [open circle symbols: experimental; solid line: DSL model].

~z2 1

0.001

0.01

I

] .....

1

0.1

10

1

p/kPa is lower for the dibranched isomer than for the linear isomer. In the low pressure region the slopes of isotherms decrease according to the sequence HEX>22DMB, indicating that the adsorption affinity of the adsorbent decreases in the presence of methyl groups branched along the carbon chain. The adsorption isotherms are fitted with the Dual Site Langmuir (DSL) model or with the Single Site Langmuir (SSL) model defined by the well-known relations: a

KIP

(DSL)

Na = Nsl 1 + KiP

(SSL)

N a =

+Na2

K2P 1 + K2P

Kp

N~ 1 + Kp

where Nas is the amount adsorbed at saturation (relative pressure p/po = 1). These models are commonly used for the adsorption of alkanes in MFI zeolites and give generally a good fit with the experimental data. The values of the Langmuir parameters are given in Table 2 even though they may lack physical significance as outlined by Ruthven 16. As shown on Figures 2,

Table 1" Comparison of amount of C6 alkanes adsorbed on ZSM- 5 under a pressure of 2 kPa and corrected diffusivities with few data found in the literature. Reference This work (Do at zero filling) Richards et al. 7 Cavalcante et al. 11

T/K 348 443 343 373 423 448 473

Wu et a117 Sun et al. 18 Millot et al. 19 Post et al. 2~

373 473 343 353 348 473 423

,,

N a /molec.uc "l HEX 22DMB 6.3 2.5 2.0 1.1 7.0 4.4 1.15 7.5 7 1.2 -

-

Do / mZ.s -1 HEX 22DMB 1.6 10 -18 2.6 10 -16 3.3 10 "16 3.3 10 15 8.9 10 -13 9 10 -16 -

1.5

-

1.0 0.7 -

4.4 10 -12 1 10 -17

1575 Table 2: Parameters of the DSL and SSL models. The maximal adsorbed amount molec.uc -~. The constants K are given in kPa -~. HEX DSL

Nasl

or

Nas2 K2

K1

SSL

348 4 70 4 0.9

Nas is expressed in

22DMB 443 5 0.36 -

348 2.89 5.48 -

443 2.8 0.34 -

3 and 4 the fit of the adsorption isotherms is reasonable. Nevertheless, Langmuir's models do not offer sufficient accuracy for estimation of the thermodynamic correction factor for the measured diffusivity values (see below). Moreover two adsorption isotherms are not sufficient to derive adsorption enthalpies and entropies derived from the Langmuir parameters. 2.2. Adsorption isobars The adsorption isobars of n-hexane and 2,2-dimethylbutane are displayed on Figures 5 and 6. They show that the adsorption process is strongly dependent on the activation temperature of the zeolite. After activation at 673 K under vacuum, the adsorption-desorption isobars of the two C6 alkanes have the same shape with reversible adsorption-desorption cycles, while after activation at 298 K a singular behavior is observed with 2,2-dimethylbutane. Indeed, during the first desorption, an unusual increase of the adsorbed amount is observed between 343 and 403 K (Figure 6). Such a phenomenon is thermodynamically inconsistent and has never been observed on ZSM-5. Above 403 K, the adsorbed amount decreases as expected and the adsorption-desorption cycles become reversible and similar to those observed after activation at 673 K. Several hypotheses can be put forward to explain this singularity as for example, an appearance of new accessible adsorption sites, an activated adsorption process or a structural change of the adsorbent. It is worth noticing that the well-known monoclinicorthorhombic phase transition of ZSM-5 occurs at a temperature closed to 343 K under vacuum 21. Even so in the state of this work this particular behavior cannot be clearly elucidated, we think that the more convenient explanation is to consider an adsorption process on two different energetic sites with a maximal adsorbed amount estimated at 3 molec.uc 1. 3 6 O

-

d 2

4-

O

~Z 2 0 200

Z 9 300

400 500 T/K

600

700

Figure 5: Adsorption-desorption isobar of n-hexane on ZSM-5 under 5 kPa [open circle: activation at 673 K; full circle: activation at 298 K].

10 200

300

400

500 T/K

600

700

Figure 6: Adsorption-desorption isobar of 2,2-dimethylbutane on ZSM-5 under 5.5 kPa after activation at 298 K [full circle: first desorption; open circles: second adsorption-desorption cycle].

1576 Both sites would be accessible in the orthorhombic phase while for steric reasons only the weak energetic sites would be occupied in the monoclinic phase. Thus, at low temperature three molecules would be adsorbed on the weak energetic sites and progressively desorbed as the temperature increases. At 343 K the phase transition occurs and the strong energetic sites become accessible. Then the molecules are preferentially adsorbed on the strongest energetic sites, which exhibit a better adsorption affinity than the others at this temperature. This would explain why the adsorbed amount increases between 343 K and 403 K. Obviously, above this last temperature the molecules adsorbed on these strong sites are desorbed in turn. Whatever the activation temperature, after a first adsorption-desorption cycle, the isobars exhibit at low temperature a plateau characteristic of the adsorption in a finite volume. The amount of alkanes adsorbed at 300 K are given in Table 3. Let assume the adsorbate as a liquid, the volume of n-hexane adsorded at this temperature is of the same order of magnitude as the microporous volume of the zeolite, n-hexane occupies almost the channel volume as opposed to 2,2-dimethylbutane, which occupies a significant lower volume. 2.3. Diffusion

Kinetics of adsorption of C6 isomers has been studied only for samples activated at 673 K under vacuum with a transient macroscopic method by means of the manometric apparatus. As adsorption experiments are performed on a thin bed of pure crystalline powder, the intracrystalline diffusion is considered as the rate limiting step of the adsorption process. The diffusion coefficients of the adsorbed species are determined from uptake kinetic curves by solving numerically second Fick's law under specific conditions. According to the SEM picture (Figure 1), the model of the zeolite particle is taken as spherical with a radius rc of 1 gm. The diffusion coefficient is assumed to be constant and isotropic over each analysis. Under these conditions Fick's law can be reduced to its spherical expression: 0Na0t = DI02Na0r 2 + r0r20Na 1 where D is the Fick diffusion coefficient, N a the adsorbed amount, r the spherical radius from the center of the zeolite particle and t the time. Fick's law is integrated over time and space using a Finite-Difference method, called Forward Time Center Space 22. The particle is divided into 1000 spherical layers of constant thickness 1 = 1 nm. The computational procedure is as follows. At the initial step, the amount of adsorbed molecules is distributed homogeneously between all the layers in equilibrium with a fixed volume of gas, Vg, under a pressure given by the adsorption isotherm at a fixed temperature. Then the zeolite particle is submitted to a higher pressure of gas according to the experimental conditions. The outer surface layer (layer 1) is assumed to be in instantaneous equilibrium with the gas phase given by the isotherm curve. Second Fick's law is then applied to the inner layers allowing the filling of the zeolite. The mass balance on the whole system gives the evolution of the Table 3: Adsorption capacities derived from the adsorption-desorption isobars at the temperature of 300 K. HEX 22DMB N a / molec.uc 1 7.3 2.81 V a / cm3.g-1 0.166 0.065 P/Po 0.22 O. 11

1577 quantity of gas in the volume Vg and the adsorbed amount. This numerical resolution of the diffusion equations requires the specification of only one boundary condition: during adsorption, the gas phase concentration on the outer surface of the zeolite particle reaches instantaneous equilibrium as soon as the adsorbent is brought into contact with the gas. This condition is fixed by the adsorption isotherm. The corrected diffusivities are calculated by means of the Darken equation: 0LnNa m D D O= D ~ 0Lnp F The thermodynamic factor F is determined by numerical derivation of the experimental adsorption isotherms. Figure 7 shows that the uptake curves are rather well described with the second Fick law. However less accurate fits have been obtained for certain loadings. These can be attributed to experimental errors. Surface barriers at the outer layer of the zeolite particle are not excluded. As shown on Table 1 the values of the corrected diffusivities are in good agreement with those given in the literature for 2,2-dimethylbutane but they are much lower for n-hexane. This could be attributed to nature of our commercial adsorbent. The corrected diffusivities are quasi-independent of the loading compare to the experimental errors (Figure 8). As expected, the corrected diffusivities increase with the temperature and are higher with the linear alkane than with the dibranched one.

3. CONCLUSION The ZSM-5 zeolite exhibits two different behaviors with n-hexane and 2,2-dimethylbutane according to the experimental conditions and the activation temperature. Under isothermal conditions and after activation at 673 K a substep is observed on the adsorption isotherm at 348 K with n-hexane. This linear paraffin probes almost the whole porosity of the adsorbent while the dibranched paraffin probes less than 40 % of the porosity. Under isobaric conditions and after activation at 298 K under vacuum, the zeolite shows a singular behavior with 2,2-dimethylbutane, which is not observed after activation at 673 K. These particular phenomena could be interpreted as the result of an adsorption mechanism on different energetic sites. On the other hand, the monoclinic-orthorhombic structural change that

011 -~

1014

0

_

HEX

"~ 0.4

~

~

i 500

1000

1500

2000

t/s Figure 7: Uptake curves for HEX and 22DMB on ZSM-5 at 348 K for the first loading,

t

22DMB 10 "20

0

~U

0

1

2

3

I

I

I

I

4

5

6

7

8

N a/molec.uc -1 Figure 8: Corrected diffusivities of HEX and 22DMB in ZSM-5 at 348 K versus loading.

1578 undergoes the ZSM-5 zeolite at 343 K is suspected to play an important role in this special adsorption process. From the kinetic point of view, the numerical resolution of the second Fick law with a Finite-Difference method allows to determine the diffusivities by fitting the uptake curves. Microporous diffusion seems to be rate limiting but surface barrier could also be significant. The diffusivities are almost constant with the loading and are more than 10 times greater for the linear alkane than for the dibranched one. A similar study with a monobranched alkane (2-methylpentane) and with linear and branched alkanes mixtures is in progress but it seems obvious that the adsorption selectivity would be governed by kinetics. The adsorption mechanism of alkanes in MFI zeolites is very complex as with aromatic or chlorinated compounds. In order to elucidate the special behaviors observed in this work, detailed studies of the adsorption of linear and branched C6 paraffins by differential calorimetry, in situ X-Ray diffraction and molecular dynamics are in progress.

REFERENCES 1. B. Millot, A. M6thivier, H. Jobic, H. Mouedded and J.A. Dalmon, Microporous and Mesoporous Materials, 38 (2000) 85. 2. R. Krishna and D. Pashek, Separation Purification Technology, 21 (2000) 111. 3. R. Krishna, L.J.P. van den Broeke, The Chemical Engineering Journal, 57 (1995) 155. 4. J; Xiao and J. Wei, Chemical Engineering Sciences 47 5 (1992) 1143. 5. C.L. Cavalcante and D.M. Ruthven, Ind. Eng. Chem. Res., 34 (1995) 185. 6. W. Zhu, F. Kapteijn and J.A. Moulijn, Microporous and Mesoporous Materials, 47 (2001) 157. 7. R.E. Richards and L.V.C. Rees, Langmuir, 3 3 (1987) 337. 8. Y. Yang and L.V.C. Rees, Microporous Materials 12 (1997) 117. 9. L. Song and L.V.C. Rees, J. Chem. Soc. Faraday Trans., 93(4) (1997) 649. 10. B. Millot, A. M6thivier and H. Jobic, J. Phys. Chem. B 102 17 (1998) 3210. 11. C.L. Cavalcante and D.M. Ruhven, Ind. Eng. Chem. Res., 34 (1995) 34. 12. B. Millot, A. M6thivier, H. Jobic, I. Clemen~on and B. Rebours, Langmuir 15 (1999) 2534. 13. D.H. Olson, W.O. Haag, R.M. Lago, J. Catalysis 61 (1980) 390. 14. D.H. Olson, G.T. Kokotailo, S.L. Lawton and W.M. Meier, J. Phys. Chem. 85(15) (1981) 2238. 15. F. Bouvier and G. Weber, J. Thermal Analysis 54 (1998) 881. 16. D.M. Ruthven, Principles of Adsorption and Adsorption Processes, John Wiley and Sons, New York, 1984. 17. P. Wu, A. Debede and Y.H. Ma, Zeolites 3 (1983) 118. 18. M.S. Sun, O. Talu and D.B. Sha, J. Phys. Chem. 100 (1996) 17276. 19. B. Millot, PhD Thesis, Universit6 Claude Bernard Lyon I, France, 1998. 20. F.M. Post, J. van Amstel, H.W. Kouwenhoven, Sixth International Zeolites Conference, Proceedings, Reno, NV, July 1983, Olson D.H. and Bisio A. Eds, Butterworth; Guilford UK, 1984, 517. 21. W.C. Conner, R. Vincent, P. Man and J. Fraissard, Catalysis Letters 4 (1990) 75. 22. W.H. Press, S.A. Teukolsky, W.T. Vetterling and B.P. Flannery, Numerical Recipes in Fortran 77,Cambridge University Press, 1992.


ADSORPTION OF INDOLE AND BENZOTHIOPHENE ZEOLITES WITH FAUJASITE STRUCTURE

1579

OVER

J.L. Sotelo, M.A. U guina, V.I./kgueda.

Chemical Engineering Department. Faculty of Chemistry. Complutense University of Madrid. 28040, Madrid, Spain. ingenieriaq@quim,ucm.es This work addresses the adsorption of indole and benzothiophene as model heteroring compounds of nitrogen and sulfur impurities, present in liquid fuels like gasoline and diesel, over zeolites with faujasite structure. It has been studied the influence of the Si/A1 molar ratio, the exchanged cation and the presence of toluene considered as a model aromatic compound present in liquid fuels. Cristallinity (by XRD), exchange level (XRF), and basic properties (CO2 TPD) of different adsorbents have also been determined. The Si/A1 molar ratio and the alkali cation exchanged have a big influence on the selective adsorption of indole and benzothiophene. Competitive adsorptives present in fuels, such as toluene, affect to the selectivity to benzothiophene adsorption rather than the corresponding to indole. Thermogravimetric analysis of the spent adsorbents shows a strong indole adsorption. Finally, a comparative study between nitrogen and sulfur adsorption confirms that nitrogen compounds could be selectively removed from fuels. 1. INTRODUCTION European legislators have introduced new specifications for low sulfur content in fuels in order to reduce exhaust emissions. For instance, a maximum sulfur level of 150 ppm and 350 ppm for gasoline and gas-oil respectively has been established. More restrictive specifications (< 50 ppm of S for both fuels) will be reached by 2005 [1]. Hydrodesulfurization (HDS) process is extensively used to remove sulfur in fuels. Deep desulfurization conditions will be required to achieve these novel specifications. For hydroprocessing, nitrogen-containing compounds are the most common poisons due to their strong adsorption on catalyst sites. The HDN of N-containing heterorings requires more severe conditions than those for other heteroring compounds [2]. Part of the nitrogen is strongly bounded to the catalyst and can not be removed during regeneration [3]. HDN produces NH3, resulting in strong deactivation of the catalyst [4]; several studies propose novel catalysts in order to decrease nitrogen compounds effects [5]. In this context, adsorption appears as a soft technology for nitrogen removal before hydrotreatment to improve HDS at milder operation conditions [6-7]. Besides, sulfur refractory compounds to HDS could be removed either before or after hydrotreatment [8-11 ]. Low silica zeolites (A and X) can be used for the selective adsorption of polar or polarizable molecules such as water, carbon dioxide or sulfur and nitrogen containing molecules [12].

1580 The aim of this work is to study the adsorption of a N-containing heteroring compound (indole) using several zeolites (FAU framework) as adsorbents. The influence of an aromatic compound (toluene) on the adsorption capacity has been studied. Finally, the competitive adsorption of indole and a S-containing compound (benzothiophene) has also been analyzed for these materials. 2. EXPERIMENTAL

2.1. Adsorbent preparation Several adsorbents based on zeolites with FAU structure with different Si/A1 molar ratio have been used. Commercial NaY and NaX zeolites were obtained from Grace Davison and Aldrich, respectively. Low silica X zeolite (LSX) was synthesised on the laboratory according to Ktihl [13], using sodium aluminate (Carlo Erba), sodium hydroxide (Merck), potassium hydroxide (Merck) and sodium silicate (Merck) as raw materials. The starting gel was previously aged and then crystallised at 343K. The solid obtained in this way was filtered and washed with 0.01M NaOH solution to avoid protonation. Alkali-exchanged LSX zeolites were prepared by conventional ion-exchange procedure: 10 ml of 0.5M aqueous solutions of sodium, potassium and cesium chlorides were added per gram of zeolite at 333K under stirring for 30 minutes. Then the zeolites were filtered and a fresh solution of alkali metal chloride was added. After the last exchange, the zeolite was washed with 0.01M solution of alkali hydroxide until no chloride ions were detectable in the filtrate. The as-synthesised NaKX zeolite (LSX) was exchanged three times with sodium chloride solutions to obtain the NaLSX sample. In the same way, KLSX and CsLSX samples were prepared from the NaLSX zeolite using potassium and caesium chloride solutions, respectively. 2.2 Characterization of the adsorbent BET surface area was calculated from nitrogen adsorption-desorption measurements at 77K using a Micromeritics Asap-2010 instrument. X-ray diffraction (XRD) patterns were performed on a Philips diffractometer (X'pert MPD) with CuK~ radiation and Ni filter. Infrared spectra were recorded on a Nicolet 510P FTIR instrument with a resolution of 2 cm -1 using the KBr wafer technique. Chemical composition was determined by X-ray fluorescence (XRF) in a Philips PW-1480 instrument. 27A1 and 29Si MAS NMR spectra were obtained on a Varian VXR-300 spectrometer equipped with a Jacobsen probe. Basic properties of the alkali ion-exchanged zeolites were determined by temperatureprogrammed desorption (TPD) of CO2. Zeolites were outgassed under a helium flow at 773K, then cooled to 373K and saturated with CO2 for lh. Afterwards, the zeolites were purged with helium at the same temperature to remove the physisorbed CO2. Finally, TPD was run at a heating rate of 10 K-min -1 to 773K. TGA of spent adsorbents were performed on a Exstar 6000 of Seiko Instruments Inc. with the module TG/DTA 6200 under a He flow of 30 cm3"min -1, with a heating rate of 10 K.min -1 up to 873K. 2.3. Adsorption experiments Adsorption experiments were carried out in closed Pyrex tubes with Vyton seals to avoid vapour losses, placed in a thermostatic bath. Initial mixture was prepared with indole

1581 (Aldrich) in anhydrous cyclohexane (Merck) with a nitrogen content of 500 ppm. 5 g of this mixture were stirred with different weights of adsorbent at 298 K during 48 h. All adsorbents were previously activated by calcination for 2 h at 723 K. After adsorption, the adsorbents were recovered by centrifugation and dried at 298 K before TGA. To determine the influence of a competitive adsorptive on the capacity and selectivity to the nitrogen compound, 5% wt of toluene (Merck) was added to the initial mixture. Nitrogen and sulfur adsorptions were compared adding benzothiophene to the mixture as a compound with the same indole structure (500 ppm of sulfur). Nitrogen concentration was measured by GC. Liquid phase sulfur analyses were performed by XRF, in a Bruker $4 Explorer wavelength-dispersive X-ray spectrometer based on multi-layer analyser, using S-Ka spectral line in 40 mm PE-HD cells, with 2.5 bun Mylar film windows in atmospheric helium mode. Acquisition time was 100 s.

3.RESULTS AND DISCUSSION 3.1. Adsorbent Properties Chemical composition, CO2 TPD, and surface area of the adsorbents studied are summarized in Table 1. Figures 1 and 2 present CO2 TPD of the adsorbents. Table 1 Properties adsobren_t of the, Si/A1

Adsorbent NaY NaX NaLSX KLSX CsLSX _

_

~

__,_~s ....................................................

,

~

,

Na K Cs (Na+K+Cs) (Na+K+Cs) (Na+K+Cs)

CO2 TPD

Tm~x BET area

(molar)

(molar)

(molar)

(molar)

(mmol-g -1)

(K)

(m2-g-1)

2.8 1.3 1.1 1.1 1.1

1 1 0.95 0.25 0.45

~ -0.05 0.75 ~

~ -m ~ 0.55

1.12"lff~ 1.49-10 -3 2.23.10 -3 1.57-10 -3 9.57.10 -4

533 593 608 583 503

844 847 765 715 507

~

Table 1 shows that the higher A1 content logically corresponds to the low silica X (LSX) zeolite samples prepared, with a Si/A1 ratio of 1.1. The CO2 TPD measurements show a foreseeable increase with the A1 content of the adsorbent, as the number of adsorption sites increases too. Lower values of CO2 TPD are obtained for cation exchanged materials due to the higher atomic weight and volume of these cations, hence the number of basic sites per gram of adsorbent decreases and the possible hindrance to CO2 adsorption should be taken into account. The temperatures corresponding to the maximum CO2 desorption (Figure 2) increase in the order: CsLSX>CsLSX.

1582 0,15 ":'. 0,14

Si02

300 250

NaY

200 150 100

b~203

50 I

I

I

0.2

0.4

0.6

1

0.8

PIPo 0.008

b)

-

0.007

Al=O3.o-'" .I~

o.oo5 o.oo4

~..

0.003 -

O

n

~),~176

~

~ o

0.006 -

="

~

~ o o

~

0.002 -

.

0.001 -

. A~~ ~ -

~:6::---'",

0.000 0

.-~176

~176

"~

~

~

..0"

SiO2 ~176176176 .~

k

,,o ~176176176176

,~~ ~

.O"~

~

.O" ~

~

~o

-A""

.~

O ..o

.~

.6 -''

..o

.oo

9

NaY

"

I

01

I

02

0.4

03

PIPo

300 O. p. r

,,,

E v

A

C)

250

zx

NaY

200

............ 00

A

." ~ .

0

1

0

02

-o . . . .

&'"

jL.dK

.A"

.A.-A" SiO2

..--'~o,~~~ ... ":.~'~

50-

~ ~176

o- o oe.ooe-e-o- o--e- ..~-,6 . . . . . . .

150 100-

~

A...--""

[

04

. ,..o'*"

1

06 t(nm)

~"

1

08

- "G'""

.. . . . . . . "" o

AI203

4

1

12

Figure 1. Adsorption isotherms (a), BET-isotherms (b) and t-plots (c) o f NaY, A1203and SiO2

1592 Table 1 Comparison of surface characteristics of various microporous materials obtained by different methods Standard t-plot Schneider's t-plot BET Harkins-Jura [ 15] modified.BET [ 16] Lecloux [ 17] Sample SBET C St Wmicro S "BET Vmicro C St Vmicro (m2g-') (m2g1) (cm3g1) (m2g1) (cm3g1) (m2g1) (cm3g1) CaA 349 -41 14 0.186 20 0.186 24 12 0.188 NaY 573 -35 25 0.290 36 0.285 65 27 0.290 USY 521 -43 115 0.215 96 0.226 34 119 0.218 ERI 274 -51 11 0.130 15 0.128 35 11 0.130 MOR 304 -48 11 0.148 20 0.145 53 11 0.149 MFI 268 -45 49 0.112 41 0.117 32 49 0.116 Table 2 Influence of C-constant value on results of Lecloux t-plot for various zeolites ERI MFI NaY USY C St Vmiero St Wmicro St Vmicro St Vmicro (m2/g) (cm3/g) (m2/g) (cm3/g) (m2/g) (cm3/g) (m2/g) (cm3/g) 20 10.1 0.131 42.2 0.119 22.7 0.293 110.7 0.224 30 10.6 0.131 45.2 0.117 24.1 0.292 1 1 7 . 3 0.219 40 10.9 0.130 47.7 0.115 25.1 0.291 121.5 0.216 50 11.2 0.130 49.8 0.114 25.8 0.290 124.4 0.214 100 11.7 0.130 56.6 0.110 27.4 0.289 1 3 1 . 3 0.209 150 Ix.

130

A

110

I--

01

"E

90

:~

7o

x ......

X

x"Xo

~~mi~='=~

o

o

0

x

....

O.-~

o - Harkins-Jura x -

50

~..~x

Lecloux

I

I

I

I

0.2

0.4

0.6

0.8

1

t(nm)

Figure 2. Comparison between t-plots Harkins-Jura and Lecloux for CaA zeolite

1593

,

..................

o

o

A

150

o

"

..-

~,

O

.....

-

0 m

,.

..x-

100

:.~---"-~-

"0"

E]- . . . .

o

0

0.2

o--

,

USY

',

:.+

! i

a.-9-n"nm -

,,

~)" " O "

o

n.

9 .O..O-

9

--

l ~ - - [ T ' " "D" " ~

......

DB-ERI

. . . . . . . . . . . .

': 0.45

9 .........

t)

Silicalite

! !

A

tx "

.n. -. .-.

:

....

,. . . . . .

: ""

o 50

,

. ~". . -

...-

:

! ,

...--n [

- .---" ...-. _ - - - -

A" ' & ' A ' "

.*

.~.

',~ . =, . : . : .

a , "*

o- -*

............

.-~

:

..-". . . . . . . . . . . . . . . .a. . . .

>

;

.N . . .a. Y : ,~ . , . . , . . . , . . , . ~ , . . , . . .

200 -

I

0.35[

, '

0.7 0.4

0.6

0.8

1

t(nm)

Figure 3. Examples of various types of t-plot shapes and range of their linearization from the shape of t-plot in the range of t=0.35-0.45, connected with the shape of adsorption isotherm. This area corresponds to the cooperative adsorption process in wider micropores at P/P0 0.02-0.2 after primary process in prinaary micropores (ultramicropores) at very low P/P0 (generally under 0.01) - this process involves enhanced adsorbent-adsorbate interactions [ 19]. The intercept of the linearization of the upper part of t-plot (above t=0.45) gives the total volume of all micropores, the volume of the secondary micropores could be estimated from the lower part of t-plot - Figure 3. The next type of shape of t-plot is also reflecting the shape of adsorption isotherm- as is was akeady published, for ZSM-5 zeolites with Si/A1 over about 50 up to silicalite a doublestep isotherm was observed, reflecting in double-step t-plot - Figure 3. The lower and upper parts of t-plot are linear generally at t=0.2-0.4 nm and t=0.5-0.8 nm, and the values of intercepts of these two linearization are in the same ratio as the liquid and solid nitrogen [20]. As we have shown, the adsorption isotherms as well as t-plots of ZSM-5 zeolites from good, microporous type for zeolites with Si/AI=14-20 via curved type changes after hydrothermal dealumination characteristic for supermicropore creation to two-steps curves for high dealuminated samples (Si/AI>45) [21 ].

4. CONCLUSIONS The t-plot method is very powerful method for the evaluation of microporous mesoporous structure of zeolites. The results showed that the Harkins Jura master isotherm is

1594 applicable for zeolites, in which the vales of 25-65 of the CBEx-COnStant of non-microporous part was determined by modified BET-isotherm. Results of Lecloux t-method on the basis of determined C-constant were very close to those of Harkins-Jura. We can to suggest to incorporation the non-linear regression of modified BET-isotherm (Eq.2) into soitwares of commercial sorption equipments to calculate C-constant for Lecloux t-method. The general range for t-plot linearization is in t=0.25-0.6, i.e. in the formation of the 2/3 of the second layer of nitrogen. For the pure zeolitic materials this linear range can be extended even to t-1 nm. Very sensitive seems to be the t-plot method to the all anomalies and irregularities in ultramicropore system (under 0.7 nm) that is typical for the most of zeolites. The curving of tplots in range oft=0.35-0.45, connected with curving of adsorption isotherms in range P/P0 up to 0.2, corresponds to the adsorption in supermicropores. In this case the linearization of the upper part of t-plot gives the total micropore volume, and from the lower part it is possible to estimate the volume of supermicropores. REFERENCES

1. J.Lynch, F.Raatz and P.Dufresne, Zeolites, 7 (1987) 333. 2. S.Cartlidge, H.-U. Nissen and R.Wessicken, Zeolites, 9 (1989) 346. 3. A.Zukal, V.Patzelovfi and U.Lohse, Zeolites, 6 (1986) 133. 4. B.Chauvin, P.Massiani, R.Dutartre, F.Figueras, F.Fajula and T.Des Courieres, Zeolites, 10 (1990) 174. 5. J.H.de Boer, Lippens B.C., Linsen B.G., Broekhoff J.C.P., van den Heuvel A. and Osinga Th.V.J.Colloid Interface Sci. 21 (1966) 405. 6. K.S.W.Sing, Chem.Ind., London 1968, 1520. 7. M.M.Dubinin, Chem.Rev., 60 (1960) 235. 8. L.E.Aneke, W.A. de Jong and P.J.van den Berg, J.Royal Neth. Chem.Soc., 99 (1980) 263. 9. G.Leofanti, P.Genoni, M.Padovan, G.Petrini, G.Trezza and A.Zecchina, Characterization of porous solids II (Eds.F.Rodriguez-Reinoso et al.), Elsevier, Amsterdam 1991,553. 10. E.F. Sousa-Aguiar, A.Liebsch, B.C.Chaves and A.F.Costa, Microporous and Mesoporous Materials, 25 (1998) 185. 11. G.Leofanti, M.Padovan, G.Tozzola and B.Venturelli, Catalysis Today, 41 (1998) 207. 12. P.Hudec, J.Novansk~, S.Silh~, T.Trung, M.Zfibek and J.Madar, Ads. Sci. Technol., 3 (1986) 159. 13. R.L.Mieville, J.Colloid and Interface Science, 41 (1972) 371. 14. M.F.L.Johnson, J.Catal. 52 (1978) 425. 15. W.D.Harkins and G.Jura, J.Am.Chem.Soc. 66 (1944) 1366. 16. Schneider P.: Applied Catalysis A: General 129 (1995) 157. 17. Lecloux A.J., Catalysis- Sci. and Technology, Vol.2, (Akad.Verlag, Berlin 1983) 172 18. P.Hudec, A.Smie~kov~, Z.Zidek, E.Rojasovfi, Stud. Surf. Sci. Catal., 130 (2000) 2903. 19. P.J.M.Carrot and K.S.W.Sing, Characterization of porous solids (Eds. K.K.Unger et al.) Elsevier, Amsterdam 1988, p.77 20. P.Hudec, A.Smie~kov~i, Z.~;idek, M.Zfibek, P.Schneider, M.Korifik and J.Koz~xlkov~i, CoECzech. Chem. Commun., 63 (1998) 141. 21. P.Hudec, A.Smie~kov~i, Z.~dek, E. Sabo and B.Lipt~kovfi, Stud. Surf. Sci. Catal., 135 (2001) 29-P-26.


1595

B i n a r y m i x t u r e a d s o r p t i o n o f w a t e r and ethanol on silicalite Y. Oumi ", A. Miyajima a, J. Miyamoto b and T. Sano a a School of Materials Science, Japan Advanced Institute of Science and Technology, Tatsunokuchi, Ishikawa 923-1292, Japan; E-mail:[email protected], Fax: +81-761-51-1625 b BEL Japan Inc., Midori, Sumida-ku, Tokyo 130-0021, Japan Single component and binary mixture adsorption behaviors of water (H20) and ethanol (EtOH) were investigated on silicalites with different crystallinity (structure-defects) by a combination of volumetric and gravimetric methods. No influence of the crystallinity of silicalite on the pure EtOH adsorption was observed. On the other hand, the amount of H20 adsorbed on the silicalite with less structure-defects was significantly smaller. The total adsorption amount in the binary mixture adsorption was strongly dependent on the EtOH/H20 ratio in the initial mixture. From the analysis of the binary adsorption data, it became clear that EtOH molecules adsorb preferentially on silicalite with less structure-defects and that more selective adsorption of EtOH takes place under the lower equilibrium pressure.

1. INTRODUCTION In recent years, zeolite membranes have attracted considerable interest due to their potential use in several industrial applications such as gas or liquid separation membranes, sensors and electrodes [1-4]. When the zeolite membranes are used as separation membranes and sensors, a detailed understanding of adsorption process of molecules is needed. Although there are a large number of literature concerning adsorption experiments with various polar and non-polar molecules, most of researches are related to the single component adsorption behavior. Some researchers have only investigated the binary adsorption behavior using the FT-IR, NMR and computer simulation techniques [5-11 ]. However, there are few studies on the simultaneous measurements of adsorption isotherms by combining volumetric and gravimetric methods. We have already reported a high alcohol permselectivity of silicalite membrane [ 12]. From single component adsorption studies of water and alcohol on powdered silicalite and ZSM-5 zeolite crystals prepared under various hydrothermal synthesis conditions, it was found that the high alcohol permselectivity is attributable to the high hydrophobic properties of silicalite crystals and that the amount of water adsorbed is strongly influenced by the number of silanol groups of zeolite crystals [13]. This suggests that the separation performance of zeolite membrane is affected by the crystallinity of crystals. From such a view point, we study an influence of the crystallinity of silicalite crystals on the adsorption behavior of binary mixture of H20 and EtOH by a combination of volumetric and gravimetric methods.

1596 2. EXPERIMENTAL

2. 1. Synthesis of silicalite crystals Synthesis of silicalite crystals was performed using colloidal silica as a silica source, tetrapropylammonium bromide (TPABr) or tetrapropylammonium hydroxide (TPAOH) as a structure-directing agent. The chemical compositions of starting synthesis mixtures prepared are as follows; (1) 0.1TPABr-0.05Na20-SiO2-40H20, (2) 0.1TPAOH-SiO2-40H20, (3) 0.36TPAOH-O.39HF-SiO2-34H20. The hydrothermal synthesis conditions were listed in Table 1. The silicalite crystals obtained were filtered, washed thoroughly with deionized water, dried at 120~ and calcined at 500~ for 20 h to remove the structure-directing agent. 2.2. Characterization The identification of zeolites obtained was achieved by X-ray diffraction (Rigaku RINT 2000). The crystal size was examined by scanning electron microscopy (Hitachi S-4100). Textural properties were determined by nitrogen adsorption (Bel Japan Inc. Belsorp 28SA). Before adsorption measurements at -196~ the powdered zeolites (ca. 0.1 g) were evacuated at 400~ for 10 h. 29Si MAS NMR spectra were recorded on a Varian UNITY INOVA400 spectrometer at 79.5 MHz with 3 kHz spinning speed and 7.6 gs pulses for 2,000 scans. DSS was used as a chemical shift reference. The IR spectra for the framework vibration were recorded on a FT-IR spectrometer (JEOL JIR-7000) with a resolution 4 cm-1 at room temperature. The sample was pressed into a self-supporting thin wafer (ca. 6.4 mg/cm 2) and was placed in a quartz IR cell with CaF2 windows. Prior to the measurements, each sample was dehydrated under vacuum at 400~ for 2 h. 2.3. Single component and binary mixture adsorption isotherms The single component and binary mixture adsorption experiments of pure H20 and EtOH vapors on the silicalites were carried out at 27~ by a Belsorp 18 and a FMS-BG adsorption apparatus (Bel Japan Inc.), respectively. Prior to adsorption measurements, the powdered silicalites (ca. 0.3 - 1.0 g) were evacuated at 400~ for 6 - 10 h. The binary mixture adsorption experiments of H20 and EtOH vapors were performed by combining volumetric and gravimetric methods. The EtOH/H20 ratio in the initial mixed vapor was varied from 100/0 to 0/100. 3. RESULTS AND DISCUSSION

3.1. Synthesis and characterization of silicalites In general, it is recognized that zeolite crystals synthesized in the presence of F- are larger as compared with zeolite crystals synthesized under a conventional hydrothermal method, and contain less structure-defects [ 14]. To clarify an influence of crystallinity of silicalite on the adsorption behavior, therefore, the silicalite crystals were synthesized with and without HF. The hydrothermal synthesis conditions are listed in Table 1. For all the obtained products, the X-ray diffraction diagrams showed no peaks other than those corresponding to MFI type zeolite structure. Fig. 1 shows the SEM images of silicalite crystals synthesized. The shape of silicalite crystals was a coffin type. The large differences in the crystal size and the aspect ratio were observed among the silicalite crystals synthesized with and without HF. By addition of HF, a large silicalite crystal with the high aspect ratio was obtained. Table 1 also

1597 Table 1

Hydrothermal synthesis conditions and characteristics of silicalites obtained.

Synthesis conditions BET Pore volume a) Chemical composition Temp. Time surface / ,cm3rl:_..:_~x ~ n q u m ) g _1 of starting synthesis gel /~ / hr area/m2g -1 0.1TPABr-0.05Na20160 24 407 0.18 SIO2-40H20 0.1TPAOH-SiO2-40H20 160 48 419 0.19 0.36TPAOH-0.39HF200 88 391 0.17 SIO2-34H20 a) Determined by the Dubinin-Radushkevich method. b) Weight loss calculated from TG curve (300~ -1100~ Sample No.

Weight lossb) /wt% 0.95 0.93 0.43

Fig. 1 SEM images of the silicalite crystals synthesized with (a) TPABr, (b) TPAOH and (c) TPAOH/HF. summarizes the characteristics of the silicalite crystals obtained at various synthesis conditions. No difference in the BET surface area and pore volume was observed. However, the large difference in the weight loss from 300 to 1100 ~ in the TG curve was observed between the silicalite synthesized with TPABr or TPAOH (sample No. 1 or 2) and the silicalite synthesized with TPAOH/HF (sample No. 3). This indicates that the number of silanol groups (structure-defects) of silicalite synthesized with TPAOH/HF is smaller than that of silicalite synthesized without HF, if the weight loss is assumed to be attributed to the dehydroxylation process (2SiOH ~ SiOSi + H20). To get further information concerning the silanol groups (structure-defects) of silicalites, the IR and 29Si MAS NMR spectra of the silicalites were measured. Fig. 2 shows the IR spectra of the silicalites in the 4000-3000 cm -1 region. The IR spectrum of the silicalite synthesized with TPABr or TPAOH exhibited two peaks at ca. 3500 and 3740 cm 1. The peaks at ca. 3500 and 3740 cm -1 are assigned to the

(c)

I

I

I

4000 3800 3600 3400 3200 3000 Wave numbers (cm- 1) Fig. 2 IR spectra of various silicalites synthesized with (a) TPABr, (b) TPAOH and (c) TPAOH/HF.

1598 hydrogen bonding adjacent silanol groups and the isolated silanol groups, respectively. In the spectrum of the silicalite synthesized with TPAOH/HF, although the peaks at ca. 3500 and 3740 cm -1 were also observed, their intensities were considerably weaker than those of the silicalite without HF. Fig. 3 shows 29Si MAS NMR spectra of the silicalites. The resonance lines between-109 and-115 ppm could be assigned to Q4 [Si(0A1)] unit [15]. As no line of Q3 [Si(1A1)] unit appears in the spectra owing to the very low A1 content of silicalite, the small line observed at ca. -103 ppm could be assigned to Q3 [Si(0A1)] ~unit belonging to SiOH defect centers in the zeolite framework. No line at c a . - 1 0 3 ppm was observed in the spectrum of the silicalite synthesized with TPAOH/HF, indicating the number of structure-defects is significantly smaller. From the above results, it may be concluded that the crystallinity of the silicalite synthesized with TPAOH/HF is higher than that of silicalite without HF.

\

3. 2. Single component adsorption of H20 and EtOH vapors on silicalites At first, the single component adsorption isotherms of pure H20 and EtOH vapors on the silicalites with different crystallinity were measured using a conventional volumetric method. Fig. 4 60

I

-90

-100

I

(a)

I

i

-110 -120 -130 -140 ppm

Fig. 3 29Si MAS NMR spectra of various silicalites synthesized with (a) TPABr, (b) TPAOH and (c) TPAOH/HF. 60

(a)

(b)

"7

"7

~o 50

5o 40

40 e~

o

30

cD

3o

,.Q

CD

"~ 2o r.r

20

9 10 c.q 7:

e=.,=a lO b o.5

P/Ps

1

0~

0

!

0'.5 P/Ps

Fig. 4 Adsorption isotherms of pure (a) H20 and (b) EtOH vapors at 27~ on various silicalites synthesized with ( 0 ) TPABr, (O) TPAOH and (A) TPAOH/HF.

1

1599 shows the adsorption isotherms of pure H20 and EtOH vapors. The amount of water absorbed was strongly dependent on the synthesis condition, whereas no difference in amount of EtOH adsorbed was observed. Namely, water adsorption on the silicalite synthesized with TPAOH/HF hardly occurred at the low P/Ps. It is well known that adsorption of water on zeolite involves the specific interaction between water molecules and the hydrophilic centers, which are silanol groups or protons associated with the framework aluminums [ 13]. Therefore, these results indicate that there is a large difference in the adsorption behavior between H20 and EtOH molecules. The EtOH adsorption on silicalite is hardly influenced by the structure-defects in the zeolite framework. Probably, EtOH molecules adsorbs on silicalite in a similar manner as non-polar molecules. 3.3. Binary mixture adsorption of H20 and EtOH vapors on silicalites To calculate the individual isotherm of each component from the adsorption data of mixed vapors, the adsorbed amount and the equilibrium partial pressure of each vapor have to be determined with high accuracy. As the dead volume of the FMG-BG instrument used for measurement of the binary adsorption isotherm with a combination of volumetric and gravimetric methods was ca. 300 ml and larger than that (ca. 180 ml) of the Belsorp 18 (for single component adsorption), the accuracy of the FMG-BG instrument was evaluated by pure H20 adsorption. It was found that there is no difference in the amount of H20 adsorbed on silicalite measured using two instruments. Therefore, next, the simultaneous measurement of the adsorption isotherms of H20 and EtOH on various silicalites was conducted by combining volumetric and gravimetric methods. Fig. 5 shows the adsorption isotherms of binary mixture of H20 and EtOH under the dosing ratio of each vapor of 50/50. EtOH molecules were found to adsorb preferentially on both silicalites synthesized with TPABr and TPAOH/HF. The total amount adsorbed on the 0.14

0.14 (a)

0.12 ~- 0.10 "~ 0.08 ra~

"7 -

9

*-, 0.06 -

-o

0.04 0.02

9

9

9

9

9

000

9

9

O

O

O

O

"~ 0.10

O

tD

"~ 0.08

_ .oO

_

9

o

0

E 0.04

I

0

O

-~ 0.06

AAAAAAAA

0

O

(b)

0.12

I

1

I

2

P/kPa

0.02

A A A I

3

0

_ . ot,? 0

^,^ ~AI_A ~

1

AA

2 P/kPa

Fig. 5 Adsorption isotherms of binary mixture of H20 and EtOH at 27~ on the silicalites synthesized with (a) TPABr and (b) TPAOH/HF. 9 Total adsorption amount, 9 Amount of adsorbed EtOH, /k Amount of adsorbed H20

3

1600 0.14

(a)

~ o 0.12

~3 [] A [] A A 4:) [] A [] A DA DA DA

o

~0 0.12

a~

e~o

~9 0.10 o

0.14

0.08 0.06 0.04 0.02

Co)

9 o" ~

0.10 ~0.08

- t& :

[]

9 o

~ 0.06 t o g /

~

o

[]

g'~ ./~

.A

0.04 0.02

0 0 1 2 3 2 3 4 P/kPa P/kPa Fig. 6 Adsorption isotherms of binary mixture of H20 and EtOH at 27~ on the silicalites synthesized with (a) TPABr and (b) TPAOH/HF. EtOH/H20 ratio in the initial mixed vapor : & 100/0, 9 80/20, F1 60/40, /k 50/50, ~ 20/80, 9 0/100

0

1

/

4

silicalite synthesized with TPAOH/HF was smaller than that on the silicalite synthesized with TPABr. However, the mole fraction of EtOH on the silicalite synthesized with TPAOH/HF was larger than that on the silicalite synthesized with TPABr. Taking into account the fact that no difference in the amount of adsorbed EtOH was observed between these silicalites as shown in Fig. 4-(b), it is suggested that the adsorption behavior of binary mixture on silicalite is strongly influenced by the crystallinity of silicalite. Additionally, to investigate an influence of the EtOH/HzO ratio in the initial mixed vapor on the binary adsorption behavior, the dosing ratio of each vapor was varied from 100/0 to 0/100. Fig. 6 shows the total adsorption isotherms of binary mixtures on the silicalites synthesized with TPABr and TPAOH/HF. For both silicalites, the total adsorption amount was strongly dependent on the EtOH/HaO ratio. Namely, the total adsorption amount decreased with an increase in the mole fraction of H20. Fig. 7 shows the relationship between the mole fraction of EtOH in the initial mixture and the mole fraction of EtOH adsorbed on silicalite. From a comparison of the mole fraction of EtOH in the adsorbed phase with the initial composition of binary mixture, it became clear that the mole fraction of EtOH adsorbed on the silicalite synthesized with TPAOH/HF was larger than that on the silicalite synthesized with TPABr, indicating more selective adsorption of EtOH on silicalite crystals with less structure-defects. It was also found that the lower the equilibrium pressure, the higher the EtOH selectivity.

4. C O N C L U S I O N S Silicalite crystals with different crystallinity were synthesized with and without HF and their single component and binary mixture adsorption behaviors of EtOH and H20 were

1601 investigated. It was found from the single component adsorption experiments that the H20 adsorption is strongly influenced by the crystallinity, namely, the amount of H20 adsorbed increased with an increase in the number of structure-defects. On the other hand, no difference in amount of EtOH adsorbed was observed. It was also found from the binary mixture adsorption experiments that EtOH molecules adsorb preferentially on silicalite with less structure-defects and that more selective adsorption of EtOH takes place under the lower equilibrium pressure. These findings suggest that in order to improve the alcohol permselectivity of silicalite membrane, an increase in the crystallinity of silicalite crystals is needed. 1.0

(a)

1.0

/-=

0.8

0.8

.==o ~ 0 . 6 ~'Y~ g0.4

.g So.2 0[

///

. ,...~

0.6

///

0.4

N 0.2

0.4

0.6

0.8

The mole fraction of EtOH in initial mixture

1.0

0.2 O[ 0

I

0.2 0.4 0.6 0.8 The mole fraction of EtOH in initial mixture

1.0

Fig. 7 Relationship between the mole fraction of EtOH in the initial mixture of EtOH and H20 and the mole fraction of EtOH adsorbed on the silicalites synthesized with (a) TPABr and (b) TPAOH/HF. Equilibrium pressure: Vq 0.1 kPa, O 1 kPa, A 3 kPa.

REFERENCES

1. J. Neel, "Pervaporation Membrane Separation Processes", R. Y. M. Huang (ed.), Elsevier, Amsterdam, (1994) 1. 2. T.Q. Nguyen and K. Nobe, J. Membrane Sci., 23 (1987) 11. 3. M.H.V. Mulder, A. C. M. Frank and C. A. Smolders, J. Membrane Sci., 23 (1995) 41. 4. J. Coronas and J. Santamar/a, Sep. Purif. Meth., 28 (1999) 127. 5. R.L. Portsmouth, L. F. Gladden and M. J. Duer, J. Chem. Soc. Faraday Trans., 91 (1995) 963. 6. S. Ashtekar, J. J. Hastings and L. F. Gladden, J. Chem. Soc. Faraday Trans., 94 (1998) 1157. 7. S. Ashteker, A. S. McLeod, M. D. Mantle, P. J. Barrie, L. F. Gladden and J. J. Hastings, J.

1602 Phys. Chem. B, 104 (2000) 5281. S.U. Rege and R. T. Yang, Chem. Engng. Sci., 56 (2001) 3781. F. Karavias and A. L. Meyers, Mol. Simu., 8 (1991) 51. L. F. Gladden, M. Hargreaves and P. Alexander, Chem. Eng. J., 74 (1999) 57. V. Lachet, A. Boutin, B. Tavitian and A. H. Fuchs, Langmuir, 15 (1999) 8678. T. Sano, H. Yanagishita, Y. Kiyozumi, F. Mizukami and K. Haraya, J. Membrane Sci., 95 (1994) 221. 13. T. Sano, T. Kasuno, T. Takeda, S. Arazaki and Y. Kawakami, Stud. Surf. Sci. Catal., 105 (1994) 1771. 14. E. Nigro, R. Mostowicz, F. Cera, F. Testa, R. Aiello and J. B. Nagy, Stud. Surf. Sci. Catal., 105 (1997) 309. 15. C. A. Fyfe, G. T. Kokotailo, G. J. Kennedy and C. de Schutter, J. Chem. Soc., Chem. Commun., (1985) 306.

8. 9. 10. 11. 12.


1603

Influence o f water adsorption on zeolite Beta C. Flego, G. Pazzueorfi, C. Perego EniTecnologie S.p.A. - Via Maritmao, 26 - 20097 S. Donato Mil. (MI) - Italy The influence of water on acidity mad catalytic performances of zeolites has been widely studied and controversial results have been proposed ha literature. The re-hyda'ation of zeolite Beta is here evaluated as a tool to recover the Bronsted acidity, that can be partially lost during thernaal treatments (e.g. regeneration). Introduction of controlled amoums of water influences both the A1 siting and the acidity distribution in zeolite Beta and consequently the densit3~ of the active sites.

1. INTRODUCTION Water strongly influences the catalytic behaviour of zeolites, modifying their catalytic performances mad lifetime [1]. Investigation of the adsorption of water is therefore of great impol~tance with respect to practical applications. Treatments with water vapour may influence the A1 distribution even at mild temperatures, with or without changes of its overall content in the zeolite [2-6]. Attention to the re-hydration of zeolite Beta has been paid in order to both verify the possible recovering of the Bronsted acidity lost during thermal treatmlents (e.g. regeneration by coke combustion causes dehydroxylation and abstraction of AI from the framework) or avoid the detrimental effects of the Lewis acid sites during reaction (e.g. selective poisoning) [5-7]. Both phenomena are responsible of modification or reduction in the number of the active sites. A series of Beta zeolite samples with similar SiO2/A1203 molar ratio were chosen in order to determine the changes in acid sites distribution related to the different locations of A1 atoms mad flaeir coordination. The conditions of water adsorption on zeolite Beta were studied (i.e. temperature and time) mad their influence on the fomaation of the active sites (i.e. in framework acid A1-OH groups [8]) was evaluated.

2. EXPERIMENTAL 2.1 Catalysts Beta zeolite samples were synthesised with SIO2/A1203 molar ratio of 20-24, according to [4]. [31 sample was mildly dealuminated to SIOJA1203=33, by post-synthesis treatmem with 0.1 M HCI (2h at 100~ giving rise to ~3 smnple. 131 sample was also thermally treated in subsequent cycles (550~ 5h, air flow), giving rise to 136, and subsequently washed wifll HC1, giving rise to [37 smnple. As a reference was synthesised one zeolite Beta (t]8) wifla B instead of AI in the framework (Si/B= 1.5). All samples were analysed in H%form, after exchange with CH~COONH4, but 134 and ~5 obtained by washing with diluted HC1 solution. Na was present in traces ( 1 t3v

1

I

I

t

T

T

i

t

l

I

•

-

-

-;

-

I

I q

-

--

E =._o

--= lO -kkX- ~,~

1

0 ffl

tO

~

f

~

I

-1~ c m 2 / s

/

. . . . . . . . . .

0 t-

t

De=0.5*10

De~0.7*10-1~ 7.........

-

O 0

o

6

0

t

l

1

I

I

I

3000

6000

I

9000

12000

15000

Time, [s]

Figure 3. Comparison between experimental and simulated data at various diffusion coefficients for the ion exchange process 2 N a + r

Cu 2+ (pH = 3).

Table 4 Values of the diffusion coefficient obtained by the two methods pH Values according to Predicted values, B oyd-Adamson, 10 ]0cm 2s ] 1010cm2s-1

Accepted as real by comparison to predicted values,

3 4 5

0.7 1.0 1.2

101Ocm2s -1

0.82 1.07 1.21

0.5 0.9 0.8

0.7 1.0 1.2

0.9 1.2 1.4

4. CONCLUSION The investigation of the influence of the initial pH of solutions on Cu(II) exchange process on chemically activated clinoptilolite showed the positive influence of pH increase on exchange capacity during time and at equilibrium. The kinetic study resulted in determination of the diffusion coefficient by two methods: B o y d - Adamson and numerical simulation. When the numerical simulation was used, the estimation of the diffusion coefficient resulted by comparing experimental data to values obtained by simulation.

1662 The numerical simulation method allowed recording of the concordance between experimental results and predicted ones by simulation all the way of the kinetic determination. The two methods led nearly equal values of the diffusion coefficient. REFERENCES

1. R.H. Perry and D.W.Green (eds.), Perry's Chemical Engineers'Handbook, Seventh Mc Graw-Hill Edition, 1997. 2. M.Cruceanu, E. Popovici, N. Balba, N.Naum, L. Vladescu, R. Russu and A. Vasile, Zeolitic Molecular Sieves (Romanian), Ed. Stiintifica ~i Enciclopedica, Bucharest, 1986. 3. H.Minato, Natural Zeolites - Sofia'95 (G. Kirov, L. Filizova and O. Petrov eds.), Pensoff Publishers, Sofia - Moscow, Bulgaria (1997) 282. 4. G. Catana, L. Frunza, D. Crisan, R. Pode and G. Burtica, Annals of"A1. I. Cuza" Univ.Iasi, Vol. I, Series of Chemistry (1992) 200. 5. D.Reichenberg, J.Am.Chem.Soc., No.75(1953) 597 6. R. Pode, A. Iovi, G. Burtica and. M. Sava, Annals of "A1. I. Cuza" Univ.Iasi, Vol. III, Series of Chemistry (1995) 85. 7. E. Popovici, G. Burtica, R. Pode, I. Bedeleanu and I. Calb, Natural Microporous in Environmental Technology (P. Misaelides et al., eds.), Kluwer Academic Publisher, Holland (1999) 345. 8. P.M. Armenante and D. J. Kirwan, Chemical Engineering Science, No. 44 (12) (1989) 2781.


1663

Dynamics of sorption columns in dewatering of bioethanol using zeolites Martin Boldig a'b, Karel Melzoch a, Jan Pokorny c, Milan Ko~i~ik b a

The Institute of Chemical Technology in Prague,Technick~. 5, 166 28, Praha 6

b j. Heyrovsl~ Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolej~kova 3, 182 23 Praha 8, E-mail: [email protected] c The Institute of Physics, Academy of Sciences of the Czech Republic, Na Slovance 2, 182 21 Praha 8 1. INTRODUCTION There is an increasing interest in using bioethanol as an additive to fuels. Classical technologies of residual water removal from ethanol via an azeotropic distillation using ternary systems suffer from numerous drawbacks. Thus, alternative methods of ethanol dewatering as sorption 1 or pervaporation techniques using membranes are searched. The aim of the present work is to test potentiality of selected zeolitic sorbents to remove residual water from ethanol-water mixtures with composition close to that of azeotrope at normal boiling point (7"6 - 78 ~ where the mass fraction WH20 in both the phases WH20 = 0.05. The emphasis is also laid on the analysis of heat effects the bed of adsorbent and a conceivable coupling of energy balance of the sorption process with that of azeotropic distillation. Another subject of interest is behaviour of the sorbent in adsorption desorption cycles and a long term stability of the zeolite in hydrothermal conditions during the bed regeneration. 2. EXPERIMENTAL SECTION

2.1. Materials Ethanol used was fine spirit from distillery B I O F E R M - LIHOVAR KOLiN a.s. with water content corresponding to the azeotropic composition i.e. 96.0 % vol.; the content of impurities was as follows: volatile acids (as acetic acid) C3 50%) because the cation positive charge is more evenly distributed over a larger number of framework oxygens.

o

A

.............

[~

site I site II site I!'

v

w

ionic r a d i u s (A)

Fig. 2. Occupancy of the phillipsite extrafmmework sites depending on the ionic radius. Lower line= not populated. Upper line=populated.

1708 ........ ,~,,,,, . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

,~

...........

t

....

: ::

:::

: :

:

. . . . . . . . . . . .

~,,o1

__

I I

I

io.l . !o., 1 |

i i

i

~

.....

s

o...I

e~

:

-

I I

!

,

Fig. 3. When the population of site II+II'ca.0.5, site II' is populated (right column). See the text for the point indicated by the arrow. It is possible to predict a priori whether the site II' was populated by the so called atomic sterical occupancy coefficient C (=crd with c and r, the atomic charge and ionic radius of the atom in site II, and d, the average cation-anion distance) [7]. It was found that when C>5.73, site II' is empty. Besides it is clear from Figure 3 that whenever the population of site II+II' exceeds ca. 0.5, the cation preferably migrates from II to II'. The point indicated by the arrow in Figure 3 corresponds to the Li-exchanged sample (1) where Li § fully occupies site II' and (Na+Ca) migrates at the window intersecting channels running along _aand b.

1,6

!

1,4

-

1,2

-

.................................

1,0-

O

o,.

t

"

. . . . . . . . . . . . . . . . . . . . . . . .

O

0,6 0,4 0,2

I .....

,

9

, ....

,

,

........

,

. . . . . . . . .

---

:,

.............

Fig. 4. Plot of the bond strength (BS) of site II vs. occupancy of site II'. Left column=not populated; right eohann=populated.

1709 The plot in Figure 4 shows that whenever the calculated bond strength of site II exceeds 0.7, site II' is not populated. On the other hand, when BS(II):

i

~/ / ,/,//,-/2,0

.//I,5 ~N

x

/'/0.5

Figure 11. Etched (010) surface in heulandite: note the very irregular shape of the dissolution figures.

X 2

2.000 vm/div 1.000 pm/div

6

~'---~

./

~'~VN

Figure 12. Naturally etched surface of the chabazite sample.

(101)

1727

3.5 Yugawaralite This zeolite has shown a dissolution behavior similar to chabazite: sulphuric acid solutions produce clear swelling and fracturing of the surface. In this case the concentration of the acidic solution had to be rather high (0.9 M) because of the remarkable resistance of yugawaralite to acid dissolution. A sequence of images showing the formation of swelling areas on the yugawaralite surface is presented in Figure 13. 4. DISCUSSION AND CONCLUSIONS The microtopographic observations provide some useful indications about the behavior of zeolite surfaces during the growth process. All three zeolites characterized by AFM microtopography show a bidimensional channel system parallel to the (010) structural plane, which is also the cleavage plane and the most developed morphological face in all these zeolite species. The suggested crystal growth mechanism is by step advance (a terrace- ledge - kink process) on steps having b/2 thickness. The observed growth spirals are essential to the process insofar they continuously provide new kink sites necessary to the crystal growth. Several kinds of growth spirals have been observed: single, double and multiple spirals with both CW and CCW sense of rotation. All step layers involved in growth spirals are b/2 thick. As far as the dissolution experiments are concerned, we observed a pronounced difference among various zeolite species. In laumontite we have observed a perfect structure-controlled dissolution, as evidenced by the perfect correspondence between etch pit geometry and face shape. Other regular dissolution figures have been observed only in naturally etched chabazite surfaces. This behaviour is strongly in contrast with the diffusion-controlled dissolution found in heulandite. A rather different behavior has been observed in yugawaralite and chabazite. The reasons causing the swelling of these surfaces are at present not clear. The fact that we observed this phenomenon only in presence of sulphuric acid and in zeolites with a significant amount of Ca, may suggest that localized supersaturation conditions just under the surface induces the nucleation of Ca sulphates with consequent surface swelling. However until now we have no evidence of nucleation of new species, and furthermore swelling is not observed in -

~

~~

Figure 13. Evolution of the surface of yugawaralite in H2SO4 (0.9 M). The arrow indicates the swelling (about 20 nm high) and the forming fracture. Images are unfiltered and were taken about 20 min apart using the deflection signal.

1728 laumontite, which is also Ca rich, albeit with a higher AI/Si ratio. Another possible cause of the deformation and fracturing of the crystal surfaces could be the stress caused by the increased volume of the leached surface layer (see Casey and Bunker, 1990). In such a case the water molecules and the AI content of the surface layer ought to be involved in the process. ACKNOWLEDGMENTS

The work has been carried out in the frame of the project "Mineral surface chemical reactions: intercalation and sorption processes" (Coordinator Prof. G. Artioli), and financed by MURST COFIN 2000. REFERENCES

1 Sugiyama Ono S., Matsuoka 0., Yamamoto S.; Microporous and Mesoporous Materials 48 (2001) 103-110. 2 Binder G., Scandella L., Schumacher A., Kruse N., Prins R.; Zeolites 16 (1996) 2-6. 3 Yamamoto S., Sugiyama S., Matsuoka O., Honda T., Banno Y., Nozoye H.; Microporous and Mesoporous Materials 21 (1998) 1-6. 4 Sugiyama S., Yamamoto S., Matsuoka O., Honda T., Nozoye H., Qiu S., Yu J., Terasaki O.; Surface Science 337-339 (1997) 140-144. 5 Yamamoto S., Sugiyama S., Matsuoka O., Honda T., Banno Y., Nozoye H.; Chemical Phisical Letters 260 (1996) 208-214. 6 Agger J. R., Pervaiz N., Cheetham A. K., Anderson M. W.; Angew. Chem. Int Ed. Engl. 35 (1996) 1210-1213. 7 Agger J. R., N. Pervaiz, A.K. Cheetham, M.W. Anderson, J. Am. Chem. Soc. 120 (1998) 10754-10859. 8 Sugiyama S., Yamamoto S., Matsuoka O., Nozoye H., Yu J., Zhu G., Qiu S., Terasaki O., Microp. Mesop. Mater. 28 (1999) 1-7. 9 Anderson M. W., Agger J. R., Thornton J. T., Forsyth N.; Angew. Chem. Ed, Ingl.35 (1996) 1210-1213. 10 Yamamoto S., Sugiyama S., Matsuoka O., Komura K., Honda T., Banno Y., Nozoye H.; J. Phys. Chem. 100 (1996) 18474-18482. 11 Bosbach D., Rammennsee W.; Geochimica et Cosmochimica Acta 58 (1994) 843-849. 12 Bosbach D., Hall C., Putnis A.; Chemical Geology 131 (1998) 143-160. 13 Putnis A., Junta-Rosso J. L., Hochella jr M. F.; Geochimica et Cosmochimica Acta 59 (1995) 4623-4632. 14 MacDugall J. E., Cox S. D., Stucky G. D., Weisenhom A. L., Hansma P. K., Wise W. S.; Zeolites 11 (1991)429-433. 15 Yamamoto S., Matsuoka O., Sugiyama S., Honda T., Banno Y., Nozoye H.; Chemical Phisical Letters 260 (1996) 208-214. 16 Komiyama M., Tsuijimichi K., Oumi Y., Kubo M., Miyamoto A.; Applied Surface Science 121/122 (1997) 543-547.

Studies in SurfaceScienceand Catalysis 142 R. Aiello, G. Giordanoand F. Testa(Editors) 9 2002 ElsevierScienceB.V. All rights reserved.

1729

O c c u r r e n c e a n d c r y s t a l s t r u c t u r e of m a g n e s i a n c h a b a z i t e E. Passaglia and O. Ferro Dipartimento di Scienze della T e r r a - Universit~ di Modena e Reggio Emilia Piazza Sant'Eufemia 19, 41100 Modena, Italy The chemical composition of three chabazite samples associated with offretite showed Mg contents as high as those found in a few other samples described in literature. The crystal structure refinement of one of these samples revealed that Mg is hosted in just one extra framework site and is totally surrounded by water molecules. 1. I N T R O D U C T I O N Chabazite is one of the most widespread natural zeolites. It may be found as micro- and macrocrystals in amygdales of massive volcanic rocks (commonly termed "hydrothermal" samples) and as submicroscopic authigenic crystals in diagenetic pyroclastic rocks (commonly termed "sedimentary" samples). According to the current zeolites nomenclature [1], chabazite is actually a zeolite series because, on the basis of the most abundant extra-framework cation, chabazite-Ca, chabazite-Na and chabazite-K species have been identified. An accurate review of the reliable chemical compositions of "hydrothermal" samples (80) from literature revealed that besides the dominant extra-framework cations (Ca, Na, K), Sr is constantly present reaching high values as 0.72 a.p.f.u., Ba is nearly absent or in very low amounts (< 0.15 a.p.f.u.). Mg, usually present in amounts lower than 0.30 a.p.f.u., reaches remarkable values in four samples out of which two exhibit a complex twinning ("herschelitic habit") and are epitaxially overgrown by fibrous crystals of offretite (a Mg-rich zeolite). In this study, three new occurrences of Mg-rich chabazite samples intimately associated with offretite are described and the structure refinement of a magnesian sample is reported. 2. E X P E R I M E N T A L

The occurrence and paragenesis of the studied chabazite samples are: a) Mont Semiol, Montbrison, France: clear-white rhombohedral crystals associated with offretite, mazzite and phillipsite; b) Gedern, Germany: clear-white lamellae overgrown by milky-white offretite fibres; c) Herbstein, Vogelsberg, Germany: clear-white flattened prisms overgrown by milky-white offretite fibres. Sample from Mont Semiol exhibits the typical and most frequent morphology of

1730 chabazite: simple pseudo-cubic rhombohedra corresponding to the shape of the unit cell. On the contrary, the morphology of the other two samples is very unusual and is, very likely, due to complex twinning resulting in a particular "herschelite" habit with large dominance of the {0001} pinacoid where the hexagonal offretite crystals can epitaxially grow. Under a binocular microscope, small crystal fragments (average 3 - 4 for each occurrence) were selected and, in the case of the samples from Germany, carefully hand-separated from the overgrown offretite fibres. The fragments were tested for mineralogy and purity by an X-ray Gandolfi camera and then enclosed in epoxy resin and polished for electron probe microanalysis. EPMA were performed on an ARL-SEMQ instrument using wavelength-dispersive mode, 20 ~m diameter electron beam size, 15 kV accelerating voltage, and 10 nA probe sample current. Reference standards were microcline (K), anorthite (Ca), albite (Na, Si, A1), olivine (Fc), diopside (Mg), Sr-anorthite (Sr) and celsiana (Ba). The paucity of pure material especially for the samples overgrown by offretite hindered the determination of their water contents. The single crystal structure refinement of the sample from Mont Semiol was performed using a Siemens P4R diffractometer with graphite monochromatized MoKa radiation ( ~ - 0.71073 A) and equipped with rotating anode generator. The unit cell parameters were derived by a least square fit using 30 medium 0 reflections. After verifying the -3m Laue symmetry, 8252 reflections were collected in 0- 20 scan mode. The intensities were corrected for absorption by ~scan system and reduced to IFI 2. 3. R E S U L T S A N D D I S C U S S I O N 3.1 C h e m i s t r y The electron microprobe point analyses for each sample (average 1 0 - 12) were highly consistent showing variations of the major elements within 3% of the estimated instrument errors indicating a high degree of chemical homogeneity within each sample. In particular, no point analysis of the chabazite samples overgrown by offretite showed Mg values as high as those known for offretite crystals confirming the X-ray analysis results about the absence of offretite zones in the analysed fragments. Fe and Ba contents were below the detection limits. The anhydrous unit cell content of the chabazites calculated by the averaged microprobe point-analyses along with the respective balance errors [2] are given in Table 1. The reliability of the chemical formulae is supported by both the framework contents (Si+A1) very close to half of the oxygen atoms and charge balance errors (E) lower than 7%. The chemical formulae allow the following remarks: concerning the framework content [R = Si/(Si+AI)], samples from Mont Semiol and Gedem are remarkably richer in Si (R = 0.72 and 0.75, respectively) than the sample from Herbstein (R = 0.65); among the extraframework cations, Ca is the dominant one in all samples, Mg shows anomalously high values up to 0.95

1731 a.p.f.u, in the sample from Herbstein, K content although variable is remarkable, Na content is negligible. Table 1 Uni t cell contents (24 Oxygens) of chabazites. E % = balance error. Sample

Mont Semiol

Gedern

Herbstein

8.71 3.32 0.46 0.80 0.02 0.07 0.56 + 4.1

8.98 3.05 0.50 0.74 0.04 0.34 + 6.6

7.85 4.19 0.95 0.96 0.04 0.17 + 4.0

Si A1 Mg Ca Sr Na K E (%)

According to the quoted Rule 5 of the zeolite nomenclature [1], all the samples can be classified as m a g n e s i a n chabazite-Ca. The high Mg contents of the studied chabazites is emphasized if compared with those from literature (Figure 1). Ca + Na

40/-

Mg

80

--

eo

-kSO

40

20 K + Sr

Figure 1. Compositional diagram showing the extra framework cation content of chabazites. D a r k area = 73 reliable analyses from literature. Crystals with "herschelitic habit" overgrown by offretite from O Adamello, Italy [3], @ Fitt~, Italy [4], @ Gedern and O Herbstein, G e r m a n y (this study). Crystals with "rhombohedral habit" associated with offretite from O Mont Semiol, France (this study) and without offretite from | P e n t l a n d Hills and | Narre Warren, Australia [5].

1732 The distribution of the points in the diagram show that the highest Mg contents are found for offretite overgrown chabazites (Nos. 1, 3 and 4). Unfortunately the complex twinning of the crystals of these samples inhibited their use for single crystal structure refinement. Therefore, this study was carried out on the sample from Mont Semiol (No. 5). 3.2 S t r u c t u r e r e f i n e m e n t The chabazite structure displays a tetrahedral framework built up by double six-membered rings in ABC sequence with fully or partially disordered (Si, A1) distribution and a large number of partially occupied extra framework sites with very irregular coordinations in which cations and water molecules are spread over [6, 7, 8]. The fully disordered (Si, A1) distribution of the Mont Semiol sample was determined by the average distances T-O for the six independent tetrahedral positions resulted in a preliminary refinement in the P - 1 space group. The structure was then refined in the space group R - 3 m using SHELX-97 [9]. The utilised atomic scattering curves were: Si for T, Ca for C2 and C4, Mg for C3, O for W. Starting coordinates for the framework atoms were taken from [7]. The extra framework cations and water molecules were localised by difference Fourier synthesis. The final reliability indices for the anisotropic model and other experimental and refinement details are reported in Table 2. Fractional atomic coordinates and interatomic distances are reported in Table 3 and Table 4, respectively. Anisotropic temperature factors were refined for framework and extra framework atoms with the exception of the extra framework sites with very low occupancy.

Table 2 Experimental and refinement details. Structural Formula Space group; Z Unit cell parameters (.~, ~ Crystal size (ram) 28 range (o) Data Range Collected reflections Unique reflections Internal R(F 2) Number of refined parameters Rhkt in the anisotropic approximation wR(F 2) for observed reflections

Apmax(e/A 3) Apmin(e/A 3)

[(Ca, K)1.51Mg0.GT][(Si, A1)120241.13.20H2O R-3m; 1 a = 9.382(2), a = 94.57(1) 0.08 • 0.09 • 0.24 4-70 -15 < h 4(~(Fo); 0.054 for 1333 0.115 0.57 -0.54

1733 Table 3 Fractional coordinates and isotropic, Uiso, or equivalent isotropic, Ueq, thermal parameters (/~2). Ueq = 1/3[Ull + U22 + U33 -b 2 (U12 + U13 +U23) cos a]. Site x y z Ueq or Uiso* Occ. (%) T .10548(4) .33341(4) .87760(5) .0121(1) O1 .2602(2) -.2602(2) 0 .0362(6) 02 .1520(2) -.1520(2) 89 .0265(4) 03 .2545(2) .2545(2) .8948(3) .0314(5) 04 .0279(2) .0279(2) .3254(3) .0294(4) C2 .2329(3) .2329(3) .2329(3) .0394(7) 39(1) C3 .4028(5) .4028(5) .4028(5) .046(3) 33(1) C4 .5850(6) .5850(6) .2366(8) .043(1)* 12(1) W1 89 89 0 .112(3) 100 W3 .221(1) .321(1) .515(1) .162(4) 54(1) W4 89 -.376(1) .376(1) .14(1)* 37(2) W5 .543(4) .543(4) .295(6) .11(2)* 14(1) W6 .268 (2) .268(2) . . . . .268(2) .13(2) 34(2) Table 4 Cation, oxygen and water molecule distances less than 3.30 C2---O3 [x3] 3.217(4) C3---C2 2.531(8) C4--O2 [• ----04 [• 2.845(4) --C3 2.90(1) ----03 --C3 2.532(9) ---C4 [• 2.943(9) ---C3 --W3 [• 2.72(1) - - W 3 [• 2.193(9) --C4 [• - - W 4 [• 2.24(2) --Wl --W5 [• 2.16(6) --W3 [• --W6 2.01(2) --W3 [• --W5 [•

2.807(5) 2.508(8) 2.941(8) 2.922(9) 2.353(8) 2.51(1) 2.88(1) 2.72(3)

The results of the structure refinement revealed the presence of three partially occupied cation sites (C2, C3 and C4) and five sites (Wl, W3, W4, W5 and W6) occupied by water molecules; sites are coded as reported by [7]. According to the chemical composition of the Mt. Semiol sample (Table 1), C2 site (along the t e r n a r y a x i s [111] near a 6-ring of the D6R cage) and C4 site (in the large cage about 2 A apart from the plane of the 8-ring window) are partially occupied by Ca and K (Figure 2 a). The shortest C4-O distance 2.508 A, compared with the shortest C2-O 2.845 A, suggests that K occupies uniquely C2 position as found in K-exchanged chabazites [7], whereas Ca is spread over both sites. C3 site (along the ternary axis [111] near the centre of the large cage) is partially occupied by Mg uniquely coordinated by water molecules (Figure 2 a). Such statement is supported by both the completely "hydrated" feature of C3 site and reliable C3--W distances.

1734

z

Z

y

c

Figure 2. Part of the chabazite cage showing cation sites (a); octahedral (b) and tetrahedral (c) coordination polyhedra of Mg in C3 site. The occupancy factor of Mg in C3 site corresponds to 0.67 atoms per. unit cell and is not in full agreement with the reported content from the chemical analysis (0.46). This excess is also observed for the Ca and K and is probably due to an underestimation of exchangeable cation by microprobe analysis as indicated by the positive balance error (+ 4.1%). Two coordinations for Mg (Figure 2 b, c) were hypothesized according with the C3--W coordination distances and W--W bond distance compatibilities. The former has a six-fold distorted arrangement (Figure 2 b), with Mg--W distances in good agreement with those observed in the other natural zeolites containing Mg as extra framework cation: offretite [10] mazzite [11], ferrierite [12]. In such six-fold coordination two m related polyhedra are possible depending on the alternated way in which Mg can result bonded to three of the six W3 and three of the six W4 (Table 4). The latter is a four-fold coordination (Figure 2 c) which is unknown for zeolites, but present in the Mg-spinel [13] and in a few silicates [14, 15, 16]. W1 is a site fully occupied by water at the centre of the 8-ring window, as reported by several authors [7, 17, 18]. The same site is found only partially occupied by other authors [19]. W3, W4, W5 and W6 site, accepted as water sites, are inside the large cage with a low occupancy [7]. The atomic coordinates of water sites slightly deviate from those reported in literature [7] as a consequence of the involvement of these water molecules in the coordination shell of a small cation as Mg. 4. C O N C L U S I O N S The usual very low and subordinate Mg content of "hydrothermal" chabazites noticeably increases up to about 1 a.p.f.u in samples associated with offretite

1735 especially in the case of crystals epitaxially overgrown by. Accordingly, the crystallization of such anomalous Mg-rich chabazite is very likely explained by the high concentration in Mg of the percolating solution needed for the growth of offretite, a Mg-rich zeolite species. The structure refinement of a crystal with about 0.5 a.p.f.u, of Mg showed this cation exclusively allocated in one extra framework site (C3) and completely coordinated by water molecules. Consequently, C3 site may show higher Mg occupancy in samples with higher Mg contents.

Acknowledgements: The work was made possible through the financial support of MIUR (Ministero Istruzione, Universith e Ricerca). The CNR (Consiglio Nazionale delle Ricerche) is acknowledged for financing the electron microprobe laboratory at the Dipartimento di Scienze delia Terra of Modena e Reggio Emilia University.

REFERENCES 1. D.S. Coombs et al. Can. Mineral., 35 (1997) 1571. 2. E. Passaglia, Am. Mineral., 55 (1970) 1278. 3. E. Passaglia and A. Tagliavini, Eur. J. Mineral., 6 (1994) 397. 4. E. Passaglia, A. Tagliavini and R. Gutoni, N. Jb. Miner. Mh., 1996 (1996) 418. 5. W.D. Birch, Mineral. Soc. of Victoria, Spec. Publ. 2 (1989) 91. 6. J.V. Smith, F. Rinaldi and L.S. Dent Glasser, Acta Cryst., 16 (1963) 45. 7. A. Alberti, E. Galli, G. Vezzalini, E. Passaglia and P.F. Zanazzi, Zeolites, 2 (1982) 303. 8. F. Mazzi and E. Galli, N. Jb. Miner. Mh., 1983 (1983) 461. 9. G.M. Sheldrick, SHELX-97: Program for the solution and refinement of crystal structures. Siemens Energy and Automation, Madison, WI, 1997. 10. A. Alberti, G. Cruciali, E. Galli and G. Vezzalini, Zeolites, 17 (1996) 457. 11. E. Galli, Soc. It. Mineralogia e Petrologia- Rendiconti, 31(2) (1975) 599. 12. A. Alberti and C. Sabelli, Z. Kristallogr., 178 (1987) 249. 13. T. Yamanaka and Y Takeuchi, Z. Kristallogr., 165 (1983) 65. 14. J.V. Smith., Am. Mineral., 38 (1953) 643. 15. N. Nguyen, J. Choisnet and B. Raveau, J. Solid State Chem., 34 (1980) 1. 16. T. Armbruster and R. Oberhaensli, Am. Mineral., 73 (1988) 585. 17. M. Calligaris and G. Nardin, Zeolites, 2 (1982) 200. 18. M. Calligaris, G. Nardin and L. Randaccio, Acta Cryst., B38 (1982) 602. 19. I.K. Butikova, Yu.F.Shepelev and Yu.I. Smolin, Crystallogr. Rep., 38(4) (1993) 461.


1737

Treatment of Urban Dump Leachates with natural Zeolite packed bed column T. Rodriguez F. a, E.Acevedo del Monte ~, G. Mori b, B.Rafuzzi b a GME. Gruppo Marbox Engineering S.r.l., Piazza della Vittoria 12/5, Genoa

b Consorzio Cuoio Depur S.p.A., San Miniato, San Romano (Pisa) It is presented the result of the treatment of Urban Dump Leachates (UDL) with a Cuban natural Zeolite (64% as Clinoptilolite-Mordenite) packed bed exchange column. The main target of the test works was to reduce conveniently the (high) content of ammonium and other contaminant elements from UDL and at the same time to establish convenient ammoniumorganic charge ratios in the treated UDL for its conventional depuration process. The mean ammonium content in the studied leachates was about 3900 mg/1 with a COD mean value of 15000 mg/l. During the pilot plant test works, ammonium, potassium and COD were removed 97, 98 and 13-22 % respectively. The Zeolite bed did not trap the heavy metals. The characteristic of the exhausted Zeolite gives the possibility to reuse it for soil amendment in agricultural applications. Relatively low volumes of urban dump leachates could be treated industrially by cationic exchange system with natural Zeolite. The combination of this process with the conventional one for UDL could optimize the last. 1. INTRODUCTION The Urban Dump Leachates (UDL) are characterized by a wide variability in its composition as well as their high content of contaminants substances. Among main contaminants we find high content of ammonium and a great content of organic substances expressed by high values of Chemical Oxygen Demand (COD), which frequently are in inconvenient ratios, making difficult to apply the conventional depuration process. For ammonium removal from UDL it has been developed technologies that consider the ammonium stripping process with the possible recovery of Ammonium salts through a chemical reaction into Sulfuric acid solution, but these technologies are not always recommendable. The high selectivity of natural Zeolites for ammonium and potassium cations makes possible to consider them as a useful solution for many environmental problems related to the presence of the already mentioned contaminants in wastewaters of diverse origin. The ammonium removal with natural Zeolites has been widely described by many authors. This particular use has been reported frequently for drinking water, aqua culture and

1738 wastewater treatment [1] where the ammonium content generally does not exceed the hundred's of mg/1. Exceptionally in some papers has been reported the use of natural Zeolites for ammonium removal through cationic exchange process in swine sewage with 1000 mg/l NH4 + [2], nevertheless it is not considered a viable method for the treatment of this kind of sewage. The aim of the present work was to study the possible application of a cationic exchange column process with a natural Zeolite packed bed for the UDL treatment. Adsorption of nutrient especies was monitored to verify the possible use of exhausted Zeolite in soil amendment for agricultural use. At the same time certain regulations were established for the NH4+/COD ratio in the UDL treated.

2. MATERIALS AND METHODS Cuban natural Zeolite (64% as Clinoptilolite-Mordenite) from Tasajeras deposit, with grain size into the range 1-3 mm, was used for the development of the experimental work in a pilot plant column. Typical chemical composition of the Tasajeras deposit's Zeolite is shown in Tablel. Table 1 Chemical composition of Tasajeras' Zeolite Comp. SiO2 A1203 CaO Fe203 Na20 MgO K20 FeO TiO2 P205 H20 % 64.3 13.7 5.0 2.7 2.2 1.2 1.2 0.8 0.4 0.1 3.4 The Zeolite content, as Clinoptilolite-Mordenite, was determined by semi quantitative XRay diffraction spectrometry method; and the total cationic exchange capacity (1.35 meq/g) was determined by cationic exchange with ammonium chloride and potassium chloride solutions alternatively for exchange and regenerative process, evaluating the effluents by atomic adsorption spectrometry. The UDL composition from the historical recorded data (from 1991 to 2001) showed that NH4§ content was into the range 2000-4000 mg/1, with mean pondered value of 3931 mg/l, while the mean pondered value of COD was very close to 15000 mg/1. The exchanger column was designed with three outlet valves to study the progress of exchange process into the column. The outlet valves were situated at different distances (A, B and C) from the bottom. The UDL was pumped through the zeolite bed ;with a size range of particles 1-3 mm from inlet at the bottom of the column, at a flow rate of 0.28 VB/h. The experimental conditions were the following: Outlets valves position Large from the bottom, cm Zeolite weight, k8

A 60 9,5

B 120 20,5

C 180 31,5

Samples of the outlet effluents A, B, C, were analyzed by Molecular Adsorption Spectrophotometry (Nessler method) for ammonium content, Atomic Adsorption Spectrophotometry was employed for metallic cations, and COD was determined by Dichromate method. The principal characteristics of UDL are shown in Table 2.

1739 Table 2. Characteristics of UDL pH Elect.Conductivity, 20 ~ l.tS/cm C.O.D., mg/l NH4+-N, mg/1 Organic Nitrogen as (N), mg/1 CI', mg/1 Na +, mg/1 K+, mg/1 Cd 2+, mg/l Cu 2+, mg/l Pb 2+, rag/1 Zn2+, mg/1

9,39 27620 14941 3435 758 4237 3228 2172 0,03 0,27 0,32 0,69

9,12 29300 16115 3305 1340 3884 2972 2136 0,04 0,23 0,28 0,45

3. RESULTS The maximum exchange capacity of the Zeolite used was 1.29 meq/g, determined at room temperature in a stirring system using an ammonium chloride solution method[4]. The breakthrough curve for NH4+ is shown in Fig 1. The main experimental results, considering 100 mg/l of ammonium as a breakthrough concentration (BTC) are summarized in Table 3 Table 3. BTC point operation parameters Outlets Volume UDL treated, 1 Effect.CEC mg/g ..A . 21.08 9.00 B 59.01 12.07 C 101.16 13.47

for NH4+ meq/g 0.50 0.67 0.75

The maximum ammonium exchange capacity obtained was 0.78 meq/g. The COD removal, during the tests, was measured and its behavior is shown in Fig. 2. The mean values obtained for the studied parameters for the UDL treated sampled at outlets B and C, at the breakthrough point, are showed in Table 4. Representative samples from a total weight of exhausted zeolite, was taken to evaluate the content of heavy metals adsorbed into the column. These results are showed in Table 5. 4. DISCUSSION

For the established experimental conditions the effective exchange capacity of natural zeolite was increased with the bed length for NH4+ and K +, up to 0.94 meq/g, with high removal efficiency of 73% for both cations. The organic material evaluated as nitrogen was significantly retained into the bed, probably due to a filtration process reaching values of 61.9-76.4 %.

1740

----g-- B --O--C --e--A

40OO ~3000 "-F

z~2000 1000 0 0

2

4

6

8 10 12 14 16 18 q['ln~ hr

Fig 1. Ammonium concentration curves of the effluents from outlets A-B-C.

+A!

70 ~60

~ B --O--C

9.- 50

O40 u

~30 o

~20 10 0c 0

2

4

6 8 Tlmeh

10

12

14

Fig. 2. Behavior of the COD for the effluents from outlets A-B-C

1741 Table 4. Mean Values of parameters for UDL at BTC point. Parameter Outlet B Removal% Outlet C pH 7,45 7,36 Elect.Conduct. at 20 ~ 12850 56,1 15300 COD, rag/1 12278 23,8 14171 Ammonium as (N), mg/1 101 96,9 268 Organic N, mg/l 316 76,4 511 Chloride (C1-), mg/1 3037 21,8 3460 Sodium (Na), mg/1 3252 0.0 3172 Potassium (K), mg/1 70 96,7 72 Cadmium (Cd), mg/1 0,08 0,0 0,04 Copper (Cu), mg~ 0,36 0,0 0,34 Lead (Pb), mg/1 0,54 0,0 0,46 Zinc (Zn), mg/1 0,45 0,0 0,36

Removal% 47,8 12,1 91,9 61,9 10,9 0.0 96,6 0,0 0,0 0,0 20,0

Table 5 Characteristics of exhausted Zeolite Parameter Values pH 8.9 Copper (Cu), mg/kg 21.4 Lead (Pb), mg/l 6.7 Nickel Ni, mg/kg 0.78 Cadmium (Cd), mg/kg 0.06 Chromo total mg/kg 3.5 Zinc (Zn), mg/kg 48.8 Mercury Hg, mg/kg 1.9 Ammonium NH4 +, % 1.1 Phosphorous total, % P 0.03 Potassium % 0.70 The COD removal by the exchange column was more effective during the first four hours of test work. During that lapse, a relatively influence of bed length on COD removal was observed up to reach a maximum value of 65 %. Nevertheless the mean effective removal value was reduced to 22 and 13 % at outlets B and C respectively. This retention can be attributed to mechanical forces inherent to the nature of the filtration process Natural zeolite used showed a preferential adsorption for NH4+ and K § cations with removal of 97-98 %, while heavy metals were not practically adsorbed by the zeolite bed as it is showed in Table 4 and Table 5. During the pilot plant test it was demonstrated that the exchange natural zeolite packed column was capable to adsorb the main nutrient species contained in the UDL, converting it in an enriched nitrogen and potassium subtract.

1742 An appropriated conventional depuration process, of course, is needed to conclude the UDL decontamination, but it can be considered the possibility to combine it with natural zeolite treatment can give several advantages. An adequate technological process is currently in progress in terms of engineering design to apply the results showed in this paper. 5. CONCLUSIONS On the basis of the high selectivity of natural zeolites for NH4+ and K § cations, an appropriated volume of UDL can be treated by cationic exchange process obtaining a reusable product for agricultural applications as its has already studied. The effective cationic exchange capacity for NH4+ and K + was 0.93 meq/g, with a exchange efficiency of 73 %. At the same time a COD removal up to 22 % was obtained. REFERENCES

1. D.B.Shah, O.Talu, D.T. Hayhurst, and X.-C Lu "Ammonium removal from industrial wastewater by ion exchange with packed-bed Clinoptilolite". Zeolite'93, Boise, Idaho, June20-28, 1993. 2. E. Passaglia, S. Azzolini. "Italian Zeolite in wastewater purification: Influence of zeolite exchangeable cations on NH4+ removal from swine sewage. Materials Engineering 1994,Vol.5 No2, pp 343-355. 3. Howard Sherry. The cation exchange properties of Zeolite. A review. Zeolite'93, Idaho, June 20-28, 1993. 4. T.Rodriguez, G.Mori. Report about the pilot plant test of urban dump leachates depurations with Cuban natural zeolite. Italy, 2001.(not publicized)


1743

Phosphorus removal from wastewater by bioaugmented activated sludge with different amounts of natural zeolite addition J. Hrenovic and D. Tibljas Faculty of Science, University of Zagreb, Zagreb, Croatia The influence and performance of different amounts of NZ additive in enhanced biological phosphorus removal (EBPR) system with different initial concentration of phosphorus (1-100 mg T-P L -1) were investigated. The efficiency of P-removal was in correlation with the amount of NZ additive used. The improvement of P removal in regards to the EBPR was lower with 0.5 % NZ addition than with 1.5 % NZ addition for each range of starting P load in wastewater. The amount of P removed increased with increasing starting P load. Significant stronger decrease of COD has been achieved in reactors with NZ, and stronger by larger NZ additive used. The application of NZ addition in EBPR process enables increase in sludge concentration (increase of MLSS) in the system and decreased volume of excess sludge (lower SVI). The probable mechanism of P-removal in EBPR system with NZ addition is a combination of adsorption of P onto NZ particles, increased metabolic activity of activated sludge, P uptake by P-accumulating bacteria biosorbed on the NZ particles and suspended in solution.

1. INTRODUCTION Enhanced biological phosphorus removal (EBPR) in conventional activated sludge wastewater treatment is based on the enrichment of activated sludge with phosphateaccumulating organisms. These microorganisms (also called P-removing or poly-P bacteria) are able to store intracellular phosphorus as polyphosphate. The requirement to achieve phosphorus removing bacterial population in an activated sludge system is exposure of activated sludge to anaerobic and aerobic conditions [1 ]. Bacteria from Acinetobacter genus, such as A. calcoaceticus, have become the model organism for biological phosphorus removal since it was isolated from a phosphorus-removing activated sludge plant [2]. Under anaerobic conditions P-removing bacteria transport volatile fatty acids (e.g. acetic, propionic acid) present in or produced from the intermediate metabolism of heterotrophic populations in wastewater, into the cell and subsequently convert and store these as polyhydroxy-alkanoates (PHA, e.g. poly-beta-hydroxy-butyrate). The energy for this transport and storage is supplied by hydrolysis of intracellularly stored polyphosphate to orthophosphate, which is released from the cell to the liquid. Under aerobic conditions, anaerobically formed PHA are catabolized using oxygen as electron acceptor to generate energy for cell growth, poly-P synthesis, glycogen formation and maintenance, resulting in the uptake of phosphate [3,4]. Natural zeolites (NZ) are the main absorptive, low-cost material used in agricultural and industrial situations [5]. Jardin and Popel [6] mention the impact of zeolites as an aluminium source for phosphate fixation by adsorption. The presence of high levels of Ca, A1 and Fe

1744 oxides in adsorptive materials such as zeolite suggests that these materials can absorb phosphorus and are potentially valuable for the use in the removal of phosphorus from wastewater [5]. Significant degree of phosphate elimination from domestic water has been achieved with addition of NZ [7]. Tertiary treatment of the effluent from wastewater stabilization pond in column with NZ tuff showed high efficiency of phosphate removal [8]. Successful reduction of phosphorus from dairy [9] and pig slurry [10, 11] with addition of NZ has been observed. Zeolite may be used alone or in combination with soils to improve phosphorus removal in constructed wetlands [5]. Improved phosphorus removal in EBPR system has been achieved using NZ as a support material [12]. The efficiency of a zeolite-containing material in most of the applications, such as ion exchange, selective sorption or catalysis, is in correlation with the amount, structure and composition of the zeolitic component itself. Similar correlation could not be observed when clinoptilolite-containing tufts were used as additive in water treatment or for water filtration [13, 14]. One of the promising approaches to improve the efficiency and increase the capacity of biological wastewater treatment plants (hydraulically or biologically overloaded plant), without increasing size, is based upon application of NZ in the aeration basin. Although the additions of zeolites in wastewater treatment evidently reduce the final concentration of phosphorus, the mechanism of this reduction is unclear. The objective of this study was to investigate the influence and performance of different amounts of NZ addition in EBPR system with different initial concentration of phosphorus.

2. EXPERIMENTAL SECTION

2.1. Natural zeolite The 0.25-0.80 mm fraction of NZ tuff (Aegean Region, Turkey) was used in this study. NZ consists of more than 70 % clinoptilolite - heulandite + mordenite, minor quartz and opal-CT, estimated by X-ray powder diffraction method. The chemical composition of the NZ (wt %) is: SiO2 68.5; TiO2 0.07; A1203 11.18; Fe203 1.21; MnO 0.02; MgO 0.71; CaO 1.98; Na20 0.33; K20 3.88; P205 0.01; H20 5.51; H20 + 5.78. The NZ tuff was washed three times with demineralised water and then dried at 105~ for 16 h before using for experiments. 2.2. Experimental operation Laboratory-scale batch experiments in alternated 24 h anaerobic/24 h aerobic stages were carried out for the synthetic medium, used to simulate the sewage, with the following composition (in mg Ll): Na-acetate, 500; Na-propionate, 40; glucose, 40; peptone, 100; MgSO4, 10; CaC12, 6; KC1, 30; yeast extract, 20; KH2PO4, variable. The concentration of KHzPO4, the only sole source of P in tests, varied from 4 up to 440 mg L 1 to obtain a concentration of P in the wastewater ranging from 1 to 100 mg L-1. The fresh activated sludge was obtained from the aeration tank of a municipal wastewater treatment plant and acclimatized for two weeks in the mineral solution with mixing and aerating at room temperature. The synthetic wastewater was inoculated with activated sludge bioaugmented with polyphosphate-accumulating bacteria Acinetobacter calcoaceticus DSM1532. The pH of the experimental reactors was regulated at 7.0+0.1 pH units with 1M NaOH or 1M HC1 only at the start of each run. Temperature was kept at 20~ After 24 h anaerobic stage the each

1745 reactor volume was divided in two reactors. In one reactor 5 g L -1 or 15 g L 1 of NZ was added and the other reactor was left without NZ addition. The aeration (about 4 L min l ) was provided by aquarium pumps.

2.3. Analytical methods The control parameters in reactors were: pH, total phosphorus concentration (T-P), chemical oxygen demand (COD), mixed liquor suspended solids (MLSS), sludge volume index (SVI) and plate counts of A. calcoaceticus. Samples were taken from the reactors three times per each anaerobic and aerobic stage. The samples were filtered before measurements through the nitrocellulose filters Sartorius pore diameter 0.2 gm. All measurements were done according to the Standard Methods for the Examination of Water and Wastewater [ 15]. pH-values were measured with Crison micro pH 2000 pH-meter. T-P concentration in water was measured after persulfate oxidation by stannous chloride method in a Cary UV-visible spectrophotometer at 690 nm. T-P concentration in activated sludge was determined after perchloric acid digestion. COD was determined by open reflux method. MLSS were determined after drying at 105~ h. SVI was calculated after 30 min sludge settlement. Bacterial number of A. calcoaceticus was determined as colony forming units (CFU) on the nutrient agar. Serial dilutions (10 -1 t o 10-8) of the one mL sample were prepared. Dilutions (0.1 mL) were plated (spread plate method) onto nutrient agar to obtain a viable cell count. Plates were incubated at 30~ for 72h. After period of incubation, colonies were counted and CFU L -1 was calculated. Dissolved oxygen and temperature were controlled with Jenway 9071 dissolved oxygen meter. 2.4. Calculations On the basis of measured parameters following values were calculated: Percentage of released phosphorus: P-released (%) = (B - A) / B x 100 Percentage of phosphorus removal: P-removal (%) = (A - C) / A x 100 Phosphorus release ratio per MLSS: P-release ratio (mg g-i) __ (B - A) / a Phosphorus uptake ratio per MLSS: P-uptake ratio (mg g-l) = (A - C) / b Phosphorus release ratio per cell A. calcoaceticus: P-release ratio (mg cell -1) = (B - A) / c Phosphorus uptake ratio per cell A. calcoaceticus: P-uptake ratio (mg cell -1) = (A - C) / d Where: A - phosphorus load at time zero (T-P m g L "l) B = phosphorus load at the end of anaerobic stage (T-P m g L "l) C = phosphorus load at the end of aerobic stage (T-P mg L -1) a = MLSS at the end of anaerobic stage (g L l ) b = MLSS at the end of aerobic stage (g L -1) c = CFU L -1 after incubation at the end of anaerobic stage d = CFU L 1 after incubation at the end of aerobic stage

3. RESULTS AND DISCUSSION Activated sludge enriched with polyphosphate-accumulating bacteria A. calcoaceticus shows EBPR characteristics [ 16]. Significant improvements of P removal in EBPR system have been achieved by addition of NZ (Table 1, 2). Whereas P eliminations in EBPR system were

1746 similar in both control experiments, P elimination increased increasing NZ additive. The efficiency of P-removal was in correlation with the amount of NZ additive used. The improvement of P removal in regards to the EBPR was lower with 0.5 % NZ addition (0.23; 1.12; 3.38; 6.28 mg L -~) than with 1.5 % NZ addition (0.59; 2.20; 11.16; 60.61 mg L -~) for each range of starting P load in wastewater. The amount of P removed increased with increasing starting P load. Just for the highest P load (load 4) in experiment with addition of 0.5 % NZ the amount of P removed was lower than for load 3, both in reactor with and without NZ additive (Table 1). It was due to the decay of phosphate-accumulating bacteria in reactors, indicated by low numbers of A. calcoaceticus and low pH. Higher anaerobic P release resulted in higher aerobic P uptake. Table 1 The performance of bioaugmented activated sludge system using 5 g L 1 NZ* additive and without NZ additive by different initial phosphorus loads

Period Influent T-P (mg L I) COD (g 02 L 1) MESS (g E -1) 109 CFU L "1 pH- value Anaerobic stage(end) T-P released (mg L -1) COD (g 02 L -1) MESS (g L 1) 101~CFU L -1 pH- value P-re lease (%) Aerobic stage(end) T-P removed (mg L -1) COD (g 02 L "l) MESS (g E l ) 1011CFU L "1 pH- value P-removal (%) Aerobic stage(end)* T-P removed (mg L "l) COD (g 02 L l) MESS (g L 1) l0 ll CFU L -1 p H - value P-removal (%)

Load 1

Load 2

Load 3

Load 4

1.93 2.05 1.23 2.3 7.00

13.24 2.10 1.41 12.3 7.02

49.35 2.05 1.11 3.3 7.01

98.48 2.10 0.62 12.3 7.01

1.59 1.37 2.91 4.8 6.46 82.38

6.62 0.65 1.83 6.5 6.51 50.00

28.64 0.90 2.47 5.0 6.64 58.03

15.33 0.86 1.69 8.0 6.82 15.57

1.40 0.77 2.92 6.0 8.68 72.54

7.12 0.51 2.55 5.4 8.52 53.78

18.28 0.49 2.66 7.2 8.60 37.04

12.77 0.46 2.34 2.0 8.40 12.97

1.63 0.42 3.03 1.8 8.59 84.46

8.24 0.36 2.70 2.0 8.66 62.24

21.66 0.31 2.70 3.0 8.56 43.89

19.05 0.28 2.84 1.2 8.05 19.34

1747 Table 2 The performance of bioaugmented activated sludge system using 15 g L l NZ* additive and without NZ additive by different initial phosphorus loads Period Influent T-P (mg L -1) COD (g 02 L "l) MESS (g E -1) SVI (ml g-l) 109 CFU L "1 pH- value Anaerobic stage(end) T-P released (mg L "l) COD (g 02 L "l) MLSS (g L-1) SVI (ml g-l) 10 l~ CFU L "1 pH- value P-re 1ease (%) Aerobic stage(end) T-P removed (mg L -1) COD (g 02 E -1) MESS (g L l) SVI (ml g-l) 1011CFU L "l pH- value P-removal (%) T-P increase in activated sludge (%) Aerobic stage(end)* T-P removed (rag L1) COD (g 02 L "l) MESS (g g "1) SVI (ml g-l)

l011CFU L-1

pH- value P-removal (%) T-P increase in activated sludge (%)

Load 1

Load 2

Load 3

Load 4

2.34 2.25 1.75 23.19 3.2 7.03

9.89 2.25 1.49 26.86 2.2 7.02

64.49 2.36 1.29 24.81 5.5 7.02

157.69 2.36 1.04 30.77 4.0 7.03

1.95 1.40 1.89 20.11 5.4 6.40 83.33

5.00 0.73 1.59 25.16 4.3 6.41 50.56

13.99 0.82 1.66 24.09 4.8 6.41 21.69

13.51 1.27 1.32 30.30 3.6 6.50 8.57

1.62 0.41 1.95 20.06 6.0 8.66 69.23 19.23

4.76 0.53 1.71 24.07 5.9 8.62 48.13 20.56

26.32 0.60 1.80 23.77 5.9 8.57 40.81 28.31

26.18 0.70 1.42 28.50 7.8 8.61 16.60 20.02

2.12 0.19 3.27 17.58 3.9 8.63 90.60 3.67

6.96 0.39 3.08 16.26 3.3 8.72 70.37 7.57

37.48 0.30 2.02 20.08 3.2 8.63 58.12 27.41

86.79 0.29 2.31 24.53 3.4 8.49 55.04 36.02

In all cases, numbers of A. calcoaceticus increased for one order of magnitude at the end of anaerobic stage, and than for one order of magnitude at the end of aerobic stage. Numbers of A. calcoaceticus in reactors without NZ additive were similar and significantly (p16 mm 4.7 4 6 16 mm > O >2 mm 46.3 45 48 O 40%.

1779 5. GEOMECHANICAL PROPERTIES Measurements of solid unit weight for zeolitized tufts carried out with the standard procedures generally provide not reliable results and, above all, hardly reproducible data. This behaviour is due to the presence of zeolites whose capability of water absorption-desorption, even at temperatures lower than 110~ is well known. This behaviour is also displayed by the different lithotypes of Campanian Ignimbrite studied in the present paper as evidenced by Figure 2. Specimens were weighted soon after drying at 110~ and then at different times, until 24 hours. The adsorbed water content Wzwas calculated with reference to the initial weight. The adsorption is very fast in the early stages of the process, especially in the case of the yellow facies where more then 20% of the desorbed water is adsorbed in the first half hour. Adsorption then continues at a rate which rapidly decreases and the equilibrium is reached at 24h. Thus, the usual procedure that encompasses the reference weight useful for the measurement of dry and solid unit weight can not be applied to zeolite-bearing materials. In this case, a re-equilibration of at least 24h at controlled R.H. conditions is required. This measurement allows to correctly define another important physical property such as porosity. This parameter can provide useful information on the mechanical behaviour of the rocks and in particular on the Uniaxial Compressive Strength. The porosity values determined in this way once again evidence an overall grouping of the samples, as previously mentioned, with decreasing values of porosity as the zeolite content increases (Figure 3). As far as the comparison between mineralogical composition and geotechnical properties is concerned it can be best performed in terms of zeolite content and uniaxial compressive strength (Figure 4).

10 I-1 Yellow Yellow-Grey A Grey-Yellow C Grey

8! o

6 - -

d[ II.

>0 or a ~

~

2 ~

,(

ab

) 0

r, '

0

4

i

i

i

8

12

16

20 Time (h)

Figure 2. Typical adsorption curves for Felette tufts.

24

1780 0,65

[21Yellow

0,61

u

O Yellow-Grey 0 () C)

= 057

A Grey-Yellow

AA C

O Grey

~D A

o

o 0,53

O~O

1,5 mbar; and d) after evacuation 15 min at RT; e) after 16 h at RT.

1788 Fe-NbMCM-41 (TIE)

,,.~18191752

'A

Fe-MCM-41 (TIE)

B 1818

a

34 d e

22100'

20100'

18100'

16100 '

Wavenumbers, cm 1

1410 0

2200

2000 1800 16.100 Wavenumbers, cm

1400

Fig 2. FTIR spectra of mesoporous molecular sieves after NO adsorption at RT of: a) 0,5 mbar; b) 1 mbar; c) >1,5 mbar; and d) atter evacuation 15 min at RT; e) atter 16 h at RT The FTIR spectra of NO adsorbed at RT on Fe-NbMCM-41 (TIE) and Fe-MCM-41 (siliceous) look similar in the range of 1800 - 1900 cm"] (Fig. 2 A,B). The difference is observed in the 1620-1650 cm"] region where NO2 species absorbs. The bands in this region are more intensive when NO is adsorbed on Fe-NbMCM-41. The behaviour of the band at 1819 cm"~ is like that at 1814 cm"~ on Fe-ZSM-5 zeolite, i.e., with the increasing of NO doses its intensity increases and a short evacuation at room temperature causes the decrease of its intensity. Simultaneously, the intensity of the 1752 cm"~band increases. Therefore, one can assigned the first band (1819 cm'~), which is accompanied by a small band at 1916 cm "1, to dinitrosyl [Fe2+(NO)2], whereas that at 1752 cm"] to mononitrosyl species. When Fe-NbMCM-41 sample was contacted with NO at RT for 20 hours (>1.5 mbar of NO) (Fig. 3) the IR bands became more intensive. Moreover, a shoulder at --1876 cm"1 is visible. It is due to mononitrosyl species located on a different iron sites than that responsible for 1752 cm1 band. The heating of the sample with the 1817 increasing temperature causes the decrease of 1817 and 1752 cm"~ bands intensities and the band at 1876 cm"] from mononitrosyl [Fe2+(NO)] becomes more pronounced. a 1986 1752 1 ,4 Therefore, one can suggests that this complex b is more stable at higher temperatures. It is worthy to notice the formation of nitro, c nitrito, nitrato species characterised by the bands at 1624 cm~ and also in-~1400 cm1 region (Fig. 3). These bands were discussed by Sachtler et al. [11, 15] and we assigned them I , I = I = I , I 2200 2000 1800 1600 1400 to the complexes formed on the species V~Ive'UTI:e~, cm"1 included oxo or peroxo ions. As the bands at --1400 cm] were observed only when NO was Fig. 3. The effect of heating of Feadsorbed on Fe-NbMCM-41 material, one can NbMCM-41 (TIE) in the presence of suggest that they are due to the presence of NO (>1.5 mbar) after: a) 20 h at RT; b) niobium in the sample. In fact, our previous 0,5 h at 473 K; c) 0,5 h at 673 K; d) 0,5 h study [16] showed such bands after NO at 723 K adsorption on the parent NbMCM-41 material.

1789 As concerns the 1 6 0 0 - 1650 crn"1 region, the highest intensity (taking into account the same experimental conditions) of the bands in this range is noted on aluminosilica micro- and mesoporous molecular sieves containing iron (Fig. 1 A,B). This band most probably covers more than one component. It might be due to the nitro groups bonded to iron ions existing at a different location, as concluded by Sachtler et al. [11] for NO adsorbed on Fe-ZSM-5. A shoulder at a lower wavelength, better visible on Fe-AIMCM-41, might be assigned to NO2 weakly adsorbed on the ferric oxide nano clusters. The same band is located at N1620 cm 4 when NO is adsorbed on Fe-NbMCM-41. It seems interesting that the nitro groups are formed slowly in the presence of NO and that its intensity increases upon evacuation or/and a longer contact time (16-20 h). Some authors [11] speculate that a superoxide ion (02") or peroxo species may be involved in the formation of nitro groups. This species can be formed in the interaction of oxygen, generated from the decomposition of NO or traces of oxygen in the system, with the vacancy in the binuclear site [HO-Fe2+-N-Fe2+-OH]2+.

3.3. Electron spin resonance spectroscopy study Fe-ZSM-5 (IE)

The ESR experiments were conducted for the identification of the "7 paramagnetic species on the surface of ::3 the catalysts used. The ESR spectra of i,.,,.i g _ . ~ g=4 ~ ,., ._.g, Fe-ZSM-5 (IE) evacuated at 773 and ill 973 K and scanned at 77 K are shown E: t-in Figure 4 a,b. The activation at 773 K ~~--3.93 _ c leads to the appearance of two ESR signals (g = 4.26 and g = 2.003). The first signal (g = 4.26) is typical of Fe 3+ , I i I paramagnetic cations in a strong 200 400 Field [mT] rhombic distortion of the tetrahedral Fig. 4. ESR spectra (registered at 77 K) alter: a) sites [11,17]. A narrow ESR signal (g = evacuation at 773 K, 2h, b) evacuation at 973 K, 2.003) can origin from coke formed 2h, c)adsorption 1 mbar of NO and evacuation at from the residual template but also it RT for 10m in. can origin from Fe 3+ in octahedral coordination as isolated ions at cationic Fe-AIMCM-41 (TIE) positions [ 18]. Moreover, Sachtler et al. Ig=6.1 I [11] attributed a sharp line at g=2.0 to superoxide ions (02") which are associated with iron ions. The increase of the evacuation temperature to 973 K causes the appearance of a small signal at g = 6.23 assigned in the literature [11,19] to Fe3+ in the coordination of the less distorted tetrahedron. Moreover, a broad signal described by g ~ 2.0 appears. It 200 400 600 800 can be attributed to the Fe-O-Fe species Field [mT] with ferri-, ferro-, and/or Fig. 5. ESR spectra (registered at 77 K) after: a) antiferromagnetic behaviour [20]. This evacuation at 873 K, 2h; b) evacuation at 973 K, 2h. signal is better visible when the spectrum

I

1790 is scanned at RT (not shown here). The ESR spectra of Fe-AIMCM-41 (Fig. 5 a,b) are similar to that of Fe-ZSM-5. The same signals are registered in the low field region (g>3) which are attributed to isolated Fe 3+ ions in different coordination environments, as well as lines at g400 ~ sodium metal and nitrogen can be generated only from that fraction of the sodium azide, which was not converted in reaction with the hydroxyl groups or water. The loss of sodium azide can be decreased or prevented if fully dehydrated zeolites zeolites are used. Impregnation of the zeolite with sodium hydroxide suppresses the NaN3 hydrolysis. Forming large pellets quickly after mixing hinders the HN3 transport and, consequently, suppresses the hydrazoic acid release. The entrance of the NaN3 into the pores of the narrow pore zeolites is strongly hindered. These zeolites can be fully dehydrated at temperature that is lower than the decomposition temperature of sodium azide.

1800 ACKNOWLEDGEMENTS

The excellent assistance of Mrs. Agnes Wellisch is gratefully acknowledged. REFERENCES

1. R.M. Barrer and J.L. Whiteman, J. Chem. Soc. A (1967) 19. 2. D. Fraenkel, and B.C. Gates, J. Am. Chem. Soc. 102 (1980) 2478. 3. J. A. Rabo, C. L. Agnell, P. H. Kasai and V. Schomaker, Disc. Farady Soc. (1966) 329. 4. F. Schmidt, W. Gunsser, and J. Adolph, ACS Symp. Ser. 40 (1977) 291. 5. J.B. Lee, J. Catal. 68 (1981) 27. 6. R.M. Barrer, E.A. Daniels, and G.A. Madigan, J. Chem. Soc. Dalton Trans., (1976) 1805. 7. P. Fejes, I. Hannus, I. Kiricsi, and K. Varga, Acta Phys.Chem. Szeged, 24 (1978) 119. 8. I. Hannus, I. Kiricsi, K. Varga, and P. Fejes, React. Kinet. Catal. Lett., 12 (1979) 309. 9. I. Kiricsi, I. Hannus, A. Kiss, and P. Fejes, Zeolites, 2 (1982) 247. 10. I. Hannus, Gy. Tasi, I. Kiricsi, J.B. Nagy, H. Fr6ster and P. Fejes, Thermochimica Acta, 249 (1995) 285. 11. L.R.M. Martens, P.J. Grobet, and P.A. Jacobs, Nature, 315 (1985) 568. 12. L.R.M. Martens, P.J. Grobet, W.J.M. Vermeiren, and P.A. Jacobs, Stud. Surf. Sci. Catal., 28 (1986) 935. 13. L.R.M. Martens, W.J.M. Vermeiren, P.J. Grobet and P.A. Jacobs, Stud. Surf. Sci. Catal., 31 (1987) 531. 14. M. Brock, C. Edwards, H. F6rster and M. SchrSder, Stud. Surf. Sci. Catal. 84 (1994) 1515. 15. E. J. Doskocil and R. J. Davis, J. Catal. 188 (1999) 353. 16. H.K. Beyer, Gy. Onyestyfik, B.J. JSnsson, K. Matusek, and K. Lfizfir, in Procc. of 12th IZC, (eds. M.M.J. Treacy, B.K. Marcus, M.E. Bisher and J.B. Higgins) Materials Research Society, Warrendale, Pennsylvania, USA, 1999. p2875. 17. K. Lfizfir, L.F. Kiss, S. Pronier, Gy. Onyestyfik and H.K. Beyer, M6ssbauer Spectroscopy in Material Science, (eds. M.Miglierini and D. Petridis) Kluwer, Amsterdam, Netherlands, 1999. p291. 18. K. LAzAr, H.K. Beyer, Gy. Onyestyfik, B.J. J6nsson, L.K. Varga and S. Pronier, NanoStructured Materials, 12 (1999) 155. 19. Z. Gabelica, N. Dewaele, L. Maistrau, J. B. Nagy and E. G. Derouane, Zeolite synthesis, (eds. M.L. Occelli and H.E. Robson) ACS Symp. Ser. 398 (1989) 539. 20. R. Sreekumar, R. Radmakumar and P. Rugmini, Chem. Comm. 12 (1997) 1133.


1801

M o d i f y i n g t h e a c i d i c p r o p e r t i e s of P t Z S M - 5 a n d P t Y zeolites b y a p p r o p r i a t e l y varying reduction methods A. Tamfisia, K. Niesz a, I. Pfilink6 b'*, L. Guczi c and I. Kiricsi a aDepartment of Applied and Environmental Chemistry, University of Szeged, Rerrich B. t6r 1, Szeged, H-6720 Hungary bDepartment of Organic Chemistry, University of Szeged, D6m t6r 8, Szeged, H-6720 Hungary cChemical Research Center of the Hungarian Academy of Sciences, Institute of Isotopes and Surface Chemistry, P.O. Box 77, Budapest, H-1525 Hungary

PtZSM-5 and PtY catalysts were prepared by the wet ion-exchange method from their Naforms. The modified zeolites were reduced by either NaBH4 or gaseous H 2. Reduction with NaBH4 did not alter the original Lewis acidity of the samples (Lewis acidity due to the sodium ions), while reduction with

H2

generated new Bronsted and Lewis acid sites. Reduction with

NaBH4 did not modify the ZSM-5 crystal lattice, however, treatment with Hz led to some structural decomposition. Thus, Lewis sites due to extraframework alumina and Bronsted sites due to hydroxyl groups of the heeled defect sites were formed. For the Y zeolite, after reduction the platinum atoms moved out from the ion-exchange positions and started to migrate toward the supercage. On their way they aggregated and the large crystallites pushed apart portions of the zeolite crystal. The detrimental effect was of a higher extent when reduction occurred with gaseous Hz. The resulting SiO2-AI203 supported catalyst contained many "true" Lewis sites due to extraframework alumina species as well as acidic OH groups connected to defect sites, however, it has lost shape-selective properties connected to the zeolitic structure. 1. INTRODUCTION Zeolites containing transition metal in their structure offer the combination of the acidic sites of the zeolites in a constrained environment as well as the properties of the metal catalysts [1]. These trifunctional catalysts (acid-base properties, shape-selective behavior and hydrogenationdehydrogenation ability) may be used effectively in environmentally benign processes, since Research leading to this contribution was financed through a grant from the National Science Foundation of Hungary (OTKA T034184). The support is gratefully acknowledged.

1802 there are a lot of possibilities in fine-tuning their activities and selectivities, thus, facilitating catalysts for virtually waste-free chemical transformations. Carefully chosen reduction methods allow the fine modification of acidity upon creating the other vital functionality, hydrogenationdehydrogenation that is [2]. In many previous studies the particle size of the transition metal after reduction was of major concern [3] and influencing the acidic properties by varying the methods of reduction was largely neglected. Particle sizes of Pd [4, 5] or Pt [6-11] were influenced by the method of preparation as well as with the methods of reduction or oxidation-reduction. Reduction mainly occurred with H2 and its effect on varying acidity, especially types of acidity, was generally not emphasized. Comparative studies with other methods of reduction are scarce [12]. Thus, this study, in which the effects of various reducing treatments on the types of acidity of Pt-exchanged zeolites of two distinctly different structures are investigated, should fill considerable gap. 2. EXPERIMENTAL 2.1 Materials and treatments

The platinum containing zeolites were prepared by the wet ion-exchange method starting from NaZSM-5 (Si/AI=40) or NaY (Si/AI= 13.8) [13]. After the ion exchange the zeolite samples were dried and calcined at 573 K for 5 hours in air. Then, the zeolites were reduced by one of the three methods: (i) NaBH 4 (aqueous solution, room temperature, 24-hour stirring) or (ii) H2 (573 K, 4 hours, gas flow, N2/H2 mixture with gradual increase in H2 content) or (iii) the Pt-containing zeolite reduced by NaBH4 was treated by H2 under the conditions of (ii). The model compound for the catalytic reaction was 1-butene from Aldrich Chemical Co. and was used as received. 2.2 Methods of characterization The resulting samples were characterized by derivatography, powder X-ray diffractometry and BET surface area measurements. Thermal behavior of the substances was investigated by a Derivatograph Q instrument. The powdered samples were placed on a platinum sample holder and studied under the following conditions: mass sample 100 mg, heating rate 10 degree/min, temperature range 300 to 1000 K in air. X-ray diffractograms were registered on well-powdered samples with a DRON 3 diffractometer in the range of 3 o _ 43 ~ For X-ray source the K 1 line of the copper anticathode was selected by the monochromator. BET measurements were performed in a conventional volumetric adsorption apparatus

1803 cooled to the temperature of liquid nitrogen (77.4 K). Prior to measurements the host samples were pretreated in vacuum at 723 K for 1 h under continuous evacuation. The acid-base properties were studied by pyridine adsorption followed by IR spectroscopy. Self-supported wafers (10 mg/cm 2) were prepared from the powdered zeolites and placed into the sample holder of the in situ IR cell. The temperature of the wafer was slowly increased to 723 K under continuous evacuation. After 2 h the sample was cooled to room temperature and the background spectrum of the zeolite was recorded. Resolution was 1 cm 1, and 64 scans were accumulated on a Mattson Genesis I FT-IR spectrometer. For the acidity measurements 1.33 kPa of pyridine was introduced into the cell at ambient temperature, which was then heated to 473 K. After 1-h equilibration the cell was evacuated for 1 h at the same temperature. The sample was cooled to ambient and the spectrum was recorded. For calculating Bronsted and Lewis acidities bands at 1540 cm -~ and 1450 cm ~ were used. The absorbances were integrated, specific areas were calculated and compared. The acidity of the catalysts was also characterized by the double bond isomerization of 1butene in a recirculatory batch reactor. 100 mg of the catalyst was placed into the reactor and pretreated at 723 K for 1 h under continuous evacuation. The reactor was cooled clown to reaction temperature (323 K) followed by introducing 66.6 kPa of 1-butene into the reactor. The gas mixture was analyzed by gas chromatography (Hewlett Packard 5710 GC, flame ionization detector, 4.5-m-long all-glass column packed with 30% dimethylsulfolane on Chromosorb W. 3. RESULTS 3.1. N a Z S M - 5 and its modified varieties

It was found that the various heat and reducing treatments of the ion-exchanged ZSM-5 zeolite did not result in significant modifications in its original structure. The BET surface areas did not change much (Table 1, column 2). The thermal behavior also remained similar. Two types of weight losses could be observed. Both were assigned to water losing processes. The first step at around 373 K was attributed to the loss of crystal water, while the high-temperature step indicated dehydroxylation. The total weights lost for the variously treated samples are very similar as well (Table 1, column 3). The XRD spectra of the platinum-containing samples resembled that of the parent sample to a great extent, except that weak Pt reflection could be observed (Figure 1). IR spectrum taken in the range of framework vibration (400 cm-1-1400 cm 1) verifies the results of the X-ray measurements.

1804 Table 1 Characteristic data on the variously treated catalysts Catalyst

BET area (m2/g)

total weight loss

Bronsted sites/Lewis

by 1200 K (%)

sites d

NaZSM-5

322

4.90

0/0.34

PtZSM-5(B a)

310

5.17

0.06/0.42

PtZSM-5(H b)

302

5.70

0.28/0.13

PtZS M-5 (BH c)

309

5.20

0.05/0.53

NaY

595

6.51

0/0.84

PtY(B a)

410

5.85

0/0.60

PtY(H b)

265

5.97

0.25/0.54

a : reduced with NaBH4 b: reduced with gaseous H 2 c: reduced with NaBH4, then, postreduction with gaseous H2 d: ratio of integrated areas

The structure remained largely intact irrespective to ion exchange and the various heat and reducing treatments. No new band appeared at 930 cm -~, which is generally accepted as the indication of framework defects [ 14].

I

As far as acidity is concerned the parent sample displayed only negligible Bronsted acidity. The intense band at 1445 cm 1 is assigned to pyridine bonded coordinatively to the sodium ions (Figure 2, spectrum a). Upon reduction

with

NaBH4 the

overall

picture remained the same. Negligible amount of Bronsted acid sites were found and the band at 1445 cm ~ could

a

40

30 2 Theta 20

10

also be detected (Figure 2, spectrum e). If this sample was further treated with H2

gas ("postreduction"), changes did

Figure 1. XRD spectra of (a) NaZSM-5, (b) PtZSM-5 not occur (Figure 2, spectrum d). reduced with NaBH4, (e) PtZSM-5 reduced with However, the acidity of the H2-reduced gaseous H2 sample changed significantly (Figure 2,

1805 spectrum b). While the other reduction methods did not generate new type of acid sites and only slightly modified the concentration of Lewis sites, reduction with gaseous hydrogen resulted in the appearance of Bronsted acidity (Table 1, column 3). At the same time the band assigned to pyridine bonded to sodium ions diminished and a new band could be detected at 1455 cm 1 indicating the appearance of "true" Lewis acid sites, generated by the release of framework aluminum. In 1-butene isomerization the samples all showed near to one as the ratio of cis-2-

Absorbance (a.u.)

butene to trans-2-butene, indicating that the compounds

were

acidic,

indeed

[5].

However, there were significant differences in the rate of double bond isomerization between the Pt-containing zeolites reduced by the two different methods.

Initially,

the

PtZSM-5 reduced by H 2 proved to be significantly more active than the one reduced by NaBH4. The ratio of the initial rates was 29.3:1. 1550

3.2. NaY and its modified varieties

1500

1450

Wavenumber (cm")

Reduction of the platinum-exchanged NaY,

Figure 2. Acidities of the ZSM-5 samples by especially by gaseous hydrogen, caused signi- pyridine adsorption followed by IR spectroscoficant changes in the zeolite structure. The py, (a) NaZSM-5, (b) PtZSM-5 reduced with BET surface areas decreased dramatically gaseous H2, (e) PtZSM-5 reduced with NaBH4, (Table 1, column 2). The diminishing lines in (d) PtZSM-5 reduced with NaBH4, then by H 2 the X-ray diffractograms indicate significant loss of crystallinity (Figure 3). At the same time large platinum clusters were formed as shown by the intense and broad reflections around 40 degree. The method of reduction influenced the acidity of the ion-exchanged Y zeolite much the same way as was experienced with the ZSM-5 zeolite. However, the effects were larger, probably due to the lower Si/A1 modulus of the faujasite structure. The parent material only contained Lewis acid centers due to the presence of sodium ions: a band at 1445 cm 1, pyridine coordinatively attached to the sodium ions, could be detected in the case of this sample as well (Figure 4, spectrum a). Reduction with NaBH4 did not alter the acidity of the parent NaY zeolite: beside the combination band, the one at 1445 cm ~ was only found (Figure 4, spectrum c). "True" Lewis centers appeared when reduction was performed by H2. Moreover, reduction with gaseous hydrogen resulted in the appearance of Bronsted acidity (Table 1, column 3 and

1806

Figure 4, spectrum b). In

1-butene

isomerization

these

samples, similarly to those of the ZSM-5, the cis-2-butene to trans-2-butene ratios were close to one, indicating again that the compounds were acidic, indeed [15]. As far as the initial rates are concerned, PtY reduced by H2 was 15.1 times more active than the one reduced by NaBH4. 4. DISCUSSION ......

40

30

.

.

.

.

.

.

.

.

.

20

.

10

When the effects of the two reducing Figure 3. XRD spectra of (a) NaY, (b) PtY reduagents are compared, one can see significed with NaBH4, (c) PtY reduced with gaseous H 2 cant difference. While NaBH4 does not influence the acidic properties of the parent sample, 2 Theta

only Lewis centers due to the presence of sodium ions could be observed, H2 treatment generated

Asorbrcau

AI

new Lewis sites as well as appreciably amounts of Bronsted sites. Beside the reducing agent, the structure of the zeolite is also a major contributing factor in determining the final properties of the platinum-containing catalysts. The original zeolite structure is

b

preserved for ZSM-5. Irrespective to the method of reduction the majority of the platinum ions remained in or near the ion-exchanged positions even after reduction. Reduction with H2 had some detrimental effect, however, minor extent of decomposition of the ZSM-5 crystal lattice did occur and extraframework alumina took part in

1570

1530

1490

1450

Wavcnumbcr (cm-')

the formation of"true" Lewis sites. For the fauj asite zeolite, after reduction the platinum atoms Figure 4. Acidities of the Y zeolite samples by pyridine adsorption followed by IR spectmoved out from the ion-exchange positions and roscopy, (a) NaY, (b) PtY reduced with started to migrate toward the supercage. On their gaseous H2, (e) PtY reduced with NaBH 4 way they aggregated and the large crystallites pushed apart large portions of the zeolite crystal. The detrimental effect was of a higher extent

1807 when reduction occurred with gaseous H2. These events resulted in the formation of SiO2-A1203supported catalyst containing many "true" Lewis sites due to extraframework alumina species as well as acidic OH groups connected to defect sites. Obviously, the reduced fauj asite samples lost their shape-selective properties. 5. CONCLUSIONS The method of reduction significantly altered the properties of transition metal containing ZSM-5 and Y zeolites. Catalysts with Bronsted and Lewis acidities and metal functionality also (PtZSM-5 and PtY) could only be obtained when H2 was the reducing agent. The extent of reduction was higher for PtY, in turn, however, this catalyst largely lost its third functionality. Shape selectivity disappeared with the extensive disruption of the original faujasite structure. Results presented in this work open a novel way to prepare catalysts with tailor-made acidity, preserving hydrogenation-dehydrogenation activity at the same time. Combination of the reduction methods allows acidity to be fine-tuned, e.g., reduction with NaBH 4 first transforms part of the platinum ions to platinum metal clusters without the generation of acid sites. Upon further reduction in H2 the rest of the platinum ions are reduced with concomitant formation of acidity. REFERENCES 1. J. Wang, Q. Li and J. Yao, Appl. Catal. A 184 (1999) 81. 2. P. Gallezot, A. Alarcon-Diaz, J.-A. Dalmon, A.J. Renouprez and B. Imelik, J. Catal. 39 (1975) 334. 3. J. Chupin, N.S. Gnep, S. Lacombe and M. Guisnet, Appl. Catal. A 206 (2001) 43. 4. G. Koyano, S. Yokoyama and M. Misono, Appl. Catal. 188 (1999) 301. 5. B. Pommier and P. G61in, PCCP 1 (1999) 1665. 6. E.S. Shpiro, R.W. Joyner, K.M. Minachev, P.D.A. Pudney, J. Catal. 127 (1991) 366. 7. P. Gallezot, A. Alarcon-Diaz, J.-A. Dalmon, A.J. Renouprez, B. Imelik, J. Catal. 39 (1975) 334. 8. M. Guerin, C. Kappenstein F. Alvarez, G. Gianetto and M. Guisnet, Appl. Catal. 45 (1988) 325. 9. J. Wang, Q. Li and J. Yao, Appl. Catal. A 184 (1999) 181. 10. S. Ciccariello, A. Benedetti, F. Pinna, G. Strukul, W. Juszczyk and H. Brumberger, PCCP 1 (1999) 367. 11. E. Baburek and J. Nov~ikov/~, Appl. Catal. A 190 (2000) 241. 12. I. Manninger, Z. Pa~,l, B. Tesche, U. Klenger, J. Halfisz and I. Kiricsi, J. Mol. Catal. 64 (1998) 361.

1808

13. M. Guerin, C. Kappenstein, F. Alvarez, G. Giannetto and M. Guisnet, Appl. Catal. 45 (1988) 325. 14. P. Fejes, I. Hannus and I. Kiricsi, Zeolites 4 (1984) 73. 15. P. Fejes and D. Kall6, Acta Chim. Hung. 39 (1963) 213.


1809

Vibrational Studies o f Iron Phthalocyanines in Zeolites Peter-Paul H.J.M. Knops-Gerrits 1., Fr6d6ric Thibault-Starzyk2, Rudy Parton 3 1 D6partement de Chimie, Universit6 Catholique de Louvain (UCL), Batiment LAVOISIER, Place L. Pasteur n~ B-1348 Louvain-la-Neuve, Belgium Tel : (32)10 -47 29 39 Fax: 47 23 30, [email protected] 2Laboratoire Catalyse & Spectrochimie, ISMRA-CNRS, 14050 Caen (cedex) France Tel +33-231 452 810; Fax +33-231 452 822; [email protected] 3 DSM Research, P.O Box 18, 6 1 6 0 MD Geleen

Tel +31 46 4761293 ; Fax+31 46 4761173; [email protected] * To whom correspondence should be addressed. The range ofzeolites which are suitable as host for phthalocyanines is limited, because of the size of these ligands. Generally, faujasite structures are preferred. Synthesis of phthalocyanines in the internal space of zeolites is exceptional in that mostly, the ligand is itself synthesized in situ before complexation of the metal ion. Moreover, depending on the metal source (salts, carbonyl complexes or metallocenes) distinct synthesis procedures can be recognized. FT-IR and Raman data are presented for the structure of phthalocyanines encapsulated in faujasite type zeolites. A new conformational model for the encapsulation of metal-phthalocyanines in the supercage of zeolite Y resolves the apparent contradiction between molecular modelling and spectroscopy of these catalysts. In the present model, two aromatic rings of the phthalocyanine are pushing against the supercage walls. 1. INTRODUCTION The range ofzeolites which are suitable as host for phthalocyanines is limited, because of the size of these ligands. Generally, faujasite structures are preferred [1-8], but the use of VPI-5 [3] has also been claimed. Synthesis of phthalocyanines in the internal space ofzeolites is exceptional in that mostly, the ligand is itself synthesized in situ before complexation of the metal ion. Moreover, depending on the metal source (salts, carbonyl complexes or metallocenes) distinct synthesis procedures can be recognized. When salts are used, the phthalocyanines are synthesized by tetramerization of dicyanobenzene on cation exchanged zeolites at temperatures between 523 and 573 K [1-5]. Although the complexation of the TMI by the formed phthalocyanine is thermodynamically favorable, it generally proceeds only slowly. The degree of incorporation of TMI in zeolite hosted phthalocyanines decreases in the sequence Co > Ni > Cu > Fe. Therefore, after synthesis, large quantities of transition metal are still ligated by the lattice, or are present as

1810 oxides [1, 4, 6, 8]. Lattice coordinated cations can be removed by exchange with NaC1, but the exchange of the TMI with Na + is incomplete [6-9]. Moreover, considerable amounts of TMI may migrate to the surface during dehydration and Pc synthesis, as shown by XPS-analysis of [Rh(Pc)]-Y and [Rh(Pc)]-X [5]. To overcome the unfavorable kinetics of the chelation of TMI by the Pc chelate, procedures using other metal sources have been proposed. 2. MATERIALS AND METHODS 2.1. Synthesis FePcY, FeTNPcY and CoPcY were prepared as previously described [3, 5] MePcY is synthesized by mixing 5 g MeY (obtained by ion exchange from NaY zeolite Ventron, anhydrous unit cell composition: Na54 (A102)54 (SIO2)138) dried at 473 K with 3.15 g DCB (8 DCB per supercage). The mixture is heated at 453 K for 24 h. Raw MePcY is treated in a soxhlet extractor with acetone, until the extraction solvent is colorless. 2.2. FT-IR and Raman spectroscopy Raman spectra were recorded on a Renishaw Raman Microscope. The zeolite samples, in powder form, were placed on a glass microscope slide. The power on the sample was about 2mW/mm 2. The collection time varied from sample to sample and was between 15 and 20 minutes. The spectra were background corrected and a Fourier deconvolution procedure, described elsewhere 3, was applied to resolve the overlapping bands in the OH stretching region. FT-IR and Raman spectra were recorded on a Bruker IFS66 instrument equipped with a Ge detector. The zeolite samples, in powder form, were pressed in metal sample-holders. The complexes were used in KBr pellets (~-1 mg of MPc or MPcY with ~-300 mg ofKBr). FT-Raman experiments were performed with 1064 nm, 300 mW Nd-Yag laser excitation. 2.3. The molecular mechanics calculations Calculations were carried out using the software Hyperchem (Auto-desk, Inc.), using a completed version of the MM+ force field, i.e. a completed version of Allinger's MM2. Partial calculations were first performed with a reduced version of the zeolite cluster, composed of the supercage only. These results allowed us to improve the time needed for calculations with a 30 A cluster.

OH

0

FePcY

O

,Buoo. J-coo. FePcY

r

~COOH

Scheme 1 : catalytic cyclohexane oxidation with FePcY and tBuOOH.

1811 2.4. The catalytic oxidations Oxidations were carried out under stirring at 25~ using 0.5 g of catalyst, 25 mmol of each substrate, 0.5 g of chlorobenzene as internal standard for the GC, and 50 ml of acetone as the solvent. The oxidant was t-butyl hydroperoxyde (TBHP : 70 % solution in water). Analysis of the products was done by gas chromatography on a CP Sil 5 CB column from Chrompack. 3. RESULTS AND DISCUSSION One of the alternative procedures involves adsorption of TMI-carbonyl complexes on the zeolite. These complexes may or may not be decomposed prior to the in situ synthesis of the Pc ligand [7,10-12]. Decomposition can be performed thermally or photochemically and results in formation of metal clusters in the zeolite. Photochemical decomposition is preferred, in order to suppress the migration of the metal to the outer surface. As the transition metal is present as metal clusters in the faujasite supercage, and as only one phthalocyanine can be synthesized per cage, the presence of unchelated TMI is unavoidable. If, on the other hand, the decomposition step is omitted, the CO ligands are directly replaced by 1,2-dicyanobenzene. However, this procedure also leaves some unchelated TMI in the zeolite, as the rate of decomposition of the carbonyls is higher than the rate of formation of [TMI(Pc)]. Therefore, it is advisable to use more stable complexes, such as metallocenes, as precursors. Zakharov et al. first applied ferrocene and cymantrene in [TMI(Pc)]-Y synthesis [13]. The amount of unchelated TMI can be minimized by selecting a proper synthesis temperature [14]. An analogous synthesis procedure was used by Parton et al. [3 ], who claimed, based on chemical analysis, that there was almost no residual iron in their [Fe(Pc)]-Y. However, because of the high stability of ferrocene, large amounts of free base phthalocyanines are synthesized. Fortunately, at low loading, these do not interfere with the catalytic activity. Using a metallocene as TMI-source moreover allows to apply molecular sieves without cation exchange capacity as host for [TMI(Pc)]. Indeed, when ferrocene is mixed with dry VPI-5 and dicyanobenzene, [Fe(Pc)] complexes are formed inside the channels of this neutral, aluminophosphate molecular sieve [3 ]. [TMI(Cp)2]-VFI + 4 DCB + H20 --~ [TMI0~c)] -VFI + 2 Cp + 2 H + + 1/2 0 2

(1)

Cyclization ofphthalocyanines starting from dicyanobenzene is a two-electron reduction process. When TMI-carbonyl complexes are used, these two electrons are supplied by the metal, which is oxidized to the divalent state: [TMI(CO)m] + 4 DCB

--~

[TMI(Pc)] + m CO

(2)

1812 Table 1. Raman bands corresponding to the symmetric deformation of the macrocyclic ring in encapsulated and free phthalocyanines (in cm-1) 9 *(632nm excitation)

FT-Raman Resonance Raman* Symmetry group

H2Pc 720

D2h

H2Pc Y 720 718 D2h

CoPc 751 746 D4h

CoPc Y 750 748 D4h

FePc 748 744 D4h

FePc Y 748 751 D4h

In case salts or metallocenes are used as the metal source, some water must be added as an electron source [3]. The subsequent complexation results in the liberation of two protons. These protons can be trapped efficiently by cyclopentadienyl anions, but when salts are used as the metal source, the protons cause surface acidity: 4 DCB + H20

~

Hffc + [TMI(Cp)2] Hffc + TMI-Y

--~

H2Pc + 1/2 02

(3)

[TMI(Pc)] + 2 cyclopentadiene

(4)

[TMI(Pc)] + I-I2-Y

(5)

The acidic sites can interfere with the catalytic activity of the phthalocyanines and may even cause dealumination or loss of crystallinity of the zeolite [5]. There is as well some spectroscopic evidence for protonation of the coordinating nitrogen atoms. This results in a symmetry reduction from D4h to D2h and a splitting of the Q-bands in the Vis-NIR region [15]. In IR spectra of [Co(Pc)]-X, a band at 1020 cm 1 was attributed to protonation of the inner nitrogen atoms [16]. XPS measurements on [Ni(Pc)]-Y showed non-equivalency of the chelating nitrogen atoms [6]. Instead of synthesizing the phthalocyanines in sial, they can be used as a template during zeolite synthesis. Phthalocyanines are suitable for such a synthesis as they are thermally and chemically extremely stable. Consequently, no residual TMI is formed during synthesis. A major problem is, however, to keep the phthalocyanines monomolecularly dispersed in the aqueous zeolite synthesis medium. By careful control of the synthesis gel chemistry, Balkus et al. obtained a zeolite X material in which 50% of the unit cells were occupied by Pc, even after severe extractions [4] . Various [TMI(Pc)]'s were also successfully used as templates for A1PO-5 and AIPO-11 synthesis [5]. However, in view of the relative size ofthe Pc ligand and the pore system of these AIPO's, it is likely that the complexes are mainly located at structural defects. As discussed previously, [TMI(Pc)] zeolites ot~en contain free base H~Pc. Distincnon between free base and metallatedphthalocyanines is facilitated by the symmetry change upon chelation. The symmetry of free base Hffc is D2h, whereas [TMI(Pc)] belongs to Dnh. In IR spectroscopy, this symmetry difference results in the splitting of the conjugated isoindole band (at 1332 cm -a) and the C-H in-plane deformation band (at 1287 cm 1) into doublets for the free base H~Pc's (at 1336 and 1322 cm 1, and at 1304 and 1278 cm -1 respectively ; see Figure 1). Moreover, the TMIN vibrations around 900 cm 1 are also typical for metallated Pc [3 ].

~'E~ .~,~

t~

~=

=r.

z

E,.-, ,~~

C

295

~, .oo

_~92

~593 f~09 ' - - I__485

~"-':=>e.4

~----__.~682se5 ,

;>833 ~'~ 762

~21186 1 ~ 1158 ~ .... ..1108 11008

~1160-1185 1,,zQ ~---1~ . . . . ";1007 S ~-_87(,~1"

~'~---'~1341 r 1309

k _ f 1340 "1308

!>1..

Raman Absorbance (a.u.)

457"C--.__

580~ t

736 ~----~

793~ - ' ~

436--~

574"--'~

7346 ~ ~ ~ 4 ~

781 - : ~

~ 1163~ 1 1 2 0 ~ 1 1 2 0 ~ 1091 1,,10 t/ 1076-~'~--~-'"'-~ 1003

. . . . . . . . . . . . . . . . .

1399~. 1426 1334~1334--- --------~

1,,1

Figure 2. Structure of (A) NaY zeolite + (B) FePc and (C) FePc encaged in Y zeolites. diameter of a supercage is only 1.3 nm. Therefore, based on molecular graphics analysis, a

saddle-type deformation of the aromatic system was proposed. In this model, the four benzene

rings of the chelate occupy the four twelve-membered ring openings of the supercage [2,8]. Some indirect (and weak) evidence for this saddle-type deformation comes from the broadening of the Nls lines in XPS, and from a coordination number of 3.6 (instead of4)and Fe-N distances of 1.84 nm (instead of 1.83 nm), determined by EXAFS [12]. Very small shifts of the IR C=N and C=C stretching frequencies have also been associated with this saddle-type deformation [ 12]. Several other effects of occlusion on Pc properties have been documented. In the DRS spectra of occluded Pc, B- and Q-bands are red-shifted ; the latter become also weaker [5,11,12]. This behavior is analogous to the red shift of the Q-band of crystalline [TMI(Pc)] upon pressure increase. Thus, [TMI(Pc)] must be in a spatially constrained environment, which results in effects similar to those of high pressures [15]. Moreover, the optimal synthesis temperature of[Co(Pc)] in the Y supercage is 542 K, which is 15 K higher than in the absence of the zeolite [24]. The confinement of the complexes to the inner zeolite volume is even more difficult for the substituted phthalocyanines. It has e.g. been demonstrated that nitro-substituted FePc's can be exclusively located at the outer surface of zeolite Y [25]. For tetra-t-butyl substituted FePc's, some evidence in favor of encapsulation in NaY has been presented [26]. The relative intensities of the B-band (280 nm) and the Q-bands (550, 580 nm) are different for encaged and adsorbed substituted Pc's. The Q-band, which arises from 7t --~ re* transitions in the inner Pc aromatic system, is strongly suppressed for the encaged complex, whereas the intensity of the B-band,

1815 which is due to transitions in the peripheral benzene rings, is markedly enhanced. Furthermore the Q- and B-bands, as well as some IR bands are shifted to lower frequencies. Encaging of tbutyl substituted Pc reduces the EXAFS coordination number from 3.9 to 3.6 and increases the Fe-N interatomic distance from 0.185 nm to 0.190 nm. All these spectral differences are attributed to the structural deformation of the Pc plane and a slight departure of the Fe atom from the Pc plane. Catalytic results on the other hand disfavor the encapsulation hypothesis, since large substrates, such as stilbene, are epoxidized on the t-butyl substituted phthalocyanines. It is unlikely that this occurs in supercages, which are occupied by t-butyl substituted Pc's [26]. Cytoehrome

P-450:

Fe=O

H-R

A Zn +2 > Mn +2 > Cd +2. This selectivity was determined by comparison of the individual ion exchange isotherms for each of the cations tested.

1850 Besides the factors mentioned above, the exchange of a metal cation on natural zeolites is also affected by the presence of other metal cations which can also be exchanged and thus compete for the same cationic sites. The effect of competitive cations on exchange in natural zeolites has not been studied and in general, research on this subject has been mainly focused to evaluating the exchange of a single metal cation and determining its exchange isotherm. Ion exchange of a metal cation in a natural, unmodified zeolite is a multicomponent process. The metal cation is exchanged from the solution to the zeolite and the exchangeable ions (K +, Na +, Ca +2 and Mg +2) are transferred from the zeolite to the solution. In addition, the hydronium ion H30 + has also been reported as exchanging from solution to zeolite, although the magnitude of this exchange is insignificant and can be considered negligible (6). The aim of this work is to determine the simultaneous exchange isotherms for Cd(II) and Pb(II) from an aqueous solution on clinoptilolite, and to examine the effect of Cd(II) on the Pb(II) exchange isotherm and that of Pb(II) on the Cd(II) isotherm.

2. E X P E R I M E N T A L M E T H O D S

The zeolitic rock used in this study was from a deposit located in San Luis Potosi, Mexico. The zeolite sample was washed several times with deionized water and dried for 24 hours in an oven set at 110~ The zeolitic rock has a BET surface area of 22.2 m2/g and a density of 2.32 g/cm 3 (6). Identification of the crystalline species present in the zeolite-rich rock was done by X-ray diffraction analysis. X-ray diffraction patterns were obtained with a Rigaku, DMAX 200 diffractometer. Pb(II) and Cd(II) concentrations in aqueous solution were determined using atomic absorption spectrophotometric methods. Absorbance of a sample was measured with a double beam Varian SpectrAA-20 atomic spectrophotometer and metal ion concentration was determined by comparing absorbance with a previously prepared calibration curve. Experimental exchange isotherm data was obtained in a batch exchanger consisting of a 500 mL Erlenmeyer flask to which 480 mL of a solution of known initial concentrations of Pb(II) and Cd(II) and a predetermined mass of zeolite were added. The adsorber was submerged in a constant temperature water bath. The solution was kept in continuous agitation by means of a Teflon-coated stirring bar driven by a magnetic stirrer. The metal cation solution and zeolite remained in contact until both cations reached equilibrium. Samples were taken at various times to follow the progress of exchange, and metal ion concentration for each sample was determined as described above. The equilibrium was attained when two consecutive samples showed no change in metal ion concentration. Preliminary experiments revealed that 21 days was sufficient to reach equilibrium. The molar uptake of the cation exchanged was calculated by performing a mass balance for each cation.

4. RESULTS AND DISCUSSION The zeolitic rock used in this study was characterized in a previous study (6). X-ray diffraction analysis revealed that rock is composed mostly of clinoptilolite and also contains calcite, quartz and some feldspars such as microcline and albite. The experimental cation exchange capacity, CEC, evaluated by sorption of NH4 § ions is 1.55 meq/g (6). The values for all the zeolite properties are within the interval of values previously reported (2,7,8).

1851 Experimental data for the single exchange isotherms of Pb(II) and Cd(II) were fitted to the Langmuir isotherm expressed mathematically as follows" qm,iKiCi

qi = - -

I+KiC i

9

(1)

The constants for this isotherm were obtained by a method of least squares employing an optimization algorithm. Langmuir isotherm constants and average percent deviation are shown in Table 1. Average percent deviation was calculated according to the following equation: %Dev =

1F

1

~ qexp - qcalc 100% . N i=l L q exp

(2)

As shown by the average percent values reported in Table 1, a reasonable fit to the experimental data was obtained with the Langmuir isotherm. Experimental ion exchange data for Pb(II) and Cd(II) are shown in Figure 1 along with the Langmuir isotherms. The qm,i value of the Langmuir isotherm for a particular ion can be considered the maximum exchange capacity of zeolite for that ion. Comparing the qm,i value for Pb(II) to that for Cd(II) shows that the exchange capacity for the Pb(II) ion is 2.3 times that for the Cd(II) ion. This indicates that the zeolite has a larger exchange capacity for the Pb(II) than for Cd(II) ion. Similar results were reported by other authors (2,4). Selectivity is the preference that a zeolite has for one cation over others and is evaluated by comparing the exchange isotherms on zeolite for each one of the ions separately. Such a comparison fails to take into consideration that when two cations are exchanging simultaneously onto the zeolite, both ions may compete for the same cationic sites of the zeolite. The exchange isotherm for a given ion onto zeolite establishes the relationship between the cation concentration in solution and the mass of the cation exchanged onto the zeolite once equilibrium is reached. The exchange isotherm for a single ion determines the exchange capacity of the zeolite for that ion when it is the only ion present in solution. The effect of the presence of a competitive cation on the exchange isotherm of another cation can be studied by comparing the exchange isotherms of either of the ions for different initial concentrations of the competitive ion. This method is limited since the initial concentration of the competitive ion is not its equilibrium concentration and the isotherm depends on the concentration at equilibrium rather than on the initial concentration. Figures 2 and 3 illustrate the method for representing simultaneous ion exchange isotherms. Table 1 Langmuir isotherm constants for single Cd(II) and Pb(II) exchange at pH 4 and 25~ Ion

% Deviation

qm,i

Ki

(m-tool/g)

(L/m-mol)

Pb +2

0.581

23.05

20.76

C d +2

0.259

2.504

23.62

1852 0.6 "

~) rael r

o i

-

"

'

"

"

"

'

"

"

"

'

O

O '

. . . .

O

..,

"

'

"

"

"

'

"

"

"

'

"

-'

0.5 ~

0.4

I

d 0.3 ~ ' 0.2

0

I

43

1)

_~--

_ m~"'"~'~~~"

o 0.1

N

.

0.0 0.0

.

02

.

"0,,Pb(II) ] 1 " ~ Cd (II) 1

.

04

06

08

~0

12

~.4

Concentration at equilibrium, m-moles/L Figure 1. Single exchange isotherms for Cd(II) and Pb(II) onto zeolite at pH 7 and 25~ Figures 2 and 3 illustrate that the zeolite exchange capacity for Cd(II) decreased drastically when the initial Pb(II) concentration was increased and that the zeolite capacity for Pb(II) decreased very slightly when the initial concentration of Cd(II) was increased. Cd(II) and Pb(II) competed for the same cationic sites but Pb(II) was exchanged at additional sites that were not available to Cd(II), and thus the zeolite exchange capacity for Pb(II) was much greater than for Cd(II). 0.3

Initial concentration of Pb(II) (m-mol/L) Pb=0.00 "'L, Pb=0.24 "-e, Pb=0.82 ~*-, Pb=l.11 "o,. Pb=l.4 "c],,, P b = l . 9 8

O

E E |

,_v, 0.2

O r 0.1

,_.... o

o.~

~176176176176176

9

0.0 0.0

........ :i?-ii17;-i

0.2

0.4

0.6

0.8

...........

. . . . . . . . . . . . . . . . .

1.0

1.2

1.4

1.6

1.8

2.0

Cd(II) concentration at equilibrium, m - m o l / L

Figure 2. Effect of initial concentration of Pb(II) on exchange isotherm of Cd(II). It is well documented that the exchange isotherm of a cation onto zeolite can be affected by the presence of other cations since all can compete for the same cationic sites. Two or more cations exchanging simultaneously onto zeolite present a much more complex exchange isotherm than that of a single cation. The exchange isotherm for two ions can be expressed by an isotherm in three dimensions by graphing the mass exchanged for either of the two cations against the concentration of both cations to obtain an exchange surface. This method for representing the isotherm of two components has been used to describe the adsorption of metal cations onto activated carbon (9) and onto bioadsorbents or biomass (10).

1853 0.7 Ca0

--~ o

0.6

9

0. i

............

0.4

~-a~u

w" .....

9

.......

9

~ ~ . t-.r....

. .........................

......... ::::::::::::::::::::::::::::

....

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

0.3

~~0.2 ~ ,~ 0.1 ~

~ of Cd(II)(m-mol/L) ]'"J~Cd,~.0 "*,., Cd =0.089 "'m Cd=0.667 Cd=1.245 xo. Cd=1.824

0.0

0.0

0.2

0.4

0.6

0.8

1.0

1.2

Pb(II) concentration at equilibrium, m-mol/L Figure 3. Effect of initial concentration of Cd(II) on exchange isotherm of Pb(II). The exchange isotherm for two cations onto zeolite can be represented mathematically by the Langmuir bicomponent isotherm expressed below: qi =

qm,iKiCi 1 + KiC i + KjCj

(3)

"

The isotherm constant values for this model are generally considered to be the same as the Langmuir exchange isotherm constants for each cation alone (Table 1). Experimental data for competitive or simultaneous ion exchange of Pb(II) and Cd(II) were interpreted according to this isotherm. The experimental molar uptake values of Cd(II) and Pb(II) were compared to the molar uptake values of Cd(II) and Pb(II) predicted with the bicomponent Langmuir isotherm, as plotted in Figure 4. As shown in this figure, the bicomponent Langmuir isotherm underestimated the molar uptake of Pb(II) while overestimated the molar uptake of Cd(II). Percent deviations obtained using the bicomponent Langmuir isotherm varied from-117.4% to 92.3% for Pb(II) and from-159.6% to 67.9% for Cd(II), and the average absolute percent deviations estimated with equation (2) are 23.2% for Pb(II) and 44.8% for Cd(II). Thus, the bicomponent Langmuir isotherm did not interpret the experimental data for competitive exchange of Cd(II). The bicomponent Langmuir isotherm provides a reasonable fit to bicomponent exchange data when qm,i is very close to qmj, however, in this case qm,Pb is 2.3 times greater than qm,cd. This could explain why this isotherm does not adequately represent the experimental data. Garke et. al. (11) proposed that when qm,i/qm,j is greater than 1, the bicomponent Langmuir isotherm can be modified as follows: qCd =

qPb =

KcdCcd [(1 + KpbCpb)qm,cd - KpbCpbqm,Pb8 ] 1 + KcdCcd + KpbCpb + (1- 8)KcdCcdKpbCpb KmbCeb[0 + KcdCcd)qm,vb -- KcdCcdqm,cd ] 1 + KcdCcd + KpbCpb + ( 1 - 8 ) KcdCcdKpbCpb

,

(4)

,

(5)

1854 where: 8=qm,cd/qm,Pb.The parameter 8 corrects the Langmuir isotherm since qm,cd/qm,Pbis less than 1. Data for the experimental molar uptake of Cd(II) and Pb(II) were compared to molar uptake of Cd(II) and Pb(II) predicted with the modified bicomponent Langmuir isotherm. This isotherm overestimated both molar uptake of Pb(II) and molar uptake of Cd(II). The percent deviations obtained with the modified bicomponent Langmuir isotherm varied f r o m 117.4% to 75.7% for Pb(II) and from-94.4% to 67.9% for Cd(II), and the average absolute percent deviations estimated with equation (2) were 21.8% for Pb(II) and 40.3% for Cd(II). Thus, the modified bicomponent Langmuir isotherm did not adequately fit the experimental data for competitive exchange of Cd(II). The bicomponent Langmuir isotherm can be modified applying a factor to account for interaction between ions i and j and represented as rli,j. This model has been employed successfully by Ho and Mckay (12) and Leyva-Ramos et. al. (9) to represent bicomponent adsorption isotherms of Co(II) and Ni(II) onto peat and Cd(II) and Zn(II) onto activated carbon. This modified bicomponent Langmuir isotherm is expressed mathematically below. q m,CdKCd (C Cd //7Cd,Cd )

m

qcd -- 1 + Kcd(eCd/r]Cd,Cd)

+ Kpb(Cpb/r]pb,Cd)

(6)

'

q m,pbK pb (C Cd/r]pb,Pb )

(7)

q~b -- 1 + Kcd (Cc~/'TCd,~b) + K~b(C~b/'7~b,~U)

is the interaction factor of cation i on the exchange of cation j. As in the other bicomponent Langmuir models, the values of the constants are the same as the Langmuir isotherm constants for each cation alone (see Table 1). The best values for the interaction factors were obtained by fitting the experimental data according to equations (6) and (7) using a method of least squares based on an optimization algorithm. The best values of these factors are: rlCd,Cd= 4.38, qPb,Cd= 3.98, rlpb,Pb= 1.12, qCd,Pb= 3.10. w h e r e rli,j

e~

.

o 0.5

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

@

W

"

E

"•0.5

,, 0.4

0.4

~., 0.3

"~~ 0.3 =

O

"6 0.2

o~

E

.~

~176

.t

.

0.1

0.2

0.3

.

.

.

.

.

.

.

.

.

.

.

.

.

-6 0.2 E

0.1 o 0.0 0.0

.

I-~.

Cd I

0.4

0.5

Experimental molar uptake, m-mol/g Figure 4. Comparison between the experimental molar uptake and the molar uptake predicted with the bicomponent Langmuir isotherm.

o 0.1 ._ ,-o 0 ~- 0.0 r 0.0

o ~

o~ ooi~ 0.1

0

~o ........ 0.2

0.3

I'w,,.

Pbl

Cd I Ii..o." .' Fe --Ga >> B [15]. Moreover, the recent discovery of an A1 containing natural zeolite (mutinaite) with the MFI topology [ 16], makes this structure of relevance also in the mineralogical field. For all these reasons, in the last decade, a large number of studies aiming at a systematic characterization of metal-substituted MFI materials have been reported. Here we describe the crystal structure of B-silicalite and compare it with that of silicalite obtained with the same synthetic procedure in fluoride medium [7]. From the crystallographic point of view, even though the structure of the hosting MFI matrix has been satisfactorily clarified, the distribution of the heteroatoms (M) over the 12 symmetry independent T sites of the orthorhombic MFI framework is still open to debate. This is an important problem, since the localization of the M atoms may play an important role in understanding the catalytic properties of the material. The low fraction of M atoms that can be inserted into the MFI framework makes the experimental localization of these atoms by diffraction measurements very difficult. For these reason the most interesting speculations concerning the Ti distribution in TS-1 are so far based on computational chemistry results [ 17,18], and only few crystallographic studies have been reported. Among those, we shall recall synchrotron radiation powder XRD analyses on TS-1 [19,20], a synchrotron radiation single crystal XRD study on Fe-silicalite [21] and powder neutron diffraction studies on TS-1 [18,22,23]. Both theoretical [17,18] and experimental [18-23] investigations give rather contradictory results on the location of the heteroatoms. It is evident that an accurate crystallographic study of the reference material, i.e. pure and defect-free silicalite, could be of great help in validating the faint experimental evidences used by the different groups to support the attributions made in their papers. Since it is well known that template burning, even under mild conditions, causes

1893 a partial migration of the heteroatoms from the framework into extra-framework positions, the single crystal diffraction studies have been mainly carried out on the as synthesized crystals, containing the tetrapropylammonium (TPA) template. The accurate location of the highly disordered TPA in silicalite yields thus an important point of reference for the other single crystal studies. This is particularly true for our single crystal study of Fe-silicalite [21], where the location of Fem at sites T9 and T10 was inferred on the basis of: (i) an increase of the average T-O distance; (ii) the presence of two peaks (2.28 and 0.67 e/]k3) in the electron density map near T9 and T10; (iii) the higher anisotropy of the thermal displacement parameters observed for T9 and T10, which was attributed to the disorder generated by iron insertion. At a recent congress Aubert et al. [24] presented a poster illustrating a single crystal study of silicalite and their results are very close to those described in this paper. We wish to thank the authors for the stimulating discussion, the helpful exchange of ideas and the comparison of our results. 2.

EXPERIMENTAL

2.1. Sample preparation and methods Silicalite and B-silicalite samples exhibiting crystals of comparatively large dimensions (-20x30x100 ~tm3) were synthesized in fluoride medium [7]. The crystals are twins, but the twin law could be easily found from the exceptions to the systematic absences expected for the P n m a space group. The twinning is due to a 4 twin axis along the [0 0 1] direction [25,26]. Our single crystal diffraction measurements were carried out at room temperature, on silicalite and B-silicalite [27], at the Materials Science beamline I D l l of the European Synchrotron Radiation Facility (ESRF), using a Bruker CCD detector. Of the latter derivative data were collected on two crystals: an as synthesized one and a crystal which had been heated at 483 K for 3 h in an ozone/oxygen flux [28], in an attempt at removing the template without damaging the crystal. For comparison also powder data were collected on the as made silicalite sample at the Powder Diffraction beamline BM16 of ESRF [29], employing a 9 channel detector. The cell dimensions and the reduction of the single crystal data were carried out using the programs SMART [30] and SAINT [31]. In order to account for a slight spot splitting at high angles, due to the twinning, the data reduction was performed using a comparatively large integration box. The most relevant crystal data are reported in Table 1. 2.2. Structure refinements Using as starting model the structure of ZSM-5 determined by Van Koningsveld et al. [32], the refinement by full matrix least squares and difference Fourier recycling was carried out using the SXELX97 program [33], which is capable of handling data from twinned crystals. All framework atoms were refined with anisotropic displacement parameters. As expected, no direct indication of the location of the boron substituent in B-silicalite was obtained. The location of the disordered TPA § template, in all the crystals, was performed by several difference Fourier cycles and the refinement of the template atoms was carried out using geometrical restraints on bond distances and angles and isotropic displacement parameters. The occupancies of the disordered atoms were found by refining the population parameters, while keeping the thermal displacement parameters fixed. The crystal of B-silicalite heated in ozone gave diffraction data as good as those of the as synthesized crystal, but the ref'mement

1894 indicated the presence of TPA +, although with a slightly lower population. This shows that template burning under such mild conditions does not damage the crystals, but is far from being complete. For this reason we will only describe the results of the refinement of the as synthesized crystal. The Rietveld refinement of the microcrystalline silicalite sample was carried out by means of the GSAS software [34], employing as starting model the structural resuk of the silicalite single crystal refinement. Table

1 -

Summary of crystal data for silicalite and B-silicalite.

Compound a b c V Density (calc)

Silicalite 20.042(3)/~ 19.990(3) 13.414(2) 5374(1) A 3

Observed reflections Data / restraints / parameters Goodness-of-fit on IFol2 R indices [1>2 t~ (I)] R indices (all data) Largest diff. peak and hole, Weight (calculated)

3389 [I>2c(I)] 4462 [I>2o'(I)] 6348 / 36 / 410 6419 / 36 / 410 0.888 0.926 R1 = 0.0589, wR2 = 0.1452 R1 =0.0426, wR2 =0.1118 R1 = 0.1093, wR2 = 0.1673 R1 = 0.0625, wR2 = 0.1178 0.644, -0.698 e ]k-3 0.585, -0.655 e A 3 w = 1/[ cy2(lFo]2)+(P) 2] w h e r e P = (lEo]2 + 21Fc]2)13

2.036 g c m -3

B-Silicalite 19.968(3)/~ 19.955(3) 13.372(2) 5328(1) A 3 2.054 g c m -3

3. RESULTS AND DISCUSSION The location of the disordered template is not straightforward, but must be done with great accuracy in order to be sure that the remaining significant electron density peaks can be correctly interpreted. The high quality of the single-crystal data obtained at beamline ID11 of ESRF [27] allowed a good elucidation of the TPA structure, which is consistent with that found in ZSM-5 and in Fe-silicafite. The template molecules are located at the intersections of the straight and the sinusoidal channels. The N atom lies on a mirror plane and two distinct disordered images of the tetrahedral arrangement of the propyl arms are obtained (Figure 1). The presence in pure silicalite of a residual electron density peak, located near sites T9 and T10 and only slightly smaller than that found in Fe-silicalite [21], disagrees with our previous interpretation of this peak as due the to the alkaline counterion. Indeed, a recent extended NMR analysis [35] has unambiguously proven that at site T9 a live-coordinate SiO4/2F group is present in silicalite crystals prepared in fluoride media. The residual electron density peak should therefore be interpreted as due to the fluoride anion linked to the T9 site located in a four ring, where F- may stabilize the strained Si-O-Si bonds. The formation of a fivecoordinate group also induces the observed lengthening of the T9-O distances. In any case, this new interpretation is still consistent with our original indication that T9 and T10 (also in the four ring) might be the preferred substitution sites for the heteroatom.

1895

CI3E) IB') C(2B') C(3AI

C(2D) C(1A)

C(3DI

C(1E)

CriB') C(1C')

C(2A) C(1D) Gill

CI2F') 8130')

CI1F')

CriB)

0(20'1

(2E') C(3E'I

C(3F')

C(3B)

Figure l - Plot of the two disordered propyl arms of TPA in silicalite (primed atoms are generated by the space group symmetry)

1= Silicalite; 2 = Z S M 5 1.62

---.4

.-., 1.61

. . . .

-9- - -

----1

ot,-- 1.60

',

,-',

0

1.59

4=KBsilicalite

I I

o
) Cu spin density to be between 0.35 and 0.45. Moreover, it shows that a bi-coordinated Cu(II) at site III has a too small Adip value to fit with any observed EPR signal. This study also shows that the Adip values vary strongly with the model structures and underlines the necessity to know the exact structure of the Cu(II) site. Despite this difficulty, useful information can be obtained from the comparison of the experimental and theoretical results. When Cu(II) is in ideal positions, both sites II and III can be assigned to experimental signals, depending on the cluster geometries. Among the structures, those cut from a simulated solid lead to the best agreement with experiment. The model iCuYB is also a possible candidate. The results of this study indicate that both site II and site III are possible sites for Cu(II) location, with larger hyperfine coupling for site II models. The estimate of the ~ experimental >>spin density confirms that the unpaired electron is strongly delocalized on the zeolite. This result corroborates the charge tranfer effect calculated for Cu(II)-Y model [25]. 4. CONCLUSION The calculated anisotropic hyperfine coupling constants of cluster models representing sites II and III of Cu(II)-Y have shown the existence of a linear relationship with the copper spin density, for both sites II and III, independently on the calculation method. The

1906 comparison of the calculated and experiment values lead us to consider four-coordinated Cu(II) at these sites as paramagnetic species leading to the observed EPR signals. REFERENCES

1. Kieger, S.; Delahay, G.; Coq, B.; Neveu, B. J. of Catalysis 1999, 183,267. 2. Kieger, S.; Delahay, G.; Coq, B. Appl. Catal. B 2000, 25, 1. 3. Liu, S.-B.; Lin, T.-S.; T-C., Y.; Chen, T.-H.; Hong, E.-C.; Ryoo, R. J. Phys. Chem. 1995, 99, 8277. 4. Schoonheydt, R. A. CataL Rev.-Sci. Eng. 1993, 35, 129. 5. Conesa, J. C.; Sofia, J. J. Chem. Soc. Faraday Trans. 1979, 75,406. 6. Turkevitch, J.; Ono, Y.; Sofia, J. Jr. Catal. 1972, 25, 44. 7. Ichikawa, T.; Kevan, L. J. Phys. Chem. 1983, 87, 4433. 8. Yu, J.-S.; Kevan, L. J. Phys. Chem. 1990, 94, 7612. 9. Casida, M. E.; Daul, C.; Goursot, A.; Koester, A.; Pettersson, L.; Proynov, E.; St-Amant, A.; Salahub, D. R.; Duarte, H.; Godbout, N.; Guan, J.; Jamorski, C.; Leboeuf, M.; Malkin, V.; Malkina, O.; Sim, F.; Vela, A. deMon,KS3, 1996 10. Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; J. A. Montgomery, J.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; G. Liu; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; A1-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; C. Gonzalez; Head-Gordon, M.; E. S. Replogle, a. J. A. P. Gaussian98,Revision A.9, Gaussian, lnc., Pittsburgh PA: 1998 11. Vosko, S. H.; Wilk, L.; Nusair, M. Can. J. Phys. 1980, 58, 1200. 12. Perdew, J. P.; Chevary, J. A.; Vosko, S. H.; Jackson, K. A.; Pederson, M. R.; Singh, D.; Fiolhais, C. Phys. Rev. B 1992, 46, 6671. 13. Becke, A. D. Phys. Rev. A 1988, 38, 3098. 14. Perdew, J. P. Phys. Rev. B 1986, 33, 8822. 15. Lee, C.; Yan, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. 16. Becke, A. D. Jr. Chem. Phys. 1993, 98, 5648. 17. Cerius2. Molecular modeling software for materials research, Accelerys, Biosym Technologies, San Diego USA: 1993 18. Wachters, A. J. H. J. Chem. Phys. 1970, 52, 1033. 19. Munzarova, M.; Kaupp, M. J. Phys. Chem. A 1999, 103, 9966. 20. Pierloot, K.; Delabie, A.; Groothaert, M. H.; Schoonheydt, R. A. PCCP 2001, 3, 2174. 21. Maxwell, I. E.; de Boer, J. J. J. Phys. Chem. 1975, 79, 1874. 22. Carl, P. J.; Larsen, S. C. J. Phys. Chem. B 2000, 104, 6568. 23. Levi, Z.; Matar, K.; Raitsimring, A. M.; Goldfarb, D. Pure and AppL Chem. 1992, 64, 799. 24. Nacache, C.; Ben Taarit, Y. Chem. Phys. Letters 1971, 11, 11. 25. Berthomieu, D.; Krishnamurty, S.; Coq, B.; Delahay, G.; Goursot, A. J. Phys. Chem. B 2001, 105, 1149.


M o l e c u l a r M o d e l i n g : a c o m p l e m e n t to e x p e r i m e n t materials u s e d in separation t e c h n o l o g i e s b y a d s o r p t i o n

1907

for

designing

porous

S. Girard, b C. Mellot-Draznieks, a G. F6rey, a and P. Pullumbi b Institut Lavoisier (IREM), UMR CNRS 8637, Universit6 de Versailles, 45 avenue des EtatsUnis, 78035 Versailles cedex, France

a

b Air Liquide, Centre de Recherche Claude-Delorme, B. P. 126, Les-Loges-en-Josas, 78354, Jouy-en-Josas Cedex, France*

In studies of complex molecular systems like zeolites or other inorganic solids, experimental studies have preceded by far the theoretical ones. During the last decade, computational chemistry has had a favorable impact almost in all branches of crystalline microporous research ranging from phase determination to structural characterization and property prediction. An important effort has been focused on developing simulation tools to describe thermodynamic and transport properties of confined fluids in the micropores together with a realistic representation of the catalytic phenomena. In this work we show how molecular modeling coupled to experiment can be used to select/design novel adsorbent materials with better separation properties for given gas mixtures. We illustrate how various computational approaches may be combined to cover major aspects of inorganic materials: i) computational design of new inorganic structures, using the AASBU method (Automated Assembly of Secondary Building Units) with the generation of virtual libraries of crystal structures; ii) lattice energies minimizations in order to anticipate stabilities of hypothetical crystal structures built of pre-defined SBUs; iii) anticipation of calcined crystal structures upon template extraction from existing as-synthesized structures; iv) finally, Grand Canonical Monte Carlo (GCMC) calculations used to correlate microscopic features of the adsorbent/sorbate system to macroscopic properties of interest, such as adsorption isotherms and isosteric heats used in engineering process optimizations.

1. INTRODUCTION Air separation for oxygen production is an important operation in chemical processing industry as well as in energy conversion applications. Classical cryogenic air separation is gradually giving way to new technologies involving either Pressure Swing Adsorption (PSA), Vacuum Swing Adsorption (VSA) or membrane permeation. PSA and VSA technologies have become increasingly competitive and are already adopted for accomplishing small-tomedium scale oxygen production for a variety of applications in steel industry, enhanced combustion, soil and wastewater cleanup. In addition to the operating parameters of the

1908 process itself, one of the important factors influencing the performance of the production unit is the adsorption properties of the adsorbent material. The structure of the adsorbent strongly determines the shape of adsorption isotherms and has a direct impact on the capital and power consumption costs, which are the main components of the overall costs of the produced gases. For this reason the understanding of the dependence of adsorption isotherms upon the microscopic features of the adsorbent is of prime importance. The research for new adsorbents with improved selectivity and stability having more favorable geometries has been the main factor of cost reduction in separation technologies by adsorption during the last decades. The introduction of a new adsorbent material needs the re-design of the process in order to determine optimal process cycles. From this point of view the approach in which molecular modeling is used prior to experiment for generating and pre-selecting novel "virtual" adsorbent structures based on estimates of their respective adsorption properties, followed by process optimization simulations, rather than relying solely on costly experimental programs involving synthesis, structural characterization and performance assessment, can effectively reduce the cost of the research for novel adsorption-based separation processes. Three-dimensional (3D) frameworks linked by T-O-T bonds, where T is an atom in tetrahedral coordination (Si, A1, B, Ga, P, etc.) represent interesting alternatives to experimentally know ones. Molecular simulation of silicates and substituted silicate frameworks ("tectosilicates") has become amply adequate to examine possible structures of potential interest [ 1]. In this work, the generation of virtual libraries of inorganic structures using pre-defined secondary building units (SBUs) has been carried out by using the AASBU method (Automated Assembly of Secondary Building Units) [2]. Its applicability to solidstate inorganic chemistry structure prediction has been demonstrated using simple building units from known families of inorganic structures [2-4]. Refining of virtual crystal structures by minimization methods is now a routine matter, limited only by the availability of appropriate interatomic potentials. Calculations were carried out using the GULP package (General Utility Lattice Program) [5]. Unfortunately, non-empirical calculations are currently computationally too demanding to be used routinely for lattice minimizations, especially in the case of low-symmetry solids with large unit cells. Relative stabilities of minimized structures showing a clear stability to density trade-off relationship are given. One of the most powerful concepts in the synthesis of complex solids is the "template" or Structure-Directing Agent (SDA) approach. An organic molecule is used to imprint certain structural features to the solid under construction. The choice of the SDA is crucial for the synthesis of targeted open-framework architectures. It is incorporated in the final structure, compensating charges and tailoring the size and shape of micropores. The non-covalent interactions heavily control the resulting structure as in the case of the mesoporous materials which are formed in a cooperative assembly process from organic surfactants and inorganic building blocks that crosslink during synthesis [6]. The removal of the template is often a critical step and the production of the related stable open-framework structure cannot be easily anticipated. In this work we will outline the computational methodology that has very recently [7-9] been adopted in order to investigate the thermal stability upon calcination treatment anticipating the crystal structure of the related open-framework template-free material. The next step is the prediction of adsorption isotherms, heats of adsorption and Henry constants for the predicted structure in

1909 order to evaluate its performance prior to synthesis and template removal. Monte Carlo simulations are appropriate for correlating microscopic features of the adsorbent/sorbate system to macroscopic properties of interest to process engineers. Adsorption isotherms and isosteric heats of adsorption are predicted via calculations carried out in the Grand Canonical (GC) statistical ensemble in which the chemical potential, the volume and the temperature are kept fixed. For Henry's constants the simulations are performed in the Canonical (C) ensemble where the number of adsorbed molecules, the volume and the temperature are held fixed. It is to notice that the outlined methodology illustrated with different examples is not exhaustive in its search of new structures and has not been fully validated. Its interest is, however, related to its potential to rationally focus an experimental program by minimizing the range of exploring trails. 2 T H E O R Y AND C O M P U T A T I O N A L DETAILS 2.1. Predicting new inorganic structures : the A A S B U method The first step of the AASBU method is the choice of the Secondary Building Unit. In this study, the sodalite or 13-cage (truncated octahedron 4668) was extracted from the experimental sodalite structure [ 10] as well as the double-4-ring (D4R) building unit, which was extracted from the experimental cloverite [10]. Both units are shown in Figure 1. The ligand or coordinating atoms (L) are considered as linkage points in all the following steps. The interatomic potentials define the rules that control the possible assembly of the SBUs, treated as rigid bodies during the simulation, with the inter-SBU interactions parameterized on an atom-atom basis by a Lennard-Jones expression for the energy of interaction between pairs of atoms i and j:

go ~_ ~.ij [ ( f i~ /,.i;1 )12 _ 2 ( ri] F/;1 )61

(1)

The parameterized force field has a highly attractive potential well, with a minimum at a very short Li...Lj separation distance in order to glue together the SBUs at the linkage points during subsequent simulation steps. A repulsive potential between Mi...Mj pairs prevents SBUs from overlapping each other together with a small attractive potential between Li...Mj pairs used to discourage undesirable local minima corresponding to proximate but unconnected SBUs. The repulsive potential between Li...Bj pairs prevents a SBU to link to an

Figure 1. The sodalite and D4R cages. Metal atoms (M) are shown in gray, bridging atoms (B) are shown in white cylinders and ligand atoms (L) are shown in black balls. Only ligand atoms are allowed to link to other ligand atoms of a different cage.

1910 atom, which is already completely connected inside its own SBU. The force field parameters have been reported recently in the literature [2,3] together with the definition of cost function or "energy" for a given configuration of SBUs in a unit-cell. It is calculated as the sum over all SBUs in the Unit cell of the Lennard-Jones terms involving Li...Lj, Mi...Mj, Li...Mj and Li...Bj pairs, with atoms i and j belonging to two different cages: (2)

E total -" Z (E L..L + EM ...M + E L..M + E L..B ) SBUs

The magnitude of the cost function provides an estimate of the degree of connectivity of a given arrangement of SBUs. Starting with the selected SBU and applying the force field and going through the steps defined in [2,3] virtual periodic structures are generated.

2.2. Lattice energy minimization and structure prediction Energy minimizations are carried out in order to anticipate both the stability of the generated virtual structures and the crystal structure of the calcined materials starting from the knowledge of the as-synthesized structure, using the simple ionic shell model, developed by Gale and Henson for A1POs [11] and the lattice energy minimization code GULP [4]. The robustness of their formal charge shell model forcefield has been demonstrated through the accurate reproduction of experimentally determined structures of A1POs [12] together with estimations of their relative framework stabilities that are consistent with thermodynamic data. We have recently extended this type of approach to the realm of gallophosphates [ 13,14]. The total lattice energy of a structure is given by: Es,r --" ~

( qiqj k ( rc . . . . . hell) 2 + ~ { A ij e x p ( - r ij ) - C ijrij - 6 -+i i ~ j [. IOij r'ij

-Jr- ~ Kijk(Oijk -- 00) 2 i* j:xk

(3)

where qi and qj refer to the charges of the ions, and A O, Pij and C 0 are short-range potential parameters defined in [5]. 0o is the equilibrium tetrahedral bond angle, rij denotes the interatomic distance. The first sum in (3) counts the self-energy of polarizable ions arising from their deformation using the shell model defined in [ 15] in which an ion is represented by a core and a shell coupled by a harmonic spring; the second sum includes pair interactions (Coulombic term together with a short range 6-exp Buckingham potential). The third sum consists of three-body angle-bending terms. Coulombic energy is calculated using the Ewald summation; Buckingham potentials were summed over all interatomic pairs; three-body potentials were calculated only between strong bonds.

2.3. Monte Carlo Simulations to predict cations configuration and adsorption properties The simulation of adsorption properties of microporous adsorbents using GCMC calculations needs as input the structural model of the adsorbent and the force field describing sorbate/sorbent and sorbate/sorbate interactions. In the case of electrically non neutral frameworks, insertion of the required complementary cations may be carried out via a Monte Carlo packing procedure followed by structure optimization [ 16], or even via (N, V,T) Monte Carlo simulations [17]. In the Monte Carlo packing procedure only short-range non-bonded terms are used to gauge the viability of each new cation position introduced.

1911 Regarding the simulation of adsorption properties in zeolites, the interaction potential is the most important ingredient. We have adopted a simplified interaction potential [ 18] including only a dispersive-repulsive short-range potential, represented by a Lennard Jones 6-12 potential combined with electrostatic interactions between partial charges on the adsorbent and guest atoms, or more generally, multipole-multipole interactions according to:

i [Ai

9

j

rij 12

"i qiq t

rij 6

(4)

rij

where Aij is the repulsion constant and Bij the dispersion constant and qi the point partial charges located at the atomic positions of the adsorbent and sorbate molecules. The LJ potential parameters for the adsorbate-zeolite and adsorbate-adsorbate interactions as well as the magnitude of the partial charges localized at the atomic positions of the adsorbent/sorbate atoms has been detailed in [ 19]. Simulations were performed using Cerius 2 suite of softwares. [2O] 3. RESULTS AND DISCUSSION

The AASBU method has been systematically used for screening over the first 70 (resp. over 20) space groups using one sodalite (resp. one D4R) cage per asymmetric-unit for each calculation. The results obtained have recently been reported in the literature [3] and apparently new structures have been discerned. Several known structures have been obtained: for example, sodalite (SOD) was obtained using the sodalite cage as SBU, ACO and AFY were obtained using D4R cage, while LTA structure was obtained either with sodalite or D4R SBUs. The final symmetry was found to be Im-3m for SOD and ACO, P-3 lm for AFY and Pm-3m for LTA structure, in agreement with the symmetry given in [10]. Simulations in progress are being carried out with the sodalite cage to generate FAU and EMT topologies. Table 1. Predicted parameters for hypothetical models generated using the sodalite (M1-M5) or D4R (T1-T5) cages as a SBU.

Model M1 M2 M3 M4 M5 T1 T2 T3 T4 T5

Space group in

Unit cell parameters

Final space

calculation

a/~,

b/~

c/A

13/o

group

C2/c

Ama2

21.55 12.54 12.80 12.79 12.80

12.28 12.54 12.80 12.79 12.80

20.25 60.92 20.07 10.05 40.17

90.96 90.00 90.00 90.00 90.00

R-3m P63/mmc P-3ml P63/mmc

Cm P -1 P na21 P -1 P na21

6.69 9.30 17.71 19.69 12.67

13.99 14.20 10.05 9.93 19.56

28.21 7.17 8.19 11.25 8.15

90.00 90.00 90.00 114.74 90.00

C2 Pm P2/c

C2/m

Fmmm Cmmm Pnma C2/m

Pna21

1912 It is to notice that not only the space groups but also other predicted parameters (unit cell parameters, atomic coordinates) coincide well with experimentally observed ones. Space groups and cell parameters of some of the unknown generated structures using the sodalite (resp. D4R) cage as a SBU are reported as models M1-M5 (resp. T1-T5) in Table 1. It is to notice that the connection of the SBU cages produces known [21 ] or new cages in each of the structures. In order to evaluate the relative stabilities of the all-silica frameworks their lattice energies per tetrahedral unit are compared to the energy of the SiO2 quartz. Accordingly with what has been reported for known silicas [22], the calculated lattice energies of the M1-M5 structures correlate well with their densities. An as-synthesized structure usually contains templating agents, water molecules, bridging hydroxy groups as well as fluorine atoms which are included in the framework. The latter species lead to an increase of the coordination number of A1 atoms from 4 to 5 or 6. In the following we briefly describe how one can computationally proceed to the prediction of the calcined form. The method consists in removing from the experimentally-defined assynthesized structure all the species known to be removed upon calcination, thus generating a highly distorted initial structure. Upon constant pressure lattice energy minimization in the space group of the original structure, the distorted framework converges into a zeolitic topology. Its energy is then compared to that of existing structures of the same class to evaluate its relative stability. To illustrate this approach, we report here the prediction of the calcined form of the as-synthesized aluminophosphate MIL-34, [A14(POn)4OH'CaH10N].Once the crystal structure of the as-synthesized MIL-34 was determined, the calcined template-free one, namely A1PO4, could be anticipated using appropriate interatomic potentials [8], before it was obtained experimentally. The predicted open-framework compound showed an unknown zeolitic topology. The reliability of the simulation allowed the Rietveld refinement of the powder pattern of the calcined phase, validating this approach (Fig. 3). This simulation method has also been successfully applied to a series of gallophosphates [ 14].

Figure 3. a) Predicted structure of calcined MIL-34 by lattice energy minimization along [ 110]. b) Comparison between experimental (up) and simulated (down) XRD powder pattern

1913 The removal of the SDA without breaking down the framework is not always an easy task because the stability of the as-synthesized structure often depends on the stabilizing templateframework interactions as well as on the experimental procedure employed during calcinations. For these reasons it is sometimes very helpful to estimate the adsorption properties of the final material prior to its calcination. Here we report GCMC simulations to predict N2 and 02 adsorption in Lithium and Sodium Low Silica X-zeolite (LSX) but the approach is quite general and could be applied to any kind of crystalline porous adsorbent. Typical runs of 2* 10 6 Monte Carlo steps from which the first 200000 are used for equilibration and not included in the averaging are sufficient to sample configuration space. The predicted adsorption isotherms of single component simulations in NaLSX and LiLSX are reported in Figure 4 and compared to their experimental counterparts. The general trends are well reproduced with a systematic tendency to overestimate the adsorption of nitrogen for low loadings and underestimate that of oxygen. On the contrary the N2 loading is underestimated at higher pressures. This indicates that the balance between van der Waals and Coulombic interactions could still be improved by slightly modifying the adopted point charges at atomic positions. It is possible in principle, to improve the description of sorbent-sorbate interactions by using sophisticated expressions of the potential functions with higher order dispersion and induction terms recently discussed in ref. [23] but it still requires the calculation of the electrostatic field inside the zeolite, which in its turn depends on the choice of the point charges assigned to the atoms. It seems [24] that parameterization of polarization contribution to the potential energy by choosing partial charges cannot be carried out without resorting to a fitting procedure of some type.

40.00 l D "c" 30.00 -~ 0

z m 20.00

o,,, /,

C

O10"007/

g"

I~ ="

D' ' ~

" "~

--

LiLSX_O2_C NaLSX_N2_C

o

NaLSX_O2_C

0.00 ~ 1.00

LiLSX_N2_E .. LiLSX_O2_E

------- NaLSX_O2_E

o 0.50

a O

NaLSX_N2_E

/% A

0.00

LiLSX_N2_C

----O

DI I

[]

1.50

Pressure (bar)

Figure 4. Predicted (C) and experimental (E) single component isotherms for nitrogen and oxygen in NaLSX and LiLSX zeolites.

1914

4. C O N C L U S I O N The rate of scientific discovery of novel superior adsorbents could be significantly accelerated through a judicious combination of experiment with computation strategies. The successful d e v e l o p m e n t of novel materials lies in their rational design and can be achieved through an understanding of fundamental interactions at the molecular level. W e have outlined a multifaceted modelling approach that although far from being complete can help in focussing the experimental effort in the search of new adsorbents. In the particular field of the separation of N2 and O2 in zeolites, the molecular simulation approach enhances the fundamental understanding of the basic microscopic p h e n o m e n a and is appropriate for establishing correlations between the microscopic features of the sorbent/sorbate systems and their macroscopic properties such as isotherms and adsorption heats. It can be successfully used to c o m p l e m e n t experimental studies.

REFERENCES 1.

M.B. Boisen Jr, G. V. Gibbs, M. O'Keefe and K. L. Bartelmehs, Microporous Mesoporous Materials, 29

2.

C. Mellot Draznieks, J. M. Newsam, A. M. Gorman, C. M. Freeman and G. F6rey, Angew. Chem. Int. Ed.,

(1999) 219-266. 39 (2000) 2270-2275. 3.

S. Girard, P. Pullumbi, C. Mellot-Draznieks, G. F6rey, Stud. Surf. Sci. Catal., 135 (2001) 254.

4.

C. Mellot-Draznieks, S. Girard, G. F6rey, J. C. Sch6n, Z. Cancarevic, M. Jansen, Chem., Eur. J. (in press).

5.

J.D. Gale, J. Chem. Soc., Faraday Trans., 93 (1997) 629.

6.

X.S. Zhao, G. Q. Lu and G. J. Millar, Industrial Engineering Chemical Research, 35 (1996) 2075-2090.

7.

S. Girard, C. Mellot-Draznieks, J. D. Gale and G. F6rey, Chem. Commun., (2000) 1161-62.

8.

T. Loiseau, C. Mellot-Draznieks, C. Sassoye, S. Girard, N. Guillou, C. Huguenard, F. Taulelle and G. F6rey, J. Am. Chem. Soc., 123 (2001) 9642-9651.

9.

S. Girard, A. Tuel, C. Mellot-Draznieks and G. F6rey, Angew. Chem. (in press).

10. W.M. Meier, D.H. Olson, Ch. Baerlocher, Atlas of Zeolite Structure Types, Elsevier, London, (1996). 11. J.D. Gale and N. J. Henson, J. Chem. Soc. Faraday Trans., 90 (1994) 3175-3179. 12. N.J. Henson, A. K. Cheetham and J. D. Gale, Chem. Mater., 8 (1996) 664-670. 13. S. Girard, J. D. Gale, C. Mellot-Draznieks, G. F6rey, Chem. Mater. 13 (2001), 1732. 14. S. Girard, J. D. Gale, C. Mellot-Draznieks and G. F6rey, J. Am. Chem. Soc., 124 (2002) 1040-1051. 15. B.G. Dick and A. W. Overhauser, Physical Review, 112 (1958) 90-103. 16. A.M. Gorman, C. M. Freeman, C. M. K61mel and J. M. Newsam, Faraday Discuss., 106 (1997) 489-494. 17. C. Mellot-Draznieks, S. Buttefey, A. Boutin, A. H. Fuchs, Chem. Commun. (2001) 2200-2201. S. Buttefey, A. Boutin, C. Mellot-Draznieks, A. H. Fuchs, J. Phys. Chem. B, 105 (2001) 9569-9575. 18. K. Watanabe, N. Austin and M. R. Stapleton, Molecular Simulation, 15 (1995) 197-221. 19. J. Ligni~res and P. Pullumbi, Fundamentals of adsorption, 6 (1998) 719-725. 20. Cerius2, Accelrys Inc. San Diego. 21. J.V. Smith, Chem. Rev., 88 (1988) 149-182. 22. N. Henson, A. K. Cheetham and J. D. Gale, Chem. Mater., 6 (1994) 1647-1650. 23. A.H. Fuchs and A. K. Cheetham, J. Phys. Chem. B, 105 (2001) 7375-7383. 24. C.J. Jameson, H.M. Lim, A.K. Jameson, Solid State NMR, 9 (1997), 277-301.


NMR-

1915

crystallographic studies o f almninophosphate AIPO4-40

C. M. Morais a'b, C. Fernandez a, V. Montouillout a, F. Taulelle c and J. Rocha b a Laboratoire Catalyse et Spectrochimie, CNRS UMR 6506, ISMRA, Universit6 de Caen Basse-Normandie, 14050 Caen, France b Department of Chemistry, University of Aveiro, 3810-193 Aveiro, Portugal CRMN et Chimie du Solide, CNRS FRE 2423, Tectonique Mol6culaire du Solide, University Louis Pasteur, 67070 Strasbourg, France

1. INTRODUCTION In solid-state NMR of inorganic materials, double-resonance (or ~H triple-resonance) experiments often involve a half-integer quadrupolar nucleus that has the disadvantage of exhibiting second-order anisotropic broadening. However, the introduction of twodimensional multiple-quantum magic-angle spinning (MQMAS) has been the source of a tremendous improvement in solid-state NMR of half-integer quadrupolar nuclei. By correlating triple- (or higher) quantum with observable single-quantum coherences Frydman et al. demonstrated that it is possible to remove the second-order quadrupolar interaction leading to high-resolution spectra [ 1]. Taking advantage of the highly resolved dimension of the experiment, double resonance schemes such as CP [2], REDOR [3] or HETCOR [4,5] have been proposed to obtain a better insight into the connectivity of solids. Thus, MQMAS has become an important part of new pulse sequences in solid-state NMR of quadrupolar nuclei. It is also becoming widely used to solve structural problems in applied materials. More recently, Delevoye et al. [6] have demonstrated that, in aluminophosphates, the resolution in the spin 89 dimension may be improved by decoupling the quadrupolar nucleus, while acquiring the spin 89free induction decay. Microporous aluminophosphate AIPO4-40 was initially thought to possess the structure of silico-aluminophosphate SAPO-40 [7]. However, because the latter contains four distinct 31p sites and the 31p MAS NMR spectrum of AlPO4--40 exhibits more than four resonances [8] the two materials cannot have exactly the same structure. Indeed, a subsequent Rietveld refinement of powder X-ray diffraction data found that although the framework topology of both materials is the same (AFR) the AIPO4-40 symmetry is lower (space group Pc2~n) [9]. The reason for the lower symmetry is the presence of an ordered arrangement of hydroxyl groups bridging between framework A1 atoms across a fourmembered ring. As a result, and according to the proposed structure, there are eight 31p and eight 27A1 (two five-coordinated and six four-coordinated) atoms in the asymmetric unit. In the present paper we show that this recent structure refinement does not fully agree with the NMR evidence.

1916 2. EXPERIMENTAL SECTION All NMR experiments were performed on a Bruker Avance 400 (9.4T wide bore magnet) spectrometer operating at 104.3 MHz and 162.1 MHz for 27A1 and 31p nuclei, respectively. 4 mm double-resonance and triple-resonance probes were used, employing MAS rates of 10 to 12.5 kHz. 1 kW high-power linear amplifier was used for 27A1 multiple-quantum excitation and decoupling, whereas a 500 W linear amplifier enabled 31p excitation. The 4 mm double-bearing triple-channel Bruker probe allowed for high-power decoupling radio-frequency (RF) fields beyond 60 kHz on the 27A1 channel. Bruker true band-pass filters with more than 90 dB of rejection on 27A1 and 31p frequencies and with less than 0.5 dB of insertion loss were used on the respective channels. Pure absorption mode 3Q- and 5Q-MAS NMR spectra were obtained by using the Z-filter sequence [10] and hypercomplex acquisition. The 27Al_31p 3Q-HETCOR spectrum was recorded as described in [5]. Except for the 5Q-MAS NMR spectrum, the acquisition of the twodimensional data sets was performed in a rotor-synchronized fashion by advancing the evolution time tl in increments equal to the rotor period. The relevant experimental parameters are given in the figure captions. All 31p and 27A1 chemical shifts are plotted in ppm relatively to aqueous 85% H3PO4 and Al(n20)63+, respectively. Drs J. Lourengo and M. F. Ribeiro, Instituto Superior Tecnico, Lisbon, Portugal kindly provided the A1PO4-40 sample that is the same sample as studied in ref. [8]. 3. RESULTS AND DISCUSSION

3.1.31p NMR spectra In a previous report, Rocha et al. have shown that the deconvolution of the non-

decoupled 31p spectrum of AIPO4-40 requires the presence of at least nine lines with intensity ratios not very different from one [8]. However, this analysis was no trivial due to the broadening of the lines that often affects the phosphorous spectra of aluminophosphates. We have recently shown that 27A1 decoupling may be used to remove residual J-coupling interactions between 31p and 27A1 in microporous aluminophosphates [6]. Figure la and lb show the A1PO4-40 31p MAS NMR spectra recorded, respectively, without and with 27A1 decoupling during the acquisition. The latter is a highly resolved spectrum whose deconvolution requires at least sixteen peaks (Figure l c). The chemical shifts and relative intensities obtained from this deconvolution are collected in table 1. 3.2. 27A! NMR spectra As the conventional 27A1MAS spectrum of the AIPO4-40 is not resolved, it is necessary to use MQMAS. The 27A1 3Q and 5QMAS NMR spectra reveal the presence of at least fifteen resonances (Fig 2a and b). The MQMAS method not only produces highly resolved 27A1 NMP, spectra, making easy the determination of the aluminum coordination, but also allows the extraction of additional parameters such as the quadrupolar coupling constants, the isotropic chemical shifts, and the access to the distribution of these parameters.

1917 Table 1. 31p Chemical shifts and relative intensities resulting from the deconvolution of the AIPO4-40 31p MAS NMR spectrum (see Figure 1c). Peak

8 31p (ppm) -13.7 -14.4 -15.3 -16.2 -17.1 -18.0 -18.8 -20.2 -20.8 -21.2 -22.5 -24.8 -26.5 -28.4 -29.6 -30.0

1

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16

13]p (%) 3.7 5.2 8.5 2.6 2.3 7.9 2.8 2.8 3.2 5.8 5.6 12.5 11.9 19.4 3.6 2.3

a)

b)

,,",,,.,,,, ..4

-

.,.

u

i' " ' ,,,','

0

,,,,: ,, .,,

t

,

.,

I

. p ~ . S !

.

.

.

.

%q., .

.

.

.

1 0 " " ' ~1'5"'" ~20

i

-25

.

.

.

.

.

.

.

3'0

.

I

,

'

ppm

Figure 1. alp MAS NMR spectra of AIPO4-40 recorded with (a) no decoupling, low-power decoupfing; (c) simulated 2 7 Al-decoupled spectrum. A 2.5 l~S (30 ~ pulse was used. Decoupling RF field was experimentally optimized to 12 kHz. were acquired with 16 transients. The recvclint~ delay was 80 s and the M_AS rate

(b) with 27A1 31p excitation Both spectra was 10 kHz.

1918

ppm

F/

30 40

60 50

40

,

pprn

.

.

.

.

20-

30

,

.

20

,

.

.

.

10

,

.

.

0

,

,

- 1~0

.

ssb i ~~ ~iE

30-

~0~,. i

............................

F/ 40-

g~ ....

50I

'1

I

I

I'

50

40

30

20

10

plxn

F2 Figure 2 .

27A13Q

(a) and 5Q (b) MAS N M R spectra of aluminophosphate A1PO4-40.

1919 MQMAS sequence strongly depends on the quadrupolar coupling. Thus, MQMAS is not directly quantitative. However, the quantification of aluminum atoms can take full benefit of the high-resolution of the MQMAS spectra when the two-dimensional spectra are analyzed simultaneously with conventional MAS NMR spectra. We have developed a simulation program (MASAI) that allows iterative fitting of both the MAS and twodimensional MQMAS spectra, taking into account the distribution of interaction parameters [ 11,12]. The results of the best fitting of both MAS and MQMAS spectra are reported in Table 2. Relative intensities, quadrupolar coupling constants and isotropic chemical shift are given for all crystallographic A1 sites. 8 lines from 37.9 to 46.3 ppm are attributed to the tetra-coordinated aluminum. The 7 lines from 19.4 and 22.6 ppm are attributed to the penta-coordinated aluminum. As only fifteen sites are detected for aluminum instead of the sixteen sites found for phosphorous it is likely that the intensity of one of the lines is too low to be detected. 3.3. MQHETCOR NMR correlation spectra 2D MQHETCOR may be used to help the assigmnent of the 27A1 and 31p AIPO4-40 MAS NMR resonances. In this sequence, the MQMAS experiment is followed by a CP transfer in order to correlate different nuclei through dipolar couplings. Work reported recently, using the MQHETCOR sequence, display heteronuclear 27AI/31p spectra correlating neighboring nuclei [5].

Table 2.

27A1 isotropic chemical shifts, relative intensities and quadrupolar coupling constants

resulting from the MASAI refinement of AIPO4-40MAS and MQMAS spectra. Peak 1 2 3 4 5 6 7

5~o 27A1(ppm) 19.4 19.6 20.2 20.9 22.1 22.2 22.6

127A1 (%) 4.4 2.2 4.4 5.7 6.5 6.5 6.5

CQ (MHz) 4.31 1.55 2.37 4.85 2.38 3.09 2.01

8 9 10 11 12 13 14 15

37.9 39.2 39.6 40.6 43.9 44.0 45.9 46.3

6.5 6.8 10.6 5.0 8.7 10.9 10.9 4.4

2.24 2.28 3.67 2.33 1.65 2.57 3.24 4.02

1920 Obviously, optimal resolution in both dimensions is essential, in order to reach an unequivocal structure determination. While multiple-quantum excitation provides a highresolution spectrum in the 27A1 dimension, high-frequency MAS is usually sufficient to remove the dominant heteronuclear dipolar interaction in the 31p acquisition domain. Nevertheless, the 31p MQHETCOR resolution is greatly improved by using 1H/27A1 double-resonance decoupling. Figure 3 shows the 2D MQHETCOR spectrum of AIPO4-40 using the pulse sequence described by Fernandez et al. [5]. However, the correlation spectra show that the study of the AIPO4-40 A1-P connectMty is not straightforward due to the large number of lines present. A structural interpretation of these results can however be established by using the NMR crystallography technique and this work is in progress.

ppm_

!

"

20

i

25 30

--

II

9

27A1 4

40

45 1 50 55 60

I

-5

-10

I

-15

I

-20

I

-25

I

-30

I

-35

ppm

31p Figure 3. 27A1-31pcorrelation spectrum of AIPO4-40 obtained using the cross-polarization 3Q-HETCOR method. The 27A1-31pcorrelation (3Q-HETCOR) spectrum was recorded using 27A1decoupling during the acquisition.

1921 4. CONCLUSION In this communication, we show that the NMR evidence is not yet consistent with the recently proposed structure refinement for the AIPO4-40 aluminophosphate. Indeed, we detect at least sixteen 31p sites and ill'teen 27A1 sites. However, their attribution is not trivial and needs additional work. 31p31p correlation spectrum using double-quantum NMR spectroscopy [ 13] with 27A1 decoupling may help. A structural interpretation of these results by using the NMR crystallography is in progress. REFERENCES

1. L. Frydman and J. S. Harwood, J. Am. Chem. Soc. 117 (1995) 5367 2. M. Pruski, D. Lang, C. Fernandez and J-P. Amoureux, Solid State Nucl. Magn. Reson. 7 (1997) 327 3. C. Fernandez, D. Lang, J-P. Amoureux and M. Pruski, J. Am. Chem. Soc. 120 (1998) 2672 4. S.E. Ashbrook and S. Wimperis, J. Magn. Reson, 147 (2000) 238 5. C.Fernandez, C.Morais, J.Rocha and M. Pruski, Solid State Nucl. Magn. Reson., in press 6. L. Delevoye, C. Fernandez, C. Morais, V. Montouillout and J. Rocha, submitted 7. N. Dumont, Z. Gabelica, E. G. Derouane and L. B. McCusker, Microporous Mater. 1 (1993) 149. 8. J. Rocha, J. P. Louren~}o, M. F. Ribeiro, C. Fernandez and J.-P. Amoureux, Zeolites, 19 (1997) 156. 9. V. Ramaswamy, L. B. McCusker and Ch. Baerlocher, Microporous Mater. 31 (1999) 1. 10. J.-P. Amoureux, C. Fernandez, S. Steuemagel, J. Magn. Reson. 123 (1996) 116. 11. A.A. Quoineaud, V. MontouiUout, C. Fernandez, S. Gautier, S. Lacombe, in preparation 12. C. Fernandez, A.A. Quoineaud, V. MontouiUout, S. Gautier, S. Lacombe, Proceedings of the 13th International Zeolite Conference, Montpellier, July 2001, ELSEVIER, eds. A. Galarneau et al. 13. J.Gottwald, D. E. Demco, R. Graf, H.W. Spiess, Chem. Phys. Lett. 243 (1995) 314.


1923

Structural characterization of borosilicates synthesized in the presence of ethylenediamine Stefano Zanardi a, Alberto Alberti a, Roberto Millini b, Giuseppe Bellussi b, Giovanni Peregob a

Dip. di Scienze della Terra, Univ. di Ferrara, C.so Ercole I d'Este 32, 1-44100 Ferrara (Italy)

b EniTecnologie S.p.A., Via. F. Maritano 26, 1-20097 San Donato Milanese (MI - Italy) Hydrothermal treatment of an aqueous solution containing tetramethoxysilane, boric acid and ethylenediamine (EN) led to the crystallization of B-MFI (BOR-C) accompanied by small amounts of B-ferrierite. Single crystals X-ray diffraction studies were performed on a small prismatic crystal of BOR-C, unit cell dimensions a = 19.869(2), b - 19.661(3), c = 13.207(2) A, space group Pnma, and on a platy FER-type crystal with unit cell dimensions a = 18.512(2), b = 13.865(1), c - 7.317(1) A, space group Immm. Different routes (unit cell volume, T-O distances, refinement of B and Si scattering curves) reveal the incorporation of boron in the framework corresponding to 14+1 and 6.5+0.5 B atoms/unit cell in MFI and FER structures, respectively. Three different EN sites in BOR-C and two different EN sites in BFER (all with partial occupancy) were found; the total number of EN, almost one half than boron incorporated in the framework, confirms that EN should be in dicationic form to compensate the negative charge of the framework. Molecular modeling simulations confirmed that those found in the refinements of the X-ray diffraction data are really the most preferred location sites for EN molecules in the two porous systems. 1. INTRODUCTION Crystallization of high-silica zeolites strongly relies with the use of organic additives, usually quaternary ammonium cations, which act as structure directing agents (SDA' s) or, in a few cases, as real templates [1 ]. The central role of the organic additives in determining the nature and the porous structure of the crystalline products has been assessed by several experimental [2,3] and theoretical [4-7] studies, leading to the conclusion that the crystallization of novel microporous structures could be achieved only through the use of increasingly large and complex cations. Indeed, several new zeolites have recently been synthesized by using this approach [3]. The use of charged organic molecules, however, is not mandatory for the synthesis of zeolites, since they can be crystallized even in the presence of neutral molecules such as amines, which favor the formation of some interesting structures such as ERB-1 [8], MCM-22 [9] (MWW), MCM-35 (MTF) [10], as well as of several chlatrasils [2]. In spite of that, they received less attention in the literature with respect to the organic cations and their role in zeolite nucleation and growth was not considered at least until the recent studies of Rollmann et al., who related the structure of small amines to the zeolite products obtained [ 11,12]. These

1924 authors found that small amines really play a pore-stabilizing role and, in general, the zeolite predicted to be better stabilized by the non-bonded amine-framework interactions is effectively obtained. Another interesting feature concerns the Na/A1 molar ratio in the products, often lower than 1. That means that, part of the amine molecules should be protonated in order to compensate the negative framework charge and that in spite of the strong basicity of the synthesis gel (pH >12). Among the small amines considered, ethylenediamine (EN) is in our opinion one of the most interesting because it is characterized by the lowest C/N ratio. According to Rollmann et al., EN favors the crystallization of ZSM22 (TON), ZSM-23 (MTT) or ZSM-48 with SIO2/A1203 = 200 [11], while upon decreasing the SIO2/A1203 ratio to 2 0 - 40, ZSM-35 (FER) and ZSM-5 (MFI) are obtained [12]. Controversial results were reported for the borosilicate system. According to Taramasso et al., B-containing MFI (BOR-C) with an estimated B content of 9.8 atom/unit cell (the highest level of B incorporation in MFI actually reported) can be crystallized from alkali-free reaction mixtures containing EN [13]. On the contrary, Gies et al. reported the crystallization of A1free FER from a synthesis mixture containing Si(OCH3)4, H3BO3, EN and H20, and provided evidences supporting the conclusion that an EN-boric acid complex acts as a template for the crystallization of FER; the same authors concluded that B is a non-framework constituent, though some framework incorporation of B was not excluded [ 14,15]. With the aim to clarify the role of EN in the crystallization of these phases and to evaluate their real framework composition, we have undertaken the structural characterization of assynthesized B-FER and BOR-C crystals crystallized according to the recipe reported in [14]. Moreover, location of EN molecules within the FER and MFI porous structures was examined by using molecular modeling tools and compared with the results of the structural characterization. 2. EXPERIMENTAL 2.1. Synthesis Synthesis was performed according to the recipe reported in [14]. A mixture with composition 0.2B203.SiO2.4EN.110 H20 was heated at 453 K for 56 days. A solid product was recovered, mainly constituted by BOR-C together with significant amount of a glassy phase and traces of B-FER. The overall composition of the as-synthesized material, determined by elemental analysis, was Sio.949B0.05102.0.098EN. 2.2. Characterization

Single crystal X-ray diffraction data were collected at room temperature on a BOR-C prismatic crystal (dimensions 25x25x50~tm) and on a platy FER-type crystal (dimensions 100x60x20gm), using a Nonius KappaCCD diffractometer equipped with a CCD detector; the MoKc~ radiation ()~ = 0.71069 A) was used. In the case of BOR-C crystal, three sets of frames were measured: the first set of 10 images, with a q~rotation width of 1~ and an exposure time of 360 sec. per frame was used for initial cell determination; a second set of 66 images with a q0 rotation width of 2 ~ and a third set of 16 images with a co rotation width of 2 ~ both with an exposure time 2800 sec. per frame, were used for data collection. A total of 10485 reflections were collected up to 53 ~ 20, 5455 unique with t~q = 0.304. For the B-FER crystal, four sets of frames were measured: i) the first set (10 frames), with

1925 a q~ rotation width of 1o and an exposure time of 180 sec. per frame, was used for initial cell determination; ii) one q0 rotation set (total 90 frames), and iii) two co rotation sets (total 65 frames), with a rotation width of 2 ~ and an exposure time of 1410 sec. per frame, were used for data collection. A total of 7024 reflection were collected up to 55 ~ 20, 1244 unique with R e q -- 0.062. The package DENZO-SMN [ 16] was used for the refinement of unit cell parameters and the data reduction, while the SHELX-93 [ 17] program was used for structure analysis. The Si and B content of the crystals was determined by Wavelength Dispersive Spectroscopy (WDS) using an Oxford WDS-600 spectroscope interfaced with a Jeol JSM840A scanning electron microscope. BN and SiO2 were used as reference materials.

2.3. Computational details An approach based on a combination of molecular dynamics (MD), Monte Carlo packing and energy minimization techniques was used for determining the locations and the nonbonded interactions of EN within the MFI and FER porous structures [ 18]. The simulation started with a high temperature (1000 K) molecular dynamics run (0.2 ns long, with a time step of 1 fs) on the isolated EN molecule, in order to explore its conformational space and to generate 1000 conformations (extracted from the trajectory one every 200 fs). After energy minimization, these conformations were stored in an archive file for use in the subsequent Monte Carlo packing step. 12 and 4 EN conformations were randomly extracted from the archive file were packed in the MFI and FER supercells (corresponding to one unit cell, periodic boundary conditions applied), respectively. Energy minimization was then applied in order to find the lowest energy sorption sites for the EN molecules. In both cases, 100 packed structures were generated and energy minimized. The lowest energy MFI/12EN and FER/4EN conformations were successively further refined by impulse dynamics cycles. The purely siliceous frameworks, built from crystallographic data, were kept fixed during the calculations. On the basis of the results reported by Lewis et al. [7], the electrostatic interactions between the SDA molecules and the framework atoms were neglected. All the calculations were performed with the MSI Catalysis 4.0.0 and Cerius 2 software packages [19]; molecular mechanics and dynamics calculations run by Discover [20] employed the cvffforcefield. 3. RESULTS AND DISCUSSION

3.1. Structural characterization Though the synthesis was carried out according the recipe proposed by Gies and Gunawardane [ 14], the results obtained were completely different. In fact, the solid recovered was mainly constituted by a MFI-type molecular sieve accompanied by small but significant amounts of a glassy phase. Only trace amounts of the expected FER-type molecular sieve were observed in the sample, as indicated by the presence of the weak (200) reflection located at 9.54 ~ 20 in the XRD pattern. This is quite surprising, since the only difference between the two preparations was the use of a 200 ml stainless-steel autoclave instead of a 2 ml sealed silica tube. Though no clear explanations were obtained for the unsuccessful reproduction of the synthesis of FER, it is possible that a slight variation of the composition of the reactant mixture, due to the partial dissolution of the silica tube by the alkaline solution, may play

1926 some role. Optical microscope observations revealed that the solid is mainly constituted by large prismatic crystals (BOR-C), accompanied by rare large and thin platelets (B-FER). Structure refinements of B-FER and BOR-C X-ray single crystals diffraction intensities were carried out using SHELX-93, starting from the crystallographic data reported for the orthorhombic MFI [21] and FER [ 14] structures (space group Pnma and Imrnrn, respectively). Unit cell parameters were a = 19.869(2) A, b = 19.661(3) A, c - 13.207(2) A for BOR-C, and a = 18.512(2) A, b = 13.865(1) A, c = 7.317(1) A for B-FER. The refinement converged to a final R(F) factor of 0.128 [ I>4~(I)] and 0.075 [ I>5~(I)] for BOR-C and B-FER, respectively. The complete description of the structures, including fractional atomic coordinates and occupation factors, is reported elsewhere [22]. Here we focus the attention on the main features of these structures, trying to derive information about the effects of the EN molecules on the properties of the crystalline products. To do that, we have to answer to the following questions: a) is boron really incorporated in the MFI and FER frameworks and to what extent? b) is there a correlation between the number of EN molecules packed in the pores and the number of B atoms in the framework? c) does EN play any structure directing role with respect to MFI and FER? Since the sample under investigation is constituted by three different phases (MFI, FER and an amorphous one) elemental analysis does not provide any useful information about the real framework composition of the two crystalline borosilicates. Therefore, their composition was determined with four different methods: i) by WDS analysis of the crystals; ii) using the well known equation relating the unit cell volume (Vx) of a sample containing a molar fraction x of a heteroatom M to that of the pure silica parent structure (Vsi): Vx -- Vsi - Vsi[ 1- (dM.o/dsi.o)3]x (1) where dM-o and dsi-o a r e the tetrahedral M-O and Si-O bond distances, respectively [13]. For BOR-C: dB-o = 1.46 A, dsi-o = 1.595 A [21], Vsi = 5332 A 3 [21]; for B-FER: dB-o = 1.46 A, dsi-o = 1.599 ,s [23], Vsi = 1954.3/~k3 [23]. iii) from the Si and B occupancies in the T-sites, obtained from the refinement of the scattering curves; iv) from the values of the T-O bond distances, assuming a linear variation of the average T-O bond distance derived from the refinements, dT-o (1.573 and 1.571 A, for BOR-C and B-FER, respectively) with the B content; dB-o and dsi-o as in point ii. The framework compositions derived from the applications of the methods i - iv are reported in Table 2. In both cases there is a clear evidence of a remarkable substitution of B for Si, valuable 14+1 B atoms/unite cell in the case of BOR-C, and 6.5+0.5 B atoms/unit cell in B-FER. In the case of BOR-C, the B content resulting from WDS analysis is significantly lower with respect to that derived crystallographically. The WDS analysis, however, is probably affected by the interference of the glassy material, which taking into account the overall chemical composition of the sample (see above) should have a much lower amount of B with respect to the BOR-C crystals. This discrepancy was not observed for B-FER, where a nice agreement exists between the framework composition derived from WDS and crystallographic analyses (Table 2). These results contrast with the hypothesis formulated by Gies et al. about the presence of B as non-framework constituent in Al-free FER [14,15]: the trivalent metal atom is effectively and extensively incorporated in the framework while we did not observe any extraframework B species.

1927 Table 2 Framework composition of as-synthesized BOR-C and B-FER determined with the different methods. BOR-C B-FER WD S [Si845B11.50192] [ S i29.5B 6.5072] Equation (1) [Si81.4B14.60192] [Si30.1B5.9072] Refinement of the Si/B occupancy [8i81.3B16.10192] [Si29.4B6.6072] T-O bond distances [Si80.5B15.50192] [8i29.2B6.8072] The refined structures of BOR-C and B-FER are shown in Figure la and b. Three and two crystallographically independent EN molecules were found in BOR-C and B-FER structures, respectively. In BOR-C, one (EN1) is located in the sinusoidal 10-ring channel parallel to [100] direction, lying on the (010) plane, the other two (EN2 and EN3) in the straight 10-ring channel (Fig. l a). The refined occupancies of these molecules correspond to 2.80(16), 2.96(12) and 1.20(8) EN molecules/unit cell, respectively, giving an overall number of 6.96(20) EN molecules in the unit cell. Another partially occupied extraframework site, assigned to water oxygen, was found in the interception of the straight and sinusoidal channel (Fig. l a). Only EN molecules were found in the B-FER porous system: one (EN1) is located in the FER cage, lying on the (001) plane, the other one (EN2) in the 10-ring channel, lying on the (010) plane (Fig. lb), corresponding to 2.0(12) and 1.62(8) EN molecules/unit cell, respectively (total number of EN molecules per unit cell, 3.62(15)). In both sites, the EN molecules occupy statistically two crystallographic positions related to each other by an inversion center. Though unambiguously recognized as EN molecules, their geometry does not correspond to the ideal one. However, because of the small dimensions of EN molecule with respect to the MFI and FER pores, one has to admit the existence of different possible positions of the molecule close to the preferred location. Therefore, what derived from the crystallographic refinement should be considered as an average of the different situations existing in the crystals. The total number of EN molecules per unit cell found for the two structures is lower than that required for a complete pore filling (12 and 4, for MFI and FER, respectively). That can be explained with the existence of some defects in the stacking of the molecules; the corresponding voids space could be filled with other extraframework species (e.g. H20). It is worth noting that, in both structures, the number of EN molecules correlates well with the number of B atoms incorporated in the framework, since the ratio N/B is close to 1. Since the synthesis was performed in the absence of alkali metal ions, the negative framework charge should be compensated by the charged organic molecules. That implies that most of EN molecules should be in dicationic (H3N-CHzCHzNH3) 2+ form or, because of the weakness of the Bronsted acidic sites in boralites [1], they simply interact with the protons forming N'"H-O hydrogen bonds. These results are perfectly in line with what reported by Taramasso et al.: the maximum amount of B, which can be incorporated in the framework, depends on the number of charged cations hosted in the pores [ 13]. Another interesting consequence of this observation is that an EN-boric acid complex, the formation of which is highly probable because of the large excess of diamine with respect to H3BO3 in the synthesis mixture, really act as a template in the crystallization of BOR-C and B-FER, as postulated for Al-free FER [14,15].

1928

(a)

(b)

t~ .L

(c)

(d)

Figure 1. Location of EN molecules determined experimentally for BOR-C (a) and B-FER (b) and lowest energy location of the same molecules in the MFI (c) and FER porous systems (d).

3.2. Modeling studies The packing method proposed by Freeman et al. proved to be an efficient tool for determining the location and energetics of organic molecules docked or packed in microporous materials [ 18]. We used this approach both to confirm the results obtained in the structural characterization of BOR-C and B-FER, to determine the maximum loading of EN molecules in the two pore systems and to evaluate the existence of any structure directing role of EN towards the formation of these structures. Table 3 Average stabilization energies of EN molecule packed in MFI and FER pores (data in kJ'molm). The number of EN molecules/unit cell are given in parentheses. MFI FER

EN 1

EN 2

EN 3

-68.0 (2.80(16))

-65.2 (2.96(12))

-51.9 (1.20(8))

-79.4 (2.0(12))

-73.9 (1.62(8))

---

1929 Application of these method correctly predicted the location of the EN molecules in MFI and FER porous structures, as clearly shown in Figure 1. In MFI, the lowest energy conformation was found with the twelve EN molecules distributed around the three well defined sites, corresponding to those derived from the crystallographic analysis (compare Figures 1a and c). Depending on the location, the EN molecules are predicted to stabilize the system in a different way (Table 3). The lowest energy sites are located in the sinusoidal 10membered ring (10MR) channels, in the same region found for the EN 1 molecule (Figure 1a). The second site, slightly less stable than the previous one, corresponding to EN molecules located in the straight 10MR channels (EN2, Figure l a). The third EN molecule is located across the straight 10MR channels and points towards the sinusoidal 10MR channel (EN3, Figure l a). With respect to EN1 and EN2, which display similar average van der Waals energies, EN3 is significantly less stable (Table 3) and that accounts well with the lower occupancy of this site derived from the crystallographic analysis (see above). Two different preferred sites for the diamine molecule were predicted in FER (Figure 1d). The low energy sites are located within the FER cages and correspond to the positions found for EN1 molecule (Figure l b). In this case, the dimensions and shape of the cage allow several different but isoenergetic conformations for EN1 and this supports the explanation given for the distorted geometry of EN1 found in the crystallographic analysis (see above). The other sites, corresponding to EN2 molecules (Figure lb), are located in the linear 10MR channels. The position predicted for EN2 along the [001 ] direction is similar with respect to that found experimentally, but it lies on the (100) plane instead of on the (010) (Figure 1d). The difference between the average van der Waals energies of the two molecules is 5.5 kJ.mol 1 (Table 3), again in line with the relative occupancy of the two sites. Upon comparing the stabilization energies of the EN/framework systems reported in Table 3, one has to admit that FER seems to be better stabilized with respect to MFI, in agreement with the results reported by Gies et al. [ 14,15], but not with those here reported. In reality the difference observed can hardly support any preference for one or the other microporous borosilicate. It is probable that subtle differences in the synthesis procedures may influence the nature of the products. As a matter of fact, Bellussi and Perego who always obtained crystalline products constituted by mixtures of B-FER and BOR-C, with B-FER content ranging between 20 and 80 wt% [24]. 4. CONCLUSIONS Single crystal X-ray structure analysis of borosilicates with MFI and FER topology, synthesized from an aqueous solution containing TMOS, boric acid and ethylenediammine, unambiguously showed the presence of boron in the tetrahedral sites. Different routes evidenced the incorporation of 14+1 B and 6.5+0.5 atoms in MFI and FER frameworks, respectively, with a substantial disorder (Si,B) in the tetrahedra. The number of EN molecules/unit cell, determined from the refinement of the X-ray diffraction data, is lower than that expected on the basis of a full loading of the channels (12 and 4 EN/unit cell, for MFI and FER, respectively) but is related to the number of B atoms/unit cell. On the basis of this evidence, it is possible to hypothesize that the number of EN molecules found in the crystals is determined by the extent of B incorporation. That implies a role of the boric acidEN complex postulated by Gies and Gunawardane [14] in the crystallization mechanism of the two borosilicates.

1930 The location of EN derived by using molecular modeling tools, was favorably compared with that obtained by the X-ray structure refinement. The systems are stabilized by favorable host/EN nonbonded van der Waals interation energies; however, the degree of stabilization changes from a site to another It is to note that site characterized by the highest stabilization energy corresponds to the site with the highest occupancy derived by X-ray refinement. REFERENCES

1. R. Szostak, Molecular Sieves. Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1989. 2. H. Gies, B. Marler and U. Werthmann in: Molecular Sieves Science and Technology. Vol I: Synthesis (H. G. Karge and J. Weitkamp, Eds.), Springer, Berlin, 1998, p. 35. 3. S. Ernst in: Molecular Sieves Science and Technology. Vol I: Synthesis (H. G. Karge and J. Weitkamp, Eds.), Springer, Berlin, 1998, p. 64. 4. T.V. Harris and S. I. Zones, Stud. Surf. Sci. Catal., 84 (1994) 29. 5. R. G. Bell, D. W. Lewis, P. Voigt, C. M. Freeman, J. M. Thomas and C. R. A. Catlow, Stud. Surf. Sci. Catal., 84 (1994) 2075. 6. P.A. Cox, A. P. Stevens, L. Banting and A. M. Gorman, Stud. Surf. Sci. Catal., 84 (1994) 2115. 7. D.W. Lewis, C. M. Freeman and C. R. A. Catlow, J. Phys. Chem., 99 (1995) 11194. 8. R. Millini, G. Perego, W. O. Parker, Jr., G. Bellussi and L. Carluccio, Microporous Materials 4 (1995) 221. 9. M.E. Leonowicz, J. A. Lawton, S. L. Lawton and M. K. Rubin, Science 264 (1994) 1910. 10. P. A. Barrett, M. J. Diaz-Cabafiaz and M. A. Camblor, Chem. Mater. 11 (1999) 2919. 11. L. D. Rollmann, J. L. Schlenker, S. L. Lawton, C. L. Kennedy, G. J. Kennedy and D. J. Doren, J. Phys. Chem. B 103 (1999) 7175. 12. L. D. Rollmann, J. L. Schlenker, C. L. Kennedy, G. J. Kennedy and D. J. Doren, J. Phys. Chem. B 104 (2000) 721. 13. M. Taramasso, G. Perego and B. Notari, in: Proc. 5th Int. Zeolite Conf., (L.V.C. Rees Ed.), Heyden, London, 1980, p. 40. 14. H. Gies and R. P. Gunawardane, Zeolites 7 (1987) 442. 15. R. P. Gunawardane, H. Gies and B. Marler, Zeolites 8 (1988) 127. 16. Z. Otwinowski and W. Minor, Meth. Enzymol. 276 (1997) 307. 17. G. M. Sheldrick, SHELXL-93, University of G6ttingen, G6ttingen, Germany, 1993. 18. C. M. Freeman, C. R. A. Catlow, J. M. Thomas and S. Brode, Chem. Phys. Lett., 186 (1991) 137. 19. Catalysis User Guide, Release 4.0.0, Molecular Simulations, Inc., San Diego, CA, 1996. 20. Discover User Guide, Release 2.9.8, Molecular Simulations, Inc., San Diego, CA, 1996. 21. H. van Koningsveld, H. van Bekkum and J.C. Jansen, Acta Cryst. B43 (1987) 127. 22. G. Perego, G. Bellussi, R. Millini, A. Alberti and S. Zanardi, Microporous Mesoporous Mater., submitted. 23. R. E. Morris, S. J. Weigel, N J. Henson, L. M. Bull, M. T. Janicke, B. F. Chmelka and A. K. Cheetham, J. Am. Chem. Soc. 116 (1994) 11849. 24. G. Bellussi and G. Perego, It. Patent 1,203,913 (1987) assigned to Eniricerche S.p.A.


1931

Molecular dynamics simulation o f water confined in zeolites P. Demontis, G. Stara and G.B. Suffritti Dipartimento di Chimica, Universit~t di Sassari, Via Vienna, 2, 07100 Sassari (Italy) FAX" +39.79.212069 Extended nanosecond scale MD simulations of water in bildtaite, silicalite and natrolite are illustrated. These systems are representative of different kinds of absorption of water in zeolites: hydrogen-bonded linear chains of water moleculars (bikitaite), water in hydrophobic materials (silicalite) and water molecules held in fixed ordered positions (natrolite). Results are in line with the experimental results and with Car-Parrinello simulations, when available. The different microscopic behaviour of water in each considered zeolite is discussed. 1. INTRODUCTION The behaviour of water in porous media has received recently a renewed interest, both from experimental and theoretical viewpoint. For instance, the experimental evidence of possible phase transitions of water at low temperatures has been studied in ordered mesoporous silica materials [1], where the confinement lowers the freezing point, and in hydrated phyUosilicates [2]. On the theoretical side, new statistical models have been developed for studying the phases of water-like liquids in porous media [3]. Among the different approaches the Molecular Dynamics (MD) simulation technique seems to be very promising in order to gain a better microscopic description for the behaviour of water in porous materials [4,5]. A recent example is the simulation of the behaviour of water in carbon nanotubes [6] which evidenced some unexpected features like the formation of onedimensionally ordered chains of water molecules, transmission bursts of water through the nanotube and sharp two-state transitions between empty and filled states on a nanosecond time scale. In the present work, extended nanosecond scale MD simulations of water in bikitaite, silicalite and natrolite are illustrated. These systems are representative of different kinds of absorption of water in zeolites: hydrogen-bonded linear chains of water molecules (bikkitaite), water in hydrophobic materials (silicalite) and water held in fixed ordered positions (natrolite).

1932 2. M O D E L AND C A L C U L A T I O N S

Standard MD simulations with periodic boundary conditions were performed in the microcanonical ensemble. A sophisticated empirical model for simulating flexible water molecules and flexible zeolite framework developed in this laboratory [5] has been used. Moreover, new empirical potential functions have been developed for representing lithium and sodium - water interactions, as the ones previously proposed for simulating aqueous solutions containing ions did not reproduce satisfactorily the behaviour of water in zeolites. In particular, a problem in representing lithium - water interactions arise because water molecules are located in planes which are roughly perpendicular to the Li - O distance, whereas for all previously available empirical Li - water potential models the preferred orientation of water molecule was parallel to this distance, with high barrier to flap motion. The special features of Li + - H20 interactions were recently discussed by Lyubartsev et al. [7] in an ab initio MD simulation study of lithium salt aqueous solutions. Therefore, a new model fitting both ab initio [8] results for the Li + - H 2 0 system and experimental data for bikitaite was performed. The final form of the Li + - water potential function reads:

VL~o,~ (r) =

1

4rcc o

qLiqi~,o + ALio,I~ exp(--BLio,H r) --]- C~i O,H r

(1)

S(~,r)

r

where S(cy , r ) i s a switching function given by:

S(cy,r)=

1 exp(_cyr2)

/f /f

nm nm

r0.5

which is necessary because the r

(2) -2

lattice sums do not converge, as in Eqn. (1)

C ~o ~ - 2 C ~ . .

The representation of sodium - water interactions differs from that adopted in Ref. [5], because a term proportional to r -6 was added to the Na + - O pair potential function in order to fit better both the energy and the Na + - O distance. The final form is:

VNaO'g(r) = 4rt~;0

qNaqH, O + ANaO,H exp(_BNao,Hr ) _ CN6a0 + DNaO, 4 H r

r

r

(3)

The parameters included in Eqns. (1-3) are collected in Table 1. Water was assumed to interact with Si and A1 atoms via a Coulomb potential only. The potential functions for interactions between an oxygen atom of the zeolite framework (Of) and an oxygen (O) or an hydrogen (H) atom of the water molecule were represented by:

Vof

O

=

1 qQqo AQO BQO C@o 4rcc ~

r

t

r12

- - + ~ r 6

r 4

(4)

1933

1 qQqH AQH V o f H = 4~r~~ +- r r 7

B@H

(5)

r 4

and the values of the parameters are reported in Table 1. They are slightly different from those reported in Ref. [5], because they were optimised in order to fit better the experimental sorption energy in all-silica zeolites. The evaluation of the Coulomb energy was performed using the efficient method recently proposed by Wolf et al. [9] and extended in our laboratory to complex systems [10]. Runs lasting at least 1 ns (using an elementary time integration step of 0.5 fs) were carried out at different temperatures and loadings in order to compare the computed results with the available experimental data. Besides structural properties (average co-ordinates and temperature factors), the absorption energies, the vibrational spectra, the diffusion coefficients ant the time correlation functions of water molecule rotations were evaluated using standard methods. The analysis of the clustering of water molecules were also carried out. The simulation box for bikitaite was made of 3x4x3 unit cells, resulting in a pseudomonoclinic cell with a = 2.5713, b = 1.9758, c = 2.28363 nm and fl = 114.52 ~ and containing 936 atoms, including 72 water molecules. The accurate neutron diffraction structure by Stahl et al. [11] was assumed. This system is of special interest because it shows parallel straight channels where hydrogen-bonded linear chains of water molecules run along the axis of the channels, parallel to regular rows of lithium ions sticking to the channel surface. Table 1. Values of the parameters included in Eqns. (1-5). Energies are obtained in kJ/mol if the distanced are in units of 10 - l ~ m (formerly A) Charges are in units of e. Interaction

A

Li +- 0 Li +- H Na +- 0 Na +- H Of- 0 Of- H Charges

B

C

2.68 105

5.15

397.2

0.5

4.14

7.07

410.8

0.5

1.0 106

4.526

3139.5

4.897 105

7.07

10 7

8.36 105

31.0 102

2.09 103

1.8 102

qL i +

qN a +

1.0

1.0

D

cr

62.8 460.5

2.09 102

qo -0.65966

qH 0.32983

qoI

- 1.0

1934 For silicalite, two unit cells superimposed along c yielded an orthorhombic simulation box of 2.0022xl.9899x2.6766 nm 3 containing 624 atoms. Silicalite is considered a "hydrophobic" zeolite, because water - water interactions are stronger than the water framework ones. The box includes 16 water molecules, corresponding to the loading of the samples used for experimental measurement of diffusion coefficient. Its framework structure [12] comprises two different channel systems, each defined by ten-membered rings. Straight channels with an elliptical cross section are parallel to b and sinusoidal channels with nearly circular cross section run along a. The resulting intersections are elongated cavities. Natrolite is a natural fibrous zeolite, containing 16 water molecules per unit cell, which are held in fixed ordered positions by an equal number of sodium ions lined around the axis of channels running along c. It was chosen because its structure [13], vibrational spectrum [ 14], and water mobility [15] are known with good accuracy. The simulation box consisted of 3 unit cells superimposed along c, yielding a pseudo-orthorhombic simulation box of 1.8272x 1.8613x 1.9779 nm 3 including 552 atoms.

3. RESULTS AND DISCUSSION The results for bikitaite, silicalite and natrolite will be discussed separately, as the behaviour of water in the different zeolites is different. In general, the simulations were able to reproduce the available experimental data, and when the agreement was not quantitative, is at least qualitatively correct. However, in some cases the goodness of some results could be changed (either into better or into worse) on the basis of the correspondence between the simulated temperature (which is the one defined by classical statistical mechanics) and the experimental one, belonging to systems which are of quantum nature. Some suggestion were put forward by Jobic et al. [16], but the problem deserves a more throughout discussion, which is beyond the scope of the present paper. 3.1 Bikitaite

The experimental knowledge of the behaviour of water in bikitaite is given by neutron [13] and X-ray [ 17] diffraction structural data, completed by IR spectra [17] and NMR data [18], Differential Thermogravimetry (DTG) and high-temperature X-ray diffraction [19]. From neutron diffraction studies it appears that the libration motion of water molecules is relatively large even at low temperature (13 K) and, indeed, NMR experiments [18] evidenced that a flip motion of the water molecules already occurs at a relatively low temperature (224 K) with a correlation time of 10-7 s. The NMR experiments were performed in the temperature range 224 - 418 K, detecting flip motion but no diffusion. The corresponding relaxation time follows closely an Arrhenius trend with values from 10-7 s (224 K) to 10-11 s (418 K). The fitted value of activation energy for the flip motion is 30_+2 kJ/mol. Recent X-ray diffraction experiments at high temperature [19] showed that at about 375 K the dehydration process begins and the complete dehydration is achieved at about 725 K. The apparent discrepancy between X-ray and NMR results can explained by considering the different observation time, which is much smaller for NMR, so that slow diffusive motion could have escaped in fitting NMR data.

1935 Overall, the water molecule chains embedded in bikitaite show a high stability. CarParrinello MD (CPMD) studies [20] demonstrated that these chains are stabilized by a permanent electric dipole parallel to the channel direction, which is coupled to an opposite dipole of the framework, stemming from a special arrangement of the aluminosilicate tetrahedra. Previous CPMD simulations reproduced well the experimental structure and vibrational spectrum of bikitaite, but, as their duration was limited to a few ps due their computational cost, failed in detecting the flip motion of water molecules at room temperature which shows relaxation times several orders of magnitude longer. Before undertaking long MD simulations using empirical functions for the interatomic interactions, intended as an extension of the CPMD ones, we verified that our model could reproduce the experimental structure (at different temperatures) and vibrational spectra, as well as CPMD results. The structural properties computed using our model agree well with the experiment both at low temperature (13 K for experiments, 50 K for simulations, in order to mimic approximately the average zero point vibrational energy) and at room temperature (within a few tens pm). Also the computed vibrational spectrum is satisfactory. The hydrogen bonding between the water molecules was well represented both from a geometrical and energetic viewpoint. Indeed, average intermolecular energy was about 15 kJ/mol per molecule, in line with the results of simulations in carbon nanotubes [6] where similar water molecule chains are present. Once the good quality of the potential model is ascertained, one can examine the long time scale results. By following a well-established theory [21], a quantitative comparison with NMR relaxation times is achieved by considering the second order rotational correlation function:

c: (t) =

[.(0)..(t)l>

(6)

where u(t) is the versor of the HOH plane and P2 is the second-order Lagrange Polynomial. Using Eqn. (6), the relaxation times for flip motion of water molecules were evaluated. They are compared with the experimental values in Table 2. They follow an Arrhenius trend, and computed activation energy is 26 kJ/mol. Therefore, the simulations underestimate the relaxation times by about one order of magnitude, as the energy barrier is too low. However, as recalled above, a discussion has arisen about the adequacy of the temperature computed from classical mechanics simulations for the comparison with experimental data. Therefore the computed relaxation times could compare more favourably with experiment. Finally, no diffusion was detected in the simulated system (fully hydrated infinite crystal), even at the highest temperature. However, it is possible that, in real (finite) crystals water can escape from the free ends of the channels, inducing a defect-driven stepwise diffusion, which eventually leads to dehydration, as experimentally observed [19]. This problem deserves further investigations, which are in progress.

1936 Table 2. Experimental (NMR) [21] and calculated relaxation times (s) for the flip motion of water molecules in bikitaite T (K) Exp. Cal. a

316 7+5 10 1~ 1.7 10 -11

424 4+2 10 -11 2.27 10 -12

611 3+ 1 10 "12 a 2.55 10 -13

800 7+2 10 -13 a 6. 10 -14

Extrapolated from the Arrhenius fit of the experimental data

3.2 Silicalite Experimental data about the diffusion of water in silicalite at room temperature and with a loading of water in the range 4 - 12 molecules per unit cell have been obtained from Pulsed Gradient Field NMR spectra by K ~ g e r and co-workers at 300 K [22]. They obtained a value of 4• 10-9m2s-1 nearly independent of loading, to be compared with 8.6 10-9m2s1 resulting from simulations. Although experimental data are still lacking at lower temperatures, the simulations may suggest a possible behaviour of the sorbed water. Thus the mobility of the water molecules was checked at different temperatures: 100 K, 130 K, 160 K, 200 K, 250 K, 300 K and 350 K. The runs lasted 3 ns (using a time step of 0.5 fs). In Fig. 1 the mean square displacements (MSD) of water molecules vs. time are reported. At room temperature the MSD shows a linear dependence on time. At 250 K the diagonal components of the diffusion tensor, which usually follow the ordering Dz< Dx < Dy, begin to change this ordering and Dx becomes close to Dy, until, at 200 K, Dx > Dy. For lower temperatures (100 - 200 K) the MSD's do not show a linear dependence on time, at least in the considered time scale. Moreover, they are nearly independent of temperature. Nevertheless, the sorbed fluid does not appear completely frozen, as the mean linear displacement is of the order of 0.3 nm after 800 ps and it is still growing (the thermal vibration amplitude is - 0.1 nm at these temperatures). From the distribution of the water molecules in the channels (not shown), it appears that by lowering the temperature from 350 K to 200 K the distribution of the water molecules is spread over all or almost all the available space of the micropores. For lower temperatures (100 - 200 K) the water molecules seem to group around some stable positions, mostly in the intersections between straight and sinusoidal channels. At 100 K a diffuse distribution along the straight channels emerges again. The analysis of the lifetime of the water molecule clusters shows that by lowering the temperature from 350 K to 200, in correspondence with "normal" diffusion the molecules spend most of the time as monomers or dimers, with lifetimes of the order of one ps, at most. In the temperature range 130 - 200 K, long-lived clusters made of 5 - 6 water molecules and lasting some tens of ps are formed. On the basis of the average distributions in the channels one may assume that these clusters are located in the intersections, hindering each other the diffusion, thus giving rise to the observed irregular, single-file like diffusion behaviour. Finally, at 100 K the longlived clusters are formed by 7-8 water molecules, and become so large than they cannot be accommodated in the intersections, but line up in the straight channels, showing a low mobility because of the low kinetic energy available.

1937 //]-4000

]

| 9

,

..

:,

.::~

": ' :

'

:'

400

600

time (ps)

!~: 300 K

O0 K

160K 200

I

~::"i'~

I/ ~

' 200

~

~ 400

~

"

/

225 K

20o K

600

time (ps)

Figure 1. Mean square displacement of the centre of mass of the water molecules v s . temperature. Left: low temperatures, showing an irregular trend. Right: high temperatures. In this case the trend is linear, and the slopes, corresponding to the diffusion coefficient, follow an Arrhenius behaviour.

The formation of long-lived clusters could be interpreted as a phase transition (freezing) of the adsorbed water in the temperature range 200 - 225 K. These findings suggest extending the investigation to different loadings in order to understand better the behaviour of water in silicalite. 3.3 Natrolite We shall shortly mention the results of preliminary simulations of natrolite. The optimised potential functions improved the reproduction of both experimental structure and vibrational spectrum. In particular, the more accurate representation of the w a t e r framework interactions was able to detect the flip motion of the water molecules at room temperature, in agreement with the experimental evidence [23]. The measured activation energy is 36 + 3 kJ/mol, so that very long simulations will be necessary for a reliable comparison between the model and the experiment, as well as for the study of the diffusion of the adsorbed water leading to dehydration at high temperature. 4. CONCLUSIONS The example which are briefly illustrated in this paper show that MD simulations can be an effective tool for studying the behaviour of water confined in the pores of zeolites. Different distributions and mobilities have been demonstrated, depending on the structure, on the kind of extraframework ions and on temperature. Further investigations are in progress in order to refine the models and to extend the simulations to different systems. The final goal is to achieve some general rules for the behaviour of water in confined geometries.

1938 AKNOWLEDGMENTS

This research is supported by Italian Ministero dell'Universit~t e della Ricerca Scientifica e Tecnologica (MURST) and by Universit~t degli studi di Sassari. REFERENCES

1. S. Sklari, H. Rahiala, V. Stathopoulos, J. Rosenholm, P. Pomonis, Micropor. Mesopor. Mater., 49 (2001) 1 2. R. Bergman and J. Swenson, Nature (London), 403 (2000) 283 3. T.H. Truskett, P.G. Debenedetti and S. Torquato, J. Chem. Phys. 114 (2001) 2401 4. P. Demontis, G.B. Suffritti, Chem. Rev., 97 (1997) 2845 5. P. Cicu, P. Demontis, S. Spanu, G.B. Suffritti and A. Tilocca, J. Chem. Phys. 112 (2000) 8267 6. G. Hummer, J.C. Rasaiah and J.P. Noworyta, Nature (London), 414 (2001) 188 7. A.P. Lyubartsev, K. Laasonen and A. Laasonen, J. Chem. Phys. 114 (2001) 3120 8.X. P6riole, D. Allouche, J. P. Daudey, and Y.H. Sanejouand, J. Phys. Chem. 101 (1997) 5018 9. D. Wolf, P. Keblinki, S.R. Phillpot, and J. Eggebrecht, J. Chem. Phys., 110 (1999) 8254 10. P. Demontis, S. Spanu, G.B. Suffritti J. Chem. Phys. 112 (2001) 8267 11. K. Stahl, A. Kvick and S. Ghose, Zeolites, 9 (1989) 303 12. H van Koningsveld, J.C. Jansen and H. van Bekkum, Zeolites, 10 (1990) 235, 13. G. Artioli, J.V. Smith and A. Kvick, Acta Crystallogr. C 40, 1658 (1984) 14. G. Gottardi and E. Galli, Natural Zeolites, Springer-Verlag, Berlin, 1985 15. N.K. Moroz, E.V. Khopolov, I.A. Belitsky, B.A. Fursenko, Micropor. Mesopor. Mater., 42 (2001) 113 16. H. Jobic, K. Smimov, D. Bougeard, Chem. Phys. Lett., 344 (2001) 147 17. S. Quartieri, G. Vezzalini, A. Sani, E. Galli, E.S. Fois, A. Gamba and G. Tabacchi, Micropor. Mesopor. Mater., 30 (1999) 77. 18. K.Larsson, J. Tegenfeldt and A. Kvick, J. Phys. Chem. Solids, 50 (1989) 107 19.G. Vezzalini, O. Ferro, S. Quartieri, A.F. Gualtieri, G. Cruciani, E. Fois, C. Ceriani, and A. Gamba, in: Recent Research Reports, 13th International Zeolite Conference, French Zolite Group, Montpellier, 2001, 09-R-04 20. E.S. Fois, G. Tabacchi, S. Quartieri and G. Vezzalini, J. Chem. Phys., 111 (1999) 355 21. R.G. Gordon in: Advances in Magnetic Resonance, J.S. Waugh (ed.), Academic Press, New York, 1968, Vol. 3, pag. 1 22. J.K~rger and D.M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, John Wiley & Sons, New York, 1992 23. R.T. Thomson, R.R. Knispel, and H.E. Petch, Can. J. Phys., 52 (1974) 2164


1939

E X A F S and optical s p e c t r o s c o p y characterisation o f silver w i t h i n zeolite matrices S. G. Fiddy a, N. E. Bogdanchikova b, V. P. Petranovskii b, J.S. Ogden a and M. AvalosBorja b aDepartment of Chemistry, University of Southampton, Southampton SO 17 1BJ, United Kingdom bCentro de Ciencias de la Materia Condensada UNAM, Apdo. Postal 2681, 22800 Ensenada, B.C., Mexico

The study of silver-zeolite samples by a combination of EXAFS and Diffuse Reflectance UV-Visible spectroscopy has been utilised to discover the effect of the structural type and Si/A1 ratio of zeolite matrix as well as influence of reduction temperature on the size, structure and oxidation of silver species stabilised inside zeolite voids. The preparation of silver-zeolite samples at high temperature using mordenite with medium Bronsted acid strength or at low temperature using mordenite with high acid strength leads to fast oxidation of reduced silver species. After reduction at medium temperatures silver clusters interpreted as Ag8~ and Ag8~+ are formed in mordenites with intermediate Bronsted acid strength. The reduction of silver samples at high temperature utilising mordenite with a very small concentration of weak Bronsted acid sites leads to the formation of amorphous Ag particles. Deformation of the eight-atom clusters is more prominent in the erionite cavities, where space is more severely restricted than in mordenite channels. It is proposed that the charged Ag8~+ clusters are more deformed than neutral Ag8~ clusters.

1. INTRODUCTION The study of the physical and chemical properties of small metal particles and clusters is very important for many scientific and technological fields such as solid state physics, surface science and heterogeneous catalysis. Zeolites are perfect matrices for the stabilisation of metal clusters due to the wellordered zeolite intracrystalline environments. However, zeolite pore structure is not the only characteristic that helps to stabilise the metal clusters. Such parameters as the SIO2/A1203 ratio and the location of the acid sites on the internal and external zeolite surface in line with the reduction conditions can all have a major influence on the stabilisation of metal species. Ag-zeolite materials are active in various catalytic reactions such as NOx decomposition [1, 2] NO2 reduction in exhaust lean-burn combustion [3], NOx reduction with hydrocarbons [4 and references therein]. Therefore, the characterisation of active silver states in catalytic reactions and the development of methods for the regulation of the contributions of silver in different states is a very important area of chemistry.

1940 In our previous work, it was shown that variation of the 8iO2/A1203 ratio and the reduction temperature lead to changes of the silver environments in mordenite [5]. The contribution of silver cations Ag +, charged and neutral clusters Agnm+, silver subcolloidal particles with size ca. 1 nm and larger metal silver particles change under the variation of these parameters [5, 6]. For the interpretation of silver species with prominent bands at 322 and 293 nm in the UV/VIS spectra of Ag-erionite, EXAFS characterisation was used. The results support an interpretation given previously that the absorption peaks at 322 and 293 nm in the optical spectra belong to cluster species Ag8~ and Ags 5+ in the channels of the crystalline framework [7]. Subsequent ab initio calculations also confirmed these conclusions [8]. EXAFS is a very valuable technique for the characterisation of the structure of metal clusters with a number of publications concerning the EXAFS study of silver species in different matrices for example in SiO2 [9], solid argon [ 10], zeolite 4A [11 ], zeolite ZSM5 [12, 13]. Other physico-chemical methods such as ESR, single crystal X-Ray diffraction, UV/VIS spectroscopy, NMR, etc. are also successfully used for the characterisation of the structure of silver clusters [ 14]. This work represents a continuation of the search for factors, which are responsible for variation of silver states in zeolites. Within this presentation, a number of different silverzeolite samples are characterised by EXAFS and Diffuse Reflectance UV-Visible spectroscopy. By a combination of these two techniques, important information can be concluded about the effect of the structural type and the 8iO2/A1203 ratio of the zeolite matrix on the size, structure and oxidation of silver clusters stabilised inside zeolites. 2. EXPERIMENTAL In this work, the mixed K+-,Na+-erionite sample with SIO2/A1203 molar ratio 8 and protonated forms of mordenites with MR from 10 to 128 were used as the zeolite matrix. The silver exchanged forms were obtained by ion exchange in 0.1 N aqueous solution of AgNO3. The excess solution was removed, samples were dried in vacuum a t - 3 4 5 K and were heated in a H2 flow at a fixed temperature ranging from 20 to 500~ for 4.5 h. The samples are abbreviated as AgM and AgE for Ag-mordenite and Ag-erionite, respectively, followed by the value of MR and reduction temperature in ~ (e.g., AgM15100). The silver content, measured using an X-ray Fluorescence Spectrometer SEA 2010, was 0.6 - 2 wt. % for AgM samples and 14 wt. % for AgE. Diffuse Reflectance UV-Visible spectra were recorded under ambient conditions on a Perkin Elmer 330 spectrometer with a standard diffuse reflectance unit, using undoped erionite and mordenite as a references. Silver K-edge EXAFS data were recorded at room temperature using the synchrotron source at Daresbury, UK, operating at 2 GeV, and samples were studied in both transmission and fluorescence mode, with data sets extending typically to k = 14 ~-1. The Ag edge position was calibrated using silver foil, and background subtractions were carried out using standard polynomials within the program PAXAS [15]. Subsequent curve-fitting utilised the single scattering curved wave theory incorporated in EXCURVE, with phaseshifts and backscattering factors calculated by normal ab initio methods [ 16]. Fourier transformation was performed on k3-weighted EXAFS oscillations in the range of 2 - 12A. Transmission electron microscopy (TEM) characterisation was carried out using a JEOL 2010 instrument with a point-to-point resolution better than 0.19 nm.

1941 3. R E S U L T S AND D I S C U S I O N

For this study, six Ag-zeolite samples were chosen. The variation of SIO2/A1203 molar ratio leads to the change in the strength of Bronsted acid sites that determine the stability of Ag8~ and Ag85+ clusters in H+-mordenites [6]. The strength of acid sites decreases in the following sequence: M15 > M10 > M30 > M72 > M128 [6]. The mixed K +-, Na +erionite sample does not contain proton sites. Results of EXAFS measurements are summarised in Fig. 1 and Table 1. In Table 1, CN represents coordination number; R - interatomic distance, ~2 -Debye-Waller factor, Rfactor- accuracy parameter of EXAFS calculations, AEo - the difference between the calculated Fermi energy level and the known values for that element. AEo is typically between -10 to 10eV, for exact models reaching to 0. Errors derived from EXCURV98 are given in parentheses. The true accuracy of bonded and non-bonded interatomic distances is considered to be 1.4% and 1.6% respectively. Precision on 1 st shell coordination numbers is estimated to be ca. 5-10% and between 10-20% for non-bonded shells. For all samples R-factor (accuracy parameter of EXAFS calculations) is < 50 %. Spectra of diffuse reflectance spectroscopy of all studied samples are presented in Fig. 2. Table 1 Ag K-edge EXAFS (AFAC = 0.8 and Ak = 2.9 - 10.3 A ~) derived structural parameters for Ag-zeolite samples Sample AgM30-500 AgM15-100 AgM10-100 AgM72-200 AGE8-100 AgM 128-300

Shell

CN

O

2.6(+0.3)

Ag

R (/k)

(fit2)

2or 2

2.26(•

0.025(•

-

-

-

O

2.6(+0.2)

2.28(+0.01)

0.028(+0.002)

Ag

0.4(+0.2)

2.73(+0.04)

0.034(+0.009)

O

2.0(+0.3)

2.34(+0.02)

0.030(+0.006)

Ag

3.6(+0.3)

2.83(+0.01)

0.034(+0.003)

O

2.2(+0.3)

2.34(+0.02)

0.024(+0.005)

Ag

3.0(+0.2)

2.82(+0.09)

0.027(+0.002)

O

2.4(i0.2)

2.35(+0.01)

0.028(+0.002)

Ag

2.0(+0.2)

2.78(+0.09)

0.028(+0.002)

O

2.0(i0.3)

2.32(+0.02)

0.021(i0.005)

Ag

2.4(+0.3)

2.83(+0.01)

0.022(+0.002)

R-factor

(%)

AE0

36.9

3.3(+0.9)

31.0

4.2(+0.7)

50.0

-4.5(+0.9)

45.9

-4.4(+0.8)

35.6

-2.4(-t-0.7)

49.3

-3.1 (-1-0.9)

1942

':,

'

/ "

6

9 A v tO .

..'

,

.

i

"i

5

,/; ";

5

m

-.

.

t-

';

i:.i,./

4.

,i

,

6

' ,

"

~'

~

4

%

:i

2

3

4

5

6

7

8

9

10 k(A_11

o

l

2

3

,

3

4

s

R(A)

Figure 1. Ag K-edge EXAFS and Fourier Transform of samples: 1 - AgM30-500, 2 AgM15-100, 3 - AgM10-100, 4 - AgM72-200, 5 - AGE8-100, 6 - AgM128-300. 6

5 C ..0 s,.,. 0 ..0

200

300

400

500

Wavelength,

600

700

800

nm

Figure 2. Diffuse reflectance UV-Vis. spectra of samples: 1 - AgM30-500, 2 - A g M 15100, 3 - A g M 10-100, 4 - AgM72-200, 5 - AGE8-100, 6 - A g M 128-300.

6

1943 The results reveal that the studied samples could be divided into three distinct groups. 3.1. Samples with dominance of silver oxidised forms For samples of the first group including AgM15-100 and AgM30-500, the EXAFS and Fourier transform spectra indicate a high proportion of Ag-O distances. In both cases, approximately 2.6 oxygen atoms at a distance of 2.26 - 2.28 A could be fitted successfully. These results are consistent with previous data on Ag§ oxygen) bonds in zeolite (ca. 2.25 A [14]). The contact between Ag + cation and water oxygen is characterised by longer distances (2.4 - 2.5 A) [ 17]. Ag+-framework oxygen interactions were further confirmed by the fitting of an Ag-Ag distance. For the AgM30-500 sample, Ag-Ag fitting only lead to worse EXAFS fitting parameters suggesting that only Ag-O was present, whereas for the sample AgM15-100 it produced an improved fit but with a low Ag-Ag coordination number (0.4) and low distance (2.73 A). It has been proposed previously that the metal-metal bond distance gradually contracts with the decrease of metal cluster size [ 18]. In bulk metallic silver the first (Ag-Ag) shell lies at 2.89 A~. Ag-Ag distances measured for eight-atom silver clusters lies in the range 2.78 - 2.83 A [7 and see the results below]. Obtained coordination numbers and distances suggested that Ag + cations were the main species present in both samples, however in sample AgM15-100 there also appears to be a small contribution from silver dimers. Optical measurements are consistent with the EXAFS interpretation. For both samples the peaks agreeing with Ag + cations were observed (~ < 230 nm). For AgM15-100 a large peak at ~ = 310 nm, associated with Ag2+ dimers and weak peaks at 240, 260 and 390 nm assigned to Agnm+, Ag42+ clusters and subcolloidal particles, respectively [6], were also observed (Fig. 2). The dominance of silver oxidised forms is due to strong metal-support interaction occurring at high temperatures (500~ for the AgM30-500 sample and the very high strength of Bronsted acid sites for the AgM15-100 sample [6], which leads to fast oxidation of reduced silver species. 3.2. Samples with dominance of Ag8 ~ and Ag8 ~ clusters The samples of the second group including AgM10-100, AgM72-200 and AGE8-100 were prepared at medium temperatures in zeolites with Bronsted sites possessing intermediate or low strength [6]. Fourier transforms of all these samples contain two prominent features related to the presence of Ag-O and Ag-Ag distances. The Ag-O distance for each sample can be typically found within the range 2.34 - 2.35 A, that is ca. 0.08 A higher than that observed for the cationic silver zeolite species, suggesting that these distances are due to physical contact between clusters occluded within the zeolite cavities/channels and framework oxygen from the zeolite matrix. Ag-Ag distances are measured in the range 2.78 - 2.83 A. In bulk metallic silver the first (Ag-Ag) shell lies at 2.89 A. For these samples formation of Ag8 clusters is suggested from the results of theoretical ab initio calculations [8]. On the basis of the analysis of the variation within coordination numbers, Ag-Ag and Ag-O distances, and optical measurements (peaks at ca. 290 and 320nm, Fig. 2), the difference in Ag8 clusters structure could be suggested. To fully understand these differences it is important to compare the pore structure of mordenite and erionite. Mordenite represents a two-dimensional system of crossing channels, which are characterised by different elliptic cross-section with axes: (a) 0.29 0.57 nm; (b) 0.67 - 0.70 nm [19]. Erionite has elongated cavities with diameter 0.63 nm and length ca. 1.5 nm connected by small windows with a diameter 0.25 nm. The cavities

1944 of neighbouring channels are interconnected with twisted 8-member rings (with sizes 0.36 x 0.52 nm) [19]. Hence, mordenite and erionite have pores with very similar crosssection (0.67 - 0.70 nm in mordenite and 0.63 - 0.63 nm in erionite), but these pores are represented by channels in mordenite and by elongated cavities in erionite. The similarity of the pore size leads to the formation of silver cluster with alike electron structure (peaks at 322 and 293 nm for AGE8-100 and peaks at 320 and 285 nm for AgM72-200 in optical spectra). The two peaks observed in the visible range for the AGE8-100 sample have been assigned to electron transfer between the clusters within the zeolite structure [7]. Results show that Ag8 clusters stabilised in relatively spacious mordenite channels possessing medium acid strength (AgM10-100) are more deformed than Ag8 clusters stabilised by weaker protons (AgM72-200). The increase in cluster deformation for AgM72-200 is characterised by a decrease in coordination number for Ag-Ag interactions (from 3.6 to 3.0) while Ag-Ag distances do not change (ca. 2.83 _~). Deformation of the Ag8 clusters increases significantly (coordination number for Ag-Ag interactions diminish to 2.0 and Ag-Ag distances decrease to 2.78 A) when they are stabilised in the cavities of erionite, where space is greatly restricted. It is interesting to note that silver clusters stabilised in 2-propanol solutions were characterised by peaks at 325 and 295 nm [20]. The positions of these peaks related to silver clusters are closer in position to the peaks for clusters in erionite (322 and 293 nm) than for clusters in mordenite (320 and 285 nm). In the solution, silver clusters are found in an isotropic environment. That suggests that in the tiny erionite cavities, silver clusters are in more isotropic surrounding that in the wider long mordenite channels. In the optical spectrum of AgM72-200 sample, the two peaks at 322 and 285 nm are assigned to the cluster species Ag8~ and Ag8~+, respectively [6]. In the spectrum of AgM10-100, the peak at 290 nm (attributed to Ag8~+) practically disappeared and only the peak at 323 nm (assigned to Ag8~ is observed. For these samples Ag-Ag distances are practically identical, but coordination number for Ag-Ag bond for sample possessing selectively Ag8~ clusters (3.0) is less that that for the sample with mixture of charged and neutral eight-atom cluster (3.6). This implies that Ag8~+ cluster is more deformed than Ag8~ According to Ref. [6] neutral Ag8~ clusters are stabilised by weak acid sites. The charged Ag8~+ clusters are stabilised by strong acid sites, which induce electron density transfer from silver cluster to acid site. This strong interaction between strong acid site and cluster could induce more significant deformation of cluster than weak acid site.

3.3. Sample with dominance of Ag particles The AgM128-300 sample represents the third group. The AgM128-300 sample is prepared at high temperature with mordenite possessing a very small concentration of weak Bronsted acid sites. According to optical data, it contains small Ag particles on the external mordenite surface. In Fig. 2, a very broad peak with maximum position ca. 400 nm was observed. This kind of broad peak is typical for plasma surface resonance band for small particles. Particles with size c a . 1 - 30 nm are observed in micrograph of AgM 128-300 sample (Fig. 3). EXAFS results showed that Ag-Ag distance for these particles (2.83 A) is high and comparable with that observed for Ag8 clusters in mordenite but the coordination number for Ag-Ag interactions is low (2.4). These results suggest that these Ag particles are amorphous. These EXAFS parameters could also be possible if the contribution of silver clusters in the sample is high but could not be registered by optical spectroscopy due to the high fraction coverage of the zeolite surface by Ag particles "masking" smaller silver clusters

1945 located inside pores. However, TEM charcterisation (Fig. 3) shows that the fraction coverage of the zeolite surface by Ag particles is not high for AgM128-300 and therefore, the second hypothesis is not proposed to be highly unlikely.

Figure 3. TEM micrograph of sample AGM128-300 4. CONCLUSIONS The characterisation of a range of silver species formed within zeolite matrices by a combination of EXAFS and Diffuse Reflectance UV-Visible spectroscopy has allowed conclusions to be drawn concerning the effects that the structural type, SIO2/A1203 ratio of zeolite matrix and reduction temperature exerts on the size, structure, deformation and oxidation of silver clusters stabilised inside zeolites. 1. The preparation of silver-zeolite sample using mordenite with medium Bronsted acid strength at high temperature (500 ~ or using mordenite with the very high strength at low temperature (100 ~ leads to fast oxidation of reduced silver species mainly into Ag§ cations. 2. After reduction at medium temperatures silver clusters interpreted as Ag8~ and Ag8~§ are formed in mordenites with intermediate Bronsted acid strength. 3. The reduction of silver samples at high temperature utilising mordenite with very small concentration of weak Bronsted acid sites leads to formation of amorphous Ag particles on external zeolite surface.

1946 4. Deformation of the eight-atom clusters is higher in the erionite cavities, where space is greatly restricted, than in mordenite channels. The charged Ag88+ clusters are more deformed than Ag8~ ones. ACKNOWLEGEMENTS

This work has been supported by CONACYT through grant No 31366-U and UNAM through grant PAPIIT-UNAM No IN 115800. The authors thank J.M. Corker for the performance of EXAFS experiments, and E. Flores, I. Gradilla, F. Ruiz, y G. Vilchis for technical assistance in optical and TEM studies. REFERENCES

1. M. Anpo, M. Matsuoka, H. Mishima, H. Yamashita, Res. Chem. Intermed. 23 (1997) 197. 2. M. Anpo, M. Matsuoka, Y. Shiyona, H. Yamashita, E. Giamello, C. Morterra, M. Che, H.H. Patterson, S. Webber, S. Ouellette, M.A. Fox, J. Phys. Chem. 98 (1994) 5744. 3. J. A. Martens, A. Cauvel, A. Francis, C. Hermans, F. Jayat, M. Remy, M. Keung, J. Lievens, P.A. Jacobs, Angew. Chem. Int. Ed. 37 (1998) 1901. 4. Y. Traa, B. Burger, J. Weitkamp, Micropor. and Mesopor. Mater. 30 (1999) 3. 5. N.E. Bogdanchikova, E.A. Paukshtis, M. Dulin, V.P. Petranovskii, Y. Sugi, T. Hanaoka, T. Matsuzaki, X. Tu, S. Shin, Inorg. Mater. 31 (1995) 487. 6. N.E. Bogdanchikova, V.P. Petranovskii, R. Machorro, Y. Sugi, V.M. Soto and S. Fuentes, Appl. Surf. Sci. 150 (1999) 58. 7. J.S. Ogden, N. Bogdanchikova, J.M. Corker, V.P. Petranovskii. Europ. J. of Phys. D. 9 (1999) 605. 8. V.S. Gurin, N.E. Bogdanchikova, V.P. Petranovskii, J. Phys. Chem. B 104 (2000) 12105. 9. T. Yokoyama, T. Ohta, Jap. J. Appl. Phys. 29 (1990) 2052. 10. P.A. Montano, J. Zhao, M. Ramanathan, G.K. Shenoy, W. Schulze, Z. Phys. D atoms, Molecules and Clusters 12 (1989) 103. 11. T. Miyanaga, H. Hoshino, H. Endo, H. Sakane, J. Synchrotron Rad. 6 (1999) 442. 12. S. Bordiga, C. Lamberti, G. Turnes Palomino, F. Geobaldo, D. Arduino, A. Zecchina Micropor. Mesopor. Mater., 30 (1999) 129. 13. S.M. Kanan, M.A. Omary, H.H. Patterson, M.Matsuoka, M. Anpo, J. Phys. Chem. B, 104 (2000) 3507. 14. T. Sun, K. Serf, Chem. Rev. 94 (1994) 857. 15. N. Binsted, "PAXAS": Microcomputer program for pre- and post-edge background subtractions. University of Southampton. UK, 1988. 16. S.J. Gurman, N. Binsted, I. Ross, J. Phys. C 17 (1984) 143; C 19 (1986) 1845. 17. G.J. Herdman, G.W. Neilson, J. Mol. Liquids 46 (1990) 165. 18. A. Balerna, E. Bernieri, P. Picozzi, A. Reale, S. Santucci, W. Burattini, S. Mobilio, Phys. Rev. B 31 (1985) 5058. 19. D.W. Breck, Zeolite Molecular Sieves. Structure, Chemistry, and Use (A WileyInterscience Publication, John Wiley & Sons, New York-London-Sydney-Toronto, 1974). 20. B.G. Ershov, E. Janata, A. Henglein, J. Phys. Chem. 97 (1993) 339.


1947

M o l e c u l a r d y n a m i c s s i m u l a t i o n s of s t a t i c a n d d y n a m i c p r o p e r t i e s of w a t e r a d s o r b e d in c h a b a z i t e S. Jost, S. Fritzsche, R. Haberlandt University of Leipzig, Institute for Theoretical Physics, Augustusplatz 10-11, D-04109 Leipzig, Germany As pores of zeolites under normal conditions are always filled with water, it is of great interest to get information about the structure and the dynamics of the adsorbed water molecules. Molecular dynamics (MD) simulations are used to get structural informations which are experimentally not accessible and to examine the diffusional process of water inside a zeolite. It is found, that the water molecules build up a partial hydration shell at the calcium atoms, which are attached to the oxygen atoms of the zeolite walls on their other sides. In the case of full water loading there are more water molecules than free places in the first hydration shells. Therefore, there is a part of water molecules which is able to move, leading to a diffusion coefficient, which increases with increasing number of guest molecules for a certain range of loadings. The special structure (trigonal space group R3m [1,2]) causes diffusion anisotropy, which is examined in further detail. The results for the diffusional part of the study are compared with experimental data.

~'::':!:i~:..':.i:~:!:ii:~ ~~'..~k.~.~ :.

.:!:i::

":':'~'::::i~'''''"

iiilii , i ~ i ~

i!iiiiii:iii!ill

.;iiiii~i:~i,!ili!:i:ii.:~:

9.':::"::: "".::::::::..

::::i:!:!:?:'....... ::" ~:..'.i::~:~i.'.':i:':~'"":~ ~:~ ~

:"::::::::::::"

Z • Figure 1. Left: A typical example of natural chabazite crystals, found in t~ep~ice (Czech republic). The size of the crystallites is approximately lcm. Right: Scematic view of the chabazite framework.

1948 Table 1 Partial charges of the atoms in units of e. OL1 denotes an oxygen atom between two silicon atoms, OL2 an oxygen atom between one silicon and one aluminum atom q(H) q(OH~.O)q(OL1)q(OL2) q(Si) q(A1) q(Ca) +0.33 -0.66 -1.025 -.12 +2.05 +1.75 +2.0

1. T H E M O D E L / C O M P U T A T I O N A L

DETAILS

The interatomic potentials are modelled as a combination of empirical and quantum chemical potentials from the literature. Besides the intramolecular water potential, all potentials are pair potentials. These potentials consist of one short ranged part of LennardJones or Buckingham type and a long ranged Coulombic part. As the calculations need very long computational times, we had to make a compromise in using sophisticated models and decided to ommit lattice flexiblility except for the cations, which are free to move. For the same reason we truncated all forces with a shifted forces correction at the cutoff r o u t = l lJ~. To improve the computational efficiency we used the RESPA algorithm [3], a multiple time step algorithm, with three different time levels. The intramolecular forces were calculated every 0.25 fs, the short ranged part of the interatomic forces every 0.5 fs and the long ranged part every 2 fs. With these technics the runs took approximately two months on our HP J5600 workstation, because for the diffusional part of this study calculations of up to 10.000.000 long ranged time steps (20 ns) were necessary. The physical situations under study vary from partially loaded zeolite (1 quarter of the possible loading) to the fully loaded zeolite, i.e. 13 water molecules per unit cell, and from room temperature (here: T = 300 K) up to very high temperatures (T = 600 K) to make diffusion faster. For the water-water interaction the BJH potential [4] is used, a well tested empirical model of valence force type. This model was initially develloped to fit the internal viTable 2 The pair potentials, without the Coulombic part, energies in kJ/mol, distances in A. The Ca-O potential is used for the oxygen of the lattice as well as for the oxygen of the water molecules. = l12010/(r s's6) - 1.0465. e x p [ ( - 4 . 0 ) . ( ( r - 3.4)2)] Vo-o 1.0465 9exp((- 1.5) 9 ((r - 4.5)2)) ( H20-H20, only) -26.1/r 9a - 41.86/(1 + exp[40.0 9 (r - 1.05)]) VO-H -16.744/(1.0 + exp[5.493 9 (r - 2.2)]) = 418767/(1 + exp[29.9 9 (r - 1.968)]) VH-H = - 1 5 7 2 / r 2 + 259700 9exp(-3.49 9r) VCa-O -

VCa-H Vca-ca gAl,Si-O Vo-o (nur H20-Gitter)

=

626/

= =

5560/r 2 + 1217632. exp(-6.79 9r) - 5 6 . 5 / r 6 + 1028/r 12 - 3 1 0 0 / r 6 + 3500000/r 12 + 15/(r 4)

+ 120200 9

xp(-6.79 9

1949 0.7

3.25 Mol./cage, T=30()K ,' 6.5 Mol./cage, T = 5 0 0 K .......... 13 Mol./cage, T = 6 0 0 K ..... ,~....

0.6 0.5 .o (3L

o

13..

X

,"\

//~".,.

0.4

//

0.3

, ," ",.~".,

,,'""

,, ,.

,

";

0.2

0.1

.--~(

,-

it...

. ._-.--';.: ............. .-"

,

,

, ~

0

3

4

5

1

2

........ 6

Figure 2. Probability distribution to find a calcium ion with a defined number of water molecules in its first hydration shell for different physical situations

brations, but was even succesfully used for hydration problems. It is combined with a potential for the interaction with calcium [5], which was fitted to quantum chemical calculations. The potentials for the framework guest interactions are taken from [6], except for the electrostatic charges, where we used the ones suggested in [7], because this choice ensures electrostatic neutrality for zeolites of general chemical compositions. 2. R E S U L T S A N D D I S C U S S I O N S

2.1. The hydration shell of the ions One major point in the structural analysis of the system is the hydration shell of the extra framework cations (calcium). The coordination number of calcium with respect to water molecules is influenced rather by the total amount of water in the system than by the temperature (see Fig. 2). For the fully loaded zeolite, the mean coordination number is 5 with a none vanishing part of ions with 3, 4 and 6 water molecules around them. For the half loading, not all possible places around the ions can be used, so the mean coordination number is is only 3 and the values, which occur vary from 1 to 5. In all cases the distance between the calcium ions and the oxygen atoms of the nearest water molecules is Ar ~ 2.4/~ (Fig. 4). Very interesting is the situation for the lowest situation under study, with a mean loading of 3.25 molecules per unit cell. In this case there is a big amount (~ 33%) of calcium ions without any water molecules in its direct neighbourhood. This amount is too big to be explained just by a random occupancy of equivalent cations. Indeed, further structural analysis shows, that these water free cations occupy the centers of the hexagonal prisms. These prisms connect the cages in the z-direction and are too small for a water-ion-complex. If we take into account, that the cations with 1 water molecule in the first hydration shell are likely to be placed closer to the border of the hexagonal

1950

0.9

i

i

i

0.5

1

1.5

0.8 0.7 0.6 ~

0.5

o 12.

0.4 0.3 0.2 0.1 0

0

2

Figure 3. Probability distribution to find a water molecule with a defined number of calcium ions in its immediate neighbourhood for different physical situations

prisms, but inside as well, we find, that almost all hexagonal prisms are occupied (There are two calcium ions per unit cell, but only one hexagonal prism.) Altogether there are two preferred types of sites, which are preferably occupied by the calcium ions: In the case of partially dehydratrion, the ions occupy the sites in the hexagonal prisms and then in the 8 rings of the framework. These site are not in the center, because the calcium ions are too small to bridge the 8 rings, but at the outer part. There the ions are usually attached to an oxygen atom of the framework, which has one aluminum atom as a direct neighbour. This behaviour is found quite similar in X-ray diffraction studies [8]. 2.2. T h e s t r u c t u r e of t h e a d s o r b e d w a t e r The other major point of intererest is the structure of the water molecules. The most important interaction with the framework system takes place with the calcium cations.

'

'

goo' goh .......... ghh ...........

6 _

'

5

i

i i

'

'

4

6

+o0.~'

ghda .......... ndca ...........

I

"I /

4 3 2

i.il 0

2

1 I

!

I

4

6

8

10

0

0

V//

2

8

10

Figure 4. Left" Radial distribution functions of water. Right: Radial distribution functions and running integration numbers of water with calcium. Both at T - 600 K and for 6.5 water molecules per unit cell

1951

u z ......)4....... rj~ L.M v-,

E

"T O

I

T"v

a 0.1

300

350

400

450

500

550

600

T/K

Figure 5. The main elements of the diffusion tensor for different temperatures and the half maximum loading.

Although the aluminum and the silicon have partial charges in the same order of magnitude as the calcium, their charges are screened by the neighbouring oxygens. For the lower loadings under study, almost all water molecules have exactly one neighbouring cation, only a few are stuck between two ions and very few have no cation in their immediate neighbourship. At the full loading, the situation has changed. Still most water molecules are attached to exactly one cation, but approximately 38 % have no cation as a direct neighbour. As these water molecules are relatively loose bound, they form a free phase of water inside the zeolite, which can easily diffuse. Another question was, weather the water molecules are able to build up a liquid like network. It turned out, that the structure is disturbed by the zeolite but most significant features are still there. The water can still build up hydrogen bonds, but the number is remarkably reduced to an average number of no more than one hydrogen bond per molecule. This implies, that the other hydrogen atoms are bound somewhere else, at the oxygen atoms of the lattice. 10

t'Xl

E

"T, O v

(:3 0.1

I

1.6 1.8

I

2

I

I

I

I

2.2 2.4 2.6 2.8

I

3

I

,.ft.

3.2 3.4

I000 K / T

Figure 6. Arrhenius plot of the mean diffusion coefficient for 6.5 molecules per unit cell

1952 30

|

,

4

5

,

,

,

7

8

,

,

,

,

25

:

E

20 15

--..- 10 a "1'-"-'

5

3

6

9

10 11 12 13

Water molecules per unit cell Figure 7. The mean diffusion coefficient versus loading at T = 600 K

2.3. Diffusion Due to the symmetrie of the crystal, diffusion in chaba~ite is anisotrop. It can be deduced from the the structure using a jump model, that diffusion in the x-y-plane should be a little bit faster than in the z-direction. If there are no further diffusion barriers Dz/D=y = 0.8 was predicted by B~r et al.[9]. But in the same study, they measured the diffusion coefficients with two different P FG NMR methods, a powder diffraction probe and a measurement with an oriented single crystal. The two methods agreed quite well and yielded Dz/D=y ,~ 0.4. This difference between theory and experiment was explained by the assumption of an additional diffusion barrier in z-direction in the middle of the cages. Diffusion of water in chabazite is a very slow process on the time scale of molecular dynamics simulations, so we had to heat up the system to get reliable diffusion coefficients up to T--- 600 K. At this temperature we found the following behaviour(fig. 7)" For a very low loading there was very little diffusional motion, leading to a very small diffusion coefficient. With increasing loading, the diffusion coefficient increases, up to approximately 75 percent of the maximium loading. For higher loadings the diffusion coefficients decrease with increasing loading. The explanation of this anormal diffusional behaviour is straight forward. A small number of water molecules is bound very strong to the cations and therefore not able to move. When almost all places in the first hydration shells are occupied, the remaining molecules can move faster. With further increasing number of water molecules, the number of mobile molecules increases and this leads to an increasing average movement. On the other hand, more molecules means more possible collision partners, decreasing the average displacement of one molecule. At approximately 10 molecules per unit cell this effect gains control over the complete process, leading to a decrease in the diffusion coefficients with further increase in the number of molecules. The anisotropie coefficient (fig. 5) varies in a range 0.7 < Dz/D=y < 1, in all situations,

1953 Table 3 The mean residence time of water in the free and the adsorbed phase for different loadings (in water molecules per cavity) and different temperatures. free phase bound phase Loading 3.25 6.5 13 3.25 6.5 13 >10ns >10ns >10ns 300K >10ns >10ns >10ns 400K 5ns 0.8ns 500K 0.4ns 3.5ns 600K > 10ns 0.14ns 0.57ns > 10ns 1.03ns 0.94ns

in which the diffusion coefficient can be calculated with sufficient reliability. This is very close, clearly within the errors of this study, to the theoretical prediction of B~ir et al.[9] Dz/Dxy = 0.8. But there is a significant deviation to the experimental value reported in the same study to be Dz/Dxy ,~ 0.4. The agreement is a little bit better with the tracer diffusion experiments[10], where no significant deviation from isotropy was reported. Unfortunately, we cannot confirm the assumption of an additional diffusive barrier with this study. But as it is impossible to make these calculations for room temperature, there might be such a low barrier, so it has no importance at these temperatures anymore.

2.4. Adsorption / desorption at the ions The other important dynamic process which is examined, is the exchange between the two phases of water molecules in the chabzite. As shown above, the water molecules in the first hydration shells of the calcium ions and the free water molecules are clearly distinguished. The question, whether there is an exchange betwewn these phases is important for the mechanism of diffusion, which can be observed. If there is no exchange, the ions with the hydration shells would block a lot of free space especially directly in the windows between the cages. If there is a frequent exchange, the ions can serve as a temporary station in the diffusion path. For room temperature adsorption and desorption to the first hydration shell is a very rare event in MD time scales, because there occur less than 0.05 events per particle and nanosecond, leading to less than 50 events in total, so these numbers cannot be evaluated. For T - 600 K we find residence times in the order of 1 ns. For our system, this means that exchange between the two phases is an improtant element in the diffusioal process, because the water molecules interchange between the two phases quite often. In those situations, when no exchange takes place, the diffusion coefficient is very low. 3. C O N C L U S I O N S The used model describes very reasonable the static properties of the system. The places for the cation agree well with experimental studies for a partially dehydrated zeolite as well as for a completely hydrated zeolite. In our model system, the water molecules build up a hydrogen bondend network even under these confined circumstances. They form a partial hydration shell around the calcium ions, which are attached to oxygen atoms of the walls on the other sides. We found a diffsion anisotropy in the order of magnitude, which is caused by the symmetrie of the crystal, but we found no evidence for an additional

1954 diffusional barrier. The exchange between the first hydration shell of the calcium ions and the free water is an essential process for the diffusion, as there is no measurable diffusion without it. REFERENCES

1. W.M. Meier and D. H. Olson. Atlas of Zeolite Structure types. Butterworths, London, second edition, 1987 2. J.V. Smith and F. Rinaldi. Acta Cryst., 16(1963)45 3. M. Tuckerman, B.J. Berne, G.J. Martyna, J. Chem. Phys. 97(1992)1990 4. P. Bopp, G. Jancso, K. Heinzinger, Chem. Phys. Lett. 98(1983)129 5. M. M. Probst, T. Radnai, K. Heinzinger, P. Bopp, B.M. Rode, J. Phys Chem 89(1985)753 6. P. Cicu, P. Demontis, S. Spanu, G.B. Suffritti, A. Tilocca, J. Chem. Phys. 112(2000)8267 7. Eugenio Jaramillo, Scott M. Auerbach, J. Phys. Chem B 103(1999)9589 8. W.J. Mortier, J.J. Pluth, J.V. Smith, Mat. Res. Bull. 12(1977)241 9. N.K. B~ir, J. K~rger, H. Pfeiffer, H. Sch~ifer, W. Schmitz, Mic. Mes. Mat. 22(1998)289 10. S. V. Garyainov, I.A. Belitsky, Phys. Chem. Min. 22(1995)443


1955

Correlations in Anisotropic Diffusion of Guest Molecules in Silicalite-1 S. Fritzsche a, J. Kgrger b aUniversity Leipzig, Institute for Theoretical Physics, Augustusplatz 9-11, D-04109 Leipzig, Germany bUniversity Leipzig, Institute for Experimental Physics I, Linnstr. 5, D-04103 Leipzig, Germany Analytical expressions for the correlation between the components of the diffusion tensor in the case of anisotropic diffusion of methane in silicalite-1 have been derived. The asymmetry in the different jump probabilities is taken into account. For comparison the same sytem is investigated by Molecular Dynamics simulations. Comparison of the derived dependencies with the results of the MD simulations yields satisfactory agreement. 1. I N T R O D U C T I O N

,< r

II

X Figure 1. Topology of the channels in silicalite-1

The diffusion of guest molecules is of crucial importance for many technical applications of zeolites as catalysts or molecular sieves [1]. Most popular applications are the use as ion exchangers in phosphate-free washing agents and the use as catalysts in the gasoline production. The migration of particles inside the zeolite is usually the process that controls the time scale and therefore the efficiency of such applications. Therefore, diffusion of guest molecules in zeolites is the subject of many experimental and theoretical investigations. Overviews can be found in [2-8]. The influence of the anisotropy of the channel systems in some zeolites (e.g. silicalite-1) on diffusion is examined in the present paper. The geometrical structure of the channel system in this zeolite is visualized in Fig. 1 (the

1956 more realistic shape can be seen in fig. 2) showing three unit cells including the axes of the channel system, straight channels in y- and zig-zag channels in x-direction. A consequence of this structure is that e.g. long-range movements of a particle in z-direction are only possible as sequences of moves both in in the straight and zig-zag channels. Therefore, the components of the diffusion tensor are correlated [9]. Similarly, an interrelation between the diffusivities in different directions has been predicted to occur in zeolites of type ZSM-11/silicalite-2 [10] and chabazite [11]. In earlier papers the memory effects in the sequence of random jumps have been neglected [12], or have been taken into account only partially by introduction of a two-step model of diffusion [13-15]. If such effects are neglected, the principle values of the diffusion tensor, i.e. the diffusivities in x-, y- and z-direction are determined by the topology and they are related to each other by simple reciprocal addition [9-12] Dz

=

Dx

+

Dv

with l~, ly and lz denoting the unit cell extensions in x-, y- and z-direction. Eq. (1) served as a guide for correlating the data of diffusion anisotropy in numerous experimental [16,17] and simulation studies [13-15,18-21] Though in many simulation studies in particular for small guest molecules the correlation between the diffusivities as provided by eq. (1) proved to provide a reasonable estimate, there were also notable deviations. In ref. [18] these deviations were quantified by introducing a memory parameter fl =

(2)

l~/Dz

It is obviously equal to 1 if eq. (1) is valid, i.e. in the case of totally negligible memory. 2. A N A L Y T I C A L T R E A T M E N T The parameters Pv,v, Pv,-v, Pv,x = Pv,-~ (and Px,~, P~,-x, P~,v - P~,-u), denote the probabilities that a displacement from intersection to intersection along a straight channel (zig-zag channel) is followed by a displacement in the same direction, in the opposite direction and along the other type of channels, respectively. Since the sum of all probabilities to proceed from a given intersection must be equal to one, the relations P~,~ + P~,-~ + 2Px,v -- Pv,v + Pv,-v + 2Pv,x - 1

(3)

are valid. The probability Pv (P~) that an arbitrarily selected segment of the diffusion path is along the straight (zig-zag) channels has to obey the stationarity condition =

+

(5)

yielding Pv

=

p~

=

2 - (Px,x "+"Px,-x "+"Py,y -k py,-y) 2 Pv,x 2 - (Px,x + P~,-~ + Pv,v + Pv,-v)

(6) (7)

1957 or equivalent expressions resulting by application of eq. (3). For particles without memory (i.e. the case considered in refs. [9-12]) we have p~,~ =

;~,_~ = p~,~ = p~/2

(8)

;~,~

; ~ _ ~ = ;~,~ = p~/2,

(9)

=

The mean square displacement of a particle in a given direction, e.g. the x-direction follows from the single shifts in that direction as

with n~ denoting the total number of displacements between adjacent intersections along the zig-zag channels and the respective displacements lxi being equal to + a (of. fig. 1). The component D~ of the diffusion tensor can be calculated from this quantity by the Einstein relation for large times t Dx = < x2(t) > (11) 2t ' with < x2(t) > denoting the molecular mean square displacement during t. Similar relations hold for the y- and z-directions where the displacements are lyi - i b and lzi = =l=c, respectively. The number of displacements in z-direction is the same as that in x-direction, since both displacements occur only simultaneously (see fig. 1). The fraction of the number of displacements in y-direction and that in x-direction must converge in the long-time limit to the ratio of the probabilities py and px

lim ---n~ PY--. t-,~r nx Px The mean square displacements are calculated [22,23]:

(12 )

l~

= nl ~ + 2 ~ < Z,l~+~ > + 2 ~ < l,l,+~ > + . . . (13) i--1 i=l with n standing for nx or ny or nz, I for a, b, or c and li for l~i, lyi or lzi, respectively. The mean values of the products of subsequent displacements < l~l~+l > have to be treated

separately for displacements within one channel type (i.e. in x- and y-direction), and for z-direction. As soon as a molecule changes from one channel type into the other, previous displacements along this channel are not correlated anymore with future ones along the same channel. Subsequent displacements in x- and y-direction are thus easily found to obey the relations < lxilx(i+j) >

-

(Px,x - Px,-x) j a 2,

< lyily(i+j) >

=

(py,y - py,-y)J b~.

(14) (15)

Inserting these relations into eq. (13) yields, for sufficiently large values of nx and ny --

lxi

=

n~

1

n--y

aS 1 + ( P x , z - P~,-~) 1 - (;~,~ - ;~,_~)'

lyi

__

b2 1 + ( p y , y - py,_y)

1 - (py,y - py,_y)"

(16)

(17)

1958 While the probabilities px,~ and p~,_~ (py,y and p~,_y) that subsequent displacements along the crystallographic x- (y-) coordinates are parallel or antiparallel directed, are given by the very model applied, the equivalent probabilities p+ and p_ for displacements in zdirection have to be provided by additional consideration. It may be deduced from fig. 1 that subsequent displacements in z-direction are antiparallel (parallel) oriented if they are separated by an even (odd) number of displacements along the straight channels. Summing over all probabilities belonging to either of the two cases yields OO

(18) i=0 O0

p+

=

4p~,ypy,~

~[(py,y + py,_y)2]i.

(~9)

i-0

The first two terms on the right hand side of eq. (18) stand for the cases of continuation of propagation along the zig-zag channels, while the sum including the prefactor considers all cases of even-numbered subsequent displacements along the y-channels. In all these cases the displacements in z-direction are opposed to each other. Eq. (19) runs over the odd numbers of intermediate displacements along y-direction and thus stands for the probability that subsequent displacements in z-direction are parallel. By summing up and making use of eq. (3) eqs. (18) and (19) may be transferred into p-

=

2(1 - p~,~ - p~,~) 1 + py,y + py,_y

P+

=

1 § py,y + py,_y"

(20)

2p~,~

(21)

With the corresponding replacements (px,~ or py,y by p+, p~,_x or py,_y by p_ and a or b by c) one finally obtains from eqs. (16) and (17) in the limit nz --+ oc

!

l~

= c~ 1 + (p+ - p _ ) = c~p_+

nz

1 - (p+ - p _ ) ---

2 C

Px,y

1 - Py,x - P~,y'

p_

(22)

where in the second equation we have made use of p+ § p_ - 1. Finally by use of the Einstein relation (eq(ll)) and the expressions for the mean square displacements as given by eqs. (16), (17) and (22) the memory parameter (eq. (2)) results to be

/~ =

p~c py A + Px B

(23)

with the notations A =

1 - (p~,~ - p~,_~) 1 + (p~,~ - p~,_~)

(24)

B

=

1 - (pu,y - py,_y) 1 + (py,y- p~,_y)

(25)

C

=

1-py,x-p~,y. Px,y

(26)

1959

yz plane

xy plane

Figure 2. Definition of the intersection regions. The circles mark those regions that are treated as intersection regions. The lowest energy values connected with small minima outside of the intersections are about -18 kJ/mol while the maximum in the center of the intersection is at about -9 kJ/mol. The energy difference between adjacent lines in the picture amounts to 1 kJ/mol. The outmost isopotential lines correspond to-5 kJ/mol.

With the eqs. (8) and (9), eq. (23) may be easily shown to fullfill the condition 3 = 1 for molecular propagation without memory. 3. M D S I M U L A T I O N S The relevance of the obtained analytical expressions has been investigated in extensive MD simulations, which are described in detail in ref. [24]. The simulations have been

1960

Table 1 Results of MD simulations for different radii r of intersection regions. The D values are in 10 -8 m2/s. The notations no-mem, mem and MD refer to data analysis without memory effects, with memory effects and the MD data. r/A 2.0 2.5 3.0 5.0 0.19345 0.16695 0.13477 0.04369 P~,~ 0.32031 0.43876 0.56070 0.87158 0.24078 0.19379 0.14724 0.04161 Px,y 0.37762 0.33034 0.26652 0.08525 Py,y 0.34844 0.4351210.54964 0.86070 Py,-y 0.13891 0.11877 0.09365 0.02751 Py,x 0.63348 0.61635 0.60295 0.59967 Py 0.36547 0.37775 0.38350 0.39653 P~ Ttx 3634 4 768 6 661 30 353 6443 ny 8 007 11023 47611 0.921 nx (no mem) 1.21 1.68 7.64 0.714 D~ (mem) 0.691 0.677 0.720 D~ (MD) 0.718 0.718 0.718 0.718 Dy (no mem) 1.61 1.99 2.74 11.81 Dy (mem) 1.70 1.62 1.53 1.49 Dy (MD) 1.68 1.68 1.68 1.68 (-o mere) 0.263 0.338 0.469 2.09 (mere) 0.160 0.152 0.146 0.153 Dz (MD) 0.17 0.17 0.17 0.17 1.40 1.42 1.42 1.41 fl (mem, eq. (23)) 1.33 1.33 1.33 (MD, eq. (2)) 1.33

carried out for a rigid lattice of pure silicalite-1 with methane as the - spherically shaped guest molecule at a loading of one molecule per channel intersection (corresponding to four molecules per unit cell) and a temperature of 300 K. The interaction parameters for the methane/lattice interaction have been taken from the spherical model potential derived in [25]. In the simulation procedure we have essentially followed our previous studies of the same host-guest system [19,26,25]. In the present study we have considered runs with an unperturbed evaluation part (after thermalization) of 5.106 simulation steps. The temperature was adjusted in the thermalization part of the run using a procedure described in [27,25] which enables runs in the microcanonical ensemble with a predefined value of the temperature. As the time step was 5 fs the length of the examined trajectory corresponded to a total time of 25 ns. A declaration of the limits of the channel intersection regions is not free from arbitrariness. One possible choice is illustrated by fig. 2. It shows the isopotential lines for the centre of a single methane molecule in three planes: a cut through the straigth channel in the yz-plane at x - 0, a cut through the zig-zag channel in the xz-plane at y = 0 and a cut through the straigth channel in the xy-plane at z = 0. The circles correspond -

1961 to cuts through spherical regions of radius 3 _~ which can be interpreted as intersection regions. The simulations have been carried out for different radii of such spheres. Table 1 provides a summary of the obtained simulation results. It particularly includes numerical values for all probabilities introduced in this study. For illustrating the data scattering, the values of both Px,y and Px,-y are presented which are found to differ by several per cent though they should coincide. The D values obtained from MD have been calculated from 4 moments of the displacement as described in [28] for the isotropic and in [25] for the anisotropic case. In addition to these diffusivities from the MD simulations, tab. 1 also contains the diffusivity data which would result by use of the Einstein relation together with eqs. (16), (17) and (22) with the indicated probabilities (case 'with memory' marked by (mem) in the table) and with their simplifications by eqs. (8) and (9) (case 'without memory' marked by (nomem) in the table). 4. C O N C L U S I O N S Summing up infinite series containing the conditional probabilities of passages through the straight and the zig-zag channels it is possible to derive formulas for the components of the diffusion tensor for methane in the silicalite-1. The results do not only mirror the topology of the channel system but, they also take into account memory effects. The derived analytical expressions presented in this paper yield values that are much closer to MD results than those from all earlier analytical treatments of the problem. Nevertheless, the agreement can probably be improved by further examinations. The interpretation of the results also remains a task for more detailed studies. 5. A C K N O W L E D G E M E N T The authors thank the Deutsche Forschungsgemeinschaft (SFB 294) for financial support.

REFERENCES 1. R. Haberlandt, S. Fritzsche, and H. L. Vhrtler, Simulation of microporous systems: Confined fluids in equilibrium and diffusion in zeolites, in Handbook of Surfaces and Interfaces of Materials, edited by H. S. Nalwa, volume 5, pages 358-444, Academic Press, San Diego, London, Boston, New York, Sidney, Tokyo, Toronto, 2001. 2. J. K~rger and D. M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley, New York, 1992. 3. N. Chen, J. T.F. Degnan, and C. Smith, Molecular Transport und Reaction in Zeolites, VCH, New York, 1994. 4. D. N. Theodorou, R. Snurr, and A. T. Bell, Molecular dynamics and diffusion in microporous materials, in Comprehensive Supramolecular Chemistry, Edt. G. Alberti and T. Bein, volume 7, pages 507-548, Pergamon, Oxford, 1996. 5. P. Demontis and G. B. Suffritti, Chemical Reviews 97, 2845 (1997). 6. S. Bates and R. van Santen, Adv. Catal. 42, 1 (1998). 7. F. Keil, R. Krishna, and M.-O. Coppens, Chem. Engin. Journal 16, 71 (2000). 8. R. Haberlandt, S. Fritzsche, and H. L. Vhrtler, Simulation of microporous systems:

1962 Confined fluids in equilibrium and diffusion in zeolites, in Handbook of Surfaces and Interfaces of Materials, edited by H. S. Nalwa, volume 5, pages 358-444, Academic Press, San Diego, London, Boston, New York, Sidney, Tokyo, Toronto, 2001. J. K~rger, J. Phys. Chem 95, 5558 (1991). 10. J. K/irger and H. Pfeifer, Zeolites 12, 872 (1993). 11. N.-K. B/ir, J. K/irger, H. Pfeifer, H. Sch/ifer, and W. Schmitz, Microporous Mesoporous Mater. 22, 289 (1998). 12. D. Fenzke and J. K/irger, Z. Phys. D 25, 345 (1993). 13. J. K/irger, P. Demontis, G. B. Suffritti, and A. Tilocca, J. Chem. Phys. 110, 1163 (1999). 14. P. Demontis, J. K/irger, G. B. Suffritti, and A. Tilocca, Phys. Chem. Chem. Phys. 2, 1455 (2000). 15. P. Demontis, G. B. Suffritti, and A. Tilocca, J. Chem. Phys. 113, 7588 (2000). 16. J. Caro et al., J. of Phys. Chem. 97, 13685 (1993). 17. J. Caro, M. Noack, K. KSlsch, and R. Sch/ifer, Microporous and Mesoporous Materials 38, 3 (2000). 18. E. J. Maginn, A. T. Bell, and D. N. Theodorou, J. Phys. Chem 100, 7155 (1996). 19. S. Jost, N.-K. B/ir, S. Fritzsche, R. Haberlandt, and J. K/irger, J. Phys. Chem. B .

lO2, 375 (199s).

20. 21. 22. 23. 24. 25. 26. 27. 28.

T. J. H. Vlugt, C. Dellago, and B. Smit, J. Chem. Phys. 113, 879 (2000). F. Jousse, S. M. Auerbach, and D. P. Vercanteren, J. Chem. Phys. 112, 1531 (2000). J. R. Manning, Phys. Rev. 116, 819 (1959). A. R. Allnatt and A. B. Lidiard, Atomic Transport in Solids, Cambridge University Press, Cambridge, 1993. S. Fritzsche and J. K/irger, J. Phys. Chem., in preparation. S. Fritzsche, M. Wolfsberg, and R. Haberlandt, The Importance of Several Degrees of Freedom for the Diffusion of Methane in Silicalite-1, submitted to Chem. Phys. S. Fritzsche, R. Haberlandt, S. Jost, and A. Schiiring, Molec. Sim. 25, 27 (2000). S. Fritzsche, Untersuchung ausgew~hlter Nichtgleichgewichtsvorg~nge in Vielteilchensystemen mittels statistischer Physik und Computersimulationen, Habilitation Thesis, University of Leipzig, 1998. S. Fritzsche, R. Haberlandt, J. K/irger, H. Pfeifer, and K. Heinzinger, Chem. Phys. Lett. 198, 283 (1992).

Studies in Surface Science and Catalysis 142 R. Aiello, G. Giordano and F. Testa (Editors) 9 2002 Elsevier ScienceB.V. All rights reserved.

1963

A combined anomalous XRPD, EXAFS, IR, UV-Vis and photoluminescence study on isolated and clustered silver species in Y zeolite C. Prestipino 1'2, C. Lamberti 1'2'3, A. Zecchina 1'3, S. Cresi 1, S. Bordiga 1'3, L. Palin4, A. N. Fitch4, P. Perlo ~ and G. L. Marra6 1 Department of Inorganic, Physical and Material Chemistry, Via P. Giuria 7, 10125 Turin (I) 2 INFM Unit~ di Torino Universit~ (I) 3 FNSTM Unith di Torino 4 ESRF, Diffraction Group, BP 220, F-38043 Grenoble Cedex, France 5 Centro Ricerche FIAT, Torino (I) 6 Polimeri Europa, S.p.A., Istituto Guido Donegani, Via G. Fauser 4, 1-28100, Novara (I) In this contribution we report on structural (XRD and EXAFS), optical (UV-Vis DRS and photoluminescence) IR (adsorption of CO) characterization of a virtually homoionic Ag-Y zeolite (Si/A1 = 2.63). Our study shows that the zeolite is a virtually 100% exchanged silver faujasite, showing almost isolated Ag+ counterions (EXAFS estimates that clustered species represent less than 2% of the whole silver). Synchrotron radiation XRPD measurements (ESRF, BM16), performed at the Ag-K edge [2` = 0.486103(2) A], just before [E -0.486093(2) A] and far away [2, = 0.491153(2) A], allowed us to locate the nearly totality of the expected Ag+ counterions: 52.0(4) out of 52.9 per unit cell, located in five different sites. Two out of the five are located in the supercage, and thus accessible to small ligand molecules, as detected by IR spectroscopy that singled out the presence of two distinct Ag+-'-CO adducts. A subsequent EXAFS (ESRF, BM29), UV-Vis DRS and photoluminescence spectroscopic study on the aggregation of Ag clusters upon thermal reduction of Ag-Y zeolite will be also briefly reported. 1. INTRODUCTION Noble metal-exchanged zeolites are active catalysts in economically important processes. In particular, Ag(I)-exchanged zeolites show high activity in several catalytic and photocatalytic processes [1-4] which have been performed by exploiting the presence of both isolated Ag+ ions and aggregated Agn clusters. Among them we can mention the photochemical dissociation of H20 into H2 and 02 [5,6], the disproportionation of ethylbenzene [7], the selective reduction of NO by ethylene [8] and the photocatalytic decomposition of NO [9]. In this contribution we will report on structural (XRD and EXAFS), optical (UV-Vis DRS and photoluminescence) IR (of CO) characterization of a virtually homoionic Ag-Y zeolite (Si/A1 -- 2.63). Our study shows that we are dealing with a virtually 100% exchanged silver faujasite showing only isolated Ag+ counterions (EXAFS estimate that clustered species represent less than 2% of the whole fraction of silver). The effect of sample activation at increasing temperature on the aggregation of silver clusters will also be briefly commented.

1964 2. EXPERIMENTAL 2.1. Materials and activation procedures Ag+-Y was prepared starting from a NH4-Y sample (Si/AI = 2.63), synthesized in Istituto Guido Donegani through a conventional exchange with a solution of AgNO3. A nearly total exchange has been obtained (corresponding to one Ag + ion for every AI framework atom), as monitored by the total erosion of the absorption bands of internal bridged Bmnsted -Si-(OH)-AI- groups centered at 3643 and 3547 cm q The first step in the characterization of a catalyst, and thus of a zeolite, concerns a thermal treatment able to remove all the molecules pre-adsorbed on the catalytic active centers coming from the ambient atmosphere (activation process). This process is needed in order to guarantee the study of well defined systems [10]. Once this step has been achieved, measurements can be performed, in situ, either on the as activated sample (i.e. zeolite under vacuum conditions) or~ after having dosed a well defined amount of high purity gas on the sample. Now, the achievement of the activation process is very critical for silver exchanged zeolites because an increase of the activation temperature (suitable to remove the most strongly bonded molecules) has the disadvantage of facilitating the aggregation of Ag + ions into Agn clusters [11,12]. Such clusters, needed for some catalytic application (vide supra), are undesired if the aim of the study is the location of isolated Ag + ions. Based on our previous study on the Ag-ZSM-5 zeolite [13,14], an activation temperature of 120 ~ has been adopted. Conversely, part of the as exchanged powder has been subjected to thermal treatments at increasing temperature (200-400 ~ with the aim to study the progressive aggregation into silver clusters. 2.2. Methods We have performed Ag-K edge x-ray absorption measurements, using the synchrotron radiation emitted by the bending magnet of the BM29 beamline at the ESRF (Grenoble, F), equipped with a Si(311) monochromator detuned to avoid harmonics. Each EXAFS spectrum, measured in transmission mode, was recorded three times under the same experimental conditions, and extracted z(k) have been averaged before the EXAFS data analysis. Standard deviation calculated from the averaged spectra was used to estimate the weight of statistical noise in the evaluation of the error associated with each structural parameters. The EXAFS data analysis has been performed following standard procedures [15] and using Michalowicz's programs [16]. The angle/energy calibration has been obtained by measuring the edge position of the corresponding metal foil (measured simultaneously with the sample by means of a third ionization chamber). Powder diffraction patterns were collected at room temperature on the powder diffraction beam line BM16 at the European Synchrotron Radiation Facility (ESRF), Grenoble, France. The beam line was used with a collimating mirror before the Si(111) water-cooled, double-crystal monochromator, set to deliver the desired wavelength [;~ = 0.486103(2), or 0.486093(2) or 0.491153(2) A], in all cases calibrated using the NIST Si standard 640b. The optional focusing mirror after the monochromator was not used. The sample capillary was spun on the axis of the diffractometer, while the detector bank was scanned from 20 = 0.5 to 20 = 32 ~ at a rate of 0.5~ at low angle and at a rate of 0.1~ at high angle. Data were collected in a continuous scanning mode, with the electronic sealers and the 20 encoder reading around 6 times per second. High-angle regions were scanned more than once to improve the statistical quality of the pattern. The total data acquisition lasted ca. 8 h for each sample. For powder XRD measurements,

1965 zeolite powder, activated under dynamic vacuum at 120 ~ for lh, was transferred (in vacuo) into a boronsilicate capillary with a l-ram diameter. The capillary was sealed and then mounted on the sample spinner on the axis of the diffractometer, which maximizes the number of crystallite orientations presented to the incoming radiation and minimizes the effect of any preferred orientation in the sample. More details on the XRPD acquisition and Rietveld refinement strategies have been reported elsewhere [17-20] where the same instrument has been used to characterize other zeolitic systems. For IR measurements, we have used an IR cell, designed to allow in situ high-temperature treatments, gas dosage and low-temperature measurements. The IR spectra were recorded, at 2 cm "~ resolution, on a BRUKER FTIR-66 spectrometer equipped with a MCT detector. UV-Vis diffuse reflectance experiments have been performed with a Varian CARY5 spectrophotometer. Photoluminescence spectra have been recorded on a SPEX Fluorolog-2 spectrofluorometer equipped with a xenon UV-Vis-NIR excitation lamp, whose light is filtered before reaching the sample by an excitation monochromator; the photoluminescence emission is than selected by a second monochromator (emission monochromator) before reaching the photomoltiplier. This photoluminescence apparatus allows thus to perform both emission and excitation scans, the former by fixing the excitation and scanning the emission monochromator, the latter by fixing the emission and scanning the excitation monochromator [21]. Both monochromators allow a sampling step up to 1 nm in the 2001000 nm (50000-10000 cm "~) spectral range. All the reported spectra have been collected with an integration time of 1.0 s per point and a sampling step of A~ = 1.0 nm. Both diffuse reflectance spectroscopy (DRS) and photoluminescence experimental set-up allow to measure the samples under controlled atmospheres. 3. RESULTS AND DISCUSSION 3.1. Isolated Ag + cations: anomalous XRPD, EXAFS and IR study The tridimensional structure of zeolite-Y is generated by connecting sodalite units with hexagonal prisms to give a framework characterized by big empty cavities (supercages) with a diameter of about 13 A. It is recognized that the cations are mainly located in a few well-defined sites [22], see Figure 1. Site I is located in the center of the hexagonal prism, closely surrounded by six oxygens of the two bases of the prism. Cations located in this site are almost totally inaccessible to guest molecules. Cations in site I' are located on the external basis of the hexagonal prisms, just inside the sodalite cage; these cations are surrounded by three oxygens of the basis of the prisms and are accessible only to molecules able to penetrate through a six-membered ring from the supercages into the sodalite cavities. This penetration is not possible even for small molecules like CO [23]. Owing to Coulombic repulsion, the simultaneous occupation of adjacent I and I ~ sites is forbidden. Sites I r and II are located in the middle of the six-membered ring forming the frontier between the supercage and the sodalite cage, just inside the sodalite cage and the supercage, respectively. For the same reason mentioned before, adjacent sites cannot be simultaneously occupied. Sites I, r , i r and II are coordinated to the framework oxygen of the zeolitic walls and are peculiar of dehydrated Y. In presence of water molecules cations are solvated and occupy positions located in the middle of the cages. Figure 1 also reports the position of the five different extraframework Ag + sites obtained from the Rietveld refinement, labelled as sites I (8.2), r(17.4), Ha (6.6), Hb (15.2) and I'm(4.6) (in parenthesis the occupancy per unit cell is given). It is worth noticing that two different sites II have been found and arbitrarily labelled as IIa and IIb. Four of them occupy positions typical of dehydrated cations. On the contrary I'~ is in the middle of the sodalite cage: this implies that such ions must be coordinated to undesorbed water molecules. In fact, in

1966

Fig. 1. Representation of the Y zeolite framework and cation location. In the fraction of the framework located in the upper fight part of the figure O atoms are represented in dark grey an the T atoms (Si or A1) in light grey. For clarity in the remaining part of the framework only light grey sticks have been adopted. Ag + cations are represented as spheres and are labelled with I, I', lla, lib and l'm in different greys scales. The supercage cavity, where guest molecules can be hosted, can be noted in the centre of the figure. anhydrous conditions, the positively charged cations are in contact with the zeolitic walls, where they interact with the negatively charged oxygen atoms of the framework. The fact that only less than 5 out of 52 cations are solvated by coordinated water molecules (being 48 in form of anhydrous cations coordinated to the zeolite framework) proofs that the activation procedure was carefully selected. Figure 2 shows the high resolution XRPD pattern collected at ~, = 0.491153(2) A and at ~, = 0.486103(2) A. Of interest is the change in relative intensity between some of the observed peaks by moving across the Ag K-edge. The Rietveld refinement, performed simultaneously on the three data sets, allows to obtain the zeolite framework and to locate the nearly totality of the expected (on the basis of the Si/A1 ratio) Ag + counterions: 52.0(4) out of 52.9 per unit cell. This is a rather remarkable result if compared with what obtained for the cases of Cu+-Y [20] and of Rb§ [18] (also BM16 data) systems, where we were able to locate "only" 41.0(5) and 48(1) cations respectively. This noticeable improvement is ascribed to the following arguments: (i) the higher scattering power of Ag; (ii) the simultaneous use of three independent frames; (iii) the benefit of the anomalous effect.

1967 i

I

I

4O 0 r

! i

i i

-

+ +

i I ~~

_

o

'

!

~

~

-

!+ ; ; t _

-

I

II

I I

"~"~""F

I I

III I I

~"~ "~

I I II

.......

"r-'- ................................................

,,,I

I

10

20

I

--

'"

-

,

,

I 30

i

"i

...... ...........

+ +

~+ 0 0

+i

§

o (J

_

I

II

I I

I I IU IIII

II I I I I I I I I I I I

II ••••••••••••••••••••••••••••••••••••••••••••••••u•••••••••••M••••i•••••••••••••••••••••

__

j. L,,~,..~,.I~.~,I ~,..~ L ............................................................

I

I . . . . . . . . .

I

10

20 2-Theta

,,,

I 30

(Degrees)

Fig.2. Observed, calculated and difference profiles and reflection positions of the AgLY zeolite. Top: Z, = 0.491153(2) A; bottom: ~, = 0.486103(2) A. We have so used the output of the Rietveld refinement to simulate the EXAFS data reported in Figure 3. In theory we have to simulate 5 EXAFS signals representing the contribution to the overall signal coming from the Ag absorbers located in the 5 different sites found by XRPD. The contribution coming from the hydrated I'm site has been ignored due to the facts that: (i) less than 10% of the absorbers occupy this site and (ii) a rather high Ag-OH2 Debye-Waller factor is expected. Moreover, since sites I' and lib show a similar local environment, (three oxygen atoms at very close distance: 2.46 against 2.47 A), the corresponding contributions were merged in a single one. As a result, 3 different contributions have been used to simulate the experimental EXAFS signal. For each contribution, the Ag-O distance was fixed to the crystallographic value (2.31 A for lla,

1968 2.61 A for I and 2.465 A for I' and Hb), while the coordination number was obtained from the "true" number of first shell O neighbours multiplied by a weighting factor obtained form the relative occupancies given in the Rietveld refinement output. Ref. [20] describes the procedure for the Cu+-Y case in great details. The two superimposed spectra at the bottom of Figure 3, represent the experimental EXAFS signal, filtered on the Ag-O peak (vide infra vertical dotted arrows in Figure 5a), and the best fit obtained by adding the three contributions described above, where the optimized parameters were one common AE and three different Debye-Waller factors. The quality of the EXAFS fit, obtained with only 4 independent parameters, on a so complex sample, represents the definitive prove of the quality of the Rietveld refinement of the XRPD data.

IIa

0.5 a.u.

I o.o5A I'+ IIb

ir

J

...... exp.

IIa +I', + IIb+ !

|

4

.

.

.

.

.

8 k (A') 12

w

'

16

Fig. 3. Best fit and simulated EXAFS spectra of the contribution to the overall signal of silver cations located in, from top to bottom: site IIa (I' and IIb merged) and I and sum of the three simulated contributions (full line) superimposed with the experimental first shell filtered kg(k) function.

2225

2200 2175 2150 2125 Wavenumber (cm-~)

Fig. 4. Room temperature IR spectra collected at increasing CO equilibrium pressures dosed on Ag+-Y zeolite. Two Ag+CO adducts are clearly observed, corresponding to CO adsorbed on IIa and IIb sites: low and high frequency bands respectively.

1969 Figure 4 shows the IR spectra of increasing doses of CO on Ag+-Y zeolite. Two distinct CO stretching bands are clearly observed at 2195 and 2186 em"t, which reflect the two Ag+CO adduets formed on the two cationic sites available in the supereage cavity, i.e. the only two accessible to CO. The high frequency band is ascribed to the adduet formed on lib sites owing to the higher polarization power of silver cations which are less shielded by framework oxygen atoms owing to the higher Ag+-O distance (2.465 A vs. 2.31 A of the IIa site). We can so conclude this section by underlying how the high resolution XRPD study has been able to explain both EXAFS and IR evidences.

3.2. Clustered silver species: preliminary EXAFS, UV-Vis and luminescence evidences

Clear evidences of the progressive clustering undergone by Ag-Y upon increasing the activation temperature is reported in Figure 5 with EXAFS, photolumineseenee and DRS UV-Vis spectroscopy.

(a)

",

.... ;---

~,

I I', I

I ; /

/_\

I /

I OA

:i.'/~1 .'.,

I!

\'~J

i ~

I

'

_

'

,/',

. :.+

,.-.,

~ ~

-,

I

'

'

~1

,"

I

,''

! ~

.

~1~

,lf

~176

.....

v~ I f

-~ ~[ I I~

,

'

A II

.... 250~ ---400 ~

L?SoP.

i+.+t .

I

/ ~'

/,

Wavelenght(nm)

(C)

+

' 1

-",~2o~

II

~

,' '

t, ~'t,,~.7,...,,-, ~ +,:,+,+,

2

(b)

~

~/~

,-- , . .

-'

'--

: .~ .I ~'.l

0

t

:""

!

II:

120 ~ x 20 250 ~ x 4 400~

\,

I

!

40000

,

"...

30000

,,

,.

20000

W a v e n u m b e r cm -1 Fig. 5. Effect of increasing the activation temperature of Ag-Y zeolite on the kS-weighted, phase uncorrected, FT of the EXAFS signal, on the excitation (250-400 nm range) and emission (400-650 nm range) photoluminescence spectra and on the DRS UV-Vis spectra, parts (a), (b) and (e) respectively.

1970 The Ag-O and Ag-Ag contribution are well resolved in the FT of the EXAFS data since occurring in the 1.35-2.15 A and 2.15-2.90 A ranges respectively. Due to the much higher scattering power of Ag with respect to O neighbors the FT of Ag-Y activated at 120 and 250 ~ have been magnified by a factor of 20 and 4 respectively. For these two samples only a first shell Ag-Ag contribution is observed, reflecting the presence of dimers, trimers and tetramers clustered species. Conversely the EXAFS signal of the sample activated at 400 ~ is dominated by the typical feature of the metal in FCC crystals reflecting the much higher size of silver clusters. As dearly shown in parts (b) and (c) of Figure 5, the progressive clustering of silver atoms has strong influences of the optical properties of the material [11,24,25,]. Photolumineseence spectra exhibit excitation components at 270, 310 and 335 nm and emission bands at 435 and 534 nm which relative intensities change upon changing the sample activation temperature. The same holds in the UV-Vis DRS spectra, where the sharp and well defined absorption at 35115 cm "1 (285 nm), typical of isolated silver species in site I is progressively reduced upon increasing of the absorption edge at 29000 cm l (345 nm) typical of silver metal. REFERENCES 1. T. Sun and K. Serf, Chem. Rev., 94 (1994) 857, and refs. therein. 2. D. Lai, J. Li, P. Huang and D. Wang, J. Mater. Gas. Chem. 3 (1994) 211. 3. Y. Inoue, K. Nakashiro and Y. Ono, Mieroporous Mater. 4 (1995) 379. 4. K. I. Hadjiivanov, Mieroporous and Mesoporous Mater., 24 (1998) 41. 5. P.A. Jacobs, J.B. Uytterhoeven and H.K. Beyer, Chem. Commun., (1977) 128. 6. G. Calzaferri, S. Hug, T. Hugentobler and B. Sulzberger, J. Photochem, 26 (1984) 109. 7. T. Baba and Y. Ono, Zeolites, 7 (1987) 292. 8. S. Sato, Y. Yu-u, H. Yahiro, N. Mizuno and M. Iwamoto, Appl. Catal., 70 (1991) L 1. 9. M. Anpo, M. Matsuoka and H. Yamashita, Catal. Today, 35 (1997) 177. 10. J.M. Thomas, Chem. Eur. J. 3 (1997) 1557. 11. L.G. Gellens, W.J. Mortier, R.A. Schoonheydt and J.B. Uytterhoeven, J. Phys. Chem., 85 (1981) 2783. 12. L.G. Gellens, W.J. Mortier, J.B. Uytterhoeven, Zeolites 1 (1981) 11. 13. S. Bordiga, C. Lamberti, et al. Microporous Mesoporous Mater., 30 (1999) 129. 14. S. Bordiga, G. Turnes Palomino, D. Arduino, C. Lamberti, A. Zecchina and C. Otero Are~n, J. Mol. Catal. A 146 (1999) 97. 15. F. W. Lytle, D. E. Sayers and E. A. Stem, Physica B 158 (1989) 701. 16. A. Michalowicz, J. Phys. IV France 7 (1997) C2-235. 17. G.L. Marra, A.N. Fitch, A. Zecehina, G. Ricchiardi, M. Salvalaggio, S. Bordiga and C. Lamberti, J. Phys. Chem. B, 101 (1997) 10653. 18. C. Lamberti, S. Bordiga, A. Zecehina, A. Carati, A.N. Fitch, G. Artioli, G. Petrini, M. Salvalaggio and G.L. Matin, J. Catal., 183 (1999) 222. 19. G.L. Marra, G. Artioli, A. N. Fitch, M. Milanesio and C. Lamberti, Microporous Mesoporous Mater., 40 (2000) 85. 20. G. Turnes Palomino, S. Bordiga, A. Zecchina, G. L. Marra and C. Lamberti, J. Phys. Chem. B, 104 (2000) 8641. 21. C. Lamberti, S. Bordiga, M. Salvalaggio, et al. J. Phys. Chem. B, 101 (1997) 344. 22. J.V. Smith, Adv. Chem. Ser. 101 (1971) 171. 23. S. Bordiga, D. Scarano, G. Spoto, A. Zecchina, et al. Vib. Spectrosc., 5 (1993) 69. 24. R. Kellerman and J. Texter, J. Chem. Phys. 70 (1979) 1562. 25. G.A. Ozin and F. Hugues, J. Phys. Chem. 87 (1983) 94.


1971

D F T a n d I R studies on c o p p e r sites in C u Z S M - 5 : s t r u c t u r e - r e d o x c o n d i t i o n s - d e n o x activity r e l a t i o n s h i p E. Broclawik a, j. Datka b, B. Gil b and P. Kozyra b aInstitute of Catalysis, PAN, ul. Niezapominajek 8, 30-239 Krakow, Poland bFaculty of Chemistry, Jagiellonian University, ul. Ingardena 3, 30-060 Krakow, Poland In this work we have studied by quantum chemical (DFT) modelling electronic and geometrical properties of copper centres in 13 position in ZSM-5. The results were compared with our earlier data concerning oc sites. It was evidenced that the properties of both Cu 2+ and Cu + in c~ sites differ from those in [3 sites. This is the most pronounced for Cu + the positive charge of which is to the largest extent neutralised by framework oxygens when located in c~ sites. Consequently Cu + in ct site has the highest energy of HOMO orbital, it is therefore the best electron donor to ~* antibonding orbital of NO and activates the most the adsorbed molecule. 1. I N T R O D U C T I O N CuZSM-5 zeolite was intensively studied due to its exceptional properties and activity in deNOx (for review see ref. 1). Nevertheless, detailed information on probable positions and coordination of exchanged copper cations in the context of their activity towards NO is still missing. It is known e.g. from photoluminescence that among many possibilities two positions: c~ and 13 are prevailing while one of them (c~) is suggested to be catalytically active [2,3]. Thus we have undertaken DFT calculations for representative models of both copper sites in CuZSM-5. The aim of this study was to provide information on structural and coordination properties of copper cations in various sites and oxidation states, and on their interaction with NO, in hope to find factors responsible for activity differences. The framework of the ZSM-5 zeolite provides three kinds of exchangeable cationic sites denoted as ec, [3 and 3' [3,4], amongst them the two first are more abundant and preferentially occupied by copper ions. This gives rise to distinct speciation of copper positions reflected in its adsorption properties and reactivity. In addition, Cu + sites are produced from Cu 2+ exchanged cations by the process of self-reduction during dehydration and a number of hypotheses have been proposed as to the structure and coordination of both copper forms [5,6]. The specific geometrical and electronic structure of self-reduced Cu species is primarily related with the reduced coordination along with the supramolecular effects due to the zeolite framework acting as a generalised ligand. This leads to their exceptional ability to switch from electron-acceptor to electron-donor properties and, in consequence, to activate adsorbed NO. This study concerns the interaction of Cu 2+ and Cu + in ZSM-5 zeolite with NO molecules. The focus has been devoted to elucidation of speciation and molecular structure of the intrazeolite adducts formed upon adsorption and self-reduction. These processes have been investigated by DFT method and the results were compared with IR data.

1972

1.1. Quantum chemical modelling DFT calculations were carried out for cluster models by Dmol software of MSI 9 [7]. Dmol code is the implementation of numerical scheme for solving Kohn-Sham equations. We have chosen standard calculational parameters e.g. local VWN exchange-correlation potential and numerical DNP basis set. Inner core orbitals were frozen during calculations. This choice was promoted by the compromise between computational efficiency and expected accuracy. It is believed that for pure DFT methods (not based on the Hartree-Fock scheme) the results are only moderately dependent on the basis set, and local approximation to exchange-correlation potential, although approximate, is clearly derived from basic physical principles. On the other hand cluster modelling of the solid is always burdened with some degree of arbitrariness. In principle, there exist several embedding schemes to simulate remaining part of the solid, which range from boundary atom freezing to very sophisticated methodologies mixing quantum chemical calculations with semi-classical simulations. We did not use more elaborated embedding schemes for the sake of easy access to molecular properties, including full vibrational analysis. The properties to be discussed here are geometrical parameters, charge distribution, one-electron energy levels and stretching frequencies of NO obtained from diagonalisation of the Hessian matrix. Thus the approach is in principle qualitative and the discussion is focused on differences between the sites hosting copper cations in oxidised or reduced forms. Models utilised in our calculations were cut off the MFI structure taken from MSI databases included in the software. Two models were selected: i) basket (M7) model composed of two fused 5T rings forming a 6T ring which simulated framework environment for copper cations in ~ position on main channel wall and ii) deformed 6T ring (Z6) postulated as framework environment for 13 copper position in sinusoidal channel. The first model (M7) has already been elaborated by us and studied in more detail previously [8-11]. Here we attempt to describe the other site in a parallel manner and compare properties of the both with emphasis put on subtle differences which could shed some light on their difference in activity towards NO. In M7 and Z6 parent structures two Si cations were substituted with A1 atoms with two protons compensating negative charge of substituted system. Geometry of all clusters has been optimised with constrained protons in terminal OH groups to keep MFI structure. In the next step Cu 2+ cation was substituted for two protons or Cu + for one proton to mimic the site after cation exchange and self-reduction. Initial position of the copper cation was selected in the centre of 6T ring in both models, final geometry of the system was the optimised one. Similar approach has also been pursued by many authors, within both semiclassical [12] and more rigorous quantum chemical regime [13-16]. We have taken the advantage to merge our experience with that of other groups, which added credibility to selected models. On the other hand, our approach enabled extending the study with explicit examination of the interaction between the site and the sorbed NO molecule and discussion of its effects on the molecule on electronic level. Thus, the most important objective of our study was the interaction between both copper forms located in the two specified models of ZSM-5 framework and NO. In each case the molecule was put in approximate bonding distance from the proper copper site and the final geometry of the complex was determined from calculations along with the details of electronic structure and properties. In the last step full vibrational analysis was performed to calculate NO stretching frequency which is a measure of the activation of NO and could be compared with IR spectra.

1973 2. R E S U L T S AND DISCUSSION Figure 1 shows schematically the structure of the MFI framework taken from MSI databases. Parts of the framework selected as M7 and Z6 models of ct and [3 sites, respectively, are marked with bold lines. After exchanging two silicons, which were separated by two other Si centres but were closest in the distance, with A1 centres these models were subjected to saturation and optimisation procedures according to the rules described in methodology. Results concerning properties of copper centres in tx position have already been published [10,11,17], here we concentrate on details of Z6 model of the 13 site for copper centre and, what is the most important, on comparative analysis of the two. Figure 2 shows already optimised geometrical structure of Cu 2§ (a) and Cu + (b) centres in Z6 model. Only the atoms forming the basal six-ring are labelled apart from the extra proton compensating the charge which has been added to the terminal AI(OH) group in the case of Cu + centre. It can be clearly verified that while divalent copper forms four bonds with 9

.

"'," -. . . . . .

"

,

,

,

,

~',-.-,, -" /

.

Figure 1

",....

+

..-

.... ...

,.,

"./

.

.

..... ",

. . . .

....:.

.

......

.

,

.

.

..

\:i", ' ~

~

M7 model for tx site (left) and Z6 model for 13 site (right) in ZSM-5

t

~ g..22o

Figure 2

i

o

z

A,v

=A

194t o

Coordination of C u 2 + (left) and Cu + (right) in Z6 model of 13 site in ZSM-5 (all distances given in ,~)

1974 Table 1. Bond distances between copper cations and bridging oxygens in M7 (c~, data taken from refs. 10,11,17) or Z6 (p) cluster models (below 2.6 A, index sT denotes oxygen in 5T ring of M7, otherwise in 6T ring), mean framework distortion (RMS) and charges on exchanged copper. Centre / Site

Cu 2+/Z6(13)

Site

Site with NO Rcu-o [A]

Qcu

RMS

Rcu-o [a]

Qcu

1.94, 1.96, 2.20, 2.25

+0.48

0.376

1.95, 1.97, 2.14

+0.45

CuZ+/]VI7(ot) 2.03, 2.03, 2.05, 2.05

+0.50

0.242

2.02, 2.03, 2.105T

+0.43

Cu +/Z6([3)

1.90, 1.94, 2.60

+0.32

0.268

2.01, 2.16, 2.23

+0.37

Cu +/M7(cz)

1.93, 2.01, 2.035T

+0.28

0.178

2.02, 2.06, 2.065T

+0.36

bridging oxygens, the reduced form makes two strong and one weak Cu-O bonds. The cluster undergoes substantial distortion to keep Cu + bonded in its centre in variance with more symmetrical or more rigid six-ring fragments of the framework where copper migrated after reduction towards more packed structural fragments [ 10,11,17]. The distance between two A1 atoms diminishes from 5.91A for p site in Cu2+ZSM-5 to 5.81 A for Cu+ZSM-5. Next the interaction of both copper centres (p site in Cu2+ZSM-5 and Cu+ZSM-5) with nitrogen oxide (II) has been examined within the same paradigm. Table 1 shows the comparison of structural and selected electronic properties of Cu 2+ and Cu + centres in our models of cz and ]3 sites in ZSM-5, before and after NO sorption. In both models (cz and 13) Cu 2+ has roughly fourfold coordination and it becomes lowered to at most triple coordination by copper reduction from Cu 2+ to Cu + and/or NO sorption. In the cz site, however, copper coordination is much more symmetric with respect to Cu-O bond lengths but reduction of coordination is always accompanied by the cation migration towards more packed five-ring fragment of the model. In the case of Z6 model of the p site, asymmetry of copper coordination is much higher and clusters distortion after copper exchange is larger than those calculated for the M7 model. The penalty paid for keeping Cu + bonded by the six-ring is its strong deformation reflected by high value of distortion parameter defined as mean square root deviation between cartesian coordinates of the atoms forming the ring (RMS). In principle structural and electronic properties of Cu 2+ and Cu + in cz and p sites are very similar, there are, however, subtle but systematic differences between both sites. Along with already discussed changes in coordination, we observed also the changes of copper charges. We have already proposed that framework oxygens acting as generalised ligand making partly covalent bonds with copper cations reduce the charge on Cu 2+ and Cu +. General conclusion may be drawn that the more packed is oxygen environment, the less positive is the charge on copper cation. Cu 2+, which in ZSM-5 acquires charge around 0.5, shows still prevailing tendency to withdraw electrons from environment. On the contrary, Cu +, which has actual charge around 0.3, may act apparently as an electron donor. Tables 2 and 3 show properties of the systems composed of a or 13 copper sites and NO molecule. NO becomes strongly bonded by all copper centres in a tilted position, the effect of its interaction with the copper cation dramatically depends on copper oxidation state while subtle but systematic differences dependent on the site structure may be also observed.

1975 Table 2 Calculated properties of adsorption complexes between NO molecule and c~ sites (data taken from refs. 10,11,17) or 13 sites in CuZSM-5: adsorption energy, equilibrium Cu - NO bond distance and angle, intramolecular NO bond length. Centre / Site Cu2+-NO

Cu+-NO

Eads kcal/mol

RCu-NO A

Cu-N-O angle (deg)

RNO ft,

Z6(13)

41

1.85

124

1.14

M7(oQ

40

1.80

128

1.15

Z6([3)

17

1.84

132

1.18

M7((x)

24

1.84

132

1.19

C u 2+ in a or 13 sites binds NO molecule more strongly than Cu +, in the letter case also dependence of adsorption energy on the site structure may be noticed. From the data contained in Table 2 one can develop the impression that the et site has enhanced ability to activate NO when hosting Cu + (larger increase of RNO) while the effect of Cu 2+ in 13 position shows in bigger strengthening of the NO bonding. This property correlates with the tendency to donate or withdraw electrons illustrated in Table 1. The information listed in Table 3 summarises our findings concerning properties of copper centres in both oxidation states in two distinct sites in ZSM-5 and the effect of their interaction with NO. It may significantly aid our understanding of geometrical and electronic factors, which are responsible for activation ability of copper centres. The data listed in Table 3 indicate clearly that donor-acceptor properties of the copper centre which coincide with its activation ability towards NO have well documented source in its bonding scheme within ZSM-5 framework. Unique tendency of copper to self-reduction is obviously the most important factor of its activity. The second factor, however, seems to be partly covalent interaction of Cu + with oxygen ligands in ZSM-5: the more packed is oxygen environment, the higher is Fermi level of the site and the stronger is tendency of copper to donate electrons to NO molecule which causes its activation. This tendency increases uniformly down the table. It is seen when considering calculated properties: the position of HOMO orbital, change of charge on both Cu cation and NO molecule and change of NO bond

Table 3. Effect of NO interaction with et or 13 sites in CuZSM-5" energy of the HOMO orbital of the cluster, change of charge on copper and on NO caused by adsorption, changes in NO bond lengths (ARNo), calculated and measured NO stretching frequency (vNO). Centre / Site

EHOMO

AQcu

AQNo

[eV] Cu2+-NO

ARNo A

VNO (DFT)

VNO (IR)

cm -1

cm -I

Z6(13)

-6.482

-0.03

+0.30

-0.02

1912

1913 a)

M7(oQc)

-6.443

-0.04

+0.25

-0.01

1904

1895 a)

Z6(13)

-5.317

+0.05

-0.03

+0.02

1778

M7(oQc) -5.170 +0.08 -0.03 a) from ref. 3 b) from ref. 9 c) from refs. 10,11,17

+0.03

1754

Cu+-NO

1809 b)

1976 distance, as well as properties which were parallel measured and calculated such as NO stretching frequency. IR spectrum of NO adsorbed at relatively low coverage on Cu-ZSM-5 shows two distinct bands of Cu2+-NO at 1905 cm 1 and of Cu + NO at 1809 cm -1. The former one is split and Wichterlova reported [3] two submaxima at 1895 and 1913 cm 1. According to the data presented in ref. 3 it may be supposed that 1895 cm -~ band corresponds to NO bonded to Cu 2+ in ct sites and 1913 cm 1 band corresponds to Cu 2+ in [3 sites. These experimental values agree well with our values calculated by DFT (1904 and 1912 cm 1 for sites c~ and [3, respectively). Unfortunately, the IR band of NO bonded to Cu + at 1809 cm ~ is not split. Maybe such a splitting could be seen when recording the spectra of adsorbed NO at liquid helium temperature (what distinctly narrows IR bands). Such experiments will be done in our laboratory in the future. As mentioned above, our DFT results (Table 3) showed that Cu + in cz sites activates the most NO molecule what is evidenced by the most distinct N-O distance elongation and the lowest NO stretching frequency. This result agrees very well with the experimental data presented by Wichterlova [3]. 3. C O N C L U S I O N S 1. CH 2+ in ct sites has square planar coordination, whereas Cu 2+ in [3 sites form two shorter and two longer bonds to framework oxygens. The coordination of Cu 2+ in [3 positions deforms the framework stronger than in tx sites. 2. Cu + in c~ sites form three strong bonds to framework oxygen, whereas in [3 sites it forms two strong and one much weaker bond. Similarly as in the case of Cu 2+, the distortion caused by Cu + in [3 sites is much larger than in c~ sites. 3. The location of both Cu 2+ and Cu + in zeolite framework diminishes strongly the positive charge on cations to: 0.48-0.50 for Cu 2+ and 0.28-0.32 for Cu +. This is due to strong influence of framework oxygens. Cu + in c~ sites has the lowest positive charge (0.28) due to higher number of oxygens in close vicinity than in 13 sites. 4. Cu 2+ in both ct and [3 sites binds NO with similar strength. The bonding of NO to Cu + sites is weaker than to Cu 2+ - it is the weakest for Cu + in 13 sites. 5. NO molecule interacting with Cu 2+ donates electrons to the cation, therefore the cation looses some positive charge. This strengthens the NO bond as evidenced by NO distance shortening (by 0.01 - 002 A). This effect is more pronounced in [3 sites than in ct sites (shorter NO bond and higher NO stretching frequency). The values of NO stretching frequencies calculated by DFT agree very well with experimental values. 6. NO molecule interacting with Cu + acquires electrons from the cation, therefore the cation gains positive charge. This weakens the NO bond as evidenced by NO distance elongation (by 0.02 - 003 A). This effect is much stronger in-ct sites than in [3 sites (longer NO bond and lower NO stretching frequency). 7. Our DFT calculations showed that Cu + in ot sites activate the most NO molecule, what agrees well with experimental suggestions of Wichterlova [3].

Acknowledgement This study was sponsored by the grant of KBN 3 T09A 010 17.

1977 REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17.

M Iwamoto, in Studies in Surface Science and Catalysis, vol. 130, Proceedings of the 12th ICC, Granada, A. Corma, F. V. Melo, S. Mendioroz and J. L. G. Fierro (Eds.), 2000, Elsevier, p. 23. B. Wichterlova, J. D6d6cek, Z. Sobalik, A. Vondrova and K. Klier, J. Catal. 169 (1997) 194. B. Wichterlova, J. D6d6cek, Z. Sobalik, Proc. 12th Int. Zeolite Conference in Baltimore, M. M. J. Treacy, B. K. Marcus, M. E. Bisher, and J. B. Higgins, Editors, MRS, 1999, p. 941. J. D6d6cek, D. Kaucky, B. Wichterlova, Micropor. Mesopor. Mater., 35-36 (2000) 483. W.K. Hall, J. Phys. Chem. B, 101 (1997) 1979. G . T . Polomino, P. Fisicaro, S. Bordiga, A. Zecchina, E. Giamello and C. Lamberti, J. Phys. Chem. B, 104 (2000) 4064. DMol, Insight II release 96.0, User Guide, San Diego: Molecular Simulations, 1996. E. Broclawik, J. Datka, B. Gil and P. Kozyra, Phys. Chem. Chem. Phys., 2 (2000) 401. E. Broclawik, J. Datka, B. Gil, W. Piskorz and P. Kozyra , Topics in Catalysis, 11/12 (2000) 335. E. Broclawik, J. Datka, B. Gil and P. Kozyra, Topics in Catalysis, in print. E. Broclawik, J. Datka, B. Gil and P. Kozyra, J. Phys. Chem., submitted. D. C. Sayle, C. Richard, A. Catlow, J. D. Gale, M. A. Perrin and P. Nortier, J. Phys. Chem. A, 101 (1997)3331. D. Nachtigallova, P. Nachtigall, M. Sierka and J. Sauer, Phys. Chem. Chem. Phys., 1 (1999) 2019. M. J. Rice, A. K. Chakraborty and A. T. Bell, J. Catal., 194 (2000) 278. P. Nachtigall, D. Nachtigallova and J. Sauer, J. Phys. Chem. B, 104 (2000) 1738. D. Nachtigallova, P. Nachtigall and J. Sauer, Phys. Chem. Chem. Phys., 3 (2000) 1552. E. Broclawik, J. Datka, B. Gil and P. Kozyra, Int. J. Mol. Sci., 2002, in print.



1979

Diffusion o f W a t e r in Silicalite b y M o l e c u l a r D y n a m i c s Simulations: A b Initio based interactions C. Bussai a'b, S. Hannongbua a, S. Fritzsche b, and R. Haberlandt b aDepartment of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand.* bDepartment of Molecular Dynamics/Computer Simulations, Institute for Theoretical Physics (ITP), Faculty of Physics and Geoscience, University of Leipzig, Augustusplatz 10 - 11, 04109, Leipzig, Germany. The silicalite-1/water potential function has been developed using quantum chemical calculations at the Hartree-Fock level using the 6-31G* basis sets. The silicalite-1 crystal structure is represented by three fragments, in which the chemical compositions are OloSiloH20, O308i22H44 and O355i29H58. Ab initio calculations have been performed for 1,032 fragment-water configurations where water coordinates are generated inside the fragments. The intermolecular silicalite-1/water potentials developed from those data points have been used in the molecular dynamics simulations. The obtained diffusion coefficients at 298 K of 3.3x10 9 m2.s -1 and at 393 K of 6.7x10 9 m2.s "1 are in agreement with those of the PFG-NMR measurements. 1. INTRODUCTION Ze01ites are microporous crystalline solids with well-defined structures. Generally they contain silicon, aluminium and oxygen in their framework and cations, water and/or other molecules within their pores. Due to their unique porous properties, major uses are in petrochemical cracking, ion exchange (water softening and purification), and in the separation and removal of gases and solvents [1]. To study these, several experiment tools have been achievable on one hand, on the other hand, theoretical tools, e. g. Molecular Dynamics simulations, become more feasible [2]. Such theoretical methods require knowledge of the interaction potentials. A crucial method to obtain such potentials is the use of ab initio calculations. Several function modification attempts and great success have been made by Sauer et al., with e. g., the QMPot method [3 ]. In this study, an alternative choice in deriving potential function parameters is proposed. Numerous silicalite-1/water energy points have been generated using quantum chemical calculations at the Hartree-Fock (HF) level using the 6-31 G* basis sets. Molecular dynamics simulations have been performed using the newly developed ab initio potential and diffusion coefficients for water molecules in the silicalite-1 have been investigated.

1980 2. CALCULATION DETAIL

2.1. Development of the intermolecular pair potential To develop intermolecular potential :)-7 ,!)" functions representing the interaction between two molecules in all configurations, numerous coordinations of the second molecule around the first one have to be generated. The interaction ... energies of all configurations have to be calculated and the obtained data points must then (c) be fitted to an analytical form. Due to the size of the silicalite-1 lattice, in Figure 1. Schematic representations of which a Crystallographic cell [4] consists of 96 Si the (a) linked domain, (b) straight and and 192 O atoms, it is rather difficult to take into sinusoidal channels, (c) intersection. account the whole lattice in the quantum chemical calculations. Therefore, the silicalite-1 crystal structure was represented by three fragments (Figures la-lc), for simplicity, named single, intersection and double rings with their chemical compositions O10Si10H20, O30Si22H44 and O35Si29H58, respectively. More details of the classification have been given elsewhere [5]. Ab initio calculations at the HF level with the extended 6-31G* basis sets have been performed for all water configurations generated inside those silicalite-1 fragments. Experimental geometries of water [6] and silicalite-1 [4] have been used and kept constant throughout. All calculations are performed using the G98 program [7]. More than 1,000 ab initio data points were fitted to an analytical function of the form

[8]:

AE(w, s)

-

cab q q}

i

j [ ro6

+ 7~7- + - - ~ + r~ij

rij

rij

'

,

(1)

where 3 and 288 denote the numbers of atoms in a water molecule (w) and the silicalite-1 (s) unit cell, :respectively. The constants A ij , Bij and C/j are fitting constants and rij is the distance between atom i of water and atomj of silicalite-1. Also, qi and qj are the atomic net charges of atoms i and j in atomic units, as obtained from the population analysis of the isolated molecules in the quantum chemical calculations. Superscripts a and b on the fitting parameters have been used to classify atoms of equal atomic number but different environmental conditions, for example, oxygen and silicon atoms of silicalite-1 in the different channels. The third polynomial term (Cjr3ig) was added in order to obtain better numerical fitting. The silicalite-1/water fitting parameters were summarized in Table 1. Concerning an assignment of a negative or positive value to the fitting parameters, physical meaning,of the atomic-based pair potentials is not achieved. Instead, it is a one-to-one correspondence between the predicted (by the potential function) and the observed (by the ab initio calculation) interaction energies. An advantage of this approach is that it is a one-to-one correspondence between the predicted and the calculated interaction energies. Analogously, as well as for better numerical fitting, the third polynomial term (CJr3,g) was added and not considered separately. Some examples are those in references [9-11 ].

1981 Table 1 Optimization parameters for atom i of water interacting with atom j in each channel of the silicalite-1 lattice. Subscripts sd and st denote sinusoidal (zig-zag) and straight channels, respectively.

O O O O i H H H H

2.2.

Sisa Sist Osa Ost j Sisd Sist Osa O~t

Molecular

qi

qj

-0.87 -0.87 -0.87 -0.87

1.57 1.67 -0.78 -0.84

qi

qj

0.43 0.43 0.43 0.43

1.57 1.67 -0.78 -0.87

Dynamics

A 6 1 (A kcal.mol ) - 9043.97 - 4159.83 1371.19 - 110.79

B (A12kcal.mo1-1) 1161167.97 989963.68 -21045.58 51208.44

C 3 (A kcal.mol 1) 1418.92 617.02 -351.61 -110.82

A (A kcal.mo1-1) 3724.97 2077.13 -406.18 34.87

B (A12kcal.mo1-1) -4314.90 -8925.29 689.37 32.84

C (A3kcal.mol 1) -792.37 -415.82 222.32 102.59

6

Simulations

The crystallographic cell [4] of Silicalite-1 contains 288 atoms (Si960192), with lattice parameters a = 20.07 A_, b = 19.92 A_ and c = 13.42 A_. Simulations have been carried out at 298 K and 393 K with the time step of 0.5 fs for the system consisting of 2 silicalite-1 unit cells. The box contains 2 water molecules per intersection of the silicalite-1. Periodic boundary conditions have been applied. The potential proposed by Bopp, Jancso and Heinzinger [12] was employed to describe 9 = . . ~...~,' , . water-water interactions while the newly . 2 developed potential shown in eq. (1), with the m 0 optimal parameters summarized in Table 1, =0 u was used to represent the silicalite-1/water interactions. According to [ 13.14] the use of -~9 -2 Ewald summations can be avoided in systems with tot:al charge zero if shifted force m AEsc~~".-.. /~' "n,D|1 |,== 9 m~ll/ potentials are applied instead. The equations D AEFIT "~ .... ~ of motion are solved by means of the , I , I , I , Velocity-Verlet algorithm. The evaluation -2 -1 0 1L/~ 2 part of each run corresponds to trajectory Figure 2. Silicalite-water interaction energies length of 10 ns after 0.5 ps thermalization during which the total energy is adjusted to a (AE) obtained from the ab initio calculations (AEscF) with the extended 6-31G* basis sets value that leads to the wished average kinetic and from the potential function (AEFIT) All energy. Therefore, the evaluation part can be done in the microcanonical ensemble without ab initio and fitted data points were also compared in the insert. perturbing the trajectories.

1982 3. RESULT AND DISCUSSION 3.1. Quality of the Silicalite-1/Water Potential With the analytical potential shown in eq. (1), the lattice-water interactions in the straight channel have been calculated and plotted in Figure 2. Here, the oxygen atom of the water molecule moves from one perpendicular surface to the opposite side along the vector r (see Figure 2), its dipole moment points parallel to vector r and its molecular plane is parallel to the window of the lattice. The ab initio interaction energies at the same lattice-water configurations have been calculated and given also for comparison. Good agreement between the two curves clearly illustrates the reliability and quality of the fit. This conclusion was, again, confirmed by the plot shown in the insert of this Figure, where all 1,050 ab initio and fitted energies have been compared. Some comments could be made concerning the quality of the ab initio interaction energies given in this study. Discrepancies and reliabilities of the data points due to the size of the fragments, the calculated methods and the basis sets used as well as an error due to the unbalance of the basis set, basis set superposition error, have been intensively examined and discussed in some previous papers [5,8]. 3.2. Characteristics of the Silicalite-1/Water Potential To visualize characteristics of the silicalite-1/water potential function, the interaction energies ~for different orientations of the water molecule in the straight channel have been computed according to eq. (1). The changes of the energies as a function of the distances along r were plotted in Figure 3. Curves 1 and 2 in Figure 3 show the minima at L < 0, and the interaction energies for L > 0 increase more slowly than those for L < 0. This occurrence can be clearly understood as water molecules in these configurations (at the right of this Figure) approach the surface at L > 0 by pointing hydrogen atoms toward the oxygen atoms of the lattice, i.e., attractive Coulomb interactions between the hydrogen atoms of water and the oxygen atoms of the surface compensate the water-surface repulsion. This leads to a slow increase of the interactio, n energy and hence an asymmetry of the lattice-water potential. The difference between the shapes of the two curves indicates how sensitive the obtained function is. That means it is able to classify the two orientations of the water molecule which differ only by rotating the molecule by 90 ~ around its dipole vector. The situation is very similar for curves 2 and 5, in which the 0 minima take place at L > 0 and the interaction energies for L < 0 increase 2-- ) faster than those for L > 0. For curves 3 and 6, the shapes are much more ~'~ -4 % symmetric than the other curves. The 5 - 4 - ~':~ reason is that the water molecule in these configurations approaches the 6-'" ~ 0 1 -2 -1 2 lattice, both for L > 0 and L < 0, by pointing its dipole vector parallel to the Figure 3. Silicalite-water interaction energies surface. Curve 6 is broader than curve (AE) obtained from the potential function 3 because in curve 3 water moves according to eq. (1) for different orientations of a toward the surface in configurations for water molecule which distances from the surface to the

i!!

1983 two hydrogen atoms are identical. For curve 6, at the same position of water as in curve 3, one hydrogen atom is closer to the surface than the other (see legend of Figure 3). This fact confirms the ab initio interaction energies reported [5]. The sensitivity of the silicalite-1/water potential to different environments has been clearly monitored, in addition to that due to water orientation as shown in Figure 3. The difference between the interactions in the straight and the zig-zag channels is consistant with the energy data analyzed intensively in our previous work [5,8]. 3.3. Diffusion coefficients

The self-diffusion coefficients are calculated from the particle displacements. In [1517] the process of self-diffusion was quite generally related to the moments of the propagator. The propagator p (r, r 0 ,t ) represents the probability density to find a particle at position r at time t when it was at relation [15] In

--

ro at time

t = 0. The n th moment of the propagator is defined by the

(2)

_

P ( r , r 0 ,t ) is the solution of the diffusion equation for the initial concentration C (r,t = 0)= 5(r - r 0). In the case of isotropic diffusion and of a homogeneous system the propagator results to be

P(r'r~ )- (4~Dt )-~ exp{ - (r4Dt -roY)

.

(3)

Although zeolites are not homogeneous the propagator can be represented in this way if the displacements exceed the size of the inhomogeneities [16]. Then p ( r , r 0 ,t ) depends only on the difference ~, -r01. For shorter times this is not true. As the transition time to the Gaussian behavior and the final D values were the quantities of main interest in the present paper an averaging over r 0 has been carried out. The resulting propagator depends only upon ~, -r0l for all times. But, it attains the shape shown in eq. (3) (or its equivalents for the different components of the diffusion tensor in the anisotropic case, see below) only for sufficiently long times. The first four moments can be calculated from eqs. (2) and (3) in the case of isotropic diffusion [ 15] and of the anisotropic system, the corresponding equations for each direction, corresponding to the x-, y- and z-axes [18]. In this case, the diffusivity D is one third of the trace of the diffusion tensor:

D = I~3(Dx + Dy + D~)

(4)

The good agreement (within the range of fluctuations) of the final D values calculated for 298 K using different moments indicates that the diffusion time used in the evaluation procedure exceeds the correlation time. The self-diffusion coefficients calculated in this way at 298 and 393 K are summarized in Table 2.

1984 Table 2 The self-diffusion coefficients calculated in this way at 298 and 393 K are summarized. Temp (K)

MD Simulation Dx (m 2s'')

Dy (m2s "1)

Dz (m2s q )

298

2.6•

-9

6.5x10 9

7.9•

393

5.7•

.9

1.3•

1.4x10 -9

.8

-1~

D(m2s -1) 3.3•

"9

6.7•

9

It can be seen from these results that the largest component of the diffusion tensor is Dy values are about two times larger than Dx at both temperatures and about seven times larger than Dz at 298 K and even larger at 393 K. This is consistent with the physical structure of the silicalite-1 crystal, which consists of zigzag channels lying in the xz-plane and the straight channels lying parallel to the y-axis. This causes the significant difference of the elementary diffusion rates in different directions.

Dy. The

Considering the diffusion through silicalite-1 as a random walk of independent steps between the channel intersections, the main elements of the diffusion tensor may be shown to be correlated by the relation [ 19] c2 Dz

=

a 2. b 2 ~

Dx

(5)

Dy'

where a, b, and c are the unit cell lengths. Eq. (5) implies that the correlation time of propagation is shorter than the mean time it takes a molecule to travel from intersection to intersection. Possible deviations from this case, i.e. correlated motion between the channel intersections, may be accounted for by introducing a parameter (6)

The case/3 = 1, obviously represents the above considered case of completely random steps. /3 > 1 indicates preferential continuation of the diffusion path along one and the same channel, while/3 < 1 stands for molecular propagation with interchanges between the two channel types more probable than at random. The values o f f l calculated in this study are equal to 1.04 at 298 K and to 1.25 at 393 K. In agreement with the behavior found for alkanes, e.g. [20], where/3 = 1.2 and 1.3, a tendency is observed that the xenon molecules and the methane molecules, respectively, in silicalite-1 prefer to remain in the same type rather than to change into a segment of the other channel type at a channel intersection.

1985

3.4. Radial distribution function

4.0

g(r)

/

n(r) 4.0

The oxygen-oxygen radial distribution functions g for the water molecules at the two 298 K 3.0 3.0 temperatures have been calculated and displayed in 2.0 2.0 Fig. 4. In inhomogeneous systems, g(rl,r2) depends upon rl also and is not simply g(r) with r=lrl-r2l . 1.0 1.0 But, as a first approximation, we have done the evaluation of g(r) like in a homogeneous isotropic 0 2 4 6 8 10 system. This is equivalent to an averaging over the sites rl taking as a weight function the relative Figure 4 Oxygen-oxygenR/raAdial number of events when the rl are found during the distribution functions and MD run. Note, that due to the asymmetry of the corresponding running integration silicalite-1 lattice the function g(r), defined in this numbers for water molecules obtained way, does not converge to 1.0 for distances of the from the simulations at 298 K and 393 order of 10 A. g(r)=l would correspond to a K. homogeneous distribution in space that can be observed in systems with a structure on molecular level only at a length scale that is larger than the size of the inhomogenities i.e. the channel structure in the present case. The radial density distributions show a first maximum at 3.5 A followed by a pronounced shoulder centered at 4.4 A. In order to see the number of neighbors the integral n(r) of g(r) is also displayed in Fig. 4. It can be seen that e.g. within a distance of 7 A around a given water molecule there are in average only two other water molecules. Although, the first minimum is not well-defined, it can be concluded that the simulations did not show any clustering of water molecules in the silicalite-1 channels for the examined temperatures and concentrations of guest molecules. 4. CONCLUSION To justify the quality of the model in representing a real system, MD simulations have been performed at 298 K and 393 K for a loading of 2 water molecules per intersection of the silicalite-1. The diffusion coefficients have been calculated according to the method described in [21] from different moments of the particle displacements. The results obtained at 298 K and 393 K are 3.3x10 9 and 6.7x10 -9 m2.s-1, respectively. These values are in satisfactory agreement with those from PFG-NMR measurements [22] at the same loading and temperature. ACKNOWLEDGEMENTS Computing facilities provided by the Austrian-Thai Center for Chemical Education and Research at Chulalongkorn University, the National Electronic and Computer Technology Center, Bangkok, Thailand and the Computing Center at Leipzig University are gratefully acknowledged. This work was financially supported by the Thailand Research Fund (TRF) and the Deutscher Akademischer Austauschdienst (DAAD). C. B. acknowledges a DAADRoyal Golden Jubilee Scholarship, Grant No. A/99/16872, and a Royal Golden Jubilee Scholarship, Grant. No. PHD/0090/2541. R.H. and S.F. also thank the DFG (Sonderforschungsbereich 294) for financial support.

1986 REFERENCES

D. W. Breck, Zeolite Molecular Sieve, Wiley, New York, 1974; J. K~irger and D. M. Ruthven, Diffusion in Zeolites and Other Microporous Solids, Wiley-Interscience, New York~ 1992. M.P. Allen and D. J. Tildesley, Computer simulation of liquids, Clarendon Press, Oxford, 1990. U. Eichler, M. Brandle and J. Sauer, J. Phys. Chem., 101 (1997) 10035. 4. D. H. Olson, G. T. Kokotailo, S. L. Lawton, W. M. Meier, J. Phys. Chem. 85 (1981) 2238. 5. C. Bussai, S. Hannongbua, R. Haberlandt, J. Phys. Chem. B 105 (2001) 3409 6. W. S. Benedict, N. Gailar, E. K. Plyler, J. Chem. Phys. 24 (1956) 1139. 7. M. J. Frisch, G. W. Trucks, M. Head-Gordon, P. M. W. Gill, M. W. Wong, J. B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Replogle, R. Gomperts, J. L. Andres, K. Raghavachari, J. S. Binkley, C. Gonzalez, R. L. Martin, D. J. Fox, D. J. Defrees, J. Baker, J. J. P. Stewart, J. A. Pople, Gaussian 98, Revision A, Gaussian, Inc., Pittsburgh, P A, 1998. C. Bussai, S. Hannongbua, S. Fritzsche, and R. Haberlandt, Chem. Phys. Lett. in press. W. L. Jorgensen, M. E. Cournoyer, J. Am. Chem. Soc. 101 (1978) 4942. 10 G. Karlstr6m, P. Linse, A. Wallqvist, B. J6nsson, J. Am. Chem. Soc. 105 (1983) 3777. 11 S. Udonsub, S. Hannongbua, J. Chem. Soc. Faraday Trans. 93 (1997) 3045. 12 P. Bopp, G. Jancso, K. Heinzinger, Chem. Phys. Lett. 98 (1983) 129. 13 D. Wolf, P. Keblinski, S. R. Phillpot, J. Eggebrecht, J. Chem. Phys. 110 (1999) 17. 14. H. Dufner, S. M. Kast, J. Brickmann, M. Schlenkrich, J. Comput. Chem. 18 (1997) 15 S. Fritzsche, R. Haberlandt, J. K~irger, H. Pfeifer, K. Heinzinger, Chem. Phys. Lett. 198 (1992) 283. 16. J. K~rger, H. Pfeifer and W. Heink, "Principles and Application of Self-Diffusion Measurements by Nuclear Magnetic Resonance", in Advances in Magnetic Resonance, Vol. 12, Academic Press: New York, 1988. 17. R. Haberlandt, J. K~irger, Chem. Eng. J. 74 (1999) 15. 18. S. Fritzsche, M. Wolfsberg, R. Haberlandt, submitted to Chem. Phys. 19. J. K~irger, J. Phys. Chem. 98 (1991) 5558. 20. S. Jost, N. K. B~ir, S. Fritzsche, R. Haberlandt, J. K~irger, J. Phys. Chem. B 102 (1998) 6375. 21. S. Fritzsche, R. Haberlandt, J. K~irger, H. Pfeiffer, M. Wolfsberg, K. Heinzinger, Chem. Phys. Lett. 198 (1992) 283. 22. C. Bussai, H. Liu, S. Vasenkov, S. Fritzsche, S. Hannongbua, R. Haberlandt, J. K~irger, Appl. Catal. A-Gen, in press. .

.


1987

C o m p a r i s o n of s m a l l size a l u m i n o - and borosilicates o p t i m i s e d by periodic Hartree-Fock A.V. Larina and D.P. Vercauteren Laboratoire de Physico-Chimie Informatique, Facult6s Universitaires Notre Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium, e-mail: a!m:'m__Ca!s-cf:Nn_dp.ac.be apermanent address: Department of Chemistry, Moscow State University, Moscow, B-234, 119899, Russia 1. INTRODUCTION From a practical point of view, borosilicates present an interest as low acid catalysts; promising applications have, for example, already been tested for N-methylaniline synthesis from aniline [1] and Beckmann rearrangement [1-3]. So far, their activity has been interpreted via few theoretical studies mainly limited at the level of isolated clusters [4]. However, borosilicates also offer several advantages for studies at the level of periodic Hartree-Fock (PHF) approaches [5], owing mainly to the small number of atomic orbitals per unit cell as compared to the respective aluminosilicates. The possibility to optimise small aluminosilicate H-forms with PHF, supported by comparison of quadrupole coupling constants (CQQ) of 2H, 170, and 27A1, measured in 2Hy and 2HZSM-5 [6], has indeed already been shown [7, 8]. Important structural distinctions between the dehydrated Na- and H-form borosilicates upon A1 B replacement, i.e., the conservation of the BOa tetrahedra or its transformation to BO3, respectively, was shown both experimentally [9] and theoretically [4]. The transformation is accompanied by the appearance of "internal" silanol groups and by a stronger volume change for the H-form v e r s u s the Na-fonn, as could be verified by PHF calculations considering long range stabilisation effects. The different trigonal BO3 and tetrahedral BO4 moieties can be distinguished by sharp differences of the CQQ values which can serve as "fingerprints" for quantitative determination of the ratio between the two B coordination types in boron containing glasses [10]. But no theoretical PHF assignment of the CQQ values of liB in zeolites has, to our knowledge, been realised yet. 2. COMPUTATIONAL DETAILS Our optimisation strategy of the H-form borosilicates includes three stages: dehydration, A1/B replacement, and Li/H replacement (Figure 1). LiABW (Si/B = 1) and LiEDI (Si/B = 1.5) borosilicates were first optimised (Table 1) at the STO-3G level with PHF [5] as realised for aluminosilicates [8]. Final H-form borosilicate structural parameters are presented in Table 2. Then, single point calculations were considered at the ps-21G*(A1, Si)/6-21G*(H, B, O) level.

1988

a)

Table 1. Symbol, number of atoms , of different B, Si, and O types, of atomic orbitals (AO) per unit cell (UC) b~, and s~nnetr~ ~roup of the cationic forms of borosilicates Name S~rrnbol AtomsBdC nB/nsi/no AO/UC S)rmmetry LiABW LiABW 28/40 1/1/4 352 Pna21 Edingtonite a)

LiEDI

for dehydrated/hydrated forms;

32/b)

1/2/5

434

P21212

dehydrated form at the ps-21 G* level

Initial XRD hydrated LiABW aluminosilicate model ......... ,1, -HaO Optimisation with CRYSTAL code at the STO-3G basis set level

]

Single point testing with ps-21G*(AI, Si)/6-21G*(Li, O) basis set $ AI---) B replacement Optimisation with empirical force field [11]

[

I

Optimisation with CRYSTAL code at the STO'3G basis set level

I

$

$

$

Single point testing with ps-21G*(Si)/6-21G*(Li, B, O) basis set

$

Li--) H replacement

I Calculation of initial H coordinates on the basis of literature for the bridge Si-O(H)-B]

$

Optimisation with CRYSTAL code at the STO-3G basis set level

..]

$

Single point testing with ps-21G*(Si)/6-21G*(H, B, O) basis set Calculation of structural parameters and electronic properties Figure 1. Three-steps optimisation procedure of the H-form borosilicates To characterise each particular atom within each coordination sphere, we considered the quadrupole interactions of each asymmetric nucleus and calculated the CQQ values using a precise estimation of the electrostatic potential and its derivatives with the CRYSTAL code [5]. CQQ values (in MHz) were obtained using the electrostatic field gradient tensor (EFG) elements at the tensor principal axes: CQQ = 2.3496• 102•215

(1)

where the coefficient in the right hand side corresponds to the diagonal element VEz~ of EFG expressed in exau 3, and the nuclear quadrupole moments q are 0.00286, -0.03, -0.0355, and0.02558, barn (1 barn = 10 -28 m 2) for 2H, 7El, 11B, and 170, respectively. The EFG anisotropy ~1 = (IVEyyl - IVExxl)/lVEzzlvalues will also be given below for completeness.

1989 3. R E S U L T S AND D I S C U S S I O N 3.1 S t r u c t u r a l and electrostatic properties The comparison of the electrostatic properties of the H-form alum,no- and borosilicates is an instructive way to explain the lower acidity of borosilicates because they possess the lowest (A1) and highest (B) proton affinities thru the series of substituted frameworks with T = Be, Fe, Ga [ 13]. As observed from Table 2, the favoured H localisation in borosilicate A B W is the same as in aluminosilicate ABW; H is located at the 0(2) outside of the 4-membered (4T) ring window. The energy gaps between the favoured site and the next ones are however larger for the H positions in the borosilicate. Table 2. Geometry of the BrOnsted sites (distances in ~, angles in o) for alum,no- and borosilicate A B W optimised with STO-3G varying the coordinates of H, B, O, A1, and Si. Relative energies (in kcal/mol) are also 1G*(A1, Si)/6-21G*(H, B, O). Parameters 0(2) 0(3) 0(4) O(1) Si-O-A1 initial a) 139.45, 124.87 124.84 143.34 , Si-O-A1opt,raised 139.06, 126.37 126.22 143.32 O-H 0.981 0.979 0.980 0.982 Si-O 1.745 1.768 1.764 1.790 A1-O 1.809 1.828 1.832 1.861 A1-H 2.350 2.457 2.480 2.371 Oc) 9.7 10.4 3.5 16.3 .

.

.

.

.

|

|

i

|

,,,

|

,,,,

,,

H,

Si-O-H A1-O-H AUSTO_3G/AUps_21G. Si-O-B initial u) Si-O-B optimised O-H Si-O B-O B-H ~c) Si-O-H B-O-H AUSTO_3G/AUps_21G, i

,

,,,,

|

,,,,,

,,,,

i

110.0 113.7 112.0 , 118.9 0.0/0.0, 8.810.9 135.29 i'24.i"5 135.56, 130.1..9 0.981 0.985 1.724 ' 1.758 1.647 1.631 2.086 2.223 3.4 ' 16.2 110.8 113.6 113.1 114.1 0.0/0.0i 64.7/59.3 |

,

,, ,,

,,,,,

,,

,

,

.

,,.,

,.,

113.1 j 120.6 i 12.3/3.0 ! 124.40 , 128.61 0.982 1.709 ! 1.653 2.283 i 17.7 |

i

.,

|

|

,

|

,

|

106.1 109.1 22.8/41.4 149.74 147.00 0.973 1.764 1.743 2.202 1.3

114.4 108.2 117.0 104.8 35.9/48.2 75.9/115.4

") coordinates from [12]; b~ coordinates of LiABW first optimised with GLASSFF 2.01 [11] and then partly with CRYSTAL [5]; c~ [3 is the angle of H deflection from the Si-O-A1 plane. The validity of the H-form aluminosilicates optimised with our approach is supported by the structure recently optimised by PHF with Schlegel algorithm [ 14] of the Si-O(H)-A1 moiety in HEDI as well as by comparison of the calculated and experimental CQQ for a series of 2H, 1~O, a~A1 framework atoms [6]. The structural parameters and CQQ of the obtained cationic forms are presented in Tables 3 and 4, respectively. Only for one O in LiABW, we observe a slightly overestimated Si-O-B angle of 148.4 ~ as compared to the data obtained experimentally for borosilicate minerals [ 15-18].

1990 Table 3. Si-O-B angles (in o) and B-Si distances (in/~) in dehydrated borosilicates optimised with STO-3G compared to experimental data for several borosilicates Structure [ Si-O-B [ B-Si Calculated LiABW 123.0, 124.2, 135.8, 148.4 2.85, 2.86 LNDI 122.8, 125.1,140.7, 141.0 2.86, 2.93 Experimental Ca-danburite, B/Si = 3 ") 125.9, 132.2 2.79, 2.83 Howlite b) 127.9, 128.0, 128.1,135.8 2.80, 2.81, 2.81, 2.87 Reedmergnerite ~) 125.0, 135.4, 140.5, 143.1 2.71, 2.85, 2.88, 2.92 Searlesite a) 130.2, 142.6 2.81, 2.92 a) ref. 15; b) ref. 16; c) ref. 17; a) ref. 18 ............... Table 4. Quadrupole coupling constants CQQ (in MHz) and EFG anisotropy 1] for 7Li and ~IB atoms calculated with ps-21G*(A1, Si)/6-21G*(H, B, O) in ABW alumino- and borosilicates Atom ] Position, geometry ] Cqq r] , Calculated 7Li . LiABW(B), LiO3 L -0.361 0.082 LiABW(A1), LiO3 i -0.478 0.728 LiABW(A1) H20, LiO4 , -0.056 0.997 ~ " "-~xper-~entaJ ) LiKSOfl ), LiO4 type 0.025 0.15 LiKSO4 "), LiO4 type 0.0358 0.0 Calculated lXB HABW(B-O2H), BO4 1.265 0.079 HABW(B-O3H), BO4 1.220 0.045 HABW(B-O4H), BO4 1.416 0.030 HABW(B-O1H), BO4 1.674 0.090 HABW(B-OzH), BOab) 2.526 0.141 LiABW(B), BO4 0.282 0.627 ExperimentaF ) Li203 B203, BO4 0.527 0.53 PbO 2B203, BO4 0.805 0.09 PbO 2B203, BO4 0.960 0.05 H-boralite (MFI type) d) . 2.55 0.0-0.2 CaO 2B203, BO3 2.56 0.54 Li20 2B203, BO3 2.6225 0.1653 33LIO 67BZO3, BOa 2.61 0.14 a) ref. 19; b) intermediate silanol structure; ~)ref. 10; d)ref. 9 ,

,

An interesting example of CQQ(~Li) shift by one order of magnitude upon dehydration in the LiABW case is demonstrated for one Li which changes its coordination from tetrahedral LiO4 to planar LiO3 in the same manner as for B upon the ~bridge to silanob~ transformation in boralite (zeolite of the MFI type) [9]. The fourth O atom in the coordination sphere is the one

1991 of a water molecule as confirmed by XRD [12]. PHF optimisation only contracts the LiO4 tetrahedron, i.e., Li-O distances shift from 1.913, 1.942, 1.968 (water), and 1.981 A to 1.814 (water), 1.878, 1.942, and 1.942 ~. Unfortunately, relevant experimental results are related to the dehydrated a3Na forms and cannot thus confirm this effect at the moment. CQQ values in the optimised structures are compared for 170 and 2H atoms in Figures 2 and 3, respectively. For two observed moieties in ABW, one can note a slight increase of CQQ at the Si-O(H)-B position as compared to the ones of Si-O(H)-A1 in aluminosilicates and a stronger increase in absolute value at the Si-O-B moiety with respect to St-O-A1. This difference between the CQQ variations at Si-O(H)-B and Si-O-B correlates well with the Si-O and T-O (T = A1, B) bond length differences. The latter are between 0.18 and 0.24 A for the ABW borosilicates and between 0.122 and 0.21 ~ for LiEDI (not shown in Figure 2) at the SiO-B moiety. For St-O-A1 in aluminosilicates [8], the differences are smaller, 0.10 - 0.15 ~. At the bridged position, the Si-O and T-O bond length differences are closer, i.e., 0.06 - 0.07 for HABW aluminosilicate and 0.02 - 0.13 A for HABW borosilicate (Table 2). I

'

I

lO -

9

9

NAT ~

ABW,,

8 N

li

..

g

--'~

'

I

Si-O(H)-B,,,.,~

V

~i'=

o

.e~,". , o

si-ol.i-A.

~

9 9 =

"~ C-5 -6

~7

9~

EDI

........................................

9

i f

9

Si-O.A! experiment

9

tx

tx

{

9

iAK. . . . . . . . . . . .

9

................... We," @ ........................... ~ Z ............... ~ I " - U - I : I t l I ~ I ~ W

\'"

120

1

J

J

.v ..... ~ ................................. r. ........ .-I

Si-O-Si e x p e r i m e n t 9

" I Si'O'B/LiABW'

-] r_

.~

"r

4

"l

"

I 140

"

x

'

Si-O-T angle, degrees

r

I

"I"

J

7

160

Figure 2. CQQ(I~o) v e r s u s Si-O-T angle (T = B, St, Ai) in H-form aluminosilicates (triangles, squares, circles), ABW borosilicate (stars, diamonds), and LiABW (crosses). Experimental boundaries are for aluminosilicates (dashed lines). Opposite to CQQ(170), absolute CQQ(ZH) values are found to decrease with respect to those of aluminosilicates (Figure 3), which is in agreement with experiment after OH anharmonicity corrections [6]. This behaviour does not correlate with the Si-O and T-O (T = A1, B) bond length differences whose variations explain only the CQQ(170) increase.

3.2 ~> transformation The drastic variations of all structural parameters predicted in [4] upon A1 --> B replacement in the H-forms as compared to the those of the initial Al-framework present a problem for the PHF optimisation. A series of H-form borosilicates was hence constructed to guess a possible favoured structure. The A1 ~ B replacement leads to an anisotropic decrease of the a, b, and c cell vectors, by 6.8, 3.4, and 3.4 %, respectively, in aluminosilicate LiABW. This cell variation was assigned to a change by 5.9 % of the average 1%OI bond lengths (T = B, Si, A1) between the B- and Al-frameworks of LiABW. On the basis of an analogous evaluation of the cell

1992 parameters, we tested two possibilities of the zj-1 zl zl + 1 zj

+ nsi

pnsit

X,i =1

s i t _l lZnj s-i t _2zl=l z j-- l nKs i t -K i t(p n7S ilt) [ 1+ in~l t K / np+ Y. ... z j > 5-' .K ) / nJS+]I-]K k,,i = 1 i zl zl+l zj \ i =1 at low pressure this equation may be reduced to"

2006

nsit K.p ~ ~ cZ i=ll+K.P

Ki c 1

1+

\i=1

)

(8)

Ki p

by means of equation 8 is possible to reproduce the adsorption isotherms at low pressure like monosite Langmuir model with K equal to sum Of single Ki. Besides, the limiting slope derived from equation 8 it is equal to equation 6. 3. QM/MM CALCULATIONS. Both considered zeolites, Ca-LSX (low silica X) and Ca-A, have a Si/Al=l and contain 48 calcium cations per unit cell. Whereas the LTA structure accommodates 6 cations per sodalite cage, located in the middle of six-membered rings, the LSX zeolite has only 32 possible calcium ions in six-membered rings (site II), the remaining 16 cations being distributed in sites I and I'. The zeolites structures have been generated by MM simulation. In the present study we have used the simulation protocols recently reported [11] to determine the out off framework cation configuration. The results of the Monte Carlo packing procedure carried out using the aluminosilicate cvff_aug forcefield in conjunction with the Discover 3.2 package for energetic optimizations, compares well with the new grid-based algorithm for cation location within the Cerius2 interface [12]. QM/MM method has been recently used for studying adsorption of gaseous molecules in zeolitic frameworks [8,9,13]. In this study we have adopted a different methodology from the recently reported in [13]. Here we calculate adsorption energy of the gas molecules interacting with each accessible cation. The embedded cluster approach consists of a Ca>-molecule (N2 or 02) system treated quantum chemically, surrounded by the specific environment of each cationic site, i.e., point charges simulating the A1, Si, O, and Ca 2§ zeolite ions. This procedure allows us to take into account long-range effects, which may differ from one cation to another. The QM calculations have been performed within the framework of density functional theory and by using the Becke [14] -Perdew [15] exchange-correlation potentials. Computational details are extensively described in ref. [ 16, 8]. As discussed in these papers, this method needs as input the values of the charges representing the zeolite atoms. At first sight, this is not a trivial problem, since these charges are not directly measurable properties. The embedding charges used in this study, i.e. Si (+2.4), A1 (+1.4), O (-1.2) and Ca (+2.0), are those used for MM simulations. The QM calcium cations were also assumed to bear a +2 charges, i.e. neglecting any zeolite to cation charge transfer. The strategy used in this work aims to evaluate Henry constants and the molar enthalpy of adsorption by using the equations (6) and (8). The multisite Langmuir model, allows us to express the global surface coverage as a sum of individual contributions. Each local equilibrium constant is evaluated as a ratio of partition functions which, written respect to the minimum of interaction energy, gives: K = i

qm,i (MS) q m (M)

e

_ AEi/RT

with

K

o, i

=

qm,i (MS) q m (M)

(9)

2007 qm,i(MS) is the local molar partition function at each cationic site for the adsorbed system, while qm(M) describes the partition function for the gaseous state. More details about the calculation of the qm (M) and qm,i (MS) are described in the ref. [8]. It is worth noting that in this study the partition functions are evaluated quantum chemically, assuming Morse potential for frustrated movements of adsorbed molecules. The thermodynamical functions qm,i and qm depend explicitly on the temperature and their ratio is temperature dependent. The Ki values are the individual equilibrium constants associated with every local isotherm. These constants depend on the Ko,i and the adsorption strength of the site according to equation (9). If all sites are equivalent, the surface is homogeneous and adsorption can be described with a monosite Langmuir model, where the global equilibrium constant K is equal to each equivalent Ki. When the surface is heterogeneous, the global equilibrium constant K could be estimated through the equation (8). Unlike our previous publication [8], in which the global constant K is achieved by fitting the theoretical multisite isotherms, in this work K is calculated analytically. As mentioned above, AH~ and K vary with T, choosing an appropriate AT, around a particular temperature, allows the plot of lnK(T) against 1/T to be linear, yielding the isosteric heat of adsorption in this domain. It is worth underlining that in this approach the accuracy of Henry constants and molar adsorption enthalpy depends on the accuracy of adsorption energies and local Ko,i, and all range of temperatures can be used to evaluate the isosteric heats of adsorption at zero coverage. Moreover, the simplicity of equation 8 allows to understand better what kind of corrections is needed to improve the QM/MM model used. In the temperature range AT, where AH~ is assumed to be constant, the logarithmic term of Ko,i should be constant within a good approximation. This term reflects the different response of the adsorbed molecule to temperature changes, with respect to gaseous molecule. It is clear that an increase of temperature will affect differently the vibrational states of the adsorbed species and the degrees of freedom of the isolated system. Although at high temperatures the adsorbed and gaseous states become less different and Ko,i may be assumed constant, generally this term is not constant for all temperatures. 4. RESULTS AND DISCUSSIONS. The calculated adsorption energies, for N2, 02 and Ar at all accessible sites have been reported in table 1. Compared to the energies obtained in our previous calculations, these are corrected on average of 1.5 kcal/mol in absolute value. The corrections of the QM/MM model necessary to obtain these energies will be the subject of a future publication. Table 1. Maximum and Minimum Adsorption Energies (in kcal/mol) for N2, O2 and Ar calculated for the Accessible Cations of Ca-A and Ca-LSX. Molecule Ca-A Ca-LSX Maximum Minimum Maximum Minimum N2 -8.5 -7.2 -9.5 -5.9 02 -6.2 -5.1 -7.5 -4.3 Ar -3.9 -3.4 -5.2 -2.8 Although the spread of the energies for Ca-LSX is larger than for Ca-A, due to the nonsymmetrical distribution of the 16 no accessible site I/I' cations, we have distinguished,

2008 in the range of 0.4 kcal/mol, four groups of adsorption energies corresponding to the cations in a CaLSX cage. The Henry constants of the same gases estimated according to equation 8 and 9 and using QM/MM adsorption energies reported in table 1, are presented in table 2. Table 2. H emy Constants for N2, 02 and Ar adsorption on Ca-A and.~Ca-LSX (in 10.6 mol/k!~Pa). Molecule Ca-A Ca-LSX Theory Exp. a Theory Exp. a N2 10.4 9.2 24.4 26.0 02 1.7 1.6 3.2 1.9 Ar 1.0 1.4 1.9 1.3 ;'Te-fe~en c ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The experimental reported values [17] have been derived from chromatographic study of the N2, 02 and Ar adsorption on cation exchanged A and X zeolites. Comparison of the calculated Henry constants of table 2 with the experimental ones shows that our approaches give good results for the adsorption of N2 on both zeolites. The Henry constant corresponding to the adsorption of oxygen molecule on Ca-A, agree well with the experimental one, contrary to the constant of 02 on Ca-LSX which is less satisfactory. Finally the calculated Henry constant associated to adsorption of Ar on Ca-LSX can be considered satisfactory, whereas the constant for Ca-A is half of the experimental value. From these results and the energies corrections necessary to obtain the values in table 1, it is possible to assert a partial conclusion: the QMM/MM model adopted in this work has some problems to reproduce the interactions energies lower than 6 kcal/mol, however it reproduces better the interactions higher than 6 kcal/mol (e.g. N2 on Ca-A) with the exception of many heterogeneous surface sites (some sites of CaLSX). However, to confirm this conclusion, a more accurate study of theoretical Koi is necessary. In fact, as reported in figure 1a and b, which show the influence of temperature on the calculated preexponential factors, the accuracy of Koi can be important. Ca-A Oxygen 0.0033 -1.2

.......................................

0.00335

0.0034

, ........................................

0.00345

~. . . . . . . . . .

,

. . . . . . . . . . . . . . .

0.0035

0.00355

, ...................................

0.0036

, ...............................

, .............................

0.00365 ,

0.0037

.......................

-1.4 .... -1.6 ,r

..~

l j ~ t ~ ~

-1.8 -2

..E -2.2 -2.4

........

-~ 11

~---/--~f~~-

-2.6

1/T Ik)

Figure l a Henry constant for Ca-LSX, 02. Koi as a function of temperature (triangles) and Koi calculated at 293 K (circles).

2009 Ca-LSX

0.00325 -1.5

0.0033

0.00335

............................ ~. . . . . . . . . . . . . . . . . . . . . .

,

Argon 0.0034

. . . . . . . . . . . . . . . . ,. . . . . . . . .

0.00345 ,

0.0035 ..........

0.00355

,

........

-1.6 -1.7 -1.8 "'

-1.9

~'

-2

J

-=

-2.1 -2.2 -2.3 -2.4

lrr (K)

Figure 1b. Henry constant for Ca-LSX, Ar. Koi as a function of temperature (triangles) and Koi calculated at 293 K (circles). Through an analysis of theoretical Koi values and effective pre-exponential factor, obtained by fitting the experimental first points of adsorption isotherms and using theoretical Henry constants and isosteric heats at zero coverage, it is possible to see that the theoretical Koi of CaA-N2, CaA-O2 and CaA-Ar agree with these effective pre-exponential parameters but for Ca-LSX systems there are some discrepancy. For this reason more tests are necessary to confirm the previous conclusion. Comparison of calculated isosteric heats at zero coverage and experimental values, reported in table 3, shows that our approach give good results. The experimental values have been derived from two methods: from slopes of van't Hoff plots using Henry constants derived from chromatographic measurements [17], and from application of Clausius-Clapeyron equation with volumetric adsorption measures [4]. Table 3. Computed and experimental isosteric heats at zero coverage (in kcal/mol) for N2, 02 and Ar adso,~ion on CazA and Ca-LSX. Molecule Ca-A Ca-LSX Theory Exp. a'b Theory. Exp:,~. . . . . . N2 7.3 6.7 a 7.5-t; 8.4 9.1 02 4.5 5.0 a 3.4 b 6.0 6.7 Ar 2.9 4.3 a 3.2 b 3.9 5.7 a ~ [ ] 7], refere-n~ce[ - [ ~ . . . . . . . . . . . . . .

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

In the first method, the plot of K with respect to 1/T can be easily fitted with a linear function but the error done by using a linear regression over a range of 70 Kelvin can easily reach 1 kcal/mol, according to the system. With the second method the values obtained depend, apart from the temperature range, on the coverage level considered. This precision and approximation being kept, a comparison of calculated and experimental results shows a good agreement for N2 adsorption. The agreement is not so good for Ar, with heats of adsorption derived from chromatographic experiments, whereas the other results are within the acceptable range of error. It is worth noting that if the isosteric heats

2010 at zero coverage are calculated with Koi which do not depend on temperature, as reported in figure 1, a systematic increase of the values is observed, according to the system. 5. CONCLUSIONS. We have presented a method to evaluate Henry constants and molar enthalpy of adsorption for gaseous molecules in zeolites based on the simple Langmuir equation, i.e. equation 8, to reproduce adsorption isotherms at low pressure. The method is based on the QM description of the interaction between the molecule and the cationic site, modeled as an embedded cation. Most of the models used to interpret adsorption isotherms are based on site equilibrium constants, where Koi and AEi are parameters adjusted to reproduce experimental isotherms. The Koi terms, often called affinity parameters, are generally considered to be independent of T and AE. The fact that we do not use empirical parameters and apply the multisite Langmuir model based on simple statistical thermodynamics derivations allows us to retain the physical significance of the thermodynamic quantities involved in the adsorption equilibrium.

Acknowledgements.The work was supported by Universit~t della Calabria. G. De Luca permanent address: Istituto di Ricerca su Membrane e Modellistica di Reattori Chimici, IRMERC-CNR, Via P. Bucci, 1-87030 Rende, Italy. REFERENCES. 1. J.A. Dunne, R. Mariwala, M.Rao, S.Sicar, R.J.Gorte and A.L. Myers, Langmuir, 12 (1996) 5888. 2. H.A. Boniface and D.M. Ruthven, Gas Sep. Purif., 7 (1993) 183. 3. G.A. Soriel, W.H.Granville and W.O.Daly, Chem. Eng. Sci., 38 (1883) 1517. 4. G.W. Miller, K.S. Knaebel and K.G. Ikels, AIChE J., 33 (1987) 194. 5. S. Sicar, Ind. Eng.Chem. Res., 30 (1991) 1032. 6. P. M. Mathias, R. Kumar, J. D. Moyer, J. M. Schork, S. R. Srinivasan, S. R. Auvil and O. Talu, Ind. Eng. Chem. Res., 35 (1996) 2477. 7. I. Langmuir, J. Am. Chem. Soc., 40 (1918) 1361. 8. G. De Luca, A. Arbouznikov, A. Goursot and P. Pullumbi, J. Phys.Chem. B, 105 (2001) 4663. 9. P. Pullumbi, J. Lignieres, A. Arbouznikov, A. Goursot, In Metal-Ligand Interactions in Chemistry, Physics and Biology ; Russo, N. and Salahub D.R. (eds), Kluwer, Dordrecht, 2000. 10. W. Rudzinski and D. H. Everett, Academic Press, Adsorption of Gases on Heterogeneous Surface, 1992. 11. J. Ligni6res, J.M. Newsam, Microporous and mesoporous materials, 28 1999 305 12. Cerius2 9Molecular modeling software, Accelrys Inc., San Diego, USA. 13. F. Yielens and P. Geerlings, J. of Molecular.Catalysis A, 166 (2001) 175. 14. A. D. Becke, P, Phys. Rev. A, 38 (1988), 3098. 15. J. P. Perdew, Phys. Rev. B., 33 (1986), 8822. 16. A.Goursot, V. Vasilyev and A. Arbuznikov, j. Phys. Chem., 101 (1997) 6420. 17. J.A. Martens, D. Ghys, M.Van de Voorde, H. Verelst, G.Baron and P. A. Jacobs, E. F. Vansant (Ed)., Separation Technology, Elsevier, Amsterdam, 1994.

2011

AUTHOR INDEX

Aboukais A. 699, 1197 Acevedo del Monte E. 1737 Acosta D.R. 1039 Agger J.R. 93 Agren P. 159 Aguado J. 77 Agueda VI. 1579 Ag6ndez J. 1267 Aiello R. 45, 423,469, 1165, 1427, 1891 Aiello S. 383 Aizawa T. 739 Akita T. 611 Alberti A. 151, 1923 Alifanti M. 823 Aliyev A.M. 787 Aloi D. 469 Alvarez A.M. 525, 1339 Anastasescu C. 1213 Anderson M.W. 93, 327, 1149 Andrade H.M.C. 343 Armaroli T. 975 Armendfiriz-Herrera H. 1039 Arribas M.A. 1015 Artioli G. 45, 1721 Artok L. 799 Asencio I. 731 Auroux A. 1091 Avalos-Borja M. 1939 B.Nagy J. 279, 287, 295, 469, 477, 503, 541, 927, 1395, 1427 Baerns M. 1141 Baldi M. 967 Bandyopadhyay R. 15 Baran P. 439 Barth J.-O. 69 Basaldella E. 1411 Batista M.S. 983 Battaglia G. 1751, 1759 Bauer E 303 Bazzana S. 117

Bechara R. 1133 Bein T. 223, 1465 Beland E 125 Bellat J.P. 1101, 1571 Bellussi G. 61, 1923 Bengoa J.E 525, 1339 Bentrup U. 1323 Berber-Mendoza S. 1849 Bergaoui L. 903 Bernauer B. 303, 1513, 1521 Berthomieu D. 1899 Berti D. 61 Beta I.A. 1647 Bevilacqua M. 975 Beyer H.K. 1347 Bi Y. 1205 Bilba N. 295 Birjega R. 1331 Bischof C. 911 Bispo J.R.C. 517 Biswas D. 771 Biswas G.K. 771 Bitter J.H. 573 Blanc A. 1395, 1473 Blanco C. 1253 Blasco T. 1331 Blin J.-L. 1687 Bogdanchikova N. 815, 1939 B6hlig H. 1647 B6hlmann W. 1355 BShringer W. 635 Boisen A. 109 Boldi~ M. 1663, 1713 Bonelli B. 143 Bonino E 3 Bonneviot L. 125 Bordiga S. 3, 199, 1963 Borello L. 143 Bortnovsky O. 533, 1505 Botas J.A. 1671 Botella P. 651

2012

Bourdon X. 951 Brabec L. 1505, 1521 BrandSo P. 327 Broctawik E. 453, 1971 Broersma A. 573 Bronic J. 423 Brorson M. 109 Brouca-Cabarrecq C. 1371 Broyer M. 1101 Briickner A. 1141 Brunel D. 1371, 1395, 1473 Bruno M. 279, 287 Bucci M. 1751, 1759 BiJlow M. 1995 Bulut H. 799 Buondonno A. 1751, 1759 Burgio G. 1023 Burtic~ G. 1655 Bus E. 573 Busca G. 967, 975 Bussai C. 1979 Cadoni M. 343 Cagnoli M.V. 525, 1339 Cai Q. 1205 Calabr6 V. 1561 Calcaterra D. 1775 Caldeira C. 359 Calleja G. 1671 Campelo J.M. 781, 1299 Cafiizares P. 707 Cappelletti P. 1775 Caputo D. 1611 Carati A. 191 Carluccio L.C. 61 Carotenuto L. 45 Carpentier J. 1197 Casci J.L. 1149 Catanzaro L. 287 Caullet P. 951 Cavallaro G. 1165 Cavani E 565, 831 Ceglie D. 1759 (~ejka J. 23, 247, 627 Centi G. 477, 503 (~erven~, L. 627 Chelaru C. 951 Chen W. 1237

Chen Y. 795 Chenevarin S. 335 Cheng L.-B. 1007 Choi S. 595 Chu Y.E 595, 887 Chun Y. 175 Ciahotn~, K. 1713 Ciambelli P. 1031 CibriLn S. 847 Clark L.A. 643 Colella A. 1751, 1759 Colella C. 45, 1611, 1751, 1759, 1841 Coluccia S. 1419 Comite P. 967 Constantin C. 1213 Coppola E. 1751, 1759 Corma A. 487, 651 Cormier W.E. 887 Corr~a M.L.S. 517 Cremades Ja6n E 549 Cresi S. 1963 Crisafulli C. 1023 Cruciani G. 151 Curcio S. 1561 Cuypers M. 263 Czuryszkiewicz T. 1117 Czyzniewska J. 335 d'Espinose J.-B. 1173 da Rosa R.G. 461 Dal Pozzo L. 565 Dalea V. 1825 Dalloro L. 191 Danilczuk M. 311 Datka J. 439, 445, 453, 1971 Davido% M. 101 de Castro B. 879 de Jong K.P. 573 De Luca G. 2003 De Luca P. 279, 287 de Lucas A. 723, 731 de Oliveira E.C. 461 D e P a o l a M . 1561 De Rossi S. 715 De Simone P. 1775 de' Gennaro B. 1841 de' Gennaro M. 1775 Decrette A. 1571

2013

Decyk P. 1785 D6de~ek J. 23, 1817 Dellarocca V. 1419 Demontis P. 1931 Derewinski M. 1157 Dexpert-Ghys J. 1371 D i R e n z o E 1057 Diaz G. 815 Diaz I. 1083, 1267, 1275 Dimitrov M. 1245 Dong J. 431, 1553 Dorado E 707, 723, 731 Duc~r6 J.M. 1899 Dumitriu E. 167, 951 Dumrul S. 117, 1497 Egger C.C. 1149 Ei6 M. 1611 Elanany M. 1867 Elangovan S.P. 911 Eltekov A.Y. 1537 Eltekova N.A. 1537 Engler C. 1647 Erdmann K. 755, 1631 Ernst S. 23, 549 Escola J.M. 77 Evmiridis N.P. 807, 839 Fajula E 581, 1057 Fan W. 1553 Fayon E 383 Fechete I. 167, 951 Fejes P. 477, 503 F6rej G. 1907 Ferey G. 1091 Fernandes A. 1371 Fernandez C. 383, 391, 1915 Ferraris G. 715 Ferreira R. 879 Ferro O. 1729, 1877 Fiddy S.G. 1939 Fieres B. 223 Fierro J.L.G. 517 Figueiredo J.L. 879 Fila V. 1513 Finocchio E. 967 Fiorilli S. 143 Fitch A.N. 1963

Fitch ER. 1995 Flego C. 1603 Fleith S. 415 Flytzani-Stephanopoulos M. 1031 Fois E. 1877 Frache A. 151, 343 Freire C. 879 Fricke R. 1323 Fritzsche S. 1947, 1955, 1979 Fr6ba M. 1245, 1695 Frontera P. 45 Frunza L. 1323 Frunza S. 1323 Fry~ovh M. 1521 Fuentes S. 815 Fujiwara M. 1307, 1379 Fukushima T. 1833 Gabelica Z. 215, 1101 G/tbov/t V. 1817 Galarneau A. 1057, 1371 Gallegos N.G. 525, 1339 Galli E. 1705 Gamba A. 1877 Gammon D.W. 619 GaneaR. 1331 Garcia-Sanchez M. 959 Garrone E. 143, 207, 215, 1395 Gaudino M.C. 1031 Gautier S. 391 Gazzoli D. 715 G6d6on A. 1173 Geidel E. 1647 Gener I. 1679 Geobaldo E 143, 215 Gervasini A. 1091 Ghisletti D. 61 Ghorbel A. 903, 935, 943 Giammona G. 1165 Gianotti E. 1419 Gierczyfiska M. 351 Gil B. 439, 453, 1971 Gilles E 1687 Giordano G. 39, 351,477, 503,581,831, 1561 Girard S. 1907 Giuffrida V. 1023 Gleizes A. 1371 Golembiewski R. 1631

2014

GomezA. 815 Gonzfilez E 1253 Gonzfilez-Pefia V. 1283 Gonzfilez-Velasco J.R. 847 Goursot A. 1899 Gramlich V. 415 Grange P. 823 Griboval-Constant A. 1133 Grizzetti R. 45 Gualtieri A.E 1705 Guczi L. 1801 Guenneau E 1173 Guerrero-Coronado R.M. 1849 Guimon C. 167 Gulbifiska M. 1221 Guo H. 999 Gurin V. 815 Guti6rrez EJ. 1671 Guti6rrez-Ortiz J.I. 847 Guzmfin-Castillo M . L . 1039 Haberlandt R. 1947, 1979 Haddad E. 1173 Halfisz J. 927 Hailer G.L. 1261 Han D. 1237 Han X.W. 175, 1481 Han Y.-J. 1109 Hancs6k J. 587, 863 Hannongbua S. 1979 Hannus I. 927 Haouas M. 31 Hartmann M. 375, 549, 911 Haruta M. 611 Hayakawa M. 675 Hecht T. 549 Hedlund J. 1437 Heintz O. 1101 Hensen E.J.M. 271, 959 Hernadi K. 85, 541 Hernando M.J. 1253 Hervieu M. 1091 Hobzovfi R. 1521 Holgado M.J. 1387 Hol16 A. 863 Horvfith W.I. 587 Hoshino M. 53 Hosoda S. 871

Hrabfinek P. 1513, 1521 Hradil J. 1513 Hrenovic J. 1743 Hrubfi I. 247 HsiaoL. 117 Huang W.Y. 1481, 1545 Huang Y. 135 Hudec P. 855, 1587 Hulea T. 951 Hulea V. 951 Hunger B. 1647 Hunger M. 659 Huwe H. 1245 Ibrahim K.M. 1767 Ilhan Y. 159 Imre B. 927 Inagaki S. 53 Iorio G. 1561 Iovi A. 1655 Ito S. 557 Ivanova I.I. 659 Jackson D.B. 1125 Jacobsen C.J.H. 109 Jacquemin J. 699 Jale S.R. 1995 Jaramillo H. 1291 Jentys A. 1619 Jirglovfi H. 1713 Johannsen K. 109 Jolima~tre E. 1571 Joly G. 1679 Josien L. 415 Jost S. 1947 Joyner R.W. 511 Kabe T. 795 Kaliaguine S. 125, 367, 823 Kall6 D. 587, 863, 1047 Kameoka S. 557 Kanda Y. 739 Kang J.-K. 683, 895 Kantarll i.C. 799 Karetina I.V. 1627 K~ger J. 1955 Katada N. 815 Katovic A. 39, 477, 503, 1561

2015 Katranas T.K. 839 Kessler H. 951 Kevan L. 763 Khaddar-Zine S. 935 Khedher I. 943 Khodakov A.Y. 1133 Khvoshchev S.S. 1627 Kikot A. 1411 Kikuchi E. 53 Kikuchi S. 1833 Kim D.-S. 683, 895, 919 Kim S.-C. 683, 895, 919 Kim S.-J. 895, 919 Kim Y.S. 595, 887 King B.L. 1125 Kirby C. 135 Kiricsi I. 85, 541,927, 991, 1801 Kiss J. 541 Kita K. 557 Kitayama Y. 675 Kiyozumi Y. 231, 239, 255 Klass L. 117 KleitzE 1117 Klepel O. 1355, 1647 KnappM. 117 Knops-Gerrits P.P. 263, 1809 Knyazeva E.E. 659 Ko~ifik M. 303, 1505, 1513, 1521, 1663, 1713 Kodama T. 675 K6hn R. 1245, 1695 Komatsu T. 667 Koningsberger D.C. 1885 K6nya Z. 541, 927 Kooli E 255 Kornatowski J. 69, 399, 755, 1631 Ko~ovfi G. 247 Kosslick H. 1141, 1323 Kovaleva N.Yu. 691 Kowalak S. 39, 351 Kozyra P. 445, 1971 Krystl V. 1513, 1521 Kub~nek P. 533 Kubo M. 1867 Kubota Y. 15 Kuliyev A.R. 787 Kumar E 1229, 1307, 1379 Kunimori K. 557 Kuraoka K. 1307, 1379

Kurosaka T.

739

Labfidi I. 85 Lacombe S. 391 Ladygina T.A. 691 Lafont E 781 Lambert J.-E 903 Lamberti C. 3, 199, 1891, 1963 Lamonier J.-E 699, 1197 Langella A. 1751, 1759, 1775 Laniecki M. 1221 Lappas A.A. 807 Larin A.V. 1987 Laversin H. 1197 Lfiz~ir K. 477, 503, 1347 Lee D.-K. 683, 895, 919 Lee G.-S. 683 Lee J.H. 595, 887 Lee S.I. 887 Lemaire E. 1571 Leon R.M. 1299 Lercher J.A. 69, 399, 1619 Lewandowska A. 1785 Leyva-Ramos R. 1849 Lezanska M. 755, 1631 Li J. 1553 Li X. 795 Li Y. 1529 Li Z. 999 Liang Y. 999 Liguori B. 45, 1841 Lim B.S. 595 Lin Z. 319 Lind6n M. 1117 Liu G. 431, 1553 Liu L. 1489 Liu Q. 1007 Liu Y. 1075 Liu Z. 603 L6nyi E 991 L6pez-Fonseca R. 847 Luna D. 1299 Ma L.L. 1489 Macario A. 1561 Machowska M. 1157 Macquarrie D.J. 1125, 1395, 1473 Madeo G. 1427

2016

Magusin P. 959 Mal N.K. 1229, 1307, 1379 Manoilova O.V. 207 Mao Y. 1857 Marchese L. 151, 343, 1419 Marchetti S.G. 517, 525, 1339 Marinas J.M. 781, 1299 Mfirquez-Alvarez C. 1267, 1283 Marra G.L. 1891, 1963 Martinez A. 1015 Martinez E 1109 Martinez H. 167 Martucci A. 151 Marzke R.E 815 Mascarenhas A.J.S. 343 Maselli L. 565 Massiani P. 359 Matsukata M. 53 Mazzei Justin E. 967 M6hn D. 85 Melenovfi L. 1713 Melero J.A. 1109, 1181 Mellot-Draznieks C. 1907 Meloni D. 167 Melzoch K. 1663 Mendoza-Barron J. 1849 Meshkova I.N. 691 M&hivier A. 1571 Mezzogori R. 565 Mhamdi M. 935 Miao W.-R. 1007 Michalik J. 311 Michel D. 1355 Mignard S. 1679 Mikailov R.Z. 787 Mikulcovfi K. 627 Milanesio M. 1891 Millini R. 61, 191, 1923 Min K. 1481 Minchev C. 1245 Minic6 S. 1023 Mintova S. 223, 1465 Mitov I. 1245 Mitsuhashi M. 667 Miyadera T. 557 Miyajima A. 1595 Miyamoto A. 1867 Miyamoto J. 1595

Miyazaki T. 871 Mizukami E 231, 239, 255 Mizusima T. 675 Mohino E 1275 M611er K.P 635 Monaci R. 167 Montouillout V. 383, 391, 1915 Montoya de la E, A. 1039 Moon G. 635 Morais C.M. 1915 Morales G. 1181 Moreau C. 581 Moret M. 1721 Moretti G. 715 Moil G. 1737 Murai S. 675 Muraoka M. 675 Nachtigall P. 101 Nachtigallovfi D. 101 Nakajima H. 239, 871 Namba S. 739 Nastro A. 279, 287, 295 Navarro M.T. 487 Nawata S. 1833 Naydenov V. 1449 Nenu C. 1331 Niesz K. 1801 Nishide T. 239 Nobukawa T. 557 Nossov A. 1173 Novfik P. 1505 Novokshonova L.A. 691 Nowak I. 1363 Oberlinger M. 23 O'Connor C.T. 635 Ogden J.S. 1939 Oh S.H. 595, 887 Oh S.-S. 895 Oliveira A.C. 517 Oliveira E.C. 1419 Omegna A. 31 Onida B. 143, 215, 1395 Onyestyfik Gy. 1047, 1639, 1793 Oszk6 A. 541 Otero Arefin C. 199, 207 Oumi Y. 871, 1229, 1595, 1833

2017 Ovejero G.

1109

Pachtova O. 303 Pak C. 1261 Palacio L.A. 1291 P~l-Borb61y G. 1347 Palin L. 1891, 1963 P~ilink6 I. 85, 1801 Paneva D. 1245 Pang W. 1205 Papp H. 1355 Parker, Jr. W.O. 61 Parton R. 1809 P~rvulescu V.I. 367, 823, 1213, 1403 Pasqua L. 469, 1165, 1427 Passaglia E. 1705, 1729 Pastore H.O. 343, 461, 1419 Patarin J. 415 Paulidou E.G. 839 Paulin C. 1101 Pavel C.C. 295 Pazzuconi G. 1603 Pefiarroya Mentruit M. 207 Perathoner S. 477, 503 Perego C. 191, 1603 Perego G. 1923 P6rez-Pariente J. 1083, 1267, 1275, 1283 Perlinska J. 311 Perlo P. 1963 Perri C. 279 Pesquera C. 1253 Pestryakov A. 815 Petitto C. 581 Petkov N. 1465 Petranovskii V. 815, 1939 Philippot E. 1371 Piccolo C. 45 Pierro P. 1165 Pietrzyk P. 453 Pinnavaia T.J. 1075 Pino E 477, 503 Pirngruber G. 31 Pistarino C. 967 Pitsch I. 1323 Pizzio L. 1411 PodeR. 1655, 1825 Pode V. 1825 Pokorny J. 1663

Poladly P.E 787 Pomakhina E.B. 659 Ponomoreva O.A. 659 Pop Gr. 1331 Popovici E. 1825 P6ppl A. 375 Prestipino C. 3, 1963 Prihod'ko R. 271 Prins R. 31 Proke~ovh P. 627 Pullumbi P. 1907, 2003 Qiao w. 999 Quartieri s. 1877 Quoineaud A.A. 391 Radovet R. 1655 Rafuzzi B. 1737 Rains J.A. 117 Rangel M.C. 517 Ranjit K.T. 763 Rathouslo) J. 1067, 1457 Rees L.V.C. 1639 Renard G. 1473 Renaud A. 1679 Reschetilowski W. 1315 Rey E 651 Ribeiro M.E 359 Richer R. 135 Ricchiardi G. 3 Rigoreau J. 1679 Rives V. 1387 RizzoC. 191 Rocchia M. 1395 Rocha J. 319, 327, 1915 Rodriguez Delgado M. 207 Rodriguez E, T. 1737 Rodriguez J.M. 77 Rohlfing Y. 1067 Rojasov~ E. 855 Rombi E. 167 Romero A. 723 Romero A.A. 1299 Romero R. 707 Rosenholm J. 1117 Rozwadowski M. 755, 1631 Russo N. 2003 Rybarczyk P. 1141

2018

Sabo L. 855 Sacco, Jr. A. 117, 1497 Sadlo J. 311 Saha S.K. 771 Salas-Castillo P. 1039 Salou M. 231, 255 S~inchez P. 707, 723, 731 Sannino D. 1031 Sano T. 871, 1229, 1595, 1833 Sarijanov E.E. 787 Sariofglan A. 787 Sarv P. 1157 Sasata K. 1867 Sastre E. 1267, 1275, 1283 Sastre G. 1015 Sauer J. 643 Sauerbeck S. 549 Schenkel R. 69 Schmidt I. 109 Schmidt W. 159, 1857 Schneider P. 1587 Sch6nhals A. 1323 Schiith E 159 Schwieger W. 407 Scir6 S. 1023 Selvam T. 407 Seong K.H. 595, 887 Serban S. 1331 Serrano D.P. 77, 1671 SerreC. 1091 Shakhtakhtinsky T.N. 787 Shen D. 1995 Shi J. 1529 Shimizu K. 675 Sierka M. 643 Sierra L. 1291 Siffert S. 699, 1197 ilhanM. 101 Silva M. 879 Simon J.M. 1571 Simon-Masseron A. 415 Sinha A.K. 611 Siska A. 541 Sitkei E. 85 Smie~kovfi A. 855, 1587 Sobalik Z. 533 Sojka Z. 453 Solcov/t O. 1505, 1587

Sotelo J.L. 1109, 1579 Sponer J.E. 533 Spoto G. 3 Stara G. 1931 Staudte B. 439 Stein E.M. 117 Sterte J. 183, 1437, 1449 Stievano L. 359 Stocchi B. 191 Stockenhuber M. 511 Stucky G. 1109 Su B.L. 1213, 1403, 1687 Suboti6 B. 423 Suffritti G.B. 1931 Sugi Y. 15 Sugioka M. 739 Sullivan M.J. 117 Sychev M. 271 Sysel P. 1521 Szauer Gy. 863 Szegedi A. 1347 Szyrnkowiak E. 351 Tabacchi G. 1877 Takami S. 1867 Tamfisi A. 1801 Tanaka H. 1619 Tanaka S. 557 Tanchoux N. 1057 Taouli A. 1315 Taulelle E 1915 Testa E 423, 469, 1165, 1427, 1891 Thibault-Starzyk E 335, 1809 Thommes M. 1695 Thomson S. 159 Tibljas D. 1743 Tiddy G.J.T. 1149 Tkachenko O.P. 511 Tobdn-Cervantes A. 1039 Todinca T. 1655, 1825 Tolvaj G. 587 Tomishige K. 557 Torracca E. 1841 Tosheva L. 183, 1437, 1449 Toufar H. 1687 Trejda M. 1785 Trens P. 1057 Triantafillidis C.S. 807, 839

2019 Trifir6 E 831 Trombetta M. 975 Trujillano R. 1387 Tsagrasouli Z.A. 807 Tsiatouras V.A. 807, 839 Tsoncheva T. 1245 Tsubota S. 611 Tsyganenko A.A. 207 Tuel A. 943 Tun9 Sava~gi 6. 787 Tuoto C.V. 287 Turnes Palomino G. 199, 207 Tvarfi~kovh Z. 533 Uemichi Y. 739 Uguina M.A. 1579 Ulkii S. 799 Uozumi T. 871 Urquieta-Gonzfilez E.A. Ushakova T.M. 691 Utting K.A. 1473 ..

983

Valange S. 215, 1101 Valencia S. 651 Valente A.A. 327 Valkai I. 863 Valverde J.L. 723, 731 Valyon J. 991, 1639 van Bokhoven J.A. 1885 van der Eerden A.M.J. 1885 van Donk S. 573 van Grieken R. 1181 van Santen R.A. 271,959 van Steen E. 619 van Veen J.A.R. 271 Varga Z. 587 Vasalos I.A. 807 Vfizquez P. 1411 Vfizquez-Rodriguez A. 1039 Vercauteren D.P. 1987 Vezzalini G. 1877 Violante D. 287 Visinescu C.M. 367 Viterbo D. 1891 Vlessidis A.G. 839 Voltolini M. 1721 Vondrov~i A. 533 Vuono D. 279, 295

Wang A. 795 Wang G. 603 Wang G.-R. 999, 1007 Wang J. 1529 Wang Q. 747, 1545 Wang Y. 175, 1489, 1545 Wang Y. 795, 1007 Wark M. 1067, 1457 Warzywoda J. 117, 1497 Watson A. 1125 Wei C. 1205 Wei Y. 603 Weidenthaler C. 1857 Weitkamp J. 659 Wellmann H. 1457 West C.M. 117 Wichterlov~i B. 533, 1817 Wilkenh6ner U. 619 WlochJ. 755 W6hrle D. 1067 W6jtowski M. 1221 Wolf G.-U. 1141 Woltermann J. 595, 887 Woo J.Y. 117 Wu Z. 747 Xie G. 175 Xie P. 603 Xie S. 747 Xu L. 603, 747 Xu Y. 1489 Xue C.E 431 XueJ. 175, 1489 Yamada H. 311 Yamana K. 1229 Yan D. 1529 Yan X. 1237 Yan X.W 175, 1189, 1481 Yang X. 1205 Yang Y.K. 683 Yao N. 1261 Yao P. 795 Yashima T. 667 Yeramifin A.A. 525, 1339 Yllmaz S. 799 Yokosuka T. 1867 YuQ. 1545 Yuschenko V.V. 659

2020

Zadrozna G. 399 Zahedi-Niaki M.H. 125, 367, 823 Zanardi S. 1923 Zecchina A. 3, 199, 1963 Zemljanova G.Ju. 1627 Zhang A.M. 1237 Zhen K. 1205 Zheng S. 1619

Zhilinskaya E. 699 Zhu J.H. 175, 1189, 1481, 1489 Zibrowius B. 159 Zidek Z. 855, 1587 Zikfinova A. 1505, 1513 Zilkovfi N. 247 Ziolek M. 1785 Zukal A. 1067

2021

SUBJECT

ab initio calculation 1979 Acetonitrile 335 Acetophenone 667 Acetylene hydration 1047 Acid catalysis 715 Acid sites 383, 439, 1331 Acid sites characterization 247 Acidic zeolite catalysis 643 Acidity 167, 1157, 1315 Acidity of OH groups 1419 Acrylamide 511 Activated clinoptilolite 1655 Acylation 627 Ag exchanged ferrierite 1031 Agglomeration 707 Ag-zeolites 1963 A1 coordination 31 A1 distribution 1817 A1, BZSM-5 279 A1,B-LZ 287 AI/Ce mixed pillars 1253 27A1 MQ MAS NMR 31 Albumin 1537 AI-Cu pillared clays 683 Aldol condensation 667 Alkali metal ion 239 Alkylamine route 215 A1-MCM-22 23 A1-MCM-41 catalysts 1299 A1-MCM-48 755, 1229, 1631 A1PO-34 151 A1PO4-11, crystallization 135 A1PO4-40 1915 A1SBA, Hyperpolarized Xenon 1173 Aluminium coordination 1885 Aluminium distribution 23 Aluminosilicate, mesostructured foams Aluminum species in zeolites 391 A1-ZSM-12, synthesis 247 Aminoacids 1083 Aminopropyl 1395

INDEX

Ammonia 1687, 1713 Ammonia treatment 549 Ammonium 1737 Analcime 423 Anchoring 461 Aniline conversion 1299 Aniline methylation 659 Anomalous XRD 1963 Anthraguinone synthesis 1007 Aromatics 595, 887, 1213 Aromatics hydrogenation 1015 Aromatization 855 Atomic Force Microscopy 93, 1721 Au/Ti-MCM-41 611 Automotive exhaust gases 1611

1075

Base Catalysis 1473 Basic MCM-48 1481 Basic Pt-supported BEA 359 Basic zeolites 549 BEA 469 Beckmann rearrangement 191 Benzene 143 Benzene adsorption 1631 Benzene oxidation 477, 503 Benzothiophene adsorption 1579 Benzoyl chloride 1427 Beta zeolite 799, 1603, 1833 Beta-zeolite catalyst 1007 B-FER 1923 Bifunctional catalysis 603 Bikitaite 1877 Bimetallic catalyst 581 Bimetallic clusters 911 Binary adsorption 1595 Biological phosphorus removal 1743 B-LZ 287 B-MFI 1923 Borosilicate 1987 Boyd Adamson equation 1655 Br6nsted acidity 31,975

2022 BrSnsted sites 3 Brownian motion 1505 B-Silicalite 1891 Butane isomerization 715 Butene skeletal isomerization 573 Butenes oligomerization 831 CA-A zeolite 2003 Calcium oxide 175 Calorimetry 1101, 1627 CA-LSX zeolite 2003 Carbenium ion 643 Carbon nanotubes 541, 1237 Carbonates 359 Catalyst deactivation 715 Catalytic activity 675 Catalytic combustion 1023 Catalytic cracking 77 Catalytic cumene cracking 755 Catalytic oxidation 847, 1205 Catalytic wet oxidation 683 Catanionic Surfactant Self-assembly 1189 Cation selectivity 1841 CCVD 541 Cd ion exchange 1849 Cd-clinoptilolite 1047 CdS and CdSe nanoparticles 1457 Chabazite 1729, 1947 Chemically modified aluminum alkoxide 1283 Chemisorption 1545 Chlorinated VOCs 847 Chlorobenzene 1023 Cigarette 1489 Clay binder 707 Clinoptilolite 1663, 1713, 1849 Clusters 1939 CO adsorption 207 Co exchanged ferrierite 1031 CO hydrogenation 919 Coal Fly Ash 1229 Cobalt catalysts 1133 Co-Fe-Cu mixed oxide 1197 Colloidal faujasite 1267 Competitive ion exchange 1849 Computational methods 1867 Computer modelling 93 Confined liquid crystals 1323 Coordination of iron in MCM-41 1347

Copper 723, 815, 1101 co-template 399 Covalent anchorage 1067 Co-ZSM-5 453, 935 Cr-Beta 1449 Cr-MFI 839 Crosslinking 159 Crystal chemistry 1705, 1729 Crystal growth 93, 117, 223 Crystal growth modelling 423 Crystal structure 1729, 1891, 1923 Crystallization Kinetics 45 Cs-MCM-41, support 525, 1339 Cu-MCM-22 343 Cumene, cracking 1075 Cu-zeolites 1899 Cu-Zn complex 85 CuZSM-5 375, 445, 1971 Cyclohexane oxidation 879 Deactivation by coking 167 Dealumination 1817, 1833 Deep hydrodesulfurization 795 Defect 1595 Defect-free MEL, synthesis 61 Dehydrogenation 807, 839 Dehydroisomerization of n-butane 603 Delaminated zeolites 651 Delamination 69 Denitrogenation 1579 DeNOx 823, 1971 Density functional theory calculations 2003 Desertification 1751 Design of zeolitic materials 1867 Desulfurization 863, 1579 DFT 1899 Diesel cetane 1015 Diffusion 109, 1931, 1955 1,4-dimethylpiperazine 415 Dissolution reactions 1721 Drug delivery systems 1165 Dry gel 15 Dry gel conversion 53 Dyes in Si-MCM-41 1067 Dynamic light scattering 45, 223 Effective diffusivity coefficient Electron spin resonance 763

1825

2023 Eley-Rideal mechanism 557 Enzime modeling 85 Epoxidation 611,943 EPR spectroscopy 453, 699, 1899 ESR 1785 Esterification 787 Ethanol dewatering 1663 Ethylene 1627 Ethylene ammoxidation 935 Ethylenediamine 1923 EXAFS 1939, 1963 Extraframework iron 167 FAB-MS coke analysis 781 Fatty acids esterification 1275 Faujasite 879 Fe- introduction 477, 503 Fe ion-exchanged zeolite 557 Fe species 983 Fe-(A1)MFI 477, 503 Fe,A1-PILC 991 Fe/Ga,A1-MCM-41 1331 Fe/ZSM-5 983 Fe-BEA zeolite 469 Fe-containing molecular sieves 1785 FeH-zeolites 533 Fe-MCM-41 1347 Fe-montmorillonite 991 Ferrierite 199, 573 Fe-ZSM5 511 Field emission scanning electron microscopy 117 Fischer Tropsch synthesis 1133 Fischer-Tropsch reaction 525, 1339 Fluid Catalytic Cracking 807 Fluoride contribution 367 Framework flexibility 1885 Framework vibrations 263 Frequency-response spectra 1639 Friedel-Crafts acylation 799 FT-IR 3, 199, 207, 215, 975, 1419, 1687, 1785, 1809 Functionalization 1181, 1411 71Ga NMR 959 Ga/H-ZSM5 959 Galliation 1833 Gallium phosphates

367

Gallophosphate 415 Gas oil 587 Gas separation 1907 Gold surface 1497 Growth kinetics 1513 Guanidines 1473 Guest-host interactions 1323 ~H / 27A1 decoupling

1915

2H NMR 1687 H2-D2 exchange 959 H202 683 Hafnium silicate 319 H-Beta 383 H-bonding 143 Heats of adsorption 1995 Heavy metals 1825 Heterocyclic compounds 1647 Heteropolyacids 1291 Heteropolycompounds 1411 Hexene 747 Hierarchical porosity 1057 H-MCM-22 635 HY zeolite 967 Hybrid catalyst 771 Hybrid zeolite disc 231 Hydrazoic acid 1793 Hydrocarbons (C6) 1679 Hydrocarbons oxidation 1403 Hydroconversion of aromatics 581 Hydrocracking 771 Hydrodechlorination 927 Hydrogenolysis of tripropylamine 1221 Hydroisomerization 707 Hydrolisis of triglycerides 1561 Hydrophobicity 1039 Hydrotalcite 1197 Hydrothermal Synthesis 1379 Hydrothermal transformation 407 Hydrotreating 587 Hydroxymethylation 565 IMEC 1315 Immobilized complexes 85 In situ 13C MAS NMR 659 In situ Synchrotron XRPD 45 In situ-EPR/UV-Vis spectroscopy Indole adsorption 1579

1141

2024 115In MAS NMR 1355 In-MCM-41 1355 In-situ XAS 1885 Interaction strength 1647 Iodine Indicator Technique 1521 IR spectroscopy 143, 335, 439, 445, 453, 573, 927, 1603 Iron containing, SBA-15 1109 Iron in MCM-41 1347 Iron oxide 1237 Iron phthalocyanines in zeolites 1809 Iron supported catalysts 1339 Iron-doped mesoporous material 517 Iron-doped zeolites 517 Isobutane 823 Isomerization 747, 863 Isomorphous substitution 327, 1331 Isopropanol decomposition 39 ITQ-2 69 Jordan faujasite

1767

K10 montmorillonite 943 Kanemite 255 Kinetic model 303 Kinetics 787 Kinetics of ammonia sorption 1639 Knoevenagel condensation 549 Lanthanides 1371 Large pore zeolites 627 Layer growth 1505 Layered silicate 231, 239 Layered zeolites 69 LDH 1387 Levynite 287 LiLSX 1995 Lipase immobilization 1561 Lithification 1775 Location and motion 1687 Long chain olefins 999 Luminescence 1371 M41S materials 1695 Macrostructures 1437 Macrotemplate 183 Magnesia 1481 Manganese (III) complexes

879

Mass Spectroscopy 903 Mass transport limitations 1793 MCM-22 951 MCM-22, synthesis 53 MCM-36 951 MCM-41 795, 871, 911, 1083, 1101, 1221, 1237, 1315 MCM-41 mesoporous material 1261 MCM-41, modified 1245 MCM-48 1205 MCM-48, modified 1245 Mesoporosity 159 Mesoporous Aluminophosphates 1419 Mesoporous aluminosilicate 1157 Mesoporous films 1457, 1465 Mesoporous materials 1091, 1411, 1671 Mesoporous materials as support 1165 Mesoporous materials, catalysis 487 Mesoporous materials, reactivity 1427 Mesoporous molecular sieves 651 Mesoporous SBA material 1323 Mesoporous silica 1291, 1371 Mesoporous silica SBA-15 739 Mesoporous solid 1173 Mesoporous supports 1561 Mesoporous zeolites 1267 Mesostructured materials 1057 Metallic Cd introduction 1047 3-methoxy-4-hydroxybenzyl alcohol 565 2-Methoxynaphthalene 799 2-methoxyphenol 565 MFI catalyst 831 MFI zincosilicate, catalysis 351 MFI zincosilicate, synthesis 351 MFI zincosilicates 215 MFI-nanocrystals 159 Micelle Templated Silicas 1125, 1395, 1473 Microporosity 1587 Microporous alumina 1283 Microporous materials, catalysis 487 Microporous molecular Sieves 651 Microporous silica 255 Microwave radiation 1481 Middle distillate 771 M-MCM-41 1403 Mo-based sulfides 795 Modelling 1149 Modified crystals 1437

2025 Molecular dynamics 1877, 1931 Molecular modeling 61, 1907 Mono- and bimetallic MCM-41 1213 Monte Carlo simulation 1995 Montmorillonite 1253 Mordenite 335, 407, 731,815 Morpholine 151 Morphology control 399 MQHETCOR 1915 MQ-MAS 391 m-xylene 1267 N20 decomposition 557 Na-magadiite 407 Nanocomposite 191 Nanocrystalline ZSM-5 77 Nanoparticles diffusion 1513 Nanosized zeolites 223 Naphthalene 627, 999 Natural zeolites 1751, 1759 Nb sources 1363 NbMCM-41 1363 n-decane isomerization 911 n-hexane 855 Ni/Y-zeolite 919 Ni-A1 hydrotalcite 1387 Nickel metal distribution 919 Nitrogen adsorption 1587, 1631 Nitrous oxide decomposition 343 N-methylation 1299 NMR 391,469, 1173 N-Nitrosamines 1489 NO 895 NO adsorption 343, 375 NO reduction 983, 991 Noble metal loading 739 Non-hydrothermal synthesis 271 Non-steroid antinflammatory drugs NOx abatement 1031 Numerical simulation 1655

1165

Organic functionalisation 1125 Organised mesoporous alumina 1283 Ortho-benzoyl benzoic acid 1007 Oxide dispersion 1245 Paraffins 1571 Para-selectivity 635

Pb ion exchange 1849 Pd/H-ZSM-5 847 Pd-Fe/Beta zeolite 699 Pellets 279 Pentasil zeolites 1817 1-pentene isomerization 781 Periodic Hartree-Fock 1987 Periodic mesoporous silicas 1133 Phenol 619 Phenol methylation 635 Phenyl functionalised 1275 Phillipsite 1705 Phosphates 823 Phosphomolybdic heterpolyacid 1221 photocatalysis 1457 Photoreduction 763 Pillared clays 903, 1253 ct-pinene transformation 1291 Platinum 1023 Polyimides 1521 Polymerization 691, 871 Polyolefin 77 Pore condensation 1695 Pore expansion 1117 Pore size 1057 Porous aterials 1907 Potential function 1979 Preferred orientation 1465 Propane 839 Propane oxidative dehydrogenation 1141 2-Propanol conversion 755 Propene 611 Propylene 871 Protonic zeolites 975 Pt nano particle 1261 Pt/H-mordenite 863 Pt-Pd/USY 587 PtSn nano particle 1261 PtY PtY-zeolites 927 PtZSM-5 1801 QM/MM embedding 643 Quadrupole coupling constants 1987 Quantum chemical modelling 1971 Quanttma chemical molecular dynamics Quantum mechanics 1809

1867

2026 Radiolysis 311 Raman spectroscopy 263, 1809 Redox activity 533 Reduction by H2 or NaBH4 1801 Reformate 595 Re-hydration 1603 Rho 311 Rhodium 461 Rietveld method 1705 Ring opening 1015 Rock texture 1775 SAPO catalysts 781 SAPO-11 603 Saponite 271 SBA-1 1149 SBA-15 1109, 1181 SCR of NOx 723, 731 Secondary mesopores 1083 Secondary synthesis 1307 Selective catalytic reduction 895 Selective oxidation 1213 Self-transformation of glass 1553 Sepiolite 675 Shape-selective adsorption 1679 Si/A1 815 Silicalite 1595, 1679, 1891 Silicalite-1 191,303, 1521, 1979 Silicalite-1 layer 1513 Silicalite-1 membranes 1505 Silver 1939 Silver clusters 311 Simulation 1947, 1955 Single molecule spectroscopy 1067 Small-pore framework 319 Sodalite 255, 311 Sodium azide in zeolites 1793 Soil amendment 1759 Soil reconstruction 1751 Solid state transformation 231 Solid strong base 175 Solid-state ion exchange 935 Solid-state NMR 135, 383 Solid-state transformation 239 Sorption hysteresis 1695 Spectroscopic Investigation 1395 Spectroscopic techniques 3 Spin coating 1465

Stability 367 Steam reforming of HVOC 967 Steam-assisted conversion 15 Steamed HZSM-5 439 Structural changes 1801 Structural characterization 151 Structural stability 1857 Styrene production 517 Sulfate ion 1039 Sulfated Zirconium 903 Sulfidation 511 Sulfonic groups 1181 Sulfonic MCM-41 1275 Superbasicity 1545 Supported iron catalysts 525 Supported vanadia catalysts 1141 Surface Microtopography 1721 Surface modification 1427 Surface structure 117 Surfactant 1387 Synthesis modification 399 Synthesis optimization 39 Synthetic clays 271 Synthetic phillipsite 1841 TAPO's 125 Template removal 303 Templates 263 Textural properties 831 Thermal Stability 1157 Thiophene hydrodesulfurization activity Thioresistance 581 Time resolved IR spectroscopy 1619 Tin phosphates 1091 Ti-PILC 723 Ti-Sn-MCM-41 1379 Titanium oxo phosphate 1117 Titanium pillared clay 895 titanium-silicalite 109 Titanosilicates 327, 619 Titanosilicates, ionic Exchange 295 Titanosilicates, synthesis 295 TMI coordination 101 Toluene alkylation 951 Toluene oxidation 699, 1197 Toxic elements 1759 TPD 445, 1647 t-plot 1587

739

2027 TPR 359 Transalkylation 887 Transition metals 1205, 1403, 1857 Trichloroethylene decomposition 967 TS-1, mesoporous 109 Urban dump leachates 1737 UV spectroscopy 1603 UV-Vis spectroscopy 23, 199 UV-Vis-NIR spectroscopy 1419 Vanadium 327 Vanadium-K10 943 Vapor phase transport 431 Vesicular mesoporous silica V-MCM-41 1307 VOCs recovery 1671 VPI-7 431

1189

Wastewater treatment 1743, 1767 Wastewater, P removal 1743 Water adsorbed in chabazite 1947 Water adsorption 1631 Water and SO2 resistance 731 Water confined in zeolites 1931

W-band ESR spectroscopy

375

XANES 125 XPS 125 XRD 1785 Zeolite 3A 1663 Zeolite films 1437, 1497, 1553 Zeolite macrostructures 1449 Zeolite particles 1639 Zeolite Seeds, assembly of 1075 Zeolite X 763 Zeolite Y 461 Zeolites as additive 1489 Zeolite-supported magnetite 1825 Zeolitization 1775 Zeotype support 541 Zincophosphate 415 Zirconia 1545 Zirconocene 691 Zn-MFI 39 ZnZSM-5 855 ZSM-5 101,207, 279, 691, 807, 1497, 1571, 1619 ZSM-5 zeolite membrane 1529 ZSM-5, spheres 183


2029

STUDIES IN SURFACE SClENCEAND CATALYSIS Advisory Editors: B. Delmon, Universit6 Catholique de Louvain, Louvain-la-Neuve, Belgium J.T.u University of Pittsburgh, Pittsburgh, PA, U.S.A. Volume 1

Volume 2

Volume 3

Volume 4

Volume 5

Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Volume 11

Volume 12 Volume 13 Volume 14 Volume 15

Preparation of Catalysts I.Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings ofthe First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, RA. Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts II. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedingsofthe Second International Symposium, Louvain-la-Neuve, September 4-7,1978 edited by B. Delmon, R Grange, RJacobs and G. Poncelet Growth and Properties of Metal Clusters.Applications to Catalysis and the Photographic Process. Proceedings of the 32nd International Meeting of the Soci6t~ de Chimie Physique,Villeurbanne, September 24-28,1979 edited by J. Bourdon Catalysis by Zeolites. Proceedingsof an International Symposium, Ecully (Lyon), September 9-11,1980 edited by B. Imelik, C. Naccache,Y. BenTaarit, J.C.Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium,Antwerp, October 13-15,1980 edited by B. Delmon and G.E Frornent New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis,Tokyo, June 30-July4,1980. PartsA and B edited by T. Seiyama and K.Tanabe Catalysis by Supported Complexes by Yu.l.Yermakov, B.N. Kuznetsov andV.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyfie, September 29-October 3,1980 edited by M. Ldzni~:ka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium,Aix-en-Provence, September 21-23,1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16,1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, R Meriaudeau, R Gallezot, G.A. Martin and J.C.Vedrine Metal Microstructures in Zeolites. Preparation - Properties-Applications. Proceedings of aWorkshop, Bremen, September 22-24,1982 edited by RA. Jacobs, N.I. Jaeger, RJir~ and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. B6nard Vibrations at Surfaces. Proceedings of theThird International Conference, Asilomar, CA, September 1-4,1982 edited by C.R. Brundle and H. Morawitz Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets

2030

Volume 16

Volume 17 Volume 18 Volume 19 Volume 20 Volume 21 Volume 22 Volume 23 Volume 24 Volume 25 Volume 26 Volume 27 Volume 28 Volume 29 Volume 30 Volume 31


Preparation of Catalysts III. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings oftheThird International Symposium, Louvain-la-Neuve, September 6-9,1982 edited by G. Poncelet, R Grange and RA. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16,1983 edited by G.M. Pajonk, S.J.Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by RA. Jacobs, N.I. Jaeger, R Ji~,V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, RQ., September 30-October 3,1984 edited by S. Kaliaguine andA. Mahay Catalysis byAcids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier,Y. BenTaarit and J.C.Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure,Technology andApplication. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. HoEevarand S. Pejovnik Catalytic Polymerization of Olefins. Proceedingsofthe International Symposium on FutureAspects of Olefin Polymerization,Tokyo, July 4-6,1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985.Proceedingsofthe Fourth International Conference, Bowness-on-Windermere, September 15-19,1985 edited by D.A. King, N.M.Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerven# New Developments in Zeolite Science andTechnology. Proceedings of the 7th International Zeolite Conference,Tokyo,August 17-22,1986 edited by Y. Murakami, A. lijima and J.W.Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis andAutomotive Pollution Control. Proceedings of the First International Symposium, Brussels, September 8-11,1986 edited by A. Crucq andA. Frennet Preparation of Catalysts IV. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-laNeuve, September 1-4,1986 edited by B. Delmon, R Grange, RA. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by RWissmann Synthesis of High-silicaAluminosilicate Zeolites edited by RA. Jacobs and J.A. Martens Catalyst Deactivation 1987.Proceedings of the 4th International Symposium, Antwerp, September 29-October 1,1987 edited by B. Delmon and G.E Froment Keynotes in Energy-Related Catalysis edited by S. Kaliaguine

2031

Volume 36 Volume 37 Volume 38 Volume 39 Volume 40 Volume 41

Volume 42 Volume 43 Volume 44

Volume 45 Volume 46



Methane Conversion. Proceedingsof a Symposium on the Production of Fuelsand Chemicals from Natural Gas,Auckland,April 27-30,1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S.Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17,1987 edited by RJ. Grobet,W.J. Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987.Proceedings ofthe 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22,1987 edited by J.W.Ward Characterization of Porous Solids. Proceedings of the IUPACSymposium (COPS I), Bad Soden a.Ts.,Apri126-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings ofthe Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17,1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. P6rot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Padl Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings oftheWorldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium,W~rzburg, September 4-8,1988 edited by H.G. Karge and J.Weitkamp Photochemistry on Solid Surfaces edited by M.Anpo andT. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14,1989. PartsA and B edited by RA. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting,Washington, DC, November 27-December 2,1988 edited by M.L. Occelli and R.G.Anthony New SolidAcids and Bases.Their Catalytic Properties by K.Tanabe,M. Misono,Y. Ono and H. Hattori RecentAdvances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and RJ. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8,1989 edited by D.L.Trimm, S.Akashah, M.Absi-Halabi andA. Bishara Future Opportunities in Catalytic and Separation Technology edited by M. Misono,Y. Moro-oka and S. Kimura

2032

Volume 55

New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22,1989 edited by G. Centi and F.Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts,Tokyo, October 23-25,1989 edited by T. Keii and K. Soga Volume 57A SpectroscopicAnalysis of Heterogeneous Catalysts. Part A: Methods of SurfaceAnalysis edited by J.L.G. Fie~ro Volume 57B SpectroscopicAnalysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-6,1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Pdrot, R. Maurel and C. Montassier Volume 60 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals,Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba andT.Tatsumi Natural Gas Conversion. Proceedings ofthe Symposium on Natural Gas Volume 61 Conversion, Oslo, August 12-17,1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPACSymposium (COPS II),Alicante, May 6-9, 1990 edited by E Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Preparation of CatalystsV. Scientific Bases for the Preparation of Heterogeneous Volume 63 Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6,1990 edited by G. Poncelet, RA. Jacobs, R Grange and B. Delmon NewTrends in COActivation Volume 64 edited by L. Guczi Catalysis andAdsorption by Zeolites. Proceedings of ZEOCAT90, Leipzig, Volume 65 August 20-23,1990 edited by G. Ohlmann, H. Pfeifer and R. Fricke Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Volume 66 Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonf~ired, September 10-14,1990 edited by L.I. Simdndi Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Volume 67 Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, Apri122-27,1990 edited by R.K. Grasselli andA.W. Sleight Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Volume 68 Evanston, IL, June 24-26,1991 edited by C.H. Bartholomew and J.B. Butt Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Volume 69 Prague, Czechoslovakia, September 8-13,1991 edited by RA. Jacobs, N.I. Jaeger, L. Kubelkovd and B.Wichtedovd Poisoning and Promotion in Catalysis based on Surface Science Concepts and Volume 70 Exper;'ments by M. Kiskinova

2033

Volume 71 Volume 72

Volume 73 Volume 74 Volume 75 Volume 76 Volume 77

Volume 78

Volume 79 Volume80 Volume81 Volume82

Volume83 Volume84

Volume85 Volume86 Volume87

Catalysis andAutomotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13,1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10,1991 edited by R Ruiz and B. Delmon Progress in Catalysis. Proceedings ofthe 12th Canadian Symposium on Catalysis, Banff, Alberta, Canada, May 25-28,1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission.Theory and CurrentApplications edited by S.D. Kevan New Frontiers in Catalysis, PartsA-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, E Solymosi and RT~t~nyi Fluid Catalytic Cracking: Science andTechnology edited by J.S. Magee and M.M. Mitchell, Jr. NewAspects of Spillover Effect in Catalysis. For Development of HighlyActive Catalysts. Proceedings of theThird International Conference on Spillover, Kyoto, Japan, August 17-20,1993 edited by T. Inui, K. Fujimoto,T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings ofthe 3rd International Symposium, Poitiers, April 5- 8,1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. P(~rot and C. Montassier Catalysis: An IntegratedApproach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, RW.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals ofAdsorption, Kyoto, Japan, May 17-22,1992 edited by M. Suzuki Natural Gas Conversion II. Proceedings oftheThird Natural Gas Conversion Symposium, Sydney, July 4-9,1993 edited by H.E. Curry-Hyde and R.F.Howe New Developments in Selective Oxidation II. Proceedings of the SecondWorld Congress and Fourth European Workshop Meeting, Benalm&dena, Spain, September 20-24,1993 edited by V. Cort6s Corberdn and S.Vic Bell6n Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited byT. Hattori andT.Yashima Zeolites and Related Microporous Materials: State of theArt 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22,1994 edited by J.Weitkamp, H.G. Karge, H. Pfeifer andW. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12,1993 edited by J.Rouquerol, E Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger

2034

Volume88 Volume89

Volume90 Volume91

Volume92

Volume93 Volume94 Volume95 Volume96

Volume97 Volume98

Volume99 Volume 100


Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5,1994 edited by B. Delmon and G.I-. Froment Catalyst Design forTailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design forTailor-made Polyolefins, Kanazawa, Japan, March 10-12,1994 edited by K. Soga and M.Terano Acid-Base Catalysis I1. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4,1993 edited by H. Hattori, M. Misono andY. Ono Preparation of CatalystsVI. Scientific Basesfor the Preparation of Heterogeneous Catalysts. Proceedings ofthe Sixth International Symposium, Louvain-La-Neuve, September 5-8,1994 edited by G. Poncelet, J. Martens, B. Delmon, RA. Jacobs and R Grange Science andTechnology in Catalysis 1994. Proceedings of the SecondTokyo Conference on Advanced Catalytic Science andTechnology,Tokyo, August 21-26,1994 edited by Y. Izumi, H.Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.EVansant, RVan DerVoort and K.C.Vrancken Catalysis by Microporous Materials. Proceedings of ZEOCAT'95, Szombathely, Hungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals andAIIoys by V. Ponec and G.C. Bond Catalysis andAutomotive Pollution Control II1.Proceedings of theThird International Symposium (CAPoC3), Brussels, Belgium, April 20-22,1994 edited by A. Frennet and J.-M. Bastin Zeolites: A RefinedTool for Designing Catalytic Sites. Proceedings of the International Symposium, Qu6bec, Canada, October 15-20,1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J.Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. Dqbrowski andV.A.Tertykh Catalysts in Petroleum Refining and Petrochemical Industries 1995. Proceedings ofthe 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26,1995 edited by M.Absi-Halabi, J. Beshara, H. Qabazard andA. Stanislaus 11th International Congress on Catalysis - 40th Anniversary. Proceedings ofthe 11th ICC, Baltimore, MD, USA, June 30-July 5,1996 edited by J.W. Hightower, W.N. Delgass, E. Iglesia andA.T. Bell RecentAdvances and New Horizons in Zeolite Science andTechnology edited by H. Chon, S.l.Woo and S.-E. Park Semiconductor Nanoclusters - Physical, Chemical, and CatalyticAspects edited by RV. Kamat and D. Meisel Equilibria and Dynamics of Gas Adsorption on Heterogeneous Solid Surfaces edited by W. Rudzi~ski,W.A. Steele and G. Zgrablich Progress in Zeolite and Microporous Materials Proceedings ofthe 11th International Zeolite Conference, Seoul, Korea, August 12-17,1996 edited by H. Chon, S.-K. Ihm andY.S. Uh

2035

Volume 106

Volume 107

Volume 108

Volume 109 Volume 110


Volume 113

Volume 114


Volume 117

Volume 118

Volume 119

Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 1st International Symposium / 6th European Workshop, Oostende, Belgium, February 17-19, 1997 edited by G.F. Froment, B. Delmon and P.Grange Natural Gas Conversion IV Proceedings ofthe 4th International Natural Gas Conversion Symposium, Kruger Park, South Africa, November 19-23,1995 edited by M. de Pontes, R.L. Espinoza, C.R Nicolaides, J.H. Scholtz and M.S. Scurrell Heterogeneous Catalysis and Fine Chemicals IV Proceedings ofthe 4th International Symposium on Heterogeneous Catalysis and Fine Chemicals, Basel, Switzerland, September 8-12,1996 edited by H.U. Blaser,A. Baiker and R. Pdns Dynamics of Surfaces and Reaction Kinetics in Heterogeneous Catalysis. Proceedings ofthe International Symposium,Antwerp, Belgium, September 15-17,1997 edited by G.I-. Froment and K.C.Waugh ThirdWorld Congress on Oxidation Catalysis. Proceedings oftheThirdWorld Congress on Oxidation Catalysis, San Diego, CA, U.S.A., 21-26 September 1997 edited by R.K. Grasselli, S.T. Oyama,A.M. Gaffney and J.E. Lyons Catalyst Deactivation 1997. Proceedings of the 7th International Symposium, Cancun, Mexico, October 5-8,1997 edited by C.H. Bartholomew and G.A. Fuentes Spillover and Migration of Surface Species on Catalysts. Proceedings ofthe 4th International Conference on Spillover, Dalian, China, September 15-18,1997 edited by Can Li and Qin Xin Recent Advances in Basic and Applied Aspects of Industrial Catalysis. Proceedings ofthe 13th National Symposium and Silver Jubilee Symposium of Catalysis of India, Dehradun, India, April 2-4,1997 edited by T.S.R. Prasada Rao and G. Murali Dhar Advances in Chemical Conversions for Mitigating Carbon Dioxide. Proceedings ofthe 4th International Conference on Carbon Dioxide Utilization, Kyoto, Japan, September 7-11,1997 edited by T. Inui, M.Anpo, K. Izui, S.Yanagida andT.Yamaguchi Methods for Monitoring and Diagnosing the Efficiency of Catalytic Converters. A patent-oriented survey by M. Sideris Catalysis andAutomotive Pollution Control IV. Proceedings ofthe 4th International Symposium (CAPoC4), Brussels, Belgium, April 9-11, 1997 edited by N. Kruse, A. Frennet and J.-M. Bastin Mesoporous Molecular Sieves 1998 Proceedings of the 1st International Symposium, Baltimore, MD, U.S.A., July 10-12,1998 edited by L.Bonneviot, F.B61and,C. Danumah, S. Giasson and S. Kaliaguine Preparation of Catalysts VII Proceedings ofthe 7th International Symposium on Scientific Bases for the Preparation of Heterogeneous Catalysts, Louvain-la-Neuve, Belgium, September 1-4,1998 edited by B. Delmon, RA. Jacobs, R. Maggi, J.A. Martens, R Grange and G. Poncelet Natural Gas ConversionV Proceedings ofthe 5th International Gas Conversion Symposium, Giardini-Naxos, Taormina, Italy, September 20-25,1998 edited by A. Parmaliana, D. Sanfilippo, F.Frusteri,A.Vaccari and EArena

2036

Volume 120A Adsorption and its Applications in Industry and Environmental Protection. Vol I: Applications in Industry edited by A. Dabrowski Volume 120B Adsorption and its Applications in Industry and Environmental Protection. Vol I1:Applications in Environmental Protection edited byA. Dabrowski Volume 121 Science andTechnology in Catalysis 1998 Proceedings of theThirdTokyo Conference in Advanced Catalytic Science and Technology,Tokyo, July 19-24, 1998 edited by H. Hattori and K. Otsuka Volume 122 Reaction Kinetics and the Development of Catalytic Processes Proceedings ofthe International Symposium, Brugge, Belgium,April 19-21,1999 edited by G.F. Froment and K.C.Waugh Volume 123 Catalysis: An Integrated Approach Second, Revised and Enlarged Edition edited by R.A. van Santen, RW.N.M. van Leeuwen, J.A. Moulijn and B.A.Averill Volume 124 Experiments in Catalytic Reaction Engineering by J.M. Berty Volume 125 Porous Materials in Environmentally Friendly Processes Proceedings of the 1st International FEZAConference, Eger, Hungary, September 1-4,1999 edited by I. Kiricsi, G. PdI-Borb61y,J.B. Nagy and H.G. Karge Volume 126 Catalyst Deactivation 1999 Proceedings ofthe 8th International Symposium, Brugge, Belgium, October 10-13,1999 edited by B. Delmon and G.F.Froment Volume 127 Hydrotreatment and Hydrocracking of Oil Fractions Proceedings of the 2nd International Symposium/Tth European Workshop, Antwerpen, Belgium, November 14-17,1999 edited by B. Delmon, G.F.Froment and R Grange Volume 128 Characterisation of Porous SolidsV Proceedings ofthe 5th International Symposium on the Characterisation of Porous Solids (COPS-V), Heidelberg, Germany, May 30- June 2,1999 edited by K.K. Unger, G. Kreysa and J.R Baselt Volume 129 Nanoporous Materials II Proceedings of the 2nd Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30, 2000 edited byA. Sayari, M. Jaroniec andT.J. Pinnavaia Volume 130 12th International Congress on Catalysis Proceedings ofthe 12th ICC, Granada, Spain, July 9-14, 2000 edited byA. Corma, F.V.Melo, S. Mendioroz and J.L.G. Fierro Volume 131 Catalytic Polymerization of Cycloolefins Ionic, Ziegler-Natta and Ring-Opening Metathesis Polymerization byV. Dragutan and R. Streck Volume 132 Proceedings of the Intemational Conference on Colloid and Surface Science, Tokyo, Japan, November 5-8, 2000 25th Anniversary of the Division of Colloid and Surface Chemistry, The Chemical Society of Japan edited byY. Iwasawa, N. Oyama and H. Kunieda Volume 133 Reaction Kinetics and the Development and Operation of Catalytic Processes Proceedings ofthe 3rd International Symposium, Oostende, Belgium,April 22-25, 2001 edited by G.E Froment and K.C.Waugh Volume 134 Fluid Catalytic Cracking V Materials and Technological Innovations edited by M.L. Occelli and R O'Connor

2037 Volume 135

Volume 136


Volume 142

Zeolites and Mesoporous Materials at the Dawn of the 21st Century. Proceedings of the 13th International Zeolite Conference, Montpellier, France, 8-13 July 2001 edited by A. Galameau, F. di Renzo, F. Fajula and J.Vedrine Natural Gas ConversionVl. Proceedings ofthe 6th Natural Gas Conversion Symposium, June 17-22, 2001, Alaska, USA. edited by J.J. Spivey, E. Iglesia and T.H. Fleisch Introduction to Zeolite Science and Practice. 2ndcompletely revised and expanded edition edited by H. van Bekkum, E.M. Flanigen, RA. Jacobs and J.C. Jansen Spillover and Mobility of Species on Solid Surfaces. edited byA. Guerrero-Ruiz and I. Rodriguez-Ramos Catalyst Deactivation 2001 Proceedings ofthe 9th International Symposium, Lexington, KY, USA, October 2001. edited by J.J. Spivey, G.W. Roberts and B.H. Davis Oxide-based Systems at the Crossroads of Chemistry. Second International Workshop, October 8-11, 2000, Como, Italy. Edited by A. Gamba, C. Colella and S. Coluccia Nanoporous Materials III Proceedings of the 3~ International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002 edited by A. Sayari and M. Jaroniec Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium Proceedings of the 2nd International FEZA (Federation of the European Zeolite Associations) Conference, Taormina, Italy, September 1-5, 2002 Edited by R. Aiello, G. Giordano and F. Testa


Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium (Studies in Surface Science and Catalysis)

Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure

Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure

Smart Materials and New Technologies

New Frontiers in Catalysis, Parts A-C (Studies in Surface Science and Catalysis)

Adsorption on New and Modified Inorganic Sorbents (Studies in Surface Science and Catalysis)

New Developments in Selective Oxidation by Heterogeneous Catalysis: Proceedings (Studies in Surface Science and Catalysis)

Catalysis by Zeolites: International Symposium Proceedings (Studies in surface science and catalysis)

Photochemistry on Solid Surfaces (Studies in Surface Science and Catalysis)

Catalytic Polymerization of Olefins (Studies in Surface Science and Catalysis)

Synthesis of High-Silica Aluminosilicate Zeolites (Studies in Surface Science and Catalysis)

Preparation of Catalysts VII (Studies in Surface Science and Catalysis)

Astronomy and Astrophysics in the New Millennium

Zeolites and Microporous Crystals: Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, August 22-25, 1993 (Studies in Surface Science and Catalysis)

Neem: Today and in the New Millennium

Astronomy and Astrophysics in the New Millennium

Characterization of Porous Solids VI (Studies in Surface Science and Catalysis Series), Vol. 144

Characterisation of Porous Solids V (Studies in Surface Science and Catalysis)

Characterization of Porous Solids III: Proceedings of the Iupac Symposium (Studies in Surface Science and Catalysis) (v. 3)

Catalysts for Upgrading Heavy Petroleum Feeds, Volume 169 (Studies in Surface Science and Catalysis) (Studies in Surface Science and Catalysis)

Development Connections: Unveiling the Impact of New Information Technologies

Global News Access: The Impact of New Communications Technologies

The Cosmos: Astronomy in the New Millennium

The New Communications Technologies, Fifth Edition: Applications, Policy, and Impact

The Beginning - New Spring

New Developments in Selective Oxidation (Studies in Surface Science and Catalysis vol55)

The Beginning - New Spring

The Beginning - New Spring

Balkans in the New Millennium, The

New Trends in Co Activation (Studies in Surface Science and Catalysis)

Reaction Kinetics and the Development of Catalytic Processes (Studies in Surface Science and Catalysis)

Impact of Zeolites and other Porous Materials on the New Technologies at the Beginning of the New Millennium (Studies in Surface Science and Catalysis)

Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure

Chemistry of Zeolites and Related Porous Materials: Synthesis and Structure

Smart Materials and New Technologies

New Frontiers in Catalysis, Parts A-C (Studies in Surface Science and Catalysis)

Adsorption on New and Modified Inorganic Sorbents (Studies in Surface Science and Catalysis)

New Developments in Selective Oxidation by Heterogeneous Catalysis: Proceedings (Studies in Surface Science and Catalysis)

Catalysis by Zeolites: International Symposium Proceedings (Studies in surface science and catalysis)

Photochemistry on Solid Surfaces (Studies in Surface Science and Catalysis)

Catalytic Polymerization of Olefins (Studies in Surface Science and Catalysis)

Synthesis of High-Silica Aluminosilicate Zeolites (Studies in Surface Science and Catalysis)

Preparation of Catalysts VII (Studies in Surface Science and Catalysis)

Astronomy and Astrophysics in the New Millennium

Zeolites and Microporous Crystals: Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, August 22-25, 1993 (Studies in Surface Science and Catalysis)

Neem: Today and in the New Millennium

Astronomy and Astrophysics in the New Millennium

Characterization of Porous Solids VI (Studies in Surface Science and Catalysis Series), Vol. 144

Characterisation of Porous Solids V (Studies in Surface Science and Catalysis)

Characterization of Porous Solids III: Proceedings of the Iupac Symposium (Studies in Surface Science and Catalysis) (v. 3)

Catalysts for Upgrading Heavy Petroleum Feeds, Volume 169 (Studies in Surface Science and Catalysis) (Studies in Surface Science and Catalysis)

Development Connections: Unveiling the Impact of New Information Technologies

Global News Access: The Impact of New Communications Technologies

The Cosmos: Astronomy in the New Millennium

The New Communications Technologies, Fifth Edition: Applications, Policy, and Impact

The Beginning - New Spring

New Developments in Selective Oxidation (Studies in Surface Science and Catalysis vol55)

The Beginning - New Spring

The Beginning - New Spring

Balkans in the New Millennium, The

New Trends in Co Activation (Studies in Surface Science and Catalysis)

Reaction Kinetics and the Development of Catalytic Processes (Studies in Surface Science and Catalysis)

Recommend Documents