Progress in Zeolite and Microporous Materials

Studies in Surface Science and Catalysis 105 PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS Studies in Surface Science ...

Author: H. Chon | S.-K. Ihm | Y.S. Uh

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Studies in Surface Science and Catalysis 105 PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS

Studies in Surface Science and Catalysis 105 PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS

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Studies in Surface Science and Catalysis A d v i s o r y E d i t o r s : B. D e l m o n a n d J.T. Yates Vol. 105

PROGRESS IN ZEOLITE AND MICROPOROUS MATERIALS

PART A Proceedings ofthe 1lth International Zeolite Conference, Seoul, Korea, August 12-17, 1996 Editors HakzeChon Son-Ki Ihm Korea Advanced Institute of Science and Technology, Taejon, Korea

Young Sun Uh

Korea Institute of Technology, Seoul, Korea

1997 ELSEVIER Amsterdam -- Lausanne -- New York-- Oxford -- Shannon -- Tokyo

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

ISBN 0-444'82344-1 © 1997 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A. - This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. 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. This book is printed on acid-free paper. Printed in The Netherlands

Contents Part A xxxi

Preface Organizing Committee of the 11th IZC

XXXII1

International Advisory Board to the 1lth IZC

xxxvi

Financial Supports

xxxix

I. Synthesis Directed Synthesis of Organic/Inorganic Composite Structures G.D. Stucky, Q. Huo, A. Firouzi, B.F. Chmelka, S. Schacht, I.G. VoigtMartin and F. Schiith Incorporation and Stability of Trivalent Cations in Mesoporous Silicas Prepared Using Primary Amines as Surfactant S. Gontier and A. Tuel

29

Synthesis ofLamellar Aluminophosphates via the Supramolecular Templating Mechanism A. Sayari

37

Synthesis and Hydrothermal Stability of a Disordered Mesoporous Molecular Sieve R. Ryoo, J.M. Kim, C.H. Shin and J. Y. Lee

45

Preparation of Silica-Pillared Molecular Sieves from Layered Silicates S.-Y. Jeong, O.-E Kwon, J.-K. Sub, H. Jin and J.-M. Lee

53

A New Synthetic Route and Catalytic Characteristics of Pillared Rectorite Molecular Sieves J. Guan, Z Yu, Z Chen, L. Tang and X. Wang

61

Textural Control ofMCM-41 Aluminosilicates F. Di Renzo, N. Coustel, M. Mendiboure, H. Cambon and F. Fajula

69

New Routes for Synthesizing Mesoporous Material Y. Sun, W. Lin, £ Chen, Y. Yue and W. Pang

77

Synthesis and Characterization ofFeSiMCM-41 and LaSiMCM-41 N.-Y. He, S.-L. Bao and Q.-H. Xu

85

Synthesis of Titanium-Containing Mesoporous Molecular Sieves with a Cubic Structure K.A. Koyano and T. Tatsumi

93

Short Range Order of MCM-41 and Mesostructured Aluminiumphosphate C. Pophal, R. Schnell and H. Fuess

101

Syntheses ofMesoporous Aluminosilicates from Layered Silicates Containing Aluminum S. lnagaki, Y. Yamada and Y. Fukushima

109

Structure Descriptors for Organic Templates Employed in Zeolite Synthesis R.E. Boyett, A.P. Stevens, M.G. Ford and P.A. Cox

117

Quantitative Aspects in the Crystallization of Zeolites H. Lechert, T. Lindner and P. Staelin

125

A Computational 'Expert System' Approach to Design Synthesis Routes for Zeolite Catalysts T. Selvam, D.N. lyer, R.C. Deka, A. Chatterjee and R. Vetrivel

133

A New Method for Enhancing Zeolite Crystallization by Using Oxyacids/Salts of Group VA and VIIA Elements as Promoters A. Bhaumik, A.A. Belhekar and R. Kumar

141

The Influence of Mixed Organic Additives on the Zeolites A and X Crystal Growth V.P. Petranovskii, Y. Kiyozumi, N. Kikuchi, H. Hayamisu, Y. Sugi and F. Mizukami

149

Studies of the Crystallization of ZSM-5 under High Gravitational Force Field W.J. Kim, D.T. Hayhurst, S.A. Lee, 3/1.C. Lee, C. IV..Lira and J.C. Yoo

157

vii Structure Directing Role ofNa ÷ and TMA + Cations in 18-Crown-6 Ether Mediated Crystallization of EMT, MAZ and SOD Aluminosilicate Zeolites E.J.P. Feijen, B. Matthijs, P.J. Grobet, J.A. Martens and P.A. Jacobs

165

Synthesis of High-Silica FAU-,EMT-,RHO- and KFI-Type Zeolites in the Presence of 18-Crown-6 Ether T. Chatelain, J. Patarin, E. Brendl~, F. Doughier, J.L. Guth and P. Schulz

173

Synthesis of Zeolites in a Microwave Heating Environment J.P. Zhao, C. Cundy and J. Dwyer

181

Synthesis of Octahedral Molecular Sieves C.-L. O'Young and S.L. Suib

189

Syntheses and Crystal Structures of Two "Organozeolites" K. Maeda, J. Akimoto, Y. Kiyozumi and F. Mizukami

197

ERS-8: A New Class of Microporous Aluminosilicates G. Perego, R. Millini, C. Perego, A. CaratL G. Pazzuconi and G. Bellussi

205

Synthesis and Characterization of Levyne Type Zeolite Obtained from Gels with Different SiO2/Al203Ratios C.V. Tuoto, J.B. Nagy and A. Nastro

213

Synthesis ofETS-10 Molecular Sieve from Systems Containing TAABr Salts P. De Luca and A. Nastro

221

Synthetic Clinoptilolite and Distribution of Aluminum Atoms in the Framework of liEU Type Zeolites M. Kato, S. Satokawa and K. ltabashi

229

Preparation of Ultramarine Analogs from Zeolites S. Kowalak, M. Str6zyk, M. Pawlowska, M. Miluska and J. Kania

237

Exploration of Non Conventional Routes to Synthesize MFI Type Titanoand (Boro-Titano)- Zeolites M. Shibata, J. G~rard and Z Gabelica

245

Synthesis, Characterization and Catalytic Properties of Vanadium Containing VPI-5 K. Chaudhari, T.Kr. Das, A.J. Chandwadkar, J.G. Chandwadkar and S. Sivasanker

253

viii Crystallization of Titanium Silicalite-1 from Gels Containing Hexanediamine and Tetrapropylarrm,onium Bromide A. Tuel

261

Synthesis and Characterisation of Gallium and Germanium Containing Sodalites G.M. Johnson and M. T. Weller

269

Synthesis and Characterization of Chromo, Ferro, Mangano and Vanadio Silicates with MTW Structure M.L.S. Corrda, M. Wallau and U. Schuchardt

277

Improved Synthesis of (Ga)- and (Ga, Al)- Faujasites Z. Gabelica, K Norberg and T. Ito

285

A Study on the Crystallization of Binderless Zeolite X X. Luo, X.. He and J. Shen

293

Synthesis of Large Crystals of Molecular Sieves-A Review S. Qiu, W. Pang and R. Xu

301

Synthesis and Characterization of ZSM-5 in Fluoride Medium: The Role of NH4+ and K ÷ Cations E. Nigro, R. Mostowicz, F. Crea, F. Testa, R. Aiello and J.B. Nagy

309

Three-Dimensional Real-Time Observation of Growth and Dissolution of Silicalite Crystal A. lwasaki, L Kudo and T. Sano

317

Synthesis of Mordenite and ZSM-11 Zeolites from Very Dense Systems" Formation of Self-Bonded Pellets P. De Luca, F. Crea, R. Aiello, A. Fonseca and J.B. Nagy

325

Studies on Crystallization of ZSM-12 Type Zeolite A. V. Toktarev and K. G. lone

333

Synthesis of Nanocrystalline Zeolite Beta in the Absence of Alkali Metal Cations M.A. Camblor, A. Corma, A. Mifsud, J. P~rez-Pariente and S. Valencia

341

Synthesis of Zeolite Beta with Low Template Content M. W.N. C. Carvalho and D. Cardoso

349

Synthesis and Characterization of Sn- Containing ZSM-48 Type Molecular Sieves Using Different Templates N.K. Mal, V. Ramaswamy, B. Rakshe and A. V. Ramaswamy

357

Inorganic Cations in AIPO4 Synthesis E. Halvorsen, A. Karlsson, T. Haug, D. Akporiaye and K.P. Lillerud

365

New Insights into the Study oflndiumphosphate Molecular Sieves L.L. Koh, Y. Xu, H. Du and W. Pang

373

Synthesis and Characterization of Novel Open-Framework Cobalt Phosphates from Aqueous-Alcoholic Systems J. Yu, Q. Gao, J. Chen and R. Xu

381

A Family of Unusual Lamellar Aluminophosphates Synthesized from NonAqueous Systems Q. Gao, J. Chen, S. Li and R. Xu

389

Synthesis of Various Indium Phosphates in the Presence of Amine Templates H. Du, J. Chen and W. Pang

397

Steric-Electronic Model of Templating Effect Z. Liu and R. Xu

405

II. Characterization The Synthesis and Characterization ofUTD-I: The First Large Pore Zeolite Based on a 14 Membered Ring System K.J. Balkus Jr., M. Biscotto and A. G. Gabrielov

415

The Nature of the Acid Sites in Mesoporous MCM-41 Molecular Sieves A. Liepold, K. Roos, W. Reschetilowski, R. Schmidt, M. StOcker, A. Philippou, M. W. Anderson, A.P. Esculcas and J. Rocha

423

Solid Mesoporous Base Catalysts Comprising ofMCM-41 Supported Intraporous Cesium Oxide K.R. Kloetstra and H. van Bekkum

431

Non-Framework Aluminium in Highly Dealuminated Y Zeolites Generated by Steaming or Substitution W. Lutz, E. LOftier, M. Fechtelkord, E. Schreier and R. Bertram

439

Spectroscopic Studies of a Magnesium Substituted Microporous Aluminophosphate DAF- 1 S.J. Thomson and R.F. Howe

447

MASNMR Chemical Shit, s and Structure in Frameworks M.T. Weller, S.E. Dann, G.M. Johnson and P.J. Mead

455

A New Method for the NMR-Spectroscopic Measurement of the Deprotonation Energy of Surface Hydroxyl Groups in Zeolites E. Brunner, J. Ka'rger, M. Koch, H. Pfeifer, H. SachsenrOder and B. Staudte

463

~70 NMR Studies of Siliceous Faujasite L.M. Bull andA.K. Cheetham

471

Deuteron Magnetic Resonance Studies of Ammonia in AgNaY-Zeolites M. Hartmann and B. Boddenberg

479

Spectroscopic Studies of 170 and 180 Labelled ZSM-5 Zeolites F. Bauer, H. Ernst, E. Geidel, Ch. Peuker and W. Pilz

487

Anisotropic Motion of Water in Zeolites EMT, L and ZSM-5 as Studied by D- and H-NMR Line Splitting A. Wingen, W.D. Basler and H. Lechert

495

EXAFS and NMR Studies of the Incorporation of Zn(II) and Co(II) Cations into Tetrahedral Framework Sites of AIPO4 Molecular Sieves N.N. Tusar, A. Tuel, 1. Arcon, A. Kodre and K Kaucic

501

Si, AI Solid Solution in Sodalite: Synthesis, 298i NMR and X-Ray Structure M. Sato, E. Kojima, H. Uehara and M. Miyake

509

Substitution of Silicon and Metal Ions in Small Pore Aluminophosphate Molecular Sieves with Chabazite Structure: Synthesis and MASNMR Study D.K. Chakrabarty, S. Ashtekar, A.M. Prakash and S. K K Chilukuri

517

Inclusion of Sodium Chloride in Zeolite NaY Studied by 23NaNMR Spectroscopy U. Tracht, A. Seidel and B. Boddenberg

525

Spectroscopic Investigation of the State of Aluminium in MCM-41 Aluminosilicates S. Viale, E. Garrone, F. Di Renzo, B. Chiche and F. Fajula

533

Boiling-Point Elevation of Water Confined in Mesoporous MCM-41 Materials Probed by 1H NMR E.W. Hansen, R. Schmidt and M. StOcker

543

In Situ Studies of Catalytic Reactions in Zeolites by Means of PFG and MAS NMR Techniques J. Karger and D. Freude

551

Vibrational Study of Benzene Adsorbed in NaY Zeolite by Neutron Spectroscopy H. Jobic and A.N. Fitch

559

Infrared Holeburning Spectroscopy in Acid Zeolites M. Bonn, M.J.P. Brugmans, H.J. Bakker, A.W. Kleyn and R.A. van Santen

567

Exploring the Sites of Adsorbed Pyrrolidine Derivatives in Y Zeolites by Joined Infrared Spectroscopic and Computer Simulation Studies E. Geidel, K. Krause, J. Kindler and H. FOrster

575

Preparation and Characterisation of Ru-Exchanged NaY Zeolite: An Infrared Study of CO Adsorption at Low Temperatures S. Wrabetz, U. Guntow, R. SchlOgl and H. G. Karge

583

New Insight into the Mechanism of Zeolite Catalyzed Nucleophilic Amination Via In Situ Infrared Spectroscopy C. Griindling, V.A. Veefkind, G. Eder-Mirth and J.A. Lercher

591

Coke Formation in Zeolites Studied by a New Technique: Ultraviolet Resonance Raman Spectroscopy C. Li and P. C. Stair

599

Preparation of Titanium-Containing Large Pore Molecular Sieve from H-AIBeta Zeolite X. Guo, X. Wang, G. Wang and G. Li

607

Syntheses and Raman Spectroscopic Study of Bis- and Tris-(1,10Phenanthroline) Manganese(II) Complexes Encapsulated in Faujasite-Y B. Zhan and X. Li

615

Chemometric Analysis of Diffuse Reflectance Spectra of CoA Zeolites: Spectroscopic Fingerprinting of Co2+-Sites A.A. Verberckmoes, B.M. Weckhuysen and R.A. Schoonheydt

623

xii Raman Characterization of the Selenium Species Formed inside the Confined Spaces of Zeolites V. V. Poborchii

631

Determination of Basic Site Location and Strength in Alkali Exchanged Zeolites D. Murphy, P. Massiani, R. Franck and D. Barthomeuf

639

A Spectroscopic Study of the Initial Stage in the Crystallization of TPASilicalite-1 from Clear Solutions B.J. Schoeman

647

Characterization and Catalytic Properties of the Galliumphosphate Molecular Sieve Cloverite R. Fricke, M. Richter, H.-L. Zubowa and E. Schreier

655

Preparation of Titanosilicate with Mordenite Structure by Atom-planting Method and Its Catalytic Properties for Hydroxylation of Aromatics P. Wu, T. Komatsu and T. Yashima

663

Characterization of Zeolite Basicity Using Iodine as a Molecular Probe S.Y. ChoL Y. S. Park and K.-B. Yoon

671

Ship-in-Bottle Synthesis of Pt and Ru Carbonyl Clusters in NaY Zeolite Micropore and Ordered Mesoporous Channels ofFSM-16; XAFS/FTIR/TPD Characterization and Their Catalytic Behaviors M. Ichikawa, T. Yamamoto, W. Pan and T. Shido

679

Characterization and Reactivity ofNi,Mo-Supported MCM-41 Catalysts for Hydrodesulfurization J. Cui, E-H. Yue, E Sun, W. -Y. Dong and Z Gao

687

Probing the Hydrophobic Properites of MCM-41-Type Materials by the Hydrophobicity Index R. Glaser, R. Roesky, T. Boger, G. Eigenberger, S. Ernst and J. Weitkamp

695

Characterisation of Acid- Base- and Redox- Type Sites in ZSM-5 Zeolites by Sorption Rate "Spectroscopy" Gy. Onyesty6k, J. Valyon and L. V. C. Rees

703

A Picosecond Spectroscopic Study on the Proton Transfers of 6Hydroxyquinoline in Zeolite Cages H. Yu, J. Park, N.W. Song and D.-J. Jang

711

xiii Ethylene Dimerization in Nickel Containing SAPO Materials Studied by Electron Spin Resonance and Gas Chromatography: - Influence of the Channel Size M. Hartmann and L. Kevan

717

The Electronegativity Equalization Method(EEM) as a Promising Tool for the Analysis of Zeolite Catalyzed Reactions G.O.A. Janssens, H. Toufar, B.G. Baekelandt, W.J. Mortier and R.A. Schoonheydt

725

Synthesis and Characterization of Iron Modified L-Type Zeolite Y.S. Ko, W.S. Ahn, J.H. Chae and S.HMoon

733

Synthesis, Characterization and Catalytic Properties of VS-2 H. Du, G. Liu, Z. Da and E. Min

741

Preparation and Characterization of Manganese Bipyridine Complexes in Zeolites with Different Pore Architectures S. Ernst and B. Jean

747

Metal Substituted ATS Aluminophosphate Molecular Sieves D. Akolekar and R.F. Howe

755

The Modified Hydrophobicity Index as a Novel Method for Characterizing the Surface Properties of Titanium Silicalites J. Weitkamp, S. Ernst, E. Roland and G. F. Thiele

763

The Thermal Stability of GaUophosphate Cloverite W. Schmidt, F. Schiith and S. Kallus

771

Electron Spin Resonance Studies of 02" Adsorbed on Aluminophosphate Molecular Sieves S.B. Hong, S.J. Kim, Y.-S. Choi and Y.S. Uh

779

The Affinity Order of Organics on Hydrophobic Zeolite Silicalite-1 Studied by Thermal Analysis Y. Long, H. Jiang and H. Zeng

787

Chemistry of CoAPO-11 and VAPO-5" ESR Studies of Molecular Oxygen Adducts C. Naccache, M. Vishnetskaya and K.J. Chao

795

Cupric Ion Species in Cu(II)-Exchanged Gallosilicate K-L and Comparison with Aluminosilicate K-L J.-S. Yu, S.B. Hong and L. Kevan

801

xiv

Part B 111. Catalysis - First Part Structure-Reactivity-SelectivityRelationships in Reaction of Organics over Zeolite Catalysts P.B. Venuto

811

Conversion of Propan-2-01 on Zeolite LaNaY Investigated by in situ M A S

853

Structure and Catalytic Activity of Co-Based Bimetallic Systems in NaY Zeolite: Low Temperature Methane Activation L. Guczi, Z. Koppany, K. V. Sarma, L. Borkd and I. Kiricsi

86 1

The Aromatization of Methane over MokIZSM-5 Zeolites without Using Oxidants M. Xie, X Yang, W. Chen, L. Tao, X Wang, G. Xu, L. Wang, Y.-D. Xu,S.Liu andX-X Guo

869

Properties of PtSn/KL Catalysts for n-Hexane Aromatization J.H. Chae and S.H. Moon

877

A New Process of Light Naphtha Aromatization Using a Zeolite-Based Catalyst with Long-Time Stability S.Fukase, N Igarashi, K. Aimoto, H. Inoue and H. On0

885

Preparation of Zeolites Incorporating Molybdenum Sulfide Clusters with Unusual Carbon Number Distribution in c 0 - H ~ Reactions M. Taniguchi, Y. Ishii, T. Murata, M. Hidai and T. Tatsumi

893

CO Hydrogenation over Pd/HZSM-5 Catalysts: Temperature-Programmed Desorption, 13CO/C180Isotope Analysis, and in-situ Infrared Spectroscopy S.-K. Ihm, J-K. Jeon and D.-K. Lee

901

Reaction Mechanisms of Heptane Isomerization and Cracking on BifunctionalPt/H-Beta Zeolites E. Blomsma. LA. Martens and P.A. Jacobs

909

NMR Spectroscopy under Continuous-Flow Conditions M. Hunger, T.Horvath andJ Weitkump

XV

Pt/Zeolite Catalysts for Hydrocracking: A Comparative Study on FAU and EMT K Zholobenko, A. Garforth, F. Bachelin and J. Dwyer

917

Acidity in Working Zeolites: Use of a Stabilised Carbenium in a New Route for the Synthesis of Secondary Amides on ZSM-5 F. Thibault-Starzyk, M.M. Bettahar, J. Saussey and J.-C. Lavalley

925

Temperature Effects on Deactivation Rate and on Nature of Coke Formed from Propene over Mordenites C.A. Henriques, J.C. Afonso, P. Magnoux, M. Guisnet and J.L.F. Monteiro

933

Catalytic Degradation of High Density Polyethylene by HZSM-5 Zeolite KJ. Fernandes Jr., A.S. Araftjo and G.J.T. Fernandes

941

The Use of Cyclohexanol Dehydration, Isobutane Cracking and 2,6-DIPN Synthesis over Dealuminated Mordenite to Probe Acidity A.W. O'Donovan and C.T. O'Connor

949

Characterization and Reactivity Study of Rhenium-Impregnated Zeolite Y Catalysed Metathesis of Olefins H. Hamdan and Z. Ramli

957

Propene Oligomerization over Dealuminated Mordenite 1. Gigstad and S. Kolboe

965

The Induction Period in Ethylbenzene Disproportionation over Large Pore Zeolites U. Weifl, M. Weihe, 3/1. Hunger, H.G. Karge and J. Weitkamp

973

Effect of Ga on the Hydrogen Transfer Activity of Zeolites with the Offretite Structure P.-S.E. Dai, C.M. Tsang, R.H. Petty, M.. Somervell, B. Williamson andM.L. Occelli

981

TPR and XPS Studies of Iron-Exchanged Y Zeolites and Their Activity during Dibenzothiophene Hydrodesulfurization M.. Nagai, O. Uchino, J. Okubo and S. Omi

989

Hydroconversion of Aromatics on Metal-Zeolite Catalysts L.P. Poslovina, V.G. Stepanov, L. V. Malysheva, E.A. Paukshtis, L.A. Vostrikova and K. G. lone

997

xvi Low Temperature Hydrocracking of Paraffinic Hydrocarbons over Hybrid Catalysts 1. Nakamura, K. Sunada and K. Fufimoto

1005

A Comparative Study of Titanium-Containing Aluminophosphate Molecular Sieves TAPO-5, TAPO-11 and TAPO-36 M.H. Zahedi-Niaki, P.N. Joshi and S. Kaliaguine

1013

MCM-41-Type Molecular Sieves as Carriers for Metal-Phthalocyanine Complexes S. Ernst, R. Glaser andM. Selle

1021

Application of CoAPO-5 Molecular Sieves as Heterogeneous Catalysts in Liquid Phase Oxidation of Alkenes with Dioxygen H.F. ~ J . van Breukelen, M.E. Gerritsen, KM. Ummels, J.S. Broens and J.H. C. van Hooff

1029

Selective Oxidation of Aromatic Hydrocarbons over Copper Complexes Encapsulated in Molecular Sieves R. Raja and P. Ratnasamy

1037

Cation Effects in the Oxidation of Adsorbed Cyclohexane in Y Zeolite: An in situ IR Study D.L. Vanoppen, D.E. De Vos and P.A. Jacobs

1045

Influence of Extra-Framework Alumina in H-[AI]ZSM-5 Zeolite on the Direct Hydroxylation of Benzene to Phenol J.L. Motz, H. Heinichen and W.F. HOlderich

1053

The Stability of Chromium in Chromium Molecular Sieves under the Conditions of Liquid Phase Oxidations with tert-Butyl Hydroperoxide H.E.B. Lempers and R.A. Sheldon

1061

Oxidation of Olefins and Alkanes with Various Peroxides, Catalyzed by Triamine Containing Manganese Faujasites D.E. De Vos, J.L. Meinershagen and T. Bein

1069

Characterization and Reversible Reduction/Oxidation of Zeolite-Hosted Tiand V-Oxide Dispersions G. Grubert, M. Wark, W. Griinert, M. Koch and G. Schulz-Ekloff

1077

Zirconium Containing Mesoporous Silicas: New Catalysts for Oxidation Reactions in the Liquid Phase S. Gontier and A. Tuel

1085

xvii Synthesis of Aluminium Free Titanium Silicate with the BEA Structure Using a New and Selective Template and Its Use as a Catalyst in Epoxidations J.C. van der Waal, P. Lin, M.S. Rigutto and H. van Bekkum

1093

Incorporation of Vanadium into ZSM-5, Mordenite and Y Type Zeolite and Their Catalytic Properties G.-J. Kim, J.-H. Kim and H. Shoji

1101

Catalytic Tuning of the Olefin Epoxidation with Hydrogen Peroxide and Faujasite Y Occluded Mn Bipyridine Complexes P.-P. Knops-Gerrits, H. Toufar and P.A. dacobs

1109

The Selective Oxidation of Cyclohexane Using Iron(Ill) Incorporated Zeolite Y and Iron(III) Supported Zeolite Y in a Heterogeneous Biomimetic Oxidation System C.-H. Park, S.-S. Nam, S.B. Kim, S.-B. Kim, K.-W. Jun and K.-I4(. Lee

1117

Solid Catalysts for the Hydroxyalkylation of Aromatics with Epoxides. Intermolecular Hydroxyalkylation versus Intramolecular Hydroxyalkylation ~A. Elings, R.S. Downing and R.A. Sheldon

1125

Heterogeneously-Catalyzed Hydroalkoxylation of Limonene and alphaPinene in the Presence of Beta Zeolite K. Hensen, C. Mahaim and I~.F. HOlderich

1133

Photochemistry of Alkyl Ketones Included within the Zeolite Cavities: The Effect of Ion-exchanged Alkali Metal Cations and Types of Zeolites H. Yamashita, N. Sato, M. Anpo, T. Nakafima, M. Hada and H. Nakatsufi

1141

Acylation of Phenol with Acetic Acid. Effect of Density and Strength of Acid Sites on the Properties ofMFI Metallosilicates F. Jayat, M. Guisnet, M. Goldwasser and G. Giannetto

1149

Structural Design for Y-type Zeolite on Large Molecule Conversion. Alkylation of Phenol with Long Chain Olefin. X-W.. Li, M. Han, X.-Y. Liu, Z.-F. Pei and L. She

1157

Zeolite-Catalysed Rearrangement of lsophorone Oxide J.A. Elings, H.E.B. Lempers and R.A. Sheldon

1165

Vapour-Phase Beckmann Rearrangement Using B-MFI Zeolites J. ROseler, G. Heitmann and I'EF. HOlderich

1173

xviii Catalytic Properties of Fluorinated CeY S. Kowalak, M. Laniecki, M. Pawlowska, K.£ Balkus Jr. and A. Khanmamedova

1181

Evaluation of Catalyst Deactivation on Beckmann Rearrangement over Indiosilicate Modified with Noble Metals M.N.A. Nasution, T. Takahashi and T. Kai

1189

Selective Fries Rearrangement of Phenyl Acetate into Hydroxy Acetophenones Catalyzed by High-Silica Zeolite NCL-1 M. Sasidharan and R. Kumar

1197

Selective Hydrogenation of Cinnamaldehyde to Cinnamyl Alcohol on LZeolite Supported Catalysts G. LL T. Li, Y.-D. Xu, S. Wong and X.-X Guo

1203

Zeolite Catalysts for the Friedel-Craffs Alkylation of Methyl Benzoate, a Strongly Deactivated Aromatic Substrate B. Janssens, P. Carry, R. Claessens, G. Baron and P.A. Jacobs

1211

Methanol Amination over Small-Pore Zeolite Catalysts K. Segawa and M. C. Ilao

1219

Synthesis of Aniline from Phenol and Ammonia over Zeolite Beta N. Katada, S. lo'ima, H. lgi and M. Niwa

1227

Cyclodimerization ofBicyclo[2.2.1 ]Hepta-2,5-Diene in the Presence of Rhodium-Containing Zeolite Catalysts N.F. Goldshleger, B.L Azbel, Ya.I. Isakov, E.S. Shpiro and Kh.M. Minachev

1235

Vinyl Chloride Synthesis on Zeolite Catalysts: The Role of Strong Lewis Acid-Base Pair Sites E.B. Uvarova, L.M. Kustov, I.L Lishchiner, O.V. Malova and V.B. Kazansky

1243

IV. C a t a l y s i s - S e c o n d P a r t

Influence of Zeolite Pore Structure on Catalytic Reactivity A. van de Runstraat, P.£ Stobbelaar, J. van Grondelle, B.G. Anderson, L.£ van IJzendoorn and R.A. van Santen

1253

xix Regio Selectivity in the Hydrogenation of Geraniol over Platinum Containing Zeolites D. Tas, R.F. Parton, K. Vercruysse and P.A. Jacobs

1261

Zeolite Catalyzed Regioselective Synthesis of lndoles P.J. Kunkeler, M.S. Rigutto, R.S. Downing, H.J.A. de Vries and H. van Bekkum

1269

Shape-Selective Zeolite Catalysed Synthesis of Monoglycerides by Esterification of Fatty Acids with Glycerol E. Heykants, W.H. Verrelst, R.F. Parton and P.A. Jacobs

1277

Selective Key-Lock Catalysis in Dimethylbranching of Alkanes on TON Type Zeolites W. Souverijns, J.A. Martens, L. Uytterhoeven, G.F. Froment and P.A. Jacobs

1285

Kinetics Study ofEthylbenzene Disproportionation with Medium and Large Pore Zeolites N. Arsenova, W.O. Haag and H. G. Karge

1293

Application of a Kinetic Model for Investigation of Aromatization Reactions of Light Paraffins and Olefins over HZSM-5 D.B. Lukyanov

1301

Conversion of n-Pentane to Benzene Toluene and para-Xylene over Pore Size Controlled Ga203 Incorporated ZSM-5 Zeolite KS. Bhat, J. Das and A.B. Halgeri

1309

The Influence of Reagents on Shape-Selective Alkylation of Biphenyls over H-Mordenite M. Matsumoto, X. Tu, T. Matsuzaki, T. Hanaoka, E Kubota, E Sugi, J.-H. Kim, K. Nakajima, A. Igarashi and K. Kunimori

1317

Selective Benzene Isopropylation over Fe-Containing Zeolite Beta A.V. Smirnov, F. Di Renzo, O.E. Lebedeva, D. Brunel, B. Chiche, A. Tavolaro, B.V. Romanovsky, G. Giordano, F. Fajula and 1.1. Ivanova

1325

Para-Selective Gas Phase 02 Oxidations of Alkylaromatics over CVD Fe/Mo/Borosilicate Molecular Sieve J.S. Yoo, P.S. Lin and S.D. Elfline

1333

A Model Study on Zeolite Composites for Improved Catalyst Selectivity N. van der Pull, LB. Janto-Saputro, H. van Bekkum and J.C. Jansen

1341

XX

Toluene Disproportionation over ZSM-5 Catalysts Covered with Silicalite Shell C.S. Lee, T.-J. Park and IE.Y. Lee

1349

Ethylation of Ethylbenzene to Produce para-Diethylbenzene X. Wang, G. Wang, H. Guo and X. Wang

1357

Mechanisms of the Skeletal Isomerization of n-Butene over a HFER Zeolite. Influence of Coke Deposits. M. Guisnet, P. Andy, N.S. Gnep, C. Travers and E. Benazzi

1365

Reaction of n-Butene over H-ZSM-5 Zeolite. Influence of the Acid Strength on the Isobutene Selectivity P. M&iaudeau, T. Vu. Anh, H. Le Ngoc and C. Naccache

1373

Skeleton Hydroisomerization ofHexene-1 in the Presence of Synthesis Gas on Zn-Cr/HZSM-5 Catalyst V.M. Mysov, V.G. Stepanov and K. G. lone

1381

Skeletal Isomerization of 1-Butene over Zeolite Catalysts: A Computational Study R. Millini and S. Rossini

1389

Studies of Crystallization of SAPO-11 Molecular Sieve and Applications for Catalytic Skeletal Isomerization of Linear Butylenes H. Tian and C. Li

1397

Pt-Cu Bimetallics in H-Y Zeolite: Microcalorimetric Study of the Effect of Copper on the Catalyst Acidity A. Auroux, Y. Ben Taarit, M. Lokolo, P. Meriaudeau and C. Naccache

1405

La-EMT, a Promising Catalyst for Isobutane/2-Butene Alkylation H. Mostad, M. StOcker, A. Karlsson, H. Junggreen and B. Hustad

1413

Skeletal Isomerization of n-Butenes on Modified ZSM-35 Catalysts B.S. Kwak, J.H. Jeong and S.H. Park

1423

Selectivity to the Skeletal Isomerization of 1-Butene over Ferrierite(FER) and ZSM-5 (MFI) Zeolites G. Seo, H.S. Jeong, J.M. Lee and B.J. Ahn

1431

xxi

V. Environment High Potential of Novel Zeolitic Materials as Catalysts for Solving Energy and Environmental Problems T. Inui

1441

Modification and Stabilization of Cu-ZSM-5 by Introduction of a Second Cation A. V. Kucherov, C.P. Hubbard, T.N. Kucherova and M. Shelef

1469

Reactivity of Adsorbates in the Decomposition of Nitric Oxide over CuZSM-5 Catalysts S.S. C. Chuang and B. Lopez

1477

Quantum Chemical Investigation of Reactants in Selective Reduction of NOx on Ion Exchanged ZSM-5 M. Yamadaya, H. Himei, T. Kanougi, E Oumi, M. Kubo, A. Stir#ng, R. Vetrivel, E. Broclawik and A. Miyamoto

1485

The Role of Water for NO Reduction by Hydrocarbons over Copper IonExchanged Mordenite Type Zeolite Catalysts M.H. Kim, I.-S. Nam and E G. Kim

1493

Catalytic Reduction of Nitrogen Monoxide by Methane over Pd-Loaded ZSM-5 Zeolites. Roles of Acidity and Pd Dispersion M. Misono, Y. Nishizaka, M. Kawamoto and H. Kato

1501

Zeolites in the Environmental Protection - Decomposition of Chlorofluorocarbons over Zeolite Catalysts Z. K6nya, L Hannus and 1. Kiricsi

1509

Preparation of Cu-Na-ZSM-5 Catalysts by Thermal Spreading Techniques W. Griinert, T. Liese and C. Schobel

1517

Preparation, Characterization and Catalytic Activity Towards Lean NOx Reduction of Over-exchanged Cu-ZSM-5 Catalysts G. Moretti, G. Minelli, P. Porta, P. Ciambelli, P. Corbo, M.. Gambino, F. Migliardini and S. Iacoponi

1525

Factors Controlling Catalytic Activity of H-Form Zeolites for the Selective Reduction of NO with CI-L A. Satsuma, M. Iwase, A. Shichi, T. Hattori and Y. Murakami

1533

xxii CO Oxidation and NO Reduction by CO on Differently Prepared CuO/Mordenites K.-H. Lee and B.-H. Ha

1541

Steam Deactivation of Transition Metal MFI Zeolite Catalysts for NOx Reduction P. Budi, E. Curry-Hyde and R.F. Howe

1549

ln-situ IR Studies of Surface Species during the Selective Catalytic Reduction(SCR) of NO by Propene over Cu-ZSM-5 Zeolites D.H. Kim, I.C. Hwang and S.I. Woo

1557

Reduction of NO by CO Using a Zeolite Catalyst Obtained from Fly Ash E. L6pez-Sa#nas, P. Salas, L Schifter, M. Mor6n, S. Castillo and E. Mogica

1565

ln-situ FT-IR and Catalytic Studies of the Selective Reduction of Nitric Oxide by Carbon Monoxide over Au/NaY Catalysts: Effect of Adding Hydrogen to the Reaction Gas Mixture T.M. Salama, R. Ohnishi and M. Ichikawa

1571

Role of Oxygen in the SCR of NOx with Propane over Co/ZSM-5: Reaction and TPD Study A. Yu. Stakheev, C. W. Lee, S.J. Park and P.J. Chong

1579

Sharp Contrast in Thermal Stability between MFI-Type Metallosilicates and Metal-Ion-Exchanged ZSM-5 and Their Catalytic Performances for NO Removal S. Iwamoto, S. Kon, S. Yoshida and T. Inui

1587

Formation of Active Sites for Reduction of NO2 with Methane by Solid State Exchange of ln203 into H-Zeolites M. Ogura, N. Aratani and E. Kikuchi

1593

Purification of NOx on Pt-ZSM-5 and Mg-Cu-ZSM-5 Catalysts under LeanBurn Engine Emission Conditions S. Choung, B. Shin and J. Bae

1601

The In-situ Characterization of Titanium Oxides Prepared in the Zeolite Cavities and Framework and Their Photocatalytic Reactivities for the Direct Decomposition of NO into N2 at 275K Y. IchihashL H. Yamashita and M. Anpo

1609

xxiii Photocatalytic Decomposition of Trichloroethylene over Aluminosilicate Zeolites with Isomorphous Incorporation of Titanium I.H. Cho, J.H. Kwak, R. Ryoo, W.S. Ahn, K.Y. Jung and S.B. Park

1617

Catalytic Decomposition of Organic Sulfur Compounds - Effect of Zeolite Acidity M. Ziolek, P. Decyk, J. Czyzniewska and H.G. Karge

1625

Use of Natural Clinoptilolite for the Optimization of Mineral Clay Liners for Waste Deposits M. W. Upmeier and K.A. Czurda

1633

Removal of Highly Concentrated Ammonium Ions by Natural Mordenite S. Noda

1641

Palladium Ion-Exchanged SAPO-5 for a Low Temperature Combustion of CI-h Y. Takita, T. lshihara, H. Nishiguchi and H. Sumi

1647

Kinetics of CH4 Complete Oxidation on CuH-ZSM-5 Catalyst A. V. Kucherov, N. V. Nekrasov, A.A. Slinkin, E.A. Katsman and S.L. Kiperman

1655

Ion-Exchange Behavior of Zeolite NaA and Maximum Aluminum Zeolite NaP E.v.R. Borgstedt, H.S. Sherry and J.P. Slobogin

1659

Zeolite MAP: the New Detergent Zeolite C.J. Adams, A. Araya, S.W. Carr, A.P. Chapple, K.R. Franklin, P. Graham, A.R. Minihan, T.J. Osinga and J.A. Stuart

1667

Part C VI. Adsorption and Diffusion Zeolites as Adsorbents and Catalysts. The Interactive System Encaged Molecule/Zeolite Framework D. Barthomeuf

1677

Methanol Adsorption and Activation by Zeolitic Protons S.R. Blaszkowski and R.A. van Santen

1707

xxiv Carbon Dioxide Adsorption Kinetics in the Presence of Light Paraffins on NaA and CaA Zeolites A. Khodakov and L. V.C. Rees

1715

Specific Adsorption from Aqueous Phase on Apolar Zeolites C. Buttersack, 1. Fornefett, J. Mahrholz and K. Buchholz

1723

Adsorption Studies on Ordered Mesoporous Materials (MCM-41) J. Jdinchen, M. Busio, M. Hintze, H. Stach and J.H.C. van Hooff

1731

Rapid-Scanning FT-IR Study on the Adsorptions of Methanol and Water on H-ZSM-5 Zeolite F. Wakabayashi, M. Kashitani, T. Fujino, J.N. Kondo, K. Domen and C. Hirose

1739

Onthe Sorption of Ethylbenzene in ZSM-5 R. Schumacher, P. Lorenz and H. G. Karge

1747

Ethylene Adsorption on HNaZSM-5: Kinetic Study S.N. Vereshchagin, N.P. Kirik, N.N. Shishkina and A. G. Anshits

1755

Adsorption of Acetylacetone on Layer Silicate Containing Various Interlayer Cations JR. Sohn and S.I. Lee

1763

Sorption of Water Vapor on HZSM-5 Type Zeolites T. Sano, T. Kasuno, K. Takeda, S. Arazaki and Y. Kawakami

1771

~H-NMR Relaxation Times of Water and Benzene Adsorbed in Zeolite Beta S. Sardar, W.D. Basler and H. Lechert

1779

Adsorption of Sulfur Dioxide on Y-Type Zeolites Y. Teraoka, Y. Motoi, H. Yamasaki, A. Yasutake, J. Izumi and S. Kagawa

1787

Vanadium Derivatives of MFI Type Molecular Sieves Investigated by Sorption and Catalytic Tests J. Kornatowski, M. Sychev, M. Rozwadowski and W. Lutz

1795

Desiccant Selection Criteria for Enthalpy Exchange Systems M.P.F. Delmas, W.D. Holeman, C.N. Blystad, W.A. Belding and J.H.D. Tantet

1803

XXV

Atomistic Mechanism of the Adsorption of CFCs in Zeolite as Investigated by Monte Carlo Simulation K. Mizukami, H. Takaba, Y. Oumi, M. Katagiri, M. Kubo, A. Stirling, E. Broclawik, A. Miyamoto, S. KobayashL S. Kushiyama and K. Mizuno

1811

The Crystal Structures of Dehydrated Fully Cd2+-Exchanged Zeolite X and of Its Carbon Monoxide Sorption Complex S.B. Jang, J.H. Kwon, S.H. Song, Y. Kim and K. Serf

1819

Structural Property of Methane(CD4) and Hydrogen(D2) Sorbed Phases on MCM-41 (0=25A) J.P. Coulomb, C. Martin, Y. Grillet, P.L. Llewellyn and J. Andr~

1827

Composition, Location, Modes of Formation and of Removal of Coke Deposited on a 5A Adsorbent P. Magnoux, M. Misk, G. Joly, S. Jullian and M. Guisnet

1835

Self-Diffusion and Diffusive Transport in Zeolite Crystals J. Kdirger and D.M. Ruthven

1843

Simulation of Hydrocarbon Diffusion in Zeolites E.J. Maginn, R.Q. Snurr, A.T. Bell and D.N. Theodorou

1851

Methane Diffusion in Zeolites of Structure Type LTA in Dependence on Physical and Chemical Parameters- An MD Study S. Fritzsche, 3/1. Gaub, R. Haberlandt, G. Hofmann, J. Kgirger and M. Wolfsberg

1859

Study of the Molecular Diffusion in the Internal Porosity of ZSM-5 and HMOR Zeolites L.C. de M~norval, J.G. Kim and F. Figueras

1867

Single File Counterdiffusion in Pores of Infinite and Finite Length J.M..D. MacElroy and S.-H. Suh

1875

Hydrogen Separation by Two-Bed PSA Process J. Yang, J.-H. Lee, C.-H. Lee and H. Lee

1883

Pressure Swing Adsorption of Organic Solvent Vapors on Mesoporous Silica Molecular Sieves S. Namba, N. Sugiyama, M. Yamai, I. Shimamura, S. Aoki and J. lzumi

1891

xxvi

VII. Modifications Post-Synthesis Modification of Microporous Materials by Solid-State Reactions H. G. Karge

1901

A New Layered (Alumino) Silicate and Its Transformation into a FER-Type Material by Calcination L. Schreyeck, P. Caullet, J.C. Mougenel, J.L. Guth and B. Marler

1949

From the Keggin Complex Containing Solution to Pillared Layer Clays - A Comprehensive NMR Study J.B. Nagy, J.-C. Bertrand, L Pdlink6 and L Kiricsi

1957

Inactivation of Acid Sites on External Surface of Zeolites with Methoxytripropylsilane J.-H. Kim, M. Okajima and M. Niwa

1965

Generation of Acid Sites by Incorporation of Cobalt in the AFR Structure J.P. Lourengo, M.F. Ribeiro, F.R. Ribeiro, J. Rocha, Z Gabelica, B. Onida and E. Garrone

1973

Acidic Properties of Galliosilicate Molecular Sieves with the Offretite Structure M.L. Occelli, H. Eckert, C. Hudalla, A. Auroux, P. Ritz and P.S. Iyer

1981

The Thermal Expansion of the Zeolites MFI, AFI, DOH, DDR and MTN in Their Calcined and as Synthesized Forms S.H. Park, R.-W. Grofle Kunstleve, H. Graetsch and H. Gies

1989

Activity Enhancement of H-Zeolites by Ag Ion-Exchange and Sulfiding with Hydrogen Sulfide M. Sugioka and L. Andalaluna

1995

Effect of Pre-dealumination by (NI-h)zSiF6 on the Properties of ZSM-5 with Steam Aging Treatment L. Huang, Q. Li, Z. Xue, G. Ding, G. Niu, Z. Li and Z. Shi

2003

Effect of the Dealumination Procedure on Surface Properties and Catalytic Performance of UHP-Y Zeolite Z. Chang, C. Ruan and G. Tong

2011

xxvii Formation of Alkali Nanoparticles in NaY Zeolite Cages and in A1PO4-5 Molecular Sieves: NMR Studies L. C. de M~norval and F. Rachdi

2019

Iridium in Pentasil: Redox Behavior and Reactivity T. V. Voskoboinikov and E.S. Shpiro

2027

Para-Chlorination Activity of Solid-State Ion-Exchanged Zeolite KL Catalysts J. ~ Yoo, D.S. Kim, J.-S. Chang and S.-E. Park

2035

Solid-State Ion Exchange of Fe(III) into Y Zeolite under Deep-Bed Conditions P. Hudec, A. Smieskovd, Z. Zidek, ~ Jorik, 3/1. Miglierini and J.B. Nagy

2043

Preparation, Characterization, and Catalysis of lntrazeolite Iron Oxide Clusters Y. Okamoto, H. Kikuta, Y. Ohto, S. Nasu and O. Terasaki

2051

Zeolite Pore Size Engineering by Chemical Liquid Deposition Y.-H. Yue, Y. Tang and Z. Gao

2059

Incorporation of Molybdenum into Mesoporous MCM-41 S.D. Djajanti and R.F. Howe

2067

Chemical Vapor Deposition on Basic Zeolites Y. Chun, Q.-H. Xu, A.-Z Yan and X. Ye

2075

Metal-Oxide Interactions and Catalytic Behaviors of La203- and V2OsPromoted Rh/NaY Catalysts K. KunimorL K. Yuzaki, T. Yarimizu, fill.. Seino and S. Ito

2083

VIII. Novel Materials Modification of a Pyroelectric Detector by Controlled Electrocrystallization of Thin Zeolite Layers G.J. Klap, M. Wiibbenhorst, J. van Turnhout, J.C. Jansen and H. van Bekkum

2093

Fabrication, Luminescence and Photoacoustic Spectroscopic Studies of the Semiconductor Nanoclusters in Zeolites G. Tel'biz, 1. Blonskij, S. Shevel and E Voznyi

2101

xxviii Zeolites as Sensitive Materials for Organic Vapour Detection: An Exploratory Study C. CantalinL M. Pelino, M. Pansini and C. Colella

2109

Adsolubilization Equilibrium of Rhodamine B by Zeolite/Surfactant Complexes K. Hayakawa, A. DobashL Y. Miyamoto and L Satake

2115

Investigations into the Engineering of Inorganic/Organic Solids: Hydrothermal Synthesis and Structure Characterization of One- and TwoDimensional Molybdenum Oxide Polymers Y. Xu, L.L. Koh, L.H. An, D.G. Roshan and L.H. Gan

2123

Synthesis and Structure of Cadmium Chalcogenide Beryllogermanate Sodalites S.E. Dann andM.T. Weller

2131

Optical and Magnetic Properties of Na-K Alloy Clusters Incorporated into LTA T. Kodaira, E Nozue, O. Terasaki and H. Takeo

2139

Growth of Oriented Molecular Sieves on Organic Layers S. Feng and T. Bein

2147

Factors Affecting the Porosity of ZSM-5 Layer on the Surface of Stainless Steel Z. Shah, E. Min and H. Yang

2155

Synthesis of Films of Oriented Silicalite-1 Crystals Using Microwave Heating J.H. Koegler, A. Ararat, H. van Bekkum and J. C Jansen

2163

News from AiPO4-5: Microwave Synthesis, Application as Medium to Organize Molecules for Spectroscopy and Nonlinear Optics, Material for One-Dimensional Membranes J. Caro, F. Marlow, K. Hoffmann, C Striebel, J. Kornatowski, 1. Girnus, M. Noack and P. KOlsch

2171

Silylation of Silicalite Membrane and Its Pervaporation Performance T. Sano, K. Yamada, S. Ejiri, M. Hasegawa, Y. Kawakami and H. Yanagishita

2179

Vertically-Aligned MeAPO4-5 Crystals Grown on Anodic Alumina Membrane K.£ Chao, CN. Wu, H.C Shih, T.G. Tsai and Y.H. Chiou

2187

xxix Synthesis ofFER Membrane on an Alumina Support and Its Separation Properties N. Nishiyama, K. Ueyama and M. Matsukata

2195

Synthesis of Ultra Thin Films of Molecular Sieves by the Seed Film Method J. Hedlund, B.J. Schoeman and J. Sterte

2203

Synthesis of SAPO-34/Ceramic Composite Membranes L. Zhang, M. Jia and E. Min

2211

Synthesis of ZSM-5 Zeolite Membrane on the Inner Surface of a Ceramic Tube H.-S. Oh, M.-H. Kim and H.-K. Rhee

2217

Synthesis of Oriented Zeolite Film on Mercury Surface Y. Kiyozumi, F. Mizukami, K. Maeda, T. Kodzasa, M. Toba and S. Niwa

2225

Growth of Oriented Zeolite Crystal Membranes M. Cheng, L. Lin, W. Yang, Y. Yang, Y.-D. Xu and X. Li

2233

Preparation of Fibrous Titanium Silicalite-l(FTS-1) from Nano TS-1 Crystals K.T. Jung, J.H. Lee, J.H. Hyun, D.S. Kim, J. G. Kim and Y.G. Shul

2241

IX. Theory Structure of the Linear Na32÷ Cluster in Zeolite X W. Shibata and K. Serf

2251

Supralattices: Another Dimension in Materials Science- Theoretical Investigation A.A. Demkov and O.F. Sankey

2259

Experimental and Theoretical Studies of Siliceous Zeolites A.K. Cheetham, L.M. Bull and N.J. Henson

2267

Charge-Transfer Molecular Dynamics ofProtonated Faujasite L.J. Alvarez, P.B. Giral, C.Z. Wilson andJ.E. Sdnchez-S6nchez

2275

Quantum-Chemical Study of Hydride Transfer in Catalytic Transformations of Paraffins on Zeolites KB. Kazansky, M. K Frash and R.A. van Santen

2283

XXX

Computer Modelling of the Structure and Synthesis of Microporous and Mesoporous Materials D. l~. Lewis, R.G. Bell, P.A. Wright, C.R.A. Catlow and J.M.. Thomas

2291

Ab Initio Computer Modelling of Zeolite Frameworks I. Modelling of Basic Clusters M. Sato and H. Uehara

2299

Intrinsic and Enhanced Broensted Acidity in Zeolites J. Dwyer, K Zholobenko, A. Khodakov, S. Bates and M.A. Makarova

2307

Molecular Sieve Effect of Chemically Modified Na-A Type Zeolite and Its Molecular Dynamic Simulation J. lzumi, A. Yasutake, N. Tomonaga, N. Oka, H. Ota, N. Akutsu, S. Umeda andlVl. Tafima

2315

Computational Study of Structural and Thermal Properties of the Microporous Titanosilicate ETS- 10 M.E. Grillo, J. Lujano and J. Carrazza

2323

Complete Redox Exchange of Indium for TI÷ in Zeolite A. Synthesis and Crystal Structure of Fully Indium-Exchanged Zeolite A N.H. Heo, H.C. Choi and K. Serf

2331

Location and Orientation of Pyrrole and Acetaldehyde Molecules inside Siliceous Faujasite as Predicted by Electronic Structure Calculations A. Chatterjee, R. Vetrivel, M. Kubo and A. Miyamoto

2339

Commensurate Freezing of Hydrocarbons in Silicalite IJ(.jM. van Well, J.P. Wolthuizen, B. Stair, J.H.C. van Hooff and R.A. van Santen

2347

Phase Transition Types Observed During the Sorption of van der Waals Gases on Model Zeolites: Silicalite I and AIPO4-5 J.P. Coulomb, C. Martin, P.L. Llewellyn and Y. Grillet

2355

Author Index

2363

Subject Index

2375

xxxi

Preface The 11th International Zeolite Conference was held successfully in Seoul, Korea, from August 12 to 17, 1996. The main conference was preceded by the Pre-Conferenee Summer School on Zeolite held in Taeduck Science Town and followed by the PostConference Symposium on Catalysis related to Environmental Application of Zeolite in Kyoungju, Korea. We would like to thank members of the IZA Council, liaison to the council, Dr. Ono, and members of the International Advisory Board for their active support for the conference. The number of registrants totaling about 500 was a little bit less than the organizing committee hoped for. Considering the shortened interval between the successive International Zeolite Conference from three to two years, this number nevertheless reflected quite a favorable response to this conference. During the 6 day meeting of the main conference 5 plenary lectures and 113 papers were presented orally in 40 sessions and 165 full papers as well as 127 recent research reports were presented as posters. Ever increasing interest and continuous developments in the zeolite science and technology are reflected in the number of submitted contributions in various areas of research and application. We would like to thank all those who have submitted the papers. It is regrettable, however, we could not accommodate many contributions of high quality due to the limited time and space available for presentation. We also had to make certain consideration on the balance among the different disciplines. Each manuscript for the proceedings was reviewed by two reviewers and in case of a conflicting reviewing results a third referee was asked to recommend as to the acceptance of the paper for publication in the proceedings. The organizing committee is most grateful to the experts who spared their precious time to review abstracts as well as the full manuscript for the proceedings. Our special appreciation goes to Dr. David Olson who headed the third referee group. The Proceeding of the 1l th Zeolite Conference is published in three volumes containing 5 plenary lectures and 274 full papers. Part A contains Synthesis and Characterization (99 papers), Part B Catalysis, Environment (102 papers), and Part C Adsorption and Diffusion, Modifications, Novel Materials and Theory (78 papers).

xxxii Although 161 papers out of 279 are presented as posters no difference is made whatsoever between the oral and poster papers in publishing the full manuscript in the proceedings. Zeolite catalysis has been and continues to be an area of major interest. Growing interest in the synthesis, and the characterization of zeolite and microporous materials is reflected in the large number of contributions. Other area of growing interest is novel materials. Adsorption, theory and modeling remain as attractive areas. Our ambitious original plan to publish the proceedings before the conference had to be modified and three volume proceedings will be published by the end of this year. Generous donations were received from a number of organizations whose names are given in the sponsor's list. The organizing committee is most grateful to them for their support. Finally we would like to express our sincere appreciation to the authors for their fine papers and all the reviewers who made careful reviewing of the submitted abstracts. We are also grateful to the members of the scientific committee who have spent so much time and effort to select the papers, especially to Prof. Gon Seo, Prof. Yang Kim, Prof. Jong Rack Sohn, Dr. Sang-Eon Park, and Prof. Ryong Ryoo. We would like to thank also Mr. Sang lck Lee and Dae-Chul Kim who helped in the preparations of the proceedings.

Seoul, August, 1996

Hakze Chon Son-Ki Ihm Young Sun Uh

xxxiii

Organizing Committee of the 1 lth IZC Chairman Hakze Chon

Korea Advanced Institute of Science & Technology, Taejon, Korea

Vice-Chairmen Hanju Lee Baik-Hyon Ha Wha Young Lee

Yonsei University, Seoul, Korea Hanyang University, Seoul, Korea Seoul National University, Seoul, Korea

Secretary Young Sun Uh

Korea Institute of Science & Technology, Seoul, Korea

Finance Sub-Committee Chairman Hanju Lee

Yonsei University, Seoul, Korea

Members Hyun-Ku Rhee Ki Woong Sung Bong H. Chang

Seoul National University, Seoul, Korea. DaeLim Industrial Co., Ltd., Seoul, Korea AeKyung-PQ Advanced Materials Co. Ltd., Seoul, Korea

Program Sub-Committee Chairman Sang Heup Moon Members Wha Seung Ahn Suk-Jin Choung Sung Hwan Han Suk In Hong Kyung Lim Kim Ho-ln Lee

Seoul National University, Seoul, Korea

Inha University, Inchon, Korea Kyunghee University, Suwon, Korea Korea Institute of Science & Technology, Seoul, Korea Korea University, Seoul, Korea Yonsei University, Seoul, Korea Seoul National University, Seoul, Korea

xxxiv Hyun Ryul Park Tae-Jin Park Kee Jun Yoon

Chungang University, Seoul, Korea Korea Institute of Science & Technology, Seoul, Korea Sungkyunkwan University, Suwon, Korea

Scientific Sub-Committee Chairman Son-Ki lhm


Vice-Chairman Gon Seo Members Byung Joon Ahn Hee Kwon Chae Paul J. Chong Jong Shik Chung Kee Sung Ha Chong Soo Han Nam Ho Heo Suk Bong Hong Seon-Yong Jeong Wha Joong Kim Jae Chang Kim Man Hoe Kim Yang Kim Chul Wee Lee Dong-Keun Lee Jae Sung Lee Jung-Min Lee Tae Jin Lee Hee Moon Sang Sung Nahm In-Sik Nam Kyung Tae No

Chonnam National University, Kwangju, Korea

Chonbuk National University, Chonju, Korea Hankuk University of Foreign Studies, Yong-ln Korea Korea Research Institute of Chemical Technology, Taejon, Korea Pohang University of Science and Technology, Pohang, Korea Pusan National University of Technology, Pusan, Korea Chonnam National University Kyungpook National University Korea Institute of Science & Technology, Seoul, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Konkuk University, Seoul, Korea Kyungpook National University, Taeku, Korea Air Force Academy, Chongju, Korea Pusan National University, Pusan, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Kyeongsang National University, Taeku, Korea Pohang University of Science and Technology, Pohang, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Yeungnam University, Taeku, Korea Chonnam National University, Kwanju, Korea Korea Research Institute of Chemical Technology, Taejon, Korea Pohang University of Science and Technology, Pohang, Korea Soongsil University, Seoul, Korea

XXXV

Korea Advanced Institute of Science & Technology. Taejon, Korea Seung Bin Park Korea Research Institute of Chemical Technology, Taejon, Korea Sang-Eon Park Korea Advanced Institute of Science & Technology, Taejon, Korea Ryong Ryoo Yonsei University, Seoul, Korea Yong Gun Shul Kyungpook National University, Taeku, Korea Jong Rack Sohn Korea Institute of Science & Technology, Seoul, Korea Dong-Jin Suh Hongik University, Seoul, Korea Sung-Sup Suh Korea Advanced Institute of Science & Technology, Taejon, Korea Seung lhl Woo Ajou University, Suwon, Korea Jae Eui Yie Kyung Byung Yoon Sogang University, Taejon, Korea Jong-Sung Yu Hannam University, Taejon, Korea

Pre-Conference Summer School on Zeolites Chairman

Seung lhl Woo


Co-Chairman

Sang-Eon Park

Korea Research Institute of Chemical Technology, Taejon, Korea

Members

Byung Joon Ahn Oh Bong Yang Sang Sun Nahm

Chonbuk University, Chonju, Korea Chonbuk University, Chonju, Korea Korea Research Institute of Chemical Technology, Taejon, Korea

Post-Conference Symposium on Catalysis Chairman

Young Gul Kim

Pohang University of Science and Technology, Pohang, Korea

Member

In-Sik Nam Kyung Hee Lee Jong Shik Chung Jae Sung Lee

Pohang University of Science and Technology, Pohang, Korea Pohang University of Science and Technology, Pohang, Korea Pohang University of Science and Technology, Pohang, Korea Pohang University of Science and Technology, Pohang, Korea

xxxvi

International Advisory Board A. Alberti J.R. Anderson J.N. Armor R. von Ballmoos

T. Bein A.T. Bell H. van Bekkum G. Bellussi H.K. Beyer D.M. Bibby M. Btilow K.-J. Chao A. Corma E.G. Derouane J. Dwyer F. Fajula

F. Fetting E.M. Flanigen P. Gallezot D. Goldfarb L. Guczi X. Guo A.B. Halgeri W. H01derich J. van Hooff R.F. Howe T. Inui K. lone P.A. Jacobs K.-J. Jens

S. Kaliaguine

University of Ferrara, Italy Monash University, Victoria, Australia Air Products & Chemicals Inc., USA Engelhard Corp., Ohio, USA Purdue University, Ind, USA University of California, Berkeley, USA Delft University of Technology, The Netherlands Eniricerche, Milano, Italy Hungarian Academy t?fSciences, Hungary The New Zealand Inst. For Ind. Res. & Dev., New Zealand The BOC Group Technical Center, USA National Tsing Hua University, Hsinchu, Taiwan Valencia University of Technology, Spain University of Namur, Belgium University of Manchester, UK ENSCM, Montpellier, France Darmstadt University of Tech., Germany UOP, Tarrytown, USA Institut de Recherches sur la Catalyse, France Weizmann Institute of Science, Israel Hungarian Academy of Sciences, Hungary Chinese Academy of Sciences, China Indian Petrochemicals Corp., India RWTH Aachen, University of Tech., Germany Eindhoven University of Tech., The Netherlands University of New South Wales, Australia Kyoto University Kyoto, Japan Boreskov Inst. of Catalysis, Novosibirsk, Russia Katholieke Universiteit, Leuven, Belgium Statoil Petrochemicals and Plastics, Norway Universit~ Laval, Canada

xxxvii

J. Kiirger

Hungarian Academy of Sciences, Hungary Fritz Haber Institute, Max Planck Society, Germany University of Leipzig, Germany

V. Kaub,i~,

University of Ljubljana, Slovenia

H. Kessler

University of Haute Alsace, France Ryukoku University, Japan Woodbury, NJ, USA Helsinki University of Technology, Finland University of Twente, The Netherlands University of Kiel, Germany Worcester Polytech. Inst., USA Katholieke Universiteit Leuven, Belgium ETH-Zentrum, Ziirich, Switzerland ETH-Zentrum, Zi~rich, Switzerland Res. Inst. of Petroleum Processing, China The Nishi-Tokyo University, Japan CNRS, Villeurbanne, France Biosym Technologies Inc., USA University of Cape Town, South Africa Mobil R&D Corp., USA Tokyo Inst. of Tech., Japan University of Leipzig, Germany National Chemical Lab., Pune, India University of Edinburgh, UK University of Maine, USA Northwestern University, Ill., USA Eindhoven University of Tech., The Netherlands Gunma University, Japan Sophia University, Japan Russian Academy of Sciences, Russia University of California, Santa Barbara, USA University of Connecticut, Conn., USA University of Clark Atlanta, Ga., USA University of Tokyo, Japan

D. Kall6 H.G. Karge

M. Koizumi G.T. Kokotailo A.O.I. Krause J.A. Lercher F. Liebau

E. Ma J. Martens L.B. McCusker W.M. Meier E. Min S. Namba C. Naccache J.M. Newsam C.T. O'Connor D.H. Olson Y. Ono H. Pfeifer P. Ratnasamy L.V.C. Rees D.M. Ruthven W.M.H. Sachtler R.A. van Santen

M. Sato K. Segawa E.S. Shpiro G.D. Stucky

S.L. Suib R. Szostak T. Tatsumi

xxxviii J.M. Thomas R.P. Townsend J.W. Ward J. Weitkamp T.E. Whyte, Jr T. Yashima R. Xu K.I. Zamaraev M. Zi61ek

The Royal Institution of Great Britain, UK Unilever Research, Bebington, UK UOP, Brea, USA University of Stuttgart, Germany The PQ Corporation, Conshohocken, USA Tokyo Inst. of Tech., Tokyo, Japan Jilin University, Changchun, China Boreskov Inst. of Catalysis, Novosibirsk, Russia Adam Mickiewicz University, Poznan, Poland

Liaison to the 11 th IZC Y. Ono

Tokyo Institute of Technology, Tokyo, Japan

Members of the IZA Council Roland von Ballmoos

President

Jens Weitkamp

Vice President

Koos Jansen

Secretary

J. Michael Bennett

Treasurer

T. Bein, G. Bellusi, K.-J. Chao, H. Chon, T. Inui, H. Karge, L.B. McCusker, W.J. Mortier, D.E.W. Vaughan, T. Yashima, S.I. Zones

xxxix

Financial Support The Organizing Committee gratefully acknowledges support by the following institutions and companies: (As of August 30, 1996) Korea Science and Engineering Foundation, Seoul, Korea Korea Research Foundation, Taejeon, Korea LG-Caltex Oil Corporation, Seoul, Korea YukongLimited, Seoul, Korea SsangYong Oil Refining Co., Ltd., Seoul, Korea DaeLim Industrial Co., Ltd., Seoul, Korea AeKyung-PQ Advanced Material Co., Ltd., Seoul, Korea Samsung Fine Chemicals Co., Ltd., Seoul, Korea Korea General Chemical Corporation, Seoul, Korea Zeobuilder Co., Ltd., Seoul, Korea Isu Chemical Co., Ltd., Seoul, Korea Cosmo Industrial Co., Ltd., Cheong-ju, Korea


I.

Synthesis


H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.

DIRECTED SYNTHESIS OF ORGANIC/INORGANIC COMPOSITE STRUCTURES

Galen D. Stucky, Qisheng Huo, Ali Firouzi, and Brad F. Chmelka Departments of Chemistry, Materials, and Chemical Engineering University of California, Santa Barbara, CA 93106, U.S.A.

Stefan Schacht, I. G. Voigt-Martin, and Ferdi Sehiith Institut ftir Anorganische Chemie, Frankfurt University, Frankfurt, Germany

1. INTRODUCTION The impact of the discovery of the synthesis of periodic mesoporous material using amphiphilic surfactants 1,2,3,4,5,6 is readily apparent from the demographics of papers that have appeared in this area, as illustrated in Figure 1 for MCM-41 publications. In addition to providing for the first time access to high surface area, monodispersed mesopores (lnm to > 10nm), the research has provided, in a more generic sense, a new synthesis paradigm on how to bring together spatially distinct, nanostructured organic and inorganic arrays into two- and threedimensional periodic, composite structures. Designed materials synthesis and properties based on the molecular level interplay of the kinetics and energetics of organic and inorganic domain and interface

200-,,,,

assembly are of vital interest to many areas including biomineralization, conducting and optical display polymer composites, chemical sensors, fine chemical and bio-catalysis, and the creation of composite phases with useful

mechanical

and

.o 150 .~ "~ ~ 100 "~, 50

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

J

i

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

~ .................

~ .................

~ .................

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

~ .................

~.................

~ .....................................................

thermal

properties for insulation and packaging applications. It is clear that this discovery is a major entry not only in the "breakthrough" library of zeolite and molecular sieve syntheses, but also

o

....

1991

*

....

....

i

~

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

~................

. . . . . . . . . . .

,

1 9 9 2 1993 1 9 9 4 1995 1 9 9 6 1997 Year

Figure 1 MCM-41 publications as found by STN of the American Chemical Society. The value for 1996 is an extrapolation based on the first five months.

in advancing the field of materials synthesis in general. In this paper, I will focus on some observations concerning this composite synthesis paradigm. The presentation is admittedly only a very selected sampling, with some brief excursions into implications related to biomineralization and porous materials design at longer (10 nm to 1000nm) length scales. Much progress has been made 7,8,9,10,11,12,13,1't,15,16 in what has been appropriately described as the "lofty goal of molecular sieve synthesis by design ''17. For these microporous ( OO, II > OO).

Charge matching of the available charge/unit area of the organic with that of the I Inorganic Molecular Species ~

HO

+ n[~

~

~}

~

~ O ~ Organic ' ~ - ~ Molecules/Arrays X"

Interface .S~ssembly

N ~N

OO

-

O > Ol II "Biomimetics" ' ~ . . ~

/

%~-~o

.. ~ SEQUENr~L

FLMGROWTH

OO,II OO

~ ~'. ~~'

Organic/l~t~organic Array A.,sembly ~,~,~ ~ q~

Inorganic Polymerization On OrganicArrays MESOPOROtSTmN FILMS

/

/

EMULSION"TEMPLArING~' Silica Acid Synthesis

MOLECULAR SIEVES

~

~.MESOPOROI.S MATERIALS

//~v

Silica Base Synthesis

Inorganic Polymerization Figure 2 Schematic illustrating organic-organic (OO), organic-inorganic (OI) interface, and inorganic-inorganic (II) control of composite materials synthesis. The ordering processes kinetically and/or thermodynamically determined and also must include solvation and cosolvent effects

encapsulating inorganic is required. Water molecules of solvation play a key role in this assembly process, both in terms of solubilizing the SDA and as an entropic thermodynamic driving force provided upon release of the water molecules as inorganic-organic assembly takes place. Taking into account these various factors, rigid, bulky and relatively short (Ilor

O O > O I , II.

Again

should

it

emphasized

that

be these

Covalent

I / o-

I

o-

~

Silicate Anion Oligomer

ordering processes can be Bilayer (Base Synthesis) kinetically thermodynamic determined.

and/or ally

,o,o. 0

0 \ O" NSi/ Si" O/ ~ O / \ O \ H

Direct Framework Surfactant Inclusion Figure 3 Strong ionic 1-6 and covalent 20-22 interface bonding in silica mesostructure assembly.

The first situation with OI > OO; OO,II 9) synthesis of mesoporous silicate phases using colloidal silica and cetyltrimethylammonium cations as the surfactant amphiphile23,24; and by the use of covalently bound organosilicon or transition metal 25 organometallic precursors in mesoporous phase synthesis. The second situation (OO > OI, II Figure 2) means that an organized organic array controls the assembly and also defines the ultimate configuration of the composite phase. In fact, this is the basis for what has been described as a central tenet of biomineralization 26,27,28,29,30,31that states that nucleation, growth, and the final morphology of biominerals are determined by the existence of a preorganized assembly of organic molecules. "Biomimetic" approaches and modelling of biomineralization have relied on this paradigm for experimental design and have accordingly focused on the use of known stable organic arrays or stabilization through, for example, covalent attachment to a substrate or crosslinking of the organic groups. Some examples include (i) hollow submicro-diameter silica cylinders obtained by depositing silica onto phospholipid tubules 32, (ii) bulk inorganic iron oxide deposited on charged bio-lipid substrates 33, and (iii) ceramic thin film processing by deposition of bulk inorganic phases on surfaces functionalized with ionic organic surfactants 34. A well known biological example of a strongly bound organic array whose surface is used as a template for inorganic oxide aggregation is apoferritin. Since composite assembly is determined by the

Hydrogen Bonding

relative strengths of the thermodynamic driving forces and the relative rates of the kinetic processes, stabilized organic arrays do not have be used if the OI binding interaction is weak. An example is the tri-layer (S§

35 hydrogen

bonding shown in Figure 4 that is obtained by combining cationic surfactants with cationic silica species at acidic pH values below the aqueous silica isoelectric point (SIp)36,37. In those cases where one wishes to retain or only slightly

N

N

R/+/~R

R/I~+~

CI" I

CI-

CI- CIi

I

I

modify the organic morphology that exists in the absence of

silica S+ X - I + trilayer (Acid Synthesis)

the silica, the (S§ but not the (S§ 3 base synthesis structure directing approach has been demonstrated to be successful. Examples include the synthesis of mesoporous

Figure 4 Structure direction by hydrogen bonding36,37 in mesostructure synthesis.

thin films on inorganic substrates 38 and at the air-water interface 39, mesostructure templating using

preformed organic liquid crystals 4~ and in the creation of micron scale shaped mesoporous structures by using amphiphilic surfactants embedded in the oil phase of oil-water microemulsions or oil water interfaces (see below).

2. SURFACTANT CONSIDERATIONS It is important to emphasize that the two situations described above (OI > OO > II, and OO > OI, II) are two different synthetic strategies, that not unexpectedly lead to composite and porous materials that have distinctly different properties. These are further described later in this paper. The common denominator for both routes is the dominating role that the surfactants have in determining the overall structural symmetry of the final product, a role also reflected in the observation that the same relatively small group of space group symmetries are obtained for conventional amphiphilic, hydrophilic, and polymer-based surfactant systems even though the underlying compositions, molecular structures, chemical and physical properties differ substantially 4i,42. Among the structures41,43 that can be observed in surfactant or lipid containing lyotropic systems, hexagonal (2d, p6m) and lamellar phases are the two most common mesophases. Six lyotropic cubic phases, in which Pm3n and Ia3d are two typical mesophases, may be found in many surfactant and lipid systems (for a recent review, see reference 42). Several intermediate phases can be formed44,45. Similar systems follow the same succession of phases, but not all of the phases are always present. However, the formations of additional new lyotropic mesophases are also possible, as indicated by the fact that new cubic phases, which consist of micelles of type I (oil-in-water), appear to be present in some surfactant systems46,47,48, 49. However, at this time no clear conclusions about the exact nature of the new phases have been obtained since the quality of the diffraction data is not adequate to determine their structures. Inorganic mesophases with good long-range ordering quality and excellent stability are helpful in characterizing new liquid crystal-like structures. In the conventional charged surfactant-water mixture at a given composition and temperature, from a molecular point of view the micellar shape or packing of the surfactant is determined by a balance between three general types of free energy contributions. One is associated with the tendency of the alkyl chains to minimize their water contact and maximize their inter-organic interactions. The second involves the coulombic and dipolar interactions among the charged headgroups and their associated anions. This contribution determines the mean area-perhead-group, a 0, that is available to each surfactant head group in an aggregate. In most classical discussions of liquid crystal aggregates, the counter-ion of the surfactant is implicitly included in

a o. The third type of free energy contribution includes solvation energies that arise from the presence of water, alcohol, or organic molecules in the hydrophilic, intermediary hydrophobichydrophilic

"palisade",

and

hydrophobic regions. Because

the

synthesis

intermediates and final composite structure are a consequence of the organization of spatially distinct organic and inorganic nanophase regions, the structure can also be readily defined in terms of surfaces generated by the above collective interactions (Figure 5). The mathematical perspective of the structure becomes one based on a differential calculus description of the spatial continuum described by the

Figure 5 Cartoon illustrating hypothetical cross section of mean surface continuums defined by collective interactions of organic surfactants and inorganic species. Curvature is defined by incompatible local packing requirements and structure frustration. The resulting modulated structures may exist in "lamellar", "hexagonal", etc., structured phases.

surfaces rather than the usual matrix algebra that is commonly used to describe a structure consisting of a collection of atoms located at discrete points in spaceSO,51. In this description, the inorganic and organic arrays of these mesostructures meet at an interface surface. The interface curvature is energetically defined so as to optimize charge repulsion and van der Waals interactions, resulting in a minimal surface structure for MCM-48 52,53,54. Phase transitions are associated with changes in the curvature of interface and may be understood phenomenologically as a competition between the elastic energy of bending the interfaces and energies resulting from the constraints of interfacial and charge separation 55. The different entropic and interaction energies in the nanoscale organic, inorganic and interface regions result in structure frustration with incompatible local packing constraints that forbid an optimal geometry where the energy is everywhere minimized. The inorganic/organic structures therefore readily undergo structural changes or transformations 56 to relieve this stress through rotational displacements of the surfaces (disclination defect). This structural modification is experimentally driven by entropic changes in the organic array (temperature), silica polymerization which modifies local density and interface charge properties23,24, 52 or through coordinated solvent (water) and organic cosolvent displacement21,23,24,73.

Because the mesostructured composite phases are frustrated structures, it is not surprising that one finds phases that for example would be classified as lamellar from the standpoint of the relatively crude average structure given by low-angle X-ray diffraction, but in fact on closer inspection by electron microscopy are found to have in-plane modulation, and a periodic rippled lamellar structure 24. BET (Brunaer-Emmett-Teller) measurements of this calcined "lamellar" phase show a high surface area and monodispersed pores. Similarly, there is considerable evidence to expect 3,4,5,56,57 that the honeycomb p6m (MCM-41) structure defined by X-rays very likely includes structures with modulated pores such as those indicated in Figure 5. The implication is that it should be possible to kinetically tune not only pore dimensions3,4,5, 21,37,58 for structures such as MCM-41, but also wall and pore morphologies. The characterization of these modulated structures with periodic necking of the honeycomb pores in a structure like MCM41 requires careful TEM and organic guest absorption/desorption isotherm studies. It is particularly important to note that the symmetries determined by the Bragg peak positions of X-ray or even electron diffraction are best viewed as "average" symmetries and a careful analysis of the diffraction intensities, freeze fracture or cryomicroscopy, solid state NMR, and adsorption or desorption data are required in order to characterize the details of the cage, wall and pore structures. Because of the limited amount of scattering data available for materials with high void densities and large unit cells, diffraction modelling must be done on the continuum surface basis and not at the discrete atom level used for zeolite structure determination. The continuum surface model has been particularly useful in understanding structures and phase transformations of materials in which surfactants play a dominating role in determining the overall structural symmetry, and there is every reason to believe that it applies equally well to inorganic mesostructured composites. However, the chemist begins with precursors defined on a molecular basis so a key question is how to relate these surfaces to surfactant molecular structure. Fortunately several investigators have shown that it is possible to mathematically relate molecular size, charge, and shape to the more global surface curvature, bending energies and morphology5~

The classical and contemporary molecular description of surfactant organization

in amphiphilic liquid crystal arrays has been described in terms of the local effective surfactant packing parameter 59,60, g __ V/a J, where V is the total volume of the surfactant chains plus any co-solvent organic molecules between the chains, a o is the effective head group area at the micelle surface defined above, and 1 is the kinetic surfactant tail length or the curvature elastic-energy55. The interface surface bending energy can be written in terms of g, the actual surfactant packing

10 parameter adopted by the aggregating chains in the phase 5~

The counterion in this classical

model is not explicitly included. It is not immediately clear that this relatively simple molecular model can be used as a first approximation to explain and predict product structure and phase transitions for the inorganic mesostructures.

Our preliminary goal was a very pragmatic one, to determine whether the

molecular packing parameter model used in liquid crystal chemistry is useful in designing inorganic/organic composite mesostructures. In classical micelle chemistry, as the g value is increased above critical values, mesophase transitions occur. The expected mesophase sequence as a function of the packing parameter is : 43,50 Packing Parameter, g Mesophase Example These

1/3

1/2

1/2- 2/3

1

Cubic (Pm3n)

Hexagonal (p6m)

Cubic (Ia3d)

Lamellar

transitions

reflect

a g =

decrease in surface curvature from cubic (Pm3n)

through

lamellar.

For

V/a o i

i,,'j MCM-4 (Pm3n) ,!(P63/mmci!j (p6m) li ;:~

surfactants to associate in a spherical structure, the surface area occupied by the surfactants polar head group should be large.

~i(la3d) i~ i~ sBA-5 ~ SBA-4

!i (R3c)i-!

_~~

If the head groups are

base ~'cltalyzed ~ ' ~

permitted to pack tightly, on the other

I I

I

EinSteinbehavior| Cluster-like I i~ I

hand, the aggregation number will

r..-

Im3m? ti Vj SBA-1 ~SBA-7 i~ SBA-3 (Pm3n) ~(P63/mmc)~,.,~ (pSm~

increase, and rod or lamellar packing will be favored. The values of g (between 1/2 and 2/3) for cubic (Ia3d) phase

~,~

depend upon the volume fraction of surfactant chains 50.

I

For the systematic investigation of

the

formation

~

~ ~ i~

. ;Lamellarl L,i I

~

~,~

acid ~.* -------~ catalyzed i'i

~Li Su "~ be vi r

of mesoporous

materials, we selected a series of surfactants (Figures 7, 8), with and without organic additives, which favor a

Figure 6 Mesophase structures obtained by systematically varying V, ao and 1 of packing parameter.

11

Figure 7. Examples of surfactants used in this research III

IIII

I IIIII

Name

I

Structure Example

ALKYLAMMONIUM CnNR1R2R3 = CnH2n+ 1NR1R2R3 n = 10, 12-16, 19, 20, 22 R = H, Car~2m+ 1

cn3 cH3

m = 1, 2, 3

GEMINIAMMONIUM Cm-s-m CmH2m+IN(CH 3)2(CI-I2)sN(CH3)2CmH2m+I m - 12, 14, 16, 18, 20, 22 s - 2 -12

~

N+

| ~CH3 CH3

DIVALENTS URFACTANT

Cn-s-I CnH2n+1N(CH3)2(CH2)sN(CH3)3 n= 12, 13, 16, 18,20,22 s=2, 3, 6

~H3+

CH3 l+

. N . ~ , , ~ N ~ CH

HYDROXY- FUNCTIONALAMMONIUM CnH2n+IN(CH 3)m[(CH2)POH]3_m n = 16 m = O , 1,2,3 p = 0 , 1,2,3

[§ N~OH

~H3

CH3

BENZALKONIUM

CnH2n+1N(CH3)2(CH2)mC6H5 n = 14, 16, 18,20

m = 1,2,3

BI-CHAIN AMMONIUM CnH2n+ 1N(CH3)2CmH2m+I n = 12, 16, 18 m

cH3 = 2-6,

12, 16, 18

~

\cH3

cH3

~cH3

CnH2n+1[N(CH3)2]m(CI-I2)pSi(OCH3)3

l+

~i__OCH3

n = 14, 18

~H3

ORGANOSILANE

m=0, 1

p=0, 3

cn3

ZWlTrI~RIQN

CnH2n+IN+(CH3)2RXn = 12 - 20 RX-= sulfonate, carbonate,

OCH3

etc

~H~ (Crb.)3-SO3-

12 range of g values when used as structure directing agents to synthesize silica-based mesophases in different reaction conditions. An examination of a large number of surfactants synthesized in our laboratory, some of which are shown in Figure 8, coupled with a study of the effects of cosolvents, has confirmed that to a first and relatively good approximation the molecular packing parameter model can be used in a predictive way to generate structures analogous to those found in conventional liquid crystal chemistry (Figure 6) 36,37,21. For example, if one wishes to create a silica structure in which there is considerable wall curvature and possible

cage

structure,

R1

the

effective head group area can be

R2

R3

Product

R1

H CH 3 CH 3 SBA-3 (p6m) CH 3 CH 3 CH 3 SBA-3 (p6m) CH 3 CH 3 CiH 5 SBA-3 (p6m) I CH 3 CiH 5 CiH 5 SBA- 1 (Pm3n) R3 CiH 5 CiH 5 CiH 5 SBA-1 (Pm3n) Figure 8 Headgroup (ao) definition of mesostructure phase at pH values below the isoelectric point

modified by simple head group I_1. substitution (Figure 8) or with the C16H33 - - N ~ R 2 Cn_s_l surfactants (Figure 7). Cn_s_l surfactants

have

charge

and

density

high large

headgroups, and therefore favor globular micellar aggregates 47. Thus, in an acid media p6m hexagonal phase with symmetry Pm3n are obtained with methyldiethyl or triethyl substitution of the head group (Figure 8), but p6m structures result if dimethyl substitution is used with hydrogen, methyl or ethyl in the third position21,36, 37. Larger surfactants are required in basic media62, and have given unit cells as large as 180/~ on edge, with cell volumes of over 5.8 million/~3. BET data and preliminary modelling of the X-ray diffraction data of the silicate phase 63 suggest a cage structure for Pm3n, similar to those proposed for a conventional liquid crystal Pm3n phase by Sadoc 64 and for the much smaller

Figure 9 Proposed Pm3n structurc for mesosilicate (SBA- 1)

(13.4/~) high temperature unit cell of melanophlogite 65 (Figure 9). A second more subtle and variable way to fine tune the surfactant molecular shape and charge is by using multicharged oligomeric units. This has been done in our laboratory with bi(gemini), tri, tetra and polymeric cationic systems with considerable success. In this discussion,

13 we will briefly review the gemini and divalent surfactants (Figure 7). "Gemini surfactant", Cm.s_m, is a name assigned to a family of synthetic amphiphiles possessing, in sequence, a long hydrophobic chain, an ionic group, a spacer, a second ionic group, and another hydrophobic tail 66,67,68,69. The divalent quaternary ammonium surfactant, Cn_s_ l, may be considered as a end member of gemini surfactant or a highly charged large headgroup surfactant. These surfactants are particularly interesting from a fundamental point of view: their structure can be considerably modified by acting independently on the length and nature of either side chain and the spacer group. The relative positions and distances of headgroups of conventional mono(quaternary ammonium) surfactants are determined primarily by electrostatic interactions and also by the packing requirements of disordered alkyl chains. Formally, the Cm_s_m surfactants

may

be

considered

as

dimers

(double-headed)

of

the

two-chain

CmHEm+I(CHE)s/EN+(CH3)2 surfactants. For bis(dimethylalkyl-ammonium) surfactants, two quaternary ammonium head group species CmHEm+IN+(CH3)2 are chemically linked through an adjustable polymethylene spacer (CsHEs). The presence of the spacer makes it possible to fine tune the distance between the head groups and thereby control the effective head group size, a o , as a function of charge (for detail see reference 70). By this means we can change V/aol of a surfactant by adjusting its spacer length 69. We used the surfactants in this family as structure directing agents in order to synthesize a variety of silica-based mesophase products. Their structure directing behavior is similar to that in surfactant-water binary system 68,69,70,71 and give the structures expected for charge density matching. Small s (2 or 3) surfactants favor MCM-50, medium s (5 or 6) surfactants favor MCM41. C16.12_16 gives MCM-48 at both room temperature and high temperature (100~

while C12 -

12-12gives MCM-41 at room temperature. The latter observation illustrates the ability to fine tune with the individual tail lengths. Note that aqueous solutions of C 12-12-12 remain micellar over the entire range of composition and do not form lyotropic liquid crystal phases 68. This circumstance demonstrates the importance of the inorganic species in the cooperative structure directing mechanism for the concentration region in which the syntheses are carded out. If the tail of one of the two surfactant head groups is eliminated to give a Cn_s_ 1 molecular shape, the effective head group area relative to total hydrophobic tail volume (V) and length (1) is effectively doubled, greatly decreasing g, the packing parameter. This puts us back in the cage

14 side of the structural phases. While a phase with P63/mmc symmetry had not been previously reported for conventional liquid crystal phases72, we found that both liquid crystal surfactant and silicate phases with symmetry P63/mmc73,21 can be obtained with varying unit cell and cage sizes by using different chain lengths (n) of the form Cn_s_1 over a wide synthesis range (from cell parameter c = 77/~ for C12_3_1 to c = 108/~ for C20_3_1). SBA-2 (below SIP) and SBA-7 (base synthesis) with P63/mmc 3-d hexagonal symmetry are readily synthesized using divalent quaternary ammonium surfactants, Cn_s_1 (e.g., C 12-3-1, C 14-3-1, C16-2-1, C 16-3-1, C 16-6-1, C 18-3-1, C 18-6-1, C20-3-1) in both basic and acidic media. X-ray, AFM and TEM experimental results show that SBA-2 has 3-d hexagonal symmetry, space group P63/mmc (No. 194), and is derived from a hexagonal close packing of globular surfactant/silicate arrays 73. The crystal growth is plate-like and excellent for making thin films and membranes at either an air-water interface 74 or an oil-water interface75 with the sixfold axis normal to the sheet direction. As expected for this geometry, the unit cell parameter c/a ratio is about 1.62. After calcination, the large cage structured mesoporous silica framework remains. The structure directing agent in SBA-2 can be removed by calcination at high temperature (500-600~

This material is thermally stable up to 8000C. The

calcined SBA-2 has a N 2 BET surface area of 500-800 m2g 1. The N 2 adsorption-desorption isotherms is type IV with a H2 hysteresis for even small pore SBA-2 (< 25 A). Thus by systematically varying the surfactant molecular structure as prescribed by the simple packing parameter model, the structural phase space associated with conventional surfactants can be extended to silicate mesoporous structures (Figure 6). Undoubtedly other symmetries and fine details of the nature of the modulated versions of these structures will be forthcoming in the near future.

2.1

M I X E D SURFACTANTS21, 73

The effect of mixing unlike surfactants can be thought of as a simple average of two surfactant packing parameters. For example, a mixture of C16_12_16 and C16_3_1 is used in silicate mesophase synthesis. The products vary from MCM-48 to SBA-2 through MCM-41 as the fraction of C16_3_ 1 increases in the mixture. The mixture of C n-3-1 and CmTMA+ can result in the formation of good quality MCM-41. The MCM-41 easily gives five or more well defined XRD peaks. It is worth noting that high-

15 quality MCM-41 still can be obtained when n = 22, while single surfactant CmTMA § (m > 20) favors the lamellar phases and does not give MCM-41 at 100*C. The high-quality MCM-41 obtained using a mixture of surfactants is thermally stable and the calcined sample still has at least 5 XRD peaks. As for MCM-41 obtained from CnTMA § surfactant, small pore calcined materials are more hydrothermally stable than large pore ones. For example, MCM-41 calcined (at 500~

from

a C12TMA § synthesis system gives a good XRD pattern after 3 hrs heating in water at 100~ while a large pore (-- 55/~) material loses its structure under the same condition. High temperature calcination can increases the hydrothermal stability of these materials. The calcined (at 800~ large pore (~ 55 A) sample shows a clear MCM-41 XRD pattern (5 or more peaks) after 2 hrs heating in water at 100~ When a swelling agent (e.g., TMB) is introduced into this synthesis system, the product MCM-41 has a large unit-cell (dl00 > 60/~) and shows good XRD patterns (4 or more narrow peaks). Our synthesis results indicate the CnTMA § is a good, but not ideal structure directing agent for the formation of MCM-41, even though CnTMA § (e.g., C14TMABr ) itself can give a high quality lyotropic liquid crystal hexagonal phase with five or more sharp reflections 76. CnTMA + has only one charge per hydrophobic chain. More charges (from Cn.3_l, two charges per chain) in the surfactant headgroups apparent are more favorable for the formation of high quality MCM-41. We have found in general for all structural phases that we have investigated that it is possible to fine-tune the synthesis and the quality of the phase even further by using a mixed surfactant approach 73.

2.2 COSOLVENTS

Org an ic c o- s o 1v e n t s are particularly effective in controlling phase

ao

(

palisade ~ \ regi~ FI~-'~'

and interface geometry during the synthesis of both mesoporous inorganic solids 2,3,21,73 and lyotropic mesophases

headgroup

V/aol < 1/3

~ ~

P63/mmc (SBA-2) hydrophobic core

Figure 10 Solvation regions accessible to of surfactant-solvent binary systems77,78. molecules with different dielectric and polar The control comes from being able to properties. The example of g < 1/3 leads to hexagonal close packed geometries. "solvate" the interface head group,

16 palisade and hydrophobic regions associated with the organic surfactant arrays (Figure 10). When a hydrophobic, apolar solvent such as trimethylbenzene is added, it seeks the most hydrophobic region (Figure 11) which is at the tail end of the surfactant array and swells the micelle size. Both V and 1 are affected and the net result can be either a phase change and/or an increase in the effective pore or cage diameter. Thus, when trimethylbenzene (TMB) is added as a swelling agent relatively large pore size changes are observed. This approach has been used in large pore MCM-41 synthesis 2,3,37 and frequently, but not always, works for other mesostructure

phases. C16.3.1 gives SBA-2 (P63/mmc) with a = 62/~, c = 100/~ when TMB/TEOS = 1.1; without TMB, a cell with a = 54/~, c = 87/~ is obtained. In both of these examples, the result is as if a longer chain surfactant (increased 1) has been used to increase the pore size. However, phase changes in some instances are also induced. C16TMA+ favors MCM-41 over a wide range of reactant compositions. At moderately high pH values, if TMB is added, the MCM-41 is replaced by a lamellar phase suggesting that the increased surfactant tail volume is more important. If this lamellar phase sample is heated before silica condensation, it reverts back to the hexagonal trimethylbenzene

configuration 24.

1 increases

A suitable polar additive is able to enter the hydrophobic-hydrophilic palisade region (first

V/aol < 1/3 P63/mmc large cell (SBA-2) Figure 11 Addition of hydrophobic, apolar molecule to surfactant array.

few carbon atoms) of the micelle, with a relative increase in the volume of the hydrophobic core to form surfactant molecule aggregates with a lower curvature surfaces, e.g.,from sphere to rod. Thus when t-amyl alcohol, a polar additive, is added into the synthesis mixture at basic pH, the SBA2 product is replaced by MCM-41 (Figure 12). In the acid synthesis one can make SBA-1 (Pm3n) using C16H33N+(C2Hs) 3 as template without t-amyl alcohol, but SBA-3 (p6m) if t-amyl alcohol is used 21. In our experience the effect of addition of polar solvents is quite predictable and one can very effectively use this to generated desired phases.

~ ~ - ~

t-amyl alcohol increases

i73

J ~

~

Hexagonal (MCM-41)

Figure 12 Phase change from P63/mmc to MCM-41 induced by addition of polar alcohol as solvent

17

2.3 HYDROXY-FUNCTIONALIZED SURFACTANTS The hydroxyl group in the functional surfactant, CnH2n+IN+(CH3)2(CH2)mOH, decreases the hydrophobicity of the headgroup and the headgroup charge is more shielded by water of solvation79 (or silicate or other anions in solution), thus decreasing the affective cationic headgroup area a o. It therefore plays an important role in the entropic and enthalpic contributions of water organization to structure direction. The hydroxyl headgroup surfactants favor formation of mesophases with low surface curvatures such as p6m or lamellar due to the smaller effective a o 37. The product is a lamellar phase when C16H33N+(CH3)2(CH2)2OH is used, while a similar structured surfactant with a smaller headgroup, C16H33N+(CH3)2C2H5, gave only a relatively high surface curvature mesophase, MCM-41. A hydroxyl group in the hydrocarbon chain of the surfactant, e.g., g-substituted, C14H29CH(OH)CH2N+(CH3)3, also has a small effective headgroup surface area. A highly ordered lamellar silicate (to sixth order in Bragg reflections) is obtained by using C14H29CH(OH)CH2N+(CH3)3. In addition, high-angle diffraction peaks are observed that are characteristic of the hydrocarbon chain packing within organized surfactant layer structuresS~

82. The use of zwitterionic surfactants 36 and other functionalized head groups is a

promising area of investigation. 3. INORGANIC CONDENSATION The processes that drive the co-assembly of organic and inorganic units into a bicontinuous composite with spatially distinct organic and inorganic regions of nanostructure are strongly correlated, which

a priori

makes the separation of the various contributing factors difficult to

resolve. Certainly, one would expect that the process of mesophase organization would be strongly coupled to the time-dependent polymerization kinetics of silicate species at the inorganic-organic interface 24. In order to separate the effects of silica polymerization from the thermodynamics of mesophase self-assembly, we have used low temperatures and careful pH control (within 0.1 pH units) to control silica polymerization relative to the overall mesophase assembly. This approach has been used to show that in the absence of inorganic polymerization, these mesophases have liquid crystalline properties, similar to those of conventional aqueous lyotropic liquid crystal systems 24. In order to maintain these liquid crystal-like properties and optimize long range composite ordering during polymerization of the inorganic species, the inorganic and organic domains must

18 be able to reorganize on the same kinetic time scale into mutually compatible configurations. Khushalani et a183 have recently shown that at high temperatures (to 150 ~

and high pH values

(base synthesis mother liquor), one can very nicely get restructuring of the silica phase with a systematic increase in pore size. Under these conditions there is considerable organic thermal disorder, but the kinetic molecular volumes are still effective in generating monodispersed pores. Our approach 21 to optimizing long range ordering and structure has been to maximize the ordering influence of the organic surfactants during the initial polymerization of the silica walls, and then to reduce OI (the organic-inorganic interface interaction) so that the organic and inorganic self assembly are less strongly coupled I

(Figure 13). We chose to do this by 1) using low temperatures to minimize

0.5 to 2

organic disorder and short reaction times

I

hours Iwater 1 powder / o~ | ~product

to kinetically create only partially condensed silica frameworks that can structurally

follow

the

organic

organization and minimize interphase frustration 37, 2) "annealing" the air-dried product at room temperature to further

filter,

as-made sample

air .dry

25~

optimize long range order, and then 3) carrying out the silica polymerization in deionized water (pH -7). The latter greatly reduces the silica charge relative to what it would have in the mother liquor so that

add to H20 at pH - 7

............................. I~ ~7 days, 1 0 0 o c

I =o,., I I -"'" ' a ~ , , , , ~

final product

the organic-inorganic (OI) interactions are correspondingly reduced. This and the

Figure 13 Low temperature, low pH, synthesis of large pore mesostructured phases.

reduced solubility at that pH makes it possible to retain the templating introduced at low temperatures by the more organized organic to the partially polymerized silica. For example, when silicon alkoxides are used as starting reagents at a surfactant-to-silicon ratio o f - 0.1, pH -12, at room temperature or lower, polymerization of the silica begins and a precipitate rapidly forms 37. Silica polymerization of this partly condensed phase is interrupted by using short reaction times (0.5 to 2 hours depending on pH) and then "ripening" the filtered, air dried, solid product at room temperature for 6-10 hours. The solubility of amorphous silica

19

minimizes in water at approximately a pH of 7 to 8, and is more than an order of magnitude less than that at the normal mother liquor pH used in MCM-41 synthesis 84. The X-ray diffraction patterns for a large unit cell (a= 77A calcined) for these MCM-41 phases show seven to eight peaks (Figure 14) and retain their structure on calcination with about a 3% cell shrinkage. N 2 BET measurements reveal that this material has a BJH pore size of 60/~,

Surfactants

5.7 105

22-3-1 + C18TMA+ 100C a = 79.6/~

a pore volume of 1.6 cma/g and a surface O

area of 1086 mE/g. An important feature is that the apparent wall thickness based on

FWHM 0.2 2 0

cps

the absorption isotherm and X-ray data is 17 A,, which is substantially greater than

0

l_2 v,-I

that obtained from conventional MCM-41 synthesis (--8-10/~). This is not surprising in view of the reduced charge associated

01

....

I,,,

2

,k~l~,,~u

3

l-,.,

4

. ! ,_=.~;-~_:--t..

5

6

,

7

8

with the silica phase at the lower pH. Three well-distinguished regions of the adsorption

isotherm

are

noticed:

1.2 106 calcined at 500 C a = 77.0/~

monolayer-multilayer adsorption, capillary condensation, and multilayer adsorption on the outer surface. In contrast to N 2

cps

adsorption results 85,86 of MCM-41 with pore size less than 40/~, a clear type H1 hysteresis

loop

in the

adsorption-

desorption isotherm is observed and the capillary condensation occurs at high relative pressure, consistent with the large pore 87.

0 , ~ .... - : ! . - - . I . . . . 1 2 3 4 5 6 7 8 Figure 14 a) X-ray diffraction pattern after treatment ol room temperature prepared sample at 100 C with wate] at pH 7 and b) after calcination

In all cases that we have examined the treatment improves the structural ordering, however in some cases there is no significant expansion of the unit cell. In the above MCM-41 example, using the indicated mixed surfactants the as made cell has a cell dimension of 54.5/~. Treatment at 2 weeks at 100 ~ in distilled water, pH = 7, gave a cell of 78.2/~, and two additional weeks of treatment gave a cell of 79.6/L However, no significant expansion of the unit cell is observed for MCM-41 and MCM-48 containing a single CnTMA+ molecule, although seven to eight peak

20 diffraction patterns are obtained for the MCM-41 and about twenty peak patterns are generated with MCM-48. The kinetic matching of inorganic and organic ordering during assembly and silica polymerization is critical to the morphological, structural and property design of mesophase materials. 4. INTERFACE CONSIDERATIONS One of the early possible models suggested for mesostructured materials synthesis using surfactants was that of coating preassembled organic arrays with the inorganic phase and then assembling these coated organic arrays into a 3-d periodic structure 88,89. There are features of this model that make it attractive, it gives a direct explanation for the analogous symmetries of the silicate structures to those of liquid crystal chemistry, and it is consistent with what at that time was the paradigm for biomimetic synthesis: first create an organized organic array, and then condense an inorganic phase on the preorganized organic surface 28. At this time, there is convincing evidence 90,91,92 that while stable organized organic arrays are an important part of inorganic nucleation and phase formation in biomineralization, total control by such an array is an extreme condition that is never completely realized. Complete structural phase changes can in fact be induced by soluble proteins 91. Generally, mutually induced structural modifications of the organic and inorganic phases are required to create the higher-order complex microstructures. A more general biomimetic approach must take into account the dynamic balance of solvation, soluble proteins and other soluble organics, organic array assembly, inorganic polymerization and the corresponding interface chemistries. Nevertheless, using pre-organized organics to control morphology and nucleation is a potentially powerful approach to composite materials synthesis, particularly in terms of macroscale shaping, as a template that can be created with the required acid-base or molecular structure characteristics, and as a liquid support phase for bulk processing or synthesis. The concept of using an organized organic array as a template is a statement that the most important free energy and/or kinetic contribution to biphase composite formation is the organization of the organic array. Inorganic deposition and subsequent polymerization do not significantly perturb that array morphology. Several possible ways to approach that goal are 1) to strengthen the organic intraarray coupling by cross-linking; 2) to stabilize the organic array by interfacing it to an inorganic substrate and 3) to decrease the organic-inorganic (OI) interface interactions relative to the organicorganic (O-O) interactions (Figure 2). As far as 3) is concerned, using hydrogen bonding at the interface (Figure 4) rather than

21 ionic or covalent interface interactions (Figure 3) is obviously a step in the right direction. A neutral synthesis route using uncharged (dodecyl amine) or nonionic surfactants (polyethylene oxide) has been explored by Pinnavaia and co-workers93,94 in the near neutral pH range where silica charges on oligomeric silica species (pK a ~ 6 -7) are greatly reduced from that expected at high pH. This is an especially intriguing study since the question of how biosilicates such as diatoms are so exquisitely assembled in nature in this pH region remains unanswered. With neutral surfactants, the primary forces that drive the self assembly of the composite are hydrogen bonding, van der Waals and dipole interactions. Monodispersed porous structures have been obtained in these investigations; however long-range order of the pores is lacking. In this pH regime above the SIP (silica isoelectric point), the silica species are still slightly negatively charged with a relatively high condensation rate. Item 2) above can be achieved by covalent linking to the inorganic substrate 95, so that the patterning of the organic array and the orientation of the organic molecules are defined by the connecting sites to the substrate. In this case the organic array is strongly bound so that its integrity is preserved upon addition of other inorganic phases. Alternatively, cationic organic amphiphiles can be bound to the substrate either by ionic (e.g. mica) or charge

image

and

van

der

Waals

bonding

(graphite) 96,1~ In this case one generates organized, periodic arrays of hemicylinders of CTAB (cetyltrimethylammonium bromide) on graphite and cylinders on mica, in both cases with the long axis of the CTAB organic array parallel to the substrate

Figure 15 Hemicylinders of CTAB formed on graphite. Measurements were made using continuous flow AFM cell with surfactant concentration near CMC 1 concentrations 91.

surface 96 (Figure 15). The advantage of this approach over the covalent attachment of surfactants is clear in that the organic phase is given more freedom and is able to organize in a variety of known liquid crystal geometries. Can these organized organic arrays be used as templates for silica thin film mesostructure nucleation? The answer to this question is yes 97,98, but in the published examples to date only when the mesostructure is formed at pH values below the SIP 36,37 (Figure 4) with the weak OI hydrogen bonding and indirect structure direction. Like the surfactant, silica films prepared on mica have their pore directions oriented parallel to the film surface. Syntheses carried out at high pH values2, 3 are less likely to be successful because of the strong OI interface interactions that can

22 be expected to disrupt the organic array assembly. The divalent surfactants used to create the P63/mmc structures form more stable organic arrays, and are much easier to convert into mesostructured films, even at basic pH values 74. The same situation applies to oriented thin silica films prepared at the air-water interface. Beautiful flexible sheets of periodic mesostructured films with p6m or subgroup symmetry (SBA-3) and the channels parallel to the film plane are formed with silica chemistry carried out at pH values below the SIP 99, but similar thin films of the MCM-41 phase have not so far been successfully made at basic pH values. Similarly, Attard and co-workers attempts to use liquid crystal structural phases formed in concentrated solutions of surfactants as templates required low pH values below SIP 100. In these examples the mechanism does not necessarily involve the simple coating of the organic array, since the hydrolysis of the silicon alkoxide used as a precursor may change or initially disrupt the organic phase structure. The important point however, is that with a weakly interacting inorganic-organic interface, the thermodynamic and kinetic factors that gave the original organic assembly its structural properties are still dominant and can control the composite organization. It should be emphasized that the acid(below SIP) and base synthesized silica mesophases have little in common other than sometimes the same space group symmetry. They do not have the same composition since mesophase samples synthesized below the silica isoelectric point require a counter anion, generally a halide anion, for each surfactant molecule that is present. Terminal Si-Ogroups are protonated so that the bulk compositions of M41S and acid prepared materials (APM) made with the same surfactants are completely different in hydrogen and halide ion content. The ion-pair surfactants of the APM materials are readily removed by washing with distilled water/ethanol at --70 ~

since the wall charge is neutral or slightly positive. Removal of

surfactant from M41S samples requires ion exchange by refluxing with acidic ethanol because of the negatively charged terminal oxygen atoms. Absorption and desorption properties similarly differ substantially. The ultimate periodic symmetry is determined in both cases by the nanophase surfactant packing requirements, so that similar space group and lattice symmetries may be observed by x-ray diffraction transmission electron micrographs. However the Bragg peaks of the two phases for a given surfactant have clearly different diffraction intensities, indicating different pore and wall structure. As pointed out by Brinker and Scherer 101, silica hydrolysis below SIP results in Huggins or chain like polymerization while M41S silica polymerization conditions lead to Einstein or cluster

23 like configurations with extensive cross-linking so that different silica wall structures are expected for the acid and base synthesized structures. The difference in wall and pore structure is evident in BET absorption isotherm measurements that show that APM exhibit a very different sorption behavior from M41S samples with a step in the isotherm at appreciably lower P/P0 values (p, pressure) than samples synthesized from alkaline media with a similar lattice spacing. These data and diffraction results show that the acid silica walls are effectively thicker than those of the corresponding M41S phases prepared at pH values above 10. Nevertheless, BET surface areas calculated for such samples can be a factor of two higher, indicating the presence of micropores or highly ruffled pore surfaces. Even taking into account the limitations of BET analysis for such materials, the difference in the values obtained indicates a major difference in the pore and wall structure of APM and MCM-41 type materials.

5. MACROSCALE STRUCTURES WITH PERIODIC MESOPORES75,102 Organic-inorganic hybrids are ubiquitous in nature as biominerals and inherently offer many opportunities for the creation of new materials with unusual features. As diphasic structures, they can be made shaped and multifunctional. The key to the integration of the organic and inorganic components is to use low temperature chemistry to control the kinetics of assembly of the organic and inorganic components, and an integral part of that process lies at the interface between the spatially distinct organic and inorganic regions. The extent to which the organic and inorganic domains have properties and structure that are characteristic of the corresponding bulk phases depends on the strength of the interface inorganic-organic (IO) interaction. The significance of this in hybrid organic-inorganic materials design has been recognized by Clement 103, who divides hybrid organic-inorganic materials into two distinct classes. In Class I, only weak bonds (e.g., hydrogen, van der Waals) give cohesion to the whole structure. In Class II the organic and inorganic components are linked together by strong chemical bonds (e.g., covalent or ionocovalent bonds). This differentiation is fundamental to the property design of organic-inorganic hybrid materials for a wide range of applications. The differences in the properties of the mesostmctured silica phases that are generated by base synthesis, an example of Class II (Figure 3) organic/hybrid materials, from the Class I silica mesostructures made at pH values below the aqueous silica isoelectric point (Figure 4) support this view. As another example of how weak OI interactions can be applied to materials synthesis, we have used the combination of long-range oil-in-water emulsion and oil-water interface physics with shorter-range molecular assembly of silica and surfactants at the emulsion interface to create

24 ordered composite mesostructured phases that are also macroscopically structured and shaped75,102. Microemulsions and emulsions occupy a special place in the hierarchy of structures, in that their formation involves long-range forces with an energy of assembly, including shape fluctuations and interaggregate interactions, about the same as the thermal energy kT 104. Hydrodynamic, long-range forces can therefore be used to define emulsion morphology and the configuration of the emulsion oil-water interface. With self-assembly energies approaching kT, emulsions are often close to the limit of stability. In order to stabilize the emulsion phase, current commercial practice is to use surfactants. Thus, oil-in-water emulsions are stabilized by short range (10 -7 m), and relatively weak van-der Waals interactions between the hydrophobic tails of amphiphilic surfactants and the emulsion organic phase. This combination provides an ideal starting place for periodic mesostructured inorganic phase synthesis. If an oil-in-water interface is used as an inorganic growth medium with the growth direction into the aqueous phase, morphological control of the resulting inorganic/organic composite assembly can be achieved at micron and longer length scales. The morphology of the preorganized organic liquid phase is preserved during the periodic mesopore synthesis by working at acid pH below the SIP, making use of the weak organic-inorganic interactions shown in Figure 4 (S§

+ structure directing during synthesis).

Mesoporous silica fibers, diatom-like hollow

spheres, and thin films are some of the morphologies that have been synthesized using conventional emulsion hydrodynamics, p6m, P63/mmc and Pm3n wall structures can be made. Surface areas of the calcined products are greater than 1000 m2/gm with narrow pore distributions. The results are of interest with regard to packaging, perhaps in slow-release applications. Hydrodynamics and emulsion technology are highly advanced commercially and the connection of this to inorganic/organic hybrid materials synthesis presents new process possibilities for hierarchically structured composite phases. 6. S U M M A R Y

As indicated in the Introduction, the discovery of periodic mesoporous structures is a major advance in composite organic-inorganic materials synthesis. In the short presentation given here, much has been omitted concerning our growing knowledge in this very rapidly expanding field, but hopefully it is clear that the organic and inorganic phases can be synergistically integrated in a pre-designed fashion. It also seems possible to carry out this co-assembly of organic and inorganic species using fluid mechanics to define macroscale shapes. Of particular interest are the

25 increasingly sophisticated studies of the dynamic processes and organic-inorganic interfaces that are present in biomineralization. These investigations seem to be converging with those being made on synthetic hybrid organic-inorganic materials, and the result could be an even more rapid evolution of technological applications. The confluence of these two trains of thought is certain to enhance an already exciting era in materials synthesis. REFERENCES

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26 24. Firouzi, A.; Stucky, G.D.; Chmelka, B.F. Synthesis of Microporous Materials, M.L. Occelli and H. Kessler, Eds., Marcel Dekker (in press). 25. Antonelli, D.M.; Ying, J.Y. Angew. Chem. Int. Ed. Engl. 1995, 34, 2014. 26. Weiner, S. Crit. Rev. Biochem., 1986, 20, 365. 27. Weiner, S.; Traub, W. Phil. Trans. Royal Society of London, 1984, B304, 428. 28. Mann, S.; Archibald, D.D.; Didymus, J.M.; Douglas, T.; Heywood, B.R.; Meldrum, F.C.; Reeves, N.J. Science, 1993, 261, 1286. 29. Mann, S. Nature, 1993, 365, 499. 30. Archibald, D.D.; Mann, S. Nature, 1993, 364, 430. 31. Heywood, B.R.; Mann, S. Adv. Materials, 1994, 6, 9. 32. Baral, S.; Schoen, P. Chem. Mater, 1993, 5, 145. 33. Archibald, D. D.; Mann, S. Nature, 1993, 364, 430. 34. Bunker, B.C.; Rieke, P.C.; Tarasevich, B.J.; Campbell, A.A.; Fryxell, G.E.; Graff, G.L; Song, L.; Liu, J.; Virden, J.W.; McVay, G.L. Science, 1994, 264, 48. 35. This notation refers to the solution species used in the synthesis, S = surfactant or surfactant precursor, X = the acid anion which is usually C1-or Br- in syntheses carried out below the aqueous silica isoelectric point (SIP) (pH ~2), and I§ or I- is used to designate the charge of the solution inorganic species that are present. Neutral silica species are also present, with a concentration that varies with pH; however in our experience the condensation rate and the quality of the mesostructure that is formed varies inversely with the neutral silica concentration in the pH region immediately below the SIP. 36. Huo, Q.; Margolese, D.I.; Ciesla, U.; Feng, P.; Gier, T.E.; Sieger, P.; Leon, R.; Petroff, P.M.; Schtith, F; Stucky, G.D. Nature, 1994, 368, 317. 37. Huo, Q.; Margolese, D.I.; Ciesla, U.; Demuth, D.G; Feng, P.; Gier, T.E.; Sieger, P.; Chmelka, B.F; Schtith, F; Stucky, G.D. Chem. Materials, 1994, 6, 1176. 38. Yange, H.; Kupermann, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. Nature, 1996, 379, 703. 39. Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G.A. Nature, 1996, 381,589. 40. Attard, G.S.; Glyde, J.C.; Goltner, C.G. Nature,1995, 378, 366. 41. Tiddy, G.J.T. Physics Reports, 1980, 57, 1 42. Luzzati, V.; Vargas, R.; Mariani, P.; Gulik, A.; Delacroix, H. J. Mol. Biol., 1993, 229, 540. 43. Henriksson, U.; Blackmore, E.S.; Tiddy, G.J.T.; Soderman, O. J. Phys. Chem., 1992, 96, 3894. 44. Husson, F.; Mustacchi, H.; Luzzati, V. Acta Crystallogr., 1960, 13, 668. 45. Hagslatt, H; Soderman, O; Jonsson, B. Liquid Crystals, 1994, 17, 157. 46. Jahns, E.; Finkelmann, H. Colloid Polymer Sci., 1987, 265, 304. 47. Hagslatt, H.; Soderman, O.; Jonsson, B. Langmuir, 1994, 10, 2177. 48. Kratzat, K.; Finkemann, H. Colloid & Polymer Science, 1994, 272, 400. 49. Gulik, A.; Delacroix, H.; Kirschner, G.; Luzzati, V. J. Phys. H, 1995, 5, 445. 50. Hyde, S.T. Pure and Applied Chemistry, 1992, 64, 1617. 51. Hyde, S.T.J. Phys. Chem., 1989, 93, 1458. 52. Monnier, A.; Schtith, F; Huo, Q.; Kumar, D; Margolese, DT; Maxwell, R.S.; Stucky, G.D.; Krishnamurty, M.; Petroff, P.; Firouzi, A.; Janicke, M.; Chmelka, B.F. Science, 1993, 261, 1299. 53. Stucky, G.D.; Monnier, A.; Schtith, F.; Huo, Q.; Margolese, DT; Kumar, D.; Krishnamurty, M.; Petroff, P.; Firouzi,A.; Janicke, M.; Chmelka, B.F. Mol. Cryst. Liq.Cryst., 1994, 240, 187. 54. Alfredsson, V.; Anderson, M. W. Chem. Mater, 1996, 8, 1141.

27 55. Gruner, S.M.J. Phys. Chem., 1989, 93, 7562. 56. Landry, C.C.; Stucky, G.D., submitted for publication. 57. Garc6s, J.M. Adv. Mater., 1996, 8, 434. 58. Khushalani, D.; Kuperman, A.; Ozin, G.A.; Tanaka, K.; Garces, J.; Olken, M.M.; Coombs, N. Adv. Mater., 1995, 7, 1156. 59. Israelachvili, J.N.; Mitchell, D.J.; Ninham, B.W. J. Chem. Soc., Faraday 2, 1976, 72, 1525. 60. Israelachvili, J.N.; Mitchell, D.J.; Ninham, B.W. Biochim. Biophys. Acta, 1977, 470, 185. 61. Fogden, A.; Hyde, S.T.; Lundberg, G. J. Chem. Soc., Faraday Trans., 1991, 87, 949. 62. Huo,Q.; Stucky, G.D. Materials Research Society Meeting, San Francisco, April 1996. 63. Preliminary modelling was done in collaboration with MSI and Biosym, John Newsam and Clive Freeman. 64. Sadoc, J.F., et al., J. Phys. France, 1988, 49, 52. 65. Gies, H. Zeit. Kristallogr., 1983, 164, 247. 66. Alami, E.; Beinert, G.; Marie, P.; Zana, R. Langmuir,1993, 9, 1465. 67. Menger, F~M.; Littau, C.A.J. Am. Chem. Soc., 1993, 115, 10083. 68. Alami, E.; Levy, H.; Zana, R. Langmuir, 1993, 9, 940. 69. Zana, R.; Talmon, Y. Nature, 1993, 362, 228. 70. Danino, D.; Talmon, Y.; Zana, R. Langmuir, 1995, 11, 1448. 71. Zana, R.; Benrraou, M.; Rueff, R. Langmuir, 1991, 7, 1072. 72. During the writing of this manuscript an article (M. Clerc_"A New Symmetry for the Packing of Amphiphilic Direct Micelles") that describes Pm 3 n and P63/mmc phases for the C12EOs/water binary system appeared in J. Phys. H France, 1996, 6, 961. 73.Huo, Q." Leon, R.; Petroff, P.M.; Stucky, G.D. Science, 1995, 268, 1324. 74.Tolbert, S.; Huo, Q.; Stucky, G.D., submitted for publication. 75.Schacht, SI,~ Huo, Q." Voigt-Martin, I.G.; Stucky, G.D." Schtith, F. Science, August 199~ 76.McGrath, K.M. Langmuir, 1995, 11, 1835. 77.DeLisi, R.; Milioto, S. Chem. Soc. Rev., 1994, 23, 67. 78.Rosen, M.J. Surfactants and Interfacial Phenomena (Wiley-Interscience,New York, 1989). 79.Zana, R.; Levy, H. J. Colloid lnterface Sci. 1995, 170, 128. 80. Harlos, K. Biochim. Biophys. Acta, 1978, 511, 348. 81.Janiak, M.J.; Small, D.M.; Shipley, G.G., Biochemistry, 1976, 15, 4575. 82.Watts, A.; Harlos, K.; Marsh, D., Biochim. Biophys. Acta, 1981, 645, 91. 83. Khushalani, D.; Kuperman, A; Ozin, G.A.; Tanaka, K.; Garces, J.; Olken, M.M.; Coombs, N. Adv. Mater. 1995,7,842. 84. Iler, R.K., "The Chemistry of Silica", John Wiley and Sons, NY, 1979, p 42. 85. Schmidt,R.; Hansen,E.W.; Stocker,M.; Akporiaye,D.; Ellestad,O.H., J. Am. Chem. Soc., 1995, 117, 4049. 86.Branton,P.J.; Hall,P.G.; Sing,K.S.W.; Reichert,H.; Schuth,F.; Unger,K.K., J. Chem. Soc. Faraday Trans., 1994, 90, 2965. 87. Llewellyn,P.L.; Grillet,Y.; Schuth,F.; Reichert,H.; Unger,K.K., Microporous Mater., 1994, 3, 345. 88. Chen, C.; Li, H.; Davis, M.E. Microporous Materials, 1993, 2, 17. 89. Chen, C.; Burkett, S.L.; Li, H.; Davis, M.E. Microporous Materials, 1993, 2, 27. 90.Belcher, A.M.; Wu, X.H.; Christensen, R.J.; Hansma, P.K.; Stucky, G.D.; Morse, D.E. Nature, 1996, 381, 56. 91. Zaremba, C.M.; Belcher, A.M.; Fritz, M.; Li, Y.; Mann, S.; Hansma, P.K.; Morse, D.E.;

28 Speck, J.S.; Stucky, G.D. Chem. Mater., 1996, 8, 679. 92. Falini, G. et al., Science, 1996, 271, 67. 93. Tanev, P.T.; Pinnavaia, T.J. Science, 1995, 267, 865. 94. Bagshaw, S.A.; Prouzet, E.; Pinnavia, T.J. Science, 1995, 269, 1242. 95. Bunker, B.C.; Rieke, P.C.; Tarasevich, B.J.; Campbell, A.A.; Fryxell, G.E.; Graff, G.L.; Song, L.; Liu, J.; Virden, J.W.; McVay, G.L. Science, 1994, 48. 96. Manne, S.; Cleveland, J.P.; Gaub, H.E.; Stucky, G.D.; Hansma, P.K. Langmuir, 1994, 10, 4409. 97. Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G.A. Nature, 1996, 379, 703. 98. Aksay, I.A., et al., Materials Research Society Meeting; Boston, Massachusetts, November 1994; European Science Foundation Symposium on Biomineralization; Granada, Spain, September 1995; International Symposium on Synergistic Synthesis of Inorganic Materials, Tagungsst~itte Schloss Ringberg der Max-Planck-Gesellschaft, March 1996. 99. Yang, H.; Coombs, N.; Sokolov, I .; Ozin, G.A. Nature, 1996, 381,589. 100. Attard, G.S.; Glyde, J.C.; G61tner, C.G. Nature, 1995, 366. 101. Brinker, C.J.; Scherer, G.W.J. Non-cryst. Solids, 1985, 70, 301. 102. Schacht, S., Diploma Thesis, Mainz, 1995. 103. Sanchez, C.; Ribot, F. New. J. Chem., 1994, 18, 1007. 104. Israelachvili, J. Colloids and Surfaces A: Physicochemical and Engineering Aspects 1994, 91, 1. 105. Manne, Srinivas; Gaub, Hermann E. Science,1995, 270, 1480.

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.

29

Incorporation and Stability of Trivalent Cations in Mesoporou_s Silicas Prepared using Primary Amines as Surfactant S. Gontier and A. Tuel Institut de Recherches sur la Catalyse. C.N.R.S. 2, av. A. Einstein 69626 Villeurbanne Cedex France

Abstract A series of trivalent metal (A13+, Ga 3+, Fe 3+, B 3+) containing mesoporous silicas (Me-MS) have been synthesized using hexadecylamine as organic templating surfactant. All materials possess mesopores of about 37/~ diameter and surface areas above 900 m2/g. A solvent extraction has been used to remove the template from the solids. As compared to a conventional thermal treatment, this procedure preserves the mesopore structure and the coordination of the cations. Extracted samples are thermally stable and can be calcined in air at high temperature without observing changes in the cation coordination.

1. I N T R O D U C T I O N The family of silica-alumina based mesoporous molecular sieves M41S has received considerable interest over the last years because of their praticularly attractive characteristics like very high surface areas and regular mesopores whose diameter can be varied between 20 and 100/~ [1]. MCM-41, the well-known hexagonal member of this family is usually prepared with cetyltrimethylammonium (CTMA) cations and possesses mesopores in the 35-40/~ range [2]. As for zeolites, several trivalent cations can be incorporated in MCM-41, whose composition can be varied within a quite large domain of Si/Me ratios. Recently, Tanev et al. [3] have reported the synthesis of Hexagonal Mesoporous Silicas (HMS) by a neutral templating route using primary alkylamines in C 8 to C18 as surfactant. These materials are very similar to pure silica MCM-41, but differ by the arrangement of the mesopores. The neutral templating route offers several advantages with respect to the conventional preparation using ammonium cations. In particular, the synthesis is performed at room temperature and the template can be removed by ethanol extraction. The removal of organics by solvent extraction is not only interesting from an environmental point of view but it also preserves the mesoporosity of the samples, which is not always the case upon thermal treatment at high temperature

[3].

30 In the present paper, we report the synthesis of various trivalent cations (A13 +, Ga 3+, Fe 3+ and B3+) containing mesoporous silicas using hexadecylamine as surfactant. Template-free materials were obtained either by calcination in air at 650~ or by a solvent extraction. The influence of both procedures on the preservation of the mesoporosity and on the nature of the trivalent metal coordination is discussed.

2. E X P E R I M E N T A L In a typical synthesis, a solution containing 1 tool of tetraeth)~l' orthosilicate (TEOS), 6.5 mol of ethanol and 1 tool of isopropyl alcohol is mixed to a second solution containing 0.3 tool of hexadecylamine in 36 mol H20. Depending on its nature, the trivalent metal precursor is introduced either in the first solution (aluminium isopropoxide, tributyl borate) or with the amine (aluminium nitrate, gallium nitrate, iron nitrate or boric acid). The gel is vigorously mixed at room temperature for about 30 min and aged under static conditions for 12 h. The solids are then recovered, washed abundantly with distilled water and air-dried. For the solvent extraction of the organics, 1 g of dried solid is dispersed in 100 ml ethanol containing 1 g NaC1 and the suspension is refluxed for 1 h. The template-free samples are then dried at 80~ for 12 h. Samples are characterized using X-ray diffraction (CGR Theta 60 diffractometer using Cu Ka radiation), N 2 adsorption/desorption (Catasorb apparatus) and Solid State NMR (Bruker MSL 300). EPR spectra are obtained on a Varian E9 spectrometer. Chemical analysis are performed by atomic absorption after solubilization of the samples in HF-HC1 solutions.

3. RESULTS AND DISCUSSION The chemical composition of the various samples is given in Table 1. For all samples (except B-containing materials) the amount of metal in the solid corresponds approximately to that introduced in the synthesis gel. As a general trend, the yield in solid decreases with the metal content. The case of boron is interesting as it is the single example where the boron content is lower in the solid than in the gel, particularly for samples prepared with low Si/B ratios. Moreover, it is possible to prepare samples with low Si/B ratios in relatively high yields. Comparison of samples 1 and 2 or 14 and 15 shows that the nature of the precursor does not greatly influences the metal content in the final product. The maximum metal incorporation-is approx, the same for A1, Ga and Fe and corresponds to Si/Me ratios of about 10 in the solid phase. When syntheses are performed with lower ratios in the gel, the yield in solid is usually very low and spectroscopic characterization shows that as-synthesized samples contain non-framework octahedrally coordinated species.

31 Table 1 Chemical composition of the different samples Si/Me No

Sample

Gel

Product

Yield (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

A1-MS(100) A1-MS(100) a A1-MS(50) A1-MS(20) A1-MS(15) Ga-MS(100) Ga-MS(50) Ga-MS(20) Ga-MS(15) Fe-MS(100) Fe-MS(50) Fe-MS(20) Fe-MS(15) B-MS(100)_ B-MS(100) ~ B-MS(50) B-MS(15) B-MS(3) B-MS(l)

100 100 50 20 15 100 50 20 15 100 50 20 15 100 100 50 15 3 1

88 97 38 21 8 86 48 14 13 85 55 18 15 150 166 95 27 17 7

86 89 85 63 50 92 83 56 30 89 81 63 48 86 88 82 97 96 81

aThe sample was prepared with aluminium ethoxide, bThe sample was prepared using boric acid. As-synthesized samples are characterized by a single broad X-ray line around 2.2 ~ (20), as already reported for HMS [4]. After calcination in air at 650 C, all samples exhibit high surface areas but Horvath-Kawazoe pore size distributions show that the mesopore system partially collapses upon thermal treatment, particularly for samples containing high metal contents (Table 2). This was also confirmed by X-ray diffraction. Solid state NMR characterization of as-synthesized samples shows that A13+, Ga 3 + and B 3 + are tetrahedrally coordinated, at least for samples with Si/Me > 10. For low A1 contents, spectra of both as-synthesized and calcined samples show a single line at-about 52 ppm, as already reported for MCM-41 [5] (Fig. 1). For high A1 contents, the spectra of calcined samples show an additional peak around 0 ppm attributed to octahedrally coordinated A1 species resulting from a partial removal of framework aluminium. This removal probably occurs during the collapse of the mesopore structure.

32 Table 2 Characteristics of various samples Si/Me No

Sample

Gel

Product

S(m2/g)

~p(/~)a

1 4 5 7 11 14 17 18

AI-MS(100) AI-MS(20) AI-MS(15) Ga-MS(50) Fe-MS(50) B-MS(100) B-MS(15) B-MS(3)

100 20 15 50 50 100 15 3

88 21 8 48 55 150 27 17

1215 (1166) 983 (1079) 1252 929 700 (935) (1056) 1237 (935)

30 (36) 32 (37) 22 28 22 (37) (36) 30 (38)

,

a~p is the pore diameter obtained from N 2 isotherms. Valuesbetween parentheses have begn obtained on solvent extracted samples.

52

52

I

A (

CALCINATION

I

~ 650~

m

"

o-'

o'

, 6o

5 0'

o' - 5 0

ppm/Al(H20]8 s§ Fig. 1 27A1 NMR spectra of samples 1 (a) and 4 (b) before (left) and after (right) calcination in air

The 11B NMR spectra of B-MS materials show a single line at -1.5 ppm, characterisitc of B3+ in tetrahedral sites [6]. Upon calcination, the spectrum is modified and a second resonance, assigned to BO 3 units, is observed (Fig. 2). The 71Ga NMR spectra of Ga-MS materials show a broad line at 167 ppm, already observed on Ga-containing zeolites and assigned to tetrahedrally coordinated Ga species [7]. The EPR spectra of FeMS solids are very similar to those obtained on Fe-containing zeolites and show a sharp peak at -~+ge-- 4.3 usually attributed to F cations in a tetrahedral coordination [8].

33

-1.5

I

= 0.24

6

I

_

-

-

-

~

.

.

.

.

(b)

(a)

40'

'

2 "0

'

0'

. - 2. 0 . . - 4 0

ppm/Et2OBF3

Fig. 2. 11B NMR spectra of sample 18 as-synthesized (a) and calcined (b).

'60

-8

-100 -1 0 ppm/TMS

-140

Fig. 3. 29Si NMR spectra of sample 3. As-synthesized (a), extracted (b) and calcined in air (c)

29Si MAS NMR spectra of as-synthesized samples show 3 distinct lines at -90, -98 and -110 ppm, attributed to Q2, Q3 and Q4 species, respectively. After calcination, the spectrum is less well resolved and a deconvolution shows that approx. 50 % of the silanol groups have been removed with respect to as-synthesized products (Fig. 3). For pure silica materials, Tanev et al. [3] have reported that washing the mesoporous silica with boiling ethanol resulted in the total removal of the amine from the mesopores. That could be possible because of the weak interactions (hydrogen bonding) between the neutral organic micelle system and the inorganic framework. However, when trivalent cations are tetrahedrally coordinated in a silica matrix, they create a defect charge that has to be compensated by cations. When no cations are present in the gel, the charge is balanced by templating molecules, usually tetraalkyla .mmonium cations. We have first followed that recipe, i.e. the dried samples were refluxed in ethanol for about 1 h. The procedure removes nearly all the organics, as evidenced by the strong decrease of specific absorption bands in i.r. spectra. However, absorptions characteristic of the template are still present in the i.r. spectrum after 3 ethanol extractions. The 13C

34 NMR spectrum of the extracted sample is quite similar to that of as-synthesized samples but shows additional bands, particularly one at 41 ppm/TMS, attributed to carbon atoms directly bonded to primary ammonium cations [9]. These cations, that are not removed by ethanol extraction, are more likely in the proximity of Me 3+ cations and serve to maintain the electric neutrality of the samples. Therefore, the incorporation of trivalent cations in mesoporous silicas necessitates the protonation of a small amount of the primary amine. We thus tried to add inorganic cations to the extraction solvent to exchange primary ammonium cations. The procedure was similar to that previously described except that NaC1 was preliminary dissolved in ethanol (lg/100 ml). I.r. and NMR spectroscopies show that all organics are then removed, even in samples containing high metal contents. However, new absorptions are observed in the i.r. spectrum around 2900 cm -1, characteristic of Si-O-C2H 5 species formed after partial esterification of the samples, which was confirmed by a significant decrease of the OH band at 3745 cm "1. Chemical analysis of the samples show that the extraction does not modify the composition of the samples. After removal of the template, all samples exhibit high surface areas and pore sizes of about 37,1. (Table 2). 52

i

(~

1

I

50

I

0

I

-50

ppm / AI (H20)O*

Fig. 4. 27A1 NMR spectra of sample 4. As-synthesized (a), calcined in air (b), extracted (c) and extracted and calcined in air at 500~ (d).

35 Fig. 4 compares the 27A1 NMR spectra of sample 4 as-synthesized, calcined at 650~ in air and submitted to the solvent extraction. As clearly demonstrated in the figure, the solvent extraction preserves the coordination of AI3+ , as n o octahedrally coordinated species are detected. The spectrum is very similar to that of the assynthesized sample. Moreover, the spectrum is unchanged when the solvent extracted sample is submitted to a calcination in air at 500 ~ which confirms that-the formation of octahedrally A1 species more likely occurs during the decomposition of the organics at high temperature. Similarly, the liB NMR spectra of solvent extracted B-MS samples are very different from those of the corresponding calcined samples (Fig. 5).

-1.5

I

'

40

I

20

I

0

I

!

-20

-40

.

ppm/Et2OBF 3

Fig. 5. llB NMR s~ectra of sample 18. As-synthesized (a), extracted (b) and extracted and calcined at 500 C in air (c).

Only one line is observed, as for as-synthesized samples, and characterizes B(OSi)4 units. As for aluminium containing materials, a subsequent calcination does change the NMR spectrum. EPR spectroscopy shows that the solvent extraction also preserves the coordination of Fe 3 + cations in mesoporous silicas. EPR spectra are identical to those of as-synthesized samples and not modified by a subsequent calcination in air.

36 The 29Si NMR spectra of solvent extracted samples are strictly similar to those of as-synthesized samples (Fig. 3). The fraction of silanol groups are the same as for starting materials, thus confirming once more the preservation of the framework. However, a small amount of the species observed around -100 ppm are probably Si-OC2H 5 moities due to the partial esterification of silanol groups during the extraction.

4. CONCLUSION We have shown that trivalent metal containing mesoporous silicas could be prepared using a primary amine as surfactant molecule. The physical properties of the materials, in particular their surface area, did not significantly change with the amount of metal incorporated. Following this recipe, mesoporous silicas containing tetrahedrally coordinated cations could be synthesized with Si/Me ratios as low as 10 without observing the presence of octahedral species. The totallity of the organics could be removed from the mesopores using a solvent extraction. The procedure preserved not only the mesoporosity of the materials but also the coordination of the trivalent cations. Extracted samples were thermally stable and could be calcined in air at 500~ without modification of the cation coordination. This calcination had the advantage to remove ethoxy groups bonded to the silica framework and formed upon the extraction process.

5. R E F E R E N C E S

C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli ande J.S. Beck, Nature, 359 (1992) 710. 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. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. A. Corma, V. Fornes, M.T. Navarro and J. Perez-Parient6, J. Catal., 148 (1994) 569. A. Sayari, I. Moudrakovski, C. Danumah, C.I. Ratcliffe, J.A. Ripmeester and K.F. Preston, J. Phys. Chem, 99 (1995) 16373. Y.X. Zhi, A. Tuel, Y. Ben Taarit and C. Naccache, Zeolites, 12 (1992) 138. D.H. Lin, G. Coudurier and J.C. Vedrine, in P.A. Jacobs and R.A. Van Santen (Eds), Zeolites: Facts, Figures and Future, Elsevier Science Publishers, B.V. Amsterdam, 1989, p. 1431. G. Boxhoonn, R.A. Van Santen, W.A. Van Erp, G.R. Hays, N.C.M. Alma, R. Huis and A.D.H. Clague, Proc. 6th Int. Zeolite Conf. (Reno, 1983), Butterworth, Guilford, 1985, p. 694.


37

Synthesis of lamellar aluminophosphates via the supramolecular templating mechanism Abdelhamid SAYARI Department of Chemical Engineering and CERPIC, Universit~ Laval, Ste-Foy, Qc, Canada G1K 7P4. The liquid-crystal templating approach for the synthesis of mesostructured materials was extended to aluminophosphates. Long chain primary and tertiary amines were used as templates. The molar gel composition was varied in a systematic manner over a wide range. Samples were thoroughly characterized using XRD, TEM, TGA, and 3~p, 27AI, 15N and lsC solid state NMR. Several lamellar phases with doo~distances in the 2 to 4 nm range were obtained. However, no three dimensional structures were detected. The gel composition was found to have a strong effect on the connectivity of aluminum and phosphorus in the final "crystalline" phase, as well as on their doo~ distances. TEM showed that some samples exhibit extended areas with unique structural features. They consisted of coaxial cylinders of alternating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders had an overall diameter of ca. 150 nm. They were aggregated into a hexagonal-like structure.

1. INTRODUCTION The crystalline mesoporous materials designated as M41S [1] have been for the last few years the subject of increasing attention. These materials are prepared hydrothermally via a supramolecular templating technique in the presence of surfactants. Synthetic methods using anionic, cationic, gemini or neutral surfactants, under either very basic or strongly acidic conditions [2-4] were developed. Thermally stable structures, particularly the so-called MCM-41 hexagonal structure have promising applications as catalysts and as advanced materials. Potential catalytic applications of such materials were reviewed recently [5]. Early investigations focussed on silicate and aluminosilicate materials [1]. Further work dealt with the incorporation of other metal cations such as Ti [6], V [7] and B [8] into MCM-41 silicates. In addition, Huo et al. [2] first reported on open-structure networks of a number of metal oxides like W, Sb, Zn, Pb, Mg, AI, Mn, Fe, Co, Ni and Zn oxides. Most of these oxides exhibited lamellar structures, except for W (hexagonal and lamellar), Sb (hexagonal and cubic) and Pb (hexagonal and lamellar) oxides. None of these oxide mesophases including the hexagonal phases was stable upon calcination. More recently, we were able to synthesize lamellar and hexagonal ZrO2

38 [9] and to stabilize the hexagonal phase [10]. Stable hexagonally packed mesoporous titania was also synthesized [11]. Abe et al. [12] prepared hexagonal vanadiumphosphorus oxides, but no information regarding their thermal stability was provided. Aluminophosphates (AIPO4) are crystalline microporous materials prepared hydrothermally, mostly in the presence of amine templates [13]. Several AIPO4s were also prepared using linear alkylene diamines [14] or cyclic diamines [15]. Recently, we extended the so-called liquid crystal templating mechanism to the synthesis of lamellar AIPO4s with d-spacings in the nanometer range [16,17]. Lamellar AIPO4s prepared in the presence of surfactants were also the subject of two other reports [18,19]. In this paper we present an overview of our findings with particular emphasis on samples prepared with the following gel composition P2Os : 0-2 AI203 : C~2H2sNH2 : 60 H20. 2. E X P E R I M E N T A L

Several series of AIPO 4 materials were prepared hydrothermally using the gel composition: x P205 : Y AI208 : z R-NR' 2 : w H20, where x = 1.0 (or 0 for P free samples), y = 0 to 2.0, z = 0.125 to 2.0 and w = 60 to 300. The template R-NR' 2 was a primary (R' = H) or a tertiary (R'= CHs) amine with a long alkyl chain (R = CnH2n§ with n = 8 to 16). However most samples were prepared in the presence of dodecylamine. These samples will be referred to as AIPO4-x:y:z:w. The following is a typical synthesis procedure of a AIPO4 sample with a molar gel composition: P2Os : AI203 : C~2H25NH2 : 60 H20. A suspension of 2.42 g of alumina (72 % pseudoboehmite alumina, Catapal B from Vista) in 5 g of water was mixed with 4 g of phosphoric acid (Fisher Scientific, 85 %) diluted with 13 g of water and stirred for about 1 h. Finally 3.2 g of dodecylamine surfactant was added to this mixture and stirred for one additional hour. The gel was then heated under autogenous pressure at 100 ~ for 24 h in a Teflon lined autoclave with no stirring. X-ray diffraction measurements were carried out on a D5000 Siemens diffractometer (CuKec radiation, ~, = 0.15418 nm). Transmission electron micrographs were obtained as reported elsewhere using a Philips CM20 instrument operated at 200 kV [17,20]. Thermogravimetric measurements were performed on a Mettler TG50 thermobalance in a flow of air. The temperature was raised at a rate of 10 ~ up to 600 ~ s~p and 27AI MAS NMR spectra were obtained on a Bruker AMX-300 (magnetic field 7.05 T, Larmor frequencies 78.18 and 121.47 MHz, respectively) and a Bruker AMX600 (magnetic field 14.1 T, Larmor frequencies 156.36 and 242.95 MHz) spectrometers. Typical MAS speeds of rotation were 10-14 kHz, and the delay times were set at 60 s for 3~p and 0.3 s for 27A1. A conventional one-pulse sequence in combination with high power proton decoupling (40 kHz) was used for both nuclei. Very short radiofrequency pulses were employed for 27AI (0.6 l~S) in order to obtain spectra for quantitative measurements [21]. A 5 mm High Speed Probe and a 5 mm Ultrasonic Speed Probe, both from DOTY Scientific were used on the AMX-300 and AMX-600, respectively. 85% H3PO4 and a 1 M solution of aluminum nitrate were used as external references. All values for the 27AI chemical shifts reported here were corrected for the second order quadrupolar interactions [22].

39 ~3C and ~SN CP MAS spectra were collected on a Bruker AMX-300 spectrometer (Larmor frequencies 75.5 and 30.1 MHz, respectively). The speed of rotation was within 3-3.5 kHz, and the CP contact time was 2 and 5 ms for ~3C and ~SN, respectively. Signals from tetramethylsilane (TMS) and the NO3" group of solid NH4NO3 were used as external references. 3. RESULTS AND DISCUSSION

Tanev et al. [6a] were the first to use long chain primary amines as supramolecular templates for the synthesis of pure and Ti-modified hexagonal mesoporous silicates. The same technique was extended to V-modified silicates [7] and to aluminophosphates [16,17]. Recently, Oliver et al. [19] prepared lamellar AIPO4s using decylamine in a non-aqueous tetraethylene glycol solvent. In the present study both primary and tertiary amines were used. ~SN and ~3C CP MAS NMR showed that amines occluded in the as-synthesized materials were actually protonated. The ~SN signal shifted from -346.0 ppm for pure dodecylamine to -340.0 + 0.4 ppm for the occluded molecule. Likewise, the ~3C chemical shift was 43.1 and 40.2 + 0.4 ppm for pure and occluded dodecylamine, respectively. The magnitude of these shifts corresponds to protonation. The effects of AI2OJP205, C~2H2s-NH2/P2Osand H20/P205 ratios as well as the effect of the alkylamine chain length were investigated systematically. All our AIPO4 materials had lamellar structures, and consequently collapsed upon high temperature calcination. The lamellar nature of these phases was inferred from the presence of only ( 001 ) XRD peaks, and also from direct TEM observations. Figure 1 shows a series of XRD patterns for AIPO4-1:y:1:60, with y = 0 to 1.8. Samples with low AI content (y = 0 to 0.4) exhibited a lamellar phase with doo~ = ca. 22.5 A. As seen below, 8~p NMR data of these samples are consistent with the occurrence of dodecylammonium dihydrogen phosphate. Recently, Oliver et al. [20] used a similar procedure to synthesize decylammonium dihydrogen phosphate. Gels with A I / P ratios higher than 0.6 gave a lamellar AIPO4 phase with doo~= 32.5 + 0.5 ,~ (Table 1). Figure 2, a representative micrograph of AIPO4-1:1:1:60, shows alternating dark and light fringes, indicative of the occurrence of layers viewed edge-on. The electron diffraction pattern is also consistent with the presence of a layered structure with a primary repeat distance of 31 A, in good agreement with XRD measurements. In addition, as shown in Figure 3, some samples exhibited extended areas with unique structural features. These areas consist of disks with an overall diameter of ca. 150 nm aggregated into a hexagonal-like army. A close-up (Figure 4) shows that each disk is comprised of alternating dark and bright concentric rings. The primary distance between dark rings was 31 A, indicating that this new mesophase is related to the layered structure shown in Figure 2. The central tubule of the disk had a diameter of ca. 36 A, consistent with the presence of a surfactant rodlike micelle. The concentric growth of rings to form self-organized, large disks with comparable diameters was interpreted as follows [17]. Because of their small head, long chain alkylammonium surfactants tend to self-organize into planar bilayers [23]. Consequently, in the presence of inorganic species, the formation of lamellar structures is strongly favored.

40

y=1.8 1.6

1.2 1.0

5

: ".';-:,.':~Y.. ~:.'Y/',,d,~ c ?;,.-x./lx.,r, fl.fJ'~ 9 . ..... ;11 ~

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Fig re 5.27AI MAS-NMR spectra of AIF ~)4-1:y:1:60. Values of y are shown on he left-hand side.

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100

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510

Chemical Shift, ppm

Figure 6. 3~p MAS-NMR spectra of AIPO4-1 :y:1:60. Values of y are shown on the left-hand side.

42 However, in the present system, the occurrence of concentric growth suggests that the system has a tendency to form some rodlike micelles, but not enough to selfaggregate, for example into a hexagonal structure. These rodlike micelles play the role of nuclei for further concentric growth of alternating rings of inorganic AIPO4 materials (dark rings) and cylindrical vesicles of surfactant (bright rings). This unique morphology is to be regarded as an example of the occurrence of new surfactantinorganic mesophases which have no lyotropic surfactant liquid crystal counterparts. Using gemini surfactants, Huo et al. [24] also discovered a surfactant-silicate mesophase with three dimensional hexagonal symmetry which has no analog among known surfactant liquid crystal structures. Likewise, Oliver et al. [19] while studying the synthesis of lamellar aluminophosphates in the presence of decylamine in a nonaqueous tetraethylene glycol solvent, found that parts of their samples exhibit remarkable morphologies and surface patterns akin to the naturally occurring silicious skeletons of diatoms and radiolaria. Figures 5 and 6 show the 27AI and 31p NMR spectra of AIPO4-1:y:1:60 samples. Detailed data are given in Table 1. Figure 5 shows that most samples exhibit three different 27AI NMR signals. Based on literature data [25], the signal at ca. 47 ppm was assigned to tetrahedral aluminum (species A) bonded to four P atoms via oxygen bridges. In agreement with Rocha et al. [27] who found that 27AI in AI(OP)4(OH2)2 resonates between -9.5 and -12 ppm, the peak at -6 to -10 ppm was attributed to framework octahedral aluminum (species B) coordinated with water and PO4 groups. Samples with low AI content (y = 0.2 and y = 0.4) exhibited only one sharp 27AI NMR peak at ca. -10 ppm corresponding to the hydrated six-coordinated AI in AIPO4 framework. Upon vacuum treatment of the samples at room temperature, the peak of species A decreased in favor of species B. This indicates that (i) both species are related to each other, and (ii) species B is coordinated to at least two water molecules. At higher AI loading (y > 0.8) a third signal with a chemical shift of ca. 10.3 ppm developed. As seen, the amorphous P free sample exhibits only one 27AI NMR signal at 10.3 ppm. It is therefore inferred that the 10.3 ppm peak observed for AI rich samples corresponds to extraframework alumina. Figure 6 shows the 31p MAS-NMR spectra of the same AIPO4-1:y:1-60 series of samples. The aluminum free sample exhibited two 3~p NMR signals at 2.3 (40 %) and 0.6 (60 %) ppm. The anisotropy (AS = -75 + 5) and the asymmetry parameter (11 = 0.3 __. 0.1) were very similar for both species. These parameters, common for acid ammonium phosphates, were assigned to two non equivalent PO2(OH)2 anions belonging to dodecylammonium dihydrogen phosphate. Samples with AI to P ratios in the range 0.8 to 1.6 exhibited a broad 3~p NMR peak centered at -13 ppm, thus excluding the presence of P sites with P in their second coordination shells. This peak was attributed to tetrahedral P bonded to (4 - X) aluminum tetrahedra and X hydroxyl groups (where X = 1 or 2). The chemical shifts of 3~p in microporous AIPO4s generally fall in the range of -19 to -30 ppm [27]. The downfield shift observed for our samples may due to several factors, particularly for the hydrophillic nature of the materials [28]. The origin of the peak broadening is most likely attributable to the occurrence of a distribution of P sites with similar but not identical environments. This conclusion stems from the fact that at higher field (14.1 T) the resolution of the peak hardly improved. The first derivative of the 31p NMR signal

43 indicates the presence of at least five subgroups of P sites (Figure 6, inset). In addition to the -13 ppm 31p signal, samples with the highest levels of AI displayed a low intensity (< 4%), sharp peak at ca. -3.4 ppm attributed to an impurity phase. For samples with very low AI contents (y = 0.2 and y = 0.4) there was a sharp peak at -19 ppm in addition to the Sip peak close to 0 ppm observed in the AI-free sample. This -19 ppm 31p peak together with the -10 ppm 27A! peak observed for the same sample may correspond to variscite: AIPO4-2H20 with 5(~IP) = -19.5 ppm and ~(27AI) = -11 ppm [29]. If this assignment is correct, the variscite phase must be highly dispersed not to be observed by XRD. As seen in Table 1 (column 7), the overall AI/P ratios of the samples are comparable to those of the corresponding gels. The framework P to AI ratios shown in the last column represent ratios of the sum of AI species A and B to P calculated based on chemical analysis, quantitative AI NMR data and assuming complete retention of phosphorus. It is seen that AI/P is usually below one. As inferred from NMR data, this indicates the occurrence of some P-O +NH3-C12H2slinkages. Additional data on the effect of other synthesis parameters will be published elsewhere [30].

4. CONCLUSIONS A variety of lamellar aluminophosphates with doo~distances in the range of 2-4 nm were synthesized via the supramolecular templating mechanism using long chain primary and tertiary alkylamines as templates. The effects of other synthesis parameters were also studied. The occurrence of lamellar phases was inferred from XRD data and by direct TEM observations. 81p and 27AIdata were consistent with the presence of aluminophosphate. Even though the synthesis variables had strong effect on the quality of the products formed and on the connectivities of AI and P, they did not favor the formation of three dimensional AIPO4 structures. In addition to the main phase with planar lamellae, some samples exhibited extended areas consisting of coaxial cylinders of altemating inorganic aluminophosphate material and organic surfactant bilayers, all wrapped around a central rodlike micelle. Such coaxial cylinders which were aggregated into a hexagonal-like structure had an overall diameter of ca. 150 nm.

Acknowledgments Partial funding by the Natural Sciences and Engineering Research Council (NSERC) of Canada is acknowledged. I wish to thank I.L. Moudrakovski, J.S. Reddy, V.R. Karra, C.I. Ratcliffe, J.A. Ripmeester, K.F. Preston, A. Chenite and Y. Le Page for significant contributions to this work.

REFERENCES (a) C.T. Kresge, M.E. Leonowicz, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710.

44

.

=

4. 5. 6.

.

.

.

10. 11. 12. 13. 14.

15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30.

Q. Huo, D.I. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B. Chmelka, F. Sch0th and G.D. Stucky, Chem. Mater., 6 (1994) 1176. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science, 269 (1995) 1242. A. Sayari, Chem. Mater. (1995), submitted for publication. (a) P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. (b) A. Corma, M.T. Navarro and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1994) 147. (c) A. Sayari, V.R. Karra and J.S. Reddy, Mat. Res. Soc. Symp. Proc., 371 (1995) 87. (a) K.M. Reddy, I.L. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun., (1994) 1059. (b) J.S. Reddy and A. Sayari, J. Chem. Soc., Chem. Commun., (1995) 2231. (d) J.S. Reddy and A. Sayari, Appl. Catal., (1995), submitted for publication. (a) A. Sayari, C. Danumah and I.L. Moudrakovski, Chem. Mater., 7 (1995) 813. (b) A. Sayari, I.L. Moudrakovski, C. Danumah, J.A. Ripmeester, C. Ratcliffe, C. and K.F. Preston, J. Phys. Chem., 99 (1995) 16373. J.S. Reddy and A. Sayari, Catal. Lett., (1996), in press. J.S. Reddy, P. Liu and A. Sayari, 1996 Spring Meeting of the Materials Research Society, San Diego, (1996). D.M. Antonelli and J.Y. Ying, Angew. Chem. Int. Ed. Engl., 34 (1995) 2014. T. Abe, A. Taguchi and Iwamoto, Chem. Mater., 7 (1995) 1429. (a) S.T. Wilson, B. Lok, C.A. Messina, T.R. Connan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. (b) S.T. Wilson, Stud. Surf. Sci. Catal., 58 (1991) 137. (a) R.H. Jones, A.M. Chippindale, S. Natarajan and J.M. Thomas, J. Chem. Soc., Chem. Commun., (1994) 565, and references therein. (b) B. Kraushaar-Czametzki, W.H.J. Stork and R.J. Dogterom, Inorg. Chem., 32 (1993) 5029. P.A. Barrett and R.H. Jones, J. Chem. Soc., Chem. Commun., (1995) 1979. A. Sayari, V.R. Karra, J.S. Reddy and I.L. Moudrakovski, J. Chem. Soc., Chem. Commun., (1996), in press. A. Chenite, Y. Le Page, V.R. Karra and A. Sayari, J. Chem. Soc., Chem. Commun., (1996), in press. C.A. Fyfe, W. Achwieger, G. Fu and G.T. Kokotailo, Prepr., A.C.S. Div. Petrol. Chem., 40 (1995) 266. S. Oliver, A. Kuperman, N. Coombs, A. Lough and G. Ozin, Nature, 378 (1995) 47. A. Chenite, Y. Le Page, Y. and A. Sayari, Chem. Mater., 7 (1995) 1015. P.P. Mann, J. Klinowski, A. Trokiner, H. Zanni and P. Papon, Chem. Phys. Left., 151 (1988) 143. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester, 1987. J.N. Israelachvili, Intermolecular and Surface Forces, Academic Press, NY, 1991. Q. Huo, R. Leon, P.M. Petroff and G.D. Stucky, Science, 268 (1995) 1324. D. Muller, E. Jahn, G. Ladwig, G. and V. Haubenreisser, Chem. Phys. Lett., 109 (1984) 332. J. Rocha, W. Kolodziejski, H. He and J. Klinowski, J. Am. Chem. Soc., 114 (1992) 4884. I.L. Moudrakovski, V.P. Schmachlova, N.S. Katsarenko and V.M. Mastikhin, J. Phys. Chem. Solids, 47 (1987) 335. L.S. de Saldarriaga, C. Saldarriaga and M.E. Davis, J. Am. Chem. Soc., 109 (1987) 2686. C.S. Blackwell and R.L. Patton, J. Phys. Chem., 92 (1988) 3965. A. Sayari et al., Chem. Mater. (1996), submitted for publication.


45

Synthesis and Hydrothermal Stability of a Disordered Mesoporous Molecular Sieve Ryong R y o o a, J. M. Kim', C. H. Shinb and J. Y. Lee c

"Departmem of Chemistry, KAIST, Taejon, 305-701, Korea; bCatalysis Research Division, KRICT, Taejon, 305-606, Korea; CDepartment of Materials Science and Engineering, KAIST

A noncrystalline mesoporous molecular sieve has been synthesized by hydrothermal polymerization of silicate and aluminate anions surrounding the molecular organization of hexadecyltrimethylammonium chloride in the presence of various organic polyacids. The noncrystalline molecular sieve is very similar to the well-known mesoporous molecular sieve MCM-41 in the aspect of the high specific surface area and uniform pore sizes, but the channel arrangement interconnected in a three dimensional disordered way distinguishes it most conspicuously from the MCM-41 which exhibits hexagonal arrangement of straight channels. The disordered structure has remarkably high hydrothermal stability and thermal stability, compared with MCM-41 and MCM-48.

1. Introduction In recent years, there have been dramatic advances in the concept of molecular sieves exhibiting uniform pore sizes. The structures of early molecular sieves were typified by crystalline microporous materials such as zeolites and AIPO4 in which the arrangement of channels (or pores) and the arrangement of framework atoms are ordered over crystallographically long range. Later, the discovery of crystalline mesoporous molecular sieves such as MCM-41 and MCM-4813 has opened a new class of molecular sieves in which only the channel arrangement is crystallographically ordered while the atomic arrangement is disordered similar to amorphous silica. Since then, a few silicate materials exhibiting disordered arrangement of mesopores with very high specific surface areas of approximately 1000 m2g1 began to attract our attention. One such material was synthesized by Chela et al. 4 using hydrothermal reaction of Na-kanemite and hexadecyltrimethylammonium (HTA) chloride. The material gave a N2 adsorption isotherm similar to that for MCM-41, which indicated that the pore size distribution was similar to that for the MCM-41. The structure of the material was illustrated by the authors with straight mesoporous channels which were entangled randomly. Bagshaw et al. s obtained another disordered mesoporous material designated MSU=I using tetraethylorthosilicate and nonionic surfactants. Guo 6 also reported a similar material. The pore size distribution curves for these two mesoporous silicate materials showed peak widths greater than 0.9 nm, which was about three times broader at the

46 half height than that for MCM-41. Although the pore structures of the materials were still very heterogeneous compared with the pore sizes for the crystalline MCM-41 and MCM-48, their discovery showed a possibility of finding noncrystalline molecular sieves with uniform pore sizes. Very recently, efforts to obtain such noncrystalline molecular sieves with truly uniform pore sizes have succeeded by the present authors using hydrothermal polymerization of silicate and aluminate anions surrounding the molecular organization of hexadecyltfimethylammonium chloride in the presence of various organic polyacids. 7 The disordered molecular sieve thus obtained is very similar to the MCM-41 in the aspect of the high specific surface area and uniform pore sizes, but the channel arrangement interconnected in a three dimensional disordered way distinguishes it most conspicuously from the MCM-41 which exhibits a hexagonal arrangement of straight channels. Besides, the disordered structure has higher thermal stability and hydrothermal stability than MCM-41 structure. Here, we describe details of the synthesis method and the hydrothermal stability of the disordered mesoporous molecular sieve exhibiting uniform pore sizes.

2. Experimental A fully disordered surfactant-silicate mesostructure has been obtained using sodium silicate, alkyltrimethylammonium (ATA) halide (CnHz~+IN(CH3)3X, n = 12 - 18, and X = Cl or Br) and sodium salt of organic polyacid. Typical procedures to obtain the disordered mesostructure were as follows: a clear solution of sodium silicate with a Na/Si ratio of 0.5 was first prepared by combining 46.9 g of 1.00 M aqueous NaOH solution with 14.3 g of a colloidal silica, Ludox HS40 (39.5 wt% SiO2, 0.4 wt% Na20 and 60.1 wt% H20, Du Pont) and heating the resulting gel mixture with stirring for 2 h at 353 K. The sodium silicate solution was dropwise added to a polypropylene bottle containing a mixture of 0.29 g of 28 wt% aqueous NH3 solution, 23.8 g of ethylenediaminetetraacetic acid tetrasodium salt (Na4EDTA), 20.0 g of 25 wt% HTACI solution and 28.0 g of doubly distilled water, with vigorous magnetical stirring at room temperature. The resulting gel mixture in the bottle had a molar composition of 4 SiO2 91 HTACI : 4 Na4EDTA : 1 Na20 : 0.15 (NH4)20 : 350 H20. After stirring for 1 h more, the gel mixture was heated to 370 K for 2 d. The resulting mixture was cooled to room temperature. Subsequently, pH of the mixture was adjusted to 10.2 by dropwise addition of 30 wt% acetic acid with vigorous stirring. The reaction mixture after the pH adjustment was heated again to 370 K for 2 d. This procedure for pH adjustment to 10.2 and subsequent heating for 2 d was repeated twice more. The precipitated product was filtered, washed with doubly distilled water and dried in an oven at 370 K. The product was calcined in air under static conditions using a muffle furnace. The calcination temperature was increased from room temperature to 823 K over 10 h and maintained at 823 K for 4 h. The calcined product is designated KIT-1. The product yield was more than 90%, based on the silica recovery. Aluminum-containing KIT-1 (AIKIT-l) samples with Si/Al as high as 5 have been obtained by adding 5 wt% aqueous solution of sodium aluminate (Strem, 99.9% on metal basis) during the formation of the above disordered surfactant-silicate mesostructure. The aluminate solution was added to the reaction mixture at room temperature following the second heating

47 step to 370 K, dropwise with vigorous mixing. The second pH adjustment o f the reaction mixture with acetic acid was carried out after the resulting surfactant-aluminosilicate gel mixture was heated for 2 d. The remainder of the synthesis procedure was the same as the preparation of the above pure silica K/T-1. Hydrothermal stability of the samples was investigated by measuring the intensity decrease in powder X-ray diffraction (XRD) pattern during heating in doubly distilled water. The sample to water ratio was fixed as 1 g.L q. Samples at~er the heating in water were filtered, washed in doubly distilled water, and immediately placed in oven to dry for 2 h at 400 K. XRD patterns were obtained from the samples with a Cu K~ X-ray source using a Rigaku D/MAXIII (3 kW) instrument.

3. Results and Discussion

Characterization of a disordered molecular sieve. Figure 1 shows XRD patterns for the disordered HTA-silicate mesostructure obtained using Na4EDTA The surfactant is easily

5

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|

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20

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I

2

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6 Two theta

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Figure 1. Powder X-ray diffraction patterns for a fully disordered molecular sieve, KIT-l, synthesized using HTAC1 as a template. Inset, the relationship between dl0o spacing and number of carbon atoms in the surfactant chain: (0) as-synthesized and (r-l) calcined.

48 removed from the mesostructure by calcination in air under static conditions at any temperatures between 823 - 1173 K. Strict conditions using gas flows are not required for the calcination. Both the as-synthesized product and the calcined material exhibit same XRD patterns with three broad peaks indexed to (100), (200) and (300) diffraction with dl00 = 4.0 4.2 nm. The XRD intensity increases approximately 3 times upon calcination due to the removal of the surfactant. The dl00 spacing decreases very slightly (___0.1 nm) upon the calcination. The three-line XRD pattern is similar to the XRD pattern for a layered material. But, the KIT-1 is not a layered material since the removal of the surfactant by the hightemperature calcination does not lead to the structure collapse. The pore size distribution for the calcined KIT-1 material has been obtained from N2 adsorption-desorption isotherms at liquid N2 temperature following the Horvath-Kawazoe analysis, s The pore-size distribution curve in Figure 2 shows a mesopore with the pore diameter 3.4 nm at maximum of the distribution. The specific surface area for the KIT-1 has been obtained to be 1000 megq by the BET method, which is similar to MCM-41. It is remarkable that the mesopore structure of the KIT-1 (~ 0.3 nm peak width) is as uniform in pore size as the crystalline MCM-41 (~ 0.3 nm peak width) la on the basis of the width at half maximum for the pore size distribution curve. Thus, the individual pore widths are truly uniform inside the disordered structure of the KIT- 1.

-

~, 700 ~

600-

500 400 300 200 lO0 0 0

0.2

0.4

0.6

0.8

1.0

Relative pressure (P/P0)

I

I

I

I

I

I

I

I

I

I

1

2

3

4

5

6

7

8

9

10

Effective pore size (nm) Figure 2. Pore size distribution curve obtained by the Horvath-Kawazoe analysis for a disordered molecular sieve, KIT-I, alter calcination. Inset, the corresponding N2 adsorption-desorption isotherms.

49 As shown by a transmission electron micrograph (TEM) in Figure 3, the disordered surfactant-silicate mesostructure is similar to a bicontinuous structure arranged in a threedimensional disordered way. The calcined product KIT-1 also gives essentially the same transmission electron micrograph as the as-synthesized form. No long range structural order has been found from the electron diffraction pattern. Compared with the hexagonal structure of MCM-41, it is believed that the organic polyacids anions causes fluctuation in the surfactant micellar arrangement, giving rise to the formation of a stable isotropic labyrinth or disordered sponge phase that is similar to the so called L3 phases known in surfactant solutions. 9 However, the arrangement of the mesoporous channels in the KIT-1 structure is distinguished from the L3 phase by the intercormection of the mesoporous channels by numerous branches.

30nm

Figure 3. Typical transmission electron micrograph: (a) MCM-41 and Co) fully disordered molecular sieve, KIT-1. We have performed the following two experiments in order to investigate if the structure of the KIT-1 consists of one-dimensional mesoporous channels entangled in a disordered way, or the structure has very short channels interconnected by numerous branches. In one of the experiments, aluminum has been incorporated within the KIT-1 framework over the range of Si/AI = 5 - oo using sodium aluminate during synthesis, and the experimental conditions for the removal of surfactant by calcination have been compared with those for AIMCM-41. Our results show that the synthesis of AIMCM-41 leads to progressive decreases in the XRD intensity and increases in the line width, due to losses in the structural order as the AI content ihcreases beyond 15 Si/A1.~~ Moreover, calcination of the as-synthesized products with high AI content leads to the formation of coke due to the surfactant decomposition, which causes

50 channel blockage. The coke formation can be prevented by washing a large fraction of the surfactant using an ethanol-HCl mixture prior to the calcination. However, significant dealumination occurs during the washing. The resulting A1MCM-41 gives low ion exchange capacity, and furthermore much of the structural order is lost during the cation exchange due to weak hydrothermal stability of the MCM-41 samples, a~ Compared with the AIMCM-41, calcination of the AIKIT-1 can be performed using air under static conditions without washing with the ethanol-HCl mixture, which is indicative of facile diffusion of gases through numerous branches in the three dimensional channel structure during calcination. In the second experiment, Pt clusters about 3 - 4 nm in diameter have been supported inside the mesoporous channels of MCM-41 and KIT-1 following an impregnation technique using HzPtCI6. Catalytic activity of the Pt-supporting samples for hydrogenolysis of ethane with 1-12 has been measured using a batch recirculation reactor. The Pt clusters are large enough to cause multiple pore blockage in the MCM-41 channels, and thus the surface atoms on the Pt clusters located inside the one dimensional channel of the MCM-41 are not accessible for the catalytic hydrogenation, n On the other hand, the Pt clusters supported inside the three dimensional channel structure of the KIT-1 are fully accessible for ethane hydrogenolysis with H2. Thus, the catalytic activity for the Pt/KIT-1 is proportional to the Pt loading. It is clear that the presence of the three-line XRD pattern for the KIT-1 comes from a short range structural order with very uniform pore sizes. Similar products can be obtained if the Na4EDTA is substituted by sodium salts of adipic acid, other polycarboxylic acids and polysulfonic acids. The products obtained with different polyacids give similar XRD patterns with three broad diffraction bands. The line widths and line shapes can be somewhat dependent on the different polyacids and also on the salt concentration. This is probably due to differences in the density of the branches interconnecting the channels and the distance between adjacent branches, which determine the local structure of the KIT-1. It is believed that the channel disorder in the KIT-1 structure can be controlled by the nature and concentration of the chemical agents used to induce the fluctuation in the micelle arrangement. Likewise to the MCM-41,1'2 it is also possible to tailor the channel widths by use of surfactants with suitable sizes. The dl00 spacings for the calcined materials are plotted against the number of carbon atoms in the ATA surfactant in Figure 1.

Hydrothermal stability of mesoporous molecular sieves. MCM-411a and other mesoporous molecular sievess'6,a3 found recently have opened new possibilities as a support for adsorption and catalysis, and also as a template for architecting nanosize materials. The mesoporous material, MCM-41 has excellent thermal stability up to 1170 K or higher in air and 02. The stability is not affected considerably by the presence of' water vapor up to 2.3 kPa in the 02 flow. 14'as Furthermore, there is a report that the MCM-41 constructed with silica framework can be stable even in a 100%-steam flow under atmospheric pressure at 820 K. t~ However, contrary to the good stability at high temperatures, the MCM-41 is reported to lose the structure easily during storage in humid air and aqueous solutions at relatively low temperatures around 370 K. 11 The loss of the structure involves silicate hydrolysis as shown by Kim and Ryoo using XRD and magic angle spinning 29Si ~ spectroscopy, at The loss of the structure makes it difficult to obtain high levels of ion exchange with MCM-41. as In

51 addition, although MCM-41 has been reported to be useful for many catalytic applications, l~qs it is expected that the loss of the structure can lead to a rapid decrease in the catalytic activities with time under experimental conditions containing water or saturated with water vapor. Thus, the poor hydrothermal stability of the MCM-41 in water can be a critical problem limiting the applications. Therefore, improvement of hydrothermal stability is a target for ultimate successful uses of the MCM-41 type materials. We have obtained pure silica MCM-41 and MCM-48 samples, following hydrothermal synthesis procedures reported in the literature, 1-3 and compared hydrothermal stability of the

KIT-1

MCM-48

MCM-41

C

~

C

b ~~_______

b

!

2

4

6

8

Two theta

2

4

6

8

Two theta

i

i

i

i

2

4

6

8

10

Two theta

Figure 4. Powder X-ray diffraction patterns for two crystalline mesoporous molecular sieves, MCM-41 and MCM-48, and a fully disordered molecular sieve, KIT-l, exhibiting the transmission electron micrograph in Figure 3: (a) calcined samples, (b) heated in water at 343 K for 12 h, (c) heated in boiling water for 12 h, and (d) heated in boiling water for 48 h. samples in boiling water with that of KIT-1. Figure 4 displays XRD patterns for the mesoporous materials against heating temperatures in water. The XRD patterns for the calcined MCM-41 samples before the heating in water show four dif~action lines characteristic of the hexagonal structure of the MCM-41. The XRD pattern for the MCM-48 sample agrees with the cubic 1,3d structure known in the literature. 13 No distinct changes in the structures are indicated by the XRD lines during 12 h in distilled water at room temperature. As the water temperature increases to 343 K, the intensity of the XRD patterns decreases conspicuously. Moreover, the decrease in the XRD intensity depends considerably on details of the synthesis procedures. 11 The stability difference is consistent with previous conclusion lsa9 that the stability of MCM-41 can be enhanced by repeating pH adjustment to around 11 during hydrothermal synthesis, due to an equilibrium shift of the synthesis reaction

52 condensation. 16~~ However, all the structures disappeared completely during the heating at 373 K. Compared with the weak hydrothermal stability of the MCM-41 and MCM-48 samples as shown in Figure 4, it is remarkable that the structure of the KIT-1 is stable in boiling water for 48 h. We have confirmed that the structure of the KIT-1 does not change under 100%-steam flow for 2 h at 1020 K. The disordered structure is also stable during heating with air flow containing 2.3 kPa water vapor for 2 h at 1220 K. In summary, the disordered mesoporous material, KIT-l, found in the present work belongs to a new class of molecular sieve in which the channels are interconnected in a three dimensional, fully disordered way. The structure of the material corresponds to a disordered bicontinuous phases known in surfactant solutions, in contrast to the hexagonal structure of MCM-41. Since the disordered structure, compared with the ordered MCM-41, has remarkable advantages due to the three dimensional diffusion and high hydrothermal stability, our findings of the new class of molecular sieves are expected to provide new opportunities for rational design of heterogeneous catalysts, adsorbents and other related materials requiring the three dimensional pore structure. References

1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. J.S. Beck, J. C. Vartuli, W. J. Roth, M. El 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. 2. A. Monnier, F. Schi~th, 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. 3. Q. Huo, R. Leon, P. M. Petroffand G. D. Stucky, Science, 268 (1995) 1324. 4. C.-Y. Chen, S.-Q. Xiao and M. E. Davis, Microporous Materials, 4 (1995) 1. 5. S.A. Bagshaw, E. Prouzet and T. J. Pinnavaia, Science, 269 (1995) 1242. 6 C.J. Guo, Stud. Surf. Sci. Catal., 97 (1995) 165. 7. R. Ryoo, J. M. Kim, C. H. Ko and C. H. Shin, J. Phys. Chem., in press. 8. R. Ryoo et al., in preparation. 9. G. Horvath and K. J. Kawazoe, J. Chem. Eng. Japan, 16 (1983) 470. 10. R. Ryoo, S. J. Cho, C. Pak and J. Y. Lee, Catal. Lett., 20 (1993) 107. R. Ryoo, S. J. Cho, C. Pak, J.-G. Kim, S.-K. Ihm and J. Y. Lee, J. Am. Chem. Soc., 114 (1992) 76. 11. D. Roux, C. Coulon and M. E. Cates, J. Phys. Chem., 96 (1992) 4174. 12. Z. Luan, H. He, W. Zhou, C.-F. Cheng and J. Klinowski, J. Chem. Soc. Faraday Trans., 91 (1995) 2955 and references on AIMCM-41 therein. 13 J. M. Kim and R. Ryoo, Bull. Korean Chem. Soc., 17 (1996) 66. 14. R. Ryoo, C. H. Ko, J. M. Kim and R. Howe, Catal. Lett., 37 (1996) 29. 15. C.-Y. Chen, H.-X. Li and M. E. Davis, Microporous Materials, 2 (1993) 17. 16. J. M. Kim, J. H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 17. E. Armengol, M. L. Carlo, A. Corma, H. Garcia and M. T. Navarro, J. Chem. Soc., Chem. Commun. (1995) 519. 18. C.-G. Wu and T. Bein, Science, 264 (1994) 1757. 19. K. R. Kloetstra and H. van Bekkum, J. Chem. Soc., Chem. Commun. (1995) 1005. 20. R. Ryoo and J. M. Kim, J. Chem. Sot., Chem. Commun. (1995) 711.


53

Preparation of silica-pillared molecular sieves from layered silicates Soon-Yong Jeong a, Oh-Yun Kwon b, Jeong-Kwon Suh a, Hangkyo Jin a, and Jung-Min Lee a aChemical Engineering Division, Korea Research Institute of Chemical Technology, Yusung, P. O. Box 107, Taejon, Korea 305-606 bDepartment of Chemical Engineering, Yosu National Fisheries University, Yosu, Korea 550-749 Magadiite and kenyaite were hydrothermally synthesized in Teflon-sealed stainless steel autoclave. The intercalation of TEOS(tetraethylorthosilicate) into the interlayers of layered silicates was carried out by amine preintercalation, and the effects of acid and base catalysts during gelation of TEOS into interlayers were investigated. It was found that the samples silica-pillared by acid- and basecatalyzed reactions show well-ordered basal spacing and super-gallery heights. Also, they exhibit relatively narrow pore size distributions in the range of 18-40 ,A, and show high surface areas in the range of 533-845 m2/g, depending on the catalyst types. These results indicate that the variations in the conditions of gelation contribute to improvement in physical properties of silica-pillared molecular sieves. 1. INTRODUCTION Layered silicates have attracted widespread interest over the past 20 years due to their catalytic, adsorptive and ion-exchange properties [1]. Recently, several researchers have investigated the pillared reactions of layered silicates such as magadiite, and kenyaite [2-5]. Magadiite and kenyaite were primarily found by Eugster [6] in the lake beds of the lake Magadi in Kenya. Afterward, other occurrences of these have been continuously reported in various regions [7], and these materials were mostly found from sodium carbonate-rich alkaline lake waters. They also have been successfully synthesized under hydrothermal conditions [8,9]. Their basic structures are composed of duplicated SiO 2 tetrahedral sheets, and are similar to clay minerals except to be free of aluminum [10]. Silica-pillaring of magadiite was reported by a few researchers. Landis et al. [4] found that the pillaring of magadiite could be facilitated by using a preswelling step in which the interlayers are exposed to organoammonium ion or amine. The calcined sample obtained from TEOS [Si(OC2H5)4] pillaring exhibited a high surface area, 530 m2/g. Sprung et al. [5] reported that the pillared derivatives of magadiite

.54 can be obtained from the calcination of hydrolyzed phenyltrichlorosilane-magadiites. Daily and Pinnavaia [3] synthesized supergallery derivatives on the basis of Hmagadiite by gelation of TEOS with EtOH suspension. After calcining to remove organic compounds, pillared magadiite with surface areas of about 520-680 m2/g, depending on the amount of gelled TEOS, was formed. However, the pillaring of kenyaite was rarely reported. Recently, Landis et al. [4] prepared pillared derivatives on the basis of H-kenyaite by gelation of TEOS with EtOH suspension. The calcined sample exhibited a high surface area, 600 m2/g. In the sol-gel process, solvent such as EtOH is added to prevent the liquid-liquid separation during the initial stage of the hydrolysis reaction and to control the concentration of silicate species and water that influence the gelation kinetics [11]. However, the reaction of TEOS gelation by alcohol suspension occurs very slowly, and because TEOS is alcohol-soluble, it can be released outside the layered phases during gelation. Aelion et al. [12] observed that the rate of hydrolysis of TEOS was influenced by the strength and concentration of the acid and base catalysts. The fast gelation using the catalyst such as acid or base can minimize the release of TEOS from the layered phases during gelation. Generally, TEOS gelation by acidor base- catalyzed hydrolysis could diversify the interfacial properties of products and result in such products as bulk gel, film, fiber, powder, and catalyst support. In the present work, we report the effects of acid or base catalysts on the hydrolysis and condensation polymerization of intercalated TEOS in H-type layered silicates.

2. EXPERIMENTAL 2.1 Syntheses of Na-magadiite and Na-kenyaite Synthetic Na-magadiite and Na-kenyaite were prepared by the reaction of NaOH/Na2CO3-SiO2 system under hydrothermal conditions using methods analogous to those described by Fletcher and Bibby [9]. Materials used were silica gel (Wakogel Q-63) and analytical reagent grades of NaOH and Na2CO3. Namagadiite was synthesized in a stainless steel autoclave without stirring at 150~ for 72 hrs under autogenous pressure, using mole ratios of SiO2 " NaOH NaCO3 9 H20 9 = 15 19 29 300. 9 Na-kenyaite was synthesized at 150-160 ~ for 70-80 hrs under autogenous pressure, using mole ratios of SiO2 NaOH 9 Na2CO3 9 H20 9 = 9 19 29 9 600. The products were filtered, and washed with deionized water in order to remove excess NaOH or Na2CO3, and dried at 40~ 2.2 Preparation of silica-intercalated layered silicates H-magadiite and H-kenyaite was prepared by titration of Na-kenyaite with 0.1 N HCI using the method of Beneke and Lagaly [8]. A suspension of 40g of Nakenyaite per 500ml of deionized water was stirred for 1 hr. The suspension was then titrated with 0.1 N HCI to a final pH of 2.0, and then maintained at the same value for one week in a refrigerator. H-magadiite and H-kenyaite was recovered by filtering, and washing with deionized water until CI-free and then dried in air at 40 ~

55 Octylamine/octylammonium-magadiite gel was reacted for 24 hrs at room temperature by adding 5.0 g of excess octylamine to 1.0 g of air-dried H-magadiite. An organic pillar precursor 20g of TEOS, was added to octylammonium-magadiite gel and then stirred for 24 hrs at room temperature. TEOS was then absorbed into the organophilic interlayer region. The TEOS-intercalated magadiite was separated by centrifugation from the mother liquid. Also, octylamine/octylammonium-kenyaite gel was formed by allowing air-dried H-kenyaite (0.86g, 0.57 mmol) to react at room temperature with excess octylamine (2g, 15 mmol) for 48 hrs. During octylamine addition, H-kenyaite absorbs the liquid amine, immediately forming a gelatinous mixture that will not flow. Silica-intercalated derivatives of kenyaite were prepared by the reaction of excess TEOS (15g, 72 mmol) with a gel composed of octylammonium-kenyaite solvated by excess octylamine for 24 hrs at room temperature. The TEOS-intercalated product was separated by centrifugation from the mother liquid. Gelation of the intercalated TEOS without catalyst was carried out by drying EtOH (10ml) suspension of TEOS-intercalated products at 40~ in air. EtOH was mixed with 3 N NH4OH and 0.1 N HCI in order to examine the effects of base and acid catalysts during the gelation. The compositions of acid and base catalysts are shown in Table 1. Gelation was conducted with stirring for 20 min. after addition of 10 ml of each catalyst to the TEOS-intercalated magadiite and kenyaite at room temperature. The stoichiometry and methodology of gelation of TEOS are wellknown, and the physical characterization of gelled silicate has been studied by several researchers [11, 13]. The gelled samples were filtered from the mother liquid, and dried in air, and then calcined at 538~ for 4 hrs in air to remove water, intercalated organoammonium ion, and organic byproducts from TEOS hydrolysis. Basal spacings of samples were determined from the 00~ X-ray powder diffraction using a Rigaku diffractometer equipped with CuK(z radiation. Nitrogen adsorption/desorption isotherms were determined by Micromeritics ASAP 2000 at 77K. All samples were outgassed at 300~ under a vacuum for 4 hrs. Surface area was determined by the BET equation. Micropore volume was obtained from t-plot methods [14], and the pore size distributions of silica-pillared products were determined by the BJH equation [15]. Table 1 Compositions of catalysts (wt%) EtOH

H20

HCl

NH3

No catalyst

95.0

5.0

Base-catalyst

16.5

79.2

-

4.3

Acid-catalyst

64.8

35.2

0.01

-

56

3. RESULTS AND DISCUSSION 3.1 Syntheses of magadiite and kenyaite The basic hydrolysis of silica gel at 150~ according to the method of Fletcher and Bibby [9] produced well-crystallized Na-magadiite and Na-kenyaite. The X-ray powder diffraction patterns of the air-dried products, shown in Fig. l(a, c) exhibited several 00~ reflections corresponding to a basal spacing of 15.6 A for Na-magadiite and 20 A for Na-kenyaite. The peak positions for this synthetic product agree closely with values reported previously [8]. The slow titration of Na-magadiite and Na-kenyaite with 0.1 N HCI resulted in the exchange of sodium ions for protons in the layered structure. The X-ray of powder diffraction patterns of H-magadiite and H-kenyaite exhibited 00~ reflections corresponding to a basal spacing of 12.6 A for H-magadiite and 18.0 A for H-kenyaite (Fig. 1 (b, d)), in agreement with earlier work [9]. The decrease in basal spacing indicated a loss of interlayer H20 upon replacement of Na* by H*.

. =,..,

==

C

=.=.=

. =,,=,

,=..=

n,' d I

I

I

I

I

I

i

I

I

I

I

I

i

I

I

I

I'

I

I

I

I

I

5 1015202530354045505560

5 1015202530354045505560

20

20

Figure 1. X-ray diffraction patterns of (a) Na-kenyaite (b) H-kenyaite (c) Namagadiite and (d) H-magadiite.

3.2 Preparation of silica-intercalated layered silicates 3.2.1 Silica-pillared magadiite The X-ray diffraction patterns and basal spacings of the calcined silica-pillared magadiites are shown in Fig. 2 (a, b, c) and Table 2. The silica-pillared magadiites gelled by base- and acid-catalyzed reactions indicate a large increase in basal spacing of 39.2 and 33.3 A, compared with the basal spacing of EtOH gelled product (17A). Table 2 shows the physical properties of the porous silica-pillared magadiites. The surface area of the sample gelled by EtOH suspension was 587

57 m2/g, coinciding with the result of Daily and Pinnavaia [3]. The sample produced by base or acid catalyst has a higher surface area and larger total pore volume than that gelled by no catalyst. These results can be explained by the point that the hydrolytic polycondensation of intercalated TEOS by acid and base catalyst could form silica clusters of more highly branched and stiff network structure. Pillared silica clusters expand the space between layered phases and affect the development of micropocity and the increase of surface area. These effects are most evident when intercalated TEOS is not released outside the layered phase during gelation. Figure 3(a) shows that the pore size distributions of silica-pillared magadiites. The sample treated with the base-catalyzed gelation has more microposity (diameter < 20 A ) and shows a sudden increase in mesopore volume near 36 A with a narrow pore size distribution. These results indicate that abrupt gelation by base catalyst is closely related to the formation of a more uniform pore. On the other hand, the sample gelled by the suspension of EtOH has a broad pore distribution. In case of gelation by acid catalyst, microporosity decreases and mesoporosity increases between 40 and 100 A compared with gelation with no catalyst.

IN,

t~ t-'

E

i

1 e

n,,

I

2

'

1'2

Degree 2e

I

18 20

4 6 8

I()121416

1'820

Degree 20

Figure 2. X-ray diffraction patterns of the calcined silica-pillared layered silicates: (a) magadiite gelled by EtOH suspension (b) base-catalyzed magadiite (c) acidcatalyzed magadiite (d) acid-catalyzed kenyaite (e) base-catalyzed kenyaite (f) kenyaite gelled by EtOH suspension.

.58 Table 2 Physical properties of silica-pillared magadiites Basal spacinga(A) 17.0

Gallery heightb(A) 5.8

Surface area (m2/g) 587

Total pore volume c (cc/g) 0.60

Base catalyst

39.2

28.0

845

0.73

Acid catalyst

33.3

22.1

648

0.62

Catalyst No catalyst

a : Sample calcined at 538~ b : Gallery height = Basal spacing - 11.2 A (thickness of H-magadiite) [16]. c : Total pore volume obtained from Gurvisch rule [17] of nitrogen adsorption isotherm at 77K.

3.2.2 Silica-pillared kenyaite

The gelation of intercalated-TEOS by catalyst produces siloxane-intercalated derivatives with well-ordered basal spacings as well as the expansion of gallery height. The X-ray diffraction patterns and basal spacings of the products gelled by EtOH suspension, base-catalyzed reaction, and acid-catalyzed reaction are shown in Fig. 2(d, e, f), and Table 3, and these products exhibit reflections corresponding to basal spacings of 29.5, 39.9,, and 39.5 A, respectively. The silica-pillared products gelled by base- and acid-catalyzed reactions exhibits a large increase in the gallery height of 22.2 and 21.8 A, compared with the gallery height of the product gelled by EtOH suspension (11.8 A). A distinctive increase of the gallery height is related to the size and the structure of pillared silica, which could be associated with the amount of intercalated TEOS, the gelation condition (catalyst type, solvent composition, pH etc.), and the rate of gelation. Gelation by base and acid catalysts could minimize the release of TEOS outside the layered phase during the gelation of the intercalated TEOS, because gelation time is markedly reduced. The hydrolytic polycondensation of intercalated TEOS by acid or base catalyst could form silica clusters of highly branched and stiff network structure. These conditions could derive the effective gelation of intercalated TEOS in the interlayer, and contribute to develop the size and structure of pillared silica clusters which bring about the large expansion of the gallery height of pillared kenyaite. The adsorption/desorption isotherms of nitrogen were obtained at 77 K. Several pore characteristics calculated from them are listed in Table 3. The specific surface areas were calculated by BET equation from the adsorption isotherm below P/Po-0.1. The specific surface area of H-kenyaite shows the low specific surface area of 84 m2/g. The calcined silica-pillared products have high surface areas between 533-606 m2/g, depending on the catalyst types. The total pore volume of

59 Table 3 Physical properties of silica-pillared kenyaites Basal spacing a (A)

Galler~ height u (A)

Surface area (m2/g)

Total Micropore surface area c (m2/g)

Total pore volume (cc/g)

H-ke nyaite

18.0

0.3

84

56

0.10

No catayst

29.5

11.8

606

509

0.49

Acid catalyst

39.5

21.8

584

490

0.46

Base catalyst

39.9

22.2

533

427

0.52

Items

a : Sample calcined at 538~ b : Gallery height = Basal spacing - 17.7 A (thickness of H-kenyaite) [8]. c : Total micropore surface area obtained from t-plot of the nitrogen adsorption isotherm at 77K. base-catalyzed sample is the largest among three pillared samples, indicating that the average pore size of base-catalyzed sample is the largest. Fig. 3(b) shows the pore size distributions of silica-pillared kenyaites. The basecatalyzed sample shows a sudden increase in mesopore volume near 22A with a narrow pore size distribution relatively. The acid-catalyzed sample shows a narrow pore size distribution with a sharp peak near 18A. The abrupt gelation of acid and base catalysts is closely related to the formation of a more uniform pore. On the other hand, the sample gelled by the suspension of EtOH has a broader pore distribution and more microporosity (diameter < 20A). It is interesting that acid- and base-catalyzed products ae very similar in physical properties to surfactanttemplated mesoporous silica. 4. CONCLUSION The acid- or base-catalyzed reaction of hydrolysis and condensation polymerization of TEOS into a layered silicate gallery could affect the physical properties of silicapillared magadiite and kenyaite. The samples that were silica-pillared by acid- and base-catalyzed reactions shows a large increase in basal spacing. Also, they exhibit relatively narrow pore size distributions in the range of 18-40A and show high surface areas in the range of 533-845 m2/g, depending on types of the catalyst and layered silicate. These results indicate that the variations in the conditions of gelation contribute to improvement in physical properties of silica-pillared molecular sieves.

60

[ No Catalyst ~ -- - Base Catalyst - - - Acid Catalyst

o~ ~2

a

......... Acid - - - - EtOH Base

o'~' 1.0 "~u m0.8 ~ 0.6

N o 0.4 ~ 0.2 o i

20

i

i

i

I

40 60 80100

i

200

Pore Diameter, [.~,]

(a) Silica-pillared magadiite

0.0

i

i

10

i

i

i

i

i i I

I

I

i

1

100

Pore Diameter(~) (b) Silica-pillared kenyaite

Figure 3. Pore size distributions of silica-pillared products.

REFERENCES 1) K. H. Berke, W. Schwieger, and M. Porsch, Chem. Tech., 39 (1987) 459. 2) O.Y. Kwon, S. Y. Jeong, J. K. Suh, H. Jin, and J. M. Lee, J. Colloid and Interface Science, 177, JCIS PN 3928 (1995). 3) J. S. Daily, and T. J. Pinnavaia, Chem. Mater., 4 (1992) 855. 4) M. E. Landis, A. B. Aufdembrink, P. Chu, I. D. Johnson, G. W. Kirker, and M. K. Rubin, J. Am. Chem. Soc., 113 (1991) 3189. 5) R. Sprung, M. E. Davis, J. S. Kauffman, and C. Dybowski, Ind. Eng. Chem. Res., 29(1990) 213. 6) H. P. Eugster, Science, 157 (1967) 1177. 7) J. McAtee, R. House, and H. P. Eugster, Amer. Mineral., 53 (1968) 1026. 8) K. Beneke and G. Lagaly, Amer. Mineral., 68 (1983) 818. 9) R.A. Fletcher, and D. M. Bibby, Clays and Clay Minerals, 35 (1987) 318. 10) A. Brandt, W. Schweiger, and K. H. Bergk, Rev. Chem. Miner., 24 (1987) 564. 11) C. J. Brinker, and G. W. Scherer, "Sol-Gel Science The Physics and Chemistry of Sol-Gel Processing." Academic Press, London, (1990), P 97-160. 12) R. Aelion, A. Loebel, and F. Eirich, J. Am. Chem. Soc., 72 (1950) 5705. 13) M. Nogami, and Y. Moriya, J. Non-Crysralline Solids, 37 (1980) 191. 14) J. H. Boer, B. G. Linsen, T. V. D. Plas, and G. J. Zondervan, J. Catalysis, 4 (1965) 649. 15) P. B. Barrett, L. G. Joyner, and P. P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 16) G. Lagaly, and K. Beneke, American Mineralogist, 50 (1975) 650. 17) L. Gurvitsch, J. Phys. Chem. Soc. Russ., 47 (1915) 805.

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials

Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.

61

A New Synthetic Route and Catalytic Characteristics of Pillared Rectorite Molecular Sieves

Jingjie Guan Zhiqing Yu Zhenyu Chen Liwen Tang and XieqingWang Research Institute of Petroleum Processing, China Petro-Chemical Corporation

This paper presents a new synthetic route and catalytic characteristics of pillared rectorite molecular sieves (PR-MS).

The PR-MS was prepared with aluminum pillaring

agent improved by poly (vinyl alcohol) according to route of mixing, drying to take shape, washing, and calcination.

It has wide pore structure with basal spacings of 27-30 ,~.

Under same conversion their activity for cracking heavy oil is 3-4 wt % higher and total yields of gasoline and light cycle oil are 6-7 wt % higher than that of commercial cracking catalysts containing USY zeolites. The catalysts developed from the PR-MS and Y-zeolites have high catalytic activity and low bottom yield. The PR-MS has also good light olefin selectivity. The catalysts containing the PR-MS and ZSM-5 type molecular sieves are a class of cracking catalysts for maximizing olefin production.

1. INTRODUCTION It is an important way for developing new energy resources to convert more heavy oil into liquid petroleum gas, gasoline and light cycle oil products. Although Y-type zeolites are commercially widely used as cracking catalysts their pore sizes are within the limits of less then 9,~. The pore sizes are not effective for cracking reactants with large molecule sizes. There is a need for new molecular sieves that are able to crack heavy oil and are easy to be commercialized in catalyst manufactory industry. Al-pillared rectorites developed by RIPP in 1985 are a class of new pillared clay molecular sieves [1]. The molecular sieves have excellent hydrothermal stability, solid acid sites and greater versatility than that of the faujusite [2]. These characteristics are advantageous for cracking heavy oil feedstock. However the prior pillared clay molecular sieves were prepared according to route of pillaring reaction of thin clay slurry and A1pillaring agent, washing, filtering, drying and calcination [3]. In general the clay particles are less than 2 mi'cra that are difficult to be filtered in commercial scale. In 1991 to tackle

62 the filtering problem a synthetic method without filtering operation of thin clay slurry was provided. However catalytic activity of the catalysts prepared by the method is not as good as Y-zeolite catalysts. For example, under same evaluated conditions by riser pilot unit the conversions of the two catalysts are 63 wt % and 67 wt % respectively [4] Up to now the pillared rictorite catalysts have not been used in FCC process. The object of this paper is to present a new synthetic route and catalytic characteristics of PR-MS with high catalytic activity. On the basis of direct research possibility of developing the PR-MS into commercial molecule sieves and microspheric cracking catalysts is studied. 2. EXPERIMENTAL 2.1 Synthetic routes of samples

Raw clays used for preparing samples are naturally rectorites with regularly interstratified mineral structure. Their quality specifications are conformable to properties in Table 1-2. Table 1 Physicochemical properties ofthe rectorites D001 by X-ray method /~

Cation exchange capacity meq/100g ....

Phase transformation temperature oC

23-24

40-60

1050

Table 2 Chemical composition of the rectorites Item Contents

_

Na20

CaO

Fe203

A!203

SiO2

wt %

1.2-2.0

3.5-6.0

< 1.5

39-43

43-51

Pillaring-bonding agent used for preparation of samples is Al-sol solution improved by poly (vinyl alcohol) (abbreviated PVA). Its 27NMR spectra are shown in Fig. 1. The PR-MS and catalysts containing PR-MS and other zeolite components were prepared by synthetic route A in Fig. 2 according to operation conditions reported in the literature [5]. The prior pillared interlayer rectorites (PIR) were prepared by conventional method [6] in accordance with synthetic route B in Fig. 2.

63

~.~ lOO

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

........ 50

0

-50

ppm

Figure 1 27NMR spectra of AI- pillaring-bonding agent

=---H ....

Rectorites Pillared-bonding agent Other composition ,.

H

Mixingand drying to take shape

Wa,shingFiltering of microspheric

m

Drying ..Samples Calcination

A: Synthetic route of the PR-MS and related catalysts

Rectorites-~

H Filteringof H Washing, .... FilteringU.... Drying

Pillaring

Pillaring agent..I I reaction

clay slurry

of clay slurry

..Samples

] i Calcination

B: Synthetic Route of the prior pillared rectorites Figure 2 Principle scheme of sample preparation It can be seen that in the prior PIR sample preparation special pillaring reaction is involved and the filtertion of clay slurry is difficult. However in the PR-MS preparation the special pillaring reaction operation required in prior art has been omitted because pillaring reaction can be completed in presence of PVA during mixing process prior spray drying. Also the pillaring agent improved by PVA is not only good pillaring agent used in thick clay slurry but also good bonding agent. The reaction mixture of the pillaring agent and clay is directly dried to take shape without filtering operation for fine clay slurry. It is easy to be produced in commercial scale. Besides this, it can contain other bonding agents required for preparing microspheric samples. Therefore, the new synthetic route can be used for preparation of microspheric cracking catalysts containing the PR-MS and other molecular sieves as well as the PR-MS

2.2. Physicochemical analysis 27NMR spectra were obtained by using Bruker Am-300 with operation conditions of SW-25000HZ SI=4K DE-300 LB=30 AQ-0085 RD--0.25 PW=3.

64 X-ray diffraction measurement was obtained by using Geigerflex D-9C X-ray diffractometor at a scan rate of 2~ 0/min and with monchromatic Cuka radiation. Surface areas and pore volumes were measured by using BET method from nitrogen adsorption isotherms. 2.3. Catalyst testing

Microactivity test was used for evaluating catabr characteristics of samples. The samples were deactivated at 800~ for 4 hours with 100% steam before evaluation. Catalytic activity for cracking light gas oil (235-337~ operation conditions of reaction temperature of 500 ~ (WHSV) of 16hr-1 and catalyst to oil ratio (c/o) of 3.2. Catalytic activity for cracking heavy oil (330-520~ conditions of reaction temperature of 520~

was obtained according to weight hourly space velocity

was evaluated by

operation

WHSV of 16hr-: and C/O of 3.0.

3. RESULTS AND DISCUSSION 3.1. Evaluation of new synthetic route

The chemical composition, physico-chemical properties and X-ray diffraction pattern of the PR-MS samples are shox~ Table 3-4 and Fig. 3. Table 3 Chemical composition of the PR-MS Contents Samples . . . .

wt %

Na20

CaO

Fe203

A1203

SiO2

The PR-MS

1.5

3.9

0.9

49.8

44.6

The prior P,IR

1.5

3.3

1.3

48.5

42.5

Table 4 Surface areas and pore volumes of the PR-MS Samples

Surface areas m2/g Fresh Steamed at

pore volumes ml/g Fresh Steamed at

800~ for 4hrs

800~ for 4hrs

The PR-MS

145

127

0.12

0.10

The prior PIR

144

101

0.13

0.10

65

29A

29A

=o cD

27/~

.>.

..>

3

I~

l

I

2 Theta

2 Theta

A: Sample prepared by present work

B: Sample prepared by prior method

Figure 3. X-Ray diffraction patterns of pillared rectorites 1" fresh sample. 2" sample steamed at 800~ for 4 hrs The results from Table 3-4 and Fig.3 indicate that the PR-MS prepared by new synthetic route in present work has physicochemical properties similar to prior PIR. The PR-MS has basal spacing of 29/~ for fresh samples and basal spacing of 27,~, for samples steamed at 800~ for 4 hrs that is the same as prior PIR, Especially, the height of d 001 peak for the PR-MS is higher than that of prior PIR indicating that a class of good pillared clays can be obtained by the simplified preparation method in present work. The new synthetic route of pillared clays and related catalysts has a bright future for commercial production. Catalytic activity of the PR-MS as compared with prior PIR is listed in Table 5. Table 5 Catalytic activity of samples for cracking light gas oil The PR-MS at present work

The prior PIR

Samples

Commercial USY catalyst

Microactivi~ %

75

71

72

66 The data in Table 5 indicate that PR-MS has microactivity of 75 wt % versus microactivity of 71 wt % by prior PIR and microactivity of 72 wt % by commercial USY catalysts.

It

means that catalytic activity of the PR-MS prepared by the new synthetic route is higher than that of prior PIR and commercial USY catalysts. It fully proves that the new synthetic route provided in present work is successful for preparing

pillared clays and related

catalysts.

3.2. Catalytic properties of the PR'MS and catalysts containing the PR-MS 3.2.1 Reaction characteristic for cracking heavy oil The activity and selectivity for crackling heavy oil with the PR-MS and catalysts containing the PR-MS and Y zeolites as compared with commercial USY catalysts are shown in Table 6. Table 6 Activit), and selectivity of th e PR-MS for cracking heavy oil Conversion Samples

Yield ,,.

wt

%

LCO

Bottom

wt %

Gas

Coke

Gasoline

USY catalysts The PR-MS

77.2

21.3

2.1

53.8

12.9

9.9

.at present work

77.4

16.1

4.7

56.6

16.4

6.2

The catalyst containing PR-MS and Y zeolites

81.8

22.3

3.6

55.9

13.8

4.4

Commercial

The data from Table 6 indicate that in cracking reaction with commercial USY catalysts a 9.9 wt% of bottom are remained but in the same operation conditions with the PR-MS and their catalysts only 6.2 wt % and 4.4 wt % of bottom are remained respectively. Obviously the PR-MS and its catalysts have catalytic activity for cracking heavy oil much better than that of the commercial USY catalysts. Also under almost same conversion level (77.2-77.4 wt %) the PR-MS yields gasoline of 56.6 wt % and light cycle oil of 16.4 wt % versus gasoline of 53.8 wt % and light cycle oil of 12.9 wt % by commercial USY catalysts indicating that the PR-MS has good gasoline and light cycle oil selectivity. The results are corresponding to wide pore structure of the PR-MS which is favored for cracking reactants with large size molecules.

67 3.2.2 Reaction characteristic for maximizing olefin production. The olefin selectivity of the PR-MS catalysts as compared with commercial REHY catalysts are listed in Table 7. Table 7 Light olefin selectivity of the PR-MS and for cracking heavy oil Samples Evaluation conditions

Conversion

wt %

Catalyst-1 REHY+Matrix

REHY+PR-MS

520~ C/O 3 WHSV 16-1

520~ C/O 3 WHSV 16-1

71

.Catalyst-3 ZSM-5+PR-MS 520~ C/O 4.5 WHSV 16-1 J

73

71

20.2 2.8 47.9 15.3 13.8

17.5 4.1 51.2 16.8 10.4

38.1 2.4 30.5 16.9 12.1

C~

0.8

0.6

1.7

C4

4.1

4.4

14.5

C5 C2 "C5

5.4

6.5

15.0

3.4 13.7

5.5 17.0

8.5 39.8

1.2

1.7

6.1

2.0

3.1

5.8

3.2

4.8

11.9

Product yield wt % Gas Coke Gasoline Light cycle oil Slurry Olefin yield wt%

iC] iC~

yield wt %

iC~-iC5

The data from Table 7 indicate that the PR-MS has not only high capability for converting heavy oil but also good light olefin selectivity. Under almost same conversion level (71-73 wt %) the catalyst-2 containing the PR-MS and the REHY zeolites has C:-C5 yield of 17 m

wt % and iC4-iC5 yield of 4.8 wt % but catalyst-1 containing only the REHY zeolites as active composition has C, -C5 yield of 13.7 wt % and iC4-iC~ yield of 3.1 wt %. It means that the light olefin selectivity of the PR-MS components is better than that of the Y-type

68 zeolites. Catalyst-3 developed from the PR-MS and the ZSM-5 type zeolites have the best light olefin selectivity in the three samples. It has C2 "C5 yield of 39.8 wt % and iC~-iC~ yield of 11.9 wt %. Obviously, the catalysts containing the PR-MS and ZSM-5 zeolites are a class of cracking catalysts for maximizing olefin production. The results above mentioned have clearly demonstrated that the PR-MS is both active components like molecular sieves and high activity matrix composition of catalysts. It can be used for preparation of cracking catalysts to convert more heavy oil .into light olefins gasoline and light cycle oil products.

4. CONCLUSION 1 The pillared rectorite molecular sieves (PR-MS) and related catalysts with high catalytic activity can be obtained by special Al-pillaring agent according to synthetic route of mixing, drying to take shape, washing and calcination. The preparation procedures are easy to be put into effect in commercial scale. 2 The PR-MS and microspheric cracking catalysts developed from the PR-MS and Yzeolites have high catalytic activity for cracking heavy oil and good selectivity of gasoline and light cycle oil. It is much better than that of commercial USY microspheric catalysts. 3 The PR-MS has better selectivity of light olefins than that of Y-zeolites. The catalysts containing the PR-MS and ZSM-5 type zeolites are a class of new cracking catalyst for maximizing light olefin production. REFERENCES !. 2. 3. 4. 5. 6.

Jingjie Guan; Enze Min and et al, Proceeding 9th International Congress on Catalysis Vol. 1, p104-111. Calgary, Canada ,(1991 ). Jingjie Guan and Thomas J. Pinnavaia, Abstract Proceeding International Symposium on Soft Chemistry Routes to New Materials in Nantes,France (1993). N. Lahav, U. Shani and J. Shabtai, Clays and Clay Minerals, Vol. 26, No. 2, p107-115 (1978). Jingiie Guan and et al , Proceeding of the International conference on Petroleum Refining and Petrochemical Processing, Beijing, P.R.China Vol. 3, p1255 (1991). C.N. 92114024 X (1995) U.S.Patent 4,757,040 (1988)


69

Textural Control of M C M - 4 1 Aluminosilicates Francesco Di Renzo, Nicole Coustel, Miren Mendiboure, H616ne Cambon and Franqois Fajula Laboratoire de Mat6riaux Catalytique et Catalyse en Chimie Organique, URA 418 du CNRS, Ecole Nationale Sup6rieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34053 Montpellier, France, fax (33)67144349

The thickness of the walls between mesopores in MCM-41 sieves can be controlled by modifying the solubility of silica and aluminosilicate species, notably by changing the alkalinity of the synthesis system. This effect strongly influences the thermal stability of aluminosilicate MCM-41 in activation conditions.

1. INTRODUCTION The disclosure of the properties of micelle-templated mesoporous silicas [1-3] has represented a twofold breakthrough. From the point of view of the materials scientist, it has marked the starting point of a blossoming research on the selforganization of inorganics and surfactant molecules [4-6]. From the point of view of the catalysis and sorption technologist, it has fullfilled a long expectation for solids with custom-tailored uniform pores of size larger than the micropores of zeolites [7]. The pore size is easily controlled by choosing a surfactant able to form micelles of the required diameter. For instance, the diameter of the hexagonally-packed cylindrical pores of MCM-41 silica can be adjusted at any value in the 18-37 A range by incorporating alkyltrimethylammonium ions with hydrocarbon chains ranging from 8 to 16 carbons [2]. Less attention has been paid to another textural parameter of the MCM-41 honeycomb: the thickness of the silicate walls which separate the micelles of the as-synthesized material and the mesopores of the activated sieve [8]. Molecular

70 dynamics simulations have shown this parameter to control the stability of the MCM-41 framework [9]. This communication deals with the correlations between the synthesis and activation parameters and the texture of the final mesoporous sieve, and the informations thereby provided about the mechanism of self-assembly of cationic surfactant and silicates.

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

MCM-41 aluminosilicate samples were prepared from synthesis systems of composition xNa20 2.8CTMA20 A1203 55SIO2 1800H20, where CTMA stands for cetyltrimethylammonium. The samples were numbered from 1 to 6 in the order of increasing

alkalinity,

expressed

as

[(Na++CTMA+-AI(OH)4-Br-2SO42")/SiO2].

Alkalinity values of the synthesis mixtures are reported in Table 1. Reagents were mixed with stirring at 70~

in the order:

deionized water,

NaOH

(SDS),

AI2(SO4)318H20 (Rh6ne-Poulenc Prolabo), CTMABr (Aldrich), precipitated silica (Rh6ne-Poulenc Zeosil 175MP, 175

m2/g, grain

wt%). The gel was heated up to 120~ autoclave, kept 15 minutes at 120~

size 2-201.t, Na 0.07 wt%, AI 0.17

in 1 hour in a 130 ml stirred stainless steel

and rapidly cooled down.

Table 1 Alkalinity of the synthesis medium and properties of mesoporous aluminosilicates.

Sample OH-/SiO2

a~ A

a550 A

AW373K p/po

Vmp

SBET

Woo

ml/g

mE/g

Woo

AI (Si+AI)

1

0.02

no MCM-41 formed

2

0.14

57

52

0.08

0.39

0.38

0.49

630

0.031

3

0.29

54

48

0.12

0.56

0.36

0.66

860

0.042

4

0.40

51

47

0.05

0.59

0.33

0.73

960

0.050

5

0.52

50

44

0.10

0.74

0.32

0.82

1090

0.069

6

0.65

49

44

0.12

0.77

0.28

0.55

940

0.083

71 The products,

washed with water and ethanol and dried at 80~

were

characterized by powder X-ray diffraction (XRD, CGR Th6ta 60 diffractometer, 0.25 mm slits, monochromated Cu Kt~ radiation), scanning electron microscopy (SEM) and electron probe microanalysis (EDX) (Cambridge Stereoscan 260 apparatus), thermal gravimetry (TG, Setaram B85 instrument, air flow, 15 mg samples, heating rate 5 K/rain), and N2 sorption at 77K (Micromeritics ASAP 2000C, 250~ 850~

outgassed samples) after calcination at temperatures in the 550-

range (air flow, heating rate 1 K/rain, 8 hours isotherm).

3. RESIILTS In Figure 1 a typical XRD diagram is reported, showing a peak slightly above 1 ~

with a width at half height of 0.4 ~

and a broad band centered around 2 ~

The two features correspond, respectively, to the 100 line and to the convolution of the 110 and 200 lines of a MCM-41 structure with low long-range order [10, 11]. The parameter a of the hexagonal lattice of all samples as synthesized and after calcination at 550~

are reported in Table 1. A shrinking of the hexagonal

cell with calcination is observed, as a decrease of the lattice parameter for the samples prepared at higher alkalinity. Intensities of the XRD lines significantly increase with calcination, probably due to the better contrast which results from

Figure 1. X-ray diffraction diagram of sample 3 after calcination at 550~

,,~1,,,i,,,i,~,1,~,1,,,i,,,i,,,i,,,i,,~1,,,

1

2

Theta Degrees

72

the extraction of the organic phase. The TG patterns are in good agreement with literature reports [3] which allow to attribute the weight loss below 100~ above 100~

to water desorption and the weight loss

essentially to the degradation and extraction of the organic phase.

Weight losses below and above 100~

expressed as fractions of the final mass are

reported in Table 1 for all MCM-41 samples. The weight loss corresponding to the surfactant decomposition is larger for samples prepared at higher alkalinity. In Figure 2 the isotherms of N2 sorption at 77 K are reported for a typical sample after calcination at 550 and 850~

(Figures 2a and 2b, respectively).

Sorption curves are typical type IV isotherms [12] without any hysteresis below p/po 0.9. The sorption step in the p/po range 0.3-0.4 is sharp for the samples calcined at 550~

(Figure 2a), indicating a narrow pore size distribution. The step

of the isotherm is less sharp and shifted to a lower pressure for the samples

700

Figure 2. Nitrogen sorption

600 n CO

400

o

300

O W

m

nt (D c0 Q

.2 0

/

500

c~

%, 0

-f

200 100

f

isotherms at 77K for sample

J

3 calcined at 550~

a) and 850~ (curve b). (+) adsorption, (*) desorption.

0

600 500 400 300

-//

f

200 100

0

~

0.0

~

I

T

0.2 RELATIVE

(curve

t

0.4

1

T ~ T

r

0.6

PRESSURE

0.8 ,

(P/Po)

I

73 calcined at higher temperature (Figure 2b), indicating a broader size distribution of mesopores with a smaller average diameter. The isotherm of sample 6 already features a less sharp sorption step after calcination at 550~ The partial pressure of the sorption step is reported in Table 1 for all samples calcined at 550~

as the mesopore volume evaluated at the top of the step and the

surface area from the BET equation for P/P~

The partial pressure of the

sorption step slowly decreases as the alkalinity of the synthesis system increases. Mesopore volume and surface area increase at increasing alkalinity up to a maximum at a OH/SiO2 0.5, and decrease for further increases of alkalinity. The c parameters of the BET equation are 102+5 for samples calcined at 550~ and 62+10 for samples calcined at 850~

A significant sorption takes place

beyond the filling of mesopores, in the partial pressure range 0.4-0.9, suggesting a large outer-surface area for the MCM-41 grains. The t-plot of the data of Figure 2a is reported in Figure 3. It indicates that no micropores are present (absence of downward deviation) [13] and confirms that capillary condensation takes place in mesopores with narrow size distribution. The gentle slope of the t-plot beyond the capillary condensation step corresponds to sorption on the outer surface of the mesoporous grains. SEM of all samples shows large aggregates of less than 100 nm grains. The aluminium mole fraction measured by EDX is reported in Table 1 for all samples. The aluminium content increases at increasing alkalinity of the synthesis medium.

EL i-O3

000 00

400 121 \ o o

0 W

m [~ o LO n CE

-

Figure 3. t-plot for the N2 sorption isotherm at 77K of sample 3 calcined at 550~

200

1 lZllZl

_J 0

IZl

-

!

IZI

1

u... t-HARKINS

i

i

i

1

4 ,~, J U R A

b ~.

(A

74 4. DISCUSSION The alkalinity of the

synthesis

system affects

several properties

of the

surfactant-silicate assembly. For any alkalinity value high enough to satisfactorily dissolve the source of silica, materials corresponding to the definition of MCM-41 sieves [14] are formed. The width of the XRD peaks does not correspond to a wide pore size distribution, suggesting that the lumping of the 110 and 200 peaks in one broad signal corresponds to a lack of long-range order, in agreement with the observed size of the MCM-41 grains, no larger than a score of lattice patterns. The small grain size can be accounted for by heterogeneous nucleation of MCM41 on colloidal silica [ 15]. In Figure 4 the values of the lattice parameter a and the CTMA content of the as-synthesized MCM-41 samples are reported as a function of the alkalinity of the synthesis medium. As alkalinity increases, the lattice parameter a, corresponding to the distance between micelle axes, decreases from 57 to 49 /~, while the CTMA/SiO2 mass ratio increases from 0.39 to 0.77. A simultaneous increase in the organic fraction and decrease of the distance between micelle axes corresponds to a lower volume fraction occupied by silica, hence to thinner walls between micelles. A simple geometric model, assuming hexagonal pore section [16], CTMA density equal to the density of hexadecane (0.77) and amorphous silica density 2.2 [17], allows to calculate the average wall thickness t. The calculated t values, reported in Figure 4, regularly decrease at increasing alkalinity and account for the observed decrease of the lattice parameter a. The micelle diameter of the as-

e.....

Fig. 4. Lattice parameter a ( 9 ), 1.0

50

.
80, a lamellar phase or a MCM-41 sample (designated MCM-4 l-A) with a well-defined XRD pattern showing the (100), (110), (200), and (210) reflection is. readily formed. However, when the H20/SiO2 falls into the range between 10-70, a mesoporous material (denoted MCM-41-B) which exhibits a strong broad XRD reflection at a 20 angle lower than 2 ~ and a weak one at around 3.5 ~ was obtained (Fig. l b). The typical gel composition for synthesis is shown in Table 1. By varying the synthetic

500~ C 400"C 250~ MCM-41-A 175~ without treatment MCM-41-B

2.0

4.0

6.0 20, o

Fig. 1 XRD patterns of MCM-41-AI and B2.

8.0

2.0

4.0

6.0

8.0

10.0

20, ~ Fig. 2 XRD patterns of typical solid precursor treated at different temperature for l h.

80 Table 1. Gel compositions and synthetic conditions for MCM-41 materials Sample

Gel composition a

Reaction

pH

temp(~

Reaction time (Hs)

MCM-4 I-A 1

0.06AlzO3:SiO2:0. 5( 16Br):0.18Na20:107H20

140

11

72

MCM-4 i -A2

0.05A1203:SIO2:0.25(16CI):0.23Na20:107H20

140

!1

72

MCM-4 ! -B 1 MCM-4 ! -B2

0.04AlzO3:SiO2:0.25(l 2CI):0.30Na20:30H~O

150

12

72

0.04A1203:SIO2:0.47(16CI):0.65Na20:23H~O

150

11

72

MCM-41-Cl MCM-4 ! -C2

SIO2:0.09(16C!):0.13(TMA)20: ! 8H20 SIO2:0.09(16CI):0.13(TMA)20:0.4MES: 18H20

150 ! 50

13 13

1 I

MCM-41-D1

TEOS:0.1 (16Br):(0.23-0.47)NazO: 118H20

-25

>10

0.5-10

MCM-41-D2

TEOS:0. i 3(16Br): 12HCI:94H20

-25

SiMCM-41 "; A1SiMCM-41.

e ~

.ill~,

"" i

l..,, IJ

__

i

2

4

6

2 | (degrees)

8

2

4

6

8

2 | (.degrees)

Figure 6 The XRD patterns for calcined (a) sample 1 and (b) sample 3, solid lines befbre calcination, dashed lines after calcination at 1153 K in air for 2 h. It was reported that the framework of A1SiMCM-41 collapsed at 963 K in stea~n treatmemt[3]. In our study, AISiMCM-41 (sample 9) steamed at 933 K lost its XRD pattten~ almost completely, which indicated its poor hydrothermal stability. However, FeSiMCM41(sample 3) and LaSiMCM-41(sample 6) still remained a stronger (100) peak and a high equih'bdum adsorption capacity for benzene under the same treatment conditions. Compared XRD values of Ill0 and c/c0 listed in Table 3, the sequence of the hydrothermal stability of samples is, FeSiMCM-41 = LaSiMCM-41 > SiMCM-41 > LaSiMCM-41 > AISiMCM-4 I.

91 From the above results, we suggest that the introdoction of Fe or La into the SiMCM-4 I increase both the thermal and hydrothermal stab'dities, while AI decrease them. Table 3 Hydrotherm__al stability of samples Before steam treatment

Sample

d,oo(nm)

1 3 6 8 9

4.03 4.46 4.08 4.19 4.15

After steam treatment

co(g/g)

dl0o(llm)

0.680 0.605 0.548 0.465 0.655

3.72 4.12 3.72

c(g/g) 0.381 0.408 0.348 0.234 0.278

1/I0(%) 54 81 83 very weak peak very weak peak

c/c0(%) 56 67 64 50 42

I0, I : The intensities of(100) peaks before and after steam treatment respectively. co, c : The adsorption capacity for benzene at P/P0=0.5 before and after steam treatment respectively. Figure 7 shows the EPR spectra ofFeSiMCM-41 at different calcination time. It was found that the color of FeSiMCM-41 samples changed from white to brown by increasing the calcination time, accompanied by decreasing the signal intensity at g=4.3 and increasing that at g=2.0 (Figure 7 b-d), suggesting that the tetrahedrally coordinated Fe(III) in as-synthesized sample FeSiMCM-41 progressively converted to octahedrally coordinated Fe(Ill) as hmreashlg the calcination time. The Mossbauer spectra also shows the same change of the coordination

8 a

b

d

Figure 7 EPR spectra of calcined sample 3. a--as-synthesized, b--1 h, c--4 h, do-8 h calcined at 813 K in air

92 enviroment of Fe(III) before and after the calcination (Figure 3). Thus, we suggest that the octahedrally coordinated Fe( Ill ) species in FeSiMCM-41 may increase the thermal and hydrothermal stabilities of the channel wall ofmesoporous material 4. CONCLUSION The hexagonal mesoporous FeSiMCM-41 and LaSiMCM-41 have been synthesized using sodium silicate as silica source. Fe( III ) and La( III ) species may impove the thermal and hydrothermal stabilities of SiMCM-41 but AI(III) destabilizes it. It seems that octahedrally coordinated Fe(Ill) species led to increase of thermal and hydrothermal stabilities of the channel wall ofmesoporous FeSiMCM-41. REFERENCES

1. C.T.Kresge, M.E. Leonowicz, W.J.Roth, J.C.Vartuli and J.S.Beck, Nature, 359 (1902) 710. 2. J.S.Beck, J.C.Vartuli, W.J.Roth, M.E.Leonowicz, C.T. Kresge, K.D.Schmitt, C. T-U. Chu, D.H.Olson, E.W.Sheppard, S.B. McCullen, J.B.Higgins and J.L.Schlenker, J.Am. Chem Soc., 114(1992) 10834. 3. A.Monnier, F.Schuth, Q. Huo, D. Kumar, D. Margolese, R.S. Maxwell, G.D. Stucky, M. Kishnamurthy, P. Petroff, A.Firouzi, M. Janicke and B. F. Chmelka, Science, 261 (1993) 1299. 4. C.Y. Chen, S.L. Burkett, H. X. Li and M. E. Davis, Microporous Materials, 2 ( 19q3 ) 27. 5. Q. Huo, D.I. Margolese, U. Ciesia, P. Feng, T.E. Gler, P. Sieger, IL Leon, P. M. Petroft~ F. Schuth, and G. D. Stucky, Na~-~, 368 (1994) 317. 6. P. T. Tanev, M. Chibwe and T. J. Pinnavala, Nature,368 (1994) 331. 7. P. L. Llewellyn, U. Ciesla, H. Decher, R. Stadler, F. Schuth and K.K. Linger, Stud. Surt~ Sci. Catal., 84 (1994) 2013. 8. A. Corma, M. T. Navarro, J. Perez-Pariente and F. Sanchez, Stud. Sure Sci. Catal., 84 (1994) 69. 9. O. Franke, J. Rathousky, G. Schulz-Ekloff~ J. Starek and A. Zukal, Stud. Surf Sci. Catal.. 84 (1994) 77. 10. S.B.McCullen, J.C. Vartttli and W. Chester, Method for Stabili~ng Synthetic Mesoporous Crystalline Material, U.S Patent No. 5 516 829(1992). 11. N. Coustel, F. Di Renzo and F. Fajula, J. Chem. Sot., Chem. Commun., (1994) 967. 12. R. Ryoo andJ. M. Kim, J. Chem. Sot., Chem. Commtm., (1995) 711. 13. Z.Y. Yuan, S.Q.Liu, T. I-L Chen, J.Z. Wang and H.X. Li, J.Chem.Soc., Chem. Commum.. (1995) 973. 14. 1LSzostak, V.Nair and T.L.Thomas, J.Chem.Soc.Faraday Trans.I.(1987) 487. 15. P. Ratnasamy and 1L Kumar, Catal. Today, 9 (1991) 329.



93

Synthesis of titanium-containing mesoporous molecular sieves with a cubic structure K. A. Koyano and T. Tatsumi Engineering Research Institute, Faculty of Engineering, The University of Tokyo, Yayoi, Tokyo 113, Japan 1. INTRODUCTION Recently a new family of mesoporous molecular sieves named M41S was discovered by the researchers at Mobil [1, 2]. The M41S family is classified into several members: MCM-41 (hexagonal), MCM-48 (cubic) and other species. These materials have uniform pore, and the pore size are able to be tailored in the range 16/~ to 100/~ through the choice of surfactant, auxiliary chemicals and reaction conditions. Because of the favorable uniformity and size of the pore, synthesis and utilization of mesoporous materials have been investigated by numbers of researchers. Some circumstantial reports about the synthesis have been made [3-5]. Adsorption property and catalytic activity of silica-based and Al-containing MCM-41 have also been reported [6, 7]. Ti-, V-, B-, Fe- and Mn-substituted MCM-41 and Ti-substituted hexagonal mesoporous silica (Ti-HMS) have also been synthesized [8-15]. These Ti- and V-substituted mesoporous molecular sieves pioneered the potential to oxidize bulky molecules which cannot enter into the micropores of zeolites such as TS-1, TS-2 and Ti-beta. MCM-48, characterized by a three-dimensional channel system, may have several advantages over MCM-41 with a one-dimensional channel system when applied to catalytic reactions: for instance, the three-dimensional pore system should be more resistant to blockage by extraneous materials than the one-dimensional pore system. Here we report the synthesis of Ti-MCM-48 and its use as a catalyst for epoxidation of alkenes and unsaturated alcohols. The effects of gel composition and the gel preparation method on the structure of mesoporous materials are also reported. 2. EXPERIMENTAL

Mesoporous materials were synthesized under hydrothermal conditions at 373 K in a static Teflon bottle for 10 days. The procedures of gel preparation were as follows. For the preparation of pure silica mesoporous materials, an aqueous solution of cetyltrimethylammonium chloride / hydroxide (CTMACI/OH, CI/OH - 70/30) was added dropwise to tetraethyl orthosilicate (TEOS) under vigorous stirring at 278 K. After stirred for 1 h, the mixture was heated at 358 K for 4 h to remove the ethanol produced in the hydrolysis of TEOS. For the synthesis of Ti-containing mesoporous materials, two types of hydrolysis method were employed. Ti-MCM-48(1) was prepared by a one-stage hydrolysis method: TEOS and tetrabutyl orthotitanate (TBOT) were hydrolyzed simultaneously after being mixed for 30 minutes at 298 K. Ti-MCM-48(2) was prepared by a two-stage method : To a 44% solution

94 of TEOS in propan-2-ol, CTMAOH in methanol and water (water / TEOS (molar) = 2) were added to partly hydrolyze TEOS at 278 K. After 1 h, a propan-2-ol solution of TBOT was added to this resultant mixture very slowly under vigorous stirring. The.mixture was then stirred for 1 h, when the aqueous solution of CTMAC1 was finally added. When water-glass (SiO 2 = 28 - 30%, Na20 = 9 - 10%) was used as the Si source, sulfuric acid was added to the mixture in order to adjust the pH to 11.6 and CTMAC1 was used as the template. The molar compositions of the gels subjected to hydrothermal synthesis (373 K, 10 days) were as follows : SiO 2. xTiO 2 9yNa20 9CTMA 9zH20, where 0 _< x _ 4. For low ] OH -I , / ISiO_! correspono~ng~h high Si/AIratios in the products the function o ~ ' o n thisz , ~ / b e c o m e s uniform for" all n. For lower values of n, k decreases distinctlg. In this region k is proportional to I OH - Isol * ISi02 ! sol with some accuracg. Both relations can be brought into a common equation bg k

=

or"

k

=

C 9( I OH -

- I SiO2 I=o )'1 sio2]~o t ( 1 + I SiO 21 sol ) 2

C *9 I AI(OH)4 - I~ol * ISiO 2 I sol )2 ( 1 + I Si02 I~ot

(g)

(10)

130

c-

E I

O tO r"

o

"-U

o

~-

c~i

4.0

+

Si/AI = 1.4

!

, ~ d

3.0 J 2.0

i

-

1.0

aJ

........................i.....................

--'i .......

i-"

121(

',

0.0 -2.0

2.0

i

1

I

6.0

10.0

14.0

(I NaOH l-IS~O= I)/I s~o= I

9

1.5

9

2.0

a

3.0

,,

4.0

o

5.0 9 6.0-9.9

9

>

10.0

Fig. 3 Rate constant of linear growth k in dependence on the ratio

IOH-Iso~/ISi021so~ c-

4,0

E I

o ,,I,--.

tO t/} tO u

o L_

, !

3,o

I

Si/AI = 1.4

i

i i

L -T--'

i io@O

o 9

2,0 ~- .... : ........... ,.-.-~s-; .............. 1,0

t

i

I~

~

i

1

i

0,0

0,2

0,4

!

=

i i

2.0

v

3.0

+

4.0

o

5.0 9 6.0-9.9

o,o -0,2

1.5

0,6

9

>

10.0

(I NaOH I-ISiO2 I)*1 s~o= I/(] + I SiO2 I) = Fig. 4 Rate constant of linear growth k in dependence on the relation given bg Eq.g

131

because it is [AI(OH) 4 ]-sol "" l OH-J,ol over a wide range of concentrati0ns. The data corresponding with Eq.9 are demonstrated in Fig.4. It can be seen that the data are described by this equation with good accuracy. Eq.9 is together with Eq. 10 similar to a LANGMUIR-HINSHELWOOD -equation for the formation of a structure of the kind (11) - Si(OH)2 - 0 - AI (OH)2 This means that the concentration dependence of the crystallization rate can be understood by a model in which the formation of a surface structure as e.g. a fourmembered ring is a rate determining event [14]. For a decision whether the ring closure or the attachment of the silicate or aluminate may be rate determining a more detailed discussion with the more extended relations of HOUGHEN and WATSON [15] has been carried out For the attachment of aluminate as rate determining step can be written

C'.( I AI(OH)4- I~o~ - SS/(ISiO21.ot*K) )

k =

(1+

CAt

AI(OH)4 -I ~ot * cslSiO

2

Isol }2

(12)

SS is the synonym for the surface structure and K the equilibrium constant of the formation of the ring structure from neigbouring - Si(OH) and -AI(OI-i) - 3 3 groups at the surface. K can be regarded to be large taking into account the low solubility of the aluminosilicate. Eq.12 becomes different from Eq.10. It can be shown that a uniform description of the experimental data is not possible by Eq. 12. Therefore, it can be seen that the dependence of k on the concentrations in the solution phase can be described by the formation of the rings given by (11) - or possibly structures formed in a consecutive reaction - as rate determining step. From Eq.10 follows that k is for the whole range of faujasites proportional to the concentation of the aluminate in the solution. The crystallization rate should, therefore, be increased with the concentration of the aluminate.. In two separate papers [16,17] the effect of the addition of fluoride and a series of complexing agents for the aluminate was thoroughly studied. It could be seen that the crystallization rate can be appreciably increased, especially for the crystallization of Y-zeolites supporting the foregoing arguments.

5. A

C

~

~

The authors thank for the support of this work by the "Deutsche Forschungsgemeinschaft.

132

6. R E F E ~ E S 1. P. Caullet and J. L. Guth, ACS Symp Series, 398 (1989) 83. 2. G. Harvey, L. S. Dent Glasser, ACS Symposium Series, 398 (1989) 49. 3.A.T. Bell, ACS Symp. Series, 398(1989)66. 4. S.P. Zhdanov and N.N. Samulevich Proc. 5th Intern. Conf. on Zeolites L. V. C. Rees Ed., Heyden, London 1980, p. 75. 5.M. Avrami, J.Chem.Phys., 9 (1941) 177. 6. R.W.Thompson, Zeolites, 12 (1992) 680. T. H. Kacirek and H. Lechert, J. Phys. Chem. 79 (1975) 1589. 8. H.Kacirek and H. Lecher-t, J.ChemPhys. 80(1976) 1291. 9 H.Lechert, P. Staelin, M. Wrobel and U. Schimmel, Studies in Surface Science and Catalysis 84A (1994) 147. 10. H.Lechert, P. Staelin, and Ch Kuntz, Zeolites 16 (1996) 149. 11. S.P. Zhclanov, S.S. Khvoshchev, and N.N. Feoktistova, "Synthetic Zeolites", Gordon and Breach Science Publishers, New York, Philadelphia, London, Paris, Montreux,Tokyo, Melbourne, 1990. 12. D. W. Breck, "Zeolite Molecular Sieves" John Wiley, New York, 1974 13. H. Lechert, H. Kacirek, and H. Weyda, in "Molecular Sieves" M.L. Occelli and H. Robson Eds., Van Nostrand Reinhold, New York, 1992, p. 494. 14. H. Lechert, Zeolites (in press). 15. O. A. Houghen and K. M. Watson "Chemical Process Principles" Part III Wiley & Sons, New York, 1948. 16. T. Lindner and H. Lechert, Zeolites, 14 (1994) 582. 17. T. Lindner and H. Lechert, Zeolites, 16(1996) 196.


133

A computational 'Expert System' 'approach to design synthesis routes for zeolite catalysts T. Selvam, D.N. Iyert , R.C. Deka, A. Chatterjee* and R. Vetrivel* Catalysis Division, National Chemical Laboratory, Pune-411 008, INDIA. Fax: +91-212-334761 /330233.

We have developed a knowledge based database system on zeolites for IBM-PCs operating on MS-DOS. The system contains a database of physico-chemical and structural properties of all the molecular sieves. The data could be retrieved from the database by choosing answers to questions from a menu driven sot~ware and hence no background knowledge of computers is assumed. Further, the wide experience in the synthesis of molecular sieves, particularly zeolites, ZSM-5, ZSM-11, AIPO4-5 and AIPO4-11 available in the literature are collected and conserved. An 'expert system' approach is developed to derive a set of conditions to achieve one's goal in the synthesis of zeolites. The most suitable conditions of synthesis are decided based on the published data in the literature or based on the correlations derived from the reported data. This system includes the graphical display of the powder diffraction pattern for various molecular sieve structures, as well as their mixtures. By pattern matching procedure, it is possible to identify and analyze the impurity phases and yield in the synthesis batches.

1. INTRODUCTION Molecular sieve materials which comprise zeolites, ALPOs, etc., are the single class of catalysts having maximum commercial applications. The application of zeolite catalysts has penetrated into the areas of environmental catalysis and fine chemical synthesis, in addition to petroleum and petrochemical industries. Nearly 100 molecular sieves with established structures are reported [1] and more than 200 hypothetical molecular sieve structures are predicted [2]. In proportion to their scientific and technological importance, knowledge and information on existing as well as new zeolite structures are accumulating over the years. The design of a new catalyst involves logical application of all the available information. It is becoming a difficult task to manage the available information on molecular sieves by human experts. , Present address : Dr. Reddy's Research Foundation Bollaram Road, Hyderabad 500 138, India * Present address : Tohoku National Industrial Research Institute, AIST - MITI, 4-2-1, Nigatake, Miyagino-ku, Sendai 983, Japan * Author for correspondence.

134 Earlier attempts in the direction of systematically storing and effectively utilizing the information on catalysts include: i) a data base on C~ catalytic chemistry by Ito et al [3] ii) computer aided design of catalysts for oxidation reaction using expert systems approach [4,5] and iii) expert system approach to design a catalyst [6]. Computerization of information on zeolite catalysts have also been reported and different databases concentrate on specific properties [7-10]. They contain extremely useful information as shown in Table 1. In this study, we have attempted a systematic information technology approach and an Expert System (ES) approach to computationally design the synthesis routes for zeolites. Table 1 Existing; databases containing; information on zeolitic system. Database Hardware Information Content Advantages Platform Zeofile Macintosh Physico-chemical Simulation of powder property, diffraction pattern for a crystallographic known crystal structure is information, possible. Any textual and structure and graphical information topology of all could be exported to Word materials in Ref. 1. processor files.

Reference 7, 10

Zeopak

VAX system

Powder diffraction patterns of synthetic and natural zeolites,

Identification and quantification of zeolite phases. Characterization is possible by pattern matching.

8

Zeobase

IBM-PC AT 286 and above

Crystalstructure of Entries on as many as 1300 various modifications materials are available. Xof zeolites, ray patterns and molecular models of zeolitic materials can be viewed on screen, plotted on printer or plotter.

9

2. METHOD OF APPROACH

Our approach is to develop an user-friendly software system which can aid the design of molecular sieve catalysts. The task of 'molecular sieve design' can be resolved into many smaller tasks and the smaller tasks could be completed in a step-wise manner. The smaller tasks in catalyst design are listed out as follows:

135 i) propose the list of required physico-chemical properties in a zeolite for a specific end-use, ii) screen out zeolites by searching for a known material possessing all required properties, iii) if one is available with all desired properties the search is completed successfully, which is most unlikely. Alternatively if few molecular sieve materials are available whose properties are reasonably closer but not exactly as needed, find out the ways of modifications to improve or bring it to the level of satisfaction. iv) list out the side-effects or unfavorable properties that may get introduced due to modifications and find ways of eliminating them and v) design a viable and efficient synthesis route for the zeolite proposed in step (iii), taking into considerations of the points listed in step (iv). When the above tasks are completed, the number of experiments needed to design a molecular sieve catalyst could be restricted to a limited number and many trial and error process could be minimized. As recently pointed out, systematic correlations are derived between the critical factors that influence the zeolite synthesis and the kinetics, the quality as well as the yield of the zeolites formed [11]. The availability of all information as a single source, that too in a convenient electronic media is an asset, in our own experience. The retrieval of information on any specific known molecular sieve or on all molecular sieves with specific property is straightforward with our well structured database. An inference engine using logical computer language such as PROLOG is developed to make decision from the data provided in the form of mathematical relations. Thus an efficient research tool which reduces the burden involved in the above mentioned steps in the molecular sieves catalyst design is developed. Additionally, this shall be an effective educational tool for any beginner in the-field of molecular sieves. A 'question-answer' interactive session between system and user is useful as a tutorial for novice in the field of molecular sieves.

3. APPLICATIONS The structure of the system is designed to perform three salient functions as shown in Fig. 1. The first feature provides access to a large database of physico-chemical properties and crystallographic information of all reported zeolites [1]. The second feature provides interactive access to the expert system for the synthesis of zeolites. At the end of the session, the most logical route for the synthesis of a desired zeolite structure is provided. The third feature is a graphic tool box application to simulate X-ray powder diffraction patterns for zeolite phases with different amounts and nature of purity. 3.1.

Source of information

The salient information which are incorporated in the database for different zeolites are given in Table 2. Molecular sieves can be sorted out based on specific value or a range of values of a property. For example, one or all molecular sieves having a framework density of 14.0 T/1000/I,3 or in the range 10.0 to 15.0 T/1000A 3 could be sorted out. Additionally, a combination search for molecular sieves could be performed. This allows one to choose all materials which have a desired value or range of values for multiple properties. For example it is possible to select all molecular sieves having a framework density values in the range 10.0 to 20.0; bidimensional channel architecture; small pore system and orthorhombic symmetry could

136

OPENING

MENU

| EXPERT SYSTEM

DATA B A S E

ZEOLITE- b

KNOWLEDGE F,

WISE

BASE

'ROPERTIES R-C--I'PRO~~T O y(

GRAPHICS TOOL

WISE

QUESTION -

'

INFERENCE ENGINE

PDP FOR PURE PHASES

/~A ETERM INESX ZEOLITE WITH DESIRED PROPERTIES

M INE$ 7 DAETER LOGICAL ROUTE FOR SYNTHESIS

I" ~

II~IPHASES WITH III I I M P U R I T Y

INTERACTIVE SESSION

ANSWER SESSION

/

h

INTERACTIVE SIMULATION ~ THETERMIN ES" E P HASE COMPOSITION

Figure 1. Outlook of the sot~ware

Table 2 Salient information on each molecular sieve stored in the data base General 3 letter code Expansion of the code - full name P h y s i c o - c h e m i c a l properties Structural properties Framework density (Fwd) Secondary building unit Dimensionality of the channel (Dim) Crystal symmetry (Xst) Pore size Crystal space group (Xsg) Pore dimension Unit cell dimensions Isotypic framework structure Atomic co-ordinates of unit cell Miscellaneous

Catalytic properties Dimension of reactant molecules Dimension of template molecules Template, synthesis conditions and reference

137 Table 3 A typical sample screen is shown for a combination search performed. INPUT SELECTION Data Entr~ Menu

I SearchMenu ] Reports Exit IF i

[,Zeolite

I

Starting Material Exit Zeolite

Property [ Combination Properties

I

Exit Framework Density Between : 10.0 and 20.0 Dimensionality 92

Pore Size Crystal Structure

: SP : ORT

OUTPUT PRODUCED Zeolite

Fwd

Dim

Xst

APC APD ATT MON

18.00

2

19.80

2

16.70 18.10

2 2

ORTHORIIOM ORTHORHOM ORTHORHOM ORTHORHOM

Total

Xsg Pbca

Pca21 P212121 Fdd2

4 zeolites

be sorted out as shown in Table 3. A boolean search with 'and', 'or' and'not' options can be performed on zeolite names, zeolite structure codes, framework density, keywords, space group, crystal system, unit cell volume, catalytic performance and authors of the work. The output always appears on the screen and there is option to store the information to the hard disk of the computer as a file for future reference or to output as a hard copy in the printer for the usage on a WYGlWYS ( what you get is what you see) basis. There is always a hot-key (F l) help facility i.e., whenever F 1 key is pressed, a help information regarding the menu items on the screen are displayed.

138 A list of reactions catalyzed by a certain zeolite is also included. The dimensions of many hydrocarbon reactant molecules are part of the data base. Hence it is possible to get a first hand information above the suitability of a molecular sieve to adsorb reactant molecules. Sorption capacity of molecular sieves from experimental reports for various molecules are also added.

3.2. Designing a synthesis route by ES approach Nearly 75 different conditions of synthesis for ZSM-5 [12] and 25 different conditions of synthesis for each of ZSM-11, AIPO4-5 and AIPO4-11 were collected from the literature. These different "conditions" include the variations in the templating organic molecule, temperature, time, static or stirring, composition and source of hydrogel as well as pH. Templates were arranged in the order of descending efficiency. Inverse linear correlations were observed between temperature and time of crystallization as well as between efficiency of the template and time of crystallization. The dependence of time of crystallization on the above mentioned conditions were plotted and fitted into suitable mathematical relations. The system initially starts by enquiring the user for the final "output" of preferred phase, yield, crystallinity and morphology. The database is searched to see if these requirements could be met by match-making with one of the existing reports in the literature. In the absence of a synthesis route in the reported literature, the system chooses the conditions from the mathematical relations already established. Here, it is assumed that the data published in the literature are correct and they could be extrapolated or interpolated to valid regions, where there are gaps in the literature. The conditions are varied within a predefined range, so that finally all conditions fit into the mathematical relations between the "conditions" and "output", within an error count of 10%. The system registers an error message, in case the required output cannot be achieved within 10% error of variation in conditions. Thus the system chooses optimal conditions for the synthesis to satisfy the requirement of the user. The system solves problems which are often poorly defined or understood. At each decision making point, the system starts by providing an introductory note about the conditions being varied as well as typical case studies which are available in the literature.

3.3. Analysis of phase purity in synthesized samples Simulated powder diffraction patterns (PDP) of all the molecular sieve materials are presented in the form of a graphic display or in the form of a table of 20 and relative intensity. It is possible to simulate PDP of mixture of more than one zeolite. When PDP of more than one structure is displayed, the pattern for each structure is displayed in different color. This facility is useful to analyze the phase purity of any synthesized sample. For example, it is well known that ZSM-11 phase occurs as impurity while synthesizing ZSM-5 phase and vice-versa. In Fig.2a and 2b, the PDP of ZSM-5 and ZSM-11 are shown, respectively. Fig.2c shows the PDP for a 50:50 mixture of ZSM-5:ZSM-11, which is a typical example. If the PDP of synthesized sample matches with Fig.2c, the phase concentration could be inferred. Similarly PDP can be simulated for different proportions of the individual component, till it matches with the experimentally observed pattern. As an another typical example, the PDP of AIPO45, AIPO4-11 and the simulated PDP for 75:25 mixture of AIPO4-5:AIPO4-11 are shown in Figs. 3a, 3b, and 3c, respectively. If a sample obtained from AIPO4-5 synthesis batch showed a pattern as in Fig.3c instead of Fig.3a, then 25% yield of A1PO4-11 could be inferred. Thus using this facility, the impurity phases and the selectivity in the synthesis could be identified.

28

28

28

Figure 2. The PDP generated by the system for ZSM-5 (a), ZSM-11 @) and 5 0 5 0 ZSM-5:ZSM-11 100. F

- 100

100.

(a 1

-

80

-

-

80 60

60

-

1

w40-

40

40

-

320-

20

20

-

Ero E60

I

(c 1

W

a

t

0

1

5

I

10

L

IS 28

a

20

1L

25

30

0

5

10

IS 28

20

2530

0

I

1

5

10

Figure 3. The PDP generated by the system for AlP04-5 (a), M 0 4 - 1 I (b) and 75:25 ~ 0 4 - 5 : M 0 4 - 1 1

I

I l A

IS 2 0 2 5 28

30

140 4. SUMMARY Molecular sieves literature today is vast and versatile. Although one may remember that an information is available in the literature, it becomes a hard or sometimes impossible task to search and retrieve that information from the books and journals. However, one should recognize that the 'information science' has benefited largely by the advent of powerful computers and efficient software packages. Desktop personal computers which have attained the status of'essential equipment' of any researcher are good enough to handle vast amount of information efficiently. We felt that one massive attempt to collect all the information, classify and store them systematically as well as to create a 'user-friendly system' to retrieve will help the whole molecular sieve research community. The database is further upgraded as an ES to design a logical route for the synthesis of zeolite. The routes are decided from either well documented synthesis procedures or from correlations derived from the reported data. Thus the feasibility of ES approach has been tested and shown as powerful tool for computer aided design of catalysts.

ACKNOWLEDGMENTS The authors (TS, RCD & AC) thank UGC and CSIR for financial support in the form of Research Fellowships. We thank Dr. S. Krishnan and Dr. P. Ratnasamy for initiating the building up of the database reported here. We acknowledge the programming expertness provided by S. Mani, S. Hajamis, S. Ganorkar and V. Dhayagude. Many people in Catalysis Division, National Chemical Laboratory actually used the system in its primitive stage and suggested improvements which made it user-friendly. The work was partly funded by UNDP.

REFERENCES

1. W.M. Meier and D.H. Olson, in: 'Atlas of zeolite structure types', Butterworth Heinmann, London 1992. 2. J.V. Smith, Chem. Rev., 88 (1988) 149. 3. T. Ito, H. Hamada, Y. Kintaichi and M. Sasaki, Catal. Today 10 (1991) 223. 4. S. Kito, T. Hattori and Y: Murakami, Appl. Catal., 48 (1989) 107. 5. T. Hattori, H. Niwa, A. Satsuma, S. Kito and Y. Murakami, Appl. Catal., 50 (1989) L 11 6. T. Hattori and S. Kito, Catal. Today 10 (1991) 213. 7. J.M. Newsam and M.M.J. Treaey, 9th Int. Zeolite Conf., Extended Abstracts and Program, No.RP 193 (1992). 8. D.K. Smith, S.Q. Hoyle and G.G. Johnson, Jr., 9th Int. Zeolite Conf., Extended Abstracts and Program, No.RP 192 (1992) 9. W.H. Baur and R.X. Fischer, 9th Int. Zeolite Conf., Extended Abstracts and Program, No.RP 216 (1992). 10. J.M. Newsam and M.M.J. Treaty, Zeolites 13 (1993) 183. 11. E.J.P. Feijen, J.A. Martens and P.A. Jacobs, Stud. Surface Sr Catal., 84A, 1 (1994). 12. A. Chatterjee and R. Vetfivd, J. Chem. Sor Faraday Trans., 91 (1995) 4313.



141

A new method for enhancing zeolite crystallization by using oxyacids / salts of group VA and VIIA elements as promoters Asim Bhaumik, A.A.Belhekar and Rajiv Kumar Catalysis Division, National Chemical Laboratory, Pune - 411 008, INDIA. ~ A novel and general method for highly efficient, fast synthesis (with 4-6 fold increase) of zeolite and related materials has been developed using a new concept of addition of some promoters (oxy anions like C104, PO43, AsO43, C103,etc in the form of salt or acid). The materials

produced using promoters exhibit similar, if not better, yield, crystallinity, morphology and catalytic properties compared to those obtained by conventional method. 1. INTRODUCTION Microporous, crystalline alumino- or metallo-silicate molecular sieves / zeolites [1-4] exhibit unique catalytic, adsorption and ion-exchange (viz. as detergent builders) properties. Zeolites are largely used as catalyst in various chemical, petroleum refining, petrochemical, bulk chemical and fine chemical processes [4-10]. Although, the nature of surface and catalytic properties of zeolites and related materials is relatively better understood, their synthesis continues to be intriguing and challenging. Zeolites are generally synthesised hydrothermally from an alkaline aluminosilicate reaction mixture which may or may not contain organic guest molecule (template), at autogenous pressure for few days to few weeks time. Inui [ 11 ] has pointed out various drawbacks of long crystallization time (few days or more) extensive labor coupled with delays and expense, low reproducibility and formation of larger crystals with inhomogeneous particle size distribution mainly because of secondary crystallization. However, the optimisation of recipes for each structure is done on case to case basis. Hence, ever increasing efforts are going on to reduce the synthesis time of these materials and thus to save energy and time. Now, we report, for the first time, a general method using some promoters (like C104, PO43, AsO43, C103", BrO 3" etc. in the form of acids or their Na/K salts) for highly efficient, fast

1 AB thanks CSIR, New Delhi for granting a senior research fellowship.

142 synthesis (with 4-6 fold or more increase) of zeolite and related materials with similar, if not better, yield, crystallinity and catalytic properties compared to those obtained by conventional method. A variety of zeolite structures representing small (NU-1 & FER), medium (ZSM-5 & ZSM-48) and large (Beta & ZSM-12) pore zeolites as well as aluminous / low silica (Si/A1 = 1-5, e.g. zeolite Y) and silicious / high silica (Si/A1 > 5: such as ZSM-5, ZSM-12, Beta etc.) molecular sieves have been chosen as representatives to demonstrate the generality and wide applicability of our present method.

2. EXPERIMENTAL All synthesis experiments were carded out in 200 ml capacity stainless steel autoclaves with teflon coatings under static conditions. In a typical synthesis, silica source was stirred with required amount of template and alkali dissolved in water for 1 h. Then the aluminium source (NaA102 for most of the cases) taken in water was added into it. Finally required amount of promoter (added in the form of oxyacid or their sodium / potassium salt of their corresponding oxyanions) taken in the rest amount of water was added slowly to the stirring gel. Stirring was continued for another 1 h and then the resulting gel was autoclaved. Na § / SiO2 molar ratio was kept constant for a particular zeolite by adding appropriate amount of NaC1. Gel compositions in terms of moles of oxides are given in Table 1" Table 1" Starting molar gel compositions of various zeolites Zeolite

Molar gel composition

NU-1

40

S i O 2 " A1203 " 5.0

FER

60

S i O 2 9A1203 "

ZSM-5

40

S i O 2 " A1203 " 5.0

ZSM-48

90

S i O 2 " A1203 " 9 . 0 N a 2 0

pH

Na20 " 10 (TMA)20 91000

H20"

18 N a 2 0 " 7.5 Pyrrolidine 92400 Na20" 5 (TPA)20 91200 9 15

DIQ-6 92700

12.2 _+0.2

H 2 0 " 6P

12.0 _+0.2

4P

10.8 _+0.2

H20"

n20"

4P

9P

11.6 _+0.2

ZSM-12

120

Beta

30SIO2 9A1203 "Na20 " 0.5K20 97.5(TEA)20 9600H20 93P

12.5 _+0.2

Y

9SiO 2 9A1203:4 Na20" 270 H20 9P

13.4 _+0.2

SiO 2 " AI203 " l0 Na20"

12 DIQ-6 " 3600

H20 "

12P

10.8 _+0.2

Where P = promoter chosen from HC104, NaCIO 4, H3PO 4, NaH2PO 4, Na2HPO4, Na3PO4, Na2HAsO4; TEA = tetra ethyl ammonium, TPA = tetra propyl ammonium, TMA= tetramethyl ammonium, DIQ-6 = hexamethylene bis (benzyl dimethyl ammonium), P =

C 1 0 4 , P O 4 3 , AsO43-

143 or none. The crystallisation temp (~

was = 170 (NU- 1, ZSM-48), 160 (FER, MFI, MTW) and

140 (BEA). In case of zeolite Y the standard seeding procedure reported by Ratnasamy et al.[ 12] was followed. 3. RESULTS AND DISCUSSION In Table 2,effect of various promoters used in the synthesis of different zeolite structures is shown. The use of small amount of promoters signigicantly enhances the overall crystallisation process in the order : C10 4- > PO43 > AsO43- >> none, independent of structures. Some stabilising effect of certain neutral sodium salts of some anions (e.g. NaC1, NaNO 3, Na2SO 4, NaC103 etc.) during hydrothermal transformation of kaolinite into sodalite and cancrinite has been observed [ 13] where the inclusion of these salts in the sodalite / cancrinite structure was found to be the main cause of stabilising a particular phase. In the present case of direct synthesis no such inclusion of promoters (anions or their salts) in crystalline molecular sieves was obtained. Table 2: Effect of various promoters on zeolite synthesis Zeolite

Cryst.

Si/AI

Crystallization time, h

Temp.~

(solid) a

CIO 4"

PO43"

msO43"

None

NU- 1

170

17-19

28.0

30.0

36.0

120.0

FER

160

10-15

18.0

26.0

26.0

60.0

ZSM-5

160

15-17

6.0

8.0

12.5

36.0

ZSM-48

170

30-38

40.0

44.0

40.0

108.0

ZSM- 12

160

45-50

36.0

40.0

42.0

132.0

Beta

140

12-14

30.0

32.0

36.0

156.0

Y

100

2.4-2.8

4.0

4.0

5.0

11.0

a: Obtained by chemical analysis of various zeolites using different promoters In Fig. 1, the crystallinity of zeolite Y (curves x and y) and ZSM-5 (curves a-d) is plotted against their corresponding crystallization time. The synthesis time was considerably reduced by the addition of the promoters in both the cases. However, in the case of zeolite Y, not only the crystallization rate but also the stability of fully crystalline material in the mother liquor was found to be more in the presence of a promoter (PO43-). While, following the conventional / standard method (without using promoter) zeolite P, a common impurity in Y synthesis [12], was detected immediately after 30 - 60 min. of the complete formation of Y (curve y, Fig. 1), in our method (curve x, Fig. 1) zeolite P was not observed even after 2 hours of the complete crystallization of zeolite Y. It is again clear from Fig. 1 that under otherwise same synthesis

144 conditions, the presence of promoters enhances the nucleation and crystallisation in the order: ClO 4" > PO4 3" > AsO4 3" >> none. The polarisability of the central element of the promoter (i.e.

charge / radius, Z/r value) also decreases in the same order. Further, from the nature of crystallisation curves (a-c vis-a-vis d in Fig 1), it can be inferred that the presence of promoters enhances the process of both nucleation and crystallisation.

lOOt

.~ 80-

w~'='e=w~n~=~J

,'

!

X

a

,,~r[

b

I iJY

I

I

>"

I

I

_~60-

I

I

i

I

Z

"J

I

I

"J 4 0 "

t

I

j=

I

l

U')

-/,'

-

l

I

,

'

I

~ 2O-

I !I

, '

/

4

I'

12

16

"

CRYSTALLIZATION

I

I

5

TIME , h

Fig. 1" Crystallization kinetics of zeolite Y (curve x with Na3PO4 and y with no promoter, arrow indicates the appearance of zeolite P) and ZSM-5 (curves a,b,c & d represent the use of HC104, H3PO4, Na2HAsO4 and none, respectively as promoter). For gel composition see Table 1. After having acertained the utility of various promoters, in zeolite crystallisation, it was thought worthwhile to study the effect of concentration of a particular promoter on a particular zeolite structure. NaH2PO 4 and ZSM-5 were chosen as representative. It was found that with

145 increasing the amount of promoter, the crystallisation time decreases upto Si / promoter molar ratio = 5. By still increasing the amount of promoter no further enhancement in crystallisation was observed (Table 3). Table 3: Effect of phosphate promoter concentrations on crystallisation of ZSM-5 T = 160 ~

gel composition is given in Table 1.

Si / Promoter molar ratio

Cryst. time, h a

oo

20

10

5

2.5

40

12.5

8.0

7.0

6.5

a: Time taken for obtaining fully cryStalline material, calculated from crystallisation kinetic curves

Hence, above mentioned data clearly demonstrate that a considerable increase in the nucleation and crystallisation processes of various zeolites can be achieved by adding a small amount of certain oxyacids (or their salts) of group V and VII A elements. After achieving the goal of faster synthesis of zeolites, next obvious question is to varify the quality of these materials. All samples were thoroughly characterised through XRD, SEM, EDX and weight chemical analysis. It is pertinent to mention that in all the cases comparable or better crystallinity and yield along with better uniformity of particle size distribution was obtained with the use of promoters over that prepared in the absence of promoter. The scanning electron micrographs of zeolite Y (Fig.2, a and b) and ZSM-5 (Fig.2, c and d) taken as representative, exhibited that the crystallite size of the samples synthesised in the presence of promoter (e.g. PO43, a and c) vis-a-vis its absence (b and d) is smaller along with better uniformity of particle size distribution. The crystallite size of zeolite Y was 0.6 to 0.8 Ixm and 0.8 - 1.5 l.tm in the presence and absence of Na2HPO4 respectively. Corresponding values for ZSM-5 were 0.8 - 1.0 ~tm and 1.0 - 2.0 ktm, respectively in the presence and absence of promoter. Same observations are obtained for other topologies also. Smaller and uniform crystallites size distribution indicates the absence of secondary crystallisation. When both nucleation and crystallisation processes are quite fast, the secondary crystallisation is expected to be eliminated or suppressed. The n-hexane cracking reaction was carried out over H form of ZSM-5 samples represented by Fig.2C and Fig.2d. The value of moles of n-hexane converted

146 per mole of A1 per h was 18.0 and 17.2 respectively for H-ZSM-5 samples synthesised in the presence and absence of promoter (T = 623 K) clearly suggesting that the material synthesised with promoters is quite comparable, if not better, in quality.

"'7

r

Fig.2: SEM photograph of zeolite Y ( a: with and b: without promoter, PO43-) and ZSM-5 (c: with and d: without promoter, PO43-) One of the most fundamental basis of the hydrothermal synthesis of zeolites is the mineralising property of water, which is greatly assisted by free OH concentration in the solution/gel. Apart from this basic requirement of mineralisability, other factors like, Si/A1 ratio, pH, aging at low temperature, crystallisation temperature and time etc. influence the type and quality of the crystalline material in rather specific ways. For example, in the crystallization of aluminous zeolites (Si/A1 = 1-5) the synthesis becomes faster with decreasing Si/A1 ratio while for high silica/silicious molecular sieves (Si/A1 > 5) the reverse is true [13-15]. Further, the range of

147 synthesis temperature for low and high silica zeolites is 80-120~ and 120-200~

respectively.

Similarly, organic bases (templates) play particularly significant role only in the synthesis of high silica zeolites [ 13-15]. However in our present method of using oxyacids / oxysalts of Gr VA and VIIA as promoters the enhancement in nucleation and crystallization is observed for all aluminosilicates zeolites independent of silica alumina ratio, pore size of the structures and temperatures of the synthesis. Although, the role of the promoters at mechanistic level is not clear, a direct correlation is found between Z / r of the central cation of the promoter with the decrease in crystallisation time. 4.

CONCLUSIONS The addition of small amount of some oxyacids (or their Na / K salts) of group V and VII

A elements (like C104, PO43", AsO43", C10 3- etc.) significantly promotes the nucleation and crystallisation of zeolites. Further, this method is applicable to all types of low, medium and large pore zeolites. Nearly 4-6 fold reduction in crystallision time could be achieved in all the cases. The quality (crystallinity, morphology, catalytic activity) of the samples obtained using promoters was comparable, if not better, than those synthesised by standard recipies without using any promoter. Promoters with more polarisability are more effective in enhancing the crystallisation. REFERENCES 1.

D.W.Breck,"Zeolite Molecular Sieves", Weley, New York, 1974.

2.

R.M.Barrer, "Hydrothermal Chemistry of Zeolites", Academic Press, NewYork, 1982.

3.

R.Szostak, "Molecular Sieves : Principle of Synthesis and Identification" Van Nostrand, Reinhold, New York, 1989. H.G.Karge, and J.Weitkamp, (Eds.), "Zeolites as Catalysis, Sorbents and Detergent Builders : Applications and Innovations," Elsevier, Amsterdam, 1989.

4. 5.

W.Holderich, M.Messe, and F.Naumann, Angew.Chem.Int.Ed.Engl. 27 (1988) 226.

6.

P.B.Venuto, Microporous Materials 2 (1994) 297.

7.

P.Kumar, R.Kumar and B.Pandey, Synlett. (1995) 289.

8.

A.Bhaumik and R.Kumar, J.Chem.Soc.Chem.Commun. (1995) 349.

9.

P.G.Schultz, Angew.Chem.Int.Ed.Engl. 28 (1989) 1283.

10.

M.E.Davis, Acc.Chem.Res. 26 (1993) 111.

11.

T.Inui, in "Zeolite Synthesis" (M.L.Occelli & H.E.Robson, Eds.) ACS Symp. Ser. 398, Am.Chem.Soc., Washington, D.C., 1989, ch.33, pp. 479-492.

12.

P.Ratnasamy, A.N.Kotasthane, V.P.Shiralkar, A.Thangaraj and S.Ganapathy, in "Zeolite Synthesis" (M.L.Occelli & H.E.Robson, Eds.) ACS Symp. Ser. 398, Am.Chem.Soc., Washington, D.C., 1989, ch.28, pp. 405-419.

148 13.

R.M.Barrer, in "Zeolites : Synthesis, Structure, Technology and Application," (B.Drzaj, S.Hocevar and S.Pejovnik, Eds.), Elsevier, Amsterdam, 1985, pp 1-26.

14.

P.A.Jacobs and J.A.Martens, Stud.Surf.Sci.Catal. 33 (1987) 58.

15.

J.S.Reddy, R.Kumar and S.M.Sciscery, J.Catal. 145 (1994) 73.



149

The influence of mixed organic additives on the zeolites A and X crystal growth V. Petranovskii', Y. Kiy0zumib, N. Kikuchib, H. Hayamisub, Y. Sugi~, F. Mizukamib "Institute of Physics, UNAM, Ensenada, B.C. 22800 Mexico" bNational Institute for Materials and Chemical Research, AIST, Tsulmba, Ibarald 305, Japan

~

University, Cfifu,~01-11 Japan

The state of Al in the initial solutions for gel preparation is found to be important for the results of zeolite synthesis. Polynuclear complex of AI with chemical shift 8 = 57 ppm in the 27AI NMR spectra influences significantly the size and the shape of NaA and NaX crystals. The concentration of this complex depends on the composition of organic additives. 1. INTRODUCTION Zeolites have wide applications in diverse areas. Nevertheless, the mechanism of their growth is not completely clear [1]. A model that includes "secondary building units" (SBU) packing was suggested [2]. The problem of this model is to explain formation of these SBU from initial compounds. Another question is connected with the role of organic molecules in the zeolite synthesis. Organic compounds are frequently used as templates for zeolite growth [3, 4]. Usually it is supposed that the organic molecules direct crystal structures as space-filling agents. Joining these two models, a "can-and-cement" model of nucleation in zeolite synthesis was developed [5]. The formation of inorganic-organic composite structures was proposed as the explanation of the mechanism directing the structure [6, 7]. Chamell [8] found that the addition of triethanolamine (TEA) in the reaction mixture resulted in the growth of large single crystals of zeolites A and X. TEA molecules are not included in their crystals during the growth process. Hence TEA can not be the real template. It plays the role of a complex forming ligand. Only the polynuclear complex of tetrahedral surrounded Al with the chemical shift 8 = 62.6 ppm in the r~Al NMR spectra was detected in the initial solution [9, 10]. Such complexes (for example, similar to alumoxanes [11 ] or to well-known silicate ions [SisO-x]s" [12]) can be the prototypes of SBU. The aging of the initial solutions increases the size of NaX single crystals from 0.15 mm [8] to 0.25 mm [13]. The small concentration (less than 1%) of a new complex of AI with 8 = 57 ppm appears in the process of aging [10]. The same complex was found to be formed in the Al containing solutions in the presence of diethanolamine (DEA) or diisopropanolamine (DIPA), but other AI complexes were different [14].

On leavingfrom A.F. IoffePhysicalTechnical Institute, RAS, Saint Petersburg, 194021,Russia

150 Transformation of the shape of NaA crystals was observed in the case of their growth in DEA- or DIPA-containing gels. The reason for the shape change can be the variation of the structure of different aluminosilicate ions (potential SBU) in the presence of unlike ligands [ 14]. On the base of the data described above it is possible to expect that the simultaneous addition of di- and trialkylamines to the solution will lead to the generation of two complexes (with 8 -- 62.6 ppm and 6 = 57 ppm) at the same time. A variation of the ratio of organic additives can change the relative concentration of different complexes of aluminum in the solution and thus controls the results of synthesis. The aim of this work is to investigate the influence of the composition of organic additives on the process of crystal growth ofzeolites A and X. 2. EXPERIMENTAL SECTION

2.1. Preparation of solutions Sodium aluminate and sodium metasilicate ermeahydrate were used. The solutions with the compositions 0.42 mol NaAIO2 + 0.95 mol TEA + 55.51 mol H20 for NaX synthesis, 0.84 mol NaAIO2 + 0.95 mol TEA + 55.51 mol H20 for NaA synthesis and 0.44 mol Na2SiO3 + 0.95 mol TEA + 55.51 mol H20 for both cases were prepared following the procedure described by Charnell [8]. TEA was replaced in these solutions by the equivalent amount of DEA, DIPA or triisopropanolamine (TIPA). The concentration of dialkylamine (DEA or DIPA) in mixture was chosen to save total number of - - R - - O H groups. For that reason their molar concentration was half as much again as the concentration of TEA or TIPA. Mixtures of additives ( T I P A - DIPA, TIPA - DEA, T E A - DIPA, T E A - DEA) that contain 0%, 50%, 80%, 90%, 98% and 100% of NHR2 were selected to perform the zeolite synthesis. Analytical grade reagents (produced by Wako Pure Chemical Industries Ltd., Japan) were used. 2.2. 27A! NMR spectra Measurements of 27A1 NMR spectra for clear aluminate solutions were carried out on a JEOL GSH-200 spectrometer, operating at 51.90 Mt-k, using a 10 mm probe tube. The observations of 2~AI chemical shiRs were quoted relative to AI(I-I20)63+ (8 = 0 ppm). The references were placed in capillaries; the last ones were coaxially inserted in the NMR tubes. The measurements were done with a 90 ~ pulse of length 28.0 I~s, 256 scans. Spectra were recorded at 293 K. 2.3. Zeolites synthesis and samples examination The gels were prepared using TEA-, TIPA-, DEA- and DIPA-containing solutions, indicated above. The same silicate solutions in combination with different aluminate solutions were used for synthesis of NaA and NaX zeolites. Equal amounts of silicate and aluminate solutions were mixed at room temperature. The crystallization process was held at 75 ~ at static conditions. The size and shape of zeolite single crystals were detected with an optical microscope Olympus B061 and a scanning electron microscope Hitachi S-800.

151 3. RESULTS AND DISCUSSION

3.1. Influence of additive nature on the aluminum complexes in the solutions Only the monomer four-coordinated ions AI(OH)4" exist in the clear aluminate solution in the lack of any organic additives (Fig. 1, a). The chemical shift of AI in this compound is equal to 79.8 ppm. Addition of tri-substituted amines (TEA or TIPA) to the solution results in almost complete disappearance of the monomer ions. Simultaneously the polynuclear complex with 8 = 62.6 ppm appears (Table 1, Fig. 1, e, f). In the presence of DEA or DIPA the ion AI(OI-I)4" is kept in the solutions side by side with a new complex with 8 - 57 ppm (Fig. 1, c, d). The concentration of the last complex is small (Table 1). This is the same complex that was found for aged solutions [10]. Table 1 Relative intensity of different peaks in 27A1 NMR spectra for initial solutions with changed organic additives. Intensity of peaks (%) with 8:

Compounds

79.8 ppm

62.6 ppm

57 ppm

< 1

> 99

< 1"

< 1

> 99

-

95

-

5

90

-

10

100

-

-

2-Diethylaminoethanol (C2Hs)2NCH2CH2OH Tds(Hydroxymethyl)aminomethan

100

-

-

100

-

-

Triethylamine

100

-

-

2,2',2" =Nitrilotriethylamine

100

-

-

N(CH2CH2NH2)3 3,3',3"'-Nitrilotdpropiorfic N(CH2CH2COOI--I)3

100

-

-

100

-

-

Triethanolamine (TEA) N(CH2CH2OH)3 Triisopropanolamine (TIPA) Diethanolamine (DEA)

[CH3CH(OH)CH2]3N

HN(CH2CH2OH)2 Diisopropanolamine (DIPA) [CH3CH(OH)CH2]2NH Monoethanolamine (MEA)

H2NCH2CH2OH

(HOCH2)3CNH2 (C2Hs)3N

Nitrilotdacetic acid ~

acid ~r

N(CH2COOH)3 Glycine ~ 100 H2NCH2COOH Nitrilotds~ethylenephosphonic Acid) ~ 100 N(CH2POaH2)a *For aged solution The acids were neutrahzed by NaOH, and corresponding sodium salts were used '~

~

9

~

-

152 An aluminum ion interacts with TEA through its alcohol termination. The complex of AI with 5 = 62.6 ppm contains aluminum as ~AI--O---AI-- bridges only [9]. Moreover it is interesting that the number of identical alkyl groups connected with a nitrogen atom influences so significantly the structure of AI complexes in the solutions (see Fig. 1, b, c, e). Specifically, monodentate ligand MEA can not form polynuclear complexes of AI. Bidentate ligands, such as DEA or DIPA, form complex with 5 = 57 ppm as well as monomer AI(OH)g. Tridentate ligands (TEA or TIPA) convert all AI in the solution into the polynuclear complex with 5 = 62.6 ppm.

23 __....~ ~

11

C

j |

9'0"

23

a

'7'0'''50

!

: - _

s

ppm

9

9'0' ' '70

I

a

i

l

1

.

'

5'0 ppm

.

90'' 70

1

23 9b"

'7b"

b "5o' ppm"

_ilL_2& 9'0''''70''

3 i

I

'

i

I

'

50 ppm

3

d

'50''ppm"

,,,

90'' ' '70'' '5'0''ppm'

Figure 1.2~AI NMR spectra of the solutions for NaA synthesis: a - without organic additives; with addition of: b - MEA; c - DEA; d - DIPA; e - TEA; f - TIPA. Concentrations of additives are as described in "Experimental". Not only the number of the alkyl groups is important, but also their nature. Consideration of the data, summarized in Table 1, results in the conclusion that only in the case of two or three - - C - - C - - O H groups bonded to a nitrogen atom, does the aluminum form the polynuclear complexes. For example, in contrast to TEA, 2-diethylaminoethanol and triethylamine do not form any complexes. The same is correct for tris(hydroxymethyl)aminomethan that contains three alcohol groups, but they are not so flexibly connected with a nitrogen atom as those ones in TEA. Most likely, the structure of the complexes of AI with tri-substituted amines is similar to atranes [15] or pro-atranes [16]. However, substitution of oxygen by nitrogen (in the case of 2,2',2"-nitrilotriethylamine) leads to the disappearance of any form of the aluminum polynuclear complexes.

153 Results of interaction depend also on the nature of groups that substitute for the hydrogen atoms in the alkyl radical. Methyl-substituted derivatives (DIPA and TIPA) produce the same complexes (Fig. 1, c, d and e, f respectively). There are only the ions of monomer AI(OH)4" in the clear solutions with additives, which content carboxyl groups (see Table 1). Aluminate solutions with low Na20/AI203 ratio are usually unstable. An alumina precipitate appears after keeping them for several days at room temperature. The TEA containing solution was stable under the same conditions for at least several years. The structure of aluminum - TEA complex is still not clear. TEA influences significantly the chemistry of aluminum in the solutions. Thus, during the preparation of alumina from gels with different additives, only a gel synthesized with TEA remains amorphous up to 650 ~ [17]. It was supposed that TEA plays the role of a nucleation suppressant during the zeolite synthesis [ 18]. Probably all these phenomena are connected with the properties of the complex with 8 = 62.6 ppm. TIPA-, DIPA- and DEA-containing aluminate solutions were stable also. In contrast, the precipitates appeared after several days of storage for all other solutions (see Table 1). 3.2. Influence of additive mixtures on aluminum complexes in the solutions The simultaneous addition of di- and trialkylamines to the solution decreases the monomer ion concentration and increases the concentration of both complexes (with 8 = 62.6 ppm and 8 - 57 ppm). Synergism of their action is observed. Half amount of TEA in the presence of DEA or DIPA connect all AI in the complex with 8 - 62.6 ppm (see Fig. 2, a and Fig. 3, a).

'"|

90

9

it''

.'

k'

70

"-"

" 5'0

l~pm

Figure 2. 27A1NMR spectra of solutions for NaA synthesis with mixed TEA - DEA additive: a - 50% and b - 90% of DEA.

:i

90

-

u

10

9

|

.

|

50

,

~

9

___~

ppm

Figure 3.27A1 ~ spectra of solutions for NaA synthesis with mixed TEA - DIPA additive: a- 50% and b - 90% of DIPA.

3.3. Influence of additive mixtures on the zeolite A and X growth Variations of crystal size during zeolite growth in bath with mixed additives were investigated. Results are Summarized in Table 2 and in Fig. 4. Different dependencies were found for growth of NaA and NaX zeolites.

154 Table 2. Size of zeolite crystals grown in the presence of mixture of additives. Zeolite type

~ e

Crystal size, tim, for NHR2 concentration, % 50 80 90 98

composition

0

NaA

TEA + DEA TEA + DIPA TIPA + DEA TIPA + DIPA

24 24 16 16

28 26 21 21

37 29 27 23

32 20 29 24

47 38 23 15

39 33 22 19

NaX

TEA + DEA TEA + DIPA TIPA + DEA TIPA + DIPA

26 26 22 22

45 35 39 33

27 20 31 17

28 21 31 19

41 30 44 28

68 59 68 59

100

70

b 40 i

10

I

g

40 10 0

40

80

0

40

80

0

th 40

80

O

t 40

80

Figure 4. Dependencies of crystal size (ttm) on additive composition (% of NHR2 in mixture NR3 - NHR2) for NaA (a - d) and NaX (e - h) zeolites: a, e - TEA - DEA; b, f - TEA - DIPA; c, g - TIPA- DEA; d, h - TIPA- DIPA. For NaA zeolite the concentration dependencies of the crystal size go through a maximum when NHR2 concentration changes. Positions of these maxima depend on the nature of NR3. For TEA and TIPA they correspond approximately 98% and 80% of NHR2 in the mixture, respectively (Table 2, Fig. 4, a - d). For NaX zeolite these dependencies also have wellpronounced maxima for the 1:1 ratio of additives. Nevertheless the biggest crystals were grown in the case of pure N t ~ 2 additives (Table 2, Fig. 4, e - 11). In the all cases TEA and DEA show better results than more bulky TIPA and DIPA, both unmixed and in mixtures. Common features of these dependencies confirm that, for the results of the synthesis, the chemical state of framework components in the solutions is more important than the origin of the individual organic compounds. The chemical shift 8 = 57 ppm is known for solid state MAS ZTAlNMR spectra of the solid phase of gels and zeolites A and X [19]. Thus the action of dialkylamine changes the state of AI in the solutions to that state found in the zeolite crystal

155 lattices. The simultaneous action of TEA leads to the slow release of AI due to properties of TEA - AI complex with 8 = 62.6 ppm. The variation of the results of synthesis for NaA and NaX zeolites may be due to their growth with the participation of different SBU. At the same time, the shape of NaA crystals varies essentially. The contribution of the planes { 110} increases with increase of NHR2 concentration for all systems examined. This dependence is illustrated for NaA crystals grown in the presence of TEA - DEA mixture (Fig. 5). In contrast, the shape of NaX crystals does not change in all cases.

Figure 5. The NaA crystals grown in the presence of TEA- DEA mixture with content of DEA: a-0%; b - 50%; c - 80%; d- 90%; e- 98%; f-100%. Evidently, the synthesis results are strongly influenced by the local chemical composition of the aluminosilicate gel network. They are defined by the proportion and the mutual position of AI and Si in - - O - - S i O---At O - - chains. When a gel is formed, its structure and composition are controlled by the structure and the composition of the initial solutions. Hence the results of zeolite synthesis depend strongly on the structure of AI complexes in the initial solutions. R is well known that Lowenshtein's rule is not violated for zeolite frameworks [2]. Nevertheless, the clear solutions that contain polynuclear complexes of aluminum give the best results for the particle size of NaA and NaX zeolites [9, 10, 13, 14].

156 4. CONCLUSIONS It is shown that amines used as organic additives for NaA and NaX zeolite growths play the role of complex-forming ligands. The structure and the number of alkyl groups in an amine molecule determine the kind of the formed aluminum complex. The changes of the concentration ratio of three different forms of AI complexes influence the zeolite A and X crystal growth. These concentrations depend on the ratio of di- and trialkylamines in the mixture of organic additives. The most important complex for zeolite growth is characterized by the chemical shift 8 = 57 ppm. This complex appears in the presence of DEA or DIPA. Synergism of (NHR2 + NR3) mixture action is observed. The concentration of NHR2 influences the shape of NaA crystals for all systems examined. In contrast, the shape of NaX crystals does not change. ACKNOWLEDGMENTS The authors thank Dr. N. Bogdanchikova for fruitful discussion, and Dr. A. Slavin for careful reading of manuscript. This work was supported by AIST, MITI, Japan. REFERENCES 1. M.E. Davis and R.F. Lobo, Chem. Mater., 4 (1992) 756. 2. D.W. Breck, Zeolite Molecular Sieves, A Wiley Interscience Publ.: New York, 1974. 3. P.A. Jacobs and J.A. Martens, Synthesis of High-Silica Ahminosilicate Zeolites (Stud. Surf. Sci. Cat., Vol. 33), Elsevier: Amsterdam, 1987. 4. S.I. Zones and R.A. Van Nordstrand, in Novel Materials in Heterogeneous Catalysis (Eds. R.T.K. Baker and L.L Murrell) ACS Symp. Set., 437 Am. Chem. Soc.: Washington, DC, 1990, Chap. 2, p. 14. 5. G.O. Brunner, Zeolites, 12 (1992), 428. 6. S.L. Burkett and M.E. Davis, Chem. Mater., 7 (1995) 920. 7. S.L. Burkett and M.E. Davis, Chem. Mater., 7 (1995) 1453. 8. J.F. Charnell, J. Cryst. Growth, 7 (197 l) 29 I. 9. A. Efimov, V. Petranovskii, M. Fedotov, M. Khrepoun and L. Myund, J. Structural Chem., 34 (1993) 548. 10. V.P. Petranovskii, in Proceedings of 9th Zeolite Research meeting, Chem. Soc. of Japan: Tottori, 1993, p. 6. 11. A.W. Apblett, A.C. Warren and A.R. Barron, Chem. Mater., 4 (1992) 167. 12. M. Wiebcke, M. Grube, H. Koller, G. Engelgardt and J. Felshe, Microporous Mater., 2 (1993) 55. 13. V.N. Bogomolov and V.P. Petranovskii, Zeolites, 6 (1986) 418. 14. V.P. Petranovskii, Y. Sugi. Unpublished results. 15. M.G. Voronkov and V.P. Baryshok, J Organomet. Chem., 239 (1982) 199. 16. J.G. Verkade, Ace. Chem. Res., (1993) 483. 17. H. Tayaa, A. Mosset and J. Galy, Europ. J. Solid State Inorg. Chem., 29 (1992) 27. 18. E.N. Coker, P.S. Hees, C.H. Sotak at al., Microporous Mater., 3 (1995) 623. 19. L.V.C. Rees and S. Chandraseckhar, Zeolites, 13 (1993) 528.


Studies of the Crystallization Gravitational Force Field

157

of ZSM-5

Under High

W. J., Kim1, D. T., Hayhurst2, S. A., Lee1, M. C., Lee1, C. W., LimI and J. C., Yoo1 1Department of Industrial Chemistry, Kon Kuk University, Seoul, Korea 2 D e p ~ e n t of Chemical Engineering, University of South Alabama, Mobile,AL, USA

The 23 factorial method was applied to the crystallization of large ZSM-5 under high gravity to optimize the synthesis condition. The optimum composition was determined and the growth rate, size distribution, and morphology were studied. The activation energies for various systems under high gravity were calculated and found to be consistently lower for elevated gravity synthesis. In addition, the effect of mixing order on the crystallization of ZSM-5 under high gravity was investigated. 1. INTRODUCTION The synthesis of large zeolite crystals has received much attention in both the open and patent literature. Among various zeolites, much attentions have focus~ on the pentasil zeolite, in particular ZSM-5/silicalite. The largest ZSM-5/silicalite are reported to range up to 420 Vm in length[i,2]. Most reports, however, have focused on the optimization of the synthesis mixture. Since the effect of high gravity on the crystallization of silicalite was relx)rted by Hayhurst et al.[3], several investigators[4-8] have reported on high gravity synthesis. According to these reports, high gravity has affects on crystal size, yield and morphology. Although most reports focus on silicalite, there are no report on the preparation of ZSM-5 under high gravity. This is perhaps due to the significant differences in synthesis chemistry between ZSM-5 and silicalite due to aluminum content in ZSM-5. In this research program, the composition of the reaction mixture for synthesis of large ZSM-5 crystal was optimized using 23 factorial method. Experiments were performed at 1 and 30 g. The effects of gravity on the percent crystallization, size distribution and morphology were evaluated.

2. EXPERIMENTAL 2.1. Synthesis The reactants used in this study were a colloidal silica,Ludox AS-40(Du Pont),

158 reagent-grade tetrapropylammonium bromide(Tokyo Chemicals), aluminum nitrate nanohydrate, aluminum hydroxide, 50 wt.% sodium hydroxide solufion(Junsei Chemical Co.) and deionized water. The reaction mixture had the oxide formula, aNa~-bA1203-100SiOz-5TPABr-cI-hO-dGravity where a, b, c, and d were varied by 23 factorial method. The synthesis was designed to perform in four different ways. In scheme I, the reactant solutions were prepared in two beakers. A half of water required , AI(NO3)a.9H20 and 50 wt.% NaOH solution were mixed in beaker I and the remaining water, Ludox AS-40 and TPABr were mixed in beaker If, respectively. After mixing these two solutions separately, they were mixed together and sitrred for enough time to give rise to homogeneous solution. Upon completion of mixing, the reactions were carried out at 180~ under i and 30 g for 24 hours. Scheme II followed exactly the same p r i e s as that of scheme I except that the concentration of Na20 was fixed at 8 moles while the concentration of A1203 was varied from 0.5 moles to 1 mole. In scheme IE based on the 23 factorial experimental results in schemes I and IL 4Na~-100SiO2-3.8TPABr-0.3A12033500H20-xGravity(x = 1 and 30 g) was used as an optimum composition to produce large crystals. The reactions were carried out at 170~ 180~ and 190~ up to 7 days under 1 and 30 g. In scheme IV, the effect of mixing order on crystal growth under high gravity was studied using the same batch composition as an optimized scheme 11I. Unlike schemes I, 1I, and Ill where total NaOH required was added into the alumina solution(be~r I), the amount of NaOH required was divided into two portions by 50%, 70%, and 100%, respectively. It was then added into the alumina solution(beaker I) and the silica solution(beaker II) finally mixing them together. Upon completion of each reaction, linear growth rates were obtained at three different temperatures and the activation energies were calculated. 2.2 Characterization Powder X-ray diffraction analysis(Rigaku Model D/Max-II~) was performed to identify crystallinity and phase. In order to measure crystal size and to investigate morphology, image analyzer(KanImager) and SEM(Shimazu Alpha 25A) were used.

3. RESULTS AND DISCUSSIONS 3.1. Scheme I Following the e ~ m e n t a l procedures described in previous section, the reactions were carried out at 180~ for 24 hours under the same conditions. The compositional ranges for three main factors, namdy, Na~3, H20, and gravity, were varied by 23 factorial method. Thus, each factor was varied from 4 moles, 2 ~ moles, and 1 g, to 8 moles, 3500 moles, and 30 g, res~:tively. Table 1 shows the effect of each combination on crystal size and the combination(bc) has the most significant positive effect while the combination(a) has the most significant negative effect. The negative effect means the decrease in crystal size while the positive effect enhances the crystal size. Aspect ratio(length divided by width) of product crystals was not significantly influenced by gravity.

159 Table 1 Compositional combinations for the effects of different factors on crystal size Crystal size(pm) Effect Significance Combination Factors A B C I II Mean a b ab c ac bc abc

8 4 8 4 8 4 8

2800 3500 3500 2800 2800 3500 3500

1 1 1 30 30 30 30

40.8 50.7 41.1 47.3 24.9 73.6 42.4

43.8 52.3 40.6 44.1 24.1 74.5 39.8

42.3 51.5 40.8 45.7 24.5 74.1 41.1

-20.5 8.9 - 1.3 - 2.2 - 6.5 13.6 - 4.6

+ + -

+ " singinficant at 99 % SEM of ZSM-5 crystals obtained from combinations (a) and (bc) are shown in Figure 1.

a

,~~

Ilc

,..~2

Figure. 1. SEM of ZSM-5 obtained from combinations (a) and (bc) Conversely, the crystallinity decreased slightly with high levels of I-~O and gravity. In order to identify the phase of solid product and crystallinity, x-ray powder diffraction analysis was performed and the crystallinity for each combination was calculated by the area between 22.5~ and 25~ in 29. Figure 2 shows XRD pattern of product obtained from combination (b) and no other phase except ZSM-5 was found. The crystallinity for each combination was summarized in table 2. Increasing the concentrations of I-IzO and gravity resulted in a slight decrease in crystallinity due to the majority of nucleation and crystallization occuring at the interface between the top-liquid phase and the segregated solid gel. As a conclusion, the gravity does not have significant effects on crystallinity while it enhances the crystal size significantly. Furthermore, it is interesting that the effect of gravity on the size distribution strongly depends on the concentration of I-hO as shown in Figure 3. At the low concentration of I-I20, the gravity resulted in positive effects on size distribution(narrow size distribution) while the gravity showed a negative effect on size distribution at the high concentration of I-I20.

160

GO 13.. 0

5.00 10.00

20.00

20

30.00

40.00

5000

Figure 2. XRD pattern of product obtained from combination (b) Table 2 Compositional combinations for the effects of different factors on crystallinity Effect Significance Combination Factors Crystallinity A B C I II Mean a b ab c ac bc abc

8 4 8 4 8 4 8

2800 3500 3500 2800 2800 3500 3500

1 1 1 30 30 30 30

96.9 98.9 98.2 96.5 98.4 96.4 72.8

96.9 96.9 97.7 100.0 97.7 96.4 82.8

96.9 97.9 98.0 98.3 98.1 96.4 82.8

-

4.6 5.0 4.6 4.7 4.8 6.1 4.6

+ -

+ " significant at 99 % 3.2. Scheme II In scheme II, AlzOa, I-hO and gravity were the parameters that were varied from 0.5 moles, 2800 moles and 1 g to 1.0 mole, 3500 moles and 30 g, respectively. In this case, the concentration of Na20 was fixed at 8 moles and the reaction was carried out for 1 day. As shown in table 3, an interesting result is that the crystal size was increased with aluminum content under high gravity while vice versa under 1 g. The effect of gravity on the crystal size seemed to become significant as aluminum content increased; that is, regardless of I-hO content, the crystal size decreased by 6 to 38% upon increasing aluminum content at 1 g while it was significantly enhanced at 30 g by 17% to 47%. This is attributed to the greater consumption of aluminum for crystal growth rather than for nucleation due to the liquid-solid segregation resulting from applying gravity. In case of crystallinity, combination(bc) in which I-hO content and gravity were high shows a negative effect on crystallinity indicating that gravity is not important as shown in table 4.

161 (a)

25

I

w

~,20

I

i

.:""~-->30G

o 10

....m.,':

7=0

(b)

/

/~\-I

"'" . ".. / /

35 _ ,

,

30

~'25 "6 20 '- 15 E 10

0

--1

=

Z

1 .i i t 40 50 60 Crystal Size(um)

5

0

! --

~

1GlG I

--

'~

I H

,,-

t

-

40 50 60 Crystal Size(urn)

Figure 3. Comparisons of the size distributions for the samples obtained from xNa20-0.5AI~O3-100SiO2-5TPABr-yH20 under 1 and 30g, respectively" (a) x=4, y=2800, (b) x=8, y=2800, (c) x=4, y=3500, and (d) x=8, y=3500. Table 3 Compositional combinations for the effects of different factors on crystal size Effect Significance Crystal size(pm) Combination Factors A B C I II Mean a b ab c ac bc abc +

1.0 0.5 1.0 0.5 1.0 0.5 1.0

2800 1 3500 1 3500 1 2800 30 2800 30 3500 30 3500 30

28.1 41.1 38.8 24.9 34.7 42.4

47.1

24.6 40.6 37.5 24.5 38.0 39.8 48.9

26.4 40.9 38.2 24.7 36.4 41.1 48.0

- 0.03 9.6 2.1 0.6 9.3

m

+ +

4.4

-

4.5

-

9significant at 97.5%

3.3. Scheme I n Based on the results obtained from schemes I and IL the optimum composition for large crystal was determined. It was for NaOH and alumina as low as possible and for I-hO and gravity as high as possible. The molar composition was

162 Table 4 Compositional combinations for the effects of different factors on crystallinity Combination Factors Crystallinity Effect Significance A B C I II Mean a b ab c ac bc abc

1.0 0.5 1.0 0.5 1.0 0.5 1.0

2800 1 3500 1 3500 1 2800 30 2800 30 3500 30 3500 30

84.0 98.2 100.0 98.4 89.9 72.8 90.4

87.3 97.7 81.5 97.7 94.8 82.8 79.4

85.7 97.9 90.8 98.1 92.4 77.8 84.9

- 4.3 - 5.4 4.2 -4.5 5.0 -8.5 2.2

+ " significant at 97.5 % 4Na20-100SiO2-3.8TPABr-0.3AI2(h-~H20. The reactions were carried out at three different temperatures up to 7 days. Figures 4 (a) and (b) show the aveyage crystal size with reaction time at 1 and 30 g.

(a) 1 G

v

(b) 3 0 G

E

E -!

v

w U oO

w

oo

~< .J

.J

Q: O

rr (3

0

1

2

3

4

5

REACTION TIME(HRS)

6

7

0

1

2

3

4

5

6

7

REACTION TIME(HRS)

Figure 4. Average crystal size vs. reaction times for the crystal obtained at various temperatures under (a) 1 g and (b) 30 g. The average crystal size with time at 190~ was significantly fluctuated in case of 1 g while the average crystal sizes with time were increased in proportion to temperature for 30 g. It also can be realized that the size distribution of crystals obtained at individual time interval for 30g was much narrower than that for 1 g. In addition, the maximum crystal sizes at each individual time for 170~ 180~ and 190~ were shown in Figures 5 (a) and (b) to investigate the reaction kinetics for crystal growth under 1 and 30 g. As shown in these Figures, the crystal sizes of ZSM-5 for three different temperatures show linear growth rotes up to 24 hours. The activation energies were calculated from the crystallization curve slope where a linear growth rate is observed. The activation energies of 63.19 kJ/mol and 56.54

163 (b) 30 G

(a) 1 G 160

19o"c

9

o-180='c

E

160

~ 120-

O''''C

.._.1

~ 80

~80.

D 40

~ 40.

>-. nO

Z~:lgO~ O:180~ O:170~

X

< ~

0

0

4

8

12

16 20 24

REACTION TIME(HRS)

0

4

8

12 16 20 24

REACTION TIME(HRS)

Figure 5. Maximum crystal size vs. reaction time for the crystal obtained at various t e ~ a t u r e s under (a) 1 g and (b) 30 g. kJ/mol were obtained for 1 g and 30 g, respectively. Lower activation energy for 30 g might be attributed to the fast crystal growth due to tl~ limited crystallization mainly at liquid-solid interphase. The value, 63.19 kJ/mol for 1 g is fairly consistent with 64.5 kJ/mol reported by Feokfistova et all9]. 3.4. S c h e m e IV

In scheme IV, the effect of mixing procedure in preparing reactant solution on crystal growth rate was studied using the same molar composition as that of scheme HI.. Unlike schemes I, ]l, and Ill where total NaOH solution required added into alumina s o l u f i o n ( ~ e r I), the reactant solutions were prepared in three different ways ; those are, 50%, 70% and 100% of 50 wt.% NaOH solution required were added into the silica solution(beaker II), respectively. The reactions were carried out at 170~ 180~ and 190~ under 1 and 30 g up to 24 hours. The logarithmic plots of the linear rate of ZSM-5 crystallization with respect to reciprocal temperature for 1 and 30 g were shown in Figures 6 (a), (b) and (c). In case of adding a half of 50 wt.% NaOH solution into silica containing solution, the activation energies for 1 and 30 g are 68.17kJ/mol and 64.85 kJ/mol(Fig.6(a)). However, as the addition of 50 wt.% NaOH solution into silica solution increased to 70% of total NaOH required(Fig.6(b)), the difference in the activation energies for 1 and 30 g became larger than that of the previous case. The activation energies for 1 and 30 g are 61.52 kJ/mol and 51.55 kJ/mol, respectively. This might be attributed to the more dissolution of highly stable silica particles upon addition of NaOH solution. On the other hand, the activation energies for the case of adding total NaOH solution required into the silica solufion(Fig.6(c)) increased to 66.51 kJ/mol and 59.85 kJ/mol for both 1 and 30 g compared to the second case. This seems to be caused by the formation of much more silicate species which could require more activation energies. It is interesting, however, to note that the activation energy under high gravity is smaller than under normal gravity regardless of mixing procedures.

164 (a)

(b)

(c) 1.85

1.85

1,85

,,r

~= 1.35

_=1.35

-%.,o .

lIT X IO00(K)

%

2~

lIT X IO00(K)

2ao

0 ~

I-

2.10

:

.~

.

220 230 1/T x IO00(K)

Figure 6. Logarithmic plots of the linear rate of ZSM-5 crystallization vs. reciprocal temperature for 1G and 30G.

4. CONCLUSIONS Several conclusions were obtained through this work. The gravity does not have significant effects on % crystallization while it enhances the crystal size significantly. In addition, the results suggest that the effect of gravity on the size distribution strongly delxmds on the concentration of I-hO. At low concentration of H ~ , the gravity resulted in narrow size distribution while it caused broad size distribution at the high concentration of I-hO. Unlike normal gravity, aluminium content shows positive effects on crystal size under high gravity. Finally, regardless of mixing procedures, a high gravity gives rise to lower activation energy than a normal gravity.

REFERENCES

1. D.T. Hayhurst and J.C. Lee, in New Developments in Zeolite Science and Technology(Fxts. y. murakami, A. Iijima and J.W. Ward) Kodansha, Tokyo and Elsevier, Amsterdam(1986),239. 2. J. Komatowsld, J. Zeolites 8(1988), 77. 3. D.T. Hayhurst, P. J. Melling, W. J. Kim and W. bibby, Zeolite Synthesis, ACS Symp. Set. No. 398(Eds. M..L. Occelli and H.E. Robson)(1989), 233. 4. W.J. Kim, Ph.D. Dissertation, Cleveland State University(1989). 5. W.J. Kim and J. Lee, J. Korean Ind. & Eng. Chem. Vol. 2, No. 2(1991), 97. 6. H. Zhang, S. Ostrach and Y. Kamotani, Trans~rt Phenomena in Materials Processing and Manufacturing, HTD-Vol. 196(1992). 7. H. Zhang, S. Ostrach and Y. Kamotani, 31st Aerospace Sciences Meeting and Exihibit, 1(1993), 11. 8. H. Zhang, S. Ostrach and Y. Kamotani, Processings of 10th International Conference on Crystal Growth(1993). 9. N.N. Feoktistova and S.P. Zhdanov, Zeolites Vol. 9(1989), 5.

H. Chon, S.-K. Ibm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 1997 Elsevier Science B.V. All rights reserved.

165

Structure directing role o f N a + and T M A + cations in 18-crown-6 ether mediated crystallization o f EMT, M A Z and SOD aluminosilicate zeolites E.J.P. Feijen, B. Matthijs, P.J. Grobet, J.A. Martens and P.A. Jacobs Centrum voor Oppervlaktechemie en Katalyse, KU Leuven Kardinaal Mercierlaan 92, B-3001 Heverlee (Leuven), Belgium

Summary

Zeolite crystallizations are performed in the system 10 SiO2; 1 A1203; 0.97 18-crown-6 ether; x Na20; y TMA20; 135 H20. EMT, MAZ and SOD type phases are obtained depending on the relative concentrations of sodium and TMA in the gel. The nature of the phases obtained is rationalized by the extended-structure approach with specific structure directing roles for the different cations in the formation of extended structures as well as in their mutual condensation.

1. INTRODUCTION Although much progress has already been made in the identification of precursors and template effects, the crystallization mechanisms responsible for the formation of zeolites are not yet fully understood [1]. According to the extended-structure approach of zeolite crystallization, zeolites grow by condensation of extended-structures (ES) whereby primary cations template the formation of ES units, and secondary cations organize the condensation of ES units [2]. Recently, this approach has been validated in the crystallization of FAU and EMT phases and their intergrowths in the presence of crown-ethers [3]. A mechanism of condensation of faujasite sheets decorated with the crown-ether molecules rationalizes and quantitatively predicts the exact nature of the faujasite polytype with respect to intergrowth pattern and phase composition. In this work, a hydrogel leading typically to the crystallization of the EMT phase was modified by adding tetramethylammonium cations (TMA) and varying the concentration of sodium. From these systems, EMT, MAZ and SOD zeolite phases were obtained. The nature of the zeolite phases obtained can be rationalized based on the known primary and secondary structure directing roles of sodium, TMA and sodium-crown-ether complexes. 2. EXPERIMENTAL Hydrogels were prepared with the following standard molar composition: 10 SiO2; 1 A1203; 0.97 18-crown-6 ether; x Na20; y TMA20; 135 H20. The Na and TMA contents of the gel are summarized in table 1. The 18-crown-6 ether (1,4,7,10,13,16-hexaoxacyclooctadecane, Janssen Chimica) was added to a colloidal silica source (Ludox HS-40). Gibsite (Fluka) was dissolved in an aqueous solution of NaOH and TMAOH under heating at 353K. The latter solution and the silica sol were combined and stirred for 15 minutes. The gels were transferred

166 into teflon lined autoclaves and aged at room temperature for 3 days. Crystallization was interrupted after 9 days of heating at 373K. The solids were recovered by centrifugation at 39 kG and washed with deionized water until the pH of the wash water was lower than 9. X-ray diffraction patterns were recorded on an automated Siemens D5000 diffractometer. Infrared spectra were taken on a Nicolet 730 FTIR spectrometer using the KBr pellet technique. Scanning electron micrographs on gold-coated samples were obtained using a Jeol superprobe 733 instrument. Thermogravimetric analyses and differential thermoanalysis profiles were recorded on a Setaram TGA 92 thermobalance in oxygen/helium (20/80 vol/vol) atmosphere. 13C MAS NMR with proton decoupling was performed on a Bruker 400 MSL spectrometer at 100.6 Mhz., with a pulse length of 4gs, a repetition time of 10s and a spinning rate of 4kHz. Quantitative 27A1 MAS NMR spectra were recorded on the same spectrometer at 104.2 Mhz, with a pulse length of 0.61 gs, a repitition time of 0.1 s and a spinning rate of 14 kHz using zeolite samples with the same topology and known A1 content as references.

3. RESULTS AND DISCUSSION 3.1. Gel composition and crystallization products The crystalline phases present in the products, as determined by XRD, are summarized in Table 1. In a first series of experiments (series A: sample 1 to 5), the influence of the addition of TMA cations to the gel was studied. The experiment in absence of TMA (sample 1), is a typical 18-crown-6 ether mediated crystallization of the hexagonal faujasite phase (EMT). Gradual addition of TMA cations yields the co-crystallization of more MAZ phase next to traces of SOD. XRD studies of series A showed, however, that the crystallization of MAZ which is most abundant in sample 2, (Figure 1) is suppressed at increasing TMA levels, the SOD phase becoming more abundant (sample 5, Figure 2). The weakly intense diffraction lines at a 2e value of 6 ~ in sample 2 indicate the presence of traces of EMT (Figure 1). The SEM picture in Figure 5 shows that the MAZ phase in sample 2 consists of spherulitic crystals, with a diameter of 1 to 2 gin. The spherulitic morphology is typical of MAZ type zeolite crystals grown under conditions of high supersaturation [4].

Table 1 Na20 (x) and TMA20 (y) concentrations in the standard hydrogel and XRD-visible crystalline phases I

Sample

x

y

Phases in product

Sample x

y

Phases in product

2.04

0.48

MAZ>EMT, SOD

8

1.92

0.48

MAZ>EMT

9

2.30

0.24

MAZ>EMT, SOD

2.21

0.24

MAZ > EMT, SOD

11

2.16

0.24

MAZ> EMT

MAZ>EMT, SOD Ij 12

1.68

0.72

SOD

!

1

2.40

0.00

EMT

i7 I

2

2.40

0.24

MAZ>EMT, SOD

3

2.40

0.48

MAZ>EMT, SOD I

4

2.40

0.72

MAZ>EMT, SOD . 10 I

5

2.40

0.96

SOD>MAZ I

6

2.28

0.48

167

I

5

l

I

l0

l

I

,

I

I

.

t

15 20 25 30 35 40 45 50 55 2O

Figure 1: XRD pattem of sample 2.

1'o1'5 ' ~o' ~5' ~o' ~5' ~o' ~5' go 55 2O Figure 2: XRD pattern of sample 5.

1 5

10 15 20 25 30 35 40 45 50 55 20 Figure 3: XRD pattem of sample 10.

' lb' l'5' J.o ~

~o ~5 do ,is go' 55 2O

Figure 4: XRD pattern of sample 12.

In order to further examine the influence of sodium and TMA cations, additional series of experiments were set up with variable amounts of sodium at a constant TMA content (series B: sample 6 to 8; series C: sample 9 to 11). For series B, all samples contain a considerable amount of MAZ and SOD, and possibly traces of EMT. In series C, three phases (EMT, MAZ and SOD) were unambiguously identified as illustrated with the XRD pattern of sample 10 (Figure 3). The SEM picture of sample 11 in Figure 6 clearly shows physical mixtures of hexagonal crystals of the EMT phase and spherulitic crystals assigned to the MAZ phase. Abundant TMA addition results eventually in the crystallization of the dense SOD phase only (sample 12, Figure 4). The Si/A1 ratio of the SOD phase of sample 12 is 6.3 according to quantitative 27A1 MAS NMR. For these experiments it is concluded that from Na and 18-crown-6 ether containing hydrogels, the crystallization of EMT is readily suppressed by addition of TMA. Instead, the MAZ phase is preferred at intermediate TMA contents in the hydrogel, while from TMA-rich hydrogels, a pure SOD phase crystallizes under the conditions investigated.

168

Figure 5: SEM picture of sample 2.

Figure 6: SEM picture of sample 11 (after 6 days of heating).

3.2. Quantification of phases in zeolite mixtures

During zeolite crystal growth, TMA cations are often occluded in cavities such as gmelinite and sodalite cages. Such occlusion phenomena of TMA have been observed previously for the crystallization of mazzite and sodalite type zeolites [5,6]. Thermoanalysis profiles confirm the presence of TMA cations in the present samples (Figures 7 and 8). The oxidative decomposition of TMA in gmelinite and sodalite cages is observed at 820 and 870 K, respectively. From the TG profile in Figure 8, the TMA content of sample 12 containing only SOD could be estimated. It was found that approximately each sodalite cage contains 1 TMA. The presence of these cations in gmelinite and sodalite cages was confirmed by Hdecoupled 13C MAS NMR. Indeed, for sample 2, the spectrum in Figure 9 shows resonance lines at 58.8 and 57.9 ppm, indicating the presence of sodalite cage and gmelinite cage occluded TMA cations, respectively [7]. Furthermore, a weak resonance is observed at 70 ppm, originating from Na-18-crown-6 ether complexes in the EMT structure [3,8]. These findings are in good agreement with the presence of three phases, viz. MAZ, SOD and EMT according to the XRD analyses (Table 1 and Figure 1). The amount of MAZ and SOD derived from the amount of TMA decomposed at 820 and 870 K, respectively, is plotted in Figure 11 against the Na20/(Na20 + TMA20) ratio in the synthesis gel. The amount of EMT in the samples (Figure 11) was estimated based on the intensity of the Double 6 Ring (D6R) vibration in the IR spectra, relative to that of the TObending vibration (450 cm 1) [3]. For sample 11, the IR spectrum is displayed in Figure 10.

169 The D6R vibration (585 cm l ) is clearly present as a shoulder at lower frequency on the Single 6 Ring vibration band (621 cm 1) of the MAZ structure.

3.3. Crystallization mechanism It is evident from Figure 11 that there is a relationship between the fraction of the Na/(Na+TMA) cations in the synthesis mixture and the formation of a particular structure type. Indeed, only intermediate fractions yield MAZ type zeolites, while from Na-rich and TMA-rich gels preferentially the EMT and SOD type zeolites are crystallized, respectively.

75

0

0

TG

150 50

-5

100~

~ ~ -lO

-10

-15 t t a Flow

0

'

~

'

460

'

660

'

-20

'

200

'

Terrp (~

Ternp (~

Figure 8:TG-DT analysis profile of sample 12.

Figure 7" TG-DT analysis profile of sample 11.

0.6

0.5

i~ 0.3

32' ~0' gS' 6'6 ' 6'4 ' g2 ' 6'0 ' 5'8 ' 5P6 (ppm)

O.

1000

~ Wa~nt~

Figure 9" 13C MAS NMR profile of sample 2

1~

~

~

5(~0 ' 4t~

(crab

Figure 10: IR spectrum of sample 11.

170

9

9 9

80 o~

~,,,,,,

9

40

20

.

",,,,.

/ .

mA

0.75

0.-80

- 0.85

0.-9ff

0.95

1

Na20 / Na20 + TM~O Figure 11" Influence of the Na and TMA content of the synthesis mixture on the crystallization of EMT, MAZ and SOD.

Following the extended-structure approach [2,3], the Na and TMA cations are classified as primary cations, responsible for the formation of extended structures (ES) during crystallization. For the formation of EMT and MAZ phases, these ES units are faujasite and mazzite sheets, respectively (Figure 12). The secondary cations that control ES condensation are Na-crown-ether complexes [3] and Na ions (Figure 12), respectively. Indeed, only Na and TMA cations were found in the MAZ materials, the latter cations being exclusively present in the gmelinite cages being part of the ES units. The crystallization pathways drawn in Figure 12, with each pathway controlled by specific primary and secondary cations, now fully explain the formation of the different zeolite types (Figure 11). Indeed, in absence of TMA, only pathway A is available, and EMT formation dominates. In presence of Na and TMA, pathway B becomes possible as mazzite sheets can be formed. Indeed, Na acts as primary as well as secondary cation for MAZ, and is assisted by TMA and its templating effect for gmelinite cages. If TMA cations become more abundant compared to Na, the structure directing potential of TMA for sodalite cages leads to SOD phases built from condensed sodalite cages. For reasons of charge compensation in the lattice, these TMA containing sodalite cages cannot be organized into an 18-crown-6 ether containing EMT lattice [3]. Pathway B, yielding MAZ, also becomes unfavourable under these conditions, as the concentration of sodium, i.e. a primary as well as secondary cation, is too low.

171

SOD

mazzite sheet

fauj asite sheet

+

i.

TMA Na+ _. TMA+

Na+1

( ~

Na+

[ [Na-18-crown-6]

g/

MAZ

EMT

Figure 12: Crystallization routes for EMT, MAZ and SOD.

4. CONCLUSIONS The crystallization of EMT, MAZ and SOD type aluminosilicate zeolites is controlled by the relative amounts of Na and TMA in the synthesis hydrogel. Their effect on the nature of the crystalline products is explained by their specific structure directing contribution in the formation of extended structures and the further organization of these units into a crystalline zeolite lattice.

ACKNOWLEDGEMENTS

This work is sponsored by the Belgian Ministry of Science Policy in the frame of an IUAP-PAI program and by the Flemish N.F.W.O. EJPF acknowledges KU Leuven for a postdoctoral fellowship, JAM and PJG the Flemish NFWO for a research position.

REFERENCES

1 E.J.P. Feijen, J.A. Martens and P.A. Jacobs, Zeolites and Related Microporous Materials: State of the Art 1994 (part A), Studies in Surface Science and Catalysis, Vol. 84., J.

172

2 3 4

5 6 7 8

Weitkamp, H.G. Karge, H. Pfeifer and W. H/51derich (Editors), Elsevier Science B.V., Amsterdam, (1994) p.3. D.E.W. Vaughan, Catalysis and Adsorption by Zeolites, G. Ohlmann, H. Pfeifer and R. Fricke (Editors) Elsevier Science B.V., Amsterdam (1991) p.275. E.J.P. Feijen, K. De Vadder, M.H. Bosschaerts, J.L. Lievens, J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Am. Chem. Soc., 116 (1994) 2950. F. Di Renzo, F. Fajula, F. Figueras, S. Nicolas and T. Des Courieres, Zeolites" Facts, Figures, Future, Studies in Surface Science and Catalysis, Vol. 49 A., P.A. Jacobs and R.A. van Santen (Editors), Elsevier Science B.V., Amsterdam, (1989) p. 119. R.M. Barrer, Hydrothermal Chemistry of Zeolites, Academic Press, London (1982) p. 166. C. Baerlocher and W.M. Meier, Helv. Chim. Acta., 52 (1969) 1853. S. Hayashi, K. Suzuki, S. Shin, K. Hayamuzi and O. Yamamoto, Chem. Phys. Let. (1985) 368. F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites 10 (1990) 546.

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 9 i997 Elsevier Science B.V. All rights reserved.

173

Synthesis of high-silica FAU-, EMT-, RHO- and KFI-type zeolites in the presence of 18-crown-6 ether. T. Chatelaina, J. Patarina, E. Brendl~a, F. Dougniera, J.L. Gutha and P. Schulzb a Laboratoire de Mat~riaux MinOraux URA-CNRS 428 ENSCMu - UHA 3 rue Alfred Werner, 68093 Mulhouse Cedex, France b Centre de Recherche Elf Antar-France de Solaize, Chemin du Canal, BP22, 69360 Saint Symphorien d'Ozon, France FAU-, EMT-, RHO- and KFI-type zeolites were synthesized by heating an aqueous alkaline aluminosilicate gel containing 18-crown-6 ether as an organic template with either sodium, or sodium and cesium, or potassium and strontium cations. The products were characterized by elemental analysis, scanning electron microscopy , powder X-ray diffraction , thermal analysis, solid-state nuclear magnetic resonance spectroscopy, and n-hexane adsorption. The 18-crown-6 ether is incorporated in each structure. Its presence turns out to be necessary to crystallize EMT-type materials and, in the other cases, it allows reproducible preparations of well crystallized materials with high Si/AI ratios (3.7-4.6). The retrieval of the 18-crown-6 ether from the EMT-type zeolite is possible with solvothermal treatments. Keywords

Synthesis, high-silica zeolites, 18-crown-6 ether

1. INTRODUCTION

Recently, in our laboratory the use of crown-ethers in an aluminosilicate gel containing sodium cations led to the crystallization of high-silica FAU- and EMT- type zeolites [1]. Later, other studies have been published on these materials [2-4]. Knowing the industrial interest for these high-silica zeolites [5], an extensive optimization work has been performed in order to decrease their synthesis cost. Moreover, as other zeolites have pores which are potentially able to accomodate crown-ether-cation complexes, a synthesis study was undertaken with different alkaline cations associated with 18-crown-6 ether. The aim was to increase the Si/AI ratio of zeolites already known and to take advantage of the structure-directing effect of crown-ether for developing more specific synthesis procedures for these zeolites. 2. EXPERIMENTAL 2.1. Reactants

The reactants were 18-C-6 ether (1,4,7,10,13,16-hexaoxacyclo-octadecane, > 98%, Lancaster), sodium hydroxide (purum, > 98%, Fluka), potassium hydroxide (normapur, > 86%, Prolabo), cesium hydroxide (50wt% CsOH, 50wt% H20, Aldrich) and strontium nitrate (normapur, >99%, Prolabo). The silicon and aluminum sources

174

were colloidal silica (40wt% SiO2 in water, CecasoI,Ceca or Ludox AS40, Du Pont de Nemours ) and sodium aluminate (56wt% AI203, 37wt% Na20, Carlo Erba) or aluminum hydroxide (purum, 65 wt% AI203, Fluka).

2.2. Synthesis procedure for the FAU-, EMT- and RHO-type materials

The samples were obtained by hydrothermal synthesis at 110 ~ optimization, the molar composition of the starting mixtures was as follows:

After

10 SiO2 : 1 AI203 : x Na20 :y Cs20:0.4-0.5 (18-crown-6) : 100 H20 where x = 2.6 and y = 0 for FAU-type zeolite; x = 2.1 and y = 0 for EMT-type zeolite; x = 1.1-1.8 and y = 0.3 for RHO-type zeolite. In all three cases, the starting mixture was prepared according to method II described elsewhere [6,7]. The resulting gel was aged at room temperature for 24 hours in a closed polypropylene bottle under continuous stirring. The crystallization was carried out under static conditions in PTFE-lined stainless-steel autoclaves during 2 to 15 days. The solids obtained were filtered, washed with distilled water until the pH of the filtrate was neutral and then dried at 80 ~

2.3. Synthesis procedure for the KFI-type materials

The samples were obtained by hydrothermal synthesis at 150~ composition of the starting mixture was:

The molar

10 SiO2 : 1 AI203 : 1.8-2.3 K20:0.10 SrO :0.5-1.0 (18-crown-6) : 160-220 H20 The synthesis procedure was similar to that developed by Verduijn [8]. A mixture of KOH and AI(OH)3 with a portion of water was boiled, under continuous stirring, until a clear solution A was formed and then cooled to room temperature. Sr(NO3)2 and 18-crown-6 were successively dissolved in another portion of water and colloidal silica was slowly poured in the thoroughly stirred solution before adding solution A. Before heating under static conditions in PTFE-lined stainless-steel autoclaves during 4 to 5 days, the resulting gel was stirred for about 30 minutes. After crystallization, the products were treated as previously described. A classical KFItype zeolite sample, i.e., without organic species, was also prepared according to the procedure described by Verduijn [8].

2.4. Chemical analysis

Si, AI, Na, K, Cs and Sr analysis was performed by atomic absorption spectroscopy. The amount of water and organic species of the as-synthesized materials was determined by thermogravimetry. Carbon analysis was performed by coulometric determination after calcination of the samples at 1050 ~ under air.

2.5. Powder X-ray diffraction

The powder patterns were obtained on a Philips PW 1800 diffractometer equipped with a variable divergence slit (CuKo0. For the FAU-type materials the relative XRD intensity was determined according to the ASTM D3906-85a procedure [9]. A similar procedure was set up for the EMT samples (18 peaks in the 2e range 1530~ and for the RHO-[7] and KFI-type materials.

2.6. Thermal analysis

Prior to analysis, the solids were equilibrated over a saturated aqueous solution

175

of NH4CI (p/po=0.85). Thermogravimetry (TG) was performed on a Mettler 1 thermoanalyzer by heating in air at 4~ -1. Differential thermal analysis (DTA) was carried out in air on a BDL-Setaram M2 apparatus between 20 and 750 ~ at a heating rate of 10 ~

2.7. Adsorption measurements The sorption studies were carried out on protonated samples, obtained according to a procedure previously described for FAU and EMT zeolites [10]. The adsorption capacity measurements were performed by using a computerized thermogravimetric equipment TG 92 from Setaram. About 100 mg of the calcined solid was activated at 450~ under flowing dry N2 (heating rate : 5~ After 1 hour at 450 ~ the sample was cooled to room temperature during 1 hour. Then the solid was subjected to a flowing mixture of dry nitrogen and n-hexane, during 10 hours. The relative pressure P/PO of n-hexane was 0.5. 2.8. 13 C,27AI, 29Si solid-state MAS and CP MAS NMR spectroscopy The spectra were recorded on a Bruker MSL 300 spectrometer. The recording conditions of the CP MAS and MAS spectra are given in ref.7. 2.9. Determination of the occluded organic species by liquid 1H NMR A known amount of the as-synthesized zeolites (~ 80 mg) was dissolved into 1.5 cm3 of a 40 wt% aqueous HF solution. Thereafter 200 mg of a lwt % dioxane-D20 solution was added as internal standard to the dissolved zeolites. After centrifugation, ,,, 0.5 cm 3 of the liquid was transferred with an equivalent volume of pure D20 in a PTFE tube. The latter was then placed in a classical glass tube for the NMR analysis. The spectra were recorded on a Bruker AC spectrometer. The recording conditions were: frequency =250.13 MHz; recycle time =8 s; pulse width =2 ms; pulse angle = 30 ~ 2.10. Retrieval of 18-crown-6 ether from EMT zeolites The retrieval of the 18-crown-6 ether was performed by a solvothermal treatment of a suspension of the as-synthesized zeolitic samples in water or in an alcohol with or without a salt at a temperature ranging from 180~ to 200~ 3.RESULTS AND DISCUSSION 3.1.Synthesis, crystal morphology, chemical composition and thermal analysis In the absence of Cs + cations and for x= 2.1, a pure EMT zeolite could be synthesized with a lower amount of the expensive 18C6 ether. Indeed, the previously used stoichiometry was reduced from 0.7 [1-4] to 0.4 (Table 1, sample D). Under these new synthesis conditions, no 18C6 remains in the mother liquor. A scale-up study has shown that with this new gel composition, batches producing several kilograms of very pure EMT-type zeolite were easy to reproduce [11].For x = 2.6, the soda content is too high for the 18C6 ether to direct the crystallization towards the EMT structure-type, the FAU phase was obtained (see Table 1, sample A ). For intermediate values ( 2.1<x< 2.6), intergrowths or overgrowths (FAU/EMT) of the two structural types were produced (Table 1, sample C). For instance, according to Treacy et al.[12], an (x value close to 0.5 was found for x= 2.4. In the absence of 18C6 ether, the materials were amorphous(see sample B) and no EMTtype zeolite could be obtained.

176

Table 1 Typical synthesis conditions of FAU-, EMT-, or RHO-type zeolites (starting molar gel composition : 10 SiO2 : 1 AI203 : x Na20: y Cs20 : z18-crown-6 : 100 H20). Heating time Sample x y z at 110~ XRD results (estimated crystallinity) a (days) A 2.6 0.0 0.4 8 FAU (100%) B 2.6 0.0 0.0 8 Amorphous C 2.4 0.0 0.4 8 FAU/EMT (or,~ 0.5) D 2.1 0.0 0.4 8 EMT (100%) E 1.8 0.3 0.5 8 RHO (100%) F 1.8 0.3 0.5 2 RHO (90%) G 1.8 0.3 0.0 2 RHO (45%) + CHA b + ANA b + amb H 1.1 0.3 0.5 15 RHO (90%) a : The reference samples are A for FAU materials and E for RHO materials. b : CHA = chabazite, ANA = analcime, am : amorphous material. The partial substitution of cesium for sodium in the gel composition (1.1< x< 1.8) gave rise to the crystallization of a pure high-silica RHO -type zeolite [7]. Here also the use of crown-ether seems to be necessary for obtaining a pure phase with good reproducibility. As a matter of fact, without the crown-ether molecule in the starting mixture, only a poorly crystalline RHO material was obtained with some chabazite and pollucite [( Na, Cs) analcime] as by-products (compare samples F and G). As it has been observed above for the RHO-type solids, the introduction of 18-C6 in the starting mixture of a KFI-type zeolite led also to a pure material (compare samples I and M, Table 2). Whereas, in the case of RHO zeolite, the soda content can be decreased to x=1.1, the K20 amount (k) has to be close to 2.3- 2.5 otherwise no crystallisation occurs after 5 days (see Table 2, sample L). Table 2 Typical synthesis conditions of KFI-type zeolites. Starting molar gel composition : 10 SiO2 : 1 AI203 : k K20:0.10 SrO :t 18-crown-6 : w H20 Heating time Sample k t w at 150~ XRD results (estimated crystallinity) a (days) Ib 2.3 0 160 5 KFI (90%) + (n.i.)c J 2.3 1 220 5 KFI (100%) K 2.3 0.5 160 5 KFI (90%) L 1.8 1 160 5 Amorphous M 2.3 1 160 5 KFI (100%) a : The reference sample is sample M b : Sample prepared according to the procedure described in ref. 8 c : (n.i.) Traces of a unidentified impurity The FAU-type crystals display a typical octahedral morphology (Figure l a), whereas the EMT-type samples consist of fairly hexagonal platelets (Figure lb). In the latter case, a decrease of the 18-crown-6 ether content (0.4 instead of 0.7) [1-4] and of the water content (100 H20 instead of 140 H20) [1-4] led to a significant

177

decrease of the crystal size (1-21~m instead of 3-4pm) which should enhance the activity of this zeolite in catalytic craking reactions. The RHO-type samples prepared in the presence of crown-ether display a sphere-like shape with an average size of ll~m (Figure lc). Whereas, aggregates of cubic crystals with a size close to 3-41~m were obtained for the KFI-type materials (Figure 1d).

(b)

(c)

(d)

Figure 1. Scanning electron micrographs of high-silica zeolites: (a) FAU sample A; (b) EMT sample D; (c) RHO sample E; (d) KFI sample M The chemical composition of some samples is reported in Table 3. The framework Si/AI molar ratios determined by 29Si MAS NMR spectroscopy are in good agreement with the values obtained from chemical analysis. This confirms that there is no extra-framework aluminum species in the as-made samples as checked by 27AI NMR spectroscopy and that no unreacted gel is present. Zeolites, whose synthesis is possible without crown-ether, i.e., FAU-, RHO- and KFI-type zeolites, show higher Si/AI ratios when the synthesis is performed in the presence of 18C6 ether. There are probably three reasons for this: -for a given Si/AI ratio in the gel, crystallization is generally possible with a lower alkalinity which does not favor the AI incorporation. -the incorporation degree of the bulky 18C6-cation complex is lower and consequently the substitution of AI for Si. -the 18C6-cation complex has better stabilization interactions than hydrated cations when the hydrophobicity of the framework increases (higher Si/AI). It can be seen from the analytical results in table 3, that there is a significant amount of organic matter incorporated in the zeolite. A strong signal close to 70ppm

178

on all 13C MAS n.m.r, spectra gives evidence of the presence of the crown-ether ( between 1 and 8 molecules per unit cell). The Rietveld refinement of the crystalstructure of an EMT-type sample showed that each large cage, i.e., both the hypocage and the hypercage contains one 18C6-cation complex [13]. It can be assumed that in the RHO- and KFI-type materials the crown-ether complexes are occluded in the large LTA-type cavities. But according to the chemical analysis only 50 to 70% of these cavities are occupied. In the case of the FAU-type zeolite the occupation factor of the supercage is close to one. The d.t.a.results show that the thermal decomposition of the crown-ether occurs at much higher temperature for the RHO-and KFI-type materials ( between 300 and 380~ than for FAU- and EMT-type materials ( between 160 and 280~ This difference can be related to the size of the apertures which are circumscribed by 8 membered rings in the former and by 12 membered rings in the latter. Table 3 Chemical composition (wt%) of some zeolite samples in their as-synthesized form (a) wt % Sample Struct. SiO2 AI203 Na20 K20 Cs20 SrO H20 Organic species type TG a CA b NMR c A FAU 46.8 14.3 8.6 / / / 17.0 12.4 12.1 n.d. D EMT 52.7 11.8 7.7 / / / 16.5 12.3 12.2 n.d. E RHO 54.4 11.9 5.2 / 10.6 / 12.5 6.0 5.9 5.9 H RHO 57.9 10.6 4.5 / 9.1 0 9.3 9.1 9.0 n.d. I KFI 51.5 12.6 / 12.1 / 1.2 n.d. / / ./ M KFI 57.1 12.8 / 10.3 / 1.2 16.4 3.1 3.1 3.0. (b) Si/AI molar ratios and 18-crown-6 per unit cell Sample Si/AI molar ratio 18-crown-6 CAb NMR d per unit cell A 2.8 3.0 ~8.5 a determined by thermogravimetry b determined by chemical analysis D 3.8 3.7 ~4.0 E 3.9 3.9 ~1.0 c determined by liquid 1H NMR H 4.6 4.6 ~1.4 d determined by 29Si MAS NMR I 3.5 3.6 / n.d. 9not determined M 3.8 4.0 ~1.0 .

.

.

.

3.4. A d s o r p t i o n

measurements An original result is observed for the RHO-type material. Indeed, the n-hexane sorption capacity (Table 4) is high and the corresponding porous volume is equal to 0.26 cm31iq.g-1.

Table 4 Adsorption of n-hexane Sample Structure type D E M

EMT RHO KFI

Adsorption capacity (wt%) 19.5 17.1 13.7

Porous volume (cm31iq.9-1) 0.29 0.26 0.21

179

This value, which is very close to the theoritical one ( 0.33 cm31iq.g -1114].) is larger than that observed for a classical RHO material (prepared in the absence of 18-crown-6), where only 50% of the theoritical volume is accessible [14]. This higher sorption capacity can be related to the non-distorded high-silica RHO framework [7]. 3.5. R e t r i e v a l of 1 8 - c r o w n - 6 e t h e r f r o m E M T z e o l i t e s

Given the high cost of the crown ether molecule, it is particularly relevant to find a procedure leading to the recovery of the intact macrocycle, and possibly allowing further recycling in synthesis. A few studies dealing with the removal of the organic templating agents from molecular sieves without calcining the samples are reported in literature. In 1986 Gelsthorpe and Theocharis [15] succeeded in extracting triethylamine from AIPO4-5 and SAPO-5 by treating these materials with a solution of methanol and hydrochloric acid at 243~ This method was further used and extented to other porous aluminophosphates by Malla and Komarneni ([16]. The EMT-type zeolite samples were treated in autoclaves by using various solutions as shown in Table 5. Table 5 Extraction media and yields (%) (extraction conditions" liquid/solid ratio 5-50, salt concentration 1-3 mol.I -t, pH 3- 7) Extraction medium Extraction yields (%) Solvent alone H20, C1 -C4 alcohols 15- 40 Solutions of salts of small cations Solutions of salts of large cations

e.g., K+, NH4 +. -protonated amines: e.g.,Et2NH, Et3N, Pr3N -quaternary ammonium salts: e.g., Me4N+ , Et4N+

30- 60 60- 95

According to the nature of the extraction medium, three extraction levels were reached: - with a solvent alone (H20 or alcohols from C1 to C4 or H20-alcohols mixture), the extraction yields did not exceed 40%. - with a solution containing small cations like alkaline or ammonium cations, the yields increase to 60%. - with a solution containing large cations like protonated amines or quarternary ammonium cations, almost all the crown-ether could be extracted. This is in agreement with the conditions which have to be fulfilled by extraction medium: it has to be a good solvent for the crown-ether or its cationic complex and it has to replace in the zeolite,with similar interaction, the extracted ether or its cationic complex.Thus water or alcohols are good solvents for the crown ether, but as guests in the silica-rich zeolite their interaction with the sodium cations is weaker than for the crown-ether. The presence of cations in the solvent helps probably the extraction because the crown-ether is stabilized in solution in the form of complexes. Moreover there can be a simultaneous cation exchange. When the cations present in the zeolite after exchange are similar to the sodium cations, this exchange does not improve the extraction. Only when the new cations exhibit stabilizing interactions

180

and volumes comparable to those of the crown-ether cation complexes, the exchange has a beneficial effect on the extraction. This is the case of bulky and relatively hydrophobic alkylammonium cations which seem to be the most suitable to mimic the 18C6-Na + complex. Among those the protonated triethylamine led to the highest extraction yield (95%). 4. CONCLUSION

FAU-, EMT-, RHO- and KFI-type zeolites were prepared by using the 18-crown-6 ether in an aluminosilicate gel composition. The addition of such an organic species to the starting mixture brought about some significant improvements. The first of them is purity since in a completely inorganic medium usually unwanted by-products appeared. Moreover, until now, the EMT structure-type cannot be obtained, even as traces, in the absence of 18-crown-6. The second outcome was a better crystallinity of the desired products. The use of the crown-ether molecule also resulted in an increased framework Si/AI molar ratio. This result can be related to the fact that the crown-ether is always incorporated and acts as a specific stabilizing template 5. ACKNOWLEDGEMENTS

The authors would like to thank Dr. H. Kessler for fruitful discussions and S. Einhorn for taking the photographs. Financial support of this work was provided by the European Union (Brite Euram program). REFERENCES

1. 2. 3. 4.

F. Delprato, L. Delmotte, J.L. Guth and L. Huve, Zeolites, 10 (1990) 546. S.L. Burkett and M.E. Davis, Microporous Materials, 1 (1993) 265. C. N. Wu and K. J. Chao, J. Chem. Soc. Farad. Trans., 91 1 (1995) 167. E.J.P. Feijen, K. de Vadder, M.H. Bosschaerts, J.L. Lievens, J.A. Martens, P.J. Grobet and P.A. Jacobs, J. Am. Chem. Soc., 116 (1994) 2950. 5. T. Des Couri~res, J.L. Guth, J. Patarin and C. Zivkov, U.S. Pat.5 273 945 (1993). 6. T. Chatelain, J. Patarin, M. Soulard, J.L. Guth and P. Schulz, Zeolites, 15 (1995)90. 7. T. Chatelain, J. Patarin, E. Fousson, M. Soulard, J.L. Guth and P. Schulz, Microporous Materials, 4 (1995) 231. 8. J.P. Verduijn, U.S. Pat. 4 944 249 (1990). 9. American Society of Testing Materials, ASTM Designation D 3906-85a (1985). 10. F. Dougnier, J. Patarin, J.L. Guth and D. Anglerot, Zeolites, 12 (1992) 160. 11. D. Anglerot, F. Fitoussi, P. Schulz, T. Chatelain, F. Dougnier,J. Patarin and J.L. Guth, ACS meeting, Anaheim 1995, accepted. 12. M.M.J. Treacy, J.M. Newsam and M. W. Deem, Proc. Roy. Soc. Ser. A, 433 (1991 ) 499. 13. C. Baerlocher, L.B. McCusker and R. Chiapetta, Microporous Materials, 2 (1994), 269. 14. W.H. Flank, ACS Symp. Ser., 40 (1977) p.43. 15. M.R. Gelsthorpe and C.R. Theocharis, J. Chem. Soc., Chem. Commun., (1986) 781. 16. P.B. Malla and S. Komarneni, Zeolites, 15 (1995) 324.


181

Synthesis of zeolites in a microwave heating environment Jing Ping Zhao, Colin Cundy and John Dwyer Centre for Microporous Materials, Department of Chemistry, UMIST, P.O. Box 88, Manchester M60 1QD, UK

ZSM-5, zeolite /3 and hexagonal Y (EMT), are successfully synthesised using microwave heating. NH 4 form [Ti]ZSM-5 and [A1]ZSM-5 are also obtained under microwave heating conditions in the presence of fluoride anions. NaY can be very rapidly crystallised in the microwave environment. High temperature (150 ~ C) can be used in NaY synthesis without inducing crystalline impurity, demonstrating the tendency of the microwave procedure to provide and maintain phase purity under favourable conditions. By comparison with conventional heating, microwave methods can significantly shorten the overall nucleation and crystal growth periods for most of the zeolites investigated, but the effect is greater in some cases than in others. In a microwave environment, a higher temperature can often be used in zeolite synthesis to gain extra benefits.

1. INTRODUCTION Zeolites are usually synthesised under hydrothermal conditions in a period from a few hours to a few days depending on the nature of the zeolite, mixture composition and the synthesis temperature. Frequently, low temperatures must be used to reduce the formation of undesired phases. However, working at a low temperature necessitates longer synthesis times which may prove too costly or impracticable for industrial application. Therefore, reducing the crystallisation time may be one of the most important targets for commercial synthesis. Recent research shows that microwave techniques present new possibilities for achieving such a target [1,2]. This paper examines the syntheses of various zeolites under microwave heating conditions and discusses the possible contribution of microwave energy to the formation of zeolites. [AI]ZSM-5, zeolite/3, EMT (Hexagonal Y) and NaY have been prepared using hydroxide ion as mineraliser. NH4 form [Ti]ZSM-5 and [A1]ZSM-5 are also synthesised from mixtures containing fluoride anion. By comparison with a conventional heating system, microwave heating can significantly reduce overall crystallisation time. It is also observed in the synthesis of Na-Y that microwave heating can significantly accelerate the formation of the FAU structure, and with a much reduced occurrence of P-type (GIS) impurity. Because of this "preference" or "selectivity", a higher temperature can be applied in NaY synthesis, making the process faster and potentially more cost effective.

182 2. EXPERIMENTAL The raw materials used were sodium aluminate (AlzO3=32.75 %, NazO=28.5%, BDH), NH4F (BDH), NaF (BDH), NaOH (BDH), 18-Crown-6 (BDH), Tetrapropylammonium bromide [TPABr] (Aldrich Chemicals), Silica sol (Ludox AS 40, SiO2=40%, Aldrich Chemicals), Fumed silica (BDH), Tetraethyl-ammonium hydroxide (20% solution, Aldrich) and TIC13 (BDH). The microwave equipment used in this work was a CEM MDS-2100 sample preparation system using a fibre optic temperature controller and a pressure controller and an adjustable power output (maximum 950W at 2450 MHz). Teflon PFA autoclaves (100 ml, unstirred), were used. The samples synthesised were characterised using 295i MAS N M R , XRD, SEM and EDAX.

3. RESULTS AND DISCUSSION 3.1 [Ti]ZSM-5 and [A1]ZSM-5 [A1]ZSM-5 was synthesised from reaction mixtures of composition xNa20-yA120360SiO2-zTPABr-nHaO, where x=4.5-7, y=0-2, z=4-8 and n=800-1600, prepared according to the following procedure. First, a silica-containing mixture (mixture A) was made by mixing silica sol (Ludox) and template (TPABr) with 1/3 of the total water. Secondly, mixture B was prepared by mixing NaA102 and NaOH with a further 1/3 of the calculated water. Both mixtures were stirred for 30 minutes. Finally, mixture B was slowly added to mixture A with vigorous stirring and the reaction gel was continuously stirred for 2 hours. The final composition was then transferred into autoclaves and heated by microwave. The temperature was controlled by a programme which in essence contains two stages which are D E temperature elevation and C temperature maintenance. The first stage usually takes 1-2 minutes and the A B second stage depends on the reaction temperature and mixture composition. 5 15 25 35 5 Twotheta Using a reaction mixture with a typical composition 5.0NazO-0.2AlzO3-60SiOz- Figure 1. XRD patterns of [A1]ZSM-5 synthesised from 4.0TPABr-900H20, a an alkaline system under microwave heating. very crystalline ZSM-5 A) 1.65 h B) 2.0 h C) 2.35 h D) 2.65 h E) 3.0 h

183 can be obtained at 170~ without seeding in 2-3 hours (Figure 1). The XRD patterns of the [A1]ZSM-5 show that the nucleation takes less than 1.65 hours and the crystal growth needs only about 1 hour. This demonstrates that [A1]ZSM-5 can be synthesised very rapidly from a conventional reaction mixture using microwave heating. The particle size of the [A1]ZSM-5 is 3-4 25 35 45 5 15 ~tm and no TwoTheta crystalline impurity phases are observed. Figure 2. XRD patterns of [A1]ZSM-5 synthesised from a In order to fluoride system under microwave heating. study the effects of 1) 7.0 hours; 2) 8.0 hours 3) 9.0 hours microwave heating on a different synthesis system, the method based on fluoride anion was chosen. The reaction mixtures were prepared using reported procedures [3,4]. Two typical reaction mixtures were used to study the effects of microwave energy on the formation of [Ti]ZSM5 and [A1]ZSM-5 in the presence of fluoride anions. Their compositions were 1.0NH4F0.03TiCI3-1.0SiO2-0.5TPABr-30H20 and 2.0NH4F-0.14A1C13-1.0SiO2-0.5TPABr-30H20. Both used silica sol (Ludox AS 40) as silica source. The reaction mixtures are usually aged overnight, but such a step is not essential. On microwave I ~ l heating, the MFI structure can be detected by XRD from the reaction mixtures after about 5 15 25 35 Two "Fheta 45 6-7 hours at 170~ C. The crystallisation will then take a Figure 3. XRD patterns of [Ti]ZSM-5 synthesised from a fluoride further 1-2 hours to system under microwave heating, complete. The XRD 1) 7.0 hours 2) 8.0 hours 3) 9.0 hours !

184 patterns of NHa-[AI]ZSM-5 and NH4-[Ti]ZSM-5 are given in Figure 2 and Figure 3 which show that they are both very good crystalline products. As can be seen the nucleation takes about 7 hours and the crystal growth needs only about 1.5 hours which is similar to that in an alkaline system. This suggests that microwave energy has no unique influence on the crystal growth in either the fluoride or in the alkaline synthesis system. It is also appearent that nucleation is the major factor controlling the overall crystaUisation time of the MFI structure in the fluoride system. Under conventional heating, the crystallisation takes about 20 hours at 175~ C using comparable reaction compositions [4], demonstrating an effective acceleration of 2-3 times using the microwave method.

3.2 Syntheses of zeolite/3 and hexagonal NaY (EMT) Zeolites 13and EMT can also be synthesised in a microwave heating environment. For zeolite/3, the reaction mixture was prepared by adding a solution containing NaA102 and NaOH to a mixture of fumed silica and TEAOH. A seeding slurry was then B optionally added. The final mixture was aged at room temperature overnight. The seeding slurry has a similar composition to the main reaction mixture but has been pre-heated 5 1~5 25 35 in a conventional oven Two Theta to nucleate. In the seeding slurry, zeolite XRD patterns of zeolite/3 synthesised at 140~ /3 can be detected using Figure 4. under microwave heating. XRD analysis. A A) 10 hours B) 14 hours typical reaction mixture has the composition 2.5Na20-1.0A1203-40SiO2-6.0(TEA)20-560H20. Usually, the amount of seeding slurry is between 4 % and 8 % based on SiO2 + A1203 in the mixture. Figure 4 gives the XRD pattern of the product after 14 hours microwave heating at 140~ C. The wellcrystallised zeolite has a Si/A1 ratio of 14 as measured by 29 Si NMR spectroscopy. Hexagonal Y (EMT) is one of most difficult aluminous zeolites to synthesise, normally taking 4-12 days at 110~ C by conventional heating [5]. However, it can be obtained in one day using microwave energy by working at a higher temperature. The composition of synthesis mixture was 2.4Na20-0.5NaF-(0.85-1.0)A1203-10SiO2-(0.5-0.8)[18-Crown-6]140H20, prepared as described previously [5]. At a lower temperature (110 ~ C), EMT can be detected after microwave-induced crystallisation for about 1.8 days and the crystal !

!

185 growth takes a further 2 days, which is nearly as long as in a conventional hydrothermal synthesis at that temperature. It is clear that the crystallisation of the EMT under these conditions is very difficult. When a higher temperature ( 115~ C) is used, a very crystalline EMT can be obtained in about 2.5 days (Figure 5). The c fast increase of crystallisation with a very small rise in temperature may suggest that the formation of EMT is very sensitive to temperature in a microwave environment. With further increase of ! 0 1~0 20 Two Theta 30 40 the temperature (130150~ C), EMT can be obtained between 820 hours depending Figure 5. XRD pattern of EMT synthesised without seeding on the reaction under microwave heating. t e m p e r a t u r e . A) 3.0 days at 110~ B)l.8 days at 115~ C)2.5 days at 115~ However, small amounts of amorphous or other impurity phases are generally unavoidable. If a seeding technique similar to that used in standard zeolite Y synthesis is used, the purity of the product can be improved, particularly at a higher temperature (150~ C).

3.3 NaY synthesis Highly crystalline NaY was synthesised in a very short time using microwave heating. The reaction mixtures were prepared by adding a solution of NaA102 and NaOH to a diluted silica sol. The compositions employed were xNazO-1.0A1203-ySiO2-240H20 , where x=4-7 and y = 10-20. When y = 10 and x=6.2, a very crystalline NaY can been obtained at 110~ in less than 1.5 hours under microwave heating without seeding. Using the same reaction mixture, the synthesis takes 9-10 hours at the same temperature under conventional heating conditions in order to achieve a good crystalline product. The synthesis time is about 6 times shorter using microwave heating. Zeolite P is an impurity phase frequently obtained in conventional NaY preparations, usually rendering NaY production impossible at higher temperature. Therefore, most industrial processes use a lower temperature (usually ca. 100~ At this temperature, the crystallisation usually takes from 10 to 30 hours. In order to study the formation of zeolites at different temperatures in a microwave environment, the NaY synthesis system was chosen. It is found that, using microwaves,

186 there is far less conversion to P-type zeolite even at higher temperatures (150 ~ C). At such a high temperature, formation of P-type zeolite is usually unavoidable by conventional heating. For example, heating the same reaction mixture at 150~ C by conventional means, produced a P-type zeolite resembling Gobbinsite [6] without any NaY. At 150~ C, the FAU structure can be detected by XRD after about 5 minutes microwave heating and the crystal growth takes a further 10-15 minutes (Figure 6). The crystalline product had an average crystal size of about 0.5-1.0/~m without any detectable crystalline impurity. Continuing the microwave heating for a total of 120 minutes yielded no evidence of any other crystalline phase. This suggests that microwave energy has made an effective and apparently selective contribution to the formation of the FAU structure D which is also stable in the microwave C reaction environment. Again, it appears that the use of B microwave /" energy may have a particular , , i 310 i advantage in 0 lb ' 2'0 Two Theta 40 encouraging the .

.

.

.

.

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nucleation of a Figure 6. XRD patterns of NaY synthesised at 150 ~ C without s in g 1e p h a s e seeding under microwave heating. under favourable A) 5 min. B) 10 min. C) 15 min. D) 20 min. conditions rather than exerting any unique influence upon crystal growth. The effect of microwave heating on the synthesis of high-silica NaY has also been studied. When Na20=5.8 and SiO2 = 15, a very crystalline NaY can be obtained in less than 1.5 hours, with seeding, at 150~ C. A typical product had SiO2/AlzO 3 = 4.7 as calculated from the unit cell parameter [7]. This demonstrates that a fairly siliceous NaY can also be obtained very rapidly at a higher temperature without crystalline impurity. Unfortunately, the true comparison of microwave heating with conventional heating at higher temperatures cannot be made because pure NaY cannot be obtained from oven-heated autoclaves under these conditions.

4. CONCLUSIONS The syntheses of [AI]ZSM-5 and [Ti]ZSM-5 can be accelerated in a microwave heating environment without seeding using either traditional alkaline media or in the presence of fluoride ions. With microwave heating, [A1]ZSM-5 can be obtained from a normal OH-

187 based composition in 2.5 hours. In most cases, crystaMsation can be speeded up by 2-3 times by using microwave heating as compared with conventional heating. Very crystalline zeolite/3 can be crystallised in 14 hours from the standard NaOH/TEAOH system using microwaves. Highly crystalline EMT can similarly be obtained at 115~ in 2.5 days without seeding. The formation of EMT is very sensitive to the synthesis temperature. The most notable contribution so far observed for microwave energy input is found in NaY synthesis. By using microwave heating, under given conditions, a reduction in overall synthesis time of up to 6 times can be achieved compared to conventional methods. NaY can be crystallised in times ranging from 10 minutes to 1.5 hours depending on the composition of the reaction mixture. In NaY synthesis, it is found that for approximately every 15 degree increase in reaction temperature, the crystallisation time is reduced by a half. Microwave heating can use this to advantage by limiting the formation of impurity phases (particularly zeolite P) and providing, in effect, selectivity towards NaY. A much higher temperature can therefore be applied in microwave-mediated NaY synthesis than in conventional synthesis procedures. Consequently, there is considerable scope for the improvement of existing procedures in terms of time and cost and perhaps also for the development of new continuous processes.

ACKNOWLEDGEMENTS We thank the EPSRC for their financial support (GR/K06877) of this work and R J Plaisted for helpful discussions.

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

A. Arafat, J. C. Jansen, A. R. Ebaid and H. van Bekkum, Zeolites 13 (1993), 162-5. P. Chu, F. G. Dwyer and V. J. Clarke, Eur. Pat. 358 827 (1990). J. Zhao, J Dwyer and D Rawlence, Proceedings of the 9th International Zeolite Conference. Montreal (1992), 155-61. J. Zhao and J. Dwyer, paper in preparation. K. Karim, J. Zhao, D. Rawlence and J. Dwyer, Microporous Materials, 3 (1994) 695-698. R. von Ballmoos and J. B. Higgins, Collection of Simulated XRD Powder Patterns for Zeolites, Zeolites, 10(5), (1990). D . W . Breck, Zeolite Molecular Sieves, Wiley, New York, p49, 1974.



SYNTHESIS OF OCTAHEDRAL

MOLECULAR

189

SIEVES

Chi-Lin O'Young' and Steven L. Suibb 'Texaco Inc. P.O. Box 509, Beacon, NY 12508 USA ~Dept. of Chemistry~ U. of Connecticut, Storrs, CT 06269-3060 USA A large number of families of manganese oxide octahedral molecular sieves (OMS) and their precursors, octahedral layered materials (OL), have been synthesized and characterized by various methods. The materials include OMS-1, OMS-2, and OL-1; they all use MnO6 octahedra as the basic structural unit to form (3X3) tunnels, (2X2) tunnels, and layered structures, respectively. Different metal cations can incorporate into the OMS and OL structures through framework, tunnel, and interlayer substitutions. These materials can be synthesized by the reflux, hydrothermal, precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gd methods. The materials have been characterized in detailed by a variety of techniques to study the structure, composition, stability, and morphology. This paper will discuss and review the different methods used to synthesize OMS and OL materials and the resultant changes in physical and chemical properties. 1. INTRODUCTION Synthesis of novel zeolites and molecular sieves such as intersecting 10- and 12-ring pore zeolites [1] and mesoporous materials [2] have been a major focus of several research groups in the past few years. Unique structural materials that result in porous and open l~amework systems such as vanadium phosphates with 18.4 A tunnels [3] and porous TiO2 materials [4] are continually being discovered. The synthesis, characterization, and applications of todorokite [5] and cryptomelane (or hollandite) materials [6-8] have been reported. These OMS materials use MnO~ octahedra as the basic structural unit to form mono-directional tunnel structures. The synthetic todorokite (OMS-1) and cryptomelane (OMS-2) have (3x3) and (2x2) tunnels with pore sizes of 6.9 x 6.9 A and 4.6 x 4.6 A, respectively (Figure 1). The precursors of OMS, bimessites or octahedral layered materials (OL-1), consist of layers of edge and comer linked MnO6 octahedra with water molecules and metal cations in the interlayer voids and have an interlayer distance of 7 to 10 A. Certain metal cations can incorporate into the OMS and OL structures through framework, tunnel, and interlayer substitutions [7,9]. The framework substitutions of OMS and OL are represented as [M]-OMS and [M]-OL, while the tunnel and interlayer substitutions are represented as M-OMS and M-OL, respectively. These OMS and OL materials can be synthesized by the reflux, hydrothermal, precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gel methods and result in materials that

190 have different physical and chemical properties. We will discuss and review these various methods used to prepare OMS and OL materials and resultant changes in physical and chemical properties.

OMS-1 Todorokite

OL-1 Birnessite

. . . . .

H20

G

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9

G

~

|

.,

OMS-2 Hollandite

.....o

M n+

9

9

r//.

.

Mn2~ Co2§ , Ni2§ Cu 2., Zn2§

(sx3)

(2xe)

Cd 2+, Mg2+ Figure 1. Structure of OL-1, OMS-1, and OMS-2 T.~...b.Le..~..~.~.~.m.~!.~...?.f..~s...~..~..~L~.~..a.t.e.~..gr..e.p...~.r.e...~....~r.~y.~..d?...".t..~rth.~.~.

Structure 1. K-OMS-2

Reactants KMn04, MnSO,

2. K-OMS-2 3. K-OMS-2 4. K-OMS-2 5. M-OMS-2 6. [M]-OMS-2

12. Mg-OMS- 1

KMn04, MnSO4 K-OL-1 from #8 KMnO4,maleic acid K-OMS-2 form # 1, 2 # 1, 2 & dopants added to initial solution MnCI2, NaOH, Mg0VlnO4)2 MnSO4, KOH, 02 KMnO4, sugar Na-OL- 1 from #7 #7 & dopants added to initial solution Mg-OL- 1 from # 10

13. M-OMS- 1

M-OL-I from #I0

14. [M]-OMS- I

[M]-OL- I from # I I

7. Na-OL-1 8. K-OL-1 9. K-OL-1 10. M-OL- 1 1 !. [M]-OL-1

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

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

Method/Conditions Hydrothermal, autoclave 100 ~ 17 h, low pH's Reflux 100 ~ 17 h, low pH's Calcination, 600 *C, 17 h Sol-gel, 450 *C, 4 h Ion-exchange Isomorphous substitution, reflux or hydrothermal Precipitation, 25 ~ aging, high pH's

Precipitation, 25 ~ high pH's Sol-gel, 450 ~ 2 h Ion-exchange, 25 ~ Isomorphous substitution, 25 ~ aging, high pH's Hydrothermal, autoclave 175 *C, 20 h Hydrothermal, autoclave 175 ~ 20 h Ion-exchange, hydrothermal, ~.t..o..~ay~..!.7...5....*...C..,...~O.~

References 6

7 7 8 7 7 5 7 10 5, 9

5 9

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

191 2. EXPERIMENTAL SECTION Table 1 shows fourteen families of OMS and OL materials have been prepared by the reflux, hydrothermal, precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gel methods.

2.1. Synthesis of OMS-2 with (2X2) Tunnel Structure K-OMS-2 have been synthesized by the hydrothermal [6], reflux [7], calcination [7], and sol-gel [8] methods (Methods 1 - 4). A typical hydrothermal or reflux synthesis was as follows: 100 mL of 0.4 M KMnO4 was added to a mixture of 30 mL of 1.74 M MnSO4 and 3 mL of concentrated HNO3. The final mixture was autoclaved or refluxed at 100 *C for 17 h; the product was filtered, washed, and dried. A standard calcination preparation was as follows: 200 mL of 3.12 M KOH was added to 200 mL of 0.89 M MnSO4. Oxygen was bubbled (12 L/min) through the solution for 4 h. The black product was filtered, washed, and calcined at 600 *C for 17 h. A typical sol-gel preparation was as follows: 3 m mole of maleic acid was added to 100 mL of 0.1 M KMnO4 and the mixture was stirred for 30 min. A dark brown sol was formed at room temperature which started to gel in 30 rnin. The resultant gel was filtered, washed, and calcined at 450 *C for 4 h. The tunnel substitution of K-OMS-2 was prepared by the ion-exchange method (Method 5) [7]. The framework substitution of K-OMS-2 was prepared by adding metal dopants (a total concentration of 0.01 M) into the initial solution, followed by reflux or autoclave treatment as described before (Method 6) [7]. 2.2. Synthesis of OL-I The precursor of OMS-1, Na-OL-I, was prepared by reacting MnCl2 with NaOH to form pyrochroite, Mn(OH)2, followed by adding a solution of Mg(MnO4)2 to the suspension to form NaOL-1 (Method 7) [5]. KMnO, or NaMnO, also could be used to prepare the same quality of precursor by adding an equivalent amount of Mg 2+ into the mixture. A typical preparation was as follows: 50 mL orS.0 MNaOH solution was added to a mixture of 40 mL of 0.5 M MnCl2 and 0.1 M MgCl2 in a plastic bottle with stirring. 40 mL of 0.2 M KMnO4 or NaMnO4 was then added to the suspension. The mixture was aged at room temperature for 7 days. The precipitate was filtered and washed thoroughly with water. XRD of the wet precipitate showed peaks at 10.1, 5.0, and 3.33 A. The interlayer peak shined from 10.1 to 7.1 Jk after drying. MgCl2 also could be mixed with the KMnO, or NaMnO4 solution, and the results were the same. The precursor of OMS-2, K-OL-l, was prepared by reacting MnSO4 and KOH with oxygen bubbling through the solution as described previously (Method 8) [7]. The sol-gel K-OL-1 was prepared by reacting KMnO4 and simple sugars (glucose or sucrose) to form a brown gel, followed by drying and calcining at high temperatures (Method 9) [10]. A standard preparation was as follows: 50 mL of 0.38 M KMnO4 was added to 20 mL of 1.4 M glucose to form a brown gel, and the gel was then washed, dried, and calcined at 450 *C for 2 h. The interlayer substitution of Na-OL-1 was prepared by the ion-exchange method at room temperature (Method 10) [9]. For the l~amework substitution, [M]-OL-1 was prepared by adding metal dopants (6 mL of 0.1 M metal salts) into the initial solution as described previously (Method

II).

192 2.3. Synthesis of OMS-I with (3X3) Tunnel Structure Mg-OMS-1 was prepared by the rearrangement of Mg-OL-1 under hydrothermal conditions (Method 12) [5]. The tunnel substitution of OMS-1 was prepared by the ion-exchange of Na-OL-1 with metal ions (Method 10), and consequently convened to M-OMS-1 by the hydrothermal treatment (Method 13) [9]. The fiamework substitution of OMS-1 was prepared by adding metal dopants into the initial solution to form [M]-OL-1 (Method 11), followed by the ion-exchange with Mg2§ then converted to [M]-OMS-1 by the hydrothermal treatment (Method 14). 2.4. Characterization Techniques Various characterization techniques have been applied to study the structure, composition, morphology, and thermal stability of these materials. They included X-ray powder diffraction (XRD), microanalysis, transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermal analysis, temperature programmed desorption (TPD) and reduction (TPK), cyclic voltammetry (CV), and others. Details of the procedures have been disclosed in the literature [512].

3. RESULTS 3.1 Synthesis The hydrothermal and reflux preparations of K-OMS-2 involved reactions between MnO4" and Mn2§ solutions. The size and concentration of counter cation, pH, and temperature were identified as important preparation parameters. The optimal conditions are: pH's < 2, 80 to 140 ~ and [KMnO4]/([KMnO4] + [Mn2"]) > 0.4. The template effects of counter cations were clearly observed in both methods [6,7]. The calcination method involved the heat treatment of the layered precursor, K-OL-1, at 6OO~ The sol-gel method involved reactions between KMnO4 and pure maleic acid. The critical preparation parameters included the concentration of permanganate, nature of the counter cation and the organic acid reducing agent, temperature of gelation and calcination. A mixture of OMS-2 and OL-I was formed if the concentration of permanganate was too high (0.35M). KMnO4 and CsMnO4 yielded most crystalline and thermally stable (800 oC) sol-gel OMS-2. NaMnO4, LiMnO4, Ca(MnO4)2, and Mg0VInO4)2leaded to less stable and impure products. The optimal conditions are: a concentration of KMnO4 around 0.1 M, pure and flesh maleic acid, reaction temperature around room temperature, and calcination temperature around 400 ~ [8]. Several critical parameters for the synthesis of Mg-OMS-1 and the precursor, Na-OL-1, were identified; they included the pH, ratio of MnO4" to Mn2§ Mg2+concentration, time and temperature of aging, time and temperature of autoclaving. The optimal conditions for the synthesis of OMS-1 are: a MnO4"/Mn2§ ratio of 0.3 to 0.4, pH of 13.8, aging at 25 to 35 ~ for 1 to 7 days, and autoclave treatment at 160 to 180 oC for 4 to 7 days. The effects of pH, MnO4"/Mn2+ ratio, and autoclave conditions have been discussed in the literature [5]. The crystaUinity and thermal stability of OMS-1 strongly depended on the crystallinity of the precursor. Table 2 shows the effects of aging temperature (5 to 35 *C) and time (1 to 7 days) on the crystallinity of Na-OL-1. The integrated intensity of the d(001) peak at 10.1 A was measured and used as an index of the crystaUinity. The results show that the crystallinity of Na-OL-1 is quite sensitive to the aging time and temperature. For example, aging at 35 *C for 1 day has about the same crystallinity as aging at 5

193 oC for 7 days. Aging at higher temperature (80 *C) significantly increases the crystallinity; however, the resultant Na-OL-1 loses the ion-exchange capacity and cannot form Mg-OL-1 and OMS-1. Table 2. Effects of aging time and temperature on crystallinity of Na-OL-I Aging Temperature Aging Time ~ Days 5 20 5941 9050 1 7345 11100 2 11982 14700 4 13069 15170 7 Values are the integrated intensity of the d(001) peak at 10.1 K

35 13571 16254 22100 24486

The Mg2§ concentration is another important preparation parameter. The most crystalline and thermally stable OMS-1 materials are prepared by Mg(MnO4)z. Using NaMnO4 or KMnO4 yields less stable and impure products. However, the same quality of OMS-1 can be formed when an equivalent amount ofMg 2+is added into the initial solution. For the sol-gel synthesis of K-OL- 1, the counter cation of permanganate plays an important role in reactions of MnOi with sugars. Different cations influence both gel formation and the nature of the final product. Only KMnO, yields sol-gel and pure products; NaMnO4 and Mg(MnO,)2 do not produce sol-gel K-OL- 1 products [ 10]. 3.2 Ion Exchange Properties K-OMS-2 prepared by the reflux and hydrothermal methods exhibit some ion-exchange properties. At room temperature, about 70% of tunnel K§ can be exchanged with Rb§ Li§ and Na + cannot exchange K § easily [7]. At 80 *C, K-OMS-2 can be exchanged with NH4+, Co2+, Cu2+, and Ni2+to form tunnel substituted [M]-OMS-2. The percentage of exchange capacity shows the order: NI-I4+ (36%) > Co2§ (18%) > Ni 2+(5.8%) ~ Cuz+(4.4%). Na-OL-1 prepared by Method 7 has very good ion-exchange capacity when it is wet. However, dehydration of Na-OL-1 decreases the interlayer distance from 10.1 to 7.1 ~ and loses the ionexchange capacity. Mg 2§ Ni2+, Co2+, Cu2+, and Zn2+ can be exchanged easily at room temperature with wet Na-OL-1 to from M-OL-1 and consequently converted to M-OMS-1 under the hydrothermal conditions [9]. Direct ion-exchange of Mg-OMS,1 to form tunnel substituted MOMS-I has not succeeded. Sol-gel K-OL-1 doesn't have any ion-exchange capacity at room temperature. 3.3. Framework Substitution The isomorphous substitutions of cations intothe OMS and O L l~amework structures involved addition of metal dopants into the solution precursors prior to any precipitation or reaction. A variety of cations, such as Cr~§ Fe3+, Co2§ Ni~§ Cu2+, and Zn2+ have been incorporated into the OMS-2 framework. At 0.01 M of dopant concentration and the standard conditions, the degree of substitution was between 1.5% to 1.7%. Divalent cations with ionic diameters similar to Mn2§ such as Mg 2§ Co2§ Niz§ Cuz§ and Zn2§ have been incorporated into the framework of OMS-1 by Method 14. Results of compositional

194 analysis showed that all doped cations (about 2%) were totally retained in the corresponding OMS1 materials. 3.4 Characterization K-OMS-2 prepared by the four different methods all have the same XRD patterns; however, they have quite different properties of crystaUinity, thermal stability, and morphology (Table 3). The reflux, autoclave, and sol-gel materials have higher average manganese oxidation states than the calcination material. XRD of the calcination and sol-gel materials show sharper peaks than those of the reflux and autoclave materials. The reflux and autoclave materials are thermally stable up to 600 ~ whereas the calcination and sol-gel materials are stable up to 800 *C. The morphology of the reflux and autoclave materials are needle-like; however, the morphology of the calcination and solgel materials are clump and irregular. Temperature programmed desorption (TPD) data for 02 evolved show that the reflux and autoclave materials have much higher oxygen loss than the sol-gel material. Table 3. Comparison of OMS-2 prepared by Reflux, Autoclave, Calcination, and Sol-Gel Methods Parameter Reflux Autoclave Calcination Sol-Gel broad broad sharp sharp XRD 3.80 3.96 3.68 3.80 Ave. Mn Oxidation state 600 ~ 600 ~ 900 ~ 800 ~ Thermal Stability needles needles clumps irregular SEM 9.41 of16 O 9.41 of16 O N.A. 0.48 of16 O Oxygen Desorption (TPD)

Tunnel cations have profound effects on the thermal stability and morphology ofM-OMS-1 [9]. Results of TGA and XRD show that Mg-OMS-1 is thermally stable up to 600 ~ Co-OMS-1 and Ni-OMS-1 are stable to 500 ~ and Cu-OMS-1 and Zn-OMS-1 are stable to 300 *C. SEM results show that the crystal morphologies of M-OMS-1 can be plates, needles, or fibrous shapes, depending on the nature of the cations. TPD results show that the tunnel cations do not markedly affect the evolution of TPD peaks, but remarkably influence TPR in both H2/Ar and CO/He with respect to both emerging temperature and population. Cu-OMS-1 appears to process more oxygen species that are especially reactive with 1-12and CO at low temperatures [ 11]. Results of XRD and TGA show that framework substituted [Zn]-, [Co]-, [Ni]-, and [Cu]OMS-1 are all thermally stable up to 400 ~ which are different from the tunnel substituted OMS1. The strongest evidence for fi-amework substitution comes from cyclic voltammetry measurements [12,13]. Results of CV studies show Cu2§ migration out of the OMS-1 tunnels for Cu-OMS-1 with most Cu 2§ in the tunnel, while no such Cu2§ migration is found for [Cu]-OMS-1, indicating the existence of Cu2§ in the framework. Results of electron paramagnetic resonance (EPR) and X-ray photoelectron spectroscopy (XPS) also indirectly support the framework substitution [ 13]. 4. DISCUSSION OMS-1 and OMS-2 have very similar tunnel structures; however, they are synthesized by quite different methods and conditions. Scheme 1 shows two different preparation routes to OMS. Both routes involve redox reactions between MnO; and Mn2+ at different pH's. Low pH's are used for

195 Route 1 tO form OMS-2. XRD of the flesh precipitate shows an amorphous phase. OMS-2 is formed by selecting suitable templates and temperatures. K§ is the major template for the synthesis of OMS-2. Attempts to synthesize OMS-1 by using larger templates are not s u ~ . High pH's are used for Route 2 to form OL-1 as precursors to OMS. XRD of OL-1 prepared by this route show either 10 or 7 A layered phases. However, the critical preparation parameters and conditions for the precursors of OMS-1 and OMS-2 are different. OMS-1 prepared by NaMn04, KMn04, or oxygen [14] are less thermally stable and impure. The effects of Mg2+suggest that a small amount of Mg2§ is in the ~ o r k of layered precursor and OMS-1, which stabilizes the OMS-1 structure. Mg2§ is the major template for the synthesis of OMS-1. K+ and high temperature are essential for the formation of OMS-2 by this route (Method 3). The rearrangement of layered silicates to mesoporous ahminosilicates also has been reported [2]. Scheme 1. Two Different Preparation Routes to OMS

Route 1:

Route 2:

Mn 2+ + MnO4

Mn 2+ +

MnO4 or Oxygen

Low pH's -"

Amorphous Phase

Hi pH's

Temperature ~ OMS-2 Template

Temperature ,~- OL-I

P

Template

OMS-I and

OMS-2

The synthesis" of OMS with (2X5) tunnel structure has been reported in the literature [16]. It was prepared by reacting pyrolusite, (1X 1) tunnel structure, with RbOH solution in a gold capsule under hydrothermal conditions at 350 ~ and 200 Mpa. Another known OMS structure with (2X3) tunnel structure, psilomelane, has not been synthesized. It is still not clear why so different procedures and conditions are needed to synthesize these OMS. Sol-gel methods for the preparation of OMS and OL materials provide several advantages, such as easy incorporation of dopants and template agents directly into the sol. Preparation of thin films via spin coating techniques that might be used in electrochemical or sensor application are also possible. The sol-gel synthesis involves reactions between MnO4" and organic reducing agents. KOL-1 prepared by the sol-gel method is very stable compared to the OL-1 materials prepared by the precipitation, hydrothermal treatment at 160 ~ for 2 days or calcination at 800 ~ for 2 h has essential no effect on its XRD pattern. 5. SUMMARY We have reviewed here the various routes that can be used tO prepare an extensive family of OMS and their precursors, OL. The syntl~c methods include the reflux, hydrothermaL precipitation, calcination, ion-exchange, isomorphous substitution, and sol-gel techniques. The materials compri~ OMS-1, OMS-2, and OL-1. The different synthetic methods have been compared, the important preparation parameters and reaction conditions have been identified. Various characterization techniques have been used to study the structure, comlx~sition,

196 morphology, and ~ stability of these materials. Both tunnel ( i o n - e x ~ e ) and framework substituted (dopants in solution precursors) materials have been reported. We have shown quite different physical and chemical properties of these matenals and how they are related to the resultant strucaae and to particular synthetic methods used to prepare such materials. REFERENCES

1. Lobo, R.F., Pan, M; Chan, I., Li, H.X.; Medmd, R.C., Zones, S.I.; Crozier, P.K, Davis, M.E. Science, 1993, 262, 1543-1546. 2. (a) Kresge, C.T.; Leonowicz, ME.; Roth, W.J., Vartuli, J.C., Beck, J.S. Nature, 1992, 359, 710712. (b) Inagaki, S.; Fukushima, Y.; Kuroda, IC J. Chem. Soc. Chem. Comm. 1993, 680. 3. Soghomonian, V.; Chen, Q., Haushalter, R.C.; Zubieta, J. Angew. Chem. Int. Ed Engl., 1993, 32, 610~11. 4. Tanev, P.T., Chibwe, M.; Pinnavaia, T.J. Nature, 1994, 368, 321-323. 5. Shen, Y.-F.; Zerger, I~P.; [kGuznum, R.N.; Suib, S.L.; McCurdy, L.; Potter, D.I.; O'Young, C.-L. Science, 1993, 260, 511-515. 6. O'Young, C.-L. in Expanded Clays and Other Microporous Solids; Occeili, M.L., Robson, H., Eds. Vol. II 333-340, Van Nostrand Reinhold, NY 1992. 7. DeGuzman, R.N., Shen, Y.-F., Neth, E.J., Suib, S.L., O'Ytmng, C.-L., ~ , S., Newsam, J. M., Chem. Mater., 1994, 6_,815-821. 8. Dumt, N., Suib, S.L., O~roung, C.-L. J. Chem. Soc. Chem. Comm. 1995, 1367-1368. 9. Shen, Y.-F., Suib, S.L., O~Young,C.-L., J. Amer. Chent Soc., 1994, 116, 11020-11029. 10. Ching, S.; Landfigan, J.K, Jorgensen, M.L., Duan, N., Suib, S.L.; O'Young, C.-L. Chem. Mater. 1995, 7, 1604-1606. 11. Yin. Y.-G., Xu, W.-Q., DeGuzma~ R.N., Shen, Y.-F., Suib, S.L., O'Young, C.-L., in Zeolites and Related Microporous Materials: State of the Art 1994, J. Weitkamp et al. Eds. pp. 1671-1676, Elsevier, 1994. 12. DeCnmnan, R.N., Shen, Y.-F., Shaw, B.I~, Suib, S.L.; O'Yom~ C.-L. Chem. Mater., 1993, 5, 1395-1403. 13. Shen, Y.-F.; Suib, S.L.; O'Young, C.-L. submitted for publication. 14. Golden, D.C., Chen, C.C., Dixon, J.B. Science, 1986, 231,717-719. 15. Tamada, O., Ymnan~to, N. Minerlt~c~ J., 1986, ~ 130-140.


197

S y n t h e s e s a n d c r y s t a l s t r u c t u r e s of t w o " o r g a n o z e o l i t e s " K. Maeda, J. Akimoto, Y. Kiyozumi, and F. Mizukami National Institute of Materials and Chemical Research, Tsukuba, Ibaraki 305, Japan

Synthesis conditions of two isomeric aluminum methylphosphonates were examined. A1MepO-~t crystallized from a well-dispersed mixture of pseudo-boehmite, methylphosphonic acid and water on hydrothermal reaction. Neutral or acidic organic additives also favored formation of A1MepO-[~ with crystal size enlargement in most case. Static mixing followed by aging or use ofglycolic solvent instead of water caused to form AlMepO-a. Single crystal X-ray structural analysis of the two products revealed that both compounds have unidimensional channels lined with methyl groups. Also two new phases, designated A1MepO-~ and -5, were found. 1. I N T R O D U C T I O N Much attention has been paid to metal organophosphonate with a lamellar s t r u c t u r e as i n t e r c a l a t i o n hosts or d e s i g n e d l a y e r e d m a t e r i a l s [1]. Organophosphonic acid (RPO3H2) have an organic group (R) covalently bonded with the phosphorus center and the structure of metal organophosphonate generally has similarities with that of related phosphate [2,3]. However, structures other t h a n layered type were rare so far. Aluminum [4] and copper [5] methylphosphonates, and zinc aminoethylphosphonate [6] reported recently possess three-dimensional neutral frameworks and unprecedented organically lined tunnel structures. Such three-dimensional frameworks will provide possibilities for designed channel structures. The copper and zinc compounds have so small space surrounded by organic moieties that even nitrogen molecules perhaps cannot intrude. On the other hand, both of the isomeric aluminum methylphosphonates, A1MepO-a [7] and A1MepO-[~ [4,8], reported by us have as _large channels as can adsorb 2,2-dimethylpropane with a kinetic diameter of 6.2/~. They can be called the first "organozeolites" with molecular sieving properties. Their frameworks are different from existing aluminum phosphate molecular sieves [9] because P/A1 ratio should be 1.5 to build neutral framework and each phosphorus center should be connected to only three aluminum centers via oxygen atoms. We report here the synthesis condition and comparison of these two isomeric structures.

198 2. EXPERIMENTAL 2.1. S y n t h e s i s a n d c h a r a c t e r i z a t i o n Aluminum source was pseudo-boehmite powder (PURAL SCF, Condea Chemie, 74.4 wt.% A1203, 25.6 wt.% water). Methylphosphonic acid was obtained from Aldrich. Other organic reagents were obtained from Tokyo Kasei and used without further purification. Fundamental synthesis procedures were as follows: ten mmol of pseudo-boehmite powder and 15 mmol of methylphosphonic acid were dispersed in 400 mmol of water. The mixture (1.0A1 : 1.5P : 40H20) was stirred at ambient temperature for 1 h. The suspension was hydrothermally treated using an autoclave with a teflon sleeve at 160 ~ for 48 h in an thermostated oven under an autogenous pressure. The solid product was filtered, washed with water, and air-dried. In some runs, 5 mmol of an organic additive listed in Table 1 were added to the starting mixture or 10 g of organic solvents listed in Table 2 were used instead of water. Static mixing, namely gentle pouring of water onto boehmite covered with methylphosphonic acid followed by static aging of the mixture, was also examined instead of stirring in the mixing procedure. X-ray powder diffraction analyses (XRD) were performed with a MAC Science MXP18 diffractometer. Scanning electron microscopy (SEM) images were taken on a Hitachi S-800 microscope. 2.2. X-ray c r y s t a l l o g r a p h y Large crystals suitable for single crystal X-ray diffraction study were obtained from a different composition of mixture (1.0A1 : 1.0P : 40H20, reacted at 220 ~ for 48 h) for A1MepO-a and by addition of dioxane (1.0A1 : 1.5P : 40H20 : 0.5dioxane) for A1MepO-f3. X-ray diffraction data were taken on Rigaku AFC-4 diffractometer for A1MepO-a and Rigaku AFC-7 for A1MepO-f3. Crystallographic data for A1MepO-a: space group, trigonal P31c; a=13.9949(13), c=8.5311(16), Z=6; for A1MepO-~: space group, trigonal R3c; a=24.650(2), c=25.299(5), Z=18. Refinements were based on F2; the final Rw(F2) were 0.1081 (for 1795 reflections, 104 parameters) for A1MepO-a and 0.2115 (for 2514 reflections, 312 parameters) for A1MepO-f~.

3. R E S U L T S AND DISCUSSION

3.1. Synthesis conditions Figure 1 shows XRD of samples prepared with no additive (la) and aqueous ammonia (lb). With no additive the product gives XRD pattern with an intense reflection at d=12.30 corresponding to A1MepO-~. A1MepO-~ was obtained also at 130 ~ whereas a different compound, designated A1MepO--8, giving the strongest reflection at d=9.59 became the major product above 200 ~ Addition of aqueous ammonia caused to form another product, designated AlMepO-~, of non-porous layered structure with the composition AI(OH)(O3PCH3)'H20 [10]. Products obtained with various high-boiling organic additives were listed in

199 Table 1. Most of carboxylic acids, alcohols and ethers, namely acidic and neutral additives, gave A1MepO-[3 as the main product. Glycolic acid and oxalic acid did not give A1MepO-[3 probably owing to their strong chelation of aluminum. Basic additives like q u a t e r n a r y ) 9 9 b ammonium hydroxide, aqueous ammonia always gave A1MepO-~. Quaternary ammonium halide had no effect on product. These results revealed that quaternary ammonium 5 ' io ' a'o ' 4'0 hydroxide does not work as 20 structure-directing agent like in A1PO4 system [11] but works simply as base. Figure 1 XRD of A1MepO-[3 and -~ prepared with Hydrophobic interaction be- no additive (a), and aqueous ammonia (b), respectween methyl groups must tively. assist formation of the chan- 9 : A1MepO-[3, A : A1MepO-~ nel structure of A1MepO-[3. When the starting materials were mixed statically and left standing before the hydrothermal reaction, products changed depending on standing time. When the starting mixture was hydrothermally treated immediately after the water addition, the products contained mainly A1MepO-[5 (Figure 2a). On standing of the starting mixtures for 24 or 48 h A1MepO-a was mainly formed with small amount of A1MepO-[3 ,-~, -5 (Figure 2b and 2c). The maximum A1MepO-a content of the

I

9

9

.[

9

qp

.

9

9

9

Table 1 A1MepO products obtained with various organic additives Additive

Product

no additive [3>>~ AcOH [3>>~ AcOH (1.0) ~>~ PhCOOH (5 CH3(CH2)loCOOH ~ HOCH2COOH ~,Uk (COOH)2 (0.25) Am HOOC(CH2)4COOH* ~

Additive

Product

1-BuOH [3 2-BuOH [3>>~ t-BuOH [3>~ Ethylene glycol [3>~ 1,4-HO(CH2)4OH [3>~ Dioxane [3 Dioxane (1.5) [3 18-crown-6 [3>Uk

Additive

Product

NH3 NH3 (1.5)

[N(CH3)4]O H [N(C2Hs)4]OH [N(C3H7)4]OH [N(C4H9)4]O H [N(C4H9)4]Br

Additive/A1 = 0.5 unless ratio is given in parenthesis, *Additive/A1 = 0.25 Uk: Unknown phases, Am: Amorphous,

%>>Uk

[3

200 Table 2 Products from nonaqueous solvents. Solvent

Product

Ethanol a,~ Ethylene glycol a>>~ Diethylene glycol a,~ Tetraethylene glycol 1,2-Propanediol a>~ 1,3-Propanediol a,~ 1,4-Butanediol Glycerol a>>~ Dioxane Am>~ Water Am: Amorphous

o 9

J

9 li

1

9? 9

9

..

9

,"

t--

Lt

c-

purest sample so far obtained • 9 9 is c a . 9 1 % according to 27A1 9 9A 9 A 9 9 9 A~A 9 9 J MAS-NMR. Pure and large crystals of A1MepO-a were obtained from starting mixture ,, . /~ 1 .. , a) of the composition 1.0AI : 1.0P : 40H20 reacted at 220 ~ for 1'o ' go ' 3b ' 4'o S 48 h (Figure 2d) as described 2(1 above. Careful s e p a r a t i o n from boehmite-derived mass, Figure 2 XRD of products obtained with however, was necessary and different aging time in static mixing. a) 0 h, b) 24 h, c) 48h, d) single crystals the yield was very low. Nonaqueous organic solvents prepared from composition 1.0A1 : 1.0P : 40H20 have been used to produce at 220 ~ novel materials and large crys9 : A1MepO--a, 9 : A1MepO-~, • : AIMepO--5, tals in zeolite-related material A : A1MepO-~ synthesis [12,13]. The effect was claimed to be owing to reducing amount of water in the reaction media. In our system, alcohol was effective as solvent to obtain microporous products; non-hydric dioxane gave small amount of crystalline product. Many alcohols listed on Table 2 tended to give product containing A1MepO-a in spite of mixing by stirring, although static mixing is necessary for formation of A1MepO--a in the case of water solvent. Especially, ethylene glycol and glycerol was most suitable to obtain A1MepO-a. With no additive the dimension of A1MepO-~ needle crystals ranges c a . 30-300 ~tm in length and less than 10 ~tm in width (Figure 3a). Sometimes small amount of AlMepO-~ was observed as plate crystals together with A1MepO-~. A1MepO-~ synthesized with aqueous ammonia was aggregates of plate crystals (Figure 3c). Instead of no templating ability of the acidic and neutral additives, most of them stimulate growth to give larger crystals; among them dioxane was most effective to

J

201

a)

V. I l l l l l l

b)

Him

V. ! ! [1111

el)

Figure 3 SEM Image of A1MepO products a) A1MepO-[3 with no additive, b) A1MepO-[3 with dioxane, c) A1MepO-~ with aqueous ammonia, d) A1MepO-a by static mixing.

202 give approximately ten times larger crystals than no additive (Figure 3b). Elemental analysis revealed that a considerable amount of dioxane remains in the crystals even after thorough washing with water, although a trace amount remain in products from other additives. Good fit of dioxane molecules into channels probably encourages crystal growth along the channels, i.e. in the direction of the needle. As can be seen in some SEM images the needle crystals were often observed as radial aggregates typically in spherical or conical shape or as needles twinned on the side. This indicates that nuclear growth starts from small boehmite particles dispersed in water followed by collapse of the aggregates into separate needle crystals. Crystals of AlMepO--a synthesized by static mixing were also long needles similar to A1Mel~O-~ of 50-1000 ~m in length (Figure 3d). Optimum synthesis conditions of A1MepO-a suggest that less dispersed boehmite in reaction media favors formation of this phase. Anyhow, further study is necessary to obtain full understanding of the reaction mechanism. 3.2. C r y s t a l s t r u c t u r e s Both structures have the same framework composition A12(CH3PO3)3 and contain both tetrahedral (/kiTh) and octahedral aluminum (A1oh) in the ratio of 3 : 1 as also confirmed by 27A1 MAS-NMR. The frameworks are composed of vertex-shared [A1ThO4], [A1ohO6] and [CH3PO3] polyhedra. The arrangement of Aloh is similar between the two frameworks. Each Aloh is surrounded by eight nearest A1Oh; two are located almost along the c-axis and the other are along the channel walls. The connectivity among A1Ohis different between the isomers as shown in Figure 4. In AlMepO-a [7] three-fold rotation axes parallel to the c-axis run through the positions Where every A1Oh (All in Figure 4a) locates. Therefore, there are only two independent phosphorus sites (P1 and P2). The connectivity along the c-axis is

I a

I ~ b

Figure 4 A1-P linkage of A1MepO-a and-[5. a) left, viewed along [110], b) right, viewed along [120] white circle: A1oh, black circle: methyl group

a

C

203 9

9

~l

eQ.

.~

.\

Io

\X'"

T ,

9 ;

9

Figure 5 Stereoplots of AlMepO-a (top) and-13 (bottom) viewed along [001]. whitecircle: A1oh, black circle: methyl group based on the stacking of 6343 polyhedra sharing vertex A1Oh(Figure 4a). Along the channel wall two A1Ohare connected by single linkage. In A1MepO-~ [8], three-fold screw axes parallel to the c-axis run beside the positions where every Aloh (All in Figure 4b) locates. Each A1oh is connected with two AlOh almost along the c-axis by linearly fused triple four-rings in which the two extreme rings are on the same side of the central four-ring (Figure 4b). Each AlOh is connected with further two Aloh among the remaining six neighbors by similar triple four-rings in which the two extreme rings are on the opposite sides of the central ring. Figure 5 showed stereo drawing of the frameworks along the c-axes. Unidimensional straight channels are running along the c-axes. The oxide frameworks surrounding the main channels contain triangular 18-ring as large as VPI-5 [14], a representative large pore aluminophosphate. So far reported phosphonates [5,6] and phosphites [15,16] with channel structures lined with organic groups or hydrogen atoms connected to phosphorus centers contained 12-rings at largest. The channels of both compounds are lined with methyl groups. In A1MepO-a and -~, large channels composed of 18rings allow to leave still large space where the inner wall are covered with methyl groups.~ A cross section of a channel of A1MepO-a looks triangle, a side of which is c a . 7.0 A. In A1MepO-~ a channel section is of similar size as A1MepO-a and looks

204 rather rounded because the channel wall are more rugged and twisted along the triangular channel than A1MepO-a.

CONCLUSIONS Optimization of synthesis conditions including organic additive, reaction solvent, mixing and aging conditions enables selective synthesis of two structural isomers of aluminum methylphosphonate "organozeolites". Organic additives do not work as template like A1PO4 synthesis but affect crystal size. Channel structures lined with organic groups are confirmed by single crystal X-ray structural analysis.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

T.E. Mallouk and H. J. Lee, Chem. Edu., 67 (1990) 829. Y.P. Zhang and A. Clearfield, Inorg. Chem., 31 (1992) 2821. G. Cao, H.-G. Hong and T. E. Mallouk, Acc. Chem. Res., 25 (1992) 422. K. Maeda, Y. Kiyozumi, F. Mizukami, Angew. Chem. Int. Ed. Engl., 33 (1994) 2335. J. Le Bideau, C. Payen, P. Palvadeau, B. Bujoli, Inorg. Chem. 33 (1994) 4885. S. Drumel, P. Janvier, D. Deniaud, B. Bujoli, J. Chem. Soc., Chem. Commun., (1995) 1051. K. Maeda, J. Akimoto, Y. Kiyozumi, F. Mizukami, Angew. Chem. Int. Ed. Engl., 34 (1995) 2335. K. Maeda, J. Akimoto, Y. Kiyozumi, F. Mizukami, J. Chem. Soc., Chem. Commun., (1995) 1033. J.A. Martens and P. A. Jacobs, in Advanced Zeolite Science and Applications, eds. J. C. Jansen, M. StScker, H. G. Karge and J. Weitkamp, Elsevier, Amsterdam, 1994, p. 653-685. K. Maeda, to be published. E.M. Flanigen, B. M. Lok, R. L. Patton and S. T. Wilson, inNew Developments in Zeolite Science and Technology, eds. Y. Murakami, A. Iijima and J. W. Ward, Elsevier, Amsterdam, 1986, p. 103. Q. Huo, R. Xu, S. Li, Z. Ma, J. M. Thomas, R. H. Jones and A. M. Chippindale, J. Chem. Soc., Chem. Commun., (1992)875. A.F. Kuperman, S. Nadimi, S. Oliver, J. Garces, M. M. Olken and G. A. Ozin, Nature, 365 (1993) 239. M.E. Davis, C. Saldarriaga, C. Montes, J. Garces and C. Crowder, Nature, 331 (1988) 698. R.E. Morris, M. P. Attfield and A. K. Cheetham, Acta. Crystallogr. Sect. C, 50 (1994) 981. M. Sghyar, J. Durand, L. Cot and M. Rafig, Acta. Crystallogr. Sect. C, 47 (1991) 2515.


205

ERS-8: a N e w Class of Microporous A l u m i n o s i l i c a t e s

Giovanni Perego, Roberto Millini, Carlo Perego, Angela Carati, Giannino Pazzuconi and Giuseppe Bellussi Eniricerche S.p.A., Via F. Maritano 26, 1-20097 San Donato Milanese (Italy)

The synthesis of micro-mesoporous aluminosilicates via gelation of a reaction mixture containing Si(OC2H5)4, AI(OC3H7)3, C2HsOH, H~O and alkali-free NR4-OH (R = C3H7, C41-I9, CsH m C8H~3) is described. X-ray amorphous mesoporous aluminosilicates (MSA) or microporous alominosilicates, characterized by a broad peak in the low angle region of XRD pattern (ERS-8), are obtained, depending on the NR4-OI-I/SiO2molar ratio and on the number of C atoms in the R groups. A structural model is proposed concerning the arrangement of the NR4§ cations within the gel, which accounts for the experimental data.

1. INTRODUCTION M41S constitutes a well known class of ordered mesoporous materials which is formed in hydrothermal conditions, starting from an aqueous solution containing silica and alumina sources together with CiH2i§ N§ cations (i > 7; usually 12 or 16) [ 1,2]. The same procedure can be used for preparing mesoporous materials containing oxides of V [3], Ti [4], Mn [5], W [6], Sb [6]. A mechanism of formation of M41S has been proposed, based on the templating effect of a liquid crystal structure formed in the reaction mixture by tetraalkylammonium cations, due to their surfactant properties [1,2]. For the MCM-41 derivative, Stucky et al. [7] suggested the initial formation of a lamellar structure of surfactant molecules interposed between sheets of silica oligomers, which transforms into the hexagonal mesophase as polymerization of silica proceeds. After the discovery of M41S-type compounds many attempts have been made to synthesize new ordered porous materials. We claimed the possibility to synthesize mesoporous X-ray amorphous aluminosilicates (MSA) with a narrow pore size distribution [8,9]. The procedure initially adopted was based on the gelation, eventually followed by a hydrothermal treatment at 180~ of an alkali-free mixture of silica and alumina sources in the presence of (C3H7)4N-OH. Successively, an easier and more reproducible procedure was adopted, based on

205 refluxing a hydroalcoholic solution of the silica and alumina sources, in the presence of (C3H7)4N-OH as gelating agent [10]. Though it is known that the textural properties of amorphous aluminosilicates can be controlled by gel formation in acidic [11] or slightly basic media [12], the synthesis of MSA is performed under strongly basic conditions in order to stabilize tetrahedral A1 [13]. This gives MSA interesting properties in acid-catalyzed reactions [8-10]. Starting from the formation mechanism postulated for MSA, we investigated the synthesis parameters which in principle could modify the arrangement of NR4§ cations in the gel, arriving at the discovery of a new class of materials (named ERS-8). The paper deals with the synthesis and characterization of ERS-8's compared to MSA's. 2. EXPERIMENTAL The materials investigated were prepared using Si(OC2H5)4 (Dynasil-A, Nobel), AI(i-OC3H7) 3 (Fluka) or AI(8-OC4H9) 3 (Fluka), aqueous alkali-free NR4-OH (R - C2H5, C3H7, C4H9, C5Hll, C6H13), ethanol and distilled water. The molar ratios EtOH/SiO 2 = 8, H20/SiO 2 = 8, SiO2/A1203 - 50 were kept constant for all the runs, varying the molar ratio NR4+/SiO2 in the range 0.03 to 0.40. In a typical preparation, AI(/-OCsH7) 3 is dissolved in aqueous NR4-OH at 60~ to which an ethanolic solution of Si(OC2H5)4 is added under vigorous stirring. When using (C6H13)4N-OH (40% wt in water), this is diluted with EtOH and then with the required amount of water before being mixed with a solution of Si(OC2Hs) 4 and AI(s-OC4H9) 3. In both procedures the initial clear solution becomes a viscous gel in a few minutes. After about 15 hours aging at room temperature, the gel is dried at lOO~ and calcined at 550~ during 8 hours in air flow. The solid products were characterized by: - X-ray powder diffraction (XRD) on a computer controlled Philips diffractometer using CuKa radiation (~ = 1.54178 A). In the angular range 1< 2(} < 10 ~ the data were collected stepwise with 1/6~ receiving slit set. The position and breadth of the Bragg peak were accurately determined by means of the FIT routine contained in the software package DIFFRAC (from Siemens), assuming a Split-Pearson VII function for the peak profile. - Thermogravimetric analysis (TGA), with a Mettler TG50 thermobalance controlled by a Mettler TC 3000 microprocessor, running in the range 25 - 900~ heating rate 10~ and 300 ml/min air flow. - Nitrogen physisorption on a computer controlled Fisons Sorptomatic 1990 system. The calcined samples were degassed under high vacuum (~ 10.5 torr) at 300~ for 4 hours. The data were analyzed with the Horvath-Kawazoe method [14]. 3. RESULTS AND

DISCUSSION

As previously reported [8-10], X-ray amorphous aluminosilicates (MSA), characterized by a narrow pore size distribution, can be obtained by using

207 (C3H7)4N-OH as a gelling agent. The same type of materials can be obtained by using other NR4-OH compounds (R = C4H9, C5Hll, Cell13), provided the synthesis is carried out with low NR4-OH/SiO 2 molar ratio [8,10,15]. When performing gelation with higher NR4-OH/SiO 2 molar ratios, we obtained materials different from MSA, characterized by the presence of a peak in the low angle region of the XRD pattern and by a very narrow pore size distribution, with pore radius constantly lower than 20 A [15]. Table 1 Characteristics of the synthesized materials. Sample a NR4-OH/SiO 2 Vol. fraction (%)b

Phase type

d (A) ~

dried d

calcined e

C3(1)

molar ratio 0.403

NR4§ 75

SiO 2 25

ERS-8

19

56

C3(2)

0.253

65

35

ERS-8

20

54

C3(3)

0.197

60

40

ERS-8

18

51

C3(4)

0.153

53

47

ERS-8

19

47

C3(5)

0.113

45

55

MSA

C3(6)

0.105

42

58

MSA

C3(7)

0.093

40

60

MSA

-

C6(1)

0.256

78

22

ERS-8

25

4O

C6(2)

0.108

60

40

ERS-8

24

32

C6(3)

0.082

53

47

ERS-8

26

31

C6(4)

0.068

48

52

ERS-8

25

32

C6(5)

0.062

45

55

MSA

C6(6)

0.047

41

59

MSA

C6(7) C6(8)

0.038 0.034

35 33

65 67

MSA MSA

C6(9)

0.032

30

70

MSA

C6(10) 0.030 28 72 MSA (a) The labels C3(X) and C6(X) stand for materials prepared with R = C 3 H 7 and C6H13, respectively. (b) Referred to dry gels; assumed mass densities of 0.9 and 2.0 g/cm 3 for NR4+ and SiO2, respectively. (c) Bragg distance in XRD pattern. (d) At 100~ in air. (e) At 550~ in air during 8 hours. Starting from these results, a systematic varying both the NR4-OH/SiO 2 ratio and the the other synthesis parameters constant (see volume fractions of SiO 2 and organic matter

investigation was carried out by chain length of R groups, keeping Experimental). By referring to the in the dried materials (derived by

208 assuming for both components reasonable values of mass density, see Table 1), it appears very clearly that a threshold exists around 50% volume fraction of NR4§ Below this threshold, MSA is formed while above this ERS-8 is formed, independent on NR4§ used in the synthesis (Figure 1) In a previous paper, a detailed characterization was given for MSA prepared with (C3H7)4N-OH [9]. To better describe the properties of ERS-8's compared to those of MSA's, attention will be focused on the materials prepared with R = C3H 7 and Cell13 (Table 1) considering that the same conclusions are valid for materials obtained with R = C4H9 and CsHlr The only exception is R = C2H5 which gives MSA over the whole range examined. A peak is observed in the XRD pattern of as-synthesized, 100~ and calcined ERS-8's; however, the position of the peak varies significantly as a function of the thermal history of the sample (Figure 2). i

9

i ERS-8

MSA

D

!i5

I,m i

3

A

l'o 2~ 3'0 4~ 50 6'0 7o ~'o ~, ioo

0

NR4 Vol.fraction(%)

Figure 1. Relationship between the NR4+ volume fraction in wholly dried precursor and the type of material obtained ~ I S A , @ERS-8).

4

2

6

8

2-Th eta [o]

10

Figure 2. Low angle region of XRD patterns for as-synthesized (A), dried at 100~ (B) and calcined at 550~ (C) ERS-8 (sample C6(2)), compared to that of MSA (D) (sample C6(7)). 40

N

"

i /

\

:x

/

i

"|

""

l '~-

34

~:ir-dried

.~

32

-o

263028 24

-

-

-

i

-

-

i

.

.

.

.

f

.

-

-

i

"

-

Figure 3. TG ( - - ) and DTG (.... ) curves of dried C6(2) sample.

as-synthesized gel

38

36

22

gel

~""B'"r"~

1()0

-

2()0

3()0

Temp. (~

400

5(}0

600

Figure 4. Variation of d as a function of calcination temperature, for ERS-8 sample C6(2).

209 The FWHM (3 - 4 ~ 20) of the peak is quite large and certainly indicative of very low structural order; however, it is significantly lower than the FWHM (8 10 ~ 20) of the peak (observed in both MSA's and ERS-8's) at ca. 23 ~ 20, due to the Si(A1)/Si(A1) pair correlation. Whether the low angle peak has to be considered a Bragg peak may be matter of discussion. In any case, it seemed reasonable to consider the related Bragg distance, d. The trace of the TGA curve, shown in Figure 3 (the same features were observed in all the dried gels), is characterized by several steps of weight loss, which are interpreted as in the following. Below 150~ residual H~O and C2HsOH are lost, while the organic matter is eliminated in two well defined steps. The first one (at ca. 270~ corresponds to the elimination of NR4-OH molecules simply "embedded" in the aluminosilicate matrix; in the second one (at ca. 350~ the elimination of NR4+ acting as counterions to the tetrahedrally coordinated A1 ions occurs. As a matter of fact, both in MSA's and in ERS-8's, the moles of NR4§ lost at higher temperature nearly correspond to the total moles of AI present in the material. The slight weight loss observed at T > 450~ probably corresponds to the combustion of some coke formed during the TG analysis and/or to the elimination of H20 deriving from condensation of surface silanol groups. Portions of C6(2) dried gel were treated in the T G furnace at the temperatures indicated in Figure 3. The X R D peak is observed for all the samples and the related Bragg distance, d, varies as a function of the treatment tempoerature (Figure 4). The value of d decreases from ca. 38 A (wet gel) to ca. 24 A in the totally dried sample (150~ and increases up to ca. 32 ik after calcination at 550~ It is worthy to note that, for dried samples, the value of d is practically independent of the NR4-OI-I/SiO 2 molar ratio, but depends on the number of C atoms of the R groups (Table 1).

9 o

9

o

9

MSA

9

o 0"6

~

0.4

L/.~':":"...... ~RS~

~

.eo.eO-eo''

9

9

9

ERS-g

,,P o

04

o g

o o

o

9

MSA

mO~ 0 0 ~0.2 o~d~

. . . . . . . .

i i0

.

.

.

Pore radius

. (~)

.

. 100

.

.

. . . . . . .

. 1000

o~

| 10

,j 100 Pore radius

. . . . . . . . 1000

(,~)

Figure 5. Cumulative pore volume of MSA Figure 6. Cumulative pore volume of MSA (sample C3(5)) and ERS-8 (sample C3(3)) (sample C6(7)) and ERS-8 (sample C6(2)) synthesized with (C3H7)4N-OH. synthesized with (CsH13)4N-OH. The pore size distribution is narrow in both materials. However, while in MSA there is a predominance of mesopores, in ERS-8 only micropores (r in the range 3

210

10/k) are present, independent on NR4 § used (Figures 5 and 6). Both MSA's and ERS-8 are characterized by specific surface area in the range 700 - 1000 m~/g. Pore volume of M S A is ranging from 0.5 to 0.7 cmS/g (C3 series) and from 0.4 to 0.5 cm3/g (C6 series);for ERS-8, values of 0.3 - 0.4 cm3/g (C3 series) and 0.4 - 0.5 cmS/g (C6 series) are measured. The following model is given, which reasonably accounts for the experimental data. In the gelation step, the formation of clusters of NR4 § cations is postulated. The picture shown in Figure 7 represents a possible structure for these clusters, where the alkyl chains in liquid-like conformation are arranged parallel to one another; the O H anions solvated by ethanol and water molecules are placed on the border of the alkyl chains assembly. In MSA-forming mixtures, the growth of these clusters is limited because of the excess of silica (volume fraction exceeding that of NR4 § see Table 1) which tends to form a three-dimensional aluminosilicate structure all around them (Figure 8a). As a consequence, the ethanol- and water-containing NR4 § domains become the pore precursors of M S A (Figure 8b) and their dimensions control the pore size of the material. In an ERS-8-forming mixture, due to the excess of NR4 § component with respect to silica(see Table 1), the NR4 § clusters are expected to grow much more in the directions perpendicular to that of the elongation of the alkyl chains. Practically, the formation of monolayers of NR4 § solvated by ethanol and water molecules, is admitted, on the surface of which aluminosilicate sheets grow. According to this hypothesis, the ERS-8 precursor has to be considered essentially as a layered-type material. The layers so formed should be embedded in ethanol and water (which constitute about 90% of the total volume of the gel) with occasional, if any, layers stacking. Therefore the d value of 38 A would represent the average distance between the aluminosilicate sheets grown on the two surfaces of each NR4 + layer (Figure 8c). It is interesting to note that the formation of a layered-type intermediate has been proposed also for M C M - 4 1 [7]. After drying, the structure should collapse with a resulting packing of the layers roughly parallel to one another. The decrease of d value for the dried gels is well accounted for by the removal of most of ethanol and water molecules from the interlayer region (Figure 8d). Consistently with the different length of the alkyl chains of NR4 + cations, the d values observed for C3 samples (18 - 20/k) are lower than those observed for C6 samples (24 - 26/k, see Table 1). However, according the poor order of the structure, only occasional correlation exists among the layers along the direction parallel to the stacking direction (no correlation at all in the other directions). As the organic matter is removed during calcination, the two aluminosilicate sheets grown on each NR4 § layer will condense forming double sheets; these constitute the building blocks which form a three-dimensional structure (through inter-sheet condensation of silanol groups), maintaining the plane of the sheets roughly parallel to each other. Due to the likely difference in the dimensions (mainly length and width) of the sheets, a regular assembly cannot occur and pores are inevitably formed. In Figure 8e a simplified, very schematic picture of the 3D structure proposed for ERS-8 is shown. According to this model, pore -

211

dimensions in the direction perpendicular to the plane of the sheet (arrows in Figure 8e) is controlled by the double-sheet thickness, which is expected to be much more regular than length and width (for this reason, a broad distribution of pore dimensions is expected along the directions parallel to the sheet plane). Since the average thickness of a single sheet is estimated to be around 5 A (by considering the d value together with the volume fractions of

- Nl~luster

MSA

ERS4~

3s~ a

I dryingand ! calcination

~ dr~.g

d

Figure 7.

Proposed structure for a

solvated (C2Hs-OH + H20)(C6H13)4 N+

/calcination

Figure 8. Schematic model proposed for pores formation in MSA and ERS-8

clusters (number of molecules arbitrary). NR4§ and S i O 2 ) , w e should expect pore dimensions of ca. 10/k (one double sheet), ca. 20/~ (two double sheets), etc. The curves of Figures 5 and 6 really agree with the presence of pores of ca. 10 and ca. 20/~ in ERS-8's. Inter-sheet distances can be identified within the structure (arrows in Figure 8e), not correlated to each other, which account for the broad Bragg peak observed in calcined ERS-8's. These distances correspond to multiples of the average thickness of the double sheet. The d values observed for calcined ERS-8 (Table 1), agree with an average of 3-4 double sheets (C6 series) and 4-5 double sheets (C3 series). Because the pore size distribution is practically identical for the two materials, the larger spacing observed for C3 series may be accounted for by a more dense

212 packing of the sheets. The lower value of total pore volume observed for ERS-8 in C3 series, relative to that in C6 series, agrees with the above hypothesis. 4. CONCLUSIONS Gelling a solution containing silica and alumina sources with an alkali-free tetraalkylammonium hydroxide (NR4-OH, R = C3H 7, C4H 9, CsH n, C8H13) leads to the formation of mesoporous (MSA) or microporous (ERS-8) aluminosilicates. In particular, the formation of ERS-8 occurs when the volume fraction of NR4 § exceeds that of silica-altunina, independent on the length of R alkyl chain. ERS-8's represent a new class of aluminosilicates characterized by a narrow pore size distribution in the range of micropores. Due to their acidic properties, these materials are suitable for applications in acid-catalyzed reactions. REFERENCES 8

Q

~

*

5. 6. 0

0

1

10. 11. 12. 13. 14. 15.

C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. 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. I~M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Soc., Chem. Commun., 1059 (1994) P.T. Tanev, M. Chibwe and T.J. Pinnavaia, T.J., Nature 368 (1994) 321. D. Zhao and D. Goldfarb, J. Chem. Soc., Chem. Commun., 875 (1995) Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R., Leon, P.M. Petroff, F. Schuth and G.D. Stucky, Nature, 368 (1994) 317. A. Monnier, F. Schuth, 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. G. Bellussi, M.G. Clerici, A. Carati and F. Cavani, US Patent No. 5 049 536 (1991). G. Bellussi, C. Perego, A. Carati, S. Peratello, E. Previde Massara and G. Perego, Stud. Surf. Sci. Catal. 84 (1994) 85. C. Perego, S. Peratello and R. Millini Eur.Patent No. 659,478 A1 (22.12.1993) T. Pecoraro, U.S. Patent No. 4,988,659 (1991). M.R. Manton and J.C. Davidtz, J. Catal. 60 (1979) 156. J. Livage, Stud. Surf. Sci. Catal. 85 (1994) 1. G. Horvath and I~ Kawazoe, J. Chem. Eng. Jpn. 16 (1983) 470. G. Pazzuconi, G. Bassi, C. Perego, G. Bellussi, R. Millini and G. Perego, It. Patent No. 94A001399 (1994).


213

S y n t h e s i s a n d c h a r a c t e r i z a t i o n of l e v y n e t y p e zeolite o b t a i n e d f r o m gels w i t h different SIO2/A1203 ratios C. V. Tuoto a, J. B. Nagyb and A. Nastro a aDepartment of Chemical Engineering and Materials, University of Calabria, Arcavacata di Rende, 87030 Rende (CS), Italy bUnit~ de R.M.N., University of Namur, Rue de Bruxelles 61, B-5000 Namur, Belgium. Levyne type zeolites have been successfully synthesized from gels 4.5Na206MeQI-xAI203-30SiO2-500H20 with 0.6_<x_60

at the end of crystallization

use gel 1

use gel 2

use gel 1

use gel 2

0. 7% 90. 6 %

0. 7 ~ 96.5 ~ 1.8~ I. 0 ~

1.1~ 80. 3 ~

4.1~ 88, 7 1.0~

6.5~ 2. 2~

9.5~ 9. 1~

6.2~

The results showed that more than 900~ samples kept original particle size at the end of aging stage and that more than 80 ~ product kept original particle size at the end of crystallization when the gel 1 usedas silica source (Most of our experiment results in this work related to the ease using gel 1). That is one of the reason to be eonsidered that most of the amorphous gel had not passed a dissolution-repreeipitation in their crystallization processes. 3.3. Chemical analysis results The chemical analysis in one of the series of experiments gave the concentrations of AlzO3, Na20 and SiOz in the solution at different stages of synthesis. The contents of AlzO3,NazO and SiOz are indicated as the percentages of the amount of every ingredient related respectively to that in the starting synthesis systems. All the results are shown in Table 2 and Fig 1. The chemical analysis results showed that the content of AlzO3 in the solution reduced quickly at the beginning of the aging stage. Before the crystallizationstarted at an elevated temperature, almost 9 8 ~ AlzOa had attached to the solid gel phase, which was X-ray amorphous at that time. Throughout 8 hours crystallization period the content of A1203 in the solution did not increase and did not fluctuate, just reduced a little. It indicated that no detectable aluminosilicate gel dissolved into the solution. At the end of 8 hours crys~lh'zation, the crystallinity of the product was higher than 90 ~ .

296 Table 2 The contents of AI,O3,NazO,SiO2 in the solution at different stages of synthesis Aging stage Time(hr. ) A1203~

Crystallization stage

NazO~

SiOz~

Time(hr. ) A12Os~

NazO~

SiOz~

1

27.4

74. 7

2.5

O. 5

I. 9

68. 4

22. 2

2 4 7 ll 16

15. 6 8. 3 2. 6 2. 3 2.2

68. 4 66.9 65.4 64. 8 61.7

I. 7 18. 1 21.9 22. 0 22. 7

1 2 4 6 8

I. 7 1.3 O. 9 O. 8 O. 8

67.7 68.2 69. 3 69.8 68. 5

22.5 23.0 23. 6 24. 0 25.7

o o

/.

/ l AI,O3~

r\ .0

so,, ;

:

"

,q

SiO,~

9_ 03

-20.0

"

8O. 0

Na,O~

10.

O"--O-

60.0

---O

.

.

.

.

,-- - - 0

.

.

.

.

.

.

0--

I0.0

- - - - .-- - - I J

A1,03% 4

8

12

16

2

4

6

8

Aging time, hour---~ Crystallization t i m e , hour m - ~ Figure 1. The contents of AlzO3,NazO,SiOz as functions of time during synthesis The fact can be served as another reason to be considered that the transformation of the amorphous gel to zeolite crystal p r o ~ e d through rearrangement, depolymerization and condensation in the solid phase (including on its surface). It can hardly be proved that the solid gel spheres, which obviously can be seen from the beginning to the end of crys~llizadon, had changed thoroughly in an equilibrium state from amorphous phase to 90 ~ crystallized phase through dissolving and reprecipitation just in a period of time which was not long. If the dissolution-reprecipitation happened in a very thin surface diffusion layer, which was an interface between the network of the gel and the liquid phase, tO detect the changes is really a difficult problem now. According to the chemical armJysis results, the content of SiOz in the solutions increased rapidly at the initial stage of aging. In the crystallization stage the content of the SiOz in the solution increased not very much. About 220~ SiOz dissolved into the solution at the end of aging stage. About 25 0~ SiO, existed in the solution when crystallization proceeded to 8 hours. Since the SiO,/AI,O3 ratio in the mixture of starting materials was much higher than that in the product zeolite X, so part of SiOz, which did not attend the arrangement of

297 Si--O--AI framwork, dissolved in the alkaline solution. The dissolution of SiO2 did not results in most of the spheres fallen-apart in the synthesis conditions of this work. Most of the spheres essentially retained the origial particle size. It meant that some kinds of silica-alumina framwork appeared and maintained from the early stage to the end of synthesis process. The cherrdcal analysis results indicated that about 70 ~ of NazO remained in the solution when the crystallization had proceeded for 8 hours. 3.4. The results of solid state Z~Al NMR experiments Figure 2 ,represented the solid state ~,AI NMR experiment results in this work, which gave the information about the local environment of AI nuclei in the solid samples at different stages of the synthesis.

(d)

(a) It

IIIIIII

I t l l l l l l l t l l l l l t t l l l

II

100 0 100 0 ppm ppm Figure 2. real NMR spectra of solid samples (a)after 2hr. aging (c)after lhr. crystallization (b) after 16hr. aging (d) after 8hr. crystallization We tried to use acetylacetone (acac)impregnation method ca-263 to obtain the signals of aluminium, which resided in the low syrmnetry environments, together with the signals of aluminium in the fourfold oxygen coordinated environments. According to the reference :2s3 38 ~ (vol.) acac solution in ethanol does not affect the tetrahedrally coordinated aluminium including the aluminium existed in the same coordination state in an amorphous silica matrix. (In this aspect further investigation is in progress now) The aluminium in the low symmetry environments, like extra-framwork aluminium in the dealuminated zeolites, was converted with acac into AI (acac) 3 complexes. From the investigation of solid state ~AI NMR it was known that the signals at about 60 ppm chemical shift are attributed to fourfold oxygen coordinated AI indicating the building units AI (OSi)4. The AI (acac)3 complexes exhibit a chemical shift of about 0 ppm as a narrow peak in the ~ AI NMR spectra. The relative area of the peaks at about 60 ppm

298 proportional to the quantities of the fourfold coordinated A1 in the samples Ez~3. Fig. 2 showed that the quantity of fourfold coordinated AI increased obviously along with the synthesis process. Fig. 2 also showed that at the initial aging stage (2hr.) fourfold oxygen coordinated aluminium already appeared in the gel. Engelhardt et alE,s:! had also detected this signal at 59 ppm in the precursor gel. At the end of aging stage (after 16hr. aged) in our experiments, a large number of fourfold coordingated A1 existed. Accxrrding to the results it was considered that the coordinated environment of aluminium at the end of aging somewhat approached to the environment in the ordered assemblages of tetrahedra in the zeolite crystal framwork. Maybe the fourfold oxygen coordinated A1 served as structure directors. So the crystallization proceeded rather fast when the temperature raised. The results of ~ AI NMR experiments were consistent with the assumption that the transformation of the amorphous gel to crystal zeolite proceeded in-situ by rearrangement of the atoms and building units in the solid phase. 3.5. The effect of the alkaline solution on the crystallization

For understanding the effect of the solution around the gel on the crystallization, another four experiments had been carried out. Every one started with the same formulation and aged in the same conditions. At the end of aging stage, the solution were decanted as far as possible, and the solid alumina-silica gel was left in the wet state. The f'rrst solid sample was washed to approach neutral. Then the sample was separated to two parts. One part of the sample was put in 383K for 48 hours. Another part was put in the room temperature for 50 days. XRD patterns showed that all these two parts of the sample were X-ray amorphous. Let the second sample (which was in the wet state and coexisted with a small amount of original solution) to crystallize at 363K for 24 hours with no additional solution added in. XRD pattern showed that zeolite X with crystaUinity of 85 ~ formed. In the third sample a NaOH aqueous solution was added to instead of the original solution which had been decanted. The concentration and volume of the NaOH solution was the same as the original one. Let the sample to crystallize at 363K for 24 hours. XRD result indicated the zeolite X pattern with crystallinity of 80 o/~. The fourth sample was washed with water one time. The solution was der.anted again. Just a small m o u n t of solution with lower pH value remained in the sample. Let the fourth sample to crystallize at 363K for 24 hours. XRD pattern showed the zeolite X with crystallinity of 52 ~ . The impurity of zeolite A and P was much more than that in the second and third samples. It meant that the solution with lower pH value was disadvantageous for crystallization. The above listed results showed that without the alkaline solution coexisted with the solid phase, the amorphous gel can hardly be transformed to crystal zeolite, although there was some crystal nuclei on the gel at the end of aging stage. According to the facts it was considered that when an alkaline solution coexisted with the amorphous gel, the atoms and the structural units in the network of the gel and on its surface had a larger amplitudes of thermal vibration and a lower activation energies for diffusion than that in the network of the gel without coexistance of alkaline solution. According to the reference Ez93it was also considered that in the crystallization process OH- ions in the solution were necessary for condensa-

299 tion proceeded in the gel, and that some HzO produced in condensation. The coexistance of the alkaline solution with solid gel was favourable to remove this kind of H,O and unnecessary SiOz and NazO in the gel. The alkaline solution promoted the transformation of the gel to zeolite framwork. 3.6. On the SEM photographs it was shown that the crystal size of zeolite X on the surface of the preformed spheres was always bigger than that inside the spheres. This is one of the fact to be noted. It can be expected that the atoms and structural units on the interface diffused and rearranged more easily than that inside the spheres. Another fact is that in the crystallization process, small part ( < 10 ~ ) of the spheres became smaller than that in the beginning of the crystallization stage. According to that two facts it was considered that in the synthesis system investigated a small amount of the zeolite crystals formed and grew probably through dissolution of the gel and reprecipitation on the interface. 4. CONCLUSION 1. Preformed binderless zeolite X with crystallinity higher than 90 ~ were synthesized by using spherical SiO2 gel as starting material. 2. It was considered that most of the amorphous alumina-silica gel in the synthesis system investigated transformed to crystal zeolite X in the solid phase. 3. In the solid phase, fourfold oxygen coordinated aluminium AI (OSi)4 increased along with the synthesis process, The existence of fourfold oxygen coordinated A1 in the amorphous gel before crystallization proceeded indicated a suitable environment in the solid phase for in-situ transformation to ordered framwork. 4. The crystaUizadon process can hardly be able toproceed without coexistence of alkaline solution with the gel. 5. Small amount of the zeolite X crystal in the system investigated probably formed and grew through dissolution of the gel and repreeipitation on the interface.

Admowledgement The authors wish to express their thanks to Li shanan, Ma Jian and their colleagues (Nanjing Jinling Petro-Chemieal Corporation Refinery), Hu Cheng (Laboratory of Solid State Microstruetures, Nanjing University) for their assistance during the experiments and their helpful discussions. REFERENCES 1. R.M. Barter, in "Zeolite Synthesis" (Eds. M . L . Oeeelli and H. E. Robson), ACS Syrup. Ser. 398, Am. Chem. Soc., Washington, D.C. (1989) P. 11. 2. B.D. McNicol, G.T. Pot-t, K. R. Loos and N. Mulder, Chem. Sex. , 121(1973)152. 3. E. M. Flanigen, Adv. Chem. Sex. 121(1973)119. 4. H. Khatami, E. M. Flanigen, unpublished work. 5. B. Fahlke, P. Starke, V. Seefeld, W. Wicker and K. P. Wendlandt, Zeolites 7 (1987) 209. 6. R. Aiello, R. M. Barrer and I. S. Kerr, Adv. Chem.Ser. , 101(1971)44.

300 7. S. P. Zhdanov, Adv. Chem. Ser. 101(1971)20. 8. S. P. Zhdanov, and N. N. Samulevieh, "Proe. Fifth Int. Conf. on Zeolites" (Ed. L. V. C. Rees) Heyden, London (1980) P. 75. 9. Ali Culfaz and L.B. Sand, Adv. Chem. Sex. 121(1973)140. 10. C. L. Angell and W.H. Flank, in "Molecular Sieves I " (Ed. J.R. Katzer) ACS Syrnp. Sex. 40, Am. Chem. Soe., Washington, D.C. (1977) P. 194. 11. Peter M. Budd, Graham J. Myatt Colin Price, and Smart W. CarT, Zeolites 14(1994) 198. 12. S. Ueda, N. Kageyama and M. Koizumi, Proc. 6th Int. Zeolite Conf. in 1983 (Ed. David Olson and Attilio Bisio), Butterworth, UK (1984) P. 905. 13. H. Lechert and H. Kacirek, Zeolites 11(1991)720. 14. H. Lechext and H. Kacirek, Zeolites 13(1993)192. 15. H. Kacirek and H. Lechert, J. Phys. Chem. 80 (1976)1291. 16. Ryozi I-Iino, Ryohei Matuura and Kenzi Toki, Bull. Chem. Soc. ]pn. 56(1983)3715. I7. Kenneth E. Hamilton, Eric N. Coker, Albext Sacco, Jr. Anthony, G. Dixon and Robert W. Thompson, Zeolites 13 (1993) 645. 18. W. Meise and F. E. Schwochow, Adv. Chem. Sex. 121 (1973) 169. 19. E. Michalko, US Patent No. 3,428,574(1969). 20. K. D. Vesely, US Patent No. 3,492,089(1970). 21. Carl V. McDaniel, Philip K. Maher and Joseph M. Pilato, US Patent No. 3,472,617 (1969). 22. Johannes P. Verduijn, WO Patent No. 92/12928. 23. T. Kawamoto, Y. Taga, I. Tosawa, T. Nishimura and W. Inaoka, Jpn Kokai Tokkyo Koho 92,198,011. 24. P. J. Grobet, H. Geerts, J. A. Martens and P. A. Jacobs, J. Chem. Soc. , Chem. Commun, (1987)1688. 25. P. J. Grobet, H. Geerts, M. Tielen, J. A. Martens and P. A. Jacobs, Studies in Surface Science and Catalysis 46(1989)721. 26. V. Bosacek, D. Freude, T. Frohlich, H. Pfeifer and H. Schmiedel, J. Colloid and Interface Science 85(1982) 502. 27. D. Freude, T. Frohlieh and H. Pfeifer, Zeolites 3(1983)171. 28. G. Engelhardt, B. Fahlke, M. Magi and E. Lippmaa, Zeolites 3(1983)292. 29. Fred Roozeboom, Harry E. Robson and Shirley S. Chan, Zeolites 3(1983)321.



301

S Y N T H E S I S OF L A R G E C R Y S T A L S OF M O L E C U L A R S I E V E S - A R E V I E W Shilun Qiu, Wenqin Pang and Ruren Xu Key Laboratory of Inorganic Hydrothermal Synthesis, Department of Chemistry Jilin University, Changchun 130023, P. R. China More than 50 kinds of single crystals of molecular sieves (50-1000 um) including aluminosilicates, aluminophosphates, gallophosphates and indiumphosphates were crystallized from the systems of organic solutions, weak acid, aqueous solutions and fluoride ions, respectively. 1. I N T R O D U C T I O N There is considerable interest in synthesis of large crystals of molecular sieves since they are useful in many research studies, development of advanced zeolite materials and industrial applications[ 1]. However, molecular sieves are normally metastable phases, large and perfect crystals are often difficult to obtain. Here we report the synthesis of more than 50 kinds of large crystals (some of them are single crystals) of molecular sieves including zeolite A, Mordenite, Beta, ZSM-5, -39,-48, cancrinite, aluminophosphate AIPO4-5,-11,-17, JDF-20, gallophosphate -LTA, Cloverite, GaPO4-C3, - C 4 , - C 7 , indiophosphate as well as aluminoarsenate, some of them are novel phases. They were prepared via, mainly, the following four novel routes: in organic solutions, in a weak acidic medium, in aqueous solutions, in the presence of fluoride ions. 2. E X P E R I M E N T A L The large crystals of the molecular sieves are generally crystallized by hydrothermal synthesis, except for the synthesis from nonaqueous solutions, in which alcohols and / or amines are employed as the solvent. The reaction mixture is added to a Teflon-lined autoclave (10-1000 ml in volume) and heated at 100-200 ~ for 1-30 days. The large crystals were recovered by filtering, washing, sonicating (if necessary) and drying. They have the sizes of 50-1000 ~tm. 3. R E S U L T S A N D D I S C U S S I O N

3.1. Synthesis in organic solutions In 1985, Bibby and Dale[2] first reported the synthesis of pure-silica sodalite using organic solvents. Following years saw the synthesis of zeolites and metal-substituted alurninophosphate microporous materials from this system in Jilin University. To extend the study, various non-aqueous solvents are successfully applied for the preparation of Silica-

302 Sodalite as well as other kind of zeolites and phosphate microporous materials (e.g., ethanol amine and ethylenediamine-glycerol media) by Xu and coworkers[3]. A significant phenomenon is found in this system that single crystals of the molecular sieves are often obtained. Further investigation indicated that the crystallization rate is rather slow than in hydrothermal synthesis owing to the lower dielectric constant of alcohols and/or amines than those of water. Fig. l a shows crystallization curves for AIPO4-21 by hydrothermal and solvothermal synthesis, respectively. It can been seen that crystallization rate in solvothermal system is much more slower than in hydrothermal system, although the nucleation rate in solvothermal system is faster. The lower polymerization and slow crystallization rate of the reactants allows the growth of large crystals of the molecular sieves. On the contrary, in hydrothermal synthesis the fast crystallization, due to the quick exhaustion of the nutrient for crystallization, impedes the crystal growth, as a result, small crystals are often obtained in this system. Different solvent plays a different affect in crystallization period which depends mainly upon the molecular configuration and polarity of the solvent molecules. It is found that large crystals of zeolites and microporous materials prefer to grow from the alcoholic medium like ethyleneglycol, butyl alcohol, hexanol and others. The crystals resulted from the gel by using small amines are likely to grow to large dimension. The following single crystals are prepared from the alcoholic systems, Si-zeolite (such as Si-sodalite from ethylene glycol, EG)[4], Si-Sodalite (Fig. 2a)[5], Si-ZSM-48 (EG)[6], AIPO4-n like AIPO4-CI (chain structure, BuOH)[7], A1PO4-CA (EG)[8], AIPO4-CC (EG)[9], A1PO4-F (HexOH) [9], AIPO4-H (BuOH) [9], AIPOa-B (EG) [9] and GaPO4-C3 (EG or HEXOH)[9]. By reason of slow crystallization in the medium, a favorable circumstance is provided for the growth of the single crystals. Some single crystals for novel microporous materials are prepared, such as gallophophate-LTA[ 10] (Fig. 2b), ZnPO4, CoPO4-1, -2, -3[ 11 ], as well as JDF-20 [9, 12, 13], which contains 20-ring channel. The preparation of JDF-20 single crystals is carded out in the 1.0 A1203 : 1.8 P205 : 4.7 Et3N : 18 TEG (Triethylene glycol) system at 180 ~ for 5 days. In addition, single crystal of cancrinite is crystallized from butane-l, 3-diol solvent[14]. Adsorption measurement shows that it possesses type I adsorption isotherms. Its adsorption capacity to water, hexane and cyclohexane are 13.7, 9.5 and 9.2%, respectively. These values are different from those of the natural cancrinite, and the cancrinite synthesized from an aqueous medium. In fact, the cancrinite obtained from the system does not exhibit adsorptive properties since the channels are blocked by intercalated salts or due to stacking faults. (%)1oo (%)100

,

/ X

.

// 0

0

5

10

15

20(x1000 s)

i

o

i

2

'

~X

Z/

~

L..

J ....

L.__L

_

4

6

8

l 0 (d)

Time

a

b

Fig. 1 Crystallization curves for AIPO4-21 at 200 ~ (a), (I) in water, (II) in HOCH2CH2OCH3, (HI) in O(CH2CH2OH)2; and for Mordenite at 145 ~ (b), (A) Silica sol as a silica source, (x) Silica sol and aerosil as "double silica sources".

303

b

V

d

e

f

Fig.2. SEM images of single crystals of Si-Sodalite (a), GaPO4-LTA (b), GaPO4-C4 (c), A1AsO4-1(d), InPO4-C11 (e) and InPO4-C12 (f).

304 3.2. Crystallized from acidic medium A novel microporous family containing the third and fifth element, M0]~X(V)O4, has been discovered by R. Xu[15]. Single crystals of the microporous MOI~X(V)O4 prefer to grow from the acidic medium, pH values of the starting mixture being 3-6. Among over 30 species of the M([II)X(V)O4 family, more than six have been grown single crystals from the acidic medium containing templates. These include GaPO4-C3[15],-C4 (Fig. 2c) [16],C7[ 17], AIAsO4-1 (Fig. 2d)[ 18], -2[ 19] and GaAsO4-2[20]. Morerecently, Y. Xu, et al.[21 ] have synthesized a novel microporous indium phosphate, InPO4-1 single crystal, from acidic medium. The structure determination indicated that the In and P atoms have octahedral and tetrahedral coordination respectively bridged through oxygen atoms. Followed by Du et al. [22], they successfully synthesized 4 kinds of large crystals of microporous InPO4 from acidic medium. They are InPO4-7, -9 (trigonal, Fig. 2e), -12 (monoclinic, Fig. 2f). According to our experience for the synthesis of single crystals of microporous M(I]I)X(V)O4, the stronger metal the M(I]I) is, the lower pH value is needed. In contrary the stronger nonmetal the X(V) is, the higher pH value is needed. 3.3 Crystallized from aqueous solutions The molecular sieves are normally crystallized from a reaction mixture gel. In this way, the concentration and solubility in the gel is very difficult to control at crystallization temperature. Along with the increasing of the crystallization rate, the nutrient for crystallization is rapidly exhausted, so it is very difficult to grow up large crystal. Crystallization from homogeneous solution is a good way to obtain single crystal since the supersaturation of the reactants can be easily controlled. By using this method, single crystals of zeolite A [23] was crystallized by adding more sodium hydroxide in the mixture to get homogeneous solution. Similarly, HF was added to the mixture of AIPO4 and diethylamine to create an aqueous solution, single crystal A1PO4-5124], FAPO-5125] wereobtained. Growth of single crystals of zeolites mordenite(Fig. 3 (a)), Beta (Fig. 4 (a)) and ZSM-5 has been successfully accomplished by using "two silica sources" technique. Pang and coworkers [26] have studied the preparation of large single crystals of zeolite mordenite from the aqueous solutions in the absence of templates. During the preparation stage, sodium silicate solution and aerosil are jointly used as silicon sources. This results in mordenite single crystals of large dimension. In the usual case, the crystallization of zeolite mordenite is completed in a short period which will result in small particles. With the use of the mixed silicon sources, the crystallization rate is reduced which is believed to be due to the different reactivity of sodium silicate solution and aerosil. Using the "two silica sources" technique, Pang and coworkers [27] also prepare large single crystals of zeolite Beta and ZSM-5, each using tetraethylarnmonium cation and n-butylamine as templates. The synthetic conditions and products are shown in Table 1 a and b. 3.4 Synthesis in the presence of fluoride ions. Guth et al. [26] developed a novel route to preparing single crystal of ZSM-5 in a nonalkaline medium in the presence of flouride and tetrapropylammonium bromite (TPABr) was employed as a template. To extend the investigation, Qiu et al. [27] studied the crystallization mechanism and reported for the first time the growth of perfect single crystals of boron and titanium substituted ZSM-5 (Borozeosilite and Titanozeosilite) (Fig. 4b and 4c). It is believed that silicon, boron and titanimn complex fluorides are formed during the beginning stage of

305

Table la Crystallization from aqueous solutions Reaction comp. (mol) SiO2 Na20 Me Cod A1203

H20

Temp (~

Time (d)

Reaction mixture

Crystal size/pro

Product

550 550 550 550 550 550

150 150 150 150 150 145

15 15 15 5 5 25

c c c c c g

185x125 27x30 85x50 8x3 2xl 110x55

MOR MOR MOR MOR MOR MOR

e

A B C D E F

1 1 1 1 1 1

60a§

b

60"+ 15b 60%15b 75a§ b 0a§

b

60a+50b

15 15 15 15 15 15

4NaC1 4NaAc 4KC1 4NaC1 4NaCI 4NaC1

a: silica from aerosil; b:silica from sodium silicate; Me: amount of salt used; c:clear solution; g: gel. Table 1 b Crystallization from aqueous solutions Gel composition (mol ratio)

Temp

No.

A1203

SiO2

Re20

Na20

H20

NaC1

A B C D

1 1 1 1

79a§ b 70a+63b 35a§ b

16TEA 27TEA 71NBA 76NBA

36 36 23 23

1466 1466 1466 1466

12 12 10 10

43a+40b

Time

(~

(d)

140 140 155 155

12 12 15 15

Crystal size/~n 28 20 63 52

Product Beta Beta MFI MFI

a: silica from aerosil; b: silica from sodium silicate; TEA: tetraethylammonium hydroxide; NBA" n-butylamine. crystallization and then slowly hydrolyzed to other less fluorinated silicates, borates and titanates, respectively, which supply, slowly and continuously, the nutrients for the growth of the single crystals [27]. Besides, Zhao and coworkers [28] successfully prepared large crystals of ZSM-5 using seven different organic templates in fluoride system. They are triethylamine, tetraethylammonium, diethylamine, ethylenedimine, choline, piperazine and 1,4-diazabicycle(2,2,2) octane, respectively. Single crystals of ZSM-12 and ZSM-39 (Fig. 4d)[29] were also crystallized from this medium. Qiu and coworkers [30] reported for the first time the growth of single crystals of aluminophosphate molecular sieves by adding fluoride to the starting mixture. A big and perfect single crystal (1000 um in length) of A1PO4-5 (Fig. 3b and 3c) was obtained. Using the similar technique, Si-, Li-, B-AIPO4-5 (Fig. 4e) and AIPO4-1 l(Fig. 40 single crystals [31 ] can be crystallized from fluoride system. Based on experimental observations, the controlling parameters for the growth of single crystals are summarized [24]. They are (1) low temperature aging of the hydrogel, (2) low temperature crystallization, (3) high water content, (4) presence of fluoride ions in proper amount and (5) pre-treatment of vessels to remove the trace amount of impurities from the surface before crystallization. By controlling these parameters, the nucleation and crystal growth processes can be controlled for the preparation of single crystals of certain dimensions.

306

b

c

Fig. 3 SEM images of single crystals of mordenite (a), and A1PO4-5 (b), (c). 4. CONCLUSIONS Novel routes to synthesis of large crystals of molecular sieves have been developed, which are crystallization in the media of (a) organic solution, (b) weak acid, (c) aqueous solution and (d) fluoride ions. ACKNOWLEDGEMENT This work was supported by National Natural Science Foundation of China and Changchun Center of Applied Chemistry.

307

a

b

c

d

e

f

Fig. 4 SEM images of single crystals of Beta (a), B-ZSM-5 (b), Ti-ZSM-5 (c), ZSM-39 (d), SAPO-5 (e) and AIPO4-11 (f).

308 REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 10. 20. 21. 22. 23. 24. 25. 26. 27. 28. 20. 30. 31.

L.B. Sand, in Proc. of the 5th Intemational Zeolite Conference on Zeolites, 1980, P. 1. D.M. Bibby and M.P. Dale, Nature, 317 (1985), 157. Q.S. Huo, S.H. Feng and R. R. Xu, Acta Chimica Sinica, 48 (1990), 639. S.H. Feng, J.N. Xu, R.R. Xu, G.D. Yang, C. G. Chen and G.P. Li, Chem. J. Chinese Univ., 4 (1988), 9. S.H. Feng, Ph. D Thesis, Jilin University, PRC, 1986. Q.S. Huo, S.H. Feng and R.R. Xu, Zeolites: Facts, Figures, Future, eds, P.A. Jacobs and R.A. van Santen, Elsevier, Amsterdam, 1989, P291. R.H. Jones, J.M. Thomas, R. Xu, Q. Huo, Y. Xu, A.K. Cheetham and D. Bieber, J. Chem. Sot., Chem. Commun., 1170 (1990). R.H. Jones, J.M. Thomas, R. Xu, Q. Huo, .A.K. Cheetham and A.V. Powell, J. Chem. Soc., Chem. Commun., 1266 (1991). Q.S. Huo, PH.D. Thesis, Jilin University, PRC, 1992. J. Yu, J. Chen and R. Xu, Microporous Mater., 5 (1995), 333. J. Yu, J. Chen and R. Xu, Poster Presentation in this Conference. Q.S. Huo and R.R. Xu, in Proc. 9 th International Zeolite Conference (Montreal, Canada), Butterworth-Heinemann, 1992, P. 279. R.H. Jones, J.M. Thomas, R.R. Xu, J.Chen, Q.S. Huo, S.G. Li and Z.G. Ma, J. Solid State Chem., 102 (!993), 204. S.G. Li, C.H. Liu and R.R. Xu, J.Chem. Soc., Chem. Commun., 1645 (1993). J. Chen, Ph.D Thesis, Jilin University, PRC, 1989; R. Xu, J. Chen, S. Feng, Chemistry of Microporous Crystals (Kodansha-Elsevier), 1990, 63. S.H. Feng, R.R. Xu, G. Yang and H. Sun, Chem. J. Chinese Univ. (English edition), 4 (1988) 1. T. Wang, G.Tang, S. Feng, C.Shang and R. Xu, J. Chem. Sot., Chem. Commun., 948 (1989). G.Yang, L.Li, J.Chen and R.Xu, J. Chem. Soc., Chen. Commun., 810 (1989). L.Li, L.Wu, J. Chen and R. Xu, Acta Crystallor., Sect. C, 286 (1991). J.Chen, L.Li, G.Yang and R.Xu. J. Chem. Soc., Chem. Commun., 1217 (1989). Y.Xu, L.L.Koh, L.H.An, S.Qiu and Y.Yue, in Proc. 10th International Zeolite Conference (Garmisch-Partenkirchen), Elsevier, 1994, P. 2253. H.Du and W.Pang, to be published. W.Pang, S. Ueda, and M. Koizumi, in Proc. 7th International Zeolite Confenrence. , Tokyo, Elsevier, 1986, P. 177. S.Qiu, Ph. D Thesis, Jilin University, PRC, 1988. W.Pang, S.Qiu, Q. Kan, Z. Wu, S. Peng, in Proc. 8th International Zeolite Conference, Amsterdam, Elsevier, 1989, P. 281. J.L.Guth, H.Kessler and R.Weg, in Proc. 7th IntemationalZeolite Conference, Tokyo, Elsevier, 1986, P137. S.Qiu, W.Pang and R.Xu, Chem. J. Chinese Univ.(English Edition), 5(1989) 8. D.Zhao, S.Qiu and W.Pang, Zeolites, 13.(1993) 478. D.Zhao, S.Qiu and W.Pang, J. Chem. Soc., Chem. Commun., 1313(1990). S.Qiu, W. Pang, H. Kessler and J.L. Guth, Zeolites, 9 (1989) 440. S.Qiu, W. Tian, W. Pang, T. Sun, D. Jiang, Zeolites, 1-1(1991) 371.

H. Chon, S.-K. lhm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials

Studies in Surface Science and Catalysis,Vol. 105 © 1997ElsevierScience B.V. All rights reserved.

309

S y n t h e s i s a n d c h a r a c t e r i z a t i o n of Z S M - 5 in f l u o r i d e m e d i u m : t h e role of N H 4 ÷ a n d K ÷ c a t i o n s

E. Nigro a, R. Mostowicz b, F. Crea a, F. Testa a, R. Aiello a and J. B.Nagy c aDipartimento di Ingegneria Chimica e dei Materiali, Universit~ della Calabria, 87030 Rende (CS), Italy bIndustrial Chemistry Research Institute, 01-793 Warsav, Poland cUnit~ de R.M.N., Facult6s Universitaires Notre Dame de la Paix, B-5000 Namur, Belgium

The role of NH4 + and K ÷ cations in the synthesis of ZSM-5 using fluoride anions as mineralizing agents was investigated as a function of the amount of aluminium and of the cations (9 < MF < 24 and (0.16 < SiO2/AI(OH)3 _ 1). The potassium containing system is the most effective in the incorporation of aluminium into the zeolite framework while NH4 is the least effective one. The crystallization curves and morphology of the crystals are also strongly influenced by the nature of fluoride salt added in the reaction mixtures.

1. I N T R O D U C T I O N The first synthesis of zeolites with MFI structure obtained in fluoride medium was reported in 1986 [1]. Guth and his co-workers patented different synthesis methods for the preparation of both purely siliceous (silicalite-1) microporous materials and of the MFI-type zeolites, with Si partly substituted by T m elements (T=B, A1, Fe, Ga) [2], by utilizing N I ~ F and HF. The effectiveness of other fluorides (NaF, KF and CsF) in the synthesis of silicalite-1 [3,4], silicalite-2 [5] and zeolite Beta [6] opened new routes for the preparation of zeolites in non-alkaline media. MFI type zeolites with boron [7] and iron [8] incorporated into the framework were successively synthesized using different fluoride salts. These studies clearly demonstrated that the presence of the different alkali cations has a marked influence on the amount of heteroatom included in the structure, rate of crystallization, morphology and crystal size. In this paper we report the synthesis and characterization of [A1]ZSM-5 type zeolite obtained in the presence of various fluoride salts.

310 2. EXPERIMENTAL P A R T The initial batch composition was: 10Si02-xMF-yAl(OH)3-1.25TPABr-330H20 with x=9, 15, 24 and y=0.16, 0.5, 1, with M=NH4 and K. The reactants were mixed in the following order: tetrapropylammonium bromide (TPABr, Fluka), distilled water, MF (M=NH4 and K, Carlo Erba), AI(OH)3 (Pfaltz and Bauer) and fumed SiO2 (Sigma). The syntheses were carried out in Morey-type PTFE-lined 20 cm 3 autoclaves at 170_+2°C for a prefixed time, without stirring, and under autogeneous pressure. At the end of the reaction the products were filtered, washed with distilled water and dried overnight at 105 °C. The nature of the solid phase and the degree of crystallinity were determined by using powder X ray diffraction. For the calculation of the crystallinity, the intensity of the main peak at d= 3.85/~ (or 23.10 ° 20) were compared with the intensity of a reference sample, for each system, purified through ultrasound treatment. The amount of aluminium and alkali cations in the crystals were determined by atomic absorption analysis. The amount of TPA + cations and water trapped into the crystals was obtained using TG analysis. DSC curves were used to evaluate the path of decomposition of the organic molecule. TG and DSC analyses were carried out in an N2 atmosphere with a heating rate of 10°C/min. Morphology and crystal size were determined by SEM. The MAS-NMR spectra of the samples were recorded on either a Bruker MSL 400 or a CXP 200 spectrometer. For 29Si (39.7 MHz) a 6.0 ~s (0=~/2) pulse was used with a repetition time of 6.0 s. For 27A1 (104.3 MHz) a 1.0 ~ts (0=x/12) pulse was used with a repetition time of 0.1 s. The number of accumulations varied between 8.000 and 15.000 to obtain a good signal-to-noise ratio.

3. RESULTS AND DISCUSSION Preliminary experiments showed that in systems where xl ZSM-5 crystallization does not occur. MFI-type zeolite was the only phase obtained when 9y~xGa>AI>Si and for ZTAI AI>Ge>Si. In considering the use of MASNMR in determining

0.3 0.2

~0.1 ""

0

N -0.1 -0.2

-0.3 -0.4

k

127

129 131 133 T-O-T angle / o

Figure 5 I~B Chemical shifts as a function of B-O-B angle in borate sodalites

462 the distributing of dopant ions in a framework the effects of changing bond angles and electrostatic influences may now be deconvoluted. These results show that, for example, replacing Si(OAI)4 by Si(OGa)4 in a particular sodalite Ms[SiTO4]6.X2, with a certain M and X, changes the 298i chemical shift by ~6.5ppm. Of this the bond angle change (typical 60 for the larger gallium replacing aluminium) alters the chemical shift by 3 ppm and the electrostatic contribution to the change is 2 - 4 ppm depending on the bond angle range. At present this analysis takes no account of variations in the T-O distance. It is noteworthy that the Si-O distance in sodalites varies between 1.65 and 1.59A and this too should be correlated with chemical shift. This analysis requires very accurate determinations of Si-O distances and this work is currently in progress.

5. ACKNOWLEDGEMENTS We thank the EPSRC for grants in the support of this work including the provision of MASNMR facilities. We also thank Johnson-Matthey for a CASE studentship for GMJ and Dr M.E.Brenchley for the synthesis of the borate sodalite samples.

6. REFERENCES [1 ] J.Klinowski. Progress in NMR Spectroscopy 16 (1984) 237. G.Engelhardt [2] G.Engelhardt and W.Veeman. J.Chem See. Chem Comm. (1993) 622. [3] S.Hayashi, K.Suzuki,S.Shin,K.Hayamizu and O.Yamamoto. 58 (1985) 52. [4] J.Newsam. J.Phys Chem 91 (1987) 1259. [5] G.Engelhardt, P.Sieger and J.Felsche. Analytica Chimica Acta 283 (1993) 967. [6] P.J.Mead and M.T.Weller Zeolites 15 (1995) 561 [7] M.E.Brenchley and M.T.Weller. Chemistry of Materials 5 (1993) 970 [8] A.C.Larson and R.B.Von Dreele. GSAS Generalised Structure Analysis System MS-H805 Los Alamos NM 1990. [9] S.Ramdas and J.Klinowski. Nature(London) 308 (1984) 521 [10] R.Radeglia and G.Engelhardt. Chem. Phys. Lett. 114 (1985) 28 [11]P.Meaudeau, G.Sapaly, G.Wicker and C.Naccache, Catal. Lett 27 (1994) 143. [12] M.T.Weller, M.E.Brenchley, D.C.Apperley and N.A.Davies. Solid State Nuclear Magnetic Resonance 3 (1994) 103. [13] G.Engelhardt, H.Koller, P.Sieger, W.Depmeier and A.Samoson. Solid State Magnetic Resonance 1 (1992) 127.

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.

463

A new m e t h o d for the N M R - s p e c t r o s c o p i c m e a s u r e m e n t of the deprotonation e n e r g y of surface h y d r o x y l groups in zeolites E. Brunner, J. K~ger, M. Koch, H. Pfeifer, H. SachsenrOder and B. Staudte Universit~t Leipzig, Fakult~it fSr Physik und Geowissenschaften, Abteilung Grenzfl~ichenphysik, Linn6str~e 5, D-04103 Leipzig, Germany A new 1H MAS NMR-spectroscopic method for the determination of the deprotonation energy AEDpof surface hydroxyl groups in zeolites is described. This method is based on the measurement of the induced ~H NMR chemical shift A 8 caused by the interaction between the surface hydroxyl groups and weakly basic probe molecules such as C2C14 or CO. The new ~H MAS NMR-spectroscopic method is compared with the formerly established methods based on the measurement of the induced wavenumber shift Av of the O-H stretching vibration caused by the adsorption of weakly basic probe molecules [1] or on the measurement of the ~H NMR chemical shift 8H in activated samples [2,3].

1. INTRODUCTION Surface hydroxyl groups especially the bridging hydroxyl groups in zeolites can act as catalytically active Bronsted acid sites. The catalytic activity of H-zeolites with respect to Bronsted acid catalyzed reactions such as the cracking of n-paraffines [4] or the disproportionation of ethylbenzene [5] is determined by (i) the concentration, (ii) the accessibility and (iii) the strength of acidity of the bridging hydroxyl groups. 1H MAS NMR spectroscopy allows the measurement of the concentration of the different types of surface hydroxyl groups since the intensity of the corresponding 1H NMR signals is directly proportional to the concentration of resonating nuclei. The accessibility of the different types of surface hydroxyl groups can be studied on samples loaded with suitable probe molecules. The present contribution is devoted to the determination of the strength of acidity of surface hydroxyl groups by 1H MAS NMR spectroscopy. The strength of gas phase acidity of a surface hydroxyl group TO-H is defined as the inverse value of the Gibbs free energy change AGDp of the deprotonation reaction TO-H~

TO-

+ H

+

.

(1)

It could be shown [6] that ~GDp is the sum of the deprotonation energy AEDp(heterolytic dissociation energy) and a constant contribution for surface hydroxyl groups in zeolites. Therefore, AEDp is a convenient measure for the strength of gas phase acidity. However, the spectroscopic measurement of the deprotonation energy AEDpis still a subject of discussion.

464 On the basis of experimental results it was suggested [2] to use the 1H NMR chemical shift 8H of surface hydroxyl groups in evacuated samples as a measure for the strength of acidity. In complete agreement with this suggestion recent ab initio quantum chemical calculations [7] revealed a linear correlation between the deprotonation energy AEDp and the chemical shift ~H for surface hydroxyl groups which are responsible for the Bronsted acidity of catalysts. These hydroxyl groups are bound to atoms (B, AI, Si or P) whose first coordination sphere consists of oxygen atoms only. The slope of AEDp amounts to - (84 ___ 12) kJ mol -~ ppm -~. Therefore, the difference AE = AEop - t~EDpsi°H between the deprotonation energy of the considered surface hydroxyl groups and non-acidic SiOH groups can be calculated according to AE

kJmo1-1

--- 84

6H -

$iOH ~H

(2)

ppm

where I~HSiOH denotes the 1H NMR chemical shift of SiOH groups which amounts to (2.0 + 0.1) ppm. Since it was possible [8] to calculate AEDpsi°H with a relatively high accuracy (AEDsi°H = (1400 __ 25) kJ mol-1), eq. (2) can be used to determine the absolute value of the deprotonation energy of surface hydroxyl groups from their 1H NMR chemical shift [3]. Since chemical shift differences can be measured with an experimental error of + 0.1 ppm it is possible by this method to determine differences in the deprotonation energy of surface hydroxyl groups with an accuracy of _ 8 kJ mol -~. It has however to be mentioned that the application of this method is restricted to free surface hydroxyl groups, i.e., surface hydroxyl groups which are not influenced by a hydrogen bond or an additional electrostatic interaction with the zeolite framework which may be a critical restriction, e.g., if surface hydroxyl groups located in pores of different diameter are compared. The mobility of the protons in bridging hydroxyl groups was recently studied by ~H MAS NMR spectroscopy at elevated temperatures [9,10]. Two remarkable phenomena could be observed for temperatures where the mean residence time xc of the protons on a certain framework oxygen atom approaches (2~tVr) -1. Here, v~ denotes the sample spinning rate. In agreement with the predictions of extended calculations [11] one observes (i) a characteristic broadening of the central line and (ii) a continuous decrease of the relative intensity of the spinning sidebands, i.e., an increase of the relative intensity of the central line. Sarv et al. [10] have observed that xc for protons of bridging hydroxyl groups in H-ZSM-5, H-mordenite and H-Y follows the sequence ~c(H-ZSM-5) < ~c(H-mordenite) < ~c(H-Y) at a given temperature, i.e., the proton mobility seems to be correlated with the strength of acidity. On the other hand, Baba et al. [9] have found a continuous decrease of xc for HZSM-5 with increasing A1 concentration. That means, the proton mobility increases with decreasing average distance between the framework A1 atoms although the strength of acidity of the bridging hydroxyl groups in the silicon-rich H-ZSM-5 is known to be approximately constant. In ref. [10] it is also discussed that the proton mobility may be correlated with the average proton affinity difference between the four oxygen atoms surrounding a framework AI atom. Further investigations are necessary in order to elucidate the interdependence between the proton mobility and the strength of acidity. An IR spectroscopic method for the determination of differences in the deprotonation energy of TO-H groups was developed by Paukshtis and Yurchenko [ 1] which is based on the

465 measurement of the induced wavenumber shift Av = V O H . . . M - V OH , where V O H . . . M denotes the wavenumber of the stretching vibration of these surface hydroxyl groups which form a hydrogen bond with the adsorbed probe molecules M. Provided that lay[ ~ 400 cm -~ (weak hydrogen bonding) the difference AE = A E D p - A E D p S i O n (see above) can be calculated according to the formula AE kJmo1-1

=-

1

l°g

A

[Av[

(3)

]A v sionl

with A = 0.00226 [1]. The deprotonation energy of different types of surface hydroxyl groups in zeolites was determined successfully by this method using CO as the probe molecule M [12,13]. On the other hand, it is known that the formation of hydrogen bonds leads to a considerable broadening of the stretching vibration bands of surface hydroxyl groups. For bridging hydroxyl groups in H-ZSM-5 zeolites Makarova et al. [14] have found a linear correlation between the induced wavenumber shift and the line width of the stretching vibration band. Denoting the full width at half maximum of this band by a one can write [14]

a= ao ( 1 . O.Olo[Avl-1

(4)

where ao denotes the full width at half maximum for the unperturbed bridging hydroxyl groups. An induced wavenumber shift of ca. 300 cm -1 which is caused by the adsorption of CO on bridging hydroxyl groups therefore leads to an increase of the line width by a factor of four. This results in a relatively large experimental error for Av which limits the accuracy of the measurement especially of small differences in the deprotonation energy.

2. EXPERIMENTAL The investigated zeolite H-ZSM-5 was kindly provided by Degussa. The total Si/AI ratio of 14 was determined by chemical analysis which was performed in the laboratory of Dr. H.G. Karge, Fritz Haber Institute of the Max Planck Society, Berlin. It was also proved by chemical analysis that the proton exchange degree is higher than 98%. The framework Si/AI ratio of 17 was determined by 27A1 and 29Si MAS NMR according to the methods described in ref. [15]. The hydrothermal treatment was performed in an apparatus mainly consisting of a horizontally arranged quartz glass tube surrounded by a furnace. About 1.5 gramme of the zeolite was placed in the tube (bed depth: 1 mm) and heated up to 813 K (heating rate: 10 K/h) under a pressure of ca. 10 Pa. Steaming was conducted over a period of 2.5 h with 13 kPa water vapour pressure using nitrogen as carrier gas (flow rate: 40 l/h). Zeolite Na-Y was provided by Chemie AG Bitterfeld-Wolfen and ion exchanged in the Department of Chemistry of the University of Leipzig to an ammonium ion exchange degree of 30 %. The framework Si/AI ratio of 2.5 for this 0.3 NH4Na-Y zeolite was determined by

466 295i MAS NMR spectroscopy. NMR and diffuse reflectance FTIR measurements have been carried out on identical samples which were prepared in the following manner: Glass tubes were filled with the hydrated zeolite (bed depth: ca. 8 mm) and heated up to 673 K with a heating rate of 10 K/h under permanent evacuation. At this temperature the samples were further evacuated for 24 h at a final pressure of 10.2 Pa. Then the samples were cooled to 77 K and loaded with definite amounts of C2C14. After loading the samples were sealed. MAS NMR spectroscopic investigations have been carried out on a Bruker MSL 500 spectrometer at low temperatures (down to 130 K) with a sample spinning rate of 5 kHz. Zeolite 0.3 NHaNa-Y is denoted as 0.3 HNa-Y after the activation since the ammonia is then removed. All NMR chemical shifts are given relative to tetramethylsilane (TMS).

3. RESULTS AND DISCUSSION It is known (see, e.g., ref. [16]) that the interaction between surface hydroxyl groups and hydrogen bond forming probe molecules M causes an induced 1H NMR chemical shift A8 = bia M - 8H, where 8HM denotes the chemical shift of the surface hydroxyl groups influenced by the probe molecules. It could be shown [17] that bH and VOH are linearly correlated at least in limited ranges. The slope of 8H amounts to - 0.0147 ppm/cm -~ for surface hydroxyl groups in zeolites and to - 0.0092 ppm/cm -1 for hydrogen bonded protons in various solids. It can therefore be supposed that the correlation between A8 and a v is given by

/x~

B Imvl

ppm

em -1

~5)

with B-values between 0.0092 and 0.0147. Provided that this is true it should be possible to make use of A8 and A SsioH instead of Av and AvsioH in eq. (3). The induced 1H NMR chemical shift A b caused by the adsorption of hydrogen bond forming probe molecules on surface hydroxyl groups can strongly be influenced by rapid thermal motions and/or exchange processes of the probe molecules (see, e.g., ref. [18]). Therefore, in most cases it is necessary to carry out the corresponding 1H MAS NMR measurements at low temperatures. Fig. 1A shows the 1H MAS NMR spectrum of zeolite 0.3 HNa-Y. The spectrum exhibits an intense signal at 3.9 ppm (line (b)) caused by bridging hydroxyl groups in the large cavities. Two further, relatively weak signals occur at 4.9 ppm (line (c)) and 2.0 ppm (line (a)) which are due to bridging hydroxyl groups in the small cavities and SiOH groups, respectively. The total concentration of bridging hydroxyl groups amounts to ca. 17 OH per unit cell (u.c.). More than 90 % of the bridging hydroxyl groups are placed in the large cavities. Figs. 1B and 1C exhibit the 1H MAS NMR spectra of zeolite 0.3 HNa-Y loaded with 8 molecules C2C14 per unit cell measured at 293 K and 130 K, respectively. At a temperature of 293 K the signal due to bridging hydroxyl groups in the large cavities is broadened and completely shifted from 3.9 ppm to ca. 4.5 ppm despite the fact that the coverage is considerably lower than the concentration of bridging hydroxyl

467 groups in the large cavities. Assuming that the C2C14molecules exchange rapidly between bridging hydroxyl groups in the large cavities one expects only one signal at a mean position given by 6 = 6H +pMA6

(6)

where PM denotes the probability that a bridging hydroxyl group in the large cavities is occupied by a probe molecule (C2C14).PM is given by the ratio NM/Nbwhere NM denotes the concentration of probe molecules (C2C14)adsorbed on bridging hydroxyl groups in the large cavities and Nb is the concentration of bridging hydroxyl groups in the large cavities. Since ArM is less than or equal to the coverage it follows PM ~ 0.5. Using ti = 4.5 ppm and 8H = 3.9 ppm eq. (6) yields A8 ~ 1.2 ppm. For the limiting case of slow exchange, i.e., for sufficiently low temperatures one therefore expects a line at 8H...M ~ 5.1 ppm besides the signal at 3.9 ppm due to unperturbed bridging hydroxyl groups in the large cavities. In fact, the spectrum measured at temperatures below 150 K exhibits two well resolved signals at ~iH = 3.9 ppm and 8H...M = 5.5 ppm (see Fig. 1C). The existence of these two signals indicates that the limiting case of slow exchange is reached for temperatures T ~ 150 K. Therefore, the measurement of A8 should be carried out at temperatures below 150 K in order to suppress the influence of the exchange processes upon the spectra.

Figure 1. 1H MAS NMR spectra of zeolite 0.3 HNa-Y. Unloaded sample measured at 293 K (A) and sample loaded with 8 molecules C2C14 per unit cell measured at 293 K (B) and at 130 K (C). The spectra shown in Figs. 1B and 1C are enlarged by a factor of 2.

_ _ _ _ ~ L-.___A

B

C 6a / ppm

468 Fig. 2 shows the low-temperature ~H MAS NMR spectra of H-ZSM-5 loaded with different amounts of C2C14. The spectrum of the unloaded sample (see Fig. 2A) exhibits the wellknown signals at 2.0 and 4.2 ppm which are caused by SiOH and free bridging hydroxyl groups. Furthermore, a broad signal at ca. 7 ppm occurs which could be assigned [19-21] to bridging hydroxyl groups influenced by an additional electrostatic interaction with the zeolite framework (bridging hydroxyl groups of type 2 [19,20]). It should be mentioned that these species give rise to a broad IR band at ca. 3250 cm -~ [22]. A quantitative analysis yields the following concentrations of the different types of surface hydroxyl groups" (0.9 + 0.2) SiOH per u.c., (3.4 ___0.4) free SiOHA1 per u.c. and (1.8 ___0.4) SiOHA1 of type 2 per u.c. A part of the signal due to free bridging hydroxyl groups is shifted from 8H = 4.2 ppm tO 8HM = 6.1 ppm after loading with 2 C2CI4/u.c. (see Fig. 2B). It could furthermore be shown that C2C14molecules are adsorbed on bridging hydroxyl groups of type 2 for coverages higher than the concentration of free bridging hydroxyl groups. The corresponding complexes give rise to a "shoulder" at ca. 6.2 - 6.4 ppm nearby the above described signal at 6.1 ppm in the 1H MAS NMR spectrum (see Fig. 2C). That means that the bridging hydroxyl groups of type 2 form similar complexes with C2C14as the free bridging hydroxyl groups. The same behaviour could be found by IR and 1H MAS NMR spectroscopy for the interaction between bridging hydroxyl groups of type 2 and other probe molecules [19,20,22]. Furthermore, it is remarkable that the SiOH groups are completely shifted from 2.0 ppm to 2.8 ppm.

Figure 2. ~H MAS NMR spectra of H-ZSM-5 measured at 130 K. Unloaded sample (A), sample loaded with 2 molecules C2C14per unit cell (B) and with 12 molecules C2C14per unit cell (C). The spectra shown in Figs. 2B and 2C are enlarged by a factor of 2.

~ A

C i

i

i

i

,

i

+,

i

,

i

i

i

l

1

0

8

8H ] ppm

Tab. 1 summarizes values for the induced 1H NMR chemical shift A8 and the induced wavenumber shift Av of various free surface hydroxyl groups in zeolites. It could be found that A 6 and Av are linearly correlated as it was supposed above (see eq. (5)). The slope B amounts to 0.01 ppm/cm -1. Therefore, AE can be calculated according to eq. (3) by using A b instead of Av. It can be seen from Tab. 1 that the values calculated for AE by using A8 and Av are in reasonable

469

agreement. The experimental error of this method will be discussed and compared with the method based on the measurement of 8H (see eq. (2)) for the following example. For bridging hydroxyl groups in the large cavities of zeolite 0.3 HNa-Y values of A~i = (1.6 + 0.1) ppm and Av = -(155 _ 15) cm-'were found. Using the induced wavenumber shifts Av and AVsioH eq. (3) yields AE = - (127 _ 35) kJ mol -~. Replacing Av and AvsioH in eq. (3) by the induced 'H NMR chemical shifts A8 and A SSiOH, respectively, it follows AE = - (146 _ 30) kJ mo1-1. Both these values are in reasonable agreement with AE = - (160 + 8) kJ mol -~ which follows from (SH - 8Hsi°H) = (1.9 _ 0.1) ppm by the use of eq. (2).

In summary, it has to be stated that the experimental error of AE calculated from ~, according to eq. (2) is still considerably smaller than the experimental error of AE determined from A ~ (new method) or A v (method of Paukshtis and Yurchenko) by the use of eq. (3) if C2Cl 4 is chosen as the probe molecule M. A reduction of the experimental error, i.e., an enhancement of the sensitivity of the latter methods requires probe molecules M causing higher shifts. The measurement of A ~ (i.e., the new method) should then be preferred instead of the measurement of Av (method of Paukshtis and Yurchenko) since the broadening of the IR bands due to the hydrogen bond formation leads to larger relative experimental errors for t~v than for AS. A promising candidate for such investigations is CO [23]. It could however be shown for CO molecules adsorbed on zeolites that the suppression of their rapid thermal motions and/or exchange processes requires measurement temperatures below 60 - 80 K [24] under magic angle spinning conditions. These temperatures can only be achieved by using a helium cooled MAS NMR device which is now available in our laboratory. Table 1 Values for the induced wavenumber shift A v and the induced ~H NMR chemical shift A~i of free surface hydroxyl groups in zeolites using C2C14 as the probe molecule M. The difference ZXE = AEDp- AEDpsi°H between the deprotonation energy of the corresponding surface hydroxyl groups and SiOH groups in SiO: was determined from Av and from t~8 (instead of ~v) by using eq. (3). Values of ASsioH = (0.75 + 0.07) ppm and AvsioH = - (80 _ 7) cm -~ could be determined for SiOH groups in SiO2. H-ZSM-5 HT denotes a hydrothermally treated H-ZSM-5.

sample

OH group

0.3 HNa-Y

SiOHA1 (HF)

H-ZSM-5

H-ZSM5 HT

•v/cm-'

A ~/ppm

- 155

1.6

SiOH

- 80

0.8

SiOHA1 (free)

- 175

1.9

SiOH

-

75

AE/kJmo1-1 (from A v)

AE/kJmol-' (from A b)

-

127

-

146

-

150

-

177

0.8

A1OH

- 105

1.1

- 52

- 74

SiOHA1 (free)

- 185

1.9

- 161

- 177

470 ACKNOWLEDGEMENT Financial support by "Deutsche Forschungsgemeinschaft" (SFB 294 "Molek~ile in Wechselwirkung mit Grenzfl~ichen") is highly appreciated.

REFERENCES .

2. 3. 4. 5. 6. 7. .

9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24.

E.A. Paukshtis and E.N. Yurchenko, Usp. Khim., 52 (1983) 426. H. Pfeifer, NMR Basic Principles and Progress, Vol.31, Springer, Berlin 1994, p. 31. E. Brunner and H. Pfeifer, Z. Phys. Chemie, 192 (1995) 77. W.O. Haag, Stud. Surf. Sci. Catal., 84 (1994) 1375. H.G. Karge, J. Ladebeck, Z. Sarbak and K. Hatada, Zeolites, 2 (1982) 94. J. Sauer, J. Mol. Catal., 54 (1989) 312. U. Fleischer, W. Kutzelnigg, A. Bleiber and J. Sauer, J. Am. Chem. Soc., 115 (1993) 7833. J. Sauer and J.-R. Hill, Chem. Phys. Lett., 218 (1994) 333. T. Baba, Y. Inoue, H. Shoji, T. Uematsu and Y. Ono, Microporous Materials, 3 (1995) 647. P. Sarv, T. Tuherm, E. Lippmaa, K. Keskinen and A. Root, J. Phys. Chem., 99 (1995) 13763. D. Fenzke, B.C. Gerstein and H. Pfeifer, J. Magn. Reson., 98 (1992) 469. L. Kubelkov~., S. Beran and J.A. Lercher, Zeolites, 9 (1989) 539. M.A. Makarova, A. Garforth, V.L. Zholobenko, J. Dwyer, G.J. Earl and D. Rawlence, Stud. Surf. Sci. Catal., 84 (1994) 365. M.A. Makarova, A.F. Ojo, K. Karim, M. Hunger and J. Dwyer, J. Phys. Chem., 98 (1994) 3619. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, Chichester 1987. J.L. White, L.W. Beck and J.F. Haw, J. Am. Chem. Soc., 114 (1992) 6182. E. Brunner, H.G. Karge and H. Pfeifer, Z. Phys. Chemie, 176 (1992) 173. M. Koch, E. Brunner, D. Fenzke, H. Pfeifer and B. Staudte, Stud. Surf. Sci. Catal., 84 (1994) 709. E. Brunner, K. Beck, M. Koch, H. Pfeifer, B. Staudte and D. Zscherpel, Stud. Surf. Sci. Catal., 84 (1994) 357. E. Brunner, K. Beck, M. Koch, L. Heeribout and H.G. Karge, Microporous Materials, 3 (1995) 395. L.W. Beck, J.L. White and J.F. Haw, J. Am. Chem. Soc., 116 (1994) 9657. V.L. Zholobenko, L.M. Kustov, V.Yu. Borovkov and V.B. Kazansky, Zeolites, 8 (1988) 175. E. Brunner, Stud. Surf. Sci. Catal., 97 (1995) 11. M. Koch, E. Brunner, H. Pfeifer and D. Zscherpel, Chem. Phys. Lett., 228 (1994) 501.


471

170 N M R STUDIES OF S I L I C E O U S F A U J A S I T E

L.M. Bull and A.K. Cheetham Materials Research Laboratory, University of California, Santa Barbara, CA 93106, U.S.A. ABSTRACT By using a combination of magic angle spinning (MAS) and double rotation (DOR) NMR techniques, and also various enrichment methods, different oxygen sites in the asymmetric unit of a zeolite (siliceous faujasite) have been observed for the first time. 1. I N T R O D U C T I O N 170 nuclear magnetic resonance (NMR) is potentially a very powerful technique for studying zeolites because the oxygen atoms in the framework are intimately involved in adsorption and catalytic processes. 170 has a nuclear spin of I=5/2 and so the observed NMR lineshape of the central transition and its relative peak position are determined by the quadrupole coupling constant, Cq, the asymmetry parameter of the electric field gradient tensor, 11, and the chemical shift tensor, all of which are dependent upon the coordination of the oxygen and the type of bonding in which it is participating. However, there are a number of reasons why relatively few 170 NMR studies have been performed on zeolites: 170 is only 0.037% in natural abundance and it is expensive to isotopically enrich, its relative NMR receptivity is poor, and the Cq parameters are believed to be large. In addition, many zeolite structures have large numbers of symmetry inequivalent oxygen sites, and these sites may be locally disordered due to the distribution of aluminum in the framework and the presence of charge compensating cations. The potential of 170 NMR to study silicatesland zeolites 2 has been demonstrated by Oldfield and coworkers using magic angle spinning (MAS) and variable angle spinning techniques. Chemically distinct sites such as Si-O-Si and Si-O-A1 were distinguished in zeolites A and Y, and from empirical correlations and theoretical considerations based on the Townes-Dailey model, 30ldfield and Schramm4 were able to predict the Size of Cq from the Si-O-Si or Si-O-A1 bond angle. More recently, high resolution 170 NMR spectra of condensed silicatesS,6,7,8 have been obtained by the implementation of techniques such as double rotation (DOR), 9 dynamic angle spinning (DAS) 1° and satellite transition spectroscopy (SATRAS), ~ methods that average the second-order effects of the quadrupole interaction. Narrow spectral lines for distinct oxygen sites in the asymmetric unit have been

472 obtained, and Grandinetti et al., 7 again using a Townes-Dailey model for predicting the effective field gradient at the oxygen from the bridging oxygen bond angle, were able to completely assign the spectrum of coesite to the 5 distinct oxygen sites in the asymmetric unit. No correlation with the bond angle at the oxygen was found for the 170 NMR chemical shift tensor, unlike that for 29Si.12 However, quantum mechanical cluster calculations are becoming more accurate for predicting 170 chemical shifts in solids. 13 Using a variety of NMR techniques, and also various enrichment methods, we have studied a prototypic zeolite system, siliceous faujasite (Sil-Y), which has four oxygen sites in the asymmetric unit and no aluminum. Figure 1 shows the structure of this zeolite determined from neutron diffraction, 14highlighting the four distinct oxygen sites. The insert tabulates the bond angles and bond lengths for each oxygen. Site Si-O-Si angle Si-O length (degrees) (/~) O(1)

138.4(2)

1.607(2)

0(2)

149.3(2)

1.597(2)

0(3)

145.8(2)

1.604(2)

0(4)

141.4(2)

1.614(3)

Figure 1. The structure of siliceous zeolite Y (SiI-Y) showing the four oxygen sites in the asymmetric unit determined from neutron diffraction.

473

2. EXPERIMENTAL Sil-Y, prepared according to Hriljac et al., 14 was 170 enriched with both 1702(g ) and H2170. The first method involved evacuating the SiI-Y for 1 hour at room temperature in a quartz tube before adding 1 atmosphere of 1702(g). The tube was then sealed and placed in a furnace and heated to 750oc. The sample was left at this temperature for 24 to 120 hours. The percentage enrichment for the sample was estimated to be --11% from the 170 MAS NMR spectrum of a non-enriched sample. The second method of enrichment was to treat the Sil-Y hydrothermally in H2170 at 95°C for 1 to 3 days. The crystallinity of the ~esulting 170 enriched samples was examined by X-ray diffraction and 29Si MAS NMR (Figure 2). 170 NMR experiments were performed at magnetic field strengths of 11.7 T and 9.4 T using Chemagnetics CMX spectrometers. DOR data were collected using a Chemagnetics probe. Recycle times of at least 10 s were found to be necessary in order to accurately quantify the relative intensities of the 170 signals. Rotor synchronization in the DOR experiments was used to remove the odd spinning sidebands. ~5 X-ray powder diffraction (XRD) data were acquired on a Scintag PAD X using Cu-Ka radiation and a liquid nitrogen cooled germanium solid-state detector.

(a) (b) CPS

1166.41036.890"/.2-

648.0" 518.4-

i , , , , i , , , , i , , ,", i , , ,

-100

ppm

-150

259"i129

oZ

Figure 2. 1702(g) enriched SiI-Y examined by: (a) 29Si MAS NMR (spinning speed = 6kHz, recycle delay 180 s, reference to TMS at 0ppm), and (b) X-ray diffraction.

474

3. R E S U L T S A N D D I S C U S S I O N Figures 2(a) and 2(b) show, respectively, tile 29Si MAS NMR spectrum aud X-ray diffi'action pattern of SiI-Y after being 170 enriched by the gaseous method described previously. Tile si~lgle narrow resonance observed in the 29Si MAS NMR spectrum confirms the high crystallinity and low almninum content of the sample after enrichment. The X-ray diffraction pattern consists of two components, one crystalline, consistent with SiI-Y, and the otller amorphous. The amorphous phase is thought to arise from the sample degrading under tile extreme temperature and pressure conditions used during the gaseous enrichment procedure. This phase is predicted to give a broad 170 NMR spectrum that may not be narrowed under MAS or DOR conditions because of the large distribution in local enviromnenls around the oxygens. It will not therefore be considered further in the analysis of the NMR data. Figure 3 colnpares the 170 MAS and DOR NMR spectra of SiI-Y enriched by 1702(g ), collected in an 11.7 T magnetic field. The MAS NMR spectrmi.l'(Figure 3(a)) shows a broad, fairly featureless pattern that arises from the second order quadrupolar broadening that is not averaged completely by rapid splinting around 54.7 o, the magic angle. The DOR techuique partially averages the second order quadrupolar broadening resulting in the significantly narrower sl~eCtl'Um shown in Figure 3(b). Three resonances and many spinning sidebands are observed, in ratios of approximately I:1:2, in accordance with 4 sites of equal occupancy observed fiom neutron diffraction. 14

b)

*

....

1200

I I00

I 0

I -100

I -200

-

'I'

200

i

100

'"

l

0 ppm

i

-t00

ppm

Figure 3. 170 NMR spectra of SiI-Y enriched with 1702(g) collected at l l.7T using (a) magic angle spinning (rotor speed = 6.5 kHz), (b) double rotation, rotor synchronized with all outer rotor speed of 710 Hz (isotropic lines are indicated by *). Both spectra are refelenced to 1120 at 0 ppm.

475 The observed shift from a quadrupolar nucleus is a combination of the chemical and the quadrupolar shifts. ~6 In order to separate the contributions of the two shifts to the observed shift, experiments need to be performed at two different magnetic field strengths as the quadrupolar shift is field dependent. In order to extract the chemical shifts for each site in the Sil-Y we recorded the DOR spectrum at 11.7 T and 9.4 T. Figure 4 compares the two spectra. It can be seen that the spectrum measured at 11.7 T is shifted to higher frequency than that measured at 9.4 T, in accordance with the quadrupolar shift being inversely dependent on the external magnetic field. The chemical shifts calculated from the spectra collected at the two fields are shown in Table 1, together with the Cq and rl parameters extracted from simulating the anisotropic dimension of a dynamic angle spinning experiment (data not shown here). ~7Again, only three resonances are observed at the lower field strength, with no significant broadening of any of the peaks. We conclude that two oxygen sites in SilY have vel3' similar chemical and quadrupolar shifts. This is not very surprising because all the sites have very similar Cq and 11 parameters, consistent with the small dispersion in the bridging oxygen bond angles. Clearly, the 170 NMR chemical shift is unpredictable, with factors, as yet unknown, contributing to its value. The chemical shifts derived from this work are within the range of those previously observed for silicates. Measurements at 14.09 T are cungntly in progress to obtain more precise values for the chemical shifts in Sil-Y.

(a)

50

(b)

0

ppm

51t

0

ppm

Figulg 4. 170 DOR NMR spectra collected at (a) 11.7T and (b) 9.4 T. Both sets of data are collected with rotor synchronization and are referenced to H20 at 0ppm.

476 Table 1. The observed shifts at 11.7 T and 9.4 T, referenced to H20 at 0ppm, and the extracted chemical shifts, quadrupolar coupling constants, Cq, and asymmetry parameters, rl, for each resonance, t7 Observed shift (ppm)

Chemical

@ 11.7 T

@9.4 T

shift (ppm)

Cq (MHz)

Peak 1

0.7

-18.1

33.5

5.1_+0.1

0.23_+0.1

Peak 2

10.9

-4.7

37.1

4.7-&-0.2

0.105:0.1

Peak 3

15.3

-2.1

44.1

4.9-L-9.2

0.03+0.1

We also investigated the possible effect of different oxygen sites having different rates of 170 exchange by heating Sil-Y at 750oc in 1 atmosphere of 1702(g) for 5 days. The 170 DOR spectrum showed no significant deviations from the spectrum shown in Figure 2, taken from the a sample heated for only 1 day at 750oc. This result confirms that an exchange equilibrium between the 1702(g) and the framework oxygen is reached fairly rapidly at 750°C. H2170 enrichment was also investigated as it is known that O2(g) may not access the small cages in the FAU structure, giving rise to the possibility of site selective enrichment based on steric arguments. Sil-Y enriched by suspending the zeolite in isotopicaUy enriched 170-water and heating to 95°C gave a similar 170 DOR NMR spectrum to that obtained from the gas enriched sample, with no additional resonances or measurable broadening of the peaks. Hydrothermal treatment of the Sil-Y at higher temperatures Was also attempted, but even though the rate and percentage of 170 enrichment increased, severe structural degradation of the sample occurred, to the point where virtually no reflections could be seen in the X-ray diffraction pattern. We have also investigated the effect of adsorbates on the 170 DOR NMR spectrum of Sil-Y to see if different oxygen sites would interact with the molecules in different ways, and thus increase dispersion of the peaks in the spectrum. Water and hexane were introduced separately into the sample, but no differences in the spectra were observed. This can be attributed to the weak interaction of the siliceous host with adsorbates, as previously noted from studies on adsorbed benzene. TM

477

4. C O N C L U S I O N S We believe that all the evidence presented above is consistent with the conclusion that the 170 NMR spectrum of Sil-Y has three resolvable peaks, with each site having similar quadrupolar coupling constants and asymmetry parameters. Two of the oxygen sites in the asymmetric unit have the same chemical shift. Assignment of the spectra using correlations of Cq or rl with Si-O-Si bond angle does not seem to be possible for this sample where the quadrupolar parameters are, within the error of the simulations, identical. This result is consistent with the small dispersion in Si-O-Si bond angles determined with high precision from neutron diffraction. ACKNOWLEDGEMENTS This work was supported by the MRL Program of the National Science Foundation under award No. DMR-9123048.

REFERENCES a N. Janes and E. Oldfield, J. Am. Chem. Soc., 108 (1986) 5743. 2H. Kyung, C. Timken, G.L. Turner, J.-P. Gilson, L.B. Welsh and E. Oldfield, J. Am. Chem. Soc., 108 (1986) 7231. 3C.H. Townes and B.P. Dailey, J. Chem. Phys., 17 (1949) 782. 4 S. Schramm and E. Oldfield, J. Am. Chem. Soc., 106 (1984) 2502. s K.T. MueUer, Y. Wu, B.F. Chmelka, J. Stebbins and A. Pines, J. Am. Chem. Soc., 113 (1991) 32. 6 K.T. Mueller, J.H. Baltisberger, E.W. Wooten and A. Pines, J. Phys. Chem., 96 (1992) 7001. r p.j. Grandinetti, J.H. Baltisberger, I. Farnan, J.F. Stebbins, U. Werner and A. Pines, J. Phys. Chem., 99 (1995) 12341. 8 C. J~iger, R. Dupree, S.C. Kohn and M.G. Mortuza, J. Non-Cryst. Solids, 155 (1993) 95. 9 A. Samoson, E. Lippmaa and A. Pines, Mol. Phys., 65 (1988) 1013. lo K.T. MueUer, B.Q. Sun, G.C. Chingas, J.W. Zwanziger, T. Terao and A. Pines, J. Magn. Reson., 85 (1990) 470. u C. J~iger, J. Magn. Reson., 99 (1992) 353. 12 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. 13 j. Sauer and B. Bussemer, private communication. 14 J.A. Hriljac, M.M. Eddy, A.K. Cheetham, J.A. Donohue and G.J. Ray, J. Solid State Chem., 106 (1993) 66. as A. Samoson and E. Lippmaa, J. Magn. Reson., 84 (1989) 410. 16 B.F. Chmelka and J.W. Zwanziger, NMR: Basic Principles and Progress, B. Bltimich (ed.) Springer-Verlag, Berlin, 33 (1994) 80. 17 L.M. Bull, J. Shore, S. Gann, Y. Lee, R. Dupree, A. Pines and A.K. Cheetham, in

preparation. 18 L.M. Bull, N.J. Henson, A.K. Cheetham, J.M. Newsam, S.J. Heyes., J. Phys. Chem., 97 (1993) 11776.



Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.

479

Deuteron Magnetic Resonance Studies of Ammonia in A g N a Y Zeolites

M. Hartmann* and B. Boddenberg

Lehrstuhl ~ r Physikalische Chemic II, Universitat Dortmund, D-44221 Dortmund, Germany

The adsorption of ammonia is investigated in silver exchanged AgNaY zeolites. Silverincorporation into NaY increases the ammonia adsorption probably due to the formation of silver-diammine-complexes. 2H-NMR spectra show fast isotropic reorientations of the ammonia at room temperature, which transform into a rigid lattice behavior with decreasing temperature. Comparison with Ag(ND3)2+-complexes ion-exchanged into NaY show the same dynamic behavior giving additional evidence for a silverdiammine-complex formation upon adsorption of ammonia.

1. INTRODUCTION Transition metal ions are catalytically active in a variety of chemical reactions [1]. Incorporation of transition metal ions into zeolite cavities or channels may result in catalysts with unique properties. 2H nuclear magnetic resonance (NMR) spectroscopy can be used to investigate the dynamics of adsorbed deuterated molecules as well as their interaction with different adsorption sites. This technique has been successfully used for studies of adsorbed benzene and propene in zeolites [2]. Ammonia may be assumed to interact specifically with the silver sites in zeolites since Ag(ND3)2+-complexes are known in solution, rendering this molecule a well-suited candidate for this study on the locations and the properties of silver ions in Y zeolites.

2. EXPERIMENTAL SECTION Starting from NaY (Union Carbide LZY-52, Si/AI =2.4) silver exchanged zeolites Ag(x)NaY with x = 14, 28, 50 and 100 % were prepared with aqueous solutions of different * Present address" Institute of Chemical Technology I, Universit~it Stuttgart, D-70550

Germany.

Stuttgart,

480 AgNO 3 concentration. The silver contents of the samples were determined by. electron microprobe analysis (EMPA). The samples were dehydrated under vacuum (p < 10-5 hPa) for 18 h at 420 °C and subsequently oxidized for 6 h at the same temperature. The ammonia adsorption isotherms were measured volumetrically at 298 K. Atter completion of the first adsorption isotherm, the ammonia was pumped off and the sample was evacuated at 298 K for 18 h (p < 10-5 hPa). Subsequently a second isotherm was measured. For the 2H-NMR experiments the samples were pretreated as described above and ammonia-d3 (MSD, Montreal, Canada) was adsorbed up to a pressure of 100 hPa. Then the sample was evacuated overnight, sealed under cooling in liquid nitrogen and stored in the dark. The 2H-NMR spectra were recorded at a resonance frequency c00/2rt = 52.72 MHz using a Bruker CXP 100 spectrometer. The spectrometer as well as the measuring procedures applied have been described elsewhere [3]. For comparison an Ag(ND3)2Y zeolite was prepared by exchanging the complex ion Ag(ND3)2 + into the NaY zeolite. Under dry nitrogen ammonia-d3 was introduced into a solution of AgNO 3 in D20. Under stirring the addition of ammonia was performed until the solution became transparent. Now the AgfND3)2 + complex has been formed in solution. Adding a calibrated amount of NaY and additional stirring in the dark formed an Ag(ND3)2Y zeolite with an exchange degree of 55 %. The zeolite was then separated from the solution and the sample was subsequently carefully dehydrated, sealed and stored in the dark.

3. RESULTS Figure 1 shows the adsorption isotherms at 298 K of ammonia in the zeolites NaY, Ag(14)NaY, Ag(28)NaY, Ag(50)NaY and AgY (Ag(100)NaY). These isotherms were obtained for the zeolites activated as described above. The isotherms obtained after 18 h ambient temperature evacuation are only displayed for the zeolites AgY, Ag(28)NaY and Ag(14)NAY. Generally, for each zeolite the pairs of isotherms steeply increase at low pressure and run almost parallel to each other yielding difference amounts Nirr that are collected in Table 1. In comparison to NaY, Nirr is enlarged considerably up to a factor of about two in the case of the most highly silver exchanged zeolite AgY.

Table 1 Irreversible adsorbed amounts of ammonia at 298 K. sample NaY

Nirr/(NH3/u. c. ) 30

Ag(14)NaY

30

Ag(28)NaY Ag(50)NaY AgV

30 35 63

481

[

N/(NH3/u. c.)

120

.

.

.

.

.

.

.

.

.

.

.

~

x

'

"

,oo

X=50 ~

+ ,X

80

60

0

2. adsorption

4O

+

20

NaY

- 0 - Ag(14)NAY 0 0

10

20

30

I 40

I

50

-

-I

60

I

70

80

1

90

100

p/hPa

Figure 1. Ammonia adsorption isotherms at 298 K in silver exchanged Y zeolites Figure 2 shows the 2H-NMR spectra of ammonia in NaY zeolite at selected temperatures between 293 and 80 K. With decreasing temperature the spectra develop from singlets of increasing width into solid state powder patterns of the Pake-Type with Av = 51 kHz prominent edge splitting. T he spectral shape transition range extends from about 200 K to 100 K. The 2H-NMR spectra of (a) AgY loaded with ammonia (Nirr = 63/u.c.) and (b) Ag(ND3)2Y at selected temperatures between 290 and 80 K are compared in Figure 3. In both cases, the spectra develop in the same fashion with decreasing temperature from sing,lets of increasing widths into temperature-independent solid state powder pattern. From the edge sI~litting of Av = 53 kHz the deuteron quadrupolar coupling constant is readily calculated as e"~qQ/h = 71 kHz. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bonding except the C3-axis rotation proceed very slowly. The breakdown of the solid state Pake pattern into temperature dependent Lorentzian-type singlets occurs in both samples in the temperature interval between 167 and 235 K.

482

250 K

293K

1~

1~

~

0

~0

-1~

-1~

150

1IX)

50

150

143K

....

-100

-150

i .... 100

i . . . . . . . . . . . . . . . . . . . 50 0 -50 -100

-1,50

+ ....

..+,5o

125 K

•,+o . . . .

100K

.,,;o . . . .

,; . . . .

- ; o

+...,~

-~

-1~

+

80 K

(P-IPo)/kHz

4 (P-Po)/kHz 1~

-50

167 K

200 K

1~

0

~

0

-50

-1~

-1~

1~

1~

~

0

Figure 2.2H-NMR spectra of ammonia adsorbed in NaY zeolite (Nirr = 30 NH3/u.c. )

-1~

483

(a)

I---25 k Hz

(b)

I----! 25; k Hz

L

.

.

.

.

.

.

.

•

~

_

~

:

. . . .

J _

,

_

290K

290K

222K

222K

167K

162K

.

.

.

___/ 80K

80K

Figure 3.2H-NMR spectra of ammonia in (a) AgY (63 ND3/u.c.) and (b) Ag(ND3)2Y

484 4. DISCUSSION The adsorption of ammonia in NaY zeolites can be increased by ion-exchange of Ag(I) ions. With increasing silver content the overall adsorption increases (Figure 1). The amount of strong adsorbed ammonia molecules Nirr also increases with the degree of exchange. In NaY 30 NH 3 molecules can not be removed from the sample due to their adsorption on strong adsorption sites. Due to absence of strong Lewis or Broensted acid sites only the sodium cations in the supercages are able to adsorb ammonia strongly [4,5]. The sodium cations are located on site SII and adsorb ammonia with an adsorption enthalpy of about 44 kJ/mol, which is close to typical chemisorption enthalpies [6,7,8]. With increasing degree of silver exchange the amount of strongly adsorbed ammonia molecules also increases, showing that silver cations are able to adsorb more than one ammonia molecule. It is well known that Ag + form silverdiammine-complexes Ag(ND3)2 + in solution and solids [9]. It was shown previously that Cu 2+ and Ni 2+ are also able to form amine complexes in solution [10] and zeolites [11,12]. Therefore, it is very likely that Ag + also forms silverdiamminecomplexes in zeolites [11], but no experimental evidence could be presented so far. The increase in ammonia adsorption in Ag(x)NaY zeolites can very well be explained by the formation of Ag(NH3)2 + complexes in the supercages. At a low level of silver-exchange, the increase is very small, showing that most of the silver-cations are no__!tlocated in the supercage. This is in excellent agreement with X-ray and xenon adsorption data, which show a preference of Ag+ for the SI position in the double six ring [ 13]. It can be concluded from our data that not all silver ions are accessible for ammonia and only some silver ions migrate into accessible sites. Therefore the irreversible adsorbed amount of ammonia Ni~ does not increase linearly with the degree of silver exchange. In AgY 63 ammonia molecules were found to be strongly adsorbed corresponding to about 32 Ag + cations on the SII positions in the supercages. X-ray crystallographic data assign between 25 and 30 silver cations to sites in the supercage [14]. This was also confirmed by xenon adsorption and xenon NMR data [ 15]. The formation of silverdiammine-complexes in zeolites can be confirmed by our NMR data. Investigation of the ion-exchange with silverdiammine-complexes indicate that these complexes can only replace all sodium cations in the supercage, but are not able to enter the 13cage [16]. Therefore, the situation in AgfND3)2Y should be comparable to the AgY after adsorption of 64 ammonia molecules. In fact, we observed that the spectra looked almost identical at all temperatures. In NaY the spectral shape develops very slowly in a temperature interval of about 100 K. This transformation interval shortens with increasing degree of silver exchange down to 68 K for AgY and Ag(ND3)2Y. In all samples a low temperature powder pattern is observed, which is characteristic for an axially symmetric electric field gradient (EFG) tensor being operative. The appearance of this rigid pattern indicates that in comparison with the characteristic NMR time XNMR all motions of the N-D bond except the C3-axis rotation proceeds very slowly [17]. The effective deuterium quadrupole coupling constant (DQCC) is 69 kHz for NaY and 71 kHz for AgY. Single crystal 2H-NMR measurements of the [Ag(ND3)2]Ag(NO2)2-complex show a DQCC of 71.6 + 0.5 MHz at room temperature [9]. Therefore, it is most likely that at least at low temperatures AgfND3)2+-complexes are also present in zeolites.

485 With increasing temperature some molecular motion runs the spectrum shapes from the slow into the fast oriental exchange limits measured on the time scale XNMR. At present it is still unclear whether this motion involves only the ammonia molecules or the Ag(ND3)2 + complex.

5. CONCLUSIONS The exchange of Ag + ions into NaY zeolites leads to an increase in ammonia adsorption most likely due to the formation of Ag(NH3)2+-complexes in the supercage of zeolite Y. The 2H-NMR-results show that the dynamic properties of the silverdiammin-complexes formed by ion-exchange or adsorption are almost identical. They show a fast isotropic reorientation at room temperature and a rigid lattice behavior at low temperature.

ACKNOWLEDGMENTS

Support of this research is gratefully acknowledged by "Fonds der Chemischen Industrie".

REFERENCES

[ 1] I. E. Maxwell, Adv. Catal. 31 (1982) 1. [ 2] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 488. [ 3] B. Boddenberg and R. Burmeister, Zeolites 8 (1988) 480. [ 4] V. Kanazirev and N. Borisova, Zeolites 2 (1982) 23. [ 5] V. P. Shiralkar and S. B. Kulkami, J. Colloid and Interface Sci. 1.08 (1985) 1. [ 6] R. M. Barrer and R. M. Gibbons, Trans. Farad. Soc. 59 (1963) 2569. [ 7] K. Morishige, S. Kittaka and S. Ihara, J. Chem. Soc. Faraday Trans. 1 81 (1985) 2525. [ 8] V. B. Shiralkar and S. B. Kulkarni, J. Colloid. Interface. Sci. 109 (1986) 115. [ 9] H. M. Maurer and A. Weiss, J. Chem. Phys. 69 (1978) 4046. [ 10] A. F. HoUemann and N. Wiberg, Lehrbuch der Anorg. Chemic, Walter de Gryter: Berlin, 1985. [11] B. Coughlan and J. J. McEntee, Proc. A. Ir. Acad. 76B (1975) 473. [12] A. Gedeon, J. L. Bonardet and J. Fraissard, J. Chem. Soc. Faraday Trans. 86 (1990) 413. [ 13] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 11. [ 14] L. R. Gellens, W. J. Mortier and J. B. Uytterhoeven, Zeolites 1 (1981) 85. [ 15] R. Grol3e, J. Watermann, A. Gedeon, J. Fraissard and B. Boddenberg, Zeolites 12 (1992) 909. [ 16] P. Fletscher and R. P. Townsend, J. Chromat. 201 (1980) 93. [ 17] S. W. Rabideau and P. Waldstein, J. Chem. Phys. 46 (1966) 4600.



487

Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.

Spectroscopic studies o f

170 and 180 labelled

Z S M - 5 zeolites

F. Bauer a, H. Ernst a, E. Geidel b, Ch. Peuker c and W. Pilz ° University of Leipzig, Permoserstr. 15, 04303 Leipzig, Germany b University of Hamburg, Bundesstr. 45, 20146 Hamburg, Germany Humboldt University Berlin, Rudower Chaussee 6, Geb. 19.5, 12489 Berlin, Germany Using mild hydrothermal conditions zeolite ZSM-5 was labelled with 170 and 180 and studied by multi-nuclear MAS NMR, Raman and IR spectroscopy. The 170 NMR experiments gave a quadrupole coupling constant of about 5.5 MHz. In the case of 180 substitution frequency shifts of Raman and IR bands up to 50 crn"1, in good agreement with model calculations, were found. The IR spectrum of coke deposits on ~80-labelled HZSM-5 showed no isotope shitS. 1. INTRODUCTION In addition to the well established characterisation of zeolites by 1H, 27A1 and

29Si MAS

NMR the labelling of zeolite ZSM-5 with the isotope 170 allows NMR studies with an framework element which is otherwise not accessible for NMR investigations. Shiits of Raman and IR bands due to 180 isotope substitution may be used to characterise the zeolite framework and facilitate the assignment of observed bands to the normal modes. Furthermore, detailed information on ZSM-5 framework may be obtained by the comparison of the experimental 180 band shitts with frequency shifts calculated for various framework cluster models. BaUmoos [ 1] published fundamental investigations of 180 isotope exchange and dealumination of zeolite ZSM-5 using D2~80. Exchange of 180-labelled carbon dioxide and zeolite A was studied by Takaishi et al. [2]. The introduction of metal cations lowered the temperature required for isotope exchange between gas phase oxygen and framework oxygen of zeolite ZSM-5 [3]. But, only few infrared studies have been published of isotope substituted zeolites [4,5]. Significant shifts of the IR bands up to 51 cm~ were found in the vibrational spectra of the two SiO2 phases, tridymite [6] and quartz [7]. In this work, labelled zeolites were also used to study the formation of carbonaceous materials deposited on acid zeolites during hydrocarbon reactions. The intensity of the socalled 'coke' IR band around 1600 cm1, which is usually attributed to carbon-carbon stretching in hydrogen deficient ring structures, proved to be a suitable measure for the coke content [8]. Bands around 1360

-

1390 cm~ and 1440 - 1490 cm~ may be assigned to CH bending modes

488 of paraffinic species. However, Eischens [9] observed after acetylene exposure to A1203 or Pt/AI203 at 523 K that the bands at 1580 and 1460 cm1 grow at the same rate, as would be expected for a pair of bands from a single species. Both of the bands showed a shift of about 20 cm 1 when oxygen in the alumina was replaced with 1SO [10]. This shift has been taken as proof that the origin of these bands are vibrations with significant oxygen displacements, such as for carboxylate. The existence of carboxylate or acetate-like complexes was also assumed on Pt/AI203 [11] and on dealuminated HY [12]. Therefore, studies with oxygen-labelled zeolites may be also helpful for the elucidation of the surface-bounding of coke residues on zeolites. 2. EXPERIMENTAL SECTION A commercial NaZSM-5 zeolite (Chemiewerke Bad K6stritz GmbH, Germany), with a Si/AI ratio of 19 (measured by 29Si MAS M R

and X-ray fluorescence analysis), was ion

exchanged three times with aqueous solution of NH4NO3 (0.1 N)and heated in air for 12 hours at temperatures up to 823 K in order to yield the H-form. Both NaZSM-5 and HZSM-5 samples were treated with vaporized H2170 (24 % 170, Isotec, USA) and H21sO (98 % 180, Chemotrade, Germany) at 723 K for one hour. Changes in the Si/AI ratio resulting from hydrothermal treatment were checked by

29Si MAS NMR. For coking studies, n-hexene was fed at

693 K on HZSM-5 yielding 4.1 wt.% coke (H/C ratio of 1.8). IR measurements were performed on a IFS 66 spectrometer (Bruker). Samples were pressed to pellets with KBr or polyethylene to obtain mid-IR transmission spectra. The DRIFT measurements were carried out on the same spectrometer, equipped with a Nz cooled MCT detector, using a Praying Mantis DRIFT attachment (Harrick) connected with a heated vacuum cell (Harrick). The samples were measured at temperatures up to 873 K under a vacuum better than 10-5 mbar. Raman spectra were taken using a DILOR XY instrument with a microscope. The 514.5 nm line of an Ar + laser (Carl Zeiss, Jena) with a power of 50 mW was used. Only NaZSM-5 samples could be measured because of the strong background of all protonated zeolites. The IR and Raman bands were measured with a resolution of 2 cm1. The calculation of normal modes was carried out by a procedure using the method of normal coordinate analysis and is described elsewhere [13]. 1H, 170, 27A1 and Z9Si MAS NMR spectra of hydrated samples were obtained at 7.0 and 11.7 T on a Bruker MSL 500 spectrometer. For the quantitative detection of Bronsted, extraframe-work and silanol species by ~H MAS NMR activated and sealed samples were used. The estimation of the Si/AI ratio and the determination of the framework and extra-framework species as well as the 170 NMR experiments were done with hydrated samples.

489 3. RESULTS AND DISCUSSION Especially the interaction of water with zeolites may affects catalytic and structural properties. As demonstrated by von Ballmoos [1] in ~SO exchange studies with zeolite ZSM-5 and water at temperatures as low as 368 K the framework of zeolite ZSM-5 is considerably less inert than usually assumed. Under steaming conditions at 873 K nearly complete exchange was found within one hour which shows the reactivity of the Si-O-Si and the Si-O-AI bridging oxygen in a temperature range typical for hydrocarbon processing. The temporary cleavage of T-O-T bonds may bring about both a widening of the zeolite windows and the generation of reactive hydroxyl groups. Takaishi and Endoh [2, 14] showed with C~sO2 that the T-O-T linkage in zeolite A was not so easily broken as in X- and Y, type zeolites. AIPO-5 was far less reactive than the aluminosilicates. All zeolite samples, irrespective of their composition, contained few, extremely reactive site for oxygen exchange. Such sites may be ascribed to amorphous (colloidal) impurities [2] and/or defects within the silicate framework [ 15]. For the study of defects in zeolites, combined use of 1SO exchange with infrared and MAS NMR spectroscopy is essential. 3.1. MAS NMR spectroscopy

During the hydrothermal oxygen isotope exchange of HZSM-5 dealumination of the framework occurred which is indicated in the ~H, 27A1, 29Si MAS NMR, and IR spectra. The 27Al MAS NMR spectrum of the hydrated sample gives hints to non-framework aluminium

A

Fig. 1: 170 MAS NMR spectra of 170-labelled NH4ZSM-5 (A) and HZSM-5 (B). (.) Spinning side bands

. . . . . . . . . . . . . . . !00

. .....

. . . . . . . . . . . . . . . . . . . . . . . . . 60

20

0

-20

-60

, .......

.,., -100

8 (ppm) species due to peaks at 33 ppm and 0 ppm. The ~H N~_S ~

spectrum of the activated

sample shows a line at 1.8 ppm and a shoulder at 2.5 ppm due to silanol and non-framework Al, respectively [ 16]. The Si/AI ratio of the HZSM-5 sample increased by water treatment at

490 800 K from 19 to 65, whereas no dealumination of NaZSM-5 was observed during the hydrothermal oxygen exchange. The results of the 170 NMR experiments (Fig. 1) give, in agreement with Timken et al. [ 17] and Yang et al. [18], a quadrupole coupling constant of about 5.5 MHz and an electric field gradient tensor asymmetry parameter of about zero. The 170 MAS NMR spectra of the zeolites NaZSM-5 and NH4ZSM-5 are identical. 3.2. Infrared and Raman spectroscopy IR and Raman frequency shifts caused by oxygen isotope substitution were detected in the spectra of NaZSM-5 and HZSM-5 for framework and hydroxyl vibrations as well as for the combination tones [ 19]. The largest shifts of about 50 cm"~ were found for the most intense IR bands at about 1200 cm~ (Table 1), which are attributed to asymmetric T-O-T stretching Table 1" Experimental 180 shifts oflR and R a m a n framework bands of NaZSM-5. Sample

Wavenumber [cm"1]

NaZSM-5

1222 1091 824

797

795

548

453

381

380

360

292

180-NaZSM.5

1166 1064 817

789

790

537

444

364

370

350

285

8

5

11

9

17

10

10

7

isotope shift

56

27

7

modes, and for the strongest Raman band at 381 cm"~ (Fig. 2), which is assigned to a symmetric bending vibration whereby the oxygen atoms move along the bisecting line of the T-O-T angle. This finding reflects the significant participation of oxygen atoms in these vibrational

Fig. 2: Raman spectra of NaZSM-5 and 180-labelled NaZSM-5

o

L)

.Os l

.

,

. . . ' -. ~ ~ v ¢ ' ,

-NaZSM-5 |

1000

500

W a v e n u m b e r (cm -1)

491 modes. Only slight shifts were observed between 825 and 785

cm"l (Fig.

3). This can be

understood by assuming a nearly symmetric T-O-T stretching mode with small displacement of oxygen atoms.

=~

e:i

i1

.e-

I

,," ,,,i,,

180-ZSM.5

~\ I~,. ~,O'ZSM'5

0 ¢..)

ms._,

.40 r,,n '~

Fig. 3" IR spectra of hydrated HZSM-5 samples

,,'.,,K \",

.2-

_--'~/'

"-'7,

1300

i.i" ,

\ ',.

,

.--'-~_

,

1 2 0 0 1100 1000

900

800

_4A~'_~" 700

600

600

'i~l

1

400

Wavenumber (cm -1) In addition, since dealuminated HZSM-5 samples show a slight shift to higher wavenumbers [20], this shill by dealumination have to be added to the observed isotope shift compared with the untreated sample. The frequency of acid hydroxyl groups (Fig. 4) were shifted from 3596 to 3587 cm"1 while no 180 shift was found in terminal SiOH groups (3735 cm'l). Indicated by the appearance of the band at 3650 cm"~ non-framework AI species were formed during the hydrothermal

,l Fig. 4 OH bands in the DRIFT spectra.

O O

£,

HZSM-5

.1

3600

37"50

3T'O0

3650

36'00

3550

W a v e n u m b e r (cm -1) treatment. Similarly to the terminal SiOH groups, no 180 substitution was detected for the OH groups bound to these aluminium species. These findings may be explained by an easy exchange of both hydroxyls with ambient water even at room temperature. This view is

492 supported by the results, due to von Ballmoos [1] and Takaishi et al. [2], that amorphous impurities and modified silanol groups are extremely reactive sites for the exchange of oxygen isotopes. 3.3. Calculations

Calculations of vibrational frequencies by normal mode analyses for framework cluster models with different oxygen isotopes were carried out. Lorentzian lineshapes with a halfwidth of 50 cm 1 and without intensity weighting are assumed in the density of states for each calculated normal mode. The calculated density of states for the pentasil unit Si13034 with all 160, 170, and 180 atoms are shown in Fig. 5. Calculated frequencies above 1250 cm 1 and near

//~O-ZS.M-5 sO.ZSM.5

15-

=

• w,,,l r,¢2

= o

Fig. 5" Calculated density of vibrational states of the pentasil unit with 160, 170, and 180 atoms.

105-

o

c,.)

O-

14100

12100

lOlO0

800

600

W a v e n u m b e r (cm -1) 900 cm 1, which arise from model artifacts due to terminal oxygen atoms [13], have been omitted. Frequency shifts 180 vs. 160 up to 50 cm"1 were obtained for the asymmetric Si-O-Si stretching modes near 1100 cm1. This seems to be caused by the high degree of oxygen displacement in these vibrational forms in the high frequency region. The lowest calculated shift was found in the region near 800 cm1, which was also shown experimentally. Displacements of oxygen atoms are found to be low during the calculated normal modes in these region. The comparison of Fig. 3 and Fig. 5 confirms that calculated frequency shit, s for the pentasil unit are in good agreement with the experimental ones in nearly all regions of the framework spectrum. Therefore, model calculations with ~So substitution may be used to test the suitability of the force field under study, but the problem of lack of experimental data to adjust force constants for zeolite lattices can not be overcome in this way.

493

3.4. Coking studies For the elucidation of the bounding of coke residues on the surface of zeolites, MAS NMR and IR studies with oxygen isotope substituted zeolites at low coke content may be very helpful. The IR spectra of carbonaceous deposits formed during n-hexene conversion at 693 K on HZSM-5 and on the ~sO-labelled sample are shown in Fig. 6, and they are typical for coke

Fig. 6: IR spectra of coke formed during n-hexene conversion on HZSM-5 at 693 K.

HZSM-5 .16

d.) .1

o

r./3

KY>RbY>CsY. The same effect was found for the ND stretching band of PY-dl. The results are summarized in Tab. 1. Table 1 Experimental wavenumbers of characteristic IR bands of the probe molecules in the gas phase and adsorbed in alkali metal Y zeolites (the dominant band of NMP is marked bold) Pyrrole I PY NMP I v(CH) [cm "1] I v(NH) [cm "l] I v(ND) [cm "11 v(NH) I v(ND) IR

[cmll

2496 2438 2442 2451 2464

3358 3282 3292 3297 3311

ivapour

NaY KY RbY CsY arb. units

NaY+PY

i

x,,.. I

300

I

I

200 100 wavenumber (cm"1) Figure 3. Far-IR spectra of NaY and KY and of PY adsorbed in NaY and KY.

2754, 2771, 2780 2768, 2800, 2812 2762, 2787, 2806 - ,2783, 2805 - ,2780, 2804

3532 3397 3327 3327 3322

2621 2538 2490 2490 2485

The fact that the Na + cation with its high polarizing power yields the largest shift indicates a direct interaction of the lone pair electron of the strong basic PY with the cations. For comparison the NH(D) frequencies measured for pyrrole under the same experimental conditions are also given in Tab. 1. In this case the sequence of the observed shifts is inversed. Thus an interaction of the NH group of pyrrole with the framework oxygens of the zeolite under formation of hydrogen bonds is supposed. This is in agreement with the results obtained by Barthomeuf [13] from which pyrrole was concluded to be a wellsuited probe to investigate sites of different basicity. Also in the far-IR spectra changes were detected upon adsorption of the probe molecules. As an example far-IR spectra of PY adsorbed in NaY and KY in comparison with the spectra of the pure zeolites are shown in Fig. 3. On the one hand no additional band for the interaction of probe molecules with the cations could be

580 observed. Only a small additional shoulder at 65 c m -1 o c c u r s in some adsorbate spectra. Because of an absorption at the same position in the far-IR spectra of the free probe molecules - assigned to the ring puckering m o t i o n - this shoulder should not be interpreted as an indication for host-guest interaction. On the other hand the comparison between the pure zeolite spectra and adsorbate spectra shows broadenings and shifts of nearly all bands. Usually the absorptions below 250 cm in the far-IR spectra of alkah metal zeohtes are assigned to vibrations of cations at specific sites [ 14]. However, no correlations between cation sites and vibrational frequencies were found in theoretical investigations where the far-IR bands were interpreted as simultaneous motions of cations in all sites coupled with framework vibrations [15, 16t. The spectra in Fi.~. 3 support the latter assignment. Especially the shoulder at 159 cm- (NAY) and 104 c m ( K Y ) - assigned to cations at SI position following the "site concept"- are remarkably changed during adsorption. This is in contradiction to the fact that cations at site I are not accessible by the PY molecule. The diameter of PY is about 4 A while the free diameter of the 6-ring window is only 2.8 A. Hence the concept of Ozin [14] assigning each band in the far-IR spectra of zeolites to the motion of a cation on a distinct site seems to be an oversimplification. The results of MC and MD calculations can be summarized as follows: In all MC calculations sites of minimal interaction energy are located near the 12-ring window of the zeolites. The minimum energy found was -73,1 kcal/mol. For NaY a strong orientation of the nitrogen atom to the cation at site II was observed for PY as well as for NMP. The conformations with the lowest interaction energies were determined at distances between sodium at site II and the nitrogen atoms of the guest molecules of about 3.2 A for NMP and of about 2.9 A for PY. The orientation of NMP in NaY at the low energy sorption site is illustrated in Fig. 4. •

-1

.

.

.

.

O D

\

"

Figure 4. Low energy sorption site of NMP in a NaY lattice section• The sodium ions are represented by large gray spheres, the distance between the nitrogen at the nearest cation is indicated by a dotted line.

581 MD simulations confirm the strong interaction between the probe molecules and the sodium cations at site II. It was found that the probe molecules preferentially slide along the cage surface. This is illustrated in Fig. 5 for NMP in NaY. At the chosen temperature of 300 K the sorbate molecules are confined to regions close to the adsorption site calculated by MC.

j1-~

f\

•

•

Figure 5. Single molecule trajectory of NMP in NaY. Our results demonstrate that pyrrolidine derivatives are well-suited probes for the investigation of host-guest interactions especially due to their sensitive Bohlmann band. For both PY and NMP adsorbed in HY and in alkali metal Y zeolites two different types of interaction can be discriminated. The good agreement between experiment and theory in the case of NaY encourages the application of computer simulation techniques to study the sorptive behaviour of the guest molecules. 5. ACKNOWLEDGEMENT The financial support by the Deutsche Forschungsgemeinschaft (Ge 783/1-1) and by the Graduiertenf6rderung der Universit~it Hamburg is gratefully acknowledged. We thank Dr. F. Bauer (University of Leipzig) for preparing the deuterated NMP derivatives.

582 REFERENCES

1. 2. 3.

.

.

6. 7.

8. 9. 10. 11. 12. 13.

14. 15. 16.

E.B. Wilson Jr., J.C. Decius and P.C. Cross, Molecular Vibrations, McGraw-Hill Book Company, Inc., New York (1955). R.N. Jones, Computer Programs for Infrared Spectrophotometry- Normal Coordinate Analysis - N.R.C.C. Bulletin No. 15, Canada (1976). G. Pfafferott, H. Oberhammer, J.E. Boggs and W. Caminati, J. Am. Chem. Soc., 107 (1985) 2305, G. Pfafferott, H. Oberhammer and J.E. Boggs, J. Am. Chem. Soc., 107 (1985) 2309. F. Billes and E. Geidel, Proc. 10th Intern. Conf. on Fourier Transf. Spectrosc., Budapest (1995) A9.3. Catalysis Version 2.3.6, Biosym Technologies, San Diego (1993). K.-P. Schr6der, J. Sauer, M. Leslie, C.R.A. Catlow and J.M. Thomas, Chem. Phys. Letters, 188 (1992) 320. J.R. Maple, U. Dinur, A.T. Hagler, Proc. Nat. Acad. Sci. USA, 85 (1988) 5350, J.R. Maple, T.S. Thatcher, U. Dinur, A.T. Hagler, Chemical Design Automation News, 5(9) (1990) 5. J.-R. Hill and J. Sauer, J. Phys. Chem., 99 (1995) 9536. F. Bohlmann, Chem. Ber., 91 (1958) 2157. D.C. McKean, Chem. Soc. Rev., 7 (1978) 399. B. Hunger and M. v.Szombathely, Stud. Surf. Sci. Catal., 84 (1994) 669. M. Ackermann, J. Nimz, M. Kudra and E. Geidel, Proc. 5th German Workshop on Zeolites, Leipzig (1993) PC9. D. Barthomeuf, Spectroscopic Investigations of Zeolite Properties, in: E.G. Derouane et al. (eds.), Zeolite Microporous Solids: Sythesis, Structure, and Reactivity, Kluwer Acad. Publ., Netherlands (1992). M.D. Baker, G.A. Ozin and J. Godber, Catal. Rev.-Sci. Eng., 27 (1989) 591, J. Godber, M.D. Baker and G.A. Ozin, J. Phys. Chem., 93 (1989) 1409. K.S. Smirnov, M. Le Maire, C. Br6mard and D. Bougeard, Chem. Phys., 179 (1994) 445. K. Krause, E. Geidel, J. Kindler, H. F6rster and H. B6hlig, J. Chem. Soc., Chem. Comm., (1995) 2481-82.


583

Preparation and Characterisation of R u - E x c h a n g e d N a Y Zeolite: A n Infrared Study of C O A d s o r p t i o n at L o w T e m p e r a t u r e s S. Wrabetz, U. Guntow, R. Schl6gl and H. G. Karge Fritz Haber Institute of the Max Planck Society, Faradayweg 4-6, D-14195 Berlin, Germany

RuNaY zeolite was prepared by ion exchange of NaY with an aqueous solution of [Ru(NH3)6]C13. The resulting complex was decomposed and reduced. The IR spectra of CO adsorbed at 110 K on the highly dispersed Ru ° clusters inside the supercages (1-1.5 nm) and on the sintered clusters at the external surface (5.5, 10-15 and 30 nm) display a broad band at ~ 2040 cm-x and a low frequency shoulder at ~2000 cm~. The former corresponds to linearly bonded CO species and the latter to CO adsorbed on the Ru atoms with low coordination numbers. The autoreduced sample produced an additional shoulder at 2098 cm1 which is assigned to the CO species weakly o-bonded to the residual Ru x+ ions in the zeolite or to CO adsorbed on single Ru atoms perturbed by surrounding oxygen atoms. The band width was found to be sensitive to reduction conditions and hence particle sizes. 1. INTRODUCTION Ru-containing NaY zeolite is an active and temporally stable catalyst for the synthesis of ammonia and for CO hydrogenation [1-3]. Samples of [Ru(NH3)6]NaY were prepared by ion exchange of NaY with an aqueous solution of [Ru(NH3)6]C13. In our efforts to study the structure-activity relationship of Ru clusters in catalysis we apply low temperature adsorption of CO as a molecular probe monitored by IR and complement the data by techniques such as TP1L TEM and XRD. The effect of Ru particle size on the spectrum of CO adsorbed at RT on alumina-supported Ru has recently been discussed by Dana Betta [4]. For particles 90 A in diameter only a single band at 2028 cm -1 was observed, while for particles of 60 A in diameter and smaller ones, three bands in the vicinity of 2140, 2080 and 2040 cm1 were noted. The exact positions of the three bands changed with decreasing particle size. The low-frequency band was assigned to CO adsorbed on low-index planes of Ru, while the two higher-frequency bands were associated with CO adsorbed on low-coordinated edge and comer metal atoms. These band assi~ments have been questioned, however, by Brown and Gonzalez [5]. Based upon their own studies performed with silica-supported Ru, they noted that CO adsorption on a reduced sample produced a strong band at 2030 cm1 and weak bands at 2150 and 2080 cm1, whereas CO adsorbed on an oxidized sample produced a strong band at 2080 cm1 and bands of medium intensity at 2135 and 2030 cm1. The low-frequency band was assigned to CO adsorbed as Ru-CO. The high- and medium-frequency bands were assigned to CO adsorbed on a surface oxide and CO adsorbed on a Ru atom perturbed by a surrounding oxygen atom, respectively. Miessner [6], in a study of CO adsorption on RuNaY at temperatures at or above room temperature, observed multiple CO adsorption on Ru metal. Gelin and Yates, Jr. [7] have shown that during adsorption below 300 K, a linear CO species may extensively form and

584 that this adsorption process is accompanied by a stoichiometric conversion of preexisting bridged CO species to linear CO species. In this work, the influence of preparation and activation conditions on size, location and micromorphology of the Ru clusters is investigated. The aim of this paper is to selectively influence the linearly bonded CO on Ru metal and then to compare our IR spectra obtained from differently reduced RuNaY with Ru/SiO: and single crystals from the literatur [8, 9]. 2. EXPERIMENTAL NaY was obtained from DEGUSSA, KM-390. [Ru(NH3)6]NaY samples were prepared by ion exchange of NaY with an aqueous solution of [Ru(NH3)6]C13 (HERAEUS) at 298 K (charge 1) and 333 K (charge 2) [3] for lh. Alter the exchange reaction, the sodium and ruthenium contents were determined by AAS and UV-VIS. Some characteristics of the samples are shown in Table 1. Table 1 Characteristics of samples used Sample Degree of Exchange AAS [%] UV-VIS [%1 theoretical [%] NaY charge 1 49 50 51 charge 2 47 50 51 * refined cell parameter of Y-zeolites (Fd3m)by XRD

colour

a* [A]

white pink violet

24.651 (__ 0.003) 24.634(_+0.002) 24.672 (_+ 0.002)

The samples used for the IR measurements were pressed into self-supporting wafers (57mg/cm2) and transferred to a low-temperature IR cell connected to a contamination-free highvacuum apparatus. The low-temperature IR cell was described by Karge et al. [ 10]. The samples were reduced with the same heating rate at 5 K/rain as shown in chart 1. The IR spectra were recorded using a Perkin Elmer 580B spectrometer. The spectral resolution was 2.3 c m "1. CO ( purity 99.997 %) and 02 ( purity 99.998 %) were purchased from Messer Griesheim, Germany and used without further purification. Hydrogen (99.999 %, Linde, Germany) was purified using an Oxisorb and a Hydrosorb trap from Messer Griesheim~ Chart 1 Summary of reduction conditions Autoreduction Ru(NHa)6NaY Dehydrated, deammoniated (723 K or 823 K, 12h) A utoreduced and CO-reduced Dehydrated, deammoniated (673 K, 12 h) CO adsorption (298 K, 5-10 mbar, 1 h) Desorption of CO (823 K, 12 h)

A utoreduced and H2_-reduced Dehydrated, deammoniated (673 K, 12 h) Flow (50 cm3/min) of H2 (673 K, 3 h) Desorption of H20 (673 K, 12 h) A utoreduced and oxidized Dehydrated, deammoniated (673 K, 12 h) Flow (50 cm3/min) of HE (673 K, 3 h) O2 adsorption (673 K or 773 K, 1000 mbar, 3 h) Evacuation of 02 (673 K, 12 h) Re-reduction with HE Desorption of H20 (673 K, 12 h)

585 The TPR (IMR-MS 100) and high-temperature in-situ IR experiments provided insight into the autoreduction process. The high-temperature in-situ IR spectra were recorded using a Perkin Elmer 2000 FTIR spectrometer (4 cm"1 resolution, average of 20 scans). The transmission electron micrographs were obtained on a Philips EM 400 T microscope ( beam energy: 100 kV). TEM images enabled us to estimate the Ru particle size and to locate the Ru particles. The crystallinity of the samples was characterized using XRD carded out in the transmission mode on a Stoe Stadi P using monochromatized Cu radation. The mean diameter of the sintered Ru particles was estimated by XRD from line broadening. The catalytic activity was checked by NH3 synthesis [3]. 3.

RESULTS AND DISCUSSION

3.1 Charactedsation of two differently prepared R.u(NHa)6 NaY zeolites.

Figure 1 shows the TPR spectra of references [(NHa)sRum-O-RuW(NHa)4-O-Rum(NHa)~]CI6 and [Ru(NH3)6]C13 and of NaY exchanged with Ru amine complexes. The N H 3 profiles of charges 1 and 2 are very broad. Contrary to RuNaY, the TPR profiles of the reference compounds exhibit narrower peaks with maxima at 563 and 543 K. The maxima of the charges 1 and 2 occurred at 593 and 613 K, respectively. The differing maxima indicate Ru complexes with various binding energies. The profile of both charges exhibits a plateau in the 433 - 463 K range. The N H 3 evolution of charges 1 and 2 was detected up to about 720 and 790 K. Figures 2 and 3 show the infrared (IR) spectra in the OH, NH, NO and H20 region of charges 1 and 2 during the autoreduction process. The spectra of Ru(NI-I3)6NaY degassed at 723K showed bands at 3550 cm] and 3641 cm1 (Brensted-acid sites) and a small terminal SiOH band at 3742 cml( insets from figures 2 and 3 ). For charges 1 and 2, three intense bands are visible: (a) -1643 cm"1 (8(asyn~) of coordinated NH3, 8(synl) mode of zeolitic H20), (b) -1451 cm1 (carbonates from the NaY, 8(NH) of the NH4+ ions) and (c) -1327 cm1 105 ~

(~ /~

/

104 "

,,:. /'" aLaJ ,

;'

::i

,'# ," I I I

Im/e = 17

',1 °'1 ',l ® .....~"1"'.

3740

i ,.i

/A,l_o.

y,l

~, 1.2

:1,

~, ,I Iii

,,

3641

1.6 1.4

' ....."~,~,4)

: .......... r

102

"644

"

/-,~

1

".

"~.. '"-

14sl I//F~I

< 0.6

!

1327

~

.

0.2

¢°f 0.2

101 ,',', ,',,,i 400

i ( ,3

~ 0.8

0.4 !''

I

I

I

I

500 600 700 Temperature [K]

Figure 1. NI-Ia-TPRprofiles of the autoreduction of (a) Ru red, (b) [Ru(NH3)6]CI3, (c) charge 1 and (d) charge 2.

2000

1800

1600

Wavenumber

1400

[cm"1]

Figure 2. In-situ IR spectra of charge 1 deammoniated at temperatures from 298 to 973 K in steps of 50 K (top to bottom) in high vacuum.

0

2000

1800 1600 1400 Wavenumber [cm "1]

Figure 3. In-situ IR spectra of charge 2 deammoniated at temperatures from 298 to 973 K in high vacuum.

586 (~(sy~) mode of coordinated NH3 in the [Ru(NH3)6] 3+ complex) [ 11, 12]. The intensity of the band at -1451 cm1 decreased sharply upon heating in the 423 - 473 K range indicating that it is mainly due to NH species. The change in the IR spectrum of the N-H vibrations is typical of the reduction of Ru m. For both charges, one band was visible at -1869 cm1 which decreased as a new band appeared at -1930 cm1. These infared bands indicated two forms of Ru-nitrosyl complexes. For charge 1 changes in this part of the IR spectrum started above 523 K and for charge 2 above 573 K. Heating above these temperatures attenuated the nitrosyl bands while a new band appeared at about 2030 cm"1 due to a very stable [Ru(Ozeol)3(Nn3)y(NO)]complex formed at site II [ 11]. This complex was completely decomposed at 773 K for charge 1 and at 823 K for charge 2. The results from the high temperature IR investigations are in excellent agreement with the results from our TPR experiments. The results indicate that the intercalation complex ion [Ru(NH3)6]C13 exhibited a rich redox and ligand exchange chemistry. Both techniques show that charge 1 is obviously easier to reduce. Ion exchange at 298 K and 333 K leads to a significant change in the cell parameter compared to NaY (Table 1). The TEM images of charges 1 and 2 after autoreduction revealed the presence of Ru particles in the supercages ranging from 1 - 1.5 nm particle size. Furthermore, charge 1 contains some 3 - 5 nm large Ru particles. The Ru particles seem to agglomerate on the external surface. Therefore, it appears that for cell parameters greater than for NaY, the Ru complexes are located inside the zeolite. For smaller cell parameters the Ru complexes are most probably outside the crystallites. Catalytic testing by NHa-synthesis of both charges showed only insignificant differences between the two, so that in the following we present only the results obtained from charge 2. 3.2 Dimension and location of the metal particles (charge 2) The mean diameter of the reduced Ru particles (Chart 1) in charge 2 were evaluated by XRD, TEM and CO chemisorption methods (Table 2). The CO chemisorption method is described in section 3.3.3. The typical TEM micrographs of autoreduced, HE reduced, CO reduced and oxidized samples are shown in figure 4. Autoreduction of charge 2 produced particles ranging from 0.7- 1.5 nm. Reduction under an H2 flow produced finely dispersed Ru particles (1 - 1.5 nm). CO reduction resulted in Ru species aggregated to apparent TEM sizes of 2 - 4 nm which is larger than one supercage. After oxidation, 5.5 - 15 nm and several --35 nm large Ru particles were found. Smaller particles migrated to the exterior surface of the zeolite, where they aggregated. Detailed inspection of the TEM image of sintered particles provided evidence that elongated fibres, circular particles and hexagonal platelets were favoured. Table 2 Ru particle dimensions obtained b~¢three different techniques Teetmique Mean diameter dp(Ru) [nm] autoreduction HE reduction XRD TEM CO chemisorption CO/Ru = 1 CO/Ru = 0.75

CO reduction

oxidation

2-4

36(loo), 23(002), 32(lOl) 5.5- 35

0.7-1.5

1- 1.5

-

1.2 + 0.3

1.6 ± 0.3

10.8 + 0.3

-

1.6 ± 0.3

2.1 ± 0.3

14.4 ± 0.3

587

Figure 4. TEM micrographs of (A) autoreduced, (B) H2-reduced, (C) CO-reduced and (D) sintered (oxidized) samples. cell parameters [ Figure 5 displays XRD patterns of difcell parameters a (fa) [A] I samples samples a (fg) [A] 1.5"104 NaY (Fd3m) 24,65-1(-~,003) 1073 K, UHV 24.717 (0.002)] ferently reduced Ru(NH3)6NaHY and NaY Ru[NH3)sNaY 24,672 (0,002) 673 K, H2 24,667(0,001)1 samples. Their comparison shows that the 723 K, UHV 24,651 (0,075) 823 K, CO 24,615 (0,004)1 823 K, UHV 24,656 (0,027) 673 K, 02 24,636(0,006)l background intensity increases with ther973 K, UHV 24,731 (0.001) 773 K, 02 24,622(0,OO6)I 1.2.104 Rull01) mal treatment. We note a loss in crystallinity of a fraction of the zeolite material • Ru(O02)][ caused by local damage of the zeolite ma- "g A~ Ru(lO0) I,I trix [ 13]. Ion-exchanged charges exhibited ._&,. 9000 a characteristic change in the intensity distribution of the reflections. The inten- -= 6000 sity ratio of the reflections 2® = 10° and 2 0 = 11 ° was completely reversed como , A AI,'i ~ ' - ~ - ¢ ~ - ~ - ~ pared to NaY. The series of patterns 3000 shows that at the end of the sequence of autoreduction at lower temperatures, autoreduction at higher temperatures, H2 0 10 20 30 40 50 2 Theta reduction, CO reduction and oxidation, this intensity ratio approximated the value of NaY. It appears that this ratio is pre- Figure 5. XRD transmission patters of zeolite NaY, dominatly influenced by Ru complex ions Ru(NH3)6NaYand differently reduced RuNaHY samples. and Ru particle sizes. All reduced samples except the CO reduced and sintered sample show cell parameters greater than observed with NaY. Comparison of X-ray diffraction and TEM results suggests a location of Ru particles (< 2 nm) inside the zeolite. Larger particles are located on the outer zeolite surface. The di~action pattern of the sintered sample additionally exhibits Ru metal reflections. For Ru(100), the average diameter was consistently larger than for Ru(002) and Ru(101) faces (Table 2).

3.3 IR spectra of adsorbed CO 3.3.1 CO adsorption experiments at 298 - 75 K Figure 6 shows the spectra obtained after adsorption of CO (0.13 mbar) on autoreduced samples of charge 2 in the 298 - 75 K range. Spectra at 298 K showed bands due to Ru(CO)3 (2154, 2098 and 2086 cml), Ru(CO)2 (2090 and 2054 cm1) and linear (2017 cm1) forms of

588

CO adsorbed on ruthenium The intensity of the Pxu(CO)3 bands decreased as the adsorption temperature is lowered from 298 K to 185 K. The most dominant band in the 185, 175 and 135 K spectra showed a maximum around 2060 cm ~ with a shoulder at 2093 cm 1 due to Ru(CO)2. Further cooling diminishes the intensity of these two bands. Bridged CO (~1855 cm -1) was formed during cooling down to 175 K. Furthermore, a single band due to linearly bonded CO (Ru-CO) at 2017 cm 1 with a high frequency shoulder due to CO adsorbed on Ru x+ ions [8] around 2093 cm ~ and a shoulder at ~2000 cm ~ was observed above 110 K. On cooling, these bands undergo a continuous shift to higher frequencies. Their intensities decrease sharply below 110 K. Bands observed in this study between 2200 and 2100 cm -1 following low-temperature CO adsorption are assigned to CO molecules adsorbed on true Lewis acid sites (2174 cm 1) [14] and physisorbed CO to form sioI-rs+.-.co (~2160 cm 1) [15]. The former species are known to be formed during dehydration [ 14]. 3.3.2 CO adsorption on differently reduced samples at 110 K In figure 7, IR spectra of differently reduced samples are compared; the spectra were normalized and corrected for dispersion. The inset presents the IR spectra of adsorbed CO at increasing coverage on an autoreduced sample (823 K). The spectrum of CO on this autoreduced sample (823 K) consists of a broad band (Av~-- 94 cm 1) with a component at about

J

.2174 2160

l

l 75 K

2086

2035

~

O

..Q ° ,
95%) and MMA was the favored reaction product (> 85% amine selectivity) over all catalysts. Quantitaitve analysis of the i.r. spectra of the working catalysts, however, revealed a quite different distribution for the adsorbed phase (see Table 2).

Table 2: Concentration of the species sorbed on mordenites and the rate of MMA formation under differential reaction conditions (p(MeOH) = 5.103 Pa, p(NH~) = 5.10 JPa, T = 633 K) Catalyst

Conv.

Conc. chemisorbed species (10 -4mol/g)

Rate MMA

TOF MMA

(%)

NH3

MMA

DMA

TMA

TET

(mol/g.s)

(molec./s.H+)

HMOR20-M

6

1.1

3.8

0.8

2.0

3.1

5.2"10.6

4.7"10.3

HMOR20

5

2.5

6.4

2.7

0.2

1.2

2.4"10.6

1.9"10.3

HMOR15

4

1.7

7.1

0.2

3.1

4.5

1.1"10.5

6.7"10.3

HMOR10-E

8

4.4

6.4

2

2.8

4.4

1.3"10.5

6.4"10.3

HMOR10

9

4.1

10

2.3

1.6

2.8

6.8"10.6

3.2"10.3

This clearly indicates that the relative surface concentrations of reactants and products in the mordenite pores is not determined by an adsorption-desorption equilibrium of the substances in the reactor.

I 0.2

0.0 1.0 2.0

3.0 4.o

o O

TI~

!

T

3500

3000

2500

1650

1500

1350

Wavenumber (era"l)

Figure 1: I.r. spectra of HMOR20 under reactive conditions (5.103 Pa methanol and 5.103 Pa ammonia at 633 K) with increasing time on stream [13]

594 First, the formation of surface species under non reactive conditions was probed by coadsorption of ammonia and methanol at ambient temperature. A coadsorption complex formed by an ammonium ion (ammonia protonated at the Bronsted acid sites of the catalyst) and a methanol molecule was found to exist. The interaction in such a complex, however, was rather weak. Since all nitrogen orbitals are engaged in chemical bonds, the alkylation of the ammonium ion is speculated to involve the protonation of the alcohol by the ammonium ion, followed by a rapid release of water and the formation of a C-N-bond (see Fig.2 [13]). To probe the role of intermediates, a series of transient experiments was performed and will in the following be described in detail for HMOR20. In a typical experiment (see Fig. 1), the activated mordenite was saturated with ammonia at 633K. All hydroxyl groups of the zeolite were interacting with ammonia by forming quantitatively ammonium ions. When methanol was added to the reactant gas stream, protonated methylamines were rapidly formed and, with time on stream, replaced the ammonium ions. The amines increased in concentration sequentially from MMA, via DMA and TMA up to TET. Sorbed methanol was not observed. This suggests that not the formation of sorbed amines, but rather their displacement from the acid sites is rate determining. To test this hypothesis, a methanol containing stream of He was passed over the ammonium form of H-mordenite at 633 K. Upon contact, all methylamines were formed rapidly in the zeolite pores, again in sequential order. At steady state (which was quickly reached), the methylammonium ions MMA, DMA, TMA and TET were present in approximately equal surface concentrations. However, none of the amines formed under such conditions (T=633K) was able to desorb from the zeolite pores and dimethylether was the only product observed in the gas phase. This is in line with the higher base strength of the substituted amines compared to ammonia [ 11,12]. Temperature programmed desorption experiments ofsorbed methylamines suggest that even at higher temperatures these amines rather decompose than desorb from the strong Bronsted acid sites. However, when ammonia was passed over HMOR20 loaded with alkylammonium ions at 633K, the amines were able to desorb from the active sites. It should be noted that their rate H

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Figure 2: Proposed reaction mechanismfor the formation of sorbed methylammonium ions

595 ofdesorption was much lower than their rate of formation (as measured in the above described transient experiment). The individual rates of displacement of the amines by ammonia do not correspond to their gas phase basicities of the amines, i.e., TMA and TET disappear completely before the surface concentrations of MMA and DMA (which attain an almost constant level at moderate time on stream) decrease [13]. However, in the gas phase mainly MMA and DMA were observed as reaction product. Whereas the amine selectivity to DMA was initially high, MMA was the favored product as the concentration ofmethylammonium ions inside the zeolite pores decreased. The formation of TMA in the gas phase was low under these reaction conditions. It is interesting to note that the initial rate of MMA formation in the gas phase in such an experiment was approximately equal to the rate of MMA formation under normal methylamine synthesis conditions (when both reactants, ammonia and methanol, are present).This gives a strong indication that the removal of the chemisorbed methylammonium ions from the active sites is the rate determining step for the overall amine synthesis reaction. (a) Adsorption assisted desorption

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Figure 3: Proposed reaction mechanism for the removal of methylamines from Bronsted acid zeolites via adsorption assisted desorption (a) and methyl scavenging (b) Two mechanisms might be proposed to explain these observations: (i) ammonia helps the amines to desorb (adsorption assisteddesorption) (see Fig. 3a) or (ii) ammonia scavenges methyl groups from surface bound amines by leaving a lower substituted amine behind (methyl scavenging mechanism) (Fig. 3b).Whereas both pathways seem feasible to explain the decrease of surface bound MMA and DMA, for the highly substituted amines (TMA, TET), the scavenging mechanism seems to prevail. The high importance of the methyl scavenging mechanism is underlined by the linear correlation of the total number of methyl groups present inside the zeolite pores (in form of the methylammonium ions sorbed on the Bronsted acid sites) and the rate of MMA formation under differential reaction conditions over the investigated mordenites (Fig. 4).

596 Table 3: Amine selectivities over investigated mordenite catalysts (p(MeOH) = 5.10 ~ Pa, p ( N H ) = 5.10 ~ Pa, T = 633 K)

Catalyst

W H S V(11"l)

2.8

HMOR20-M

Conv.(%)

35

Amine selectivity (mol%) Total

MMA

DMA

TMA

96

67

30

3

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76

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20

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i',',!',i',i', Selectivity in methylamine synthesis In order to relate the physico-chemical properties of the mordenites to the selectivity in methylamine synthesis, the amine selectivities are compared at two methanol conversions (i.e., at 35% and 90%). The data are summarized in Table 3. Note that the catalysts, which showed the highest rates for the formation of MMA at low conversions (i.e., HMOR10-E and HMOR15), also exhibited the highest activity at intermediate and high conversion (compare space velocities required for the same conversion over the different samples). This indicates that measuring the rate of MMA formation under differential conditions is a valid tool to compare catalytic activity in methylamine synthesis. Upon variation of the conversion, similar trends were observed for all catalysts. With increasing conversion, the selectivity towards MMA decreased, whereas the selectivity towards TMA markedly increased. For HMOR20-M, however, the formation of TMA in the gas phase (but not in the zeolite pores) was almost completely suppressed. In accordance with the proposed reaction mechanism, this increase in the selectivity to the higher substituted amines is not attributed to the sequential formation of chemisorbed amines (which did not change markedly in their relative concentration as the conversion increased) but to an increasing contribution of methylamines as scavenging agents. Despite these general similarities, the selectivity towards the various methylamines strongly depended on the catalyst composition and pore volume. To give an example, a high selectivity towards TMA (55%) was observed at 90% conversion over HMOR20 (which is close to the thermodynamic equilibrium distribution) whereas over HMOR10 the selectivity towards TMA was only 24% at the same conversion. In general, the selectivity towards MMA increased and the selectivity towards TMA decreased in the order HMOR20 < HMOR15 < HMOR10-E < HMOR10. This trend can be correlated to the increase in concentration of Bronsted acid sites and the decrease in the micropore volume. If one considers that all acid sites are covered under reaction conditions, both effects lead to a restriction in void space inside the zeolite pores. Along the same line, Abrams

597

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et al. [ 14] showed over a series of RHO type zeolites that the yield in TMA decreased in parallel with the gravimetrically determined amount of TMA inside the zeolite pores. The decrease in the micropore volume apparently causes spatial constraints to form T M A via methyl scavenging by DMA. Alternatively, the constraints could also lead to a restriction of the diffusion of the bulkier methylamines which would favor transalkylation to chemisorbed methylammonium ions. The most prominent example for a selective catalyst due to severe diffusional constraints to the bulkier products is HMOR20-M. Although a high concentration of higher methylated ammonium ions (i.e., TMA and TET; compare Table 2) was found inside the pores of this catalyst, hardly any TMA molecules (amine selectivity < 5 %) could leave the zeolite pores, even at high methanol conversion (90 %). Poisoning of non-selective sites on the outer surface by the modification procedure can be excluded, because the rate of amine formation was higher over the HMOR20-M as compared to the parent sample (see Table 2 and Table 3). Similar results were obtained by Segawa et al. [3] who used a mordenite modified via chemical vapor deposition with SIC14 for methylamine synthesis. From their catalytic data and ditfusivity measurements (the diffusivity of the amines over the modified catalyst decreased in the order MMA > DMA > TMA), they concluded that the observed improvement in the selectivity towards the lower substituted products, MMA and DMA, could only be attributed to a narrowing of the pore openings of the mordenite channels.

CONCLUSIONS All Bronsted acidic mordenites investigated are highly active in methylamine synthesis. The principal mechanism for the formation of amines was the same for all samples studied. Methylammonium ions (including tetramethylammonium ions) are rapidly formed on the Bronsted

598 acidic sites of the zeolites via a bimolecular complex of a chemisorbed (methyl)ammonium ion and hydrogen bonded methanol. However, the release of the amines into the gas phase is the rate determining step. At low methanol conversion, this removal is proposed to occur via (i) ammonia adsorption assisted desorption or (ii) scavenging of a methyl group of chemisorbed amines by gas phase ammonia. The activity in methylamine synthesis (expressed as the initial rate of formation ofmonomethylamine) can be directly correlated to the total concentration of surface methyl groups present in the zeolite pores. This suggests that the methyl scavenging mechanism is the more important route. At high conversions, formed methylamines take over the role of ammonia as scavenging agent. Consequently, the high initial selectivity towards monomethylamine decreases in favor of the secondary (dimethylamine) and tertiary (trimethylamine) products with increasing methanol conversion for most catalysts. Despite this general trend, the amine selectivity differs drastically over the various mordenite samples at high methanol conversion. As the highest selectivity towards the lower substituted amines was observed for the catalyst with the smallest micropore volume (and as it decreased with increasing micropore volume of the different mordenites used), limitations in the rate of methyl scavenging by methylamines and/or transport limitations for the bulkier trimethylamine are concluded to cause the increase in selectivity with increasing aluminum concentration. The very high selectivity towards the lower substituted products obtained over the silylated mordenite sample is concluded to be a direct consequence of the narrowing of the pore openings by silylation. Although all methylamines can be formed inside the pores of thiscatalyst, TMA is retained in the channel system, and undergoes rapid transmethylation to form the lower methylated amines, which are able to diffuse out of the zeolite pores. ACKNOWLEDGEMENTS The authors are indepted to the Christian Doppler Society and NIOK for partial support of this work. REFERENCES

1. M.G. Turcotte and T.A. Johnson, in J.I. Kroschwitz (Ed.), Kirk-Othmer Encyclopedia of Chemical Technology, 4th edition, John Wiley & Sons, New York, 1992, Vol.2, p.369. 2. A.B. van Gysel and W. Musin, in B.Elvers, S. Hawkins, and G. Schultz (Eds.), UUmann's Encyclopedia of Industrial Chemistry, 5th edition, VCH, Weinheim, 1990, VoI.A16, p.535. 3. K. Segawa and H. Tachibana, J.Catal., 131, 482 (1991). 4. T. Kiyoura and K. Terada, Eur.Patent Appl. 593.086, 1994. 5. Y. Ashina, T. Fujita, M. Fukatsu, K. Niwa and J. Yagi, Stud. Surf. Sci. Catal., 28, 779 (1986). 6. F. Weigert, J. Catal., 103, 20 (1987). 7. A. Kogelbauer, Ch. GrOndling, and J.A. Lercher, Stud. Surf. Sci. Catal., 84, 1475 (1994). 8. A. Kogelbauer, Ch. GrOndling, and J.A. Lercher, J. Phys. Chem., accepted for publication. 9. M. Sawa, M. Niwa, and Y. Murakami, Zeolites, 10, 532 (1990). 10. G. Mirth, F. Eder, and J.A. Lercher, J.Appl. Spectrosc., 48, 194 (1994). 11. N. Cardona-Martinez and J.A. Dumesic, J. Catal., 128, 23 (1991) 12. D.J. Parrillo, R.J. Gorte, and W.E. Fameth, J. Am. Chem. Soc., 115, 12441 (1993). 13. Ch. GrOndling, G. Eder-Mirth, and J.A. Lercher, Res. Chem. Intermed, accepted 1995. 14. L. Abrams, D.R. Corbin, and M. Keane, Jr., J. Catal., 126, 610 (1990).


599

Coke formation in zeolites studied by a new technique: ultraviolet resonance Raman spectroscopy Can Li ° and Peter C. Stair b

"State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, P.O. Box 110, Dalian 116023, China bCenter for Catalysis and Surface Science, Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, IL 60208, USA

An ultraviolet (UV) Raman spectrometer was recently set up with the goal of improving Raman spectroscopy for catalysis and surface science studies. Using UV Raman spectroscopy, coke formation and oxidation in ZSM-5 and USY have been investigated. The coke species were generated by the reaction of propylene in the zeolites at temperatures from 297 to 773 K. The gaman bands of coke species can be clearly resolved since the fluorescence interference is successfully avoided in UV Raman spectra. There are three groups of strong Raman bands observed at 1360-1400, 1580-1640, and 2900-3100 cm"t. Various carbonaceous species like olefinic, polyolefinic, aromatic, polyaromatic and pregraphite species can be discriminated based on the positions and relative intensities of these Raman bands. At lower temperatures, olefinic and aromatic species are dominant for both zeolites, and these species desorb or partly convert into polyaromatic and pregraphite species at high temperatures. The coke species formed at high temperatures are quite different for the two zeolites: polyolefinic and aromatic species are predominant in ZSM-5, and polyaromatic and pregraphite species are the major species in USY. I. INTRODUCTION A common problem with zeolite catalysts used in hydrocarbon conversion is deactivation by coke deposition [ 1]. A detailed study of the chemical nature of coke species in zeolites, as they are forming, can contribute importantly to the understanding of coke formation mechanism that is absolutely necessary to improving the catalytic performance of petrochemical processes. There have been extensive studies of coke formation, but the nature and mechanism of coke formation are still not clear[2]. In principle, vibrational spectroscopy can offer the opportunity to study the coke formation mechanism. Raman spectroscopy has recently received considerable attention in the field of catalysis because it offers a number of potential advantages over other methods of vibrational spectroscopy in the characterization of real catalysts [3,4]. For example, Raman spectroscopy can potentially obtain information about both surface adsorbed species and the structure ofc~slysts under working conditions. But Raman spectroscopy has gencndly not lived

600 up to its potential because the Raman scattering cross section is inherently small for many catalysts, and the Raman signal is often overwhelmed by strong fluorescence from the catalyst surface. The mrface fluorescence problem becomes particularly serious when the catalyst is contaminated with carbonaceous species. Consequently normal Raman spectroscopy using visible laser as the excitation source can not be applied in many situations involving catalytic hydrocarbon conversions. A few normal Raman spectroscopic studies have been conducted to examine the coke on catalysts[5-7], however, for most coked catalysts it is difficult to measure the Raman spectrum because of strong fluorescence interference. We have recently performed ultraviolet resonance Raman spectroscopy using a continuous wave ultraviolet laser as the excitation source [8,9] with the purpose of avoiding fluorescence and enhancing the Raman intensity. This technique has been applied to catalyst characterization and it was found that by using an ultraviolet wavelength below 260 nm not only is the Raman scattering enhanced, but the fluorescence background is avoided. In this paper, UV Raman specua are presented for coke species formed in ZSM-5 and USY zeolites which are difficult or impossible to measure by normal Raman spectroscopy. The results indicate that LN Raman spectroscopy can identify different coke species formed in zeolites and is capable of characterizing coke formation under working conditions.

2. EXPERIMENTAL The ultraviolet laser beam for exciting UV Raman spectra was generated by frequency doubling the 514.5 nm output of a 12-watt Ar+ ion laser to 257.2 nm using a temperature-tuned KDP crystal. The power of the 257.2 nm line can be as large as 30 mW, but in our studies the power delivered to the sample was kept below 5 mW to avoid heating. The Raman scattering from the sample surface was collected by an AIMgFz coated ellipsoidal reflector using a back-scattering geometry, and then focused into a 0.32 m single grating spectrograph through a notch filter. A 3600 groove/mm holographic grating was used to disperse the Raman scattered light onto a pyroelectrically cooled imaging multichannel photomultiplier tube (IPMT) with a spectral coverage of 2100 cm"~. A quartz reaction cell was specially fabricated for the UV Raman spectroscopic studies. A detailed description of this cell is given in a previous paper[10]. The cell consists of an outer part and an inner part which are connected and sealed by an o-ring firing. The outer part is surrounded by a furnace for pretreatment and reaction, and terminates at one end in a spherical bubble, and its center is located at the focus point of the collection mirror. The inner part has a sample holder which is movable between the furnace and the measurement position without exposure to air. The sample can be treated over a wide temperature range, 293-1200 K, under various atmospheres. ZSM-5, USY, and coked ZSM-5+SiO, catalysts were provided by Amoco Oil Co. The powder samples were pressed into wafers for Raman measurements. He (99.99%) was used as carrier gas. 02 (99.5%) and C3Hs (> 99%) were used for pretreatment and coke formation reaction, respectively. The coke generation was carried out in a C3I-I~(25%)+He(75%) flow with a flow rate of 150 ml/min. The oxidation of coke was performed in 02 flow with a flow rate of 100 ml/min.

601 3. RESULTS AND DISCUSSION 3.1. U V and normal R a m a n spectra of ¢oked samples

Figure 1 shows spectra recorded by normal Raman and UV Raman spectroscopy for ZSM-5 + SiO2 coked at 773 K with naphtha. No distinct Raman bands are detectable in the normal Raman spectrum even at a laser power of 100 mW. The Raman bands are buried by an intense background due mainly to fluorescence from the surface, indicating that the normal Raman signals are typically obscured by strong fluorescence. By contrast, Raman spectra of surface coke species can be clearly and easily detected using UV Raman spectroscopy In the 600-2200 cm"t region there are two groups of Raman bands observed near 1375 and 1610 cmt which are characteristic bands of coke species in olefmic and aromatic forms. It is obvious that UV Raman spectroscopy effectively avoids the fluorescence background. We have compared normal and UV Raman for a number of catalysts. In every case the UV Raman spectra are clear and strong while only strong fluorescence is observed by normal Raman spectroscopy. 3.2. C o k e formation in Z S M - 5 and U S Y at elevated temperatures

Coke formation in two typical zeolites, ZSM-5 and USY, were compared using UV Raman spectroscopy[ 10]. UV Raman spectra were recorded for ZSM-5 and USY treated in C3I-I6+ He

A: Normal Raman B: UV Raman

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1000

i

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1500

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2000

Raman shift/cm-1 Figure 1. Normal Raman and UV Raman spectra of ZSM-5+SiO2 catalysts ¢oked with naphtha at 773 K. (Normal: 514.5 rim, 100 mW; UV: 257.2 rim, 5 roW).

602 flow at 300 K and 773 K and the time for each temperature is about 1 h. For the sake of brevity, the observed Raman bands and their a s s i s t are summarized in Table 1. At room temperature, the adsorbed species for the two zeolites are olefms and the spectra are almost identical, meaning that the adsorbed species formed in the two zeolites at room temperature are similar. However, as the reaction temperature increased, the spectra for the two samples become quite different. At 573 K, the species in ZSM-5 still keep the identity of olefin but the species in USY tend to convening into polyolefm and aromatic species because their bands at 1390 and 1635 cm"1 shift down to 1380 and 1610 cm4, respectively. In addition, the bands near 3000 cma hardly change for ZSM-5 but almost vanish for USY. This clearly shows that the coke formation reactions in the two zeolites are different in nature. Obviously, for USY some of the adsorbed species desorb, and some of them convert into highly dehydrogenated carbonaceous species since the bands of CH stretching vibrations are attenuated dramatically. The striking shift of the band in the 1600 cm"~ region is also indicative of chemical changes in the adsorbed hydrocarbon species. These changes suggest that the olefinic species convert into polyolefm and aromatic, and this conversion appears to be easier in USY than in ZSM-5. When the temperature was increased to 773 K, the bands near the 3000 cm~ region of C-H stretch vibrations disappear but the spectrum in lower frequency region still shows the feature of olefinic species as indicated by the bands at 1375 and 1620 cm~. This means that part of the olefmic species in ZSM-5 transform into polyolefinic species through polymerization and/or dehydrogenation. The band at 1620 cm"~ slowly shifts to lower frequency if the reaction is prolonged, eventually shifts to 1610 cm~ which is in the characteristic region of aromatic species. Therefore, it appears that the coke formation in ZSM-5 is initiated from adsorbed olefinic species that p r ~ through polyolefms and terminate with aromatic species. By contrast, for USY the Raman bands at 1380 and 1610 cm1 shift respectively down to 1365 and 1595 cm~ , indicating the formation ofpolyaromatic and pregraphite species because the observed Raman bands are very close to the characteristic frequencies of polyaromatic and pregraphite species [11,12].

Table 1 UV Raman bands of coke species derived from the reaction of CsI-I~ in ZSM-5 and USY at different temperatures in C~-I~+ He flow for 1 h

Zeolite

ZSM-5 USY ZSM-5 USY ZSM-5 USY

Temperature (K)

297 297 573 573 773 773

(w): weak band.

Raman shift (cm"l)

1390, 1390, 1390, 1380, 1375, 1365,

1560, 1635, 2970, 2990 1635, 2980, 3010 1560, 1630, 2970, 2990 161O, 2960(w) 1620, 2970(w) 1375, 1 5 9 5

Assignment

olefin olefin olefin polyolefin+aromatic polyolefin polyaromatic+pregraphite

603 3.3. Coke formation in ZSM-5 and USY with time The evolution of coke species not only depends on the reaction temperature, but also varies with reaction time[2]. Figure 2 shows the UV Raman spectra recorded for different reaction times of propene with ZSM-5 at 773 K. At the beginning the spectrum resembles that at room temperature, and the observed bands at 1395 and 1630 cm"t are primarily due to adsorbed olefm species. After a reaction for 3 11, the two bands shift down to 1390 and 1620 cm"t respectively. These bands continue shifting down to 1375 and 1610 cm"~for another 3 h as seen in Figure 2D. This slow change proves that the adsorbed olefm species gradually convert into polyolefin species and finally into aromatic species because the band at 1610 cm"t is close to the band of aromatic species[13]. The 1610 cm"t band no longer shifts even after a further prolonged reaction at this temperature. Meanwhile it is interesting to note that the Raman band intensities decline slightly rather than develop with reaction time. This may be interpreted as evidence for no further coke accumulation after a certain amount of coke has formed since the channels in ZSM-5 are not large enough to host bigger coke particles. Figure 3 presents the UV Raman spectra for USY reacted with propene at 773 K. The coke band at 1610 cm"~grows considerably in the first hour and keeps on developing, indicating that

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1600 2400 3200 Raman shift/cm-1 Figure 3. UV Raman spectra of coke species formed in USY with propene at 773 K for. different reaction time.

604

the coke species build up in the USY with reaction time. Another noticeable change is that with a prolonged reaction at 773 K, the band at 1610 cm"t shifts to 1585 cm"xwhich is very close to the characteristic band of graphite at 1575 cm"~[12] and the band at 1375 crn"~ shifts to 1360 cm~ which is due to edge defects of the graphite [14], These remits strongly suggest that pregraphite species are formed in USY at 773 K, and the coke particle becomes bigger with longer time. Apparently, the coke formation in USY is quite different from ZSM-5 where mainly polyolefin and aromatic species are dominant at this temperature. 3.4. Oxidation of coke species in ZSM-S and USY The chemical nature of coke species could be distinguished through oxidation of the coke because the different coke species may show different reactivity towards oxygen. UV Raman spectroscopy was used to follow the coke species lett in the coked ZSM-5 and USY after different stages of oxidation treatment in 02 flow. The coke species were formed at 773 K in a C3I-I6+ He flow for more than 3 h and then the sample was purged with He alone for 30 min. Figure 4 exhibits the Raman spectra recorded for coked ZSM-5 oxidized at various temperatures. The spectrum is scarcely altered when the coked ZSM-5 is exposed to 02 at room temperature. When the sample was treated at 573 K, a dramatic decrease of band intensities at 1375 and 1610 cm"t

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Raman shift/cm-1 Figure 4. UV Raman spectra of coked ZSM-5 treated in 02 flow.

1600

2400

3200

Raman shift/cm-1 Figure 5. UV Raman spectraof cokmi USY treated in Oz flow.

605 is clearly observed from Figure 4A to 4B. This can be attributed to a removal of coke species. Further removal of coke is more difficult as can be seen by the persistence of the coke band through spectra 4C and 4D. When the sample was treated in O5 flow at 873 K for 3 h, the coke species are finally removed except for a tiny/band at 1610 cm"~still remained no matter how long the oxidation was continued. The two st~es of coke oxidation probably manifest two kinds of coke species, polyolefm and aromatic formed in ZSM-5. The former is reasonably easier to oxidize than the latter. The slow oxidation of aromatic species might be explained in terms of slow diffusion of oxygen in the channels of ZSM-5. Figure 5 shows the Raman spectra of coked USY treated in O~ flow at 773 K and 873 K. No evident change in the spectrum was observed when the oxidation treatment was carried out at temperatures lower than 773 K. This implies that the coke species in USY are chemically inert towards oxygen, consistent with the assignment that the coke species in USY are mostly in the form ofpregraphite. A remarkable attenuation ofthe bands at 1365 and 1585 cm4 occurred at 773 K(Figure 5B), and the band intensity was reduced further with increasing temperature to 873 K(Figure 5 C). Two residue bands at 1605 and 1615 cm"~ survived from the oxygen treatment at 873 K. By comparing with the coke in ZSM-5, the coke species in USY seems easier to remove. This may be due to the bigger pores of USY which allows a faster diffusion of 05. A very important phenomenon in Figure 5 is that the band position at 1585 cm~ shifts up to above 1600 cm"~when the majority of the coke species was removed. It can be assumed that the pregraphite is removed by gradual oxidation at the edge of the graphite sheets. As a consequence, the particle becomes smaller and smaller which produces the up-shifting of the coke band from 1585 to 1610 cm-~. , 3.4. Mechanism of coke formation

The coke formation in the ZSM-5 and USY are different especially at high temperatures. The fact that the Raman spectra of adsorbed propene at room temperature are similar for the two zeolites suggests that the species are mainly adsorbed propene or/and polypropene. At elevated temperatures, the adsorbed olefin is dehydrogenated and polymerized, resulting in the polyolefin and aromatic species. Because of the limitations set by the pore size, the polyolefin and aromatic species can not grow fluter in ZSM-5. For USY, with increasing temperature, the polyolefin and aromatic species gradually aggregate into pregraphite species which mainly accumulate in the cages of the zeolite. It is also assumed that the difference in coke formation in the ZSM-5 and USY is not only due to the pore structures but also due in part to the acidity of the two zeolites. A study of the relationship between coke formation and acidity is under way.

4. SUMMARY UV Raman spectroscopy has been demonstrated to be a powerful tool for characterizing coke formation in zeolite catalysts. The sensitivity of Raman spectroscopy is improved significantly owing mainly to avoiding fluorescence interference. Coke formation in zeolites is initiated with adsorbed olefinic species and terminated with polyolefin and aromatic species in ZSM-5, but proceeds to pregraphite in USY. This difference is attributed to the pore structure and acidity of the two zeolites.

606 ACKNOWLEDGMENT We gratefully acknowledge Frank Modica and Jeffrey l~_tller for providing the zeolites and coked industrial catalysts. The Raman spectrum of figure 1A was measured by Maritoni Litorja. Acknowledgement is made to the Donors of the Petroleum Research Fund, administered by the American Chemic~ Society for partial support of this research. This project was also supported by the Center for Catalysis and Surface Science of Northwestern University.

REFERENCES

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

M. Guisnet and D. Magnoux, Appl. Catal., 54(1989)1. H. G. Karge, Studies in Surface Science and Catalysis, 58(1991)531. G. J. Hutchings, A. Desmartin-Chomel, R. Oiler and J.-C. Volta, Nature, 368(1994)41. J. Miciukiewicz, T. Mang and H. Knozinger, Appl. Catal. A: General, 122(1995)151. P.D. Green, C. A. Johnson and K. M. Thomas, Fuel, 62(1983) 1013. C. A. Johnson and K. M. Thomas, Fuel, 63(1984) 1073. D. Espinet, H. Depert, E. Freund and G. Martino, Appl. Catal., 16(1985)343. C. Li and P. C. Stair, Catal. Lett., 36(1996) 119. C. Li and P. C. Stair, Pro¢. of Inter. Congr. Catal., 1996, Baltimore, USA. C. Li and P. C. Stair, Catal. Today, in press. P. Kwizera, M. S. Dresselhaus and G. Dresselhaus, Carbon, 21 (1983) 121. M. Nakamizo, Carbon, 29(1991)257. D, Lin-View, N. B. Colthup, W. G. Fateley and J. G. Grasselli, The Handbook of Infrared and Raman Characteristic Frequencies of Organic Molecules, Academic Press, Inc., Boston, 1991. 14. P. Lespade, R. AI-Jishi and M. S. Dresselhaus, Carbon, 20(1982)427.


607

Preparation of Titanium-containing large pore molecular sieve from HAl-Beta zeolite Guo Xinwen, Wang Xiangsheng, Wang Guiru and Li Guangyan Institute of Industrial Catalysts, Daliam University of Technology, Dalian 116012, P.R. China ABSTRACT Ti-AI-Beta zeolite has been synthesized by gas-solid isomorphous substitution with H-AI-Beta zeolite as precursor. X-ray diffraction pattern showed that its structure was similar to that of zeolite H-AI-Beta. The only difference between their IR spectra was an extra adsorption band appearing at 960cm-~. The study of IR and UV-Vis spectra showed that Ti was incorporated into the framework of Beta zeolite and Ti was tetrahedrally coordinated as part of the framework. After the precursor was treated with acid, the incorporation of titanium became easy. With increasing acid concentration, the content of Ti into the framework increased. In addition, increasing substitution time, the content of Ti into the framework also increased. Key words Gas-solid isomorphous substitution, H-AI-p zeolite, Ti-AI-~zeolite. 1. INTRODUCTION In recent years isomorphous substitution of silicon or aluminium by titanium in the lattice of molecular sieves has attracted a considerable attention, especially the syntheses of TS-1 ~J and TS-2 [=3zeolites, which possess interesting catalytic properties. However, in the field of fine

608

chemicals, it is sometimes required to oxidize large molecules that can not penetrate in the narrow pores of the MFI structure. Ti-Beta zeolite has less steric constraints than TS-1 for oxidation of cycloalkanes because of its large pore. In 1992, Camblor Es~reported the synthesis of I-Ti,Al']-Beta in the hydrothermal system, then, Rigutto E4] reported the secondary synthesis of Ti-~ zeolite from boron-Beta. However, little information on the secondary systhesis of Ti-p zeolite from H-AI-Beta zeolite was available in the literature. In this paper. We present that Ti has been introduced by secondary synthesis into the structure H-AI-Beta zeolite.

2. Experimental 2. 1 Materials and Methods Thestarting material AI-Beta (SiOz/AI203 -- 25) synthesized with organic template (TEAOH) Es3was calcined for 6 hours at 540°C. The Hforms of AI-Beta were prepared as follows= 10g of the freshly calcined powder was treated with 200ml of 0. 4N aqueous solution of ammonium nitrate (4h at 80°C), and then deammoniated by heating at 540°C for 4h (sample I ). Another portion of the freshly calcined powder was treated with 2N (sample I ) or 5N HCI solution (sample I ) at 80°C for 4hr. "Concentration" of zeolite in the suspension was 50g/dm s of HCI solution. The secondary synsthesis method was performed by reacting H-AIBeta (predried at 450"C) at 500°C in a flow of dry N2 saturated with TiCI4 vapour.

2. 2 Characterization X-ray powder diffraction patterns were recorded on a XR-3A diffractometer using the CuKo radiation. UV-Vis spectra were collected on a UV-240 spectrometer, and Framework IR spectra were obtained on a IR-435 spectrometer using the KBr method (1 w t ~ zeolite).

609

3. RESULTS AND DISCUSSIONS 3. 1 Characterization of TI-AI-Beta zeolite

Powder X-ray diffraction patterns in Fig. 1. Show that the samples kept pure BEA structure after their transformation into H-form (spectrum a) and subsequent treatment of the H-form with TiCI4/Nz(spectrum b). After reacting H-AI-Beta with TiCI~, the crystallinity of the sample does not decrease and there is no variation inthe spacing of the (600) plane. Framework IR spectra indicates that the zeolites are highly crystalline (See Fig. 2). Moreover, the presence of an IR band at ~60cm-~ strongly suggests the incorporation of Ti atoms as a framework element. Eel The chemical composition is obtained by AAS after dissolution of the sampies (Table 1). a

f .

5

.

.

,, .

|.

.

10

L

15

.

.

L~

.=

20

25

211

,,.,

!.5oo

•

..

iooo

500

Wavenumber (ore- 1) Fig. 1 XRD patterns of sample I (a) and the corresponding Ti-Al-j3(b) zeolites

Fig. 2 IR patterns of sample I (a) and the corresponding Ti-Al-j3(b) zeolites

Table 1. Chemical composition of sample 1 and the corresponding Ti-AI-p sample Sample

Composition (mol ~ ) SiO~.

AI=O~

Ti02

H-AI-~

99.05

0. 88

0. 07

Ti-AI-~

96.80

1.00

2.20

610

The equivalence between remained SiO2 and incorported TiO2(see Table1) indicates that Ti is incorporated into the framework by mainly isomorphous exchange with framework Si, From (Fig3),

UV-Vis

spectra

it can be observed

that curve a shows a weak peak in the 210-230nm range because of a small amount of T i in the sample, and curve b shows a intense band in the 210-230nm range and a shoul-

~ 8

der at -~, 270nm. The band at ,~-225nm in the UV-Vis pectrum of calcined hydrated TS-1, has

a

b

been assigned to the ligand to metal charge transfer (CT) involving isolated Ti atoms in

190

290

390

._,

Wavelength(nm)

490

octahedral coordination rTl. De- Fig. 3 UM-vis spectra of sample ! (a) and the hydration of this sample shifts correspondingTi-AI-19(b)zeolites the band to ca. 205nm, characteristic for a CT transition involving tetra coordinated Ti (IV) in the FTiO,I or [TiOsOHl structure cSJ. Taking into account these assignments, we could conclude that in these Ti-AI-p samples, most of titanium exists i~ the form of isolated tetra coordinated Ti species.

3. 2 The effect of various treatment condition.

611

a

1500. . . .

,,

~

.=

~_

1000

j~...

t__=

500

Wavenumber (cm - * )

Fig. 4 IR framework spectra of Ti,AI-beta zeolites which precursors treated under different conditions • a. no treatment b. c. 2NHCI d. SNHCI

NH,NO=

190

290

390

490

590

Wavelength (nm) Fig. 5 UV-Vis spectra of Ti-AI-Beta zeolites which precursors treated under different condition: a. no treatment b. NH4NOs c. 2N HCI d. 5N HCI

Fig. 4 shows that the intensity of 960cm -~ increases with increasing acid concentration, and the increase of the intensity of 960cm -~ indicates that the content of titanium which is incorporated into the framework increases. From Fig. 4, we can s e e , when using as synthesed powder as precursor, there is a small peak at about 960cm -~ because of a large amount of AI 3+ and Na + existing in it. When precursor was treated with NH4NO3 solution, the intensity of 960cm -~ increases slightly. Although Na + is removed through ion exchange, there is a large amount of AI 3+ in the precursor. When the precursor was treated with 2N HCI, the

612

intensity of 960cm -~ obviously increases. Inereasing acid concentration, the intensity of 960cm -~ continues to increase. The reason is that acid treatment produces hydroxyl nests because of dealumination, which leads to a more efficient incorporation of titanium into the frameworkCg- ~01. Fig. 5 shows that the intensity of the band at

d

220nm increases with increasing treatment degree, this indicates that the content of Ti into the framework increases. It is in total agreement with the IR result. We can also see, when using as-synthesed powder as precursor (curve small

a),

there is a

amount

of

TiOz

J..;.

1500

1000

..

500

(anatase) in the product. Wavenurnber (cm -~) This also indicates that Fig. 6 IR spectra of Ti-AI-p prepared with AI s+ and Na + in the predifferent substitution time (sample ! cursor are detrimental to as precursor) a)0 b) 6h c)18h d)26h the incorporation of titanium and make titanium exist in the form of extra framework titanium. When the precursor is treated with NH,NOs or HCI, the amount of extra framework Ti decreases. The framework spectra of Ti-AI-Beta zeolites show that the content of Ti into the framework increases with increasing substitution time (Fig. 6). 4, CONCLUSION Titanium-containing large-pore molecular sieves with the BEA struc-

613

ture can be prepared by reacting H-AI-Beta with titanium chloride at 500°C. Dealumination ~ d s to a more efficient incorporation of titanium into the beta framework. AOKNOWLEDGE~NTS The authors express their sincere thanks to professor Z. H. Zou (Department of applied chemistry, Dalian University of Technology) for his help in registering the UV-Vi= diffuse reflectance spectra.

REFERENCES El] M. Taramasso, G. Perego and B.Notari, US Pat. ,4410501(1983). [-2-] J. S. Reddy and R. J. Kumar, J. Catal., 130(1991)440. E3] M. A. Camblor, A. Corma,J. Perez-Pariente,zeolites, 13(1993)82. E4-1 M. S. Rigutto, et al., Studies in Surface Science and Catalysis, 84 (1994)2245. C5-] R. L. Wadlinger, G.T. Kerr and E. J. Rosinski, US. Pat. 3308069 (1967) [6"] M. R. Boccuti, et al., Stud. Surf. Sci. Catal. ,48(1989)133. E7] A. Zecchina, G. Spoto, S. Bordiga, et al., Studies in Surface Science and Catalysis,69(1991)251. E8~ F. Geobaldo, S. Bordiga, A. Zecchina, et al., Catal. Lett., 16 (1992)109. E9-] R. M. Barrer and M. B. Makki, Canadian Journal of Chemistry, 42 (1964)1481. ElO-] Guo Xinwen, Ph. D Thesis, Dalian University of Technology,China, 1994.



615

S y n t h e s e s a n d R a m a n s p e c t r o s c o p i c s t u d y of bis- a n d t r i s - ( 1 , 1 0 - p h e n a n t h r o l i n e ) m a n g a n e s e ( I I ) c o m p l e x e s e n c a p s u l a t e d in f a u j a s i t e - Y B.-Z. Zhan and

X.-Y. Li

Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ABSTRACT We report the syntheses of three different types of occluded Mn(II)-l,10phenanthroline complexes in faujasite-Y. We present a detailed characterization of the encapsulated complexes by Raman spectroscopy and by other analytical techniques. We show Raman evidences for a strong interaction between oxygen atoms of the supercages and metal ion of the occluded complexes. We illustrate that the relative intensity ratio between Raman marker peak of metal complexes and that of zeolite framework can be use to estimate the occlusion concentration quantitatively. Finally, the preliminary results of the title guest-host composites as selective allylic hydroxylation catalysts are presented. 1. INTRODUCTION The lasting extensive interests in zeolite encapsulated transition metal complexes lie in their established applications in catalysis, gas adsorption and separation, and in their potentially useful electrochemical and photochemical properties. The Y-type faujasite is particularly attractive because their well organized "supercages" have an internal diameter of 13/~ and are sufficiently large to host redox-active metal complexes of such multidentate ligands as phthalocyanine[1], schiff base[2] and polypyridine[3]. The main difficulties in the study of molecular sieve confined chemistry, however, are the characterization of guest molecules in the host-guest composites, the assessment of the effect of cage environment on the guest, as well as the in situ monitoring of the behaviors of guest molecules in a chemical process[4]. In this paper, we report (a) the synthesis of three different t y p e s of occluded Mn(II)-l,10-phenanthroline (Phen) complexes in faujasite-Y, and (b) a detailed characterization of the encapsulated complexes by R a m a n spectroscopy. We demonstrate that Raman-scattering is a very powerful technique for characterizing and monitoring the guest molecules occluded in the zeolite cages. We show the Raman evidences for a strong interaction between the supercages and the occluded complexes. We illustrate that the relative intensity ratio between Raman marker peak of metal complexes and that of zeolite framework can be use to estimate the occlusion concentration quantitatively. Guest molecules having visible or near ultraviolet absorption

616 can be s t u d i e d by resonance R a m a n s c a t t e r i n g , while those d i s p l a y i n g interference of intramolecular fluorescence can be studied by using the n e a r i n f r a r e d excited F o u r i e r t r a n s f o r m R a m a n technique[5]. F i n a l l y , t h e p r e l i m i n a r y r e s u l t s of the title g u e s t - h o s t composites as h y d r o x y l a t i o n catalysts are presented. 2. E X P E R I M E N T A L 2.1. Synthesis M n ( P h e n ) 2 2 + / Y , M n ( P h e n ) 2 ( O z ) 2 ~ , a n d Mn(Phen)32+/Y : The sodium form of faujasite-Y (NAY) with Si/AI = 2.6 was obtained from H u n a n Petro Co., China. All chemicals were purchased from Aldrich and used as received. The synthesis of the title complexes in NaY was carried out by the following steps. First, Na + cations in NaY were exchanged by Mn 2+, at a loading of I Mn 2+ pel; two s u p e r c a g e s , in a vigorously s t i r r e d aqueous solution c o n t a i n i n g the stoichiometric a m o u n t s of NaY and Mn(OAc)2 at 85°C for 3 hours u n d e r N2 atmosphere. The pH value of the solution was retained at ~ 6 during cation exchange by t i t r a t i n g with 2M HC1 solution. Second, the suspension after cation exchange was filtrated and w a s h e d by redistilled w a t e r a n d t h e n dehydrated at 150°C for 3 hours under vacuum. Third, MnY thus obtained was thoroughly mixed with methanol solution of P h e n in a specified ligand-tometal ratio at room t e m p e r a t u r e . The methanol was t h e n removed at 80°C using an oil bath. Each template synthesis(the diffusion of ligands into the supercages) was conducted in a sealed v a c u u m glass tube for 24 hours. A specific type of occluded complex was obtained by controlling the ratio of r e a c t a n t s (ligand-to-metal) and the reaction temperature, as listed in Table 1. Upon the completion of reaction, the powder s a m p l e was extracted by 10% NaC1 aqueous solution, acetonitrile and dichloromethane respectively in order to remove the unreacted ligand and surface adsorbed complexes. Table 1 The specifications of the synthesized samples Sample Label

Loada

Reactant Ratio b

Reaction Product Temp(°C) Color

Product Ratioc

Product Assignmentd

Mnl00211

4

2.1:1

100

gray

2.16

Mn(Phen)22+/Y

Mn200211

4

2.1:1

200

pale pink

2.05

Mn(Phen)2(Oz)2~

Mn200351

4

3.5:1

200

pale pink

2.83

Mn(Phen)32+[Y

a. number of Mn2+ per unit cell; b. the mole ratio of Phen-to-Mn2+ in reactants; c. the mole ratio of Phen-to-Mn 2+ in products from elemental analysis(for C,N) and atomic absorption analysis(for Mn2+); d. see the text for discussion. C/s-Mn(Phen)2Cl2 and Mn(Phen)3(CIO4)2 were synthesized according to the l i t e r a t u r e methods[6,7] and were further confirmed by elemental analysis. Calcd. for cis-Mn(Phen)2C12:C,59.2; H,3.29; N,11.5%. Found: C,58.3; H,3.15;

617 N,11.2%. Calcd. for Mn(Phen)3(C104)2: C,54.4; H,3.02; N,10.6%. Found: C,55.0; H,2.95; N,11.2%. R a m a n spectrum of Mn(Phen)2C12 confirms t h a t it is cis isomer[6b] . 2.2. C h a r a c t e r i z a t i o n Atomic absorption analysis(A.A.A.) of Mn 2+ content was conducted with a Model PU9100X(Philips) AA spectrometer equipped with a Mn lamp. Elemental analysis(E.A.) was carried out for C, N elements for each sample. FT-Raman spectra were collected on a Bruker IFS 100 spectrometer equipped with a CW Nd:YAG laser(1064 nm excitation) and a Ge detector cooled at liquid N2 temperature. All Raman spectra were collected with 180 ° scattering geometry and ~ 4 cm -1 spectral resolution. Typically, a laser power of 200mW was used to i r r a d i a t e onto a loosely packed powder sample held in a aluminum holder. Usually, 2000 scans need to be averaged in order to reach a reproducible signal-to-noise ratio. FT-IR spectra were collected using a Bruker IFS66 spectrometer with the sample being thoroughly mixed in a KBr pellet (sample/KBr ratio = 1:100 by weight ). 3. RESULTS AND DISCUSSION 3.1. R a m a n s c a t t e r i n g v e r s u s IR abs or pt i on: N e a r i n f r a r e d excited F o u r i e r R a m a n mmttering is a m u c h s u p e r i o r a n d a v e r y sensitive p r o b e of

ill

NaY ....

,

~

,

Mn100211

Mn200351

-J-

1700

i

J

I

I

1250 lOOO 750

Wavenumber

(A)

cm-1

I

400 1700

I000

Wavenumber (B)

'1

500

cm-1

I00

Fibre-1 : FT-IR spectra(A) and FT-Raman spectra(B) of NaY and its two occlusion oom~ Mn(Phms)22+/Y and Mn(Phen)32+/Y. See text for the experiments] conditions and ~scussion8.

618 For the purpose of comparison, we displayed in Figure 1 the FT-IR (A) and FTRaman(B) spectra of NaY and its two occlusion composites Mn100211 and Mn200351, respectively. As can be clearly seen from the IR spectra(A), NaY itself absorbs strongly in almost full mid-IR region, leaving only a narrow window between 1250-1550cm "1 for the analysis of the encapsulated molecules. Of particular disappointing is the finger-printing region below 800 cm "1 where strong absorption of faujasite-Y obscured any hope to extract useful information about the occluded molecules. FT-IR spectra of MLx/Y composites, in spite of its very limited information, do indicate the successful occlusion of Mn(Phen)x 2+ complexes within the faujasite-Y supercages as evidenced by the peaks at 851, 1432, 1473, 1523, 1545 cm -1, respectively. It is almost impossible, for example, to structurally distinguish the occluded complexes in the samples Mnl00211 and Mn200351 from their IR spectra. R a m a n spectra, as illustrated in Figure-lB, provide a much superior probe than FT-IR for the occluded complexes. First of all, NaY itself, like most of the SiA10 zeolites, is a poor light scatterer. Its Raman spectra is very simple and quite well-defined. Therefore the complication from the internal vibrations of zeolite framework in the Raman spectra of guest-host composite can be easily identified and removed. Secondly, almost whole mid-IR and far-IR regions can be used to study the occluded molecules. This is of particular significance because the subtle differences between the occluded complexes prepared under different conditions can thus be studied using both the functional group and the finger-printing regions. Thirdly, a well-defined zeolitic internal Raman peak at -500 cm "1 ( mainly the skeleton's T-O-T bending character[8] ) provides an ideal internal standard to study such quantitative information as the occlusion concentration and hydration level of the zeolite, etc. 3.2. Mn(Phen)22+/Y v e r s u s Mn(Phen)32+/Y: R a m a n m a r k e r s for d i f f e r e n t ligand-to-metal ratio. E.A. and A.A.A. of the two samples prepared under very different conditions, Mn100211 and Mn200251, show that Phen-to-Mn ratios are -2:1 and -3:1, corresponding to the occluded complexes of Mn(Phen)22+ and Mn(Phen)32+, respectively. At least four set of Raman bands are identified that clearly mark the differences of the ligand number in the occluded complexes. For the occluded Mn(Phen)22+, these band are located at 1600 (shoulder), 1302, 722, 276 cm -1, respectively, while for the occluded Mn(Phen)32+, they are found at 1592 (sharp),1314, 727, 286 cm -1 respectively. The sensitivity of both Phen internal modes at high frequency and the Mn-L mode at low frequency to the change of coordination number is expected in t h a t they reflect different strengths of complex-cage interaction due to different shapes and sizes, as well as different strengths of Mn-Phen interaction[9-11]. To further confirm the observations made above, we have synthesized homogeneous cis-Mn(Phen)2C12 and Mn(Phen)3(C104)2. The Raman spectra of the homogeneous complexes were acquired and compared with those of occluded complexes in the finger-printing region (Figure-2). Indeed, the Raman spectra show remarkable similarity for the same type of complex no

619 m a t t e r it is in the occluded or homogenous forms. While for complexes with different n u m b e r of ligands, the differences in t h e i r R a m a n spectra are clearly visible. For bis-complex, the characteristic peaks were observed at ~ 419(with shoulder) and 177/152 (doublet) cm -1, respectively, while for tris-complex, the features are at 423/411(doublet) and 162 cm -1, respectively.

c/s-Mn(Phen)2Cl2

tJ

3.3 Mn(Phen)22+fY v e r s u s M n ( P h en)2(Oz)2fY: R a m a n evidences for the d i r e c t i n v o l v e m e n t of the s u p e r c a g e o x y g e n a t o m s in m e t a l c o o r d i n a t i o n sphere. A striking observation was made on a p a r t i c u l a r p r e p a r a t i o n of the I I I I I sample Mn200211 which was made 55O 4OO 300 2O0 I00 with a starting Phen-to-Mn ratio o f - 2 , but with the synthesis being carried Wavenumber c m ' l Figure-2 : Raman spectra of the fingerout at 200°C. E.A. and A.A.A. results printing region for homogeneous and indicate that the occluded complexes occluded Mn(II)-Phen complexes. indeed has a Phen-to-Mn ratio of ~ 2, in consistence with the expected occlusion of Mn(Phen)22+ complex. Yet, its Raman spectrum bears remarkable similarity to that of Mn(Phen)32+ftr, with m a r k e r bands at 1592(sharp), 1315, 727, 286cm -1, respectively, indicating that the occluded Mn 2+ is in a six-coordinated state ( F i g u r e - 3 ) . Several new Raman bands were also observed at 1401, 673, 567cm -1, respectively.

Mn(Phen)2(Oz)2/Y~

1500

i

I

z

i

l'

l

l

i

1250 750 700 650 600 550 500 450

~I

390

Wavenumber cm'l

Figure-3 • Comparison of Raman spectra of Mn(Phen)22+/Y, Mn(Phen)2(Oz)2/Y, and Mn(Phen)32+/Y. See text for the experimental conditions.

620

We are therefore compelled to conclude that, at high temperature of synthesis, the oxygen atoms of supercages break away from the zeolite framework, and start to strongly interact with the occluded Mn 2+ ion or Mn(Phen)22+ complex. We denote this sample by Mn(Phen)2(Oz)2~ where Oz is the oxygen atom from the zeolitic supercage. This idea, together with the occlusion composites synthesized under other conditions, can be expressed in the Scheme-1. The new band observed at 673 cm -1 is presumably due to the Si(Al)-O- stretching of the broken Si-O-Al skeleton. The other two new bands at 1401 and 567 cm -1 are attributable to the splitting of the nearby Phen band due to strong steric distortion of the complex.

s Si

W,

o.AI"

..Si

,,

MnNaY ""'1~

A

..~

y

'o

c

M'n .....O-si'

•

Scheme-1 • Three types of faujasite-Y occluded Mn2+-Phen complexes. 3.4. T h e g u e s t v e r s u s t h e h o s t : R s m a n i n t e n s i t y r a t i o a s a q u a n t i t a t i v e estimation of occlusion concentration.

For a given type of occluded complex, the intensity ratio between a well-defined characteristic Raman band of the guest molecule and that of the host matrix should correlate linearly to occlusion concentration (the average number of complexes per gram of sample). This is indeed what we observed by using four different levels of loading in Mn(Phen)3 2+ /Y. In Figure-4, the intensity ratio of 1050 cm -1 ( guest mode ) over 500 cm -1 ( host mode ) peaks was plotted against the concentration of the guest. An excellent linear relationship was obtained.

I

~

0.4 0.3-

'0.2

0.1

8 o.o

I 0.0

0.5

I 1.0

Iloso / I soo PiJmm-4 : 110601 ~ ss a m s r k ~ for the E m l t ~ m c e n t r a t i ~ in J&m(l~n)$2+/Y.

621 3.5. Catalysis The oxidation of cyclohexene was used as a reference reaction to study the catalytic properties of the three types of occluded Mn-Phen complexes. They show different catalytic behaviors, in agreement with the composition/ structure studies reported in the previous sections. Two competitive pathways of oxidation were observed with one mainly leading to epoxide and the other to allylic hydroxylated products. We have achieved the selection of one pathway over the other by using different oxidants. The optimization of the catalytic conditions is currently in progress. 4. CONCLUSIONS We have demonstrated that the type and the structure of the occluded molecules depend not only on the ligand-to-metal ratio used as reactants, but also on the t e m p e r a t u r e applied during the synthesis. We have shown that Raman spectroscopy is a very sensitive probe for the structure of the occluded molecule, as well as for the interaction between the guest molecule and the supercage. We i l l u s t r a t e d t h a t R a m a n spectroscopy can be used to quantitatively estimate the occlusion concentration, and therefore be utilized to optimize the synthesis of catalyst with desired concentration of catalytic site. Optical fiber guided and time-resolved Raman spectroscopy will enable us to study and monitor the reaction intermediates formed during catalysis. Finally, the title occluded complexes were shown to be good catalysts for the selective allylic hydroxylation of alkenes. ACKNOWI,~EMENTS

We acknowledge the Research Grant Council, Hong Kong and the Hong Kong University of Science and Technology for the financial support (to XYL). R~'ElCENCES 1. (a) V. Yu. Zakharov and B. V. Romanovsky, Vestn. Mosk. Univ., Ser. 2: Khim., 18 (1977) 142 [Eng. Trans. in Sov. Mosc. Univ. Bull., 32 (1977) 16]; (b) B. V. Romanovsky, R. E. Mardaleishvili, V. Yu. Zakharov, and O. M. Zakharova, Vestn. Mosk. Univ., Ser. 2: Khim., 18 (1977) 232; (c) V. Yu. Zakharov and B. V. Romanovsky, Vestn. Mosk. Univ., Ser. 2 KhJm., 18 (1977) 348; (d) G. Meyer, D. Wohrle,D. Mohl and G. Schultz-Ekloff, Zeolites 4 (1984) 30; (e) T. Kimura, A. Fukuoka, and M.Ichikawa, Shokubai, 31 (1988) 357; (f) R. F. Patton, L.Utytterhoeven, and P. A.Jacobs, Stud. Surf. Sci. Catal. 59 (1991) 395; (g) E.Paez-Mozo, N.Gabriunas, F. Lucaccioni,D.D. Acosta, P. Patrono, A. L. Ginestra,R.Ruiz,and B.Delmon, J. Phys. Chem., 97 (1993) 12819; (h) R. F. Parton, I. F. J. Vankelecom, M.J.A. Casselman, C. P. Bezoukhanova, J. B. Utytterhoeven, and P. A. Jacobs, Nature, 370 (1994) 541; (i) K. J. Balkus, Jr., A. G. Gabrielov, S. L. Bell, F. Bedioui, L. Rouk, and J. Devynck, Inorg. Chem., 33 (1994) 67. 2. (a) D. Chatterjee, H.C. Bajaj, A. Das, and K. Bhatt, J. Mole. Catal., 92 (1994) L235; (b) D.E. Devos, F. Thibault-Starzyk, and P. A. Jacobs, Angew.

622

3.

4. 5.

6. 7. 8. 9. 10. 11.

Chem. Int. Ed. Engl., 33 (1994) 431; (c) F. Bedioui, L. Roue, E. Briot, J. Devynck, S. L. Bell, and K. J. Balkus, J. Electroanal. Chem., 373 (1994) 19. (a) K. Maruszewski, D.P. Strommen, and J.R. Kincaid, J. Am. Chem. Soc., 115 (1993) 8345; (b) K. Maruszewski and J.R. Kincaid, Inorg. Chem., 34 (1995) 2002; (c) P.P. Knopes-Gerrits, D.D. Vos, F. Thibault-Starzyk, and P.A. Jacobs, Nature, 369 (1994) 543. S.L. Suib, Chem. Rev., 93 (1993) 803. (a) D. E. De Vos, D. L. Vanoppen, X.-Y. Li, S. Libbrecht, Y. Bruynseraede, P.P.Knopes-Gerrits, and P.A. Jacobs, Angew. Chem.: Chem. Eur. J., 1(2) (1995) 144; (b) P.P.Knopes-Gerrits, E. Feijen, X.-Y. Li, and P.A. Jacobs, Angew. Chem.: Chem. Eur. J., in press(1996). (a) B. P. Sullivan, D. J. Salmon, and T.J. Meyer, Inorg. Chem., 17 (1978) 3334; (b) R.E. Morcom and C. F. Bell, J. Inorg. Nucl. Chem., 35 (1973) 1865. A.A. Schilt and R.C. Taylor, J. Inorg. Chem., 9 (1959) 211. (a) P. K. Dutta, K.M. Rao, and J.Y. Park, J. Phys. Chem., 95 (1991) 6654; (b) C. Bremard and M. Le Maire, J. Phys. Chem., 97 (1993) 9695; (c) A. J. M. de Man and R. A. van Santen, Zeolites, 12 (1992) 269. (a) N. Abasbegovic, N.Vukotic and L. Colombo, J. Chem. Phys., 41 (1964) 2575; (b) E.R. Lippincott and E. J. O'rielly, Jr., ibid., 23 (1955) 238; (c) A. A. Schilt and R.C. Taylor, J. Inorg. Nucl. Chem., 9 (1959) 211. K. Krishnan and R.A. Plane, Spectrochim. Acta, 25A (1969) 831. K. Nakamoto, B. Hutchinson, and J. Takemoto, J. Am. Chem. Soc., 92 (1970) 3332.

H. Chon, S.-K. Ihm and Y.S. Uh (Editors)

Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.

623

Chemometric Analysis of Diffuse Reflectance Spectra of CoA Zeolites: Spectroscopic Fingerprinting of Co2+-Sites An A. Verberckmoes*, Bert M. Weckhuysen and Robert A. Schoonheydt

Centrum voor Oppervlaktechemie en Katalyse, Departement lnterfasechemie, K.U.Leuven, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium 1. ABSTRACT A new method for spectroscopic fingerprinting is proposed for CO 2+ in zeolite A. The method is based on the use of different mathematical (GRAMS) and chemometrical techniques (PCA and SIMPLISMA) which were applied on series of diffuse reflectance spectra of CoA zeolite as a function of the Co-content and taken after dehydration at 400°C. Two Co 2÷ species could be determined, which were assigned to trigonal and tetrahedral coordination at the hexagonal windows. Their relative concentrations as a function of the Coloading were determined.

2. INTRODUCTION The coordination of TMI on surfaces is characterized by low symmetry and incomplete coordination. Different sites can be occupied simultaneously, which can lead to overlapping spectra. This is also the case for zeolites. However, the sites are crystallographically known and therefore well-defined. Here, we present an analysis method of such overlapping spectra 2+ with the most simple example Co A, where according to XRD only one site is present: the oxygen six-ring. Diffuse reflectance spectroscopy is ideally suited for probing the coordination environment via d-d transitions measured in the VIS region. The DRS spectra of CoA zeolites as a function of Co-content are unraveled with chemometrical techniques, like Principal Component Analysis (PCA) 1'2 and SIMPLe-to-use Interactive Self-Modeling 35 Analysis (SIMPLISMA) -. Chemometric analysis is not yet generally used in spectroscopic 67 investigations of catalysts. ' Here, the results on the number of components and the obtained spectroscopic signatures of the different coordination sites of Co 2+ will be discussed.

3. EXPERIMENTAL SECTION

3.1. Sample preparation and spectroscopy Zeolite A (HMV 7) was exchanged with COC12.6H20 to obtain cobalt zeolites with variable cobalt content. 6 The Co 2+ contents of the samples as determined by atomic absorption spectroscopy (AAS) after acid dissolution of the solids were 0.25, 0.5, 0.86, 1.32, 1.85, 2.18, 2.48 and 2.72 Co2+/UC. The samples were dried, granulated (0.25-0.40 mm) and calcined at

624 400°C in a DRS flow cell during 24h in an oxygen stream. Diffuse reflectance spectra were taken on a Varian Cary 5 UV-VIS-NIR spectrophotometer at room temperature in the 2002500 nm region. The spectra were recorded against a BaSO 4 standard (KODAK). Before applying mathematical/chemometrical techniques, the spectrum of the NaA support after dehydration at 400°C was subtracted from each of the CoA spectra and a baseline correction was performed.

3.2. Mathematical and chemometrical techniques (i) The spectra were decomposed in Gaussian bands with a Grams 386 software package of Galactic. Industries. Corp.. The band positions were estimated by eye and were kept almost constant when decomposing the series of spectra. Also the bandwidths were kept almost constant. (ii) P C A 1'2 is a factor analysis method and the model is:

A=T.B+EA A (mxn) is the spectral data matrix of m samples taken at n digitized wavelengths. B is a hxn matrix with h the number of PCA basis vectors, also called the loading vectors or loading spectra. T is a mxh matrix with in the columns the intensities ('scores') of the h loading vectors for the m samples. The columns in T are orthogonal. E A is the matrix of the spectral residuals. PCA describes the spectral information via principal components. This means that each component maximally describes the spectral variation in A. The PCA analysis is performed by using the Chemometric toolbox of MATLAB. (iii) SIMPLISMA (SIMPLe-to use Interactive Self-Modeling Analysis) 35 is a method to resolve the spectral data matrix A (mxn) in pure component spectra. The method is based on the principle of the pure variable. This is a variable, in this case a given wavelength, at which the intensity comes from one component only. The model is:

-jr= C.S T is the transpose of the spectral data matrix. S is a (kxm) matrix with the unknown pure spectra in the mixture and C a (nxk) matrix with in the columns the fractional contributions of the pure spectra, k is the number of pure spectra. When using in C the observed intensities m

of the pure variables, S can be resolved by the method of least-squares. The SIMPLISMA software has been developed by Willem Windig of KODAK and runs under MATLAB.

4. RESULTS In order to gather information on the speciation of cobalt, different techniques have been applied on the DRS spectra of Co2+A and the visible region has been selected because of the resolution. A flowshart of the applied methods is shown in figure 1. With the Grams decomposition method the spectra were systematically resolved in Gaussian bands. In addition to the mathematical fitting of spectra, chemometrical techniques were introduced. First PCA was used to obtain the number of components. This number is necessary in the

625 SIMPLISMA analysis. The latter method results in pure component spectra and intensities of the pure components in the individual spectra, which allows spectroscopic fingerprinting.

0.5 0 -0.5

500

550

600

650

700

750

800

WAVELENGTH (nm)

Chemom

Mathemati

DECOMPOSITION

PCA

_ component number prediction

,b

SIMPLISMA

_ pure component spectra 56O

60e

~de

_ intensity profiles

WAVELENGTH(nm)

Figure 1. Flowshart for the method followed to obtain information on the speciation of C o 2+ in zeolites. The upper figure contains the DRS spectra of CoA with increasing Co-content. The figure left below is the decomposed spectrum of Co2.48A. Figure 2 gives the overall spectrum of Co].85A after dehydration at 400°C. In the visible region a band is present around 390 nm. A triplet can be observed with maxima around 538, 580 and 637 nm with a shoulder at 733 nm. In the near infrared a broad overlapping region of cobalt exists with at 1385 nm and 2200 nm respectively the overtone and combination bands of hydroxyls.

626

tm-

Nanometers

Figure 2. DRS spectrum of Con 85A after dehydration at 400°C. 4.1. S p e c t r a l d e c o m p o s i t i o n

The visible region of the CoA spectra was decomposed in Gaussian bands at 505, 538, 580, 637, 692 and 733 nm. The band positions were kept constant with a small variation of + 3 nm. Figure 3 shows the intensity courses of the band areas of the band decomposition of CoA dehydrated at 400°C. The band evolutions are shown for the separate bands at 505, 692 and 733 nm. A summation of the band areas has been taken for the triplet bands around 538, 580 and 637 nm. There is a global increase of the band areas with increasing Co2+-content, except for the band at 733 nm. The upper curve in figure 3 is the overall intensity of the visible region (sum of the band areas of the six bands). The intensity course is not strictly linear, which is an indication that more than one component contributes. 480

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# Co/UC Figure 3. Band areas of the separate bands at 505, 692 and 733 nm; of the sum of the bands at 538, 580 and 637 nm; and of the sum of the six bands, all as a function of Co content for CoA dehydrated at 400°C. 4.2. P r i n c i p a l C o m p o n e n t A n a l y s i s

PCA is a chemometrical tool for the determination of the number of principal components. It is an explorative technique and can be used as a predictive step before u

SIMPLISMA analysis. Because normalization of the spectral data in A is a necessary step in the SIMPLISMA procedure, the same data pretreatment must be performed for a well

627 matched PCA analysis. The normalization formula of SIMPLISMA for a set of spectra (j = 1...m) taken in a wavelength region with n wavelengths (i = 1...n) and equal intervals, is: _

X~

Xu

_

IIx,ll

~ +~t~)

with ~i and ~ti respectively the standard deviation and mean at wavelength i of the m spectra. This normalization procedure can be simulated by variance scaling (VARSCALE) the data, which is an optional function of the chemometrics toolbox of MATLAB. Determining how many of the principal components to keep is a crucial step in factor-based techniques like PCA. The indicator function PCAREV calculates the Reduced Eigenvalues (REV) according to the method of Malinowski. 8 It looks at the eigenvalues associated with each eigenvector and is proportional to the amount of variance in the data. Tabel 2 gives the reduced eigenvalues for the CoA data. From the REV% values in table 2, it is derived that approximately 97% of the variance can be explained with two eigenvectors or PCA components. If more than two factors are kept, one is in danger of overfitting the data and adding noise. Table 2 Reduced Eigenvalues (REV) of the VARSCALED spectral CoA data RANK REV REV% 1 0.1206 94.88 2 0.0025 1.9669 3 0.0012 0.944 4 0.0012 0.944 5 0.0007 0.0055 6 0.0005 0.0039 7 0.0003 0.0024 8 0.0001 0.0008

4.3. SIMPLISMA Taking into account the PCA prediction of two components for CoA, SIMPLISMA can highlight the pure spectra and their intensity profile. -3 A. -3 B. xlO xl0 lO ~, 011 5

o 700

600

500

WAVELENGTH (nm)

700

600

500

WAVELENGTH (nm)

Figure 4. Pure component spectra of component 1 (4A) and component 2 (4B).

628 Figures 4A-B show the pure spectra. The first component has two main absorption bands at 666 and 616 nm, accompanied by a band at 512 nm. The second component has three bands at 635,582 and 546 nm and a small band at 738 nm. Figure 5 gives the intensity contributions of the pure spectra to the individual CoA spectra. Both components increase with increasing C o 2+ c o n t e n t .

O

200

comp. 2,..-" ~Z L)

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# Co/UC Figure 5. Intensity contributions of component 1 and 2 to the individual DRS spectra of CoA. Table 3 gives the P3ure variables and the corresponding weight, purity and purity-corrected standard deviation. The values in the third row of table 3 are almost zero, which is an indication that only noise is left after two components were selected. Table 3 Relative total intensities of the weight, purity and purity-corrected standard deviation. Pure variable Weight Purity Stdev selected -

100

100

100

688 552

1.5371 0.0102

1.2587 0'0068

1.2512 0.0030

4.4. Comparison of the different techniques

The best fit decomposition of the CoA spectra gives six Gaussian bands. The band positions and widths are given in the first and second column of table 4. The third and fourth column of table 4 give respectively the absorption maxima of the pure spectra of component 1 and 2, resolved with SIMPLISMA. There is a good agreement between the band positions at 538, 580, 637 and 733 nm decomposed with Grams and the absorption maxima of component 2 of SIMPLISMA. Correspondence also exist between the intensity contributions of the sum of the three bands in figure 3 and component 2 in figure 5.

629 Table 4 Band positions and widths of the Gaussian bands in which the CoA spectra are decomposed and positions of the absorption maxima of the pure spectra obtained with SIMPLISMA. Gaussian bands Gaussian bands component 1 component 2 (SIMPLISMA) (SIMPLISMA) L/nm width L/nm L/nm L/nm 512 505 + 3 40 + 4 546 538 + 3 38 + 3 582 580 + 3 46 + 6 616 637 + 3 70 635 666 692 + 3 551+ 6 733 + 3 20 738

5. DISCUSSION

The three spectral analysis techniques (decomposition in Gaussian bands, PCA and SIMPLISMA) point together to the presence of two Co 2+ species in CoA dehydrated at 400°C. One of the components (component 2, figure 4B) closely matches the experimental spectra both in band position and in intensity course. For proper use of SIMPLISMA the following must be considered: (1) the components must be pure, (2) the different components may not be correlated and (3) the law of Lambert-Beer must be valid, which means that spectra in the non-linear absorption regime can't be used for the analysis. Klier proposed a single nearly trigonal symmetry of Co 2+ in CoA, calcined at 350°C, where Co 2+ is coordinated to three framework oxygens almost in the plane of the six-ring. 9 Heilbron and Vickerman suggested the existence of a pseudo-tetrahedral Co(Ox)3 O2 or Co(Ox)3OH- species (Ox=lattice oxygen) after dehydration at 400°C and the development of trigonal CoO3 at higher temperatures, l° From our chemometrical methods a co-existence of two coordinations after dehydration at 400°C is most probable. The bonding of cations with non-lattice oxygens is common for polyvalent cations such as C 2+, La 3+ and Ce 3+ in X- and Y-type zeolites. 1113 These two components are also present in Co2+-exchanged faujasite-type X- and Y-zeolites, as was found in a recent study. 6 The question is how to assign the coordination types to real sites in zeolite A. There are three six-ring sites in zeolite A: in the cubo-octahedron, in the plane of the six-ring and in the supercage. 14'15 When located in the plane of the six-ring, the coordination is trigonal. In the two other cases the coordination is pseudo-tetrahedral if a fourth extra-lattice ligand is present. We suggest that the pure spectrum of the first component with two absorptions in the 610-680 region and one at 525 nm (figure 4A) corresponds to pseudo-tetrahedral symmetry and that the pure spectrum of the second component with three bands at 635,582 and 546 nm and with a small band at 730 nm (figure 4B) corresponds to trigonal symmetry. Pseudotetrahedral cobalt is thus assumed to make up part of the coordination when CoA is fully dehydrated, but the component which matches best the experimental spectrum is trigonal. For the exact interpretation of the pure spectra calculations of theoretical spectra of the coordination of Co 2+ at the six-rings are in progress.

630 6. CONCLUSIONS A combination of band decomposition, PCA and SIMPLISMA applied on DRS spectra, has proved to be useful for the determination of Co 2+ coordinations in zeolite A. After dehydration at 400°C two components have been identified, which were assigned to trigonal and pseudo-tetrahedral symmetry of cobalt at the six-ring sites of zeolite A. Both components are common to two of the three earlier defined components in faujasite-type X-and Y-zeolites. Future work will be directed towards an extension of the chemometric techniques which can aid in the spectroscopic investigation of zeolites and of heterogeneous catalysts in general. A.A.V. acknowledges a grant of the I.W.T. (Belgium) and B.M.W. a grant as research assistant of the National Fund for Scientific Research of Belgium (N.F.W.O.). This work was financially supported by the Geconcerteerde Onderzoeksactie (GOA) of the Flemish Government and by the Fonds voor Kollectief Fundamenteel Onderzoek (FKFO) under grant no. 2.0050.93.

REFERENCES

1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.

S. Wold, Chemometrics and Intelligent Laboratory Systems, 2 (1987) 37. E.V. Thomas, D.M. Haaland, Anal. Chem., 62 (1990) 1091. F. Cuesta Shnchez, D.L. Massart, Analytica Chimica Acta, 298 (1994) 331. W. Windig, C.E. Heckler, F.A. Agblevor, R.J. Evans, Chemom. Intell. Lab. Syst., 14 (1992) 195. W. Windig, S. Markel, J. of Molecular Structure, 292 (1993) 161. A.A. Verberckmoes, B.M. Weckhuysen, J.A. Pelgrims, R.A. Schoonheydt, J. Phys. Chem., 99 (1995) 15222. B.M. Weckhuysen, A.A. Verberckmoes, A.R. De Baets, R.A. Schoonheydt, submitted to J. Catal.. E.R. Malinowski, J. ofChemometrics, 1 (1987) 33. K. Klier, R. Kellerman, P.J. Hutta, J. Chem. Phys., 61 (1974) 4224. M.A. Heilbron, J.C. Vickerman, J. Catal., 33(1974) 434. M.L. Costenoble, W.J. Mortier, J.B. Uytterhoeven, J. Chem. Soc., Faraday Trans. 1, 74 (1978) 466. P.P. Lai, L.V.C. Rees, J. Chem. Soc., Faraday Trans. 1, 72 (1976) 1809. P.P. Lai, L.V.C. Rees, J. Chem. Soc., Faraday Trans. 1, 72 (1976) 1827. R.A. Schoonheydt, Catal. Rev., Sci. Eng., 35 (1993) 129. W.J. Mortier, Compilation of Extra-Framework Sites in Zeolites, Butterworths, Guildford, 1982.


631

R a m a n characterization of the selenium species formed inside the confined spaces of zeolites V.V.Poborchii Ioffe Physico-Technical Institute, St.Petersburg 194021, Russia Institute for Materials Research, Tohoku University, Sendai 980-77, Japan Five types of zeolites containing adsorbed Se species have been studied by Raman scattering. A strong influence of the confinement geometry of the zeolite pores on the structure of the Se species have been observed. New cyclic molecules 5e12 have been found to be stabilized in the large cavities of the zeolite A. Six-membered cyclic Se molecules have been observed in the chabazite and mordenite. In the mordenite, these molecules have been found to coexist with the helical Se chains. Amorphous-like array of the irregular Se chains has been found in the zeolite X. Linear chain of the interacting Se22 anions has been found in the cancrinite channels, chain properties being influenced significantly by the one-dimensional incommensurability between the chain and the cancrinite matrix. 1. INTRODUCTION Zeolites possessing regular system of cavities or channels with diameters -1 nm are very attractive for preparation of arrays of microclusters in the cavities or 1-dimensional chains in the channels. Selenium can be easily injected into the zeolites by adsorption, and some kinds of species can be stabilized in zeolites. Se species confined in the zeolite pores have been studied during almost 20 years. However, this subject is rather complicated, and many questions about the structure and properties of the zeolite-confined selenium are under discussion till now. Bonding of atoms in neutral Se species is organized by orbitals hybridized from s and p atomic orbitals. Each Se atom has two nearest neighbors distanced at 0.23-0.24 nm, the bond angle being 101-106 °. Most stable species of bulk selenium are helical chains in trigonal selenium and Ses molecules in monoclinic selenium [1] . The dihedral angle in selenium structures is quite flexible and its value and sign can be varied in different species. It is helpful to consider chain-like fragments (no change of the sign of the dihedral angle) and ring-like fragments (with change of the sign of the dihedral angle) (fig. 1). In the confined spaces of zeolites, we can expect stabilization of a variety of combinations of these fragments, because the sign of the dihedral angle can easily be changed according to the topology of the zeolite framework. The main purpose of the present work is to determine the structures of Se species confined in the pores of a variety of zeolites, namely: chabazite (Ch), mordenite (M), A, X, and cancrinite (C) using Raman spectroscopy. Why so many different zeolites have been examined? The first reason for this is a possibility to demonstrate the influence of the confinement geometry and of the interaction with the zeolite framework on the structure of the stabilized Se species. The second reason isa possibility to identify some species confined in different zeolites by means of comparison of their Raman spectra (RS).

632 In this work, single crystals and powder samples of zeolites containing adsorbed selenium (A-Se, X-Se, Ch-Se, M-Se, C-Se) have been studied. Structural models for zeolite-confined Se are proposed. This work is a systematic study, which includes critical analysis of some published data (Ch-Se [2] ; A-Se [3-5]; X-Se [3-5]; M-Se [4,6-8]; C-Se [4,7,9,10]). This is a first attempt to combine Raman data of many different zeolites in one paper to deduce logical scheme of material design (selenium is only suitable example) inside zeolite pores with different framework topology, the spectra in this work being presented with some new details compared to the published ones, new interpretations being given. 2. EXPERIMENTAL Synthetic zeolites A (Na12A1125i12048), X (Na92A192Si1000384),and hydrocancrinite and natural chabazite (Caa.6Na0.4A13.6Sis.5021.6) and mordenite (Ca2Na4Si40A18096) have been used. The sizes of crystals were from 30- 50 ~tm (zeolite A) to 2-3 mm (cancrinite along c-axis). Chalcogens have been adsorbed into the zeolites at the temperatures 450 - 550 ° C during several days after the hydration of zeolites. It is not so easy to make Raman measurements of microcrystals such as mordenite using only macrooptical Raman devices. In this work, RS have been studied using the microoptical equipment as well as the traditional macrooptical technique. Usage of a microoptical Raman device consisting of a microscope optically connected with a double or triple monochromator, allowed to find easily microcrystals, to choose high quality crystals or high quality part of a crystal, and using microobjective, to collect effectively the light scattered from the few microns area excited by laser microprobe. Triple Dilor-Z, and double DFS-24 monochromators have been used. 647.1 nm line of the Kr-laser and 514.5 nm line of the Ar-laser have been used for the excitation of RS. The laser light probe power was 1-20 mW and the size was 10-30 l.tm. It is well known that RS of zeolite matrices, which are excited by the visible light, are much weaker than the spectra of the adsorbed chalcogens. The bands of the zeolite vibrations have been weaker than the noise level in RS of all the samples examined. (NasSi6A16024(OH)2)

3. RESULTS AND DISCUSSION. 3.1 IRREGULAR Se CHAINS IN THE ZEOLITE X. The spectrum of X-Se displays a broad band with the maximum at 258 cm -1 in the bondstretching mode region and very weak features in the bond-bending mode region. The spectrum of X-Se is similar to that of amorphous Se (a-Se) [ 11 ] (fig.2). This means that the Se species, which are stabilized in the zeolite X, are similar to the Se species in a-Se, namely,. irregular chains consisting of combinations of the ring-like and chain-like fragments and some portion of the ring molecules. This is not surprising, because the large cavities of the zeolite X (diameter-- 1.3 nm) are associated through the wide windows with diameters -~0.7 nm. Within this confinement geometry, Se atoms can easily construct quite long chains, which penetrate from one large cavity to another one through the windows. Thus, adsorbed species can be associated into the 3-dimensional continuum similar to the bulk amorphous solid. The feature at-~330 cm "1 can be assigned to the charged selenium molecules, probably to Se2, which can be located not only in large cavities, but also in sodalite cages. RS of low-loaded X-Se displays the 330 cm -1 band intensity comparable with the intensity of the 258 cm 1 band.

633

Fig.1. Selenium chain-like (a) and ring-like (b) fragments.

o~-Se8 __/

a-Se

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L /

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25 28

I !

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100

RAMAN SHIFT, cm-1 Fig.2.Structural fragments and Raman spectra of o¢-monoclinic selenium (o~-Se8) [11], amorphous selenium (a-Se) [11], X-Se and A-Se (k0=514.5nm).

634 3.2 STABILIZATION OF THE Sel2 RINGS IN THE LARGE CAVITIES OF THE ZEOLITE A. In contrast with the zeolite X, large cavities of the zeolite A (diameter - 1.14 nm) are connected through the narrow windows with the diameter- 0.42 nm. This is a good condition for the stabilization of separate clusters in the cavities. RS of A-Se (fig.2) displays specific bands, which are not similar to the spectra of known Se species. (Some versions of RS of A-Se are also presented in ref. [3-5]. The relative intensities of the bands seem to be more reliable in the spectrum presented in fig.2) The spectrum doesn't depend on the concentration of selenium. It is not so difficult to show that the Se species responsible for the spectrum differ from the Ses molecules, which are usually considered as species stabilized in the zeolite A (see review [12]). The Ses molecules should display a strong band associated with the symmetric bond-bending mode. This band is clearly seen at 112 cm 1 in the spectrum of the a-monoclinic Se [ 11 ] (fig.2), consisting of the Ses molecules. However, this band is absent in the spectrum of A-Se. On the other hand, one can find strong bands in the spectrum of A-Se at lower frequencies (fig.2). RS of A-Se can be explained, if we consider ring clusters larger than Ses. Let us consider the molecule Sel2 (fig.2, A-Se) with the structure similar to that of the cyclo-dodecasulfur S12 [13]. This molecule consists of alternating ring-like and chain-like fragments and possesses D3a symmetry. Six Se atoms in this molecule occupy positions in the middle plane (black circles in fig.2, A-Se), and six others occupy three positions in the plane higher (white circles) and three positions in the plane lower (gray circles) than the middle plane. The structure of the Sel2 molecule is compatible with the structure of the large cavity of the zeolite A. In fact, six Se atoms from the middle plane can occupy the positions near 4membered rings of (si,ml)O4 tetrahedra (fig.2, A-Se), the molecule being oriented by the threefold axis along the threefold axis of the zeolite. Probably, the orientations of the rings in the neighboring cavities are not correlated. We can estimate roughly the expected frequency of the symmetric bond-bending mode of the molecule Se12 v(Se12) using data for the frequencies of symmetric bond-bending modes of v(Ses)=112 cm "1 [1], v(512)=128 cm1 [14], v(Ss)=218 cm 1 [14]. v(Se12)-v(S12)xv(Ses)/v(Ss) = 128xl 12/218 -~ 65 cm ~. This value is quite close to the frequency 55 cm ~ of a strong band in RS of A-Se, and so we can attribute this band to the symmetric bond-bending mode of 5e12. A strong band at 28 cm "1 can be attributed to the libration of the Se~2 ring in the cavity. More careful calculations of the frequencies and Raman intensities of the Se~2 molecule vibrations [5] show reasonable agreement with RS of A-Se, calculated frequency of the symmetric bond-bending mode being equal to the experimental one 55 cm ~. The assignment of A-Se Raman bands to the vibrations of Se~2 molecule is supported by the A-Se loading density data. The loading density-10.5 Se atoms per cavity is determined in the work [5]. This value corresponds well to the expected value 12 atoms per cavity. 3.3 Se6RING MOLECULES IN THE CHABAZITE CAVITIES. One of the most important problems in the studying of zeolite-confined Se species is to find simple system among zeolites with selenium, which can be used as a base for characterization of more complex systems. The chabazite is a good candidate for the preparation of some kind of simple Se clusters in its cavities. The chabazite cavity sizes

635 (0.67nm x 0.67nm xl.0nm) (fig.3) are too small for Ses ring molecules (Ses diameter is -0.75nm). We can expect stabilization of smaller ring molecule Se6 (fig.3). The symmetry of the Se6 molecule D3d and its size (diameter -- 0.65nm) are compatible with the symmetry and size of the chabazite cavity. According to RS of rhombohedral selenium consisting of Se6 molecules [15], there are four Raman-active internal modes of the Se6 molecule with the frequencies 102 cm l (E e - bond-bending), 129 cm l (Ale- bond-bending), 221 cm 1 (E e - bondstretching), 247 cm ~ (Ale- bond-stretching). RS of Ch-Se (fig.3) displays bands correlating with the Se6 modes. Obviously, the band at 104 cm"1 should be attributed to the E e bondbending mode, the band at 135 cm "l to the Ale bond-bending mode, the band at--220 cm ~ to the E e stretching mode. It should be noted that the band at --220 cm -~ is clearly seen in contrast with the spectrum presented in ref. 2. It is due to the change of the excitation wavelength from 647.1 nm to 514.5 nm and to more sensitive detection. The band at 274 cm "l should be attributed to the Ale bond-stretching mode, but the spectral position of this band differs significantly from the position of the corresponding band in RS of the rhombohedral selenium 247 cm "~. The reason for this is a strong interaction of the Se6 molecules in the rhombohedral selenium, which influences the internal bond strength. If we extrapolate data on the dependence of the frequency of the Ale bond-stretching mode of the rhombohedral selenium on the intermolecular interaction [ 16], we can expect that the frequency of the Ale stretching mode of a separate Se6 molecule should be higher than that of the rhombohedral selenium, the frequency 274 cm ~ being quite reasonable. The band at 135 cm l displays a shoulder at 145 cm "l. It can be attributed to the combination of the Ale bending mode with the libration of the molecule in the cavity or to the forbidden in RS A2u bond-bending mode which become active due to distortion of the molecule. 3.4 HELICAL Se CHAINS AND Se6 RINGS INSIDE MORDENITE CHANNELS. Mordenite channels (elliptic cross section 0.67nm x 0.7nm) formed by 12-membered tings of (Si, AI)O4 tetrahedra are attractive for the preparation of 1-dimensional structures. One can expect that Se atoms form single chains inside channels. In many works, arguments for the stabilization of single Se chains inside mordenite channels have been found. However, the structure of the chains is unclear. Moreover, in all the previous studies (see review [ 12] ) all the observed phenomena had been considered with the assumption that the mordenite-confined selenium forms only the chains. In this section, another point of view is proposed and experimental evidence is given for stabilization of two types of species in the mordenite channels, namely helical Se chains and Se6 ring molecules. If we consider polarized RS of M-Se (fig.3), we can distinguish two types of the bands. We attribute the bands active only for cc-polarization (256 cm l and the low frequency broad band) to the first type and other bands (104, 135, -0220, 274 cm "1) to the second type. The first type bands should be attributed to the Se chain, high Raman activity of the bands in the ccpolarization being associated with the resonant enhancement due to the absorption of the Se chain for the light polarized parallel to the chain. Obviously, the 256 cm l band should be attributed to the symmetric bond-stretching mode, and the low-frequency broad band centered at-- 40 cm "1 to the acoustic-like mode active due to the finite chain length (according to our calculations this band corresponds to a set of 10-20-atomic Se chains). The structure of the chains is probably close to that of trigonal one. In the works [4,7] another chain structure has been proposed to explain anisotropy of RS in the a-b-plane, but as it is shown below and in ref.

636

I

'l

104

o

~hain.

If,~~I eh~ a~

7-

M-Se

tb t

=18.13

~

300 b=20.29

--~

~/

200

c=7.50

100

RAMAN SHIFT, cm'l

Fig.3. Structural fragments and Raman spectra of Ch-Se (2,0=514.5nm) and M-Se (~0=647.1nm); "aa", "bb", and "cc" indicate polarizations of the incident and scattered light beams in respect to the mordenite axes./~ l

220K C-Se

100K y

b q a=12.67 c=5.165

, 300

, 200

_ 1()0

RAMAN SHIFT, cm-1 Fig.4. Structural fragments and Raman spectra of C-Se (~0=514.5nm) for the polarizations of incident and scattered light beams parallel to the c-axis of the cancrinite. Linear dimerized Se chain and periodic (period is equal to c/2) potential of the cancrinite in the center of channel are schematically shown.

637 [8], the a-b-anisotropy of RS is associated with another kind of Se species in the mordenite channels. It is clear that the second type bands coincide with the bands of Ch-Se. Obviously, the mordenite channel contains the same molecules as the chabazite cavity contains, namely Se6. The polarization dependence of RS of M-Se corresponds to the Se6 molecule oriented by the threefold axis along the b-axis of the mordenite crystal. In fact, the bands at 104 cm 1 and 135 cm ~, which can be assigned to the Eg and A~g bond-bending modes, should be less active, when the incident and scattered light beams are polarized parallel to the threefold axis of the molecule, Eg mode being forbidden in this geometry. Alg symmetric bond-bending mode is not forbidden, but it is much more active for the polarizations of the incident and scattered light beams parallel each other and perpendicular to the threefold axis of the molecule (our calculations show that the Raman activity of the Axg bond-bending mode is negligible for this polarization). To summarize two types of Se species, namely helical chains and Se6 ring molecules are stabilized in the mordenite channels, the rings being oriented by the threefold axis along the baxis of the mordenite. It is interesting to note that the intensity ratio of the chain Raman bands to the Se6 bands is almost the same for. different M-Se samples prepared under different conditions. Probably, it means that some kind of regular arrangement of chains and Se6 rings occurs.

3.5 LINEAR CHAIN OF INTERACTING Se22" ANIONS IN THE CANCRINITE CHANNELS AND INCOMMENSURABILITY BETWEEN THE CHAIN AND THE CANCRINITE MATRIX. Cancrinite channels as well as mordenite ones are formed by the 12-membered rings of (Si,A1)O4 tetrahedra. However, free space of the cancrinite channel is smaller than that of mordenite, because there are 2 Na ÷and 2 OH per unit cell in the cancrinite channel [9]. RS of C-Se (fig.4) at the temperatures 50-400 K displays dominant band at ---246 cm 1 (this value is determined at 300 K) which practically doesn't change when the temperature changes, but the spectrum at lower frequencies changes significantly. (RS of C-Se for smaller temperature interval are also presented in ref. [4,7,9].) According to the x-rays diffraction data [9,10] Se atoms occupy positions in the center of the channel (fig.4) and display wide distribution along the c-axis of the cancrinite. All these data can be explained, if we suppose existence of the interacting selenium dimers in the channels, linear dimerized chain of Se atoms being formed. In fact, the 246 cm 1 band can be attributed to the internal dimer mode. The wide distribution and the temperature dependence of RS can be explained, if we suppose incommensurability between the chain and the cancrinite matrix along the c-axis. A misfit between the lattice parameter of the chain and that of the cancrinite depends on the temperature, and so the arrangement of dimers in the chain depends on the temperature also. Our examination of the C-Se x-rays photoelectron spectra (ESCA) shows that Se22 anions are stabilized in the cancrinite. The frequency 246 cm "1 is quite reasonable for the vibration of Se22". During adsorption, probably, 2OH" are substituted by one Se22 in the channel. In this case, interaction between Se22 and closely connected with the cancrinite framework 2Na ÷ should be quite strong. This interaction determines coupling between the chain and the incommensurate cancrinite matrix. Such a system can be basically described as a

638 1-dimensional incommensurate system. Molecular dynamics simulation of the temperature dependence of the structure and RS of linear dimerized chain under the action of incommensurate periodic potential [ 10,17] show qualitative agreement with the experimental data. 4. CONCLUSION A variety of structures from Se atoms have been experimentally designed in free spaces of different zeolites and corresponding RS have been studied. The structural models for zeoliteconfined Se species are proposed. Most important conditions, which influence the structure of the Se species, are the topology of the zeolite framework and the interaction with the host zeolite lattice, incommensurability being important. Se species, which are unstable in other conditions, have been found to be stable inside zeolite pores, giving rise to new types of the zeolite-based solids. Acknowledgments. The author is grateful to V.N.Bogomolov for supplying mordenite and cancrinite, to V.P.Petranovskii, S.G.Romanov, and Y.A.Barnakov for the sample preparation, to A.V.Shchukarev for the ESCA of C-Se and to the International Science Foundation (grant R4P300) for the partial support of the work. REFERENCES 1. R.M.Martin, G.Lucovsky, K.Helliwell, Phys.Rev.B 13 (1976) 1383. 2. Yu.A.Barnakov, V.V.Poborchii, A.V.Shchukarev, Phys. Solid State 37 (1995) 847. 3. V.N.Bogomolov, V.V.Poborchii, S.V.Kholodkevich, JETP Lett. 42 (1985) 517. 4. V.V.Poborchii, Proc. of the 1-st Japanese-Russian Meeting "Material Design Using Zeolite Space", Kiryu, Japan, 1991, p. 1. 5. V.V.Poborchii, M.S.Ivanova, V.P.Petranovskii, Yu.A.Bamakov, A.Kasuya, Y.Nishina, Materials Science & Engeneering A, in press. 6. V.N.Bogomolov, V.V.Poborchii, S.G.Romanov, S.I.Shagin, J.Phys. C: Solid State Phys. 18 (1985) L313. 7. V.V.Poborchii, J.Phys.Chem.Sol. 55 (1994) 737. 8. V.V.Poborchii, Chem.Phys.Lett., in press. 9. V.N.Bogomolov, A.N.Efimov, M.S.Ivanova, V.V.Poborchii, S.G.Romanov, Yu.I.Smolin, Yu.F.Shepelev, Sov.Phys.Solid State 34 (1992) 916. 10.Yu.A.Barnakov, A.A, Voronina, A.N.Efimov, V.V.Poborchii, M.Sato, Inorganic Materials, 31 (1995) 748. 11. A.Mooradian, G.B.Wright, "The Physics of Selenium and Tellurium" (Pergamon, London, 1969), p.269. 12. G.D.Stucky, J.E.MacDougall, Science 247 (1990) 669. 13. A.Kutoglu, E.Hellner, Angew. Chem. 78 (1966) 1021. 14. R. Steudel, Spectrochimica Acta, 31A (1975) 1065. 15. K.Nagata, K.Ishibashi, Y.Miyamoto, Jap.J.Appl.Phys., 20 (1981) 463. 16. K.Nagata, K.Ishibashi, Y.Miyamoto, Jap.J.Appl.Phys., 22 (1983) 1129. 17. V.V.Poborchii, A.N.Efimov, to be published.


639

D e t e r m i n a t i o n of basic site location and strength in alkali e x c h a n g e d zeolites. D.Murphy, P.Massiani*, R.Franck and D.Barthomeuf Laboratoire de Rdactivitd de Surface, URA 1106 CNRS, Universitd Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France ABSTRACT IR studies using adsorbed pyrrole were used to characterise the basicity of alkali exchanged X, Y and EMT. Specific IR bands due to pyrrole adsorbed on the basic framework oxygens adjacent to alkali cations at sites I', I and II were identified. The relative strengths and various locations of the basic sites were determined for the first time. The inherent heterogeneity of basic site strengths is shown to vary significantly from site to site for a given cation. Within the series NaEMT, NaY and NaX the relative strength of the basic sites increases for the framework oxygens associated with Na + cations from sites I' to I to II. 1. I N T R O D U C T I O N The basicity of alkali exchanged zeolites has received growing attention of late because of the intrinsic catalytic properties of these materials [1,2]. Recent studies on basic zeolites have focussed on the characterisations of the basic sites themselves in order to obtain more accurate information on the strength and density of these sites [3-6]. Although various techniques have been used to study basicity in zeolites, on an experimental basis no single technique is currently able to locate exactly which oxygens are basic in the framework [2]. Adsorption of IR spectroscopic probes is now widely used to characterise basicity on oxides and zeolites. Pyrrole (C4H4NH) has in particular been fruitfully used for more than a decade as an acid probe ofbasicity in zeolites [3-6, 8-11]. The H-donor pyrrole molecule interacts with the basic framework 0 2- sites (Lewis bases) and the vNH stretching frequency of the NH .... O hydrogen bridge is then used as a measure of the overall basic strength in the zeolite [2-4]. In all these previous IR studies of adsorbed pyrrole, the Vmax of the broad vNH stretching band was taken as the measure of the overall basicity in the zeolite. The idea t h a t pyrrole may be used to probe not only the overall basicity in a

640 zeolite but also the localised basicity was first suggested in the results of Scokart and Rouxhet [4]. This idea was also developed by Kaliaguine based on FTIR, XPS and microcalorimetric studies of the adsorbed pyrrole in different alkali exchanged zeolites [5,7,9]. It was concluded that the basic sites in alkali exchanged faujasites are the framework oxygens adjacent to the alkali cations, and the basicity is determined mainly by the local environment [5,9]. A recent publication by our group using pyrrole confirmed that the basicity in alkali EMT (a hexagonal faujasite) is indeed determined by the local environment and that the basic framework oxygens are those adjacent to the alkali cations at specific extraframework sites (i.e., sites I, r and II) [11]. In other words the pyrrole was employed to probe not only the local basicity but furthermore it permitted the identification of basic sites with different strengths and their specific locations in the zeolite framework. The aim of the present contribution is to extend our previous work on alkali EMT to the detailed characterisation of basicity in faujasites. It will be shown that by deconvolution of the IR spectra of adsorbed pyrrole and comparison with known cation populations (assuming no cation migration), both the locations and relative strengths of the individual basic sites may be determined. 2. E X P E R I M E N T A L

NaX and NaY zeolites were supplied by Union Carbide and NaEMT by Elf Co. Alkali exchanged Y zeolites were prepared from NaY by liquid exchange with a cationic chloride solution as described elsewhere [12]. Unit cell compositions were determined by atomic absorption spectroscopy (Table 1). The zeolites were pressed into self supported wafers for IR analysis. Dehydration of the zeolites was performed as follows: slowly heated (rate=lK min" 1) under a flow of dry air to 773K, followed by evacuation (pressure=10 "3 Pa) for 15 hours at the same temperature. Pyrrole (supplied by Aldrich) was stored over molecular sieves and distilled under vacuum before use. Admission of a known amount (0.7 moYsupercage (s.c.) for Y and 0.62 mol/s.c, for X zeolite) of the probe onto each sample was performed as described previously [11]. The FTIR spectra were recorded on a Bruker IFS 66V spectrometer with a spectral resolution of 2 cm "1. Deconvolution of the spectra were performed using a standard Bruker Opus/IR soi~ware program.

641 TABLE 1. Chemical composition of the zeolitesamples % EXCHANGED CATION

SAMPLE

FORMULA

LiY

Li37Na18(A102)55(SiO2) 137

NaY

Na56(A102)56(SiO2) 136

100% Na +

KY

K54Na2(A102)56(SiO2) 136

96%

K+

RbY

Rb45Na9(A102)54(SiO2) 138

83%

Rb +

CsY

Cs45Na9(A102)54(SiO2)138

83%

Cs +

NaEMT

Na20(A102)20(Si02)76

100% Na +

NaX

Nas6(A102)86(Si02)106

100% Na +

67%

Li +

3. R E S U L T S Figure 1(a-e) shows the IR spectra in the 3600-2750 cm -1 region of pyrrole adsorbed on dehydrated Li, Na, K, Rb and CsY respectively. The broad band located between 3450 and 3200 cm -1 is generally assigned to the vNH stretching frequency of the NH .... O hydrogen bridge of chemisorbed pyrrole interacting with a basic site [3,5,10]. The complete IR spectrum of pyrrole adsorbed on basic zeolites is quite complex and has been reported elsewhere [3-6,10,11]. The complex series of narrow bands in the low wavenumber region have been recently discussed in detail [10]. Figure 2 shows the analogous spectrum of pyrrole adsorbed on dehydrated NaX. It can be clearly seen from these figures that the vNH band profile changes dramatically from the LiY to CsY series and to NaX, evidencing an inherent complexity in the strength and heterogeneity of the basic sites. In order to better understand this complexity and to identify the various intrinsic components of the broad vNH band, a band simulation program was used to deconvolute the experimental spectra in Figures 1 and 2. The upper trace in each figure represents the experimental spectrum while the lower trace represents the computer fitted spectrum. The accuracy of the fit was determined from the RMS error which was less than 0.0005 in each case. The individual vNH component bands are plotted in Figures 1 and 2 (the narrow combination bands are not shown). For all samples the position (a) and relative integrated intensities (b) of each component band are listed in Table 2 together with the available percentage cation distributions at specific extraframework sites as reported in the literature. The analogous IR spectra and deconvoluted bands for NaEMT have been presented elsewhere [11], so that only the resulting data are listed in Table 2 for completeness.

642

,

1

i .....

a

tq

IM

d

u~d

d

3500

:~50

3000

IJavenumber ca -I

2750

3500 I

1

3250 " 3000 gavenumber cm"l

I

2250

C

d

.e.,i

..ac~

d

. . . . . .

1

"

_

3500

3250

3 )oo

Javenumber ~'

,1

d~

.~,~

275o !

.,,.4

:S

d

t5

\

_

d 3500

3250 3000 14aventeber ol -I

2750

3500

3250 3000 l~avenumber cm-1

2750

Figure 1. Experimental and fitted IR spectra of pyrrole adsorbed on dehydrated (a) LiY, (b) NaY, (c) KY, (d) RbY and (e) CsY showing the vNH component bands.

643 T A B L E 2. (a) Wavenumbers of deconvoluted IR bands (cm -1) assigned to basic sites adjacent to alkali cations in inner or supercages sites and (b) relative i n t e g r a t e d i n t e n s i t y of the deconvoluted band together with the (reported

percentage alkali cation distributions per unit cell from the literature). SAMPLE

I n n e r sites

S u p e r c a g e sites

Ref.

I! I H 3432 3415 3293 (b) 7,9 % (7.9%) (13) 48.0% (45.0%) 23, !% (23.2%) NaX? .... (a) 3380 3319 3269 (b) 30,1% (31.3%) 5,2% (3.2%) 35,4% (33.4%) (14) LiY (a) 3445 3403* 3359* 3325 3228 (b) 45.6% (57.9%) 24.3% 10.4% 12.9% (12.8%)6.8% (15) NaY (a) 3405 3356 3273 (b) 29.2% (28.3%) 9.3% (5.7%) 62.5% (66.0%) (16) KY (a) 3383 3303 3246 36.1% (36.0%) 12.3% (10. 7%) (b) (17) 51.6% (53.3%) RbY (a) 3403* 3354* 3283 3180 5,1% 43.5% 40.8% (b) 10,5% CsY (a) 3399* 3359* 3280 3173 2,0% 47.3% 37.4% (b) 13,3% * Component bands associated with unexchanged Na + cations, t R e m a i n i n g cations located at other specified framework sites.

N a E M T t (a)

4. D I S C U S S I O N 4.1 C h a r a c t e r i s a t i o n of basic sites in alkali Y zeolites The bathochromic shift in the vNH stretching vibration of adsorbed pyrrole is used to monitor the oxygen framework basicity in zeolites [3-5]. Component bands with different vNH frequencies should then suggest the presence of various basic sites with different relative strengths. In alkali exchanged EMT, a heterogeneous distribution of basic sites was observed and related to the localised nature of the basicity [11]. Since the charge on the framework oxygen will depend not only on the SiOA1 angle and T-O distance, but also on the M+-O distance (where M + is the alkali cation), the basicity of the oxygens adjacent to these exchanged cations will vary from site to site as the M +O distance varies. In EMT, each individual component band of the deconvoluted vNH band was assigned to the basic framework oxygens adjacent to an alkali cations at sites r, I and II. This assignment was proposed based on the similarities between the relative integrated intensities of the different component bands and the known distribution and population of cations per unit cell [11]. A similar interpretation of the component bands in Figure 1 can be made for the alkali Y zeolites. The percentage distribution of Na + cations per unit cell of dehydrated NaY are 28.3, 5.7 and 66.0% Na + at sites I', I and II respectively, as

644

1

_,_1

. . . .

I

_

vNH/cm-I 3450 r

|

~ S i t e

32503300~ 0,32

I

[ _

0,37

0,42

Negative Oxygen Charge

0

dJ

3500

~ 3250 ~

°'~

2750

@NaEMT

~NaY

@NaX

F i g u r e & Relationship of oxygen F i g u r e 2. Experimental and fitted IR dmrge and vNH frequency for pyrrole ads. at basic sites in EMT, Y and X. spech-a of pyn~le on dehydrated NaX. determined by 23Na NMR [16] in the absence of adsorbate (Table 2). The relative integrated intensities of the three IR component bands at 3405, 3356 and 3273 cm-1 were 29.2, 9.3 and 62.5% respectively. Based on the similarities with the above percentage Na + distribution, the three IR bands can be assigned to the basic framework oxygens adjacent to Na÷ cations a t sites I', I and II. In addition it is well known that the cations of sites I and F are connect~ to oxygens 0(3) while the II cations are linked to 0(2) oxygens. Since adjacent I' and I sites are not occupied simultaneously, this menn~ that three different types of"potential basic" oxygens e ~ in NaY, in agreement with the above pyrrole results. Confirmation of the above assignments for basic sites in NaY can be obtained from the LiY, RbY and CsY deeonvoluted spectra. In these three samples unexdmnged Ha + cations (Table 1) are present which should be in inner cavities (I and/or I') since the supercage II cations are more easily exchanged. Therefore evidence of basic sites associated with inner cavity Na + cations should be apparent in these IR spectra. Component bands are indeed visible at 3403 and 3359 cm-1 in LiY, RbY and CsY which were also visible in the NaY spectnnn and assigned to the basic sites associated with the Na + cations at sites I' and I. Moreover chemical analysis reveals that 17% of the Na + cations remain unexchanged in RbY and CsY while 33% remain unexchanged in LiY. In agreement with these values the relative intensities of the Na + related bands (3403 and 3359 cm-1) in RbY, CsY and LiY were 15.6, 15.3 and 34.7% respectively. Based on the comparison between the percentage integrated intensities of t h e remaining deconvoluted bands in LiY and the reported % Li+ cation populations, the various component bands in LiY can be identified as the basic

645 sites adjacent to the Li + cations at sites r and II (Table 2). A minor band visible at 3465 cm -1 in LiY (and also observed in NaY) can be assigned to pyrrole adsorbed on a Lewis acid site as discussed previously [11]. The two r e m a i n i n g bands in RbY and CsY may be assigned to framework oxygens associated with Rb + and Cs + cations in the supercages. It was recently proposed t h a t some Cs + cations may occupy inner cavities (I and I') [18] and the consequences of this on our present findings is currently under investigation. In KY the a g r e e m e n t between component band intensities and K + cation distribution is quite good (Table 2), so t h a t the deconvoulted bands in Figure lc may be identified as the framework oxygens associated with K + at sites r, I and II.

4.2. D e p e n d e n c e of basic strength on alkali cations at specific sites. It is well known t h a t the overall basicity of the framework oxygens increase with an increase in the electropositivity of the countercation. This dependence also occurs at localised sites as evidenced from the present results. In Y zeolite all the exchanged alkali cations occupy site II. (In RbY and CsY the majority of the Rb + and Cs + cations also occupy supercage sites II). The vNH frequency for the site II related component bands (where two "supercage" bands are observed a weighed average of the two was taken) shifts to lower cm -1 from LiY (3299 cm -1) to NaY (3273 cm -1) to KY (3246 cm -1) to RbY (3233 cm -1) to CsY (3230 cm-1). In other words for the same basic site adjacent to the alkali cation at site II, the relative basic strength increases as the alkali cation is exchanged from Li + to Cs +. 4.3. Comparison of localised basic sites in NaEMT, NaY and NaX. The location of the basic sites in N a E M T [11] and NaY (section 4.1) have been identified. The same comparative procedure when applied to NaX containing adsorbed pyrrole, again assuming no cation migration, also enables the identification of the basic site locations in this zeolite (Figure 2 and Table 2). N a E M T is a hexagonal faujasite and the "types" of cations sites in this zeolite are the same as those in the NaY and NaX faujasite structures (i.e., the r, I and II sites are structurally equivalent in all three zeolites). The basic sites adjacent to the Na + cations at these specific sites I', I and II have been identified in all three zeolites. The values of negative oxygen charge calculated from the Sanderson electronegativity equalization principle [19] for NaEMT, NaY and NaX are plotted in Figure 3 as a function of the vNH frequency of pyrrole adsorbed on basic oxygens adjacent to site r, I and II Na + cations. The basicity in alkali exchanged zeolites is well known to depend on the alkali cation present and the Si/A1 ratio. For a given cation (Na +) as the Si/A1 ratio is increased from X to Y to EMT the

646 respective negative charge on the oxygens increases. This graph clearly illustrates the further dependence of zeolite basicity on specific local environments and the distribution of basic site strengths depending on the site of the adjacent cation. The trends of increasing basicity for the basic sites adjacent to the Na + cations from sites r to I to II are similar in all three zeolites (X, Y and EMT). 5. CONCLUSION Pyrrole was used to characterise the basicity of alkali exchanged zeolites. Using a curve fitting program, the broad vNH band of pyrrole adsorbed on framework basic sites of alkali Y and NaX zeolites was deconvoluted into several component bands. Based on the comparison between cation site populations and integrated intensities of these deconvoluted bands, the locations of the different basic sites can be identified and their relative strengths determined. The basicity in these alkali zeolites depends therefore not only on the Si/A1 ratio or the nature of the alkali cation but also on the localisation of the basic sites themselves.

Acknowledgements. Financial assistance from the EU under the HCM network (contract no. CHRX-CT94-0477) is gratefully acknowledged.

REFERENCES 1. Hathaway, I.E. and Davies, M.E., J.Catal., 116 (1989) 263. 2. Barthomeuf, D., Catal.Rev., (1996) submitted. 3. Barthomeuf, D., J.Phys.Chem. 88 (1984) 42. 4. Scokart, P.O. and Rouxhet, P.G., Bull.Soc.Chim.Belg., 90 (1981) 983. 5. Huang, M. and Kaliaguine, S., J.Chem.Soc., Faraday Trans., 88 (1992) 751. 6. Xie, J., Huang, M. and Kaliaguine, S., Catal.Lett., 29 (1994) 281. 7. Huang, M., Adnot, A. and Kaliaguine, S., J.Catal., 137 (1992) 322. 8. Akolekar, D.B., Huang, M. and Kaliaguine, S., Zeolites, 14 (1994) 519. 9. Huang, M., Kaliaguine, S., Muscas, M., Auroux, A., J.Catal., 157 (1995) 266. 10. Binet, C., Jadi, A., Lamotte, J. and Lavalley, J.C., J.Chem.Soc., Faraday Trans., (1995) in press. 11. Murphy, D., Massiani, P., Franck, R. and Barthomeuf, D., J.Phys.Chem., (1996) submitted. 12. Prasad Rao, P.R.H., Massiani, P. and Barthomeuf, D., Stud.Surf.Sci.Catal., 84 (1994) 1449 13. Lievens, J.L., Verduijn, J.P., Bons, A-J., Mortier, W.J., Zeolites, 12 (1992) 698. 14. Olson, D.H., Zeolites, 15 (1995) 439. 15. Franklin, K.R., Townsend, R.P., Whelan, S.J. and Adams, C.J., in Proceedings of 7th International Zeolite Conference, Eds., Y.Murakami, A.Iijima and J.W.Ward, (1986) 289. 16. Engelhardt, G., Hunger, M., Koller, H. and Weitkamp, J., Stud.Surf.Sci.Catal., 84 (1994) 421. 17. Mortier,W.J., Bosmans,H.J., Uytterhoeven, J.B., J.Phys.Chem., 76 (1972) 650 18. Koller, H., Burger, B., Schneider, A.M., Engelhardt, G., Weitkamp, J., Micro. Mater., 5 (1995) 219. 19. Sanderson, R.T., Chemical bonds and bond energies, Academic Press, NY, 1976


A spectroscopic s t u d y of t h e i n i t i a l s t a g e in crystallization of TPA-silicalite-1 from clear solutions

647

the

B r i a n J. S c h o e m a n

Department of Chemical Technology, Lule~i University of Technology, S-971 87 Lule~i, Sweden Discrete sub-colloidal (2-4 nm) particles have been identified in TPA-silicalite-1 precursor solutions and isolated from aqueous solution by extraction. The powdered extract sample was shown to possess microporosity, entrapped TPA + cations and a short range order by Raman and FT-IR spectroscopy and electron diffraction. An in-situ light scattering study shows that the sub-colloidal particles are subject to an Ostwald ripening mechanism and the evolution of a second particle population is detected on the sub-colloidal size range.

1. INTRODUCTION Numerous studies dealing with the crystallization of zeolite from apparently clear homogeneous solutions have dealt primarily with the events occurring in solution during the intermediate and latter stages of crystallization. Information on the initial stage in the crystallization event is normally extracted by extrapolating growth kinetics data to zero time according to the method of Zhdanov and Samulevich [1]. One of the difficulties associated with the direct analysis of the initial stage in zeolite crystallization lies in the fact that the crystal sizes are in the sub-colloidal size range, 1-20 nm, as well as that the crystals co-exist with macroscopic amorphous phases. It is generally accepted that the amorphous phase undergoes dissolution thus supplying the solution phase with species that form the first zeolitic structures that grow to the desired crystalline phase. The amorphous silica phase in the crystallization of TPAsilicalite-1 is typically one of several silica sources, for example silicic acid or a colloidal silica sol and the templating agent may be tetrapropylammonium (TPA) hydroxide, TPAOH/TPABr or a mixture of TPABr/NaOH [2,3]. Alternatively, the silica source may be derived from the hydrolysis of tetraethoxy silane with TPAOH [4] thus creating a reaction mixture free from macroscopic amorphous material. Such solutions are idealy suitable for light scattering studies and furthermore, the hydrolysis of silanes is a well investigated topic [5].

648 The purpose of this report is to describe the nature of the TPA-silicalite-1 precursor solution prepared via the hydrolysis of tetraethoxy silane with TPAOH and the events taking place during the initial stage of crystallization in the subcolloidal size range.

2. EXPERIMENTAL 2.1. Materials and preparation of the precursor sol A solution with the molar composition 9TPAOH 25SIO2 480H20 100Ethanol was made up by hydrolyzing a dilute solution of tetrapropylammonium hydroxide, TPAOH (Sigma, 1M in water, 143 ppm Na, 4200 ppm K, < 10 ppm A1) and tetraethoxy silane, TEOS (Merck, > 98%) for at least 24h at room temperature. The hydrolyzed TPA-silicate solution was pre-filtered through a Gelman Sciences Supor Acrodisc membrane filter, 0.2 ~m pore size, whereafter the solution was passed through a Millipore ultrafiltration membrane (Ultrafree-PF) with a nominal molecular weight limit of 100 000. 2.2. Extraction of sub-colloido! precursor particles Sub-colloidal silica particles were extracted [6,7] from the precursor sol prior to hydrothermal treatment. The pH of the silicate solution was reduced to ca. 2 by adding a strong cationic ion-exchange resin (Dowex HCRS-E, duPont) in the hydrogen form whereafter the resin was separated from the acidic sol and a hydrogen-bonding agent, t-butyl alcohol (Riedel-de-Haen, p.a.) was added with stirring. The organic phase was salted out by adding NaC1 (Merck, p.a.) and the organic phase containing the polymeric silica was separated and freeze dried to a powder for further analysis. 2.3. Spectroscopic analyses Diffuse reflectance FTIR (DRIFT) analyses on undiluted freeze dried samples of extract-powder and Ludox® SM (duPont) were performed with a Perkin-Elmer FT-IR 1760X spectrometer. Raman spectr a of uncalcined samples of the extract powder and a reference sample of well crystallized TPA-silicalite-1 synthesized according to the method given in reference 4 were obtained using a Perkin-Elmer PE1700X NIR FT-Raman spectrometer and N2 adsorption data on outgassed (100°C) samples was collected with a Micromeritics ASAP 2010 instrument. Electron diffraction patterns from the extract powder were collected using a JEOL 2000EX transmission electron microscope (TEM) in the diffraction mode.

649

2.4. ln-situ synthesis Quasi-electric light scattering spectroscopy (QELSS) was used to monitor the crystallization of TPA-silicalite-1 with a Brookhaven Instruments BI-200SM goniometer couPled to a Lexel Ar laser operating at 514.5 nm and a laser output effect of 500 roW. F u r t h e r details concerning these analyses are reported elsewhere [8]. The hydrolyzed synthesis solution, ca. 17 ml, was hydrothermally treated i n - s i t u at 70°C. The alignment of the optics was confirmed once the temperature of crystallization was reached.

3. RESULTS AND DISCUSSION The fact that the TPA-silicate solution passes freely through a Millipore ultrafiltration membrane with a nominal molecular weight limit of 100 000 would appear sufficient evidence to term the solution as being a "homogeneous" clear solution. Analysis of the undiluted (viscosity 6.4 cP at 22°C) solution prior to hydrothermal treatment with QELSS shows however that sub-colloidal particles are present in solution as an essentially monodisperse population with an average particle size of 2-3 nm. The particle size as estimated by the reaction of monomeric silica with molybdic acid [7] yields a size of 2.8 nm, in good agreement with the QELSS results. The particle size and narrow distribution has also been confirmed with Cryo-TEM (ca. 2 nm) [8]. A Raman spectroscopic analysis of the aqueous sol was undertaken to identify the presence of structurally entrapped TPA-cations [9] which could support the notion that the sub-colloidal particles possess a structure resembling that of the MFI phase. As shown in Figure la, only the prominent bands of the free TPAOH in the solution phase are visible. In order to detect structurally entrapped TPA+, the free TPA as well as TPA associated with the particle surface should be removed. For this reason, the polymeric silica was e x t r a c t e d from the solution phase - a process entailing removal of free and surface TPA by deionization and separation of the particles from smaller silicate species by hydrogen bonding of tbutyl alcohol with the particle surface. The resulting particle is lyophobic in a saturated saline solution thereby resulting in a phase separation - the organic phase containing the polymeric silica. Raman spectra, Figure 1 of the reference material, TPA-silicalite-1, crystal size 60 nm, shows the charactersitic peaks due to entrapped TPA cations [9]. Note, essentially no other forms of TPA are present in this sample. This spectrum may be compared to that of the freeze dried powder

650 containing the polymeric silica, Figure 1. Since the free and surface associated TPA is essentially absent in this sample, these peaks may be assigned as being due to entrapped TPA in the silicate structure. Since TPA cations are too large to enter the channel structure after the formation of the channels, they must be incorporated during the formation of the sub-colloidal particles, i.e. during the polymerization of silica species released as a result of the base (TPA) catalyzed hydrolysis of TEOS.

°,

(ii)

I

1415

"

?

/\~

I

1300

l\~

I

I

"

I

1200 1100 1000 FREQUENCY SHIFT (¢m-l)

~

!

I

I

I

900

800

740

Figure 1. R a m a n spectra of i) the aqueous synthesis solution showing the absorption bands primarily due to free TPAOH, ii) the structurally entrapped TPA present in the extract powder sample and iii) the structurally entrapped TPA present in XRD crystalline silicalite crystals following deionization. The peaks marked by * are the peaks of interest. DRIFT analysis of the extract powder yields the result that the absorption band at ca. 560 cm -1 assigned to highly distorted double six rings present in the MFI structure [10] is present as shown in Figure 2. The presence of both the 560 cm -1 and the absorption band at ca. 450 cm -1 can be indicative of the presence of the MFI phase [11] although the absorption band at ca. 1220 cm -1 normally present in the DRIFT spectra of well crystallized TPA-silicalite-1 ( all be it a very weak band ), is not evident in the DRIFT spectra of the extract powder. Calcining the powder at 480°C for a time as short as 4 minutes results in the

651 disappearance of the absorption band at 560 cm -1 yielding a DRIFT spectra more similar to truly amorphous silica than that of the MFI phase as seen in Figure 2.

I 1200

800 (cm"l)

6O0

Figure 2. DRIFT spectra of a freeze dried powder (i) containing extracted subcolloidal silicate particles, (ii) of the extract material following calcining at 480°C, 4 minutes and (iii) of truly amorphous silica particles, Ludox~ SM. TEM micrographs of the extract powder show large aggregates of siliceous (as shown by EDX analysis) material as well as a few areas with apparently discrete particles with sizes less than 5 nm. The reason that aggregates are present is due firstly to the extraction and freeze drying process as well as to the method of the TEM sample preparation. A light field image of such a discrete particle is shown in Figure 3a and the diffraction pattern due to this particle is shown in Figure 3b. These diffraction spots correspond to the d-spacings 1.45, 2.11, 2.65, 2.79, 3.853,91, 5.16, 5.43 and 5.61A which are similar to certain peaks in the XRD pattern for crystalline TPA-silicalite-1 in the 2-theta range 16-66 °. This result may seem surprising since it has been stated that precursor particles believed to be nuclei, presumably the size of a few unit cells, contain too few repeat units to yield electron diffraction patterns [12]. No details concerning the minimum particle size detectable by electron diffraction were given. The same particle, analyzed with EDX-analysis, was shown to be a silica particle thus indicating that the

652 siliceous particle may possess an ordered structure similar to that of the MFI phase. N2 adsorption data shows that the freeze dried powder contains microporous material with a pore diameter in the range 4-8 A comparable to t h a t of TPAsilicalite-1 (pore diameter 5.5 A). The BET specific surface area of the powder is 212 m2/g. According to Scholle et.al. [13], microporosity may be detected by N2 adsorption even though TPA is present since the structures are solids with many defects. It is also possible t h a t the observed microporosity arises from the intraparticle cavities that are formed upon sample drying. Caution in the interpretation of these results is therefore necessary.

a)

b)

ii;ii

......................................... ". . . . . . . . . . . . . .

Figure 3. a) A light field image of aggregated siliceous particles imaged with TEM and b) the diffraction p a t t e r n of this particle showing diffraction spots corresponding to short interplanar distances. A critical appraisal of the results presented above indicate t h a t it is most probable that these sub-colloidal particles are highly defected structures and the DRIFT, N2 adsorption data and Raman results (and possibly the electron diffraction data) can not solely support the view that the X-ray amorphous particles possesses a form of short range order similar to that of the MFI phase. In order to reach a final conclusion in this respect, further detailed investigations are necessary. An in-situ QELSS study of the hydrothermally treated sol was performed at 70°C, a relatively low temperature which was chosen so that the slow kinetics would allow for the accumulation of reliable data. The intensity of the scattered light due to the particles in solution as a function of crystallization time is shown in Figure 4a for the initial stage of the crystallization. The appearance of the

653 curve in the interval 0-40 minutes is particularly interesting - the intensity of the scattered light decreases initially and after 40 minutes, it increases almost exponentially. This observation may seem to be unexpected since the temperature increase during the s~mple heating period, ca. 25 minutes, will result in the increase in the Brownian motion of the particles and thus an increase in the intensity of the scattered light should be observed. The temperature increase will however result in a higher solubility of the siliceous particles and a redistribution of silica will take place via an Ostwald ripening mechanism, i.e. smaller silica particles will depolymerize and soluble silica will be deposited onto the larger particles [14]. The number of particles will thus decrease and the net result is a reduction in the intensity of the scattered light. The results of the particle size analysis are shown in Figure 4b. "~

a)

300

-

~ m Z

b) 250 -

200 -

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0

0

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0

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2

4

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CRYSTALLIZATION TIME (h)

o

o

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lo

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Figure 4. a) The scattered light intensity as a function of crystallization time and b) the increase in the average particle size with crystallization time. The average particle size increases initially from 2-3 nm, at room temperature, to 3.5 nm at 70°C. The particle size continues to increase to ca. 6 nm during the first 12 hours of hydrothermal treatment during which period, the particle size distribution (PSD) is monomodal. After ca. 12 hours, a second particle population appears, the PSD changes to a bimodal PSD and the average particle size of the small size-fraction reverts to the original size of 3.5 nm. A reasonable interpretation of these results is that the monomodal PSD's initially observed actually represent the average of two separate particle populations that are not resolved by the light scattering technique. Once the technique is able to resolve the two populations, the PSD of the small size-fraction reverts to its original state as stated above.

654 4. CONCLUSIONS Spectroscopic studies of the polymeric silica in a hydrolyzed silicalite-1 precursor solution indicate that the sub-colloidal particles may possess a short range order but the defect structures require further characterization before their role in the crystallization of silicalite may be determined. The use of a high effect laser scattering system allows one to monitor the events taking place on the subcolloidal size range thus enabling the study of the initial stage in the crystallization of silicalite. 5. ACKNOWLEDGEMENTS

The financial assistance by the Swedish Research Council for Engineering Sciences (TFR) is gratefully acknowledged. REFERENCES

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

S.P. Zhdanov and N. N. Samulevich, Proc. 5th Int. Conf. on Zeolites, (Ed. Rees, L. V. C.), Heyden, London, (1980) 75. J . J . Keijsper and M. F. M. Post, "Zeolite Synthesis" - ACS Symposium Series 398, (Eds. Occelli, M. L. and Robson, H. E.), Washington, DC, (1989) 28. C.S. Cundy, B. M. Lowe and D. M. Sinclair, J. Crystal Growth, 100 (1990) 189. A.E. Persson, B. J. Schoeman, J. Sterte and J-E. Otterstedt, Zeolites, 14 (1994) 557. See for example E. P. Plueddemann, Silane Coupling Agents, Plenum Press, 2nd ed., New York, 1991. R.K. Iler, "Soluble Silicates"- ACS Symposium Series 194, (Ed. Falcone Jr., J. S.), Washington, DC, (1982) 95. B.J. Schoeman, To be submitted for publication. B.J. Schoeman and O. Regev, Submitted to Zeolites for publication. P.K. Dutta and M. Puri, J. Phys. Chem., 91 (1987) 4329. P.A. Jacobs, E. G. Derouane and J. Weitkamp, J. Chem. Soc., Chem. Commun., (1981) 591. G. Coudurier, C. Naccache and J. C. Vedrine, J. Chem. Soc., Chem. Commun., (1982) 1413. J. Dougherty, L. E. Iton and J. W. White, Zeolites, 15 (1995) 640. K . F . M . G . J . SchoUe, W. S. Weeman, P. Frenken and G. P. M. van der Velden, Applied Catalysis, 17 (1985) 233. R.K. Iler, The Chemistry of Silica, Wiley, New York, (1979).


655

Characterization and catalytic properties of the galliumphosphate molecular sieve cloverite R. Fricke~, M. Richter a, H.-L. Zubowa ~ and E. Schreierb alnstitute for Applied Chemistry and bHumboldt-University Berlin, Rudower Chaussee 5, D-12484 Berlin, Germany

Modification of the galliumphosphate cloverite by various heteroelements has been carried out. In the case of Ti-cloverite at least partial incorporation of titanium into the galliumphosphate lattice could be shown. Catalytic properties of both the pure and the Ti- and Si-modified cloverite catalysts show remarkable activity and selectivity in the etherification of isobutene by methanol and ethanol producing MTBE and ETBE, respectively. Modification with titanium influences the Catalytic properties in a negative way whereas Si-modification shows nearly no effect. The nature of acid centers is discussed.

INTRODUCTION During the last years attempts have been made to synthesize molecular sieves with super wide pores. Following the aluminophosphate structure VPI-5 having 18-membered pore openings (18MR) the synthesis of the 20MR type JDF-20 with the same composition and the gaUiumphosphate cloverite (20MR) were the most recent results (not counting the mesoporous M41S system). Cloverite is the only molecular sieve with a three-dimensionally arranged super wide pore system having pores of 13.2 A and large cavities of about 30 A of diameter. In addition, small pores (< 4 A) not intersecting the large pores are present [ 1]. The characterization is mainly concentrated on the template containing samples because the structure easily collapses after detemplation under the influence of moisture. This might be the reason why only limited information on the catalytic properties of cloverite are available. Recently it has been shown for the first time that cloverite, following appropriate in situ decomposition of the template, catalyzes the gas phase etherification of isobutene to methyl tert-butyl ether (MTBE) under atmospheric flow conditions in the temperature range from 363 to 383 K with good performance [2]. The examination of the formation of ethyl tert-butyl ether (ETBE) is a continuation of the former work, despite the fact, that at present the replacement of methanol by ethanol is of minor commercial relevance but could become an alternative as the efficiency of the exploitation of biomass will be improved. As dehydration of ethanol to diethyl ether (and water) is a more facile process than that of methanol we are concerned with the extent of this side reaction and with the influence of the higher gas phase concentration of water on the stability of the cloverite molecular sieve. The present contribution deals also with physico-chemical and catalytic properties of cloverite samples containing Si and Ti as heteroatoms.

656 EXPERIMENTAL Molecular sieves

The synthesis of cloverite was carried out according to [3] but in a microwave oven at 443 K for 1 h. The gel composition was as follows: 1 Ga20 3 : 1 P205 : z HF : 6.0 Q : x EIO : y H20 (Q" quinuclidine; EIO: heteroelement; x = 0-0.3; y = 70-600; z = 1 1-4.1). Catalytic measurements were performed in a flow reactor with a catalyst volume of 1.9 cm 3 containing 90 wt.% cloverite and 10 wt.% SiO 2 binder at a flow rate of 0.9 cm3s -1 and a molar alcohol to iso-butene ratio of 1. The weight hourly space velocity (alcohol and isobutene) was 5.9 h -1. Detemplation was performed in situ under a flow of either nitrogen or air. This procedure is described in detail in Ref. [2]. The fluoride content in the spent cloverite samples was determined after removal of organic residues by calcination of the samples at 773 K in air for 2 h. Alter dissolving the material in diluted H2SO 4 and neutralization with NaOH the F- content was measured with a F- ionselective electrode (Mettler-Toledo). Characterization

Infrared spectra were taken with a Specord M 85 (Carl Zeiss, Jena) FTIR spectrometer. Self-supporting wafers of the samples placed in an IR cell were used for measurements. UV-vis measurements were carried out with a Perkin-Elmer Lambda 19 spectrometer. BaSO 4 was used as standard. The reaction

H3C~ H3c/C~---CH2 + ROH ~

H3C~ /OR H3c/C~cH3

(1)

The etherification reaction of isobutene and relevant side reactions are shown in R = -CH3; -CH2CH3 Figure 1. Equation (1) describes the main reaction leading to the corresponding tertiary 2 ROH ~-~ R20 + H20 (2) ethers MTBE or ETBE. The dehydration of the alcohols to symmetric dimethyl ether H3C~ H3C,,x _jOH (3) H3c/C~CH2 + H20 ~ H3c~C~cH3 (DME) or diethyl ether (DEE) is one undesired side reaction consuming the alcohol and producing water (eq. (2)). Isobutene can be converted to tert-butanol by reaction with H3C~~CH,g---C~CH 2 H3C~ ,7 water (eq. (3)) and/or can oligomerize to CH3 (4) predominantly the two isomers 2,4,4 2 m H3 ~ H3 trimethyl pent-l-ene and 2, 4, 4 trimethyl pentH3c/CmCH2"~ H3C----~mCH--~-~CmCH3 2-ene (eq. 4). As will be shown in a forthcoming paper, CH3 the trimethylpentenes are also etherified by H~ H3C~ ~OR the alcohol according to eq. (5). In the data C~---CH2+ ROH ~ RC,,,,C~cH3__ (5) given in the tables, this reaction is not considered. However, because it additionally consumes ethanol, this side reaction has to be R 1= -CHg--H3 taken into account when comparing the x3 experimental overall ethanol consumption with calculated thermodynamic values which are based exclusively on the reaction Figure 1. Etherification of isobutene with methanol and ethanol. For explanation see text. according to eq. (1). i..x

657

Activity and selectivity Because the reaction proceeds with volume reduction, conversion values were calculated on the basis of the gas phase composition at the reactor outlet taking into account the reaction scheme given in Figure 1. Accordingly, the alcohol concentration at the reactor inlet is given by the sum of unreacted alcohol found at the reactor outlet plus the percentage of tertiary ethers formed and the percentage of symmetric ethers, the latter multiplied by 2 due to stoichiometry. The dehydration of the alcohols leads to 2-3 vol.% DME and ETBE, respectively, at the highest reaction temperature (403 K) but does practically not proceed to a substantial extent in the low temperature range (353-393 K). The selectivity to the tertiary ethers is referred to the conversion of isobutene. Relating the selectivity of these ethers to the conversion of the alcohols would yield values of 100% due to the low percentage of side reactions consuming additional alcohol. Thus, the selectivity to the tertiary ethers is expressed as SE =

100 CE/(CE + 2CDIB) (%)

where cE is the molar ether concentration and CDiB is the molar concentration of diisobutenes (2, 4, 4 trimethyl pent-l-ene and 2, 4, 4 trimethyl pent-2-ene). Higher oligomers when present are considered correspondingly to their carbon number.


Modification by heteroatoms In a former investigation a series of heteroatoms (Ti, Si, AI, Ni, Co, Fe, Mg) has been introduced into the synthesis gel in order to modify properties of cloverite [3]. When applying microwave heating the products have been found to be of enhanced crystallinity. The influence of the heteroatoms on the XRD pattern (compared to pure cloverite) was small. Ti cloverite (Ti-Clo) showed the highest crystallinity and adsorption capacity of all modified samples. For the sake of comparison a highly crystalline Si-Clo sample is additionally involved into this study. In the case of Ti-Clo the possible incorporation of titanium into the cloverite lattice has been proven by ESR measurements of the reduced samples as well as by UV-vis measurements. After reduction in hydrogen at 773 K the Ti-Clo sample shows an axial ESR spectrum of Ti 3+ ions which is temperature dependent ( ~ = 1.93, g tl = 1.88). In particular, . the temperature dependence is usually taken as evidence for (distorted) tetrahedral coordio nation of the Ti 3+ ions. This 200 400 6(30 BOO would mean that it is possible to WovelenQth (nr~) reduce Ti4+ incorporated into Figure 2. UV-vis spectrum of Ti cloverite. the cloverite lattice to the three-

A v-N

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,

,

658 valent state. On the other hand, it still seems difficult to distinguish between titanium located on lattice or extra-lattice positions on the basis of these ESR results alone. However, further evidence for tetrahedral coordination of titanium is obtained by UV-VIs measurements (Figure 2). Well-resolved bands at 214 and 247 nm were shown in the spectra as well as an additional shoulder at about 289 nm. Following an interpretation of UV-vis spectra of Corma et al. [4] for Ti-containing MCM-41 the band at 214 nm should be caused by titanium in low coordination (probably tetrahedral). It should be mentioned at this occasion that the position of this band slightly depends on the conditions of the synthesis, i.e. on the HF concentration used for gel preparation. The band at 247 nm and the shoulder show that "Ti clusters" are also present, i.e. Ti in higher coordination and/or aggregation. Summarizing the ESR and UV-vis results it may be concluded that at least part of titanium is located on lattice positions. At present it is, however, not known which consequences the different location of titanium might have concerning catalytic properties because, up to now, this question has not been investigated in detail. Catalytic results 1. Formation o f MTBE and ETBE over pure cloverite Without any side reaction, conversion data for the alcohol component and isobutene should be the same for the molar reactant ratio of one. This is indeed observed in good approximation for both alcohols at reaction temperatures up to 363-368 K due to the marginal extent of side reactions occuring below 373 K. Therefore, the selectivity of the tertiary ethers is high (95.8 % MTBE and 98.5 % ETBE at 363 K). At higher reaction temperatures the isobutene conversion grows considerably due to the onset of oligomerization reactions. Consequently, the selectivity of the ether formation deteriorates, because it is referred to the isobutene conversion. The use of ethanol for the etherification instead of methanol is thermodynamically less favourable since the possible maximum conversion is generally lower than that for methanol (values are given in parentheses). Practically, at a reaction temperature of 373-383 K, the ethanol conversion reaches its thermodynamic equilibrium value which is as low as 24 % at 373 K and 18 % at 383 K. In case of MTBE formation the methanol equilibrium conversion is 64.4 % at 373 K and 54.1% at 383 K. Characteristically, the conversion of ethanol on cloverite was found higher than allowed by thermodynamics if the temperature exceeded 383 K. This is attributed to the additional consumption of ethanol by the formation of DEE (one mole of DEE formed consumes two moles of ethanol) and by the etherification of diisobutene isomers. The extent of these two side reactions corresponds to 4.5 % of the ethanol consumption at 403 K (Table lb, last row), so that the actual conversion of ethanol to ETBE is reduced to 7.5 %. This agrees excellently with the thermodynamic prediction and underlines that under the applied reaction conditions the thermodynamic equilibrium of the ETBE formation is reached. 2. Formation of ETBE over pure and modified cloverite Adequate data are given for Ti-containing and Si-containing modifications of the cloverite material. The performance of catalysts cannot be appropriately compared at the point of thermodynamic equilibrium. Considering the conversion at 353 and 363 K, it is striking that the Ti-cloverite is significantly less active than the other two samples, whilst the non-modified and the Si-modified cloverite are not largely different in their activity.

659 Table la Catalytic data for the MTBE reaction over non-modified cloverite after in situ oxidative detemplation T/K

Conversion/%

Selectivity/%

MeOH

Isobutene

MTBE

IB dimers

363

35.8 (37.1)

36.0

95.8

4.2

368-

38.0 (69.0)

38.6

91.9

8.1

373

36.7 (64.4)

40.7

83.4

16.6

383

30.5 (54.1)

44.0

57.6

42.4

Methanol conversions in parentheses are the calculated equilibrium values. Table lb Catalytic data for the ETBE reaction over non-modified cloverite after in sire oxidative detemplation T/K

Conversion/%

Selectivity/%

EtOH

Isobutene

ETBE

I]3 dimers

353

8.2 (54.0)

7.9

99.5

0.5

363

15.6 (34.0)

15.1

98.5

15

373

19.7 (24.0)

19.6

97.3

2.7

383

18.6 (18.0)

19.9

91.5

8.5

393

13.9 (12.0)

16.6

80.5

19.5

403

11.0(8.0)

13.4

72.1

27.9

Ethanol conversions in parentheses are calculated equilibrium values. Table 2 Catalytic data for the ETBE reaction over Ti-modified cloverite after in situ oxidative detemplation T/K

.

Conversion/%

Selectivity/%

EtOH

Isobutene

ETBE

IB dimers

353

1.7

1.6

98.9

1.1

363

3.0

2.9

97.0

3.0

373

4.7

4.8

93.5

6.5

383

6.9

7.6

87.3

12.7

393

8.9

8.7

88.5

11.5

403

9.5

8.7

88.2

11.8

660 Table 3 Catalytic data for the ETBE reaction over Si-modified cloverite after in sire oxidative detemplation T/K

Conversion/%

353

Selectivity/%

EtOH

Isobutene

ETBE

IB dimers

10.9

10.9

99.3

0.7

363

19.5

19.7

98.4

1.6

373

21.0

21.5

96.3

3.7

383

19.3

21.2

86.6

13.4

A c i d centers

It is well acknowledged that the etherification reaction of iso-butene by alcohols requires strong acid sites. The industrial production of MTBE is performed (in liquid phase) over

Q

0.2 "

~

"~-"~~

d

1612

I

0.2

1448 1448

ls4o t,jb .~~,...~./

1800

.

~

1600

1400

Wavenumber/cm- 1

1800

1600

.~j~a '

i 400

Wavenumber/cm- 1

Figure 3. IR spectra of the pure clovefite (left) and Ti-cloverite (fight) after (a) vacuum treatment at 753 K, (b) adsorption/desorption of pyridine (4 kPa) at room temperature, (c) after annealing at 423 K (18 h) and (d) 673 K (1 h).

661 sulphonated ion exchange resin catalysts with high acidity. The question arises, therefore, of what nature the acid centers on the ¢loverite catalysts are. In the literature there are some suggestions concerning the acidity of pure ¢loverite [5,6]. Already the very first IR measurements of the OH region have shown that ¢loverite exhibits two bands at 3670 and 3700 ¢m-1 [7]. Following the structure of ¢loverite which shows that it contains two structtiral OH groups these bands have been attributed to P-OH and Ga-OH groups, respectively. In a recent paper Mtiller et al. [6] have identified these groups to be of moderate (P-OH) and high (Ga-OH) acid strength. In particular the Ga-OH groups are considered to be responsible for a concerted BrOnsted and Lewis type interaction with adsorbed polar molecules, for instance methanol. However, no catalytic data are given in their paper. In the present study these high (HF) and low frequency (LF) hydroxyl bands are also observed, usually the I-IF band with much lower extinction than the LF band. Surprisingly, no difference in the IR spectra were obtained for the pure or Ti-modified cloverite catalyst although Ti-eloverite is much less active than pure doverite in the formation of ETBE (Tab. lb, 2). Comparative adsorption of pyridine on samples that have been evacuated for 1 h at 753 K, oxidized in 0 2 and again evacuated shows about the same concentration of Lewis (1448 and 1612 cm-1) as well as Br6nsted (1540 ¢m-1) centers on both pure and Ti-eloverite catalysts. Increasing the desorption temperature to 423 K shows, however, that pyridine desorbs faster from the pure doverite samples suggesting that Ti-cloverite should possess stronger Lewis and Br/3nsted acid sites. This conclusion does, however, not fit the expectations connected with the lower catalytic activity of the Ti-cloverite catalyst. There are several patents which enclose the post-synthesis modification of zeolites by hydrogen fluoride, fluorsulphonic acid, fluorphosphoric acid and other highly acidic media [8]. Following a completely different idea, it seems, therefore, reasonable to assume that residual fluoride ions within the cloverite solid aPter in situ detemplation might be responsible for the good catalytic performance of the cloverite catalysts in the etherification reactions studied. In a first attempt to confirm this approach a pure cloverite and a Ti-cloverite sample have been analyzed with respect to their F- concentration after reaction. The result shows that both cloverite catalysts indeed contained fluoride but the pure cloverite sample had a higher fluorine concentration than the Ti-containing one. This might point to a certain contribution of the fluoride ions to the catalytic activity which would be in coincidence with the acidification of industrial catalysts by fluorine compounds. However, at the present stage of investigation, it cannot evidently be shown how fluoride ions come into action when located within the cloverite lattice. According to the proposed structure aider synthesis, F- ions should be located in the four-ring subunits where they act as counter-ion to the quinuclidine cation Q+ [1]. After detemplation the residual fluoride ions could modify the acidity of adjacent hydroxyl groups. However, no direct evidence for this idea is available at present; further studies concerning this question are under investigation. Nevertheless, it is obvious that the question whether fluorine contributes to the overall acidity, i.e. also to the catalytic activity, in a dominating or insignificant way is crucial. Taking into account that the amorphous galliumphosphate sample that has been synthesized in completely the same way as cloverite (i.e. also with the same concentration of HF) exhibited a distinctly lower catalytic activity for the formation of MTBE than a crystalline cloverite catalyst [2] it has to be concluded that the cloverite structure is an essential property with respect to the catalytic appearance.

662 Acknowledgement. The authors kindly acknowledge analytical and technical assistance of Mrs. E. Lieske, Mr. R. Eckelt, and Mr. U. Marx. R. F. and M. R. are indebted to the qTonds der Chemischen Industrie' (VCI) for financial support. REFERENCES 1. M. Estermann, L.B. McCusker, C. Baerlocher, A. Merrouche, H. Kessler, Nature, 352 (1991) 320. 2. M. Richter, H.-L. Zubowa, R. Eckelt and R. Fricke, Microporous Materials, in press. 3. H.-L. Zubowa, E. Schreier, K. Jancke, U. Steinicke and R. Fricke, Collect. Czech. Chem. Commun., 60 (1995) 403. 4. A. Corma, M.T. Navarro and J. P6rez Pariente, J. Chem. Soc., Chem. Commun., 1994, 147. 5. A. Janin, J.C. Lavalley, E. Benazzi, C. Schott-Darie and H. Kessler, Proc. ZEOCAT '95, Szombathely (Hungary), July 9-13, 1995, H.K. Beyer, H.G. Karge, I. Kiricsi and J. B. Nagy (editors), Elsevier Sci. Publ. Amsterdam, 1995, 124. 6. G. MOiler, G. Eder-Mirth, H. Kessler and J.A. Lercher, J. Phys. Chem., 99 (1995) 12327. 7. T.L. Barr, J. Klinowsky, H. He, K. Alberti, G. MOiler and J.A. Lercher, Nature, 365 (1993) 429. 8. US Patent No. 5,364,981 (1994).


663

Preparation of Titanosilicate with Mordenite Structure by Atomplanting Method and Its Catalytic Properties for Hydroxylation of Aromatics Peng Wu, Takayuki Komatsu and Tatsuaki Yashima Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152, Japan Titanium mordenites (Ti-M) with different titanium and aluminum content have been prepared by the solid-gas reaction at elevated temperatures (atom-planting method) between highly dealuminated mordenites and TiCI4 vapor. Ti-M samples were characterized with MAS NMR, IR and UV spectroscopies, which indicates that Ti atoms have been incorporated tetrahedrally into the mordenite framework through the reaction of TiCI4 with hydroxyl nests composed of SiOH groups. The catalytic properties of Ti-M for the liquid phase hydroxylation of various aromatic substrates having different molecular sizes with 1-1202were studied by comparing with those of MFI-type titanosilicate (TS-1). Ti-M showed comparable specific activity to that of TS-1 for the hydroxylation of smaller substrates which can diffuse easily into the pores of both catalysts. However, Ti-M was more active than TS-1 in the hydroxylation of bulkier aromatics, because Ti-M has larger window size of pore than TS-1. 1. INTRODUCTION MFl-type titanosilicate, TS-1, has opened new possibilities for using zeolites as oxidation catalysts under liquid-phase conditions. TS-1 has been shown to be a remarkable catalyst for the selective oxidation of a large family of organic substrates using H202 as an oxidant under mild conditions, i.e., hydroxylation of alkylbenzenes and phenol [1], epoxidation of olefins [2], oxidation of paraffins to the corresponding alcohols and ketones [3], oxidation of alcohols [4] and ammoximation of ketones [5]. These successes on TS-1 have induced subsequent researches on the synthesis of other Ti-containing zeolites, e.g., TS-2 (MEL) [6] and Ti-ZSM-48 [7]. These titanosilicates show unique catalytic activity due to isolated and tetrahedrally coordinated Ti atoms in the framework. However, they are restricted to the oxidation of relatively small substrates because of their medium-pore structures. It is not ambitious to say that the synthesis of large-pore titanosilicates is one of the main research subject in the field of developing oxidation catalysts. These motives have led to the synthesis of titanosilicates with 12-ring channels such as Ti-Beta [8], TAPSO-5 [9] and Ti-ZSM-12 [10]. More recently, a mesoporous Ti-MCM-41 material has been synthesized and found to have an advantage over TS-1 and Ti-Beta in the

664 oxidation of large organic molecules [11]. The hydrothermal synthesis of Ticontaining zeolite with mordenite structure, however, has been reported seldom. Atom-planting method, i.e., a treatment of highly siliceous zeolites with metal chloride vapor at elevated temperatures, has been proved to be a useful way for preparing metallosilicates with MFI a n d M O R structures [12-14]. In this study, we have performed the incorporation of Ti atoms into the mordenite framework by the atom-planting method to prepare Ti-containing mordenite and compared its catalytic properties for the hydroxylation of various aromatics with those of TS-1. 2. E X P E R I M E N T A L H-Mordenites, M ( l l ) (framework Si/AI atomic ratio of 11) and M(8.2) were used as starting material for the dealumination to obtain various dealuminated mordenites with Si/AI ratios of 41-325. The dealumination was carried out by the calcination in air at 973 K followed by HNO3 reflux, as described in detail elsewhere [14]. The atom-planting procedure with TiCI4 vapor was similar to the alumination treatment [14]. After dehydration at 773 K for 4 h, 2 g of dealuminated mordenite was treated with TiCI4 vapor (1.7 kPa) in a flowing helium at 673 K for a prescribed process time (5 min-4 h). The sample was then purged with pure helium at 673 K for 1 h. After cooling it to the room temperature, the TiCl4-treated sample was washed with deionized water and dried at 383 K for 24 h to obtain Ti-containing mordenite, Ti-M(n), where n was the Si/AI ratio of the parent dealuminated mordenite. The reference catalyst, TS-1 (Si/Ti=104)was synthesized hydrothermally according to the patent [1]. 29Si MAS NMR (Bruker MSL-400), IR (Shimadzu FTIR-8100) and diffuse reflectance UV-visible (Shimadzu MPS-2000) spectroscopies were used for the characterization of Ti introduced into mordenite structure. Hydroxylation of aromatics with H202 was performed in a 50 ml flask with a magnetic stirrer at 363 K. In a typical run, 50 mg of catalyst, 2 ml of water, 20 ml of aromatic substrate and 1 ml of H202 (30 wt%) were mixed in the flask. The reaction was then carried out under vigorous agitation for 2 h. The reaction mixture was analyzed with gas chromatography using p-ethylphenol or 2,5-xylenol as an external standard. 3. RESULTS AND DISCUSSION 3.1 Preparation and characterization of Ti-M The incorporation of Ti by the atom-planting treatment with TiCI4 vapor at elevated temperature was first investigated to know where Ti atoms were located. The effect of the process time of TiCI4 treatment was investigated using M(71) as a parent. As shown in Fig. 1, the amount of Ti introduced (bulk Ti) increased rapidly with increasing the process time to 1 h. The longer process time than 1 h did not make the amount of Ti increase further. The amount of bulk AI changed very slightly during the TiCI4 treatment. As a result, Ti-M with the amount of Ti larger than that of

665

|

"6 0.6 Bulk Ti • P,,,i

O'2 .~ 0.4

ofo

< •[--. ; 0.2

--------U 3

o
99 %) together with trace amounts of benzaldehyde and benzyl alcohol

667 resulted from the side-chain oxidation. The activity of the incorporated Ti was "6 found to be dependent greatly on the oOI.o-O-°--o-o composition of the parent dealuminated / mordenites. The specific activity o / (turnover number per Ti atom) for the 20 "6 o hydroxylation of benzene ring to yield corresponding cresols increased with Z ' I I 0 0 J ~ Si/AI ratio from 11 to 200, then it only [--, 100 200 300 changed slightly with further dealuminaSi/AI ratio tion (Fig. 6). This indicates that a low AI content in Ti-M is favorable for the high Figure 6. The specific activity of toluene specific activity. Similar behavior has hydroxylation as a function of Si/AI ratio been observed on hydrothermally synin Ti-M. Reaction conditions: eat., 50 mg; thesized [AI, Ti]-Beta in the olefin oxidatemp., 363 K; substrate, 20 ml; H202 (30 tion [8]. As all of Ti-M catalysts with wt%), 1 ml; H20, 2 ml; time, 2 h. various AI contents exhibited only the 220 nm band in their UV spectra and showed the characteristic band at 963 cm-1 in their IR spectra, the lower activity observed over Ti-M with Si/AI ratio below 200 cannot be due to the presence of nonframework Ti species. The dealumination is suggested to increase the hydrophobicity of the zeolites, which may result in higher catalytic activity, since the reaction was performed under aqueous condition. Furthermore, the electronic density around Ti sites is reported to be altered by the AI atoms nearby [8]. Therefore, the lower AI content is expected to result in M-free Ti sites in Ti-M, which generates the higher activity of Ti atoms. Hydroxylation of various aromatics. The catalytic properties of Ti-M sample were studied by comparing with those of TS-1 for the hydroxylation of various aromatics with different molecular sizes. Ti-M(245) was used to lower the influence of AI on the activity assuggested by Fig. 6. The Ti content in Ti-M(245) was almost the same as that in TS-1 (Si/Ti=104). For both TS-1 and Ti-M, products generated by three reaction paths were observed, that is, the direct hydroxylation of benzene ring to cresols (phenol for benzene), the substitution of hydroxyl group for side-chain alkyl groups to yield phenol and the oxidation of side-chain alkyl groups to corresponding alcohols, aldehydes and ketones. The amounts of alcohols, aldehydes and ketones through the third path were generally small (< 1%) and comparable to those obtained without catalysts. Thus, the oxidation of side-chain alkyls is a noncatalyzed reaction and is not considered for the activity comparison between TS-1 and Ti-M. Figure 7 compares the catalytic activity and selectivity of TS-1 (a) and Ti-M (b) for the hydroxylation of aromatic substrates with single alkyl group. It can be seen that TS-1 and Ti-M showed comparable turnover number in the case of the hydroxylation of the smallest substrate, benzene. The activity of TS-1 for toluene decreased to less than half of that for benzene, and decreased further when bulkier substrates were

668

~

Benzene ring hydroxylation Side-chain substitution

6O

16

>-5_0

).,, 12

"6 4O 3o

20 o

1o

r-I a

b

Benzene

a

b

a

gxl b

T E Substrate

0 b a

a

C

b t-B,

Figure 7. Hydroxylation of various aromatics over TS-1 (a) and Ti-M(b). Cat.: TS-I(104), (Ti: 0.151 mmol g-l); Ti-M (245), (Ti: 0.150 mmol g-l). Reaction conditions: see Figure 6. T=toluene; E= ethylbenzene; C=cumene; t-B=t- butylbenzene.

a b p-Xy

a b a b o-Xy m-Xy Substrate

Figure 8. Hydroxylation of xylene isomers over TS-1 (a) and Ti-M (b). reaction conditions: see Figure 6.

used. TS-1 was completely inactive for t-butylbenzene. On the other hand, Ti-M showed a little higher activity for toluene than that for benzene. Although the activity of Ti-M decreased gradually for larger substrates, Ti-M was still active in the case of t-butylbenzene hydroxylation. There could be two factors dominating the reactions of these alkyl aromatics on zeolite catalysts. Electron-donating alkyl groups attaching to the benzene ring would increase the electrophilicity of substrates, and subsequently promote the ring hydroxylation in the order: -C(CH3)3 >-CH(CH3)2 >-CH2CH3>-CH3. The bulkier alkyl groups, however, are expected to retard the reaction rate due to the diffusion limitation and/or to steric hindrance for transition states. The reactions of the bulkier substrates on the medium-pore TS-1 catalyst might be dominated by the second factor. Therefore, the activity of TS-1 decreased to zero in the hydroxylation of t-butylbenzene. In the case of large-pore Ti-M, the first factor might dominate slightly in the hydroxylation of toluene to make its activity a little higher than that for benzene hydroxylation. For the substrates larger than toluene, the second factor~ the diffusion limitation, becomes to play a leading role in the reaction, resulting in the decrease in activity. Nevertheless, Ti-M always showed higher activity for ring hydroxylation of alkylbenzenes than TS-1, indicating that Ti-M is a potential catalyst especially for the hydroxylation or oxidation of bulky molecules. As shown in Fig. 7, Ti-M showed surprisingly high activity yielding phenol from cumene. Phenol was produced accompanied with a similar amount of 2-propanol and acetone, just like the commercial process of cumene

669

oxidation for phenol formation. Figure 8 compares the catalytic activities of Ti-M with those of TS-1 in the hydroxylation of xylene isomers. The main products were xylenols (>99 %) over both catalysts. Ti-M showed comparable specific activity to that of TS-1 for pxylene, while it showed much higher activity than TS-1 for bulkier o- and m-xylene. The lower activity for p-xylene than for o- and m-xylene over Ti-M must be due to the lowest reactivity of p-xylene itself among the three isomers. Therefore, these three isomers may have comparable diffusion rates within the 12-ring channels of mordenite. On the other hand, TS-1 was almost inactive for o- and m-xylene, indicating o- and m-xylene have much lower diffusion rates than p-xylene within the 10-ring channels of TS- 1. Competitive hydroxylation of toluene with other aromatics. In order to clarify the diffusion of aromatics inside the zeolite channels, the competitive hydroxylation of toluene with ethylbenzene, cumene and t-butylbenzene was carried out on both Ti-M and TS-1. Figure 9 shows the results obtained over Ti-M. When the competitive reaction of toluene with ethylbenzene was performed, the specific activity for toluene decreased to lower than half of the activity observed for the independent hydroxylation of toluene and was almost the same as that for ethylbenzene. When the competitive molecules were changed to cumene and tbutylbenzene, the activity for toluene hydroxylation decreased further to be similar to those for cumene and t-butylbenzene, respectively. It is indicated that all of the substrates used here are able to diffuse into the mordenite channels. When two kinds of molecules are present simultaneously within the channels of zeolite, the diffusion

60

20 Ti-M

"-' 50

-r

TS-I 15

o

"~30 "6 20

~1o o

Z 5 O 0 T

T

E T C Substrate

L.~

r'-I

T

t-B

Figure 9. Competitive hydroxylation of toluene with other aromatics over Ti-M. Reaction conditions: toluene, 10 ml; competitive substrate, 10 ml; others, see Figures 6 and 7. Abbreviations are the same as those used in Figure 7.

0 T

T

[--I ,---. E T C T Substrate

t-B

Figure 10. Competitive hydroxylation of toluene with other aromatics over TS-1. Reaction conditions: toluene, 10 ml; competitive substrate, 10 ml; others, see Figures 6 and 7.

670 rate of the smaller molecule is controlled by the larger molecules having lower diffusion rate [15]. Consequently, the activity for toluene was lowered by the presence of larger molecules. In the case of TS-1, the hydroxylation of toluene was retarded by presence of ethylbenzene and cumene but was little affected by tbutylbenzene (Fig. 10). The molecules of ethylbenzene and cumene with very low diffusion rate would hinder the diffusion of toluene in the channels of TS-1, while tbutylbenzene hardly diffusing into the channels would not hinder the diffusion of toluene significantly. 4. CONCLUSION Ti-containing mordenites, Ti-M, with tetrahedrally coordinated Ti atoms in the framework sites can be prepared by the atom-plating method using dealuminated mordenites and TiCI4 vapor. Ti-M with a low AI content is an active catalyst for the hydroxylation of aromatic substrates with 1-1202. For the reactions without serious steric restrictions, Ti-M shows a specific activity comparable to that of TS-1. On the other hand, the large-pore Ti-M is more effective for the reactions of bulkier aromatics than the medium-pore TS-1. REFERENCES

1. T. Taramasso, G. Perego and B. Notari, US Patent No. 4 410 50(1983). 2. C. Neff, A. Esposito, B. Anfossi and F. Buonomo, Eur. Patent, Appl. 100 119(1984). 3. T. Tatsumi, M. Nakamura, S. Negishi and H. Tominaga, J. Chem. Soc. Chem. Commun., (1990)476. 4. G. Bellusi and M. S. Rigutto, Stu. Surf. Sci. Catal., 85(1994)177. 5. A. Thangaraj, S. Sivasanker and P. Ratnasamy, J. Catal., 131(1991)394 6. J.S. Reddy, R. Kumar and P. Ratnasamy, Appl. Catal., 58(1990)L1. 7. D.P. Serrano, H.-X. Li and M. E. Davis, J. Chem. Soc. Chem. Commun., (1992) 745. 8. A. Corma, M. A. Camblor, P. Esteve, A. Martfnez and J. P6rez-Pariente, J. Catal., 145(1994)151. 9. A. Tuel, Zeolites, 15(1995)228. 10. A. Tuel, Zeolites, 15(1995)236. 11. T. Blasco, A. Corma, M. T. Navarro and J. P6rez-Pariente, J. Catal., 156(1995)65. 12. K. Y amagishi, S. Namba and T, Y ashima, Stu. Surf. Sci. Catal., 49(1989)459. 13. P. Wu, T. Nakano, T. Komatsu and T. Yashima, Stu. Surf. Sci. Catal., 90(1994)295. 14. P. Wu, T. Komatsu and T. Yashima, J. Phys. Chem., 99(1995) 10923. 15. S. Namba, K. Sato, K. Fujita, J. H., Kim and T. Yashima, in" Proceedings of the 7th International Zeolite Conference", Y. Murakami, A. Iijima and J. W. Ward (eds.), Kodansha, Tokyo, (1986)661.


671

C H A R A C T E R I Z A T I O N O F Z E O L I T E B A S I C I T Y U S I N G IODINE AS A MOLECULAR PROBE S. Y. Choi, Y. S. Park and K. B. Yoon* Department of Chemistry, Sogang University, Seoul 121-742, Korea The visible absorption band of iodine adsorbed on zeolite blue shifted with the increase in the electropositivity of the counter cation and the aluminum content in the framework. Since the visible band of iodine in solution has been known to blue shift with the increase in the basicity (donor strength) of the solvent, the blue shift over zeolite was attributed to the increase in the basicity of the zeolite framework, from the consideration of zeolite as a solid solvent. The framework structure, moisture content and the degree of NH3 loss from NH4+-exchanged zeolites also greatly affected the absorption band of iodine. The results established the charge-transfer interaction between the adsorbed iodine and the zeolite oxide surface which allow iodine to be used as a novel molecular probe for the systematic evaluation of the zeolite basicity. 1. I N T R O D U C T I O N

Iodine has long been known as a prototypical solvatochromic compound whose color, hence the visible absorption band changes dramatically depending on the (electron) donor strength of the solvent[i,2]. For instance, it is violet in carbon tetrachloride as in the vapor, red in benzene, various shades of brown in alcohols and ethers, and pale yellow in water. The visible absorption of free iodine arises from the transition of an electron from n* to o*. However, it has been proposed that the relative energy of the latter is significantly perturbed in electron-rich solvents because of the donor-acceptor interaction between iodine and solvents[3] Thus the stronger the donor strength of the solvent, the higher the energy level of o" orbital shifts up resulting in the hypsochromic shift of the visible absorption band. Accordingly, the visible absorption bands of iodine can be related to the donor strengths of various solvents. In this abstract, we present the dramatic shifts of the visible bands of iodine adsorbed on various zeolites, with the change in the Si/A1 ratio, the electronic nature of the cation, the dehydration temperature, and the framework structure, and the interpretation of the shifts in terms of the change in the donor strengths of zeolites, from the consideration of zeolites as electrolytic solvents[4]. 2. E X P E R I M E N T A L

LTA(1.0), FAU(2.6), LTL(3.3), MOR(21), MOR(34), ZSM-5(900) were purchased from Union Carbide. FAU(1.2) was purchased from Strem. LTA(x) (x -- 1.4, 2.1, and 2.3), FAU(x) (x = 1.0 and 3.4), ZSM-5(x) (x = 50 and 150) were obtained from S. B. Hong of the Korea Institute of Science and

672 Technology. MAZ(4.2) was obtained from D. R. Corbin of the DuPont Company. ZSM-5(x), x -- 14 and 28, were kind gifts from ALSI-PENTA Zeolithe GmbH. All the organic templates included within zeolites during the preparations were removed by heating at 500 °C under flowing oxygen. The ion exchange was carried out by refluxing the zeolites in an aqueous solution of concentrated (0.5-1 M) salts at least 5 times to ensure the complete or maximum exchange. H*-exchanged zeolites were prepared from the corresponding NH4-forms~ The ion-exchanged samples were rigorously dried at 300 °C in uacuo ( 250 °C) observed over FAU(1.2) than over FAU(2.6) to reach a constant wavelength also reflected the higher •

•

•

•

o

•

Z+

•

•

•

•

•

2+

~-~-

•

677 Table U. The Visible Absorption Band of Iodine Adsorbed on Na +and NH4*FAU. a Effect of Dehydration Temperature.

Temp b

Na + - FAU

NH4 + - FAU

Si/Al

SVAI

1.2

2.6

1.2

2.6

25

-- 440¢

491

-- 443 ¢

-- 472 ¢

50

-~ 440 ¢

491

-- 432 ~

---471 ¢

1O0

437

486

-~ 432 ~

462

150

431

482

447

466

200

424

478

469

479

225

422

470

488

250

420

470

490

300

421

472

492

485

491

487

492

478

350 400

421

479

450 500 a ~ max

492 420

480

487

492

in nm. bin °C. ~Shoulder band.

hydrophilicity of FAU(1.2) than FAU(2.6), due to the greater numbers of cations and anionic sites in the framework. By contrast, however, the iodine band on NH4*-FAU(2.6) progressively red shifted at Td between 150 and 300 °C followed by an initial blue shift due to water loss until Td reached --100 °C. This contrasting behavior was ascribed to the generation of highly electronegative H ÷ from NH4÷ by the gradual loss of NH3 at Td between 150 and 300 °C. This result further revealed that deamminafion of NH4*-FAU(2.6) starts above ---100 °C. A similar trend was observed from NH4+-FAU(1.2). However on this zeolite, a sudden, discontinuous red shift of iodine band was observed at Td > 300 °C. An independent X - r a y powder diffraction analysis revealed that serious breakdown of the framework occured at Td > 300 °C. By the s~n+e analogy, the unusually blue shifted iodine band observed from M g " - e x c h a n g e d FAU(2.6) which was d ~ y d r a t e d at 300 °C (see Table I) was attributed to the strong hydration of Mg ~÷ owing to its extremely high positive charge density, since hydration would decrease the electronegativity of Mg z+ in the same way NH3 did to H + in the above experiment. In conclusion, the results established that iodine can serve as a convenient and highly efficient molecular probe for the quantitative evaluation of zeolite

678 basicity (donor strength) which varies depending on the Si/A1 ratio, the nature of cation, the structure, and the dehydration temperature. ACKNOWLEDGMENT

We thank the Aimed Basic Research Program of the Korea Science and Engineering Foundation (KOSEF), the Ministry of Education (MOE), Korea, and the Center for Molecular Catalysis (CMC) of Seoul National University for financial support. We also thank S. B. Hong and D. R. Corbin for providing some of the zeolite samples and the referees for the valuable comments. REFERENCES

1. J. H. Hildebrand, Science, 150 (1965) 3695. 2. E. M. Voigt, J. Phys. Chem., 72 (1968) 3300. 3. R. S. Drago, Physical Methods for Chemists, Saunders College Publishing: Ft. Worth, 1992, 2nd ed. p. 134. 4. J. A. Rabo, C. L. Angell, P. H. Kasai and V. Schomaker, Discuss. Faraday Soc., 41 (1966) 328. 5. (a) R. M. Barrer and S. Wasilewski, Trans. Faraday Soc. 57 (1961) 1153. (b) R. M. Barrer and S. Wasilewski, ibid, 1140. 6. Although the controlled amounts were very small compared to the maximum adsorbed amounts ( ) 500 mg per g of zeolite)[~], we believe most df them are incorporated within the pores, since the external surface area corresponds to only - - 1 % of the total surface area, and a much lesser amount was adsorbed on K*-LTA whose pore opening is smaller than the kinetic diameter of iodine. 7. The terms used for the zeolites used in this report were mostly taken from their structure codes since some zeolites have different names depending on the Si/A1 ratio, despite the identical structures. 8. Y. Okamoto, M. Ogawa, A. Maezawa and Imanake, T., J. Catal., 112 (1988) 427. 9. (a) M. Huang, A. Adnot and S. Kaliaguine, J. Am. Chem. Soc., 114 (1992) 10005. (b) M. Huang, A. Adnot and S. Kaliaguine, J. Catal. 137 (1992), 322. 10. (a) T. L. Barr and M. A. Lishika, J. Am. Chem. Soc., 108 (1986) 3178. (b) T. L. Barr, Zeolites, 10 (1990) 760. 11. V. K. Kaushik, S. G. T. Bhat and D. R. Corbin, Zeolites, 13 (1993) 671. 12. J. Stoch, J. Lercher and S. Ceckiewicz, Zeolites 12 (1992) 81. 13. (a) D. Barthomeuf, J. Phys. Chem. 88 (1984) 42. (b) See also D. Barthomeuf, in Acidity and Basicity of Solids, Fraissard, J.; Petrakis, L., Eds. NATO ASI Series C 444, Kluwer Academic, 1994, p. 181. 14. M. Huang and S. Kaliaguine, J. Chem. Soc., Faraday Trans. 88 (1992) 751. 15. W. J. Mortier and R. A. Schoonheydt, Prog. Solid St. Chem. 16 (1985) 1. 16. K. Serf and D. P. Shoemaker, Acta Cryst. 22 (1967) 162.


679

Ship-in-Bottle Synthesis of Pt and Ru Carbonyl Clusters in NaY Zeolite Micropore and Ordered Mesoporous Channels of FSM-16; XAFS/FTIR/TPD Characterization and Their Catalytic Behaviors Masaru Ichikawa, Takashi Yamamoto, Wei Pan#, and Takafumi Shido Catalysis Research Center, Hokkaido University, Sapporo 060, Japan

Abstract Ru3 (CO)I 2, H4Ru4(CO)I 2, [Ptg(CO)18] 2- and [Pt12(CO)24] 2- were synthesized in NaY cages by "ship-in-bottle" technique. Thermostable robust platinum clusters [Ptl5(CO )30] 2- combined with R4 N+ (R=Me,Et,Bu and Hex) and MV2+ Were encapsulated in the ordered hexagonal mesoporous channels of FSM-16(27.5A diameter) which were characterized by FTIR, EXAFS and HRTEM methods. They remain their flexibility of duster frameworks and exhibited remarkedly higher catalytic activities for the water-gas-shift reaction, compared with Pt9-Ptl2 clusters restricted in NaY micropores(12A). Highly dispersed Pt aggregates (ca 15A size) in FSM-16 channels were uniformally prepared from the Ptl5 carbonyl clusters by the evacuation at 323-473K to controlled removal of CO. The coordinatively unsaturated Pt aggregates in FSM-16 exhibited catalytic activities in the hydrogenation of ethene and butadiene selectively towards butenes. 1. INTRODUCTION Zeolites are aluminosilicate crystallines consisting of microporous cages of molecular dimensions(8-12 A) which are interconnected with smaller windows and channels(6-8A). Such micropores can supply "templating" circumstances for the selective synthesis of some bulky metal carbonyl clusters which fit the interior cages as ultimate "nanometer reaction vessels". This is coined with "ship-in-bottle" synthesis and may open new opportunities for the rational design of tailor-made catalysts[l] of discrete metal/alloy dusters having uniform sizes and metal compositions and with sufficient stability against a metal sintefing and leaching under the prevailing reaction conditions. Previously, uni- and bimetal carbonyl clusters such as Rhf_xIrx(CO)16(0-6) [2], [Pt3(CO)6]n2-(n=3,4)[3], [Ru6(CO)1812-[4], [Fe2Rh4(CO)1512-[5] and [HRuCo3(CO)12] [6] were synthesized in NaY and NaX zeolite cages, which were characterized by EXAFS/ XANES, FTIR,129Xe NMR, HRTEM and Raman spectroscopies. They are useful for preparing discrete metal/alloy clusters(less than 10A size) which catalyze the alkane hydrogenolysis[2], CO hydrogenation towards C 1-C5 alcohols[2,6] and olefin hydroformylation reaction[5]. There are current interests in mesoporous materials such as MCM-41[7] and FSM-16[8] having their honeycomb structures with ordered enormous channels of 20100 A diameters, which are larger than microporous cavities of conventional zeolites such as NaY, ALPO4-5 and ZSM-5. They are potential hosts for inclusion of bulky organic and inorganic species for new application to design of tailored metal catalysts[l] acx~ssible for larger substrates. In view of the relative interests in their mobility/flexibility and reactivity of #Permanent address: DeI~mment of Chemical Engineering, Chinghua University, Beijing, China.

680 the intrazeollitic clusters, we have extended to proceed a "Ship-in-Bottle" synthesis of Ru and robust Pt carbonyl clusters in NaY micropores and the ordered mesoporous channels of FSM -16. Their structmal flexibility and thermostabilities of metal clusters in micro and mesoporous constraint have been discussed in conjunction with their catalytic behaviors in 13CO exchange reaction, water-gas-shift reaction(WGSR) and hydrogenation of ethene and butadiene in terms of reactivities and selectvities.

2. EXPERIMENTALS AND PROCEDURES 2.l.Catalyst Preparation Ru3(CO)12/NaY (2-4 wt% Ru) was prepared by vapor deposition method. The mixture of Ru3(CO)12 and NaY was heated at 354K for 4 days, where sublimation of Ru3(CO)12 proceeded onto NaY. Ru3(CO)12/NaY was converted by the reaction with with H2 at 343K into H4Ru4(CO)12/NaY(2082m, 2064s, 2032m, 2021m and 2001w cm1). 2.0-5.0 wt% pt2+/NaYwere prepared by cation-exchange of NaY(LZY-52 from TOSO Chem. Co.; Si/AI=5.6) with Pt(NH3)4CI 2 at 363K in aqueous solution. [Pt12(CO)24] 2/NaY (v(CO)=2080vs and 1841s cm-1; 290, 445 and 640 nm) and [Pt9(CO)18] 2/NaY(2056vs and 1798s cm-1; 435 and 710 nm) were synthesized from pt2+/NaY and Pt(NH3)4/NaY(4 wt% Pt) by the reductive carbonylation with CO/H20 from 288-373K, respectively[3]. The host FSM-16 was synthesized using a polysilicate (Kanemite; NaHSi2053H20) and trimethyl-hexadecylamine chloride as a surfactant template according to published proceAures[8]. After calcination at 823K, the resulting material (surface area,950 m 2/g) presents ordered hexa-gonal channels(27.SA) with silanol groups(3745 cm1) characteristic of the X-ray powder patterns in low angle region(20=2.26, 3.44, 4.50 and 5.90) and the TEM observation. The NaY/FSM-16 encaged Ru and Pt carbonyl clusters were subjected to the controlled evacua-tion at 323-423K and 10-4 torr to remove CO, followed by the reduction in H2 flow(1 bar, 40 ml/min) at 293-673 K. 2.2 Reactivity Study The reactions were carried out using a dosed circulating microreactor in which 150-350 mg of the catalyst was charged prior to the catalytic probing reactions such as WGSR and olefin hydrogenation. Product analysis was performed by a periodical sampling of the effluent gas, which was quantitatively analyzed by TCD and FID gas chromatography using silica/Porapak Q and VZ-10 columns and on line with Gc-mass (Parkin-Elmer ; 50 m capillary columun) for isotope-labbeled products. 2.3. Infrared and UV-vis experiments The catalysts were pressed into self-supporting wafers(15 mm i.d., 12-20 mg/cm2) which were placed into an infrared cell equipped with CaF2 windows. Infrared and diffuse reflectance UV-vis spectra were measured at various coniditions using a bouble-beam Fourier transform IR with a resolution of 2 crn"1 and Hitachi-330 spectrophotometer. For the isotope-labelling experiments 13CO (90% enriched) was purchased from MSD Isotope Co. 2.4. EXAFS Spectroscopy The samples(2-5 wt%Ru and Pt) were charged under N2 in an in-situ EXAFS cell with KAPTON film windows(500 mm) to prevent exposure to air. EXAFS(Extended X-Ray Absorption Fine Structures) measurements were carried out similarly as reported[2-6] at SOR beam line 10B with a Si(311) channel-cut monochrometor in the Photon Factory in the National Laboratory for High Energy Physics(KEK-PF) using synchrotron radiation with an electron energy of 2.5 GeV at current of 250-360 mA. The EXAFS spectra were measured at the Ru K edge(22120 ev) and Pt L3(11549 ev).

681 3. RESULTS AND DISCUSSION 3.1. Ship-in-bottle Synthesis of Ru3 and Ru4 Carbonyl Clusters in NaY About 1.0 gr of NaY (evacuated at 673K to remove water) was mixed with Ru3(CO)12 crystal (Strem Chemicals Co.99.% purified from n-hexane solution) under nitrogen atmosphere, which was heated at 354K for 4 days, where sublimation of Ru3(CO)12 proceeded onto NaY. The XRD(x-ray diffraction, Cu-K) patterns suggested that the crystal-line phase of Ru3(CO)I 2 at 20 =12.60, 16.44, and 30.42) mixed with NaY zeolite phase was broaden and diappeared after the thermal heattreatment at 354K, leaving only those of NaY crystals. The Ru contents of the aging samples were determined by using an inductively coupled plasma-atomic emission(ICP) spectrometer to evaluate the residual Ru laoding which was almost invariant before and after the heattreatment of the mechanical mixture. This suggests that a gaseous Ru3(CO)12 is diffused and highly dispersed into the NaY supercages by sponteneous monolayer dispersion, similar to [Au(I)CI]/NaY[9]. The IR spectra gave intense bands at 2068s and 2028s crn-1, which quite resemble those of Ru3(CO)12 (2060s, 2026s and 2002m tan -1) in hexane solution. The resulting orange-colored material was very reactive with H2 (100 torr) using a closed circulating system by removal of a trace of water with a liq.N2 trap at 323-363K led to the formation of hydridocarbonyl cluster in NaY(2082m, 2064s, 2032m, 2021m and 2001w crn-1), which resemble those of H4Ru4 (CO)12 in cyclohexane solution (2081m, 2067s, 2031m and 2024m cm-1). To obtain more insight into the structure of Ru carbonyl clusters formed in NaY zeolites, EXAFS spectra of Ru K-edge were measued for the samples of Ru3(CO)1.2/NaY (I) and H4Ru4(CO)12/NAY (II) under N2 atmosphere at 300K. The Fourier transform(FT) parameters for the sample of (I) CN(Ru-Ru)=2.0,R= 2.86A) and (II) (CN(Ru-Ru)=3.2,R= 2.80A) are very similar to those of Ru3(CO)12 and H4Ru4(CO)12 diluted in boron nitride. 3.2. Ship-in-bottle synthesis of Ptl$(CO)302" Channel Host and FTIR/EXAFS Characterization.

Encapsulated

in

FSM-16

A sample contaning 5.0 mass% Pt was prepared by impregnation of FSM-16 with an aqueous solution of H2PtCI6. This sample was exposed to CO(200 torr) in a closed circulating system by ramping the temperature from 300 to 323K, resulting in the IR bands at 2188, 2149 and 2119cm -1, as presented in Fig.l(a). From the analogy of the previously reported Pt carbonyl species[10], the IR bands at 2188, 2149 and 2119 cm "1 can be ascribed to cis-Pt(CO)2C12(2188 and 2149 cm -1) and Pt(CO)C13(2119 em-1), respectively. The Pt carbonyls were converted by subsequent admission of H20 vapor(15 torr) onto the CO atmosphere to make an olive-green product(sample A) exhibiting a steady-state spectrum (Fig. l(b)) of carbonyl bands(2081s and 1880m cm -1) and UV-vis reflectance (~,max;452 and 855 nm). The final specman closely resembles those of [E~N]2[PtlS(CO)30] in MeOH (2056s(terminal) and 1872m(bridged) cm -1, Z.max; 408 and 697 nm) and crystal[ll], but with shifting of both carbonyl bands to higher wavenumber(Av=29-10 cm -1) and the visible absorption peaks to higher wave length, respectively. These shifts are consistent with the formation of contact ion pairs[8] between the Pt carbonyl anions and proton or Lewis acid sites on the wall of FSM-16 channels. Attempts to extract the platium carbonyl species from sample A with THF(tetrahydrofuran) and MeOH were tmsucgesful, but with [(Ph3P)2N]C1 in THF proceeded, which gave specifically an appreciable amount of [(Ph3P)2N]2[Ptl5 (CO)30] (2056s and 1872m cm -1; kmax; 405 and 702 nm in T H e . This is consistent with an uniform entrappment of [Ptl5(CO)30] 2- in mesoporous channels of FSM-16, which was prepared by a reductive carbonylation of H2PtCI6/FSM-16, analogous to its synthesis in solution[ll]. It was found that [Ptl5(CO)30] 2- in FSM-16(sample A) is relatively unstable,

682 and evacuation of. the sample A at 10-4 torr and 300-323K led to the irreversible transformation owing to partial removal of CO to give the brownish product (2063s and 1820w cm'l), which resembles those of higher nuclearity Pt carbonyl dusters reported as [Pt55(CO)x] n- .and [Pt38(CO)4412-(2060-2043s and 1832-1820w cm"1) in THF solution [12]. As presented in Fig.2, the electron micrograph of the evacuated sample A showed that platinum aggregates having ca.20 A diameter were uniformly distributed along the ordered

0.2

0

0

c~ /

|

\ct

/

i

2200 2000 18'00 1600 14'00 Wavenumber / cni 1

Fig. 1. In-situ FI'IR spectral changes in reductive carbonylation of H2PtCI6 /FSM-16 with CO and CO+H20 at 323K

Fig.2. TEM of [Pt15(CO)3012-/ FSM- 16 after evacuation(343K)

mesopomus channels of FSM-16 crystals with a negligible formation of the external particles. This was in good agreement of the observed EXAFS parameters (C.N.=8.6;R(Pt-Pt) =2.74A) for the sample A after evacuation at 323K. On the other hand, the thermostable [Pt15(CO)30] 2- in FSM-16 was successfully prepared by using the FSM-16 which was coimpregnated with H2PtCI6 and quartemary alkyl ammonium salts (R4NX;R=Methyl, Ethyl, Butyl and Hexyl; X = CI, Br and OH) and methyl viologen chloride([MV]2Cl) ([CH3N N CH3]2CI') from each aqueous solution. The reductive carbonylation of each coimpregnated sample resulted in an olive-green product (UV-vis reflect-ance AAA and 859 nm), showing the intense CO bands which appeared at 2075-2079s and 1875-1884m cm'I(R4NCI; R=Methyl, Ethyl, Butyl and Hexyl), relatively shifted to higher frequencies by using the larger alkyl ammonium cations. It is worthy to note that those organic cations stabilize the robust Ptl5 carbonyl cluster dianion and increase their thermostabilities in the FSM-16 channels by varying the used quaternary ammonium cations(R4N + and MV2+); None Aic, '

(5)

for A~° 0), while the radius of the circle is proportional the amount of electron density displacement I Matrix algebra states that the sum of the diagonal elements of a matrix before and after diagonalization is identical. In the specific case of the hardness matrix this means that: n

n

~2'rl~- Lhi (z=l i=1

(4)

The molecular geometry will directly influence the principal hardnesses. Some will shift to lower values, some remain constant, while others increase in value, if the composition is constant. Identification of the polarization channel(s) which favor a reaction pathway as a first step and then systematically manipulating the geometrical configuration of the catalysts so as to lower the principal hardness(es) of the reaction channel(s) provides us with a sensitive design tool for the formulation of structure/composition activity/selectivity relationships. The normal representation of the energy expression: dE

- j~'n°rdQT

+ ~-l ~ h i=l

i

dQ~

(5)

is obtained upon orthogonal transformation of the matrix equivalent of the EEM energy expression: dE - z*dq T + dqrldq T

(6)

It is a particularly useful reactivity criterion, since the most important PNM's, which describe a charge reorganization process along the reaction path, can be selected from an energetical viewpoint. In eq 5 Z n°r is a row vector consisting of the normal electronegativities, %nor = c~

aQi

(7)

728 which measure the force behind the inflow or outflow of electrons via the independent electron population channels Q. In our recent work the electronic normal mode analysis of the H-exchange reaction of methane on a zeolite cluster has been presented, using the transition state proposed by Kramer et al. [6]. It was shown that the PNM's can be divided into three groups. When the charge redistribution mainly involves the atoms of methane, we define them as molecular modes. Cluster modes are concentrated on the cluster. Interaction modes involve charge rearrangements on methane and on the cluster. On the basis of the energy criterion (Eq. 5) the softest interaction mode was selected as the polarization channel responsible for driving the reaction, with a contribution of more than 80 % to the energy at the transition state. Figure 1 shows the softest interaction mode, referred to as the reaction mode, and the associated nuclear displacements.

Figure 1. The reaction polarization channel and the reaction coordinate The proton of the bridging hydroxyl group decreases its electron population, or increases its net positive charge and thus its acidity. At the same time, the neighboring bridging oxygen decreases its electron density, hereby stimulating an increase in the electron population of the proton connected to the carbon, consequently, weakening the C-H bond. Both electron displacements favor a proton transfer from the cluster to the methane and simultaneously from the methane to the cluster. The difficulty of the charge rearrangement of the reaction mode is reflected in the value of the reaction mode hardness. We can now safely use the reaction mode hardness as a reactivity parameter, where a more reactive situation is connected with a lowering of the hardness. 3. RESULTS 3.1 The impact of structural effects on the reaction mode hardness As the geometry directly impacts on the value of the reaction mode hardness, the importance of structural differences between zeolite crystals can be envisaged. The typical test cases are faujasite (FAU) and ZSM-5 (MFI). Faujasite is a highly symmetric zeolite with one unique tetraheder site surrounded by four different oxygen types. The unit cell comprises 192 T-atoms. MFI is a much more complex structure consisting of 12 topologically different Tatoms and 26 O-sites. When the size of the unit cell is doubled, it also contains 192 T-atoms. Both zeolite structures FAU and MFI are converted into idealized crystals with prescribed distances (r(Si-O) =1.61~ r(A1-O) = 1.73 A) and bond angles (O-T-O = 109.47 °, T-O-T = 145 ° with T = Si or A1), using the DLS program [17]. To avoid compositional influences only

729 one Si atom is substituted with an A1 atom in the P1 unit cell. Calculations thus refer to a Si/A1 ratio of 191 for both FAU and MFI . . . . . The transition state geometry of the CH5 moiety has been taken from the literature [6]. The methane carbon atom and the two exchanging H-atoms are placed in the O-AI-O plane, with H-O distances identical to the reported ones. The sterically accessible pathways for MFI and FAU have been determined by graphical analysis, using the Hyperchem software [ 18]. The obtained reaction mode hardnesses are given in Table 1. For MFI the atom numbers refer to the structure determined by Olson et al.[19] It is also indicated whether the methane molecule is situated in the Straight channel (S), the Zigzag channel (Z), or at the intersection. For FAU O 1 and 04 belong to the 12-ring. /

Table 1 Structural influence on the reaction mode hardness position CH56+ MFI

FAU

A1 T2 T3 T4 T5 T6 T8 T9 T10 T11 T12 T12 T

O O1 02 04 05 05 07 O18 O15 O11 Oll 020 O1

O 02 020 O 17 O21 O19 08 025 026 022 020 / 024 04 /

/

Z-channel x x x

S-channel x x x x x

....

x x x x x

h (V/e) 12.5808 12.5500 12.5698 12.5282 12.5935 12.5644 12.5521 12.5596 12.5882 12.6085 12.5897 12.5554

/

The reaction mode hardness varies from 12.53 V/e to 12.61 V/e due to topological differences only. Three T-sites of MFI (T3, T5 and T9) are more reactive (lower hardness) compared to the T-site of FAU. They are situated, respectively, at the intersection of the two channels (T3), in the straight channel (T5) and in the zigzag channel (T9). There is no relation between the reaction mode hardnesses and the channel type (Z or S). Both the most reactive (T5_O5-O21) and the least reactive (T12_Oll-O20) methane are situated in the straight channel. 3.2 The impact of AI distribution on the reaction mode hardness To study the influence of the AI distribution on the reaction mode hardness, we restrict ourselves to the most reactive site on the basis of the topology, i.e. T5 of MFI. An AI atom is inserted at each of the 12 Next Nearest Neighbors (NNN) positions ofT5 by substitution of Si while'keeping the geometry fixed. The accompanying O-H bond is placed in the AI-O-Si plane, bisecting the AI-O-Si angle. The length is set at 0.96 A. The protons are positioned on the oxygens, bridging the Nearest Neighbors (NN) to the NNN's. Figure 2 shows the labels of

730 the NNN T-sites relative to T5. The reaction mode hardnesses are shown in Figure 3 and Table 2.

y

r.

12.59.

T3

O16 T4

~ 12.58. T2

T2

12.57. 06_T2 • ,.~

-

•

12.56.

01

T2

O18- T9

"~ 12.55. T3

019 T3

015 TIO

•

.~ 12.54. O22T7

12.53. 12.52

Figure 2. Transition state of H-exchange of methane at T5 of MFI.

: ..............

VVV

m

Ol1_T12 O10_T10 O17 37 m

0

,

2 4 6 8 10 12 i

,

i

•

i

,

i

•

i

•

i

Figure 3. Impact of AI distribution on the reaction mode hardness.

Table 2 Influence of A1 distribution on the reaction mode hardness Si_NN H-pos'i'tion ................ AI_NNN T6 06 T2 O18 T9 O19 T3 Tll Oll T12 022 T7 O10 T10 T1 O1 T2 O16 T4 O15 T10 T4 O16 T1 O 17 T7 03 T3 . . . . . . . . . . . . . . . . . . . . .

O16 T1 O3_T2

m

VVV

, . . . . . . . . . . . . . . .

h (V/e) 12.5676 12.5610 12.5704 12.5295 12.5303 12.5282 12.5618 12.5875 12.5554 12.5283 12.5282 12.5283 ,,

,,

,,

The data clearly show that A1 can harden the reaction, but not in all of the NNN positions. This depends whether the A1 is bonded to a Si_NN tetrahedron, which contains one of the two oxygens that take part in the H-exchange. If A1 is connected to S~6 and Sil, the reaction mode hardness increases. This variation is in the same order of magnitude as the reactivity differences due to the topology. For all other AI positions the influence is negligible.

731 3.3 The impact of AI content on the reaction mode hardness

The compositional influence on the reaction mode hardness is probed by a systematic increase of A1 at the NNN positions of T5, while keeping Si at the other T-sites in the unit cell. Taking into account the rule of Loewenstein (no A1-O-A1), a maximum of 8 A1 atoms can be inserted. Figure 4 shows the dependence of the reaction mode hardness on the number of A1 atoms that have been added. The labels correspond to those mentioned above and indicate the T-site which has been substituted, keeping the A1 distribution of the previous A1 content fixed.

13.0-

O6T2

~ 12.9.

O15_T10~

12.8.

O1%T1

Compositional influence

12.712.6.

/ O18_T9 O1T:201~_T12 •

12.5- ~

Structural influence

6 ~ ~ ~ 8

# AI at NNN-positions

Figure 4. Impact of the A1 content on the reaction mode hardness The results clearly show that the A1 content directly influences the hardness of the reaction mode. The higher the A1 content, the harder it becomes, and therefore the more difficult the H-exchange reaction will be. The plateaus that appear in the relation can be explained by the details of A1 distribution, which are superposed on the results of the AI content. When the newly added A1 atoms are connected to Si4 (O16_T1) or Sil 1 (O1 l_T12 and O 17_T7), we obtain no significant increase in the reaction mode hardness. The composition affects more drastically the reactivity, compared to the structural differences, as is evidenced by the large variation in the reaction mode hardness. 4. DISCUSSION In this paper the H-exchange between CH4 and a bridging hydroxyl in the faujasite and the ZSM-5 structure has been studied. In order to include the long range electrostatic effects, we combined the ab initio optimized transition state geometry proposed by Kramer et al. [6] with the EEM-CSA approach. In this way topologically different systems can be compared, excluding any cluster size effects. The hardness of the reaction polarization channel provides

732

us with a sensitive tool to probe the influence of both structural and compositional properties of the zeolite catalysts. Using idealized structures, which contain one A1 per unit cell (Si/A1 = 191), the different zeolite topologies-can be compared directly. It is interesting to see that the structural properties o f MFI tend to make it more active than FAU, especially if one assumes that the most reactive site determines the overall activity. From experimental studies, it is known that the number of A1 atoms at the N N N positions of the reaction center has a tremendous impact on the reactivity. According to the N N N principle, the maximal intrinsic acidity is obtained when no A1 is present in the second coordination sphere of the acid proton. The here proposed findings theoretically verify the importance of the NNN's. The higher the number of A1 atoms at the N N N positions, the harder the reaction will be. However, an increase of A1 at the NNN sites, not connected to the bridging oxygen atoms that take part in the reaction, will not result in a lower reactivity. In conclusion, both structural effects and the chemical composition (Aluminum distribution and Si/A1 ratio) determine the hardness and therefore difficulty of the H-exchange reaction, compositional effects being dominant. G.O.A.J. thanks the Flemish Institute for the Support of Scientific-Technologic Research in Industry (I.W.T.). The authors acknowledge financial support from the Belgian State Secretariat for Scientific Research in the form of a Concerted Research Action (G.O.A) REFERENCES

1. P.A. Jacobs and J.A. Martens in Introduction to Zeolite Science and Practic (Eds. H. van Bekkum, E.M. Flanigen and J.C. Jansen), Stud. Surf. Sci. Catal. No 58, Elsevier, Amsterdam, 1991, p. 445. 2. W.J. Mortier, J. Sauer, J.A. Lercher, H. Noller, J. Phys. Chem. 88 (1984) 905. 3. G.J. Kramer, R.A. van Santen, J. Am. Chem. Soc. 115 (1993) 2887. 4. A. Redondo, P. Jeffrey Hay, J.Phys.Chem 93 (1993) 11754. 5. V.B. Kazansky in Advanced Zeolite Science and Applications (Eds. J.C. Jansen, M. St0cker, H.G. Karge and J. Weitkamp), Stud. Surf. Sci. Catal. No 85, Elsevier, Amsterdam, 1991, p. 251. 6. G.J. Kramer, R.A. van Santen, C.A. Emeis, A.K. Nowak, Nature 363 (1993) 529. 7. E.M. Evleth, E. Kassab, L.R. Sierra, J.Phys.Chem 98 (1994) 1421. 8. J.A. Lercher, R.A. van Santen, H. Vinek, Catal. Lett. 27 (1994) 91. 9. I.N. Senchenya, V.B. Kazansky, Kinet. Catal. 35 (1994) 61. t0. S.R. Blaszkowski, A.P.J. Jansen, M.A.C. Nascimento, R.A. van Santen, J.Phys.Chem 98 (1994) 12938. 11. G.J. Kramer, R.A. van Santen, J. Am. Chem. Soc. 117 (1995) 1766. 12. J. Sauer, Chem. Rev. 89 (1989) 199. 13. a) W. J. Mortier, S. K. Ghosh, S. Shankar, J. Am. Chem. Soc. 108 (1986) 4315.;b)B.G. Baekelandt, W.J. Mortier, J.L. Lievens, R.A. Schoonheydt, J. Am. Chem. Soc. 113 (1991) 6730.;c) G.O.A. Janssens, B.G. Baekelandt, H. Toufar, W.J. Mortier, R.A. Schoonheydt, J. Phys. Chem. 99 (1995) 3251.;d) H. Toufar, B.G. Baekelandt, G.O.A. Janssens, W.J. Mortier, R.A. Schoonheydt, J. Phys. Chem. 99 (1995) 13876. 14. R.F. Nalewajski, Int. J. Quantum Chem. 56 (1995) 453. 15. a) G.O.A. Janssens, B.G. Baekelandt, H. Toufar, W.J. Mortier, R.A. Schoonheydt, Int. J. Quantum Chem. 56 (1995) 317.;b) B.G. Baekelandt, G.O.A. Janssens, H. Toufar, W.J. Mortier, R.A. Schoonheydt and R.F. Nalewajski, J. Phys. Chem. 99 (1995) 9784. 16. R.F. Nalewajski, J. Korchowiec, Z. Zhou, Int. J. Quant. Chem.: Quant. Chem. Symp. 22 (1988) 349. 17. C. Baerlocher, A. Hepp, W. M. Meier, DLS-76: A Program for the Simulation of Crystal Structures, ETH: Zurich, Switzerland (1978). 18. HyperChem, Release 3 for Windows, Molecular Modeling System; Hypercube, Inc., and Autodesk, Inc.: Waterloo, Ontario (1993). 19. D.H. Olson, G.T. Kokotailo, S.L. Lawton, W.M. Meier, J. Phys. Chem. 85 (1981) 2238.


S y n t h e s i s and Characterization of I r o n m o d i f i e d L - t y p e

733

Zeolite

Y. S. Koa, W. S. a h n a, J. H. Chae b, and S. H. Moonb aDepartment of Chemical Engineering, Inha University, Inchon 402-751, Korea bDepartment of Chemical Engineering, Seoul National University, Seoul 151-742, Korea

Iron modified L-type zeolite was synthesized hydrothermally, partially substituting iron atoms for the framework aluminum. XRD, SEM, IR, EPR, XAS, TG/DTA were performed confim~g the isomorphous substitution of iron into the zeolite structure. Catalytic studies reveal that benzene selectivity in n-hexane aromatization reaction is enhanced for the Fe-subsfituted L.

I. INTRODUCTION Isomorphous substitution of iron into the framework of zeolites would induce changes in both acidity distribution and pore size resulting in a modification of the catalyst selectivity. Trace amounts of non-framework iron in the zeolites has also been shown to contribute to the overall catalytic activity[l]. The replacement of A13. by Fe 3. in the framework of zeolite ZSM-5 has been reported by various workers[2-4]. McNicol and Pott[5] have shown the existence of Fe 3. impurity in the faujasite and mordenite framework using EPR, phosphorescence and chemical studies. Szostak and Thomas[6] were the first to deliberately synthesize a sodalite with significant quantities of Fe 3÷ in the framework. Over the years, a wide range of iron analogues such as MEL[7], MOR[8], MTT[9], MTW[10], NU-I[ll], and TON[12] were reported and techniques such as ESR, EXAFS, UV-visible diffuse reflectance, and Mossbauer spectroscopy were employed to demonstrate the tetrahedral coordination of iron. Though the methods of synthesis of iron-substituted L-type zeolites have been recently investigated[13,14], further work on the synthesis of iron substituted L-type zeolites with extensive characterization would be beneficial to the understanding of the system. In this work, we present the details of

734 synthesis and report the results of a wide range of characterization techniques including XRD, SEM, IR, EPR, XAS, and TG/DTA. Catalyst selectivity for n-hexane aromatization is also briefly discussed.

2. E X P E R I M E N T A L 2.1. S y n t h e s i s The reagents used in preparing the substrate were Ludox HS-40 (Dupont Co., 40% SiO2), potassium hydroxide (Tedia Co., 85%), aluminum hydroxide (Junsei Co., 51.1% AbO3), nanohydrate ferric nitrate (Shinyo Co., 98%) and distilled water. The synthesis of A1- and Fe-modified L-type zeolites was carried out following the method of Verduijn[15], partially Fe-substituted L-type zeolite samples were synthesized from substrates having the following compositions : 2.35 I~O - x Fe2Os - A12Os - 10 SiO~ - 160 H~.O, where x = 0, 0.015, 0.03 and 0.06. The reaction mixture was transferred to a 100ml teflon-lined stainless steel autoclave and maintained in an air oven at 170°C under unstirred conditions. Autoclaves were removed at different time intervals from the oven and were quenched immediately in cold water for identification. The solid products were separated by suction-filtration. Excess alkali was washed with distilled water, and :the products were dried in an air oven at 120°C for 12h. 2.2. Characterization X-ray diffraction patterns of the different crystalline samples were determined by X-ray diffractometer (Phillips, PW-1700) using Ni-filtered monochromatic CuKa radiation. Unit cell parameters were obtained by a least-squares fit. The crystallite size and morphology of the crystalline phase were examined using a scanning electron microscope (Hitachi, X-650) after coating with a Au-Pd evaporated film. Framework i.r. spectra of samples were recorded in air at room temperature on a Perkin Elmer 221 spectrometer with wafers of zeolites mixed with dry KBr. Electron spin resonance spectra were measured with a Bruker E-2000 spectrometer in the temperature range of 100K--400IL The XAS spectra were measured above the Fe K-edge at beamline 1 0 B , Photon Factory of National Laboratory for High Energy Physics in Tsukuba, Japan. Thermogravimetric analysis was made on a Dupont 2000 thermal analyzer in the temperature range 298--873K at a heating rate of 10K min -1. Chemical analysis was performed using ICP (Jobin Yuon-JY-38 VHR),

735

2.3. Catalysis For n-hexane aromatization, catalysts containing 2wt% Pt were used. The n-hexane aromatization was performed under atmospheric pressure at 773K in a conventional fixed bed microreactor system with 0.2g catalyst. The reaction products were analyzed by gas chromatograph (Shimadzu, GC-14A) equipped with flame ionization detector using Porapack-Q and SE-30 columns.

3. RESULTS AND DISCUSSION The chemical compositions of A1- and Fe-modified L-type zeolites are given in Table 1. For each sample, an increase in the iron contents cause the A1 contents of the crystalline material to decrease, which suggests that Fe a÷ and A13÷ could perform a similar role in the synthesis of L-Wpe zeolite and compete for its incorporation into the framework. The X-ray diffraction patterns of the crystals are shown in Figure 1. Virtually identical diffractograms of L-type zeolite were obtained irrespective of the iron contents of the sample. Figure 2 shows the influence of iron contents on the unit cell volume, V. The unit cell volume of the Fe-modified L-type zeolite increased linearly with increases in iron contents. These results strongly support that the F e 3÷ ions are incorporated in the silicate framework.

tt Q

~9

i

10

i

I

,

20

I

30

i

i

40

2 Theta Figure 1. XRD patterns of (a) A1- and (b) Fe-modified L-type zeolite.

736 Table 1 Chemical composition of A1- and Fe-modified L-type zeolites Samples (X) 0.000 0.015 0.030

SiO2 60.1 61.5 60.8

0.060

60.7

Chemical composition (wt%) A1203 Fe203 17.1 0.000 16.3 0.308 16.1 0.645 15.7

1.231

I~O 15.9 15.4 15.2 15.3

Figure 3 shows the SEM photograph of the as-synthesized Fe-modified L-type zeolite. Morphology appeared sensitive to the Fe2Oa/Al203 ratio of the synthesis mixture : A distinctive characteristic of iron addition was the formation of cylindrical shaped crystals accompanied by a decrease in crystal size as the iron contents increased. On the other hand, clam shaped crystals of a domed basal plane was synthesized in the absence of iron. IR spectra of normal A1- and partially Fe-substituted L-type zeolite with different iron contents are shown in Figure 4. Partially Fe-substituted L-type

2212 I

~ 2208[

:>

"~=~ 2204 t

2200 t / ° 21961~

, !

i

I

0.000 0.015 0.030 0.045 0.060 Fe203 / AI203 molar ratio Figure 2. Unit cell volume of the Fe-modified L-type zeolites.

Figure 3. S EM photograph of Fe-modified L-type zeolite.

737 zeolites showed a pattern similar to the normal L-type zeolite. As the amount of iron increases, IR absorption bands near 1027 and 1099 cm -1 shift progressively towards the lower frequency region compared with the absorption band of iron free L-type zeolite. The shift to lower frequency in the spectra is due to the presence of heavier Fe a+ existing in tetrahedral sites. In the partially Fe-substituted L-type zeolite, new Si-O-Fe bond vibrations could be found at near 668cm -1, which is absent in the i.r. spectrum of normal L-type zeolite, and its intensity increased with increasing iron contents.

(d)

1200

11 O0

1000

900

800

700

600

Wavenumbev (cm "1) Figure 4. IR spectra of A1- and Fe-modified L-type zeolites. (a) x = 0, ( b ) x = 0.015, ( c ) x = 0.03, ( d ) x = 0.06

Figure 5 shows EPR spectra of as-synthesized partially Fe-subsitiuted L-type zeolites. The EPR spectra of the samples reveal two main signals at g=2.0 and 4.4, and the sharp signal at g=4.4 assigned to tetrahedral Fe(m)[16], increased in intensity with increasing iron contents. According to the X-ray absorption spectroscopy(XAS), the Fe-O distance and Fe-O coordination number for Fe-modified L-type zeolite(Feg:)a/A12Oa = 0.06) were 1.85A and 4.2, respectively. The XAS results showed conclusively that the majority of the iron was substituted into the L-type zeolite framework.

738

g=4.4

(a)

I

,

1000

,

I

2000

,

3000

I

,

4000

5000

B (Gauss) Figure 5. EPR spectra of Fe-modified L-type zeolites at 100K. (a) x = 0.015, ( b ) x = 0.03, ( c ) x = 0.06

Figure 6 shows thermogravimetfic analysis patterns of the as-synthesized A1- and partially Fe-substituted L-type zeolite. The weight loss is due to dehydration of physically sorbed or occluded water, and TGA shows that Fe-modified samples are more hydrophobic than normal L-type zeolite.

100 98

o

94

} 92

(a)

L

(b) 8

,

0

i

100

•

!

•

200

I

300

•

I

400

,

I

500

•

600

Tcmlpel'at~e (°C) Figure 6. TGA patterns of (a) A1- and (b) Fe-modified L-type zeolite.

739 Pt supported on zeolite L has been commercially applied as a catalyst for n-hexane aromatization, and in Table 2, a comparison was made on benzene selectivity for the L-type zeolites with or without Fe 3+ in the framework. Significant enhancement in selectivity to benzene was obtained for the partially iron-substituted L-type zeolites. However, large reduction in conversion was also observed. This may be a consequence of Fe 3. released from the zeolite structure blocking the 1D pores at high reaction temperature. Further investigation is in progress. Table 2 Products distribution in n-hexane aromatization Samples (X) C1 = C5

Selectivity C6 isomer

MCP

C6I-I6

Conversion

0.000

3.66

20.97

1.63

73.74

90.47

0.015

2.22

9.19

1.10

87.49

35.70

0.030

4.31

4.77

1.05

89.87

34.27

0.060

3.36

1.92

2.05

92.67

32.02

Reaction conditions : temp. 773K, atmospheric pressure, Hz/HC=6, TOS 2h

4. CONCLUSIONS Fe-modified L-type zeolite was synthesized hydrothermally, partially substituting Fe 3÷ for the framework aluminum of L-type zeolite from substrates having the following compositions : 2.35 K20 - x Fe~O3 - A1203 10 SiO2 - 160 H20, where x - 0, 0.015, 0.03 and 0.06. The unit cell volume of the Fe-modified L-type zeolite was found to increase linearly following the increases in the iron contents of the substrate. The crystal morphology also changes depending on the iron contents of the sample. The mid range i.r. spectra shows a band shift to lower frequencies as iron incorporation into the lattice increases, and new S i - O - F e bond vibration was located at near 668cm -1. The EPR spectra shows a signal at g =4.4, 'which can be assigned to Fe 3. isomorphously subsitituted in the tetrahedral positions. For n-hexane aromatization reaction, enhanced benzene selectivity could be achieved by introducing Fe 3. into the zeolite L.

740

Acknowledgements We would like to thank Dr. Nomura of photon factory, Tsukuba, Japan for helping us using the synchrotron radiation source at beamline 10B and Pohang Accelerator Laboratory in Korea for providing the travel expanses. Professor R. Ryoo's group in KAIST for their assistance in analyzing EXAFS data is also acknowledged. YSK and WSA thank Inha university for providing the research fund in 1994.

REFERENCES 1. R. Szostak, Molecular Sieves, Principles of Synthesis and Identification, Van Nostrand Reinhold, New York, 1989. 2. R. Szostak and T. L. Thomas, J. Catal., 100 (1986) 555. 3. A. Meagher, V. Nair and R. Szostak, Zeolites, 8 (1988) 3. 4. R. B. Borade, Zeolites, 7 (1987) 398. 5. B. D. McNicol and G. T. Port, J. Catal., 25 (1972) 223. 6. R. Szostak and T. L. Thomas, J. Chem. Soc., Chem. Commun., (1986) 113. 7. J. S. Reddy, K. R. Reddy, R. Kumar and P. Ratnasamy, Zeolites, 11 (1991) 553. 8. A. J. Chandwadkar, R. N. Bhat and P. Ratnasamy, Zeolites, 11 (1991) 42. 9. R. Kumar and P. Ratnasamy, J. Catal., 121 (1990) 89. 10. Zhao Yanan and Li Xi Hexuan, Shiyou Xuebao, Shiyou Jiagong, 6 (1990) 33. 11. G. Bellusi, R. MiUini, A. Carati, G. Madinelli and A. Gervasini, Zeolites, 10 (1990) 642. 12. R. B. Borade, A. Adnot and S. Kaliagine, Zeolites, 11 (1991) 710. 13. P. N. Joshi, S. V. Awate and V. P. Shiralkar, J. Phys. Chem., 97 (1993) 9749. 14. V. A. Duke, K. Latham and C. D. Williams, Zeolites, 15 (1995) 213. 15. J. P. Verduijn, Exxon Chemicals, Int. Pat. WO 92/13799 (1992). 16. B. Witcherlova, Zeolites, 1 (1981) 181.


741

Synthesis, characterization and catalytic properties of VS-2 Hongwei Du, Guanghua Liu, Zhijian Da and Enze Min Research Institute of Petroleum Processing P.O.Box. 914-28 100083, Beijing, ElL China Microporous vanadium silicalite with MEL structure (VS-2) has been synthesized by a new method. XRD, IR, and ESR examinations show that vanadium ions are atomically and immobilely dispersed in V-Si molecular sieves, and exist in the form of V +4 and V +5 in assynthesized and calcined san~les, respectively. It is found that the incorporation of vanadium makes the pore structure of VS-2 more uniform and regular. A tentative mechanigic explanation is given for the increase of BET area upon vanadium insertion. The results of phenol oxidation with aqueous H:O: show that VS-2 molecular sieves have good catalytic activity and selectivity. 1. INTRODUCTION Vanadium-silicalite-2 (VS-2) is a new class of oxidation catalyst with good activity and selectivity in the oxidation of a variety of organic substances by diluted H202 [1-3]. However, since the electronic structure of vanadium element is quite different from that of silicon element and the ratio of electronic charge to radius of vanadium ion is higher , the insertion of vanadium into zeolite t~amework is very difficult. Up to now, the effective method reported in literature [4-5] for synthesizing VS-2 is the hydrothermal crystallization using organic materials. In the method, organic tetraethyl orthosilicate was used as silicon source, large amount of expensive organic base, tetrabutyl ammonium hydroxide (TBAOH), was used as base sourse and template , and the synthesis procedure is rather critical and complicated. All these make VS-2 a high cost product. In the present paper, a cheap and simple method for preparing vanadium-silicalites-2 will be introduced, in which inorganic SiO2 pellets are used as silicon source, and V205 is used as vanadium source. Expensive TBAOH is effectively utilized by greatly reducing the amount of water in the system The synthesized VS-2 molecular sieves are characterized by various physicochemical methods and tested by probe reaction of phenol oxidation with H202. 2. EXPERIMENTAL Vanadhm~silicalite-2 was synthesized as follows: a certain quantity of V205 was dissolved in a solution of TBAOH (40% aqueous solution) and the mixture was stirred for 0.5-1h. Dried micro spheric silica with a definite surface area and pore size was added to the above mixture and stirred uniformly. The typical molar composition of the gel was as follows: SiO2: x VO2: 0.14TBAOH: 4H20 (x = 0.005--43.03)

742 The resultant mixture was tranferred into a Telflon, lined autoclave and crystallized at 413K for 5 days. After crystallization, the product was filtered, washed with deionized water and dried at 383K for 8h. The organic template was removed by calcining the product at 813K for 5h. The samples of VS-2 molecular sieves were characterized by XRD (D/max-IliA, D/ max-TA), IR spectroscopy (Perkin-Elmer-580, FT-IR, Bruker-IFS-113V ), ESR measurement (E200-D), ICP analysis (Plasma Spectrovac) and adsorption techniques (ASAP-2400, Micromeritics ). Phenol oxidation by H202 ( 30% ) was carried out in a batch glass reactor and the products were analyzed by gas chromatography (Varian 3400) using OV-01 capillary column (30mx0.25mm). 3. R E S U L T S AND D I S C U S S I O N 3.1. The incorporation of vanadium ions into zeolitic framework

The X-ray powder diffraction patterns are presented in Fig.1. It can be seen that the XRD pattern of VS-2(Si/V=78) is similar to that of silicalite-2, showing the typical MEL structure. The absence of peaks at 20 = 9.06 ° and 24.06 ° indicates that there is no MFI type impurity in the product. The unit cell parameters given in Table 1 show that the unit cell volume of vanadium-silicalite-2 is larger than that of silicalite-2, and increases with the vanadium content to a certain extent. This can be explained by the fact that larger vanadium ions are incorporated into zeolitic framework of MEL structure.

b

--

10

20

30

40

20 Fig. 1. XRD pattems of silicalite-2 (a) and VS-2 (Si/V=78) (b). Table 1 Unit cell parameters of vanadium-silicalite-2. Sample Si/V molar ratio Unit cell Gel Product a Silicalite-2 oo oo 2.002 VS-2 200 378 2.003 137 224 2.002 78 127 2.004 33 82 2.004

=

b 1400

1000

600

400

Wavenumber(cm.t) Fig. 2. IR spectra of silicalite-2 (a) and VS2 (Si/V=78) (b).

parameter/nm b c 2.002 1.336 2.003 1.337 2.003 1.338 2.004 1.339 2.004 1.338

Volume/nm 3 5.354 5.364 5.369 5.378 5.376

743 The insertion of vanadium ions in the framework is further confirmed by IR experiments. The IR spectra of vanadium-silicalite-2 and silicalite-2 are shown in Fig. 2. The main peaks of VS-2 shift slightly to lower wavenumber, which is due to that the length of V-O bond is longer than that of Si-O bond and also the mass of vanadium atom is larger than that of silicon atom. In addition, a new absorption band at around 970 cm-1 appears in the spectrum of vanadium-silicalite-2, which is absent in the spectrum of vanadium-free silicalite-2. This can be accounted for by the influence of the incorporation of vanadium ions on the asymmetric stretching vibration of Si-O8+ "V ~ [6]. The above experimental results prove that the larger vanadium ions are incorporated into the framework position. 3.2. The chemical state of vanadium ions

The ESR spectra of both as-synthesized and calcined VS-2 samples have been recorded at room temperature. As shown in Fig. 3a, the spectrum of as-synthesized VS-2(Si/V=78) is composed of 8-splitting lines arising from the b hyperfine interactions of the d electrons of the V +4 ion with the I=7/2 spin of the 5Iv nucleus. i This kind of ESR spectrum is characteristics of atomically dispersed and immobile V ÷4 ions [7]. After calcination of the as-synthesized sample in air at 813K for 5h, no ESR signals are observed (Fig. 3b), which indicating that V ÷4 ions ( d 1 ) are completely oxidized to V +5 ions Fig. 3. ESR spectra of VS-2 (Si/V=78) (dO). After reducing the calcined sample in H2 at as-synthesized (a), calcined (b) and 753K for 4h, the typical ESR spectrtun of reduced (c). atomically dispersed V ÷4 ions with multisplitting lines appears again, showing that the reversibility of the transformation between V +s and V +4 and that V +5 ions are still linked to the silicalite structure in calcined sample. The above results indicate that vanadium ions in V S-2 zeolites exist as atomically and immobily dispersed V +4 and V +5 state in as-synthesized and calcined samples, respectively. The transformation of V +4 ¢:> V +5 is completely reversible, showing the good redox properties of vanadium-silicalite-2.

1

3.3. Pore distribution and surface area

The pore distribution profiles of silicalite-2 and VS-2 measured by N 2 adsorption and calculated on the basis of BJH methods are shown in Fig.4 and Fig.5, respectively. It can be seen that both the meso pore with 4nm diameter in average and the large pore about 55nm diameter in average exist in silicalite-2, whereas only the meso pore with 3.2nm diameter in average exists in vanadium-silicalite-2. Moreover, the pore volume of vanadium-silicalite-2 (0.025ml/g) is less than that of silicalite-2 (0.044ml/g). It seems that the introduction of vanadium plays a role in reducing the volume of medium pore as well as inhibiting the formation of larger pore, which leads to a more regular and uniform pore structure of vanadium-silicalite-2. This may be the main reason causing the increase of BET area after the insertion of vanadium (Table 2). When too much vanadium is added to the synthesis system,

744 the BET area of vanadium-silicalite-2 decreases. This is probably due to the formation of extra-framework vanadium cluster which might block the pore of VS-2.

•

-

0.40

..

-I--~ :'

.

A

''i

4nm_..:. . . . A n m

~:::"

~ 0.30

.~ 0.20 ;~ 0

o t~

.

'

./

,..-:....

"-,-:-:.i-z i

•

~

'.

."

:ii!iiifil if:ii!!! ..

l

'.

.. , t . . :

• J-:

":

I • ::'"

0.10 t--~=~-~.---v--t, 55nm ---~'"i ]._.,

0.00

:"

'

T ...... ~ : . 2 _ 5 I. ---. -'. -.,...., - . . . . . . . . .

1

~

"

10

0.28

!ii

~ :'

• A

"~ 0.20 E o

area ofvanadium-silicalite-2. BET surface area/mZ.g4 co 267 200 489 504 517

,

~ !

"i

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

. . . . . . i ............................ I ...............

.

:

.

! " t ..... ..........

"" . ....

"

.a .........

I -."

• ._:_

0

0.00

-

1

Pore diameter(nm)

Table 2 BET surface Sample Si/V VS-2

..

.......

;> 0.10

: .....

i

0

-I ........ ..

Fig. 4. Pore distribution curve of silicalite-2.

• '

|

1. I . . . . . . . . . . . . . . . .

:"' . . . . .

,,...

-

100

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

'ell

1, ~..

. ,,

.~. . ,

_--

i-~

.--z---r-

"_.~.-.'._...__.z~

.

j

• ,,..,:..,

10 100 Pore diameter(rim)

Fig. 5. Pore distribution curve of VS-2 (SifV=78).

137 531

78 538

33 511

3.4. Adsorption properties The data of adsorption measurements are listed in Table 3. It can be seen that the sorption capacity over VS-2 decreases with the adsorbates in the following order: n-hexane> cyclohexane > water. The RAL values of VS-2 with different Si/V molar ratios are in the range of 1.81 to 1.90, exhibiting the obvious hydrophobicity. The hydrophobical property of vanadium-silicalite-2 makes it possible to use low concentration aqueous HzO2 (30%, safe and cheap) as oxidant and water as solvent in catalytio reactions, which makes the reaction process both convenient and cost-effective. Comparing with the case of silicalite-2 (Table 3), the adsorption capacity of H202 over vanadium-silicalite-2 is enhanced evidently, and it increases with the increase of vanadium content, which can be accounted for by the cl~emical absorption of H202 on vanadium-silicalite-2.

Table 3 Adsorption properties ofvanadium-silicalite-2 Sample Adsorption capacity/wt.% n-Hexane cyclo-Hexane Water Silicalite-2 12.9 7.89 6.68 VS-2.(200) 13.1 8.12 6.91 VS-2(137) 13.2 8.23 7.11 VS-2(78) 13.1 8.09 7.38 VS-2(33) 13.4 8.16 7.27

H2Oz 6.4 8.5 9.2 11.1 11.3

RAL= n-Hexane(wt. %) / Water(wt.%) . 1.93 1.90 1.87 1.81 1.85

745 3.5. Surface acidity NH3-TPD profiles of different samples are shown in Fig.6. The absence of the significant high temperature peak on VS-2 ( curve b) reveals the lack of strong acid sites. Fig. 7 illustrates the IR. spectrum of pyridine adsorption on VS-2 (Si/V=78) after evacuation at 302K. Three absorption bands at 1548cm-1, 1447cm-1 and 1492cm-1, which can be assigned to Bronsted acid sites, Lewis acid sites, both Bronsted acid sites and Lewis acid sites, respectively, are identified in the spectrum. It is calculated that the ratio of the number of Bronsted acid sites to that of Lewis acid sites on VS-2 (Si/V=78) is approximately 0.62.

I

i

|

l

i

I

!

373 573 773 Temperature(K)

Fig. 6. NH3-TPD profiles of silicalite-2 (a) VS-2(Si/V=78) and ZSM-5(Si/Al=65)

1700

t

1600 1500 1400 Wavenumber(cm"~)

Fig. 7. FTIR spectrum of VS-2(Si/V=78) after pyridine adsorption. "

3.6. Oxidation of phenol with H20 2 The results of phenol oxidation by diluted H202 over VS-2 with different Si/V molar ratio in water solvent are given in Table 4. The samples of VS-2 were treated with 0.5M ammonium acetate solution at room temperature and then calcined at 773K for 6h before use in the catalytic reaction. Reaction conditions were as follows: catalyst/phenol-4.8wt.%; phenol/H202 =3.0mol/mol; HEO/phenol=5:l(wt.); temperature(K)=353; reaction time=8h. It can be seen that vanadium-silicalite-2 samples are catalytically active in hydroxylation of phenol to catechol and hydroquinone. Phenol conversion over VS-2 (Si/V=78) could reach a level of 18.6% with a selectivity of 99.1%. The main products are catechol (58%) and hydroquinone (41%). Para-benzoquinone is obtained in an amount less than 1%.

Table 4 Catalytic properties of phenol hydroxylation over vanadium-silicalite-2 Sample Phenol Phenol H202 Products distribution / mol% conversion/% selectivity/% efficiency/% CAT HQ PBQ VS-2(33) 18.2 99.3 54.5 58.8 40.3 0.89 VS-2(78) 18.6 99.1 55.3 58.2 41.0 0.83

746 4. CONCLUSIONS Vanadium-silicalite-2 has been synthesized by a low cost method. XRD and IR experiments confirm that vanadium ions are incorporated into the zeolitic l~amework. ESR spectra indicate that vanadium ions are atomically and immobily dispersed respectively in as-synthesized and calcined samples. The transformation of V+4c:~V+5 is found to be completely reversible, showing the good redox properties of VS-2. BET, NH3-TPD, FTIR and sorption measurements reveal that vanadium-silicalite-2 possesses regular and uniform pore structure, surface hydrophobicity and medium acid strength with a ratio of Br&asted to Lewis acid sites of 0.62. The synthesized VS-2 molecular sieve exhibits good catalytic activity and selectivity in phenol oxidation by aqueous H202 solution. REFERENCES

1. P. 1~ Hari Prasad Rao, A. V. Ramaswamy and P. Ratnasamy, J. CataL,141(1993)604. 2. P. P,. Hari Prasad Rao and A. V. Ramaswamy, AppL Cata[ A: General, 93(1993)123. 3. P. P,.Hafi Prasad Rao, K.Ramesh, A. V. Ramaswamy and P. Ratnasamy, Stud. Sure Sci. CataL, 78(1993) 385. 4. Hari Prasad Rao, A.A. Belhekar, A.V. Ramaswamy et al., J. CataL, 141(1993)595. 5. P. lk Haft Prasad Rao, Rajiv Kumar, A. V. Ramaswamy and P. Ratnasamy, Zeolites, 13(1993)663. 6. R. M. Boccuti, M. K. Rao, et aL, Stud. Sure Sci~ CataL, 48(1989)133. 7. ~ Takahashi, M. Shiotani~ H. Kobayashi and J. Sohma, J. CataL 14(1969)134.

H. Chon, S.-K. Ihm and Y.S. Uh (Editors) Progress in Zeolite and Microporous Materials Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All fights reserved.

747

Preparation and characterization of manganese bipyridine complexes in zeolites with different pore architectures S. Ernst and B. Jean Institute of Chemical Technology I, University of StuRgart, D-70550 Stuttgart, Germany

[Mn(bpy)2]2+-complexes have been synthesized and immobilized in the supercages of zeolite Y and, for the first time, in the large intracrystalline cavities of zeolite MCM-22. No complex formation could be observed with zeolite EU-1 as host material, most probably due to steric constraints. The synthesized host/guest compounds are characterized by physico-chemical methods and their catalytic properties are tested in the liquid phase oxidation of cyclohexene using aqueous hydrogen peroxide.

I. INTRODUCTION Zeolites and related mieroporous materials are currently under intensive study as inorganic host materials for the immobilization of catalytically active guests, i. e., transition metal complexes [1-3]. In many eases, zeolite inclusion is applied to avoid the formation of polynuclear aggregates and to strongly reduce the self-oxidation tendency of the complexes. Typical organic ligands used for the synthesis of zeolite encapsulated transition metal complexes are phthalocyanines, the Sehiff base salen and 2,2"-bipyridine (bpy) [1-3]. With very few exceptions (e. g., metal phthaloeyanines in AIPO4-5 [4,5], VPI-5 [6] or in zeolite EMC-2 [7]), only faujasite-type zeolites have so far been explored as host materials. There are, however, several additional framework topologies currently available having large intracrystalline voids which render them promising candidates for the immobilization of transition metal complexes. Among these framework topologies are those of zeolite MCM-22 and zeolite EU-1. Zeolite MCM-22 contains two independent pore systems: One consists of large supereages (ca. 0.71 x 1.82 rim) which are accessible and interconnected via 10-membered ring windows. The second channel system is made up from two-dimensional, sinusoidal channels with 10-membered ring openings [8]. The pore system of zeolite EU-1 essentially consists of unidimensional, non-interconnected 10-membered ring channels with very large side pockets of ca. 0.65 nm in diameter and 0.8 nm deep [9]. Hence, these two zeolitic materials could potentially act as hosts for transition metal complexes of suitable size. Here we report on our attempts to synthesize manganese bipyridine complexes in the intracrystalline voids of zeolites MCM-22 and EU-1. For

748 comparative purposes, zeolite Y is also included in the present study. Manganese-bisbipyridine complexes encapsulated in the supercages of faujasite-type zeolites were recently reported to catalyze selective oxidation reactions of alkenes in the liquid phase [10], in particular oxidation of cyclohexene to adipic acid using tertiarybutylhydroperoxide as the oxidant [11 ].

2. EXPERIMENTAL SECTION Zeolites MCM-22 (nsi/nAl = 21, [12]) and EU-1 (nsi/nAl = 20, [13]) were synthesized according to published procedures. The as-synthesized materials were calcined at 540 °C in air in order to remove the organic templates and then extensively ion exchanged at room temperature with a 0.1 n aqueous solution of NaC1. The sample of zeolite NaY used in this study was a commercial product supplied by Union Carbide Corp., Tarrytown, N. Y., USA, and had a nsi/nAl-ratio of 2.6. Predetermined amounts of Mn 2+ were introduced into the three zeolites with different pore architectures by ion exchange with Mn(CH3COO)2 in aqueous suspension at room temperature. For complex formation, the Mn 2+ exchanged and dried zeolites were mixed in a glove box with 2,2"-bipyridine ligand with a ratio of nbpy/nMn2+ - 2.5. The mixture was then transferred to a glass ampoule, sealed and kept in an oven at 90 °C for a period from two to three days. The obtained products were soxhlet-extracted with dichloromethane to remove uncomplexed bipyridine ligand. After hydrothermal synthesis and after each further modification step it was ascertained by X-ray powder diffraction (Siemens D 5000) that the crystallinity of the samples remained virtually unchanged. The prepared materials were further characterized by chemical analysis using atomic emission spectroscopy with inductively coupled plasma (AES/ICP), UV/VIS-spectroscopy in the diffuse reflectance mode, solid-state IR-spectroscopy using the KBr pellet technique and simultaneous thermogravimetry/differential thermal analysis (TGA/DTA). The catalytic properties of the prepared host/guest compounds were tested in the liquid phase oxidation of cyclohexene using hydrogen peroxide (35 wt.-%) as the oxidant.

3. RESULTS AND DISCUSSION After the complex formation and soxhlet extraction steps, the obtained materials exhibit a slight pink color in the cases of zeolite Y and zeolite MCM-22, whereas zeolite EU-1 remained white. This is a first indication that complex formation occurred in the former zeolites but not in the latter. Most probably, the space in the side pockets of zeolite EU-1 is not large enough to allow for the bipyridine complexes to form. The positions of the absorption maxima in the UV/~S-spectra of the Na +- and Mn2+exchanged zeolites and after treatment with bipyridine and soxhlet extraction are summarized in Table 1. Replacing part of the initially present Na+-cations by Mn 2+cations results in a shift of the absorption maximum from 225 nm to 235 nm for zeolite Y and from 250 nm to 260 nm for zeolites MCM-22 and EU-1. For the samples of

749 zeolites Y/l, Y/2 (two samples with different Mn2+-contents were prepared from zeolite Y) and MCM-22 which had been treated with bipyridine, new absorption bands appear which can be attributed to the formation of [Mn(bpy)2]2+-complexes. In particular the absorption maxima at 495 nm and 530 nm (519 nm for [Mn(bpy)2] 2+MCM-22) can be attributed to the typical metal-to-ligand charge transfer. The absorption maximum around 360 nm is most probably due to ligand-to-metal charge transfer. No new absorption bands can be observed for the bipyridine-treated sample of zeolite MnEU-1. As already suspected from the unchanged color of this zeolite, obviously no complex formation had occurred. At present it is tentatively assumed that this is due to the steric constraints in the intracrystaUine voids of zeolite EU-1. The UV/VIS-spectra of the prepared host/guest compounds are depicted in Figure 1. As expected, the absorption bands for the faujasite-type zeolite with a higher concentration of Mn2+-cations are more intense. This indicates a higher density of [Mn(bpy)2]2+-complexes in the former sample. The weak shoulder around ca. 360 nm (ligand-to-metal charge transfer) almost disappears for the samples with lower complex content. UV/WIS-spectroscopy of the solutions recovered after soxhlet extraction reveals that during the extraction step only uncomplexed 2,2"-bipyridine is removed from the zeolite. The Mn2+-containing complexes are obviously efficiently retained in the zeolites, i. e., they are encapsulated.

Table 1. Positions of the absorption maxima in the solid-state UV/VIS-spectra of the Na +- and Mn2+-exchanged zeolites and after the treatment with 2,2"-bipyridine and soxhlet-extraction. Sample

Positions of the absorption maxima, nm

NaY NaMCM-22 NaEU-1

225 250 250

MnY MnMCM-22 MnEU-1

235 260 260

300 300

247 247 245 260

295 295 295 300

[Mn(bpy)2]2+-y/1 [Mn(bpy)2]2+-y/2 [Mn(bpy)212+-MCM-22 [Mn/bpy)E]E+-EU- 1

356 356 361

495 495 495

530 530 519

750

Fourier transform infrared spectra of the encapsulated complexes reveal only minor frequency shifts as compared to the homogeneous [Mn(bpy)2]2+-complex (the latter was prepared as described in [14]). Knops-Gerrits et al. [10] reported that in faujasitetype zeolites the cis-bipyridine complex is preferentially formed over the transconfiguration, which is indicated by the occurrence of two absorption bands at ca. 757 cm-1 and ca. 772 cm-1. A single band around 772 cm-1 would be indicative for the trans-configuration. The host/guest compounds prepared in the present study possess a dublett with absorption bands at ca. 757cm-1 and 772 cm-1. Hence, cis-[Mn(bpy)2] 2+ seems to be the preferentially formed complex in the cavities of both, zeolite Y and zeolite MCM-22. It is known from the free homogeneous complex that oxygencontaining ligands may favor the formation of the cis-species. In a zeolite, this role could be played by oxygen ions of the zeolite lattice. This would also result in an enhanced retention of the complex in the zeolite.

356 Z

•

0

530

i

Ifl_ n,"

0

cO nn

T

A41

8B49 7

d

A35

O

6

I

,

I

40

20

,

I

60 nsi/nTi

80

100

- RATIO

Figure 2. Influence of the titanium content and the mode of preparation of titanium silicalites on their Modified Hydrophobicity Indices.

I

'

I

"

I

'

.....

1,0

z 0 F-

.....

A48 A35 B49 A30

0

co 0,5 m
80), both synthesis methods probably yield products with a comparable density of lattice defects because they produce materials with similar Modified Hydrophobicity Indices. Upon successive incorporation of titanium atoms into the silicate framework, the materials become more and more hydrophilic which is indicated by a decreasing HI*. To account for this decrease in HI* it is assumed that water molecules can coordinate to framework Ti-atoms. Hence, samples with nsi/nTi > 40 show a decrease in HI* with increasing titanium content. From the l.W/VIS-spectmm of sample A30 (cf. Figure 3) it can be deduced that beside titanium in tetrahedral coordination in the silicate framework a separate crystalline titanium-containmg phase is present. It is assumed that the latter phase adsorbs practically no water. Hence, HI* of sample A30 is between those of samples A40 and A48, which presumably contain a comparable titanium content in the zeolite lattice. Sample A35 and those samples prepared acx~rding to method B for which the presence of a second titanium species is suggested by their UVNIS-spectra, are significantly more hydrophilic than those samples synthesized after method A which possess only tetrahedrally coordinated titanium in the ffamenwork At present, the following structural models can be envisaged for these hydrophilic titanium species in sample A35 and the samples synthesized according to method B: (i) titanium dioxide with a partide diameter in the nanometer range and an unusually large number of surface Ti-OH groups, (ii) an amorphous hydrophilic titanium silicate with titanium in octahedral coordination and, (iii) the incorporation of titanium species into defect sites of the silicate framework. If this occurs in octahedral coordination (titanium incorporation as "framework satellite"), hydrophilic hydroxyl nests are created which are analogous to those formed upon silicon removal from the t~amework Based on the experimental data presently available, no dear decision can be made as to which of the structural models discussed above comes closest to reality. The catalytic behavior of selected titanium silicalite samples was tested in the liquid phase hydroxylation of phenol with aqueous hydrogen peroxide. Typical results obtained with sample A35 as catalyst are depicted in Figure 4. After co. 70 minutes the peroxide initially added to the reaction mixture is completely converte~ The yields of the hydroxylation products, i.e., hydroquinone and catechol reach almost 40 % each. Hence, a total yield of hydroxylation products of nearly 80 % is obtained at the end of the experiment. These results are in principal agreement with published data obtained under comparable reaction conditions with a Euro-TS-1 sample [4]. The sample used in the present study (A35) was prepared in a similar manner as Euro-TS-1 and its nsi/nTi-ratio was similar as well. The catalytic behavior of selected additional samples (A30, A48, B49) prepared in this study is compared in Figure 5 with that of sample A35. As a measure for the quality of the _cat___al_ysts, the total yield of hydroxylation products (viz. the sum of the yields of hydroquinone and catechol) was choserL Titanium silicalite A30 contains amorphous

768

o-e,

>_a ._1 LU .=.

100 80

>-

•

YHydroquinone

•

Yc=t,~ol

~

m T='t

/ ~

n,

O x

/

60

Z O

= 0.5 g = 100oc

nH2o/nphenol =

1/4

solvent -

acetoneI H20 J

_

40

00

n, LU >

20

Z

O (3

,

I

20

I

I

,

I

,

40 60 REACTION TIME, min

I

=

80

Figure 4. Hydroxylation of phenol on titanium silicalite sample A35 at 100 °C.

80 •

A 35

• •

A48 B 49

•

o"9, 60

~-~--

................e~

tO ",~

m=,t

=

0.5 g

T

=

100 °C

nH202/ nR,.,~

solvent

= •

114 acetonelH=O

I

~" 20

0

20

40

REACTION T I M E ,

60

80

min

Figure 5. Yield of hydroxylation products over TS-1 smnples prepared ac~rding to different synthesis procedures and with different nsi/nTi-ratios.

769 titanium dioxide; samples A48 and B49 possess similar titanium contents but most probably do not contain amorphous titanium dioxide (cf. Figure 3). It can be seen from Figure 5 that the rate of hydroxylation is somewhat lower for catalyst A48 than for sample A35. This is in agreement with the lower titamum content of the former sample. At the relatively high reaction temperature applied in the present study, hydrogen peroxide is always consumed to a certain extent by thermal decomposition. Therefore, the maximum yield of hydroxylation products increases with increasing ratio of the rate constants of hydroxylation and decomposition. Hence the maximum yield which can be achieved with sample A48 is somewhat smaller than with sample A35. Catalyst B49 contains approximately the same overall amount of titanium as sample A48. However, hydroxylation yields are dearly lower (cf. Figure 5). The relatively broad absorption in the UV/VIS-spectmm of B49 suggested the presence of an additional titanium species. We anticipate that this species, regardless of its exact nature, possesses a somewhat lower catalytic activity than tetrahedrally coordinated titanium atoms in framework positions. Titanium silicalite A30 possesses the highest titanium content of all catalysts prepared in the course of this study. However, it exhibits by far the lowest hydroxylation activity due to the presence of large amounts of amorphous titanium dioxide. From the results presented here it can be deduced that synthesis method A allows to incorporate more titanium into framework positions than synthesis method B. In addition, for a comparable overall content of titanium, the former possess a considerably higher catalytic activity than the latter.

4. CONCLUSIONS It has been shown that samples of titanium silicalite-1 having different nsi/nTi-ratios and prepared according to two different synthesis procedures can differ significantly in their hydrophobic/hydrophilic surface properties as revealed by the Modified Hydrophobieity Index HI*. For nsi/nTi-ratios above ca. 40, HI* decreases linearly with increasing titanium content which has been attributed to the increased formation of polar Si-O-Ti bridges in the zeolite framework For TS-1 samples with higher titanium content, HI* strongly depends on the method of preparation and is considerably influenced by the formation of additional titanium-containing species in extra-framework positions. From the results of the catalytic characterization in the hydroxylation of phenol it can be concluded that, in particular, TS-1 samples with a high titanium content (i.e., nsi/nTi below 40) are the more active, the lower their Modified Hydrophobicity Index. Hence, the determination of HI* seems to be a useful method for the characterization of titanium silicates which furnishes valuable structural information especially in combination with other suitably selected characterization techniques.

770 ACKNOWLEDGEMENTS J. W. and S. E. gratefully acknowledge financial support by Deutsche Forschungsgemeinschaft, Fonds der Chemischen Industrie and Max-Buchner-Forschungsstithmg,

REFERENCES

1. M Taramasso, G. Perego and B. Notari, US Patent 4,410,501 (1983). 2. G. Perego, G. Bellussi, C. Como, ~ Taramasso, F. Buonomo and/L Esposito, in: New Devdopments in Zeolite Science and Technology, Y. Murakami, A fijima and J. W. Ward (eds.), Kodansha, Tokyo, and Elsevier, Amsterdam, 1986, pp. 129-136. 3. B. Notari, in: Innovation in Zeolite Materials Science, P. J. Grobet, W. J. Mortier, E. F. Vansant and G. Schulz-Ekloff (eds.), Studies in Surface Science and Catalysis, Vol. 37, Elsevier, Amsterdam, 1988, pp. 413-425. 4. B. Kraushaar-Czametzky and J. H. C. van Hooff, Catal. Lea., 2 (1989) 43-47. 5. J. A~ Martens, P. L. Buskens, P. A. Jacobs, A~ van der Pol, J. H. C. van Hooff, C. Ferrini, H. W. Kouwenhoven, P. J. Kooyman and H. van Bekkum, Appl. Catal. A, 99 (1993) 71-84. 6. A~ Zecchina, G. Spoto, S. Bordiga, A~ Ferrero, G. Pelrini, G. Leofanti and M Padovan, in: Zeolite Chemistry and Catalysis, P./L Jacobs, N. I. Jaeger, L. Kubelkovfi and B. Wichterlovfi (eds.), Studies in Surface Science and Catalysis, Vol. 69, Elsevier, Amsterdam, 1991, pp. 251-258. 7. S. P. Mirajkar, A, Thangaraj and V. P. Shiralkar, J. Phys. Chem., 96 (1992) 30733079. 8. J. Weitkamp, P. Kleinschmit, A~ Kiss and C. I-L Berke, in: Proceedings from the Ninth International Zeolite Conference, 1L von Ballmoos, J. B. Higgins and ~ M J. Treacy (eds.), Part II, Butterworth-Heinemmm, Boston, 1992, pp. 79-87. 9. /k Thangaraj, M J. Eapen, S. Sivasanker and P. Ratnasamy, Zeolites, 12 (1992) 943950. 10. D. Trong On, A. Bittar. A~ Sayari, S. Kaliaguine and L. Bormeviot, Catal. Lett., 16 (1992) 85-95. 11. G. Bellussi and ~ S. Rigutto, in: Advanced Zeolite Science and Applications, J. C. Jansen, M. St6cker, H. G. Karge and J. Weitkamp (eds.), Studies in Surface Science and Catalysis, Vol. 85, Elsevier, Amsterdam, 1994, pp. 177-213.


771

T h e T h e r m a l S t a b i l i t y of t h e G a l l o p h o s p h a t e C l o v e r i t e W. Schmidt, F. Schiith, S. Kallus Institut fiir Anorganische Chemie und Analytische Chemie, Johann Wolfgang Goethe-Universitiit, Marie Curie-Stral3e 11, D-60439 Frankfurt, Germany

The thermal stability of cloverite, [GasPsO31(OH)2](QF)2(H20)n, was studied by in situ x-ray diffraction (XRD), in situ infrared spectroscopy (IR), thermogravimetric analysis combined with differential thermal analysis (TG/DTA) and nitrogen adsorption measurements. During the heating process the framework remained stable to at least 500°C in air as well as in vacuum. Cooling down to room temperature in ambient air led to a structure break down, while under vacuum the cloverite structure remained stable during the cooling process. When the calcined cloverite is kept at temperatures above 100°C its framework remains intact while storage under aliphatics produces a material similar to cloverite.

1. I N T R O D U C T I O N The gallophosphate cloverite has gained much attention due to its unique pore system consisting of three different types of cages including an extremely large supercage with approximately 3 nm in diameter [1]. All cages are connected and accessible via windows consisting of at least 8 T-atoms. Thus, unique adsorption properties should be expected from this material, which consists mainly of large and open cavities. During the synthesis water molecules and organic template molecules are enclosed inside the pore system of cloverite, which have to be removed prior to any application of that material as molecular sieve. Attempts to activate cloverite by calcination in ambient air failed and only amorphous material remained after the samples had been taken out of the furnace [2]. The instability of the calcined cloverite makes it useless for any application. The aim of our investigations was, and still is, to get insight into the behavior of cloverite during thermal treatment and, perhaps, find a possibility to preserve the crystalline structure of the material, even after the calcination process, in

772 order to make the material amenable to adsorption studies and for the use of cloverite as host material for organic and inorganic guest molecules. Several research teams work on very similar topics [3,4], which shows the interest t h a t activated cloverite gains.

2. E X P E R I M E N T A L All cloverite samples were synthesized in teflon lined autoclaves, which were placed in an oven at 150°C for 24 hours. HF (Merck) and N a F (Aldrich) were used as fluoride sources. Both of them can be used to synthesize cloverite, the products only show differences in the morphology of the crystals and slightly differences in their weight loss during the calcination. The molar compositions of the reaction gels were Ga203 : P205 : 6 Q : 0.75 HF : 71.3 H20 and Ga203 : P205 : 6 Q : 0.75 NaF : 71.3 H20 (Q = quinuclidine, Fluka), respectively. The amount of water in the reaction gel was fixed by the water content of the employed gallium sulphate solution (8 wt.% Ga, VAW AG), the phosphoric acid (85 wt.%, Merck), and the hydrofluoric acid (40 wt.%, Merck). For the preparation of the reaction mixture the phosphoric acid was poured into the gallium sulphate solution. Then the quinuclidine was added slowly which results in a highly viscous gel. The gel became less viscous in the case of HF and remained as gel in the case of N a F after t h e addition of the fluoride source. The reaction mixture was stirred during the whole preparation. The homogeneous reaction mixture was poured into the teflon lined autoclaves. For in situ XRD measurements in ambient air and under vacuum CUK~ radiation and a Paar HTK 10 heating chamber were employed. The powder samples were prepared on a platinum sample holder serving as a resistance heater. IR experiments were carried out in a heatable vacuum cell [5] using a SpectraTec Research Plan Microscope attached to a Nicolet 5SBX optical bench. DRIFT measurements were performed on the same spectrometer using a heatable DRIFT cell (Harrick). Samples were heated with a rate of 5°C/min for in situ XRD m e a s u r e m e n t s and data sets were recorded every 50°C up to 500°C. One m e a s u r e m e n t took about 20 minutes. In order to treat the samples in the IR cell in a similar way they were heated with the same heating rate using the same t e m p e r a t u r e steps. The samples were kept at this temperatures for 20 minutes before they were measured. TG/DTA experiments were performed on Linseis L81 and Setaram TG-DTA 92-16 thermobalances in an air flow of 10 lPa with a heating rate of 10°C/rain. Adsoi'ption isotherms were recorded on a Micromeritics ASAP 2010 adsorption instrument using nitrogen as adsorbent at 77 K. The samples were activated at 350°C under vacuum.

773 3. R E S U L T S AND D I S C U S S I O N According to the synthesis routes mentioned above cloverite was obtained in good quality without amorphous impurities. The crystals obtained by using NaF as fluoride source were much larger than those from the reaction mixtures containing HF. They were occasionally elongated up to 200 ~m in one direction as shown in figure lb). Besides those elongated rods one finds typical cubeoctahedrally shaped cloverite crystals [6] with 40 ~m in sizes in the NaF system. Figure 1. SEM images of a) cloverite crystals obtained using HF as fluoride source scale b a r - 20 ~m

b) a cloverite crystal elongated in one direction obtained using NaF as fluoride source scale b a r - 100 ~m

The crystals obtained from the HF containing reaction mixtures have the same cube-octahedraUy shape but only sizes of 10 ~m or are even smaller (figure la)).

774 The different sizes of the crystals from both synthesis systems can be explained by two effects. The pH of a mixture containing NaF is higher than that of those containing HF. The dissolution of the reaction gel is slowed down and during the nucleation period only a small number of crystal nuclei is formed, which then grow much larger since the gel is consumed by only a few crystals. The NaF containing reaction mixture has a gel like consistency and the crystals remain longer in the growth area of the reactor. The HF mixture is a clear solution and once the growing crystals reach a distinct size their sedimentation starts. At the bottom of the reaction vessel the growth of the crystals is restricted, since the number of growing crystals is much higher and, thus, the concentration of molecules with the ability to condense on the crystal surfaces is much lower than in the upper parts of the vessel. TG/DTA measurements in flowing air show that the calcination behavior of crystals from both systems is similar up to a temperature of 900°C. Using a heating rate of 10°C/rain water is desorbed in an endothermic process up to 200°C. The desorption of the quinuclidine, followed by its combustion, takes place in four distinct exothermic steps in the temperature range between 300-700°C (maxima of exothermic peaks: 340°C, 430°C, 490°C, 540°C). In general the samples contain 6-10 wt.% water and approximately 13-14 wt.% quinuclidine. At temperatures above 900°C the samples crystallized in the NaF system show an additional weight loss of about 2-3 wt.%. It might be due to the combustion of an impurity of only little crystallinity which is not detectable by XRD. Since this additional weight loss did not appear with samples synthesized in the HF system, we used only material synthesized with HF for the investigations described below.

~lL~l~.~~' ~ Ib

~

211ela

~o

~

Figure 2. In situ XRD pattern of a) Cloverite, recorded during heating up to 500°C and cooling down to 20°C under vacuum

~~~i.

~~.~,~,

2111ela

b) Cloverite, recorded during heating up to 500°C and cooling down to 20°C under vacuum and exposure to ambient air

XRD patterns of cloverite, recorded during the heating and cooling down period, are shown in figure 2. Diffraction patterns were measured in steps of

775

50°C. For clearness not all patterns which were recorded are presented and only the range up to 30 ° 2 theta is shown. When cloverite is exposed to a vacuum of 10 .4 mbar at room temperature its XRD pattern has a much lower underground t h a n an as synthesized sample. Figure 2a) reveals t h a t once the vacuum is stable the diffraction patterns remain basically unchanged when the sample is heated up to 500°C. The intensities of some peaks change slightly, but the structure of cloverite remains stable and the crystallinity of the sample does not decrease. Cooling down to room temperature under vacuum does not change the XRD pattern. Heating in ambient air also does not affect the crystallinity and the structure of cloverite. The crystal structure can withstand t e m p e r a t u r e s up to 500°C without getting damaged and the XRD pattern is basically the same as under vacuum at t h a t temperature. Differences occur during the cooling in air as shown in figure 3. While no structural changes are detectable down to 100°C a rapid amorphization of the crystalline cloverite is observed within a few minutes when the temperature is below 100°C. This amorphization is not observed when the sample which was heated under vacuum is exposed to ambient air. Figure 2b) presents the diffraction patterns of that sample after its exposure to air.

1oo

_r

I

,,iw,~ku iuiiiw,11~~]~A" " - ~

,, ] ~ , A ~

IF vkJ v ~' ~ k.~L,~.'"'-.,,~,~,,,J ~ 200L . . . . . ~ .... o

"

~)

~

~0

"%. ~ - ~ ~k ~

~

~

"~'

\/.~ _ v\ k

/

20°C 100"6

~'~',",,,,,,,~~13oo'c,m , o3n:L

, ~"~'"~1 ~s

'B~m'aoom i 'lX~mb.oon£ nm.n~o0ra

2theta

Figure 3. In situ XRD p a t t e r n of Cloverite, recorded during heating in ambient air up to 500 °C and cooling down to 50°C in air

3600

3400

3200

3000

Wavenumbers (crn "1)

2800

2600

Figure 4. In situ IR spectra of cloverite, recorded at different t e m p e r a t u r e s in air

IR experiments can help to explain the differences in the t h e r m a l stability of cloverite under vacuum and in air. IR spectra were recorded in steps of 50°C up to 500°C. In figure 4 parts of the spectra at four different t e m p e r a t u r e s are shown. At 20°C a broad band in the range form 2500-3700 cm -1, caused by water in the material, is superimposed with three additional bands. The one at about 3175 cm -1 can be assigned to a quinuclidiunium NH vibration, proving the protonated state of the template. The bands at 2886 cm "1 and 2948 cm "1 are due to symmetric and asymmetric CH streching vibrations of the template. At a temperature of 100°C the broad water band disappeared, indicating t h a t no more

776 water is present inside the cloverite channels at this temperature. The template molecules can withstand a temperature of 300°C. At higher temperatures the desorption and decomposition of the template takes place and at 450°C no more NH and CH streching vibrations were observed. A part of the template desorbs physically at temperatures above 3000C. When we used a heatable DRIFT cell in static air quinuclidine condensed at the colder KBr windows as a blank measurement showed. The above mentioned experiment had to be performed either using an air stream or under vacuum where the desorbed molecules were carried away from the sample. Therefore, we employed a heatable cell for an IR microscope in which, due to its dimensions and to the gas flow or vacuum, respectively, no condensation of the quinuclidine occured. Measurements under vacuum showed that the template does not desorb totally from the sample. It remains ~nside the cloverite structure even at temperatures of 500°C. The template inside the pores of cloverite seems to stabilize the structure when the sample is cooled down and then exposed to ambient air. Calcination in air leads to a total combustion of the template and the structure collapses at temperatures below 100°C, probably by interaction with water molecules from the humid air. Bedard et al. [4] assume the formation of defect sites during the calcination process caused by the loss of the template. Once these defect sites are formed water molecules might interact with them and initiate the disintegration of the crystal structure. A large number of defect sites should affect the XRD pattern of the material, which was not observed. The half widths and the intensities of the reflection peaks did not change essentially during the heating periods. The long range order of the crystal structure still exists. 1074 1046 11

Figure 5. IR spectra of cloverite ~

"-J

a

1070

o c ,.o o L-

1154 ~

1200

982

1000

800

Wavenumbers

600

a) as synthesized, b) calcined 20 h in ambient air at 500°C, c) heated" in a DRIFT cell to 500°C in dry air, d) same as c) then cooled down in dry air to 300°C, e) same as c) then cooled down in dry air to room temperature in dry air * Heating rate = 5°C/rain

(ore"1)

Alterations were observed in the short range order. The IR spectra differ significantly between 500-1300 cm -1 during the heating period in air as shown in figure 5 . Due to vibrations of the crystal framework as synthesized cloverite exhibits six distinct bands in that region at about 594, 633, 665, 1046, 1074, and

777

1198 cm -1. At 500°C the bands at 594, 633, and 1046 cm -1 disappear, while the intensity of the band at 665 cm -1 increases. Three broad new bands at 774, 982, and 1154 cm -1 were observed. The bands at 982 and 1154 cm -1 appear as shoulders of the one at about 1070 cm -1 and become more pronounced when the sample is cooled down below 350°C. After calcination in ambient air these three bands become one broad one and cannot be resolved any longer. Additionally, a broad band occurs at about 640 cm -1 as shown in figure 5b). The band at 774 cm -1 also disappears. The two broad single bands after the calcination were also observed by Bradley et al. [3]. They concluded t h a t the long range order as well as the short range order were lost after the exposure of calcined cloverite to ambient air. The IR spectra recorded at higher t e m p e r a t u r e s during the calcination show t h a t the short range order changes, but is still intact. We have to keep in mind t h a t the XRD patterns recorded at those t e m p e r a t u r e s have not changed dramatically. One can conclude t h a t the long range order inside the crystal is not affected by the thermal treatment, the crystal structure is still intact, but the nearest neighbour interaction between the T-atoms changes during the heating period. In order to stabilize the structure of calcined cloverite there seem to be two possibilities. Either keeping the material at elevated t e m p e r a t u r e s to avoid water adsorption from the ambient air or a blocking of the pore system with hydrophobic adsorbents. By keeping the material at t e m p e r a t u r e s above 100°C after its calcination in air it was possible to achieve a type 1 isotherm, typical of microporous materials, in a nitrogen adsorption experiment. The calcination of the sample for the adsorption measurement was performed at 450°C in ambient air. In the glass sample holders no higher temperatures could be achieved. The sample contained about 4.5 wt.% coke which burnt off at 530°C in an exothermic process in an TG/DTA experiment. The coke seems to block the pore system, since the pore volume, detected by the nitrogen adsorption, is less t h a n expected. Further experiments are in progress to obtain adsorption isotherms of pure cloverite. Nevertheless, the samples obtained in the above experiments were microporous without any detectable mesopores. Figure 6. XRD p a t t e r n of cloverite a) as synthesized b) after 16 h at 550°C in air at 20°C c) calcined at 550°C in air and exposed to ambient air at room t e m p e r a t u r e d) calcined and stored in decane e) calcined and stored in hexane f) calcined and stored in hexane for one day () .... ' " ' i O ' ....

.... 2 0 " " 2 theta

.... 3 0 ' " '

'

''

'1

40

778 When calcined cloverite, still at temperatures above 100"C, is poured into hexane or decane a material is obtained which exhibits XRD patterns similar to those of cloverite as shown in figure 6. The intensities of the peaks are different but the positions of the peaks are similar to those from cloverite. The material was dried and stored in ambient air for several month without changing its XRD pattern. The aliphatics seem to be adsorbed inside the pore system, since weight losses of about 13-14 wt.% were observed in the range of 100-3000C by TG/DTA experiments. For a sample, stored under decane over night and dried in ambient air, a weight loss of 13.6 wt.% was detected which corresponds to a pore volume of 0.19 cma/g which agrees with pore volumes found by Merouche et al. by adsorption measurements on cloverite using aliphatic and aromatic hydrocarbons as adsorbates at 25°C [2]. Nitrogen adsorption experiments at 77 K proved that the material stored under aliphatics is still microporous, even when the micropore volume is only half of that of a sample calcined in air at 450°C. Mesopores could not be found in any of the samples. Thus, the aliphatics must be adsorbed in the micropores.

4. C O N C L U S I O N S Our experiments showed that the cloverite structure can be kept intact in ambient air at elevated temperatures. Once the material is calcined one has to keep it above 100°C for further experiments. The long range order of the structure is not affected by the thermal treatment while changes of the short range order are observed during the calcination process. Storage in aliphatics leads to a material which seems to be very similar to cloverite. This material is microporous and the micropore volume is still remarkable. Further investigations on the real structure of that material must show whether a structure conversion took place.

REFERENCES 1 2 3 4 5 6

M. Estermann et al., Nature 352 (1991) 320 A. Merrouche et al., Zeolites 12 (1992) 226 S.M. Bradley et al., Solid State Nucl. Magn. Reson. 2 (1993) 37 R.L. Bedard et al., J. Am. Chem. Soc. 115 (1993) 2300 F. Schfith et al., J. Am. Chem. Soc. 116 (1994) 1090 J. Patarin et al., Proc. 9th Int. Zeolite Conf., Montreal 1992, Eds. R. von Ballmoos et al., (1993) by Butterworth-Heinemann, 263


779

E l e c t r o n spin r e s o n a n c e studies of 02- a d s o r b e d on a l u m i n o p h o s p h a t e m o l e c u l a r sieves Suk Bong Hong ~, Sun Jin KiIn a, Young-Sang Choib and Young Sun U h a aKorea Institute of Science and Technology, P.O. Box 131, Cheongryang, Seoul 130-650, Korea bDepartment of Chemistry, Korea University, Seoul 136-701, Korea Electron spin resonance (ESR) spectra of the superoxide ion, 02-, generated in dehydroxylated aluminophosphate (A1PO4) molecular sieves and related materials are presented. Among the molecular sieves studied here, the extra-large-pore VPI-5 shows the largest spin concentration of O2-, which corresponds to 2.9 x 10~8 ions per gram. Analysis of the g values of 02- in AIPO4 molecular sieves reveals that the crystal field splittings, A, of the adsorbed O2- are in the range of 1.32-1.60 eV and suggests that the crystal field interaction is dependent on the pore size of the molecular sieves.

1. INTRODUCTION Aluminophosphate (AIPO4) molecular sieves were first synthesized by Wilson et al. in the early 1980s [1,2]. Although these microporous solids exhibit remarkable diversity in the framework structure, the use of AIPO4 molecular sieves as catalysts and separation media has been severely limited. This may be due to the lack of Bronsted acidity, which originated from the neutrality of their framework. The A1 and P atoms in A1PO4 molecular sieves occupy tetrahedral framework positions in strict alteration, with AI/P=I for a perfect framework. However, actual AIPO4 materials cannot be crystallographicaUy perfect because of framework defects generated during the synthesis and/or post-synthesis treatment steps. It has been recognized that many important properties of molecular sieves can be influenced by the presence of these defect sites [3]. Therefore, it is of fundamental interest to accurately identify the nature of these defect sites. There are a number of investigations on the defect sites in zeolites. However, no attention has been directed toward the characterization of structural defects in AIPO4 molecular sieves. Here we present the results obtained from the ESR spectra of the superoxide ion, 02-, adsorbed on the defect sites of a wide variety of AIPO4 molecular sieves and related materials. The crystal field interaction between the adsorbed 02- ion and the A1PO4 framework is also discussed.

780 2. E X P E R I M E N T A L The molecular sieves AIPO4-5, A1PO4-11, AIPO4-17, A1PO4-18 and A1PO4-20 were synthesized according to the procedures described in the Union Carbide patent [2]. VPI-5 was prepared by the procedures given by Davis and Young [4]. A1PO4-8 was synthesized by heating hydrated VPI-5 at 473 K for 24 h. A1PO4 tridymite was prepared by the procedures given by Cheung et al. [5]. Amorphous A1PO4 (AI/P=I) was obtained by heating a layered A1PO4 material in air at 873 K for 24 h. CoAPO-5, SAPO-5 and SAPO-34 molecular sieves were synthesized by the procedures described in the literature [6,7]. As-synthesized molecular sieves were calcined in 02 at 823 K for 16-24 h to remove the structurally incorporated organic structure-directing agents. The calcined samples were refluxed in water for 4 h and then dried at room temperature. Structural information on A1PO4 molecular sieves and related materials prepared in this study is given in Table 1. All the molecular sieves were phase-pure and show good crystallinities, as seen by powder X-ray diffraction (XRD) using a Rigaku D / M a x - H A diffractometer (Cu Ir~ radiation). This can be fftrther supported by the nitrogen BET surface area measurements, which were performed on a Micromeritics ASAP 2000 analyzer (see Table 1). Chemical analysis for CoAPO and SAPO materials was performed by a Jarrell-Ash Polyscan 61E inductively coupled plasma (ICP) spectrometer. The ESR spectra at 77 K and room temperature were obtained on a Bruker ER-200D Spectrometer operating at X-band (~ 9.45 GHz) with 100-kHz field modulation. Prior to ESR measurements, approximately 30 mg of the samples were placed into a quartz tube of 3-mm inner diameter, slowly heated in a vacuum of 10-5 Torr to 773 K, and then kept at this temperature for 2 h. After cooling to room temperature, the dehydroxylated samples were exposed to O~. at a pressure of about 10 Torr for 5 min. Finally, the gas-phase O~. was removed by evacuation to better than 10-4 Torr. The spin concentration of 02- adsorbed on molecular sieves was determined by comparing the intensities of the doubly integrated superoxide spectra with those obtained from various weighted amounts of DPPH. The estimated error in spin concentration is _+50%. The spectra at 4 K were measured on a Bruker ESP-300 Spectrometer in combination with an Oxford ESR 900 cryostat.

3. R E S U L T S AND DISCUSSION Figure 1 shows the ESR spectra at 77 K developed after adsorption of 02 on A1PO4 molecular sieves with different structures dehydroxylated under a vacuum' of 10-5 Torr at 773 K for 2 h. Very recently, we have found that the structural defect present in hydrated AIPO4 molecular sieves can change into the paramagnetic centers when the molecular sieves are dehydroxylated at high temperatures. Distinct differences in the position and number of the

781 Table 1 Physical data of AIPO4 molecular sieves and related materials used in this study Sample

Structure Ring Anhydrous unit Molecular wt. Surface area type size cell composition per unit cell (m2.g -1)

tridymite AIPO4 AIPO4-20 A1PO4-17 A1PO4-18 SAPO-34( I ) SAPO-34(II) AIPO4-11 AIPO4-5 SAPO-5 CoAPO-5( I ) CoAPO-5(II) AIPO4-8 VPI-5 amorphous A1PO4

SOD ERI AEI CHA CHA AEL AFI AFI AFI AFI AET VFI .

6 6 8 8 8 8 i0 12 12 12 12 14 18 .

AlzP208 A16P6Ou AllsP18072 AluP24096 A]I8.oPI6..6Sil.4072 AIIs.oPI5.8Si2.20v2 AI2oP2oOso AII2P12048 AIn.3PIo.sSil.90~ AIn.gP12.oCoo.IO~ Aln.oP12.oCoLoO~ Ala6Pa6Om AllsP18072 . .

244 732 2195 2927 2191 2189 2439 1463 1461 1467 1495 4390 2195

458 568 604 639 185 337 302 331 329 62 409 14

observed ESR signals from dehydroxylated AIPO4 molecular sieves reveal that the nature of the paramagnetic defects formed is significantly different in the structure type of the molecular sieves. We are investigating this fln-ther by using variable-temperature ESR studies, and the results will be given elsewhere (Hong et al., in preparation). The introduction of 02 into dehydroxylated AIPO4 molecular sieves resulted in an immediate loss of the ESR signals from paramagnefic defect centers. As seen in Figure 1, however, three new ESR signals are observed. Many papers on the physicochemical properties of the charged dioxygen species in various metal-ion-exchanged zeolites have been published, and comprehensive ESR studies of the adsorbed dioxygen are available [8,9]. A comparison of the ESR spectra in Figure 1 with the data report~ in the literature reveals that the paramagnetic oxygen species formed in dehydroxylated A1PO4 molecular sieves is 02- [9]. All the ESR spectra in Figure 1 were not significantly broadened by the presence of 10 Torr of 02, indicating that the spin-spin interaction between the chemisorbed 02- ion and physically adsorbed O~. is negligible. However, they were destroyed when the molecular sieves were exposed to 10 Torr of water vapor. The dehydroxylation of hydrated A1PO4 materials at the condition stated earlier and the subsequent adsorption of 02 at room temperature give rise to the complete regeneration of the 02- ESR spectra. Therefore, it is most

782 likely that the formation of 02- in A1PO4 molecular sieves is reversible. On the other hand, the absence of any hyperfine structure in the ESR spectra of Figure 1 suggests that no hyperfine interaction between the unpaired electron in 02- and the A1 atoms (•=5/2) in the A1PO4 framework exists. This may be result of the localization of the unpaired electron onto the adsorbed 02 molecule only. To more accurately ensure this speculation, we have performed ESR measurements on the 02 adsorbed on A1PO4 molecular sieves at 4 K. As expected, the 02- spectra obtained at this temperature are the same as those in Figure 1 and no hyperfine structures in the spectra are observed.

20G (i)

g=z = 2.01 ~ .

~

9

9

2.0198

(g) .

.

x 100

w

-.~=f

.

_

.

x

(f) (e) _

.

_

(d) .u.z.__

.

2.0231 ,,.,~ .

.

.

.

.

.

.

.

--.~

~-====-~--

2.0236

x

40 __

-

x ~

--

2

_

_...,.~ k

x.

,

~

j~,~.._

.._ . . =

L

L , = _

,,__

1 _

__

~ L

V

Figure i. ESR spectra at 77 K of O~- adsorbed on (a) tridymite A1PO4, (b) AIPO4-20, (c)A1PO4-17, (d)AlPO4-18, (e)AlPO4-11, (f)ALP04-5, (g) AIPO4-8, (h) VPI-5 and (i) amorphous A1PO4 dehydroxylated under a vacuum of 10-5 Ton" at 773 K for 2 h.

783 Table 2 g Values, spin concentrations and crystal splittings, A, at 77 K of 02- species adsorbed on AIPO4 molecular sieves and related materials Spin g Value concentration A" (eV) Sample gzz

tridymite AIPO4

A1PO4-20

AIPO4-17 A1PO4-18 SAPO-34( I ) AIPO4-11 AIPO4-5 SAPO-5 CoAPO-5( I ) CoAPO-5( 11) AIPO4-8 VPI-5

amorphous AIPO4

2.0209 2.0201 2.0236 2.0231 2.0216 2.0214 2.0215 2.0216 2.0215 2.0211 2.0202 2.0198 2.0199

g~

2.0103 2.0104 2.0106 2.0106 2.0103 2.0103 2.0102 2.0104 2.0102 2.0104 2.0105 2.0109 2.0101

g~

2.0033 2.0041 2.0035 2.0033 2.0036 2.0033 2.0039 2.0039 2.0038 2.0037 2.0041 2.0045 2.0034

(1016.g-1) 1.3 2.4 2.4 0.8 5.4 15.1 163 65 85 0.9 17.5 293 0.6

1.51 1.57 1.32 1.35 1.45 1.47 1.46 1.45 1.46 1.49 1.57 1.60 1.59

"Calculated using the simplified equation g ~ = g~ + 2k/A [12]. k has been taken equal to 0.014 eV so that comparison with earlier results can be made

[9]. Another interesting observation obtained from the ESR spectra in Figure 1 is that the relative intensity of the 02- signals differs significantly according to the structure type of the molecular sieve where the 02- ions are adsorbed. The spin concentrations of the 02- formed in A1PO4 molecular sieves and related materials studied in this work are listed in Table 2. In general, the larger pore size the molecular sieve has, the higher concentration of the adsorbed 02- ions it shows. For example, the concentration of the 02- ions formed on the small-pore AIPO4-20 is much small as compared to that of the extra-large-pore VPI-5. This result can be due to differences in the paramagnetic defects generated in dehydroxylated A1PO4 molecular sieves. However, a linear relationship between the concentration of 02- ions and the pore size of the molecular sieves was not observed, indicating that the formation of 02- in AIPO4 molecular sieves is more complicated than our expectation. Figure 1 and Table 2 also show that the 02- ion can be formed on amorphous and tridymite A1PO4 phases, although its concentrations on these nonmicroporous A1PO4 materials are very small as compared to those on the AIPO4 molecular sieves. Therefore, it appears that microstructure is not necessary to produce 02- ions in A1PO4 materials, but it plays an important

784 role in achieving high concentrations of 02- ions. The substitution of heteroatoms such as Si or Co into the AIPO4 framework gives rise to a significant decrease in the intensity of the 02- ESR spectra. Figure 2 shows the ESR spectra at 77 K of the 02- adsorbed on SAPO-34 with low and high Si contents. The 02- ESR spectrum from SAPO-34( I ) with Si/A1 = 0.08 exhibits three peaks at g = 2.0216, 2.0103 and 2.0036 while no noticeable 02- signals are observed for SAPO-34(II) with Si/A1 = 0.12. The same trend was also observed from C o A I ~ - 5 samples with different Co contents (see Table 2). Therefore, it is clear that the concentration of the 02- ions formed in AlPO4-based molecular sieves decreases as the heteroatom content in their framework increases. This can be directly related to the decrease in concentration of paramagnetic defect centers in these molecular sieves. The g values of the O~.- adsorbed on A1PO4 and related materials are listed in Table 2. These data reveal that all the 02- ESR spectra obtained here exhibit only one g ~ value, which is in contrast to 02- formed on T-irradiated alkaline-earth-exchanged zeolites where several different gz~ values can be observed [10]. Therefore, it appears that each molecular sieve contains only one type of 02- sites. This can be further support~ by the fact that the average g values of the 02- ions on A1PO4 molecular sieves at 77 K are quite similar to those obtained from the 02- ions on the corresponding materials at room temperature. The g values listed in Table 2 also show that the g,, values are different in the structure type of AIPO4 molecular sieves, while g ~ and g= remain almost unchanged. Figure 3 illustrates the electronic energy diagram of 02-. The crystal splitting, A, of the / / ~ level of 02- can be calculated from the g values. The theoretical expressions for calculation of the

20 G (b)

x 150

gzz= 2.0216 ~

(a) ,

,

J _

x 40 .

.

.

.

Figure 2. ESR spectra at 77 K of 02- adsorbed on (a) S A P O - 3 4 ( I ) and (b) SAPO-34(II). The signal intensity is referenced relative to the ESR specmm~ of 02- on VPI-5 in Figure lh.

785

IA

rq rg rg

E

z

Figure 3. The simplified energy level diagram for 02- in the ground state. A and E indicate the crystal field splitting and the energy difference of the lowest and highest occupied energy levels, respectively.

A and E energy splittings were derived by Kanzig and Cohen [11] and simplified by Kasai [12]. For the case E > A >> k, the expressions were simplified to g~, ~ g~ + (2k/A)

(1)

S~ ~ g~ + (2k/E) - (k2/A2) - ( k 2 ~ )

(2)

s=

(3)

= g,-

(X2/a~) + (X~/E A)

where k is the spin-orbit coupling constant = 0.014 eV and ge is the g value of the free electron. The calculated A values of the 02- in AlPO4 molecular sieves and related materials studied in this work are in the range of 1.32-1.60, as seen in Table 2. These A values are larger than any reported values from 02- ions on alkaline- or alkaline-earth-exchanged zeolites, and are comparable to those of 02- species on TiO2 or Ti3+-exchanged Y zeolites [9]. This suggests that 02- is more strongly held in AIPO4 molecular sieves than in zeolites. It is interesting to note that the A values of the 02- ions on AIPO4-8 and VPI-5 materials are quite similar to those of the 02- ions on the exterior surfaces of tridymite and amorphous AIPO4 phases, and AIPO4-20. This can be attributed to the large interior surface areas of these extra-large-pore materials. Finally, the A values listed in Table 2 clearly demonstrate that the crystal field interaction gets stronger when the pore size of an AIPO4

786 molecular sieve is larger. This suggests that the nature of 02- sites present in each A1PO4 molecular sieve is not the same. 4. CONCLUSIONS

The results presented here show that the 02- ion can be formed by dehydroxylation of AIPO4 molecular sieves under a vacuum of 10-5 Torr at 773 K for 2 h and then adding 02. This is in contrast to most of the ESR studies of O2- on zeolites in that the formation of 02- ions in AlPO4 molecular sieves is possible only by dehydroxylation, without 7 or UV irradiation of the molecular sieves. This property enables us to suggest that AIPO4 molecular sieves may find possible applications in catalysis and molecular separation technology. ACKNOWLEDGMENT

This work was support~ by the Technology under contract No. 2N13723.

Korea Institute

of Science

and

REFERENCES

1. S.T. Wilson, B.M. Lok, C.A. Messina, T.R. Cannan and E.M. Flanigen, J. Am. Chem. Soc., 104 (1982) 1146. 2. S.T. Wilson, B.M. Lok and E.M. Flanigen, US Patent No. 4 310 440 (1982). 3. G. Engelhardt and D. Michel, High-Resolution Solid-State NMR of Silicates and Zeolites, Wiley, New York, 1987. 4. M.E. Davis and D. Young, Stud. Surf. Sci. Catal., 60 (1991) 53. 5. T.T.P. Cheung, ICW. Willcox, M.P. McDaniel and M.M. Johnson, J. Catal., 102 (1986) 10. 6. B.M. Lok, B.K. Marcus, L.D. Vail, E.M. Flanigen and S.T. Wilson, European Patent No. 159 624 (1985). 7. B.M. Lok, C.A. Messina, R.L. Patton, R.T. Gajek, T.R. Cannan and E.M. Flanigen, US Patent No. 4 440 871 (1984). 8. J.H. Lunsford, Adv. Catal., 22 (1972) 265. 9. M. Che and A.J. Tench, Adv. Catal., 32 (1983) 1. 10. K.M. Wang and J.H. Lunsford, J. Phys. Chem., 74 (1970) 1512. 11. W. Kanzig and M.H. Cohen, Phys. Rev. Lett., 3 (1959) 509. 12. P.H. Kasai, J. Chem. Phys., 43 (1965) 3322.


787

Studies in Surface Science and Catalysis, Vol. 105 © 1997 Elsevier Science B.V. All rights reserved.

THE

AFFINITY

ZEOLITE

ORDER

SILICALITE-1

OF

ORGANICS

STUDIED

BY

ON

HYDROPHOBIC

THERMAL

ANALYSIS

Yingcai Long *, Huiwen Jiang, and Hong Zeng Department of Chemistry, Fudan University, Shanghai 200433, People's Republic of China

Abstract The AT values determined by TG/DTG are used to compare the affinity order of 27 organics adsorbed in the hydrophobic zeolite silicalite-1, which possesses a perfect Si-O-Si micropore surface without cations and silanol defect. Silicalite-1 has a strong affmity for some organic compounds with high polarity. The physical meaning of the AT values, and the nature of two type supermolecular host/guest interactions are discussed. Keywords" aff'mity order of organics; thermal analysis; silicalite-1 1. I N T R O D U C T I O N Silicalite-1 is a hydrophobic zeolite and very useful as a separation medium, such as an adsorbent or a membranes. Its hydrophobic/organophilic character is based on the interaction between the guest molecules adsorbed and the zeolite framework. A hydrophobicity (h) [1J has been def'med as the ratio of the weight loss at 150 °C to the weight loss at 400 °C for a dealuminated zeolite Na-Y. A hydrophobicity index(HI) [2'31 has been introduced. It was defined as XtoluenefXwater where X is the loading, i.e., the mass of adsorbed compound per mass of dry adsorbent (such as ZSM-5, silicalite-1, and zeolite Y). The hydrophobicity index can be used to compare the hydrophobic properties and the aff'mities for hydrocarbons. Actually, the sorption and desorption behavior of an organic compound with different functional groups is strongly influenced by small amounts of cations and defects t4-61. There is a perfect Si-O-Si micropore surface in the hydrophobic zeolite silicalite-1. The temperatures for complete desorption of most compounds adsorbed within silicalite-1 are usually lower than 300 °C in a narrow region without catalytic effects. A new concept AT, the aff'mity of the perfect micropore surface in silicalite-1 to an organic compound adsorbed was recently developed. AT = T a- Tb was defmed, where T d is the temperature of the weight loss peak in the DTG curve and Tb the boiling point of the compound [61. In this paper we will use the AT values to compare the atTmity order of 27 organics, and reveal the nature of the host/guest interaction. * Author for correspondence. This work was supported by the Science Fund of the Chinese Academy of Science.

788 2.

EXPERIMENTAL

2.1. Preparation of zeolite samples The zeolite samples used in this study were hydrothermally synthesized in an amine-Na20SiO2-H20 system by using water glass as a silicon source [7]. The as-synthesized products were washed, filtered, dried by IR lamp, and then calcinated in an oven to remove the organic template at about 600 °C for 2 hr. After treated with a 0.5N HCI solution at 95 °C for 4 hr, it was washed, filtered, dried and calcinated at 550°C for 2 hr to get H-ZSM-5. The H-ZSM-5 sample was calcinated at 800 °C in a quartz tube for 100 hr using an air flow, which was saturated at room temperature by water steam, to get the silicalite-1. The flow rate of water steam was controlled by a micro pump at 105 _+5 ml/h.

2.2. Adsorption and Determination of The AT Value Reagents used as adsorbates were chromatographic grade or analytically pure. Adsorption of vapors of a guest compound was carried out in a fixed bed of the zeolite sample at room temperature for more than 24 hr in order to fully saturate the sample. When the guest compound was a solid at room temperature, the adsorption was done at higher temperature to get vapor from the melt. TG/DTG/DTA measurements were carried out by using a PTC10A thermal analyzer with an air flow of 70 ml/min at a rate of 5 °C/min from room temperature to 700 °C. About 10 mg of a zeolite sample were used in a test. The sensitivities of TG and DTA used were 0.0ling and +25gV respectively. The adsorption data were calculated from TG, and Td data from the temperature of the weight loss peak in the DTG curves. Tb is the boiling point of the guest compound at standard condition and can be found in any handbook of physical chemistry. If there are two weight loss peaks appearing in a DTG curve, the higher one is taken as Td to calculate the AT value.

3.


3.1. Structural Characterization of the Silicalite-1 Sample SEM showed that the as-synthesized MFI zeolites were in a prismatic form of single crystals with a size of 6 x 15 ~tm. The XRD patterns indicate that the samples were pure MFI phases with a high crystallinity. The silicalite-1 Sample has monoclinic symmetry according to the diffraction peaks doublets at 24.4 °, 29.2 °, and 48.6°(20). 29SiMAS NMR spectra show that the Si-OH peak at -103 ppm and the Si(1A1) peak at -106 ppm, which appear in the patterns of H-ZSM-5 sample, disappear in the pattern of silicalite-1. The small amount of framework aluminum in the H-ZSM-5 sample came from the silica source as an impurity in the water glass. The high resolution 29Si resonance with 20 peaks indicates a symmetry transition from orthorhombic to monoclinic, accompanied by a perfection of the framework upon steam treatment N. In the process of preparing the sample of silicalite-1, the dealumination and desilanation effect of steam treatment can also be detected by the FT-IR spectra. A [TO4] external asymmetrical stretching vibration at 1226 cm 1 and an internal

789 asymmetrical stretching vibration at 1095 cm 1 move to higher frequencies. At the same time the vibrations of silanol groups at about 3730 and 3450 cm ~ disappear in the spectra of silicalite- 1. All these facts indicate that the sample of silicalite- 1 used in this study possesses a perfect crystalline structure, whose framework is constructed by Si-O-Si without Si(1A1) The and Si-OH defects. This is the typical character of silicalite-1 [ 8 - 1 2 ] . hydrophobic/organophilic properties greatly change with the content of framework defects. Silicalite-1 has high hydrophobicity, its adsorption of water vapor at room temperature is less than lwt%.

3.2. The Thermal Desorption Behavior Typical TG/DTG/DTA spectra of paraffins, aromatics, alkyl-alcohols, multivalue hydroxyl alcohols, alkyl-amines, and organic acids (see Fig. 1) indicate that the temperatures for complete desorption of these compounds are lower than 300 °C, and that the desorption occurs in a narrow region without catalytic effects, so far no exothermic effect in the DTA curves is found. There is no significant thermal effect on desorption for most hydrocarbons. An endothermic effect can be found from the DTA curves of silicalite-1 for p-xylene, alkylalcohols, multivalue hydroxyl alcohols, alkyl-amines, and formic acid respectively. The hydrophobicity of the siliceous zeolite is based on the absence of exchangeable cations and the substitution of aluminum atoms in the framework by silicon atoms. HOwever the influence of the structural defects on the hydrophobicity is not negligible. The hydrophobic/organophilic property is a characteristic of the siliceous zeolite and is a generalization of the difference between organics and water upon adsorption and desorption. It is a reflection of an interaction between the micropore surface of the zeolite and certain organic molecules. In order to reveal the nature of the interaction, it is necessary to exclude an influence of cations and structural defects of Si-OH. The behavior of thermal desorption can be used to characterize the interaction in silicalite-1 since there is no the influence [13] . That is why we used the sample of silicalite-1 to determine the AT values, and investigate the host/guest interaction.

3.3. The Physical Meaning of the AT Values Considering that there are no strong electrostatic sorption centers on the micropore surface of the perfect silicalite-1 framework, the adsorption of guest molecules can approximately be treated as a physical capillary condensation. If a liquid is immersionally wetting(i.e., philic) a capillary surface, the capillary ascent phenomenon will cause the boiling (i.e., violent desorption) temperature of the liquid condensed within the capillary to be higher than its boiling point in the free state at normal atmospheric pressure. Obviously, if a liquid is nonimmersionally wetting (i.e., phobic) a capillary surface, the capillary depression phenomenon will cause the boiling temperature of the liquid within the capillary to be lower than its boiling point at normal conditions. It is reasonable to believe that the AT value is positive if the micropore surface of silicalite-1 is playing a role of the philicity with the guest molecule. The AT value is negative if the micropore surface is phobic with the adsorbate. No unit for the AT values is required while comparing the philicity or the phobicity of the interaction.

790

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\,

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138

141 I

200

I

400 °C

I

200

I

400 °C

Figure 1. TG/DTG/DTA spectra of some organics on zeolite silicalite-1, a) pentane, b) ethylbenzene, c) p-xylene, d) 1-pentanol, e) glycerol, f) ethylamine, g) acetic acid, h) ethylacetate; . . . . . . TG ....... DTA, ~ DTG, $ endothermic effect

791

3.4. The AT Values of Organics The AT values and the adsorption data of organics are listed in Table 1. The AT values are 60~90 for the saturated hydrocarbons, which indicate the organophilic property of silicalite-1. The AT values are 6~13 for benzene, toluene, ethyl-benzene and p-xylene, but -91 for naphthalene. The facts indicate that silicalite-1 has weaker aff'mity with the unsaturated hydrocarbons than that with the saturated hydrocarbons. The AT value from -4 for ethanol increases to 13 for 1-propanol. The AT values decreases by about 55 as increasing one hydroxyl group on an alkyl-alcohol with the same bulk structure, such as from 1-propanol to 1.2-propanediol, from 1.3-propanediol to glycerol, and from ethanol to ethylene glycol as well. The AT values decreases by about 40 from benzene to phenol. The negative AT values for alkyl-alcohols with multivalue hydroxyl groups indicate that silicalite-1 is usually phobic to the organics with hydroxyl groups. The AT values are 160, 127 and 116 respectively for methylamine, ethylamine and acetaldehyde, which are much higher than that for other alkylamines and organics with oxygen atom. It is possible that there is a double molecule associate existing in the channel intersections of the zeolite. 3.5. The Sub-Hydrogen Bond Between Hydrocarbons and Framework 0 2. It is a simple way to investigate the degree of the interaction between a zeolite(i.e., the host) and an adsorbate(i.e., the guest). During temperature programming, the temperature for mass desorption of a guest molecule from micropore(i.e., Td- the temperature of the weight loss peak in the DTG curve) is related to the interaction between the surface of silicalite-1 and the molecules. The framework of silicalite-1 is constructed by [SiO4] tetrahedra linked by sharing 02. . Since the radius of oxygen atom is much larger than that of silicon atom, the whole surface of the framework is actually covered with 02-. In the framework of silicalite1 the distance between two neighboring oxygen atoms is 0.149 nm, which is very close to 0.154 nm, the distance between two neighboring carbon atoms in saturated hydrocarbons. The bond angle of C-C-C or H-C-H in saturated hydrocarbons is 109°23 ", which is equal to the bond angle of O-Si-O in the framework of silicalite-1. The C-H groups of saturated hydrocarbons can freely rotate along the C-C axis. It provides a greater opportunity to play a role of the interaction between the mass hydrogen atoms of saturated hydrocarbons and 02. in the framework with a sub-hydrogen bond. It leads Td to be much higher than Tb for most saturated hydrocarbons adsorbed as a monomolecular layer within the micropore of the zeolite. The influence of the number of carbon atom on the interaction is not obvious. The weak sub-hydrogen bond increases the AT values, but can not induce a visible thermal effect of desorption.

792 Table-I Adsorption and the AT Values on Silicalite-I guest molecule

M

molecule size(nm)

Au /u.c.

Tb

(b.p.) °C

Td weightl o s s

AT (Td-Tb)

103 79, 127 63, 185

67 58 87

peak temp.°C

pentane n-hexane heptane

72.15 86.18 100.21

.90 1.03 1.16

8.3 8.5 6.5

36.1 69 98.4

benzene toluene ethyl-benzene p-xylene naphthalene

78.12 92.15 106.17 106.17 128.19

.58 .91 1.03 1.32 1.16

7.8 8.0 5.5 7.8 3.1

80.1 110.6 136.2 138.3 218

90 124 149 94, 144 127

10 13 13 6 -91

ethanol 1-propanol 1-pentanol phenol

46.07 60.11 88.15 94.11

.69 .82 1.07 .84

15.0 12.4 9.2 6.9

78.5 97.4 137.3 181.7

75 110 141 78, 144

-4 13 4 -38

ethylene glycol 1,2-propanediol 1,3-propanediol glycerol

62.07 76.11 76.11 92.11

.85 .82 .99 .99

15.6 9.9 10.9 4.0

198.9 189 213.5 290

104, 144 134 166 193

-55 -55 -48 -97

31.06 45.07 59.11 87.11 101.19

.50 .63 .75 1.10 1.13

19.8 15.6 10.8 9.1 7.7

-6.3 16.6 47.8 104.4 130

154 72, 144 94 121 107, 194

160 127 46 15 64

formic acid acetic acid

46.03 60.05

.36 .69

28.0 17.5

100.7 117.9

86 128

-15 10

acetaldehyde ethyl-ether acetone ethylacetate

44.05 74.12 58.08 88.12

.44 .85 .62 1.18

17.6 8.6 12.5 8.8

20.8 34.5 56.2 77.1

137 87 95 138

116 53 39 61

methylamine ethylamine n-propylamine pentylamine n-hexylamine

The number of hydrogen atoms in an aromatic molecule is less than that of in a saturated hydrocarbon with the same number of carbon atoms. An aromatic ring is in a plate form. The hydrogen atoms on the ring can not rotate along C-C axis. The opportunity of playing the role of the interaction between the framework 0 2- and the molecules adsorbed with subhydrogen bond for aromatics is much less than that for saturated hydrocarbons. It leads to a lower AT value for aromatics. The AT values decrease to negative because of a stronger polarity of the hydroxyl groups in molecules for multivalue hydroxyl alcohols, which are

793 repulsed by the framework O 2". organics with hydroxyl groups.

This is the reason that silicalite-1 is usually phobic to

3.6. The Effect of Associate on the AT Value Restricted by micropore size, the molecules of p-xylene can form an associate with hydrogen bond in the channel intersections of MFI zeolite [~31. A hydrogen atom of the methyl group in the molecule of p-xylene combines with an aromatic ring in a neighbor molecule of pxylene to form the associate with hydrogen bond. The situation is similar to that in the pure crystal of p-xylene, and may cause the visible thermal effect on the desorption in the DTA curve [~41 . The plate molecules of naphthalene are easier to form a tight crystal structure in the free state. The boiling point of naphthalene is 218 °C, much higher than that of nnonane (C9H20, b.p. = 150.7 °C), which has a similar molecular weight. The dimension of the naphthalene molecule is 0.58 x 1.16 nm. The adsorption of naphthalene is about three molecules per unit cell. It is impossible for the molecules to form an associate with each other within the micropores of silicalite-1. The situation leads Td to decrease to 127 °C, and the AT value decreases to -91. The endothermic effects on desorption for alcohols or phenol possibly come from a deassociation of the associate, which is formed in a combination with a neighboring molecules with hydrogen bond within the channel intersections.

3.8. Abnormal AT Values Amine groups are also polar, and their polarities are weaker than that of hydroxyl groups. So the AT values of alkyl-amines are usually positive. The molecular size of methylamine and ethylamine are 0.50 and 0.63 nm respectively. It is easy to form a double molecule associate by combining -NH2 groups with an adjacent molecule with stronger hydrogen bond in the channel intersections. The double molecule associate may lead the AT value to be abnormally high for methylamine and ethylamine, and cause the visible endothermic effect on the desorption. The molecules of pentylamine can not form double molecule associate in the channel intersections because of their larger molecular size. The hydrogen bond, which exists between two neighboring molecules in the free liquid state, diminishes since the molecules of pentylamine are isolated by the zeolite channels. The diminution of the hydrogen bond makes the Td close to the boiling point of n-hexane, which has a similar molecular weight with pentylamine. The endothermic effects of desorption for alkyl-amines are a sign of forming the associate in the channel intersections with hydrogen bond. The AT value is abnormally high for acetaldehyde. It is also possibly caused by a double molecule associate forming with hydrogen bond at the channel intersections. The tendency of the AT values is abnormal for heptane (1 = 1.16 nm), 1-pentanol (1 = 1.07 nm) and pentylamine (1 = 1.13 nm) in comparison with n-hexane, 1-propanol and npropylamine respectively. There are four channel intersections per unit cell of silicalite-1. The space of the intersection can be seen as a cross of four channels with length of 1.05 nm and cross section of 0.54 x 0.56 nm. If a molecule has a length < 1.05 nm, the molecule is adsorbed in the intersections with higher probability. Otherwise, a molecule with the length > 1.05 nm is not easy to be adsorbed at the intersections. In this situation, the interaction

794 between the alkyl groups and the framework O 2", and the interaction between the polar functional groups of the molecules will change more. It leads the Av values to become abnormal. CONCLUSION TG/DTG/DTA can be used for quick analysis of affmity on zeolites. The AT values compare the order of host/guest interaction and are useful for separation practice. The difference in the A7 values and the thermal effects of desorption are brought by different interactions between the micropore surface and the functional groups of organics. It is also influenced by the different situations of the associates located at the channel intersections. These facts are a reflect of different type supermolecular interaction for the host/guest system.

REFERENCES

1. M.W. Anderson, and J. Klinowski, J. C. S., Faraday Trans., 1, 1986, 82, 1449. 2. C.H. Berke, A. Kiss, P. Kleinschmit and J. Weitkamp, J. Chem. -Ing. -Tech., 1991, 63, 623. 3. J. Weitkamp, P. Kleinschmit, A. Kiss and C. H. Berke, Proceedings from the 9th IZC, Montreal 1992, Ed. R. Von Ballmoos et al, 1993 by Butterworth-Heinemann, VII, P79. 4. N.B. Milestone, and D. M. Bibby, J. Chem. Tech. Biotechnol., 1983, 34A, 73. 5. N.B. Milestone, and D. M. Bibby, J. Chem. Tech. Biotechnol., 1981, 31,732. 6. Long Y-C., Jiang H., and Zeng H., J. Fudan Univ., 1994, 33(1), 101. 7. Long Y-C., Sun Y-J., Wu T-L., Wang L-P., Qian M., and Fei L., CN apply No. 92 1 13807.5. 8. Sun Y-J., Huang Y-F., Wu T-L., Wang L-P., Fei L., Yang H., and Long Y-C., ACTA CHIMICA SINICA, 1994, 52, 573. 9. E.M. Flanigen, R. L. Patton, USP, 4073 565, (1978). 10. C.A. Fyfe, J. H. O'Brien, and H. Strobl, Nature, 1987, 326(19), 281. 11. D.H. Olson, W.O. Haag, and R.M. Lago, J. Catal., 1980, 61,390. 12. E.L. Wu, S.L. Lawton, D.H. Olson, A.C. Rohrman, Jr., and G.T. Kokotaillo, J. Phys. Chem., 1979, 83(21), 2777. 13. Zeng H., Jiang H., Long Y-C., Sun Y-J., Wu T-L., Wang L-P., ACTA PHYSICO CHIMICA SINICA, 1995, 11(3), 242. 14. Y-C. Long, H. Zeng, Y-J. Sun, T-L. Wu, L-P. Wang, "Two Types of P- Xylene / Silicalite-1 Associate and Their Formation Studied by XRD, TG/DTG/DTA, 29Si and 13C MAS NMR", to be published.


795

Studies in Surface Science and Catalysis, Vol. 105 1997 Elsevier Science B.V.

CHEMISTRY OF CoAPO-11 AND MOLECULAR OXYGEN ADDUCTS.

VAPO-5

• ESR

STUDIES

OF

C. NACCACHE 1, M. VISHNETSKAYA 2 and Kuei-Jung CHAO 3 • hstitut de gecherches sur la Catalyse, CNKS, Villeurbanne, France. 2. Moscou University, Chemical Department, Moscou, Russia. 3. National Tsing Hua University, Department of Chemistry, Hsinchu, Taiwan. 1. ABSTRACT CoAPO-11 and VAPO-5 were synthesized in the presence of diisopropylamine. XRD indicates high crystalline materials with the isotopic structure of respectively ALPO-11 and ALPO-5. CoAPO-11 UV-spectra and the existence of acidic OH revealed by infrared, indicate isomorphous substitution of divalent cobalt for aluminium Removal of the template by 02treatment at 773 K produced a diamagnetic oxo-type cobalt Co = O. This species activates molecular oxygen only after H2-treatment at high temperature. The esr spectra of the oxygen adduct formed upon 02 adsorption on H2-reduced CoAPO-11 correspond to the paramagnetic species Co(2+)-O-O'. The amount of this peroxo cobalt adduct increases with the severity of the H2-treatment. One concludes that Co = O is converted into an active Co(2+) species by H,treatment. Co(2+)- O-O" is very reactive toward H2, hydrocarbons, and is regenerated by cyclic 02 adsorption. At high temperature about 573-773 K Co(2+)-O-O" is converted into Co = O oxo species non active to activate molecular oxygen. VAPO-5 was studied by esr. The esr spectra revealed the presence of V 4* both in the sample as synthesized and in the 773 K O2-treated sample to remove the template. Tetravalent V is rather stable in O2 at high temperature which indicates that V 4+ ion substitutional framework position is resistant towards oxidation. Like CoAPO-11, oxygen adducts V- O-O" are formed, as revealed by esr, only aiter H2-treatment at high temperature. The esr results are interpreted similarly to those on CoAPO- 11. 2. INTRODUCTION There has been much interest in the synthesis of isomorphously substituted aluminophosphates (1). It has been established that aluminophosphates containing cobalt possess redox properties which contribute to the catalytic properties for oxidation (2). In addition the isomorphous substitution of Co(2+) into the framework of ALPO generates acidic properties associated with P-OH sites (3). Although the Bronsted acidity linked with P-OH is generally weak, CoAPO exhibited interesting catalytic properties in reactions catalyzed by acid such as isomefisation ofbutene (4) and isomefisation of alkanes (5). The aim of this work was to investigate the redox properties of CoAPO-11 and VAPO-5 in relation to their interaction with oxygen and/or hydrogen. Esr technique was employed to investigate these interaction

796 3. EXPERIMENTAL SECTION Preparation of CoAPO- 11 and VAPO-5. The CoAPO- 11 was synthesized using a gel having the following molecular composition : H3PO4 (85%) = 1, A1203 = 0.475, COSO4, 71-120 = 0.05, diisopropylamine = 0.75 1-120 = 40 The crystallization was carried out in a teflon bottle in an autoclave at 463 K for 2 days. Highly crystalline CoAPO-11 was obtained following the above procedure as indicated by XRD analysis (4). The crystals of CoAPO-11 with the template were thoroughly washed, filtered and dried in air at 373 K. The CoAPO-11 showed a deep blue color. The organic template was removed by calcination in air at 773 K. The sample turned green and is designated by Ox-CoAPO-11. Ox-CoAPO-11 samples which have been reduced by H2 in the temperature range 473-773 K are designated by Redu-CoAPO- 11. The gel composition for the synthesis of VAPO-5 was the following in molar ratio • P r 3 N " A1203 : 1.9 H3PO4 : 0.05 V205 : 401-120. The crystallization was carried in an autoclave at 423 K during two days. The n-tripropylamine template was removed by calcination of VAPO-5 in air at 773 K (Ox-VAPO-5). The sample turned yellow. Upon 1-12reduction at 773 K (ReduVAPO-5) the VAPO-5 returned to almost its initial green color. XRD spectra of the assynthesized, oxidized, reduced VAPO-5 s a b l e s are in agreement with those of A1PO-5 published in the literature (4). Esr spectra were recorded with an X-band Varian E9 spectrometer operating with 100 KHZ frequency modulation. g-values were determined using a dual cavity and DPPH (g = 2.0036) as reference. All esr spectra were recorded at 77 K unless mentioned in the text. Prior esr measurement, samples were outgassed directly in the esr quartz tube. Oxygen adsorption was performed on samples having experienced an activation procedure which will be specified in the text. 4. RESULTS AND DISCUSSION As indicated in the experimental section the as-synthetized blue CoAPO-11 showed well characterized x-ray line pattern of the isostructural A1PO- 11. The x-ray line pattern was almost unchanged upon removal of the organic template by calcination in air. CoAPO-11 sample was formed of spherical particles with 10 Bm size. The infrared spectrum of the ox- CoAPO-11 in the OH stretching vibration showed band at 3640 cm~ attributed to P-OH. The acid character of these P-OH groups was demonstrated by their reaction with a base like pyridine. The IR band at 3640 cm"1 disappeared on adsorption of pyridine. Simultaneously bands at 1550 and 1490 cmt characteristic ofpyridinum ions and at 1490 and 1450 cmt characteristic of pyridine coordinated with Lewis centres appeared. According to these in~ared results the oxidized CoAPO-11 sample shows some Bronsted and Lewis acidity. Similarly the H2-reduced CoAPO- 11 sample exhibited almost the same infrared features as the ox- CoAPO- 11 that is IR band at 3640 cm ~ which disappeared upon pyridine adsorption with the subsequent formation of IR bands at 1540, 1490 and 1450 cmt indicating the Bronsted and the Lewis acid character of Redu-CoAPO- 11. The aluminophosphate molecular sieves have neutral framework. Isomorphous substitution of divalent cations Co 2+ for trivalent A13+ in A1PO generates negative framework charge, neutralized by H +. CoAPO-11 will show Bronsted acidity for divalent Co ~-+. By contrast substitution of trivalent cations for A1 3+ will leave neutral the framework. CoAPO- 11 where cobalt is at + 3 oxidation state should not exhibit any no proton acidity. The infrared results presented above have indicated that calcined ox-CoAPO-11 and

797 reduced Red- CoAPO-11 exhibited almost the same proton acidity. Therefore one must necessary conclude that the charge of the substituted cobalt in these samples is always + 2 since no Bronsted acid site would form for Co 3+ substitution. It is tempted to conclude in agreement with recent work based on esr and uv studies of CoAPO-5 that no oxidation of Co 2+ into Co 3+has occurred upon calcination ofCoAPO- 11 (5). The esr spectrum at 77 K of the as synthesized blue sample CoAPO-11 consists of a very broad asymmetrical signal which spreads within more than 2000 gauss. The approximative gvalues are around g~ = 4.5 g2 = 2. The signal was previously ascribed to Co 2+ (5,6). The esr signal of ox- CoAPO-11 calcined at 773 K has a considerable lower intensity and apparently is much more broad than the previous one. These changes in esr spectra were interpreted either in terms of cation oxidation Co 2+ to Co a+ (6) or in terms of lattice distortion of the molecular sieve, therefore the local tetrahedral field symmetry at Co 2+ ions was changed (5). The calcined ox- CoAPO-11 sample apparently did not activate molecular oxygen. Indeed no paramagnetic oxygen species visible to esr appeared upon 02 adsorption at 77 and 293 K on ox- CoAPO-11 outgassed in the temperature range 293-473 K. However upon H2 treatment in the temperature range 473-773 K the Red- CoAPO- 11 became active towards Ozadsorption. Indeed as Oz (1 torr) was allowed to react with Red-CoAPO-11, previously outgassed at 473 K, a strong esr signal with anisotropic g-values g~ = 2.0210 g2 = 2.0093 g3 2.0024 was observed (figure 1). The species responsible for this 3-g values esr signal were unambiguously identified to the cobalt superoxide (or peroxide) ions. It was observed that the superoxide esr signal intensity (hence the amount of paramagnetic Co-O2 adduct) increased as the temperature of the initial H2-treatment was higher, the Hz-treatment at 773 K producing the highest quantity of superoxide ions. The thermal stability of these superoxide ions was found to depend on the sample temperature. At about 423 K under outgassing conditions the esr signal of the paramagnetic oxygen adducts disappeared within few minutes. However re-adsorption of molecular oxygen on this sample regenerated immediately the superoxide species. Within these experimental conditions (Red- CoAPO- 11 never contacted with O~ at high temperature) the Red-CoAPO- 11 sample can perform several cycles of oxygen activation. By contrast as the temperature of the Co-O2 adduct was raised up to 773 K, the esr signal of the superoxide ions disappeared, and a subsequent adsorption of molecular oxygen did not generate, as previously, the superoxide esr signal. This 02 treated at 773 K, was rendered again active to oxygen adsorption only after it was H2-treated in the temperature range 473-773 K. The superoxide species were found reactive towards hydrocarbons, particularly olefins and cycloolefins, the esr signal of the paramagnetic Co-O2 adducts disappearing upon contacting the sample with for example cyclohexene. From the known structure of AIPO-11 it is logical to assume that if the Co 2÷ ions, in cobalt substituted AIPO, are part of the aluminophosphate lattice, they will be in a tetrahedral environment, replacing partially some lattice A13÷ ions. The resulting tetrahedral crystal field experienced by Co 2÷ would give to this ion the high spin 3/2 electron configuration which for powdered samples produce very broad esr signal as observed with CoAPO-11. The negative excess charge, resulting from the substitution of A13÷ for Co 2+ is in the as-synthesized sample compensated with the protonated di-isopropylamine. The removal of the template by calcination would leave protons only if the overall charge of the A1 substituted cation species is at the 2+ oxidation state. As stated above the increase of the oxidation state of Cobalt from 2+ to 3+ would render the CoAPO framework neutral. The presence of proton acid sites on the ox- CoAPO-11 suggests that upon calcination Co(2+) forms with oxygen a cobak oxo species such as Co = 0 where the two p orbitals beating one impair electron form with d orbitals of Co the Co = 0 bonds. The total charge of the oxoadduct would be (Co = 0) 2+ while the cobalt ion =

798 would be virtually at 4+ oxidation state. Such cobalt-oxo adduct apparently could not further activate molecular oxygen. Co = 0 oxo complexes were stable towards decomposition up to 773 K, However upon H2-treatment at high temperature (473-773 K) the oxygen ad-atom is removed following the reaction • (Co = 0) 2+ + H2 ~ Co z+ + He0. The above discussion explains how and why Oe-treated and H2-treated CoAPO-11 exhibited the same proton acidity. The He-treated CoAPO-11 adsorbed oxygen in the form of a paramagnetic superoxo or peroxo like cobalt species. On the He-reduced sample tetrahedral Co(2+) would form bonds with 02 molecule either via electron transfer between Co(2+) and 02 (ionic bonding) forming Co (3+) - O2" species or via coupling of one unpaired d electron of Co (2+) and one unpaired electron of molecular oxygen forming the covalent peroxo cobalt adduct Co-O-O'. The relatively high thermal stability of the cobalt peroxo adduct, and the relatively high ionization potential for Co (2+) ~ Co (3+) render the covalent structure Co-O-O" more favorable. One can predict that the oxidation mechanism over CoAPO catalysts will be of radical type. The Co-O-O" adducts react with hydrogen donor molecules, H2, RH in a catalytic cycle following 2He Co (2+) + 02 --~ Co-O-O" ) Co (2+) + 2H20 At high temperature, 773 K, the peroxocobalt adducts are converted into the cobalt oxospecies Co-O-O" -~ Co=O + ½ 02 (T = 773 K) which is inactive toward the activation of molecular oxygen. The solid recovered its activity toward 02 after the oxo species were removed by H2 (or hydrocarbons) at 773 I~

12.02

I DPPH

..___

IDPPH I~ 2.009

..2.0093 H lOLgau,ss

1

i

I 2.003

I 2.0024

Figure 1. esr spectrum of Co-O-O"in CoAPO-11

Figure 3. esr spectrum of V-O-O"in VAH3-5

799

[DPPH 100 gauss

Figure 2. esr spectrum at 77 K of V(+4) in VAPO-5 Esr study of VAPO-5. The esr spectrum of the as-synthesized VAPO-5 is shown in figure 2. The esr spectra recorded at 77 K and at 293 K extfibited identical esr features (g-values and hyperfme splittings). By decreasing the temperature the esr line intensities increased following the 1/T Curie Law. The magnetic parameters of this esr signal are" gx = 1.94 g:= 1.99, A//= 185 gauss A± = 72 gauss. These parameters are typical for V (4+) ions in distorted axial symmetry. V (4+) has a 3d ~ configuration. This ion in a tetrahedral symmetry such as experienced for T site in ALPO-5 framework will present a very short spin-lattice relaxation time, and since the line width is related to the inverse of the relaxation time, the esr spectrum of V (4+) in a non distorted tetrahedral crystal field will be broadened beyong detection at 77 and 293 K. Indeed V (4+) in VCI4 compound was not observed by esr (7). Similarly (nor)4 V has no detectable esr signal at temperatures down to 143 K (8). Below this temperature the esr signal of V (4+) with" gl = 1.984 g2 = 2.036 AI = 40 g and A¢/= 120 gauss becomes discernible. This was attributed to V (4+) species in a tetrahedral crystal field with a tetragonal distortion. The detection of an esr signal at 293 K attributed to V (4+), and the relatively large hyperfme splitting (A//= 185 gauss, A± = 72 gauss) therefore indicate that if vanadium ions present in VAPO-5 are in substitutional position, then the symmetry around V (4+) is considerably distorted. It is likely that the symmetry around V (4+) observed by esr in VAPO-5 is provided by three identical VO bonds and one short V-O bond. Suggesting that V (4+) replaces P (5+) in AIPO-5 then the local structure will be (02)3 V-O(H)...AI(O2)3. The results have also shown that the V(4+) esr signal observed on the as synthesized VAPO5 remained almost unchanged alter calcination. It is clear that V (4+) in VAPO-5 was stable

800 toward oxidation into V (5+) which is diamagnetic. It appeared that the crystal energy of the lattice was able to stabilize V (4+) ion. The calcined VAPO-5 does not form paramagnetic oxygen species by 02 adsorption on the outgassed sample. However the sample submitted to H2-treatment at 773 K, adsorbed O2 to form the vanadium peroxospecies V-O-O" whose esr spectrum is shown in figure 3. The esr gvalues are g~ = 2.021 g2 = 2.009, g3 = 2.002. The mechanism for the formation of this peroxospecies is apparently similar to that proposed for CoAPO sample' (02")3 V-O(H)

773 K (OZ')3V 02 (02")3 V-O-O" H2 > ) At high temperature (773 K) the (02")3 V-O-O" species is again converted into : (O2")3V-O(H) 5. REFERENCES E.M. Hanigen, B.M. Lok, ILL. Patton and S.T. Wilson, in Y. Muzakami, A. figima and J.W.Ward (Editors), Studies in surface Science and Catalysis, New Developments in Zeolite Science and Technology, Kodansha Elsevier (1986) p. 103. B. Kraushaar-Czametzld, W.G.M. Hoogervorst and W.H.J. stork in J. Weitkamp, H.G. Karge, H. Pfeifer and W. HSldefich eds Zeolites and Related microporous Materials • State of the Art 1994 Studies in Surface Science and Catalysis Vo184, (1994) p. 1869. J. Jiinchen, M.J. Haanepen, M.P.J. Peeters, J.H.M.C. van Wolput, J.P. Wolthuizen, J.H.C. van Hooffin J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich eds, Zeolites and related microporous Materials • State of the Art 1994. Studies in Surface Science and Catalysis vo184, (1984)p. 373. J.M. Bennett, J.P. Cohen, E.M. Hanigen, J.J. Pluth, J.V. Smith, ACS Symposium Series, 218 (1983) 109. V. Kurshev, L. Kevan, D.J. Parillo, C. Pereira, G.T. Kokotailo and ILJ. Gorte, J. Phys. Chem, 98 (1994) 10160. 6.

L.E. Iton, I. Choi, J.A. Desjardins, V.A. Maroni, Zeolites, 9 (1989) 535.

7.

J.C.W. Chien and C.IL Boss, J. Phys. Chem. 83 (1961) 3767. B.K. Bower, M. Findlay, J.C.W. Chien, Inorg. Chem., 13 (1974) 759.


Cupric Ion Species in Cu(II)-Exchanged and Comparison with Aluminosilicate K-L

801

Gallosilicate

K-L

Jong-Sung Yu. a Suk Bong Hongb and Larry KevanC aDepartment of Chemistry, Han Nam University,Taejon, Chungnam, 300-791,Korea bKorea Institute of Science and Technology, P.O.Box131,Cheongryang,SeouI,Korea CDepartment of Chemistry, University of Houston, Houston, TX, 77204-5641,USA

The locations and adsorbate interactions of Cu(II) in Cu(II)-exchanged gallosilicate with the zeolite L channel structure were investigated by electron spin resonance(ESR) and electron spin echo modulation (ESEM) spectroscopies, and compared with those in Cu(II)-exchanged aluminosilicate K-L zeolite previously studied. Similar results to those in aluminosilicate CuK-L zeolite are observed in its gallium analog, suggesting that gallium substitution has little effect on the interaction of Cu(II) with adsorbates in the L framework structure. 1.

INTRODUCTION

Isomorphous substitution of elements other than Si and AI into T-site framework positions in zeolite has been a long standing interest in zeolite chemistry as this can provide a route to modify the framework characteristics and geometries. Gallosilicate is one such important material where zeolitic aluminum atoms can be replaced by gallium atoms[I-4]. The gallium analogs of zeolites usually possess physical and chemical properties different from their aluminum analogs[1,2]. Transition metal ions on zeolite surfaces are increasingly being exploited as controllable catalytic active sites[5]. This requires knowledge of both the location and adsorbate interactions of the transition metal ion. Various experimental techniques have been used in the elucidation of these properties. In particular, electron spin resonance (ESR) has been widely used in delineating information concerning the number of different species, oxidation state and coordination environment of a paramagnetic metal center such as Cu(II)[6-8]. A type of pulsed ESR, known as electron spin echo modulation (ESEM) spectroscopy can provide additional quantitative information concerning the number of surrounding adsorbate nuclei and their interaction distance[9]. In earlier work, the location and adsorbate interactions of Cu(II)in Cu(II)exchanged K-L a!uminosilicate zeolite was investigated by electron spin resonance techniques[10,11]. Zeolite L is a channel type zeolite with a main channel diameter of about 0.75 nm[12]. The replacement of AI by Ga in the framework

802

may change the type of cation coordination and its reaction properties. In the present work, the interaction of paramagnetic Cu(II) with various adsorbates in Cu(II)-exchanged K-L gallosilicate is studied by electron spin resonance(ESR) and electron spin echo modulation (ESEM) spectroscopies to deduce the locations and coordination geometries of Cu(rl)with adsorbates in this material. These results are compared to those in Cu(II)-exchanged K-L aluminosilicate zeolite. 2. EXPERIMENTAL SECTION Synthetic K-L zeolite was obtained from Union Carbide Corp. A gallium analog of L zeolite was synthesized according to the procedure described by Newsam and Vaughan[1,2]. The structure of the materials was determined by X-ray diffraction. These K-L materials were then exchanged at room temperature for 12 h by dropwise addition of a 10 mM solution of cupric nitrate (Alfa Products) according to the procedure described earlier[13]. Copper exchange was 0,1-0.4 % by weight of the K-L sample, assuming complete exchange. Dehydration of the sample was carried out with the sample in a Suprasil quartz ESR tube (2 mm i.d. by 3 mm o.d.)reactor by degassing at room temperature following evacuation and oxidation with oxygen at at 400 oC according to the procedure described in the earlier work[10,11] In particular, gallosilicate is known to have a lower thermal stability than the corresponding aluminosilicate[3]. Thus no evacuation was usually made at temperatures higher than 210oc in this work. This heat treated sample with oxygen is termed as a dehydrated sample. After dehydration, adsorbates as gases were admitted at room temperature to the sample tubes and left to equilibrate. ESR spectra were measured at 77 K on a modified Varian E-4 spectrometer interfaced to a Tracor Northern TN-1710 signal averager. ESEM spectra were recorded at 4.5 K with a Bruker ESP 380 pulsed ESR spectrometer. Three-pulse echoes were measured by using a 90°- x-90 °- T90 ° pulse sequence with the echo measured as a function of T to analyze the deuterium modulation from deuterated adsorbates. B o t h the theory and methods used for simulation of the data are described in detail elsewhere[9]. 3. RESULTS AND DISCUSSION Figure 1 shows the ESR spectra of fresh hydrated CuK-L aluminosilicate zeolite at room temperature and 77 K. At room temperature the ESR spectrum shows an almost isotropic signal at giso = 2.17 as shown in Figure la. The fresh hydrated samples of gallium anologs of CuK-L also give a similar ESR spectrum mainly consisting of a broad isotropic line at ambient temperature. Such a broad ESR signal at room temperature is indicative of a mobile species which is rotationally unrestricted on the ESR timescale. Analysis of the three-pulse ESEM spectrum of zeolite K-L which has been rehydrated with D20 and which exhibited an identical ESR spectrum to the fresh zeolite indicates a water solvation number of six around Cu(II), i.e. [Cu(H20)6] 2+ located in the main channel for both gallium and aluminum analogs of CuK-L. At 77 K this [Cu(H20)6] 2+ complex becomes immobilized and gives rise to an asymmetric spectrum with two species, denoted as species A and B, typical of Cu(II) axial powder spectra as shown in Figure lb.

803

Fresh Hydrated CuK-L

giso=2.166

RT ESR

~

b

I

X8

'

200 G

I

'

I

'

gilA=2.412

'

I

!

77K ESR I

I

gllS=1.943

i

Figure 1. ESR spectrum of fresh, hydrated CuK-L zeolite recorded (a) at room temperature and (b) at 77 K. Interestingly, species B, which is a minor species, shows ESR parameters with reversed g values of gll = 1.94, All = 97 x 10 -4 cm -1 and g_L= 2. 16. From a consideration of the symmetry expected in L zeolite and the analysis of the deuterium modulation obtained for species B, the Cu(II) species is suggested to be a diaquo complex [ C u ( O z ) 3 ( H 2 0 ) 2 ] with trigonal bipyramidal geometry. Interestingly, the minor Cu(II) diaquo species seen in aluminosilicate is not observed in the gallosilicate. Similar changes in ESR profile were observed during evacuation in the gallosilicate and aluminosilicate analogs of CuK-L. When the samples are evacuated at room temperature, the isotropic components of the ESR signal recorded at ambient temperature decrease and the ESR signal is no longer broadened at room temperature. This is indicative of the copper losing some water ligands and becoming immobilized by coordination to several lattice oxygens. The three-pulse ESEM results indicate that Cu(II) is now coordinated to only three water molecules. Upon further evacuation at increasing temperature Cu(II) ion moves from the main channel towards recessed sites. Complete dehydration produces one major Cu(II)species

804 assigned to the center of a hexagonal prism in a six-ring channel based on a lack of broadening of its ESR lines by oxygen for both gallium and aluminum analogs. Interestingly, water in Cu(II)-exchanged K-L gallosilicate is removed more easily than in the corresponding Cu(II)-exchanged aluminosilicate. This may be verified by the temperatures at which no further change of the ESR signal is observed during evacuation which may indicate the point of complete dehydration. This is around 2 0 0 o c for CuK-L gallosilicate and around 350 oC for CuK-L aluminosilicate. Adsorption of molecules such as water, alcohols, ethylene, benzene, ammonia, pyridine and dimethyl sulfoxide causes changes in the ESR spectrum of the Cu(II) indicating migration from a recessed site toward cation positions in the main channels where adsorbate coordination can occur. Table I summarizes the ESR parameters of Cu(II) in CuK-L gallosilicate compared with those of CuK-L aluminosilicate zeolite after various sample treatments. Table [. ESR parameters at 77 K of Cu(II) in CuK-L gallosilicate aluminosilicate zeolites observed after various sample treatments Gallosilicate

Treatment

glla

fresh/RT ESRc

2.17d

fresh

2.400

Aiib

and CuK-L

Aluminosilicate

g_La

glla

Allb

g_La

2.166d 134

2.08

2.412

137

2.08

97 159

2.157 2.05 2.08

dehydrated

2.338

153

2.07

1.943 2.334

+CH3OH

2.381

132

2.09

2.389 2.394

134 134

+CH3CH2OH

2.381

128

2.09

2.382

134

2.08

+ CH3CH2CH3OH

2.380

131

2.08

2.384

133

2.08

+C2H4 + NH3

2.344 2.254

148 175

0.07 2.05

2.343 2.255

159 177

2.07 2.05

+ pyridine

2.255

187

2.06

2.260

181

2.06

+ benzene + (CH3)2SO

2.350 2.394

140 129

2.07 2.09

2.341 2.392

161 117

2.05 2.06

aEstimated uncertainty is +_ 0.008. bThe unit of All is 10 -4 cm -1 and the estimated uncertainty is + 6 x l 0 -4 cm -1. CESR measured at room temperature, dgiso value. Figure 2a shows the ESR spectrum at 77 K observed after adsorption of ethanol on dehydrated CuK-L gallosilicate samples. Adsorption of methanol produces a new ESR spectrum with its g value shifted from 2.338 to 2.381 and a decrease in the All coupling. Similar ESR spectra are observed upon

805

adsorption of ethanol and propanol. The corresponding three-pulse ESEM spectrum with adsorbed CH3CH2OD is shown in Figure 2b. The simulation indicates interaction with two deuterium nuclei, .i.e. two ethanol molecules, with a Cu(U)-D distance of 0.28 nm. Cu(II) also forms complexes with two molecules of methanol and propanol based on ESEM analyses. Cu(II) in CuK-L alumin0silicate forms similar complexes with two molecules of methanol, ethanol and propanol, respectively. Cu(II) also forms new complexes with one molecule of ethylene and benzene based on ESR and ESEM data for both gallium and aluminum analogs. Figure 3 shows the ESR spectra after adsorption of NH3 and pyridine onto dehydrated CuK-L. CuK-L gallosilicate with adsorbed 15NH3 shows five hyperfine lines centered at gll = 2.054 and split by 18 x 10-4 cm -1 which are shown in the expanded second-derivative spectrum in Figure 3a. Since 15N has a nuclear spin of 1/2, the five lines indicate four ammonias coordinated to the Cu(II). A similar ESR spectrum with five hyperfine lines was observed in dehydrated CuK-L aluminosilicate upon 15NH3 adsorption. In this case, the Cu(lI) species is suggested to be located in the center of 12-ring main channel coordinating to four ammonias in a square-planar geometry. However, ESR spectra with different nitrogen hyperfine interactions in dehydrated CuK-L gallosilicate and CuK-L aluminosilicate measured after

b. CH=CH=OH I ~ 1.0 z

c6 0.8 nv

X8

~

V !

,

-a'°e ,

gll --2.381

0.6

oo z 0.4 u.I I.z O 0.2 ..r. O u.I

,

CuK-L gallosilicate

==,=.

a. C H 3 C H 2 O D

0

1

2

3

4

5

T, lU= Figure 2. (a) Experimental(---)and simulated ( - - - ) three-pulse ESEM spectrum recorded at 4 K. The best simulation indicates N = 2, R = 0.28 __.0.01 nm and Also = 0.28 MHz. (b) The corrresponding ESR spectrum at 77 K of dehydrated CuK-L gallosilicate with adsorbed CH3CH2OD.

806

a. ~ S N H 3 / G a - C ~

~..... f g=2.052 . b. P y r i d i n e / G a - ~ , ,

,

, , .... ~,

,

f ~

c. P y r i d i n e / A I - - C u ~

o,,

if

("''""r" V g=2.055

Figure 3. ESR spectra at 77 K of (a) dehydrated CuK-L gallosilicate with 70 Torr of 15NH3 added at room temperature, (b) dehydrated CuK-L gallosilicate equilibrated with pyridine containing 14 N and (c) dehydrated CuK-L aluminosilicate equilibrated with pyridine (the g±region is expanded as the second derivative to more clearly show the hyperfine structure).

807

pyridine adsorption are shown in Figures 3b and 3c. A new cupric ion species due to complex formation with pyridine is observed with at least nine hyperfine lines centered at g_L =2.055 and split by 15 x 10 -4cm -1 and seven lines centered at g_L = 2.055 and split by 16 x 10-4 cm-1 as shown in the expanded second-derivative spectra for CuK-L gallosilicate and CuK-L aluminosilicate, respectively. Since 14 N in pyridine has a nuclear spin of 1, the nine and seven lines indicate four and three pyridine molecules directly coordinated to the Cu(H), respectively. Thus, the Cu(II) species in K-L gallosilicate is suggested to be located in the center of a 12-ring main channel coordinating to four pyridines in a square-planar geometry, while Cu(II) species in CuK-L aluminosilicate is located near an eight-ring window of a main channel coordinating to four framework oxygens and three pyridine molecules. Dimethyl sulfoxide also produces new Cu(II) ESR signals. The analysis of three-pulse ESEM data for CuK-L gallosilicate with adsorbed deuterated dimethyl sulfoxide(D3CSOCD3) shows no deuterium modulation as shown in Figure 4. Since no modulation is usually observed at a distance beyond about 0.5 nm, the new Cu(II) species may be due to Cu(II) indirectly interacting with dimethyl sulfoxide at a longer distance. Interestingly, we observed strong deuterium modulation for aluminosilcate CuK-L zeolite with adsorbed deuterated dimethyl sulfoxide. The simulation indicated that the Cu(II) in aluminoslicate CuK-L zeolite directly interacts with one molecule of dimethyl sulfoxide with a Cu(II)-D distance of 0.37 nm. 1.0

CuK-L + SO(0D3)2

z q.8

E" 1.o z

~d 0.8

n-

CuK-L gallosilicate

" 0.6

0.4

Z I

CuK-L aluminosilicate

0 0.2

'-1tO uJ

0

1

2

3

4

5

T, 14s Figure 4. Experimental ( ~ ) and simulated (- - -) three-pulse ESEM spectra recorded at 4.5 K of dehydrated CuK-L gallosilicate and CuK-L aluminosilicate with adsorbed D3CSOCD3. The simulation parameters for CuK-L aluminosilicate are N = 6, R = 0.37 + 0.01 nm and Aiso = 0.02 MHz.

808

4.

CONCLUSIONS Rather similar results to those for CuK-L aluminosilicate were observed in CuK-L gallosilicate but there are also significant differences. The main Cu(II) species in hydrated CuK-L aluminosilicate and gallosilicate is an octahedrally coordinated hexaaquo species [Cu(H20)6] 2+ which resides in the main channel with rotational freedom at room temperature. The minor Cu(II) species with reversed g values assigned to a diaquo complex in hydrated CuK-L aluminosilicate is not observed in the gallosilicate. Upon partial dehydration at room temperature, the fully hydrated cupric ion loses some of its coordinated water and becomes anchored to the zeolite lattice by partial coordination to zeoliticoxygens. The coordinated water is removed more easily in the gallosilicate than in the aluminosilicate. When completely dehydrated, the cupric ions are located in cation sites recessed from the main channels. Absorption of adsorbate molecules such as water alcohols, ammonia, pyridine, dmethyl sulfoxide, benzene and ethylene causes migration of Cu(II) into the main channel and coordination with the molecule. Cu(II) forms complexes with two molecules of alcohols and one molecule of ethylene and benzene based on ESEM data for both the gallium and aluminum analogs. Cu(II) also forms complexes with four molecules of ammonia in the center of twelve-membered ring main channels based on resolved nitrogen superhyperfine. However, Cu(II) coodinates with four molecules of pyridine in K-L gallosilicate and only three molecules of pyridine in K-L aluminosilicate based on resolved nitrogen superhyperfine. Cu(II) also interacts directly with dimethyl sulfoxide in KL aluminosilicate, but only indirectly at a longer distance with dimethyl sulfoxide in K-L gallosilicate based on ESEM data. 5.

ACKNOWLEDGMENTS

This research was supported by the Korea Science and Engineering Foundation (951-0303-046-2), the U.S. National Science Foundation and the Robert A. Welch Foundation. REFERENCES °

2. 3. .

5. 6. 7. 8. 9. 10. 11. 12. 13.

J. M. Newsam and D.E.W. Vaughan, Stud. Surf. Sci. Catal. 28 (1986) 457. J. M Newsam, Mat. Res. Bull. 21 (1986)661. R. Sz0stak, Molecular Sieves, Principles of Synthesis and Identification; Van Nostrand Reinhold: New York, 1989; p.212. J. M. Thomas and X.-S. Liu, J. Phys. Chem. 90 (1986) 4843. I.E. Maxwell, A d v . Catal. 31 (1982) 1. M. Narayana, S. Contarini and L. Kevan, J. Catal. 94 (1985)370. R. G. Herman and D.R. Flentge, J. Phys. Chem. 82 (1978) 720. J.S. Yu and L. Kevan, J. Phys. Chem. 95 (1991)3262. L. Kevan, In Time Domain Electron Spin Resonance; L. Kevan and R. N. Schwartz, Eds. ; Wiley- Interscience: New York, 1979; Chapter 8. J. S. Yu, J.M. Comets and L. Kevan, J. Phys. Chem. 97 (1993) 11047. J. S. Yu and L. Kevan, J. Phys. Chem. 98 (1994) 12436. A. Sayari, J. R. Morton and K. F. Preston, J. Phys. Chem. 93 (1989) 2093. J.S. Yu and L. Kevan, J. Phys. Chem. 94 (1990) 5995.

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