Studies in Surface Science and Catalysis 129 NANOPOROUS MATERIALS II
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Studies in Surface Science and Catalysis Advisory Editors: B. Delmon and J.T. Yates Vol. 129
NANOPOROUS MATERIALS II Proceedings of the 2nd Conference on Access in Nanoporous Materials, Banff, Alberta, Canada, May 25-30,2000
Edited by Abdelhamid Sayari Laval University, Departmentof Chemical Engineering, Ste-Foy, Quebec G1K 7P4, Canada MietekJaroniec Kent State University, Department of Chemistry, Kent, OH 44242, USA Thomas J. Pinnavaia Michigan State University, Department of Chemistry, East Lansing, Ml 48824, USA
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CONTENTS
Preface
xv
Organizing Committee
xvi
International Advisory Committee
xvi
I. Synthesis of Mesoporous Silicas A Novel Approach to Polymer-template Mesoporous Molecular Sieves Kai Schumacher, Sabine Renker, Klaus K. Unger, Ralph Ulrich, Alexander Du Chesne, Hans W. Spiess and Ulrich Wiesner
1
Synthesis of Uniform and Stable Millimeter-Sized Mesoporous Silica Ropes by the Addition of Polymer and Ammonia Hydrothermal Treatment Hong-Ping Lin, Lu-Yi Yang, Chung-Yuan Mou, Huang-Kuei Lee and ShangBin Liu
7
The Synthesis and Characterization of Mesoporous Molecular Sieves MCM-41 with Interconnected Channels Hong-Ping Lin, She-Tin Wong, Shang-Bing Liu, Chung-Yuan Mou and ChihYuan Tang
15
Synthesis of Mesoporous Silica Molecular Sieves via a Novel Templating Scheme Xiaoming Zhang, Zhaorong Zhang, Jishuan Suo and Shuben Li
23
Preparation of Spherical Micrometric MSU-X Mesoporous Silica Particles for Chromatography Applications Cedric Boissiere, Andre Larbot and Eric Prouzet
31
Synthesis of Nanometer-sized Mesoporous Silica and Alumina Spheres Qian Luo, Li Li, Zhiyuan Xue and Dongyuan Zhao
37
Assembly of Nanoporous Silica via Amphoteric Surfactant Templating Scheme Jing Xin, Xiaoming Zhang, Zhaorong Zhang and Jishuan Suo
45
Formation of Integrated MCM-41 Mesostructure in Fluoride Medium : An Improvement of Hydrothermal Stability Q.-H. Xia, K. Hidajat and S. Kawi
49
New Way to Synthesize MCM-41 and MCM-48 Materials with Tailored Pore Sizes J.L. Blin, G. Merrier, C. Otjacques and Bao-Lian Sii
57
Poly(oxyethylene) Oleyl Ethers as Templating Agents for the Synthesis of Large Pore Mesoporous Materials J.L. Blin, G. Merrier and Bao-Lian Su
67
Pore Size Engineering of Mesoporous Silicas Using Alkanes as Swelling Agents J.L. Blin, C. Otjacques, G. Merrier and Bao-Lian Su
75
Improvement of Hydrothermal Stability of Mesoporous Molecular Sieves of MCM41 Type Debasish Das, Chou-Mei Tsai and Soofin Cheng
85
Improvement on Thermal Stability and Acidity of Mesoporous Materials with Posttreatment of Phosphoric Acid Limin Muang and Quanzhi Li
93
In situ Synthesis of Micro- and Mesoporous Al-MFI / MCM-41 like Phases with High Hydrothermal Stability Arne Karlsson, Michael Stocker and Karin Schafer
99
Microwave Synthesis of Micro-Mesoporous Composite Material D.S. Kim, S.-E. Park and S.O. Kang
107
Preparation of Y/ MCM-41 Composite Materials Ruifeng Li, Weibin Fan, Jianming Ma, Kechang Xie
117
Supported Crystallization of MFI- and FER-type Molecular Sieves on Porous Glasses W. Schwieger, M. Rauscher, R. Monnig, F. Scheffer and D. Freude
121
Supercritical Fluid Extraction of Amine Surfactant in Hexagonal Mesoporous Silica (HMS) S. Kawi and A.-M. Goh
131
Performance of Tetraalkylammonium Ions during the Formation of Zeolites from Tetraethylorthosilicate C.E.A. Kirschhock, R. Ravishankar, K. Truyens, F. Verspeurt, P.A. Jacobs and J.A. Martens
139
Study of Interactions between Silicate Species and Surfactant Micelles in the Synthesis of Organized Mesoporous Materials Jorn Frasch, Benedicte Lebeau, Michel Soulard, Joel Patarin and Raoul Zana
147
II. Synthesis of Framework-Modified Mesoporous Silicas Novel Ordered Mesoporous Materials with Hybrid Organic-Inorganic Network in the Frameworks S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki
155
Synthesis and Catalytic Application of Organically Modified Ti-MCM-41 Type Materials Naoko Igarashi, Satoshi Kidani, Rizwan-Ahemaito and Takashi Tatsumi
163
Influence of Silylation on the Catalytic Activity of Ti-MCM-41 during Epoxidation of Olefins A. Corma, J.L. Jordd, M.T. Navarro, J. Perez-Pariente, F. Rey and J. Tsuji
169
Synthesis and Modification of Ti-containing Catalysts for Epoxidation of Alkene Jie Bu and Hyun-Ku Rhee
179
Synthesis and Adsorption Properties of Cerium Modified MCM-41 Antonio S. Araujo and Mietek Jaroniec
187
Microwave Synthesis of Zr (Ti)-Si-Al HDN Catalytic Material Sun Wanfu, Ma Bo, Suo Jisuan, Li Shuben, Luo Xihui and Jiang Zongxuan
195
Characterization of Mesoporous and Microporous Molecular Sieves Containing Niobium and Tantalum Martin Hartmann, Stefan Ernst, A.M. Prakash and Larry Kevan
201
Direct Incorporation of Al in SB A Mesoporous Materials : Characterization, Stability and Catalytic Activity Y.-H. Yue, A. Gedeon, J.-L. Bonardet, J.B. d'Espinose, N. Melosh and J. Fraissard
209
Post-Synthesis Alumination of MCM-41 by A1(N03)3 (I): Improvement in Acidity for Purely Siliceous MCM-41 S. Kawi and S. C. Shen
219
Post-Synthesis Alumination of Si-MCM-41 by A1(N03)3 (H): Enhancement of Hydrothermal, Mechanical and Chemical Stabilities S. Kawi and S. C. Shen
227
Siting of Co(II), Zn(II) and Cu(I) Ions in (A1)MCM-41 J. Dedecek, N. Zilkovd and J. Cejka
235
Reversible Transition of the Coordination of Al in MCM-41 H. Kosslick, H. Landmesser, R. Fricke and W. Storek
243
III. Synthesis of Surface-Modified Mesoporous Silicas Functionalised Mesoporous Materials for Green Chemistry James H. Clark, Duncan J. Macquarrie and Karen Wilson
251
Peculiarities of Alkyl-modification of Ordered Mesoporous Materials : A Singlestep Treatment on Uncalcined MCM-41 Involving Template Removal and Surface Functionalization Valentyn Antochshiik and Mietek Jaroniec
265
New Organically Modified Hexagonal Mesoporous Silicas : Preparation and Applications in Catalysis Duncan J. Macquarrie, Dominic B. Jackson, Stephane Taillaud, Karen Wilson and James H. Clark
275
Organo-functionalized Surface Modified MCM-41 Type Mesoporous Materials Having Various Organic Functional Groups Priyabrata Mukherjee, Subhash Laha, Deendayal Mandal and Rajiv Kumar
283
Phenyl-functionalized Silicate Mesophases with Hexagonal or Cubic Symmetries: Influence of Synthesis Parameters Valerie Goletto, Marianne Imperor and Florence Babonneau
287
Covalent Attachment of Dye Molecules to the Inner Surface of MCM-41 Yven Rohlfing, Dieter Wohrle, Michael Wark, Giinter Schulz-Ekloff, Jiri Rathousky and Arnost Zukal
295
Preparation and Characterization of Metal-Chalcogenide/MCM-41 Complexes CM. Kowalchuk, Y. Huang and J.F. Corrigan
303
Studies on Immobilization of Co(II)-La(III) Schiff Base Complex in MCM-41 Binbin Fan, Ruifeng Li, Zhihong Liu, Jinghui Cao and Bing Zhong
311
The Use of Alkylchlorosilanes as Coupling Agents for the Synthesis of Stable, Hydrophobic, Surfactant Extracted MCM-48/VOx Catalysts P. Van Der Voort and E.F. Vansant
317
Epoxidation over Niobium and Titanium Grafted MCM-41 and MCM-48 Mesoporous Molecular Sieves M.P. Kapoor and Anuj Raj
327
Titanium Iso-propoxide Grafting on M41S Type Hosts : Catalytic and Adsorption Study KK Kang, CS. Byun and W.S. Ahn
335
Ternary Transition Metal Oxides within Mesoporous MCM-48 Silica Phases: Synthesis and Characterization R. Kohn, F. Brieler and M. Froba
341
The Inclusion of Polymeric Carbon in Channels of the Siliceous MCM-41 Mesoporous Molecular Sieve J. Hlavaty, L. Kavan, J. Rathousky and A. Zukal
349
IV. Synthesis of Other Nanoporous and Nanostructured Materials On the Way to New Nanoporous Transition Metal Oxides OlafMuth and Michael Froba
357
First Synthesis of Mesostructured Hexagonal Germanium Sulfides Using Gemini Surfactants Nadine Oberender and Michael Froba
367
Synthesis and Characterization of Mesostructured Molybdenum Sulfides with Intercalated Cationic Surfactants Jie-Sheng Chen, Ying Wang and Ru-Ren Xu
375
Synthesis and Characterization of Novel Mesostructured Tungsten Sulfides Charles (Chibiao) Liu, Amy Ferryman, Julia E. Fulghum and Songping D. Huang
383
The Mesopores Developed during Boronation of Zeolites p Chun Yang and Qinhua Xu
391
Mesostructured Clay Catalysts: a New Porous Clay Heterostructure (PCH) derived from synthetic saponite Mihai Polverejan, Yu Liu and Thomas J. Pinnavaia
401
Al-Modified Porous Clay Heterostructures with Combined Micro- and Mesoporosity P. Cool, J. Ahenach. O. Collart and E.F. Vansant
409
Mesoporous Synthetic Clays : Synthesis, Characterizationand Use as HDS Catalyst Supports K.A. Carrado, L. Xu, CL Marshall, D. Wei, S. Seifert, CA.A. Bloomquist
417
Techniques for Tailoring the Pore Structure of Si02-Ti02 Sol Pillared Clays H. Y. Zhu, Z. Ding and G.Q. Lu
425
Porous Smectite-type Materials Containing Catalytically Active Divalent Cations in Octahedral Sheets M Shirai, K. Aoki, Y. Minato, K. Torii and M. Aral
435
LDH-Surfactant Composite Nanoribbons P.C. Pavan, L.P. Cardoso, E.L. Crepaldi and J.B. Valim
443
Synthesis and Characterization of a New Sn-incorporated CoAl-layered Double Hydroxide (LDH) and Catalytic Performance of Co-spinel Microcrystallites in the Partial Oxidation of Methanol S. Velu and K. Suzuki
451
Construction Strategies for New Generation Micro-porous Solids Ian D. Williams, Stephen S-Y. Chui, Samuel M-F. Lo, Mingmei Wu, John A. Cha, Teresa S-C. Law, Herman H-Y. Sung, Fanny L-Y. Shek, Jenny L. Gao and Tolulope M. Fasina
459
Preparation Effects on Titania-sulfate Aerogel Morphology J. Mrowiec-Bialon, L. Pajak, A.B. Jarzebski and A J. Lachowski
467
Distribution of Pt Clusters in Si02 and Ti02 Nanotubes Michael Wark, Christina Hippe and Gilnter Schulz-Ekloff
475
Catalytic Formation of Carbon Nanotubes on Fe-loading Molecular Sieves Materials : An XPS Study N.-Y. He, C Yang, P.-F. Xiao, G.-H Wang, Y-J Zhao, Z.-H Lu and C-W. Yuan
483
V.
Characterization of Nanoporous Materials
Probing the Pore Space in Mesoporous Solids with NMR Spectroscopy and Magnetic Resonance Microimaging S.R. Breeze, S.J. Lang, A. V. Nosov, A. Sanchez, LL. Moudrakovski, CL Ratcliffe and J.A. Ripmeester
491
Characterization of Mesoporous Molecular Sieves : Differences between M41S and Pillared Layered Zeolites Wieslaw J. Roth, James C. Vartuli and Charles T. Kresge
501
Magnetic Resonance Microimaging Studies of Porous Petroleum Coke Eric B. Brouwer, Igor Moudrakovski, Keng H. Chung, Gerald Pleizier, John A. Ripmeester and Yves Deslandes
509
Effect of Pore Size on the Adsorption of Xenon on Mesoporous MCM-41 and on the '^^Xe NMR Chemical Shifts: a Variable Temperature Study Wen-Hua Chen, Hong-Ping Lin, Jin-Fu Wu, Sung-Jeng Jong, Chung-Yuan Mou and Shang-Bin Liu
517
What Does TEM Tell Us about Mesoporous Silica W. Zhou
525
Transmission Electron Microscopy - an Indispensable Tool for the Characterisation of M41S-type Materials Patricia J. Kooyman, Michel J. Verhoef and Eric Prouzet
535
SEM and TEM Investigations of Macroporous and Toroidal Mesostructured Transition Metal Oxides D. Antonelli and M. Trudeau
543
'H, ^H and ^^Si Solid State NMR Study of Guest Acetone Molecules Occupying the Zeolitic Channels of Partially Dehydrated Sepiolite Clay M.R. Weir, G.A. Facey and C Detellier
551
In-situ Small Angle X-ray Scattering (SAXS) Studies on the Formation of Mesostructured Aluminophosphate / Surfactant Composite Materials M Tiemann, M. Froba, G. Rapp and S.S. Funari
559
Thermogravimetric Characterization of Mesoporous Molecular Sieves Michal Kruk, Abdelhamid Sayari and Mietek Jaroniec
567
Self-consistent Determination of the Lamellar Phase Content in MCM-41 Using Xray Diffraction, Nitrogen Adsorption and Thermogravimetry Michal Kruk, Mietek Jaroniec, Yong Yang and Abdelhamid Sayari
577
Recent Advances in Adsorption Characterization of Mesoporous Molecular Sieves Mietek Jaroniec, Michal Kruk and Abdelhamid Sayari
587
Calculations of Pore Size Distributions in Nanoporous Materials from Adsorption and Desorption Isotherms Peter L Ravikovitch and Alexander V. Neimark
597
Determination of Pore Size Distribution of Mesoporous Materials by Regularization CO. Sonwane and S.K Bhatia
607
The Sorption of «-Butyl and /er/-Butyl Alcohols by Phenyl-Modified Porous Silica Claire M. Bambrough, Robert CT. Slade and Ruth T. Williams
617
Change of Reorientational-vibrational Relaxation upon Capillary Condensation in Silica Mesopores Hideki Tanaka, S. Inagaki, Y. Fukishima and Katsumi Kaneko
623
Characterisation of Microporous Materials by Dynamic Sorption Methods Frank Thielmann, David A. Butler, Daryl R. Williams and E. Baumgarten
633
Diffusion of High Molecular Weight Hydrocarbons in MesoStructured Materials of theMCM-41 Type D.S. Campos, M. Eic and M.L. Occelli
639
Modeling Single-Component Permeation Through A Zeolite Membrane from Atomic-scale Principles David S. Sholl
649
Adsorption and Transport of Polyatomic Species in One-dimensional Systems : Exact Forms of the Thermodynamic Functions and Chemical Diffusion Coefficient A.J. Ramirez-Pastor, F. Roma, A. Aligia, V.D. Pereyra andJ.L. Riccardo
655
Mechanical Strength of Micelle-Templated Silicas (MTS) Delphine Desplantier-Giscard, Olivier Collart, Anne Galarneau, Pascal Van Der Voort, Francesco Di Renzo and Franc^ois Fajida
665
Structural Analysis of Hexagonal Mesoporous Silica Films Produced from Triblock-Copolymer-Structuring Sol-Gel D. Grosso, A.R. Balkenende, P.A. Albouy and F. Babonneau
673
On Structure/Property Relations in Nanoporous Semiconductors of the Cetineitetype U. Simon, J. Jockel, F. Starrost, E.E. Krasovskii, W. Schattke, B. Marler, S. Schunk, M. Wark and H. Wellmann
683
Structural and Textural Properties of Zinc (Il)-Chromium (III) Spinel Oxides Prepared Using a Hydrotalcite-like Compound E.L. Crepaldi, P.C. Pavan, W. Jones andJ.B. Valim
691
New Porous Composite Material - Characterization and Properties A.N. Scian, M. Marturano and V. Cagnoli
701
Stabilized Cluster Formation of Supercritical Xe in Carbon Nanopores M. Aoshima, T. Suzuki and K. Kaneko
711
VI. Applications of Nanoporous and Nanostructured Materials Adsorption of Halocarbons in Nanoporous Materials: Current Status and Future Challenges C Mellot Draznieks, J. Eckert and A.K. Cheetham
721
Synthesis and Applications of Functionalized Nanoporous Materials for Specific Adsorption J. Liu, G.E. Fryxell, S. Mattigod, T.S. Zemanian, Y. Shin and L.-Q. Wang
729
Non-electrostatic Surfactant Assembly Routes to Functionalized Nanostructured Silica: Prospects for Environmental Applications L. Mercier
739
The Use of Mesoporous Silica in Liquid Chromatography 747 Karl W. Gallis, Andrew G. Eklund, Sara T. Jull, James T. Araujo, Joseph G. Moore and Christopher C. Landry Pressure Swing Adsorption of Butanone on Silica MCM-41 S. Namba, M. Aikawa, K. Takeuchi, D. Yomoda, Y. Inoue, S. Aoki and J. Izumi
757
Mercury-Sorption Characteristics of Nanoscale Metal Sulfides G.A. Moore, P.J. Martellaro and E.S. Peterson
765
New Chiral Hybrid Organic-Inorganic Mesoporous Materials for Enantioselective Epoxidation D. Brunei, P. Sutra and F. Fajula
773
The Direct Enantioselective Synthesis of Diols from Olefins using Hybrid Catalysts of Chiral Salen Cobalt Complexes Immobilized on MCM-41 and Titaniumcontaining Mesoporous Zeolite Geon Joong Kim, Dae Woon Park, Wha Seung Ahn and Dong Wha Park
781
Nano-Clusters, Enantioselective Catalysis and Molecular Recognition Contrast Agents in MCM-41 - Part I Douglas S. Shephard
789
Nano-Clusters, Enantioselective Catalysis and Molecular Recognition Contrast Agents in MCM-41 - Part II Douglas S. Shephard
797
Photoactive Characteristics of Rhenium Complex Encapsulated in AlMCM-41 by Ion-exchange Method S.-E. Park, KM. Sung-Suh, D.S Kim and J Ko
807
Physico-chemical and Catalytic Properties of MCM-41 Mesoporous Molecular Sieves Containing Transition Metals (Cu, Niand Nb) M Ziolek, I. Nowak, I. Sobczak, A. Lewandowska, P. Decyk and J. Kujawa
813
Activity Enhancement of Mesoporous Silicate FSM-16 by Metal Ion-exchange and Sulfiding with Hydrogen Sulfide for Acid-catalyzed Reactions M Sugioka, L. Andalaluna and J.K.A. Dapaah
823
Application of Disordered Mesoporous Molecular Sieve KIT-1 as a Support for Energy/environmental Catalysts S.Y. Ryu, as. Byun, N.K. Kim, D.H. Park, W.S. Ahn, JM. Ha and K.J. Park
831
Radical Type Catalytic Sites on Mesoporous Silica T. Hattori, T. Ebigase, Y. Inaki. H. Yoshida and A. Satsuma
837
Tungstate and Molybdate Exchanged Layered Double Hydroxides (LDHs) as Catalysts for Selective Oxidation of Organics and for Bleaching Bert F. Sels, Dirk E. De Vos and Pierre A. Jacobs
845
Mediating Effect of CO2 in Base-Catalysis by Zeolites Tawan Sooknoi and John Dwyer
851
Effective Sol-gel Adsorbents of Water Vapor Prepared Using Ethyl Silicate 40 as a Silica Precursor J. Mrowiec-Bialon, A.I. Lachowski, M. Kargol, J.J. Malinowski andA.B. Jarzebski
859
Photochromism of an Azobenzene in a Nanoporous Silica Film M. Ogawa, J. Mori and K Kuroda
865
Silica-CTAB-Water Phase Diagram at 150°C: Predicting Phase Structure by Artificial Neural Network Y. Yang, L. Belfares, F. Larachi, B. Grandjean and A. Sayari
871
Author Index
879
Subject Index
885
PREFACE
The first symposium on Access in Nanoporous Materials was held in Lansing, Michigan on June 7-9, 1995. The five years that have passed since that initial meeting have brought remarkable advances in all aspects of this growing family of materials. In particular, impressive progress has been achieved in the area of novel self-assembled mesoporous materials, their synthesis, characterization and applications. The supramolecular selfassembly of various inorganic and organic species into ordered mesostructures became a powerful method for synthesis of mesoporous molecular sieves of tailored framework composition, pore structure, pore size and desired surface functionality for advanced applications in such areas as separation, adsorption, catalysis, environmental cleanup and nanotechnology. Over 2000 papers have been published on self-assembled mesoporous materials since 1992 and more than 90% of these papers have appeared in just the last five years. The growth in this area of materials research has been truly remarkable. In addition to mesostructured metal oxide molecular sieves prepared through supramolecular assembly pathways, clays, carbon molecular sieves, porous polymers, sol-gel and imprinted materials, as well as self-assembled organic and other zeolite-like materials, have captured the attention of materials researchers around the globe. Clays, zeolites and solgel materials are still very popular because of their extensive and expanding applications in catalysis and separation science. Novel carbons and polymers of ordered porous structures have been synthesized. There are almost unlimited opportunities in the synthesis of new organic materials of desired structural and surface properties via self-assembly or imprinting procedures. The contents of the current volume presents a sampling of more than 150 oral and poster papers delivered at the Symposium on Access in Nanoporous Materials II held in Banff, Alberta on May 25-28, 2000. The selected papers cover the three main themes of the symposium: (i) synthesis of mesoporous silicas, framework-modified mesoporous silicas, and surface-modified mesoporous silicas, (ii) synthesis of other nanoporous and nanostructured materials, and (iii) characterization and applications of nanoporous materials. About 70% of the papers are devoted to the synthesis of siliceous mesoporous molecular sieves, their modification, characterization and applications, which represent the current research trend in nanoporous materials. The remaining contributions provide some indications on the future developments in the area of non-siliceous molecular sieves and related materials. Although the present book does not cover all topics in the area of nanoporous materials, it reflects the current trends and advances in this area, which will certainly attract the attention of materials chemists in the 21^^ Century.
January 20, 2000
Abdel Sayari Mietek Jaroniec Thomas J. Pinnavaia
ORGANIZING COMMITTEE Chairman A. Sayari
Laval University, Quebec, Canada
Vice-Chairmen M. Jaroniec T.J. Pinnavaia
Kent State University, OH, USA Michigan State University, East Lansing, MI, USA
Members B. Grandjean S. Hamoudi M. Kruk F. Larachi W. Zhang
Laval University, Quebec, Canada Laval University, Quebec, Canada Kent State University, OH, USA Laval University, Quebec, Canada Michigan State University, East Lansing, MI, USA
INTERNATIONAL ADVISORY COMMITTEE C.J. Brinker D. Brunei C.R.A. Catlow M. Camblor K. Chao C.G. Coe A. Corma C. Detellier H.C. Foley M. Froba S.Inagaki K. Kaneko S. Komameni C.T. Kresge R. Kumar K. Kuroda J.A. Lercher J. Liu Th. Maschmeyer J. Olivier E. Prouzet J.R. Ripmeester D.M. Ruthven M. Stocker G.D. Stucky T. Tatsumi K.K. Unger
University of New Mexico, NM, USA Ecole Nationale Superieure de Chimie de Montpellier, France The Royal Institution of Great Britain, London, UK Universidad Politecnica de Valencia, Spain Tsinghua University, Hsinchu, Taiwan Air Products and Chemicals, Inc., Allentown, PA, USA Universidad Politecnica de Valencia, Spain Ottawa University, Canada University of Delaware, DE, USA University of Hamburg, Germany Toyota Central R&D Laboratories, Inc., Nagakute, Japan Chiba University, Japan The Pennsylvania State University, PA, USA The Dow Chemical Co., Midland, MI, USA National Chemical Laboratory, Pune, India Waseda University, Tokyo, Japan Technische Universitaet Muenchen, Germany Pacific Northwest National Laboratory, Richland, WA, USA Delft University of Technology, The Netherlands Micromeritics, Inc., GA, USA Ecole Nationale Superieure de Chimie de Montpellier, France National Research Council, Ottawa, Canada University of Maine, USA SINTEF, Oslo, Norway University of California, Santa Barbara, USA Yokohama National University, Yokohama, Japan Johannes Gutenberg Universitaet, Mainz, Germany
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
A novel approach to polymer-template mesoporous molecular sieves Kai Schumacher ^, Sabine Ranker ^, Klaus K. Unger ^, Ralph Ulrich ^, Alexander Du Chesne ^, Hans W. Spiess ^, Ulrich Wiesner ^ ^ Institut fuer Anorganische Chemie und Analytische Chemie Johannes Gutenberg Universitaet Duesbergweg 10-14, 55099 Mainz, Germany ^ Max-Planck-Institut fuer Polymerforschung Postfach 3148, 55021 Mainz, Germany ^ Materials Science & Engineering, Cornell University 320 Bard Hall, Ithaca, N.Y. 14853-1502, USA
A novel synthesis route was developed to produce spherical silica particles. The synthesis is based on a modified Stoeber method and the room-temperature synthesis of MCM 41Smaterials applying tetraethoxysilane, alcohol, water, ammonia and homopolymers as template. The specific surface area, the specific pore volume and the average pore diameter were varied in the following ranges: 5 - 1,000 mVg; 0.1 - 1.0 cmVg and 2 - 5 0 nm. With respect to catalytic applications hetero-atoms e.g. Al were incorporated into the silica framework.
1. INTRODUCTION The pioneering work on the synthesis of ordered mesoporous silicas of the type M41S by Mobil researchers has led to the design of periodic porous silicas with pore sizes between 2 and 10 nm using low-molecular weight templates e.g. n-hexadecyltrimethylammonium bromide [1]. For the use of polymer as a template several pathways are reported in the literature. Pinnavaia et al. [2] prepared mesoporous molecular sieves using polyethylenoxide as a surfactant. Wiesner et al. [3] used block copolymer phases for the synthesis of aluminosilicate mesostructures with different morphologies. Triblock copolymers were employed as structure-directing agents by Stucky [4] and his co-workers resulting in hexagonal ordered mesoporous silicas. Mercier et al. sythesized organically functionalized mesostructered materials using non-ionic polyethylenoxide surfactant [5]. MSU-X mesoporous silicates prepared from non-ionic polyethylenoxide were reported by Bagshaw [6]. We have combined these approaches of pore size engineering with another synthesis route where uniform spherical particles are obtained in the micron and submicron size range [7]. The latter procedure allows one to adjust the particle size and size distribution of the silica particles by the reaction conditions.
2. EXPERIMENTAL 2.1 Synthesis The template was dissolved in a mixture of alcohol and water at room temperature. An appropriate amount of ammonia was added. Tetraethoxysilane was finally added and the reaction mixture was left or stirred. After 8 hours the resulting solid was recovered by filtration, washed with deionized water and dried in air at ambient temperature. The template was removed by calcination at 823 K for 6 hours or by solvent extraction. 2.2 Characterisation Nitrogen sorption measurements were performed on a Quantachrome Autosorb 6B (Quantachrome Corporation, Boynton Beach, FL, USA). All samples were degassed at 423 K before measurement for at least 12 hours at 10'^ Pa. Mercury-porosimetrie has been measured on a Porosimeter 2000 (Carlo Erba Instruments) Scanning electron micrographs were recorded using a Zeiss DSM 962 (Zeiss, Oberkochen, Germany). The samples were deposited on a sample holder with an adhesive carbon foil and sputtered with gold. The ^^Si Magic-Angle Spinning (MAS) spectra were recorded on a Bruker ASX-500 spectrometer with an ^^Si frequency of 99.35 MHz. The silica spectra were recorded at spinning speeds of 5 kHz using a 45° angle pulse and a recycle delay of 120 s. The ^^Al Magic-Angle Spinning (MAS) spectra were recorded on a Bruker ASX-500 spectrometer with an Al frequency of 130.32 MHz. The aluminium spectra were recorded at spinning speeds of 14 kHz using a small tip angle pulse (1.2 Q) and a recycle delay of 100 ms. X-ray fluorescence analyses were performed on a Philips PV1400 x-ray fluorescence spectrometer using fused sample/polyvinylalcohol disks. FTIR spectra were collected using a Mattison Instruments Galaxy 2030 Series FTIR spectrometer (Mattison Instruments, Madison, USA). UV/VIS spectra were obtained from a Zeiss Spectralphotometer DM 4 (Zeiss, Oberkochen, Germany).
3. RESULTS AND DISCUSSION Based on the original Stoeber synthesis which leads to nonporous silica particles we developed a novel route to synthesize mesoporous silica particles. All syntheses were carried out in an homogeneous solution of alcohol/water/ammonia. To control the porosity of the obtained particles, polymers were used as templates. The adsorption and desorption isotherms of nitrogen on each sample show the typical type IV isotherm according to the lUPAC nomenclature [8]. At the adsorption branch, the adsorbed amount increased gradually with an increase in relative pressure by multilayer adsorption. A sudden uptake of the adsorbed amount was observed over a narrow range of relative pressure (p/po) depending on the pore size. The isotherms show a HI-hysteresis. For larger pores mercury-intrusion was used to determine the pore size and the pore volume. The porosity parameters could be varied over the whole mesopore seize range. Typical parameters of some examples are given in the following table (Table 1).
relative pressure p/pg Figure la: Nitrogen isotherm of a polymer templated material at 77 K (sample 5)
Pore diameter [nm]
Figure lb: Pore size distribution of a polymer templated material calculated from the isotherm in figure la (BJHoes). Scanning electron image of the samples prepared by new synthesis are shown in figure 2. It is seen that the particles are uniform in size and do not agglomerate. The samples have also been characterised by means of solid state NMR. ^^Al chemical shifts have been shown to be sensitive to the coordination number and ^^Si chemical shifts are strongly influenced by nearest neighbour effects.
Table 1 Properties of polymer templated silicas Sample 1 2 3 4 5
As (BET) M'/g 35 83 255 534 220
Vp (Gurvitch) cm^/g 0.43 0.48 0.89 0.54 0.66
Pd (BJH) N2 -sorption, nm 55.1 23.2 70.5 3.9 9.0
Pd Hg-porosimetry, nm 39 16.2 60.1 -
Figure 2: SEM of polymer templated material of an average particle diameter of 200 nm Uncalcined samples show three different ^^Si-NMR peaks which can be assigned to Q^, Q^ and Q silicon species. After calcination Q"* environments are formed at the expense of Q^ and Q . Figure 3 shows two ^^Si-NMR spectra of a sample before and after calcination. For catalytic application it is necessary to incorporate hetero-atoms into the silica framework. Several samples have been synthesised using different aluminia precursors. The metal content was determined by X-ray fluorescence analysis, UV-VIS spectra, IR spectra and solid state NMR spectroscopy, respectively. X-ray fluorescence analysis provides information about the metal content of the samples. By variation of the metallic precursor concentration the metal content of the product could be enhanced up to 10 % w/w. Figure 4 shows the Al NMR spectrum of a calcined sample. There are three peaks visible, a peak due to octahedrally coordinated aluminum (Oh), a peak due to tetrahedrally coordinated aluminum (Td) and a peak in between due to highly distorted tetrahedral sites. The tetrahedrally coordinated aluminum can be assumed to be incorporated into the aluminosilicate network while the octahedrally coordinated aluminum is occluded in the pores or exists as an amorphous by product.
Q4 SiO
k
OSi
Q3 SiO
a) before
/
/
OSi Q2
SiO
y
o
O ' .84 ' -88 ' - « ' - «
^O? " -^^
-^*
;j/3
J«
ja?
J»
/1\
-^» -133
b) after
o
/ .84
-S8
-93
-96
V -tOa
-iOt -iCB
\-ia
-lU
-IX
-13t
-tS
-132
Figure 3: ^^Si spectrum a) before and b) after calcination
a
200
160
120
80
40
0
-40
-80
-120
Figure 4: ^^Al NMR spectra of a calcined sample It should be mentioned that IR and UV-VIS spectra provide no evidence on the valency state of heteroatoms. However, the results clearly indicate that the metals are incorporated into the Si-framework during the synthesis.
4. CONCLUSION A novel synthesis has been introduced to control the morphology and the porosity of micron size silica particles. Homo-polymers were used as templates, tetraethoxysilane as a silica source, alcohol/water mixtures as solvents and ammonia as a catalyst. The particle size could be adjusted in a range of 100 - 250 nm and the pore diameter between 2 - 5 0 nm. Suspensions of these spherical particles are used for spray drying to produce large agglomerates which are used as packings for various separation techniques such as High Performance Liquid Chromatography (HPLC) or Supercritical Fluid Chromatography (SFC). They also serve as supports for catalysts.
ACKNOWLEDGEMENT The authors would like to thank the Bundesministerium fiier Bildung, Wissenschaft, Forschung und Technologic, Bonn (BMBF No. 03D0068B5) for financial support and Merck KgaA, Darmstadt for supplying chemicals. Special thanks to Dr. B. Mathiasch for his help with XFA, IR and UV-VIS measurements. Prof K.S.W. Sing for discussing the nitrogen sorption isotherms, S. M. De Paul for helpful discussions concerning the NMR spectroscopic measurements and Dr. K. F. Krebs, Merck KgaA, Darmstadt for mercury porosimetry measurements. REFERENCES 1. C.T.Kresge, M.E.Leonowicz, W.J.Roth, J.C.Vartuli, J.S.Beck, Nature, 359, 1992 2. S.AiBagshaw, E.Prouzet, T.J.Pinnavaia et al.. Science, 269, 1995 3. M.Templin, A.Franck, A.Du Chesne, H.Leist, Y.Zhang, R.Ulrich, V.Schandler, U.Wiesner, Science, 278, 1997 4. D.Zhao, J.Feng, Q.Huo, N.Melosh, G.Fredrickson, B.Chmelka, G.Stucky, Science, 279, 1998 5. R.Richer, L.Mercier, Chemical Communications, 16, 1998 6. S.A.Bagshaw, Chemical Communications, 3, 1999 7. W.Stoeer, A.Fink, E.Bohn, Journal of Colloid and Interface Science, 26, 1968 8. S.J.Gregg, K.S.W.Sing, Adsorption, Surface Area and Porosity, Academic Press, 1982, sec. Edition
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
Synthesis of uniform and stable millimeter-sized mesoporous silica ropes by the addition of polymer and ammonia hydrothermal treatment Hong-Ping Lin*, Lu-Yi Yang*'^ Chung-Yuan Mou^ Huang-Kuei Lee'' and Shang-Bin Liu"* "Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei, Taiwan 106, R.O.C. ''Institute of Materials Science and Manufacturing, Chinese Culture University, Taipei, Taiwan 111, R.O.C. ""Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, R.O.C. The addition of water-soluble polymers such as polyethylene oxide (PEO) or polyvinyl alcohol (PVA) into the synthetic mixture of the C JMAX-HNO3-TEOS-H2O system (n = 16 or 18; X = Br or CI) under shear flow is found to promote uniformity and elongation of rope-like mesoporous silica. The millimeter-scaled mesoporous silica ropes are found to possess a three-level hierarchical structure. The addition of water-soluble polymer does not affect the physical properties of the silica ropes. Moreover, further hydrothermal treatment of the acidmade material under basic ammonia conditions effectively promotes reconstruction of the silica nanochannels while maintaining the rope-like morphology. As a result, a notable enhancement in both thermal and hydrothermal stability is foimd.
1. INTRODUCTION The M4IS molecular sieves, first disclosed by Mobil researchers in 1992 [1], are primarily synthesized via an alkaline route, which is known to produce mesoporous materials with highly condensed and compact structures [2-5]. However, since the quaternary ammonium surfactants (S^) and negative-charged silica oligomers (I) are organized by strong S^I' electrostatic interactions under alkaline conditions, the size and morphology of the synthesized mesoporous products can not be easily tailored. Owing to the increasing interests and potential applications in optical and electronic devices for these mesoporous materials, there exists an increasing demand in synthesizing large-sized mesoporous aluminosilicates. Consequently, another synthetic route, namely the acidic route (pH < 2) must be invoked [6]. In this case, the dominant forces between surfactant molecules and silica oligomers are weak electronic or hydrogen bonding interactions (for example, S^f, S'^X" T or S'^X" I^). The existence of such weak interactions thus facilitates the tailoring of the size and morphology of the synthesized products. Many interesting morphologies and large-sized mesoporous materials have been synthesized by varying either the physical (e.g., shear flow or
electric/magnetic field) or chemical (e.g., properties of the source materials or additives) parameters [7-11]. Since most of the synthesized aluminosilicate mesoporous materials are organic surfactant templated, their morphology and nanostructure are mostly governed by the surfactant micellar properties. In the acid route especially, there must exist counterions (X) between quaternary ammonium surfactants (S"") and silica framework ( r or 1°). Thus, factors that influence the micellar structure of S^X' are crucial to the structural properties of the synthesized mesoporous products. In this paper, we report the effects of applying shear flow [12-14] and the addition of water-soluble polymer [15] for promoting the formation of millimeter-sized mesoporous silica ropes. In addition, a post-synthesis-ammonia hydrothermal treatment was provided to restructure the periodic order and improve the thermal and hydrothermal stability of the acid-made mesoporous materials without altering the original rope morphology. The combination of neutral polymer with cationic surfactant takes advantage of both the S*X r and S^I° templating synthesis of mesoporous silica. The former gives strong interaction and ion exchange ability and the latter S°I° interaction leads to long fibers [16,17].
2. EXPERIMENTAL The millimeter-sized, rope-like mesoporous silica was synthesized via the acid route [10]. Organic surfactants, namely CjgTMACl (Tokyo Chemical Industry) or CjgTMAB (Acros), were dissolved with proper amount of water followed by the addition of polymers (PEO or PVA) and nitric acid (Acros). Then, tetraethyl orthosilicate (TEOS, Acros) was added to the clear solution under stirring at temperature of 40 or 32 °C. The molar ratio of the resultant gel solution is: 1 surfactant: (5.0-10.0) TEOS : (20.0-40.0) HNO3 : (0-30.0) PEO repeating unit: (700 - 1500) H2O. Finally, the gel solution of pH < 1 was allowed to stir for 8-48 h. The final products were filtered, washed with water and then dried in air. To improve the ordering of the mesostructure, 1.0 g of the dried acid-made sample was first mixed with 50.0 g 1.0 M NH4OH aqueous solution (pH --11.0), then sealed in an autoclave and maintained at 100 °C for 2 days. The resultant sample was then calcined in air at 560 °C for 6 h to remove the organic directing agents. X-ray powder diffraction (XRD) was performed on a Scintag XI diffractometer using Cu K„ radiation (k = 0.154 nm). Nj adsorption-desorption isotherms were obtained at 77 K on a Micrometrics ASAP 2010 apparatus. Scanning electron microscopy (SEM) was done on a Hitachi S-800 machine operating at an accelerating voltage of 20 keV. The ultra-thin transmission electron micrographs (TEM) were obtained on a Hitachi H-7100 operated at 100 keV. Elementary analysis data were taken from the Perkin-Elmer 2400. The FT-IR spectra were taken on the Perkin Elmer 1600. The solid state MAS ^^Si NMR spectra were recorded at room temperature on a Bruker MSL-300P spectrometer.
3. RESULTS AND DISCUSSION SEM micrographs of the mesoporous materials prepared from CigTMAB-HNOj-TEOS-HjO composition, with and without the addition of PEO-6000 polymer, are shown in Fig. 1. In the absence of this polymer additive, mesoporous silica with gyroidal particulate morphology (Fig. lA) was formed even under the shear flow conditions. On the other hand, the addition of this polymer promotes the formation of mesoporous silica materials with millimeter-sized, rope-
like morphology (Fig. IB). The assembly and structure of the silica ropes can be visualized more explicitly by the magnified SEM and TEM micrographs. The latter was obtained by microtome method. It is obvious that the silica ropes are bundles of hexagonal micro-sized silica fibers (Fig. IC). The microtome TEM micrograph in Fig. ID provides a sliced, crosssectional view of the silica fibers, showing that the silica fiber consists of hexagonal arrays of nanochannels parallel to the long axis. These mesoporous material ropes are therefore of three-level hierarchical structure: millimeter (in length) rope, micrometer (in diameter) fiber and nanometer pores/channels. The addition of water-soluble polymers during synthesis thus promotes the formation of second-order hierarchical structure of the mesoporous silica.
Figure 1. The SEM and microtomed TEM micrographs of the mesoporous materials obtained from C,6TMAB-HN03-TEOS-H20 mixture at 32 °C, (A) without and (B) with the addition of PEO-6000 polymer. PEO repeating unit/CjJMAB = 10. (C) Large-magnification SEM of the rope end of the sample in B (D) the cross section TEM micrographs of the silica rope in B. As our previous report, the silica ropes also can be directly obtained from a CigTMAClHNO3-TEOS-H2O mixture, but the length is not uniform and less than 2.0 mm. When PEO-
10 6000 polymer is added, the mesoporous silica rope becomes uniform (Fig. 2A) and the longest silica ropes are extended to as long as ca. 10 mm. It may be concluded that a longer surfactant chain length (i.e. greater hydrophobicity) therefore promotes the formation of lengthened micelles and thus longer silica ropes [10]. Closer examination of the silica ropes (Fig. 2B) confirmed that the constituted micrometer silica fibers are closely paced and have little defect cavities. Thus the addition of the proper amount of polymer with longer chain length surfactants can provide a way for synthesizing millimeter-sized silica fibers, that do not have packing defects within them. This kind of mesoporous silica fiber might be useful for optical or electronic devices [18].
Figure 2. (A) SEM micrograph of the mesoporous silica ropes prepared by adding PEO-6000 into the C,8TMAC1-HN03-TE0S-H20 mixture at 40 T , PEO repeating unit/C,JMACl = 5.2 (B) a magnified cross-sectional view of the silica fiber. Similar mesoporous silica ropes were obtained when different types of water-soluble polymers (such as PEG and PVA) were added into the C JMAX-TEOS-HNO3-H2O synthetic mixture. The physical properties of these mesoporous materials synthesized from the CnTMAX-TEOS-HNOj-polymer-HjO systems were listed in Table 1. Both the d,oo spacing and the pore size of the mesoporous silica products are found to increase only with the chain length of the surfactant used, and are independent of the molecular weight and type of the polymer additives. All samples have a similar B.E.T. surface area of ca. 1,000 mVg, indicating that while the addition of water-soluble polymers play a prominent role in tailoring the morphology of the mesoporous silica, it has little influence on their structural and physical properties. It is known [19] that the addition of water-soluble polymers promotes the formation of silica ropes. This can be attributed to the flexibility of the polymers, which may be readily aligned by the applied shearing flow. As a result, these flow-aligned polymers assist the silicasurfactant micelles (S^X r ) to align co-axially along the direction of the flow. The silica species then gradually condense on the surface of the aligned micelles and self-assemble to form the millimeter-sized silica ropes. Moreover, the addition of polymer also increases the viscosity of the gel solution and thus forms a homogenous flow field which in turn favor the formation of the long silica ropes.
11 Table 1 Physical properties of the mesoporous silica ropes synthesized from the CnTMAX-TEOSHNO^-polymer-H^O system BET n BJH d,oo (nm) Surface Surfactants Polymer Pore size area (CJMAXy Additives'' As-Synthesized Calcined (mVg) 1182 C.JMAB 2.15 3.98 3.43 1182 2.15 C.JMAB PEO-2000 3.98 3.43 2.09 1163 CJMAB PEO-6000 3.39 3.91 1143 CJMAB 2.08 PEO-100000 4.02 3.42 1032 2.13 CJMAB 4.02 PEO-300000 3.39 1083 2.18 C,JMAC1 PVA 4.03 3.45 1058 2.63 CJMACl 4.36 3.83 1058 2.63 CJMACl PEO-6000 4.36 3.83 982 2.70 C.JMACl 4.41 PEO-100000 3.90 2.68 978 CJMACl PVA 4.40 3.87 'The synthetic temperature for the (C,8, C,6)TMAX-HN03-TEOS-polymer-H20 systems (X = Br or CI) were at 40 and 32 °C, respectively. ''Different types of polymers with varied molecular weights (as indicated by the index numbers) were examined, namely; polyethylene oxide (PEO); polyvinyl alcohol (PVA). The greater index number would indicate a larger molecular weight. ""Calculated from the adsorption curve of the N2 adsorption/desorption isotherm.
*i
^ >* u a 1
X
j
900
B
d
•A
I Sample 11. in 100 "C water for 12 h jg3i^/^M^/^^^^^^^^»^^^^H
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Sample I. in 100 °C water for 3h
•m—After ammonia treatment —^
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3
4
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6
7
8
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0
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Figure 3. (A) XRD patterns and (B) N2 adsorption/desorption isotherms of the calcined mesoporous silica ropes obtained from the C,6TMAB-HNO3-TEOS-(PEO-6000)-H2O system before and after the post-synthesis ammonia treatment at 100 °C and a further hydrothermal stability test in water at 100 °C.
12 As indicated by XRD patterns, there exist just 2-3 broad peaks in the calcined acid-made materials (Fig. 3A). Moreover, the Nj adsorption/desorption isotherm shown in Fig. 3B, the calcined acid-made mesoporous silica indeed possesses a broad capillary condensation at the partial pressure p/p^ of ca. 0.2-0.4, indicating a broad pore size distribution with a FWHM ca. 1.0 nm calculated from the BJH method. This is attributed to the occurrence of partial collapse of the mesostructure during the high temperature calcination. The hexagonal structure completely collapsed when subjected to further hydrothermal treatment in water at 100 °C for 3 h. Mesoporous silica materials synthesized from the acid route are commonly believed to be less stable than those from the alkaline route [6, 7]. To improve the meso-structural order and stability of the mesoporous silica ropes, a postsynthesis ammonia hydrothermal treatment (at 100 °C) was invoked. As indicated by the XRD profile in Fig. 3A, 4-5, sharp features are readily observed in ammonia hydrothermal treated samples. Moreover, after the post-synthesis ammonia treatment, the sample also possesses a sharp capillary condensation at p/po~0.35(Fig. 3B) corresponding to a much narrower BJH pore size distribution of ca. 0.12 nm (at FWHM). In other words, the mesostructures are not only more uniform but also more stable when subjected to the post-synthesis treatment. The morphology of the silica ropes remained unchanged during the ammonia hydrothermal process. The mesostructures remain intact under hydrothermal at 100 °C in water even for extended reaction time (> 12 h). To further characterize the effect of the ammonia hydrothermal treatment, we compared elemental analysis data and IR spectra before and after ammonia hydrothermal treatment to quantitatively disclose the role of counterion between the silica framework and surfactants. In Table 2, the N/C molar ratio of the mesoporous materials prior to the ammonia hydrothermal treatment is nearly twice of that after the treatment. Moreover, the IR band at 1383 cm', which arises from the NO3' stretch bending mode, completely disappears after ammonia hydrothermal treatment [20]. These results verify that the existence of nitrate counterion (the nitrate/surfactant » 1) between surfactant molecules and silica framework in the acid-made mesoporous materials. The bridging counterion NO3' was completely removed after ammonia hydrothermal treatment.
Table 2 The elemental analysis data and IR adsorption band at 1383 cm' of the mesoporous silica ropes synthesized from the C JMAX-TEOS-HNO3-PEO-6OOO-H2O system before and after the ammonia hydrothermal treatment. Q3/Q4 ratio Samples^ IR band N/C ratio' at 1383 cm' C16-UAT C16-AT CI8-UAT CI8-AT
0.116 0.060 0.098 0.051
Presence Vanish Presence Vanish
0.80 0.63 0.96 0.59
"UAT: before the ammonia hydrothermal treatment; AT: after the ammonia hydrothermal treatment at 100 °C. The N/C ratio of CjJMAB and C.gTMACl is 0.061 and 0.055
13 A possible mechanism of the ammonia hydrothermal treatment for the acid-made sample is shown below. The predominant interaction between the silica wall and the surfactant of the acid-made products is the weak hydrogen bond interaction through an intermediate counterion (i.e. NOj"). Such weaker interaction eases the removals of organic template by hot water or organic solvent [6]. Thus, when the acid-made materials are subjected to the ammonia hydrothermal treatment, the interactions between the surfactant and silicate framework would be transformed as: \
NH^OH Si-OH NO,-(5>--—
Weak hydrogen bond interaction
\ •
— i
Strong electrostatic interaction
The stronger electrostatic interactions between the negatively charge silicate and the surfactant's cationic headgroup prevent the organic surfactant from being entirely eliminated by hot water during the hydrothermal reaction. In Table 2, the Q3/Q4 ratio decreases upon the ammonia hydrothermal treatment. Accordingly, the treatment would promote the less condensed silica species in acid-made materials to subsequently condense into more wellordered silica structures, which is similar to that directly prepared from the base route. Such ammonia hydrothermal treatment is also applicable to mesoporous silica materials synthesized under different conditions, e.g., different acid source or temperature. This and other interesting issues will be reported later. 4. CONCLUSIONS We have demonstrated that a mesoporous silica with high-level hierarchical rope-like morphology can be synthesized under acidic conditions and in the presence of water-soluble polymers. The present work should provide a pathway in realizing the biomimetic high-order hierarchical structures for the other porous materials. Further enhancement of the product size and/or the hierarchical structure order could, in principle, be manipulated by the other physical or chemical parameters, e.g. variations in the rheology of the water solutions, application of homogeneous shear flow or increase the chain length of surfactant etc. The post-synthesis ammonia hydrothermal treatment is found to effectively enhance the ordering of the mesostructures, and the thermal and hydrothermal stability of the mesoporous silica synthesized via the acid route. The synthesis of such stable, ordered, millimeter-sized, mesoporous silica ropes should have some potential applications in nanoporous solid templates, wave-guides or other optical/electrical devices.
ACKNOWLEDGMENTS The authors wish to thank Mr. Chin-Yuan Tang for technical assistance in SEM and TEM microtome measurements and thank Profs. Soofm Cheng and Ben-Zu Wan for helpful discussions. This research has been partially supported by a grant from the Chinese Petroleum Corporation (87-S-032) and by the National Science Council, ROC. (NSC88-2113-M-002-027 to CYM; NSC88-2113-M-001-008 to SBL).
14 REFERENCES 1. J.S. Beck, J.C. Vartuli, WJ. 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. 2. (a) C. F. Cheng, W. Z. Zhou, J. Klinowski, Chem. Phys. Lett., 263 (1996) 247; (b) W. Z. Zhou, J. Klinowski, Chem. Phys. Lett., 292 (1998) 207. 3. M. Kruk, M. Jaroniec, A. Sayari, J. Phys. Chem. B, 103 (1999) 4590. 4. (a) J.M. Kim, J. H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem., 99 (1995) 16742; (b) R. Ryoo, S. Jun, J. Phys. Chem. B, 101 (1997) 317. 5. C.Y. Chen, H.X. Li and M. E. Davis, Microporous Mater., 2 (1993) 17. 6. Q. Huo, S. L Margoleses, U. Ciesla, R Feng, D. E. Gier, R Sieger, B. R Chmelka, R. Leon, R M. Petroff, R Schuth, G.D. Stucky, Nature, 368 (1994) 317. 7. (a) S. Mann and G. A. Ozin, Nature, 382 (1996) 313. (b) H. Yang, N. Coombs, L Sokolov and G. A. Ozin, Nature, 381 (1996) 589. 8. S. Schacht, Q. Hou, LG. Voigt-Martin, G.D. Stucky, F, Schuth, Science, 273 (1996) 768. 9. R T. Tanev, Y. Liang and T.J. Pinnavaia, J. Am. Chem. Soc, 119 (1997) 8616. 10. H. R Lin, S. B. Liu, C.Y. Mou and C.Y. Tang, Chem. Commun., 583 (1999). 11. D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. R Chmelka and G. D. Stucky, Science, 297 (1998) 548. 12. M. Linden, S. Schacht, R Schuth, A. Steel and K.K. Unger. J. Porous Mater., 5 (1998) 177. 13. H. W. Hillhouse, T. Okubo, J. W. V. Egmond and M. Tsapatsis, Chem. Mater., 9 (1997) 1505. 14. K.J. Edler, RA. Reynolds, A.S. Brown, T. M. Slaweck, J.W. White, J. Chem. Soc, Faraday Trans., 94 (1998) 1287. 15. R J. Bruinsma, A.Y. Kim, J. Liu, S. Baskaran, Chem. Mater., 9 (1997) 2507. 16. Q. Huo, D. Zhao, J. Feng, K. Weston, S.K. Buratto, G.D. Stucky, S. Schacht and R Schuth, Adv. Mater., 9 (1997) 974. 17. R Schmidt-Winkel, R Yang, D. L Margolese, B. R Chmelka and G. D. Stucky, Adv. Mater., 11(1999)303. 18. R Marlow, M. D. McGehee, D. Zhao, B. R Chmelka, G. D. Stucky, Adv. Mater., 11 (1999) 632. 19. C. A. Finch (eds.). Industrial Water Soluble Polymer, Hartnolls Ltd, Bodmin, Cornwall, UK., 1996. 20. A. Corma, Chem. Rev., 97 (1997) 2373.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
15
The Synthesis and Characterization of Mesoporous Molecular Sieves MCM-41 with Interconnected Channels Hong-Ping Lin\ She-Tin Wong^ Shang-Bing Liu' Chung-Yuan Mou^* and Chih-Yuan Tang'^ a. Institute of Atomic and Molecular Sciences Academia Sinica, P. O. Box 23-166, Taipei, Taiwan 106. b. Department of Chemistry, National Taiwan University, Taipei, Taiwan, 106.
c. Department of Zoology, National Taiwan University, Taipei, Taiwan, 106.
The mesoporous MCM-41 materials with highly connected nanochannels have been obtained from the aluminosilicate-C^TMAX-HjO systems by a delayed neutralization procedure. The Nj adsorption-desorption isotherm of the materials exhibits a large type B hysteresis at p/po~ 0.5. The hysteresis effect is dependent on the synthetic condition and the Si/Al ratio. In spite of the chain length of the surfactant, this hysteresis takes place around the same p/po value. From the TEM micrographs of the ultrathin section, one can clearly find that these mesoporous materials contain many structure defects, which are irregular shaped and the size distributes between 5.0-30.0 nm. Combining the above two evidences, we interpret the neighboring channels are interconnected through defects in channels walls. The diffusion of molecules inside these highly defective MCM-41 materials becomes more effective. We show that it leads to a better catalyst support material using ethylbenzene dehydrogenation reaction as an illustration. 1. INTRODUCTION Since the discovery of the mesoporous molecular sieves MCM-41,[1] these materials have been extensively investigated in many applications as catalysis, catalyst support, adsorbent, electronic and optical devices. [2-4] Especially there exists a strong motivation to use the MCM-41 materials as catalytic supports, which have large surface area('-1000 mVg), high
16 thermal stability and a tunable pore size(1.5 - 10.0 nm). With larger pores than typical zeohtes, it could deal with bigger molecule. It is well known that the accessibility of the mesoporous materials would play an important role in catalysis, and catalytic activity depends on the free diffusion of the reactants, intermediates and products [5]. Thus, a mesoporous molecular sieve with better mass transport pore would be a more suitable candidate for some industrial catalytic reactions of the largesized hydrocarbons in dehydrogenation, hydrocracking or hydrodesulfurization. According to the results of many investigations [6], mesoporous MCM-41 materials commonly has a uniform 1-dimensional channel structure with the absence of any significant micropores. However the highly ordered 1-dimensional MCM-41 channels would make the intra-channel transportation ineffective especially for large-sized reactants or products. Thus, for exploring the application of the MCM-41 materials as catalytic support, it is highly desirable to find a procedure and compositions to obtain a highly accessible mesoporous material. In principle the bicontinuous 3-dimensional network structure of MCM-48 would act as a good catalytic support. [7] However, its lower hydrothermal and thermal stability has led to much less application of MCM-48 in catalysis. Recently, a family of mesoporous molecular sieves (denoted as MSU-G) with vesicle-like hierarchical structure, worm-like mesoporous structure and bicontinuous nano-porous silica had been synthesized.[8-10] It was proposed that highly accessible mesoporous materials could be obtained through different synthetic procedure and composition. Previously, we reported a convenient delayed-neutralization method for the formation of the aluminosilicate mesoporous MCM-41 materials with tubular and hollow spherical morphology. We find the same method leads to local structural defects, where the nanochannels of the MCM-41 become effectively intra-connected and could readily provide better transport for large molecules between inter-channels. We will denote in this paper the highly defective MCM-41 materials as Def-MCM41. Here we combined the result of nitrogen adsorption-desorption isotherm and the microtome TEM technique to provide the methods for characterization. The higher conversion of ethylbenzene dehydrogenation over tubular DefMCM41 fiirther revealed that the mesoporous aluminosilicate materials with structural defects possess highly connected channels with large surface area. 2. EXPERIMENTAL: The synthetic procedure for preparing the MCM-41 mesoporous materials is based on the delayed-neutralization process reported in our previous paper.[ll] The silica source is sodium
17 silicate (27% SiOj, 14% NaOH) from Aldrich. The source of aluminum is sodium aluminate from Riedel-de Haen. The quaternary ammonium surfactants, CnH2n+,(CH3)3NX (n = 10-18, CnTMAX, X = CI or Br), are purchased from Tokyo Chemical Industry, Aldrich or Acros and used without further purification. The molar composition of the resultant gel is: 1.0 C^TMAX : (2.10-1.30) SiOj: (1.63 - 1.20) NaOH : (0.67 - 0.40) H2SO4: (50 - 500) HjO. The gel mixture was heated at 100 °C for 48 h in a static autoclave. The as-synthesized product was filtrated and washed with deioned water, then calcined at 560 °C in air for 6 h to remove the organic templates. To synthesize the Def-MCM41, the same process was used except that a suitable amount of sodium aluminate was mixed with the solution of surfactant. The x-ray powder diffraction (XRD) patterns of the synthesized samples were collected on a Scintag XI diffractometer using Cu K^ radiation. Nj adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus. The transmission electron micrographs (TEM) were taken on a Hitachi H-7100 micrometer operated at 75-100 keV. Scanning electron microscopy (SEM) was performed on Hitachi S-800 operated at an accelerating voltage of 20 keV. All the Def-MCM41 samples used in the catalytic reaction study were the tubular form previously reported. We use Def-MCM41 and the regular defect-free MCM-41 as catalyst supports. The catalysts were prepared by physically mixing the aluminosilicates or silica MCM-41 with 6 weight percent of molybdenum trioxide. For comparison silica gel was also used. All the catalysts were pre-calcined at 500 °C for 6 h before used. Catalytic reactions were carried out at 500 °C in a continuous flow micro-reactor system. The reactions were started by injecting the ethylbenzene, continuously (2.72 ml/h) into the nitrogen carrier gas stream (effluent flow rate = 30 ml/min) and the reaction product (gas and liquid) was analyzed off-lined by a Shimadzu GC-7A gas chromatograph. Liquid product was collected by a condenser (10°C) positioned at the outlet of the reactor and the components were separated with a packed column (5% SP-1200 + 1.75% Bentone 34 on 100/120 Suplecoport, 6 fl). Catalyst regeneration was done at 500°C for 1 day under an air flow of about 75 ml/min. 3. RESULTS AND DISSUCTION Fig lA shows the XRD pattern of the calcined MCM-41 sample synthesized from CigTMAB-silica and aluminosilicate systems with the Si/Al = oo(sample I) and 37(sample II) by using the delayed neutralization process. In both materials, there exist at least 4 sharp XRD peaks, which indicate well-ordered hexagonal structure of MCM-41. It means that the incorporation of aluminum into silica framework could not have significant effect on the arrangement of MCM-41 mesostructure. When the nitrogen adsorption-desorption isotherms
18 of these samples were examined, one could clearly find that both of the samples have the typical sharp capillary condensation at p/po = 0.32, corresponding to a pore size of about 2.6 nm and narrow pore size distribution with full width at half maximum (FWHM) = 0.12-0.14 nm.[12] However, the aluminosilicate(II) possesses an additionally large hysteresis at p/po = 0.5 to 1. There is a large jump of the adsorbed volume of about 200 cmVg S. T. P in the desorption branch. The hysteresis is of type B according to de Boer's classification. The existence of this hysteresis indicates that aluminum incorporation into the silica framework might induce the formation of some defective structures.
100 .
B
000 .
— o — Desorption
0k
H
900 .
—•—Adsorption
/
100 .
1 1 ^
600 . 300 . 400 . }00 .
B 3
e
> 26/deeree
200 •
100 .
j^
r
30CI.C»on UJ
2:
Figure 1. SEM images of as-synthesized mesoporous silica spheres.
3 4 5 8 20 (degree) Figure 2. X-ray diffraction (XRD) patterns of as-synthesized and calcined mesoporous silica spheres.
40
Figure 3. TEM images of calcined mesoporous silica spheres with magnification (a) 85000; (b) 212500. It is flirther confirmed by TEM images. As shown in Figure 3a, calcined silica sphere is retained with uniformed size (~ 200 nm). Both bead morphology of silica spheres and the near hexagonal mesoporous arrays can be observed in TEM images (Figure 3b). Almost entirely products are uniformed spheres. Mesoporous arrays can be observed along the surface of the
uuu -
400-1 T O - —
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0.02
< >
1000- — 0.0
a. H
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i
0.06
1 10 100 Pore diameter (nm) I — 1
0.2
1
1
0.4
1
1
0.6
1
1
0.8
Relative Pressure, (P/PQ)
300-
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10
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10.
REFERENCES [I] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Krege, 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] S. Biz and M.L. Occelli, Catal Rev. -Sci. Eng., 40 (1998) 329. [3] C.Y. Chen, H.X. Li and M.E. Davis, Micropor. Mater., 1 (1993) 217. [4] J.M. Kim, J.H. Kwak, S. Jun and R. Ryoo, J. Phys. Chem. 5, 99 (1995) 16742. [5] J.M. Kim and R. Ryoo, Bull. Korean Chem. Soc, 17 (1996) 66. [6] R. Ryoo and S. Jun, J. Phys. Chem. B, 101 (1997) 317. [7] T.Tatsumi, K. Koyano andN. Igarashi,J. Chem. Soc, Chem. Commun.,325 (1998). [8] A. Corma, Q. Kan aad F. Rey, J. Chem. Soc, Chem. Commm., 579 (1998). [9] J.F. Pan, S.Y. Liu, K. Hidajat and S. Kawi, J. Inst. Eng. Sing., 38 (1998) 55. [10] F.H.P. Silva and H.O. Pastore, J. Chem. Soc, Chem. Commun., 833 (1996). [II] C.J. Brinker and G.W. Scherer, Sol-Gel Science, Academic Press, Ix)don, p.l07 and p.644, 1990. [12] Q.H. Xia, K. Hidajat and S. Kawi, Mater. Lett., in press (1999). [13] Z. Luan, C. Cheng, H. He and J. Klinowski, J. Phys. Chem. B, 99 (1995) 10590. [14] X.Y. Chen, L.M. Huang and Q.Z. Li, J. Phys. Chem. B, 101 (1997) 8460. [15] K.A. Koyano and T. Tatsumi, Chem. Lett., 5 (1997) 469.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
57
New way to synthesize MCM-41 and MCM-48 materials with tailored pore sizes J.L. Blin, G. Herrier, C. Otjacques and Bao-Lian Su* Laboratoire de Chimie des Materiaux Inorganiques, I S I S , Universite de Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium The present work shows that by adjusting the molar ratio of two swelling agents : decane and TMB, the crystallization temperature and time, either the large pore hexagonal SiMCM-41 or cubic Si-MCM-48 can be obtained. The characterization of these two structures by SEM, TEM, nitrogen adsorption-desorption analysis is reported. Their thermal stability has been also evaluated. It has been found that the decane and TMB concentration and the crystallization temperature can affect strongly the final structure of the products. The phase transition of cubic MCM-48 to hexagonal MCM-41 with increasing of TMB or decane concentration is explained by considering the surfactant packing parameter g (V/aslc). However, to explain the MCM-41 to MCM-48 transition with increasing crystallization temperature, some complementary studies should be made. Keywords : MCM-41, MCM-48, tailored pore size, decane/TMB, swelling agents, phase transition, thermal stability 1. INTRODUCTION In 1992 a new family of highly ordered mesoporous inorganic compounds has been discovered by Mobil scientists. Three different structures, hexagonal MCM-41, cubic MCM48 and lamellar MCM-50 have been identified [1-2]. But because of its bad thermal stability, the MCM-50 attracts less research attention. Moreover, the MCM-41 and the MCM-48 structures are excellent candidates for catalysis and separation processes. MCM-41 has already widely been used as catalysts and catalyst support in various reactions [3-6]. Although fi-om a catalytic point of view (i.e. shape selectivity), MCM-48 is more desirable than MCM41 for industrial applications, the three-dimensional channel network of MCM-48 has been more rarely investigated. This is probably due to the difficulty of its synthesis that is usually performed with gemini surfactants [7-8] and the pore diameter remained less than 6.0 nm. In the context of treatment of large molecules, much effort has been devoted to enlarge the pore size of mesoporous siliceous materials. To reach this goal, the hydrothermal post-synthesis treatment can be done [7-9], surfactants of different chain lengths [10], polymers such as triblock copolymers [11] can be used as templates. On the other hand, the swelling agents such as 1,3,5-trimethylbenzene [1, 12], tetralkylammonium cations (TAA^) [13], amines [14] or alkanes [15-16], in particular decane [17], can be incorporated to expand the pore size of compounds. Recently, using gemini surfactants Gn-s-m, where n and m are the alkyl chain * Corresponding author
58 length and s is the spacer, Van der Woort et al [8] have obtained either the hexagonal MCM41 or the cubic MCM-48 structure. They reported that the pore size depends on the alky! chain length whereas the spacer length determines the crystallographic phase. Spacer length of 10-12 carbon atoms yields the cubic MCM-48 phase whereas smaller spacer leads to the formation of the hexagonal MCM-41. Whatever is the final structure, the pore diameter is less than 3.0 nm. The present work provides a new original and significant synthesis method using jointly alkane and 1,3,5-trimethylbenzene as swelling agents to enlarge the pore size. In this way, we obtained not only the large pore MCM-41 (9.0 nm) but also large pore MCM-48 (> 7.0 nm). Synthesis of MCM-48 with such a pore diameter has not been reported before. We have investigated the effect of the molar ratio of the two swelling agents, the crystallization time and temperature on the synthesis. The final products were characterized by Scanning Electron Microscopy, Transmission Electron Microscopy, microdiffraction, and nitrogen adsorption-desorption analysis. 2. EXPERIMENTAL 2.1. Synthesis All syntheses were made according to the following scheme. Sodium silicate solution
H2SO4
•--..,,^^ stirring H2S04
pH adjustment
["
stirring
^ y ^
Micellar solution of surfactant + Swelling agents : decane and TMB
i
Gel
1
Hydrothermal treatment at different temperatures and times
i Solvant extraction
i Drying \r Powders Synthesis scheme of large pore mesoporous (Si)-MCM-41 and (Si)-MCM-48.
59 The surfactant, cetyltrimethylammonium bromide, was dissolved in water to obtain a clear solution, in which the first swelling agent (decane) was then added drop by drop with stirring. After ten minutes stirring at room temperature, the second swelling agent (TMB) was introduced. Sodium silicate was added to the obtained micellar solution and the pH value was adjusted with sulfiiric acid. The pH value and surfactant/silicium molar ratio was fixed at 10 and 0.62 according to the protocol established previously for conventional MCM-41 synthesis [18]. The chemical composition of the gel is 1.0 CTMABr, 0.63 Si02, x C10H22, y TMB, 102 H2O. Different alkane/TMB molar ratios have been used and x/y varies in a range of 0.40 to 1.33. After stirring for several hours at room temperature, the homogenous gel was placed in a Teflon autoclave and heated. The crystallization temperature and time vary respectively from 80°C to 100°C and from 1 day to 11 days. The final products were obtained after ethanol extraction and calcination under nitrogen and then air atmosphere at 500°C for 18 hours. 2.2. Characterization Micrographs of the obtained intermediate and final phases were made from Philips XL-20 Scanning Electron Microscope (SEM) using conventional sample preparation and imaging techniques. The transmission electron micrographs were taken using a 200kV CM20 microscope. Samples for HRTEM observations were prepared by dispersing the powder of samples in methanol. A drop of this slurry was dispersed on a holey carbon film placed on a Cu grid. Nitrogen adsorption - desorption isotherms were obtained at -196 °C over a wide relative pressure range from 0.01 to 0.995 from a volumetric adsorption analyzer ASAP 2010 manufactured by Micromeritics. The samples were degassed in vacuum for several hours at 250°C. The pore diameter and the pore size distribution were determined by the BJH method [19]. Structures were identified by microdiffraction and TEM. 3. RESULTS AND DISCUSSION 3.1. Characterization of the final products obtained using two different decane/TMB molar ratios For a preliminary study, two different decane/TMB molar ratios of 0.66 (x = 1 and y = 1.5) and 1.33 (x = 2 and y = 1.5) have been used. The crystallization time and temperature employed are respectively 4 days and 80°C. The two products obtained after extraction and calcination are referred to as A and B. 3.1.1. Determination of the crystallographic phase by TEM Because of the high value of pore diameter of these two samples, it was not possible to obtain a XRD pattern with conventional XRD diffractiometer. The crystallographic structure of the samples was therefore identified by using the HRTEM, the microdiffraction or the Fourier Transformation of TEM images. Figure 1 reports the TEM micrographs (Fig. la) and the microdiffraction (Fig. lb) of the 110 zone of sample A and Figure 2 shows the TEM micrographs (Fig. 2a) and its Fourrier Transform (Fig. 2b) of sample B. The pore size determined by TEM is around 7.0 nm for sample A and 8.5 nm for sample B. Both Figures 1 and 2 show the highly organized structure. The microdiffraction of the 110 zone of the sample A, shown in Fig. lb and being rarely observed of the material exhibits the channels in cubic arrangement [20]. This indicates that sample A should be a cubic MCM-48 type material. For sample B, it is quite difficult to obtain a microdiffraction pattern. Its crystallographic structure was determined with the help of the Fourier Transform of its TEM image (shown in Fig.2b).
60
Figure 1 : TEM micrograph (a) and microdiffraction (b) of the 110 zone of sample A.
50 nm Figure 2 : TEM image (transversal view) (a) and its Fourier transformation (b) of sample B. The Fourier Transform picture exhibits a sixfold symmetry and the measured angles between two bright spots are very close to 60°. This suggests that the channels of sample B have a hexagonal arrangement and sample B is a hexagonal MCM-41 type material. 3.1.2. Nitrogen adsorption-desorption analysis Figure 3 depicts the isotherms (A) and the pore size distribution (B) of these two samples. Both samples exhibit a type IV nitrogen isotherm (Figure 3Aa and 3Ab), characteristic of mesoporous materials. The BET surface area was found to be 900 m /g and 658 mVg for samples A and B, respectively. The sharp increases in the adsorbed volume of N2 due to capillary condensation are found to occur at high relative pressure p/po (0.65 for MCM-48 sample and 0.70 for MCM-41 sample). This indicates that the pore diameter is rather large. The difference in capillary condensation pressure between these two samples suggests that these two materials have different pore diameters since the p/po position of the inflection point is related to the pore diameter. The part of the curve corresponding to the capillary condensation is almost vertical reflecting the homogeneity of the samples. It can also be seen that the pore diameter distribution obtained from the BJH method (Figure 3Ba and 3Bb) is quite narrow and centered at 7.5 nm for MCM-48 sample and 9.0 nm for MCM-41 sample. These values are very similar to those obtained by HRTEM.
61
B 1200-
b
0015.
800(Resorption 600-
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^ — - " ' ^ ' ^ 200. 1700.
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adsorption
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11
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adsorption
400200-
1M \ ! \ \
0015. 0010. 0.005.
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0000.
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0.2
0.4
J
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1 . ^ l""-?-
50
60
0.6 Pore diameter (ran)
Relative pressure p/p^
Figure 3 : N2 adsorption isotherms at -196°C (A) of MCM-48 (a) and MCM-41 (b) and the pore size distribution (B) obtained from BJH method for MCM-48 (a) and MCM-41 (b). 3.1.3 Crystals morphology No significant difference can be noted in crystals morphologies for MCM-48 and MCM-41 materials. Both exhibit variable size and form. The surface of these crystals is quite porous. The crystal morphology of MCM-48 bears some analogy with the one reported by Schumacher et a/ [21] for MCM-48 samples prepared via a hydrothermal treatment.
Figure 4 : Crystals morphologies of MCM-48 (a, b) and MCM-41 (c, d) obtained from the present work.
62 3.1.4. Thermal Stability This study was performed on two samples, which are different of those used until now. The molar ratio between the two swelling agents, crystallization time and temperature are respectively 1.33 (x = 2 and y = 1.5), 4 days and 100°C for the MCM-48 material and 0.80 (x = 2 and y = 2.5), 4 days and 100°C for the MCM-41. To evaluate their thermal stability, these samples were calcined at different temperatures: 500°C, 550X, 600°C and 700°C. The heating rate, duration and atmosphere are the same as those described above for a 500°C calcination temperature. Figure 5 depicts the variation of the surface area of MCM-41 (curve a of Fig. 5) and MCM-48 (curve b of Fig. 5) as a function of calcination temperature. From Figure 5a, for the MCM-41 material, we can notice a slight increase of the value of the specific surface area after calcination at 550°C compared to calcination at 500°C. This can be explained by the fact that a small quantity of organic compounds (surfactant and swelling agent) still remained in the pores after extraction and calcination at 500°C. They can be completely removed only after calcination at higher temperatures. For this sample, the specific surface area after calcination at 500°C is probably underestimated. From 500°C to 600°C, the samples maintain pratically their high surface area (more than 600 mVg), even if from a general point of view the value decreases slowly. For both MCM-41 and MCM-48 materials, the value of specific surface area decreases dramatically after calcination at 600°C. The maximum volume of nitrogen adsorbed by samples decreases from 1200 cmVg for calcination at 500°C to 180 cmVg for calcination at 700°C for the MCM48 materials and from 1130 cm^/g to 200 cm^/g for the MCM-41 one. These observations indicate that a calcination temperature superior to 600°C can destroy almost completely the structure of MCM-41 and MCM-48 materials. 3.2 Optimization of crystallization time and temperature. For a molar ratio of decane/TMB equal to 0.66 (x = 1 and y = 1.5), the effect of the crystallization time and temperature on the formation of mesoporous materials has been studied.
^
900-
800-
1000
\ ^
700-
^
s
600-
•
J^
500-
^
400-
• >. b
ml
300-
^ I 200-
500
550
600
650
700
Calcination temperature (°C)
Figure 5 ; Variation of the specific surface area with calcination temperature a : MCM-41 sample, b : MCM-48.
1
1 —— 1
1
—1
1—
.—
1
Crystallization time (days)
Figure 6 : Variation of the specific surface area with crystallization time and temperature a : 80X, b : 100°C.
63 From Figure 6 it is clear that whatever is crystallization temperature, 80°C or 100°C, the crystallization time should not excess 4 days. After this delay, the amorphisation of the material is completely reached, the value of the specific surface area drops sharply and no homogeneous pore size distribution is obtained. Lower crystallization temperatures, for example 60°C, should be studied. It should be noted that for a given molar ratio of decane/TMB the variation of crystallization temperature and time can lead to the formation of both MCM-41 and MCM48:We would like to show here only the effect of crystallization temperature and time on the formation of mesoporous materials. We neglect at the moment which kind of mesoporous materials is formed at a given crystallization temperature and time. This will be discussed in the following section. 3.3 Phase transition of MCM-41 to MCM-48 or MCM-48 to MCM-41 with molar ratio of decane/TMB and crystallization temperature The synthesis conditions have been varied to study their influence on the value of pore diameter and on crystallographic structure. From Table 1, which sums up the synthesis conditions investigated, the crystallographic structure, the value of the pore diameter, specific surface area and pore volume of obtained products. It is clear that both large pore MCM-41 and MCM-48 can be obtained depending on the synthesis conditions used. It is very clear from the results reported in Table 1 that the concentration of decane and TMB and the crystallization temperature are the factors dominating the final phases. For a defined molar ratio of decane/TMB, at low temperature range, MCM-41 is obtained and increasing the crystallization temperature favors the transition from hexagonal MCM-41 to cubic MCM-48. At high crystallization temperatures, MCM-48 is pratically the only structured product. While for a given crystallization temperature, the increase of the decane or TMB concentration leads to the transformation of cubic MCM-48 to hexagonal MCM-41 structure. For example, at 80°C, with a decane/TMB molar ratio of 0.66 (x = 1.0 and y = 1.5), the MCM-48 structure with a specific surface area of 882 mVg and an average pore diameter of 7.5 nm is synthesized. Whereas with a decane/TMB molar ratio of 1.33 (x = 2.5 and y = 2.0), the hexagonal MCM-41 with a specific surface area of 658 mVg and an average pore diameter of 9.0 nm is obtained. Stucky et al [22] have shown that the molecular packing model used to describe the water-surfactants systems could be extended to mesoporous silicate structures. From this point of view, we can explain why the phase transition from cubic MCM-48 to hexagonal MCM-41 occurs with the increase of decane or TMB concentration. The surfactant packing parameter g is defined as V/as Ic, where V is the volume of the hydrophobic chain of the molecule, as is the headgroup area and U is the critical length of the hydrophobic tail. Small values of g stabilize more curved surfaces such as MCM-41 (1/3 < g < 1/2), while larger values of g stabilize structures with less curvature such as MCM-48 (1/2 < g < 2/3) and lamellar structure (g = 1). According to Kunieda et al. [23], swelling effect represents the effect of added compounds to increase the volume of the lipophilic part of the micelle without expanding as, whereas the penetration effect means the effect of compound to expand as without increasing the volume of the aggregates. So when the concentration of decane or TMB is raised and if the quantity of sweUing agent is not too high to destroy the formed micelles, the swelling effect occurs when the value of both V and U increases, as remains constant and the value of g changes. The transition of MCM-48 to MCM-41 therefore takes place. Meaning that g decreases and that the value of Ic varies more important than that of V.
Table1 Synthesis conditions, specific surface area, pore volume, pore diameter and structure of obtained compounds
Nb Nb mole of mole of decane TMB
R
Crystallization temperature ("C)
Crystallization Time (days)
Specific surface area (m21g)
Pore volume (cm3/g)
Pore diameter (nm)
Identified structure
1
2.5
0.40
80
1
662
1.1
6.5
1
2.5
0.40
80
4
293
0.2
*
Hexagonal MCM-4 1 Almost amorphous
1
2.5
0.40
100
1
334
0.2
*
Almost amorphous Almost amorphous
1
1.5
0.66
80
1
414
0.3
*
1
1.5
0.66
80
4
882
1.6
7.1
Cubic MCM-48
1
1.5
0.66
100
1
551
0.9
6.1
a
1
1.5
0.66
100
4
632
1.1
6.9
a
2
2.5
0.80
80
1
252
0.3
5.6
Almost amorphous
2
2.5
0.80
80
4
608
1.1
7.5
a
2
2.5
0.80
100
1
635
1.2
7.4
a
2
2.5
0.80
100
4
756
1.6
7.9
Hexagonal MCM-4 1
2
1.5
1.33
80
4
658
1.5
8.8
Hexagonal MCM-41
2
1.5
1.33
100
1
706
1.1
6.0
Cubic MCM-48
2
1.5
1.33
100
4
88 1
1.7
7.5
Cubic MCM-48
R is the molar ratio of decane1TMB
* : No homogeneous pore size distribution was obtained a : Structure of these materials was not yet investigated
65 The effect of temperature on g is difficult to predict because effects such as solvatation, entropic thermodynamic have to be taken into account. Thus the phase transition of MCM-41 to MCM-48 can not be explained by using the packing parameter g when crystallization temperature increases. Some complementary studies (synthesis at lower and higher temperatures, XRD or SAXS measurements...) should be made to understand and explain the mechanism of phase transition. 4. CONCLUSION Incorporating jointly decane and 1,3,5-trimethylbenzene during the micellar solution preparation allows us to expand further the pore size, which can be adjusted up to 9.0 nm. Using only alkane [16] and in particular decane [17] as swelling agent the maximum pore size archived was 5.0 nm. The present work shows that by adjusting the molar ratio between the two swelling agents either large pore hexagonal MCM-41 or large pore cubic MCM-48 can be obtained. If the concentration of decane or TMB increase, the phase transition of cubic MCM-48 toward MCM-41 occurs. This transformation can be explained by considering the surfactant packing parameter g. Increasing temperature favors the formation of the MCM-48, we failed to explain the phase transition of MCM-41 to MCM-48 with increasing crystallization temperature. More studies should be made to understand this phenomena. ACKNOWLEDGEMENT: This work has been performed within the framework of PAI/IUAP 4-10. Gontran Herrier thanks the FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship. Authors thank Mr. Bart Pauwls and Prof G. Van Tendeloo for useful discussions and help in the TEM analysis. REFERENCES 1. 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, SB. McCullen, J.B. Higgins and J.L. Schenker, J. Am. Chem. Soc, 114 (1992) 10834. 2. C.T. Kresge, ME. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 3. PL. Llewellyn, Y. Ciesla, H. Decher, R. Stadler, F. Schuth and K.K. unger. Stud. Surf. Sci.Catal, 84(1994)2013. 4. J. Aguado, DP. Serrano, M.D. Romero and J.M. Escola, J. Chem. Soc, Chem. Comm., (1996) 765. 5. A. Corma, M. Iglesias and F. Sanchez, Catal. Lett., 39 (1996) 153. 6. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. 7. Q. Huo, D.I. Margolez and G.D. Stucky, Chem. Mater., 8 (1996) 1147. 8. P. Van Der Voort, M. Mathieu, F. Mess and E.F. Vansant, J. Phys. Chem. B, 102 (1998) 8847. 9. A. Sayari, P. Liu, M. Kruk, M. Jaroniec, Chem. Mater, 9 (1997) 2499. 10. A. Sayari, V.R. Karra and J. Sudhakar Reddy, Symposium on Synthesis of Zeolites, Layered compounds and other Microporous Solids, 209^ National Meeting, Am. Chem. Soc. Anaheim (1995). 11. D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Fredrickson, B.F. Chmelka and G.D. Stucky, Science, 279 (1995)548.
66 12. D. Desplantier-Giscard, A. Galarneau, F. Di Renzo and F. Fajula, ISeme reunion du Groupe Frangais des Zeolithes, Carry Le Rouet (1999). 13. A. Corma, Q. Kan, M.T. Navarro, J. Perez-Pariente and F. Rey, Chem. Mater., 9 (1997) 2123. 14. A. Sayari, M. Kruk, M. Jaroniec and I.L. Moudrakovski, Adv. Mater., 10 (1998) 1376. 15. N. Ulagappan, C N R Rao, Chem. Comm., (1996) 2759. 16. J.L. Blin, C. Otjacques, G. Herrier and Bao-Lian Su, submitted to Langmuir for publication, (1999). 17. J.L. Blin, C. Otjacques, G. Herrier and Bao-Lian Su, to be published. 18. J.L. Blin, G. Herrier, C. Otjacques and Bao-Lian Su, accepted for publication in Stud. Surf. Sci. Catal., (1999). 19. E.P. Barret, L.G Joyner, and P.P. Halenda, J. Am. Chem. Soc, 73 (1951) 37. 20. M.K. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 103 (1999) 4590. 21. K. Schumacher, M. Gain and K.K. Unger, Microporous and Mesoporous Mater., 27 (1999)201. 22. G.D. Stucky, Q. Huo, A. Firouzi, B.F. Chmelka, S. Schacht, I.G Voigt-Martin anf F. Schuth, Stud. Surf. Sci. Catal., 105 (1997) 69. 23. H. Kunieda, K. Ozawa and K-L. Huang, J. Phys. Chem. B, 102 (1998) 831.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
67
Poly(oxyethylene) oleyl ethers as templating agents for the synthesis of large pore mesoporous materials J.L. Blin, G. Herrier and Bao-Lian Su* Laboratoire de Chimie des Materiaux Inorganiques, I S I S , Universite de Namur, 61, rue de Bmxelles, B-5000 Namur, Belgium The synthesis of siHca mesoporous materials using polyoxyethylene alkyl ether and in particular decaoxyethylene oleyl ether C18H35 (CH2CH20)io as templating agent has been reported. Characterization of final products by nitrogen adsorption analysis shows that the value of pore diameter depends on several factors such as heating temperature and time, stirring time of micellar solution and pH of the gel. The expansion of pore size with increasing temperature was explained by taking into account the change of surfactant conformation with temperature. 1. INTRODUCTION The synthesis of pure silica mesoporous materials requires a surfactant. The surfactant molecules pack together to give first isolated spherical, then cylindrical micelles and finally highly-ordered phases in aqueous solution. Depending on the weight percentage and the type of surfactant and the temperature of micellar solution preparation, self organization of the template molecules can occur and give different structures such as normal hexagonal phase (Hi), normal bicontinuous cubic phase (Vi), lamellar phase (La), reversed bicontinuous cubic phase (V2) or reversed hexagonal phase (H2). When the inorganic source of silica is added to the micellar solution and the value of pH adjusted, the condensation and polymerization of silica around the micelles of surfactant can take place and lead to the final mesoporous materials. Properties of final compounds will be affected by surfactant/silicium ratio, kind of surfactant, pH value, stirring time, heating temperature and time. The mesoporous products can be synthesized via electrostatic pathway, based on a supramolecular assembly of charged surfactant (S^, S) with charged inorganic precursor (l\ I) Sl", S^Xt (X = CI", Br), S'M'^T (M^ = Na\ K^) [1-3] or via neutral pathway S^I^ [4-6]. Due to numerous remarkable properties such as high specific surface area and pore volume, the adjustable pore size, the high thermal stability and the ease of surface modificafion, this kind of materials has a wide range of potential applications including separation [7] and catalytic processes [8] or used as host matrix [9]. Until now, cethyltrimethylammonium bromide (CTMABr), with which materials having pore diameter around 2.6 nm and specific surface area over 700 mVg were obtained, was the surfactant often employed. As the potential applications of mesoporous materials require large pore diameter, to enlarge the pore size we have modified our synthesis protocol of conventional mesoporous materials [10] to incorporate either only alkanes [11] or jointly alkanes and 1,3,5 trimethylbenzene [12] as expanders. By this way, we have obtained both MCM-41 and MCM-48 materials with adjustable pore size up to 10 nm, which is the maximum pore size we can achieve.
68 The new solution to expand further the pore diameter of mesoporous materials is to find surfactants which can form bigger micelles than cethyltrimetylammonium bromide. Polyoxyethylene alkyl ethers have shown to be potential surfactant for mesoporous materials synthesis [13-15]. In this work, we have thus performed synthesis using Hquid crystalline solutions of the decaoxyethylene oleyl ether as templating agent for the pore size engineering of mesoporous materials. We have studied the effect of silicium source, pH value, stirring time of micellar solution, heating temperature and time on the formation of mesoporous materials and on the value of mean pore diameter. The compounds are mainly characterized by nitrogen adsorption-desorption analysis. 2. EXPERIMENTAL 2.1 Choice of surfactants The new surfactants have to answer several criterions : 1- lower cost and commercially available, 2- possibility of self-assemble into rod-like micelles with dimension larger than those formed by CTMA^ and controllable phases. Polyoxyethylene alkyl ethers are excellent candidates for this application. Although the phase diagram of this kind of templating agent was widely reported in the literature [16-17], only few of them can satisfy all the required criterions (mainly the low cost). Polyoxyethylene oleyl ether and in particular decaoxyethylene oleyl ether Ci8H35(CH2CH20)io, noted as Ci8(EO)io, is one of them. In this study we have investigated the ability of using this template instead of CTMABr as surfactant. 2.2. Synthesis Ci8(EO)io phase diagram [17] indicates that the temperature domain of the normal hexagonal phase Hi is between 0°C and 60°C for a 50 weight percentage of surfactant. The micellar solution with a 50% (Wt) of Ci8(EO)io surfactant was thus prepared by dissolving the surfactant at 55°C in aqueous solution, of which the pH value was then adjusted either with NaOH to 10 or H2SO4 to 2.0. The obtained solution was stirred from hours to several days at 55°C before adding drop by drop the silica source such as tetramethoxysilane, or sodium silicate. The surfactant/silicium molar ratio is fixed at 2.48. The obtained gel was sealed in Teflon autoclaves and heated (or left at room temperature). The final products were obtained after extraction with a soxhlet apparatus during 30 hours and calcination under nitrogen and then air atmosphere at 500°C for 18 hours. 2.3. Characterization Nitrogen adsorption - desorption isotherms were obtained from a volumetric adsorption analyzer ASAP 2010 manufactured by Micromeritics. The samples were first degassed for several hours at 350°C. The measurements were then carried out at -196°C over a wide relative pressure range from 0.01 to 0.995. The average pore diameter and the pore size distribution were determined by the BJH method from the adsorption branch of isotherm [18]. 3. RESULTS AND DISCUSSION 3.1. Effect of pH value and silicium source on mesoporous materials formation The pH value will control the condensation of silica, so the pH value should be adjusted to form monomer then oligomer of silica in order to obtain its condensation and polymerization around the micelles of surfactant. Under acidic conditions, silica source such as
69 tetramethoxysilane (TMOS) hydrolysizes to silicic acid and positively charged oligomeric intermediates. The alkoxide and silanol groups are easily protonated for example to =Si(0H2)^ [19]. Whereas under basic conditions, the intermediates species are negatively charged SiO". To study the influence of the silica source on the mesoporous materials formation, we have performed synthesis by adding different source of silica : neutral TMOS and ionic sodium silicate to the micellar solution. Syntheses were made either under acidic (pH = 2) or basic (pH = 10) conditions for TMOS. Table 1 contains essential information of the obtained products. From Figure lA which depicts the isotherm of some samples and on the basis of the results reported in Table 1, it is clear that mesoporous materials can be formed both under acidic and basic conditions with TMOS. The obtained isotherms are type IV according to the BDDT classification [20], characteristic of mesoporous materials. They can be decomposed in three parts : the formation of the mononolayer, a sharp increase characteristic of the capillary condensation of nitrogen within the mesopores and finally a plateau indicating the saturation of the samples. The capillary condensation step is quite evident and it occurs at high relative pressure. This indicates that samples are homogeneous and that the pore diameter is rather large since the p/po position of the inflection point is related to the pore diameter. The high value of pore diameter is confirmed by the pore size distribution determined by BJH method using the adsorption branch of the isotherm (Figure IB). When sodium silicate is used as silica source, no structured mesoporous material is formed at pH value of 10. With our synthesis conditions, no electrostatic interactions between the sodium silicate and the neutral surfactant could occur. Only amorphous silica is the final product as indicated by the shape of the isotherm (Figure 1 A, c) and the low volume of nitrogen adsorbed. In this case, the isotherm is characteristic of macroporous materials without a defined pore size. When the silica source is TMOS, the templating pathway is the neutral one SV, the mesoporous material is achieved by hydrogen bonding between the neutral decaoxyethylene oleyl ether surfactant (S^) and the inorganic precursor (I^). However, we will not go further here to understand why the pore size of materials obtained at pH value equal to 2 is larger than that obtained at pH 10. 3.2. Influence of stirring time, heating time and temperature on mesoporous materials formation For this study, the pH value is fixed to 10. As the specific surface area can be used to express the crystallinity of materials, the scheme represented in Figure 2, which describes the different steps observed during zeolite synthesis, has been also found in the synthesis of Table 1 Synthesis conditions, specific surface area (SBET), pore volume (V) and pore diameter (0) of products obtained by adding different source of silica to the micellar solution Silica source
pH
Heating time (days)
Heating temperature
TMOS 10 1 100°C TMOS 2 2 100°C Sodium Silicate 10 2 60°C a : the value is quite small b : no homogeneous pore size distribution was obtained
V (cm^/g)
0 (nm)
(m^/g) 615 509 133
1.3 1.8 a
7.8 13.2 b
SBET
70
B
desorption ^
^
/ • • ' /
adsorption 1200 1000
! !
desorption / /
adsorption 0.0
0.2
0.4
0.6
0.8
Pore tfaneter (Bn)
Relative pressure p/p.
Figurel. Nitrogen adsorption isotherms (A) and pore size distributions (B) of samples synthesized with different silica source, a : TMOS, pH = 10, b : TMOS, pH =2 and c : sodium silicate, pH= 10. mesoporous materials [10] and is used here to explain the variation of the specific surface area with heating temperature and time. This analysis can help us to adjust the synthesis parameters. In the case of the synthesis of zeolite crystals, four steps corresponding respectively to the nucleation (I), the crystallization (II), the growth of crystals (III) and the amorphisation (IV) have been observed. In the formation of mesoporous materials, the first step (I) is related to the hydrolysis of inorganic silica source in aqueous solution and no mesostructured solid phase can be obtained. This step is rarely observed due to the high rate of polycondensation of silica source around the micelles and that of formation of mesoporous materials. The resulting solid phase, in this step, should be amorphous and the specific surface area should be quite low. The progressive polycondensation of the source of silica around highly organized micelles occurs at the step (H) and the mesostructured phase is progressively formed. After removal of the surfactants, the ordered solid with surface area higher and higher can be obtained. In the third step, the surface area and pore volume stop growing. However, the thermal stability of obtained solid phase increases. This indicates that during this step, the plycondensation of silica source continues and the thickness of the wall separating two adjacent pores increases with heating time as we have previously observed [10]. The last step (IV) corresponds to the amorphisation. The high temperature and long heating time lead often to the formation of the amorphous phase both in the case of synthesis of zeolites [21] and mesoporous materials [10].
71
B
a C/3
2
Reaction rate Figure 2. Different steps observed during zeolite synthesis.
4
6
Crystallization time (days) Figure 3. Variation of the specific surface area with heating time and temperature a: 60°C,b: 100°C.
Figure 3 depicts the variation of the specific surface area with heating time and temperature. The stirring time of the micellar solution before addition of teramethoxysilane is 0.75 h. Neither at 60°C or 100°C, the steps (I) and (H) are observed. The value of the specific surface area of materials obtained at 60°C is higher than those at 100°C. After 1 day at 100°C or 2 days at 60°C, the specific surface area drops dramatically and the amorphisation step is achieved. For the synthesis conditions reported above, heating time should not exceed 2 days at 60°C and low heating temperatures are recommanded. For all other compounds obtained, the synthesis conditions and the porosity characteristics are listed in Table 2. The resuhs reported in this table demonstrate the success in the synthesis of the homogeneous mesoporous materials, showing pore size from 3.0 nm up to 14.7 nm with very high specific surface areas up to 1030 mVg. The higher pore diameter, the broader the pore size distribution. It is observed that the pore diameter depends not only on the heating time, temperature but also on the stirring time. Two evident tendencies can be drawn. Firstly increasing heating temperature favors the formation of large pore mesoporous materials but too long heating time leads to the amorphisation of compounds. This is in agreement with what observed by Pinnavaia et al. On the other hand it is found for the first time that long stirring time leads to the reduction of the pore size of the obtained solid phase. Sierra et al [22] have reported the synthesis of mesoporous materials using polyethyleneglycol-4-terocylphenylether with 9-10 ethoxy groups (Triton XI00) as surfactant and sodium silicate as silica source. They have concluded that the pore size depends on the polycondensation of the silica, which is increased with temperature, and the surfactant conformation. The expansion in the pore size with heating temperature observed in present work can be explained using the interpretation proposed by Sierra et al [19], who justify the increase of the pore diameter pH values are changed from 2 to more than 8.5. At low heating temperature, in an aqueous solution of polyoxyethylene oleyl eher, the hydrophilic part of the surfactant exhibits a contracted conformation because of the hydrogen-bonded water of molecules present around the contracted hydrophilic heads of surfactant (Figure 4a). Contacts and interaction of the ethoxy oxygens with the silanol groups of the silica are not favored by this configuration. If heating temperature is raised, the bonded water molecules disappear and a more extended conformation is expected (Figure 4b). This
72
Table2 Synthesis conditions and porosity characteristics : specific surface area and pore diameter (0) of final products Stirring time (hours)
Heating temperature (°C)
Heating time (days)
0.75
48 96 96
25 25 25 25
7 7 7 10
1029 1019
96 96
50 50
0.75 0.75
48 48 64 0.75 0.75 0.75
48
(SBET),
pore volume (V)
SBET (mVg)
V (cmVg)
0 (nm)
980 942
1.15 0.90 0.73 0.50
4.7 3.9 3.4 2.9
2 4
852 860
0.57 0.98
3.3 4.5
60 60 60 60 60
1 2 1 2 4
718 700 759 632 639
1.33 1.43 1.26 1.17 1.49
7.0 7.6 6.2 7.0 8.8
100 100 100 100
0.5 1 2 1
644 615 434 539
1.40 1.30 1.65 1.56
8.1 7.8 14.7 10.9
conformation allows more interactions with silica but needs a larger area on the silica walls to be achieved. The consequence is an increase in the pore diameter. Finally, if the heating temperature is further raised, the surfactant/silica interface becomes less important and the size of the mesopores increases with the stretching of the surfactant molecules. Another tendency that increasing stirring time of the micellar solution before adding silica source leads to the formation of mesoporous materials with lower pore diameter, can not be easily interpreted. However, we can imagine that long stirring time can lead to the formation of small size micelles. In fact, the long stirring time can resuh in the reorganization of the formed micellar phase to get more stable micelles in small size. On the other hand, contact between water molecules and the hydrophilic head can be enhanced and as a consequence, the contracted conformation of the hydrophilic head predominates and the materials with small pore size are obtained. Nevertheless, the complementary studies (synthesis of compounds with different stirring time for a given heating time and temperature) should be performed to better understand what happens. 3.3 Synthesis mechanism consideration Two mechanisms have been proposed by Mobil's scientists to explain the formation of mesoporous materials [1]. In the first route, the hexagonal micelles are formed and direct the growth of the mesoporous materials. When surfactant is Cethylttrimethylammonium bromide, (CTMABr) this route has been abandonned because hexagonal MCM-41 materials can be synthesized even though the weight percent of surfactant is less than CMC2 (Critical Micelle Concentration for which rod micelles of CTMABr packed together to give a hexagonal array). In
73
H jO i
1
lipophilic^—o.
"C H 2
CH2
^C H 2
CH5
^XCHH. ,
CH,
H lipophilic
^o^
H 2O I
O^ CH2
J^Hj. ^O
^o ^ ^C H ,
i
H ,0 •
^6H C H,
I
.CHj^ ^0>^ ^^^2 CH2 CH2
-PH^
O
J^^^7
- O CH2
CH2
J^H,
O
^OH CH2
Figure 4 Polyoxyethylene alkyl ether conformation a : in aqueous solution, b : in oil. the second route, silica species interact with rod micelles of surfactant to form the mesoporous compound. This pathway explains why the synthesis of hexagonal MCM-41 can be performed with the weight percent of CTMABr less than CMC2. In present study, where polyoxyethylene alkyl ether is employed as surfactant, the templating agent concentration is located in the existence range of the normal hexagonal phase (Hi), so the synthesis route in this case is rather the first pathway than the second one. At the beginning of the synthesis, the surfactant adopts a contracted conformation in an aqueous solution (Figure 4a), which favors contact between the hydrophilic head group of Ci8(EO)io and water molecules but disfavors the contact with silanol group of silica. When the heating temperature is increased, the interaction between oxygen atom of the ethoxy and water molecules becomes weaker and weaker. Whereas that between these oxygen atoms and silanol group of TMOS becomes stronger and stronger. The conformation of surfactant changes and the stretching of molecules of surfactant takes places (Figure 4b). The size of micelles consequently grows. The large pore mesoporous materials are formed. Some syntheses have been performed with lower surfactant weight percent, but always in composition range where normal hexagonal phase (HI) is present according to the phase diagram. As shown in Table 3, the values of pore diameter are higher than those obtained with a 50 weight percent of Ci8(EO)io, nevertheless a bimodal pore size distribution can be observed. The effect of surfactant weight on the formation of mesoporous materials will be further studied. 4. CONCLUSION Polyoxyethylene alkyl ether and in particular decaoxyethylene oleyl ether C18H35 (CH2CH20)io can be used as templating agent for silica mesoporous materials formation. The synthesis can be performed under both acidic or basic conditions with tetramethoxysilane (TMOS) as silica source while no mesoporous compound was obtained with sodium silicate. Tableau 3 Synthesis conditions and porosity characteristics : specific surface area (SBET), pore volume (V) and pore diameter (0) of final products obtained with lower surfactant concentration Surfactant (WT%)
Stirring time (days)
Heating temperature
Heating time (days)
V (cm^/g)
0 (nm)
(mVg)
3 3 1
684 690 759
2.02 1.96 1.26
6.4/15.3 5.9/15.0 6.2
SBET
(!C) 40 45 50
1 1 2
60 60 60
74
It is observed that the value of mean pore diameter, which is three times larger than those obtained with CTMABr, depends strongly on several factors such as heating time and temperature and stirring time. The expansion of pore size is noted when the heating time is increased. This variation was explained by the change of surfactant conformation with temperature. From first results, it seems that long stirring time of micellar solution leads to the formation of materials with smaller size. Further investigations will be made to understand this phenomena. We have observed also that reducing the pH value of the gel leads to large pore size of materials. The structural characterization by XRD and TEM will be reported elsewhere. ACKNOWLEDGEMENT : This work has been performed within the framework of PAI/IUAP 4-10. G. H. thanks the FNRS for a FRIA scholarship. The helpful assistance of Miss C. Otjacques is acknowledged. REFERENCES 1. 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. Schenker, J. Am. Chem. Soc, 114 (1992) 10834 2. C.T. Kresge, ME. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710 3. 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 4. A. Sayari, Stud. Surf. Sci. Catal., 102 (1996) 1 5. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865 6. P.T. Tanev and T.J. Pinnavaia, Chem. Mater., 8 (1996) 2068 7. PL. Llewellyn, Y. Ciesla, H. Decher, R. Stadler, F. Schuth and K.K. Unger, Stud. Surf. Sci. Catal., 84(1994)2013 8. P.T Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321 9. C.G. Wu and T. Bein, Chem. Mater., 6 (1994) 1109 10. J.L. Blin, G. Herrier, C. Otjacques and Bao-Lian Su, Stud. Surf. Sci. Catal, (1999) 11. J.L. Blin, G. Herrier, C. Otjacques and Bao-Lian Su, to be published 12. J.L. Blin, G. Herrier, C. Otjacques and Bao-Lian Su, to be published 13 S.A. Bagshaw, E. Prouzet and T.J. Pinnavaia, Science, 269 (1995) 1242 14 S.A. Bagshaw, Chem. Comm., (1999) 1785 and (1999) 271 15 W. Zhang, B. Glomski, T.R. Pauly, and T.J. Pinnavaia, Chem. Comm., (1999) 1803 16. D.J. Mitchell, G.J.T. Tiddy, L. Waring, T. Bostock and MP. Mc Donald, J. Chem. Soc. Faraday Trans. I, 79, (1983) 975 17. H. Kunieda, K. Shigeta, K. Ozawa and M. Suzuki, J. Phys. Chem. B, 101 (1997) 7952 18. E.P. Barret, L.G. Joyner, and P.P. Halenda, J. Am. Chem. Soc, 73 (1951) 37 19. J.C. Bunker, G.W. Scherer, Sol-Gel Science, Academic Press, San Diego (1990) 20 S. Brunauer, L.S. Deming, W.S. Deming and E. Teller, J. Amer. Chem. Soc, 62 (1940) 1723 21 D.W. Breck, Zeolite Molecular Sieves, John Wiley & sons, New York, (1974) 22 L. Sierra and J.L. Guth, Microporous and Mesoporous Mater., 27 (1999) 243
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
75
Pore size engineering of mesoporous silicas using alkanes as swelling agents J.L. Blin, C. Otjacques, G. Herrier and Bao-Lian Su* Laboratoire de Chimie des Materiaux Inorganiques, I S I S , Universite de Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium Alkanes have been used as swelling agents and incorporated during the micellar solution preparation in order to synthesize large pore mesoporous materials. The introduction sequence of the swelling agent, the effect of the number of carbon atoms of the alkane on the structural and textural properties of products were investigated. Final compounds were intensively characterized by several techniques (XRD diffraction, SEM, TEM, nitrogen adsorption-desorption analysis). 1. INTRODUCTION Since their discovery in 1992 by Mobil scientists [1-2], highly ordered mesoporous materials MCM (Mobil Crystalline Materials), in particular hexagonal MCM-41 ones have opened a wide field of potential applications in catalysis and separation processes [3]. For example Tanev et al have reported an exceptional activity of Ti-HMS materials for the hydroxylation of benzene with H2O2 in the presence of acetone as solvent [4]. Al-MCM-41 has been used for the cracking of polyethylene, n-heptane and vacuum gas oil [5], etc.... For all these applications, materials should have sufficient large pore diameters with uniform pore size distribution. If the synthesis and the characterization of conventional mesoporous materials with pore size of around 1.8-3.0 nm are well known, the synthesis of mesoporous materials with tailored pore sizes remains, however, a challenge for scientists. Post-synthesis treatments [6-7], surfactants of different chain lengths [8], polymers such as triblock copolymers [9] used as templates or incorporation of swelling agents such as tetraalkylammonium cations [10], amines [11], triisoprobylbenzene [12] and 1,3,5trimethylbenzene[13-14] can be employed to enlarge the pore size. However, the synthesis and sometimes the reproduction are difficult. Recently, Kunieda et al [15], on the basis of a study on the effect of oil on the structure of liquid crystals in polyoxyethylene dodecyletherwater systems, found that the penetration tendency is very predominant for alcohol and aromatic hydrocarbons such as m-xylene with no significant change in the micelle size, whereas the swelling tendency is preponderant force for saturated hydrocarbons such as decane and squalane which lead to large size micelles. The alkanes have already been used in the synthesis of mesoporous materials [16]. In the present work, we have modified our synthesis protocol of conventional mesoporous materials [17] in order to incorporate alkanes as expander.
76 2. EXPERIMENTAL 2.1. Synthesis Cetyltrimethylammonium bromide (CTMABr) was first dissolved in water with stirring at 40°C to obtain a clear micellar solution. Sodium silicate and alkane were then added drop by drop. The pH value of the gel obtained was adjusted with sulfuric acid. After stirring for several minutes at room temperature, the homogeneous gel with the molar composition of 1 CTMABr, x CnH2n^2, 0.63 Si02, 102 H2O (5 < n < 12, x varies from 0.5 to 4.0. However, in this paper we present only the results obtained with molar ratio of CTMABr/ CnH2n+2 equal to 1.0) was sealed in Teflon autoclaves. The final products were obtained after ethanol extraction with a soxhlet apparatus during 30 hours. It was found that after 30 hours solvant extraction, a small quantity of surfactant still remained in the pores of materials. A supplementary calcination under nitrogen and then air atmosphere at 500°C for 18 hours was made. The sequence of introduction of alkane and the effect of the alkane chain length on the final phase were studied. 2.2. Characterization Powder XRD patterns of obtained materials were recorded with a Philips PW 170 dififractometer, using CuKa (1.54178 A) radiation, equipped with a thermostatisation unit (TTK-ANTON-PAAR, RUBER HS-60). The scanning micrographs of obtained phases were made from a Philips XL-20 Scanning Electron Microscope (SEM) using conventional sample preparation and imaging techniques. For TEM observations, the samples were prepared using two different methods. First, sample powders were embedded in epoxy resin and then sectioned on an ultramicrotome. The thin films were supported on copper grids previously coated by carbon to improve the stability and reduce the accumulation of the charges. Other samples were prepared by dispersing the powder products in ethanol. The slurry was then dried on a holey carbon film placed on a Cu grid. The transmission electron micrographs were taken on a Philips EM 301 microscope equipped with a tungsten gun using an accelerating voltage of lOOkV. Nitrogen adsorption - desorption isotherms were obtained from a volumetric adsorption analyzer ASAP 2010 manufactured by Micromeritics. The samples were first degassed for several hours at 350°C. The measurements were then carried out at 196°C over a wide relative pressure range from 0.01 to 0.995. The average pore diameter and the pore size distribution were determined by the BJH method from the adsorption branch of isotherm [18]. 3. RESULTS AND DISCUSSION 3.1. Effect of the introduction sequence of the swelling agent The introduction of the swelling agent could affect strongly and orient the formation of the micelles and consequently the formation of final phase and the pore size of the materials. The effect of the introduction sequence of the swelling agent was studied to find the best way to incorporate the swelling agent into the formed micelles based on CTMABr in an aqueous solution. For this goal, decane used as probe was added before and after the introduction of sodium silicate and at different temperatures. Table 1 gives the synthesis conditions and the characterization results of obtained products by BET as well. From Table 1, it is evident, compared to the sample obtained without introduction of decane with the pore size of 2.6 nm, that the introduction of decane during the synthesis has a beneficial effect on the pore diameter. However the largest pore size is obtained if the swelling agent is added during the preparation of the micellar solution. In fact, if the decane is
77
Table 1 Specific surface area (SRET), total pore volume (V), determined by the BJH method and pore diameter (0) of products obtained from different synthesis conditions (decane/surfactant molar ratio equal to 10 and a crystallization time of 8 days at 100°C were used) Sample
SBET(mVg)
V(cm^/g)
0(nm)
A : without using decane
ll(f{115f
0.465^(0.452)'
2.6' (2.4)'
B : decane incorporated during the micellar solution preparation
66 r (750)'
0.908' (0.908)'
4.9' (4.9)'
5ir
0.627'
C : decane incorporated at room temperature after preparation of micellar solution before silica
4.5'
0.661' D : decane incorporated at room 640' 4.1' temperature after preparation of micellar solution after silica a : Obtained on the samples only after extraction by solvant and drying in vacuum without calcination. b : Obtained on the samples calcined after extraction by solvant added during the preparation of micellar solution, it can be easily incorporated into the core of the micelles to form aggregates and the swelling effect occurs as indicated by Kunieda et al [15]. The large micelles and finally the large pore materials are obtained. When decane is introduced after formation of micelles or especially after adding silica, the micelles are already formed, only part of decane added can penetrate into the center of the micelles under our oreoaration conditions, the swelling effect is less pronounced. It should be noted that characterization results were obtained from the 5.9 nm samples only extracted by ethanol and after drying Sample D in vacuum at 100°C during 24 hours. It is observed that the treatment used can not eliminate completely all surfactants and swelling molecules incorporated. The specific surface area can be 1 ^ underestimated. An additional calcination is necessary to remove all organic compounds and to liberate the pores of materials. The results given in Sample A Table 1 (in parentheses) show clearly the increase 4.2 nm in surface area but inchanged pore volume and pore 1 diameter after extraction plus calcination treatment. Figure 1 reports only the XRD patterns of samples A and D since samples B and C give -. . , similar XRD patterns to sample D. The hexagonal 2e(») MCM-41 structure is clearly identified by the Figure 1. X-ray diffraction presence of the three peaks characteristic of the patterns of obtained products. 100, 110 and 200 reflections respectively located at 29 - 1.48°, 20 = 2.56°, 29 = 2.97° for sample D. '^'>vw«V
u
78
This part of study indicates that the introduction of decane during the synthesis can highly enlarge the pore diameter but the best way to incorporate decane is during the preparation of the micellar solution. The final products have a high specific surface area and a high pore volume in spite of their underestimation due to the presence of remaining surfactant and swelling agent in the pores. In all following studies alkane will therefore be incorporated during the micellar solution preparation and to liberate completely the pore of materials after extraction, samples will be fiirther calcined. 3.2 Effect of the alkane chain length on the structure and the pore diameter of samples Different alkanes from pentane to dodecane were incorporated in the micellar solution of CTMABr. In order to investigate the influence of the alkane chain length on the structural and textural properties of the formed mesoporous materials, as mentioned above, all syntheses were performed using a molar ratio of alkane/surfactant equal to 1.0. The crystallization time and temperature are respectively 8 days and 100°C. 3.2.1. Structural characteristic and crystal morphologies 2.76 nm
g
3
S
2 17 npm m 3 77 nm \
2 02 nm
—I—
—T"
12
6
14
2e(°) Figure 2. Variation of the XRD patterns of the samples synthesized with different alkanes a : Cs, b : C6, c : C?, d : Cg, e : Cio, f: Cn and g : Cn
79 Except the sample obtained with nonane, for the compounds synthesized using pentane (Fig. 2a), hexane (Fig. 2b), decane (Fig. 2e), undecane (Fig. 2f) and dodecane (Fig. 2g) as swelling agent, the peaks characteristic of the 100, 110 and 200 reflections of the hexagonal MCM-41 structure with a well-ordered channel array are observed. The channel arrangement is further confirmed by TEM micrographs (figure 3). For the materials obtained with heptane (Fig. 2c) and octane (Fig. 2d), only a very weak peak concerning the 100 reflection is detected and the two others having too low intensities to be pointed with precision. Only an amorphous phase is observed from the sample obtained using nonane as swelling agent. Figure 4 depicts the variation of the unit cell (ao = 2 dioo/(3) ) with the number of carbon atoms of the alkane. If we do not consider the pore diameter value of the sample obtained with nonane, a linear relationship can be observed and the value of ao increases from pentane to decane, meaning that the wall thickness or the pore diameter or both increase. But from undecane, this value decreases sharply. The fact that MCM-41 phase is not observed either by TEM or XRD, suggests that using nonane as swelling agent can not lead to the synthesis of highly ordered MCM-41 structure. Crystals morphologies (Figure 5) of our samples are analogous with those reported in literature for conventional MCM-41 materials [17, 19-20]. WT'
a 1 ^1 :
mm ^^Ir SOi»«
Figure 3. TEM Micrographs (longitudinal view) of some samples synthesized using different alkanes as expanders, a : undecane b : dodecane.
Number of carbons atoms
Figure 4. Variation of the cell parameter with the number of carbon atom of the alkane incorporated as expander.
lOnin
Figure 5. Crystal morphology of the samples obtained with alkanes a : Cg, b : Cio, c : Cii and d : Cn as expander.
80 3.2.2. Textural characteristics Figure 6 shows the isotherms of the samples using different alkanes as expander. Except the sample obtained with nonane, the adsorption-desorption isotherms of all other compounds are type IV, characteristic of mesoporous materials according to the BDDT classification [21]. Isotherms can be decomposed in three parts : the formation of the monolayer, a sharp increase characteristic of the capillary condensation of nitrogen within the mesopores and finally a plateau indicating the saturation of the samples. From pentane to decane the relative pressure at which the capillary condensation occurs, increases from 0.30 to 0.60, indicating that the value of the pore diameter increases when the alkane chain length is raised since the p/po position of the inflection point is related to the pore diameter. From undecane, this value decreases to reach 0.40 for dodecane. We can conclude that the value of the pore diameter drops from decane to dodecane. For nonane even though a series of trials were further made, the sharp increase due to the capillary condensation is not observed. The material is not a structured compound and no homogeneous pore size distribution is obtained (Figure 7e). Some complementary studies will be done to explain this phenomena. This compound will not be taken into account in following analyses.
b
SBET = 411m7g
«
SBET = 576 m /g
i
8
SBET = 405 m'/g
f
SBET = 750 m7g
e
SBET = 489 m7g
I
Relative pressure p ^
Relative pressive p/pg
Figure 6. Nitrogen adsorption-desorption isotherms of the samples obtained by incorporation of different alkane as expander : a : C5, b : Ce, c : C?, d : Cg, e : C9, f: Cio g : Cn and h : Cn.
81 d
^^3.7 nm
^^3.5 nm
c
i b
^^2.6 nm 5
a
JBl; (A) thickness f^J; 0
calcined 1070 0.83 water-treated 705 0.23 0.6 calcined 950 0.70 water-treated 880 0.70 1.0 calcined 930 0.68 water-treated 885 0.65 1.4 calcined 1030 0.76 water-treated 1030 0.70 2.0 calcined 740 0.52 water-treated 750 0.54 ' The pore size distribution is broad and irregular.
27 25 27 28 28 27 27 27 27 27
27 —* 27 24 26 24 27 24 25 23
46.1 41.9 45.7 44.9 45.1 43.9 46.0 44.6 44.7 45.9
19.1 __* 18.7 20.9 19.1 19.9 19.0 20.6 19.7 22.9
41 samples prepared with and without TPA* are shown in Fig.2. It clearly shows that in case of TPA"" free MCM-41 -sample, the mesopore structure was completely destroyed after the hydrothermal treatment. On the other hand, with the addition of TPA^ the structure retains most of its mesopore structure even after 4 days of hydrothermal treatment. However, the sharp inflection observed in the isotherm at p/po= 0.35 for the calcined sample has been slightly broadened after hydrothermal treatment, indicating that the pore size distribution has been changedfi^oma very narrow range to slightly extended range. Table 1 lists the BET surface areas and other physical properties of the calcined and hydrothermally treated samples. It can be seen that these samples possess very high surface area in the range of 800-1000 mVg. Pore wall thickness of the calcined samples was found to be about 19 A and no increase in wall thickness was noticed by adding additional TPA^ ions. However, the wall thickness was found to increase slightly after hydrothermal treatment due to pore wall restructuring during such treatment [12,13]. 3.2 Effect of Other Cations Effect of cations other than TPA" has also been studied and the results are given in Table 2 and Fig.3. The molar ratio of additional cation/CTA* was kept at 1.4. It can be seen that cations like tetramethylammonium or tetraethylammonium ions or even sodium also gives highly ordered MCM-41 structure. The BET surface area of these samples was found to be >1000 mVg. Fig.3 shows that the MCM-41 structures obtained with these additional cations have improved hydrothermal stability. The long range ordering was unaffected by 4 days of hydrothermal treatment. Moreover, the surface area and pore volume of the water-treated samples was only marginally lower than that of the calcined samples. Mesopore size distribution analysis showed a very narrow distribution with a peak pore diameter of about 27 A for the calcined samples. It can be seen that the pore wall thickness of the calcined samples was in the 18-19 A and did not change much by adding additional cations. The increased hydrothermal stability observed was, therefore, not due to any increase in pore wall thickness. For the water-treated samples, the adsorption isotherms and pore size distribution curves showed that after hydrothermal treatment the pore size distribution was
89 Table 2 Physical properties of MCM-41 samples prepared with different additional cations Cation
_typ^ TPA" TEA" TMA" Na"
Treatment
ABET
VRJH
DBJH
Peak pore
a^
calcined water-treated calcined water-treated calcined water-treated calcined water-treated
(m'/g) 1030 1030 1010 940 1045 960 1020 920
(cm^/g) 0.76 0.70 0.80 0.76 0.87 0.79 0.80 0.72
(A) 27 27 28 27 28 27 28 27
diameter (A) 27 24 27 24 27 24 27 24
{A) 46.0 44.6 45.5 45.4 46.4 44.8 45.0 43.3
Pore wall thickness (A) 19.0 20.6 18.5 21.4 19.4 20.8 18.0 19.3
sUghtly broadened, however, the mesopore structure was unaffected. The peak pore diameter also slightly shifts to a lower value due to increase in pore wall thickness. ^^Si MAS NMR spectra of the uncalcined MCM-41 samples synthesized normally and with T P A " and Na" are shown in Fig.4. It was observed that the ratio of Q4/Q3 peaks was higher in samples synthesized with additional cations. The effect was most pronounced with TPA" as the additional cation. The higher Q4/Q3 ratio indicates that the silicate polymerization during the formation of the mesostructure was enhanced by the presence of the additional cations. Upon calcination, the free silanol groups are forced to condense to form Si-O-Si bond and ^^Si MAS NMR of the samples showed predominantly Q4 peak. However, these
(b)
3
20 (degree)
4
5
6
7
20 (degree)
Fig.3 XRD patterns of MCM-41 samples synthesized with different additional cations; (a) calcined samples, (b) afler hydrothermal treatment (cation/surfactant = 1.4).
90
(b)
Fig.4 Si MAS NMR spectra of uncalcined MCM-41 samples with different cations; (a) no cation, (b) TPA^ and (c) Na^. Si-O-Si bonds are likely to be under strain and will easily be opened up in the presence of water. It appears that during the hydrothermal treatment water molecule readily hydrolyses these bonds thereby affecting the mesostructure of the MCM-41 samples. However, samples with higher Q4/Q3 ratios are less susceptible to such attack by water molecule and thereby retain their mesostructure unaffected by hydrothermal treatment. The improvement of hydrothermal stability of MCM-41 samples was possibly due to alteration of the electrostatic interaction between the cationic surfactant micelles and the surrounding anionic silicate species by the presence of additional cations. It has been reported that the presence of TAA* cations in aqueous silicate solutions enhances the abundance of symmetric, cage-like polysilicate anions and impedes the hydrolysis of the anions by forming a protective hydrophobic shell [14]. Presence of alkali metal cations also found to stabilize polymerized silicate anions [15]. These additional cations possibly interfere with the rapid condensation of the silanol groups during the acidification process. Rapid condensation usually leads to structures containing more number of defect sites, in this case Q3 sites. In contrast, slow condensation of the silanol groups results in a structure that has more Q4 sites and hence higher hydrothermal stability. 3.3 Synthesis with C14 and Cis Surfactants This method of synthesizing MCM-41 with improved hydrothermal stability is applicable with surfactants of different chain length also. XRD patterns of the calcined samples synthesized with surfactants Cuand C18 chain length with TPA" as additional cation and without any cations are shown in Fig. 5. XRD patterns of the hydrothermally treated samples are also included for comparison. It can be seen that when no TPA* was added to the synthesis
91
0^
3
26 (degree)
4
5
6
29 (degree)
Fig.5 XRD patterns of MCM-41 samples synthesized with C M and Cig surfactants without any additional cation (a) and (b), and with TPA^ as additional cation (c) and (d). Calcined samples (a) and (c), water-treated samples (b) and (d).
gel the MCM-41 structures formed were easily degraded by hydrothermal treatment. Although the 100 peak was detected in the XRD, its intensity was drastically reduced. Also the reflections from other higher order planes like 110, 200 and 210 were either disappeared or became very weak after hydrothermal treatment. In contrast, the samples prepared with TPA^ were found to be quite stable to hydrothermal treatment. The intensity of the 100 peak was found to be very strong even after 4 days of hydrothermal treatment. Reflections from other higher order planes are also clearly visible. The nitrogen adsorption-desorption isotherm of calcined MCM-41 samples synthesized with Ci4 surfactants showed a sharp inflection at p/po = 0.3 characteristic of capillary condensation within uniform pores. The average pore diameter was found to be ca. 23 A. Samples prepared with Ci8 surfactants, on the other hand, showed a sharp inflection at a higher p/po value of 0.4 indicating presence of bigger pores with an average pore diameter of ca. 30 A. The BET specific surface area, mesopore volumes, and pore wall thickness of the calcined and water-treated samples are given in Table 3. BET surface area of the samples prepared with CM surfactants were found to be less affected by hydrothermal treatment. When the samples synthesized without TPA^ subjected to hydrothermal treatment the sharp inflection in the isotherm became very broad indicating wide distribution of pores. In contrast, the mesopore distribution of the samples prepared with TPA' was found to be less affected by hydrothermal treatment. For the samples prepared without TPA^ the mesopore volume was found to decrease sharply and the pore diameter was broadened over a large range indicating loss of the mesopore structure. Addition of TPA* was found to minimize the structural collapse and thereby helps to preserve the mesoporosity
92 ACKNOWLEDGEMENr D. Das is grateful to National Science Council, Taiwan for a post-doctoral fellowship. Authors wish to thank China Petroleum Corporation, Taiwan for financial assistance. REFERENCES 1. C.T. Kresge, M.E.Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 1992, 359, 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc, 1992,114, 10834. 3. C.Y. Chen, H.X. Li and M.E. Davis, Microporous Mater., 1993, 2, 17. 4. J.M. Kim, J.H. Kwak, S. Jun and R . Ryoo., J. Phys. Chem., 1995, 99, 16742. 5. R. Ryoo, J.M. Kim, C.H. Ko and C.H. Shin, J. Phys. Chem., 1996,100, 17718. 6. L.Y. Chen, S. Janicke and O.K. Chuah, Microporous. Mater., 1997,12, 323. 7. R. Ryoo and S. Jun, J. Phys. Chem., 1997,101, 317. 8. K.A. Koyano, T. Tatsumi, Y. Tanaka, S. Nakata, J. Phys. Chem. B, 1997,101, 9436. 9. X.S. Zhao and G.Q. Lu, J. Phys. Chem. B, 1998,102, 1556. 10. H.R Lin, C.Y. Mou and S. Cheng, Microporous Mater., 1997, 10, 111. 11. D. Das, C-M. Tsai and S. Cheng, J. Chem. Soc. Chem. Commun., 1999, 473. 12. N. Coustel, F. Di Renzo and F. Fajula, J. Chem. Soc. Chem. Commun., 1994, 967. 13. L. Chen, T. Horiuchi, T. Mori and K. Maeda, J. Phys. Chem. B, 1999,103, 1216.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
93
Improvement on thermal stability and acidity of mesoporous materials with post-treatment of phosphoric acid L.-M. Huang and Q.-Z. Li Department of Chemistry, Fudan University, Shanghai 200433, P. R. China
The thermal stability and acidity of mesoporous materials can be greatly improved by the impregnation of phosphoric acid. Because of high surface area and medium acid strength, H3P04-modified AlMCM-41 materials show potential applications in cracking of long-chain alkanes for producing gas olefins, especially iso-butene.
1. INTRODUCTION The discovery of a new family of highly ordered mesoporous materials with uniform pore size in the range of 1.6-30 nm has greatly expanded the capacities of heterogeneous catalysis. ^•^ It has been reported that the incorporation of Al species to siliceous MCM-41 can result in potential acid catalysts. However, because of the amorphous character of mesoporous materials, the disadvantages of AlMCM-41 — lack of thermal or hydrothermal stability and lack of strong acidity — would greatly limit their applications in acid catalysis."* Up to now, there are several routes related to the improvement on the thermal stability of mesoporous materials, such as pH adjustment,^ ion-exchange,^ salt effect^ and surface silylanization.^ Through these methods, the thennal stability of MCM-41 is largely improved due to the increase of aluminosilicate polymerization degree and the decrease of surface hydroxyl groups. The recent synthesis of mesoporous material (SBA-15) also shows great improvement in thermal stability, due to its increased wall thickness up to 60 A and less weakness in the pore wall.^ In general, because of the amorphous character of AlMCM-41 material, the improvement of its acidity still remains a great challenge. Some efforts have been exerted on the re-crystallization of the mesopore wall. ^ In this paper, we put forth a simple and feasible route to increase both thermal stability and acidity of mesoporous materials. Many facts have shown that the frameworks of mesoporous materials are soft and can be shrunk after removal of surfactants, different from the rigid frameworks of crystalline zeolites. By studying the kinetically destroying process of mesoporous materials during thermal or hydrothermal treatment, it is found that under high temperatures, dioo values gradually decrease before the total collapse of the mesostructures. That is to say, the pore channels of mesoporous materials gradually shrink before collapsing because of the severe dehydration of surface hydroxyl groups followed by dealumination. Thus, we can imagine that, the mesostructures would be greatly preserved by reacting with the abundant hydroxyl groups in order to resist the kinetic shrinkage of mesostructures. It is found that H3PO4 is such an
94 excellent candidate/^'^^ On one hand, P-OH bonds in H3PO4 molecules can easily react with surface OH groups, therefore reducing the further condensation of the OH groups. Likewise, H3PO4 molecules can also fill up the weakness in mesopore wall. On the other hand, H3PO4 molecules can be polymerized to polyphosphoric acid with network structures at high temperatures. These network structures, which are tightly attached to the surface of mesostructures, can effectively resist the shrinkage of pore channels during thermal or hydrothermal treatment. In present paper, H3PO4 - impregnated MCM-41 materials was prepared and its thermal stability, acidity and cracking activity was compared with those for a commercial zeolite catalyst and AlMCM-41 before impregnation.
2. EXPERIMENTAL 2.1 Materials H3P04-modified mesoporous materials were prepared as follows. Firstly, NaAlMCM-41 (Si/Al=14.2) was synthesized according to the procedure by Ryoo et al.^ The product was calcined at 550 °C to remove surfactant cetyltrimetylammonium bromide (CTAB). Then, the calcined NaAlMCM-41 was impregnated with the calculated amount of H3PO4 solution at pH = 1-3 and 30 °C before dried at 100 °C. The products were allowed for calcination at 550 °C for 4 h. The final samples were denoted as PiM41 and P8M41, according to the H3PO4 impregnation amount of 1 wt% and 8 wt% (P2O5 wt%), respectively. For comparison, HAlMCM-41 was prepared from ammonium-exchanging NaAlMCM-41 at 70 °C for 2 h followed by calcination at 550 °C for 6 h. Thermal and hydrothermal stability tests were conducted at 1000 °C for 0.5 h in dry Ar flow and at 800 ^C steaming (100 % H2O) for 2 h, respectively. 2.2 Characterization XRD patterns were obtained with a Rigaku D/MAX-IIA diffractometer system equipped with Ni-filtered Cu-Ka radiation. N2 adsorption/desorption isotherms were measured vsdth a Micromeritics ASAP 2010 system. NH3-TPD was conducted under He flow of 30 ml/min and a heating rate of 20 °C/min. 2.3 Reactivity The cracking reactions of normal alkanes such as n-C?®, n-Cio°, n-Ci2° and n-Ci6° were performed in a pulse microreactor at 500 °C with N2 flow rate of 15 ml/min and pulse amount of 0.5 ul. 100 mg of catalysts were put into a quartz tube v^th diameter of 4 mm. For n-Ci6° cracking, similar conversions were obtained by varying the amount of the catalysts used. 3. RESULTS AND DISCUSSION Fig. 1 shows well-defined reflection peaks (100), (110), (200) and (210) for H3PO4modified AlMCM-41, compared with calcined NaAlMCM-41. Only a little decrease is observed for dioo value and (100) peak intensity in P8M41. BET surface areas decrease from 939 m^/g to 850 m^/g then to 830 m^/g and pore sizes also decrease from 3.1 nm to 2.9 nm with the increase of H3PO4 impregnation amount (Table 1). The results indicate that the
95 impregnation of H3PO4 has only a few changes in the mesostructural integrity and pore parameters of AlMCM-41 samples. However, HAlMCM-41 shows larger contraction of pore channel because of the dealumination during the ammonium-exchanging process and the subsequent calcination. When H3PO4 loading increases up to 8 wt %, XRD profile of P8M41 still presents at least three diffraction peaks representative of well-ordered hexagonal mesostructure even after severe thermal treatment at 1000 °C. Whereas HAlMCM-41 totally lost its mesostructure under the same condition (Figure 1). Likewise, under hydrothermal treatment at 800 ^'C, the percentage of pore channel shrinkage is only 13.0 % for P8M41, compared with 19.5 % for NaAlMCM-41 and 22.3 % for HAlMCM-41. Especially for P8M41, it still has BET surface area of 568 m^/g after strong hydrothermal treatment (Table 1). The above results indicate that H3PO4 impregnation can effectively resist the pore shrinkage, thus resulting in less destroy of the mesostructure.
Figure 1. Effect of thermal and hydrothermal treatment on various AlMCM-41 materials. (1) NaAlMCM-41, (2) HAlMCM-41 and (3) P8M41. (a) before treatment, (b) thermal treatment at 1000 °C for 0.5 h and (C) hydrothermal treatment at 800 °C and 100 % for 2 h. It has been demonstrated that phosphoric acid can fill up the weakness in mesopore wall by the interaction of P-OH bonds with surface hydroxyl groups. Moreover, network polyphosphoric acid can be formed at high temperature, which help maintain the mesostructure by resisting the pore channel shrinkage. In addition, it is estimated for P8M41 that, the number of H3PO4 molecules distributed on MCM-41 surface (-8x10^%) is greatly lower than the number for a single layer loading (~8xlO^Vg). Therefore, the low H3PO4 loading, which has few changes to the mesostructural property, is more effective to improve the thermal stability of mesoporous material.
96 Table 1 Effect of thermal and hydrothermal treatment on various AlMCM-41 materials Sample NaAlMCM-41 HAlMCM-41 PiM41 Before SBET/m7g 939 870 850 stability
P8M41 830
Pore size/nm
3.1
3.1
2.9
2.9
test
dioo/nm
4.01
3.76
3.84
3.84
Steaming
dioo/nm
3.23
2.92
3.26
3.34
treatment
SBEi/m /g
450
dioo/nm
3.46
568
at 800 °C Calcination
-3.0
3.46
at 1000 °C
In addition to the improvement in thermal stability, H3PO4 impregnation also afford mesoporous material with improved acidity. The NH3-TPD results of H3P04-modified AlMCM-41 show that with the post-treatment of H3PO4, not only the acid strength has been greatly improved, but a little increase in acid number as well (Figure 2 and Table 2). It also suggests that the network polyphosphoric acid be formed at 550 °C and low loading amount of 8 wt % H3PO4. Although we have no direct evidence for the formation of polyphosphoric acid, we can estimate that, if one H3PO4 molecule is impregnated on the surface of MCM-41 and the neigboring H3PO4 molecules do not react with each other, one additional acid site is
s 08
^CA C 0
Q. w
^ ^
>X\^^'^^--^H
y/ O ^ ^ . ^^^ ^^*^ ^^** / // V X^ ^^^b ^^^ / ^^^-^^ y 1 1 ^^^^\ ^1 1——1 U
/ ^
100
/
^fc.
300 500 Temperature ('C)
Figure 2. NH3-TPD profiles of mesoporous materials, (a) NaAlMCM-41, (b) HAlMCM-41 and (c) P8M41
97 Table 2 NH3-TPD results of various mesoprous materials Sample Acidity Acid Strength (^C)
Acidnumber(x 10^^/g)
NaAlMCM-41
^210
4.1
HAlMCM-41
-250
6.0
P8M41
-'250
4.8
expected to be produced. However, the acid number in this paper does not increase so much as expected. Therefore, it may suggests that the neighboring H3PO4 molecules would be polymerized to form polyphosphoric acid during calcination. Moreover, as shown in Table 1 and Figure 1, the preparation of HAlMCM-41 from ammonium-exchanging NaAlMCM-41 resuhs in a great loss of thermal stability. While H3PO4 can be directly impregnated on NaAlMCM-41 without further ammonium exchange, and the resulting P8M41 possesses the similar acidity and reactivity for cracking n-alkane to HAlMCM-41 (Table 3). Although P8M41 is less active for n-heptane cracking, it shows the similar activity to a commercial ZSM-5 catalyst for n-hexadecane cracking which requires only weak and medium acid sites. Compared with ZSM-5 catalyst, less gas products and more gasoline and kerosene (C9-C14) are produced for P8M41 due to its relative medium acid strength. Moreover, large surface area and small acid number (i.e. low acid site density) could result in the decrease of hydrogen transfer reaction. So, good selectivity can be achieved toward gas olefins (up to 85 % in gas products) for long-chain alkane cracking. ^^ In addition, among gas olefins C2~-C4~, the proportion of butene is as high as 54 % compared with 40 % for commercial ZSM-5 catalyst. Therefore, the cracking reaction shows better selectivity toward iso-butene, which is an important feed for producing MTBE — one of excellent gasoline additives. Therefore, H3P04-modified NaAlMCM-41, which has high thermal stability, medium acid strength and low acid site density, is promising catalyst for long-chain alkane cracking for producing gas olefins, especially iso-butene.
4. CONCLUSION In conclusion, by studying the kinetically destroying process of mesoporous materials, we find that the modification of H3PO4 is an effective route to improve the mesostructural stability by resisting the pore shrinkage during thermal treatment. It has several specific advantages indicated above. The effect of the loading amount of H3PO4 on the properties of MCM-41 is still underway. Moreover, the idea of this method can also be expanded to the mesoporous materials other than MCM-41. The improvement of both thermal stability and acidity would make mesoporous materials more promising to act as acid catalysts.
98 Table 3 Comparison of cracking performance of n-alkanes over various samples (reaction temperature: 500 "C) Sample
' ZSM-S""^
P8M41
HAlMCM-41
Cracking
n-C7°
10.3
3.3
3.2
activity/%
n-Cio°
32.7
14.2
16.3
Sample weight:
n-Ci2°
67.0
24.8
32.5
n-Ci6°
99.2
88.5
99.6
n-C,6^- 90% 62.8 75.8
66.7
24.2
37.2
33.3
C2~ - C4 ~
62.9
53.5
56.7
iC4^
16.8
18.6
18.1
Selectivity in
Olefin/%
83
85
85
gas products
C2 1C3 :C4
10:50:40
3:44:54
3:47:50
C4 /C4
0.19
0.15
0.16
iC4^/TC4"
0.67
0.65
0.64
100 mg
Conversion of Gas (C1-C4) Selectivity/%
Liquid (C5-C14)
* aged with 100 % H2O steam at 800 °C for 4 h. ACKNOWLEDGMENT
This work is supported by NSFC (Grant No. 29733070).
REFERENCES 1 J. S. Beck et al, J. Am. Chem. Soc, 114 (1992) 10834. 2 D. Zhao, J. Feng, Q. Huo, N. Melosh, G. H. Fredrickson, B. F. Chmelka, and G. D. Stucky, Science, 279 (1998) 548. 3 S. hiagaki, Y. Fukushima, and K. Kuroda, Stud. Surf. Sci. Catal., 84 (1994) 125 4 A. Corma, V. Fomes, M. T. Navarro, and J. Perez-Pariente, J. Catal, 148 (1994) 569. 5 R. Ryoo and J. M. Kim, Chem. Commun., (1995) 711. 6 J. M. Kim, J. H. Kwak, S. Jun, and R. Ryoo, J. Phys. Chem, 99 (1995) 16742. 7 R. Ryoo and S. Jun, J. Phys. Chem. B, 101 (1997) 317. 8 T. Tatsumi, K.A. Koyano, Y. Tanaka, and S. Nakata, Stud. Surf. Sci. Catal., 117(1998) 143 9 K. R. Kloetstra, H. van Bekkum, and J. C. Jansen, Chem. Commua,(1997) 2281. 10 B. Viswanathan and A. C. Pulikottil, Catal. Lett., 22 (1993) 373. 11 W. W. Kaeding and S. A. Butter, J. Catal., 61 (1980) 155. 12 X.Y. Chen, L. M. Huang, G. Z. Ding, and Q. Z. Li, Catal. Lett., 44 (1997) 123.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
99
In situ Synthesis of Micro- and Mesoporous Al-MFI / MCM-41 like Phases with High Hydrothermal Stability. Ame Karlsson^*, Michael Stocker^ and Karin Schafert>, a SINTEF Applied Chemistry, P.O. Box 124 Blindem, N-0314 Oslo, Norway. ^ Grace GmbH, In der Hollerhecke 1, D-67545 Worms, Germany *Corresponding author Micro- and mesoporous aluminum containing MFI / MCM-41 like composite phases were synthesized in situ using mixtures of C6Hi3(CH3)3NBr and Ci4H29(CH3)3NBr templates / surfactants in the ratios of 85%/15% and 75%/25% in the synthesis gels and tested in 100% steam for 5 hours at temperatures of 650 and 815°C, respectively. An improved hydrothermal stability was observed for some of the aluminum containing composites obtained using these mixtures of surfactants, the more interesting samples formed using synthesis temperatures from 168 to 175°C. The two most interesting types of samples were composites consisting of approximately 60%/40% mixtures of MFI/MCM-41 like structures, and of a 95% MFI and 5% mesophase, respectively. The surface area retention for the first type after heating in 100% steam for 5 hours at 815°C was in the range of 45-55% (N2-BET surface area (BET-SA) after steaming ca. 215 m^/g), while the second type showed a surface area retention of almost 85% (N2-BET surface area after steaming ca. 340 m^/g). These materials are potentially interesting candidates in ZSM-5 catalyzed processes in which diffusional restrictions have been a limiting factor.
1. INTRODUCTION The recently discovered mesoporous materials can be synthesized with regular pore diameters in the range of at least 15-100 A [1-3]. Consequendy, these materials expand dramatically the range of materials available for applications in size selective processes or in processes where diffusional restrictions have been a limiting factor using the tradidonal zeolites. One interesting applicadon would be as a catalytic material in the conversion of large hydrocarbon molecules. Unfortunately, the hydrothermal stability of these materials has so far been insufficient to withstand commercially relevant operadng conditions, and most samples collapse readily when exposed to 100% steam at 600°C [3], We reported earlier [4] results obtained by exploring in situ I simultaneous synthesis of siliceous "zeolite / MCM-41" materials using a mixture of C6Hi3(CH3)3NBr and Ci4H29(CH3)3NBr surfactants (referred to below as Q and Cy^, respectively) under various synthesis conditions. Interesdngly, treatment
100
in 100% steam at temperatures up to 815°C for 5 hours showed that some of the composites exhibited a very high hydrothermal stability [5]. Characterization results revealed the formation of fairly complex aggregates of MFI and MCM-41 type material [4]. The enhanced hydrothermal stability obtained for the siliceous MFI and MCM-41 composite samples indicated an integration of the two structures beyond a simple physical mixture, in which case aluminium containing analogues would have the potential of a unique combination of acid properties and integration of microporous and mesoporous structures. In the present paper we present recent synthesis and characterization results of the aluminum containing composite analogues.
2. EXPERIMENTAL The syntheses of purely siliceous MFI/MCM-41-type composites (described in detail in ref. [4]) were obtained by optimising template concentrations and reaction temperatures using the general molar gel composition: 0.5 (Q+C;^): Si02 : 0.28 Na20 : x H2O X varied from 44 to 52 depending on the C^Ci4 ratio. It appeared that the relative amounts of the two structures in the final composite samples were simple functions of the ratio of the templates and the synthesis temperature: a high C^Ci4 ratio and high synthesis temperature favored a high MFI/MCM-41 ratio in the final product and vice versa, the most interesting samples were obtained using C^Ci4 ratios of 85%/15% and 75%/25%, respectively, and synthesis temperatures in the range of 150 to 175°C. For the synthesis of the aluminum containing analogues, we attempted to approximate the gel and synthesis conditions closely, in order to maintain this predictable behavior if possible. Therefore, a fairly high Si/Al ratio of 20 was chosen as a first approach using 0.43 g of sodium aluminate (54% AI2O3, 41% Na20, Riedel de Hahn) in the usual recipe. The general molar gel ratios can now be summarised as: 0.5 {C6+Cj4): 0.025 AI2O3: Si02 • 0-3 Na20 : x H2O The sodium aluminate and appropriate amounts of surfactants/templates were dissolved in water using C^Ci4 ratios of 85%/15% and 75%/25%, respectively, and the mixture was aged over night (16-20 hours) according to ref [6]. Subsequently, the usual gel preparation described earlier [4] were followed with no further changes, then the gels were transferred to PTFE-lined autoclaves and heated at 150-175*'C for 6 days, quenched in cold water, washed 3-4 times in liberal amounts of water, decanted, filtered and dried overnight in ambient air. The samples obtained by this procedure, containing various ratios of zeolite and mesoporous materials, were tested in 100% steam for 5 hours at temperatures of 650 and 815°C, respectively, in a fixed bed reactor. Prior to the tests the samples were calcined at 550°C in N2 (1 hour) and dry air (4 hours). Synthesis and characterization data for the most interesting samples with regard to improved hydrothermal stability are listed in Tables 1 and 2.
101 Table 1. Synthesis and characterization data of the parent samples A-C. The samples were obtained using combinations of C6Hi3(CH3)3NBr and Ci4H29(CH3)3NBr in the gels and various synthesis temperatures. Synthesis time 6 days. Si/Al^^, ratio: 20. Sample
Sample A Sample B Sample C Reference^
C6
75 75 85 0
Ci4
25 25 15 100
Synth, temp. [°C]
175 168 168 150
Type of product' [%] MFI
MCM-41
60 60 95 0
40 40 5 100
d[100] [A]
Si/Al
61 48 45 40
16.6 20.7 9.42 0
1. Subjective estimates from XRD peak intensities. 2. Aluminum distribution inhomogeneous 3. Siliceous pure MCM-41 synthesized by the same procedure (see ref [4, 5] for further details).
Table 2. N2-BET surface area (BET-SA [mVg]) of the calcined (550°C) and steamed (100% steam at 650 and 815°C for 5 hours) samples A-C and a siliceous pure MCM-41 referenced BET-SA [mVg] Sample Sample A Sample B Sample C Reference 1
550°C, calcined
650°C, 100% steam
815°C, 100% steam
397 493 410 928
334 416 387
213 215 341 69
1. Siliceous pure MCM-41 synthesized by the same procedure (see ref. [4, 5] for further details).
102 After treatment at various temperatures the samples were analysed by XRD (Siemens D 5000 diffractometer with a Ge monochromator and CuK^^ radiation, step time 7.0 sec. (26>region 17**) and 1 sec. (20 region 3-40°), step size 0.020° 20, wave length: 0.15406 nm) and nitrogen adsorption isotherms (Carlo Erba Sorptomatic 1800). The calcined samples were investigated by Infrared Spectroscopy (Perkin Elmer Model 2000 in the 4000-370 cm^ range; KBr tablets) and the Si/Al ratios were determined using a Cameca Microbeam electron microprobe (EPMA) as an average of five points (area analysis 25x25 ^m).
3. RESULTS AND DISCUSSION In our previous synthesis of the siliceous samples [4], interesting MFI/MCM-41-type composites with high hydrothermal stability, having MFI/MCM-41 ratios of 0.05 to 0.1, were obtained using C / Q ^ ratios of 85%/15% or 75%/25% at synthesis temperatures of 150°C and 175°C, respectively. In the present study, introducing aluminum into the gels, the similar synthesis conditions lead to almost pure MCM-41 like phases, and to predominately MFI-like phases, respectively, and only small improvements in hydrothermal stability. Therefore, the intermediate temperature range was investigated closer. The more interesting synthesis temperatures turned out to be in the range of 168-175°C. For this temperature region, the synthesis procedure is somewhat labile and produces composites with comparable amounts of MFI and MCM-41 as well as almost pure MFI-type or MCM-41 like structures. The amount of MFI- and MCM-41-type material were estimated from XRD peak intensities. For the samples containing only small amounts of MFI, the presence of non-XRD detectable structures were identified for all samples by weak intensities in the infrared spectra near a frequency of 550 cm-^ which is characteristic for a skeleton bending mode for the MFI type samples [7]. The hydrothermal stability obtained also varied considerably for the samples, in the following, only three samples revealing the best hydrothermal stability will be discussed in detail. Synthesis and characterization data for the selected samples A, B and C, are summarized in Tables 1 and 2. The corresponding data for a siliceous pure MCM-41 sample synthesized by the same procedure [4,5], have been included for comparison reasons. The N2-BET surface areas and surface area retention measured after calcination at 550°C and steaming at 650, and 815°C are plotted in Figure 1 (left and right, respectively). The XRD patterns in the low angle region important for mesoporous structures are displayed in Figure 2, right, for samples A, B and C. The characteristic low-angle peak for the mesoporous structure may be identified for all the calcined samples and the first few peaks in the diffractogram of the MFI structure are visible as well. The N2 adsorption isotherms are displayed in Figure 2, left, for the same samples. The pure siliceous MCM-41 sample (reference) synthesized earlier by the same procedure [4, 5] showed the typical high surface area, well resolved [100], [110], [200] and [210] diffraction peaks in the XRD pattern and an N2 adsorption isotherm (lUPAC type IV) revealing a sharp inflection in the curve at ca. p/po=0.33 due to pore condensation typical for a narrow pore size distribution around a value of 28 A. The siliceous composite samples obtained, using combinations of the C^ and C;^ templates and different synthesis
103
1000
CM
100
Ref(Si-MCM-41)
750
E
0)
o
500
Sample C
75
" n
50
Sample C Sample A Sample B
2
3 (0 IUJ CM
g, c o c
250 -|
n o u Sample i
•S
25 H
3
Sample B
Ref(Si-MCM-41)->- A
Ref(Si-MCM-41)-^ A 550
650
750
Temperature (C)
850
550
650
750
850
Temperature (C)
Figure 1. N^-BET surface areas (left) and surface area retention (right) of samples A, B,C and the reference sample (Si-MCM-41) calcined at SSO^'C and steamed at 650 and 815°C, respectively. temperatures, showed lower N2-BET surface areas and less well resolved features in the XRD and N2-adsorption curves, and the present aluminum containing composite analogues display similar characteristics (Tables 1 and 2, Figure 2). Figure 2 also displays the XRDs and N2adsorption isotherms obtained after steam treatment, allowing the evaluation of the structural integrity as a function of the steam temperature for the various samples. The N2-adsorptiondesorption curves reveal two inflections in the curves, at ca. 0.2-0.3 and 0.5 p/po, and two peaks in the pore size distribution (PSD) curves, indicating a bimodal PSD character in these samples. The first inflection in the curves (difficult to observe for samples A and B in the condensed presentation of Figure 2), corresponding to pore diameters in the 21-28 A range, becomes more visible subsequent to steaming. The second inflection is most pronounced in the desorption branch from which pore diameters around 40 A may be derived. The hysteresis loops observed in the 0.5 - 1 p/po region, might be explained by imperfections or "bottlenecks" in the channel system [8,9] and/or textural mesoporosity [10]. Most samples reported in the literature collapse readily when exposed to 100% steam at 600°C [3]. The composite samples A, B and C in the present investigation retain a considerable part of the surface area even after steaming at 815°C (55, 45 and 83% for samples A, B and C, respectively (Figure 1). A closer inspection of the N2- isotherms and XRDs for samples A and B (Figure 2) reveals that the typical features for mesoporous structures are quite well preserved after steaming at 650°C, while reduced considerable at the higher steaming temperature. Sample C may be considered as a MFI-type material containing a small amount of meso-structure, roughly estimated to 5%, a fact that explains partly the
104 250
7000
125 +
3500
(0
o tn -a
HUU
Q. I-
T3
7000 Sample B 1 550°C, c ^ A 650°C,s N^^^Ji^^jeweeeawe^^
200
0/^^^
o
Sl^^Cs^^^
V)
0 0.5 P/Po
7000
P/Po
Figure 2. N2-adsorption-desorption isotherms (left) and X-ray diffraction patterns in the low angle region, important for mesoporous structures (right) for samples A-C, calcined at 550°C and steamed at 650 and 815°C, respectively, c, calcined; s, steamed.
105
extremely good hydrothermal stability. It is interesting to note, however, that the characteristics of a mesophase in the N2- isotherms is maintained to a very large extent for this sample even after steaming for 5 hours at 815°C. If the XRDs recorded after high temperature steaming are consulted (Figure 2), it is difficult to identify the low-angle «meso-peak», indicating that the amount of mesoporous structure present is too small for XRD detection or that the regularity of the remaining mesoporous structure is too low. The most stable purely siliceous composite material reported earlier [4] had well preserved characteristics after steam treatments. The composition was different, however, ca 10% MFI and 90% mesophase, allowing the mesoporous content to be assessed easier. Previous characterisation (SEM, HREM) of the purely siliceous composite materials [4] revealed the formation of fairly complex aggregates of MFI and MCM-41 type material. The data were not sufficient to assess a more "intimate" integration of the two structures, although the enhanced hydrothermal stability obtained [5] would indicate the presence of a closer interaction between the two phases than just a physical mixture. The actual structure of the present samples has not been studied in further detail so far. Taking into account the considerable potential within catalytic applications for structures which may possess unique integrations of acidity and structural properties, the synthesis, modification, catalytic activity and structure of these materials should be studied closer.
4. CONCLUSIONS Micro- and mesoporous aluminum containing MFI / MCM-41 like composite phases were synthesized in situ using mixtures of C6Hi3(CH3)3NBr and Ci4H29(CH3)3NBr templates / surfactants in the ratios of 85%/15% and 75%/25% in the synthesis gels and tested in 100% steam for 5 hours at temperatures of 650 and 815°C, respectively. An improved hydrothermal stability was observed for some of the aluminum containing composites obtained using these mixtures of surfactants. The more interesting samples formed, using synthesis temperatures from 168 to 175°C, were composites consisting of approximately 60%/40% mixtures of MFI/MCM-41 like structures, and of a 95% MFI and 5% mesophase, respectively. Characterization results indicate the presence of meso-structure maintained in the samples even after 5 hours in 100% steam at 815°C, leading to MFI type materials with a small content of structure in the meso region. These materials are potentially interesting candidates in ZSM-5 catalyzed processes in which diffusional restrictions have been a limiting factor. Although the increased hydrothermal stability of the Al-containing MFI/MCM-41 like phases was not as pronounced as for the corresponding Al-free composites, we regard this observation as an improvement, taking into account the general rule of less hydrothermal stability when introducing Al into mesoporous materials.
ACKNOWLEDGEMENT The authors acknowledge financial support from the European Commission in the framework of the Non Nuclear Energy Program JOULE-THERMIE.
106 REFERENCES 1. 2.
3. 4. 5.
6. 7. 8. 9. 10.
C.T. Kresge, M.E. Leonowicz, W.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.TU. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. S.Biz and M.L.Occelli, Catal. Rev.-Sci. Eng., 40 (1998) 329 and references cited therein. A. Karlsson, M. Stocker and R. Schmidt, Microp. and Mesop. Mater., 27 (1999) 181. A. Karlsson, M. Stocker and K. Schafer in I. Kiricsi, G. Pal-Borbely, J.B Nagy and H.G. Karge (Eds.), Porous Materials in Environmentally Friendly Processes, Stud. Surf. Sci. Catal., 125(1999)61. R. Schmidt, D. Akporiaye, M. Stocker and O.H. Ellestad, J. Chem. Soc, Chem. Commun., (1994) 1493. P. A. Jacobs, E. G. Derouane and J. Weitkamp, J. Chem. Soc, Chem. Commun., (1981)591. G.C. Bond, Heterogeneous Catalysis, 2. edition, Oxford Science Publications, Clarendon Press, Oxford, 1987. M. Kruk, M. Jaroniec and A Sayari, Microp. and Mesop. Mater., 27 (1999) 217. P.T. Tanev and T.J. Pinnavaia, Chem. Mater. 8 (1996) 2068.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
[07
Microwave Synthesis of Micro-Mesoporous Composite Material D,S. Kim', S.-E. Park'*, and S.O, Kang^ 'Industrial Catalysis Research Laboratory, Korea Research Institute of Chemical Technology (KRICT), Taejon 305-606, Korea ^Department of Chemistry, Korea University, Choongnam 339-700, Korea Synthesis of micro-mesoporous composite material was attempted by using two different templates, [(CH3CH2CH2)4NBr (TPABr) and C,4H29(CH3)3NBr (MTAB)] under the irradiation of microwave. Two precursor solutions for mesoporous and microporous phases were prepared using MTAB micelles and silicalite solution containing TPABr, respectively. The ratio of micro-mesophase was strongly influenced by the pretreatment conditions of silicalite1 solution. To obtain composite materials having micro- and meso phases, nucleation step was needed for silicalite-1 solution prior to mixing with MTAB micelles. Microwave irradiation accelerated nucleation in pretreatment step and rapid crystallization. Synthetic process of these composite materials was monitored by TG-DTA and photoluminescence spectroscopy with pyrene probe. During the crystallization, the supramolecular interaction between MTA^ micelles and SiOVTPA^ was observed and as the crystallization proceeds, the structure of composite materials changed from bimodal mesopore to multiple micromesopore, 1. INTRODUCTION Since the discovery of the M41S materials with regular mesopore structure by Mobils scientists [1], many researchers have reported on the synthetic method, characterization, and formation mechanism. Especially, the new concept of supramolecular templating of molecular aggregates of surfactants, proposed as a key step in the formation mechanism of these materials, has expanded the possibility of the formation of various mesoporous structures and gives us new synthetic tools to engineer porous materials [2]. Recently, microwave induced heating technique has been considered as a new tool for the zeolite synthesis considering several fascinating advantages in comparison with conventional hydrothermal heating. To date, several type of zeolites such as AIPO4-5, zeolite A, zeolite Y, ZSM-5, and MCM-41 have been prepared by microwave irradiation [3-6]. Recently, we reported that the addition of ethylene glycol into the MCM-41 precursor gel under microwave irradiation provides a new way to control the crystallinity and morphology of MCM-41 [7]. However, there have been only a few reports about the synthesis of the micro-mesoporous materials. Bekkum group have reported that the FAU zeolite overgrown with small content of
108
mesoporous MCM-41, which induced ion exchange capacity of FAU by sodium contents [8]. This material has demonstrated potential applications in adsorption and catalysis such as refinery conversion processes and fine chemicals synthesis [8,9]. Recently, Karlsson et al. reported that in-situ formation of both the micro- and mesoporous structures (MFl and MCM-41, respectively) was obtained using synthetic gel containing two templates [10]. For these materials, it might be expected to introduce a 'zeolite type' wall structure into the amorphous pore wall of mesoporous material, indicating that mesoporous superstructure can be synthesized with dramatic improvements in the physicochemical properties. Generally, the mechanism proposed for structure direction and self-assembly in the synthesis of Si-ZSM-5 involves the formation of an ordered, hydrophobic hydration sphere around the TPA cation [11], The assembly process of MCM-41 is controlled by electrostatic interaction between silicate species and charged surfactant head groups [12]. Based on those propositions mentioned above, we tried to design a mesoporous material having micro crystalline wall by controlling the ratio of Q"* silicate species formed around TPA and Q^'^ silicate species interact with the micelles. To synthesize micro-mesoporous composite material through the control of Q^'^ and Q"* groups, two different templates were used and nucleation step of microporous material was introduced prior to the crystallization. And also we have attempted to monitor microenvironment of micro-mesoporous composite materials during the nucleation and crystallization steps using TG-DTA and photoluminescence with pyrene probe.
2. EXPERIMENTAL Micro-mesoporous composite materials was prepared in a CEM microwave oven (MDS2000) by using 50 - 100% of the maximum power of the oven (Wmax = 630 watts, frequency = 2.45 GHz, ?max = 200 psi). The percentage power of microwave was programmed in percent increments to control the rate of heating. The fiber optic probe with a type of phosphor sensor was used for controlling the temperature of microwave oven. In the microwave synthesis, firstly the synthetic gel of silicalite-1 with ITEOS : 0.5NaOH : O.lTPABr : 55.6H2O was prepared, then mixed with surfactant micelle (0.137MTMAB). In the synthesis of micro-mesoporous composite materials, TPABr was first dissolved in aqueous 5 wt% NaOH solution with stirring at room temperature for 30 min. The template solution was added TEOS (Aldrich). Subsequently, diluted HCl solution was added dropwisely to the resulting mixture till the pH of the gel reached about 12, followed by stirred vigorously for 2 h. This resulting precursor gel was loaded in a microwave oven equipped with a Teflon autoclave, and irradiated to be controlled at 165°C for 60 min under 300 - 600 W of microwave power. And as a second treatment, the pre-treated gel solution was slowly added into 10 wt% aqueous solution of MTAB with stirring at room temperature for 1 h. The final mixture was again loaded into a Teflon autoclave and then irradiated to be maintained its temperature of 165°C with varying synthetic time under 300-600 W. The resuUing solid product was isolated by filtering, washing with deionized water, and drying in air at 100°C for 10 h. To remove the organic templates, the as-synthesized samples were calcined at 550 °C for 6 h in air. The prepared samples were characterized by several instrumental analysis techniques. X-
109 ray powder diffraction (XRD) patterns were obtained on a Rigaku diffractometer using Cu Ka radiation. Scanning electron microscopy (SEM) was performed with a JEOL scanning electron microscope (model JSM 840). BET measurements including surface area and pore volume were performed by Micromeritics sorption analyzer (model ASAP-2400). Nitrogen adsorption-desorption isotherms were also measured at 77 K with a Micromeritics instrument using a conventional volumetric technique. The samples were degassed at 300°C for 3 h. The mean pore sizes of samples were calculated using the BJH procedure. Thermal properties was analyzed by thermogravimetric/differential thermal analysis (TG/DTA) (SETARAM TGDT92). A continuous flow of air was maintained at 30 ml/min and temperature was increased at 10 °C/min up to 873 K. In order to confirm the formation of the micromesoporous composite materials, photoluminescence (PL) spectroscopy was adopted by using pyrene (2 jiM) as fluorescence probe. The PL spectra were recorded on a Shimadzu spectrophotometer RF-5301PC with quartz cubic cell at room temperature.
3. RESULTS AND DISCUSSION 3.1. Characteristics of microwave syntheses of MCM-41 and silicalite-1 Figure 1 shows XRD patterns of MCM-41 and silicalite-1 prepared by microwave irradiation. The material prepared at 120-100°C for 40 min by microwave irradiation exhibits four well defined peaks in Fig. la, which can be assigned to hexagonal mesoporous MCM-41. The BET surface area and pore volume of MCM-41 were 1020 mVg and 0.86 cmVg, respectively. The material synthesized at 165°C for 2.5 h under microwave irradiation, shows the XRD in Fig. 1(b), which confirms that single phase of silicalite-1 is formed. It was previously demonstrated that microwave irradiation in the synthesis of MCM-41 exhibited a significant effect on shortening of synthesis time and control of its crystallinity and morphology [13]. The short 53 crystallization time was ascribed to relatively fast dissolution of the gel upon microwave irradiation as compared to conventional hydrothermal heating [6]. The shorter synthesis time by microwave heating could be explained by two different mechanisms, i.e., 20 30 40 50 the rapid heat-up of the sample and superheating by a better heat transfer which 20 (degree) results in rapid and sufficient heating of the synthesis mixture [14,15]. Therefore, it Fig. 1. XRD patterns of (a) MCM-41, and seemed that microwave heating not only (b) Silicalite-1 synthesized by microwave increases the rate of crystallization but also irradiation. directs the crystallization mechanism.
110 3.2. Synthesis of micro-mesoporous composite materials via microwave irradiation The micro-mesoporous composite materials are synthesized with two-step treatment using two different kinds of templates such as TPABr and MTAB. The relative portion and properties for microphase and mesophase of the final products in terms of changing microwave irradiation time are shown in Fig. 2 and Table 1. For sample I prepared by silicate gel containing TPA, amorphous phase was obtained. During the second treatment for mixture of sample I and MTAB micelles, sample II-IV were transformed from mesophase (MCM-41) to microphase (silicalite-1) with increasing the time of microwave irradiation. By irradiating microwave for 10 min at 165°C, the mixture of MTAB micelles and the sample I produces an ordered mesoporous material (Fig. 2b) before calcination. The XRD pattern of Figure 2b exhibits four distinct diffraction peaks, indicating the synthesis of hexagonal MCM-41 phase, with ^\^^ IT Port D I a a t i c r ( A )
1
0.0
0.4
'
x: P«rc DUBCIcr ( A )
1
0.8
^
T
0.0
0.4
'
'
\
0.8
.,_ 0.0
Relative Pressure (p/po) Fig. 4. N2 adsorption-desorption isotherms of (a) sample II, (b) sample III, and (c) sample IV, and pore size distribution of (a') sample II, (b') sample III, and (c') sample IV of micro-mesoporous composite materials. from the mesophase to micro-mesophase due to the collapsed mesophase or separation of MFI nuclei from mesoporous wall formed as the silicate species and MFI nuclei. Additionally, the microwave treatment during the crystallization process at high temperature may cause the metastable mesophase to collapse into the denser or amorphous phase in synthetic mixture as well as provide the favorable condition for the formation of silicalite-1. A summary of parameters obtained by nitrogen sorption is shown in Table 2. In Table 2, pore diameters of major peaks (*) for sample II-IV are increased from 2.5 to 2.87 nm as extending the microwave irradiation. It implied that the additional space created in the mesoporous channels, as a consequence of the pore size enlargement, that is filled by extra water [16]. In principle, the micro-mesoporous composite materials can be synthesized by the
113 microwave irradiation, in order to design novel porous material, through the physicochemical improvement of the mesoporous materials with supramolecular templating mechanism. Our main goal in this work is to design a new material consisting of a mesoporous material having crystalline wall of the interacted silicate species (Q"*) that is supposed to be formed around TPA^. And then these silicate species (Q^'^) interact around the MTAB surfactant micelles. In addition to the proposed Si-ZSM-5 synthetic mechanism, the hydrophobic hydration sphere formed around TPA^ is partially or completely replaced by silicate species when a sufficient amount of soluble silicate species is available [11] and then formed the silica enclathrated TPA species [17]. Bekkum group have reported that the overgrowth of MCM-41 on faujasite was observed. It could be deduced that an important driving force is the exchange of CTA-cations with Na-cations [9]. Thereby, we attempt to apply driving force through the ion-exchange of MTA^ with Na^ having MFI nuclei enriched with Na^ and enhancing the electrostatic interaction of formation mechanism of mesoporous material. Therefore, micro-mesoporous composite material is obtained instead of mesopore having crystalline wall. Especially, in our experiment (not shown), controlling parameters of irradiation times, temperatures, and aging times applied by microwave irradiation or hydrothermal heating, more stable mesoporous materials might be prepared. 3.3. TG-DTA and PL analysis of micro-mesoporous composite materials The DTG curves of as-synthesized samples (II-IV) are shown in Fig. 5. At least three distinguishable peaks are observed in the differential thermogravimetric (DTG) curves. Those are supposed to be related with water and MTAB cations interacted with silicate species and TPA cations trapped in the channels of silicate-1, respectively. There are four distinct stages of weight loss in the DTG data shown: 23-1 SOT (due to the desorption of water), 150-300°C (removal of the MTAB species: P„ Pj), 320T (related with water losses via condensation of silanol groups that form siloxane bonds: P3) and 388T (TPA cations occluded in the crystalline structure: P4). The DTG results imply that it is transformed from mesoporous structure to micro-mesoporous material with increasing microwave irradiation time as the increase of TPA species are accompanies with the decreasing amount of MTAB species. These phenomena are accompanied with the increase of condensation of terminal hydroxyl groups lining of the wall with silica channels, related with the P3 peak. 200 300 400 500 600 The DTG curve in Fig. 5a is obtained both P, (211°C) and P2 (244"C). On the other Temperature (°C) hand, in the DTG result (not shown) of Fig. 5. DTG curves of as-synthesized microneatly synthesized MCM-41 by microwave mesoporous composite materials: (a) sample irradiation, these peaks are observed at 240II, (b) sample III, and (d) sample IV. 260°C. The peak of P2 was known to be due
114
to interact was ascribed with siliceous species and intact surfactant aggregates [18]. Also, the peak of Pj was ascribed to the interaction between siliceous species (silicalite-1 nuclei) and surfactant micelles. Chen et al. have reported the discrimination in the DTG curves for the transformation from MCM-41 to ZSM-5 [18]. They described also that, in the DTG curve of ZSM-5 formed from MCM-41 prepared under hydrothermal heating at 165°C, most of the template was removed by heating of 212°C. Due to the interaction between Cj^TMA surfactant micelles and preformed ZSM-5 gel, the decomposition temperature of surfactant molecules, which is related to P2 peak, has been mentioned as the catalyzer for the formation of ZSM-5 [18]. Furthermore, the micro-mesoporous composite materials having multipores (Table 2) could be formed by the separation from amorphous mesoporous wall to MFI nuclei interacting with its wall. Pyrene shown a number of photophysical features that made it an attractive fluorophore to probe the microenvironment in micellar aggregates [19]. For the peaks of pyrene PL, two important peaks at about 373 nm and 390 nm among the five dominant peaks of pyrene fluorescence were numbered as I and III, respectively [20]. It has been known that intensity ratio of peak III to I (III/I) increased as the polarity at the solubilization site of pyrene decreases. Figure 6 shows fluorescence spectra (X^^ = 310 nm) of pyrene in precursor gel containing TPA and I-IV samples denoted as (a), (b), (c), (d) and (e), respectively. The value of III/I of pyrene does not change under silicalite-1 gel due to no formation of micelle. However, in the Fig. 6d (sample II), III/I ratio is rapidly increased, while sample III and IV are decreased slightly again. Previously, Park et al. have reported that III/I ratio of pyrene for MCM-41 materials was higher than 0.9 [21]. The PL using pyrene probe is applied to monitor the rapid condensation of 0.80 MCM-41 wall under microwave irradiation. The increase of the III/I ratio demonstrates 0.75 that the MTAB micelle surface becomes hydrophobic due to the condensation of the 0.70 silicate species to form the mesoporous MCM-41 structure. Also, the III/I ratio 0.65 could be interpreted as the degree of the compactness of the head group region and 0.60 the extent of surface charge [22,23]. Therefore, the increase in the III/I ratio of 0.55 pyrene of Fig. 6d can be regarded as the (e) (a) (b) (c) (d) increase of the compactness of the head group region of the micelle, resuUing from formation encapsulation and Fig. 6. Variation of III/I ratio in pyrene PL the polymerization of MFI nuclei/silicate Spectra from the various micro-mesoporous gels: (a) TPA solution, (b) sample I, (c) species around micelle surface to build the mixture of (b) and micelle, (d) sample II, and silica walls of sample II. It seems also to be monitored the electrostatic interaction (e) sample IV. between MTAB surfactant micelles and (A: microporous gel, B: mesoporous gel, C: MFI nuclei through the increase of III/I micro-mesoporous gel) ratio of pyrene PL. The decrease of III/I
115 ratio of Fig. 6e sample after microwave irradiation seems to be attributed to the decrease of its polymerization or deformation of wall due to the transition from silicate species to MFl nuclei species. By monitoring the pyrene PL in micro-mesoporous gels, it is expected to form micelle enclathrated crystalline wall with the increase of III/I. However, the DTG and PL analysis results indicated the evidence for the transformation from the mesoporous wall containing MFI nuclei to the partially separated phase of microporous and mesoporous walls.
4. CONCLUSIONS This study demonstrated that the micro-mesoporous composite materials could be synthesized with two-step treatment by microwave using two different templates system with TPABr and MTAB. This formation was controlled by the self-assembly formation of supramolecular templates between MTA* micelles and SiO'/TPA" gels. As varying microwave irradiation time of micro-mesoporous materials, gradually transition from the mesophase to micro-mesophase was occurred. These materials have higher ^,00 spacing of mesoporous materials and lead to transition from mesophase to micro-microphase by an increment of synthetic time, while the calcined products is formed with bimodal and trimodal pore size distribution under microwave irradiation within 3 h. From TG-DTA and PL analysis, the self-assembly formation of supramolecular templates between MTA^ micelles and SiO' /TPA^ gels were monitored.
ACKNOWLEDGMENTS This work was supported by a grant from the Ministry of Science and Technology of Korea. REFERENCES 1. J.S. Beck, J.C. Vartuli, W.J. Roth, M. E. Leonowicz, C.T. Kresge, K.O. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem.Soc, 114(1992)10834. 2. J.C.VartuU, S.S. Shin, C.T. Kresge and J.S. Beck, Stud. Surf. Sci. Catal., 117 (1998)13. 3. P. Chu and F.G. Dwyer, US Patent No. 4 778 666 (1988). 4. A. Arafat, J.C. Jansen, A.R. Ebaid and H. van Bekkum, Zeolites, 13 (1993) 162. 5. I. Gimus, K. Jancke, R. Vetter, J. Richter-Mendau and J. Caro, Zeolites, 15 (1995) 33. 6. C.-G. Xu and T. Bein, Chem. Commun., (1996) 925. 7. D.S. Kim, J.-S. Chang, W.Y. Kim and S.-E. Park, Bull. Korean Chem. Soc, 20 (1999) 408. 8. K.R. Kloeststra, H.W. Zandbergen, J.C. Jansen and H. van Bekkum, Microporous Mater., 6(1996)287. 9. K.R. Kloeststra and H. van Bekkum, J. Chem. Soc, Chem. Commun., (1995) 1005. 10. A. Karlsson, M. Stocker and R. Schmidt, Microporous & Mesoporous Mater., 27 (1999) 181.
116 11. S.L. Burkett and M.E. Davis, J. Phys. Chem., 98 (1994) 4647. 12. 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. 13. S.-E. Park, D.S. Kim, J.-S. Chang and W.Y. Kim, Catal. Today, 44 (1998) 301. 14. D.M.P. Mingos and D.R. Baghurst, Chem. Soc. Rev., 20 (1991) 1. 15. RM. Slangen, J.C. Jansen, H. van Bekkum, Microporous Mater., 9 (1997) 259. 16. B.D. Khushalani, A. Kuperman, G.A. Ozin, Adv. Mater., 7 (1995) 842. 17. M.E. Davis, Stud. Surf. Sci. Catal., 97 (1995) 35. 18. X. Chen, L. Huang and Q. Li, J. Phys. Chem. B, 101 (1997) 8460. 19. J.K. Thomas, Chem. Rev., 80 (1980) 283. 20. M. Almgreen, R Grieser and J.K. Thomas, J. Am. Chem. Soc, 102 (1980) 3188. 21. S.-E. Park, D.S. Kim, J.-S. Chang and W.Y. Kim, Stud. Surf. Sci. Catal., 117 (1998) 265. 22. A. Galameau, D. Lemer, M.R Ottariani, RD. Renzo and R Fajular, Stud. Surf Sci. Catal., 117(1998)405. 23. H. Itoh, S. Ishido, M. Normura, T. Hayakawa and S. Mitaku, J. Phys. Chem., 100 (1996) 9047.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
117
Preparation of Y/MCM-41 composite materials Ruifeng Li* , Weibin Fan, Jianming Ma, Kechang Xie Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024, China A composite material (denoted as Y/MCM-41) composed of a core of zeolite Y particle and a thin layer of MCM-41 have been prepared by the crystallization of the reaction mixture of MCM-41 and zeolite Y particles. The Y/MCM-41 particle size increases with the increase of the Si02/Al203 ratio of MCM-41. Introduction of hydroxymethyl fiber into the zeolite Y particle favors the significant increase of its strength, but zeolite p easily formed. The adsorption property of Y/MCM-41 is different from those of zeolite Y and MCM-41. H(Y/MCM-41) as a catalyst is highly selective to C4-C5 hydrocarbons and slowly deactivated in the cracking of n-heptane compared to the mechanical mixture particles of HY and HMCM-41 (designated as H(Y+MCM-41)). 1. INTRODUCTION In the last decades, mesoporous materials MCM-41 S have caused a booming research. This is mainly because they basically increase the accessibility of large molecules to the external opening of the pores in comparison with zeolites [1]. Moreover, introduction of transition metals in the walls of MCM-41 will give catalytic redox properties which are of use in selective oxidation as well as for air pollution abatement, isomorphous substitution of Si(rV) by Al(III) can generate Br(t)nsted acid sites [1,2]. The large pores combined with acidity are specially useful for carrying out catalytic cracking of large molecules. In the reaction of the cracking of gas oil, Al-MCM-41 produces more liquid fuels and less gases and coke than amorphous silica-alumina, while more selective toward diesel formation and give less gasoline compared with USY [3]. A new catalytic material, MCM-41 coated USY, has shown its inherent property to convert heavy products in the cracking of vaccum gas oil[4], and predicted its potential application in adsorption and catalysis. In this paper we report the preparation of Y/MCM-41 composite materials comprising a core of zeolite Y particle and a thin shell of MCM-41, with a bimodal pore size distribution. 2. EXPERIMENTAL 2.1 Sample preparation The as-synthesized zeolite Y (Si02/Al203 =3.1) was pelleted and crushed to 40-60mesh, which was mixed with the reaction mixtures of MCM-41 prepared in terms of the ratio of 0.2Na2O:SiO2:xAl2O3:0.15 CTAB:50H2O (x = 0-0.05), and then, sealed into an autogenous autoclave under static condition at 120 °C for 3 to 15 days. When the crystallization reached the desired time, the autoclave was cooled with cold water. The product was fiilly rinsed in a sieve (80 mesh). Some samples were calcined at 550 °C for 3h so as to remove the organic molecules. If we want to increase the zeolite Y particle strength, a little amount of hydroxymethyl fiber aqueous solution could be uniformly added to zeolite Y when it was
118 pelleted. H (Y/MCM-41) was prepared according to the following steps: 1) Ig of the calcined Y/MCM-41 composite material(Na form) was statically exchanged with 30ml of lmlo/1 NH4NO3 aqueous solution for four times, and each time lasts for 12h; 2) the obtained NH4 (Y/MCM-41) was dried at 90 °C overnight, and calcined at 550 °C for 4 h. 2.2 Sample characterization The crystalline phase of the as-prepared product was identified by a Dmax/yA x-ray diffractometer with CuKa radiation and Ni filter. The crystallite morphologies and uniformity and thickness of MCM-41 layer overgrown on zeolite particle Y were determined on a JEOL JSM-35C scanning electron microscopy. The N2 adsorption of the composite sample at 77 K was measured on a ASAP 2000 adsorption instrument. The catalytic performances of the composite H(Y/MCM-41) were investigated by using cracking n-heptane in hydrogen atmosphere as model reaction, and the product was analyzed by a GC-9A gas chromatography equipped with a ionized flame detector. 3. RESULTS AND DISCUSSION 3.1 XRD analyses Figure 1 shows the XRD patterns of the as-synthesized samples. It was found that there only contains the characteristic peaks of MCM-41 and zeolite Y in Figure 1(a), indicating that the product is composed of these two phases without other crystalline impurities. However, when a certain amount of hydroxymethyl fiber was introduced into zeolite Y particles, zeolite P was easily formed in the products(Figure 1(b)) although the particle of Y/MCM-41 significantly strengthens; moreover, the special peak (100) of MCM-41 is also very weak, even nearly undistinguishable, perhaps this is because there is only a much thinner layer overgrown on the surface of zeolite Y particles, compared with that of zeolite Y particles without containing hydroxymethyl fiber, revealed by the sizes of the as-prepared samples. 3.2 Scanning electron microscopy The SEMs of the as-prepared Y/MCM-41 (the SiOs/AbOs ratio of MCM-41 is about 100) composite materials are given in Figure 2. It shows that zeolite Y particles, composed of a lot of octahedral crystallites, are closely encircled by a uniform amorphous MCM-41 layer of about 40 |am thick. By contrast, on the surface of hydroxymethyl-fiber-containing zeolite Y particles, only a 5-8|im thick layer overgrows; moreover, it is clear that a few of zeolite P crystallites appear, which are corresponding with the XRD measurement. Nevertheless, the strength of the particles of the Y/MCM-41 composite materials strongly increases. At present time, why the shell of MCM-41 overgrown on hydroxymethyl-fiber-containing zeolite Y particles is much thinner than that on pure zeolite Y particles is not very understandable, possibly the difference of surface roughness, caused by their stiffness, of two samples cannot be neglected. It was also found that the lower the Si02/Al203 ratio of the reaction mixture for preparing MCM-41 is, the thicker the MCM-41 shell is, which can be clearly seen from the dimensions of the product particles. The longer crystallization time may be responsible for this since crystallization time increases with the decrease of the Si02/Al203 ratio when other ratios are kept constant. This could also be directly confirmed by the following fact: for the preparation of the same Y/MCM-41 composite material, with the increase of the crystallization time, the product particles grow larger.
119
(a)
(b)
Figure 2. The scanning electron micrographs of the as-prepared samples (a) without hydroxymenthyl fiber, (b) containing hydroxymenthyl fiber. 3.3 N2 adsorption properties The adsorption isotherm of the Y/MCM-41(the Si02/Al203 of MCM-41 is 100) composite materials for N2 at 77 K is illustrated in Figure 3. It was found that the isotherm, like that of MCM-41, is the type IV in the lUPAC classification. Some papers reported that the adsorption isotherm of MCM-41 for N2 at 77 K has no adsorption-desorption hysteresis [5,6]. However, in our experiment there is a big hysteresis loop, containing two obvious sections, between 0.2 and 1.0 of p/p"". The first section has two steep branches and can be related to capillary condensation occurring in the cylindrical pores [7,8], and the second section may be due to mixed pore systems [7]. Furthermore, it can also be found that in the isotherm of the Y/MCM41 a sharply rising initial portion concave to the pressure axis is like those of zeolites, exhibiting Langmuir shape because micropores are filled up before a monolayer can be established on the other surface, which indicates strong interaction between adsorbate and adsorbent. These show that the differences between the adsorption isotherm of Y/MCM-41 and MCM-41 are caused by their pore size distribution. The former has bimodal pores, while the latter has only unimodal pores. 3.4 Catalytic properties of Y/MCM-41 Cracking of n-heptane by H(Y/MCM-41(SiO2/Al2O3=100)) and H(Y+MCM-41)(its exchange condition is similar to Y/MCM-41 except that the mechanical mixture of NaY and NaMCM-41 powder was used and then pelleted and crushed to 40-60mesh) were performed at 350-600°C in hydrogen atmosphere show that H(Y/MCM-41) exhibits an obviously higher selectivity towards C4-C5 hydrocarbons; moreover, the deactivation of Y/MCM-41 is also greatly slower than that of HY although they are both deactivated within an hour. The much larger pores of MCM-41 than of zeolite Y is responsible for the slow deactivation. However, the total cracking conversion is lower about 40% compared to H(Y+MCM-41); Nevertheless, with the increase of the reaction temperature the total conversions of them are both significantly increase. It was shown that zeolites not only have the larger amount of Br(t)nsted acid sites than mesoporous materials, but also the acid sites are also stronger[l]. With regard to H(Y+MCM-41), the particle surface contains a lot of stronger Br(t)nsted acid sites due to the presence of zeolite Y, while on the surface of H(Y/MCM-14) particles there are a smaller amount of relatively weaker Br(|)nsted acid centers because zeolite Y is encircled by MCM-41. As a result, the catalytic activity of the former is much higher than that of the latter.
120 ^
600
Figure 3. The adsorption isotherm of N2 on Y/MCM-41 without hydroxymethyl fiber at 77K. 4. CONCLUSION A composite material of Y/MCM-41 was prepared by the overgrowth of MCM-41 on zeolite Y particles with a uniform layer which thickness increases with the decrease of Si02/Al203 ratio of MCM-41. Addition of hydroxymethyl fiber into zeolite Y particles can greatly increase the strength of the product particles, but zeolite P easily formed. The adsorption property for N2 is different from those of MCM-41 and zeolite Y It exhibits a unique property combining the mesoporous and microporous adsorption characteristics. The catalytic performances of cracking of n-heptane of Y/MCM-41 are obviously different from those of the mechanical mixture of zeolite Y and MCM-41. Compared to the letter, the former exhibits a higher selectivity of C4-C5 hydrocarbons and a lower deactivation rate, but its catalytic conversion decreases about 40%.
REFERENCES 1. A. Corma, Chem. Rev., 97(1997)2373. 2. W.A. Carvalho, P.B. Varaldo, M. Wallau and U. Schuchardt, Zeolites, 18(1997)408. 3. A. Corma, M.S. Grande, V. Gonzalez-Alfaro and A.V. Orchilles, J. Catal., 159(1996)375. 4. K.R. Kloestra, H.W. Zandbergen, J.C. Jansen and H. van Bekkum, Microporous Mater., 6(1996)287. 5. RJ. Branton, RG. Hall, K.S.W. Sing, H. Reichert, F. Schueth and K.K. Unger, J. Chem. Soc. Faraday Trans., 90(1994)2965. 6. RJ. Branton, RG. Hall and K.S.W. Sing, J. Chem. Soc. Chem. Commun., 1993,1257. 7. R.Sh. Mikhail and E. Robens(eds.), Microstructure and Thermal Analysis of Solid Surface, Wiley Heyden Ltd., 1983. 8. S. Komaraeni, V.C. Menon and R. Pidugu, J. Porous Mater., 3(1996)115.
121
Supported Crystallization of MFI- and FER-type Molecular Sieves on Porous Glasses W. Schwieger, M. Rauscher, R. Monnig, F. Scheffler, D. Freude* Lehrstuhl fur Technische Chemie I, Friedrich-Alexander-Universitat Erlangen-Niimberg, EgerlandstraBe 3, 91058 Erlangen *) Fachbereich Physik, Fakultat fiir Physik und Geowissenschaften, Universitat Leipzig, LinnestraBe 3-4, 04103 Leipzig We report in this paper about a crystallization on or into the matrix of porous glass materials focused in particular on the transformation behavior of the glass matrix in the zeolitic material. As a result biphasic silicates with a bimodal pore system have been prepared by optimizing the hydrothermal treatment process of a macroporous glass under autogenous conditions. The products descripted here contain and combine both, the properties of an amorphous glass matrix and the microporosity of crystals with MFI- or FER-structure. The products were characterized by chemical analysis, XRD and SEM investigations. Their catalytic activities were tested in a n-hexane cracking reaction.
1. INTRODUCTION Porous silicates such as porous glasses, silica gels, and zeolites play an important role as adsorbents, catalysts and catalyst supports. New applications of porous silicates currently under development include hydrogen storage, optical information storage, media for chemical reactions, and formation of thin crystallized membranes [1,2]. The development is accelerated by a more detailed knowledge of the silicate formation conditions, and by the combination of different preparation techniques [3, 4]. The knowledge gathered here allows to "tailor" a porous material for specific purposes. Recently, we proposed a preparation route for such bisilicatic materials which could be regarded as a supported crystallization process [5]. The resulting materials have a high potential as catalysts, adsorbents and membranes. These new investigations focused on the effect of different crystallization methods on the tranformation behavior of a macroporous glass material into a partially or fully crystallized product. 2. EXPERIMENTAL 2.1. Synthesis The conversion of porous glass into the zeolites with MFI- and FER-structure has been performed following various synthesis routes, with or without a template addition. Tetrapropylammoniumbromide (TPABr) or propylamine (PA) have been used as a socalled structure directing template. Starting reaction mixtures expressed in mole ratios of the oxides are given below: 28 Na20/ 6 B2O3/ AI2O3/ 69 Si02/ 56 TPABr/15428 H2O in case of using tetrapropylammoniumbromide and
122 X NajO/ 6 B2O3/ AI2O3/ y SiOi/135 PA/ 15428 H2O with x= 17-28 and y=H-ll in case of using propylamine. Additionally a template-free synthesis route has been followed in order to obtain pure ZSM-5 or ZSM-35 products. The batch compositions were the following: X NazO/ 6 B2O3/ AI2O3/ y Si02/15428 H2O with X = 21 - 28 and y = 22 - 72. At first, an aluminum containing alkaline solution has been prepared by adding a Al2(S04) X 18 H2O solution to a NaOH solution thus the resulting mixture is still basic. Optional the template TPABr (solved in H2O) or propylamine have been added (all chemicals were obtained from Fa. Merck). In a second step the porous glass granules (Trisopor®, Schuller GmbH) were added to this solution. Afterwards the whole reaction mixture was degassed under vacuum. The syntheses were carried out in stainless steel autoclaves (v=50 cm^) at 448 K. The moving of the synthesis mixtures was realized by rotating the autoclaves. The autoclaves were removed succesively from the crystallization oven after particular times. The as-synthesized products were dried at 383 K for 16 h, rehydrated in air and analyzed. Selected samples have been modified for the catalytic testing by a calzination step at 823 K for 5 h and by a proton exchange in a 0.1 M HCl solution for 24, 16 and 8h. These samples were also dried at 383 K for 16 h and rehydrated in air. 2.2. Characterization The rehydrated products were characterized by chemical analysis (ICP: Plasma 400 Perkin Elmer), powder-XRD measurements (URD 63 Seifert) in a range from 4° to 40° (20), N2-Adsorption (Sorptomatic 1900 Porotec), Hg-porosimetry, He-density and Scanning Elektron Microscopy (SEM). Catalytic activities for n-hexane cracking were performed using an isothermally operated flow reactor. The feed stream of nitrogen was saturated at 3°C with hexane. With the help of a bypass it was possible to determine both the reactor inlet and outlet concentration of hexane using a gas chromatograph (Varian Star 3400) with FID-detector. Before starting the measurements the catalysts were treated in synthetic air at 823 K for Ih. The hexane conversion rates on the catalysts were determined at 673 K , 723 K and 773 K after a steady state at each temperature had been reached. 3. RESULTS 3.1 Synthesis: kinetic investigations The general preparation route describing the supported crystallization on and into porous glasses are published earlier [5]. On the base of this idea and outgoing from the results presented in these paper, we focused on the investigation of the transformation behavior of the amorphous glass matrix into the crystalline products applying different crystallization routes. As a general result the experiments can be summarized as following: All preparation routes with templates TPABr and propylamine as well as the inorganic route without any organic template addition can be optimized in a way that the crystallization process will lead always to partially or ftilly crystalline MFI-products. Figure 1 shows typical kinetic curves of crystallization for different preparation routes.
123
1. Synthesis with TPABr _ a _ N a 2 0 / S i 0 2 = 0.33
2. Syntheses with propylamine —#—Na20/Si02 = 0.40 —V—Na20/Si02 = 0.30 —•—Na20/Si02 = 0.25
3. Templatfree syntheses —o—Na2O/SiO2 = 0.40 —V—Na20/Si02 = 0.35 50
75
100
—D—Na2O/SiO2 = 0.30
Crystallization time / h
Figure 1: The kinetic courses of the ZSM-5 formation in porous glass, performed following various synthesis routes (with and without template) and varied alkalinity (Si02/Al203=69). Taking the crystallinity parameter QM (external standard [6]) as a measure for content of crystallized zeolite. Starting Si02/Al203-ratio in the reaction mixture was 69. The QAIvalues of about 1 are representing a tranformation of 100% into the zeolitic material. Figure 1 shows that the phase transformation, the crystallization rates as well as the length of the induction periods, depends drastically on the kind of template and the alkalinity. One has to take into account that all other synthesis parameters have been kept as constant as possible. The crystallization in the TPABr containing reaction systems is very fast in comparison to the other two systems. Comparing just the synthesis runs with the similar Na20/Si02ratio of about 0.33 the length of the induction period (the time for the seed formation) is shortened by the factor of five from 125 h (templatfree) to 25 h (PA) to 5 h (TPABr). An other observation should be mentioned. The courses of the crystallization are not characterized by the typical S-shape like it is known from many hydrothermal zeolite crystallizations [7,8]. After the first crystalline fraction could be detected by XRDmeasurements one can observe a steeper increase of the crystallinity value in comparison to the decreasing effect at the end of the crystallization process. An explanation can not be given at the moment. However, it might be due to a special situation in more dense surrounding which we also faced in the crystallization processes in porous glasses where at least a hindrance of the material transport has to be taken into account. For the crystallization in present of TPABr a maximum crystallinity is reached already after 10 h with a surprising high value of about 1.6 pretending a crystalline content of more
124 than 100%. However, considering the QAi-value of about 1.15 after template removal by calzination at 550°C for at least 3 hours, one has to conclude that this high value of about 1.6 represents a real crystallinity effect due to a well ordered template phase (molecule) in the pore system. A similar decrease of the QAi-values (from 1.0 to 0.89) has been observed for the propylamin containing but not for the templatefree synthesized zeolites. In addition, the level of crystallinity (crystalline content) which can be reached seems to be different as well. The QAi-values decrease from 1.6 via 0.8 to just 0.6 for the TPABr, the propylamine and the templatefree syntheses, respectively. The syntheses with propylamine and those without templates have been carried out with various amounts of NaOH, in order to examine the influence of the alkalinity on the crystallization rate. This is also shown in figure 1. It is obvious that (i) the crystallization with propylamine is performed faster in comparison to the template free crystallization, and (ii) the crystallization rates decrease with a decreasing amount of NaOH in the reaction mixture for both synthesis pathways similarly. Both tendencies are described for the conventional hydrothermal zeolite crystallization processes from gel-like reaction mixtures as well [9]. However, considering the real measurable pH-values in the starting reaction mixtures, we found e.g. in the propylamine containing mixture with the Na20/Si02 ratio of 0.3 a pH-value of 12.76 which is even lower than the pH value in the template-free mixture with a Na20/Si02 ratio of 0.4 of 12.86. Therefore, this indicates that the conversion rate of the porous glass into ZSM-5 is more influenced by the components of the starting composition, especially the used template molecules in the synthesis mixture, than by the total alkalinity of the overall reaction mixture. 3.2. Synthesis - variation of the SiOi/AhOa-ratios Further investigations which have been performed in the propylamine system showed that it is possible to direct the conversion of the used macroporous glass into zeolitic materials either with MFI or PER structures. One of the most decisive factors is the aluminum content of the starting synthesis mixture (table 1). By varying the Si02/Al203 ratio systematically one can obtain both structures, the MFI zeolite at the higher ratios (larger than 65) or the FER-type zeolite at the lower ratios (smaller than 50). Table 1: Structure directing effects by changing the Si02/Al203 ratios in the starting synthesis mixture (MFI / FER) with the starting composition of the reaction mixture of 28 Na20 / 6 B2O3 / z AI2O3/ 69 Si02/ 135 PA /15428 H2O (z=0.95-3.20) Synthesis Product
Reaction Mixture
Structure
Si02/Al203 ratio
Si02/Al203 ratio
72.1
71.4
MFI
68.9
68.4
MFI(QAI=0.81)
46.0
24.6
FER (98%crystallinity)
21.6
16.2
FER (95%crystallinity)
(QA,=0.89)
125
ZSM-35
Cu - K^ / grd20
Figure 2: Powder XRD-pattems of several ZSM-35 composites, starting material and products, after different crystallization times (tk : 0 h, 23 h, 36 h and 65 h) It is shown that at a Si02/Al203 ratio in the starting mixture between 68.9 and 46.0 in the synthesis products leads to a change in the crystalline structure from pure ZSM-5 (MFI) to pure ZSM-35 (PER). In figure 2 and figure 3 typical powder XRD-pattem are given to characterize the crystallization process of FER-type and MFI-type zeolite. The above given values of %crystallinity for the ZSM-35 containing samples are a relation between the diffraction peaks area of an external standard in the range of 20 24.65° to 26.30° to the samples peak area. The powder XRD patterns for the PER crystallization are shown for the crystallization times of 23 h, 36 h and 65 h. The pattern labeled with tk = 0 h represents the amorphous character of porous glass like it was employed as the raw material for the crystallization. At the crystallization time of 23 h, no crystalline structures are observable, however the shape of the amorphous background changed drastically indicating that a transformation process is already in progress. At a crystallization time of 36 h spikes showing up on top of a broad amorphous halo. This process is due to an PER formation, corresponding to the (002) and (022) PER reflection at 9.36 and 25.83 degree 20. After a crystallization time of 65 h a well generated PER pattern with narrow reflection lines is recognizable. No additional lines have been observed, indicating the high purity of the material. The powder XRD-pattems of a typical ZSM-5 formation series are shown in figure 3. Again, the pattern labeled tk = 0 h is due to the starting porous glass. At a crystallization time of 46 h the typical MPI reflections are just detectable on top of the broad amorphous background. The crystallinity degree was at this crystallization stage QAI=0.03, which
126
ZSM-5
Figure 3: Powder XRD-pattems for several ZSM-5 composite materials for the starting material and at different crystallization times (tk = Oh, 46 h, 54 h and 58 h) means that the seeding process has just been started. At this stages both crystallizations, the course of the FER and the MFI formation, behave similar. However, in contrast to the FER crystallization, the shape of the amorphous background has not changed so drastically during the MFI seed formation. This is surprising, because the same raw material has been employed and both, the FER and the MFI zeolites, are not that different structurally. At crystallization times of 54 h and 58 h the MFI structure was well established with different values of the crystallinity parameter QAI is about 0.36 and 0.82, respectively. No other byproduct could be observed. Considering the higher porosity of such as-synthezised material compared to a powdered MFI zeolite, both, the crystallinity parameter of about 0.8 and the fact that no by-product was observable, lead to the conclusion that the product is a pure MFI phase even when the QAi-value is lower than 1. 4. CHARACTERIZATION 4.1. Porosity The formation of ZSM-5 and ZSM-35 is also apparent in the changes of the porosity. The data of the porosity subdivided in the values of the macroporosity and the microporosity of the crystallization products are listed in table 2. For comparison the data of starting porous glass are given as well. It is obvious from this data that the macropore volume remains approximately constant during the crystallization process. An increase of the micropore volume from the low crystallized products at tk = 23 h and 36 h to the fully crystallized ZSM-35 at tK=65 h was estimated. From both observations one has to conclude that, in consistence to the above given XRD-pattems, a micropore system has been generated in a macroporous
127
Table 2: Porosity values of the starting porous glass, partial and fully crystalline composite materials with FER (ZSM-35) and MFI (ZSM-5) structure. He-density
Macropore volume
Micropore volume
[ml/g]
[ml/g]
tk =0 h (porous glass)
1.007
0.014
tk =23 h (amorphous)
1.154
0.010
tk = 36 h (5% crystallinity)
1.212
0.011
tk =65 h (100% crystallinity)
0.998
0.148
2.287 1 2.140 1
tk=0 h (porous glass)
1.007
0.014
2.206
tk=56h(QAi=0.8)
1.063
0.153
2.090
ZSM-35 - composite material
2.206 1 2.304 1
ZSM-5 - composite material
surrounding. The He-density, the real density, decreases with the increasing crystallinity. If one consider that the shape and so the volume of the porous glass particles remains intact during the crystallization even it is transformed, this effect might be caused by the lower density of zeolite in comparison to a glassy matrix. The difference in the He-density between the fully crystallized FER-product (tk=65 h) and the MFI-product (tk=56 h) is caused by the structural differences and reflects the different framework density of both zeolites. 4.2. Scanning Electron Microscopy To prove that the shapes of the porous glass granules could be preserve scanning electron microscope photographs have been taken from products at every crystallization stage. The imagines of the starting porous glass and the fully crystalline ZSM-5 and ZSM35 products are shown in figure 4 with different magnifications. Figure 4a shows the starting porous glass granules with their typical glossy shape. The products shown in figure 4b and 4c are fully into ZSM-5 (4b) and ZSM-35 (4c) converted glass granules. It is evident that the physical shape and the contours of the materials remains intact during the crystallization process even when the surface of the granules seems a little more rough. However, almost no single powdered crystals were observable in an appreciable amount. These facts lead to the conclusion that the crystallization indeed only took place in the matrix of the porous glass particles. Figures 4d and 4e show the outer surface of the (completely) crystallized granules consisting of ZSM-5 and ZSM-35, respectively. In figure 4f the primary single ZSM-5 crystals, which are recognizable at this magnification, are of a nearly coboid shape with rectangular very smooth faces. The size is with approximately 100-200 nm very small. In figure 4g the particles with the ZSM-35 structure are shown at the same magnification. In spite of this high magnification no single crystals could be detected at the surface of this aggregates. The smallest visible particles are needle-like objects with the size of about 150 nm by 100 which may not be the primary crystals. At least they, the primary crystals, are not so perfectly formed due to a not completely finished crystallization stage when this crystallization process is interrupted. In comparison, the FER-type crystals are much more smaller (1 order of magnitude) than the
128
Figure 4a. Porous glass
398435-10* Bicli
Figure 4b. ZSM-5 granules
ra8B7 SE TSBk MO/l
18.89.1936
Figure 4d. Surface of a ZSM-5
I M18/t ia.B9.199e
Figure 4f ZSM-5 crystals on the surface
Figure 4c. ZSM-35 granules
398435-1000 beta
Figure 4e. Surfaceof a ZSM-35 grain
)9»aS-3M00 fcch
Figure 4g. ZSM-35 agglomerates on the surface
Figure 4: SEM-photographs of totally to ZSM-5 (figures 4b, 4d, 4f) and ZSM-35 (figures 4c, 4e, 4g) converted porous glass (figure 4a)
129 already very small primary MFI-crystals forming the aggregates. Here one has to mention again, the size of the whole aggregates are the same as the size of the granules of the starting porous glasses. 4.3. Catalysis Such particles could be used as catalyst or catalysts supports directly without any pressing or extruding procedures. Powdered zeolites can act as a solid state acid. After a forming process they are applied in many reactions [10] e.g. the H-MFI and the H-FER for the skeletal isomerization of olefins [11,12]. Therefore the partially and fiilly crystalline ZSM-5 composites have been tested in an acidic catalytic test reaction, the cracking of hexane. The composite materials were modified by a thermal treatment to remove the organic template and by a H-ion exchange step to uncover their acidic character. The results of the catalytic investigations are summarized in figure 5. The conversion rates of n-hexane are shown as a function of the crystallinity parameter QAI for different temperatures. We found that the catalytical activity increases simultaneously with the increased crystallinity of the composites, the crystallization products. According this linear correlation it can be concluded that the catalytical active sites, the acidic centers in the zeolitic framework, are always, independent of the crystal content of the composite material, accessible for educt of the test reaction, the n-hexane molecules. This leads to the assumption that the crystallization must start on the interface (at the phase border) between the solution (contains the alkalinity and the template) and the solid (porous glass) surface and has to carry on to the volume phase of the glass resulting finally in complete transformed granules.
,
1 7*=;
•^
1.50-
1
r
^ 1.25•.—"
3.00-
* CO
1 ll
GHSV = 10000 h"'
1
k^ / cm'*(g*s)-^ rate constant (mass related) • 773 K A 723 K • 673 K
1 1 1 1
i),75•^.50-
-/
-
:: ^
"
0.4
0.6
0.251
00 0,00-
0.2
0.8
•
I
•
1
1.0
Figure 5: n-hexane cracking activity of MFI composite materials as aftinctionof the crystallinty parameter QAI
130 5. CONCLUSIONS The investigation can be summarized as follow. • Bi-phasic porous silicates containing amorphous and crystalline components can be prepared as stable pellets even if different crystallization routes known from the conventional crystallization processes are employed. Thus the inorganic template-free crystallization route and the crystallization in presence of propylamine could be applied and optimized for the crystallization on and into porous glasses, the so-called supported crystallization. •
•
•
MFI and FER containing composites and fully crystalline materials of both stuctural types could be realized if the propylamine route was achieved. By varying the Si02/Al203-ratio in the starting reaction mixture the crystallization could be directed to the FER or MFI products. The crystallization could be carried out in a way to preserve the macroporous character of the pellets even after they have been completely transformed in the microporous material. This could be shown by investigations of both direction of crystallization as well as for the "inorganic" and the "propylamine" crystallization routes. Catalytic test reactions (n-hexane cracking) prove that partially crystallized biphasic ZSM-5 containing silicate materials are catalytically active composites. The activity of the prepared biphasic silicates is related to the crystalline fraction in the pellets.
Acknowledgement: The authors grateftilly acknowledge the support of the Deutsche Forschungsgemeinschaft in form of a research grant (Schw 478/8-1) and the financial support from Forschungsministerium of the Land Sachsen-Anhalt (projekt-number: 2175A/0085B). References: [ 1 ] G. A. Ozin, Adv. Mater. 4 (1992) 612 [2] J. Caro, G. Finger, J. Komatowski, J. Richter-Mendau, L. Werner, B. Zibrowius, Adv. Mater. 4 (1992) 273 [3] M.-H. Khim, H.-X. Li, M. E. Davis, Microp. Materials 1 (1993) 191 [4] S.L. Suib, Chem. Rev. 93 (1993) 803 [5] W. Schwieger, M. Rauscher, F. Scheffler, D. Freude, U. Pingel, F. Janowski, Proceedings of the 12^*" International Zeolite Conference, Baltimore (1998) 1849 [6] G.T. Kerr, J. Phys. Chem. 70 (1966) 1047 [7] R.W. Thompson, T.-C. Ying, Zeolites 4 (1984) 353 [8] W. Schwieger, K.-H. Bergk, D. Freude, M. Hunger, H. Pfeiffer, ACS Symp. Series 398(1988)275 [9] R. Szostak, "Molecular Sieves - Principles of Synthesis and Identification" van Nordstrand Reinhold, New York (1988) [10] I. Wang, C. T.-J. Chem, K.-J. Chao, T.-C. Tsai, J. Catal. 60 (1979) 140 [11] H. Mooiweer, K.P. de Jong, B. Kraushaar-Czametzki, B.C.H. Krutzen, Stud. Surf. Sci. Catal. 84(1994)2327 [12] I.D. Harrison, H. Leach, H. Frank, D. A. Whan, Zeolites 7 (1987) 21
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
131
Supercritical Fluid Extraction of Amine Surfactant in Hexagonal Mesoporous Silica (HMS) S.Kawi*andA.-H. Goh Department of Chemical and Environmental Engineering, National University of Singapore, Singapore 119260, Republic of Singapore
ABSTRACT Supercritical carbon dioxide modified with 10 vol% methanol has been employed for the removal of the amine surfactant in hexagonal mesoporous silica (HMS). The effects of temperature and pressure on the extraction efficiency have been extensively studied. It has been found that within an hour, as high as 96% of the amine surfactant can be extracted at a relatively mild condition of 85°C and 100 bar. At constant pressure, high extraction efficiencies are obtained at 50 and 85°C while at constant temperature, high efficiencies occur at 100 bar and 250 bar. This work establishes the feasibility of using supercritical fluid extraction (SFE) for the removal of the amine surfactant. In fact, it has been discovered that SFE produces HMS of more enhanced mesoporosity as compared to that of calcination.
1.
INTRODUCTION
In 1995, Tanev and Pinnavaia [1] have reported the synthesis of a new type of mesoporous molecular sieve designated as the hexagonal mesoporous silica (HMS). Instead of using the ionic inorganic precursor and surfactant as in the case of MCM-41 [2], HMS is manufactured by hydrolysis reaction between a neutral inorganic precursor, tetraethylorthosilicate (TEGS) and a neutral primary amine surfactant (8-18 carbons). HMS possesses numerous favourable characteristics, but, like MCM-41, its synthesis process can only be concluded by the removal of the surfactant. This was reportedly done either by calcination at 630°C or by warm ethanol extraction [1]. However, both methods are deemed as rather unsatisfactory. This is because calcination is rather time consuming and during the process, valuable, but rather toxic, surfactant is being combusted away, thereby emitting some amount of noxious gases. In addition, it has been found that calcination leads to substantial amount of pore contraction and collapse [3] in HMS. Extraction, on the other hand, utilises huge amount of liquid solvent 750ml of ethanol per gram of as-synthesised HMS [1] - and this will make eventual solvent disposal a rather challenging chore. * Corresponding author; Tel: (65)8746312; Fax: (65)7791936; E-mail:
[email protected] This research work is funded by the National University of Singapore.
132 As such, supercritical fluid extraction (SFE) has been proposed for the removal of the amine surfactant. In this work, 1.8 ml/min of CO2 modified with 0.2 ml/min of methanol has been employed for the extraction purpose. Through SFE, recycling of surfactant can be accomplished while generating minimal liquid solvent (methanol) for disposal. Supercritical CO2, which forms the solvent bulk, can be easily separated from the extract (and methanol) via depressurisation and the resultant gas can then either be recycled or be released into the atmosphere. 2.
EXPERIMENTAL
2.1
Synthesis of as-synthesised HMS and calcined HMS As-synthesised HMS was prepared using the method prescribed by Tanev and Pinnavaia [1,3]. 0.27 mol of dodecylamine was dissolved in 9.09 mol of ethanol and 29.6 mol of water. 1 mol of TEOS was then added to the mixture under vigorous stirring. The reaction mixture was allowed to age under room temperature for 18 hours. After ageing, the solid reaction product was washed with deionised water and recovered from the aqueous mixture by filtration. The moist solid was subsequently air-dried and sieved into the desired particle sizes using mesh no. 40 (0.425mm) and 60 (0.250mm). Some amount of the as-synthesised HMS sample was then sent for calcination at 630°C for 4 hours. 2.2
SFE of amine surfactant The SFE process was carried out in a JASCO system. For each run, which lasted for an hour, 0.5g of the as-synthesised HMS was being loaded into the extraction cell housed in an oven. The system uses a HPLC and a syringe pump for pumping liquid CO2 and the modifier (methanol) respectively so as to build up the system pressure. The desired system pressure was set and controlled by a back pressure regulator while the system temperature was set and controlled by the temperature controller attached to the oven. The extracted amine surfactant is collected in a vial placed at the outlet of the back pressure regulator. 2.3
Post extraction analysis Thermogravimetric analysis (TGA) and differential thermal analysis (DTA) were performed simultaneously, using DTG-50 (Shimadzu), on the HMS samples after SFE. The mass loss between 150 to 300°C can be attributed to the decomposition and combustion of the amine template [3]. Hence, by expressing the mass loss of the samples in this temperature range as a percentage of the mass loss of the as-synthesised sample in the same temperature range, the extraction efficiency can be determined. 2.4
N2 adsorption and XRD The surface areas and pore properties of both the supercritical fluid extracted and the calcined samples were analysed by nitrogen physisorption at 77K using a Quantachrome Auto-Sorb 1 analyser. The powder X-ray diffraction patterns of both samples were recorded using a SHIMADZU XRD-600 powder X-ray diffractometer, where Cu target Ka-ray was used as the X-ray source.
133
3.
RESULTS AND DISCUSSION
3.1
Extraction using pure CO2 Extraction of the amine surfactant using pure CO2 at 65°C and 150 bar has been attempted, but it has been found from TGA that there is completely no extraction at all. In 1985, Dandge et al [4] have reported that the solubility of amines in supercritical CO2 generally decreases with increasing basicity (Kb). The threshold basicity, above which the compound becomes insoluble in CO2, was found to be about 10'^. As such, it is a foregone conclusion that the surfactant dodecylamine (Kb^lO"*) is insoluble in pure supercritical CO2. This explains the zero extraction observed and thereby justifies the need of a modifier (methanol in this case). 3.2
Temperature Effect Figure 1 below shows the graph of extraction efficiency vs. temperature at 100 and 250 bar. As mentioned in the experimental section, efficiency of the SFE process is calculated from the TGA results. The TGA of the HMS samples having undergone SFE at 100 bar are shown in Figures 2(a) and (b). By comparing Figure 1 to Figures 2(a) and (b), one can see that the larger tiie mass loss within the temperature range of 150-300°C, the lower will be the extraction efficiency.
-100 bar -250 bar
45
55
65
75
95
85
Temperature (°C) Figure 1. Graph of extraction efficiency vs. temperature for constant pressures.
150
325
0
Temperature ( C)
500
Figure 2(a). TGA for 45-50°C, 100 bar
150
_
325
.0^.
500
Temperature ( C) Figure 2(b). TGA for 60-85°C, 100 bar
134 For this work, a 9:1 volumetric flow ratio of liquid CO2 and methanol has been employed and this corresponds to a molar flow ratio of 8.5:1.5. From literatures [5], the critical parameters of such a mixture are about 50°C and 94 bar. From Figure 1, one can see that the two other isobars exhibit two maximas - one in the subcritical and the other in the supercritical region. It is a well-known fact that SFE processes are controlled either by solubility or mass transfer limitations [6]. As such, the shape of these isobars has to be explained in term of these two limitations too. The critical temperature of pure CO2 is 31°C [7]. For the subcritical range of 31-50°C, the fluid entering the extraction cell will consist of two phases - a liquid methanol phase and a supercritical phase. It has been reported that the difiusivity of liquid is about 10-100 times smaller than that of the supercritical fluid [6] and this implies that the difficulty of mass transfer associated with the former is also magnified by the same factor. In an extraction process, mass transfer occurs during 1) the fluid's penetration of the matrix's pores and 2) the subsequent transport of the analyte (solute) from the matrix into the bulk fluid [6]. The presence of entrained liquid methanol droplets will thus greatly mcreases the amount of mass transfer resistance present in the system. Such resistance is reduced upon an increase in temperature and this accounts for the rise in extraction efficiency observed in the temperature range of 45-50°C. The process is obviously mass transfer controlled within the mentioned temperature range. Increasing the temperature has three positive effects on extraction efficiency and they are listed as follows. 1. It increases the rate of solvent transfer mto the matrix's pores and the subsequent migration of the analyte to the bulk fluid. 2. The analyte needs to desorb from the martix's pore wall before it can be dissolved in the solvent. Alexandrou et al [8] has highlighted the need of thermal energy to overcome the energy barrier before desorption can take place. As such, increasing the temperature will allow more potentially feasible desorption to take place. 3. It decreases the amount of methanol m the liquid phase and thus decreases correspondingly the magnitude of mass transfer resistance existmg in the system. However, a perpetual increase in temperature beyond 50°C eventually brings about a downfall in the extraction efficiency because of the occurrence of the retrograde solubility phenomenon [9]. Such phenomenon arises in the vicinity of the critical temperature and it is characterised by high compressibility of the fluid, which causes the fluid's density to drop sharply as temperature increases. Since the solvating power of a fluid is known to be an increasing function of its density [7,10], the drastic fall in extraction efficiency after 50°C is thus accounted for. This is consistent with the reports that maximum extraction fluid density at high pressures occurs at a temperature just above the critical temperature of the fluid [10,11]. In addition, it must be commented that though high extraction efficiencies have been obtained at 50°C, it is still rather unwise to operate near the critical point due to the drastic drop in efficiency occurring upon slight deviation from the optimum point. In the supercritical phase, both temperature and pressure play a significant role in determining the extraction efficiency. After the short-lived retrograde solubility effect subsides at about 55-60°C, a transition of the system back to the mass transfer controlled situation will take place where increasing temperature will, once again, bring about a surge in the extraction efficiency. In fact, for the supercritical phase,
135
mass transfer limitation will dominate up to the supercritical maxima occurring at about 85®C as can be seen in Figure 1. It is vital to note that the density and hence, the solvating power of a supercritical fluid increases with increasing pressure but with decreasing temperature. Hence, after 85°C when the solubility limitation begins to dominate, one can see that the extraction efficiency begins to deteriorate with increasing temperature. It has been reported that for systems involving strong matrixanalyte interaction, temperature is generally more important than pressure in the promotion of the extraction efficiency [12,13]. This is probably due to the need of high temperatures to overcome the high activation energy barrier of desorption. However, such discovery cannot probably be extended to this work because, unlike MCM-41 [2], the bonding that exists between the pore wall and the surfactant in HMS is merely hydrogen bond [3] instead of ionic bond. 3.3
Pressure Effect Figure 3 illustrates the graph of extraction efficiency vs. pressure at 80°C. This isotherm exhibits two minimas and one maxima just like a fourth power polynomial fimction. As can be seen, the isotherm shows a sharp drop in the extraction efficiency as pressure increases beyond 100 bar. According to Shaw et al [14], in the vicinity of the critical point (-89 bar), there will be a great decrease in the partial molar volume - the change in the volume of the system with the addition of the solute. This means that near the critical point, the solvent molecules will move toward and cluster around the solute molecules so strongly that the total volume of the solution is drastically reduced. Such clustering effect may have induced a favourable partition of the analyte from the matrix mto the supercritical solvent. In addition, a drastic drop in the partial molar volume implies a great increase in the density and hence, the solvating power of the supercritical fluid. Hence, all these serve to explain the high extraction efficiencies observed at 100 bar. After 100 bar, the "critical-point or clustering effect" begins to wither, thereby causing the efficiency to drop acutely. In fact, the efficiency decreases perpetually till 150 bar after which the diminishing clustering effect gets progressively compensated by increasing pressure (implying increase solvating power).
73
X
150
200
Pressure (bar) Figure 3. Graph of extraction efficiency vs. pressure at 85°C.
136
After the first minima at 150 bar, one would expect the extraction efficiency to increase continuously as a result of better solvating power associated with increasing pressure. However, the efficiency actually decreases first at about 180 bar before it rises again at 225 bar. This is similar to the solubility (of solutes in a supercritical solvent) vs. pressure diagram plotted and published in the literature [10]. The author [10] explains this unusual phenomenon in term of the repulsive forces "squeezing" the solute out of the solution. This explanation is rather plausible in the sense that as the pressure increases, the solvent and solute molecules become more closely packed together. As a result of dense packing, the repulsion between the solute and the solvent particles also increases correspondingly, causing the partition coefficient to become more in favour of the matrix rather than the supercritical solvent. In addition, under such "overcrowding" circumstance, the solute may experience greater degree of mass transfer resistance as it movesfi"omthe pores to the bulk fluid. However, it seems that the magnitude of the repulsive force do not increase significantly as the pressure increases so that eventually, it becomes subdued by the continuous increase in the solvating power causing the overall extraction efficiency to increase once again after 225 bar. After 225 bar, we believe that the efficiency will increase continuously. This is substantiated by an earlier work of Lai [15] where the effect of pressure on the extraction efficiency of the quaternary ammonium ion fi-om the MCM-41 matrix has been studied. In that work, the maximum pressure studied is 350 bar and it was found that the efficiency increases as the pressure increases in the range of 225-350 bar. 3.4
Characterisation of HMS samples From Figures 1 and 3, one can see that at a rather mild condition of 85°C and 100 bar, about 96% of the amine surfactant can be removed via SFE. However, it is vital to perform some characterisation studies on the mentioned SFE sample. N2 adsorption study and XRD were being performed on both the SFE and the calcined samples. The results obtained are shown in Figures 4 and 5 below.
(D
E
o Q-
0.00
Figure 4(a). N2 adsorption isotherm
1.00
50.00 Pore Size (A) Figure 4(b). Pore size distribution
100.00
137
1.00
3.50
6.00
8.50
11.00
2e/° Figure 5. XRD patterns of SFE and calcined HMS samples. From the N2 adsorption study, one can see that SFE produces HMS samples exhibiting similar, if not better, isotherm as compared to that of calcination. In addition, it was found that the former sample possesses comparable specific surface area (-1200 m^/g), but larger pore size (28 A) than that of the latter (25 A). This justifies the fact that calcination causes pore contraction in the HMS sample, which is consistent with what is being reported in the literature [3]. The XRD results given in Figure 5 further prove that SFE produces HMS sample of more enhanced mesoporosity. One can also see that tiie XRD pattem of the SFE sample peaks at a smaller 20 value than that of the calcined sample, implying a larger inter-pore distance for the former. From the 20 values, the inter-pore distance for the SFE and the calcined samples are calculated to be 47.2 and 43.2 A respectively. The pore wall thickness can then be calculated from the difference between their inter-pore distance and their pore diameter and they are 19.2 and 18.2 A respectively. The thicker pore wall of the SFE sample implies that SFE produces sample of higher thermal stability [3] than that prepared by calcination. Hence, all these results establish SFE as a feasible technique used for surfactant removal. 4. CONCLUSIONS Temperature and pressure are the two most important physical parameters in SFE because together, they define the density and hence, the solvating power of a supercritical fluid. As such, there is an imperative need to research on the effects of these two parameters so that SFE of the amine surfactant can always be carried out at the optimum conditions. It has been discovered that at 85°C and 100 bar, as high as 96% of the surfactant can be removed within an hour. At constant pressure, high extraction efficiencies can generally be obtained at 50°C and 85°C while at constant temperature, satisfactory efficiencies occur at 100 and 250 bar. Like liquid extraction [3], SFE produces HMS of more enhanced mesoporosity as compared to that of calcination.
138 REFERENCES [I] [2] [3] [4] [5] [6] [7] [8] [9] [10] [II] [12] [13] [14] [15]
P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359(1992)710. P.T. Tanev and T.J. Pinnavaia, Chem. Mater., 8 (1996) 2068. D.K. Dandge, J.P. Heller and K.V. Wilson, Ind. Eng. Chem. Prod. Res. Dev., 24 (1985)162. E. Brunner, W. Hultenschmit and G. Schlichtharle, J. Chem. Thermodyn., 19 (1987)273. J.R. Dean, Applications of supercritical fluids in industrial analysis, 1^^ ed., Florida: Chapman & Hall (1993) Chp.l. Larry T. Taylor, Supercritical Fluid Extraction, New York: John Wiley & Sons, Inc. (1996) 18. N. Alexandrou, M.J. Lawrence and J. Pawliszyn, Anal. Chem., 64 (1992) 301. P.O. Debenedetti and S.K. Kumar, AIChE J., 3 (1988) 645. S.A. Westwood, Supercritical Fluid Extraction and its use in Chromatographic Sample Preparation, 1'^ ed., Florida: Chapman & Hall (1993) Chp.l. S. Bowadt and S.B. Hawthorne, J. Chromatogr. A, 703 (1995) 549. S.B. Hawthorne and D.J. Miller, Anal. Chem., 66 (1994) 4005. J.J. Langenfeld, S.B. Hawthome, D.J. Miller and J. Pawliszyn, Anal. Chem., 65 (1993)338. R.W. Shaw, T.B. Brill, A.A. Clifford, C.A. Eckert and E.U. Franck, Chem. & Eng. News (Dec. 23,1991) 26. M.W. Lai, Supercritical fluid extraction of surfactant, M.Eng. Thesis, National University of Singapore (1998).
Studies in Surface Science and Catalysis 129 A. Sayarietal. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
139
Performance of tetraalkylammonium ions during the formation of zeolites from tetraethylorthosilicate C.E.A. Kirschhock, R. Ravishankar, K. Truyens, F. Verspeurt, P.A. Jacobs and J.A. Martens Centrum voor Oppervlaktechemie en Katalyse, Kardinaal Mercierlaan 92, B-3001 Heverlee, Belgium Tetraethyl orthosilicate was hydrolyzed at room temperature in concentrated aqueous solutions of tetraalkylammonium hydroxide (TEAOH, TPAOH, TBAOH). The formation of silicate species was monitored with gel permeation chromatography (GPC) and IR. The silicon polycondensation process in presence of the three types of tetraalkylammonium cations is very similar and proceeds via a stepwise aggregation mechanism involving occlusion of the tetraalkylammonium cation in specific ring structures. Significant differences in the kinetics of the individual aggregation steps are observed depending on the tetraalkylammonium cation. For the precursor species, these kinetic differences can be explained in terms of stabilization by hydrophobic interactions. For the larger species identified as nanoslabs, the kinetic differences can be explained based on differences in interaction potentials, estimated using extended DLVO theory.
1. INTRODUCTION The molecular steps and the silicate species involved in the formation of Silicalite-1 zeolite (MFI topology) starting with tetraethylorthosilicate (TEGS), tetrapropylammonium hydroxide (TPAOH) and water were determined using a combination of gel permeation chromatography (GPC), X-ray scattering (XRS), IR and ^^Si liquid NMR [1-4]. The first steps in the TPAOH mediated polycondensation process of TEOS at room temperature (Fig.l) are the consecutive formafions of bicyclic pentamer (1), pentacyclic octamer (2), tetracyclic undecamer (3) at the interface of TEOS and the aqueous TPAOH solution. Tetracyclic undecamers each sheathing a hydrophobic propyl groupfi-omthe aqueous solution can dimerize in two ways (4) and (4') and condense with a third tetracyclic undecamer into the precursor occluding a TPA (5). Specific dimer (6) and trimer (7) structures of this precursor are involved in the formation of nanoslabs (8), consisting of twelve precursors coupled three by four and with dimensions of 1.3 nm X 4 nm x 4 nm in the crystallographic a, b and c directions, respectively, of Silicalite1. The nanoslabs show a tendency to form linkages with their small sides in the b and c direction resulting in sheets of two by two nanoslabs (9). Further condensation requires * This work was sponsored by the Belgian Federal government (lUAP-PAI programme); R.R. acknowledges K.U.Leuven for a postdoctoral fellowship.
140
^^^.^^^1^
Figure 1. Molecular steps of the formation of Silicalite-1 in the TEOS-TPAOH-water system: bicyclic pentamer (1); pentacyclic octamer (2); tetracyclic undecamer (3); dimers of tetracyclic undecamer (4) and (4'); precursor (5); dimers of precursor (6) trimer of precursors (7); nanoslabs (8); sheets of nanoslabs (9); intermediates (10) and final large Silicalite-1 particles (10) [1-4].
141 heating to 100°C, whereupon nanoslabs aggregate into intermediates (10) and finally into large particles showing Bragg diffraction (11). The growth of nanoslabs and intermediates proceeds through an aggregation mechanism governed by the interaction potentials of the different faces decorated with the propyl groups of occluded tetrapropylammonium cations. The molecular steps encountered in the crystallization of Silicalite-2 (MEL topology) in the TEOS-TBAOH-H2O system are similar to those encountered with TPAOH, although the coupling mode of tetracyclic undecamers and the kinetics of the individual steps are different [3]. It is known from literature that the use of TEA instead of TPA leads to the crystallization of ZSM12 and ZSM8 [5,6]. The latter is thought to be a relative to the MFI-MEL family [5]. In the present paper we compare the silicon polycondensation process in presence of TPAOH and TBAOH with TEAOH.
2. EXPERIMENTAL Experiments were typically performed using 10 g TEOS. Aqueous solutions of TPAOH(40%), TBAOH(55%) and TEAOH (40%) from Alfa and TMAOH.5H2O (Aldrich) were mixed with TEOS respecting a molar TEOS/TXAOH ratio of 0.37 (X=B,E,P,M) and a H2O/TEOS ratio of 6, respectively. These 'concentrated' mixtures were intensively agitated with a magnetic stir bar to provoke emulsification. Interruption of the stirring resulted in phase separation. The volume of aqueous phase increased with stirring time. With TPAOH and TBAOH, the TEOS layer disappeared typically after 30 min. The TEOS hydrolysis was significantly slower with TEAOH, whereas with TMAOH, there was no spontaneous hydrolysis at all. After hydrolysis, more water was added to reach a molar H2O/TEOS ratio of 16 and stirring continued. The extraction procedure of silicate species from the aqueous solutions was explained earlier [7]. An amount of 5 ml of solution was quickly poured into 15 ml of a stirred 0.5 N HCl solution. An amount of 20 ml tetrahydrofuran (THE) was added and stirring continued for 30 min. Addition of 10.7 g NaCl resulted in a phase separation. The THE layer containing the silicate species was separated from the aqueous layer. The extraction with THE of the aqueous layer was repeated and the THE solutions combined to maximize extraction efficiency. Finally THE was evaporated in a rotavap at 20°C. The final products from the concentrated solutions were gel like. The extracts from the diluted systems were powdery. The experimental details on the gel permeation chromatography (GPC) and IR spectroscopy can be found in refs. [2] and [3], respectively.
3. RESULTS AND DISCUSSION 3.1. From TEOS to precursors in concentrated solutions In the concentrated TPA system at room temperature, the silicon polycondensation processes lead to the formation of precursors and their oligomers (5)-(7) (Fig.l). It was previously shown by XRS and GPC that in the TBA system very similar species are formed [3]. Based on the crystallographic differences between the Silicalite-1 (MFI topology) and Silicalite-2 (MEL topology), it is mandatory that the linking of the tetracyclic undercamer units in (4) and (5) and all larger silicate species be different. The structure directing effect of TPA versus TBA resides in the alternative coupling of two tetracyclic undecamers, resulting
142
in structures with inversion center holding a TPA (4), and with a mirror plane when holding a TBA cation. The molecular mechanism of this structure directing effect of TPA versus TBA merits deeper investigations. The kinetics of the different condensation steps till (7) (and equivalent structures in the TBA system) are similar for TPA and TBA. This is illustrated using GPC data in Fig.2. The GP chromatograms of extracts after 5, 10, 15 and 45 minutes of hydrolysis illustrate the consecutive formation of species identified as dimers of tetracyclic undecamer (4) and (4'), precursor (5), dimers of precursors (6) and trimer of precursors (7). The assignment is based on the TPAOH retention volumes and previous identification with ^^Si liquid NMR for 45 min ^ ^ the TPA system [2]. The GPC traces show that the transformation of species of type (4), (4') and (5) into (6) and (7) 15 min __, is slower with TBA than with TPA. This coupling of precursors seems to 10 min be hindered by the long butyl chains of occluded TBA. The largest silicate species in the ^ 5 min extracts of the concentrated TEA 5 4' 4 6 7 system after 45 min, detected with 8,5 9,0 7,5 8,0 GPC as a shoulder on the main peak, 7;o has a retention volume characteristic TBAOH for dimer of tetracyclic undecamer 45 min (Fig.2). This indicates that the first steps of the TEOS polycondensation processes till the formation of (4) and 15 min (4') occur with TEA as well. This is in agreement with the proposed model in 10 min which TEOS hydrolyses at the TEOSaqueous interfaces in the vicinity of the alkyl groups of the 5 min tetraalkylammonium cations (Fig.l). 5 4' 4 7 6 These alkyl groups favor the formation 9,0 7,5 8,0 8,5 of the hydrophobic silica surfaces 7;o encountered already in the smallest retention volume species (l)-(3). The absence of trimers Fig.2 Gel permeation chromatograms of extracts taken after the indicated times. I
.
I
.
1
11
1
,
1
143 of tetracyclic undecamers (5) after 45 min in the TEA system (Fig.2) suggests that the ethyl chains are too short for a simultaneous strong interaction with three undecamers to form precursors (5) (Fig.l). The formation of MEL-MFI intergrowths with TEA [5,6] suggests that TEA has no clear structure directing influence. This is now explained by the non specific coupling of tetracyclic undecamers (3) (Fig.l) in presence of TEA. 3.2. From precursors to nanoslabs in diluted systems The aggregation of the silica species in solution continues upon dilution with water [3]. With TPA, the main final products at room temperature are nanoslabs (8), counting 396 Si atoms and composed of twelve precursors (5) (Fig.l). With TPA, it was experienced that the timing of the water addition was not very critical. The yield of Silicalite-1 nanoslabs on silica basis is typically 80% [1]. With TBA, there is a violent hydrolysis and gel formation when the water is added from the beginning. To avoid this, it is preferred to add water after complete hydrolysis. The MEL nanoslabs with occluded TBA rapidly dimerize at room temperature to resuh in the formation of slabs with dimensions of 1.3 x 4.0 x 8.0 nm [3]. The product yield on silica basis is similar to the TPA system. With TEA, the amount of extractable silicate was much smaller. IR spectra of the extracts recorded after 24 h stirring of the solutions are shown in Fig.3. % T
TBAOH
4000
3500
3000
2500 2000 (cm-^)
1500
1000
Fig.3. IR spectra of extracts from the diluted synthesis mixtures after 24 h. The broad band centered around 3400 cm"^ is due to hydroxyl groups. The asymmetric and symmetric C-H stretching at 2960-2995 cm"' and 2875-2895 cm'\ respectively, and the C-C and C-H bending at 1485-1510 cm"^ and 1380-1395 cm"\ respectively, are very weak in the TEA sample. The intense IR bands of Silicalite-1 and -2 are the internal asymmetric stretching of the silicate tetrahedron at ca. 1080 cm"\ and the Si-0 bending at ca. 450 cm"^ [8]. These two bands are structure insensitive [9] and occur at a same frequency in the three
144 samples. The structure sensitive external asymmetric stretching vibrations of the framework tetrahedra occurring in Silicalite-1 and -2 crystals at 1220 cm'^ is substantially broadened in and shifted to lower frequencies. The symmetrical stretching vibration are found at the usual frequency of ca. 800 cm'V The band at 960 cm'^ is assigned to silanol groups, abundantly ^-i present. The 570 cm' band is ascribed to the framework five-ring vibration [2]. The shift by 20 cm'^ with respect to the frequency observed in bulk Silicalite-1 and -2 (550 cm"') is explained by a particle size effect [2]. In the three samples, this band occurs at the same frequency, suggesting the presence of Silicalite nanoslabs of a similar size. The 570 cm'^ band is less intense in the TEA sample. Thermogravimetric analysis (TG) under oxidizing atmosphere reveals a high-temperature exothermic weight loss of 1.5 wt.% for the TEA sample, compared to 6.6 and 9.3 wt.% for TPA and TBA nanoslabs, respectively. From IR and TG it is concluded that nanoslabs are formed in presence of TEA, but the lower TEA content and the lower intensity of the double five-ring vibration indicate that the solid contains also less structured silica. 3.3. From nanoslabs to colloidal Silicalite upon heating The aggregation sequence of TPA-nanoslabs into aggregates and colloidal Silicalite-1 upon heating to 100°C was previously studied by XRS [4]. Upon heating, the system with TEA undergoes aggregation much quicker than the TPA system, which is aggregating faster than the TBA system. However, the material gleaned fi'om the ethyl derivative lacks the high crystallinity inherent to the colloidal Silicalite-1 formed with TPA and Silicalite-2 with TBA. This indicates that in the presence of TEA, the mechanism follows rather a less directed path. This agrees with the previous observation that TEAOH is not partaking in the hydrolysis of the whole silicon source so that asides of pentasil specific units also other siliceous species are formed. To understand the behavior of the crystallization and especially the question why aggregation occurs only at higher temperatures, a temperature dependent calculation of the potential energy of the interfaces as function of particle distance has been undertaken, using the DLVO formalism as before [4]. The XRS studies have shown that the crystallization starts with the formation of intermediates by condensation of the nanoslabs along the a direction. Therefore the energy calculations were performed for the be plane. The results are shown in Fig.4 for the TPA and TBA systems. The energy planes per surface and thermal energy have in common that large barriers exist for small distances seemingly preventing aggregation of the particles along a. However, it is most intriguing that just in front of these barriers minima exist. Moreover the minimum in the TPA system at 100 °C is in the order of magnitude of the thermal energy when multiplied with the size of the be plane of the intermediates. This means that particles not only undergo random encounters but remain trapped at a mutual distance of about 0.7 nm at 100 °C. Sufficient time is available to allow alignment mandatory for the subsequent fusing. The fusion itself can not be described by the DLVO theory because now chemical rather than physical interactions govern this process. With increasing temperature the minimum deepens and, therewith, the contact times increase. This is one factor responsible for speeding up of the crystallization with temperature. Comparison to the system with TBA immediately reveals that the interparticle distance is larger than in the TPA case. Also the depth of the energy well is comparatively shallow, which indicates shorter contact times of the particles. This directly accounts for the slower crystalHzation of Silicalite-2 compared to Silicalite-1. The same calculations performed for TEA revealed a surprise. In this case no barrier at low distances is encountered. The interfacial energy at elevated temperatures just keeps on
145
TPAOH
\
r^^
340 ^t 360 It 380 :^ 4 0 0 ^
7/y;i Sji'f^—r—±5 f" 0.5
1.0 d/nm
/
/
440 ^ 460 480
,
1.5
2.0
TB AOH
^-^ • /
/
' 0.8
1.0
-,2
^4 d/nm
!
/
'
^ '
I
. . - ^ - - 1' 1.6 1.8
.
•,': 2.0
^ 340 360 r 380 ^
f 440 460 * 480
Fig.4. Potential energy per thermal energy and surface area as a function of the distance to a nanoslab with occluded TPAOH and TBAOH and the temperature.
146 dropping with decrease of distance. This means that upon encounter between particles with random orientation, these can fuse randomly. This result explains why with TEA very quickly a solid material with low internal ordering is obtained in absence of a substantial barrier for the mutual approach of the particles. 4. CONCLUSIONS The molecular mechanism of formation of colloidal Silicalite zeolites from TEGS and TPAOH, TBAOH and TEACH is very similar. The structure directing effect of the tetraalkylammonium cation occurs at the stage of the coupling of tetracyclic undecamers. TPA favors the formation of dimers with inversion center, TBA with mirror plane, while with TEA there is no preference. The tetraalkylammonium ions have a strong influence on the kinetics. With TEA, all steps beyond the formation of tetracyclic undecamers are hampered by the short size of the ethyl group not reaching far enough outside the precursor species. The yield of Silicalite nanoslabs is low and the product disordered. The slower crystallization of colloidal Silicalite-2 compared to Silicalite-1 can be rationalized based on significant differences of the surface potentials of aggregating nanoslabs, estimated with extended DLVO theory.
REFERENCES 1. R. Ravishankar, C.E.A. Kirschhock, P-P Knops-Gerrits, E.J.P. Feijen, P.J. Grobet, P. Vanoppen, F.C. De Schryver, G. Miehe, H. Fuess, B.J. Schoeman, P.A. Jacobs and J.A. Martens, J. Phys. Chem. B, 103 (1999) 4960. 2. C.E.A. Kirschhock, R. Ravishankar, F. Verspeurt, P.J. Grobet, P.A. Jacobs and J.A. Martens, J. Phys. Chem. B, 103 (1999) 4965. 3. C.E.A. Kirschhock, R. Ravishankar, L. Van Looveren, P.A. Jacobs and J.A. Martens, J. Phys. Chem B, 103 (1999) 4972. 4. C.E.A. Kirschhock, R. Ravishankar, P.A. Jacobs and J.A. Martens, J. Phys. Chem. B (1999) submitted. 5. P.A. Jacobs and J.A. Martens, Stud. Surf Sci. Catal., 33 (1987) p.l91. 6. Z. Gabehca, E.G. Derouane and N. Blom, Appl. Catal. 5 (1983) 109. 7. R. Ravishankar, C. Kirschhock, B.J. Schoeman, D. De Vos, P.J. Grobet, P.A. Jacobs and J.A. Martens, Proceed. 12^^ Int. Zeolite Conf, Ed. M.M.J. Treacy, B.K. Marcus, M.E. Bisher, J.B. Higgins, Materials Research Society, 1999, 1825. 8. ref5,p.43. 9. E.M. Flanigen, Zeolite Chemistry and Catalysis, Ed. J.A. Rabo, Adv. Chem. Ser., 171 (1976)21.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
147
Study of interactions between silicate species and surfactant micelles in the synthesis of organized mesoporous materials Jom Frasch*, Benedicte Lebeau^ Michel Soulard^, Joel Patarin^ and Raoul Zana*". ^ Laboratoire de Materiaux Mineraux (UPRES-A-7016, CNRS), ENSCMu, 3, rue Alfred Werner, 68093 Mulhouse Cedex, France ^ Institut Charles Sadron (CNRS-ULP), 6, rue Boussingault, 67000 Strasbourg, France
We report investigations into silicate/surfactant assembly in the preliminary stages of the formation of ordered mesoporous siliceous MCM-41 materials. These materials were obtained in the presence of cetyltrimethylammonium bromide (CTAB) or chloride (CTAC) as surfactants by decreasing the pH of clear basic silicate solutions (waterglass solution or octameric silicate species SigOzo*'). The influence of the surfactant molecules on the nature of silicate species was studied by ^^Si liquid state NMR spectroscopy. Pyrene time-resolved fluorescence quenching experiments were used to characterize surfactant micelles in the presence of silicate species and/or NaOH. The resuhs obtained suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers.
1. INTRODUCTION The formation of ordered mesoporous siliceous MCM-41 (Mobil Composition of Matter) materials was first described by the Mobil Oil Company [1,2]. These exciting materials consist of amorphous silica walls with an ordered hexagonal arrangement of mesopores. Since their discovery, the mechanism of formation of these soUds synthesized from cationic surfactants in the presence of anionic silicate species has been much discussed [2-5]. Several studies were performed using in situ probing spectroscopies [6-9]. One of the main contribution was that of Stucky and coworkers [4], who proposed that these mesostructures result from a cooperative organization process involving electrostatic interactions between the positively charged quaternary ammonium micelles (S^) and the negatively charged silicate or aluminosilicate
148 framework (I). The first step of most of the mechanisms proposed thus far is based on the anionic exchange between surfactant counterions and siHcate anions. In this study, pyrene time-resolved fluorescence quenching experiments were used to characterize systems made up of cetyhrimethylammonium bromide (CTAB) or chloride (CTAC) and large excesses of silicates (waterglass solution or octameric silicate species Si802o^', the so-called D4R (double four ring)) in strongly basic media, with overall compositions similar to those used when preparing organized mesoporous materials. SiHcate surfactant mixtures were also characterized by ^Si liquid state NMR spectroscopy to show the influence of the surfactant on the nature of silicate species present in solution.
2. EXPERIMENTAL SECTION
Aqueous surfactant solutions were prepared at room temperature. The experiments were performed at a fixed concentration of 0.1 M CTAB or CTAC by decreasing the pH (by addition of HCl or replacing OH" by CI' with NaCl) of clear basic 0.5M silicate solutions (sodium silicate (Na2Si307) solution (waterglass) or D4R species stabilized in an aqueous tetramethylammonium hydroxide/methanol solution [10]). ^^Si liquid state NMR spectroscopy was performed on a Bruker DSX 400 spectrometer under the follov^ng conditions: frequency = 79.489 MHz, pulse width (90°) = 10 |is, recycle delay = 30 s, number of scans = 500 to 2000, temperature = 303K. The siHcate-containing surfactant solutions was introduced in a 8mm glass tube which is inside a 10 mm glass tube. In the latter tube, monomeric silicate species (high alkaline waterglass solution) were introduced as external reference (6 ~ -70ppm/ TMS (tetramethylsilane)). The probe molecule pyrene (-10"^ M) was used in time-resolved fluorescence quenching experiments using a single photon counting apparatus, cetylpiridinium chloride (CpyC, -10' M) being introduced as a quencher of the pyrenefluorescence[11-13]. All the experiments were performed at 303K. From thesefluorescencestudies the micelle aggregation number (N) and the pyrenefluorescencelifetime (x) were obtained [14]. X-ray diffraction experiments were performed on a STOE STADI-P diflfractometer (CuKai radiation X = 1.5406 A) equipped with a linear position-sensitive detector. The solutions and the solids were introduced in a 0.3mm capillary Lindemann tube (Debye-Sherrer geometry).
3. RESULTS and DISCUSSION 3.1. Starting mixtures at high pH The main experiments performed with the two different silica sources are reported in Table 1 and the ^Si liquid state NMR spectra of experiments 3, 4, and 11 (Table 1) are given in Figure 1.
149 Table 1 Systems CTAB/Si02/NaOH, CTAC/Si02/NaOH, and CTAB/SiOz/TMAOH/MeOH: Values of the pyrenefluorescencelifetime (T) and micelle aggregation number (N). No.
Systems (stoechiometry in mol/L)
x (ns)
N
systems with waterglass as silica source (1)*
O.ICTAB
165
145
(2)*
O.ICTAB: 0.5 NaOH
177
149
(3)*
0.1 CTAB : 0.5 Si02 : 0.9 NaOH
180
161
(4)
0.1 CTAB : 0.5 Si02 : 0.4 NaOH : 0.6 TMAOH : 21vol% MeOH 260 260
(5)
O.ICTAC
155
100 103
(342*) (101*)
(6)
O.ICTAC: 0.5 NaOH
165
106
(7)
0.1CTAC:0.5SiO2:0.9NaOH
190 113 (342*) (116*)
systems with D4R as silica source (8)*
0.1 CTAB : 21vol% MeOH
195
81
(9)*
0.1 CTAB : 1.0 TMAOH
196
127
(10)*
0.1 CTAB : 1.0 TMAOH : 21 vol% MeOH
247
77
(11)*
0.1 CTAB : 0.5 Si02 : 1.0 TMAOH : 21 vol% MeOH
272
96
* deaerated systems
The spectrum of the CTAB / waterglass solution (experiment 3, Figure la) is characterized by one resonance at -71ppm (reference TMS) corresponding to monomeric species (Q ) and several signals located between -78 and -83ppm; the latter being assigned to small oligomers e.g., dimeric and trimeric species (Q^ and Q^ units). Broad and less intense components attributed to more condensed species are also observed at around -88ppm (Q units).
150
*'*-«ft«^'*^1M*w«4,VrtA^^^ b)
^A'lVi^iASI^WllM'^^
c)
* I
-70
I
-90
-80
-100
Sppm/TMS Figure 1 Si liquid state NMR spectra of surfactant-silicate mixtures: a) waterglass 0.5M Si02 : O.IM CTAB : 0.9M NaOH, experiment 3, Table 1, b) octameric silicate species (D4R) 0.5M Si02 : O.IM CTAB : l.OM TMAOH : 21vol% MeOH, experiment 11, Table 1 and c) waterglass 0.5M SiOz : O.IM CTAB : 0.4M NaOH : 0.6M TMAOH : 21vol% MeOH experiment 4, Table 1. (* external reference)
The ^^Si liquid state NMR spectrum of experiment 11 (Figure lb) displays mainly one sharp and intense line at -99.4ppm corresponding to the D4R units. It is worthy to note that in the presence of a large amount of sodium cations (experiment 4), the concentration of D4R species considerably decreases (see Figure Ic), such a result being already mentioned in the literature [15]. Under our experimental conditions, no significant influence of the CTAB polar head group on the nature of the silicate oligomers is observed. Indeed, the spectra of experiments 3 and 11 are similar to those of the corresponding silicate solutions free of surfactant (spectra not reported) [10,16].
151 From time-resolved fluorescence quenching experiments, it seems that the micelle aggregation number N is significantly larger for CTAB than for CTAC solutions (see experiments 1 and 5). However, the variations of N upon addition of NaOH and of NaOH + Si02 are qualitatively similar for the CTAB and CTAC solutions and in all cases correspond to quasi-spherical micelles. Thus, the addition of 0.5M NaOH brings about a very small increase of N which is likely the result of two antagonistic effects: (i) a decrease of N due to the exchange of a small part of micelle-bound bromide ions by added hydroxyl ions; (ii) an increase of N associated with the increased ionic strength of the system due to the added NaOH. The values of the pyrenefluorescencelifetime x in deaerated solutions of CTAB and CTAC (systems 1 and 5 in Table 1) are very diflferent, 165 vs 342 ns, owing to the efficient dynamic quenching of the pyrene fluorescence by the micelle-bound bromide ions, micelle-bound chloride ions having no quenching effect [17,18]. Micelle-bound hydroxyl ions also have no quenching effect on the pyrene fluorescence since i has nearly the same value in solutions of CTAC and of cetyltrimethylammonium hydroxide (CTA0H)[19]. The x value for system 7 in Table 1 shows that silicate anions also do not quench the pyrene fluorescence. Indeed, the x values in deaerated solutions of CTAC and of CTAC + NaOH + sodium silicate (systems 5 and 7 in Table 1) are identical (342 ns) whereas, any quenching by silicate anions would have resulted in a lower value of x in system 7. The quenching of the pyrene fluorescence by the bromide ions can thus be used to detect a possible exchange of micelle-bound bromide ions by added hydroxyl or silicate anions (or by any non-quenching counterion for that matter). Indeed, when such an exchange occurs, the concentration of micelle-bound bromide ion decreases and the pyrene lifetime increases from the value x(Br) = 165 ns in the absence of exchange to x(Cl) = 342 ns in the case of complete exchange upon addition of hydroxyl, silicate, or chloride ions. From the x values reported in Table 1 (experiment 3) and according to reference [20] the maximum of the exchange is around 16%. In the presence of D4R species as silica source (second set of experiments Table 1), the aggregation numbers, with and without silica (experiments 11 and 10, respectively), are characteristic of spherical micelles. Moreover, the addition of D4R units has no effect on the pyrene fluorescence lifetime (x), which means that there is no BrVsilicate exchange in this micelles-containing system. As it is well known [21], the presence of methanol leads to a decrease of the aggregation number (compare experiments 9 and 10). Under our experimental conditions no CTA^-based mesophase was evidenced by XRD analysis.
3.2. Study of the pH lowering In Table 2 are reported the diflferent experiments performed in the system CTAC/SiOz (waterglass)/NaOH. The pH was decreased by addition of HCl or replacing part of OH" by CI' v^th NaCl. Clear solutions are obtained for pH higher than 11.5. When the pH is lower, a precipitate occurred, which was identified as poorly organized mesoporous silica by X-ray diffraction analysis (Figure 2).
152
Table 2 Systems CTAC/Si02/NaOH/(NaCl/HCl): Values of the micelle aggregation number (N), and ^ No. Systems (stoechiometry in mol/L) N pH
(7)
0.1 CTAC : 0.5 Si02 : 0.9 NaOH
113
13.2
acidification by replacing NaOH by NaCl (12)
0.1 CTAC : 0.5 Si02 : 0.6 NaOH : 0.3 NaCl
154
12.6
(13)
0.1 CTAC : 0.5 Si02 : 0.5 NaOH : 0.4 NaCl
157
12.1
(14)
0.1 CTAC : 0.5 Si02 : 0.4 NaOH : 0.5 NaCl
152
11.6
acidification by adding HCl (15)
0.1 CTAC : 0.5 Si02 : 0.9 NaOH : 0.3 HCl
143
12.5
(16)
0.1 CTAC : 0.5 Si02 : 0.9 NaOH : 0.5 HCl
148
11.6
From time-resolvedfluorescencequenching experiments performed on clear solutions (pH higher than 11), the pH lowering mainly leads to a small increase of the aggregation number and that whatever the procedure used (addition of HCl or replacing part of NaOH by NaCl). For instance, N increasesfi-om113 to 154 as the pH is decreasedfi-om13.2 to 12.6 (Table 2, experiments 7 and 12). However, such N values correspond to only slightly elongated micelles. Therefore, under these experimental conditions no drastic change is observed before the polymerization of silica.
2 4 6 8 2e(deg) Figure 2. Powder XRD pattern (k= 1.5406 A) of the solid obtained in experiment 7 after lowering of the pHfi-om13.2 to 11.
153 4. CONCLUSION
The preliminary formation steps of organized mesoporous materials was investigated by different techniques such as ^^Si liquid state NMR spectroscopy and time-resolved fluorescence quenching experiments. Under the experimental conditions used, no significant influence of the polar head group of the surfactant is observed on the nature of the silicate oligomers present in the starting solution. Time-resolved fluorescence quenching measurements using pyrene as probe molecule showed that only a smallfi"actionof micellebound bromide ions is exchanged by hydroxyl and silicate anions. Moreover, the micelle aggregation number in CTAB and CTAC-containing systems increases only slightly in the presence of these additives or by lowering the pH of the mixture. Its value indicates that there is hardly any change in the micelle structure under the conditions used. These results suggest that interactions between silicate species and surfactant micelles are weak in the precursor solution. The absence of any organization in the system prior to precipitation seems to indicate that the most important step in the process is the formation of siliceous prepolymers. The interaction of these prepolymers with surfactants could be responsible for micelle growth and subsequent reorganization of the silica/micelle complexes into ordered mesoporous structures. Such a hypothesis might be confirmed by preliminary potentiometric measurements using a bromide ion-specific electrode the amount of fi*ee bromide anion increasing at pH around 11 when the polymerization of silica starts.
REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth and J.C. Vartuli, US Patent No.5102643 (1992). 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. 3. C.Y. Chen, S.L. Burkett, H.X. Li and M.E. Davis, Microporous Mater., 2 (1993) 27. 4. Q. Huo, D. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schuth and G.D. Stucky, Chem. Mater., 6 (1994) 1176. 5. A. Firouzi, D. Kumar, L.M. Bull, T. Besier, P. Sieger, Q. Huo, S.A. Walker, J.A. Zasadzinski, C. Glinka, J. Nicol, D. Margolese, G.D. Stucky and B.F. Chmelka, Science, 267(1995)1138. 6. Y.S. Lee, D. Surjadi and J.F. Rathman, Langmuir, 12 (1996) 6202. 7. A. Firouzi, F. Atef, AG. Oertli, G.D. Stucky and B.F. Chmelka, J. Am. Chem. Soc, 119 (1997) 3596. 8. DC. Calabro, E.W. Valyoscsik and F.X. Ryan, Microporous Mater., 7 (1996) 243. 9. A. Galameau, F. Di Renzo, F. Fajula, L. Mollo, B. Fubini and M.F. Ottaviani, J. Colloid Interface Sci., 201 (1998) 105. 10. I. Hasegawa, S. Sakka, Y. Sugahara, K. Kuroda and C. Kato, J. Chem. Soc, Chem. Commun., (1989) 208. 11. R.G. Alargova, I.I. Kochijashky and R. Zana, Langmuir, 14 (1998) 1575.
154 12. R.G. Alargova, I.I. Kochijashky, M. Sierra and R. Zana, Langmuir, 14 (1998) 5412. 13. R. Zana, M. In, H. Levy and G. Duportail, Langmuir, 13 (1997) 5552. 14. R. Zana, In Surfactant Solutions, New methods of Investigation, R. Zana (ed), M. Dekker Inc., New York, Chap. 5 (1987) 241. 15. R.Thouvenot, G. Herve, J.L. Guth and R. Wey, Nouveau J. Chim., 10 (1986) 479. 16. G. Engelhardt and D. Michel (eds.) High Resolution Solid State NMR of Silicates and Zeolites (1987). 17. E. Abuin, E. Lissi, N. Bianchi, L. Miola and F.H.; Quina, J. Phys. Chem., 97 (1983) 5166. 18. E. Abuin and E. Lissi, J. Colloid Interface Sci., 143 (1991) 97. 19. P. Lianos and R. Zana, J. Phys. Chem., 87 (1983) 1289. 20. R. Zana, J. Frasch, M. Soulard, B. Lebeau, and J. Patarin, Langmuir, 15 (1999) 2603. 21. R. Zana, Adv. Colloid Interface Sci. 57 (1995) 1.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
155
Novel Ordered Mesoporous Materials with Hybrid Organic-Inorganic Network in the Frameworks S. Inagaki% S. Guan% Y. Fukushima% T. Ohsuna^ 0. Terasaki'^ Toyota Central R&D Labs., Inc., Nagakute, Aichi, 480-1192, Japan ^Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan ^Department of Physics, Graduate School of Science and CREST, JST, Tohoku university, Sendai, 980-8578,Japan Pore-wall structure and structural order of novel hybrid organic-inorganic mesoporous materials were compared with those of organic-free siliceous MCM-41 and organic-grafted MCM41 materials. NMR spectra showed that the hybrid mesoporous materials were composed of SiOj ^-CH^-CH^-SiO^ ^ unit, which is quite different from the pore-wall structure of the organicgrafted MCM-41. The hybrid mesoporous materials have higher ordered pore-arrangement structure and sharper pore size distribution than the other mesoporous materials. The improvement of the mesoscopic order results in the formation of well-defined external morphologies reflecting two and three-dimensional hexagonal symmetries of pore arrangement. They have higher thermal stability than the organic grafted MCM-41 material because organic groups in the hybrid network are sandwitched by the inorganic species. 1. INTRODUCTION The synthesis procedure, using self-assembly of surfactant-inorganic molecular composite, has given us various ordered mesoporous materials with different compositions, pore structures and morphologies. The framework composition, which is extremely important to be used for catalysts and adsorbents, has been extended from silicates in the first synthesized mesoporous materials MCM-41 ^^ and FSM-16^^ to the transition metal oxides such as, AI2O3, ZrO^, Ti02 etc.^^ and pure metal ?f\ These extension of the composition makes the mesoporous material usefixl as not only acid catalysts but also photo- and electro-catalysts, sensors and electrodes. Recently, introduction of organic groups in the mesoporous materials have attracted attentions because the organic groups have many variety and peculiar functions that are not expected for inorganic
156
Inorganic oxide Organic group
i^s> Novel hybrid mesoporous material
Inorganic oxide Organic group
^ ^^ Conventional organic-grafted mesoporous material
Figure 1. Schematic models of the novel hybrid mesoporous material and conventional organicgrafted mesoporous material. materials. Mesoporous silica possessing thiol groups onto the pore surface showed high adsorption efficiency for heavy metals such as Hg, Ag, Cr ions^\ Mesoporous silica grafting sulfonic acid groups showed high catalytic activity for selective formation of bulky organic molecule 2,2-bis(5-methylftiryl)propane^^ These organic grafted mesoporous materials are prepared typically by two methods. The first one is post-synthesis treatment in which pore-wall surface of inorganic mesoporous material is modified with organosilane compounds. Another method is direct synthesis from organosilane compounds by the condensation reaction in the presence of surfactant. The organosilane compounds with one silyl group, CgH,^-Si(0Et)3^\ C^H5-Si(OEt)3^\ NH2(CH2)3-Si(OEt)3«), C2H3-Si(OEt)39\ HS(CH2)3-Si(OEt)3'«) have been used for the direct synthesis of the organic-functionalized mesoporous materials using the mixture of organosilane and tetraethylorthosilicate [Si(OEt)^]. However, highly ordered mesoporous material is difficult to be formed from those organosilane compounds because the regular micelle structure is disturbed by the organic groups coexisting in the pore space during the formation process. These organic-grafted mesoporous materials have a heterogeneous pore-wall structure in which organic groups are grafted on the inorganic mainframework,that has the disadvantage of making catalytic sites on the inorganic framework inactive through coverage with organic groups. Recently, we have reported the syntheses of ordered mesoporous materials, in which ethane fragments (-CH^CH^-) and Si203 species are distributed homogeneously at the molecular level in theframeworkand form a covalendy bonded network"^ They are the first periodic mesoporous materials that contain organic materials in the inorganic main framework as an essential component. These materials are quite different from conventional organic-grafted ordered mesoporous materials. Another group also reported almost the same mesoporous materials with hybrid structure of ethane fragments and Si203 species after our publication'^^ In addition, our hybrid
157 mesoporous materials have highly ordered two- (2D) and three- (3D) dimensional hexagonal symmetries with well-defined external morphologies"^. These hybrid materials have been synthesized by condensation of 100% organosilane compound with two silyl groups [(CHfi)^CH2CH2-(OCH3)3] in the presence of alkyltrimethylammonium surfactant. Here, we report structures of the new hybrid mesoporous materials, pore-wall and structural order, and these results are compared with siliceous MCM-41 and organic-grafted MCM-41. 2. EXPERIMENTAL 2.1. Sample preparations Three types of mesoporous materials, siliceous MCM-41, organic-grafted MCM-41 and hybrid mesoporous materials were prepared. The organic-free MCM-41 material was prepared by condensation of tetramethylorthosilicate [Si(OCH3)4: TMOS] in the presence of hexadecyltrimethylammonium chloride (HDTMA) at 25 °C for 48 h. The molar ratio of TMOSiHDTMAiNaOHiHp was 1:0.09:0.5:117. The organic-grafted MCM-41 was prepared by co-condensation of TMOS and ethyltrimethoxysilane [C2H5-Si(OCH3)3: ETMS] in the presence of HDTMA at 25 °C for 38 h, followed by hydrothermal treatment at 70 °C for 8 h after pH adjustment at 8.5. The molar ratio of TMOS:ETMS:HDTMA:NaOH:H20 was 4:1:0.6:3:580. The hybrid mesoporous materials were prepared by condensation of 1,2bis(trimethoxysilyl)ethane [(CH30)3Si-C2H4-Si(OCH3)3: BTME] in the presence of octadecyltrimethylammonium chloride (ODTMA). The molar ratios of BTME:ODTMA:NaOH:H20 were 1:0.57:2.36:353 and 1:0.12:1:231 for the hybrid mesoporous materials with 2D- and 3D-hexagonal symmetries. The temperatures of condensation are 95 and 25 °C, respectively. The detail synthesis condition is reported previously"\ The surfactants of the as-synthesized mesoporous materials were removed by calcinations at 550 °C for the MCM-41 material and by solvent extraction for the organic-grafted MCM-41 and the hybrid mesoporous materials. 2.2 Characterizations ^^Si NMR spectra were recorded on a Bruker MSL-3000WB spectrometer at 59.62 MHz by using trimethylsilane for reference of chemical shift at 0 ppm. XRD patterns were obtained with a Rigaku RINT-2200 diffractometer using Cu Ka radiation. Scanning electron microscope (SEM) images were obtained on a JEOL JSM-890 with a field emission gun. The adsorption isotherms of nitrogen were measured using a Quantachrome Autosorb-1 system. The sample were outgassed for 2 h at room temperature before the measurements.
158 3. RESULTS AND DISCUSSION 3.1. Pore-wall structure In Figure 2 ^^Si NMR spectrum of the hybrid mesoporous material is compared with those of siliceous MCM-41
and
the
organic-
functionalized MCM-41 materials. The hybrid mesoporous material has two signals due to T^ and T^ type Si species [T^: -57.0 ppm, SiC(0Si)2(0H), V: -66.0 ppm, SiC(0Si)3], with Si bond to carbon atom covalently, and no signal d u e to Q t y p e Si s p e c i e s [Si(OSiX(OH)^.^, n=l-4], which have no Si bond to carbon atom. Although, the siliceous and organic-grafted MCM-41 materials have Q^ Q^ and Q^ s i g n a l s [Q^: - 9 2 . 0 , -90.7 p p m , SiCOSiyOH)^, Q^: -101.4, -100.8 ppm, Si(0Si)3(0H), Q^: -110.7, -109.5 ppm,
50
0
-50 -100 -150 Chemical shift (ppm)
-200
Si(0Si)4] due to Si species with no bond to carbon atom and the organic-grafted MCM-41 material had also T^ and T^
Figure 2. ^^Si MAS NMR spectra of (a) siliceous MCM-41, (b) C2H3-grafted MCM-41 and (c) C2H^-Si203 hybrid mesoporous materials.
signals in addition to the Q type signals. '^C NMR spectrum of the hybrid mesoporous material showed only one signal assigned to the carbons of Si-CH2-CH2-Si. So, the hybrid material can be concluded that a framework structure was made by hybrid network composed of SiO, 3-CH2-CH2-SiO, 5 unit, and the material is quite different pore-wall structure from the siliceous MCM-41 and the organic-grafted MCM-41 materials as shown in Figure 3. This is the first mesoporous material whose framework has a completely uniform distribution of organic fragments incorporated within the inorganic oxide framework at the molecular level, and has a periodic pore-arrangement structure. This is also the first organic-containing mesoporous silicate material that has only T type Si species without Q type Si species. Organic-grafted mesoporous silicate with only T type Si species has never synthesized previously. The organosilane compound with only one alkoxyl group at the end of the organic group [R-Si(0R)3] was difficult to form mesoporous material
159
0
0
0
0
\ \AAX)^
^
0
0
I
I
wvo^
^
0
I
0
0
wvX)
0
0
/
\
r2'^4
/
I
0
C2H5 'VO-Si-C^H.-Si^"^
Q
U\AA/
Siliceous MCM-41
Organic-grafted MCM-41
Hybrid mesoporous material
Figure 3. Schematic drawings for pore-wall structures of siliceous MCM-41, organicgrafted MCM-41 and hybrid mesoporous materials. when 100 % of the organosilane compound were used. Silane compound such as tetraethylorthosilicate and tetramethylorthosilicate is necessary to be mixed with the organosilane compound to form a stable framework because the organic groups are located onto the internal pore surface and do not construct the main framework. The mixture ratio of organosilane/silane compounds should be under 0.25 to form a mesostructure^^^. In contrast, organosilane compound with two or more alkoxyl groups can form highly ordered mesoporous material even though 100% of the organosilane is used. The organosilane compound with two or more alkoxyl groups can make stableframeworkwithout mixing silane compound because the alkoxyl groups at the both side of organic fragment condense each other to form a stable three-dimensional network containing organic fragment in the network. Furthermore, the regular micelle arrangement is not disturbed during the formation of hybrid mesoporous material. The hybrid mesoporous material has some silanol groups corresponding to the T^ type Si species. The peak intensity ratio of the TVT^ signals of the hybrid mesoporous material is almost equivalent to the QVQ"^ signals of siliceous MCM-41 material. This indicates that silanol density is almost same to the siliceous MCM-41 material. Adsorption isotherm of water vapor showed that the hybrid mesoporous material has more hydrophobic surface than siliceous MCM41. This fact supports that the organic fragments are exposed on the surface and increased the surface hydrophobicity. 3.2. Pore-arrangement structure The introduction of organic groups within the framework of the mesoporous material has improved the periodicity of the pore-arrangement with the symmetries. Figure 4 shows the XRD patterns of the three types of mesoporous materials with 2D-hexagonal symmetry (p6mm).
160 The diffraction peaks of the hybrid mesoporous material are sharper than those of MCM-41 and organic grafted MCM-41 materials. Further, the higher ordered diffractions, (300), (220) and (310) were observed for the hybrid mesoporous material, indicating a high degree of mesoscopic order. Their higher ordered diffractions have not been usually observed for previously reported mesoporous materials such as MCM-41 and FSM-16. The hybrid mesoporous material with 3D-hexagonal symmetry also showed the well-defined XRD pattern''^ the peaks in which are separated more distinctly than those 4 5 6 7 of previously reported 3D-hexagonal 2e(degree) mesoporous materials, SBA-2'^^ and SBAFigure 4. XRD patterns of (a) siliceous MCM41, (b) organic-grafted MCM-41 and (c) hy12'^^ It suggests that the hybrid mesoporous brid mesoporous materials with p6mm. material has also a higher ordered mesoscopic order. The organic grafted mesoporous materials usually have poorer mesoscopic order than MCM-41 as shown in Figure 4 , b. The high resolution transmission electron micrograph showed highly ordered pore arrangement from the hybrid mesoporous material"\ The electron diffraction spots were observed up to sixth order with hexagonal symmetry, suggesting excellent long-range order in the mesophase*'^ ' These hybrid mesoporous materials have well-defmed external morphologies as shown in the scanning electron micrographs (Figure 5). They showed rodlike particles with a hexagonal cross-section for 2D- and spherical particles for 3D-hexagonal symmetries, respectively. Almost all of the produced particles have same well-defined external morphologies*'\ These external morphologies are ideal particle shapes reflecting the symmetries of pore-arrangement. To our knowledge, the formation of the highly ordered mesoporous material with ideal hexagonal rodlike shape reflecting the 2D-hexagonal pore-arrangement symmetry is the first case although the mesoporous materials with highly curved'^^ and fibrous'^^ morphologies have been reported previously. This suggests that these mesoporous materials have highly ordered mesoscopic order and a low density of defects. The introduction of organic groups to the inorganic framework relax the stress existing in the rigid inorganic silicate network and improved the
161
(a)
(b)
Figure 5. SEM photographs of hybrid mesoporous materials with (a) 2D- and (b) 3D- hexagonal symmetrie. pore-arrangement symmetry. The hybrid mesoporous materials have very uniform pore sizes. The nitrogen adsorption isotherm showed sharp increase in adsorption at P/Po=0.36-0.39 due to a capillary condensation of nitrogen in the mesopores (Figure 6). The sharpness is higher than those of the MCM-41 and organic grafted MCM-41 materials, indicating narrower pore size distribution. The pore sizes and BET surface areas of the MCM-41, the organic grafted-MCM-41 and the hybrid mesoporous materials areas are 1.8 nm and 890 mVg, 1.5 nm and 720 mVg and 3.1 nm and 750 mVg, respectively. The hybrid mesoporous material 600 have higher thermal stability than the organic-grafted MCM-41. Thermal gravimetry analysis of the hybrid mesoporous material under air showed gentle weight loss from 400 to 700 °C due to the decomposition of ethane fragment in the pore wall, whose temperature was higher than the decomposition temperature (250500 °C) of ethyl groups on the pore wall of the organic-grafted mesoporous material. The hybrid materials are also stable in boiling water. The higher thermal and hydrothermal stability of the hybrid mesoporous materials is attributable to the hybrid pore-wall structure, in which or-
P/Po Figure 6. Nitrogen adsorption isotherms of (a)sili^eous MCM-41, (b) organic-grafted MCM-41 and (c) hybrid mesoporous material with p6mm.
162 ganic groups are fixed at the both side in the inorganic network. The mesoscopic structure of the hybrid mesoporous materials were preserved after the decomposition of the organic groups in the framework. Although large shrinkage of the lattice constant was observed during calcination, the calcined materials showed XRD patterns with 2D- and 3D-hexagonal symmetries and uniform pore-size distribution. 4. CONCLUSION The hybrid mesoporous materials have quite different structure in pore-wall from siliceous MCM-41 and organic-grafted MCM-41 materials. Organic fragments and inorganic oxide moieties are distributed homogeneously at the molecular level in the framewrok, forming a covalently bonded network. They have highly ordered mesoscopic order and well-defmed external morphologies. The hybrid mesoporous materials are expected to be used as novel catalysts, adsorbents and hosts for nanocluster synthesis because unique surface property is expected from the pore-wall structure, both organic and inorganic active sites are exposed on the surface.
REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710. ; J. S. Beck et al., J. Am. Chem. Soc, 114 (1992) 10834. 2. S. Inagaki, Y. Fukushima, K. Kuroda, J. Chem. Soc., Chem. Commun., (1993) 680.; S. Inagaki, A. Koiwai, N. Suzuki, Y. Fukushima, K. Kuroda, Bull. Chem. Soc. Jpn., 69 (1996) 1449. 3. R Yang, D. Zhao, D. I. Margolese, B. F. Chmelka, G. D. Stucky, Nature, 396 (1998) 152. 4. G. S. Attard et al.. Science, 278 (1997) 838. 5. X. Feng et al.. Science, 276 (1997) 923. 6. W. M. V. Rhijin, D. E. D. Vos, B. F. Sels, W. D. Bossaert, R A. Jacobs, Chem. Commun. , (1998)317. 7. S. L. Burkett, S. D. Sims, S. Mann, Chem. Commun., (1996) 1362. 8. D. J. Macquarrie, Chem. Conmiun., (1996) 1961. 9. M. H. Lim, C. F. Blanford, A. Stein, J. Am. Chem. Soc., 119 (1997) 4090. 10. C. E. Fowler, S. L. Burkett, S. Mann, Chem. Commun., (1997) 1769. 11. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna, O. Terasaki, J. Am. Chem. Soc, 121 (1999) 9611. 12. B. J. Melde, B. T. Holland, C. F Blanford, A. Stein, Chem. Mater., 11(1999) in press. 13. M. E. Lim, C. F. Blanford, A. Stein, Chem. Mater., 10 (1998) 467. 14. Q. Huo, R. Leon, R M. Petroff, G. D. Stucky, Science, 268 (1995) 1324. 15. D. Zhao, Q. Huo, J. Feng, B. F Chmelka, G. D. Stucky, J. Am. Chem. Soc, 120 (1998) 6024. 16. H. Yang, N. Coombs, G. A. Ozin, Nature, 386 (1997) 692. 17. G. D. Stucky et al.. Adv. Mater., 9 (1997) 974. 18. We thank H. Kadoura for FE-SEM observation and N. Suzuki and Y Seno for discussion on TEM. O. T. & T. O. thank CREST, Japan Science and Technology Corporation (JST) for the support.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
163
Synthesis and Catalytic Application of Organically Modified Ti-MCM-41 Type Materials Naoko Igarashi,^ Satoshi Kidani,^ Rizwan-Ahemaito'' and Takashi Tatsumi^ ^Department of Applied Chemistry, Graduate School of Engineering, The University of Tokyo, Hongo, Tokyo 113-8656, Japan ''Division of Materials Science & Chemical Engineering, Faculty of Engineering, Yokohama National University, Tokiwadai, Hodogaya-ku, Yokohama 240-8501, Japan Organically modified mesoporous titanium-substituted MCM-41 materials (TiMCM-41-R, R= CgHg, CH3) have been synthesized. These materials show higher hydrophobicity than unmodified Ti-MCM-41. This high hydrophobicity has a strong influence on the activity improvement in the oxidation of alkenes with HgOg. Furthermore, hydrothermal treatment during synthesis has increased titanium incorporation.
1. INTRODUCTION Since the recent discovery of M41S family [1,2], numerous studies have been performed extensively on modifications of the MCM-41 synthesis to increase the potential applicability of these materials. Titanium-substituted MCM-41 is capable of oxidizing larger molecules with H2O2 than titanium-substituted zeolites [3]; however, it showed much lower activity than either TS-1 or Ti-beta for the oxidation of small molecules probably due to the presence of large number of silanol groups [4]. In this regard, our strategy to enhance the activity of Ti-MCM-41 in the liquid phase oxidation is to increase its hydrophobicity. We have already reported that the activity of titanium-containing mesoporous molecular sieves in oxidation of alkenes and alkanes with HgOg was remarkably enhanced by trimethylsilylation of silanol groups [5]. Here we report a one-step synthesis of organically modified mesoporous titanium-substituted MCM-41, which has proved to be highly active in the oxidation of alkenes with Hfi^. Additionally, to increase the titanium incorporation, the hydrothermal treatment has been applied.
164
2. EXPERIMENTAL 2.1 Synthesis Ti-MCM-41-R materials (6-19% organic incorporation) were synthesized from a mixture of two different silica sources, tetraethyl orthosilicate (TEOS)(80%) and organoalkoxysilanes (20%) such as phenyltriethoxysilane (PTES) and methyltriethoxysilane (MTES), and tetrabutyl orthotitanate (TBOT) as a titanium source. An aqueous solution of NaOH / hexadecyltrlmethyl-ammonium bromide (C^JMABr) was added to a mixture of TEOS and either PTES or MTES in an ice bath under vigorous stirring, and after 1.5 hour stirring TBOT was added dropwise. Finally, the mixture (1.0 TEOS : 0.25 organosilane : 0.15 C^eTMABr: 0.38 NaOH : 125 H2O : 0.02 TBOT) was stirred for 2 days at room temperature. The samples made by this method are denoted as Ti-MGM-41 -R [A]. In addition to the standard one-step synthesis method, the hydrothermal treatment at 87 °C fori day which followed the 2-day stirring at room temperature has been also conducted for comparison to give Ti-MCM-41-R [B]. The organic templates and other alkaline cation residues were removed by an acid treatment method using 1M HOI solution in ethanol at 80°C for 16h. 2.2. Analytical procedure X-ray powder diffraction (XRD) patterns were collected on a Mac Science diffractometer Model No. 1030 M3X using Cu-K„ radiation. Inductively coupled plasma (ICP) analyses were performed on a Shimadzu ICPS-8000E analyzer for titanium. Thermogravimetric analyses (TGA) were carried out on a Ulvac 9000 thermogravimetric analyzer. The samples were exposed to moisture over a saturated aqueous solution of NH^CI at room temperature for 24 hours before the measurements, and were heated in Ng gas flow (120 ml/min) at a heating rate of 10 °C/min. The nitrogen adsorption isotherm measurements were carried out using a Belsorp 20A apparatus. Solid-state silicon nuclear magnetic resonance (^^Si NMR) were recorded on JEOL JNM-GX270 spectrometer using magic angle spinning (MAS) at a frequency of 53.54 MHz. Chemical shifts were referenced to external tetramethylsilane (TMS). Spinning rates of 3.5 kHz, recycle delay time of 7.0 s, pulse widths of 6.0 ^is, and 15000 to 20000 scans were taken for ""Si. Ultraviolet-Visible (UV-VIS) spectroscopy was performed on a Hitachi 340 spectrometer with a diffuse reflectance mode. Infrared (IR) spectra were obtained with a Perkin Elmer 1600 series FT-IR using a KBr pellet method with 64 scans. Atypical oxidation run used 50 mg of a catalyst suspended in a mixture of 25 mmol of substrate and 5 mmol of Hfi^ (31% aqueous solution). The mixture was vigorously stirred at 323 K.
165 3. R E S U L T S A N D D I S C U S S I O N The XRD patterns of the organically functionalized Ti-substituted samples from which the surfactant was removed by the acid treatment exhibited d^oo, d^^o ^'^^1 dgoo reflections, indicating a hexagonal arrangement of channels. UV-VIS spectra confirmed tetrahedral titanium incorporation around 220 nm (Figures 1 and 2). In addition, 260 nm band was observed for Ti-MCM-41-Ph, suggesting the incorporation of the phenyl groups (Figure 1), whereas only a broad shoulder around 220-300 nm was observed for Ti-MCM-41-Me (Figure 2). There was no difference in the UV-VIS spectra between the samples synthesized at room temperature and hydrothermally treated. IR spectroscopy also indicated qualitatively that organosiloxanes were incorporated into the silica framework. The appearance of peaks at 3050 cm"* and 1430 cm"* for Ti-MCM-41-Ph is due to the presence of phenyl groups (data not shown) while a peak at 2987 cm"" for Ti-MCM-41-Me (data not shown) corresponded the presence of aliphatic C-H bonds. Various physical data for the Ti-MCM-41 and organically functionalized Ti-MCM41 samples are shown in Table 1. For the phenyl group-containing Ti-MCM-41 samples, contraction of pore diameter was observed as a result of the interaction between the phenyl groups and the hydrophobic alkyl groups of the micelles suggested by Richer and Mercier [6]. The presence of phenyl groups is likely to result in the deeper penetration of the organosiloxane-derived molecules within the micelle than do the TEOS-derived molecules. The amount of organic functionalities present In the framework was determined by ^^Si MAS NMR, which showed 22-27 mol% of phenyl group incorporation and 9-14 mol% of methyl group incorporation. The amount of titanium incorporation has been increased from the Si/Ti ratio of 490 to that of 56 for Ti-MCM-41-Ph and from 570 to 52 for Ti-MCM-41-Me when the hydrothermal treatment at 87*'C was applied during the synthesis. Ti-incorporation into the framework was favored in the hydrothermal treatment.
200
300 400 Wavelength / nm
500
Figure 1. UV-VIS spectrum of TIMCM-41-Ph [A].
200
300
400
500
Wavelength / nm Figure 2. UV-VIS spectrum of TiMCM-41 Me [A].
166 Table 1. Physical data for Ti-MCM-41 and organically functionalized Ti-MCM-41 -R Pore Diameter^[A]
Surface Si/Ti Area[m2/g] Product^Starting)
c, R^ [mol%]
Ti-MCM-41
23.2
1015
123
(80)
-
Ti-MCM-41-Ph[A]
19.0
979
490
(50)
28
T I - M C M - 4 1 -Ph
[B]
19.0
1016
56
(50)
22
Ti-MCM-41-Me [A]
21.2
1040
570
(50)
8.9
Ti-MCM-41-Me [B]
23.2
1146
52
(50)
14
a : determined by Ng adsorption isotherm(D-H method) b : determined by elemental analysis c : determined by ^^Si NMR analysis
From the TGA of the moisture-equilibrated samples, the amount of water adsorbed onto the surface of titanium-substituted mesostructured silica materials was measured as the wt. % loss around 100°C (Figure 3), obviously the presence of organic groups resulted in a significant decrease in the amount of adsorbed water. This strongly suggests that the organically functionalized Tisubstituted mesoporous materials are much more hydrophobic than nonfunctionalized materials. This trend was also observed for organo-group containing mesoporous materials which showed the stability towards water and mechanical pressure [7]. In TGA, the difference in the amount of water adsorbed was not noticeable between the phenyl and the methyl substituents. 25 100 200 300 400 500 600 The catalytic activity of the organiTemperature / °C cally functionalized Ti-substltuted samples for the oxidation of cycloFigure 3. Weight loss (wt%) in TGA hexene with HgOg as an oxidant was analyses of (a) Ti-MCM-41, (b) remarkably Increased by one or two organically functionalized Ti-MGMorders of magnitude (Table 2). This 41-Ph and (c) Ti-MCM-41-Me. activity enhancement is attributed to conducted in the flow of N,. the increased hydrophobicity caused
167
Table 2. Oxidation of cyclohexene with H2O2 over Ti-MCM-41 and organically functionalized Ti-MCM-41 -R
r ^
KJ^
cat.
H2O2 •
u •i 5 * o< ^ 1
3
2
TON Conv. (mol% of max)(mol/Ti-mol •h) 1
rr^ ^
< * 4
0.7
1.8
Selectivity (%) 2 3 0 30.0 15.2
8.5
170
37.3 36.6
26.1
0
0
13.0
27
14.5 16.0
5.0
64.5
8
Ti-MCM-41 -Me [A] (570)
4.0
50
6.0 16.5
70.1
7.4
17
Ti-MCM-41 -Me [B] (52)
18.0
30
19.6 15.4
5.3
59.7
19
Catalyst (Si/Ti ratio) Ti-MCM-41 (123)^
Ti-MCM-41 -Ph [A] (490) Ti-MCM-41-Pli[B] (56)
H2O2 Decomp. 4 (%) 54.7 58
conditions : cat. 50 mg, substrate 25mmol, H2O2 5mmol, 323K, 2h.(a : 3h.) * Numbers in parenthesis denote SiATi ratio.
by the presence of organic groups; we have observed similar enhancing effect for the Ti-containing MCM-41 post-synthetically trimethylsilylated [5]. Furthermore, nonproductive decomposition of HgOg has been inhibited by the organic functionalization. Recently Corma et al. reported the high oxidation activity (with tert-butyl hydroperoxide as an oxidant) of organically functionalized Ti-MCM-41 prepared by one-step direct-synthesis method [8]. For the samples prepared by the standard one-step synthetic method denoted as [A], a large increase in the selectivity of epoxidation was observed. Although the samples synthesized by the synthetic method [B] resulted in the higher conversion, diol was the major product. The diol is produced by an acid catalyzed reaction; thus, the increased Ti-incorporation may contribute to the acidity increase of the sample. Results of oxidation of unsaturated alcohols are shown in Table 3. Both 2penten-1-ol and 3-methyl-2-buten-1-ol exhibited higher reactivity than cyclohexene. A decrease around 20-50% in catalytic activity of organically functionalized samples has been observed. This is probably due to the inhibition of access of the rather hydrophilic substrates to the Ti-active sites surrounded by the organic groups of increased hydrophobicity. It is noteworthy that the epoxidation was favorable for the organically functionalized samples whereas the alcohol oxidation was retarded.
168
Table 3. Oxidation of unsaturated alcohols With H2O2 over Ti-MCM-41 and organically functlonallzed Ti-MCM-41-R Catalyst (Si/Ti ratio)
Conv. TON (mol% of max)(mol/mol-Ti • h)
2-penten-1-ol oxidation Ti-MCM-41 (123) Ti-MCM-41-Ph[A](490) Ti-MCM-41 -Me [A] (570)
32.4 25.5 16.0
121 496 277
3-methyl-2-buten-1 -ol
Ti-MCM-41 (123) Ti-MCM-41-Ph[A](490) Ti-MCM-41-Me[A](570)
Selectivity (%)
2-pentenal
epoxide
81.0 60.4 49.0
19.0 39.6 51.0
3-methyl -2-butenal 17.7 12.0 10.0
66.2 234 173
84.7 59.8 75.3
Decomp. (%)
0.0 8.0 0.0
epoxide
15.3 40.2 24.7
37.9 14.0 34.0
Cat. 50 mg, substrate 25 mmol, H2O2 5 mmol, 323 K, 2h. * Numbers In Parenthesis denote Si/Ti Ratio.
In summary, the organically functlonallzed Ti-substituted MCM-41 materials have been successfully synthesized by one-step synthesis method with a varied TlIncorporatlon of the Si/Ti ratio from 50 to 600. The hydrothermal treatment resulted In the Increase of Tl-lncorporatlon. The epoxidation selectivity was Improved by organic functionalizatlon than alcohol oxidation probably due to the Increased hydrophoblclty nearby the Ti-active sites. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992)710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.TU. 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. K.A. Koyano and T. Tatsumi, Microporous Mater., 10 (1997) 259. 4. T. Blasco. A. Corma, M.T. Navarro and J.P. Parlente, J. Catal., 156 (1995) 65. 5. T. Tatsumi, K.A. Koyano and N. Igarashi, Chem. Comm., (1998) 325. 6. R. Richer and L Mercler, Chem. Comm., (1998) 1775. 7. N. Igarashi, Y. Tanaka, S. Nakata and T. Tatsumi, Chem. Lett, (1999) 1. 8. A. Corma, J. L Jorda, M. T. Navarro and F. Rey, Chem. Comm., (1998) 1899.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
169
Influence of silylation on the catalytic activity of Ti-MCM-41 during epoxidation of olefins. A. Conna,^* J.L. Jorda,^ M.T. Navano,^ J. Perez-Pariente,'' F. Rey^ and J. Tsuji' ^Institute de Tecnologia Quimica (C.S.I.C. - U.P.V.) Avenida de los Naranjos s/n, 46022-Valencia, Spain ^Institute de Catalisis y Petroleoquimica (C.S.I.C.) Campus Universitario de Cantoblanco, 28049 - Madrid, Spain ^Sumitomo Chemical Co., Ltd., Petrochemicals Research Laboratory 2-1 Kitasode Sodegura City, Chiba Pref, Japan Ordered mesoporous titanium containing MCM-41 type materials have been silylated with a suite of organosilanes. We found that catalytic activity for epoxidation of cyclohexene using tercbutylhydroperoxide as oxidant as well as the selectivity to the desired epoxide strongly depend on the hydrophobic properties and on the number of free silanol groups of Ti-MCM41 catalysts, which can be nicely controlled by modifying the number and/or the nature of the organosilane groups bounded onto the walls of Ti-MCM-41 catalysts.
1. INTRODUCTION The development of new Ti containing zeolites and modifications of those already known are of great importance in the filed of liquid oxidation reactions using heterogeneous catalysts. Since the discover of the earlier Ti-containing zeolite, TS-1, by Enichem [1], a constant effort have been made to extend the capabilities of new materials having Ti isomorphically substituted into the siliceous framework of a variety of zeolites. Then, Ti has been successftiUy incorporated in several zeolites, such as TS-2 [2], Beta [3], ZSM-48 [4], ZSM-12 [5] and also in different zeotypes AlPO-5, AlPO-ll [6], AlPO-36 [7] and SAPO-5 [8]. Recently, the use of liquid crystals as templating agents for the synthesis of new materials has led to new catalysts having a well ordered pore system with pore size ranging from 1.5 to 10 nm [9,10]. The isomorphical substitution of Ti in the silica framework has allowed to carry out catalytic processes using heterogeneous catalyst in which very bulky reactants are involved [11,12]. This opened a new field of research of great interest not only from the point of view of scientific knowledge, but also for its implication in industrial productions of fragrances, food additives, etc. However, when small size molecules, which can freely diffuse either in Beta zeolite and MCM-41, are used a lower turnover is observed on Ti-MCM-41 catalysts compared to that obtained on Ti containing Beta zeolite [13]. This low activity can be attributed to that some of the Ti sites in MCM-41 type of catalysts are buried on the silica walls, being non-accessible to the reactants and also, to the very different adsorptive properties of Ti-MCM-41 and Ti-
170 Beta catalysts. Indeed, the extremely high concentration of silanol groups located at the external surface of the pores of Ti-MCM-41 gives to these catalysts a relatively hydrophilic character [14]. It has been shovm previously that the catalytic activity for epoxidation reactions depends strongly on the adsorption properties. Indeed, the most hydrophobic catalysts, the highest catalytic activity is obtained [15,16,17]. These observations have moved us to carry out the systematic study of the effect of silylation on the catalytic activity of Ti-MCM-41 materials. Here, we have varied the loading of alkylsilane groups and the steric volume of the organosilane moieties bounded on the surface of the Ti-MCM-41 walls. 2. EXPERIMENTAL SECTION 2.1. Materials Ti-MCM-41 sample was prepared from a gel having the following molar composition: Si02: 0.0015 Ti(0Et)4 :0.12 CTABr : 0.26 TMAOH : 24.3 H2O were CTABr is cetyltrimethylammonium bromide (from Aldrich), TMAOH is tetramethylammonium hydroxide (from Aldrich). The silica source was Aerosil-200 from Degussa and Ti(0Et)4 was supplied by Alpha Products. The crystallization was performed at 100°C for 48 hours in Teflon lined stainsteel autoclaves. The solid was recovered by filtration and exhaustive washing with distilled water until neutral pH in the filtrate was obtained. Then, the Ti-MCM-41 was dried at 60°C for 24 hours. The occluded surfactant was removed by calcination at 540T in nitrogen for 1 hour and subsequently, for 5 hours in air. Typically surface silylation was performed as follows. 1 g of calcined Ti-MCM-41 was outgassed at 300°C for 2 hours. Then, 10 g of a solution containing the appropriate amount of hexaalkyldisilazane (obtained from ABCR GmbH & Co) in toluene was added under Argon on the dehydrated Ti-MCM-41. The resulting mixture was refluxed under inert atmosphere at 120°C for 2 hour. Then, the silylated sample was filtrated and washed with 250 ml of toluene. The different hexaalkyldisilazanes (HADS) and the SiRa/SiOi molar ratios employed in this study are listed in table 1. The SiR3/Si02 was calculated assuming that each HADS molecule gives two silyl groups (SiRs) and the composition of the mesoporous Ti-MCM-41 material is Si02 (i.e. Ti and silanol groups contributions to the final composition were neglected). Table 1 Silylating agents and SiR3/Si02 molar ratios employed for surface modification of Ti-MCM41 catalyst. Sample
Silylating agent (silyl group)
SiR3/Si02 molar ratio
0.05Me-TiMCM41
Hexamethyldisilazane (SiMes)
0.05
0.10Me-TiMCM41
Hexamethyldisilazane (SiMes)
0.10
0.15Me-TiMCM41
Hexamethyldisilazane (SiMes)
0.15
0.20Me-TiMCM41
Hexamethyldisilazane (SiMes)
0.20
0.25Me-TiMCM41
Hexamethyldisilazane (SiMes)
0.25
0.30Me-TiMCM41
Hexamethyldisilazane (SiMes)
0.30
171
Sample
Silylating agent (silyl group)
SiRs/SiOi molar ratio
0.30Pr-TiMCM41
Dipropyltetramethyldisilazane (SiPrMe2)
0.30
0.30Ph-TiMCM41
Diphenyltetramethyldisilizane (SiPhMe2)
0.30
0.30Ph2-TiMCM41
Tetraphenyldimethyldisilazane (SiPh2 Me)
0.30
2.2. Characterization X-ray powder diffraction patterns were collected on a Phillips X'Pert MPD diffractometer equipped with a PW3050 goniometer (using the Cu Ka radiation, graphite monochromator). ^^Si-MAS-NMR spectra were measured at ambient temperature on a Varian VXR 400S WB spectrometer at 79.459 MHz, using a high speed CP/MAS Varian probe with zircona rotors (7 mm in diameter). The spectra were recorded with pulses of 4.5 |LIS of l/37i rad and a recycle delay of 50 s. The ^ Si chemical shifts were referenced to tetramethylsilane. The ^^Si spectra were deconvoluted using the Felix Gaussian fitting program. Thermogravimetric and differential thermal analysis were performed on a Nezst STA-409 EP thermobalance using a heating rate of lO^'C/min and an air flow rate of 100 ml/min. Difftise Reflectance UV-vis spectra were collected on Gary 5 Varian spectrometer equipped with a Trying Mantis' attachment from Harric. The Ti content was determined by atomic absorption on a Varian Spectra A-10 Plus spectrometer. G, H and N analysis were performed on a Fissons EA-1108 elemental organic analyzer. 2.3. Catalytic activity The catalytic activity of the silylated and the non-silylated Ti-MGM-41 materials was tested in epoxidation of cyclohexene using tercbutylhydroperoxide (TBHP) as oxidant. In a typical catalytic run 56 mmol of olefin were mixed with 14 mmol of TBHP (olefin/TBHP ratio = 4) at the reaction temperature, 60°G. Under these reactions conditions the water content was 2 wt.%. Then, 30 mg of catalyst (0.5wt% catalyst) were added to the reaction medium. This instant was taken as time zero of reaction, and aliquots of the reaction media were withdrawn at different reaction times and subsequently analyzed by Gas Chromatography using a 5 % phenylsilicone column (HP-5) of 25 meters length. 3. RESULTS AND DISCUSSION 3.1. Study of the silylation degree. The effect of amount of silyl groups linked to the surface of Ti-MGM-41 on the physicochemical properties and on the catalytic activity was studied using hexamethyldisilizane (HMDS) as silylating agent which yields to two trimethylsilyl groups (SiMes) per molecule of HMDS able to react with the silanol groups located into the MGM-41 channels. The chemical composition of the trimethylsilylated Ti-MGM-41 catalysts is reported in table 2. The degree of silylation, i.e. SiMe3/Si02 ratio in the solid, was calculated from the G content assuming that the G/Si stoichiometry of the anchored organosilyl group is 3. It is clear from the results showed in table 2 that the Ti content is nearly constant upon silylation, indicating that Ti does not leach out from the mesoporous catalyst during silylation procedure. Also, the structural integrity of the pore array is preserved as it is deduced from the XRD patterns (figure 1) which remains unchanged after silylation.
172 Table 2 Chemical composition of trimethylsilylated Ti-MCM-41 catalysts. Sample TiMCM41
C (wt%) n.d.
H (wt%) n.cl.
N (%wt) n.d.
Ti (wt% Ti02) 2.10
(SiMe3/Si02)soiid 0
0.05Me
2.66
1.02
0.08
2.01
0.049
0.1 OMe
4.89
1.47
0.06
1.98
0.102
0.15Me
6.92
1.87
0.06
2.05
0.160
0.20Me
7.55
1.97
0.07
1.94
0.181
0.25Me
7.94
2.04
0.07
2.00
0.195
0.30Me
8.13
2.05
0.06
1.97
0.201
0.20
0.30Me 025Me 0.20Me 0.15Me
OIOM? O.OSMe Ti-MCM-41
4
6 26 (degrees)
8
Figure 1. XRD patterns of trimethylsilylated Ti-MCM •41 catalysts.
0.00 0.05 0.10 0.15 0,20 0.25 0,30 (SiMej/SiOaoiution
Figure 2. Degree of silylation of Ti-MCM-41 catalysts using HMDS as silylating agent.
The plot of the SiMe3/Si02 ratios used for silylation in the reaction mixture versus the C content incorporated into the Ti-MCM-41 materials indicates that there is a maximum of silylation which yield to 8 wt% C incorporated into the mesoporous Ti-MCM-41 solid. This value corresponds to a SiMe3/Si02 ratio into the solid close to 0.15. It is notorious that the effectiveness of trimethylsilylation is nearly 100% when SiMe3/Si02 ratios lower than 0.2 are used in the silylating solution, decreasing strongly when the ratio is upper this value. This result clearly indicates that the maximum surface coverage of trimethylsilyl groups bounded to the Ti-MCM-41 walls is close to a SiMe3/Si02 ratio of 0.15. This conclusion is further supported by ^^Si-MAS-NMR spectroscopy. Indeed, figure 3, which shows the ^^Si-MAS-NMR spectra obtained for the some trimethylsilylated samples, clearly indicates that the resonance appearing at -100 ppm, assigned to Si(0Si)30H (Q3) species, decreases as the SiMe3/Si02 ratio in the silyating solution increases. Simultaneously, a new signal at 14 ppm appears which is assigned to Me3Si—(OSi) species [14,18,19]. These
173 results strongly suggest that silanol groups reacts with HMDS yielding trimethylsilyl moieties bounded to the Si atoms located into the walls through oxygen bridges. 0.25-
« 0.20 •
>^
J
«M
Q 0.15(0
^n
1 0.10(O
^
^y^
0.05 -
0.00- %•
20 10
-80
-100 6(ppm)
-120
-140
Figure 3. ^^Si-MAS.NMR spectra of trimethylsilylated Ti-MCM-41 catalysts.
0
1
1
2
1
1
1
1
4 6 C content (wt%)
1
1 — '
8
Figure 4. Carbon content of trimethylsilylated Ti-MCM-41 samples versus SiMe3/Si02 ratio calculated by ^^Si-MAS-NMR spectroscopy.
The quantitative analysis of the deconvoluted ^^Si-MAS-NMR spectra permits to calculate the SiMe3/Si02 ratios which give a close value to that obtained by chemical analysis as it is shown in figure 4 indicatingtiiatthere is no unreacted HMDS located within the pores of TiMCM-41. The hydrophobicity of the trimethylsilylated Ti-MCM-41 was estimated from the weight loss of the hydrated samples at 150°C, since this weight loss is generally attributed to physisorbed water on the surface of the mesoporous solids. It was found that there is a nearly linear correlation between amount of water adsorbed on the fully hydrated trimethylsilylated catalysts and the amount of trimethylsilyl groups bounded to the surface as it is shown in figure 5. The Ti environment in trimethylsilylated Ti-MCM-41 catalysts with different degrees of silylation was studied by means of diffuse reflectance UV-vis spectroscopy. The UV-vis spectra of trimethylsilylated samples are compared with the calcined material in figure 6. Silylated samples show a similar UV-vis spectra to that observed in the calcined Ti-MCM41 catalyst, presenting a very narrow band centered at 220 nm characteristic of Ti sites surrounded by four silicon atoms through oxygen bridges [20,21]. It is noticeable that the UVvis band at 220 nm becomes slightly narrower as the degree of silylation increases. This could be related to the decrease of the amount of physisorbed water with the degree of silylation. Therefore, tetrahedral Ti sites will be unable to expand their coordination with water molecules upon silylation, and consequently, the UV-vis band appearing at 220 nm will be shifted to lower wavelengths and will be narrower than the band observed in the spectrum of the fully hydrated Ti-MCM-41 [21]. It is remarkable that the *in-situ'-spectrum of the dehydrated Ti-MCM-41 sample at 150°C under vacuum (spectrum not shown) closely resembles to that observed for the 0.30Me catalyst, which is the most hydrophobic catalyst presented here. These results suggest that Ti sites in the silylated Ti-MCM-41 catalysts are in tetrahedral coordination with no water located in theirfirstsphere of coordination.
174
5 y//
1
(0
ii
4
N
o :£ 2
75 E
t. o z
5 ^0
I,
0
,
1
2
•
1
•
1
4 6 C Content (wt%)
.
\ \ \ \
Ti-MCM-41
0.05Me
/ / ' / ^ \ A /// \ \ \ // \ \ / \
O.IOMe 0.15Me 0.20Me! 0.25Me 0.35Me
1 —
8
\^
/y//X\\ VV
200
\ ^ ^ ^ ^ X ^
300
500
400
Wavelength (nm)
Figure 5. Variation of the hydrophobicity, calculated as the weight loss at 150°C by thermogravimetry, with the degree of trimethylsilylation of Ti-MCM-41 catalysts.
Figure 6. Uv-Vis spectra of trimethylsilylated Ti-MCM-41 catalysts with different degrees of silylation.
The catalytic activity of the trimethylsilylated Ti-MCM-41 samples for epoxidation of cyclohexene with tercbutylhydroperoxide (TBHP) is reported in figures 7a and 7b. 100
\^WJ^
•
• •^
*>< o 98 a o> o
*
»
> 96 . \ •V..^__
•
"" •• •
0) CO
•
QA
2 3 4 Time of reaction (h)
b
/•
0.30Me + O.IOMe 0.25Me X 0.05Me 0.20Me • Ti-MCM-41 0.15Me — •
20
1 —
>
40 60 Conversion (%)
Figure 7. Effect of trimethylsilylation degree on the catalytic performance of Ti-MCM-41. a) Catalytic conversion of cyclohexene (referred to the maximum considering the ratio cyclohexene/TBHP = 4). b) Selectivity to the epoxide. The catalytic activity strongly increase with the degree of silylation of Ti-MCM-41 catalysts. However, It is notorious that low level of trimethylsilylation does not affect to the catalytic conversion. This effect is more clearly showed in figure 8 where the initial reaction rate, calculated assuming a pseudo-first order of reaction, is plotted against the C content of the silylated Ti-MCM-41 catalyst. Therefore, it can be deduced from figures 7 and 8 that there is a limit of surface coverage by trimethylsilyl groups above the active Ti sites of Ti-MCM-41 increases greatly their activity. This effect can not be directly attributed to a change in the Ti environment, but may be related to the increase of hydrophobicity of the catalyst as the surface coverage by silyl groups increases. Previously, we have reported that catalyst
1
80
—
175
deactivation is probably due to the irreversible adsorption of glycols, which are produced by the oxirane ring opening reaction with water. This reaction is also responsible of the decrease of selectivity to the desired epoxide [16]. Silylation of Ti-MCM-41 produces a twofold benefit. Firstly, silylation increases greatly the hydrophobicity of TiMCM-41 catalysts and therefore, water concentration on the surface is reduced, and the subsequent glycol formation is nearly avoided. Moreover, silylation 4 6 8 decreases the number of silanol groups C content (%) (and very probably Ti-OH groups). These groups posses a weak acid character, but Figure 8. Effect of the degree of strong enough to catalyze the undesired trimethylsilylation on Ti-MCM-41 on the reaction of oxirane ring opening. initial reaction rate for catalytic epoxidation Therefore, the glycol formation will also of cyclohexene with TBHP. be decreased upon silylation due to the reduction in the number of silanol groups located at the surface of the Ti-MCM-41 catalysts. In figure 7b , it is shown that the selectivity to the epoxide increases with the degree of silylation, and therefore a lower catalyst deactivation should be expected. Notoriously, the selectivity to the epoxide starts to increase even at very low level of coverage, but probably, the amount of glycol produced is still enough to produce catalyst deactivation. This is specially true, taking into account the very low catalyst/reactants (0.005 wt/wt) used in this work. Therefore, we conclude that surface modification of Ti-MCM-41 materials by trimethylsilylation yields to catalysts up to nine times more active for epoxidation of olefins using organic hydroperoxides as oxidants than analogous non-silylated Ti-MCM-41 catalysts.
.-=^20
3.2. Effect of the steric volume of the anchored silyl groups The influence of the steric volume of the silyl groups anchored in the surface of Ti-MCM41 was studied by grafting a suite of alkyldimethyldisilazanes (see table 1) along this study the number and volume of the silyl groups was modified. Similarly to that observed during trimethylsilylation, no leaching of Ti was detected after surface modification using bulky silylating agents. Also, the XRD patterns of the silylated mesoporous Ti-MCM-41 remain unchanged respect to the non-silylated material, indicating that the ordering of the mesoporous array is not modified upon silylation with large silylating agents. The chemical analysis of silylated-Ti-MCM-41 catalysts is shown in table 3. The carbon content permits to calculate the degree of silylation for the different silylating agents, expressed as (SiRMe2/Si02)soiid ratio. It was found that this ratio becomes smaller as the steric volume of the silyl groups increases. This resuh could be explained be taking into account that the maximum number of silyl group able to be allocated into the external surface of the Ti-MCM-41 catalyst will depend on the steric volume of the anchored groups, being this value higher as smaller the silyl group is. The steric volumes of the different silyl groups used in this work vary as follows: SiMea < SiPrMe2 « SiPhMe2 < SiPh2Me
176
which is exactly the same trend than that found for the degree of silylation as it is shown in table 3. Table 3 Chemical composition of alkyldimethylsilylated Ti-MCM-41 catalysts. Sample TiMCM41
C (wt%) n.d.
H (wt%) n.d.
N (%wt) n.d.
Ti (wt% Ti02) 2.10
(SiRMe2/Si02)soiid 0
0.30Me
8.13
2.05
0.06
1.97
0.162
0.30Pr
11.91
2.77
0.08
1.97
0.149
0.30Ph
18.27
2.31
0.08
1.98
0.154
0.30Ph2
15.34
1.71
0.11
1.99
0.091
0
-20-80
-100
-120
6 (ppm)
Figure 9. ^^Si-MAS.NMR spectra of silylated Ti-MCM-41 catalysts.
-140
Ti.MCM41 0.30Me 0.30Pr 0.30Ph O.SOPhj Silylated TI.MCM-41 catalysts
Figure 10. Variation of the hydrophobicity, calculated as the weight loss at 150°C by thermogravimetry, for different silylated TiMCM-41 catalysts.
The ^^Si-MAS-NMR spectra of the silylated samples are shown in figure 9. Similarly to the results observed for trimethylsilylation, the intensity of band appearing at -100 ppm, assigned to silanol groups strongly decreases after silanization and new resonances appear in the range between 15 to -7 ppm, which are characteristic of moieties containing Si-C bonds [22], indicating that silyl groups have been grafted onto the external surface of the pores of Ti-MCM-41. The differences in the chemical shifts suggest that the structure of the different silyl groups have been preserved upon anchoring them on the Ti-MCM-41 catalyst. The maximum decrease in the intensity of the Q3 resonance was observed for the 0.30HTiMCM-41 sample, which is the catalyst that posses the highest degree of surface coverage by silyl groups as was stated by chemical analysis, while the minimum decrease was found for sample 0.30Ph2, similarly to the observed SiR3/Si02 ratio. These ^^Si-MAS-NMR resuks are in good correlation to the chemical analysis and strongly support that the silylating agents react with the silanol groups located at the pore system to yield silyl groups bounded onto the Ti-MCM-41 .
177
The hydrophobicity of the different silylated Ti-MCM-41 has been estimated from the weight loss at 150°C (figure 10). It was found that upon silylation all the samples becomes strongly hydrophobic as deduced from the little amount of water adsorbed on the silylated samples compared to the parent Ti-MCM-41 catalyst. Also, it is notorious that the hydrophobicity of the silylated materials becomes higher as the steric volume of the silyl group increases. Sample 0.30Ph2, which contains the bulkier silyl group, does not follow the above trend. However, this result may be explained by taking into account that the effectiveness of silylation procedure in this sample is very low as deduced from its Si-MASNMR spectrum (figure 9), leaving a large number of unreacted silanol groups that provide a remarkable hydrophilic character to the material. Consequently, 0.30Ph2 catalyst show a lower hydrophobicity than that expected from the steric volume of the silyl group anchored in its surface. The initial reaction rate and the selectivity to the epoxide for cyclohexene epoxidation with TBHP versus the degree of silylation for the different silylated Ti-MCM-41 catalysts is presented in figure 11. It is clear that the highest the silylation degree, higher the catalytic activity and the selectivity to the epoxide are. Taking into account that the hydrophobicity was quite close for the different silylated materials, one could deduce that 0,05 0,10 0,15 unreacted free silanol groups (SiRMe^SiO,)3,,^ pointing to the pore channels of the Figure 12. Initial reaction rate and selectivity to Ti-MCM-41 are responsible of the epoxide during epoxidation of cyclohexene with TBHP formation of glycols, which poison the catalytically active Ti sites of the on silylated Ti-MCM-41 catalysts. mesoporous structure. Therefore, the conclusion raised from the above results is that in addition to the hydrophobic properties of Ti-MCM-41 catalysts, the number of silanol groups present in the surface of the mesopores plays an important role in the final catalytic behavior of Ti containing mesoporous materials. 4. CONCLUSIONS Silylation of Ti-MCM-41 materials produces highly active and selective for epoxidation ol olefins using organic hydroperoxides as oxidants. It has been found that the controlling parameters of the final catalytic activity of silylated Ti-MCM-41 materials are the hydrophobicity and the concentration of free silanol groups on the external surface of the mesopores that built up the Ti-MCM-41 structure. ACKNOWLEDGMENTS Financial support by the Spanish MAT97-1207-C03-01 and MAT97-1016-C02-01 is gratefiiUy acknowledged. We thank Sumitomo Chemical Co. for supporting this research. J.L.J, thanks to the Ministerio de Educacion y Ciencia for the Doctoral fellowship.
178 REFERENCES 1. M. Taramasso, G. Perego and B. Notari, US Pat. 4 410 501 (1983). 2. J.S. Reddy, K. Kumar and O. Ratnasamy, Appl, Catal., 58 (1990) LI. 3. M.A. Camblor, A. Corma, A. Martinez and J. Perez-Pariente, J. Chem. Soc, Chem. Commun., (1992) 589. 4. D.P. Serrano, H.X. Li and M.E. Davis, J. Chem. Soc, Chem. Commun., (1992) 745. 5. A. Tuel, Zeolites, 15 (1995) 236. 6. N.Ulagappan and V. Krishnasamy, J. Chem. Soc, Chem. Commun., (1995) 373. 7. M.H. Zahedi-Niaki, P. Narahar and S. Kaliaguine, J. Chem. Soc, (1996) 47. 8. A. Tuel and B. Taarit, J. Chem. Soc, Chem. Commun., (1994) 1667. 9. C.T. Kresge, M.E. Leonowicz, W.J.Roth, J.C. Vartulli and J.S. Beck, Nature, 359 (1992) 710. 10. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schimitt, C.T.W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCulien, J.B. Higgens and J.L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. 11. A. Corma, M.T. Navarro and J. Perez-Pariente, J. Chem. Soc, Chem. Commun., (1994) 197. 12. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature 368 (1994) 321. 13. A. Corma, M.T. Navarro, J. Perez-Pariente and F. Sanchez, Stud. Surf. Scien. Catal., 84 (1994)69. 14. K.A. Koyano, T. Tatsumi, Y. Tanaka and S. Nakata, J. Phys. Chem. B, 101 (1997) 9436. 15. T. Blasco, M.A. Camblor, A, Corma, P. Esteve, J.M. Guil, A. Martinez, J.A. Perdigon and S. Valencia, J. Phys. Chem. B, 102 (1998) 75. 16. A. Corma, M. Domine, J.A. Gaona, J.L. Jorda, M.T. Navarro, F. Rey, J. Perez-Pariente, J. Tsuji, B. McCulloch and L.T. Nemeth, Chem. Commun., (1998) 2211. 17. T. Tatsumi, K.A. Koyano and N. Igarashi, Chem. Commun., (1998) 325. 18. D.W. Sindorf and G.E. Marciel, J. Am. Chem. Soc, 105 (1983) 3767. 19. X.S. Zhao, G.Q. Lu, A.K. Whittaher, G.J. Miliar and H.Y. Zhu, J. Phys. Chem. B, 101 (1997)6525. 20. T. Blasco, M.A. Camblor, A. Corma and J. Perez-Pariente, J. Am. Chem. Soc, 115 (1993)11806. 21. L. Marchese, T. Masschemeyer, E. Gianotti, S. Coluccia and J.M. Thomas, J. Phys. Chem. B, 101 (1997)8836. 22. R. Anwander, C. Palm, J. Stelzer, O. Groeger and G. Engelhardt, Stud. Surf. Scien. Catal., 117(1998)135.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
179
Synthesis and Modification of Ti-containing Catalysts for Epoxidation of Alkene Jie Bu and Hyun-Ku Rhee* School of Chemical Engineering and Institute of Chemical Processes Seoul National University, Kwanak-Ku, Seoul 151 -742, Korea
The effects of catalyst surface properties on the selectivity have been investigated for cyclohexene epoxidation with aqueous H2O2 over Ti(Al)-beta and Ti-MCM-41 molecular sieves. Low Al-containing Ti-beta was synthesized and found to enhance the production of epoxide and suppress the formation of diol. The hydrophobic Ti-MCM-41 was obtained by silylation with a NEW silylating agent N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) and N,0-bis(trimethylsilyl)trifluoroacetamide (BSTFA). FT-IR spectra and the weight loss determined by TGA indicate that MSTFA is a more efficient silylating agent and reacts with silanol groups at room temperature to yield a large amount of-SiMcj group on the surface of Ti-MCM-41. Over the silylated Ti-MCM-41, the yield of epoxide was improved while the selectivity to by-products decreased. On the basis of experimental results, a presumable reaction mechanism with two parallel and competitive paths is proposed for cyclohexene epoxidation with H2O2 over Ti-containing molecular sieves.
1. INTRODUCTION Since the first synthesis of TS-1 in 1983 [1], considerable efforts have been devoted to the synthesis of titanium-containing zeolites [2, 3]. Recently, Ti-beta, a large-pore molecular sieve, has been extensively studied [4, 5]. Owing to its unique large-pore channel system, Tibeta seems to be more active than the medium-pore TS-1 catalyst for the oxidation of cyclic and branched alkenes with aqueous hydrogen peroxide. Under the usual synthesis conditions, however, Ti-beta crystallizes with some Al as a framework constituent [4]. This leads to the presence of acid centers, which may have a detrimental effect on the activity or selectivity of this type of catalyst. Since 1992, the discovery of a new family of mesoporous molecular sieves has received much attention [6,7]. Because of their mesoporous nature (20-100A), the Ti-MCM-41 zeolites may be useful as oxidation catalysts for larger molecules [8]. In this • Address for correspondence:
[email protected] FAX: 82-2-888-7295.
Tel: 82-2-880-7405.
180 regard, Corma et al. [9] observed that the hydrophihc/hydrophobic property of Ti-MCM-41 plays an important role in their activity for liquid phase oxidations. Thus, the silylation of hydrophilic silanol has become the subject of recent studies [10, 11]. In addition, it was also reported that Ti02-Si02 mixed oxides could catalyze the epoxidation of alkenes [12]. Many authors have observed that the Ti-containing catalysts of various types gave different selectivities to epoxide for alkene epoxidation with aqueous H2O2 or tert-butyl hydroperoxide. However, the effects of surface properties, Br0nsted acid and the silanol, on the formation of by-products are yet to be reported in detail. In the present work, it is aimed to examine the influences of modified surface natures of Ticontaining catalyst on the olefin epoxidation. For this purpose, the Ti(Al)-beta, low Alcontaining Ti-beta and Ti-MCM-41 were prepared. Especially, after synthesizing Ti-MCM-41, we used a new silylating agent, N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA), to increase the hydrophobicity of the catalyst. The substitution of trimethylsilyl groups for the silanol H atoms was confirmed by FT-IR, while the hydrophobicity was characterized by TGA technique. Subsequently, these prepared catalysts were applied to the epoxidation of cyclohexene with H2O2 and their catalytic behaviors were investigated.
2. EXPERIMENTAL 2.1 Synthesis and modification of catalysts The Al-containing Ti-beta sample, TB-1, was synthesized according to the methods proposed by Camblor et al. [4]. Aerosil 200 (Degussa), titanium ethoxide, tetraethylammonium (TEAOH 40 wt%, Alfa), and aluminum nitrate nonahydrate were used as reagents. The ratios Si/Ti and Si/Al were adjusted as needed. To synthesize Ti-beta with extremely low Al-content (TB-2), the gel solution excluding Al was first prepared by following the procedure of Camblor et al.[4]. The synthesized TB-1 was then used as seed and added to the gel under stirring (4 g of seed per 100 g of total Si02). Crystallization was carried out in a rotated PTFE-lined stainless steel autoclave at 140 °C for 14 days. The ratios of Si/Ti and Si/Al in the samples are listed in Table 1. The Ti-MCM-41 sample, TM-1, was synthesized according to the method of Koyano and Tatsumi [8]. The hydrophobic Ti-MCM-41 catalyst, TM-2, was prepared by silylation of TM1 with N,0-bis(trimethylsilyl)trifluoroacetamide (BSTFA). The silylation was carried out following the procedure described by D'Amore and Schwarz [11]. The procedure for the silylation of catalyst with N-methyl-N-(trimethylsilyl)trifluoroacetamide (MSTFA) is as follows: A solution of 1 g MSTFA (99%, Aldrich) in 10 g of toluene was added to the sample TM-1 (0.5 g). The mixture was stirred at room temperature for 2h, then the treated catalyst was filtered, washed with toluene and dried in air for 8 h to give the sample TM-3.
181 2.2. Characterization X-ray powder diffraction (XRD) patterns were collected on Rigaku model D/Max-3C using CuK a radiation. UV-vis spectroscopic measurements were recorded on a Varian GARY 3E double beam spectrometer using dehydrated MgO as a reference in the range of 190-820 nm. The elemental analyses of the samples were performed by inductive-coupled plasma atomic emission spectroscopy (ICP-AES). The surface area and mean pore diameter of Ti-MCM-41 samples were obtained from the adsorption isotherms of nitrogen at -196 T on a Micromeritics ASAP 2010. The amount of water adsorbed on the sample was determined by Thermogravimetric Analyzer (FERKFN ELMET TGA-7) using a heating rate of 5 *^C/min (to 150 °C) and an Nj flow rate of 20 ml/min. Framework FT-IR spectra were taken on the Nicolet Impact 410 FT-IR instrument. The sample dispersed in KBr with the ratio of catalyst to KBr of 1/40 was pressed to wafer. Table 1 Characterization of Ti(Al)-beta and Ti-MCM-41 Structure
___^
Catalyst •'
Si/Ti
Si/Al
^^T surface Area(m/g)
^^/^ ^. Weight loss (%) Diameter (A) ^
BETA
TB-1 TB-2'
76.5 57.1
39.1 400
D
D
MCM-41
TM-1 TM-2' TM-3'=
60.3 66.7 69.4
D D D
1405 1213 1153
26.1 23.1 22.4
O
D
D D 16.5 9.12 6.39
' Using TB-1 as seed to synthesize TB-2. ^ Silylation of TM-1 with BSTFA. ' Silylation of TM-1 with MSTFA. 2.3. Catalytic experiments The epoxidation of cyclohexene with H2O2 was carried out at 70 °C in a magnetically stirred three-necked flask equipped with a condenser. In practice, 0.05 g of catalyst was dispersed in the solution containing 0.02 mol of cyclohexene and 20 ml of acetonitrile (solvent). The mixture was then heated to 70 °C under stirring and 0.01 mol of H2O2 (35 wt.% aqueous solution) was introduced in one lot. The sample was periodically collected and analyzed by gas chromatography (HP 5890 Series II) equipped with FID and HP-1 capillary column.
3. RESULTS AND DISCUSSION 3.1. Characterization The XRD patterns of synthesized Ti(Al)-beta and Ti-MCM-41 are found to be in good
182 agreement with those reported in the literature [4, 8]. The UV-vis spectra of all the samples give no signal for anatase phase and indicate the presence of isolated Ti and oligomeric (Ti0)n [13]. As shown in Table 1, the Si/Al ratio of TB-2 is significantly increased, and this indicates that the Ti-beta with extremely low Al content is successfully synthesized by the seed method. The BET surface areas and pore volumes were measured for the three samples and are listed in Table 1. Silylated samples have lower surface area and pore volumes than the parent sample TM-1. It is in agreement with earlier reports by Tatsumi [10] and D'Amore [11] that the surface area and pore volume decreased as the silanol groups inside the pore of TiMCM-41 were trimethylsilylated by silylation agents. Corma et al. [22] proposed a convenient method to determine the silylation degree from the weight loss determined by TGA and found that there is a linear correlation between the hydrophobicity and the carbon content or the silylation degree. Their method was employed here to estimate the silylation degree. As may be noticed in Table 1, the least weight loss of TM-3 indicates that the sample TM-3 obtained by silylation with MSTFA is more hydrophobic than TM-2 prepared by silylation with BSTFA.
0)
o c
03 -Q L_
O
(/)
400 c o Z. 300 o -D 200 2!
0.0 0.2 0.4 0.6 0.8 1.0 P/Po
-.on
0.0
0.2
0.4
0.6
0.8
1.0
P/Po
Figure 4. N2 adsorption isotherms of PSM and AMM-5 before and after compression at 200 MPa. However, AMM-5 could maintain a higher surface area than PSM when they are subjected to the same compression under a range of pressures from 200 to 800 MPa. Figure 4 shows the deformation of the N2 adsorption isotherms of PSM and AMM-5 samples after compression at a pressure of 200 MPa. It can be observed that the effect of high pressure compression on PSM is more detrimental than on AMM-5. The N2 adsorption amount on PSM has decreased tremendously at all ranges of P/Po = 0-1.0. The results indicate that the externally introduced Al species has improved the mechanical stability of MCM-41. The enhanced mechanical stability shown here may be attributed to the effect of recrystallization (coming from post-synthesis alumination), which act to heal defect sites in the mesoporous structure of MCM-41 [12]. The mesopores of purely siliceous MCM-41 are suggested to be as fragile as dumped ceramic tubes. When the structural defects are healed by post-synthesis alumination, the "healed" mesopores may then become more resistant to the high pressure compression, hi addition, N2 adsorption and XRD results show that AMM materials have mesopores with thicker pore walls than PSM. The combmation of these effects shows that AMM can resist compression better than PSM. 3.4. Stability test in solution at high pH The XRD patterns of PSM and AMM samples after treatment in NaOH solution (pH = 11) for 12 h are shown in Figure 5. Both AMM-1 and AMM-5 samples still exhibit well defined XRD patterns, showing good preservation of the textural uniformity of AMM samples in a strong basic solution, hi contrast, no XRD peak is detected for PSM sample under the same treatment conditions, showing that the uniform mesoporous structure of PSM has been completely destroyed by NaOH solution in 12 h. This resuh is not surprising as it has been well reported that PSM was unstable in basic solution [4]. The results of this investigation show that the chemical stability of MCM-41 material in basic solution can be substantially improved by the external introduction of Al species onto its surface.
234
1
d OJ
/ //
>s
U) c
J u /
I
AMM-5 AMM-1 PSM
1.0 2.0 3.0 4.0 5.0 6.0 7.0 8.0 9.0 29 n Figure 5. XRD patterns of PSM and AMM samples after treatment in basic solution 4.
CONCLUSIONS The hydrothermal stability of MCM-41 mesoporous materials in boiling water or in high temperature steam has been found to be significantly enhanced by the external introduction of Al species. The surface Al species is found to be effective in preventing the mesoporous structure of MCM-41 from hydrolysis during long duration treatment in boiling water. The mechanical stability of MCM-41 has also been improved using post-synthesis alumination by preventing the uniform mesoporous structure to be broken under high pressure compression. Furthermore, the externally introduced Al species substantially improve the chemical stability of MCM-41 in high pH solution. REFERENCES [I] J.M. Kim, J.H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem., 99 (1995) 16742. [2] R. Ryoo, J.M. Kim, C.H. Ko, C.H. Shin, J. Phys. Chem. 100 (1996) 17718. [3] A. Corma, M.S. Granda, V. Gonzalez-Alfaro, A.V. Orchilles, J. Catal., 159 (1996) 375. [4] D.T. On, S.M.J. Zaidi, S. Kaliaguine, Microporous Mesoporous. Mater. 22 (1998) 211. [5] R. Ryoo and S. Jun, J. Phys. Chem. B, 101 (1997) 317. [6] D. Das, CM. Tsai, S. Cheng, Chem. Commun., (1999) 473. [7] N. Igarashi, Y. Tanaka, S.I. Nakata, T. Tatsumi, Chem. Lett., (1999) 1 [8] K. Koyano, T. Tatsumi, Y. Tanaka, S. Nakata, J. Phys. Chem. B, 101 (1997) 9436. [9] Z. Luan, C-F. Cheng, H. He, J. Klinowski, J. Phys. Chem. 99 (1995) 10590. [10] Z. Luan, H.He, W. Zhou, C-F. Cheng, J. Klinowski, J. Chem. Soc. Faraday Trans. 91(1995)2955. [II] D. Das, CM. Tsai, S. Cheng, Chem. Commun. (1999) 473-474. [12] R. Mokaya and W. Jones, Chem. Commun. (1998) 1839. [13] W. Lutz, W. Gessner, R. Bertran, I. Pitsch, R. Fricke, Microporous Mater. 12 (1997)131. [14] E.Galleiand D. Eisenbach, J. Catal., 37 (1975) 474. [15] G.L. Woolery, L.B. Alemany, R.M. Dessau, A.W. Chester, Zeolites 6 (1986) 14.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
235
Siting of Co(II), Zn(II) and Cu(I) ions in (A1)MCM-41 J. Dededek, N. Zilkova and J. Cejka J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic; e-mail
[email protected] Siting of metal ions (Zn^^, Co^^ Cu^) in the cationic sites of (A1)MCM-41 matrix has been investigated employing UV-VIS-NIR DR spectroscopy and UV-VIS emission spectroscopy. Four types of cationic sites were identified in dehydrated (A1)MCM-41. Divalent (Zn^^or Co^^ ions are accommodated only in two sites. Cu^ ions in reduced, Cu^^ ion exchanged (A1)MCM-41, occupy four types of cationic sites. Two sites are accessible for divalent cations, other two only for monovalent cations. Distribution of cations among sites depends on the metal ion loading in molecular sieve.
1. INTRODUCTION New types of catalysts based on metal ions loaded zeolites have attracted a great attention in the last decade. Unusual cation coordinations together with strong localized electrostatic field can significantly alter the chemistry of cations in individual exchange sites. Thus, these cations especially in "high" silica zeolites (Si/Al > 10) possesses significantly different properties compared to cations on other supports and exhibit unique catalytic properties [1]. The catalytic applications of metal loaded zeolites has been up-to now restricted to zeolite materials with their pore diameter lower than lOA. The discovery of mesoporous molecular sieves of the M41S family [2] significantly enlarged a possibility to explore the unique catalytic properties of the metal ions in cationic sites also in reactions of large organic molecules, which cannot enter usual zeolite channel systems. Catalytic activity of metal ions coordinated to the framework of mesoporous molecular sieves atracted attention at first. Recently, catalytic activity of the metal ions incorporated into extraframework positions of the MCM-41 in various reactions was also reported [3]. But, the site geometry, coordination and distribution of the metal ions in the extraframework sites of the MCM-41 host matrix are not understood, and only a few papers have dealt with this problem. For dehydrated Mn-(A1)MCM-41, only one type of single cation was reported [4]. As it was shown in the case of pentasil zeolites (MOR, FER, MFI), UV-VIS-NIR DR spectroscopy and UV-VIS emission spectroscopy appear to be extremely powerflill tools for the characterization of transition metal ions in molecular sieve matrices [1,5]. The aim of this study is to employ this promissing and powerflill technique for the characterization of siting of metal cations exchanged into the (A1)MCM-41 matrix.
236 2. EXPERIMENTAL 2.1. Synthesis of (A1)MCM-41 (A1)MCM-41 molecular sieves of chemical composition (in wt.%): 96.1 Si02, 3.9 AI2O3 (Si/Al 21) and 98.0 Si02, 2.0 AI2O3 (Si/Al 42.5) were synthesized from sodium silicate, hexadecyl-trimethylammonium bromide, ethyl acetate and aluminium hydroxide modifying the procedure described in Ref [6]. Structure of the synthesized, calcined and ion exchanged (A1)MCM-41 was checked using X-ray powder diffractometer Siemens D5005 in the BraggBrentano geometry arrangement with CuKa radiation. The diffraction patterns calcined and ion-exchanged (A1)MCM-41 are depicted in Figure 1. Adsorption isotherm of nitrogen (Figure 2) was recorded at 77 K with an Accusorb 2100E instrument (Micromeritics) praiding a surface area (BET) of 980 m^g'^ and pore size of 33 A. The sample was activated at 350 °C for about 20 h at the pressure 10"^ Pa before the measurement.
1.
3^ CO
1 /J I /4
c c
h. /U
^KJ^ s^-^
s
J\
\ ^ v^"^
h
b —
a — 1
'
1
2
'
1
•
1—'—r—'—'
4 6 2theta
Figure 1. X-ray diffraction patterns of(Al)MCM-41 (a) calcined, (b) ion-exchanged.
0.0
0.2
0.4 P/Po
0.6
Figure 2. Nitrogen isotherm on(Al)MCM-41at77K.
2.2. Preparation of Co^^-, Zn^^- and Cu^-(AI)MCM-41 Zn^^, Co^^ and Cu^^ ions were incorporated into calcined (A1)MCM-41 via ion exchange performed using very diluted Zn^^and Co^^ nitrate and Cu^^ acetate solutions at ambient temperature. Samples were carefully washed by distilled water, dried at ambient temperature and grained. Detailed conditions of the sample preparation and chemical composition of Me -(AI)MCM-41 are given in Table 1. Prior to monitoring of the Cu^ and Zn^^ photoluminescence spectra, the Cu^^ and Zn^^ molecular sieves were calcined in an oxygen stream at 620 K for 3 hrs with a heating rate of 1 K/min. For monitoring emission spectra of Zn^^- and absorption spectra of Co ^-(Al)MCM41, samples were then dehydrated at 750 K under vacuum of 7x10"^ Pa in a silica flask connected with a optical cell. Dehydration was carried out with a heating rate of 5 K/min in two steps: 370 K for 30 min and 750 K for 3 h. For monitoring spectra of Cu^-(A1)MCM-41, samples were dehydrated at 750 K for 1 h, subsequently reduced in a stream of carbon
237
monoxide for 40 min at 750 K and than evacuated for 30 min. After dehydration/reduction, the sample was cooled down to ambient temperature and transferred under vacuum into the optical cell and sealed. Table 1 Chemical composition of Me-(A1)MCM-41 and conditions of their preparation. Molecular sieve
Si/Al
Co-(Al)MCM-41 42.5 Co-(Al)MCM-41 42.5 Co-(Al)MCM-41 42.5 Zn-(A1)MCM-41 21.0 Zn-(A1)MCM-41 21.0 Cu-(A1)MCM-41 42.5 Cu-(A1)MCM-41 42.5 Cu-(A1)MCM-41 42.5 Cu-(A1)MCM-41 42.5 * two step ion exchange
Me/Al
Me concentration in solution (M)
0.08 0.11 0.26 0.05 0.24 0.04 0.15 0.22 0.39
0.05 0.05 0.05 0.03 0.10 0.0007 0.002 0.04 0.05
volume of solution per Igof zeolite (ml/g) 6 20 2 X 140* 12 2x100* 20 20 20 45
time of exchange (h)
12 12 2x12* 3 2x12* 5 5 5 5
2.3. UV-VIS-NIR DR spectroscopy UV-VIS-NIR diffuse reflectance (DR) spectra were measured using a Perkin-Elmer UV-VIS-NIR spectrometer Lambda 19 equipped with a diffuse reflectance attachment with an integrating sphere coated by BaS04. Spectra of sample in 5 mm thick silica cell were recorded in a differential mode with the parent zeolite treated at the same conditions as a reference. For details see Ref [5]. The absorption intensity was calculated from the Schuster-Kubelka-Munk equation F(Rco) = (l-Roo)V2Roo, where R* is the diffuse reflectance from a semi-infinite layer and F(Roo) is proportional to the absorption coefficient.
2.4. UV-VIS emission spectroscopy Cu^ emission spectra were recorded using a nanosecond laser kinetic spectrometer (Applied Photophysics). Cu^-zeolites were excited by the laser beam of the XeCl excimer laser (Lambda Physik 205, emission wavelength 308 nm, pulse width 28 ns, pulse energy 100 mJ). The 320-nm filter was situated between 2 mm thick silica cell and monochromator. Emission signal was detected with the photomultiplier R 928 (Hamamatsu), recorded with the PM 3325 oscilloscope and processed by a computer. All the luminescence measurements were carried out at room temperature. The Cu^ emission spectra were constructed from the values of luminescence intensity at the individual wavelengths of emission in selected times after excitation (2, 5, 10, 20, 50, 100 and 200 jis). For details see Ref [7].
238 3. RESULTS AND DISCUSSION 3.1. Co^^-(Al)MCM-41 The absorption in VIS region is characteristic for the Co^^ ions of various coordinations in molecular sieve matrices [1,8,9]. Dehydrated Co-(Al)MCM-41 samples exhibit weak adsorption in VIS region in the range 13 000 - 23 000 c m \ characteristic for Co^^ ions, and they are white or very pale blue depending on the Co loading. Normalized DR VIS spectra of dehydrated Co-(Al)MCM-41 of various Co concentration are shown in Figure 3a. Three main bands with maxima at 14 600, 17 300 and 20 000 cm"^ are present in the spectrum. A shoulder on the high energy edge indicates a presence of another band between 20 000 and 23 000 c m \ Significant changes in the normalised spectra with increasing Co loading (substantial increase in a band at 14 600 cm'^) indicate a presence of two types of Co^^ species in the molecular sieve. The absence of the bands at 5 260 and 7 120 cm'^ (8+v and 2v combination vibration bands of water molecule) in the NIR spectrum reflects the complete dehydration of the Co^^ molecular sieve. The presence of only one OH (2v) combination vibration band at 7 320 cm"^ both in dehydrated H- and Co-(Al)MCM-41 indicates that Co-OH groups were not formed during dehydration. Comparision of NIR spectra of hydrated and dehydrated (A1)MCM-41 and dehydrated Co-(Al)MCM-41 is given in the Figure 4. It is concluded that only bare Co^^ ions are present in dehydrated Co-(Al)MCM41 being located in two different coordinations, i.e. there exist two cationic sites og Co^^ ions in(Al)MCM-41.
15000
20000
25000 15000 wavenumber (cm'^)
20000
25000
Figure 3. a) Normalized DR VIS spectra of dehydrated Co^^-(Al)MCM-41. Co/Al 0.08 (—), 0.11 (- - -), 0.26 (• • •). b) Decomposition of the VIS spectrum of the Co^^-(Al)MCM-41 (Co/Al 0.26) to the Gaussian bands. Experimental data (O), fit (—), Gaussian bands ( — ) . To attribute absorption bands to the individual Co^^ sites, second derivative mode analysis (not shown here) and decomposition of the spectra to the Gaussian curves was used (details of methods see in Refs [5,7,10]. The decomposition of the VIS absorption of the
239 Co^^-(Al)MCM-41 to the Gaussian curves is illustrated in the Figure 3b. Two types of Co^^ ions were identified. Ions of the type a are reflected in the VIS spectrum by a single band with maximum at 14 600 cm"\ Cationic site corresponding to this type of the Co ^ ions is occupied preferentially at high Co loadings. Co^^ ions of the type P correspond to the spectrum composed from four bands at 15 850, 17 450, 19 970 and 21 700 cm ^ These Co^^ ions predominate in the Co-(Al)MCM-41 with low Co loading. One coordination of bare Mn^^ ions was reported by Kevan et al. in (A1)MCM-41 [4]. Because Co^^ spectra of type a and P are very different from 2+ 8 the spectrum of tetrahedral Co ion, none Co^^ ions were incorporated into framework position. Thus, the discrepancy in the number of reported cationic sites in Ref [4] and in this work should reflect different metal loading in molecular sieve or differences in its chemical composition (Si/Al ratio). As it was already 5000 6000 7000 8000 9000 10000 mentioned, population of transition wavenumber (cm"'') metals in individal cationic sites depends on the metal loading. The Figure 4. Comparison of NIR spectra of hydrated effect of the Si/Al ratio was not studied (• • •) and dehydrated (- - -) (A1)MCM-41 and for MCM-41 matrix, but is well known dehydrated ( ) Co-(Al)MCM-41 (Co/Al 0.11). for pentasil containing zeolites [1]. It is necessary to mention that observed VIS spectra of the Co ^ ions are similar to those reported for pentasil containing zeolites (with deformed six-member rings present in the framework) and different from the spectra of the Co^^ ions located in A, X and Y zeolites [1,5,8,9]. It indicates that deformed six-member rings are present in the structure of MCM-41, but the confirmation of this suggestion requires further detailed study. 3.2. Zn^^-(AI)MCM-41 Zn ion is isoelectronic with Cu^ ion and their luminescence reoresents optical transition from the lowest excitet state (3d^4s^ triplet) to the ground state (3d* singlet). In this case, luminescence center is characterized by a single fosforescence band. Emission spectra of dehydrated Zn^^-(A1)MCM-41 samples are shown in Figure 5a. Changes in the spectrum with Zn loading and with time of the spectra recording after excitation pulse indicate the presence of several emission bands in the spectrum. According second derivative mode analysis (not shown here) and decomposition of the spectra to the Gaussian curves, shown in Figure 5b, the spectrum is composed fi^om three bands with maxima at 390, 445 and 510 nm. Weak band at 390 nm reflects emission of the (A1)MCM-41 host (not discussed in this paper). Thus, only bands at 445 and 510 nm correspond to Zn^^ emission. Dependence of the relative intensity of these bands on the Zn concentration confirms the assignement of these bands to two different emission centers. NIR DR spectra (not shown here) does not indicate presence of water molecules or Zn-OH groups. Moreover, ion emission is extremely sensitive
240
to the presence of extraframework ligands, which represent luminescence quenchers. Thus, only bare Zn^^ ions are present in dehydrated Zn^^-(A1)MCM-41. It can be concluded that two cationic sites are occupied by Zn^^ ions in dehydrated MCM-41 molecular sieve, site characterised by the emission at 445 nm is occupied preferentially at low Zn loading. This is in agreement with results obtained for Co^^ ions. Thus, we should generalize that divalent metal cations occupy two different cationic sites in dehydrated (A1)MCM-41 molecular sieve.
375
500
625 375 wavelength (nm)
500
625
Figure 5. a) Normalized VIS emission spectra of dehydrated Zn^^-(A1)MCM-41. Zn/Al 0.24, spectra recorded at 2 (—) and 50 ( ) |is after excitation; Zn/Al 0.05, spectrum recorded at 2 (• * *) |is after excitation, b) Decomposition of the emission spectrum of the Zn'^ -(A1)MCM-41 (Zn/Al 0.24) to the Gaussian bands. Experimental data (O), fit (—), Gaussian bands ( — ) . Spectrum recorded 2 jis after excitation. 3.3. Cu^-(AI)MCM-41 Cu^^-(A1)MCM-41 reduced in CO exhibits blue or green emission depending on the Cu loading and reduction conditions. Moreover, emission spectrum changes with time of spectra recording. All this indicates the presence of several Cu^ emission centers in (A1)MCM-41. Second derivative mode analysis and decomposition of the spectra to the Gaussian curves applied on the spectra of Cu^-(A1)MCM-41 of different Cu loading and recorded at different time indicate presence of five emission bands in the spectrum. A weak band at 390 nm corresponds to the emission of the (A1)MCM-41 host. Bands at 430, 470, 540 and 580 nm represents four different coordinations of the Cu^ ions in (A1)MCM-41. Decomposition of spectra to these bands is illustrated in Figure 6. Emission spectra of Cu^-(A1)MCM-41 are similar to those reported for Cu^ containing zeolites of MOR and MFI structure [1,7]. But, because the emission wavelength is not
241 unambigeous characteristic of the Cu^ ion, statements on the similarity of the Cu^ siting in the (A1)MCM-41 and zeolites requires fiither studies. Cu ions are incorporated into molecular sieve as divalent (Cu^^(H20)6)^^ or monovalent (Cu^^X'(H20)5)^ complex cations [11]. Thus, after dehydration and subsequent reduction, Cu^ ions are placed in viscinity of one or twoframeworkaluminium atoms [1]. As two cationic sites with two close aluminium atoms are present in (A1)MCM-41 (cf Chapters 3.1. and 3.2.), four different coordinations of the Cu^ ions in (A1)MCM-41 represent two cationic sites with two close aluminium atoms and two cationic sites balanced by a single aluminium atom. Thus, besides two cationic sites accommodating divalent cations, two other cationic sites being enable to accommodate only monovalent cations are present in the (A1)MCM-41 molecular sieve. Because only low exchange degree can be reached for divalent cations (cf Table 1), sites with isolated aluminium atoms represent majority of cationic sites in (Al)MCM-41 with Si/Al > 20.
o T5 0) N (0
/
/
E k.
/
cl
/ ' «% ^&\ Ssk\ '
/ * » '\
// '*
% MB&.\
'
/ - I
tAHLx
r
/.'. M\
o c 1
^
^1
400
*
i
1
500
^
• •t \\ *\ II
\
%•
•
%\
1
i\ ii
%\ %\
jpaML
^
'
600
1 '
'
I
700 400 500 wavelength (nm)
600
700
Figure 6. Emission spectra of Cu'^-CAl)MCM-41. a) Cu/Al 0.03, 2 ^s after excitation, b) Cu/Al 0.09, 2 ^s after excitation, c) Cu/Al 0.16, 100 ^is after excitation, d) Cu/Al 0.35, 2 ^s after excitation. Emission spectrum (—), Gaussian bands (—).
242
4. CONCLUSIONS Four types of cationic sites were identified in dehydrated (A1)MCM41. Divalent cations are accommodated only in two sites. One of them is occupied preferentially, second one only at high loadings and represents minority of cationic sites. Two other sites accommodate only monovalent cations. However, monovalent cations occupy also cationic sites accessible for divalent cations. Distribution of monovalent cations among these four sites also varies with the metal ion loading. Acknowledgement This work was supported by the Grant Agency of the Czech Republic (No. 104/99/0840) and by Grant Agency of the Academy of Sciences of the Czech Republic (No. A 4040707). The authors thank to Dr. T. Grygar for chemical analysis. REFERENCES 1. B. Wichterlova, J. Dedecek and Z. Sobalik, Proc. 12th IZC, Baltimore 1998, (Eds. M.M.J. Treacy et al.). Materials Research Society, (1999) 941, and references therein. 2. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C Vartuli and J.S. Beck, Nature 359 (1992) 710. 3. Proceedings of the 1st International Symposium on Mesoporous Molecular Sieves, Baltimore 1998, (Eds. L. Bonneviot et al). Stud. Surf. Sci. Catal. 117, Elsevier, 1998. 4. J. Xu, Z. Luan, T. Wasowicz and L. Kevan, Microporous Mesoporous Mater. 22, (1998) 179, and references therein. 5. J. Dedecek and B. Wichterlova, J. Phys. Chem. B, 103, (1999) 1462. 6. G. Schultz-Eklofif, J. Rathousky and A. Zukal, J. Inorg. Mater. 1 (1999) 97. 7. J. Dedecek and B. Wichterlova, Chem. Phys. Phys. Chem. 1 (1999) 629, and references therein. 8. R. Kellerman and K. Klier, Surface and Defect Properties in Solids, Chem. Soc. London, 4 (1975) 1, and references therein. 9. A. A. Verbeckmoes, B.M. Weckhuysen and R. A. Schoonheydt, Microporous Mesoporous Mater. 22 (1998) 165, and references therein. 10. J. Dedecek and B. Wichterlova, J. Phys. Chem. 98 (1994) 5721. 11. J. Dedecek and B. Wichterlova, J. Phys. Chem. B, 101 (1997) 10233.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
243
Reversible Transition of the Coordination of Al in MCM-41 H. Kosslick, H. Landmesser, R. Fricke and W. Storek^ Institute of Applied Chemistry, Richard-Willstatter-Str. 12, D-12484 Berlin, Germany ^Federal Institute of Materials Research and Testing, 12489 Berlin, Richard-Willstatter-Str. 5
The influence of calcination and different hydrothermal treatments on the coordination of aluminum in MCM-41 molecular sieves is investigated by ^^Al MAS NMR and FTIR spectroscopy using ammonia as probe molecule. Samples are further characterized by XRD and ^^Si MAS NMR spectroscopy. It is found that non-tetrahedral Al species formed during calcination can be reversible transformed into tetrahedral coordinated framework Al by hydrothermal treatment. „Re-inserted Al„ gives rise to Bronsted acidity. INTRODUCTION The development of mesoporous materials with more or less ordered and different connected pore systems has opened new access to large pore high surface area zeotype molecular sieves. These silicate materials could be attractive catalysts and catalyst supports provided that they are stable and can be modified with catalytic active sites [1]. The incorporation of aluminum into framework sites of the walls is necessary for the establishment of Bronsted acidity [2] which is an essential precondition for a variety of catalytic hydrocarbon reactions [3]. Furthermore, ion exchange positions allow anchoring of cationic transition metal complexes and catalyst precursors which are attractive redox catalytic systems for fme chemicals [4]. The subject of this paper is the examination of the influence of calcination procedures, of soft hydrothermal treatment and of the Al content on the stability of the framework aluminum in substituted MCM-41. The impact on the Bronsted acidity is studied. EXPERIMENTAL Synthesis Al-MCM-41 samples [5] with varying contents of aluminum were synthesized from gels of the molar composition 13 Na20 * x AI2O3 * 96 Si02 * 30 TEAOH * 14 HDTMACl * 1585
244
H2O, with x = 0.4 to 14 (synthesis time: 2-3 d, temperature: 116°C). Silica sol and sodium aluminate solution freshly prepared by dissolving Al pellets in NaOH were used as Si or Al sources, respectively. The as-synthesized materials were calcined at 600°C and NH4exchanged with 0.5 M NH4NO3 at 80°C. Then samples were dried in air or calcined at 450°C to obtain the H-form. Characterization XRD pattern were obtained on a STOE powder diffraction spectrometer. The resolution was nearly 0.02 deg. ^^Si and ^^Al MAS NMR spectra were recorded on a Bruker DMX-400 spectrometer (Bo = 9.4 T) under magic angle spinning conditions (13-14.5 kHz) at a resonance frequency of 104.3 MHz. The external reference standards for ^^Si and ^^Al MAS NMR were TEOS (5=0) or A1(H20)6 (5=0) in a IM aqueous A1(N0)3 solution, respectively. As sample holder a 4 mm outer diameter Zr02 rotor was used. For the TPD and FTIR experiments the samples were purged in a helium stream at 120°C for 1 h or in vacuum for 30 minutes at 100°C, respectively. Subsequently, samples were loaded with ammonia at 100°C. After second purging to remove the physisorbed NH3 (2-3 h at 100°C in He (TPD) or 0.5 h at 100°C in vacuum (FTIR)) the conventional TPD runs were performed at a heating rate of 10 K/min and a helium flow of 0.5 ml/min. The desorbed amount of ammonia was analysed continuously using a thermal conductivity cell. FTIR spectra were recorded on a Biorad FTS 60A spectrometer with an MCT detector. The transmission IR studies were performed with self-supporting wafers using special infrared cells made from quartz-glass for in-situ measurements. This cell was connected to a high vacuum and gas dosing system. The heating rate during FTIR NH3 desorption experiments was 5 K/min. For comparison of infrared data of the different samples the infrared spectra were corrected by its corresponding wafer „weight„ (5-8 mg/cm^). RESULTS Characterization The XRD patterns of as-synthesized and calcined samples show the typical reflections of MCM-41 molecular sieves. Four low angle reflections are observed. One main peak arises at 20 = 2 deg. which is assigned to the {100} reflection. The {110}, {200} and {210} reflections are of lower and very weak intensity. The patterns could be indexed on a hexagonal lattice due to the hexagonal arrangement of pores. The appearance of the XRD pattern is maintained with the increasing aluminum content up to a total Si/Al ratio of 6.8 (9.8 gel). Only the resolution
245
of the peak doublet between 2G = 4 to 5 deg decreases somewhat with increasing aluminum content. Further increase of the Al content leads to a drastic decrease of the reflection intensities. The order of the MCM-41 framework is lost.
as-synthesized
100
50
(ppm)
-50
Figure 1: ^^Al MAS NMR spectra of Al-MCM-41 after different treatment The ^^Si MAS NMR spectra of as-synthesized samples show a weak signal at -90 ppm and two intense lines at ca. -100 and -110 ppm. designated as Q^, Q^ and Q^ lines, respectively. They are caused by silicon nuclei of 2-fold connected geminal (Si-0-)2Si(OH)2 groups, 3-fold connected silicon in (Si-0-)3SiOH groups, and 4-fold connected silicon - Si(-0-Si)4, respectively. The connectivity of Si04-tetrahedra, i.e. the percentage of SiOH groups that condensate to Si-O-Si bridges in the framework, which is 0% for Si(0H)4 and 100% for Si(0Si)4 groups, was calculated from the relative intensities of the three ^^Si NMR signals. It reaches ca. 40% in the as-synthesized Al-MCM-41. Calcination causes a condensation of silanol groups in the walls of MCM-41. The intensity of the Q"^ signal at -110 ppm increases on the expense of the Q^ and Q^ line. The framework connectivity increases to ca. 66%.
246 The ' Al MAS NMR spectra of as-synthesized Al-MCM-41 samples show a main signal of tetrahedral coordinated aluminum at ca. 53 ppm. The aluminum is nearly entirely incorporated into the framework. As generally observed, calcination leads to the formation of penta- Al^^^ and octahedrally coordinated aluminum Al^^^ on the expense of the tetrahedral framework Ar \ In the ^^Al MAS NMR spectra of calcined samples appear additional lines at ca 30 ppm and 0 ppm, respectively (Fig. 1). Only ca. 1/3 of the Al atoms remain in tetrahedral framework positions even after thermal treatment at 600°C.
- - NH4-Form _
. calcined rehydrated
O C
ca
o
BF3 /Base
BaseH^
solvent >
Figure 2 Preparation of supported boron trifluoride complex Characterisation of the supported BF3/Si02 catalysts was performed using DRIFTS in conjunction with pyridine titration which show that all the catalysts exhibit both Lewis and Bronsted acidity. This is determined by the absorption bands observed in DRIFT spectra at 1445 and 1461 cm'^, (Lewis sites), 1638 and 1539 cm-l (Bronsted sites) and 1611 and 1489 cm-1 (combined Lewis/Bronsted sites). There is a striking difference in the nature of the acid sites depending on catalyst preparation, with the catalysts prepared in ethanol exhibiting higher concentrations of Bronsted acid sites than those prepared in toluene (Figure 3). Thus both choice of BF3 precursor and solvent can influence the acidity of supported boron trifluoride catalysts. On heating the Bronsted sites are diminished (Figure 4).
255
BF3.0Et2/SiO^ inPhMe BF3.0B2/SiOb inEtOH BF3(htO)2/SiO^ inPhMe
inEtOH
1900 1900
1700
1500
1300
1800
1700 1600 1500 Wavenumbercm"^
1400
1300
Wavenumber (cm ) Figure 3
Effect of reaction mixture composition on the acidity of supported boron trifluoride complex as measured by spectrosopic titration with pyridine.
Figure 4 Effect of heating on the acidity of boron trifluoride complex
The origin of the acid sites on the catalysts prepared from ethanol was investigated using thermogravimetric analysis coupled with evolved gas FTIR (TGIR), which allows molecules desorbing from the catalyst during thermal analysis to be identified by their vibrational spectrum. Heating both catalysts above 100°C results in significant weight loss and the observation of ethanol desorption in the IR. However the differential mass lost indicates that the ethanol desorption temperature from BF3(H20)2/Si02 is 10°C higher than from BF3.0Et2/Si02, and approximately twice the amoimt of ethanol is evolved. The uptake of short chain alcohols can be used as an indication of the strength and concentration of Bronsted acid sites on zeolites. These results therefore suggest that BF3(H20)2/Si02 possesses a higher coverage of stronger Bronsted acid sites compared to BF3.0Et2/Si02. Further heating beyond 400°C results in an additional weight loss which is accompanied by the evolution of HE from the catalyst. The large reduction in surface area on heating to > 400°C can be explained by the formation of borosilicate (Table 1). The evolution of ethanol above 100°C coupled with the loss of Bronsted acidity indicates that Bronsted acid sites in the BF3(H20)2/Si02 catalyst may arise from the binding of ethanol to supported BF3 centres resulting in the formation of a [SiOBF3]' [EtOH2]+ complex. Further evidence in support of this model comes from ^H MAS
256 NMR of the as prepared catalyst which show resonances at 1.34, 4.01 and 8.16 ppm which are consistent with CH3, CH2 and OH2"*' of protonated ethanol.
1
Table 1
Sample KlOO Si02 Catalyst A as prepared Calcined 150°C Calcined 400°C Calcined 60Q°C
Surface Area m^g-l 300 259 258 267 56
J
Typical surface areas of a supported boron trifluoride complex catalyst heated to different temperatures.
The catalytic activity of these supported BF3 samples was tested using the reaction of 1-octene with phenol (performed at 85°C using 0.05 M of each reactant, in 100 ml of 1,2 dichlorethane with Ig of supported BF3 catalyst). Table 2 shows the phenol conversion and selectivities towards octyl-phenyl ether obtained after 23 hours reaction time. It is clear that the activity of the BF3(H20)2/Si02 catalyst prepared in ethanol is superior to the other samples. The activity can thus be correlated with the number and strength of Bronsted acid sites identified on these catalysts using TGIR. Following reuse of BF3(H20)2/Si02 samples, a decrease in conversion and selectivity towards ring alkylation products is observed relative to the fresh catalyst. The loss of activity on recycling the catalyst may result from organic residue deposited on the catalyst during reaction causing pore blocking and/or poisoning of active sites. 1 Catalyst BF3(H20)2/Si02 (EtOH) 1 BF3(H20)2/Si02 - recycled BF3(H20)2/Si02 (PhCHa) BF3(OEt2)/Si02 (EtOH) 1 BF3(OEt2)/Si02 (PhCHs) Table 2
Phenol Conversion (%) 30 6 4 3 90% Average of seven separate peroxyacid preparations Table 5
Solid peracids prepared by sol-gel methodology via the cyanoalkyl substituted materials and solid carboxylic acids.
The materials show excellent shelf lives, maintaining over 90% of their activity after several weeks. On heating they decompose at 150 °C. They are also ignited by impact. After use they can be recovered as the solid acid and recycled with fresh hydrogen peroxide. In a typical reaction, the epoxidation of cyclooctene, a batch of solid peracid gave yields of 98,100 and 93% of the epoxyproduct after 2-4h on the 1^* run, 1^* recycle and 2^^ recycle respectively (peroxyacid site loadings of 2.45, 2.45 and 2.40 respectively as determined by titration). Numerous other reactions have been successfully carried out using these novel reagents and will be reported elsewhere.
5.3.
Solid sulphonic acid In a similar manner to the solid carboxylic acid and the solid peracid, solid sulphonic acids can be prepared by a modification to a published method [16]. The precursor organosilane in this case is the trimethoxy(thioalkyl) silane which gives good quality reproducible gels with varying amounts of tetrethoxysilane. The solid mercaptan can be easily oxidised to the solid sulphonic acid with hydrogen peroxide (Figure 10).
263
(EtO)4Si+
(MeO)3Si(CH2)3SH
^Q^^X^^™"^
Figure 10 Preparation of solid sulphonic acid based on a hexagonal mesoporous silica. Key features of the nnaterials are: 1. High loadings of sulphonic acid groups (2-3 mmole g) 2. Spectroscopic titration with pyridine reveal strong Bronsted acid groups 3. High surface areas (ca. 300 m^g'^) The solid acids are highly active in typical Bronsted acid catalysed reactions including esterifications. Details of these will be reported elsewhere.
6.
CONCLUDING REMARKS
A wide range of solid analogues of important inorganic and organic reagents and catalysts can be made by chemically modifying mesoporous solids. Through new hexagonal mesoporous silicas for example, it is possible to achieve loadings that are so high that materials with weight-for-weight activities equivalent to conventional homogeneous reagents can be made. The solid analogues possess many important advantages over the traditional liquid or soluble reagents. They are easier and safer to handle, they can be easily recovered from the application and
264
reused, they can have greater stability and/or shelf life, they can give significantly higher selectivities on application. While the focus of this article has been on reagents and catalysts it is clear that the same principles could be applied to other applications such as adsorbents for trapping and transport and for controlled release uses. The advent of new mesoporous materials will add potential to the area through wider availability of alternative support materials offering different capacities and activities.
7.
ACKNOWLEDGEMENTS
We are indebted to the many postgraduate and postdoctoral researchers who have contributed to the work in these areas notably Peter Price, Kate Shorrock, Dominic Jackson, James Mdoe, John Rafelt and Sjack Elings. We also thank our many sponsors notably the EPSRC, Royal Academy of Engineering, the Royal Society, the European Union and UK and European Industry.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.
J. H. Clark, Green Chem., 1 (1999) 1. P. Tundo and P.T.Anastas (eds). Green Chemistry: Challenging Opportunies, Oxford Science, Oxford, 1999. J.H.Clark and D.J.Macquarrie, Chem.Commun., (1998) 853. J.H.Clark, K.Martin, A.J.Teasdale and S.J.Barlow, Chem. Commun., (1997) 2037 J.H.Clark,P.M.Price, K.Martin, D.J.Macquarrie and T.W.Bastock, Eur.Pat.AppL, 1998. J.H.Clark,P.M.Price, K.Martin, D.J.Macquarrie and T.W.Bastock, J.Chem.Res., (1997),430. P.M.Price, DPhil.thesis, University of York, UK ,1999. K.Wilson and J.H.Clark, Chem.Commun., (1998) 2135. K.Wilson and J.H.Clark, Eur.Pat.AppL, 1999. I.CChisem. J.Rafelt, M.T.Shieh, J.H.Clark, D.J.Macquarrie, C.Ramshaw and K.Scott, Chem. Commun., (1998) 1949. J.Rafelt, D.Phil thesis. University of York, UK, 1999. J.E.Mdoe, J.H.Clark and D.J.Macquarrie, Synlett., (1998) 625. D.J.Macquarrie and D.B.Jackson, Chem.Commun., (1997) 1781. D.J. Macquarrie, D.B. Jackson, J.E. Mdoe and J.H. Clark, New J. Chem., (1999) 539. J.A. Elings, R.Ait-Meddour, J.H. Clark and D.J. Macquarrie, Chem. Commun., (1998) 2707. W.M. Van Rhyn, D.E. DeVas, B.F. Sels, W.D. Bossaert and P.A. Jacobs, Chem. Commun., (1998), 317.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
265
Peculiarities of alkyl-modification of ordered mesoporous materials: A single-step treatment of uncalcined MCM-41 involving template removal and surface functionalization Valentyn Antochshuk and Mietek Jaroniec Department of Chemistry, Kent State University, Kent, Ohio 44242, USA The purpose of this work was to explore a new procedure of ligand grafting on the surface of ordered mesoporous silicas such as MCM-41 synthesized via templating procedure. This procedure allows to synthesize ordered mesoporous silicas of hydrophobic properties via one-step treatment of uncalcined MCM-41 samples with trialkylchlorosilanes. This treatment causes a simultaneous removal of the surfactant template from mesopores and attachment of trialkylsilyl groups. In the current work a series of samples modified via one-step procedure was compared with those prepared via conventional modification. Trimethylsilyl and octyldimethylsilyl ligands were attached to the surface of mesopores by reaction with monochloroalkylsilanes of the general formula ClSiMe2R. The attachment of alkyl groups to the silica surface, the structure of the bonded layer and changes in the mesopore volume of materials upon their functionalization were studied by means of elemental analysis, high resolution thermogravimetry, nitrogen adsorption, ^^C and ^^Si solid-state NMR. The one-step procedure shows some advantages, e.g., higher surface coverage and better ordering of bonded ligands, in comparison to the conventional modification, especially when longer alkylsilanes are used.
1. INTRODUCTION Preparation of MCM-41 materials [1] usually includes a high temperature calcination step that in fact opens mesopores by removing structure directing molecules (template) as well as results in the structure shrinkage due to high temperature condensation of silanol groups [2]. Functionalization of mesoporous materials with organosilanes is often employed to synthesize materials of desired surface properties for advanced adsorption and catalyUc applications [3] as well as to improve their stability [4]. There are two most popular procedures used to functionalize the mesopore surface: 1) post-synthesis attachment of functional groups by treating mesoporous silicas with organosilanes [5], and 2) one-pot synthesis involving co-hydrolysis of different silanes [69]. The first procedure usually employs calcined samples and does not affect significantly their mesopore structure. However, this procedure has some disadvantages such as low surface coverage of the resulting materials, which is associated with relatively high amount of residual silanols. In contrast, the one-pot synthesis seems to be an attractive way to functionalize mesoporous silicas because it does not require their prior calcination, but its
266 shortcomings lie in providing materials of broad pore size distributions and lower structural ordering [8, 9]. Although these shortcomings can be reduced or even eliminated by proper selection of organosilanes, as has been nicely shown by Inagaki et al. [10], the one-pot synthesis remains to be less flexible to introduce a complex surface functionality in comparison to the post-synthesis functionalization. However, there is no doubt that the onepot synthesis is extremely promising for preparation of novel organic-inorganic hybrid materials of uniformly distributed organic segments in the entire inorganic framework [10,11]. A typical procedure for post-synthesis functionalization of ordered mesoporous materials with organic ligands involves reaction of silanes with calcined samples either in the presence of solvent or without solvent [12,13]. Although organosilane functionalization of MCM-41 materials has been most often performed on calcined samples, a few attempts to modify uncalcined materials have been reported too [14-16]. The first two references signalize only about modification of uncalcined ordered mesoporous silicas. Ref [14] was published before the well-known Mobil's work, which initiated a tremendous interest in ordered mesoporous materials. It deals with modification of the alkyltrimethylammoniumkanemite complex, which in the light of our current knowledge can be considered as a uncalcined ordered mesoporous material. Ref [15] is from the Mobil group and contains a short paragraph related to silanization of uncalcined MCM-48 and related materials. Both these reports do not provide details about the pore size changes upon modification, surface coverage and degree of surfactant removal. The other report [16] is focused on a stepwise selective functionalization of MCM-41 materials, which is targeted on an initial modification of their external surface, followed by opening of mesopores via HCl/ethanol extraction and their further modification. In the light of our recent studies [17], a selective modification of the external surface and mesopores interior as that proposed in [16] seems to be difficult to achieve. The idea of the aforementioned functionalization of ordered mesoporous materials is based on a direct exchange of the surfactant template in uncalcined samples by ligand species, which interact strongly with the silica surface [17]. For instance, hexagonally ordered silica-cationic surfactant mesophases are self-assembled due to relatively week coulombic interactions and the surfactant template can be easily exchanged by other cations interacting more strongly with the negatively charged silica surface. In fact, this approach has been often used to remove and/or exchange the surfactant template from ordered mesopores [18,19]. Thus, it is not surprising that this idea was recently utilized to use various metal cations as efficient carriers for delivery of bifunctional ligand molecules into mesopores [20]. Namely, the cationic surfactant template in uncalcined mesoporous silica can be exchanged by metal complexes with aminosilanes, the siloxy groups of which condense with surface silanols allowing to remove metal cations by extraction. The ionexchange method of the MCM-41 functionalization [20] seems to be promising for attachment of specific ligands, which are able to form metal complexes. This method is not applicable for attachment non-specific groups, e.g., alkylsilyl ligands, which are very popular in many applications. Independently, we reported [17] that the surfactant template can be directly exchanged by organochlorosilanes, which form a strong covalent bonding with the silica surface. A major advantage of this approach in comparison to the ion-exchange method [20] lies in its
267 generality, e.g., non-specific ligands can be attached too. In addition, the one-step modification of uncalcined MCM-41 samples with trialkylchlorosilanes allows to achieve several goals: 1) removal of surfactant molecules fi-om mesopores, 2) avoidance of high temperature calcination and structure shrinkage or solvent extraction procedures, 3) preparation of mesoporous materials with complex surface functionality. This method can be especially attractive for low temperature synthesis of catalytic materials and adsorbents with high coverages of attached ligands and high degree of pore uniformity. The aim of the current study was to fiirther demonstrate the advantage of the use of uncalcined MCM-41 samples instead of calcined ones for surface functionalization. For this purpose two series of modified samples were synthesized via either modification of uncalcined and calcined MCM-41 materials in order to compare their surface and structural properties. Extensive characterization of these samples was done by elemental analysis, nitrogen adsorption, highresolufion thermogravimetry and '^C and ^^Si solid-state NMR.
2. MATERIALS AND METHODS 2.1. Materials Trimethylchlorosilane was fi-om Petrarch System hic. (Bristol, PA), whereas «-octyldimethylchlorosilane, pyridine (anhydrous, water content below 0.003%) and solvents were fi-om Aldrich Chemical Co. (Milwaukee, WI). All materials were used without additional purification. The MCM-41 sample used in the current study was a previously reported largepore MCM-41 prepared by post-synthesis hydrothermal restructuring [21]. The uncalcined and unmodified sample is denoted as MCM-U. This uncalcined sample, i.e., sample with template molecules inside mesopores, was funcfionalized via one-step process that includes simultaneous trialkylsilanization and extraction of the template. Two modified samples were prepared with trimethylsilyl and octyldimethylsilyl groups attached to the mesopore walls and denoted as MCM-UM and MCM-UO, respecdvely. A typical synthesis [17] was carried out as follows: about 0.2 g of the uncalcined MCM-41 was dispersed in 10 ml of trialkylchlorosilane, and refluxed for 36 hours (modification scheme is shown on Figure 1). Subsequently, about 5 ml of anhydrous pyridine was added to the mixture and refluxed for next 18 hours. After cooling down, the mixture was filtered and washed several times with small portions of ethyl alcohol, mixture of ethyl alcohol and
(CH3)3SiCI
Figure 1. Schematic representation of a single-step process that involves a simultaneous template removal from the MCM-41 mesopores and their surface functionalization [17],
268 n-heptane, and finally with n-heptane to remove the excess of silane and pyridine as well as possible products of hydrolysis. Finally, the modified mesoporous material was dried overnight in an oven at 95-100°C under vacuum. For the purpose of comparison, the MCM-U sample was calcined (denoted as MCM-C) and subsequently modified with trimethyl- and octyldimethylchlorosilanes using a conventional procedure 112]. The resulting samples were denoted as MCM-CM and MCMCO, respectively. 2.2. Characterization methods. The content of carbon, nitrogen and hydrogen in all modified samples was determined using a LECO Model CHNS-932 elemental analyzer (St. Joseph, MI). For each sample three measurements were done with the relative error of less than 0.1%. TA Instruments model TA 2950 (New Castle, DE) analyzer was used to carry out highresolution thermogravimetric analysis. All thermogravimetric measurements were done in a nitrogen atmosphere. The maximum heating rate in all cases was 5°C/min. over a temperature range from 25 to 1000°C. The accuracy in the weight change measurements was 0.1%. Nitrogen adsorption measurements were done using a Micromeritics model ASAP 2010 adsorption analyzer (Norcross, GA). Adsorption isotherms were measured at -196°C over the interval of relative pressures from 10'^ to 0.995 using nitrogen of 99.998% purity. Before each analysis the sample was degassed for 2 hours at 150°C under vacuum of about 10'^ Torr in the degas port of the adsorption apparatus. Specific surface areas of the materials under study were calculated using the BET method [22, 23]. Their pore size distributions were evaluated from adsorption branches of nitrogen isotherms using the BJH method [24] with the corrected form of the Kelvin equation for capillary condensation in cylindrical pores [25, 26]. In addition, adsorption energy distributions (AED) were evaluated from submonolayer parts of nitrogen adsorption isotherms using the algorithm reported in Ref [27]. ^^Si single pulse magic angle spinning (MAS) and ^^C CP-MAS NMR experiments were performed on a Bruker NMR spectrometer model Avanchi 400DMX (Bruker Instrument Inc., San Jose, CA) operating at resonance frequencies of 79.49 and 100.54 MHz for ^^Si and '^C, respectively. Each sample was spun at the magic angle with the spinning frequency of 2-2.5 kHz in a 7 mm zirconia rotor. Measurements were performed at room temperature using air as driving and bearing gas. A high-power decoupling was used during data acquisition to remove heteronuclear coupling. Chemical shifts were externally referenced to TMS (tetramethylsilane). Saturation-recovery experiment was performed to find spin-lattice relaxation times (TO for different silicon sites. The relaxation delay between pulses in the ^^Si NMR experiment was 600 seconds (it was taken as 5Ti for slowest relaxing site). ^^Si spectra deconvolution was performed in the absolute intensity mode and line broadening of 30 Hz was applied.
3. RESULTS AND DISCUSSION Contrary to Ref [16], longer reaction times of alkylchlorosilanes with MCM-41 and addition of pyridine to the reaction mixture resulted in a complete removal of surfactant
269 Table 1. Structural features of the "parent" MCM-41 and materials functionalized with trialkylchlorosilanes. Sample
SBET (m
MCM-U
Ci,g. (mmol-g'' SiO.)
dejH ( n m )
Vn.(cm'-g')
~3
8.2 (0.4f
-
-
MCM-C
915
4.9^
5.65
1.06
MCM-UM
540
2.81
5.25
0.69
MCM-CM
570
2.76
4.95
0.65
MCM-UO
385
2.83
4.20
0.38
MCM-CO
395
2.15
4.10
0.39
•g-')
- BET surface area; Cng. - surface coverage; dejH - pore diameter; Vm - volume of mesopores ^ - amount of single and geminal (in brackets) silanol groups estimated from ^^Si NMR data; - amount of silanol groups estimated from '^^Si NMR data. SBET
molecules (template) from mesopores and preparation of the material with densely grafted ligands. Nitrogen adsorption isotherm on the uncalcined MCM-41, which prior adsorption measurements was degassed at 40°C and pressure -10'^ Torr, shows no evidence of porosity and complete inaccessibility of mesopores (see Table 1). The external surface area of the MCM-U sample was in the order of magnitude typical for nonporous materials. In fact, it was even difficult to assess such small surface area very precisely due to the small amount of the sample. However, adsorption studies of the MCM-UM and MCM-UO samples, which were obtained via one-step functionalization of the uncalcined MCM-U material with alkylchlorosilanes, revealed opening of the mesopores during modification process. The pore diameter of the resulting materials, their specific surface area and mesopore volume depend on the size of attached ligands (see Table 1). The presence of bonded alkylsilyl groups (appropriate ratio C:H) and a successful removal of all surfactant molecules and pyridine (complete absence of nitrogen) was shown by means of the elemental analysis. Table 1 contains also the surface coverage of the bonded ligands. From the deconvolution of the ^^Si NMR spectroscopy data it was found that the amount of silanol groups that are present in the MCM-U and MCM-C samples are different. The uncalcined sample has more silanol groups (8.2 and 0.4 mmol/g Si02 single and geminal silanols, correspondingly) than calcined sample (4.9 mmol/g Si02 single silanols and the lack of geminal one). These facts, as well as difference in the pore diameter for the samples with the same amount of grafted ligands (see discussion below), indicate the condensation of silanols upon calcination (this is a common effect for mesoporous silicates) that results in the pore contraction. Attachment of alkylsilyl ligands to the silica surface via chemical bonding was proved through the ^^C CP-MAS solid state NMR. Line assignments were done based on the previous studies [28]. The observed chemical shifts are in the range 0-40 ppm that corresponds to the signal(s) from carbon atoms of attached alkylsilanes. In addition, the ligand grafting was proved by the ^^Si NMR spectroscopic studies that showed a decrease in
270 700
100
0.2
0.4
0.6
0.8
Relative pressure, P/P^
1.0
10-^
10-5
10"*
10-'
10-
Relative pressure, P/PQ
Figure 2. Nitrogen adsorption isotherms (A) and their low-pressure parts (B) for the MCM-C (O), MCM-U (•), MCM-UM (A), MCM-CM (A), MCM-UO (•) and MCM-CO (D) samples. Note that the amount adsorbed on MCM-U (•) is very small. the amount of silanol groups Q^ and Q~ with a simultaneous increase in the amount of siloxane sites (Q'*) (signals at -98, -89 and -107 ppm, correspondingly). An additional signal from alkylsilyl groups attached to the surface in a monomeric fashion, (M), was observed at 12-15 ppm [12]. The unmodified calcined sample, MCM-C, shows a very pronounced step of the capillary condensation at the relative pressure p/po-0.5 (Fig. 2A) on the nitrogen adsorption isotherm, which shifts gradually to lower pressures for the modified samples. The size of the hysteresis loop decreases with the increasing size of the grafted ligands. In contrast, the amount of nitrogen adsorbed on the uncalcined MCM-41 material is very small, and only the low-pressure part of the isotherm is shown in Fig. 2B. The single-step modification of this sample either with trimethyl- or octyldimethylchlorosilanes gives materials that show behavior characteristic for uniform pore system. Adsorption isotherms on modified MCM-U and MCM-C samples as well as the surface coverages are similar (see Table 1). The surface properties of the corresponding modified samples are analogous as shown in Fig. 3, which presents a comparison of the adsorption energy distributions evaluated from submonolayer nitrogen adsorption data. These distributions resemble those reported in Ref [12, 13] for modified calcined MCM-41 materials. It can be noticed that in the case of octyl bonded phases, in spite of their slightly different bonding densities, both modification methods gave materials of essentially identical AEDs, which are much narrower than AED for the calcined MCM-41. It is known that attachment of relatively long octyl ligands results in substantial depletion of high-energy sites (reactive silanols) characteristic for unmodified silica surface [12]. It is interesting that in the case of octyl ligands this depletion is analogous for both types of modifications. Of course, the depletion effect is much smaller for methyl ligands as shown in Fig. 3A, especially for the MCM-CM sample, indicating a higher nonuniformity of the bonded layer in this sample in comparison to MCM-UM.
271
0.25
A
•—>
0.25
^
^
-3 0.20
o 0-20
ec ^-^
£c
.2 0.15
.§ 0.15
^
s.
3
."2 0.10
."3 0.10
T3
T3
>. i
>-> 0.05
g 0.05
onn < 0
5
10
15
Adsorption energy (kJ mol')
20
Adsorption energy (kJ mol' )
Figure 3. Adsorption energy distributions for the MCM-C (0), MCM-UM (A), MCM-CM (A), MCM-CO (O) and MCM-UO (•) samples. However, the mesopore sizes of the samples prepared by conventional modification are smaller than the sizes of the corresponding samples prepared from uncalcined material because of the shrinkage of the former one during calcination. Note that the difference in the mesopore sizes of the modified samples (MCM-UM and MCM-UO vs. MCM-CM and MCM-CO) is determined not only by the framework shrinkage but also by the differences in the length of bonded ligands as well as their surface coverages. The latter effect is clearly visible in the data reported in Table 1. For instance, the pore diameter for the sample modified with trimethylchlorosilane via conventional procedure is about 0.30 nm smaller than the size calculated for the sample modified with the same silane via one-step procedure. This relatively big difference in the mesopore sizes of the MCM-CM and MCM-UM samples can be related mainly to the framework shrinkage during calcination because the surface coverages of both samples are similar. However, this is not case for the samples with attached octylsilyl ligands, i.e., MCM-CO and MCM-UO (Fig. 4). Although these samples were prepared from the same uncalcined and calcined MCM-41 materials as those used to prepare the MCM-CM and MCM-CU samples, their surface coverages are quite different (see Table 1). In this case the difference in the pore diameter is smaller (about 0.1 nm) because of the two competing factors: higher surface coverage (about 30% higher in comparison to the sample modified via conventional procedure) and no shrinkage when uncalcined sample is used. This example indicates that the functionalization of uncalcined MCM-41 samples is advantageous because it allows synthesize ordered materials with high surface coverages and the pore sizes are larger then those for the corresponding samples prepared from calcined materials. Finally, note that the observed changes in the pore size of the calcined MCM-41 due to its modification with octyldimethyl- and trimethylchlorosilanes (1.5 and 0.7 nm, correspondingly) are normal [12, 13] for conventional modification of mesoporous materials.
272
^
A
1.2
A
'B -=
B
1.0
W)
C. "J \
3 0.8 c o •-§ 0.6
^
1 0.4
1
D//yl 1
E
< u N ^ 0.2 < u u. O
jzp^]^^ . V N ^ ^ ^ , 3.0
4.0
5.0
6.0
7.0
dBjH (nm)
Figure 4. Pore size distributions for the MCM-C (A), MCM-UM (B), MCM-CM (C), MCM-UO (D) and MCM-CO (E) samples.
0
200
400
600
800
Temperature (°C) Figure 5. TG and DTG curves for the MCM-C (dotted lines), MCM-CO (dashed lines) and MCM-UO (solid lines) samples.
Modification of the mesoporous silica with alkylchlorosilanes resulted in the increase of the weight loss on the TGA curves (Fig. 5). Thermogravimetric studies revealed a great similarity of the samples modified via conventional and one-step procedures. As can be seen from Fig. 5 the sample with attached longer alkylsilane prepared via one-step procedure has much higher surface coverage than that obtained via conventional modification. A notable difference in the shape of the DTG curves for the MCM-UO and MCM-CO samples could be attributed to the presence of the different conformations of the attached alkyl groups [12]. In conclusion, a much higher amount of the attached octyldimethylsilyl groups can be achieved in the one-step modification in comparison to the conventional procedure. In spite of the increase in the size of the attached ligands (from trimethylsilyl to octyldimethylsilyl), the amount of grafted ligands in the one-step synthesis is almost constant (Table 1). Probably, the maximum number of the attached groups in the proposed one-step modification is limited by the size of the -Si(CH3)2CH2R anchoring groups. Also, the curvature of mesopores and their size can limit the maximum amount of anchored groups.
4. CONCLUSIONS A simple and effective one-step modification-extraction procedure discussed above makes possible a simultaneous removal of the surfactant template from mesopores and their grafting with organic ligands. Such procedure gives several advantages, e.g., higher surface coverage of grafted ligands, avoidance of the silica framework shrinkage and high temperature calcination or solvent extraction. A much higher degree of grafting alkylsilyl groups can be achieved in the proposed one-step procedure. The maximum surface coverage remains rather constant for a given type of anchoring groups and seems to depend mainly on
273
the curvature of mesopores and their size. Elimination of the calcination step before modification of MCM-41 reduces the pore shrinkage in the resulting organic-inorganic hybrids and provides alkyl-bonded MCM-41 materials of larger pore sizes in comparison to those for conventionally modified samples. In addition to simplicity of the proposed one-step procedure, it allows for synthesis of ordered mesoporous materials of higher hydrophobic properties than parent materials. In fact, a high coverage of the grafted alkylsilyl groups stabilizes MCM-41 with respect to various environmental factors, as it was mentioned elsewhere [4]. Also, this study shows that the proposed previously procedure for stepwise modification of uncalcined MCM-41 samples [16] requires further studies in order to carry out a selective functionalization of the external surface and mesopore walls of these materials
ACKNOWLEDGMENT The authors thank Dr. A.Sayari for providing the MCM-41 sample. The donors of the Petroleum Research Fund administrated by the American Chemical Society are gratefully acknowledged for partial support of this research.
REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. C.-Y. Chen, H.-X. Li and M.E. Davis, Micropor. Materials, 2 (1993) 17. 3. X. Feng, G.E. Fryxell, L.-Q. Wang, A.Y. Kim, J. Liu and K.M. Kemner, Science, 276 (1997)923. 4. T. Tatsumi, K.A. Koyano, Y. Tanaka and S. Nakata, Stud. Surf. Sci. Catal., 117 (1998) 143. 5. A. Cauvel, G. Renard and D. Brunei, J. Org. Chem., 62 (1997) 749. 6. K. Moller and T. Bein, Stud. Surf. Sci. Catal., 117 (1998) 53. 7. J.H. Clark and D.J. Macquarrie, Chem. Comm., 1998, 853. 8. C.E. Fowler, S.L. Burkett and S. Mann, Chem. Comm., (1997) 1769. 9. M.H. Lim, C.F. Blanford and A. Stein, J. Am. Chem. Soc, 119 (1997) 4090. 10. S. Inagaki, S. Guan, Y. Fukushima, T. Ohsuna and O. Terasaki, J. Am. Chem. Soc, 121 (1999)9611. 11. B.J.Melde, B.T. Holland, C. F. Blanford and A. Stein, Chem. Mater., 11 (1999) 3302. 12. V. Antochshuk and M. Jaroniec, J. Phys. Chem. B, 103 (1999) 6252. 13. C.P. Jaroniec, M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 102 (1998) 5503. 14. T. Yanagisawa, T. Shimizu, K. Kuroda and C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 1535.
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15. J.C. Vartuli, K.D. Schmitt, C.T. Kresge, WJ. Roth, M.E. Leonowicz, S.B. McCullen, S.D. Hellring, J.S. Beck, J.L. Schlenker, D.H. Olson and E.W.Sheppard, Chem. Mater., 6(1994)2317. 16. M. Park and S. Komarneni, Micropor. Mesopor. Mater., 25 (1998) 75. 17. V. Antochshuk and M. Jaroniec, Chem. Comm., 1999, 2373. 18. A.-R. Badiei and L. Bonneviot, Inorg. Chem., 37 (1998) 4142. 19. W.J. Roth, Method of M41S Functionalization of Potentially Catalytic Heteroatom Centers into As-synthesized M41S with Concomitant Surfactant Extraction, US Patent No. 5 925 330(1999). 20. S. Dai., Y. Shin, Y. Ju, M.C. Burleigh, J.-S. Lin, C.E. Barnes and Z. Xue, Adv. Mater., 11 (1999) 1226. 21. A. Sayari, P. Liu, M. Kruk and M. Jaroniec, Chem. Mater., 9 (1997) 2499. 22. S. Brunauer, P.H. Emmett and E. Teller, J. Am. Chem. Soc, 60 (1938) 309. 23. J. Roquerol, D. Avnir, C.W. Fairbridge, D.H. Everett, J.H. Hayness, N. Pernicone, J.D.F. Ramsay, K.S.W. Sing and K.K. Unger, Pure Appl. Chem., 66 (1994) 1739. 24. E.P. Barrett, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc, 73 (1951) 373. 25. M. Kruk, M. Jaroniec and A. Sayari, Langmuir, 13 (1997) 6267. 26. M. Kruk, V. Antochshuk, M. Jaroniec, A. Sayari, J. Phys. Chem. B, 103 (1999) 10670. 27. M.v. Szombathely, P. Brauer and M. Jaroniec, J. Comput. Chem., 13 (1992) 17. 28. K. Albert and E. Bayer, J. Chromatogr., 544 (1991) 345.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
275
New Organically Modified Hexagonal Mesoporous Silicas: Preparation and Applications in Catalysis Duncan J Macquarrie*, Dominic B Jackson, Stephana Tailland, Karen Wilson and James H Clark Department of Chemistry, University of York, Heslington, York, YOlO 5DD, England The ability to prepare organically functionalised, highly ordered silicas with control over texture and porosity is of great benefit to catalysis. A one-step method for the preparation of such materials has recently been developed, leading to Hexagonal Mesoporous Silicas (HMS), which display typical wormhole pore structure, and contain organic groups. High surface areas, and tight pore size distributions can also be obtained. Additionally, very high loadings of organic groups can be incorporated. The paper will describe the scope of the synthesis procedure, and indicate some applications of the materials obtained.
1. INTRODUCTION The discovery of the M41S class of materials in 1992 has led to enormous strides in the development of catalysts and supports with high degrees of control over porosity and structure being possible. [1,2] Pore sizes of ca. 2-5 nm are easily achieved, and larger pores are also possible with the help of auxiliary micelle swelling agents such as mesitylene.[l,2] Such materials can be used directly as acid catalysts[3], or can be further functionalised by grafting of organic groups [RSi(0R')3] to provide catalysts with activity in a range of reaction types[4]. Advantages of these grafted materials over traditional amorphous silica based analogues include higher surface areas, tightly defined porosity, and the ability to graft higher quantities of organic functions per gram of catalyst. However, the synthesis of MCM-41-type materials requires the use of quaternary ammonium catalysts, which have to be removed at high temperature, leading to destruction of the template. A more elegant route, particularly in terms of template recovery, and mildness of removal, is the ST route, discovered by Pinnavaia, which leads to the related Hexagonal Mesoporous Silicas, materials which contain wormhole-like framework mesopores.[5] We have extended this method to provide a convenient and versatile method for the preparation of organically modified HMS materials in one step.[6] This involves the co-condensation of two silanes, tetraethoxysilane (TEOS) and the substituted trialkoxy silane (RSi(0R')3 in the presence of long chain amines in aqueous ethanol. We have recently extended this approach to three-silane couplings. [7] This paper deals with the preparation, properties and catalytic activity of some of these novel materials.
276
2. PREPARATION OF THE CATALYSTS The catalysts are prepared as described previously. [6] The templating solution consists of a solution of w-dodecylamine (5.09 g) in aqueous ethanol (53 ml water and 46 ml ethanol). To this is added a total of 0.1 mol silane. The reaction is allowed to proceed for 18 h at room temperature. After filtration and extraction of template with ethanol, the material is filtered and dried. The filtrate fi-om the preparation is generally free of unreacted silanes, indicating that all the silanes are condensed, a fact borne out by the excellent agreement between theoretical and experimental composition. The template and the ethanol can both be recovered pure (the template with 99% efficiency). Both can be reused, as can the templating solution. This means that the process is essentially waste-free[7]. Figure 1. Preparation of Organically Modified Hexagonal Mesoporous Silicas.
We have found that the ratios of silanes used can be varied within wide limits, and more than two silanes can be used. For example, TEOS, y-aminopropyl trimethoxysilane (AMPsilane) and phenyl trimethoxysilane can be co-polymerised to give a material containing both amino groups and phenyl groups. Some representative examples of materials synthesised by this procedure are detailed in Tables 1 and 2. As can be seen, amine-containing materials tend to have surface areas somewhat lower than the parent HMS materials (ca. 1000 m^ g"^), whereas neutral organic groups tend to have higher surface areas than the unflinctionalised material. Pore sizes are relatively constant at around 2.6 nm, with the exception of the biftinctional materials, which have substantially lower pore sizes. All the materials display a very narrow pore size distribution, up to an organic loading of 2.3 mmol g ^ Indeed it is sometimes possible to achieve true HMS materials at loadings up to 2.7 mmol g\ Such loadings are much higher than achievable by grafting silanes onto silica (by a factor of 3-10, depending of the silane) and higher than grafting onto HMS materials (by ca. 50%). Table 2 contains data relating to higher loading materials, which do not have the tightly controlled pore structure, but do have very high loadings of organic groups. The surface areas achieved with the amine template are typically much higher than those obtained in its absence. Table 2 shows that it is possible to produce a wide range of high loading materials using the templating technique. Despite the lack of HMS-type structure, the template allows the production of materials with high surface areas. Loadings are much higher than those achievable using grafted methods. These materials are amorphous, and tend to have surface areas and pore structures similar to amorphous silicas. However, those prepared with cyanoethyl groups attached still have very high surface areas and predominantly narrow pores, albeit within an amorphous structure. The reason for this is not apparent. Incorporation of silane is excellent, with essentially complete incorporation even at 1: 1 ratios.
277
Table 1 Physical Parameters of Templated Materials RSi(0R")3 + R'Si(0R")3 + TEOS R
R'
^ [HMS]—R,R'
Loading Specific Surface Area Pore Diameter^ nm m^g-^ mmol g'^ 2.6 756 9 1.1 H2N(CH2)3 2.6 745 2:0 8 2.3 H2N(CH2)3 2.7 731 1 .0 9 1.1 MeHN(CH2)3 2.5 925 1 :0 9 1.1 Me2N(CH2)3 2.5 707 2:0: 8 2.3 Me2N(CH2)3 2.1 Ph 947 1: 1 8 1.1, 1.1 H2N(CH2)3 1.9 790 Me 1 : 1:8 1.1, 1.1 H2N(CH2)3 2.7 1607 1 :0 9 1.1 C1(CH2)3 2.6 1356 1:0:9 1.2 NC(CH2)2 2.4 1490 1:0:9 1.2 HS(CH2)3 2.4 890 2:0:8 2.5 HS(CH2)3 2.3 H2C=CH 1056 1:0:9 1.1 a - pore diameter D measured from D = 4V/S, where V is theframeworkpore volume (measured in the range 0 < ?/?« < 0.5 to exclude textura I porosity) and S is the specific surface area. ratio R : R' : TEOS
nr
Table 2 Physical Parameters of Amorphous Materials RSi(0R')3 + TEOS
-^ [ H M S ] - R
Loading Specific Surface Area Pore Diameter* nm m^g-^ mmol g"^ 8 132 4.0 H2N(CH2)3 n"2 10 249 4.9 1 : 1 H2N(CH2)3 9 118 1 :2 3.0 MeNH(CH2)3 10 323 1 :2 3.8 Me2N(CH2)3 2.1 1600 1 :2 3.8 NC(CH2)2 2.3 603 4.7 1: 1 NC(CH2)2 10 440 1 :2 3.9 HS(CH2)3 10 140 1: 1 4.7 HS(CH2)3 a - pore size distribution broad and centred on thefigurequoted. R
ratio R: TEOS
We have recently found that changing the solvent mixture used in the preparation of the aminopropyl materials described above has a significant effect on the properties of the material. In the case of 9 : 1 TEOS : AMP-silane with «-dodecylamine as template we have changed the composition of the solvent stepwise from 25 vol% water : 75 vol% ethanol to 75 vol% water : 25 vol% ethanol. Several significant trends can be observed, and the results are summarised in Table 3. We have taken the pore volume filled at P/Po < 0.5 to be due to framework pores (the pore size distribution at aromatic ketones) is thus completely changed, with ketones being much more reactive than benzaldehyde. Thus, whereas the grafted amorphous silica analogues of the catalyst will complete this conversion in around 10 minutes at 80 ""C, the newer materials require seven days at 110 °C to go to completion. The reasons for this behaviour are not yet clear, but may be related to differences in the exact nature of the amine groups attached to the surface. Reaction of AMPS with benzaldehyde results in a rapid (20-30s) formation of the corresponding imine. Such a reaction takes several days in the case of the HMS material. Similarly, the alkylation of the nitrogen with e.g. benzyl chloride is far more rapid (by a factor of several hundred) in the case of the grafted material. Thus, the amine groups in the new materials do not appear to display any significant nucleophilicity, despite their similar basic
281 activity in base-catalysed reactions. Titration of these materials[6b] indicates that AMPS is more basic, and has a range of different base strengths, whereas the HMS material has a definite inflection around pH 6-6.5. This value is close to that for the pK of silica itself [13] This may suggest that the active site in AMPS is essentially NH2, but that in aminopropylHMS it may be better described as NHs^ SiO'. Tables Knoevenagel reaction of ketones with ethyl cyanoacetate
R
H*
R' loading mmol g'^
ketone
CN
R
< C02Et yield
R' time
solvent
CN
+ H,0
)=={
w^
COjEt turnover number^
1.2 2.5 4.0 4.9 1.2 2.5 1.2 2.5
cyclohexanone 92% 2h 2400 toluene cyclohexanone 94% 1.5h toluene 2550 cyclohexanone 96% l.Oh toluene not measured cyclohexanone 94% 1.5h toluene not measured 3-pentanone 1127 95% 18h toluene 3-pentanone 1305 97% 4h toluene acetophenone 49% 72h toluene 55 acetophenone 48% 36h toluene 47 (a) tumover number is measured as number of moles product per mole of active sites As might be expected for a different active site, the poisoning mechanism appears to differ from AMPS to AMP-HMS. AMPS is poisoned in these reactions by a slow nucleophilic attack of the amine on the ester of the C-acid, leading to formation of amide, and reduction in basicity.[14] This does not happen with the HMS materials, consistent with a nonnucleophilic amine group. A consequence of this is that the HMS materials have tumover numbers 4-5 times higher than the grafted materials. The poisoning mechanism appears to be due to the selective adsorption of one of the reaction components, blocking access to the active site of the catalyst. Table 6. Reaction of ethyl cyanoacetate with selected ketones using aminopropyl/phenyl containing HMS. Loading ketone yield time solvent tumover AMP Ph number 1.15 1.1 5900 98% 0.5h toluene cyclohexanone 215 1.15 1.1 86% 24h toluene acetophenone 3600 96% 1.15 1.1 3-pentanone 2.5h toluene Reactions conducted at reflux with continuous removal of water. Loadings in mmol g' Further improvements can be made to the performance of the HMS materials by preparing materials containing two different functional groups. By co-condensation of TEOS, aminopropyl trimethoxysilane and phenyl triethoxysilane, a templated material can be
282 produced which contains both amine groups and phenyl groups on the surface. This increases the reaction rate significantly, and also improves the turnover number by a factor of 2.5-4. Results are shown in Table 6 for the reaction of ethyl cyanoacetate with selected ketones.
6. CONCLUSIONS The use of a one-pot synthesis procedure for Organically Modified HMS materials has been shown to lead to a range of materials of use as reagents and catalysts. The flexibility of the method allows a significant degree of control over the nature of the materials and their activity can be tuned by modification of preparation conditions or by incorporation of additional groups.
7. ACKNOWLEDGEMENTS DJM thanks the Royal Society for a University Research Fellowship, JHC thanks the Royal Academy of Engineering / EPSRC for a Fellowship.
REFERENCES 1. C T Kresge, M E Leoniwicz, W J Roth, J C Vartuli and J S Beck, Nature, 359 (1992) 710 2. J S Beck, J C Vartuli, W J Roth, M E Leoniwicz, 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 3. K. R. Kloetstra and H van Bekkum, J. Chem. Res (S), (1995) 26; E Armengol, M L Cano, A Corma, H Garcia and M T Navarro, Chem. Comun, (1995) 519 4. D E De Vos and T Bein, Chem. Comm (1996) 917; M Lasperas, N Bellocq, A Martin, D Brunei Tetrahedron Asymmetry 9 (1998) 3054; M Lasperas, T Lloret, L Chavez, I Rodriguez, A Cauvel and D Brunei, Stud, Surf. Sci.Catal, 108 (1998) 75 5. P Tanev and T J Pinnavaia, Science, 267 (1995) 865 6. (a) D J Macquarrie, Chem. Commun., 1916 1996; (b) D J Macquarrie, D B Jackson, J E G Mdoe and J H Clark, New J. Chem., 23 (1999) 539 7. D J Macquarrie, Green Chemistry, 1 (1999) 195 8. T R Pauly, Y Liu, T J Pinnavaia, S J L Billings and T P Rieker, J. Amer. Chem. Soc, 121 (1999) 8835 9. (a) W Zhang, J Wang, P T Tanev, T J Pinnavaia, Chem. Commun (1996) 253; (b) J S Reddy, P Lui, A Sayari, App. Catal A, 7 (1996) 148 10. S A Elings, R Ait-Meddour, D J Macquarrie and J H Clark, Chem Commun (1998) 2707 11. (a)W M van Rhijn, D E De Vos, B F Sels, W M Bossaert, P A Jacobs. Chem Commun, (1998) 313 (b) W M Bossaert, D E deVos, W M van Rhijn, J Bullen, P J Grobet and P A Jacobs, J. Catal., 182 (1999) 156 12. (a) K R Kloetstra and H A van Bekkum, Chem Commun (1995) 1005; (b) M Lasperas, T Lloret, L Chavez, I Rodriguez, A Cauvel and D Brunei, Stud. Surf. Sci., Catal, 108 (1997) 75 (c) B M Choudary, M Lakshmi Kantam, P Sreekanth, T Bandopadhyay, F Figueras and A Tuel, J. Mol Catal, A, 142 (1999) 361 13. Characterisation and Chemical Modification of the Silica Surface, ed. E F Vansant, P Van der Voort and K C Vrancken, Elsevier, Amsterdam 1995, ch 9 14. D J Macquarrie, J H Clark, A Lambert, A Priest and J E G Mdoe, React. Funct. Polym 35 (1997)153
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
283
Organo-fiinctionalized surface modified MCM-41 type mesoporous materials having various organic functional groups Priyabrata Mukherjee, Subhash Laha, Deendayal Mandal and Rajiv Kumar Catalysis Division, National Chemical Laboratory, Pune-411 008, India
Various organo-fiinctionalized MCM-41 type pure-silicate and alumino-silicate mesoporous materials were synthesized using cetyltrimethylammonium bromide as the template. Mono- and bi-fiinctional organo-MCM-41 materials such as SH-MCM-41, NH2MCM-41, vinyl-MCM-41, SH-Al-MCM-41, SH-NH2-MCM-41 etc. were studied. X-ray diffractogram, chemical analysis, BET surface area measurement etc. of the samples reveal that ordered organo-MCM-41 type mesoporous materials were prepared. There is tremendous scope in these area as the organo-MCM-41 materials can be used as solid ligand for various metal ions or even metal complexes, including one with a chiral moiety. Such composite solid materials may exhibit interesting catalytic properties.
1. INTRODUCTION One of the major advantage of mesoporous materials is that they can be used as catalyst or support when the molecules of larger dimension are used as the reactants. In fact the difficulties faced with the larger molecules offer a major thrust for the discovery of mesoporous materials which can effectively be used as a catalyst for larger molecules. The discovery of M41S materials has overcome the pore size constraint (< 20 A) which can be tuned in a range of 20-100 A by carefiilly optimizing the synthesis conditions [1,2]. MCM-41 is most extensively studied member of the M41S family because of its hexagonal array of unidimensional pore architecture. In addition to catalysis, MCM-41 type mesoporous materials are increasingly being explored for a variety of different applications, such as support, as sensors / carriers, surface modification etc.
Address for correspondence: Dr. Rajiv Kumar, Catalysis Division, National Chemical Laboratory, Pashan Road, Pune-411 008, India (Fax: +91-20-5893761 / -5893355; e-mail:
[email protected]). P. M., S. L. and D. M. gratefully acknowledge C. S. I. R., New Delhi for granting them research fellowships.
284
We have recently reported an improved method for the synthesis of MCM-41 type materials [3] using oxy anions like phosphate as promoter [4] The surface modification of MCM-41 can effectively be carried out by direct synthesis using alkoxysiloxane and organosiloxane precursors in a templating environment [5-10]. These organofunctionalized MCM-41 type materials can be used as solid ligands for metal ions, forming anchored metal complex. Bifunctional organo groups incorporated in MCM-41 walls can potentially be used to prepare mixed-metal type class of metal complexes in an inert matrix. Here we report the synthesis and characterization of monoflinctional and bifunctional organo-MCM-41 type pure-silicate and alumino-silicate mesoporous molecular sieves. 2. EXPERIMENTAL 2.1 Synthesis The syntheses of various organo-MCM-41 materials were carried out as follows. For the synthesis of 3-aminopropyl- and 3-mercaptopropyl-functionalized-MCM-41 materials, 3-X-propyltrimethoxysilane (XPTS) was used along with TEOS in a 1: 2.5 molar ratio as the silica sources (X = NH2 or SH). The gel composition was 1.0 XPTS: 2.5 TEOS: 0.42 CTMABr : 0.96 NaOH : 272 H2O : 66 MeOH. Methanol was used in the initial gel mixture to reduce and control the fast hydrolysis of XPTS. Vinyl-MCM-41 was also prepared using the gel composition as: 1.0 TEOS: 0.1 VTES: 0.15 CTMABr: 0.38 NaOH : 125 H2O. The bifunctional SH-NH2-MCM-4I and 3-mercaptopropyl functionalized Al-MCM-41 were prepared in a same way as the corresponding monofunctional MCM-41 materials, using the gel composition given in Table 1. For the SH-Al-MCM-41 sample Si02: AI2O3 molar ratio was 1: 0.025. The reaction mixture was first stirred at room temperature for 12 h and then heated in autoclave at 95 ^C for 36 h under static condition. The product obtained was filtered, washed several times first with distilled water, and then with acetone. The product was dried at 95 ^C for 4 h. The surfactant was removed by solvent extraction with a mixture of solvents containing methanol, water and HCl. In a typical extraction process, 85 g of methanol and 3.25 g of HCl (35.4 %) were used for 24 h under reflux for the extraction / removal of surfactant from 1.0 g of the solid product. 2.2 Characterization The samples were characterized by X-ray diffraction, chemical analysis, FTIR spectroscopy, BET surface area and pore size distribution measurement. The X-ray diffractograms of the samples were recorded on a Regaku D MAX III VC diffractometer with CuK. radiation between 2 and 10^ (26) with a scanning rate of 2^/min. Chemical analyses were done on a Carlo Erba EA1108 elemental analyzer. The specific surface areas of the extracted samples were determined by BET method using the adsorption of N2 measured with Omnisorb CX-100. Prior to the adsorption experiment, all the samples were activated at 100^ C for 6 h at 10 "^ Torr. FTIR spectra in the 400-4000 cm"' range were recorded on and Shimadzu FTIR-8201 PC (in Nujol on KBr disc technique). Solid state '^C CP MAS NMR experiments were carried out at a Bruker's high resolution MSL-300, which works at 7.5 MHz for '^C nucleus. The 71 / 2 pulse of 5 |a second was used. Magic
285 angle spinning measurements were done at 2.49 KHz and Hartmann - Hann match conditions were adjusted using adamantane. Chemical shifts of the various carbon resonances were then assigned with respect to adamantane. Table 1 : Chemical Composition of the Synthesis Gel Mixture of Organo-MCM-41 Samples Sample A : HS-MCM-41 B:: H2N-MCM-4I C;: HS-H2N-MCM-4I D : SH-Al-MCM-41 E: Vinyl-MCM-41
TEOS 2.5 2.5 1.0 1.0 1.0
APIS 1.0 0.125 -
Molar Gel Composition MPTS VTES CTMA NaOH 0.96 0.42 0.96 1.0 0.42 0.38 0.125 0.17 0.38 0.1 0.17 0.38 0.1 0.15
H2O 272 272 109 109 125
MeOH 66 66 26 26 -
3. RESULTS AND DISCUSSION XRD pattern of all the samples exhibited weak 110, 200 lines along with main 100 reflection. This indicates that all the samples have high degree of hexagonal ordering. The Table 2 contains the theoretical and as well as the observed C, H, N and S values of all the samples. The theoretical C, H, N and S values were calculated according to the initial gel composition of the samples. The chemical analysis of the respective samples, after extracting out surfactant, confirms the presence of respective organic functionalities in the respective organo-MCM-41 mesoporous materials. Further, it may be inferred from the data given in Table 2 that the % incorporation of organic ftmctionalities in the solid ranges between 80-90 % of the initial moles taken in the gel. However, the mole percent distribution of C, H and S/N in the solid was found to be comparable with corresponding theoretical values indicating that the organic group remains intact during synthesis. Further, it is interesting to note that the molar ratio of 3-mercapto-propyl and 3-aminopropyl remains same (1:1) in the solid also indicating that there is no appreciable impact of the nature of the organic ftinctional group on their incorporation in the solid product. The surface area and the pore size distribution of organo-MCM-41 samples are given in Table 3. All the samples have surface area in the range 510 - 840 m^g'V The highest surface area was observed in the case of the sample-A (SH-Si-MCM-41, 840 m^g'^) and lowest was obtained in the case of sample-C (SH-NH2-MCM-4I, 510 m^g') with all other sample having their surface area in the intermediate range. The pore size distribution shows that they have pore diameter in the range of 20 - 40 A. All materials were reproducible with respect to ordering, pore size and surface area under the synthesis conditions. The ^^C CP MAS spectra of the sample-A show the presence of the most prominent peak at 27.2 ppm for Ci carbon atom adjacent to the SH group and C2 carbon atom of the 3-mercaptopropyl group. There was another minor intensity broad peak at around 22.3 ppm which is assigned to Ci and C2 carbon atoms of the dipropyl disulfide. The presence of dipropyl disulfide in the sample A may be due to oxidative dehydrogenation of two adjacent thiol groups leading to the formation of disulfide (S-S) group. An unresolved shoulder down field to the C3 carbon of the thiol was observed for the C3 carbon of the dipropyl disulfide. Similar results were obtained by Lim et al. [6].
286 However, sample-B exhibited three '^C peaks at 9.8, 20.8 and 42.6 ppm due to the presence of three different carbon environments of n-propyl-NH2-MCM-41. Table 2: Theoretical and observed organic chemical composition of the samples Samples A: SH-MCM-41 B: NH2-MCM-4I C: SH-NH2-MCM-4I D: SH-Al-MCM-41 E: Vinyl-MCM-41
Theoretical, mmole per g!solid S C H N 10.52 3.54 24.6 11.19 3.73 29.8 8.18 1.36 1.36 20.5 3.94 1.31 9.2 2.91 4.4
Experimental, mmole per g solid C H N S 22.5 3.21 9.62 26.1 3.24 9.69 16.5 1.09 1.10 6.54 8.0 1.13 3.37 4.1 2.55
Table 3 : BET surface area and pore size distribution of organo-MCM-41 samples Samples A : SH-MCM-41 B : NH2-MCM-4I C : SH-NH2-MCM-4I D : SH-Al-MCM-41 E : Vinyl-MCM-41
Pore Size (A) 40 24 35 38 35
BET Surface Area (m^g'^) 840 550 510 673 775
4. CONCLUSIONS In this communication we have reported the synthesis and characterization of different types of surface modified MCM-41 type pure-silicate and alumino-silicate materials. Simultaneous incorporation of thio- and amino-propyl groups was also achieved. There is tremendous potential in this field as these materials itself can be used as supra-molecular ligand in complexation with a metal ion. They can also be potentially used as host for different nano-cluster stabilization. REFERENCES 1. C.T.Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olsen, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J.Am. Chem. Soc, 114 (1992) 10834. 3. P. Mukherjee, R. Kumar and U. Schuchardt, Stud. Surf. Sci. Catal., 117(1998) 351. 4. R. Kumar, A. Bhaumik, R.K. Ahedi and S. Ganapathy, Nature, 381(1996) 298. 5. M. H. Lim, C. F. Blanford and S. Stein, J. Am. Chem. Soc, 119 (1997) 4090. 6. M. H. Lim, C. F. Blanford and S. Stein, Chem. Mater., 10 (1998) 467. 7. S.C. Laha, P. Mukherjee and R. Kumar, Bull. Mater. Sci., 22 (1999) 623 8. S.L. Burkett, S.D. Sims and S. Mann, Chem. Commun. 1367 (1996) 9. D. J. Macquarrie, Chem. Commun., (1996) 1961. 10. L. Mercier, T. J. Pinnavaia, Adv. Mater., 9 (1997) 500.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B. V. All rights reserved.
287
Phenyl-functionalized silicate mesophases with hexagonal or cubic symmetries: influence of synthesis parameters. Valerie Goletto\ Marianne Imperor^, Florence Babonneau^* 'Chimie de la Matiere Condensee, UPMC-CNRS, 4 place Jussieu, 75252 Paris cedex 05, France (*E-mail:
[email protected],fr) "Physique des Solides, Bat. 510, Universite Paris-Sud-CNRS, 91405 Orsay Cedex, France
Organically modified porous silicates have been prepared under acidic conditions, by direct reaction of a mixture of phenyltriethoxysilane (PTES) and tetraethoxysilane (TEOS), and an aqueous solution of cetyltrimethylammonium bromide (CTAB). For a 1:4 molar ratio between PTES and TEOS, the hexagonal (2d, p6m) phase, but also a cubic phase analogous to the already reported SBA-1 phase (PmSri), can be prepared. The surfactant can then be efficiently removed by calcination at 350°C, leading to phenyl-functionalized microporous silicates with two types of architecture. The influence of several parameters (PTES/TEOS ratio; ethanol content) that affect the organization of the samples, will be discussed.
1. INTRODUCTION Many applications of silica-based nanoporous materials such as adsorption, ion exchange, catalysis and sensing, require specific surface properties. Since the discovery of the MCM series of mesoporous materials [1], two main methods have been used to functionalize their large internal surface [2]. The first one consists in a covalent grafting of organic entities, R, via organotrialkoxysilanes (RSi(0R')3), on the pore walls of a silica-based mesostructured network. It was already used to incorporate sensing [3,4] as well as catalytic functionalities [5,6] into MCM supports. One main advantage of this method is a good control of the organization of the silicate phase, based on published procedures which have been established to prepare well-ordered hexagonal MCM-41 [7] or cubic MCM-48 [8] type phases. But, one disadvantage is a possible low efficiency of the grafting and inhomogeneous distribution of the R groups, which will depend on the reactivity of the precursors and the accessibility of the silanol groups. The second method consists in the direct introduction of the organic functions during the synthesis of the templated network, via cocondensation of organotrialkoxysilanes (RSi(0R')3) and tetraalkoxysilanes (Si(0R')4), in presence of the structuring agent. An increasing number of publications are now dedicated to this one-pot synthesis method. A variety of organic functionalities have been successfully introduced such as phenyl [9-11], vinyl [12], mercaptopropyl [13,14], aminopropyl [14,15], methacryloxypropyl [16] and glycidoxypropyl
288 [14] groups. Introduction of two functionalities has even been reported [17]. High loading (R/Si up to 0.25 molar ratio) and homogeneous distribution of organic functions can be reached. One may however face some difficulties during sample preparation: (i) the removal of the surfactant should not affect the R groups. Calcination techniques can only be used in specific cases, when the Si-C bond is thermally stable above 400°C; this is the case of phenyl groups as already shown by '^C CP MAS NMR [10]. Most of the time, solvent extraction techniques are used, that may disrupt the ordered mesostructure [9,10,14]. An interesting study of the effect of various template removal methods on the organization of mesoporous methacrylate hybrid systems has been published [16]. (ii) the organic functions may not be necessarily all located at the pore surface, especially for high loading of functional groups. However, results of bromination reactions performed on vinyl-[12] and methoxyloxypropyl-[16] functionalized samples suggest a very good accessibility of these groups. Small angle neutron scattering experiments, using contrast matching techniques, seem also to confirm the location of the vinyl groups on the internal channel surfaces [18]. This may be due to the hydrophobic character of most of the R groups, that will develop specific interactions with the amphiphilic template molecules, and be pushed to react at the surfactant/silicate interface. Consequently, the organic functions introduced via an organotrialkoxysilane may play an important role in the self-assembly mechanism. This was recently illustrated with phenylmodified silicates prepared under acidic conditions (HCl) with CTAB as template. The experimental conditions were those published for the synthesis of the hexagonal (2d, p6m) SBA-3 phase [19]. When phenytriethoxysilane (PTES) and tetraethoxysilane (TEOS) in a 1:4 molar ratio, are first prehydrolyzed in presence of ethanol, and then reacted with an acidic solution of cethyltrimethylammonium bromide (CTAB), the expected 2d hexagonal phase is obtained [10]. But when the mixture of PTES and TEOS directly reacts with the CTAB solution, another phase is obtained [20], whose X-ray pattern matches the one already reported for the SBA-1 cubic silicate phase [21,22] and assigned to the PmSn space group. Most of the one-pot syntheses of organically functionalized mesoporous silicates have been done under basic conditions. Only hexagonal phases (2d, p6m) were reported so far, except one very recent example of a phenyl-functionalized cubic phase [17], analogous to the bicontinuous MCM-48 phase (Ia3d) [8]. The cubic phase prepared under acidic conditions from PTES and TEOS is indeed related to a different type of cubic mesophases, micellar mesophases, reported in the literature for various surfactant/solvent systems [23] as well as for lipid-containing systems [24]. In this paper, we will investigate different synthesis parameters (PTES/TEOS and EtOH/Si ratios) to see how they influence the architecture of the mesophase. 2. EXPERIMENTAL SECTION Preparation A : TEOS (Fluka) and PTES (Fluka) in a 4:1 molar ratio are mixed with an acidic solution of CTAB (CTAB/Si = 0.12; HCl/HjO/Si = 9.2/130/1), and stirred for several hours. The precipitate is then filtered, rinsed with distilled water and dried at 100°C.
289 Preparation B : TEOS and PTES in a 4:1 molar ratio are first pre-hydrolyzed for 1 h at room temperature in ethanol (EtOH/Si = 1:1; H20/Si = 1; pH (HCl) = 1.2). Then the solution is reacted with the solution of CTAB (same as for A). The following steps are similar. The low angle X-ray diffraction patterns were recorded either at LURE, the French synchrotron facility, with a wavelength of X = 1.2836 A in a transmission mode, or with a Philips diffractometer using the CuK„ wavelength in a reflection mode. 3. RESULTS 3.L Identification of the mesophases Two procedures have been used to prepare phenyl-functionalized silicate mesophase under acidic conditions. The only difference between them, is the pre-hydrolysis step, which was added in preparation B to initiate the hydrolysis and condensation reactions of the two precursors, prior to reaction with the CTAB solution. This step may cause the formation of co-condensed species between PTES and TEOS, and thus prevent phase separation [25]. The X-ray patterns of the samples obtained for PTES/TTEOS = 1:4 with these two synthesis procedures are presented in Figure 1. For preparation A, the pattern is similar to that reported for the cubic SBA-1 phase and assigned to the PmSn space group [19]. All the peaks can be indexed, and lead to an average cell parameter of 83.8 A. Preparation B gives the usual pattern due to a two-dimensional {p6m) hexagonal phase, with a cell parameter of 39.5 A. Presence of the two Si sites in a 1:4 ratio has been confirmed by ^^Si MAS-NMR [10,20]. (b) - Preparation B
(a) - Preparation A
A, 3
*10
4 2 theta (')
Figure 1. X-ray diffraction patterns of the as-synthesized phenyl-functionalized silicate mesophases obtained from preparations A (a) and B (b) (PTES/TEOS = 1:4). Data recorded with synchrotron radiation (k = 1.2836 A). The formation of the cubic phase is related to the presence of phenyl groups: under the same synthesis conditions, with TEOS as unique inorganic precursor, only hexagonal phases have been prepared. But it also depends on other parameters since for a given PTES/TEOS molar ratio, the hydrolysis step plays a major role. We have thus tried to better understand the parameters that control the formation of the cubic phase, by changing the PTES/TEOS as well as the EtOH/Si molar ratios.
290
Figure 2: X-ray diffraction patterns of samples obtained with various PTES/TEOS ratios, following preparation A. Data recorded with a conventional Xray source.
Figure 3: X-ray diffraction patterns of samples obtained with various added EtOH amounts, following preparation A. Data recorded with a conventional X-ray source.
3.2. Influence of the PTES/TEOS molar ratio Samples have been prepared with PTES/TEOS molar ratios ranging from 1:10 to 5:1, and following method A. ^^Si MAS-NMR spectra have confirmed the incorporation of phenyl siloxane units in the silicate framework, according to the starting stoichiometry. For the two highest PTES amounts (2:1 and 5:1), gel-like samples are formed, which do not exhibit any long-range order. For PTES/TE0Sl, the Xray pattern shows one main peak, that could be assigned to a hexagonal phase. Indeed, one can observe a continuous increase in the amount of hexagonal phase with increasing EtOH/Si ratio. 3.4. Influence of alcohol Ethanol when added directly to the PTES/TEOS mixture is thus highly favoring the formation of a hexagonal phase with respect to the cubic phase. However, even if ethanol is not directly added to the precursor mixture, it is produced during the synthetic procedure via hydrolysis and condensation reactions of PTES and TEOS. One can now wonder which results will be obtained if methoxysilanes rather than ethoxysilanes are used. A sample has been prepared starting from phenyltrimethoxysilane (PTMS) and tetramethoxysilane (TMOS) in a 1:4 molar ratio. The corresponding X-ray pattern shows the presence of a hexagonal phase with no evidence of cubic phase formation. These results suggest that the presence of methanol is favoring the unique formation of hexagonal phases. 4. DISCUSSION The results obtained in the present study are summarized in Table 1. The amount of phenyl groups introduced in the silicate framework and the nature and amount of alcohol present in the reaction mixture play a major role in the formation of the cubic versus hexagonal phase.
292 Table 1 Results of the X-ray preparation A. Alkoxide precursors TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS PTES TEOS TMOS PTMS TMOS
investigations performed on various samples obtained according to
1:10 1:5 1:4 1:3 1:2 1:1 2:1 2:1 1:4 1:4 1:4 1:4 1:4 1:4
Solvent None
ROH/Si
EtOH EtOH EtOH EtOH EtOH None None
0.25:1 0.5:1 0.75:1 1:1 2:1
Phase Hexagonal Hexagonal Hexagonal + Cubic Cubic Hexagonal + Cubic Hexagonal Hexagonal Amorphous Amorphous Hexagonal + Cubic Hexagonal + Cubic Hexagonal + Cubic Hexagonal Hexagonal Hexagonal Hexagonal
In the case of pure silicates, this cubic phase corresponding to the Pm3n space group, had not been obtained with CTAB, but with cetyltriethylammonium bromide (CTEAB) in HCl media and called SBA-1 [19,26]. It has been described as a cage-like structure with open windows, providing three-dimensional channel connectivity. For certain applications, this architecture is more attractive than the one-dimensional channels found in hexagonal mesophases. Cubic mesophases of space group PmSn have been reported for lipid systems and their structure was largely discussed in the literature [27]. A study by freeze-fracture electron microscopy agreed with a micellar model consisting of two sets of micelles [24,28]. However, a direct transfer of this model for the silicate mesophases is not straightforward. Elimination of the surfactant requires the micelles to be connected, and a precise description of the channel connectivity has not yet been reported. These micellar cubic mesophases require large surface curvature and low charge density. Their formation is thus favored by the use of surfactant molecules with large polar head group, and acidic conditions under which the charge density at the silicate/surfactant is always limited. The fact that this phase can be prepared with CTAB when PTES is present, suggests the existence of specific interactions between the phenyl groups and the polar head of the surfactant molecules. It was indeed reported that benzene molecules are preferably located at the hydrophilic-hydrophobic interface [29]. When the PTES/TEOS ratio exceeds 1:4, a hexagonal mesophase is again formed. SEM observation of the 1:1 powder shows two types of grain morphology: aggregates of facetted
293 particles and smooth spherical grains. Their well-defined round shape suggests that a melting phenomenon may have occurred during the sample preparation. This should be related to the behavior observed for the X-ray amorphous samples prepared with high concentration in phenyl groups (2:1 and 5:1 compositions), which melt at 100°C. The two grain morphologies might be due to a phase separation between an amorphous phenyl-rich phase and a hexagonal phenyl-poor phase. Further investigation will be performed to confirm this hypothesis. The role of ethanol in the formation of the cubic phase is intriguing. Increasing amount causes the change from cubic to hexagonal phase. Indeed similar effect has already been reported for pure silica phase [19]. Addition of 'AmOH to the synthesis procedure of SBA-1 gives a hexagonal phase. The authors point out the fact that these hydrophobic organic compounds co-dissolve with the surfactant assemblies and contribute most strongly to the hydrophobic chain volume. Ethanol, as polar organic additive, is solubilized by the micelle and located in the palisade layer, between the head group and the hydrophobic core [19]. Increasing amount results in the formation of the low surface curvature mesophase. The use of TMOS instead of TEOS did not lead to any cubic phase. This may be due to the different properties of methanol compared to ethanol. This solvent is highly polar and hydrophobic and does not penetrate the micelle surface [19]. 5. CONCLUSION Organically-modified porous silicates have been prepared under acidic conditions, by direct reaction of phenyltriethoxysilane (PTES) and tetraethoxysilane (TEOS) with an aqueous solution of cetyltrimethylammonium bromide (CTAB). For a 1:4 molar ratio between PTES and TEOS, cubic SBA-1 type phase with the PmSn space group has been prepared. A lower or higher amount of phenyl groups causes a change to hexagonal mesophases. The cubic SBA-1 type-phase had never been obtained before with CTAB. This micellar phase, characterized by a high surface curvature, is stabilized by the presence of a large polar head. These results suggest the existence of specific interactions between the phenyl groups and the polar head of the surfactant molecules. Increasing amount of ethanol introduced in the synthesis procedure leads also to the formation of hexagonal phases. Ethanol, as polar organic solvent, may co-dissolve with the surfactant assemblies, contribute to an increase of the chain volume and favor the formation of low surface curvature mesophase. 6. ACKNOWLEDGMENTS The authors would like to acknowledge Prof. B.F. Chmelka (Dept. of Chemical Engineering, UC Santa Barbara) and Prof. G.D Stucky (Dept. of Chemistry, UC Santa Barbara) and their groups for very helpful discussions. This work is part of a CNRS/NSF collaborative exchange program.
294 REFERENCES 1.
J.S. Beck, J.C. Vartuli, WJ. 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) 0834 . 2. K. Moller and T. Bein, Chem. Mater., 10 (1998) 2950. 3. L. Mercier and T.J. Pinnavaia, Adv. Mater., 9 (1997) 500. 4. J. Liu, X. Feng, G.E. Fryxell, L.-Q. Wang, A.Y. Kim, M. Gong, Adv. Mater., 10 (1998) 161. 5. A. Cauvel, G. Renard, D. Brunei, J. Org. Chem., 62 (1997) 749. 6. Y.V. Subba Rao, D.E. De Vos, P. A. Jacobs, J. Chem. Soc. Chem. Comm., (1997) 355. 7. M. Griin, K.K. Unger, A. Matsumoto, K. Tsutsumi, Microporous and Mesoporous Materials, 27 (1999) 207. 8. J. Xu, Z. Luan, H. He, W. Zhou, L. Kevan, Chem. Mater., 10 (1998) 3690. 9. S.L. Burkett, S.D. Sims, S. Mann, J. Chem. Soc. Chem. Comm. 1996, 1367. 10. F. Babonneau, L. Leite, S. Fontlupt, J. Mater. Chem. 9 (1999) 175. 11. CM. Bambrough, R.C.T. Slade, R.T. Williams, J. Mater. Chem. 8 (1998) 569. 12. M. H. Lim, C.F. Blanford, A. Stein, J. Am. Chem. Soc, 119 (1997) 4090. 13. M. H. Lim, C.F. Blanford, A. Stein, Chem. Mater., 10 (1998) 467. 14. C.E. Fowler, S.L. Burkett, S. Mann, J. Chem. Soc. Chem. Comm. (1997) 1769. 15. D.J. Macquarrie, D.B. Jackson, J.E.G. Mdoe, J.H. Clark, New J. Chem. 23 (1999) 539. 16. K. Moller, T. Bein, R.X. Fischer, Chem. Mater., 11 (1999) 665. 17. S.R. Hall, C.E. Fowler, B. Lebeau, S. Mann, J. Chem. Soc. Chem. Comm. 1999, 201. 18. M. Lim and A. Stein, Mat. Res. Soc. Symp. Proc, 519 (1998) 89 19. Q. Huo, D.I. Margolese, G.D. Stucky, Chem. Mater. 8 (1996) 1147. 20. V. Goletto, V. Dagry, F. Babonneau, Mat. Res. Soc. Symp. Proc. (in press) 21. Q. Huo, D.I. Margolese, U. Ciesla, D.G. Demuth, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schuth, G.D. Stucky, Chem. Mater., 6 (1994) 1176. 22. M.J. Kim, R. Ryoo, Chem. Mater., 11 (1999) 487. 23. X. Auvray, M. Abiyaala, P. Duval, C. Petipas, I. Rico, A.Lattes, Langmuir, 9 (1993) 444. 24. H. Delacroix, T. Gulik-Krzywicki, P. Mariani, V. Luzzati, J. Mol. Biol., 229 (1993) 526. 25. L. Delattre, F. Babonneau, Mat. Res. Soc. Symp. Proc. 346 (1994) 365. 26. M.J. Kim, R. Ryoo, Chem. Mater., 11 (1999) 487. 27. J. Charvolin, J.F. Sadoc, J. Phys. France, 49 (1988) 521. 28. V. Luzzati, R. Vargas, P. Mariani, A. Gulik, H. Delacroix, J. Mol. Biol. 229 (1993) 540. 29. A. Firouzi, D.J. Schaefer, S.H. Tolbert, G.D. Stucky, B.F. Chmelka, J. Amer. Chem. Soc. 119(1997)9466.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
295
Covalent attachment of dye molecules to the inner surface of MCM-41 Yven Rohlflng^ Dieter Wohrle^ Michael Wark^ Gunter Schulz-Ekloff*^, Jiri Rathousky^ and Amost Zukal'^ ^ Institute of Organic and Macromolecular Chemistry, University Bremen, D-28334 Bremen, Germany b
Institute of Apphed and Physical Chemistry, University Bremen, D-28334 Bremen, Germany J. Heyrovsky Institute of Physical Chemistry, Academy of Science of the Czech Republic, CZ-182 23 Prague 8, Czech Republic
Ordered mesoporous silicas with functionalized surfaces open the way for covalent bonding of organic molecules. As host material pure silica of the MCM-41 type was used. The Si-MCM-41 molecular sieves were synthesized by the recent method of homogenous precipitation yielding a highly uniform particle shape and pore ordering. In a first step the silanol groups on the outer surface of the particles were saturated with diphenyldichlorosilan. As a precursor for covalent bonding 3-aminopropyl-triethoxysilan (APTES) was grafted on the inner surface of the host material. Subsequently dye molecules (rhodamine B sulfonylchloride, 4'-dimethylaminoazobenzene-4-carbonic acid) were anchored into the host material. The covalent bonding to the amino group was activated by pyridine and dihexyldicarbodiimide, respectively. The progress in pore modification was pursued by nitrogen sorption measurements and by diffuse reflectance UV/vis and FTER spectroscopy. 1. INTRODUCTION Dye molecules encapsulated in host materials have interesting application potential, e. g., in data storage, quantum electronics and non-linear optics, due to their novel properties resulting from host-guest interactions, an increased diffusion stability and a higher degree of organization of molecular dipoles. Compared with sol-gel monolithic or organic polymeric matrices, inorganic porous frameworks, such as zeolites and mesoporous molecular sieves, do not suffer from phase separation and thermal relaxation problems and offer a high chemical and mechanical stability. In small pore zeolites with cage structure, e. g., faujasites, dye molecules encapsulated by in situ synthesis or crystallization inclusion are stable against extraction.^' ^ However, these methods fail for MCM-41 due to the channel structure and the wider pore diameter (3 nm) of the host material. Covalent bonding of guests is necessary to obtain diffusion stability. Therefore, anchoring of organic molecules with catalytic functions into MCM-41 by covalent bonding was recently reported by Brunei et al.^
296 This investigation aims for the first time at the stabilization of photoisomers of photochromic dye molecules in molecular sieves by modification of the MCM-41 surface and covalent bonding of dye molecules. Diphenyldichlorosilane is utilized for ;7r^-silylation to warrant an anchoring of the dyes exclusively inside the MCM-41 channel structure. 2. EXPERIMENTAL SECTION 2.1. Synthesis of siliceous MCM-41 26.1 g of n-cetyltrimethylammonium bromide (CTABr, ALDRICH) were dissolved in 4 L of bidistilled warm water in a 5 L polypropylene vessel. After about 30 min of stirring a clear solution resulted. 26.7 g of sodium metasilicate (Na2Si03, ALDRICH) were added and the whole mixture was stirred until it became homogeneous. Subsequently 40 mL of ethyl acetate (FLUKA) were quickly added under intense stirring. After 15 sec the stirring was stopped, the vessel was shut and remained without further agitation for 20 h at ambient temperature and 40 h at 363 K. After filtration of the hot reaction mixture the obtained precipitate was extensively washed with bidistilled water and ethanol. The solid was dried and the o^anic component was removed by calcination in air at 873 K for 20 h (heating rate: 2 K/min). 2.2. Silylation of MCM-41 All the silylation reactions were carried out with freshly dried MCM-41 material under nitrogen atmosphere, and all solutions were distilled over desiccants under inert gas. 2.2.1. Pr^-Silylation of the external surface of MCM-41 Approximately 1 - 1.5 g of MCM-41 were given in a flask and capped with a stopper. Subsequently the flask was evacuated (10'^ mbar) for 2 h and filled with nitrogen. The molecular sieve sample was suspended in 30 - 50 mL dry tetrahydrofurane (THF), and 25 fiL - 600 |iL of diphenyldichlorosilane (Ph2SiCl2) were added under stirring. After 45 min the solid was filtered and extensively washed with THF and dichloromethane. Afterwards it was dried in a heating box and evacuated (10"^ mbar) for 2 h. 2.2.2. Functionalization of the inner surface of MCM-41 The functionalization was performed either in dichloromethane or in toluene depending on the desired reaction temperature. Silylation in dichloromethane suspension. The samples (0.5 - 1.5 g) were suspended in 25 - 50 mL of dichloromethane and this suspension was precooled in an ice/NaCl mixture (258 K). Silylation was obtained by adding 0,01-7 mmol/g MCM-41 of APTES at 258 K for 2 h. Afterwards the reaction mixture was allowed to reach ambient temperature. After further stirring for 22 h the surplus APTES was removed by extensive washing of the solid with dichloromethane and diethylether. The obtained solid was dried as described in 2.2.1. Silylation in toluene suspension. The procedure is similar to the one described in the preceding paragraph with the exception that the toluene suspension was heated to reflux under stirring for 1 h. For vigorous washing toluene and diethylether were used. 2.3. Covalent bonding of dye molecules Anchoring of 4'-dimethylaminoazobenzene-4-carbonic acid 1. An amount of 3aminopropylsilyl-MCM-41 (0.5 - 0.7 g) was mixed in a flask with a desired quantity of 1 (0.001 - 1 mmol), dried in the same way as described before and suspended in 30 mL dichloromethane. The suspension was cooled in an ice/NaCI mixture (258 K) and stirred for
297 1.5 h. Dicyclohexylcarbodiimide (DCC) in dichloromethane was added as a catalyst for the activation of 1. The reaction mixture was allowed to reach ambient temperature and after 20 h of stirring it was filtered and vigorously washed with dichloromethane to remove DCC and non-reacted 1. The obtained modified MCM-41 was thoroughly washed with ethanol in a Soxhlet apparatus for 3 d and dried as described before. Anchoring of rhodamine B sulfonylchloride 2. Anchoring the rhodamine dye was carried out under similar conditions as in the preceding paragraph. A suspension of 3aminopropylsilyl-MCM-41 and 2 in 40 mL dichloromethane was precooled and after 1.5 h an excess of the catalyst pyridine was added. The reaction was finished after 20 h of stirring and the recovered solid extensively washed and subjected to a Soxhlet treatment. 2.4. Characterization Texture parameters were investigated by nitrogen sorption measurements with a MiCROMERmcs ASAP 2010. The external and internal surface area and mesopore volume were determined using a comparison plot. The mesopore size was calculated by a geometrical method.^ The samples were degassed overnight at 473 K. XRD patterns were recorded on small-angle X-ray diffractometer PHILIPS X'pert Alpha 1. Diffuse reflectance UV/vis spectra were obtained with a PERKIN-ELMER Lambda 9 spectrometer. The samples were also examined by DRIFT (diffuse reflectance infrared fourier transform) spectroscopy with an BioRAD FTS-60A instrument equipped with a homemade reflection chamber containing an open sample holder. 3. RESULTS AND DISCUSSION 3.1. Dye anchoring Precursors for the grafting of the dyes can be anchored by different ways. In one route they are added to the structure directing surfactant and are encapsulated during the condensation process of the MCM-41 material.^ In an alternative route a preformed MCM-41 is modified by silylation.^ This paper reports an anchoring via the latter route. Since the target of the study is the anchoring of the chromophores at the internal surface exclusively, a /7r^-silylation of the external surface is carried out, based on the experience of a preferred reactivity of the external silanol groups for this process. The selective silylation can result from (i) diffusion limitation and (ii) various nature of silanol groups on internal and extemal surface. With respect to the size of Ph2SiCl2 it can be supposed that there is a difference in inner and outer surface.
Scheme 1. Silylation reactions on MCM-41 surface: a) silylation of the outer surface by diphenyldichlorosilane in THF at ambient temperature and b) functionalization of the inner surface with APTES in CH2CI2 at ambient temperature and toluene at reflux. APTES is not neccessarily bonded on three silanol groups.
298
Scheme 2. Functionalization of the inner surface of MCM-41: a) grafting of 1 in CH2CI2, activated by DCC, and b) grafting of 2 in CH2CI2, activated by pyridine. The preceding silylation has to be selected such, that these groups are blocked prior to functionalization (Scheme 1).^ Depending on the structure of the dye molecule, which shall be immobilized, the grafting precursor has to contain an active group which enables the desired coupling reaction. For this purpose silane with an amino ligand has been chosen. For the amide coupling (Scheme 2) the carbonic acid group in 1 has to be activated with DCC and the reaction with sulfonylchloride 2 with pyridine.
Figure 1. XRD patterns of MCM-41: a) calcined material, b) pr^-silylated and c) functionalized with APTES. 3.2. Characterization Low-angle powder X-ray diffractometry. In Figure 1 XRD reflection patterns of a non silylated calcined Si-MCM-41 (a), an one which is /7r^-silylated (b) and an one silylated with APTES (c) are compared. The calcined and the outer surface silylated MCM-41 show the characteristic reflexions. The intensities of the reflections in the functionalized sample are reduced as expected from an increased scattering for filled pores.^ The unessentially changed d spacings confirm the preserved structure.^^
299 FTIR-spectroscopy. In Figure 2 the DRIFT spectra of differently treated MCM-41 samples are shown. The sharp absorption band of the parent material (a) at 3745 cm'^ is attributed to free silanol groups whereas the broad band at ca. 3540 cm'^ is ascribed to hydrogen-bonded silanol groups.^^'^^ ^ Due to modification the free silanol tu groups disappear almost completely, whereas the band of hydrogen-bonded silanol groups is shifting to lower wavenumbers. Thus, the free silanol groups are more susceptible to silylation. Spectrum b) in Figure 2 indicates 5000 4500 4000 3500 3000 2500 2000 1500 the specific silylation of the outer -1 Wavenumber cm surface with diphenyldichlorosilane, Figure 2. DRIFT spectra of MCM-41: a) calcined, since even after addition of high b) outer surface silylated, c) silylated in amounts of the silylation agent the dichloromethane and d) azo dye grafted. intensity of the band at 3745 cm' decreases only slightly, indicating that this agent does not react with the inner surface silanol groups. The anchoring of organic moieties is characterized by the sharp absorption at ca. 2940 cm' and the shoulder peak at ca. 2875 cm'^ being ascribed to the aminopropyl function.^^ Also the peaks in the range 1600 - 1500 cm'' are signifying the presence of amine and amide functions. Nitrogen adsorption. MCM-41 source material as well as silylated or dye-anchored samples are exhibiting isotherms of the type IV according to lUPAC nomenclature (Figure 3). The isotherms are showing mono- and multilayer adsorptions on the pore walls (p/po < 0.2), a reversible step at p/po = 0.3 and p/po = 0.24, respectively, and multilayer adsorption on the outer surface of MCM-41 (p/po > 0.35). Because the isotherms are very flat in the last region Table 1 Texture parameters from adsorption isotherms, i. e, total surface area (Stot), external surface area (Sext), mesopore surface area(Sme), mesopore volume (Vme) and mesopore diameter (Dn^e).
MCM-41 pr^-silylated APTES-functionalized dye-grafted
:[mV']
Sext[mV']
1138 1151 764 717
89 115 110 89
Sn
: [mV^]
: [cmV^]
Dme [nm]
1049 1036 654 628
0.780 0.773 0.407 0.391
3.0 3.0 2.5 2.5
300
0,2
0,7
0,3
Relative pressure / p/p^ Figure 3. Nitrogen adsorption measurement: a) calcined MCM-41, b) outer surface silylated, c) APTES-functionalized material and d) grafted rhodamine dye. it can be concluded that the external surface is very small.^"^ Figure 3 and Table 1 demonstrate the typical changes in the isotherms and the texture parameters in dependence on several steps leading to dye anchoring. Two groups of isotherms can be distinguished belonging to (i) parent or pr^-silylated and (ii) APTES-functionalized and dye-anchored materials. Silylation of the outer surface with diphenyldichlorosilane does practically not influence the texture of MCM-41. As previously indicated by TEM micrographs only the external surface is effected.^ It is noteworthy, too, that the pr^-silylation is independent on the offered amount of silane (Table 2) and, thus, does not result in a bonding at the inner surface at ambient temperature. A shrinking of pore diameter by utilizing diphenyldichlorosilane is only observed at higher reaction temperatures. Table 2 Resistance of texture parameters to /7r^-silylation with diphenyldichlorosilane at ambient temperature: Silane loading (x), total surface area (Stot), external surface area (Sext), mesopore surface area (Smc), mesopore volume (Vme) and mesopore diameter (Dme)> X [nLg-^]
Stot [mV^l
Sext [mV^]
Sme [mV^]
Vn^ [cm^g']
Dme [nm]
0 67 150 600
1248 1176 1199 1173
95 89 72 70
1153 1087 1127 1103
0.828 0.793 0.786 0.775
2.9 2.9 2.8 2.8
301 "a
1.0-
b
550,^
553-p='4^
0.8-
/ / C 558 \\ d 565-A>. \\
0.6-
^
III
0.4-
U^ 0.2-
308
352 ^ ^
0.0-0.2-
-~:\
353
/
''^'^\
-0.4-
1
'
1
^ or*
g
./' 1 ^^^—' / 1
1
500
o
V.
1
1
600
Wavelength / nm Figure 4. Diffuse reflectance UV/vis spectra of MCM-41 a) with adsorbed rhodamine dye (0.0075mmol/g MCM-41), b) with covalently grafted rhodamine dye (0.01 m mol/g MCM41; 0.5 mmol APTES/g MCM-41) compared to rhodamine B sulfonylchloride c) in ethanol and d) in water. A significant alteration of pore parameters is found after covalent bonding of APTES. Since the aminopropyl groups decorate the inner surface, a shrinkage of the pore diameters from 3.0 nm to 2.5 nm is noticed. Furthermore, the mesopore filling step becomes less sharp with increasing organic content indicating a widening of the pore size distribution. Due to low loading the fmal dye grafting results only in a slight decrease of the pore volume. The results prove that functionalization of MCM-41 and anchoring of dye molecules result in a pore narrowing but that a high degree of ordering is being preserved. UV/vis reflectance spectroscopy. The main absorption band of the anchored rhodamine dye at 553 nm or of the azo dye at 441 nm, determined by the UV/vis reflectance spectroscopy, represents a blue-shift in comparison to spectra in aqueuos solution. In water (pH 7) the azo dye provides a band at 468 nm and the rhodamine dye a main absorption band at 565 nm. The blue-shift results from the chemical environment inside the pores of MCM-41, which is dominated by residual silanol and, especially, non reacted amino groups, and thus more basic compared to that in solution. The interaction of the dye molecules with the host is further confirmed by a broadening of the absorption bands of anchored dyes (e.g. rhodamine B sulfonylchloride, Figure 4) in comparison to the main bands of the free chromophores in solution. Blue-shifted and broadened spectra results also after physisorption of the dyes on functionalized MCM-41. In this context it has to be noticed that the reversible adsorption of the azo dye on non modified Si-MCM-41 leads to a red-shift of the main band to 503 nm. This indicates the importance of the chemical environment on the host-guest interactions. At low concentrations of the anchored rhodamine dye, the samples exhibit a strong fluorescence, which indicates the presence of individual, free chromophore moieties. To avoid
302
the formation of dye aggregates in case of the rhodamine dye, the loading has to be limited to 0.0075 mmol dye/g MCM-41. 4. CONCLUSIONS The surface silanol groups of MCM-41 silica allow a successful anchoring of organic dye molecules. The anchoring has no effect on the ordering of the MCM-41 material. After local pr^-silylation of the outer surface an exclusive functionalization of the pore surface conducting to a decrease of pore diameter is obtained. The dyes are homogeneously distributed at low loading, cannot be removed by Soxhlet extraction and interact strongly with the host material. 5. ACKNOWLEDGEMENT The authors gratefully acknowledge founding from the Bundesministerium fur Bildung und Forschung (BMBF, TSR - 085 - 97), Germany. We thank Dr. M. Wendschuh-Josties (Department of Crystallography, ¥B 5, University of Bremen) for the XRD measurements.
REFERENCES C. Schomburg, D. Wohrle, G. Schulz-Ekloff, Zeolites 1996, 77, 232. M. Bockstette, D. Wohrle, I. Braun, G. Schulz-Ekloff, Microporous and Mesoporous Mater. 1998, 23, 83. (a) D. Brunei, A. Cauvel, F. Fajula, F. DiRenzo, Stud. Surf. Sci. Catal 1995, 62, 749; (b) P. Sutra, D. Brunei, Chem. Commun. 1996, 2485. (a) G. Schulz-Ekloff, J. Rathousky, A. Zukal, Microporous Mesoporous Mater. 1999, 27, 273; (b) J. Rathousky, M. Zukalova, A. Zukal, J. Had, Collect. Czech. Chem. Commun. 1998,65, 1893. J. Rathousky, G. Schulz-Ekloff, A. Zukal, Microporous Mater. 1996, 6, 385. C.E. Fowler, B. Lebeaus, S. Mann, Chem. Commun. 1998, 1825. H. van Bekkum, K. R. Kloetstra, Stud. Surf. Sci. Catal. 1998, 777, 171 D. S. Shephard, W. Zhou, T. Mashmeyer, J. M. Matters, C. L. Roper, S. Parsons, B. F. G. Johnson, M. J. Duer, Ang^w. Chem. 1998, 770, 2847. (a) A. Ortlam, J. Rathousky, G. Schulz-Ekloff, A. Zukal, Microporous Mater. 1996, 6, 171; (b) B. Marler, U. Oberhagemann, S. Vortmann, H. Gies, Microporous Mater. 1996, 6, 375. T. Tatsumi, K. A. Koyano, Y. Tanaka, S. Nakata, Stud. Surf. Sci. Catal. 1998, 777, 143. J. Chen, Q. Li, R. Xu, F. Xiao, Angew. Chem. 1995, 707, 2898. X. S. Zhao, G. Q. Lu, A. K. Whittaker, G. J. Millar, H. Y. Zhu, J. Phys. Chem. B 1997, 707, 6525. X. S. Zhao, G. Q. Lu, J. Phys. Chem. B 1998, 702, 1556. C. P. Jaroniec, M. Kruk, M. Jaroniec, J. Phys. Chem. B 1998, 702, 5503.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
303
Preparation and Characterization of Metal-Chalcogenide/MCM-41 Complexes C. M. Kowalchuk, Y. Huang, J. F. Con-igan Department of Chemistry, The University of Western Ontario, London, Ontario, N6A 5B7, Canada. With the discovery of MCM-41 there has been much interest in exploring the materials chemistry of these nanoporous open frameworks. Our investigations into this area have focused on introducing the high nuclearity copper-tellurium cluster Cu6(TePh)5(PPh2Et)5 i into the hexagonal array of channels of MCM-41 with diameters of -3.0 nm. Thermogravimetric analysis, IR and UV-vis spectroscopy and powder X-ray diffraction have been performed to provide details of impregnation. Upon heating these materials at high temperature the loss of TePh2 and PPh2Et causes condensation of the cluster frame. Powder X-ray diffraction provides proof of this condensed material and copper telluride formed inside the channels of MCM-41. This process may lead to the formation of novel nanocomposite materials with new electric properties.
1. INTRODUCTION The production of semiconducting devices based on molecular electronics instead of bulk materials will potentially provide an enormous increase in computing speed and storage capacity [1,2]. The independent development of the chemistry of mesoporous silicates and metal chalcogenide clusters over the past decade has seen explosive growth and our interest lies in the union of these two fields; the encapsulation of novel metal chalcogenide clusters and their subsequent condensation into size limited semiconducting materials utilizing the nanopores of MCM-41 as a template. Much work has been carried out in the synthesis of semiconductor nanoclusters and colloids [3]. The use of the bissilylated chalcogen reagents E(SiMe3)2 (E = S, Se, Te) has proven to be an elegant and powerful entry into both colloidal and nanocluster materials [4]. Work by Fenske has demonstrated that a variety of metal-chalcogenide particles, stabilized with a tertiary phosphine ligand sphere, can be generated and structurally characterized. Monosilylated selenium and tellurium reagents, RESiMea, have also been shown to offer an entry into nanometer sized metal-chalcogenide particles via controlled chalcogenolatechalcogenide condensation reactions, via the elimination of ER2 [5]. It has recently been demonstrated that photochemical condensation of the copper tellurolate complex Cu6(TePh)5(PPh2Et)51 through the loss of TePh2 and phosphine ligands leads to the formation of condensed copper telluride cluster [Cu5o(TePh)2oTei7(PEtPh2)8]'*" [6]. Our efforts in this area are to exploit this method within the channels of the nanopouous MCM-41. MCM-41, a well investigated member of the M41s family, has one dimensional channels whose lengths are much greater than their diameter. These hexagonally symmetrical channels may be tailored in size from 1.5 to 10 nm diameter by changing the chain length of the surfactant used to prepare the materials [7]. The use of these channels to restrict growth
304
reactions to one direction has been demonstrated elegantly with the generation of polyanaline filaments [8] and ferrocenophane polymers [9]. 2. EXPERIMENTAL 2.1. Instrumentation Thermogravimetric analyses were performed on a Mettler Toledo TGA/SDTA851 analyzer using dynamic temperature program of 25-750°C with a heating rate of 10°C/min. Infrared spectra were recorded on a Bomem MB 100 FT-IR spectrometer. Powder X-ray diffraction patterns were obtained on a Rigaku diffractometer using Co Ka radiation (X= 1.799260 A). Single crystal X-ray diffraction data were obtained with an Enraf-Nonius Kappa CCD diffractometer. UV-vis spectra were recorded on a Gary 100 Bio Spectrophotometer. 2.2. Preparation of starting materials MCM-41 was prepared under hydrothermal conditions using literature procedures [7], with a cetyltrimethylammonimum bromide as a template. The identity of the product was confirmed by powder XRD. Cu6(TePh)6(PEtPh2)5 was prepared using the literature method [6]. The structure was verified using single crystal X-ray crystallography. 2.3. Atmospheric impregnation of the Cu-cluster I Calcined MCM-41 was dehydrated at 120°C under 10'^ torr for three hours. Different amounts of the copper tellurolate cluster 1 were then mixed with dehydrated MCM-41. The mixture was mechanically stirred under nitrogen at atmospheric pressure until a yellow homogeneous powder was obtained. TGA, PXRD and IR studies were then carried out. 2.4. Dynamic vacuum impregnation Homogeneous samples of two different cluster/MCM-41 ratios (1:4, 1:2) were obtained using the atmospheric procedure in 2.3. These samples were then heated at various temperatures (70, 110 and 120°C) at 10"^ torr. TGA, PXRD and IR were obtained. 2.5. Static vacuum impregnation Prepared homogeneous samples (2.3) were heated at various temperatures under an initial vacuum of 10'" torr and heated at 70, 110 and 120°C. TGA, PXRD, and IR measurements were performed. 2.6. Annealing copper telluride inside MCM-41 A sample of the highest loading percentage (19%) was prepared and then heated at 550, 750 and 950''C separately with a 0.5 hour duration under nitrogen and atmospheric pressure.
305 PXRD data were obtained at high angle to monitor the formation of copper telluride.
3. RESULTS AND DISCUSSION 3.1. Molecular structure of Cu6(TePh)6(PEtPh2)5 The molecular structure of 1 is illustrated in figure 1 [6]. Cluster 1 consists of six Cu and six Te centers that are shielded with a phosphine and aryl organic shell.
3.2. Thermogravimetric analysis TGA has proven to be an effective tool in monitoring the introduction of Cu6(TePh)6(PEtPh2)51 (Figure 2) in to MCM-41 and is our primary technique. Decomposition curves provide information about the loading features and percent impregnated. The channel diameter in MCM-41 was calculated from the dioo spacing to be approximately 3.4 nm, compared to cluster diameter of 1.6 nm obtained from single crystal X-ray diffraction data [6]. Characteristic loading decomposition curves of 1 are shown in Figure 2. For the pure cluster sample (Figure 2a), only one decomposition curve is observed. This result suggests that TePh2 and PPh2Et ligands of i are eliminated at the same temperature, accounting for the 70.8% weight loss observed (calculated 71.5%). For the purpose of comparison a mixture of copper cluster and silicalite-1 was also prepared. Since the channel sizes of silcalite-1 (0.5 nm) are much smallerthanthediameterofthecluster, Cu6(TePh)6(PEtPh2)5 can only be adsorbed on
Figure 1. Molecular structure of 1 in the crystal. Te atoms are illustrated as spheres with diagonal hashing, Cu atoms as spheres v^th horizontal hashing and P with vertical hashing.
306 the external surface of silcalite-1. Since no strong interactions between the outer surface of the host and copper cluster are expected, the decomposition curve should be very similar to the pure cluster. This was indeed observed (Figure 2b). TGA analysis of a sample of MCM-41 loaded with Cu6(TePh)6(PEtPh2)5 at 10°C under dynamic vacuum is shown in Figure 2c. The weight loss at low temperature is due to the cluster adsorbed on external surface since the observed decomposition temperature (IStf^C) is the same as that for pure 1 and as the cluster/siIicalite-1 mixture. The weight loss at higher temperature (250''C) results from the copper cluster loaded inside the MCM-41 channels. The TGA results indicate some impregnation at 70°C and under dynamic vacuum conditions, with cluster 1 being only partially loaded inside of MCM-41. A comparison of dynamic and static impregnation techniques 2d, 2e respectively suggests that a small quantity of 1 remains outside of the pores (2d) while with the former technique Figure 2e illustrates the MCM-41/cluster complex prepared by heating at llO'^C under static vacuum. It contains only one decomposition curve suggesting that all of 1 is loaded inside the MCM-41 channels with little on the external surface of MCM-41. The percent loading may also be calculated from these curves (Table 1). It is observed that loading is relatively time independent. Reaction temperature and the ration of 1:MCM-41 control the loading percent if a minimum heating time of 12 hours is allowed to elapse. In the
(a) (b)
^
(c)
(d) (e)
Figure 2. TGA decompositions 25-550^ C 107min (a) Pure Cu6(TePh)6(PPh2Et)5, Cu6(TePh)6(PPh2Et)5/silicalite-l heated at 70^C under dynamic vacuum, Cu6(TePh)6(PPh2Et)5/MCM-41 treated at 70T under dynamic vacuum, Cu6(TePh)6(PPh2Et)5/MCM-41 treated at 110°C under dynamic vacuum, Cu6(TePh)6(PPh2Et)5/MCM-41 treated at 1 lO^'C under static vacuum.
(b) (c) (d) (e)
307
Table 1 Percent Cu6(TePh)5(PPh2Et)5 Impregnation into MCM-41 (mass % Cu cluster) Atm. Temp. (% loading*)
70° C (% loading)
llO^C (% loading)
120° C (% loading)
4.4 5.1
-
-
-
-
9.1 10.9
10.1 11.2
18.8 19.0
-
15.0 16.1
18.2 19.1
19.2 19.4
Atmospheric 25% cluster 50% cluster Dynamic Vacuum 25% cluster 50% cluster Static Vacuum 25% cluster 50% cluster
5 pores of MCM-41 case of atmospheric N2 conditions, a maximum of 5% loading was observed and can be attributed to a small quantity of cluster at the entrance of the pores. Under dynamic vacuum (lO'"^ torr), impregnation increases to 11%. With an increase in temperature, higher loading was achieved for both the, dynamic and static vacuum methods with a maximum loading of 19% attained at 120°C.
3.3. Spectroscopic analysis Infrared spectroscopy can be used to probe the guest cluster inside the MCM-41 host. 3046 ^^^^ 3002
2963
2927 ^
(a)
3050 3000 2950 2900 2850 cm" Figure 3. IR C-H stretching region of cluster and cluster/MCM-41 complexes: (a) pure Cu6(TePh)6(PPh2Et)5, (b) Cu6(TePh)6(PPh2Et)5/MCM-41 treated at 110°C under dynamic vacuum
308 In particular, the C-H stretching vibrations should be most sensitive to the environment of the cluster because the C-H bonds of external phenyl and ethyl groups are located on the surface of the cluster sphere and may interact with the walls of the MCM-41. However, the frequencies of v(C-H) modes of Cu-cluster loaded inside MCM-41 are almost identical to those of pure i, indicating that the interaction between 1 and the MCM-41 host is weak. Despite having only weak interactions, clusters i do not desorb from their MCM-41 host under a purge of N2 gas, at room temperature (60 hours). The IR results also imply that the structural integrity of the cluster is maintained during impregnation as the same characteristic stretching pattern is observed for all samples. Photolysis of 1 and the i/MCM-41 complex provides evidence of cluster condensation in the pores of MCM-41. A darkening of the sample and a bathochromic maxima shift are observed for both samples. This parallels the solution chemistry of 1 whereby the photochemical reaction of 1 leads to the nanocluster [Cu5o(TePh)2oTei7(PEtPh2)8]'^'[6]Although it is unlikely the same reaction path is followed in our (solid) studies, the UV data are consistent with the formation of higher nuclearity species.
Degrees 26 Figure 4. PXRD patterns (all intensities are normalized to the dioo reflection intensity of calcined MCM-41. Low angle diffractions (2-15° 20): (a) pure MCM-41 aoi 4.44nm dioo: 3.84nm) (b) 2x magnification of pure Cu6(TePh)6(PPh2Et)5, (c) Cu6(TePh)6(PPh2Et)5/MCM-41 treated at TO^'C under dynamic vacuum (ao: 39.4nm dioo: 3.41nm), (d) Cu6(TePh)6(PPh2Et)5/MCM-41 treated at 110°C under static vacuum (ao: 4.21nm dioo: 3.65nm)
309
3.4. PXRD analysis Low angle PXRD (Figure 4) is a powerful technique for the analysis of both MCM-41 and Cu6(TePh)6(PEtPh2)5. The powder pattern of 1 (3b) displays strong reflections (hm and h202) indexed from the calculated powder pattern. MCM-41 has the characteristic diffraction pattern (hioo, hno, h2oo) with a calculated dioo of 4.4 nm. The mixture of 1 and MCM-41 (figure 3a) gives rise to both characteristic diffraction patterns. The observed hm and h202 reflections of 1 are due to the clusters adsorbed on the external surface of MCM-41. With the cluster/MCM-41 complex (figure 4d, the same sample as figure 2d) only the diffraction pattern of MCM-41 is observed. Thus we conclude that the structural integrity of MCM-41 is maintained during impregnation and the cluster is dispersed in the channels with no long range ordering. A decrease in the intensity of the dioo diffraction and smaller d-spacing are observed. The smaller d-spacing in 4a versus 4c and 4d suggests the loading process causes a decrease in the unit cell. TEM observations may provide additional structural information. A sample (figure 2d) of cluster/MCM-41 complex annealed at high temperature gives rise to high angle reflections (Figure 5). Patterns (a) and (b), pure of MCM-41 and the cluster/MCM-41 complex, respectively exhibit no high angle reflections. However, when a sample of l/MCM-41 is heated to SSO^'C (figure 5c) additional reflections can be observed. The pattern resembles those reported for Cuo.64Teo.36 [10], crystallizing with orthorhombic symmetry. If heated to TSO^'C a different diffraction pattern is observed (figure 5d) consistent with the second separate orthorhombic phase of Cuo.64Teo.36 [10]. Heating at QSO^'C produces a pattern (figure 5e) closely resembling that of copper telluride's high temperature phase, which crystallizes with a hexagonal habit [10].
c (L>
fNM'^^^
(b)
(d)
(e) —I
20
1
1
1
1
1
1
1
r
25
30
35
40
45
50
55
60
Figure 5. PXRD high angle (15-70** 20) patterns of annealed cluster/MCM-41 complexes: (a) pure MCM-41, (b) cluster/MCM-41; no annealing, (c) annealed at 550°C 0.5 hours, (d) annealed at 750''C 0.5 hours, (e) annealed at 950''C 0.5 hours.
310 As the annealing temperature increases, the intensity of the high angle diffractions increases relative to the hioo reflection of MCM-41.
4. CONCLUSIONS The loading of Cu6(TePh)6(PPh2Et)5 into MCM-41 may be accomplished by several methods. Using TGA as a probe, the nature o f ! loaded into MCM-41 can be determined as well as the percent loading. It was found that the static vacuum loading procedure was the most successful with a maximum weight loading of 19%. Through the use of IR spectroscopy and PXRD further information may be obtained. IR studies suggest that the interaction of 1 with the MCM-41 framework is minimal and the cluster remains intact during loading. From PXRD, the framework of MCM-41 was also shown to be stable under the loading conditions. Through the thermal elimination of TePh2 and PPh2Et, copper telluride may be thermally produced inside the channels of MCM-41. At 950°C, PXRD suggests a phase change of copper telluride from orthorhombic to a hexagonal unit cell. UV-vis data indicate that the condensation of 1 can also be induced by photolysis. The formation of semiconducting copper telluride inside the spatially confined pores of MCM-41 may be a step towards the formation of semiconducting nanowires. Acknowledgements We thank The University of Western Ontario, NSERC and the CFI for financial support.
REFERENCES 1. 2. 3. 4. 5. 6. 7.
A. Aviram, J. Am. Chem. Soc, 110 (1988) 5687. C. Carter, Molecular Electronic Devices, New York 1982. G. Schmid, J. Chem. Soc. Dalton Trans., (1998) 1077. S. Dehnen, D. Fenske, Chem. Eur. J., 2 (1996) 1407. M. Semmelmann, D. Fenske, J. F. Corrigan, J. Chem. Soc. Dalton Trans., (1998) 2541. J. F. Corrigan, D. Fenske, Angew. Chem. Int. Ed. Engl., 36 (1997) 1981. 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. Higgens, J. L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10834. 8 . C. Wu, T. Bein, Chem. Mater., 6 (1994) 1109. 9 . M. J. MacLachlan, P. Aroca, N. Coombs, I. Manners, G. A. Ozin, Advanced Materials, 10 (1998) 144. 10 . R. Blachnik, M Lasocke, U Walbrecht, J. Solid State Chem., 48 (1983) 431.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
311
Studies on immobilization of Co(II)-La(III) schifFbase complex in MCM-41 Binbin Fan^"^, Ruifeng Li^*, Zhihong Liu^, Jinghui Cao^, Bing Zhong^ ^Institute of special Chemicals, Taiyuan University of Technology, Taiyuan 030024, China ^State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, The Chinese Academy of Sciences, Taiyuan 030001, China Immobilization of heteronuclear Co(II)-La(III) salen (denoted as (CoLa)salen ) complex into the pores of MCM-41 matrix was studied and characterized by element analysis, TGDTA, FTIR and UV-vis. The results showed that the heteronuclear complex may undergo dissociation during immobilization step due to the strong guest/host interaction. Nevertheless, the obtained product(designated as (Co,La)salen/MCM-41) exhibits higher conversion than Cosalen/MCM-41 and Lasalen/MCM-41 and high stability for the oxidation of styrene. In addition, the effects of the reaction time, reaction temperature, and solvent on the catalytic properties of (Co,La)salen/MCM-41 in oxidation of styrene were also investigated. 1. INTRODUCTION Transition metal complexes encapsulated in the channel of zeolites have received a lot of attention, due to their high catalytic activity, selectivity and stability in field of oxidation reactions. Generally, transition metal complex have only been immobilized in the classical large porous zeolites, such as X, Y[l-4]. But the restricted sizes of the pores and cavities of the zeolites not only limit the maximum size of the complex which can be accommodated, but also impose resistance on the diffusion of substrates and products. Mesoporous molecular sieves, due to their high surface area and ordered pore structure, off*er the potentiality as a good host for inmiobilizing transition complexes[5-7]. The previous reports are mainly about molecular sieves encapsulated mononuclear metal complex, whereas the reports about inmiobilization of heteronuclear metal complex in the host material are few. Here, we try to prepare MCM-41 loaded with binuclear Co(II)-La(III) complex with bis-salicylaldehyde ethylenediamine schiff base. 2. EXPERIMENTAL 2.1 Preparation of (CoLa)salen (CoLa)salen was prepared as in Ref.[8]. To a 20ml isopropanol solution of Immol salen, a solution of 0.5mmol Co(N03)2 6H2O and 0.5mmol La(N03)3 in 5ml isopropanol was added. The mixture was stirred at room temperature for Ih under nitrogen atmosphere and a brown yellow precipitate was formed. After filtration the precipitate was washed for several times with isopropanol and ether, and ftirther dried under vacuum. Cosalen and Lasalen were prepared according to the method described in Ref.[8] and Ref [9] respectively.
312
2.2 Complex impregnation (Co,La)salen/MCM-41 were prepared by stirring 2g calcined MCM-41 and a solution of 0.36g (CoLa)salen in 30ml isopropanol at room temperature for 24h. The solid fraction was filtered, extracted with acetone, and finally dried at 333K for 5h. Cosalen/MCM-41 and Lasalen/MCM-41 were also prepared by impregnating 2g calcined MCM-41 with 30ml isopropanol solution of 0.36g Cosalen or Lasalen by using the similar method as preparation of(Co,La)/MCM-41. 2.3 Sample characterization Metal content was determined by a LABTAM 8401 inductively coupled plasma spectrometer. X-ray powder diffraction was carried out on a Rigaku 2304 diffractometer with CuK« radition(Ni filtered). IR and UV-vis spectra of the solid samples were recorded on a PE FTIR 1760 spectrometer and a PE Lambda Bio 40 instrument respectively. TG-DTA was performed on a CN8076E(Rigaku) thermal analysis instrument. 2.4 Catalytic measurement Styrene oxidation reactions were carried out in sealed batch reactors (100ml) at 333K for lOh. The reactant was composed of 3ml styrene, 2ml 30wt.% H2O2, 10ml acetone and O.lg catalyst. The products were analyzed on a GC-9A gaschromatography. 3. RESULTS AND DISCUSSION X-ray powder diffraction patterns of the calcined and immobilized (Co,La)salen MCM41 samples are presented in Fig.l. It can be seen that MCM-41 does not undergo structure change during the preparation of the catalyst and exhibits the hexagonal phase with at least two resolved peaks((100) and (110)) in the diffraction patterns. Upon immobilization and extraction, the obtained sample has light brown yellow color, suggesting that the complex be fixed in the MCM-41 host material. Elemental and thermogravimetric analyses were carried out to determine the complex content loaded on the MCM-41. Elemental analysis is given in table 1. In the isopropanol solution of (CoLa)salen the molar ratio of La/Co is 1.0, but fi-om table 1 it can be seen that the La/Co molar ratios are all less than 1.0 in the obtained products. With the increase of the Si02/Al203 ratio of the MCM-41, although the La content dramatically decreases, the Co content has not obvious change. According to the weight loss during 250-500°C, corresponding to the decomposition of salen ligand, and the metal content in the samples, it can be concluded that the molar ratios of (Co+La)/salen are nearly 1.0. These indicate that most of the (CoLa)salen complex may dissociate into mononuclear complex of Cosalen or Lasalen, possibly due to the strong interactions of (CoLa)salen complex and Si-OH in the wall of MCM-41 [7], and Cosalen is easier to be loaded on MCM41 than Lasalen. Table 1 Co and La content loaded on MCM-41 with different Si02/Al203 Si02/Al203 50 100 200
Co (wt.%) 0.9248 1.0915 0.8506
La (wt%) 0.6849 0.4933 0.1132
La/Co(mol.) 0.313 0.1918 0.0565
313
2000
1225 Wavenumbers, cm"
Figure 2, FTIR spectra of (Co, La) salen (a), (Co, La) salen/MCM-41 (b) and the calcined MCM-4I (c)
450
200
300
400
500
600
700
800
Wavelength, nm Figure 3. The diffuse UV-vis spectra of (Co,La)salen (a), Lasalen/MCM-41 (b), (Co,La)salen/MCM-41 (c) and Cosalen/MCM-41 (d)
The FTIR spectra of (Co, La)salen loaded/MCM-41, free (CoLa)salen and the parent MCM-41 material are depicted in Fig.2. From Fig.2, it can be seen that complex free MCM41 has no absorption bands in the wavenumber region between ca. 1300cm' and 1500cm . In the spectrum of the metal salen containing MCM-41, even if the (CoLa)salen may dissociate during the immobilization step, the characteristic adsorption bands of metal salen complexes are clearly visible, indicating that the metal salen complexes are indeed loaded on the mesoporous material and cannot be removed by acetone extraction. This can be confirmed by the reflectance spectra of different samples in Fig.3. From Fig.3, it can be seen that for the samples loaded metal salen an absorption band around 400nm ascribed to metal to ligand charge transfer[3], can be seen respectively. Generally, immobilization of metal complexes in the wall of mesorporous materials is attributed to the electrostatic interaction between anion oxygen sites on the channel walls of and positively charged complexes [5]. Comparing the IR spectra of MCM-41 loaded with (Co,La)salen and free (CoLa)salen, it can be seen that the band at 1380cm'' attributed to NO3' [8] is still remained, which indicates that the positive charge is balanced by NOs" instead of by the anionic host framework. Metal salen complexes are not held by ionic interaction between guest and the host framework, but by the strong guest/host interactions, mainly between the aromatic ring of the complex and the internal surface silanol groups of the walls of mesoporous[7].
314 The catalytic properties of the sample were tested with the oxidation of styrene. From table 2, it is obvious that (Co,La)salen/MCM-41 exhibits higher catalytic activity than Lasalen/MCM-41 and Cosalen/MCM-41 under the identical reaction conditions. This result suggests that there may be a synergism between two complexes containing different central ions, which makes it favorable for the oxidation of styrene. Table 2 Oxidation of styrene over different catalysts Conv.
Products PA BA % Cosalen/MCM-41 21.6 2.77 91.8 Lasalen/MCM-41 16.6 89.1 4.53 (Co,La)salen/MCM-41 30.7 1.55 92.5 BA = benzaldehyde; PA = phenlactaldehyde; SO = styrene oxide Catalyst
SO 5.48 6.33 5.93
Table 3 Recycling of (Co,La)salen/MCM-41 for styrene oxidation Oxidant
Number of
Conv.
H2O2
recycling 1st 2nd
% 30.7 27.8
H2O2
BA 92.5 88.74
Products PA 1.55 4.23
SO 5.93 7.03
Because (Co,La)salen is immobilized on the mesopore wall of MCM-41,it is anticipated that this catalyst would exhibit high stability for catalyst recycling. Therefore, the (Co.La)salen/MCM-41 sample was recovered by filtration and used again for oxidation of styrene. The activities for (Co,La)salen/MCM-41 for two successive oxidation of styrene are listed in table 3. The catalytic activity obtained for the second run is over 90% of that for the first run. This result indicates that (Co,La)salen is firmly immobilized in MCM-41 and has high stability in oxidation styrene. Figure 4 displays the catalytic properties of (Co,La)salen/MCM-41in the oxidation of styrene at room temperature. It shows that with the increase of reaction time, the conversion of styrene increases, the selectivity for benzaldehyde varies slightly, and the content of phenlacetaldehyde and styrene oxide in the products decrease and increase respectively, hi the experiment range, the (Co,La)salen exhibits high stability and no deactivation is observed. The influence of temperature on the conversion and product distribution in the oxidation of styrene is shown in table 4. At low reaction temperature, the conversion of styrene is very low with benzaldehyde and phenlacetaldehyde as the main products. With the increase of reaction temperature, the conversion of styrene significantly increase and styrene oxide is formed, at the same time the formation of phenlacetaldehyde is suppressed.
315
Figure 4. The catalytic activity and selectivity of (Co,La)salen/MCM-41 in the oxidation of styrene as a function of time Table 4 Influence of reaction temperature on styrene conversion over (Co,La)salen/MCM-41 Reaction temperature
Conv.
X
% 0.47 2.66 15.6 30.7
25 40 50 60
BA 85.4 91.8 89.1 92.5
Products PA 15.0 6.59 3.40 1.55
SO 6.12 6.56 5.93
Table 5 Effect of solvent on catalytic performance of (Co,La)salen/MCM-41 Solvents
Conv.
Products SO PA
Others BA % 9.23 Tetra-chloromethane 6.72 0.61 8.51 75.5 2.68 Acetonitrile 1.80 13.1 8.69 82.4 6.79 Water 4.14 3.98 4.61 84.5 Acetone 30.7 1.55 5.93 92.5 Others = benzene-1,2-ethanediol, benzoic acid and other unidentified high boilers The effects of solvent on the oxidation of styrene are illustrated in table 5. The conversion is highest when acetone was used as solvent. When acetonitril, water and tetrachloromethane were used as solvents respectively, not only is the conversion very low, but also in the products benzoic, benzene-1,2-ethane diol and other unidentified high boilors appear.
316 4. CONCLUSION Immobilization of heteronuclear Co(II)-La(III) salen complex on mesoporous materials was tried. Due to the strong interaction of CoLasalen and the mesoporous material, mainlybetween the aromatic rings of the complex and the internal surface silanol groups of the mesoporous, the heteronuclear complex may undergo dissociation during immobilization. Metal salen complexes are firmly loaded on the MCM-41 host material and exhibit high stability in the oxidation of styrene, no matter whether they exist as mononuclear or as heteronuclear. There may be a synergism between two different complexes, which makes (Co,La)salen/MCM-41 has higher conversion than Cosalen/MCM-41 and Lasalen/MCM-41 for the oxidation of styrene.
REFERENCES 1. K.J. Balkus Jr., M. Eissa, and R. Levado, J. Am. Chem. Soc, 117(1995)10753 2. S.B. Ogunwumi and T. Bein, Chem.Commun., (1997)901 3. C.R. Jacob, S.R Varkey and R Ratnasamy, Microporous and Mesoporous Mater., 22(1998)465 4. R Thibault-Starzyk, R.R Parton and R A. Jacobs, Stud. Surf. Sci. Catal. 84(1994)1419 5. S.S. Kim, W.Z. Zhang and T.J. Pinnavaia, Catal. Lett. 43(1997)149 6. M. Eswaramoorthy, Neeraj and C. N. R. Rao, Chem.Commun., (1998)615 7. L. Frunza, H. Kosslick, H. Landmesser, E. Hoft, and R. Fricke, J. Mol. Catal. A: Chemical, 123(1997)179 8. G. Lu, K. M. Yao and L.R Shen, Chinese Journal of Applied Chemistry, 15(1998)1 9. G. Lu, K.M. Yao, W.G. Chen, L.R Shen and H.Z. Yuan, Chinese Journal of Applied Chemistry, 15(1998)1
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
317
The use of alkylchlorosilanes as coupling agents for the synthesis of stable, hydrophobic, surfactant extracted MCM-48A^Ox catalysts. P. Van Der Voort* and E.F. Vansant University of Antwerp (U.I.A.),Dept. of Chemistry,Laboratory of Adsorption and Catalysis, Universiteitsplein 1, B-2610 Wilrijk, Belgium The use of dimethyldichlorosilane as a coupling agent for the grafting of VOx structures on the MCM-48 surface, produces a material that is simultaneously hydrophobic (inmiscible with water) and very active (all V-centers are accessible, even for water molecules and the catalytic activity for methanol oxidation has increased). The VOx surface species are grafted by the Molecular Designed Dispersion of V0(acac)2 on the silylated surface, followed by a calcination in air at 450°C. These hydrophobic MCM-48 supported VOx catalysts are stable up to 500°C and show a dramatic reduction in the leaching of the V-centers in aqueous media. Also the structural stability has improved enormously. The crystallinity of the materials does not decrease significantly, even not when the samples are subjected to a hydrothermal treatment at 160°C and 6.1 atm. pressure. 1. INTRODUCTION The discovery of the mesoporous M41S materials has expanded significantly the possibilities for processing bulky molecules for catalytic and adsorption purposes [1,2]. In the early years (1992-1996), main emphasis was on the optimization of the synthesis of the MCM materials. Especially the reproducible synthesis of MCM-48 has been an important difficulty. In the last few years, several research groups, including ourselves [3], have found elegant ways to synthesize high-quality MCM-48. Moreover, numerous reports have appeared lately on the successftil activation of the mesoporous, pure silica M41S materials, either by hydrothermal incorporation of active hetero-elements or by surface grafting [4]. However, major drawbacks still limit the use of such materials on a large scale : (1) the 'burning out' of the expensive surfactant imposes both economical and ecological problems, (2) the stability of the MCM walls in hydrothermal conditions or thermal conditions needs to be improved and (3) incorporated and / or grafted transition metal ions leach out of the structure in aqueous solutions, rendering aqueous phase catalysis virtually impossible. These three problems will be dealt with in this presentation: the MCM-48 support is prepared by a controlled extraction of the cationic gemini surfactant, in such a way that no thermal post-treatment step is required. Secondly, we present an approach of selective, partial hydrofobization of the silica walls, using dimethyldichlorosilane (DMDCS), rendering it essentially hydrophobic to withstand the water attack, but creating simultaneously sufficient active sites for a subsequent grafting of the surface. Finally, VOx surface species are grafted on the silylated MCM-48 surface, in such a way that leaching is almost completely suppressed.
318
2. EXPERIMENTAL Pure silica MCM-48 was prepared using the gemini 16-12-16 surfactant (general formula [CnH2n+i>r(CH3)2" (CH2)s - lsr(CH3)2CmH2m+i].2Br), as described previously [3]. In this case, the surfactant was not burnt-out, but extracted by a 9:1 vol% MeOH/conc. HCl mixture. The material was silylated using appropriate amounts of Cl2Si(CH3)2 (dimethyldichlorosilane, DMDCS) and N(C2H5)3, dissolved in toluene. After filtration and vacuum drying at 300°C, the silylated MCM-48 was hydrolyzed by stirring in water for 2 h and dried at 300°C in a regular furnace. V0(acac)2 was anchored on the silylated surface using the gas phase Molecular Designed Dispersion method[l 1]. Finally, the sample was calcined at 450°C. Infrared Spectra were measured on a Nicolet 5DBX spectrometer, equipped with a MTEC photo-acoustic detector. X-Ray Diffractograms were recorded on a Philips PW1840 powder diffractometer, using Ni-filtered Cu Ka radiation. Porosity and surface area studies were performed on a Quantachrome Autosorb-1-MP automated gas adsorption system. The calcined samples were degassed for 17 h at 200°C. Gas adsorption occurred using nitrogen as the adsorbate at liquid nitrogen temperature. Surface areas were calculated using the well known BET method, pore size distribution was calculated using the method of Barret, Joyner and Halenda [5]. TGA measurements were recorded on a Mettler TG50 thermobalance. Raman spectra were taken on a home built system, composed of Spectra-Physics 2020 series lasers, coupled with a Dilor XY-800 triple spectrometer and a Whight Instruments nitrogen cooled CCD. All samples were measured at room temperature in a backscattering configuration, with 514.53 nm Ar^ laser excitation. The laser power was tuned between 1 mW and 30 mW. UV-VIS diffuse reflectance spectra were taken on a Varian Gary 5 spectrophotometer, equipped with a specially designed Praying Mantis difftise reflectance attachment of Harrick. 3. RESULTS AND DISCUSSION 3.L The synthesis of MCM-48, using gemini surfactants and controlled surfactant extraction. Surfactant extractions have been attempted previously, but mainly for HMS materials (mesoporous silicas, prepared using neutral amines as the surfactant). Due to the much weaker S^I® interaction, compared to the S^F interaction, these surfactants can be extracted relatively easily [6]. Some reports have been published on the extraction of the cationic surfactant, but the resulting material is mostly inferior to the calcined one and in most cases, still a posttreatment at high temperatures is required [7]. We describe an extraction procedure for MCM48, that does not require a post-treatment and that produces materials with a better quality than the calcined ones. The efficiency of extraction is determined by thermogravimetric analysis and the extracted surfactant is recrystallized from acetone and re-used for subsequent syntheses. TGA measurements showed that 90-95w% of the surfactant is extracted by this procedure. Elemental analysis, X-Ray diffraction and infrared analysis proved that the recrystallized extracted surfactant is identical to the original one, and we have re-used it several times without any decrease in the quality of the obtained MCM materials.
319 The extracted and the calcined materials show clearly different characteristics. This is exemplified by the pore size distributions, as calculated by the conventional BJH method, in Figure 1. (a): MCM-48, extracted with MeOH and HCI S(BET) = 1350 rrF/g, V(p) = 1.3 ml/g (b): MCM-48, calcined S(BET) = 1260 nf/g. V(p) = 0.98 ml/g (c): MCM-48, extracted and subsequently calcined S(BET) = 1180 nf/g, V(p) = 0.84 ml/g (d): Sample (a), after reaction with DMDCS S(BET) = 1050 m^/g , V(p) = 0.82 ml/g
'T
I—
15
20
25
30
Pore radius (A)
Figure 1 : Pore size distribution of different MCM-48, as described in the legend. The MCM-48, prepared by surfactant extraction, has a larger pore size and a more narrow pore size distribtion, in comparison to the calcined samples. It can be speculated that the oxidative decomposition of the surfactant at high temperatures causes very high local temperatures, resulting in a slight structural collapse and thus a slightly broader pore size distribution. Moreover, several authors [8] - including ourselves [3] - have observed a shrinkage of the crystallographic unit cell and a reduction of the overall pore size upon calcination. The soft extraction procedure does not suffer from these two effects: the resulting pore size distribution is more narrow and the maximum is situated at a higher pore radius. XRD analysis confirms these observations: the cubic unit cell for the calcined MCM-48 is calculated to amount 0.804 nm, whereas the unit cell for the extracted MCM-48 is 0.854 nm. 3.2.
Modification ofthe MCM-48 walls with dimethyldichlorosilane.
Some researchers have tried to stabilize the MCM wall by a complete hydrofobization ofthe surface, replacing every silanol group with a trimethylsilyl group, using e.g. trimethylchlorosilane of hexamethyldisilazane [9]. Although this treatment is very effective in se, it yields a surface that is completely unreactive towards subsequent grafting of transition metals. We therefore present a silylation procedure with dimethyldichlorosilane (DMDCS), which allows - upon hydrolysis - a recreation of surface silanols. The reaction of (alkyl)chlorosilanes with a silica surface has been discussed and reviewed in great detail in literature [10]. Although 5 different reactions are possible with di-, tri- or tetrachlorosilanes, basically two important surface species are created. The first is a monodentate silyl group, created by the monomolecular reaction of 1 silanol with 1 chlorosilane, according to reaction (A) {cfr. Figure 2). The second surface specie is a bidentate silyl group, created either by a bimolecular reaction (B) or by a consecutive reaction (C). We have reported previously [11] that the surface of MCM-48, prepared by the gemini 16-12-16 surfactant, possesses 0.9 OH/nm^.
320
MONODENTATE SPECIES
SI-OH
^^ C I - S I - CI
+
Me
/
NEt3 >
-HCI
= SI - o -1
Me
(A)
BIDENTATE SPECIES iSi-OH
CU Me .SI Me CI
* SI-OH
NEt, -2 "CI
=SI-0^
Me
=si-0
Me
(B)
Consecutive reaction
SI-OH
/
CI
NEt3
-Si-O •
S i - 0 - S I - Me
-HCI
Me
(C)
SI
=Si-0
Me
\ CI
Figure 2 : Most important reaction mechanisms of the reaction of dimethyldichloro-silane with the surface of MCM-48. Figure 3 shows that room temperature stirring of MCM-48 with DMDCS, using NEt3 as a catalyst, removes all silanols and leaves the surface covered with silyl groups. Chemical analysis established that surface is covered with 20% of bidentate species ((Si-0)2Si(CH3)2, which are completely inert towards further reaction) and 80 % of monodentate species (Si-0Si(CH3)2Cl). This sample is hydrolyzed by stirring in liquid water to yield Si) 3740 3720 3700 0-Si(CH3)20H surface groups, W a v * n u m b* r (cm -1) which will act as anchors for the vanadium grafting. Figure 3: Infi-ared spectra of (a) blank After hydrolysis (Figure 3 (cY), a MCM-48; (b) after reaction with DMDCS; small shoulder at 3747 cm — •" (c) after subsequent hydrolysis restored, but the main band positioned around 3738 cm"V This band is assigned to the hydroxyls that are created by the hydrolysis of the Si-Cl groups. EMPA (Electron Micro Probe Analysis) confirms that no residual chlorine groups remain on the surface after stirring with water at room temperature for 2 h. Already at this point, the MCM materials are extremely hydrophobic and are no longer miscible with water. This is ftirther evidenced by measuring the water adsorption isotherms of a non-treated MCM-48 (Fig. 4 (a)) and the same sample, after reaction with DMDCS and
321 subsequent hydrolysis (Fig. 4 (b)). Although the total water adsorption remains approximately the same, the condensation of water vapor in the pore of the blank MCM-48 sample already commences at p/pO < 0.4, whereas in the case of the silylated MCM-48, the vapor condensation is postponed untill relative pressures of about 0.7, which clearly shows the strong repulsive behavior of the silylated sample towards water vapor. The MCM-48 samples have retained the long-range ordening of the pores and still possess a high surface area and pore volume. X-Ray Diffractograms and pore size distributions will be discussed in more detail underneath.
3.3.
Figure 4 : Water adsorption isotherms of (a) blank MCM-48; (b) after reaction with DMDCS; (c) after VOx grafting.
Molecular Designed Dispersion of VO(acac)2 on the silylated MCM-48.
V0(acac)2 is grafted on the hydrolyzed, silylated MCM-48 surface using the gas phase Molecular Designed Dispersion method [11,12,13,14]. In principle, the complex is anchored to the hydroxyl groups of the support by either a hydrogen bonding or by a ligand exchange mechanism. The adsorbed complex is called the precursor. A treatment in air at elevated temperatures converts the adsorbed acetylacetonate complex into metal oxide species, that are chemically bonded to the surface. 3800 3300 2800 2300 1800 1300 The infi-ared spectrum (Fig. 5b) shows that all recreated silanols have reacted and that Figure 5 : Infrared spectra of (a) silylated, characteristic bands appear in the 1600 - hydrolyzed MCM-48; (b) after reaction 1300 cm** region, due to the acac ligand with V0(acac)2; (c) after calcination. [13]. Chemical analysis reveals that the ratio of acac ligands to V centers on the surface is 1, which means that the reaction has followed a ligand exchange mechanism:
Si-OH + V0(acac)2 -> Si-O-VO-(acac) + Hacac In a final step, this precursor is calcined at 450°C. The infrared spectrum (Fig. 5 c) clearly shows the V-OH bands appearing at 3660 cm'* [15], although apparently also a fraction of the silanols has been restored. Furthermore, the presence of the C-H vibrations (around 3000 cm'*) and the absence of the acac vibrations indicate that the acac ligands have decomposed completely, but that the methylsilyl groups are stable towards calcination at 450°C.
322
Further information on the structure of the grafted surface species was obtained by Raman spectroscopy. Figure 6 shows the Raman spectrum of the calcined, silylated VOx-MCM. The shoulder at 1060 cm'^ and the broad band at 800 cm"* are due to the silica [16,17]. The strong band at 1040 cm'* is characteristic for the stretching vibration of terminal V=0 bonds in monomeric, tetrahedral vanadia surface species. The presence of crystalline V2O5 can be excluded, since even traces of these species produce a very strong band at 996 cmV The 1150 1050 950 850 750 broad band around 920 cm"* is assigned to Raman Shift (cm') the stretching vibrations of terminal vanadyl groups within a two-dimensional surface Figure 6 : Raman spectrum of the phase. The Raman spectrum of Fig. 6 silylated, VOx catalyst. therefore suggests that the majority of the surface V-species are present as monomeric, tetrahedral species but that a fraction of these species has clustered to form surface polymers. This clustering of a small fraction of the V-species is consistent with the formation of some silanols in the infrared spectrum. Figure 7 presents the overall, idealized reaction mechanism. The surface of MCM-48 contains 0.9 OH / nm^ which react completely with DMDCS in the liquid phase, if NEta is used as a catalyst. The majority of the silanols react monofunctionally but a small fraction also reacts ftirther, according to reaction (3) to yield inert, bidentate species. All chlorine ftinctions on the surface are converted towards hydroxyls upon hydrolysis. The V0(acac)2 is reacted in a gasphase reactor with this silylated, hydrolyzed surface. All recreated silanols react with the V0(acac)2 in a 1:1 stoichiometry, following a ligand-exchange mechanism. Upon calcination at 450°C, the acac ligands are decomposed but the methylsilyl ftinctions remain intact. Most of the V-species are converted into isolated, tetrahedral VOx species, as indicated in Figure 4. However, a small fraction clusters to form surface oligomers, hereby recreating a fraction of the silanols. 3.4.
Catalytic evaluation
Preleminary experiments have been performed to evaluate the catalytic activity of the MCM48 supported VOx catalysts, with equal V-loading, to study the effect of the coupling reagent. Table 2 summarizes the catalytic activity and selectivity of the catalysts for the gas-phase oxidation of methanol, which was used as a model reaction. MCM-48/DMDCSA^Ox catalyst 1 Table 1 MCM-48A^Ox catalyst iTemp. 300°C 350°C 400X 300°C 350°C 400X IConversion 48% 80% 84% 24% 38% 55% 21% 44% 58% Selectivity Formaldehyde 60% 85% 85% 67% 52% 30% Selectivity Dimethylether 15% 8% 5% [Selectivity Methylformate 20% 5% 2% 2% 0% 0%
323
I-OH [-OH
- O ^ +Cl2Si(CH3)2
.CH3 HjO
N(Et), - O ^
CH3 /CH3 -O—Si-Cl
I-OH
CH3 -O
^
CH3
- O ^
/^"^
^CH3
^
^HJ
l-O—Si-OH
S i - 0 —
CH3
j, V-(acac)
CH3 i-O^
/CH3
O2, H2O i-o-^
CH3
^ H ;
^^OH
-O—Si-O— V=
\
•CH3
\
0 OH
Figure 7 : Idealized reaction scheme for the grafting of VOx surface species on MCM-48, using DMDCS as a coupling agent. It can be inferred from table 1 that the activity of the catalyst with the coupling agent is the highest. The selectivity for formaldehyde has decreased however and a larger amount of demethylether is formed. This is an indication for a higher surface acidity, probably caused by the V-OH groups, surrounded by an hydrophobic environment. Further studies are currently performed, both in gas-phase and in liquid-phase catalytic reactions. These results will be the subject of a subsequent publication. 3.5.
Stability of the silylated VO,/MCM-48 catalysts.
3.5. J. Leaching in liquid water Table 2 presents the results of leaching experiments. A non-silylated MCM sample, and a MCM sample, silylated with Me2SiCl2 (DMDCS) were grafted with VOx and stirred with water for 1 hour. Upon silylation with DMDCS, the material is no longer miscible with water and the stability of the V centers towards leaching is improved dramatically.
1 Table 2 non silylated MCM-48, 1 DMDCS-MCM-48
n(V) on sample 1.4 mmol/g 1.2mmol/g
n(V) leached out 1.2 mmol/g 0.2 mmol/g
%V leached out 1 86 %
17%
1
3.5.2. Water adsorption It is important to stress that, in spite of the high hydrofobicity of the samples, the V^^ centers are still accessible to water adsorption. This is evidenced by the color change from bright white to dark orange upon standing in ambient air. This color change is caused by water adsorption on the V-centers, shifting the position of the L -> M charge transfer bands to the visible region, as illustrated in Figure 8. Several authors have speculated on the mechanisms behind this dramatic color change. The tendency of isolated pseudotetrahedral V^ compounds
324 to increase their coordination sphere by coordinating water molecules has been mentioned in literature [18]. It is now generally agreed that the color change is due to the coordination of ambient water, in which the tetrahedral V^ center coordinates with two water molecules, resulting in a (pseudo) octahedral structure. The complete accessibility of all V-centers is further evidenced by the water adsorption isotherms of Figure 4. Comparison of the water Wavelengm (rvT| adsorption isotherm (b) and adsorption isotherm (c) shows that the Figure 8 : UV-DRS spectra of (a) dry and difference in water uptake at p/pO = (b) hydrated sample. 0.6 (just before water condensation in the pores) is 2.8 mmol/g. If one assumes that one tetrahedral V center adsorbs two water molecules, the concentration of V-sites would be 2.8/2 = 1.4 mmol/g. The experimental value is 1.2 mmol/g, which is a good indication that all V sites are tetrahedral centers and are accessible to coordinate with two molecules of water. 3.5.3. Hydrothermal stability The silylated materials show an unusual structural stability in hydrothermal (high temperature and pressure) conditions. The silylated samples were put in a closed stainless steel vessel on a perforated grid. Underneath the samples, liquid water was introduced and the vessel was tightly closed, after which the entire reactor was heated in a furnace from 100°C up to 150°C and kept at this temperature for at least 24 h. The X-Ray Diffractograms of the samples after this hydrothermal treatment are presented in Figure 9, evidencing that the silylated samples, even after a treatment at 150°C and 4.7 atm, still show a remarkable crystallinity, in contrast to a non-treated sample (curve d) that has lost completely its crystallinity after a treatment at 120°C. The same conclusions can be drawn from the pore size distributions in figure 10. The pore size distributions of the silylated samples do not change significantly upon hydrothermal 2theta treatments, whereas a blank MCM-48 has lost its mesoporosity after a hydrothermal Figure 9 : XRD of (a) silylated MCM-48, (b) after hydrothermal treatment at 120°C treatment at 120°C. Curve (e) of the nonsilylated MCM-48 in Figure 10 coincides with and 1.95 atm., (c) 150°C and 4.7 atm. (d) the X-axis of the figure over almost the entire non silylated MCM at 120°C and 1.95 atm. pore region and is very difficult to see. It is
325 noteworthy that upon hydrothermal treatment at 160°C (curve (d)), the pore size distribution seems to sharpen again. This phenomenon has been observed several times and is the subject ofa further study.
>
Pore radius (A)
Figure 10 : Pore size distributions of (a) original silylated MCM-48; after hydrothermal treatment at (b) 120°C, (c) 140°C and (d) 160°C; (e) non-silylated MCM-48 after hydrothermal treatment at 120°C. 4. CONCLUSIONS The preparation of pure silica MCM-48, using cationic gemini surfactants, followed by a controlled extraction of the surfactant results in materials that have a more narrow pore size distribution than the ones in which the surfactant is removed by calcination. Since no calcination is needed, there is no unit cell contraction and the extraction surfactant can be recrystallized and re-used. The use of DMDCS as a coupling agent for the grafting of VOx species on the surface of MCM-48, results in hydrophobic materials, with a high stability towards leaching and structural collapse, but with the V-centers still in accessible positions. The reaction of pure silica MCM-48 with dimethyldichlorosilane and subsequent hydrolysis resuhs in hydrophobic materials with still a high number of anchoring sites for subsequent deposition of vanadium oxide structures. The Molecular Designed Dispersion of V0(acac)2 on these silylated samples results in a V-loading of 1.2 mmol/g. Spectroscopic studies evidence that all V is present as tetrahedral V^ oxide structures, and that the larger fraction of these species is present as isolated species. These final catalysts are extremely stable in hydrothermal conditions. They can withstand easily hydrothermal treatments at 160°C and 6.1 atm pressure without significant loss in crystallinity or porosity. Also, the leaching of the V in aqueous conditions is reduced with at least a factor 4. ACKNOWLEDGEMENTS The authors thank the FWO (Fund for Scientific Research - Flanders, Belgium) for financial support. This work was sponsored by a FWO research grant, nr. G.0446.99. Mrs. Fabiana Quiroz, Mrs. Mariska Mathieu and Mr. Kristof Cassiers are acknowledged for the aid in the experimental work.
326 REFERENCES 1 2
3 4 5 6
7
8
9
10
11 12 13 14 15 16 17 18
Microporous and Mesoporous Materials (special issue), vol. 27, 1999. Mesoporous Molecular Sieves, Studies in Surface Science and Catalysis 777, L. Bonneviot, F. Beland, C. Danumah, S. Giasson and S. Kaliaguine eds, Elsevier Science Publishers, Amsterdam, 1998. P. Van Der Voort, M. Mathieu, F. Mees, and E.F. Vansant, J. Phys. Chem. B, 102 (1998), 8847. A. Tuel, Microporous and Mesoporous Materials, 27 (1999), 151 and references therein. E.P. Barret, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc, 73 (1951), 373. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995), 865; R. Mokaya and W. Jones, J. Mater. Chem., 8 (1998), 2819; P.T Tanev and T.J. Pinnavaia, Chem. Mater., 8 (1996), 2068. C.Y. Chen, H.X. Lu and ME. Davis, Microporous Mater., 2 (1993), 17; S. Hitz and R. Prins, J. Catal., 168 (1997), 194; F. Babonneau, L. Leite and S. Fontlupt, J. Mater. Chem., 9(1999), 175. A A. Romero, M.D. Alba, W. Zhuo and J. Klinowski, J. Phys. Chem. B, 101 (1997), 5294; C.F. Cheng, W. Zhuo, D.H. Park, J. Klinowski, M. Hargreaves and L.F. Gladden, J. Chem. Soc. Faraday Trans., 93 (1997), 359. C.P. Jaroniec, M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem. B, 102 (1998), 5503; X.S. Zhao and G.Q. Lu, J. Phys. Chem. B, 102 (1998), 1556; K.A Koyano, T. Tatsumi, Y. Tanaka and S. Nakata, J. Phys. Chem. B, 101 (1997) 9436. E.F. Vansant, P. Van Der Voort and K.C. Vrancken, Characterization and Chemical Modification of the silica surface, Studies in Surface Science and Catalysis, 93, Elsevier Science Publishers, 1995. P. Van Der Voort, M. Morey, G.D. Stucky, M. Mathieu and E.F. Vansant, J. Phys. Chem. B, 102 (1998), 585. P. Van Der Voort, K. Possemiers and E.F. Vansant, J. Chem. Soc. Faraday Trans., 92 (1996), 843. P. Van Der Voort, I.V. Babitch, P.J. Grobet, A.A. Verberckmoes and E.F. Vansant, J. Chem. Soc. Faraday Trans., 92 (1996), 3635. P. Van Der Voort, M.G. White and E.F. Vansant, Interface Science, 5 (1997), 179. P. Van Der Voort, M.G. White, M B . Mitchell, A.A. Verberckmoes and E.F. Vansant, Spectrochimica Acta A:Molecular Spectroscopy, 53 (1997), 2181. M. Schraml-Marth, A. Wokaun, M. Pohl and H.L. Krauss, J. Chem. Soc. Faraday Trans., 87 (1991), 2635. W.P. Griffith, inSpectroscopy of Inorganic Based Materials, vol. 14, R.J.H. Clark, R.E. Hester, eds., John Wiley, New York, 1987 M. Anpo, M. Sunamoto and M. Che, J. Phys. Chem., 93 (1989), 1187.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
327
Epoxidation over niobium and titanium grafted MCM-41 and MCM-48 mesoporous molecular sieves. M. P. Kapoor' and Anuj Raj ^ ""Osaka National Research Institute, Department of Energy and Environment, Synthetic Chemistry Section, 1-8-31 Midorigaoka, Dceda, Osaka-563, Japan* ^National Institute of Materials and Chemical Research, Tailored Nanostructures Group, 1-1 Higashi, Tsukuba, Ibaraki-305-8565, Japan
Highly dispersed niobium or titanium containing molecular sieve catalysts were prepared by post synthesis modification of anchoring at surface silanol groups, using direct grafting of niobium and titanium compounds onto the totally accessible inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Different niobium and titanium compounds are used to accomplish the grafting. Catalytic epoxidation activity of niobium or titanium grafted mesoporous molecular sieves were studied using H2O2 as an oxidant. These niobium and titanium containing mesoporous molecular sieves exhibit higher epoxidation activity and better H2O2 selectivity.
1. INTRODUCTION Today's environmental concerns demand the streamlining of the catalytic processes for the production of fine chemicals. The utility of H2O2 as an oxidant is significant step in this direction due to the fact that its by-product is only water. Titanium incorporation in several silicalites such as silicalite-l(2), ZSM-12, ZSM-48 and zeolite-p are well documented and found to be effective for the selective oxidation of alkanes, the hydoxylation of phenols and the epoxidation of alkenes using H2O2 [1-6]. Later the synthesis and catalytic activity of titanium containing aluminophosphate molecular sieves viz. TAPO-5, TAPO-11, TAPO-31 and TAPO36 [7-8] were reported. The mesoporous silicas with large channel diameters (25-lOOA) could be used as selective oxidation catalyst to extend the capabilities to molecules with larger size [9-12]. The discovery of titanium containing siliceous mesoporous materials MCM-41 and MCM-48, where titanium is incorporated in the framework of mesoporous Si02, leads to remarkable catalytic performance utilising both diluted hydrogen peroxide and organic peroxides. These reactions are of prime importance to the fine chemical as well as pharmaceutical industries. Currently there is a great interest in titanium containing molecular sieve catalyst, in addition to these the incorporation of other metal ions (e.g., V, Cr, Co, Fe) by isomorphous substitution of Si has also been reported. Niobium is substitution in lattice position is less extensively studied but the use of niobium oxide as a support, promoter and as a solid acid has been studied in detail. Niobium containing materials in the ammoxidation of
328 propane and the oxidative dehydrogenation of propane is reported [13]. Recently, the synthesis of niobium silicalite [14] and niobium containing mesoporous molecular sieves [15] are reported with convincing evidence for isomorphous substitution of niobium ion. Another report describes the synthesis of a stable hexagonally packed mesoporous niobium oxide molecular sieve through a novel ligand-assisted templating mechanism [16]. Recently, the synthesis of niobium anchored in p structure was reported [17]. Also, the niobium containing materials has shown the significant activity in the large number of reactions [18-20]. Niobium and titanium incorporation in a molecular sieve can be achieved either by hydrothermal synthesis (direct synthesis) or by post-synthesis modification (secondary synthesis). The grafting method has shown promise for developing active oxidation catalyst in a simple and convenient way. Recently, the grafting of metallocene complexes onto mesoporous silica has been reported as alternate route to the synthesis of an active epoxidation catalyst [21]. Further the control of active sites, the specific removal of organic material (template or surfactant) occluded within mesoporous molecular sieves during synthesis can also be important and useful to develop an active epoxidation catalyst. Thermal method is quite often used to eliminate organic species from porous materials. However, several techniques such as supercritical fluid extraction ( S r e ) and plasma [22], ozone treatment [23], ion exchange [24-26] are also reported. In the present work the synthesis of highly dispersed niobium or titanium containing mesoporous molecular sieves catalyst by direct grafting of different niobium and titanium compounds is reported. Grafting is achieved by anchoring the desired compounds on the surface hydroxyl groups located on the inner and outer surface of siliceous MCM-41 and MCM-48 mesoporous molecular sieves. Catalytic activity was evaluated in the liquid phase epoxidation of a-pinene with hydrogen peroxide as oxidant and the results are compared with widely studied titanium silicalites. The emphasis is directed mainly on catalytic applications of niobium or titanium anchored material to add a more detailed view on their structural physicochemical properties.
2.
EXPERIMENTAL
The mesoporous silica MCM-41 and Ti-MCM-41 samples were prepared using the procedure by Trong On et al. [27]. Similarly MCM-48 and Ti-MCM-48 were synthesized using the method reported by Tatsumi et. al. [28]. In the synthesis of Nb-MCM-41 and NbMCM-48 mesoporous molecular sieves, tetraethyl orthosilicate (TEOS) and niobium oxalate were used as sources of Si02 and NbjOs respectively. For Nb-MCM-41 the chemical composition of gels were SiO^: 0.02 N b P s : 0.13(CiJMA)2O: 0.13(TMA)2O: 0.13(NH4)2O : 55H2O. Oxalic acid was used to adjust the pH. For Nb-MCM-48, first a dispersed micellar solution of cetyltrimethylammonium bromide/hydroxide (Ci^TAMBr/OH) was prepared mixing a CigTAMBr solution (29 wt.% in water, Aldrich) with a hydroxide for halide exchange resin (IRA-400 (OH), Aldrich). The percentage exchange of bromide by hydroxide was 31.4 %. Another solution containing tetraethyl orthosilicate (TEOS) and niobium oxalate was prepared by stirring together for about 30 min. This solution was then added drop wise to the dispersed micellar solution under vigorous stirring at room temperature during a period of 1 h. The gel composition was Si02: 1.37 C^gTMA: 0.02 Nb.O,: 64 H2O. The gel was then transferred into a
329 Teflon-lined autoclave and healed to 373 K for 72 h for hydrothermal crystallisation. Solid products were recovered by filtration, washed thoroughly with distilled water and dried in air at 353 K. Finally the solids were calcined under continuous air tlow at 813 K for 6 h. 2.1. Template removal by solvent exchange process In a typical procedure, as made MCM-41 or MCM-48 (1.0 g) sample was added to 200-ml solution of 5% HCl in ethanol. The mixture was kept in closed container and stirred for 3 h at room temperature. The change in pH was constantly monitored. Sample was recovered by filtration and washed with distilled water and acetone. Finally, the samples were dried at 343 K overnight and calcined in air at 623 K for 2 h. These samples are designated as exchanged mesoporous MCM-41 and MCM-48 molecular sieves. 2.2. Liquid grafting procedures Titanium or niobium was grafted on both calcined as well as exchanged mesoporous molecular sieves MCM-41 and MCM-48. A desired amount of titanium butaoxide was first dissolved in methanol and the calcined or exchanged mesoporous MCM-41 and MCM-48 molecular sieves sample were add to the solution and kept for the 6 h at 323 K. Sample were recovered by filtration and initially dried at 373 K and finally calcined at 813 K for 6 h. In the other two steps procedure of titanium grafting, the weighed amount of titanocene dichloride was first diluted in chloroform and grafted on calcined or exchanged mesoporous molecular sieves sample. Solution was allowed to diffuse into pores of sample for at least 8 h at 333 K. The samples were then completely dried at 373 K for 5h. In the second step, the grafted samples (red color) were treated with triethylamine to activate the surface sites of the mesoporous sieves. The sufficient time (about 4 h) was allowed for the color of suspension to change from red to yellow. This confirmed the well establish substitution of the chloride with alkoxyl/siloxyl ligands had occurred. Samples were thoroughly washed with chloroform and remaining organic component were removed by calcination at 813 K for 6 h, leaving the white powdered mesopx^rous catalyst with 1.9 wt% grafted titanium. Niobium ethoxide or niobium oxalate was used to graft niobium on calcined as well as exchanged mesoporous MCM-41 and MCM-48 molecular sieves. Similar procedure was used as described for titanium grafting using titanium butaoxide. 2 3 . Characterization The X-ray diffraction (XRD) patterns of the sample were measured using Rigaku D-Max.II VC X-ray diftactometer using nickel filtered Cu Ka (X.= 1.5406 A) radiation. The specific BET surface area and average pore sizes were determined by Nj adsorption-desorption isotherms at 77 K using an Omnisorp-100. Diffuse reflectance UV-spectra were obtained using Perkin_Elmer Lambda 5 spectrophotometer using mesoporous silica MCM-41 or MCM-48 as a standard. The details are already reported earlier [27]. 2.4. Catalysis a-Pinene epoxidation reaction in a glass batch reactor under continuous stirring and reflux was performed using hydrogen peroxide as an oxidant over powdered titanium or niobium containing MCM-41 or MCM-48 catalyst prepared by direct hydrothermal synthesis and grafting route. Typical reaction procedure and related details are described elsewhere [27]
330 3. RESULTS AND DISCUSSION The powder X-ray diffraction patterns of the MCM-41 and MCM-48 samples are consistent with the XRD pattern of such material reported in literature and positively confirm the identity of the material [28-31]. The chemical composition and some important textural properties are given in Table 1 and 2. The pore radius was decreased on framework incorporation as well as grafting the niobium or titanium onto pure MCM-41 and MCM-48 but essentially were in similar range on grafting with different niobium or titanium compounds studied. The BET surface area of pure MCM-41 and MCM-48 calcined siliceous mesoporous were 1280 and 1310 m7g respectively and comparable to the ones previously reported for these materials. The surface area of the pure MCM-41 and MCM-48 sample where template was removed by solvent exchange method followed by calcination was lower than pure calcined samples but leads to larger pore size. Comparatively lower surface area for these samples is likely due to re-adjustment of the long-range order of the mesopores on removal of the template, into the silica framework. Further, both niobium and titanium grafted samples again showed lower surface area compare to the samples obtained by direct niobium or titanium incorporation by the hydrothermal synthesis. Table: 1 Descriptions, chemical composition and textural properties of MCM-41. Catalyst Pore radius, BET Surface area A m "g '^
cl.joo spacmg A
MCM-41' MCM-41 ^
18.0 19.0
1280 1158
36.5 37.1
2.0 wt% Nb-MCM-4r 2.0 wt% Ti-MCM-4r
14.2 16.5
1067 1230
31.8 32.7
1.9wt%Nb-MCM-^l='*' 1.9 wt%Nb-MCM-41 ^*^ 1.9 wt%Ti-MCM-41'*^ 1.9 wt%Ti-MCM-41 ^"^
13.1 13.5 15.5 16.0
924 908 1162 1146
30.6 30.5 31.3 314
1.9wt%Nb-MCM-4l'*' 1.9 wt%Nb-MCM-41'"^ 1.9wt%Ti-MCM-41'^' 1.9wt%Ti-MCM41'"s
13.2 13.5 15.6 16.0
902 891 1092 1073
30.9 30.9 31.5 31.7
* template removed by calcination. ^ template removed by solvent exchange and then calcined. ^' hydrothermal synthesis. ^' grafted with niobium ethoxide. " grafted with niobium oxalate. ^ grafted with titanium butaoxide. ^ grafted with titanocene dichloride. Diffuse reflectance UV-spectra showed the framework incorporation of titanium and niobium in the all sample studied. The detailed characterisation of the materials will be presented in the subsequent paper. The catalytic properties are studied by the epoxidation of apinene, using H2O2 as an oxidizing agent. The framework 2.0wt % Ti-MCM-41 and 2.0wt % Ti-MCM-48 mesoporous molecular sieves samples synthesized by direct hydrothermal route.
331 surface areas 1230 m'/g and 1191 mVg respectively, exhibit relatively fair conversion and the poor H2O2 efficiency with a epoxide as a sole reaction product. (Table 3). The presence of framework niobium showed an increase in activity and H2O2 efficiency compared to framework titanium. Over niobium containing mesoporous molecular sieves 1,2 pinane diol was also observed as the result of epoxy ring cleavage. Table 2 Descriptions, chemical composition and textural properties of MCM-48. Pore radius, BET Surface area d.^oo spacing Catalyst A m^g"^ A MCM-48' 35.3 16.3 1310 MCM-48 ^ 36.8 17.5 1124 2.0 wt% Nb-MCM-48'^ 2.0 wt% Ti-MCM-48'^
12.2 14.7
998 1191
30.1 30.9
1.9 wt%Nb-MCM-48 ^"^'^ 1.9wt%Nb-MCM^8^"'= 1.9wt%Ti-MCM^8^"' 1.9 wt%Ti-MCM-48 ^"s
10.6 11.1 12.4 13.5
972 847 1026 1008
29.2 29.4 30.1 30.0
1.9wt%Nb-MCM-48'*' 1.9 wt%Nb-MCM-48^"' 1.9wt%Ti-MCM-48^^' 1.9 wt%Ti-MCM-48'"2 Key as illustrated in Table 1.
10.5 10.8 12.5 13.4
892 801 966 913
29.6 29.6 30.0 29.8
Table 3 a-Pinene epoxidation over calcined niobium or titanium containing mesoporous molecular sieves prepared by direct hydrothermal synthesis a-Pinene Products (mol.%) H202(mol.%) Catalyst
ccpinene oxide 92.6 100
1,2 pinane diol 7.4
conversion
efficiency
2.0wt % Nb-MCM-41 2.0wt % Ti-MCM-41
conversion ("^^^•^^) 8.5 6.1
46.8 44.2
16.4 11.6
2.0wt % Nb-MCM-48 2.0wt % Ti-MCM-48
9.1 7.8
90.7 100
9.3
43.3 50.0
19.3 13.1
Reaction conditions: catalyst, O.lg; a-pinene, 0.037 mol; H2O2 (30% aqueous solution), 0.044 mol; reaction temperature, 328 K; reaction duration, 5 h. Acidity of niobium containing MCM-41 mesoporous materials, as reported by Ziolek et. al [15], are sufficient to provide the relatively mild acidic sites necessary to cleave the epoxy ring leading to diols. The a-pinene conversions and H2O2 selectivities were always higher with MCM-48 when compared with MCM-41 molecular sieves. This is probably due to the
332 topology, MCM-41 is comprised of unidimensional array of hexagonally arranged pore system (hexagonal), which consist of straight tube like channels while MCM-48 contains two independent three dimensional pore systems, which are interwoven and situated in a mirrorplane position to each other (cubic). Table 4 a-Pinene epoxidation on niobium or titanium grafted mesoporous molecular sieves where the template was removed by calcination. Products (moL%) a-Pinene H A ( mol.%) Conversion " Catalyst conversion efficiency 1,2 pinane a-pinene (mol.%) diol oxide 43.4 1.9wt % Nb-MCM-41' 18.8 9.2 8.9 90.8 1.9wt % Nb-MCM-41' 42.9 11.8 20.7 88.8 9.4 1.9wt % Nb-MCM-48' 22.5 49.7 13.7 86.3 11.7 1.9wt % Nb-MCM-48 ^ 16.9 51.3 25.3 83.1 13.2 1.9wt%Ti-MCM-41'= 1.9wt%Ti-MCM-41' 1.9wt%Ti-MCM-48'= 1.9wt % Ti-MCM-48'
6.9 7.7 8.6 9.4
100 97.9 100 97.3
— 2.1 — 2.7
42.3 45.6 45.7 44.8
13.7 14.5 15.8 18.1
Reaction conditions as described in Table 3. "" grafted with niobium ethoxide. ^ grafted with niobium oxalate. *" grafted with titanium butaoxide. ** grafted with titanocene dichloride. Catalytic data shown in Table 4 indicates that on systematic incorporation of niobium or titanium (~1.9wt % by ICP) onto calcined MCM-41 and MCM-48 molecular sieves by grafting results in an increase in the a-pinene conversion as well as improves the HjOj efficiency. However, the activity and the product selectivity differ with the type of niobium or titanium compound used for grafting. When niobium oxalate was used as grafting agent, the higher diols formation and maximum a-pinene conversions and H2O2 efficiency were observed. Very little diols formation was also seen when titanocene dichloride was used for grafting. This is likely due to the presence of traces of chloride ion, which could provide mild acidic sites and which are responsible for the ring opening of the epoxide. While the samples grafted with niobium ethoxide or titanium butaoxide also showed a reasonable increase in catalytic activity and H2O2 efficiency. Again the activity and selectivity of the niobium-grafted samples was always higher than that of titanium grafted samples. Table 5 lists the results of a-pinene conversion obtained over niobium or titanium grafted MCM-41 and MCM-48 samples where the template was first removed by solvent exchange method followed by calcination. The catalytic activities are comparatively higher than ones obtained over niobium or titanium grafted MCM-41 and MCM-48 samples where template was removed using a conventional thermal method (direct calcination). A similar trend for the H2O2 efficiency was noticed.
333 Table 5 Epoxidation over niobium or titanium grafted mesoporous molecular sieves where template was removed by solvent exchange method prior to calcination Products (mol.%) a-Pinene H A ( mol.%) conversion a-pinene Catalyst 1,2 pinane conversion efficiency (mol.%) diol oxide 1.9wt % Nb-MCM-41 ^ 19.6 43.4 10.1 89.9 9.2 1.9wt % Nb-MCM-41 ^ 22.1 42.4 12.6 9.9 87.4 1.9wt % Nb-MCM-48 ^ 24.5 53.6 14.9 13.6 85.1 1.9wt % Nb-MCM-48' 27.2 55.2 18.3 15.1 81.7 1.9wt 1.9wt 1.9wt 1.9wt
% Ti-MCM-41 ^ % Ti-MCM-41"" % Ti-MCM-48"" % Ti-MCM-48""
7.3 8.0 9.1 9.6
100 97.1 100 95.8
2.9 4.2
40.4 38.9 38.4 40.8
15.2 17.8 19.9 20.6
Key as illustrated in Tables 3 & 4. Significantly higher catalytic activity of niobium or titanium grafted molecular sieves compared to sample prepared by direct hydrothermal route where niobium or titanium in framework, is likely due to the presence of surface sites which could be isolated and tetrahedral in nature to provide better performance. However, the presence of dimers or oligomers could be responsible for decomposition of the peroxide. The octahedrally co-ordinated titanium, which is usually inactive for the epoxidation of alkenes as it lacks free co-ordination sites [21,32]. Also, the high concentration of the silanol groups as well as hydroxy 1 groups present on the wall surface is responsible for the decomposition of H2O2. Therefore, in case of grafted mesoporous molecular sieves, probably the average structure of the catalytic site is mainly tetrahedral and/or the site might be composed of a mixture of different tetrahedral species. Further, the template removal through solvent exchange procedure is much more effective as evident, for complete removal of organic template as compared to standard calcination method. From general point of view the removal of organic species from mesoporous molecular sieves through calcination usually results water, decomposed hydrocarbons along with some forms of nitrogen and bromine compounds. The possibility of irreversible adsorption of decomposed material onto the inner wall of mesopores may cause the reduced pore size. However, in the case of template removed by solvent exchange procedure almost every organic species washed out from the mesopore and rendered the comparatively bigger pore size that eventually may enhance the catalytic activity.
4. CONCLUSION In agreement with catalytic results it is clear that upon direct grafting, a very high dispersion of isolated tetrahedral centres may be generated on the walls of mesoporous MCM-41 and MCM-48. This in turn allows for the possible tuning to improve the catalytic activity while preserving the mesoporous framework intact. Epoxidation with samples where template was removed by solvent extraction proceeds at better rate than with other mesoporous samples.
334 Further studies to clarify the exact nature of possible active sites are worth pursuing to design novel catalysts.
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Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
335
Titanium iso-propoxide grafting on M41S type hosts : catalytic and adsorption Study K.K.Kang, C.S.Byun, and W.S.Ahn* School of Chemical Science and Engineering, Inha University, Inchon, Korea 402-751 Titanium incorporated mesoporous molecular sieves were prepared by grafting TIPOT (Ti-isopropoxide) on various pure silica M41S type host materials. These catalysts were characterized by using XRD, Nj physisorption, UV-vis DRS, and FT-IR spectroscopies. The Ti-grafted mesoporous derivatives were catalytically active for the selective oxidation of 2.6-DTBP (2,6-di-tert-butylphenol) with HjOjand the catalytic activity decreased in the order of MCM-48>HMS>KIT-1>MCM-41>SBA-1. Apparently 3 dimensional channel system of MCM-48 and HMS with small particle size and textual mesoporosity proved to be advantageous in liquid phase reaction. For adsorption of MIBK (methyl isobutyl ketone), pure silica mesoporous materials had far superior sorption capacity (0.23-0.3 g/g-catalyst) to ZSM5. HMS showed the best sorption capacity among them, and titanium grafting enhanced the MIBK adsorption to a minor extent. Interestingly, however, 2% Ti-HMS in which the metal precursor is introduced with the silica source during synthesis, produced a significant increase of ca. 20 % in sorption capacity than pure silica analogue. 1. Introduction Recently, synthesis and characterization of different types of mesoporous silicates such as MCM-41 [1], HMS [2], MCM-48 [1], KIT-1 [3] and SBA-1 [4] have been reported. These materials possess high surface areas and uniform pores, the size of which can be tuned between 20-100 A by varying the preparation conditions. A large number of applications of these materials are envisaged in the area of catalysis, separation, adsorption and advanced materials. Among these, redox molecular sieves effective in large organic molecule transformations in liquid phase reactions had emerged by incorporating various redox metal species such as titanium into the mesoporous M41S structures. Usually Ti-containing mesoporous molecular sieves were prepared by introducing a suitable titanium precursor during hydrothermal synthesis [5]. Various post-synthetic procedures were also reported such as titanium butoxide or titanocene grafting or atom planting technique using TiCl4 vapor [6]. In this study we used pure silica MCM-41, MCM-48, KIT-1, HMS, and SBA-1 as hosts for titanium isopropoxide grafting to obtain a series of catalysts with redox activity. This paper reports comparison in performance of these materials for 2,6-DTBP oxidation and MIBK, a typical volatile organic compound (VOC), adsorpdon.
336 2. Experimental The silica forms of MCM-41, MCM-48, KIT-1, HMS, and SBA-1 were prepared by hydrothermal synthesis following the methods reported in the literature [1,2,3,4,7]. The calcined mesoporous materials were slurried in absolute isopropyl alcohol solution containing TIPOT (10wt%) with magnetic stirring at room temperature for 3h. The solid products obtained were then washed with absolute isopropyl alcohol, dried and calcined in air at 823K
m. The powder X-ray diffraction(XRD) patterns of the grafted samples were measured on a Phillips PW3123 diffractometer using CuKa radiation {X =1.5406 A). The morphology of the samples was examined by SEM (Hitachi, S-4200) and TEM (Philips, CM 220). X-ray energy-dispersive spectra (EDS) were obtained using an attached EDAX (coupled with Hitachi S-4200) spectrometer. The specific surface areas and average pore diameters were determined by nitrogen physisorption using a Micromeritics ASAP 2000 automatic analyzer. FT-IR spectra of samples were recorded at ambient temperature on BOMEM MB 104 spectrometer (in the range of 500 ~ 4500cm"') equipped with a diffuse reflectance cell. UVvis diffuse reflectance spectroscopy was performed on a Varian GARY 3E double beam spectrometer using dehydrated MgO as a reference. The catalytic activities of all samples were tested for the liquid phase oxidation of 2,6-DTBP to quinone using HjO^ as an oxidant. Reactions were carried out under vigorous stirring in a two-neck glass flask equipped with a condenser and a thermometer. The oxidation of 2,6 DTBP was conducted using 10 mmol of substrate, 100 mg of catalyst, 10 g acetone as a solvent, and 30 mmol of 35 wt% H2O2. The reaction was performed at 333 K for 2 h. The products were analyzed using a GC equipped with a HP-5 capillary column and a FID. MIBK adsorption was carried out at atmospheric pressure and 297K. A quartz tube (15mm i.d.) containing lOOmg adsorbent in the middle was exposed to 1000 ppm MIBK in air prepared using a saturator at a space velocity of 2400 h'. The adsorption unit was placed inside an isothermal water bath controlled by a mechanical circulator. The adsorbent was freshly calcined at 623 K prior to adsorption runs. Adsorption capacity was measured by a gas chromatograph by integrating the breakthrough curve. 3. Results and discussion All mesoporous titanosilicates obtained by the Ti-isopropoxide grafting exhibit the same XRD patterns and the intensity remained almost constant as those for their pure silica analogues as shown in Figure 1. These confirm that structural integrity of the mesoporous materials remained intact after TIPOT grafting treatment. Table 1 shows the structural properties of the titanium-incorporated mesoporous molecular sieves. The surface areas of all samples were over 750 mVg, being typical of M41S group materials. After grafting, surface area and total pore volume decreased [8]. UV-vis DRS spectra of the calcined titanium mesoporous materials are shown in Figure 2. An absorption band centered at ca. 220 nm is observed in all the samples, and this band is usually assigned to a low-energy charge-transfer transition between tetrahedral oxygen ligands and central Ti"*^ ions [9, 10]. The shoulder at 270 nm probably corresponds to partially polymerized hexa-coordinated Ti species [10], and some polymeric species are suspected to co-exist with the isolated Ti sites in all the mesoporous samples prepared. However, anatase-like Ti02 phase at 330 nm was absent in these samples.
337
Table 1. Pore diameter and BET surface area of calcined materials
1
Ti-MCM-41
^^
11
7
Ti-MCM-48
Ti-KIT-1
1\ ^ 0
1
2
—1
(A)
Surface area (m-7g)
Pore dia.
Si-MCM-41
0.73
30.0
785
Ti-MCM-41 '
0.53
28.3
749
Si-MCM-48
1.25
27.4
1370
Ti-MCM-48'
1.20
27.0
1376
Si-KIT-1
0.92
32.2
955
Ti-KIT-1 '
0.92
30.0
955
Si-HMS
0.93
38.8
834
Ti-HMS'
0.83
38.7
801
Ti-HMS'
1.20
31.0
758
Ti-HMS
Ti-SBA-1 1
Total pore volume (ml/g)
.1
1
4
6
.
1
;
20 Figure 1. XRD patterns of titanium mesoporous materials
^ titanium grafting, ^ titanium source introduced with the silica source during synthesis.
In the FT-IR spectra of M41S type mesoporous materials, the band at 3737cm'' is usually assigned to isolated SiOH groups [11]. The TIPOT grafting of mesoporous materials showed the decreasing of the intensity of IR band at 3747 cm"', which indicates that grafting reaction is taking place between isolated silanol and the TIPOT [12]. Table 2 reports the catalytic activities of the catalysts prepared for 2,6-DTBP oxidation. All the titanium grafted materials were active as catalysts for liquid phase oxidation of 2,6-DTBP, and catalytic activity decreased in the order of MCM-48 (24.5% conversion) > HMS (22.8%) > KIT-1 (16.0%) > MCM-41 (14.3%) > SBA-1 (5%). Apparently, 3 dimensional channel system of MCM -^8, and HMS with small particle size and textual mesoporosity proved to be useful in liquid phase reaction [1,2,3]. Chemical analysis of the titanium-grafted SBA-1 by EDX showed far less titanium at the surface than the others; it seems surface nature of SBA-1 synthesized in acidic medium is different from the rest. All Tigrafted samples suffered from titanium leaching during the liquid phase oxidation; HMS host resulted in over 4 % loss in metal content while the rest showed 2%.
338
TiO.
Ti-MCM-41 Ti-MCM-48 Ti-KITTi-HMS Ti-SBA-1 200
300
400
500
600
700
W ave length (nm ) Figure 2. UV-vis spectra of titanium mesoporous molecular sieves
0
50
100
150
200
250
300
time (min) — ZSM-5 (SiOv'Al,O,= 100. 250mg) — Si-HMS (lOOmg) — Ti-HMS (grafting. lOOmg) — Ti-HMS (Ti source introduced with the silica source during synthesis. lOOmg)
Figure 3. Breakthrough curves of mesoporous materials; 297K. lOOOppmMIBK
Great potential of M41S type mesoporous materials as adsorbents for VOC's have recently been demonstrated for TCE (trichloroethylene) adsorptive removal [13]. In addition, introduction of transition metal elements such as Cr, Ti and Zr were reported to affect the adsorptive properties of mesoporous materials [12. 13, 14]. Titanium introduced by post-synthetic grafting can decrease the terminal silanol population, rendering it relatively hydrophobic [12]. On the other hand, Occelli et al [14], based on Ti- or Zrincorporated MCM-41 samples prepared hydrothermally, stated that sorption site strength for organic compounds increases in the order of Si-OH < Zr-OH < Ti-OH. MIBK adsorption result for each sample is also shown in Table 2. Pure silica mesoporous materials (except SBA-1) had far superior adsorption capacity (0.20.3g/g-catalyst) to ZSM-5. Mesoporous materials seem to have about 6-7 times more adsorption capacity than ZSM-5. In general, the surface area and pore volume are important parameters in adsorption, and mesoporous materials offering much larger BET surface area and pore volume should offer less diffusion restrain and higher adsorption capacity [15]. Among the mesoporous materials, HMS showed the best sorption capacity possibly due to the textual porosity reported [16]. Titanium grafting enhanced the MIBK adsorption to a minor extent. Interestingly, Ti-HMS, in which titanium source was introduced with the silica source during synthesis, showed a significant enhancement in MIBK adsorption over the pure silica analogue as shown in the breakthrough curves in Figure 3. A subtle difference seems to exist in surface property among the Ti-HMS depending on the preparation methods.
339 Table 2. Catalytic and adsorption properties of mesoporous molecular sieves
Catalyst
Adsorbed MIBK amount per unit weight of adsorbent (g/g)
Titanium concentration -EDS / amount of leaching- ICP (wt%)
2,6-DTBP conversion (%)
Selectivity to 2,6-di-tertbutyl quinone (%)
ZSM-5 (SiO2/AlA=100)
0.0316
-
-
-
Si-MCM41
0.2296
-
-
-
Ti-MCM41 '
0.2306
2.72/2.1
14.3
>99.0
Si-MCM48
0.2550
-
-
-
Ti-MCM48 '
0.2743
2.97/2.0
24.5
>99.0
Si-KIT-1
0.1975
-
-
-
Ti-KIT-1 '
0.2242
1.87/2.0
16.0
>99.0
Si-HMS
0.2871
-
-
-
Ti-HMS '
0.2992
2.13/4.4
22.8
>99.0
Ti-HMS '
0.3860
2.68/5.6
23.7
>99.0
Si-SBA-1
0.0302
-
-
-
Ti-SBA-1 '
0.0375
>0.5/-
5
>99.0
Reaction conditions: 10 mmol 2,6-DTBP, 30 mmol H202(35wt% in aqueous solution), 100 mg catalyst, 10 g acetone (as solvent), 337 K, 2 h. ' titanium grafting , ''titanium source introduced with the silica source during synthesis. 4. Conclusion XRD analysis of the titanium-grafted samples confirmed that high structural order was maintained after grafting. Incorporation of the titanium grafted to the mesopore structure was confirmed by UV-vis spectroscopy. Chemical analysis of the titanium-grafted SBA-1 by EDX showed far less titanium at the surface than the others; it seems surface nature of SBA-1 synthesized in acidic medium is somewhat different from the rest. After titanium grafting treatment of mesoporous materials, the band intensity at 3747 cm"' decreased significantly, which indicates that the grafting reaction is taking place between isolated silanol and TIPOT. All the titanium grafted materials were active as catalysts for liquid phase oxidation of 2,6DTBP, and catalytic activity decreased in the order MCM-48 > HMS > KIT-1 > MCM-41 > SBA-1. All samples showed titanium leaching; HMS host resulted in ca. 4 % loss in metal
340
content while the rest showed ca. 2% leaching. In adsorption test, pure silica mesoporous materials had shown far superior sorption capacity to ZSM-5. Ti-HMS in which the titanium source was introduced with the silica source during synthesis, showed the best sorption capacity among the tested materials, and titanium grafting enhanced the MIBK adsorption only to a minor extent. Acknowledgement This work has been supported by the fund provided by Inha University (1999). References 1. C. T. Kresge. M. E. Leonowiz, 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. Leonwicz, C. T. Kresge, K. D. Schmitt, C. T-W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCllen, J. B. Higgins and J. L. Schlenker, J. Am. Chem. Soc, 114 (1992) 10834. 2. P. T. Tanev, M. Chilbwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 3. R. Ryoo, J. M. Kim, C. H. Ko and C. H. Jun, J. Phys. Chem., 100 (1996) 17718. 4. Q. Huo, R. Leon. R M. Petroff and G. D. Stucky, Science, 268 (1995) 1324., M. H. Kim and R. Ryoo, Chem. Mater., 11 (1999) 487. 5. T. Blasco, A. Corma, M. T. Navarro and J. R Pariente, J. Catal., 156, (1995) 65., O. Franke, J. R. Rathousky, G. Schulz-Ekloff and A. Zukal, Stud. Surf. Sci. Catal., 91 (1994) 309., R T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321., W. Zhang and T. J. Pinnavaia, Catal. Lett., 38 (1996) 261. 6. T. Maschmyer, R Rey, G. Sanker and J. M. Thomas, Nature, 159 (1995) 378., R Wu and M. Iwamoto, J. Chem. Soc. Faraday, Trans., 94 (1998) 2871., W. S. Ahn, D. H. Lee. T. J. Kim, J. H. Kim, G.Seo and R. Ryoo, Appl. Catal. A: General., 181 (1999) 39. 7. R. Ryoo, S. Jun, J. M. Kim and M. J. Kim, J. Chem. Soc, Chem. Commun., (1997) 2225., 8. R. Mokaya and W. Jones, J. Chem. Soc, Chem. Commun., (1997) 2185. 9. W. Zhang, M. Froba, J. Wang, P. T. Tanev, J. Wong and T. J. Pinnavaia, J. Am.Chem. Soc, 118(1996)9164, 10. K. A. Koyano and T. Tatsumi, J. Chem. Soc, Chem. Commun., (1996) 145. 11. A. Jentys, K. Kleestorfer and H. Vinek, Microporous and Mesoporous Mater., 27 (1999) 321. 12. T. G. Kang, J. H. Kim, G. Seo and H. C. Park, HWAHAK KONGHAK, 36 (1998) 364. 13. S. Kawi and M. Te, Catal. Today, 44(1998) 101. 14. M. L. Occelli, S. Biz and A. Auroux, Appl. Catal. A: General., 183 (1999) 231. 15. M. Suzuki (eds.), Adsortion Engineering, Japan, Tokyo, 1990. 16. B. J. Aronson, C. R Blanford and A. Stein, Chem. Mater., 9 (1997) 2842.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
341
Ternary transition metal oxides within mesoporous MCM-48 silica phases: Synthesis and characterization R. Kohn, F. Brieler, M. Froba* Institute of Inorganic and Applied Chemistry, University of Hamburg, Martin-Luther-King-Platz 6, D-20146 Hamburg, Germany
Ternary cobalt(II) iron(III) oxide nanoparticles were synthesized within mesoporous MCM48 silica phases by applying wet impregnation technique, drying, and calcination procedures at different temperatures. X-ray diffraction and EXAFS measurements revealed the existence of small particles without the destruction of the MCM-48 host structure. Nitrogen physisorption and low-angle X-ray diffraction data confirmed the preservation of the mesopores of the nanostructured host/guest compound. The measured reduction of the inner surface and pore diameter indicated the coating of the inner surface of the mesopores of the MCM-48 silica phase with cobalt iron oxide. XANES investigations proved the existence of cobalt(II) and iron(III) within the slightly disordered nanoparticles.
1. INTRODUCTION Since 1992, the year of the first report on ordered mesoporous molecular sieves [1] (denoted as M41S phases), these materials have gathered a lot of interest in materials science. Due to their regular pore arrangement and narrow pore size distribution, they extended the range of ordered microporous molecular sieves, e.g. zeolites, to the lower-end region of mesopores. One of the M41S materials the cubic MCM-48 silica phase (space group Ia3d [2,3]) with its two interwoven but unconnected three-dimensional channel systems is of great interest for size and shape selective applications, e.g. molecular sieves [4], catalysis [5], or as host structure for nanometer-sized guest compounds [6]. These materials can be also regarded as a suitable mesoporous model adsorbent for testing theoretical predictions of pore condensation [7,8]. The difficult synthetic access to MCM-48 molecular sieves was the reason that most interest on mesoporous silica has been focused almost exclusively on the MCM-41 type of materials in the past. Only few attempts have been made to synthesize highly ordered MCM-48 phases [9-15]. Previously, we could report the first in-situ formation of binary iron oxide nanoparticles within MCM-48 silica phases [16]. In this study we present first results on ternary cobalt iron oxide nanoparticles within mesoporous silica host structures.
to whom correspondence should be addressed (email:
[email protected])
342
2. EXPERIMENTAL SECTION The pristine MCM-48 silica phase has been synthesized by standard procedures described elsewhere [1]. Wet impregnation technique was used (1.6 molar aqueous solutions of cobalt(II) nitrate (Co(N03)2-6H20) and iron(III) nitrate (Fe(N03)3-9H20) with a ratio of lCo:2Fe) to introduce cobalt iron oxides into the mesoporous MCM-48 molecular sieve. After impregnation the material was calcined at 575° C for 6 hours, followed by a calcination at 600° C for 72 hours (product A) or at 650° C for 72 hours (product B), respectively. The reference material CoFe204 was obtained by calcination of a mixture of cobalt(II) nitrate and iron(III) nitrate with the same ratio as above lCo:2Fe. The calcination was carried out at 650° C for 72 hours. Phase purity was controlled by powder X-ray diffraction (P-XRD). P-XRD data were recorded on a Bruker AXS D8 advance diffractometer (Cu-Ka) in 0/0 geometry with a secondary monochromator. The BET surface areas and pore diameters were determined by nitrogen adsorption/ desorption isotherms at 77 K using a static volumetric technique (Quantachrome Autosorb 1). Before the physisorption measurements the samples were outgassed at 100° C for 15 hours under vacuum. X-ray absorption spectroscopic measurements were carried out at the storage ring DORIS III (HASYLAB@DESY, Hamburg, Germany) at the EXAFS II beam line, which was equipped with a Si (111) double-crystal monochromator. All spectra were recorded at room temperature in a step-scanning mode. For data analyses the program WinXAS [17] was used.
3. RESULTS AND DISCUSSION 3.1 Powder X-ray diffraction The P-XRD patterns of the host structure in compounds A and B show a reduction of the reflections in comparison to the pristine MCM-48 silica material (figure 1). 210
40
50
60
70
pristine MCM-48 silica B (calcination at 650° C) A (calcination at 600° C) -
I .
. • I
• >
.1
•
10 6 20 n Figure 1: X-ray diffractograms of the intercalated cobalt iron oxides A and B and pristine MCM-48 silica; inset: enlargement of the high-angel region of phase A and B.
343
After the treatment of the pristine MCM-48 silica phase with the aqueous solution and following drying all hkl reflections are extinct. They reappear during the transformation of the nitrate to the oxide in the course of calcination [16]. The growing of the reflections as a function of temperature may be interpreted as an annealing of the particles within the pores. The d-spacings of the 211 (d = 33.1 A) and 220 (d = 28.6 A) reflections of the pristine MCM-48 silica are observable in the host/guest compounds whereas the higher order reflections (20 : 4-6°) disappear. In addition, the 113/021 (d = 2.53 A; 1=100%) and 208/220 (d = 1.48 A; 1=36%) reflections of the inverse cobalt iron structure appear in sample A and B. In contrast to the bulk material of CoFe204 (figure 2) synthesized under exact the same conditions as the phases A and B, the reflections of the host/guest compounds are much broader and weaker in intensity , indicating the occurrence of very small particles. 113/021
208/220
20
30
40
26 n Figure 2: X-ray diffractogram of the inverse spinel cobalt ferrite CoFe204. Applying the Scherrer formula to the 113/021 reflections reveal a particle size of approximately 5-6 nm. This size, which is larger as the mean pore diameter of the host structure, can either be advised to elongated particles within the mesoporous network or by particles on the outer surface. Against the latter point speaks the fact that an undisturbed crystal growth of unconfined particles outside of the mesopores, would lead to larger particle sizes and therefore to much higher peak intensities of the corresponding XRD pattern (compaiQfigure1 dind figure 2).
3.2 Nitrogen physisorption The nitrogen adsorption/desorption isotherms show typical type IV profiles (lUPAC classification [18]) for the pristine MCM-48 and the Co/Fe/O/MCM-48 silica materials A and B {figure 3).
344
T
0.4 0.6 relative pressure p/p^
Figure 3: Nitrogen adsorption/desorption isotherms (taken at 77 K) of the pristine MCM-48 silica phase and the cobah iron oxide containing MCM-48 silica materials A (calcination at 600° C) and B (calcination at 650° C). The sharp steps in the isotherms at a relative pressure p/po = 0.28 in case of the pristine MCM-48 silica and 0.23 for the transition metal containing silica reveal the high order of the mesoporous systems. The hysteresis starting at a relative pressure of p/po = 0.47 arises from a second porous system, which will be discussed elsewhere. Applying the BJH theory to the adsorption isotherms of all three samples the pore diameter distributions can be calculated. The pore diameter distribution of the pristine MCM-48 material shows a sharp maximum with a mean pore diameter of 2.49 nm (figure 4).
pristine MCM-48 A B
2
3
4
pore diameter [nm] Figure 4\ Pore diameter distribution of the pristine MCM-48 and the Co/Fe containing materials A and B.
345
We are aware of the fact that the BJH theory, which is based on the Kelvin equation, underestimates the pore diameter by ca. 1 nm. Nevertheless it provides valuable information on the relative changes of the pore diameter. For the Co/Fe oxide containing silica the mean pore diameter of the slightly broader distribution is decreased to 2.25 (A) and 2.24 nm (B). The reduction of the adsorbed gas volume as well as the smaller mean pore diameter is a first evidence that the Co/Fe oxide is located within the pores of the MCM-48. It is remarkable that the different calcination temperatures do not affect the pore system. In contrast to the small changes in the XRD patterns the differences in the adsorption/desorption isotherms are marginal. To ensure that the Co/Fe oxide nanoparticles have been formed mainly within and not outside the pores, the pore diameter was also determined by applying another method based on geometrical calculations. By assuming cylindrical pores the average pore diameter Dh can be calculated according to Dh = 4-Vp/As(BET) [8]. As(BET) is the specific surface of the mesoporous channel system obtained by the BET method and Vp is the pore volume at the relative pressure p/po < 0.40. Table 1 shows respective data of the pore diameters, the specific surface, and the pore volume for the pristine MCM-48 silica phase and the transition metal oxide containing phases A and B.
Table 1: Characterization of the pristine MCM-48 silica and the host/guest compound A and B with respect to specific surface area, pore volume and mean pore diameter. As(BET) [m^/g]
Vp[10-^m^/g]
dejH [nm]
Dh [nm]
Pristine MCM-48
1217
0.81 (p/po=0.40)
2.49
2.66
A
569
0.35 (p/po=0.38)
2.25
2.46
B
564
0.34 (p/po=0.38)
2.24
2.41
BET specific surface area As(BET); total pore volume Vp; BJH mean pore diameter dejH; average pore diameter Dh = 4 Vp/As(BET). The reduction of the inner surface and pore volume is comparable to the reduction that is found for the incorporation of iron oxides within MCM-48 silica phases [16]. The reduction of the mean pore diameter obtained is of the same magnitude. This is a strong evidence for the incorporation of the ternary transition metal oxide within the mesopores of the MCM-48 molecular sieve. If the oxide would be on the outer surface the pore diameter dejH would not change going from the pristine MCM-48 to the impregnated products A and B, but the calculated pore diameter Dh must change because the reduction of the inner surface and pore volume would be still observable. Both parameters are normalized to the weight.
3.3 X-ray absorption spectroscopy X-ray absorption spectroscopic (XAS) measurements were performed at the Fe K-edge and Co K-edge in order to obtain information on the structure and oxidation state of the Fe and Co within the transition metal oxide nanoparticles. Bulk a-Fe203 and CoFe204 were used as reference compounds for the Fe K-edge and C03O4 and CoFe204 for the Co K-edge. The
346 XANES (X-ray absorption near edge structure) region of the Fe K- and Co K-edge are compared m figure 5 a and h. CoK
FeK
/^X
®
/ ' ^^N X\ ^ / ' • • ^ -• ^^ -^^ / / .' * . ^ ^^""^^^ /
c o
+-»
/
oC/5
r ' '"'\ ''-' ""'^"^ /
'
/
•
/ / • ' /
11
-D cd J
—
O
• •
"
^" •"• "•
^
a-Fe,Oj
----CoFeA
c --""•' /'/
A
'\._^.y
B
7.11
" • • • -
'
/*/ •, 1/
^^:/
7.10
-
" ' ^ -
7.12
7.13
7.14
7.70
7.71
7.72
7.73
keV keV Figure 5: Comparison of the Fe K-edge (a) and the Co K-edge (b) XANES spectra of the nanostructured host/guest compounds A and B and reference materials. Considering the energy position of the Fe K-edge (figure 5 a) the structured host/guest compounds appear at the same position as the bulk reference materials CoFe204 and a-Fe203, indicating a valence shift characteristic for iron(III). In addition, the overall shape that can be seen as a fingerprint of the structure shows strong similarities to the XANES of CoFe204. All features are a little bit smeared but, which is typical for slightly disordered nanoparticles [16,19]. In contrast, the XANES of the Co K-edge (figure 5 b) of the C03O4 is much more complex compared to the nanocomposites and the reference material CoFe204. CoFe204 is an inverse spinel with cobalt(II) only while C03O4 is a 1:2 mixture of cobalt(II) and (III). Again the X A N E S of the host/guest compounds indicates the existence of cobalt(II) only and therefore the formation of CoFe204. CoO can be excluded because it requires much higher synthesis temperature.
CoK
FeK f\ / \ / \
® ^^
4"'
,s
^ ^ "-rCzOs CoFe.O, \
A
v. ^ \X \
^
/ ,' • / • '. - - - : i'\
'! \
-
-
-
•
'
/
/ 0
1
\! 2
\ 3
4
5
6
7
8
R[A] Figure 6: Comparison of the Fe K-edge (a) and the Co K-edge (b) Fourier transformations of the nanostructured host/guest compounds A and B and reference materials.
347
First qualitative analyses of the Fourier transformations (FTs) of the respective EXAFS (extended X-ray absorption fine structure) oscillations of the reference compounds (figure 6) show peaks up to 5.5 A while the incorporated transition metal oxides A and B show only peaks up to 3.3 A. A comparison of the first shell (oxygen shell) between 1 and 2 A shows no difference in the magnitude for the reference CoFe204 and the products A and B. In contrast, the higher shells, which represent the metal-metal distances, are strongly reduced for the products A and B compared to the bulk reference compounds, which is another indication for small particles within the pores. Further quantitative EXAFS analyses are currently in progress.
4. CONCLUSIONS In this study we present the first in-situ formation of a ternary transition metal oxide within the pores of MCM-48 silica. XRD measurements showed the preservation of the host structure as well as the formation of 5-6 nm small particles. In addition, the analysis of nitrogen physisorption data revealed the existence of mesopores with smaller pore diameters and surface areas in comparison to the pristine phase, which can be attributed to the introduction of the Co/Fe/0 phase into the pores. First qualitative XANES and EXAFS analyses support the formation of CoFe204 nanoparticles. TEM investigations on particle size and structure are in progress. To learn more about the properties of the oxide nanoparticles Mossbauer and magnetic measurements have to be carried out, which are planned for the future. Summarizing one can say that in case of these complex nanostructured host/guest compounds only the application of several complementary characterization techniques give reasonable results on the structure of the system.
4. ACKNOWLEDGEMENTS The Deutsche Forschungsgemeinschaft (Fr 1372/1-1 and Fr 1372/2-1) and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank HASYLAB@DESY for allocating beamtime and Dr. M. Tischer (HASYLAB) for help during the XAS measurements. Finally, we would like to thank Dr. M. Thommes (Quantachrome, Germany) for valuable discussions and assistance in the analysis of the physisorption data.
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. 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.
348
2.
3.
4. 5. 6. 7.
8. 9. 10. 11. 12. 13. 14. 15. 16.
17. 18. 19.
R. Schmidt, M. Stocker; D. Akporiaye, E. H. Torstad, A. Olsen, Microporous Mater., 5 (1995) 1; V. Alfredsson, M. W. Anderson, T. Ohsuna, O. Terasaki, M. Jacob, M. Bojrup, Chem. Mater. 9 (1997) 2066; V. Alfredsson, M. W. Anderson, Chem. Mater., 8 (1996) 2066 A. Monnier, F. Schuth, Q. Huo, D. Kumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, P. Petroff, A. Firouzi, M. Janicke, B. F. Chmelka, Science, 261 (1993) 1299. U. Ciesla and F. Schuth, Microporous Mesoporous Mater., 11 (1999) 131. A. Corma, Chem. Rev., 97 (1997) 2373. K. Moeller and T. Bein, Chem. Mater., 10 (1998) 2950. P. I. Ravikovitch, S. C. O. Domhnaill, A. V. Neimark, F. Schuth, and K. K. linger, Langmuir, 11 (1995) 4765; P. Ravikovitch, D. Wie, W. T. Church, G. L. Haller and A. W. Neimark, J. Phys. Chem. B, 101 (1997) 3671; J. P. Branton, P. G. Hall, K. S. W. Sing, H. Reichert, F. Schuth, and K. K. Unger, J. Chem. Soc, Faraday Trans., 90 (1994) 2965. M. Thommes, R. Kohn and M. Froba, Stud. Surf. Sci. Catal. (1999) in press; M. Thommes, R. Kohn and M. Froba J. Phys. Chem. B (1999), submitted. A. A. Romero, M. D. Alba, W. Zhou, J. Klinowski, J. Phys. Chem. B, 101 (1997) 5294. P. Behrens, A. Glaue, C. Haggenmuller, G. Schechner, Solid State Ionics, 101-103 (1997), 255. K. W. Gallis, C. C. Landry, Chem. Mater., 9 (1997) 2035. A. Corma, Q. Kann, F. Rey, Chem. Commun. (1998) 579. F. Chen, L. Huang, Q. Li, Chem. Mater., 9 (1997) 2685. J. Xu, Z. Luan, H. He, W. Zhou, L. Kevan, Chem. Mater., 10 (1998) 3690. J. M. Kim, S. K. Kim, R. Ryoo, Chem. Commun. (1998) 259. M. Froba, R. Kohn, G. Bouffaud, O. Richard, G. van Tendeloo, Chem. Mater., 11 (1999) 2858; R. Kohn, G. Bouffaud, O. Richard, G. van Tendeloo, M. Froba, Mat. Res. Soc. 5v/w.Proc. 547(1998)81. T. Ressler, J. Synchrotron Rad., 5 (1998) 118. K. S. W. Sing, D. H. Everett, R. A. W. Haul, L. Mouscou, R. A. Pierotti, J. Rouquerol, T. Siemieniewska, Pure &Appl. Chem. (1985) 57. M. Froba and O. Muth, Adv. Mater., 11 (1999) 564.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
o^g
The inclusion of polymeric carbon in channels of the siliceous MCM-41 mesoporous molecular sieve J. Hlavaty, L. Kavan, J. Rathousky and A. Zukal J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic, Dolejskova 3, CZ-182 23 Prague 8, Czech Republic l,4-Diiodo-l,3-butadiyne and l-iodo-l,3,5-hexatriyne were prepared and used as precursors for the generation of polymeric carbon containing polyyne chain segments. The polymerization of the former precursor was initiated by the UV irradiation, while that of the latter one by a concentration rise due to stripping off the solvent. To protect the polyyne chains the polymerization was performed in the channels of the siliceous MCM-41 mesoporous molecular sieve. The FTIR spectra of polymeric carbons confined in the charmels showed a broad band at 2090 - 2180 cm"', which proved the presence of linear polyyne sequences. The localization of the polymeric carbon in the channels of the MCM-41 was confirmed by X-ray diffraction and nitrogen adsorption.
1. INTRODUCTION Linear polymeric carbon chains containing conjugated triple bonds called carbynes or polyynes have been a topic of a number of recent studies [1-4]. Owing to their extraordinary optical, electronic and mechanical properties, these materials are very promising for applications in molecular electronics and materials science [3]. Because of the intrinsic instability of the polyyne chains, several methods have been suggested for their protection, such as (i) capping their ends with attached spacious groups [3], (ii) the separation of chains using spacious substituents, (iii) including substituted aromatic rings in the polyyne chains [5] and (iv) the encapsulation of these chains in the voids of porous inorganic hosts, like microporous or mesoporous molecular sieves. Since the encapsulation of several conducting polymers, such as polypyrrole, polyaniline and pyrolyzed polyacrylonitrile, in the well-defined porous systems of both mentioned types of molecular sieves has been already reported [6-10], this procedure seems to be a promising way for the protection of polyyne chains. There are principally two pathways to encapsulation of polyyne chains in the voids of the porous host, viz. the intraporous polymerization of a suitable monomeric precursor and the intercalation of soluble oligoynes followed by their propagation. The former way includes the penetration of the porous structure with a soluble monomeric precursor and the subsequent intraporous polymerization, which can be started, e.g., by the UV irradiation. The latter one
This research was supported by the Grant Agency of the Czech Republic via the Grant No. 203/98/1168.
350
involves the impregnation of the porous host with an oHgomer, which then spontaneously polymerizes in the host pores by a radical mechanism without the evolution of a low molecular side-product. The present study aims at the development of procedures for the generation of polyyne chains in the mesopores of the siliceous MCM-41 and the characterization of materials obtained. As precursors, l,4-diiodo-l,3-butadiyne (C4I2) and l-iodo-l,3,5-hexatriyne (CeHI), were used.
2. EXPERIMENTAL 2.1. Synthesis of precursors The siliceous MCM-41 mesoporous molecular sieve used as a host was prepared according to [11]. C4I2 and C6HI were prepared according to the recipes given in [12]. The stability of both precursors is limited. While C4I2 can be stored in small amounts in a freezer at -30 °C for several months, CeHI is stable only in its n-hexane-ether solution. 2.2. The polymerization of C4I2 and CeHI in the channels of the MCM-41 The assembly for the inclusion of polymeric carbon in the channels of the MCM-41 consisted of two quartz ampoules connected to a vacuum pump (Fig. 1). A solution of C4I2, prepared by dissolving of 100 - 200 mg of this precursor in 6 ml of petroleum ether at 40 °C, was put into the ampoule 1, cooled by liquid nitrogen and evacuated. Then the assembly was sealed off at the narrowing A. MCM-41 host (200 mg) was dried in the ampoule 2 at 480 - 510 ""C under vacuum of 10'^ Pa for 10 hours. Then the assembly was sealed off at the narrowing B. Dried molecular sieve was transferred through the broken bubble valve into the precursor solution in the ampoule 1, which was held at room temperature. After mixing, the solvent was slowly stripped off by cooling down the ampoule 2 by liquid nitrogen. Then the ampoule 1 was sealed off at the narrowing C and irradiated with a 100 W UV lamp for 48 - 60 hours. The pale yellow powder-like material turned gradually brown and finally black. When the irradiation was completed, the ampoule was opened and the material extracted with toluene. Then it was flushed with petroleum ether and carefully dried in vacuum of 150 - 200 Pa at room temperature. 1 - 7 ml of the n-hexane-ether solution of CeHI was mixed with 200 mg of the MCM-41 pre-treated in the described assembly (Fig. 1). After stripping off the solvent, a brown-yellow material remained in the ampoule. It turned dark brown during 24 hours when kept at room temperature in daylight, which showed the completeness of the polymerization. Then the ampoule was opened, evacuated for 4 hours (150-200 Pa) and subsequently extracted with toluene. After flushing with petroleum ether, the rest of the solvent was removed in vacuum at room temperature. The materials obtained were studied by FTIR spectroscopy, physical adsorption and X-ray diffraction. Their composition was determined by elemental analysis. 2.3. Methods FTIR spectra were recorded with an Impact 410 (Nicolet) spectrometer. Powder X-ray diffraction data were obtained on a Siemens D 5005 diffractometer in the Bragg-Brentano geometry arrangement using CuKa radiation. Adsorption isotherms of nitrogen at -196 °C
351
VACUUM
Figure 1. Assembly for the inclusion of the polymeric carbon in the channels of the MCM-41. were measured with an Accusorb 2100E instrument (Micromeritics). Each sample was degassed at 200 °C for at least 20 hours until a pressure of 10"^ Pa was attained. As the samples containing polymerized C4I2 and CeHI released practically all the iodine in vacuum at 200 ^C, they were pre-evacuated separately in a special apparatus.
3. RESULTS AND DISCUSSION 3.1. The structural parameters of the MCM-41 The structural parameters of the parent MCM-41 were determined using X-ray diffraction and nitrogen adsorption. The X-ray diffractogram is typical for the well-ordered MCM-41 structure, five Bragg reflections being clearly discernible (Fig. 2, diffractogram 1). The lattice constant, calculated assuming P6 symmetry, equals 4.2 nm. Fig. 3 shows the nitrogen adsorption isotherm on the MCM-41. While the first increase in the amount adsorbed occurring up to p/po « 0.05 corresponds to the multilayer coverage of the surface, the second one at p/po « 0.3 - 0.4 to the volume filling of mesopores. The total surface area Stot ofll09m^/g, external surface area oext of 110m7g and mesopore volume Vme of 0.806 cm^/g were evaluated from the nitrogen adsorption isotherm using the comparison plot method [11]. The size of the MCM-41 hexagonal channels can be characterized by their hydraulic diameter Dme, whose size of 3.2 nm was calculated according to the formula Dme = 4 . 2 Vme / Sme = 4 . 2 V ^ e / (Stot ' Sext) ,
(1)
where Sme is the surface area of mesopores. The pore wall thickness 5 = 1.0 nm was assessed
352
Figure 2. X-ray diffractograms of the parent MCM-41 (1) and sample B3 (2).
Figure 3. Nitrogen adsorption isotherms on the parent MCM-41 (1) and sample A2 (2). Open and solid points denote adsorption and desorption, respectively.
from Dme and the lattice constant using a geometrical model of the MCM-41 structure [13]. This model, which is valid only for a well-ordered MCM-41 structure, enables also to calculate the total length of pores: U e = V(3).S„,e'/24.Vn,e;
(2)
for the MCM-41 sample used Lme equals 89.4 x 10^ m/g. According to the variation in the pore diameter and the total pore length, structural changes due to the encapsulation of polymeric carbon in the channels of the MCM-41 will be assessed. 3.2. The formation of the polymeric carbon The following reaction scheme of the formation of the polymeric carbon can be assumed (according to [12]). The iodobutadiyne radical forms after the photochemical homolytic splitting of the C-I bond: hv IC^C-C^CI
•C=C-C=CI + Yih
This radical dimerizes forming diiodooctatetrayne, a very reactive intermediate, which spontaneously polymerizes:
353
2•C=C-C=CI
^
IC=C-C=C-C=C-C=CI
n IC^C-C=C-C=C-C=CI
-^
=[IC-C=C-C=C-C=C-CI]n= -[IC=C=C=C=C=C=C=CI]n-
The released iodine is slowly added to C4I2 yielding C4I6: 2I2 + IC=C-C=CI
->
I2C=CI-CI=CI2
A mechanism anticipating a radical polymerization was suggested for the polymerization of C6HI at room temperature due to a concentration rise [8]. This mechanism leads to an unstable polymer: n IC=C-C=C-C=CH
->
=[IC-C=C-C=C-CH]n=
^
-[IC=C=C=C=C=CH]n-
After explosive decomposition of this polymer, a polymeric carbon is formed, which does not display any polyyne bands in the FTIR spectrum. The iodine, which was released by the decomposition of the polymer, could be added to C6HI yielding finally the dimer C12H2I8: 2 IC=C-C=C-C=CH + I2
->
(IC=C-CI=CI-CI=CH)2.
3.3. The formation of polymeric carbon in the channels of the MCM-41 host FTIR spectra of the polymeric carbon formed in the channels of the MCM-41 by both the photochemical polymerization of C4I2 and the polymerization of CeHI due to a concentration rise exhibit a broad polyyne band at 2090 - 2180 cm'* (Fig. 4, Tab. 1). Consequently the mesoporous structure of the MCM-41 protects the reactive polyyne chains. Moreover, the confining of C6HI in the channels of the MCM-41 eliminates the explosive behaviour of the polymeric product. The elemental composition of MCM-41 samples with encapsulated polymeric carbon is presented in Tab. 1. All these materials contain relatively large amounts of iodine, which probably improves the stability of polyyne chains [14]. Despite the protective role of the MCM-41 porous structure, the stability of polyyne sequences in carbon chains against the heat exposition is limited. When all the materials prepared are heated in air at 100°C, the polyyne band in the IR spectrum diminishes. The heating at 160-180 ""C in vacuum of 10"'Pa leads to the release of iodine and the disappearance of the polyyne band. With samples containing small amounts of polymeric carbon (especially samples Al, A2, A3 prepared from C4I2), the MCM-41 structure was completely preserved, as their X-ray diffractograms were practically identical with that of the parent MCM-41. However, the diffractogram of sample B3, containing the largest amount of polymeric carbon, is characterized by only four Bragg reflections and less distinct resolution (Fig. 2, diffractogram 2). These changes can be ascribed to a slight damage of the MCM-41 porous structure and to the decrease in the electron density contrast between the pore walls and pore voids. All features of the adsorption isotherm of the parent MCM-41 are preserved also with samples containing polymeric carbon. (As a typical example, the isotherm on sample A2 is
354
Table Polymeric carbon generated in the channels of the MCM-41
Sample
Precursor/MCM-41 ratio^
Elemental composition (wt. %) H I
C
FTIR polyyne band (cm-^) Si02
Diiodobutadiyne precursor 6.18 8.31 8.10
0.52 0.40 0.95
18.60 19.70 20.80
74.70 71.59 70.15
2090-2180 2160-2180 2165-2180
8.60 14.35 16.90
0.63 0.80 0.80
11.50 19.20 26.20
79.27 65.65 56.10
2140-2180 2160-2180 2166-2180
150/200 200/200 200/200
Al A2 A3
lodohexatriyne precursor 1/200 5/200 7/200
Bl B2 B3
^ Expressed as mg of C4I2 / nig of the MCM-41 with samples Al A3, ml of ether - n-hexane solution of C6HI / mg of the MCM-41 with samples Bl - B3.
diols -> ether the d-value of the nanostructure steadily increases from 32.6A to 37.6A which is probably caused by e.g. a thicker wall structure. The strong shift of the d-value going from diols to the ether may be explained by the fact that this ligand substitutes only one alkoxide ligand in contrast to the diols and for that reason leaves more alkoxide ligands unchanged which are prone to easy hydrolysis and condensation reactions resulting in a faster growing of the walls. Equidistant bragg reflections in the powder pattern and transmission electron microscopic investigations (figure 2) indicate the presence of a lamellar phase for the product synthesized with 2,4pentanedione. The other three products show only rather broad reflections in the P-XRD. In addition, TEM investigations (not depicted) indeed reveal the same features as for the former product but with a lower degree of order and smaller domains. The d-value which is obtained
modifier 2,4-pentanedione 1,5-pentanediol 1,3-propanediol EGmBE
d[A] 32.6 33.0 33.6 37.6
Figure 1. P-XRD pattern of products synthesized with different modifiers under otherwise comparable conditions (80°C, 10% dodecylphosphate, 5d, Ti.modifier/l.T)
Figure 2. TEM picture of a titania / dodecylphosphate composite synthesized at a low surfactant concentration (10%) with 2,4-pentanedione
360 from TEM-pictures confirms within margins of error the values obtained by P-XRD. EXAFS investigations of the mesostructured composites at the Ti K-edge reveal the condensation of the Ti(0'Pr)4 precursor within the mesostructure [8]. The short range order for the first tw^o coordination shells (Ti-0, Ti-Ti) is almost the same compared to the bulk phase anatase, although the bond distances are slightly longer. In addition, thermoanalysis of the pure surfactant and the mesostructured products showed that there is no evidence for significant amounts of ligands which could have been retained in the titanium precursor. The structure and d-values of the products corroborate the results of former groups using 2,4-pentanedione as modifier [1,14]. Unfortunately all titania products including the products synthesized according to literature procedures [1] could not be calcined or extracted without a complete loss of the structure according to P-XRD, indicating almost exclusive building of lamellar phases. Physisorption measurements complement these results showing BET surface areas < 50mVg for the calcined or solvent extracted samples. Interestingly at high surfactant concentration of about 30%wt and higher a modifier is not necessary anymore. This could be due to the less amount of water, which is required for a fast hydrolysis leading to condensed phases. In figure 3 powder diffraction patterns of two products synthesized with and vdthout 2,4-pentanedione at 40%wt surfactant concentration are shovm. The d-value of both products reveales almost the same but the order inside the mesostructure seems to be slightly better for the product synthesized without modifier. A transmission electron microscopic picture of this product is shown in figure 4. It can be seen that domains revealing the lamellar structure are relative small. Part of them show a significant degree of curvature mimiking fingerprints like it has been shown earlier for other materials, e.g. zirconium oxide sulfate [4]. Nevertheless, these products cannot be calcined or extracted without the formation of condensed phases.
with 2,4-pentanedione -without 2,4-pentanedione
Figure 3. P-XRD pattem of products synthesized with and without modifier at a dodecylphosphate concentration of 40%wt (80°C, 5d)
Figure 4. TEM picture of a titania / dodecylphosphate composite synthesized at a high surfactant concentration (40%) without modifier
361 4. NANOSTRUCTURED RHENIUM DIOXIDE In our search for suitable precursors for the synthesis of mesoporous rhenium oxides we found an extraordinary organometallic compound which fulfilled the requirements needed. It had to be stable to a high degree in water; but reactive enough to undergo hydrolysis and condensation reactions. In addition, it should be able to interact with a suitable structuredirecting lyotropic template during the synthesis. Methyltrioxorhenium (MTO) complies with all of these demands and is easy to synthesize [15]. It is well soluble in water and polymerises only at elevated temperatures around 80°C building the only known polymeric organometallic oxide to date. The use of primary long chain alkylamine solutions as structure-directing agents is appropriate due to the fact that MTO readily forms adducts with a great variety of amines. In figure 5 four conceivable pathways are depicted. All of them have been tested for the synthesis of mesostructured rhenium oxides. The left pathway comprising first the isolation of the pure base adduct and afterwards the application of hydrothermal treatment failed already in the first step. The base adducts of C12-, C14- and Ci6-amine could not be obtained in pure form. In any case redox reactions took place in advance of fiill isolation of the adduct similar to results obtained by Herrmann et al. during the synthesis of various amine adducts [16]. On the mid left side pathway, poly-MTO should first be isolated, which then would be hydrothermally treated with a surfactant solution to yield nanostructured poly-MTO or rhenium oxide. Due to incompleteness of the first reaction step this way is also not favourable.
r~^ A\
CH,
kj
on\
Jr
MTO base adduct o
Miquid^ crystal
/^cooperative >^ mechanism PolyMTO
^ 'Y^TT''
nanostructured Poly-MTO nanostructured Re02/Re03
LJ:
nanostructured Poly-MTO nanostructured ReO^/ReOg
Figure 5. Conceivable synthesis strategies for nanostructured rhenium oxides
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426 various techniques and also suggested that the silica/titania particles stack in the interlayer space of clay sheets. Furthermore, the pillared layered structure in this solid can be altered radically by supercritical drying (SCD) (3) and treatment with surfactant octadecyltrimethylammoium (OTMA) (5). In contrast, the change in the pore structure of other pillared clays due to SCD and the treatment with surfactants is limited, according to our results. The pore structure of sol pillared clay appears quite versatile and can be readily tailored. Meanwhile, the relative large pore size and pore volume in the sol-pillared clay provide room for further modification to enhance the shape selectivity of the solid when used as adsorbents or catalysts. These features may lead to a wide range of applications of the solpillared clay. In the present work, three approaches: supercritical drying, templating with surfactants of quaternary ammonium salts and a two-step modification are applied to the sol pillared clay to tailor the pore structure. Mechanisms involved in these processes are discussed in detail.
2. EXPERIMENTAL 2.1. Preparation of sol-pillared clays Sodium bentonite with a cation exchange capacity (CEC) of 75 meq/100 g of clay, supplied by Commercial Minerals Ltd., Australia, was used as starting clay material, to prepare samples for SCD and surfactant treatments. Besides, sodium montmorillonite (Kunipia G), from Kunimine Industrial Company, Japan, was used as the starting clay for samples of pore opening modification. CEC of this clay is 100 meq/100 g of clay. The sol-pillared clay was prepared following the procedures of Yamanaka et al (1). The mixing ratio of Msi (mol)/A/Ti(mol)/CEC equivalent of clay was 30/3/1. Si02-Ti02 sol pillared montmorillonite, subjected to a calcination for 4h at 773 K, was labeled as sol-PILM. Portions of the wet cake prepared with bentonite were subjected to SCD or treatment with surfactants as described below. 2.2. Sample drying and calcination Supercritical drying (SCD): The wet cake of sol pillared bentonite (sol-PILB) was dispersed into absolute ethanol and filtrated, washed with ethanol for several times to replace the water by ethanol. The cake was transferred to a cartridge and the ethanol was extracted by supercritical fluid (CO2) for 3 h under 3000 psi, at 323 K. The flow rate was controlled around 2.5 ml/min. The sample was then calcined at 773 K for 12 h and labeled as sample SCD. 2.3. Templating with quaternary ammonium salt surfactants A portion of the wet sol-intercalated clay was mixed with a surfactant of quaternary ammonium salts [CH3(CH2)n-i N(CH3)3Br] by stirring for 2 hours. 15.75 mmol of surfactant was added to each gram of the starting bentonite clay. The resultant mixture of clay and surfactant was transferred into an autoclave and kept in an oven at 100°C for 3 days. The wet cake was washed with water to Cf ions free and the solid was recovered by filtration. The solid was dried in room temperature and calcined at 773 K for 4 h. The calcined products were labeled as sol-PILB-Cn, where n denotes the number of carbon atoms in the alkyl chain of the surfactants used. Four samples were prepared: sol-PILB-C12, -C14, -CI 6 and -CI 8.
427
2.4. Loading of Ca^^ ions into the pillared clay and modification with TEOS The calcined sol FILM powder (2.0 g) was dispersed in an aqueous solution of Ca(N03)2 (100 ml, 0.1 M). The pH of the dispersion is about 3.2. It was adjusted to 7 -8 by adding of a diluted solution of NaOH (0.02 M). Since the pH value of the dispersion decreased gradually with time, a small dose of NaOH solution was added each time so that the pH was fixed at a certain value. After stirring the dispersion for about 12 hours the solid was separated from the suspension by filtration, washed with deionized water and dried at 323 K overnight. This sample is hereafter labeled as Ca^"^ doped sol-PILM (Ca-STPILM). 0.5 g of Ca-STPILM was equilibrated with 10 ml of TEOS at 343 K for 2 days. The solid product was separated by filtration, washed with ethanol twice and with water once, dried overnight and calcined at 673 K for 2 h. The product is named as modified sol-PILM. 2.5. Characterization Chemical composition of the samples was analyzed on an inductively coupled plasmaatomic emission spectrometer (ICP-AES). Fusion method (6) was used to prepare samples and standards. X-ray diffraction patterns of sample powder were recorded on a Philips PW 1840 powder diffractometer with cobalt A^a radiation at 40 mA and 100 kV and a nickel filter. The N2 ads-desorption isotherms of samples were measured at liquid nitrogen temperature using a gas sorption analyzer (Quantachrome, NOVA 1200). The samples were degassed at 323 K for 3 h prior to the measurement. The surface area was calculated by the BET equation and the external and micropore surface areas were determined through the /-plot method of Lippens and De Boer (7). The ads-desorption isotherms of organic vapors were measured at 295 K on the samples degassed similarly using a gravimetrical rig with quartz springs as the micro-balance elements. The liquid densities of the organics (in g/cc) are: toluene, 0.865; pxylene, 0.866; w-xylene, 0.868 and mesitylene, 0.864.
3. RESULTS AND DISCUSSIONS
0.4
3.1. Framework of sol pillared clay The diffraction peaked at about 5.3 ° is After calcination the most obvious one in the XRD pattern of the calcined sol-PILB (Fig 1). The broad 0.2 peaks reflect some short-range aggregation of I B e f o r e calcination clay sheets. The long-range aggregation of clay layers could be in a poor order. This result is similar to that reported by Occelli et al (3). The basal spacing is thus estimated to 5 10 15 20 be 1.96 nm and the free spacing, 1.0 nm (= 26, degrees 1.96 nm - 0.96 nm, where 0.96 nm is the thickness of a single silicate layer of Figure 1 X-ray diffraction (XRD) patterns montmorillonite clay), which is consistent of Si02-Ti02 sol-PILB. with the mean pore width of 1.02 nm, obtained from nitrogen adsorption data (Table 1). N2 ads-desorption isotherm of the sample also indicates evidently that the sample is a microporous solid (Figure 2). No obvious peaks are observed on the XRD patterns of SCD solid and the sol-PILB-Cn sample. However,
428
distinct changes in pillared layered structure, caused by the SCD and surfactant treatments, are observed from nitrogen ads-desorption data. oo ^ f i N2 ads-desorption isotherms of solsol-PILB-C16^,,^ ji H 400 PILB (the normal sample), supercritical dried c/o y'^^ |i y^^ 1 B sample (SCD) and sol-PILB-C 16 are shown in /^'^ / o Figure 2. The isotherm for sol-PILB is of the c o character of Type I, generally observed for > ^ SCD s a m p l e / y B" 200 oC/3 microporous solids (8,9), except for the -T3 obvious hysteresis. The isotherm of sol-PILB< p normal sol PILB C16 is of Type IV, with a remarkably large adsorption capacity. Similar situations are observed for other sol-PILB-Cn samples. This 0.0 0.2 0.4 0.6 0.8 1.0 suggests that the surfactant treatment create P/PQ substantially large amount of pore volume in Figure 2 N2 ads-desorption isotherms of the sol-pillared clays. SCD treatment results in the clay samples. Solid symbols represent a significant increase in adsorption at high relative pressures {P/Po >0.7), whereas some adsorption and empty ones, desorption. adsorption increase at low relative pressures (P/PQ < 0.2) is observed. The isotherm exhibits mainly the character of Type II isotherms, which are usually observed on nonporous or macroporous solids. The radical changes in the isotherms suggest that SCD and surfactant treatments bring about significant change in the framework of sol pillared clay, r-plots of the samples were constructed using N2 adsorption data to calculate various parameters of the pores in the samples. The results are summarized in Table 1. ^ r f ^
j/*^
n
/'
/ /
!
1
H
Table 1 Porosity parameters of the samples estimated from N2 adsorption data sol PILB SCD sample sol-PILB-C 16 BET surface area (m^/g) Total pore volume (cm^/g) Surface area of micropores (m'^/g) Surface area of framework pores (m^/g) Volume of micropores (cm^/g) Volume of framework pores (cm^/g) Mean width of micropores (nm) Mean radius of framework pores (nm)
302.6 0.214 227.4 0.116 1.02 -
400.2 0.746 178.2 0.101 1.13 -
750.2 0.876 475.9 0.331 1.39
The mean pore width (or radius) is derived from the ratio of pore volume to the surface area of these framework pores (or micropores) when geometry of the pores is assumed. According to the results in Table 1, SCD creates larger surface area at the expense of substantial loss in the surface area and volume of micropores. Sample SCD has a quite low apparent density, compared with the sample dried in air. Evidently, SCD treatment results in a significant change in the framework of the sample. The pillared layered structure which is
429 associated with formation of micropores within the galleries between clay layers does not prevail in the sample. In the slurry of clay and the pillaring solution, the clay platelets are oriented relatively randomly. When the slurry is filtered and the washed cake is dried in air, strong interfacial force of water and large aspect ratios of clay sheets favor the aggregation of face-to-face (FF) orientation (3). Therefore, in the sample dried in air, the micropores formed within the interlayer space have the most important contribution to the pore volume of the sample. If a solvent of weak interfacial tension, such as ethanol, replaces water in the cake, the driving force to FF orientation is greatly reduced. During SCD the silicate layers and pillars will keep their wet-cake structure that means more macropores and less regular pillared layered structure. Such a structure has a large surface area of non-micropores and exhibits a large adsorption at high P/PQ, as we observed for the SCD sample. The framework of this sample is so-called three-dimensional house of card structure. It is also believed that strong interfacial force of water causes collapse of micropores during drying process because smaller pores have higher interfacial tension and easier to collapse. In the SCD process, the interfacial tension is reduced dramatically, thus the micropore collapse is avoided, resulting in an increase in the total surface area. We also found that SCD results in an obvious increase in micropore volumes of Al-PILC and clay pillared with mixed oxides of aluminum and lanthanum (LaAl-PILC). An interesting point is that the SCD treatment does not alter the pillared layered framework so much for Al- and LaAl-PILC as it does for the sol-pillared clay (more details will be published elsewhere). The reason for the phenomenon that the pillared layered framework in sol-pillared clay is more ready for change should be the unique pillars, the sol particles combined with titanium cations. Choy et al (4) found that the stacking of sol particles within the interlayer space could be rearranged at moderate experimental conditions. This property is also observed when the wet cake treated with a quaternary ammonium surfactant. The samples treated with the surfactants have a very high porosity, with pores concentrated in the region of small mesopores and supermicropores. Because both silica sol and surfactant are involved in the MCM-41 synthesis (10), which has been extensively investigated in recent years, similarities in the pore formation between samples treated with the surfactants and in MCM41 may be anticipated. The pore volume and surface area of sol-PILB-Cn samples are closer to those of the corresponding MCM41 solid prepared with silica fume and the same surfactant used in the preparation of sol-PILB-Cn, rather than to that of sol PILB. For example, the total pore volume and BET specific surface area of sol-PILB-C16 are 0.84 cmVg and 756 m^/g, respectively, much larger than those for sol-PILB, 0.24 cmVg and 404 m^/g, but comparable to those for MCM41-C16, 0.94 cmVg and 790 m^/g. These resuhs suggest that treatment with surfactants of quaternary ammonium salts alters the structure of the sol pillared clay radically. Recently, Galameau et al (11) reported a successful synthesis of mesoporous solids from layered clays using quaternary ammonium surfactants as template agents. In their approach layered clays were intercalated with surfactants. TEOS was then allowed to hydrolyze and condense surrounding the surfactants in the galleries. An open-framework of silica formed in the galleries after the removal of the surfactants by heating and was termed porous clay heterostructure. The formation of the mesopores in this approach is analogous to that in MCM41 synthesis. An obvious difference between our treatment with surfactants from that reported by Galameau et al is that the surfactant was introduced into the system where the sol particles existed already in the interlayer galleries. Nevertheless, it is possible for the surfactant treatment that surfactant molecules form micelles in the galleries, if the sol particles
430
are readily to be rearranged. The 40 surfactant micelles, thus, act as template as in the synthesis of MCM-41 materials (10) and sol particles rearrange and further condense, surrounding the micelle together with the clay layers. 30 According to this mechanism, the size of the mesopores will be in direct proportion to the chain length of the sol-PILB quaternary ammonium surfactants. The 20 sol P I L B - C n relation between the mean diameter of M CM 4 1 - C n the framework pores in sol-PILB-Cn samples and the alkyl chain length of 1 5 1 : 12 the surfactants, expressed by the number Cn of carbon atoms in the alkyl chain of the surfactants (Cn), is shown in Figure 3. The pore sizes of sol-PILB-Cn Figure 3 The relation between the mean samples are larger than that of the diameter of the framework pores in solnormal sol-PILB (given on the vertical PILB-Cn samples and the alkyl chain length axis) and increases with Cn. The of the surfactants, expressed by the number variation trend of the pore dimension of carbon atoms in the surfactants (Cn). This with Cn is very similar to that for relation for MCM41 samples prepared using MCM41 samples. This supports the the same surfactants and silica fume are template mechanism suggested above. given for comparison. It is noted that both the micelle and the sol particles modified titanium cations are positively charged. However, the inorganic species can still condense surrounding the micelle through some pathways described by Huo et al (12). We also found that surfactant treatment did not work on Al- and LaAl-PILBs. The pillar precursors of these pillared clays hold the clay layers strongly. In contrast, the forces in the modified sol particles to hold the clay layer are weak, allowing radical change in the framework when the wet cake of the sample is subjected to SCD or surfactant treatment. This unique property leads to various derivative materials of different pore structures. 3.2. Modification of pore openings During the calcination at 773 K, the sol particles were converted into oxide particles within the interlayer region and rigid pillared layered framework formed in the sol-PILM sample. A two-step procedure is proposed to modify the micropore openings of the solid to enhance the ability of recognizing different molecules. The XRD patterns are essentially unchanged by Ca^^ ion exchange and the subsequent modification with TEOS, indicating that the pillared layered framework remained stable. N2 ads-desorption isotherms of the three samples have similar shapes (not shown). Loading of Ca^^ cations resulted in an apparent reduction in N2 adsorption capacity mainly in a region of P/Po < 0.2, reflecting that most of the loss in pore volume comes from fine micropores. A slight drop in the adsorption caused by the subsequent TEOS modification is observed. This fact is consistent with the small increase in Si02 content of the modified product, as shown in
431
Table 2. Compositions of the three samples, represented in molar ratios, are also summarized in Table 2. Calcium contents in the Ca^^ doped sol-PILM sample and the TEOS modified product are obviously greater than that of the sol pillared clay, indicating significant amount of Ca^^ ions was loaded into sol-PILM host during the cation exchange. Si02 content of the modified product increased, with respect to that of the precursor Ca^^ doped sol-PILM, by a small amount. TEOS equilibrated with Ca^^ doped sample for two days is the only resource of the extra Si02 in the product. Therefore, it is suggested that TEOS molecules hydrolyzed within the pores of the clay in the presence of Ca^^ cations and their coordinating water. The hydrolyzed products were then converted to silica during the subsequent heating treatment. The small amount of silica thus formed leads to a significant change in the adsorption selectivity of the clay as shown below. Table 2 Composition of sol-PILM and its derivative samples (in molar ratio) elements sol-PILM Ca ^^ sol-PILM modified product Si Ti Al Mg Fe Ca
10.74 2.16 1.71 0.28 0.08 0.02
10.25 1.99 1.71 0.25 0.07 0.37
10.77 1.98 1.71 0.25 0.07 0.37
3.3 Adsorption of organic vapors Adsorption-desorption isotherms of toluene, p- and m-xylene and mesitylene vapors by the sol-PILM and the modified product are compared in Figure 4. As anticipated, a decreasing tendency of uptake of each adsorbate is observed from sol-PILM to the final modified product due to the decrease in pore volume. It is also not surprised that the capacity of vapor adsorption by each sample decreases as the molecular size of the adsorbate increases, following the order: toluene >p- xylene > m-xylene > mesitylene. The reason for less uptake of larger adsorbate is the accessibility of the pore system in the solids. There must be some micropores in sol-PILM, rejecting mesitylene molecules, but uptaking toluene, xylene isomers. Moreover, some micropores reject mesitylene and mxylene but uptake toluene and/7-xylene. For the parent sol-PILM and the Ca^^ ions doped solid, the ;7-xylene uptake in the micropores is slightly greater than that of w-xylene, the uptake ratio of p- to w-xylene is slightly greater than unity. After the second step of the modification with TEOS, this ratio increases to about 1.7, while the difference between the molecular sizes of the two isomers is about 0.1 nm. Apparently important changes in the structures of micropores are resulted from the modification. The TEOS modified product exhibits significant molecular recognition properties (Fig 4c and d). Besides, the hysteresis on the isotherms of xylene isomers and mesitylene of this solid extends to very low pressure region, reflecting the difficulty in
432
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Figure 4 Ads-desorption isotherms of organic vapors by sol -FILM, Ca^"^ doped sol-PILM and the TEOS modified product. From top to bottom are: (a) toluene; (b)/7-xylene; (c) w-xylene and (d) mesitylene.
desorption process. This is normally caused by the micropores with an entrance of similar dimension of the adsorbate molecules. Because the increase in silica content (Table 2) of the TEOS modified product with respect to those of the Ca^^ doped sample is small, the modification mainly reduces the entrance size of micropores. This is supported by the fact that the TEOS modification causes a slight decrease in micropore volume (0.001 cmVg). The micropores of small entrance (inkbottle shape) reject large adsorbate molecules such as mesitylene and mxylene, but are accessible to smaller adsorbates such as toluene and pxylene. The fact that modified product has a high adsorption capacity for toluene and the uptake ratio of toluene over mesitylene is 2.7, supports this argument. In Fig 5, the structures of the sol-PILM, Ca^^ ions doped PILM and TEOS modified product are illustrated schematically. The structure model of the sol-PILM shown in Fig 5 was first proposed by Yamanaka et al (1). They observed a diffraction peak at lower angle, which corresponds to a basal spacing of about 3.8-4.0 nm. The diffraction peak we observed at 5.3 "" is close to the secondary diffraction reported by them. If the peak is the secondary diffraction, the corresponding basal spacing is thus estimated to be 3.92 nm and the free spacing, 2.96 nm. However, Yamanaka et al (1) argued that the free spacing of about 3 nm could not reflect the size of the preponderant portion of the micropores in the pillared clay. Their adsorption also indicates that most of the surface area
433
and pore volume of the solid come from micropores (pore size 99% (Merck), SOS =95% (Aldrich), SDBS >80% (Aldrich), and SOBS =97% (Aldrich). The distilled water used in the experiments was deionised in a Millipore MilliQ System. The Mg-Al-COa-LDH used as adsorbent and sorbent was prepared with an Mg:Al ratio of 2:1 by the coprecipitation method at variable pH [6]. The material obtained was characterised by powder X-ray diffraction (PXRD, using a Siemens D-5005 X-ray diffractometer), and elemental and thermal analyses. The material showed the characteristic lamellar structure with a basal spacing of 7.6 A, specific surface area of 87.1 m^ g\ determined by the N2-BET adsorption isotherm, and an approximate minimum molecular formula: [Mg,,2Al,(OH)5g4}(c03)o5 23l(H20). The size distribution and the average size of the LDH particles were determined by light scattering, using a Zetasizer 4 from Malvern. Table 1 Surfactant equilibrium concentrations of the adsorbed-materials analysed by SEM. Higher Concentration Surfactant Lower Concentration CMC (mol dm"^) (mol dm'^) (mol dm"^) SOS 2.0x10'^ 1.15x10"^ 1.0x10-^ SDBS 1.0x10-^ 3.28x10"^ 5.0x10-^ SOBS 1.5x10-^ 1.12x10'^ 2.0x10'^
445
Adsorption was performed using the batch method by placing a constant mass (200 mg) of the powdered adsorbent in Erlenmeyer flasks. The solid was sonicated with an appropriate amount of water (30 cm^) in order to homogenise and decrease the particle size. Surfactant solution (20 cm^) was then added to produce a final concentration ranging from one order of magnitude below the respective critical micelle concentration (CMC) to a concentration about three times higher than the CMC. According to the standard procedure adopted, the suspension was shaken at 25 °C for 72 hours, and centrifuged (10,000 x g) and the remaining solid was drained and dried under vacuum at room temperature. The dried adsorbed material was powdered and sprinkled on the sample holder of a Zeiss DSM 960 Digital Scanning Microscope which was then submitted to gold sputtering with a Sputter Coater Balzers SCD 050. Even though the adsorption of SOS, SDBS, and SOBS was performed in the same way as described above, only the solids obtained at two surfactant concentrations, according to their respective CMC were analysed by SEM. Table 1 shows the concentrations utilised as well as the corresponding CMC for these surfactants determined by conductance measurements. EDS measurements for the LDH adsorbed with SDS were carried out using the same microscope device. In this case the material was submitted to carbon sputtering to avoid a gold signal. The signals collected were those corresponding to the Mg, Al, S and Na Ka transitions. Other tests were also performed with the SDS-adsorbed material. One test concerned the deposition process, in which the material was deposited on the sample holder by the dipcoating method. The influence of a rapid washing of the material (containing the nanoribbons) by immersion in pure water was also tested. Finally, the dried material (containing the nanoribbons) was thermally treated at 300 °C for 2 hours and then sprinkled on the sample holder.
3. RESULTS Figure 1 shows the typical topography presented by the LDH particles. A heterogeneous surface can be identified and a deeper analysis will also show the largest pores. This topography pattern was also observed in the SDS-adsorbed material when the surfactant equilibrium concentration was lower than the corresponding CMC (8.2x10'^ mol dm'^). No other different image patterns were observed. The SDS-adsorbed material obtained fi-om the CMC upwards, presented a distinct SEM image pattern, as can be seen in Figure 2. These ribbon-like images seemed to be anchored on the surface of the LDH. These ribbons, with a thickness around 50 nm, width around 1 |im, and a variable length, will be simply called nanoribbons. These nanoribbons were very abundant in the range of surfactant concentration analysed, being found in about 80% of the particles present in the sample-holder. The thickness of the nanoribbons, determined by SEM, is in agreement with the size of the smallest LDH particles, determined by light scattering. The average diameter of the particles was about 380 nm, determined fi-om the particle diameter distribution rangingfi-om20 to 5000 nm. Figure 3 shows SEM micrographs of pure SDS obtained under the same conditions of deposition on the sample holder. The morphology and the topography of the particles are quite different from those observed for the LDH and the nanoribbons shown above.
446
Figure 1. SEM micrographs of the Mg-AlCO3-LDH prepared and used as adsorbent (20 kx) The nanoribbons were present on the material adsorbed with SDS, when sampled as powder and when deposited on the sample-holder by dip-coating. The material washed with water also showed the nanoribbons, however with less abundance. On the other hand, thermal treatment of these materials resulted in the vanishing of the nanoribbon pattern. The EDS measurements indicated that amounts of Mg, Al and Na were quite constant in all particles and regions analysed, including the restricted regions over the nanoribbons pattern. The amount of S increased with increasing surfactant concentration in the adsorption process, but remained almost constant in the solids obtained at the adsorption plateau surfactant concentrations. Analysis of the pattern observed over the nanoribbons showed an S amount very similar to that observed in regions outside the nanoribbons.
Figure 2. SEM micrographs of the SDS-adsorbed material obtained at the following SDS concentrations (a) 8 mmol dm"^, lOkx; (b) 10 mmol dm'^, 5kx; (c) 26 mmol dm'^, lOkx.
447
(a)
(b)
Figure 3. SEM micrographs of pure SDS (a) Ikx and (b) lOkx. As can be seen in Figure 4, AFM was also used to characterise the nanoribbons, but only the bulk phase was observed. The main problem in this kind of analysis was that the nanoribbons were not fixed in the bulk phase, not easy to be found, and not rigid. With the maximum resolution achieved, a grain boundary map corresponding to either particle aggregates or crystallites showed grains of about 70 nm in diameter. The SEM images of the material adsorbed with SOS, SDBS, or SOBS did not present nanoribbons at lower surfactant concentrations. At high concentrations (> CMC) a different behaviour was observed for each case. The SDBS-adsorbed material did not show nanoribbons, even at high surfactant concentrations. Nanoribbons were observed for both the SOS- and SOBS-adsorbed materials, as can be seen in Figures 5 and 6. However, their incidence in the SOS-adsorbed material reached only 5% of the particles analysed, while in the SOBS-adsorbed LDH this ribbon-like images were observed in about 90% of the particles.
W.O0O HN/Ai*
W».9B0 tm/*ti
Figure 4. AFM micrograph of an SDSadsorbed material.
448
^^^ ^ ^
(a) (b) Figure 5. SEM micrographs of the SOS-adsorbed material obtained at high surfactant concentration (a) Ikx, and (b) 5kx.
4. DISCUSSION Based on the amount of surfactant adsorbed and on that present in solution, we concluded that it is unlikely that these nanoribbons consist of pure crystallised surfactant. On the basis of the approximate size of the SDS anions, we also ruled out the possibility that this image pattern resulted from aggregates of surfactant adsorbed onto the LDH's surface [11].
(a) (b) Figure 6. SEM micrographs of the SOBS-adsorbed material at high surfactant concentration (a) Ikx, and (b) 5kx.
449 The above assumptions are supported by the results obtained by EDS, since a composition similar to the bulk phase was found just over the nanoribbons. Moreover, SEM micrographs of pure SDS showed different morphology from that of the nanoribbons. The results observed when the SDS-adsorbed material was washed or heated indicated that SDS molecules are present in the nanoribbons, since a quick water washing did not remove the nanoribbon images. The results also indicated that the alkylic chain length and the hydrophobic group did not affect the formation of the nanoribbons. Their occurrence cannot be related to the maximum amount of surfactant adsorbed, i.e., 20, 9.5, 25, and 25% (m/m), for SDS, SOS, SDBS, and SOBS, respectively. The surfactants that presented a considerable amount of nanoribbons were SDS and SOBS, and the only plausible relation between them is that these surfactants have the closest CMCs (0.82 and 1.12 mol dm"^, respectively), again indicating that the presence of micelles in solution is essential to the formation of the nanoribbons. According to the results shown here, we may conclude that the composition of these nanoribbons is related to a composite formed by the association of the smallest LDH particles (perhaps crystallites, as shown by AFM) and the surfactant. These adsorbed LDH particles may be oriented in the c axis, and linked by the adsorbed surfactant layer and surfactant molecules. A schematic representation of such composite is shown in Figure 7.
• • ••
S^^^^ // (
\/ -
• Counter-ion - Na* ajrfactant-SDS
" ^ ^ 0 0^ :.Q^ocr,^:?o,p
I^ XMJut .
^^T«3nfB" A A * >.'!Ju'.
A a A
1 Figure 7. Simplified schematic representation of the composition and formation process of the LDHsurfactant composite, according to the proposed model.
450
The preferred orientation would be possible due to the crystallographic arrangement of the LDH layer, and should result in the long lengths observed. The nanometric thickness is very similar to the grain boundary size observed by AFM, and this monodispersion of particles may be responsible for the apparent flexibility and smoothness of the nanoribbons. Each surfactant, which would act as the bridge between the adsorbed particles, achieves its CMC producing micelles with specific amount of unimers in a specified arrangement. Thus the micelle features and the CMC values should be important factors in the formation of the ribbon-like composite.
5. CONCLUSIONS The hypothesis formulated to explain the results shown here considers the nanoribbons as an extremely organised (oriented) hybrid organic-inorganic nanocomposite. The size control as well as the isolation of these nanoribbons, which can eventually be done using a polymerisable surfactant, may be feasible for give more information about the composition and the structure of such nanoribbons.
ACKNOWLEDGMENT The authors wish to thank the Brazilian agencies Fundagao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP, grants no. 96/12373-9 and 96/06030-1), Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq/PADCT) and Conselho de Aperfei9omanto de Pessoal de Ensino Superior (CAPES) for financial support.
REFERENCES 1. J. Liu, A. Kim, L. Q. Wang, B. J. Palmer, Y. L. Chen, P. Bruinsma, B. C. Bunker, Adv. Coll. Interf.Sci., 69 (1996) 131. 2. N. K. Raman, M. T. Anderson, C. J. Brinker, Chem. Mat., 8 (1996) 1682. 3. G. S. Attard, J. C. Clyde, C. G. Goltner, Nature, 378 (1995) 366. 4. S. Mann, S. L. Burkett, S. A. Davis, C. E. Fowler, N. H. Mendelson, S. D. Sims, D. Walsh, N. T. Whilton, Chem. Mat., 9 (1997) 2300. 5. B. Ammundsen, D. J. Jones, J. Roziere, G. R. Bums, Chem. Mat., 9 (1997) 3236. 6. A. de Roy, C. Forano, K. el Malki, J.-P. Besse, Anionic Clays: Trends in Pillaring Chemistry, in Synthesis of Microporous Materials, vol II, M.L. Occelli, H.E. Robson, (Eds.), Van Nostrand Reinhold, New York, 1992, chapter 7, p. 108. 7. I. Pavlovic, M. A. Ulibarri, M. C. Hermosin, J. Comejo, Fresenius Envir. Bull., 6 (1997) 266. 8. K. Esumi, S. Yamamoto, Coll. Surf. A: Phys. Eng. Asp., 137 (1998) 385. 9. S. P. Newman, W. Jones, New J. Chem (1998) 105. 10. P. C. Pavan, G. A. Gomes, J. B. Valim, Micropor. Mesopor. Mater., 21 (1998) 659. 11. P. C. Pavan, E. L. Crepaldi, J. B. Valim, Coll. Surf. A: Phys. Eng. Asp., 154 (1999) 397.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
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Synthesis and characterization of a new Sn-incorporated CoAI-layered double hydroxide (LDH) and catalytic performance of Co-spinel microcrystallites in the partial oxidation of methanol S.Velu and K.Suzuki Ceramics Technology Department, National Industrial Research Institute of Nagoya, 1-1 Hirate-cho, Kita-ku, Nagoya 462-8510, Japan. M(II)AlSn-LDHs with M(II) being Mg, Ni or Co were synthesized by a coprecipitation method. The influence of Sn on the thermal transformations and redox properties were investigated in detail using XRD, TG/DTA, SEM, TPR, "^Sn-MAS NMR and UV-visible diffuse-reflectance (DR) spectroscopy methods. Some of these samples calcined at 450 "C were tested as catalysts in the partial oxidation of methanol (POM) reaction. In this paper we discuss briefly the effect of Sn-incorporation on the structural features and reducibility of CoAl-LDH. The catalytic performance of Co-spinel microcrystallites derived from CoAl-, and CoAlSn-LDHs was also evaluated. 1
INTRODUCTION Layered double hydroxides (LDHs), also known as anionic clays, are an important class of material currently receiving increasing interests owing to their potential applications as catalyst precursors and microporous materials. They consists of positively charged brucite [Mg(0H)2]-like M(II)-M(III) hydroxide layers which are separated from each other by an interlayer composed of anions and water of crystallization. These compounds are represented by the general formula: [M(II)i.xM(III)x(0H)2]'l(A")x/n m.H20]", where M(II) and M(III) are divalent and trivalent cations, respectively and A"' is a charge compensating anion. We are currently interested in the synthesis of a new series of M(II)-M(IV)-LDHs by a coprecipitation method. We have shown recently the partial substitution of Ap^ by M(IV) cations such as Zx^^ in the MgAl-LDHs and their catalytic performance in the liquid phase hydroxylation of phenol [1]. The incorporation of Sn'*^ in the M(II)A1-LDH matrix is being investigated currently. Our detailed study [2] using XRD, SEM, FT-IR, "^Sn- and ^^Al-MAS NMR techniques revealed that, about 30 atom % of the Al^^ in the Mg-Al layer could be isomorpously substituted by Sn"^^ to obtain a new MgAlSn-temary-LDH. As a continuation of our study, we have synthesized the similar Ni^^ /Co^^ containing analogues, since, LDHs containing transition metal ions such as Ni^^ or Co^^ are found to possess unique physicochemical properties for electrochemical, magnetic and catalytic applications. The importance of Snincorporation can be justified by the fact that the Sn-incorporated mesoporous zeolite-like materials have attracted much interest in recent years because of their potential applications in catalysis [3]. Furthermore, several Sn-containing mixed oxide systems have been widely employed as a powerful catalyst in various industrially important reactions [4]. In the present study we demonstrate the formation of a new CoAlSn-LDH, and describe briefly the effect of
Phase obtained
(A\
Uncalcined
LDH
.-C
--
f! d
ea
h
V1 + e
h C V) I
9
=!
Y
m
rh &
&
U
U
Co-spinel + Co2sno, CS
10
20
30
40
50
2 Theta (Degrees)
60
70
10
20
30
40
50
60
70
2 Theta (Degrees)
Figure 1. XRD patterns of (A) CoAI-LDH and (B) CoAISn-LDH calcined at different temperatures for 5h; CS = cobalt spinel, CT = CozSnOl inverse spinel
453
Sn-incorporation on the structural and redox properties of the CoAl-LDHs. The catalytic activity of spinel catalysts obtained from these LDH precursors has been tested in the partial oxidation of methanol (POM) to H2 fuel for fuel cells. 2
EXPERIMENTAL The detailed experimental procedure on the synthesis of Sn-incorporated analogues, physicochemical characterization and the catalytic POM reaction can be obtained from our earlier reports [1, 2, 5]. Lattice parameters of the samples were calculated from the XRD data collected between 5 to TO"" 26 employing a scan speed of O-S"" 29/min. 3 3.1
RESULTS AND DISCUSSION XRD and DRS study The XRD patterns of CoAl-LDH and their Sn-incorporated counterparts are depicted in Figure 1. Similar to the MgAl-, and NiAl-LDH systems [2] the uncalcined samples of CoAl- and CoAlSn-LDH also exhibit a single phase corresponding to the LDH. The chemical composition and lattice parameters of CoAl- and CoAlSn-LDH are presented in Table 1 together with those of Mg- and Ni-containing analogues. It can be seen that the 'a' parameter increases (by about 0.03 A) while the 'c' parameter decreases (by about 0.30 A) when Sn"^^ co-exists in the CoAl-LDHs. It can also be noted from the Table that the M(II):A1 atomic ratio in each pair of the M(II)A1-LDH [M(II) - Mg, Ni, Co] is kept constant while adding the required amount of Sn salt during preparation. Hence, the expansion in the 'a' parameter clearly indicates the incorporation of Sn"^^ in their LDHs framework, because of the difference in the ionic radii. The observed contraction in 'c' parameter could be explained on the basis of an increase in attractive force between the brucite-like layer and the interlayer. This is because of an increase in charge density of the brucite-like layer since a tetravalent cation Sn "^ occupies the layer framework. Table 1 Chemical composition and lattice parameters of M(II)A1-, and M(II)AlSn-LDHs LDH 'M(II):Al:Sn M(II)/ M(II)/ Lattice 'FWHM 't(A) Atomic ratio Al (Al+Sn) parameters (A) (26) a c MgAl-LDH 3:0.70:0.00 4.29 4.29 3.053 23.543 1.22 65 MgAlSn-LDH 3:0.70:0.29 4.29 3.03 3.067 23.349 1.64 48 NiAl-LDH 3:1.00:0.00 2.88 2.88 3.046 23.280 1.45 55 NiAlSn-LDH 3:0.95:0.40 3.16 2.22 3.064 22.754 1.97 41 CoAl-LDH 3:0.86:0.00 3.49 3.49 3.074 23.047 1.05 76 CoAlSn-LDH 3:0.93:0.36 3.23 2.33 3.103 22.750 1.18 68 ' Determined by X-ray fluorescence spectroscopy; ^ Full width at half maximum (FWHM) of (003) plane; ^ Crystallite size (t) calculated from (003) plane using Debye-Scherrer equation The DR spectra of CoAl-LDH and the Sn-containing counterpart exhibit (Figure 2) a band around 540 nm with a weak shoulder around 370 nm for Co^^ in an octahedral coordination. No absorption band that can be accounted for Co^^ was noticed, indicating that under the preparation condition almost all the Co ions are existing as Co^^. The CoAlSn-LDH shows an additional strong band around 210 nm, due to the existence of Sn"^^ in the CoAlLDH matrix.
454
The XRD patterns of CoAl-, and CoAlSn-LDHs calcined at various temperatures are also included in Figure 1 itself. They show the presence of a non-stoichiometric spinel phase similar to C03O4, C0AI2O4 or C02AIO4 at all the calcination temperatures. However, the XRD pattern and the lattice constant of the C03O4 spinel are comparable with those of C0AI2O4 or C02AIO4 Ca' parameters of C03O4, C0AI2O4 and C02AIO4 reported in the literature are 8.084, 8.103 and 8.086 A, respectively). Best fit was observed for C0AI2O4 and C03O4. In fact, in an earlier study [6] on the similar CoAl-LDH, it has been reported that C03O4 is the only crystalline phase formed in the calcination temperature 238-320 ""C, while a mixture of C03O4, C02AIO4 and C0AI2O4 phases could be formed at higher calcination temperatures. Our XRD data, however, revealed that a mixture of C03O4 and C0AI2O4 or a solid solution of these two spinels would have been formed. Taking into account the initial composition of Co and Al (atomic ratio of 3:0.86) in the parent LDH precursor, it is reasonable to assume that the spinel phase would correspond to a non-stoichiometric material whose composition can be either Co ^Al2-xCo^\04 or Co^^Co^^2-xAlx04. The observed lattice constant of the resulting Cospinel at all the calcination temperatures was considerably less than that of the C0AI2O4 and more or less similar to that of C03O4 spinel. These results reveal that Co^^ possibly substitutes for Al^^ in C0AI2O4 to form a non-stoichiometric spinel of the composition, Co^^Al2-xCo^^x04. The sample calcined at 1100 °C is highly crystalline. Assuming that there is no loss of Co or Al during calcination, the value of x ~ 1.33 has been calculated from the initial composition of Co and Al. 40-r
I §00 600 900 Wavelength (nm) Figure 2. Diffuse-reflectance spectra of C0AI-, and CoAlSn-LDHs calcined at different temperatures; (a) CoAl-uncalcined, (b) CoAlSn-uncalcined, (c) CoAl-450, (d) CoAlSn-450, (e) CoAl-700, (f) CoAlSn-700, (g) CoAl-900, (h) CoAlSn-900, (i) CoAl-1100 and (j) CoAlSn-1100^C/5h.
455
The Sn-incorporated counterparts also exhibit similar XRD patterns in the temperature range 300-700 °C. However, the lattice parameters of the resulting Co-spinel phase is higher than that derived from the non Sn-containing samples, indicating that some amounts of Al^"^ or Co^"^ are isomorphously substituted by Sn'*'^ in the non-stoichiometric spinel. The resulting material can thus be represented as Co^^Al2.(x+y,Co'\Sn'^^y04^y/2 or Co^To^^2-(x+y)AlxSn'*^y04+y/2A further calcination of CoAlSn-LDH at 900 T develops additional XRD lines, whose intensity increases with further increasing calcination temperature to 1100 °C. The XRD patterns of the additional phase corresponded to that of Co2Sn04 inverse spinel. The lattice constant of Co2Sn04 (8.578 A) is lower compared to that of the literature value (8.638 A), implying that Al"^"" isomorphously substitutes a part of Sn"^^ in the Co2Sn04 inverse spinel. 3.2
TPR study The effect the Sn-incorporation on the reducibility of CoAl-LDHs was investigated by temperature programmed reduction (TPR) experiments. Three main reduction regions can be envisaged from the TPR profiles of CoAl-, and CoAlSn-LDHs calcined at various temperatures (Figure 3). They are region-I, in the temperature range 250-450 ""C, region-II between 450 and 550 °C and region-Ill above 550 °C. These regions should correspond to the presence of three different Co species in the sample. Region-I (TPR peak between 250 and 450 ^Q: Iht first TPR peak in all the samples of CoAl-LDH calcined above 300 °C appears in this region. It should be noted that the Co^^ and Co^^ in the reference samples of CoO and/or C03O4 are also reduced in the same region. However, the presence of CoO is unlikely, and XRD of the samples showed the existence of a non-stoichiometric spinel with a composition either Co^^Al2-xCo^\04 or Co^^Co^^2-xAlx04. This spinel phase can also be considered as a mixture of C03O4 and C0AI2O4, because, the Co species in the above non-stoichiometric spinel will have chemical environments similar to both C03O4 and C0AI2O4. Hence, the reduction in this region would correspond to the reduction of Co^'^-Co^^ species in the non-stoichiometric spinel. This assignment is supported by the DR spectra, (see Figure 2) which exhibits a strong absorption in the range 600 to 800 nm due to the presence of Co^^ species and a weak band around 450 nm for Co^^ species in the tetrahedral coordination [7]. The increase in TPR reduction temperature as a fiinction of precalcination temperature (see the inset in Figure 2) is usually ascribed to an increase in the crystallinity of the samples as evidenced from an increase in the sharpness of the XRD peaks. It can also be noticed from the TPR profiles that the peak width and the intensity oixht first TPR peak decreases while the intensity of the second peak increases with increasing precalcination temperature. This implies that the amount of Co^^-Co^^ like species decrease at the expense of the formation of Co species in the region-Ill. Region-in (TPR peak above 550 "C): All the samples of CoAl-LDH calcined in the temperature range 300 to 900 °C as well as the uncalcined sample exhibit TPR peak above 550 °C. The reduction of C0AI2O4 crystallites has already been reported to take place around 800 ^C. Although, the LDH structure gets collapsed before being reduced in TPR, the Co^^ ions are certainly surrounded by Al^^ ions because, in the CoAl-LDH structure, Al^^ occupy the layer framework.. Hence, the reduction in this region can be attributed to the reduction of Co^'^-Al^"^ species, wherein, the Co^^ ions having a large number of 0-Al ligands. The resulting species behave chemically like C0AI2O4 spinel. This assignment is in line with the chemical formula of the non-stoichiometric spinel suggested for these samples. Furthermore, the tetrahedrally coordinated Co^^ in the C0AI2O4 spinel develops a broad triplet band above 550 nm, which is superimposed on the Co^^ band in the present study. The results, therefore.
0
100
200
300 400 500 Temperature (c)
600
700
0
100
200
300 400 500 Temperature (c)
600
Figure 3. TPR profiles of (A) CoAl-LDH and (B) CoAlSn-LDH calcined at different temperatures for 5h; (a) Uncalcined, (b) 300, 450, (d) 700, (e) 900, (f) 1 I00 OC; The inset in Figure 3 A shows the variation of the position of TPRfirst peak with respect to the calcination temperature
457
reveal that the Co304-Uke species present in the non-stoichiometric spinel are reduced in the region-I while the CoAl204-like species are reduced in the region-Ill. Furthermore, a nonstoichiometric spinel whose composition could be viewed as a mixture of C03O4 and C0AI2O4 are present even at 300 °C. For the sample calcined at 900 "^C, the peak becomes more intense, probably indicating the transformation of Co^^-Co^^ species into Co^^-Al^"^ species. The TPR peak in this region is completely absent and, it is shifted toward the region-I for the sample calcined at 1100 °C. Since, the XRD of the sample showed the presence of a well-crystallized and well-defmed solid solution having a formula Co^^Ab-xCo x04 with x = 1.33, this peak can be attributed to the reduction of Co^^-Co^^ species diluted by Ap^ [8]. Region-n (TPR peak between 500 and 550 ""Q: CoAl-LDH calcined at 700 °C is the only sample wherein a broad shoulder around 475 ""C is clearly discemable. This indicates that there is, possibly, another Co-containing specie exists, which is reduced in this region. Taking into account the fact that a part of Co^^ being oxidized to Co^^ during calcination in the intermediate temperatures (around up to 700 ^C), it is reasonable to assume that this intermediate Co specie is due to the formation of a novel mixed Co^^-Al^^ oxide. The DRS Kubelka-Munk function is the highest for sample calcined at 700 T , corroborating the formation of a large quantity of Co^^ species. This assignment is also in line with an earlier TPR study on the coprecipitated Co-Al sample calcined at 650 ^C, wherein the reduction of similar Co^'^-Al^^ mixed oxide of formula C03AIO6 has been suggested based on the thermodynamics of the Co ions in the Co-containing spinels [8]. The TPR profiles of CoAlSn-LDH calcined at various temperatures (Figure 3B) also show at least three reduction regions as those of the non Sn-incorporated analogues. However, the profiles are even more complex especially in the reduction region-Ill, above 550 ^C. It is interesting to note that the reducibility of these species diminished upon Sn-incorporation (see the inset in Figure 3A). This is in contrast to the results observed in the Ni-containing analogues wherein the reducibility of Ni^^ species is greatly enhanced by the presence of Sn"^^. It should be recalled that the XRD data indicated the possibility for the isomorphous substitution of Sn"^^ for Ap^ or Co^^ to form a non-stoichiometric spinel of the formula Co^"'Al2.(x+y)Co^^Sn'*\04+(y/2) or Co^^Co^^.(x+y)AlxSn'^\04+(y/2). The Co^^-Co^^ species, which are reduced in the region-I in CoAl-LDH derived materials, are associated with Sn"^^ in addition to Al^"^ in the Sn-containing analogues thereby increasing the reduction temperature because of the enhanced polarization of Co-0 bonds. Such an interaction would also facilitate the electronic transition in DR spectra, as the Kubelka-Munk function of all the bands are doubled in the Sn-containing samples (see Figure 2). The TPR peaks corresponding to Co^^Al^^ species, which are reduced in the region-Ill, are more intense. The reduction of Sn"*"^ coincides with the reduction of Co^^-Al^^ species in the region-Ill (above 550 ^C). Several shoulders are also detected in this region especially for the sample calcined at 700 and 900 °C. However, it is unclear if these shoulders are due to the reduction of Co^^-Al^^ species or of Sn"^^ in different chemical environments. Similar to the non Sn-containing counterpart, the sample calcined at 1100 ^C exhibits an intense TPR peak around 410 ""C with a shoulder around 350 °C due to the reduction of Co^^-Co^^ species diluted by both AP^ and Sn"^^. Besides, a strong peak around 540 ^C also appears for the reduction of Sn"^^ -^ Sn^. 3.3
Catalytic partial oxidation of methanol The use of CI sources in applied catalysis research has grown enormously in the past few years. The fact, that methanol can be conveniently synthesized from methane or coal, has promoted research into the reactions involving methanol. Partial oxidation of methanol to H2
458 and CO2 has recently been suggested to be an alternative route to produce hydrogen fuel with lower amount of CO, useful for fuel cells [9]. Table 2 ^Partial oxidation of methanol over C0AI-, and CoAlSn-LDHs calcined at 450 "C/5h ^Catalyst H/4C Temp. MeOH Carbon Hydrogen conversion Selectivity (mol %) Selectivity (mol %) CO (mol %) H2O CO CO2 H2 CoAl-LDH 1.14 81.0 200 19.0 34.1 35.3 64.7 250 52.9 1.06 47.1 48.1 44.3 55.7 1.03 300 12.9 87.1 31.3 35.2 64.8 CoAlSn-LDH 200 92.8 1.11 7.2 43.4 39.9 60.1 94.4 1.13 250 5.6 47.0 65.4 34.6 300 0.0 100.0 0.97 74.6 57.3 42.7 ' MeOH space velocity (WHSV) = 0.4 mol h" g" r Catalysts were reduced at 700 ""C for 2 h. in a stream of H2 (10 cc/min) The results of catalytic partial oxidation of methanol over the spinel catalysts derived from CoAl- and CoAlSn-LDH are presented in Table 2. A methanol conversion of 30 to 50 mol % was obtained over catalyst derived from CoAl-LDH. The products obtained were H2, H2O, CO and CO2. Other products such as formaldehyde, methyl formate or dimethyl ether was not observed under the present experimental conditions. The selectivity of H2O was very high (« 40 to 60 %), probably because of the involvement of the complete oxidation of methanol over these catalysts. It is interesting to note from the Table that the methanol conversion rate and the selectivity of CO2 increased over the catalyst derived from the Sncontaining analogue. The observation that only traces of CO is produced in the Sn-containing catalyst, is attractive for the development of catalyst for POM reaction to produce H2 for fuel cell applications. The only inconvenience is the higher selectivity of H2O by complete oxidation, probably because of the higher Co content in the sample. In conclusion, a new Sn-incorporated CoAl-LDH has been synthesized. The effect of Sn-incorporation on the thermal transformation into spinels and their reducibility are investigated. Incorporation of Sn diminishes reducibility of Co species because of the enhanced polarization of Co-0 bonds. The Sn-containing spinel exhibits better catalytic performance in the partial oxidation of methanol to produce H2 and CO2 useful for fuel cells. REFERENCES 1. S.Velu, D.P. Sabde, N. Shah and S.Sivasanker, Chem.Mater. 10 (1998) 3451. 2. S.Velu, K.Suzuki, M.Okazaki, T.Osaki, S.Tomura and F.Ohashi, Chem.Mater., 11 (1999) 2163. 3. K.G.Severin, T.M.Abdel-Fattah and T.J.Pinnavaia, Chem.Commun., (1998) 1471. 4 . S.Stork, W.F.Maier, I.M.M.Salrado, J.M.F.Ferreria, D.Guhl, W.Souverijns and J.A.Martens, J.Catal., 172 (1997) 414. 5. S.Velu, K.Suzuki and T.Osaki, Catal.Lett., (1999) in print. 6. M.A.Ulibarri, J.M.Femandez, F.M.Labajos and V.Rives, Chem.Mater., 3 (1991) 626. 7. A.J.Marchi, J.I.Di Cosimo and C.R.Apesteguia, Catal.Today, 15 (1992) 383. 8. P.Amoldy and J.A.Moulijn, J.Catal., 93 (1985) 38. 9. L.Alejo, R.Lago, M.A.Pena and J.L.G.Fierro, Appl.Catal.A 162 (1997) 281.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
459
Construction Strategies for New Generation Micro-porous Solids Ian D. Williams, Stephen S-Y. Chui, Samuel M-F. Lo, Mingmei Wu, John A. Cha, Teresa S-C. Law, Herman H-Y. Sung, Fanny L-Y. Shek, Jenny L. Gao and Tolulope M. Fasina Department of Chemistry, Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, China Approaches to the formation of three new types of micro-porous materials that complement zeolites will be discussed. In each case, whether metal coordination polymers, metal-linked ceramic oxide clusters, or new hybrids containing both coordination and ceramic components, engineering of the Secondary Building Unit (SBU) is of critical importance. Successful examples of these approaches include the first thermally stable 3-D micro-porous coordination polymer with chemical functionallzability [Cu3(TMA)2(H20)3]n, as well as a 3-D micro-porous cluster based material [Vi2Bi806oH8{Cd(en)(H20)}3]"". 1. BACKGROUND Zeolites and other traditional molecular sieve materials are a great technological success story with a wide range of commercial applications, which seems even more remarkable from a chemists' point of view, since they have a limited compositional basis. Herein we report our approach to the design and construction of several new 'families' of micro-porous solid, namely metal coordination polymers, cross-linked metal oxide cluster materials and hybrid 'ceramic-organic' solids. (Figure 1) These have various potential advantages over zeolites in terms of flexibility of design, engineering of cavity size and shape, as well as more facile chemical functionalization and introduction of catalytic centers or binding sites. Along with these advantages come a different set of attendant problems and we will discuss how some of these may be addressed. 2. IVIETAL COORDINATION POLYMERS Over the last few years there has been an explosion of interest in this area and compounds displaying selective absorption, ion exchange and even catalytic properties have been reported.[1-3] Metallo-organic polymers do offer greater potential for chemical and structural diversity than traditional zeolite micro-porous solids, but are plagued by low dimensionality, lattice interpenetration and framework instability. Experience in constructing such materials can address most of these problems. We are grateful to ttie Research Grants Council, (HK) for financial support (grants 681/96P, 6061/98P, 6148/97P and 6188/99P). IDW thanks Dr. E.M. Ranigen and Prof. R. Xu (Jilin) for prompting this work through their insight and enthusiasm for micro-porous solids.
460
\ /^'=="\ /^=^^ Z'^^. /
n
n
1
t
Metal Coordination Polymer
Metal Cross-Linked Cluster Solid
Ceramic-Organic Hybrid Solid
Figure 1. New Classes of Micro-porous Solid 2.1 Polymer Dimensionality Compared to zeolite formation, crystal growth of metal coordination polymers Is a relatively rapid process and low dimensional 'kinetic' products, which represent incomplete polymer condensation, are often formed. In general these materials have large numbers of terminal ancillary ligands, such as water, which effectively block further cross-link formation. We have consistently found that hydrothermal (or solvothermal) conditions
offer the best approach to enhancing
dimensionality and promoting formation of 3-D network polymers.
polymer
The high temperatures and
pressures allow more rapid thermodynamic equilibrium in the system and extended time or higher temperatures favor thermodynamic rather than kinetic products.
461
Figure 2. Ancillary Ligation and Control of Polymer Dimensionality. From top, 1-D [Er(TMA)H20)5], 2-D [E(TMA)(H20)3] and 3-D [ErfTMA)]
A particularly good example of the control of polymer dimensionality by use of hydrothermal approach is provided by the polymers formed in the Er^*/TMA system, shown in Figure 2, above (TMA is benzene-1,3,5-tricarboxylate).
462 Polymer crystals grown from silica gel at room temperature contain 1-D chains of [Er(TMA)(H20)5]n,[4] The use of a highly controlled 'layer diffusion' method gives 2-D [Er(TMA)(H20)3]n. In this material loss of two ancillary aqua groups allows pendant carboxylates of adjacent 1-D polymer strands to connect and create a sheet. Finally the use of hydrothermal conditions (180°C, autogenous pressure, 3 days) allows formation of anhydrous [Er(TMA)]n. This has a 3-D network with no simple topological relationship to the hydrated forms.[5]
2.2 Channel Engineering A major problem In zeolite and micro-porous solids is that of pore-size engineering. Although with the advent of meso-porous solids, it is clear that large pores can be accessed, the ability to fine-tune pore sizes below 15A is difficult in zeolitic chemistry. Further, it is not possible to design specific pore shapes or alter pore-lining functionalities. In the case of metal coordination polymers, the use of variable size 'spacer" units in chemically similar ligands can, in favorable cases, allow pore size expansion of othenA/ise homostructural arrangements.[6] In addition the use of particular ligand combinations can create channels with engineered shapes, such as rectangular, diamondoid, triangular, elliptical or hexagonal. Two approaches to the construction of designed pores comes from i) the use of 'supra-molecular" assembly and the templating effect of either guest molecules, or pendant ancillaries which can later be removed. In the case of [CuafTMAjgCpyjg] a 2-D assembly is formed with large hexagonal shaped 48atom rings (6 Cu and 6 TMA bridges). The cavities have three pyridine moieties directed to the ring centers.(Figure 3a) Replacement of these pyridines by smaller ancillaries such as water would thus give access to large channels, and avoid interpenetration problems that are often encountered. A second approach which might be called 'super-molecular" assembly involves the use of expanded ligands.
By analogy with TMA we synthesized
the
poly(benzoate)
ligand
hexakis-{p-
benzoicacid}benzene, [C6(C6H4COOH)6], this forms 3-D porous solids with metal cations such as Pb^*. The larger ligand size leads to larger pores (ca 13.5A in this case) as shown in Figure 3b.
2.3 Framework Stability The high thermal stability of zeolites and related micro-porous solids Is one of their most attractive features.
Whilst it is clear that materials with organic components cannot withstand ultra-high
temperatures, quite respectable compos/f/ona/stability can be achieved. Thus the [Er(TMA)] polymer mentioned above shows no weight loss in its TGA curve before 550°C. However for porous solids another key issue is that of structural stability. Many open framework coordination polymers lose their crystalline structure upon mild heating, or even evacuation, through loss of guest molecules. Recently we formed a coordination polymer [Cu3(TMA)2(H20)3]n which shows framework stability to 225°C as demonstrated by high temperature single crystal diffractometry.[7] This compound has cubic symmetry and an open framework with 10A intersecting pores (Figure 4). The key to its success is that it is built up of rigid octahedral shaped Secondary Building Units which are of ca. 1nm dimension. The rigidity stems from use of planar benzoate ligands in which the COO groups all bridge [CU2] dimers, a well-known and thermodynamically stable motif in copper carboxylate chemistry. With metal-ligand bonding optimized and locked in-place the compound can withstand both loss of channel solvent molecules, as well as loss of the axial aqua groups on the Cu centers.
463
Figure 3. a) [Cu3(TMA)2pyJ b) [Pb(Hexacid)] Open Networks
464
The resulting [Cu3(TMA)2] framework can then be re-functionalized with other ligating groups such as pyridine. Such chemical modification of pore-linings has clear implications for the possibility of engineering highly selective guest-host binding.
3.
CROSS-LINKED OXIDE CLUSTER SOLIDS
In attempting synthesis of micro-porous phases of transition metal borates we and others found that in the case of vanadium a variety of anionic metal borate clusters were formed.[8-10] The 'cluster* type SBU found in the [Cu3(TMA)2] suggested that the vanadoborate clusters might also be useful SBUs in formation of porous phases. Two soluble clusters, [VgBgoOsoHg]^ and [VigBiaOsoHg]^^*, allowed us to explore exchange of the existing [enHg] or Na countercations for metals, eg. Ca or Ln which could serve as cross-linking agents for the formation of porous networks. This was a partial success but resulting crystal size was small. We then found that presence of such metal cations during the initial hydrothermal synthesis can form the networked materials in a one step reaction. Actually the addition of the second metal can have four outcomes. First, a metal-amine complex may be formed and serve as countercation to the cluster, second the metal can be bound to the outside of the cluster surface through chelation to borate groups, as found for [M6(en)i2Vi2Bi806oH8] M = Zn, Co, Ni. Next it can be incorporated into the cluster itself, as In a [ZnioVioB28] phase we found, and finally it can serve as a cross link between clusters. Through use of Cd as the cross-linking agent we have found the first micro-porous vanadoborate materials Two cubic phases with 3-D micro-channels are formed, one with a primitive lattice and the other body centered. The structure of the body centered form is shown in Figure 5. The Cd bridges are chelated to borate groups on each side. Single crystal X-ray diffractometry shows this phase is stable up to 300°C, after which point the organic component is decomposed. In repeated heatingcooling cycles to 300°C the phase shows a reversible weight loss of 11% water. Upon dehydration this can be used for absorption of equivalent amount of a variety of organic solvents. The cavity size Is ca. 20% volume of the material. These class of materials holds much promise for ion exchange, selective absorption or catalysis. In the latter case this could take several modes of action, either from the vanadium oxide cluster, which are well known as REDOX active species, or through the metal bridges, either cadmium and related ions acting as Lewis acid sites, and the prospect of other metal bridges such as platinum metals which might allow a variety of reactions to take place within the micropores. Secondly pore-size expansion should prove possible in this class of compound, either through the use of more extended and elaborate bridging entities, and also since larger clusters such as [V12B32] are known and we are actively pursuing these directions.
4.
FUTURE PROSPECTS - HYBRID SOLIDS
A final class of materials can be mentioned here which combine some of the advantages of the metal coordination polymers in terms of design flexibility with the higher stability of the cluster cross linked materials. New hybrid solids containing ceramic and organic components are now being made and offer an almost unlimited range of possibilities.[11,12]
In our work we have found several phases
which involve metal hydroxide cores cross linked by organic moieties. These are prepared through extended hydrothermal synthesis in some coordination polymer systems. Their formation Is probably via intermediates with bridging aqua groups which deprotonate to form ^-hydroxide.
465
Figure 4. Structure of [Cu3(TMA)2(H20U A Chemically Functionalizable Micro-porous Coordination Polymer
Figure 5. [V,,B,30eoH3{Cd(en)(H,0)}3r A Micro-porous Framework Solid from Metal Cross-linked Clusters
466 Hydroxide ligands have three electron pairs for binding and can serve as either double or triple bridges. Thus stable SBUs which are metal hydroxide clusters or 1-D polymers can form. These are thermally stable components which can then be cross linked by polybenzoate or other system ligands to create open frameworks with high structural integrity. These can withstand ancillary ligand loss and guest host binding as in [Co5(OH)2(1245)2(H20)J, (1245 = pyromellitate). We have recently found that this compound can selectively bind 4,4'bipyridine across its 10A channels. (Figure 6)
Figure 6. A Ceramic-Organic Hybrid [COsCOH)^ (1245)2(H20)J
REFERENCES 1.
C. RobI, Mater. Res. Bull, 27, 99 (1992).
2.
B.F. Abrahams, B.F. Hoskins, D.M. Michail and R. Robson, Nature, 369, 727 (1994).
3.
M. Fujita, Y.J. Kwon, S. Wahizu, K. Ogura, J. Amer. Chem. Soa, 116, 1151 (1994).
4.
Z.B. Duan, G.C. Wei, Z.S. Jin, Z. Ni, J. Less Common Metal, 171, LI (1991).
5.
S.S-Y. Chul, Ph.D. Thesis, HKUST (1999).
6.
S. Kitagawa et al, Angew. Chem., Int Ed. Engl., 38, 140 (1999).
7.
S.S.Y. Chui, S.M-F. Lo, J.P. Charmant, A.G. Orpen, I.D. Williams, Science, 283, 1148 (1999).
8.
I.D. Williams, M. Wu, H.H-Y. Sung, X.X. Zhang, J. Yu, Chem. Commun., 2463 (1998).
9.
J.T.Rijssenbeek, D.J. Rose, R.C. Haushalter, J. Zubieta, Angew. Chem. Int., 36, 1008 (1997).
10.
C.J. Warren, D.J. Rose, R.C. Haushalter and J. Zubieta, Inorg. Chem., 37,1140 (1998).
11.
S.O.H. Gutschke, A.M.Z. Slawin and P.T. Wood, Chem. Commun., 2197 (1995).
12.
D. Hagrman, R.P. Hammond, R. Haushalter and J. Zubieta, Chem. Mater., 10, 2091 (1998).
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
467
Preparation effects on titania-sulfate aerogel morphology J. Mrowiec-Bialon% L. Pajak*", A.B. Jarz^bski'* and A.I. Lachowski" •Institute of Chemical Engineering, Polish Academy of Sciences, 44-100 Gliwice, Baltycka 5, Poland •"Silesian University, Institute of Physics and Chemistry of Metals, 40-007 Katowice, Bankowa 12, Poland ''Institute of Chemical and Process Engineering, Faculty of Chemistry, Silesian Technical University, 44-100 Gliwice, Ks. M. Strzody 7, Poland The presence of sulfate ions markedly affects the nanopore structure of titania-sulfate aerogels. In Ti02-S04^' materials, unlike in zirconia-sulfate aerogels, the larger sulfate load stimulates formation of a more consolidated structure. The XRD analysis shows that even a crystalline phase (anatase) may be present in fresh, dry aerogels, which, perhaps, is the first observation of this phase in sol-gel titania obtained from the low temperature drying process. 1. INTRODUCTION The sulfate promoted transition metal pxides focussed considerable attention in recent years due to attractive catalytic properties. Most of the research carried out to date centered on sulfated zirconias,^'^ not suq^risingly perhaps, as they exhibit the highest surface acidity (Ho < -16.04) among the members of this family of materials and appear to be able to initiate isomerization reactions in temperatures as low as 298 K. Far less interest attracted sulfated porous titanias, mainly owing to a lower surface acidity,^ although it may be a useful property in many catalytic situations. Thus closer inspection of the preparation procedures for sulfated titanias may be of interest, in particular as the reports on preparation and properties of these materials are scarce and we are not familiar with any work dealing with titania-sulfate aerogels. The sulfated transient metal oxides can be easily prepared by precipitation of hydroxides from aqueous solutions of metal salts using a base, followed by impregnation of the hydroxide using sulfuric acid. An alternative procedure makes use of the sol-gel method. In that case the structural properties of the final materials depend on the type of alkoxide and solvent used, their concentrations, amount of water and drying regime. A dilute sulfuric acid is added to alkoxide precursor and the resulting sulfated alcogel is dried to give a sulfated porous material.
* Corresponding author, e-mail:
[email protected] 468 The conventional drying of wet gels gives xerogels, while the supercritical drying, e.g. using supercritical carbon dioxide, affords aerogels.^ Morphologies of these two distinct classes of solid porous materials differ dramatically so as their properties. The morphology of aerogels closely portrays the solid skeleton of wet gels, while that of xerogels is a complex product of the structure collapse and gel shrinkage, which occur during the drying process. Consequently, the structure of aerogels can be tailored to the specific demand by means of tools of the sol-gel method, what is hardly possible in case in xerogels. Typically, the morphology of aerogels is very open and porosity markedly larger than that of xerogels and this bears on the specific activity, significantly higher in case of aerogels. This fuels the interest in aerogels, shown by catalyst engineers in the past decade.^'^
2. EXPERIMENTAL 2.1. Preparation of the samples Six samples, in two series, of titania-sulfate aerogels, were prepared with a titania content in the sol-gel system of 0.5 mol of Ti/1 of butanol (samples A...) or 0.25 mol/1 (samples B...) to obtain materials with a nominal 10, 15 or 20 mol% of S04^'in wet samples (designated A(B)10,..,A(B)20). An additional series of three samples containing 1 wt% platinum was also prepared with a 0.5 mol/1 titania content. These samples were designated: A10Pt,..,A20Pt. Titanium butoxide, (TiB, 99% Aldrich) was titania precursor and chloroplatinic acid was used as a platinum source. The molar ratio TiB/H20 was equal to 4 in all samples. Alcogels were prepared as follows. First two solutions were prepared: C consisted of half of the total butanol content, sulfuric acid (96%), and TiB, whereas D consisted of remaining butanol, water and chloroplatinic acid (if added). Then solution D was added to C under vigorous stirring at room temperature. Gelation occurred in 0.5- 90 min. Alcogel samples (50 ml) were dried for 5 h in a stream of supercritical carbon dioxide (343 K, 12 MPa). 2.2. Characterisation Prior to examinations the as-obtained samples were ground into powder and dried at 383 K for 2 h to remove remaining volatiles. Textural properties of materials: SBET» mesopore volume VpN2(BJH method, desorption branch) and micropore volume V„,i (Harkins-Jura t-plot method) were determined from the nitrogen adsorption experiments at 77 K performed using a Micromeritics ASAP 2(X)0 instrument. Before these measurements the samples were additionally degassed at 353 K for 8 h with the final pressure ca. 0.1 Pa for at least 1 min. Morphology was investigated using the methods of small angle scattering of X-rays, SAXS (Kratky type camera, CuK„ radiation) and transmission electron microscopy, TEM (JEOL 2000 SX). Crystal structure was determined by XRD. After investigations all fresh samples were calcined in static air at 753 K for 2 h and characterised in the same way. 3. RESULTS AND DISCUSSION As can be seen from Table 1 all fresh samples exhibited a remarkable porosity; the specific surface area, SBET was between 160 and 480 mVg and the mesopore volume, VpN2 was in the range of 0.7- 1.4 cmVg and those volumes appeared to be the largest in Pt doped samples. Micropore volumes were virtually insignificant in all samples as can be seen from t-plots
469 Table 1. The characteristic parameters of porous texture c
b
c a •^BET
^P(N2)
mVg
mVg
cmVg
cmVg
AlO
384
157
1.22
1.32
A15
323
151
1.27
1.37
A20
158
125
0.82
0.70
BIO
292
135
0.75
0.54
B15
297
171
0.92
0.89
320
326
174
1.07
0.67
AlOPt
477
132
1.25
0.57
A15Pt
482
162
1.44
0.74
A20Pt
412
140
1.44
0.69
Sample
V
*
^P(N2)
- before calcination, * after calcination (Fig. 1) and for this reason are not given in Table 1. The effect of sulfate concentration on the porous texture of aerogels proved to be more complex. While in samples of A..series the larger sulfate content appeared to suppress porosity in the range of small mesopores this was observed neither in aerogels of B.. series nor in those doped with platinum (A..Ft series). This is displayed by the pore size distributions (PSD) given in Fig. 2a and 3a. Although the scattering curves from the corresponding samples are quite similar in shape (cf. Fig. 4a), yet they markedly differ in the values of slopes, especially of those from AlO and A15 and that
//
A20Pt
/ /
1
"]
•
/
/o^'" ,/ o
1/ JZ
0.6
Figure 1. t-plots analysis of titania-sulfate aerogels; ((•) points taken for linear regrresion)
470
0.06
0.04 A15 > 0 . 0 3 •]
Ji0.02 i
,0.01 o 0.00
10 100 Pore diameter, nm
10 100 Pore diameter, nm
Figure 2. Pore size distributions in titania-sulfate aerogels before (a) and after calcination (b). of A20, which indicates a strong difference in a nanostructure of these samples. For q-values from the range of 0.025-0.35A"^ (i.e. in a broad range of small scales) the constant slope of the scattering curve from A20 corresponds to surface fractal dimension D, = 2.2, which indicates a fairly smooth surface, and hence devoid of smaller pores. *^'^* For q ^W*-^''.'^*'^i»'v-vr,\'v-,-,'A-/—•*—
V
283K
303K
A ••>,
300
200
ppm
293K
100
Figure 4. Variable temperature flowing HP *^^Xe NMR MAS spectra of xenon adsorbed in montmorillonite pillared with pyridinium ions. The XQ/NJ/HQ mixture flow rate was 300 cc/min. The sequence of temperature steps progresses from bottom to top. The spectrum at 373K was after drying in a stream of flowing helium for 3 hrs at 373K. some large pores, perhaps partially filled with water. After heating the sample for some 3 hours in flowing helium at 100°C, a line at ~ 170 ppm becomes visible, and this is characteristic of xenon in nanopores. As the sample is cooled, there is the gradual appearance of a line at -90 ppm, attributable to xenon in the mesopore space of the clay. A line in this position also is visible for xenon in contact with dry sodium montmorillonite, where the mesopore space becomes available for xenon adsorption once the water is removed, but the interlayer space is too small to accommodate xenon. The weakening of the nanopore xenon line, and its ultimate disappearance can be taken to mean that the interlayer nanopore space is no longer accessible to the flowing gas, probably because of the shrinking of the interlayer space. As the xenon can no longer flow into the favoured nanopore space, of necessity it shifts to the mesopore space. The spectra recorded in the temperature region where both
497 nano- and mesopore lines are observable may be affected to a certain degree by exchange of xenon between the two sites. Thus xenon is an accurate reporter on the state of the void space in the pillared clay during the dehydration step and the subsequent cooling that adjusts the geometry of the pillared galleries. In the experiments illustrated above, a remarkable amount of information becomes available rather quickly and easily when flowing HP xenon gas is used as a probe material. Although some of the information could have been obtained with thermally polarized xenon, such experiments would have to be carried out on sealed, pressurized samples, but to obtain information on the temperature-programmable interlayer void space would be far more difficult if not impossible. 3.4 Imaging void space with hyperpolarized xenon There have been few attempts to image materials with thermally polarized xenon. In 1998, samples of an aerogel were imaged on the three observed chemical shifts in the spectrum [30], one for the gas, the other for two distinct regions within the aerogel. The sample was prepared under -30 atmospheres of xenon, and the time required to produce an image was more than 20 hrs., however, the experiment did illustrate the feasibility of using the large chemical shift dispersion of xenon for the imaging of spatial inhomogeneities. Fig. 5 shows the image produced by flowing hyperpolarized gas in a phantom consisting of a hollow Vycor cylinder filled with NaY zeolite. The spectrum (fig.5, bottom left) shows lines characteristic of the gas near 0 ppm, the xenon in Vycor at 76.1 ppm and
C.S., ppm
120100 80 60 40 20 0 -20 Cherrtcal Shift, ppm 129^
Figure 5. HP Xe Chemical Shift Imaging (left and centre) of a phantom consisting of a 7 mm porous Vycor tube filled with NaY zeolite and placed inside an open 9 mm ID glass tube (right). Images from Xe in the three different chemical shift environments can be clearly separated. The NMR spectrum is shown bottom left.
498 NaY zeolite at 60.1 ppm. The image was obtained for a 3mm slice with full chemical shift imaging (note that for thermally polarised Xe this type of imaging experiment would be far more demanding in terms of experimental time even than chemical shift resolved imaging, as practiced for the Aerogel samples[30]), and was obtained in -30 min. Thus, the improvement in imaging with HP xenon over thermally polarized xenon is impressive, and indicates that there are real prospects for applications in the characterization of materials. 4. CONCLUSIONS The ^^^Xe chemical shift - pore size relationship was extended to the mesopore region with a number of controlled pore glasses with pores in the 75-385 A range. There is a correlation of the pore diameter with the inverse of the chemical shift. For small pores there is markedly different behaviour with the switch occurring near the nanopore - mesopore transition at 10-20A. The development of optical pumping methods for producing highly polarized xenon have increased the sensitivity of Xe NMR experiments by factors up to 10^ Both batch and continuous production of hyperpolarized xenon have the prospects of revolutionizing the way that NMR spectroscopy can be used to characterize materials. Applications include the characterization of pore space in bulk solids, the measurement of diffusion constants, and the following of in-situ processes such as dehydration and activation of sorbents and catalysts, the study of adsorption - desorption processes. Finally, the introduction of flowing HP Xe over porous samples allows chemical shift imaging in a reasonable length of time.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15.
T. Ito and J. Fraissard, J. Chem. Phys., 76 (1982) 5225. J. Ripmeester, J. Am. Chem. Soc, 104 (1982) 289. C. I. Ratcliffe, Ann. Rep. NMR Spectr., 36 (1998) 124. D. Raftery and B. F. Chmelka, NMR Basic Principles and Prog., 30 (1994) 111. P. J. Barrie and J. Klinowski, Progr. NMR Spectrosc, 24 ^992) 91. J. -L. Bonardet, J. Fraissard, A. Gedeon and M.-A. Springel-Huet, Catal. Rev.-Sci. Eng.,41 (1999) 115. J. A. Ripmeester, C. I. Ratcliffe and J. S. Tse, J. Chem. Soc. Faraday Trans. I, 84 (1988)3731. J. A. Ripmeester and C. I. Ratcliffe, J. Phys. Chem., 94 (1990) 8773. J. A. Ripmeester and C. I. Ratcliffe, Energy and Fuels, 12 (1998) 197. M. A. Springuel-Huet and J. Fraissard, Zeolites, 12 (1992) 841; C. J. Tsiao, J. S. Kauffmann, D. R. Corbin, L. Abrams, E. E. Carroll, Jr. and C. Dybowski, J. Phys. Chem., 95 (1991) 5586. J. A. Ripmeester and C. I. Ratcliffe, J. Phys. Chem., 94 (1990) 7652. J. A. Ripmeester and C. I. Ratcliffe, Analytica Chimica Acta, 283 (1993) 1103. C. J. Jameson, A. K. Jameson, B. I. Baello and H.-M. Lim, J. Chem. Phys., 100 (1994) 5965. C. J. Jameson, A. K. Jameson, H.-M. Lim and B. I. Baello, J. Chem. Phys., 100 (1994) 5977. C. J. Jameson and H.-M. Lim, J. Chem. Phys., 103 (1995) 3885.
499 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.
C. J. Jameson, A. K. Jameson and H.-M. Lim, J. Chem. Phys., 104 (1996) 1709. C. J. Jameson and H.-M. Lim, J. Chem. Phys., 107 (1997) 4373. J. A. Ripmeester and C. I. Ratcliffe, J. Phys. Chem., 99 (1995) 619. I. L. Moudrakovski, C. I. Ratcliffe and J. A. Ripmeester, Appl. Magn. Reson., 10 (1996)559. J. Fraissard and J. Karger, Zeolites, 9 (1989) 351. T. Ito, J. Fraissard, J. Karger and H. Pfeifer, Zeolites, 11 (1991) 103. J. Karger, H. Pfeifer, F. Stallmach, N. N. Feoktistova and S. P. Zhdanov, Zeolites, 13 (1993)50. J. Karger, H. Pfeifer, T. Wutscherk, S. Ernst, J. Weitkamp and J. Fraissard, J. Phys. Chem., 96 (1992) 5059. W. Heink, J. Karger, H. Pfeifer and F. Stallmach, J. Am. Chem. Soc, 112 (1990) 2175. J. Karger, H. Pfeifer, F. Stallmach and H. Spindler, Zeolites, 10 (1990) 288. I. L. Moudrakovski, C. I. Ratcliffe and J. A. Ripmeester, Appl. Magn. Reson., 8 (1995)385. B. F. Chmelka, D. Raftery, A. V. McCormick, L. C. de Menorval, R. D. Levine and A. Pines, Phys. Rev. Lett., 66 (1991) 580. C. J. Jameson, A. K. Jameson, R. Gerald II and A. C. de Dios, J. Chem. Phys., 96 (1992) 1676. I. L. Moudrakovski, C. I. Ratcliffe and J. A. Ripmeester, J. Am. Chem. Soc, 120 (1998)3123. D. M. Gregory, R. E. Gerald II and R. E. Botto, J. Magn. Res., 131 (1998) 327. B. Driehuys, G. D. Gates, E. Miron, K. Sauer, D. K. Walter and W. Happer, Appl. Phys. Lett., 69 (1996) 1668. D. Raftery, E. MacNamara, G. Fisher, C. V. Rice and J. Smith, J. Am. Chem. Soc, 119(1997)8746. M. Haake, A. Pines, J. A. Reimer and R. Seydoux, J. Am. Chem. Soc, 119 (1997) 11711. R. Seydoux, A. Pines, M. Haake and J. A. Reimer, J. Phys. Chem. B, 103 (1999) 4629. E. Brunner, M. Haake, L. Kaiser, A. Pines and J. A. Reimer, J. Magn. Res., 138 (1999) 155. M. Hunger and T. Horvath, J. Chem. Soc. Chem. Commua, (1995) 1423. J. Crank, "The Mathematics of Diffusion", Oxford Univ. Press, N.Y., (1967).
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Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
501
Characterization Of Mesoporous Molecular Sieves: Differences Between M41S And Pillared Layered Zeolites. Wleslaw J. Roth, James C. Vartuli and Charles T. Kresge Mobil Technology Company, Paulsboro, NJ 08066. Two major classes of mesoporous molecular sieves are currently of great interest: M41S materials prepared by direct synthesis and pillared materials synthesized by swelling and/or pillaring of layered solids. The pillared sieves derived from layered zeolite precursors and represented by MCM-36 complement the extensively studied M41S class and are of interest due to their much stronger acid activity. Both types are prepared under similar conditions (high pH, presence of surfactants) and share some common characteristics. This presents a problem of differentiating between them, especially because of possible contamination of the pillared product with M41S and the parent zeolite. The pillared zeolite case is particularly critical and requires proof that mesoporoslty is not the result of M41S impurity. The typical methods employed to characterize both classes of mesoporous materials are X-ray diffraction, microscopy, static and dynamic adsorption/desorption techniques and catalytic testing. No single detemnination appears to provide an unambiguous answer concerning purity of the pillared phase. The absence of significant contaminants and identity of the product can be detennined by combining results from all the mentioned techniques. The evidence of mesoporoslty and pillaring are found by static sorption measurements and X-ray diffraction, respectively. Sorption isothemris and dynamic sorption in conjunction with microscopic images provide further indications of successful exfoliation of the layered zeolite precursor. 1. INTRODUCTION Two categories of mesoporous solids are of special interest: M41S type materials and pillared or delaminated derivatives of layered zeolite precursors (pillared zeolites in short). The M41S family, first reported in early 1990's [1], has been extensively studied [2,3]. These materials exhibit broad structural and compositional diversity coupled with relative ease of preparation, which provides new opportunities for applications as catalysts, sorption and support media. The second class owes its existence to the discovery that some zeolite crystallizations can produce a lamellar Intemriedlate phase, structurally resembling zeolites but lacking complete 3-dimensional connectivity in the assynthesized fonn [4]. The complete zeolite framework is obtained from such layered zeolite precursor as the layers become fused, e.g. upon calcination. The layers posses zeolitic characteristics such as strong acidity and microporoslty. Consequently, mesoporous solids derived from layered zeolite precursors have potentially attractive characteristics different from M41S and the zeolite species
502
itself. MCM-36 obtained from the MCM-22 precursor by swelling and pillaring, is the first known representative of the pillared zeolite family [5,6]. A recent report claims preparation of delaminated product designated ITQ-2 [7] obtained by an alternative delamination of the swollen MCM-22 [5]. This paper addresses two issues concerning practical aspects of synthesis and utility of pillared zeolites. First it shows how to demonstrate that the observed mesoporous attributes of a particular preparation are not due to undesired M41S contamination. This possibility arises because of the similarity of synthesis regime in both cases: aluminosilicate substrates treated with cationic surfactant at high pH and temperature. Second issue concerns the benefits of pillaring zeolite precursor as manifested via improved catalytic perfomriance. Herein we compare the properties of MCM-41 and the pillared zeolite MCM-36 obtained with cetyltrimethylammonium cation as the swelling/templating surfactant. 2. EXPERIMENTAL The synthetic procedures have been described elsewhere [1,5,6]. Characterizations were carried out using the following equipment: X-ray diffraction - Scintag diffractometer; static sorption - Macromeritics ASAP 2000 and 2400; dynamic sorption [8] - DuPont 951 TGA interfaced to a PDP 11/44 computer. 3. RESULTS AND DISCUSSION 3.1. Preparation and identification of pillared zeolite MCM-36 The conversion of MCM-22 precursor to the pillared zeolite MCM-36 is achieved via a three step procedure illustrated in Figure 1. The first step involves breaking of the apparent non-covalent interlayer bonding, which requires relatively severe conditions of high base concentration, elevated temperature and presence of cationic surfactant. There is a concomitant expansion of the interlayer separation by ca. 25 A, caused by surfactant insertion in the interlayer region. The swelling step is critical and once accomplished successfully is followed by rather routine treatments: pillaring and calcination. The fomier is introduction of pillar precursors, i.e. silicate moieties, which become pemrianent props in the 3^^ step calcination, which also eliminates organic residue (including surfactant) and generates a porous structure. The efficiency of swelling of the zeolite precursor is detemriined primarily based on changes in the X-ray diffraction pattern before and after treatment. The treatment preserves the layers but alters their relative distance and/or orientation. The peaks corresponding to in-layer reflections (hkO) remain unaffected while the others undergo changes such as shifting, broadening or disappearance. The Xray pattern of MCM-22(P) and its swollen derivative could not be indexed and assigned unambiguously due to broad peaks and extensive peak overlap [6].
503 MCM-22 Precursor
Swollen MCM-22 Precursor
MCM-36
repeat c = >50 A
repeat c = -50 A
repeat c = 27 A
Swelling
pH>12,>90C
"^
MCM-22 - MWW repeat c = 700
>700
15 10 15
10-30 >25 >25
>10 >40 >40
Dynamic sorption uptake/rafe, pi sorbate/g sorbent, ^l/gs^^ 3-methylpentane 105/34 80/30 cyclo-hexane 56/75 36/6 2,2-dimethylbutane 58/4 57/15 p-xylene 83/7 62/5 o-xylene 28/4 61/5 40/2 1,3,5-trimethylbenzene 8/1
42/12 22/7 31/9 21/2 24/3 22/1
N2 isotherm type hysteresis
IV none or HI
1 none
IV H3 or H4
structure as well, but to a lesser extent than MCM-41. The effect of delamination is particularly evident with the adsorption capacity for the bulky 1,3,5trlmethylbenzene. The observed 5-fold increase upon transition from MCM-22 to MCM-36 proves much increased accessibility. Dynamic adsorption of 2,2-dimethylbutane into MCM-22 expressed as amount sorbed vs. time showed a peculiar three step uptake profile [6]. This was interpreted as reflecting adsorption into different pore regions. MCM-36 showed similar three step plot but with enhanced capacity for the first, fast uptake stage. This is again a reflection of pillaring, which modified some pore features while not affecting others. MCM-41 exhibited much lower dynamic sorption capacity for 2,2-dimethylbutane than both MCM-22 and MCM-36 and would therefore produce a reduction in the overall sorption value if present.
507 700 600 500 MCM-41
Q.
So O
400 MCM-36
o
I 300 3
§
200 100
MClyl-22 1• • 1 • • i l i i * "
L n » ••"
""
* 1 • • • »• • • • •*
0.2
0.4
0.6
0.8
P/Po
Figure 3. N2 isotherm for MCM-22, MCM-36 and MCM-41. The overlap of MCM-36 and MCM-41 plots at lower pressures is coincidental. 3.3. Improved catalytic activity of MCM-36 The primary reason for preparation of the pillared species is a potential improvement or modification of the catalytic behavior compared to the parent zeolite. MCM-36 has shown advantage over MCM-22 in the Isobutane alkylation process [10,11], which Is catalyzed by strong acids. The perfomriance of MCM-22 was characterized by low activity and poor yield of alkylate while with MCM-36 the conversion was complete and the yield approached its theoretical value ~2. The observed improvement with MCM-36 can be attributed to a more open structure, achieved by delaminatlon/pillaring. The significance of this effect is emphasized by the fact that MCM-36 contains up to 50 % by weight of inert silica pillars, which must result in 'dilution' of the active centers. It was reported that the single layer sheets of the MCM-22 precursor contain 12-ring pockets [12]. In MCM-36 these pockets are exposed and readily accessible, which may be one of the reasons for the observed enhanced perfomriance. Of course, M41S with its acidity comparable to the silica-alumina catalyst is not expected to show meaningful perfomiance in the isobutane alkylation process. 4. CONCLUSIONS The successful preparation of pillared or delaminated zeolite is confimried using X-ray diffraction and TEM, which also allow detection of M41S impurities if
508
present in significant amount. MCM-36 sorptive characteristic are between MCM22 and MCM-41 and some distinct features are observed, strengthening differentiation between all three materials. Pillaring of zeolite precursor may result in improved activity, e.g. MCM-36 in alkylation, justifying the additional preparative steps to modify the as-synthesized zeolite precursor. MCM-36 complements the more common and easier to produce M41S materials as a mesoporous catalysts because of high zeolitic activity.
REFERENCES 1. (a) C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. (b) J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L Schlenker, J. Am. Chem. Soc., 144(1992) 10834. 2. J.C. Vartuli, W.J. Roth, J.S. Beck, S.B. McCullen and C.T. Kresge, in Molecular Sieves Science and Technology, H.G. Karge and J. Weitkamp (eds.). Springer, 1998, Vol. 1 (Synthesis), 97-120. 3. S. Biz and M.L. Occelli, Catal. Rev.-Sci. Eng., 40 (1998), 329. 4. (a) M.E. Leonowicz, J.A. Lawton, S.L. Lawton, M.K. Rubin, M. K. Science, 264 (1994) 1910. (b) S.L. Lawton, A.S. Fung, G.J. Kennedy, L.B. Alemany, CD. Cheng, G.H. Hatzicos, D.N. Ussy, M.K. Rubin, H.C. Timken, S. Steuetrnagel and D.E. Woessner, J. Phys. Chem., 100 (1996) 3788. 5. C.T. Kresge, W.J. Roth, K.G. Simmons, J.C. Vartuli, Crystalline Oxide Material, US Patent No. 5 250 277 (1993). 6. W.J. Roth, C.T. Kresge, J.C. Vartuli, M.E. Leonowicz, A.S. Fung, M.B. McCullen, in Studies in Surface Science and Catalysis , H.K. Beyer, H.G. Karge, I. Kirlcsi, J.B. Nagy (eds.), Elsevier, 1995, Vol. 94, 301. 7. A. Comfia, V. Fornes, S.B. Pergher, T.L.M. Maesen and J.G. Buglass, Nature, 393(1998)353. 8. E.L. Wu, G.R. Landolt, A.W. Chester, in Studies in Surface Science and Catalysis, Y. Murakami, A. lijima, J.W. Ward (eds.), Elsevier Science, 1986, Vol. 28, 547. 9. Reporting Physisorption Data for Gas/Solid Systems, lUPAC Recommendation, Pure Appl. Chemistry, 57 (1985) 603. 10. Y.J. He, G.S. Nivarthy, F. Eder, K. Seshan and J.A. Lercher, Microp. Mesop. Mat., 25(1998)207. 11. E.J.A. Schweitzer and P.F. van den Oosterkamp, Microp. Mesop. Mat., 20 (1998)393. 12. S.L. Lawton, M.E. Leonowicz, R.D. Partidge and M.K. Rubin, Microp. Mesop. Mat., 23(1998) 109.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) 2000 Elsevier Science B.V.
509
Magnetic Resonance Microimaging Studies of Porous Petroleum Coke Eric B. Brouwer,^'^ Igor Moudrakovski,^ Keng H. Chung,'^ Gerald Pleizier,^ John A. Ripmeester^ and Yves Deslandes^ ^Institute for Chemical Process and Environmental Technology, National Research Council, Ottawa, ON, Canada Kl A 0R6 ^Steacie Institute for Molecular Sciences, National Research Council, Ottawa, ON, Canada K1A0R6 ^Syncrude Research Centre, 9421-17 Ave., Edmonton, AB, Canada T6H 1N4 Magnetic resonance microimaging (MRM) is used to characterize petroleum coke formed in the upgrading of Athabasca bitumen. Formation of large coke materials poses processing difficulties, and so the study of the structure and formation mechanism is important. The larger, "peas and beans" coke (approximately spherical, 4.1 mm diameter) possesses significant porosity, and is ideal for MRM. Coke is soaked in cyclohexane, and then 72-, diffusion-, and density-weighted ^H images are collected with an unprocessed image resolution of 24 |im pixel'^ Scanning electron microscopy indicates that the coke is surrounded by a dense, 100 |im-thick outer shell. MRM shows that this shell is permeable to cyclohexane, and that a significant pore volume is present. The cyclohexane diffusion coefficient (Dmax = 1.4 x 10'^ m^ s'*) is roughly one-half of the Dmax in pure solution indicating that the cyclohexane inside the coke is liquid-like. The MRM images indicate that the coke is formed by an agglomeration mechanism from smaller fluid coke particles (nominal diameter 150 |im), which in turn posess microporosity. MRM shows great promise as a non-destructive, structural and dynamic characterization method for petroleum coke. Strategies to enhance resolution will be presented and discussed.
1. INTRODUCTION Petroleum coke is a highly carbonized industrial material that originates from a variety of petrochemical refining and oil upgrading processes [1]. The Athabasca oil sands of northem Alberta, Canada contain extensive deposits of bitumen, which can be separated from the sand and other materials and subsequently upgraded to a synthetic crude oil. The upgrading of Athabasca-derived bitumen occurs in large coker reaction vessels operating at 524 °C in which bitumen is sprayed, along with stream, onto a fluidized coke bed which effects the cracking of the bitumen to lighter fractions. The nominal size of the fluidized coke particles is
510 150 |im, and have been shown to be microporous by both porosimetry (28(7) A average BET pore diameter) and ^^^Xe NMR (8iso = 100 ppm) measurements. Fluid coke can agglomerate to form significantly larger materials in the order of centimetre dimensions. These larger coke materials pose significant problems for the transport of the fluidized bed within the coker. Furthermore, coke deposition on reactor walls requires costly maintenance shutdowns of the upgrading facilities. To summarize, while fluidized coke is integral to the bitumen upgrading process, coke deposition and agglomeration processes are detrimental. Consequently, the structure and formation mechanisms of petroleum coke are fundamental to the understanding of the formation, role and behavior of coke in bitumen upgrading processes. The structure of petroleum coke varies widely according to the feed and coking conditions, and thus encompasses a wide range of structural motifs and morphologies. In this study, we focus on coke materials, commonly called "peas and beans" coke, which are approximately spherical in shape, range in size from 4 to 9 mm, and contain significant pore volume. From a processing perspective, these coke beans are too large for efficient mechanical transport within the coker and thus their formation is to be avoided. The pertinant scientific questions that arise include: (a) what is structure and (b) what is the formation mechanism of these coke beans. The structure of petroleum coke has been extensively investigated by diffraction, scanning and transmission electron microscopy (SEM, TEM), porosimetry, optical microscopy, solidstate nuclear magnetic resonance (NMR) and thermogravimetric methods [2]. These techniques give 1- and 2-dimensional, static spectra or images of the coke material. However, it is difficult to correlate the chemical structural resolution to the distribution of the chemical properties in the coke. Recently we have applied magnetic resonance microimaging (MRM) to obtain 3-dimensional spatial characteristics of the void spaces in petroleum coke [3]. MRM is able to provide various images of the coke weighted according to the physical properties of the molecule probing the void space. For the probe hydrocarbon molecule cyclohexane, images weighted according to r2-relaxation, density and diffusion are obtained, and gives information that is dynamic, rather than static, in nature. In this contribution, we continue to explore the application of MRM to the characterization of petroleum coke structure. 2. RESULTS AND DISCUSSION Figure 1 shows several cross-section SEM images of a 4.1 mm diameter bean coke. The first image at low magnification shows that the interior of the coke material consists of many smaller coke particles of a nominal diameter of 150 jim. There are significant void spaces in the interior of the coke bean. The highlighted area in the upper left comer is shown with higher magnification in Figure 1(b), and emphasizes the shell that encloses the 150 |im-fluid coke particles. This shell varies in thickness from -50 to 125 jim, with an average thickness of approximately 100 jim, and appears to be much more dense that the interior of the coke bean. The outer surface smooth without any prominent features. The third SEM (Figure Ic) gives further detail of the cross-section of the shell. The SEM images indicate that the structure of the larger coke beans is consistent with a formation mechanism involving the agglomeration of 150 |im-fluid coke particles. The growth or agglomeration events, as indicated by the particle diameter, appear to be terminated
511
Figure 1. Scanning Electron Microscopy (SEM) images of a 4.1 mm diameter coke bean; cross-section. Magnification: (a) x75; (b) xl50, expansion of the outlined area in (a) to focus on the shell; (c) x200, with further detail of the exterior shell. The SEM images were collected on a Jeol JSM-5300 scanning microscope.
by the deposition of a higher density, approximately 100 jim-thick layer of coke material which displays very little porosity relative to the interior. This shell may arise from bitumen spray which locks in a loose agglomerate of fluid coke particles into the resulting larger material. Some questions which arise from the SEM study involve the characteristics of the shell, its behavior (and still, the mechanism of formation) as well as the nature of the significant void space present in the interior of the coke bean. The coke material was also examined by MRM (prior to the SEM imaging) in which the spatial properties of the void space are probed by the ^H NMR signal of cyclohexane located in the core pore system. Cyclohexane is chosen because it has a single ^H NMR chemical shift value, and possesses a relatively short T\ relaxation value of ~2 s. The coke bean was impregnated with cyclohexane in an ultrasonic bath to minimize the amount of trapped air bubbles. The coke was then loaded into either a standard 5 or 10 mm NMR tube depending on the size of the coke bean. Space around the coke was filled two materials saturated with cyclohexane in order to immobilize the coke and to prevent evaporation from the coke. Both materials give different signal from the area outside the coke: gypsum minimizes the
512 cyclohexane signal from outside the coke, whereas zeolite HSY maximizes the cyclohexane signal. It is the latter material which gives the MRM images with the better contrast. The MRM images were acquired using a Bruker DSX-400 NMR spectrometer (9.4 T magnetic field, 400 MHz ^H resonance frequency) equipped with a Bruker MR2.5 microimaging probe with either a 5 or 10 mm coil. All images were acquired with spin-echo sequences optimized for shortest spin-echo time TE. Diffusion-weighting was introduced by a pair of diffusion gradient pulses located between the excitation and refocussing pulses applied along the slice-selection direction. Slice selection was obtained using truncated Gaussian pulses, with a slice gradient of 32 G cm'\ Images were acquired in a square field of view of 5 mm in a 208 x 208 matrix, which was expanded to 256 x 256 during processing, or with a square field of view of 10 mm in a 208 x 208 matrix, which was expanded to 256 x 256 during processing. Intensity-weighted and r2-weighted images are results of fitting the images acquired with different TE with the single-exponential function / = IoQxp{'TE/T2). Intensities in the resulting images are proportional to spin-density /o and spin-spin relaxation time T2, respectively. Intensity in the diffusion-weighted image is proportional to the diffusion coefficient of cyclohexane in the coke framework. Figure 2 shows a multiple-echo, intensity-weighted cyclohexane MRM image of a 300 \im cross-sectional slice of a 4 mm diameter coke bean. The density-weighted ^H images are collected with an unprocessed image resolution of 24 pim pixel'^ Since the coke is packed in wet gypsum, the area outside the coke shows very little cyclohexane intensity and appears black. Several feature arise from this image which complement the SEM images in Figure 1. Firstly, a dark band at the perimeter of the coke corresponds to the exterior shell depicted in deal in Figure Ic. This dark feature reflects the absence of cyclohexane and thus indicates the
Figure 2. Multiple-echo, intensity-weighted cyclohexane MRM image of 4 mm diameter coke bean, with 300 jim slice thickness. The coke is packed in wet gypsum. The highest intensity corresponds to the area of greatest cyclohexane intensity. Note the dark band of high-density (low C6H12 intensity) at the coke perimeter. On the right is the intensity profile of the region indicated by the grey box; the dashed lines indicate the edges of the coke bean. The regions of low intensity immediately inside the coke perimeter indicate the high density coke shell.
513 high density nature of the shell. Although the SEM images of the shell might have indicated that the interior void space is sealed by this shell, the presence of significant cyclohexane intensity in the interior proves that the shell is penetrable by cyclohexane. A second feature worth noting is the nature of the void space: it is both randomly distributed and interconnected. The lack of any significant dark regions in the interior indicates that the probe molecule has thoroughly peneterated the entire interior. The distribution of this intensity also shows no apparent ordering or organization. The diffusion- and Ti-relaxation weighted images are shown in Figure 3. The diffusionweighted image (left) shows how the cyclohexane diffusion constant D varies spatially in the same coke bean as in Figure 2. The cyclohexane diffusion constant in pure solution at room temperature is 2 x 10"^ m^ s"^ The median and maximum intensity values corresponds to Z) = 0.5 - 1.4 X 10'^ m^ s'\ and indicate that the cyclohexane is in a liquid-like environment. The r2-relaxation weighted image (right) shows variation in the cyclohexane r2-relaxation constant from 9-65 ms. A superposition of the D-, intensity- and r2-relaxation-weighted images indicates that the signal intensity corresponds to identical spatial locations. Examination of four larger (--8.9 mm diameter) and visibly similar coke beans by cyclohexane-MRM demonstrates the power of this technique. Figure 4 shows four MRM density-weighted, 300 |im cross-sectional images of coke beans packed in zeolite HSY, and collected using a 10 mm coil. Since the density is much higher in the zeolite, the background gives a bright background which better illustrates the coke bean edges than the previous images. Since the images have identical intensity increments, a quantitative comparison of the volume of cyclohexane in the void spaces can be calculated. The images are calibrated to the inner diameter of the 10-mm NMR tube, which is 8.9 mm [4]. The total volume for each cross-sectional slice is determined by first calculating the area of the coke bean, and then multiplying the area by the 300 jim slice thickness. The mean intensity was calculated, and the image intensity calibrated internally such that the brightest
4.4
'-
'''
' *
^ •. .
. • «
Figure 3. Diffusion {left) and r2-relaxation (right) weighted images of a 300 )im-slice of the same coke bean as in Figure 2.
514 (255) and darkest (0) intensities correspond to voxel fully occupied and unoccupied, respectively, by cyclohexane. The volume of the coke framework, Vcoke, was calculated by subtracting the volume of the cyclohexane in the pores, Vpore, from the volume of the slice, Vsiice; the relative densities for each image were then calculated by taking the ratio Vcoke / Vshce- These calculations are summarized in Table 1.
BBfftfil
^niBB .M'^/^v, ; - ^
Figure 4. MRM images of cyclohexane in four, randomly selected types of bean coke. The coke was placed in a 10-mm o.d. NMR tube (8.9 mm i.d.), packed with zeolite HSY and soaked with cyclohexane. The intensity gradients of the four images are identical. The unprocessed image resolution is 28 jim pixel'^
515 Table 1 Coke image density calculations Sample Area (mm^)
Mean Intensity (std. dev.)
Vsiice (mL)
Vpore
(mL)
Vcoke (mL)
V^coke ' •^slice
(a)
53.13
20.04 (22.40)
15.94
1.25
14.69
0.922
(b)
41.02
34.00(56.10)
12.31
1.63
10.68
0.867
(c)
42.42
45.24 (57.25)
12.73
2.25
10.48
0.823
(d)
51.74
19.36(26.91)
15.52
1.17
14.35
0.924
The values of the relative densities confirm what is visually apparent: cokes (a) and (d) are more dense than are cokes (b) and (c). It should be noted that these calculations are relative since the intensity has not been externally calibrated. Work is in progress to calibrate the cyclohexane signal intensity in the zeolite HSY surrounding the coke beans. The low-density coke images (b) and (c) warrant further comment. As in the smaller coke bean image in Figure 2, these show, in general, a random distribution of pore volume across the slice area. Both images show a thin band of higher coke density at the perimeter of the coke which is consistent with that observed for the coke in Figure 2. Coke (c) shows a faint band of higher coke density originating near the arrow at the right, sweeping first left and up across three-quarters of the coke width, then down and terminating at the arrow at the bottom. This c-shaped band indicates that this coke bean was initially smaller (with a high-density coke shell), but expanded radially with a low density agglomeration of -150 jim fluid coke particles before being finally enclosed with a second, terminal shell of higher-density coke. 3. CONCLUSIONS In conclusion, the structure of petroleum coke can be successfully investigated using magnetic resonance microimaging techniques, and holds the potential for successfully characterization of other industrial materials. In this work we have developed preliminary strategies for optimizing conditions for data acquisition, as well as directions for image processing and analysis. The advantages of MRM over other techniques such as SEM are that both chemical and dynamic images of the probe molecule within the pore space can be investigated without sample destruction. However, the resolution of MRM is lower than SEM, and furthermore, SEM looks at the coke framework rather than the pore system itself Thus MRM and SEM can be described as complementary structural techniques, and together, provide a structural description more complex than what is singly provided by either technique. The MRM images show that the pore system within the coke is interconnected, and that generally, the pores are distributed randomly throughout the coke interior. The exception to this statement is the evidence for secondary growth as illustrated in Figure 4(c). The
516 mechanism of formation is consistent with agglomeration of the smaller, 150-mm fluid coke particles followed by deposition of a high density layer which seals and terminates the coke growth. Measurements such as r2-relaxation and diffusion indicate how the probe molecule is influenced by the coke framework. The images with different weightings show a high spatial correlation. Finally, a significant limitation to the MRM technique is the lack of spatial resolution. Since a 3-dimensional image is collected, a two-fold improvement in resolution at the same signal-to-noise ratio requires an increase in time of (2^/ = 64. Currently we are investigating the use of other hydrocarbon probe molecules which possess long 72-relaxation time constants which allow the collection of multiple-echo signals to improve resolution without the time penalty. REFERENCES 1. H. Onder and E. A. Bagdoyan, Everything You Always Wanted to Know About Petroleum Coke, Svedala Industries Kennedy Van Saun, Danville, PA, 1997. 2. (a) J. M. Jimenez Mateos, E. Romero and C. Gomez de Salazar, Carbon, 31 (1993) 1159. (b) M. Pruski, B. C. Gerstein and D. Michel, Carbon, 32 (1994) 41. (c) F. Fortin and J. Rouzaud, 73 (1994) 795. (d) A. R. Pradhan, J. F. Wu, S. J. Jong, T. C. Tsai and S. B. Liu, Appl. Catal. A 165 (1997) 489. 3. E. B. Brouwer, I. Moudrakovski, K. H. Chung, G. Pleizier, J. A. Ripmeester and Y. Deslandes, Energy & Fuels, 13 (1999) 1109. 4 All image processing was carried out with the public domain Windows version of Image/J version 1.06a, developed by W. Rasband at the National Institutes for Health and obtainable from http://rsb.info.nih.gov/ij/.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
517
Effect of pore size on the adsorption of xenon on mesoporous MCM-41 and on the ^^^Xe NMR chemical shifts: a variable temperature study Wen-Hua Chen/ Hong-Ping Lin,' Jin-Fu Wu/ Sung-Jeng Jong,'-^ Chung-Yuan Mou^ and Shang-Bin Liu*-* ''Institute of Atomic and Molecular Sciences, Academia Sinica, P. O. Box 23-166, Taipei, Taiwan 106, R.O.C. ^Department of Chemistry, National Taiwan University, Taipei, Taiwan 106, R.O.C. A comprehensive study of the effect of pore size on the adsorption of xenon on mesoporous MCM-41 molecular sieves and *^^Xe NMR chemical shifts has been made. '^^Xe NMR spectra of MCM-41 samples (Si/AI = 37; pore size 1.8-3.0 nm) with varied xenon loading were obtained at different temperatures (140-340 K). The observed ^^^Xe NMR chemical shifts were fitted by regressional nonlinear least-squares fitting based on a two-site exchange model. As a result, the temperature variation of '^^Xe chemical shifts at zero xenon loading, i.e. 8Xp = 0) which arise mainly from xenon-wall interactions, were obtained. The pore size (d) and 5^ can be correlated by an empirical relations: 5s(T, d) = A(T)/(^ + B(T)). The two parameters, A(T) and B(T), are found to have nearly the same temperature dependence. At low temperature (T < 190 K), the two parameters both increase abruptly with decreasing temperature. Whereas at high temperature (T > 250 K), they were found to slowly decrease with increasing temperature.
1. INTRODUCTION The dynamics of the molecules adsorbed in confined geometry is one of the most common and important research subject which has received much attention in the past few decades. Owing to the large polarizability and the chemical inert nature of the monoatomic xenon, the '^^Xe NMR chemical shift is very sensitive to its environment and thus provides an ideal probe for the investigation of the structure of porous materials [1]. There have been numerous publications in this area since the pioneering works by Ito and Fraissard [2] and by Ripmeester [3], and several reviews have been attributed to related subjects [4-7]. Recent developments of the mesoporous MCM-41 materials [8] have also drawn intense attention due
^Present address: Union Chemical Lab, Industrial Technology Research Institute, Hsinchu, Taiwan 300, R.O.C. 'Corresponding author.
518 to their potential applications as catalysts or catalyst supports. The structure of MCM-41 consists of a hexagonal array of one-dimensional channels of uniform mesopores with pore diameter in the range 1.5-10 nm, depending on the nature of the template and synthesis conditions [9]. The straight, unconnected channels of controllable pore size make MCM-41 an ideal model adsorbent for both theoretical and experimental investigation of critical phenomena of molecules in a confined space [10-12]. '^^Xe NMR provides an excellent probe to investigate the morphology and topology of MCM-41. In particular, at the limit of low coverage, the interactions between a Xe atom (adsorbate) and the MCM-41 adsorbent deserve further investigation, in part, because of the amorphous nature of the wall structure which are still not well known at the present [8, 9]. It is the purpose of the present paper to study the effect of pore size on the adsorption of xenon on mesoporous MCM-41 molecular sieves. In particular, much attention will be focused on the temperature variation of '^^Xe NMR chemical shifts at low Xe loading to realize the characteristics of the Xe-wall interactions. 2. EXPERIMENTAL Powdered, particulate MCM-41 molecular sieves (Si/Al = 37) with varied pore diameters (1.80, 2.18, 2.54 and 3.04 nm) were synthesized following the conventional procedure using sodium silicate, sodium aluminate and C„TMAB (n = 12, 14, 16 and 18) as the source materials for Si, Al and quaternary ammonium surfactants, respectively [13]. Each sample was subjected to calcination in air at 560 °C for 6 h to remove the organic templates. The structure of the synthesized material was confirmed by powder X-ray diffraction (XRD) and by scanning/transmission electron microscopy. Their average pore sizes were deduced from the adsorption curve of the Nj adsorption-desorption isotherm obtained at 77 K by means of the BJH method (Table 1). Calcined sample (ca. 0.1 g) was first introduced into a designed sample cell then subjected to dehydration by gradual heating (1 °C min"') from room temperature (25 °C) to 350 °C for at least 40 h under vacuum. After sample dehydration, a known amount of Xe (70% ^^^Xe isotope-enriched) was introduced into the sample and then the sample cell was sealed off with a mini-torch while Xe was trapped at the bottom of the sample cell by liquid Nj. Special cares were taken in minimizing the dead volume of each sample cell during glass sealing. For practical purposes, the amount of Xe adsorbed, p, is expressed as the number of Xe atoms per
Table 1 Characteristics of the MCM-41 samples Pore Diameter AE3^(exp) (nm) ^2Z (kcal/mol)' 1.80 2.07 37 2.18 2.09 37 2.54 37 2.10 3.04 37 2.05 ^Obtained from the experimental results shown in Fig. 3. ^'Results obtained from data fittings. ''Obtained from data fitting by Eq. 4.
AE^^sCfit) (kcal/mol)' 2.71 2.87 2.57 2.78
6oa (ppm)^
107 105 105 107
519
|iiii|iiii|iiii|imiiiii|iiii|
200
100
0
tiiii|iiii|iiii|iiiniiii|i
200
CHcrtnical
100
shift
0
CpprnJ
Figure 1. Temperature variations of '^^Xe NMR spectrum for an MCM-41 sample (pore diameter 3.04 nm) at assorted Xe loading, p, of (a) 12, (b) 119, and (c) 243 amagat. effective free volume of anhydrous MCM-41 at room temperature. As a result, samples with varied Xe loading ranging from 2-245 amagat (i.e. the gas density at 0 °C and 1 atm) were prepared. One should bear in mind that the Xe loading so determined represents the apparent Xe density (Xe in the overall sample region). The true or ejfective density (Xe adsorbed within the pore of MCM-41) only when at low enough temperature to which the amount of gaseous Xe (outside of the pore) is negligible. The effective Xe density for each sample was calibrated by xenon adsorption isotherm done at designated temperatures (see below). '^^Xe NMR measurements were performed at 138.326 MHz on a Bruker MSL-500P NMR spectrometer. The free-induction-decay signals were accumulated typically with a relaxation delay of 0.3 s. All '^^Xe chemical shifts were referred to gaseous Xe at zero density [15]. The experimental temperatures are believed accurate to ± 2 °C. Detailed sample preparation and experimental procedures have been described in an earlier report [10]. 3. RESULTS AND DISCUSSION 3.1. *"Xe NMR chemical shifts: Xe-wall interactions (SJ Typical '^^Xe NMR spectra for Xe adsorbed on MCM-41 are depicted in Fig. 1 for assorted Xe loading and temperatures. A decrease in the NMR linewidths with temperature were observed with decreasing temperature except for the high Xe loading samples at T < 190 K (Fig. Ic) which can be ascribed due to condensation of Xe within the pores of MCM-41 (yide infra). The additional peak at ca. 245 ppm in Fig. Ic resembles the chemical shift of bulk liquid Xe. Typical variations of '^^Xe chemical shift with apparent Xe loading are depicted in Fig. 2 for Xe adsorbed on MCM-41 with pore diameter of 2.54 nm. Similar results were found for the other MCM-41 samples with different pore sizes. To obtain the correlation between the '^^Xe chemical shift with ejfective Xe loading, the observed chemical shifts must be calibrated by the curves shown in Fig. 3, which were deduced from the xenon adsorption isotherms at selected temperatures. The results after such calibration are presented in Fig. 4. It
520 is noted that, regardless of the pore size, the curves in Fig. 3 plateau at T < 190 K indicating a complete adsorption of Xe within the pores of the MCM-41. It is noted that, at low loading, the chemical shift show a parabolic-like curvature (Figs. 2 and 4). Such behavior was not found in the silica-form (Si/Al = oo) MCM-41 sample [10] and hence may be ascribed as due to the presence of Al in the wall of MCM-41. Such paraboliclike chemical shift behavior has also been observed in amorphous alumina and silica-alumina at low Xe loading, to which Cheung et al. [16] ascribed as due to the presence of a broad distribution of pore sizes. MCM-41 is known, thus far, to possess partially crystalline structure analogous to amorphous silica [8]. Results obtained from our SEM/TEM and N2 adsorption-desorption isotherm indeed indicated that, unlike the silica-form MCM-41, the Alcontaining MCM-41 synthesized via the delay neutralization method [13] tends to possess an additional broad distribution of large defect cavities typically in the order of 10-20 nm. It is noted that, the existence of such pore-intersecting cavities favors the transport of the adsorbates within the hexagonal array of one-dimensional pores of MCM-41 and hence should be beneficial for use of the material as catalyst or catalyst support. Ito and Fraissard [2] expressed the room temperature '^^Xe NMR chemical shift of Xe adsorbed in porous adsorbent as 5 = 6^ + 5s(p = 0) + a,p, where 5^= 0 is the chemical shift reference, d^p = 0) represents the interaction between a Xe atom and the wall of the adsorbent. The last term represents the contribution arising from binary Xe-Xe interactions which, at moderate loading, is linear with p. In the present study, the observed '^^Xe NMR chemical shifts can be expressed as the ftinction of Xe loading and temperature as: 5(p, T) = 5,(T) -f a,(T)p + a,(T)p' + ....
(1)
It has been demonstrated that the temperature and density dependence of the '^^Xe chemical shift can be expressed as weighted average between two sites in rapid exchange [10]. That is, 5(p, T) = P3(T)5,(p3, T) + P,(T)5,(pg, T),
(2)
where P^and Pg are the probabilities of finding the adsorbed and gaseous xenon, respectively; Pa and Pg are the density of the adsorbed and non-adsorbed Xe; p = pa+ pg- The probability of finding a Xe atom at the wall is given by [17]: P^ = pjp =TJ(T^ + Tg); Pg = Pg/p = 1- Pa, where Ta is the average time xenon spent on the wall, and r^ us the reciprocal of the xenon collision rate with the surface. The average xenon sticking time on the wall is given by r^ = r^ exp(AEads//?T), where T^ is the preexponential factor and AE^d^ is the energy of adsorption (Table 1), and R is the gas constant. For practical purposes, we express the adsorbed and gaseous chemical shift contributions as second-order polynomials: 5a(Pa, T ) « 8^ + a,.(T)p. + 250 K), the two variables were found to slowly decrease with increasing temperature. For a more quantitative analysis of the data, we express the two parameters by simple polynomial functions: A(T) = A, + A,T + AjT' + AjT^ + ...; B(T) = B, + B,T + BjT^ + B3T' + ...
(6)
523 140
-T
'
1
'
1
*
MCM-41 120 100
?
a 80
53(T.d) = A(T)/(d+B(T))
1
6.0x10^
1
0:1:5:::! i ^ • O - O - O 200 ^ • A - A - -A 220 A-A. . . ^
60
2.000
\
T(K)
1
1 X) r -
r-
20
\ \
1 1.500
15
,o3
2.0x10'
40
\
500 200
250
o 300
1
1
i
-O-A(T)
\
-X- B{T)
^^*^^
400 00
5
J
1
1 00
\ < 1.000
4.0x10^ 1
J600
350
H
1 200
Zeolite 5. = 49.9/(D,han„el-0.2346)
0.0
20 1 2 3 Pore Size (nm)
Figure 6. Variations of 5^ and pore size obtained from various MCM-41 samples at different temperatures.
300
200
T{K) Figure 7. Temperature variations of the two constant variables A(T) and B(T) defined in Eqs. 5 and 6 (see text).
The results obtained from the regressional polynomial fittings are listed in Table 2. Such quantitative data should be helpful in estimating the value for 5^ at any temperature if the pore size of the MCM-41 is known, or vise versa. Table 2 Temperature coefficients of A(T) and B(T) which correlate Xe-wall contribution of '^^Xe NMR chemical shift (5,) and the pore size of MCM-41 molecular sieves (Eg. 5)^ B B B. IL. 20467.67 0.00162 -119.80 0.1788 189.82 -1.0904 ^Obtained by regressional polynomialfittings(Eq. 6) to the second order. 4. CONCLUSIONS We demonstrate that the physical properties of Xe adsorbed in mesoporous MCM-41 molecular sieves can be deduced from the analysis of the variable temperature '^^Xe NMR chemical shift data. For example, the interactions between the adsorbed Xe and the wall of the adsorbent, b^. Our results indicate that the interactions arise from Xe adsorbed in mesoporous MCM-41 deviates significantly from not only the bulk Xe, but also from Xe adsorbed on microporous adsorbents or polymer surfaces. At a given temperature T, the pore size dependence of 6^ can be described by the empirical relation: 65(T, d) = A(T)/(d + B(T)). The two temperature-dependence parameters were expressed by polynomial functions whose temperature coefficients were also revealed explicitly to the second order.
524
ACKNOWLEDGMENTS The authors thank Profs. Soofin Cheng and Ben-Zu Wan for helpful discussions. This research has been partially supported by a grant from the Chinese Petroleum Corporation (87S-032) and by the Nation Science Council, R. O. C. (NSC88-2113-M-OO1-008 to SBL).
REFERENCES 1. J. Reisse, Nouv. J. Chim., 10 (1986) 665. 2. (a) T. Ito and J. Fraissard, in: L.V.C. Rees (ed.), Proc. 5th Inter. Zeolite Conf Heyden, London, 1980, p. 510; (b) ibid, J. Chem. Phys., 76 (1982) 5225. 3. (a) J.A. Ripmeester, ISMAR-Ampere Inter. Conf on Magn. Reson., Delft, Netherlands, 1980; (b) ibid, J. Am. Chem. Soc, 104 (1982) 289. 4. J. Fraissard, Zeolites, 8 (1988) 350. 5. C. Dybowski, N. Bansal and T.M. Duncan, Ann. Rev. Phys. Chem., 42 (1991) 433. 6. P.J. Barrie and J. Klinowski, Progr. NMR Spectrosc, 24 (1992) 91. 7. D. Raftery and B.F. Chmelka, in: P. Diel et al. (eds.), NMR Basic Principles and Progress, Vol. 30, Springer-Verlag, Berlin, Heidelberg, 1994, p. 111. 8. (a) C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710; (b) 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. 9. H.P. Lin, Y.R. Cheng, C.R. Lin, F.Y. Li, C.L. Chen, S.T. Wong, S. Cheng, S.B. Liu, B.Z. Wan, C.Y. Mou, C.Y. Tang and C.Y. Lin, J. Chin. Chem. Soc, 46 (1999) 495. 10. S.J. Jong, J.F. Wu, A.R. Pradhan, H.P. Lin, C.Y. Mou and S.B. Liu, Stud. Surf. Sci. Catal., 117(1998)543. 11. E.W. Hansen, E. Tangstad, E. Myrvold and T. Myrstad, J. Phys. Chem., 101 (1997) 10709. 12. K Morishige and K. Nobuoka, J. Chem. Phys., 107 (1997) 6965, and references therein. 13. H.P. Lin, S. Cheng and C.Y. Mou, Microporous Mater., 10 (1997) 111. 14. S.B. Liu, L.J. Ma, M.W. Lin, J.F. Wu and T.L. Chen, J. Phys. Chem., 96 (1992) 8120; J. Phys. Chem., 98 (1994) 4393. 15. C.J. Jameson, A.K. Jameson and S.M. Cohen, J. Chem. Phys., 59 (1973) 4540; J. Chem. Phys., 62 (1975) 4224. 16. T.T.P. Cheung, CM. Fu and S. Wharry, J. Phys. Chem., 92 (1988) 5170; T.T.P. Cheung, J. Phys. Chem., 93 (1989) 7549; T.T.P. Cheung and CM. Fu, J. Phys. Chem., 93 (1989) 3740. 17. D. Raftery, L. Reven, H. Long, A. Pines, P. Tang and J.A. Reimer, J. Phys. Chem., 97 (1993) 1649. 18. (a) Q.J. Chen and J. Fraissard, J. Phys. Chem., 96 (1992) 1809; (b) T.T.P. Cheung, J. Phys. Chem, 94 (1990) 376. 19. A. Labouriau, T. Pietrass, W.A. Weber, B.C. Gates and W.L. Earl, J. Phys. Chem. B, 103 (1999)4323. 20. M.W. Maddox, J.P. Oliver and K.E. Gubbins, Langmuir, 13 (1997) 1737. 21. J. Demarquay and J.P. Fraissard, Chem. Phys. Lett., 136 (1987) 314.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
525
What does TEM tell us about mesoporous silicas W. Zhou "-^ * Department of Chemistry, University of Cambridge, Cambridge CB2 lEW, United Kingdom ^ School of Chemistry, University of St. Andrews, St. Andrews KY16 9ST, United Kingdom
The role of transmission electron microscopy (TEM) in characterisation of silica-based mesoporous molecular sieves is discussed. It is demonstrated that TEM can not only serve as a supporting technique as presented in many relevant reports, but also give us a lot of valuable structural information which could not be gained from other experiments.
1. WTRODUCTION The new synthetic mesoporous silicas, first reported in 1992 [1,2], possessing regular pores with pore diameters from 1.5 nm to over 10 nm, exhibit remarkable features including welldefined pore size, high thermal and hydrolytic stabilities, high degree of pore ordering, etc. There are real prospects of using these materials in the chemical and petroleum industries, pharmaceutical industry and manufacture of nanoelectronic devices. In military use, they might be developed to new materials for storing up and/or absorbing biotoxic gases. In the last few years, synthesis and the formation mechanism of these materials, chemical substitution in the framework and introduction of metal clusters into the mesopores have been extensively investigated. However, our knowledge of the structures of these mesoporous molecular sieves from X-ray-based methods is still very limited, chiefly because of the paucity of the reflection peaks that they yield. The specific features of these mesoporous materials in comparison v^th the conventional microporous zeolites are not only of their large pore sizes, but also of their amorphous walls instead of crystalline walls in the latter. Strictly speaking, they are not "crystals" which must have short range ordering, although the term "crystals" is still commonly used for these materials because no better words available. Consequently, in a powder X-ray diffraction (XRD) pattern of MCM-41, the most extensively studied mesoporous silica with a hexagonal unit cell, only one strong peak corresponding to the (100) diffracted beam at very low angle of 2G and a few extremely weak peaks in a higher angle range are visible. In this report, the advantages of applying transmission electron microscopy (TEM) in this field are demonstrated. For example, it allows us to observe directly the mesopore systems, to detect the local structures such as surface structures, local defects and the morphologies of the particles, to image directly ordered and partially ordered metal nanoparticles loaded inside the mesopores and to identify possible new phases in a multiphasic specimen.
526 2. THE TEM METHOD TEM has its own disadvantages. Firstly, several limitations on the resolution must be considered if images are to be interpreted successfully Secondly, strong interaction of electron with the specimen resuhs in multiple scattering of electrons and the image contrast becomes very complicated. Computer image simulations are often needed to confirm proposed models and the multislice approximation [3,4] is a common method of calculating multiple scattering in systems where many diffracted beams are present. Finally, only 2D images can be obtained. On the other hand, the wavelength of an electron is very short. For example, when the accelerating voltage is 200 kV, the wavelength X = 0.00251 nm which is much shorter than the wavelength of X-rays used in conventional diffraction experiments (e.g. X of Cu Ka is 0 154 nm). Furthermore, unlike X-rays and neutrons, focusing of an electron beam can be readily achieved using electromagnetic fields: hence magnified images of objects may be produced in a similar way as with light but with much higher resolution due to the shorter wavelength. TEM is therefore indisputably the single most effective tool for detecting local structures of the mesoporous materials. In comparison with conventional microporous zeolites, the mesoporous materials have less problems in absorption of moisture, partly because the inner surface of these mesopores is usually lyophobic and partly because the pore sizes are large, allowing a fast desorption and diffusion of water molecules. The specimens are therefore less sensitive to the electron beam irradiation. In addition, since the principal d-spacings of the structures are large, relatively low magnifications can be used in recording the images without losing any detectable information Most TEM images presented in this report were obtained from a Jeol JEM-200CX electron microscope operating at 200 kV with a modified specimen stage with objective lens parameters Cs = 0.41 mm and Cc = 0.95 mm, giving an interpretable point resolution of 0 185 nm [5]. Mesoporous sample for the TEM studies was prepared by crushing the particles between two glass slides and spreading the powder on a holey carbon film supported on a Cu grid. The sample-deposited grid was then transferred into the specimen chamber. The images were recorded at magnifications of 24,000X to 49,000X. Computer simulations of the TEM images using proposed models were performed according to the multislice method, using CERIUS HRTEM programme developed by Cambridge Molecular Design Ltd.
3. DHIECT OBSERVATION OF THE MESOPORES A good example of direct observation of mesopores is TEM imaging of MCM-41 as shown in Figure 1 viewing down the pore axis and a direction perpendicular to the pores. The images along these directions have been often used to identify hexagonal MCM-41-type phases and, aided by the selected area electron diffraction (SAED) method, to confirm that there is no crystallographic ordering along the pore axis. The disks of light contrast in Figure lb correspond to the channels, while the dark network is the image of the silica wall. Therefore, the materials with different distances between pore centres can be easily distinguished by direct measurements on the images [6,7]. However, the disk size can be changed gradually and the contrast pattern can be even converted by changing
527
10 nm k ^
y.»?;';-"X .^-^"
Figure 1. (a) TEM image of MCM-41 silica viewed down the [100] direction, showing no ordering along the pore axis (horizontal). The interline distance is about 2.6 nm, corresponding to the (100) d-spacing of the hexagonal unit cell with a = 3.0 nm. The insets show a corresponding SAED pattern (bottom) and a structural model (top), (b) TEM image of MCM41 viewed dovm the pore axis, showing the hexagonal arrangement of the mesopores
the value of lens focus as being often observed experimentally and confirmed by image simulations. Consequently, the disk size in Figure lb is not necessary to be the real diameter of the pores. It is also difficult to detect the pore size, therefore the wall thickness,fi-omthroughfocus images, since there are several other facts that could raise uncertainty on the image contrast. One is that the columnar pores are often bent. Another problem is that the materials suffer fi-om severe vibration under the electron beam irradiation. The pore size of MCM-41 is usually obtained by a much more realistic adsorption-desorption method [8]. It is relatively easy to image directly the mesopores of MCM-41 because the material contains only hexagonally arrayed cylindrical pores. The cubic MCM-48 is an analogue of a lyotropic liquid crystal consisting of two interweaving non-intersecting channels. The wormlike mesopores are not parallel to any principal axis of the unit cell and the contrast patterns of the imagesfi-omMCM-48 become complicated [9]. In another hexagonal mesoporous phase, SBA-2, there are two types of pores, straight pores along the [100] and zigzag pores along the [001] directions. The former is similar to the pores in MCM-41 and can be easily imaged. The latter can only be revealed when a wide range of focus values is tried and the pores are more likely to be imaged under the over-focus condition [10].
4. OBSERVATION OF MORPHOLOGY AND LOCAL DEFECTS TEM can serve as a unique technique for the studies of mesoporous materials, revealing the morphologies of particles and the local structures in a nanometer resolution. We often obtained some extraordinary TEM images showing various morphologies of the mesoporous particles, e.g. the hexagonal morphology from MCM-41 particles [11,12], cubic morphology from MCM-48 [13], spherical morphology from SBA-2 and flat particles from STAC-1 specimens
528 [14]. These morphologies are closely related to the crystal structures of the materials, i.e. the detailed arrangements of the mesopores. Therefore, the formation of a specific morphology may start fi-om very beginning of the crystal growth. As shown in the inset of Figure 2, a very small particle of MCM-41 consisting of only about 35 channels has also a hexagonal morphology. Imaging the local structures of the crystals enables us to find defects and to understand the detailed formation mechanism. If we have a close look at the images of Figure lb and Figure 2, it is not difficult to see that the former shows a perfect hexagonal symmetry, while the hexagonal arrangement of pores m Figure 2 is distorted and the pore sizes are not uniform. A careful layer-by-layer measurement on Figure 2 indicates that the distance between pore centres close to the centre of the particle is about 5.8 nm and it increases continuously to about 6.4 nm in the near surface layer. It is believed that the particle with a gradient of pore size is in an intermediate state of channel growth and this observation allows us to propose that the formation of cylindrical surfactant/silica aggregates relies on an interaction between silicate ions and the surfactant, and must be diffusion controlled [15]. The silicate ions enter the particle fi-om the surface parallel to the c axis of the aggregates, while the molecules of the surfactant must enter along the aggregate cylinders simultaneously. Thus at short synthesis times the channel wall is thin, the density of the surfactant/silica aggregates is low and the diameter of the surfactant rods is short. As the synthesis time is increased, more silicate ions arrivefi-omthe solution, with the result that the walls of channels located near the perimeter of the particle become thicker than those near the centre. As more surfactant molecules penetrate the surfactant/silica aggregates, the organic content and consequently the diameter of the channels increase. Again, this proceeds more easily near the surface of the particle, where lattice strain is lower than that at the centre. This detailed mechanism can be used to explain many experimental observations about the pore size tuning according to the synthesis conditions and has enabled us to improve dramatically the crystal growth of MCM-41 [16]. Surface properties of mesoporous materials are sometimes important and TEM surface profile imaging is often used to investigate the surface structures of these materials. The advantages of this method are that it can be used to study the surfaces of small crystallites of ahnost any morphology, that the specimen preparation is as simple as that for the studies of bulk structures without requiring any special treatment and that, unlike scanning tunnelling
Figure 2. TEM image of MCM-41 viewed down the pore axis. The sample was prepared with reaction time of 24 hours. The distortion due to a gradient of distance between pore centres is highlighted by a white curve. The inset shows a small MCM-41 crystalfi^oma specimen prepared with reaction time of 1 hour. Two dislocated channels are indicated by arrows.
%'
10 nm
529
Figure 3. TEM surface profile images of MCM-41 specimens overheated at 165 °C for (a) 96 h and (b) 48 h. The view directions are along (a) the [100] direction and (b) the pore axis.
Figure 4. TEM image of MCM-41 "stained" with [RU6C(CO)l4(Tl'-C6H4C,oH2o06)] showing strong contrast on the surface of the particles, which indicates the clusters are bound predominantly to the outer surface of the derivatized MCM-41 particles.
microscopy, TEM provides a profile image not only of the surface layer but also of the underlying bulk structure, hence giving information relating two [17]. Figure 3 shows two TEM surface profile images from overheated MCM-41 specimens with surface collapse. It is often observed that the decomposition under the electron beam irradiation occurs in the whole particle rather than just the surface region. Therefore, it can be concluded that the mechanisms of decomposition of MCM-41 during calcination and under electron beam irradiation are different. In addition, the observation of surface collapse at the ends of channels (Figure 3a) is important because such a collapse resuhs in the seal of the channels and would stop diffusion of the molecules in the future apphcation of these materials [7]. When introducing metal clusters into the mesopores, it is essential to make sure that the metal clusters actually enter the pores instead of depositing on the outer surface of the particles. In the latter case, the coating layer of metal atoms can be easily detected by TEM as shovm in Figure 4. This information has helped us to improve our experimental method by protecting the outer surface of MCM-41 particles before introducing metal clusters into the channels [18].
530
5. DETERMINATION OF THE 2D MESOPORE SYSTEM IN SBA-2 The pore arrangement in MCM-41 could be determined by XRD due to its relatively simple structure. For other mesoporous phases with much more complicated structures, such as SBA2, determination of a complete mesopore system by XRD becomes extremely difficult. SBA-2 was first reported in 1995 [19] and was believed to consist of discrete large cages obeying the symmetry of space group P63/mmc [20,21]. However, the pore system connecting these supercages had not been determined until the TEM technique was appHed [10]. TEM images revealed that, in some particles of SBA-2, the linked supercages were stacked indeed in the "ABAB " sequence characteristic of hexagonal close-packed (hep) structure. However, many particles contained layered defects resuking in "ABC" stacking that signified the presence of polytypic intergrowths of the cubic close-packed (ccp) structure (Figure 5). The phase having this ccp structure was designated STAC-1 and discussed in the next section. A complete mesopore system was determined by analysis of through-focus TEM images. When viewed down the a axis of the hexagonal unit cell, large white dots seen in the structural image (under-focused) change into black ones in the over-focused image (Figure 5), implying a group of straight pores (type I) similar to those in MCM-41. No further details of the pore system can be seen in the under-focused images However, in the over-focused images, zigzag
Figure 5. Over-focus TEM imagefi-omSBA-2. The close packed layers are marked, indicating the ABAB stacking with a few layered defects. A pathway of the channels along the c axis is highlighted by a white zigzag line. All three types of channels are visible, i.e. straight channels along the view direction of the projected image (type I), zigzag channels along the c axis in the hep region (type II) and channels run straight in the ccp region (type III). A combination of type I and III channels forms a new phase (STAC-1) as shown in Figure 6. The insets show simulated images based on a proposed model (right) with specimen thicknesses of 9.8 nm (top) and 4.9 nm (bottom), and lens focuses of 220 nm (top) and 200 nm (bottom).
531 stretches along the c axis appear in the thicker hep regions, strongly indicating that there is a second group of channels (type II) which intersects the type I channels. A 2D mesopore system (right side of Figure 5) was therefore proposed and confirmed by examination of images on other projections and by computer image simulations. Consequently, the ID type I channels and the zigzag type 11 channels lying on the c axis uncovered by TEM indicate that symmetry of the SBA-2 structure must be lower than P63/mmc previously suggested based on XRD. This work demonstrates how powerful the TEM technique is in detecting unusual channels in the mesoporous materials.
6. IDENTIFICATION OF NEW PHASES Another characteristic of the TEM method is that individual particles are examined. Its disadvantage is that the images recorded may not be typical On the other hand, it is easy to identify new phases in a multiphasic specimen In an SBA-2 sample [10] ccp instead of hep stacking dominates in some particles, indicating a new phase, STAC-1. Using the through-focus technique as applied in the determination of
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Figure 4. Nitrogen adsorption isotherms of: left, the hexagonal MSU-4 silica; right, the hollow spheres.
3.2. Morphology Morphological information, such as the presence of hexagonal particles or of spheres, can be obtained by scanning electron microscopy (figure 3). In some specific cases, there is a close relationship between the specific nanostructure of the material and the particle morphology . For example figure 3 (left) displays particles of MSU-4 silica that exhibit a morphology with 120° edges that reflect the hexagonal local nanostructure of the porous framework.^ In TEM (figure 5, left) both the 120angle and the hexagonal ordering of the pores can be observed directly. However, other preparations of MSU (figure 3, right)^ do not allow direct interpretation of the local symmetry from SEM images. Nitrogen adsorption/desorption isotherms give the most accurate information about the size and shape of the pores that are present in the material. For the
Figure 5. TEM images of left, the hexagonal MSU-4 silica; right, the hollow spheres.
539 hexagonal material depicted on the left of figures 3-5, the nitrogen isotherm is of type IV (figure 4, left), characteristic of a narrow mesopore size distribution and a regular pore shape. However, the isotherm of the other material exhibits a huge desorption loop that can hardly be related to any classical isotherm type. It indicates a bottlenecked pore shape with a large remaining adsorbed volume. The information that can be extracted from this curve is that nitrogen is trapped in large pores with small apertures (the desorption at p/po = 0.42 may not be assigned to the pore size but to the nitrogen catastrophic desorption). This makes sense when from TEM we can see (figure 5, right) that the mesoporous material forms hollow spheres with walls made of the expected 3D worm-hole porous framework characteristic of MSU materials. During the nitrogen adsorption step, nitrogen can condense into these spheres but it will not be allowed to desorb until the pressure reaches the desorption step for the porous walls (ie below p/po = 0.42). 3.3. Loading with HPA For modified M41S-type materials, TEM is one of the characterisation methods of choice as well. The loading of heteropolyacid (HPA) onto MCM-41 initially leads to the formation of particles of heteropolyacid inside the mesoporous channel system. As with any loading of l\/ICM-41 with foreign substance (even template), a decrease in intensity of the higher order X-ray reflections relative to the zero-order reflection is observed. However, TEM shows that the hexagonally ordered structure is not destroyed by such a loading. This decrease can be caused by a decrease of the overall regularity of the structure due to non-ordered adsorption, or by a decrease of the electronic density difference between the network and the pores before and aftpr loading (void exchanged by HPA).
Figure 6. TEM images of HPA loaded onto MCM-41: a) ordered phase, viewed side-on with respect to the pores; b) disordered phase.
540
Figure 7. TEM images of the used catalyst, clearly showing the large particles of clustered HPA on the external surface of the MCM-41. The average size of the HPA particles can be determined from XRD only if the particles are large enough, whereas both large and small particles can be characterised properly using TEM. TEM yields information on the particle size distribution as well as on the location of the particles (ie inside the channel system or on the external surface). For a PW12HPA/MCM-41 catalyst containing 33 wt% HPA, the HPA is present inside the MCM-41 channel system after preparation of the catalyst. However, after use as a catalyst for the liquid-phase esterification of hexanoic acid with 1-propanol (solvent: toluene), the HPA particles have sintered into large particles on the external surface of the mesoporous material.'^ Figure 6 shows two morphologies of MCM-41 freshly loaded with HPA. Both morphologies are present in the same sample of catalyst. No particles of HPA have been found using TEM. The only difference between the pure MCM-41 and the HPAloaded MCM-41 in XRD (see figure 8) is a decrease in the relative intensity of the higher-order diffraction lines. EDX elemental analysis in the TEM does show that tungsten is present throughout the MCM-41. These observations lead to the conclusion that the HPA is present as very small particles inside the channel system of the MCM-41, too small to be determined even using TEM. After use as a catalyst the HPA has clearly sintered, as can already be seen from XRD (figure 8). TEM (figure 7) shows that the large particles of HPA are mainly located on the external surface of the MCM-41.
541
14000
Figure 8. XRD patterns, from top to bottom: * pure HPA; * starting MCM-41; * freshly loaded HPA/MCM-41; *usedHPA/MCM-41.
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40
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3.4. Partial recrystallisation in the partial recrystallisation of MCM-41 to MFI, the development of very small entities with a high density or crystallinity can be followed using TEM before MFI can be observed using X-ray diffraction. Moreover, the formation of the MFI as
Figure 9. TEM image of MCM-41 after the partial recrystallisation to MFI.
542
W
Figure 10. XRD pattern of MCM-41 after the partial recrystallisation to MFI.
separate particles or as a real intergrowth with MCM-41 can only be determined using TEM. The preferential location of the dark entities is in and on the disordered mesoporous material.^ Figure 9 shows a TEM image of MCM-41 after the partial recrystallisation treatment. The image clearly shows dark dots that were not present before the partial recrystallisation treatment. The corresponding XRD pattern showing a small signal for MFI is shown in figure 10. No separate crystals of MFI have been found using TEM.
4. CONCLUSION In the characterisation of mesoporous materials, TEM Is an Indispensable tool to complement information gathered using XRD, nitrogen adsorption and SEM.
5, ACKNOWLEDGMENTS C. Boissiere is gratefully acknowledged for providing the hexagonal MSU material, and NWO for financial support.
6. REFERENCES 1) J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am. Chem. Soc. 114 (1992) 10843. 2) C. Boissiere, E. Prouzet, unpublished results. 3) F. Cot, P.J. Kooyman, A. Larbot, E. Prouzet, in: Mesoporous Molecular Sieves 1998, eds. L. Bonneviot, F. Beland, C. Danumah, S. Giasson and S. Kaliaguine, Studies in Surface Science and Catalysis 117 (1998) 231. 4) M.J. Verhoef, P.J. Kooyman, J.A. Peters, H. van Bekkum, Microp. Mesop. Mat. 27(1999)365. 5) M.J. Verhoef, P.J. Kooyman, J.C. van der Waal, J.A. Peters, H. Van Bekkum, in preparation.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
543
SEM and TEM investigations of macroporous and toroidal mesostructured transition metal oxides D. Antonelli' and M. Trudeau^ ''Department of Chemistry and Biochemistry, University of Windsor, Windsor, Ontario, N9B-3P4, Canada* ^Emerging Research Technologies, Hydro-Quebec Research Institute, 1800 Boul. LionelBoulet, Varennes, Quebec, J3X ISl, Canada The synthesis of toroidal and continuous macroporous metal oxides with ordered mesoporosity by a ligand-assisted vesicle templating strategy involving is described. These niobium and molybdenum oxide based materials have pore sizes in the 200-800 nm range as determined by SEM studies and roughly 20 A mesopores in the same plane as the macropores as determined by TEM and XRD. In this study the forces governing the formation of toroids versus continuous macroporous structures is investigated and a unique TEM-induced topological transformation in which structures consisting of smaller toroids encapsulated by a larger toroid rearrange into an even larger single toroid is discussed. 1. INTRODUCTION Metal oxide materials with ordered pores in the meso- and macro- size regirne are predicted to be useful as catalytic supports,''^ adsorbents, chromatographic materials,'-' light weight structural materials,'^' and in optical,'"^'^' and electronic applications'^' where the pore size and overall particle morphology are crucial to performance. By judicious selection of liquid ciystal template and manipulation of solution conditions metaJ oxide-based materials can be fabricated with uniform pores in the 5-100 A size regime.''^"' Manipulation of nucleation and growth of the liquid crystal embryo and surface curvature in mesoporous materials can lead to materials with complex form and surface patterns on the micron level, and thus control over two levels of dimension - the meso (2-50 nm) and the macro (501000 nm) - can be achieved in the same material."^•'^' In 1997 Pine and Imhoff showed that macroporous transition metal oxide materials can be synthesized by an emulsion templating strategy in which uniformly dispersed oil droplets and surfactants were used to achieve a templating interaction with the inorganic precursor."*^' These materials had hexagonally packed pores from 50 nm to several microns in dimension and had evidence of randomly ordered and irregularly-sized mesopores below 50 A. More recently, macroporous silica'"^^' with highly ordered pores on both the macroporous and mesoporous scale was synthesized by templating onto submicron sized Latex spheres. A related approach was used to synthesize macroporous niobium oxide, however no evidence of regular mesoporosity in this material was provided.'-" This feature is crucial in catalytic"^process where substrate diffusion on the nanometer level must be controlled.
* This work was supported by NSERC and The University of Windsor. John Robinson and Julian Thorpe are acknowledged tor their work in electron microsconv.
544 In order to extend the control of pore size and orientation down into the mesopore regime we employed a combination of ligand-assisted templating with niobium ethoxide and amine surfactants - successful in the synthesis of stable transition metal oxide mesoporous^'"*^ and microporous''^' materials - and vesicle templating. Lin et al showed that by adding salt to a synthesis mixture of MCM-41 mesoporous silicate vesicle templating of the individual mesotubes into randomly oriented free-standing macroporous tubules could be achieved.^"' By using a related approach we recently demonstrated that free standing molybdenum oxide toroids roughly 200-800 nm in diameter with mesopores aligned perpendicular to the toroidal plane could be synthesized.''^^ This approach was also extended to synthesize a continuous macroporous niobium oxide structure with mesopores aligned perpendicular to the macropore axis.'~^' In this study, we use scanning electron microscopy and transmission electron microscopy to study the macropore structure of these materials and a unique topological transformation reaction. 2. RESULTS AND DISCUSSION When Nb(OEt)3 and dodecylamine were combined at a 0.6-1 ratio and treated with salt water (0.5-1.0 g of NaCl per gram of Nb ethoxide) a heavy white precipitate formed. The mixture was allowed to stand for 24 h before aging at 80 °C, 95 °C, and 150 °C. The solid was then collected by filtration and dried in an oven overnight. Scanning electron microscope images (SEiM) oi: the while Nb-based material revealed large uniform macroporous sheets and particles. Figure 1 shows a side-on view of a region in which the macropore structure shows a continuous array oi conjoined tubules. This contrasts to the pore structure of materials made with Latex spheres as a templating agent which consists of highly ordered spherical voids.'"^^-'' The size of the macropores ranged from 200 nm to 300 nm and were roughly uniform within a specific region. This compares to those values
Figure 1. SEM image of macroporous mesostructured Nb oxide showing (A and B) the tubular macropore structure and (C) a region oi lower order. obtained previously for macroporous metal oxides and hollow channeled macroporous mesolamellar metal phosphates with shapes and surface morphologies which mimic
545 biological forms.^'^^ This new material displayed an X-ray diffraction peak at d = 32 A, typical of mesoporous niobium oxide synthesized from dodecylamine at this metal-tosurfactant ratio. This data confirms the ordered mesostructured nature of this new macroporous material. Transmission electron microscope (TEM) images of macroporous regions of microtomed samples of this material'""*^ further supports our claim that this
M = Nb
M(0Et)5 NH
1) EtOH/H20/NaCl 2) Heat
500 nm 1) EtOH/H20/NaCl
2) Heat
M = Mo Figure 2. Synthetic scheme for macroporous transition metal oxides. Double templating is achieved by the co-existence of rod-like micelles and vesicles or microbubbles. material is indeed mesostructured and that the amine-occluded mesopores lie along the same vector as the macropores. This structural feature is also observed in samples of freestanding silica nanotubules^^"*' and mesostructured molybdenum oxide toroids.^'"^^ An
546 illustration of our synthetic strategy is outlined in the top part of Figure 2. By controlling the phase balance such that mesolamellar niobium oxide (Nb-TMS3) formation is inhibited in favor of mesoporous Nb-TMS 1 while still allowing formation of surfactant vesicles, normally associated with the layered region of the phase diagram, a double meso-macrotemplating interaction can be achieved. Treatment of the as-synthesized material with one equivalent of p-toluene sulfonic acid with respect to the amine in ethanol followed by extensive washing gave a template-free material with no trace of a C-H stretch in the infrared (IR) spectrum. The XRD pattern of this material showed a single broad peak at d = 32 A (Cu Ka) and a nitrogen adsorption isotherm typical of mesoporous materials, demonstrating that the mesoporous structure was retained after acid treatment. The B.E.T. (Brunauer, Emmett, Teller) surface area of this material ranged from 600-900 mVg and the Horvath Kawazoe (HK) pore size 23 A. This compares closely to similar values for previously synthesized Nb-TMSl.'""^' The SEM studies of this material demonstrates that the macroporous structure was not affected by acid treatment. Acid treatment of as-synthesized materials not aged for long enough periods or at high enough temperatures gave macroporous materials with no XRD pattem but high surface areas and a narrow mesopore size distribution. When the synthesis procedure described above is conducted with Mo(OEt), in place of Nb(0Et)3 a ne^w bronze material is formed.'"'^ This material has an XRD d-spacing centered at 32 A. Attempts to remove the surfactant by acid washing led to complete loss of stmcture as determined by nitrogen adsorption and XRD. Attempts to stabilize the material by aging at higher temperature in order to promote further condensation in the walls led to a new blue material with a lamellar XRD pattem (d(lOO) = 34 A) that also showed peaks in above 20 ° 20. TEM studies revealed that this material was a phase mixture of Mo (IV) oxide and a layered Mo (IV) oxide.'--' TEM studies of microtomed samples of the bronze material showed it consisted of roughly ninety percent of toroids from 200-800 nm in diameter as shown in the lower portion of Figure 2. These rings were free standing as opposed to fused, as in the case of the Nb-based materials, and were only
Figure 3. SEM image of a sample of toroidal mesostructured molybdenum oxide bronzes showing lack of any extended order in the structure.
547 50-100 nm in depth. SEM images (Figure 3) of this material showed only regions of amorphous macroporosity and no evidence of the extended macroporous materials observed in the case of the Nb materials.
Figure 4a. Toroidal mesostiuctured Mo oxide bronze before electron beam-induced transformation. Micrograph recorded at 300 KV on a Hitachi H-9000 STEM. The reason for the difference in structure from the Mo materials to the Nb materials can be rationalized on the basis of comparative rates of nucleation and growth."^^ Mo (V) is less Lewis acidic than Nb (V) and hence the rate o( hydrolysis for molybdenum alkoxides at a
Figure 4b. Toroid after electron-beam induced transformation in which the smaller toroids have broken up and become incoiporated into the larger toroid.
548
given pH value is slower than seen for the analogous niobium alkoxide. This means that in the case of the synthesis of the macroporous niobium oxide, the ratio of the rate of condensation and propagation of the structure relative to rate of nucleation, which should be the same for both systems due to the analogous solution conditions and surfactant ratios employed, is greater than that observed in the Mo system. This leads to a threedimensional network of extended tubules in the case of the Nb materials in which condensation within and between of individual mesotubes and macrotubes is at a higher level of completion than in the free-standing mesostructured Mo toroids. This slower rate of condensation relative to nucleation leads to toroidal rings in which the individual mesotubes are not veiy stable and the forces between mesotubes both within and between individual toroids are weaker than those forces holding the macroporous Nb mesostructure together. This is further reflected in the instability of the Mo oxide toroids to surfactant removal. Also consistent with the low-degree of condensation between mesotubes within the Mo oxide toroidal structure is a unique TEM-induced rearrangement of cell-like structures consisting of toroids within toroids into one larger toroid. This is shown above in Figure 4a and Figure 4b on the previous page. This process occurs at accelerating voltages greater than 150 KV and takes approximately 5-10 seconds to go to completion. Close up images of the structure after rearrangement show that the individual mesotubes have retained their integrity. This suggests that the forces holding together the individual mesotubes are stronger than those holding together the toroids, and that the cell-like structures are unstable kinetic products while the larger hollow toroids are the thermodynamically preferred stiTJcture under the conditions observed in this system. The favoring of one degree of curvature over another within a system where all other parameters are held constant has previously been attributed to surface charge on the species in question."^^ A greater degree of surface charge leads to less curvature due to Coulombic repulsion and a lesser degree leads to species with higher curvature due to the predominance of Van der Walls attractive forces between neighboring surface points. In summary, we have shown how two veiy similar Mo and Nb oxide mesostructured systems leads to formation of either continuous macroporous structures with extended tube lengths or free-standing torroids v\ith limited extension in the third dimension. We attribute this difference to relative rates o\' condensation versus nucleation in each system. A unique TEM-induced reaction involving evolution of shape was also discussed and demonstrates that many of the new mesostructures with order on the macroscopic level may also show dynamic behavior.
REFERENCES [1] [2] [3] [4] [5] [6] [7] [8] [9]
M. P. Harold et al. Catalysis with inorsanic membranes. MRS Bull. 1994, 79, 3439. R. R. Bhave, Inorganic membranes synthesis, characteristics, and applications (Van Nostrand Reinhold, New York. 1991) M. X. Wu, T. Fujiu, G. L. Messing, / Non-Crxst. Solids 1990, 727, 407-412. E. Yablonovitch, J. Opt. Soc. Am, ^ 1993, 10, 283-295. J. D. Joannopoulos, R. D. Meade, J. N. Winn, Photonic Crystals: Molding the Flow of Light (Princeton Univ. Press, 1995). C. Soukoulis, Photonic Band Gap Materials (Kluwer, Dordrecht, 1996). P. L. Flaugh, S. E. O'Donnel, S. A. Asher, Appl. Spectrosc. 1984, 38, 847-850. P. Singer, Semicond. Int. 1996, 79, 88-96. C. T. Ki-esge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature 1992,359,710-712.
549 [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24]
Q. Huo, D. I. Margolese, U. Ciesla, P. Feng, T. E. Gier, P. Sieger, R. Leon, P. M. Petroff, F. Schuth, G. Stucky, Nature 1994, 368. 317-321. M. E. D'dvi^, Nature 1993,364, 391-393. P. T. Tanev, M. Chibwe, T. J. Pinnavaia, Nature 1994, 368, 321-323. D. M. Antonelli, J. Y. Ying, Angew. Chem. Int. Ed. Engl. 1996, 35, 426-430. D. M. Antonelli, J. Y. Ying, Cur. Opin. Coll. & Int. Sci. 1996, 7, 523-529. T. Sun, J. Y. Ying, Nature 1997, 389, 704-706. H. Yang, N. Coombs, G. A. Ozin, Nature, 1997, 386, 692-695. S. Mann, G. A. Ozin, Nature 1996, 382, 313-318. O. Scott. A. Kuperman, N. Coombs, A. Louoh, G. A. Ozin, Nature 1995, 378, 47-50. A. Imhof, D. J. Pine, Nature 1997, 389, 948-951. B. T. Holland, C. F. Blanford, T. Do, and A. Stein, Chem. Mater. 1999, 77, 795805. Yang et al. Science, 1998, 282, 2244. H.-P. Lin, S. Cheng, and C.-Y. Mou, Chem. Mater. 1998, 10, 581-589. D. M. Antonelli and M. Trudeau, Angew. Chemie Int. Ed. 1999, 38, 1471-1475. D. M. Antonelli, Microporous and Mesoporous Materials, /// press.
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Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
551
^H, ^H and ^^Si solid state NMR study of guest acetone molecules occupying the zeolitic channels of partially dehydrated sepiolite clay M. R. Weir, G. A. Facey and C. Detellier Ottawa-Carleton Chemistry Institute, Department of Chemistry, University of Ottawa Ottawa, Ontario, Canada KIN 6N5 SepioUte clay ( ca. 5 nm, the experimental desorption branch corresponds to the equilibrium evaporation, while the experimental capillary condensation branch corresponds to the spontaneous (spinodal) condensation. Moreover, the NLDFT predictions of equilibrium and spontaneous capillary condensation transitions for pores wider than 6 nm are well approximated by the macroscopic equations of the Derjaguin-Broekhoff-de Boer theory [10,11], while the results of the traditional Cohan equations (the BJH method) are significantly in error. Two kernels of theoretical isotherms in cylindrical channels have been constructed corresponding to the adsorption and desorption branches. For a series of samples [2-4], we show that the pore size distributions calculated from the experimental desorption branches by means of the desorption kernel satisfactory coincide with those calculated from the experimental adsorption branches by means of the adsorption kernel. This provides a convincing argument in favor of using the NLDFT model for pore size characterization of nanoporous materials provided that the adsorption and desorption data are processed consistently.
2. NONLOCAL DENSITY FUNCTIONAL THEORY OF ADSORPTION HYSTERESIS In the density functional theory, the structure and thermodynamics of confined fluids are predicted from the intermolecular potentials of the fluid-fluid and solid-fluid interactions. To
599 model nitrogen adsorption, we employ a version of the NLDFT based on the Tarazona's smoothed density approximation [19]. A detailed description of the theory has been given in our previous publications (see e.g. [23] and references therein). Predictions of the density functional theory depend largely on the correct choice of the parameters of intermolecular interactions. Parameters of the Lennard-Jones potential describing the fluid-fluid interactions have been optimized to provide an accurate description of the two-phase equilibrium in bulk nitrogen, including the surface tension of the liquid-gas interface [23]. Interactions between solid and fluid are approximated using the potential in an infinite cylindrical pore [25]. Parameters of solid-fluid interactions have been chosen to provide the best possible fit to the standard nitrogen isotherm on nonporous oxides [26]. A comparison of the calculated excess adsorption isotherm in a large cylindrical pore of 107 nm with the standard nitrogen isotherm is presented in Figure la. The steps on the calculated isotherm are caused by the structureless pore wall model used. On average however, the calculated adsorption isotherm agrees well with the experimental data as it is seenfi*omthe corresponding t-plot (Figure lb). The adsorption and desorption isotherms have been calculated for the N2 sorption at 77K in cylindrical pores in the range 2-100 nm. The points of equilibrium and spinodal transitions are plotted in Fig. 2 in comparison with the adsorption and desorption points calculated according to standard Cohan's equations using the same nitrogen standard isotherm [26]. There are several features worth noting. The line of equilibrium capillary condensation asymptotically approaches the Kelvin equation for the spherical meniscus and the line of spontaneous capillary condensation asymptotically approaches the Kelvin equation for the cylindrical meniscus. This asymptotic behavior is in agreement with the classical scenario of capillary hysteresis [12]: capillary condensation occurs spontaneously after the formation of the cylindrical adsorption film on the pore walls while evaporation occurs after the formation of the equilibrium meniscus at the pore end. As the pore size decreases, the surface forces come forefi"ont and deviations fi'om the classical picture become significant even for pores as large as 10-20 nm. Much better agreement has been found v^th the results of the Deijaguin-Broekhoff'-de Boer theory [10,11] (Figure 3, top). For pores wider than ca. 6 nm the equilibrium and spontaneous capillary condensation transitions predicted by the NLDFT are well approximated by the semiempirical equations of Broekhoff'and de Boer [10,11]. In smaller pores, the deviations are substantial (Figure 3, bottom). The NLDFT predicts the critical point for capillary condensation phase transition (capillary critical pore size) at ca. 2 nm, which is approximately the minimum pore size in which capillary condensation is experimentally observed [21,27]. However, the theory fails to predict the disappearance of the hysteresis loop for pores smaller than ca. 4 nm (hysteresis critical point) [20,15]. It should be noted that the theory of Broekhoff" and de Boer fails to predict both critical points unless some additional semi-empirical corrections are made [16]. Recent Monte Carlo simulations of N2 in cylindrical pores fijUy support the results of the NLDFT calculations [28]. Thus, it appears that the failure of the NLDFT to predict the disappearance of the hysteresis loop at relative pressures below ca. 0.4 and pores smaller than ca. 4 nm is of a fiindamental nature and cannot be explained by approximations made in the theory.
600 0.08 •^ 0.07 o 0.06 F E c" 0.05
D NLDFT isotherm in a 107 nm cylindrical pore — Standard nitrogen isotherm
0
Q. O (A (0
004 0 03 0.02
o X
UJ
0.01
0.5 1 Standard t-curve, nm
O^nm 0 6 P/Po
1.5
Figure 1. (a) Comparison of the NLDFT isotherm in a 107 nm diameter cylindrical pore with the standard nitrogen isotherm on nonporous oxides [26]. (b) corresponding statistical film thickness plot.
• NLDFT equilibrium transition n NLDFT spinoda) condensation — BJH (desorption)
I—BJH I (adsorption)
i 10 Pore size, nm
100
Figure 2. Capillary hysteresis of nitrogen in cylindrical pores at 77 K. Equilibrium desorption (black squares) and spinodal condensation (open squares) pressures predicted by the NLDFT in comparison with the resuhs of Cohan's equation (the BJH method) for spherical (crosses and line) and cylindrical (line) meniscus.
601
• NLDFT equilibrium transition a NLDFT spinodal condensation -»«-Broekhoff -de| Boer (desorption) — Broekhoff -del Boer (adsorption) 10 Pore size, nm
100
• NLDFT equilibrium transition D NLDFT spinodal condensation -*«-Broekhoff -de| Boer (desorption) ! — Broekhoff -del Boer (adsorption) 3 4 5 Pore size, nm Figure 3. Capillary hysteresis of nitrogen in cylindrical pores at 77 K. Equilibrium desorption (black squares) and spontaneous condensation (open squares) pressures predicted by the NLDFT in comparison with the results of the Broekhoff and de Boer theory [10, 11].
602 3. COMPARISON WITH EXPERIMENTS AND CALCULATION OF PORE SIZE DISTRIBUTIONS From our earlier studies, we have made the following general conclusions regarding capillary condensation in cylindrical pores [20, 22, 23]. Reversible isotherms in sufficiently narrow pores and desorption branches of the hysteretic isotherms in wider pores correspond to the equilibrium transitions predicted by the NLDFT. The adsorption branches of hysteretic isotherms lie inside the theoretical hysteresis loop. These conclusions were made based on analyses of limited experimental data on reference MCM-41 materials with pores < 5nm. Sayari et al. [2-4] have recently synthesized enlarged MCM-41-type samples with pore diameters from 5 to 10 nm. The N2 isotherms on two of these samples are presented in Figs. 45 in comparison with the theoretical hysteresis loops for cylindrical pores of an average size, formed by the metastable adsorption branch and the equilibrium desorption branch. The experimental and theoretical hysteresis loops are in good qualitative agreement. To calculate the pore size distributions we have constructed two kernels of theoretical isotherms in cylindrical channels corresponding to the metastable adsorption and equilibrium desorption branches. These kernels were employed for calculating pore size distributions from experimental isotherms following the deconvolution procedure described elsewhere [21, 24]. In Figs. 6-7 we present the pore size distributions of the enlarged MCM-41 samples [2-4] calculated from the experimental desorption branches by means of the desorption kernel and the pore size distributions calculated from the experimental adsorption branches by means of the adsorption kernel. The pore size distributions obtained from the desorption and adsorption branches practically coincide, which confirms that the NLDFT quantitatively describes both branches on the adsorption-desorption isotherm. Structural parameters of the MCM-41 materials calculated by means of the NLDFT method are listed in Table 1. We note very good agreement between the results obtained from the desorption and adsorption branches of the isotherms, especially for samples #1 - #3. It is worth noting that the pore wall thickness (1.2-1.8 nm) of wide-pore MCM-41 materials is larger than that usually obtained for conventional MCM-41, and tends to increase with the pore diameter.
4. CONCLUSIONS The non-local densityfiinctionaltheory (NLDFT) with properly chosen parameters of fluidfluid and fluid-solid intermolecular interactions quantitatively predicts both adsorption and desorption branches of capillary condensation isotherms on MCM-41 materials with pore sizes from 5 to 10 nm. When pore-blocking (networking) effects are insignificant, the pore size distributions calculated from the adsorption and desorption branches of the experimental isotherm are in good agreement. For materials with a wide hysteresis loop of lUPAC's type H2 [14], which is usually attributed to pore blocking [29,30], the use of the adsorption branch may yield more reliable results, provided the kernel of metastable adsorption isotherms is employed. For samples with smaller pores (< 5 nm), the equilibrium desorption branch has the advantage of being theoretically more accurate. In this case we recommend using desorption isotherms for estimating pore size distributions in mesoporous materials of the MCM-41 type.
603
0.04 i i i j I B H i h 1 • 1fcI • • • • '1
• • ^
6 i
c 0 •5
':'
0.03 -
^
i E
1
0.02 -
M
inside the pores by KN,<X> = E,I3 (3) where No is the Avogadro's number and A^ is a constant. Here this approach can be used for cylindrical micropores using the Saito-Foley potential. The mean potential, O, for a cylindrical micropore system is given by [7] 4
dl
(4)
^ k+\
where NA and A^£ are number of oxygen atoms in the surface of zeohte and molecules/atoms in adsorbent respectively, do is the diameter of the adsorbent molecule and a* and fik are given by r(-4.5) (5) ' r{-4.5-k)r{k + \) „,/,__ r(-i.5) (6) r ( - i . 5 - ^ ) r ( ^ + i) Using Eqs. (3)-(6), the values of E {=PEo) are obtained in terms of the pore diameter as described by Chen and Yang [17]. 2.2 Mesopores In a recent article [5] we have proposed a new hybrid isotherm for adsorption in nonporous materials. The isotherm combines our recent molecular-continuum model for higher pressures, with other widely used models such as the Unilan model for the lowpressure region \-\-bPe' Q(P) = - ^ l n \ + bPe~ Is where Cm is the monolayer capacity and the constants are given by
-r^)J
C^b («• 2s _(l + ^.Fe~-^i\+bpe'
=
Pd$ /dt_
(7)
(8)
2sC„
e
p{e' -ke-' )
(9)
611 The hybrid isotherm uses our recent molecular-continuum model for higher pressures [18,19]. The model is extended here for adsorption in mesoporous materials. The equilibrium thickness t of the adsorbed layer at pressure P is given by ^ y vUr-t) c/>(t,r)^\v^dP = ^-^-^ ^ (10) Po (r-t-A/2/ where r is radius of the pore and (/)(t,r) is the position dependent incremental local potential due to the solid. The integral (second term) was obtained using the B WR equation of state. The fluid-solid interaction potential parameters were obtained by fitting the condensation pressures satisfying the stability boundary d^^_r^vJ(r-t + A/2) dt (r-t-X/lf
^^^^
to the MCM-41 data. The pressure at which the capillary evaporation occurs is given by ^^ ly vUr-t?(i>(r,r)^ \vdP-. 1^^ ^ = =0 (12) Po ' [(r-t)(r-t-A) + Acj,,/4] Consistent with Equation 1, with different adsorption regimes as shown in Figure 1, the integral can be split as r^
rp(p)
C„{P)= lp„X':P)f{>:P)dr+
r^^^^
\pXr.P)f{r.P)dr+
\ pXr.P)f{r.P)ir
(13)
permitting different forms of p{r, P) in each integral. Here p^ (r, P) represents the effective density for pores in which capillary condensation occurs, and p^ {r, P) that for pores in which multilayer surface coverage occurs. In the present case for the multilayer region
pc{'-'P)=r(
^—FTT7
\
2, n^^
^^^^
and for low pressure (15) with the effective density in the micropores given by pSr^P)=Pi expl- [RT\n{PjP)lPEXr)Y\
(16)
lUPAC defines the lower limit of mesopores as 2 nm [ 1 ] which was considered as the limit below which the adsorption will occur by volume filling. However, in our recent article, based on the tensile stress hypothesis, we have shown that this limit is different than lUPAC limit. Using the mechanical stability criterion for the cylindrical meniscus (during adsorption), the critical size is obtained from
P +r >
^
n?)
612 along with the thermodynamic stability condition (18)
{r-t-A/iy 3. RESULTS AND DISCUSSION
The above model was applied to our nitrogen isotherm data for different MCM-41 samples to extract pore size distributions (PSD's). Inversion of the adsorption integral was accomplished using the regularization package of Bhatia [10]. Figure 2 presents the PSD results for pure MCM-41 samples, along with the XRD estimates of the pore size. The details of the calculation of pore size from the XRD and gas adsorption have been given elsewhere [14,20]. Figure 3 depicts the results for a 1:1 (w/w) C12/C18 MCM-41 mixture.
30
40
50
60
pore diameter (A)
Figure 2. Comparison of pore size distribution of MCM-41 samples by regularization with the XRD pore diameter In the PSD calculations, LJ parameters of the silica-nitrogen (afs = 3.586 A and €fs/k = 66.6 K) and nitrogen-nitrogen ((afr=3.681 A, Gff/k=91.5 K) interactions were taken from our previous article [20]. The fluid was represented by the BWR equation of state. The properties of the adsorbate fluid (nitrogen) were taken as the bulk saturated liquid properties. For each isotherm, the values of the small regularization parameter were varied and the standard deviation, 5, was estimated. The final value was chosen in such a way that the standard deviation was constant and a stable non-negative PSD was obtained as described earher [10]. In the present model, the lower mesopore limit was taken as 2.4 nm which was estimated from our model describing stability of the adsorbate meniscus in mesopores. The D-R isotherm was applied for all the pores below 2.4 nm. Although the present calculations
613 show that the sample do not contain micropores, the applicability of the D-R equation up to 2.4 nm pore size, needs to be studied. Further, artifacts may arise at the transition between the different isotherms for the micropores and mesopores, but for the predominantly mesoporous MCM-41 this effect is not of significance. 0.15
30
40
30
50
40
50
pore diameter (A)
pore diameter (A)
0.10
E cv 0.05
0.00 30
40
pore diameter (A)
Figure 3. Comparison of pore size distribution of mixture of MCM-41 materials by regularization with the XRD pore diameter, as well as pore size distribution estimated from pure components: (a) C12+C18, (b)C12+C16,(c)C10+C14 The adsorption branch of the isotherm was used for the present calculations. It can be observed from Figure 2 that the predicted PSD is very close to the pore size from XRD except
614 for C8. This could be due to the random and disordered nature of the C8 sample, which was confirmed by HRTEM and fractal analysis in our previous work [20]. Consequently the XRD pore size of this material may not be meaningful. Mixtures were also prepared by physical mixing of different samples [12] and, as expected, showed peaks close to those of the pure components (c.f Figures 3 (a), (b) and (c)). This confirms that the present model provides a unique way of presenting the real pore structure of the system. The pore size distribution obtained by averaging the individual pure component PSD's of C12 and C18 is also shown in Figure 3 (a). Although the height of the peak for CI2 sample is higher by this averaging method, it should be noted that the area under the curve (pore volume) is the same. Similar results were obtained for the C12+C16 and C10+C14 mixtures, as shown in Figures 3 (b) and (c), with the result for the mixture PSD being very close to that of the average as well as the XRD method. Comparison of the pore size distribution determined by the present method with that from the classical methods such as the BJH, the Broekhoff-de Boer and the Saito-Foley methods is shown in Figure 4. Figure 5 shows a close resemblance of the results of our method with those from the recent NLDFT of Niemark et al. [16], and XRD pore diameter for their sample AMI. The results clearly indicate the utility of our method and accuracy comparable to the much more computationally demanding density functional theory. There are several other methods published recently (e. g. [21]), however space limitations do not permit comparison with these results here. It is hoped to discuss these in a future publication. 0.4
u.^o
. XRD
BJH fi 0.3 ]\
—
1 1 1
1 BdB 1 H 1 1 l|
0.20
p
Present theory
— NLDFT results
\ \
0.15 1 1 Ml
Q
5 0.2
Q
>
\\ present work
0.10
l\
1\ /\'^
0.1
'/ / // // // //
0.05
1V \ 0.0 1
^
20
, ,^A/.( 30
, ^
I' A \
40
, 1•
"
50
-^
60
PORE DIAMETER (A)
Figure 4. Comparison of pore size distribution of C18-MCM-41 sample determined by regularization with that from classical theories and the XRD pore diameter
0.00
yj 30
^^^r——.____ 40
50
PORE DIAMETER (A)
Figure 5. Comparison of pore size distribution of sample AMI of Niemark et al. [16] determined by current method with that from regularization with NLDFT, and the XRD pore diameter.
615 4. CONCLUSIONS A new model for determining the pore size distribution of micro and mesoporous materials from gas adsorption isotherm has been successfully proposed and tested. The present model was found to be successful in predicting the pore size distribution of pure as well as binary physical mixtures of MCM-41. 5. ACKNOWLEDGEMENTS The authors wish to acknowledge Mr. Russell Williams for his help with the computations. REFERENCES 1. Gregg, S.J. and Sing, K. S., Adsorption, Surface Area and Porosity, Academic Press, New York (1982). 2. Kaneko, K., J. Membrane Sci., 96 (1994) 59. 3. Sonwane, C. G. and Bhatia, S. K., Langmuir, 15 (1999) 2809. 4. Rouquerol, J., Avnir, D., Fairbridge, C. W., Evertt, D. H., Haynes, J. H., Pemicone, N., Ramsay, J. D., Sing, K. S. W. and Unger, K. K., Pure Appl. Chem., 66 (1994) 1739. 5. Sonwane, C. G. and Bhatia, S. K., submitted (1999). 6. Horvath, G. and Kawazoe, K., J. Chem. Eng. Jpn., 16 (1983) 470. 7. Saito, A. and Foley, H. C , AiChE J., 37 (1991) 429. 8. Do, D. D., Adsorption Analysis: Equilibria and Kinetics, Imperial College Press, London, (1998). 9. Bhatia, S. K., Chem. Eng. Sci., 53 (1998) 3239. 10. Inoue, S., Hanzawa, Y. and Kaneko, K., Langmuir, 14 (1998) 3079. 11. Morishige, K. and Shikimi, M., J. Chem. Phys., 108 (1998) 7821. 12. Sonwane, C. G. and Bhatia, S. K., Langmuir, 15 (1999) 5347. 13. Maddox, M. W., Olivier, J. P., Gubbins, K. E., Langmuir, 13 (1997) 1737. 14. Kruk, M., Jaroniec, M. and Sayari, A., J. Phys. Chem. B, 101 (1997) 583. 15. Sayari, A., Yang, Y., Kruk, M. and Jaroniec, M., J. Phys. Chem. B, 103 (1999) 3651. 16. Neimark, A. V., Ravikovitch, P. I. and Unger, K. K., J. Colloid Interface Sci., 207 (1998) 159. 17. Chen, S. G. and Yang, R. T., Langmuir, 10 (1994) 4244. 18. Bhatia, S. K. and Sonwane, C. G., Langmuir, 14 (1998) 1521. 19. Sonwane, C. G. and Bhatia, S. K., Chem. Eng. Sci., 53 (1998) 3143. 20. Sonwane, C. G., Bhatia, S. K , and Calos, N., Ind. Eng. Chem. Res., 37 (1998) 2271. 21. Kruk, M., Jaroniec, M. and Sayari, A., Langmuir, 13 (1997) 6267.
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Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
617
The Sorption of «-Butyl and tert-Butyl Alcohols by Phenyl-Modified Porous Silica. Claire M. Bambrough", Robert C.T. Slade*' and Ruth T.Williams'*. ' Department of Chemistry, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK. ^Department of Chemistry, University of Surrey, Guildford, GU2 5XH, UK. The sorption of «-butyl alcohol and tert-butyl alcohol on phenyl modified MCM-41 type sorbent having pores of approximately 20 A diameter {i.e. in the microporous range), has been studied. Comparison of butanol sorption with nitrogen, water, and benzene sorption data indicates that steric hindrance significantly affects the sorption of «-butyl alcohol by the microporous silica, far more so than for tert-butyl alcohol. The different shapes of the isotherms obtained on the microporous material (Type 1 for /er/-butyl alcohol, Type IV for nbutyl alcohol) suggest that the preferred mechanism for adsorption of tert-butyl alcohol is via organic interactions with surface phenyls, whereas for «-butyl alcohol, a mechanism of polar interaction is more likely.
1. INTRODUCTION Since the synthesis of the mesoporous aluminosilicates designated M41S in 1992 , the sorption characteristics of these materials have been of great interest. The high surface area and uniform pore structure of these solids makes them ideally suited to applications as sorbents and catalysts. Early attempts to incorporate catalytically active guest species in the channels of MCM-41, however, resulted in low guest loadings, probably due to the absence of specific interactions between host and guest. This problem was alleviated by the synthesis of organically-modified porous silica by the hydrolysis and co-condensation of a siloxane and organosiloxane in the presence of a surfactant.^ Our studies of nitrogen, water vapour and benzene sorption on a phenyl-modified MCM-41 type material^ indicated that it was microporous (average pore diameter « 20 A), suggesting that the incorporation of phenyl groups resulted in a reduction in pore diameter. In this paper we report the sorption of/er/-butyl alcohol and «-butyl alcohol on phenylmodified porous silica. Butanol adsorption is of interest for two reasons. The first is that by
618 using the two isomers we can explore whether the adsorption process is shape sensitive (nbutyl alcohol can be thought of as a straight chain whereas /^r/-butyl alcohol approximates to a spherical shape). The second is given the bifiinctional nature (alkyl and hydroxyl) of butanol, we attempt here to elucidate whether adsorption occurs through hydroxyl interactions with polar surface groups or via butyl interaction with the pendant phenyl functions. Our earlier studies of water, and benzene sorption on the microporous phenyl-modified silica pointed to the material being hydrophobic with some indication of a degree of steric hindrance in the sorption of benzene. The sorption of butanol isomers (303 K) enables us to further investigate the sorption characteristics of the phenyl-modified silica.. The adsorption isotherms presented here were measured using a gravimetric technique and are compared with previously reported nitrogen, water and benzene sorption data.^
2. EXPERIMENTAL 2.1 Synthesis The microporous phenyl-modified silica was synthesised via the hydrolysis and cocondensation of phenyltriethoxysilane and tetraethoxysilane in the presence of hexadecyltrimethylammonium bromide using a previously reported procedure. 2.2 Instrumentation Adsorption of the two butanol isomers at 303 K was performed using a McBain-Bakr gravimetric balance built in-house (details reported previously^). The sample was outgassed at 373 K under vacuum for several hours to remove physisorbed vapour prior to adsorption. Isotherms are presented as plots of amount sorbed (mmol g'^) versus relative pressure, p/p . 3. RESULTS AND DISCUSSIONS The isotherms are presented in Figure 1 and the results are summarised in Table 1 together with nitrogen, water and benzene isotherm data^ to aid comparison. 3.1 n-Butyl Alcohol Sorption «-Butyl alcohol adsorption on the microporous phenyl-modified silica yielded an isotherm which appears, upon preliminary examination, to display Type IV characteristics (Figure 1). A sharp knee evident at p/p^ « 0.05 corresponds to a BET monolayer capacity, n^ = 0.55 mmol g\ much lower than those given by other adsorbates (nitrogen, water, benzene) for this sample.^ A specific surface area, Ssp = 103 m^ g ' calculated from the n^ value, is much lower than the specific surface areas obtained for this material using other adsorbates (Ssp(N2) = 882 m^ g'^)^ It is important to note that calculation of the specific surface area was
619 performed using a value for the molecular area a^ = 31 A^, obtained from the liquid density (a^ = 1.091 (M / p L)^^^)."* This equation assumes spherical molecules and hexagonal-packing, which is inaccurate in the case of «-butyl alcohol (a straight-chain alcohol). For this reason, it is the monolayer capacity which is of real interest here. It is worth noting that the shape of the isotherm bears some resemblance to the water sorption isotherm (303 K)^. If it assumed that «-butyl alcohol adsorbs on the surface of this material via polar interactions with surface hydroxyls, a notional value for the number of surface hydroxyl groups per unit surface area may be calculated if it is also assumed that one A2-butanol molecule will adsorb on one surface hydroxyl. Using the monolayer capacity (nm = 0.55 mmol g" ) and the nitrogen specific surface area (Ssp(N2) = 882 m^ g ^ , a value of 0.38 OH groups per nm^ is obtained. This is considerably lower than that calculated from the water isotherm (0.9 nm"^),^ which suggests that widespread localised bonding is occurring during nbutyl alcohol sorption (i.e. a monolayer is not being formed). If the sample is assumed (from water sorption) to have 0.9 OH groups per nm , then it appears that «-butyl alcohol is occupying just over a third of the available bonding sites. This would occur if adsorbed alcohol molecules block the entrance to a pore channel or block access to adsorption sites by lying across them, thus preventing further adsorption. The total pore volume, Vp = 0.23 cm^ g\ calculated from this isotherm is considerably lower than those given by other adsorbates for this material (see Table 1) including, as will be shown later, tert-buty\ alcohol (Vp(/^r/-butyl alcohol) = 0.33 cm^ g"^). This confirms that complete surface coverage has not occurred during «-butyl alcohol adsorption and also that steric effects have influenced the structure of the adsorbed layer.
1
_A
1^0
\±14±}
S^
.^ A ^
A
EA
oo o
o
^ o
1
c
i
6 0.0
A
^
A
^
o
o
B
adsorption of «-butyl alcohol adsorption of tert-buty\ alcohol adsorption of benzene
o o
OQO
^
r-
r-
0.1
0.2
0.3
1
1
1
0.4
0.5
0.6
1—
0.7
0.8
relative pressure, p/p^ Figure 1. Adsorption isotherms of n- and tert-buty\ alcohol (303 K) and benzene (293 K) measured on a microporous phenyl-modified silica.
620 3.2 tert-Butyl Alcohol Sorption /^r/-Butyl alcohol sorption at 303 K on the microporous phenyl-modified sample yielded an isotherm displaying mainly Type I characteristics (Figure 1). At low relative pressures there is a large uptake of adsorbate before the isotherm levels off at p/p^ « 0.2. BET analysis yields a monolayer capacity of 2.12 mmol g' , considerably larger than that obtained from /i-butyl alcohol sorption (n^ = 0.55 mmol g'). The obtained n^ value gives a specific surface area, Ssp = 664 m^ g\ which is slightly lower than would be expected for this material (Ssp (N2) = 882 m^ g'). As the isotherm resembles Type I, Langmuir analysis was also performed and this yielded a monolayer layer capacity of 3.70 mmol g' , corresponding to a specific surface area, Ssp = 742 m^ g"' very similar to that given by benzene sorption (750 m^ g" ).^ Values of the adsorptive cross-sectional area, ac = 45 A (obtained using the BET monolayer capacity and the N2 specific surface area) and ac = 38 A (calculated using the Langmuir n^ value) are considerably lower than that obtained for «-butyl alcohol on this material (ac = 284 A^) suggesting that localised adsorption is not occurring in this case. There is some uncertainty when interpreting Type I isotherms as to whether the "knee" represents the monolayer capacity or the pore volume. The value obtained for the total pore volume (taken from the plateau of this isotherm), Vp = 0.33 cm^ g', is identical to that given by the water isotherm and very similar to that given by the benzene isotherm (0.31 cm g" ). It is, however, considerably larger than that obtained from «-butyl alcohol sorption (0.23 cm g" ) which suggests that the tert-isomtr (approximately spherical) actually packs more efficiently than the w-isomer (linear) in the pore channels. The fact that Vp(N2) = 0.45 cm g' , suggests that tert-butyl alcohol is somehow packed in a less-dense state than its liquid form. This may be due to localised bonding on surface hydroxyls (although the bulky nature of the tert-isomer may preclude polar interactions between the alcohol fiinctional group and surface hydroxyls). However, since smaller total pore volumes were also observed with both benzene and water adsorption, steric hindrance of the sorbate is a more likely explanation of this feature. Low pressure hysteresis was observed in the desorption branch of the isotherm (not shown), which suggests that some degree of rehydroxylation is occurring. Further comparison with benzene sorption on this sample yields striking results if the two isotherms are plotted on the same axis, as shown in Figure 1. The fact that these two isotherms may be completely superimposed suggests that the mechanism of tert-butyl alcohol sorption is similar to that observed in benzene sorption. This indicates that the sorption mechanism of tert-butyl alcohol on this material is more influenced by organic interactions with the surface phenyl groups than by polar interaction with surface hydroxyls. (It should be noted, however, that a small amount of rehydroxylation is indicated by the low pressure hysteresis.) Further evidence that H-bonding to surface hydroxyls is not the preferred sorption mechanism in this case is highlighted when a value for the number of surface hydroxyls is obtained. The value calculated from the monolayer capacity of the rer/-butyl alcohol isotherm (1.45 nm'^) is much greater than that obtained from either the w-butyl alcohol isotherm (0.38 nm"^), or the water isotherm (0.9 nm'^). If the value calculated from the water isotherm is taken to be the most accurate (due to the small dimensions of the unhindered water molecules) it becomes evident that the tert-butyl alcohol molecules are not undergoing localised adsorption.
621 4. SUMMARY AND CONCLUSIONS A summary of the features ofn- and tert-buty\ alcohol sorption on the phenyl modified silica sample is presented in Table 1, together with information on nitrogen, water and benzene sorption for comparison.^ Table 1 Summary of butanol, water, benzene and nitrogen sorption data Adsorptive
«-BuOH
tert-BuOH
H20
C6H6
N2
Physical Quantity
Value for phenylmodified silica
nm / mmol g" Ssp/m'g-^ Vp/cm^g-' nm / mmol g"^ Ssp/m'g-^ Vp/cm^g-*
0.55 103 0.23 2.12 (3.70)^ 644 (742) 0.33
nm / mmol g"^
1.32
Ssp/m'g'
83
Vp/cm^g-^
0.33
Urn / mmol g"'
3.00 (4.7)^
Ssp/m^g-^
750 ± 5 0 (1217)
Vp/cm^g-'
0.31
im / mmol g"
9.04
Ssp/m'g-^
882
Vp/cm'g-^
0.45
nm - monolayer capacity, Ssp - specific surface area, Vp - total pore volume. * Figures in parentheses are calculated by applying the Langmuir model. ^ These values are calculated from Type I isotherms and are therefore more likely to reflect the micropore volume rather than the monolayer capacity.
622 Our results show that there is a clear difference between the sorption of tert-buty\ alcohol by the microporous phenyl-modified silica compared with the sorption of «-butyl alcohol. Comparison of total pore volumes and monolayer capacities with those for other sorptives leads us to conclude that sorption of the straight-chained isomer is significantly more sterically hindered than its branched analogue. This may be explained by the difference in the shapes of the two molecules. The "spherical" /er/-butyl alcohol molecule is able to easily penetrate into the micropores of the unswollen sample without blocking the pore-entrance, while the straight-chain «-butyl alcohol molecule may block the pores upon adsorption. The low monolayer capacity of w-butyl alcohol on the microporous silica and similarity in shape of the isotherm with that for water suggests that sorption occurs via hydrogen-bonding between the alcohol functional group and surface hydroxyls. However, in the case of /er/-butyl alcohol sorption on the microporous sample, given the similarity with the benzene isotherm, it appears that adsorption is via organic interactions with surface phenyls rather than hydrogen-bonding with surface hydroxyls.
ACKNOWLEDGEMENTS Authors thank S. Mann (University of Bristol), S.D. Sims and S.L. Burkett (University of Bath), for supplying the microporous phenyl-modified silica. CMB thanks the Open University for financial support, and the University of Exeter for use of its facilities
REFERENCES 1. Kresge, C.T.; Leonowicz, H.E.; Roth, W.J.; Vartuli, J.C; Beck, J.S. Nature, 359 (1992) 710. 2. Burkett, S.L.; Sims, S.D.; Mann, S., J. Chem. Soc, Chem. Commun., (1996) 1367. 3. Bambrough, CM.; Slade, R.C.T.; Williams, R.T.; Burkett, S.L.; Sims, S.D.; Mann, S., J. Coll. Inter. Sci., 201 (1998) 220. 4. Emmett, P.H.; Brunauer, S., J. Amer. Chem. Soc, 60 (1938) 309. 5. Gregg, S.J.; Sing, K.S.W., Adsorption Surface Area and Porosity, 2""^ Edition, Academic Press, London, 1982.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
623
Change of reorientational-vibrational relaxation upon capillary condensation in silica mesopores H. Tanaka, S. Inagaki^ Y. Fukishima' and K. Kaneko Physical Chemistry, Material Science, Graduate School of Natural Science and Technology, Chiba University, Inage, Yayoi, Chiba, 263 - 8522, Japan *Toyata Central R&D Labs. Inc., Yokomichi, Nagakute, Aichi, 480-11, Japan The adsorption isotherm of acetonitrile on FSM-16 (pore-width = 4.6 nm) was measured volumetrically, showing the presence of a clear adsorption hysteresis. The infrared spectra of adsorbed acetonitrile on FSM-16 were measured at 303 K along the adsorption and desorption branches. In the C = N stretching v, region, two bands were observed at 2265 cm^ and 2254 cm' (V2a and v^P band), respectively, assigned to hydrogen-bonded molecules on surface hydroxyls of FSM-16 and physisorbed molecules in mesopores. We analyzed the V2P band of the physisorbed molecules precisely and determined the changes of the peak position and the half-band width in the course of adsorption and desorption. The peak position did not seriously change with adsorption and desorption, but a slight difference between adsorption and desorption was observed. The half-band width vs. PIP^ relation had a marked hysteresis, corresponding to the adsorption hysteresis. Therefore, it was shown that the molecular reorientation-vibration states of acetonitrile molecules upon adsorption and desorption are different from each other and they are also different from that of the bulk liquid at the same temperature. 1. INTRODUCTION Capillary condensation has been used to evaluate the pore size distribution of mesopores. Various adsorption studies on regular mesoporous silica such as MCM-41 or FSM showed the limitation of the classical capillary condensation theory [1-9]. In the case of the evaluation of the pore size distribution, we assumed that condensates in mesopores are liquid. Recent systematic studies on structures of molecules confined in micropores
624 showed that the molecular assembly in micropores is not an ordinary liquid, but has a specific structure depending on the pore width and pore-wall chemistry [10-13]. Hence the molecular states of molecules adsorbed even in mesopores should be examined by molecular spectroscopy. Acetonitrile is a symmetric top molecule and has a very large dipole moment (3.92 D), which plays a key role in the reorientational relaxation. Hence acetonitrile can be a nice probe molecule to elucidate the molecular motional state of molecules condensed in mesopores. Authors applied the time correlation analysis for the band shapes of infrared vibration-rotation spectra to elucidate the molecular state of acetonitrile in mesopores of MCM-41 (pore-width w = 3.2 nm) at 303 K [14,15]; the C = N stretching v. band of the infrared spectra of adsorbed acetonitrile on MCM-41 was analyzed to show that before capillary condensation, the relaxation time x is smaller than that of the bulk liquid, suggesting weakly hindered rotation and after capillary condensation, x is slightly longer than that of the bulk liquid. These results indicated that this time correlation analysis is quite effective to understand the slight difference of molecular motional state in mesopores. Although the adsorption isotherm of acetonitrile on the MCM-41 (w = 3.2 nm) had no adsorption hysteresis, the information on changes of the molecular motion along the adsorption hysteresis should be valuable for elucidation of the adsorption hysteresis from the molecular level. This article describes the changes in reorientational-vibrational relaxation of acetonitrile in wider mesopores of FSM-16 along the adsorption hysteresis at 303 K. 2. EXPERIMENTAL FSM-16 samples were prepared by Toyata Central R&D Labs. The pore structure of FSM-16 was gravimetrically determined by N. adsorption at 77K. The surface area, pore volume, and pore width were 804 m' g ', 0.77 ml g\ and 4.6 nm, respectively. Acetonitrile of spectroscopic grade reagent from Wako Pure Chemicals was used. The adsorption isotherm of acetonitrile was measured at 303K using a computer-controlled volumetric apparatus [16]. The infrared spectra of acetonitrile adsorbed on FSM-16 were measured at 303K using an in situ IR cell with KRS-5 windows with the aid of a FT-IR spectrometer (JASCO FT/IR-550). The infrared spectrum was measured with the summation of 256 consecutive scans and a resolution of 1 cm \ The FSM-16 powder was uniformly coated on the KBr disk under the pressure of 5MPa, which does not induce the destruction of pore structures [17].
625
3. RESULTS AND DISCUSSION 3.1. Adsorption hysteresis and infrared spectral change with adsorption. The adsorption isotherm of N. on FSM-16 at 77 K had an explicit hysteresis. As to the adsorption hysteresis of N^ on regular mesoporous silica, the dependencies of adsorption hysteresis on the pore width and adsorbate were observed; the adsorption hysteresis can be observed for pores of w ^ 4.0nm. The reason has been studied by several approaches [58]. The adsorption isotherm of acetonitrile on FSM-16 at 303K is shown in Fig. 1. The adsorption isotherm has a clear hysteresis; the adsorption and desorption branches close at PIP(^ = 0.38. The presence of the adsorption hysteresis coincides with the anticipation of the classical capillary condensation theory for the cylindrical pores whose both ends are open. The value of the BET monolayer capacity, n^, for acetonitrile was 3.9 mmol g ^ By assuming the surface area from the nitrogen isotherm to be available for the adsorption of acetonitrile, the apparent molecular area, a^, of adsorbed acetonitrile can be obtained from n^. The value of a^ for adsorbed acetonitrile (0.35 nm") was quite different from the value (0.22 nm^) from the liquid density under the assumption of the close packing. Acetonitrile molecules on the mesopore surface are packed more loosely than the close packing. The later IR data will show that acetonitrile molecules are adsorbed on the surface hydroxyls in
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
P/P„
Fig.l. Adsorption isotherm of acetonitrile on FSM-16 at 303 K. Open and solid symbols denote adsorption and desorption.
626
1
rV
0.8
/ 0.6
^
+ V
V 3
4
1/
jy
^ - ^
0 2300
11
1
1
2290
2280
2270
X
0.68 0.63 0.55
\ '\
/y
0.4 0.2
0.70
//\
^
\
0.19
A ^
0.02
^x-^/C\
i
^
^^^^^^i^^^ J
2260
2250
2240
2230
2220
Wave n u m b e r / c m
Fig.2. IR spectra of adsorbed acetonitrile on FSM-16 in the C ^ N stretching v^ region.
the monolayer adsorption stage. Fig. 2 shows the infrared spectra of adsorbed acetonitrile in the wave number region of 2220 to 2300 cm^ over the range of PIP^, = 0.02-0.70 in the course of adsorption. The amount adsorbed at PjP^^ = 0.19 goes over the monolayer capacity. Hence the absorption bands below PjPf^ = 0.19 are mainly assigned to acetonitrile molecules in the monolayer. According to the preceding studies [14,15], the band at 2265 cm^ can be associated with the C = N stretching V2 band of the acetonitrile molecules interacting with surface hydroxyls of FSM-16; these molecules are in the monolayer. The C = N stretching V2 band at 2265 cm^ shifts upward by 11 cm ' from the bulk liquid phase value due to a greater C = N force constant of the acetonitrile molecule hydrogen-bonding with surface hydroxyls after Purcell [18]. Above F/F^ = 037, a new band is explicitly observed at a low-frequency side of the 2265 cm' band. This band grows with the increase of PlPi^ (after capillary condensation). Also the band position of the lower frequency band briefly agrees with that of the bulk liquid acetonitrile, suggesting that the band at 2254 cm^ can be assigned as molecules physisorbed in mesopores. The observed spectra in the C = N stretching V2 region were carefully deconvoluted in a similar way to the method published earlier[15]. The one example of the deconvolution for the band at P|P^^ - 0.70 is shown in Fig. 3. Here we designate the V2 band of hydrogenbonded acetonitrile at 2265 cm ' as the V2a band and that of physisorbed acetonitrile 2254 cm^ as the V2P band. Also we corrected the contribution by the hot band transitions of V2^'
627 and V2^^ Therefore, we can determine precisely the band width and peak position of V2P bands. 3.2. Band shape analysis. It is well known that the v^ band of liquid acetonitrile is significantly asymmetric due to an overlap of hot band transitions in the low frequency side. A study of gas phase rotationvibration spectrum [19] showed that the hot band transition from the first exited state of the degenerated C-C = N bending v^^ mode, v.^^ = v. + Vj^ - v^, has its center at 4.944 cm ' lower than that of the fundamental transition, v.*. Also the presence of v.*"' = v. + 2v^ - 2v^ transition is expected. The careful study on the v. band of liquid acetonitrile by Hashimoto et al [20] provided the reorientational and vibrational relaxation times of liquid acetonitrile molecule. They corrected the contribution by the hot band transition using the Boltzmann population law and approximated the v/, v.^', v^^^ and V3 + V4 bands by Lorentzian curves. Hence, we adopted their analytical way and extended their method to analyze the V2P bands. It is necessary to resolve the overlap of those transitions by least-squares fitting in order to obtain the widths of the component bands. For the least-squares procedure, we have to determine analytical functions for fundamental transitions, v.'a, v.'P, and for hot band transitions; the V2P band is reproduced as a sum of three Lorentzian curves of the V2'P, v.^'^P, and V2^^p. We also took into account the presence of the V2^^a and V2^~a band for the V2a band. The observed spectra in the 2320-2220 c m ' range were deconvoluted using
2300
2280
2260
2240
2220
Wave number / cm
Fig.3. Resolved infrared absorption spectra of adsorbed acetonitrile in the C = N stretching V2 region at PIP^^ = 0.70.
628
Fig.4. Changes in absorbance of the v^P band with adsorption and desorption. Open and solid symbols denote adsorption and desorption, respectively.
observed band profile at P/P„ = 0.02 for v.a band, three Lorentzian curves for v/p, V2*'^p, and V2^^P bands, one Lorentzian curve for V3 + V4 band of physisorbed acetonitrile, and a horizontal baseline. The detailed procedures were published in the preceding paper [14,15]. Figure 4 shows the changes in absorbance of the v.^P band with adsorption and
2254.4
2253.6 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Fig.5. Peak shifts of the V2P band with adsorption and desorption. Open and solid symbols denote adsorption and desorption, respectively.
629
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
P/P 0
Fig.6. Changes in half-band width of the v.p band with adsorption and desorption. Open and solid symbols denote adsorption and desorption, respectively.
desorption. The absorbance upon desorption is greater than that upon adsorption. The absorbance vs. P/P,, is similar to the adsorption isotherm. That is, the absorbance vs. P/P,, has an explicit hysteresis. The absorbance vs. P/P„ relation closes at P/P^, = 0.37, agreeing with that of the adsorption hysteresis. Figure 5 shows the peak position of the v.^p band in the course of adsorption and desorption. Here the peak position of the bulk liquid acetonitrile is shown by the solid horizontal line. The peak position does not change significantly; the peak position shifts slightly to a smaller wave number with the increase of P/P„ upon adsorption, approaching to that of the bulk liquid. On the other hand, the peak position of the band upon desorption is situated at a smaller wave number, coinciding with that of adsorption at P/P,, = 0.37. The shift to a smaller wave number indicates the increase of the intermolecular interaction due to the dipole-dipole interaction. Hence, acetonitrile molecules should be associated with each other even prior to the capillary condensation. The condensed molecular layer is slightly stabilized due to the dipole-dipole interaction in the course of desorption, compared with the condensates upon adsorption. However, the absolute change in the peak position is too small to make a definite conclusion on the mechanism. The correlation between the half-band width (half width at half-maximum heights) of the V2^P band and P/Pf, were plotted in Fig. 6. Here, the half-band width of the band of the bulk liquid is shown by the horizontal line. The half-band width varies from 2.8 to 3.5 cm \
630
The half-band width is constant below PIP,, - 0.37, and then suddenly drops with the increase of PIP,,. The relationship between the half-band width and PIP,, has a clear hysteresis corresponding to that of adsorption isotherm. The half-band width upon desorption is smaller than that upon adsorption. 33. Molecular motional picture of adsorption hysteresis. If the infrared band profile of single vibrational transition is given by Lorentzian, the band profile corresponds to the reorientational and vibrational time-correlation functions of exponential form, the relaxation time is expressed by,
T = (2;rcAv,J-'
(1)
^'•=X;'+T:\
(2)
where Av, ^ is the half-band width (cm '), x^ is the reorientational relaxation time, and x, is the vibrational relaxation time. Therefore, we can obtain the information about the reorientational-vibrational relaxation of an acetonitrile molecule from the half-band width of the V2^P band. Consequently, the half-band width vs. f/F^^data provide the information on the molecular motional state. Therefore, data of Fig. 6 show the change of the reorientational-vibrational relaxation time of acetonitrile molecules confined in mesopores upon adsorption and desorption. Before the capillary condensation, the relaxation time is smaller than that of bulk liquid, whereas it is greater than that of the bulk liquid after condensation. The difference of molecular motion between precondensation and postcondensation states is not significant, but this work can show clearly the presence of such a difference. If vibrational and reorientational relaxation processes are dominated by molecular collisions, the molecular reorientation is more rapid before condensation and it becomes slower than that of the bulk liquid with the progress of the capillary condensation, which indicates the formation of a weakly organized molecular assembly structure in mesopores. Even the mesopore can affect the state of the condensates through a weak molecular potential. The organized state should be stable in mesopores, because the relaxation time is almost constant above the condensation PIP,,. The relaxation time upon desorption is greater than that upon adsorption at the same PIP,, in the hysteresis region. This distinct difference provides an important information. The half-band width is almost constant from PIP,, = 0.70 to PIP,, - 0.59 on the course of the desorption and then it increases gradually with the decrease of PIP,,. Consequently, the
631 weakly organized molecular assembly formed upon condensation is stabilized in mesopores and has a structure different from that of the bulk liquid. The once-formed organized structure is still stable under the PIP,, region prior to the perfect condensation. Therefore, desorption should proceed by the molecular evaporation from this organized assembly without collapsing the whole assembly structure. The organized assembly should be divided into small islands in the course of further desorption, and then finally it disappears at the closing point of the hysteresis. Thus, the fundamental structure of the weakly organized assembly which is formed upon perfect condensation should remain during desorption. Although the interaction potential of a molecule with the mesopore wall is not great, molecules in condensates can sense it and form a slightly different structure from the bulk liquid. Therefore we can control intermolecular structures using the molecular field of mesopores. In future we are planning to extend this approach to another system to understand a general nature of molecular assemblies formed in mesopores. At the same time, we used to understand the molecular assembly confined in pores whose width is on the boundary of micropores and mesopores, as unique molecular structures and properties have been found in micropores by these authors [11,13, 21-24]. Acknowledgment. This work was funded by the Grant-in-Aid for Scientific Research B from Japanese Government. REFERENCES 1. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T. W Chue, D.H. Olson. E.W. Sheppard, S.B. MacCullen, J.B. Higgins and J.L. Schelender, J. Am. Chem. Soc, 14 (1992) 10824. 2. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc. Chem. Commun., (1993) 680. 3. RJ. Branton, RG. Hall and K.S.W. Sing, J. Chem. Soc. Chem. Commun., (1993) 1257. 4. RL. Llewellyn, Y. Grillet, F. Schuthe, H. Reichert and K.K. linger, Microporous Mater., 3 (1994) 345. 5. RL Ravikovitch, S.C.O. Domhnaill, A.V. Neimark, R Schuth and K.K. Unger, Langmuir, 11(1995)4765. 6. K. Morishige, H. Fujii, M. Uga and D. Kinukawa, Langmuir, 13 (1997) 3494. 7. M. Kruk, M. Jaroniec and A. Sayari, J. Phys. Chem., 101 (1997) 583. 8. C.G. Sonwane, S.K. Bhtia and N. Calos, Ind. Eng. Chem. Res., 37 (1998) 2271.
632 9. S. Inoue, Y. Hanzawa and K. Kaneko, Langmuir, 14 (1998) 3041. 10. Z. -M. Wang and K. Kaneko, J. Phys. Chem. 102 (1998) 2863. 11. K. Kaneko, J. Suzuki, and C. Ishii, Chem. Phys. Lett. 282 (1998) 176. 12. T. Ohkubo, T. liyama, K. Nishikawa, T. Suzuki, and K. Kaneko, J. Phys. Chem. 103 (1999) 1859. 13. K. Kaneko, Carbon, in press. 14. H. Tanaka, T. liyama, N. Uekawa, T. Suzuki, A. Matsumoto, M. Grun, K.K. Unger and K. Kaneko, Chem. Phys. Lett., 292 (1998) 541. 15. H. Tanaka, A. Matsumoto K.K. Unger and K. Kaneko, Characterization of Porous Solids V, in press. 16. T. liyama and K. Kaneko, Chem. Phys. Lett, (in preparation). 17. Y.G. Vladimir, F. Xiaobing, B. Zimei, L.H. Gary and A.O. James, J. Phys. Chem. 100 (1996)1985. 18. K.F. Purcell and R.S. Drago, J. Am. Chem. Soc, 88 (1965) 919. 19. I. Suzuki, J. Nakagawa and T. Fuzikawa, Spectrochim. Acta. 33A (1977) 689. 20. S. Hashimoto, T. Ohba and S. Ikawa, Chem. Phys. 138 (1989) 63. 21. K. Kaneko, Supramolecular Sci., 5 (1998) 267. 22. K. Kaneko, A. Watanabe, T. liyama, R. Radhakrishan, K.E. Gubbins, J. Phys.Chem. 103(1999)7061. 23. M. Aoshima, T, Suzuki and K. Kaneko, Chem. Phys. Lett. 310 (1999) 1.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
633
Characterisation of Microporous Materials by Dynamic Sorption Methods F. Thielmann", D.A. Butler\ D.R. Williams''^ E. Baumgarten' '
Surface Measurement Systems Ltd., 3 Warple Mews, Warple Way, London W3 ORF, United Kingdom ^ Department of Chemical Engineering and Chemical Technology, Imperial College of Science, Technology and Medicine, London SW7 2BY, United Kingdom ^ Department of Physical Chemistry, Heinrich-Heine-Universitaet, 40225 Duesseldorf, Germany Finite concentration IGC is a useful tool to investigate surface and pore properties. A novel combination of fmite concentration IGC and thermal desoqDtion provides the possibility to separate micropore adsorption from surface and mesopore adsorption. This allows the calculation of BET values with physical relevance for highly microporous materials and the consideration of molecular sieve effects.
1. INTRODUCTION The most versatile surface characterisation methods are based on gas or vapour sorption and these techniques can provide physico-chemical information such as enthalpies, surface energies and diffiision constants but also surface area and pore size distributions. Vapour and gas sorption measurements can be performed with static or dynamic methods, either of which can provide information on equilibrium behaviour. Furthermore, the measurements can be performed using gravimetric or volumetric based instrumentation. The most common flow methods are inverse gas chromatography (IGC) [1] for volumetric studies and dynamic gravimetric instrumentation [2]. The difference between IGC and conventional analytical gas-solid chromatography is the adsorption of a known adsorptive mobile phase (vapour) on an unknown adsorbent stationary phase (solid state sample). Depending on experiment setup, IGC can be used at finite or infinite dilution concentrations of the adsorptive mobile phase. The latter method is excellent for the determination of surface energetics and heat of sorption of particulate materials [3]. With IGC at finite dilution, it is possible to measure sorption isotherms for the determination of surface area and porosity [4]. The benefits of using dynamic techniques are faster equilibrium times at ambient temperatures. Apart from the described advantages, there is still one problem m the analysis of highly microporous materials. The micropores have a completely different adsorption mechanism compared to the mesopores and the outer surface area. Whereas the latter can be described by a monolayer mechanism according to the BET equation [5] the adsorption in smaller
634 micropores takes place as a so-called volume filling process [6]. This means an immediate condensation of the vapour coming from stronger adsoqjtion inside the micropores because of the local geometric and energetic situation. This means that for BET calculations from isotherms of highly microporous materials the obtained values have no physical meaning because the assumed sorption mechanism is wrong. In such cases a separate consideration of the mono and multilayer sorption and the micropore contribution becomes necessary.
2. METHODS Calculation of the isotherm can be done by the method of Cremer and Huber [7] for pulse measurements or by the approach of James and Phillips [8] for frontal analysis. The separation of micropore and mesopores plus the outer surface area was done by the combination of elution and flash thermodesorption [9]. With the latter it is possible to determine the contribution of the micropores to sorption separately as micropore desorption requires a higher activation energy because of the above mentioned effects. After injection of an organic vapour or a selection of a particular concentration of the gas mixture, adsorption took place on the sample in the column. The following carrier gas eluted the adsorbate and the peak was recorded. For thermodesorption, the sample was heated immediately to 473 K after the elution peak reached the baseline again. The peak which occurred had the same shape as the elution peak and was analysed again with the above mentioned methods. 3. RESULTS AND DISCUSSION The results will be illustrated by three examples. The first example shows an alumina (Degussa, type C) that is non porous and shown in Figure 1.
Cyctohexaneat29eK Hexane at 293 K
Q-2D0H
t/sec Figure 1. Chromatograms of cyclohexane adsorption on alumina
635 The very weak thermodesorption peak near 600 sec. supports this assumption. Therefore the elution isotherm should be very similar to the one that results from a static gravimetric measurement. This is shown in Figure 2 .
Alumina-C
o 5
^2^ H IGC (desorption) gravim. (adsorption) gravim. (desorption)
0.Q2
0,04
0,06
0.08
0.10
0,12
0,14
p/po
Figure 2. Comparison of grav.-static measured isotherms and IGC elution isotherm for alumina The second example is the measurement of two zeolites of different size. This should illustrate the other extreme of adsorption behaviour, a material that has a negligible surface but a strong micropore adsorption. In Figure 3 two cyclohexane experiment with a 3A and a 13X zeolite is shown. 800
600
Zeolite 13X Zeolite 3A
s. 200 H
smi I inxSmii U.iJ.i Ltttii IM W :J.I luiiu 1
200
•
1
400
«
1
600
•
1
800
•
1
1000
•
1
1200
'
1
1400
•
1
1600
'
1
1800
•
1
2000
t/sec
Figure 3. Chromatograms of cyclohexane adsorption on zeolite 3A and 13X
636 The 13X shows a strong thermodesorption peak whilst the 3A shows none. The explanation is found in the size of the cyclohexane which has a critical diameter of 6 A. Therefore it has access to the pores of the 13X with 10 A diameter whilst there is no access in the pores of the 3 A that has a diameter of 3 A. In this case the thermodesorption isotherm should be similar to the static gravimetric obtained one as shown in Figure 4.
Q(Dt8 QOOK QODM QGOQ Qomo QODDB-I
Ihermodesoipticn Isotherm Go/imetric Isothefm
QOaOB QODW-I QODDB-j QGOQO
p/po
Figure 4. Comparison of static-grav. measured isotherm and IGC thermodesorption isotherm for zeolite 13X This suggests the method is also as a useful tool to display the molecular sieve effect in a simple way. Another application is the measurement of activated carbon. Two different types of activated carbon were used for this investigation. One standard carbon with a rather small surface area of 75 mVg (manufacturer's data) and one typical carbon supplied by Norit (GAC) with a surface area of 1100 m^/g (manufacturer's data). 700600 500
Standard-actjvated cartnri GAC-activated caitxxi
400
i 300-1 200 100 0
t/sec
Figure 5. Pulse chromatograms for cyclohexane adsorption on activated carbon
637 Figure 5 shows the chromatograms of both carbons in the case of a pulse measurement with cyclohexane. It is especially easy to recognize that thermodesorption shows a completely different picture for both sanq)les. This suggests a difference in micropore structure. This is confirmed by Figure 6 that shows the micropore size distribution calculated from the second peak isotherm.
0.0010
0.08-
0.0009
N.
0.0008
Q06H
0.0007
—°— Standanck activated cartxxi
L'fi"omthe original lattice L into an effective lattice L\ where each empty site of L turns into an empty one of L', while each set of A: sites occupied by a kmer in L is represented by an occupied site in I'. Thus, the total number of sites in I ' is M' = M-¥(kA)N
(1)
and the coverage 9' = ^ = ^ . M' k
k
(2)
657 The canonical partition functions in the original and effective lattice must be equal. Accordingly, the Helmholtz free energies per site in L and L\ F and F\ respectively, are related through
{k-\) N
k
(3)
F'
This relationship makes complete the mapping from the original problem of ^-mer adsorption on L to an effective Ising-like one (monomer adsorption) on Z,'. F' is exactly known[17-19], hence, the exact form of F is obtained F = \\^ — a k
^In^ + ( l - 0 ) l n ( l - e ) - 2 a l n a k k
(4)
e -a Inl 0 -a - ( l - e - a ) l n ( l - 0 - a )
•kj
where a, b and A are given by 20(1-0)
; b=-
(^-Oo kA
^-Q'i
; ^ = [l-exp(-wA^r)]-'
(5)
k Then the chemical potential \i and the entropy per site S are straightforward from
kJ
kJ
•ln[A:(6-l+e)+e]+()t-l)ln
\^'-%.t
-}-{k -\)\nk - k\n[k{b
-\-\-Q)-Q] (6)
^ =^ \n^ kg k k
-^{\-Q)\n{\-e)-2a\na-
-a In
-(l-0-a)ln(l-0-a)
(7)
The Eq. (6) represents the exact form for the adsorption isotherm of interacting /r-mers in one dimension. For non interacting adsorbates (vv=0) we obtain from Eq. (6) the isotherm equation [20]
^'--(it-Oln kJ
k
+ ln0-A:ln(l-0)-lnA:
(8)
which constrast significantly with the known Flory-Huggins (FH) [21] approximation for linear chains on a one-dimensional lattice,
658
kj
:ln0-A:ln(l-0)
(9)
because of the strong effect traced to the factor {k'l)\n[l-{k-l).^/k] that is absent in the latter one. The differences observed in the adsorption isotherm are also qualitatively and quantitatively significant for the entropy. It has been recently shown that the isotherm of adsorption of an ideal adsorbate on a heterogeneous surface can be appreciably improved by taking into account the exact form of 5 from Eq. (7) instead of the approximate one arising from F-H theory [22]. Results for the coverage dependence of the chemical potential (adsorption isotherm) and entropy per site are shown in figs. 1-2 for various A:-mer's sizes and interaction energies [attractive (w0)]. l.Of
Figure 1. Adsorption isotherms for interacting (repulsive as well attractive) dimers and 10mers; (a) k = 2,w/kBT= -10; (b) k = 2,w/kBT=0; (c) k = 2,w/kBT= +10; (d) /: = 10, w / kBT= +10. The frill circles represent results from Monte Carlo Simulation in the lattice-gas model. Comparison between experimental isotherm of CH4 from ref [11] (open triangles, T = 77.35 K; open diamonds, T= 96.50 K) and theoretical isotherm [from Eq. (6)] of dimers in the lattice-gas approximation (frill triangles, T= 77.35 K, w = 0.61 Kcal/mol; full diamonds T = 96.50 K,w = 0.61 Kcal/mol). The adsorption isotherms for attractive k-mers become more steeped the stronger the lateral interaction. No significant qualitative changes are observed as the ^-mer size increases. However, the curves have a pronounced plateau at 0 = k/{k+I) for strongly repulsive interactions, which smoothes out for already WABT = +2. This type of isotherm has been reported very recently for Kr and CH4 adsorbed in AIPO4-5, where, very likely, the mismatch between the equilibrium separation of the intermolecular interaction and the lattice constant along the nanochannels give rises to repulsive interactions. MC simulations of alkanes adsorbed in AIPO4-5 show that double-steeped isotherms are the consequence of a rearrangement of molecules within the tubules in such a way that, although ads-ads interaction energy increases (in absolute values) upon rearrangement, ads-adsorbent interaction energy diminishes, giving a net repulsive effect in the adsorption isotherm as well as in the isosteric heat of adsorption [23,24]. In figure 1 experimental isotherms of CH4/AIPO4-5 at r = 77.35 K
659 and T = 96.5 K from Martin et al. are shown. The plateau at the coverage 0 = 2/3 is the beginning of a relocation of methane within the unit cell of AIPO4-5 in order to step from 4 to 6 molecules per unit cell. From simulation studies [23] it arises that the isosteric heat of adsorption increases from zero coverage up to 6 « 2/3. This increase is mainly due to attractive interactions between neighboring methane. This attraction favours very much the adsorption of pair of methane molecules as small clusters (dimers). The structure of this quasi-one-dimensional phase is essentially determined by the local minima in the gas-solid potential. hi order for the fiill coverage to be attained a relocation of molecules within the unit cell must occur. In the high density phase, even though the ads-ads interactions increase, the gassolid interaction energy decreases owing to the stronger repulsion in the new equilibrium positions, giving a net repulsive decrease in the isosteric heat. The full triangles and diamonds in fig. 1 correspond to theoretical adsorption isotherms of dimers at the same temperatures that ones in the experimental results. The value of the nearestneighbor interaction energy w = 0.61 kcal/mol was taken be equal to the decrease of the isosteric heat at 0 = 2/3 from the molecular simulation of ref 23. Since CH4 presumably adsorbs as a entire unit on the adsorption sites of AIPO4-5 (thus corresponding to monomer adsorption rather than dimer adsorption), the experimental isotherm are much steeped at low and high coverages than the ones for dimers as expected. However fair agreement of the slope and broadness of the plateau is observed. The main reason of the comparison is to highlight that the physical nature of the plateau both in experiments and the model are the same, other than the matching of CH4 to dimer adsorption in this case. Additional isotherms for dimers with stronger interactions are shown in circles. It is worth noticing that although the double-steeped isotherm may be indicative of the second-order phase transitions (as speculated in Ref 15 for Kr in A1P04-5), they may be not for an adsorbate whose size is comparable to the nanotube diameter that behaves as a one dimensional confined fluid. MC simulations in the grand canonical ensemble (shown in fiill circles) fully agree with the predictions from Eq. (6). Novel general features of the coverage dependence of the entropy per site arise (Fig. 2): for attractive interactions S is symmetrical with a maximum at 0 = 0.5 for interacting as well as for noninteracting monomers (^=1). For interacting /:-mers (^>1) the maximum shifts to higher coverages 0 > 0.5). However, given a ratio w / kgT < 0 the maximum shift to higher coverages such a way that the larger k the more apart the maximum gets from 0 = 0.5. This result is distinct from the one-dimensional limit of noninteracting ^-mers (w / ^^1=0) in the FH approximation for which the maximum of S shifts to lower coverages as k increases. Given a A:-mer size, the stronger the interaction the smaller the shift of the maximum from 0=0.5. For repulsive interactions, the entropy develops a local minimum at 0 = k/(k-^I) > 0.5 that gets sharper as the ratio w / ksT increases, and shifts to higher coverages as the k-mQT size increases. The position of this minimum seems to embody valuable information about the collective transport of particles, since (as shown latter in Fig. 3) relates to the maximun of the chemical diffusion coefficient of repulsive y^-mers. The coverage dependence of the entropy per site was also compared with MC estimations by thermodynamics integration [25]. For any coverage, the exact value of the entropy in a reference state is taken from the infinite temperature limit of Eq. (7), S (oo,0) / kg.
660
0.5 ^
0.4 r
0.3
2
^)
/^^^^ \c)
\ f J^^
0.2 h
c !05.' 0.1 h
&V^"^^
0.0
0.0
0.2
(a) 0.4
1
I
0.6
0.8
T
^"^
1.0
0, coverage Figure 2. Entropy per site, 5(0), versus lattice coverage, 0. The curves denoted (a) to (d), corres-pond one-to-one to the cases displayed in fig. 1; (e) A: = 2, M; / ksT - +2; (f) A: = 10, w / ksT- -2. Results of thermodynamic integration by Monte Carlo Simulation in the canonical ensemble are shown in symbols for the respective cases. The agreement obtained is remarkable for weak as well as strong lateral interactions, regarding the intrinsic difficulties of entropy calculation for polyatomic species at low temperatures. The shallow as well as the sharp minima for repulsive dimers in the cases (d) and (c) of Fig. 2 are accurately reproduced by this calculation. 3. COLLECTIVE SURFACE DIFFUSION Concerning diffusion of interacting /:-mers, we consider the process to be thermally activated jump to nearest-neighbors positions, (i.e, the ^-mers can move over a distance of only one lattice constant either to left or right if the final sites are empty). It is worth mentioning that a very interesting alternative mechanism of diffusion in one-dimensional systems has recently been proposed [26], where the molecules move along one direction in concerted motion involving jump of admolecules clusters rather than by individual jumps, because of the strong mismatching between the equilibrium separation for the lateral interaction of two isolated molecules and the lattice spacing. Although this mechanism may account for the high mobility observed for single-file diffusion in AIPO4-5 [27], the results are still hypothetical. It should be also mentioned that also nearest-neighbor repulsive interactions owing to the mistmatch between the molecular size and lattice spacing also lead to an increase of the chemical difdision coefficient at finite coverages. There exists evidence of this type of diffusion mechanism for polyatomic molecules adsorbed in nanocylindrical pores [15]. Although the migration of large molecules can, in many cases, show other jump mechanisms, the fact that exact prediction of the coverage dependence of the diffiision constant can be obtained under this assumption justifies the following analysis. It is known that the displacement of single particle in single-file diffusion follows the time dependence rV() «=^'^^. Hence the tracer diffiision coefficient vanishes at long times for diffusion on an infinitely long lattice. However, MC simulations show that the collective
661 motion of monomers in one dimension at finite coverages, represented by the center of mass's mean-square displacement obeys a time dependence R^(t) cct, where the proportionality constant Dj(Q) is coverage dependent and called jump (or phenomenological) diffusion coefficient [28]. This time dependence have been analytically predicted by mathematicians [29,30]. It is straightforward to show that this behavior also holds for A:-mers which perform elementary jumps of lenght equal to one lattice constant in one dimension. From the GreenKubo formalism, the chemical diffusion coefficient is D(Q) = Dj(B)Th(Q), where
Th(e) =
(11)
a(ln0) J
is called the thermodynamic factor and it can be obtained from Eq. 8 by differentiation. D(%) can be alternatively written as
D(0) = r(e).
a[exp(^/A:,r)]
(12)
ae
where r(0) = Z)j(0)j9.exp(- \ilkgT)
is called the effective jump rate and physically gives the
rate of jumps of/:-mers into nearest neighbor empty sites at coverage 0 [31 ]. Thus,
r(0) = y (^"-^^
(13)
where P^^ and Y^^ are the probability that a A:-mer havey /:-mers as nearest-neighbor, and its jump rate, respectively. In the quasi-chemical approximation (that becomes exact in one dimension).
_ (ney
po) _
(14)
(1+riey
where r| = expC-u- / kaT) and E = [Afc-/t+*(/t-/)e+29]/2rie. The jump rate is defined by V^' = r(0).Ti'^. Finally
D(e) =
(1 + e)
d{\ilkj)
(assuming r(0) = 1 arbitrarily).
(15)
662 16
[ Repulsive interaction 12
(c)
8 4
0.0 0.2 0.4 0.6 0.8 6, coverage Figure 3. Relative chemical diffiision coefficient D(Q) /Do versus lattice coverage 9 (where Do = lime-»o D(Q)). The curves for repulsive and attractive interactions are shown in the upper and lower half of the figure, respectively. Labels correspond to the parameters specified in Fig. 2. It is worth noting the limit case of noninteracting A^-mers for which w - 0 and
D(e) =
(16) k
This gives a much weaker coverage dependence for D than the one predicted by using the chemical potential from the FH approximation, DpH(e)cx
(^-1) 0
(17)
The coverage dependence for the chemical diffusion coefficient D(0) of dimers and 10mers is depicted in Fig. 3 for repulsive (top) and attractive interactions (bottom). The general behavior for D(0) is that a repulsive interaction lead to an increase of D(0) for all 0. For large w / kBT»\, Did) varies very significantly within an interval of 0 around the one corresponding to the plateau in the adsorption isotherm shown in Fig. 1. It reaches a maximum, and then becomes approximately coverage independent up to full coverage owing to a cancellation effect between /)(0) (that decreases on 0) and Th{d) (that increases on 0). This characteristic of D(0) can be traced to the minimun of 5(0) at the same coverage. As pointed out before, the net effect of repulsive interaction is the overall increase of chemical diffusion at high coverages. It should be mentioned that this situation may occur in
663 quasi-one-dimensional pores where the mismatch of the adsorbate size and lattice constant can give rise to repulsive forces between nearest-neighbor units. Attractive interactions, in turn, decrease £)(6) for all 0, with respect to noninteracting /:-mers, leading to a smooth coverage dependence, except for G close 0 and 1. 4. CONCLUSIONS A comprehensive description of thermodynamic functions and their dependence on parameters as the type of interactions, adsorbate size, temperature and surface coverage was given through their exact forms. Remarkable differences with F-H approximation were shown and discussed. The transport properties on this model system have been presented by mean of the calculation of the chemical diffusion coefficient representing the macroscopic relaxation of density fluctuations. Analytical isotherms were compared with experimental ones in nanocylinders with essentially one-dimensional character. Features of the diffiision coefficient can be traced to similar ones of the thermodynamic functions, as the maximun in D(6) that relates to the minimun in 5 (0). The statistical thermodynamics analysis of k-mcrs adsorption in a one-dimensional lattice provides an intuitive approach to linear molecules confined in quasi-one-dimensional nanotubes. More elaborated analytical solutions that incorporate nearest and next-nearestneighbors between A:-mer's ends can be obtained by applying the mapping proposed in the present work. The results presented here are intended to provide a simple analytical framework to interpret experiments about equilibrium and transport in narrow nanocylinders. This work was partially supported by the CONICET (Argentina). The authors are gratefuly indebted to M.W. Cole for helpful discussions. REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.
S. lijima. Nature 345 (1991) 56. S. lijima and T. Ichihaschi, Nature 363 (1993) 603. P. M. Ajayan and S. lijima. Nature 361 (1993) 333. D. S. Bethune, C. H. Kianig, M. S. de Vries, G. Gorman, R. Savoy, J. Vasques and R. Beyers, Nature 363 (1993) 605. T. W. Ebbensen, Science 265 (1994) 1850. G. Stan and M. W. Cole, Surf. Sci. 395 (1998) 280. G. Stan, M. Boninsegni, V. H. Crespi and M. W. Cole, J. Low. Temp. Phys. 113 (1998) 447. R. Radhakrishnan and K. E. Gubbins, Phys. Rev. Lett. 79 (1997) 2847. M. W. Cole, V. H. Crespi, G. Stan, J. M. Hartman, S. Moroni and M. Boninsegni, Anisotropic Condensation of Helium in Nanotubes Bundles, Priv. Communication, 1999. M. W. Maddox and K. E. Gubbins, Langmuir 11 (1995) 3988. J. C. Martin, N. Tosi-Pellenq, J. Patarin and J. P. Coulomb, Langmuir 14 (1998) 1774. V. R. Choudhary and S. Mayadevi, Langmuir 12 (1996) 980.
664 13. B. L. Newalkar, R. V. Jasra, V. Kamath and S. G. T. Baht, Microporous Matter. 11 (1997) 195. 14. A. C. Dillon, K. M. Jones, T. A. Bekkedahl, C. H. Kianig, D. S. Bethune and M. J. Heben, Nature (London) 386 (1997) 377. 15. J. P. Coulomb, C. Martin, Y. Grillet and R. Khan, Fundamentals of Adsorption 6, edited by F. Meunier, Elsevier, Amsterdam, 1998, p. 177. 16. F. Rittner, B. Boddenberg, M. J. Bojan and W. A. Steele, J. Chem. Phys. submitted, (1998). 17. Kikuchi,Phys.Rev. 81 (1951)988. 18. A. Aligia, Phys. Rev. B 47 (1993) 15308. 19. T. L. Hill, An Introduction to Statistical Thermodynamics; Addison-Wesley Publishing Company: Reading, Mass. 1960. 20. A. J. Ramirez-Pastor, T. P. Eggarter, V. D. Pereyra and J. L. Riccardo, Phys. Rev. B 59 (1999)11027. 21. P. Flory, J. Chem. Phys. 10 (1942) 51. 22. A. J. Ramirez-Pastor, V. D. Pereyra and J. L. Riccardo, Langmuir 15 (1999) 5707. 23. V. Lachet, A. Boutin, R. M. Pellenq, D. Nicholson and A. Fuchs, J. Phys. Chem. 100 (1996)9006. 24. T. Maris, T. J. H. Vlugt and B. Smit, J. Phys. Chem. B, 102 (1998) 7183. 25. K. Binder, Journal of Computational Physics 59 (1985) 1. 26. D. S. Scholl and K. A. Fichthom, Phys. Rev. Lett. 79 (1997) 3569. 27. K. Hahn and V. Kukla, Phys. Rev. Lett. 76 (1996) 2762. 28. F. Bulnes, V. D. Pereyra and J. L. Riccardo, Phys. Rev. E 58 (1998) 86. 29. T. E. Harris, J. Appl. Prob., 2 (1965) 323. 30. F. Spitzer, Adv. Math., 5 (1970) 246. 31. D. A. Reed and G. Erlich, Surf. Sci. 102 (1981) 588.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
665
Mechanical Strength of Micelle-Templated Silicas (MTS) Delphine Desplantier-Giscard^ Olivier Collart^, Anne Galameau^ Pascal Van Der Voort^, Francesco Di Renzo^ and Fran9ois Fajula^ ^Laboratoire de Materiaux Catalytiques et Catalyse en Chimie Organique, Ecole Nationale Superieure de Chimie de Montpellier, 8 rue de FEcole Normale, 34296 Montpellier cedex 5, France ^University of Antwerpen (UIA), Department of Chemistry, Laboratory of Adsorption and Catalysis, Universiteitsplein 1, B-2610 Wilrijk, Belgium
The mechanical properties of Micelle-Templated Silicas (MTS) are very sensitive items for industrial process applications which might submit catalysts or adsorbents to relevant pressure levels, either in the shaping of the solid or in the working conditions of catalysis or separation vessels. First studies about compression of these highly porous materials have shown a very low stability against pressure. These results concern these specific materials tested. In this study, we show very stable MTS with only a loss of 25% of the pore volume at 3 kbar. The effects of several synthesis parameters on the mechanical strength are discussed.
1. INTRODUCTION Since the disclosure by Mobil of Micelle-Templated Silicate structures called MCM-41 (hexagonal symmetry) or MCM-48 (cubic symmetry) [1,2] many other structures have been synthesized using different surfactants and different synthesis conditions. All of these MicelleTemplated Silicas (MTS) have attracted much interest in fields as diverse as catalysis, adsorption, waste treatment and nanotechnology. MTS materials possess a high surface area ('-lOOO m^/g), high pore volume (~1 mL/g), tunable pore size (18-150 A), narrow pore size distribution, adjustable wall thickness (5-20 A). The silica walls can be doped with different metals for catalytic applications, like Al or Ti, for acidic or oxydation reactions, respectively. In most spectroscopic studies, the solids to be studied are usually compressed to form pellets under pressures around 1.5-2 kbar. From an academic point of view, the stability of MTS towards pressure is very important, since most spectroscopic studies of lattice groups or adsorbed probes might be affected by a degradation of MTS during compression. For industrial applications compaction is crucial to handle the powder. Thus the mechanical properties of MTS are a very sensitive topic if we think about the future of these materials. Solids with such high porosity and small wall thickness are very likely to be crushed. Previous studies point out a very weak mechanical strength of MTS [3,4J which can jeopardize further industrial development. It has been demonstrated that these materials have the lowest mechanical stability among the
666 adsorbents like zeolites, alumina, mica, illite, kaolin, halloysite, and silica gels [3]. It was suggested that moisture in the atmosphere under which the solid had been compressed was responsible for its destruction by siloxane hydrolysis and that a compression under nitrogen was less critical [4]. A compaction on the as-synthesized material followed by calcination gives also better stability [5]. Also the silylation of the surface allowed one to totally protect the MTS [5], a subsequent calcination to remove the organic part gives also a material as stable as the one compressed under nitrogen or packed with the former surfactants. Tatsumi et al. 15] have also shown that MCM-48 is less stable than MCM-41. They suggest that the mechanical stability should be due to pore wall thickness and to aluminium content. To control this sensitive point we compared the stability of MTS synthesized under a wide range of conditions. In this study we have examined different parameters which can influence the stability of MTS: the wall thickness (from 5 to 15 A), the pore size (from 20 to 80 A), the topology (MTS with hexagonal and cubic symmetry), the amount of Al in the silica walls. The decrease of porosity can be followed from nitrogen sorption isotherms at 77 K.
2. EXPERIMENTAL The preparation of hexagonal MTS was performed using cetyltrimethylammonium bromide (CTAB), Aerosil Degussa as silica source, sodium aluminate, 1,3,5-trimethylbenzene (TMB) in molar ratio: 1 Si02 / 0.1 CTAB / 0.1 - 0.4 NaOH / 0 - 0.035 NaA102 / 20 H2O / 0 1.3 TMB. The mixture were stirred during 30 minutes and sealed in autoclave at 115°C for 24 h. The solids were then filtered, washed with distilled water, and dried at 80°C overnight. The surfactants were removed by calcination at 550°C for 8 h under air. The preparation of cubic MTS of Ia3d crystallographic structure was carried out in the presence of a gemini surfactant with the general formula [Ci6H33N'^(CH3)2-(CH2)i2N+(CH3)2Ci6H33].2Br, abbreviated as 16-12-16. The source of silica is tetraethoxysilane (TEOS). The gel of the synthesis presented a composition of 1 TEOS / 0.62 NaOH / 0.06 1612-16/148 H2O. The mixture was sealed in autoclave at 115°C for 3 days. The solid was then filtered and sealed again in autoclave with water for 20 h at 115°C. The solid was then filtered, washed with distilled water, and dried at 80°C overnight. The surfactants were removed by calcination at 550°C for 8 h under air. Mechanical stability of hexagonal and cubic MTS was investigated as follows: 0.2 g of the samples were compressed in a steel die of 2.5 cm^ using a hand-operated press, for 1 min under air. Compressing the powder for longer time (during 1 h) had no further effect on the pore volume, the surface area, the pore size and the X-ray diffraction pattern. Only the interparticles porosity was sligthly affected.
3. RESULTS AND DISCUSSION 3.1 Changes of hexagonal MTS by compression The XRD pattern of calcined hexagonal MTS HI, as shown in Figure 1, exhibited a strong peak at d-spacing of 44 A followed by a second order shallow peak. This kind of XRD
667
pattern is usually attributed to materials named M41s which are presumably ordered on a shorter scale than MCM-41. The intensities of the diffraction peaks progressively decreased with increasing pressure without changing the d-spacing.
I 1
2
3
4
5 6 7 8 26 / degree Figure 1. X-ray diffraction pattern of hexagonal MTS HI compressed at different pressure.
,
T
1—r
1
J
800 700 ! « « h
2 500 u
1 I
400 h
3 >
300
J
_n
a
f
^r'---^'^^^^
J
^^
r
j ^ ^ ' ^ ^ ^
-^
-j
1
^_f>-::rf
J
200 \
h 100 L 0C 0.
i
j^^^
H J . __ x_
.
1
0.2
.
.
1
0.4
.
.
, P/Po
.
L
0.6
.. . .a. -^
1
0.8
^
- _J
]
1.0
Figure 2. Nitrogen sorption isotherm at 77 K of the hexagonal MTS HI compressed at: (a) 0, (b) 1.6,(c)2.6,(d)4.7kbar.
668 The nitrogen sorption isotherm of this sample, shown in Figure 2, was characteristic of MTS materials with a sharp step at p/po around 0.35 due to the filling of uniform pores. The pore volume and the surface area were 0.70 mUg and 950 m-/g, respectively. The pore size determined by the Broekhoff and De Boer method was 35 A and the pore wall thickness determined by a geometrical method [6] was 8.6 A. This sample exhibited also a second porosity at higher p/po due to the stacking of small particles. The particle size of this sample was less than 100 nm as measured on SEM pictures. The mechanical stability was tested under various pressure levels from 0.8 kbar to 4.7 kbar. At 0.8 kbar the structure was not affected by the compression. At higher pressure, p > 1.2 kbar, the volume and the surface area decreased progressively as shown in Figure 2 and Table 1. At 4.7 kbar, the volume loss was 38%. The pore size of this material was unchanged under compression and no microporosity was detected by tplot analysis. The unchanged d-spacing and pore size of this MTS material probably indicated that some parts of the material stayed intact while other parts were totally crushed giving rise to non porous amorphous fraction which contributed to the decrease in pore volume and surface area. Furthermore the interparticles porosity was strongly modified. An hysteresis loop appeared, which was characteristic of pores among spherical particles, as particles were becoming closer to each other. At higher pressure, the volume of this interparticles porosity decreased and the hysteresis loop broadened resulting, at 4.7 kbar, in ink bottle pore shapes. 12r (lOh
0
1
2
3
4
5
p(kbar) Figure 3. Change of particle stacking under compression The pattern of particle packing was analyzed by calculating the evolution of the particle density p: p = 1 / (Vp + Vs)
(1)
The porous volume Vp (textural volume) was estimated by the volume adsorbed at the end of the pore filling (end of the step) and the silica specific volume Vs was taken at 1/2.2 where 2.2 is the density of amorphous silica. This calculation confirmed that for pressure higher than 0.8 kbar, the particle density strongly increased: from 0.89 without compression to 1.15 at 4.7 kbar. The
669 particles were slowly crushed under compression. The interparticular void fraction Ej was also examined: Ei = Vi / (Vi +Vg)
(2)
The interparticular void volume Vi was evaluated as the difference between the total porous volume (textural + interparticular) and the textural pore volume Vp. The particle specific volume Vg (volume of grain) was the reciprocal of the particle density (equation (1)). The interparticular void fraction decreased for pressure > 0.8 kbar, the particles getting closer and closer progressively. In a rigid sphere model, the void fraction is related to the coordination number of the spheres. In a model with spheres of identical size, the stacking of the particles increased from a coordination number of 6 at p = 0.8 kbar to the maximum of 12 at 3 kbar as shown in Figure 3. At higher pressure the particles started to lose the spherical shape giving rise to the particular shape of the hysteresis loop at 4.7 kbar seen in Figure 2, characteristic of restrictions to the opening of the interparticular pores. The size and volume of the intergranular porosity decreased with the increase of the coordination number among grains with pressure. 3.2 Changes of cubic MTS by compression Table 1 Effects of compression on volume and surface area of hexagonal and cubic MTS Pressure hexagonal MTS HI cubic MTS / kbar S (m^/g) Vp (mI7g) S (m^/g) Vp(mL/g) 0 950 0.93 0.70 1460 0.8 0.85 980 0.70 1380 1.2 910 0.64 1.6 0.79 890 0.62 1330 2.0 910 0.63 2.7 0.70 860 0.57 1220 2.75 820 0.55 3.15 0.61 1140 4.72 725 0.43 The XRD pattem of the calcined cubic MTS gave the first peak indexed as (211) at a dspacing of 30.3 A which corresponded to a unit cell of 74.2 A. As for the hexagonal MTS, the XRD pattem of the cubic MTS remained unchanged during the compression, only the intensity was slightly decreasing. The pore volume and the surface area of the cubic MTS decreased slowly under pressure as shown in Figure 4 and Table 1. This decrease was slightly larger than for the hexagonal MTS. For the cubic MTS, less interparticles porosity was observed during compression. By SEM analysis the particle size was found to be around 5 jim larger than for hexagonal MTS. The
670 effect of stacking for such large particles would be more difficult to observe by nitrogen sorption, but crushing of particles and stacking should occur in a similar way. 1000,
,—,
r-
-1
0.2
0.4
0.6
1
I
I
1
I
0.8
P/Po Figure 4. Nitrogen sorption isotherm at 77 K of the cubic MTS compressed at: (a) 0, (b) 0.8, (c) 1.6,(d)2.7,(e)3.2kbar. 3.3 Comparison with literature data The results obtained in this work were compared with the literature data (Figure 5) in term of VA^O, VQ being the pore volume of the uncompressed material. The decrease in pore volume of MTS synthesized during this work was much lower than the previous literature results. The MTS tested in this work were much more resistant than those of literature, notwithstanding the atmosphere of the compression was untreated air, a condition shown to be especially unfavorable. Various authors found the cubic MTS much less stable than the hexagonal one. In this work only a small difference was observed. Tatsumi et al. [4,5] have improved the mechanical stability of their samples by using different methods, such as compressing under nitrogen. The results are shown in Figure 5, but their best samples are less stable than our samples. The other methods they used was the silylation or compression of as-synthesized MTS followed by calcination. Silylated materials were very stable, no loss of pore volume was noticed. For the solids silylated and then calcined and for solids compressed as-synthesized, the results were close to the compression under nitrogen. The assumption given by the authors was the hydrolysis of the structure by the water contained in the pores which impair the structure and accounted for the difference between compression under air and nitrogen. The better stability observed for samples silylated and calcined was attributed to thicker walls. According to these authors MTS with thinner walls and
671
aluminium-doped should be less stable. The wall thicknesses of their samples were between 10 and 12 A. These parameters are analyzed in the next section. B ' I ' I ' I ' i '
Cub-MTS
0.2
0
1 2
3 4 5 p (kbar)
6
I I I I I I I
0
1 2 3 4 5 p (kbar)
6
0
0
I
I I I I I 1 I
1 2 3 4 5 6 p (kbar)
Figure 5. Pore volume decrease in function of the pressure for cubic (Cub-MTS) and hexagonal (Hex-MTS) MTS. (A) Hexagonal MTS from: • this work, • Gusev [3] and +, X, • , • Tatsumi [4,5]: • compressed under air; + silylated, compressed and calcined; X compressed with surfactants and calcined, • compressed under nitrogen. (B) Cubic MTS from: n this work and Tatsumi [4] compressed under O air and under V nitrogen. (C) Comparison of • hexagonal and G cubic MTS from this work. 3.4 Influence of different parameters on mechanical stability: wall thickness, Al content, pore size For a same pore size of 35 A, an aluminium-containing MTS was found as stable as a pure siliceous MTS, as shown in Table 2. Aluminium does not seem to weaken the structure as suggested in literature [5]. MTS with different wall thicknesses were prepared according to Coustel et al. [7] who have shown that decreasing the alcalinity of the synthesis gel increased the wall thickness. Different MTS with wall thicknesses between 8.6 A and 14.5 A were synthesized, but no differences in stability were observed (Table 2). Wall thickness in this range of size does not seem to be a crucial parameter and does not explain the difference of stability between our synthesis and literature results. Probably the field of variation of porosity was to small (±25%) (Table 2) to induce significant changes of stability. MTS with larger pores (80 or 110 A) were synthesized using TMB as swelling agent and compared with MTS of 35 A pore size. These solids presented a much larger porosity and were found to be much less stable than the medium-pore solids of this work (Table 2). Increasing the wall thickness by decreasing the alcalinity decreased the void fraction of about 30% but not enough to improve the stability.
672 Table 2 Features of different hexagonal MTS with different wall thickness (e) and pore diameter (D) synthesized with or without Al, and different alcalinity. Initial pore volume (VQ) and surface area (SQ), pore volume (Vi.6) and surface area (Si.6) for samples compressed at 1.6 kbar. Samples HI* H2* H3* H4 H5 H6* H7*
Si/Al 30 pure Si pure Si 30 30 30 30
D Ik 35 35 35 35 35 80 110
e Ik 8.6 11.7 11 14.5 9 11 15
OH/Si02 0.26 0.26 NH3 0.10 0.40 0.26 0.10
Vo /mlVg 0.70 0.52 0.62 0.54 0.81 1.86 1.45
So Infilg 950 730 820 720 1100 970 660
VL6
/ml7g 0.62 0.46+ 0.56 0.43 0.61 0.99 0.72
S1.6 /m2/g 890 670+ 750 640 940 835 560
V1.6/V0 0.86 0.88+ 0.90 0.79 0.75 0.53 0.50
+Sample H2 compressed at 2.7 kbar instead of 1.6 kbar. *Samples synthesized under stirring. H3 synthesized from ref [8]. 4- CONCLUSION The difference of mechanical strength among the different samples showed that the same label, MCM-41 or MCM-48, covers very different materials. The large differences observed among the solids tested by different research groups suggest that usually overlooked properties like defect concentration, grain size, orientation and aggregation, can significantly affect the mechanical strength. The wall thickness, the aluminium content or the geometry of the pore system and the atmosphere of tests are less critical parameters than expected. Only large variations of the porosity have a clear influence on the stability of the pore system. The mechanical stability of MTS is good enough for major industrial applications like extrusion or chromatography. For spectroscopic studies or for other applications needing pelletization at high pressure, care should be taken to check that no modification of the porosity and the nature of the solid had taken place. Testing MTS in real applications is just beginning and the successful preparation of stable custom-tailored materials can strongly influence the future of MTS in the industry. REFERENCES 1- C. T. Kresge et al., Nature, 359 (1992) 710. 2- J. S. Beck et al., J. Am. Chem. Soc, 114 (1992) 10834. 3- V. Y. Gusev et al., J. Phys. Chem., 100 (19%) 1989. A- T. Tatsumi et al., Chem. Lett., (1997) 469. 5- K. A. Koyano et al., J. Phys. Chem. B, 101 (1997) 9436. 6- A. Galameau et al., Microporous and Mesoporous Materials 27 (1999) 297. 7- N. Coustel et al., J. Chem. Soc, Chem. Commun., (1994) 967. 8- F. Di Renzo et al., Microporous Materials, 10 (1997) 283.
Studies in Surface Science and Catalysis 129 A. Sayari et ai. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
673
Structural Analysis of Hexagonal Mesoporous Silica Films Produced from Triblock-Copolymer-Structuring Sol-Gel D. Grosso*", A. R. Balkenende ^ P. A. Albouy ' and F. Babonneau' ^ Laboratoire Chimie de la Matiere Condensee, Universite Pierre et Marie Curie, 4 Place Jussieu, 75252 Paris Cedex 05, France. E-mail :
[email protected] * ^^ Philips Research Laboratories, Prof. Holstlaan 4, 5656 AA Eindhoven, Netherlands. ^Laboratoire de Physique des Solides, Universite Paris-Sud, 91405 Orsay Cedex, France. The increasing interest in mesoporous materials in various fields of application has motivated the investigation of novel high-technology thin films with pore size, structure and organisation controlled by the presence of directing agents. The design of optical coatings made by this method is described in this work. Silica thin films with Id-hexagonal mesoporous structure were deposited on silicon and glass substrates from a silicate precursor solution, containing a non-ionic triblock copolymer (EO106PO70EO106 : F127) as structuring agent. 70 to 700 nm thick films, with an excellent optical quality, were prepared by single dipcoating at a constant withdrawal rate. The surfactant was removed from the coatings by various treatments (i.e. thermal treatment, washing, ultra-sonic bath or sohxlet extraction). XRD investigations showed that shrinkage or collapsing of the film network can take place upon removal of the surfactant. Profiles of the relative amounts of surfactant, present in the pores, were measured by Rutherford Back Scattering. Refractive index, thickness and porosity of untreated and treated coatings were deduced from ellipsometry measurements. Each treated film exhibits an uni-axial anisotropy. The highest porosity (46%) and the lowest structural degradation are obtained for samples pre-treated at 400 K before extraction of the surfactant by sohxlet technique, as a result of the rigidification of the silica matrix. 1. INTRODUCTION In the last decade, the design of nanostructured mesoporous materials has gained an increasing interest for a variety of applications (i.e. separation, catalysis, encapsulation, chemical sensing, low-dielectric coatings, optics). Many efforts are made to produce mesoporous systems with large pores and high accessibility for potential encapsulation of polymers, chelated metals, ions or for grafting functional groups [1]. On the other hand, such high porosity would be suitable for the design of low dielectric constant films, and antireflective coatings. Low reflective thin films can be made with various ratios of tetraethosysilane (TEOS) to methyltriethoxysilane (MTES) [2], leading to refractive indices down to 1.27, and pore dimensions between 30 and 120 nm. However, these are not
674 structured nanomaterials. The most common method used to produce ordered materials consists mainly in adding a certain amount of ionic or non-ionic surfactant to a solution of silicate precursor (or any other material precursors) prior to condensation. At a specific concentration, a particular surfactant mesophase is formed (e.g. hexagonal, cubic or lamellar) and the rigid network condensed around the surfactant micelles upon solvent evaporation. This gives rise to materials containing organised cavities of homogeneous dimension filled up with the organic templating phase [3-4]. The removal of this latter phase leads to structured mesoporous materials with accessible pores. For the processing of films, the initial solution is usually very diluted and the formation of the mesophase occurs via evaporation-induced selfassembly [5]. Hexagonal, cubic and lamellar silica materials with pore size, ranging from 15 to 35 A, have been produced with ionic surfactants such as cetyltrimethylammonium bromide or chloride (CTAB or CTAC), or double headed gemini quaternary ammonium salts, with various alkyl chain lengths [6-10]. They were performed as bulk and thin films. It is also possible to produce mixed meso/macroporous materials, using a multiphase separation technique [11]. Recently, templated materials, synthesised with non ionic block copolymer surfactants (e.g. EOxPOyEOx), have shown a high degree of organisation, a high porosity and the presence of larger pore dimensions (20 to 300 A for bulk materials, and 20 to 100 A for films) [12-15]. The use of these large organic molecules to organise inorganic porous matrices is one of the most promising routes in terms of high porosity, large pore diameters, high degree of ordering, low cost and low toxicity involved. Surfactants are usually removed by a thermal treatment that could entirely or partly destroy the network structure, thus reducing the porosity and decreasing the degree of organisation [16]. In addition, the removal step is generally accompanied by a pore shrinkage due to further condensation of the silica matrix. Kundu et al. also observed a gradual shrinkage during pyrolysis of mesoporous silica films prepared with cetyltrimethylammonium chloride (CTMACl) [16]. Certain coated systems may not accept high temperature often required for the decomposition of ionic surfactants (e.g. 573 K for CTAB). In this respect, the advantage of using EO106PO70EO106 lies in the fact that its thermal decomposition starts already at 423 K, a temperature at which many systems do not suffer too much from annealing. In the present work, we have analysed the structure of Id-hexagonal mesoporous silica films, deposited by dip-coating on silicon wafers and glass plates. Initial solutions were prepared via acidic hydrolysis-condensation of TEOS in ethanol in the presence of Pluronic F127 (EO106PO70EO106). Coatings were treated according to various procedures before being analysed by RES (Rutherford Back Scattering), VASE (Variable Angle Spectroscopic Ellipsometry), LAXRD (Low Angle X Ray Diffraction) and TEM (Transmission Electronic Microscopy). 2. EXPERIMENTAL A prehydrolysed solution was prepared by refluxing at 342 K for 1 h an ethanolic solution, containing TEOS, water and hydrochloric acid with the following molar ratios: TEOS: EtOH: HCl: H2O, 1: 3: 5 10"^: 1. The amount of F127 was dissolved in ethanol and added to the prehydrolysed solution together with the additional water and HCl. The final solution was then stirred for 24 h at room temperature before deposition. The typical final molar ratios were TEOS: EtOH: HCl: H2O: F127, 1: 20: 0.004: 5: 0.005. In order to favour
675 the deposition of homogeneous coatings with good adhesion, glass and silicon substrates (typically 20mm x 70mm) were successively washed with 2M HNO3, H2O, EtOH and dried after rinsing with acetone. Thin films with excellent optical quality were then deposited by dip coating at a constant withdrawal rate. Varying the deposition rate and the dilution allowed for the obtention of thickness, ranging from 70 to 700 nm. The layer thickness at the edge of the samples was about 20% less than at the centre. In order to create porosity by removing the organic phase, different treatments were applied to the dried coatings. TGA analysis performed under O2 on corresponding bulk xerogels show exothermic phenomena from 430 K to 500 K, suggesting that the EOxPOyEOx surfactant decomposes within this temperature range. In respect to this observation, the following treatments were used: as-prepared coating, washed in ethanol at 298 K, thermally treated at 430 K in air for I h, thermally treated at 400 K in air for 1 h + sohxlet extraction in hot ethanol for Ih, thermally treated at 400 K for 1 h + ultrasonic extraction in ethanol for 30 min, calcined at 620 K in air for 1 h (ramp at 10 K / min). Coating structures were deduced by LAXRD in reflection mode with a Philips diffractometer, using the Cu Ka source. Sample F (calcined at 620 K in air for 1 h) was analysed by TEM (JEOL 100 CX II apparatus) after the film was scratched off the substrate, redispersed in ethanol and deposited on a carbon coated copper grid. Ellipsometry measurements were carried out with a Woollam VASE instrument. Optical properties were deduced by the fitting of the obtained \|/ and A values, measured between 400 and 1400 nm, for 53°, 56°, and 59° incident angles. The model used for fitting consisted of a glass substrate with a porous SiOi film on top. The thickness and the refractive index of the top layer were the only fitted parameters. The refractive index was varied by assuming a certain volume of voids in silica (using a Bruggeman effective medium approximation with a depolarization of 1/3). It should be noted that within certain boundaries the effect of varying the pore volume is fully correlated to varying the depolarization factor (which is related to the pore geometry). In the case of uni-axial anisotropy the refractive index was split into an in-plane and a normal component. The areal amount of C, Si and O in the films were determined by the depth profiling RBS technique, using a 2.5 MV Van de Graaf accelerator and a normal incident beam of 2 MeV "^He"" ions. The energy of the back scattered He"" was analysed using a silicon surface barrier detector positioned at 10° from the incident beam. The spectra are given as normalized yield (i.e. counts at the detector per mC He dose) versus energy. Spectra were quantified by simulations, using a stack of layers with varying amounts of the relevant elements. 3. RESULTS The as-prepared coating (A) and those treated via B, C, D, E, and F methods exhibited good uniformity and good optical quality. 3.1 Low angle X-ray diffraction XRD patterns of the as-prepared and treated coatings are shown in Figure 1. The asprepared film (Figure la) gave an intense diffraction peak at 20 = 0.72 ° and a much less
676 intense one at 26 = 1.41 °. These were indexed as the {100} and the {200} Bragg peaks, attributed to a Id-hexagonal structure of lattice parameter a = 140.9 A with rod-like micelles aligned parallel to the substrate surface. C, D and F treated coatings (Figures lb, Id, and If) exhibited the {100} Bragg peaks with reduced inter-planar distances, resulting of the network shrinkage. Coatings treated via methods B and E (Figure le) showed no organisation by XRD, suggesting that the Id-hexgonal organisation was lost after such surfactant removal steps. The direct removal of F127 via washing with ethanol at 298 K (B) induced a dramatic collapsing of the network attributed to the low degree of condensation.
I
as-prepared
100 \
Figure 1: XRD patterns obtained for asprepared and treated F127/Si02 Id-hexagonal structured films; (a): as-prepared film (d{100} = 122 A) , (c): film thermally treated at 430 K under air for 1 h (d{100} = 98 A), (d): film thermally treated at 400 K under air for 1 h followed by extraction in hot ethanol for Ih (d{100} = 76 A), (f): film calcined at 620 K in air for 1 h (d{100} = 70 A), (e): film thermally treated at 400 K for 1 h followed by ultrasonic extraction in ethanol for Ih (no organisation).
200
*50
(a)
100
430 K (Ih)
300
"r'Hfc,v- '.
*200
400 K + sohxlet *50
>.VM>t,^.^
620 K (Ih) *50
••^"V.VA.^
400 K + Ultrasound
0.5
1.0
1.5
2q/°
2.0
2.5
*50
3.0
A thermal pre-treatment at 400 K or higher was performed to improve the network rigidity and therefore to make it more prone to accept the surfactant removal without inducing nanoscopic change distortion. Such a thermal pre-treatment did not destroy the organisation but was accompanied by an increase of the {100} peak intensity (Figure Ic), and by the appearance of both the second {200} and third {300} order harmonic peaks, indicating that the organisation had become better defined after heating 1 h at 430 K. However it was accompanied by a shrinkage expressed by the ^{100} reduction of 20%. In spite of the rigidification, the so pre-heated coating lost its organisation upon ultrasonic extraction of F127 in a bath of ethanol (Figure le). The mechanical agitation may have been too strong, inducing collapsing of the network. On the other hand, when this pre-heated coating was posttreated by sohxlet extraction in hot ethanol, the hexagonal phase was retained and a shrinkage corresponding to 38% of ^{100} was observed (Figure Id). Calcining the coating directly at 620 K did not alter the organisation of the pores and induced a shrinkage of 43% of
677 d{ 100}(Figure If). Diffraction peaks present for A, C, D and F coatings are characterised by linewidth broadness (A6), measured at half maximum intensity, smaller than 0.07 °, suggesting that the average dimension of the organised domains are quite large and remained so after treatment. By applying the Scherrer formula [17] for AO = 0.07 ° = 1.22 10'^ rad., one would end up with an average dimension of organised domains of t = (0.9 X) I {AO cos 6) = 1150 A. As this value is just above the limit for the use of such a relation (i.e. t < 100 nm), the extend of organisation is taken to exceed 100 nm. 3.2. Transmission electronic microscopy Micrographs of the film calcined at 620 K (F) are shown in Figure 2. The patterns look like finger prints described as swirling patterns by Brinker et al. [5]. Both pictures represent pore channels aligned in the same plane with no evident preferred direction, characteristic of a Id-hexagonal structure. Figure 2b represents a domain containing two distinct distances between channels, measured as Ly = 122 A and L2 = 82 A. In Figure 2a, the domain contains only one periodic distance between channels measured to be Lj = 80 A. (a)
Figure 2: TEM micrographs of two different pieces of a calcined at 620 K coating, scratched from the substrate and redispersed in ethanol (sample F).
678 3.3. Ellipsometry The optical properties of the films deposited on silicon wafers were deduced from ellipsometry measurement and are given in Table 1. The thickness non uniformity in the measurement area was less than 3.5 %. The as-prepared coating has a high refractive index (i.e. 1.48 at 550 nm) close to that of dense SiOi, suggesting that the pores are filled with the organic phase. After treatment, both the refractive index and the film thickness decrease due to the combined effect of surfactant removal and network condensation. The measured data could not be described correctly assuming an isotropic coating. In the present cases, \|/ and A plots recorded for all treated samples were best fitted when assuming an uni-axial anisotropy. In other words, once the surfactant is removed, films show birefringence with the lowest refractive index (n) measured in the direction normal to the substrate surface and the highest one (n') found in the plane of the surface (see Table 1). Geometrical effects probably account for the anisotropy of the samples. The main axis of the rod-shaped F127 micelles is randomly oriented in the plane of the surface, leading to different in-plane and normal refractive indices. The lowest refractive index of n = 1.241 was obtained with the D treated film, followed closely by n= 1.264 measured on the F treated film. Film (C), that was only thermally treated at 430 K, exhibits a refractive index of n = 1.402, indicating a porosity of about 10 % explained by a partial removal of the organic phase (TGA on bulk xerogel showed a gradual removal of F127 at 430 K in air, that is total after 5 h). The coating that underwent the treatment E is thinner and denser as a result of structure collapsing in agreement with the lack of organisation shown in XRD. The pore volumes, given in Table 1, are obtained from the refractive indices, assuming an effective medium approximation with spherical voids in the silica matrix. Note that a different geometry and orientation of the pores may lead to values of the pore volume that differ by about 5%. 3.4. Rutherford back scattering RBS depth profile analysis of the A- and D-treated samples are shown in Figure 3, where lines correspond at the maximal energy from which the C, O, and Si atoms at the surface are detected. 50
i240 c
Figure 3: RBS plots obtained with the (A) asprepared (a) and the D-treated (d) coatings.
3 O
1V
^
0
Si
O20 <jn
E
(a)\ W
0 10
120
420
720
Energy/KeV
1020
1
1320
679 Coating A consists of a layer containing Si, O and C, while in the case of coating D, only Si and O are present on top of a silicon substrate (the areal amounts obtained from simulations are listed in Table 1), implying that the organic phase is effectively removed. The distribution of Si, O, and C atoms is uniform, indicating that no composition gradient exists in the layer and that the distribution of organic domains was homogeneous within the coatings. For the asprepared sample, the composition ratio C/Si = 2.9 close to the value of 3.17 calculated from the concentrations used for the initial solution. Taking into account the initial ratio 0/C = 0.44, calculated in F127, and the fact that the silicate network should not be fully condensed at this stage (0/Si > 2), the areal density of O found for the untreated coating by RBS should be superior to 0.44*C + 2*Si = 1200. In the present investigation, the areal density of O is much smaller than this value (O = 780). The difference may be induced by the He^ bombardment: in certain cases, He^ ions can transfer enough energy to break covalent bonds, leading to decomposition of especially organic materials. Table 1: Values of interplanar spacing (d{ 100}), film thickness (h), refractive index at 550 nm (n and n'), porosity (Vp), areal density of atoms (±5 %), obtained for as-prepared and treated films by XRD, ellipsometry and RBS. (A: as-prepared coating, B: washed in ethanol at 298 K, C: thermally treated at 430 K in air for 1 h, D: thermally treated at 400 K in air for 1 h + sohxlet extraction in hot ethanol for Ih, E: thermally treated at 400 K for 1 h -i- ultrasonic extraction in ethanol for 30 min, F: calcined at 620 K in air for 1 h (ramp at 10 K / min)).
Treatment A C D E F
d{100} h /A /nm 122 736 98 514 76 458 371 70 462
Normal to surf. n Vp/% 1.479 1.402 1.241 46 14 1.386 1.264 41
In-plane of surf. Vp' / % n' 1.417 42 1.261 9 1.413 36 1.287
RBS/ xlO^^ atoms cm'^ C 0 Si 1055 780 367 355 0 739 -
No carbon was recorded for the D-treated film. The 0/Si composition ratio was found to be 2.08 and is attributed to the extent of condensation as the organic phase has been removed completely. Based on the amount of Si for sample D and assuming a density of 2.3 g cm"^ for amorphous Si02, the top layer would correspond to a thickness of 154 nm, if a dense layer is assumed. As the actual layer thickness is 458 nm, this would imply a porosity of 66%. Here a considerable discrepancy with the porosity obtained from ellipsometry is evident. In this respect it should be noted that the RBS measurement was done more to the edge of the sample than ellisometry, where the thickness is smaller than in the centre. Further, the refractive index determined with ellipsometry is very accurate. However, the relation of porosity with refractive index depends on the model used. 4. DISCUSSION Both XRD and TEM investigations show that the incorporation of F127 (ratio F127/Si = 0.005) in a prehydrolysed solution of TEOS leads to the formation of highly ordered composite films, when deposited by dip coating. Once F127 is removed, the pores consist of channels aligned preferentially parallel to the substrate surface in random directions. Peaks
680 appearing in the XRD patterns could suggest that the structure is lamellar, but TEM pictures show no lamellar organisation, confirming the fact that pores are channels arranged in a characteristic Id-hexagonal structure. 4.1. The origin of the structure anisotropy From the results obtained by XRD, ellipsometry and TEM on the calcined (F) coating, one can deduce the structural change occurring during heat treatment. In the present condition of XRD investigation, only planes parallel to the surface of the substrates diffracted. Supposing that the single peaks, appearing in the pattern (Figure la and If), correspond to d{ 100}, a profile of the film structure in the {001} planes could be modelled as in Figure 4. In the untreated coatings (Id-hexagonal organisation), one expects |a| = |b| = (4/3) x ^{100} = 141 A, and close neighboured pores are therefore all separated by this unique distance |a|. la'l = L2JbM=L7
di20
OOQQp 620 K As prepared coating
Calcined mesoporous coating
Figure 4: Model of the nanopore structure (cut through a (001) plane), evolution of the inter pore distances with treatment at 620 K, involving shrinkage in the direction normal to the substrate surface. After Calcination at 620 K (treatment F), the reduction of ^{100} to 70 A is due to the condensation-induced shrinkage of the structure. As a result of the adhesion forces existing at the interface substrate/film, the shrinkage in the direction normal to the substrate surface is expected to be greater than the in-plane shrinkage. In the present case, such an uni-axial shrinkage would lead to a considerable reduction of d{ 100}, but only to a slight alteration of d{l20} (see Figure 4). This suggests that pores can now be separated by two different characteristic distances depending on the direction. Indeed, two distances were observed on the TEM micrographs and were measured as Ly = 122 A and L2 = 82 A. Ly and L2 are attributed to the distances separating the pore channels in the {hOO} planes and in the {OkO} planes respectively. Therefore, L; = |b'| and L2 = |a'|. From this deduction, d'{\00] is calculated as ^'{100} = (L2^-(L/)^)'^ = 55 A. The difference between both d'{lOO] values, deduced from TEM and observed by XRD, is probably due to the measurement uncertainty often met with micrographs of mesoporous oxides (the measurement could be underestimated because of focus conditions, electronic bombardment effects, and tilting of the structure with respect to the imaging electron beam). For instance, Weindenhof et al. reported a d-spacing that was 20 % lower by TEM than by XRD for mesoporous Ti02 and Zr02 materials [18].
681 Another way of confirming the hypotheses of uni-axial shrinkage is to compare (h-h') /h = 0.37 and {d{lOO} - d'{100}) /d{lOO} = 0.43. Both reduction of thickness and inter-planar distance are almost similar and therefore consistent with a shrinkage occurring mainly in the direction normal to the surface, that led to the refractive index anisotropy recorded for the F treated samples. The same anisotropy was observed by ellipsometry for the C, D and E treated films, suggesting that the uni axial shrinkage applied for each treatment. 4.2. Influence of the treatments We have seen that heating the coating at 430 K for 1 h (treatment C) led to a highest degree of organisation by XRD, and a noticeable reduction of the thickness. This structural improvement may be due to the softening of F127, combined with the simultaneous condensation of the network. A partial removal of the surfactant also occurs as a porosity of around 10 % is measured after treatment. For the D-treated coating, the thermal pre-treatment was fixed at 400 K to prevent the partial removal of F127. After extraction in hot ethanol, the silica coating was highly, but not fully, condensed as the ratio of the areal densities of oxygen to silicium atoms, measured by RBS, was found to be 2.08. In addition, the D treated coating exhibited no carbon content, the highest porosity (e.g. 46 %), and no alteration of the structure. Hence, pre-treatment at 400 K is beneficial in the sense that it strengthens the network so that the removal of the surfactant can take place without dramatic damage. For the E-treated film, the extraction in hot ethanol was replaced by ultrasonic extraction in an ethanol bath for 30 min. This method led to thinner and denser coatings with no XRD organisation. In spite of the rigidification of the network, this treatment was not appropriate to obtain organised porosity with the present system. The direct calcination of the coating (treatment F) led to an organised mesoporous condensed structure with a cfflOO} and a porosity of 40 %, slightly lower than for the D-treated coating. 5. CONCLUSION For the presently studied TEOS/F127 system, the calcination at 620 K treatment is advantageous over the other ones as it is a straight forward technique, inducing simultaneously the total removal of the organic phase, the condensation of the network, and the formation of organised porosity. However, it could be applied only to films that are deposited on substrates accepting such high temperatures. On the other hand, the treatment D method involves a less disturbing temperature for the system and leads to slight less shrinkage of the structure. Both methods produce coatings with an organised mesoporosity close to 40% and a uni-directional anisotropy characteristic of birefringence. Also, both treatments lead to organised domains exceeding 100 nm. Acknowledgement The authors would like to acknowledge the support of Y. Tamminga and H. Snijders of Philips CFT for the RBS analyses. BASF is greatly thanked for providing the Pluronic F127 surfactant. REFERENCES 1. 2.
K. Moller and T. Bein, Chem. Mater., 10 (1998) 2950. K. Makita, Y. Akamatsu, A. Takamatsu, S. Yamazaki and Y. Abe, J. Sol-Gel Sci. and
682
3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
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Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
6o3
On structure/property-relations in nanoporous semiconductors of the cetineite-type U. Simona), J. Jockel^), F. Starrostb), E. E. Krasovskiib), W. Schattkeb), B. Marler^), S. Schunk^), M. Wark^), H. Wellmanne) ^) Institut fiir Anorganische Chemie, Universitat GH Essen Schutzenbahn 70, D-45127 Essen* b) Institut fiir Theoretische Physik, Universitat Kiel Leibnizstr. 15, D-24118 Kiel c) Institut fur Mineralogie, Ruhr-Universitat Bochum, D-44780 Bochum d) Institut fiir Anorganische Chemie, Johann Wolfgang Goethe Universitat Frankfurt, Marie-Curie-Str. 11, D-60439 Frankfurt/Main e) Institut fiir Angewandte und Physikalische Chemie, Universitat Bremen, D-28334 Bremen The optical properties of cetineite-type chalcogenoantimonates, a new class of nanoporous semiconductors, are investigated experimentally and theoretically. By varying the chemical composition of the synthetic isotype structures we find a chemical trend in the optical gaps ranging from 2.0-2.4 eV. The cetineites display a larger gap for Na-" than for K^ as well as a larger one for S2- compounds as compared to Se^-. In newly synthesized mixed phases with variable Na:K-ratio the band gap depends on the Na.K-ratio. This gives an insight into structure/property-relations and shows that the optical properties can be tuned chemically in this novel class of nanoporous semiconductors.
1. INTRODUCTION In the search for a new class of nanomaterials, i.e. crystalline nanoporous semiconductors, a lot of efforts are made to synthesize new compounds with a zeolite-like open framework structure consisting of typical semiconductor elements like As, Sb, Se, etc.^i-^] jj^ 5^995 Wang and Liebau reported the preparation and structure of oxoselenoantimonates (III) with a zeolite-like channel structuret^-^^, which are related to the natural mineral cetineite.^'^l This mineral with composition (K;Na)3+x(Sb203)3(SbS3)(OH)(2.8-x)H20 with x - 0.5, first described in 1987 by Sabelli and Vezzalinit^], was found in Le Cetine mine in Tuscany, Italy. The cetineite-phases have the general formula A6(Sbi20i8)(SbX3)2(6-mxy)H20x(Bn^^(0H)m)y, with A = Na^ K^ Rb^ Sr2^ Ba2^ X = S2-, Se2-, and B = Na^ Sb3^ CO32-.
684 In the course of our studies we use the abbreviation (A;X) for the A-cation and Xanion position (see table 1). In contrast to zeohtes, i. e. the traditional molecular sieve materials which are electrical insulators, it has been shown that cetineites are crystalline nanoporous materials with a photosemiconducting host lattice^^l The present paper reports on the optical properties of the phases with A = Na, K and X = S, Se from an experimental and theoretical point of view.
1.1 Crystal structure The crystal structures, Fig. 1, have space group symmetry P63 or PGa/m. Tubes of composition [Sbi20i8] are formed by linking [SbOa] pyramids. The electron lone pairs of their Sb(2) and Sb(3) atoms are perpendicular to the tube walls. The tube arrangement can be described as an hexagonal rod packing. Single [Sb(l)X3]3- pyramids are located between the tubes. Their lone pairs are oriented parallel to the tube axes. The interior of the tubes, whose free diameter is approximately 0.7 nm, may be occupied by chains of face-sharing [H20]6 octahedra or by [(Na,Sb),(OH, H20)6] octahedra extended along the tubes. Figure 2 shows transmission electron micrographs of the regularly arranged channel structure of (K;Se) of the a x b plane (a) and of the a;b x c plane (b).
Table 1 Chemical composition and abbreviation of cetineite-phases Chemical composition abbreviation (A;X) Na6(Sbi20i8)(SbSe3)2(Nai,86Sbo,i4)((OH2.28(H20)4.o2)
(Na;Se)
Na6(Sbi20i8)(SbS3)2(Nai.2(OH)i.2(H20)4.8)
(Na;S)
K6(Sbi20i8)(SbSe3)2(H20)6
(K;Se)
K6(Sbi20i8)(SbS3)2(Sbo.i4(OH)o,42(H20)5.6)
(K;S)
685
Figure 1. Structure of (Na;Se): projection onto the a x b plane, the channel filling ions and water molecules are left out in the plot. For (K;Se) it has been shown by means of infrared spectroscopy that the water can be removed at room temperature under high vacuum within a few secondsf^l In (Na;Se), (Na;S), and (K;S) the center of the [(H20,OH)6] octahedra is partially or fully occupied by Na+ and/or Sb^-" ions in the D-position in statistical disorder.
Figure 2. Transmission electron micrograph of (K;Se) showing the a x b plane^^^ (a) and the a;b x c plane (b).
686 The lattice constants of (Na;Se), (Na;S), and (K;S) are a;b = 14.423(3), 14.152(3), and 14.318(3) A, and c = 5.565(1), 5.5758(7), and 5.633(1) A, respectively. (K;Se) exhibits a 2x2x1 superstructure with spacegroup symmetry P63 and lattice constants of a;b = 29.260(7) and c = 5.6164(7) A.^^.e] 1.2 Preparation The preparation of cetineite phases is based on a hydrothermal reaction of elementary Sb, Se or S, H2O and NaOH or KOH. In the case of (Na;Se) and (K;Se) the presence of CH3NH2 or CH3(CH2)2CH(CH3)NH2 as a template is needed, but in both cases the template is not incorporated into the structure, which is proven by elementary analysis. The educts are charged in a 25 ml teflon bottle, sealed, and heated in a steel autoclave for 3-4 days at a temperature between 473 - 493 K. The batch contains well-formed single crystals besides large formations of smaller intergrown crystals and polyphase material. In the case of (Na;Se) ruby red single crystals with a maximum length of 2 mm and a hexagonal cross section of 3.1 10-2 mm^ are obtained. Crystals of the other phase range from 1 - 1.2 mm in length. The products are characterized by powder X-ray diffraction data (Siemens D 5000) in the range of 20 = 5° - 65°. All crystals are large enough for the electrical and optical measurements, and they are stable in air, water, methanol, ethanol, and acetic acid.
2 OPTICAL PROPERTIES To determine the optical band gap UV/Vis transmission spectra between 220 and 750 nm (1.5 eV - 2.95 eV) of single crystals are measured at room temperature by use of a micro spectrophotometer Leica MSP-SP. Figure 3 shows the UVA/'is spectra of the four cetineite-phases (spectral resolution 1 nm). The absorption edge is determined from the intersection point of the energy axis and the extrapolated line of the linear slope of the transmission spectra. The ruby red colour of the (Na;Se) and (K;Se), and the orange red colour of (Na;S) and (K;S) correspond with their absorption edges. In comparison to the cetineites with X = Se the sulphur phases reveal a larger band gap. Changing the cation from A = Na+ to A = K"" a red shift is observed. This is a first attempt to tune the optical properties of the isostructural cetineite-phases, in a range of 0.35 eV, by varying the chemical composition.
687 80 70 £ 60-1
I ^0 1 J^
(K;S)\\
\
2.29eV \ ^
\
I 40 g 30 \
1
(K;Se) \ \(Na;Se) \2.i2eV 2.03eV \
\/
20
\
(Na;S) \
2.38eV
\
\
10 j 0 1.5
1.7
1.9
2.1 2.3 energy [eV]
2.5
2.7
2.9
Figure 3. Representative UVA^is-spectra of (Na;S), (K;S), (Na;Se) and (K;Se) single crystals, measured in transmission.
To verify whether the optical excitation energy of the photoconductivity corresponds to the optical band gap, the conductivity is measured at simultaneous variation of the wave length of the irradiating light. A 300 W xenon lamp combined with a lattice monochromator, which has a spectral resolution of 10 nm, is used. Figure 4 shows the current at 10 V measuring voltage along c versus the photon energy of the irradiating light in the range of 1.55 - 2.95 eV (200 - 800 nm). The onset of photoconductivity is determined from the intersection point of the X-axis (dark current) and the extrapolated line from the linear part of the conductivity. All spectra show a pronounced onset of photoconductivity at 2.40 eV (Na;S), 2.25 eV (K;S), 2.15 eV (Na;Se), and 2.10 eV (K;Se), respectively, which is found to be in remarkable agreement with the optical gap. This proves that the photoconductivity is a bulk property instead of a surface conduction phenomenon. The theoretical description of the electronic structure has been obtained by means of the LAPW method on an ab-initio basis^i^] The electronic potential is determined self-consistently for the elementary cell of the bare host structure, which consists of 44 atoms. More complicated systems, where the tubes are filled with water molecules are also taken into account. Recent self-consistent fullpotential calculations (FLAPW) are performed to refine the results^iil.
688
1.5
2.0
2.5
3.0 3.5 4.0 e n e r g y [eV]
4.5
5.0
1.5
2.0
2.5
3.0 3.5 4.0 e n e r g y [eV]
4.5
5.0
1.5
2.0
2.5
3.0 3.5 4.0 e n e r g y [eV]
4.5
5.0
1.5
2.0
2.5
3.0 3.5 4.0 e n e r g y [eV]
4.5
5.0
Figure 4. Photocurrent at 10 V along c direction vs. photon energy of the irradiating Hght.
The band structure and the optical extinction coefficient are calculatedf^^] i^ local density theory the gaps may be underestimated as usual, and the direct band gap, in contrast to the high maximum of the dielectric function, which we take to represent the optical band gap, is significantly smaller than the experimentally obtained values. Disregarding this uncertainty in the interpretation of the absolute values the chemical trend itself shows remarkable agreement (Table 2) with the experimental results. It follows the ionisation energy of the alkali atoms, which is larger for sodium than for potassium. The larger gaps for the sulfur compounds as compared to the selenium ones reflect a trend, which is met in various covalently bound crystals of II-VI or III-VI compounds and most probably indicates the strength of the Sb(l)-X binding.
689 Table 2 Experimental and theoretical optical gaps for cetineites with A = Na+, K"*^ and X = S2- Se2-
Phase Na;S K;S Na;Se K;Se
exp. optical band gap (eV) 2.38 2.29 2.12 2.03
exp. onset photoconduction (eV) 2.40 2.25 2.15 2.10
theo. optical band gap (eV) 2.18 1.98 1.77 1.62
3 (Na,K;S) CETINEITE MIXED PHASES New cetineite phases with variable K/Na ratio are synthesized by hydrothermal reaction of elementary Sb and S in an aqueous mixture of NaOH and KOH following the same experimental route. Orange-red single crystals with a maximum length of 1.2 mm and a hexagonal cross section of 910-2 inin2 are obtained. The K/Na ratio is determined from single crystals via EDX (Zeiss DSM 950/Tracor Voyager II). Comparison of the potassium content of the cetineite crystals to the composition of the synthesis mixture shows a preferred incorporation of potassium into the structure. Although this compound with 50 atoms per unit cell is a highly complex structure, it seems to follow the empirical Vegard's rule similar to homogeneous mixed crystals and for intermetallic phases of the Laves type as well as to microporous tin(IV)thioselenides of type SnSi-xSex-i.^^^^ As it was expected the optical gap follows the chemical composition in the same way as the lattice constants. Thus, the optical properties of cetineites can be tuned to a certain extent. Figure 5b shows the dependence of the optical band gap on the lattice constants of the cetineite crystals.
4 CONCLUSIONS In conclusion we showed how the optical excitation energies in isostructural cetineites, the experimental ones as well as the theoretical values, depend on the chemical composition. Based on this, in mixed phase (Na,K;S) the band gap can be tuned by varying the Na:K-ratio. These results give an insight into the structure/property-relation and show that the optical properties can be tuned chemically in this novel class of nanoporous semiconductors.
690 lattice constant a;b [Ajo
14.45 + O o< 14.40
5.65^
14.22
(Na;S)
O 14.25
+ 5.50 o
0)
. | 14.20
5.45 '^
14.15 • 14.10 20
—I— 40 60 80 mole % K
5.40 100 5.35
14.32
1-
2.39-
>
I
a 2.37I * M> +5.55 t« K>. C
14.27
— h -
1
,5.60 ^ 1K;S)5
^ 14.30-1-
-
14.17
b)
5.70
Si 'ci 14.35
S
14.12 2.41-
5.75
14.50
73
C 2.35a
S
• 11
"g 2.33•4^
a ® 2.31. 2.29•
I 1 —
5.60
I 1-
5.65
K
5.70
5.75
lattice constant c [A]9
Figure 5. Dependence of lattice parameters a;b and c on the composition (a) and dependence of the optical band gap on the lattice constants a;b and c (b) of the cetineite mixed phases (Na,K;S). ACKNOWLEDGEMENTS This work was supported by the "Deutsche Forschungsgemeinschaft" under contracts SI 609/2-1 and SCHA-360/14-1. The TEM image in Fig. l b was taken by Th. Sawitowski, which is gratefully acknowledged. REFERENCES [1] G. A. Ozin, Adv. Mater., 4 (1992) 612 [2] R. L. Bedard, L. D. Vail, S. L. Wilson, E. M. Flanigen, US Patent No. 4,880,761 (1989) [3] W. Sheldrick, M. Wachhold, Coord. Chem. Rev, 176 (1998) 211 [4] M. G. Kanazidis et al., J. Am. Chem. Soc, 117 (1995) 1294 [5] F. Liebau and X. Wang, Beih. z. Eur. J. Mineral, 7 (1995) 152 [6] X. Wang, Z. Kristallog., 210 (1995) 693 [7] C. Sabelh, G. Vezzalini, N. Jb. Miner. Mh, 9 (1987) 419 [8] C. Sabelh I. Nakai, S. Katsura, Amer. Mineral, 73 (1988) 398 [9] U. Simon, F. Schuth, S. Schunk, X. Wang, and F. Liebau, Angew. Chem. Intern. Ed. Engl, 36 (1997) 1121 [10] F. Starrost, E. E. Krasowskii, W. Schattke, J. Jockel, U. Simon, X. Wang, and F. Liebau, Phys. Rev. Lett., 89 (1998) 3313 [11] F. Starrost, E. E. Krasowskii, W. Schattke, J. Jockel, U. Simon, (unpublished results) [12] U. Simon, J. Jockel, F. Starrost, E. E. Krasovskii, W. Schattke, Nanostructured Materials, Proceedings of NANO'98 (1999), 447 [13] G. Ozin, in L.U. Interante, L. A. Casper, A. B. Ellis (Eds.), Materials Chemistry - An Emerging Discipline, Advances in Chemistry Series 245, ACS, Washington DC (1995)
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
691
Structural and textural properties of zinc(II)-chromium(III) spinel oxides prepared using a hydrotalcite-like compound E.L. Crepaldi,^'^ P.C. Pavan," W. Jones*' and J.B. Valim' ^Depto. de Quimica, Faculdade de Filosofia, Ciencias e Letras de Ribeirao Preto, Universidade de Sao Paulo, Av. Bandeirantes, 3900, Ribeirao Preto, SP, 14040-901, Brazil* ^Dept. of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1E W, UK Spinel oxides are important industrial catalysts, possessing good thermal and chemical stability and an ability to maintain catalytic activity at high temperatures. The study reported here is concerned with the properties of Zn(II)-Cr(III) spinel oxides prepared from a carbonate containing LDH precursor. For this study the spinel oxide was separated, by treatment with dilute mineral acid, from the oxide mixture (ZnO and ZnCr204) obtained by the thermal decomposition of LDH. For comparison, Zn(II)-Cr(III) spinel oxides were also prepared by more standard synthesis methods. Structural and textural properties were evaluated, using PXRD, TG/MS, FT-IR and N2 adsorption.
1. INTRODUCTION Spinel oxides with a general formula AB2O4 (i.e. the so-called normal spinels) are important materials in industrial catalysis. They are thermally stable and maintain enhanced and sustained activities for a variety of industrially important reactions including decomposition of nitrous oxide [1], oxidation and dehydrogenation of hydrocarbons [2], low temperature methanol synthesis [3], oxidation of carbon monoxide and hydrocarbon [4], and oxidative dehydrogenation of butanes [5]. A major problem in the applications of this class of compound as catalyst, however, lies in their usually low specific surface area [6]. Numerous methods of spinel synthesis can be found in the literature. Amongst these, the preferred method is based on the solid-state reaction of a mixture containing M " 0 and M'^203 in the appropriate ratio [6,7,8]. The mixture is heated to temperatures generally higher than 800 °C in order for the spinel structure to be formed. Another method is based on the precipitation of both cations, from their nitrates or chlorides, as a mixed hydroxide or a mixed organic salt (e. g. citrate, oxalate) [6,7,9,10]. In this case, the very good dispersion of phases containing the metal cations improves the formation of the spinel oxide, although high calcination temperatures are nevertheless frequently used (> 800 °C). Layered double hydroxides (LDHs), with a hydrotalcite-like structure, are a class of materials which have received considerable attention in the last decade. The structure of LDHs is based on the stacking of metal cation hydroxide (brucite-like) layers, with a positive charge on the layers resulting from the isomorphous substitution of some of the bivalent
692 cations by trivalent. To maintain electroneutrality an appropriate number of anions, usually hydrated, occupy the interiayer space. A general formula representing this class of materials is •nH20. A wide variety of metal cation combinations, as well as a wide variety of anions, have been reported. The M'VM"' ratio can vary in a large range, reportedly from 1 to more than 10, although values from 2 to 4 are more common [11,12,13]. The thermal decomposition of such materials, at temperatures high enough to produce the metal oxides, usually results in two well-dispersed phases: M " 0 and M^M"'204 [14,15]. Exceptions include cases involving thermally stable anions (i.e. sulphate, polyoxometalates, etc intercalated anions) when other phases are formed [16,17]. The formation of spinel oxides from LDHs occurs at relatively low temperature [18], as a result of the very good dispersion and the metal cation proximity in the hydrotalcite-like layer [19]. Specifically for Zn-Cr-COsLDHs, there are three important studies concerning thermal decomposition. The first one was that of Lai and Howe [20], following the thermal decomposition of Zn-Cr-LDHs intercalated with carbonate, chloride, bromide, fluoride and nitrate. No structural or chemical characterisation of the thermal decomposition products has been reported, however. Fuda and co-workers [21] reported a more complete study, with special attention to the structure of the thermal decomposition products, as well as to the oxidation of Cr(III) species during heating treatment. These authors observed the formation of a spinel oxide at temperatures above 400 °C, the crystallinity of which increased with temperature up to 700 °C, the highest temperature studied, del Arco and co-workers [17] presented the most complete study, with the textural properties of the thermal decomposition products also investigated. The results showed that the specific surface area increased with temperature from room temperature to about 230 °C, when it reaches a maximum of approximately 140 m^ g'', decreasing quickly thereafter to around 60 m^ g*' at 400 °C. Moreover, extensive characterisation of both structure and composition of the oxide phases was reported, including X-ray absorption and TPR measurements. Although the thermal decomposition of Zn-Cr-COs-LDHs has, therefore, been studied in detail, a specific characterisation of the actual spinel oxides obtained from the LDHs, separated from the mixed oxide phase, has not been reported either for Zn-Cr-LDHs or for other cation combinations. Although the structure of the spinel oxides formed from LDHs has been reported occasionally, the effect of separating the spinel phases from the whole thermal decomposition product in the properties of the spinel oxide has not been evaluated. Here we report on the properties of the spinel oxides produced by the thermal decomposition of a ZnCr-COs-LDH. For comparison, spinel oxides phases were synthesised by the two other methods described above, and treated in a similar way to those obtained by LDH decomposition.
2. EXPERIMENTAL 2.1. Synthesis of Zn-Cr-COa-LDH precursor Zn-Cr-COs-LDH was synthesised by an adaptation of the coprecipitation method described in the literature [11]. A solution containing 0.100 mol of Zn(N03)2-6H20 (Merck >99%) and 0.033 mol of Cr(N03)3-9H20 (Merck >98%) in 70 cm^ of water (3:1 Zn:Cr ratio) was added in a solution containing 0.300 mol of NaiCOs (Merck > 99.9%) in 280 cm^ of water under vigorous stirring. A hydrothermal treatment using a stainless steel reactor vessel Parr 4522
693 was used to improve the crystallinity of the LDH. The treatment was performed at 100 °C for 18 hours, at 3 bar, adjusted with nitrogen, after which the solid material was separated by filtration, washed with water, and dried under vacuum in the presence of activated silica gel. Chemical analysis gave Zn 42.5%, Cr 11.7%, C 1.4%, O 41.7%, and H 2.8%. 2.2. Synthesis of mixed hydroxide precursor Mixed hydroxide was prepared by a method previously described [9]. A mixture of 10% metal nitrate aqueous solution with a Zn:Cr ratio of 0.5 was prepared. This solution was heated to 70 °C, while a 5% ammonia (Merck) aqueous solution was added drop wise under constant stirring, with the pH maintained at approximately 7.0. The mixture was digested for another 2 hours at 80 °C in order to complete the precipitation. The precipitate was filtered, washed and dried in air at 110 °C for 12 hours. Chemical analysis yielded Zn 21.4%, Cr 34.1%, O 41.9% and H 2.6%. 2.3. Synthesis of ZnCr204 from solid solution of ZnO and Cr203 To prepare the spinel oxide by solid-state reaction of a mixture of the individual metal cation oxides, the following procedure was used [8]: a mixture of powdered ZnO (Aldrich >99%) and Cr203 (Aldrich >99%) (0.5 Zn:Cr ratio), was carefully grounded for 10 minutes in order to obtain a good dispersion of the compounds in the solid mixture. This mixture was then heated at 900 °C for 12 hours in a N2 atmosphere. 2.4. Thermal treatment of layered double hydroxide (LDH) and mixed hydroxide (MH) Thermal treatment for both materials was performed in a tubular horizontal ftimace, in a N2 atmosphere, for 2 hours at the defined temperature. The heating rate was 5 °C min"', and the samples were removed from the ftimace after cooling to room temperature. Samples were treated at temperatures from 100 to 1200 °C, in intervals of 100 °C. 2.5. Post-treatment with mineral acid (HCl) All samples were submitted to post-treatment with mineral acid. The purpose of the acid post-treatment was to eliminate the ZnO phase in the materials obtained from the thermally treated LDH and a similar procedure was also performed on the other samples for comparison purposes. Hence, 400 mg of each sample was added under stirring to 50 cm^ of a 1 mol dm'^ HCl (Merck) solution at 25 °C under stirring. After a contact period of 2 hours, the residual solid material was separated and washed, centrifuged, and dried at 120 °C for 12 hours. 2.6. Characterisation PXRD measurements were performed using a Siemens D5005 X-ray diffractometer, with graphite monochromator to select Cu Ka radiation, in 2-70° 20 range, with step of 0.02° s ^ Simultaneous TG/MS (thermogravimetry coupled with mass spectroscopy) analysis was performed using a Polymer Laboratories PL-TGA 1500 coupled to a Leda-Mass Mini Lab mass spectrometer by a quartz capillary transfer Hne. The line was heated to 120°C. The ramp rate used was 10 °C min"^ with a N2 flow rate of 25 mL min"\ The TGA apparatus was at atmospheric pressure, and the mass spectrometer at a working pressure of 6x10'^ torr and an electron energy of 70 eV. Specific surface area and average pore size were determined by adsorption of nitrogen at -196 °C, using the BET and BJH methods, respectively. The samples were outgassed at 110 °C for 1 hour before the measurements. Seven adsorption and six desorption points were acquired for each sample. Fourier-transform infrared (FT-IR)
694 spectra were recorded on KBr pellets (a 2% solid solution) with a Nicolet 205 instrument, with 80 scans per sample. Carbon, nitrogen and hydrogen amounts were determined by elemental analysis using a Perkin-Elmer Elemental Analyzer 2400 CHN instrument. Water content was determined by TG analysis with a heating rate of 1 °C min"' using a TGA/DTA simultaneous analyser from TA Instruments. Atomic absorption spectroscopy was used to determine Zn and Cr contents, using a Varian AA-175 instrument. 3. RESULTS AND DISCUSSION 3.1. Synthesis, composition and thermal behavior of LDH and MH The PXRD patterns for both the LDH and MH are shown in Figure 1. The LDH showed a PXRD pattern with very intense and sharp peaks, characteristic of a very well ordered hydrotalcite-like compound. The basal spacing obtained was 7.68 A, very close to the reported data [17,20,21] for carbonate containing LDH. From the chemical analysis we obtain the following molecular formula: Zn2 9Cr(OH)7 8(C03)o5-2.3H20 (normalized to Cr = 1). The Zn:Cr ratio obtained, 2.9, was slightly lower than the one expected, 3, indicating a preferential solubilisation of Zn(II) cations, a feature widely reported in the synthesis of LDHs [11]. The mixed hydroxide (MH), however, gave no significant PXRD reflections. The identified formula for this compound was Zni oCr2(OH)8 o (normalized for Cr = 2), with a Zn:Cr ratio of 0.5, exactly as expected, showing the complete precipitation of the metal cations.
20
30
40
50
60
70
26/degrees
Figure 1. PXRD diagrams for (a) Zn-Cr-MH and (b) Zn-Cr-COs-LDH (Miller indices following [12]).
695 a
1001
90-
g
\
00-
b
----H,0
\ /'
C0,(5x)
1
----H,0
II
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/. /
1 801
\
\ ^ .'!
'.
SSN'^
8070^
^ ~T
200
400
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r—
600
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1
800
TemperaturerC
'
1
1—
1000
1200
r
1
1
1
600
.'
1
800
'
1 —
1000
1200
Temperature/°C
Figure 2. TG/MS data acquired in Ni atmosphere for (a) Zn-Cr-COs-LDH, and (b) Zn-CrMH. Figure 2 shows the TG/MS data for both the obtained precursors materials. For LDH (Figure 2a), only water and carbon dioxide were detected. The data indicated that carbonate decomposed in the 200-400 °C range, with the first peak for water loss, around 180 ° attributable to the loss of interlamellar water. After which the decomposition of the layer hydroxyl groups commences and continues up to approximately 700 °C. At temperatures higher than 700 °C the mass remains almost constant, showing the complete conversion to oxides. In the thermal treatment of the MH (Figure 2b), only water was detected. The decomposition occurred from room temperature to 700 °C, after which the mass was kept constant showing the complete conversion to the oxides. 3.2. Structural characterisation of the thermally treated materials The PXRD pattern for the spinel oxide prepared by solid-state reaction of a ZnO-Cr203 gave an a parameter for the cubic unit cell of 8.308 A (the average of all a parameters calculated by the formula a = ^(h^ + k^ + /^)'^^ for each observed reflection), close to the reported value of 8.3275 A [22]. The PXRD pattern for this compound showed intense and sharp peaks, indicating high crystallinity. Post-treatment with acid did not show any significant change in the PXRD pattern. Thermal treatment between 100 and 200 °C of the LDH resulted in a progressive reduction in crystallinity and basal spacing. At 300 °C the material obtained was amorphous, showing no significant reflection in the PXRD pattern. From 400 to 1200 ° C two phases were observed, ZnO and ZnCr204 [22], (see Figure 3 for selected PXRD patterns). The crystallinity of these phases increased with temperature. It was clearly observed that the formation of these oxides coincided with the end of carbonate decomposition (see Figure 2a). The FT-IR spectra (data not shovm) indicated that the materials treated at 400 °C or higher were carbonate free, since the band at 1360 cm"^ characteristic of carbonate, disappeared. Moreover, the spectra showed a reduction in the intensity of the band around 3400 cm'\ attributed to the layer hydroxyl groups and water. This band disappeared at 700 °C. As shown by the FT-IR data, materials treated ft-om 400 to 700 °C retained residual hydroxyl groups. No reflections for
696 phases containing these anions were present in the PXRD patterns, indicating that these phases were very well dispersed in the structure of the obtained oxides. These results are in agreement with the reported data for the thermal decomposition of Zn-Cr-COs-LDHs [17,20,21]. As can be seen from Figure 3, treatment with mineral acid completely eliminated the ZnO phase from the obtained materials, resulting in a pure spinel oxide phase. The mixed hydroxide precursor (MH) was treated at 500, 900 and 1200 °C. Again a progressive increase in crystallinity was observed with the increase of the temperature. FT-IR spectra showed a band around 3400 cm"* for the sample treated at 500 °C, which was absent in samples treated at 900 and 1200 °C. The explanation follows that given for the LDH system. As observed for the spinel oxide prepared by the solid-reaction method the treatment with mineral acid did not affect the PXRD pattern.
c 0
29/degrees Figure 3. PXRD patterns for materials obtained by the thermal treatment of the Zn-Cr-COs-LDH. • ZnCr204 and O ZnO. TA means post-treatment with mineral acid.
697 8,35
8.33
900 °C), and submitted to acid post-treatment, did the a parameter fall to values obtained for materials prepared by the other methods. Vaccari and co-workers reported that the size of the unit cell of spinel oxides obtained by heating of LDHs is dependent on the M " : M " ' ratio, the greater the ratio, the greater the unit cell [15]. This behaviour was attributed to the increasing of the relative amount of M(II) cations in the material, resulting in a non-stoichiometric spinel oxides. The additional M(II) cations occupy octahedral sites in the structure, yielding structures intermediate between the spinel and rock-salt, with a predominant character of spinel. The results described above agree with these observations. The decreasing in the a parameter with the temperature observed here can be attributed to the progressive segregation of Zn(II) cations from the structure, combined with the loss of hydroxyl groups (at intermediate temperatures, from 400 to 700 °C). The decreasing of the a parameter with acid post-treatment indicated that the octahedral sites occupied by Zn(II) cations in the spinel structure were accessible to acid attack. It is possible, therefore, to see that the synthesis of spinel oxides through a LDH precursor (in the used Zn:Cr ratio) conducted to nonstoichiometric compounds, with an excess of Zn(II) cation in its structure. Moreover, this excess of Zn(II) can be partially (or even totally) removed by a post-treatment with mineral acid, as indicated by the evolution of the unit cell a parameter.
698 a 80-
R \
—•—LDH —O— LDH TA —A—MH —A—MHTA • ZnO-Crp, ° ZnO-CrjO,TA
\ \ \
60-
°-——-_
E m 40-
20-
^^V ^
^
^
b
v ^ ^ ^ ^ ^ = ^
0400
600
800
Temperaturerc
1000
32 600
800
1000
Temperature/'C
Figure 5. Change in the textural properties of the materials with the temperature: (a) specific surface area; and (b) average pore size. 3.3. Textural properties The evolution as a function of temperature of the specific surface area (SSA) and average pore size (APS) of the materials is shown in Figure 5. Materials obtained from the LDH show a reduction in SSA with the temperature as reported in the literature. This reduction can be attributed to the crystallisation of the material [15,17]. However, post-treatment with mineral acid was an increase in the SSA for all temperatures. It is possible to attribute this increase to two combined effects, which can both increase the porosity of the materials, as well as yield more active adsorption sites: (i) the elimination of ZnO; and (ii) the elimination Zn(II) cations occupying octahedral sites in the spinel oxide structure. Even though the SSA had varied sensibly, the average pore size (APS) remained fairly constant with temperature. Acid treatment increases the APS value for all temperature tested, although the effect was very small (Figure 5b). Comparison of the materials obtained by the different synthesis methods showed that spinel oxides obtained from the LDH presented greater SSA values than those obtained by other methods, principally after the posttreatment with mineral acid. On other hand, the treatment with acid had little influence on the textural properties of the spinel oxides obtained by the other methods. Comparing the SSA for materials obtained at 900 °C, it is possible to note that the spinel oxide obtained from a LDH precursor (after elimination of the ZnO) showed the greatest surface area. The value obtained, 24.5 m^ g'^ was even greater than the reported data for a similar material prepared from an MH precursor treated at 800 °C, 12.94 m^ g"^ [9]. The difference become more significant when we utilise for comparison the value obtained for the LDH treated at 800 °C (after a treatment with acid), 32.7 m^ g"^
4. CONCLUSION The results obtained in this work indicated that the use of a LDH as precursor to prepare spinel oxides is viable. Separation of the spinel from the oxide mixture can be achieved by a simple treatment with mineral acid. The materials obtained by this method present an excess of bivalent cations in the structure, due to the starting material possessing a M":M'" ratio
699 of bivalent cations in the structure, due to the starting material possessing a M":M'" ratio different to that of the spinel oxide. Moreover, it was shown that this excess was progressively segregated from the structure with the increasing of the temperature, as well as by the treatment with acid. Spinel oxides obtained by LDHs showed greater specific surface area than those prepared by the classical methods at the same calcination temperature. The segregation of the Zn(II) from the structure, as well as the elimination of the ZnO increase both the SSA and the APS. Oxides obtained from an LDH or a MH precursor at relatively low temperatures (from 400 °C) present very high specific surface area for a spinel oxide (reaching more than 80 m g" ). Although these materials have low crystallinity, they present chemical stability (since these materials were not decomposed by the post-treatment with acid, pH = 0) and are thermally stable at least to the temperature of preparation. Therefore, these materials may be very usef 4 in catalytic applications.
ACKNOWLEDGEMENT The authors wish to thank the Brazilian agencies Funda9ao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP, processos 96/06030-1 e 96/12373-9), Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq/PADCT) and Funda9ao Coordena9ao de Aperfei9oamento de Pessoal de Nivel Superior (CAPES) for financial support. The authors also thank Prof Dr. M. Rosolen for the N2 adsorption measurements.
REFERENCES 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14.
C. Angeletti, F. Pepe, P. Porta, J. Chem. Soc, Faraday Trans., 74 (1978) 1595. V.I. Fedecva, I.D. Voinov, Kinet. Katal., 19 (1978) 625. V.Yu. Prudnikova, React. Kinet. Catal. Lett., 14 (1980) 413. A.T. Baricevic, B. Brbic, D. Jovanovic, S. Angelov, D. Mehandziev, C. Marinova, P. Kinilov-Stefanov, Appl. Catal., 47 (1989) 145. J.R. Hightower, Chem. Eng. Educ, 16 (1982) 148. M.R. Tarasevich, B.N. Efremov, in Studies in Physical and Theoretical Chemistry 11, S. Trasatti (Ed.), Elsevier Scientific Publishing, Amsterdam, 1981, chapter 5, p. 221. C.N.R. Rao, B. Raveau, Transition Metal Oxides, VCH Publishers, New York, 1995, Part III, p. 289. G.C. Allen, M. Paul, Appl. Spectroscopy, 49 (1995) 451. N.J. Jebarathinam, M. Eswaramoorthy, V. Krishnasamy, Bull. Chem. Soc. Jpn., 67 (1994) 3334. W.F. Shangguan, Y. Teraoka, S. Kagawa, Appl. Catal. B, 8 (1996) 217. W.T. Reichle, Solid State Ionics, 22 (1986) 135. A. de Roy, C. Forano, K. el Malki, J.-P. Besse, in: Synthesis of Microporous Materials vol. II, M. L. Occelli, H. E. Robson (Eds.), Van Nostrand Reinhold, New York, 1992, chapter 7, 108. T. Lopez, P. Bosch, E. Ramos, R. Gomez, O. Navaro, D. Acosta, F. Figueras, Lagmuir, 12 (1996)189. W.T. Reichle, S.Y. Kang, D.S. Everhardt, J. Catal, 101 (1986) 352.
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15. 16. 17. 18. 19. 20. 21. 22.
F. Cavani, F. Trifiro, A. Vaccari, Catal. Today, 11 (1991), 173. V.R.L. Constantino, T.J. Pinnavaia, Inorg. Chem., 34 (1995) 883. M. del Arco, V. Rives, R. Tmjillano, P. Malet, J. Mater. Chem., 6 (1996) 1419. T. Hibino, Y. Yamashita, K. Kosuge, A. Tsunashima, Clays Clay Miner., 43 (1995) 427. M. Vucelic, W. Jones, G.D. Moggridge, Clays Clay Miner., 45 (1997) 803. M. Lai and A.T. Howe, J. Solid State Chem., 39 (1981) 368. K. Fuda, K. Suda, T. Matsunaga, Chem. Lett., (1993) 1479. JCPDS-ICDD, PDF Database, 1996, No. 36-1451 (ZnO); 22-1107 (ZnCr204).
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
701
New porous composite material - characterization and properties A.N.Scian*, M.Marturano* and V.Cagnoli** CETMIC: Cno. Centenario y 506 - (1897) C.C.49 Gonnet - Bs. As.- Argentina •CETMIC-CONICET-UNLP **CINDECA-CONICET-UNLP ABSTRACT TEOS, phenol-formaldehyde resin and ethyl alcohol were mixed and gelled at room temperature. Afterward, the gel was dried and submitted to thermal treatment at high temperature (1550 "C) in a strongly reducing atmosphere and transformed into a composite of general formula Si02-C. This composite consists on an amorphous porous silica network intercrossed with another carbon network that has carbonaceous microdomains of high activity; this new material has a sharpened pore size distribution near 1000 A diameter mainly assigned to the silica network. Both mentioned networks are self supported and independent among them, so one from the other can be isolated without losing the original shape and volume of the starting composite; then this new material has the property of generating two other different porous solids. The new composite was characterized by: LOI, XRD, IR, BET surface area, pore size distribution and SEM. Its adsorption rate was tested with methylene blue solutions by using different techniques, and the results were compared with those obtained for a commercial high performance activated carbon. Experiments with different gelling and drying temperatures were performed in order to control the range in which the composite shows the sharpened pore size distnbution. The catalytic activity was tested and compared on FISCHER-TROPSCH process (hydrogenation of CO) on doped iron composite and on doped iron isolated carbon network. Other potential uses of the new composite are presented. 1. INTRODUCTION The development of new porous materials that could be used as adsorbents, catalysts, catalyst supports, molecular sieves, etc. [I], are very well discussed by several authors [2-9], describing interesting properties and characteristics of materials such as: MCM-41, MCM-48, M41S, FSM16, lamellar phases, intercalation products, special CMS (carbon molecular sieves), fullerenes, carbon nanotubes, etc.; being some of them silica based materials, and carbon based the others. Respect to the silica based materials, researchers focused their attention on the control of the microstructure, on the facility of the obtainment process, and especially in the development of materials with open pores bigger than 100 A diameter.
702
On the other hand, forty years ago a lot of properties of carbon based materials coming from pyrolytic processes were well known [10-14]; but it was in the last years where new kinds of CMS, fullerenes and carbon nanotubes [15-17] were discovered and studied. These materials have the limitation of their small open pore size, which in general is near 10 A diameter, and then, diffusional and esteric problems may be present when they must interact with molecules of equal or bigger size within the mentioned pore diameter. In general, both types of materials (silica and carbon based) have different technological applications, mainly, by their different pore size range and by their different physicochemical surface properties. This work is about the development of a new porous composite material [18] which consists of a silica network intercrossed with a carbon network of high activity, being both networks independent and self supported. This new composite was obtained by the sol-gel method, it has an open pore size distribution curve characterized by a big sharpened zone near the 1000 A diameter, and it possess a high thermal stability. 2. EXPERIMENTAL 2.1. Synthesis 40 g of TEOS (Tetraethylortosilicate) (SILBOND 40: AKZO Chemicals Argentina) were stirred in a flask together with 20 g of fully liquid phenol-formaldehyde resin (72-76 % wt/wt residual solids after polymerization, 8-12 % water and 0.4-1.2 % free formol) (RL-628; ATANOR Argentina). Enough quantity of commercial ethanol (44 ml) was added during stirring until the emulsion became clear (only one phase). The clear solution was then kept at room temperature in a sealed flask until a clear gel is produced (48 h). Then the gel was dried during several days at room temperature and cured at 180 "C until the resin setting. The solid so obtained was introduced in two different sealed refractory crucibles and immersed in a coke bed. One of the crucibles was submitted to the following thermal treatment in an electric furnace: 20-1000 X at 5T/mm. heating rate, followed by 3 h soaking time at this temperature, obtaining a porous solid composite called SC-100. The other crucible was treated in equal conditions but varying the final temperature to 1550 T , obtaining in this case another porous composite called SC-155. The SC-155 showed a little volumetric expansion respect to the SC-100 material. (The names SC-100 and SC-155 means: S=^ silica; C^ carbon; and the number is the treatment temperature in T/10). The SC-155 material calcined in oxidizing atmosphere at 1000 T and until constant weight produced a self supported porous silica network (the carbon was burnt out) which was called S-155; this silica structure maintained the same shape and volume as the original composite. On the other hand, the SC-155 treated with 20 % HF aqueous solution until the elimination of Si02 produced a self supported carbon network called C-155, and this carbon structure maintained the same shape and volume as the original composite. 2.2. Characterization The new composite (SC-155) and some of its precursors and derivatives were characterized by: LOI (loss on ignition), XRD ( X ray diffraction), IR (infrared spectra), BET specific surface area, nitrogen adsorption desorption isotherms, pore size distribution (mercury porosimetry), dynamic methylene blue adsorption and SEM (Scanning Electron
703
Microscopy). Different conditions of gelling and drying the precursor were tested, and the corresponding pore size distributions of the materials so obtained were evaluated. Finally, catalytic activity was tested and compared on Fischer-Tropsch process (hydrogenation of CO), on doped iron composite (Fe/SC-155) and on doped iron isolated carbon network (Fe/C-155). The reaction parameters were: temperature = 270 T ; R(H2/CO)=3 and pressure =^ latm. In all the cases a commercial activated carbon of high activity called AC-ref (Activated Carbon- reference) was used as a standard of comparison; an exception is the Fischer-Tropsch test, in which the reference was called CON (Conventional Activated Carbon) by its different characteristics with respect to AC-ref 3. RESULTS AND DISCUSSION As the TEOS partial hydrolysis (this case) forms partially ethoxylated complex silicic acid, and after condensation (gelling) develops a coherent gel structure based mainly on hydrated silica, it suggest that the liquid phenol-formaldehyde resin will be trapped in the gel as intricate and connected high viscosity liquid thin films. These two systems will perform independently during drying, curing, and firing in strongly reducing atmosphere, resulting in a porous material composed by a silica network intercrossed with a carbon network. The obtained S-155 and C-155 materials confirms this hypothesis. The loss on ignition for the SC-155 material was 26.34 %, while the value for AC-ref was 96.80 %; then the composite showed 3.67 times less carbon than the reference. Infrared spectrum of SC-155 showed exclusively the characteristic bands corresponding to silica. X ray diffractogram of SC-155 showed a big band centred at d = 4.07A assigned to the amorphous silica, and other two bands (d = 3.48 A and d "= 2.07 A centred) assigned to carbon pseudo structure. (XRD bands are observed instead of peaks when amorphous phases or short order atomic arrangement are present). The AC-ref sample instead showed only two bands centred at the same values observed for the SC-155 carbon bands (3.48 and 2.07 A). The 3.48 A band has an intermediate value between the corresponding to the graphite basal plane -3.34 A- and the turbostratic carbonaceous structures, that according with Foley et al. [5] is 3.82 A. The 2.07 A band corresponds to the 2.03 A calculated by Foley for the inplane carbon-carbon bond, indicating then that the carbonaceous structure of both materials would be associated with an expanded graphitic structure, but in the case of SC-155 the carbonaceous structure is accompanied by an amorphous silica structure. The specific surface area measured through BET method at liquid nitrogen temperature for AC-ref, SC-100, SC-155 and S-155 are shown in Table 1. This table shows that the value obtained for AC-ref is higher than that obtained for the SC-155 sample, and the corresponding SC-155 value is higher than those obtained for SC-100 and S-155. The first comparison is related with the carbon level of the samples, but the second one (SC-100 and SC-155) is related with the expansion of the carbon microdomains because of the higher treatment temperature. The low value observed for S-155 denotes that the carbon network contributes with the major part of the specific surface area. Figure 1 shows the adsorption-desorption isotherms of AC-ref, SC-100 and SC-155 materials, showing all of them a hysteresis loop that corresponds to the H3 type [19], and formed by very wide pores having narrow short openings, or by pores formed by parallel
704
Table 1 BET specific surface area (m^/g) for AC-ref; SC-100; SC-155 and S-155 AC-ref SC-lOO SC-155 BET Specific Surface Area 745 143 196 (mVg) _
S-155 51
plates at some distance like graphite structure. The curve corresponding to AC-ref shows the greatest amount of adsorbed nitrogen due to its higher carbon content (as it is observed in LOI test). The thermal treatment of SC series did not altered the pore shape, but when the synthesis temperature was increased from 1000 to 1550 "C the adsorbed volume was increased, showing more increment with the increment of p/po. Then, from the point of view of this technique, the increase of the synthesis temperature in the SC series produced an increase in the total pore volume because of the expansion of carbon microdomains. Figure 2 shows the cumulative pore volume vs. pore radius for AC-ref; SC-100 and SC-155 obtained by mercury intrusion technique. The curve corresponding to AC-ref shows a wide pore radius distribution; instead, the curves assigned to SC-100 and SC-155 showed sharpened zones with maximum slope in 459A and 524A respectively, denoting a small increase of these values with the increase of the synthesis temperature. This phenomenon is probably produced by the growing of the big pores of the silica network at the expense of the
300-
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• • •• •
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Figure 1. Adsorption-desorption nitrogen isotherms for AC-ref; SC-100 and SC-155 Adsorbed volume (cmVg) vs. p/po .
705
'oi
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-
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Pore radius [A]
Figure 2. Cumulative pore volume vs. pore radius for AC-ref; SC-100 and SC-155 Mercury intrusion porosimetry. disappearance of the smallest ones as the temperature is increased; simultaneously the expansion of the carbon microdomains occurs. The shape of the curves below the 400 A radius is quite similar for the three samples, indicating that this region corresponds to the internal mesoporosity of the carbon network itself Then, the mentioned sharpened zone in the 500 A region corresponds to the meso-macroporosity developed between carbon network and silica network walls. It was mentioned that when a piece of SC-155 was calcined at 1000 T in oxidizing atmosphere, the obtained product was a white porous amorphous silica material with the same shape and external volume as the starting one, and when a piece of the same material was submitted to acid attack by 20 % HF aqueous solution, a self supported carbon network was obtained, also with the same shape and external volume as the original. These experiences show that the silica network and carbon network are self supported structures and independent one from the other. In Figure 3, the adsorption performance of AC-ref, SC-100 and SC-155 on 10 mg/1 methylene blue aqueous solution eluded through a bed of 100 mg of the materials is presented. Each test was performed with materials of the same granulometry and at the same flowing conditions (60 ml/h). Figure 3 shows that SC-100 has lower kinetic adsorption performance than AC-ref and SC-155, and that SC-155 has higher kinetic adsorption performance than the AC-ref in spite of its lower carbon level. As commercial activated carbons are obtained at temperatures near lOOOT, then AC-ref and SC-100 perhaps have quite the same texture of carbon microdomains, but as it is above mentioned the carbon level of the two materials is very different, then, this justifies the lower performance of SC-100.
706 10 9
• -SC.155 • -SC.100 A-AC.ref
300
600
Eluded ml [Methylene Blue 10 mg/l]
Figure 3. Eluded methylene blue concentration (mg/l) vs. ml of methylene blue 10 mg/l passed through a bed of 100 mg material (AC-ref; SC-100 and SC-155) at 60 ml/h. The case of SC-155 is different due to its more expanded silica and carbon microdomains, then, the access of the methylene blue molecules to the activated sites is enhanced respect to the other materials tested, producing the observed differences. In an experiment carried out with 100 mg/l methylene blue concentration the behaviour was the same as described before, but, there was a time in which SC-155 reached the saturation and the material stopped the adsorption; the AC-ref instead continued the adsorption at longer times due to its higher carbon contents. Then, the great difference in adsorption kinetics observed between SC-155 and AC-ref is justified by the more expanded structure of carbon microdomains of SC-155 than the reference, and by the higher radius of meso-macropores observed for the SC-155; the last point provides an easy access of molecules to be adsorbed into the grains of the material, minimizing diffusional problems. Figure 4 presents the SEM microphotographs of SC-155 [(A) scale bar 1mm and (B) scale bar 10|am ]; and, AC-ref [(C) scale bar 1mm and (D) scale bar lOjam]. Comparing the microphotographs (A) and (C) (SC-155 and AC-ref with low magnification respectively) it can be seen that the grains of SC-155 have a smooth texture and conchoidal fracture like glassy phases, while AC-ref shows a high rugosity surface. In microphotographs (B) and (D) (SC-155 and AC-ref with high magnification respectively) it is possible to observe the porosity of SC-155 material because the magnification of the photograph is in the order of magnitude of the macropores, instead, the microphotograph of AC-ref shows fractured grains with smooth texture because the order of magnitude of the carbon micropores is not in the range of the photograph magnification.
707
Figure 4. Microphotographs of: (A) SC-155 (scale bar 1 mm), (B) SC-155 (scale bar 10 ^m) (C) AC-ref (scale bar 1 mm), (D) AC-ref (scale bar 10 ^m) C-155 and S-155 materials showed a sponge like structure as the microphotograph of figure 4-B, but when S-155 was submitted to methylene blue adsorption test it did not show adsorption properties; C-155 material instead showed a high adsorption activity. The last observation demonstrates that carbon structure is the responsible of the adsorptive behaviour and that the silica structure in this case acts only as an inert skeleton, but at the same time, silica was the responsible of the expanded carbon network developed during the synthesis of the composite. There is a possibility of controlling the texture of this new composite by varying the precursor synthesis conditions, especially by the control of gelling and drying temperatures. This is another important aspect of this material to be taken into account Figure 5 shows the cumulative pore volume vs. pore radius for SC-155 gelled at 40 T and dried at different temperatures (20, 40 and 60 ''C). Symbols as "G40.60" mean: Gelling temperature 40 ""C, drying temperature 60 X; and so on. The figure shows that a change of the mean radius of the sharpened zone of the distribution curves is obtained with a change of the drying temperature. On the other hand, comparing the SC-155 curve of Figure 2 (the conditions were G20.20) with the SC-155 (G40.20) of Figure 5, it can be observed that the gelling temperature also modifies the mean radius of the sharpened zone.
708 1400
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400-
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a
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SC-155 -•—G40.20 -•— G40.40 -A— G40.60
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1000
10000
Pore radius [A]
Figure 5. Cumulative pore volume (cmVg) vs. pore radius (A) for SC-155: (•)G40.20; (•)G40.40 and (A)G40.60. The properties of these new materials as catalyst support were tested on FischerTropsch process (CO-H2 reaction) in a fixed bed differential reactor. Three materials were tested: a) CON, a conventional activated carbon; b) SC-155 (G40.60) and c) C-155 (G20.20). All of them were previously iron doped until 5% metallic iron wt/wt was reached. The test conditions were: Reaction temperature =270T; H2/CO ratio=3, pressure = latm. The main properties of the tested catalyst supports and their performance in the first hour test are shown in Table 2. SC-155 (G40.60) and C-155 (G20.20) were selected for this test in order to compare materials with near the same specific surface area but with different structural composition, and CON was selected because it is of common use and has very different texture characteristics respect to the other two materials. Table 2 shows that SC-155(G40.60) and C-155(G20.20), which have comparable values of specific surface area, have very different CO conversion values. Both materials were produced under different gelling and drying conditions, and as a consequence the C-155 precursor (SC-155/G20.20) was different in pore size distribution than the SC-155(G40.60), but that difference has not sense when silica was eliminated for to produce C-155, having the last one other new characteristics; then the observed differences in CO conversion are mainly attributed or to the higher value of specific pore volume of C-155, or because some of the metallic iron were on silica surface of the SC-155 diminishing its catalytic activity, but not attributed to the different gelling and drying conditions. CON material, in spite of its low specific surface area and its low specific pore volume is a fully carbon material like C-155 is, then its lower performance in CO conversion is attributed to the specific surface and pore characteristics.
709 Table 2. Properties and performance on Fischer-Tropsch process for: conventional activated carbon (CON), SC-155 (G40.60) and C-155 (G2Q.2Q) used as catalyst supports. Fe/CON
Fe/SC-155(G40.60)
Fe/C-155(G20.20)
Specific surface area [mVg]
22
464
494
Fe [%J
5
5
5
Specific pore volume Icm'/gl
0.67
1.26
6.00
CO conversion |%|
1.58
1.61
33.19
2.07x10'^
3.52 X 10'^
6.18 X 10'^
31
56
61
2.02
0.22
0.009
Hydrocarbon produced per g of catalyst in Isec. [mole/g s| CH4 /total hydrocarbons [%] olefins / paraffins
The high values of CH^total hydrocarbons ratio observed for C-155 and SC-155, suggest that when CO and H2 reach the catalyst active sites and the reactions proceed CH4 is kinetically the first product obtained, and it can leave immediately the catalyst due to the big dimensions of the catalyst pores, denoting a low diffusional resistance for this process. Respect to olefms/paraffms ratio, it can be mentioned that the n electron bonds present in the unsaturated hydrocarbons can interact with the surface electrons of the carbon microdomains; these surface electrons act as hydrogenation catalyst in the same way that Platinum surface electrons act in conventional catalytic hydrogenation processes. The observed values of olefms/paraffms ratio decrease from CON to C-155, and practically olefins are not present as a product when C-155 is used, suggesting for the new developed materials the presence of high electronic densities surrounding the carbon microdomains. The characteristics and properties of these new materials provide an interesting field of technological applications. A lot of work must be done to acquire a comprehensive knowledge about the processes to obtain particular textures and desired properties of these materials. 4. CONCLUSIONS • A new porous composite material with Si02-C generic formula and with sharpened pore size distribution in the range of 400-600 A radius is obtained. • The new composite consists of an amorphous silica network intercrossed with a carbon network of high activity. • The new material is obtained through a sol-gel method followed by a thermal treatment at very high temperatures, so the material has a high thermal stability.
710
• As an adsorbent, the new material has similar properties as the activated carbons but with better kinetic performance. • Both mentioned intercrossed networks are independent and self supported, and they can be isolated one from the other obtaining two other porous materials that can be applied to several uses (catalysis, environment, etc.). • The texture of the composite and its derivatives can be modified by the modification of the gelling and drying temperatures of the precursor. • The composite and its isolated carbon network tested as a catalyst support on FischerTropsch process shows big differences respect to the performance of a standard catalyst support. REFERENCES 1. Thomas J. Pinnavaia and M.F.Thorpe (eds.). Access in Nanoporous Materials, Plenum Press, New York, 1995. 2. S.B. McCullen, J.C. Vartuli, C.T. Kresge, W.J. Roth, J.S. Beck, K.D. Schmitt, M.E. Leonowicz J.L. Schlenker, S.S. Shih and J.D Lutner, ibid., pp M2. 3. P.T. Tanev and T.J. Pinnavaia, ihid., pp 13-28. 4. M.T. Anderson, J.E. Martin, J. Odinek and P. Newcomer, ihid., pp 29-38. 5. H.C. Foley, M. S. Kane and J. F. Goellner, ihid., pp 39-58. 6. C.J. Brinker, S.Wallace, N.K. Raman, R. Sehgal, H. Samuel and S. M. Contakes, ihid., pp 123140. 7. B.C. Dave, B. Dunn and J. I. Zink, ihid., pp 141-160. 8. G.CCoe,//?/J.,pp 213-230. 9. U. Cielsa, M. Grun, T. Isajeva, A.A. Kurganov, A. V. Neimark, P. Ravikovitch, S. Schacht, F. Schuth, and K.K. Unger, ihid., pp 231-240. 10. S. Mrozowsky, The nature of artificial carbons, Soc. of Chem. Industry (eds.), London, 1958. pp7-18. 11. D.J.E. Ingramand and D.E.G. Austen, ibid., pp 19-25. 12. A.F. Adamson and HE. Blayden, ibid., pp 28-35 13. A.R.G. Brown and W. Watt, ibid., pp 86-100. 14. F.H. Winslow, W. Matreyek and W.A. Yager, ibid., pp 190-196. 15. Staff writer. Techno Japan, 23 N^' 3 (1996) pplO-17. 16. E.J. Baran, Quimica de los FuUerenos, Exacta-La Plata (eds), Argentina, 1996. 17. M. Endo, S.Iijima and M.S. Dresselhaus (eds). Carbon Nanotubes, Pergamon, 1996. 18. A.N. Scian, Material Compuesto Poroso y su Procedimiento de obtencion. Argentine Patent under tramitation. H" P 98-0-06222. 19. S.J. Gregg and K.S.W. Sing, Adsorption Surface Area and Porosity, Academic Press London, 1982, p 303.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
711
Stabilized cluster formation of supercritical Xe in carbon nanopores
M. Aoshima*, T. Suzuki, and K. Kaneko Physical Chemistry, Material Science, Graduate School of Natural Science and Technology, Chiba University, Inage, Yayoi,m Chiba 263 -8522, Japan
The cluster formation of Xe molecules in a slit-shaped graphitic micropore was studied by Grand Canonical Monte Carlo(GCMC) simulation and adsorption measurement at 300K above the critical temperature of Xe using activated carbon fibers which have considerably unifoiTn micropores of slit-shape. The GCMC simulated and experimental adsorption isothemis indicated the presence of a strong Xe-Xe interaction inducing the cluster formation. We applied the cluster analysis to the simulated adsoiption isotherm for the pore width w=0.9nm. Just after the steep rising at 75.5kPa, a wide distribution of the cluster size was observed. It was shown that the growth of small Xe clusters provides the steep rising of the Xe adsoiption isotherm. The geometrical structure of Xe clusters was estimated from the simulated radial distribution function.
1. INTRODUCTION Recently various kinds of porous materials have been developed and their properties and structures have been gathering great concerns in science. There are two types of pores of intraparticle pores and interparticle ones[l]. The intraparticle pores are in the primary particle itself, while the interparticle pores originate from the interparticle void spaces. Zeolites are the most representative porous solids whose pores come from the structurally intrinsic intraparticle pores. The pore geometry can be evaluated by their crystallographic data. The carbon nanotube of which pore wall is composed of graphitic sheets is also the •Present address: Department of Intelligent Mathine System, Akita Prefecture University Ebinoguchi 84-4, Tuchidaniaza, Honnjyoushi, Akita, 015-0055, Japan
712
structurally intrinsic
intraparticle
adsorbents are obtained by
pore [2].
Activated carbons of the most popular
the activation reaction of carbon materials using H2O or CO2.
Also activated carbon has intrinsic intraparticle pores, although crystalline.
particles are not necessarily
On the other hand, silica gels and carbon aerogels are representatives of
interparticle porous materials.
Recently developed regular mesoporous silica should be an
itermediate between the intraparticle and interparticle porous materials. Because the pore wall is composed of fine primary particles to form a long range ordered structure having regular mesopores.
This regular mesoporous silica has given a great stimulation to a variety of
sciences. We have an excellent activated carbon of fiber morphology, so called activated carbon fiber ACF[3].
This ACF has considerably uniform slit-shaped micropores without mesopores,
showing characteristic adsorption properties. The pore size distribution of ACF is very narrow compared with that of traditional granular activated carbon.
Then, ACF has an aspect similar
to the regular mesoporous silica in particular in carbon science.
Consequently, we can
understand more an unresolved problem such as adsorption of supercritical gas using ACF as an microporous adsorbent. There are many important supercritical gases such as O2, N2, CH4, NO, and H2 which are deeply associated with energy, environmental, food, and medical technologies. understanding of supercritical gas adsorption is requested to support
Further
important technologies.
Ahhough vapor adsorption on micropores, which is called micropore filling, is considerably understood, micropore filling of a supercritical gas has not been studied
sufficiently
irrespective of its importance. This is because supercriUcal gas cannot be abundantly adsorbed in micropores[4]. Kaneko et al studied micropore filling of supercritical NO in micropores of iron oxide-dispersed activated carbon fiber, proposing the transition of supercritical gas to quasi-vapor in the micropore and supercritical Dubinin-Raduskevich analysis[5-7]. We need more fundamental understanding of supercritical gas adsorption. The crifical and boiling temperatures of the Xe gas are 289.6K and 165.9K,
respectively.
The size of a spherical Xe molecule is 0.396nm and the Xe-Xe interacfion energy is 217K[8,9]. Accordingly a considerable amount of Xe can be adsorbed in micropores above the critical temperature only by the dispersion interaction. However, no study on adsorption of supercritical Xe in carbon micropores was reported as for as we know, although Xe gas has a potential for important applications such as anesthetic and Xe lamp.
In the preceding
papers[10,l 1], we reported study on Xe clusters in a model graphite micropore using molecular simulation and empirical analysis of
Xe adsorption isotherms under supercritical conditions.
In this work, the relationship between micropore filling of supercritical Xe in micropores of ACF at 300 K and cluster size distribution by cluster analysis is described.
713 2. EXPERIMENTAL Pitch-based ACFs (P5, PIO, and P20) were used. The micropore structures of ACFs were determined by the No adsorption isotherm at 77K using the gravimetric method after the preevacuation of ACF samples at 383K. The Nj adsorption isotherm was analyzed by use of the subtracting pore effect (SPE) method for the a^-plot with the reference of the standard Nj adsorption isotherm of nonporous carbon black [3,12]. The high purify Xe gas (99.99%, Takachiho) was adsorbed on ACF samples at 300K after pre-evacuation of ACF samples at 383K and ImPa for 2 h. The Xe adsoiption isotherm at 300 K was also measured gravimetrically.
3. GCMC SIMULATION AND CLUSTER ANALYSIS The established grand canonical Monte Carlo simulation procedure was used. The random movement of molecules makes new configurations and they are accepted according to Metropolis's sampling scheme[13,14]. The pressure P for a chemical potential was directly calculated from the molecular density using GCMC simulation without the wall potential. The radial distribution function (RDF) was calculated. The intensity of RDF at a distance r was obtained from the average number of molecules which are coordinated at the distance between r and r+Ar (Ar = 0.0 Inm) for all Xe molecules in 1000 snapshots of the equilibrium state. It was divided by 27rr, because we analyzed only monolayer adsorption region. We used the 12-6 Lennard-Jones potential for the fluid-fluid interaction: 12
6~
(1)
't'A^„) = ^^tr
Here e^y and a^ are the Xe-Xe potential well depth and effective diameter. They are e^/k = 276.17K and a^ = 0.396mii[8,15]. r,j is the intermolecular distance. The interaction potential (t)^^ of a Xe molecule with a single graphite slab is given by Steele's 10-4-3 potential function[16]. Kfi^) = A
(cy.y 3A(0.61A + z)'
(2)
where A is 271(^/6 ./pA^ ^ is the vertical distance of the molecule from a graphite surface layer, p is the carbon atomic number density, A is the interlayer distance, and 8^^ and o^^ are fitted parameters of the Xe-carbon potential well depth and effective diameter, respectively, s^, and a3, were obtained with the use of the Lorentz-Berthelot rules. We used an established technique
714 of the slit-shaped unit cell in x and y directions[17-19]. The size of the rectangular cell was /X /Xw, where / and w are the unit cell length and slit width, respectively. The rectangular box is replicated two-dimensionally to form an infinite slit shaped micropore. Here, the w is not equal to the physical width of H, which is defined as the distance between opposite carbon atom layers, but w is the empirical slit width which is the pore width from the molecular adsorption experiment. The M> is associated with H by eq. 3 [20]. w = H - (2zo-aJ,
Zo-0.856a,,
where ZQ is the distance of closest approach, of w = 0.90 and 1.00 nm were calculated.
(3) hi this work, only model graphite model pores
In the cluster analysis, local molecular configurations of low energy in the equilibrium are presumed to be clusters.
The cluster distribution is obtained using the equilibrium snapshots,
when the following function F({n,}) is a minimum. We calculated F({n,}) and determined the cluster distribution using the Metropolis method.
f ({A7 • }) = U {{rn }) - TS{rn }) , ^(^^. j) ^ ^ ^ . ,
S({n,}) = k(lnN!-I In n ,!)
(4)
Here the number of molecules in the /th cluster is n„ the total number of molecules N, and the number of ways of allocating N molecules to a given partition {n,}.
Uj is the cluster
formation energy of the cluster j , which consists of nj molecules; Uj is the sum of each intermolecular potential.
S({n,}) is the allocation entropy of partition [21].
4. RESULTS AND DISCUSSION 4.1. Adsorption isotherms of Nj vapor and supercritical Xe Figure 1 shows adsorption isotherms of N2 at 77K on three kinds of ACFs. Although the equilibrium pressure of N2 in the adsorption isotherms is expressed by the logarithm scale, the N2 adsorption isotherms are of Type I, indicating the presence of uniform micropores. The N2 adsorption isotherm was analyzed by the a^-plot with the subtracting pore effect (SPE) method. Both a^-plots gave an predominant upward deviation below as=0.5 due to the enlianced adsorption. The micropore parameters from these ttj-plots for the N2 adsorption isotherms were determined by use of the SPE method, as given in Table 1. The theoretical basis for the SPE method was given by use of GCMC simulation in the preceding report [12]. Here, the pore volume was detemiined using the bulk liquid N2 density (0.808 g-ml"'). Consequently, the average pore width can be d e t e r m i n e d from t h e geometrical relation u s i n g t h e pore volume a n d surface a r e a . The average pore w i d t h of t h e s e ACFs is in t h e r a n g e of 0.75 to 1.05 n m , corresponding to the model g r a p h i t e pore.
715 1.00
-7
0.80
I
T3 4^
^ C 3 O
0.60
0.40
B
= 0.90 and 1.00 nm systems. The cluster analysis evidenced the presence of clusters in the snapshots, giving the cluster size distribution. Fig. 4 shows the histograms of clusters in both
718
pores
at
different
Xe
which correspond to the
pressures characteristic
points of the adsorption isotherm. In case of w = 0.90 nm pore system, before the rising of t h e isotherm (at 33.7 kPa), 62% of adsorbed Xe molecules
w=0.9nm
are monomers,
b u t we can fnid 27% of the dimers there; at 50.5 kPa, the percentage of Xe dimers is 30% and even 16% of the t r i m e r s are formed. J u s t after the steep rising at 75.5 kPa, there is the wide distribution of the cluster size; the molecular number in the cluster is in the r a n g e up to 12. At 113 k P a the percentage of t h e monomer drops to 23%, because small Xe clusters merge into
greater
above
the
clusters.
Therefore,
even
critical
temperature
in
w=1.0nm
micropores Xe molecules are associated with each other to form great clusters which can be a precondensed state. This fact s u p p o r t s the idea t h a t predominant adsorption of supercritical gas needs the stable
cluster
adsorbate
formation
molecules.
between
Also
similar
cluster formation is shown in the wnm
pore.
However,
1.00
population
of
clusters is m u c h smaller t h a n t h a t of w^ = 0.90
nm.
Therefore,
concentrated
cluster
the
formation
from the deep well of the potential
of a
graphitic pore. quite
sensitive
Xe molecule
highly stems
interaction with
the
The potential depth is to
determine
population of t h e clusters.
the
Figure4. Cluster size distributions of Xe m a graphite pore of w^= 0.90 nm and w- 1.00 nm at 300 K as a function of Xe pressure.
719
4.4 Structure of Xe clusters Fig. 5 shows both of the whole RDFs of all Xe molecules including monomers and the intra-cluster RDF that indicates the Xe-Xe distance only in the formed cluster. Here Xe pressure is 75.5 kPa for both pore systems and solid and broken lines denote the whole RDF and the intra-cluster RDF. The difference between both RDFs provides the information on the cluster structure. The intra-cluster RDF has a very short peak at 0.44nm corresponding to the dimer at 33.7kPa for both pores. The peak at 0.44 nm indicates the presence of dimers and trimers of regular triangle shape. On the other hand, the intra-cluster RDF of the 0.90 nm pore system has a week peak at 0.76 nm, suggesting the presence of the complex structure of the regular triangles,
Figure 5. Radial distribution function(RDF) of whole Xe molecules and clusters for w = 0.90 and 1.00 nm
Accordingly, the starting unit of
pores at 75.5 kPa.
the Xe cluster should be dimer
lines denote RDFs of the whole Xe
and the trimer of the triangle
molecules and clusters,
Solid and dotted
form should be the elementary structure for great clusters. In case of the 1.00 nm pore, the concentration of the clusters having the complex structure giving the peak at 0.76 nm should be nil. Thus, Xe molecules form more and greater clusters having the complex geometrical structure are formed in narrower pores under supercritical conditions.
720
Acknowledgment This work was funded by the NEDO project from Japanese Government.
References 1. K. Kaneko, J. Membrane Sci., 96 (1994) 59. 2. S. lijima. Helical microtubes of graphitic carbon, Nature, 354 (1991) 56. 3. Kaneko, C. Ishii, N. Nagai, Y. Hanzawa, N. Setoyama, and T. Suzuki, Advances Colloid Sci.76-77 (1998) 295. 4. K. Kaneko, and Murata, K., Adsoiption, 3 (1997) 197. 5. K. Kaneko, Langmuir, 3(1987)357. 6. K.Kaneko, Colloid Surf., 37 (1989)115. 7. Z.M.Wang,, T.Suzuki,, N.Uekawa, K.Asakura, and K.Kaneko, J. Phys. Chem., 96(1992)10917. 8. J.O.Hirschfelder, C.F.Curtiss, R.B.Bird, Molecular Theory of Gases and Liquids, Wiley, New York, 1954. 9. A. J.Stone, The Theory of Intermolecular Forces, Clarendon press. Oxford, 1996. 10. M. Aoshima, T. Suzuki, and K. Kaneko, Chem. Phys. Lett.310(1999) 1. 11. M. Aoshima, K. Fukazawa, and K. Kaneko, J. Colloid Inteface Sci. in press. 12. N. Setoyama, T. Suzuki, and K.Kaneko, Carbon, 36(1998) 1459. 13. N. Metropolis, A. W. Rosenbluth, M. N. Rosenbluth, A. H. Teller, J. Chem. Phys. 21 (1953)1084. 14. M. P. Allen, D. J. Tildesley, Computer Simulation of Liquids, Oxford University Press, Oxford, 1987 15.A. J. Marks, J. N. Murrell, A. J. Stace, J. Chem. Soc. Faraday, trans., 87 (1991) 831. 16. W. A. Steele, Surf. Sci., 36 (1973) 317. 17. K. E. Gubbins, Molecular Simulation, 2 (1989) 223. 18. R. F. Cracknell, D. Nicholson, N. Quirke, Mol. Phys., 80 (1993) 885. 19. T. Suzuki, K. Kaneko, K. E. Gubbins, Langmuir, 13 (1997) 2545. 20. K. Kaneko, R. F. Cracknell, D. Nicholson, Langmuir, 10 (1994) 4606. 21. G. N.Coverdale, R. W.Chantrell, G. A. R.Martin, A.Bradbury, A.Hart, and D. A.Parker, J. Magnet. Magnet. Matter., 188 (1988) 41.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. Ail rights reserved.
721
Adsorption of halocarbons in nanoporous materials: current status and future challenges. C. Mellot Draznieks^, J. Eckert and A. K. Cheetham^ ^Institut Lavoisier, Universite de Versailles Saint Quentin, Versailles Cedex 78035, France.* Los Alamos National Laboratory, Los Alamos, New Mexico, 87545, USA. ^Materials Research Laboratory, University of California, Santa Barbara, CaUfomia 93106, USA.
1. INTRODUCTION An increased awareness of environmental issues relating to ozone-depleting chlorofluorocarbons (CFCs) [1] and to the removal of chlorinated solvent residues from contaminated ground water and soils [2] is driving the need to develop new separation and catalytic conversion processes for halocarbons. Zeolites and related nanoporous materials have been recently recognized as interesting alternatives to other media such as activated carbons for the sequestration and conversion of halocarbons, such as methyl chloride [3], trichloroethylene [4], and various hydrofluorocarbons [5], using ZSM-5 or faujasite-type zeolites. Obviously, aluminosilicates and related molecular sieves offer a range of potential advantages: they offer the possibility of fme-tuning separation processes by utilizing polarity differences between various halocarbons and of tailoring separation processes by harnessing chemically specific host-guest interactions. Unlike the situation with hydrocarbons in zeolites, relatively few experimental and simulation data concerning the behavior of halocarbons in zeolites and analogous nanoporous materials have been reported so far. They include calorimetric [6] and isotherms measurements [7,4], FTIR/Raman [8] and NMR [9] spectroscopies studies, diffraction work [10], and generalized forcefield simulations [11] on a variety of sorbate/sorbent systems. However, there remains a great deal to be done in this emerging field. In view of the growing interest in the field, we have undertaken a program aimed at developing reliable computer-simulations procedures that can be used to complement and enhance the analysis of experimental data, such as thermodynamic, spectroscopic and structural measurements. In the context of chlorocarbons, for example, the main vehicles for our study have been chloroform, trichloroethylene and tetrachloroethylene, these molecules being the most abundant in ground water and aquifer contamination. To date, we have focused primarily on halocarbons in faujasite (FAU) systems, a choice based upon (i) the availability of FAUs in a variety of Si/Al ratios and cation forms, (ii) their large apertures (^7.5 A) and sorption capacities, and (iii) their commercial availability at modest cost. We have used a combination of various techniques for probing the influence of parameters such as the Si/Al ratio, the cation content and the sorbate loading: energy minimizations and
722
canonical Monte Carlo simulations, calorimetric measurements, powder neutron diffraction and vibrational spectroscopies (Raman and inelastic neutron scattering) were used. 2. FORCEFIELD DEVELOPMENTS In the first step, we have developed a new forcefield for chlorocarbon-type molecules in zeolites [12], where both host-guest and guest-guest interactions are considered, since intermolecular interactions need to be taken in account for simulating the coverage dependence of adsorption properties. A Lennard-Jones potential is used to describe the nonbonding interactions, together with a coulombic term accounting for the interactions between the dipole moment of the guest and the electrostatic field generated by the zeolite host: ^ Lennard-Jones = 5:ij (Ay/r
- Bij/r ) = lij 8ij [(r ij/rij)
- 2 (r ij/rij) ]
E Coulombic = ^ij QiOj/rij
(1) (2)
where Aij is the repulsive constant and, Bij, the dispersive constant, with eij = B ij/4Aij, r*ij = (Aij/Bij) . Short-range parameters between (C,H) atoms and the (0,Na)zeoiite atoms were taken from simulation studies reporting good predictions of hydrocarbons adsorption in various zeolites. Short-range parameters involving CI atoms were derived from those of argon in zeolitic structures, taking in account the difference in their atomic polarizabilities. More details on the derivation of forcefield parameters for chlorocarbons are reported in ref 12. Recently, the forcefield has been successfully extended to treat fluorocarbons and chlorofluorocarbons in faujasite zeolites [13]. Typically, parameters for short-range interactions between F atoms and (0,Na)^^^,j,^ atoms were obtained from our values for the Cl...(0,Na)^^,jj^ parameters, taking into account their differences in atomic polarizabilities and atomic radii (for further details, see ref 13). Table 1 shows our complete set of zeolitehalocarbon short-range parameters. We wish to stress that no fitting to experimental data on halocarbons was used in the derivation of these parameters. Table 1 Lennard-Jones parameters used for chloro- and fluoro- and chlorofluoro-carbons in zeolites.
0...C 0...H 0...C1 0...F Na...C Na...H Na...Cl Na...F
^,.(K)
r.*(A)
87.06 90.53 165.4 41.35 13.24 11.41 212.48 52.12
3.25 2.70 3.43 3.03 3.69 3.1 2.9 2.5
C...C C.Cl C...F C...H C1...C1 C1...H C1...F F...F F...H H...H
£..(K)
r.*(A)
25.86 55.65 13.91 26.73 119.8 57.53 34.67 29.7 14.38 27.63
3.75 3.79 3.39 3.36 3.82 3.39 3.70 3.02 2.99 2.96
723
Table 2 Partial aiomic charges for a selection of chloro-, fluoro- and chlorofluoro-carbons. C CHCI3 C2HCI3 CHF3 CF, CF3CI CFA CFC13
F
CI
H
-0.102 -0.026 +0.18 C,>0.064; C,:+0.036; Cl,:-0.068; Cl„:-0.054; CU,:-0.022; H,:+0.172 +0.719 -0.245 ' " +0.016 +0.800 -0.200 +0.614 -0.176 -0.086 +0.210 -0.086 -0.019 -0.039 -0.033 +0.024
Atomic charges on the guest molecules were obtained from first principles Hartree-Fock calculations, fitting the electrostatic potential surface (EPS), then scaled up or down in order to reproduce the experimental dipole moments. Table 2 gives partial charges of typical molecules considered in our work. 3. CHLOROCARBONS IN ZEOLITES 3.1. Key features of host-guest interactions As a case study, the structures and energetics of CHCI3 binding sites in NaY zeolite were studied by energy minimization calculations and were compared with inelastic neutron scattering and Raman measurements on the same system [12]. All favorable binding sites were found in the 12-ring windows (Figure 1). In contradiction with previous X-Ray diffraction work on chloroform in NaY [10], simulations based upon our new forcefield drew our attention to the importance of hydrogen bonding in chloroform/zeolite interactions as well as in other systems involving halocarbons with C-H bonds. This interaction was clearly confirmed (i) by inelastic neutron scattering and Raman spectroscopies, which showed a typical softening of the C-H stretching mode (v,) and a hardening of the H-C-Cl bending mode (vj involving the H atom of the chloroform molecule, and (ii) very recently, by a H/D pair distribution function obtained by inelastic neutron scattering on the same system [14]. Two important additional components of the total host-guest interactions are clearly revealed: (i) short-range interactions between chlorine atoms and framework oxygens (ii) electrostatic interactions between chlorine atoms and accessible Na ions of the supercages. 3.2. Thermodynamics Calorimetric studies aimed at evaluating the quality of our forcefield and providing a better understanding of chlorocarbon adsorption in zeolites. Figure 2 shows the calorimetric heats of adsorption of chloroform in the three faujasite-type zeolites, siliceous FAU, NaY and NaX as a function of sorbate loading (open symbols). Excellent agreement is observed with the predictions of {N,V,T) Monte Carlo simulations based upon our forcefield (filled symbols) [15]. Experiments together with simulations provide invaluable insight concerning the trends of heats of adsorption as a ftinction of host and guest polarity, with three dominant features: (i) the heats of adsorption increase with increasing polarity of the zeolite host (siliceous FAU < NaY < NaX), underlining the importance of the dipolar nature of the guest molecules and of their interactions with the electrostatic field generated by the extraframework cations (ii) the
724
predominance of short-range interactions upon electrostatic interactions (iii) a systematic increase in short-range interactions upon loading. Indeed, as the loading increases, this additional contribution arises from intermolecular attractions between chloroform molecules. In the special case of NaX, this increase is unexpectedly cancelled out by a decrease in the electrostatic term, leading to a flat profile in the adsorption heat. Similarly, our forcefield works equally well for unsaturated halocarbons. For example, calorimetric heats of adsorption for trichloroethylene in the same three faujasite zeolites are in excellent agreement with our (N,V,T) Monte Carlo simulations [16]. Our results at "zero" loading suggest, unlike hydrocarbons, an analogy between the adsorption processes of saturated and unsaturated halocarbons. kJ/mol J.HU.U
0 experimental ^simulation D experimental • simulation 0 experimental ^simulation
CHCl3/NaX CHCl3/NaY
120.0-
CHCl3/sil. Y 100.0-
80.0-
# 0 ^ #:)tD 9^
•
0
60.0-
•^
• o ^
40.0^
N(molecules)/unit-cell 20.0-1
0.0
Figure 1. Binding geometry of CHCI3 adsorbed in NaY from our computer simulations [12].
— 1
10.0
1
20.0
—
I
30.0
1
40.0
SO.O
Figure 2. Observed and simulated heats of adsorption of CHCI3 at 300K in the three faujasite-type zeolites [15]. See text.
3.3 Location of chlorocarbons molecules in zeolite cavities The precise knowledge of the sorbate location in the zeolite cavities is of ftindamental importance because it gives direct insights into the nature of host-guest interactions. Such information is normally obtained by performing low temperature diffraction studies, usually with powder. With benzene in zeolite NaY, for example, it is well established by powder neutron diffraction [17] that benzene sits on the 3-fold axis where it is bound face-on to an SII cation by a 7c-Na" interaction. The situation with halocarbons turns out to be more difficult to characterize: the molecules tend to occupy positions of lower symmetry within the cavity, leading to a disordered situation in which the scattering density associated with the molecule is smeared out thinly around the cavity. In such cases, simulations provide an important bridge between thermodynamics and structures, for example through the analysis of pair fiinctionsfromMonte Carlo simulations or the analysis of favorable binding sites from energy minimizations.
725 In the case of chloroform in faujasite-type zeolites, {N,V,T) simulations show that at the "zero" loading limit, sorbate molecules adsorbe in the 12-ring windows. As the loading increases, intermolecular attractions between chloroform molecules are detected in the CI CI pair function, although the local environment of each chloroform molecule is largely maintained. This aggregation effect, arising from the high polarizability of chlorine atoms, leads to an inhomogenous and disordered distribution of the sorbate molecules. Interestingly, the interactions between zeolites and unsaturated chlorocarbons like trichloroethylene (TCE) are found to be strikingly different from those between zeolites and unsaturated hydrocarbons (i.e. ethylene and benzene). Both our simulations and our spectroscopic results on the adsorption of TCE in faujasites show that interactions between the 7c electrons and the cations, which dominate in the case of hydrocarbons, are replaced by interactions between the chlorine atoms and the cations [18]. Figure 3 shows typical positions of TCE in NaY zeolite as predicted by energy minimizations. This is a consequence of the different charge distribution in hydrocarbons and halocarbons.
Figure 3. Binding geometries of trichloroethylene in NaY as predicted by our energy minimizations. Left: bridging position. Right: bidendate position.
726 4. FLUORO- AND CHLOROFLUORO-CARBONS IN ZEOLITES 4.1. Key features of fluoro- and chlorofluoro-carbons adsorption The forcefield has been successfully extended to treat fluorocarbons and chlorofluorocarbons in faujasite zeolites [13]. {N,V,T) Monte Carlo simulations on the adsorption of a series of fluoro-, chlorofluoro- and hydrofluoro-carbons (CF^, CF3CI, CF.Cl,, CFCI3, CHF3) in siliceous Y and NaY zeolites were performed and compared with available calorimetric data on the same host-guest systems. They predict adsorption heats with good accuracy (Table 3), while yielding a first validation of our forcefield parameters. Table 3 Comparison between experimental and simulated heats of adsorption for various host-guest systems (at the "zero" loading limit). Exp. (kJ/mol) CF4 / siliceous Y CF3CI / siliceous Y CF^Cl, / siliceousY CF^Cl^ / NaY CFCI3 / siliceous Y CHF3/NaY CHCI3 / NaY
-12.4 -21.5 -25.3 -33.4 -24.6 N/A -53.2
Simul. (kJ/mol) -12.2 -18.5 -23.0 -29.1 -27.9 -45.8 -48.8
Temp. (K) 423 423 423 278 423 300 300
Interestingly, the results reveal some striking differences between zeolite-fluorocarbon and zeolite-chlorocarbon interactions. In siliceous Y, host-guest interactions are driven primarily by F...0 and C1...0 van der Waals interactions, and H...0 hydrogen bonding in the case of hydrogen-containing fluorocarbons. When cationic zeolites are considered, such as NaY, additional electrostatic interactions with Na cations of the supercages are clearly revealed in the pair functions. In line with recent NMR/diffraction studies by Grey et al. [9,10], our simulations show that F Na electrostatic interactions are crucial in the adsorption process of fluorocarbons in cationic zeolites and control the orientation of the sorbate molecules in the supercages. Also, {N,V,T) simulations have enabled us to compare the behavior of CHF3 with that of CHCI3, in NaY zeolite. At the "zero" loading limit, both sorbates have similar adsorption heats in NaY at 300K (-45.8 and -48.8 kJ/mol, respectively). However, the striking difference between CHF3 and CHCI3 adsorption in NaY is that the relative contributions of the dispersive and electrostatic interactions are exactly reversed, fortuitously leading to similar adsorption heats for both sorbates. The predominance of the electrostatic interactions in the CHF3/NaY system is easily understood on the basis of the higher dipole moment of CHF3 (1.65 D) compared to that of CHCI3 (1.06 D). In contrast with CHCI3 adsorption in NaY, (N,V,T) simulations have shown that CHF3 adsorption heats show a flat profile with loading in this zeolite, as a consequence of the constancy of both the electrostatic and dispersive terms. This relies on the much lower atomic polarizability of F atoms in comparison with CI atoms.
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4.2. Location of CFClj in NaY; evidence for cation migration We have completed a careflil neutron diffraction study of CFCI3 in zeolite NaY [19]. Regarding extraframework cations, the refinement clearly revealed important modifications of the cations distribution up adsorption. While SII sites are hardly affected by the adsorption of CFCI3 (i.e. 32 ions per unit-cell), SI sites are occupied with ~7 ions per unit-cell, and SI' sites, although they offer a favorable environment for Na cations and are significantly occupied in bare NaY, were found empty. This is to be considered as a result of a migration process of Na cations occuring within the sodalite cages or towards the supercages. The CFCI3 molecules were found in the 12-ring windows. The whole molecule faces one of the three 4-ring windows of the supercage, at a middle distance of two 6-rings occupied by Na cations in SII sites. One of the chlorine atoms is close to framework oxygens at typical van der Waals distances. The two other chlorine atoms point towards the 6-rings of the supercages, each CI atom pointing towards a Na cation in site II. The F atom was found close to framework oxygens of the 12-ring window. The location of the sorbate molecule is in very good agreement with our simulations if we take account of a redistribution of the sodium cations during adsorption, leading to favorable F Na electrostatic interactions in the 12-ring windows. These results are in agreement with recent NMR measurements on similar systems by Grey et al. [10].
5. FUTURE CHALLENGES Concerning the thermochemical studies and simulations of halocarbons, the current situation is an outstanding agreement between our {N,V,T) Monte Carlo simulations and available calorimetric data. The next validation step of our forcefield concerns the comparison between adsorption isotherms and grand canonical Monte Carlo simulations. At present, we are using a forcefield that uses a Lennard-Jones to describe the short-range interactions together with a coulombic term for long-range interactions. More complex short-range functions that included explicit treatment of host-guest induction energy shall be also explored. We know from both our Monte Carlo simulations and our neutron diffraction studies that the potential energy surfaces in the halocarbon systems are relatively flat, even though the adsorption heats are high. Further molecular dynamics and NMR measurements will allow the exploration of sorbate dynamics. Also, the exploration of other zeolite systems is of interest. The work to date has focused primarily on halocarbons on faujasite systems. Other architectures, such as the so-called MFI structure, e.g. ZSM-5 and silicalite, make an interesting comparison with faujasite and are likely to attract interest in the future because of their commercial potential applications that require an organophilic/organophobic sorbent.
REFERENCES 1. L. E. Manzer, Science, 249 (1990) 31. 2. G. J. Hutchings, C. S. Heneghan, I. D. Hudson and S. H. Taylor, Nature, 384 (1996) 341. See also: H. Mukhopadhyay and E. C. Moretti, Current and Potential Future Industrial Practices for Reducing and Controlling Volatile Organic Compounds, Am. Inst, of Chem. Engineers, Center for Waste Management, (1993) New York. 3. A. S. Zarchy, R. T. Maurer and C. C. Chao, U.S. Patent No. 5 453 113 (1994).
728 4. L. Alvarez-Cohen, P. L. McCarty and P. V. Roberts, Environ. Sci. Technol., 27 (1993) 2141. G. Weber, O. Bertrand, E. Fromont, S. Bourg, F. Bouvier, D. Bissinger and M. H. Simonot-Grange, J. Chim. Phys., 93 (1996) 1412. 5. D. R. Corbin and B. A. Mahler, World Patent, W.O. 94/02440 (1994). 6. H. Stach, K. Sigrist, K.-H. Radeke and V. Riedel, Chem. Technik., 5 (1994) 278. 7. S. Kobayashi, K. Mizuno, S. Kushiyama, R. Aizawa, Y. Koinuma and H. Ohuchi, Ind. Eng. Chem. Res., 30 (1991) 2340. T. Kawai, Yanagihara and K. Tsutsumi, Colloid. Polym. Sci., 272 (1994) 1620. 8. J. Xie, M. Huang and S. Kaliaguine, React. Kinet. Catal. Lett., 58 (1996) 217. P. S. Chintawar and H. L. Greene, J. Catal., 165 (1997) 12. M. K. Crawford, K. D. Dobbs, R. J. Smalley, D. R. Corbin and C. P. Grey, J. Phys. Chem., 103 (1999) 431. 9. C. P. Grey and D. R. Corbin, J. Chem. Phys., 99 (1995) 16821. H. P. Lim and C. P. Grey, Chem. Commun., (1998) 2257. T. T. P. Cheung, J. Phys. Chem., 96 (1992) 5505. 10.1. Gameson, T.Rayment, J. M. Thomas and P. A. Wright, J. Phys. Chem., 92 (1988) 988. A. Z. Kazkur, R. J. Jones, J. W.Couves, D. Waller, C. R. A. Catlow and J. M. Thomas, J. Phys. Chem. Solids., 52 (1991) 1219. A. Z. Kazkur, R. J. Jones, D. Waller, C. R. A. Catlow and J. M. Thomas, J. Phys. Chem., 97 (1993) 426. C. P. Grey, F. I. Poshni, A. F. Gualtieri, P. Norby, J. C. Hanson and D. R. Corbin, J. Am. Chem. Soc, 119 (1997) 1981. 11. A. R. George, C. M. Freeman and C. R. A. Catlow, Zeolites, 17 (1996) 466. J. B. Parise, L. Abrams, J. C. Calabrese, D. R. Corbin, J. M. Newsam, S. Levine and C. Freeman, Stud. Surf. Sci. Catal., 98 (1995) 248. 12. C. F. Mellot, A. M. Davidson, J. Eckert and A. K. Cheetham, J. Phys. Chem., 102 (1998) 2530. 13. C. F. Mellot and A. K. Cheetham, J. Phys. Chem., 103 (1999) 3864. 14. J. Eckert, C. F. Mellot and A. K. Cheetham (in preparation). 15. C. F. Mellot, A. K. Cheetham, S. Harms, S. Savitz, R. J. Gorte and A. L. Myers, J. Am. Chem. Soc, 120(1998)5791. 16. C. F. Mellot, A. K. Cheetham, S. Harms, S. Savitz, R. J. Gorte and A. L. Myers, Langmuir, 14(1998)6728. 17. A. N. Fitch, H. Jobic and A. Renouprez, J. Phys. Chem., 90 (1986) 1311. 18. A. M. Davidson, C. F. Mellot, J. Eckert and A. K. Cheetham (in preparation). 19. C. F. Mellot, D. Cox, J. Rodriguez-Carvajal, R. Papoular and A. K. Cheetham (in preparation).
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) 2000 Elsevier Science B.V.
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Synthesis and Applications of Functionalized Nanoporous Materials for Specific Adsorption J. Liu,* G. E. Fryxell, S. Mattigod, T. S. Zemanian, Y. Shin, and L.-Q. Wang Pacific Northwest National Laboratory, Battelle Boulevard, Richland, WA 99352, USA 1. ABSTRACT Surface chemistry is one of the most important properties of mesoporous materials for many applications. There are several approaches to functionalize mesoporous materials in order to tailor the surface chemistry: one step synthesis by co-condensation, direct silanation of partially hydroxylated surface, controlled hydrolysis and condensation, and silanation using supercritical fluid as the reaction medium. Varying the amount chemically and physically adsorbed water can systematically tailor the quality and the population density of functional groups. Furthermore, using supercritical fluid as the reaction medium, organic molecules can be effectively delivered and attached to the internal surfaces of pores less than 1 nm in diameter. The ability to construct high quality functional monolayers allows rational design of molecular recognition and binding sites in mesoporous materials, and has led to the development of very efficient adsorbing materials. One approach to form a host structure that matches the size and shape of the target species is to take advantage of the coordinate chemistry between the functional molecules and metal ions. Highly selective bindings of the target species have been observed against competing species of similar sizes and shapes. More sophisticated surface sites can be constructed for the recognition of complicated molecules and species using large pore mesoporous materials. 2. INTRODUCTION Ordered mesoporous materials (1) based on surfactant liquid crystalline templates have great potential in environmental and industrial processes, including separation, catalysis, and sensing. The key to these applications lies in the successful construction of functionalized molecular monolayers with target specific sites that are accessible to the targets. Several approaches have been followed to incorporate functional groups into mesoporous materials, mostly for catalytic applications (2). These studies have been extensively reviewed by Moller and Bein (3). The most straightforward method involves direct silanation of partially hydroxylated mesoporous silica, which depends on the population density of hydroxyl groups existing on the surface (4). The hydroxylation problem can be partially solved by using uncalcined mesoporous silica, which was prepared using a neutral surfactant. The surfactant was subsequently removed by solvent extraction techniques (5). The co-condensation method is a one step process in which the functional molecules were incorporated into the materials during the preparation of the mesoporous materials (6,7). This paper will focus on the formation of high density, high quality molecular monolayers constructed by purposely introducing physically adsorbed layers of water molecules before silanation (8), and by using supercritical fluid as the delivery and reaction media (9).
730
The ordered mesoporous materials were synthesized using surfactant micellar structures as templates, and can be prepared under various experimental conditions. Many excellent papers have been published in this area (10). More recently, the templates were extended to include block copolymers (11). The high surface area and the controllable pore shape are very desirable for many applications. Forming organized molecular monolayers in mesoporous supports is fundamentally similar to the formation of self-assembled monolayers on two-dimensional substrates, which are widely explored for engineering the surface and interfacial properties of materials, such as wetting, adhesion and friction (12). These monolayers are also used to mediate the molecular recognition processes (13) and to direct oriented crystal growth (14). In this approach, bifunctional molecules containing a hydrophilic head group and a hydrophobic tail group adsorb onto a substrate as closely packed monolayers. The tail group and the head group can be chemically modified to contain specific functional groups. The hydrocarbon tails provide the driving force (van der Waals interaction) for the self-assembly of the molecules into close packed arrays on the substrate. The organosilane end of the molecule is covalently bonded to the oxide surface and crosslinked to adjacent silanes through hydrolysis and condensation reactions of the hydroxyl groups. The main driving force for short alkyl chains to pack on the substrate is chemical bonding through condensation reactions. The quality of the functional monolayers on the mesoporous materials is gready affected by the population of silanol groups and adsorbed water molecules on the mesoporous silica surface. The silanols are needed to anchor the organic molecules to the silica surface, and physically adsorbed water is required for the hydrolysis reaction. However, excess free water from capillary action is also detrimental to the efficient formation of a clean monolayer, due to polymerization of organic molecules in the solution. 3. MOLECULAR MONOLAYERS IN MESOPOROUS MATERIALS High quality, close packed monolayers can be formed on the mesoporous supports through the introduction of several layers of physically adsorbed waters on the mesoporous surface. The role of the water molecules is to physically confine all the hydrolysis and condensation reactions of the organosilianes at the interface. Experimentally, this approach is accomplished by adding the requisite amount of water to a suspension of mesoporous silica in toluene and stirring the mixture for an hour to allow complete dispersal of the aqueous phase across the ceramic interface. When the mesoporous ceramic interface is properly hydrated, construction of the monolayer is accomplished by adding one equivalent (or slight excess) of the desired alkoxysilane (based on available surface area), stirring the mixture, and heating it in toluene reflux for several hours. Currendy, we can systemadcally vary the population densities of functional groups on the mesoporous materials from 10% up to 100% of the full surface coverage. Figures la and lb compare the ^'^Si NMR spectra obtained from the mesoporous silica functionalized with tris(methoxy)mercaptopropylsilane (TMMPS) with different degree of hydration. Based on NMR and other studies, we can conclude that with a low degree of hydration, the functional molecules only cover part of the surface (25% coverage). The siloxane groups can adopt three different conformations: (i) isolated groups that are not bound to any neighboring siloxanes, (ii) terminal groups that are only bound to one neighboring siloxane, and (iii) crosslinked groups that are bound to two neighboring siloxanes. Among these three groups, the terminal conformation (ii) is dominant. With a higher degree of hydration, a high surface population density can be achieved. NMR spectra
731
for ^^Si show the predominance of only crosslinked bonding conformation for the siloxanes, rather than a distribution of isolated, terminal, and crosslinked groups. (iii) Crosslinked (ii) Terminal (i) IsoloteJV Partially hydrated, 25% coverage
Polymeric / siloxaife / 50
I \
100
c) 20 A pore 150
PP"^
' ' ' ' ' ppm 0 -50 -100 -150 -200 Figure 1. ^^Si NMR spectra of functionalized mesoporous silica, (a) Large pore mesoporous silica (50 A) with low degree of hydration and low coverage (25%). (b) Large pore mesoporous (50 A) with a higher degree of hydration and a higher coverage, (c) Small pore mesoporous silica (20 A). We have also observed that when the pore size become smaller (less than 2 nm), the quality of the monolayer degrades (Figure Ic). The ^^Si peaks corresponding to the siloxane groups are broad, indicative of polymeric siloxanes with heterogeneous chemical environment. At the same time, the peak corresponding to the bulk silicon (from mesoporous silica) has a pronounced Q3 component, suggesting the siloxanes are not chemically bonded to the substrate. 4. SUPERCRITICAL FLUID DEPOSITION OF MOLECULAR MONOLAYERS When the pore size becomes too small, the diffusion and the delivery of the functional molecules become difficult. This will explain the poor quality of the monolayers in mesoporous materials with pore less than 2 nm. This problem can be solved by using a supercritical fluid (SCF), instead of an ordinary solvent, as the delivery and the reaction medium. Supercritical fluid techniques have been used widely in aerogel chemistry, surface treatment and in extraction of natural products (15). SCF has also been used to deposit metals on a substrate (16), and to prepare polymer metal nanocomposites. SCF fluids offer a unique environment to perform chemical reaction because of the liquid-like solvation properties and gas-like physical properties (viscosity, diffusivity) (17). The low density, low viscosity, high diffusivity, and low surface tension of SCF fluids make them ideal media for performing silanation of the internal surfaces of porous materials. CO2 is environmentally benign, nontoxic, non-flammable, and inexpensive. The mild critical conditions for CO2 (Tc = 31.1 °C, Pc = 73.8 bar) can be easily attained, and are unlikely to cause degradation of the porous substrates. Due to direct pressure pumping, the silanes are readily delivered to the internal pore surface. Similarly, when the pressure is decreased, the unreacted silanes and by-products are forced out of the inner volume of the porous substrate. In principle, the silanation process can be accomplished in a few minutes. Another advantage is that no secondary organic waste
732
is generated because the supercritical process does not use an organic solvent. Since the supercritical method can be directly applied to commercial zeolites, it provides a valuable alternative approach to functionalized microporous materials through post treatment.
Normal monolayi
' ' ' I
\SCF monoljiyer
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I
I
I
50
I
I
I
I
I
I
100
I
I
I
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15
Figure 2. (a) ^'Si NMR spectrum of functionalized mesoporous Silica using supercritical CDj as the reaction medium, (b) Solubility of the functional molecules as a function of pH. The SCF process further improved the quality and the chemical stability of the monolayers on mesoporous silica. Figure 2a is the ^'Si NMR spectrum of the supercritically functionalized mesoporous silica. Compared to Figure lb, the peak corresponding to the crosslinked siloxanes is much more pronounced, indicating a high degree of crosslinking. Both regularly functionalized mesoporous silica and the supercritically functionalized mesoporous silica were tested for the hydrolytic stability at different pH. At high pH, the siloxane groups at the silica monolayer interface can be subjected to hydrolysis reaction if defects exist in the monolayer. Figure 2b shows the concentration of monolayer molecules dissolved in solutions at different pH after functionalized mesoporous materials were equilibrated with the solution over 24 hours, as measured by the sulfur concentration in ppm (parts per million). The solubility of the regular monolayer began to increase at pH 10, while the supercritically processed monolayers showed very high chemical stability and low solubility in the whole pH range. The supercritical process can be used to functionalize microporous materials. It is usually difficult to deliver and deposit functional molecules in microporosities without blocking the pore channels by external depositions. In this study, commercial zeolite beta (from Zeolyst) is used. Zeolite beta is a high silica microporous material with parallel pore channels made up of 12 membered silicate rings (18). The zeolite was functionalized with TMMPS in supercritical CO2 (SCCO2) environment. For comparison, TMMPS functionalized mesoporous silica was also studied for the catalytic reaction. The thiol groups on zeolites and mesoporous silica were oxidized into SO3H groups with HjOj. Following the methods developed by Jones et al. (19), the catalytic ketalization of cyclic ketone by ethylene glycol is used to demonstrate the size selectivity of the SCCO2 sulfonated zeolite beta. The catalytic reaction of cyclohexanone (HEX) and glycol forms 2,2-pentamethylene-l,3-dioxolan (cyclic ketal), while the reaction of pyrenecarboxaldehyde (PYC) with glycol forms the
733
corresponding acetal (20). The catalytic reactions were conducted in glass reactors at 70 °C. The size selective catalytic properties of the supercritically modified zeolite compare well with similar materials prepared by an in-situ silanation process reported by Jones et al. The active functional groups were delivered to the internal pore surface of the microporous material and remain accessible to molecules that can enter the pore channels. The supercritical process provides an alternative approach to functionalize microporous materials because of the enhanced diffusivity of the functional molecules in the micropore channels and accelerated reaction kinetics. Furthermore, the supercritical process simplifies the materials preparation and may open up new opportunities for commercial zeolites.
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Figure 3. (a) Schematic of functionalized zeolite beta, (b) Conversion of PYC over functionalized zeolite, (c) Poisoning effect of amines on zeolite. Figure 3a is a schematic of the functionalized zeolite beta. Figure 3b plotted the catalytic conversion of HEX and PYC over 6 A zeolites as a function of time. For sulfonated zeolite (Z-SO3H), more than 60 % HEX was converted in 4 hours, and nearly complete conversion was observed over 12 hours. On the other hand, PYC, which has a large molecular size and cannot enter the microporosity, showed less than 8 % conversion over extended reaction time with same Z-SO3H as catalyst. Both HEX and PYC were alsoreactedover pure zeolite beta (Z), and the TMMPS functionalized zeolite (Z-SH) before it was treated with H2O2. Pure zeolite and Z-SH showed low catalytic activity, and only a small fraction of either HEX or PYC was converted. Further evidence of the size selectivity is provided when amines of different sizes are used to poison (neutralize) the acid sites (19). As shown in Figure 3c, the
734
addition of triethylamine ((C2H5)3N, or TEA), a small amine molecules that can enter the pore channels, completely stopped the reaction. Under the same condition the addition of methyldioctylamine [(CH3(CH2)7)2NCH3, or MDOA], a large molecule that can not enter the pore channels, instead of TEA, did not have any effect on the conversion of HEX over ZSO3H. 5. ADSORPTION BY FUNCTIONALIZED MESOPOROUS MATERIALS The adsorption behavior of thiol (TMMPS) functionalized mesoporous silica towards mercury and other metal ion species has been extensively tested. Mercury and heavy-metal contamination is a serious problem at waste contaminated sites of the Department of Energy (21). Industrial and civilian sources deposit a large amount of mercury into the environment every year (22). The functionalized mesoporous materials have many desirable properties, including high metal loading, up to 0.64 g Hg/g adsorption materials, high selectivity without significant interference from other abundant cations such as alkali and alkaline earth, and anions such as CI", CN", CO3'-, SO/", and PC/" in wastewater, pH stability from 2 to 10, high affinity to reduce the mercury concentrations in salt solution and in groundwater to below a 10 ppt level, and fast kinetics. wi '"^'-^ " 'bl) g 6O0.« - " t
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Equilibrium concentration (mg/L) Figure 4. Adsorption isotherms of mercury by thiol functionalized mesoporous silica, (a) HgNOg. (b) CHgHgOH. The mercury absorption of HgNOa in 0.1 M NaNOj solutions exhibited a typical Langmuir isotherm curve, as shown in Figure 4a. The maximum loading is 635 mg/g (or 3.2 mmol Hg/g). Similar adsorbing behavior is observed for other mercury species, such as
735
methylmercury, CHa-Hg-OH (Figure 4b). Methylmercury, the most toxic form (22), is formed mainly by methylation of mercury by the methanogenic bacteria that are widely distributed in the sediments of ponds and in the sludge of sewage beds. Methylmercury can accumulate in fish in contaminated waterways. Mercury poisoning symptoms in humans include digestion disturbances, emaciation, diarrhea, speech stammering, delirium, paralysis of the arms and legs, and death by exhaustion. However the adsorption behavior of methyl mercury deviates from Langmuir behavior when the equilibrium mercury concentration is higher than 75 mg/L. The maximum adsorption is close to the results for mercury ions. Other soft metals, such as silver, showed very similar behavior (Figure 5a). However, as the "softness" decreases, the adsorption isotherms deviate significantly from the ideal Langmuir curves (Figure 5b).
Equilibrium concentration (mg/L)
Equilibrium concentration (mg/L) Figure 5. (a) Adsorption of Ag ions by thiol functionalized mesoporous silica, (b) Adsorption of Pb ions by thiol functionalized mesoporous silica. The ability to construct high quality monolayers makes it possible to systematically tailor tiie surface chemistry and design more sophisticated molecular recognition sites in mesoporous materials. The design of anion selective mesoporous silica is a good example (23). This research is motivated by the recent reports of the crisis caused by arsenic contamination of drinking water in Bangladesh and other parts of the worid (24,25). In Bangladesh alone, health officials estimated 50 to 70 million people could be affected by drinking water contaminated by natural arsenic sources. Arsenic, along with other toxic
736
metals like chromium, and selenium, are included in the U. S. Environmental Protection Agency's list of priority pollutants. These contaminating species, unlike many heavy metals
oooooo Figure 6. Construction of nK)lecular recognition sites in mesoporous silica. The first step is the formation of close packed nwnolayers. The second step is the incorporation of a transition metal ions, followed by coordination of the metal ions with the ligands to form host sites that match the shape and size of the targets.
200 400 600 Equilibrium concentration (mg/L)
0
800
200 400 600 Equilibrium concentration (mg/L)
Figure 7. Adsorption isotherms for anions by EDA-Cu functionalized mesoporous silica, (a) Chromate. (b) Arsenate. 2-
and transition metals, can exist in nature as tetrahedral oxyanions (arsenate ions HASO4 , H2As04^*, and chromate ions HCi04", Cr04^) (26,27). In many cases, trace amounts of arsenate and chromate need to be removed from waste solutions containing high concentrations of competing anions, sulfate, and chloride in particular. Currently, the
737
development of effective anion binding materials is an important subject in chemistry, biochemistry, materials and environmental science (28,29). Many anions are very similar in size and shape, and therefore difficult to differentiate. We synthesized and used metal chelated ligands immobilized on mesoporous silica as an efficient anion binding material for both arsenate and chromate. The mesoporous silica was functionalized with an ethylenediamine (EDA) terminated silane [(2 aminoethyl,)-3-aminopropyl trimethyl silane]. Cu(II) ions were bonded to the EDA monolayer with a 3 to 1 EDA to Cu ratio, forming an approximately octahedral Cu(EDA)3 complex structure. Computer modeling indicates the complex structure consists of an electrophilic basket with C3 symmetry that forms an ideal host for a tetrahedral anion. This approach is schematically illustrated in Figure 6. The adsorption isotherms for removing arsenate and chromate from contaminated water are plotted in Figure 7. Nearly complete removal of arsenate and chromate has been achieved in the presence of competing anions for solutions containing up to 100 ppm toxic metal anions under a variety of experimental conditions. Good selectivity between chromate (or arsenate) and sulfate ions can be achieved at high anion concentrations. Anion loading is more than 120 mg (anion)/g of adsorption materials. The anion loading capacity of this material is comparable (on a molar basis) to the heavy metal loading capacity achieved with the best cation sorbent materials (functionalized mesoporous silica) discussed earlier, when the stoichiometry of binding and the atomic/molecular weight of the target species are taken into consideration. This approach is especially promising considering the rich chemistry that can be explored with monolayers, with mesoporous silica, and the possibility of designing better anion recognition ligands. ACKNOWLEDGEMENT This work is supported by the Office of Basic Energy Sciences of the Department of Energy. Pacific Northwest National Laboratory is operated by Battelle Memorial Institute for the US Department of Energy under Contract DE-AC06-76RL01830. REFERENCES 1.
2. 3. 4. 5. 6. 7. 8. 9.
(a) 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. (b) C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and J. S. Beck, Nature 359 (1992) 710. A. Sayari, Chem. Mater., 8 (1996) 1840. K. Moller and T. Bein, Chem. Mater, 10 (1998) 2950. D. Cauvel, G. Renaid, D. Brunei, J. Org. Chem, 62 (1997) 749. (a) L. Mercier and T. J. Pinnavaia, Advanced Materials, 9 (1997) 500. (b) L. Mercier and T. J. Pinnavaia, Envir. Sci. TechnoL, 32 (1998) 2749. S. L. Burkett, S. D. Simms, S. Mann, Chem. Comm., 1996,1367. M. H. Lim, C. F. Blanford, A. Stein, Chem. Mater., 10 (1998) 467. (a) X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, K. Kemner, and J. Liu, Science, 276 (1997)923. (b) J. Liu, X. Feng, G. E. Fryxell, L. Q. Wang A. Y. Kim, and M. Gong, Advanced Materials, 10 (1998) 161. Y. Shin, T. S. Zemanian, G. E. Fryxell, L.-Q. Wang, anf J. Liu, submitted to Microporous and Mesoprous Materials, 1999.
738 10. (a) J. S. Beck, and J. C. Vartuli, Cur. Opin. Sol. St. Mater. Sci. 1, 76, 1996. (b) J. Liu, A. Y. Kim, L. Q. Wang, B. J. Palmer, Y. L. Chen, P. Bruinsma, B. C. Bunker, G. J. Exarhos, G. L. Graff, P. C. Rieke, G. E. Fryxell, J. W. Virden, B. J. Tarasevich, L. A. Chick, Adv. Colloid. Interface. Sci. 69, 131, 1996. (c) N. K. Raman, M. T. AndersonC. J. Blinker, Chem. Mater., 8,1682,1996. (d) A. Corma, Chem. Rev., 97 (1997) 2373. 11. P. Yang, D. Zhao, D. I. Margolese, D. I.; B. F. Chmelka, G. D. Stucky, Nature, 396 (1998) 152. 12. (a) G. M. Whitesides, Scientific American, 273 (1995) 146. (b) A. Ulman, Chem. Rev, 96 (1996) 1533. 13. K. D. Schierbaum, T. Weiss, E. U. Thoden van Velzen, J. F. J. Engbersen, D. N. Reinhoudt, W. Gopel, Science, 265 (1994) 1413. 14. B. C. Bunker, P. C. Rieke, B. J. Tarasevich, A. A. Campbell, G. E. Fryxell, G. L, Graff, L. Song, J. Liu, J. W. Virden, and G. L. McVay, Science, 264 (1994) 48. 11. See articles in "Supercritical Fluids," Chemical Reviews, 99, R. Noyori (Eds), 1999. 16. J. J. Waticins, J. M. Blackburn and T. J. McCarthy, Chem. Mater, 11 (1999) 213.\ 17. J. J. Waticins and T. J. MaCarthy, Chem. Mater., 7 (1995). 18. (a) J. C. van der Waal, M. S. Rigutto, H. van Bekkum, J. Chem. Soc. Commun., (1994) 1241. (b) M. C. Camlor, A. C. Corma and S. Valencia, Chem. Commun., (1996) 2365. 19. C. W. Jones, K. Tsu and M. Davis, Nature, 393 (1998) 52, andtiiereferences cited. 20. T. W. G. Solomons, in Organic Chemistry, John Wiley & Sons, 3rd edition, 1984, p.720-725, 21. U.S. Department of Energy (DOE), Mixed Waste Focus Area, Technical Baseline Results, World Wide Web: Http://wastenot.inel.gov/mwfa/results.html (1996); U.S. Department of Energy (DOE). FY91 Waste and Hazard Minimization Accomplishments, DOE Report MHSMP=-91-37, Pantex Plant, Amarillo, TX 79177, (1991); J. E. Klein. R&D Needs for Mixed Waste Tritium Pump Oils (U), Westinghouse Savannah River Company Inter-Office Memorandum, SRT-HTS-94-0235 (July 11, 1994). 22. S. Mitra. Mercury in the Ecosystem (Trans Tech Publications, Lancaster, PA,1986). 23. G. E. Fryxell, J. Liu, T. A. Hauser, Z. Nie, K. F. Ferris, S. Mattigod, M. Gong, and R. T. Hallen, Chem. Mater., 8 (1999) 2148. 24. R. Nickson, J. McArthur, W. Burgess, K. M. Ahmed, P. Ravenscroft, M.Rahman, Nature, 395 (1998) 338. 25. http://bicn.com/acic. 26. C. F. Jr. Baes, R. E. Mesmer, in The Hydrolysis of Cations; John Wiley & Sons: New York, p 215 and p 366-368,1976. 27. E. A. Woolson, in Biological and Environmental Effects of Arsenic, edited by B. A. Fowler, Elsevier: New York, p 51-120,1983. 28. J. L. Sessler, P. I. Sansom, A. Andriewvsky, V. Krai, In SuperMolecular Chemistry of Anions, edited by A Bianchi, K. Bowman-James, K. and E Garcia-Espana E., WileyVCH: New York, p 355-420,1997. 29. J. L. Atwood, K. T. Holman, J. W. Steed, Chem. Commun., 1401,1996,
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
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Non-electrostatic surfactant assembly routes to functionalized nanostructured silica: prospects for environmental applications L. Mercier Department of Chemistry and Biochemistry, Laurentian University, Sudbury, Ontario P3E 2C6, Canada The assembly of functionalized nanostructured silica with uniform pore channels using neutral alkylamine surfactants (S^I^ -^ HMS) and non-ionic alkylpolyethyleneoxide surfactants (N^f -^ MSU-X) provides many advantages over conventional electrostatic assembly pathways (S"^!', etc.). In contrast with electrostatically assembled MCM-41-type materials, mesostructured adsorbents produced by non-electrostatic assembly methods typically possess pore channel structures and particle morphologies which improve their ability to interact with targeted adsorbate species. Moreover, non-electrostatic assembly pathways are well-suited for the direct synthesis of functionalized mesostructured silica by one-step preparation processes under ambient temperature, neutral pH conditions. The environmental application of such materials for the treatment of mercury-contaminated water is also demonstrated. 1. INTRODUCTION l.I Mesostructure functionalization methods: grafting and direct incorporation The recent discovery of mesostructured oxides such as MCM-41^ has spurred much research interest in the field of nanoporous materials synthesis and design. For nearly a decade, much emphasis has been put on structural and mechanistic investigations relating to these compounds,"^'^ although current research trends are increasingly focussing on the inclusion chemistry of these materials.^ Mesostructured materials are now beginning to find practical applications in many areas, including catalysis^"^^ and environmental remediation. "^ Thus, a fundamental cornerstone of research in this area consists of the preparation of chemically functionalized derivatives of mesostructured oxides. Two strategies can be considered for the preparation of functional mesostructure derivatives in which the functional groups are covalently anchored to the oxide framework: grafting and direct incorporation. Grafting entails the anchoring of functional groups on the pore walls of a preformed mesostructure.'^'^ This implies a process requiring two steps: first the mesostructure preparation, then the anchoring of the functional group). Typically, a grafted mesostructure can be produced by treating the non-functional nanoporous oxide with an organosilane reagent of the type Si(0R)3R' (where R' is a functional group). Tethering of this coupling agent to the surface of the mesostructure channels occurs by the condensation of the reagent to surface Si-OH groups on the mesostructure. Direct incorporation refers to the one-step assembly of functional mesostructures.'^"'^ This can be achieved by adding the organosilane functional group directly into the mesostructure synthesis mixture and allowing
740
the functional silane to incorporate itself into the mesostructure framework. Since the organic functional groups are typically hydrophobic in nature, successful preparation of these organicinorganic hybrids proceeds by the self-assembly of these molecules at the water-micelle interface (i.e., the lipophilic organic chains "dive into" the micelle). 1.2 Mesostructure synthesis modes: electrostatic and non-electrostatic assembly Because the way in which a mesostructure is prepared affects the structure, morphology and surface chemistry of the product obtained, the mesostructure formation mechanism is also expected to impact the preparation of functionalized derivatives. Mesostructure synthesis pathways can be subdivided into two general categories: electrostatic and non-electrostatic pathways. Electrostatic assembly occurs when the structure-directing surfactant used to prepare a mesostructure possess charge, such as cationic alkyltrimethylammonium (S^)^'^ or anionic alkylsulfonate (S")^'^ surfactants. Since charge matching between the surfactant molecules (S) and the inorganic precursors (I) is a prerequisite for mesostructure formation (S^I", S"r) , the framework thus formed will also bear a charge. The relatively strong ionic interaction between the assembly surfactant molecules and the inorganic framework thus necessitates the use of high temperature calcination (>600°C) to remove the surfactant and induce porosity in the mesostructure. Although extraction techniques involving ion exchange are reported, the harsh conditions needed (i.e., strongly acidic or alkaline pH's) often result in lattice destruction.^'^ Electrostatically-assembled mesostructure typically exhibit a high degree of crystallographic ordering. Non-electrostatic assembly implies mesostructure formation using structure-directing surfactants which do not bear charge, including alkylamines (S^)^ and non-ionic alkylpolyethyleneoxide (H^f surfactants. Thus, the framework-forming inorganic precursors now consist of neutral entities (I^) which interact with the surfactants by way of hydrogen bonding. A consequence of this much weaker interaction (in contrast to electrostatic assembly) is the typical formation of worm-like pore channels for neutrally-assembled mesostructures, resulting in a low degree of ordering as observed by X-ray diffraction. A comparison between typical properties of mesostructures produced by both electrostatic and non-electrostatic assembly is shown in Table 1. This paper will present a comparative assessement between the preparation of functionalized silica mesostructures by grafting and direct incorporation methods. The use of both electrostatic and non-electrostatic surfactant assembly routes in relation to these functionalization modes will also be discussed. 2. EXPERIMENTAL 2.1 Grafted mesostructures Pure silica mesostructures were prepared using both electrostatic surfactant (cetyltrimethylammonium bromide, CTAB) assembly and neutral alkylamine (octylamine and dodecylamine) assembly. Thus, tetraethoxysilane (TEOS) was added to solutions of framework-forming surfactants and stirred for 24 hours. The exact conditions for the synthesis procedures are described in a previous publication.'^^ Although Soxhlet extraction over ethanol was used to remove the framework-bound surfactant molecules from the amineassembled mesostructures, 650°C calcination was necessary to remove the CTAB from the
741 electrostatically-assembled material. Table 2 gives the nomenclature of the compounds prepared using the different surfactants. Each mesostructure (1 g) was then refluxed with 3-mercaptopropyltrimethoxysilane (MPTMS) (2 g) in anhydrous toluene (25 ml) for 24 hours. The resulting product was filtered, washed with toluene followed by ethanol, and washed by Soxhlet extraction over ethanol.^^ All of the prepared materials (both pristine and flinctionalized mesostructures) were characterized by X-ray diffraction, N2 sorptometry, ^^Si MAS-NMR and chemical analysis. The physico-chemical properties thus obtained for each material are shown in Table 2. Table 1 Typical characteristics of electrostatically and non-electrostatically assembled mesostructures NON-ELECTROSTATIC ELECTROSTATIC ASSEMBLY (SV, N^f) ASSEMBLY (S^l) MATERIALS SURFACTANTS ASSEMBLY CONDITIONS SURFACTANT REMOVAL SURFACE CHEMISTRY PORE CHANNEL / CRYSTAL STRUCTURE TEXTURAL PROPERTIES
M41S(MCM-41) Quaternary alkylammonium halides Hydrothermal Alkaline or Acidic Calcination (>600°C)
HMS, MSU-X Primary amines Non-ionic surfactants Room temperature Neutral pH Solvent extraction
Sparse surface OH groups Hexagonal, lamellar, cubic (ordered) Large particles (>10 |im) Fibrous morphology
Abundant surface OH groups Wormhole (disordered) Ultrafme particles (95 >90 >85 >95 >95 >95 >95
TBHP; /er/-butylhydroperoxide, reaction temp;30°C, reaction time: 36hs The sample (B) in Fig.3 was used as a catalyst for epoxidation (Si/Ti ratio=160) and Salen (A) ^ obtained using a pure siliceous MCM-41 by the anchoring method as shown in Scheme 1.
Fig. 6 shows the diffuse reflectance UV-visible spectra of typical salen Co(in) complexes immobihzed on MCM-41, the homogeneous salen complex of the same structure, and a pure Si-MCM-41. The chiral salen ligands of Co(III) form showed the bands at near 250 and 370 nm on the UV spectra. But the pure siliceous MCM-41 no absorption peak at all. This broad band is probably due to the charge-transfer transitions between metal and ligand. This result indicates that the successful immobilization of chiral salen ligands was achieved. The vibrational circular dichroism(VCD) spectroscopy can be used to elucidate the stereochemistries of chiral molecules, including the accurate estimation of enantiomeric excess and their absolute configrations[20]. Optically pure samples as well as a racemic sample(c) were used as a reference to compare the VCD spectra. Three VCD spectra are shown in Fig. 7: a spectrum of 99 % ee R(-)-l-phenyl 1,2-ethanediol(a) and that of 99 % ee S( + )-l-phenyl 1,2-ethanediol(b) obtained from Aldrich Co., and the other is that of the product obtained on the Ti-MCM-41/chiral Co(in) salen catalyst(d). The VCD spectra of opposite configration, such as R(-) and S( + ) , exhibited the reverse absorption peaks as shown in Fig. 7. It is very useful to determine the absolute configuration and %ee value by these VCD spectra for the asymmetric reactions. In conclusion, the chiral salen Co(III) complexes immobilized on Si-MCM-41 colud be synthesized by muhi-grafting method. The asymmetric synthesis of diols from terminal olefins was applied with success using a hybrid catalyst of Ti-MCM-41/chiral Co(III) salen complexes. The olefins are readily oxidized to racemic epoxides over Ti-MCM-41 in the presence of oxidants such as TBHP, and then these synthesized diols are generated sequenfially by epoxide hydrolysis on the salen Co(III) complexes. This catalytic system may provide a direct approach to the synthesis of enantioselective diols from olefins.
788
1200
VNfave length(nnn)
Fig.6. UV-visible spectra of Co(in) Salen complex immobilized on MCM-41 (b), the homogeneous salen complex of the same structure (a), and a pure SiMCM-41(c).
1250
1300
1350
1400
1450
1500
V\fe^/enurTt)e^(cm^)
Fig.7. Vibrational circular dichroism(VCD) spectra of R(-)-l-phenyl 1,2-ethanediol, S( + )-l-phenyl 1,2-ethanediol and racemic 1-phenyl 1,2-ethanediol.
REFERENCES C. Neri, A. Esposito, B. Anfossi and F. Buonomo, Eur. Pat. 100119. C. Neri, B. Anfossi and F. Buonomo, Eur. Pat. 190609. A. Corma, Chem. Rev., 97 (1997) 2373. A. Tuel, Studies in Surface Science and Catalysis, 117 (1998) 159. K. A. Koyano and T. Tatsumi, Studies in Surface Science and Catalysis, 105 (1997) 93. S. Namba and A. Mochizuki, Res. Chem. Intermed. 24 (1998) 561. R. Ryoo and J. M. Kim, J. Chem. Soc. Chem. Commun. (1990) 71. Namba, A. Mochizuki and M. Kito, Studies in Surface Science and Catalysis, 117 (1998) 257. 9 H.S. Roh, J. S. Chang and S. E. Park, Korean J. Chem. Eng. 16 (1999) 331. 10. E. N. Jacobson, W. Zhang, A. R. Muci, J. R. Ecker and L. Deng, J. Am. Chem. Soc, 113 (1991)7063. 11. F. Minutolo, D. Pini and P. Salvadori, Tetrahedron Letters, 37 (1996) 3375. 12. S. B. Ogunwumi and T. Bein, Chem. Commun., (1997) 901. 13. P. Piaggio, P. McMom, C. Langham, D. Bethell, P. C. Bulman-Page, F. E. Hancock and G. J. Hutchings, New J. Chem., 22 (1998) 1167. 14. G.-J. Kim and S.-H. Kim, Catalysis Letters, 57 (1999) 139. 15. L. Frunza, H. Kosslick, H. Landmesser, E. Hoft, R. Fricke, J. of Molecular catalysis, 123 (1997) 179. 16. R. Ryoo, J. M. Kim, C. H. Shin and J. Y. Lee, Studies in Surface Science and Catalysis, 105(1997)45. 17. B.Kraushaar and J.H.C. Van Hooff, Catalysis letters, 1,(1988), 81. 18. M.R. Boccuti, K.M. Rao, A. Zecchina, G. Leofanti and G. Petrini, Stud. Surf. Sci. Catal. 48(1989) 133. 19. M. Tokunaga, J.F. Larrow, F. Kakiuchi and E. N. Jacobsen, SCIENCE, 277 (1997) 936. 20. K.M. Spencer, S.J. Cianiciosi, J.E. Baldwin, T.B. Freedman and S.A. Nafie, Applied Spectroscopy, 44 (1990) 235.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
789
Nano-Clusters, Enantioselective Catalysis and Molecular Recognition Contrast Agents in MCM-41. Part I. Douglas S. Shephard The University Chemical Laboratories, Lensfield Road, Cambridge, CB2 lEW, U.K. These papers describe the production and study of several high-performance materials based on MCM-41. First, bimetallic nanoparticle catalysts derived from precursor metal-cluster carbonylates anchored inside the mesoporous channels of silica. In-situ X-ray absorption and FT-IR spectroscopies as well as ex-situ high-resolution scanning transmission electron microscopy have been used to chart the progressive conversion, by gentle thermolysis, of the parent carbonylates to the denuded, bimetallic nanoparticle catalysts. These bimetallic catalysts exhibit no tendency to sinter, aggregate or fragment into their separate component metals during use and display good performance in the catalytic hydrogenation of hex-1-ene a detailed kinetic study. Secondly, the ordering of these particles within the channels at high loading and their conductivity on denuding. Thirdly, how the internal surface of mesoporous silica may be selectively fiinctionalised with propyl ammonium groups, and their presence and position may be directly imaged by high resolution electron microscopy (HRTEM) after 'staining' with the cluster-cluster crown compound [Ru6C(CO)i4(h6-C6H4CioH2006] • Fourthly, it is demonstrated that a chiral ligand (derived from 1,1'-bis-diphenylphosphinoferrocene (dppf)), bound to an active metal (Pd) and anchored via a molecular tether of appropriate length to the inner walls of a mesoporous silica support {ca. 30A diameter) yields a degree of catalytic regioselectivity as well as an enantiomeric excess that is far superior to those of the same free (i.e. homogeneous) catalyst in the synthetically useful process of ally lie amination. Fifthly, how a new reuseable heterogeneous oxidation catalyst immobilised within MCM-41 has been prepared and used in clean organic synthesis, with molecular oxygen as cooxidant. 1. INTRODUCTION In recent years, with the advent of mesoporous silicate materials, the science of oxide supports (especially siliceous ones) has undergone a revolution. These silicates have a highly regular structure composed of channels (diameter 20 to lOOA) which provide huge porosity and surface areas (>lOOOm^/g). The large pore sizes of these materials (c./ microporous zeolites) offer the possibility of creating catalytically active sites within the silicate framework, and the topological restraints produced by the confinement of solvent, substrate and reactant may be expected to give rise to greater efficiency and selectivity in the reaction process. 2 . BIMETALLIC CATALYSTS Allied to this, current interest in the use of bi-metallic catalysts is largely due to the enhanced activity and selectivity that may be achieved.'"^ The break-through came when a
790 catalytically active metal was used in conjunction with another that was significantly less active, but worked in a complementary manner. At Exxon Sinfelt' et al studied the Pt-Re and Ru-Cu systems for catalytic reforming and showed highly encouraging results for these systems. However, despite considerable effort, the preparation of these systems is limited by the rather crude method of metal salt deposition (metal salts are deposited onto an amorphous support material {e.g. alumina, carbon, silica) and then calcined under O2 before reduction to the active low valent species with H2). This method has many drawbacks, of which, the greatest being precise control over size, morphology and homogeneity of the bi-metallic particles cannot be achieved; these factors and the random nature of the support lead to loss of selectivity/activity of the catalyst - a wasteful and costly problem in industry. Several mixed-metal molecular cluster carbonyls that have recently been used as precursors for heterogeneous catalysts (e.g. Nasher et al? and Shephard et al, ^ In the former -i
^
2-
mstance, where the cluster anion was [Re6C(CO)i8{m -Re(C0)3}{m -Ir(C0)2}] , it was found that upon thermolysis the metals segregated into separate entities, possibly because of the nature of the oxide support used. In the latter, where the cluster anions were [Ag3RuioC2(CO)28Cl]^" and [Ru6C(CO)i6Cu2Cl]22-, there was no evidence for segregation and sintering, and we believe this to arise for two reasons. First, the mesoporous silica support, onto which the cluster carbonylate is initially anchored, is replete with silanol groups that interact strongly with the carbonylate anion. Second, the relative oxophilicity of the silver/copper atoms makes them ideal bonding centres securing the bimetallic cluster to the oxide support. Active Catalyst protective sheati capable of physisorption to^ support
and M^ + M^ known exactly
MVM^
and M U M^ is known exactly -highly disperse -MVM^
cluster deposition MCM41 (mesoporous silicate)
>
-highly regular -huge surface area sheath removal (activation + anchoring)
bimetallic particle
Scheme 1. Cluster deposition and activation.
2.1. Preparation and Characterisation When considering the more precise route of depositing and thermolysing carbonyl clusters for the production of supported nanoparticles, several important criteria concerning the choice of bimetallic cluster precursor have to be borne in mind."^ First, the protective sheath surrounding the organometallic precursor must be readily removable (we find that mild thermolysis suffices). Second, interactions with the surface must be stronger than those involved in solvation or between the precursor species so that aggregation into small molecular crystallites and subsequent sintering on the surface is suppressed upon removal of the CO sheath. Anionic cluster carbonyl species,^ typified by [Ru6C(CO)i6Cu2Cl]2[PPN]2
791 and [Ag3RuioC2(CO)28Cl][PPN]2, fulfil these criteria as their interaction with the MCM41 surface is of the Si-OH^"'"--d-0-C-M type^ and intermolecular Coulombic repulsion prevents their aggregation prior to thermolysis. A representation of this approach is given in scheme 1.
Figure 1. STEM images of the RuI2Cu4 catalyst in MCM-41 a) before and b) after vitrification by the electron beam. Inset labelling of particles a,b,c,d shows how they remain anchored in place during this process. Apart from gaining deep insights into the nanostructures and morphology of the resulting catalysts using a combination of annular dark-field (ADF) and bright field (BF) high resolution scanning transmission electron microscopy (see figure 1)7 we have tracked the precise structural details of the conversion of this precursor material into its active catalytic state principally by using in-situ X-ray absorption techniques. We were able to gain accurate local structural information for the catalyst by the application of these element specific (e.g. Cu and Ru) techniques. The X-ray absorption near edge structure (XANES) provides the electronic state and qualitative local structural information, whereas the extended X-ray absorption fine structure (EXAFS) establishes the precise local structural details, thus, revealing the internal structure of the nanoparticle (see figure 2 for example). Confirmation of the loss of the protective carbonyl sheath during thermolysis was found by in-situ FT-IR and ftirther corroborative evidence was established by thermogravimetric analysis. 2.2 Catalytic Evaluation The catalytic performance of the supported bimetallic nano-particles in the hydrogenation of unsaturated molecules was tested on a wide variety of unsaturated species: hex-1-ene, phenyl acetylene, diphenyl acetylene, trans-stilbene, cis-cyclooctene and D-limonene. The highly efficient hydrogenation of hex-1-ene was accompanied by the isomerisation reaction to cisand trans-hex-2-ene, which were subsequently hydrogenated (albeit at a much slower rate) as reaction ensued. Phenylacetylene is completely converted to ethylbenzene under the reaction conditions used. No hydrogenation of the phenyl group was detected. This shows a considerable degree of selectivity of the catalyst. This selectivity was ftirther illustrated in the hydrogenation of diphenylacetylene which gave both stilbene (predominently trans-) and bibenzyl.^ Careful kinetic studies at 20 bar hydrogen and 373 ^C show an induction time of 60 minutes and an
792 overall turnover frequency of 25,700 mol[Hex]mol[Cu4Rul2]-lh-l. The kinetic details for this catalyst and for a new Ag4Rul2 system are summarised in Fig. 3.
Figure 2. The formation of a Pd-Ru bimetallic nanoparticle catalyst from a single source bimetallic molecular carbonylate anion [Pd6Ru6(CO)24]2-. •H
95 Con.AgRu
-•
H2 (bar) AgRu
••
n-Hex«ne AgRu
^-
Cis-Hex-2-en* AgRu
•
Trans-Hex-2-#r>* AgRu
—
% Con.CuRu H2 (bar) CuRu
n-Hexane CuRu
Cis-H*x-2-*r>e CuRu
200
300
Time (mim)
Figure 3. A comparrison of the hex-1-ene hydrogenation behaviour of the two nanoparticle catalysts Ag4Rul2 and Cu4Rul2. Note how the silver containing catalyst is significantly superior in terms of TOF in the early stages of the reaction (i.e. no induction time).
793 Together these results demonstrate that our initial strategy of removing the stabilising CO sheath from a mixed metal cluster to produce a well-defmed metal nanoparticle and anchoring the more oxophilic second metal to the MCM-41 surface has met with success. This work also reveals that there is abundant scope for further exploitation of bimetallic metal-cluster carbonylates as precursors for other supported nanoparticle catalysts. Moreover, a wide range of catalytic reactions besides hydrogenation awaits study. 3. SUPRAMOLECULAR ORDERING In the course of our work on the exploitation of mesoporous silicas for the production of novel supported metallic catalysts we have discovered a method of producing ordered arrays of nanoparticles, in this case anionic ruthenium cluster carbonylates, [Ru6C(CO)i6]^' and [H2Ruio(CO)25]^" interspersed with bis(triphenylphosphino)iminium (PPN"^) counterions. Thus providing a new methodology for the production of ordered platinum group metal nanoparticles. The nature of these materials has been probed by high resolution electron microscopy (HREM both real and reciprocal space),^''^ FT-IR and other techniques. We have found that, in the case of the Ru6 and Ruio cluster carbonyls accommodated inside MCM-41 mesoporous silica, the metal cluster anions are repeated at ca. 17.0A and 26.6A respectively, along the axis of the mesopores (30+2A internal diameter), the inner surfaces of which are essentially structurally disordered. The formation process and structure of these one dimensional crystals and their inter-relation in the three dimensional framework will be discussed. In view of the intense current interest in the properties of nanoelectronic materials, ranging in dimentionality from zero (quantum dot) to two (i.e. ID and 2D), much effort has recently been expended,^' in developing novel ways of producing ordered arrays, including linked cluster networks of metallic nanoparticles in a size range down to ca. 15A. The only prior comparable preparation of linear arrays of metallic nanoparticles (apart from the special instances of "decorated" atomic steps on graphite'^ and molybdenite'^ surfaces with nanoparticles of the coinage metals and the insertion of materials inside carbon nanotubes)'"* is that of Schmid and Homyak, who recently reported that the 500A diameter pores of alumina membranes could be packed with 130A diameter, ligand stabilised Au55 gold colloids by vacuum induction. Figure 4 reveals the regular nature of the mesopores of the MCM-41 silica (inset Fourier shows spots in 100 direction); absence of 001 spots in the Fourier transform shows that there is no crystallographic order in the direction of the pore axis. Figures 5 and 6 show typical HREM and STEM bright field images (together with their Fourier transforms) of the MCM-41 loaded with the carbonylate salts. These images show regular repeats along the pore axis [001] in both Ru6.1 MCM-41 with d-spacings, derived from their Fourier transforms, of c^. 17 and 27A, respectively. An interpretation of the structural features contained in figures 5 and 6 may be afforded by examination of the individual cluster carbonylate salts and the possible packing motifs available to them. Bearing in mind charge compensation and CO stretching frequencies the individual hexa-ruthenate clusters may be viewed as being hydrogen bonded to the pore wall {via several interactions of the Si-0-H-O-C-Ru type), flanked by two PPN counterions. Given an internal pore diameter of 30±2A, the centre to centre distance between two hexaruthenate clusters along the pore axis is 17A at an angle (q) of 48 deg. (figure 7) and 1=25.5A.
794 This suggests that the clusters are most likely to pack such that they sit alternately on opposite sides of the MCM-41 channel, forming a zig-zag arrangement in the pore axis direction. In the case of MCM-41/[H2Ruio(CO)25][PPN]2 the clusters may also be assumed to be bound to the silica surface and separated by two PPN moieties with an ideal inter-cluster distance of ca. 21.ik (as found in the crystal structure). The average d-spacing calculated from the well defined diffraction spots [001] is about 26.6A which is very close to the linear model distance (figure 7). Consequently we may describe the packing of the deca-ruthenate clusters as approximating to linearity with 0° < 0 < 5°. This essentially linear arrangement of the deca-ruthenate clusters may be ascribed to their larger Van der Waals radii over that of the hexa-ruthenate anion and possibly their differing spatial linkages with the PPN counterions (as notedfi-omtheir crystal structures). A model detailing the packing of the [H2Ruio(CO)25]^" anion together with its [PPN]"^ cations inside a single mesopore which is consistent with all the facts retrievedfi-omFTIR and HREM (and FT) is given in figure 7.
.'jjii^&ii^.
Figure 4. HRTEM image of pure silica MCM-41, with its Fourier transform (inset), viewed perpendicular to the pore axis ([001] direction, indicated by the arrow). Scale bar, 10 nm.
Figure 5. HRTEM image of MCM-41 loaded with [Ru6C(CO)16 ][PPN]2, with its Fourier transform (inset). The average repeat distance derived from the reciprocal space (002) spots of the Fourier transform is 2.1 nm, corresponding to a projection of the average repeat distance on the [001] axis for the clusters. Scale bar, 10 nm.
Although much work remains to be done on these novel materials, many possible applications may be envisaged for these 'constrained' arrays of mono-disperse nanoparticles. It is already clear from additional experiments that it is possible to denude the clusters of their carbonyls by gentle heat treatment in vacuo thus producing nanoparticles of ruthenium metal. Further work, in this highly promising area, involves using clusters and counterions of different sizes to tailor the intercluster distance and examining the electronic, magnetic and optical properties. Indeed we have very recently examined the conductivity of an MCM-41 strand ca. 1 pores packed with denuded Cu4Rul2 clustrers. The \N graph thus produced shows 'electron hopping' behaviour with a bias voltage of ca. 2.5V which is perfectly consistent with the calculated capacitance of these small particles.
795
*»:nnp»]ainp»ininp' ,•
30A±2
A^=^niax-^ii
B
L2
^*i?>ait5i^t'i.+.
•30A±2
• dmin ' ^location of arc El = cation (PPN) ( ^ = cluster dianion (Ru6C(CO)i6)^- or (H2Ruio(CO)25)^Figure 6. STEM bright-field image (24)ofMCM-41 loaded with [H 2 Ru 10 (CO)25 ][PPN]2 (II) show-ing highly regular features along the pore axis, with its Fourier transform (inset). The repeat distance derived from the reciprocal space (001) spots of the Fourier transform is 2.95 nm. Scale bar, 20 nm.
Figure 7. A schematic diagram showing how the maximum and minimum d spacings can be derived from geometrical packing considerations. (A) Packing that gives the maximum d spacing dmax ; (B) packing that gives the minimum d spacing for a cluster carbonyl of given van der Waals radius. Because the intercluster spacing , is constant, the relative position of B to A is determined by angle q. Therefore, B must lie somewhere on an ellipse (in a cylindrical channel), and the d spacing dmin is determined by dmin =l(cos q). The observed packing may be expected to lie within the limits 0 350 nm) at room temperature and 77 K. To confirm the encapsulation of [Re(I)(C0)3(bpy)(py)]^ into the AlMCM-41 pore, Xe-NMR spectra of AlMCM-41 have been measured before and after the [Re(I)(C0)3(bpy)(py)] encapsulation. The quenching of photoluminescence from [Re(CO)3(bpy)(py)]7AlMCM-41 by CO2 has been studied to apply this system to photoinduced activation and reduction of CO,.
1. INTRODUCTION Among the mesoporous M41S molecular sieves synthesized recently by Mobil's scientists, MCM-41 molecular sieves show a regular hexagonal array of uniform mesopores with diameter from 15 to 300 A , a high surface area of about 1000 mVg, and a pore size distribution nearly as sharp as that of zeolites [1-3]. The MCM-41 materials with these unique properties have promising utility for catalysis and separation of bulky molecules and various research areas. The MCM-41 materials have been synthesized by hydrothermal heating at about 100 °C for several days or prolonged reaction at room temperature. Recently, Bein et al. and our group reported that the mesoporous MCM-41 materials can be synthesized in short crystallization time using microwave irradiation instead of hydrothermal heating [4,5]. Based on host-guest interaction, microporous zeolites have been used as heterogeneous host for encapsulation of metal complexes and organometallic fragments. For zeoliteencapsulated photosensitizer, the steric and electrostatic constraint imposed on the complexes within the channels or cages of zeolites can alter the photochemical and photophysical properties of the guest complexes and diminish the photodegradation and undesirable electron transfer reactions [6]. But, the pore sizes (-13 A) of microporous zeolites are too small for
808 encapsulation of bulky photosensitive guest. Therefore, it has been reported that mesoporous MCM-41 materials are used as heterogeneous host for the encapsulation of bulky photosensitizers such as triphenylpyrylium [7], porphyrin [8], phthalocyanin [9], and rhodamindye [10]. Recently, the interest in Re (I) complexes has been increased due to their potential utility for the activation and reduction of CO2 into CO and C03^' in a purpose of construction of artificial photosynthetic systems [11-13]. Rhenium Complexes such as ReX(C0)3(bpy) (X=C1, Br) and Re(CO)2(bpy)[P(OEt)3]2have been used as photocatalysts for CO2 reduction to CO in solvent mixture of triethanolamine/dimethylformamide [12,13]. Most of the research on photochemical activation and reduction of CO2 using Re(I) complexes have focused on the homogeneous solution systems. There are few reports concerned about the encapsulation of rhenium complexes into molecular sieves and their photochemical application to the photochemical reduction of CO2. In this study, we focus on the encapsulation of [Re(I)(C0)3(bpy)(py)]^ into mesopore of AlMCM-41 and its photophysical characterization using XRD, FTIR, Xe-NMR, diffuse reflectance (DR) UV-visible, electron spin resonance (ESR), and photoluminescence spectroscopy with photoirradiation and CO2 adsorption.
2. EXPERIMENTAL For the synthesis of AlMCM-41 (Si/Al = 30), myristyltrimethylammonium bromide [MTAB, C,4H29N(CH3)3Br] was used as a quaternary ammonium surfactant. A sodium silicate solution was prepared by mixing aqueous NaOH solution with Ludox HS 40 (39.5 wt % Si02, 0.4 wt % Na20 and 60.1 wt % HjO, Du Pont) as a colloidal silica source with stirring in water bath (60 °C) for 30 min. A1(N03)3 was used as an Al source. This sodium silicate solution was slowly added to 25 wt % aqueous MTAB solution with vigorous stirring at room temperature for Ih. The pH of the precursor gels was adjusted to 10 by adding the dilute sulfuric acid. The molar composition of the final MCM-41 precursor gel was Si02: AI2O3 : MTAB : NaOH : H2O = 1.0 : 0.033 : 0.167 : 0.5 : 40.5. The precursor gels were heated in a oven at 100 °C for 3 days. The MCM-41 solid products were filtered, washed with de-ionized water, and dried in air at 100 °C for 10 h. The as-synthesized sample was calcined at 550°C for 6 h in air. [Re(CO)3(bpy)(py)]^PF6" was used as a photosensitizer. The bulky [Re(C0)3(by)(pi)]" complex was encapsulated into the mesoporous AlMCM-41 (Si/Al = 30) molecular sieve by ion-exchange method using the aqueous solution of [Re(CO)3(bpy)(py)]TF6". The encapsulated rhenium complex, [Re(CO)3(bpy)(py)]7AlMCM-41, was evacuated (< 10"'bar) at 100 °C for 16 h. Then, [Re(CO)3(bpy)(py)]7AlMCM-41 was characterized by XRD, FTIR, diffuse reflectance (DR) UV-visible, ESR spectroscopy with photoirradiation (X > 350 nm). Photoluminescence spectra were measured with photoirradiation and CO, adsorption .
3. RESULTS AND DISCUSSION Figure 1 shows a schematic feature of the encapsulation of [Re(C0)3(bpy)(py)]" into mesoporous AlMCM-41. Figure 2 shows XRD patterns of AlMCM-41 and [Re(C0)3
809
hv(k
> 350 nm)
[Re(I) (C0)3(bpy)(py)]^ /AlMCM-41
Figure 1.
Encapsulation of photosensitive
[Re(I) (CO)3(bpy)(py)]"into AlMCM-41.
Figure 2.
XRD patterns of (a) AlMCM-41
and (b) [Re(I) (C0)3(bpy)(py)]" /AlMCM-41.
166.21 ppm
[Red) (C0)3(bpy)(py)]^ /AlMCM-41
(a)AlMCM-41 58.67 ppm (b) [Re(C0)3(bpy)(py)]^ /AlMCM-41
[Red) (C0)3(bpy)(py)]Chemical shift
2100
2050
2000
1950
1900
1850
Wavenumber/ cm"^
Figure 3.
Xe-NMR spectra of (a) AlMCM-41
and (b)[Red) (CO)3(bpy)(py)]VAlMCM-41.
Figure 4. FTIR spectra with encapsulation of [Re(I) (C0)3(bpy)(py)]'into AlMCM-41,
810 (bpy)(py)]7AlMCM-41. With the encapsulation of [Re(CO)3(bpy)(py)]Mnto AIMCM-41 by ion-exchange method, there is a Httle change in d,oo but the overall intensities of the two XRD patterns are almost same. This result indicates that there is no significant change of AIMCM41 structure and that the AlMCM-41 framework is stable enough to act as heterogeneous host for the encapsulation of complexes. Figure 3 shows Xe-NMR spectra of AlMCM-41 and [Re(CO)3(bpy)(py)]7AlMCM-41. The difference of chemical shifts of these two Xe-NMR spectra supports that the rhenium complex is encapsulated inside the mesopore of AlMCM-41 [14]. FTIR spectra in Figure 4 also support the encapsulation of [Re(C0)3(bpy)(py)]^ into the mesopore of AlMCM-41. The frequencies of three CO ligands of [Re(C0)3(bpy)(py)]" are changed after encapsulation into AlMCM-41. This frequency change due to the encapsulation seems to be ascribed to the steric hindrance and electronic interaction exerted to [Re(C0)3(bpy)(py)]" restricted inside the AlMCM-41 mesopore. Both Xe-NMR and FTIR spectra provide the evidences for the encapsulation [Re(C0)3(bpy)(py)]" into the AlMCM-41 mesopore. To study for the photochemistry of [Re(I)(CO)3(bpy)(py)]7AlMCM-41, ESR, DR UV-visible, FTIR, and PL spectra were measured with photoirradiation and CO2 adsorption. After photoirradiation, [Re(CO)3(bpy)(py)]7AlMCM-41 gave ESR spectrum in Figure 5 which can be assigned to b p y radical in [Re(CO)3(bpy')(py)]/AlMCM-41 [15]. The DR UV-visible absorption spectra of [Re(I)(CO)3(bpy)(py)]7AlMCM-41 are shown in Figure 6. With photoirradiation, the new absorption bands appeared at 380-530 nm as shown in Figure 6(b). These new bands are assigned to the [Re(I)(C0)3(bpy')(py)] radical [16,17]. The spectrum in Figure 6(b) is similar to UV-visible absorption spectrum of [Re(I)(C0)3(bpy')(py)] radical photoinduced in the solution of [Re(C0)3(bpy)(py)]^ and TEOA (triethanolamine) in DMF. It has been known that TEOA acts as an electron donor 1.0
MLCT (metal-to-ligand charge transfer) absorption
0.5
/ g = 2.0029 AHpp=14G AH t o t a l s 120 G
Figure 5. ESR spectrum of [Re(C0)3(bpy)(py)]^ with photoirradiation at room temperature.
0.0-i 300
400
500 600 W a v e l e n g t h / nm
Figure 6. DR UV-vis spectra of [Re(I) (CO)3(bpy)(py)]VAlMCM-41 after (a) evacuation at 100 °C, (b) photoirradiation (A. > 350 nm), and (c) 20 torr CO2 adsorption.
811 [16]. In [Re(I)(CO)3(bpy)(py)]7AlMCM-41, the [Re(I)(C0)3(bpy-)(py)] radical was produced with photoirradiation in the absence of an electron donor such as TEOA. FTIR spectra in Figure 7 also supports the formation of [Re(C0)3(bpy")(py)] radical in AlMCM-41 with photoirradiation. The frequencies of v(CO)'s of [Re(CO)3(bpy)(py)]7AlMCM-41 shift to lower values as shown in Figure 7, which indicates the formation of the [Re(C0)3(bpy")(py)] radical [18]. It has been known that the reduction of the metal-carbonyl complexes is accompanied by a lowering of v(CO) in the reduced complexes due to back-donation [18b]. FTIR spectra in Figure 7 indicate that [Re(C0)3(bpy)(py)]" in AlMCM-41 is reduced into [Re(C0)3(bpy")(py)] with photoirradiation. It has been known that the frameworks of the aluminosilicate zeolites show the electron-donating property to proper electron acceptors encapsulated in their pores and that the steric hindrance stabilizes the generated radical species restricted in the pores [6]. From the results of ESR, DR UV-visible, and FTIR spectra, it can be supposed that the aluminosilicate framework of AlMCM-41 may act as an electron donor in the photoinduced formation of [Re(I)(C0)3(bpy')(py)] radical in AlMCM-41. [Re(CO)3(bpy)(py)]VAlMCM-41 showed photoluminescence (PL) at 77 K due to MLCT (metal-to-ligand charge transfer) band as shown in Figure 8(a). With CO, adsorption and photoirradition, the PL intensity decreased greatly as shown Fig. 8(c), which indicate that CO2 interacts with MLCT band of [Re(C0)3(bpy-)(py)] complex [19]. The result suggest that [Re(C0)3(bpy)(py)]" in AlMCM-41 is reduced into the radical of [Re(C0)3(bpy-)(py)] by photoirradiation in the presence of CO2 which may act as an active species in photoinduced activation and reduction of C0>
evacuated at 100 °C
1
I
I
2100
2050
2000
1950
1900
1850
1800
Wavenumber/ cm" Figure 7. FTIR spectra of [Re(I) (CO)3(bpy)(py)]VAlMCM-41 with photoirradiation (>. > 350 nm).
450
500
550
600
Wavelength (nm)
Figure 8. Photoluminescence spctra of [Re(I) (CO)3(bpy)(py)]7AlMCM-41 after (a) evacuation at 100 °C, (b) photoirradiation, (c) and CO2 adsorption at 77 K.
(
812
4. CONCLUSIONS In this work, we have discussed the encapsulation of bulky [Re(I)(CO) 3(bpy)(py)]^ into the mesoporous AlMCM-41 and the spectroscopic characterization using XRD, Xe-NMR, FTIR, ESR, DR UV-visible, and PL spectroscopy with photoirradiation and CO, adsorption. Xe-NMR spectra provide the evidences for the encapsulation of [Re(I)(C0)3(bpy)(py)]* into AlMCM-41. With photoiradiation over [Re(I)(C0)3(bpy)(py)]' /AlMCM-41, the formation of [Re(I)(C0)3(bpy-)(py)] radical was observed by ESR, DR UV-visible, and FTIR spectroscopy. In [Re(CO)3(bpy)(py)]VAlMCM-41 system, it seems that the aluminosilicate framework of AlMCM-41 may act as an electron donor for the photoinduced formation of [Re(C0)3(bpy)(py)] radical which is an active species for activation and reduction of CO2. ACKNOWLEDGMWNTS This work was financially supported by the Ministry of Science and Technology in Korea and partly by the Korea Science and Engineering Foundation (KOSEF).
REFERENCES 1. J. S. Beck and J. C. Vartuli, Current Opinion in Solid State and Mater. Sci., 1 (1996) 76. 2. (a) C. T. Kresge , M. E. Leonowicz, W. J. Roth, J. C. Vartuli, and J. S. Beck, Nature, 359 (1992) 710. (b) J. S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. O. 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. 3. J. C. Vartuli, S. S. Shih, C. T. Kresge, and J. S. Beck, in Mesoporous Molecular Sieves 1998, Stud. Surf. Sci. Catal., L. Bonneviot, F. Beland, C. Danumah, S. Giasson and S. Kaliaguine, Eds., Vol.117, p. 16, Elsevier, Amsterdam (1998). 4. C. -G. Wu and T. Bein, Chem. Commun., (1996) 925. 5. S.-E. Park, D.S. Kim, J.-S. Chang and W.Y. Kim, Catal. Today, 44 (1998) 301. 6. K. B. Yoon, Chem. Rev., 93 (1992) 321 and references therein. 7. M. L. Cano, A. Corma, V. Fomes, H. Garcia, M. A. Miranda, C. Baerlocher, and C. Lengauer, J. Am. Chem. Soc.,118 (1996) 11006. 8. H. M. Sung-Suh, Z. Luan, and L. Kevan, J. Phys. Chem. B, 101 (1997) 10455. 9. I. Honma and H. S. Zhou, Chem. Mater., 10 (1998) 103. 10. H. Yamashita, A. Tanaka, M. Nishimura, K. Koyano, T. Tatsumi, and M. Anpo, Stud. Surf. Sci. Catal., 117(1998)551. 11. M. Halmann, Nature, 275 (1978) 30. 12. S. Tazuke and N. Kitamura, Nature, 275 (1978) 301. 13. J. Hawecker, J.-M. Lehn, and R. Ziessel, Helv. Chim. Acta, 69 (1986) 1990. 14. J.-F. Wu, T.-L. Chen, L.-J. Ma, M.-W. Lin, and S.-B. Liu, Zeolites, 12 (1992) 86. 15. (a) W. Kaim and S. Kohlmann, Inorg. Chem., 29 (1990) 2909. (b) W. Kaim and S. Kohlmann, Chem. Phys. Lett., 139 (1987) 365. 16. H. Hori, F. R A. Hohnson, K. Koike, O. Ishitani, and T. Ibusuki, J. Photochem. Photobiol. A: Chem., 96 (1996) 171. 17. K. Kalyanasundaram, J. Chem. Soc. Faraday Trans., 2 (1986) ,2401. 18. G. J. Stor, F. Hartl, J. W. M. van Outersterp, and D. J. Stufkens, Organomet., 14 (1995) 1115. 19. M. Anpo and K. Chiba, J. Mol. Catal., 74 (1992) 207.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
813
Physico-chemical and catalytic properties of MCM-41 mesoporous molecular sieves containing transition metals (Cu, Ni, and Nb) M. Ziolek, I. Nowak, I. Sobczak, A. Lewandowska, P. Decyk and J. Kujawa A. Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, 60-780 Poznan, Poland This paper reviews our recent works concerning Nb-containing mesoporous materials of MCM-41 type, their modification with Ni, Cu, and NH4^, characterization and possible application in the following catalytic reactions: (i) decomposition of NO, (ii) hydrosulfunzation of methanol, (iii) dehydrosulfurization of thiols and sulfides, and (iv) oxidation of thioethers with H2O2. The properties of M-NbMCM-41 sieves are compared with those of their A1-, V-, and Ti-analogous. Cu-NbMCM-41 materials show a higher resistance to SO2 than CuAlMCM-41 in the NO decomposition. Nb-containing samples exhibit active oxygen that takes part in the catalytic oxidation. In addition they present LAS and BAS pairs useful for thiol formation from alcohol and H2S. 1. INTRODUCTION Since the first synthesis of siliceous mesoporous molecular sieves described in the literature in 1992 [1], several mesoporous materials possessing various T atoms together with Si in the lattice have been prepared. The synthesis and properties of niobium- and siUceous-containing mesoporous sieves of MCM-41 type were first time described by our group [2,3] and almost parallely, Nb-doped mesoporous sieves were synthesized by Zhang and Ying [4]. The potential catalytic application of mesoporous molecular sieves is addressed to the adsorption and transformation of bulky compounds which are not capable of diffusing in the micropores of zeolites. In the case of Nb-containing mesoporous molecular sieves, their unique oxidizing properties [5] make them interesting also for the reactions of smaller molecules. This paper partially sunmiarizes our recent works [2,3,5-13] concerning the physicochemical and catalytic properties of Nb-containing mesoporous sieves. The behavior of NbMCM-41 materials is compared with the one of AlMCM-41 molecular sieves. Moreover, we discuss how the mesoporous matrices (i.e. aluminosilica (AlMCM-41) and niobiosihca (NbMCM-41)) affiect the properties of the protons-, nickel-, and copper-exchanged materials. The choice of the transition metals incorporated to the mesoporous sieves depended on the fixture use of the materials either as sorbents or catalysts in the transformation of sulfiir compounds (Nb- and Ni-materials), or in the catalytic redox reactions (Cu- and Nb-sieves). 2. EXPERIMENTAL AlMCM-41, NbMCM-41, VMCM-41, and TiMCM-41 materials with vanous Si/T ratio (T = Al, Nb, V, Ti) were synthesized according to the procedures described in [2,14].
814 Hydrogen forms of these materials were obtained via exchange of Na^ for NH^^ m an aqua solution, followed by the deanmionation at 673 K under vacuum or in the He flow. Copper modified NbMCM-41 and AlMCM-41 sieves were prepared in three ways: i) by the cation exchange with copper acetate solution, ii) by the impregnation with Cu(N03)2, and in) by the incorporation of Cu^* during the synthesis. In the case of the ion exchange, the obtained samples, after filtration, were calcined at 673 K for 4 h. Nickel nitrate solution was used for both the impregnation and the cation exchange in the mesoporous matrices. The cation exchange was conducted at room temperature (RT). In the impregnation procedures various time of mixing at RT prior to the evaporation was used. The catalysts are denoted by symbols where the number after the name of sieves descnbes Si/T ratio, whereas the next one stands for the degree of the cation exchange. The physico-chemical properties were studied by means of N2 adsorption/desorption (at 77 K with Micromeritics 2010 apparatus), XRD (TUR 42 diffractometer with CuKa radiation), ESR, H2-TPR and FTIR techniques. The ESR measurements were conducted after evacuation of the catalysts at vanous temperatures (RT to 723 K) and after NO adsorption at RT. ESR spectra were recorded at 77 K on a RADIOPAN SE/X 2547 spectrometer. The patterns were obtained at V^SR = 8.9 GHz FTIR study was performed using a VECTOR 22 (BRUKER) spectrometer. The self supported discs of -10 mg cm'^ of the catalysts were activated under vacuum at 723 K Pyndine was admitted at RT and after saturation the samples were degassed at RT, 423, 523, and 623 K in vacuum for 30 min. Temperature-programmed reduction (H2-TPR) of the samples was earned out using H2/Ar (10% vol) as reductant (V=32 cm^ mm'^). The sample (40 mg) was treated in the He flow at 673 K for 1 h and cooled to RT. Then it was heated at a rate of 10 K min'^ to 1300 K under Hj/Ar. H2 consumption was measured by a TCD detector in the PulseChemiSorb 2705 apparatus. The catalytic reaction between thioethers and hydrogen peroxide was conducted at 303 K using 35% H2O2 (2 mmoles), thioether (96% n-BuzS - 2 mmoles) and methanol as a solvent. The decomposition of organic sulfiir compounds was studied in a pulse micro reactor in the temperature range 523-623 K with the GC analysis of reactants and products. The reaction between methanol and hydrogen sulfide was conducted in a flow system at 623 K, using a mixture of H2S and CH3OH with the 2:1 molar ratio. The decomposition of NO was measured in a flow system at temperatures 473-923 K. The reaction conditions are described in [11]. 3. RESULTS AND DISCUSSION 3.1. Physico-chemical properties of mesoporous matrices i./.i. Nitrogen adsorption/desorption and XRD studies The most rehable information about the mesoporous structure of sohds comes fi-om lowtemperature nitrogen adsorption isotherms which enable the calculation of the specific surface area, pore volume, and pore size distribution. Figure 1 shows the N2 adsorption isotherms of the purely siUceous MCM-41, niobium-containing MCM-41, and AlMCM-41. They are typical of reversible adsorption type IV and at relative low pressures (p/po < 0.3) are accounted for by monolayer adsorption of nitrogen on the walls of the mesopores. As the relative pressure increases (p/po > 0.3), the isotherm exhibits a sharp inflection, characteristic of the capillary condensation within uniform mesopores, where the p/po position of the inflection point is
815
0,0 H
0,2
0,4
0,6
800
0,8
1.0
1,2
1,4
1,6
B
AIMCWM1-16
E 600 o -a
•e
400H
\
o 0) T3 (0 0)
AIMCM-41-32 200H
E 0,2
0.4
0,6
0,8
1.0
Relative pressure, p/p
1.2
1.4
2
4
6
8
Pore radius, nm
Figure 1. N2 adsorption/desorption isotherms and pore size distribution of MCM-41 containing various T-atoms. related to the diameter of the mesopores. The hysteresis loop is caused by the capillary condensation in the secondary interparticle mesopores, and the steepest part is in the p/po range close to the saturation pressure. The biggest hysteresis loop and the poorest XRD pattern (shown in Figure 3) are observed for NbMCM-41 with Si/Nb = 16. The BJH plot of the N2 physisorption on the purely siliceous MCM-41 (Figure IB) leads to a remarkably narrow pore size distribution with a pore radius of ca. 1.5 nm and a very high surface area of 1140 m^ g\ The extra peak in the pore size distribution in NbMCM-41-16 at ca. 1.8 nm can be ascribed to the intraparticle pores, and is similar to the ones in vanadium- and titanium-containing MCM-41 [15,16]. The highest homogeneity in the pore size distribution of the Nb-containing materials is registered in NbMCM-41 with Si/Nb = 64 (Figure 1 A). The incorporation of high amount of niobium results in the formation of an additional pore system and the achievement of an incoherent structure. A stirring with water (at RT, for 8 h) leads to the well ordered homogeneous material, confirmed by the adsorption/desorption isotherm and the pore size distribution (Figure 1 A). The extra pore system in AlMCM-41 (both with Si/Al = 16 and 32) is evident due to the pore size distribution plot (Figure IB). At the higher content of aluminum, not only was a bimodal pore distribution registered, but also a main XRD peak at 2.0 nm was broadened. 3,1.2, Hydrothermal and mechanical stability Some authors reported that the structure of MCM-41 is very sensitive to moisture [17,18].
816 Their results demonstrated the collapse of the pore structure of AlMCM-41, which occurred upon rehydration of the sample at room temperature. This phenomena was due to the hydrolysis of the bare Si-O-Si(Al) bonds in the presence of water vapor [18]. When the AlMCM-41 sample was left in contact to air for 3 months, its structure completely collapsed [19]. NbMCM-41 materials exhibit relatively high hydrothermal stabihty, much higher than AlMCM-41 sieves. 60 r AJMCM-41-16
Figure 2. Influence of the hydrothermal treatment on the structure of NbMCM-41 and AlMCM-41 - XRD patterns of a) a fresh sample (after calcmation at 773 K), b) after treatment in water vapor at 373 K, or c) at 573 K, or d) at 723 K, and e) after final calcination at 773 K for 5 h.
The exposition of NbMCM-41 molecular sieves to the atmosphere, for 4 months, does not change significantly their XRD patterns, which indicates their resistance to humidity from the atmosphere. Heating of the sample in the presence of water vapor only shghtly influences the XRD pattern (Figure 2). It is due to the incorporation of water into the pores that decreases the X-rays scattering. Contrary to the results noted on AlMCM-41-16 the calcination at 723 K of hydrotermally treated NbMCM-41 was nearly able to restore the well resolved structure (Figure 2A e). The hydrolysis of Si-O-Al bonds must occur in the AlMCM-41 material, which causes the irreversible changes in the sample. Niobium-containing MCM-41 sieves exhibit much higher mechanical stability than their aluminum analogous [2]. That was evident in XRD patterns obtained after pressing of the samples under increasing pressure, which was described in our earher work [2]. 3.2. Physico-chemical properties of NbMCM-41 and AlMCM-41 modified with NB/, copper, or nickel 3,11, The structural properties The physico-chemical properties of nickel loaded mesoporous materials depend on the nature of the matrix. The modification of Al-containing MCM-41 samples with Ni via cation exchange does not change significantly the N2 adsorption/desorption isotherms, the pore size distribution, and an ordering of the material, whereas, NbMCM-41 is not resistant to the Nimodification. As evident in Figure 3, the N2 adsorption/desorption isotherms of Ni-NbMCM41 exhibits shape characteristic of non-porous or macroporous materials (type II). The Niexchanged sample does not show mesopores. The XRD pattern of the Ni-NbMCM-41 material reveals one not intensive peak (100). The modification of AlMCM-41 by Ni via impregnation causes the significant decrease of both the surface area and pore volume. The adsorption isotherm of this sample shows smaller volume of nitrogen adsorbed in mesopores in comparison with the two other Al-containing materials presented in Figure 3. The same behavior of disordering the hexagonal structure was observed in copper-
817
exchanged niobium-containing samples as shown in Figure 3. The introduction of copper mto the mesoporous molecular sieves during the synthesis (sample denoted CuSiMCM-41) does not move the N2 adsorption/desorption isotherms towards type IV. Ni-AIMCNM1^2-46 AIMCM^1-32
1-41-32
0.4
0.8
12
1,6
RelatK^ pressiie, p/p
0.0 02
0,4 0,6 0,8 1,0
Relative pressure, p/p
1,2
1,4
4 20,
Figure 3. N2 adsorption/desorption isotherms (A) and XRD patterns (B) of nickel- and coppercontaining AlMCM-41 and NbMCM-41. 3,2,2. The reducihility of nickel and copper cations in Nh- and Al-containing MCM-41 It is known that transition metal cations, which occupy extra lattice positions in zeolites, are reduced while activated under vacuum or in an inert gas flow [20]. The same occurs while applying mesoporous matrices for metal cations. The species formed during the auto reduction depend on the nature of a matrix, the conditions of cation modification, and activation They can be identified by H2-TPR, ESR, and NO/FTIR measurements. As example. Figure 4 displays H2-TPR profiles of AlMCM-41 mesoporous sieves modified in various ways with nickel [12]. Two samples were impregnated by Ni(N03)2 (profiles a and b), and the third was modified by the cation exchange. If a isolated soUd wetted with the solution was mixed for 1 h at RT under vacuum to obtain a homogeneous mixture, pnor to an evaporation at higher temperature, two peaks were registered in H2-TPR (profile a). A low temperature (LT) peak is assigned to NiO -> Ni® reduction, whereas a high temperature (HT) one is due to the reduction of Ni-cations. This suggests that during the impregnation procedure a partial cation exchange has occurred. However, when the mixture after wetting was inmiediately evaporated, the H2TPR profile (Figure 4b) exhibited only one peak because of the reduction of NiO species. The broad peak registered for 600 SCO 1000 1200 the cation-exchanged sample originsfi*omthe reduction of Temperature, K Ni^^ and/or Ni* isolated cations (Figure 4c). The partial Figure 4. H2-TPR profiles of reduction of Ni^^ to Ni^ during the activation was confirmed a) Ni/Al-MCM-41-32, b) by the adsorption of NO followed by the ESR measurements Ni/AlMCM-41-16, and c) The FTIR study after NO adsorption on the activated NiNi-AlMCM-41-32-45 [12]. AlMCM-41 material indicated the presence of Ni^^ cations.
818
Thus, Ni-exchanged materials after activation under vacuum or in helium flow possess both Ni^^ and Ni^ cations. This determines their catalytic activity. The auto reduction of copper in the Cu-exchanged MCM-41 materials depends on the nature of a matrix and occurs according to: 2[Cu''0H"]' :^ Cu' + Cu'^O" + H2O (1) The followmg species were identified on the activated CuAlMCM-41 mesoporous sieves [5,11,13]: Cu^ Cu\ and Cu'^O; They were concluded on the basis of NO/FTIR, NO/ESR, and H2-TPR studies. As example. Figure 5 [5,11] shows the reduction of Cu^^, the change of its coordination due to the activation temperature, and the effect of NO adsorption 3500 measured by ESR spectroscopy. In the sample evacuated at RT the ESR signals indicate the presence of Cu^^ cations coordinated Figure 5. ESR spectra of with six H2O molecules forming octahedral structure described CuAlMCM-41.32-132 by gir2.37 and A||=138 G. That was also reported by Kim et al. evacuated at: a) RT, A - [21], The evacuation at a temperature above 373 K causes the expanded intensity scale, transformation of this structure to Cu-tetrahedral coordinated, b) 573 K, c) 723 K and d) confirmed by gir2.31 and A||=164 G. NO adsorption on the after NO adsorption [5]. sample evacuated at 723 K gives rise to a weak ESR signal, which can origin fi-om Cu>10 or Cu^^OTsfO complexes (both paramagnetic) [22]. The reduction of Cu^^ in Cu-NbMCM-41 occurs easier than in Cu-AlMCM-41. That was concludedfi-omthe ESR signal due to Cu^^ which significantly diminishes while Cu-NbMCM41 is evacuated at 523 K and it completely disappears after evacuation at 723 K [5]. 3,2,3. Acidic and redox properties Depending on niobium location, the Nb-containing catalysts can reveal Bronsted acid, Lewis acid, or redox properties. Niobium oxide cationic species (NbOn(^'^°)^), which occupy the extra lattice cation positions, play the role of the Lewis acid sites and may exhibit the redox properties. Nb localized in the firamework of mesoporous MCM-41 sieves provides the Lewis acidity [3,4] and the oxidizing properties [5,12]. Nb-containing MCM-41 sieves represent Lewis 1700 1600 1500 14( acidity proven by FTER study conducted after Wavenumber, cm"'' pyridine adsorption [3,4]. Hydrogen forms of Figure 6. FITR spectra after desorption niobium-containing MCM-41 materials exhibit lower of pyridine at RT of a) H-AIMCM-41- Bronsted acidity than that in hydrogen aluminosilicate 16 and b) H-NbMCM-41-16 [3]. mesoporous molecular sieves (see the band at 1549 cm'* in Figure 6 [3]). The dehydroxylation of H-NbMCM-41 samples causes the formation of the following lattice species: Py:L 1450
S i - O ^ ^1
^0-Si •
0
Si-0^
Si-0'
^0-Si- •
0
Si-0'
^NbCT
^ 0 - Si
(2)
819
The number of Lewis acid sites (lattice or extra lattice niobium species), measured from the absorbance of the IR band due to pyridine adsorbed on Lewis acid centers (a band at -1450 cm'^), is much higher on H-NbMCM-41-16 than on the hydrogen form of alummosilica molecular sieve of MCM-41 type [3]. :^Nb-0' species (denoted M-Nb-0" m the text below) could play a role of the Lewis base or the oxidizing center. The existence of the latter species was proven by ESR measurements [5]. The paramagnetic centers were observed in the ESR spectra of all niobium-containing mesoporous molecular sieves. Their character depends on the evacuation temperature. The evacuation at 573 K gives rise to a signal (g = 2.031 and 2.005) Uke that described in the hterature for Nb205 doped Ti02 [23], which was assigned to the oxygen species formed by photo-irradiation of Nb=0 species, changed into Nb-0" species. The following evacuation of the sample at 723 K reveals the arising of a sharp signal with g = 1.997. Such a signal is characteristic of a hole center generated by oxygen present in the semiconductors. It was also described for niobium oxide evacuated at 773 K and interpreted as a hole localized mainly on an oxygen atom and near a niobium atom [24]. 3.3. Catalytic properties of mesoporous molecular sieves 3,3.1 The catalytic decomposition of NO The easier reduction of copper in the NbMCM-41 material could result in the higher conversion of NO to N2 and O2. On the other hand, the negative charge on oxygen in the M-Nb-0' species interacts more strongly with all copper cationic species than the matrix of AlMCM-41. Thus, the copper species in the NbMCM-41 matrix does not adsorb NO too strongly, which causes its lower activity in the NO decomposition than that registered on CuAlMCM-41 [5,8,13]. Although the Cu-NbMCM-41 materials exhibit lower activity in this reaction, they are resistant to SO2 poisoning [13]. It seems that this behavior of niobium matnx could be exploited. 3,3,2, Hydrosulfurization of methanol The reaction between methanol and hydrogen sulfide is very demandmg, and many features can change its selectivity. This reaction can lead to the formation of thiol and/or sulfide: 100
c .2 {2 eo
> §404 I
^^_^^^^^^_^,^ 80 2. l+M)MCiyM1-16 CD l+NbMCNMI^! 60. H-NbMCI^I-GA o I+AIMCN441-16 R2S02 sulfone
(5)
The high activity to sulfoxide can be reached when the mitial activity of the catalyst is high because the formation of sulfone from sulfoxide is a slow reaction. The conversion of n-Bu2S versus a reaction time is plotted in Figure 8. All niobium-containing mesoporous sieves present high activity in the n-dibutyl thioether conversion, and the reaction occurs NbMCM-41-16 without an induction period [5,12]. One should pomted out H-NbMCM-41-16 T1MCM^1-16 that the highest selectivity to sulfoxides is on the Nb- and HNbMCM-41 catalysts (-99%), whereas other materials 0 100 200 300 studied (Nb205, TiMCM-41) show a lower selectivity to Time, min R2SO compounds. It is due to a lower activity in the first Figure 8. The activity of reaction period. Thanks to that the second reaction to R2SO2 different catalysts in oxidation has a chance to occur. Vanadium MCM-41 also exhibits high of n-dibutyl sulfide with activity, comparable wdth that of NbMCM-41 matenals. hydrogen peroxide. However, because of vanadium leaching from the catalyst during the reaction, it cannot be regenerated. The NbMCM-41 catalysts, even after few regenerations, still exhibit high activity in the thioethers oxidation [5,12]. 4. SUMMARY • The heterogeneity of the pore size distribution in both matrices (AlMCM-41 and NbMCM41) has been observed. It was more evident when Si/T ratio was lower. • Nb-containing mesoporous sieves are hydrothermally and mechanically more stable than AlMCM-41. • Dehydroxylated (H)NbMCM-41 molecular sieves reveal radical oxygen (M-Nb-O) which plays a role of the strongly oxidizing center. • Modification of NbMCM-41 sieves with Ni or Cu via a cation-exchange procedure causes the transformation of meso- to non porous or macroporous structure. • The structure of AlMCM-41 materials is resistant to Cu- and Ni-modification. • Ni-AlMCM-41 materials activated under vacuum or in He flow at 673-723 K exhibit Ni^^ and Ni^ species. • Cu-AlMCM-41 samples, activated as above, possess Cu^^, Cu^, and Cu^^O' species. Cu ^ cations are easier reduced if NbMCM-41 matrix is appUed. • H-NbMCM-41 sieves appeared to be useful catalysts in the hydrosulfurization of methanol towards methanethiol thanks to the presence of LAS and BAS pairs, low strength of acidic sites (side reactions are minimized), and the absence of dissociative adsorption of H2S • Nb-containing MCM-41 materials reveal the very high activity in the oxidation ofthioethers to sulfoxides. Moreover, their activity does not change after few regenerations, which suggests that Nb is not leached, or leaching is negligible.
822 • There is not evident influence of a kind of mesoporous nickel matrix on the dehydrosulfurization of thiols and sulfides. Ni-impregnated materials are less active than Niexchanged mesoporous molecular sieves.
ACKNOWLEDGMENT This work was partially supported by the Pohsh Committee for Scientific Research (KBN) under grant: 3 T09A 099 12. I. Nowak would like to thank for the grant fi-om Foundation for Polish Science. Hanna Poltorak is acknowledged for her experimental work in the field of dehydrosulfiirization. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. M. Ziolek and I. Nowak, Zeolites, 18 (1997) 356. 3. M. Ziolek, I. Nowak, J.C. LavaUey, Catal. Lett., 45 (1997) 259. 4. L. Zhang and J.Y. Ying, AIChE J., 43 (1997) 2793. 5. M. Ziolek, I. Sobczak, I. Nowak, P. Decyk, A. Lewandowska and J. Kujawa, Microporous and Mesoporous Mater., in press. 6.1. Nowak and M. Ziolek, in: Proc. 3rd Polish-German Zeohte Colloquium, M. Rozwadowski (Ed), Nicholas Copernicus University Press, Torun 1997, p. 161. 7. M. Ziolek, J. Kujawa, J. Czyzniewska, I. Nowak and M. Kubiak, in: Proc. 3rd Pohsh-German Zeohte Colloquium, M. Rozwadowski (Ed), Nicholas Copernicus University Press, Toruh 1997, p. 181. 8. M. Ziolek, I. Sobczak, P. Decyk and I. Nowak, Pohsh J. of Environmental Studies, 6 (1997) 47. 9. M. Ziolek, I. Nowak, P. Decyk and J. Kujawa, in: "Mesoporous Molecular Sieves 1998", L. Benneviot et al. (Eds.), Elsevier, Amsterdam 1998, Stud. Surf. Sci. Catal, 117 (1998) 509. 10. M. Ziolek, I. Nowak, P. Decyk, O. Saur and J.C. LavaUey, in: Proc. 12th International Zeolite Conference, M.M.J. Treacy et al. (Eds), Materials Research Society (1999) 833. 11. M. Ziolek, I. Sobczak, P. Decyk and I. Nowak, Stud Surf. Sci. Catal., 125 (1999) 633. 12. M. Ziolek, I. Nowak, H. Pohorak, A. Lewandowska and I. Sobczak, Stud. Surf. Sci. Catal, 125 (1999)691. 13. M. Ziolek, I. Sobczak, I. Nowak, M. Daturi and J.C Lavalley, Topics in Catalysis, in press. 14. J.S. Beck, J.C Vartuh, W.J. Roth, ME. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T-W. Chu, D.H. Olson, E.W. Sheppard, SB. McCuUen, J.B. Higginsand J.L. Sdilaiker, J. Am. Chem. Soc., 114(1992)10834. 15. S. Gontier and A Tuel, Micropor. Mater., 5 (1995) 161. 16. M.D. Alba, A.I. Becerro and J. Khnowski, J. Chem. Soc. Faraday Trans., 92 (1996) 849. 17. V.Y. Gusev, X. Foig, Z. Bu, G.L. HaUer and J.A. O^brioi, J. Phys. Chem., 100 (1996) 1989. 18. N. Igarashi, Y. Tanaka, Sch.-I. Nakata and T. Tatsumi, Chem. Lett., (1999) 1. 19. X.S. Zhao, F. Audsley and G.Q. Lu, J. Phys. Chem. B., 102 (1998) 4143. 20. B. Wichterlova, J. Dedecek and Z. Sobahk, in: Proc. 12th International Zeohte Conferoice, M.M.J. Treacy et al. (Eds), Materials Researdi Society (1999) 941. 21. J.Y. Kim, J.S.Yu and L. Kevan, Molecular Physics, 95 (1998) 989. 22. E. Giamello, D. Murphy, G. Magnacca, C. Monterra, Y. Shioya, T. Nomura and M. Anpo, J. Catal., 136(1992)510. 23. H. Kokusen, S. Matsuhara, Y. Nishino, S. Hasegawa, K. Kubcmo, Catal. Today, 28 (1996) 191. 24. D. de A.B Filho, D.W. Franco, P.P.A. Filho and O.L. Alves, J. Mater. Sci., 33 (1998) 2607. 25. AV. Mashkina, Russian Chemical Reviews, 64 (1995) 1131. 26. M. Ziolek, J. Czyzniewska, J. Kujawa, A. Travert, F. Mauge and J.C Lavalley, Microporous and Mesoporous Mater., 23 (1998) 45.
Studies in Surface Science and Catalysis 129 A. Sayarietal. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
823
Activity enhancement of mesoporous silicate FSM-16 by metal ion-exchange and sulfiding with hydrogen sulfide for acid-catalyzed reactions M. Sugioka, L. Andalaluna and J K A. Dapaah Department of Applied Chemistry, Muroran Institute of Technology, 27 -1 Mizumoto-cho, Muroran 050-8585, Japan.
The modification of mesoporous silicate FSM-16 by metal ion-exchange and sulfiding with hydrogen sulfide was studied through the isomerization of 1-butene, cis-2-butene and cyclopropane. It was revealed that the catalytic activities of MeFSM-16 were remarkably enhanced by sulfiding wdth hydrogen sulfide due to the formation of new Bronsted acid sites Keywords: Mesoporous sihcate (FSM-16), Activity enhancement, Sulfiding; Hydrogen sulfide; Isomerization; n-Butene; Cyclopropane
1. INTRODUCTION Mesoporous silicates such as MCM-41 [1] and FSM-16 [2] are potentially available for treating bulky chemicals and synthesizing large molecule materials. However, these new materials show low acidity and low catalytic activity for acid-catalyzed reactions. Thus, efforts for incorporation of aluminium leading to the formation of acid sites have been made. In our previous works, we have reported that the sulfiding of metal ion-exchanged silica gel and zeolites with hydrogen sulfide resulted in remarkable activity enhancement for some acid-catalyzed reactions [3-5]. The catalytic performance of silicate FSM-16 might also be improved by modification using metal ion-exchange and sulfiding with hydrogen sulfide. In this study, the effect of sulfiding of metal ion-exchanged FSM-16 (MeFSM-16, where Me = Ag, Cd, Cu and Ni) with hydrogen sulfide was studied using the isomerization of 1-butene, cis-2-butene and cyclopropane as model reactions. Infi-ared spectroscopic measurement of pyridine adsorption over MeFSM-16 before and after sulfiding were performed by use of FTIR in order to clarify the mechanism of activity enhancement of MeFSM-16 by sulfiding.
824 2. EXPERIMENTAL The isomerization of 1-butene, cis-2-butene and cyclopropane were carried out in a conventional closed circulating reactor by employing 0.05 g, 0.035 g and 0.035 g of catalyst, at 25°C, 75T and 150T, respectively. The initial pressure of reactant in each case was 40 Torr. A gaschromatograph equipped with TCD and a propylene carbonate column (4 m) cooled in ice was employed for the products analysis FSM-16 sample employed was synthesized by Toyota Central R&D Labs Japan, using kanemite (layered sodium silicate) as the source of sihca [2]. MeFSM-16 were prepared by ion-exchange of silicate FSM-16 (channel diameter = 2.7 nm; surface area =1190 m^g"^) using 0.2 mol/1 of AgNOs, CdCl2, CuCl2 and NiCl2 aqueous solutions. All catalysts were calcined in air at 500T for 4 hours. The degree of metal ion-exchange in AgFSM-16, CdFSM-16, CuFSM-16 and NiFSM-16 were 3.13x10"^ 2.35x10"^, 1.59x10"^ and 0.9x10"^ wt%, respectively, as determined by inductively coupled plasma (ICP) analysis. Catalysts were evacuated at 500°C for 2 hours prior to the reaction and sulfided with 40 Torr of hydrogen sulfide at 100-500^C for 1 hour followed by evacuation at the same temperature for 0 5 hour. Infrared spectroscopic measurement was performed by Jasco FT-IR 230S using an in-situ cell. Hydrogen sulfide adsorption was carried out by introducing 40 Torr of hydrogen sulfide into the cell at 200^C, followed by evacuation at the same temperature for 0.5 hour. Pyridine adsorption was performed by introducing 10 Torr of pyridine vapour into the cell at 150^C, followed by evacuation at the same temperature for 0.5 hour.
3. RESULTS AND DISCUSSION 3.1. Activity enhancement of MeFSM-16 in the isomerization of 1-butene The activity enhancement of MeFSM-16 by sulfiding with hydrogen sulfide in the isomerization of 1-butene is shown in Table 1. Mesoporous siUcate FSM-16 and MeFSM-16 (Me = Ag, Cd, Cu) showed low activity for 1-butene isomerization before sulfiding. On the other hand, NiFSM-16 has high activity in the isomerization of 1-butene. The activities of MeFSM-16, except that of NiFSM-16, were enhanced remarkably by sulfiding with hydrogen sulfide and the order of activity enhancement was AgFSM-16>CdFSM-16>CuFSM16>NiFSM-16. Furthermore, the enhanced activities of MeFSM-16 were higher than that of HY zeolite and that of AgFSM-16 was about twice the activity of HY zeolite. The activity of NiFSM-16 was decreased by sulfiding with hydrogen sulfide. It is assumed that the decrease of the activity of NiFSM-16 is attributed to the transformation of oxide form of nickel into considerably small amount of the sulfide form.
825
Table 1. The activity enhancement of MeFSM-16 by sulfiding with hydrogen sulfide in the isomerization of 1-butene at 25°C Catalysts
Activity (%/g. min) Before Sulfiding After Sulfiding
cis/trans ratio Before Sulfiding After Sulfiding
FSM-16
2.8
3.1
2.02
2.04
AgFSM-16 CdFSM-16 CuFSM-16 NiFSM-16
1.3 10.6 3.0 47.8
38.2 34.0 24.7 29.4
2.09 1.36 1.66 0.78
1.28 1.23 149 1.17
HY
19.1
1.06
Catalyst weight: 0.05 g, Sulfiding temp. : 200°C, Reaction temp. : 25°C. It is generally known that selectivity of product (cis trans ratio) in 1-butene isomerization which proceeds on Bronsted acid site is close to one. In the case of HY zeolite which has wide pore (0.78 nm), selectivity of product of 1.06 was obtained. As shown in Table 1, it was observed that selectivities close to one were obtained for sulfided MeFSM-16. Therefore, it might be inferred that Bronsted acid sites were generated over MeFSM-16 by the sulfiding with hydrogen sulfide and those sites might be attributed to activity enhancement Table 2 shows the activity enhancement of MeFSM-16 for the isomerization of cis-2Table 2. Activity enhancement of MeFSM-16 by sulfiding with hydrogen sulfide in the isomerization of cis-2-butene at 75T
Catalysts
Activity (%/g. min) Before Sulfiding After Sulfiding
trans/i ratio Before Sulfiding After Sulfiding
FSM-16
4.2
5.9
0.97
0.75
AgFSM-16 CdFSM-16 CuFSM-16 NiFSM-16
2.1 13.9 3.4 13.1
30.5 29.3 25.9 20.2
1.42 1.08 0.98 1.23
2.24 2.21 1.75 1.38
Catalyst weight: 0.035 g, Sulfiding temp. : 200''C, Reaction temp. : 75''C
826 butene by sulfiding. It is interesting to note that the trans-2-butene to 1-butene ratio (trans/1) was increased by the sulfiding, indicating that the acid strength of MeFSM-16 is also increased by the sulfiding treatment. Figure 1 shows the effect of the sulfiding temperature on the catalytic activity and selectivity of MeFSM-16 in the isomerization of 1-butene. It was found that the catalytic activity of sulfided MeFSM-16 depended strongly on the sulfiding temperature and it attained maximum at 300T for AgFSM-16 and CdFSM-16 and at 200°C for CuFSM-16, respectively. It was revealed that the selectivity in cis/trans ratio of 2-butene formed in the isomerization of 1-butene over MeFSM-16 was almost constant at the various sulfiding temperatures except at lOOT for AgFSM-16 and CuFSM-16.
1
CdFSM-16>CuFSM-16>NiFSM-16 This result is in good agreement with our previous study in the activity enhancement of Me""*"/Si02 by sulfiding with hydrogen sulfide [4]. Furthermore, it was revealed that the enhanced activities were as high as half of that of the activity of HY zeolite Lower activities of sulfided MeFSM-16 than that of HY zeolite is assumed to be due to the smaller amount of the active site generated over MeFSM.16 Figure 2 shows the effect of sulfiding temperature on the catalytic activity of MeFSM-16 in the isomerization of cyclopropane. It was revealed that the catalytic activity of sulfided MeFSM-16 also depended on the sulfiding temperature and the maximum activity was attained at almost the same temperature as in the case of the isomerization of 1-butene. The maximum activities of AgFSM-16 and CuFSM-16 were observed at 200T, whereas that of CdFSM-16 was at 300°C. It can be assumed that the changes of the activities of MeFSM-16 against the sulfiding temperature are related to the amount of the Bronsted acid sites formed on the MeFSM-16 by the sulfiding with hydrogen sulfide That is to say, the Bronsted acid sites on MeFSM-16 formed by the sulfiding are unstable and very sensitive to the high temperature compared to those of H-zeolites By these facts, sulfided MeFSM-16 catalysts
828
E
> o
40
Z
-
•
M
,'
A
>0
M.^
\
200
.
,> -'
1
250
.
1
:iOO
.
> z
-4
T
1
.
)
.
'
•
/. A.
—•--Pt'MCM-4l — • — Pt MCM-48 -.A—Pt AlKIT-l -T-
Pt HMS
— • - Pt r-AI.O. -4^
PtZSM-5
• « ,^
^ ^ 5 -
0
Figure 4. Proposed mechanism for the enhanced alkylation acliviiy by reversible carboxylaiion process during the intervention of carbon dioxide in the alkylation of acetonitrile
In addition, the cyanoacetate, formed by carboxylation of acetonitrile, appears to be relatively stable as pure cyanoacetic acid is found to decompose thermally at 160 °C [21]. Therefore, the a-hydrogen of the carboxylated acetonitrile would be much more acidic than the a-hydrogen of the original acetonitrile since the molecule consists of two strongly electron withdrawing groups, namely -CN and COOH, and, hence, is more easily abstracted by the active basic sites. This will effectively enhance the formation of a carbanion intermediate and facilitate alkylation with appropriate electrophiles. Since the carboxylation process is reversible, the alkylated cyanoacetate species could then undergo decarboxylation to give the carbon dioxide and the alkylated acetonitrile in a manner similar to that observed for CNCH2Cu(I). A mechanism for these phenomena can be proposed in Figure 4 where, presumably, methanol is the alkylating agent. Table 1 Amounts of water in the product mixture from the alkylation of acetonitrile
Catalyst
Before contact with CO2 (^'rweight)
During the contact with C62 C^rweight)
After contact with CO2 (9fweight)
CsNaX-CsOH CsNaX
Hi 1.85
2M 5.48
932 8.81
Although, alkylated products were increased, it was noticed that relatively small amounts of water were produced during the intervention of carbon dioxide (Table I) but larger amounts were obtained in the period following removal of carbon dioxide.lt was suggested here that the bicarbonates can be formed during the contact with carbon dioxide, by reaction with water eliminated from the alkylating agent. This especially takes place over catalysts with cesium cation "clusters" since more water was consumed over CsNaX-CSOH, compared with CsNaX. As the partial pressure of carbon dioxide decreases on removal, bicarbonates begin to decompose to water and carbon dioxide. Subsequently, the carbon dioxide from bicarbonates may facilitate further enhancement in the next period providing high activity after the interfering carbon dioxide is removed. This is in particular over CsNaX-CsOH (Table 2) because the additional basicity arising from excess cesium cation "clusters" would facilitate the formation of bicarbonates better than the exchangeable cesium cations. Therefore, synergy of carbon dioxide in the activity was observed in the alkylation of acetonitrile both during the intervention and also in the period following removal of carbon dioxide from the gas stream.
856 Table 2 Intervention of carbon dioxide in the alkylation of acetonitrile Catalyst CsNaX-CsOH
CsNaX
Canier gas He He He CO2 He He
Time on stream 30 60 90 120 150 180
Acetonitrile Conversion 15 11.4 10.1 24.1 16.9 16.5
S He
30 60 90 120 150 180
\23 14.2 12.5 17.7 14.2 12.1
He CO2 He He
Propionitrile
Acrylonitrile
lOT 8.6 7.3 16.9 12.1 11.2
__
TZ 2.8 2.6 4.4 2.9 2.7
2.2 1.9 6.3 4.6 2.8 83 9.3 8.6 13.0 11.3 8.6
Reaction temperature -350 °C, W/F -40 g.h.niol-l. Carrier gas flow rate -35 nil/niin. Material balance -95-98%
The results over CsNaX-CsOH show a greater increase in the alkylated yield than that over the CsNaX (Table 2) but selectivity of the products over both catalysts, when carbon dioxide is used as carrier gas, remains similar to that observed in the conventional reactions. It is clear that yield of propionitrile is considerably increased during the intervention of carbon dioxide over CsNaX-CsOH (~ 2 times) supporting the conclusion that the direct alkylation with methanol is mainly promoted by the catalyst with excess cesium cation "clusters" [15]. In contrast, the reaction over CsNaX gives significant yield of acrylonitrile. This is consistent with the previous report [15, 23] that formaldehyde serves as an alkylating agent in the reaction over cesium exchanged catalyst with no excess cesium "cluster" (CsNaX). The increased yield of acrylonitrile when carbon dioxide is used as carrier gas, again, reveals that framework oxygens are sufficiently basic in cesium exchanged zeolite X to promote proton abstraction from acetonitrile and the carbanions formed over these sites appear to exist within the polar framework leading to the reactions discussed earlier. Therefore, it seem clear that the alkylation activity of acetonitrile with both methanol (to form propionitrile) and formaldehyde (to form acrylonitrile), can be enhanced by the incorporation of carbon dioxide. Since the intervention of carbon dioxide has only a small effect on the adsorption of the highly electrophilic substrate (acetonitrile); at the reaction temperature, cesium carbonates (CS2CO3) would not be readily formed by carbon dioxide over CsNaX-CsOH in the presence of acetonitrile. Consequently, the catalyst remains active and still provides a high activity after carbon dioxide is replaced by helium. If the additional cesium carbonates were formed during the admission of carbon dioxide, then after carbon dioxide was withdrawn, the catalyst activity should be reduced in a manner similar to that observed over the fully carbonated CsNaX-CsOH (Figure 5). This is demonstrated by the alkylation of acetonitrile over fully carbonated CsNaXCsOH, obtained by passing carbon dioxide over CsNaX-CsOH at 350 °C prior to the reaction. It was found that a reduced activity was obtained as shown in Figure 5. However, the activity of fully carbonated catalysts indicates that the cesium carbonate species are sufficiently basic and active to promote the alkylation of acetonitrile, but in somewhat lower than fresh CsNaXCsOH. According to the observed high activity after removal of carbon dioxide and the above discussion, it seems clear that formation of cesium carbonate during carbon dioxide intervention is not the case. To clarify the speculation, an overall mechanistic pathway for the reaction over active basic sites in zeolites is proposed in Figure 6, where methanol is the alkylating agent.
857
Conversion of acetonitrile (%moI) 20-1
m n
13-
p"^ •
10-
Xs
H.
60
Fresh
Carbonated
120
I
0^ :
CH I
.CN
Figure 5. Catalytic activity of fully carbonated CsNaX-CSOH,compared to the fresh one
^
0 0
J L .CN
„_
0 :
"P
CH I
I
-CO;
HCHO
0
H.C— C— CN
180
Time on stream (min)
C
I
0
JL
,Cs
H.
H,C=C--CN H
-H,0
0
-co,
: Xs
X
^CN CH
I CH^
z-
Figure 6. Mechanistic pathway for the proposed enhancing effect of carbon dioxide in the alkylation of acetonitrile over the active basic sites
The promotion of activity by carbon dioxide, shown above, would not be expected in the side-chain alkylation of toluene when the reaction occurs in the presence of carbon dioxide. This is attributed to, (i) as discussed earlier, carbon dioxide is more electrophilic than toluene, consequently, the preferential adsorption of carbon dioxide would inhibit formation of the benzyl carbanion, (ii) if any benzyl carbanion is generated, bonding between the benzyl carbanion and the cesium counter ion would have very limited ionic character because the delocalised 7i-electrons in the aromatic ring would tend to be co-ordinated with the large cesium cations, inhibiting the insertion of carbon dioxide observed in cyanomethyl copper(I) or acetonitrile over CsNaX, and (iii) if, rarely, carboxylation takes place, the resulting phenyl acetate-like species should be quite stable and difficult to undergo decarboxylation [24] to give products. Accordingly, the intervention of carbon dioxide is found to suppress the alkylation of the less electrophilic substrate, (toluene) and, conversely, to enhance the alkylation of the highly electrophilic acetonitrile. CONCLUSION As carbon dioxide is more electrophilic than toluene, the suppression by carbon dioxide of the side-chain alkylation of toluene with methanol seems to result from preferential adsorption of carbon dioxide over the active basic sites. In contrast, acetonitrile is more electrophilic than carbon dioxide, because of the strongly electron withdrawing group (-CN group), therefore, carbon dioxide does not readily displace acetonitrile from the basic sites and, consequently, does not appear to inhibit the alkylation of acetonitrile with methanol. Moreover, carboxylation of the carbanion intermediates, formed from acetonitrile, is believed to take place, forming a strongly electrophilic species which enhances the secondary proton abstraction. This results in an increase in the alkylation activity of acetonitrile (with both methanol and formaldehyde) and, because the intermediates can be readily decarboxylated, a high product selectivity to propionitrile or acrylonitrile (depending on the catalysts) is obtained. However, this observed enhanced activity may be applicable only to the reaction of highly electrophilic substrate which provides a reversible carboxylation.
858 Accordingly, the strong interaction of the alkylated substrates which derive from their electrophilicity and polarity, appears to be highly consequential for reaction over basic zeolite catalysts, especially reactions involving carbanion intermediates. In addition to an effect on the rate of reaction, electrophilicity and polarity reflects an important role in determining the catalytic pathway. It is believed that the enhanced alkylation activity, observed during the presence of carbon dioxide, derives from characteristic reactions of the carbanion intermediate and no reaction with free radical species is involved in these phenomena. This is because a high product selectivity, together with a low rate of catalyst deactivation, reveals a specific reaction pathway to form certain products, namely acrylonitrile and propionitrile, which is seldom observed when the reaction proceeds via radical intermediates. Together with the significance of the electrophilicity and polarity of the substrate, it seems clear that the mechanism involving carbanion intermediates represents the most likely reaction pathway over these basic zeolite catalysts. REFERENCES 1. G. Zhang, H. Hattori and K. Tanabe, Appl. Catal., 36 (1988) 189. 2. M. He and J. Ekerdt, J. Catal., 90 (1984) 17. 3. F. Ma, D. Lu and Z. Guo, J. Mol. Catal., 78 (1993) 309. 4. K. Tanabe, H. Hattori, T. Yamaguchi and T. Tanaka, Acid Ba.se Catalysis, Proceedings of the International Symposium on Acid-Base Catalysis, Kodansha Ltd., Tokyo (1989) 72. 5. L. Rees and J. Hampson, C02-Zeolite Reaction for Gas Separation, ed. C. Pradier and J. Paul, Carbon Dioxide Chemistry : Environmental Issues, Proceedings of the International Symposium on C02 Chemistry, The Royal Society of Chemistry, Cambridge (1994) 250. 6. T. Sooknoi, S. Barri, A. Garforth and J. Dwyer, unpublished paper, UMIST, 1995. 7 G. Kaye and T. Laby, Tables of Physical and Chemical Constants, Longman Scientific and Technical, New York, 1986. 8. R. Weast and M. Astle, CSC Handbook of Chemistry and Physics, CRC Press INc, Boca Raton, 1982. 9. G. Kim, D. Cho, K. Kim and J. Kim, Catal. Lett., 28 (1994) 41. 10. T. Inui, K. Kitagawa, T. Takeguchi, T. Hagiwara and Y. Makino, Appl. Catal. A, 94 (1993)31 11. M. Fujiwara and Y. Souma, J. Chem. S o c , Chem. Commun., 10 (1992) 767. 12. J. Jeon, K. Jeong, Y. Park and S. Ihm, Appl. Catal., 124 (1995) 91. 13. B. Bartos, H. Freund, H. Kuhlenbeck, M. Neumann, H. Lindner and K. Muller, Surf. Sci., 179(1987) 59. 14. P. Hathaway and M. Davis, J. Catal., 116 (1989) 263. 15. T. Sooknoi and J. Dwyer, Stud. Surf. Sci. Catal., 97 (1995) 423. 16. M. Burgers and H. van Bekkum, Catal. Lett., 25 (1994) 365. 17. M. Burgers and H. van Bekkum, Stud. Surf. Sci. Catal., 84 (1994) 1981. 18.T. Tsuda, T. Nakatsuka, T. Hirayama and T. Saegusa, J. Chem. S o c , Chem. Commun. (1974)557. 19. T. Tsuda, Y. Chujo and T. Saegusa, J. Chem. S o c , Chem. Commun. (1976) 415. 20. T. Tsuda, Y. Chujo and T. Saegusa, J. Amer. Chem. S o c , 100 (1978) 630. 21. D. Darwnsbourg, J. Chojnacki and E. Atnip, J. Amer. Chem. S o c , 115 (1993) 4675. 22. Arno Behr, Carbon dioxide Activation by Metal Complexes, VCH Publishers, Weinhiem, 1988. 23. T. Sooknoi and J. Dwyer, Role of Substrate's Electrophilicity in Base Catalysis by Zeolites, to be published. 24. S. Inoue and N. Yamazaki, Organic and Bio-Organic Chemistry of Carbon Dioxide, Kodansha Ltd., Tokyo, 1982.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
859
Effective sol-gel adsorbents of water vapor prepared using ethyl silicate 40 as a silica precursor J. Mrowiec-Bialon", A.I. Lachowski% M. Kargol*, J.J. Malinowski* and A.B. Jarz^bski*** ^Institute of Chemical Engineering, Polish Academy of Sciences 44-100 Gliwice, Baltycka 5, Poland ^'Institute of Chemical and Process Engineering, Faculty of Chemistry, Silesian Technical University, 44-100 Gliwice, Ks. M. Strzody 7, Poland Effective composite sol-gel adsorbents of water vapor can be prepared in a one-step procedure using ethyl silicate 40 as a cost-effective silica precursor. Adsorption properties exhibited by the materials thus obtained are comparable with those of the samples obtained from the more complex procedure developed previously which made use of tetraethoxysilane.
Keywords: water adsorption, sol-gel materials, ethyl silicate 40, nanocomposites
1. INTRODUCTION The sol-gel silicas doped with hygroscopic compounds: calcium chloride or lithium bromide, which were devised more recently^ ^ appeared to be very effective adsorbents of water vapor with sorption capacities reaching 100 wt%. They were prepared using a conventional two-step preparation procedure, initially proposed by Brinker et al."*, with tetraethoxysilane (TEGS) as a silica precursor. Although tetraalkoxysilanes are usually applied as silica precursors in sol-gel procedures, the oligomeric type precursors can also be used instead. Ethyl silicate 40 (ES 40) is the most common commercial form of ethoxypolysiloxane available at prices competitive to those of TEOS/TMOS. Hence its application in the synthesis of composite adsorbents could be economically attractive provided that this would has little or no adverse effect on their final properties. The objective of this work was to test an alternative, cost-effective preparation procedure making use of the low-cost reagents: ES 40 and 95wt% EtOH and to compare the properties of the adsorbents obtained with those of their counterparts similar in composition, yet synthesized with the use of TEGS.
Corresponding author, e-mail:
[email protected] 860 2. EXPERIMENTAL 2.1. Preparation of the samples Five samples of the xerogel composite adsorbents were prepared in a one-step, base catalyzed procedure to obtain a nominal 10, 20 or 30 wt% content of calcium chloride and 20 or 30 wt% of lithium bromide in dry samples (labeled CalO - Ca30 and Li20 - Li30). The molar ratio ofthecompounds in all samples was SiiEtOHrHiOiNHa = l:8:3:8xial Wet gels were prepared as follows. First two solutions were prepared at room temperature: solution A contained ES 40 and half of the total ethanol content whereas solution B consisted of remaining ethanol, water, ammonium hydroxide and calcium chloride or lithium bromide. Then solution A was added to B under stirring and the resulting sol was heated to 50°C. Gelation took place in 3-4 h or 0.3-0.5 h for CaClj or LiBr doped samples, respectively. The alcogel samples were dried slowly under cover at room temperature to obtain solid adsorbents of water vapor of the xerogel morphology. For comparison a pure silica xerogel sample (designated SiOj) was also prepared as described above, yet using no dopant, and it was investigated and characterized in the same way. 2.2. Characterization of porous texture, morphology and adsorption properties Nitrogen adsorption isotherms were measured at 77 K by a Micromeritics ASAP 2(XX) instrument to obtain specific surface area SBET ^ ^ pore-size distributions, evaluated using the BJH method. Assessments of microporosity were made from t-plots using the Harkins-Jura correlation. Morphology of the samples was examined using transmission electron microscopy (JEOL 2000 SX). Water vapor adsorption isotherms were measured volumetrically using a standard system.''^ The measurements were isobaric and adsorption was carried out at 298K. Thermal analysis (TG, DTG, DTA) of the samples was performed using an OD-102 instrument (Budapest) at the heating rate of 2K/min. The composites were ground and fine powders were saturated with water vapor prior to heating up to 573K.
3. RESULTS AND DISCUSSION Both the nitrogen adsorption experiments (cf. Table 1, Fig. 1) and TEM analysis (Figs. 2 and 3) indicate a remarkable presence of mesopores of the volume Vp(N2) located in pores of a mean diameter d,„ ranging from 9 to 14 nm. This nanoporosity, fairly similar in both families of materials, consistently decrease with increase in the chloride/bromide content. More importantly perhaps, the detected mesopore volumes (0.7-1.5 cmVg) well exceed the values recorded in the similar samples yet prepared using TEOS (0.4-0.6 cmVg).''^ The same trend is also observed in the conventional silica xerogel sample. Thus the use of ethyl silicate 40, and hence of the pre-polymerized precursor, results in materials with nanoporosity larger than in the corresponding samples but prepared using TEOS/TMOS. These results corroborate earlier reports on the effect of pre-polymerization of silica precursors on the resulting porous texture of sol-gel materials.^ While all samples showed a type IV isotherms, a sign of a significant mesopore volume, t-plots analysis indicated either a null or a minute microporosity, ¥„„ (cf. Table 1). Apparently this bears on the abilities to adsorb water vapor in general and in particular on the characteristics of the adsorption process against relative vapor pressures, p/po.
861 Table 1. The characteristic parameters of porous texture BET
^P(N2)
2/
m^/g
cmVg
nm
cm^/g
Ca30
254
0.73
8.9
0.04
Ca20
304
1.10
10.1
0.04
CalO
410
1.50
10.1
0.04
Li30
225
1.00
14.8
0.02
Li20
330
1.20
12.5
-
SiOj
669
1.10
6.4
-
Sample
As expected, the water adsorption experiments demonstrated the strong affinity for water exhibited by all composites, markedly stronger by those with the larger dopant content (cf. Fig. 4). On the whole water adsorption isotherms displayed in Fig. 4 appear to be similar to those previously reported for the corresponding samples but prepared using a two-step procedure with TEOS and bone dry EtOH as reactants.^'^ Similarly as before the samples doped with bromide show larger 100 Pore diameter, nm sorption capacities than those containing chloride in the relative pressure range of 0-0.2 Figure 1. Pore size distribution in adsorbents samples. (Fig. 5). This is due to a strong difference in the affinity for water showed by these two compounds. Note that one may expect, similarly as before ^, a fair similarity and overlap of water adsorption/ desorption isotherms owing to the physical nature of water adsorption on the composites prepared. The derivative (DT, DTG and DTA) spectra, for samples Ca30 and Li30 are displayed in Figs. 6 and 7. As can be seen the bulk of water can be removed from adsorbents by heating at 378-383 K. From the calcium chloride doped xerogels all water can be removed by heating at 450 K whereas from those with lithium bromide by the treatment at 473 K.
862
Figure 2. TEM image of Ca30 sample.
0.0
0.2 0.4 0.6 0.8 Relative p r e s s u r e
Figure 4. Water vapor adsorption isotherms at 298 K.
Figure 3. TEM image of Li30 sample.
0.00
0.05 0.10 0.15 Relative p r e s s u r e
0.20
Figure 5. Water vapor adsorption isotherms at 298 K,
863
DTG
DTG ^"-^.^
x^ -__JDTA
^--^--^_J)TA - ^
/
•' 0
*^ 10g20-
0
\ \ \
«4050290
340
y'
** 10"20.c3 0 -
— 1
1
\ TG
^40-
TC
390 440 490 540 Temperature, K
\,
50-
590
Figure 6. Thermal analysis of Ca30 sample.
290
340
390 440 490 540 Temperature, K
590
Figure 7. Thermal analysis of Li30 sample.
Note that these values are about 20 K lower than those detected in the corresponding (xerogel) samples synthesized previously.^'^ Thus the xerogel adsorbents obtained from a novel, cost-effective process also appear to be quite attractive as regards the regeneration temperatures. A cycle of repeated adsorption-desorption experiments is currently under way. It aims to determine the long-term operational characteristics of adsorbents and the stability of adsorption/structural properties. The results already obtained appear to be quite promising; the properties of adsorbents are comparable with those shown by the adsorbents synthesized in a two-step procedure with TEOS as a silica precursor.
4. CONCLUSIONS The hybrid xerogels exhibiting remarkable adsorption capacities of water vapor can be prepared in a cost-effective way using the modified one-step, sol-gel procedure with ethyl silicate 40 as silica precursor. Preliminary experiments indicate that water adsorption properties of these materials are equally attractive as those shown by the adsorbents obtained from the more complex and expensive procedures with TEOS as a silica precursor. Acknowledgements. The authors gratefully acknowledge the Polish State Committee for Scientific Research for the financial support for this work under Grant 3T09C 019 15.
864 REFERENCES 1. J. Mrowiec-Bialon, A.B. Jarz^bski, A.I. Lachowski, J.J. Malinowski, Yu.I. Aristov, Chem. Mater., 9(1997)2486. 2. J. Mrowiec-Bialon, A.B. Jarz^bski, L. Pajak, Langmuir, 15 (1999) 6505. 3. Pending Patent Applications - Germany: 198 55 475.3, Russia: 97120422, Poland: P319261. 4. C.J. Brinker, K.D. Keefer, D.W. Schaefer, R.A. Assink, B.D. Kay C.S. Ashley, J. NonCryst. Solids, 64 (1984) 45. 5. J. Mrowiec-Bialon, A.I. Lachowski, A.B. JarzQbski, L.G. Gordeeva, Yu.I. Aristov, J. Colloid Interface Sci., 218 (1999) 500. 6. C.J. Brinker, G.W. Scherer, Sol—Gel Science, The Physics and Chemistry of Sol-Gel Processes, Academic Press, New York 1990. 7. J. Mrowiec-Bialon, Doctoral dissertation, IICh-PAN, Gliwice 1998.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
865
Photochromism of an azobenzene in a nanoporous silica film M. OGAWA"'^ J. MORF and K. KURODA^' ^ PRESTO, Japan Science and Technology Corporation ^ Department of Earth Sciences, Waseda University, Nishiwaseda 1-6-1, Shinjuku-ku, Tokyo 169-8050, Japan. "" Department of Applied Chemistry, Waseda University Ohkubo 3-4-1, Shinjuku-ku, Tokyo 169-8555, Japan. "^ Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, Nishiwaseda 2-8-26, Shinjuku-ku, Tokyo 169-0051, Japan
The incorporation of a cationic azobenzene derivative, p-( co -dlmethylethanolammonioethoxy)-azobenzene bromide, into nanoporous silica films and the photochemical reactions of the adsorbed dye were investigated. The nanoporous silica films were prepared from tetramethoxysilane and octadecyltrimethylammonium chloride by the rapid solvent evaporation method which we have reported previously. The adsorption of the cationic azo dye was conducted by casting an ethanol solution of the dye onto the nanoporous silica films. Upon UV light irradiation, trans-azobenzene isomerized photochemically to the c/s-form and photochemically formed c/s-form turned back to the Jrans-form upon visible light irradiation. The nanoporous silica films were proved to be an excellent reaction media to Immobilize organic photocromic species.
1. INTRODUCTION The preparation of inorganic-surfactant mesostructured materials in a controlled morphology is a key issue for the application of inorganic-organic mesostructured materials.[1] The processing of the silica-surfactant mesostructured materials as thin film is the subject of current interest, since the films might be applied to sensors, optical and electronic materials, etc. to which powder samples do not have an access. Accordingly, thin films of mesoporous silica have been prepared on solid substrates[2-13] and at air-water interface.[14] Among possible synthetic approaches, the rapid solvent evaporation method, which we have developed,[2,3,15,16] is a promising way for the preparation of the silica-
866
uv
\
^ -9-P Vis, A
frans-form Scheme I.
c/s-form
C/s-fransisomerization of azobenzene
surfactant mesostructured materials in a controlled morphology, since the reaction is very simple and the resulting films are highly transparent and homogeneous. The immobilization of photoactive species into the silica-surfactant mesostructured materials is worth investigating toward future photofunctional materials. Photochemistry on solid surfaces is a growing new field which yields a wide variety of useful application such as sensitive optical media, reaction paths for controlled photochemical reactions, molecular devices for optics, etc. [17] Along this line, the incorporation of organic dyes into silica-surfactant mesostructured materials [17-20] as well as nanoporous silica films[3] have been reported so far. We now report the photochromic reaction of an azobenzene in the nanoporous silica film. Since the photochromic behavior is environmentally sensitive, photochromism of organic substances in solid matrices has been investigated to understand as well as to modify the photochromic behavior.[21] Photochromism of azobenzene and Its derivatives due to cis-trans isomerization (Scheme I) has widely been investigated. Photocontrol of chemical and physical functions of various supramolecular systems has vigorously been studied by using photochemical configurational change of azobenzene derivatives.[22,23] 2. EXPERIMENTAL 2-1. Materials Tetramethoxysilane (abbreviated as TMOS) and octadecyltrimethylammonium chloride [(Ci8H37)(CH3)3N"CI'; abbreviated as C^gTAC] were obtained from Tokyo Kasel Industries Co., and used without further purification. A cationic azobenezene derivative [abbreviated as AZ] (the molecular structure is shown in Scheme II) was prepared by the reaction between 4-phenylazophenol, 1,2dibromoethane, and 2-dimethylaminoethanol, obtained from Tokyo Kasei Industries Co.
/
V.-N=N_/
\ _ 0 ( C H 2 ) 2 - NMCH2)20HBr"
Scheme li. p-(a;-dinnethylethanolammonioethoxy)- azobenzene bromide [AZ]
867
2-2. Sample preparation The thin film of silica-surfactant mesostructured material was prepared by the reactions of TMOS and C^gTAC, as reported previously[3]. The film was calcined in air to prepare nanoporous silica films. The adsorption of the dye onto the nanoporous silica film was conducted either by immersing the calcined film into an ethanol solution of the dye or casting the solution onto the film. 2-3. Characterization X-Ray diffraction was performed on a MOSX-HF^^ diffractometer (MAC Science) using Mn filtered Fe Ka radiation. The thickness of the films was determined with a surface profilometer (Kosaka Laboratory Co., SE 1700). Absorption spectra of the films were recorded on a Shimadzu UV-3100PC spectrophotometer. Nitrogen adsorption isotherm was obtained on a BELSORP TCV (BEL Japan Inc.) system. 2-4. Photochemical reaction The photochemical reaction of the adsorbed azobenzene was conducted by UV and visible light irradiation with a 500 W super high pressure Hg lamp (USHIO USH-500D). A band pass filter. Toshiba UV-D35; the transmittance centered at 350 nm, was used for isolating the UV light. For the cis-Xo-trans backward reactions, a sharp cut filter, HOYA L42 (cut off wavelength is 420 nm) was used to obtain visible light. The reactions were monitored by the change In the absorbance of transisomer of the azobenzene.
3. RESULTS AND DISCUSSI ON 3 - 1 . Prepration of nanoporous silica films By spin coating the mixture containing the prehydrolyzed TMOS and a 0.2 M aqueous solution of C^gTAC (pH=2, at the molar ratio of TMOS:Ci8TAC=9.2:1), a transparent thin film formed on the substrate. The X-ray diffraction pattern of the film (Figure 1 a) showed a sharp diffraction peak with the c/value of 4.6 nm, which accompanied the 2nd order reflection. In order to remove the surfactants from the substrate to obtain porous silica films, the as coated film of the silica-surfactant mesostructured material was calcined in air at 450 °C. Sharp diffraction peak was observed in the XRD pattern of the calcined film
4 8 12 2 0/degrees (FeKa) Figure 1. XRD patterns of (a) the silica-CisTAC composite film, (b) after calcination, (c) after adsorption of AZ
868
(Figure 1b), showing that the ordered microstructure was retained even after the removal of surfactants. The cf value of the calcined film was 4.1 nm. The SEM image of the film surface (data not shown) also indicates that the film is continuous and crack free. These observations are well consistent with those reported in the previous paper[2], showing the formation of silica-surfactant mesostructured material and the successful transformation of the as coated film into a nanoporous silica film. Nitrogen adsorption isotherm of the calcined film on a glass substrate was type IV, showing that the film is mesoporous. From the isotherm, the pore size was determined to be 3.7 nm. Since the Isotherm was obtained for the film supported on the substrate, it was impossible to determine the exact weight of the porous film. Therefore, the surface area and the porosity of the film cannot be determined at the present stage. 3-2. Introduction of the azo dye into the nanoporous siiica film Guest species can be incorporated into the preformed porous silica films by impregnation in a similar way for the introduction of guest species into the crystalline inorganic host materials. Host-guest as well as guest-guest interactions in porous silica films are expected to control the states of guest species to lead novel functional supramolecular systems. We have already reported the incorporation of cationic dyes, methylene blue, 1,1'-diethyl-2,2'-cyanine, tetrakis-(N-methyl-4-pyridinio) porphyrin by immersing a nanoporous silica film in aqueous solutions of dyes.[3] The adsorbed amounts of dyes can be controlled by changing the reaction conditions, i.e. dye concentration and reaction period. For the introduction of AZ, the nanoporous silica film was immersed in an ethanol solution of AZ. However, the adsorbed amount of AZ was very small even when the reaction period was prolonged or the concentrated AZ solution was employed. Therefore, an ethanol solution of AZ was casted on the nanoporous silica film and dried in air. The visible absorption spectrum of the film after the reaction with the dye showed an absorption band at around 340 nm, which is ascribable to the JI-JI* transition of trans-AZ. The absorption maximum observed for the film was consistent with that of a dilute ethanol solution of the dye. Considering the fact that the absorption spectra of aggregated AZ such as crystals and the AZ adsorbed on a layered silicate [24] are different from that of the present system, the AZ cations were adsorbed and dispersed molecularly on the surface of the nanoporous silica. The relatively high absorbance was achieved without significant spectral shifts, showing that the AZ was adsorbed in the nanopore not only on the external surface. Since the reaction was conducted by just casting the AZ solution on the nanoporous silica film, the interactions between the inner surfaces of the nanopore and the AZ were thought to be very weak. The adsorbed AZ was desorbed almost completely by careful washing with ethanol, indicating that the dye-surface interactions are weak. The AZ cations are thought to interact with the surface of the nanopore by hydrogen bonding through counter anions (Br) and/or through hydroxyl groups of AZ. Efforts are being made in our laboratory to attach azobenzene moieties on the nanopore with stronger interactions.
869
3-3. The photochemical reaction of the adsorbed dye The change in the absorption spectrum of the AZ adsorbed film is shown in Figure 2. By irradiation of UV light for 1 min., the band due to the fcrans-isomer (at 340 nm) decreased (spectrum (b) in Figure 2). Irradiation for a longer period did not cause further spectral change. By the visible light irradiation for 1 min, the absorption spectrum was recovered. Reversible spectral change was observed repeatedly. The ratio of the c/s-isomer formed by 200 300 400 500 600 the UV irradiation at the wavelength / nm photostationary state at room temperature was roughly estimated to Figure 2. Absorptbn spectra of AZ adsorbed be no less than ca. 70 % from the nanoporous film after (a) Vis irradiation, and (b) UV irradiation absorbance change in the absorption band at 340 nm due to the transisomer. Thus, the adsorbed dye exhibits reversible photoisomeri-zation in the nanopores. The absorption maximum as well as the photochemical reaction confirmed the adsorption of AZ molecularly in the nanopore. The loading amounts of the dyes, the pore size and surface modification are expected to affect the photoprocesses of the adsorbed dyes. In order to construct molecularly designed functional host-guest systems from nanoporous silica films, further study on the adsorption and the photoprocesses of the dyes Is now undenvay and will be reported subsequently. 4. CONCLUSIONS A cationic azobenzene derivative, p-((^-dimethylethanolammonioethoxy)azobenzene bromide, has been successfully introduced into the nanoporous silica films and the adsorbed dye exhibited photochemical reactions. Upon UV light Irradiation, frans-azobenzene isomerized photochemically to c/s-form and the photochemically formed c/s-form turned back to frans-form upon visible light irradiation. The nanoporous silica films were proved to be immobilizing and reaction media for organic photochemical reactions. 5. ACKNOWLEDGEMENTS This work was partially supported by Waseda University as a special research project.
870
REFERENCES 1. S. Mann and G. Ozin, Nature, 382 (1996) 313. 2. M. Ogawa, Chem.Commun., (1996) 1149. 3. M. Ogawa, H. Ishikawa, and T. Kikuchi, J.Mater.Chem.,8 (1998) 1783. 4. H. Yang, A. Kuperman, N. Coombs, S. Mamiche-Afara, and G.A.Ozin, Nature, 379 (1996)703. 5.1.A. Aksay, M. Trau, S. Manne, I. Honma, N. Yao, L. Zhou, P. Renter, P.M. Eisenberger, and S.M. Gruner, Science, 273 (1996) 892. 6. K.M. McGrath, D.M. Dabbs, N. Yao, I.A. Aksay, and S.M. Gruner, Science, 277 (1997)552. 7. H.W. Hillhouse, T. Okubo, J.W. van Egmond, and M. Tsapatsis, Chem.Mater., 9 (1997) 1505. 8. S.H. Tolbert, T.E. Schaffer, J. Feng, P.K. Hansma, and G.D. Stucky, Chem.Mater., 9(1997) 1962. 9. Y. Lu, R. Ganguli, C.A. Drewien, M.T. Anderson, C.J. Brinker, W. Gong, Y. Guo, H. Soyez, B. Dunn, M.H. Huang, and J.I.Zink, Nature, 389 (1997) 364. 10. M.T.Anderson, J.E. Martin, J.G.Odinek, P.P. Newcomer, and J.P. Wilcoxon, Microporous Mater., 10 (1997) 13. 11. J.E. Martin, M.T.Anderson, J.G.Odinek, and P.P. Newcomer, Langmuir, 13 (1997)4133. 12. A. Ayral, C. Balzer, T. Dabadie, C. Guizard, and A. Julbe, Catal Today, 25 (1995) 219. 13. R. Ryoo, C.H. Ko, S.J. Cho, and J.M. Kim, J.Phys.Chem. B, 101 (1997) 10610. 14. H. Yang, N. Coombs, I.Sokolov, and G.A.Ozin, Nature, 381 (1996) 589. 15. M. Ogawa, J.Am.Chem.Soc, 116 (1994) 7941. 16. M. Ogawa and T. Kikuchi, Adv.Mater., 10 (1998) 1077. 17. Ramamurthy, V. (ed.). Photochemistry in Organized & Constrained Media, VCH Publishers, Inc., New York, 1991. 18. M. Ogawa, Langmuir, 11 (1995) 4639. 19. M. Ogawa, T. Igarashi and K. Kuroda, Chem.Mater., 10 (1998) 1382. 20. M. Ferrer and P. Lianos, Langmuir, 12 (1996) 5620. 21. H. Durr, and H. Bouas-Laurent, (eds.), Photochromism -Molecules and Systems, Elsevier, Amsterdam, 1990. 22. G.S.Kumar, and D.C. Neckers, Chem.Rev.,89 (1989) 1915. 23. M. Irie, Photophyical and Photochemical Tools in Polymer Science, M.A. WInnik (ed.), Reidel Pub., Dordrecht 1986. 24. M. Ogawa, manuscript in preparation.
Studies in Surface Science and Catalysis 129 A. Sayari et al. (Editors) © 2000 Elsevier Science B.V. All rights reserved.
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Silica-CTAB-Water Phase Diagram at 150 ^C: Predicting Phase Structure by Artificial Neural Network Y. Yang, L. Belfares, F. Larachi, B.P.A. Grandjean and A. Sayari* Department of Chemical Engineering and CERPIC Universte Laval, Ste-Foy, Qc, GIK 7P4, Canada A silica mesophase diagram was explored via the synthesis of 31 samples using mixtures with the following compositions 1.0 Si02 : 0.317 TMAOH : x CTAB : y H2O with 0.042 < x < 0.67 and 21.7 < y < 298 . All samples were prepared at 150 °C for 40 h. Synthesis mixture compositions which afford hexagonal (MCM-41), cubic (MCM-48) and lamellar phases were delimited. In particular a relatively wide range of compositions were found to give rise for the first time to good quality MCM-48 without using polar additives. The artificial neural network approach was shown to be capable of predicting the structure of mesophases based on their TMAOH-CTAB-water composition.
1. INTRODUCTION Since the discovery of the so-called FSM-16 [1] and M41S [2] ordered mesoprous silicas in the early nineties, research in this area underwent unprecedented growth. New silica phases and morphologies such as SBA-2, SBA-8, SBA-15, MSU-V, MSU-n and KIT were discovered, and numerous non silica mesoporous inorganic materials were synthesized [3, 4]. A large number of potential applications, particularly in catalysis were also proposed [5, 6]. Among the surfactants used as structure directing agents, cetyltrimethylammonium bromide (CTAB) was the most predominant. However, this extensive effort was somewhat erratic as the number of different recipes grew tremendously. The objective of this investigation was to study in a systematic way the effect of the amount of water and CTAB relative to silica under otherwise identical conditions. An artificial neural network approach was designed to predict the nature of the mesophases based on the composition of their synthesis mixtures.
2. EXPERIMENTAL 2.1. Materials Samples were prepared using the following overall mixture composition 1.0 Si02 : 0.317 TMAOH : x CTAB : y H2O. The relative amounts of CTAB (x) and water (y) were varied in the ranges 0.042 - 0.67 and 21.7 - 298, respectively. In a typical preparation corresponding to X = 0.45 and y = 67, an amount of 3.85 g of TMAOH (25 w%) was diluted with 37.1 g of water before adding 5.886 g of CTAB under vigorous stirring. After 15 min, 2 g of Cab-0-Sil
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silica was added. The gel obtained after stirring for an additional 30 min was transferred into a Teflon lined autoclave, and heated statically under autogenous pressure for 40 h at 150 °C. The obtained materials were filtered, washed extensively, dried and calcined at 540 °C, first in flowing nitrogen, then in air. 2.2. Measurements X-ray diffraction (XRD) spectra were obtained on a Siemens D5000 diffractometer using CuKa radiation (k = 0.15418 nm). Scanning electron microscopy (SEM) images were recorded on a JEOL 840A microscope operated at an accelerating voltage of 10 kV. Nitrogen adsorption measurements were performed using a Coulter Omnisorp 100 gas analyzer. Before analysis, the samples were degassed under vacuum (ca. 10'^ torr) at 300 °C for 1 h. 2.3. Methods The specific surface area, SBET, was calculated using nitrogen adsorption data and the BET (Brunauer-Emmett-Teller) method [7]. Pore size distributions (PSDs) were calculated using the recently developed KJS (Kruk, Jaroniec, Sayari) approach [8]. This method uses the BJH (Barrett-Joyner-Halenda) procedure [9] based on adsorption data and calibrated specifically for ordered mesoporous silica. The pore diameter corresponding to the maximum of PSD will be denoted as WKJS. For pure MCM-41 hexagonal phases , the size of primary mesopores, Wd, was also calculated using a simple geometric model which consists of an infinite array of hexagonally packed cylindrical pores [10]. The relationship between Wd, the primary mesopore volume Vp, and the dioo distance obtained from XRD data is as follows: i w. = cd
pVp —
ii + pv,^ where c = (8/(3'^^7i))'^^ is a constant and p is the density of the silica walls taken as equal to 2.2 cmVg. An artificial neural network (ANN) model was developed to predict the structure of the mesoporous materials based on the composition of their synthesis mixtures. The predictive ability of the networks was tested throu^ comparison of the mesophase structures predicted by the model and those actually determined by XRD. Among the various ANN models available, three-layer feed-forward neural networks with one hidden layer are known to be universal approximators [11,12]. The neural network retained in this work is described by the following set of equations that correlate the network output S (currently, the structure of the material) to the input variables Uj which represent here the normalized composition of the synthesis mixture : 1 , ., 1 S= and Hj = 1 + exp -Zco^A 1 + exp I is the number of input variables, J is the number of nodes in the hidden layer to be optimized. The model output S was set to 1 for the cubic MCM-48 structure, 2 for the MCM41 hexagonal form and 3 for the lamellar form. The input variables Ui and U2 were the normalized weight fractions of CTAB and TMAOH, respectively. Hj+i and Ui+i are the bias constants set equal to 1, and coj and coij are the fitting parameters. The NNFit software
873 developed at U. Laval [13] was used to build the ANN correlation. Parameters identification, known as the training of the neural correlation, was performed using non-linear least-squares regression where a quadratic objective function was minimized by means of the quasi-Newton Broyden-Fletcher-Goldfarb-Shano algorithm [14]. The learning set on which the minimization and weights identification were performed consisted of 16 randomly selected (Ui, U2, S) sets, representing 70 % of the available data. The ANN correlation thus derived was then applied on the remaining 12 sets (Ui, U2, S) for testing the network predictive capabilities.
3. RESULTS AND DISCUSSION A total of 31 samples were synthesized. Table 1 shows for each sample, the Si02-CTABwater molar composition used, the nature of the obtained phases and the physical properties obtained from nitrogen physisorption data. As shown by the XRD data for non calcined materials, depending on the composition, lamellar (L), MCM-41 hexagonal or MCM-48 cubic phases were obtained in pure or mixed forms. The range of composition giving rise to different mesophases is shown in Figure 1. Notice that there is a relatively wide range of compositions corresponding to the formation of essentially MCM-48 silica with a pore size of ca. 4 nm. To our knowledge, this is the first time that MCM-48 silica has been synthesized using Cab-0-Sil silica without the use of polar additives [11]. In all previous reports dealing with the synthesis of MCM-48, addition of ethanol either directly or through the use tetraethylorthosilicate was reported to be required for this mesophase to occur [12-14]. The effect of water may be illustrated by samples prepared using a constant CTAB to Si02 ratio of 0.45. As seen in Figure 1, six samples were synthesized using H2O to silica ratios in the range 67 - 298. The XRD patterns for four of these samples are shown in Figure 2. The lowest H20/Si02 ratio afforded a lamellar with dooi = 3.40 nm. This phase consisted of monodipersed spheres of ca. 13 ± 1 jam in diameter phase with very rough surfaces (Figure 3a). As expected, upon calcination, the XRD pattern disappeared, but the material obtained exhibited quite high surface area ( 433 m^/g) and pore volume (0.40 cm^/g). At higher water to silica ratio, i.e. 100, a cubic MCM-48 phase developed. SEM images (Figure 3b) showed the occurrence of small particles with ca. 1 [xm diameter on top of the previously mentioned 13 jam spheres. This particular sample exhibited a surface area of 846 m^/g and a total pore volume of 0.87 cm^/g. At even higher water content, the lamellar phase persisted, and a hexagonal MCM-41 phase with dioo = 5.26 nm formed. SEM images showed that this material was comprised of very tiny particles without specific shape located on top of the 13 ^m spheres corresponding to the lamellar phase. The use of a large amount of water (H20/Si02 = 298) afforded almost pure MCM-41 silica with a pore size of 5.9 nm. Consitent with XRD data, the rough spheres disappeared (Figure 3d). The overall morphology was dominated by small particles below 1 jam in dimension. Figures 4a and 4b depict selected Nitrogen adsorption-desoprtion isotherms and pore size distributions (PSDs) for the same series of samples. As seen here and also in Table 1, all hexagonal phases exhibited pore sizes mostly above 5 nm, while typical pore sizes of MCM41 silica prepared in the presence of CTAB under more common temperatures, i.e., 80 - 120 °C, have 3.5 to 4 nm pores [5, 19]. Earlier work showed that direct synthesis or postsynthesis hydrothermal restructuring in the mother liquor at high temperature, e.g. 150 °C gave rise to
874 Table 1. Effects of composition on the structure of silica mesophase Sample No. AS421 AS460 AS461 AS462 AS465 AS466 AS467 AS470 AS471 AS472 AS474 AS478 AS479 AS480 AS481 AS482 AS484 AS489 AS490 AS491 AS492 AS504 AS505 AS506 AS507 AS508 AS515 AS516 AS517 AS518 AS519
CTAB H2O Si02 SiO^ Phase" 0.45 ~67 ~L 0.17 67 H 0.67 67 L+H 0.45 133.3 H+L 0.083 67 H 0.25 67 C 0.33 67 L 0.45 100 C+L 0.45 116.7 H 0.042 67 H+L 0.29 67 C+L 0.025 41.3 H 0.025 29 H 0.025 21.7 H 0.082 41.3 H 0.082 25.3 H 0.165 51.7 H+C 0.082 30.5 H 0.165 29 H 0.25 45.8 C+L 0.33 43.5 L 0.45 83.3 C+L 0.165 41.3 H 0.45 195 H+C 0.33 124.7 C+L 0.165 105 C+L 0.45 298.3 H 0.165 161 H+C+L 0.082 120.5 H 0.33 53 L 0.25 33.7 L
"D^ (nm) 3.40 5.35 3.48/5.19 5.26/3.49 6.02 4.52 3.42 4.28/3.41 4.38 5.81 4.26/3.35 6.66 6.65 7.11 5.34 5.68 5.28/4.62 5.68 5.03 4.28/3.23 3.32 4.41/3.37 5.53 4.71 4.22/3.38 4.64/3.63 4.95 5.18/4.76 4.77 3.55 3.54
SBE/
Pore size
(m'/g)
COKJS (nm)
~433 - ^ 1482 6.6 nd 570 807 6.6 6.6 705 5.5 792 4.8 591 4.8 846 5.2 840 5.3 449 4.6 898 5.6 356 5.8 439 5.5 465 5.8 761 6.7 808 4.6 921 6.3 802 5.9 955 4.5 936 4.7 566 1020 5.0 5.9 806 1157 5.3 1163 4.9 4.9 751 1059 5.9 6.0 923 5.3 877 5.2 611 nd 718
Vt^
(cmVg)
(cmVg)
0.40 2.13 0.50 0.94 0.89 1.02 0.48 0.83 0.79 0.54 1.06 0.39 0.49 0.52 0.88 1.05 0.94 0.97 1.14 1.01 0.48 1.85 0.83 1.22 1.13 0.67 1.13 0.98 0.88 0.51 0.60
0.45 2.14 0.57 0.99 0.91 1.08 0.53 0.87 0.84 0.56 1.10 0.41 0.52 0.55 0.90 1.07 0.96 0.98 1.16 1.04 0.54 1.91 0.87 1.25 1.15 0.72 1.15 1.00 0.89 0.55 0.76
(a) L: lamellar (MCM-50), H: hexagonal (MCM-41), C: cubic (MCM-48); (b) XRD (100), (001) and (211) interplanar spacings for H, L and C phases, respectively; (c) BET surface area; (d) primary mesopore volume; (e) total pore volume; (f) not determined.
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H,0 t^^ MCM-41, hexagonal ^ MCM-48, cubic ^ MCM-50, lamellar
25 CTAB Figure 1. Phase diagram for mesophase silica prepared at 150 °C for 40 h using the following synthesis mixtures: 1.0 Si02 : 0.317 TMAOH : x CTAB : y H2O with 0.042 < x < 0.67 and 21.7
