Nanoporous Materials III

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Studies in Surface Science and Catalysis 141 NANOPOROUS MATERIALS III

This Page Intentionally Left Blank

Studies

in S u r f a c e

A d v i s o r y Editors:

Science

and Catalysis

B. Delmon and J.T. Yates

Vol. 141

NANOPOROUS

MATERIALS !11

Proceedings of the 3 '~ International Symposium on Nanoporous Materials, Ottawa, Ontario, Canada, June 12-15, 2002

Edited by A. Sayari

University of Ottawa, Department of Chemistry, Ottawa, Ontario K1N 6N5, Canada

M. Jaroniec

Kent State University, Department of Chemistry, Kent, Ohio 44242, USA

-

¢

2002

ELSEVIER

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PREFACE Since the Breck Award winning discovery of the so-called M41S silica mesostructures in the early nineties, the field of ordered mesoporous materials has grown so dramatic that it has developed into a distinct research area. Remarkably, in the last ten years, over 3000 papers have been published on such materials. Impressive progress has been achieved in the use of self-assembly approaches and supramolecular templating techniques to generate a Wide variety of ordered inorganic, organic and hybrid mesostructures with tailored framework composition, pore structure, pore size, morphology and surface properties. The range, of synthesis conditions and the variety of templating surfactants and oligomers that have been explored is truly remarkable. Many fascinating discoveries have been made not only in the rational design of such materials at the molecular level, but also in a wide range of potential applications, for example in adsorption, catalysis, separation processes, environmental cleanups and optoelectronics. Among the most recent developments in this area is the extension of the amphiphile selfassembly techniques to the synthesis of ordered mesoporous organosilicas, which demonstrated that there are almost unlimited opportunities in tailoring surface and structural properties of mesoporous materials. Another important discovery is the use of ordered nanoporous silica and colloidal crystals to create new periodic mesoporous and macroporous materials, including carbons, polymers, metals, and alloys. Combination of different synthesis approaches such as amphiphile, colloidal crystal or microemulsion templating, micromolding and soft lithography led to materials with hierarchically ordered structures. International Symposia on Nanoporous Materials are intended to bring together investigators to discuss complementary approaches and recent advances concerning not only materials synthesized through supramolecular templating, but also a variety of other nanoporous materials such as clays, carbon molecular sieves, porous polymers, sol-gel and imprinted materials as well as self-assembled organic and organometallic zeolite-like materials. Judging from the remarkable success of the previous symposium "Nanoporous Materials II" (May 2000, Banff, Canada), and from the wide range of high quality abstracts and manuscripts submitted to the current meeting, the Organizing Committee is confident that the Nanoporous Materials III symposium will achieve its objective of gathering scientists interested in sharing their valuable findings related to a large variety of nanoporous materials. The contents of the current volume presents a sampling of more than 160 oral and poster papers that will be presented at the Symposium on Nanoporous Materials III held in Ottawa, Canada on June 12-15, 2002. The selected papers cover the three main themes of the symposium: (i) synthesis of mesoporous silicas and related materials (ii) synthesis of other nanoporous and nanostructured materials, and (iii) characterization and applications of nanoporous materials. Compared to the proceedings of the previous symposium, the current volume contains more contributions related to catalytic and environmental applications, which is a very positive trend. Although the present book does not cover all topics in the area of nanoporous materials, it reflects the current trends and advances in this field, which will certainly continue to attract the attention of materials scientists around the globe. Finally, on behalf of the Organizing Committee, we gratefully acknowledge the generous support of the Faculty of Science (University of Ottawa), the National Research Council of Canada (NRC), the Steacie Institute for Molecular Sciences (SIMS) and the University of Ottawa's Centre for Catalysis Research and Innovation (CCRI). Abdel Sayari February 18, 2002 Mietek Jaroniec


vii

ORGANIZING COMMITTEE Chairman

Abdel Sayari

University of Ottawa, Ottawa, Ontario, Canada

Vice-Chairman

Mietek Jaroniec

Kent State University, Ohio, USA

Members

Markus Antonietti JeffBrinker Christian Detellier Kazuyuki Kuroda John Ripmeester

Max-Planck-Institute of Colloids and Interfaces, Germany University of New Mexico, New Mexico, USA University of Ottawa, Ottawa, Ontario, Canada Waseda University, Tokyo, Japan National Research Council, Ottawa, Canada

INTERNATIONAL ADVISORY COMMITTEE D. Antonelli G. Attard A. Cheetham J.H. Clark C. Crudden E. Derouane M. Fr6ba A. Galameau S. Inagaki K. Kaneko S. Komameni M. Kruk R. Kumar B. Lebeau Th. Maschmeyer C.Y. Mou G. Q. Max Lu A. Neimark E. Prouzet H.-K. Rhee W.J. Roth D.M. Ruthven R. Ryoo F. Schtith A. Stein M. St6cker B.-L. Su T. Tatsumi O. Yaghi M. Ziolek

University of Windsor, Ontario, Canada University of Southampton, United Kingdom University of California, Santa Barbara, Califomia, USA University of York, York, England University of New Brunswick, NB, Canada University of Liverpool, United Kingdom Justus-Liebig-University, Giessen, Germany Ecole Nationale' Sup6rieure de Chimie de Montpellier, France Toyota Central R&D Laboratories, Inc., Nagakute, Japan Chiba University, Chiba, Japan Pennsylvania State University, University Park, PA, USA Kent State University, Ohio, USA National Chemical Laboratory, Pune, India Universit6 de Haute Alsace, Mulhouse, France Delft University of Technology, The Netherlands National Taiwan University, Taipei, Taiwan The University of Queensland, Brisbane, Australia TRI/Princeton, New Jersey, USA Ecole Nationale Sup6rieure de Chimie de Montpellier, France Seoul National University, Seoul, Korea Exxon-Mobil Research and Engineering Co., New Jersey, USA University of Maine, Orono, Maine, USA KAIST, Taejon, Korea MPI ftir Kohlenforschung, Mtilheim, Germany University of Minnesota, Minneapolis, Minnesota, USA SINTEF, Oslo, Norway The University of Namur, Belgium Yokohama National University, Yokohama, Japan University of Michigan, Ann Arbor, Michigan, USA A. Mickiewicz University, Poznan, Poland


ix

CONTENTS

Preface Organizing Committee

vii

Intemational Advisory Committee

vii

I.

Plenary Lectures

Recent Developments in the Synthesis and Chemistry of Periodic Mesoporous Organosilicas Tewodros Asefa, Geoffrey A. Ozin, Hiltrud Grondey, Michal Kruk, and Mietek Jaroniec Porous Materials: Looking Through the Electron Microscope O. Terasaki, T. Ohsuna, Z. Liu, M. Kaneda, S. Kamiya, A. Carlsson, T. Tsubakiyama, Y. Sakamoto, S. Inagaki, S. Che, T. Tatsumi, M. A. Camblor, R. Ryoo, D. Zhao, G. Stucky, D. Shindo and K. Hiraga

27

Molecular Imprinting - A Way to Prepare Effective Mimics of Natural Antibodies and Enzymes Giinter Wulff

35

II. Synthesis of Mesoporous Silicas Plugged Hexagonal Mesoporous Templated Silica: A Unique Micro- and Mesoporous Material with Internal Silica Nanocapsules P. Van Der Voort, P. I. Ravikovitch, A. V. Neimark, M. Benjelloun, E. Van Bavel, K.P. De Jong, B. M. Weckhuysen and E.F. Vansant

45

Imprinting of the Surface of Mesoporous Silicates using Organic Structure Directing Agents Kaveri R. Sawant and Raul F. Lobo

53

Synthesis and Characterization of Polymer-Templated Ordered Silica with Cagelike Mesostructure J.R. Matos, M. Kruk, L.P. Mercuri and M. Jaroniec

61

The Modeling of Wall Structure of Siliceous MCM-41 Based on the Formation Process Yasunori Oumi, Kazuhiko Azuma, Takuji Ikeda, Shintaro Sasaki and Tsuneji Sano

69

Pore Size Adjustment of Bimodal-mesoporous Silica Molecular Sieves Xiaozhong Wang, Tao Dou, Dong Wu and Bing Zhong

77

Alcothermal Synthesis of Large Pore, High Quality MCM-48 Silica Jihong Sun and Marc-Olivier Coppens

85

Studies of MCM-41 Obtained from Different Sources of Silica Icaro S. Paulino and Ulf Schuchardt

93

Synthesis and Characterization of Hexagonal Mesoporous Materials Using Hydrothermal Restructuring Method Kyoung-Ku Kang and Hyun-Ku Rhee

101

Synthesis of Highly Ordered Mesoporous Compounds with Control of Morphology Using a Non-ionic Surfactant as Template A. LOonard, J.L. Blin and B. L. Su

109

Towards a Better Understanding on the Mechanism of Mesoporous Formation via an Assembly of Cn(EO)mTMOS J.L. Blin, A. L~onard, G. Herrier, G. Philippin and B.L. Su

117

Mesoporous Silicas via Organic-Inorganic Hybrids Based on Charged Polymers Graham M. Gray and John N. Hay

127

Mesoporous Silicas of Hierarchical Structure by Hydrothermal SurfactantTemplating under Mild Alkaline Conditions Zhong-Yong Yuan, Wuzong Zhou, Bao-Lian Su and Lian-Mao Peng

133

III. Synthesis of Framework-Modified Mesoporous Silicas Synthesis and Characterisation of Super-microporous Aluminosilicates Prepared via Primary Amine Templating E. Bastardo-Gonzalez, Robert Mokaya and William Jones

141

A1-MCM-41 Synthesis Studies Using Al-Isopropoxide as A1 Source R. Birjega, R. Ganea, C. Nenu, Gr. Pop, A. Jitianu

.151

Mesoporous Aluminosilicates from Coal Fly Ash P. Kumar, N. Mal, Y. OumL T. Sano and K. Yamana

159

xi New Route for Synthesis of Highly Ordered Mesop0rous Silica with Very High Titanium Content Xiang-Hai Tang, Xin Wen, Shi-Wei Sun and Hai-Yan Jiang

167

Synthesis and Characterization of Ti-containing Mesoporous Alumina Molecular Sieves Chun Yang and Xi Li

173

IV. Synthesis of Surface-Modified Mesoporous Silicas Organizing One-Dimensional Molecular Wires in Ordered Mesoporous Silica Zongtao Zhang, Douglas A. Blom and Sheng Dai

183

Synthesis and Catalytic Properties of Organically Modified Ti-HMS Yong Yang and Abdelhamid Sayari

189

Synthesis and Characterization of Methyl- and Vinyl-Functionalized Ordered Mesoporous Silicas with High Organic Content Michal Kruk, Tewodros Asefa, Mietek Jaroniec and Geoffrey A. Ozin

197

Polyfunctionalized Silica Adsorbents Obtained by Using Dodecylamine as Template Inna V. Mel'nyk (Seredyuk), Yuriy L. Zub, Alexey A. Chuiko, Mietek Jaroniec and Stephan Mann

205

Characterization of Mesoporous Thin Films Formed with Added Organophosphonate and Organosilane Michael A. Markowitz, Eva M. Wong and Bruce P. Gaber

213

Improving the Hydro-Stability ofMCM-41 by Post-Synthesis Treatment and Hexamethyldisilazane Coating Jing Yang, Antje Daehler, Michelle L. Gee, Geoffrey W. Stevens and Andrea J. O'Connor

221

Adsorption of CO on Zn-Cu(I)/HMCM-41 Qihong Shi, Nongyue He, Fei Gao, Yibing Song, Yang Yu and Huilin Wan

229

V.

Synthesis of Mesoporous Metal Oxides

Design of Transition Metal Oxide Mesoporous Thin Films Eduardo L. Crepaldi, Galo J. de A. A. Soler-Illia, David Grosso, PierreAntoine Albouy, Heinz Amenitsch and ClOment Sanchez

235

xii Mesoporous Alumina as A Support for Hydrodesulphurization Catalysts Jiri Cejka, Nadezda Zilkovd, Ludgk Kalu$a and Miroslav Zdra~il

243

Preparation and XAFS Spectroscopic Characterization of Mesoporous Titania with Surface Area more than 1200 m2/g Hideaki Yoshitake, Tae Sugihara and Takashi Tatsumi

251

Mesoporous Zirconium Oxides: An Investigation of Physico-chemical Synthesis Parameters J.L. Blin, L. Gigot, A. L~onard and B.L. Su

257

Single Crystal Particles ofMesoporous (Nb, Ta)205 Junko N. Kondo, Tomohiro Yamashita, Tokumitsu Katou, Byongjin Lee, Daling Lu, Michikazu Hara and Kazunari Domen

265

VI. Synthesis of Other Nanostructured Materials and Nanoparticles Preparation of Exfoliated Zeolites from Layered Precursors - The Role of pH and Nature of Intercalating Media Wieslaw J. Roth and James C. Vartuli

273

Control of Mesopore Structure of Smectite-type Materials Synthesized with a Hydrothermal Method Masayuki Shirai, Kuriko Aoki, Kazuo Torii and Masahiko Arai

281

Synthesis, Characterization and Catalytic Application of Mesoporous Sulfated Zirconia Young-Woong Suh and Hyun-Ku Rhee

289

Synthesis of Mesoporous Silicoaluminophosphates (SAPO) Erica C. de Oliveira and Heloise O. Pastore

297

Synthesis and Characterization of Mesostructured Vanadium-Phosphorus-Oxide Phases Moises A. Carreon and Vadim V. Guliants

301

Novel Macroporous Vanadium-Phosphorus-Oxides Arrays of Spherical Voids Moises A. Carreon and Vadim V. Guliants

309

with Three-Dimensional

Engineering Active Sites in Bifunctional Nanopore and Bimetallic Nanoparticle Catalysts for One-Step, Solvent-Free Processes Robert Raja and John Meurig Thomas

317

xiii Using Au Nanoparticles-Surfactant Aqueous Solution for a Convenient Preparation of Mesoporous Aluminosilicates Containing Au-Nanoparticles Yu-Shan Chi, Hong-Ping Lin, Chinn-Nan Lin, Chung-Yuan Mou and BenZu Wan

329

The Use ofTemplated Mesoporous Materials as Tempates for the Development of Odered Arragements of Nanowire and Nanorods of Electronically Important Materials J. D. Holmes, T. R. Spalding, K. M. Ryan, D. Lyons, T. Crowley and M. A. Morris

337

Synthesis and Adsorption Properties of Novel Carbons of Tailored Porosity Z. Li, M. Kruk and M. Jaroniec

345

Flexible Metal-Organic Frameworks with Isomerizing Building Units D. V. Soldatov and J. A. Ripmeester

353

Dynamic Porous Frameworks of Coordination Polymers Controlled by Anions Shin-ichiro Noro and Susumu Kitawaga

363

Mesoporous Polymeric Materials Based On Comb-Coil Supramolecules Sami Valkama, Riikka Miiki-Ontto, Manfred Stamm, Gerrit ten Brinke and Olli Ikkala

371

VII.

Characterization of Nanoporous Materials

Electron Microscopic Investigation of Mesoporous SBA-2 Wuzong Zhou, Alfonso E. Garcia-Bennett, Hazel M. A. Hunter and Paul A. Wright

379

A Study of Morphology of Mesoporous Silica SBA-15 Man-Chien Chao, Hong-Ping Lin, Hwo-Shuenn Sheu and Chung-Yuan Mou

387

SBA- 15 versus MCM-41: Are they the same Materials? Anne Galarneau, HOlOne Cambon, Thierry Martin, Louis-Charles De M~norval, Daniel Brunel, Francesco Di Renzo and Franfois Fajula

395

Comprehensive Characterization of Iron Oxide Containing Mesoporous Molecular Sieve MCM-41 Zhong-Yong Yuan, Wuzong Zhou, Zhaoli L. Zhang, Q. Chen, B.L. Su, and Lian-Mao Peng

403

xiv Mesoporous Molecular Sieves of MCM-41 Type Modified with Cs, K and Mg Physico-Chemical and Catalytic Properties Maria Ziolek, Aleksandra Michalska, Jolanta Kujawa and Anna Lewandowska

411

Meso-ALPO Prepared by Thermal Decomposition of the Organic-Inorganic Composite. A FTIR Study Enrica GianottL Erica C. Oliveira, Valeria Dellarocca, Salvatore Coluccia, Heloise O. Pastore and Leonardo Marchese

417

Organic - Inorganic Phase Interaction in A1SBA-15 Mesoporous Molecular Sieves by Double Resonance NMR Spectroscopy Jean-Baptiste d'Espinose, Elias Haddad and Antoine G~dOon

423

Adsorption of Nitrogen on Organized Mesoporous Alumina Jiri Cejka, Lenka Vesel6, Jiri Rathousk~ and Arnogt Zukal

429

The Use of Ordered Mesoporous Materials for Improving the Mesopore Size Analysis: Current State and Future Michal Kruk, Mietek Jaroniec and Abdelhamid Sayari

437

Sorption Properties and Hydrothermal Stability of MCM-41 Prepared by pH Adjustment and Salt Addition Nawal Kishor Mal, Prashant Kumar and Masahiro Fujiwara

445

Acidity Characterization ofMCM-41 Materials Using Solid-State NMR Spectroscopy Qi Zhao, Wen-Hua Chen, Shing-Jong Huang, Yu-Chih Wu, Huang-Kuei Lee and Shang-Bin Liu

453

Acidity of Calcined AI-, Fe-, and La-containing MCM-41 Mesoporous Materials: An Investigation of Adsorption of Pyridine Nong-Yue He, Chun Yang and Zu-Hong Lu

459

Acid Properties of Ammonium Exchanged A1MCM-41 with Different Si/A1 Ratio Antonio S. Arafijo, Cristiane D.R. Souza, Marcelo J.B. Souza, Valter J. Fernandes Jr., and Luiz A. M. Pontes

467

Kinetic Evaluation of the Pyrolysis of High Density Polyethylene over HA1MCM-41 Material Antonio S. Arafijo, Valter J. Fernandes Jr, Sulene A. Araujo and Massao Ionashiro

473

Electrorheological Response of Mesoporous Materials under Applied Electric Fields Min S. Cho, Hyoung J. Choi, Wha-Seung Ahn and Myung S. Jhon

479

XV

VIII. Catalytic Applications of Nanoporous Materials Synthesis and Characterization of TiO2 Loaded Cr-MCM-41 catalysts E.P. Reddy, Lev Davydov and Panagiotis G. Smirniotis

487

Photocatalytic Ethylene Polymerization over Chromium Containing Mesoporous Molecular Sieves Hiromi Yamashita, Katsuhiro Yoshizawa, Masao Ariyuki, Shinya Higashimoto and Masakazu Anpo

495

Catalytic Reduction of Nitric Oxides on A1- containing Mesoporous Molecular Sieves W. Li, Y. Zhang, Y. Lin and X. Yang

503

Catalytic Oxidation of alpha-Eicosanol to alpha-Eicosanoic Acid Over Ti, Zr and Mn Doped MCM-48 Molecular Sieves Changping Wei, Yining Huang, Qiang CaL Wenqin Pang, Yingli BL and Kaiji Zhen

511

Preparation of Pd/A1-MCM-41 Catalyst and Its Hydroisomerization Properties for Long Chain Alkane Compounds Shui Lin, Han Ning, Sun Wan-Fu, Liu Wei-Min and Xue Qun-Ji

517

Alkylation of Phenol with Methyl tert-Butyl Ether over Mesoporous Material Catalysts Xiang-Hai Tang, Xin-Liang Fu and Hai-Yan Jiang

525

Isopropanol Dehydration over Nanostructured Sulfated MCM-41 Antonio S. Araujo, Joana M.F.B. Aquino, Cristiane D.R. Souza and Marcelo J.B. Souza

531

Effect of Si/A1 Ratio and Pore Size on Cracking Reaction over Mesoporous MCM-41 Wen-Hua Chen, Qi Zhao, Hong-Ping Lin, Chung-Yuan Mou and ShangBin Liu

537

Hydrogenation and Mild Hydrocracking of Synthetic Crude Distillate by Ptsupported Mesoporous Material Catalysts Hong Yang, Craig Fairbridge, Zbigniew Ring, Randall Hawkins and Josephine M. Hill

543

Carbon-Carbon Bond Forming Reactions Catalyzed by Meso- and Microporous Silicate-Quaternary Ammonium Composite Yoshihiro Kubota, Yusuke Nishizaki, Hisanori Ikeya, Junko Nagaya and Yoshihiro Sugi

553

xvi A Selectivity of Zeolite Matrices in the Cu(II) Reduction Process Vitalii PetranovskiL Valerij Gurin, Nina Bogdanchikova, Miguel-Angel Hernandes and Miguel A valos

561

Reduction of Binary Silver-Copper Ion Mixture in Mordenite: an Example of Synergetic Behavior Vitalii Petranovskii and Nina Bogdanchikova

569

Preparation, Characterization and Catalytic Properties of CuPC/Y Nanocomposite Huaixin Yang, Ruifeng Li and Kechang Xie

575

IX.

Environmental Applications of Nanoporous Materials

Environmental Applications of Self-Assembled Monolayers on Mesoporous Supports (SAMMS) Glen E. Fryxell, Yuehe Lin, Hong Wu and Kenneth Kemner

583

A Possible Use of Modified Mesoporous Molecular Sieves in Water Treatment Processes Izabela Nowak, Barabara Kasprzyk, Maria Ziolek and Jacek Nawrocki

591

Organized Mesoporous Titanium Dioxide - A Powerful Photocatalyst for the Removal of Water Pollutants Jiri Rathouslc~, Mark~ta Slabovd, Katerina Macounovd and Arnogt Zukal

599

Mesoporous Materials for Heavy Metal Ion Adsorption Synthesized by Displacement of Polymeric Template V. Antochshuk, M. Jaroniec, S.H. Joo and R. Ryoo

607

Organically-modified Mesoporous Silica Spheres with MCM-41 Architecture as Sorbents for Heavy Metals M. Etienne, S. Sayen, B. Lebeau, and A. Walcarius

615

NO and NO2 Gas Sensors Based on Surface Photovoltage System are Fabricated by Self-ordered Mesoporous Silicate Film Hao-Shen Zhou, Takeo Yamada, Keisuke Asai, Itaru Honma, Hidekazu Uchida and Teruaki Katsube

623

xvii

XO

Other Applications of Nanoporous Materials

Polymerisations in Mesoporous Environments James H. Clark, Duncan Macquarrie, Valerie Sage, Katie Shorrock and Karen Wilson

631

Incorporation ofNano-sized zeolites into a Mesoporous Matrix, TUD-1 Z. Shah, W. Zhou, J.C. Jansen, C. Y. Yeh, J.H. Koegler and Th. Maschmeyer

635

Formation and Stabilization of Gold Nanoparticles in Organo-Functionalized MCM-41 Mesoporous Materials and their Catalytic Applications Chitta Ranjan Patra, Anirban Ghosh, Priyabrata Mukherjee, Murali Sastry and Rajiv Kumar

641

Entrapment and Stabilization of Cadmium Sulphide (CdS) Nanoclusters Formed Inside Propylthiol Functionalized MCM-41 Mesoporous Materials Anirban Ghosh, Chitta Ranjan Patra, Priyabrata Mukherjee, Murali Sastry and Rajiv Kumar

647

SnO2 Nanoparticles in the Pores of Non-structured SiO2 and of Si-MCM-41: Comparison of their Properties in Gas Sensing Yuecel Altindag, Andrei Jitianu and Michael Wark

653

Spontaneous Nitride Formation in the Reaction of Mesoporous Titanium Oxide with Bis(Toluene) Titanium in a Nitrogen Atmosphere. M. Vettraino, X. He, Michel Trudeau and David Antonelli

661

Isolation and Characterization of Amorphous Solids from Oil Sands Fine Tailings Abdul Majid, Steve Argue, Irina Kargina, Victor Boyko, Gerry Pleizier and Jim Tunney

669

Author Index

675

Subject Index

679


Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Recent developments in the synthesis and chemistry o f periodic m e s o p o r o u s organosilicas

9

a*

Tewodros Asefa, a Geoffrey A. Ozln, ' Hiltrud Grondey, a Michal Jaroniec b

KIRlk, b

and Mietek

a Materials

Chemistry Research Group, Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada b Department of Chemistry, Kent State University, Kent, Ohio 44242, USA

Synthetic routes have recently been developed to an entirely new class of organicinorganic hybrid nanocomposite materials called periodic mesoporous organosilicas (PMOs) containing bridging organic groups integrated within a well-ordered mesoporous silica-based framework structure. Some of the interesting properties of these types of materials have been demonstrated but real challenges still remain including the scope of the synthesis approach and breadth of the compositional domain, reactivity and stability of the materials, their chemical and physical, electrical and mechanical properties, as well as potential applications. In this review, we will describe our past and current research concerning new synthetic strategies, unique properties and advantageous features of alkane, alkylene, aromatic, heteroatom-containing, chiral, and star-like organic functional group containing PMOs as advanced materials for diverse applications. Some of the effects of size and kind of organic groups on the order and integrity of the structure of the materials are discussed. Various synthetic routes, such as lithiation, Grignard, hydroboration, Pd-catalysed Heck coupling and alcoholysis reactions that we used to make the molecular poly(trialkoxysilyl)organic precursors are also briefly described.

I. INTRODUCTION The synthesis of periodic mesoporous organosilica (PMO) materials containing organic groups in the framework of ordered and high surface area mesostructures is drawing increasing attention recently [1-30]. One of the main driving forces behind the synthesis of various PMOs is the traditional interest in the synthesis of ordered hybrid organic-inorganic nanocomposites wherein beneficial properties of one of the components are enhanced or new properties uncharacteristic of the individual components are created. Furthermore, the presence of organic functional groups in such ordered nanoporous materials also offers additional advantages that make the materials potentially useful as catalytic and chromatographic supports, chemical and biological

sensors and membranes. Consequently, research in the field of organic-inorganic hybrid nanocomposite materials has remained an active area of investigation for the last few decades and is likely to expand in the years as well. Enormous advances made since the first types of classical organically modified silicas (ORMOSILs) and organically modified ceramics (ORMOCERS) were reported [31-33]. The successes in coupling organic and inorganic groups at a molecular level for the synthesis of hybrid organicinorganic xerogels and amorphous materials [34-40] have also led to many advances in recent years and proved to have advantages over thesimple physical mixing of the constituents in their bulk states. However, until recently, there have been no welldeveloped approaches to create uniform pores of controlled size in these materials. In fact, their porous structure is highly dependent on the synthesis temperature and drying conditions [39,40]. With the work of Mobil scientists in 1992 and the first report on mesoporous silica (MCM-41) materials [41-44], a new research direction in organicinorganic nanocomposite materials emerged. The synthesis of these materials is carried out using inorganic precursors and surfactant templates and the method of supramolecular self-assembly. In many cases, the supramolecular templates can be removed without the collapse of the ordered composite, thus rendering ordered mesoporous structures. After the first papers on materials with siliceous frameworks [41-44], synthesis techniques and compositions of mesoporous materials developed further to include various other kinds of nanocomposite and nanoporous materials. The methodology, for instance, was modified to include ordered hexagonal, cubic and lamellar structures as well as disordered structures with uniform pores [41-50] while the composition field was expanded to include Pt, TiO2, M/Ge4Sl0, etc. [51-57]. Furthermore, by judicial choice of templates and swelling agents, control over the size, porosity and structure of the materials have been achieved [41,49,50,58,59]. The literature in the past 10 years also contains demonstrations on potential applications of these materials in catalysis, nanoelectronics, separation, host-guest chemistry and sensing [60-66]. However, many of these applications were not achieved with the periodic structure alone. The presence of electroactive, optically active or reactive functional sites within these high surface area and ordered mesoporous materials were required for these applications to be realized in practice. The introduction of terminal organic functional groups into periodic mesoporous silicas either through direct or indirect (post-synthetic) synthetic approaches has been successfully used as a way to functionalize this class of materials in the past few years [67-71 ]. However, these approaches have often resulted in materials with lower degree of structural ordering and low or moderate loading of organic groups. The recent approach of introducing organic functional groups into the framework of periodic mesoporous organosilicas (PMO) provided a way to overcome these drawbacks [ 1-30]. PMOs are a new class of organic-inorganic hybrid nanocomposites with uniformly distributed organic functional groups in the framework of the materials. They are synthesized like MCM-41 materials through in a one-pot surfactant-templated supramolecular self-assembly procedure but from the hydrolysis and condensation of poly(trialkoxysilyl)organic precursors ([(R'O)3Si]xR, x = 2 ,3) [1-30]. The enormous choice of polysilylated molecular precursors having various types of bridging organic functional groups with for example electroptic, catalytic and hydrophilic/hydrophobic

properties and the self assembly of these with various kinds of supermolecular templates are providing researchers with large varieties of periodic mono- and multi-functional organic-inorganic nanocomposite materials. The ability for molecular integration of organic and inorganic groups in the framework of these materials may have advantages over direct and indirect (grafting)methods that may create non-uniformly distributed organic groups protruding into the void spaces. Benefits include tailorable physical properties [ 19,20], uniform distribution of functional groups, possibility of controlling the loading of the functional groups using co-condensation approach, and unique chemical properties. Some of these bridging organic groups have also proven to be accessible for chemistry and can be further transformed chemically [1,18]. Interesting chemical differences between organics in the framework and in the channels has been reported and used to have advantages in the preparation of a new sub-class of bifunctional and multifunctional PMOs [8]. The dependence of thermal, mechanical, dielectric and adsorptions properties on the nature of the organic groups in these hybrid materials will likely result in new products, processes and devices made out of PMOs in the near future [3,19-21 ]. The ability to synthesize film and various curved PMO morphologies [7,19-21] may lead to advances in catalysis, chromatography and membrane science and technology. Advances made in the synthesis and characterization of the properties of PMO materials in just less than two years have inspired investigations into new kinds of functionalized PMO materials and a search for commercial applications. However, only a few types of organic functionalized PMOs have been reported so far. They include methylene, ethane, ethylene, acetylene, thiophene, benzene and bithiophene containing PMOs, [ 1-30]. There is also a notable scarcity of detailed investigations of the structural integrity, and thermal and chemical stability of Si-C bonds for various kinds of organic groups in PMOs [3,8]. Herein, we review some developments in our research group mainly concerning the synthesis of various PMO materials and possible applications. Particular attention will be given to PMO materials having heteroatom and side-arm starlike organic groups, which are envisioned to enhance the reactivity and accessibility of bridging organic groups in PMOs enabling judicious surface modification that leads to the tailoring of function. 2. E X P E R I M E N T A L 2.1. Materials. HC1 and NH 3 solutions were obtained from BDH. Methanol was supplied by ACP. Hexanes, pentanes, diethylether and tetrahydrofuran (THF) were purchased from ACP and were dried with Call2 and molecular sieves before every use. The THF and the diethylether were further distilled over Na/benzophenone. All commercially available bis(triethoxysilyl)organic compounds were obtained from Gelest. All other chemicals were received from Aldrich. 2.2. PMO PRECURSORS 2.2.1. Commercially available PMO precursors. Scheme 1 shows most of the commercially available precursors (either triethoxysilylated or trichlorosilylated) that we

obtained from Gelest and used for the preparation of PMOs. The precursors were used as received without further purification.

Scheme 1. Commercially available PMO precursors used for the synthesis of PMO materials (Si denotes -Si(OEt)3 or-SIC13).

Si~Si

Si~si

1

Si~"x,~"X, Si

2

3

4

~---Si S

~

S i ~ S i

S i ~ N H

Si Si 6

S i ~ N H 7

2.2.2. Synthesis of PMO precursors.

We have utilized various synthetic routes to prepare polyalkoxysilylated PMO precursors that are not commercially available. Representative synthetic routes are shown in Scheme 2 and some examples of PMO precursors and PMOs that we have prepared from these precursors are shown in Scheme 3. Some of these precursors have been synthesized before and used to prepare hybrid org~ic-inorganic xerogels (materials with a relatively broad pore size distribution) by many groups over the past few years [34-40]. Most of the aromatic precursors were synthesized using Grignard reactions while the methine precursor is made through silylation-alcoholysis and the anthracene and ferrocene precursors through lithiation. 2.2.3. Synthesis of organomethylene PMO precursors. (Scheme 4 and 5). Bis(trialkoxysilyl)organic precursors containing methylene groups in the backbone with side-arm functionality have been prepared through lithiation of a bis(triethoxysilyl)methane precursor under nitrogen atmosphere in a freshly distilled anhydrous solvent followed by coupling reactions of the resulting carbanion with various electrophiles (Scheme 4) [72]. [CAUTION: t-BuLi is strongly pyrophoric and should be handled extremely carefully]. This route resulted in a new class of side-arm bridging organosilane precursors. A representative synthetic procedure for the preparation of bromomethylene (BM) and 1,1-bis(triethoxysilyl)-2-(p-bromophenyl)ethane (or pbromobenzyl-methylene) (BBM) PMO precursors are given below. Similar reactions were applied or could be applied to the synthesis of the other precursors shown in Scheme 5. Crude products were used as precursors when distillation resulted in decomposition.

Synthesis of [1,1-bis(triethoxysilyOmethyl] lithium salt (la). A commercially available bis(triethoxysilyl)methane (BTM) (1) was lithiated following a literature procedure [72] with a slight modification. Typically, to 150 mL freshly distilled dry THF was added

bis(triethoxysilyl)methane (BTM, 1) (3.0 g, 8.8 mmol) and the solution was stirred under nitrogen for 5 minutes. Then 5.2 mL of 1.7 M t-BuLi (8.8 mmol BuLi) was added to the above solution dropwise over 10 minutes a t - 78 ~ under a nitrogen atmosphere. The solution was stirred for 1 hr a t - 78 ~ The resulting carbanion lithium salt (la) was subsequently quenched with bromine or 4-bromobenzyl bromide (see below) a t - 78 ~ (Scheme 4 and 5). Scheme 2. Synthetic routes to polysilylated PMO precursors. 1) Grignard Mg / XSi(OR')3 r_

X--R--X

(R'O)3Si~R--Si(OR')3

2) Alcoholysis R'OH

X3Si~R--SiX 3

=

(R'O)3Si--R--Si(OR')3

3) Hydrosilation

~--R---~

(R'O)3Si~

HSi(OR')3,.

, ~R~si(OR')3

H2PtC16 4) Pd-coupling Heck Reaction '----"

X-Ar--X

+ 2

Pd (I) / NEt3

~

\Si(OR,)3

(- HX)

5) Hydroboration

(R'O)3Si-----~__ Ar .v_-

OR)3 BH3.THF

3

or

( ~Si(OR,)3 )

~___ Si(OR')3

l

i(OR')3

H~-----'--~Si(OR,)3

~R'O)3SiJ ~ S i ( O R ' ) 3 6) Lithiation (R'O)3Si~Si(OR')3

t-BuLi / THF .~ -78 C

(R'O)3S i ~ S i ( O R ' ) 3 Li §

R"X

r- (R'O)3Si'~Si(OR')3

R"

7) Silylation of acid halides (acyl halides) C13Si~/SiC13 R~COC1

+ 2 HSiC13

r-

I R

EtOH

(EtO)3Si~/Si(OEt)3

R

Scheme 3. Commercially unavailable precursors used for PMO synthesis (Si stands for

Si(OEt)3). Si Si I Si H

Si~Si

8

9

10

Si Si

Si~ ~-S- S i S

s i ~ S / ~ -Si

11

Si

Si [

Si

.... Si ~/] 12

Fe ~

Si~-.~Si

13

14

Si Si~.v,,'~/'~Si

Si 15

Si H3C~~~ L

H3C

Si H3CO~

--CH3 16

si 20

Si

Si

17

18

F

F

F

F

s. si 21

y "OCH3 Si 19

s si 22

Synthesis of 1,1-bis(triethoxysilyl)bromomethane[(EtO)3SiCHBrSi(OEt)3](BM). To

the carbanion solution (la) was added, a slight excess of bromine (1.5 g, 9.4 mmol) a t - 78 ~ The solution was stirred a t - 78 ~ for 30 min and then at room temperature for 2 hrs. The solvent was removed and the residue was extracted with dry hexane and then filtered. After pumping the solvent off, the residue was vacuum distilled giving bis(triethoxysilyl)bromomethane (3): bp 110-112 ~ / 0.04 mm Hg; IH NMR (300 MHz,

CDC13) 6; 1.21-1.25 (t, 18 H, CH3), 6; 2.18 (s, 1H, CHSi), 6; 3.90-3.96 (q, 12H, CH20); 13C NMR (75.48 MHz, CDC13) 6; 10.13 (CHSi), 6; 18.38 (CH3), 6; 59.62 (CH20); El-MS (m/z) 419 (23%, M+), 375 (25%, [M+- 44]), 163 (100%, [(EtO)3Si+]). Scheme 4. Lithiation of bis(triethoxysilyl)methane and subsequent coupling of lithiated carbanions with organic electrophiles. EEtOXs EtO .... iV

si~Eo Et .....OEt

t-BuLi/THF_ -78 ~

(1)

(la)

(1)

EtO QEt EtO~.\qi &OEt EtO....... ~S_ ......OEt L1

EtEtOksi si~EoEt EtO...... V + .....OEt Li

R'X ~

EtO\ ~Et EtO--~:i ~:i--,OEt EtO....... " ~ ......OEt R'

(2)

(la)

EtO\ EtO~si EtO...... V

~)Et X2 EtO\ ?Et Si~.,OEt .._ EtO,..-. . . . . OEt .....OEt "- EtO......~ly~l""OEt__ Li+ X

R ' M g X EEtONsi Si~.EOEt ._ EtO...... " ~ .....OEt R'

(3)

(la)

R' = D (lb); Br (lc); p-CH2-C6H4Br (ld); p-CH2-C6H4-Si(OMe)3 (le); COOH / COO-Li + (lf); CH2CH2CH2NH2 (lg); Br-camphor (lh); C6F5 (li) (Grignard reaction was also used after lithiation); CH2(CH2)n-i(CF2)mCF3 (lj); CH2NHC6H4NO2 (lk) (See Scheme 5 also).

Synthesis of 1,1-bis(triethoxysilyl)-2-(p-bromophenyl)ethane (p-bromobenzylmethylene precursor) [(EtO)3SiCH(CH2-p-C6H4BOSi(OEt)3] (BBM). To la was added a THF (10mL) solution ofp-bromobenzylbromide (2.2 g, 8.8 mmol) a t - 78 ~ After similar work-up as above, BBM was obtained: bp 144-148 ~ / 0.04 mm Hg; 1H NMR (300 MHz, CDC13) 6 0.55-0.58 (t, 1 H, CHSi), 8 1.18-1.23 (t,18H, CH3), 8 2.89-2.92 (d, 2H, CHzPh), fi 3.78-3.82 (q, 12H, CH20), ~ 7.17-7.20 (d, 2H, ArH), 6; 7.36-7.39 (d, 2H, ArH); ~3C NMR (75.48 MHz, CDCI3) fi 10.80 (CHSi), 6; 18.46 (CH3), 6;29.47 (CHEPh), 8 58.66 (CH20), 6130.84, 131.10 (CH aromatic); EI-MS (m/z) 508 (5%, M+), 464 (100%, [M +- 44]).

Scheme 5. Synthesis of PMO precursors with organomethylene groups in the framework. lh

(R'O)3.Si~Si(OR')3

(R'O)3Si'~Si(OR')3

~'H2

3c'a%dpLboTm~

]

(R'O)3Si~ISi(OR')3

lo

O~---O-Li§

Si(OMe)3 CO2 C1CH2

(R'O)3Si~-~Si(OR')3 [

/Si(OMe)3

(~U2Br !(R' !................................... O)3Si~Si(OR' [ )3 ] 1CH NH"(R'O)3Si'~Si(OR')3 i "+ [ C( 2)3 2 CH2(CH2)2NH2

~~1

[.................Li..........~p...j

CF3(CF2)m(CH2)nC1 /

D20

/ NHCH2CI / ~

Br2~

(R,O)3Si~.~Si(OR,)3

~I CH2(CH2)n-1 (CF2)mCF3 (R'O)3Si Si(OR')3 lj

(R'O)3Si~-Si(OR')3 NO2 ~FsMgBr CH2 I IH (R,O)3Si~...Si(OR,)3 (R'O)3Si'~Si(OR')3 D F~F lk lb li F F NO2 F 2.3. Self-assembly of BM and other PMOs under acidic conditions For a typical synthesis under acidic conditions, a solution of cetyltrimethylammonium bromide (0.34 g, 0.93 mmol), HC1 (7.18 g, 36 wt%, 70.8 mmol) and water (13.4 g, 0.74 mol) was prepared at room temperature (Scheme 6). To this solution was added 3.85 mmol (or 7.70 mmol Si) of the required precursor and the mixture was stirred for 30 mins. After aging at 80 ~ for 4 days, the product was isolated by filtration, washed with copious amounts of water, and dried under ambient conditions. (For bromomethine a light brownish powder was obtained with a typical yield of 0.56 g).

2.4. Self-assembly of B B M and other P M O s under basic conditions

A solution of cetyltrimethylammonium bromide (0.67g, 1.84 mmol), ammonium hydroxide (14.18 g, 35 wt%, 0.14 mol) and water (26.73 g, 1.48 mol) was prepared at room temperature (Scheme 6). To this solution 2.94 mmol (or 5.88 mmol Si) of precursor was added (for 50% BBM PMO, 2.94 mmol BBM and 1.47 mmol of TEOS) and the solution was stirred for 30 minutes during which time a precipitate formed. After aging at 80 ~ for 4 days, the product was filtered, washed with copious amounts of water, and dried under ambient conditions resulting in a fine powder (typical yield of 0.49 g of white powder was obtained for bromobenzyl-methylene PMO). Scheme 6. Synthesis and solvent-extraction of BM and BBM PMOs

EE~~si si~EoEt EtO " x ~ .....OEt

CTABr/ +/H20 . . . . . .

oS

Lo

O""Si~ Si''''O

Br

Br

lc

lc'

EtO\ ~Et EtO.---Si qi-,-OEt EtO"" x ' ~ ......OEt +

02,

.OEt EtO~S,i"OEt

CTABr/ OH-/H20

HCI / MeOH ..... =

BM PMO

......o

H2

HC1/ MeOH = BBM PMO

OEt

Br ld

Br ld'

2.5. Surfactant-extraction of P M O s

The surfactant was removed from the samples using solvent-extraction in an HC1/methanol solution. Typically ca. 0.3 g of as-synthesized powder was stirred for 6 hrs at 55 ~ in a solution of 4 g of conc. (36 wt %) HC1 and 170 g methanol. The product was then isolated by filtration, washed with methanol, and dried in air. A typical weight loss of ca. 30 % was obtained for each sample after a single solvent extraction. The surfactant removal was confirmed by solid-state 13C CP MAS NMR. 2.6. Instrumentation and characterization techniques

The as-synthesized and surfactant extracted materials were characterized using powder X-ray diffraction (PXRD), transmission electron microscopy (TEM), 13C, 29Si CP MAS (29Si MAS as required) and 79Br MAS NMR, N2 adsorption, thermogravimetric analysis (TGA), and elemental analysis (EA).

10 Powder X-ray diffraction (PXRD) patterns were measured with a Siemens D5000 diffractometer using Ni-filtered Cu-K~ radiation with )~ = 1.54178 A. The high temperature in-situ measurements were done as reported in Ref. [8]. TEM images were recorded on a Philips 430 microscope operating at an accelerating voltage of 100 kV. Nitrogen adsorption measurements were carried out at 77 K on a Micromeritics ASAP 2010 volumetric adsorption analyzer. Before the measurements, the samples were degassed under vacuum at 100 or 140~ Weight change curves were obtained under nitrogen or air atmosphere on a TA Instruments TGA 2950 thermogravimetric analyzer (TGA) using a high-resolution mode with a maximum heating rate of 5 ~ min -1. Solution phase nuclear magnetic resonance spectra were taken with a Varian VXR 300 spectrometer. Solid-state NMR spectra 13C CP-MAS (100.6 MHz), 29Si CP-MAS (79.5 MHz) and 79Br MAS NMR (100.3 MHz) were obtained with a Bruker DSX400 spectrometer. Experimental conditions: 13C CP-MAS NMR (6.5 kHz spin rate; 2.5 ms contact time; 3 s recycle delay; 10,000-20,000 scans); 13C NQS (non-quaternary suppression) CP-MAS NMR experiment, 6.5 KHz spin rate, 3 s recycle delay, 50 ~ts dephasing delay, 5,000-10,000 scans; 29Si MAS NMR (6.5 KHz spin rate; 100 s recycle delay; 800-1000 scans). 29Si CP MAS NMR experiments: 6.5 kHz spin rate, 3 s recycle delay, 10 ms contact time, zr/2 pulse width of 6.0-7.5 ~ts, 200-5,000 scans. 79Br MAS NMR (6.5 KHz spin rate; 0.1 s recycle delay; 50,000-100,000 scans). 3. R E S U L T S A N D D I S C U S S I O N 3.1. Precursor synthesis Since the reports on the first PMOs [1,2,13,18]; we have been actively investigating the possibility of incorporating other organic functional groups into the framework of PMOs. Some of the challenges are briefly described here. As most PMO precursors were not commercially available, their synthesis under inert atmosphere was routinely required. Most of the poly(trialkoxysilyl)organic precursors discussed in this article were prepared through lithiations and Grignard reactions while some are commercially available. We found that the synthesis through lithiation usually resulted in higher yields than the analogous Grignard reactions. The required precursors were usually isolated and purified by distillation under vacuum to avoid possible decomposition due to their rather high boiling points under standard conditions. The reaction of lithiated carbanion solution (la) with electrophiles was found to be quite favorable and resulted in good yields when the reaction was carded out rigorously under an inert atmosphere. 3.2. Synthesis of PMOs 3.2.1. Linear unsaturated and saturated organic bridge-bonded PMOs So far, the literature reported PMOs have rigid and/or short organic groups. These were found to result in rather well ordered mesoporous structures. For instance, we reported the synthesis of the shortest organic bridgebonded PMOs (1) with 2-D hexagonal mesostructures (Figure 1) containing methylene bridges that are isoelectronic with oxygen atoms in MCM-41 materials [3]. We also recently expanded the synthetic route to include cubic [10] and biphasic (hexagonal and cubic together in the same material) methylene PMOs. Figure 1 shows the powder X-ray diffraction (PXRD) pattems and TEM images for various methylene PMO structures. The thermal

11 transformation of bridging methylene into terminal methyl groups in these PMO materials is particularly interesting and provides a novel route to new organic functionalized mesoporous materials with high loadings of functional groups. Similarly, we prepared an ordered mesoporous methine PMO (8) with the methine groups bridging three silicate units in a mesoporous structure (Scheme 3). The precursor for this PMO (8) was prepared by silylating chloroform in the right stoichiometry followed by alcoholysis as reported by Corriu et al. [72]. (A) Methylene PMO (hexagonal and cubic)

(C) Hexagonal methylene PMO

(B) Methylene PMO (biphasic)

(D) Cubic methylene PMO

Figure 1. PXRD patterns of hexagonal, cubic and biphasic methylene PMOs (A, B) and TEM images hexagonal and cubic methylene PMOs (C, D) PMOs with two carbon bridging organic groups were among the first to be reported. They included ethane (2), ethylene (3) and acetylene (12) PMOs [1,2,13-18]. 2D hexagonal, 3-D hexagonal and cubic structures were reported for 2 [ 13-16], both 2-D

12 hexagonal and disordered structures were reported for 3 [1,18], while most of the Si-C bonds were found to cleave for 12 [2]. The ability to vary both the composition and the structures of the materials will offer numerous opportunities for applications. Ethane PMOs were also reported to adopt various curved morphologies [25,26], which will likely be useful in separation applications. Furthermore, accessibility and reactivity of ethylene group in an ethylene PMO were demonstrated with bromination [1,18] and hydroboration [ 12] reactions giving more opportunities for further functionalization. For instance, the 13C NMR and 29Si CP MAS NMR spectra for ethylene PMO before and after hydroboration (Figure 2) indicate that some of the ethylene carbons are available for reaction with the retention of the periodic structure (see PXRD patterns in Figure 2A).

(B) t3c CP MAS NMR spectra

(A) PXRD patterns lOOOl~,

- Jll, ~, ~

"

Solvent-extracted

~

//

Hydroborated (Spin i~'ate= 8"()KHz

l] 1

tlydmborated (SpinRate = 6.5 KHz)

Hydroborated

20(0) (C) '~Si CP MAS NMR spectrum

/

II~

e

S~

borated " " * " ' ~ ' ( " ' ' " ' 1

150 I

";"

"

I

. . . . .

|

. . . .

....

100

~'1

. . . .

50

I'"

0

I

o -so -too .1so Chemical shift (ppm)

Chemical shift (ppm)

Figure 2. A) PXRD pattems of ethylene PMO before and after hydroboration; B) 13CCP MAS and (C) 29Si (CP) MAS NMR spectra of ethylene PMO before and after hydroboration. PMOs containing a bridging group with one or two carbon atoms as well as other rather short and rigid organic groups were observed to self-assemble quite readily and form well-ordered structures. Further investigations in our group revealed that poly(trialkoxysilyl)organic precursors having bridging organic groups with four or more carbon chains did not self-assemble and gave only amorphous gels and disordered materials. For instance, 2-butylene (16), hexylene (5), 1,4-diethylbenzene (6), and N,N'dipropylethylenediammine (7) bridge bonded materials when templated with surfactants were all found to be amorphous with no low angle Bragg reflections. However, three

13 carbon chain precursor materials with bridging organic groups like 1,1dimethylvinylidene (4) were found to give partially ordered PMO materials. These observations indicate that the more flexible and non-polar bridging organic groups partially or fully prevent the development of mesoscopic order during surfactant templated self-assembly. Attempted synthesis of 2-butylene with large head group surfactants (cetyltriethylammonium bromide) [73] also failed to produce well-ordered PMO materials.

3.2.2. Aromatic and organometallic bridge bonded PMOs various kinds of aromatic and organometallic PMOs [2,9] with ordered mesoporous structures and some of these with a certain degree of additional spatial ordering of the aromatic groups in the pore walls due to re-re stacking, (as indicated by Xray diffraction data [9]) have been synthesized and reported. For instance, we have prepared 1,4-phenylene (9) [2], 1,3,5-phenylene (14) 2,5-thiophene (10) [2], 2,2'bithiophene (11) [2], 1,4-xylyl (17) [9], 1,4-(2,5-dimethylphenylene) (18) [9], 1,1'ferrocene (13) [2], and 1,4-(2,5-dimethoxyphenylene) (19) PMOs [9]. In addition, ptoluyl (20), 1,4-perfluoroaryl (21) and 2,5-pyridine (22) PMOs have also been prepared but their structures were not well defined and their stability was poor. The synthesis conditions for most of the aromatic PMOs should be chosen carefully in order to avoid or minimize cleavage of Si-C bonds. For example, significant cleavage of Si-C bonds was observed in thiophene, bithiophene and ferrocene PMOs. Aromatic groups within these PMOs are potentially useful as good supports and carriers for metal or organometallic guest molecules, which may ultimately be useful as catalysts (for example pyridine is a good ligand for anchoring metals). Some of the organics are also known to have high affinities for many other organic molecules, a property that could make this class of PMOs useful for separation and environmental remediation applications. Moreover, conjugated aromatic and organometallic containing PMOs based on anthracene (15) and ferrocene (13) could find applications as sensors as they contain optically active and electroactive moieties. 3.2.3. Organomethylene bridge bonded PMOs Recently, we have prepared a new class of PMOs with star-like organic bridging groups based on a methylene backbone. It is known that methylenesilica PMOs were amongst materials with well ordered mesoporous structures and interesting properties and we have chosen these groups as platforms to attach various functional groups (Schemes 4 and 5). These reactions were carried out by taking advantage of the lithiation reactions that bis(trialkoxysilyl)methane undergo resulting in the corresponding carbanion (la) allowing one to replace one of the protons by other active organic molecules. This opened an avenue to a diverse group of functionalized PMOs including heteroatom containing organic groups. Some examples of PMOs whose synthesis was attempted by this route include bromomethylene (lc), p-bromobenzyl-methylene (ld), p-benzylmethylene (le), carboxylic acid-methylene (If), (+)-bromocamphor-methylene (lh), and perfluoaryl-methylene (li) PMOs. The synthesis of PMOs with these groups would afford some of the first mesoporous materials with highly reactive bridge bonded organic groups in the framework and protruding into the void spaces. Each of these materials is anticipated to have their own special properties and particular applications. For example,

14 the (+)-bromocamphor is a chiral group and could be useful for chiral reactions, catalysis and separations. The benzyl-methylene PMO is a new type of aromatic PMO with three surrounding silicate units unlike most of the previously discussed aromatic PMOs. The perfluoraryl (li) and perfluoroalkyl (lj) functional groups will make the PMO materials more hydrophobic and "Teflon-like" and could lead to a new class of low dielectric constant materials. Our preliminary investigation on perfluoraryl-methylene PMOs (li) was promising but more work is needed to determine their stability and properties. The other reactions in Scheme 5 that will not be discussed in detail here but are worth mentioning involve those carrying basic (lg), acidic (If) and optically non-linear (lk) functional groups on the methylene bridging group. Each of these PMOs would be interesting as an advanced material for various applications. (A) Methine PMO (solvent-extracted)

(B) p-Benzyl-methylene PMO (solvent-extracted)

~

IlL

9 .... 2

~.~t,~ ~ 4

.

. 6

.

.

.

t

. 8

.

l(i

0 5

1o

20 (degrees)

10

15

20

2A

.~0

2e (degrees)

(C) Other PMOs (solvent-extracted) 3OO0

ga, 2~m .,.., ~ l~D.~v~. I ,

/

Bromocamphor-methylene PMO

[

-Dime, y vm leae. , o

0 .....

-.~-~.~..-~,o.~.. ,. . . . . 5

,

..

.

!0

..

, 15

-~ ............. 20

20 (degrees)

Figure 3. PXRD pattems of various PMOs with organomethylene groups in their frameworks. The synthesis of the aforementioned PMOs was performed and PXRD patterns of most of the resulting materials after template extraction using acid/methanol washing showed at least one low angle Bragg peak indicating the presence of an ordered mesoporous structure (Figure 3). For some of these materials, the porous structure was additionally studied using nitrogen adsorption. The sample synthesized using the

15 bromocamphor-methylene precursor was clearly mesoporous (see adsorption isotherm in Figure 4A) and exhibited primary mesopores of diameter about 3.6 nm, a very large BET specific surface area (1310 m 2 g-l) and a significant total pore volume (0.99 cm 3 g-l). However, as will be discussed below in more detail, this PMO was found to contain only a small amount of bromocamphor-methylene bridging groups, whereas its predominant bridging group was methylene. The presence of very limited amount of bromocamphor groups can be inferred from a small magnitude of TGA weight loss (Figure 4B). Therefore, this sample can be referred to as methylene/bromocamphor-methylene PMO. The attempted synthesis of benzyl-methylene PMO (le) in which a bridging organic group connected to three silicon atoms resulted in a mostly microporous material with the BET specific surface area of 670 m 2 g-i and a total pore volume of 0.41 cm 3 g-1. The surfactant-extracted sample exhibited a small weight loss related to the decomposition of the residual surfactant at about 200~ followed by a very large weight loss that can be primarily related to the decomposition and thermodesorption of large organic groups in the PMO structure. The synthesis of a 1,1-dimethylvinylidene PMO (4) was also attempted and the resultant solvent-extracted sample was mesoporous (pore diameter of about 2.6 nm) and exhibited an appreciable adsorption capacity despite the fact that a certain amount of surfactant was not removed, as seen from TGA. As will be shown below, an appreciable degree of cleavage of Si-C bonds took place during the synthesis of this material, and therefore it is expected to have a significant amount of pendent rather than framework organic groups (the retention of organic groups can be inferred from TGA data; additional evidence for preservation of Si-C linkages is presented below) The presence of bridging organic groups in these PMOs, were investigated using solid-state NMR spectroscopy. The 13C CP MAS NMR (Figure 5), for instance, indicates the presence of small percentages of bromocamphor-methylene and a large quantity of methylene not containing camphor groups probably because the crude product was used as a precursor in this case. Further, large percentages of functional groups were observed in methine (8) and 1,1-dimethylvinylidene (4) PMOs. The methine PMOs also showed similar thermal~ stability to those of methylene PMOs and the surfactant template in these materials was easily removed with calcination in air at 350 ~ Template removal in acid/methanol solution for 1,1-dimethylvinylidene PMO causes hydrochlorination of the vinyl carbons unless carried out under very mild acidic solution. The 29Si CP MAS NMR spectrum (Figure 6) of the sample synthesized using the bromocamphor-methylene precursor showed essentially only T sites, thus revealing no Si-C bond cleavage. However, most of the other materials exhibited a cleavage of Si-C bonds, which was significant for instance for 1,1-dimethylvinylidene and methine PMO, in which case as much as about 50% of these bonds could have been cleaved. In what follows, we will focus on similarly prepared bromo- and p-bromobenzylfunctionalized methylene PMOs where bromo and organobromo sites are attached to bridging methylene groups. Organohalides are well known to undergo various reactions such as lithiation, Grignard and metal-catalyzed coupling and the organohalide functional groups in PMOs are likely to undergo similar reactions allowing further surface, chemical and physical modifications and making catalytic activity, ion-exchange and easily surface-modifiable materials possible. This property makes these materials the first examples of highly reactive PMOs due to the ease of substitution of bromides by other interesting functional groups and ligands.

16 100 -'-' 700 f melhylenei ' i A) " '~0 Lbr~176176 600 / m e t h y l e n e ~ l , 1-dimethyl- " r.~ ~ 500

90 80

..~methyiene/bromocamphorlene

~

._~ 1,1-dimethyl"x,.X.... . .vinylidene ................

(B)

400 "~

300

PMO \ \

60

200 t~ 100
radius of gyration 5- 15nm molecular weight 30000 - 500000 =:> soluble

Figure 3

b. Intramolecularly crosslinked macromolecules (nanogels, microgels) =:>radius of gyration of less than 15 nm in solution molecular weight 50000 - 200000 =:> soluble

c. Macrogels obtained by usual crosslinking polymerisation Three-dimensional infinite network insoluble

lntramolecularly crosslinked macromolecules.

The problem in this synthesis is that usually under these conditions under intermolecular crosslinking three-dimensional infinite networks of macrogels are obtained (Fig. 3c). In special solvents (e.g. cyclopentanone) at low monomer concentration (e.g. 1%) it is possible to obtain highly crosslinked (nominal degree of 70%) soluble microgels with molecular imprinting. They were characterized through GPC, viscosimetry, and membrane osmometry, and were found to be highly crosslinked macromolecules with a molecular weight comparable to the one of proteins (see Table 2). Molecular recognition experiments clearly pointed out the presence of selective functionalized cavities within the microgels. Recognition experiments can be performed in homogeneous solution, after which the microgels are conveniently separated by ultracentrifugation or by precipitation. At present experiments are undertaken with transition state analogue imprinted soluble microgels which are imprinted with a complex of 6 and 2 and which are catalytically active in hydrolyzing the diphenylcarbonate (7) [38].

43 Table 2

Synthesis and characterization of molecularly imprinted nanogels [37]

Solvent

% yield Mw Mn Mw/M. Mn(osm) q a-value monomer % mixture Cyclopentanone 1 88 4 . 0 104 9 . 2 103 4.3 3.5 " 10~ 7.9 1.1 Cyclopentanone 2 91 5.1 " 105 2 . 2 104 23 n.d. 11.8 1.2 Cyclopentanone 3 92 Partially gelated nld. 1.4 DMF 1 76 4.2104 1.2104 3.4 4.9105 5.5 1.2 ACN/toluene 1:1 1 85 8.5 " 104 1.3 " 104 6.8 7 . 0 105 n.d. n.d. Monomer mixture consisting of 70% ethylene dimethacrylate, 25% methyl methacrylate, and 5% monomer 1. Radically initiated polymerisation ( AIBN ) at 80~ for 4 days. Mw = weight averaged- Mn = number averaged molecular weight from GPC. Mn(osm) = membrane osmometry; r I = intrinsic viscosity; a-value = equilibratio of the microgel after template removal with the racemate of phenyl a-mannopyranoside. Mimicking natural enzyme action is quite a demanding task. Molecular imprinting has brought quite some progress in this direction. Typical enzyme properties like Michaelis-Menten kinetics, competitive inhibition, induced fit etc. were observed. The catalytic activity of natural enzymes, though, is much higher by several orders of magnitude but the catalysts obtained are rather stable and can be easily prepared. Soluble nanogels in which each particle possesses one active site will be of special interest since enzyme analogy will be relatively high in this case.

5. ACKNOWLEDGEMENT This work was supported by Deutsche Forschungsgemeinschafi and Fonds der Chemischen lndustrie.

REFERENCES

[1] [2]

G. Wulff, Angew. Chem. 101 (1989) 22; Angew. Chem. Int. Ed. Engl., 28 (1989) 21. G. Wulff, in: A. D. Schliiter, Ed., Synthesis of Polymers, Wiley-VCH Verlag, Weinheim, 1998, pp. 375-401. [3] G. Wulff, B. Heide, G. Helfmeier, J. Am. Chem. Soc. 108 (1986) 1089. [4] G. Wulff, Angew. Chem. 107 (1995) 1959; Angew. Chem. Int. Ed. Engl. 34 (1995) 1812. [5] G. Wulff, A. Sarhan, K. Zabrocki, Tetrahedron Lett. (1973) 4329. [6] G. Wulff, W. Vesper, R. Grobe-Einsler, A. Sarhan, Makromol. Chem. 178 (1977) 2799. [7] E. Fischer, Ber. Dtsch. Chem. Ges. 27 (1894) 2985. [8] G. Wulff, M. Minarik, J. Liquid Chromatogr. 13 (1990) 2987. [9] G. Wulff, S. Schauhoff, J. Org. Chem. 56 (1991) 395. [10] S. Mallik, S. D. PlunkeR, P. K. Dhal, P. D. Johnson, D. Pack, D. Shnek, F. H. Amold, New J. Chem. 18 (1994) 299.

44 [ 11] P.K. Dhal, in: B. Sellergren, Ed., Molecularly Imprinted Polymers - Man Made Mimics of Antibodies and Their Application in Analytical Chemistry, Elsevier, Amsterdam, 2001, pp. 185-201. [12] B. Sellergren, M. Lepist6, K. Mosbach, J. Am. Chem. Soc. 110 (1988) 5853. [ 13] K. Mosbach, O. Ramstr6m, Biotechnology 14 (1996) 163. [ 14] R.A. Bartsch, M. Maeda, Eds., Molecular and Ionic Recognition with Imprinted Polymers, ACS Symposium Series, Vol. 703, Washington, 1998. [15] B. Sellergren, Ed., Molecularly Imprinted Polymers. Man-Made Mimics of Antibodies and Their Application in Analytical Chemistry, Elsevier, Amsterdam, 2001. [ 16] G. Wulff, in: F. Diederich, P. J. Stang, Eds., Templated Organic Synthesis, Wiley-VCH, Weinheim, 1999, pp. 3 9 - 73. [17] K. Mosbach, Trends Biochem. Sci. 19 (1994) 9. [ 18] K.J. Shea, Trends Polym. Sci. 2 (1994) 166. [19] T. Takeuchi, J. Matsui, Acta Polym. 47 (1996) 471. [20] M.J. Whitcombe, E. N. Vulfson, Adv. Mater. 13 (2001) 467. [21] R.A. Lerner, S. J. Benkovic, P. G. Schultz, Science 252 (1991) 659. [22] P. G. Schultz, Angew. Chem. 101 (1989) 1336; Angew. Chem. Int. Ed. Engl. 28 (1989) 1283. [23] D. Robinson, K. Mosbach, J. Chem. Soc., Chem. Commun. (1989) 969. [24] B. Sellergren, K. J. Shea, Tetrahedron Asymmetry 5 (1994) 1403; B. Sellergren, R. N. Karmalkar, K. J. Shea, J. Org. Chem. 65 (2000) 4009. [25] K. Ohkubo, K. Sawakuma, T. Sagawa, J. Mol. Cat. A 165 (2001) 1; and earlier papers of this group. [26] G. Wulff, Chem. Rev. in press. [27] G. Wulff, A. Biffis, in: [ 15], pp. 71 - 111. [28] G. Wulff, K. Knorr, Bioseparation, in press. [29] G. Wulff, T. Gross, R. Sch6nfeld, Angew. Chem. 109 (1997) 2049; Angew. Chem. Int. Ed. Engl. 36 (1997) 1961. [30] A.G. Strikowski, D. Kasper, M. Grfin, B. S. Green, J. Hradil, G. Wulff, J. Am. Chem. Soc. 122 (2000) 6295. [31] P. Wentworth, A. Datta, S. Smith, A. Marshall, L. J. Partridge, G. M. Blackburn, J. Am. Chem'. Soc. 119 (1997) 2315. [32] J.-M. Kim, K.-D. Ahn, G. Wulff, Macromol. Chem. Phys. 202 (2001) 1105. [33] K. Hosoya, K. Yoshizako, N. Tanaka, K. Kimata, T. Araki, J. Haginaka, Chem. Lett. (1994) 1437. [34] L. Ye, P. A. G. Cormack, K. Mosbach, Anal. Commun. 36 (1999) 35. [35] K. Landvester, N. Bechthold, F. Tiarks, M. Antonietti, Macromolecules 32 (1999) 2679. [36] A. Strikowski, B. S. Green, G. Wulff, unpublished results. [37] A. Biffis, N. B. Graham, G. Siedlaczek, S. Stalberg, G. Wulff, Macromol. Chem. Phys. 202 (2001) 163. [38] B.-O. Chong, G. Wulff, unpublished results.


45

Plugged Hexagonal Mesoporous Templated Silica: A unique micro- and mesoporous material with internal silica nanocapsules. P. Van D e r Voort a'! , P. I. Ravikovitch b, A.V. Neimark b, M. Benjelloun a, E. Van Bavel a, K.P. De Jong c, B. M. Weckhuysen c and E.F. Vansant a. a University of Antwerp (UIA), Dept. of Chemistry, Universiteitsplein 1, B-2610 Wilrijk, Belgium. i Corresponding author; email [email protected]

b Center for Modeling and Characterization of Nanoporous Materials, TRI Princeton, P.O. Box 625, Princeton, NJ 08542, USA. $cUniversity of Utrecht, Dept. of Inorganic Chemistry and Catalysis, Debye Institute, Sorbonnelaan 16, 3508 TB Utrecht, The Netherlands. Following the development of purely mesoporous templated silicas, it is a desirable next step to create innovative catalytic support materials, consisting of a stable composite matrix with combined micro- and mesoporosities and a sufficient stability to withstand most industrial treatments. We show in this paper the development of a hexagonal plugged material, with combined micro- and mesopores and a tunable amount of both open and inkbottle mesopores. The ratios of these different pore types are variable in a wide range. The obtained materials are much more stable than the conventional micellar templated structures known so far. 1.

INTRODUCTION

Following the pioneering publications on the synthesis of mesoporous, semi-crystalline silicas [1-4], intensive research efforts have been devoted to the development of new mesoporous support materials of ordered structure. The research is motivated by the fact that such materials fill the gap in catalytic chemistry between the crystalline microporous zeolites and amorphous, disordered mesoporous supports like silica gel [5]. Due to their controlled pore size and a very narrow pore size distribution, the ordered mesoporous materials have a large potential as catalytic support in fine chemistry [5], pharmaceutical industry [7], as well as for the production of special polymer materials [6]. Heterogenizing the synthetic procedures in these fields of chemistry forms an important tool in achieving the goals of green, sustainable production processes and end-of-pipe waste reduction [7]. It is desirable to create innovative catalytic support materials, consisting of a stable composite matrix with combined micro- and mesoporosities. Such materials will offer significant supplementary advantages of an improved diffusion rate for transport in catalytic processes (faster reactions); better hydrothermal stability [8]; synthesis of multifunctional catalysts, which can process a large variety of feedstocks; capabilities of encapsulated waste in the micropores; controlled leaching rates for a constant and gradual release of an active component, etc. Here, we present a very simple synthesis procedure of a plugged hexagonal mesoporous material with very thick walls, high stability and controllable and tunable micro- and mesoporosities.

46 2.

EXPERIMENTAL

A plugged MTS material is prepared by dissolving 4 g of Pluronic P123 (non-ionic triblock copolymer, EO20POToEO20)in an acidic water/HC1 solution. Subsequently, an amount of TEOS (between 5 and 25 g) is added. The solution is stirred for 4-8 hours at a fixed temperature between 40 and 80~ and subsequently aged at ambient pressure for 17 h at 80120~ The white solid was filtered, washed and calcined at 550~ Detailed experimental conditions can be found in [9]. 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~ TGA measurements were recorded on a Mettler TG50 thermobalance. Mechanical pressing tests were performed in a unilateral press with a typical 13 mm dye (Specac). Hydrothermal tests were performed by placing the sample on a grid in an autoclave, which is filled with liquid water underneath the grid. The entire system is placed in an oven for 17 h in the temperature range 120-160~ exposing the sample to steam at autogeneous pressure. Other hydtrothermal experiments were performed using a fixed bed reactor, using a nitrogen flow, saturated with a certain percentage of water vapor.

3. RESULTS AND DISCUSSION

3.1. Nitrogen isotherms Changing the synthesis parameters in a controlled way allows the reproducible synthesis of a broad variety of materials. The adsorption-desorption isotherms of three distinctly different materials are shown in Figure 1. The isotherm in figure 1A is typical for the SBA-15 material [3], a two-dimensional p6mm structure formed by open cylindrical mesopores > ca. 5 nm in diameter. The desorption isotherm corresponds to the vapor pressure of the equilibrium meniscus in the open cylincrical pore, while the adsorption isotherm corresponds to the limit of stability of the adsorption film [ 10]. It should be noted that the material contains significant amounts (up to 30% of the porosity) of intrawall micropores (< ca. 3 nm) located in the pore walls, as evaluated by the NLDFT method [11]. The isotherm in Figure 1C shows the isotherm of a material with regular cylindrical pores that are accessible only through permeable microporous plugs. This is evident from the desorption branch of the isotherm and the shape of the hysteresis loop. If a pore is plugged, desorption is delayed until the vapor pressure is reduced below the desorption pressure from a pore aperture (ink-bottle efffect). However, if the pore aperture is below a critical diameter, decrease in the vapor pressure causes the fluid in the larger pores to become thermodynamically unstable before the desorption pressure for the pore aperture is reached [ 12]. For nitrogen at 77 K this instability occurs at p/p0: 0.42-0.45. The isotherm in Figure 1B is remarkable. It exhibits the following characteristic features: 1) adsorption in intrawall micropores at low relative pressures; 2) multilayer adsorption in regular mesopores and capillary condensation in narrow intrawall mesopores; 3) a one-step capillary condensation, indicating uniform mesopores; 4) a two-step desorption branch indicating the pore blocking effects (sub-step at the relative pressure of ca. 0.45). The adsorption-desorption behavior is consistent with a structure comprising both open and closed cylindrical mesopores. This interpretation is fully supported by the non-local density functional theory (NLDFT) of adsorption and hysteresis in cylindrical pores [ 10]. The mesopore size distribution and the total amount of micropores are calculated from the

47 adsorption branch of the isotherm by the NLDFT method [11]. The fractions of open and closed mesopores, as indicated schematically in Figure 2, have been determined from the pore size distributions (see Table 1). Details of calculations will be presented elsewhere.

Figure I : Nitrogen adsorption-desorption isotherms of (A) SBA-15, all open mesopores, (B) plugged material, with combined open and closed mesopores and (C) material with exclusively closed mesopores.

Figure 2. Nitrogen adsorption-desorption isotherm (77K) of a plugged hexagonal mesoporous templated silica and the mesopore size distribution calculated by the NLDFT method [ 10].

48 3.2. X-Ray Diffractogram The X-Ray Diffraction pattem in Figure 3 shows the characteristic reflections for a 2D hexagonal pore ordering in the p6mm space group [3]. The plugged mesoporous material therefore has the same structure as the SBA-15 hexagonal material.

Figure 3 : X-Ray Diffractogram of the plugged hexagonal mesoporous silica sample.

3.3. Plugged hexagonal mesoporous templated silica The data in Figures 1-3 point towards a composite material with a combined micro- and mesoporosity, as schematically represented in Figure 4. The rather thick walls ( - 4 nm) of the large cylindrical mesopores are perforated with micropores. Moreover, the cylindrical mesopores themselves are 'plugged' with amorphous silica nanocapsules, which are also microporous. These nanocapsules are created by large excess of the silica source (TEOS) that is used in the synthesis and by rapid hydrolysis of the silicon alkoxide at the very low pH used in the synthesis. The micropores in the silica walls can be explained by the penetration of hydrophilic poly(ethyleneoxide) chains of the triblock copolymer in the silica wall, as already suggested by Kruk et al [13]. The microporosity of the plugs may have a different origin. It is known that Pluronic triblock copolymers are in fact polydisperse mixtures of several triblock copolymers with a wide range of molecular weights, and that they contain appreciable amounts of diblock copolymers and even free PO chains. Some of these components, especially the low molecular weight ones, may not be involved in the actual templating of the mesopores, but still act as templates for the disordered nanocapsules, inducing a complementary porosity. The mesopores themselves are created by the so-called charge compensating templating mechanism of the entire triblock copolymer. The most important characteristics of these materials are summarized in Table 1. The table evidences the large variety in sample characteristics that can be obtained. The thickness of the mesoporous walls is typically 3-4 nm, which is excellent, compared to a typical wall thickness of 1 nm for the well-known MCM-41 structure. Extremely high total pore volumes can be obtained. The contribution of micropores (with contributions of both micropores in the walls and micropores in the silica nanocapsules) has an unprecedented high value. Micropore volumes up to 0.3 cm3/g can be obtained (40% of the total pore volume), which is

49 considerably higher than the micropore volumes of any composite material known so far. Both the ratio micropore/mesopore volume as the ratio open/closed mesopores is tunable in a wide range.

Microporous pore wall Microporous silica plugs 9

9

./ 9

9

............. 9...................................

......

9

9

9...... 9...............

9 9 ................................ 9

9

9 9

9

9

9

9

~

.

_

9

:l

6-8 nm

~ 1 7 96

9

3-4 nm

9

Open mesopore Closed mesopore Figure 4 9Schematical representation of the plugged hexagonal mesoporous templated silica (PHMTS) Table 1" Structural characteristics of 4 selected samples, PHMTS : Plugged Hexagonal Mesoporous Templated Silica, a0- lattice spacing, Vtot- totfil pore volume (micropores and mesopores), gmi- micropore volume, Vine- mesopore volume, Dads-- pore diameter from the adsorption branch, Does- - pore diameter from the desorption branch, Dgeom-- pore diameter from geometrical considerations using Vm,, Vine and 2.2 g/cm 3 for the silica skeleton density, hw pore wall thickness, hw = a0 - Daos; Vine(open)- volume of open mesopores, Vine (closed)volume of closed mesopores Sample

a0

(nm) SBA-15

ll.31

PHMTS-1

Vtot

SBET

Vmi

Vine

Dads

Ddes

Dg.... hw (ads) Vme(open) Vme (closed)

(cm3/g) (mVg)(cm3/g)(cm3/g)(nm)

(nm) (nm)

1.25

(nm)

(cm3/g)

(cm3/g)

950

0.14

l.ll

7.3.

7.59

7.76

4.01

l.ll

0

I 1.08 1.03

1040

0.29

0.74

6.79

7 . 0 3 7.12

4.30

0.23

0.51

PHMTS-2

9.58

0.71

880

0.26

0.45

6.08

no

5.89

3.50

< 0.01

0.45

PHMTS-3

10.16 0.83

945

0.30

0.53

7.03 7.03

6.94

3.13

0.38

0.15

.........

50

3.4. Transmission electron microscopy Using a Philips CM200 microscope, we have investigated extensively the PHMTS and SBA-15 samples in bright field transmission mode. In figure 5 we show representative images for these materials. Both micrographs provide side-on views of the ordered mesopore system. While"the mesopores in SBA-15 run smoothly over several micrometers of length, the PHMTS displays smaller domain sizes for the ordered mesopores. Moreover, the wall thickness yaries more strongly for the latter material, which may be caused by the presence of silica plugs inside the mesopores. Most recently, 3D-TEM techniques have been developed to image mesopores in three dimensions [14,15]. In a future paper, we will present evidence from 3D-TEM for the different pore systems in SBA-15 and our novel material PHMTS [9].

Figure 5" TEM images of SBA-15 (left) and PHMTS (fight).

3.5.

Stability

Table 3 presents the intrinsic thermal, mechanical ~and hydrothermal stabilities of some of the most important MTS materials [16]. Table 3 reveals that all materials are poorly resistant to (mild) hydrothermal treatments. The SBA-15 is the best. resistant of the conventional mesoporous silicas ; the PHMTS material is by far the most stable. It still has a very significant surface area and pore volume after 5 days of hydrothermal treatment or after a 24 h treatment in an autoclave in pure steam (sample placed above the water on a grid). Most materials collapse after a thermal treatment at 750~ with two exceptions: MCM-41 and PHMTS. Resistance toward mechanical (unilaterial) pressure is again best for PHMTS, followed by SBA-15. The thick walls of SBA-15 are further stabilized and supported in the PHMTS structure by the silica plugs, resulting in an extremely high mechanical resistance. The reported 10 tons/13 mm 2 was the highest pressure that could be obtained in our press. Pure silica based materials are obviously stable in neutral and acid conditions, but decompose in alkaline conditions.

51

Table 3 : Intrinsic stabilities of MTS materials ; SA = surface area (mZ/g), PV = pore volume (ml/g). Thermal stabilities after treatment in furnace, ambient atmosphere for 17 h at indicated temperature. Hydrothermal stability at x% water vapour at y temperature for z hours of treatment. Mechanical pressure, structure is collapsed if the XRD peak < 25% of the original peak and / or the typical diffraction peaks are no longer present. Chemical resistance : stirring for 24 h in an aqueous solution with indicated pH. l. Pressures are expressed as tons per 13 mm 2 pellet ; 1 ton/13 mm 2 corresponds to 740 bar. Treatment

MCM- 41 [PV SA

Themaal T :550~ 1027 T :650~ 970 T :750~ 879 T :850~ 795 Hydrothermal 25%/400/50 892 25%/400/120 864 100%/100/24 106 Mechanical 25% pressure I "ftonsI Chemical pH= 1 + pH = 7 + pH= 13 .

MCM- 48 [PV SA

HMS SA [ PV

SBA- 15 SA I PV

0.90 0.76 0.68 0.53

1433 1248 108 -

1.14 0.73 !';;z:-.-,. /

'ta~-.-~ *.~

I

..,._..]-atlT...: ........l~."..:

. . . ~ .~.

integration of several unit

"

"

~

t ===============_===,==,=~ ~ . + %. .,"

Fig. 1 Modeling ofMCM-41 structure. (a) random model, (b) layer model, (c) phased layer model 3.2. X-ray diffraction pattern of the framework structure of MCM-41 The XRD simulations of the three constructed models were performed. Fig. 2 shows the XRD patterns of the constructed models and the synthesized MCM-41 as a reference. The XRD pattern of the synthesized MCM-41 presented a typical four-peak' pattern with a very strong peak at a low 20(d (100)) and three weaker peaks at a higher 20 (d (110), d (200), d (210)). Layer models exhibited a typical XRD pattern with four peaks. However, the peak broadening of the 100 reflection was observed. As it is considered that the intensity of the 100 reflection is very sensitive to the relative contrast between wall and mesopore densities, the observed broadening is probably due to imperfections in pore arrangement than to a finite particle size. On the other hand, in the case of the random model, intensities of the 110 and 210 peaks were relatively week. From the fact that the experimental XRD was

100

1

(a)

110

200

210

b)

~ 9

_=

I

2

t

I

3

I

I

I

I

I

I

4 5 6 2 0 (degree) Fig. 2 Simulated X-ray diffraction patterns of MCM-41. (a) random model, (b) layer model, (c) phased layer model, (d) experiment

73 fitted better to simulated pattern by both layer models than the random model, it found that the peak intensities are influenced by the atomic location in the wall structure. 3.3. 29Si MAS NMR spectra of MCM-41 In order to obtain information about the chemical state of Si in the MCM-41 wall structure, the 29Si MAS NMR spectra were simulated using the phased layer model. Fig. 3 shows 29Si MAS NMR spectra of the synthesized MCM-41 and the phased layer model. In the 29Si MAS NMR spectrum of the synthesized MCM-41, the broad peak a t - - - - l l 0 ppm, assigned to strongly polymerized Q4 Si species, as well as the weak shoulder peak a t - -100 ppm, assigned to Q3 Si species, was observed. In the simulated 29Si MAS NMR spectrum from the phased layer model, the broad peak centered a t - 1l0 ppm was observed. This suggests that there is no difference in the chemical state of Si between the synthesized MCM-41 and the phase layer model. 3.4.

4

-50

Coordination of Si in the framework of

MCM-41 The radial distribution function between Si and O atoms in the MCM-41 wall structure was investigated in order to clarify the difference in the

- 1O0 ppm

Fig. 3 29Si MAS NMR spectra of MCM-41. (a) experiment, (b) phased layer model

/y--

lO 40

(c)

50/

i

- 150

/

0 o

~2 30 0

r(c)

,.Q

~ 2o . ,...(

~

~10

0 1.5

l

1.6

1.7

1.8

1.9

Distance between Si and O (A)

2.0

0

I

1

2

I

I

4

I

6

Distance between Si and O (A)

Fig. 4 Radial distribution function between Si and O Fig. 5 Coordination number of Si. atoms. (a) random model, (b) phased layer model, (a) random model, (b) phased layer (c) MFI type silicalite model, (c) MFI type silicalite

74 coordination state of Si. As shown in Fig. 4, sharp peaks were observed at 1.68 and 1.71 _Ain the radial distribution function for MFI type silicalite. On the other hand, in the radial distribution functions for the random model and the phased layer model, the broad peak was observed around 1.7/~. The bond distances, l(Si-O), evaluated by both models were longer that than from MFI type silicalite. Fig. 5 shows that the coordination number of O surrounding Si atom calculated from the radial distribution function. The coordination number of Si increased with an increase in the Si-O distance. In the l(Si-O) ranging from 1.65 A to 3.0A, the coordination number of Si in MFI structure was almost constant 4, indicating the coordination state of Si in the MFI structure is tetrahedral, while the coordination number for the random model and the phased layer model was not constant. This strongly suggests that the coordination state of Si in the MCM-41 wall structure is not uniform. 3.5. Electron density image of the framework structure of MCM-41 The electron density images were calculated using the integral intensities of Bragg reflections. Fig. 6 shows the electron density maps for the synthesized MCM-41. It is noted that the electron density is not continues. The electron density at the intersections of wall was lower than that at the other parts. Fig. 7 shows that the electron density images calculated using the random model, the layer model and the phased layer model. The wall electron densities for both layer models were not continuous, whereas the image for the random model was continuous. The wall electron densities for both layer models were qualitatively similar to the simulated one using the synthesized MCM-41. The non uniformity of the wall electron density ofMCM-41 have been reported by Solovyov et al. [13-14].

(a)

(b)

(c)

Fig. 6 The electron density projections for synthesized MCM-41. (a) 2-D, (b) 3-D, (c) mosaic

4. Conclusion From all above results, it was found that the peak intensity of XRD pattem derived from the hexagonal structure of MCM-41 is dependent on the modeling method of the wall structure. It was also found that the wall structure ofMCM-41 is not uniform.

75

(a)

(A)

(B)

(C)

Fig. 7 The electron density projections for (A) random model, (B) layer model and (C) phased layer model. (a) 2-D, (b) 3-D, (c) mosaic

REFERENCES 1. C, T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, J. S. Beck, Nature, 359 (1992) 710. 2. J.S. Beck, J. C. Vartuli, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, J. B. Higgins, J. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 3. U. Cisela, F. Schtith, Microporous Mesoporous Mater., 27 (1999) 131. 4. A. Corma, Chem. Rev., 97 (1997) 2373. 5. A. Sayari, Stud. Surf. Sci. Catal., 102 (1996) 1. 6. P. Behrens, Angew. Chem. Int. Ed. Engl., 35 (1996) 515. 7. B.P. Feuston, J. B. Higgins, J. Phys. Chem., 98 (1994) 4459. 8. M.W. Maddox, J. P. Oliver and K. E. Gubbins, Langmuir, 13 (1997) 1737. 9. R.D. Oldroyd, J. M. Thomas, G. Snakar, J. Chem. Soc., Chem. Commun., (1997) 2025. 10. S. Schacht, M. Janicke, F. Schtith, Microporous Mesoporous Mater., 22 (1998) 485. 11. C. A. Koh, T. Montanari, R. I. Nooney, S. F. Tahir, R. E. Westacott, Langmuir, 15 (1999) 6043. 12. R. G. Bell, in 9M. M. J. Treacy, B. K. Marcus, M. E. Bisher, J. B. Higgins (Eds.), Proceeding of the 12th IZC, Baltimore, Materials Research Soc., (1999) 839. 13. L. A. Solovyov, S. D. Kirik, A. N. Shmakov, V. N. Romannikov, Microporous Mesoporous Mater., 44-45 (2001) 17. 14. V. B. Fenelonov, A. Yu. Derevyankin, S. D. Kirik, L. A. Solovyov, A. N. Shmakov, J.-L. Bonardet, A. Gedeon, V. N. Romannikov, Microporous Mesoporous Mater,, 44-45 (2001) 33.

76 15. J. M. Kim, L. H. Kwak, S, Jun, R. Ryoo, J. Phys. Chem., 99 (1995) 16742. 16. T. Ikeda, A. Nisawa, M. Okui, N. Yagi, H. Yoshikawa, S. Fukusima, to be submitted. 17. L.A. Castonguay, A.K. Rappe, J. Am. Chem. Sot., 114 (1992) 5832. 18. A.K. Rappe, K.S. Colwell, Inorg. Chem., 32 (1993) 3438. 19. The Zeoliotes and Aluminophosphates force field of Erik de Vos Burehart, Ph.D. Thesis, (1992) 'Studies on Zeolites: Molecular Mechanics, Framework Stability and Crystal Growth', Table I, Chapter XII. 20. K. Kwamura, in 9Moleucular Dynamics Simulations, eds. F. Yonezawa, Springer, (1990) 88. 21. L. Verlet, Phys. Rev., 98 (1967) 159. 22. P. Ewald, Ann. Phys., 64 (1921) 253. 23. R. Miura, H. Yamano, R. Yamauehi, M. Katagiri, M. Kubo, R. Vetrivel, A. Miyamoto, Catal. Today, 23 (1995) 409.

Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

77

Pore size adjustment of bimodal mesoporous silica molecular sieves Xiaozhong Wang, ab* Tao

D o u , a Dong

Wu band Bing Zhong b

Institute of Special Chemicals, Taiyuan University of Technology, Taiyuan, 030024 China e-mail" wanuxiaozhonu~.tvut, edu.cn

a

...

,....._..

f

bState Key Laboratory of Coal Conversion, Institute of Coal Chemistry, the Chinese Academy of Sciences, Taiyuan, 030001 China Bimodal mesoporous silica (BMS) molecular sieves were synthesized using quaternary ammonium surfactant at room temperature with lower pH values, and their pore sizes were tailored using a simple method by controlling the size of the structure-directing surfactant or incorporating an auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) into the reaction systems, which has been used previously to control the pore sizes of MCM-41 mesoporous materials. It was shown that as the surfactant alkyl chain length or the amount of TMB used was increased, the enlargement of the primary mesopore size of BMS materials was accompanied concurrently by the decrease of its secondary mesopoe size, and the degree of the primary mesopore size enlargement (ca. 1.0nm) is far smaller than that of MCM-41 mesoporous materials prepared under similar synthesis conditions, but in contrast the degree of its secondary mesopore size decrease is more sharp (ca.6.4nm). These results may be in connection with the characteristic framework structure of our BMS materials. When the amount of TMB used exceeded a certain degree, the primary mesopore structure of BMS materials was still present but its secondary mesopore structure got collapsed. INTRODUCTION The synthesis of inorganic frameworks with specified and organized pore networks is of potential importance in catalysis[l], separation technology[2] and biomaterials engineering [3]. Since the first synthesis of mesoporous MCM-41 materials[4.5], there has been intense activity in the design and synthesis of a variety of mesoporous solids with different structural features. Such features as pore size, pore size uniformity, interparticle porosity, and stability (thermal and hydrothermal) of these mesoporous molecular sieves were shown to be controlled by a proper choice of synthesis conditions[4-9]. At present, the surfactanttemplated synthetic procedures have been extended to include a wide range of compositions, and a variety Of conditions have been developed for exploiting the structure-directing functions of surfactant. These solids allow fasten diffusion of large organic molecules than the zeolitic and aluminium phosphate-based microporous sieves. These structural characteristics

78 make them potentially useful as catalysts for fluidized catalytic cracking and for the manufacture of fine chemicals. However, the information feedback from the practical industrial process shows that the catalyst used in the large molecules reaction requires a reasonable distribution of two-grade or multi-grade pores, and therefore over a long period of time direct synthesis of inorganic porous materials with two-grade or multi-grade pore distribution is researched for by zeolite chemists. In earlier investigations, we showed that careful controlling alkalinity affords a novel porous materials with well-defined bimodal mesopore size distribution at ambient conditions [11~.! 1]. The materials contain randomly distributed hexagnoal and stripe-like mesoporous channels with uniform pore size and exhibit very large surface areas and pore volumes. The secondary mesopore structure of BMS materials may be formed via the development of incomplete condensation of SiO2species around the adjacent surfactant micelle. The bimodal mesoporous structure of thus formed should be able to be tailored by applying the similar methods with which were used usually in the pore size adjustment of M41S materials. So far, relatively little work has been reported for the pore size mediation of bimodal mesoporous silica mesostructures and no convincing mechanism for such bimodal mesoporous structure formation has been put forward. Undoubtedly, the ability to control framework bimodal mesoporous distribution can be of great value in designing BMS materials as catalysts, ' adsorbents and sensor materials. Accordingly, in the present work we have examined the effect of the structure-directing surfactant size and the addition of auxiliary organic solvent such as 1,3,5-trimethylbenzene on the pore size characteristic framework cross-linking of BMS molecular sieves. At the same time we give a full account of the trend of pore size adjustment of BMS materials and extend on our hitherto proposed formation mechanism. 2. EXPERIMENTAL SECTION

2.1. Synthesis The synthesis procedure for BMS materials was described elsewhere[10.11]. For the purposes of probing the effect of surfactant alkyl chain length and auxiliary organic solvent such as 1,3,5-trimethylbenzene (TMB) on pore size distribution of BMS materials, these samples were prepared typical using CI4H29N(CH3)3Br(C14) and C16H33N(CH3)3Br(C16) as templates. Tetraethylorthosilicate (TEOS) was used as a source of silica, and the pH values of the reaction mixture was adjusted with aqueous ammonia. In each case the reaction mixtures had the following molar composition: 1.0 SiO2:0.2 CI4H29N(CH3)3Br : 0.09 NH3" H20 : 115H20 1.0 SiO2:0.2 CI6H33N(CH3)3Br : 0.3 NH 3"H20 : (0-0.84) TMB : 115H20 The number of molar of ammonia in each reaction mixture was varied relying on surfactants alkyl chain length. When TMB was used as an auxiliary structure director, it was added to the surfactant solution and stirred for 15min before the addition of TEOS. All of the BMS reaction products were washed repeatedly with distilled water in a centrifuger, dried in air at 353K and finally calcined in air at 2K min"~to 823K for 6 h to remove the template.

79 2.2. Characterization

The powder X-ray diffraction pattems (XRD) were recorded using a D/max-2500 powder diffractometer with Cu-K a radiation (40kV, 100mA),0.02~ size and 1 s step time over the range 1~ 0 200 ~ The characteristic structural element of MCM-41 is a hexagonal array of the pores. The powder X-ray diffraction (XRD) patterns obtained from the non-calcined MCM-41 is typical of a well ordered structure and shows four Bragg peaks at low reflection angles between 2 and 7~ (20) that can be indexed to a hexagonal lattice as (100), (110), (200) and (210). The XRD patterns indicate that there are no crystalline phases present, because no reflections at higher angles are observed. However, as suggested by Cheng et al. [ 14], the term crystalline can be used only with respect to the periodic array of channels that represent the single element of order in the material. We performed some studies varying the reaction time to verify the structural development of MCM-41 with time. In studies of reaction times, Ortlam et al. [ 15] compared the temporal development of the X-ray reflection intensities of as-synthesized and calcined MCM-41 materials. They observed that the hexagonal structure of MCM-41 may be formed after reaction times of about lh at 104~ and that 90% of the hexagonal channel arrangement is formed after 5-6 h, whereas the fully developed mesopore structure requires 72 h. The XRD patterns obtained in this work from non-calcined MCM-41 recorded after different times are shown in Figure 2. We can observe that the structural arrangement of MCM-41 is formed after 15 min and maintained with prolonged reaction times. A rapid hydrolysis process of TMOS can explain this behavior TEOS and TPOS, assisted by the controlled pH and an efficient condensation under these synthesis conditions. However, these materials had demonstrated a very poor thermal stability, since the structure collapsed during the calcination process 9 Only crystallization times higher than 2 h allow the formation of MCM-41 that presented high thermal stability. Another feature of these diffractograms is the increasing of peaks intensity with reaction time and their shift to lower 20 values, indicating an growth of the unit cell.

96

However, when the MCM-41 samples were submitted to the calcination process, the peak intensities of the XRD patterns decrease and the 20 positions shift to a higher value indicating a contraction of the lattice. This process is caused by the removal of the surfactant from the channels, and subsequent condensation of silanol groups on the walls. We can observe the disappearance of peak (210); this is probably due to a reduction of the order of the structure. The XRD patterns for the MCM-41 as-calcined are shown in Figure 3. The observed 20 angles, the relative Miller indices (hkl) and the unit cell parameter ao calculated by linear regression of equation (1) are listed in Table 1. ao2= 4d2(h 2 + k 2 + hk)/3

(1)

(100)

20h 2h lh 0.5h 0.25 h

;

'

~

'

~

'

;

'

~

'

1'0'

1'2

2O Figure 2. XRD patterns of the as-synthesized samples with different reaction times, using TPOS as silica source.

TPOS TEOS

0)

TMOS

;

'

~

'

~

'

;

'

~

'

1'0

2O Figure 3. XRD pattems of the calcined MCM-41 using TMOS, TEOS and TPOS as silica source.

97

Table 1 X-ray data of MCM-41 synthesized with TMOS, TEOS or TPOS as silica sources. Source of silica TMOS

TMOS-calc.

TEOS

TEOS-calc

TPOS

TPOS-cal

hkl

Observed 20 (o)

dhkl (nm)

100 110 200 210 100 110 200 210 100 110 200 210 100 110 200 210 100 110 200 210 100 110 200 210

2.26 3.86 4.46 5.86 2.50 4.28 4.96

3.91 2.29 1.98 1.54 3.53 2.05 1.78

2.20 3.80 4.38 5.80 2.44 4.22 4.86

4.01 2.32 2.02 1.52 3.62 2.09 1.82

2.16 3.76 4.34 5.72 2.42 4.18 4.84

4.09 2.35 2.03 1.54 3.65 2.11 1.83

Unit cell parameter (XRD) ao (nm) 4.50

4.08

4.63

4.17

4.71

4.21

It is important to observe that an increase of alkoxy group shifts of the 20 positions to smaller values, and consequently lager unit cell parameters are obtained. After the calcination process of the samples an expected contraction of 9 % (Aao = 0.4 - 0.5 nm) of the unit cell parameter was observed. This unit cell contraction is near to that observed by Cai et al. [ 16] in similar synthesis conditions. Nitrogen adsorption measurements were carried out for the calcined samples as another method to confirm the highly ordered MCM-41 structure. The adsorption and desorption isotherms of nitrogen of each sample show the typical type IV isotherm, as defined by the International Union of Pure and Applied Chemistry (IUPAC) [ 17]. However, the isotherm obtained for MCM-41 synthesized using TMOS as silica source can be considered type I, since it presents very small inflection characteristics of capillary condensation processes. The nitrogen adsorption isotherms obtained at 77 K for calcined MCM-41 are shown in Figure 4. We observe three stages of nitrogen adsorption and desorpfion in the isotherms. The adsorptions at very low relative pressure, p/po, correspond to monolayer-multilayer adsorption on the pore walls with no pressure transitions and no inflection points indicating complete absence of micropores [ 18]. The increase of the absorbed volume at low pressures is followed by a steep with an inflection point at intermediate relative pressures, which is due to capillary condensation inside the mesopores. The last stage is a plateau at high relative pressures

98 associated with multilayer adsorption on the extemal surface of the crystals [19]. This indicates that the mesoporous were completely filled. The fact that inflections of the isotherm (TPOS) are sharper indicates that the MCM-41 synthesized with TPOS presentes a narrow pore size distribution. This can be related to the fact that TPOS is hydrolyzed more slowly in comparison to TEOS and mainly to TMOS. 700 600

TPOS ,.0~9~0~4--0~- 0 --0--0--0 - - 0 - 0 ~ 0

500 E

~

TEOS

400 300-

< 200"6 > 100-

~~"

~

c)_..~o

o

TMOS /o--9--9--2~-P~' +

-I-

T

[__~ Adsorption DesorptionI

0

0,0

'012'014 0',6 ' 0',8 RelativePressure(P/Po)

'

1,0

Figure 4. Nitrogen adsorption isotherms obtained at 77 K of MCM-41 calcined using TMOS, TEOS and TPOS as silica source. An important characteristic of nitrogen adsorption is that the specific pore volume, specific surface area and average pore diameter become larger with increasing carbon chain length of the alkoxy group as shown in Table 2. The MCM-41 synthesized using TMOS as silica source, presents the smallest specific pore volume, specific surface area and average pore diameter. On the other hand, the MCM-41 synthesized using TPOS as silica source, presents the largest specific pore volume, specific surface area and average pore diameter. The Brunauer-Emmett-Teller (BET) method [20] was used to calculate the specific surface area. Considering that the MCM-41 channels are cylindric, the diameters D of the mesoporous were calculated following the equation 2 given by Gurvitsch method: D4v/S = 4 Vmes/SBET

(2)

where Vines is the mesoporous volume estimated from the N2 adsorption isotherm and Sser is the BET surface area. The most used method to calculate the pore size distribution is based on the BarrettJoiner-Halenda (BJH) model [21 ]. However, we did not apply this model to calculate average pore diameter due to an underestimation of the pore diameter. This is occurred due to the instability of the liquid nitrogen meniscus inside the mesopores. The wall thickness of the MCM-41 was calculated by the difference between unit cell parameters ao, determined by X-ray diffraction, and the pore diameter (eq. 2) obtained by equation 3. e:ao-D

(3)

99 The values of wall thickness are in agreement with the literature, except for MCM-41 synthesized with TMOS that presented e = 1.83 nm. However, this value is only approximation, since there is no type IV isotherm. We therefore must be cautious with the values of e obtained using different analysis techniques. Same of the properties of the calcined MCM-41 samples (prepared by the use of different tetraalkoxisilane) obtained by nitrogen sorption, X-ray diffraction and density analysis are shown in Table 2. Table 2 Properties of the MCM-41 samples as-calcined prepared by use of different tetraalkoxisilane obtained by nitrogen sorption and X-ray diffraction. Source of silica

Specific pore volume

Specific surfacea r e a

c m 3 g-i

m 2 g-i

0.39 0.72 0.84

685 1002 1127

TMOS TEOS TPOS

Average pore diameter(N2sorption) n m 2.25 2.86 2.99

Wall thickness (nm)

Density (g cm_3)

1.83 1.31 1.22

2.00 1.96 1.81

Scanning electron microscopy was used to determine the particle size, particle morphology and the particle size distribution of the synthesized MCM-41. The particle size of all samples range from 0.5 gm to 2.0 gm with an average size of 1.1 pan. While the particles of MCM-41 synthesized with TEOS and TPOS presents a morphology more defined, the MCM-41 synthesized with TMOS presents fused particles. This can to be attributed to the fast hydrolysis process of TMOS, which may cause the particle coalescence. The scanning electron micrographs are shown in Figure 5.

(a)

(b)

(c)

Figure 5. Scanning electron micrographs of MCM-41 as-calcined using (a) TMOS, (b) TEOS and (c) TPOS as silica source.

4. CONCLUSIONS In this work we report the synthesis of MCM-41 type mesoporous materials via an efficient and rapid method at room temperature using the three different silica sources, TMOS, TEOS and TPOS. We observe that the structural arrangement of MCM-41 is formed after 15 min, but only after crystallization times of more than 2 h MCM-41 with high thermal stability is formed. We observe that with the the increase of chain alkoxy group a higher unit cell parameter, specific surface area, specific pore volume and average pore diameter is obtained.

100 Particularly, it appears clear that the use of TPOS produces an improvement in the structure of MCM-41. Because of these characteristics, the MCM-41 is an excellent support for various 9 catalysts, where are used in transesterification, oxidation and polymerization reactions of olefins, in our research group.

5. ACKNOWLEDGEMENTS The authors thank FAPESP and CNPq for financial support for this work (grant number 99/02649-5) and Profs. Mafia do Carmo Gongalves and Heloise de Oliveira Pastore for assistance.

REFERENCE

1. T. Yanagisawa, T. Shimizu, K. Kuroda e C. Kato, Bull. Chem. Soc. Jpn., 63 (1990) 988. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.P. Schmitt, C.T.W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins e J.L. Schlenker; J. Am. Chem. Soc. 114 (1992) 10834. 3. C.T. Kresge, M.E. Leonowicz, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 4. C. Lee, W. J. Lee, Y. K. Park, S. Park, Catalysis Today, 61 (2000) 137. 5. S. Wong, H. Lin, C. Mou, Applied Catalysis A: General, 198 (2000) 103. 6. M.A. Camblor, A. Corma, P. Esteve, A. Martinez and S. Valencia, Chem. Commun., 795 (1997). 7. W.A. Carvalho, P. B. Varaldo, M. Walau and U. Schuchardt, Zeolites, 18 (1997) 408. 8. W.A. Carvalho, M. Walau and U. Schuchardt, J. Mol. Catal. A, 144 (1999) 91. 9. Y.S. Ko, T. K. Han, J. W. Park e S. I. Woo, Macromol. Rapid Commun., 17(1996) 749. 10. I. S. Paulino, A. P. de Oliveira Filho, J. L. de Souza and U. Schuchardt, Stud. Surf. Sci. Catal., 130 (2000) 929. 11. S. Biz and M. L. Occelli, Catal. R e v . - Sci. Eng., 40 (1998) 329. 12. G. Cent, S. Parathoner, F. Trifir6 A. Aboukais, C. F. Aissi and M. Guelton, J. Phys. Chem., 96 (1992) 2617. 13. M. D. Alba, Z. Luan and J. Klinowski, J. Phys. Chem., 100 (1996) 2178. 14. C-F. Cheng, W.Zhou, D. H. Park, J. Klinowski, M. Hargreaves and L. F. Gladden, J. Chem. Soc., Faraday Trans. 93 (1997) 359. 15. A. Ortlam, J. Rathousky, G. Schulz-Ekloff and A. Zukal, Microporous Mater., 6 (1996) 171. 16. Q Cai, W-Y Lin, F-S Xiao, W-Q Pang, X-H Chen and B-S Zou, Microporous Mesoporous Mater. 32 (1999) 1. 17. K. S. W. Sing, D. H. Everett, R. A. Haul, L. Moscou, R. A. Pierotti, J. Rouquerol and T. Siemieniewska, Pure Appl. Chem., 57 (1985) 603. 18. S. Storck, H. Bretinger and W. F. Maier, Appl. Cat. A: General, 174 (1998) 137. 19. P. J. Branton, P. G. Hall and K. S. W. Sing, J. Chem. Soc., Chem. Commun., 1257 (1993). 20. S. Brunauer, P. H. Emmett and E. Teller, J. Am. Chem. Soc., 59 (1937) 1553. 21. E. P. Barrett, L. G. Joyner and P. P. Halenda, J. Am. Chem. Soc., 61 (1951) 373.

b t U d l e s i n b u r t a c e :::iclence a n a t s a t a l y s l s 1/41

A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

Synthesis and characterization of hexagonal hydrothermal restructuring method

101

mesoporous

materials

using

Kyoung-Ku Kang and Hyun-Ku Rhee School of Chemical Engineering and Institute of Chemical Processes Seoul National University, Kwanak-ku, Seoul 151-742, Korea

Pure siliceous MCM-41 samples were prepared by the usual hydrothermal synthesis method and also by the hydrothermal restructuring method. The hydrothermal restructuring procedure was carried out by the pH control and the rehydrothermal treatment. The restructuring method gave almost 90% of yield of calcined Si-MCM-41 on the basis of the weight of silica in the reaction mixture. All the samples were characterized by using X-ray diffraction (XRD), TEM, and N2 physisorption. The XRD patterns of all the samples exhibited the well-defined reflections and the (100) reflection of the restructured samples showed no shift after calcination because the wall of Si-MCM-41 was densely packed by the restructuring procedure. During the restructuring procedure, the values of d-spacing and unit cell parameter of Si-MCM-41 were increased. TEM analysis revealed that the restructured sample has a highly ordered hexagonal array. According to the N2 physisorption results, the restructured samples possessed a small pore size compared with that of the sample without being treated. Both the pH control and the rehydrothermal treatment have exercised influences on the structure of Si-MCM-41.

1. INTRODUCTION Mesoporous materials (M41S) have been synthesized by researchers at Mobil in recent years [1]. These materials consist of three different types of structure; hexagonal arrayed structure of MCM-41, 3D arrayed structure of MCM-48 and lamellar arrayed structure of MCM-50. These mesoporous materials exhibit unique characteristic properties. First, they possess a uniform pore size in the nano range (3-10 nm). Secondly, it is easy to control the pore size by using alkyl chain structure directing agents of different lengths [1 ] or micelle swelling agent (like trimethylbenzene) [2]. Mesoporous molecular sieves have also attracted much attention because of their unique properties [1]. Since the discovery of mesoporous materials, these materials have been applied as catalyst supports, adsorbents, column materials for separation, and hosts for large molecules [3]. The research interest in this field has been focused on their synthesis mechanism, development of synthesis procedure such as morphology control, enhancement of stability, characterization, synthesis of new materials based on MCM-41 synthesis concept, and technical applications [1,3-5]. Among them, the development of synthesis method is considered to be one of the most important subjects.

102 The aim of this work is to improve the textural properties of Si-MCM-41. To achieve this goal, we have developed a hydrothermal synthesis using pH control and rehydrothermal treatment and investigated the effect of hydrothermal restructuring method by applying various analysis techniques. As a result, it is found that both the rehydrothermal treatment and forced pH control result in an increase in the wall thickness. Therefore, the hydrothermal restructuring method is proven effective to improve the structural properties of mesoporous materials.

2. EXPERIMENTAL

2.1 Synthesis The Si-MCM-41 sample was prepared by using the usual hydrothermal synthesis method at 383 K. A sodium silica solution was prepared by combining aqueous NaOH solution with Ludox HS-40 (SiO2 40 wt% colloidal silica in water, Dupont). The resulting mixture was heated under stirring until clear. The template solution was then prepared with distilled water and cetyltrimethylammonium bromide (CTMABr) at 303K in an isothermal water bath. The sodium silicate was slowly added to the template solution under vigorous stirring. The composition of the resultant gel was SiO2 : 0.25 CTMABr : 0.7 Na20 : 60 H20. The gel obtained was stirred at room temperature for 1 h. The CTMA-silicate mixture was heated in an autoclave reactor without stirring to 383 K for 24 h. The precipitated product was hotfiltered, washed with distilled water and dried in an oven at 373 K overnight. The product was calcined in air at 823 K for 5 h by using a muffle furnace.

2.2. Hydrothermai restructuring method The restructured Si-MCM-41 samples were prepared with SiO2 : 0.25 CTMABr : 0.7 Na20 : 60 H20 by following the same mixing procedure as before and the hydrothermal reaction proceeded for 1 day. The value of pH was monitored by a digital pH meter. The reaction mixture was then cooled to room temperature, and the pH of the mixture was adjusted to -8 using strong acidic solution under vigorous stirring for more than 1 h. The resultant gel became homogeneous white solution. After stirring for lh, the pH of the mixture was adjusted to -10 using thick NaOH solution under vigorous stirring for 1 h and distilled water of 1'0 % by volume was added under vigorous stirring. This mixture was heated again at 383 K for 24 h. The pH adjustment and subsequent heating were repeated two more times. Finally, the solid product was hot-filtered, washed and dried in an oven at 373 K for overnight. The product was calcined in air at 823 K for 5 h by using a muffle furnace. 2.3. Characterization The yield of Si-MCM-41 is defined by the ratio of the weight of pure silica (SiO2) phase to the total weight of SiO2 in the reaction mixture. The weight of Si-MCM-41 was measured after calcination process. The phase identification of the solids were performed by using Xray diffractometer (Rigaku, D/MAX-II A) equipped with an Ni-filtered monochromatic Cu Ka (;~=1.54056 A) radiation from a tube at 30 kV and 40 mA. The morphology of the samples was examined by TEM (Jeol model JEM-2000EXII). The specific surface area and the average pore diameter were determined by nitrogen physisorption with the BET method at the liquid nitrogen temperature using a Micrometrics ASAP 2010 automatic analyzer.

103

Usual Synthesis Procedure ..

,"*

:

I

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

] #

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

] "**,

Cooling to room temperature, pH control with acid to about 8

1 "

IL :

~

9

Stirred at room temperature for 1 h

ID4'I, i l l l l l l i

:

~i pH control with NaOH solution and addition of water

,millER

~]

:"

:...: :.......:....:.::......:....w :.:E.: :: :.: :...

- ..................... Kept in a convection oven maintained at 383 K lEniN

: 9

IL

: ...: :.: :..:..: :.:.....:: :

]

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

:

:.

pH control

ii l l l l l l l l l l l l l l , I l l i l l

i

~ rehydrothermal " treatment

Ilillll,llllIIIlllllllllllllllllllllllllll*

Filtration, washing and drying at 373 K for 24 h i

.....Calcination at823 K in air for 5 h ..... Fig. 1. Procedure for the hydrothermal restructuring method

3. RESULTS AND DISCUSSION The yield of Si-MCM-41 was calculated by the weight of calcined sample divided by the amount of SiO2 in the reaction mixture. As the restructuring procedure was repeatedly carried out, the yield of Si-MCM-41 was increased. In case of the usual hydrothermal synthesis, the product yield was about 65 %. The unreacted silica could be actually observed in the filtration solution. In case of the restructured samples, however, the yield of product reached the level of above 90 %. Such a high yield is attributed to the forced pH control which brings about a shift in the reaction equilibrium. It is well recognized that Si-MCM-41 is synthesized in basic medium. In this study, the usual synthesis procedure was carried out under basic condition with a pH value of about 11 [1,4]. The pH of the mother liquor, however, was adjusted to -8 by adding strong acidic solution. When the pH is-8, the solid product disappeared from the reaction mixture and the reaction gel became sticky. This phenomenon caused by the change in pH was similarly observed in the sol-gel process. This viscous solution was not maintained long and turned soft. This indicates that the forced pH control brought about a shift in the reaction equilibrium and promoted the progress of Si-MCM-41 synthesis reaction. XRD patterns of the Si-MCM-41 samples synthesized in the present work are presented in Fig. 2. The patterns for all Si-MCM-41 samples consist of three distinguishable peaks, which can be indexed to different (hkl) reflections of a hexagonal structure [1]. The XRD patterns in parts (c) and (d) of Fig. 2 consist of one very intense line, three weak lines, and one very

104 weak line, which can be indexed to (100), (110), (200), (210) and (300) diffraction lines, respectively, and these represent the characteristics of the hexagonal structure of MCM-41. Figure 2 (a) shows the XRD lines for the Si-MCM-41 samples which were obtained after heating the initial reactant gel mixture in an autoclave at 383 K for 24 h. Here one can clearly observe a strong XRD line broadening and lattice contraction after calcination. These changes are similar with those reported previously by other research groups [6]. In general, the XRD patterns of mesoporous materials shift to the region of higher angle and concurrently the value of d-spacing of mesoporous materials decreases after calcination. This phenomenon is caused by desorption of water, condensation of silica, and loss of structural uniformity. However, the lattice contraction and peak broadening in the samples synthesized by applying the restructuring method were negligible or did not occur at all. The presence of a (300) diffraction line for restructured Si-MCM-41 samples indicates that the high structural uniformity was maintained after calcination. Therefore, it is evident that the restructured SiMCM-41 sample has a high structural stability and the silica species would be completely condensed during the course of restructuring.

(a)

(b)

A~

~ 2

/

._~

calcination

4

6

8

10

2

4

2

4

2O

(c)

6

8

6

8

2O

10

(d)

% cr

#. 2

4

6

20

8

10

20

Fig. 2. XRD patterns of Si-MCM-41 samples: (a) Si-MCM-41 before applying the restructuring procedure; (b) Si-MCM-41 after applying the restructuring procedure once; (c) Si-MCM-41 after applying the restructuring procedure twice; (d) Si-MCM-41 after applying the restructuring procedure three times 9 The as-synthesized samples were washed with distilled water and dried in oven at 373 K, and the calcination was performed in air at 823 K.

105 Figure 3 shows the pore size distributions for the mesoporous samples obtained by the nitrogen adsorption isotherm at liquid nitrogen temperature using the Barrett-Joyner-Halenda (BJH) analysis [1,2,4]. Type IV isotherm, typical of mesoporous materials, is observed and as the relative pressure increases (P/P0>0.25), each isotherm exhibits a sharp inflection, characteristic of capillary condensation within the mesoporous [ 1]. This feature indicates that both samples possess a good structural ordering and a narrow pore size distribution, and also that there has been neither structural nor phase change during restructuring. Furthermore, it is noticed that the restructuring treatment results in a shift in the pore size distribution to the region of lower pore size.

(a)

./'

600

~0

~ 500 400

o 200

ni)i null ,

0.0

"U--n__l__l--l--l--I--l--l--l--I i

0.2

,

i

0.4

Relative

(b)

,

Pressure

i

,

0.6

i

0.8

i

i

i

,

1.0

i ,I

100

(P/Po)

Pore diameter (,h)

600

~

~o 5o0

.~

~

p ~ - i

9

9

I

ii

400

300

i

200

lO0

~n-i-n- 9149

o , 0.o

i

0.2

,

l

i

0.4 Relative

I

i

0.6 Pressure

(P/Po)

i

0.8

,

i

i

1.0

10

,

,

1

,

,

i

| il

,

100 Pore

diameter

i

i

,

,

, ,

(A)

Fig. 3. Nitrogen sorption isotherms and pore size distributions of calcined Si-MCM-41 samples" (a) Si-MCM-41 before applying the restructuring procedure, (b) Si-MCM-41 after applying the restructuring procedure three times. The results of TEM imaging of Si-MCM-41 are shown in Fig. 4. Part (a) shows the image taken in the direction perpendicular to the pores and part (b) the image of viewing down the pore axis. The images along these directions have often been used to identify the hexagonal MCM-41 type phases. These images of Si-MCM-41 revealed a highly mesoporous structure consisting of cylindrical pores arranged on a hexagonal lattice.

106 (a)

(b)

Fig. 4. TEM images of the calcined sample after applying the restructuring procedure three times. The view directions are along (a) the (100) direction and (b) the pore axis. Table 1 presents the structural data of Si-MCM-41 samples. The surface areas of both samples are larger than 800 m2/g, being typical of MCM-41 materials. As given in Table 1, the interplanar spacing dl00 for the calcined parent sample, which did not undergo any restructuring procedure, is 33.82 A and this value ig typical for MCM-41 silicates synthesized using cetyltrimethylammonium bromide as a templating surfactant [ 1, 2]. For the restructured sample which is synthesized with the same reactant composition, the increase in d-spacing was larger than that for the sample synthesized by the usual hydrothermal method. In the present work, the value of 2 0 was reduced by 0.47 while the d-spacing was increased by 7.43 A. This result is related to the shift of XRD patterns to the left-hand side. Of particular relevance to the present study is the pore wall thickness of the Si-MCM-41 materials; the average pore diameter and the pore wall thickness calculated by using the BJH method are given in Table 1. In case of the restructured sample, desorption and adsorption average pore size was decreased whereas the wall thickness was increased. Concerning the textural properties, the pore diameter increases in general if the XRD line shifts to the left. When surfactants of different alkyl chain lengths and different micelle swelling agents are used for the synthesis of Si-MCM-41, the samples synthesized would have different pore diameters [1,2]. As the pore size increases, the (100) reflection shifts to the left (i.e., the value of 2 0 decreases) and the d-spacing increases. The textural data from XRD register an increase after the hydrothermal restructuring treatment. The nitrogen sorption data, however, shows that the pore size was decreased after the restructuring treatment. This was certainly caused by the forced pH control and the subsequent hydrothermal treatment. During the step of pH control, the unreacted silica species in mother liquid are dissolved and during the reheating process, these dissolved silica species take part in the MCM-41 synthesis reaction again.

107 Table 1. Structural properties of Si-MCM-41. Si-MCM-41: usual systhesis method

Si-MCM-41: restructuring procedure three times

Surface area (m2/g)

955

878

20

2.61

2.14

d- value (dl00;A)

33.82

41.25

Unit cell parameter (a0A)

39.05

47.63

BJHdes average pore diameter (A)

32.2

29.8

BJHdes wall thickness(A)

6.85

17.83

BJHads average pore diameter (A) BJHads wall thickness (,~)

33.9

33.6

5.15

14.03

ao = the lattice parameter, from the XRD data using the formula ao = 2dlo o~f3, wall thickness = ao - pore diameter

4. CONCLUSIONS The Si-MCM-41 prepared by the usual hydrothermal synthesis method has been treated by the restructuring method, which consists of the forced pH control step and the subsequent rehydrothermal treatment step. It is found that the restructuring treatment developed in this study can substantially improve not only the yield but also the quality of Si-MCM-41. Indeed, the XRD patterns and the nitrogen sorption data of the treated samples present textural properties different from those of the parent sample. The lattice contraction and peak broadening was negligible or disappeared after calcination. The values of both d-spacing and unit cell parameter are increased and the pore size is decreased. These results were indirect evidence for the effectiveness of hydrothermal restructuring method. The hydrothermal restructuring method gives rise to an improvement in textural properties of mesoporous materials, which is achieved by the condensation of silica species within the pore wall, leading to an increase in the pore wall thickness. In the rehydrothermal procedure, it is evident that the time for the hydrothermal crystallization is extended and the pore. wall is strengthened into a thicker condensed silica frame as a result of the increase in the amount of dissolved silica species during the pH control step. In brief, the forced pH control brings about a shift in the equilibrium of Si-MCM-41 synthesis reaction through the additional dissolution of unreacted silica species. On the other hand, the reheating process promotes the condensation of silica. Therefore, it is obvious that the two steps in the restructuring procedure have a synergistic effect for the increase in the wall thickness.

108 ACKNOWLEDGMENT

This work was supported by Grant No. 2000-1-30700-002-3 from the Basic Research Program of the Korea Science & Engineering Foundation and also partially by the Brain Korea 21 Program of the Ministry of Education.

REFERENCES 1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359, (i997) 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. Olsen, E. W. Sheppard, S. B. McCullen and J. L. Schlenker, J. Am. Chem. Soc., 114, (1992) 10834., C. T. Kresge, M. E. Leonowicz, W. J. Roth and J. C. Vartuli (Mobil Oil Corp.), U.S. Patent 5098684, (1992). 2. J. S. Beck, U.S. Patent 5057296, (1991)., N. Ulagappan and C. N. R. Rao, J. Chem. Soc. Chem. Commun. (1996) 2759. 3. N. Ulagappan and C. N. R. Rao, J. Ame. Chem. Soc., 116, (1996) 10785., R. Burch, N, Cruise, D. Gleeson and S. C. Tsang, J. Chem. Soc. Chem. Commun., (1996) 951., T. M. Abdel-Fattah and T. J. Pinnavaia, J. Chem. Soc. Chem. Commun., (1996) 665., J. Chui, Y. Yue, Y. Sun, W. Dong and Z. Gao, Stud. Surf. Sci. Catal., Vol. 105, (1997) 69., U. Junges, W. Jacobs, I. Voigt-Martin, B. Krutzsch and E Schuth, J. Chem. Soc. Chem. Commun., (1995) 2283., K. R. Kloestra and H. Van Bekkum Stud. Surf. Sci. Catal., Vol. 105, (1997) 431. 4. J. S. Beck, J.C. Vartuli, G. J. Kennedy, C.T. Kresge, W. J. Roth and S. E. Schramm, Chem. Mater., 6, (1994) 1816., A. Monnier, E Schuth, Q. Huo, D. Kumar, D. Kumar, D. Margolese, R. S. Maxwell, G. D. Stucky, M. Krishnamurty, E Petroff, A. Firouzi, M. Janicke and B. E Chmelka, Science, 261, (1993) 1299., G. D. Stucky, A. Monnier, E Schuth, Q. Huo, D. Margolese, D. Kumar, M. Krishamurty, E Petroff, A. Firouzi, M. Janicke and B. F. Chmeka, Mol. Cryst. Liq. Cryst., 240, (1994) 187., A. Firouzi, F. Atef, A. G. Oertli, G. D. Stucky and B. F. Chmelka, J. Am. Chem. Soc., 119, (1997) 3596. 5. N. Coustel, F. D. Renzo and F. Fajula, J. Chem. Soc. Chem. Commun. (1994) 967., D. Khushalani, A. Kuperman, G. A. Ozin, K.Tanaka, J. Garces, M. M. Olken and N. Coombs, AdV. Mater. 7, (1995) 842., A. Sayari, P. Liu, M. Kruk and M. Jaroniec, M. Chem. Mater. 9, (1997) 2499. 6. C. Y. Chen. S. L. Burkett, H.-X. Li and M. E. Davis, Microporous Mater., 2, (1993) 27.

Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

109

Synthesis o f highly ordered mesoporous compounds with control o f m o r p h o l o g y using a non-ioni~z surfactant as template A. L6onard #, J.L. Blin and B.-L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur, 61, rue de Bruxelles, B-5000 Namur, Belgium phone : +32-81-72-45-31, Fax: +32-81-72-54-14, e-mail : [email protected] Highly ordered hexagonal mesostructures (CMI-1 compounds) can be obtained under mild acidic conditions by working at low concentrations of non-ionic decaoxyethylene cetyl ether [C16(EO)i0]. The present work shows that it is possible to gain control at the nanometer scale over the packing symmetry of the channels, as well as at the micrometer level over the morphology of the particles by varying the surfactant / silica molar ratio and the hydrothermal treatment conditions. Very high loadings of silica precursors typically afford highly ordered hexagonal CMI-1 compounds whereas an increase of the surfactant / silica molar ratio results in materials with a more disordered channel array. In a parallel way, very low molar ratios of surfactant / silica lead to ropes, gyroids and toroids whereas spheres are the most stable shape with the lower quantities of silica. From this point, it appears thus that not only the structure but also the morphologies encountered for MCM-41 type mesoporous silica can be reproduced with a non-ionic templating agent. 1. INTRODUCTION A more environmental-friendly way to prepare large-pore mesoporous materials consists in using polyoxyethylene alkyl ether surfactants as templates because of their lower toxicity and good biodegradability [ 1-6] with respect to their ionic analogues like for example cetyltrimethylammonium bromide, the template generally used in the preparation of MCM-41 [7,8]. Besides, it appears that the recovery of the template is easier and so a further reutilization could be envisaged. Until short ago, the use of these surfactants afforded only disordered wormhole-like structures unless working in very strong acidic media [2] or adding transition metallic cations to the micellar solution [9]. Another way to proceed was to remove the methanol released from the hydrolysis of TMOS by using a rotary evaporator like proposed by Attard et al.[10]. We however recently showed that it was possible to obtain directly highly ordered hexagonal structures of channels (CMI-1 compounds) under mild acidic conditions by working at low concentrations of decaoxyethylene cetyl ether [Cl6(EO)10] [11]. These materials possess very uniformly-sized openings, specific surface areas exceeding 900 mVg and consist of spheres with 1-2 ~tm diameter. A LCT-type cooperative mechanism was proposed to explain the formation of these molecular sieves. It is important to control the structure of the materials. Indeed, if they are to be applied in catalysis, the 3-dimensional structure of MSU is most appropriate whereas the # :FRIA fellow *: Corresponding author

110 production of low-branched polyethylene fibres [12] or the fabrication of semi-conducting wires [13] would require a regular array of long straight channels. Besides this, the morphology turns out to be crucial also. Indeed, spherical particles are the most suitable in chromatographic applications [14]. This work shows that it is possible to gain control over these two aforementioned factors by adjusting the Cl6(EO)10 / silica molar ratio and the hydrothermal treatment conditions. Different characterization techniques (SEM, TEM, XRD and nitrogen adsorption-desorption analysis) have been used to shed light on the morphological, structural, and textural features of the prepared CMI-1 compounds. 2. EXPERIMENTAL 2.1. Synthesis

A 10 wt.% micellar solution was prepared by dissolving 6.67 g of decaoxyethylene cetyl ether [Cl6(EO)10, Brij 56 | in 60 ml water. Sulfuric acid was then added to decrease pH to a value of 2. After homogenization at 70~ TMOS was added dropwise, the quantity depending on the desired surfactant / silica molar ratio (from 0.25 to 3.50). After further stirring during 1 hour, the synthesis gel was poured in teflon-lined cartridges sealed in stainless steel autoclaves. Hydrothermal treatment was performed at 40, 60 and 80~ during 3, 2 and 1 days respectively. The recovered gel was then extracted using a Soxhlet apparatus, dried under vacuum at ca. 60~ and calcined at 550~ under nitrogen and oxygen. 60 ml bidistilled (

water

~

6.67 g C16(EO)lo, [ (Bri~6| y

H2804

Micellar solution (pH 2, 70~ Gel Hydrothermal treatment at 40, 60 and 80~ during 3, 2 and 1 day(s) I

surfactant / TMOS molar ratio = R = 0.25-3.50

Ethanol extraction Drying[ Synthesis scheme of Calcination at 550~

I=:~1 Powders [

orderedcMi.1 materialsmeS~176176

2.2. Characterization

Information about structure was obtained by X-ray diffraction measurements with a Siemens D-5000 diffractometer and transmission electron microscopy was performed on a Philips Techna'f microscope with an acceleration voltage of 100kV. The powdery samples were embedded in an epoxy resin and sectioned with an ultramicrotome before being

111 deposited on carbon, coated copper grids. The textural properties of our compounds were assessed by nitrogen adsorption-desorption measurements. Analysis took place over a wide range of relative pressures on a Micromeritics ASAP 2010 or Tristar 3000. The pore diameter and the pore size distribution were determined by the BJH method [15] although it is well known that this method gives underestimated pore size values and that some new interesting methods have been developed recently by Jaroniec et al. [ 16]. However, this will not affect our systematic comparison as the same method was used for all of the experimental results. Morphological features have been investigated with the use of a Philips XL-20 scanning electron microscope. For conductivity purposes and in order to enhance the yield of secondary electrons, powders were first covered by a thin layer of gold by metallization. 3. RESULTS AND DISCUSSION 3.1. Information about structure 3.1.1. Determination of the arrangement by XRD

Only the lowest surfactant / silica molar ratios (R) have been investigated by XRD measurements because of the very small quantity of materials obtained as the molar ratio increases. As the walls of the mesoporous compounds are amorphous, the quality of the materials will be reflected by the regular repetition of the pore to pore distance which is characterized by a very strong feature at low angles. If the packing symmetry of the channels is regular in space, secondary reflections will appear on the diffractograms. For example, in the case of MCM-41, besides the sharp 100 peak, additional 110, 200, 210,...features will be visible on the diffractogram. In that case, it is possible to determine the cell parameter a0 = 2d~00/3 ~r2,which represents the sum of the pore diameter and the wall thickness. 1 day at 80~

2 days at 60~

3 days at 40~

'II- 5.3 nm

4 5.7 nm

5.7 nm

V••'l •-

I

R = 1.50

//1~0 7 9 nm

t~ 3.1nm

,,

~-" 5.3 nm

.==

~-5.4 nm

~1-- 5.5 nm

.t=t

R = 0.50

3.2nm

3.8nm ',~ l~2.7nm ~ _

_

2

_

/ 3.1nm

. . . . ,

4

.

.

.

6

.

20 (o)

.

-

8

10 2

.

.

.

.

20 (o)

.

.

8

lo

2

.

.

4

.

:

6

20 (o)

.

:

8

.

10

Figure 1" XRD patterns of samples prepared at different hydrothermal treatment conditions and with 2 surfactant / silica molar ratios (R) The multi-peak pattern characteristic of hexagonal materials can be clearly evidenced for the sample prepared at 80~ with a surfactant / silica molar ratio of 0.50. Using the Bragg law to calculate the d-spacings, the unit cell parameter can be determined to be equal to 6.2 nm. Increasing the surfactant / silica molar ratio does not influence the aspect of the diffractograms. However, if hydrothermal treatment is performed at 60~ the secondary

112 reflections are less well resolved and their intensity drops with the amount of added silica. If R exceeds 1.00, no secondary peaks can be evidenced any more, suggesting the appearance of a disordered network like MSU materials. At 40~ the 110 and 200 reflections drop in intensity with increasing R suggesting the appearance of a less ordered channel array. The hydrothermal treatment conditions as well as the surfactant / silica molar ratio (R) do not seem to have an effect on the unit cell parameter which remains between 6.1 and 6.8 nm. However, lower amounts of added silica have a strong influence on the structure of the samples. As R augments, the regular structure is progressively lost and materials are more likely to belong to the family of MSU rather than CMI-1. 3.1.2. TEM observations

Figure 2 shows the TEM pictures of the compounds that were obtained under different conditions of hydrothermal treatment and at molar ratios ( R ) o f 0.50 and 1.50. The molar ratios that were studied ranged from 0.25 to 3.50. From a general point of view, it appears that the compounds are well ordered if the R value remains below 1.50. The hexagonal "honeycomb-like" arrangement of the channels characteristic of CMI-1 is clearly visible on the TEM micro graphs . The inserted Fourier Transforms show hexagonally dispersed spots, confirming the hexagonal packing symmetry that was suggested from XRD measurements. However, if the surfactant / silica molar ratio increases beyond this value, the regular empilement is progressively lost, more rapidly at 60 than at 80 or 40~ Even if arranged channels can still be found on the grid, these zones become much more sparse when the amount of added silica decreases. This phenomenon is the most amplified at 60~ It appears thus that there is coexistence between regular CMi~-1 material and disordered MSU-type for the higher molar ratios. I day at 80~

2 days at 60~

3 days at 40~

R = 1.50

R = 0.50

Figure 2" TEM pictures and inserted Fast Fourier Transforms of samples prepared at different hydrothermal treatment conditions and with 2 surfactant / silica molar ratios (R) In all cases, the compounds evolve towards wormhole-like disordered structures as the added silica becomes less. Interestingly, the materials are not as well ordered at 60 than at 80 or 40~ whatever the value of R is, according to the results obtained by XRD and TEM. From all of these observations, it can be postulated that the amount of added inorganics plays a key role in the organization of our materials. Indeed, for low surfactant / silica molar ratios, i.e. for high contents of tetramethoxysilane, all the isolated micelles are covered by a shell of inorganic material and these supramolecular entities can self-assemble in

113 order to form the regular structure through a cooperative mechanism in agreement with our previous results [11 ]. The higher the ratio or the lower the silica content, the less regular the organization. One could imagine that there are not enough silica oligomers present in solution to perform a complete condensation of all these silica rods. In this case, the regular hexagonal packing symmetry, though still present at some places, does not prevail over the whole extend of the material. Places of disordered wormhole-like materials are then formed when the channels move one from each other in order to form a continuous silica framework. The first regular organization of CMI-1 materials was reported for samples prepared at a surfactant / silica molar ratio (R) of 1.50. Above results show that a very regular array of channels can also be obtained at a R value as low as 0.50 and so, the higher concentration of methanol released in this case (1.30 mol/1 compared to 0.44 mol/1 for the original preparation of CMI-1) seems not to disturb the formation of the regular structure. (Since it was previously reported that the threefold larger amount of released methanol could play a role of liquid crystal breaker [ 1,10]) Present observations strongly confirm our previous proposition that it is the interaction between the hydrolyzed silicic species and the hydrophilic heads of the surfactant that will determine the final structural geometry of the pores.

3.2. Nitrogen adsorption-desorption analysis Figure 3 depicts the isotherms and the pore size distributions (inserts) of materials obtained for R values of 2.50 and 0.50 for different hydrothermal treatment conditions. At 80~ all the isotherms are type IV, characteristic of mesoporous compounds. The capillary condensation step locates at around p/p0 = 0.54, whatever the molar ratio R is, suggesting constant pore diameters. This step however seems to be more steep for the lower values of R suggesting a better homogeneity in the pore sizes. This is verified on the pore size distributions which are very narrow and centered at 4.2 + 0.4 nm. For all of the amounts of added silica, the specific surface areas of mesoporous compounds are very high (Table 1). Table 1 : Textural and structural features of the samples as a function of surfactant / silica molar ratio (R) and hydrothermal conditions. Hydrothermal treatment

SBET(m2/g) 0.50 2.50 0.50 2.50

920 830 1182 2 days at 60~ 616 o15o 1096 3 days at 40~ [ 2.50 913 n.d. "no data, - "not observed, * 9from XRD data. 1 day at 80~

. ,

Pore diameter (nm) 4.1 4.3 3.4 3.8 3.8

Cell parameter ao (nm)* 6.2 n.d. 6.4 n.d. 6.1 n.d.

If treatment is performed at 60~ isotherms are type IV only for the molar ratios below 1.5 and get a shape located between type I and IV for the higher ratios, characteristic of supermicroporous compounds. If the surfactant / silica molar ratio varies from 1.50 to 3.50, the maximum of the pore size distributions passes from 3.4 to a value less than 2.0 nm (Fig.3). This shift toward supermicroporosity could be explained by a rearrangement of the micellar solution. Indeed, polymerization at 60~ is not as extended as at 80~ and so, to maximize interactions between silica species, a rearrangement could occur in the synthesis gel forming smaller micelles leading to a continuous silica network with shrinked openings. This phenomenon is accompanied by a loss of the regular ordering of the channels. As already

114 discussed in literature, the characteristics of the final framework result from an interplay between inorganic and organic species present in solution [1,11]. Evolution at 40~ is a bit more particular in the sense that secondary mesoporosity appears for the molar ratios that exceed 0.50. Indeed, isotherms are type IV for the high loadings of added silica whereas the adsorbed volume at high relative pressures strongly I day at 80~

800 o.o~ oo

R = 2.50 ~ 600 ~400

40011

2 days at 60~

3 days at 40~

0.03

1 oo!

3o0 o0

1000

0o,

200 800

600

800

200 /-

200 1/ "

200.

0.0 0.2 0.4 0.6 0.8 1.0

Relative pressure p/po

0.0 0.2 0.4 0.6

0.8 1.0

Relative pressure P/Po

0.0 0.2 0.4 0.6 0.8 1.0

Relative pressure p/Po

Figure 3 :Nitrogen adsorption isotherms and inserted pore size distributions of samples prepared at different treatment conditions for 2 surfactant / silica molar ratios (R). increases when there are less silica species in solution. The higher the R value, the more secondary mesoporosity appears. The extend of polymerization increases with temperature. For example, the masses of powdery materials obtained at 40, 60 and 80~ for R being equal to 0.25 are 1.30, 1.82 and 2.00 g respectively. For the higher surfactant / silica molar ratios at 40~ a rearrangement of the micellar solution like observed at 60~ would not be the best way to obtain maximum polymerization and instead of this, holes would remain in the structure. This hypothesis could explain the appearance of secondary mesoporosity and the constant pore diameters at 40~ Indeed, the maximum of the pore size distributions remains practically constant (3.5-3.8 nm). Nevertheless, rearrangements of the channels are likely to occur leading to more disordered wormhole-like structures. This explains the less good organization for the higher surfactant / silica molar ratios.

3.3. Morphological features The morphologies of the samples prepared at different surfactant / silica molar ratios and at variable conditions of hydrothermal treatment are shown in Figure 4. At 80~ if the loading of added silica is very high (R = 0.25), the majority of the sample is made of hexagonal-shaped ropes with a length of several microns. For a R value of 0.50, the toroids and gyroids prevail and when the surfactant / silica molar ratio is equal to 1.00, we can observe a mixture of toroids and spheres. Beyond this value, only spheres are present. There is thus a clear evolution from ropes to gyroids and finally towards spheres as the surfactant / silica molar ratio increases. These peculiar morphologies have already been encountered by Ozin and his coworkers who used an ionic templating agent and TEOS as inorganic source [17,18]. The syntheses were carried out at room temperature or 80~ and the preparations were done in a

115 quiescent state as agitation led to the same morphologies, but with more broken forms. They proposed that a silicate liquid crystal embryo with a hexagonal cross-section evolves into several morphologies with degrees of curvature that depend on the initial reaction conditions. 1 day at 80~

2 days at 60~

3 days a t 4 0 o c

R = 2.00

R = 0.50

R = 0.25

Figure 4 : SEM pictures of samples prepared at 80, 60 and 40~ and with surfactant / silica molar ratios of 0.25, 0.50 and 2.00. A lower acidity or an increase in temperature favour the preparation of spheres rather than gyroids [19,20]. They pointed out that higher acidic quiescent conditions afford rapid growth and polymerization of a silicate liquid crystal seed where polymerization induces local rigidification effects that dictate the curvature. In this case, there is a smooth and continuous deposition of silicate-micellar species on specific regions of the liquid crystal seed, which results in the formation of gyroids. When pH value is increased or when the syntheses are performed at 80~ there is a slower global silicification and the curvature results from surface tension forces. The slower polymerization at lower acidity makes thus surface tension the overriding shape-controlling factor and spheres minimize surface area and surface free energy. In our present study, pH value remained constant throughout all of the syntheses, but only the surfactant / silica molar ratio as well as the hydrothermal treatment conditions were changed. However, for the lower molar ratios, there are a lot of silica oligomers present in solution. So, we could deduce that, the more silica present in solution, the more polymerization will induce local rigidification effects, resulting in the specific local deposition of silica species affording ropes or gyroids. When the amount of inorganics present in solution is progressively decreased, i.e. for the higher molar ratios, the growth process will not be controlled by the polymerization any more. The more preferential mechanism will be the minimization of the surface tension at the surfactant / silica interface and thus the most stable resulting shape is the sphere. A similar evolution is observed at 60~ although the morphologies are not as well defined as at 80~ Spheres are the only morphology that exists at higher values of R. At 40~ the evolution of morphology is a bit more particular. For high loadings of tetramethoxysilane, toroids and ropes can be detected, just like at 60 and 80~ When the ratio is increased beyond 1.00 however, the characteristic morphologies can still be found but

116 have no smooth surfaces any more but rather a more broken appearance. At a ratio of 2.00, the surfaces of the particles become more and more broken and the shapes, though still suggesting the toroidal and gyroidal morphologies, start to be more stochastic. These observations are consistent with the results of the adsorption-desorption measurements. The fact that the particles are not as smooth any more on their surfaces is coherent with the appearance of secondary mesoporosity as the surfactant / silica molar ratio is increased. The more spongy appearance of the samples preiaared at 40~ comes from a less advanced polymerization at lower temperatures and, above all, at higher surfactant / silica molar ratios. 4. CONCLUSION The influence of the amount of added silica on the internal as well as external morphologies of mesoporous compounds has been evidenced. From a general point of view, the packing symmetry of the channels tends towards a wormhole-like one if less silica species are present in solution. A high concentration of TMOS typically leads to "exotic" morphologies already encountered for MCM-41 type materials. Also, hydrothermal treatment conditions have a drastic influence on structure, texture and morphology. ACKNOWLEDGEMENTS

This work has been performed within the framework of PAI/IUAP 4-10. Alexandre L6onard thanks FNRS (Fonds National de la Recherche Scientifique) for a FRIA scholarship. REFERENCES

1. G.S. Attard, J.C. Glyde and C.G. GSltner, Nature, 378 (1995) 366. 2. D. Zhao, Q. Huo, J. Feng, B.F. Chmelka and G.D. Stucky, Jr. Am. Chem. Soc., 120 (1998) 6024. 3. E. Prouzet and T.J. Pinnavaia, Angew.Chem.Int. Ed. Engl., 36(5) (1997) 516. 4. E. Prouzet, F. Cot, G. Nabias, A. Larbot, P. Kooyman and T.J. Pinnavaia, Chem., Mater., 11 (1999) 1498-1503. 5. S.A. Bagshaw and T.J. Pinnavaia, Angew. Chem.Int. Ed. Engl., 35(10) (1996) 1102. 6. P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. 7. 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. 8. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 9. W. Zhang, B. Glomski, T.R. Pauly and T.J. Pinnavaia, Chem.Commun.,(1999) 1803. 10. N.R.B. Coleman and G.S. Attard, Microp. and Mesoporous Mater., 44-45 (2001) 73-80. 11. J.L. Blin, A. L6onard and B.L. Su, Chem. Mater. 13(10) (2001) 3542. 12. K. Kageyama, J.I. Tamazawa and T. Aida, Science, 285 (1999), 2113. 13. C.G. Wu and T. Bein, Chem. Mater., 6 (1994) 1109. 14. C. Boissi6re, A. van der Lee, A. E1 Mansouri, A. Larbot and E. Prouzet, Chem. Commun., (1999) 2047. 15. E.P. Barret, L.G. Joyner and P.P. Halenda, J. Am. Chem. Soc., 73 (1951) 373. 16. M. Jaroniec, M. Kruk and A. Sayari, Stud. Surf Sci. Catal., 129 (2000) 587. 17. H. Yang, N. Coombs and G.A. Ozin, Nature 386 (1997) 692. 18. G.A. Ozin, H. Yang, I. Sokolov and N. Coombs, Adv. Mater. 9(8) (1997) 662. 19. G.A. Ozin, C.T. Kresge and H. Yang, Stud. Surf. Sci. Catal. 117 (1998) 119. 20. H. Yang, G. Vovk, N. Coombs, I. Sokolov and G.A. Ozin, Jr. Mater. Chem., 8(3) (1998) 743.


117

Towards a better understanding on the mechanism o f mesoporous formation via an assembly o f Cn(EO)m and T M O S J.L. Blin, A. L6onard #, G. Herrier #, G. Philippin and B.-L. Su* Laboratoire de Chimie des Mat6riaux Inorganiques, ISIS, The University of Namur (FUNDP), 61, rue de Bruxelles, B-5000 Namur, Belgium The present work deals with a systematic study of mesoporous materials synthesis. A series of polyoxyethylene alkyl ether surfactants such as Cl3(EO)n (n = 6, 12, 18), C16(EO)10, CI8(EO)I0 have been used. It is revealed that the surfactant conformation changed with the heating temperature. Indeed, at higher temperatures, a more extended molecular conformation can be obtained, which leads to materials with larger pore sizes. We have also shown that the interaction between template and silica disturbs the hexagonal array of micelles in solution leading to the formation of DWM-1 or DWM-2 compounds for concentrated micellar solution. We have also correlated the structural characteristics of the recovered mesoporous molecular sieves with the VH/VL ratio of the template.

1. INTRODUCTION Owing to their large internal surface area, open three dimensional structure, and adjustable chemical properties, microporous materials such as zeolites have widespreadly been used in chemical and petrochemical industry. However, because of their limited pore sizes, the treatment of more bulky molecules requires new solids able to provide catalysts or adsorbents with larger openings. The synthesis efforts have culminated in 1992 when Mobil researchers reported the preparation of several mesoporous silicates with unique pore structures [1, 2]. Tunable openings have been obtained by using cationic surfactants with variable hydrocarbon tail lengths as templating agents and by adding some auxiliaries like, for instance, organic swelling molecules. The synthesis of mesoporous molecular sieves consists of the condensation and polymerization of an inorganic precursor around the micelles of surfactant. Until now, a large series of cationic, anionic, gemini and neutral surfactants have been used in the synthesis of ordered mesoporous silicas or non-silicas and materials labeled MCM, HMS [3], TUD [4], SBA [5], MSU [6] and CMI [7] have been obtained. The synthesis of the last ones is achieved through a neutral N~ ~pathway, in which hydrogen bondings are responsible for the cohesiveness between the non-ionic recoverable and biodegradable surfactant (N~ and the inorganic precursor (I~ The first syntheses of mesoporous molecular sieves achieved by using such non-ionic polyoxyethylene alkyl ether [Cm(EO),] surfactants were reported by Attard et al. [8] with octaethylene glycol monododecyl ether [CI2(EOs)] and octaethylene glycol monohexadecyl ether [Cm6(EOs)]. However, the regular mesoporous obtained by this group [9] was only owing to the gentle removal of the large amount of methanol released from the hydrolysis of TMOS used as silica precursor. Indeed they found # : FRIA Fellow * : Corresponding author

118 that methanol played a role of liquid crystal breaker destroying the hexagonal H1 phase formed by the surfactant molecules in aqueous solution. Nevertheless, recently, via a new pathway [(N~ ~ which involves the formation of hydrogen bonds between a cationic metal (M "+= Li+, Co 2+, Mn2+, and Zn2+) complex form of a non-ionic polyoxyethylene surfactant (N ~ and the neutral inorganic precursor (I~ Pinnavaia et a/. [ 10] have successfully oriented the structure of the final silica compounds working at a very low concentration of around 1.8 wt.% in neutral media. Cubic SBA-11 and hexagonal SBA-!2 were obtained using CI6(EO)10 and Cls(EO)10 respectively at a weight percentage of 4-6, the syntheses being performed at room temperature in strong acidic media (pH 0.2) normally observed for mesoporous materials. In Figure 2 we have, for comparison, also included sorption isotherms for equivalent (w.r.t. synthesis gel Si/A1 ratio) materials, which have been synthesised without the A1 dissolution step. Such materials

~, 400 ~

400

o~~ 300 i

300

~

200 J

i

1~ 100

"~

o

200 100

0.0

0.5

Partial pressure

1.0

(PIPo)

o

0.0

0.5

Partial pressure

1.0

(PIPo)

0.0

0.5

Partial pressure

1.0

(PIPo)

Figure 2. Nitrogen sorption isotherms of primary amine templated mesoporous aluminosilicates (top) and super-microporous aluminosilicates (middle), prepared at gel Si/A1 ratio of (a) 40, (b) 20, (c) 10. The bottom isotherms are for the super-microporous aluminosilicates after they were subjected to post-synthesis grafting of A1.

147 typically possess 15-20% less A1. As shown in Figure 2, the lower A1 content materials have larger pores and in some cases (top isotherm Figure 1(a)) exhibit a clearly defined mesoporefilling step. Post-synthesis grafting of A1 was found to increase the microporous character of the A1-MMS samples. The effect of Al-grafting on pore size and porosity is illustrated in Figure 2; the super-micropore filling step is shifted to lower partial pressures (lower pore size) after grafting. As shown in Table 2, the pore size of A1A1-MMS samples is lower than that of A1MMS materials by 2 to 3 A. This reduction in pore size is accompanied by a decrease in surface area and pore volume, which in turn leads to an increase in the proportion of micropore surface area and volume. Indeed sample A1AI-MMS 10 is virtually microporous, with a pore size of 14.5 A and a proportion of micropore surface area and volume above 75%. It is likely that the extra A1 is grafted onto the inner pore walls of the AI-MMSX materials thus reducing the pore size. This does not however affect the basal spacing (see Table 2) since the pore shrinkage occurs within existing pores. We note that structural ordering (as indicated by powder XRD - see Figure l(b)) is largely unaffected by the grafting process. Only modest reductions in the intensity of the basal peak are observed. We however do not think that this reduction in the intensity of the basal peak is due to degradation of structural integrity. It is more likely caused by changes in the scattering domain size (SDS), i.e., the SDS reduces during grafting. Indeed using scanning electron microscopy (SEM), we have observed that the particle size of A1A1-MMSX samples is smaller than that of the AIMMSX sample from which they are derived. This is illustrated in Figure 3 for A1-MMS20 and A1A1-MMS20 samples. It is noteworthy that although the particle size reduces, the spherical particle morphology is maintained after grafting. A uniform particle size is also retained after grafting. (a) A1-MMS20

(b) A1A1-MMS20

Figure 3. Representative scanning electron microscopy (SEM) micrographs of (a) A1-MMS20 and (b) A1A1-MMS20 illustrating the effect of post-synthesis Al-grafling on the particle size. Scale bar = 10 gm. In addition to tailoring the pore size, Al-grafiing also increases the acid content and catalytic activity as shown in Table 1. The increase in acid content after A1 grafting is greatest

148 for A1A1-MMS40 (-~ 60%) and lowest for AIAI-MMS 10 (- 12%). The introduction of extra AI clearly creates new acid sites. This is possible since the AI-MMS materials possess exposed and accessible silanol groups, which can act as anchoring points for the extra AI [ 16,17]. For extra acid sites to be generated, some of the extra A1 must be incorporated into the framework. We note that 27A1 MAS NMR spectroscopy indicated that the majority of AI in the A1A1-MMSX samples is in tetrahedral coordination. The proportion of octahedrafly coordinated (extra-framework) AI in A1A1-MMSX samples is however generally higher than that for corresponding AI-MMSX samples indicating that some of the extra AI is not incorporated into the framework. It is interesting to note that sample A1-MMS 10 which has the highest cation exchange capacity (CEC = 42 mEq/100g) takes up less 'extra AI' than AIMMS40 with a CEC of 12 mEq/100g. This implies that the take up of extra AI is not entirely an ion exchange process. Rather it appears that the extra AI is taken up chiefly via a grafting process that occurs on silanol groups [16,17]. This is consistent with our previous observation that the concentration of silanols in A1-MMS type materials decreases with A1 content [11]. It is therefore expected that A1-MMS40 would have more silanol groups (capable of anchoring greater amounts of extra A1) compared to AI-MMS10. Indeed under similar grafting conditions the pure silica material incorporates at least twice as much A1 as the A1-MMS samples. The solid acid catalytic activity of AI-MMS (AI-HMS) materials is a well studied topic. Here the discussion on catalytic activity is restricted to the differences between A1-MMS and AIAI-MMS materials. As shown in Table 1, all samples present considerable catalytic activity for the conversion of cumene. The percentage of cumene cracked (predominantly to benzene and propene) depends directly on the Si/A1 ratio and the acid content. Comparison of the activity at 50 and 150 min. indicates that the rate of deactivation is comparable for all samples. It is also clear that A1AI-MMS samples are more active than their corresponding AIMMS samples. The increase in activity, on grafting, is highest for A1-MMS40 and lowest for AI-MMS 10. The increase in activity for cumene conversion after grafting therefore mirrors the percentage (and absolute) increases in A1 content and acidity. Furthermore since the cumene conversion to benzene and propene occurs on Bronsted acid site, we may infer that there is an increase in the number of Bronsted acid sites after grafting. The catalytic activity of Bronsted and Lewis acid sites can be separately evaluated using the alkylation of toluene. Bronsted acid activity was tested using the alkylation of toluene with benzyl alcohol and Lewis activity was tested using the alkylation of toluene with benzyl chloride. In both cases 1,2 and 1,4 methyldiphenylmethane are the main products and the activity is pseudo-first-order rate for both reactions. In all cases Al-grafting was found to increase the extent of alkylation. In Figure 4, the effect of gratting extra AI is illustrated using sample A1-MMS20. The extent of alkylation over AIA1-MMS20, for both reactions, was found to be consistently higher at all reaction times. The variation of rate of alkylation with time suggests that Alkylation over A1A1-MMS20 may approach equilibrium quicker than happens for A1-MMS20. We note that similar trends were observed for A1-MMS40 and A1MMS10 and their grafted derivatives. The catalytic data in Figure 4 therefore shows that grafting of extra AI has beneficial effects not only for Bronsted acid catalysis but also for Lewis acid catalysed reactions.

149

30

(a)

(b)

25 o~ tO i,..

(D > c"

O (O

20 15 10

,

,

,

i

0 1 2 3 4 5 6

0 1 2 3 4 5 6

Time (hour)

Time (hour)

,

,

Figure 4. Conversion as a function of reaction time in the alkylation of toluene using (a) benzyl alcohol and (b) benzyl chloride on A1-MMS20 (dark symbols, 9 and o) and AIA1MMS20 (open symbols, [] and o). Primary amine templating of mesostructured aluminosilicates is known to provide solid acid catalysts whose activity is generally higher than that of materials prepared via quaternary ammonium ion templating [11,12,18]. The improved activity has been variously attributed to a number of factors including improved accessibility to acid sites, a unique pore structure and textural mesoporosity, which allow easy diffusion of molecules [11,12,15,18]. Recently, it has also been shown that the wormhole-like structure of primary amine templated aluminosilicates has 3-dimensional connectivity [19]. Three-dimensional connectivity is an important factor with respect to catalytic activity. This report therefore represents the first synthesis of super-microporous aluminosilicates with 3-dimensional connectivity. These super-microporous materials combine all the advantages of primary amine templated aluminosilicates and also offer the potential for size and shape selective catalysis of large substrates which is not possible for mesoporous materials. ACKNOWLEDGEMENTS EBG would like to thank CONICIT for the Venezuelan financial support. COT and ORS scholarships are also acknowledged. R.M. is grateful to the EPSRC for an Advanced Fellowship. REFERENCES

1. A. Corma, Chem. Rev., 97 (1997) 2373. 2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710.

150 3. 4. 5. 6. 7.

J.Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. S.A. Bagshaw and A. R. Hayman, Chem. Commun., (2000) 533. X.S. Zhao, G. Q. Lu and X. Hu, Chem. Commun., (1999) 1391. T. Sun, M. S. Wong and J. Y. Ying, Chem. Commun., (2000) 2057. D.P. Serrano, J. Aguado, J. M. Escola and E. Garagorri, Chem. Commun., (2000) 2041. 8. E. Bastardo-Gonzalez, R. Mokaya and W. Jones, Chem. Commun., (2001) 1016. 9. R. Mokaya and W. Jones, Chem. Commun., (1996) 981. 10. R. Mokaya and W. Jones, Chem. Commun., (1996) 983. 11. R. Mokaya and W. Jones, J. Catal., 172 (1997) 211. 12. R. Mokaya and W. Jones, Catal. Lett., 49 (1997) 87. 13. R. Mokaya and W. Jones, J. Mater. Chem., 8 (1998) 2819. 14. H. H. P. Yui and D. R. Brown, Catal Lett., 56 (1998) 57. 15. W. Z. Zhang, T. R. Pauly and T. J. Pinnavaia, Chem. Mater., 9 (1997) 2491. 16. R. Mokaya and W. Jones, Phys. Chem. Chem. Phys., 1 (1999) 207. 17. R. Mokaya and W. Jones, J. Mater. Chem., 9 (1999) 555. 18. T. J. Pinnavaia and W. Z. Zhang, Stud. Surf. Sci. Catal., 117 (1998) 23. 19. J. Lee, S. Yoon, S. M. Oh, C. H. Shin and T. Hyeon, Adv. Mater., 12 (2000) 359.

Studies in Surface Scienceand Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 ElsevierScienceB.V. All rightsreserved.

151

A 1 - M C M - 4 1 synthesis using Al-isopropoxide as A1 source R. Birjega, R. Ganea, C. Nenu, Gr. Pop and A. Jitianu ZECASIN, Spl.Independentei 202, PO-Box 12 304, 77206 Bucharest, Romania. Fax: 40 1 3125241, e-mail: zecasin@ com.pcnet.ro *Institute of Physical Chemistry "I.G.Murgulescu", Bucharest, Romania. 1. INTRODUCTION Since a new family of mesoporous materials designated as M41S was first introduced to the scientific community by the scientists from Mobil Corporation [1,2] a great deal of work was devoted to the possibility of isomorphous substitution of heteroatoms, especially aluminum, into the silica network, in order to modify the composition of the material. The aluminum substitution in silica MCM-41 is of particular interest, both for an academic or technological approach, as the expected acid properties and ion exchange capacity modifications, allowing the design of catalysts or catalyst support materials for further active species grafting. Numerous preparation strategies were described with a variety of chemical systems and different raw materials [3-10]. The synthesis of the ordered mesoporous materials implies a complex array of reactions in media where reactants organic and inorganic species and conditions can be continuously changed. In this work we focused on a series of A1-MCM-41 an aim to establish an appropriate Si/A1 atomic ratio domain of the reaction mixture to be used in relatively low concentration surfactant syntheses. The synthesis conditions were selected bearing in mind a possible application at large scale of such materials. The surfactant used was cetyltrimethylammonium bromide (C16TMABr). As silicon source a combination of alkali silicate, organic silicate and pure silica powder, was used. Aluminum isopropoxide, a monomeric alumina precursor was selected as alumina source. Janicke et al. [3] claimed that aluminum isopropoxide, as aluminum source is efficient in controlling Al-framework incorporation, while different results were reported by Luan et al. [7]. Janicke et al considered that syntheses employing aluminum isopropoxide yielded products required gentle heat treatment in an inert atmosphere of flowing nitrogen followed by calcination in flowing oxygen. However, we performed all the calcinations in air with low heating ramp l~

152 2. EXPERIMENTAL SECTION 2.1 Synthesis A series of mesoporous aluminosilicate A1-MCM-41 have been synthesized using cetyltrimethylammonium bromide (C16TMABr, Fluka) as surfactant. A1-MCM-41 syntheses of different Si/A1 mole ratios (5-120) at low surfactant concentration (CI6TMABr/SiO2 mole ratio= 0.15 ) using similar procedures to those reported [1-3] were performed. The nature of the silicon precursor is considered to be an essential factor for obtaining a high-quality MCM-41 mesoporous material. Although, it has been used as silicon source, various raw materials, the degree of condensation of the walls and the degree of pore ordering are influenced by a proper choice of the silicon precursor in connection with the synthesis conditions. Highly basic gels favor the lamellar phase, while weakly basic gels lead to the formation of amorphous silica. The alkalinity level of the synthesis gel plays an important role in the MCM-41 formation. Consequently, a combination of alkali silicate, organic silicate and pure silica powder is preferred to be used as a silicon source [4,5]. Therefore, we used a mixture of sodium silicate, tetramethylammonium silicate and fumed silica as an adequate silicon precursor for A1MCM-41 syntheses. Aluminum isopropoxide (Merck) was the aluminum source and tetramethylammonium hydroxide solution (25% TMAOH, Aldrich) was used as mineralizer. The aluminum-containing gels were prepared as follows: Firstly, a tetramethylammonium silicate solution (TMA/Si=0.5, 10% SiO2) was prepared by mixing appropriate amounts of 25% TMAOH solution and fumed silica (98% SiO2, Sigma). Then, the sodium silicate solution (27% SiO2, 9% Na20, Merck), water and fumed silica were added to the teramehylammonium silicate solution, under continuous stirring. Secondly, a 15% aqueous CI6TMABr solution was added to the above silicate mixture under vigorous stirring and a well-homogenized gel was obtained. Finally, an adequate amount of aluminum isopropoxide was added into the surfactant-silicate mixture. The mole chemical composition of the aluminosilicate gel was: SiO2:0.07Na20:0.085TMAOH:0.004-0.40A1203:0.15C16TMABr:60H20 A reference synthesis using only purely siliceous synthesis gel was prepared by the same procedure, without the addition of the aluminum source. After stirring for one hour at room temperature the synthesis gel having, a pH around 12 was loaded into a 500 ml Teflon-lined autoclave and heated at 100~ for 48 hours, under continuous stirring. After cooling to room temperature, the resulting product was repeatedly washed with distilled water until the pH reached 7.5, separated by filtration and dried in air, at room temperature. The surfactant was removed from the as-synthesized product by calcination in air (static conditions) with a heating rate of I~ from room temperature to 550~ and maintained at 550~ for 6 hours.

153 2.2 Characterization

The as-synthesis and calcinated samples were characterized by X-ray powder diffraction on a DRON-3 diffractometer using monochromated CuKa radiation. Diffractions patterns were recorded from 1~ to 30~ (20) with a resolution of 0.02~ and a count time of 20s at each point. The diffraction peaks were fitted assuming a Pearson-VII function for the peak profile. SiO2 a-quartz was used as internal standard as well as for the background extraction in the low angle region. The IR spectra were recorded between 1600 cm1 to 400 cm1 with a SPECORD M80 spectrophotometer using KBr pellets technique. The BET surface areas were measured by the N2 physisorption at 77K using a Grimm BET automatic surface analyzer. A preheating in a helium atmosphere at 200~ for 2 hours was performed in order to consider the percent of water in the calculation of the BET surface areas. The overall acidity of the samples was evaluated by a NH3-TPD. The procedure consists of: the preheated of the samples at 200~ for 2 hours in argon stream, the saturation of the surface with ammonia at 120~ in a mixture of ammonia-argon gas stream and the removal of the physically adsorbed ammonia. Finally, the temperature was linearly ramped up to 600~ and the total amount of desorbed ammonia was continuously trapped in sulfuric acid solution and quantified by titration with NaOH solution. For comparison, the acid content of the samples was also evaluated using a simple method of titration with a hydroxide solution, taking into account that one equivalent of base corresponds to one equivalent of proton (H§ associated to the framework aluminum sites [11,12]. 3. RESULTS AND DISCUSSION As figure 1 shows, except for the high aluminum content synthesis (Si/Al=l.25), the XRD patterns of the as-synthesized sample exhibit well -defined peaks of pseudohexagonal structure of MCM-41. The peaks are indexed in P6 symmetry with a lattice constant ao, actually the distance between the centers of two neighbor pores, calculated as ao=2dl00/~/3. Along with the pattern of an ordered channels disposition, the broad peak appearing between 20-25 ~ (20) usually assigned to an amorphous silica or silica-alumina ,is considered. Table 1 presents the structural data of the as-synthesized MCM-41 samples. o

A I/(A

I+ S i)

0.032 ".444

0

~ 2oC

uKa(*)

Figure 1. XRD pattems of as -synthesized MCM-41 content

samples with different aluminum

154 From the data presented in Table 1 one could notice the effect of aluminum incorporation into MCM-41 materials. For low contents of aluminum the diffraction intensity of the main peak is higher than that of the pure silica analogue suggesting a tendency of an improvement of the condensation of the silica structures in the presence of the aluminum. There is no a regular increases of the lattice constant ao with the aluminum content due probably, to the low aluminum amount and a high amount of silanol groups. For the last sample, the small ao value is explained by the formation of aluminum-rich dense phase, confirmed by SEM and BET measurements and therefore a low content aluminum MCM-41 is formed in this case. There is a linear relationship between the relative intensities of the 110 diffraction peak and the relative intensities of the 210 peak to the 200 peak. This result is in agreement with the model structure for MCM-41 proposed by Feuston & Higgins [13]. In the simulated patterns in accordance with the experimental patterns, the relative intensity of this peak decreases with increasing wall thickness. In the as -synthesis forms the presence of aluminum seems not to increase perceptive the wall thickness. One should also noticed that the full width at half maximum height of the 100 reflection FWHM increased with the aluminum content as a sign of a slight distortion of the long range order induce by aluminum incorporation. However, the A1-MCM-41 samples exhibit a slight decrease of the broadness of the 100 peak after calcination in contrast with the Si-MCM-41 analogue which 100 peak is broader after calcination. The Al-rich synthesis leading to a mixture phase composition thermally unstable is not included in this trend. A broad peak suggests also small particle size, confirms by SEM micrographs for the relative larger amounts of aluminum. Table 1. XRD data of as-synthesized MCM-41 samples Sam pie 1 2 3 4 5 6 7 8 9

A1 mole fraction 0 0.008 0.013 0.016 0.020 0.024 0.032 0.063 0.167 ,,,

ao (A) 51.14 50.97 51.77 50.61 51.31 51.44 49.72 51.26 47.92 ,,,,,

,,

MCM-41 I100 FWHM100Ill0 (a.u.) ( o) (a,u.) 8.32 0.25 0.88 7.65 0.28 0.46 9.32 0.26 0.68 7.32 0.28 0.72 7.00 0.28 0.75 7.76 0.27 0.52 6.08 0.30 0.66 5.67 0.32 0.25 4.61 0.25 0.79 ,,

,

Amorphous p e a k Ill0/I1001210/1200 d I FWHM (A) (a.u.) (o) 0.106 0 . 6 1 4.15 0.262 1.12 0.061 0.89 4.18 0.268 1.10 0.073 0.78 4.17 0.263 1.12 0.099 0.62 4.15 0.239 1.06 0.107 0 . 6 1 4.15 0.236 1.04 0.067 0.96 4.18 0.259 1.14 0.109 0.60 4.13 0.228 1.09 0.043 1.05 4.18 0.216 1.17 0.108 0.44 ,,,,

,

,

,

,

,,,,,,,,

,,,,,

,,

,

,

,,,,

It could be observed that the intensity of the 100 peak decreased with the intensity of the broad amorphous peak .It means that main contribution to the intensity of this peak is provided by the disordered material of the walls. As an intrinsic property of MCM-41 material is worth to give more attention to the analysis of this peak [14]. After the calcination procedure, the XRD diffraction pattems of the samples present the already reported behavior [1-7,5,15]: an enhancement of the peak intensities and a shrinking of the lattice as a result of surfactant removal and a subsequent condensation ofsilanol groups. Upon calcination a displacement towards higher angles

155 along with a decrease of the intensity of this maxim are observed. The result is to be connected with structural water removal. The structural data are collected in Table2. Table2. Structural data of calcinated MCM-41 samples Sample

1 2 3 4 5 6 7 8

A1 mole fraction 0 0.008 0.013 0.016 0.020 0.024 0.032 0.063

Lattice contraction (%) 7.16 5.80 9.06 6.69 7.99 6.85 6.86 9.60

I100eale/I100synt__FWHM100calc/FWHM100synt Weight loss (%) 2.432 1.089 48.74 2.568 0.908 49.15 2.629 0.926 49.60 2.384 0.976 48.55 2.611 0.980 48.00 2.428 1.009 47.56 2.253 1.000 46.44 2.317 0.933 44.84

Apart from sample 9, with high aluminum content (A1 mole fraction=0.167), which proves to be thermally unstable, all the samples exhibit well-defined typical MCM41 XRD pattems. The good quality of the materials is pointed out also by the high total weight loss and also by the relatively low contraction lattice. It was already mentioned that for the Al-containing MCM-41 samples calcination improves the restructuring process of the walls reflected by a sharper 100 diffraction peak in comparison with the assynthesis forms. The result could be attributed to an initial lower silanol groups amount. The 27A1 MAS NMR spectrum of as-calcined sample 7 (figure 2) proves clearly that almost all the A1 ( 94 %) are tetrahedrally coordinated ( 53.8 ppm) . The octahedral aluminum NMR signal (0 ppm) are influenced also a by the degree the sample hydration[ 16]. .

.

m

.

m

.

.

.,,=

I

-400

-2;o

"

o'

20'0

"

4o'o

ppm

Figure 2. The 27A1MAS NMR spectrum of calcinated sample with an A1 mole fraction = 0.032

156 The IR spectra of the as-calcinated samples are similar with those reported for siliceous MCM-41 [1] with four adsorption bands at 1080 cml , 800 cm1, 960 cm1 and 450-470 cm1. The T-O-T band at 1080 cm"1 usually indicating the MCM-41 formation presents a slight shift to lower wavenumbers as the A1 content increases. The 960 cm1 maximum assigned to Si-OH groups became less well-defined also with the aluminum amount increment. For the highest A1 content sample this maximum is entirely overlapped by the 1080 cm1 one (figure 3). ~ A I I ( A I+ S i) 0.032 c In E

. o

/..~

"

o12,

~ o

40'0

60'0

8(~0

10~)0

wavenum

12100 ber

14=00

16~)0

18=00

20'00

( c r n "')

Figure 3. IR-spectra of two calcinated A1-MCM-41 samples The measurements of the acidityare presented in Table 3. Table 3. The acidity measurements Sample

Ai mole fraction

1 2 3 4 5 6 7 8 9

0 0.008 0.013 0.016 0.020 0.024 0.032 0.063 0.167

,,

,,

,

,,

,

,

Acid measurements ............ NH3-TPD (mmol/g) NaOH titr~ion(mmol/g) 0.1100 0.0843 0.2000 0.1849 0.4383 0.1874 0.2960 0.1925 0.2800 0.2000 0.2850 0.2025 0.2992 0.2575 0.5038 0.3000 0.6042 0.1875 ,,

,,,,

,,,,

,,,,,,,

,

,,,,,

,,,

The above data show a difference between both methods of acidity evaluation. The NaOH titration could be more accurate correlated with framework A1, and consequently with Broensted acidity and therefore it displays a rather good linear relationship with the A1 content. The NH3-TPD measurements imply both Broensted and Lewis acid sites and therefore the overall acidity is higher. There is no obvious linearity of the ammonia acidity with the A1 content probably due to different silanol groups amount. The rich-A1 sample

157 which is a mixture of MCM-41 and a dense amorphous phase exhibits, as expected, a low NaOH acidity due to framework A1 and higher total acidity (ammonia-TPD) provided by the dense phase. The SEM images and the BET surface areas (figure 4) proved for the formation of high quality ordered mesoporous materials up to an A1 mole fraction of 0.063. The rich A1 amount samples exhibit upon calcination, a poorly resolved XRD pattern, a less wellresolved IR spectrum, a SEM image with large portions of non-porous material and a low BET surface area.

A1/(AI+Si) = 0 ....~ !~'~,~~!ii~iI

BET=1123 m2/g

A1/(Al+Si):0.032 BET=1090 m2/g

A1/(AI+Si):0.167 BET = 332 m2/g

Figure 4. SEM micrographs of representative MCM-41 samples

158 In conclusion, our studies emphasize that it can be obtain a good-quality A1-MCM41 material, with well-resolved XRD patterns, high BET surface areas and acid properties in an range of AI mole fraction (A1/(AI+Si)) up to 0.063. It seems also that the presence of low amounts of A1 in the framework prevents 'the deterioration of the long-range structural order as in the case of siliceous MCM-41 occurred [17].

3. ACKNOWLEDGEMNTS We are grateful to Dr. Teresa Blasco ( ITQ, Valencia) for the record.

27A1MAS NMR spectrum

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) 10834. 2. K.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli, U. S. Pat., 5,098,684 (1992). 3. M. Janicke, D. Kumar, G. D. Stucky, B. T. Chmelka, Stud. Surf. Sci. Catal., 84 (1994) 243. 4. F. Schfith, Ber. Bunsenges Phys. Chem., 99 (1995) 1306. 5. C.T. Cheng, D. H. Park, J. Kinowski, J. Chem. Soc. Faraday Trans., 93 (1997) 193. 6. R. Schmidt, D. Akporiaye, M. Stk6cker, O. H. Ellestad, J. Chem. Soc., Chem. Commun., (1994) 1493 7. Z. Luan, C. -F. Cheng, W. Zhou, J. Klinowski, J. Phys. Chem., 99 (1995) 1018. 8. K.R. Kloestra, H. W. Zandergen, H. van Bekkum, Catal.Lett., 33 (1995)157. 9. H. Kosslick, G. Lischke, B. Parlitz, W. Storek, R. Frike, Applied Catalysis A: General, 184 (1999) 243. 10. W. B6hlmann, D. Michel, Stud. Surf. Sci. Catal., 135 (2001), 06p 17. 11. R. Mokaya, W. Jones, J. Catal., 172 (1997) 211. 12. R. Mokaya, W. Jones, J. Mater. Chem., 9 (1999), 533. 13. B. P. Feuston, J. B. Higgins, J. Phys. Chem., 98 (1994) 4459. 14. M. Ookawa, Y. Yogoro, T. Yamaguchi, K. Kawamura, Stud. Surf. Sci. Catal., 135 (2001) 06002. 15. M. T. Keene, R. D. M. Gougen, R. Denoyel, R. K. Harris, J. Rouquerol, P. L. Llewellyn, J. Mater. Chem., 9 (1999) 2843. 16. J. M. Kim, J. H. Kwak, S. Jun, R. Ryoo, J. Phys. Chem., 99 (1999) 16742. 17. R. Ryoo, J. M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711.


159

M e s o p o r o u s aluminosilicates from coal fly ash P. Kumar*, N. Mal, Y. Oumi~, T.

S a n o 1 and

K. Yamana

Ceramic Section of Chemistry & Food Department, Industrial Research Institute of Ishikawa, Kanazawa, Ishikawa 920-0223, Japan. E-mail: [email protected] 1School of Materials Science, Japan Advanced Institute of Science & Technology, Tatsunokuchi, Ishikawa 923-1292, Japan

For the first time supematant of the coal fly ash has been used to prepare aluminum containing MCM-48. It was found that most of the Si and A1 components in the fly ash could be effectively transformed into MCM-48 when a surfactant mixture containing cationic cetyltetramethylammonium bromide, CTMABr and tetraoxyethylene dodecyl ether, C12(EO)4 is used as template. It has been observed that the fusion plays an important role in enhancing the hydrothermal condition for synthesis of these mesoporous materials. High concentration of Na ions present in the supernatant of fused fly ash is found to be not critical in the formation of A1-MCM-48 when prepared under controlled pH condition. 1. INTRODUCTION Although there are many articles concerning synthesis of mesoporous materials [1-2] most of them have been severely biased to MCM-41 [3-4]. The bias may be attributed largely to the fact that the synthesis of MCM-48 required very specific synthesis conditions [5]. MCM-48 (cubic, space group Ia3d) with its highly branched and interwoven threedimensional networks of the mesopore channels are believed to be much more resistant to pore blockage while being used as absorbents and catalyst supports than the onedimensional channel of a hexagonal MCM-41 [1,6]. Presently however, both the economic and environmental costs for large-scale manufacture of these materials are high due to the cost and toxicities of both templates and preferred silica source. A variety of silica sources are generally used to prepare these materials including fumed silica and silicon tetraethoxide. The industrial manufacture of mesoporous materials is likely to be economically prohibitive if silicon alkoxides and fumed silica in particular are selected. Coal combustion in the world accounts for approximately 37% of the total electricity production and in turn, results in the production of a huge amount of fly ash as waste material [7]. Nearly 600 million metric tons of fly ash are produced annually in the world, with the global recycling rate being only 15% [8]. Since the major chemical compounds contained in fly ash are SiO2 and A1203 (60-70 wt% and 16-20 wt%, respectively), resource recovery is one of the most important issues of waste management at present [9-

160 11 ]. Very recently we have reported our preliminary investigation on the synthesis of A1MCM-41 and SBA-15 type of materials and their characterization [12]. In this paper we report the studies carried out on the hydrothermal synthesis condition of the aluminum containing MCM-48 type materials using coal fly ash as a silicon and aluminum source and their characterization. Synthesis of both MCM-41 and MCM-48 is related in the sense that they can be prepared under identical hydrothermal condition subject to the type and concentration of surfactant used [ 1]. In addition to that various synthesis routes to MCM-48 were developed in order to overcome the synthesis shortcomings [ 13]. These synthesis results showed that the crystallinity of the MCM-48 went through an optimum as a function of time. The MCM-48 products were obtained as an intermediate between transformations from a hexagonal or disordered surfactant-silica mesophase to a more stable lamellar mesophase [14]. One report [15] suggested that the transformation of the MCM-48 mesophase to lamellar can be quenched by adjusting the pH of the reaction mixture. Another report [ 16] claimed that the mixed surfactant approach resulted into high quality MCM-48 as an energetically favored mesophase. Finally, it has been reported that the use of gemini surfactants induce the formation of cubic structure even using fumed silica as silicon source [17]. All these studies indicate that the formation of MCM-48 type materials is possible under certain synthesis conditions. Our aim in this study is to prepare MCM-48 type materials so we focus on three factors, a) single surfactant concentration, b) mixed surfactant concentration and c) intermediate pH adjustment. 2.

EXPERIMENTAL

2.1.

Materials Coal fly ash used in this study was obtained from Nanao-Ota power plant, Hokuriku and used as obtained. The chemical composition of fly ash revealed apart from the main constituents such as silica (67.5%) and alumina (18.7%), the other impurities such as Fe203, CaO, MgO, 1(20, TiO2, Cr203, P205 Na20, K20 and SO3 with 3.6%, 2.0%, 0.7%, 0.9%, 0.8%, 0.9%, 0.3%, 0.2%, 0.4%, 0.7%, respectively. The specific surface area (BET) and cation exchange capacity (CEC) of the coal fly ash were found to be 4.5 mE/g and 0.8 meq/100g, respectively. 2.2. Synthesis of AI-MCM-41 and AI-MCM-48 The silica and aluminum source was the supernatant from fused fly ash powder [ 18]. The concentrations of Si, A1 and Na in supernatant measured by atomic adsorption spectroscopy (Perkin Elmer AS-800) are 11000, 380 and 35000 ppm, respectively. The detail synthesis procedure for MCM-41 was followed from our previous study [12]. MCM48 materials in this study were synthesized using both single surfactant and a surfactant mixture. While CTMABr was used as a single surfactant, the mixture was prepared by using CTMABr and C12(EO)4 (Aldrich). All batches were prepared using a synthesis gel with the following molar composition: CTMABr/ C12(EO)4/H20/Si = 0.35-0.55/0.150.25/100/1. The obtained gel was stirred for thirty min., the synthesis mixture was placed in a Teflon lined stainless steal autoclave and heated at 373 K for 4 days under static condition. Intermittently, the reaction mixtures were cooled to room temperature after one day at 373 K and subsequently added with acetic acid to adjust the pH at 10.2. The MCM48 product was finally filtered after an additional heating at 373 K for another 3 days. To

161 remove the surfactant, the as synthesized sample was calcined in air under static conditions at 813 K for 6 hours, with a linear temperature ramp of 0.5K / min and two plateaus of 60 minutes each at 423 and 623 K. For a comparison an aluminum containing MCM-41 and MCM-48 were prepared following the procedure as reported in the literature [1, 13]. All the materials obtained were further calcined as explained above.

2.3. Analysis and Characterization Powder X-ray diffraction (XRD) patterns were measured using CuK~ radiation by using MAX18X. cE The chemical composition was analyzed with Li]2B404 method by using X-ray fluorescence (XRF) technique (Philips PW2400), BET surface area by N2adsorption at liquid nitrogen temperature (Belsorp 28SA) and morphology by scanning electron microscopy (SEM) using Hitachi S-4100. Transmission electron microscope (TEM) image was obtained by using JEOL 2010. FT-IR spectra of the self supporting wafers were measured by JEOL JIR-7000. TG-DTA analysis was performed using Rigaku TG-8120. Solid-state nuclear magnetic resonance resonance 29 Si and 27A1 NMR spectra were obtained on a Varian VXP-400. 3. RESULTS AND DISCUSSION Because larger amounts of Si and A1 species can be dissolved by fusion method, we adopted fusion approach in this study. Figure 1 summarizes the XRD patterns of the fly ash (a) and the fused fly ash (b) at 823 K. It can be seen that the major crystalline phases present in the fly ash are quartz, mullite and aluminosilicate glass, which are present as the amorphous phase. On the other hand, a ! i m ; w w i large amount of sodium silicate exists in b the fused fly ash (Figure l b), which implies Ss that fusion is effective in extracting silicon from quartz. Most of the quartz species have reacted with the NaOH and resulted s s s into the disappearance of the respective s s s~Ms peaks in fly ash. SsS s iLlij,~slls sS Ss I ~' M S S S Figure 2 shows the XRD patterns of .,.., different MCM phases of calcined samples Q a prepared under different surfactant / silica ratio. It can be seen that low concentration of surfactant (CTMABr) results into MCM41 type materials as suggested from the Q XRD pattern (Figure 2a) clearly displaying at least four reflections that are consistent M MMM M O Q with indexing to a hexagonal cell, typical of an MCM-41 product. The observation 10 20 30 40 50 60 70 of three higher angle reflections other than 20 (degree) the d]00 indicates that the product is likely Figure 1. X-ray diffraction profiles of the to possess the symmetrical hexagonal pore fly ash (a) and the fused fly ash (b) at 823 structure typical of MCM-41. A further K. M = mullite, Q = quartz, and S = increase in surfactant concentration sodium silicate resulted into mesophases, poor in

162

Table 1 Physical properties of the raw material and the calcined mesoporous materials Sample Surf i Surf2 SBET/ Si/A1 Pore d 100 d 211 ao 3 /SiO2 /SiO2 m 2 g-i volume /nm /nm /nm

Pore size 4 /nm

/ c m 3 ~-1

Fly ash A1-MCM-41 (a) A1-MCM-41 (b)

0.20 0.35

-

4.5 842 738

2.9 13.8 18.5

. 0.75 0.57

A1-MCM-41 (c)

0.55

-

731

65.0

0.57

A1-MCM-48 (d)

0.55

0.15

639

62.3

A1-MCM-48 (e)

0.55

0.18

848

59.4

1 2 3 4

.

. 4.24 3.56

. -

4.9 4.1

3.2 2.9

3.56

-

4.1

2.7

0.55

-

3.17

7.8

2.5

0.82

-

3.04

7.4

3.0

cetyltrimethyl ammonium bromide tetraoxyethylene dodecyl ether unit cell parameter, using 2d100/~ for MCM-41 and d211 q5 for MCM-48 Dollimore-Heal method

hexagonal structural order as indicated from the gradual disappearance of diffraction peaks assigned to (110), (200) and (210) reflections (Figure 2b, 2c). The surfactant / silica ratio higher than 0.55 resulted into to a featureless XRD pattern (not shown). By increasing the concentration of CTMABr in the synthesis gel, a phase transitions from hexagonal to lamellar passing through an intermediate state of cubic structure is reported [1-4]. But using the supernatant as silica source it was not observed, in other words MCM-48 formation was not facilitated under the synthesis condition using CTMABr 211 alone. Figure 2d and 2e shows the 5 XRD patterns of the surfactant-silica mesophase obtained from the starting mixtures of CTMABr/ C12(EO)4 = 0.55/0.15 and 0.55/0.18, respectively. _= It can be seen the presence of neutral surfactant has resulted into mesophase, identical to the cubic MCM-48. We observed that the optimum condition ~ b for MCM-48 using supernatant as 110 silica source was 0.55/0.18 as it showed the sharpest XRD patterns. 4 6 8 Table 1 summarizes characteristics of 20 / degree the calcined mesoporous materials Figure 2. XRD profiles of the different obtained. calcined M C M type materials. CTMABr/SiOE 9a - 0.22, b - 0.35, c, d and e -- 0.55; C12(EO)4/SIO2: d - 0.15 and e - 0.18 9

!

9

!

,

4•

>,

e-

i

|

--

163 From this synthesis experiment the XRD pattern in Figure 2e, a highly ordered MCM-48, without any trace of lamellar phase peaks was recovered. The high ordered array of these materials could be inferred from the presence of a well def'med set of diffraction peaks between 3 ~ and 6~ the XRD patterns assigned to the (211), (220), (321), (420), (422) and (431). Gel representing higher than 0.18 of C12(EO)4 resulted either into unidentified mesophase or without any XRD pattern. The (211) reflection is found at approximately 3.6 nm for all the as-synthesized samples. This correspond to a unit cell size of 8.7 nm. For the calcined samples the same reflection occurs at-3.1 nm, which gives a unit cell length o f - 7 . 5 nm. Hence, as should be expected, there has been a shrinkage of the unit cell ( - 1 3 % ) during calcinations which probably Figure 3. N2 adsorptionis due to silanol condensation. This magnitude desorption isotherms of different of unit cell shrinkage was in the range of values samples (c, d and e), prepared normally reported in the literature using other under different conditions. silicon source, that are in the 5-15% range [1417]. The same tendency is observed for the (220) reflection suggesting that the supernatnt of coal fly ash could be used as a source material. The N2 adsorption-desorption isotherms of different samples (c, d and e) are shown in Figure 3. It corresponds to a reversible type IV isotherm which are characteristic for mesoporous materials. An inflection point is observed at relative pressures between 0.25 and 0.3. This corresponds to the filling of the mesopores, and the sharp increase in the adsorbed volume indicates a uniform pore-size distribution. It can be seen (Table 1) how the presence of neutral surfactant facilitates the formation of MCM-48. The presence of a small hysteresis loop in sample c, indicates the formation of lamellar phase which is very similar to the studies that has been reported at high surfactant/ silica ratio [19]. Figure 4 shows SEM images of the mesoporous materials prepared from the Figure 4. SEM images of fly ash and samples c ,d and e supernatant and the original fly prepared from supernatant of coal fly ash.

164 ash. Morphologically, the fly ash consists essentially of aluminosilicate glass spherules that disintegrate during fusion treatment and are converted into amorphous sodium silicate and sodium aluminosilicate. The SEM images of mesoporous materials are generally Figure 5. SEM images A1-MCM-41 (1) and A1-MCM-48 found in a range of (2) using pure chemicals. spherical top, ribbon, torous shape d particles [20]. In the present case, the particles of the materials obtained have a spherical habit at around less than 1 lxm although some agglomerates are also visible (sample c). The presence of C12(EO)4 as neutral surfactant resulted into more uniform spheres (sample d and e, respectively). In Figure 5 we have compared the SEM images of MCM-41 and MCM-48 by using pure chemicals such as TEOS and Al(NO3)3 as silicon and aluminum sources, respectively. The major particles of MCM-41 (sample 1) are constructed from elementary units that have ribbon-like habits. Formation of such ribbon-like particles are reported for MCM-41 when prepared under aged condition [21]. MCM-48 (sample 2) exhibited spherical raspberry-like pattern. Since both these samples are prepared from reaction mixtures differing in the surfactant / SiO2 ratio and content of water, this substantial difference in particle morphology are not unexpected. TEM images of micro-sectioned samples (Figure 6) also showed well ordered hexagonal arrays of mesoporous channel (sample a) and pores arranged on the cubic plane (sample e) confirming that the materials indeed possess the pore system symmetries that were inferred from the X-ray patterns and N2 isotherms. A mixed surfactant approach has been reported in the literature for the preparation of mesoporous materials [19]. In many cases, two different surfactants are completely miscible and form liquid crystalline miceller mesophase cooperatively. The structure in the micelle packing in the liquid crystalline mesophases may be determined by various effects such as their head to tail packing parameters, electrostatic interaction and hydrogen bonding between the head groups of the two different kinds of surfactant molecules [22]. This phase behavior becomes more complicated when silica and alumina sources are present in the Figure 6. TEM image of A1-MCMform of supernatant of coal fly ash. Supernatant is a highly alkaline solution of silicate and aluminate 41 (sample, a)and A1-MCM-48 (anions) and are strongly attracted by electrostatic (sample, e).

165 interaction surrounding the head groups of the CTMABr, which may lead to the high concentration of the anions on the surface of the surfactant micelles. The neutral surfactant has no strong interaction with the anions, and consequently its incorporation to the micelles will bring a dilution of the anions at the surface. This ~9 low surface concentration may further lead to a certain contraction of the micellar surface resulting in a phase transition from hexagonal to cubic. At this stage we are not advancing any explanation about the complexities of phase behavior of the fly ash supematant-surfactant mesostructures in the 10 5 0 aqueous solution, however we believe that ppm C12(EO)4 acts more as a diluents and based on our Figure 7. 27A1-MAS-NMR observation facilitated the formation of MCM-48 structure. spectra for fly ash and calcined Another factor that affected the formation A1-MCM-41 (sample a) prepared of cubic phase was the pH of the supematant- from supematant of fused fly ash surfactant mesostructure. Generally, a high pH solution. * Denotes the spinning condition is a major driving force for the side bands. transformation to lamellar [23]. In our case the pH adjustment to 10.2 during the synthesis arrested this transformation and also helped to improve the product yields. This is agreement with the report where the pH adjustment was mentioned as a means for quenching the transformation of the MCM-48 mesophase to lamellar [16]. We note that the BET surface area and pore volumes of the samples (Table 1) prepared in this work are slightly lower than those that may be obtained from more reactive silica sources. The values obtained are nevertheless similar to those reported in the literature and indicate high overall total porosity. Based on these observations, the possibility to use the dissolved silica and aluminum present in the supematant of fused fly ash as a precursor for the synthesis of MCM-48 are fairly good. But use of the high Si containing fly ash would be a better choice. One of the most important features of our study using coal fly ash is the aluminum incorporation into the framework of the synthesized materials [12, 18]. The information about the chemical state of A1 in the materials obtained using 27A1 MAS-NMR spectra of the original coal fly ash (a) and calcined A1-MCM-41 (b) were measured (Fig. 7). Fly ash did not show any peak, indicating the lack of any tetrahedrally (Td) coordinated framework aluminum. But a very strong single peak assigned to the Td framework aluminum at ca 54 ppm was observed for the sample prepared from fly ash without any Oh non-framework (0 ppm) aluminum. Even at such high aluminum concentration (Si/A1 = 13.8), absolutely no Oh peak was observed. This provides a direct evidence of A1 incorporation in the MCM-41 framework. Similarly, in the case of A1-MCM-48 (sample d and e) Td aluminum (spectra not shown) were observed though the presence of high concentration of surfactants has a dramatic effect on the aluminum incorporation as revealed from 27A1 MAS-NMR studies. ,

I

,

i

,

,

I

,

I

i

i

l

!

|

!

|

166 4. CONCLUSIONS In summary, it is shown that the supematant of coal fly ash can be used as a raw material for the synthesis of aluminum containing MCM-48. The use of surfactant mixture between cationic hexadecyl trimethylammonium surfactants and neutral tetraoxyethylene dodecyl ether surfactants has greatly facilitated the synthesis of MCM-48 performed under controlled pH condition. A high aluminum incorporation in tetrahedral position is revealed in the mesoporous materials which may be useful for certain catalytic applications. The experimental data produced here suggest that the coal fly ash could be a suitable source of silicon / aluminum with a low economy and environmentally friendly reagent for the preparation of well ordered mesoporous materials. REFERENCES

1. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. S. Inagaki, Y. Fukushima and K. Kuroda, J. Chem. Soc., Chem. Commun., 1993, 680. 3. A. Corma, Chem. Rev., 97 (1997) 2373. 4. A. Sayari, Y. Yang, M. Kruk and M. Jaroniec, J. Phys. Chem. B, 103 (1999) 3651. 5. J.M. Kim, S. K. Kim and R. Ryoo, J. Chem. Soc., Chem. Commun., (1998) 259. 6. C.L. Landry, S. H. Tolbert, K. W. Gallis, A. M. Monnier, G. D. Stucky, P. Norby and J. C. Hanson, Chem. Mater., 12 (2001) 1600. 7. C. Zevenbergen, J. P. Bradley, L. P. V. Reeuwijk, A. K. Shyam, O. Hjelmar and R. N. J. Comans, Environ. Sci. Technol., 33 (1999) 3405. 8. G. Belardi, S. Massimilla and L. Piga, Resource, Conservation and Recycling, 24 (1998) 167. 9. A. Singer and V. Berkgaut, Environ. Sci. Technol., 29 (1995) 1748. 10. S. Rayalu, N. K. Labhasetwar and P. Khanna, U.S. Patent No. 6027708 (22 February 2000). 11. N. Shigemoto, S. Sugiyama, H. Hayashi and K. Miyaura, J. Mater. Sci., 30 (1995) 5777. 12. P. Kumar, N. K. Mal, Y. Oumi, K. Yamana, and T. Sano, J. Mater. Chem., 11 (2001) 3279. 13. M. L. Pena, Q. Kan, A. Corma and F. Rey, Microporous Mesoporous Mater., 44-45 (2001) 267. 14. A. Corma, Q. Kan and F. Rey, J. Chem. Soc., Chem. Commun., (1998) 579. 15. J. Xu, Z. Luan, H. He, W. Zhou and L. Kevan, Chem. Mater., 10 (1998) 3690. 16. R. Ryoo, S. H. Joo and J. M. Kim, J. Phys. Chem. B, 103 (1999) 7435. 17. P. Van Der Voort, M. Mathieu, F. Mees and E. F. Vansant, J. Phys. Chem. B, 102 (1998) 8847. 18. P. Kumar, Y. Oumi, K. Yamana and T. Sano, J. Ceram. Soc. Japan, 109 (2001) 968. 19. G. Oye, J. Sjoblom and M. Stocker, Microporous Mesoporous Mater., 27 (1999) 171. 20. K. Schumacher, M. Grun and K. K. Unger, Microporous Mesoporous Mater., 27 (1999) 171. 21. M. Grun, K. L. Unger, A. Matsumoto and K. Tsutsumi, Microporous Mesoporous Mater., 27 (1999) 207. 22. J. L. Palous, M. Turmine and P. Letellier, J. Phys. Chem. B, 102 (1998) 5886. 23. R. Ryoo and J. M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711.

Studies in Surface Science and Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

167

N e w route for synthesis o f highly ordered mesoporous silica with very high titanium content Xiang-Hai Tang*, Xin Wen, Shi-Wei Sun and Hai-Yan Jiang College of Chemistry, Nankai University, Tianjin 300071, P. R. China

Meso-structured titanosilicates with variable amounts of titanium have been hydrothermally synthesized under static conditions via a new route. By a separate hydrolysis procedure during gel preparation, this approach effectively prevents titanium ions from formation of indissoluble titanium species in the mixture at very high titanium content. The physicochemical properties of these materials were characterized by means of XRD, FT-IR, UV-Vis DRS and catalytic oxidation of 4-methylphenol with dilute hydroperoxide. In contrast to literature reports, this novel method can lead to mesoporous titanosilicates with a highly ordered MCM-41 hexagonal phase even in a gel with n(Yi)/n(Si) up to 1/4 and a relatively low template content. It reveals that these materials are chemically homogeneous. During crystallization, titanium ions were partially incorporated into the MCM-41 framework. The addition of TBAOH resulted in a more cross-linked and ordered wall structure.

1.

INTRODUCTION

The synthesis and characterization of hetero-atom containing molecular sieves have been a hotspot in the field of zeolite synthesis since the last two decades [ 1-5]. These materials are the most promising candidate catalysts for the environmentally benign industrial processes. Among them titanium-containing molecular sieves (e.g., TS-1, TS-2 and Ti-beta) have attracted considerable attention. However, their catalytic performance in reactions involving in bulky molecules is greatly limited due to the micropore nature, as their pore size is usually less than 2 nm. Fortunately, the pore diameter of solid materials with regular voids has been largely expanded since the discovery of M41S family materials. Up to now, mesoporous titanosilicates such as Ti-MCM-41 [6], Ti-HMS [7] and Ti-MCM-48 [8] have been successfully synthesized, but titanium content in these solids is usually very low. More recently, E1 Haskouri et al. [9] used a complexing polyalcohol (2,2,2-nitrile-triethanol) as a hydrolysis retarding agent for Yi species and reported that meso-structured titanosilicates could be prepared from a gel of n(Yi)/n(Si) up to 1/4. However, the framework of these materials was lack of long-range order, the BET surface area of the product with the highest n(Ti)/n(Si) of 1/1.9 in solid was merely 595 m2/g. It seems that these materials are analogues * To whom correspondence should be addressed. Email: [email protected].

168 to Ti-HMS, and most of the titanium atoms locate outside the framework, which are lack of oxidation activity. Generally, the catalytic performance of mesoporous titanosilicate in selective oxidation reactions is dependent on the framework titanium and its amount. Hence it is of importance to develop new approaches for the preparation of high quality mesoporous titanosilicate. Here we demonstrate the synthesis and characterization of high titanium content MCM-41 titanosilicates with a highly ordered hexagonal phase. By our new approach high quality products could be prepared from a gel with n(Ti)/n(Si) up to 1/4 and a relatively low template content.

2.

MATERIALS AND METHODS

2.1.

Materials Tetraethyl orthosilicate (TEOS) and tetrabutyl orthotitanate (TBOT) were employed as silicon and titanium sources, respectively. Cetyltrimethylammonium bromide (CTABr) was used as template. Both liquid ammonia and tetrabutylammonium hydroxide (TBAOH) were used as hydrolysis agents. Firstly, TEOS, liquid ammonia and distilled water were mixed to prepare solution A, while TBOT was dropwise added to a mixture of TBAOH, ethanol and distilled water to prepare solution B. The two solutions were vigorously stirred in closed vessels respectively at ambient temperature till the solutions became homogeneous and transparent. Secondly, a gel was prepared by mixing solutions A and B, then the gel was boiled to evaporate alcohols and part of water. Finally, the gel was slowly added to a solution of CTABr, liquid ammonia and distilled water, and further stirred at ambient conditions for 3 h to obtain a mixture with a molar composition of (1-x)SiO2:xTiOz:yCTABr:O.3yTBAOH: mNH3:zH20 (where x=0-~0.2, y=0.1~0.7, z=100--520, m=6~l 1). The resultant was sealed in a PTFE-lined stainless steel autoclave and heated at 383 K for 72 h. The as-synthesized product was separated by centrifugation at 9000 rpm, washed twice with distilled water and later freeze-dried overnight to remove water from the solid product. To obtain the template free sample, the as-synthesized product was heated in air from ambient temperature to 823K at 7 K/min, kept for 4h, then raised temperature to 923K at 5 K/min and kept for lh. 2.2.

Characterization Powder X-ray diffraction (XRD) patterns were obtained in the 20 range 1-10 ~ with a Rigaku D/MAX 7A diffractometer using the Cu Ka radiation operated at 40 kV and 40 mA. FT-IR study was performed using a Bruker Vector-22 with a resolution of 4 cm ~ and 30 scans. The KBr technique with a sample to KBr weight ratio of 1"150 was used. UV-vis diffuse reflectance spectra (UV-vis DRS) were measured with a Shimadzu UV-240 spectrophotometer. Spectra were recorded in the 190-800 nm wavelength range against a MCM-41 standard. The catalytic activity of the calcined sample was tested by selective oxidation of 4methylphenol with dilute hydroperoxide. The reaction products were analyzed with a Hewlett-Packard HP G 1800A GC-MS instrument.

3. 3.1.

RESULTS AND DISCUSSION Material synthesis

169 Table 1 summarizes the gel molar compositions for preparation of various samples. Generally, hydrothermal syntheses of microporous zeolites and mesoporous materials are carried out in alkaline media. Unfortunately, most transition metal ions tend to hydrolyze and form insoluble species like metal hydroxides even in a neutral solution. To overcome that disadvantage and to increase the efficiency of metal source in the synthesis, it is essential to develop a new method to prevent the metal source from the formation of insoluble species. TBAOH, a strong base and an ionic surfactant, is commonly used as a structure-directing agent in zeolite synthesis. The hydrolysis of TBOT in a TBAOH-ethanol-H20 solution resulted in a mixture of soluble titanium species and effectively prevented or suppressed the formation of Ti(OH)4 and TiO(OH)2. Indeed, no visible precipitation occured 9000 during gel preparation even in systems with very high amounts of titanium. It a, is worthy of mention that our products are quite different from those previous ~ 6000 ~ _ F reported by E1 Haskouri et al. [9]. In our case, most of the samples exhibit at .9 least three well-resolved reflections in ~ 3000 .;> the 20 range between 2-6 ~ (Figure 1), B which can be indexed to an ordered hexagonal lattice typical of MCM-41 0 6 11 16 [10]. It reveals that following the new 20( ~) synthetic route highly ordered MCM-41 titanosilicates could be hydrothermally Figure 1. XRD patterns of the calcined samples synthesized within a relatively wide A-F in Table 1 (Offsets: vertically by 1000 CPS, composition range. horizontally by 1.0 degree). , ,,,,~

;j

Table 1. Influence of gel composition on crystallization of mesoporous titanosilicate. Sample Gel molar composition Phase Crystallinity ~ (%) n(SiO~) n(TiO?) n(CTABr) n(TBAOH) n(NH3) n(HzO) A 1.00 0.00 0.50 0.02 9.20 130 MCM-41 100 B 0.95 0.05 0.50 0.03 9.20 130 MCM-41 99 C 0.90 0.10 0.50 0.03 9.20 130 MCM-41 90 D 0.85 0.15 0.50 0.05 9.20 130 MCM-41 86 E 0.82 0.18 0.50 0.05 9.20 130 MCM-41 80 F 0.80 0.20 0.50 0.06 9.20 130 MCM-41 68 G 0.85 0.15 0.50 0.05 6.00 130 MCM-41 69 H 0.85 0.15 0.50 0.05 11.00 130 MCM-41 70 I 0.85 0.15 0.05 0.05 9.20 130 Amorphous J 0.85 0.15 0.10 0.05 9.20 130 MCM-41 72 K 0.85 0.15 0.70 0.05 9.20 130 MCM-41 76 L 0.85 0.15 0.50 0.05 5.70 80 MCM-50 M 0..85 0.15 0.50 0.05 7.10 100 MCM-41 85 N 0.85 0.15 0.50 0.05 36.80 520 MCM-41 85 a. Counted on MCM-41 phase compared to sample A after calcination.

170 A careful XRD examination of the as-synthesized and calcined products was also performed at high angle area. However, no distinct peaks were observed in the 20 range between 20-80 ~ which implies the absence of bulky crystalline TiO2 (i.e., anatase and rutile) and titanium silicate. This result suggests that these materials are probably chemically homogeneous. 3.2.

Framework IR Shown in Figure 2 is the framework IR spectrum of as-synthesized sample D. The band at 1080 cm ~ can be ascribed to asymmetric stretching of [SiO4] tetrahedra, while the bands at 459 cm ~ and 800 cm ~ can be assigned to bending of framework Si-O bonds. To our surprise, two weak bands centered at 560 cm ~ and 1237 cm ~ respectively are also observed, which are rarely seen in previous literatures concerning mesoporous materials. The former band characterizes the double-rings' vibrations of external [SiO4] tetrahedra in MFI or MEL structure [ 11 ]. In our study, TBAOH was used as a hydrolysis agent, therefore it is possible that TBA + templated the formation of MFI or MEL structure during crystallization. It suggests that at least part of the [TO4] (T=Ti, Si) tetrahedra within the walls of the assynthesized products is highly cross-linked. This has been demonstrated very recently by two groups separately but using tetraethylammonium or tetrapropylammonium hydroxide as a hydrolysis agent [12,13]. The presence of TBA + resulted in a more ordered and condensed wall structure than those of conventionally prepared ones. However, no absorption related to TiOz or TiO(OH)2 species is detected, which indicates that Ti well disperses in the assynthesized titanosilicate with very high amounts of titanium. An IR band around 960 cm ~ is also observed in Figure 2, which is often assigned to a lattice defect and is correlated with the substitution for silicon with metal ions in the silica framework [1-8]. However, recent work suggests that this band corresponds to a Si-O vibration in a Si-OR (R=H, tetraalkylammonium) group in siliceous materials (e.g., Al-beta and pure MCM-41 silica) [ 14,15]. Nevertheless, there is a controversy on the origin of this IR band, we attentively exclude its appearance as an evidence of the incorporation of Ti into the framework of MCM-41.

1.2

0.8

f ............................................................... ,'-:'. 0.9

.~ 0.6

.:~.4

E r,r

O

DO

< 0.3

~.2 0.0

I

I

I

I

I

I

I

I

0'000 Wavenumber (cm")

Figure 2. Framework IR spectrum of as-synthesized sample D

300 400 2(nm)

500

Figure 3. UV-vis diffuse reflectance spectrum of calcined sample D

171

3.3.

UV-vis DRS UV-vis diffuse reflectance spectroscopy (DRS) is an especially useful technique to characterize the local titanium environment in titanosilicates [ 16]. Figure 3 depicts the UV-vis DRS of calcined sample D. Two absorption bands can be distinguished between 190-400 nm. The band centered at 230 nm corresponds to tetrahedrally coordinated Ti species that substitute for Si in the silica framework, whereas the band around 285 nm probably arises from five- and six-coordinated Ti species [17]. As tetrahedrally coordinated Ti can be hydrated and generates five- and six-coordinated Ti species, it suggests from the UV-vis DRS that more than half amount of titanium may reside in the mesoporous framework, though we can not exclude the existance of partially polymerized Ti species in the walls. However, the absence of a distinct absorption around 330 nm indicates that no anatase-like phases are formed in such a Ti-rich mesoporous titanosilicate upon calcination [16]. 3.4.

Catalytic property The catalytic activity of these solids in the selective oxidation of 4-methylphenol in the presence of dilute hydroperoxide was tested in a glass flask. In a typical reaction, 1.0 g of calcined sample D, 1.0• .2 mol of 4-methylphenol, 1.0• .2 mol of H202 and 100 mL of distilled water were mixed and stirred at 353 K. The results obtained after 5 h of reaction indicated that the catalyst was highly selective. A conversion of 37.6% based on 4methylphenol was reached and 4-methyl catechol was the sole product. Such a preliminary result reveals that sample D is highly catalytically active, which suggests that Ti ions are tetrahedrally incorporated into the MCM-41 framework. In summary, our results unambiguously demonstrate that, by a separate hydrolysis procedure during gel preparation, titanium can be effectively prevented from formation of indissoluble titanium species in the synthesis mixture at very high titanium content, thus highly ordered MCM-41 titanosilicates can be hydrothermally synthesized even from a gel with n(Ti)/n(Si) up to 1/4 and a relatively low template content. During crystallization, titanium ions were partially incorporated into the MCM-41 framework. The addition of TBAOH resulted in a more cross-linked and ordered wall structure. With the advantages of large pore diameter and ordered framework structure, the as-synthesized mesoporous titanosilicates may find special usage in catalysis field.

4.

ACKNOWLEDGMENTS

The financial support of this research by both the National Natural Science Foundation of China (through Grants No. 29733070 and No. 50102001) and the Natural Science Foundation of Tianjin, China (through Grant No. 13608111) is gratefully acknowledged.

REFERENCES 1. G. Perego, G. Bellussi, C. Corno, M. Taramasso, F. Buonomo and A. Esposito, Stud. Surf. Sci. Catal., 28 (1986) 129. 2. M. S. Rigutto and H. van Bekkum, Appl. Catal. A, 68 (1991) L 1. 3. D.P. Serrano, H. X. Li and M. E. Davis, J. Chem. Soc., Chem. Commun., (1992) 745.

172 4. M.W. Anderson, O. Terasaki, T. Ohsuna, A. Philippou, S. P. Mackay, A. Ferreira, J. Rocha and S. Lidin, Nature, 367 (1994) 347. 5. X.-H. Tang, R.-Z. Zhu, L.-R. Pan and H.-X. Li, Chem. J. Chinese Universities, 21 (2000) 517. 6. A. Corma, M. T. Navarro and J. Perez-Pariente, J. Chem. Soc., Chem. Commun., (1994) 147. 7. P.T. Tanev, M. Chlbwe and T. J. Pinnavaia, Nature, 368 (1994) 321. 8. K.A. Koyano and T. Tatsumi, Chem. Commun., (1996) 145. 9. J. E1 Haskouri, S. Cabrera, M. Gutierrez, A. Beltr~n-Porter, D. Beltr~.n-Porter, M. D. Marcos and P. Amor6s, Chem. Commun., (2001) 309. 10. C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 11. E.M. Flanigen, H. Khatami and H. A. Szymanski, Adv. Chem. Ser., ACS, 101 (1971) 201. 12. Y. Liu, W. Zhang and T. J. Pinnavaia, Angew. Chem. Int. Ed., 40 (2001) 1255. 13. Z. Zhang, Y. Han, L. Zhu, R. Wang, Y. Yu, S. Qiu, D. Zhao and F. Xiao, Angew. Chem. Int. Ed., 40 (2001) 1258. 14. C.B. Dartt, C. B. Khouw, H.-X. Li and M. E. Davis, Microporous Mater., 2 (1994) 425. 15. T. Blasco, A. Corma, M. T. Navarro and J. Perez-Pariente, J. Catal., 156 (1995) 65. 16. A. Tuel, Microporous Mesoporous Mater., 27 (1999) 151. 17. F. Geobaldo, S. Bordiga, A. Zecchina, E. Giamello, G. Leonfanti and G. Petrini, Catal. Lett., 16 (1992) 109.


Synthesis

and

Characterization

173

of

Ti-containing

Mesoporous

Alumina

M o l e c u l a r Sieves Chun Yang a.b*and Xi Li a a College of Chemistry and Environmental Science, Nanjing Normal University, Nanjing, 210097, P.R.China u National Laboratory of Molecular & Biomolecular Electronics, Southeast University, Nanjing, 210096, RR.China

Ti-containing mesoporous alumina molecular sieves with a structure of MSU-2 have been synthesized using metal alkoxides as titanium and aluminum sources and Triton X-100 as templating surfactant, respectively. The structure of the products and the state of Ti species were characterized by XRD, TEM, N2-sorption, UV-vis DRS and Laser Raman Spectroscopy. It is found that the surface area and the pore volume of sample increase when Ti is incorporated, suggesting an effect of Ti on stabilizing the alumina framework. It is also shown that most of Ti exists in the framework as an isolated mononuclear species at Ti/Al_--before

buffer

i

b)

in M E S b u f f e r

1

/

8

o

after m MES

i

,

t

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i

o

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0

1.5

2

2.5

Pore

3

3.5

Diameter

4

(nm)

4.5

5

1.5

2

2.5

3

3.5

4

4.5

Pore Diameter (nm)

Figure 5. BJH curves of the MCM-41 before and after immersion in MES buffer for 7 days; (a) post-synthesis treated MCM-41 (sample M2c); (b) HMDS-coated MCM-41 (sample M2d).

4

CONCLUSIONS

In this work, it was found that both normal and post-synthesis hydrothermally treated MCM-41 were unstable in different buffer solutions. The post-synthesis hydrothermal treatment caused an increase in the unit cell dimension, BET surface area and the pore volume relative to the untreated MCM-41, whilst the characteristic XRD pattem and type IV nitrogen adsorption isotherm were retained. The structural stability of the materials which were both hydrothermally treated and HMDS-coated was superior to the untreated MCM-41 in aqueous solutions over extended exposure times. The combination of post-synthesis hydrothermal treatment and HMDS coating has the benefits of improvement to the silica stability plus a robust hydrophobic surface coating. This protects the silica surface from water and buffer solutions but, in contrast to HMDS coating alone, causes little change to the pore volume. Hence, in this work we have investigated the stability of normal MCM-41 as well as samples which have been post-synthesis hydrothermally treated or HMDS-coated or both.

228 Tests have been performed using distilled water and three different buffers solutions commonlyused in biochemical processes for periods of up to seven days. The HMDS coating has been demonstrated to produce the most stable MCM-41 samples.

5

ACKNOWLEDGMENTS

Jing Yang gratefully acknowledges the support of China Scholarship Council and the Ministry of Education, People's Republic of China. 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) 10834. 2. X. S. Zhao, F. Audsley, G. Q. Lu, Journal of Physical Chemistry B, 102 (1998) 4143. 3. X. S. Zhao, G. Q. Lu, Journal of Physical Chemistry B, 102 (1998) 1556. 4. T. Tatsumi, K. A. Koyano, Y. Tanaka, S. Nakata, Journal of Porous Materials, 6 (1999)13. 5. A. Stein, B. J. Melde, R. C. Schroden, Advanced Materials, 12(2000) 1403. 6. M. M. L. Ribeiro Carrott, A. J. Estevao Candeias, P. J. M. Carrott and K. K. Unger, Langrnuir, 15 (1999) 8895. 7. R. Anwander, I. Nagl, M. Widenmeyer, G. Engelhardt, O. Groeger, C. Palm, T. Roeser, J. Phys. Chem. B, 104 (2000) 3532. 8. J. Kisler, PhD thesis, Univeristy of Melbourne, Australia, 2001. 9. J. Kisler, M. Gee, G. W. Stevens, A. J. O'Connor, Chemistry of Materials, submitted. 10. C. P. Tripp, M. L. Hair, Langmuir, 8(1992) 1120. 11. C. P. Tripp, M. L. Hair, Langmuir, 8(1992) 1961. 12. L. Y. Chen, T. Horiuchi, T. Mori, K. Maeda, Journal of Physical Chemistry B, 103 (1999) 1216. 13. M. Knak, M. Jaroniec and A. Sayari, Microporous and Mesoporous Materials, 27 (1999) 217. 14. R. Ryoo and S. Jun, Journal of Physical Chemistry B, 101 (1997) 317. 15. R. M. C. Dawson, D. C. Elliott, W. H. Elliott, K. M. Jones, Data for Biochemical Research, 3rd edition, Oxford University Press, Oxford, 1986. 16. S. Brunauer, P. H. Emmett, E. Teller, Journal of the American Chemical Society, 60 (1938) 309. 17. S.J. Gregg, K. S. W. Sing, Adsorption, Surface Area and Porosity, 2nd edition, Academic Press, London, 1982. 18. P. J. Branton, K. S. W. Sing, J. W. J. White, Chem. Soc., Faraday Trans., 93(1997) 2337. 19. M. Kruk, M. Jaroniec, A. Sayari, Langrnuir, 13(1997) 6267. 20. M. Kruk, M. Jaroniec, J. M. Kim, R. Ryoo, Langmuir, 15(1999) 5279. 21. M. Kruk, M. Jaroniec, A. Sayari, Journal of Physical Chemistry B, 101 (1997) 583. 22. R. K. Iler, The Chemistry of Silica, John Wiley & Sons, New York, 1979.


229

Adsorption o f CO on Zn-Cu(I)/HMCM-41 Qihong Shia, Nongyue He, b'c* Fei Gao, a Yibing Song,a Yang Yu a and Huilin Wan d a

Department of Chemistry, Shantou University, Shantou, 515063, Guangdong, P. R. China

b Key Lab of New Materials and Technology of China National Packaging Corporation, Zhuzhou Engineering College, Zhuzhou 412008, P. R. China c Key Laboratory for Molecular and Biomolecular Electronics of Ministry of Education, Southeast University, Nanjing 210096, P. R. China d The State key Laboratory for Physical Chemistry of Solid Surfaces, Xiamen University, P. R. China Using mesoporous acidic HMCM-41 as parent, the Zn-Cu(I)/HMCM-41 catalysts, which can reversibly adsorb and release CO, were successfully prepared in laboratory scale by means of the solid-state ion exchange together with introducing Zn as an assistant to improve the dispersion degree of the active component Cu(I) on the surface. With increase of the loading amounts of Zn and CuC1 from 0% to 9.0% and 25.0% respectively, its CO adsorption amounts increased from 10.6 pmol/g to 183.0 pmol/g correspondingly. The FT-IR in situ characterization for CO adsorption demonstrated that there existed two dynamic equilibriums between surface carbonyl complexes: Cu(CO)3+Cu(CO)2++ CO and Cu(CO)2+r Cu(CO) + + CO. The equilibriums can be shifted reversibly by changing the temperature and pressure. At the same time, adsorption and release of CO is accompanied, which is the possible CO source for carbonylation reactions. Our recent research reveals that a starting alcohol was protonated at the acid sites of modified microporous Y, 13 or ZSM-5 and transformed into carbonium ions, from which the carboxylic acids (-

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.

,

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.

a

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Relative pressure p/Po

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~o 20 go ~o Pore diameter (nm)

Figure 3 9 Nitrogen adsorption isotherms (A) and pore size distribution (B) of samples obtained at 80~ a" 0.5, b" 1, c 91.5 and d 92 days. of equal size. H2 type hysteresis loop is typical for wormhole structured materials such as DWM [26]. Figure 4 depicts the variation of the specific surface area with heating time and temperature. A t 60~ (curve b of Fig. 4): After 4 days of hydrothermal treatment, the value of the specific surface area decreases from 375 to 318 m2/g. Referring to the typical 4 steps crystallization curve observed for zeolites (steps I : the nucleation, II :the growth of crystals, III : the crystallization and IV : the amorphisation) [28] or mesoporous molecular sieve synthesis (step I : the hydrolysis of inorganic source in aqueous solution, step II : the polycondensation of inorganic source around micelles, step III "the continuation of polycondensation and the formation of mesostructures and step IV : the destruction of the latter) [29] this can be attributed to the destruction of the structure, i.e. the step IV. Neither step I nor step II are detected. These two steps are already achieved during the preparation of the gel. The destruction of the structure can be related to the crystallization of the amorphous wall of the zirconia mesoporous molecular sieve.

262 600 500 N

400

.... ::::::::::::::::::::::::

........

......................... .a...,..

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......

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300 0

'-

"

200

r~

9~ O

b

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,~

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Hydrothermal time (days) Figure 4 9Variation of the specific surface area with heating time and temperature a" 40, b" 60 and c" 80~

t~ A t 80 ~ (curve c of Fig. 5) 9the variation of the specific surface area with heating time is analogous to that reported for 60~ but drops atter a very short time, indicating the destruction of the mesostructure. r At 40~ (curve a of Fig. 4): The value of the specific surface area increase from 320 to 385 mVg. This part of the curve correspond to the second step of the crystallization curve, the mesoporous zirconia begins to be formed. Then, the specific surface area remains constant to 380 mVg, step III is reached.

3.3. Discussion From the results obtained in the present study and those reported previously, an electrostatic pathway, based on a supramolecular assembly of charged surfactants with charged inorganic precursors, is employed for the preparation of zirconia mesoporous materials (Fig. 5). In the presence of sodium hydroxide, the hydrolysis and the polymerization of zirconia precursor around the preformed micelles of surfactant in aqueous solution, takes place. In the first step, the synthesized material is supermicroporous, then it becomes mesoporous if the hydrothermal treatment duration is prolonged. This transformation is favored by an increase of the temperature (step2). In a paper dealing with the use of amine as expander and postsynthesis treatment to increase the pore sizes of MCM-41 silicas, Sayari et al [30] have concluded that there is a possibility that walls between pores may break during the process of pore expansion, in such a way that pairs or triplets of adjacent pores transform into single but larger pore. In our case, we can also consider such a mechanism to explain the transformation from a supermicroporous molecular sieve to a mesoporous one. The obtained materials exhibit specific surface areas up to 400 m2/g. However, the channels are not well organized, the compounds have a structure with wormlike channels such as reported for DWM samples [26]. If synthesis time or temperature are further increased, crystalline particles of ZrO2 appear in our compounds, as is proved from the TEM micrographs. The presence of tetragonal and monoclinic zirconia is confirmed by XRD, the peaks characteristic of theses structures are pointed on the XRD pattern (step 3). Thus too long heating time or too

263 high synthesis temperatures lead to the crystallization of the mesoporous walls. The structure collapses and only an interparticular porosity remains (step 4). 4. CONCLUSION The present study reveals that zirconia molecular sieves can be obtained via an electrostatic assembly, using cetyltrimethylammonium bromide as surfactant and zirconyl chloride as inorganic precursor. The optimization of the synthesis conditions leads to the formation of mesoporous zirconia without addition of some phosphate or sulfate anions. The template was removed by ethanol extraction. The different formation steps have been clearly evidenced. A synthesis mechanism to describe the evolution of materials formed in autoclave has been postulated. In a first step the material is supermicroporous, then a breakdown of the wall leads to the formation of the mesoporous molecular sieves with high specific surface area (400 mVg). However the prepared compounds belong to the family of DWM-2, i.e. materials with a disordered wormhole-like structure and inhomogeneous channel size distribution. Finally, too high temperature or too long hydrothermal treatments are responsible of the crystallization of the walls and the structure collapses. No mesoporosity is detected any longer. Particles of crystalline zirconia are observed by TEM and tetragonal and monoclinic ZrO2 are detected by X-ray diffraction analysis Step 2 MESOPOROUS

Stepl SUPERMICROPOROUS

ZrOC12,SH20 ~-Mieellar solution of CTMABr

ads

time

polymerization ofzirconia

T~

k,

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Step 3 Begimming of the

time

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T~

Step 4

~ Crystallization

d~, gads

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Figure 5 9Proposed mechanism for mesoporous zirconia synthesis.

p/p0

264 ACKNOWLEDGEMENT

:

This work has been performed within the framework of PAI/IUAP 4-10. Alexandre L6onard thanks FNRS (Fond National de la Recherche Scientifique, Belgium) for a FRIA scholarship. REFERENCES

1.

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

28. 29. 30.

J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.TW. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schenker, J. Am. Chem. Sot., 114 (1992) 10834 C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710 P.L. Llewellyn, Y. Ciesla, H. Decher, R. Stadler, F. Schtith and K.K. Unger,. Stud. Surf. Sci. Catal., 84 (1994) 2013. J. Aguado, D.P. Serrano, M.D. Romero and J.M. Escola, Chem. Comm., (1996) 765. A. Corma, M. Iglesias and F. Sanchez Catal. Lett., 39 (1996) 153. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. A. Corma, Chem. Rev., 97 (1997) 2373. A. Tuel, S. Gontier and R. Teissier, Chem. Comm., (1996) 651. D.J. Jones, J. Jimenez, A. lopez, P. Torres, P. Pastor, E. Rodriguez and J. Rozi6re, Chem. Comm., (1997) 431. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P.M. Petroff, F. Schtith and G.D. Stucky, G.D, Nature, 368 (1994) 317. D.M. Antonelli and J.Y.Ying, Angew. Chem. ,Int. Edn. Engl., 35 (1996) 426. S.A. Bagshaw and T.J. Pinnavaia, Angew. Chem. Int. Edn. Engl., 35 (1996) 1102. Z.R. Tian, W. Tong, J.Y. Wang, N.G. Duan, V.V. Krishnan and L.S. Suib, Science 276 (1997) 926. U. Ciesla, S. Schacht, G.D. Stucky, K.K. Unger, K.K. and F. Schtith, Angew. Chem. Int. Edn. Engl., 35 (1996) 541. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka and G.D. Stucky, Nature, 396 (1998) 152. Y. Inoue Y. and H. Yamazaki, Bull. Chem. Soc. Jpn., 60 (1987) 891. A. Clearfield, Inorg. Chem., 3 (1964) 146. T. Yokoyama, T. Setoyama, N. Fujita, M. Nakajima and T. Maki, Appl. Catal. A, 4 (1992) 149. J.A. Knowles and M.J. Hudson, Chem. Comm., (1995) 2083. A. Kim, 2 P. Bruinsma, Y. Chen, L.Q. Wang and J. Liu, Chem. Comm., (1997), 161. G. Pacheco, E. Zhao, A. Garcia, A. Sklyarov and J.J. Fripiat, Chem. Comm., (1997) 491. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, B.F. and G.D. Stucky, Chem. Mater., 11 (1999) 2813. F. Del Monte, W. Larsen and J.D. Mackenzie, J. Am. Ceramic Soc., 83 (2000) 1508. G. Pacheco, E. Zhao, A. Garcia, A. Sklyarov and J.J. Fripiat, J. Mater. Chem., 8 (1998)219. J.L. Blin, R. Flamant and B.L. Su, I. J. Inorg. Mater., 3 (2001) 959. J.L. Blin, A. L6onard. and B.L. Su, Chem. Mater., 13 (10) (2001) 3542. M.M. Dubinin in : Progress in Surface and Membrane Science, 9 (D.A. Cadenhead, ed) Academic Press, New York, (1975), p. 1. D.W. Breck, Zeolite Molecular Sieves, John Wiley & sons, New York, (1974). J.L. Blin, C. Otjacques, G. Herrier and B.L. Su, I. J. Inorg. Mater., 3 (2001) 75. A. Sayari, M. Krtak, M. Jaroniec and I.L. Moudrakovski, Adv. Mater., 10 (1998) 1376.


265

Single crystal particles o f mesoporous (Nb, Ta)205 Junko N. Kondo, a Tomohiro Yamashita, a Tokumitsu Katou, a Byongjin Lee, a Daling Lu, a' b Michikazu Hara a and Kazunari Domen a, b a Chemical Resources Laboratory, Tokyo Institute of Technology, 4259 Nagatsuta, Midoriku, Yokohama 226-8503, Japan

b Core Research for Evolutional Science and Technology, Japan Science and Technology, 21-13 Higashiueno, Taito-ku, Tokyo, 110-0015, Japan

Mesoporous Nb-Ta mixed oxide, (Nb, Ta)2Os, with whormhole mesopore structure was prepared by using a block co-polymer template and metal chlorides in ethanol. The mesoporous (Nb, Ta)205 calcined at 673 K for 20 h showed 140 mZ-gl of BET surface area and 3.0 nm of pore size. The amorphous wall of the mesoporous (Nb, Ta)205 was crystallized by further calcination at 923 K for 1 h. The BET surface area and pore size estimated by N2 adsorption isotherm of the crystallized (Nb, Ta)205 were 48 m2.g-~ and 10.0 nm, respectively. Detailed analysis of transmission electron microscope (TEM) and electron diffraction (ED) revealed that each particle of sub-micron size was a mesoporous single crystal. The pore size and crystallinity observed by TEM were in good agreement with N2 adsorption-desorption isotherms and powder X-ray diffraction patterns. Similar mesoporous (Nb, Ta)205 single crystal particles were also obtained by using amorphous precursor prepared by ligand-assisted templating method. Therefore, mixing two similar elements, Nb and Ta, is suspected to be beneficial for crystallization with sustaining the mesoporores.

1. INTRODUCTION Tantalates are found to be highly active photocatalysts for overall water decomposition under UV irradiation [1]. These tantalates are prepared by solid-state reactions and are completely crystallized, where electron mobility is high. The mesoporous metal oxides with high surface area and crystallized wall structures are expected to be advantageous not only for photocatalysis but also as various catalysts. For the purpose of development of new types of photocatalyst, we synthesized some mesoporous tantalum oxides [2-5], and studied their photocatalysis. In the studies on crystallization, we found that mesoporous (Nb, Ta)205 prepared by mixing Nb and Ta metal sources was crystallized with remaining mesopores in crystallized lattice structure, although pure Nb205 and Ta205 resulted in destruction of mesopores upon crystallization. In this study, production of mesoporous (Nb, Ta)205 single crystal particles is reported, and relation of the amorphous precursors and the crystallized product is discussed.

266 2. MATERIALS AND METHODS 2.1. Materials

Mesoporous Nb2Os,Ta205 and (Nb, Ta)205 were synthesized by two methods, a method using a block co-polymer surfactant as a template [4-7], and an improved ligand-assisted templating (denoted as LAT method) [2, 3, 8]. In the block co-polymer templating method, 0.01 mol of NbC15 or TaC15, or 0.005 mol of both TaC15 and NbC15 were added to 10 g of ethanol containing 1 g of poly (alkylene oxide) block copolymer, Pluronic P-123. After vigorous stirring for 30 min, the resulting sol solution was transferred to a Petri dish for aging at 313 K for 6-10 days. The surfactant was removed by calcination at 673 K for 20 h. In LAT method, octadecylamine (6.15 mmol) was mixed in Nb(OEt)5 or Ta(OEt)5, or Ta(OEt)5 and Nb(OEt)5 (12.30 mmol in total) under Ar gas atmosphere, and warmed to 323K for 10-30 min. Then was added deionized water (25 mL). The precipitate was then washed with water and ethanol. Aging was carried out at 353 K for 1 day, 373 K for 1 day, and 453 K for 7 days, successively. The product was washed with the deionized water, ethanol, and diethyl ether. The powder was then dried at 373 K for 12 h in the atmosphere. The surfactant-containing sample (1 g) was treated with trifluoromethane sulfonic acid in dimethoxyethane at 195 K for 1 h with stirring, followed by warmed to the ambient temperature. The powder Was washed with 2-propanol, deionized water, ethanol, and diethyl ether, and then dried in evacuation at 373 K within 12 h. 2.2. Measurements

X-ray diffraction (XRD) patterns were obtained on a Rigaku R1NT 2100 diffractometer using Cu K.ctradiation. The TEM images were obtained using a 200 kV JEOL JEM201 OF. Nitrogen-gas adsorption-desorption isotherms were measured by Coulter Omnisorp 100CX and SA-3100 systems. Differential thermal analysis (DTA) and thermogravimetry (TG) were performed using a Shimadzu DTG-50 in air at a heating rate of 5 or 10 K'min -1. 2.3. Methods

The BET specific surface area was calculated in the relative pressure range between 0.05 and 0.2. The pore-size distributions were determined by BJH (Barrett-Joyner-Halenda) analysis using the adsorption branch.


Crystallization of a mesoporous transition metal oxide was first attempted by using Ta205 prepared by LAT method. The BET surface area and the pore size of the as-prepared wormhole mesoporous sample after chemical extraction of the surfactant were 410 m2"g~ and 3.3 nm, respectively [3]. The wall thickness of the as-prepared sample estimated by simple subtraction of the pore size from d(100) value was 1.1 nm, which is considerably thin. When the mesoporous Ta205 was calcined at 673 K for 20 h before crystallization, N2 adsorption isotherm as shown in Figure 1(a) was observed, which is analogous to type IV pattern. The BET surface area decreased to 330 m2.g-l, and the pore size was broadly distributed to 3.0 nm. The type IV isotherm as well as TEM observation still evaluate the sample calcined at 673 K as a mesoporous material. The crystallization condition of Ta205 sample was

267 determined by TG-DTA analysis together with XRD observation of samples calcined at various temperatures. By calcination at 1023 K for 1 h for crystallization, the BET surface area estimated from the N2 adsorption isotherm shown in Figure 1(b) decreased to only 16 m2.g-~, and the sample is no more regarded as mosoporous material, although a clear XRD pattern of orthorhombic Ta205 was obtained. 100

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Figure 2. N 2 adsorption-desorption isotherms ofmesoporous Ta20 5 (A) and (Nb, Ta)205 (B) prepared using block co-polymer template after calcination at 673 K for 20 h (a) and after calcination at crystallization temperatures (1023 K (A) and 923 K (B) for 1 h, (b)). Filled and open marks corrspond to adsorption and desorption branches, respectively. The BET surface area of the mesoporous material of pure Nb205 prepared by block copolymer templating method was remained at 45 m2.g-~ after crystallization at 873 K, and the

268 crystallized mesoporous Nb205 was once expected to be formed. However, only a ring ED pattern was obtained from a crystallized particle instead of a sharp spot ED pattern of a single crystal (Details are mentioned below). Therefore, the each particle is regarded as "an aggregates of small crystals, and the mesoporosity is attributed to the interparticle space. We then mixed Ta with Nb for the purpose of decreasing the crystallization temperature of homogeneously mixed oxides due to their same oxidation number (V) and the same ionic radii (0.78 A) in hexa-coordination in oxides. The crystallization temperatures, which were determined by exothermal DTA peak above 773 K, of Nb205 (848 K) and Ta205 (1018 K) agree well with those for non-porous materials. The gradual and continuous change in crystallization temperature was observed depending on the Nb/Ta ratio between pure Nb205 and Ta205 (not shown), which is indicative of the homogeneous mixing of the Nb and Ta in mixed oxides. The Nb/Ta ratios were also confirmed to be "as prepared" by elemental analysis of the samples before and after crystallization using TEM apparatus in c a . 5 n m ranges. Although Nb-Ta mixed oxides at different Nb/Ta ratios were prepared and studied, results are focused on thesample at Nb/Ta = 1 are shown in this study (denoted as (Nb, Ta)205 hereafter). The as-prepared (Nb, Ta)205 indicated type IV adsorption isotherm pattern typical to mesoporous materials, and the BET surface area was 140 m2.g-1 (Figure 2B(a)). The pore size distribution was centered at 3.0 nm. The presence of only (100) diffraction peak at d(100) = c a . 7.0 nm in low-angle XRD pattern (Figure 3) indicates the wormhole mesoporous structure, which is also confirmed by TEM images (not shown). '

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269

Figure 4. TEM image of a crystallized mesoporous (Nb, Ta)2 0 5 and ED patterns from the whole particle (top) and 4 different areas.

A TEM image and ED patterns from a whole particle and several areas (ca. 200 nm in diameter) indicated as dotted circles are shown in Figure 4. The wormhole mesoporous structure is observed in the image, and the sharp spots in ED pattern from the whole particle indicates that the particle is not a polycrystal but only one crystal domain exists in the particle. Furthermore, the ED patterns from several different areas are coincident with that obtained from the whole particle. This is a clear evidence that a mesoporous (Nb, Ta)205 particle is a single crystal. The presence of mesopores in crystallized lattice was clearly observed in high resolution images as shown in Figure 5 [2, 3]. The direction of lattice fringes though out mesopores was the same.

Figure 5. High resolution TEM image of mesoporous (Nb, Ta)205 single crystal particle.

50 particles in sub-micron size of the crystallized mesoporous (Nb, Ta)205 were analyzed in the same manner, and 45 particles resulted in the same images and ED patterns as those

270 observed in Figure 4. Therefore, crystallinity of the sample was estimated as ca. 90 %. The rest of the particles remained amorphous without showing any ED spots. Assuming that the mixing of Nb and Ta is effective, same strategy would be successful for LAT method. Mesoporous (Nb, Ta)205 was prepared by mixing equivalent amount of Nb(OEt)5 and Ta(OEt)5 in LAT method. 100

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The mesoporosity of the samples was evaluated by N2 adsorption-desorption isotherms as shown in Figure 6. The BET surface area of the (Nb, Ta)205 after calcination at 673 K was 206 m2.g-~, and there was no peak in the pore size distribution which spread to 4.0 nm. The similar mesoporous (Nb, Ta)205 material was prepared by LAT method after calcination at 673 K to the case of pure Ta2Os. However, the effect of mixing Ta to Nb in the mixed oxide was observed after crystallization. The sample was crystallized at lower temperature calcination (923 K) than Ta205. The N2 adsorption-desorption isotherm of the crystallized sample indicated type IV pattern typical to mesoporous material. It is noticed that the crystallized (Nb, Ta)205 material prepared by LAT method and that prepared by using block co-polymer template resulted in similar material judging from the isotherms. Therefore, we also observed TEM images and ED patterns. A TEM image in Figure 7 (middle) demonstrates the presence of mesopored in a crystallized (Nb, Ta)205 particle, and the ED pattern from the whole particle indicates that the observed particle is not a single crystal but consists of a few crystal domains. Because the ED patterns in Figures 3 and 7 are obtained along different zone axes, they should not be necessarily coincident. However, the ED pattern in Figure 7 is clearly a set of spot-patterns of a few single crystals, although the presence of mesopores in the lattice image was confirmed in high magnification image (left). It is noted that small particle (< 100 nm in diameter) were single crystals. Therefore, the size of the single crystal domains of the (Nb, Ta)205 material is considered to be smaller when it is prepared by LAT method than that prepared by block co-polymer templating method. Interestingly, considerably different mesoporous (Nb, Ta)205 with amorphous wall produced similar crystallized mesoporous material. We therefore, tentatively regard the nature of the element is an important factor rather than the preparation method in the present study. The effect of mixing Nb to Ta in oxide is probably due to the low surface tension of Nb2Os, which also decreased the surface tension of mixed oxide and prohibited the pore collapse and formation of aggregates. Similar phenomenon was observed for Nb205 ultrafine 1Mrticles [9].

271

Figure 7. High resolution TEM image and an electron diffraction pattern of crystallized mesoporous (Nb, Ta)20 5 particle (middle) prepared by LAT method. There occurred a drastic change in material appearance. The wall thickness of the as-prepared mesoporous (Nb, Ta)205 by LAT method and block co-polymer templating method were 1.1 and 4.0 nm, respectively, while that of the crystallized sample is estimated as c a . 10 nm (see Figure 5 and 7). The pore size of the amorphous mesoporous (Nb, Ta)205 prepared by both methods expanded from c a . 3 to c a . 10 nm. Therefore, the material transfer of (Nb, Ta)205 consisting the wall upon crystallization is more for the precursor prepared by LAT method, resulting in the smaller crystal domains, i.e. poor formation of single crystal particles. All the amorphous (Nb, Ta)205 precursors had wormhole mesoporous structure, and the crystallized mesoporous (Nb, Ta)205 also consisted of wormhole mesopores. Although detailed phenomena occurring during crystallization are of interest, non-ordered mesoporous structure of the material prohibited the clarification of the crystallization process. Therefore, we prepared a hexagonally ordered mesoporous (Nb, Ta)205 by optimizing the preparation method using a block co-polymer in order to proceed in-situ observation by TEM during crystallization. Briefly, the amount of metal source, TaC15 and NbC15 was decreased to 0.0025 mmol each (0.005 mmol in total), a half of the original amount. A small amount of water was added before aging for the improvement of the ordered structure of mesopores [10].

Figure 8. N 2 adsorption isotherm (A) and low-angle (B~(RD pattern of hexagonally ordered mesoporous (Nb, Ta)205.

272 The type IV adsorption isotherm was observed (Figure 8A), and the BET surface area and pore size were 193 m2-g~ and 5.5 nm, respectively. The wall thickness estimated by the pore size and the repeat distance obtained from d(100) value (Figure 8B) assuming hexagonal mesopore structure was c a . 2.0 nm. As shown in Figure 9, hexagonally ordered mesoporous structure was observed by TEM together with ED pattern (inset). The pore size and the wall thickness mentioned above agreed with the values estimated from a high resolution TEM image. Now crystallization of this material is under examination.

Figure 9. TEM image and an electron diffraction pattern of a hexaganally ordered mesoporous (Nb, Ta)20 5 particle.

4. ACKNOWLEDGMENTS This work was supported by the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology (JST) Corporation.

REFERENCES

1. 2.

H. Kato and A Kudo, Chem. Phys. Lett., 295 (1998) 487. Y. Takahara, J. N. Kondo, T. Takata, D. Lu and K. Domen, Chem. Mater., 13, (2001) 1194. 3. J.N. Kondo, Y. Takahara, T. Takata, D. Lu and K. Domen, Chem. Mater., t3, (2001) 1200. 4. B. Lee, J .N. Kondo, D. Lu and K. Domen, Stud. Surf. Sci. Catal., 135, 07-P-15 (2001). 5 B. Lee, J. N. Kondo, D. Lu and K. Domen, Chem. Commun., submitted. 6. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Nature, 396 (1998) 157. 7. P. Yang, D. Zhao, D. I. Margolese, B. F. Chmelka and G. D. Stucky, Chem. Mater. 11 (1999)2813. 8. D.M. Antonelli and J.Y. Ying, Chem. Mater., 58 (1996) 874. 9. P. Nair, J. Nair, A. Raj, K. Maeda, F. Mizulami, T. Okubo, and H. Imitsu, Mater. Res. Bull., 34 (1999) 3. 10. T. Katou, J. N. Kondo, D. Lu and K. Domen, in preparation.


273

Preparation o f exfoliated zeolites from layered precursors: The role o f pH and nature o f intercalating media Wieslaw J. Roth and James C. Vartuli Corporate Strategic Research, ExxonMobil Research and Engineering, Annandale, NJ 08801. The ability to modify interlayer separation and arrangements in lamellar solids by appropriate chemical treatment has been exploited to generate novel porous materials. Treatments such as intercalation or delamination may be quite demanding and sensitive to various factors. This is illustrated by the zeolitic layered material, MCM-22 precursor, which can be swollen with a cationic surfactant but requires a high pH environment. The resulting product, swollen MCM-22 precursor, may then be converted to the pillared mesoporous material, MCM-36, or to a delaminated solid with randomly oriented layers. This report deals with the issues associated with swelling of the layered MCM-22 precursor, which is the critical step. We focus on the problem of swelling efficiency when the required surfactant medium with high pH is a mixture of a base (NaOH or tetraalkylammonium hydroxide) and cationic surfactant chloride. Only the combination with tetrapropylammonium hydroxide produced swelling, while the methyl and ethyl homologues did not. This indicates dependence on the nature of cations present in the swelling medium. We also discuss in detail the criteria for identification and quantification of the swollen product and avoidance of potential impurities. 1. INTRODUCTION Two-dimensional (layered) materials are characterized by relatively weak interlayer bonding [ 1]. The layers can in principle be separated by appropriate treatment, for example, by intercalation of guest molecules between the layers. This potential to manipulate the interlayer separation and spatial arrangement of the sheets has been exploited to generate novel materials [2,3]. Pillared clays and layered oxides with permanent props between the layers exemplify new catalysts with increased and controlled pore sizes prepared by this approach. While zeolites are considered rigorously 3-dimensional solids, one of them, MCM-22 [4] was recognized as existing in a lamellar form, called MCM-22 precursor [5,6], prior to any treatment that chemically locks the layers into the typical rigid zeolite solid. The precursor is composed of 25 A sheets stacked in registry. Each sheet by itself can be considered a zeolite because of its MWW internal connectivity and pore system. The 3-D zeolite framework is generated as the layers condense upon calcination, with concomitant contraction of the repeat distance in the stacking direction by about 1.5 A. The existence of the layered MCM-22 precursor was exploited to make a novel material, pillared zeolite, MCM-36, which combines strong acidity with mesoporous character [6,7]. This field, initiated by MCM-22, has been recently expanded to ferrierite, as the corresponding layered precursor was discovered [8].

274 The layer separation in MCM-22 precursor by intercalation, referred to as swelling or delamination, proved very challenging due to strong interlayer bonding. It was eventually accomplished with a solution of a cationic surfactant, hexadecyltrimethylammonium chloride, having high pH equal to 13.8 as the result of partial substitution of halide for hydroxide anions [6]. This work focuses on usage of a small base/surfactant salt combination as the alternative to the initially used reagent. Apart from the attempted replacement of this exotic chemical, the reported study illustrates the difficulty and pitfalls associated with swelling MCM-22 precursors. The report describes identification of the swollen product, detection of the unswollen phase (if present), and the possible presence of mesoporous impurities. Finally we propose a rationale for the observed relationship between the nature of the base cation and the success/failure of the swelling treatment. 2. EXPERIMENTAL SECTION The synthesis procedures are described elsewhere [6,7]. Briefly, MCM-22 precursor with silica/alumina molar ratio of approximately 23/1 was synthesized hydrothermally in the presence of hexamethyleneimine as the structure directing agent. The surfactant hydroxide solution (pH = 13.8) was obtained by contacting 29 % hexadecyltrimethylammonium chloride (CI6TMA-C1) solution (Akzo) with an anion exchange resin IRA-400(OH) - about 30 % of the C1 anions were replaced. The high pH surfactant mixtures used in subsequent experiments were prepared by adding concentrated MOH solution (M is Na or tetraalkylammonium cation) to the above 29 % C16TMA-C1 solution until pH 13.8 was reached. Swelling experiments were carried out with less than 10 % solid content at 95-100~ for 1-2 days. In the case of sodium, amorphization of the zeolite occurred and the experiment was repeated at room temperature. X-ray powder diffraction scans (XRD) were recorded using a Scintag diffractometer. 3. RESULTS AND DISCUSSION 3.1 Delamination of MCM-22 Precursor with Surfactant Hydroxide, R(CH3)3N-OH The separation of layers in MCM-22 precursor was first achieved by intercalation of long chain quatemary ammonium cations, CI6H33(CH3)3N§ by a unique treatment developed for that purpose. The swelling of layered materials, like clays and silicates, is possible under relatively mild conditions with organic amines [ 1-3,9]. All of the known treatments failed with the MCM-22 precursor. The latter proved swellable only under rather severe high pH environment with no other cations present to compete with the intercalating cationic surfactant species. The appropriate reagent was obtained by performing anion exchange with the solution of Ct6H33(CH3)3N-C1, resulting in replacement of ca. 1/3 C1 with OH anions (hereto designated CI6TMA-OH).

Non-covalent but relatively strong interlayer bonding is the apparent reason for the described difficulty in swelling the MCM-22 precursor. The in-registry alignment of the layers concluded from the distinct, albeit broadened, interlayer hkl reflections in the XRD pattern also supports the existence of some relatively strong chemical linkage. Hydrogen bonding between silanols on the surface of each layer with possible involvement of the templating HMI

275 molecules is the primary candidate. This is further supported by the observation that formation of the oxygen bridges between layers upon calcination involves unit cell contraction by ca. 1.5 A, consistent with conversion of SiOH into Si-O-Si moieties. In this context the high pH requirement for successful swelling may be rationalized by the following chemical reaction: Si-O-+ H20

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30

Figure 2. Expanded X-ray powder diffraction pattem, from Figure 1, of MCM-22 precursor and the CI6TMA-OH swollen derivative showing features critical for diagnosing surfactant intercalation and layer separation.

276 In this case high pH promotes elimination of hydrogen bonding by deprotonation of silanols. The generated negatively charged SiO- centers simultaneously repel each other and attract the intercalating long chain surfactant cations. An interlayer bonding scheme involving pairs of opposing silanol groups has been proposed in the report on the structure of the layered silicate KHSi205 [ 10]. 3.2 Identification of The Swollen MCM-22 Precursor Another challenge associated with delamination of MCM-22 precursor was proving successful swelling and the development of diagnostic tools for distinguishing between complete and partial swelling. This was non-routine although X-ray diffraction seemed to provide some obvious answers. Successful swelling was found to result in expansion of the crystallographic unit cell in the c-direction by about 25-30 A (corresponding to 50-55A repeat; the expansion is consistent with thickness of the CI6TMA+ bilayer). Based on that, the following effects could be expected in the XRD pattern:

1. general hkl reflections shift to lower 20 angles or disappear altogether, 2. hk0 reflections remain invariant, 3. a prominent 001 peak emerging around 50-55 A d-spacing, possibly accompanied by higher order peaks at appropriate positions. In practice, only the last two have been rigorously obeyed (see Figure 1). The first prediction was impossible to analyze over the entire region because the XRD pattern of MCM-22 precursor and its swollen derivative appeared too complex and with broadened peaks to permit unambiguous peak deconvolution and assignment. Subsequently experience showed that the XRD region up to 10 degrees 20 is sufficient to discern the efficiency of swelling/layer separation. The diagnostic features are marked in Figure 2. The disappearance of the 002 reflection at 6.5 ~ is obvious. The merging of the 101 and 102 peaks observed in MCM-22 at 20 angles 8~ and 10~ into a broad band and a new peak at-~5.5 ~ are still empirical but consistently observed in samples deemed successfully swollen. The independent criteria corroborating swelling were TEM examination (showing exfoliated layers) and successful preparation of the pillared derivative MCM-36 (proven by XRD, TEM, unique sorption features) [6]. 3.3 Detecting Presence of Unswollen Phase The primary concern regarding the swelling of MCM-22 precursor was determining if all precursor was exfoliated. The dominating 001 reflection at ca. 55 ~ could not be to used to establish complete swelling. The peak intensity appeared too sensitive to factors such as water content, possible preferred orientation, particle size, etc, to provide anything but a qualitative measure. The criterion for estimating the amount of unswollen phase was empirical and involved judging the extent of peak separation in the range 8-10 ~ A band without a trough in the middle suggests negligible unexfoliated component. And vice versa- the magnitude of a dip, if any, in the band indicates contribution from unswollen phase. This criterion carries over to the pillared species, MCM-36, which can be appraised in the similar manner.

277

3.4 Ruling out MCM-41 and Mesoporous Impurities An obligatory feature of the XRD pattern of surfactant intercalated MCM-22 precursor is the prominent low angle 001 line, which usually occurs at d-spacing >50 A. This may be thought sufficiently distinct from 40-45 A, typically seen for MCM-41 or related materials, which may form under comparable conditions [ 11]. Nonetheless, the possible presence of these mesoporous phases cannot be dismissed outfight. The synthesis conditions are conducive to M41S generation. In particular, high pH may result in partial dissolution of the MCM-22 precursor thus supplying silicate, which may combine with the surfactant and generate M41S. The absence of M41S phase at this stage can be determined by calcination of a small portion of the swollen product. We observed that swollen MCM-22 precursor converted to MCM-22 upon calcination. When M41S impurity was present the low angle line in XRD was maintained upon calcination and/or the product had increased BET and sorption compared to MCM-22. The initial studies in this area also relied on extensive TEM examination of samples and no M41 S-like patterns were observed. 3.5 Delamination Attempts with MOH/C16TMA-CI Mixtures As discussed above the delamination of MCM-22 precursor was achieved by treatment with a cationic surfactant solution under conditions of high pH generated by partial substitution of chloride with hydroxide. Subsequent studies explored swelling using solutions obtained by mixing the surfactant chloride solution and a base, such as NaOH or tetraalkylammonium hydroxide, as the high pH source. The corresponding XRD patterns (see Figure 3 and Table 1) show that nature of the cation accompanying the hydroxide determines whether swelling occurs or not. Among the four hydroxides investigated: sodium, tetraalkylammonium - methyl (TMA; XRD not shown), ethyl (TEA) and propyl (TPA), only the last allowed swelling with the surfactant. (/) r

no trough

1E

..(3 i._ .m

016

Z" v

1=:

(1) C: m

!

"

\.

A

.A

A .....

TPA-O./OI0TM

-O,J

L-

,I,,I

m

TEA-OH/C16TMA-CI~..~j

002.--------~ ~ tl

i

1

i

5

i

trough

i

i

10

2 + (degrees)

i

i

15

i

i

20

Figure 3. XRD patterns of MCM-22 precursor after treatment with different swelling solutions.

278 This behavior may be related to the cation size in the following manner. TPA cations may be excluded from the interlayer region as too large. This would allow the surfactant molecules to migrate in with concomitant swelling. In contrast; TEA and the smaller cations appear small enough to diffuse in between the layers and to affect ability of the surfactant cations to enter and/or cause swelling. The interlayer separation in MCM-22 precursor is estimated around 5 A or greater since condensation upon calcination, which is accompanied by contraction, produces a 10-member ring aperture (4.1 x5.5 A). There is also a possibility that cation interaction with the SiO- moieties on the surface of the layers is the responsible or contributing factor. The following series reflects the ability of cations to enter the interlayer space and/or interact with SiO-moieties in the MCM-22 precursor: N a, TMA, TEA > C I6TMA > TPA Swelling is possible when interaction with CI6TMA is favored, which occurs when TPA but not the other cations are present. The product compositions shown in Table 1 indicate that a significant pickup of the organic species occurs even without swelling. Apparently, in the unswollen products surfactant molecules accumulate on the surface and possibly between the layers in a horizontal orientation. Table 1 Properties of the MCM-22 precursor before and after treatment with swelling mixtures MCM-22 Precursor

MCM-22 precursor treated with

NaOH/ TEA-OH/ TPA-OH/ C16TMA.OH C16TMA-CI C16TMA-CI C16TMA-CI unswollen

XRD features

002 peak, deg. 8-10 deg. region 5.5 deg. peak Composition, wt. % SiO2 AI203

Molar ratio

6.5 -6.2 -6.2 separated 101 and 102 peaks no no no 73.80 5.3

48.00 6.1

54.60 6.4

swollen

absent band without trough present 38.90 4.7

44.20 3.3

Na

1.4

1

0.35

0.2

0.02

N

1.85

2.12

2.13

2.29

2.34

C

9.5

26

24.2

35.6

32.2

SiO2/AI203

23.7

13.4

14.5

14.1

22.8

(Na+N)/AI C/N

1.9 6 **

1.6 14.3

1.3 13.3

1.9 18.2

2.6 16.1

12.9 0.16

32.5 0.60

30.4 0.50

43.8 1.01

39.9 0.84

Estd. organic content wt% * Organic/solid weight ratio

*Sum of wt % of C+N+H; hydrogen content approximated at twice the molar amount of carbon **Fixed based on composition and used to calculate % C.

279 The silica/alumina molar ratios of the swollen products in Table 1 show an interesting trend. All of the MOH/CI6TMA-C1 treated samples show the value -14/1, which is much lower than the original 23/1. This indicates significant dissolution of silica and probable destruction of some portion of the crystalline product. In comparison, the CI6TMA -OH swollen product retained its original composition. This aspect of the use of MOH/CI6TMA-C1 mixtures for swelling suggests the need for caution and may warrant closer attention. CONCLUSIONS The layers in MCM-22 precursor can be separated by treatment with a hexadecyltrimethylammonium (C16TMA) hydroxide. The product, consisting of alternating layers of MCM-22 and surfactant bilayer (with thickness 25 and 25-30 A, respectively) can be identified and quantified based on unique XRD features. High pH mixtures obtained by mixing the surfactant halide and sodium or tetraalkylammonium hydroxide were investigated as altemative swelling media. It was found that only the tetrapropylammonium hydroxide/ surfactant combination resulted in the swollen product. Possible reasons are exclusion of TPA from the interlayer region based on size or less favorable interaction with SiO moieties. REFERENCES

1. "Intercalation Chemistry", M.S. Whittingham and A. J. Jacobson (eds.), Academic Press, 1982. 2. A. Clearfield in "Advanced Catalysts and Nanostructured Materials, Modem Synthetic Methods", W. R. Moser (editor), Academic Press, 1996, 345. 3. K. Otsuka, Chem. Mater., 9 (1997) 2039. 4. M.E. Leonowicz, J.A. Lawton, S.L. Lawton, and M.K. Rubin, Science, 264 (1994) 1910. 5. S.L. Lawton, A.S. Fung, G.J. Kennedy, L.B. Alemany, C.D. Chang, G.H. Hatzikos, D.N. Lissy, M.K. Rubin, H.C. Timken, S.E. Steuemagel, and D.E. Woessner, J. Phys. Chem., 100 (1996) 3788. 6. W.J. Roth, C.T. Kresge, J.C. Vartuli, M.E. Leonowicz, A.S. Fung, and S.B. McCullen, in "Catalysis by Microporous Materials" (Studies in Surface Science and Catalysis, vol. 94), H.K. Beyer, H.G. Karge, I. Kiricsi, J.B. Nagy (eds.), Elsevier, 1995, 301. 7. W.J. Roth and J. C. Vartuli, in "Nanoporous Materials II" (Studies in Surface Science and Catalysis, vol. 129), A. Sayari, M. Jaroniec and T.J. Pinnavaia (eds.), Elsevier, 2000, 501. 8. (a) A. Corma, U. Diaz, M.E. Domine, and V. Fornes, Angew. Chem. Int. Ed., 39 (2000) 1499; (b) A. Corma, V. Fornes, S.B. Pergher, T.L.M. Maesen and J.G. Buglass, Nature, 393 (1998) 353. 9. M.E. Landis, B.A. Aufdembrink, P. Chu, I.D. Johnson, G.W. Kirker and M.K. Rubin, J. Am. Chem. Soc., 113 (1991) 3189. 10. J.A. Malinowskij and N. W. Below, Dokl. Akad. Nauk SSSR, 1979, 99. 11. (a) C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359, 710 (1992). (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.



281

Control of mesopore structure o f smectite-type materials synthesized with a hydrothermal method Masayuki Shirai, a Kuriko Aoki, Kazuo Torii, b and Masahiko Arai c a

Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Sendai 980-8577, Japan

a

b Supercritical Fluid Research Center, National Institute of Advanced Industrial Science and Technology, Nigatake, Miyagino, Sendai, 983-8551, Japan c Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan

Mesoporous smectite-type (MST) materials containing catalytically active magnesium or cobalt divalent cations in octahedral sheets (MST(Mg) or MST(Co)) were synthesized from water glass and metal chloride with a hydrothermal method. Mesopore structure of MST materials were controlled by the calcination of the mixture of alkylammonium chloride molecules and silicate fragments synthesized with the hydrothermal method.

1. INTRODUCTION Thermally stable mesoporous materials may be useful catalysts and supports [1 ]. Porous smectite-type materials having catalytically active divalent cations in the octahedral sheets are synthesized by a hydrothermal method without adding any template [2, 3]. Magnesium divalent cations in the lattice of the smectite-type materials synthesized showed high activities for the formation of dimethyl carbonate and ethylene glycol from ethylene carbonate and methanol [4]. Cobalt divalent cations in the lattice of the smectite-type materials synthesized showed high activities for hydrodesulfurization of thiophene [5, 6]. For the increase of the number of active sites, the enlargement of surface areas of MST materials is preferable. The enlargement of pore volumes and pore diameters is also desirable for easy diffusion of large molecules and for preventing blockage of pores by carbonaceous materials during reactions. In this paper, we report the control of pore properties (surface area, pore volume, and pore diameter) of smectite-type materials containing catalytically active species in lattice.

282 2. EXPERIMENTAL 2.1. Preparation of smectite-type materials containing Mg ~§ in octahedral sheets Smectite-type materials containing magnesium divalent cations in octahedral sheets were prepared with a hydrothermal method [7]. A Si-Mg hydrous precipitate was obtained by adding an aqueous solution of magnesium chloride to an aqueous water glass solution of controlled pH with an aqueous ammonium solution. The Si/Mg ratio was fixed at 8/5.8. After filtration and washing the precipitate with distilled water, a slurry was prepared from the Si-Mg hydrous oxide precipitate and an aqueous ammonium solution. Following a hydrothermal reaction, the resulting slurry (Slurry Si-Mg) was filtrated and calcined, and then the final product (MST(Mg)T, T: hydrothermal temperature) was obtained. We prepared other smectite materials containing magnesium divalent cations in lattice with dialkyldimethyl quaternary ammonium chloride containing 75% octadecyl, 24% hexadecyl, and 1% octadecenyl groups as alkyl groups (trade name: 2HT-75, Lion Akzo Co., Ltd.). Following the hydrothermal reaction, 2HT-75 was added to Slurry Si-Mg. The final product (MST(Mg)T+2HT75) was obtained by calcination of the mixture of 2HT-75 and Slurry Si-Mg. 2.2. Preparation of smectite-type materials containing Co 2+in octahedrai sheets Smectite-type materials containing cobalt divalent cations in octahedral sheets were also prepared with a hydrothermal method [8]. A Si-Co hydrous precipitate was obtained by adding an aqueous solution of cobalt chloride to an aqueous water glass solution of controlled pH with sodium hydroxide. The Si/Co ratio was fixed at 8/5.8. After filtration and washing the precipitate with distilled water, a slurry was prepared from the Si-Co hydrous oxide precipitate and an aqueous sodium hydroxide solution. Following hydrothermal reaction, the resulting slurry (Slurry Si-Co) was filtrated and mixed with aqueous ammonium chloride (NH4C1) solution. After filtration and calcination, the final product (MST(CO)T, T: hydrothermal temperature) was obtained. Other smectite materials containing cobalt divalent cations in lattice were prepared with dimethyldistearyl ammonium chloride (C18d). C18d was added to Slurry Si-Co after the hydrothermal treatment. The final product (MST(CO)T+C 18d) was obtained by the calcination of the mixture of C18d and Slurry Si-Co.

3. RESULTS AND DISCUSSION 3.1. Structure of MST(Mg) and MST(Mg)+2HT75 Figure 1 shows XRD patterns of MST(Mg)s23 and MST(Mg)523+2HT75 samples calcined at 873 K. Peaks at ca. 8~ 20 ~ 28 ~ 35 ~ 53 ~ and 61 ~ (the X-ray patterns were obtained with a Shimadzu XD-D 1 instrument using a Kct source (~ - 1.5418 A)) are assigned to (001), (02, 11), (003), (13, 20), (24, 31, 15) and (06, 33) peaks characteristic of smectite structure. All

283 MST(Mg) and MST(Mg)+2HT75 samples prepared in this study shows similar XRD patterns, which show that all the samples have smectite-type structures.

._~'~ (a)

"

"-"

c-

,

0

I

I

I

I

I

I

10

20

30

40

50

60

70

2 0 / degree Figure 1. XRD patterns of smectite-type materials containing magnesium divalent cations in octahedral sheets hydrothermally treated at 523 K and calcined at 873 K; (a) MST(Mg)523 and (b) MST(Mg)523+2HT75. The nitrogen adsorption-desorption isotherms were measured at 77 K on MST(Mg) and MST(Mg)+2HT75 samples and their pore diameters evaluated from desorption isotherms with the BJH method are shown in Figure 2. The MST(Mg) samples are micro- and mesoporous materials and the distribution of pores depends on the hydrothermal temperature. The MST(Mg)473 sample had much micropores than the MST(Mg)s23 sample. All smectites prepared in this study have large surface area and high pore volume even after calcination at 873 K. The surface areas of natural smectite clays are less than 20 m2g-1 [9]. Pillared clays are thermally stable above 773 K, having surface areas of 200-500 m2g1, because small oxide particles (pillars) induce interlayer porosity in montmorillonite [10]. Small fragments of smectite would intercalate between silicate layers (smectites fragments intercalated in smectite layers) in MST materials synthesized with the hydrothermal method, and the micropores would be formed between the layers [7]. The surface area and pore volume are enlarged by adding the quaternary ammonium chloride after the hydrothermal treatment. MST(Mg)+2HT75 samples were mesoporous materials and had no micropores. MST(Mg)T+2HT75 samples prepared had higher surface area and larger pore volume values than those of the MST(Mg)T samples that were prepared at the same hydrothermal temperature. The size distribution of silicate fragments in MST(Mg) would be similar to that of MST(Mg)+2HT75 because the hydrothermal conditions

284 for both samples were the same. The dispersion of silicate fragments during drying and calcination would relate to the pore structure of smectite materials synthesized. Bulky dialkyldimethyl ammonium cations would be adsorbed on the exchangeable sites on silicate layers and the orientation of the silicate fragments would be changed. After calcination the silicate fragments would become higher pillars which form higher pore volumes and make larger mesopores in MST(Mg)+2HT75 samples. There is no micropore in MST(Mg)+2HT75 samples because all silicate fragments become pillars or disperse in mesopores. Higher temperature of the hydrothermal reaction would increase the size of silicate fragments. MST(Mg)523 would have larger silicate fragments (higher pillars) compared with those of MST(Mg)473 , and then the pore size of MST(Mg)523+2HT75 became larger than that of MST(Mg)473+2HT75.

0.06

0.06 ~,

,_.
4.0. A similar formation sequence as a function of the solution pH has been reported previously for other mesostructured vanadium phosphorus oxides and associated with changes in the structure of surfactant mesophases [ 11,15]. The XRD patterns for the as-synthesized and calcined mesostructured VPO phases prepared using VOSO4 and H3PO4 as vanadium and phosphorus sources and a CH3(CH2)lsN(CH3)3Br surfactant are shown in Figure 5. The as-synthesized mesostructured VPO phase (Figure 5a) shows the presence of 3 intense reflections at low 20 angles, 2.2 ~ 3.8 ~ and 4.5 ~ corresponding to d-spacings of -39 ~ (100), 23 ,~ (110) and 19.8 A (200),

306 respectively which are in agreement with the formation of hexagonal VPO phase [9-11]. Small Angle X-Ray Scattering (not shown here) confirmed the presence of these reflections. For the calcined hexagonal VPO phase (Figure 5b), the reflection at 20 = 2.35 ~ (dspacing=37.4 A) suggests that the mesostructure is retained even after calcination in air at 673 K. A slight decrease in the d-spacing is attributed to the template removal. XRD reflections were not detected in the 10o4.0). A similar behavior has been observed for the formation of the silica mesostructures [19].

307 The TEM image of the as-synthesized mesostructured VPO prepared using VOSO4 and H3PO4 as vanadium and phosphorus sources and a CH3(CH2)IsN(CH3)3Br surfactant (Figure

6) displays a hexagonal array of cylindrical -38 ~ pores in agreement with the XRD data. The EDS elemental analysis showed a P/V molar ratio - 1.0, which is optimal for achieving superior catalytic performance in the oxidation of n-butane [ 1,2].

Figure 6. TEM image of as-synthesized hexagonal VPO phase.

The surface compositions and the average oxidation states of vanadium in mesostructured VPO phases were determined by XPS and double titration method, respectively [ 12]. For the sample prepared using VOSO4 and H3PO3 as the vanadium and phosphorus sources, respectively, and a cationic CH3(CH2)lsN(CH3)3Br surfactant at pH=2.87, a very phosphaterich surface (P/V-1.7) was obtained which is known to stabilize the +4 oxidation state of vanadium [20]. The average oxidation state was higher for the calcined (+4.3) than for assynthesized samples (+4.1) probably due to oxidizing activation conditions (air, 673K). The XPS results are in agreement with previous studies indicating a surface P/V ratio higher than the bulk value, i.e. P/V=I.0 [20]. Specific: surface areas for calcined phases were in the 45-60 m2/g range. Although these areas are higher than those reported for conventional VPO catalysts (5-20 m2/g), much higher surface areas were expected for the mesoporous phases. Remaining occluded surfactant species as well as the presence of amorphous regions were responsible for these relatively low surface areas. In fact, the XPS experiments for the sample prepared using VOSO4 and H3PO3 as vanadium and phosphorus sources and a cationic CH3(CH2)IsN(CH3)3Br surfactant at pH=2.87, confirmed the presence of the organic material in the calcined mesostructured VPO structure. According to the XPS data, the calcined mesostructured VPO phases contained-30 % of the original carbon present in the as-synthesized sample. Optimal synthesis conditions for achieving complete template removal are currently under investigation. Mesostructured hexagonal and cubic VPO phases can be prepared employing cationic, anionic and alkyl amine surfactants. The formation of well-defined mesostructures depends highly on the nature of the surfactant headgroup. Shorter hydrocarbon chain surfactants (C12C~6) favor the formation of hexagonal phases, while longer chain surfactants (C~8) lead

308 mainly to the formation of cubic mesostructures. These mesostructured VPO phases are promising as novel catalytic systems for the partial oxidation of lower alkanes.

ACKNOWLEDGEMENT The authors would like to thank Mr. A.M. Hirt (Materials Research Laboratories, Inc., Struthers, OH) for the XPS data. This work was supported by the University of Cincinnati Research Council and the Wright-Patterson AFRL/DAGSI grant. REFERENCES

1. G. Centi, Catal. Today, 5 (1993) 16. 2. V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catal. Today, 28 (1996) 275. 3. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartulli, J.S. Beck, Nature 359 (1992) 710. 4. H.P. Lin and C. Mou, Science, 273 (1996) 765. 5. P.T. Tanev, Y. Liang and T.J. Pinnavaia, J.Am. Chem. Soc., 119 (1997) 8616. 6. P. Yang, D. Zhao, D.I. Margolese, B.F. Chmelka, G.D. Stucky, Chem. Mater., 11 (1999) 2813. 7. A. Sayari, Y. Yang, J. Phys. Chem. B, 104 (2000) 4835. 8. M. Kruk, Y. Sakamoto, O. Terasaki, R. Ryoo, C. Ko and M. Jaroniec, J. Phys.Chem B, 104 (2000) 292. 9. T. Abe, A. Taguchi, M. Iwamoto, Chem. Mater., 7 (1995) 1429. 10. T. Doi and T. Miyake, Chem. Commun., (1996) 1635. 11. N. Mizuno, H. Hatayama, S. Uchida, A. Taguchi, Chem. Mater., 13 (2001) 179. 12. B.K. Hodnett, P. Permanne and B. Delmon, Applied Catal., 6 (1983) 231. 13. Q. Huo, D.I. Margolese, U. Clesla, P. Feng, T.E. Gier, P. Sieger, R. Leon, P. Petroff, F. Schuth and G.D. Stucky, Nature 368 (1994) 317. 14. A. Sayari, J. Am. Chem. Soc. 122 (2000) 6504. 15. R.G. Laughlin, The aqueous phase behavior of surfactants, Academic Press, London, 1994. 16. M.S. Wong and J.Y. Ying, Chem. Mater., 8 (1998) 2067. 17. V.V. Guliants, J.B. Benziger, and S. Sundaresan, Chem. Mater., 6 (1994) 353. 18. C.J. Brinker, G.W. Scherer, Sol-gel science, Academic Press, London, 1990. 19. J.M. Kim, Y. Han, B.F. Chmelka, G.D. Stucky, Chem. Commun., (2000) 2437. 20. P. Delichere, K.E. Bere, M. Abon, Applied Catal. A: General, 172 (1998) 295.

Studies in Surface Scienceand Catalysis 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

309

Novel macroporous vanadium-phosphorus-oxides with three-dimensional arrays of spherical voids Moises A. Carreon and Vadim V. Guliants* Department of Chemical Engineering. University of Cincinnati, Cincinnati OH, 45221-0171, USA

Macroporous vanadium phosphorus oxide phases with unique compositional, structural and morphological properties have been synthesized by employing close-packed hexagonal arrays of polystyrene spheres as a template. The macroscale-templated synthesis produced VPO phases with unprecedented high surface areas, desirable macroporous architecture, as well as optimal bulk composition and preferential exposure of the surface (100) planes of the catalytically active and selective phase VO2P207 for the partial oxidation of n-butane.

1. INTRODUCTION Mixed metal oxides possess interesting and promising catalytic properties for the selective oxidation of lower alkanes [1]. For example, Mo-V-Nb and Sb-V oxides are catalytically active in the oxidative dehydrogenation and selective oxidation of ethane and ammoxidation of propane [2]; and the vanadium-phosphorus-oxides (VPO) are selective in the oxidation of n-butane to maleic anhydride [3]. Conventional synthesis methods for mixed metal oxides, both wet chemistry and solid-state, offer very limited control over desirable structural and compositional properties such as the phase, bulk and surface compositions, preferential exposure of active and selective surface planes, surface areas and pore architectures, which define their catalytic properties in selective oxidation of lower alkanes. Therefore, there is a critical need for novel routes of assembling hierarchically designed mixed metal oxides, which display remarkable ordering on micro- (100 nm for pore architectures) scales. Macroscale-templated synthesis of nanocrystalline mixed metal oxides represents an attractive approach for the hierarchical design of catalytic materials. Several single-element macroporous oxides with very interesting structural properties have been synthesized by self-assembly using colloidal sphere templates. Stein et al. [4-8] reported the synthesis of highly ordered macroporous TiO2, ZrO 2, A1203, SiO2, Fe203, Sb406 , W O 3 , MgO, Cr203, Mn203, NiO, ZnO, CaCO 3 and Co304. Pine et al. [9,10] described the synthesis of ordered macroporous ZrO2, TiO 2 and SiO2. Velev et al. [11-13] reported the synthesis of macroporous silica via colloidal crystallization. Wijnhoven et al. [14,15] described the synthesis of ordered macroporous TiO2 and SiO2.

310 Recently, we reported the first successful example of a hierarchically designed macroporous mixed vanadium-phosphorus-oxide (macro-VPO) with desirable structural and compositional properties for selective oxidation of n-butane [ 16]. Here, we present a detailed study of these novel macroporous vanadium-phosphorus oxide phases.

2. MATERIALS AND METHODS

2.1. Materials V20~ (Aldrich) and VO[CHO(CH3)2] 3 (99 %, Alfa Aesar) were used as vanadium sources. HaPO 3 and HaPO 4 (85 %, Fisher Chemicals) were used as phosphorus sources. Ethanol and isobutanol (Aldrich) were used as solvents. NHEOHHC1 was used as reducing agent. Monodispersed polystyrene spheres (--- 400 nm diameter) were synthesized by emulsion polymerization process described elsewhere [6]. The ordered closed-packed colloidal array of spheres was obtained by centrifugation of polystyrene sphere suspensions for 12 h at-1000 rpm. 2.2. Synthesis of macroporous VPO In a typical synthesis, the close-packed array of polystyrene spheres was deposited on filter paper in a Buchner funnel under vacuum and impregnated with a phosphoric or phosphorus acid solution in anhydrous ethanol. Then a solution of vanadium source in anhydrous ethanol was added dropwise to the polystyrene spheres under suction. Then, the composite was dried in air overnight. The polystyrene spheres were removed from the assynthesized macroporous VPO composite by either calcination in air at 723 K for 12 h (heating rate=5~ or Soxhlet extraction for 5 days using a mixture of acetone and tetrahydrofuran (1:1 volume ratio). Typical synthesis compositions on weight basis were: Ethanol/Vanadium precursor =1-10, Vanadium precursor/spheres = 2-6. In all experiments P/V molar ratio was kept constant (1,1) which is the optimal bulk composition for the partial oxidation of n-butane. When V205 was used as a vanadium source the synthesis procedure for macro-VPO phases was as follows. VzO 5 was refluxed in isobutanol or ethanol for 16 h. HaPO 3 or HaPO 4 dissolved in isobutanol or ethanol and centrifuged polystyrene spheres were added to this resultant blue/green slurry. The slurry was refluxed for another 20 h. The resultant blue slurry was filtered, washed with small quantity of isobutanol and dried in air at 393 K. The synthesis conditions are shown in Table 1.

2.3. Synthesis of conventional VPO phases. For comparison, 3 VPO phases were synthesized using conventional synthesis methods. In all the syntheses the P/V molar ratio was kept at 1.1. Aqueous VPO precursor The so-called "aqueous" VPO precursor was prepared according to the synthesis procedures proposed by Yamazoe et al. [ 17]. A solution of NH2OHHC1 (5 g) and H3PO4 (85 wt.%, 13.94 g) in 150 ml of deionized water was heated under stirring to 353 K. V205 (10 g) was slowly added to this solution, and a color change from orange to blue/green due to reduction of VS+was noted. The solvent was evaporated in air and a blue-green product dried

311 at 393 K. Soluble VO(H2PO4) 2 impurity was removed from the V O H P O 4 0 . 5 H 2 0 product by boiling in water.

Organic VPO Precursor An organic VOHPO40.5H20 precursor was prepared according to a modified published procedure [18]. V205 (20 g) was reduced by refluxing in isobutanol (220 ml) for 14 h. Anhydrous orthophosphoric acid (H3PO4) (27.88 g) dissolved in isobutanol (20 ml) was added slowly over a period of 2 h to this blue/green suspension which was refluxed for another 20 h. The resultant blue slurry [VOHPO40.5H20] was filtered, washed with small quantities of isobutanol and acetone, and dried in air at 393 K.

Phosphite VPO Precursor Phosphite precursor VOHPO3.1.5H20 was synthesized according to a procedure described elsewhere [19]. V205 (10 g, 55 mmol) was refluxed in absolute ethanol (200 ml) for 16 h. A color change from orange to green indicated reduction of V 5+. The slurry was cooled to room temperature and H3PO3 (10 g) dissolved in absolute ethanol (80 ml) was added. The mixture was refluxed for another 20 h and the blue slurry obtained was cooled, filtered and washed with absolute ethanol. The solid was dried at 393 K for 16h. Each of the conventional VPO precursors described above was activated at 673 K for 8 days to obtain the "equilibrated" active catalytic phase (VO)2PzO 7 . 2.4. Characterization

Powder X-ray diffraction (XRD) patterns were recorded on a Siemens D-500 diffractometer using Cu Kc~ radiation with step size of 0.02~ The N2 BET specific surface areas were determined using a Micromeritics Gemini 2360 analyzer. Scanning electron micrographs were recorded on a Hitachi S-3200N SEM. Chemical analyses were carried out for P and V at Galbraith Laboratories, Inc., Knoxville, TN. Infrared spectra were collected on a Bio-Rad FTS-60 IR spectrometer.

3. RESULTS AND DISCUSSION Table 1 shows the typical synthesis conditions, specific surface areas and crystalline phases for conventional and macroporous VPO (macro-VPO) phases. Using conventional synthesis methods (aqueous, organic and phosphite routes) only low specific surface areas in the 7-17 m2/g range were obtained. On the other hand, macro-VPO phases showed much higher surface areas in the 44-75 m2/g range after the removal of polystyrene spheres. It is important to mention that the surface areas of the macro-VPO phases are consistent with a theoretical estimate for the 20 nm cubic crystals of (VO)2PzO 7. Our findings indicated that it was possible to obtain different crystalline VPO phases by appropriately choosing the VPO sources and a template removal method (calcination or Soxhlet extraction). There is a general agreement in the literature that vanadyl pyrophosphate, (VO)zP207, is the catalytically active and selective phase in the oxidation of n-butane to maleic anhydride over the VPO catalysts [1,3,20,21]. Table 1 shows that the macroporous VPO sample 5 contains (VO)2P207 as the only crystalline phase. However, all

312 other VPO phases (VOPO42H20 , VOHPO42H20 and ~-VOHPO42H20 ) present in macroporous samples 1-4 are immediate precursors for (VO)2P207. For instance, VOPO42H20 can be transformed to (VO)2P207 by reduction in alcohol and subsequent thermal treatment in N2. VOHPO42H20 and 13-VOHPO42H20 can be transformed to (VO)2P207 by calcination at 773 K in N2. Furthermore, the ICP elemental analysis revealed that these novel macro-VPO phases display optimal bulk compositions ( P / V - 1.05) for a superior catalytic performance in the oxidation of n-butane to maleic anhydride. Table 1. Typical synthesis conditions, crystalline phases and specific surface areas for amacro and bconventional VPO phases. For macro-VPO phases: Alcohol/Vanadium precursor ratio (wt:wt) was 1, 3, 5, 10 and 10 for samples 1, 2, 3, 4 and 5 respectively. Vanadium precursor/PS sphere ratio (wt:wt) was 2, 2, 6, 6 and 5 for samples 1, 2, 3, 4 and 5 respectively. Ethanol was used as solvent for samples 1, 2, 3 and 4 and isobutanol for sample 5. Sample

VPO sources

General Description

Crystalline Phase

Calcined 723 K

VOPO 4 2H20

Surface Area (m2/g) 64

Soxhlet extracted

WOPO4 2H20

50

Calcined 723 K

VOPO4 2H20

41

Soxhlet extracted

75

Calcined 723 K

VOHPO 4 4H20 13-VOHPQ 2H20 (VO)2P207

44

Aqueous VPO

(gO)zPzO 7

7

Organic VPO

(VO)2P207

17

Phosphite VPO

(VO)zPzO7

10

,

a

1

a

2

a3 a4 a 5

b6 b7 b8

VO[CHO(CH3)2]3 H3PO3 VO[CHO(CH3)2]3 H3PO3 V/O5 H3PO3 V205 H3PO3 V205 H3PO4 V205 H3PO4 V205 H3PO4 V205 H~PO~

Figure 1 shows the highly ordered and monodispersed closed-packed arrays of polystyrene spheres used as a template in the synthesis of macroporous VPO phases. In order to obtain these highly ordered structures, polystyrene suspensions were centrifuged at 9001000 rpm for 12-24 h. Formation of macroporous VPO phases involved two main steps: (1) the self-assembly of appropriate vanadium and phosphorus species at the surface of polystyrene spheres, followed by (2) the condensation of the inorganic framework around the spheres upon drying. Then, the template was removed from the inorganic-organic composite by either calcination or Soxhlet extraction. Figure 2 shows a typical SEM image of macroporous VPO after the template removal by calcination. An ordered pore structure displaying interconnected pores (200 nm diameter) inside spherical - 400 nm cavities left after template removal is evident. The wall thickness

313 estimated from SEM was - 90 nm. The average crystal size determined by the Scherrer equation [22] was 20 nm indicating that the macroporous wall was only four crystals thick. This relatively large size of nanocrystal building blocks is probably responsibly for somewhat less ordered appearance of the macro-VPO structures. These novel VPO phases offer a possibility to improve the transport of reactant molecules through the macroporous structure and are promising as novel partial oxidation catalysts.

Figure 1. SEM image of colloidal crystal arrays o f - 400 nm polystyrene spheres used as a template for macroVPO.

Figure 2. SEM image of macroporous Vanadium-Phosphorus-Oxide (macro-VPO) calcined in air at 723 K. Figure 3 shows the XRD patterns for macro-VPO (sample 5) and conventional VPO phases (samples 6, 7 and 8). The XRD patterns of the macro-VPO and conventional VPO phases shows the presence of (VO)2P2O 7 as the only crystalline phase. The (100) surface planes of (VO)2P2O 7 have been proposed to contain the active and selective surface sites for

314 n-butane oxidation to maleic anhydride. Previously, the intensity ratio of the interplanar (100) and (042) X-ray reflections of vanadyl pyrophosphate (I10o/Io42) has been employed as an indicator of the preferential exposure and the stacking order of the surface (100) planes [3]. The conventional VPO phases exhibited low intensity ratios (0.4, 0.9 and 1.5 for phosphite, aqueous and organic conventional VPO, respectively) indicating that the surface (100) planes were not dominant in these phases. On the other hand, the macroscale-templated synthesis yielded much higher intensity ratios, I~00/I04z= 2.5, suggesting that macroporous VPO phases expose the surface (100) planes of vanadyl pyrophosphate to much greater degree than the conventional VPO catalysts.

2.50

A

C

0.90 im

b

e~ c

....

:a

[

I

I

1

I

I

1

I

10

15

20

25

30

35

40

45

__

.

50

2 theta/degrees Figure 3. XRD pattems of a) phosphite VPO, b) aqueous VPO, c) organic VPO and d) macroporous VPO. Numbers on the left-hand side indicate the intensity ratio of the interplanar (100)* and (042)** X-ray reflections of ( V O ) z P z O 7 (I100/I042). Figure 4 shows the IR spectra for as-synthesized, Soxhlet extracted and calcined macroVPO. As-synthesized macro-VPO shows the characteristic bands for VPO phases. The stretching frequencies of V-O-V and V=O appear at 683-642 and 1044-972 cm ~ respectively [23]. The stretching vibrations for P-O-P, P=O, PO3, P-OH and P-H are present at 930, 1198, 1126-1090, 3371 and 2350-2250 cm -~ respectively [24]. The sharp peak at 1640 cm ~ has been associated with surface adsorbed water. The two bands at 1440 and 1495 cm ~ correspond to the C-H symmetric and asymmetric deformation vibrations of polystyrene, respectively. Also, C-H stretching vibrations are present at around 2920-2850 cm l.

315 Soxhlet-extracted and calcined macro-VPO phases show similar vibration modes for vanadium and phosphorus species. However, the vibration modes of the organic template (bands at 1440 and 1495 cm ] and 2920-2850 cm -~) were absent, indicating that the polystyrene spheres were completely removed.

a

A

5 0 C

.=_ E C L_

1-

3000

2500

2000

1500

1000

Wavenumber (cm-1) Figure 4. Infrared spectra of a) as-synthesized, b) Soxhlet extracted and c) calcined macroporous VPO. * Polystyrene vibration modes. The macroscale-templated route produced vanadium-phosphorus-oxide phases with unprecedented high surface areas (75 m2/g), desirable pore architecture, as well as optimal bulk compositions and preferential exposure of the surface planes of the active and selective catalytic phase for the selective oxidation of lower alkanes. The proposed method offers a possibility to control and fine tune structural, compositional and morphological properties of VPO phases that are critical for achieving superior catalytic performance. This study has demonstrated that the macroscale self-assembly route holds a great promise for the rational design of mixed metal oxides with desirable structural, morphological and compositional properties with promising catalytic properties for selective oxidation of lower alkanes.

316 REFERENCES

1. F. Trifiro, Catalysis Today, 21 (1998) 41. 2. S.A. Holmes, J. A1-Saeedi, V.V. Guliants, P. Boolchand, D. Georgiev, U. Hackler, Catalysis Today, 67 (2001) 403. 3. V.V. Guliants, J.B. Benziger, S. Sundaresan, I.E. Wachs, J.M. Jehng, J.E. Roberts, Catalysis Today, 28 (1996) 275. 4. B.T. Holland, C.F. Blanford, A. Stein, Science, 281(1998) 538. 5. B.T. Holland, L. Abrams, A. Stein, Journal of American Chemical Society, 121 (1999) 4308. 6. B.T. Holland, C.F. Blanford, T. Do, A. Stein, Chemistry of Materials, 11 (1999) 795. 7. H. Yan, C.F. Blanford, B.T. Holland, W.H. Smyrl, A. Stein. Chemistry of Materials, 12 (2000) 1134. 8. H. Yan, C. F. Blanford, B.T. Holland, M. Parent, W.H. Smyrl, A. Stein, Advanced Materials, 11 (1999) 1003. 9. A. Imhof, D.J. Pine, Nature, 389 (1997) 948. 10. G. Subramanian, Vinothan, N. Manoharan, James D. Thome, David J. Pine, Advanced Materials, 11 (1999) 1261. 11. O.D. Velev, T.A. Jede, R.F. Lobo, A.M. Lenhoff, Nature, 389 (1997) 447. 12. O.D. Velev, T.A. Jede, R.F. Lobo, A.M. Lenhoff, Chemistry of Materials, 10 (1998) 3597. 13. O.D. Velev, E.W. Kaler, Advanced Materials, 12 (2000) 531. 14. J.E. Wijnhoven, W.L. Vos, Science, 281 (1998) 802. 15. J. Wijnhoven, S. Zevenhuizen, M. Hendriks, D. Vanmaekelbergh, J. Kelly, W. Vos, Advanced Materials, 12 (2000) 888. 16. M.A. Carreon and V.V. Guliants, Chemical Communications, (2001) 1438. 17. H. Morishige, J. Tamaki, N. Miura and N. Yamazoe, Chemistry Letters, (1990) 1513. 18. H.E. Bergna, US. Patent, 4 769 477, 1988. 19. V.V. Guliants, J.B. Benziger, S. Sundaresan, Chemistry of Materials, 7 (1995) 1485. 20. G. Centi, Catalysis Today, 5 (1993) 16. 21. F. Cavani, F. Trifiro, Applied Catalysis A, General, 85 (1992) 115. 22. B.D. Cullity, S.R. Stock, Elements of X-Ray Diffraction, Prentice Hall, Upper Saddle River, NJ, 2001. 23. G. Socrates, Infrared Characteristic Group Frequencies, Wiley, New York, 1994. 24. T. Abe, A. Taguchi, M. Iwamoto, Chemistry of Materials, 7 (1995) 1429.


317

Engineering active sites in bifunctional nanopore and bimetallic nanoparticle catalysts for one-step, solvent-free processes

Robert Raja, a'b* and John Meurig Thomas a'c a

Davy Faraday Research Laboratory, Royal Institution of Great Britain, 21 Albemarle Street, London W1S 4BS, U.K (e-mail: [email protected])

b Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge, CB2 1EW, U.K c Department of Materials Science and Metallurgy, Cambridge University, Cambridge CB2 3QZ, U.K. The design, atomic characterization, performance and relevance to clean technology of two distinct categories of nanocatalysts are described and interpreted. The first category consists of extended, crystallographically ordered inorganic solids possessing nanopores (apertures, cages and channels), the diameters of which fall in the range c___~a0.4 to c__~a1.5 nm, and the second of discrete bimetallic nanoparticles of diameter 1 to 2 nm, distributed more-orless uniformly along the inner walls of mesoporous (c_._a_a3 to 10 nm diameter) silica supports. The bifunctionality of the former class of catalysts consists of Bronsted acid sites together with redox active ones. M H ions, since they have protons loosely bound to an adjacent framework oxygen atom are the loci of the Bronsted sites, and the MllI ions are demonstrably the redox active sites. These catalysts have proved effective in the one-step, solvent-free, ammoximation of cyclohexanone to cyclohexanone oxime and ~-caprolactam with a mixture of air and ammonia. A range of bimetallic nanocatalysts (Cu4RUl2, Pd6Ru6, Ru6Sn, RusPt and Rul0Pt2), encapsulated in mesoporous silica, have also been studied for the direct one-step hydrogenation of dimethyl terephthalate (to 1,4-cyclohexanedimethanol), of benzoic acid (to cyclohexanecarboxylic acid), of naphthalene (in the presence of adsorbed sulfur) to (predominantly) cis-decalin, as well as for the solvent-free, selective hydrogenation of cyclic polyenes at low temperatures.

1. INTRODUCTION More than three quarters of the organic molecular products that are manufactured industrially entail the processes of either hydrogenation or oxidation; and with the impending arrival of the so-called hydrogen economy and the parallel drive towards clean technology this fraction will inevitably rise in the near future, the most desirable agents of conversion being molecular hydrogen and air (or oxygen). But increasing use of these agents requires the further development of robust, new, highly active and selective catalysts that, ideally, should effect single-step conversions under relatively mild and solvent-free conditions [1].

318 Apart from those that, inter alia, simulate the behavior of enzymes in their specificity shapeselectivity, regioselectivity and ability to function under ambient conditions, many of these new nanocatalysts are also viable as agents for effecting commercially significant processes in a clean, solvent-free, single-step fashion. In particular, oxidations using these nanocatalysts may be effected in air; they do not require aggressive oxidants like concentrated nitric and sulfuric acids, dichromates, permanganates or periodates [2], nor cryogenic or other engineering designs to produce nitrogen-free oxygen. So far as hydrogenations are concerned, it is relevant to note that the chemical industry is turning increasingly for its feedstocks to biological molecules extracted from the plant kingdom rather than to the constituents of oil in the production of high added value materials. Because such molecules are too large to enter the inner surfaces of microporous catalysts, it is appropriate for mesoporous catalysts (dia. in range 30-100 A) to be used so as to facilitate access of reactants to, and diffusion of products away from, the catalytically active sites that are (ideally) distributed in a spatially uniform manner over the high-area solid. With microporous and mesoporous solids it is readily possible to "place" or "engineer" active centres in atomically well-defined, spatially isolated fashions at accessible locations at their internal areas, which typically fall in the range 500 to 1000 m2g-~. The greater the number of active sites per unit area, the greater, obviously, is the overall catalytic activity. And by ensuring that the sites are spatially isolated the greater are the chances of securing high intrinsic catalytic activity per site.

2. EXPERIMENTAL

2.1. Preparation of bifunctional microporous catalystsThe microporous materials were synthesized from their precursor gels with a requisite amount of a suitable organic template and a carefully chosen amount of M u ions to substitute for the A1In in the framework. The bifunctional nanopore catalysts used for the ammoximation experiments are: (MIIMm)A1PO-36, M II- Mg; M nI = C o , Mn; {conventionally represented Co0.04Mg0.04A10.92PO4-36} they are structurally well-defined [3] possessing pore apertures of 6.5 x 7.5 .~, and a surface area (overwhelmingly internal) of ca 700 m2g1. X-ray absorption spectroscopy has established [4] that of the 4 atom percent of the framework A1uI isomorphously replaced by M ions, approximately 50 percent are in the M ~ and 50 percent in the M III state. M II ions, since they have protons loosely bound to an adjacent framework oxygen atom, are the loci of Brrnsted acid active sites. The M nl framework ions, on the other hand, are 'redox' active sites, capable of activating hydrocarbons and oxygen [5]. In (MIIMIII)A1PO-18, M - Co, which we have also studied for the ]gurposes of elucidating the nature of the catalysis, all the Co ions are in the Co m state [4]; and the pore diameter is so small that only air, H202 and ammonia (or hydroxylamine when formed [6]) may gain access to the interior surface of the sieve.

2.2. Preparation of encapsulated bimetallic nanoparticle catalysts: The catalytic materials were prepared following standard procedures [7-9]. The mesoporous silica was loaded with the cluster {for example, [Ph4P]2[RusPtC(CO)Is] or [PPN]2[RuloPt2C2(CO)28]} by making a slurry in diethylether/dichloromethane, stirring for 48 h under nitrogen, and filtering. The purple {for [Ph4P]2[RusPtC(CO)15]} and brown {for [PPN]2[RuloPt2C2(CO)28]} solids were washed with dry ether and dried in vacuum. The

319 loading was calculated for 10 wt % of Ru/Pt metal core. However, a small amount of non adsorbed compound [PPN]2[Ru10Pt2C2(CO)28]was found in the filtrate. Both products were characterized by F.TIR and the clusters were shown to be intact within the pores of MCM-41. The mesopore-encapsulated clusters [Ph4P]2[RusPtC(CO)Is] and [PPN]2[Ru]0Pt2C2(CO)28] were activated by heating at 195~ in v a c u o for 2 h. As in earlier preparations [7], the FTIR spectra recorded after activation showed no residual peaks corresponding to the carbonyl stretching frequencies. 2.3. Electron microscopy The electron microscopy characterizations were carried out on a VG HB501 fieldemission STEM microscope, and the catalysts dispersed on a holey carbon film supported on a copper grid from a suspension in hexane, as described in ref. [7]. 2.4. Catalysis The catalytic reactions were carried out in a high-pressure stainless steel catalytic reactor lined with Poly Ether Ether Ketone (PEEK). Dry hydrogen (20 bar) {in the case of hydrogenations} or air (30 bar) {in the case of ammoximation} was pressurised into the reaction vessel and, using a mini robot liquid sampling valve, small aliquots of the sample were removed to study the kinetics of the reaction, without perturbing the pressure in the reactor [10]. The products were analysed (using a suitable internal standard) by gas chromatography (GC, Varian, Model 3400 CX) employing a HP-1 capillary column (25 m x 0.32 mm) and flame ionisation detector. The identity of the products was confirmed by injecting authenticated samples and further by LC-MS (Shimadzu, QP 8000) employing a 7cyclodextrin dialkyl column (Chiraldex, 20 m x 0.25 mm).

3. RESULTS AND DISCUSSION 3.1. Bifunctional catalysts for the ammoximation of cyclohexanone The conversion of cyclohexanone 1 to the oxime 2 and its subsequent Beckmann rearrangement to e-caprolactam 3 are vital stepping stones in the manufacture of Nylon-6 [ 11 ] (scheme 1). On an industrial scale, one popular procedure in converting 1 to 2 o

NOH

--(OH2) 5 I~1

. . . . . . . .

1

2

3

Scheme-1 is to employ hydroxylamine sulfate, the sulfuric acid thus liberated being neutralized by ammonia [12,13], with the consequential production of large quantities of (low value) ammonium sulfate [14]. The traditional industrial route for effecting the Beckmann rearrangement (2----~ 3) is by use of a strong mineral acid such as oleum (Scheme 2).

320

2 + (NH4)2SO4 + H20

1 + (NH2OH).H2SO4 + 2NH 3 oleum 2

~ ~

NH3 H2SO4

OH

3 + 1/2 (NH4)2SO4

Scheme-2 We have investigated the effectiveness of a bifunctional nanopore catalyst, designated MnMIIIA1PO-36 (see Fig. 1), where MIII - Co, Mn, in the framework of which a few Mg II ions have replaced some A1111ions, achieves the conversion of cyclohexanone to its oxime and e-caprolactam in a one-step, solvent-free manner in the liquid phase using a mixture of air an ammonia. The dimensions of the nanopores in MA1PO-36 are just large enough to permit ingress of any of the molecules, cyclohexanone, cyclohexanone-oxime or e-caprolactam. In MIIMUIA1PO-18, M - Co, which we also studied to elucidate the nature of the catalysts, all the Co ions are in the Co IH state; and the nanopore is so small (0.38 nm) that only air, ammonia (and hydroxylamine when formed) may gain access to the interior surface of the sieve. M!! :M:it!AI P ~ t 8

M IIM iI!AIP~ 36!

A9

!::

i:'

oMg,,Oco,,eco,,,e,i oP e o OH

B

Fig. 1A In MIIMUIA1PO-36 (M - Co, Mn), the framework M nI ions are the redox active centres (A1), whereas M II ions have associated ionizable OH bonds attached to the framework and these are the Br6nsted (B1) acid sites. Mg II ions in the framework also have neighbouring ionizable OH ions (B2 sites). Fig. 1B In MIIIA1PO-18 all the framework M Ill ions are again redox active centres: there are no Co II (or Mn II) framework sites. Mg II framework ions again have

Our designed bifunctional nanocatalysts, l~IIIMIIIA1PO-36, perform very well in consecutively converting cyclohexanone to its oxime and c-caprolactam because: 9 hydroxylamine (NH2OH) is readily formed in situ inside the pores from NH3 and 02 at the M IxI redox active site; and 9 the NH2OH converts cyclohexanone to its oxime both inside and outside the pores, and, likewise, at the BrBnsted active sites, cyclohexanone-oxime is isomerized to ecaprolactam inside the nanopores of the catalyst. Confirmation of the essential correctness of the above interpretation of the mode of operation of the nanocatalysts comes from the following facts: 9 deliberate increase in the concentration of Bronsted sites in Co(Mn)A1PO-36 significantly enhances the rate of production of caprolactam;

321 9 no caprolactam is ever produced with MA1PO-18 catalysts, even when the Bronsted active center concentration is increased, solely because the oxime is too large to gain access to these centers via the 0.38 nm pore apertures; 9 with air (or Oz) as oxidant, the smaller-pore ComA1PO-18 nanocatalyst gives higher rates of conversion of cyclohexanone to the oxime than with CoHCoIIIA1PO-36 because of the higher concentration of the redox active centers in the former; 9 when a bulky oxidant, such as tertiary butyl hydroperoxide, is used with a CoIIIA1PO-18 nanocatalyst, no conversion at all of cyclohexanone takes place, because the redox active centers are inaccessible; and 9 a kinetic study shows that NH2OH is initially formed at a rapid rate but is then converted to 2 in the presence of cyclohexanone. Furthermore, experiments carried out in the absence of cyclohexanone proved unequivocally the formation of NH2OH from NH3 and 02 at the redox (Co III) site. 25

conversion

conversion

o

,

m~m~m ---------m~'nn~ - ~

23-

-/

v / IZ

. ~

"

~"N'%

I/-/

5_] i/~"

/T----T ,jv/r"

-

NOH

5 "I~

o~hers 9~

0

5

15

10 t/h

20

NOH

25

O0

~1~~_4_~__~____, T

05

'

I

iO

'

I

15

'

I

2O

'

others I

25

~-

Fig. 2 Kinetic plot (left) showing the conversion of cyclohexanone and the formation of cyclohexanone-oxime and s-caprolactam as a function of time, in the presence of air and ammonia. The plot on the fight shows an expanded view of the initial part of the reaction, where the presence of hydroxylamine (formed in situ from NH3 and 02) at the M nI active (redox) site, is unequivocally established. We may categorically rule out the 'imine' mechanism [ 15] for the formation of the oxime 2, according to which C6H]0=NH is a necessary intermediate. Since this species is also too large to enter the pores of the ComA1PO-18 (which smoothly yields 2 from 1 with a mixture of air and NH3), the dominant mechanism entails direct conversion of 1 with NH2OH to 2; Further, peroxydicyclohexylamine (PDCA), believed by some workers [ 15] to be the key byproduct in the imine mechanistic path, was not observed. Our results also demonstrate that the MA1PO catalysts that we have developed for this and other oxidations [16,17] function in a genuinely heterogeneous manner [18] and not seemingly s o - because the active entities (e.g. Co III or Mn In ions in this instance) do not leach out and then simply adhere to the molecular sieve where they would operate as loosely bound homogeneous catalysts. If the Co III or Mn III were leached out we would have seen

322 appreciable conversion using TBHP as an oxidant with ColIIA1PO-18, yet there was none. Moreover, if the Mg II ions were leached out they would have catalyzed the Beckmann

Fig. 3 Bar chart summarizing the relative performances of the bifunctional A1PO catalysts for the ammoximation of cyclohexanone in the presence of ammonia and different oxidants (air and TBHP). Catalyst A = MnnMnnlA1PO-36; B = MgIIMnmA1PO-36; C = conlA1PO-18; D = MgIICoInA1PO-18; Reaction conditions: cyclohexanone : TBHP -= 3 : 1 (mol); catalyst - 0.5 g; A i r - 3.5 MPa; cyclohexanone : NH3 = 1 : 3 (mol); T - 328 K; cyclohexanone _---50 g; mesitylene (internal standard) - 2.5 g;

rearrangement of (2) to e-caprolactam (with MgnCoHIA1PO- 18), but again none of the latter is formed. In a separate experiment, using Mg~MnmA1PO-36, the solid catalyst was filtered off from the reaction mixture (when hot) after 4 h and the reaction was continued with the resulting filtrate for a further 16 h. No further conversion to e-caprolactam was observed, and the filtrate analyzed by ICP/AAS analysis revealed only trace amounts of Mn and Mg ( ~"

e!

Stable Frameworks ........ I

.....

desorption

3rd Generation Dynamic Channeies Responding r 5timuti

o ,o0 0

adsorption desorption Scheme 1

i~ncapusu~ating

364 generation compounds provide microporous channels with guest molecules, which are broken by the removal of all guest molecules. The second ones have rigid vacant channels formed after the removal of guest molecules. The third ones bear flexible channels, which change their own frameworks responding to external physical stimuli, such as electric or magnetic field and light, and a chemical stimulus by guest molecules. A large number of dicarboxylate- or tricarboxylate-bridged porous coordination polymers have been so far synthesized and investigated about their porous functions. These carboxylate-bridged porous coordination polymers tend to provide rigid framework because of the two site-binding mode of anionic carboxylate groups, therefore classified as the second generation compounds. Recently, several coordination polymers have been prepared, where these frameworks change reversibly on removal/clathration of guest molecules or anions [14,27-33]. The porous coordination polymers of 4,4'-bipyridine (4,4'-bpy) have relatively flexible frameworks based on the single site-binding of neutral pyridyl groups, potentially affording the third generation compounds evolving from the second generation ones [9,12,16,34]. On this background, we have challenged to develop a new type of coordination polymer chemistry of 4,4'-bpy. Recently, we have reported in syntheses and dynamic porous functionalities of a series of Cu--4,4'-bpy-AF 6 (A = Si, Ge, P) coordination polymers [35], in which a conversion of 3-D networks, {[Cu(AF6)(4,4'-bpy)2]-8H20}n (A = Si (la'SH20), Ge (2a-8H20)) (3-D Regular Grid), to interpenetrated ones, {[Cu(4,4'-bpy)2(H20)2]-AF6} n (A = Si (lb) and Ge (2b)) (2-D Interpenetration), took place by immersed in water in the solid state. Moreover, l b showed unprecedented dynamic anion-exchange properties and is classified as the third generation compound. As an key point to construct such a dynamic porous system, we noted counter anions, which have not only a role to neutralize overall charge in the solid but also to regulate frameworks, therefore we called this anion a framework-regulator. On the other hand, a pair of a metal and a ligand is regarded as a framework-builder because frameworks owes to topology and geometry of both ligands and metal cations. Cu(II) complexes could be relevant for crystal engineering by such framework-builder/-regulator, liable to undergo Jahn-Teller effect, resulting in a (4+2) coordination. In the presence of 4,4'-bpy ligand, the AF6 anions tend to sit the axial sites of the Cu(II) ion. By utilizing this tendency, the control of the framework by anions could be carried out. In this manuscript, we succeeded in synthesizing novel porous coordination polymers, {[Cu(TiF6)(4,4'-bpy)2].xH20 }, (3a.xH20) (3-D Regular Grid) and {[Cu(4,4'bpy)2(H20)z]'TiF6}~ (3b) (2-D Interpenetration), which were crystallographically characterized and investigated about dynamic porous functions.

2.

EXPERIMENTAL SECTION

2.1. Syntheses of bpyh(H2Oh]'TiF6}n (3b)

{[Cu(TiF6)(4,4'-bpy)2]-xH20}n (3a'xH20)

and

{[Cu(4,4'-

The compounds of 3a-xH20 and 3b were synthesized as follows: a hot aqueous solution

(50 mL) of Cu(BFa)2"xH20 (711 mg, 3.00 mmol) and (NH4)2TiF6 (594 mg, 3.00 mmol) was added to a hot aqueous solution (50 mL) of 4,4'-bpy (936 mg, 6.00 mmol). A color of the resultant suspension was purple and gradually changed to sky-blue. The obtained sky-blue powder of 3b was filtered, washed with acetone, and dried under vacutma to give the microcrystals (yield: 1184 mg, 66 %). The crystals of 3b suitable for the X-ray analysis were obtained as follows: a EtOH solution of 4,4'-bpy was diffused to an aqueous solution of

365 Cu(BF4)fxH20 and (NH4)TiF 6 in the straight glass tube. Sky-blue crystals of 3b were obtained together with purple crystals of 3a-xH20 after a few weeks. Although purple crystals and powder perhaps form a similar 3-D porous network to la-8H20 and 2a'8H20 from the result of the XRPD measurement, a good quality of single crystals was not obtained. The homogeneity of the powder sample of 3b was confirmed by comparison of the observed and calculated XRPD patterns obtained from the single-crystal data. This powder sample contains guest H20 molecules, because of the presence of a vacant space generated by a slight defect of the overall structure. Anal. Calcd for {[Cu(4,4'-bpy)2(H20)2]-TiF6.1.3H20}n (3b-1.3H20): C, 40.25; H, 3.59; N, 9.42. Found: C, 40.22; H, 3.81; N, 9.38. IR (KBr pellet): 3366 bin, 3106 w, 3083 w, 1645 w, 1609 s, 1536 m, 1490 m, 1413 m, 1322 w, 1221 m, 1067 m, 1012 w, 850 w, 813 m, 730 w, 680 m, 637 m, 526 s, 470 m (cml).

Table 1.

Crystallographic Data for 3b.

formula fw crystal system a,A c,A V, t13 space group Z p(calcd), g.cm3 F(000) /~(MoKcz), cm "~ diffractometer radiation (~,, A) temprature, ~ GOF no. of obsd data no. of variables R" (I > 2.00o(I)) Rwb (all data)

C2oH2oCuF6Na02Ti 573.84 tetragonal 11.301(1) 15.733(2) 2009.3(4) P4/ncc (No. 130) 4 1.897 1156.00 15.40 AFC7R 0.71069 23 1.06 570(I > 2.00o(I)) 81 0.042 0.070 b R~ -- [(Xko (IFo[-IFc])2/~,wFo=)] ~j2.

R = ~,llFol-IFcl]/~,lFo[. .

2.2.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

.

X-ray Structure Determination In compound 3b, data collections were carried on a Rigak~ AFC7R automated diffractometer with a graphite monochromated Mo-Kct radiation. Unit cell constants were obtained from a least-squares refinement using the setting angles of 25 well-centered reflections in the ranges 22.95 < 20 < 29.83 o. Azimuthal scans of several reflections indicated no need for an adsorption correction. The structure was solved by direct methods using the MITHRIL90 program [36] and expanded using Fourier techniques [37]. The nonhydrogen atoms were refined anisotropically. All hydrogen atoms were included but not refined. All calculations were performed using the teXsan crystallographic software

366 package of Molecular Structure Corporation [38]. determinations are summarized in Table 1.

3. 3.1. (a)

(c)

Crystal data and details of the structure

RESULTS AND DISCUSSION Crystal Structure of 3b An ORTEP view around a Cu(II) center of 3b is shown in Figure l(a) with numbering (b)

(d)

Figure 1. (a) ORTEP drawing around a Cu(II) center of 3b at the 30 % probability level. The hydrogen atoms are omitted for clarity. Selected bond distances (A): Cu(1)-N(1)= 2.040(4), Cu(1)-O(l*) = 2.384(6) [Symmetry Code : (*) x, y-l, z]. (b) ORTEP drawing of a 2-D network of 3b along the b-axis. The hydrogen atoms are omitted for clarity. (c) View of the interpenetration mode of 3b along the ab vector. The two types of 2-D layers lying parallel and perpendicular to the paper plane are represented by the stick and cylindrical bond models, respectively. The counter TiE62" anions and the hydrogen atoms are omitted for clarity. (d) ORTEP view showing the micropore cross section of the network of 3b along the c-axis. The counter TiF62 anions and the hydrogen atoms are omitted for clarity.

367 scheme. The Cu(II) atom has anelongated octahedral environment with four nitrogen atoms of 4,4'-bpy ligands in the equatorial plane and two oxygen atoms of 1-I20 molecules in the axial sites. The bond distances of Cu-O and Cu-N of 3b are similar to those of lb and 2b. The Cu(II) centers are bridged by 4,4'-bpy ligands to form a 2-D sheet having square grids with comer angles of ca. 89 and 91 ~ as shown in Figure 1(b). Each 2-D sheet lying in (a-b)c and (b-a)c planes affords a doubly-interpenetration mode (2-D Interpenetration) to make microporous channels with dimensions of ca. 2 A x 2 A along the c-axis (Figures l(c) and l(d)). These channels are filled by free TiF62"dianions, which interact with the coordinated 1-120 molecules by hydrogen bonds (2.665(4) A), whose value is apparently shorter than those of lb and 2b (2.702(3) and 2.686(4) A, respectively), relevant for a size of AF6 anions. This complex is isostructural with the Cu(II) and Zn(II) compounds reported previously [35,39].

3.2.

Dynamic Structural Transformation by Solvent and Anions The summary of framework transformation by solvent and anions are listed in Scheme 2. An interesting feature of this complex is that the 3-D structure of 3a-xH20 (3-D Regular Grid) are transformed into the 2-D interpenetrated structure of 3b (2-D Interpenetration) in the solid phase. When the mixture of Cu(BF4)2"xH20 and (NH4)2TiF6 reacted with 4,4'-bpy in a hot H20 solution, purple powder of 3a'xH20 immediately precipitated, where identification was carried out by the XRPD measurement (Figure 2(a)). Further stirring of suspension with purple powder of 3a-xH20 made a color changed from purple to sky-blue. The IR measurements show that a Ti-F stretching band of the sky-blue sample have the different frequency from that of the purple sample (from 570 to 526 crn~). Moreover, as shown in Figures 2(b) and 2(c), the XRPD pattern of the sky-blue powder is in good agreement with the simulated pattern calculated from the crystallographic data of 3b, clearly indicating that the 3D porous coordination polymer, 3a'xH20, is transformed into the 2-D interpenetrated network, 3b.

~ NH4PF6 NH4NO3 (NH4)2GeF6

H20

(NH4)2TiF6 2-D Interpenetration

{[Cu(4,4'-bpy)2(H20)2]'TiFs}n(3b)

~

sky-blue

3-D Regular Grid

2-D Interpenetration

{[Cu(TiFsX4,4'-bpy)2]-xH20}n(3a-xH20) purple

{[Cu(4,4'-bpy)=(H20)2]'SiFs}n (1b) sky-blue

Scheme 2

368

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(b)

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Figure 2 (left). XRPD patterns of (a) immediately obtained purple solid 3a-xH20, (b) sky-blue solid 3b obtained by long immersing pure 3a-xH20 in a hot I-I20 solution, and (c) simulation of 3b. Figure 3 (fight). IR spectra of (a) lb, (b) solid obtained by immersing lb in a H:O solution containing excess amount of (NI-{4)2TiF6,(c) solid obtained by immersing 3b in a I-I20 solution containing excess amount of (NI-{a)2SiF 6, and (d) 3b. The black and dotted arrows show SiF62 and TiF6 2" stretching bands, respectively. Several anion-exchangeable porous coordination polymers have been hitherto reported

369 [ 15,34,40], in which the microporous frameworks are maintained during the anion-exchange, so called the second generation compounds. We have investigated about the anion-exchange properties of lb, illustrating the third generation system [35]. In the same way, we also examined about the anion-exchange properties of 3b. When microcrystals of lb were immersed in (NH4)2TiF 6 (excess) solution, the color of the compound unchanged. However, as shown in Figure 3, the IR spectrum of a resultant powder clearly shows a new TiF6 2" band (525 cm"~) in addition to original SiF62 bands (746 and 483 crnl), indicating that the compound has partially undergone the anion-exchange. This compound maintains crystallinity during the anion-exchange process as illustrated by sharp peaks observed in the XRPD pattern, which is in a good agreement with that of a original sample lb. The complete exchange of the counter anion is not attained. This is possibly because the TiF6 2" anion is larger than SiF62 and is readily trapped in the channel near the surface by a strong hydrogen bonding interaction with coordinated 1-120 molecules. Therefore, interpenetration into a deeper region of the anion is prevented. Indeed, no anionexchange from TiF62 to SiF62 occurred in 3b as mentioned below. On the other hand, when 3b was immersed in aqueous solution in the presence of excess amount of NH4PF6, NH4NO3, ('NH4)2SiF6, and (NH4)2GeF 6 anions for a few days, no anion-exchange occurred. This is associated with the size of TiF62 anions: the anion'is too large to go through the small channel windows (ca. 2 A x 2 A). Moreover, hydrogen bonds with coordinated I-I20 molecules may support the strong trap ofTiF62- anions to the channels.

4.

CONCLUSION New dynamic porous coordination polymers, {[Cu(TiF6)(4,4'-bpy)2]-xH20}, (3a'xH20) and {[Cu(4,4-bpy)2(HzO)2].TiF6s, ' " (3b) (2-D Interpenetration), have been synthesized, crystallographically characterized, and investigated about their dynamic porous functions. Interestingly, the 3-D network 3a-xH20 is transformed into the 2-D interpenetrated network 3b. As compared with l b, the 2-D interpenetrated network 3b shows no anion-exchange properties, because of the larger size of the TiF6 2- anion tharl SiF62- one. On the other hand, the partial anion-exchange from lb to 3b was observed. Future works are in progress to create a novel dynamic porous coordination polymer responding to the external stimulus such as light, pressure, heat, and electric field.

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10. 11. 12. 13. 14. 15. 16. 17. 18. 9. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37.

38. 39. 40.

Engl., 36 (1997) 1725. M. Kondo, T. Okubo, A. Asami, S. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, K. Seki, Angew. Chem., Int. Ed. Engl., 38 (1999) 140. M. Kondo, M. Shimamura, S. Noro, S. Minakoshi, A. Asami, K. Seki, S. Kitagawa, Chem. Mater., 12 (2000) 1288. S. Noro, S. Kitagawa, M. Kondo, K. Seki, Angew. Chem., Int. Ed. Engl., 39 (2000) 2082. S. S.-Y. Chui, S. M.-F. Lo, J. P. H. Charmant, A. G. Orpen, I. D. Williams, Science, 283 (1999) 1148. H. J. Choi, T. S. Lee, M. P. Suh, Angew. Chem., Int. Ed. Engl., 38 (1999) 1405. B. F. Hoskins, R. Robson, J. Am. Chem. Sot., 112 (1990) 1546. M. Fujita, Y. J. Kwon, S. Washizu, K. Ogura, J. Am. Chem. Sot., 116 (1994) 1151. T. Sawaki, T. Dewa, Y. Aoyama, J. Am. Chem. Sot., 120 (1998) 8539. J. S. Seo, D. Whang, H. Lee, S. I. Jun, J. Oh, Y. J. Jeon, K. Kim, Nature, 404 (2000) 982. R. M. Barter, ACS Advances in Chemistry Series 121: Molecular Sieves, Eds. W. M. Meier and J. B. Utyyerhoeven, American Chemical Society, Washington, DC, 1974, 1. R. E. Wilde, S. N. Ghosh, B. J. Marshall, Inorg. Chem., 9 (1970) 2512. H. J. Buser, D. Schwarzenbach, W. Petter, A. Ludi, Inorg. Chem., 16 (1977) 2704. K. R. Dunbar, R. A. Heintz, Prog. Inorg. Chem., 45 (1997) 283. T. Iwamoto, Inclusion Compounds, vol. 5, Eds. J. L. Atwood, J. E. D. Davies, and D. D. MacNicol, Oxford, New York, 1991, 177. T. Dewa, Y. Aoyama, Chem. Lea., (2000) 854. T. Dewa, T. Saiki, Y. Imai, K. Endo, Y. Aoyama, Bull. Chem. Soc. Jpn., 73 (2000) 2123. T. Tanaka, K. Endo, Y. Aoyama, Chem. Lea., (2000) 1424. D. V. Soldatov, J. A. Ripmeester, S. I. Shergina, I. E. Sokolov, A. S. Zanina, S. A. Gromilov, Y. A. Dyadin, J. Am. Chem. Soc., 121 (1999) 4179. K. S. Min, M. P. Suh, J. Am. Chem. Soc., 122 (2000) 6834. K. S. Min, M. P. Suh, Chem. Eur. J., 7 (2001) 303. L. C. Tabares, J. A. R. Navarro, J. M. Salas, J. Am. Chem. Soc., 123 (2001) 383. O.-S. Jung, Y. J. Kim, Y.-A. Lee, J. K. Park, H. K. Chae, J. Am. Chem. Soc., 122 (2000) 9921. C. J. Kepert, T. J. Prior, M. J. Rosseinsky, J. Am. Chem. Soc., 122 (2000) 5158. S. O. H. Gutschke, D. J. Price, A. K. Powell, P. T. Wood, Eur. J. Inorg. Chem., (2001) 2739. O. M. Yaghi, H. Li, J. Am. Chem. Soc., 118 (1996) 295. S. Noro, R. Kitaura, M. Kondo, S. Kitagawa, T. Ishii, H. Matsuzaka, and M. Yamashita, J. Am. Chem. Soc., in press. C. J. Gilmore, MITHRIL - an integrated direct methods computer program. University of Glasgow, Scotland, 1990. P. T. Beurskens, G. Admiraal, G. Beurskens, W. P. Bosman, R. de Gelder, R. Israel, and J. M. M. Smits, The DIRDIF-94 program system, Technical Report of the Crystallography Laboratory, University of Nijmegen, The Netherlands, 1994. Crystal Structure Analysis Package, Molecular Structure Corporation, 1985 & 1999. R. W. Gable, B. F. Hoskins, R. Robson, Chem. Commun., (1990) 1667. O. M. Yaghi, H. Li, J. Am. Chem. Sot., 117 (1995) 10401.


371

M e s o p o r o u s Polymeric Materials Based On C o m b - C o i l Supramolecules Sami Valkamaa, Riikka M/~ki-Onttoa, Manfred Stammb, Gerrit ten Brinke a'c and Olli Ikkalaa Department of Engineering Physics and Mathematics, Helsinki University of Technology, P.O.Box 2200, FIN-02015-HUT, Espoo, Finland b Institut ffir Polymerforschung "Dresden e.V.", Hohe Strasse 6, D-01069 Dresden, Germany c Materials Science Center, Dutch Polymer Institute, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands

a

In this work we present a procedure to achieve nanoscale mesoporous materials. Previously we have shown using hydrogen bonded amphiphiles that polymeric comb-coil supramolecules leading to lamellar-within -cylindrical assembly allow preparation of "hairy tubes", i.e. nanoscale empty tubes with polymer brushes at the wall. Here the concept is generalized: We use comb-coil supramolecules with lamellar-within-lamellar structure based on coordination bonding. Polystyrene-block-poly(4-vinylpyridine), PS-block-P4VP, is used with zinc dodecyl benzene sulphonate Zn(DBS)2, which coordinates to the lone electron pair of pyridine nitrogen of the latter block. Due to the physical nature of coordination bonding part of supramolecular template can be removed after the structure is formed by selective dissolution resulting in empty space between lamellae with polymer brushes at the walls. 1.

INTRODUCTION

Biological systems allow several examples for functional membranes, such as the cell walls with their transport proteins. Numerous biomimetic concepts have thus been pursued. Synthetic functional membranes have major technological applications e.g. in purification and biotechnological applications. Nanoporous materials (pore size 20-500 A) have been prepared using various methods. One method is based on using block copolymers via photocrosslinking and ozonolysis[1]. Another, in turn, uses assemblies of surfactants and block copolymers in the synthesis of inorganic materials[2-4]. Different kinds of degradation processes have also been presented to obtain nanoporosity[5, 6]. A new application has been to prepare so-called low dielectric material for electronics, based on self-assembly and selective removal of materials[7, 8]. Self-organization leads to nanoscale polymeric structures based on competing interactions and incorporation of several schemes of self-organization[9-13]. Previously we have introduced a concept where amphiphilic molecules are physically bonded selectively to one block of a block-copolymer and they self-organize to form structure-within-structures [1416]. The scheme also allows the preparation of mesoporous materials[17]. The starting material has been diblock copolymer polystyrene-block-poly(4-vinyl pyridine), PS-blockP4VP, with a stoichiometric amount of pentadecyl phenol, PDP, hydrogen bonded to the latter block. The block lengths have been selected to render a lamellar-within-cylindrical morphology, where the P4VP/PDP-blocks form cylinders within the rigid glassy PS-medium

372 and where the P4VP/PDP-complexes, being of a comb-like architecture, self-organize as lamellae within the cylinders. Due to the physical nature of the hydrogen bonding the cylinders can be emptied afterwards by using selective solvent to flush the amphiphiles away resulting nanoporous material with polymer brushes at the walls [17]. The same procedure has also been successfully used to prepare polymeric nanofibers[ 18]. In this article we generalize the concept to prepare other geometries. We describe an alternative structure-within-structure morphology, i.e. lamellar-within-lamellar, and use coordination bonding instead of hydrogen bonding used previously. The underlying idea is that, in general, coordination may allow construction of supramolecules when hydrogen bonds cannot be formed. The present PS-block-P4VP is a feasible model compound as it allows to test both interactions. We use PS-block-P4VP with zinc dodecyl benzene sulphonate Zn(DBS)2, which coordinates to the lone electron pair of pyridine nitrogen of the latter block (see Scheme 1). We show that the concept used previously [17] can also be applied to cover potentially stronger interactions between polymer backbone and amphiphiles i.e. coordination bonding. Due to the physical nature of coordination bonding part of supramolecular template can be removed after the structure is formed by selective dissolution resulting in empty space between lamellae with polymer brushes at the walls.

P4VP (/CH2~cH/)n ~

:5

coordination bon~...,,~ -_---

PS (/CH2~cH/)m

6

Zn(DBS)2 Scheme 1. 2.

MATERIALS AND METHODS

2.1. Materials PS-block-P4VP (Polymer Source Inc.) had Mw = 41,400 g/mol and 1,900 g/mol, respectively, for the PS and P4VP blocks and Mw/Mn = 1.07. Dodecyl benzene sulphonic acid (DBSA) was of purity 90% (Tokyo Kasei) and the main remaining impurity consisted of different chains lengths: (CnH2n+l)(CmH2m+l)CH-Ph-SO3H, with n+m+1= 10... 14. ZnO was of purity 99.0% and acquired from J. T. Baker B. V. Zinc dodecyl benzene sulphonate Zn(DBS)2 was synthesized in ethanol from DBSA anal ZnO according to ZnO + 2DBSA --->Zn(DBS)2 + H20. The detailed description of the procedure is given elsewhere[19]. In contrast to the earlier work, the product was additionally purified by recrystallizing it three times from acetone by adding water dropwise and the purity was assured by NMR-spectroscopy. After purification, the alkyl chains do not contain branches. Finally, the Zn(DBS)2 was dried in vacuum (10 .2 mbar) at 80 ~ for 24 hours.

373 The complexes PS-block-P4VP[Zn(DBS)2]o.9o were prepared by dissolving both components, PS-block-P4VP and Zn(DBS)2, in analytical grade chloroform. Mole fraction 0.90 of Zn(DBS)2 was used. The solvent was evaporated at 60 ~ on a hot plate; thereafter the samples were vacuum dried at 60 ~ for at least 12 hours.

2.2. Dynamic rheological orientation ARES (Rheometric Scientific Inc.) rheometer was used in oscillating mode with parallel plate geometry with gap of 1 mm. The sample was heated up to 200 ~ and then annealed at 170 ~ for 1 h. The shear was performed at 170 ~ for 16 h using 0.1 Hz and 50 % strain amplitude for 7 h. More details of the rheological experiments can be found elsewhere [17, 2O]

2.3. FTIR FTIR spectroscopy was used to study the interaction between the polymer and amphiphile molecules. All infrared spectra were obtained using a Nicolet Magna 750 FTIR spectrometer. The minimum number of scans was 64 and the resolution was 2 cm -l. Analysis was made from pressed pellets, which were prepared by grinding the samples with potassium bromide.

2.4. Small angle X-ray scattering (SAXS) measurements The Bruker NanoSTAR equipment used consists of a Kristalloflex K760-8- 3.0kW X-ray generator with cross-coupled G6bel mirrors for Cu K~ radiation resulting in a parallel beam of about 1 mm 2 at sample position. A Siemens multiwire type area detector was used. The sample-detector distance was 0.65 m (for more details see [ 17]).

2.5. Preparation of "hairy objects" After the dynamic shear orientation, the SAXS intensity patterns and FTIR were measured near room temperature. The pieces of the sample were immersed into analysis-grade methanol at room temperature for at least 12 h to remove Zn(DBS)2 within the lamellae. To verify that Zn(DBS)2 has been removed, the SAXS intensity patterns and FTIR measurements were performed again and compared to the original measurements. 3.

RESULTS AND DISCUSSION

Evidence for the complex formation between Zn(DBS)2 and PS-block-P4VP was obtained based on the FTIR bands characteristic for the aromatic carbon-nitrogen stretching. Figure 1 shows the FTIR spectra of PS-block-P4VP[Zn(DBS)2]o.9o complex in the 1650-1550 cm -l region. The spectra of similarly treated pure PS-block-P4VP and Zn(DBS)2 are also depicted for reference. Due to the coordination between Zn(DBS)2 and lone electron pair of the nitrogen in pyridine ring a shifted absorption band appears at ca. 1620 cm -1 (Figure 1). Previous studies showed that the 1596- 1597 cm -~ aromatic carbon-nitrogen stretching band of P4VP is shifted to 1615 - 1617 cm -I when the pyridine group participates in metal-ligand n-bonding[ 19, 21 ].

374 I

I

~

I

~

I

'

I

'

I

a)-

1640

1620

1600

1580

Wavenumber cm

1560

-1

Figure 1. FTIR spectra for a) Zn(DBS)2 b) PS-block-P4VP[Zn(DBS)2]o.9o and c) PS-blockP4VP. In b) the aromatic carbon-nitrogen stretching band is shifted to ca. 1620 cml due to the pyridine group participating in metal-ligand n-bonding. Oscillatory shear flow was used in order to enhance the organization of nanostructures and reduce the amount of grain boundaries. In contrast to the previous studies [17, 20] the P4VP[Zn(DBS)z]0.90 complex has higher softening temperature than P4VP(PDP) probably because the Zn-pyridine coordination may cause some crosslinking between the chains. Therefore the resulting material PS-block-P4VP[Zn(DBS)2]o.9o is rather stiff even at relatively high temperatures such as 170~ Previous studies with P4VP[Zn(DBS)2] showed that the material is ordered up to 200 ~ [19] in contrast to order-disorder-temperature of 67~ for P4VP(PDP)I.0 [22]. Although the material was sheared long time in high temperatures, only small signs of macroscopic orientation was observed. However, the shear has notable effect on the material allowing more facile removal of the amphiphiles. Small angle X-ray scattering was used to analyze the mesomorphic behavior of the samples after the shear flow. The SAXS intensity pattern of unwashed sample in Figure 2 shows the first intensity maximum at ca. ql = 0.03 A 1 (corresponds the long period value of Lp = 210 A), which comes from the larger structure between the blocks of PS and P4VP[Zn(DBS)2]0.90. Also the second order intensity maximum is presented as a shoulder at ca 2q~ = 0.06 A 1, indicating that the structure is lamellae. The broad band at q2 - - 0 . 2 0 A l, in turn, corresponds to the inner lamellar structure in the P4VP[Zn(DBS)2]0.90 phase with a long period of 32 A [23]. In the corresponding homopolymer complex where a slightly less pure Zn(DBS)2 was used, the second order peak at 2q2 becomes observable [19], indicating lamellar structure.

375 . . . . . . . .

100

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. . . . . . . .

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q(1/A) Figure 2. SAXS intensity patterns for PS-block-P4VP(Zn(DBS)2)o.9 before and after amphiphile (Zn(DBS)2) removal with methanol. The larger structure is 210 A and the inner structure is 32 A. The magnitude of the scattering vector is given by q - (4~/X)sin0 where 20 is the scattering angle and X = 1.54 A. The scattering intensity is in a logarithmic scale. The advantage of using physically bonded (in this case coordinated) supramolecule template PS-block-P4VP[Zn(DBS)2]o.9oinstead of conventional block copolymer molecules is that the formed structures can be emptied easily, as part of the template, e.g. the oligomeric Zn(DBS)2, "flows" out from the inner structure in a suitable solvent. This is presented in the Figure 2, which shows SAXS intensity patterns for the sample before and after washing procedure. After washing the sample with methanol, which is a suitable solvent for both P4VP and Zn(DBS)2 but not for PS the inner lamellar structure is lost. This results in the disappearance of the SAXS intensity maximum of the inner lamellar structure (q2 = 0.20 A l) and simultaneously a strong increase in the intensity of the larger structure is observed which is a clear indication that a substantial part of the Zn(DBS)2 has been removed (Figure 2). The removal of amphiphile also results in a color change of the sample from transparent to white. Further evidence for the amphiphile removal can be observed from the FTIR spectra, which is illustrated in the Figure 3. The characteristic band for pyridine coordination at 1620 cm -1 is disappeared in the washed sample, indicating that a substantial amount of Zn(DBS)2 is removed from the material.

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1520

-1

Figure 3. FTIR for PS-block-P4VP[Zn(DBS)2]o.9o before (a) and after (b) amphiphile (Zn(DBS)2) removal with methanol. In the washed sample (b) there is no evidence of characteristic peak at 1620 cm 1 corresponding to pyridine-metal ligand interaction. The resulting structure was lamellar-within-lamellar although the volume fraction was chosen to be in the cylindrical regime of PS-block-P4VP(PDP) - phase diagram [16] (i.e. 75.3 w% of PS). This indicates that the phase diagram of PS-block-P4VP[Zn(DBS)2]x complexes is highly asymmetric. The lamellae are not highly macroscopically oriented but surprisingly we found that the removal of the amphiphiles was successful. We believe that proper shear flow conditions and further annealing can improve the macroscopic order. The search for right parameter to find structures other than lamellar and to tailor the dimensions of the mesoporous structures are under investigation. In conclusion, we have demonstrated that different kinds of physical interactions can be used for the preparation of self-organized hollow structures in a glassy rigid PS-medium. We show that also stronger interactions than hydrogen bonding [ 17], e.g. coordination bonding, between diblock copolymer and amphiphile are usable. The structures are formed by selforganization of supramolecules (Figure 4 a). Part of the supramolecular template, Zn(DBS)2, can be conveniently removed at the end after the structure has been formed (Figure 4 b), thus overcoming the need of use degradation or corresponding methods to make mesoporous materials. This concept permits a relatively easy way to increase the functionality of the material i.e. surface area per volume unit. Such materials could be further developed to nanoscale electrical or biotechnological applications.

377

a)

b) lamellar-with/n-lamellar

hairy lamellae

Figure 4. Schematic picture of the procedure towards mesoporous materials, a) Original lamellar-within-lamellar structure and b) after the removal of Zn(DBS)2 the lamellae selforganization remains due to the rigid glassy PS. Since the P4VP block can be expected to still cover the wall of the otherwise empty lamellae, we call them "hairy lamellae". 4.

ACKNOWLEDGEMENTS

Dr. Evgeny Polushkin is gratefully acknowledged for assistance with the SAXS measurements in Groningen. Dr. Roland Vogel and Dr. Werner Haselbach are gratefully acknowledged for assistance with the rheological experiments in Dresden. The work has been supported by Finnish Academy and Technology Development Centre (Finland). 5.

REFERENCES

1. S. Stewart and G. Liu, Chemistry of Materials, 11 (1999) 1048. 2. E. Kr~imer, S. F6rster, C. G61tner and M. Antoinetti, Langmuir, 14 (1998) 2027. 3. J.K. Ying, C. P. Mehnert and M. S. Wong, Angewandte Chemic International Edition, 38 (1999) 56. 4. S. F6rster and M. Antonietti, Advanced Materials, 10 (1998) 195. 5. M. Bognitzki, H. Hou, M. Ishaque, T. Frese, M. Hellwig, C. Schwarte, A. Schaper, J. H. Wendorff and A. Greiner, Advanced Materials, 12(9) (2000) 637. 6. T. Thurn-Albrecht, R. Steiner, J. DeRouchey, C. M. Stafford, E. Huang, M. Bal, M. Tuominen, C. J. Hawker and T. P. Russell, Advanced Materials, 12 (2000) 787. 7. D. Mecerreyes, E. Huang, T. Magbitang, W. Volksen, C. J. Hawker, V. Y. Lee, R. D. Miller and J. L. Hedrick, High Performance Polymers, 13(2) (2001) S11. 8. D. Mecerreyes, V. Lee, C. J. Hawker, J. L. Hedrick, A. Wursch, W. Volksen, T. Magbitang, E. Huang and R. D. Miller, Advanced Materials, 13(3) (2001) 204. 9. M. Antonietti, J. Conrad and A. Thtinemann, Macromolecules, 27 (1994) 6007. 10. M. Antonietti, A. Wenzel and A. Thtinemann, Langmuir, 12(8) (1996) 2111. 11. M. Antonietti, J. Conrad and A. Thtinemann, Trends in Polymer Science, 5 (1997) 262. 12. G. ten Brinke and O. Ikkala, Trends in Polymer Science, 5 (1997) 213. 13. C. Ober and G. Wegner, Advanced Materials, 9(1) (1997) 17. 14. J. Ruokolainen, R. M~ikinen, M. Torkkeli, T. M/ikel~i, R. Serimaa, G. ten Brinke and O. Ikkala, Science, 280 (1998) 557. 15. J. Ruokolainen, M. Saariaho, O. Ikkala, G. ten Brinke, E. L. Thomas, M. Torkkeli and R. Serimaa, Macromolecules, 32 (1999) 1152.

378 16. J. Ruokolainen, G. ten Brinke and O. T. Ikkala, Advanced Materials, 11 (1999) 777. 17. R. M/iki-Ontto, K. de Moel, W. De Odorico, J. Ruokolainen, M. Stamm, G. ten Brinke and O. Ikkala, Advanced Materials, 13(2) (2001) 117. 18. K. de Moel, G. O. R. Alberda van Ekenstein, H. Nijland, E. Polushkin, G. ten Brinke, R. M/iki-Ontto and O. Ikkala, in press Chemistry of Materials (2001). 19. J. Ruokolainen, J. Tanner, G. ten Brinke, O. Ikkala, M. Torkkeli and R. Serimaa, Macromolecules, 28 (1995) 7779. 20. R. M~ikinen, J. Ruokolainen, O. Ikkala, K. de Moel, G. ten Brinke, W. De Odorico and M. Stature, Macromolecules, 33 (2000) 3441. 21. L. A. Belfiore, A. T. N. Pires, Y. Wang, H. Graham and E. Ueda, Macromolecules, 25(5) (1992) 1411. 22. K. de Moel, R. M~iki-Ontto, M. Stamm, O. Ikkala and G. ten Brinke, Macromolecules, 34 (2001) 2892. 23. Preliminary TEM measurements confirm that the structure is lamellar-within-lamellar. To be published.

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A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

379

Electron microscopic investigation o f mesoporous SBA-2 Wuzong Zhou*, Alfonso E. Garcia-Bennett, Hazel M.A. Hunter and Paul A. Wright School of Chemistry, University of St Andrews, St Andrews, Fife, KY16 9ST, UK.

Microstructure of mesoporous SBA-2 has been investigated by using transmission electron microscopy and scanning electron microscopy. The material consists of two phases based on hexagonal close-packed and cubic close-packed supercages. These two phases coexist in domains and synthesis of monophasic specimen has not so far been achieved. Three morphologies, i.e. solid spheres, hollow spheres and flat plates, have been recorded and their formation mechanisms are discussed.

1. INTRODUCTION Mesoporous silica SBA-2 was first reported in 1995 [1]. The material was believed to consist of discrete supercages in a hexagonal close-packed (hcp) arrangement and the space group was determined to be P63/mmc. In 1998, based on transmission electron microscopic (TEM) studies, we proposed [2] that two types of mesopores, a group of straight pores along the [100] direction and another group of zigzag pores parallel to the [001 ] zone axis, connect the supercages in the hcp structure (Fig. l a). We also revealed a new phase designated STAC-1, which has a structure with cubic close-packed (ccp) supercages. The latter phase is also connected by two-dimensional mesopores (Fig. 1b). Since then, very few reports dealing with the structures of these materials have been released, and, up to date, the above two models provide the best approaches to the real structures of the hcp and ccp phases. However, uncertainty exists regarding the pore connectivity. Once well ordered materials are prepared, better structural models will be developed from electron microscopy using the so-called direct determination method [3, 4]. Nevertheless, the co-existence of the hexagonal and cubic forms is not in doubt. One of the difficulties in the structural studies of these materials is that it is hard to obtain large domains of the monophasic hcp or ccp phase. The TEM images we used in our previous report [2] show large enough monophasic domains of these two phases for image simulations in order to determine the mesopore systems. However, there are usually some stacking faults showing a mixture of the ABCABC and ABAB ordering along the c axis of the hexagonal unit cell (Fig. 2). In fact, the real structure contains much smaller domains and their orientations can be random. The present work is therefore focused on these domain structures and on the microstructure-related morphologies of the particles.

380

Fig. 1 Schematic drawing of the channel structures of (a) the hcp phase and (b) the ccp phase in mesoporous specimen SBA-2. For comparison with the hcp phase, a hexagonal unit cell is also chosen for the ccp phase.

2. EXPERIMENTAL

The synthetic method is the same as that reported previously [2]. Gemini quaternary ammonium surfactant was used as template. The ratio of surfactant, TMAOH (tetramethylammonium hydroxide), TEOS (tetraethyl orthosilicate) and water was 0.05 : 0.5 : 1 : 1.50. The reaction pH was adjusted to 11 with 1M HCI. Aider 2 h stirring at room temperature, the specimen was recovered by filtration, washed with distilled water, and dried in air at room temperature. The powder sample was calcined at 500 ~ to Fig. 2 TEM image of a large domain of the ccp phase. Stacking faults are indicated by remove surfactant molecules. Initial characterization of the specimens white arrows. was by X-ray powder diffraction (XRD) method using a Philips PW 1830 diffractometer equipped with a secondary monochromator. A 20 range from 1.5 to 8 ~ was normally scanned over 2 h. TEM images were obtained on a Jeol JEM-~)0 CX and a Jeol JEM-2010 electron microscopes, both operating at 200 kV. Specimen was prepared by spreading the powder on a holey carbon f i l l supported on a Cu grid, followed by transferring it into the chamber of the microscope. Structural images were recorded at magnifications from 24,000X to 80,000X. Scanning electron microscopic (SEM) images were recorded on a Jeol JSM-5600 scanning electron microscope operating at various accelerating voltages from 1 kV to 30 kV. The powder sample was deposited on a double-sided carbon adhesive disc sitting on a specimen stub. The specimen was then directly transferred into the SEM chamber without any coating treatments. An accelerating voltage with minimum beam charge was then chosen.


XRD profiles of the samples agreed with the previous results for SBA-2 [1 ] and may be indexed onto a hexagonal unit cell with a = 4.90 and c = 8.04 nm. However, some variation in

381 the peak intensities indicated possible existence of the ccp component [5]. Three principal morphologies were found in the specimen after calcination. One is large hollow sphere with about 50 to 150 ~tm in diameter and the thickness of shells is about 1 to 2 ~tm directly measured from the SEM images of some holes on the hollow balls (Fig. 3a, b). The second morphology is small solid sphere with the diameter in a range of 2 to 3 lxm. Some individual solid spheres can be seen on the surface of the hollow ball. The third morphology is sheet-like plate as shown in Fig. 3c. The particle becomes transparent under the electron beam, indicating that it is very thin along the incident beam direction. It was also noticed that these flat plates usually have sharp edges. Some small spherical particles, 2 to 3 ~tm in diameter, can also be seen on the surface of the plate (Fig. 3c). TEM images of most small spheres show a multi-domain structure (Fig. 4). Some domain boundaries are highlighted in Fig. 4b. It can be seen that these domains intergrow together with random orientations. The projections of domains 1, 2 and 6 can be considered to be [100] of the hcp phase. However, their c axes rotate around the a axis as shown in Fig. 4b. Domain 5 shows mainly the ccp phase with a few stacking faults, the image contrast pattern is similar to that shown in Fig. 2. Domain 4 is also a ccp phase viewed down the [110] direction of the cubic unit cell. This domain structure is beneficial to the formation of the spherical morphology.

Fig. 3 (a) and (b) SEM images of the synthesized SBA-2 specimen, showing two principal morphologies, hollow ball and solid sphere. The diameter of the hollow ball shown is about 150 ~tm and that of the small spheres is about 1 to 2 ~tm. The cross section of the shell of the hollow ball is marked by two white arrows in (b). (c) TEM image of a fiat plate obtained at a low magnification with some solid spheres on the surface.

382

Fig. 4 (a) A TEM image of part of a small spherical particle. (b) A copy of (a) with the domain boundaries marked by white lines.

Fig. 5 Enlarged TEM image of the domain 2 in Fig. 4b. The sequence of layer-packing is indicated.

Fig. 6 TEM image of a solid sphere showing a single ccp phase when viewed down the [110] direction of the cubic unit cell.

383 A close examination of individual domains in Fig. 4 reveals that stacking faults are very common inside the domains. For example, the area 2 in Fig. 4b looks like a monophasic domain with the projection along the [100] zone axis of the hcp phase. However, an examination of sequence of the layer-arrangement along the c axis enables us to find many stacking faults so that it becomes a mixed phase of the hcp phase and ccp phase. Consequently, identification of this domain to either the hcp phase or the ccp phase is not justifiable (Fig. 5). This structural feature is similar to the intergrowth of zeolites FAU/EMT [6,7]. The hcp/ccp irregular intergrowth happens often because the lattice energies of these two phases are very close. Refinement of the synthetic conditions in order to produce either pure hcp or pure ccp phase is difficult, but not impossible. In the same specimen presented above, we occasionally observed indeed some particles that seem to be monophasic. For example, Fig. 6 is a TEM image from a small solid sphere. The structure has been identified as the ccp phase and the view direction is along the [ 110] zone axis. No domain structure can be seen in this particle. Direct TEM examination on the hollow balls is difficult due to their large size and the spherical shape. To can'y out TEM structural studies, the hollow balls were selected under optical microscope. The specimen was then ground for a few minutes and most hollow spheres were crushed into fragments. TEM images of these fragments show again a multidomain property and a uniform thickness (Fig. 7). The size of domains in the particle shown in Fig. 7a is about 5 nm or more and they do not have regular shapes. The domains in Fig. 7b, on the other hand, show a regular but distorted hexagonal shape with domain size of about 2 nm in diameter. These domains are close-packed on the shell plan, forming a larger hexagonal pattern. A possible formation mechanism is that in the ab plans of the hcp phase exist some clusters of ccp phase as shown in the inset of Fig. 7b. These clusters are partially ordered in the ab plans to form hexagonal pattern. The thin particle shown in Fig. 3c is observed from a specimen before grinding. Its more regular shape and lower thickness distinguish itself from the fragments of the hollow balls. In fact, the flat plate shown in Fig. 3c was most likely to be originally a part of larger sheet. Some TEM images showed indeed much larger plates with several cracks. TEM images at a high magnification show that the sheet-like particle seems to be monophasic, although some local defects are still visible (Fig. 8). Selected area electron diffraction (SAED) pattern from an area of a few micrometer in diameter (see the inset of Fig. 8) confirms its monophasic property and shows a hexagonal pattern. Therefore the incident beam was perpendicular to either the (001) plane of the hcp phase or the { 111 } planes of the ccp phase. According to the models proposed in our first paper about SBA-2 [2], the ideal mesopore networks in the hcp and ccp phases are both 2-dimensional instead of 3-dimensional (Fig. 1). In the case of latter, the 2-dimensional network contain mesopore-connected supercages is the (111) plane of the cubic unit cell, and there are no other mesopores acting as bridges between them. Consequently, the interaction of the micellar network in between these (111) planes must be much weaker in comparison with the intraplane interaction. It is therefore not surprising to see that the flat plates are perpendicular to the [111] zone axis of the cubic unit cell.

384

Fig. 7 TEM images of some fragments from hollow spherical particles. A multi-domain structure can be easily observed. Examples of typical domains in (a) and (b) are highlighted. The inset of (b) shows schematic drawing of a ccp cluster in the hcp network.

385

Fig. 8. TEM image at high magnification obtained from a sheet-like particle as seen in Fig. 3c. The inset is the corresponding SAED pattern.

4. CONCLUSION According to the SEM and TEM observations, synthesized SBA-2 specimens have three morphologies. Most solid small spheres consist of irregular domains with random orientations. This morphology must relate to a spherical micelle packing arrangement. Although each domain shows structural homogeneity, it lacks long-range ordering and otten contains irregular intergrowth of the hcp and ccp components with size in a nanometer scale. Large domains and even single domain spheres were occasionally observed, implying that the formation of monophasic spheres is possible. We believe that hollow spherical balls of silicate form from assembly of micellar and silicate condensation on surface of some bubbles. These particles therefore can move to the liquid surface during the reaction process and were indeed observed by optical microscopy. During calcination, hollow balls undergo considerate damage, which resulted in the formation of irregular openings that enabled us to measure the thickness of the shells (Fig. 3b). TEM images revealed that the shells of the hollow particles have also a domain structure (Fig. 7). In addition to the stacking faults along the c axis of the hexagonal unit cell as shown in Fig. 5, some very small 3-dimensional domains were also observed (see Fig. 7b). The flat plates, which probably formed in the liquid/air interface, are monophasic and the orientation is well selective to be normal to the [ 111 ] axis of the cubic unit cell. If our model

386 for the STAC-1 [2] is correct, all the mesopores in the flat plates would be parallel to the planes and there would be no pores across the plates. Further studies are being carried out in these laboratories. During the preparation of this report, we have performed part of systematic investigations of the synthetic conditions for SBA-2 and found that we were already approaching the goal of producing single-phase materials. Introducing bubbles into the reaction system, we obtained much larger yield of hollow balls of silicate.

REFERENCES

1. Q. Hue, R. Leon, P. M. Petroff and G. D. Stucky, Science, 268, 1324 (1995). 2. W. Zhou, H. M. A. Hunter, P. A. Wright, Q. F. Ge and J. M. Thomas, J. Phys. Chem., 102, 6933 (1998). 3. O. Terasaki, personal communication, (2001). 4. Y. Sakamoto, M. Kaneda, O. Terasaki, D. Y. Zhao, J. M. Kim, G. Stucky, H. J. Shin and R. Ryoo, Nature, 408, 449 (2000). 5. H.M.A. Hunter, A. E. Garcia-Bennett, I. D. Shannon, W. Zhou and P. A. Wright, J. Mater. Chem., in press (2001) 6. J. M. Thomas and G. R. Millward, J. CherrL Soc. Chem. Commun., 1380 (1982). 7. J.M. Thomas, O. Terasaki, P. L. Gai, W. Zhou and J. Gonzalez-Calbet, Accounts Chem. Res., 34, 583 (2001).

~tuales m ~urrace ~clence ana ~ataiysls 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

387

A study o f m o r p h o l o g y o f m e s o p o r o u s silica S B A - 15 Man-Chien C h a o a, Hong-Ping Lin b , Hwo-Shuenn Sheu c and Chung-Yuan M o u a a. Department of Chemistry and Center of Condensed Matter Research, National Taiwan University, Taipei, Taiwan, 106. b. Institute of Atomic and Molecular Sciences Academia Sinica, P. O. Box 23-166, Taipei, Taiwan 106. c. Synchrotron Radiation Research Center, Hsinchu, Taiwan.

The mesoporous silica SBA-15 in various morphologies micrometer-sized fibers, millimeter-scaled ropes and macrospheres have been conveniently prepared by controlling the chemical composition. For reducing the size of particles of the SBA-15 materials, a delayed agitation process was found to lead to nanometer-sized fibers. We propose that it generates numerous nucleation seeds at the interface of TEOS and surfactant water solution and leads to very small fibers. With a proper aging time of 20 minutes, silica nanotubes-bundles with diameter of about 100 nm were obtained. In addition to the normal mesopores, the SBA-15 silica nanotubes possess extra textural porosity.

1. INTRODUCTION Micelle-templated mesoporous silica (MMS) [1,2] are of great interest to scientific community because of their tunable mesopore structures which lead to many applications such as catalyst supports, adsorbent and solid templates. In applications of the mesoporous silica such as catalysis, its morphology is an important controlling factor [3,4]. When a silica source is combined with a surfactant, the self-assembly process is complicated involving surfactant self-assembly in solution, mesophases transformation, and silica speciation reactions. All the factors influence the morphology of the mesoporous materials obtained. This has been amply demonstrated in MCM-41 materials [5]. Tuning the chemical composition, using proper inorganic precursors or applying physical field have achieved morphology and size controls on the mesoporous materials [6-8]. Recently the highly ordered SBA-15 [2], synthesized by using triblock copolymer EO20PO70EO20, was found to exhibit rich morphologies. [9,10] The acid-made SBA-15 particles appear to be softer(weaker surfactant/silicate interaction), stickier(more surface

388 silanol), and resulting in richer morphologies. Furthermore, the interface between the insoluble organic TEOS and aqueous copolymer solution appears to offer a new way of morphological control through multiphase assembly [ 10]. Basically, the formation process of MMS materials follows the sequence: nucleation assembling growth ~ aggregation. The particle size of MMS will be dependent on the number of nucleation seeds during and the aggregation capability of the surfactant-silica clusters. The more the nuclei, the smaller the particle sizes. On the other hand, a decrease of growth and aggregation would also help the formation of smaller particles (or fibers). Recently, Mann and coworkers devised a growth quenching procedure in the alkaline synthesis of MCM-41 to obtain nanoparticles of mesoporous silicas [ 11]. In this report, we present several methods of morphological control of the SBA-15. We tuned the TEOS/triblock copolymer ratios or added a proper amount of multivalent salts in the EO20POToEO20-TEOS-HC1-H20 reaction composites to increase the aggregating ability of the triblock copolymer-silica species nanocomposites. Thus, the SBA-15 mesoporous silicas in macro-scaled form (e.g. centimeter-sized sphere, millimeter-sized ropes and micrometer-sized fibers) were facilely prepared. Moreover, a delayed-agitation procedure was conveniently used to create rich silica nucleation seeds at the interface between the TEOS and surfactant aqueous solution. These induced the formation of nanotubes and fine microparticles of the SBA-15 mesoporous silica.

2. MATERIALS AND METHODS 2.1. Materials The tri-block copolymer is (ethylene oxide)20-(propylene oxide)70-(ethylene oxide)20, (EO20PO70EO20; P123) from Aldrich as the mesostructure-templating species. The silica source is tetraethylorthosilicate (TEOS; 98% from Acr6s), and hydrochloride (HC1, 37%) is from Acr6s. All chemical agents were used as received. 2.2. Synthesis The micrometer-sized fibrous mesoporous SBA-15 silicas were prepared according to the typical synthetic process reported by Stucky et al. [2]. 1.0 g triblock copolymers P123 and 9.44 g of 37% aqueous hydrochloride acid were dissolved in 30.0 g water to form a clear solution. Then 2.30 g TEOS was added to that solution under stirring condition then further stirred for 5-24 hr at the 40 ~ The gel chemical compositions in molar ratio is 1.0 P123:(64-160) TEOS: 555 HCI: 11584 H20. We differ from ref.[2] mainly in that higher acid concentration is used here. The millimeter-sized silica ropes were prepared according to the above procedure and same composition except for an extra addition of (2.0-4.0)g of Na2SO4 or Na3PO4. With the same synthetic procedure, the centimeter-sized mesoporous SBA-15 silica sphere was obtained from a higher TEOS content system with TEOS/EO20PO70EO20 weight ratio in the range of 3.5 to 5.0 under a stirring rate of about 500 rpm. For the preparation of SBA-15 silica nanotubes, a delayed-agitation procedure was performed. In this process, the TEOS was added into the surfactant-acid aqueous solution without agitation, and that two-phase solution (TEOS is on the upper layer) then stood statically for equal or longer than 20 minutes. After stirring the reaction mixture at high speed,

389 a white precipitate was suddenly formed. The gel solution was further stirred for 18-24 hr. The chemical composition are the same as that for silica fiber (TEOS/EO20POy0EO20 weight ratio = 2.30). While using the composition for macrosphere formation (TEOS/ EOz0PO70EO20 weight ratio = 4.0), microparticles were formed instead. After filtration, washing with water and drying at room temperature, we recovered the SBA-15 mesoporous silica products. The surfactant templates were completely removed after calcination. 2.3. Measurements X-ray powder diffraction (XRD) patterns were recorded on Wiggler-A beamline ()~ = 0.1326 nm) of the Taiwan Synchrotron radiation research center at Hsinchu, Taiwan. N2 adsorption-desorption isotherms were obtained at 77 K on a Micromeritics ASAP 2010 apparatus. Before the analysis, the calcined samples were outgases at 250~ for about 6 h under 10-3 torr condition. The pore size distribution was obtained from the analysis of the adsorption branch by using the BJH (Barrett-Joyner-Halenda) method. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were taken on the Hitachi S-800 and H-7100 with the operating voltages of 20 and 100 keV, respectively.

3. RESULTS AND DISCUSSION Figure 1 shows several representative SEM, TEM and optical microscope imagines for SBA-15 mesoporous silicas synthesized from different reaction composites. Using the reaction composites similar to that of typical one [2], the micrometer-sized silica fibers were obtained and the length is in tens micrometers (Fig. 1A). Under higher magnification (Fig. 1B), one can clearly find the fibers are nodular which seems to be formed from sticking linearly many sub-micron particles. The nodular shape is different from the rope-like domain observed in ref. [2] where [HC1] ~ 2.0 M. This could be ascribed to the higher acidity ([HC1] ~ 2.5 M) in our synthesis composites. However, stirring is also an important factor. With the addition of a proper amount of NazSO4 or Na3PO4, the fibrous mesoporous SBA-15 products in millimeter size were obtained, and the longer one is about 0.5 mm (Fig. 1C). Under a higher magnification (Fig. 1D), one can clearly see the morphology is rope-like and the SBA- 15 ropes consist of fibers of micron diameter. Using microtome TEM technique to examine the nanostructures of the fibers (Fig. 1E), it is shown the SBA-15 nanochannels are well ordered and aligned with the direction of fibers. Therefore, the millimeter-sized SBA-15 silica ropes is regarded as a hierarchical structure similar to the silica ropes synthesized by this laboratory from C~sTMAB-TEOS-HNOg-H20 composite [12,13]. Thus, we suggest that the addition of the multivalent salts promoted the elongation of the EO20POy0EO20-silica micelles. The long micelles are then shear-aligned into the millimeter-sized SBA-15 silica ropes. In contrast, the addition of univalent salts (NaC1, NaBr or NaNO3) did not help the formation of millimeter-sized silica ropes. The above explanation is further corroborated by a recent study of the effects of salts on the micellization of pluronic solution. [ 14] Pandit et al. [ 14] reported that salt solutions help the elongation of the micelles of Pluronic copolymers by increasing the hydrophobic domain. The power of the micelle formation is in the order: Na3PO4> Na2SO4>NaC1, with NaC1 solution being the least almost as effective as pure water.

390

Figure 1. The SEM, TEM and optical microscope images of the SBA-15 mesoporous silicas, synthesized from different reaction composition, in various morphologies: A. The SEM micrograph of micrometer-sized fibers (TEOS/EOz0POv0EO20 weight ratio - 2.30); B. SEM micrograph of sample A in higher magnification; C. The optical micrograph of millimeter-sized silica ropes (TEOS/Na2SOa/EOz0POv0EO20 weight ratios = 2.30/2.0/1.0).; D. SEM micrograph of sample C in higher magnification; E. Microtome TEM micrograph of sample C.; F. Photograph of the centimeter-sized sphere (TEOS/EO20PO70EO20 weight ratio = 4.O). To examine the effect of aggregation, one may use more TEOS at high acidity to promote the cross condensation between surfaces of silica particles. When the TEOS/EOz0POv0EO20 weight ratio was adjusted into the higher range of 3.5-5.0, we saw the silica-EOz0POToEO20 particles mutually aggregated together during the reaction process and then a centimeter-sized sphere was formed (Fig. 1F). We found the sphere has interestingly high elastic property and mechanical stability [ 15]. In strong acidic condition, the larger silica oligomers have greater binding strength with EOz0POv0EO20 micelles and stronger aggregation capability. However, further increasing the TEOS/EO20POv0EO20weight ratio higher than 7.0, most of TEOS were hydrolyzed and formed the template-free amorphous silicas in acidic condition [16]. The macro-sphere was no longer produced at such high TEOS content.

391 Besides the compositional adjustments on the cooperation assembly of the silica-EOz0POy0EO20 composites, controlling the number of the nucleating seeds in the gel solution is also an essential determining factor on the morphology of the mesoporous materials. According to previous reports [17,18], the silica nuclei can be progressively created at the interface of the hydrophobic TEOS and aqueous surfactant solution via a surfactant-catalyzed hydrolysis of TEOS. Based on this concept, we performed a delayed-agitation method to induce more nucleation seeds in the synthesis of SBA-15 mesoporous silicas. It is hoped that the growth and aggregation processes will be retarded relatively because of transport limitation. In Fig. 2A, we see bundles of nanotubes of SBA- 15 were obtained after the two-phase reaction mixture stood statically for a 20-minute and then followed by a sudden stirring at high speed.

Figure 2. The SEM and TEM micrographs of the SBA-15 mesoporous silicas prepared by the delayed-agitation process. A. The SEM micrograph of SBA-15 nanotubes (TEOS/EO20POToEO20 weight ratio - 2.3; aging time = 20 rain); B. TEM micrograph of sample A.; C. The SEM micrograph of microparticles (aging time - 1 hr); D. The SEM image of microparticles (TEOS/EO20POToEO20 weight ratio = 4.0; aging time = 20 rain). The TEM micrograph shows nanotubes consisting of about ten nanochannels with the diameter at about 100 nm (Fig. 2B). To our knowledge, this may be the smallest dimension SBA-15 silica ever made. Prolonging the aging time of the reaction mixture to about 1.0 hr, the

392 SBA-15 product is in inhomogeneous microparticles instead of the nanotubes (Fig. 2C). To explain the above results, we propose that the nucleation seeds of the SBA-15 be continuously generated at the interface of TEOS-EOz0PO70EO20 solution. In the early nucleation stage, the number of nucleation seeds would increase with the aging time. However, the nucleation seeds would also aggregate with each other or grow into larger ones. Thus aging-time control crucially determines the homogeneity and the particle dimension of the final SBA-15 products. From many tests on aging-time, we found 20-minute aging can produce the smallest silica nanotube-bundle. In order to show further the effect of increasing nucleation seeds on the SBA-15 particle size, the delayed-agitation process was also applied to the composites for centimeter-sized sphere as well. One could obviously find the SBA-15 morphology transformed into the microparticles (Fig. 2D) instead of the macro-sphere (Fig. 1 F). The size reducing also occurred in the system for silica fibers or ropes. Fig. 3A shows the XRD patterns of the as-synthesized mesoporous SBA-15 aforementioned in different morphologies and dimensions. All of the SBA-15 samples possess distinct 2-3 peaks indexed to the well-ordered hexagonal structure. The almost identical dspacings of 8.8 nm for these samples reflect the same reaction temperature of 40 ~ and similar composition [2]. When examining their N2 adsorption isotherms (Fig. 3B), it is clear that all samples have a sharp capillary condensation at P/P0 - 0.60-0.70 corresponding2 to pore sizes around 6.0 nm. The BET surface areas of these samples are about of 450-550 m/g. However, the adsorption behavior of the SBA-15 silica nanotubes is worth mentioning. In that sample, there exists further increase of N2 condensation at P/P0 >0.9 (indicated by an arrow in Fig. 3B), which is attributed to the filling of textural pores [3]. The textural porosity results from the aggregation of the small-sized bundles (sample IV). The samples with larger domain (sample I, II, III) show little or no textural porosity.

750 70O 650

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600 "

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550 500 " 450 -

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350 "~

300 250

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.

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.

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Figure 3. The XRD patterns and N2-adsoption isotherms of the SBA-15 mesoporous silicas in various morphologies and dimensions. A. XRD patterns; B. N2-adsoption isotherms. I. Micrometer-sized fiber; II. Millimeter-sized rope; III. Centimeter-sized sphere; IV. Nanotubes.

393 Besides of morphology control, we also used the post-synthesis hydrothermal treatment to tune the pore size and porosity of these SBA-15 mesoporous silicas [19]. After 100~ hydrothermal treatment for one day, pore size (-7.5 nm), surface area (- 650 m2/g) and porosity (---0.9 cm3/g) increased in all of the SBA-15 samples but the morphologies were still preserved. Combining with the hydrothermal treatment, the SBA-15 mesoporous materials with desired morphologies, dimensions and porosity could be easily prepared for potential applications.

4. CONCLSION In conclusion, the controls of nucleation, growth and aggregation are shown to be fundamental factors in tailoring the morphologies of the mesoporous materials. Adjusting the chemical composition or performing the delayed-agitation process can help us conveniently obtain the SBA-15 mesoporous silicas in different morphologies and dimensions. It should be a versatile mesoporous material with potential applications in catalyst, separations, sensors, and nano-materials fabrications. Besides, the control of interfacial nucleation could help one to understand other sol-gel processes such as biomineralization or in designing better methods for creating new inorganic-organic nanocomposites.

ACKNOWEDGMENTS

This research was financially supported by the National Science Council of Taiwan (NSC 89-2113-M-002-028). We also acknowledge the CTCI Foundation for supporting HR-TEM work.

REFERENCES

1. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 2. P. Yang, D. Zhao, D. I. Maargolese, B. F. Chmelka and G. D. Stucky, Science, 279 (1998) 548. 3. T. R. Pauly, Y. Liu, T. J. Pinnavaia, S. J. L. Billinge and T. P. Rieker, J. Am. Chem. Soc. 121 (1999) 8835. 4. (a) S.T. Wong, H. P. Lin, C. Y. Mou, 2000, Applied Catalyst A 198 (2000) 103 (b) H. P. Lin, S.T. Wong, C. Y. Mou, and C.Y. Tang, J. Phys. Chem. B, 104 (2000) 7885. 5. C.Y. Mou, H. P. Lin, Pure and Applied Chemistry, 72 (2000) 137. 6. P. T. Tanev, T. J. Pinnavaia, Science, 271 (1996) 1267. 7. D. Zhao, P. Yang, Q. Huo, B. F.Chmelka and G. D. Stucky, Current Opinion in Solid State and Materials Science, 3 (1998) 111. 8. Ch. Danumah, S. Vaudreuil, L. Bonneviot, M. Bousmina, S. Giasson and S. Kaliaguine, Microporous and Mesoporous Mater., 44-45 (2001) 241. 9. D. Zhao, J. Sun, Q. Li and G. D. Stucky, Chem. Mater., 12 (2000) 275. 10. D. Zhao, P. Yang, B.F. Chmelka, and G. D. Stucky, Chem. Mater., 11 (1999) 1174.

394 11. C. E. Fowler, D. Khushalani, B. Lebeau and S. Mann, Adv. Mater., 13 (2000) 649. 12. H. P. Lin, C. P. Kao, S. B. Liu and C. Y. Mou, J. Phys. Chem B, 104 (2000) 7885. 13. H. P. Lin, S. B. Liu, C. Y. Mou and C. Y. Tang, Chem. Comm., (2000) 583. 14. N. Pandit, T. Trygstad, S. Croy, M. Bohorquez, and C. Koch, J. Colloid and Interface Sci., 222 (2000) 213. 15. C. P. Kao, H. P. Lin and C. Y. Mou, J. Phys. Chem. Sold., 62 (2001) 1555. 16. J. H. Jung, K. Nakashima and S. Shinkai, Nano Lett., 3 (2001) 145. 17. Q. Huo, D. Zhao, J. Feng, K. Weston, S. K. Burano, G. D. Stucky, S. Schachi and F. Schuth, Adv. Mater., 12 (1997) 974. 18. H. Yang, N. Coombs, I. Sokolov, G. A. Ozin, Nature, 381 (1996) 589. 19. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuk and M. Jaroniec, J. Phys. Chem. B, 104 (2000) 11465.


395

SBA-15 versus M C M - 4 1 : are they the same materials? Anne Galameau, H616ne Cambon, Thierry Martin, Louis-Charles De M6norval, Daniel Brunel, Francesco Di Renzo and Francois Fajula Laboratoire de Mat6riaux Catalytiques et Catalyse en Chimie Organique, UMR 5618 ENSCM/CNRS, Ecole Nationale Sup6rieure de Chimie de Montpellier, 8 rue de l'Ecole Normale, 34296 Montpellier cedex 5 - FRANCE. e-mail: [email protected].

SBA- 15 materials have been used instead of MCM-41 in different applications for the last three years because of their large pores easy to synthesize. Large pore MCM-41 are obtained by adding swelling agents during the synthesis, which gives harder syntheses than SBA-15 due to the use of unstable nanoemulsions. But are SBA-15 the same material as MCM-41 differing only in their method of synthesis? The answer depends on the synthesis temperature of SBA-15. SBA-15 synthesized up to 100~ possess micropores which lead to an overestimation of their surface areas, whereas higher synthesis temperatures allow to eliminate micropores producing SBA-15 materials with properties close to MCM-41. Proper equations are provided to evaluate the amount of micropores, the true surface area and the true wall thickness. The micropores which interconnect the mesopores in SBA-15 have a strong influence in adsorption measurement, nitrogen adsorption, 129Xe NMR and in surface functionalization.

1. INTRODUCTION In 1998, a new synthesis of ordered hexagonal mesoporous silica, named SBA-15 [ 1], was proposed using triblock poly(ethylene oxide) - poly(propylene oxide) -poly(ethylene oxide) copolymers as templates. SBA-15 materials are large-pore (> 50 A) ordered silicas more stable and easier to form than large pore MCM-41 [2] with a pore size in the same range (same pressure of nitrogen pore filling). But have SBA-15 the same properties as MCM-41 ? MCM-41 templated by swelled alkylammonium micelles present a pore size equivalent to SBA-15 but feature much larger volume. Nevertheless, classical BET surface areas are the same for SBA-15 and MCM-41 with the same pore diameter. This indicates that the evaluation of pore size by the classical D-4V/S Gurvitch equation does not hold for SBA- 15. The presence of microporosity on SBA-15 could justify the results, although t-plot analysis gives opposite results [3]. A surface of mesopores pitted by large non-uniform micropores could render unreliable the evaluation of surface areas by classical BET equation and of microporosity by the t-plot analysis, due to the non-possibility of monolayer-multilayer adsorption in micropores and to the unvalidity of the usual reference isotherms to this kind of surface, respectively. Proper equations to evaluate microporosity, wall density and surface area of SBA-15 are provided. Calculated surface areas of SBA-15 are much lower than BET

396 surface areas for solids presenting microporosity. This difference of surface area evaluation can be a large source of errors in different applications such as catalysis or adsorption implying, for instance, grafting densities calculations. Microporosity renders also unreliable the correlation between 129Xe NMR chemical shift and pore size like it has been noticed for MCM-41 [4, 5]. The amount of microporosity in SBA-15 can be controlled by the synthesis temperature and syntheses performed at higher temperatures than temperatures used in literature (temperatures between 35 and 100~ can offer SBA-15 without microporosity and produce materials with properties close to MCM-41.

2. MATERIALS AND METHODS 2.1. Materials

SBA-15 materials have been synthesized according to the methods described in literature (for synthesis temperature of 100~ [1]: 1 g of Pluronic P123 [(EO)20(PO)70(EO)20, Aldrich] 15 g H20, 30 g HC1 2 M, 2.1 g tetraethylorthosilicate (TEOS, Aldrich). The mixture has been maintained at 35~ for 24 h and then for 2 days at a given temperature between 35 and 130~ under static conditions in a teflon-lined autoclave. SBA-15 with pores of 50, 80 and 100/~ are synthesized at 60, 100 and 130~ respectively. Reference MCM-41 materials were synthesized at 115~ by using cetyltrimethylammonium bromide (CTAB, Aldrich), 1,3,5-trimethylbenzene (TMB, Aldrich) pyrogenic silica (Aerosil 200V Degussa), sodium hydroxide (Prolabo) and deionized water in molar ratios 1 SiO2 / 0.26 NaOH / 0.1 CTAB / 20 H20 / x TMB. MCM-41materials with pore size of 37, 50 and 115 A pore size were synthesized using TMB/surfactant ratio of 0, 2.6 and 13, respectively [6]. All materials were filtered, washed with water and dried at 80~ for 24 h. The solids were then calcined in air at 550~ for 8 h. Two different types of surface functionalization leading to the best surface coverage for MCM-41 [7] were performed, using two different octylsilane as grafting agents: chlorodimethyloctylsilane and trimethoxyoctylsilane. Calcined materials were first outgassed under vacuum at 180~ The amount of grafting agent added as modifier corresponds to a density of 5 grafting agent/nm 2 of calcined materials. In type 1 surface functionalization, chlorodimethyloctylsilane was added to a stirred suspension of material (1 g) in anhydrous refluxing toluene (30 mL) containing pyridine (1 pyridine / grafting agent). The reagents were stirred for 15 h at 120~ In type 2 surface functionalization, trimethoxyoctylsilane (Aldrich) was added to a stirred suspension of material (1 g) in anhydrous toluene (30 mL) at room temperature. After 1 h of stirring in flowing nitrogen, H20 (1 H20 / grafting agent) and catalysts, p-toluenesulfonic and ammonium fluoride (0.05 / grafting agent), were added. The mixture was stirred 1 h at 20~ and 4 h at 60~ The resulting water and methanol were removed by azeotropic distillation at 120~ The solids were then recovered by filtration, washed with different solvents and dried at 80~ overnight. 2.2. Measurements

Powder X-ray diffraction (XRD) data were obtained on a CGR Th~ta-60 diffractometer with Inel drive, using monochromated Cu Kot radiation. The adsorption/desorption isotherms of nitrogen or argon at 77 K were measured using a Micromeritics ASAP 2000 instrument. Each sample was outgassed at 250~ for calcined materials or at 180~ for functionalized

397 materials until a stable static vacuum of 3x10 -3 Torr was reached. Pore diameter was measured by the Broekhoff and de Boer (BdB) method which has been demonstrated as one of the best method for MCM-41 materials [8]. Aerosil silica was used as non-porous reference material for the t-plot analyses. BET surface area and the CBET parameter were calculated using adsorption data in the relative pressure range from 0.15 to 0.26 included in the validity domain of the BET equationenon adsorption was performed at room temperature on previously in-situ outgassed (400~ materials by adding different amounts of xenon corresponding to pressures between 200 and 1000 Torr. The tubes were then sealed and 129Xe NMR spectra were recorded with a Bruker AC250 spectrometer at room temperature, at the resonance frequency of 69.19 MHz using a re/2 pulse of 18 ~ts and repetition time of of 2 s. Chemical shifts are referenced to that of gaseous xenon.

3. RESULTS AND DISCUSSION 3.1. Nitrogen sorption at 77 K The nitrogen isotherms of SBA-15 and MCM-41 materials with similar pore sizes are reported in Figure 1. In all cases the isotherms are of type IV and exhibit hysteresis loop of HI, typical of materials withpores of constant cross-section (cylindrical or hexagonal). The pore-filling step in adsorption and desorption curves is sharp, corresponding to a 600 ' ' ' I ' ' ' I ' ' ' I ' ' 'I'' " narrow pore size distribution. SBA-15 and MCM-41 materials share these overall features 400 I/ i but differ when their isotherms are 200 " (b)/ I quantitatively examined. ' The slope of the J isotherm after the low-pressure adsorption step 000 MCM-41 -'-'-I .i is much lower in the case of SBA-15, 800 corresponding to a lower surface area of the mesopore surface, and the pore volume as 600 (a)/ / measured at the top of the mesopore-filling step 400 is also much lower for SBA-15 than for MCM41. Both features strongly suggest that SBA-15 200 pores are separated by silica walls thicker than Or the walls of MCM-41. As wall thickness is 0.2 0.4 0.6 0.8 1 0 inversely proportional to surface area [9], these P/Po observations should give much lower BET surface area for SBA-15 than for MCM-41. But, Figure 1" Nitrogen isotherm at 77 K surprisingly BET surface area calculations show of MCM-41 with 50 A (a) and similar surface areas (-900 m2/g) for both 115/~ (b) pore size and SBA- 15 materials except for SBA-15 synthesized at with 50 )~ (c) and 96 A (d) pore size. 130~ where the BET surface area is -500 me/g. This unexpected trend of the BET surface area requires a careful examination of the measurement technique. The CBET parameters from the BET equation have been calculated and negative or unusual high positive values are observed for SBA-15 synthesized at 60 and 100~ whereas usual CBET values for MCM-41 (CBET "~ 90) are obtained for SBA-15 synthesized at 130~ [3]. These values reveal that BET equation is not valid for SBA-15 -

398 synthesized at 60 and 100~ and strongly suggests the presence of micropores in these two materials. T-plot analyses show effectively micropores for SBA-15 synthesized at 60~ but no micropores for SBA-15 synthesized at 100~ which is in contradiction with the previous observations. To evaluate the amount of micropores in SBA-15 and to calculate the true surface area, the true wall thickness and the true wall density, we used in the calculations unambiguous data: the cell parameter (a) from XRD, the total pore volume (Vp) taken at the end of the pore-filling step and the pore diameter (DBdB) calculated by BdB method on the desorption branch of the isotherm. For MCM-41, by using a model of hexagonal honeycomb, it has been shown that DBdB is close to the geometrically calculated pore diameter [8] and can then be expressed as: DBdB = 1.05 a [Vp/(Vp+l/Psi)] 1/2

(1)

where Psi is the density of amorphous silica (2.2 g.cm-3). For MCM-41, Wp is equal to the mesopore volume (Vmes). In the case of materials containing micropores, like SBA-15, the total pore volume (Vp) is the sum of the micropore volume (Vg) and the mesopore volume (Vmes), so equation (1) becomes: DBdB = 1.05 a [Vmes/(Vp+l/Psi)] 1/2

(2)

Hence the true mesopore and micropore volumes can be Calculated by the following equations: Vmes= (DBdB/1.05a) 2 (Vp+ 1/Psi)

(3)

Vla = Vp_Vmes

(4)

The average density Pw of the walls between mesopores of SBA-15 is the result of the contributions of micropore volume and silica volume and can be expressed as: 1/pw = V~t + 1/Psi

(5) In this model of hexagonal honeycomb structure for SBA-15, the wall thickness [8] and the surface area [9] of the mesopores are given by the following equations: t = a - 0.95 DBdB Smes = 4.104/pw t [(1-t/a)/(2-t/a)]

(6) (7)

with t and a expressed in A. By calculation (Table 1), we found that SBA-15 synthesized at 60~ exhibits a micropore volume as high as the mesopore volume, SBA-15 synthesized at 100~ features a non-

399 negligeable amount of micropores (whereas no micropores were identified by t-plot) with a micropore volume equal to 33% of total pore volume and SBA-15 synthesized at 130~ has no micropores. True mesopore surface areas Smes are much lower than BET surface areas for SBA-15 synthesized at 60 and 100~ and in good agreement with the BET surface area (-~500 m2/g) for SBA- 15 synthesized at 130~ Table 1 Total pore volume Vp, mesopore volume Vmes (eq. 3), micropore volume Via (eq. 4) (mL/g), BET surface area, mesopore surface area (eq. 7) (m2/g), wall density law (eq. 5), wall thickness (A) (eq. 6) and pore size for SBA-15 synthesized at 60, 100 and 130~ Vp SBET DBd B Vmes Via Pw t Sines 60-SBA 100SBA 130SBA

0.76 1.19

931 912

49 77

0.34 0.79

0.42 0.40

1.15 1.17

44 33

263 422

1.23

514

96

1.25

0

2.20

15

550

This confirms that the porosity of SBA-15 synthesized at high temperature corresponds to an array of constant-diameter mesopores, with no contribution from microporosity and exhibits the same features as a large pore MCM-41. The origin of the microporosity has been proposed to result from the sharing of the hydration spheres of poly(ethylene oxide) chains between micelles, which disappears at high synthesis temperature, when the hydration sphere volume of ethylene oxide chains decreases [3]. The micropores of SBA-15 are non-homogeneously organized around the mesopores (no supplementary XRD peaks have been distinguished on XRD pattern) and low pressure argon isotherms strongly suggest non-uniform pore size (no step at low pressure has been observed in argon isotherms, like for zeolites). The size of the micropores are estimated to be between 10 and 30 A [ 10]. SBA- 15 offer a new kind of surface containing the openings of a new kind of "micropores" which could imply adsorption properties significantly different of what is known and leads, for instance in this study, to the unvalidity of t-plot analysis. 3.2. Xenon adsorption and 129Xe NMR 129Xe NMR of adsorbed Xe at different pressures on various materials, like zeolites [ 11 ], has been used to characterize their porosity. The resulting Xe chemical shift (SXe) is, in first approximation, depending on Xe interaction with the solid surface and on the interaction between Xe molecules: 8Xe = 8interaction Xe-surface + 8interaction Xe-Xe The first term (Sinteraction Xe-surface) depends on the pore size and on the type of surface. It can be evaluated by extrapolation at p = 0 of the plot 8Xe as a function of Xe pressure. The second term (Sinteraction Xe-Xe) depends on the pore size and on Xe pressure. High Xe pressures and small pores increase the number of collisions between Xe molecules, hence, 8Xe increases. It has been found on microporous amorphous silica [12] that the plots 8Xe as a function of Xe

400

pressure are linear, 8Xe(p=0) and the slope decrease as pore size increases. 8Xe becomes independent of Xe pressure for pore larger than 13 A. For MCM-41, 8Xe have been plotted as a function of Xe pressure. A slight positive slope has been obtained for MCM-41 with 20/~ pore size. For larger pore size, 129Xe NMR spectra are practically independent of Xe pressure and a correlation between 8Xe and pore diameter has been established at a Xe pressure of 1000 Torr (Figure 2).

100 80

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Figure 2. 129Xe NMR chemical shift of adsorbed Xe at 1000 Torr versus pore diameter. Plain circles are representing MCM-41 materials and the curve, the resulting relationship between 8Xe and pore diameter. Empty triangles, squares and circles, are corresponding to the ~SXeof the first, second and third 129Xe NMR peaks found for each SBA-15 and have been plotted as a function of their BdB pore diameter (corresponding synthesis temperature is indicated).

For SBA-15, at 1000 Torr Xe pressure, several 129Xe NMR peaks have been observed. For materials prepared at low temperature (50-60~ three peaks have been obtained: a first small one at --90 ppm and two others a t - 8 0 and ~-70 ppm. For solids synthesized at higher temperature (larger pore size), only the second and third peaks at lower chemical shifts are obtained. The first peak should be relevant of micropores and the second and third of the mesopores. Even if the position of the first peak is not yet well understood and if the presence of the later two peaks is not clear yet, Figure 2 shows that data of SBA-15 synthesized at temperatures between 50 and 100~ significantly differ from the relationship of MCM-41, whereas data of SBA-15 synthesized at higher temperatures do. This can be explained by the presence of micropores in SBA-15 obtained at temperatures between 50 and 100~ Indeed Xe has an average residence time in all the pores of SBA-15 with a residence time of Xe in micropores higher than in mesopores, which will shift 8Xe relative to mesopores towards higher chemical shifts. Moreover these observations strongly suggest an interconnection between the micropores and the mesopores. Further works are under study. 3.3. Surface functionalization Two kinds of surface functionalization, providing the best surface coverage for MCM-41 [7], have been performed on SBA-15 materials. Two different octylsilanes were used: (1) chlorodimethyloctylsilane and (2) trimethoxyoctylsilane. For MCM-41, the surface coverage is almost independent of pore size with a grafting density of nl-l.5 grafts/nm2 (CBET~18) and n2-~1.8 grafts/nm2 (CBET"23) for method (1) and (2), respectively (Table 2). CBET is a good indicator of surface coverage on silica materials, a lower CBET is diagnostic for a better

401 coverage [7]. The maximum of grafting density has been obtained on silica gel with nl=l.9 grafts/nm 2 (CBET=16) and n2=2.5 grafts/nm 2 (CBET=23). The surface coverage is more efficient with method (1) although the number of grafted species is lower. Method (2) is a surface polymerization (horizontal) between grafted chains and requires a strict homogeneous surface to avoid anarchical polymerization (vertical) which can be evidenced by a higher CBET relative to the presence of non-grafted silanols. Table 2 Pore volume V (mUg), BET surface area SBET (mUg), CBET, grafting density n (greffons/nm 2) and n* corrected by the surface calculated by eq. 7 for methods (l) and (2), for MCM-41 with 35, 50 and 115 A pore size and SBA-15 synthesized at 60, 100 and 130~

Samples

V1

SBET1 CBETI nl

nl*

V2

SBET2 CBET2 n2

MCM-35 0.27 nd nd 1.36 0.24 nd MCM-50 0.52 560 17 1.48 0.44 574 MCM-115 1.53 562 20 1.42 1.31 544 60-SBA 0.25 275 30 0.95 3.36 0.15 204 100-SBA 0.62 424 20 1.16 2.50 0.48 412 130-SBA 0.78 327 18 1.44 1.44 0.66 336 nd: not determinated (Pore sizes are so reduced that BET equation is

nd 22 28 43 35 24 no more

1.52 1.90 1.79 1.23 1.61 2.20 valid)

n2*

4.35 3.47 2.20

For SBA-15 synthesized at 60 and 100~ the grafting induces larger decrease in BET surface areas (900 to 200 or 400 m2/g) than for MCM-41 pointing out again some problems about the validity of BET surface areas for these calcined materials. Incorrect surface areas induce false grafting densities (n) which are calculated by m 2 of initial calcined materials. Corrected values can be obtained by using the surface areas reported in Table 1 by eq. 7, but the resulting grafting densities (n*) exhibit values too high to be correct which can be explained by a part of octylsilanes grafted in the micropores. The surface coverages are lower (CBET higher) than for MCM-41 revealing non grafted silanols on the surface so a different homogeneity of surface. For SBA-15 synthesized at 130~ the grafting density and the surface coverage are analogous to MCM-41. 1

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Figure 3. Ratio of volume of grafted samples per g of silica (V) to initial volume (Vp) versus pore diameter. oCircles are representing MCM-41 samples and square SBA-15 samples. Filled points are relative to the grafting by method (1) and empty point by method (2). Filled and dotted curves are the losses of relative volume for a cylender of pore diameter decreasing 120 of I0 and 12 A, respectively.

.

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100

402

To compare all the different samples, we can normalize the volumes and calculate the volumes per g of silica (instead of g of material) reported per volume of initial materials. In this way, Figure 3 shows that MCM-41 samples grafted by methods (1) or (2) followed a similar evolution as a loss of volume of cylindrical pores where the diameter will be decreased by 10 or 12 A, respectively. SBA-15 synthesized at 130~ (D = 96 A) shows a loss of volume similar to MCM-41, with a slightly lower value revealing a barely higher grafting density than MCM-41. For SBA-15 synthesized at 60~ (D = 50 A), the losses of volume strongly differ from MCM-41, showing that some of the micropores have been grafted and some micropore volume filled. The loss of volume after grafting by method (2) suggests a large part of anarchical polymerization as evidenced by the high value of CBET (Table 2). In the case of SBA- 15 synthesized at 100~ the loss of volume after grafting by method (1) is close to MCM-41 and the surface coverage is similar (similar CBET) to MCM-41 (Table 2) showing that micropores are not blocked but are large enough to be grafted in a similar way as mesopores. This observation will induce the presence of larger micropores in SBA-15 synthesized at 100~ than in SBA-15 synthesized at 60~ With the grafting method (2), higher loss of volume and lower surface coverage (higher CBET) than MCM-41 (Table 2) are consistent with the occurence of anarchical polylerization in the pores. This later could be induced by some surface heterogeneity due to the presence of micropores on this surface which does not allow a perfect horizontal polymerization of the grafted chains due to the presence of variation in surface curvature on the surface. In conclusion, SBA-15 materials synthesized at temperatures lower than 130~ possess micropores (or small mesopores) connecting well-ordered mesopores which confer to the materials different adsorption properties and different behaviours towards grafting than MCM-41. On the contrary, strong similarities have been found between MCM-41 and SBA15 synthesized at 130~ where no micropores are produced. REFERENCES I. D.Y. Zhao, Q. S. Huo, J. L. Feng, B. F. Chmelka and G. D. Stucky, J. Am. Chem. Soc., 120 (1998) 6024. 2. J.S. Beck et al., J. Am. Chem. Soc., 114 (1992) 10834. 3. A. Galarneau, H. Cambon, F. Di Renzo and F. Fajula, Langmuir, (2001) in press. 4. R. Ryoo and J. M. Kim, J. Chem. Soc., Chem. Commun., (1995) 711. 5. 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. 6. D. Desplantier-Giscard, A. Galarneau, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal., 135 (2001) 06-P-27. 7. T. Martin, A. Galarneau, D. Brunel, V. Izard, V. Hulea, A. C. Blanc, S. Abramson, F. Di Renzo and F. Fajula, Stud. Surf. Sci. Catal., 135 (2001) 29-0-02. 8. A. Galarneau, D. Desplantier, R. Dutartre and F. Di Renzo, Microporous Mesoporous Mater., 27 (1999) 297. 9. F. Di Renzo, D. Desplantier' A. Galarneau, F. Fajula, Catal. Today, 66 (2001) 75. 10. R. Ryoo, C. H. Ko, M. Kruk, V. Antochshuck and M. Jaroniec, J. Phys. Chem. B, 104 (2000) 11465. 11. J.-L. Bonardet, J. Fraissard, A. Gedeon, M.-A. Springel-Huet, Catal. Rev. Sci. Eng, 41 (2) (1999) 115. 12. A. Julbe, L. C. de Menorval, C. Balzer, P. Davis, J. Palmeri and J. A. Dalmon, Porous Mater., 6 (1999) 41.

Studies in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) @2002 Elsevier Science B.V. All rights reserved.

403

C o m p r e h e n s i v e characterization o f iron oxide containing m e s o p o r o u s molecular sieve M C M - 4 1 Z.Y. Yuan, a'* W. Zhou, b'* Z.L. Zhang, a Q. Chen, c B.-L. Su d and L.-M. Peng a.~ Beijing Laboratory of Electron Microscopy, Institute of Physics & Center for Condensed Matter Physics, Chinese Academy of Sciences, P.O. Box 2724, Beijing 100080, China a

b School of Chemistry, University of St. Andrews, St. Andrews, Fife KY 16 9ST, United Kingdom, e-mail: [email protected] CDepartment of Electronics, Peking University, Beijing 100871, China d Laboratory of Inorganic Materials Chemistry, University ofNamur (FUNDP), 61 rue de Bruxelles, B-5000 Namur, Belgium

We have synthesized iron oxide-containing mesoporous silica MCM-41 by a direct route. This material has a significant microporousity, which might be related with the loading of iron oxide in the MCM-41 mesopores, though no crystalline phases of iron oxide were detected by XRD. Intensity deviations and extra bright intensity within the pores, as well as local superstructural phases within a large area, were observed in the TEM images. The position and area of such a crystalline phase could change during the successive electron beam exposure and resulted in a formation of a cycle. These phenomena imply the possible existence of small Fe203 nanocrystals loaded in the mesoporous silica materials with regular and oriented arrangement. EELS spectrum of the sample proves the existence of iron, but the chemical environment of iron in the MCM-41 silica should be different from that of bulk amorphous porous iron oxide. The small iron oxide crystals in the MCM-41 sample might be linked with silica wall through oxygen. Another possibility is that iron oxide reacts with silica during synthesis, causing the partial crystallization of the MCM-41 framework. In other words, microcrystals of iron oxide may exist in the silica matrix.

1. INTRODUCTION Since the exciting discovery of the novel family of molecular sieves M41S was reported by Mobil's researchers in 1992 [1,2], these materials have received considerable interest due to their potential applications in the area of catalysis, separation, and advanced materials. MCM-41, the well-known hexagonal member of this family, exhibits regular mesopores between about 1.5 and 10 nm, very high surface area (typically 1000-1200 m2/g), high hydrocarbon sorption capacity, and thermal stability. Various transition-metal atoms can be introduced into the network of MCM-41 in order to generate potential catalysts, which could be more active compared to microporous systems [3-6]. MCM-41 has also been proved to be

404 a suitable support for preparing metal or metal oxide-based catalysts [4,7,8]. The welldefined mesoporous structure of MCM-41 implies that the material could be a suitable host for quantum semiconductor structures of low dimensionality [8-10]. Iron-containing zeolite and zeolite-like molecular sieves are of great interest, because they show interesting catalytic properties. For example, iron-containing zeolite Y was used in the process for the catalytic reduction of NOx in exhaust gases [ 11]. Catalytic synthesis of carbon nanotubes, with a fullerene-like structure, has been reported with the use of a zeolite Y catalyst which contains iron or cobalt [12], or a Fe-loaded mesoporous silica [13]. The synthesis of iron-containing MCM-41 was first reported by Yuan et al. [ 14]. Fe incorporation in the silicate "framework" was evidenced on the basis of FTIR and EPR data. FeMCM-41 has also been investigated by some other groups for possible applications, exhibiting many significant catalytic properties recently [15,16]. Nanoparticles of Fe203 were claimed by Abe et al. [ 17] to encapsulate into the uniform pores of MCM-41, which had a wide bandgap from 2.1 to 4.1 eV owing to the quantum size effect. Very recently Fe203 nanoparticles were synthesized within mesoporous MCM-48 silica phases by using multiple cycles of wet impregnation, drying, and calcination procedures [ 18]. Herein a direct synthesis route for iron oxide-modified MCM-41 is described. Small Fe203 crystals exist in the channel network of MCM-41 silica, as revealed by high resolution transmission electron microscopy (HRTEM).

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

The mesoporous MCM-41 materials were prepared using cetylpyridinium bromide (CPBr) surfactant as a templating material. All chemicals were obtained from Beijing Chemicals Corp. Under stirring, tetraethylorthosilicate (6.75 ml) was added dropwisely to an aqueous solution containing 5 g of CPBr and 18.8 ml of ammonia solution (-25%) in 65 ml distilled water. After stirring for several minutes the mixture becomes cloudy, indicating the onset of some silica precipitation; a solution of 0.121 g iron(Ill) nitrate (Fe(NO3)3-H20) in 5 ml was then added, which co-precipitate with the silica and become incorporated into the silica. After stirring more than 30 min, the mixture was loaded into an autoclave and statically heated at 90 ~ for 3 days to complete crystallization of the MCM-41 material. The resultant solid product was recovered by filtration, washing with distilled water and drying in air at room temperature. In order to remove the organic species in the mesopores, the as-synthesized material was calcined in air from room temperature to 540 ~ with a rate of 1 ~ followed by a further calcination at 540 ~ for 5 h. The powder X-ray diffractograms (XRD) of the solids were recorded on a Rigaku D/max 2400 diffractometer using CuKo~ radiation (~, = 0.154 nm). N2 adsorption-desorption isotherms were obtained at liquid nitrogen temperature on a Quantachrome Autosorb-1 apparatus. The sample was degassed at 300 ~ for 10 h in vacuum prior to adsorption. The specific surface area was determined by the BET (Brunauer-Emmett-TeUer) method and poresize distribution was obtained with the N2 adsorption branch using the BJH (Barrett-JoynerHalenda) method. Local structures of the mesoporous crystals were studied using high resolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) on a Philips CM200 FEG (field emitting gun) equipped with electron energy loss spectroscopy (EELS) and working at a voltage of 200 kV. EELS experiments were performed in image mode using the GIF (Gatan imaging filter) system and a 0.5 and 1.0 eV/channel energy dispersion, and a 3 mm selected area aperture in the GIF system. The

405 specimens for the HRTEM studies were prepared by dispersing the particles in alcohol by ultrasonic treatment, and dropping onto a holey carbon film supported on a copper grid.


The X-ray diffraction pattems of the resultant materials are depicted in Figure 1. The diffractograms of both assynthesized and calcined samples exhibit four sharp diffraction peaks at low 20 range (1.5 - 10~ reflecting a typically well-aligned MCM-41 structure [1,2]. The diffraction peak of (100) did not change atter calcination except for an increase of the peak intensity, indicating no decrease of the unit cell parameter (a0 = 4.719 nm). This suggests that the nanostructure of the iron containing MCM-41 silica possesses a high thermal stability. The XRD patterns show no additional peaks at the high 20 range of 10-100 ~ indicating that no crystalline iron oxide phase has been formed outside the pore structure, even atter calcination. However, iron oxide clusters might be synthesized within the pores and too small for X-ray detection. N2 adsorption-desorption isotherm for the calcined sample and its corresponding pore size distribution curve calculated using the BJH method are presented in Figure 2. As shown in Figure 2, a typical irreversible type IV adsorption isotherm with a hysteresis loop, as identified by IUPAC [19], is observed. A sharp step occurs in P/Po range between 0.3 and 0.4, which indicative of the filling of N2 molecules in the mesopores. The P/Po position of the inflection points is clearly related to a diameter in the mesopore range and the step indicates the mesopore size distribution. From a plot of the pore size distribution in Figure 2, we can see a narrow pore size distribution centered at 3.1 nm. The pure silica MCM-41 was also

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.

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6 8 10 20 (degree) Figure 1. XRD patterns of iron oxide containing MCM-41 samples. + Adsorption --'-- D e s o r p t i o . n ~ ~

60(

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Figure 2. N2 adsorption-desorption isotherm at liquid N2 temperature for the sample and its corresponding pore-size distribution curve

(Inset). synthesized with the same condition without the

406

Figure 3. Micropore size distribution plots resulted from (a) MP method and (b) HK method. addition of iron species, and presented the similar XRD and N2 adsorptin data. BET surface area of the present FeMCM-41 sample is 981 m2/g and pore volume is 0.925 cm3/g, which are slightly lower than those of pure silica MCM-41. However, further micropore analysis by means of t-method [20], MP method [21] and Horvath-Kawazoe (HK) method [22] shows that the sample has a significant microporosity. A V vs. t (the volume of gas adsorbed versus the statistical thickness of an adsorbed film) plot with interval slopes is calculated. The micropore surface area and micropore volume from the t-method are 596 m2/g and 0.377 cm3/g respectively. Micropore analysis result from the MP method shows a micropore size distribution centred at 1.37 tun, and from the HK method, the pore width is 1.22 nm (Figure 3). Such a large microporosity might be related with the loading of iron oxide in the MCM-41 mesopores. A combination of highresolution transmission electron microscopic image processing and selected area electron Figure 4. A typical TEM image of the calcined iron oxidediffraction has been containing MCM-41 and its electron diffraction pattern (Inset).

407 proved to be a suitable method to study small crystals, which can not be efficiently measured by single-crystal X-ray diffraction for structure determination [23]. In the present study, HRTEM and electron diffraction have been used to study the possible small crystals of iron oxide implanted in mesoporous MCM-41. It is significant to notice that the samples were relatively stable under the electron beam irradiation in comparison with pure silica MCM-41 and most of other doped MCM-41 materials. Figures 4 and 5 show a series of TEM images obtained by successive imaging at the same area and their electron diffraction images. A regular hexagonal arrangement of the pore openings is observed and the pore center to pore center distance is about 4.5 ran, in agreement with the XRD results very well. Since these photographs were recorded under conditions far from optimum (Scherzer focus), the contrast is reversed. The overfocus condition, however, does not alter the symmetry of the structure observed so that the hexagonal shape of the pores in the micrographs represents the true geometry of the pores [24]. A close look of these images enables us to view intensity deviations and extra bright intensity within the pores. Fr~Sba et al. [18] believed that the existence of extra intensity modulations should be related with the doping of iron oxide in MCM-48 molecular sieve silicas. The TEM images in Figures 4 and 5 also show local superstructure crystalline phases within a large area. The position and area of such a crystalline phase could change during the successive electron beam exposure and resulted in the formation of a cycle. Under the electron beam irradiation, the possible movement with slight tilting of the particle, and the ruggedness of the particle surface might result in some changes of the image contrast pattern. However, we believe that the significant contrast change observed in Figures 4 and 5 implies the existence of iron oxide nanocrystallites within the pores or in the silica framework. They may have regular and oriented arrangement. Additional strong evidence for the Fe203 loading is supported by the selected area electron diffraction of the same particle (Figure 4) and the inverse Fourier transform pattern (Figure 5d). There are superstructure reflections reflected in the electron diffraction patterns besides many diffraction spots appeared with uniform hexagonal pattern (Figure 5d), though no any additional X-ray diffraction peaks were observed for Fe203 nanocrystallites. It is evident to indicate the possible existence of onedimensionally ordered iron oxide nanoarrays accreted with the hexagonal mesoporous silica. Figure 6 shows the TEM image of the iron oxide containing MCM-41 sample viewed along a direction perpendicular to the pore axis and its inverse Fourier transform pattern. The intensity deviations and extra intensity within the pores are also visible on the images. Many spots appear in its inverse Fourier transform pattern, revealing that Fe203 crystals present in the pore structure and its orientation is almost along with the pore axis (Figure 6). To make sure of the formation of Fe203 microcrystals, electron energy-loss spectroscopy was carried out. Transmission electron energy-loss spectroscopy has been proved to be a powerful analytical tool to investigate chemistry and electronic structures in thin solid objects [25,26]. Since the iron oxide crystals are too small and dispersed within the mesoporous silica, the signal-to-noise ratio is very low, and the EELS experiments should be done by increasing the number of scans up to 50. Figure 7 shows the EELS spectrum of the present sample, in comparison with the spectrum of amorphous mesoporous Fe203 obtained from the similar method described in experimental section. It is interesting that the Fe-L2.3 edge of the present Fe203 containing MCM-41 silica is -15 eV lower in energy than that of the bulk porous amorphous Fe203, and its intensity is lower. Only one weak peak can be observed obviously instead of at least two from the bulk mesoporous Fe203, which could be attributed to the low iron content in the sample (Si/Fe ratio of 100). The chemical environment of iron

408 in the MCM-41 silica should be different from that of bulk amorphous iron oxide. The small iron oxide crystals in the MCM-41 sample might be linked with silica wall. Another possibility is that the small iron oxide crystals react with silica during synthesis, causing the partial crystallization of the framework. These possibilities might cause the decrease of the energy of Fe L2,3edges in the present MCM-41 materials.

Figure 5. (a) - ( c ) A serious of TEM images obtained by successive imaging at the same area in Figure 4 (the crystalline phase positions are circled in white), and (d) the inverse Fourier transform pattern of image (a) (some superstructure reflection spots are pointed with white arrows).

409 4. CONCLUSIONS Iron oxide containing mesoporous silica MCM-41 has been synthesized directly and characterized with various techniques. Comprehensive application of HRTEM, SAED, and ELLS revealed that microcrystals of iron oxide might exist in the silica matrix with regular and oriented arrangement, though the chemical environment of iron in the present mesoporous silica is different from that of bulk amorphous porous iron oxide. Further investigation of this material on the relationship of its microstructure and property as well as its possible applications is still in progress in these laboratories.

Figure 6. TEM image of the sample on a direction perpendicular to the pore axis and its inverse Fourier transform pattern (inset).

5. ACKNOWLEDGEMENT This research was supported by the National Natural Sciences Foundation of China (NSFC) and Chinese Academy of Sciences.

REFERENCES

1. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.-W. Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins and J.L. Schlenker, J. Am. Chem. Sot., 114 (1992) 10834. 3. A. Sayari, Chem. Mater., 8

Figure 7. EELS iron L2,3 edges for (a) bulk amorphous mesoporous Fe203 and (b) Fe203 containing MCM-41 silica.

410 (1996) 1840. 4. A. Corma, Chem. Rev., 97 (1997) 2373. 5. K.M. Reddy, I. Moudrakovski and A. Sayari, J. Chem. Sot., Chem. Commun., (1994) 1059. 6. A. Corma, M.T. Navarro and J. Porez-Pariente, J. Chem. Soc. Chem. Commun., (1994) 147. 7. A. Corma, A. Martinez, V. Martinez-Soria and J.B. Monton, J. Catal., 153 (1995) 25. 8. J.Y. Ying, C.P. Mehnert and M.S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. 9. H. Winkler, A. Birkner, V. Hagen, I. Wolf, R. Schmechel, H. von Seggern and R.A. Fischer, Adv. Mater., 11 (1999) 1444. 10. R. Leon, D. Margolese, G. Stucky and P.M. Petroff, Phys. Rev. B, 52 (1995) 2285. 11. K. Segawa, K. Watanabe and S. Matsumoto, Jpn. 93317649, 1993 [Chem. Abstr., 120 (1993) 142982s]. 12. A. Hernadi, A. Fonseca, J.B. Nagy, D. Bernaerts, A. Fudala and A.A. Lucas, Zeolites, 17 (1996) 416. 13. W.Z. Li, S.S. Xie, L.X. Qian, B.H. Chang, B.S. Zou, W.Y. Zhou, R.A. Zhao and G. Wang, Science, 274 (1996) 1701. 14. Z.Y. Yuan, S.Q. Liu, T.H. Chen, J.Z. Wang and H.X. Li, J. Chem. Soc., Chem. Commun., (1995) 973. 15. A. Wingen, D. Anastasieviec, A. Hollnagel, D. Werner and F. Schiith, Stud. Surf. Sci. Catal., 130 (2000) 3065. 16. N. He, S. Bao and Q. Xu, Appl. Catal. A, 169 (1998) 29. 17. T. Abe, Y. Tachibana, T. Uematsu and M. Iwamoto, J. Chem. Soc., Chem. Commun., (1995) 1617. 18. M. FrSba, R. KShn and G. Bouffaud, Chem. Mater., 11 (1999) 2858. 19. K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol and T. Siemieniew, Pure Appl. Chem., 57 (1985) 603. 20. G.D. Halsey, J. Chem. Phys., 16 (1948) 931. 21. R.S. Mikhail, S. Brunauer and E.E. Bodor, J. Colloid Interface Sci., 26 (1968) 45. 22. G. Horvath and K. Kawazoe, J. Chem. Eng. Japan, 16 (1983) 470. 23. A. Carlson, T. Oku, J.-O. Bovin, G. Karlsson, Y. Okamoto, N. Ohnishi and O. Terasaki, Chem. Eur. J., 5 (1999) 244. 24. V. Alfredsson, M. Keung, A. Monnier, G.D. Stucky, K.K. Unger and F. Schiith, J. Chem. Soc., Chem. Commun., (1994) 921. 25. R.F. Egerton, Electron Energy-Loss Spectroscopy in the Electron Microscope, Plenum, New York, 1986. 26. M.M. Disco, C.C. Ahn and B. Fultz, Eds., Transmission Electron Energy Loss Spectrometry in Materials Science, TMS, Warrendale, 1991.

~stUdleS m Surtace Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) O 2002 Elsevier Science B.V. All rightsreserved.

411

M e s o p o r o u s molecular sieves of M C M - 4 1 type modified with Cs, K and M g physico-chemical and catalytic properties Mafia Ziolek, Aleksandra Michalska, Jolanta Kujawa and Anna Lewandowska

A.Mickiewicz University, Faculty of Chemistry, Grunwaldzka 6, PL-60-780 Poznan, Poland; e-mail: ziolek@amu, edu.pl Siliceous, aluminosilicate, and niobosilicate mesoporous molecular sieves of MCM-41 type were modified with Cs, K, and Mg via an ion exchange and impregnation. The impregnation with Cs-acetate leads to a partial structural distortion of mesoporous sieves used. In spite of that Cs-impregnated NbMCM-41 exhibits the highest basicity, whereas, Cs template ion exchanged materials as well as Cs-impregnated AIMCM-41 show the acid-base properties. Kimpregnated MCM-41 is only less basic than Cs/NbMCM-41, and thanks to its stability during the modification , one can recommend K/MCM-41 mesoporous molecular sieves as an effective basic catalysts. I. INTRODUCTION The discovery of mesoporous materials of MCM-41 type in 1992 has given hope for their use as matrices for basic agents. Many organic syntheses require the catalytic reactions involving basic centres. Catalysis by basic zeolites is limited to relatively small molecules. The mesoporous molecular sieves can be of assistance in this field. The impregnation or ion exchange with cesium species commonly generates basicity. However, it is well known that SiO-Si bonds readily hydrolyse in strong basic media. Recently, C.Noda Perez et al. [1] have found that even in pH _= 8.5 the MCM-41 structure is not stable. Al-containing samples were less resistant than the purely siliceous one to basic media. However, Kloestra and van Bekkum [2] concluded that an increase in the framework stability occurs with lowering of Si/AI ratios, i.e. with the growth of AI content. It can be supposed that the nature of T-atom in the MCM41 frameworks influences their resistance to basic media interaction. The aim of this study was to use various mesoporous matrices (SiMCM-41, AIMCM-41 and NbMCM-41) for the generation of basic centres with Cs, K and Mg via an ion exchange procedure or an impregnation. A very low concentrated (- 0.02M) solutions of metal salts were applied. The obtained materials were characterised with X R , N2 adsorption/desorption, F T I ~ H2-TPR, TEM, and a test reaction (acetonylacetone cyclization [3]). 2. EXPERIMENTAL

2.1. Synthesis and modification Si- Nb- and AI- containing mesoporous molecular sieves ofMCM-41 type were synthesised according to the procedure described in [4] and modified in the preparation of NbMCM-41 according to [5]. Si/T atom ratio of 32 has been applied. The Cs, K, and Mg ion-exchange

412 (IE) was performed using stirring of the calcined mesoporous solid in aqua solution of cesium acetate (0.02 M), or potassium chloride, or magnesium chloride, respectively, at room temperature (RT). After stirring, the samples were filtrated, washed with 20 cm3 of distilled water and dried at 373 K for 5 h. The template ion exchange (TIE) has been also applied using mesoporous molecular sieves containing template (i.e. before the calcination). The modified mesoporous molecular sieves were also prepared by the impregnation. In this case the calcined sieves were used as parent materials and atter the impregnation the samples were not washed, only dried at 393 K for 1 h and calcined at 773 for 14 h. The percent of metal introduced to the MCM-41 samples was obtained from AAS analyses. 2.2. Sample characterisation

N2 adsorption/desorption studies were conducted at 77 K with Micrometrics ASAP 2010 apparatus. The samples were first outgassed at 573 K for 3 h.

Powder X-ray diffraction (XRD). XRD patterns were obtained on TUR 42 difffactometer with CuK~ radiation (10kV, 40 mA) and a step size 0,02 ~

The temperature-programmed reduction (TPR) of the samples was carried out using H2/Ar (10 vol.%) as reductant (flow rate = 32 cm3 min'~). 0.03 g of the sample was filled in a quartz tube, treated in a flow of helium at 673 K for 1 h, and cooled to room temperature. Then, it was heated at the rate of 10 K min"~to 1100 K under the reductant mixture. A thermal conductivity detector in the PulseChemiSorb 2705 (Micromeritics) instrument measured hydrogen consumption. Fourier-Transform Infrared Spectroscopy ~TIR). Infrared spectra were recorded with a VECTOR 22 (BRUKER) FTIR spectrometer. The samples were prepared by diluting of 0.001 g of the mesoporous molecular sieve in KBr. The spectra were scanned in the framework range ( 4 0 0 - 1500 cm~).

Transmission Electron Microscopy~EM). JEOL 2000 transmission electron microscope was used for the TEM image registration. 2.3. Test reaction The acid and base characteristic of the catalyst were evaluated using the probe reactionacetonylacetone (AcAc) cyclization- reported by Dessau [3] and applied by Alcaraz et al. [6]. In this reaction dimethylfuran (DMF) is produced on acidic centres, whereas, basic centres are involved in the formation of methylcyclopentenone (MCP). The reaction was conducted in a pulse-flow micro-reactor in which 2 cm3 of AcAc was passed continuously over 0.05 g of the granulated catalyst at 523 K in a nitrogen flow. The reaction products were collected downstream of the reactor in a cold trap and analysed by a gas chromatography (CHROM-%, Silicone SE-30 / Chromosorb column).

413 3. RESULTS AND DISCUSSION 3.1. Texture characterisation

The data calculated from N2 desorption isotherm of cesium and potassium containing molecular sieves are shown in Table 1. All of the materials are mesoporous with a high surface area and pore volume. Both parameters decrease drastically in two samples, Cs-NbMCM41(IE) and Cs/NbMCM-41, less significantly for all other impregnated materials and only slightly in the case of cation-exchanged ALMCM-41. Table. 1. Catalysts and their characterisation Cation exchange, [%] or impreg.

Catalyst

Surface area, BET [m2gq ]

Pore volume BJH [cm3gq ]

1022 750 752 1033 1000 946 791 1034 984 558 301

1.455 1.125 1.054 1.266 1.236 1 142 0.905

The above results were confirmed by XRD patterns which showed less intensive, smiled peaks for the impregnated materials and Cs-NbMCM-41 (IE) in comparison with those for the parent samples (as example - Fig. 1). However, when template ion exchange procedure has been applied the final materials, even in the case of Nb-containing matrix, exhibited well ordered hexagonal arrangement (Fig. 2).

[wt.%] MCM-41 K/MCM-41 Cs/MCM-41 A1MCM-41 K-AIMCM-41 (TIE) Cs-AIMCM-41 (IE) Cs/AIMCM-41 NbMCM-41 K-NbMCM-41 (TIE) Cs-NbMCM-41 (IE) Cs/NbMCM-41

5 5 -

73 77 5 80 79 5

1 193 1 121

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100

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Fig. 2. XRD patterns ofNbMCM-41 modified with various cations.

It is worthy to notice that a TEM image (Fig. 3) of the material estimated on the basis of XRD and N2 adsorption/desorption isotherm as the most distorted one, i.e. Cs/NbMCM-41,

414 indicates the presence of parallel hexagonal ordered mesopores.Therefore, one can suggest that the decrease of pore volume and the intensity of XRD peaks are due to the presence of a bulky phase in the mesopores rather than to the destruction of mesopores in the material. The literature [7] described the effect of the adsorbate molecules located in the mesopores on the decrease of the XRD peak intensity. The same effect can be involved by the presence of bulky oxide species in the impregnated samples. So, one should consider rather the distortion of the mesoporous structure and not the destruction due to the modification. This distortion effect for the desired cation depends on the nature of T-element located in the framework. The presented results indicate that aluminosilica material exhibits the highest stability. The following order of the stability of modified mesoporous molecular sieves of M41S family can be presented: Me/AIMCM-41 > Me/MCM-41 > Me/NbMCM-41 where Me=K, or Cs, or Mg. Most probably two effects cause the destabilisation of the mesoporous molecular sieves. One, described earlier in [1], it is the interaction of basic alkali media with the solid. The second one is the location of a bulky phase in the pores leading to the changes of the texture parameters Therefore, the structure properties strongly Fig. 3. Transmission electron depend on the modification procedure, which, among micrographe of Cs/NbMCM-41. others, determines the species formed in the final material. It is well illustrated by the FTIR spectra shown, as example, for aluminosilica materials in Fig. 4. The impregnation followed by calcination leads to the formation of cesium oxide species which size is, of course, greater than that of cesium cation located in the extra framework position after the cation exchange procedure. The presence of a bulky phase in the mesopores of Cs/AIMCM-41 material causes the lowering of the IR band intensity in the spectrum registered in the framework range.

3.2. Surface properties

Fig. 4. FTIR spectra of aluminosilica based Materials.

We have considered the effect of the modification procedure on the surface properties of the matrix. Nb-containing support has been chosen for this study because one could observe the changes in the reduction properties of niobium. H2-TPR technique has been applied in this study. In the H2-TPR profiles the peaks at about 1000 K and higher temperatures were assigned earlier to the reduction of niobium located in the framework [8].

415 160 140

-120

f, 0

~ 80 .

-r-

60

40

200

400 600 800 Temperature, K

1000

Fig. 5. H2-TPR profiles of modified NbMCM-41 materials.

The number of H2-TPR peaks in this region corresponds to the number of various niobium species, i.e. various surroundings of No. From this observation one can indirectly conclude the uniformity of metal loading. The analysis of the TPR profiles presented in Fig. 5 suggests the highest uniformity of Cs loading in the sample prepared via the impregnation - only one main reduction peak in the TPR profile was registered. A higher number of peaks observed in the case of the other samples suggests various location of cesium cations in relation to niobium in the NbMCM-41 framework, or more precisely saying - various cationic surroundings of Nb framework species. This behaviour can determine the catalytic activity of the material discussed in the next paragraph. If cesium cations are not located near each Nb in the framework, a part of niobium species can act as active centres, increasing the acidity of the material.

3.3. Catalytic testing Table. 2. The results of the cyc!ization of acetylacetone.

In the transformation of acetonyloacetone the ratio of the Catalyst AcAc MCP/DMF conversion, selectivity ratio selectivities: methylcyclopentenone (MCP) / % dimethylfuran (DMF) determines Cs/MCM-41 21 oo the b a s i c - acidic properties of Cs-NbMCM-41 (IE) 3.5 0.4 the catalysts [3,6]. It has been Cs- NbMCM-41 (TIE) 21 0.7 stated that when MCP/DMF >> 1 Cs/NbMCM-41 5 oo the catalyst exhibits basic Cs-AIMCM-41 (IE) 26 0.2 properties, whereas MCP/DMF Cs-AIMCM-41 (TIE) 58 0.2 1), like MgO. 4. Magnesium impregnated matrices are less basic than Cs and K impregnated samples. 5. Aluminosilica material, even impregnated with cesium acetate, is not basic. Cs/AIMCM-41 reveals acid - basic properties (MCP/DMF -~1). 4. SUMMARY 9 The impregnation ofMCM-41 samples studied with Cs-acetate leads to the decrease of BET surface area and pore volume as well as the XRD peaks intensity which is the highest on NbMCM-41. However, a TEM image indicates that the honey comb structure and parallel ordered mesopores in Cs/NbMCM-41 are preserved, suggesting only a partial structural distortion. It hints that NbMCM-41 mesopores arrangement is rather stable and the surface area and pore volume decrease is due to the presence of bulky phase inside the pores. 9 The impregnation of MCM-41 with KCI or MgCI2 solutions causes only slightly changes in the ordering of the materials. 9 The traditional cation exchange procedure (IE) influences a little the mesoporous structure, whereas, the application of the template ion exchange (TIE) allow the total resistance of the hexagonal arrangement of the materials. 9 The basicity (measured by the test reaction) of the impregnated samples is higher than that one of the ion-exchanged materials and changes in the following order: Cs/NbMCM-41 ~ Cs/MCM-41 > K/MCM-41 > M g ~ C M - 4 1 9 Cs-NbMCM-41 (TIE) and the Cs/AIMCM-41 exhibit acid - base properties. The presented studies confirmed the earlier observation [1,2] that mesoporous molecular sieves of MCM-41 type modified via impregnation with Cs-acetate are highly basic. Moreover, this work indicates that the basicity of mesoporous molecular sieves can be enhanced if the N b containing matrix (thanks to its higher oxygen charge [9]) is applied instead of AIMCM-41, although the surface area and pore volume of NbMCM-41 drastically decrease after impregnation with Cs. To reduce this disadvantage, K-impregnated siliceous MCM-41 can be used. Its basicity is higher than that of Mg~CM-41 and slightly lower than that of Cs/MCM41 and Cs/NbMCM-41. REFERENCES

1. C.N. Perez, E. Moreno, C.A. Henriques, S. Valange, Z. Gabelica and J.L.F. Monteiro, Microporous and Mesoporous Materials, 41 (2000) 137 2. K. R. Kloestra and van Bekkum, Stud. Surf. Sci. Catal., 105 (1997) 431. 3. R.M. Dessau, Zeolites, 10 (1990) 205. 4. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, and J.S. Beck, Nature, 359 (1992) 710. 5. M. Ziolek, I. Nowak, Zeolites, 18 (1997) 356. 6. J.J. Alcaraz, B.J. Arena, R,D, Gillespie and J.S. Holmgren, Catal. Today, 43 (1998) 89. 7. B. Marler, U. Oberhagemann, S. Vortman, H. Gies, Microporous Materials, 33 (1999) 165. 8. M. Ziolek, I. Sobczak, A. Lewandowska, I. Nowak, P. Decyk, M. Renn and B. Jankowska, Catal. Today, 70 (2001) 169. 9. M. Ziolek, I. Sobczak, I. Nowak, P. Decyk, A. Lewandowska and J. Kujawa, Microporous and Mesoporous Materials, 35-36 (2000) 195.


417

M e s o - A L P O prepared by thermal decomposition o f the organic-inorganic composite: A F T I R study Enrica Gianotti ~a)*,Erica C. Oliveira~b),Valeria Dellarocca~a), Salvatore Coluccia ~a),Heloise O. Pastore ~b)and Leonardo Marchese ~c) Ca)Dipartimento di Chimica IFM, Universi~ di Torino, v. P. Giuria, 7, 10125, Torino- Italy ~b)Instituto de Quimica, Universidade Estadual de Campinas, CP 6154, 13083-970, Campinas, SP, Brasil. ~c)Dipartimento di Scienza e Tecnologie Avanzate, Universitfi del Piemonte Orientale, "A. Avogadro", C.so Borsalino, 54, 15100, Alessandria - Italy.

Mesoporous ALPO was synthesised using cetyltrimethylammonium bromide (CTAB) as a structure-directing agent. FTIR spectroscopy was used to follow the formation of the ALPO mesophase by thermal decomposition of the aluminophosphate/surfactant composite and NH3 was used as probe molecule to monitor the surface acidity of the product. 1. INTRODUCTION Aluminophosphate-based microporous molecular sieves are known to exist in a wide range of structural and compositional diversity. In the search for new synthesis methods, that could afford channel systems with pores in the range of mesoporosity, phosphate-based molecular sieves, like cloverite[1] and VPI-5[2], have been prepared and displayed ring systems larger than the usual 12-T-atom found in large pore zeolites. Despite the large pores, the openings in these solids are not larger than 1.2 nm limiting their use to similar microporous systems of reactants. It was not until the advent of mesoporous silicates and aluminosilicates that the possibility of preparing aluminophosphates with pore apertures larger than the ones already known turned into a reality[3]. However, the main problem of the mesostructures firstly obtained is their low thermal stability when calcined or submitted to conventional neutral or acid solvent extraction for the removal of the structure-directing agent. Recently, we reported the synthesis of aluminophosphates and magnesium-aluminophosphates using cetyltrimethylammonium bromide (CTAB) as a structure-directing agent with the aim of obtaining large-pore mesoporous materials[4]. An alkaline extraction was proposed to prevent collapse of the mesostructure while promoting simultaneous ion exchange in metal-aluminophophates. The synthesis of aluminophosphate and magnesium-aluminophosphates-based mesoporous materials using aluminum sulfate as a source of aluminum and without introduction of hydrofluoric acid as a mineralizing agent was also explored[5]. In this paper, we report on the formation of an ALPO mesophase by thermal decomposition of the aluminophosphate-surfactant composite monitored by FTIR spectroscopy. NH3 was used as molecular probe of the surface acidity of the product.

*to whomcorrespondenceshouldbe addressed,[email protected]

418 2. EXPERIMENTAL Mesoporous ALPO samples were synthesized[5] by adding solution 1, prepared by dissolving aluminum sulfate 18-hydrate in water, into solution 2, prepared by the dilution of phosphoric acid in water. After that, an aqueous suspension of CTAB was added, followed by 30 min of homogeinizing and the addition of TMAOH 25 wt % aqueous solution until the desired pH. The mixture was aged for 24h at room temperature, after which it was submitted to hydrothermal treatment at the 70~ C for 48h. The gel composition for a final pH of 8.50 was A1203 : 1.27 P2Os : 2 CTAB : 7.35 TMAOH : 410 H20. The sample was submitted to an alkaline extraction [4] followed by heating under argon until 773 K at 1K min ~ and 10h at that temperature under dry oxygen. The materials were characterized by powder X-rays diffraction (Shimadzu, XRD 6000, CuK~, 30 kV, 40 mA, 2~ 20 minl) and N2 adsorption (ASAP 2010, Micromeritics at 77K after thermal treatment at 298 K until residual pressure of 10-4 Pa). FTIR spectra on ~elletised sample were recorded using a Bruker IFS88 spectrometer at the resolution of 4 cm-, equipped with a high vacuum variable temperature infrared cell (LB-100 of the Infraspac of Novosibirsk) which was permanently connected to a vacuum line (ultimate pressure < 105 mbar). 3. RESULTS AND DISCUSSION The use of cetyltrimethylammonium bromide as surfactant, aluminum sulphate and orthophosphoric acid has allowed the preparation of mesoporous aluminophosphate with Xrays diffractogram characteristic of a hexagonal organization of pores, Fig.la. However, after extraction on alkaline solution and calcination, the samples show only the (100) diffraction in the X-ray diffractogram, Figure lb and c. N2 adsorption at 77K shows that the surface area of of the aluminophosphate is 760 m2g1 with an isotherm that is a mixture of types I and IV and a maximum pore volume of 0.32 cm 3 g-l. The study of the thermal decomposition of the template in mesoporous as-synthesized ALPO was followed by in situ infrared spectroscopy.

]~1000 cps o,.~ r~

-:i

',.._..-_..L

l

20/degrees Fig. 1 - X-rays diffractograms: (a) of as-synthesized sample used in this work; (b) extracted with alkaline solution, and (c) calcined after extraction.

419 Fig.2 shows the FTIR spectra of the, as-synthesised mesoporous ALPO after outgassing at increasing temperatures from 200~ to 500~ (curves a to e). After water desorption at 200~ a broad band in the range 3800-3200 cm -1 due to H-bonded P-OH and AI-OH groups is observed. At higher temperatures (curves b-e), dehydroxylation takes place and oxygensharing-AIO4 and -PO4 tetrahedra, along with isolated P-OH and AI-OH groups, are formed (Scheme 1). In fact, bands at 3670 cm 4, corresponding to the stretching mode of isolated POH, and bands at 3789 and 3720 cm 4, due to the stretching mode of free AI-OH groups, became more evident when the temperature was increased. Absorptions in the range of 3050-2800 cm 4 are due to the C-H stretching vibrations of the CH2 and CH3 groups of the surfactant. Bands of CH2 (2922, 2851 and 1458 cm -1) and CH3 (2964 and 2876 cm 4) of the hydrocarbons chains of the template decrease with increasing temperature up to 500~ (curve e) but disappear only after calcination at 550~ (curve f).

0 /

H..o/H_ -

H. __0 /

t

H

H

--O ~

t

-H20

I

H

0 /

,, a"-o P'o

O

O/

"a,

o / / \.-./ OYOo t)

_

_

-

~

o-O-

_

Scheme 1

. . . . .

N

,n.

/ / b

"o

":'""'/""""/""

0

5

10 15 are f (~tmol m2)

I

20

Figure 4. Comparison plots for the sample OMA/3 using standard adsorption data obtained on Degussa Aluminiumoxid C (A) and ~x-alumina (+).

0,10,0

4

6

D (nm)

I

8

Figure 5. Mesopore size distributions for samples OMA/1 (A), OMA/2 (B) and OMA/3 (C).

435 The reference isotherm obtained on alumina DC transformed to statistical film thickness curve was also used for calculations of mesopore size distributions using the Barrett-JoynerHalenda (BJH) method [ 16]. The resulting distribution curves for samples OMA/1, OMA/2 and OMA/3 are shown in Fig. 5. Table 2. Material parameters

Sample OMA/1 OMA/2 OMA/3 OMA/4 OMA/5 OMA/6

SBET

STOT

SEXT

VMESO

VBJH

DBJH

d

6

(m 2 g-l)

(m 2 g-l)

(m 2 g-i)

(cm 3 g-l)

(cm 3 g-~)

(nm)

(nm)

(nm)

489 313 223 758 397 707

475 303 222 769 382 696

2.0 1.9 2.0 36.1 25.2 5.9

0.527 0.477 0.409 0.681 0.633 0.588

0.566 0.511 0.422 0.722 0.671 (0.521)

3.3 4.4 5.1 3.7 4.7 (2.5)

5.0 7.0 8.8

1.7 2.6 3.7

The parameters of aluminas obtained by means of BET, comparison plots and BJH methods are summarized in Table 2. The values of SmT and VMESOfrom comparison plots are in satisfactory agreement with values of SBETand VBJHobtained using BET and BJH methods.

20_crrgz-~l "7 15-

O

i'

E 10t~

o 0,0

1

0,2

i

0,4

I

0,6

i

0,8

1

P/Po Figure 6. Nitrogen adsorption isotherms at - 196 ~ on samples OMA/4 (), OMA/5(v) and OMA/6 (zx). The solid points denote desorption.

In the reaction mixture of samples OMA/4, OMA/5 and OMA/6 lauric acid was replaced with stearic acid, which enabled to obtain aluminas with substantially larger surface areas exceeding 700 m2/g (Table 2, samples OMA/4 and OMA/6). The pore size of sample OMA/6 was assessed by means BJH method; as the Kelvin equation is not strictly valid for these relative fine mesopores, the pore diameter of sample OMA/6 is given in Table 2 in parenthesis. The conclusions, which can be drawn from the results obtained on the samples OMA/4, OMA/5 and OMA/6, are fully identical with those, which follow from results on samples OMA/1, OMA/2 and OMA/3. Therefore, the applicability of the standard adsorption data on the alumina DC was successfully tested on all the OMA samples.

The influence of the heat treatment of the OMA on its structure was investigated also by X-ray powder diffraction. All the diffractograms of samples OMA/1, OMA/2 and OMA/3 exhibited only one diffraction peak at very low 20. With increasing temperature of the heat treatment the position of the diffraction peak is shifted towards lower 20, which indicates some re-organization of the alumina structure connected to an increase in the pore size caused

436 by their coalescing [5]. The correlation length d corresponding to the maximum of the diffraction peak and pore wall thickness 6, which was determined by subtracting the pore diameter from the correlation length, are given in Table 2. The structure data obtained from nitrogen adsorption confirmed that the higher the calcination temperature, the larger the correlation distance and consequently the pore size. Simultaneously, the pore size increases from 3.3 to 5.1 nm. This increase in the pore size is accompanied by the increase in the pore wall thickness, which agrees with the coalescing mechanism. 4. CONCLUSIONS The standard nitrogen adsorption data on non-porous Degussa Aluminiumoxid C and a-alumina were obtained at -196 ~ in the range of relative pressures from 0.001 to 0.95. The applicability of these data was tested using the comparison plot method. Although some OMA samples were calcined at relatively high temperatures, only the standard data on Degussa Aluminiumoxid C have proved to be suitable for the analysis of nitrogen isotherms on OMA. ACKNOWLEDGEMENTS

This investigation was supported by the Ministry for Education, Youth and Sport of the Czech Republic (ME404) and by NATO (SFP-974217). REFERENCES

1. 2.

3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16.

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, 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. J.Y. Ying, C. P. Mehnert and M. S. Wong, Angew. Chem. Int. Ed., 38 (1999) 56. S.A. Bagshaw and T. J. Pinnavaia, Angew. Chem. Int. Ed. Engl., 35 (1996) 1102. F. Vaudry, S. Khodabandeh and M. E. Davis, Chem. Mater., 8 (1996) 1451. S. Cabrera, J. E1 Haskouri, J. Alamo, A. Beltrfin, D. Beltr~in, S. Mendioroz, M. Dolores Marcos and P. Amor6s, Adv. Mater., 11 (1999) 379. S. Valange, J.-L. Guth, F. Kolenda, S. Lacombe and Z. Gabelica, Microporous Mesoporous Mater., 35-36 (2000) 597. H.Y. Zhu, P. Cool, G. Q. Lu and E.F. Vansant, Stud. Surf. Sci. Catal., 135 (2001) 253. V. Gonz~les-Pefia, C. M~rquez-Alvarez, E. Sastre and J. P&ez-Pariente, Stud. Surf. Sci. Catal., 135 (2001) 204. R. Ryoo, J.M. Kim, C.H. Ko and C.H. Shin, J. Phys. Chem. 100 (1996) 17718. M. Jaroniec, M. Kruk and J. P. Olivier, Langmuir, 15 (1999) 5410. S.J. Gregg and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982, p. 90. J. Rathousk~, G. Schulz-Ekloff and A. Zukal, Micropor. Mater. 6 (1996) 385. F. Rouquerol, J. Rouquerol and K. Sing, Adsorption by Powders and Porous Solids, Academic Press, London, 1999, p. 174. B.C. Lippens, B. G. Linsen and J. H. de Boer, J. Catal., 3 (1964) 32. J.H. de Boer, B. G. Linsen and Th. J. Osinga, J. Catal., 4 (1965) 643.


437

The use o f ordered m e s o p o r o u s materials for i m p r o v i n g the m e s o p o r e size analysis: Current state and future Mietek Jaroniec, a Michal Kruk a and Abdelhamid Sayari b a

Department of Chemistry, Kent State University, Kent, Ohio 44242, USA

b Centre for Catalytic Research and Innovation, Department of Chemistry, University of Ottawa, Ottawa, Ontario K1N 6N5, Canada

In 1997 a simple approach to an accurate calculation of mesopore size distributions was proposed on the basis of model gas adsorption isotherms for ordered mesoporous materials (OMMs), whose pore diameters were evaluated independently from adsorption pore size analysis. This approach was originally developed for nitrogen adsorption at 77 K on silicas with cylindrical pores, and recently extended to argon adsorption at 77 and 87 K, as well as to adsorption on materials with hydrophobic surfaces. The method to calculate the pore size distributions (PSDs) from nitrogen data at 77 K has already attracted much interest and has been implemented by several researchers working in the field of OMMs. Herein, the current state and future perspectives in the development of accurate methods for PSD calculation using OMMs as model adsorbents are discussed. The development of these methods requires the availability of OMMs with as wide range of pore diameters as possible, and therefore the progress and challenges in the synthesis of such materials are overviewed. Recent advances in the development of reliable and independent methods for the OMM pore size assessment, necessary for elaboration of the aforementioned methods for the PSD calculation, are reviewed. The current status and prospects for the development of OMM-calibrated methods for the pore size analysis of different kinds of mesoporous materials are discussed.

1. INTRODUCTION The discovery of ordered mesoporous materials (OMMs) [1] has had a tremendous influence on the field of gas adsorption characterization of porous materials. Gas adsorption has become a key tool for characterization of OMMs [2,3], because it is a crucial method for calculation of mesopore size distributions. On the other hand, OMMs are first model mesoporous solids with pores of uniform size and well-defined shape, so they are suitable for evaluation of the applicability and accuracy of gas adsorption methods for characterization of porous solids [2-8]. The use of OMMs as model adsorbents allowed one to convincingly demonstrate that gas adsorption data actually provide an abundance of.information about structural properties of mesoporous materials. However, the studies of OMMs also strongly suggested that many currently used methods for determination of the structural parameters, such as the pore size distribution (PSD) and the specific surface area, from gas adsorption

438 data are highly inaccurate or sometimes even inherently inapplicable [2-8]. Therefore, there is a strong incentive to develop new methods for gas adsorption data analysis or, if possible, to modify the classical methods, in order to be able to fully realize the potential of gas adsorption for characterization of porous materials. To this end, the discovery of OMMs not only allowed us to find weaknesses in the hitherto known methods of adsorption data analysis, but also provided a way for refinement of some of these methods or for development of new ones. This is because OMMs can actually be used to generate model gas adsorption isotherms for mesopores of various sizes and shapes. In the case, when the pore size and geometry of the model OMMs can be accurately determined, and the degree of their structural ordering is high, the model gas adsorption isotherms generated in this way are benchmark results that can be used to assess the suitability of other methods for generating model gas adsorption data. These other methods include those based on advanced computational approaches, such as nonlocal density functional theory (NL DFT) [4] and computer simulations [9]. These methods require a largely arbitrary assignment of branches of computational adsorption-desorption hysteresis loops to the experimentally observed branches of the hysteresis loops, in addition to the necessity of using proper interaction parameters, models for the adsorbent and gas molecule structures, and so forth. The availability of experimental adsorption data for model mesoporous solids would certainly facilitate the refinement of these highly promising, advanced computational approaches and other, less sophisticated methods for adsorption isotherm modeling and PSD calculation. Herein, the current state and future challenges in the development of the pore size analysis methods with the aid of model OMMs are discussed.

2. PORE SIZE ANALYSIS BASED ON MODEL MESOPOROUS ADSORBENTS

The first practical approach for calculation of PSDs in a wide range of pore diameters using OMMs as model adsorbents was reported by us in 1997 [6]. This approach, often referred to as the Kruk-Jaroniec-Sayari (KJS) method, was based on the following ideas. The starting point is the synthesis of a series of OMMs with the same type of the porous structure, and with gradually increasing pore diameter within as wide range as possible. The subsequent step requires the determination of the pore size of these OMMs using an accurate and reliable method independent from gas adsorption methods of the pore size analysis. The final step is to acquire gas adsorption isotherms on these model materials and to use these model data to assess the feasibility of the pore size assessment from adsorption and desorption data, and to elaborate, if possible, a method to calculate PSDs from the branch or branches of isotherms that were found to be inherently suitable for the pore size assessment. This final step includes the following sub-steps: 9 the determination of the experimental relation between the pore diameter and the capillary condensation/evaporation pressures, 9 the critical assessment of the inherent suitability of these relations for the PSD determination, which leads to a selection of one branch or two branches of the isotherms as potentially suitable for the pore size analysis, 9 the approximation of the potentially useful relation(s) by Using appropriate expressions, and, if needed and possible, the extrapolation over the pore size range beyond that exhibited by the model adsorbents used,

439 9 the construction of model adsorption isotherms for pores of different sizes (which in addition to the information obtained in the previous step requires the elucidation of the statistical film thickness of the surface layer formed on the walls of pores of different sizes), 9 the implementation of the PSD calculation procedure based on the model gas adsorption isotherms generated as described above. So far, this approach has been implemented for cylindrical pores with silica-based and hydrophobic (hydrocarbon-based) surfaces in the case of nitrogen adsorption at 77 K [6,10], and argon adsorption at 87 K [11,12] and 77 K [13] (in the latter case, so far the method has been elaborated for silica-based surfaces only). Series of MCM-41 silicas with approximately cylindrical pores of size between 2 and 6.5 nm were used as model OMMs to establish the relations between capillary condensation/evaporation pressures and the pore size [6,11,13]. An equation that provides a relation between the MCM-41 pore diameter, X-ray diffraction (100) interplanar spacing, and the pore volume in a honeycomb porous structure [14,15] was used for an independent pore size assessment. In all cases, it was found that the capillary condensation pressure tends to gradually and systematically increase as the pore diameter of model OMMs increased, which provides strong evidence that adsorption branches of isotherms are suitable for the PSD calculations. On the other hand, the relation between the capillary evaporation pressure and the pore diameter was much more complicated in the pressure range of adsorption-desorption hysteresis, including much scatter of results and a systematic irregularity close to the lower pressure limit of adsorption-desorption hysteresis. In particular, there was much evidence that model materials with more uniform pores tend to desorb at higher pressures than materials of less uniform porous structure do, which suggests that the capillary evaporation is delayed by the presence of constrictions even in uniform channel-like pores of MCM-41 [6,16]. This is reminiscent of the well-known pore network effects that lead to a delayed desorption in solids with 3-dimensionally connected porous structures (see [3,16,17] and references therein). It was concluded on the basis of adsorption studies for model OMMs that adsorption branches of isotherms are suitable for PSD calculation, whereas desorption branches of isotherms are much less suitable even for channel-like pores (their unsuitability for PSD calculation for pore networks with constrictions is well known [3,16,17]). The relation between the pore diameter and the capillary condensation pressure was approximated and extrapolated by expressions similar to the well-known Kelvin equation, but with additional constant correction terms, which was intended to ensure a proper behavior for pores much larger than those of the model OMMs used [6,11 ]. The examination of the adsorption data for OMMs also led to the conclusion that the monolayer-multilayer formation before the onset of capillary condensation can be satisfactorily approximated for pores of different diameters by a common statistical film thickness curve (t-curve) based on an adsorption isotherm for a macroporous silica. This approximation is more crude for pores of diameter close to the micropore range [11,13,18], and in general less satisfactory for argon [11,13] than for nitrogen [6,19]. The aforementioned relations between the pore diameter and capillary evaporation pressure were used along with the t-curves in calculations of PSDs [6,10-13] using an algorithm based on the well-known Barrett-Joyner-Halenda (BJH) work [20]. A very good agreement was usually observed between PSDs assessed using the KJS approach discussed above for nitrogen and argon (both at 77 and 87 K). It should be noted that the use of OMMs to improve the PSD calculation methods, such as BJH, had been proposed by Naono et al. [21 ] before the development of the KJS method. However, Naono et al. did not follow many of the steps and sub-steps

440 mentioned above, which are needed for successful development of the OMM-calibrated PSD calculation procedures, and consequently, their method was found to produce significant errors in the pore size assessment [2]. The KJS procedure proposed for nitrogen adsorption at 77 K has already been implemented by several research groups working in the field of nanomaterials [22-26] in addition to its extensive use by the two groups that participated in the development of this approach [6,10-13,18,19,27]. However, the procedures based on the KJS concept have been elaborated so far only for materials with channel-like pores. So the KJS PSDs are expected to be less accurate for materials with cage-like pores. These expectations are confirmed by the results of the recently published work [28], which showed that the KJS procedure for channel-like silica pores [6] appreciably underestimates the size of cage-like pores, although it still offers a significant improvement in accuracy when compared to the standard adsorption methods of the pore size analysis for channel-like pores. In addition, there are also indications that the extrapolation of the experimental relation between the capillary condensation pressure and pore diameter for cylindrical pores over larger pore sizes using an empirical equation similar to the Kelvin equation for hemispherical meniscus [6,11] becomes less accurate as the pore diameter increases [29]. Therefore, the challenge is in the development of adsorption methods for an accurate determination of PSDs for the pore range as wide as possible (perhaps not only mesopores, but micropores and macropores), and for various pore shapes (including channel-like and cage-like structures), using OMMs and related materials as model adsorbents.

3. SYNTHESIS OF M O D E L M E S O P O R O U S ADSORBENTS

Since the time the KJS approach for cylindrical pores was developed [6], a limited progress has been made in the synthesis of OMMs with extra-large cylindrical pores. In particular, we are aware of not reports on the successful synthesis of alkylammoniumsurfactant-templated MCM-41 silica with high degree of structural ordering and pore sizes above 7 nm, that is, above the limit achieved before using hydrothermal restructuring approaches [30-32]. On the one hand, the discovery of triblock-copolymer-templated SBA-15 silicas [33] appeared to extend the upper limit of pore diameters attainable for 2-D hexagonally ordered materials with cylindrical pores to about 30 nm. However, it has soon become clear that the actual pore diameter limit for SBA-15 is most likely less than half of this value, as the larger pore materials have some i~ore structure ordering, but are foam-like [34]. In addition, SBA-15 exhibits a 3-dimensionally connected porous structure, that is, the large, uniform mesopores of this silica are connected by much narrower pores (micropores and narrow mesopores) in the pore walls [35,36], which is related to inherent properties of the templates with poly(ethylene oxide) blocks [35,36] that are capable of becoming occluded in the silica pore walls. The 3-D pore connectivity is a general feature of SBA-15 synthesized under various conditions [37]. Until recently, most of the claims about the possibility of synthesizing non-microporous (that is, with 2-D pore system) SBA-15 did not have any good basis or were in disagreement with the results obtained using reliable inverse replication methods [38], while other claims deserve further scrutiny (see discussion in [38]). In particular, there is good evidence that high-temperature calcination of SBA-15 at above 1173 K leads to the closure of the connecting pores [39]. This is accompanied by a prominent structural shrinkage, so the pore size of the resultant SBA-15 with 2-D pores tends to be in the upper range of diameters attainable for high-quality MCM-41. A way to circumvent this problem by using an SBA-15 silica with somewhat larger pores and with a small content of

441 connecting pores was proposed, and a silica was obtained with pore diameter of about 9 nm and with adsorption properties distinctly different from those of typical SBA-15 and similar to those expected from an extra-large-pore MCM-41 [38]. However, the contention about a complete elimination of pore connections for such a material still needs to be verified using inverse carbon or platinum replication methods [38]. The synthesis of SBA-15 in the presence of salts is also promising from the viewpoint of elimination of the connecting porosity in SBA-15 [23], but the final verification by using inverse replication methods is lacking. Finally, the hydrothermal treatment at 403 K was claimed to afford SBA-15 without micropores, which would imply the lack of connections between the pores [40], but SBA-15 synthesized under very similar conditions have already been found to exhibit highly connected porous system with large holes in the walls of ordered pores [41 ]. Some progress has also been made in the synthesis of small-pore silicas with 2-D hexagonal structures. In particular, the use of mixtures of surfactants with two short alkyl chains afforded highly ordered MCM-41 with the XRD (100) interplanar spacing as low as 2.7 nm [18]. Moreover, silicas that exhibit pore diameters tailorable in the micropore range and a single, but very narrow XRD peak corresponding to the (100) interplanar spacing down to as low as 2.3 nm, can readily be synthesized from commercially available reagents [42]. Further studies are required to verify whether these remarkable ordered microporous silicas exhibit well-ordered 2-D hexagonal structures, or less ordered pore structures. A significant progress has recently been achieved in the pore size tailoring of MCM-48 silica [1] that exhibits a 3-D connected structure of uniform pore channels. In particular, large-pore MCM-48 (pore diameter above 4.5 nm) was reported [43,44]. However, the pore size range attainable for MCM-48 is still much more restricted than that attainable for MCM41. Also, a noticeable progress has been made in the synthesis of OMMs with cage-like pores of tailorable size. This includes the work on SBA-1 and SBA-6 silicas with the same cubic Pm3n structure [45], whose pore diameter can be tailored in a range about as wide as that achievable for MCM-41 by choosing surfactants of various structures and different alkyl chain length, or by adding proper amount of micelle expanders [45,46]. However, SBA-1 and SBA-6 exhibit structures with two kinds of mesoporous cages of different size [45], which would make their prospective application as model adsorbents more difficult. Oligomer- and polymer-templated silicas with cage-like mesoporous structures of cubic Im3m and 3-D hexagonal P63/mmc symmetry can also be synthesized with pores of various sizes [33,47,48], and are thus promising as prospective model adsorbents. However, because of the inherent tendency of the poly(ethylene oxide) blocks of oligomer/polymer templates to be occluded in the walls of silicas synthesized using these templates, the aforementioned cage-like silicas are likely to exhibit micropores in their siliceous frameworks in a way similar to that proven for SBA-15 [35-37]. Finally, there emerges an opportunity to synthesize ordered materials with pore sizes on the borderline between the mesopore and macropore ranges using colloidal crystals as templates [49], which promises to provide model extra-large-mesopore adsorbents.

4. A S S E S S M E N T OF PORE SIZE FOR M O D E L M E S O P O R O U S ADSORBENTS

The most promising methods for an independent assessment of pore dimensions of model OMM adsorbents are those based on equations that provide relations between structural parameters in the OMMs structure [14,15], and on the electron crystallography method [45]. A geometrical equation for the pore size assessment of MCM-41 was originally reported by two research groups [14,15], whose contributions were submitted for publication in 1996

442 within just two months from one another. The derivation of this equation was based on the consideration of a relation between the (100) interplanar spacing, dl00, the primary mesopore volume, Vp, and the pore diameter, Wd, in 2-D hexagonal structure of uniform cylindrical pores. This equation assumes the form: Wd = cdl00[pVp/(l+pVp)] 1/2, where 9 is the density of the pore walls and c is a constant dependent on the pore geometry (1.213 for circular pores). Others [7,50-52] later proposed the same equation. This equation was extended to materials with 2-D hexagonal porous structure akin to that of MCM-41, but with porosity in the pore walls, in which case Wd = Cdl00[Vp/(1/p+Vp+Vmi)]1/2, where Vmi is the volume of pores in the walls [53]. Others also proposed an analogous equation [54]. This equation is suitable for the pore diameter determination of SBA-15 and MSU-H silicas [29,54]. More recently, geometrical equations suitable for the pore size determination for various OMM structures have been reported. These include equations for cubic Ia3d structure of MCM-48 [55], 3-D hexagonal P63/mmc structure [28], cubic Im3m structure of SBA-16 [28,56], other cubic structures with cage-like pores [28], and 2-D hexagonal structure of CMK-3 carbon, which consists of an array of connected rods [37]. So, it is now possible to determine the pore diameter of many OMMs on the basis of the interplanar spacing and primary mesopore volume (in some cases the volume of pores in the walls is also needed) using simple geometrical relations. It needs to be noted that in general, the geometrical relations for the OMM pore size determination are based on assumptions that the structure is infinite in two or three dimensions, and the material is composed only of an ordered phase of a given structure. In addition, the relation for a 2-D hexagonal array of rods neglects the presence of connections between these rods [37], whereas the relations for structures with cage-like pores [28,56] neglect the pore connectivity and assume the spherical pore shape. A more realistic model of 3-D OMM structures can be elucidated using the electron crystallography [45,57], which reveals the actual structural complexity and does not require assumptions about a particular simple structure type that are necessary in derivations of geometrical equations for the pore size of 3-D OMMs. However, the most convenient implementation of the electron crystallography technique for OMM has much in common with the aforementioned equations, because it requires the ratio of the pore volume to the pore wall volume in order to determine the threshold in the 3-D potential map, or in other words, the surface that divides the pores from the walls in the structure [57]. This is analogous to the case of the geometrical equations where the pore diameter/unit cell size ratio in a simplistic structural model is calculated from the pore volume/pore wall volume ratio. So far, the electron crystallography method involved the determination of the pore wall volume as a reciprocal of the wall density evaluated using helium picnometry [45,57], and in the case of polymer-templated silicas, no provisions were made for taking into account the presence of micropores in their pore walls. The microporous nature of these walls results in the difference between the actual volume of the microporous pore wall and the pore wall volume estimated from the framework density. This is expected to lead to an error in the determination of the threshold of the 3-D density map from the pore volume/pore wall volume ratio for these materials.

5. CONCLUSIONS The use of OMMs as model adsorbents for elaboration of accurate methods for the PSD calculation is a highly promising approach. Because of its simple nature and the recent advances in the synthesis and independent pore size assessment of OMMs with different

443 structure types, there emerge opportunities in extending this approach to the pore shapes different from cylindrical. On the other hand, model OMMs are still available only with pore dimensions in limited intervals of the mesopore range, which hinders the development of accurate PSD calculation methods for very large mesopores. It would be desirable to extend the synthesis of ordered materials to the largely uncharted 1.3-2.0 nm part of the micropore range, thus providing better foundations for the PSD analysis in this important pore size interval.

6. ACKNOWLEDGMENTS The donors of the Petroleum Research Fund administered by the American Chemical Society are gratefully acknowledged for support of this research.

REFERENCES

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Studies in Surface Scienceand Catalysis 141 A. Sayariand M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

445

Sorption properties and hydrothermal stability of MCM-41 prepared by pH adjustment and salt addition Nawal Kishor Mal, a'* Prashant Kumarb and Masahiro Fujiwara a a

AIST Kansai, 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, JAPAN

b Ceramic Section, Industrial Research Institute of Ishikawa, Kanazawa 920-0223, JAPAN

MCM-41 with average pore size of ca. 4 nm was prepared with intermediate pH adjusted to 10.5, 9.75 and 9.25 for three times and a salt addition. Water and benzene sorption data show that hydrophobicity of MCM-41 increased after decreasing the pH of synthesis gel and the absence of occluded SiO2 type species inside the pores of MCM-41. TG analysis predicts that hydroxyl groups in as-synthesized catalyst decreased by 48 % after lowering the pH of the synthesis gel from 10.5 to 9.25.29Si MAS NMR shows the presence of small amount of Q3 species in the as-synthesized samples. Hydrothermal stability of the samples was drastically improved after lowering the pH to 9.25, which shows at least three peaks in XRD after 8 days of hydrothermal treatment at 373 K. It seems that lowering of the pH causes further polymerization, a kind of silylation of pore walls or deposition of precipitate silica on pore walls and, therefore, decreases the hydroxyl groups and improves the hydrothermal stability.

1. INTRODUCTION The improvement of the structure and hydrothermal stability of MCM-41 materials is of great importance due to their applicability as catalyst [1,2], absorbents [3], and host for various kinds of molecules [4,5]. Following the discovery of M41S family of mesoporous molecular sieves by Mobil [6,7] many attempts have been made to improve the structure by direct hydrothermal synthesis at low temperature (i.e. 373 K) [8,9] to give smaller pore size (diameter < 4.0 nm) and at high temperature (i.e. > 423 K) to give larger pore size (diameter > 4.5 nm) MCM-41 materials [10-12]. According to Chen et al. [13], MCM-41 can maintain its structure up to 1123 K in anhydrous conditions. Kim et al. [14] also reported that MCM-41 shows stability in air and oxygen containing water vapor system at 1170 K. However, hydrothermal stability of MCM-41 is poor in water [15], especially in boiling water the structure gets collapsed within 2 days [ 16]. Recently, new variety of mesoporous silica such as SBA-15 [16], KIT-1 and MSU-G [16-18] has been synthesized, they exhibit greater hydrothermal stability due to much thicker walls compared with Si-MCM-41 but are less wall ordered. The hydrothermal stability ofMCM-41 was improved after three times intermediate pH adjustment and salt addition during crystallization [8] or post synthesis restructuring of as-synthesized samples in water [19] or mother liquor [20]. Post synthesis silylation is effective to enhance the hydrothermal stability due to increase ofhydrophobicity [21].

446 In the present investigation, we have found that hydrothermal stability of MCM-41 was dramatically improved prepared using different molar composition better than described by Ryoo et al. [8], since the stability and the characteristics of the MCM-41 are strongly affected by the synthesis conditions [ 13]. The finding is supported by the characterization of parent and hydrothermally treated samples by using XRD, sorption study, TG analysis and 29Si MAS NMR.

2. MATERIALS AND METHODS 2.1. Materials The reactants used in this study were sodium silicate (52.5% SiO2, 25% Na20, Wako chem.), cetyltrimethylammonit~m bromide (96%, Kanto Chem.) (CTMABr), tetramethylammonium hydroxide (25% aqueous, Aldrich Chem.) (TMAOH), potassium chloride (KC1, Wako Chem.) and H2SO4 (96%, Wako Chem.). In a typical synthesis, 22.86 g of sodium silicate was dissolve in 100 g of water to give clear solution under stirring. 48.86 g of TMAOH and 38.72 g of CTMABr were dissolved in 100 g of water by stirring at 308 K to give a clear solution. Both the clear solutions were mixed together and stirred for 2 h. Finally, 13.28 g of H2SO4 in 51 g of H20 was then added and stirred for 2 h to become pH ca. 10.5. The molar composition of the gel was 1 SiO2 : 0.51 CTMABr : 0.67 TMAOH : 0~46 Na20 : 0.75 KCI : (0.65 + X) H2SO4 : 80 H20 (pH = 10.5), where KC1 was added after first heating period to 373 K for 24 h and X (0.1 - 0.5) the amount of H2SO4 was added during intermediate pH adjustment for three times. The gel was then heated in a polypropylene bottle, without stirring, to 373 K for 24 h. The mixture was then cooled to room temperature and H2SO4 was added to adjust the pH ca. 10.5. i 1.83 g of KC1 was then added and heated again to 373 K for 24 h. This procedure for pH adjustment and subsequent heating was repeated twice (i.e. carried out three times in all), except no KC1 was added during second and third pH adjustment. The resultant product was filtered, washed with distilled water, dried at 378 K for 24 h and calcined at 823 K for 6 h. Two other MCM-41 samples were prepared under similar condition with different pH adjusted to 9.75 and 9.25. Catalysts prepared with pH adjusted to 10.5, 9.75 and 9.25 are denoted to be as sample 1, 2 and 3, respectively. 2.2. Surfactant extraction The surfactant (template) from as-synthesized samples was removed by treatment of 1 g of catalyst in 60 g of dry ethanol and 1 ml of HC1 (1 M) at 353 K for 24 h under vigorous stirring. After filtration samples were washed with eti,anol and dried at 378 K for 24 h. This procedure was repeated once. 2.3. Hydrothermal treatment 0.4 g of calcined MCM-41 in 400 g ofwater was heated in a propylene bottle to 373 K for 4 and 8 days. The Sample was then filtered, dried at 378 K for 24 h and calcined at 773 K for 90 min. 2.4. Characterization XRD patterns were obtained with a Shimadzu XRD-6000 diffractometer. BET surface area and pore size were obtained from N2 adsorption isot~aerms measured at 77 K using Bellsorp

447 28 instrument. Prior to N2 adsorption, the samples were degassed at 473 K for 5 h. The sorption measurement was carried out gravimetrically in a electrobalance (Chan, USA) at 298 K and at fixed p/p0 ratio of 0.5 each of water and benzene as adsorbents after equilibrium for 3 h. FT-IR spectra of template extracted samples were obtained with a JASCO FT/IR-230 using KBr pellets (3 mass% catalyst). Thermogravimetric analyses (TGA) of as-synthesized samples were obtained with a Seiko, SSC/5200. Samples were heated at the rate of 5.0 K/m from 293 to 1073 K. 29Si MAS NMR spectra were recorded at 11.75 T on a Varian INOVA 500 NMR spectrometer with a CPIMS probe. Data were acquired at 99.3 MHz and 10 s recycle delays. The chemical shifts are given in ppm using tetramethylsilane (TMS) as a standard material.

2.5. Methods The BET surface area [22] w~,,s calculate{' in the relative pressure range between 0.04 and 0.2. The primary mesopore size (WKjs) was ,~alculated using adsorption branch of isotherms according to method describe elsewhere [23] that is; WKjs = cd(pVv)V2/(1 + pVI,) 1/2, where C = 1.213, p - - 2.2 gcm -3, d is the lattice spacing of d~00, and Vv is the primary mesopore volume. Total pore volume was determined from the amount adsorbed at relative pressure of 0.99 [22]. The pore size distributions were calculated from the adsorption branches of the nitrogen adsorption isotherms using Barrett-Joyner-Halenda (BJH) method [24].


3.1. Synthesis, structure and sorption properties Absence of the C-H stretching vibration band at 2933 and 2855 cm 1, and bending vibration band at 1460 cm 1 in the FT-IR spectra of surfactant-extracted samples confirm the complete removal of surfactant. XRD profiles of all the MCM-41 samples, with pH adjusted to 10.5, 9.75 and 9.25, after cal,c,ination an~! surfactant-extraction are shown in Fig. 1. Four peaks in the XRD patterns of all samples are clearly observed, which are characteristics of long range ordering of the MCM-41 structure. Intensity of dl00 peak of surfactant-extracted samples is slightly higher than that of calcined samples. The XRD and adsorption characteristics of the calcined and surfactant-extracted samples are shown in Table 1. As clear from Table 1, lowering the pH from 10.5 to 9.25, the d~00 spacing of calcined samples were ~

~ - - - ~ ~ 4 6 20 (degree)

B

, ,,, i

8

10

2

4 6 20 (degree)

8

10

Figure 1. XRD profiles of calcined (A) anO surfactant-extracted (B) samples obtained with pH adjusted t o (a) 10.5, (b) 9.75 and (c) 9.25.

448 Table 1. XRD and adsorption characteristics of the samples under study, a Sorption capdloo (nm) Sample As-synt- Calci- ao SBET Vp WKJS acity (wt%) Vbenzene /pH hesized ned (nm) (m2g"1) (cm3g"1) (nm) H20 Benzene (cm3ga) 1 (10.5) 3.91 3.75 4.33 1020 0.82 3.65 18.6 73.8 0.85 2(9.75) 4.12 4.03 4.65 960 0.80 3.90 15.3 70.6 0.81 3 (9.25) 4.15 4.12 4.76 911 0.75 3.94 13.4 66.4 0.77 Surfactant-extracted samples 1 3.91 4.09 b 4.72 1116 0.91 4.05 32.2 75.6 0.93 2 4.12 4.23 b 4.88 1082 0.87 4.16 24.1 71.2 0.89 3 4.15 4.18 b 4.83 1021 0.78 4.03 16.2 68.2 0.81 aParenthesis indicates the repeated pH adjustment of the synthesis gel; dloo:X-ray diffraction (100) interplanar spacing; ao: unit cell parameter = 2d~oo/3V2;S~ET:BET specific surface area; Vp: Primary meosopore volume; WKjs:Averagepore size; Vb...... : pore volume measured using benzene as adsorbent at P/Po= 0.5 and 298 K. bnot calcined. increased from 3.75 to 4.12 nm, respectively. An increase in dl00 spacing after lowering the pH of the synthesis gel was reported elsewhere [25]. The lattice contraction after calcination of samples 1, 2 and 3 were 4.1, 2.2 and 0.7%, respectively. MCM-41 synthesized without pH adjustment shows lattice contraction 20-25% [13]. In contrast, lattice contraction was not observed in samples synthesized by repeated pH adjustment [26]. Unit cell expansion after surfactant-extraction of samples 1, 2 and 3 are 4.6, 2.7 and 0.7%, respectively. This result emphasized that the amount of hydroxyl groups at low pH (9.25) adjusted sample was much lower than at high pH adjusted samples. However, the surface area and pore volume of the samples were decreased after lowering the pH. It is likely that lowering of pH from 10.5 to 9.25 causes the formation of more polymerized pore wall and/or deposition of silica on the pore walls inside and outside the pore uniformly and therefore results in an increase in the dl00 spacing and a decrease in the pore volume. The sorption capacity for water shows that surfactant-extracted samples are more hydrophilic than calcined form because silanol groups were present in surfactant-extracted samples. The amount of water absorbed in surfactantextracted sample 3 (16.24 %) is 97.5% lower than surfactant-extracted sample 1 (32.21%) ~oo]

A

~i

2.8 ( n m )

B

)IW'~uA~A4 9A- 9C

:: ,0ol 300 l

o

. . ~

~

FeSiMCM-41 >LaSiMCM-41. 1. INTRODUCTION Due to the channel diameter within 1.5-10 nm, the synthesis of mesoporous molecular sieves family M41S by Mobil in 1992 threw new light on the conventional concept of synthesis of zeolites [ 1]. For its potential application, the studies on this family of novel materials have been very active in the last decade [2, 3]. It is well known that heteroatoms can modify the physico-chemical properties of zeolite molecular sieves, thus numerous papers about heteroatom-containing mesoporous materials have been published [4-13]. However, though a Fe-containing MCM-41 sample with Si/Fe molar ratio of 75 has been reported, Fe, and La, two common elements widely used in microporous molecular sieves, have not yet been studied in detail. We have successively incorporated Fe(III) and La(III) into the framework (channel wall) of MCM-41 (designated as FeSiMCM-41 and LaSiMCM-41) using CI6H33(CH3)3NBr (CTAB) as template and water glass as silicon source [14]. In our previous publications, some very interesting properties of these materials were reported. FeSiMCM-41

460 is a very active alkylation catalysts [15]; SiMCM-41, A1SiMCM-41 and LaSiMCM-41 mesoporous molecular sieve materials show a very strong photoluminescent (PL) effect [ 16]; Fe-loading SiMCM-41 not only allows the catalytic deposition of carbon to synthesis carbon nanotubes, but also can orientate the growth direction of the carbon nanotubes and control the diameter of the formed carbon nanotubes [ 17]. Recently we investigated the change of the state of Fe and La species in FeSiMCM-41 and LaSiMCM-41 before and after calcination to remove template and studied the acidity of mesoporous materials FeSiMCM-41, LaSiMCM-41, SiMCM-41, A1SiMCM-41 and HA1SiMCM-41 with microcalorimetric studies of the adsorption of ammonia and temperature programmed ammonia desorption method (NHa-TPD) [18-19]. However, the microcalorimetric studies of the adsorption of ammonia and NHa-TPD method can not differentiate Bronsted-acid from Lewis-acid. Because the surface characteristics, such as catalysis, sorption behavior and so on, of a solid material show close relationship with the nature of its surface acid sites, it is very important to investigate the Bronsted-acid and Lewis-acid of mesoporous materials. Generally, pyridine is applied as probe molecule to obtain information about Bronsted-type acid sites and Lewis-type acid sites. In this paper, we will report the Bronsted-acid and Lewis-acid properties of A1SiMCM-41, FeSiMCM-41 and LaSiMCM-41 by means of Fourier transform infrared (FT-IR) following adsorption of pyridine on these samples. 2. EXPERIMENTAL

2.1. Samples The synthesis of samples has been reported previously [ 14]. The resulting solid products were recovered by filtration, extensively washed with distilled water, and dried in air at ambient temperature, followed by calcination at 813 K in air for 6 h to remove template. 2.2. Characterization The low-angle powder X-ray diffraction patterns were recorded for calcined samples on a Rigaku (D/max-Y A) X-ray diffraction instrument with Cu-Kcz radiation to verify their hexagonal phase structure. The adsorption of nitrogen at 77 K was conducted on a Micromeritics ASAP2000 instrument to analyze the diameters of the investigated samples. The compositions of samples were obtained on a Jarrell-Ash 1100 inductively coupled plasma quantometer. For each of the investigated samples the adsorption of pyridine was conducted after activation. The samples (10 mg) were ground by hand with a pestle in a mortar for 5 min and were then pressed at 4 tons to give a self-supporting pellet (15 mm in diameter, 6 mg/cm2). The wafer was placed inside a glass cell equipped with CaF2 window. After degassed at 723 K for 5 h to reach the vacuum of 5xl0 3 Pa, the samples were cooled to room temperature, then pyridine gas was introduced into the cell. After equilibration of samples with pyridine at room temperature for 5 min, the temperature was increased to 423 K at a heating rate of 5 K/min and was remained at that temperature for 0.5 h while the samples were degassed. Then the samples were cooled to room temperature to allow the FT-IR spectra to be recorded. Similarly the samples were evacuated at 523, 623 and 723 K for 0.5 h and

461 cooled to room temperature to acquire the corresponding FT-IR spectra respectively. Spectra were recorded on a Nicolet 510P FT-IR spectrometer with a resolution of 2 cm -~. 3. RESULTS AND DISCUSSION

3.1. physieo-chemical properties of samples All the as-synthesized samples showed a XRD pattern (not shown here) matching with that first reported by Kresge et al. for hexagonal mesoporous molecular sieves materials [1]. After calcination to remove template, all the XRD patterns of investigated samples remained typical patterns of hexagonal phase with increase in diffraction strength and a shift to lower 20 direction (partiallyl shown here). Shown in Figure 1 are the XRD patterns for a few typical calcined samples, all the patterns show a very strong (100) diffraction peak and 2-3 weak peaks at relatively higher 20 positions. The composition and pore sizes of the tested samples are shown in Table 1.

Figure 1. XRD patterns of calcined (a) AISiMCM-41(118), (b) FeSiMCM-41 (124) and (c) LaSiMCM-41(147).

Table 1. Physico-chemical properties of calcined samples. .

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Samples a

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s'uri;ace

Molar composition of ca.!cined'samples 'Pore sizeb sio2/A1203 SiO2/Fe203 SiO2/La203 (nm)

AISiMCM-41 (118)

118

AISiMCM-41 (58) AISiMCM-41 (32)

area (m2/g)

3.3

l 107

58

3.3

1084

32

3.3

1049

FeSiMCM-41 (124)

124

3.3

998

FeSiMCM-41 (62)

62

3.3

947

FeSiMCM-41 (32)

32

3.3

932

LaSiMCM-41 (147)

147

3.3

974

LaSiMCM-41 (83)

83

3.3

932

LaSiMCM-41 (48) 48 3.3 893 aNumbers in parentheses are SiO2)Me ratios, Me=AI, Fe and La for the corresponding samples, respectively. b From BJH method.

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462 Compared with microporous zeolites, all the MCM-41 type mesoporous molecular sieve samples show a much larger pore size (-3.3 nm) and larger surface area.

3.2. Adsorption of pyridine NH3-TPD (Temperature-Programmed-Desorption) technique is a conventional technique to detect the acidity of a solid catalyst, but it can only acquire the overall information of the acid strength and amount and is difficult to detect the acid type and subtle change in acid density and strength. NH3-microcalorimetric measurement can investigate the subtle change in acid density and strength but still is difficult to distinguish the Bronsted-type acid from Lewis-type acid [20]. Usually, pyridine was chosen as the probe molecule since it enables a distinction between weak and strong Lewis- and Bronsted-type acid sites, i.e., hydroxy groups that interact with pyridine by hydrogen bonding or electron-pair donation and hydroxy groups that form pyridinium ions upon adorption [21-23]. In the FT-IR investigation of pyridine adsorption on samples, the bands at-1545,-~1490 and 1625 cm 1 were assigned to pyridinium ions formed on Bronsted-acid sites and those at 1600 and 1449 cm 1 hydrogen-bonded pyridine, the band at 1455 cm ~ was attributed to the the presence of a small fraction of strong Lewis-acid sites [23, 24]. In other publications, the band at-~1449 cm l was associated with the adsorption of pyridine on Me n+ (metal cations) such as Na +, K +, Cs +, La3+ and so on [25-27]. In this paper, we mainly described the changes in bands at-1545,-1490, 1455 and-1449 cm ~. Shown in Figure 2-4 are the FT-IR spectra of pyridine adsorption on AISiMCM-41, FeSiMCM-41, LaSiMCM-41 samples, respectively. At similar MezO3 species content, the acid density sequence is: A1SiMCM-41 >FeSiMCM-41 >LaSiMCM41. From Figure 2, as previously reported by Jentys and co-workers for Al-containg MCM-41 samples [23], we can find that bands assigned to pyridinium ions formed on Bronsted-acid sites (-1491, -1546 cm I) and to pyridine adsorbed on strong Lewis-acid sites (1456 cm 1) were observedclearly on all the AISiMCM-41 samples and the adsorption intensity Figure 2. Difference FT-IR spectra of pyridine increases with the AI content, meaning adsorbed on calcined (A) AISiMCM-41 (118), (B) both the Bronsted-acid and Lewis-acid AISiMCM-41(58) and (C) AISiMCM-41 (32) at densities increased with the increase in AI (a) 423 K, (lo)523 K, (c) 623 K and (d) 723 K. content.

463

Wavenumber (cm -1) Figure 3. Difference FT-IR spectra of pyridine adsorbed on calcined (A) FeSiMCM-41(124), 03) FeSiMCM-41(62) and (C) FeSiMCM-41 (32) at (a) 423 K, (lo) 523 K, (c) 623 K and (d) 723 K.

Wavenumber (cm "1)

Figure 4. Difference Fr-IR specWa of pyridine The presence of band a t - 1 4 4 9 cm -1 in adsorbed on calcined (A) LaSiMCM-41(147), (B) LaSiMCM-41 (83) and (C) LaSiMCM-41 (48) at Figure 2 is different from the observation by Jentys et a1.[23]. They reported that the 1449 (a) 423 K, (b) 523 K, (c)623 K and (d) 723 K. cm ~ band was only observed on the samples with a SIO2/A1203 ratio R-CH2-CHR'-CH2-R" R'-*CH-CH2-R" --> R-CH3 + R'-CH-CH-R" *CH2-R" --) R-CH2-CH2-R' *CH2-R" -~ R-CH3 + CH2=CH-R '

(R) (D) (R) (D)

In general, the obtained compounds by HDPE pyrolysis were paraffin, olefins and aromatics, and they were grouped as C,, where "n" represents the number of carbons. The velocity of the catalytic cracking of polymer, depends on the conversion (or), temperature (T) and time of reaction (t). In each process, the reaction velocity is given as a function of conversion f(a) and can be determined from experimental data. From thermogravimetric curves for the mixture (H-A1MCM-41/HDPE) at three different heating rate (Figure 2), graphics of degree of conversion (%) as a function of temperature, were obtained, as shown in the Figure 3. One well-defined weight change state is viewed in the degradation process. Through TG curves, were determined the initial, medium and final temperature for the HDPE pyrolysis in presence of catalyst. The use of H-A1MCM-41 solid acid catalyst to the polymer degradation requires information concerning the kinetic parameters, mainly the energy activation relative to the process. Reliable methods for determining the activation energy using dynamic integral TG curves at several heating rates have been proposed by Flynn and Wall [13], where it was demonstrated that the heating hate and the absolute temperature can be related as follows: dlogfl

~0,457~

--L--Y-J E

01)

477 and, inserting the R value 8.314 J.mol~.K ~, an expression obtained for E Clog fl E ~ -4,35 9~ 31/T

100

9

(02)

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...... 5 ~ . . . . 10 ~ - 20 ~

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80

o~

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60

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150

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300

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450

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600

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750

900

Temperature (~

Figure 2. Thermogravimetric curves of degradation of H-A1MCM-41/HDPE.

100

80

C .0_ i,.

tO

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(a)

60

(b) 40

(c) 2o

0 '

350

,t,

,

I

400

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450

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500

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550

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""

-

600

Temperature ~

Figure 3. Conversion ofH-A1MCM-41 /HDPE in function of temperature at different heating rate: (a) 5 ~ (b) 10 ~ and (c) 15 ~

478 Thus, it was calculated the activation energy related to degradation of a the HDPE in presence of the H-A1MCM-41 catalyst, using the slope of the logarithmic heating rate curves as a function of the reciprocal temperature. The activation energy observed for the polymer degradation without catalyst was 225.5 KJ.mol -~ against 184.7 KJ mol -~ in the presence of the H-A1MCM-41. These results indicate that this material may have acted as a cracking catalyst for the HDPE, enhancing the generation of light products of potential industrial use. The low value of activation energy for evidences that the H-A1MCM-41 mesoporous material is efficiency of the for the degradation process. 4. CONCLUSIONS The products resulted from HDPE pyrolysis by acid H-A1MCM-41 are distributed in a narrow range of carbon, C2-C5, C6-C9 e Cl0-Cl4, typically LPG, gasoline and medium distillate, evidencing that the pyrolysis mechanism is a function of the pore system and the acid properties. The activation energy for the process, as determined from multiple heating rate TG curves and kinetic model, decreased from 225.5 KJ.mol -I (H-A1MCM-41/HDPE) to 184.7 KJ mo1-1 (HDPE), evidencing that the mesoporous H-A1MCM-41 acted as a good catalyst for pyrolysis of polyethylene. ACKNOWLEDGEMENTS

The authors acknowledge the support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq), and Fundagao de Amparo h Pesquisa do Estado de Sao Paulo (FAPESP). REFERENCES

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

(a) Characterization of Municipal Solid Waste in the United States, EPA Report, 530-R94-042, 1994. (b) E. D. Amico and M. Roberts, Chem. Week, 4 (1995) 12. A.R. Songip, T. Masuda, H. Kuwarara and K. Hashimoto, Appl. Catal. B: Environmental, 2 (1993) 153. M.M. Taghiei, Z. Feng, F.E. Huggns and G.P. Huffman, Energy Fuels, 8 (1994) 1228. V.J. Fernandes Jr., A. S. Arafijo and G.J.T. Fernandes, Stud. Surf. Sci. Catal., 105 (1997) 941. P.N. Shrrtt, Y.H. Lin and A.A. Garforth, Ind. Eng. Chem. Res., 36 (1997) 5118. V.J. Femandes, A.S. Araujo, G.J.T. Fernandes, J.R. Matos and M. Ionashiro, J. Therm. Anal. Calorim., 64 (2001) 585. Y. Ishihara, H. Nambu, T. Ikemura, and T. Takesue, Fuel, 69 (1990) 978. X. Xiao, W. Zmierczak and J. Shabtai, Preprints of ACS - Div. Fuel. Chem., 4 (1995) 4. M.A. Uddin, Y. Sakata, A. Muto, Y. Shiraga, K. Koizumi and K. Murata, Microporous Mesoporous Mater., 21 (1998) 557. J. Aguado, J.L. Sotelo, D.P. Serrano, J.A. Calles and J.M. Escola, Energy Fuels, 11 (1997) 1225. E. Dwyer and D.J. Rawlence, Catalysis Today, 18 (1993) 487. A. Holmstrong and E.M. Sorric, J. Appl. Polym. Sci., 18 (1974) 761. J.H. Flynn. and W.A.Wall, Polym. Lett., 4 (1969) 323.


479

Electrorheological response o f m e s o p o r o u s materials under applied electric fields Min S. Cho a, Hyoung J. Choi a, Wha-Seung Ahn b, and Myung S. Jhon c a

Department of Polymer Science and Engineering, Inha University, Inchon, 402-751, Korea

b Department of Chemical Engineering, Inha University, Inchon, 402-751, Korea c Department of Chemical Engineering, Carnegie Mellon University, Pittsburgh, PA 152133890, USA

As a novel candidate for electrorheological (ER) fluids, mesoporous MCM-41 particles suspended in silicone oil, were examined. MCM-41 particles, with a well-defined channel structure analyzed by X-ray diffractometry and transmission electron microscopy, was synthesized. ER fluids were then prepared by dispersing these synthesized MCM-41 particles in silicone oil. These ER fluids exhibit viscosity enhancement and peculiar viscoelastic properties under applied electric fields. We found that the small amount of moisture absorbed in hydrophilic MCM-41 is responsible for the particle polarization at high electric field strengths. The static yield stress of ER fluids were also measured and correlated with a universal scaling function suggested by Choi et al. 1. INTRODUCTION The discovery of the mesoporous materials has opened a new class of molecular sieves exhibiting ordered arrangement of uniform, nanometer size pores. Especially, the silicabased mesoporous materials have been extensively studied in various catalytic reactions [ 1-3]. Other interesting applications, using this unique pore structure and high surface areas of mesoporous materials, are the removal of mercury or heavy metals from contaminated solutions [4], nanometer electronic material [5, 6], and immobilization of small enzymes in the mesopore structure [7]. Recently, mesoporous particles have been also used [8] as a new candidate material for electrorheological (ER) fluids. The ER fluids are typically composed of a suspension of micron-sized particles in a nonconducting fluid, and their rheological response can be changed by an imposed external electric field [9, 10]. Under an applied electric field, ER fluids make rheological property change from a liquid-like state to solid-like state; a stress is required to break the chainlike or columnar structure, and flows afterwards. This stress, referred to as a yield stress, is electric field strength dependent and is the key characteristic parameter for an ER fluid. ER fluids also exhibit increase in their shear viscosities of several orders of magnitude at low shear rates. The solidified ER fluids under an electric field are referred as a Bingham fluid. The Bingham fluid model, popular rheological model for many ER fluids, is described below [ 11 ].

480

~,=0

x<x

(1) Y

Here, x is the shear stress, Xy is the yield stress, ~, is the shear rate, and ~1 is shear viscosity. ER suspensions can be classified as either an intrinsic polarizable (anhydrous) or extrinsic polarizable (hydrous) system. The former system, which are polarized in an electric field due to electrons within the polymer molecule, usually contains semiconducting polymer panicles such as polyanilines and its derivatives [12-15], poly(acene quinone) radicals [ 16], polymer/clay nanocomposites [17], phosphate celluose [18], inorganic particles including PZT [19], and carbonaceous particle [20]. While the latter system uses additives such as water and alcohol. It is generally believed that the anhydrous ER systems are better than the hydrous counterparts in practical applications due to high thermal stability and better dispersion quality. Commercial zeolites have also been tested as a suspended panicle for ER fluids [21,22]. Recently, Cho et al. [22] examined the rheological and dielectric properties of the ER fluids prepared from commercial zeolites 3A, 5A and 13X suspended in silicone oil and found that the ER fluid using zeolite 13X showed the best ER performance. It is noted that porous solid materials including zeolite have been used as adsorbents, catalysts and catalytic supports owing to their high surface areas. However, their applications are limited due to the relatively small pore openings. The pore enlargement was one of the challenge in zeolite research [23], and a myriad of synthetic efforts to enlarge the pore size led to the success in preparation of ordered mesoporous materials with a broad range of pore sizes up to 30 nm [24]. A recent discovery of a new family of crystalline mesoporous molecular sieve materials M41S have attracted considerable interest. They are either a hexagonal phase, a cubic phase, or a unstable lamellar phase which are known as MCM(Mobil Composition of Matter)-41, MCM-48 and MCM-50, respectively [ I, 25]. In our study, the synthesized MCM-41 was dispersed in silicone oil (5 - 20 wt%) and evaluated their ER fluid properties. We also examined the mesoporous materirals as a potential candidate for a dispersed phase in ER suspensions.

2. MATERIALS AND METHODS 2.1. Materials

MCM-41 was synthesized using a 25 wt% aqueous solution of hexadecyltrimethylammonium chloride (HTAC1, Aldrich), 28 wt% ammonia (NH3) solution, and sodium silicate [26]. HTA-silicate gel with a chemical composition of 4SiO2:IHTACI: 1Na20:0.15(NH4)20:200H20 was placed into a polypropylene bottle and heated to 370 K in an oven for 3 days. EDTANa4 and acetic acid treatments were performed during the synthesis to enhance crystallinity and hydrothermal stability. The precipitated product was filtered, washed with distilled water and dried at 370 K for 12 hours in an oven, followed by calcination in 02 at 823 K for 10 hours. 2.2. Measurements

X-ray diffraction (XRD) pattern of the calcined MCM-41 sample was obtained with a powder X-ray diffractometer (Philips, PW-1700) using a CuKa radiation (40kV, 25mA).

481 Nitrogen adsorption isotherm and specific area were determined using Micromeritics ASAP 2000 automatic analyzer. A series of ER fluids were prepared by dispersing the MCM-41 particles in anhydrous silicone oil with the particle concentrations of 5, 10, and 20 wt%. ER properties of the fluid were then measured at (25 + 0.1) ~ using a rotation type rheometer (MC120, Physica, Germany) equipped with a high voltage generator. Measuring unit was a concentric cylinder having 1.06 mm gap between a set of bob and cup, and DC high electric field strength perpendicular to the flow direction was applied to the measuring unit through the cup; the inner wall of the cup is the positive electrode, the bob is grounded. Initially, an electric field was applied to the ER fluid for 3 minutes before the measurement in order to establish the equilibrium internal structure of particles. Flow curves for each ER fluid were obtained in a controlled shear rate mode. Static yield stresses (smallest stress to fluidize an ER fluid under the applied electric field) were measured in controlled shear stress (CSS) experiments.

3. RESULTS AND DISCUSSION Figure l(a) shows the XRD pattern for the calcined MCM-41. Regular hexagonal channel structure of MCM-41 was clearly implicated by the strong (100) peak followed by three [(110) (200) and (211)] peaks in the diffractogram for the 20 ranges of 2 - 7~ (Fig. 1(a)). BET surface area of the sample was ca 950 mZ/g with average BJH pore diameter of 31A. TEM image of Fig. l(b) shows uniform hexagonal shaped pore structure of MCM-41. Various particle morphology of MCM-41 is reported and surface textures exhibit a range from smooth through pitted [27]. According to our Scanning electron microscope (SEM) analysis (not shown), MCM-41 crystals prepared were mostly made of particles of 8 0 - 120 nm in diameter, and existed as agglomerated groups [8].

Figure 1. XRD pattern (a) and TEM image (b) for MCM-41 used in our experiment.

482 Figures 2(a) and (b) show the shear stresses and shear viscosities as a function of shear rates under the various applied electric field strengths for MCM-41 ER system with the particle concentration of 10 wt% in silicone oil. Typical ER characteristics, shear stress increase with electric field strength and also decrease or reaches plateau at low shear rate region [12,13,15], are observed as shown in Figs. 2. MCM-41 is hydrophilic but has no polarizable species such as electrons and ions, so that the MCM-41 ER fluid is believed to be activated by absorbed moisture. The moisture content in MCM-41 was ca 6 wt% from thermogravimetric analysis (TGA).

Figure 2. Shear stress (a) and shear viscosity (b) as function of shear rates for MCM-41 ER fluids (10 wt% MCM-41 particles in silicone oil) under different applied electric field strengths. (c) An optical microscopic photo of an ER fluid (5 wt% MCM-41) under zero electric field strength. (d) Optical microscopic photo of the same fluid as (c) under the electric field strength of 3kV/mm. Black parts in (c) and (d) are electrodes, which are separated by 0.5 mm.

483 As shown in Fig. 2(d), the variations in flow properties of ER fluids are due to the particle chain structures formed by the applied electric field. The instantaneous transition from liquid to solid structure stems from the particle microstructures. An applied deformation over certain level breaks the particular chains and these can be reformed as long as electric field is applied. The energy consumption during destruction of particle structures is responsible for the viscosity enhancement. The ER effect disappears under high deformation rate (shear rate), because there is no enough time for broken particles to reform structures at such high shear rates (Figs. 2 (a) and (b)). The particle chains are maintained under sufficiently small strain, and they are stretched and rearranged by the deformation and produces the viscoelasticity of solidified ER fluids under an applied electric field. 10 5

(a)

(b) /k

A

10 2

v v v v v v v v V v

Q.

A

L___,

~~

~ O

03 t~

0

oolO

0

I~. 104

0

0

1

(D t-C0

0

0

0

0

0

0

0

[]

10 0 10 -2

,

zX 20 wt% o 10 wt% [] 5 wt%

.

, ....

,I

10 -1

,

,

......

13

[1 I

10 0

. . . . . . .

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101

. . . . .

,,,I

10 2

Shear rate [sec -~]

. . . . . .

ul

10 a

[] []

I

10 0

. . . . . . . .

[]

V

3.0 kV/mm 2.0 kV/mm o 1.0 kV/mm a. 0.5 kV/mrn

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[] . . . . . . . .

10a0-1 1

[]

i

. . . . . . . .

101

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10 2

Frequency [sec -~]

Figure 3. (a) Shear stress as function of shear rates for MCM-41 ER fluids for three different particle concentrations (5, 10 and 20 wt%) at 3 kV/mm. (b) G' as a function of deformation frequency with strain of 0.0015 for 10 wt% MCM-41 ER fluid.

Shear stress data of MCM-41 ER fluids having three different particle concentrations (5, 10, and 20 wt%) are shown in Fig. 3(a). A sample with 5 wt% MCM-41 exhibited little ER effect ; the slope of shear stress versus shear rate in log plot was almost 1.0, (i.e., Newtonian fluid behavior). The samples with 10 wt% and 20 wt% of MCM-41 demonstrate similar flow behaviors as given in Fig. 2. An observed, large ER property above the "critical particle concentration" is noteworthy. More careful measurement on this observation will be provided in future communication. Dynamic tests employing an oscillatory shear were also conducted to study the viscoelastic properties of the solidified ER fluid under an applied electric field. Linear viscoelastic response is obtained at strain of 0.0015 from the strain amplitude sweep measurement, which measures stress as a function of sinusoidal strains at a constant frequency. The storage modulus (G'), the in-phase stress component with the strain, is observed to be larger than the loss modulus (G"), the out-of-phase stress component. These values were independent of the strain in the linear viscoelastic region. However, as we

484 increased the applied strain, G" becomes larger than G', and these moduli sharply decreased. This phenomenon can be explained in terms of the elasticity of ER fluid, which is generated by particle chain structures in an imposed electric field [28, 29]. When the fibril structures of the suspended zeolite particles sustain the applied strain, the elasticity is dominant in the linear viscoelastic region. However, as the strain is increased, the deformation begins to distort the structure, and the structure breaks down beyond a certain degree of deformation, and finally the elasticity of ER fluid disappears abruptly [20]. Martin et al. [30], reported that they could not observe any linear viscoelastic region and, therefore, claimed that the energy of ER fluid was stored in G". Based upon our experimental result, however, it seems thestrain amplitude less than 0.01 will guarantee that the measurements are in the linear viscoelastic region. Figures 3(b) shows the plot of G' as a function of frequency with small strain in linear viscoelastic region. G's either remained constant or slightly increased as deformation frequency increased up to 100 sec j. Since the relaxation time for deformation was too large, it is expected that the internal chain structures of ER fluids are not destroyed by deformation under the given conditions. The increase in G' with the applied electric field strength indicate that the ER fluid becomes more elastic with the increasing electric fields strength under linear viscoelastic conditions. 10 3

.

.

.lOW 9

10 2

Q.. '# 01

.

.

,~

10 3

20wt%

s,ooe- /

9

10 4 9 20 wt% 9 10 wt%

....

10 2

10 ~

/

t

9

10 0

10 ~ 10 -~

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10 0

I ......,d

101

10 -1 10 -1

10 0

^

101

10 2

E

E [kV/mm]

Figure 4. (a) Xs of each MCM-41 ER fluid with various electric field strengths. (b) the universal correlation. The solid line represents the calculated value from Eq. (3). The static yield stress (Xs) data of the MCM-41 ER fluids are plotted in Fig. 4(a) and these are correlated with a universal scaling relationship between electric field strength (E) and Xs via Eq. (2) suggested by Choi et al. [31 ]"

x, (E) - ~E 2

~/E/Er/

tanh ~/E/E ~:

'

(2)

485 where Gt depends on the dielectric constant of the fluid and particle concentraion, and Ec is the critical electric field strength, which is related to the particle conductivity and particle concentration. Equation (2) indicates that % is approximately proportional to E 2 for E > Ec. We simplified Eq. (3) by using a generalized scaling function [31 ]: = 1.313133/2 tanh ~ ,

(3)

where 1~- E/E c and ~ = zs(E)/'C(Ec). The Xs data ofMCM-41 ER fluids are collapsed onto a single curve using Eq. (3), as shown in Fig. 4(b). E c is 0.4 kV/mm for 10 wt% MCM-41 and 0.7 for 20 wt% MCM-41.

4. CONCLUSIONS In this study, MCM-41 mesoporous molecular sieve was synthesized and its ER characteristics was examined. The synthesized MCM-41 had well-defined channel structure from XRD and TEM analyses. Its suspension in silicone oil showed typical ER properties and moisture absorbed in hydrophilic MCM-41 is the polarization species at high electric field strengths. The static yield stresses were measured in CSS mode, and these were related to applied electric field strengths by Zs "- E TM for 10 wt% MCM-41 ER fluid and % ~ E 1"67 for 20 wt% MCM-41 ER fluid.. The linear viscoelastic properties ( G' and G") of MCM-41 ER fluid under various electric field strengths were also measured at small strain. The elasticity of solidified ER fluids increased with applied electric field strength under the linear viscoelastic condition.

5. A C K N O W L E D G M E N T S This study was supported by research grants from the Korea Science and Engineering Foundation (KOSEF) through the Applied Rheology Center (ARC), an official KOSEFcreated engineering research center (ERC) at Korea University, Seoul, Korea.

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

U. Ciesla and F. Schfith, Microporous Mesoporous Mater., 27 (1999) 131. M. ICruk, M. Jaroniec and A. Sayari, Microporous Mesoporous Mater., 35 (2000) 545. A. Sayari, Chem. Mater., 8 (1996) 1840. X. Feng, G. E. Fryxell, L. Q. Wang, A. Y. Kim, J. Liu and K. M. Kemner, Science, 276 (1997) 923. C.G. Wu and T. Bein, Science, 264 (1994) 1757. C.G. Wu and T. Bein, Science, 266 (1994) 1013. J.F. Diaz and K. J. Balkus Jr., J. Mol. Catal. B: Enzymatic, 2 (1996) 115. H.J. Choi, M. S. Cho, K. K. Kang and W. S. Ahn, Microporous Mesoporous Mater., 39 (2000) 19.

486 9. R. Tao and Q. Jiang, Phys. Rev. Lett., 73 (1994) 205. 10. H. Yamada, Y. Taniguchi and A. Inoue, Int. J. Mod Phys. B, 15 (2001) 947. 11. I. S. Sim, J. W. Kim, H. J. Choi, C. A. Kim and M. S. Jhon, Chem. Mater., 13 (2001) 1243. 12. H. J. Choi, T. W. Kim, M. S. Cho, S. G. Kim and M. S. Jhon, Eur. Polym. J., 35 (1997) 699. 13. H. J. Choi, M. S. Cho and K. To, Physica A, 254 (1998) 272. 14. H. J. Choi, J. W. Kim, M. S. Suh, M. J. Shin and K. To, Int. J. Mod. Phys. B, 15 (2001) 649. 15. H. J. Choi, M. S., Cho J. W. Kim, R. M. Webber and M. S. Jhon, Int. J. Mod. Phys. B, 15 (2001) 988. 16. H. J. Choi, M. S. Cho and M. S. Jhon, Int. J. Mod. Phys. B, 13 (1999) 1901. 17. J. W. Kim, S. G. Kim, H. J. Choi and M. S. Jhon, Macromol. Rapid Commun., 20 (1999) 450. 18. S. G. Kim, H. J. Choi and M. S. Jhon, Macromol. Chem. Phys., 202 (2001) 521. 19. W. Wen, N. Wang, W. Y. Tam and P. Sheng, Appl. Phys. Lett., 71 (1997) 2529. 20. J.W. Kim, H. J. Choi, S. H. Yoon and M. S. Jhon, Int. J. Mod. Phys. B, 15 (2001) 634. 21. D. J. Klingenberg, P. Pakdel, Y. D. Kim, B. M. Belongia and S. Kim, Ind. Eng. Chem. Res., 34 (1995) 3303. 22. M. S. Cho, H. J. Choi, I. J. Chin and W. S. Ahn, Microporous Mesoporous Mater., 32 (1999) 233. 23. M. Kruk, M. Jaroniec and A. Sayari, Microporous Mesoporous Mater., 35/36 (2000) 545. 24. Ch. Danumah, S. M. J. Zaidi, G. Xu, N. Voyer, S. Giasson and S. Kaliaguine, Micropor. Mesopor. Mater., 37 (2000) 21. 25. W. Zhang, M. Fr i5ba, J. Wang, P. T. Tanev, J. Wong and T. J. Pinnavaia, J. Am. Chem. Soc., 118 (1996) 9164. 26. R. Ryoo, C. H. Ko and R. F. Howe, Chem. Mater., 9 (1997) 1607. 27. Q. Huo, D.I. Margolese, U. Ciesla, P. Feng, T.E. Gier, P. Sieger, A. Firouzi, B.F. Chmelka, F. Schiith, and G.D. Stucky, Chem. Mater., 6 (1994) 2317. 28. M. S. Cho, Y. J. Choi, H. J. Choi, S. G. Kim and M. S. Jhon, J. Molecular Liq., 75 (1998) 13. 29. S. G. Kim, J. W. Kim, M. S. Cho, H. J. Choi and M. S. Jhon, J. Appl. Polym. Sci., 79 (2001) 108. 30. J. E. Martin, D. Adolf and T. C. Halsey, J. Colloid Int. Sci., 167 (1994) 437. 31. H. J. Choi, M. S. Cho, J. W. Kim, C. A. Kim and M. S. Jhon, Appl. Phys. Lett., 78 (2001) 3806.


487

Synthesis and characterization o f TiO2 loaded C r - M C M - 4 1 catalysts E.P. Reddy +, Lev Davydov, and Panagiotis G. Smirniotis* Reaction Engineering and Catalysis Research Laboratory, Chemical Engineering Department, University of Cincinnati, Cincinnati, OH 45221-0171, USA. + Presenter; * Corresponding Author: E-mail: [email protected] Chromium substituted mesoporous MCM-41 material (Si/Cr = 80) was synthesized by the incorporation of chromium ions during synthesis. This was then loaded with titania via sol-gel method to explore the photoactivity in visible light. This prepared catalyst was characterized by different physico-chemical techniques such as BET surface area, nitrogen physisorption, oxygen chemisorption, XRD, XPS, TPR and Raman spectroscopy. The BET surface area, however, was lower than those commonly found in our siliceous MCM-41. This is due to the partial pore breakage, as recorded by the pore size distribution analysis. The chromium metal dispersion found in the majority of the specimens studied was quite high (in the vicinity of 50%) and decreased with the loading of TiO2. The XRD analysis showed the patterns of CrMCM-41, 25%TiO2/Cr-MCM-41 similar to those of siliceous MCM-41, however, the intensity of the d~00 peak is decreased, while the loading of titania. Raman studies of CrMCM-41 and 25% TiOz/Cr-MCM-41 indicate that the chromium is well dispersed inside the MCM-41 framework, and decreasing intensity of Raman peaks at 144 cm ~, 397 cm -~, 518 cm ~ and 641 cm -~ upon loading of titania on Cr-MCM-41 indicates that the titania was interacting with chromium, and where as the peaks are associated to anatase phase of loaded titania. TPR studies revealed a change in the reduction temperature of Cr(IV) in the titanialoaded Cr-MCM-41. The reduction temperature of Cr(VI) was found to depend on the nature of the chromium species in the MCM-41 matrix. This significantly contributes to the remarkable photocatalytic activity of TiOz/Cr-MCM-41, and this does not happen in other transition metals incorporated MCM-41. The surface composition and binding energy of Cr 2p3/2 peak of the Cr-MCM-41 and 25%TiO2/Cr-MCM-41 was analyzed by XPS and showed considerable diffusion of chromium ions to the surface upon loading of titania. The binding energy value of Cr 2p3/2 was decreased upon loading of TiO2 on Cr-MCM-41, indicates that Cr is strongly interacting with TiO2. Eventually, two separate surface electronic levels corresponding to Si-O-Cr and Ti-O-Cr regions were found by XPS analysis for TiOz/CrMCM-41. 1. INTRODUCTION A great deal of recent research focused on a new family of molecular sieves, designated as MCM-41, has been discovered by Mobile scientist [1-3]. This material possessed a uniform arrangement of hexagonally shaped mesopore structure. Moreover, by changing the liquid

488 crystal template (LCT) mechanism, the pore size may be varied from 1.5 to 10 nm by changing the surfactant chain length [4]. Their high thermal and hydrothermal stability, uniform size and shape of the pores, and large surface area, make them of interest as sorbents and catalysts [ 1-4]. Pure silica possesses a neutral framework, which limits its application as a catalyst or as a support for preparing novel heterogeneous catalysts. Consequently, isomorphous substitution of silicon with transition metals was an excellent strategy in creating catalytically active sites and anchoring sites for catalytically active molecules in the design of new heterogeneous catalysts. Therefore, various transition metals such as Ti [5], V [6], Fe [7], Mn [8] and Cr [9] incorporated molecular sieves having redox catalytic properties have been synthesized by hydrothermal method. Among these materials, chromium incorporated microporous as well as mesoporous molecular sieves are particular interest because chromium compounds are widely used as stoichiometric oxidants in organic synthesis [10] and as homogeneous catalysts [11] in the presence of alkyl hydroperoxides. Moreover, Cr (VI) typically catalyzes oxidations via an oxometal mechanism in which chromyl (CrO2+2) species are the active oxidants. Only one report is available in the open literature concerning the photocatalytic oxidation of organics over chromium incorporated MCM-41 [ 12] Very recently we reported that the titania doped Cr-MCM-41 is very active catalyst for the liquid phase photocatalytic oxidation of organics at atmospheric conditions [13]. However, the exact nature of chromium in MCM-41 needs to be known. The explanation of the distribution, oxidation state, and co-ordination of these Cr species on titania doped Cr-MCM-41 is fundamental important for understanding thecatalytic role of Cr in 25%TiO2/Cr-MCM-41. In this present work, a systematic characterization study of Cr-MCM-41 and 25%TiO2/CrMCM-41 was undertaken by using different physico-chemical techniques such as BET surface area, nitrogen physisorption, oxygen chemisorption, XRD, diffuse reflectance UVVis, XPS, TPR, and Raman spectroscopy and evaluated these catalyst by visible light irradiated photocatalytic oxidation of formic acid. 2. EXPERIMENTAL SECTION

2.1. Materials The sources of silica, titania and chromium were Ludox HS-40 (Aldrich, 40 wt.-% colloidal silica in water), tetraisopropyl othotitanate (TIPOT, Fluka, p.a.) and chromium nitrate (Cr(NOa)a.9H20 Fisher, 99.97% purity) respectively. As a quaternary ammonium surfactant compound hexadecyltrimethylammonium bromide (HDTMABr, Alfa Aesar, 99 +%) was used. Other compounds of the synthesis were tetramethylammonium hydroxide (TEAOH, Fluka 40 wt.-% solution in water) and ammonium hydroxide (NH4OH, 29 wt.-% solution in water). All chemical were used without further purification. 2.2. Synthesis procedure Chromium substituted MCM-41 with an atomic Si/Cr ratio of 80 was synthesized as previously reported [14] using Ludox HS-40 as the source of silica. The following is the typical preparation procedure: 35 grams of Ludox was added to 14.55 ml of water under stirring, and 18.2 ml of 40 % TEAOH added. Independently, 18.25 g of the template was dissolved in 33 ml of water, and subsequently 7 ml of 28 % NH4OH was introduced. Finally, the above two solutions containing Ludox and template were mixed together. The

489 corresponding amount chromium nitrate dissolved in water was added drop-wise from a pipette to the resulting mixture. The final mixture was stirred together for 30 minutes, then transferred into telfon bottle and treated under autogenous pressure without stirring at 90 100~ for 3 days. The resulting solids were filtered, washed, dried, and calcined at 550~ for 10 hours under airflow. The temperature profile was 2 ~ up, 15 ~ down.

2.3. Impregnation procedure 1.5 g of Cr-MCM-4 l was impregnated with a solution of TIPOT in---100 ml of isopropanol giving Ti loading 25 wt.-%. The system was dried while stirring at ambient temperature. It was then placed in the oven to dry at 100~ for 1 hour. They were then transferred into a boat-like crucible and calcined at 450~ for 3 hours with a temperature ramp of 2 ~

2.4. Characterization The specific BET surface area of Cr-MCM-41 and 25%TiO2/Cr-MCM-41 materials were measured by nitrogen adsorption at-196~ by using a Micromeritics Gemini 2360. HorvathKawazoe maximum pore volume and adsorption average pore diameter measurements of these materials were performed with a Micromeritics ASAP 2010 using adsorption of N2 a t 196~ All samples were degassed at 250~ under vacuum before analysis. Oxygen uptake measurements of Cr-MCM-41 and 25%TiO2/Cr-MCM-41 materials performed at 370 ~ with a Micromeritics ASAP 2010 Chemi system. The powders were characterized by UV-Vis spectrophotometer (Shimadzu 2501PC) with an integrating sphere attachment ISR1200 for their diffuse reflectance in the range of wavelength of 200 to 800 nm. BaSO4 was used as the standard in these measurements. X-ray diffraction (XRD) studies were obtained on a Nicolet powder X-ray diffract meter equipped with a CuK~ radiation source (wave length 1.5406/~) to assess their crystallinity. Raman spectra were obtained at room temperature using excitation line from Coherent 906 Ar + (514.5 nm) and K-2 Kr + (406.7 nm) ion lasers, collecting backscattered photons directly from the surface spinning (-~2000 rpm) solid samples in 8-mm diameter pressed pellets. Conventional scanning Raman instrumentation equipped with a Spex 1403 double monochromator, with a pair of gratings with 1800 grooves/mm, and a cooled Hamamastsu 928 photomultiplier detector was used to record the spectra under the control of a Spex DM3000 micrometer system. Temperature programmed reduction (TPR) experiments were carried out in a gas flow system equipped with a quartz micro-reactor, using custom-made set-up attached with TCD detector. Approximately 100 mg of sample was pretreated in 23 ml/min flowing of He at 350~ for 1 h. After pretreatment, the materials were tested in 6 vol% H2/He, 25 cm3/min and increasing the temperature from 100~ to 800~ at 5~ and kept the temperature at 800~ for 2 h. XPS analyses were conducted on a Perkin-Elmer Model 5300 X-ray photoelectron spectrometer with MgK~ radiation at 300 W. Typically, 89.45 and 35.75 eV pass energies were used for survey and high-resolution spectra, respectively. The effects of the sample charging were eliminated by correcting the observed spectra for a C 1s binding energy value of 284.5 eV.

490 3. RESULTS AND DISCUSSIONS

The BET surface areas, pore volume, pore diameter and metal dispersion values of MCM41, Cr-MCM-41, 25%TiOz/MCM-41 and 25%TiOz/Cr-MCM-41 are depicted in table 1. One can clearly see that the lowering of surface area as well as increase of pore volume and pore diameter values with the introduction of chromium inside the MCM-41 framework as compared to the siliceous MCM-41. Since the same surfactant template was used for the synthesis of both siliceous and Cr substituted MCM-41 materials, one should expect to obtain nearly same pore diameter for both the materials. The above difference is mainly due to the partial blockage of the hexagonal tubular walls of the MCM-41 structure. The presence of chromium salt changes the ion strength of the gel during synthesis, which may hinder the action of the template and results in the formation of lower surface area as well as bigger pore diameter and pore volume. Cr-MCM-41 lost its surface area, pore volume and pore diameter with the 25% loading of titania. It is due to the partial blockage of the pores, as we explained in our earlier papers [ 13] smaller percentage of titania deposition shows negligible loss of the surface area, where as higher coverages lead to substantial loss of in surface area. This clearly indicates the higher loading of titania on Cr-MCM-41 fill up some of the pores leading to their partial blockage. Table 1. BET surface areas, pore volume, pore diameter and metal dispersion of MCM-41, 25%TiOz/MCM-41, Cr-MCM-41 and 25%TiOz/Cr-MCM-41 ............ Catalyst ............ i~ET SA ......Pore volume Pore diameter(nm) % of dispersion a (m2/g) (cm3/g) , (O/Cr) MCM-41 940 0.94 4.2 Not detected 25 %TiO2/MCM-41 667 0.56 3.4 0.1 Cr-MCM-41 825 1.08 5.23 54.37 25%TiOz/Cr-MCM-41 623 0.72 4.62 22.25 a %'"'Ofdispersion fraction of Cr'atoms at iiae su'rface, assuming OadjC"rs~f= 1. As shown in Table 1, MCM-41 and 25%TiO2/MCM-41 did not show any metal dispersion as expected, where as the Cr- substituted MCM-41 revealed higher Cr-dispersion when compared to the titania loaded Cr-MCM-41. This is due to the partial blockage of the Cr active sites by the TiO2 loading, making them inaccessible to co-ordinatively unsaturated sites. Davydov et al [ 13] already explained that the loading of titania on siliceous MCM-41 does not show any metal dispersion, this suggests that the loaded titania does not chemisorb any oxygen atoms, moreover, it blocks the accessible co-ordinatively unsaturated sites of transition metals incorporated MCM-41 s. UV-vis absorption spectra of Cr-MCM-41 and TiOz/Cr-MCM-41 were recorded in the range of wavelength of 200 to 800 nm (Figures not shown). Neat Cr-MCM-41 exhibit three types of absorption peaks at "-275 nm, "-380 nm, corresponding to Cr+6 and shoulders at--470 nm corresponding to and Cr+3 species [9]. The same material, but loaded with 25% TiO2 exhibits higher absorption in the UV range due to the presence of titania. All the materials still retain high absorption in visible light (up to 600 nm)and have distinct shoulder at--370 nm.

491 X-ray diffraction (XRD) patterns of MCM-41, Cr-MCM-41 and TiO2/Cr-MCM-41 recorded from 20 = 2 ~ to 7 ~ are shown in Figure 1. The XRD reflections 100, 110, 200, and 210 of Cr-MCM-41 and TiOz/Cr-MCM-41 are determined at the same location as that of siliceous MCM-41 reflections [2,3], which can be indexed to hexagonal lattice structure. The intensities of these peaks lower, when compared to the MCM-41. One can suggest that the presence of Cr ions obstructs the structure-directing action of the template by changing its ionic strength [15]. One more interesting point is that we could not detect any peaks associated to Cr or chromium oxides. This indicates that the chromium ions are either dispersed in the MCM-41 framework or stays outside the framework as an amorphous phase. The XRD of 25%TiOz/Cr-MCM-41 was also recorded in the range of 20 = 20 ~ to 50 ~ in order to assess the crystallinity of TiO2 loading on to the Cr-MCM-41. It showed the 25%TiOa/CrMCM-41 exhibited low crystallinity of titania, may be due to an intimate contact with chromium ions or uniform distribution of titania on the pore walls of the MCM-41. These XRD results are in perfectly agreement with Raman and XPS results that we explained in latter paragraphs.

144

lOO

I-.,, 5 v

A /

5

25%TiOJMCM-41 397 518

cQ}

r _.=

25%TiOJCr-MCM-41 ~ % T i O 2M / -C 4 r-M ~C

2

~

~,

~

~ 7

2e

Figure 1. XRD patterns of MCM-4 l, CrMCM-41 and 25%TiO2/MCM-41.

Cr-MCM-41

1+0

360

4~0

6~0

Raman shift (cm "1)

Figure 2. Raman spectra of Cr-MCM-41, 25%TiOz/MCM-41 and 25 O~TiO2/Cr-MCM-41

Raman Spectra of MCM-41, Cr-MCM-41, 25%TiO2/MCM-41 and 25%TiO2/Cr-MCM-41 was presented in Figure 2. Raman spectra associated to Cr-MCM-41 did not show any peaks allied to Si-O, Cr-O bending or stretching modes, shows that the Cr is well dispersed in side the MCM-41 framework. However, in the case of 25%TiOz/MCM-41 and 25% TiO2/CrMCM-41' shows fours bands at 144 cm -~, 397 cm -~, 518 cm ~ and 641 cm -l indicate the existence of titania (anatase) particles [16]. The intensity of these four peaks corresponds to 25%TiOz/MCM-41 higher than 25%TiO2/Cr-MCM-41. Therefore, One can easily understand that the loaded titania is directly interacting with the Cr ions incorporated inside the MCM-41

492 framework. The results obtained from these spectra are well agreeing with the XPS results explained in the latter paragraphs. TPR profiles measured for Cr-MCM-41 and 25%TiO2/Cr-MCM-41 are shown in Figure 3. The reduction behavior of Cr is different in both the cases, though the amount chromium is same. There are two major peaks were observed in the case of Cr-MCM-41, according to Uhm et al [ 17] the peak at 434~ corresponding to reduction of Cr(VI) to Cr(III), and the peak at 800~ was associated to the hydroxyl groups leaving the surface of amorphous silica, since it observed even in the siliceous MCM-41. The TPR profile of 25%TiO2/Cr-MCM-41 show marked difference, when titania is loaded onto the Cr-MCM-41. The reduction temperature of Cr (VI) to Cr (III) increased from 434~ to 502~ this may due to a higher degree of its interaction with the titania loading. In addition to that chromium is expected to achieve the tetrahedral coordination, when incorporated during the synthesis of MCM-41 [ 15]. Therefore, this may also contribute to the peculiar interaction of incorporated chromium and loaded titania leads to increase the reduction temperature of Cr(VI). The interaction between chromium ions and titania was clearly observed by XPS analysis, which is discussed in latter paragraphs.

/

502

~'~

800 5

G)

v

200

,

!

400

600

800 Isothermal I = Temperature (~

Figure 3. TPR profile of: a) 25%TiO2/CrMCM-41; b) Cr-MCM-41

526

528

530

532

534

536

Binding energy (eV)

Figure 4. XPS of 0 l s core level for : a) 25%TiO2/Cr-MCM-41; b) Cr-MCM-41.

The XPS spectra of O 1s core level for Cr-MCM-41 and 25%TiO2/Cr-MCM41 are shown in Figure 4. Only one type of oxygen photoelectron peak at 532.7 eV belongs to SiO2 [18] was observed for Cr-MCM-41, where as the deconvoluted spectra of O ls core level peak corresponding to 25%TiO2/Cr-MCM-41 show three types of peaks, which was attributed due to the overlapping contribution of oxygen from silica, titania and chromium. As shown in figure 4, one can clearly see that the binding energies values of O ls at 529.3 eV, 530.2 eV, and 532.7 eV are belongs to the oxygen atoms that are bound to Cr(O)x, TiO2 [ 18] and SiO2 respectively. Moreover, the peak intensity of O 1s is decreased when titania loaded onto Cr-

493 MCM-41. This shows that the titania interacting with incorporated Cr ions and inducing into the surface of MCM-41. The XPS of Si 2p core level spectra belongs to Cr-MCM-41 and 25%TiO2/Cr-MCM-41 is shown in Figure 5. The binding energy value of Si 2p is found at 103.2 eV, which agrees well with the values reported in the literature [20]. The intensity of Si 2p is very high in the case of Cr-MCM-41 when compared to that of 25%TiO2/Cr-MCM-41. This indicates that the loading of titania is covering the surface of Cr-MCM-41, at the same time, it inducing the chromium from the framework of MCM-41. As shown in Figure 6, the intensity of Cr 2p is more predominant in the case of 25%TiOJCr-MCM-41 than in the case of Cr-MCM-41, but the binding energy of Cr 2p3/2 is decreased in the former case. In the case of Cr-MCM-41 the binding energy of Cr 2p3/2 is 579.4 eV; it probably corresponds to the Cr(IV). With loading of titania, the binding energy of Cr 2p3/2 decreases from 579.4 to 577.2 eV, it is an indication of either decrease of oxidation state of chromium or may be due to the interaction between loaded titania and chromium.

Cr 2P3/2

Si 2p

,.-..,

.i >" r

a

t,-

_.=

tin

97.5

lo;.o'1o~,.~

~o;.o'1o7.5

Binding e n e r g y (eV)

Figure 5. XPS of Si 2p core level for: a) 25%TiO2/Cr-MCM-41; b) Cr-MCM-41.

575

580

585

590

Binding energy (eV)

Figure 6. XPS of Cr 2p core level for 9 a)25%TiO2/Cr-MCM-41; b) Cr-MCM-41

The relative dispersion of chromium inside and out side the MCM-41 framework was also estimated from the XPS measurements of Cr-MCM-41 and 25%TiO2/Cr-MCM-41. The surface atomic concentration ratios of Cr/Si and Cr/Ti were taken as a measure of the relative dispersion of chromium oxide; inside and outside of the MCM-41 before and after the titania loading. The ratio between Cr/Ti - 0.136 and Cr/Si = 0.236 for 25%TiOJCr-MCM-41, where as the Cr/Si ratio is 0.002 in the case of Cr-MCM-41. As a matter of fact, the Cr/Si atomic ratio values clearly indicate that the chromium ions species was well dispersed inside the MCM-41 framework in the latter case. When loading of titania, Cr/Si ratio increased from 0.002 to 0.236 and the Cr/Ti - 0.136 indicates that Cr species induced form the framework to the surface and interacted with titania in the form of Ti-O-Cr, which also was observed on

494 Cr/TiMCM-41 [21] and on Cr/TiO2 [22]. The XPS results are in perfectly agreement with XRD and Raman results explained in earlier paragraphs. 4. A C K N O W L E D G M E N T S

The authors are grateful to the Young Investigator Award of the United States Department of Army ( Grant DAAD 19-00-1-0399) and NATO Science for Pease Program (Grant SfP974209) for the their support of this work. We also acknowledge funding from the Ohio Board of Regents (OBR) that provided matching funds for equipment to the NSF CTS9619392 grant through the OBR Action Fund #333. REFERENCES ~

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495

Photocatalytic Ethylene Polymerization over C h r o m i u m Containing Mesoporous Molecular Sieves Hiromi Yamashita*, Katsuhiro Yoshizawa, Masao Ariyuki, Shinya Higashimoto, and Masakazu Anpo* Department of Applied Chemistry, Graduate School of Engineering, Osaka Prefecture University, Gakuen-cho 1-1, Sakai, Osaka 599-8531, Japan

The photocatalytic reactivities for ethylene polymerization on chromium-containing mesoporous molecular sieves (Cr-HMS) have been investigated. The characterizations with several spectroscopic measurements, such as XAFS, ESR, UV-VIS, and photoluminescence have indicated that Cr-HMS involves tetrahedral chromium oxide (Cr-oxide) moieties which are highly dispersed and incorporated in the framework of molecular sieve with two terminal Cr=O. In the presence of ethylene, Cr-HMS exhibited photocatalytic reactivity for the ethylene polymerization not only under UV light irradiation but also visible light irradiation. The photocatalytic reactivity of the Cr-HMS mesoporous molecular sieves was found to be much higher than those of the Cr-silicalite microporous zeolite and imp-Cr/HMS prepared by the impregnation method. The efficient dynamic quenching of the photoluminescence of the Cr-oxide moieties in the excited state in the Cr-HMS by the addition of ethylene molecules was found to indicate that the charge transfer excited state of a tetrahedral Cr-oxide moieties and large pore size plays a significant role in the photocatalytic reactions.

1. INTRODUCTION The highly dispersed transition metal oxides incorporated within the framework of zeolites and molecular sieves show unique reactivities not only for various catalytic reactions, but also for photocatalytic reactions under UV light irradiation [1-3]. The unique and efficient photocatalytic systems incorporating the transition metal oxides (Ti, V, Mo, etc.) have been designed and developed using the cavities and frameworks of zeolites and mesoP0rous molecular sieves [4-6]. Recently, we have found that chromium-containing mesoporous and zeolite catalysts can exhibit the photocatalytic reactivities for the NO decomposition and partial oxidation of alkanes and alkenes not only under UV light irradiation but also visible light irradiation [7-9]. The highly dispersed chromium oxide (Cr-oxide) supported on silica is an industrially important catalyst for ethylene polymerization [10,11]. Recently, it has reported that chromium acetyl acetonate complexes grafted on mesoporous molecular sieves (MCM-41) can exhibit the efficient reactivity for ethylene polymerization at higher than 373 K after the calcination at higher than 773 K [ 12, 13].

496 In the present study, we have investigated the photocatalytic reactivity of the chromium-containing mesoporous molecular sieves (Cr-HMS) for the ethylene polymerization not only under UV light irradiation but also visible light irradiation. The characterization of the local structure of the active sites and their role in the photocatalytic reaction have been investigated at the molecular level by means of dynamic photoluminescence, XAFS, ESR, UV-VIS, and XRD measurements along with an analysis of the reaction products.

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

Cr-HMS mesoporous molecular sieves (Si/Cr=50, 100, 500) were synthesized using tetraethylorthosilicate and Cr(NO3)3 9H20 as the starting materials and dodecylamine as a template [7,8,14,15]. The chromium-silicalite (CrS-1) microporous zeolite (Si/Cr=500) was hydrothermally synthesized using tetraethoxysilane and Cr(NO3)9H20 as starting materials and tetrapropyl ammonium hydroxide (TPAOH) as a template in accordance with previous literature [16]. Imp-Cr/HMS zeolite (Si/Cr=50) were prepared by impregnating HMS with an aqueous solution of Cr(NO3)3.9H20. Calcination of the sample was carried out in a flow of dry air at 773 K for 5 h. Prior to spectroscopic measurements and photocatalytic reactions, the catalysts were degassed at 723 K for 2 h, heated in 02 at the same temperature for 2 h and then finally evacuated at 473 K for 2 h to 10-6 Torr. The photocatalytic reactions were carried out with the catalysts (100 mg) in a quartz cell with a flat bottom (80 ml) connected to a conventional vacuum system (10 -6 Torr range). The photocatalytic reactions were carried out under UV light (~> 270 nm) or visible light (k>450 nm) irradiation at 273 K using a high pressure mercury lamp through water and color filters. The photocatalytic polymerization of ethylene was carried out in the presence of ethylene (3.0 mmolg-cat -~) and the formation of polyethylene was confirmed by the IR measurement. XRD patterns were obtained with a Shimadzu XD-D1 using Cu K_ radiation. XAFS (XANES and EXAFS) spectra were obtained at the BL-9A facility of the Photon Factory at the National Laboratory for High Energy Physics (KEK-PF) in Tsukuba. The Cr K-edge absorption spectra were recorded in the fluorescence mode at 295 K with a ring energy of 2.5 GeV and the Fourier transformation was performed on k3-weighted EXAFS oscillations by a procedure described in previous literature [7,17]. UV-VIS spectra were recorded at 295 K with a Shimadzu UV-2200A spectrophotometer. ESR spectra were recorded with a JEOL2X spectrometer (X-band) at 77 K. The in situ photoluminescence spectra the catalysts were measured at 77 K with a Shimadzu RF-501 spectrofluorophotometer. IR measurements were carried out at 295 K using a JASCO FT-IR 7600 spectrometer with the catalysts before and after photocatalytic reaction.


Figure 1 shows the XRD patterns of the Cr-HMS mesoporous molecular sieve and impCr/HMS. The results of the XRD analysis indicated that the Cr-HMS mesoporous molecular

497 sieve has the structure of the HMS mesoporous molecular sieve having pores larger than 20 A and the Cr-oxide moieties are highly dispersed in the framework of molecular sieves, while no other phases are formed [7,8,14,15 ]. Figure 2 shows the XAFS spectra of the treated Cr-HMS and imp-Cr/HMS. Cr-HMS exhibits a sharp and intense preedge peak which is characteristic of Cr-oxide moieties in tetrahedral coordination having terminal Cr=O [7,8,18]. In the FT-EXAFS spectrum, only a single peak due to the neighboring oxygen atoms (Cr-O) can be observed indicating that Cr ions are highly dispersed in Cr-HMS. From the curve fitting analysis of the FT-EXAFS spectrum, it has found that there are two oxygen atoms (Cr=O) in the shorter atomic distance of 1.57 A and two oxygen atoms (Cr-O) in the long distance of 1.82 A. The imp-Cr/HMS exhibits a weak preedge peak in the XANES spectra and an intense peak due to the neighboring Cr atoms (Cr-O-Cr) in the FT-EXAFS spectra, indicating that the catalyst consists of a mixture of tetrahedrally and octahedrally coordinated Cr-oxide species (Cr203like cluster).

r-HMS

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0

I

5

0

I

1'0

2e / Degree

15

20

5990

6030

Energy / eV

0

2

4

6

Distance / ,&

Fig. 1. XRD patterns. Fig. 2. Cr K-edge XANES spectra (A,B) and (a) Cr-HMS,(b) imp-Cr/HMS (Si/Cr=50). Fourier transforms of spectra EXAFS(a,b). (a) Cr-HMS,(b) imp-Cr/HMS (Si/Cr=50). R: atomic distance (A), N" coordination number.

The ESR technique was also applied to investigate the coordination state of the Croxide moieties by monitoring the Cr 5+ ions formed under UV irradiation of the catalyst in the presence of H2 at 77 K. As shown in Fig. 3, after photoreduction with H2 at 77 K, Cr-HMS exhibits a sharp axially symmetric signal at around g=l.9 (g//= 1.889, g_/=1.979), attributed to the isolated mononuclear Cr 5+ ions in tetrahedral coordination [19]. On the other hand, imp-Cr/HMS exhibits a broad signal at around g=2.02 indicating the presence of Cr203 clusters. Figure 4 shows the diffuse reflectance UV-VIS absorption spectra of the Cr-HMS catalysts. Cr-HMS catalysts exhibit three distinct absorption bands at around 270, 370, 480

498 Cr5

gl-- 1.979

1.2

(a) Cr-HMS

~ g / / = 1.889 t-

._o I

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I

100 mT

t-

0.8

U.. e,-

~; 0.4 -i V

.

0 I

200

300

I

400

I

500

I

600

I

700

800

Wavelength/nm

Fig. 3. The ESR spectra of the photoreduced Fig. 4. Diffuse reflectance UV-VIS spectra (a) Cr-HMS and (b) imp-Cr/HMS (Si/Cr=50) of Cr-HMS (A-C) and HMS (D). (A) in the presence of H2 at 77 K. Si/Cr=50, (b) Si/Cr=100, (c) Si/Cr=500.

n m which can be assigned to the charge transfer from O 2" to Cr 6+ of the t e t r a h e d r a l l y coordinated Cr-oxide moieties [20]. The absorption bands assigned to the absorption of the dichromate of C r 2 0 3 cluster cannot be observed above 550 nm, indicating t h a t t e t r a h e d r a l l y coordinated Cr-oxide moieties exist in an isolated state. Cr-HMS evacuated at 473 K exhibited a photoluminescence spectrum at around 550750 nm upon excitation of the absorption (excitation) band at around 250-550 nm. Figure 5 shows the photoluminescence spectra of Cr-HMS observed at 77 K upon the excitation at 370 nm. The photoluminescence bands upon the excitation at 280, 370 and 500 nm were observed at the same position, while the intensities of spectra depend on the wavelength of excitation; the larger intensity was observed with the excitation at 370 nm. In the excitation spectrum of Cr-HMS monitored at 640 nm, three excitation bands are observed at 270, 370 and 490 nm, which are corresponding to the absorption bands observed in the UV-VIS absorption spectra shown in Fig. 4. No change in the positions of these absorption bands is observed with changing the monitoring wavelength of photoluminescence. These results suggest that the photoluminescence occurs as the radiation decay process from the same excited state independently to the excitation wavelength. These absorption and photoluminescence spectra are similar to those obtained with well-defined highly dispersed Cr-oxides anchored onto Vycor glass or silica [21-24] and can be attributed to the charge transfer processes on the tetrahedrally coordinated Cr-oxide moieties involving an electron transfer from 0 2- to Cr 6+ and a reverse radiative decay, respectively. These results indicate that the Cr-HMS mesoporous molecular sieve involves Cr-oxide moieties in tetrahedral

499 %t~

,o 1.2 0

5

E E ~

0.8

i-" eet-

0

0.4

t~

550

600

650

700

750

D

800

0 0

Wavelength / nm

60

120

180

240

300

Reaction time / min

Fig. 5. Effect of the addition of

Fig. 6. The reaction time profiles of photocatalytic polymerization of ethylene on the Cr-HMS (a:Si/Cr=50, c:Si/Cr=500), imp-Cr/HMS (b:Si/Cr=50), and CrS-1 (d:Si/Cr=500) under UV light irradiation (

ethylene on the photoluminescence spectra of the Cr-HMS (Si/Cr=50). Amount of added ethylene: a) 0, b) 5, c) 10, d) 15 lamol'g-cat-1, e) degassed after d).

X>270nrn).

coordination having two terminal Cr=O, being in good agreement with the results obtained by XAFS, ESR and UV-VIS measurements. The estimated model for the local structure of the Cr-oxide moieties and the charge transfer excited state are shown in the following scheme.

0202~ Cr6+ ff O/

~

hv "

hv'

I~

020~Cr5( O~ %

As shown in Fig. 5, the addition of ethylene onto the Cr-HMS led to an efficient quenching of the phosphorescence in its yield, its extent depending on the amount of ethylene added. The observation of efficient quenching with the ethylene addition indicates that the charge transfer excited state of the tetrahedrally coordinated isolated Cr-oxide moieties, (Cr5+--O-) *, easily interact with ethylene under light irradiation. UV light irradiation (~>270 nm) of the Cr-HMS in the presence of ethylene led to the photocatalytic polymerization at 275 K. Figure 6 shows the reaction time profile of photocatalytic polymerization of ethylene. As shown in Fig. 6, the ethylene uptake increases almost linearly to the irradiation time. The reaction immediately stopped when irradiation was ceased. Figure 7 shows the IR spectra of the Cr-HMS in the presence of ethylene. The

500 formation of polyethylene on the Cr-HMS after the UV irradiation was confirmed by monitoring CH2 streching bands (2854 cm-', 2926 cml) of polyethylene [13]. The formation of these reaction products was not detected in the dark conditions nor in irradiation of the HMS itself without Cr-oxide. These results clearly indicate that the presence of both Cr-oxide moieties included within HMS as well as UV light irradiation are indispensable for the photocatalytic reaction to take place and the Cr-HMS can act as an efficient photocatalyst for the ethylene polymerization under UV irradiation. As shown in Fig. 6, the photocatalytic reactivity of the Cr-HMS was found to be much higher than those of the CrS-1 microporous zeolite and imp-Cr/HMS prepared by the impregnation method. These results indicate that the charge transfer excited state of a tetrahedral Cr-oxide moieties and large pore size plays a significant role in the photocatalytic reactions.

2925 cm 1

0

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0

E E ~

0.8

b 0.4

3000

2900

2800

Wavenumber / cm "1

0 0

60

120

180

240

300

Reaction time / min

Fig. 7. IR spectra of Cr-HMS (Si/Cr=50) in the presence of ethylene, a) before light irradiation, b) after UV light irradiation (

Fig. 8. The reaction time profiles of photocatalytic polymerization of ethylene on the Cr-HMS (Si/Cr=50) under a) UV light

~>270 nm)for 1 h.

irradiation ( ~,>270 nm) and b) visible light irradiation ( L>450 nm).

Figure 8 shows the reaction time profile of photocatalytic polymerization of ethylene on the Cr-HMS under visible light irradiation (L>450 nm). The Cr-HMS also shows photocatalytic reactivity even under visible light irradiation, although the reaction rate under the visible light irradiation is smaller than under UV light irradiation (L> 270 nm). These results indicate that Cr-HMS can absorb visible light and act as an efficient photocatalyst for the photocatalytic polymerization of ethylene under not only UV light but also visible light irradiation.

501 4. CONCLUSIONS It has been found that Cr-HMS molecular sieves contain tetrahedrally coordinated Croxide moieties in the framework having two terminal Cr=O and that the charge transfer excited state of the Cr-oxide moieties are responsible for the efficient photoluminescence and photocatalytic reactivities. The present results have clearly demonstrated that the Cr-HMS with mesoporous structure and tetrahedrally coordinated Cr-Oxide moieties can exhibit the efficient reactivity for the photocatalytic polymerization of ethylene under UV light irradiation. The Cr-HMS can also absorb visible light and act as a photocatalyst even under visible light irradiation. This photocatalytic system with tetrahedrally coordinated Cr-oxide moieties dispersed on mesoporous silica seems to be a good candidate to use abundant visible or solar light energy for the useful chemical synthesis.

ACKNOWLEDGMENT

This work has been supported by the Grant-in-Aid Scientific Research from the Ministry of Education, Science, Culture, and Sports of Japan (Grants 12042271 and 13650845). The XAFS measurements were performed at the KEK-PF in the approval of the Photon Factory Program Advisory Committee (Proposal No. 2001G115) with helpful advice from Prof. M. Nomura.

REFERENCES

1. B. Notari, Ad. Catal., 41 (1996) 253. 2. A. Corma, Chem. Rev., 97 (1997) 2373. 3. M. Anpo and M. Che, Ad. Catal., 44 (1999) 119. 4. M. Anpo and H. Yamashita, in "Surface Photochemistry", (ed) M. Anpo, J. Wiley & Sons, Inc., Chichester, 1996, pp. 117-164. 5. H. Yamashita, J. L. Zhang, M. Matsuoka, and M. Anpo, in "Photofunctional Zeolites: Synthesis, Characterization, Photocatalytic Reactions, Light Harvesting", (ed) M. Anpo, NOVA Science Publishers, New York, 2000, pp. 129-168. 6. S. Higashimoto, R. Tsumura, S. G. Zhang, M. Matsuoka, H. Yamashita, C. Louis, M. Che, and M. Anpo, Chem. Lett., (2000)408. 7. H. Yamashita, M. Ariyuki, S. Higashimoto, S. G. Zhang, J. S. Chang, S. E. Park, J. M. Lee, Y. Matsumura, and M. Anpo, J. Synchrotron Rad., 6 (1999) 453. 8. H. Yamashita, M. Ariyuki, S. Higashimoto, Y.Ichihashi, Y. Matsumura, M. Anpo, J. S. Chang, S. E. Park, and J. M. Lee, in Proc. 12th Intern. Zeolite Conf., (Baltimore, USA), 1998, pp. 667-672. 9. H. Yamashita, K. Yoshizawa, M. Ariyuki, S. Higashimoto, and M. Anpo, Stud. Surf Sci. Catal., 135 (2001) A28P07. 10. B.M. Weckhuysen, I. E. Wachs, and R. Schoonheydt, Chem. Rev., 96 (1996) 3327. 11. Z. Tvaruzkova, B. Wichterlova, J. Chem. Soc., Faraday Trans. 1, 79 (1983) 1591. 12. R.R. Rao, B. M. Weckhuysen, R. A. Schoonheydt, Chem. Commun., (1999) 445.

502 13. B.M. Weckhuysen, R. R. Rao, J. Pelgrims, R. A. Schoonheydt, P. Bodart, G. Debras, O. Collart, P. V. D. Voort, and E. F. Vansant, Chem. Eur. J, 6 (2000) 2960. 14. W. Zhang, P. T. Tanev, and T. J. Pinnavaia., J. Chem. Sot., Chem. Commun., 979 (1996). 15. S.G. Zhang; M. Ariyuki, S. Higashimoto, H. Yamashita, and M. Anpo, Microporous and Mesoporous Materials., 21 (1998) 621. 16. H.O. Pastore, E. Stein, C. U. Davanzo, E. J. S. Vichi, O. Nakamura, M. Baesso, E. Silva, and H. Vargas, J. Chem. Sot., Chem. Commun., (1990) 772. 17. H. Yamashita, M. Matsuoka, K. Tsuji, Y. Shioya, and M. Anpo, J. Phys. Chem., 100 (1996) 397.14. 18. M.S. Rigutto and H. V. Bekkum, Appl. Catal., 687 (1991) L 1. 19. B.M. Weckhuysen, R. A. Schoonheydt, J. M. Jehng, I. E. Wachs, S. J. Cho, R. Ryoo, and E. Poels, J. Chem. Sot., Faraday Trans., 91 (1995) 3245. 20. B.M. Weckhuysen, R. A. Schoonheydt, D. E. Mabbs, and D. Collison, J. Chem. Soc., Faraday Trans., 92 (1996) 2431. 21. B.M. Weckhuysen, A. A. Verberckmoes, A. L. Buttiens, and R. A. Schoonheydt, J. Phys. Chem., 98 (1994) 579. 22. M. Anpo, I. Yakahashi, and Y. Kubokawa, J. Phys. Chem., 86 (1982) 1. 23. M.F. Hazenkamp and G. Blasse, J. Phys. Chem., 96 (1992) 3442. 24. W. Hill, B. N. Shelimov, I. R. Kibardina, and V. B. Kazanskii, React. Kinet. Catal. Lett., 31 (1986) 315.


503

Catalytic reduction of nitric oxide on Al-containing mesoporous molecular sieves W. Li*, Y. Zhang, Y. Lin, X. Yang Department of Environmental Science & Engineering, Tsinghua University, Beijing 100084, China A series of mesoporous aluminosilicate materials were synthesized at room temperature, and tested for nitric oxide (NO) reduction by propene in the presence of oxygen on their Cu ion-exchanged forms. The experimental results revealed that NO reduction activity was not decreased but slightly increased above 400~ on Cu-A1-MCM-41 in the presence of water vapor as compared to that on Cu-ZSM-5, which indicated Cu-A1-MCM-41 was more water resistant than Cu-ZSM-5. In addition, NO reduction activity was also investigated on the Ni, Co or Mn ions doped Cu-A1-MCM-41 samples in the absence or the presence of water vapor. It was found that NO conversion on Ni doped Cu-A1-MCM-41 was increased below 350 ~ in the absence of water vapor, and NO conversion was also enhanced above 400 ~ upon introducing water vapor into the feed gas. 1. INTRODUCTION Selective catalytic reduction of nitric oxide with hydrocarbons (HC-SCR) in the presence of oxygen is one of the major challenges in the automobile exhaust after-treatment for lean-burn gasoline engines and diesel engines. Since Iwamoto et al [1] reported a high NO reduction activity with hydrocarbons on Cu-ZSM-5, a number of metal ion-exchanged zeolites have been widely investigated for reducing NOx with ethylene, propene, propane, methane and other hydrocarbons [2]. Besides, several metal oxide catalysts such as Sn/ZrO2, Cu/A1203 and A1203 were also reported to be active for HC-SCR reaction. Among all the catalysts investigated, Cu ion-exchanged ZSM-5 shows the best NO conversion. However, most zeolite-base catalysts, namely Cu-ZSM-5, Co-ZSM-5, Ga-ZSM-5, Ce-ZSM-5, Mn-ZSM-5, Fe-ZSM-5, In-ZSM-5, are very sensitive to water vapor and sulfur species and quickly deactivated, possibly because of the irreversible dealuminization of the zeolite structure and the sintering of metal active species [3]. Therefore, some new zeolite materials

*To whom correspondence should be addressed E-mail: [email protected]

504 such as Cu-exchanged IM5 [4] and Cu-exchanged SAPO [5] were also tested and found to be very active for the reaction. As we know, Besides the zeolite acidity, the pore structure was a key factor for the NO reduction activity [6]. Indeed, Tabata et al [7] proposed that the superior activity of the large-pore zeolite Co/beta could be ascribed to the ease of diffusion of reactants, products and inhibitors such as water and SO2 in its channels. By contrast, the lower activity of Co/ferrierite was found for C3H8-SCR reaction, probably because the diffusion in its small pores was hindered [7]. On the other hand, MCM-41 type materials developed by Beck et al [8] have the regular uniform mesoporous structures and high surface areas, may provide a better dispersion of active metal components on their surface and prevent the diffusion limitation for the catalytic reduction of NO. Jentys et al [9] and Shen et al [10] recently reported a high activity for NO reduction by propylene on Pt/MCM-41. In this presentation, aluminum containing MCM-41 mesoporous materials with high hydrothermal stability were synthesized, and their Cu ion-exchanged forms were tested for catalytic reduction of NO with propylene in the presence of excess oxygen. Special attention was paid on water vapor effect on the catalytic performance of those mesoporous materials. 2. EXPERIMENTAL 2.1 Materials

A1-MCM-41 was synthesized at ambient temperatures using cetyltrimethylammonium bromide (CTAB, Beijing chemical reagents company) as a template. A fixed amount of CTAB and NaOH were dissolved in deionized water under stirring and slightly heating at 50~ then a required amount of tetraethyl silicate (TEOS, Huabei special chemical reagents center) was slowly added to the above solution. After stirring for 15 min, an appropriate amount of aluminum sulfate solution was introduced into the solution under strong stirring. The reaction mixture had the following molar composition: TEOS:A12(SO4)3:CTAB:NaOH:H20 = 1:x:05:0.2:10, where x = 0.2, 0.1, 0.05, 0.02. The pH value of the suspension was then repeatedly adjusted to the value of 12 with tetramethylammonium hydroxide (TMAOH, Beijing Daxing Xingfu chemical company) solution. After the mixture gel was placed at room temperature for 24h, the resulting solid was filtered, washed with deionized water, and dried at 60~ The thus obtained samples were calcined in nitrogen at a heating rate of 2 ~ to 550 ~ and shifted to oxygen atmosphere at 550~ for another 2h. The available sample was ion-exchanged twice with an aqueous solution of NH4NO3, then dried and calcined at 550~ to obtain the acid form zeolite, H-MCM-41-y, where y is the SIO2/A1203 ratio of the sample. The above H-MCM-41-y was further ion-exchanged with a copper acetate solution for 4 times, followed by drying at 100~ and calcining at 550~ to obtain Cu-A1-MCM-41-y catalysts. Cu-ZSM-5 was prepared by ion-exchanging H-ZSM-5 (Nankai University Plant, Tianjin) with a copper acetate solution under the similar conditions. Ni, Mn and Co doped Cu-A1-MCM-41-y catalysts were prepared by impregnating the above Cu-A1-MCM-41-y sample with their nitrate salt aqueous solution.

505

2.2. Characterization of the catalysts X-ray diffraction patterns were obtained on Rigaku D/max RB X-ray diffractometer using Cu K a radiation. Nitrogen adsorption and desorption isotherms were determined at 77K by means of Quantachrome AUTOSORB-I surface area analyzer, from which BET surface areas were calculated and the pore size distributions were determined using the procedure proposed by Barrett, Joyner and Halenda (BJH). Elemental analysis was done with X-ray fluorescence analyzer on Shimadzu XRF-1700 spectroscopy. 2.3. Catalytic and adsorption measurements NOx reduction experiments were carried out in a fixed bed reactor. In a typical experiment, 0.10g of a catalyst was introduced into the reactor with a feed gas of 1000ppm NO, 3100ppm C3H6, 3% 02 and helium as balance gas, and the total flowrate of 150ml/min. 4~8% water vapor by volume was once supplied by passing helium gas through water bubbler. NOx gases from the reactor outlet was continuously analyzed by a NO/NOx Chemiluminescence Analyzer (Thermal Electronics Model 42CHL). Other reactants and products were analyzed by gas chromatography with a 5A molecular sieve column and a Porapak Q column with a thermal conductivity detector. NO conversion was calculated based on the difference between the inlet and outlet NO concentration.

A B C D 1;

1'2

20( ~ )

Figure 1. XRD pattern of A1-MCM-41 samples with the various SIO2/A1203 ratios after calcination at 550~ in oxygen. The SIO2/A1203 ratios were (A) 50; (B) 20; (C) 10; (D) 5. 3. RESULTS AND DISCUSSION

3.1 Characterization of the catalysts Figure 1 shows the small-angle XRD pattern (2-10 ~ 2 0 ) of the A1-MCM-41 samples with different SIO2/A1203 ratios after calcination at 550~ It is observed that all the samples show one peak around 2 ~ 2 0 associated with dl00 plane assigned to the typical MCM-41 materials [11 ]. The height of the peak remained high for the samples with the Si02/A1203 ratio being 50 to 10. A further decrease in the SIO2/A1203 ratio would lower the peak intensity, indicating a poor ordered wall structure, which is in good agreement with previous report [12]. The main peak shifts towards higher d-spacing with a decrease in the SIO2/A1203 ratio,

506 indicating an increase in the interplanar distance for the A1-MCM-41 material, which is due to the replacement of shorter Si-O bands (0.160nm) by longer A1-O bands (0.175nm) in the A1-MCM-41 structure. Similar results were reported by Corma et al [13]. Figure 2A and 2B show the NE adsorption/desorption isotherm and the pore size distribution calculated based on BJH method according to adsorption branch of A1-MCM-41-10 and A1-MCM-41-50, respectively. The steps of the isotherms at relative pressure (P/P0) between 0.2-0.35 in both Figure 2A and 2B are associated with the condensation of nitrogen in primary mesopores. Figure 2A shows the pore size distribution of primary mesopores based on BJH calculation method for A1-MCM-41-10, and the BET specific surface area is 1100 mE/g; Likewise, Figure 2B shows the pore size distribution for A1-MCM-41-50, and its BET specific surface area is 1150 m2/g. It is noteworthy that there is a sharp step at high relative pressure in the isotherm in Figure 2A characteristic of H1 type isotherm for slit-shaped secondary mesopores [ 14], which is caused by the condensation of nitrogen within the existent secondary mesopores formed by crystal aggregates [15]. This type of isotherms is attributed to nanostructural materials with uniform mesopores[16]. A similar isotherm was previously reported by Occelli et al [15] and Cesteros et al [17] on A1-MCM-41.

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Figure 2. N2 adsorption-desorption isotherm and pore size distribution from BJH (inset) for the sample A1-MCM-4 l- 10 (A) and A1-MCM-41-50 (B). The low thermal stability of MCM-41 type materials compared with conventional zeolites such as ZSM-5 is a critical problem that will affect the practical application of the MCM-41 type materials. Therefore, several methods have been used to improve the hydrothermal stability of the MCM-41 type materials. Here, the pH value was repeatedly adjusted in the gel solution to improve the hydrothermal stability followed the method proposed by Ryoo et al [16]. Figiare 3 shows the XRD patterns of A1-MCM-41-10 heated at 550~ in oxygen for 2h and successively treated in boiling water for 12h. The XRD pattern for the sample heated in boiling water shows only a slight decrease in its peak intensity compared to the sample calcined at 550~ in oxygen. The decrease in XRD intensity is due to the disintegration of MCM-41 structure in hot water and silicate hydrolysis. The better hydrothermal stability

507 obtained on the A1-MCM-41-10 sample is using the repeated pH adjustment, which is structure order and textural uniformity as show that no significant changes occurred acetate solutions.

.~

believed to be related by the synthesis procedure consistent with the improvement in the long range reported by Ryoo et al[16]. Further experiments for the sample being ion-exchanged with copper

~ 20(

~ ' l~, ' ,'2 ~ )

Figure 3 XRD pattern of the A1-MCM-41-10 at different treatment: (A) As-synthesized; (B) Calcined at 550~ in oxygen; (C) Calcined at 550~ in oxygen followed by heating in boiling water for 12h. Table 1 shows the results of chemical compositions determined by X-ray fluorescence analyzer. It was found that the molar ratio of SIO2/A1203 in the final solid samples except for that on A1-MCM-41-10 was only slightly higher than that in the gel precursors, because some A1 species might be lost during the synthesis procedures. Table 1. Chemical compositions of Cu 2+ ion-exchanged A1-MCM-41 and Cu 2+ ion-exchanged ZSM-5. SIO2/A1203 Cu loading 7 Sample (wt %) .......................Gel ...........a........................................................................................... S o ' i i d 'b ................... A1-MCM-41-5 5 7.4 Cu-A1-MCM-41-10 10 9.9 5.5 MCM-41-20 20 25.6 Cu-A1-MCM-41-50 50 55.7 5.7 Cu-ZSM-5-50 50 54.5 4.8 ............... ~ . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . a Calculated value of the molar ratio of SiOJAI203 in the synthesis gel. bAnalyzed value for Cu and SIO2/A1203in the final solid samples calcined at 550 ~

3.2. Catalytic experiments NO conversion into nitrogen over Cu-A1-MCM-41-10 and Cu-A1-MCM-41-50 as function of temperature in the absence or presence of water vapor was illustrated in Figure 4. It is observed that adding water vapor into the feed gas led to an increase in the activity on

508 Cu-A1-MCM-41-10 at lower temperatures; while at higher temperatures (i.e., above 400~ the NO conversions remain almost unchanged. For the sample with a higher SIO2/A1203 ratio as 50, the temperature for the maximum NOx conversion was shifted to lower temperature range, and an increase in the NO reduction activity was found at 350 ~ on Cu-A1-MCM-41-50 in the presence of water vapor as compared to that in the absence of water vapor, i.e., 48.7% viz. 37.6%. The different behaviors between the two samples are possibly due to the depression of total oxidation of propylene by water vapor at the temperature investigated, and hence provide different type or amount of organic intermediates for NOx reduction. The detailed reason is under investigation and possibly related with the different pore structures and the acid sites or acid strength on the sample. Jentys et al [9] recently reported that the similar enhancement of the water vapor on the activity of the NOx reduction by propene on Pt/MCM-41 impregnated with tungstophosphoric acid, but a decrease in the activity was still observed on Pt/MCM-41 without dopants in the presence of 2.5 vol. % water vapor. 60 50 .~ 40 o

+

A,W/O H20

+

A,With H20

---/k-- B,W/O H20 ~-

30

B,With H20

~ 2o z 10 0

~ 50

150

250

350

450

550

Temperature/oc

Figure 4. NOx conversion versus temperature on Cu-A1-MCM-41-10 (A) and Cu-A1-MCM-41-50(B) in the absence of water vapor (open symbols) and in the presence of water vapor (solid symbols). Since Cu-ZSM-5 have been widely reported as a highly active catalyst for NO reduction by propene [2], Cu ion exchanged ZSM-5 was also prepared and tested under the same conditions with that for Cu-A1-MCM-41-50. The results in Figure 5 shows the dependence of NOx conversion on water vapor over Cu-ZSM-5. Although the NOx conversion was higher on ZSM-5 without H20, NOx conversion was greatly affected by the presence of water vapor. For example, at 400~ NOx conversion was decreased from 85% to 71% upon introducing 4% water vapor into the feed gas; A further increase in water vapor content to 8%, led to more serious deactivation and a lower NO conversion of 54%, as expected. By comparing Figure 5 with Figure 4, it is clear that Cu-A1-MCM-41 was more water resistant than Cu-ZSM-5, particularly at the temperature above 400~ The further investigation for understanding the enhancement effect is in progress in our laboratory.

509 100 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

+ A ~

80

B

i .--a-c

~

60 J'

= o ~

I 40 i

Z

20 ~ 0 '

.

100

. 200

.

.

. 300

.

.

. 400

.

.

. 500

600

Temperature/~C

Figure 5. Water effect on NOx conversion over Cu-ZSM-5-50" (A) without water; (B) with 4% water; (C) with 8% water. Figure 6 shows NO conversion as a function of temperature on the Co, Mn and Ni modified Cu-A1-MCM-41-10 catalysts. By the comparison with the data in Figure 4, it revealed that doping of Co and Mn decreased NO conversion greatly, and meanwhile more NO2 gases were detected. However, the temperature corresponding to maximum NO conversion was shifted from 425 ~ for 45.4% to 350 ~ for 48.2% after doping Cu-A1-MCM-41 catalyst with Ni ions. It is clear that the maximum NO conversion was slightly increased, and NO conversion below 350 ~ was increased remarkably in the absence of water vapor. For example, NO conversion at 350 ~ was increased from 20% to 48.2% after doping Ni ions on Cu-A1-MCM-41-10. A slight decrease was also found above 400 ~ in the absence of water vapor. However, upon introducing water vapor into the feed gas NO conversion on Ni doped Cu-A1-MCM-41 was found to be enhanced above 400 ~ despite a decrease in NO conversion was observed below 400 ~

60! -~A 5o! 4O

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

A B +C

"~ g 30

z~ 20 10

100

200

300

400

500

600

Temperature/o c

Figure 6. NO conversion as a function of temperature without water vapor on Co (A), Mn (B), Ni (C) doped Cu-A1-MCM-41-10 catalysts, and on the Ni doped sample in the presence of 4% water vapor (D).

510 4. CONCLUSION A series of mesoporous aluminosilicate materials were synthesized at ambient temperatures, and their mesoporous structures were intact after calcination at 550~ in oxygen or heating in boiling water as detected by XRD, N2 adsorption-desorption analysis. NO reduction by propene was conducted on these Cu ion-exchanged mesoporous materials. It is found that in presence of water vapor NO reduction activity was not decreased but slightly increased above 400~ on Cu-AI-MCM-41 as compared to that on Cu ion-exchanged ZSM-5, which implied Cu-A1-MCM-41 was more water-resistant than Cu-ZSM-5. Furthermore, addition of Ni ions to those Cu-A1-MCM-41 samples led an increase in NO conversion below 350 ~ and a slight decrease above 400 ~ in the absence of water vapor. It is worth noting that NO conversion on Ni doped Cu-A1-MCM-41 was also enhanced above 400 ~ in the presence of water vapor. The results suggest that the Ni and Cu modified A1-MCM-41 type materials are potential catalysts for NO reduction by propene in the presence of water vapor with further attempts on improving the NO reduction activity. 5. ACKNOWLEDGMENTS National Natural Science Foundation of China (NO. 29907003) are gratefully acknowledged. REFERENCES

1. M. Iwamoto, H. Yahiro, Y. Mine, S. Kagawa, Chem. Lett., (1989) 213. 2. Y. Traa, B. Barger, J. Weitkamp, Micropor. Mesopor. Mater., 30 (1999) 3. 3. J.Y. Yang, Ct P. Lei, W. M. H. Sachtler, H. H. Kung, J. Catal., 161 (1996) 43. 4. A.E. Palomares, F. Marquez, S. Valencia, A. Corma, J. Mol. Catal. A, 162 (2000) 175. 5. T. Ishihara, M. Kagawa, Y. Mizuhara, Y. Takita, Chem. Lett, (1991) 1063. 6. C.Yokoyama, M.Misono, Catal. Today, 22 (1994) 59. 7. T. Tabata, H. Ohtsaka, L. M. F. Sabatino, (2 Bellussi, Micropor. Mesopor. Mater., 21 (1998)517. 8. J.S. Beck, J. C. Vartuli, W. J. Roth etal, J. Am. Chem. Soc., 144 (1992) 10834. 9. A. Jentys, W. SchieBer, H. Vinek, Catal. Today, 59 (2000) 313. 10. S.-C. Shen, S. Kawi, Catal. Today, 68 (2001) 245. 11. P.T. Tanev, M. Chibme, T. J. Pinnavaia, Nature 368 (1994) 321. 12. R. B. Borade, A. Cleatfield, Catal. Lett., 31 (1995) 267. 13. A. Corma, V. Fornes, M. J. Navarro and J. P. Pariente, J. Catal. 148 (1994) 569. 14. S. J. Gregg, and K. S. W. Sing, Adsorption, Surface Area and Porosity, Academic Press, London, 1982. 15. M. L. Occelli, S. Biz, A. Auroux, G. J. Ray, Mesopor. Mesopor. Mater., 26 (1998) 193. 16. M. Kruk, M. Jaroniec, and A. Sayari, Langrnuir, 13 (1997) 6267. 17. Y. Cesteros, Ct L. Hailer, Mesopor. Mesopor. Mater., 43 (2001) 171. 18. R. Ryoo, S. Jun, J. Phys. Chem. B, 101 (1997) 317.


511

Catalytic oxidation o f alpha-eicosanol to alpha-eicosanoic acid over Ti, Zr and M n doped M C M - 4 8 molecular sieves Changping Wei a*, Yining Huang b, Qiang Cai c, Wenqin pangC, Yingli Bi d and Kaiji Zhen d aDepartment of Chemistry Engineering, Jilin Institute of Technology, Changchun 130012, P.R.China bDepartment of Chemistry, The University of Western Ontario, London N6A 5B7 Canada CKey Laboratory of Inorganic Synthesis and Preparative Chemistry, Jilin University, Changchun 130023, P.R.China dDepartment of Chemistry, Jilin University, Changchun 130023, P.R.China

A series of MCM-48 mesoporous molecular sieves doped with Ti, Zr and Mn were synthesized by hydrothermal crystallization and characterized by XRD, UV, EDX and N2 adsorption. These samples were used as catalysts to perform the catalytic oxidation of a-long chain eicosanol to the corresponding a-eicosanoic acid. The experimental results show that MCM-48 molecular sieves doped with Ti, Zr and Mn can be used as a catalyst for the title reaction and have highter catalytic activity than pure MCM-48 for the conversion. 1. INTRODUCTION Since 1992, a new family of mesoporous molecular sieves has been discovered[I-2]. Because of its poor thermal stability, the MCM-50 attracts less research attention. But the MCM-41 and the MCM-48 structures are excellent candidates for catalysis and separation processes. Ti- and V-substituted MCM-41 and Ti-substituted hexagonal mesoporous silica such as Ti-HMS have been synthesized[3-7]. These mesoporous molecular sieves can be used as catalysts for oxidation of bulky molecules which could not enter the micropores of zeolites such as TS-1, TS-2 and Ti-beta. As a catalyst, MCM-48 characterized by a three-dimensional channel system has several advantages over MCM-41 which has a one-dimensional channel system. For instance, the three-dimensional pore system should be more resistant to blockage by extraneous materials than the one-dimensional pore system. Thus, MCM-48 mesoporous molecular sieves may find industrial and biochemical applications in catalysis, separation, and encapsulation[8-11 ]. In this work, we synthesized MCM-48 mesoporous molecular sieves doped with Ti, Zr and Mn by hydrothermal crystallization using surfactants, TEOS and several different transition metal salts as starting materials. The products were characterized by XRD, UV, EDX, and N2 adsorption. The catalytic performance of M (M = Ti, Zr and Mn)-MCM-48 for the oxidation of a-eicosanol to a-eicosanoic acid has been tested. * Corresponding author, E-mail: [email protected]; Fax: 86-0431-5952413.

512 2. EXPERIMENTAL

2.1 Synthesis of M-MCM-48 molecular sieves The M-MCM-48 (M = Ti, Zr and Mn) molecular sieves were synthesized[12] hydrothermally with TEOS (Tetra-Ethyl-Ortho-Silicate), transition metal salts, CTAB (Octadecyl-Trimethyl-Ammonium-Bromide), NaOH and distilled water. The procedure was following: NaOH was dissolved in distilled water, then transition metal salts and the CTAB were added. When the solution became homogeneous, TEOS was added and the resulting solution was transferred to an autoclave and heated at 373 K for three days. The products were washed with distilled water, dried at ambient temperature and calcined at 773 K for 4h. 2.2 Characterization of M-MCM-48 molecular sieves The X-ray diffraction patterns of the M-MCM-48 were recorded on a SCINAG XDS2000 Diffractometer with Cu-K~ radiation. The UV diffuse reflectance spectra were recorded on a UV-3100 (HITACHI company) spectrometer. EDX analysis were carried out on a HIACHI-8100 transmission electron microscope operated at 200 KV. Nitrogen adsorption and desorption isotherms at 77K were measured using a Micromeritics ASAP 2400 Instrument. The data were analyzed by the BJH (Barrett-JoynerHalenda) method using the Halsey equation for multilayer thickness. The pore-size distribution was obtained from the analysis of adsorption branch of the isotherm. 2.3 Test of the catalytic properties The catalytic reactions of eicosanol were carried out in a 4-neck flask equipped with a magnetic stirring bar, a thermometer, an oxygen inlet and a condenser. Reactions were carried out at 413 K for 5h. 0.1-0.2 g catalysts (100 mesh) were used. The a-eicosanol was purified before used. Conversion of ct-eicosanol and the yield of a-eicosanoic acid was calculated according to a stearic acidity. The stearic acidity was determined as following: 1.0 g products were dissoved in 70 ml hot ethanol. To the solution 6 drops of phenol phthalein were added, followed by titration with 0.2 M KOH. Then the excessive 0.2 M KOH was added, followed by titration with 0.2 M HC1. The stearie acidity was calculated based upon the titer.

3 RESULTS AND DISCUSSION

3.1 Characterization of M-MCM-48 molecular sieves The X-ray diffraction pattems of M-MCM-48 (M = Ti, Zr and Mn) (Figure 1) are in agreement with those of typical MCM-48 materials[13]. All as-syntheiszed samples exhibited a very strong low angle peak at around 2.30 ~ two weak peaks at 2.70 ~ and 4.40 ~ corresponding to diffraction planes of (211), (220) and (332), respectively. The XRD patterns of calcined M-MCM-48 looked similar to those of as-synthesized samples except that the refletion peaks shifted to the higher 20 angle slightly. The existence of metal atoms in MCM-48 framework was confirmed by UV and EDX analysis. For example, the UV spectra of the Si-MCM-48 and the Ti-MCM-48 are shown in

513 Figure 2. The band at 210 nm was assigned to isolated framework titanium in tetrahedral coordination, and the band at 230 nm was assigned to framework titamium in octahedral coordination[4]. A band at ca. 270 nm was attributed to extraframe titanium[ 14]. EDX spectrum of Ti-MCM-48 is shown in Figure 3. Both UV and EDX results indicated that the titamium atoms exist in the MCM-48 framework.

.

Before calcination

l

After calcination

g~ t-,r

!

1

3

5

7

9

2

t

t

4

~

.I

6

1

l

t

8

_

10

2O

Figure 1 XRD patterns of 2% M-MCM-48 3.2 Influence of reaction conditions on catalytic oxidation activity

We first carded out the gas phase (non-catalytic) oxidation and the results indicated that the product selectivity is low. The reaction is also uncontrollable. Over other catalysts such as simple metal oxides, the conversion of higher a-carboxylic alcohol to the corresponding carboxylic acid is also very low (20%). However, the selectivity of aeicosanoic acid was greatly enhanced when M-MCM-48 were used as catalysts. The effect of temperature on catalytic activity over Ti-MCM-48 was studied and results were given in Table 1. The optimum reaction temperature was 413K. The same conclusion can be drawn for Zr-MCM-48 and Mn-MCM-48. The product of the oxidation of a-eicosanol at 413K was extracted from the reaction system for composition analysis. The results show that after 5h, the highest yield of aeicosanoic acid was obtained (Table 2). Further increasing reaction time did not result in a higher yield. This probaly is due to decarbonation of the acid caused by heating for a longer time.

514 3.3 Influence of M content on catalytic oxidation activity The effect of M content on catalytic activity was also examined. Table 3 gives results of the catalytic oxidation of eicosanol over Ti-MCM-48. The yield of the desired product, aeicosanoic acid, increases gradually with increasing Ti content and reaches a maxium at a loading level of 1% Ti-MCM-48. Further increase in the Ti content results in a decrease in the yield. Table 4 shows the effect of M content on catalytic activety over Zr-MCM-48 and MnMCM-48. The yield and the selectivity of a-eicosanic acid both increase with increasing M content.

0.3 0 0.4

o

*m'

0.2

-t3- MCM-48 -o- TI-MCM-48

0.1 0

~

200

250

300

350

450

400

500

Wavelengths/nm

Figure 2 UV spectra of Ti-MCM-48

-siv,, ,,

Ti Ka

" L

0

'

I

I

I

1

2

3

4

"

I,

I

'

'

'

5

6

7

8

9

10

E leV

Figure 3 EDX spectrum of Ti-MCM-48 Table 1. Effect of reaction temperature on Y, CH3(CH2)IsCOOH, over 1% Ti-MCM-48 T (K) 393 403 413 423 433 Y*(%) 18.4 40.2 54.4 38.8 21.4 *" Yield of a-eicosanic acid. Reaction time: 5h.

515 Table 2. Effect of reaction time on Y ( C H 3 ( C H 2 ) I s C O O H ) o v e r 1% Ti-MCM-48 t (h) 3.0 5.0 7.5 10.0 Y* (%) 27.0 54.4 54.0 46.4 *: Yield of a-eicosanic acid. Reaction temperature: 413K. Table 3. Effect of Ti content on yield of a-eicosanic acid over xTi-MCM-48 xTi (%) 0 0.1 0.5 1.0 2.0 5.0 Y* (%) 14.9 47.8 51.6 54.4 52.2 46.8 *" Yield of tx-eicosanic acid. Reaction temperature: 413K. Reaction time: 5h. Table 4. Effect of M content on catalytic oxidation activity over xM-MCM-48 xM (%) Selectivity of tx-eicosanic acid (%) Yield of tx-eicosanic acid (%) 2.0Zr 51.3 48.4 5.0Zr 64.4 60.4 8.0Zr 87.7 84.3 1.0Mn 35.9 33.7 2.0Mn 39.3 37.6 5.0Mn 52.3 48.9 Reaction temperature: 413K. Reaction time: 5h.

q13

8

3200.0

1975.0

1250.0

762.5

400.0

Wavenumbers/cm 4

Figure 4 IR spectra of products obtained over 2% M-MCM-48 catalysts In the IR spectra of reaction products, a band at 719 c m "1 c a n be assigned to the C - O H bending vibration of cx-eicosanol. The peak at 1720 cm l can be attributed to the carboxylic

516 group in a-eicosanoic acid. The intensity ratio, 11720/ 1719 indicates qualitatively yield of the reaction. Fig.4 clearly shows that for a given M content catalytic reactivity is: Zr-MCM-48 > Ti-MCM-48 > Mn-MCM-48 > Si-MCM-48, which is consistent with the results of chemical and GC-MS analysis. In Summary, the M (M = Ti, Zr and Mn)-MCM-48 can be used as a catalyst for the selectively catalytic oxidation of a-eicosanol to a-eicosanoic acid. Further studies on the catalytic properties of M (M = Ti, Zr and Mn) are in progress.

4 CONCLUSION MCM-48 molecular sieves doped with Ti, Zr and Mn can be synthesized by hydrothermal crystallization. M-MCM-48 molecular sieves can be used as a catalyst for the oxidation of a-eicosanol to its corresponding acid. The M contents have important effect on the catalytic activity. M-MCM-48 exhibits higher Catalytic activity than pure MCM-48 for the conversion. ACKNOWLEDGEMENTS This work was financially supported by the Committee of Science and Technology of Jilin Province, China, National Nature Science Foundation of China, and China Scholarship Council. REFERENCES 1. C.T. Kresge, M.E. Leonowicz, W.J. Roth et al., Nature 359 (1992) 710. 2. J.S. Beck, J.C. Vartuli, W.J. Roth et al., J.Chem. Soc., Chem. Commun. (1994) 147. 3. K.M. Reddy, I. Mondrakovski et al., J. Chem. Soc., Chem. Commun. (1994) 1059. 4. A. Corma, M.T. Navarro and J.P. Pariente, J. Chem. Soc., Chem. Commun. (1994)147. 5. P.T. Tanev, M. Chibwe and T.J. Pinnavaia, Nature, 368 (1994) 321. 6. T. Blasco, A. Corma, M.T. Navarro and J.P. Pariente, J.Catal. 156 (1995) 65. 7. N. Vlagappan, C.N.R.Rao, J.Chem. Soc., Chem. Commun. (1996) 1064. 8. V. Alfredsson, M.W. Anderson, Chem. Mater. 8 (1996) 1141. 9. S. Anderson, S.T. Hyde, K. Larsson, Chem Rev. 88 (1988) 221. 10. M. Morey, A. Da~cidson, H. Eckert, Chem. Mater. 8 (1996) 486. 11. M.J. Hudson, J. Knowles, Chem. Mater. 6(1) (1996) 89. 12 W. Changping, C. Qiang et al., Chem. J. Chinese Universities, 19(7) (1998)1154 13. S. Kawi, M. te, Catalysis Today 44 (1998) 101-109. 14. K. A. Koyano, T. Tatsumi, Chem. Commun. (1996) 145.


517

Preparation o f Pd/A1-MCM-41 Catalyst and Its H y d r o i s o m e r i z a t i o n Properties for long chain alkane c o m p o u n d s Shui Lin a,b, Han Ning a'b, Sun Wan-Fu c*, Liu Wei-Min a and Xue Qun-Ji a a Lanzhou Institute of Chemical Physics, The Chinese Academy of Sciences, Lanzhou 730000, P R China b Great Wall Lubricating Oil Group Company, SINOPEC, Beijing 100085, P R China c Fushun Research Institute of Petroleum and Petrochemicals, Fushun 113001, P R China

The mesoporous material A1-MCM-41 with various Si/A1 ratios was synthesized rapidly by microwave under acidic conditions using cetylpyridinium bromide (CPBr) as template and tetraethylorthosilcate (TEOS) as silica source. The hydrogen forms of A1-MCM-41 were obtained via ion exchange. Pd/A1-MCM-41 catalysts were prepared by impregnation method. The catalytic activity and selectivity of as-synthesis catalysts for tridecane hydroisomerization were also evaluated.

1. INTRDUCTION The hydroisomerization of long-chain paraffins for improving the properties of gasoline (high octane number), diesel (low pour and cloud points) and lubricant base stocks (low pour points and high viscosity) is very importance, especially, for producing high quality lubricant base oil. This process usually use novel metal (Pt or Pd) supported on zeolite (Beta, SAPO-5 or Y) as catalyst[I,2], this kinds of catalyst are good for hydroisomerization of small molecule alkanes (~< C6), for hydroisomerization of long-chain alkanes, however, there exist undesirable cracking due to their relative strong acid sites. In order to suppress the cracking reaction and keep high hydroisomerization selectivity, the alternative choice is to find a new support with suitable porosity and acidity. Since the discovery of the new class ofmesoporous molecular sieves in 1992, there has been a growing interest in their potential catalytic applications. Because of their relatively mild acid sites and the possibility to vary the Si/A1 ratio in a wide range without significant changes in pore structure, these materials are very attractive model catalysts for transformation of bulky compounds, especially, for the hydroisomerization of long-chain alkanes. De Rossi et al.[3] studied the hydroisomerization of normal paraffins over a series *Corresponding author: Fushun Research Institute of Petroleum and Petrochemicals, Zip: 113001, Liaoning, P R China. Fax" 86-431-6429551" E-mail" Sunwanfu@fripp,conl,cn

518 of catalysts and found that the selectivity for isomers is higher and cracked products is lower on Pt/MCM-41 as compared to other materials. In this communication, The mesoporous material A1-MCM-41 with various Si/A1 ratios were synthesized rapidly by microwave under acidic conditions using cetylpyridinium bromide (CPBr) as template and tetraethylorthosilcate (TEOS) as silica source. The catalytic activities of Pd/A1-MCM-41 catalysts for tridecane hydroisomerization were evaluated at 3.5MPa in a fixed bed reactor packed with 20ml of catalyst. The reaction products were analyzed by gas chromatography. Comparing with microporous zeolites such as USY, Pd/A1-MCM-41 has higher hydroisomerization selectivity.

2. EXPERIMENTAL

2.1. Synthesis AI-MCM-41 Mesoporous aluminum-containing MCM-41 with different Si/A1 ratio was snthesized by microwave under acidic conditions. The preparation procedure is as follow: Preparing mixture solution A: Distilled H20, diluted H2SO4 and surfactant (cetyltrimethylammonium chloride, CTAC) were mixed together with stirring, then adding sodium aluminate at temperature about 50~ with intensive stirring until homogeneous. Mixture Solution B: Sodium waterglas(40 wt% SiO2) was dissolved by ethanol with stirring. Adding mixture B into Mixture A under an appropriate rate at different temperature with stirring. The homogeneous reaction gel was sealed in a cylindrical PTFE container and heated by a 700W microwave oven for 20 minutes. The solid product was recovered by filtration, washed with deionized water, dry at 120~ and calcinated at 560~ for 6h. The hydrogen forms of A1-MCM-41 (HA1-MCM-41) were prepared by ion exchange A1-MCM-41 with 1 M aqueous solution of NH4NO3 at 70~ followed by the deammonation at 400~ in the atmosphere. 2.2. Preparation of Pd/HAI-MCM-41 The synthesized HA1-MCM-41 samples were extruded into a strip form with a binder. Pd/HA1-MCM-41 catalysts were prepared by the impregnation with Pd (NO3)2 and dried at 120~ for 4 hours, then calcinated at 500~ for 3 hours, respectively. 2.3. Characterization of the samples Microwave oven (model CEM-2000) power is 700W with temperature programmed. X-ray diffraction (XRD) was carried out with a Ragaku D/max 2500 using Cuko radiation. Texture parameters were investigated by nitrogen sorption measurements with a Micromeritics ASAP 2400 automatic N2 adsorption instrument. The external and intemal surface area and volume were determined using a comparison plot. The pore size distribution of the synthesis samples was calculated by a geometrical method. The samples were degassed at 300 ~ for 8 hours. Differential thermal analyses (DTA) were performed on a Du Pond thermal analyzer from ambient temperature to 1000~ with 10 mg of the sample, a heating rate of 10~ and an air flow. The acid properties of HA1-MCM-41 and Pd/HA1-MCM-41 were measured using Nicolet 560 FT-IR with pyridine adsorption/desorption. Elemental compositions of the samples (the contents of A1203 and SiO2 of bulk analysis) were determined by chemical analysis. The catalytic activities for

519 tridecane hydroisomerization were evaluated at 3.5MPa in a fixed bed reactor packed with 30ml of catalyst. The catalyst first were reduced with a Hz gas at 300~ for 5 hr at 3.5MPa, then the tridecane was introduced into reactor at the rate of 45mL/h, WHSV=I.5, H2/oi1=600:1, temperature is 300~176 reaction products were analyzed by gas chromatography.

3. RESULTS AND DISCUSSION 3.1. Characterization of the support The texture and structure of the synthesized mesoporous materials were examined using XRD, BET and DTA. The XRD patterns are shown in figure 1. From figure 1 we can see, the X-ray diffractogram of A1-MCM-41 structure present typical pattern with a strong peak at low angle assigned to dlo0 reflection(figure 1a) and the structure well-preserved after calcination in air at 700~ for 5 h (figure l b). The intensity of dl00 peak of A1-MCM-41 decreased with the amount of A1 incorporated into framework of MCM-41 increased. The DTA pattern shows three distinct peaks at 1 2 3 4 5 temperatures of 100~ 280 and 900~ The peak at 2Theta [deg. ] 100~ is attributed to water evaporation in the sample. The decomposition of the template results Fig1. XRD patterns of as synthesized in middle-peak in the DTA profile, the high-temperature peak at 900~ is attributed to zeolite under 500 ~ (a) and 700 ~ (b) framework collapse of the synthesis sample. The properties of different Si/A1 ratio of A1MCM-41 are listed in table 1. ---

.

Table 1. The textural and structureal of the different Si/A1 ratio A1-MCM-41 . Sample . . 1 2 3 Si/A1 ratio 100 75 50 Surface area (mZ/g) 671.2 650.4 639.5 Pore volume (ml/g) 0.695 0.684 0.675 Mean pore size (nm) 4.2 4..2 4.0 .

.

.

.

.

,

,.

!

_.,

|

m

,,

4 25 617.7 0.631 3.7

As the amount of A1 incorporated increased, the surface area, pore volume of A1MCM-41 decreased gradually This is probably due to calcinated partial collapse of the hexagonal structure caused by the instability associated with the presence of increasing amounts of framework A1. The mean pore sizes were evaluated using N2 adsorption isotherms by BJH method. The adsorption/desorption isotherm plots and resulting hysteresis loop are shown in the

520 Figure 2. It can be seen that there is a sharp step at intermediate relative pressures typical of IUPAC type IV isotherm. The step restricted to narrow range of p/p0, implies the existence of a narrow range of pores in the vicinity of 4.0nm. The "d" value and adsorption/desorption isotherm plots are in agreement with the literature [4,5] Purely siliceous MCM-41 has no Broensted acidity, but when some trivalent cations such as A1, Fe incorporate into framework of MCM-41 [6,7], it will creates moderately acidic sites. IsothermPlot 600550500~450-

+ Adsorption o Desorpti0n

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Isotherm Plot + Adsorption o Desorption

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Relative Pressure (P/Po)

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Figure 2. Adsorption/desorption isotherm plots of different Si/A1 ratio of A1-MCM-41 (a): Si/AI=100, (b): Si/Al=75, (c): Si/AI=50, (d): Si/A1-25. These A1 incorporation amount and methods influence the nature of A1-MCM-41 such as concentration of acid sites, pore structure and surface area. The acidity and pore

521 structure of the catalysts has a major effect on hydrocracking and hydroisomerization. Lubricating oils of good quality should have lower pour points and higher viscosity indexes. Effective producing lubricating oil catalysts should have higher activity to transform n-alkanes to isoalkanes, because the isoalkanes have lower pour points. The catalysts with high hydrogenation ability and moderate acidity are desirable for hydroisomerization of long chain hydrocarbons [8]. Figure 3 gives the acid property of different Si/A1 ratio HA1-MCM-41 From Figure 3 we can know, with the content of A1 increasing, 0.25 the total amount of acidity the zeolites increase strikingly, / ~ , , , , ~ , ~ CB+L especially for Lewis acid. When 0.2 " Si/A1 ratio of the zeolite ,.........4~.-- . ~ ' ' ' ' ' ~ CL decreases to 25, the Lewis o.15 amount of acidity increases E almost 50% comparing with that E of Si/A1 ratio of the zeolite is 0.1 100. 0.05

3.2. Pd/HAI-MCM-41

HA1-MCM-41 was prepared by ion exchange A1-MCM-41 with 1M aqueous solution of NH4NO3, followed by the deammonation at 400 ~ in the atmosphere. The temperature of aqueous solution of NH4NO3 influences the exchange degree of the A1-MCM-41. Therefore, the suitable exchange temperature should be chose by the experiment. The effect of different temperature on the exchange degree of the zeolite has been investigated as shown in Figure 4. From Fig 4 we can see, as the ion exchange temperature increase, the more Na were released from the zeolite, When the steaming temperature is at 70 ~ the content of NaO of the zeolite is'only 0.12 wt%, continue rise the temperature to 90~ the content of NaO only decrease a little bit, and the framework of

CB

-

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0 100

m m

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I

I

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~-~ 0.02 z 9

,~ 0.01 O

20

I

I

I

40

60

80

Temperature o C

Figure 4. The relationship between content of NaO and ion exchange temperature

100

522 the zeolite collapsed partially due to severe desodium(according to the XRD pattern, not shown), as a result, the amorphous phase increased in the zeolite. Pd/HA1-MCM-41 catalysts with different Si/A1 ratio were prepared by the impregnation with Pd (NO3)2, the properties and composition of the Pd/HA1-MCM-41 catalysts were listed in table 2. From table 2 we can see that as the Si/A1 decrease, the total amount of acidity increase, while the surface area decrease. This is probably due to partial collapse of the hexagonal structure caused by the instability associated with the presence of increasing amounts of framework aluminum. Table2. The properties and comPos!t!on of the Pd/HA!-MCM?41 catalyst s .... Catalysts 1 2 3

4

0.35

0.35

0.35

0.35

Si/A1 ratio

100

75

50

25

Surface area m2/g

668.5

644.3

637.0

613.5

Total acid amount mmol/g

0.135

0.161

0.164

0.198

Content of Pd

wt %

3.3. Catalytic activity The performance of the Pd/HA1-MCM-41 catalysts with different Si/A1 ratio for conversions of tridecane was evaluated in a 30ml fixed bed reactor. The catalysts (1, 2, 3 and 4) hydro-conversion was shown in Figure5. From the Figure we can see: the order of conversion of tridecane of the four catalysts is 4>3>2>1. This order is the same as total acid amount of these four catalysts (see table 2), that is, the more amount of A1 incorporated into framework of the MCM-41 catalyst ~upports, the higher hydro-transformation activity of the catalysts were obtained. The product distribution data of n-tridecane hydro-transformation over Pd/HA1-MCM-41 catalysts with different Si/A1 ratio were summarized in table 3.

Figure 5. Influence of catalyst with different Si/A1 ratio on coversion of tridecane

523 From table 3 we can know, at around 60% conversions, all Pd/HA1-MCM-41 catalysts showed high hydroisomerization selectivity of more than 86 wt.% It seems that hydroisomerization reaction is favorable than hydrocracking, due to the mild acidity of HA1-MCM-41. Table3. Product selectivity for hydroisomerization of n-tridecane over different catalysts Catalyst Conversion Mono-C~3 Di- C13 Tri- C13 Branch C13 % (wt%) (wt%) (wt%) (wt%) 1 59.3 57.0 18.3 11.1 86.4 2 59.8 58.3 19.9 10.5 88.7 3 60.2 58.8 17.5 10.2 86.5 4 59.5 60.2 20.2 11.4 91.8 Among the catalysts, Pd/HA1-MCM-41 catalyst with Si/A1 ratio 25 has the highest hydroisomerization selectivity, while the catalyst withSi/A1 ratio 100 is the lowest one. In other words, the higher the total acid amount of the Pd/HA1-MCM-41 catalyst support was the higher the conversion and isomerization yield were obtained over the Pd/HA1-MCM-41 catalysts. From this point of view, Pd/HA1-MCM-41 catalyst with lower Si/A1 ratio will be good catalyst for producing high quality lubricating base oil.

4. CONCLUSIONS The mesoporous material A1-MCM-41 with various Si/A1 ratios was synthesized successfully under the conditions mentioned in this paper. The hydrogen forms of A1-MCM-41 were obtained via ion exchange. The ion exchange temperature affects the acid property of the supports drastically.The hydroisomerization of n-tridecane was carried out in a fixed bed reactor at 350~ and 3.5MPa over Pd/HA1-MCM-41 catalysts, the experimental data show that the Pd/HA1-MCM-41 catalyst with Si/A1 ratio 25 has higher hydroisomerization selectivity for transformation tridecane. The catalytic activity decreased in the order of Pd/HA1-MCM-41 (Si/Al=25)> Pd/HA1-MCM-41 (Si/AI=50)> Pd/HA1-MCM-41 (Si/Al=75)> Pd/HA1-MCM-41 (Si/AI=100). The more amount of A1 incorporated into framework of the MCM-41 catalyst supports, the higher were the reactivity and isomer yield obtained.

REFERENCES

1. R. A. Meyers, Handbook of Petroleum Refining Processes, McGraw-Hill, New York (1996). 2. C. Bischofand M. Hartmann, Stud. Surf. Sci. Catal., 135(2001). 3. K. J. Del Rossi, G. H. Hatzikos, A. Huss, US Patent 5,256.277 assigned to Mobil Oil Corp. (1993). 4. K. J. Edler and J. W. White, J. Chem. Soc. Chem. Commun. 155(1995). 5. T. Chiranjeevi, Prashant Kumar, M. S. Rana, G. Murali Dhar, and T. S. R. Prasada Rao, Stud. Surf. Sci. Catal., 135(2001).

524 6. R. Mokaya and W. Jones, J. Mater. Chem., 9(1999)555. 7. R. Mokaya, J. Catal., 186(1999)470. 8. F. Alvarez, F. R. Ribiero, G. Perot, C. Thomazeau and M. Guisnet, J. Catal., 162(1996)179.

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A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rightsreserved.

525

Alkylation o f phenol with methyl tert-butyl ether over mesoporous material catalysts Xiang-Hai Tang*, Xin-Liang Fu and Hai-Yan Jiang College of Chemistry, Nankai University, Tianjin 300071, P. R. China

Alkylation of phenol with methyl tert-butyl ether (MTBE) for the synthesis of tert-butylphenol (TBP) and 2,4-di-tert-butylphenol (2,4-DTBP) has been studied over mesoporous material catalysts with MCM-41 structure. Influence of reaction conditions as well as modification with superacid on the catalytic properties of aluminosilicate MCM-41 was evaluated. The results were compared with those of Y zeolite. It revealed that the alkylation was governed by the acidity and the pore structure of the catalyst. The distribution of products was a function of temperature. Increasing temperature promoted the selectivities toward p-TBP and 2,4-DTBP while dealkylation of 2,4-DTBP was observed at high temperature. At low temperature, the lower the space velocity was applied, the higher the phenol conversion and the 2,4-DTBP selectivity were obtained. The ratio of height to diameter of catalyst bed had almost no effect on this reaction in the range of 2-6. A mild acidity can meet the requirement for the alkylation of phenol with MTBE, while a strong acidity boasts the side-reactions such as dimerization and coke formation which results in a fast deactivation of the catalyst. Aluminosilicate MCM-41 was found to be a promising catalyst for phenol alkylation with MTBE by taking the advantage of its mild acidity and large pore diameter.

1. INTRODUCTION Alkyl phenols are valuable fine chemicals, which are vastly employed in chemical industry, pharmacy and pesticide manufacturing [1]. The syntheses of alkyl phenols by alkylation of phenol with alcohols and olefins have been extensively studied in many literatures. The most commonly used catalysts are liquid mineral acids (e.g., HF, H2SO4, H3PO4, etc.), acid solids (e.g., A1C13, ZrC14, BF3, SbC15, etc.), metal oxides and mixed oxides (e.g., 7-A1203, A1203-SIO2, TiOz-WO3, etc.) as well as resins, whereas the major disadvantages are quite obvious: (1) the exhausted catalysts usually cause pollution problems; (2) the alkylation reactions are non-selective thus undesirable by-products are produced. However, there are relatively few publications concerning zeolitic materials as catalysts in these reactions. This is particularly true in the catalytic syntheses ofbutylphenols. * To whom correspondence should be addressed. Email: [email protected]

526 Molecular sieve is widely used in petrochemical industry due to its regular pore aperture and tunable catalytic activity. MCM-41 silica is a newly discovered ordered mesoporous material. The MCM-41 structure possesses a hexagonal array of uniform mesopores in the range of 2-10 nm, a large surface area (>700 m 2 g-l) and a large pore volume [2]. By incorporation of aluminum into the silica framework, acid sites can be generated, nevertheless they are weakly acidic and can only be compared with those present in amorphous silicaaluminas [3]. However, to those reactions requiting a mild acidity, aluminosilicate MCM-41 is a catalyst of choice [4,5], moreover, it can also be used as a support for catalytically active materials. Here for the first time we demonstrate the alkylation of phenol with MTBE on aluminosilicate MCM-41 catalysts. The catalytic performances of these catalysts are also compared with those of Y zeolite.

2. EXPERIMENTAL 2.1. Materials All chemicals used were A.R. grade and commercially purchased from companies without further treatment. Aluminosilicate MCM-41 was prepared by the hydrolysis of tetraethyl orthosilicate (TEOS), aluminum isopropoxide [AI(i-OPr)3] and cetyltrimethylammonium bromide (CTAB) in ammonia solution, the final molar composition is 1.0 TEOS:0.05 AI(i-OPr)3:0.50 CTABr: 9.2 NH3:130 H20, then the mixture was hydrothermally treated at 383 K for 72 h. The solid product was filtered, washed with distilled water and dried at 393 K overnight. The assynthesized sample was calcined in a muffle at 873 K in air for 3 h to burn off the occluded organics. Finally, it was ion-exchanged twice with a 0.3 mol 1-l NH4NO3 solution at 368 K for l h to remove the extra-framework A1. Sample thus obtained is denoted as MC-0. The MC-0 powder was mixed with boehmite, dilute nitric acid and distilled water for giving a proportion MC-0:alumina of 65:35 in the final solid (MC-l) after calcination at 773 K for 3 h. A superacid-supported sample MC-2 was prepared as follows. Dried MC-1 was wetimpregnated with a 10% v/v TiC14/ethanol solution (1 g/2 ml) and exposed in a moisture air ovemight, then soaked with a 0.25 mol 1-1 (NH4)2S208 solution. After dried in air it was heated at 473 K for 2 h and later calcined at 823 K for 3 h. H-form Y zeolite (atomic ratio Si/Al=2.5) was purchased from Huahua Group Ltd., P. R. China and further calcined at 923 K for 3 h (HY-0). An alumina-bonded Y (denoted HY-1) was also prepared similarly to MC-1 with a proportion HY-0:alumina of 65:35. 2.2. Characterization Powder X-ray diffraction (XRD) pattems were obtained with a Rigaku D/MAX ),A diffractometer using the Cu Kot radiation operated at 40 kV and 40 mA. Elemental analysis was performed on a Shimadzu X-Ray Fluoresence Spectrometer VF320, data were collected and analyzed with a Data Processor DP-32 workstation. The reaction products were analyzed with a Hewlett-Packard HP G 1800A GC-MS instrument. A SE-30 capillary column (50 m, 0.2 mm I.D.) was equiped. 2.3. Catalytic testing The alkylation was carried out under atmospheric pressure in a down-flow fixed-bed

527 reactor with a 16 mm I.D. Catalysts (20-30 mesh) were loaded in the thermal static part of the reactor. In a typical run 8.0 g of catalyst was loaded and tested for 6 h on stream. A mixture of phenol and MTBE at an n(MTBE)/n(phenol) ratio of 2/1 was pumped into the reactor. The products were trapped in a condenser at the reactor outlet.

3. RESULTS AND DISCUSSION 3.I. Material characterization Elemental analysis revealed that the atomic ratio Si/A1 in MC-0 was 23, which is a little higher than that in the synthetic gel. A TiO2 loading of 8.4% and a SO42 loading of 6.2% were observed in MC-2. The XRD patterns of MC-0, MC-1 7500 and MC-2 are shown in Figure 1. All samples exhibit at least three wellc..) resolved reflections in the 20 range .~5000 between 2-6 ~ which can be indexed to

an ordered hexagonal lattice typical of MCM-41 [2]. The intensities of the reflections decrease by 40% on MC-1 2500 A~ and by 50% on MC-2 as compared to B ~ those of MC-0, respectively. This is quite in accordance with the proportion 0 of the MCM-41 in these samples. 1 4 7 10 Meanwhile, the peaks of the MCM-41 20( ~) Figure 1. XRD patterns of (A) MC-0, (B) MCphase in MC-1 as well as in MC-2 move slightly towards high angle, which is 1 and (C) MC-2. probably due to the fact that the MCM-41 framework suffer more shrinkage upon further calcination [2]. A minor widening of the reflection peaks was also observed both on MC-1 and MC.2. It indicates that the MCM-41 phase in MC-1 as well as in MC-2 is less ordered than that in MC-0. Nevertheless, the mesoporous framework sustained after calcination even for several times. A careful XRD examination was also performed at high angle area. However, no distinct peaks were observed for MC-1 and MC-2 in the 20range between 20-80 ~ which implies the absence of crystalline 7-A1203 phase in both samples and the absence of crystalline TiO2 phase in MC-2. This result suggests that the titanium species are highly dispersed in MC-2. ~

3.2. Catalytic properties MTBE is carefully chosen as an alkylating agent for phenol butylation because itself as well as its cracking product can act as a good solvent for phenols. Factors that affect the alkylation of phenol with MTBE on the catalysts were evaluated. 3.2.1. Influence of temperature The analysis results of the products revealed that the distribution of the reaction products was a function of temperature. Figure 2 depicts the influence of temperature on the reactions over MC-0 and HY-0. On both catalysts, under the conditions of feed weight hourly space

528 velocity (WHSV) of 0.8 h -I and molar ratio n(MTBE)/n(phenol) of 2.0, the MTBE conversion increased with temperature before 413 K was reached, so did the phenol conversion. This is quite easy for understanding that the first step for the alkylation requires the cracking of MTBE, and increasing temperature accelerates the formation of C4+ and the overall reaction rate. Further increasing temperature resulted in MTBE fully converted while the phenol conversion began to decrease from 423 K. Butylphenols were the main alkylation products and minor methylphenols were formed at high temperature. As temperature increased the pTBP selectivity increased whereas the o-TBP selectivity decreased, however, the selectivity toward 2,4-DTBP changed following the trend of the phenol conversion. Indeed, a higher temperature favors the formation of para isomer from a thermaldynamic point of view, meanwhile, side-reactions are also boasted. At high temperature water, dimethylether and low hydrocarbons were detected in the products, which suggests that dehydration between formed methanol molecules, dimerization of C4+ species and dealkylation of 2,4-DTBP took place on both MC-0 and HY-0. However, the results also indicate that HY-0 is more active than MC-0, which may be due to a higher density of acid sites on HY-0. 100

100-

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i

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463

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0 388

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403 418 433 Temperature (K)

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448

Figure 2. Influence of temperature on the performance of catalysts (A) HY-0 and (B) MC-0. (a: phenol conversion, b: p-TBP selectivity, c: 2,4-DTBP selectivity and d: o-TBP selectivity) 3.2.2. Influence of space velocity Tables 1 and 2 show the effect of feed space velocity on the alkylation over MC-0 and HY-0, respectively. The evaluation was performed at 413 K with a feed molar ratio n(MTBE)/n(phenol) of 2.0. The conversions of phenol and MTBE as well as the selectivities

Table 1. Alkylation of phenol with MTBE on MC-0 catalyst at various space velocity. WHSV Phenol conversion Selectivity (%) 2,4-DTBP (h -~) (%) TBP p-TBP o-TBP 0.80 92.8 45.4 40.1 5.3 53.1 1.05 86.4 51.7 35.6 16.1 45.5 1.65 74.1 61.7 36.6 25.1 36.5 2.58 72.0 62.4 36.0 26.4 35.8 3.12 64.5 63.4 33.6 29.8 34.9

529 Table 2. Alkylation of phenol with MTBE on HY-0 catalyst at various space velocity. WHSV Phenol conversion Selectivity (%) (h -~) (%) TBP p-TBP o-TBP 2,4-DTBP 0.95 91.1 55.4 48.4 7.0 42.3 1.80 85.3 60.3 51.9 8.4 37.1 2.01 83.2 65.7 56.5 9.2 31.5 2.27 71.0 70.3 61.8 8.5 29.7 toward p-TBP and 2,4-DTBP gradually decreased with increasing space velocity. This can be correlated to the change of contact time. We assume that 2,4-DTBP is a consecutive reaction product formed by alkylation of o-TBP and p-TBP with MTBE. At high space velocity sidereactions were markedly suppressed, which is supported by the observation of less multialkylated components in the products. However, the reaction mechanisms may be different on catalysts MC-0 and HY-0. The difference on selectivity toward o-TBP as well as 2,4-DTBP is significant. On MC-0 the selectivity toward o-TBP was much higher and quickly increased with increasing space velocity, whereas on HY-0 it was almost constant. The reason will be discussed in the following context. To elucidate the effect of external diffusion on this reaction, an experiment was performed by varying the height of the catalyst bed. On both catalysts MC-0 and HY-0, at 413 K with a feed molar ratio n(MTBE)/n(phenol) of 2.0 and a WHSV of 0.8 h -1, the ratio of height to diameter of catalyst bed had negligible influence on this reaction in the range of 2-6, i.e., the phenol conversion and the product distribution changed very little.

3.2.3. Influence of surface acidity To understand the mechanism of phenol alkylation with MTBE, reaction was carried out on catalysts with various surface acidity. Evaluation was performed at 413 K with a feed molar ratio n(MTBE)/n(phenol) of 2.0 and a WHSV of 0.68 h -~. The results are summarized in Table 3. Table 3. Alkylation of phenol with MTBE on different catalysts with various surface acidity. Catalyst Phenol conversion Selectivity (%) (%) TBP p-TBP o-TBP 2,4-DTBP HY-1 85.7 54.8 44.0 10.8 40.9 MC-1 84.6 49.7 37.0 12.7 48.7 MC-2 68.4 55.4 43.2 12.2 42.8 As titania was formed on the MC-1 matrix and interacted with $042"during the treatment, surperacidic sites could be generated on the surface of MC-2 [6]. On HY-1 Broensted acid sites are in the ascendant, while on MC-1 weak Lewis acid sites are dominant and a few weak Broensted acid sites are present. As previously reported [7], Broensted acid sites strongly interact with the aromatic ring while Lewis acid sites interact with oxygen to form phenolate complexes. In the former case alkylation can occur either at the oxygen or at the ring, while in the latter alkylation at the ring in the ortho position is favored. However, hardly any O-

530 alkylation products were detected. We assume that, if they were formed, they had been consumed through isomerization and transalkylation. A. Corma et al. found that, on partially ion-exchanged Y zeolite, at 303 K phenol reacted with tert-butanol in CC14 to form tert-butyl phenyl ether; at 353 K the activity increased with the strength of the acid sites, and weak strength acid sites favored the formation of 2,4-DTBP [8]. Interestingly, the selectivities toward o-TBP and 2,4-DTBP are significantly higher on the MC catalyst series than those on the HY catalyst series. It is worthy of mention that under the same reaction conditions the bondant (A1203) was much less activity than HY-0 and MC-0. Generally, cracking and isomerization require a strong acidity. The minority of o-TBP in the reaction products on Y zeolite catalysts might be due to the following reasons: (1) the consumption of o-TBP through isomerization to p-TBP and formation of 2,4-DTBP; (2) the shape-selective effect of the pore aperture on its formation. The latter factor hindered the formation of 2,4-DTBP on Y zeolite too. While on the MC catalyst series, the spacial hindrance on formation of intermediate complex and diffusion could be excluded, and the isomerization of o-TBP was less pronounced due to the weak acidity. However, it can be seen from Table 3 that phenol conversion on MC-2 was obviously lower. This can be correlated to the surperacidity on MC2. As strong acid sites are always responsible for the coke formation due to the strong interaction with the adsorbed molecules, the catalytic active sites are blocked and a fast deactivation is resulted in. Indeed, after a 6 h run, MC-2 became dark black while HY-1 and MC-1 were yellowish. It indicates that a mild acidity can meet the requirement for the alkylation of phenol with MTBE, strong acid sites are not necessary and may do harm to the catalyst. In summary, our results suggest that mesoporous aluminosilicate is a promising catalyst for the alkylation of phenol with MTBE by taking the advantages of its mild acidity and large pore diameter. Further work is still ongoing to improve the activity and the selectivities toward p-TBP and 2,4-DTBP with MCM-41 aluminosilicate.

4. ACKNOWLEDGMENTS The financial support of this research by the National Natural Science Foundation of China (through Grant No. 29873024) is gratefully acknowledged.

REFERENCES 1. T. Kirk and K. Othmer, Encyclopedia of Chemical Technology 3rd Ed., Wiley, New York, 1981. 2. C. T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992) 710. 3. A. Corma, V. Fomes, M. T. Navarro and J. Perez-Pariente, J. Catal., 148 (1994) 569. 4. A. Sayari, Chem. Mater., 8 (1996) 1840. 5. R. Mokaya, W. Jones, Z. Luan, M. D. Alba and J. Klinowski, Catal. Lett., 37 (1996) 113. 6. H. Hino, S. Kobayashi and K. Arata, J. Am. Chem. Soc., 101 (1979) 6439. 7. E. Santacesaria, D. Grasso, D. Gelosa and S. Carra, Appl. Catal., 64 (1990) 83. 8. A. Corma, H. Garcis and J. Aprimo, J. Chem. Res., (1988) 40.

Studies m ~urtace ~mence and tdatalysls 141 A. Sayari and M. Jaroniec (Editors) 9 2002 Elsevier Science B.V. All rights reserved.

531

Isopropanol dehydration over nanostructured sulfated M C M - 4 1 Antonio S. Araujo*, Joana .M.F.B. Aquino, Cristiane D.R. Souza and Marcelo J. B. Souza Federal University of Rio Grande do Norte, Department of Chemistry, CP 1662, 59078-970, Natal, RN (Brazil) The synthesis of MCM-41 and sulfate-MCM-41 is reported. The MCM-41 sample was prepared by the hydrothermal method using cethyltrimethylamine as template, and characterized by BET surface area, X-ray diffraction, infrared spectroscopy and thermogravimetry. The sulfate containing MCM-41 was prepared by the controlled impregnation of MCM-41 with 0.5 M sulfuric acid. From n-buthylamine adsorption data and thermogravimetry, MCM-41 has no considerable acidity, whereas the SO427MCM-41 presents medium acid sites. The materials were used as catalyst for isopropanol dehydration, in a fixed bed continuous flow reactor. The SO42-/MCM-41 presents catalytic activity for isopropanol dehydration at relatively moderate temperature 473 K, with selectivity to propene.

1. INTRODUCTION The newly discovery mesoporous molecular sieve MCM-41 possesses high surface area and a uniform hexagonal array [1,2], opening new opportunities in the hydrothermal synthesis and modification in order to obtain new acid materials for heterogeneous catalysis applications [3,5]. Some properties of a thermostable mesophase of basic zirconium sulfate with texture characteristics close to those of MCM-41 has been reported [6]. The peculiarities of the catalytic behavior of the mesophase are related to its acidic properties. The MCM-41 nanostructured materials present ordered channels, and disordered atomic arrangement similar to that of amorphous silica. The formation of the MCM-41 phase occurs according to the liquid crystal template (LCT) mechanism, in which tetrahedral SiO4 species react with the surfactant template under hydrothermal conditions. The number of acid sites can be modified on a wide scale by isomorphic substitution, by ion-ex-change or by treatment with acids. In this work the silica MCM-41 was treated with sulfuric acid solution in order to generate acid sites in its surface. The introduction of sulfate ions in molecular sieves has recently been studied [7] and revealed that they can be active catalyst for the synthesis of b-naphtyl-methyl-ether, from bnaphthol and methanol at 473 K. In this work, the synthesis of SO42-/MCM-41 was studied using the controlled impregnation method. The obtained material was applied to the isopropanol dehydration using fixed bed continuous flow reactor. 2. EXPERIMENTAL

The MCM-41 was firstly synthesized by the hydrothermal method of a gel with molar composition 4SIO2:1Na20:1C16H33(CH3)3NBr:200H20, with pH adjustment and salt addition.

532 The synthesis was carried out at 373 K by 4 days. Then, it was washed with distilled water, recovered by filtration and dried at 373 K for 1 day. The material was calcined at 823 K in nitrogen and then in air atmosphere. The material was characterized by XRD (Rigaku), infrared spectroscopy (Midac) and thermogravimetry (Mettler Toledo TGA/SDTA 851). BET surface area was measured using nitrogen adsorption at 77 K, on an ASAP 2010 (Micromeritics). For the sulfatation of MCM41, ca. 1 g of calcined material was treated with 30 ml of 0.5 N sulfuric acid, at room temperature for 2 h, and then heated at 343 K until complete evaporation. The sample was dried at 383 K for 10 h, in an oven, and subsequently calcined at 823 K for 5 h, under nitrogen atmosphere flowing at 30 mL.min -~. The presence of sulfate species in the MCM-41 material was verified by thermogravimetry. The acid properties were investigated by using nbutylamine as molecular probe, followed by TGA, according to procedures de-scribed in the literature [8]. The obtained material was tested as a catalyst for isopropanol dehydration, in a fixed bed continuous flow reactor [9], at temperatures of 473, 513 and 553 K, and W/F ranging from 1.3 to 4.1 g.s.mol -l, where W - mass of catalyst (g) and F = flow of reactant (mol.sl). The products were analyzed by gas chromatography using a Porapak Q-packed column. 3. RESULTS AND DISCUSSION

The thermogravimetric analysis of the uncalcined MCM-41 material in nitrogen atmosphere show three weight losses [8], in the following temperature ranges: (i) from 298 K to 443 K (4% thermodesorption of physically adsorbed water); (ii) from 443 K to 543 K (38% surfactant decomposition) and (iii) from 543 to 803 K (7% residual surfactant decomposition and silanol condensation). From the characterization of the synthesized MCM-41 by XRD, FT-IR and TG, it was verified that the hydrothermal method has been efficient to obtain the MCM mesophase. The FT-IR spectra of the Si-MCM-41 show a characteristic absorption band at 960 cm -1, due to Si-OH groups, and others at the 1080, 800 and 465 c m -1 regions, which are characteristics of the material. As shown inFigure 1, the XRD patterns for the Si-MCM-41 and SO42-/MCM41 present a very strong peak, at ca. 2.1 ~ 2 O, due to (100) index. Two weak peaks were also distinguished as peaks characteristic of the family, at 4.10 (110) and 4.8 ~ (200), suggesting hexagonal symmetry [1,2]. Thus the structure of the sulfate modified sample is still nanoporous and similar to MCM-41. The thermogravimetry measurements showed that the sulfate species interact with the MCM-41 surface, generating the catalytic acid sites. From TG curves, the sulfate groups decompose in two steps: i) from 473 to 668 K and ii) from 668 K to 774 K, generating the Bronsted and Lewis acid sites. From n-buthylamine adsorption, it was verified that pure MCM-41 has practically no acidity or very low acid sites density (0.1 mmol/g), whereas SO4-2/MCM-41 has ca. 1.2 mmol/g of total acidity. The adsorption parameters for the silica MCM-41 were: surface area of 780 m2.g~, pore volume of 0.68 cm3.g-I and pore width of ca. 3.7 nm. For the sulfated sample, a decrease in the surface area to 720 m2.g-l was obtained. However, the pore volume and width were practically the same. This confirms that stable SO4~/qVICM-41 can be synthesized by controlled impregnation methodologies.

533

Figure 1. X-ray diffraction patterns of MCM-41 and sulfate-containing MCM-41.

The proposed structure to the sulfated material is shown in Figure 2. The scheme considers that the material surface is totally dehydrated, which is obtained after calcination at 773 K, with the sulfate covalently bonded to the silicon via oxygen atoms. The negative charge of the oxygen is neutralized by one proton forming Bronsted acid sites (BA). Due to the inductive effect of the sulfate group, the strong Lewis acid sites (LA) can be generated on its surface. From the infrared analysis, the asymmetric and symmetric stretching of the S=O bond were determined in the 1215-1125 cm ~ and 1060-995 cm l regions, respectively.

Figure 2. Scheme proposed for the sulfate-containing MCM-41 material showing possible Bronsted acid (BA) and Lewis acid sites (LA). The catalytic tests shown that the MCM-41 without sulfate presents very low catalytic activity, with conversion of ca. 7% at the studied temperatures. On the other hand, the SO42 /MCM-41 was very active to the isopropanol dehydration, with ca. 78% of conversion, producing propene and isopropyl ether, as can be seen in Figure 3. The conversion attains a maximum at W/F equal to 4.1 g.s.mol -~, independently of the reaction temperature. The high activity of the SO42-/MCM-41 catalyst can be visualized as a function of the surface acidity generated by the sulfate groups. The selectivity was measured as the propene/ether ratio to

534 each temperature reaction as a function of the W / F . In Figure 4, it is observed that the selectivity to olefin is higher for low W/F values, with propene/ether ratio in the range of 1 to 1.8. For values of W/F superior to ca. 2.7, the propene/diisopropyl ether ratio decrease is around 0.8, being almost constant for the studied temperature. This is evidence that there is a relation between the contact time of the reactant with the particular pore system and diffusion associated with the acidity of the SO42/MCM-41. 90 80

o~ v ~

t--. 0 oo L_

> rO

70 60 50

- - = - - 473 K

40

- - e ~ 553 K

30

- - A ~ 513 K

'

10

i

1,5

'

i

2,0

9

i

'

2,5

i

3,0

,

i

3,5

'

i

4,0

'

4,5

W/F (g.s/mol) Figure 3. Isopropanol conversion as a function of W/F for different temperature on the SO42/MCM-41.

2.0 l~ .O t~

- - = ~ 473 K |

1.6

- - A ~ 513 K I

I

L_ L_

(1) r-

1.2

uJ t-

0.8-

Q. O --9 0.

0.4

0.0 1.0

!

'

1 15 ' 2:0

' 215

' 310

' 3'.5

' 410

' 4.5

W/F (g.s/mol) Figure 4. Propene/diisopropyl ether ratio in the isopropanol dehydration reaction as a function of the W/F at different temperatures for the S042/MCM-41.

535 Experimental kinetic data of the isopropanol dehydration over MCM-41 and 8042" /MCM-41 have been obtained in a fixed bed continuous flow reactor. For all experiments the following conditions were assumed: isothermal reaction in fixed bed, catalyst in powder form, uniform bed porosity, reactor profile as plug flow and stream in stationary state. Linking the values of residence times with conversion in a first order kinetic model [ 10], the obtained fit represents the variation of the conversion rate for a given temperature (Figure 5). The slop of this curve gives the rate constant of the process. 3.5 3.0

9 MCM-41 9 S042/MCM-41

% 2.5 ~ o "" 2.0

"i--..

1.5 1.0

. 1.32

. 1.36

.

. 1.40

. 1.44

1.48

, 1.52

103/T (103/T)

Figure 5. Arrhenius plots for determination of the activation energy for isopropanol dehydration over MCM-41 and SO42-/MCM-41. Table 1 summarizes the kinetic data obtained by the Arrhenius plot for the experimental reactor data. These results show a decrease in the activation energy to isopropanol dehydration in comparison with the pure MCM-41 with activation energy of 20 kJ.mo1-1 for MCM-41 and 36.73 for SO42-/MCM-41. The decreasing in the activation energy is affirmed by the increasing in the MCM-41 acidity by the incorporation of the sulfate groups in the mesoporous array. Table 1. Parameteres of Arhenius equation (T, k), apparent activation energy (Eat), preexponential factor (Ao) and respectives aciditiesfor the isopropanol dehydration over MCM41 and SO42-/MCM-41. MCM-41 Temperature (K) k.106 (s") 1000/T (K-') ln(k.106) Eat (kLmol:') 473 3.334 2.114 1.204 36.73 513 9.611 1.949 2.263 Acidity (mmnol/g) 553 12.667 1.808 2.539 0.1 SO4"2/MCM-41 Temperature (K) k. 106 (S-1) 1000/T (K l) ln(k. 106) Eat (kJ.mol -l) 473 5.365 2.114 1.680 20.02 513 9.737 1.949 2.276 Acidity (mmol/g) 553 11.112 1.808 2.408 1.2

536 4. CONCLUSIONS The sulfated MCM-41 material was very active to the process, with conversion to propene and diisopropyl ether. This activity is attributed to the high acidity generated by the sulfate incorporation on the MCM-41 structure generating Brrnsted and Lewis active sites. The isopropanol conversion increases with the temperature from 55 to 72 %, with the propene/diisopropyl ether molar ratio changing from 1 to 1.8. The possibility to modify the surface of the MCM-41 by treatment with sulfuric acid and subsequent calcination open new opportunities to generate strong active acid sites in stabilized nanostructured materials. ACKNOWLEDGEMENTS The authors acknowledge the support from the Conselho Nacional de Desenvolvimento Cientifico e Tecnolrgico (CNPq), and Agrncia Nacional do Petrrleo (ANP). REFERENCES

1.

C.T. Kresge, M. E. Leonowicz, W. J. Roth, J. C. Vartuli and J. S. Beck, Nature, 359 (1992). J. S. Beck, W. J. Roth, M. E. Leonowicz, C. T. Kresge, K. D. Schmitt moder, C. T. W. Chu, D. H. Olson, E. W. Sheppard, S. B. McCullen, Y. B. Higgins and I. L. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. A. Sayari. Stud. Surf. Sci. Catal., 102 (1996) 1. 4. A. S. Araujo and M. Jaroniec, J. Colloid. Interf. Sci., 218 (1999) 462. 5. X. S. Zhao, G. Q. Lu and G. J. Millar, Ind. Chem. Res., 35 (1996) 2075. 6. V. N. Romannikov, V. B. Fenelonov, B. A. Paukshtis, A. Y. Derevyankin, V. I. and Zaikovskii, Microporous Mesoporous Mat., 21 (1998) 411. W. C. Li, Y. C. Chih and N. K. Na, Appl. Catal. A: Gen., 178 (1998) 1. 8. A. S. Araujo, V. J. Femandes Jr. and S. A. Verissimo, J. Therm. Anal. Calorim., 59 (2000) 1. A. S. Araujo, M. J. B. Souza, V. J. Fernandes Jr. and J. C. Diniz, React. Kinet. Catal. Lett., 66 (1999) 141. 10. A. S. Araujo, T. B. Domingos, M. J. B. Souza and A. O. S. Silva, React. Kinet. Catal. Lett., 73 (2001) 283. .

.

.

.


537

Effects o f Si/A1 Ratio and Pore Size on Cracking Reaction over M e s o p o r o u s MCM-41 Wen-Hua Chen a, Qi Zhao a, Hong-Ping Lina, Chung-Yuan Mou b, and Shang-Bin Liu a'* a Institute of Atomic and Molecular Sciences, Academia Sinica, E O. Box 23-166, Taipei, Taiwan 106, R.O.C. b Department of Chemistry and Center of Condensed Matter Research, National Taiwan University, Taipei, Taiwan 106, R.O.C.

The hydrocracking abilities of mesoporous MCM-41 materials were studied using 1,3,5-triisopropylbenzene (1,3,5-TiPB) cracking as test reaction. Various MCM-41 samples with varied Si/A1 ratios (15 to ~ ) and pore sizes (1.57 to 3.04 nm), synthesized by the 'delayed neutralization' method, were examined. It is concluded that 1,3,5-TiPB cracking reaction over A1-MCM-41 is diffusion controlled and coking is responsible for catalyst deactivation. The roles of A1 content and pore size on the catalytic features of the samples were evaluated by the conversion of 1,3,5-TiPB, coke content and deactivation parameters.

1. INTRODUCTION The mesoporous MCM-41 materials, which consist of hexagonal arrays of uniform channels with tunable poresizes (1.5 - 20.0 nm), possess prominent properties, such as high surface area (~ 1000 m2/g), hydrocarbon sorption capacities (> 0.7 ml/g), and thermal and hydrothermal stability render many potential applications. For examples, as adsorbents during sorption/separation processes, as supports for electronic/optical devices, or as catalysts to catalyze large organic molecules whose molecular size are greater than typical pore size (ca. 7 A) of the microporous zeolites [1-4]. It is well known that the activity of a catalyst depends mainly on its acidity and mass-transport limitations. The former is normally manipulated by the concentration and distribution of A1 species contained in the catalyst, while the latter is controlled by steric constrains imposed on the structural porosity of the catalyst. In this context, A1-MCM-41 materials, being less acidic compared to most microporous zeolites and possess highly ordered mesoporous channels, are most suitable as catalysts for catalytic cracking of large molecules during which only weak acidity is required [5-8]. Nevertheless, catalyst deactivation due to coking remains as the major problem need to be resolved. Thus, in view of promoting the catalytic performance of A1-MCM-4 l, fabrication of catalysts with desirable catalytic activity while in the same time resistant to coking is an interesting task. The objective of this study is to investigate the effects of A1 content and pore size on the catalytic performances of A1-MCM-41 during hydrocracking reaction.

538 2. EXPERIMENTALS

2.1. Materials The powdered, particulate MCM-41 molecular sieves with varied Si/A1 ratios (15 -00 ) and pore diameters (1.57-3.04 nm) were synthesized by the so-called "delayed neutralization" procedure. Their structural features were confirmed by powder X-ray diffraction (XRD) and by scanning/transmission electron microscopy. The average pore size and surface area of the sample were shown in Table 1.

2.2. Cracking Reaction 1,3,5-triisopropylbenzene cracking was used as test reaction throughout this study. The reagent (1,3,5-TiPB; A.R. grade, ACROS) was used with further purification by molecular sieve 4A. All reactions were conducted in a continuous flow, fixed-bed flow reactor under the standard conditions, namely T r = 573 K; WHSV = 15.25 h-~; pressure = 1 atm; carrier gas: N2; N2/EB = 2.0 mol/mol, and time-on-stream (TOS) = 0-6 h. Palletized and pressed MCM-41 sample (10-20 mesh; ca. 1 g) was mixed with quartz (ca. 20-30 mesh) and packed into the reactor. Prior to the reaction, sample was first activated in air at 723 K for 8 h; the reactor was then cooled under N2 stream down to the desired reaction temperature. The composition of the reactor effluents was analyzed by gas chromatography (Shimadzu GC-9A) using a packed column (5% SP-1200 + 1.75% Bentone 34 on 100/120 Supelcoport, 6 fl). All products were identified using the internal standard method. The carbonaceous residues retained in the fouled samples were determined by thermogravimetric analysis measurement (TGA; Netzsch TG209). Typically, ca. 10 mg of catalyst was heated from 298 to 1173 K at a rate 10 K/min under dried air. The coke content was determined from the weight loss between 573-973 K.

3. RESULTS AND DISCUSSION The catalytic activities of various A1-MCM-41 sasmples were assessed by the conversion of 1,3,5-TiPB, which was catalyzed to produce mainly mono- and di-substituted isopropylbenzenes. In general, the yields of major products were found to obey the order: 1,3-DiPB > 1,4-DiPB > cumene > 1,2-DiPB > propene >> benzene. Only trace amount of benzene yield was observed.

3.1. Effect of catalyst Si/Al ratio Although the structures of siliceous MCM-41 materials are normally more stable than aluminosilicate MCM-41, they lack ion-exchange capability due to electrically neutral framework charge. As the result, siliceous MCM-41 materials are expected to exhibit nearly null catalytic activity. On the other hand, isomorphous substitution of the framework Si by A1 would results more negatively charged framework, which in turn render the formation of acid sites requisite for catalytic reactions. To investigate the effect of A1 content on catalytic performance during hydrocarbon cracking, AI-MCM-41 samples of similar pore size (ca. 2.6 nm) but with varied Si/A1 ratios were prepared. The catalytic activities of various samples during cracking reactions are then evaluated in terms of conversion of 1,3,5-TiPB, as shown in Fig. 1.

539

Table 1. Characteristics and catalytic properties of the MCM-41 Samples. Fouled catalysts

Fresh catalysts Samples

Si/A1

Pore size Pore volume Surface area Coke (rim) a (ml/g) b (mZ/g)b contentc

Deactivation parameters

d

Xo

tc

a

Xo + t~

M15

15

2.62

0.98

1015

4.5

29.2

38.3

0.81

67.5

M20

20

2.68

0.94

927

4.1

23.1

33.0

0.51

56.1

M37

37

2.54

1.15

1064

4.3

22.1

25.6

0.47

53.3

M46

46

2.64

1.06

1032

4.6

24.3

21.4

0.76

45.7

M60

60

2.58

1.02

1135

4.3

22.6

20.6

1 . 0 4 43.2

M120

120

2.61

0.98

1093

3.7

17.4

20.4

0.79

37.8

M177

177

2.56

1.05

983

3.1

7.8

29.3

0.77

37.1

M370

370

2.58

0.98

1027

2.6

7.7

27.8

0.87

35.5

SM

~

2.66

0.94

1074

MCM-C10

37

1.57

0.81

1191

9.6

12.9

80.7

0.75

93.6

MCM-CI2

37

1.80

0.96

1291

4.4

40.0

50.3

0.50

90.3

MCM-C14

37

2.18

0.96

1150

4.6

35.5

42.1

0.45

77.6

MCM-CI6

37

2.54

1.15

1064

4.3

22.8

30.5

0.53

53.3

MCM-C18

37

3.04

1.21

1028

3.2

17.1

0.58

28.5

11.4

aData obtained by the BJH method based on the desorptlon curve of N2 adsorption/desorption isotherms (77 K). bDetermined by N2 isotherms at P/Po = 0.96. CObtained from the fouled catalysts by TGA, in unit of wt%. dResults obtained from data fitting of Eq. 1. eRepresent initial conversion (TOS = 0 h); in unit of wt%. Except for siliceous MCM-41 (Si/A1 - ~o ) which revealed the expected null activity, the 1,3,5-TiPB conversion curves obtained from various A1-MCM-41 samples were found to decay exponentially with time-on-stream (TOS) and can be fitted by the following equation: Y , = Y o + k e ~'

(1)

where Xt represents the conversion at a given time (TOS) t, Xo and k are constants, and the exponent et is a parameter accounts for deactivation rate. The results of the fittings are shown in Fig. 1 as solid curves and related deactivation parameters derived are depicted in Tablel. Coking is the prominent reason accounts for the deactivation of the catalysts and appears to be more pronounced during the initial stage of the reaction. Overall, the catalytic activities of

540

catalysts became more stable as TOS exceeds ca. 4 h. That the coke content (obtained by TGA at TOS = 6 h) decreases with increasing AI content of the AI-MCM-41 samples indicates that carbonaceous residues are likely deposited on the acid sites of the catalysts.

A

|

70

'{

6o

,~0

50

uE,

a.m

II I

i

9 9

I 4)'

M37 M60

M17'7

~

.,0

/k

M37O |

0

0

70

|

M46 M120

i

A ~ 70~ . . . . . . .

I

A

I I

e-

o

60

> C

o

40

0

m 50

C

30

C

I11

20 ""

~ 40

lO

I/ =

0

0

=

B]

_

2

,

6

T i m e - o n - s t r e a m (h)

Fig. 1. Variations of 1,3,5-TiPB conversion against time-on-stream during cracking reaction over various MCM-41 and A1-MCM-41 samples.

0

'

!

100

"''

|-

200

'

|

300

'

t

400

SilAI ratio

Fig. 2. Correlation of 1,3,5-TiPB initial conversion with Si/AI ratio of AI-MCM-41 samples.

Furthermore, the initial conversion of 1,3,5-TiPB (i.e., Xo + k; at TOS = 0 h) deduced from Eq. 1 was also found to decrease exponentially with the Si/AI ratio of the AI-MCM-41, as shown in Fig. 2. Eventually, the initial conversions reach a plateau value of ca. 37 wt% for samples with Si/AI >__120. However, we note that this effect should depend on the contact time or WHSV applied. Presumably, an increase in WHSV will shorten the contact time and hence result in a lower 1,3,5-TiPB conversion. It is hypothesized that, upon initial reaction, reactants 1,3,5-TiPB are immediately catalyzed to form carbonaceous residues, which tend to deposit on the acid sites. Progressive formation of coke on the acid sites therefore resulted in an overall reduction of catalyst acidity. As the result, the conversion of 1,3,5-TiPB maintained at a constant level for TOS > 4 h. In this context, this observation is thus in line with the notion that hydrocarbon cracking reactions over AI-MCM-41 catalysts, which is diffusion controlled, require only weak acidity.

3.2. Effect of catalyst pore size

To explore the effect of pore size on the catalytic performance, 1,3,5-TiPB cracking reaction were carried out on different AI-MCM-41 samples with varied pore diameters (namely, 1.57, 2.18, 2.80, 2.54 and 3.04 nm) but having the same AI content (Si/AI = 37). The resultant 1,3,5-TiPB conversions against TOS are shown in Fig. 3. Again, the solid curves in Fig. 3 represent fittings of deactivation curves for various samples by Eq. 1. The results of the fittings are also summarized in Table 1 together with the coke content. The variations of the

541 extrapolated 1,3,5-TiPB initial conversion with catalyst pore size are shown in Fig. 4. 100

,

,

~

"~ C

~k(~

75

~..1 \ --

~

0 ,m

E > = 0 o I-

~

' ~ k \\

[] 50

.

~

8O

-~

i

470

~

"-2a--_,-o~

om

o z___o

[ ] x7~- - ~ -

o

o m oc. .

0 m

c

"o m (I. I---

460

,.:

450

al

40

a. I-

20

,i

Time on stream

960

e-

- -~.

c

.o m

> C

(~

o

o

480 E

.-..

,_-h,[] '.'v. \

25

100

Q.

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

\ x x

490

.=

SilAI=37 r-I MCM-CIO O MCM-C12 f MCM-C14 V MCM-C16 0 MCM-C18

. (h)

Fig. 3. Variations of 1,3,5-TiPB conversion against time-on-stream for various AI-MCM-41 samples with varied pore size durin~ crackin~ reaction.

1.5

2.0

2.5

3.0

Pore size ( n m )

Fig. 4. Correlations of 1,3,5-TiPB initial conversions and desorption temperature with pore size of AI-MCM-41 samples.

The consistent decrease in initial conversion of 1,3,5-TiPB, coke content, and deactivation rate (~) with pore size indicate that, at a given AI content, AI-MCM-41 with smaller pore diameters are more favorable in terms of their catalytic activity. Nevertheless, they also appear to deactivate more easily. The aforeobserved phenomena seem to associate with the adsorptive properties of samples. To verify this point, additional experiments were performed on samples subjected to special treatments. The samples were prepared by first adsorbing saturated amount of 1,3,5-TiPB by vapor transfer method, then placed under ambient conditions overnight to ensure homogeneous adsorbate distribution followed by TGA measurements as desorption tests. Each sample (ca. 10 mg) was heated to 1173 K at 5 K/min under dried N2, accordingly the temperature at which 1,3,5-TiPB completely desorbed can be determined. As shown in Fig. 4, the final desorption temperature was found to increase with decreasing pore size of AI-MCM-41, which is in line with the trend observed for initial conversion. The results therefore indicate that, for AI-MCM-41 with the same AI content, the increase in adsorption capacity of 1,3,5-TiPB with decreasing pore size therefore corresponds to enhanced catalytic activity observed during the initial stage of the reaction. It may be envisaged that the smaller the pore size, the greater the adsorption strength for the reactant, and consequently the more hindrance imposed on the reactant/product molecular diffusion, this promoting the catalytic activity during the cracking reaction. Finally, the notable differences observed in the overall catalytic features of the particular AI-MCM-41 sample with the smallest pore diameter (1.57 nm) deserve further discussion. Considering that the kinetic diameter of the 1,3,5-TiPB reactant is ca. 0.85 nm and the progressive deposition of carbonaceous residues during reaction, steric hindrance and possibly pore blocking are likely to occur in the channels of the mesoporous AI-MCM-41. Presumably, these effects should also be inter-connected and should be more pronounced for samples with

542 smaller pore size. As the result, the A1-MCM-41 sample with 1.57 nm pore size would be vulnerable to coking due to diffusion limitations, thus the observed high deactivation rate (ix) and total coke content (9.6 wt. %) compared to the other samples (Table 1). Moreover, except for sample with 1.57 nm pore size, the fact that the coke content and deactivation rate are nearly independent of pore size of A1-MCM-41 indicating that deactivation due to coking depends mostly on the sample A1 content.

4. CONCLUSIONS The effects of Si/A1 ratio and pore size on catalytic performances of mesoporous aluminosilicate MCM-41 molecular sieves during 1,3,5-TiPB cracking reaction have been investigated. The activity of the catalyst was found to decrease exponentially with time-on-stream regardless of the AI content and pore size possessed by the sample. Coking was found responsible for catalyst deactivation, the coke content and deactivation rate are found to depend on A1 content rather than pore size of the samples, except for the extreme case of small mesopores. The initial conversion of 1,3,5-TiPB was found to decay exponentially with Si/A1 ratio of the samples with similar pore sizes, whereas for samples with the same Si/A1 ratio, it decreases gradually with pore size. It is concluded that hydrocracking reaction over A1-MCM-41 is diffusion controlled and requires only weak acidity.

5. ACKNOWLEDGMENTS The authors thank Profs. Soofin Cheng and Ben-Zu Wan for helpful discussions. The supports of this work by the Chinese Petroleum Corporation (88-S-067) and by the Nation Science Council, R. O. C. (NSC89-2113-M-001-033 to SBL) are gratefully acknowledged.

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5tuclles in Surface Science and Catalysis 141 A. Sayari and M. Jaroniec (Editors) Crown Copyright 9 2002 Published by Elsevier Science B.V. All rights reserved.

543

Hydrogenation and mild hydrocracking o f synthetic crude distillate by Pt-supported mesoporous material catalysts Hong Yang a, Craig Fairbridge a, Zbigniew Ringa, Randall Hawkinsa and Josephine M. Hill b aNational Centre for Upgrading Technology, Devon, AB, Canada, T9G 1A8 bDepartment of Chemistry, University of Calgary, Calgary, Alberta, Canada, T2N 1N4

The hydrogenation and mild hydrocracking activities of a Pt-supported mesoporous molecular sieve catalyst and a Pt-supported mordenite catalyst were studied in a bench scale fixed-bed reactor. The reaction temperatures ranged from 240 to 360~ with a total pressure of 10.3 MPa, and LHSV of 1.0. The feed was a hydrotreated middle distillate derived from Canadian oil sands. Feed and total liquid products were characterized by ASTM standard methods for physical properties, and by the GC-MS method for chemical compositions. The detailed chemical compositional results using GC-MS, show that supporting Pt on a mesoporous material (Pt/MM-alumina) results in a catalyst that is able to hydrocrack large molecules such as 3-ring naphthenes. At the same reaction conditions, this catalyst also has a higher hydrogenation activity and better mild hydrocracking selectivity than a mordenitesupported Pt catalyst (Pt/Mor-alumina). At a similar conversion level, the Pt/MM-alumina catalyst gave a superior diesel yield and a lower naphtha yield than the Pt/Mor-alumina catalyst, likely because of its larger pore structure and lower acidity.

1. INTRODUCTION Middle distillates derived from heavy oils and oil sands contain high concentrations of aromatics and naphthenes, and low concentrations of paraffins. The middle distillates often have lower cetane number compared to those derived from conventional petroleum. Deep aromatic saturation and ring opening of naphthenic compounds by hydrogenation and mild hydrocracking reactions would improve the combustion properties of the fuels. Existing hydrotreating catalysts do not perform these functions. Fuel quality improvements, therefore, depend heavily on the development of new catalysts. Furthermore, replacing the conventional catalysts in the existing hydroprocessing units with new functionality catalysts, is the most economic approach to improve product quality and avoid large capital investment. The synthesis of mesoporous molecular sieves with tunable pore structure and surface functionality has attracted significant interest [ 1-5]. Mesostructured materials are beginning to find application in the area of adsorption, catalysis and environmental protection [6-10]. In the present work, we tested a mesoporous molecular sieve-supported Pt catalyst for the

544 hydrogenation and mild hydrocracking of a middle distillate feed, and compared the activity to that of a mordenite-supported Pt catalyst. 2. EXPERIMENTAL

2.1 Catalyst preparation Pt-supported mesoporous aluminosilicate catalyst was prepared according to a patented procedure [10] using aluminum sulfate hydrate and sodium silicate solution (27% SiO2) as alumina and silica sources, and cetyltrimethylammonium bromide (CTABr) as template. (All chemicals were supplied by Aldrich.) In a typical preparation, a solution made of 5.2 g aluminum sulfate, 208 g water and 35.8 g cetyltrimethylammonium bromide, was added to a second solution containing 56.6 g sodium silicate, 80 g water with 2.4 g sulfuric acid. The mixture was stirred for 30 min and placed in a sealed glass bottle at room temperature ovemight. A solid product was recovered by filtration, washed with deionized water, air-dried at room temperature, and oven-dried at 110~ The synthesized solid was calcined at 540~ under NE/air for 5 hours to remove the template. Afterwards, the product was reacted at 70~ with ammonium nitrate solution for 4 hours. The ion-exchanged product was washed and centrifuged to remove all traces of ammonium ion before being heated at 120~ for 1 hour and 505~ for 4 hours, under air. This product was then impregnated with platinum using an aqueous solution of Pt(NH3)4C12 (54.35wt% Pt, Alfa Aesar, Ward Hill, MA) to produce a Ptloaded mesoporous material (Pt/MM) catalyst. Pt-supported mordenite catalyst was prepared from CBV21A (Zeolyst International Valley Forge, PA) by ion exchange with Pt(NH3)4C12 in aqueous solution. The mixture was reacted ovemight at room temperature to establish equilibrium and complete the ion-exchange process. The solid was washed and centrifuged free of chloride before being air-dried at room temperature. Samples were mixed with alumina to disperse the active metal/mesoporous material into a matrix suitable for catalyst testing on a complex petroleum feed. Alumina also acted as a binder to fabric catalyst extrudates of good mechanic strength. Catalyst extrudates (0.8 mm diameter) were made by mixing the Pt/MM and Pt/Mor with 80wt% pseudo-b6ehmite (Catapal B, Condea Vista, Houston, TX.). The extrudates were calcined at 400~ for 4 hours before use. The final catalysts were coded Pt/MM-alumina and Pt/Mor-alumina.

2.2 Feedstock preparation The feedstock used for hydrogenation and mild hydrocracking experiments was prepared by fractionation and hydrotreatment of a light gas oil from an ebullated bed hydrocracker (Syncrude Canada Ltd, Fort McMurray). The lighter portion of the gas oil was first distilled off using a single-column continuous distillation unit to give an initial boiling point of approximately 260~ The strategy was to fractionate the material to obtain a somewhat heavier fraction that could be cracked down into the diesel range by mild hydrocracking. Following distillation, the heavier fraction was severely hydrotreated to reduce the sulfur and nitrogen contents to less than 20 ppm in order to protect the noble metal catalysts.

545

2.3 Catalyst activity testing procedure Evaluation of the hydrogenation and mild hydrocracking activities of the Pt/MMalumina and Pt/Mor-alumina catalysts was carried out using an automated microreactor system. The fixed-bed stainless steel tubular reactor (30.5 x 0.635 cm) was operated in the continuous up-flow mode and heated by a three-zone electric furnace. The reactor was equipped with an axial thermowell housing one moveable thermocouple, and the temperature gradient along the catalyst bed was approximately 2~ The reaction conditions were, therefore, considered isothermal. A 6 ml volume of catalyst extrudes 3 to 4 mm in length were loaded into the reactor. Pre-heating and post-heating zones were filled with quartz particles of 20-48 mesh. The catalysts were reduced in situ in a hydrogen flow (350 ml/min, 0.69 MPa) at 400~ for 14 hours. After reduction, the reactor was cooled to 200~ and feed and hydrogen were introduced to the reaction system through a mixer coil and a pre-heater that was set at 150~ The reactions were conducted at 10.3 MPa pressure, 1.0 h 1 liquid space velocity, and with a hydrogen gas rate of 600 NL/L feed. The temperature was varied in the range 240~ to 360~ Once the density of the liquid products became stable (usually after 24 hours for each change of temperature), the liquid product was collected over a set mass balance time period.

2.4 Analytical methods Chemical compositions of feed and total liquid products were determined by lowresolution mass spectrometry using a modified Robinson method [11]. The method did not require prior separation of samples into saturate and aromatic fractions. The instrument used was a Hewlett Packard GC-MS equipped with an HP 5972 mass spectrometer, HP 7673GC/SFC injector and HP 5890 gas chromatograph, with Helium as the carrier gas. The column used for the analysis was a 30m x 0.250mm x 0.25~tm HP 5MS. The software used for calculating the chemical compositions of the total liquid products was supplied by PCMSPEC [12]. The boiling range of feed and total liquid products were given by ASTM method D2887. Density (g/mL, 15.6~ aniline point (~ and kinematic viscosity (cSt, mm2/s, 40~ were also analyzed by standard ASTM methods. Ignition quality of the total liquid product was estimated by two different methods: 1) a combustion test method using the Ignition Quality Tester (IQT TM) [13]; and 2) a correlation method ASTM D976-80 ~ where the cetane index (CO was calculated using density d (15.6~ and T50, the 50% volume recovery temperature of ASTM D86 distillation method, by the equation CI = 454.74 -1641.416 *d +774.74 *d2 -0.554 *T50 + 97.803 *(Log T50) 2 T50 was obtained by converting SD50, the 50% weight recovery temperature of the ASTM D2887 simulated distillation method, using the following equation: T50 = 0.77601 (SD50) 1"0395 [14]. The equipment used for measuring the surface areas and pore size distributions of the catalysts was a Micromeritics ASAP 2010C. The BET N2 adsorption, single point, and Horvath-Kawazoe methods were used to calculate the total surface area, pore volume and median pore diameter, respectively. For the ammonia temperature programmed desorption (TPD), samples (0.2 g) were placed in a quartz flow cell and attached to a gas handling and vacuum system (Advance Scientific Designs Inc.). The samples were heated to 500~ over 1 hour in flowing helium (60 ml/min). After 2 hours at 500~ the samples were cooled to 100~ and exposed to a stream of ammonia in helium (9.79 % NH3, Praxair) for 1 h. The samples were then purged with helium for 20 min before beginning. The samples were heated

546 at 10~ to 600~ in 25 ml/min He (STP), and held at 600~ for up to 20 minutes. A fraction of the gases exiting the sample cell was directed to a quadrapole mass spectrometer (UTI 100C) through a leak valve. The pressure in the mass spectrometer was maintained at 2.0 x 10-4 Pa and calibration of the mass 17 signal was performed at the beginning of each experiment using a standard gas mixture of NH3 in He. The contents ofPt, AI and Si in Pt/MM and Pt/Mor materials were measured by inductively coupled plasma-mass spectrometry (ICPMS) at the Alberta Research Council. The catalyst samples were first acid digested under microwave heating using a QWAVE-1000 microwave sample preparation system (Questron, Mercerville, NJ, USA), equipped with temperature and pressure regulation. The ICP-MS system used for analysis was a Perkin-Elmer Elan 5000 ICP quadrupole mass spectrometer (Thornhill, ON, Canada), equipped with a GemTip cross-flow nebulizer, Ryton spray chamber, plasma torch with a quartz injector, a Gilson four-channel peristaltic pump (Model Minpuls III) and a Gilson 212B auto-sampler. A detailed analytical procedure can be found elsewhere [ 15]. 3. RESULTS AND DISCUSSION 3.1 Characteristics of the catalyst materials Catalyst pore structure, surface area and acidity are factors that affect the performance of a catalyst. Table 1 summarizes the BET surface area, pore volume and pore diameter of the as-synthesized material, Pt/MM and Pt/Mor, as well as the prepared catalyst extrudates. The Pt contentand Si/A1 ratio of Pt/MM and Pt/Mor catalysts are also indicated in Table 1. The Ptloaded mesoporous molecular sieve (Pt/MM) had larger median aPOre diameter (26A), substantially higher surface area (978 mE/g) and pore volume (0.88cm/g) than P ~ o r , which had a pore diameter of 5.4A, surface area of 387 m2/g and pore volume of 0.24 cm3/g.

Table 1. Characterization of Pt-loaded mesoporous material and mordenite catalysts Catalyst samples

SaET

(m2 g-l) Mesorpore materials (MM) 1054 Ammonium ion-exchanged MM 1001 P~M, 978 P~M-alumina 479 CBV 21 A (Mor) 346 Pt/Mor 387 Pt/Mor-alumina 315

Pore volume Median pore (cm3 g-l) 1.185 0.950 0.881 0.467 0.196 0.238 0.328

diameter (A) 32.1 26.8 26.0 27.9 5.2 5.4 32.1

Pt

Si/A1

(wt%)

(mass)

0.97

16.2

0.82

10.7

Results of the acidity measurements (NH3-TPD) of ammonium ion-exchanged mesoporous material and CBV 21A, the original mordenite, are presented in Figure 1. Three maxima were observed on the curve of CBV 21A. Two peaks at low temperature around 160 and 200~ correspond to the presence of two different weak acid sites. The third peak at high

547 temperature, around 560~ is due to strong acid sites. The quantity and the strength of the acid sites on the mesoporous material are significantly lower than those of mordenite as observed in Figure 1. In fact, the high temperature desorption peak disappeared on the mesoporous material. The amount of ammonia desorbed at lower temperatures is also much less compared to mordenite. The higher acidity of mordenite could be due to its higher A1 content (Table 1). While the low temperature peaks are commonly assigned to weak Lewis and Br6nsted acid sites, the assignment of the high temperature peak (>550~ is still under discussion [1619]. In a study of coupling the NH3-stepwise temperature programmed desorption and FT-IR, Zhang et al. concluded that both Lewis and Br6nsted acid sites were responsible for the desorption that occurred around 180 to 250~ and that only Brfnsted acid sites caused the desorption at higher temperatures [ 16]. On the other hand, Kosslivk eta/. [19] suggested that peaks at temperatures above 550~ are mainly caused by ammonia desorbed from strong Lewis acid sites. Clearly, further studies are needed to solve these contradictions, which are beyond the scope of this study. 3.5

80.0 -I..............................................................................................

3.o

7o.o 6o.o

"~

i[

50.0 1 1.5

~ 40.0

~.0

~-

0.5

0.0 .

~0.0 )

. . . . . . . . , 100 200 300 400 500 600 isotherm . Temperature (~

Figurel. Ammonia TPD curves: a)mordenite CBV21 b) ammonium-ion exchanged mesoporous material.

3.2 Hydrogenation

and mild hydrocracking

0.0

f

i ....... ,.

0.0

10.0 20.0 30.0 40.0 ,,,~4~ .o C+ Conversion%

. . . . 50.0

Figure 2. Naphtha and diesel yields as function of the conversion of 343~ (o) Pt/MM-alumina, naphtha; (ra) Pt/MMalumina, diesel; (o) Pt/Mor-alumina, naphtha; (+)Pt/Mor-alumina, diesel.

activities

Hydrogenation and mild hydrocracking experiments were carried out between 300 and 360~ for Pt/MM-alumina and between 240 and 300~ for Pt/Mor-alumina. A lower temperature was used for the Pt/Mor-alumina because of its higher cracking activity at elevated temperatures. Table 2 presents the yields of naphtha, diesel and 343~ fractions, and physical properties of the feedstock and the products at various temperatures.. Figure 2 compares the diesel and naphtha yields versus the conversion of 343~ fraction for both catalysts. These results show that Pt/Mor-alumina had significantly higher undesirable cracking activity than Pt/MM-alumina. At the same temperature (300~ the conversion of

548 the 343~ fraction was 50.1% with a naphtha yield of 46.4wt% for Pt/Mor-alumina compared to 11.8% conversion of 343~ and 6.3wt% naphtha yield for Pt/MM-alumina. At the same conversion level, Pt/MM-alumina gave significantly higher diesel yield and lower naphtha yield. By interpolation and extrapolation of the conversion-yield curves, we calculated that at 30% 343~ conversion, Pt/MM-alumina would produce 65.4wt% diesel and 13.0wt% naphtha, whereas Pt/Mor-alumina would produce 53.1wt% diesel with 25.3wt% naphtha. In a mild hydrocracking process, an acceptable level of naphtha formation might be less than 20% with a conversion of 343~ of around 30% [19-20]. Therefore, Pt/MM-alumina is a better catalyst to convert heavy oil feedstock into the diesel fraction with a mild hydrocracking process. The selectivity of the mesoporous molecular sieve catalyst towards higher diesel yields relates to its weaker acidity and mesoporous structure, as compared to the strong acidity and the microporous structure of Pt/Mor-alumina catalyst. Table 2 showed that, over both catalysts, density and viscosity of the total liquid products decreased with reaction temperature. Since these properties are often correlated with paraffinic and aromatic contents in fuel, a decrease means a reduction in aromatic content; hence hydrogenation of aromatics to naphthenes and the formation of paraffins by ring opening was apparent under the conditions used in this study. Aniline point is another important parameter in characterization of petroleum fractions. It is a measure of aromatic content and molecular weight. In this study, aniline point first increased with temperature, reached a maximum, then decreased with temperature. These results indicate that product quality is improved by hydrogenation of aromatics at lower temperatures; however, as the temperature increases, the quality is deteriorated due to the formation of lighter molecules by overcracking. Table 2. Physical properties of feed and total liquid products (hydrogen pressure 10.3 MPa, LHSV 1.0, hydrogen flow 600NL/L feed)

wt% of fractions IBP-177 (naphtha) 177-343 (diesel) 343+ Conversion of 343~ Density (g mLl) H/C ratio Viscocity 40~ (cSt, mm2 s-1) Aniline point (~ Cetane IQT Cetane D613 CI(D976-80)

Feed 3.3 65.8 30.8

Reactiontemperatures (~ Pt/MM-alumina Pt/Mor-alumina 240 260 280 300 300 320 340 360 5.2 7.0 16.9 46.4 6.3 9.2 16.6 32.0 64.9 64.0 58.5 38.2 66.5 66.4 64.4 59.0 29.9 29.0 24.6 15.4 27.2 24.4 19.0 9.0 2.9 6.0 20.2 50.1 11.8 20.8 38.5 70.9

0.8628 0.8489 0.8428 0.8277 0.7975 1.85 1.94 1.92 1.94 2.01 5.49 4.66 4.06 2.89 1.62 75.3 51.4 50.4 50.3

0.8586 0.8541 0.8364 0.7920 1.89 1.89 1.95 1.81 5.10 4.73 3.12 1.33

81.6 52.3

81.4 52.3

79.1 50.7

74.4

76.8 49.1

77.8 49.1

76.7 46.7

68.3 38.5

53.6

54.8

57.4

54.4

51.2

52.4

56.6

46.5

549 Table 3. Chemical compositions (mass%) of feed and total liquid products (Hydrogen pressure 10.3 Mpa, LHSV 1.0, Hydrogen flow 600NL/Lfeed) Reaction temperatures (~ Pt/Mor-alumina Pt/MM-alumina Feed 300 320 340 360 240 260 280 300 75.0 79.7 86.6 89.5 Saturates 74.6 94.0 94.2 95.4 98.4 ll.4 11.8 1 3 . 2 14.0 Paraffins ll.0 1 2 . 3 13.0 14.7 19.3 23.7 25.5 28.6 31.8 Monocycloparaffins 23.1 30.6 31.4 34.5 41.1 26.3 27.9 29.3 27.5 Dicycloparaffins 26.3 34.3 33.5 33.5 30.9 13.9 14.6 15.7 16.3 Tricycloparaffins 14.3 16.4 16.1 12.3 6.9 25.0 21.0 14.9 12.3 Aromatics 25.4 6.1 5.9 4.7 1.7 14.6 10.8 4.9 2.8 Monoaromatics 16.4 0.6 0.3 0.3 0.4 4.4 2.6 0.2 0.0 Benzenes 5.4 0.0 0.0 0.0 0.0 5.2 4.0 2.1 1.0 Naphthenebenzenes 6.1 0.0 0.0 0.0 0.0 5.0 4.2 2.8 2.0 Dinaphthenebenzenes 5.0 0.6 0.3 0.3 0.4 5.4 5.1 4.4 3.7 Diaromatics 4.4 2.2 2.3 1.8 0.3 2.8 2.9 3.4 4.1 Triaromatic+ 3.0 2.6 2.5 2.0 0.2 1.4 17.5 41.4 51.4 Aromatics conversion % 76.1 76.9 81.6 93.3

Hydrocarbon groups

Four saturated hydrocarbon groups (paraffins [normal plus isoparaffins], monocycloparaffins, dicycloparaffins and tricycloparaffins) and five aromatic hydrocarbons (benzenes, naphthenebenzenes, dinaphthenebenzenes, diaromatics and triaromatics+) can be effectively identified by the GC-MS method used in this work. Table 3 presents detailed chemical compositional analyses of the feedstock and the liquid products at different reaction temperatures. The results indicate that Pt/MM-alumina has a higher hydrogenation activity than Pt/Mor-alumina. At the same reaction temperature (300~ the aromatics conversion was 76.1% for Pt/MM-alumina compared to 51.4% for Pt/Mor-alumina. The aromatic conversion increases with temperature and reaches 93.3% at 360~ for Pt/MM-alumina. Pt/MM-alumina also has a much higher ability to convert diaromatics and triaromatics+ compounds than Pt/Mor-alumina. The contents of diaromatics and triaromatic+ were reduced to near zero at 360~ over Pt/MM-alumina catalyst, whereas these values were almost constant over the Pt/Mor-alumina catalyst. Since Pt/MM-alumina has substantially higher surface area than Pt/Mor-alumina, for the same level of metal loading, we may expect a better metal dispersion in the former catalyst that would promote the hydrogenation reaction. Because of its high surface area, we could increase the metal loading on Pt/MM-alumina without causing agglomeration of the Pt atoms, making it, therefore, more active at lower temperatures. It is also worth to note that the density of Pt/MM-alumina is lower than Pt/Mor-alumina, due to its higher pore volume and, therefore, for the same space velocity, a smaller amount of Pt/MMalumina (4.09 g) than that of Pt/Mor-alumina (5.01 g) was loaded into the reactor. This advantage makes the mesoporous materials more attractive for industrial scale-up since less amount of metal and catalyst support are needed.

550 As for the saturated hydrocarbon groups, the results in Figure 3a (Pt/MM-alumina) and Figure 3b (Pt/Mor-alumina) show that paraffins and monocycloparaffins contents increase with reaction temperature for both catalysts. Monocycloparaffins can be produced by a number of different reactions pathways such as hydrogenation of monoaromatics or opening one ring of a dicycloparaffin. For the formation of two ring naphthenes, maximums were observed for both Pt/MM-alumina and Pt/Mor-alumina catalyst. These maximums indicate that both catalysts are able to convert molecules with sizes comparable to two-ring naphthenes into one-ring naphthenes. Over Pt/MM-alumina catalyst, the formation of the three-ring naphthenes first increased with temperature and reached a maximum at about 300 to 320~ At temperatures over 320~ the three-ring naphthenes decreased with temperature. However, three-ring naphthenes continuously increased with temperature over the Pt~or-alumina catalyst. The pore size of the Pt/MM is 26A, which is large enough for the diffusion of threering compounds [21] to the hydrogenation and hydrocracking sites located inside the mesoporous molecular sieve portion of the catalyst extrudates. The conversions of these bulky molecules are limited by the smaller pore diameter of Pt/Mor (5.4 A) that prevents the approach of bulky molecules to the active sites. 50.0 45.0 40.0 35.0 30.0 o 25.0 20.0

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

...........b ................

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

(ae~

20.0

I,,,,,

15.0

"~ 10.0 5.0 0.0 280

D_____----o-------

10.0

~5.0

5.0 0.0

300

320 340 Temperature (~

360

380

,

220

240

,

r

260 280 Temperature (~

300

320

Figure 3. Distribution of saturated products over Pt/MM-alumina (a) and Pt/Mor-alumina (b): (+) paraffins; (o) monocycloparaffins; (o) dicyloparaffins; (t~)tricycloparaffins. The ignition quality of a diesel fuel depends mainly on its chemical composition. In general, normal paraffins have the highest cetane numbers and, for the same carbon number, isoparaffins have lower cetane numbers, followed by moncycloparaffins, alkylbenzenes, polycycloparaffins and polyaromatics. Results in Table 2 show that the Pt/MM-alumina catalyst produces products with a higher ignition quality as predicted by both IQT TM and ASTM D976-80. Higher saturate and lower aromatic contents in the case of Pt/MM-alumina are believed to be the major contributors to the higher quality liquid product.

551 4. CONCLUSIONS A Pt-supported mesoporous molecular sieve (Pt/MM-alumina) and a Pt/supported mordenite (Pt/Mor-alumina) were used as catalysts for hydroprocessing a middle distillate derived from Canadian oil sands. The catalytic testing results suggest that Pt/MM-alumina has more suitable pore structure and surface acidity for hydrogenation and mild hydrocracking of middle distillate, so as to create a better quality diesel fuel than Pt/Mor-alumina. Compared to Pt/Mor-alumina, Pt/MM-alumina produced higher aromatic conversion and diesel yield, with minimum formation of naphtha under the same reaction conditions. Detailed chemical compositional analyses of the feedstock and the total liquid products at several reaction temperatures showed that both catalysts were able to hydrocrack two-ring naphthenes to smaller naphthenes and paraffins. However, only Pt/MM-alumina could effectively convert three-ring naphthenes and three-ring aromatics. The liquid products obtained over Pt/MMalumina catalyst had better ignition quality as determined by Ignition Quality Tester and ASTM D976-80.

5. ACKNOWLEDGEMENTS Partial funding for this work has been provided by the Canadian Program for Energy Research and Development (PERD), the Alberta Research Council and The Alberta Energy Research Institute. The authors gratefully acknowledge Mr. Robert Garez for operating the catalyst testing unit and NCUT analytical laboratory staff for determining the feed and products properties. The authors are grateful to Mr. Ken Mitchell and Mr. David Sporleder, Shell Canada Limited, Calgary Research Center, for the IQT test. We wish to thank Dr. R.A. Kydd, Department of Chemistry, University of Calgary, for the TPD measurements. The authors also wish to thank Syncrude Canada for kindly supplying of the LC-Finer LGO. Hong Yang is thankful to the Natural Sciences and Engineering Research Council of Canada for partial financial support. 6. R E F E R E N C E S

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. Schlenker, J. Am. Chem. Soc., 114 (1992) 10834. 2 C.K. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli and J.S. Beck, Nature, 359 (1992) 710. 3 P.T. Tanev and T.J. Pinnavaia, Science, 267 (1995) 865. 4 A. Tuel, Microporous Mesoporous Mater., 27 (1999) 151. 5 A. Sayari, M. Jaroniec and T.J. Pinnavaia, eds., Studies in Surface Science and Catalysis, Volume 129, Nanoporous Materials II, Elsevier, Amsterdam, 2000. 6 M.R. Apelian, T.F. Degnan, Jr., D.O. Marler and D. N. Mazzone, US Patent 5,227,353, 1993. 7 K.M. Reddy, B.L. Wei and C.S. Song, Catal. Today, 43 (1998) 261. 8 C.S. Song and K. M. Reddy, Appl. Catal. A: General, 176 (1999) 1.

552 9 M.J. Cheng, F. Kumata, T. Satio, T. Komatsu and T. Yashima, Appl. Catal. A: General, 183 (1999) 199. 10 C.J. Guo, C.W. Fairbridge and J-P. Charland, US Patent, 5,538,710, 1996. 11 C.J. Robinson, Anal. Chem. 43 (1971) 1425. 12 R.M. Teeter Software for calculation of hydrocarbon types, PCMASPEC, 1925 Cactus Court, #2, Walnut Creek, CA 94595-2505, USA., 1994. 13 L.N. Allard, G.D,Webster, T.W. Ryan, G. Baker, A. Beregszaszy, C.W. Fairbridge, A. Ecker and J. Rath, SAE Paper, 3591, 1999. 14 T.E. Daubert and R.E. Jr. Pulley, Chapter 1. Division of Refining. Technical Data Bookm Petroleum Refining, fifth edition. Washington DC: American Petroleum Institute, 1992. 15 S.L.Wu, Y.H. Zhao, X. B. Feng and A. Wittmeier, J. Anal. At. Spectrom 11(1996) 287. 16 W.M. Zhang, E.C. Burckle, P.G. Smirniotis, Microporous and Mesoporous Materials, 33(1999)173. 17 A.W. O'Donovan, C.T. O'Connor and K.R. Koch, Microporous Materials, 5 (1995) 185. 18 B.L. Meyers, T.H. Fleisch, G.J. Ray, J.T. Miller and J.B. Hall, J. Catalysis, 110 (1988) 82. 19 H. Kosslick, G. Lischke, B. Parlitz, W. Storek and R. Frichke, Appl. Catal. A, general, 184 (1999) 49. 20 E.P.Dai and C.N. Campbell, Mild hydrocracking of heavy oils with modified alumina based catalysts, P 127, Catalytic Hydroprocessing of petroleum and distillates, Eds, M.C. Oballa and S. S. Shih, Marcel Dekker, Inc. 1994. 21 E. Benazzi, L. Leite, N. Marchal-George, and H. Thouloat. 17th North American Catalysis Society Meeting, Toronto, 2001. Poster program P44.


553

C a r b o n - c a r b o n b o n d forming reactions catalyzed by meso- and m i c r o p o r o u s silicate-quaternary a m m o n i u m composite Yoshihiro Kubota, Yusuke Nishizaki, Hisanori Ikeya, Junko Nagaya and Yoshihiro Sugi Department of Chemistry, Faculty of Engineering, Gifu University, Gifu 501-1193, Japan The Knoevenagel condensation of carbonyl compounds with active methylene compounds catalyzed by as-synthesized, ordered porous silicate-quaternary ammonium composite materials gave corresponding c~,13-unsaturated esters in high yields under very mild liquid phase conditions. The activity was as high as that of aminopropyl-functionalized porous silicates. In the case of other carbon-carbon bond forming reactions such as Claisen-Schmidt reaction and Michael reaction, the activity of the composite materials was much higher than that of aminopropyl- functionalized silicates. 1. INTRODUCTION High-silica, ordered porous materials including micro- and mesoporous materials have found great utility as catalysts and sorption materials [1,2]. From the environmental points of view, silica-based solid catalysts could be utilized for recyclable, waste-minimum, non-hazardous, and energy-minimum reaction systems since they can be readily separated and recovered. Solid base catalysis is less investigated than acid catalysis [3]. Knoevenagel condensation, which is catalyzed by a weak base catalyst, is one of carbon-carbon bond formation reactions. Traditionally, amines and other homogeneous bases are known to be effective catalysts for this reaction [4]. As for heterogeneous catalysis, there are examples of amino group-immobilized silicas (amorphous [5] or ordered [6]). Modified ion-exchange resins can catalyze this reaction [7,8], and in these cases catalytic active sites are also immobilized amino groups (In both cases, 'push-pull' type mechanisms are proposed.). Besides these types of catalysts, various solids such as mesoporous silicates in alkali ion form or alkali-impregnated mesoporous silicates [9], zeolites in alkali ion form [ 10], sepiolite [ 11], and hydrotalcites [ 12], are used as catalysts, although less mild reaction conditions are necessary in most cases. In the case of porous silicates, structure-directing agent (SDA)-free materials have been considered as catalysts, which is logical to utilize their large surface area inside pores. On the other hand, no attention has been paid to the catalytic activity of as-synthesized organic-silicate composites. We report here the high catalytic activity of as-synthesized mesoporous silicate (MCM-41) and large-pore microporous silicate (beta; BEA) for the Knoevenagel condensation (Eq. l) [ 13]. Additionally, the same catalysts were used for some other carbon-carbon bond forming reactions such as Claisen-Schmidt reaction (Eq. 2) [14] and Michael reaction (Eq. 3) [ 15], and proved to be active as well. R1

R3

~=:=O + (

R2

1

R4

2

catalyst=

R1

R3

R2~(

R

3

4

+

H20

(1)

554 2. EXPERIMENTAL SECTION 2.1. Materials

(HDTMA+)-[Si]-MCM-41 denotes as-synthesized pure-silica MCM-41 synthesized using hexadecyltrimethylammonium (HDTMA § cation as SDA [ 16,17]. To synthesize this composite, the procedure (2)-(b) of Ref. 16 was exactly followed. On the basis of elemental analysis, the amount of HDTMA § cation occluded in the (HDTMA§ is 1.65 mmol/gcomposite. (TEA§ denotes as-synthesized aluminosilicate beta synthesized using tetraethylammonium (TEA § cation as SDA [18]. This was synthesized from a gel having composition 1.0SiO2-0.3TEAOH-0.6TEABr-0.02NaOH-0.0084A1203-0.22N(CH2CH2OH)315H20. In a typical synthesis procedure, 11.91 g of TEABr (90 mmol) was dissolved in 14.5 g of de-ionized water. 18.93 g (45 mmol) of 35wt%TEAOH solution (Aldrich) and 0.276 g of sodium aluminate (Nacalai, 42.8%A1203, 33.7%Na20) were added with stirring. The stirring was continued for 10 min, and 22.53 g of colloidal silica (Ludox HS40, 150 mmol) and 5.01 g (33 mmol) of triethanolamine (Wako) were added to the homogeneous mixture. The gel was further stirred for 3 h to make it completely homogeneous. The mixture was then transferred to Teflon-lined autoclave (125 ml) and heated statically in a convection oven at 150 ~ for 8 d. The product was recovered by filtration and washed with deionized water, and then dried at room temperature. (TMBp2§ denotes as-synthesized pure-silica beta synthesized using 4,4'.... 9 trimethylenebis(1-methyl-l-(2-methylbutyl)plpendmmm) ( T M B P2§) cation as S D A. TMBP2§ was synthesized by the quaternization of 4,4'-trimethylenebis(1-methyl piperidine) with (S)-(+)- 1-iodo-2-methylbutane in ethyl acetate under reflux for 18 h, followed by conversion into dihydroxide form with ion-exchange resin (DIAION | SA10A(OH), Mitsubishi Chemical Co.). The hydroth_ermal synthesis was carried out statically from a gel having composition SiO2- 0.3TMBP2§ at 150 ~ for 20 days. The product was recovered by filtration and washed with deionized water, and then dried at room temperature. (TEA§ is an as-synthesized, pure-silica beta synthesized by the method of Camblor et al., in which the amount of defect site is very low [ 19,20]. This was synthesized as follows: 6.82 g (16.2 mmol) of 35%TEAOH solution (Aldrich) was gently stirred in a Teflon vessel. Then 6.25 g (30.0 mmol) of tetraethylorthosilicate (Tokyo Chemical Industry) was added and the mixture was stirred at room temperature for 18 h allowing evaporation of ethanol. To the resulting clear solution, 0.59 g (16.2 mmol) of HF (55% aqueous solution, Stella Chemifa) was added. The gel became semi-solid after the addition of HF. Manual stirring with a Teflon rod was necessary to make the gel homogeneous. The final composition of the synthesis mixture was SiO2-0.54TEAOH-0.54HF-10.7H20. The gel was divided into three parts and each part was transferred to Teflon-lined stainless-steel autoclaves (23 ml each) and heated to 150~ with rotation (66 rpm) using a convection oven equipped with a rotator. After 5 d, the product was recovered by filtration and washed with deionized water, and then dried at room temperature. Part of (HDTMA+)-[Si]-MCM-41 and (TEA§ were calcined at 550~ in air to give [Si]-MCM-41 and [A1]-BEA, respectively. Aminopropyl-functionalized [Si]-MCM-41 (denoted AP-MCM-41), which was used for comparison, was also prepared [5,21]. In a typical procedure, [Si]-MCM-41 (2.0 g) vacuum dried at 250~ for 1 h was suspended in anhydrous toluene (30 ml). To this suspension, 0.494 g (2.75 mmol) of 3-aminopropyltrimethoxysilane was added and the mixture was stirred under reflux for 2 h. Toluene containing methanol (ca. 10 ml) was distilled off and toluene (10 ml) was added again; the reflux was continued for another 0.5 h. The product was recovered by filtration and washed with deionized water, and then dried at room temperature to give 2.438 g of white powder. The content of amino group was estimated 1.31 mequiv./g based on elemental analysis.

555 2.2. Measurements X-ray diffraction data were recorded on a Shimadzu XRD-6000 diffractometer using CuKtx radiation and ~, = 1.5404 A. Elemental analyses were performed using ICP (JICP-PS-1000 UV, Leeman Labs Inc.). The scanning electron microscopy (SEM) images were recorded on a Philips XL30 microscope. ~H and 13C NMR spectra were obtained on a JEOL t~-400 FT-NMR spectrometer. 27Si MAS NMR spectra were recorded on a Varian UNITY Inova 400 FT-NMR spectrometer. Nitrogen adsorption measurements were carried out on a BELSORP 28SA gas adsorption instrument. A Shimadzu DTG-50 thermogravimetric analyzer was used to carry out the thermogravimetric analysis (TGA) and differential thermal analysis (DTA). 2.3. Reaction procedures The Knoevenagel condensation was typically carried out as follows: to a solution of a carbonyl compound (1, 2.5 mmol) and an active methylene compound (2, 2.6 mmol) in benzene (2 ml), solid catalyst (200 mg) was added and stirred for 1-6 h. After filtration, the catalyst was washed thoroughly with benzene and recovered. Ethyl ~~-cyano-ig!-phenylacrylate (3: R~=Ph, R2=H, R3=CN, Ra=CO2Et) was isolated from the filtrate by column chromatography (hexane/ethyl acetate = 10/1 ). In a typical procedure of the Claisen-Schmidt reaction, solid catalyst (120 mg) was added to a solution of aryl aldehyde (4, 1.0 mmol) and excess ketone (5, 10-68 mmol) and stirred for 6 h. After filtration, the catalyst was washed thoroughly with benzene and recovered. The products 6 and 7 were isolated from the filtrate by column chromatography (hexane/ethyl acetate=l/I). The typical procedure of the Michael reaction was as follows: under nitrogen atmosphere, solid catalyst (100 mg) was added to a solution of chalcone (8, 1.25 mmol) and ethyl malonate (9, 1.4 mmol) in benzene (2 ml), and stirred for 6 h. After filtration, the catalyst was washed thoroughly with benzene and recovered. The product 10 was isolated from the filtrate by column chromatography (hexane/ethyl acetate=8/l). All products were confirmed by means of IH, 13C NMR spectroscopy and GC.

O O Ar"~H + " ~ R 4

catalyst OH O - Ar/J"'~R

5

0 ph./~.,~L.ph + 8

O + Ar/'~"~R

6

C02Et ( CO2Et 9

(2)

7 o

,~Ph catalyst = Ph CO2Et CO2Et

(3)

10

3. RESULTS AND DISCUSSION Results of the reaction of benzaldehyde (1, R ~=Ph, R2=H) with ethyl cyanoacetate (2, R 3=CN, Ra=CO2Et) using various catalysts are listed in Table 1. (HDTMA+)-[Si]-MCM-41 showed high catalytic activity and the reaction proceeded smoothly under very mild conditions to give desired product 3 in high yields (Entries 1, 2). The reactions catalyzed by (TEA+)-[AI]-BEA and 2+ . . . . (TMBP)-[SI]-BEA gave 3 . m moderate yields, respectwely (Entries 3, 4). On the other hand, calcined, SDA-free materials such as [Si]-MCM-41 and [AI]-BEA showed no catalytic activity even at elevated temperature (Entries 5, 6). HDTMA+Br- and TEA+Br-, which are raw materials

556

for the hydrothermal synthesis, did not exhibit high activity (Entries 7, 8). When TEA+OH -, which should be stronger base than halides, was used as catalyst, the yield of 3 was still relatively low (Entry 9). These results suggest that the high activity emerges only when the silicate and quaternary ammonium parts f o r m a composite. However, (TEA+F-)-[Si]-BEA had no activity despite the fact that this is a composite material (Entry 10). Table 1 Condensation of benzaldehydewith ethyl cyanoacetate using various catalysts a Entry

Catalyst

1 2 3 4 5 6 7 8 9 10

(HDTMA+)-[Si]-MCM-41 (HDTMA+)-[Si]-MCM-41 (TEA+)-[AI]-BEA f (TMBP2+)-[Si]-BEA [Si]-MCM-41 [AI]-BEAf HDTMA+Br" g TEA+Br g TEA+OH h (TEA+F')-[Si]-BEA

Temp. (~ 20 20 20 20 80 80 20 20 20 20

Time (h) 1 6 6 6 6 6 6 6 6 6

Yield b of 3 (%) 82 97, 94c, 80a, 60e 51 49 0 0 6 6 24 0

'7 The reactionwas carriedout as describedin the text. bIsolatedyields. cThe 2nd use of catalyst, oThe 3rd use of catalyst. ~The 4th use of catalyst. f SIO2/A!203=105" g0.30mmolof eachcatalystwas used. h0.96mmol.

Solid-state 29Si MAS NMR spectra of the representative as-synthesized materials are shown in Fig. I. The resonances corresponding to Si(3-OSi, 1-OH), i._e. Q3, are obvious in the spectra of (HDTMA+)-[Si]-MCM-41, (TEA+)-[AI]-BEA and (TMBP2+)-[Si]-BEA, whereas only little Si(3-OSi, l-OH) resonance can be seen in the spectrum of (TEA+F-)-[Si]-BEA, which is consistent with the reported results [16,19,22-24]. Therefore, it is suggested that the actual catalytic sites are basic (SiO)3SiO- moieties in the composite materials. Metal oxides, hydroxide ions and organic amines are absent in this reaction system. It seems that the SiO- moiety is an effective base in a non-polar medium with the assistance of quaternary ammonium cation. This is essentially different situation from the case that hydroxide or alkoxide could be generated from metal cation in aqueous or alcoholic media and function as a base. The basic function is located on the side of parent silicate framework unlike the case in which mobile hydroxide or alkoxide could take part in the reaction mechanism as a base. Nitrogen adsorption measurement of active (HDTMA+)-[Si]-MCM-41 did not give Type-IV isotherm and BET surface area was 14 mE/g (Fig. 2b), whereas the typical Type IV isotherm and a large BET surface area (1013 mE/g) were obtained from catalytically inactive [Si]-MCM-41 as shown in Fig. 2a. This indicates that the large surface area and complete porosity are not indispensable in this reaction system. The reaction should be taking place at around pore-mouth of the silicates, not deeply inside the pore. The efficient catalysis by MCM-4 l-based material may be due to the more exposed catalytic sites at pore-mouth as compared to zeolite-based materials.

557 osi I SiO--S,i--OSi [

osi

I

k

SiO--Si--OSi

~ t

o- -,,

'

osi

f;

I s,o-s,-os,

1000

t

?si

I/ s~o ~ ~-,

800

~i-os '

% " L___

OSi

OSi

I

SiO--Si--OSi

A

oJ-'..,.

B

(a)

'7,

E

B

600

0 "O

I

SiO--Si--OSi

~.. 0

'

B

SiO--Si--OSi

]~

~

OSi

t-

SiO--Si--OSi

0

200

3

o -40

-60

-80

-100

-120

-140

-160

B B

B

E 600 nm that can be assigned to the familiar d-d transition of Cu 2+ ions [26]. This broad band in the spectra does not show any structure, and we cannot speak about difference in Cu 2+ ion positions from these data, although, they should have some distribution between the available ion-exchanged sites [10-12]. But they are not resolved in the absorption spectroscopy results, and this resolution is worse in DRS measurements. The appearance of this long-wave band is similar for all mordenites (Fig. 1a-c), making evident that their various acidity [6] do not provide noticeable changes in the positions of Cu 2+ ion absorption band. The situation is changed principally for the reduced Cu-exchange mordenites. Under the small Si/A1 ratio we see no clear features in the DRS spectra that could be associated to the metallic copper for all temperatures of reduction. However, under Si/A1--15 and 103 the absorption feature is developed in the form of plasmon resonance band for metallic copper nanoparticles (see, e.g., [27-32]). This situation could be considered as a

564 0.8-

SilAl=7. 5 ]

~I

0.6

~ "S[ iiA'=lS 1 .....

!

,,.,...~:--~..:, 5 %. 9 ,~.~ ":""

o.43

.

2

400

~

C

o.e-

o.2

J~

9

Si/AI:103 I 6 .:-~

0.8

........... .t

J~ ,_ 0.4 0

0.0

b

0.3-~

u c

0

"

i

....

0.1 i . , ~ . , ~ . I o.=

eoo

0.0

4;o

0.0 I . . . . . . . . .

860 400 eoo 800 W a v e I e n g t h, nm

e~o

Figure 1. DRS spectra of blank Mot (1), CuMor (2)and reduced at temperatures 150, 250, 350 and 450 ~ (3, 4, 5, 6 respectively),

I

8~o

rather trivial fact, if its appearance does not change with varying the matrix properties. Fig. lb,c shows that the shape of the plasmon resonance changes essentially under growth of Si/AI: from a step-like feature to the pronounced maximum, and a weak feature appears in the range . . . . . . . . . . of 400-500 nm. The latter can be attributed to the

features in the bulk copper band structure and are often easily observable in different types of ultrafine copper (particles, thin films) [32-35]. They will not be considered here in detail. The different reduction temperatures result in almost same shape of spectra, and, hence, this factor contributes only in the amount of the copper reduced. This result indicates that the type of matrix rather than the reduction conditions determines the state of the reduced copper nanoparticles. The temperature factor looks here as the kinetic one, and can be treated as occurrence of some activation barrier for the reduction reaction since we see some threshold variation under comparison of the DRS spectra for 250 ~ an those for the higher temperatures (especially, for Si/AI=103). Such temperature dependence can be quite understandable if to take into account that Cu 2§ ions are bound in zeolite matrix while H2 molecules usually unreactive under low temperatures. In the case of the Cu-exchanged reduced mordenite under Si/AI=7.5 one can point the rise of a structureless absorption and the disappearance of the long-wave band k>600 nm; i.e. the copper reduction process occurs also, however the products are difficult to decode. Those can be: i) few-atomic clusters like the species known in the case of Ag-exchanged mordenitesof similar composition [22-24]; ii) Cu § ions in different position in the mordenite lattice [36]; iii) some form of copper oxide (nanoparticles, nonstoichiometric clusters, etc.) [37]. The experimental data in Figs. 2 and 3 for the reduced forms of Cu-exchanged erionite and clinoptilolite support the above conclusion on the dominant role of the zeolite matrix in the formation of copper reduced form, and the little contribution of the reduction temperature, however the reduction is noticeable already at 250 ~ That may be associated with the lower acidity of Na §,K-enomte § " " and Na+,K§ 2§ clinoptilolite as compared with the above H § mordenites. In the case of erionite and clinoptilolite we can see intermediate shapes between the steplike one and well-pronounced maximum. No significant change of shape and position of maximum is observed for all the set of temperatures used. Under intermediate reduction temperatures (e.g. 250 and 350 ~ the initial oxidized Cu 2§ form still is observed in the spectra. The curves, corresponding to the highest reduction temperature (450 ~ already do not contain the noticeable long-wave band. That evidences the complete reduction of Cu(II)

565

1.0 0.3 -4

.--'"" "~".

..'"~, "~

4

5

.... 4

0.8

.-..,, ';

..3;"

\. "" > " " ' ~ : : . - : ....

3

.....

,

:i

er ~

..Q 0.2

o

0.6

3. 0 t*

0.1

~

/

I|

0.0

.

i

.

400

9

.

Wavelength,

1

800

600

nm

l

0.0004-

~ =2.0 o

a

0.0002

-4 o.oooo

400

600

"

800

0.0006

I ~;o = 4 . 0

: o

0.0oo4

t

b

0.0002

0.0000

400

60O Wavelength,

0.2

/

0.0

]

.

,

400

.

.

Wavelength,

Figure 2. DRS spectra of blank Eri (1), CuEri (2) and samples reduced at temperatures 150, 250, 350 and 450 ~ (3, 4, 5, 6 respectively).

o

O.4

/tI

800

nm

Figure 4. Calculated spectra of Cu particles in the media with ~0=2 (a) with size (from bottom to the top) 1, 2, 3, 5, 10 and 50 nm and with ~0=4 (b) with size (from bottom to the top) 1, 2, 5 and 10 nm.

600

800

nm

Figure 3. DRS spectra of blank Cli (1), CuCli (2) and samples reduced at temperatures 150, 250, 350 and 450 ~ (3, 4, 5, 6 respectively). under these conditions and the dominating influence of zeolite properties on the final state of reduced copper resulting in variation of the band position and the shape of this band. In order to simulate these observations we used the Mie theory for the simple case of individual spherical particles with size in the range of R

Nanoporous Materials III

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High Performance Structures And Materials III

Organosilicon Chemistry III: From Molecules to Materials

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Host-Guest-Systems Based on Nanoporous Crystals

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