Recent Advances and New Horizons in Zeolite Science and Technology (Studies in Surface Science and Catalysis, Volume 102)

Studies in Surface Science and Catalysis 102 RECENT ADVANCES AND NEW HORIZONS IN ZEOLITE SCIENCE AND TECHNOLOGY

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Studies in Surface Science and Catalysis Advisory

Editors:

B. D e l m o n

a n d J.T, Y a t e s

V o l , 102

RECENT ADVANCES AND NEW HORIZONS IN ZEOLITE SCIENCE AND TECHNOLOGY Editors H. Chon Department of Chemistry, KAIST, Yusung-ku,Taejon, 305-701 Korea

S.I.Woo Department of Chemical Engineering, KAIST, Taejon, 305-701 Korea S.-E, Park Industrial Catalysis Research Laboratory, KRICT,RO. Box 107, Taejon, 305-606 Korea

1996 ELSEVIER Amsterdam

- - L a u s a n n e - - N e w Y o r k m O x f o r d --- S h a n n o n - - T o k y o

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

ISBN 0-444-82499-5 91996 Elsevier Science B.V. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior written permission of the publisher, Elsevier Science B.V., Copyright & Permissions Department, P.O. Box 521, 1000 AM Amsterdam, The Netherlands. Special regulations for readers in the U.S.A.- This publication has been registered with the Copyright Clearance Center Inc. (CCC), 222 Rosewood Drive, Danvers, MA 01923. Information can be obtained from the CCC about conditions under which photocopies of parts of this publication may be made in the U.S.A. All other copyright questions, including photocopying outside of the U.S.A., should be referred to the copyright owner, Elsevier Science B.V., unless otherwise specified. No responsibility is assumed by the publisher for any injury and/or damage to persons or property as a matter of products liability, negligence or otherwise, or from any use or operation of any methods, products, instructions or ideas contained in the material herein. This book is printed on acid-free paper. Printed in The Netherlands

Preface Ever increasing interest and continuous developments in the zeolite science and technology have been reflected in the overwhelming response from all of the world to the 1 lth International Zeolite Conference. This book was conceived as a handbook for the 11th IZC Pre-conference summer school on zeolites, held in 1996 at Taejon, Korea. Three-day school on "Introduction to Zeolite Science and Practice" preceeding the 8th and 9th Internaional Zeollite Conference helped to improve the interaction between newcomers and experienced scientists in the field.

At the 10th IZC

Summer School, the lectures were given under the theme, "Advanced Zeolite Science and Applications" at an advanced level rather than introductory courses. Extending the concept of the 10th IZC summer school, the 11th IZC Summer School has also been organized to help those who have already actively worked on zeolite science and technology to be exposed to the latest new developments and the new horizons of zeolite science and technology for the 21 st century. The content of this book entitled, "Recent Advances and New Horizons in Zeolite Scinence and Technology" intended to give an extensive review and analysis of the important new findings of last 10 years on the synthesis, characterization and applications of zeolite materials as well as the prediction of new R&D directions for the next decade. We would like to express our appreciation to the authors who have written excellent manuscripts of their lectures in such a limited time. We sincerely hope this summer school has contributed to the advancement of the zeolite science and technology. Hakze Chon, Seong Ihl Woo, and Sang-Eon Park KAIST/KRICT, Taejon, Korea, April 1996

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vii List of contributors

T. Bein

Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA H. van Bekkum Waterman Institute of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands M.E. Dry Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa M.J. den Exter Waterman Institute of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands S. Feast

Department of Chemical Engineering, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands J. van de Graaf Waterman Institute of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands R.F. Howe Deptmant of Physical Chemistry, University of New South Wales, P.O. Box 1, Sydney 20, Australia J.C. Jansen Waterman Institute of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands S. Kaliaguine P

9

9

Departement de Genie Chimique, Universite Laval, Ste-Foy, Quebec, G IK 7P4, Canada

viii

F. Kapteijn Waterman Institute of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

S. Lee Department of Chemistry, University of Michigan, Ann Arbor, MI, 48109-1055, USA J.A. Lercher Department of Chemical Technology, Catalytic Processes and Materials, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands J.M. Newsam Molecular Simulations inc., 9685 Scranton Road, San Diego, CA 92121, USA C.T. O'Connor Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Rondebosch 7700, South Africa A. Sayari

Department of Chemical Engineering and CERPIC, Universite Laval, Ste-Foy, Quebec, G1 K 7P4, Canada Karl Serf Chemistry Deparment, University of Hawaii, 2545 The Mall, Honolulu, HI 968222275, USA E. Van Steen Catalysis Research Unit, Department of Chemical Engineering, University of Cape Town, Rondebosch, 7700, South Africa M. St~cker SINTEF Oslo, Department of Hydrocarbon Process Chemistry, P.O. Box 124 Blindern, N-0314 Oslo, Norway

ix S.L. Suib Department of Chemistry, Department of Chemical Engineering, and Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3060, USA D. Venkataraman Department of Chemistry, Cornell University, Ithaca, NY 14853, USA

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xi

Contents

Preface List of contributors

vii

Chapter 1. Periodic Mesoporous Materials- Synthesis, Characterization and Potential Applications A. Sayari Introduction Synthesis and characterization of M41S and related materials Catalytic Applications of M41S and related materials Other potential applications Concluding remarks References

1 2 27 33 33 37

Chapter 2. Synthesis, Characterization, and Catalysis with Microporous Ferrierites, Octahedral Molecular Sieves, and Layered Materials S.L. Suib Overview n-Butene isomerization with boron substituted zeolites and

47

ferrierites Synthesis of octahedral molecular sieves and octahedral

52

layered materials Characterization of octahedral molecular sieves and octahedral

55

layered materials Catalytic activity of octahedral molecular sieves and octahedral

65

layered systems References

67 69

xii

Chapter 3. Organic Zeolites ? Stephen Lee and D. Venkataraman Introduction Dianin's compound Helical tubulates of 2,6-Dimethylbicyclo[3.3.1]nonaneexo-2,exo-6-diol [Ag(1,3,5-tris(3-ethynylbenzonitrile)benzene)CF3SO3]-2CsH 6 [Ag(1,3,5-tris(4-ethynylbenzonitrile)benzene)C F3SO3]-2C6H8 Further directions References and notes

75 76 78 79 81 84 92

Chapter 4. Spectroscopic Characterization of Zeolites R.F. Howe Introduction EPR spectroscopy Infrared spectroscopy Raman spectroscopy UV-Visible spectroscopy X-ray absorption spectroscopy Mass spectrometry of zeolites References

97 98 106 123 127 130 134 136

Chapter 5. Characterization of Zeolitic Materials by Solid-State NMR -State of the Art M. St~cker Introduction High resolution solid state NMR spectroscopy- experimental techniques including 2D NMR Recent highlights about the framework characterization Pore architecture investigated by NMR In-situ NMR studies with zeolitic materials

141

Diffusion of adsorbed molecules monitored by NMR Acidity of zeolitic materials Concluding remarks

179 181 184

143 157 172 176

xiii

References

185

Chapter 6. Application of Surface Science Techniques in the Field of Zeolitic Materials S. Kaliaguine Introduction Photoelectron spectroscopy (XPS) and other surface analysis XPS of zeolites Acidity in zeolites Basicity in zeolites Active centers in zeolitic oxidation catalysts Conclusion References

191 192 204 209 217 221 225 227

Chapter 7. Computational Approaches in Zeolite Structural Chemistry J.M. Newsam Roles of simulation Structural characterization Occluded or sorbed molecules and clusters Meso- and macro-structure Dynamical behavior Sorptive behavior Intrazeolite chemistry Some five year challenge areas Conclusion References

231 233 247 250 251 253 255 255 259 260

Chapter 8. What Can Be in the Channels and Cavities of Zeolites ? Karl Serf Zeolites and their frameworks The contents of neutral zeolite frameworks

267

The contents of charged zeolite frameworks

270

Molecular and ionic sieving

274

Sorption of atoms and molecules

276

269

xiv

A single substance may react within a zeolite

279

Two different substances may react within a zeolite Encapsulation

285 286

An empty zeolite is a crystalline solvent

289

An appeal for care in the preparation of samples for study

289

Final comments

291

References

291

Chapter 9. Conjugated and Conducting Nanostructures in Zeolites T. Bein Introduction

295

Polyacetylene and derivatives in zeolites

304

Heteroaromatic conducting polymers in zeolites

305 314

Carbon-based conducting materials in nanometer channels Conclusions References

317 318

Chapter 10. New Catalytic Applications of Zeolites for Petrochemicals C.T. O'Connor, E. Van Steen, and M.E. Dry Introduction Catalytic cracking Alkylation Aromatization of alkanes/alkenes

323 325 336 344

Skeletal isomerization of 1-butene Alkene oligomerization

353

Isomerization of long-chain alkanes

353

References

355

349

Chapter 11. Synthesis of Intermediates and Fine Chemicals using Molecular Sieve Catalysts S. Feast and J.A. Lercher Introduction

363

Chemical functionalities of molecular sieves

365

Physical aspects of molecular sieve catalysis

xv

for chemical synthesis Conclusion and outlook References

396 400 404

Chapter 12. Zeolite-based Membranes, Preparation, Performance and Prospects M.J. den Exter, J.C. Jansen, J. van de Graaf, F. Kapteijn, and H. van Bekkum Introduction Dynamics of zeolite pores and consequences for adsorption

413

and permeation Small pore zeolites Medium pore zeolites Large pore zeolite-based membranes (12 membered

417 421 428

oxygen ring) Permeation through silicalite-1 membranes: examples

432

and modelling Zeolite membranes in catalytic conversions

433 446

Prospects References

450 451

Keyword index

455

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H. Chon, S.I. Woo and S.-E. Park (Editor,s) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All fights reserved.

Periodic mesoporous materials: synthesis, characterization and potential applications Abdelhamid SAYARI Department of Chemical Engineering and CERPIC, Universit~ Laval, Ste-Foy, Qc, Canada G1K 7P4

1. INTRODUCTION The design of materials with precise pore structure is of tremendous technological importance [ 1,2]. In addition to zeolites and molecular sieves, subject of our meeting, there are many other materials where the shape, connectivity and size distribution of their pores determine their usefulness. Such materials include pillared clays [3], carbon molecular sieves (CMS) [4], sol-gel materials [5], porous polymers [6,7] and other organic solids [8-10], hollow tubes of polypeptides [11,12], carbon [13,14], polymers [15], metals [16] and inorganic oxides [17,18] as well as imprinted organic [19] and inorganic [19,20] materials. For some applications such as air separation by CMS, changes in the optimum effective micropore size by only 0.3 ]k dramatically deteriorate the separating ability of the CMS [21]. As far as crystalline porous materials are concerned, zeolitic molecular sieves (zeotypes) are of particular interest not only because of their important industrial applications as adsorbents [22], ion-exchangers [23] and catalysts [24,25] but also because of theft potential applications as hosts for a variety of technologically advanced materials such as semiconductor clusters, molecular wires with tailored electronic and optical properties, enzyme mimicking organic complexes, etc. [26-30]. Zeotypes were historically divided into four groups: small, medium, large and ultra-large pore molecular sieves, with pore openings comprised of 8, 10, 12 and more than 12 tetrahedral (T) atoms, respectively. Figure 1 shows the pore size of typical zeotype materials. Before the discovery of M41S in 1991, the only ultra-large pore zeotypes were metallophosphates. These are A1PO4-8 [31], VPI-5 [32], Cloverite [33] and JDF-20 [34] with 14, 18, 20 and 20-membered rings, respectively. All microporous zeotypes have been reviewed extensively [35,36]. This paper is concerned with the particular class of molecular sieves having periodic mesoporous structure with pore sizes in the range of 2 to 10 nm. They are comprised of the M41S mesoporous molecular sieves and solids with related structures. In the first part, the preparation methods and characterization techniques will be reviewed and discussed. Silicatebased materials and non-silicate materials will be dealt with separately. In the second part of this review particular emphasis will be put on potential applications reported in both the patent and the open literature. Early progress in this field has been presented in the previous Summer School by Casci [37]. Potential catalytic applications of M41S were also reviewed r~.e~.ntlv rqRl_

Figure 1. Pore size of molecular sieves. 2.

SYNTHESIS AND CHARACTERIZATION OF M41S AND RELATED MATERIALS

The term "periodic" will be used interchangeably with the word "crystalline", because as will be shown in a later section, these materials have actually amorphous walls despite their long-range periodic structure. Such materials are prepared very much like zeolites by hydrothermal treatment of a gel, typically at 80 to 120 ~ for 24 to 48 h. The main difference between the preparation of zeolites and M41S materials is that in the former case a single cationic (e.g. alkali, organic ammonium or phosphonium cations) or molecular (e.g. amines) species is used as "template" (or structure directing agent [39-41]), whereas in the latter case an array of surfactant molecules plays the role of template.

Strictly speaking, a template is a structure (usually organic) around which a material (often inorganic) nucleates and grows in a "skin-tight" fashion, so that upon the removal of the templating structure, its geometric and electronic characteristics are replicated in the (inorganic) material. In this sense, except for the possible example of ZSM-18 [39,42], there is hardly any examples of true templating effect in zeolite synthesis. Because of geometrical constraints and the small size of species used as templates, the host-guest interactions are rather weak so that the shape of the pore most often does not reflect that of the template. This among other problems makes the "rational design" of zeolite synthesis particularly difficult [39,43]. In the early 90's Mobil researchers had the brilliant idea to use supramolecular surfactant templates for the preparation of porous silicates. This led to the discovery of the so-called M41S mesostructured molecular sieves and to a tighter control on the morphology and pore size of the desired material through simple manipulations of the synthesis conditions. The preparation of mesoporous silicates requires three ingredients in the appropriate amounts: a solvent, a source of silica and a surfactant. In addition, other reagents such as acids, bases, salts, expander molecules and cosolvents may be used. The nature and relative amount of all these ingredients may vary greatly, thus offering a high degree of flexibility for the design and preparation of periodic mesostructured materials. The Mobil group identified three main silicate phases. They consist of a hexagonal phase referred to as MCM-41, a cubic phase (space group: Ia3d) known as MCM-48, and a non stable lameUar phase (MCM-50) which can be stabilized by post-synthesis treatment in the presence of tetraethyl orthosilicate (TEOS). Among other facts, the striking similarity between these structures and those of known liquid crystal phases led Mobil researchers and others to propose the so-called liquidcrystal templating (LCT) mechanisms in order to rationalize the formation of mesostructured materials.

2.1 Synthesis Mechanisms of Surfactant/Silieate Mesophases The most studied crystalline nanoporous silicates are those prepared in aqueous medium in the presence of long chain alkyltrimethylammonium hydroxide or halide under basic conditions. It is therefore practical to deal with this case ftrst, then consider other pH conditions, surfactants and synthesis approaches. At this stage it is essential to learn about surfactant aggregation in water. A surfactant is an amphiphile molecule with a hydrophilic head group and a hydrophobic tail. Figure 2 shows a schematic phase diagram of cetyltrimethylammonium bromide (C16T/VIABI")ill water [44]. Depending on the temperature and concentration, surfactants tend to self-organize into aggregates with different shapes. At very low concentration, the surfactant molecules are randomly dispersed in solution. As the concentration reaches a critical level referred to as CMC1, spherical micelles are formed. The outer surface of the micelle is comprised of the hydrophilic heads of surfactant molecules, while the tails of these molecules are directed toward the center of the micelle. There exists a second critical concentration CMC2 corresponding to the further aggregation of spherical into cylindrical or rod-like miceUes. As the concentration increases further, a higher level of aggregation into liquid crystals takes place. There are three main liquid crystalline phases with hexagonal (H1), cubic (V 1) and lameUar (L) structures (Figure 3). The H 1 phase is the result of a hexagonal packing of cylindrical miceUes, while the L phase corresponds to the formation of surfactant bilayers.

The cubic phase may be regarded as a bicontinuous structure as suggested by Luzzati et al. [45]. To describe the tendency of an amphiphile to aggregate into a particular morphology, Israelachvili et al. [46,47] introduced the packing parameter g = v/aolc with 1c < lmax = 1.54 + 1.26 n (A) and v = 27.4 + 26.9 n (A3). Here 1c, v and n stand for the critical length, the volume and the carbon number of the hydrophobic chain, and ao for the optimum surface area of the headgroup. The packing parameter determines whether the amphiphile will form spherical micelles (g < 1/3), cylindrical micelles (1/3 < g < 1/2), vesicles or bilayers (1/2 < g < 1) or inverted structures (g > 1). The LCT mechanism as postulated by Mobil scientists [48,49] is represented in Figure 4. Along Pathway (1), the template is supposed to self-organize into a liquid-crystal phase such as H 1 before being encapsulated by inorganic species which then condense into rigid walls. This mechanism is consistent with recent data obtained in the presence of high concentrations of surfactant [50]. In the second proposed pathway, the inorganic species participate in the ordering process of the surfactant-inorganic mesophase and influence its morphology. The LCT mechanism was ftrst proposed based on (i) the close similarity between the morphology of the surfactant-inorganic mesophases and liquid crystals, and (ii) the dependence of the pore size on the surfactant chain length and on the amount of auxiliary organics such as 1,3,5 trimethylbenzene (TMB) [48,49]. Further work by the same group [5153] supports the occurrence of Pathway (2). Two main reasons were evoked, (i) M41S materials are often prepared in the presence of surfactant concentrations well below that required for the formation of a liquid-crystal phase, and (ii) the hexagonal, culSic and lamellar M4IS structures may be formed by changing only the silica concentration indicating that no preexisting liquid-crystal phase is required. Several other workers reached similar conclusions, but often with slight variations [5459]. Based on XRD, thermogravimetric analysis, 29Si and in situ 14N NMR, Davis et aL [54,55] concluded that rod-like miceUes coated with 2 to 3 monolayers of silica form before they spontaneously self-organize into a hexagonal phase, with further silica condensation during calcination. Obviously, this mechanism may be operative only if rod-like micelles are formed in the synthesis medium. To this end two requirements should be met: (i) the surfactant carbon chain should be long enough so that the formation of rod-like micelles is possible, and (ii) the surfactant concentration should be equal to at least CMC2, the minimum concentration for the generation of such micelles. While agreeing that a liquid-crystal templating mechanism initiated by silica occurs, Cheng et al. [57,58] claimed that in the presence of TEOS as the silica source synthesis takes place only if the surfactant concentration is equal to CMC1 or higher. Based on XRD and 14N NMR data, Steel et al. [59] proposed a modified LCT mechanism. They postulated that the silicate source first dissolves into the reaction medium and promotes the formation of a surfactant hexagonal mesophase. Then the silicate forms parallel layers in between the rows of cylindrical micelles. The hexagonal organic-inorganic mesophase is then formed by puckering of the silicate layers. If the concentration of silicate is high, the layers will be thicker and the puckering does not take place, thus leading to a lamellar phase. One of the most important contributions toward the elucidation of synthesis pathways of M41S and related materials was made by Stucky and his colleagues [60-66] at both Santa Barbara University (USA) and Johannes Gutenberg-Universi~t, Mainz (Germany). They fully

Figure 4. Mobil group proposed formation pathways of M41S [49].

documented the fact that the presence of preorganized liquid crystal structures or even rodlike micelles prior to adding the inorganic precursor is not required as a "static" template for nucleation and growth of the inorganic phase. This conclusion stems from several experimental observations [61]: (i) hexagonal MCM-41 phases can be prepared using C16TMABr, C16TMAC1 or C16TMAOH at concentrations much below CMC2, (ii) hexagonal MCM-41 phases can also be made in the presence of short chain surfactants such as C12TMAOH and C12TMAC1 which have not been reported to form rod-like micelles in water, and (iii) MCM-41 and MCM-48 can be prepared at temperatures above 70 ~ where rod-like miceUes are unstable. Moreover, Firouzi et al. [64] argued that addition of inorganic species to a micellar assembly of organic molecules often leads to reorganization into new morphologies depending on the electrostatic and steric interactions between organic and inorganic species. The cooperative templating mechanism proposed by Stucky and coworkers is shown schematically in Figure 5. Prior to silicate addition, the surfactant is in a dynamic equilibrium between spherical or cylindrical micelles and single molecules. Upon addition of a silica source, the predominantly multicharged silicate species ion-exchange with the OHor Br- anions to form organic-inorganic ion pairs accompanied by dissociation of the organic miceUes and aggregation of the ion pairs into a new mesophase. The multidentate interaction controls the number of surfactant molecules that can bind to a given inorganic species and determines the interface packing density and ultimately the biphase morphology. The last step is the polymerization and condensation of the inorganic species. In addition to the arguments mentioned earlier in favor of the cooperative organization of inorganic-surfactant mesophases, further support was obtained by investigating a system with very low surfactant concentration where the effects of self-assembly are decoupled from the kinetics of silica condensation. Using in situ small-angle neutron scattering, Stucky et al. [64,66] found that a 1% CTAB aqueous solution exhibits an isotropic miceUar distribution. Addition of a silicate solution leads to the formation of an inorganic-organic hexagonal array. Similar conclusions were drawn from freeze-fracture electron microscopy studies [64,67]. Moreover, in the absence of polymerization, in situ 21-1NMR of labelled surfactant showed that this mesophase referred to as a silicatropic liquid crystal (SLC) exhibits a behavior very similar to a lyotropic liquid crystal (LLC) including a reversible first-order transformation between the lamellar and hexagonal phases. Huo et al. [60,61] also demonstrated that such a cooperative organization process is not limited to ion pairs formed between cationic surfactants (S§ and anionic inorganic species (I-) but can be easily generalized to include other pathways. In addition to the S+I- route described above, three other pathways were considered. Pathway S-l+ involves the cooperative organization of a cationic inorganic solution species and an anionic surfactant. The other two routes correspond to the assembly of surfactant and inorganic ions with similar charges, mediated by small ions with the opposite charge. These pathways are referred to as S+X-I + (X- = CI-, Br-) and S-M~- (M+ = Na § K+). Typical syntheses to illustrate each of these pathways have been reported [60,61]. Fyfe and Fu [68] proceeded in two steps. They ftrst obtained a precipitate by reacting the octamer Si8028t~ with C16TMABr. The degree of condensation was then adjusted by titrating the oxoanions by acid vapor treatment during which the following sequence of structural transformations took place: layered precipitate ---) cubic ---) lamellar ---) hexagonal

mesophase. These findings strongly indicate that before extensive polymerization of the silicate species, the organic-inorganic mesophase exhibits the behavior of a liquid-crystal.

Figure 5. Cooperative templating mechanism [64]. In another important development, Pinnavaia et al. [69-72] used non ionic surfactants such as primary amines and polyethylene oxide to prepare silicates with cylindrical nanopores

referred to as HMS and MSU-n, respectively. Contrary to the case of charged surfactants where the electrostatic interactions between inorganic and surfactant ions play a key role in determining the morphology of the mesophase, in the presence of neutral surfactants hydrogen bonding becomes a predominant factor. Figure 6 represents the S~ ~ pathway in the case of primary amine surfactants. The authors postulated that the Si(OC2Hs)4.x(OH)x species formed by hydrolysis of TEOS participate in H-bonding interactions with the lone pairs of surfactant amine headgroups. This new organic-inorganic complex may be considered as an amphiphile with a very bulky headgroup which increases the likelihood for the formation of rod-like micelles. These micelles self-organize into a hexagonal packing followed by condensation of silanol groups and silica walls formation. k,-./ S~

i~

CnH2n+INH 2

+

Si(OEt)4.x(OH)x

B .# qHSN"

/ ;""e

$i o H

H~H

/?%.

!t

H

o OEt

#

"'/

--

Figure 6. S~ ~ templating mechanism [70]. Parallel to the discovery of M41S materials, Yanagisawa et al. [73-75] from Waseda University and Inagaki et al. [76-79] from Toyota prepared nanoporous silicates designated as FSM- 16 using a completely different strategy. They treated a layered kanemite polysilicate in a 0.1 M aqueous solution of C16TMABr at 70 ~ followed by filtration, drying and calcination. Further work on this method was carried out by Vartuli et al. [51] and Chen et al. [80]. As shown schematically in Figure 7A, Inagaki et al. [76] proposed a two-step mechanism for the formation of the FSM-16 material. First, the surfactant cation exchanges with Na§ and penetrates in between the silicate sheets. Subsequently, the flexible silicate layers wind around the exchanged C16TMA+ cations. Further condensation of silanol groups between adjacent silicate layers leads to a highly ordered honeycomb structure. The intermediate formation of a lameUar silica-surfactant mesophase has been observed recently by in situ XRD [75]. As represented in Figure 7B, Chen et al. [80] proposed that once

C16TMA + exchanges with Na + forming a bilayer intercalated silicate, local rearrangement of silicate species accompanied by the formation of rod-like miceUes and silanol groups condensation leads to the nanoporous structure. Table 1 Suffactant used for the preparation of mesostructured materials Cationic CnH2n+I(CH3)3N+X -

X- = CI-, Br-, OH-

n = 8-22

CnH2n+l (C2H5)3N+X -

n = 12, 14, 16, 18

(CnH2n+I)2(CH3)2N+X -

n = 10-18

C16H33N(CH3)2CnH2n+ 1

n = 1-12

Gemini

+ + [CnH2n+ 1(CH3)2N-CsH2s-N(CH3) 2CmH2m+ 1]Br2

n = 16, s = 2-12, m = 1-16

Anionic C14H29COOH, C 17H35COOH C12H25OPO3H 2, C14H29OPO3K n = 12, 14, 16, 18

CnH2n+IOSO3Na C16H33SO3H, C12H25C6H4SO3Na Neutral CnH2n+INH2

n = 10-16

CnH2n+IN(CH3)2

n = 10-16

Cll_15(EO)n

C = alkyl

n = 9, 12, 15, 20, 30

CnPh(EO) m

Ph = phenyl

n = 8 or 12, m = 8, 10, 18

(PEO) 13(PPO)30(PEO) 13

10

Figure 7. Formation methcanisms of FSM-16 according to (A) Inagaki et al. [76], and (B) Chen et al. [80]. More recently Fukushima et al. [79] carried out the ion exchange at pH = 12 followed by pH adjustment to 8.5. Elemental analysis and 29Si MAS NMR showed that at pH = 12, single sheets folded, but without condensation between them (only Q3 Si species were present in the solidphase). In addition significant amounts of silica dissolved. Upon adjustment of the pH to 8.5, an increase of Q4/(Q3 + Q4) to 0.6 was observed suggesting that dissolved SiO2 condense into walls 2 to 3 layers thick. Galameau et al. [81] used a similar approach to design porous clay heterostructures. They intercalated layered fluorohectorite by C16TMA+ cations followed by treatment in a solution of neutral amine and TEOS, then drying and calcination. In brief, the authors proposed that the interlayer galleries of the intercalated clay are further swollen by the amine followed by insertion of TEOS, formation of rod-like micelles and silica polymerization. 2.2 Synthesis Conditions of M41S and Related Mesoporous Silicates Periodic nanoporous silicates have been prepared in a wide variety of conditions. Different sources of molecular, and non molecular silica have been used. This includes TEOS, TMOS, fumed, colloidal and precipitated silicas. Depending on the synthesis conditions, particularly on the nature of the silica source, crystallization may take place in seconds at subambient temperatures [82], or at room temperature [60,61,69,72,83]. However, in most cases the crystallization temperature was set in the 80 - 120 ~ range. Liu et al. [84,85] found that the use of small amounts of colloidal particles (silica or titania) promotes the formation of ordered structures by providing nucleation seeds. The pH conditions varied from extremely acidic [60,61], to neutral [69,72] to very basic [48,49]. Ryoo and Kim [86]

11 showed that alternating hydrothermal treatments (373 K, 24 h) and pH adjustments using acetic acid (pH = 11) leads to MCM-41 products with improved crystallinity and in higher yields due to an equilibrium shift. All the surfactants listed in Table 1 have been used for the preparation of nanostructured materials, mainly for silicates. The most used surfactants are alkyltrimethylammonium hydroxides or halides. Depending on the synthesis conditions, they give rise to hexagonal, cubic or lamellar structures. Because of their high packing factors, two-tailed surfactants favor the formation of lameUar structures. On the contrary, surfactants with bulky headgroups promote the formation of mesophases with high surface curvature. For example, the use of surfactants such as alkyltriethylammonium and cetylethylpiperidinium under acidic conditions afforded a cubic phase named SBA-1 [87] with space group Pm~n [60,61]. Using gemini surfactants CnH2n+IN+(CH3)2(CH2)sN+(CH3)3 with very large headgroups, under either basic or acidic conditions, Huo et al. [87] discovered a new mesophase (SBA-2) that has three-dimensional hexagonal (P63/mmc) symmetry and apparently no lyotropic surfactant or lipid liquid mesophase counterpart. The structure of this material is derived from hexagonal close packing of globular silicate-surfactant arrays. The cage-structured mesoporous network of SBA-2 may offer advantages over the unidimensional channels of MCM-41, particularly in host-guest applications. Sayari et al. [88,89] used a series of alkyl cetyldimethylammonuim bromides, (C16H33)(CnH2n+1)(CH3)2N+Br - (n = 1 to 12), under basic conditions. For n < 9, an interesting odd-even effect was observed. Hexagonal and lamellar phases were obtained in the presence of surfactants with odd and even n values, respectively. For n >__9, the surfactant behaved as a long chain two-tailed molecule and gave only lamellar phases. These findings were related to the organization of the occluded surfactant as evidenced by 13C NMR. Two other mesoporous silicates, HMS and MSU-n, were prepared at room temperature in the presence of neutral amines, and polyethylene oxide surfactants, respectively. They have parallel cylindrical channels, but are not identical to MCM-41 [69,70,72]. The main structural difference is that the pore system of HMS and MSU-n silicates is much less ordered than that of MCM-41. The use of cosolvents may have different effects as illustrated in the following examples. Anderson et al. [82] found that the use of methanol or formamide as cosolvents enabled them to control the crystallization temperature. Moreover, the use of methanol as cosolvent and formamide as solvent allowed the fine tuning of the pore size to within 1/~. Huo et aL [87] found that the use of tert-amyl alcohol as a polar cosolvent in the presence of surfactants with bulky headgroups such as C16H33N+(C2H5)3 or CnH2n+IN+(CH3)2(CH2)sN+(CH3)3 increases the volume of the hydrophobic core and therefore the packing factor, leading to a hexagonal (MCM-41) instead of a cubic (Pm~ n) or a three dimensional hexagonal (P63/mmc) phase. However, the most known effect of co-solvents is that of non polar trimethylbenzene (TMB) which dissolves into the hydrophobic part of the micelle and acts as a swelling agent [48,49,61]. One of the most important aspects of the liquid-crystal templating strategy for the manufacture of mesoporous inorganic materials is the ability to adjust the pore size (or the interlayer distance) between ca. 2 and 10 nm. This may be achieved by using surfactants with different chain lengths (Figure 8). This method used for all three M41S phases has been extended to SBA-2 and more recently to FSM silicates [90]. Very fine pore size tuning may be achieved by using variable amounts of methanol as cosolvent [82]. For pore sizes above

12

ca. 40 A, expander molecules such as TMB may be used (Figure 9). These molecules increase the size of the surfactant miceUes by dissolving into their hydrophobic region. Another less known method [91] consists of preparing a MCM-41 silicate at relatively low temperature, e.g. 70 ~ then heating it in its mother liquor at high temperature, e.g. 150 ~ (Figure 10). This restructuring occurs via silica dissolution, transport and redeposition. Moreover, the pore size may be narrowed by post-synthesis silylation [49].

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Carbon number, n

Figure 8. Pore size of MCM-41 as a function of carbon number of RTMABr. n. ref. [49]" A: unpublished work.

Figure 9. Variation of dlo0 spacing of MCM-41 as a function of TMA/SiO 2 ratio [61].

t..r.3 c~ o. (n o o v-. "O

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6

Aging time, day

2.3 Characterization of Mesoporous Silicates Mesoporous silicate molecular sieves were characterized by an arsenal of techniques including XRD, SEM, TEM, adsorption measurements, 29Si NMR, IR, Raman and XANES. XRD patterns of all nanoporous phases are dominated by low angle peaks. Figure 11 shows typical patterns for MCM-41, MCM-48, MCM-50 and SBA-2 phases. HMS, MSU-n and some "MCM-41" exhibit only the 100 peak either because of too small scattering domain sizes [69] or because of poorly ordered pore system [55,72,80].

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Figure 11. Typical XRD patterns. (A) MCM-41, 03) MCM-48, (C) Lamellar, (D) SBA-2. TEM combined with electron diffraction has been a key technique for proper characterization of nanoporous materials [48-55,58,60,61,80,92-94] and elucidation of their structures [87,95]. An interesting application of TEM was reported by Chen et aL [80] who used the Fresnel method to determine the thickness of the FSM-16 walls. According to this method the imaged wall thickness decreases with decreasing focus and reaches a minimum which is the real thickness at zero defocus. Adsorption measurements indicate that mesoporous silicates have high porosity (0.7 -1.2 cm3/g) and very high surface area often exceeding 1000 m2/g. As shown in Figure 12 (upper part) N 2 adsorption-desorption isotherms of MCM-41 [49,96,97-99], MCM-48 [49] and FSM [90] materials are of type IV in the IUPAC classification and have a typical shape. They are usually reversible and exhibit a sharp step at P/Po in the range 0.25 to 0.5 depending on the average pore size. This step increase in N 2 adsorption corresponds to capillary condensation within uniform mesopores. The sharpness of this step reflects the uniformity of the pore size whereas its hight the pore volume. Llewellyn et al. [ 100] reported that hysteresis in the Ar

14 and N 2 adsorption-desorption isotherms does occur for materials with pore sizes exceeding 2 and 4 rim, respectively. The authors concluded that these pore sizes represent the limit between "secondary micropores" and "mesopores" for each adsorptive. Branton et al. [96,101] also found that 0 2 and Ar adsorption isotherms at 77 K exhibit significant hysteresis loops. Rathousky et al. [102] studied the adsorption of cyclopentane on MCM-41 at 243-333 K and found a hysteresis loop when the adsorption is carried out below the pore critical temperature. Nitrogen adsorption isotherms were also calculated using molecular simulation [103] and density functional theory [104, 105]. Figure 12 (lower part) shows that the SBA-2 N 2 adsorption isotherm exhibit a H2 hysteresis loop consistent with bottle-shaped pores [87]. EFFECTIVE PORE DIAMETER (nm) 1.0

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Figure 12. N 2 adsorption-desorption isotherm and pore size distribution for MCM-41 (upper) and SBA-2 (lower) silicates. Water adsorption on MCM-41 was studied by 1H NMR [94,106-108] and by gravimetry and FTIR [ 109]. Schmidt et al. [ 106,108] measured the freezing point of adsorbed water by

15 1H NMR and used it to determine the pore size distribution of mesoporous silicates. They also derived the self-diffusion coefficient of water in MCM-41 and MCM-48 [94,107]. LleweUyn et al. [109] found that MCM-41 exhibits a type V water adsorption isotherm indicating an initial repulsive character followed by capillary condensation at higher pressures. The wall thickness of hexagonally packed silicates (MCM-41 and HMS) was determined as the difference between the repeat distance ao between pore centers (measured from TEMs or calculated from XRD data with the formula ao = 2d10o/~/3) and the Horvath-Kawazoe [110] pore diameter. In most studies it was found to be in the range of 9 to 12 A. Molecular dynamics simulations show that there is an excellent agreement between experimental XRD patterns and those calculated based on _> 10/~ wall of amorphous silica models [ 111]. This rather thin wall combined with very high porosity is at the origin of the relatively low mechanical stability of these materials [112]. Tanev and Pinnavaia [69] reported that the walls of HMS silicates are always thicker than those of MCM-41 and connected this observation to differences in formation mechanisms. Coustel et al. [ 113] found that the wall thickness of MCM-41 silicates may be varied from as low as 5/~ to 17 A, mainly through control of the OH- / S i t 2 ratio, thus the silicate solubility. Magic angle spinning (MAS) 29Si NMR was also used extensively for the characterization of M41S silicates [49,114]. One of the earliest observations is that 29Si NMR spectra are broad and closely resemble those of amorphous silica, a strong indication that the material waUs are actually amorphous with a wide range of O-Si-O angles. This conclusion was further supported by FTIR [115], Raman [115] and SiK XANES [116] data. Deconvolution of 29Si NMR spectra gave essentially two peaks at -100 and -110 ppm corresponding to the so-called Q3 [Si(OSi)3OH] and Q4 [Si(OSi)4] environments. A small peak (Q2) may appear at ca. -90 ppm. The Q3/Q4 ratio reflects the degree of polymerization of silica and the density of OH groups. Vartuli et al. [53] found that in as-synthesized M41S silicates prepared by the electrostatic S+I- pathway, Q3 remains almost constant at ca. 50%, consistent with the N/Si ratio being practically constant in all three phases. On the contrary, Steel et al. [114] found that Q3/Q4 is the same (59%) for as-synthesized MCM-41 and MCM48, but much higher (96%) for the lamellar phase suggesting that the latter phase has thinner walls. Likewise, Hut et al. [60] found Q,/Q4 ratios of 55 and 120% for hexagonal and lamellar silicates, respectively regardless of whether they are prepared by the electrostatic S+I- or S+X-I+ pathway. 29Si NMR also shows that for thermally stable materials the Q3/Q4 ratio decreases upon calcination [87,114,115] as additional cross linking takes place. Sayari e t al. [96] found that Q4/(Q2 + Q3) increases upon incorporation of boron in the silicate framework indicating that the substitution occurs through interaction with hydroxyl groups.

2.4 Synthesis and Characterization of Modified Mesoporous Silicates 2.4.1. Mesoporous Aluminosilicates The synthesis of aluminum containing MCM-41 molecular sieves was reported in both the patent [117-119] and the open [48,49,92,113,115,120-129] literature. There are also some reports on A1-HMS [130] and A1-FSM- 16 [77]. MCM-41 aluminosilicates were prepared under hydrothermal conditions, typically at 70-150 ~ during 1 to 10 days. Various sources of silica and alumina were used. Janicke et al. [121] reported that aluminum isopropoxide is a much better precursor than Catapal B. In the presence of isopropoxide, they prepared samples with Si/A1 ratios down to 16, with aluminum being entirely in tetrahedral positions.

16 Luan et al. [ 125] tested a number of aluminum precursors and found that aluminum sulfate leads to total incorporation of A1 in tetrahedral sites up to very high loadings (Si/A1 = 2.5). On the contrary, other researchers prepared M-rich samples using sodium aluminate [122124,128]. These discrepancies may be explained, at least partly, on the basis that the preparation methods used by various groups were different. Fu et al. [131] prepared MCM-41 aluminosilicates using the two-step approach described earlier [68]. They first prepared well defined aluminosilicate polyanions AlxSis.x(OH)xO2o. 4 (0 < x < 4) oligomers to be used as precursors. These precursors were then precipitated with C16TMABr and treated with water vapor at 110 ~ for 3 days. This method not only affords MCM-41 aluminosilicates with variable Si/A1 ratios down to the lowest possible ratio of 1/1, but it offers additional flexibility in the design of new materials by using suitable building blocks. Incorporation of aluminum in MCM-41 silicates brings about a dramatic decrease in the intensity of the XRD [124,125] and a significant broadening of the pore distribution [120]. As mentioned before, the MCM-41 silicate walls are amorphous with a wide range of T-O-T bond angles. The presence of A1 in such a highly distorted environment in addition to the limited flexibility of the O-A1-O angle compared to O-Si-O may generate a more defective structure with a broader pore size distribution. 27A1 MAS NMR was widely used to distinguish between "framework" and extraframework aluminum. Several workers found a linear relationship between the absolute intensity of the 27A1 NMR peak and the A1 content in as-synthesized samples [ 120,124,125,132]. However, it was also reported that A1 containing MCM-41 silicates exhibit a strong tendency to dealumination during the removal of surfactant by calcination. Dealumination is mostly due to hydrolysis of framework aluminum by steam generated during the combustion of the surfactant. Corma et al. [120] found that direct calcination of Al-rich MCM-41 at 540 ~ in air gives rise to material with significant amounts of extraframework aluminum, smaller pores and lower BrCnsted acid site density compared to samples treated first in N 2 and then in air at the same temperature. The two-step activation procedure has two advantages: (i) the local temperature is lower, and (ii) much less water vapor is formed. Luan et al. [125,133] found that dealumination depends on the nature of counter cations. Calcination of as-synthesized samples does not generate extraframework A1 species, but it brings about a broadening of the 53 ppm 27A1 NMR peak due to decreased symmetry. However, proton exchanged samples were found to be prone to dealumination, presumably because the small H § cation cannot satisfy the framework charge balance efficiently. A series of A1 containing MCM-41 samples with Si/A1 ratios in the range 62 to 2.5 were synthesized in our laboratory using sodium aluminate [132]. As show in Table 2, NMR data indicated that for both as-synthesized and for calcined samples at least 90-95% of all A1 in all samples was located in tetrahedral positions. Table 2 also shows that the BET surface area decreases sharply as the Si/A1 ratio drops below 10. This is consistent with TEM observation made by Kloetstra et al. [126] who found that in samples with low bulk Si/A1 ratios, most of the aluminum was part of a separate dense phase displaying a tetrahedral environment. This indicates that the conventional one-pulse NMR technique does not discriminate between tetrahedral A1 in the MCM-41 framework and tetrahedral A1 in the so-called dense phase [126], and should be combined with adsorption and TEM measurements for proper characterization of the state of aluminum.

17 The thermal and hydrothermal stability of a sample with Si/A1 = 26 was investigated in our laboratory by heating batches for 3 h in dry air, or in pure water vapor in the temperature range 550 to 850 ~ In dry air, the extent of dealumination increases with the treatment temperature but the crystallinity and the pore structure are preserved. However, under hydrothermal conditions, extensive dealumination takes place even at 550 ~ and the structure collapses above 650 ~ Another investigation of the thermal and hydrothermal stability of ion-exchanged A1-MCM-41 (Si/A1 = 39) was carried out by Ryoo et al. [127] using dry or water vapor saturated 0 2 for 2 h at different temperatures. Under both sets of conditions, the stability was found to depend on the nature of the counter cations in the following order: y 3 + _ Ca2+ > Na+ _ as-prepared A1-MCM-41 > pure silica MCM-41. Table 2 Thermal and hydrothermal stability of AI-MCM-41 (Si/A1 = 26) (a) Temperature (b)

Thermal treatment

Hydrothermal treatment

oC

(Sa~/g)

pore vol. (c) (cc/g)

AI(T) (a) (%/

S (ma~/g)

pore vol. (e) (cc/g)

550 650 750 850

1465 1230 1252 1320

1.42 1.16 1.13 1.03

92 61 49 45

974 1005 231 170

0.78 0.75 0.30 0.22

(a) From ref. (132); (b) 3 h in dry air for thermal treatment and in water vapor for hydrothermal treatment; (c) pore volume according to the MP-method; (d) tetrahedral aluminum (%). As for the acidity of A1 containing MCM-41, ammonia TPD data indicate that it is comparable to that of amorphous silica-alumina, and much lower than the acidity of zeolites such as USY or H-mordenite [115,120,134]. This is consistent with Raman, FTIR and 29Si NMR data which indicate that despite their long range order, M41S mesoporous silicates and aluminosilicates exhibit essentially amorphous walls [48,49,115].

2.4.2. Mesoporous Titanosilieates Titanium modified mesoporous silicates were first mentioned in the patent literature [135]. In 1994, several research groups reported independently on the synthesis and characterization of Ti-MCM-41 [136-138] and Ti-HMS [139]. Corma et aL [136,140] prepared Ti-MCM-41 under hydrothermal conditions (413 K, 28 h) using fumed Aerosil silica, TMAOH (25 %), C16TMAOH/Br and Ti ethoxide. Similar preparations were also used by other workers [137,138,141]. Pinnavaia et al. [139,142] prepared Ti-HMS at room temperature using long chain primary amines instead of charged surfactants. More recently, several other groups reported on Ti-HMS [143] and Ti-MCM-41 [144,145]. In a detailed investigation Gontier and Tuel [143] studied the effect of several parameters on the synthesis of Ti-HMS at room temperature. This includes the following: (i) presence of isopropyl alcohol, (ii) synthesis time, (iii) length of the surfactant carbon chain, (iv) nature of the Ti source, (v) surfactant/SiO 2 ratio. Maschmeyer et al. [145] grafted Ti species on the pore walls

18 of MCM-41 silicate using titanocene solutions in chloroform. They obtained materials with very high density of accessible Ti sites. Bugshaw et aL [72] prepared Ti-MSU-1 in the presence of a non-ionic polyethylene oxide template. Kim et al. [146] used non aqueous formamide medium to control the Ti alkoxide hydrolysis. They were able to prepare mesoporous titanosilicate samples with very high Ti content. The XRD patterns of Ti containing samples consist of a main peak at 2 0 < 3 ~ corresponding to the 100 diffraction, sometimes accompanied by much weaker 110, 200 and 210 reflections in the 2 0 range of 4 to 7 ~ HMS based catalysts exhibit only the 100 diffraction peak because of excessive broadening of the hk0 reflections due to too small scattering domain sizes [69,71,139,142] or more likely to the presence of a poorly ordered pore system [55]. Indeed, TEM studies indicated that the pore structure of Ti-HMS is much less ordered than that of Ti-MCM-41 [147]. In addition, SEM [143] and TEM [147] show that Ti-HMS is comprised of spherical particles with 0.2-0.3 lam in diameter. N 2 adsorption isotherms obtained by Pinnavaia et al. [71,139,142] showed that in addition to the frameworkconfined mesoporosity due to the presence of parallel channels, HMS materials display a weU developed textural mesoporosity. However, other workers found that the N 2 adsorptiondesorption isotherm of Ti-HMS is reversible [143,147]. The pore distribution was broader for HMS as compared to MCM-41 materials. The Ti-MSU-1 material also exhibited only the 100 diffraction peak due to the occurrence of disordered, hexagonal-like packing channels [72]. These samples displayed reversible N 2 adsorption-desorption isotherms with no hysteresis. Several authors used diffuse reflectance UV-Visible spectroscopy to probe the local environment of Ti sites [136-138,140,141,143,147]. Sayad et al. [137,141,147] found that both Ti-HMS and Ti-MCM-41 exhibit an absorption band at 220 nm with no indication of a band at 330 nm characteristic of anatase [148]. There was however a weak shoulder at 270 nm, particularly for Ti-rich samples. Corma et al. [ 140] also found a band at 205-220 nm and a shoulder at 270 rim. The absence of anatase was also conf'mned by the absence of the characteristic 140 cm "1Raman band [138,149]. The band at 220 nm was attributed to isolated framework Ti species in interaction with water molecules [150]. The 270 nm band was assigned to partially condensed hexacoordinated Ti species belonging to a silicon rich amorphous phase [155]. FTIR spectra of Ti modified crystalline mesoporous materials (Figure 13) display a band at ca. 960 cm 1 [136,140,141,144,147]. A similar band was also found in FTIR spectra of Timodified zeolites such as TS-1 [156,157], Ti-13 [151-153], Ti-ZSM-48 [158,159] and others. It was attributed to the stretching mode of at least five different entities [ 160]: (i) SiO4 units bonded to a titanium ion (Si-O...Ti) [148], (ii) titanyl (Ti=O) groups [161], (iii) Si-O in SiO...H groups [162], (iv) T i - O in TiO4 tetrahedra [163], and (v) Si-O in SiOH...(OH)Ti defectives sites [164]. However, the presence of the 960 cm -1 band in FTIR spectra of TiMCM-41 and Ti-HMS samples may not be regarded as a firm proof for Ti incorporation because Ti-free silicates also exhibit a similar band. Nevertheless, the intensity ratio of the 960 to the 800 cm "1 band (due to the symmetric stretching vibrations of SiO4 [41]) was significantly higher for Ti containing samples than for pure silicates. The relative enhancement of the 960 cm "1 band may be attributed to Ti incorporation [133]. X-ray photoelectron spectroscopy (XPS) has also been used to gain insight into the local environment of Ti sites. Interestingly, tetrahedral Ti(IV) has a (2P3/2) binding energy (BE) more than 1 eV below that of octahedral Ti(IV) [151,165-169]. Using a BE of 103.3 eV for Si(2p) as internal reference, the BE of Ti(2P3/2) in both Ti-MCM-41 and Ti-HMS was found

19 to be 459.9 +_ 0.1 eV (Figure 14) compared to 458.6 eV for Ti in octahedral coordination as in TiO 2 [141,147]. Several groups [144,145,170,171] used X-ray absorption techniques to characterize Ti sites in Ti-MCM-41. Their data were also consistent with the occurrence of Ti species in tetrahedrally symmetrical environment.

459.9

B

&, 9

1200

t

| |

1000

I

I !

800

I

I I

"

600

Wavenumber, c m

i

400 41 ;4

"1

Figure 13. Infrared spectra of Ti-HMS samples [147]. A, B, C, D, E correspond to samples with Si/Ti ratios of .o, 145, 76, 39 and 22, respectively.

459

464

469

Binding energy, eV Figure 14. XPS spectrum of Ti2p in Ti-HMS, Si/Ti = 76 [141,147].

2.4.3. Mesoporous Vanadosilicates Periodic mesoporous vanadium modified silicates (V-MCM-41 and V-HMS) were synthesized, characterized and tested in our laboratory [93,141,172-174]. V-MCM-41 samples were prepared hydrothermally at 373 K using Cab-O-Sil fumed silica, vanadyl sulfate and dodecyltrimethylammonium bromide [172]. The preparation of V-HMS was carded out at room temperature in the presence of dodecylamine as template [173,174]. As in the case of V-free Ti containing samples, the XRD patterns of V-MCM-41 consisted of a main 100 peak and weak 110 and 200 peaks, while those of V-HMS exhibited only the 100 diffraction peak. N 2 BET adsorption isotherms with theft characteristic sharp step were fully reversible with no hysteresis loops [175]. TEM showed that V-MCM-41 particles have a highly ordered porous structure with some typical hexagonal crystals [38,93,173,176]. However, as other HMS materials, V-HMS consisted of globular particles of 0.1 to 0.4 lain in diameter with disordered channel packing [173,175]. Diffuse reflectance UV-visible spectra of V-HMS exhibited two bands in the range of 373-385 nm and 252-272 nm. Similar bands were observed at 384 and 265 nm for V-

20 silicalite-1 [ 177]. The charge transfer (CT) band at ca. 380 nm was attributed to V+5 with a short V=O bond and three longer V-O bonds, possibly in interaction with water vapor. The ca. 265 nm band was assigned to V +5 in a tetrahedral environment [175,177]. 51V NMR is one of the most suited techniques for characterizing V sites [141,172-174]. Typical 51V NMR V-M CM-41-60 V-HM S-60 spectra are displayed in Figure 15. Table 3 shows relevant NMR parameters for our samples and some closely related systems. As seen in Fig. 15, dehydrated samples exhibited signals with significant anisotropy of the chemical shift, the largest anisotropy being for V-HMS samples (Table 3). The isotropic chemical shift for dehydrated samples was in the range of-650 to -720 ppm. b Adsorption of water brought about a ~k downfield shift of the isotropic component by 100 to 150 ppm and affected the anisotropy significantly. In C the case of V-HMS, the hydration led to a large anisotropy increase, while it had -50O -1000 o -5oo - ;oo no effect on the parameter of asymmetry. The parameters of anisotropy for V-HMS Chemical shift, ppm were very close to those reported for (SiO)3V=O species in amorphous Figure 15. 51VNMR spectra of V-MCM-41-60 V/SiO 2 [178,179], indicating that V and V-HMS-60 (ref. 173). a: static NMR species in V-HMS are similar to those of dehydrated samples; b: static NMR of in V/SiO 2. Scheme 1 is a simplified hydrated samples; c: MAS NMR of hydrated representation of V site in V-HMS and samples, the rotation speed was 3.3 kHz its interaction with water. This proposal for V-MCM-41-60 and 11.2 kHz for is in full agreement with the UV-visible V-HMS-60, (*) side bands. data already discussed. In contrast, exposure of V-MCM-41 to water vapor led to almost complete disappearance of the anisotropy. Two nearly isotropic lines at-508 and -527 ppm could be resolved. The small value of anisotropy reflects the high degree of symmetry of the V species. The "symmetrization" of the V environment upon exposure to water vapor is believed to be due to interaction with OH groups as shown in Scheme 2. The active participation of hydroxyl groups and water in the stabilization of vanadium in the molecular sieve "framework" was further substantiated by 29Si MAS NMR data [173].

21 Contrary to V modified zeolites [180-184], as-synthesized mesoporous vanadosilicates did not exhibit any EPR signals at 77 K [173,174]. This was attributed to the presence of V 4+ in highly symmetrical lattice positions. Because of the electronic degeneracy and the associated very short relaxation times, lower temperatures maybe needed for the observation of such species. The presence of V +5 or clustered V+4 in the as-synthesized materials was excluded based on NMR data [173]. Recently, Tuel and Gontier [185,186] also reported on the preparation and characterization of V-HMS. At variance with our data, their as-synthesized samples exhibited an anisotropic hyperfine EPR signal at room temperature. The main conclusion of this study is that Si--O--V bonds form during calcination leading to isolated and tetrahedraUy coordinated vanadium centers which interact readily with water vapor. Morey et al. [187] prepared a series of V/MCM-48 samples by impregnation using dry hexane solutions of O=V(OiPr) 3. Their 51V NMR data for hydrated and dehydrated samples were almost identical to those reported for V-HMS [173,174], suggesting that in the absence of water, vanadium sites consist of pseudotetrahedral (SiO)3V=O grafted on the channel walls. Interactions of these species with water were also investigated by UV-visible.

2.4.4. Other Mesoporous Metallosilicates Boron was successfully incorporated in the "framework" of MCM-41 silicate in our laboratory [93,99,188]. In addition to XRD, N 2 adsorption and TEM, samples were thoroughly characterized by liB and 29Si MAS NMR. In as-synthesized materials, all boron was found to be in the four-coordinated state up to a B/Si ratio of 16%. 29Si MAS NMR data indicated that the attachment of the boron to the lattice takes place through interaction with structural hydroxyl groups. Careful calcination under dry conditions transformed a significant fraction of the boron into trigonally coordinated species with complete retention of boron in the framework up to B/Si = 8%. Calcination of samples with higher B loading generated extraframework species. Exposure of calcined samples to moist air at room temperature led to partial deboronation by hydrolysis. Boron-containing silicates were also studied briefly by other workers [130,189]. Several authors [129,130,190] reported on Fe modified MCM-41 and HMS. Yuan et al. [190] interpreted their FTIR and EPR data on the basis of Fe incorporation in the silicate "framework". Tuel and Gontier [130] found that the EPR spectrum of Fe-HMS is comprised of three signals one of which corresponds to iron-substituted framework sites. A number of literature reports dealt with Ga-modified mesoporous silicate molecular sieves [129,130,191,192]. Cheng et al. [191] synthesized Ga-MCM-41 with Si/Ga from 10 to 120. The quality of the samples was very sensitive to the pH of the gel mixture. No extraframework gallium was detected by 71Ga NMR in the as-synthesized samples. However, during calcination, the 4-coordinated gallium was partially expelled from the structure for samples with Si/Ga < 20. Galloaluminosilicate [192] as well as Ti [193] ans manganese [ 194] modified mesoporous silicates were also synthesized and characterized. 2.5 Synthesis and Characterization of Non-Silica Mesostructured Materials Huo et al. [60,61,195] first extended the LCT strategy to the synthesis of non-silica-based mesostructures, mainly metal oxides. Both positively and negatively charged surfactants and inorganic species were used. It was found that a suitable metal oxide should have the following characteristics: (i) depending on the formation mechanism, the ability to form polyanions or polycations allowing multidentate binding to the surfactant, (ii) the polyanions

22 Table 3 Parameters of 51V NMR spectra of V-MCM-41 and V-HMS (a'b) Samples

Si/V

Anisotropy A~5, ppm _ 10

Parameter of asymmetry, rl

~istat,

-650 -527 -508 -660 -527 -720 -600 -715 -590

V/MCM-41-D V/MCM-41-H

60 60

-320 --50

0.3 - 0

V/MCM-41-D V/MCM-41-H V/HMS-D V/HMS-H V/HMS-D V/HMS-H

145 145 60 60 124 124

-315 --50 -475 -640 -480 -640

0.3 - 0 0.15 0.15 0.15 0.15

Ref. (178) [(C6H11)7(Si7012)VO] 2 OV(OSiPh3) 3 WSiO2-D WSiO2-H

-398 -422 -487 -620

-

ppm +_. 10

0.05 0.05 0.05 0.13

~iiMAs, ppm _ 3 -665 -527

-530 -708 -580 -711 -576 -714 -736 -710 -609

(a) From ref. (173);

la33-ai I---iaee-ai I, I~ilX-~ii[, 15i=1/3(~i11+~i22+~33),Afi=~i33-~Si,rl=l~i22-~ill I/la33-ai (c) D: dehydrated samples, H: hydrated samples. (b)

O

O

Io

.2o

Io

7S2\oCe..~

c7S2 ",,or 0

I~'~H

Scheme 1

t o,,~ ~,o-

,-..

p.-"-~ o

v / ~,,,.. o~o.--,~

H20

. . . . . .

"--

Scheme 2

X.._ _o:r "o -[ ".~r ' o - " s , /~

I;

23 or polycations should be able to condense into rigid walls, (iii) a charge density matching between the surfactant and the metal oxide is necessary to control the formation of a particular phase. As shown in Table 4 most of the metal oxides have a strong tendency to form lamellar structures, except Sb, W, Pb. The formation pathway of these materials depends on the charge of the surfactant and that of the inorganic ion involved in the synthesis. Different mesostructured Sb and W oxides were synthesized at room temperature by controlling the pH of the system. Regardless of their structures, all the resulting materials collapsed upon calcination. Stein et al. [196,197] independently explored the applicability of the use of surfactants for the synthesis of channel structures with transition metal oxide frameworks. Vanadium, niobium, molybdenum and tungsten oxides were studied. In the case of tungsten, hydrothermal reaction of sodium metatungstate with C16TMAOH gave the salt [C16H33N(CH3)a]6(H2W1204o). Despite the apparent similarity of TEM micrographs and XRD patterns of this material to those of mesoporous silicates, the salt contained unconnected Keggin ions H2W120406-. These Keggin ions pack in a puckered layer arrangement and create roughly spherical cavities for the suffactant micelle counterions. As a result, attempts to remove the template cations and condense the inorganic portion of the structure invariably led to dense WO3. x phases. As in the case of lamellar MCM-50 [ 198], the authors prepared a stable salt-gel by reacting the surfactant niobotungstate salt with TEOS. During this treatment Nb-O-Si linkages were formed. Removal of the cationic surfactant by acidextraction resulted in porous structures with surface areas up to 265 mE/g. Following the pioneering work of Huo et al., Antonelli and Ying [ 199] prepared the first stable mesoporous transition metal oxide, TiO 2, using a modified sol-gel method. They showed that the key step to obtain a desired phase is to control the hydrolysis rate of the organometallic precursor, titanium tris-isopropoxide. Acetylacetone was added to the system to stabilize the titanium compound and to lower the hydrolysis rate. A hexagonal phase was obtained only in the presence of phosphate surfactant. After calcination at 627 K, hexagonally packed TiO 2 prepared in the presence of tetradecyl phosphate had a BET surface area of 200 m2/g and a pore distribution centered at 32 A. However, IR spectroscopy revealed a strong absorption band at 1087 cm-1, indicating the presence of phosphate ions, even after calcination. Luca et al. [200] prepared mesostructured vanadium oxide. In their synthesis, cetyltrimethylammonium vanadate was first crystallized from water solution. It was then dissolved in alcohol followed by titration with HC1 to pH = 2.2. A hexagonal phase was identified by XRD. However, the material was thermally unstable. Abe et al. [201] synthesized a hexagonal vanadophosphate using a similar templating method under hydrothermal conditions. Characterization of the resulting hexagonal phase by IR and XRD showed that the inorganic part was basically amorphous and was similar to glass V2Os-P205. Sayari et al. [202,203] extended the LCT technique to the synthesis of mesostructured zirconium oxide. The use of long chain quaternary ammonium salts or primary amines as templates led to the formation of hexagonal and lameUar ZrO 2 phases, respectively. Zr(SO4) 2 was used as zirconium source, which provided a highly acidic medium, pH < 1.5. Consistent with the synthesis conditions and EDX analysis data a S+X-I+ mechanism where the suffactant-inorganic interaction is mediated by sulfate anions was proposed. Unfortunately, both structures collapsed upon removal of the surfactant either by high temperature calcination or by solvent extraction [203]. However, the hexagonal form was successfully

24 Table 4 Typical Synthesis Results Using Different Inorganic Precursors and Surfactants inorganic precursor

surfactant

phase

XRD d spacing (A)*

Sb oxide Sb oxide

ClgH37(CHa)aNBr C 18H37(CH3)3NBr

cubic (Ia3d) hexagonal

42.9 46.0

Sb oxide

ClgH37(CHa)3NBr

lamellar

37.5

W oxide

CId-I3s(CH3)3N-Br

hexagonal

40.0

W oxide

C16Hs3(CH3)3NBr

lamellar

28.3

zinc phosphate

C.I-I2.§

lameUar

21.6(10), 23.5(12), 26.0(14), 28.2(16), 30.5(18), 32.5(20)

alumina

C 12H25C6I-I4SOaNa

lamellar

28.9

Pb 2+ Pb 2+

C 1d-IssSOsH C 1d'Iss SOsH

hexagonal lamellar

45.8 38.5

Fe 2+

C 16I-I33SOaH

lamellar

41.0

Mg 2+

C12H25OPO3H2

lamellar

31.0

Mn 2+

C 12H25OPOsH2

lameUar

28.6

F es*

C 12H25OPOaH2

lamellar

26.9

Co2+

C 12H25OPO3H2

lamellar

3 0.8

Ni 2+

C12H25OPOsH2

lamellar

31.1

Zn 2+ .Al 3+

C 12H25OPOsH2 C 12H25OPOaH2

lamellar lamellar

29.6 26.4

Gas+

C 12H25OPOsH2

lamellar

27.2

Fe 2§

CnI-I2n+lOSO3Na

lamellar

21.0(12), 23.0(14), 27.3(16), 30.3(18)

Fe s§

C~H2~§

lamellar

23.1(12), 26.0(14), 28.1(16), 28.1(18)

Co 2+

C,H2,+IOSOsNa

lamellar

20.9 and 39.7(12), 22.8(14), 41.5 and 27.4(16), 28.4(18)

Ni 2+

C,H2,+~OSOaNa

lamellar

31.8, 23.5 and 23.2(14), 43.5 and 27.5(16), 24.3(18)

Mn 2§

C~-I2,§

lamellar

23.3(14), 42.2 and 28.9(16), 24.3(18)

tin sulfide

Ca6H33(CH3)3NBr

lamellar

25.8

* Carbon numbers of the surfactant chains are shown in parentheses.

25 stabilized by post-synthesis treatment with potassium phosphate at room temperature followed by air calcination at 627 K. The final product contained significant amounts of phosphorous, and had a surface area exceeding 500 m2/g but with a broad pore size distribution [202]. Knowles and Hudson [204] also prepared a mesoporous, high surface area zirconium oxide in a basic medium. At pH = 11.4-11.7, zirconium species formed a gel. Through a scaffolding process followed by calcination at 723 K, they obtained zirconium oxide with surface area in the range 238-329 m2/g. The d-spacing of as-synthesized compounds did not change with the length of the surfactant used, which is inconsistent with the LCT approach. An alternative mechanism was proposed. The positive surfactant ion first exchanges with the ions in zirconium hydroxide gel. Subsequently, controlled heating and scaffolding condense the inorganic structure, and calcination leads to a mesoporous, large surface area material. Pinnavaia et al. [69,72] prepared a hexagonally packed alumina through the neutral templating approach. In the presence of a polymer surfactant, (PEO)13(PPO)30(PEO)13, an alumina with a d spacing of 63/~ and a surface area of 420 m2/g was obtained [72]. It was also mentioned that non-layered alumina can be synthesized using octyl or dodecyl amine as template and a neutral aluminum alkoxide precursor. Compared with metal oxides, less attention has been paid to the synthesis of mesostructured metal sulfides [61,205]. The only systematic work was reported by Anderson and Newcomer [205]. The liquid-crystal templating approach was applied to metal sulfides, such as Mo, W, Co, Fe, Zn, Ga, Sn and Sb sulfides. All of the products were lameUar and consisted of bilayers or interdigitated layers of surfactant molecules sandwiched between metal sulfide layers. As mentioned in the introduction, microporous aluminophosphates form a large and important family of molecular sieves. With the ultimate goal of preparing stable ultra-large aluminophosphates, two research groups attempted to use a supramolecular surfactant array as template. Oliver et al. [206-208] reported on the synthesis of lamellar aluminophosphates in non aqueous medium. A typical synthesis gel was: 14 TEG : 0.9 A1203 : 2.5 H20 : 1.8 P205 : 3.0 CloH21NH2 , where TEG denotes tetraethylene glycol. SEM showed that in some parts of the resulting lamellar materials, there exists micrometer-scale surface patterns, including bowl, honeycomb and quilted shapes with superimposed finer columnar, sphere, mesh, and pore-like structural features. The origin of these pattems was explained based on a proposed vesicle templating mechanism along with a "cellular" model (Figure 16). The multi-functional role of TEG was emphasized. It acted as a solvent for the surfactant, a polydentate ligand to Al3+, a co-surfactant to control the bilayer curvature, and a demixing agent to promote surfactant-TEG phase separation and patterning of vesicle bilayer, and ionchannels to facilitate the transport of (TEG)A11u ionophores through vesicle bilayers, to access reactive phosphate sites and permit mesolamellar aluminophosphate nucleation and growth. The micrometer scale patterns obtained mimic some natural microskeletons such as diatom and radolarian. Sayari et al. [209-212] followed another path to the synthesis of mesolameUar aluminophosphates. They used AI20 3, H3PO4 and primary or tertiary amines as surfactants in aqueous media. The surfactant was found to be protonated while acting as template. Effects of synthesis parameters such as A1/P, P/amine, P/H20 ratios were studied systemically by XRD and 27A1 and 31p NMR. The connectivity between A1 and P was found to be dependent on the synthesis parameters.

26

Figure 16. Model for vesicle and bilayer control of macroscopic morphology and surface patterning, and mesolamellar structure of the aluminophosphates [206]. TEM studies [210] indicated that in some samples apart from the simple lameUar packing, there were extended areas comprised of a unique hexagonal-like packing of alternating concentric dark and bright tings (Figure 17). As shown schematically in Figure 18, these circular fringes were interpreted as the edge projections of cylindrical layers of inorganic A1PO4 materials separated by cylindrical vesicles of surfactant, all wrapped around a single rodlike miceUe. This new mesophase has no equivalent among known surfactant liquid crystal phases.

Figure 17. TEM Image of as-synthesized AIPO4 [210].

Figure 18. Representation of coaxial cylindrical bilayer growth of A1PO4 [210].

27 3.

C A T A L Y T I C APPLICATIONS OF M41S AND RELATED MATERIALS

Most potential applications reported in the literature use MCM-41, HMS or FSM-16 type of materials. Patents dealing not only with the applications, but also with the synthesis of crystalline mesoporous materials are almost exclusively assigned to Mobil Oil Corporation. Extensive lists of these patents are provided as appendixes. Catalytic applications may be conveniently divided into three categories: (i) acid catalysis, (ii) liquid phase redox catalysis, and (iii) other applications. 3.1 Acid Catalysis Acid sites in mesoporous silicates can be generated either by isomorphous substitution of trivalent cations such as A1 or B for Si, or by adding an acidic component such as a heteropolyacid, an ultra stable Y (USY) or a A1 containing ZSM-5 zeolite. 3.1.1. Mesoporous Aluminosilieates A1-MCM-41 based materials were tested in a number of petroleum ref'ming processes. A NiMo impregnated Al-MCM-41 catalyst (12 wt% MoO 3, 3 wt% NiO) was tested for hydrocracking of vacuum gasoil, and found to be more efficient in hydrodesulfurization and hydrodenitrogenation than NiMo loaded on USY or on amorphous silica-alumina. The higher performance of NiMo/MCM-41 was attributed to its high and freely accessible surface area and also to the higher dispersion of catalytically active ingredients. In addition, despite its lower acidity, the NiMo/MCM-41 catalyst was also found to have higher activity in mild hydrocracking of gasoil than USY or amorphous silica-alumina based catalysts. Other investigations reported in the open literature include microactivity tests (MAT) [214], oligomerization of propene [129], cracking of cumene [134] and bifunctional hydroismerization and hydrocracking of n-hexadecane [215]. Potential catalytic applications of MCM-41 based catalysts in the petroleum refining industry were also reported in the patent literature (Appendix 2). The hydrogen form of A1MCM-41 mixed with A1203binder in a ratio of 65 to 35 wt% was used for the cracking of a straight run naphtha at 540 ~ and ca. 3 atm [216]. At the same conversion (43-45 %), the MCM-41 based catalyst produced more C3-C5 olefins (74 vs. 54 %) and much less light gas and linear hydrocarbons (11 vs. 29 %) than medium pore ZSM-5 zeolite. In addition, MCM-41 exhibited higher selectivity towards valuable isobutane and isopentanes which can be further upgraded via alkylation by olefins or via dehydrogenation into isoalkenes. A catalyst comprised of 35% Al-MCM-41 and 65 % silica-alumina-kaolin clay matrix was also tested in fluid catalytic cracking and found to be more active and more gasoline selective than a catalyst containing 35 % USY. It also displayed higher selectivity towards C5 olefins. Apelian et al. [218] compared an A1-MCM-41 based catalyst (5.8% Ni, 29.1% W on 65% alumina, 35% A1-MCM-41) with a fluorided NiW/A1203 for the hydrocracking of a heavy wax. At low conversion (< 50%), both catalysts had similar lube yields, but at higher conversion NiW/MCM-41 exhibited higher yields. Furthermore, the MCM-41 based catalyst gave lubes with higher viscosities, thus allowing operation at a much higher wax conversion while still meeting viscosity specifications. NiMo/H-MCM-41/A1203 catalysts were found to be effective for the removal of heavy metals (Ni, V, As, Fe) and asphaltenes from resid and shale oil under relatively mild conditions [219]. Combination of the mildly acidic A1-MCM-41 with a strongly acidic zeolite such as USY in the presence of an alumina binder in addition to nickel and tungsten leads to an enhanced overall hydrocracking activity of the catalyst and a decreased yield of light gas, compared to USY free catalyst [220].

28 Protonated A1-MCM-41 catalysts were also used for the cracking of olefmic feedstocks such as FCC gasoline. They gave high selectivities towards valuable isobutene and isoamylenes which can be further upgraded into high octane components via etherification with methanol [221]. Other patent applications dealt with oligomerization of propene [222], dealkylation of 1,3,5 tri-tert-butylbenzene [223], alkylation of naphthalene with long chain alpha-olefins [224], and alkylation of benzene with ethylene [225]. Isobutane alkylation with butene over H2SO4 or BF3 promoted H-MCM-41 was also investigated [226]. In all these processes, H-MCM-41 based catalysts exhibited promising performances, and in some cases may be considered as a viable alternative to currently used commercial catalysts. Armengol et al. [227] used protonated A1-MCM-41 molecular sieve for alkylation of bulky aromatic compounds such as 2,4-di-tert-butylphenol with a bulky alcohol (cinnamyl alcohol). This reaction did not occur in the presence of large pore HY zeolite indicating the importance of the mesoporous structure of the H-MCM-41 catalyst and the accessibility of active sims. Kloetstra et al. [228] obtained excellent results during the tetrahydropyranylation of alcohols and phenols over A1-MCM-41 (Scheme 3). Bulky alcohols including cholesterol, adamantan-l-ol and 2-naphthol were converted into the corresponding tetrahydropyranyl ethers in relatively short periods of time.

Scheme 3 Shinoda et al. [229] found that FSM-16 mesoporous aluminosilicate compares favorably with the currently used liquid acid BF3.OEt2 for the synthesis of meso-tetraarylporphyrins from the corresponding aromatic aldehydes and pyrrole. In addition, contrary to soluble catalysts and to K10 acid treated montmonrillonite, FSM-16 could be used repeatedly after regeneration by calcination at 500 ~ Kloetstra [230] used Na§ and Cs+ exchanged A1-MCM-41 catalysts to carry out a base catalyzed reaction, namely the Knoevenagel condensation of benzaldehyde with ethyl cyanoacetate (Scheme 4).

O

H + H2C\ /CNco2Et

-H20

= ~~CO2Et ~

CN

Scheme 4 In the presence of Na-MCM-41 as catalyst, and water as solvent, almost 100 % selectivity was achieved at 90% conversion. However, this reaction did not take place in ethanol. Instead, benzaldehyde and ethanol reacted over residual acid sites giving equilibrium concentration of diethyl acetal. In addition, both H-MCM-41 and Na-MCM-41 catalyzed the condensation of benzaldehyde with acetophenone to chalcone (Scheme 5a with R = H) and

29 other aldol condensations of bulky molecules. They were also found to catalyze intramolecular Michael addition of the a-unsaturated ketones to flavones (Scheme 5a and 5b with R = OH). The strongly basic Cs-MCM-41 was found to be an effective catalyst for the Michael addition of chalcone and diethyl malonate. o

o

R ~

o

-~~

(b)~R = OH

o

Scheme 5

3.1.2.

Heteropolyacid (HPA) Supported MCM-41 Silicate Catalysts

To enhance the acidity of A1-MCM-41 Kozhevnikov et al. [231] added 10 to 50 wt% phosphotungstic acid. The obtained catalysts were found to be more efficient than H2SO4 or bulk phosphotungstic acid in liquid phase alkylation of 4-tert-butylphenol by isobutene and styrene. Using four test reactions, Kresge et al. [232] compared phosphotungstic acid loaded MCM-41 to more conventional catalysts. The reactions were: (i) n-butane conversion, (i.i) nhexane conversion, (iii) alkylation of isobutane with 2-butene, and (iv) alkylation of benzene with 1-tetradecene. For the first reaction, at similar n-butane conversions, HPA/MCM-41 gave much higher isobutane selectivities than ZSM-5. It was reported that n-hexane conversion and the isomerization selectivity over HPA/MCM-41 are significantly higher than those obtained in the presence of the unsupported ammonium salt of phosphotungstic acid [233] or the supported salt on silica or alumina [234]. The higher dispersion of HPA on MCM-41 may be at the origin of these differences. However, HPA/MCM-41 displayed lower isobutane alkylation activity than unpromoted H-MCM-22 zeolite. In addition, the ratio of trimethylpentanes to dimethylhexanes was low indicating that the quality of the alkylate was poor. As for the alkylation of aromatics, both benzene and tetradecene conversions were higher for the supported than unsupported phosphotungstic acid.

3.2 Liquid Phase Redox Catalysis Since the discovery in the early 80's of the remarkable catalytic activity of Ti-modified silicalite-1 (TS-1) in the selective oxidation of organic substrates by dilute H20 2, the field of transition metal modified zeolites grew tremendously as shown in a number of recent reviews [156,235,236]. In addition to its hydrophobicity, the major role of the zeolite matrix is the stabilization of isolated redox centers. However, the limited accessibility of these sites precluded the use of large substrate molecules. The discovery of crystalline mesoporous silicate was immediately perceived as an ideal solution to these limitations.

30 Ti, V and Sn-modified mesoporous silicates were reported to be active in a number of liquid phase oxidation reactions. Ti-containing samples were used for the selective oxidation of large organic molecules in the presence of ten-butyl hydroperoxide (TBHP) or dilute H20 2 [71,136,137,139-141,147,186,237]. Typical data shown in Table 5 indicate that both TiMCM-41 and Ti-HMS are efficient catalysts for the epoxidation of bulky olefms such as ctterpineol and norbomene in the presence of TBHP or H20 2. Comparison with Ti-13 indicates that the accessibility of active sites plays a critical role in the liquid phase oxidation of organic molecules. Mesoporous titanosilicates also exhibited remarkable activity in the hydroxylation of 2,6-di-tert-butyl phenol (2,6 DTBP) [142,147] and the oxidation of cyclododecanol [147], naphthol [147] aniline [237] and chloroaniline [186]. However, they were disappointingly poor catalysts for the liquid phase oxidation of n-hexane and aliphatic primary amines, as well as the ammoximation of cyclohexanone [ 147,238]. Table 5 Oxidation of tx-Terpineol (a) and Norbomene (a'b) over Ti-MCM-41 and Ti-HMS Catalyst

Time

o~-terpineol

Time

Norbornene

Ref.

(h)

epoxide

others

(h)

epoxide

alcohol

TiA1-MCM-41 (c)

3 8

23.8 31.5

4.0 8.6

5 11

26.4 42.3

3.1 6.4

140 140

Ti-13(el)

3 8

4.1 7.6

2.5 5.8

5 11

10.3 18.3

6.6 12.8

140 140

Ti-MCM-41 (e)

-

-

-

2

5.6

10.2

147

Ti-HMS 0)

-

-

-

2

16.4

3.6

147

(a) Reaction conditions in Ref. 140: temperature (313 K), catalyst (100 mg per mmol of substrate), TBHP in CH2C12 to olefin ratio: 1.2; (b) reaction conditions in Ref. 147: temperature (335 K), catalyst (100 mg), norbomene (100 rag), 30 wt% H20 2 (2.36 g), acetonitfile (10 ml); (c) Si/A1 = 97, Si/Ti = 55; (d) Si/Ti = 60, Si/A1 = ca. 200; (e) Si/Ti --49; (j) Si/Ti =76. Corma et aL [239] took advantage of both redox and acidic properties of Ti/A1-MCM-41 (Si/Ti = 68 and Si/A1 = 196) aluminosilicate to carry out the multistep oxidation of linalool to cyclic furan and pyran hydroxy ethers (Scheme 6). The reaction was carried out in acetonitfile at 353 K using TBHP as oxidant. Conversions as high as 80 % were obtained. As shown in Scheme 6, it was postulated that the reaction takes place via epoxidation over Ti sites followed by acid catalyzed intramolecular opening of the epoxide ring by the 3-hydroxy group. Ti-13 zeolite gave somewhat lower conversions in addition to the preferential formation of furans over pyrans (ratio of ca. 1.5) due to shape selectivity. Ti-MCM-41 and gave furan to pyran ratios of ca. 0.9, comparable to those obtained by the epoxidase conversion of linalool.

31

.

TBHP

,..

3a

H

~ 2b

H

3b

Scheme 6 Sayari et al. [ 172,174] found that like their Ti containing analogs, V-HMS and V-MCM41 are efficient catalysts for the hydroxylation of bulky aromatic molecules such as naphthol and 2,6 DTBP. Gontier et al. [186,237] found that V-HMS has no activity in the oxidation of aromatic amines in the presence of H20 2. However, it oxidizes aniline selectively into nitrobenzene when TBHP and acetonitrile are used as oxidant and solvent, respectively. Notice that Ti-HMS is active even in the presence of H20 2 and gives a different product distribution, particularly azoxybenzene [237]. Das et al. [193] also reported that Sn-MCM-41 is effective in the liquid phase hydroxylation of phenol and naphthol. Geometric constraints and related factors including active site accessibility, steric effects of transition states and diffusion limitation of reactants and products play a crucial role in liquid phase catalyzed reactions [240]. Several examples are presented hereafter for illustration: (i) TS-1 is more active in the oxidation of linear vs. branched alcohols [241], and in the epoxidation of linear vs. cyclic olefins [153]; (ii) in the presence of TBHP, TS-1 has no activity [242], while Ti-g is less active than Ti-MCM-41 [243]; (iii) large TS-1 particles are less effective catalysts than smaller ones [244]. All the above mentioned observations stem from diffusion limitations and steric constraints due to reactants or transition-state intermediates. However, this is not the only factor that governs the activity of Ti sites in molecular sieves. Indeed when considering the epoxidation of a smaU linear olefin such as 1-hexene, the activity of Ti silicates decreases in the order TS-1 >> Ti-13 > Ti-MCM-41 [153,171,243]. Furthermore, the more demanding reactions such as the hydroxylation of benzene and the oxidation of linear alkane and aliphatic primary amines take place on TS-1 but not in the presence on Ti-13 or Ti-MCM-41. The reason for such a strong dependence of Ti catalytic properties on the nature of the silicate matrix is not well understood. Indeed, none of the spectroscopic techniques used so far including X-ray absorption [171,245] shows any difference in the local environment of Ti in various crystalline micro- and meso-porous silicates. Decreasing hydrophobicity from TS-1 to Ti-l~ and Ti-MCM-41 is a contributing factor in the parallel decrease of the intrinsic catalytic activity of Ti sites. However, as suggested earlier [137,141,147], subtle variations of some important properties such as bond angles or redox potential may be at the origin of this behavior.

32 3.3 Other Catalytic Applications Because of their extremely high surface areas MCM-41 and FSM-16 (alumino) silicates were used as supports for catalytically active materials. Bhore et al. [246] found that Ni supported HMCM-41 has higher propene oligomerization activity and improved selectivity towards trimers and tetramers than Ni on medium pore zeolites. Several authors used Pt/MCM-41 [215,247-249] and Pt/FSM-16 [250] catalysts prepared by impregnation or by ion-exchange. Del Rossi et al. [247] used 0.6% Pt/HMCM-41 for the conversion of n-hexane in the presence of H e. The isoparaff'm yield obtained at a given n-hexane conversion over Pt/H-MCM-41 was significantly higher than over 0.6% Pt on amorphous SiO2-A1203. In addition, the MCM-41 based catalyst gave much less cracked products. Inui et al. [250] used Pt supported FSM-16 mesoporous silicate downstream of H-Fe-silicate to convert in one step propene into high octane branched alkanes in the gasoline range. This catalyst showed much better hydrogenation activity than Pt on amorphous silica with a broad distribution of mesopores. 0.85% Pd supported on a 65% - 35% extruded mixture of HMCM-41 and A1203 showed higher benzene hydrogenation activity than other Pd containing catalysts such as Pd/USY, Pd/ZSM-5 and Pd/SiO2 [251]. However, the Pd dispersion was also higher for the MCM-41 based catalyst than the others. MCM-41 supported V205-TiO2catalysts (6.1% Ti and 2.5% V) were used for NO x selective reduction [252] in the presence of a gas stream containing 125 ppm NO, 125 ppm NH 3, 0.12% 0 2 in He. The MCM-41 based catalyst was found to be more active than V205TiO2/SiO2, particularly at high temperature. Several patents were also devoted to the oligomerization of 1-decene over Cr203 impregnated A1-MCM-41 catalysts [253-255]. The products obtained had much higher viscosities than those obtained over chromia on silica. Immobilization of transition metal complexes such as metaUoporphorin [256], Fe(II)penanthroline [257] and others [258] on the walls of MCM-41 silicates is an interesting recent development in this field. Chibwe et aL [256] ion exchanged A1-MCM-41 with meso-tetra(Imethylpyridinium) porphyrin COO/) complex, [Co(II)TMPyP]~ . At low loading, the turnover numbers of the supported complex in the oxidation of 2,6 DTBP by H20 2 were up to two orders of magnitude higher than for the homogeneous catalyst. This remarkable increase compared for example to the 5-fold activity enhancement obtained in the oxidation of methyl cyclohexane over Fe phtalocyanine loaded NaY [259] indicates that the improved accessibility of the active complex in MCM-41 materials plays a crucial role in the catalyst performance. Liu et aL [257] prepared Fe(II)-Phen on A1-MCM-41 (A1/Si = 20) using an alcohol solution of [Fe(Phen)3]C12 at room temperature. This catalyst was used repeatedly in benzene hydroxylation by 30% H202 without any significant loss of activity. Huber et al. [260] also encapsulated a tin-molybdenum complex, Me3SnMo(CO)a(rI-CsH5) in MCM-41 silicate. The material was characterized by EXAFS and FTIR-TPD measurements. The complex was found to attach strongly to the channel walls, and starts to decompose at ca. 200 ~ At 300 ~ sub-nanometer size Sn-Mo clusters were obtained which may have interesting catalytic properties. Helldng et al. [261] found also that A1-MCM-41 materials display a promising behavior as phase transfer catalysts. The rate of the two-phase reaction between potassium iodide and 1-bromopentane was enhanced significantly upon addition of A1-MCM-41.

33 4.

OTHER POTENTIAL APPLICATIONS

Periodic mesoporous materials may have important applications in the area of seperation of biological materials. The fabrication of composite and non composite membranes based on M41S silicates has been reported in the patent literature [262]. Another area with potential growth is the encapsulation of technologically advanced materials. Preliminary findings dealing with the following materials have been reported: election transfer photosensitizers [263] semiconductors [264] - polymer wires [265-268] conducting carbon wires [269] - sensing devices [270] - materials with non linear optic properties [271] - quantum sized clusters [271,272] -

-

-

5.

CONCLUDING REMARKS

M41S crystalline mesoporous materials have expanded the area of microporous zeolites and molecular sieves into the mesopore range. Their discovery has created new opportunities in several areas. First, by its simplicity and diversity the synthesis strategy aroused the interest of zeolite synthesis scientists in the rich chemistry of surfactant-inorganic systems. This effort has already led to several important findings, in particular (i) the design of new synthesis routes using cheap polymer surfactants, (ii) the discovery of new morphologies without lyotropic surfactant counterparts, and (iii) the synthesis of some stable non-silica based mesoporous materials. However many challenging tasks are yet to be accomplished. Pertinent examples include the development of efficient methods for the removal of templates from non-silica framework without altering the porous structure, and the the design and synthesis of three dimensional mesoporous cage-structured materials using cheap and readily available surfactants. In addition, crystalline mesoporous molecular sieves have promising properties particularly in catalysis. The main advantages of these materials are their extremely high surface areas and the great accessibility of their pore systems. Because of the amorphous nature of their pore walls, mesoporous aluminosilicates have much lower acidity than acid zeolites such as H-Mordenite, HY and ZSM-5. However, reactions that do not require very strong acid sites will benefit from the enhanced accessibility of acid sites and the high surface area of these materials. In addition, the acidity drawback may be mitigated by combination with strongly acidic ingredients such as heteropolyacids. More important than the limited strength of acid sites is the stability of mesoporous aluminosilicates in terms of both aluminum retention and structure preservation. Issues that are crucial in evaluating these materials for possible commercial applications include resistance to dealumination and mechanical stability under working conditions, as well as the long term activity maintenance. As for transition metals modified mesoporous molecular sieves, innovative applications in selective oxidation of bulky molecules relevant to pharmaceuticals and agrochemicals should be envisaged. In this context, strong collaboration between scientists working in (i) catalysis, (ii) the design and fabrication of crystalline mesoporous materials, and (iii) organic synthesis is essential to identify opportunities where heterogeneous liquid phase redox catalysis may be useful. In addition, despite many advantages of using solid catalysts for

34 liquid phase reactions, leaching of active ingredients may occur. It is worthwile to undertake an in-depth evaluation of this problem during repeated reaction - regeneration cycles. ACKNOWLEDGMENTS

The author is grateful to Y. Yang, P. Liu, C. Danumah and H. Michel and J. Desgagn, for their help with the manuscript. APPENDIXES

Appendix 1 Mobil US Patents on Synthesis of Mesoporous Materials Authors

Patent Number

Publication Date

Title

Beck et al. Calabro et al.

5334368 5308602

02.08.94 03.05.94

Beck et al. Kresge et al.

5304363 5300277

19.04.94 05.04.94

Beck et al. Kresge et al.

5264203 5250282

23.11.93 05.10.93

Beck et al.

5246689

21.09.93

Chu et al. Kresge et al. Kresge et al. McCullen et al.

5215737 5211934 5198203 5156829

01.06.93 18.05.93 30.03.93 20.10.92

Degnan et al.

5156828

20.10.92

Johnson et al.

5112589

12.05.92

Calabro et al.

5110572

05.05.92

Beck et al.

5108725

28.04.92

Chu et al.

5104515

14.04.92

Kresge et al.

5102643

07.04.92

Kresge et al. Beck

5098684 5057296

24.03.92 15.10.91

Synthesis of Mesoporous Oxide Synthesis of Crystalline Ultra-Large Pore Oxide Materials Porous Materials Synthesis of Mesoporous Crystalline Material Synthetic Mesoporous Crystalline Materials Use of Amphiphilic Compounds to Produce Novel Classes of Crystalline Oxide Materials Synthetic Porous Crystalline Material Its Synthesis and Use Synthesis of Mesoporous Aluminosilicate Synthesis of Mesoporous Aluminosilicate Synthetic Mesoporous Crystalline Material Method for Stabilizing Synthetic Mesoporous Crystalline Material Method for Manufacturing Synthetic Mesoporous Crystalline Material Method for Synthesizing Mesoporous Crystalline Material Using Acid Synthesis of Mesoporous Crystalline Material Using OrganometaUic Reactants Synthesis of Mesoporous Crystalline Material Method for Purifying Synthetic Mesoporous Crystalline Material Composition of Synthetic Porous Crystalline Material, Its Synthesis Synthetic Mesoporous Crystalline Material Method for Synthesizing Mesoporous Crystalline Material

35 Appendix 2 Mobil US Patents on Catalytic Applications of Mesoporous Authors

Patent Number

Publication Date

Title

Baker et al. A p e l i a n et al.

5468368 5451312

21.11.95 19.09.95

Beck et al.

5370785

06.12.94

Shih

5344553

06.09.94

Kresge et al.

5 3 2 4 8 8 1 28.06.94

Degnan et al.

5290744

01.03.94

Marler et al.

5288395

22.02.94

Degnan et al.

5281328

25.01.94

Marler et aL Pelrine et al. Borghard et al.

5277792 11.01.94 5270273 14.12.93 5264641 23.11.93

Apelian et al.

5264116

Bhore et al.

5 2 6 0 5 0 1 09.11.93

Aufdembrink

5258114

02.11.93

Del Rossi et al.

5256277

26.10.93

Le et al.

5232580

03.08.93

Apelian et al. Beck et al.

5227353 5200058

13.07.93 06.04.93

Kresge et al. Degnan et al. Degnan et aL

5196633 5191148 5191147

23.03.93 02.03.93 02.03.93

Lubricant Hydrocracking Process Catalyst and Process for Producing LowAromatics Distillates Hydrocarbon Conversion Process Employing a Porous Material Upgrading of a Hydrocarbon Feedstock Utilizing a Graded, Mesoporous Catalyst System Supported Heteropoly Acid Catalysts for Isoparaffin-Olefm Alkylation Reactions Hydrocracking Process Using Ultra-Large Pore Size Catalysts Production of High Viscosity Index Lubricants Hydrocracking with Ultra Large Pore Size Catalysts Production of Hydrocracked Lubricants Olefin Oligomerization Catalyst Aromatics Saturation with Catalysts Comprising Crystalline Ultra-Large Pore Oxide Materials Production of Lubricants by Hydrocracking and Hydroisomerization Catalytic Oligomerization Process Using Modified Mesoporous Crystalline Material Ultra Large Pore Cracking Catalyst and Process for Catalytic Cracking Paraffin Isomerization Process Utilizing a Catalyst Comprising a Mesoporous Crystalline Material Catalytic Process for Hydrocarbon Cracking Using Synthetic Mesoporous Crystalline Material Hydroprocessing Catalyst Composition Catalytic Conversion over Modified Synthetic Mesoporous Crystalline Material Catalytic Conversion Isoparaff'm/Olefin Alkylation Isoparaffin/Olef'm Alkylation

23.11.93

et al.

36 Appendix 2 (continued) Mobil US Patents on Catalytic Applications of Mesoporous Materials Authors

Patent Number

Publication Date

Title

L e et al.

5191144

02.03.93

Le Kresge et al.

5191134 02.03.93 5 1 8 3 5 6 1 02.02.93

Degnan et al.

5183557

02.02.93

Schipper et al.

5179054

12.01.93

Kresge et al. Beck et al.

5 1 7 4 8 8 8 29.12.92 5143707 01.09.92

Bhore et al.

5 1 3 4 2 4 3 28.07.92

L e et al.

5134242

28.07.92

L e et al.

5134241

28.07.92

Le Pelrine et al.

5118894 02.06.92 5 1 0 5 0 5 1 14.04.92

Olefin Upgrading by Selective Conversion with Synthetic Mesoporous Crystalline Material Aromatics Alkylation Process Demetallation of Hydrocarbon Feedstocks with a Synthetic Mesoporous Crystalline Material Hydrocracking Process Using Ultra-Large Pore Size Catalysts Layered Cracking Catalyst and Method of Manufacture and Use Thereof Catalytic Conversion Selective Catalytic Reduction (SCR) of Nitrogen Oxides Catalytic Oligomerization Process Using Synthetic Mesoporous CrystaI1ine Material Catalytic Olefin Upgrading Process Using Synthetic Mesoporous Crystalline Material Multistage Olefm Upgrading Process Using Synthetic Mesoporous Crystalline Material Production of Ethylbenzene Production of Olefin Oligomer Lubricants

37 Appendix 3 Mobil US Patents on Other than Catalytic Applications of Mesoporous Materials Authors

Patent Number

Publication Date

Herbst et al. Kresge et al. Olson et al.

5378440 03.01.95 5 3 6 6 9 4 5 22.11.94 5 3 6 4 7 9 7 15.11.94

Beck et al.

5348687

20.09.94

HeUfing et al.

5347060

13.09.94

Roth et al.

5238676

24.08.93

Beck et al.

5220101

15.06.93

Beck et al.

5145816

08.09.92

Withehurst

5143879

01.09.92

Title Method for Separation of Substances Supported Heteropoly Acid Catalysts Sensor Device Containing Mesoporous Crystalline Material M41S Materials Having Nonlinear Optical Properties Phase-Transfer Catalysis with OniumContaining Synthetic Mesoporous Crystalline Material Method for Modifying Synthetic Mesoporous Crystalline Materials Sorption Separation over Modified Synthetic Mesoporous Crystalline Material Method for Functionalizing Synthetic Mesoporous Crystalline Material Method to Recover Organic Templates from Freshly Synthesized Molecular Sieves

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H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All fights reserved.

47

Synthesis, Characterization and Catalysis with Microporous Ferrierites, Octahedral Molecular Sieves, and Layered Materials Steven L. Suib U-60, Department of Chemistry, Department of Chemical Engineering, and Institute of Materials Science, University of Connecticut, Storrs, CT 06269-3060 This review concerns the synthesis, characterization, and catalytic activity of microporous ferrierite zeolites and octahedral molecular sieves (OMS) and octahedral layer (OL) complexes of mixed valent manganese oxides. The ferrierite zeolite materials along with borosilicate materials have been studied as catalysts for the isomerization of n-butenes to isobutylene, which is an important intermediate in the production of methyltertiarybutylether (MTBE). The OMS materials have tunnels on the order of 4.6 to 6.9 A. These materials have been used in the total oxidation of CO to CO2, decomposition of H202, dehydrogenation of C6H14, C6H14 oxidation, 1C4H 8 isomerization, and CH 4 oxidation. The manuscript will be divided into two major areas that describes zeolites and OMS/OL materials. Each of these two sections will include a discussion of synthesis, characterization, and catalytic activity. A..Overview. In the context of microporous and mesoporous materials, IUPAC has provided a variety of recommendations for nomenclature and characterization of porous materals, that can be found in the literature. 1 Microporosity should not be based on structural data but on adsorption data. Sorption by materials that show Type 1 isotherms is an indication of a microporous material. Pore size distributions less than 20 A are related to microporous materials like zeolites. Materials having pores between 20 A and 500 A are refered to as mesoporous materials. Materials that have pores larger than 500 A are refered to as macroporous. The measurement of pore size distributions is well established. However, the use of BET surface area measurements for zeolitic materials has been called into question due to potential multiple adsorption and nonconformity of monolayer adsorption implicite in the BET theory. The type of gas used, the method of data analysis, and even the use of the term surface area for a zeolitic material has been seriously questioned lately. On the other hand, most commercial manufacturers supply a surface area determined often by a three point or even a one point procedure that some researchers feel tells something about the material.

48 For microporous materials there is considerable debate on how to interpret surface area data due to the presence of micropores. Usually BET data are reported when one purchases a zeolite from a commercial vendor. The specific value of the BET measurement may give some indication of the relative surface area of the zeolite. Several experimental methods have been developed to collect BET data and many different equations have been used to model adsorption data. There are several discussions in the literature on various methods that have been used to analyze adsorption data by Broekhoff, et al.2 and Masthan et al.3 There is considerable debate on which method is the best for measuring BET data, however, there is good consensus that pore size distributions can be measured accurately with little problem in data analysis. Synthesis of novel micropomus zeolites such as intersecting 10- and 12-ring pore zeolites 4 has been the major focus of several research groups in the past few years. One of the more recent examples of catalytic applications that exploit micropores is the conversion of n-butenes to isobutylene over 10 ring zeolites like ferrierite as will be discussed later in this manuscript. Mesoporous molecular sieves materials 5-8 designated M41S (which include the MCM-41 class of materials) have made a further major impact on the area of synthesis of porous materials. A variety of open framework structures that are mesoporous have recently been reviewed by Thomas.9 Activated charcoal, MCM-41, mesoporous tungsten oxide, and substituted MCM-41 materials are mentioned. This article primarily emphasizes potential applications of such materials and possible mechanisms of reaction. The mesoporous sysems are compared briefly to microporous materials such as zeolites, ALPOs, MeALPOs and SAPOs. MCM-41 has been prepared by a liquid crystal templating mechanism where surfactant molecules are belived to act as templates by Beck et al.10 Surfactants such as C16H33(CH3)3NOH/CI in aqueous solution have been added to silica and alumina sources such as HiSil, tetramethyl ammonium silicate, and Catapal, respectively and then heated under autogeneous pressure to temperatures near 150oc for extended periods of time such as 48 h. In such procedures, products are cooled to room temperature, filtered to remove the spent Mother liquor, washed with distilled deionized water to clean the surface of the crystallites and then dried in air. The resultant solid materials show X-ray powder diffraction peaks indicative of mesoporous materials with d-spacings on the order of 40 A. At times other peaks at larger angles are observed and these materials have generally been indexed to hexagonal or cubic structure types for the MCM-41 materials. The attractive feature of these systems is that the size of the mesopores can be controlled by controlling the length of the hydrocarbon chain. Materials ranging from about 20 A to about 200 A have been prepared in this way. Siliceous MCM-41, aluminosilicate MCM-41, and mesitylene based materials have also been reported by Beck et al.10 N-Brand sodium silicate solutions were added to acidic solutions with the subsequent addition of surfactant and generation of a gel. Siliceous MCM-41 materials resulted by mixing these gels with water and heating the mixture to temperatures of 100oc for 6 days. Similar materials with different elemental compositions were prepared by using C12H25(CH3)3NOH/CI surfactant solutions with sodium aluminate solutions. Ultrasil silica,

49 tetramethylammonium silicate solution, or tetramethylammonium hydroxide solutions were added to the surfactant aluminate solution while stirring. Mixtures were then put into an autoclave at temperatures of 100oc for 1 day. Aluminosilicate MCM-41 materials resulted in such preparations. It is clear in the synthesis of these materials that aging of the gel is an important process and that rearrangements and bond transformation s can occur in the gel after it is formed. Specifically, mesitylene MCM-41 materials were prepared by adding the mesitylene during the last step of the synthesis,. Mesitylene gels as in the other MCM-41 preparations were then heated in an autoclave and allowed to crystallize. The specific conditions include temperatures of 105oc and quenching of the autoclaved gel after 4 h of thermal treatment. The calcination of such materials is important and involves treatment in nitrogen gas at elevated temperature and subsequent thermal treatment in air at elevated temperatures for 6 h. Calcination removes the surfactant "template" and it is clear that specific calcination treatments are critical in order to avoid degradation of the MCM-41 product and to completely remove the surfactant. 10 The mesoporosity of these materials has been established by BET measurements and gas adsorption experiments. As the chain length of the surfactant was increased from C 8 to C16, the amount of adsorbed benzene was increased, indicating that there was a relationship between the size of the surfactant and the amount of gas adsorbent taken up by the MCM-41 material. In terms of a comparison to zeolite materials, experiments were done at 60 torr pressure and at 25oc. The USY zeolite sample had an uptake that was about 4 times less than that of MCM-41. The above mentioned MCM-41 materials all show pore size distributions with broad bands centered around 40 A. The pore size distribution measurements are a true indication of the size of the pores and can be used to verify the existence of mesopores. Further evidence of mesopomsity comes from X-ray powder difraction experiments which were done to determine the crystallinity of these materials. The position of the (100) reflection was found to correlate with the amount of uptake by the different materials, or in therwords, with the mesoporosity of these systems. Pores of the MCM-41 materials were shown to form in a hexagonal shape by using high resolution transmission electron microscopy data. 10 The d-spacings of the (100) reflection of the MCM-41 materials made with mesitylene and the ratio of the number of moles of mesitylene to the number of moles of surfactant show a linear relationship as evidenced by X-ray powder diffraction data. As mentioned above, it is possible to produce both hexagonal and cubic MCM-41 structure types. In the case of the mesitylene systems, when the ratio of surfactant to Si was less than one, hexagonal MCM-41 was oberved. When the ratio of surfactant to Si was greater than one, cubic MCM-41 materials phase were observed. It is clear then that the structural type of MCM-41 can be controlled by proper choice and amounts of reactants. The original reaction mechanism for the growth of M41S and MCM-41 materials was proposed by Mobil researchers. 10 This proposed mechansim involved formation of rod-like structures of micelles and concomitant formation of a hexagonal array of rods, after which an inorganic species would encapsulate the rods and surround the surfactant species. Calcination of these composite materials led to the

5o formation of either hexagonal or cubic arrays of MCM-41 as shown by work of Beck et al.,10 and Kresge et a!.11 The Mobil researchers suggested that the inorganic encapsulating species might be able to control the formation of the micellular liquid crystalline phase during synthesis. The carbon chain length on the surfactant was critical for the control of the resultant structure of the micelles which in turn was important in the control of the mesoporosity and the pore size distributions of the M41S materials. A liquid crystal template model was proposed in order to explain the various stages of the crystallization process of MCM-41. Compositional and analytical data for these MCM-41 materials clearly show that isomorphous substitution of divalent, trivalent, tetravalent, and pentavalent cations can occur in these systems as reported by Kresge et al. 12 Several of the above-mentioned materials have been used in catalytic applications especially in the area of oxidation catalysis. For example, incorporation with titanium and vanadium species into MCM-41 has led to interesting partial oxidation catalysis as reported by several groups including Sankar et al., 13 Tanev et al., 14 Reddy et al., 15 Corma et a1.,16 and other work by Corma et a1.17 A variety of other elements have been suggested to be incorporated in a series of patents by Mobil researchers: (Pelrine et al., 1-8 Pelrine et al., 19 Bhore et al., 20 Le et al., 21 Le et al., 22 Le et al., 23 Bhore et a1.,24 Le et al., 25 Shih, 26 Kresge et a1.,27 Degnan et a1.,28 Del Rosssi et a1.,29 HeUring et al.,30 and further work by Kresge et al.31). Other novel structural materials that can lead to microporous and open framework systems are the vanadium phosphate class of materials which have 18.4 A elliptical tunnels32 which have been pursued by Haushalter, Zubieta and coworkers. A particularly novel structure consisting of inorganic double helices, 33 was also prepared by this group. Propped open structures have been made by the intercalation of organic/inorganic SnS2-cobaltacene layered intercalation compounds, 34 as reported by Nicoud which are reminiscent of intercalation of dichalcogenide species that are important in high energy battery applications. Novel microporous and mesoporous materials such as TiO2 based systems35, 36 are continuing to be reported that have interesting structural and catalytic properties. A specific area of interest in all of the above-mentioned systems is the precise control of the porosity of these materials, as well as control of their acidity. 37 Systematic control of the acidity of these materials and related materials (superacids) 38 is also an ongoing area of interest and is important in the potential use of these materials in acid catalyzed reactions. The above discussion is important as regards the synthesis of both microporous ferrierite materials as well as the mixed valent maganese oxide OMS and OL materials to be discussed below. All of the above materials in general are insulating materials. In addition, they are often synthesized with charge compensation in mind. For example, A13+ substitution for Si 4+ in zeolites leads to an inherent cation-exchange capacity. This is observed in zeolites, M41 S, and other materials like clays and pillared clays. Another way to approach the generation of microporous materials is to generate mixed valency of one element in an oxidic structure which should also lead to cation exchange capacity. Mixed valent materials 39 have been the focus of considerable attention due to their role in a variety of fundamental and applied research studies, including electron transfer, photoredox systems, biological materials, magnetic

.5] materials, semiconductors, and batteries. Several types of coordination complexes, clusters, enzymes, and extended structures have been studied in this regard. One particular system that tends to allow mixed valency in oxides is that of manganese. This is likely a result of the unusually high number of multiple oxidation states that are available for this element. Most mineral phases of manganese oxide show some degree of mixed valency. Birnessite is a layered mixed valent manganese oxide material that consists of MnO 6 octahedra, however, some of the manganese ions are reduced from Mn4+ to Mn3+. Birnessite is the most abundant manganese mineral. Birnesite shows cation exchange capacity as do many other mixed valent manganese oxide minerals such as todorokite, pyrolusite, ramsdellite, and others. Syntheses of mixed valent manganese nodules in agar gels 40, spinels 41 and layered materials 42 via sol-gel methods have recently been pursued. In addition, stabilization of 10 A manganite materials 43 via low temperature hydrothermal treatment may serve as standard materials for geothermometers. Considerable recent work concerns lithiation of manganese oxides (mostly layered structures) as potential battery materials. 44 Secondary nonaqueous rechargeable batteries are the goal of this area of research. Manganese containing malachite materials have also been shown to undergo solid state redox equilibria based on XPS data. 45 Review articles concerning mixed valent manganese oxides are available. 46 It is clear that the redox properties of the solution phase precursors, of the intermediates, and the final mixed valent products are very important during nucleation and growth of these materials. Organic oxidations 47 with manganese oxide have focused on dehydrogenations (loss of H) and incorporation of electronegative species (O). The activity of manganese oxides depend on particle sizes, solvents, methods of preparation, and other factors.47,48, 49 MnO2 is often used for oxidation of allylic and benzylic alcohols to aldehydes or ketones. In addition, amines can be converted to imines, thiols to disulfides, sulfides into sulfoxides, etc.47, 48 Manganese oxides have been reported to deposit on surfaces of bacteria and play a role in the decomposition of humic substances. 50 Natural manganese oxides systems have also been shown to decompose halogenated hydrocarbons. 51 Decomposition of H202 has also been catalyzed by MnO2 .52 In most of the above cases besides the decomposition of H202, the manganese oxide materials act as stoichiometric reagents in oxidation reactions of conversions of for example alcohols to ketones. A further report of the oxidation ability of manganese nodules is that of Nitta. 53 Several reactions were carried out with natural manganese oxide nodules including oxidative dehydrogenations of aikanes and cycloalkanes, reduction of NO, total oxidation of CO, and use in the gettering of metal and mixed metal ions. For example, nodules were found to have a tremendous capacity for adsorption of heavy metals and toxic metals like Pb 2+, and Hg2+. In addition, nodules have been used to sequester metals that are present in petroleum fractions that can contain metals like V and Ni. These metals can cause degradation of the fluid cracking catalysts even at levels as low as 1 ppm. A final report showing the potential of manganese oxide nodules was that of Weisz. 54 Manganese nodules dredged from the Pacific basin were tested in a variety

52 of catalytic oxidations including the total oxidation of methane, CO, and butane. The activity of the nodules was compared to commercial oxidation catalysts like Pt on AI20 3 and CuO. In all cases, the nodules were more active than the commercial catalysts. In addition, a good correlation was found between activity and large surface area. For example, as the surface area (ranging up to 250 m2/g) was increased going from one nodule to another the catalytic activity for total oxidation increased. This is a good example of the potential of the use of high surface area manganese oxide materials. B. n-Butene Isomerization with Boron Substituted Zeolites and Ferrierites. A major recent effort in our research program has involved studies of butene isomerization catalysts. Boron ZSM-5 and ZSM-11 catalysts have been synthesized and characterized55 and studied for isomerization of n-butenes to isobutylene.56,57 The study of these catalysts with a variety of bases such as NH 3, butene, ethylene have led to a better understanding of the amounts and types of acid sites in these materials. 58 A specific model explaining the role of different catalytic sites on B-ZSM-5 and B-ZSM-11 is shown in Figure 1. Quantitative Temperature Programmed Desorption (TPD) data have been used to identify 3 types (different stedc

~

Ethylene

~ i f i e g t i o n of Acid Sites on B-ZSM-5 + B-ZSM-II: Site 0, Adsorption of NH3; Site I, Adsorption of Nt][3 + 1-C4H8; Site H, Adsorption ofNH 3 + I-C4H $ + C2]B4.

Figure 1. Three Different Sites of B-Containing Zeolites. accessibilities) of sites: (1) those that only interact with NH 3 (2) those that interact with both NH 3 and ethylene (strong acid sites that lead to oligomerization) and (3) those that interact with butene (moderately weak acidic sites) that lead to isomerization. A systematic series of synthetic, characterization and butene isomerization catalysis studies of ferrierite and ferrierite-like materials such as ZSM-22 59 ZSM23, 60 and ZSM-35,61 was undertaken to study optimization of isobutylene product. Coke deposits 62 in the pores of these materials play a key role in isobutylene formation as does the overall acidity and structure of the pore system. Such shape selective effects have been probed with TPD methods. A comparison of catalytic conversions, selectivities, yields and other properties of some ferrierite and ZSM-22 samples is given in Table I.

.53 Table I Com0adson of Catalyl;i~ Properties of ~

Like Materials."

Catalyst*

Conversion**

Selectivity**

Yield**

Polymer"

Rate***

FER

60

40

24

12.5

161

FE R/TMOS

43

40

17

2.2

115

FER/AI20 3

43

82

36

4.0

241

ZSM-22

61

51

31

10

389

ZSM-22' 61 51 31 10 389 - 420oc, 20 cc/min, 220 mg catalyst. TOS = 4 h FER, 10 h ZSM-22. FER = Tosoh Ferrierite; TMOS = tetramethyl orthosilicate, source of Si; AI20 3 mixed with 40% Dispal powder, then extruded. ' = 100 cc/min. -in %. - mmol i-C4H8/min-g catalyst. The data of Table I suggest that addition of alumina binder can have a marked influence on selectivity with minimization of polymer (> C4) formation. Deactivation via coke formation depends on the exact pore size and shape of the zeolite. The time it takes for each catalyst type to reach steady state is quite variable, i.e., 4 h for FER and 10 h for ZSM-22. The FER/AI20 3 catalyst has the best yield of all materials we have studied. 61,62 Even though the structures are relatively similar, there are important differences in overall activity and deactivation. In the case of ferrierite we observed an excellent quantitative correlation between the different types of acid sites as determined by TPD and crystallographicaily different and accessible (to n-butenes) oxygen atoms. The 8 different types of oxygen sites of ferriedte are shown in Figure 2 and distributions are summarized in Table I1. Table II Distribution of 0 2- !ons in a Unit Cell of .FER. TvDe #

1 4

;~ 16

3 16

4 4

5 4

6 8

7 8

8 12

TPD data for NH 3, 1-C4H 8 and i-C4H 8 are given in Table III for FER.62 The amount of i-C4H 8 sorbed (using a similar procedure to that described above for quantifying 3 types of sites on B-ZSM-5, Fig. 2) is 30.8% whereas 1-C4 H 8 is accessible to 74.1% of the total acid " s~tes " . The 0 2 - "~ons o f type 1,2 and 4 are located in both 10-member and 8-member rings and have the largest space around them (XL). Types 3 and 5 are located inside the channels of the 8-member rings

54

i

(a)

(b)

The 8 Different O 2- Sites of FER along, a- [001], b- [010]. 9

I

I

Figure 2. Eight Different Oxygen Sites of Ferrierite. Table III Total Amount (%) Acid i . ~ ~;amole FER

NH3 100

~

i_nFER (4.4 [H+]/Unit Cell).

IIIII

1-C4H8 74.1

i-C4H 8 30.8

and type 6 is inside channels of 8-member rings. Types 3,5,6 grouped together are of about the same size (L) but with a smaller accessibility than XL. Types 7 and 8 (S) are all located on 5-member rings which limits space around them. Molecular modeling with Biosym software was used to study the steric constraints of these different sites. 62 Statistically , the % H+(XL), H+(L), and H+(S) are the same as the 0 2distributions (data from Table il). Note that the % H+(XL) [33.3%] closely matches the

.55 amount of i-C4H 8 sorbed (30.8%, Table III). The % of H+(XL) plus H+(L) [72.2%] also quite closely matches the amount of 1-C4H8 adsorption (74.1%, Table III). In summary, acid sites on FER have different size constraints from a structural point of view. Coke (predominantly aromatic in nature) formation is limited to < 11 wt. % of the micropore volume of FER. Coke formation modifies desirable polymerization (dimerization) reactions. Such blocking produces the pore shapes and limits access to more strongly acidic sites that catalyze less significant contributions for shape selectivity for skeletal isomerization of n-butene. TPD results suggest that adsorption of NH 3, 1-C4H 8 and i-C4 H8 is shape selective. 62 The technological importance of butene isomerization and similar experiments with ferrierite by researchers at Shell Amsterdam63,64 have recently been reported. 65 Results of our studies (and scaleup studies at Texaco) have led to the commercialization of n-butene isomerization catalysts by Texaco, Inc.66 C. Svnthesis Of Octahedral Molecular Sieves and Octahedral Lavered Materials. The primary building block in octahedral molecular sieves is an octahedral unit. As shown in Figure 3, there are several ways to link these primary building blocks together, such as vertex sharing, edge sharing, or face sharing as described in the treatise by Wells.67 MnO6 octahedra in many materials prefer to link at verteces and edges. The objective in the preparation of octahedral molecular sives is to be able to control the linking of primary building blocks in order to generate porous materials. The structure of synthetic todorokite is shown in Figure 4. It consists of edge and corner shared MnO6 units. Three MnO6 units are shared on each side generating a 1 dimensional pore which measures 6.9 A on each side. The synthesis of synthetic todorokite or OMS-1 was achieved by reacting Mg(MnO4) 2 with Mn2+ salts in NaOH aqueous solutions. The first step in the synthesis is the precipitation of synthetic birnessite or OL-I. This layered material is shown in Figure 2 and consists of edge and corner shared MnO6 units as in OMS-I. However, a layer structure is produced which has exchangeable cations between the layers as well as water molecules. The second step in the production of OMS-1 is the ion-exchange of OL-1 with Mg2+ cations to form a layered material that has the buserite structure. The final step in the synthesis of OMS-1 is the autoclave treatment of synthetic buserite at elevated temperature (near 150oc) and under autogenous pressure.68,69

56

PrimaryBuildingBlock : MnO6 ~

TiO6

%

ReO6

SecondaryBuildingBlock"

Edge Sharing Figure 3. Building Blocks of Octahedral Molecular Sieves. We now know from elemental analyses and ion-exchange studies that a key ingredient in the synthesis of OMS-1 is Mg2+ ions which must be introduced in the first step of the synthesis. On the basis of charge balance and analytical data, about 3 weight % Mg 2+ goes into the framework of OL-1 which leads to stabilization of the layered structure and concomitant generation of synthetic buserite and synthetic todorokite. If no Mg2+ is present during the initial nucleation of OL-1, it is possible to generate the layered structure, however, we have not been able to convert the non Mg2+ containing OL-1 species into OMS-1.70

5"/

8yntheti c Tod oro kite

OMS-1 2+ 2+ Mn 4+ .5012 4.47.4 55 H2 0 Mgl-l. /!nl.9 4.4.4 Figure 4. Structure of Syntheitc Todorokite, OMS-1. The incorporation of Mg 2+ into OL-1 is consistent with the finding of Mg 2+ in some natural birnessite materials. The Mg2+ isomorphous substitution in octahedral sites of OL-1 is also similar to the substitution of Mg 2+ ions in octahedral layers of certain smectite clays where Mg2+ substitutes for some AI3+, such as in montmorillonite which has an ideal composition of Exx[AI2.xMgx]{Si4}O10,~OH)2, where Ex stands for exchangeable cations, the [ ] symbols signify that AI ~H" and Mg2+ an octahedral layer, and the {} symbols signify that Si 4+ is in a tetrahedral coordination. Mg2+ ions can also substitute for AI3+ in the clay mineral chlorite, and is present in octahedral layers of the clay mineral vermiculite. 71 The conversion of a layered material like OL-1 into a 1-dimensional porous material like OMS-1 is also not surprising. It is well known that certain fluid cracking three dimensional zeolite catalysts can be grown from layered clays. In addition, it is noted that birnessite is the most common manganese mineral. Its abundance may be important in geological transformation of birnessite into todorokite.68, 69 Another 1 dimensional tunnel structure material that we have studied is synthetic cryptomelane or OMS-2. The structure and composition of cryptomelane are given in Figure 5. This structure is composed of a 2 x 2 edge and corner shared structure which generates a 4.6 ,~ 1 dimensional tunnel. Cryptomelane is a K+ form of the Ba 2+ mineral hollandite. The chemical formula indicates as is the case for OL-1 and OMS-1, that cryptomelane is a mixed valent manganese oxide with waters of hydration. Cryptomelane also has ion-exchange capacity just like OMS-1 and OL-1, although the restricted size of the tunnel does not allow very large cations to be incorporated, or they are only incorporated to a small extent. 72 During the synthesis of OMS-1 and OL-1, several other phases can be produced. In particular, psilomelane, pyrolusite and ramsdellite can be formed. Pyrolusite has a composition of MnO2 which has a simple tetragonal rutile structure

58 or the b-MnO2 structure. 67 Psilomelane is a 2 x 3 phase, and ramsdellite is a 1 x 2 MnO 6 structure having slit-like pores on the order of 2.3 A by 4.6 A. In addition to these phases, reduction of MnO2 to Mn203 which is known as bixbyite [actually (Fe,Mn)20 3 in nature] or reduction even further to MnO (which as the face centered cubic structure of NaCI)67 can occur during synthesis. Another phase that needs to be avoided in order to prepare microporous materials is Mn304 or hausmannite. It is important to try to avoid nucleation of these phases, or to be able to convert these materials into a desirable phase. Finally, when ion-exchange and isomorphous substitutions are carded out, at temperatures ranging from 400oc to 800oc, it is possible to produce dense spinel phases which are quite stable.

Synthetic Cryptomelane OMS-2

KMn8 O16. nH20 Figure 5. Structure and Composition of Synthetic Cryptomelane. The composition of OMS-1 is (Mg2+,0.98.1.35Mn2+1.89.1.94Mn4+4.38. 4.54012-4.47-4.55 H20. About 3 weight % Mg ~'+ is part of the framework of the resultant OMS-1 material. The average oxidation state of manganese in OMS-1 is about 3.5 as determined by thiosulfate titrations. The thermal stability of OMS-1 is up to 500oc for degradation in vacuum or in N2 and up to 600oc when the degradation is done in the presence of 02.68,69 Synthetic cryptomelane or OMS-2 has a composition of KMn8016.nH20. In this case, there is no substitution of the framework with K + or other ions such as the Mg2+ incorporation with OMS-I. The average oxidation state of OMS-2 is about 3.9. The framework is primarily composed of Mn 4+ ions, however, some Mn3+ ions are found. The thermal stability of OMS-2 is about 800oc for decomposition in vacuum or in N2 and up to about 900oc when decomposition is done in the presence of 0 2. In both OMS-1 and OMS-2, the presence of 02 leads to healing of the structure by O atoms. Defect sites are believed to be oxygen vacancies that are formed during thermal treatment. Synthetic birnessite or OL-1 has an an octahedral layer (OL) structure and a composition of [K,Na]4Mn14027-21H20). The average oxidation state of the

59 manganese is about 3.6 to 3.7, similar to that in OMS-I. This similarity in oxidation state may be an important factor in the transformation of OL-1 into OMS-1, and implies that the transformation may not be as dependent on redox chemistry as is the initial precipitation reaction of OL-1 or in the formation of OMS-2. The thermal properties of OL-1 are similar to those of OMS-1, however, in all cases, the presence of cations, the specific synthesis procedure, and the crystallinity of the resultant OMS1 and OL-1 materials can lead to large differences in thermal properties, especially in differential scanning calorimetry studies.68,69 The structure and composition of OL-1 are shown in Figure 6.

Figure 6. Structure and Composition of OL-1. Several methods have been used to produce different types of OL-1, OMS-1, and OMS-2 materials. The materials that are produced by various methods lead to vastly different materials, that have unique chemical and physical properties. Some of the properties that can be controlled are particle size, color, morphology, average manganese oxidation state, thermal stability, ion-exchange capacity, electrical conductivity, magnetic properties, crystallinity, defect density, desorption of oxygen, and catalytic properties. Table IV summarizes 16 different classes of OMS-1, OMS-2, OL-1, and amorphous manganese oxide (AMO) materials that we have prepared. These materials are separated into different classes because they show different crystalline, chemical and physical properties. For the case of OMS-1 these materials

60 are more crystalline and thermally stable than either natural or other reported synthetic materials. Table IV Summary of !4 Classes of OMS and OL Materigls. Method 1. Reflux

Structure OMS-2

2. Calcination

OMS-2

3. Sol-Gel

OMS-2

4. Sol-Gel

OL-1

5. Precipitation

OL-1

6. Hydrothermal

OMS-1

7. Ion-Exchange

M-OMS-1

Reactants

KMnO4, MnS04~?O MnC]2,02, NaOH KMnO4, KOH#ucrose Cyclodextrin, KMnO4,

Conditions I00~ 17h

Reference 72

400~

18h

?2

gel; 500~ 2h gel, 400oc, 2h

73 73

KOH

(runnel) 8. Ion-Exchange

10. Isomorphous Substitution

M-OMS-2 (tunnel) M-OL-1 (interlayer) [M]-OMS-2 (framework)

11. Isomorphous Substitution 12. Isomorphous Substitution 13. Complexation

[M]-OL-I (framework) [M]-OMS-I (framework) AMO

14. Sol-Gel

OMS-2

'i5. Crystal Growth in Gels

OL-I

9. Ion-Exchange

KMnO4, Mn 2+, NaOH #5,Mg 2+ exchange; #6 plus ion exchanl~e #1,2 plus ionexchange #5, ion exchange

room tanperature

68,69

autoclave 175o12

68,69

room temperature

68,69,74,75

room temperature

68,69,74,75

room Temperature

68,69,74,75

#1,2 plus Dopants added to initial sol

# 1,#2, added @room temperature #5, added @ room temperature #6, added @ room temperature 800(:

68,69,74,75

0~ gel, 500~

?3

#5, plus dopants

added to initial sol #6 plus Dopants added to initial sol KMnO4, oxalic acid KMnO4, ma]eic acid Sodium silicate gel containing

1V~z+, KMnO4

Room temperature, 30 days

68,69,74,75 68,69,74,75

76

??

above hardened gel.

Each of the materials in Table IV will be described below. Entry 1 involves the precipitation of OMS-2 from solutions that are refluxed. This synthesis results in high surface area synthetic cryptomelane materials. For method 1, the resultant 3.9 oxidation state of manganese is obtained by starting with a Mn 7+ reactant (KMnO4) and a Mn2+ reactant and approaching an intermediate oxidation state. The reflux method and a subsequent ion-exchange is shown in Figure 7.

6]

KlVlnO 4 +

Mn 100Oc//

M+

Mn 2+ + NaOH + 0 2

Figure 7. Synthesis of OMS-2 Via Reflux Methods. OMS-2 can also be made by oxidation of Mn 2+ as summarized in method 2. In this case, calcination to higher temperatures (400oC versus 100oc for method 1) is needed in order to obtain crystalline OMS-2 material. The particle size of the OMS-2 prepared by calcination is larger than that prepared by reflux methods. Another method for preparation of OMS-2 that we have recently reported involves the use of sol-gel techniques. In this case, sugars like sucrose can be reacted with KMnO4 to reduce the manganese with concomitant formation of a sol. Aging of the sol followed by calcination at elevated temperatures leads to crystalline OMS-2. Sol-gel derived OMS-2 materials show much greater thermal stability than materials made by either procedures 1 or 2, and they also are of much larger particle size. As particle size is increased the number of defects also decreases. The amount of oxygen desorption from OMS-2 prepared by sol-gel techniques is significantly lower than from materials prepared by reflux or calcination methods. While other sugars besides sucrose can be used to obtain OMS-2, sucrose gives the most crystalline and pure product. Synthetic birnessite or OL-1 can be prepared by sol-gel methods such as the reaction described in method 4. In this method cyclodextrin can be oxidized by KMnO4 with formation of a sol. Aging and calcination of the resultant gel at 400oc leads to crystalline OL-1. The particle size of the OL-1 made by method 4 is larger than with other methods, and to date we have not been able to convert the sol-gel derived OL-1 material into OMS-1 by any method. Method 5 describes the precipitation of OL-I. Such reactions of KMnO4 and Mn 2+ lead to small particle size OL-1 that can be converted into OMS-1. The crystallinity of this OL-1 is lower than the sol-gel derived materials and the thermal desorption of oxygen is marked for the precipitated materials as opposed to small amounts of oxygen desorption for the sol-gel materials. More oxygen defects are present in the precipitated OL-1 that the sol-gel materials (as is the case for OMS-2 systems, and the defects are again due to oxygen vacancies. This method along with subsequent ion-exchange of OL-1 is shown in Figure 8.

62

KMnO 4 +

Mn

2-1-

+

NaOH Ru

0L-1 M-I-

Figure 8. Synthetic Scheme for Precipitation of OL-1. Method 6 involves the hydrothermal alteration of OL-1 into OMS-I. The overall reaction scheme involves the precipitation of OL-1, followed by ion-exchange with Mg2+ cations, followed by treatment in an autoclave at 175oc for several days. The resultant OMS-1 material has very small particle size, high surface area (250 m2/g) and a considerable number of defect sites. It is esssential that Mg2+ ions are present during the precipitation of OL-1 in order to end up with crystalline and thermally stable OMS-1 materials. An overall scheme depicting the synthesis of OMS-1 is shown in Figure 9. Method 7 involves the ion-exchange of OMS-I. In this case, divalent ions such as Mn 2+, Co 2+, Cu2+, Ni2+, and Zn 2+ can be exchanged into tunnels sites of OMS1. Ion-exchange at temperatures of 80oc can lead to enhanced exchange of OMS-I. Good evidence for ion-exchange of monovalent ions such as in OMS-1 also exists. The ion-exchange forms of OMS-1 will be given the acronym M-OMS-I. Ion-exchange of OMS-2 can also occur due to exchange of K+ ions out of tunnel sites. Both monovalent and divalent cations have been exchanged into OMS2. The acronym M-OMS-2 will be used to signify ion-exchange of OMS-2. Ionexchange at temperatures greater than room temperature again lead to enhanced amounts of cation-exchange with respect to room temperature exchange. This process is described in method 8. Ion-exchange of OL-1 can also occur much the same way as with OMS-1 and OMS-2 as described in method 9. The ion-exchange of OL-1 is a critical step in the formation of OMS-I. The hydrated cationic complex is believed to act as a template around which the tunnel can be formed. There is a small variation in ionic radius that allows the formation of OMS-1, with ions ranging in size from Mn2+ to Zn2+ being ideal. In addition to substitution of tunnel sites via ion-exchange, it is possible to isomorphously replace cations in the framework of OMS-I. The general synthetic scheme is reported in method 10 and involves the doping of small amounts of cations into the precursor solution, before OL-1 is precipitated. Divalent cations like Mn2+, Co2+, Cu2+, Ni2+, and Zn 2+ can be incorporated into the framework in this manner.

63 Template cations are still needed in the second step in order to produce [M]-OMS-1 materials where [M] signifies incorporation of M into the framework. Isomorphous substitution of OMS-2 is described in method 10. In this case, the dopants are added to the solution prior to precipitation and reflux treatment.

o,.1

.g2

Mg.OL.1 175~ Autoclave

io.s 1 1 M+

Figure 9. Synthesis Scheme for Preparation of OMS-1. [M]-OMS-2 materials are significantly different than M-OMS-2 materials in terms of chemical and physical properties. A similar type of nomenclature is used for describing the isomorphous substitution of OL-I. For example, [M]-OL-1 would signify isomorphous substitution in the MnO 6 layers of OL-I. This preparation method is dsecribed in method 11 of Table IV. Method 13 describes the generation of amorphous manganese oxide (AMO) materials that are made by the complexation of oxalic acid with KMnO4. This reaction is done at 80oc and leads to an amorphous gray black powder. The chemical and physical properties of AMO are very different than all the other materials listed in Table IV. K + ions are incorporated into AMO due to reduction of Mn 4+ ions. Analytical data suggest that some unreacted oxalic acid is incorporated into the AMO powder. This suggests that some Mn 4+ ions are reduced to Mn3+ creating a mixed valent species, as is the case for OL-1, OMS-1, and OMS-2. The AMO materials has been found to be an outstanding photooxidation catalyst for the conversion of alcohols to ketones such as Isopropanol to acetone. Two key features of this system concern the ability to desorb oxygen at very low temperature and the synergistic effect of AMO with other solid substrates such as MgO. Oxygen can be desorbed from AMO at room temperature during photolysis. The oxygen can be detected by chromatographic techniques and it is clear from X-ray photoelectron spectroscopy (XPS) studies that there is an enhancement of oxygen at the surface of the AMO during photolysis. The amount of oxygen desorbed at low temperaures is markedly higher than crystalline OMS and OL materials.

64 Another interesting feature of the AMO photocatalysts is the effect of diluent substrates such as MgO or activated C. Addition of substrates causes an increase in the rate of photoassisted catalytic oxidation of isopropanol. A synergistic effect is clear; specific amounts of diluent lead to an increase. Too much or too little diluent leads to a decrease in rate. The exact explanation of this synergistic effect is not known, however, it may related to the ability of species such as OH or adsorbed hydrocarbons and intermediates to travel back and forth across the AMO/substrate interface. There does not seem to be a correlation of rate with the surface area, acid base character, particle size or other physicaVchemical properties of the substrate. Method 14 involves the sol-gel synthesis of OMS-2. In this case dicarboxylic acids like maleic acid are used to reduce KMnO4. Highly crystalline low surface area OMS-2 materials can be made in this manner. The dicarboxylic acids are oxidized to CO 2 as is the case for the sol-gel sugar preparations (Method 3). It is apparent from studies of sol-gel syntheses that strong acids react very rapidly and do not generate stable sols. For this reason, weak acidic material like dicarboxylic acids, sugars, cyclodextrins and similar materials need to be used. The final method mentioned in Table IV involves crystal growth in gels. In this case, sodium silicate gel is acidified with a weak acid like acetic acid and a sol is formed. Mn2+ ions can be dissolved in the sodium silicate reactant before addition of the sodium silicate. The order of addition of acid to sodium silicate is important because addition of sodium silicate to acid causes instantaneous gelation and air and CO 2 can be trapped in the gel. When acid is slowly added to the Mn2+, a sol forms that takes about 8 h to form a gel via syneresis. The evolved water during syneresis must be allowed to vaporize, so it is important that the container is not sealed, but only lightly covered to avoid contamination from the atmosphere and dust particles. Once the gel has set, KMnO4 solutions can be added to the top of the hardened gel. At this stage it is important to stopper the reaction tube, so that the KMnO4 solution does not change its concentration. After 1 week nucleation of crystallites occurs, and after about 1 month large crystals of OL-1 (150 m) are formed. The crystal growth in gel method slows down the crystallization process and can decrease the number of nucleation events with respect to similar reactions carded out in solution. There are some other methods that might be envisioned that could lead to OMS and OL materials. One obvious direction would be to use structure directors or templates that are similar to those used in zeolite synthesi such as tetraalkylammonium halides. Unfortunately, we have observed that such structure directors and templates react with KMnO4 and get oxidized to CO2. Another seemingly obvious route would be electrochemical syntheses. Some research has been done in this area, however, it is difficult to synthesize a sizeable amount of material such as with controlled potential electrolysis. In addition, some early work showed the generation of amorphous materials that after inital formation can be heated to form spinel phases without apparently going through the OMS/OL phases. Another obvious direction would be to mix several of the methods outlined in Table IV. In this case, at least for the preparation of OMS-1, so far we have only been able to convert OL-1 which has been made by precipitation methods. This may be due to the small article size and large number of defects in OL-1 synthesized by precipitation methods (method 5). One combination that does lead to new materials is

65 the isomorphous substition of either OL-1 (method 11) or OMS-2 (method 10) followed by ion-exchange of the isomorphously substituted OL-1 or OMS-2. In these materials, two types of metal ions can be incorporated into resultant OMS and OL materials, one in the tunnel sites and one in the framework sites. A variety of combinations of metals is possible. By converting [M]-OL-1 to [M]-OMS-1 (method 9) with ion-exchanging the [M]-OL-1 precursor, it is possible to incorporate two types of metal ions into OMS-I. Literature preparations of OMS-2 types of materials such as hollandite and cryptomelane materials are quite common. A variety of other methods have been used to make hollandite type materials such as thermal treatment of mixed oxides and high presssure syntheses, however, the resultant maetrials usually are of very large particle size and have porosities and catalytic actvities that are significantly lower than OMS and OL materials described in Table IV. Exceptions to this trend are the sol gel materials of Table IV (methods 3, 4, and 13) which lead to relatively stable and large particle size systems. The crystals grown in gels (method 15) also lead to relatively inert materials that have large particle sizes. OMS-2 prepared by reflux methods has higher surface areas, smaller particle sizes, more acid sites and greater defects (primarily oxygen vacancies) than materials reported by others.78,79, 80 For the sol-gel preparations, we are not aware of use of this method to prepare OMS-2. Both tunnel (M-OMS) and framework [M]OMS substitution is possible. We are also not aware of attempts to incorporate transition metals into the framework of such materials. Note that the different preparations of OL (#4,5) and OMS-2 (#1-3,14) of Table IV yield unique materials with different particle sizes and other physical properties. D. Characterization of OM$ and OL Material~. The compositional, electrochemical, structural, mixed valency, magnetic, thermal, acidity, and surface properties of OMS and OL materials have been investigated in detail by our group.68"70, 72"77 The semiconducting nature of such systems is particularly novel.68,69,75 The ease of electron transfer,75 may allow applications in energy and electron transfer as well as battery applications. Isomorphous substitution,74, 75 of OMS-1, OMS-2 and OL-1 has led to a variety of new materials where chemical and physical properties can be controlled. For the case of magnetic environments, spin glass behavior,74 has been observed in these systems. The extreme variety in chemical composition68-70.72-77 of materials such as those outlined in Table IV is difficult to achieve in similar systems such as clays and zeolites, because many more transition metals prefer to be in octahedral coordination and can be substituted for manganese than the number of substitutions for AI3+ and Si4+ in tetrahedral sites. Acidity of OMS and OL materials can be controlled by varying the composition and structure. This is not unexpected based on studies of zeolites and clays. Tremendous adsorptive capacities of OMS and OL materials are observed68, 69 on the same order of magnitude of natural manganese nodules. The OMS and OL materials rival zeolitic uptakes and can be as high as 20 g adsorbate per 100 g OMS/OL material. Similar observations53,81 have been made for the natural

66

manganese nodule counterparts of OMS and OL systems, although such natural materials are often mixtures, less pure, less crystalline, and less readily available. A summary of electrochemical, magnetic, and conductivity properties is given in Table V. Note that these properties can be controlled by proper choice of OMS/OL framework and composition. Some specific applications of these materials are as sensors to discriminate size and charge, as regchargeable nonaqueous secondary batteries, as new semiconducting materials, and in magnetic applications. The correlation of structure and composition with properties shown in Table V has not been well developed. For example, we do not know why K-OMS-2 is a spin glass material whereas other OMS-2 materials and OMS-1 and OL-1 materials are not spin glasses. The same lack of understanding holds for the conducting properties of these systems. OMS-2 systems have conductivities that are orders of magnitude greater than OL-1 and OMS-1 systems. This may be due to the incorporation of traps in both OL-1 and OMS-1 that are divalent cations that impede electron transfer, but this is not known with certainty. The mechanism of conduction is also not clear. In certain cases, electrical conductivity is apparent whereas in other materials, ionic conductivity appears to dominate. It is likely that both types of conductivity occur in some materials. Table V Physical ProDerties and ADolications of OMS and OL Materials. PROPERTY Electrochemistry Cyclic Voltammetry _

Conductivitv OMS-1 Ea = 0.35 eV OMS-2 Ea = 0.25 eV OL-1 E,~ = 0.39 eV Maanetism Mg-OMS-1 K-OMS-2 Cu-OL-1 v

COMMENT

REFERENCE

Charge Selective, Size Selective

76

5 x 1o-5 (w-c=)-I 1 x 10 -3 (W-cm)-] 1 x 10 -6 (W-cm) "]

82

Weak Magnetism Spin Glass Weak Ma~lnetism

28

General synthetic conditions for preparing OMS and OL materials are summarized in a proceedings manuscript. 83 The synthesis of small particles of OMS2 having the cryptomelane structure as well as a Rietveld refinement have been done to verify space group assignments. 72 We have recently shown that various inorganic cations can be used as true templates to form synthetic todorokite. 84 Characterization with a variety of techniques was necessary in order to understand the changes in structure and role of the template. The resulting materials can incorporate Cu 2+, Zn 2+, Ni 2+, Co 2+ and other divalent ions in tunnel sites of these materials. Organic reducing agents such as fumaric acids have been used to develop

67 sol-gel preparations of cryptomelane (OMS-2).85 In addition, simple sugars have been used with KMnO 4 to prepare synthetic birnessite materials (OL-1).86 These solgel routes have opened up many doors for the synthesis of OMS and OL materials and are much easier to carry out than our earlier preparations. Infrared spectroscopy methods have been used in these systems to study the role of the organic reducing agents. A variety of characterization methods have been used to study the optical, electronic, surface, bulk, thermal, morphological, and magnetic properties of OMS and OL systems. Cyclic voltammetry methods have be used 87 on these systems to study charge transfer since they conduct. In addition, both dc and ac electrical conductivity measurements have been made on OMS-1, OMS-2 and OL-1, and results suggest that both electrical and ionic conductivity exist. 82 Both dc and ac magnetic susceptibility experiments have shown that some of these materials are spin glasses.74 Surface properties76, 88 have been studied with both X-ray photoelectron spectroscopy and scanning Auger microscopy and results suggest that there is good lateral and bulk homogeneity even for doped samples. X-ray absorption studies in collaboration with Steve Wasserman and Katie Carrado at Argonne National Labs89, 90 have been done to study the average manganese oxidation states of these systems, and EXAFS studies have been used to confirm the octahedral geometry and to help understand differences in structural properties of crystalline OMS and OL materials as well as amorphous manganese oxide photocatalysts. E. Catalvtic Activitv of OMS and OL Svstems.

The two major catalytic applications of OMS and OL materials involve oxidations and photooxidations. Some amorphous manganese oxide (AMO) systems have been prepared that are outstanding photooxidation catalysts for degradation of CH3Br and conversion of isopropanol to acetone. 76 Catalytic data for several OMS and OL systems are summarized in Table VI. Table Vl Catalytic Data for OMS and OL Systems. Catalysis CO Oxidation H20 2 Decomposition C6H14 Dehydrogenation C6H14 Oxidation 1-C4H 8 Isomerization CH 4 Oxidation Photocatalvsis i-C3HTO-H Oxidation III

Product . . . . CO 2, 100% S, RT H 2, 02, 90% S to 1-hexene 60% Co 80% S to 1-C6H12OH c/t-2-C4H 8 C2,C3,C4,C5,C6

Reference 68,69 91

CH3(CO)CH 3, Room T, 100% S

76

I

II

I

II

II

91 91 68,69 68,69

II

I

68 One of the most important properties of these OMS and OL systems is their ability to lose and recover oxygen.92,93,94,95 Temperature programmed desorption, reduction, Oxidation, and studies of lattice oxygen mobility and structural stability studies have been done on a variety of systems. A review of these and similar open framework structures has recently been submitted.96 The major focus of our ongoing research is catalytic studies of OMS and OL materials. Catalytic activity for both total and selective oxidations is excellent.68,69,91 Some of the reactions under investigation are the liquid phase oxidative hydrogenation of cyclohexane, gas phase oxidative dehydrogenation of cyclohexane, oxidative dehydrogention of hexane to 1-hexene, total oxidation of CO, selective oxidation of humic acid, decomposition of peroxide, and similar reactions. We are optimizing conversions, selectivities, and yields and studying the fate of the OMS and OL materials after reaction. Mechanistic studies of interactions of organic reagents with OMS and OL surfaces are also underway. We have shown that [Ni2+]-OMS-2 and [Ni2+]-OMS-1 catalyze the selective conversion of hexane to 1-hexene. Stainless steel flow reactors of 1/4" diameter containing 0.5 g catalyst, charges of 7 g n-hexane in 2 h, 1 atm pressure and temperatures of 500oc are used in these experiments. Both gas chromatography (GC) and mass spectrometry (MS) analyses are done to monitor product distributions. Conversions as high as 60% and selectivities of 90% (to the terminal olefin) have been observed for the OMS-2 system. This may be a consequence of the better shape selectivity of [Ni2+]-OMS-2 (4.6 A tunnel) versus [Ni2+]-OMS-1 (6.9 A). The latter material is not as selective or active. Systems that do not contain Ni ~'+ are totally inactive. 91 There is precedence for dehydrogenation activity of these systems since manganese nodules have been reported to be excellent catalysts for dehydrogenation of cyclohexane. 53 Unfortunately, the gas phase reaction of hexane to hexene lead to degradation of the catalysts. It is clear that the manganese is reduced predominantly to Mn 2+ in the crystalline residue which has the MnO structure. The XRD peaks are very broad indicating a highly defect type structure which still contains MnO 6 octahedral units. Due to this structural change in the catalyst we have been focusing on oxidative dehydrogenations. In this case the hope is that oxygen can be used to regenerate the mixed valent catalyst when the organic substrates are selectively oxidized. Liquid phase oxidations have been shown to preserve the structure of OMS during reaction and produce yields and conversions that are on the same order of homogeneous catalysts. The synthesis of OMS and OL materials has led to unique opportunities to study fundamental chemical and physical phenomena such as mixed valency, electron transfer, spin glass behavior, conductivity, isomorphous substitution, catalytic oxidations, photocatalysis, new synthetic methods, structural analysis, phase stability, and other areas.68-70, 72-77 Semiconducting molecular sieves are rare and are excellent materials for high resolution spectroscopic characterization (especially imaging, surface studies, etc.) and for catalytic studies for correlation of activity/selectivity and redox or electron transfer capability. The potential of OMS materials as adsorbents, catalysts and in secondary battery applications has been featured in Catalytica ,Highliqhts_,97 Scientific American98 and Science Digest.99

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H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology

Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All rights reserved.

75

Organic Zeolites? Stephen Lee Department of Chemistry, University of Michigan, Ann Arbor, M148109-1055, USA. and D. Venkataraman Department of Chemistry, University of Illinois, Urbana, IL 61801, USA. January 11, 1996

Introduction Zeolites are a class of inclusion compounds, in which the occluded guests can be removed without the collapse of the host structure. The porous structure that results upon the removal of guests (generally water) has proved to be technologically important in the areas such as shape selective catalysis, sensors and ion-exchange membranes. ~ Parallel to the developments that are taking place in the synthetic procedures of these aluminosilicates, there has been an ongoing effort for the construction of porous solids using organic compounds. It is believed that by the use of organic compounds, a greater level of control on the nature and size of the cavities may be possible. Also, these organic based materials may be more easily processed into thin films or fibers. One of the difficulties in the realization of porous materials using organic molecules is the isotropic nature of intermolecular interactions. This isotropy generally leads to closest packing. Directional intermolecular interactions such as hydrogen and coordination bonds have been used to ovemde the tendency for closest packing. If a host network has void space, then these voids may be filled in two ways: a) by inclusion of guest moieties such as solvent and/or counterions (like zeolites) and b) by the interpenetration of topologically equivalent nets - a process of self-inclusion. Generally, either mechanism results in a dense structure. Also, in contrast to zeolites, typical molecular inclusion compounds irreversibly lose crystallinity, undergo a phase change or alter their morphology upon loss of their guests. It has, however, been shown that some coordination and hydrogen bonded networks can rapidly exchange inclusions or counterions while maintaining crystal integrity. 2 The problems of closest packing and the general instability of the host lattice raises the question of whether a porous structure with zeolitic properties constructed from organic compounds i.e. an organic zeolite, can be realized? More specifically, can a porous organic solid, if realized, function like a zeolite? In this review, we shall address these questions by taking a few examples of inclusion compounds where the host structure is proven to be stable to solvent loss. We will also evaluate a future direction for these compounds in the context of building hosts for the synthesis of chiral organic molecules. These examples may be considered as the stepping stones towards the goal of an organic zeolite. 3

76

Dianin's Compound One

of

the

well-studied

inclusion

compounds

is

4-p-h y dro x yphen y 1-2,3,4-

trimethylchroman. 1, commonly known as Dianin's compound. 4 First

CH 3

prepared in 1914 by the Russian chemist A. P. Dianin, this compound has attracted considerable attention due to its ability to tenaciously hold on to certain organic solvents. A cage structure was predicted in the adducts of 1 based on space group, unit cell

H3C

dimensions and packing considerations 5 and was confirmed by detailed X-ray single crystal structure studies on the chloroform and

OH

ethanol adducts of 1 after 15 years. 6 Irrespective of the guest, the structure consists columns of independent cages of ca. 6/~ x 11 ~, running along the c-axis (see Figure 1). Six molecules of 1 link together through hydrogen bonds to form a complex such that the oxygen atoms (of the phenol) constitute an hexagon. Alternate molecules in the complex point up or down. The cage is formed when two of these complexes stack in a way that one of the hexagons of the H-bonded oxygen atom constitutes the ceiling and the other hexagon constitutes the floor of the cage. 6 The stacking of these cages along the c-axis result in columns whose topology resembles the interior of an Allihn condenser.

Figure 1: A stereoviewshowing an independentcage of in the structure of Dianin's compound (1). Based on the occupancies and thermal factors of the guest molecules in these structures, it was speculated that some of the cages might be completely empty. 7 This speculation led to the conclusion that Dianin's compound might retain its cage structure upon complete removal of the

-2-

77

guest species. A definitive proof that the cages are only partially filled and that the host is stable came from CP/MAS 129Xe and 13C NMR of the xenon occluded guests.

Ripmeester and co-

workers crystallized 1 from dodecane under varying xenon pressures. ~la In the 129Xe NMR spectra of the occluded complexes, they found two peaks, ca. 18 ppm apart, which varied in intensity as a function of the pressure of xenon used during the crystallization. The high field peak was predominant under low xenon pressures while the low field peak was predominant at higher xenon pressures. As illustrated in Figure 2, depending on the concentration of xenon, the cages can be completely

empty

(A),

partially

filled

(B)

or

completely filled (C, the maximum occupancy of a cage being two xenon atoms).

From their intensity

behavior, the high field peak was assigned to partially filled configuration (B) and the low field peak was attributed to the completely filled configuration (C). The authors of this paper mention that "it is not really clear whether the presence of the second guest in the cage is the primary reason for the shift, or whether the second guest causes a change in the configuration of the methyl groups at the neck of the cage which in turn is sensed by the xenon atom." Since they also reported similar behavior in the ethanol-xenon adducts and 13C NMR showed chemical shift changes in the methyl carbon, it is tempting to assign the 18 ppm shift to the change in configuration of the methyl groups. This NMR study attests the earlier speculation that some of the cages might be partially occupied.

In addition, although the ref'mement details remain unpublished, MacNicol and co-

workers noted that their X-ray studies confirm that even the unsolvated 1 retains its cage structure. 8 Since Dianin's compound retains its structure after the loss of solvent, can it function like a zeolite? In other words, can the adsorption and desorbtion of the guests take place without the loss of crystallinity or change of phase? Barrer and Shanson demonstrated that 1 does behave like a

I I I I I I

~ds

zeolite by studying its sorption properties using a variety of gaseous guests. 9 The guest molecules used were argon, krypton, xenon, carbon dioxide, methane, ethane, propane, n-butane, iso-butane and neo-pentane.

They found that the sorption isotherms of 1, like

-3-

P

Po

A ty.pe I adsorption isotherm

78

zeolites, were of Type I (Brunauer's classification). 1~ More recently, the dynamics of these occluded guest molecules in the cages of 1 have been studied by NMR spectroscopy. 11 Unlike zeolites, the cages in Dianin's compound are not extended.

The cavities that

constitute the ceiling and the floor of cage are ca. 2.8 ]k in diameter. This indicates that the entry of the guest molecules into the guest-free Dianin's compound may occur through some reorganization.

However, there is no satisfactory explanation to why the porous solid state

structure of 1 is stable to solvent loss. Although the host lattice may not be as rigid as zeolites, it is important to note that the structure does retains its pores after the removal of guests and has sorption properties like zeolites.

Helical tubulates of 2,6-Dimethylbicyclo[3.3.1]nonane-exo-2,exo-6-diol

An interesting inclusion compound derived from 2,6-dimethylbicyclo[3.3.1]nonane-exo-

2,exo-6-diol, 2, was reported by the group led by Bishop and Dance. 12 The crystal structure of the alicyclic diol 2 can be

HO

OH

construed as packing of helices along the c-axis. The parallel canals that result from the helical tubules have an unobstructed triangular cross-sectional area of roughly 20 A2 which are

H3C

occupied by the guests (see Figure 3). ~ The helices bear striking

"

u

CH~

2

similarities to that in the structure of a-quartz. The resemblance to a-quartz becomes important in the design strategies, as discusses in the second part of this review, to use these porous organic solids for asymmetric synthesis. 13

Figure 3: Stereoviewof the helical tubulates along c-axis in the guest adducts of 2.

-4-

79

Several guest adducts of 2 with similar helical tubuland structure have also been reported by Bishop, Dance and co-workers. ~~ The most interesting result they was reported was that guestfree sample of 2, irrespective of how it was prepared, retained its open channel structure. The guest-free samples were prepared in three ways: a) by heating the inclusion compounds under reduced pressure b) by sublimation and c) by crystallization from mesitylene. ~4 The products that were obtained by the three methods were identical by IR spectroscopy and elemental analysis. The elemental composition, C11H2002, corresponded to the pure diol 2. 13C CP/IVIAS NMR studies showed that the carbon resonances observed for the guest-free product were almost the same as for the helical tubulate compounds. The powder X-ray diffraction pattern of the guest-free product was in complete agreement with the calculated pattern from the single crystal coordinates of adducts of 2 but without the solvent. It is rather intriguing why 2 does not crystallize in a closest packed arrangement in the absence of solvent. Even though crystal structures of various guest adducts of 2 are known, TM no studies about the exchange of these guests have been reported. The examples described above show that it is possible to realize inclusion compounds which does not collapse upon removal of solvent. The host lattice in these structures is primarily held by intermolecular hydrogen bonds. Since the stability of zeolites results from the stronger SiO-AI covalent bonds, it would be of interest to explore intermolecular bonds that are reversible and have similar strength to that of covalent bonds. In this regard coordination bonds have attracted wide attention for the construction of networks. ~5 A general strategy that is being followed is the construction of molecular analogs of prototypic minerals using organic ligands and metal ions. We will discuss two coordination networks constructed by this approach~6 from the group led by Lee and Moore which retain their pores upon partial or complete loss of the included solvent.

[Ag(1,3,5.tris(3-ethynylbenzonitrile)benzene)CF3SOa].2C6H6 The

structure

of

[Ag( 1,3,5-tris(3-

CN

ethynylbenzonitrile)benzene)CF3SO3].2C6H6, 4, is a 3-connected, two-dimensional coordination network (see Figure

4 ) . 17

II

In this structure, the coordination

geometry around silver is trigonal pyramidal, with three nitriles of the network in the basal plane and a

NC

triflate counterion bound to the apical position. The orientation of the nitrile groups on 3 together with the above-mentioned silver coordination geometry gives a [12]annulene-like segment as the simplest cyclic motif

-5-

CN

1,3,5-tris(3-ethynylbenzonitrile)benzene (3)

80

of the network. These sheets are stacked in an ..-ABCD..- sequence creating channel structures that run at an oblique angle to [101 ] i.e., the layer normal (Figure 5). Difference Fourier analysis locates sixteen molecules of benzene per unit cell in the channels, four of which are disordered.

Figure 4: Illustration of the [12]annulene unit in a sheet that constitutes the structure of 6. These sheets stack at an oblique angle to[101] to generate channels of c a . 8 ,~, in diameter.

The thermogravimetric analyses (TGA) of microcrystalline powders of this complex trace revealed two discrete mass losses at 110 ~ and 145 ~ corresponding to a mass percent of four (4.5%) and twelve benzene molecules (13.5%) respectively. It was rationalized that the initial mass loss corresponded to the loss of four disordered benzene units. The mass loss at 145 ~ was attributed to the loss of the ordered twelve benzenes. Differential scanning calorimetry (DSC), optical microscopy and X-ray powder analysis showed that there was no phase change associated with the first mass loss. At 145 ~

much like an inclusion compound behavior, a solid-to-solid phase transition

occurs concomitant with the loss of the remaining benzene molecules. This high temperature solid phase eventually undergoes a melting transition at 169 ~

Upon cooling the melt, a glassy

material is obtained. DSC cooling traces indicated heat capacity jumps that could be attributed to a glass transition. No chemical decomposition of the ligands occurs up to 200 ~ as verified by redissolving the glassy material and recording its NMR spectrum. The

unit

cell

parameters

of

a

single

ethynylbenzonitrile)benzene)CFaSOa].2C6H6 heated to 110 ~

crystal

of

[Ag(1,3,5-tris(3-

for 10 minutes and subsequently

cooled to room temperature remain unchanged within the standard deviation of the original crystal. ~s TGA experiments on crystals of similar or larger dimension confirm that a mass loss

81

equivalent to four benzene molecules occurs under these conditions.

These crystals remain

optically transparent and uniformly birefringent when viewed between crossed-polarizers. Under higher magnification, interesting surface changes as a function of temperature were noted in these crystals. It could be seen from the optical micrographs that surface defects appear around 110 ~

increasing to 124 ~

However, the macroscopic crystals remained intact. It is

likely that there is a surface reconstruction after the removal of the disordered benzene molecules.

Figure 5: A stereoview showing the channels which are at an oblique angle to (101) in the crystal structure of 4. Benzene molecules occupy these cavity and are omitted for clarity. Microcrystalline samples heated to 145 ~ under vacuum and subsequently cooled to room temperature re-absorb benzene vapor in an amount that corresponds to the mass percent in the original, unheated sample. Sorption saturation is achieved in

ca.

60 h at room temperature. X-ray

powder diffraction shows that the original solid phase was reformed. This behavior is analogous to that of classical inclusion compounds.

In contrast, samples of 4 heated to 110 ~

re-absorb

benzene in an amount equivalent to four molecules of benzene in less than 45 min. without ever undergoing a phase change.

[Ag(1,3,5-tris(4~thynylbenzonitrile)benzene)CF3SO3]-2CrH6 Two polymorphs exists for the complex of 5 with silver(I) triflate. We have previously reported the crystal structure one of polymorphs, 6, and it is homeotypic with LaPtSi (ThSi2-type) structure. 2b,~9 In contrast to the LaPtSi structure where each of the atoms are roughly comparable in size, the tritopic ligand 5 is significantly larger than either the Ag(D or CF3SO3" ions. As a result, a single LaPtSi-type net constructed with the dimensions of the tritopic ligand 5 and silver

82

(I) generates large void spaces. The triflate anion is of insufficient size to fully occupy these voids. The voids along [010] and [001] are filled by the six mutually independent, interpenetrating LaPtSi-type lattices that constitute the [Ag(5)CF3SO3]-2C6H6 structure (Figure 6). 20

The

interpenetration occurs in a way to accommodate the propensity of aromatic rings to lie in a slightly staggered coplanar arrangement with an offset angle (o0 37 ~ at an interplanar distance (d) of ca. 3.3 ]k. To accommodate the geometrical requirements 21 (o~ and d) for stacking of the aromatic rings, the nets are sheared to an angle (0) of 60 ~ (0=90 ~ for an ideal LaPtSi net and 0 ~ for CaCuP (A1B2-type).

However the interpenetration leaves the channels along [100] near the

maximal size possible for this network (see Figure 7). This cavity of 15 x 22/~, is filled by the solvent molecules.

The coordinates of 12 benzene molecules/unit cell were located in the

refinement.

Earlier, we had reported the exchange of

CN

benzene with benzene-d6 in the channels of 6 without the destruction of crystallinity.

We have extended this

study to other guests like toluene, m-xylene, undecane, benzyl

alcohol,

2,6-di-tert-butylphenol

and

II

(+_)-l-

phenylethanol. 22 Cell constants determined by X-ray powder diffraction after exchange were close to the original cell constants.

Optical micrographs showed

that there was no dissolution and reformation of the crystals during the period of exchange.

NC

CN

1.3.5-tris(4-ethynylbenzonitrile)benzene(5)

Guest-free samples of 6 can be prepared by heating the guest included complex in a TGA furnace room temperature to 200 ~ under nitrogen. When the samples were heated in an open pan, no phase changes were observed in the DSC.

The X-ray powder diffraction data can be

indexed as a two-dimensional rectangular unit cell with no sufficient information along the a-axis. However the lattice constants b and c had changed only by 5%-10% from the initial single crystal model. 2~ The OkI reflections calculated from the single crystal model (orthorhombic cell) were in agreement with the observed Okl reflections (rectangular cell). Based on this model, it can be concluded that the 15 A x 22/~, pores have been retained in the guest-free samples. The guest-free samples were exposed to vapors of benzyl alcohol, benzene, m-xylene, cyclooctane, undecane, (+_)1-phenylethanol. 24 Thermogravimetric analyses showed that cyclooctane and undecane were not absorbed by the guest-free sample of 6 in 60 h. However, benzyl alcohol, (+_)-l-phenylethanol, benzene and m-xylene were absorbed with the guest to ligand stoichiometry of ca. 3.5:1.0. The lattice cell constants calculated from X-ray powder diffraction were identical to those calculated from solution exchanged samples.

83

Figure 6: A single LaPtSi-type net from the crystal structure of 6. Five more nets interpenetrate to fillthe void space created in a single net.

Figure 7: A stereoview showing the 15 ,A,x 22 ,~ channels along the a-axis in the crystal strucure of 6. The solvent molecules and the counterion occupy these channels and have been removed for clarity.

Recently Yaghi and co-workers reported a coordination network based on trimesic acid and Co(II) which retained its porous structure upon the removal of solvent molecules and selectively absorbs aromatic guests. 25 Also, Fujita, Ogura and co-workers have demonstrated that cyanosilation of aldehydes can be performed in the microchannels of a two-dimensional square material composed of cadmium(II) and 4,4'-bipyridine. 2~ From the examples described above, it is quite apparent that organic zeolites are more of a reality than a fantasy. 26 What is the future that await these porous compounds with zeolitic behavior?

84

Further Directions One future direction for organic zeolite analog chemistry will involve the synthesis of chiral organic molecules. The reasons are two-fold. First molecules in crystals are generally trapped in just a few conformations. If the host crystal environment is chiral then this chirality can be imparted to the guest. Second, there are a large number of readily available chiral organic building blocks which can be used in the preparation of chiral host crystals. This is especially true for organic molecules and hence there is a clear advantage for organic as opposed to inorganic crystal hosts. We highlight here a few studies in which the synthesis of chiral molecules has been achieved through the use of organic crystals in the hopes that this will prove a useful incentive and review. The reported studies fall into two natural categories. In the one case one starts with racemic mixtures or optically inactive compounds, crystallize these materials into chiral crystals and finally by subsequent reactions, trap this chirality in the final chemical products. In the second category one forms host-guest inclusion compounds in which the host is already an optically resolved compound. This in turn leads to the formation of optically active guest molecules. In the first class of studies the sole chiral influence derives from the asymmetric environment of the molecule in the crystal. This implies that while the initial molecular sample is either fully racemized or just not optically active in solution, the crystal is optically active and thus belongs to one of the 65 chiral space groups (out of a total of 230 possible space groups). As the starting mixture is achiral in all likelihood the crystalline product will be a mixture of the two chiral forms. However by either growing just a few large crystals out of the sample and/or by seeding the sample with previously selected crystals of a given chirality one may still obtain reasonable quantities of chiral crystals. While the hand-picking of crystals may play an important role, it has been shown that this is not necessary in imparting a given chirality to a bulk sample. One of the clearest examples of this is in the study by Wilson and Pincock on a racemic mixture of binapthyl, 7, crystals. 27

(R)-(-)-I, l'-binaphthyl

(S)-(+)-1,1'-binaphthyl

85

These authors observed that binaphthyl crystallize in two polymorphs. The one is stable at lower temperature, is centrosymmetric and is not optically active. This polymorph melts at 145 ~

The

second polymorph is stable at higher temperature but is metastable at room temperature. It is optically active and melts at 158 ~

Wilson and Pincock show that as one cycles in temperature

between room temperature and 150 ~

a sample which is initially the optically inactive low

temperature polymorph transforms to an optically active solid. After three or four cycles one achieves the maximum optical resolution which corresponds to 56% ee. The crux of the Wilson and Pincock experiment is that at 150 ~

the reaction physically resembles a solid state reaction in

which the low temperature form is melting in the near presence of high temperature polymorph crystals. These chiral crystals are therefore nucleating sites for further chiral crystal growth. As at room temperature binaphthyl retains its chirality, the resultant samples can then be dissolved with retention of stereochemistry. The studies by Wilson and Pincock are however not the first study to show chirality from an initial achiral host. In earlier work Penzien and Schmidt showed that 4,4'-dimethylchalcene, 8,

Br2 (g)

v

A

single Crystal

Ar =

~/-~CH3

r

8

~"-Br

A•COAr ....Br

enantiomer ratio, 53:47

crystallizes in the chiral space group P212121. 28 Further this central ethylenic bond undergoes addition with Br2 to yield for a given single crystal with 6% optical yield. In subsequent papers Schmidt and other workers at the Weizman Institute further explored the ability of olefins to form chiral products. '9 Many of their studies centered on [2 + 2] photoreactions between adjacent olefins. In the absence of pores in a given solid it may be seen that such photochemically induced reactions, which are either intramolecular or between neighboring molecules, are ideal as one of the reactants does not have to break through the initial solid state host, thus destroying the originally advantageous chirality. In reactions such as the olefin plus bromine addition reaction one needs the chirality to be preserved on the surface of the crystal up to the moment of the transition state. If the reactant molecule loses its initial chiral arrangement the chirality is of course not transmitted to the product. ~ Schmidt and his coworkers who had already developed great expertise in solid state photochemistry observed that if one crystallized a disubstituted olefin, the olefins could orient themselves in one of two arrangements, 9 or 10. 31

86

In the arrangement shown in 9 a subsequent photoreaction would lead to an achiral product. By contrast, the arrangement 10 would lead to a chiral substituted cyclobutane.

After studying a

a..~

number of disubstituted olefm compounds the authors discovered that such olefins tend to form in the former

\ b _ \....~

and not the latter arrangement. They therefore proposed

\b

a

two modified versions of the [2 + 2] photocyclizationf

a

a try /9

\~

In one set of studies researchers prepared a mixed

\b

crystal with two different types of disubstituted olefins. This is illustrated in 11, involving two olef'ms, one with a thiophene and the other a pure benzyl derivative. As

a

11 shows there are two possible ways for these two olefins to interact. However as there was no symmetry element relating the upper to the lower reaction of 11, a

tw

/---

b

/9

10

disproportionate amount of one reaction could and did occur with respect to the other. This led to a final optically active product. 32

Ph ~-~Ar

Th

4A

hv

axis

r ~---

Ar

Ar Th hv

PhL_

Ar

In a second approach the authors considered disubstituted diolefins as is generically illustrated in 12. 33 These disubstituted olefms tend to arrange themselves in crystals in the form shown in 12. There is therefore good potential for chiral product formation. On this note, some authors have found that mutual interaction between the a, b and X substituents (see 12) can insure

87

the overall enforcement of this stereochemistry. 33b Especially ideal are a or b units which are esters and X units which are phenyl spacers. Conversely carboxylic acids and amides tend to hydrogen bond in a symmetric fashion thus leading to a achiral crystal type. This latter method has led to a number of successes. 7 One interesting application involves the formation of chiral polymers from an initially racemic mixture of starting materials. 34 In all these experiments, a key intermediate step is the formation of a chiral crystal from an achiral starting material. Rational procedures for such a synthesis are therefore desirable. In a series of beautiful experiments Addadi, van Mil and Lahav demonstrated one such rational approach. 35 In this work they first made use of an chiral a-substituent (see 12, in this example a was a chiral sec-butyl group, X was a phenyl link and b was an ethyl group). As the initial diolefin was chiral, the resulting product was also of necessity chiral.

They then studied the resultant

structure. They found that sec-butyl groups from neighboring groups were in close proximity to one another.

They therefore noted that were they to transfer the methyl group of the sec-butyl

group to the neighboring molecule they would then have a formal mixture of 3-propyl and 3-pentyl units with nearly the same quality of packing. These latter two molecules are not optically active. They then demonstrated that these latter molecules when grown together into a co-crystal forms a unit cell somewhat like the initial crystal (in the

sec-butyl

case unit cell dimension are a = 13.17 A,

b = 6.94, c = 5.25, ot = 103.1 ~ 13 = 95.5 ~ and "/= 90.1 ~ while in the latter case a = 13.53 ,~,, b = 6.90,4,, c = 5.28 ,~,, ot = 102 ~ [3 = 104 ~ and ), = 94~

Both crystals are chiral and are in the P1

space group. (Pure 3-propyl and 3-pentyl crystals by contrast form in P-1 with rather different at cell dimensions.) The optically active crystals upon exposure to UV light do indeed react to form an optically active product mixture.

Finally in the same work they show that a crystal in P21

symmetry with a = 3-pentyl b = methyl and X = phenyl link (see 12 for the location of the a, b and X substituents) leads to a resultant chiral cyclobutane product with ee up to 100%.

t2

x hv x

t2

88

A number of fine studies have shown that high ee's can be achieved. 36 It has generally proven feasible to determine the resultant chiral form from the chiral conformation of the reactant molecules in the crystal. Generally the optical product closest in geometry to the chiral reactant is the true final stereoisomer.

Examples of this, 13 and 14, illustrate schematically the chiral

conformation of the reactants and the chiral form of the products. Some recent studies include those in reference 36. hv H3

Ar HO Ar = - - ~ C I

13

14 in (-)-crystal

14 in (+)-crystal

H3 ~ ~ - ~ P h

P H3(~ ,ON~CH3

H3~"~O CH3

CH3 hv

H3C

CH3

Ph

Ph

CH3

i i

CH3

Mirror plane 14

A second methodology for chiral synthesis is to use a host-guest method in which the initially chiral host imparts chirality to the final reacted guest product 37 Useful hosts have proven to be 15 through 18. In all cases shown here the host is not only chiral, but has two optically active phenol or alcohol groups. These alcohol or phenol units are held in a fairly rigid manner to one another and therefore can readily hydrogen bond to many guests in a stereospecific manner. Two examples, both taken from the insightful work of F. Toda illustrate (Is)

89

the practicality of such host ligands. In the first example the host 16b was co-crystallized with 19 to make a 1:1 inclusion compound. 12 While the molecule 19 is by itself is not chiral, once linked by hydrogen bonds to the chiral host, a twist is introduced into the backbone of the 19 molecular structure. Based on the X-ray structure the inclusion compound can be seen to be like that shown in 20. Further as 20 shows, 19 undergoes a photoinduced cyclization. In the inclusion compound this photoreaction proceeds with 100%ee to only one of the two possible stereoisomers. CH3 CI

~

H3C---~

HO

HO

/CH3

(S,S)-(-)17

(s, S)-(-) 16 16a R= 16b R=p-C6H5 16c R=m-C6H5 Ph.

OH

P

-0

O

O

Ph Php/\OH

"0

NMe2 19

R= - (CH3)2or cyclohcxylor cyclopentyl 18

oX

Ph

Ar

\

H

i Ph

/O

\\

.." "0

0 "/

Ph

NMe2

.H

Ph,.

OH

hv \CH3

20

90

In a related host-guest based chiral synthesis, 38 the molecule 16b was used to make a 1:1 inclusion compound with 21. Just as in the case of 19, 21 is achiral, contains

o i

two oxygens on adjacent carbon atoms and undergoes a photorearrangement.

.,r

The photorearrangement is a cyclization as is shown in 22. A stereodiagram of \

a portion of the crystal structure is shown in 23.

Recalling the Woodward-

Hoffmann rules that such reactions occur in a disrotatory manner one sees that

i

.~/C~R

J 21

of the two possible dirotatory pathways, one of the two (see 22) causes the rearranging groups to push into the neighboring host molecule. The reaction therefore proceeds in only one of the two stereochemical pathways.

Ph

HO Ph C1 l

O

O

hv

O

O

&O" "'"H

(IS, 5R)-(-)-22 R=Et

(IR, 55")-(+)-22

22

23

91

While these chiral host-guest inclusion compounds have been demonstrated to produce excellent ee's it has proven difficult to predict in an apriori fashion the direction of enantiomeric preference.

CH3

For example in the cyclization CH3

reaction of 21, host 16b produces one enantiomer in 98% ee while 16c produces the opposite enamtioner with 95% ee. 39 It should be noted that 16b and 16c are rather similar and it is therefore difficult without further 4o work to ascribe the exact cause for the different product outcome.

I

CH3

24

Another example of such difficulties is given in reference 36. One of the reasons for this lack of predictability is that in every host-guest system a new crystal must be made. It is easy to imagine that it would be difficult to transfer one stereo-orientation from one crystal to the next. It may prove in the future that the use of chiral porous organic solids as hosts will obviate this latter concern.

Acknowledgement A part of our work described here (Compounds 3-6) was done under the guidance of Prof. Jeffrey S. Moore at the University of Illinois at Urbana-Champaign.

We thank the National

Science Foundation (Grant CHE-94-23121) for financial support. A portion of our research was carried out at the Center of Microanalysis of Matierals, University of Illinois, which is supported by the U. S. Department of Energy under Grant DEFG02-91-ER45439. S.L. Thanks the J. D. and C. T. MacArthur foundation (1993-97) and the A. P. Sloan Foundation (1993-95) for fellowships. S.L. and D.V. thank Prof. Jeffery S. Moore for helpful discussions. We also thank Messrs. G. B. Gardner and Y. -H. Kiang for their contributions.

92

References and Notes

1.

(a) Newsam, J. M. Science 1986, 231, 1093. (b) Breck, D. W. Zeolite Molecular Sieves; Robert E. Krieger Publishing Co.: Malabar, FL, USA, 1974.

2.

(a) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 1546. (b) Gardner, G. B.; Venkataraman, D.; Moore, J. S.; Lee, S. Nature 1995, 374, 792. (c) Fujita, M.; Kwon, Y. J.; Washizu, S.; Ogura, K. J. Am. Chem. Soc. 1994, 369, 727. (d) Endo, K.; Sawaki, T.; Koyanagi, M.; Kobayashi, K.; Masuda, H.; Aoyama, Y. J. Am. Chem. Soc. 1995, 117, 8341. (e) Wang, X.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1994, 116, 12119.

3.

As emphasized in the text, the importance of a zeolite derives from its function. It is not only important to realize a structure which retains its cavities upon removal of the guest but should also demonstrate reversible absorption and desorption of guests, selectivity in the intake of guest molecules and stability under conditions of a typical reaction.

.

For a review of Dianin's compound and related systems see: MacNicol. D. D. Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Oxford University

Press: Oxford, 1984; Vol. 2, Chapter 1, pp 1-46. 5.

Powell, H. M.; Wetters, B. D. P. Chem. Ind. 1955, 256.

6.

Since 1971, a number of crystal structures of various adducts of 1 have been reported in the literature.

The atomic coordinates of the chloroform adduct is used as the starting

point for other adducts. For the atomic coordinates of the chloroform adduct see: Flippen, J. L.; Karle, J.; Karle, I. L. J. Am. Chem. Soc. 1970, 92, 3749. 7.

Note that as the thermal motion and occupancy factors are correlated, it is difficult to differentiate between low occupancy and high disorder.

A conclusive proof about the

occupancy of the can be obtained from the chemical shift information in NMR spectroscopy. 8.

(a) MacNicol, D. D.; McKendrick, J. J.; Wilson, D. R. Chem. Soc. Rev. 1978, 7, 65-87. (b) MacNicol. D. D. Inclusion Compounds; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Eds.; Oxford University Press: Oxford, 1984; Vol. 2, Chapter 1, p 14.

9.

Barrer, R. M.; Shanson, V. H. J. Chem. Soc., Chem Commun. 1976, 333.

93 10.

Brunauer, S. The Adsorption of Gases and Vapors; Princeton University Press: Princeton, 1965. Type IV and type V behavior have also been observed in other porous compounds. See Allinson, A.; Barrer, R. M. J. Chem. Soc. A 1969, 1718.

11.

(a) Lee, F.; Gabe, E.; Tse, J. S.; Ripmeester, J. A. J. Am. Chem. Soc. 1988, 110, 6014. (b) Pang, L.; Lucken, E. A. C.; Bemardinelli, G. J. Am. Chem. Soc. 1990, 11, 8754.

12.

(a) Ung, A. T.; Bishop, R.; Craig, D. C.; Dance, I. G. Scudder, M. L. J. Chem. Soc., Chem. Commun. 1991, 1012. (b) Ung, A. T.; Gizachew, D.; Bishop, R.; Scudder, M. L.; Dance, I. G.; Craig, D. C. J. Am. Chem. Soc. 1995, 117, 8745.

13.

For a review of helical canal inclusion networks see: Bishop, R.; Dance, I. G. Top. in Curr. Chem. 1988, 149, 137.

14.

Mesitylene is considered as too bulky a solvent to be included in the cage.

15.

(a) Hoskins, B. F.; Robson, R. J. Am. Chem. Soc. 1990, 112, 1546. (b) Moore. J. S.; Lee,S. Chem. Ind. 1994, 556.

16.

Based on this approach, few inclusion compounds have been made. see References 2a-c and 24.

17.

Venkataraman, D.; Gardner, G. B.; Lee, S. Moore, J. S. J. Am. Chem. Soc. 1995, 117, 11600.

18.

We could not collect data after 25" in 20 for the single crystal.

Hence a complete

structural solution and refinement was not possible. 19.

The second polymorph, polymorph B, can be considered as homeotypic with CaCuP (A1B2-type) structure. The structure consists of undulating hexagonal sheets. Five such nets interpenetrate to file the void space in a single net. see Venkataraman, D.; Lee, S.; Moore, J. S.; Zhang, P.; Hirsch, K. A.; Gardner, G. B.; Covey, A. C.; Prentice, C. L. Submitted for publication to Chemistry of Materials.

20.

(a) Wang, X.; Simard, M.; Wuest, J. D. J. Am. Chem. Soc. 1994, 116, 12119. (b) Duchamp, D. J.; Marsh, R. E. Acta Crystallogr. 1969, B25, 5. (c) Ermer, O.; Lindenberg, L. Helv. Chim. Acta. 1991, 74, 825. (d) Ermer, O. J. Am. Chem. Soc. 1988, 110, 3747.

21.

Gavezzotti, A.; Desiraju, G. R. Acta Crystallogr. 1988, B44, 427.

22.

Asgaonkar, A.; Gardner, G. B.; Kiang, Y. -H.; Lee, S.; Moore, J. S; Venkataraman, D. Manuscript in preparation.

94 23.

The lattice constants for the original orthorhombic cell were a -- 11.625 A, b - 19.110 ,~, c = 38.856 A. The lattice constants for the rectangular cell of the guest-free sample were b = 22.76/~ and c = 36.48 ]k.

24.

The exposure temperature was 40 ~ for benzene, 65 ~ for benzyl alcohol and 60 ~ for other guest molecules.

25.

Yaghi, O. M.; Li, G.; Li, H. L. Nature 1995, 378, 703.

26.

For other important examples of networks which does not collapse upon guest removal see: (a) Cartraud, P.; Cointot, A.; Renaud, A. J. Chem. Soc., Faraday Trans. 1 1981, 77, 1561. (b) Allinson, S. A.; Barrer, R. M. J. Chem. Soc. A 1969, 1717.

27

Wilson, K. R.; Pincock, R. E. J. Am. Chem. Soc. 1975, 97, 1474.

28.

Penzien, K.; Schmidt, G.M.J. Angew. Chem. Int. Ed. Engl. 1969, 8, 608.

29.

a) Green, B. S.; Lahav, M.; Rabinovich, D. Acc. Chem. Res. 1979, 29, 187. b) Addadi, L.; Lahav, M. Pure Appl. Chem. 1979, 51, 1269.

30.

Wudl, F.; Lightner, D. A.; Cram, D.J.J. Am. Chem. Soc. 1967, 89, 4099.

31.

Green, B. S.; Lahav, M.; Schmidt, G.M.J. Mol. Crvst. Liq. Cryst. 1975, 29, 187.

32.

a) Elgavi, A.; Green, B. S.; Schmidt, G.M.J.J. Am. Chem. Soc. 1973, 95, 2058. b) Heller, E.; Schmidt, G. M.J. Isr. J. Chem. 1971, 9, 449.

33.

a) Green, B.S.; Lahav, M.; Schmidt J. Am. Chem. Soc. 1973, 95, 2058. b) Cookson, R. D.; Franklel, J. J.; Hudec, J. Chem. Soc., Chem Commun. 1965, 16. c) Adam, G. Tet. Lett. 1971, 2030. d) Addadi, L.; Gati, E.; Lahav, M.; Leiserowitz, L. Isr. J. Chem. 1976-

1977, 15, 116. 34.

a) Addadi, L.; Cohen, M.D.; Lahav, M. Mol. Cryst. 1976, 32, 137. b) Addadi, L.; Cohen, M.D.; Lahav, M. J. Chem. Soc., Chem. Commun.1973, 471.

35.

Addadi, L.; Lahav, M. J. Am. Chem. Soc. 1979, 101, 2152. b) Addadi, L.; van Mil, J.; Lahav, M. J. Am. Chem. Soc. 1982, 104, 3422.

36.

a) Evans, S. V.; Miguel, G.-G.; Omkaran, N.; Scheffer, J. R.; Trotter, J.; Wireko, F. J. Am. Chem. Soc. 1986, 108, 5648. b) Sekine, A.; Hori, K.; Ohashi, Y.; Yagi, M.; Toda, F. J. Am. Chem. Soc. 1989, 111,697. c) Roughton, A. L.; Muneer, M.; Demuth, M.; Klopp,

I. Krtiger, C. J. Am. Chem. Soc. 1993, 115, 2085. d) Kaupp, G.; Haak, M. Angew.

95 Chem. Int. Ed. Engl. 1993, 32, 694. e) Toda, F. Synlen. 1992, 303. f) Toda, F.; Tanaka,

K.; Stein, Z.; Goldberg, I. Acta Cryst. 1995, 351,856. g) Ramamurthy, V.; Venkatesan, K. Chem. Rev. 1987, 87, 433.

37.

For reviews see (a) Toda F. Acc. Chem. Res. 1995, 28, 480, (b) Toda F. Synlett. 1992, 303 and references therein.

38.

Kaftory, M.; Tanaka, K.; Toda, F. J. Org. Chem. 1988, 53, 4391.

39.

Tanaka, K.; Kakinoki, O.; Toda, F. J. Chem. Soc., Chem. Commun. 1992, 1053.

40.

Hashizume, D.; Vekusa, H.; Ohashi, Y.; Matsugawa, R.; Miyamoto, H.; Toda, F. Bull. Chem. Soc. Jpn. 1994, 67, 985.

This Page Intentionally Left Blank

H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All rights reserved.

Spectroscopic

Characterisation

of

97

Zeolites

Russell F Howe Department of Physical Chemistry University of New South Wales Sydney, Australia 2052

1.INTRODUCTION This review is concerned primarily with optical spectroscopic methods for characterising zeolites and molecules adsorbed in zeolites. The electromagnetic spectrum spans the range from radiofrequencies to Xradiation. Spectroscopic techniques included in this range are, in order of increasing frequency, NMR, EPR, infrared, UV-VIS and Raman, XPS, XAS and Mossbauer spectroscopies. The NMR and XPS techniques are discussed elsewhere in Chapter 4 (B and C respectively). This review will give an overview of recent advances in EPR, infrared, Raman, UV-VIS and X-ray absorption spectroscopies as applied to zeolitic materials. Brief mention will also be made of new techniques recently reported for obtaining mass spectra of zeolites and adsorbed species. Each of the spectroscopic techniques has an extensive literature which cannot possibly be reviewed within the confines of a single book chapter. The reader seeking introduction to the principles of the methods and prior literature is referred to references [1-3] at the end of the chapter. It is well understood by experienced practitioners of zeolite science that no problem can be fully solved by applying a single spectroscopic or physical technique. That theme will be repeated here; complete characterisation of a zeolite system requires a combination of chemical, physical and spectroscopic experiments, and it is necessary to have a good appreciation of the advantages and limitations of each technique. Where disagreements exist in the literature, it is often because different techniques under different conditions are giving different answers.

98

2.EPR SPECTROSCOPY 2.1

Background Electron paramagnetic resonance (or electron spin resonance) was first applied to zeolites more than 30 years ago, and continues to be utilised. EPR can in principle observe any species containing one or more unpaired electrons; in the case of zeolites this means paramagnetic transition metal or rare earth ions, solid state defects which are paramagnetic, and inorganic or organic radicals. The method is highly sensitive, detecting as few as 101 2 spins in favourable circumstances.This sensitivity does impose the need for caution in interpreting weak signals, which can be caused by trace level impurities irrelevant to the chemistry being studied. EPR spectroscopy has not seen the dramatic changes in instrumentation and techniques that have characterized NMR spectroscopy over the past 20 years. Until recently, commercial EPR instruments used exclusively the continuous wave m i c r o w a v e techniques first developed 50 years ago. The advent of digital electronics and computer controlled spectrometers has made the experiment easier to do, more reliable, and arguably improving signal to noise ratios, but the principle of measurement remains that of measuring (in derivative mode) the absorption of microwave radiation at a fixed frequency as the magnetic field is continuously scanned. The physics of the continuous wave EPR experiment are summarised in the spin Hamiltonian j~

= gl3H

+

aS.I

+

D S1S2

All three terms in the Hamiltonian contain chemical information. The g tensor is determined by the ground and excited state wave functions of the atom or molecule containing the unpaired electron(s) (strictly speaking the excited state wave functions coupled to the ground state through spin orbit interactions). It is thus sensitive to factors influencing the ground and excited state wave functions, such as the crystal field in the case of transition metal ions, or the electrostatic fields in zeolite cavities for adsorbed radicals. The components of the g tensor determine the magnetic field at which resonance occurs for a given microwave frequency. Almost all zeolite studies have used polycrystalline powders rather than single crystals, which means that the observed spectra are a summation of spectra from individual crystallites at all possible orientations with respect to the magnetic field. If the principal components of the g tensor are sufficiently different, the first derivative powder spectrum will show three clearly

99 resolved features at the magnetic fields corresponding to the g t e n s o r components. More commonly, computer simulation of the powder spectrum is necessary to extract reliable parameters. The isotropic and anisotropic hyperfine coupling terms in a arise from interactions b e t w e e n electron and nuclear spins, and provide information about the nature of the orbital containing the unpaired electron and the extent to which it overlaps with orbitals on adjacent atoms. The anisotropic term can cause similar difficulties to the g tensor anisotropy in analysing spectra of polycrystalline powders; extracting coupling constants from spectra of transition metal ions or radicals in zeolites can be difficult or impossible without computer simulation. The third term in the spin Hamiltonian is the so-called zero-field splitting term which arises in systems containing more than one unpaired electron. This is often encountered in spectra of transition metal ions, and can complicate measurement and interpretation of spectra. Depending on the magnitude and sign of the zero-field splitting, signals may be broad and poorly resolved, not detected at all, or may show anomalous temperature dependence (an example is high spin Co 2+ with three unpaired electrons, which is considered below). Long range dipolar interactions between an unpaired electron and nuclear spins on adjacent atoms will not normally be resolved in conventional powder EPR spectra.The pulse technique of electron spin echo modulation (ESEM) is in favourable cases able to detect very weak hyperfine interactions not seen in CW EPR. The method measures modulation of the electron spin echo signal by dipolar hyperfine coupling in the time domain at fixed magnetic field. Until recently, pulsed EPR experiments required home built instrumentation, and in the zeolite field the method has been restricted to a small number of practitioners. Commercial pulsed EPR instruments are however now available, and the ESEM technique and variants thereof will undoubtedly grow in popularity.

2.2

EPR Spectroscopy of Adsorbed Radicals Organic and inorganic radical formation in zeolites can occur spontaneously, on adsorption of molecules into a suitably activated zeolite, or as the result of radiolysis of adsorbed species. Once a radical is formed, EPR spectroscopy can be used to follow its subsequent reactions. For example, Trifunar et al have recently described the use of variable temperature EPR to investigate reactions of olefin radical cations generated in ZSM-5 zeolites. [4] . This work shows clearly the facile rearrangement of radical cations produced by irradiation of

100 olefins adsorbed in zeolites at low temperatures. Figure 1 shows EPR spectra observed at 100K and 160K after irradiation of 1,4 cyclohexadiene adsorbed in NaZSM-5 at 77K. The initial spectrum recorded is that of the 1,3 cyclohexadiene radical cation, confirmed by the calculated spectrum shown below. The spectrum of the parent 1,4 cyclohexadiene radical cation could not be detected at all, even when irradiation was carried out at 4 K. The isomerization of the 1,4 cyclohexadiene radical cation to the 1,3 cyclohexadiene radical cation is known to be energetically favourable in the gas phase, but does not occur in low temperature inert gas matrices in the absence of photo excitation. It is remarkable that reaction occurs so readily in zeolite pores. Trifunac et al. suggest that the reaction may be driven by excess energy remaining after ionisation of the parent molecule, however radical : zeolite interactions clearly also play a role. I

b)

c)

d)

t 50,G

I

_A

F i g u r e 1. EPR spectra recorded at (a) 100K and (c) 160K after irradiation at 77K of 1,4-cyclohexadiene in NaZSM-5. Spectra (b) and (d) are calculated spectra for the 1,3-cyclohexadiene radical cation and the cyclohexadienyl radical respectively. (From r e f e r e n c e 4, with permission).

101 The spectrum obtained after warming to 160 K is that of the neutral cyclohexadienyl radical (confirmed by the computer simulation shown). Transformation of the radical cation to a neutral radical is commonly observed in condensed phases and occurs via ion 9 molecule reactions. A similar explanation is proposed for the adsorbed species in NaZSM-5" formally proton transfer from the cyclohexadiene radical cation to an adsorbed cyclohexadiene molecule will produce the cyclohexadienyl radical and a diamagnetic carbenium ion. In HZSM-5 the Bronsted acid protons can also contribute to the chemistry occurring. The conversion of 1,3 cyclohexadiene radical cation to the cyclohexadienyl radical occurs in a similar manner to that in NaZSM-5, but at higher temperatures a third signal due to the cyclohexenyl radical is detected. In this case carbenium ions generated by protonation of cyclohexadiene at Bronsted sites act as scavengers of free electrons produced during radiolysis, forming H addition type radicals. The chemistry of these low temperature reactions of radicals and radical cations is certainly highly relevant to the catalytic chemistry of hydrocarbons in zeolites and warrants further detailed study by EPR spectroscopy in parallel with other spectroscopic techniques. The recent review by Rhodes [5] should be consulted for information about other work in this area.

2.3

Metal Ion Exchanged Zeolites EPR spectroscopy has been used with good effect to follow the state of Cu in Cu exchanged MFI zeolites used as catalysts in the selective reduction of NOx with hydrocarbons in the presence of oxygen. These catalysts, first reported by Iwamoto et al. [6] and Held et al. [7], have since become widely studied because of their high activity and selectivity to nitrogen [8]. Questions addressed using EPR spectroscopy include the location of the Cu in the zeolite, the extent of clustering of Cu at high Cu exchange levels, the oxidation state of the Cu, and the extent to which all of these vary during catalyst pretreatment or use. Kucherov et al.[9] have shown that at low Cu levels, all of the Cu exchanged into the zeolite is detected as isolated Cu 2+ cations existing in two different coordination states: four coordinate square planar and five coordinate square pyramidal. Figure 2 shows a typical spectrum of Cu 2 + exchanged MFI , showing the two different Cu 2+ species.[10]. Both species are located in the main channels of the MFI structure [9]. At higher Cu exchange levels, the EPR spectra become poorly resolved due to the onset of Cu:Cu interactions. Some authors have shown that heating Cu exchanged MFI zeolites in vacuo causes spontaneous reduction of Cu 2+ to Cu + and loss of the EPR signals [11,12]. Shelef [8] argues that it is only coupled Cu species that can be spontaneously

102 reduced in vacuo. In the presence of oxygen (ie. under conditions similar to those of the working catalyst) the isolated Cu 2+ species are not reduced by hydrocarbons such as propene. Shelef et al. have shown very recently using EPR spectroscopy that steam treatment at 650 C or calcination at higher temperatures causes an irreversible transformation in the coordination state of the Cu 2+ , suggested to involve a rearrangement of the local topography of the isolated Cu 2 + sites [13]. There is disagreement as to whether this is caused by dealumination of the zeolite [14]; Sachtler [15] has suggested that a Cu aluminate phase is produced in hydrothermally treated zeolites. More in-situ EPR studies of the type conducted by Shelef et al. are needed before the remaining questions concerning the Cu MFI catalysts can be answered.

I'

!

i

2600

I

I

I

I ~

.,,

I

3000

t/

I

.

I

3400

[G]

Figure 2. EPR spectrum of Cu 2+ in CuZSM-5 NO reduction catalyst.[10] 2.4

Metal Substituted Zeolites The synthesis of zeolitic materials containing transition elements substituted into the framework has become of considerable interest. A crucial issue for such materials is the question of whether the material

103 as synthesised really does contain the transition element substituted into the framework, and whether or not it remains in the framework following activation or use in a catalytic reaction. The sensitivity of the spin Hamiltonian parameters for a paramagnetic transition metal ion to the crystal field environment means that EPR spectroscopy should be able to clearly distinguish between a metal ion in the framework with tetrahedral or distorted tetrahedral coordination and an extra framework species with octahedral or distorted octahedral coordination. The formation of extra framework metal oxide or hydroxide clusters will also be evident from the EPR spectra in that such species will contain strongly coupled spins. The recent EPR study by Kevan et a1.[16] of cobalt substituted ALPO-5 illustrates the power of the EPR technique to characterise metal substituted zeolites. CoAPO-5 as synthesised is blue in colour; the electronic spectrum is characteristic of tetrahedrally coordinated Co 2+, suggesting that Co 2+ has been incorporated into the A1PO4 lattice. The corresponding generation of Bronsted acid sites indicates that Co 2 + substitutes for A13+.The material does however change colour to yellowgreen on calcination at high temperature; this has been interpreted to mean that cobalt is partially oxidised to Co3+ under these conditions.[17] Figure 3 shows EPR spectra measured by Kevan et al. at 7 K of CoAPO-5 before and after calcination. The anisotropic signal observed is characteristic of high spin Co 2+ in tetrahedral coordination (and because of effective spin lattice relaxation in the spin = 3/2 system can only be detected below liquid nitrogen temperatures). At first sight, the decrease in signal intensity on calcination would seem to support the suggestion that Co 2+ is oxidised to Co 3+ on calcination. However, Kevan et al. showed that the apparent signal loss is in fact due to a change in the temperature dependence of the signal , as illustrated in Figure 4. When the spectrum is measured at 20K there is no difference in intensity between the as synthesised and calcined samples. The as synthesised material follows approximately the normal Curie law dependence of signal intensity on temperature, whereas the calcined material deviates from Curie law behaviour below 20K. This behaviour can be accounted for in terms of distortion of the tetrahedral symmetry of Co 2+. Such distortion can induce a negative zero field splitting between the Ms= + 1/2 and Ms= + 3/2 doublets. As the temperature decreases, the Co 2+ ions tend to populate the lower energy + 3 / 2 doublet, which is EPR silent, thus diminishing the signal intensity. The calculated curve in Figure 4 assumes a zero field splitting o f - 1 3 cm-1, and agrees well with the experimental data. The exact cause of the symmetry distortion on calcination is not yet clear, although Kevan et al.

104 suggest that it may involve coordination of molecular oxygen to the Co 2+ ions in the ALPO-5 lattice.

500 G S

,

"

-

slier ~ l ~ n s l i o n --_

--.-

g " 5.45

14 ,

12

~

8

~

4

m

2

.........

Curie Law Calculated

0 ',, X \

b.

C]

After calcination

0

As-synthesized

W

0

10

20

30

40

T,K

Figures 3 and 4. EPR spectra of Co2+ at 7K and temperature dependence of signal intensity from as synthesized and calcined CoAPO-5. Reproduced with permission from reference 16. A second example of the use of EPR spectroscopy to investigate lattice substitution in zeolites is the recent work of Schoonheydt et al. on VAPO-5 materials [18]. The EPR spectra of VAPO-5 containing low concentrations of vanadium show two overlapping signals which both have the g-tensor and 51V hyperfine parameters typical of distorted octahedral V 4+. An additional broad poorly resolved signal is detected in higher vanadium content samples. The broad signal is undoubtedly due to extra lattice vanadium oxide species in which V4+ ions are strongly magnetically coupled. The isolated octahedral V4+ species may

105

be anchored to the surface of the zeolite, but the absence of any spectroscopic signature of V4+ tetrahedrally coordinated to 4 oxide ions of the lattice suggests that isomorphous substitution of V 4+ into the lattice does not occur. 2.5

ESEM Studies of Metal Ions in Zeolites The electron spin echo modulation technique detects directly the coordination environment around a paramagnetic ion by observing the dipolar coupling to nuclei of weakly coordinated ligands. This technique has been used extensively by Kevan, for example, to examine transition metal ion exchanged and substituted zeolite materials [19].

I0

8

6 ,,i,,o w

r c: -ira

.=_ O ..g:

,., ILl

2

o

t

z

3

4

T,/~s

Figure 5. Three pulse ESEM recorded at 4 K of (a) MnAPO-5 and (b) MnAPO-11 containing adsorbed D20. Reproduced with permission from reference 20.

106 Figure 5 illustrates the method showing three pulse ESEM traces for MnAPO-5 and MnAPO-11 both containing adsorbed D20, from the work of Brouet, Chen and Kevan [20]. The modulation observed on the spin echo decay curves is caused by dipolar hyperfine coupling between the unpaired electrons on Mn 2+ and the nuclear spins of deuterium atoms in surrounding water molecules. The modulation patterns are distinctly different for the two different ALPOs. The pattern for MnAPO-5 could be fitted with a model of 4 deuteriums at a distance of 0.30nm, consistent with solvation of two D 2 0 molecules about a Mn 2+ cation in a non-framework position. For MnAPO-11, on the other hand, the modulation pattern required a model of two deuterium atoms at a closer distance of 0.24nm and two at a longer distance of 0.36nm. This model is consistent with coordination of two water molecules to a negatively charged site, where the deuterium atoms of each water molecule are not equidistant from the manganese, suggesting (albeit indirectly) that the Mn 2+ in MnAPO-11 is substituting isomorphously for aluminium in the ALPO lattice.

3. INFRARED SPECTROSCOPY 3.1

Background Infrared spectroscopy, like EPR, has been used for more than 30 years to study zeolites and adsorbed molecules. The introduction of Fourier transform techniques has made the conventional transmission measurement in the mid infrared region much easier and faster to perform , which has allowed new experiments to be performed which were either impossible or extremely difficult with wavelength scanning instruments. For example, in situ measurements of zeolite catalysts under reaction conditions are now routinely performed.Time resolved measurements on the scale of seconds or less are now feasible, and spectra can be measured routinely in the energy limited far infrared region. Another exciting new development is infrared microspectrocopy, which permits spectra to be recorded from zeolite single crystals.Some of these newer developments will be illustrated in the examples which follow. 3.1

Zeolite Lattice Modes The vibrational frequencies of the so-called lattice modes of aluminosilicate zeolites (stretching and bending modes of the T-O linkages, plus specific vibrations of discrete structural units) were first studied in detail by Flanigen more than 20 years ago [21]. The lattice modes are sensitive to both the composition of the lattice and the structure. For example, Jacobs et al. showed that the T-O stretching

107

frequencies of different zeolites correlated with the average electronegativities of the zeolite frameworks [22]. For zeolite Y, a linear correlation has been reported between the frequencies of selected lattice modes and the aluminium content of the zeolite [23]. The lattice mode frequencies are also sensitive to the aluminium content of the lattice in the case of MFI zeolites. Campbell et al. [24] have recently reported that the lattice mode frequencies can be used to monitor changes in the lattice aluminium content of HMFI catalysts after hydrothermal treatment or use in the conversion of methanol to hydrocarbons. Figure 6 shows plots from reference 24 of the frequencies for two of the lattice modes ( t h e structure sensitive 544 c m -1 band, due to deformation modes of the MFI lattice, and the 790 c m -1 band, a structure insensitive T-O-T stretching mode) versus the lattice aluminium content determined by solid state 27A1 and 2 9 S i NMR for fresh and treated MFI zeolites. The correlation in this case is not linear, but is still a useful empirical guide to the extent of lattice dealumination.

5,55

'

'

' 0

9 a , ~ ,]~:83H[-6 .,.~~.,~

T_ 5~o

~

parent

i 805 I

I

I

545

,

i

'

,

1~

coo

-

795

0

2

4-

6

lattice .~lnm~ntum Content/.~1 (uc) -1

Figure

8

790

0

2

4

6

8

Lattice Alumtnlum C o n t e n t / A1 (ue) -1

6. Dependence of lattice frequencies on lattice aluminium content for HMFI zeolites. Reproduced with permission from reference 24.

108 The sensitivity of lattice modes to structural changes is illustrated by the recent study of Mueller and Connor [25] on the effects of cyclohexane adsorption on the structure of MFI zeolites. The adsorption of molecules such as paraxylene and benzene into MFI zeolites causes a structural transition from monoclinic to orthorhombic symmetry, which has been characterized by X-ray powder diffraction and 29Si NMR spectroscopy [26]. Cyclohexane has a slightly larger kinetic diameter than benzene or paraxylene (0.60 nm compared with 0.585nm), but does not cause the same structural transition. Cyclohexane adsorption does however affect the zeolite lattice mode vibrational frequencies. Figure 7 shows spectra taken from reference 25 before and after (upper spectrum) adsorption of cyclohexane in a low aluminium MFI zeolite. The OTO bending mode at 547.5 cm -1 is shifted to higher frequency and a new band appears at 401.6 cm-1 on adsorption of cyclohexane. Mueller and Connor attribute these changes to a flexing of the MFI lattice, causing small changes in the OTO bond angles (the 401.6 cm-1 band is tentatively attributed to a pore mouth vibration shifted up in frequency from below the 350cm -1 cut off of the spectrometer used). The amount of cyclohexane adsorbed is only half that of benzene or paraxylene; it is suggested that cyclohexane adsorbs only in the linear channels of the MFI structure, perturbing the lattice modes but not inducing the structural transformation to orthorhombic symmetry.

~SM-S

(3OO)

45S.~ 454.7---.~ ~

\

o

401.6

600 I

I

560 I

I

520 I

I

480 I'

I

440 .I

I

400 380 I

Wave Number

F i g u r e 7. Infrared spectra of ZSM-5 before and after adsorption of cyclohexane . Reproduced with permission from reference 25.

109

3.3

Low Frequency Cation Vibrations The far infrared region of zeolite spectra (here defined as the frequency region below 300 cm ~ shows bands which are attributed to stretching vibrations of the zeolite cations vibrating relative to the zeolite lattice. The vibrational frequencies of the cations depend on their charge, mass and interaction with the zeolite. In the case of zeolite NaY, for example, 4 intense bands are observed which have been assigned to Na + cations in the 4 different cation sites SI, SI', SII and SIII [27]. Esemann and Forster [28] have recently used far infrared spectroscopy to follow the progress of solid state ion exchange into zeolites HY and NH4Y and to compare solid state ion exchange with conventional aqueous ion exchange. Figure 8 shows far infared spectra from reference 28 of zeolite NH4Y after solid state ion exchange by heating in the presence of solid NaC1, KC1 and RbC1 respectively. In all three cases the spectra obtained by solid state exchange are identical to those observed after aqueous ion exchange. The 4 bands due to Na + cations in sites SI (160 cm-1), SI' (106 cm -1), SII (188 cm -1) and SIII ( 90 cm -1) are all shifted to lower frequencies by the amounts expected for the change in reduced mass when Na + is replaced by K + or Rb +. In the case of Cs +, the solid state exchange produced a band due to Cs + cations occupying site I in the double six rings of the faujasite structure (at 86 cm -1) which was not formed after conventional aqueous ion exchange with Cs +. This can be explained by the inability of the large hydrated Cs + cation to enter the double six-rings.

KY .8NaY .6-

.4-

.2-

:3;0

2;0

200 1;0 Wavenumbers

1;0

F i g u r e 8. Far-infrared spectra of zeolites prepared by solid state ion exchange. Reproduced with permission from reference 28.

110 The far infrared experiments also showed that the effectiveness of solid state ion exchange depends on the starting form of the zeolite. N H 4 Y allows effectively complete ion exchange on reaction with metal halide salts, due to the initial formation of NH4C1. With HY, on the other hand, it is difficult to achieve exchange levels beyond 50% due to the immediate formation of HC1 which dealuminates the zeolite to a considerable extent.

3.4.

Infrared Spectra of Probe Molecules Infrared spectroscopy has been used for many years to probe acid sites in zeolites. Typically, strong bases such as ammonia or pyridine are adsorbed, and the relative or absolute intensities of bands due to Lewis acid adducts or protonated Bronsted acid adducts are measured. The basicity of ammonia or pyridine is however much stronger than that of most hydrocarbon reactants in zeolite catalysed reactions. Such probe molecules therefore detect all of the acid sites in a zeolite, including those weaker acid sites which do not participate in the catalytic reaction. Interest has recently grown in using much more weakly basic probe molecules which will be more sensitive to variations in acid strength. It is also important in studying smaller pore zeolites to use probe molecules which can easily access all of the available pore volume. Nitrogen and carbon monoxide are both candidates as small probe molecules which may interact only with strong acid sites in zeolites and which can be observed by infrared spectroscopy. As an illustration of this method, consider the recent work of Wakabayashi et al. on N2 adsorbed in H-mordenite [29], HY and HZSM-5 [30]. References to infrared spectra of adsorbed CO include [31-33]. Figure 9 shows spectra (from reference 29) of N 2 adsorbed in Hmordenite as a function of pressure at a temperature of l l0K. The two bands observed are both due to N-N stretching vibrations of molecularly adsorbed N 2 species; this was confirmed by recording spectra with 15N 2 and 15 N 14N mixtures. The higher frequency band at 2352 cm-1 saturated at low nitrogen pressures, whereas the 2335 cm -1 band increased in intensity with increasing nitrogen pressures up to 3 kPa. The two bands also showed a different temperature dependence; the 2335 cm -1 band decreased steadily with increasing temperature and was totally lost at 280K, whereas the 2352 cm -1 band was still present at this temperature. These observations indicate that the species responsible for the 2352 cm -1 band is more strongly adsorbed than that giving the 2335 cm -1 band. Wakabayashi et al. observed also

111 changes in the zeolite v(OH) bands during nitrogen adsorption at low temperatures, and found that the growth of the 2335 cm -1 band correlated strongly with loss of the 3616 cm -1 v(OH) band due to Bronsted acid sites and the appearance of a new v(OH) band at 3510 c m - 1 . The 2335 cm -1 band was thus assigned to N 2 hydrogen bonded to Bronsted acid sites, the hydrogen bonding causing a 106 cm -1 shift in the v(OH) frequency. There was no interaction of nitrogen with the silanol groups responsible for a v(OH) band at 3752 cm -1.

The 2352

c m -1 band was assigned to N 2 interacting with Lewis acid sites in the zeolite; this band was suppressed if the zeolite was pretreated with water vapour, but restored if the zeolite was outgassed at high temperature before exposing to nitrogen. 2335

s

2400

2300

Figure 9. FTIR spectra of nitrogen adsorbed in H-mordenite at 110K. Nitrogen pressures of (a) 0.3, (b) 0.5, (c) 1.6 and (d) 3.7 kPa. Reproduced with permission from reference 29. Similar results were found for nitrogen adsorbed in HZSM-5 and HY zeolites[30]. A 2352 cm -1 band was observed only under conditions (high temperature evacuation) where Lewis acid sites could be anticipated. The frequency of the N 2 interacting with Bronsted acid sites is identical in HZSM-5 to that in H-mordenite (2334 cm -1 c o m p a r e d with 2335 cm-1). In HY, the corresponding frequency is 2338 cm -1,

112 suggesting a stronger perturbation of the nitrogen molecule in this case (the gas phase frequency of N 2 is 2330 cm -1, measured by Raman spectroscopy). In summary, physisorbed nitrogen appears to offer several advantages as an infrared probe of acid sites in zeolites. It clearly d i s t i n g u i s h e s between Bronsted and Lewis acid sites without interference from gas phase species, it is small enough to probe sites in smaller pore zeolites, and its interaction with the zeolite is sufficiently weak and reversible to have negligible influence on the zeolite chemistry. It is not yet clear whether the method can probe variations in Bronsted acid strength. The infrared spectrum of adsorbed nitrogen can also be used to probe cation sites in zeolites. Zecchina et al [34] compared vibrational frequencies of CO and N2 adsorbed at low temperatures in mordenite containing different alkali metal cations. In both cases the vibrational frequencies could be correlated with (R x + Rm)-2, where R x is the cation radius and R m the radius of the adsorbed molecule, suggesting a simple electrostatic field explanation for the frequency shifts between different cations. The appearance of a band due to N 2 interacting with a particular zeolite cation will also mean that that particular cation is located in sites accessible to the N 2 molecule. In contrast to the enormous body of literature associated with acid catalysis by zeolites, base catalysis has until recently received little attention. Infrared spectroscopy of probe molecules can however be used to study basic sites in zeolites in an analogous manner to the experiments characterising acid sites. Barthomeuf [35] first suggested using pyrrole as an acid probe of basic sites, and showed that the (NH) frequency of adsorbed pyrrole correlated well with the negative charge on the oxygen atoms of different zeolites calculated from average electronegativities. More recently, Huang and Kaliaguine [36] have used this method to examine a series of X, Y, mordenite and ZSM-5 zeolites exchanged with different alkali metal cations. The infrared spectrum of adsorbed pyrrole is complex, and care must be exercised in identifying the v(NH) mode and eliminating contributions to the spectrum from other forms of adsorbed pyrrole besides those present at basic sites. The v(NH) frequency of the free pyrrole molecule is 3497 cm-1; adsorption in basic zeolites shifts this to lower frequency by up to 320 cm-1. In the series of zeolites LiX, NaX, KX, RbX and CsX, for example, the v(NH) frequency of adsorbed pyrrole decreases from 3295 cm -1 to 3175 cm -1. Furthermore, in zeolites containing two different alkali

113

metal cations, two different v(NH) bands can be resolved. Figure10 shows spectra of pyrrole adsorbed in respectively NaX and CsNaX containing 6, 29, and 60 Cs per unit cell. The 3280 cm -1 band associated with Na cations is gradually diminished as the Na content decreases, and the 3175 cm -1 band associated with Cs increases. Note that a band at 3375 cm -1 is present in all of the X zeolites studied and was attributed to a weak structure dependant basic site independent of the cations present.[36]

3175

c-

05

..Q 04

,::5

j

3375 I

q"l

(d)

I

3280

(c) (b) (a)

Figure 10. Infrared spectra of pyrrole adsorbed in (a) NaX, and CsNaX containing respectively (b) 6, (c) 29, and (d) 60 Cs per unit cell. Reproduced with permission from reference 36. Huang and Kaliaguine interpret spectra such as those in Figure 10 to mean that the strongly basic sites in alkali metal exchanged zeolites are framework oxygen atoms immediately adjacent to the alkali metal cations, acting as Lewis bases. Since the v(NH) frequency of pyrrole adsorbed on MgO surfaces is at 3320 cm -1, those zeolites for which the

114 corresponding frequency is below this value can be taken to have basic sites stronger than those of MgO. This is the case for all alkali metal exchanged X zeolites, CsY, RbY, Cs-mordenite and Rb-mordenite. Alkali metal exchanged ZSM-5 zeolites, on the other hand, show v ( N H ) frequencies for adsorbed pyrrole of 3370 cm -1, suggesting that these zeolites contain only weakly basic sites. Huang and Kaliaguine also attempted to improve the correlation established by Barthomeuf between the v(NH) frequency of adsorbed pyrrole and the partial charge on the zeolite oxygen atoms, by calculating a local electronegativity for a fragment of the zeolite consisting of a single 6 ring plus one or more cations. The partial charge on the oxygen atoms in the 6 ring is then determined by this local electronegativity r a t h e r than an electronegativity averaged over the entire zeolite lattice. Figure 11 compares the correlation between the v(NH) frequency and oxygen charges calculated from the local electronegativity (solid line) and the average electronegativity (dotted trace). The local description clearly gives a smoother correlation for the more basic zeolites.

3500

!

3400

!

Li

3450 I

i

I

i

I

Li

K, Nb, Cs ~ Na -

E

3350

-I-

z 3300

ZSM-5

Cs

3250

~,

mordenite

3200

3150 0.210

K ~ Li Rb ~ k, '~Na

Rbk~,

I

0.260

I

0.310

Rb !

0.360

Y

I

X

0.410 0.460

Cs

1

0.510

oxygen charge Figure 11. Correlation of (NH) frequency of adsorbed pyrrole with oxygen atom charge calculated from global (dotted line) or local (solid line) electronegativity. Reproduced with permission from reference 36.

115

3.5

Hydrogen Bonding versus Proton Transfer A key issue in the chemistry and catalysis of basic molecules reacting in acid zeolites is the extent to which proton transfer occurs from the Bronsted site to the basic molecule. For strongly basic molecules like a m m o n i a or pyridine, infrared spectroscopy clearly identifies the protonated adduct (NH4 + or PyH +) from its characteristic vibrational frequencies. For trimethylphosphine, also a strong base, both infrared and NMR evidence for complete proton transfer are convincing[37]. For molecules which are less strongly basic, the question is not so easily answered. Hydrogen bonding of adsorbed molecules to the Bronsted OH groups in acid zeolites causes a shift to lower frequency and increases in the half width and intensity of the v(OH) infrared band, and there have been many studies correlating the extent of the frequency shift with the basicity of the adsorbate or the strength of hydrogen bonding.Where the hydrogen bonding becomes very strong, the infrared spectrum becomes more complex and harder to interpret. Figure 12 shows, for example, difference spectra obtained when methanol is adsorbed in HZSM-5 zeolites containing different concentrations of Bronsted acid sites at 423K. The negative peak at 3610 cm -1 shows that the Bronsted acid sites are interacting with methanol. The intense broad bands at around 2800, 2400 and 1700 cm -1 correlate with the concentrations of Bronsted acid sites in the different zeolites, and were originally attributed to v(OH) and 8(HOH) vibrations of a protonated methanol species C H 3 O H 2 + ; ie. proton transfer was considered to occur from the Bronsted acid sites to adsorbed methanol [38,39]. Note that the spectra in Figure 12 also contain contributions from methanol reacting with silanol groups and A1OH groups associated with extra f r a m e w o r k aluminium to form respectively CH3OSi and CH3OA1 species [40]. Similar intense bands at approximately the same frequencies are observed when other molecules of comparable proton affinity are adsorbed in HZSM-5 e.g.water [41],dimethylether [38,40,41], acetone and various carboxylic acids [41]. Pelmenschikov et al. [42,43] pointed out that these bands are very similar to the so-called A,B,C triplet of OH bands characteristic of strong molecular hydrogen bonded complexes in liquid or solid phases. The most widely accepted explanation for the A,B,C triplet in hydrogen bonded systems is that due to Claydon and Sheppard [44], who suggested that the A,B,C triplet are in fact pseudobands caused by the superposition onto a very broad single (OH) band of two so-called Evans transmission windows caused by Fermi resonance between the (OH) mode and the first overtones of in-plane ( 2 6(OH) =__ 2600 cm -1) and out of plane ( 2 y(OH) ___- 1900 cm -1) bending

116 modes respectively. If this is origin of the A,B,C triplet for molecules adsorbed in HZSM-5, then proton transfer is not occurring from the Bronsted site to the adsorbed molecules.

o er

b

.t) I,..,

O

<

--Vd |

4000

3000

I

I

I

2000

F i g u r e 12. Infrared difference spectra obtained on adsorption of m e t h a n o l in HZSM-5 zeolites containing different aluminium concentrations at 423 K. Si:A1 ratios of (a) 12, (b) 27, (c) 16 and (d) 121. Data from reference 40.

Pelmenschikov et al. have recently provided direct experimental support for this alternative description of strong hydrogen bonding rather than proton transfer [45].The ~5(OH) band in HZSM-5 is obscured by the zeolite lattice vibrations between 1000 and 1300 cm -1. In deuterium exchanged DZSM-5 however the corresponding ~5(OD) vibration occurs at 894 cm -1 and can be directly observed. Adsorption of CD3CN into DZSM-5 shifts the ~(OD) band to 988 cm -1 (an increase in the ~i(OH) frequency is expected when hydrogen bonding occurs). The A and B components of the A,B,C triplet for CD3CN adsorbed in DZSM-5 occur at around 1880 cm -1 and 2060 cm -1 ; the Evans window between

117 these two bands matches exactly the frequency calculated from 2x ~(OD) (The C component of the triplet in this case is not resolved). The argument that strong hydrogen bonding rather than proton transfer is the prevailing interaction with molecules other than very strong bases is given further weight by theoretical density functional calculations which indicate that protonated methanol is not a stable species in HZSM-5146], but a transition state in the dissociation of the strongly hydrogen bonded species to generate surface bound methoxy groups and water. Similar calculations have not yet been reported for dimethyl ether, but given the similarity of the A,B,C v(OH) triplet in the infrared spectra of methanol and dimethylether adsorbed in HZSM-5 [40] it is likely that protonated dimethylether is also not a stable adsorbed species.

3.6

Time Resolved Infrared Spectroscopy As noted above, the advent of rapid scanning Fourier transform spectrometers has made time resolved in-situ studies of catalytic reactions a realistic possibility.An early example demonstrating the value of this approach is the work of Lercher et al. on xylene diffusion and isomerization in HZSM-5 [47]. These workers set up an in situ infrared cell which approximated a continuous stirred tank reactor and measured infrared spectra as a function of time at various reaction temperatures in a continuous flow of reactant (coupled with GC analysis of reaction products). Figure 13 shows time resolved difference spectra measured on exposure of HZSM-5 to flowing ortho-xylene at 473K. These show the rapid initial adsorption of the reactant onto Bronsted acid sites (note the negative peak in the difference spectra at 3610 cm-1), and the slower development of bands due to adsorbed meta-xylene. After 1 hour on stream approximately 15% of the acid sites were covered with metaxylene molecules. Under steady state conditions, all three isomers of xylene reacted at the same rate (GC analysis) and gave the same surface concentrations (FTIR analysis) at 473K ie. at this temperature surface reaction and not reactant diffusion is the rate limiting step. At higher temperatures (e.g.573 K) the diffusivity of the reactant molecules began to influence the overall reaction rate and the concentrations of adsorbed molecules increased in the order m-xylene < o-xylene < p-xylene. The results were considered to be completely consistent with a unimolecular 1,2 methyl shift mechanism for the isomerization reaction. The time resolution of these experiments was typically 20 seconds. Measurement of spectra on time scales much shorter than this is now in principle possible. The work of Lercher has demonstrated that time

118 resolved in-situ measurements of adsorbed species coupled with conventional analysis of gas phase products will greatly advance understanding of reaction mechanisms in zeolite catalysis.

Figure 13.

Time resolved difference infrared spectra of the adsorption and reaction of o-xylene with HZSM-5 at 473K under continuous flow conditions. Reproduced with permission from reference 47.

3.7

Single

Crystal

Studies

Currently available FTIR microscopes allow transmission or reflectance infrared spectra to be measured from sample areas as small as ca 20 microns (the diffraction limit for infrared radiation). Several groups are now using this technology to measure infrared spectra from zeolite single crystals. There are several reasons for performing the spectroscopy on single crystals rather than the more usual polycrystalline powders. Firstly, study of a single crystal avoids any difficulties caused by the presence of crystalline or amorphous impurities in a macroscopic sample. Second, the use of polarised radiation and oriented single crystals allows in principle information to be obtained about the orientation of adsorbed molecules within zeolite pores. Also, the common experimental problem encountered in transmission infrared measurements on pressed pellets of

119 polycrystalline powders of strongly sloping baselines, caused by scattering of infrared radiation by crystallites with dimensions comparable to the infrared wavelength, is not present in single crystal studies. Single crystal spectroscopy does of course require the synthesis of zeolite single crystals of suitable size, which is not always easy. Diffusion restrictions in large crystals may also be a problem in achieving rapid equilibrium in adsorption or desorption studies. An example of the advantages of single crystal spectroscopy in terms of well defined samples is the recent work of Muller et al. on sorption and reactivity of cloverite [48]. Cloverite is a microporous gallophosphate with 2.9 nm diameter supercages connected via 1.32nm 20-ring windows. Muller et al. carried out single crystal infrared measurements on crystals between 80 and 140 microns in size, using an in-situ vacuum cell. Figure 14 shows spectra obtained from single crystals of cloverite as synthesised and after calcination. The spectrum of the as-synthesised material is dominated by bands due to the quinuclidine template. There is also however a v(OH) band at 3675 cm-1 assigned to structural P-OH groups located in the 20-ring windows of the cloverite structure. On calcination, the quinuclidine is removed, and aside from lower frequency bands which are due to overtones and combinations of Ga-O and P-O stretching vibrations, the spectrum contains two intense v(OH) bands, the 3675 cm -1 band increased in intensity and a new band at 3702 cm -1. The 3702 cm -1 band is assigned to GaOH groups, also located on the 20-ring windows. In comparison with previously reported spectra measured on polycrystalline powders [49], the v(OH) bands from the single crystal are much more distinct and well resolved, and the assignment of the bands differs from that given previously. Muller et al. obtained support for their identification of 20-ring POH and GaOH groups from studies of the interaction of different adsorbate molecules with the hydroxyl groups. For example, both types of hydroxyl group interacted with adsorbed benzene and toluene, giving a single hydrogen bonded v(OH) frequency at about 3450 cm-1. Ammonia and pyridine adsorption gave spectra characteristic of both Bronsted and Lewis acidity; it is proposed that the Lewis acidity is generated by partial opening of Ga-O bonds adjacent to the GaOH groups. What is clear from this study is that single crystal microspectroscopy can give much better quality data than conventional transmission through pressed pellets of polycrystalline powders.

120

1.6

activated "as .synthesized" cloverite cloverite

1.4 1.2

0.8 0.6 0.4 0.2 3500

3000

2500 wavenumber [ 1/cm]

2000

! 500

Figure 14. Infrared spectra of a single crystal of cloverite before and after calcination. Reproduced with permission from reference 48. As an illustration of how orientation information can be deduced from polarisation measurements in the infrared microscope, the recent study by Howe and Jackson of ethene oligomerization in single crystals of HZSM-5 is cited [50]. Ethene reacts slowly in HZSM-5 at room temperature to form an oligomeric species which has the spectroscopic signature (infrared and 13C NMR) of a long chain aliphatic hydrocarbon. At higher temperatures the oligomer becomes more highly branched, and ultimately cyclic species and aromatic hydrocarbons are produced. Microspectroscopic measurements were undertaken on single crystals of HZSM-5 exposed to ethylene at different temperatures using either unpolarised infrared radiation or radiation polarised parallel to or perpendicular to the crystal c axis. Figure 15 shows spectra obtained with unpolarised radiation in the v(CH) region of ethene adsorbed in HZSM-5 after heating to various temperatures. At low temperatures, the spectrum is dominated by a pair of bands at 2934 cm -1 and 2860 cm -1 due to asymmetric and symmetric stretching modes respectively of CH2 groups in

121 polymethylene chains. At 473K and above there is an increasing contribution from bands at 2960 cm-1 and 2870 cm-1 due to the correspondin~ vibrations of CH3 groups as chain branching occurs.

(e)

Wavenumbers (Cm"1)

Figure 15. Infrared spectra of ethene adsorbed in a single crystal of HZSM-5 at room temperature (a) and after heating successively to 373K(b), 473K(c), 573K(d) and 673K(e) (unpolarised). Reproduced with permission from reference 50. The effects of using polarised radiation on the spectra measured after exposure of the crystal to ethene at 373K are shown in Figure 16. Polarisation parallel to the crystal c axis enhances the contribution of the CH 3 symmetric and asymmetric modes to the spectrum relative to the unpolarised spectrum or that o b t a i n e d with polarisation perpendicular to the c axis. Similar polarisation effects were found for crystals exposed to ethene at room temperature or 473K, regardless of which crystal face the infrared beam was incident upon. After heating

122 to 573K or above, however, the infared spectrum obtained independent of the plane of polarisation of the incident beam.

was

The symmetric and asymmetric stretching modes of the CH 2 groups in a polymethylene chain will be observed most strongly when the plane of polarisation of the incident beam is perpendicular to the linear axis of the chain, whereas for the terminal CH 3 groups on such a chain little difference is expected between perpendicular and parallel polarisation. The observed polarisation effects are interpreted to mean that the low temperature linear oligomer chains are formed along both the linear and sinusoidal channels of the ZSM-5 structure. Above 473K however, the shorter chain highly branched oligomers formed are randomly oriented at channel intersections and no polarisation effects are observed.

(c)

(b)

+

, . , . _ - -

3050

3000

2950

2900

2850

2800

2750

Wavenumbers(cm"1)

Figure 16. Polarised infrared spectra of a single crystal of HZSM-5 after heating in ethene to 473 K. (a) unpolarised, (b) polarised perpendicular to the c-axis, (c) polarised parallel to the c-axis. Reproduced with permission from reference 50.

123 4. R A M A N S P E C T R O S C O P Y

4.1

Background Raman spectroscopy has not attracted the same attention as infrared spectroscopy as a technique for characterising zeolites and adsorbed molecules. This can be attributed to two principal experimental problems: the inherent low sensitivity of Raman spectroscopy, and the tendency for many zeolite samples to fluoresce in the intense visible laser beam normally used for Raman spectroscopy. Despite these difficulties, many groups have reported successful use of Raman spectroscopy to study zeolites. A recent review has been given by Bremard and Bougeard [51]. Two recent instrumental advances offer hope of future progress. The Fourier transform Raman technique, which uses near infrared laser excitation and a Michelson interferometer to overcome the sensitivity loss at lower frequency, solves in principle at least the fluorescence problem. The sensitivity of conventional Raman spectroscopy has been enhanced many orders of magnitude by the development of high throughput holographic notch filters to replace monochromators for elimination of Rayleigh scattered light, and the replacement of photomultiplier detectors by CCD cameras. The new generation of Raman instruments using notch filters, a single monochromator and a CCD detector can record good quality spectra in a few seconds at laser powers orders of magnitude lower than those previously needed.

4.2

Zeolite Lattice V i b r a t i o n s The stretching and bending modes of zeolite lattices have weak Raman cross sections, which makes measuring high quality Raman spectra difficult. Laser induced fluorescence is also a common problem with dehydrated zeolites, although this can be overcome with the Fourier transform technique. As with the corresponding infrared s p e c t r a , the frequencies of the Raman active lattice modes depend on both the local structure and the composition of the zeolite lattice. From a theoretical perspective, Raman (and infrared) spectra of zeolite lattices can be calculated from first principles using the methods of molecular dynamics. In this approach, an ensemble of atoms representing the system is allowed to evolve with time under an interatomic potential describing the van der Waals, Coulombic and hydrogen bonding forces acting. Infrared or Raman spectra are then obtained by applying linear response theory to the resulting time dependant dipole moment or polarizability tensors. For example, Smirnov and Bougeard [ 52] have compared calculated and observed Raman spectra for a series of siliceous zeolites. Figure 17 shows results

124 obtained. The molecular dynamics calculations do moderately well in matching the experimental spectra below 600 cm-1, where the vibrational modes are Si-O-Si bending modes and skeletal deformation modes associated with particular structural features. The calculations do not however predict correctly the observed very low intensity Si-O stretching modes at higher frequencies. Smirnov and Bougeard suggest that this has to do with uncertainty about the magnitude of the anisotropic part of the polarizability tensor.

I

o

'

9

9 '

,

'

i

'

~5oo

,

'

t

loo6

L '

,

'

-

WAVENUMBERS CM-1

Figure 17. Comparison of calculated and experimental of siliceous zeolites.(a) sodalite (calculated), (b) zeolite A (c) zeolite A (calculated), (d) FAU (calculated), (e) FAU (f) silicalite (calculated).Reproduced with permission from

Raman spectra (experimental), (experimental), reference 51.

125 Introduction of aluminium into a zeolite lattice broadens the lattice modes, but also introduces additional bands in the Raman spectra at low frequencies due to cation vibrations, completely analogous to the far infrared bands described in section 3.3. Figure 18 shows, for example , Raman spectra taken from the work of Bremard and Le Maire [53] of zeolite Y exchanged with different alkali metal cations. The arrows indicate bands assigned to translational modes of the cations; these move to lower frequency as the mass of the cations increases, just as in the far infared spectra. I

I

!

I e

J d

J

I

,! 100

I

I 200

WAVEX .U 'M BEg(CM"1)

Figure 18. Low frequency Raman spectra of dehydrated alkali metal exchanged faujasite zeolites. (a) Cs, (b) Rb, (c) K, (d) Na, (e) Li. Reproduced with permission from reference 51. 4.3

Raman

Spectra

of

Adsorbed

Molecules

The low intensity of the Raman bands intrinsic to zeolite structures is an advantage when attempting to observe adsorbed molecules (in contrast to the situation in infrared spectroscopy, where large regions of

126 the spectrum can be blanked out by zeolite lattice bands). Many Raman studies have been reported of both inorganic and organic molecules adsorbed in zeolites, which will not be reviewed here. An interesting new development is the possibility of achieving resonance enhancement of Raman spectra of selected adsorbed species by tuning the exciting laser wavelength to match a UV-VIS absorption band of that particular species. For example, Jakupca and Dutta [54] used 4-amino pyridine to probe the weak acid sites present in NaY. The normal Raman spectrum of this molecule adsorbed in NaY shows bands due to both the neutral molecule and the protonated adduct. The UV absorption band of the protonated adduct is however shifted by 20nm from that of the neutral molecule, to 264nm. When the Raman spectrum is recorded using 266nm laser excitation, the spectrum of the protonated adduct is resonance enhanced selectively. This approach of varying the exciting wavelength to selectively enhance Raman spectra of different adsorbed species will find many other applications in zeolite chemistry as the instrumentation required becomes more widely available.

4.4

Raman

Microspectroscopy

Raman microspectroscopy using visible or near infrared laser light is performed by coupling a conventional optical microscope to the Raman spectrometer. A laser spot size of ca 1 micron can be achieved , allowing study of single crystals of zeolites. The advantages of measuring Raman spectra from zeolite single crystals are similar to those of infrared microspectroscopy; contributions to the spectra from amorphous impurities are eliminated, and information about orientation of adsorbed molecules can in principle be deduced from polarisation measurements. Because the method does focus the laser beam into a very small area, the risk of laser induced heating or decomposition of adsorbed species may be enhanced, although the new generation of single monochromator instruments can measure spectra at much lower laser powers than previously.The technique has until now not been widely exploited. Figure 19 shows a spectrum obtained in the authors laboratory from a 100 micron single crystal of as synthesised ALPO-5, using a low power (30mW) argon ion laser and a single monochromator/CCD detector spectrometer [55]. The major bands in the spectrum are those due to the triethylamine template molecule used in the zeolite synthesis. Attempts to follow the process of template decomposition in these single crystals by Raman microspectroscopy have so far failed because of the intense fluorescence induced by the visible laser in heated samples. Further work on this and other single crystal zeolite systems is in progress, but the spectrum shown illustrates the excellent signal to noise that can be achieved with current Raman microscope instrumentation.

127

9

9

Figure 19. synthesised

II

Raman [55].

5. UV-VISIBLE

5.1

9

Iooo

9

,

9

9

g

2o00

spectrum

of

9

9

9

"3060

a

single

crystal

"

"

of

"

ALPO-5,

as

SPECTROSCOPY

Background

Diffuse reflectance techniques have been used to measure UV-Visible spectra of transition metal ions in zeolites and of adsorbed molecules for nearly 30 years. There have been no dramatic changes in experimental technique in recent times, although the theoretical basis for interpretation of the electronic spectra of transition metal ions in zeolites has received considerable attention in the past 10 years. There has also been a growing realisation of the importance of combining UVVIS data with that from other spectroscopic and structural methods to fully characterise zeolite systems.

128 5.2.

UV-Visible Spectra of Transition Metal Ions in Zeolites As an illustration of the current state of the art for electronic spectroscopy of transition metal ions in zeolites, refer to the recent review by Schoonheydt of Cu 2+ in different zeolites [56]. Schoonheydt shows that experimental measurement of diffuse reflectance spectra (and in the case of Cu 2+ EPR spectra) must be combined with theoretical calculations if a complete interpretation is to be made. The exact frequencies of the d-d transitions in the electronic spectrum of Cu 2+ are independent of the zeolite structure type, the Si'A1 ratio, and the co-exchanged cations, but depend solely on the local coordination environment. Figure 20 shows the diffuse reflectance spectrum of dehydrated Cu-chabazite: the expanded portion reveals the three d-d transitions in the region around 15000 cm -1.

FIRI

o.o5

0.80 0.60 0.40 0.20

0.00 5000

20000

30000

40000

cm

-1

F i g u r e 20. Diffuse reflectance UV-VIS spectrum of dehydrated CuNa-chabazite. Reproduced with permission from reference 59. For a given site model, the d-d transitions (and the EPR parameters) can be calculated using the theoretical angular overlap model of Klier et a1.[57]. Schoonheydt et al. use this model to carry out a least squares

129

fitting of the 6 experimentally determined parameters ( three d-d transition frequencies from the electronic spectrum and g and hyperfine parameters from the EPR spectrum) for Cu 2+ in sites I, I' and II in zeolites A, X, Y and chabazite. In contrast to the d-d transitions, the frequency of the intense charge transfer band in the UV spectra of Cu 2 + in zeolites ( around 40,000 cm -1 in Figure 20) does vary with the zeolite structure and composition. In simple terms, the frequency of the charge transfer band maximum can be related to the electronegativity difference between Cu 2+ and the surrounding oxygens. (In principle, this should vary with the local coordination, but the experimentally observed charge transfer band cannot be resolved into separate components in practice). If the properties of the Cu2+ are assumed to be constant, then differences in electronegativity can be ascribed to variations in properties of oxygen atoms in different zeolites. The electronegativity equalisation method of Mortier et al. [ 58] can be used to calculate a quantity called the global softness of the zeolite, which incorporates both the chemical composition and the structure of the zeolite lattice. As shown in Figure 21, the frequency of the charge transfer transition for Cu 2+ exchanged in different zeolites correlates well with the global softness.

0 - Cu C T / e r a

o1

44 +

z~4-5

43

"~'o 42

/

c

D 0

41

4O

239

0.10

I

I

0.12

0.14

0.16

0.18

0.20

o / global softness

Figure 21. Correlation of the Cu2+-O charge transfer band frequency with the global softness of different zeolites.

130 6. X - R A Y A B S O R P T I O N S P E C T R O S C O P Y

6.1

Background X-ray absorption spectroscopy (XAS) encompasses both EXAFS and XANES. As synchrotron radiation has become more widely accessible in recent years, both of these techniques have become more popular for the characterisation of zeolites. XANES involves measuring the fine structure immediately below the X-ray absorption edge which results from electronic transitions to high lying excited states. EXAFS is concerned with the fine structure occurring above the absorption edge, caused by back scattering of the ejected photoelectrons from neighbouring atoms. In simple terms, XANES provides information about the electronic structure and environment of the atom being measured, while EXAFS gives structural information. Both XANES and EXAFS have been used for many years to study transition metal clusters dispersed in zeolite supports. More recent interest has focussed on the characterisation of substituted zeolites, and on the use of low energy Xrays to observe directly the framework elements in aluminosilicate zeolites.

6.2

Framework Substitution The question of whether an element is or is not isomorphously substituted into a zeolite lattice can in principle be answered by a combination of XANES and EXAFS. The pre- edge features appearing in the XANES spectrum of a transition element can be used to deduce the symmetry of the ligand field, since the selection rules for the bound state transitions responsible for these features are governed by symmetry. In particular, an intense pre-edge peak due to l s to 3d transitions will be observed only in the absence of inversion symmetry e.g. for tetrahedral coordination but not for octahedral coordination. For example, the appearance of this peak in the XANES of CoAPO-5 is consistent with the EPR and UV-VIS evidence for tetrahedrally coordinated Co 2+ isomorphously substituted for A13+ in the A1PO-5 lattice [60]. Evidence for isomorphous substitution also is obtained from the EXAFS, which measures the numbers and distances of nearest and next-nearest neighbour atoms. In principle, the EXAFS of an isomorphously substituted element should show not only 4 nearest neighbour oxygen atoms at a distance consistent with the known lattice structure, but also the appropriate number of next nearest neighbour framework atoms as well. In practice, second and third shells are not always detected because of disorder in the structure. In CoAPO-5, only the first shell oxygen atoms are detected; 4 at a distance of 1.93 A.

131 Corma et al. have recently used EXAFS and XANES to assist in the characterisation of Ti-MCM-41 structures[61] .Figure 22 shows Ti XANES spectra for the Ti-MCM-41 material after calcination to remove the surfactant template and exposure to air (i.e.hydrated), and following subsequent dehydration. The dehydrated sample shows an intense preedge peak whose position and intensity are consistent with the presence of tetrahedrally coordinated Ti 4+. This peak is reduced in intensity and slightly shifted in the hydrated sample; there are also some changes in the absorption edge profile. Corma et al. attribute these changes to a transition to distorted octahedral symmetry on hydration. The Ti EXAFS of the dehydrated sample (actually the Fourier transform magnitude of the weighted EXAFS) is shown in Figure 23, and compared with that of anatase.The EXAFS of the Ti-MCM-41 shows only a single coordination shell, which could be fitted with 4.3 oxygen atoms at a distance of 1.81 A, significantly shorter than the corresponding distance in anatase. There is also no evidence in the MCM-41 material of Ti-Ti distances seen in bulk anatase at larger values of r, although the small peaks seen around 3 A may be due to Ti-Si next nearest neighbours. Corma et al. conclude from the EXAFS, XANES and other characterisation studies that the Ti-MCM-41 material prepared by incorporating Ti into the MCM-41 synthesis does indeed contain Ti substituted into the silicate framework of the MCM-41.

1,s 0 t~

tN t~

o0,5 Z

I

0

I

I

40

I

!

8O

AE (eV)

Figure 22. Ti K-edge XANES of Ti-MCM-41. (a) calcined then exposed to air, (b) calcined then dehydrated. Reproduced with permission from reference 61.

132

15

I

I

I 2

1 4

r,rj

I:

~

~176

LT.

0 0

R (A)

6

F i g u r e 23.

Fourier transform magnitude of k 3 weighted EXAFS from anatase (solid line) and Ti-MCM-41 (dashed line). Reproduced with permission from reference 61. 6.3

XANES a n d E X A F S of Silicon and A l u m i n i u m The X-ray absorption edges of Si and A1 occur in the so-called soft Xray region below 2000 eV (at 1839 and 1559 cV respectively). Until recently, Si or A1 EXAFS and XANES was not feasible because of the difficulties of producing monochromatic X-rays and of detecting X-ray absorption at these energies. These difficulties have however now been overcome, and soft x-ray monochromators with fluorescence or electron yield detection of X-ray absorption are now available at several synchrotrons around the world. Koningsberger and Miller [62,63] have reported aluminium XANES and EXAFS for several different zeolites. Figure 24 shows XANES spectra of four different zeolites. There arc clear differences in the intensity of the so-called white line, the peak immediately above the absorption edge, between the four samples.

133

N3 E t_

|

0 e-

.i , t-,. ~ "

v

l'~

!.i k~ "--

2

_

/

.t.., \ - ,i,.

|: "!~"

c ~0

!~

_q

',

-...... %*"<,.

|

.. ......... "~176

0

/

.Q1

t_ !

x

•

~

0

,

-5

!,

0

,

i

L

5

10

1

15

20

Energy (eV) Figure 24. A1 K-edge X-ray absorption spectra of NaY (solid line), HY (dotted line), H-CaY (dashed line) and H-LaY (dot-dash line). Reproduced with permission from reference 63. The white line intensity, representing a l s to 3p transition, is clearly sensitive to the type of cation exchanged into the zeolite. The white line intensity is in fact a measure of the electron density on the aluminium; the greater the electron density the lower the white line intensity. The order of electron density parallels the acidity of the zeolites. The most acidic is HLaY, which has the most positively charged aluminium.(and thus the highest white line intensity).Koningsberger and Miller suggest that the higher acid strength is due to withdrawal of electron density from the hydroxyl group bound to aluminium by the adjacent La cation, consistent with the model proposed by Lunsford[64] for the enhanced acidity of REY zeolites. The corresponding A1 EXAFS data reported by Koningsberger and Miller shows that there is no change in the coordination number or A1-O distance (within experimental error) as the zeolite cation is varied. The coordination number remains 4, and the A1-O distance 1.70+0.01 .~ in all cases. The first report of Si XANES data for a zeolite is that by Fr6ba et al. [65], who examined a series of aluminosilicate sodalites containing different occluded anions. Varying the anion from chloride through to chromate changes the Si-O-A1 bond angle in sodalite from 138.2 to 148.1 degrees, as determined by X-ray diffraction. These variations in geometry caused significant changes in the post edge fine structure in

134 the XANES spectra for both Si and A1. The authors attribute the changes to an increase in the next nearest neighbour Si-A1 distance as the bond angle increases, reducing the next nearest neighbour back scattering amplitude. These conclusions are qualitative at this point; nevertheless the potential of Si and A1 XANES and EXAFS to follow subtle changes in framework geometry as a result of adsorption or other modification of a zeolite has been clearly demonstrated.

7. MASS S P E C T R O M E T R Y OF Z E O L I T E S

7.1

Background

Mass spectrometry is a standard spectroscopic technique for the c h a r a c t e r i s a t i o n of high molecular weight organic and inorganic compounds, but has until recently received little attention from the zeolite community. The surface composition of zeolites has been explored using fast atom bombardment mass spectrometry (FABMS)[66] and secondary ion mass spectrometry [67], but mass spectrometric analysis of the bulk composition of a zeolite or of adsorbed molecules has not until very recently been attempted. The practical difficulty is to vaporise the solid. Two different strategies have been proposed: laser ablation and plasma desorption.

7.2

Laser

Ablation

Mass

Spectrometry

The laser ablation technique uses a high intensity near infrared pulsed laser to simultaneously vaporise and ionise solid samples.[68]. Application of this technique to zeolites was first reported by Jeong et a1.[69], who examined positive and negative ion mass spectra obtained on laser ablation of three different zeolites, using a Fourier transform ion cyclotron resonance mass spectrometer to trap and identify the ions produced. At low laser power levels, the zeolite cations are photodesorbed from the zeolite and detected as Na + or K + gas phase ions. Under these conditions the zeolite structure remains intact. A similar photodesorption of zeolite cations has been reported by Tanaka et al.[70], although they used a time of flight technique to detect the ions. At higher laser powers (above a critical threshold power which depends on the zeolite and the extent of dehydration of the zeolite) Si + and A1+ cations are detected. At these laser powers, the zeolite is clearly destroyed. This is confirmed by observation of the corresponding negative ion spectra, shown in Figure 25. A complex array of silicate and aluminosilicate cluster anions are produced; species such as N a S i 3 0 7- (mass 219), KSi30 7- (mass 235), A1Si20 6- (mass 179), and many others. In these, and in many other zeolites studied, the

135

composition of the ions produced correlates at least semi quantitatively with the chemical composition of the zeolite (High aluminium zeolites give higher abundances of aluminium containing cluster anions, for example). There is no indication however that the cluster ions detected represent intact fragments of the zeolite lattice, i.e. that the cluster ions give information about the connectivity of atoms in the zeolite lattice. What is much more likely is that they are generated through gas phase ion molecule reactions in the laser plume produced on ablation at high laser powers, which contains a complex mixture of charged and neutral atoms, molecules and electrons. 219

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Figure 25. Laser ablation negative ion mass spectra of (a) NaMOR, (b) KLTL, (c) NaLTA zeolites. Reproduced with permission from reference 69.

136 In work not yet published the laser ablation method has also been applied to zeolites containing adsorbed species. It has been shown that the technique is able to generate molecular ions from strongly adsorbed species such as hexamethylbenzene in NaY, the tetrapropylammonium cation in as synthesized MFI zeolites, and polyethylene oligomers generated from ethylene in HZSM-5.

7.3

Plasma Desorption Mass Spectrometry An alternative method for producing mass spectra of solid samples which shows promise for the analysis of zeolites and related materials is the plasma desorption technique recently reported by Schwiekert et al. [71,72]. This technique uses a less energetic means of ionizing the solid than laser ablation, and initial indications are that negative ion spectra from materials like zirconium phosphate may reflect the connectivity as well as the stoichiometry of the solid analyzed to a greater degree than laser ablation.

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J.K.Lambert, G.S.Koerma, J.M.Macavay,J.M.Chabala and R.Levi-Selti, Appl.Surface Sci. 55 (1992) 149. D.M.Lubman,ed, "Lasers and Mass Spectrometry", Oxford University Press, New York, 1989 S.Jeong, K.J.Fisher,R.F.Howe and G.D.Willett, Microporous Mater. 4 (1995) 467. K.Ishigoh, K.Tanaka, Q. Zhuang and R.Nakata, J.Phys.Chem. 99 (1995) 12231. M.J.Van Stipdonk, M.A.Park, E.A.Schweikert, P.Sylvester and A.Clearfield, Int. Mass Spectrom. Ion Processes 128 (1993) 133. M.J.Van Stipdonk, W.R.FarreU and E.A.Schwiekert, Proc. 42nd ASMS Conference on Mass Spectrometry, 1995.

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H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology

Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All fights reserved.

141

Characterization of Zeolitic Materials by Solid-State NMR State of the Art Michael St'6cker SINTEF Oslo, Department of Hydrocarbon Process Chemistry, P.O.Box 124 Blindern, N-0314 Oslo, Norway Dedicated to my parents, Sophia and Michael Stticker, sen. Contents

5. 6. 7. 8.

Introduction High Resolution Solid State NMR- Experimental Techniques, including 2D NMR Recent Highlights about the Framework Characterization 3.1. Zeolites, A1PO4 and SAPO Molecular Sieves 3.2. Mesoporous Materials 3.3. Octahedral Molecular Sieves 3.4. Organic Molecular Sieves/Carbogenic Molecular Sieves (CMS) Pore Architecture investigated by NMR In-situ NMR Studies with Zeolitic Materials Diffusion of adsorbed Molecules monitored by NMR Acidity of Zeolitic Materials Concluding Remarks

1.

INTRODUCTION

.

2. 3.

.

Solid-state NMR spectroscopy is nowadays a well established technique for characterization of zeolites and other porous materials with respect to structure elucidation, pore architecture, catalytic behaviour and mobility properties (like diffusion). The objective of this paper is to highlight recent solid-state NMR results of zeolitic materials, based on new techniques, methods and pulse sequences. The intention is not to review recent NMR results, since a large number of such papers is easily available and one of the latest was presented during the 10th IZC Summer School on Zeolites in Wildbad Kreuth, Germany, two years ago (1). The development of solid-state NMR as a successful tool for the characterization of zeolitic and other porous materials has been tremendous during the last two decades. Considerable progress has been made with respect to higher magnetic field strengths, faster

142 spinning speed rates, sophisticated hardware and pulse sequence developments as well as improved multinuclear capabilities aiming towards enhanced resolution and sensitivity for high-resolution solid-state NMR investigations. Solid-state NMR provides some advantages compared to X-ray diffraction (XRD), since amorphous materials as well as crystalline materials can be studied. In addition, silicon and aliminium have almost the same scatterings factors, which means no discrimination between those two nuclei can be achieved by XRD. While X-ray diffraction provides information about the long-range ordering and periodicities, the NMR technique allows investigations on the short-range ordering (local environment) and structure. Powder XRD reveals only limited information with respect to zeolitic material lattice structures and only single crystal XRD could be used to elucidate the structure of zeolitic materials. As known, zeolitic material crystals are usually to small for single crystal XRD investigations. However, XRD is a very complementary technique to solid-state NMR, and the use of both methods is still a powerful combination in connection with zeolitic material characterization. The potential of high-resolution solid-state NMR has been known for a long time. However, the challenge has always been to overcome the problems in connection with recording solid-state NMR spectra with sufficient resolution. The main difference in the NMR characteristics of liquids and solids is that distinct nuclear spin interactions like chemical shift anisotropy, dipolar and quadrupolar interactions, which lead to excessive line broadening, are averaged in liquids due to the fast thermal/molecular motions of molecules, but are operative in the rigid lattice of solids (the molecules are less mobile) (2). As a consequence, the fine structure is lost since broad lines are obtained, hiding the essential information of analytical character. In addition, long Tl-values are controlling the relaxation of nuclei in solids, due to the lack of translation- and rotation motions. However, todays high-resolution solid-state NMR equipment allow the study of solids in a routine manner and provide information of analytical quality, mainly due to the above mentioned improvements. As already mentioned, a large number of review papers and books exist, covering the enormous literature available within the field of solid-state NMR related to zeolitic materials. Therefore, a certain number is listed among the references (1, 3-16), and I would like to advice the interested reader to consult those reviews for an extensive survey, since this paper will focus mainly on the highlights of recent developments. In the f'trst part of this paper a brief overview will be presented regarding the experimental techniques of solid-state NMR spectroscopy (including 2D NMR) leading to spectra with the quality of high resolution. This part will be kept on a level acceptable for experienced zeolite scientists without requirering all the fundamental knowledge of the general and/or specific solid-state NMR theory and is to a large extend based on the presentation given during the 10th IZC Summer School on Zeolites in Wildbad Kreuth, Germany, July 1994. The consecutive parts will deal with recent highlights regarding the information available from high-resolution solid-state NMR and its analytical application to investigations of zeolitic materials. The author hopes that the reader at the end of this paper will have some understanding of the present and future potential of solid-state NMR spectroscopy within this field.

143

0

HIGH RESOLUTION SOLID STATE NMR SPECTROSCOPYEXPERIMENTAL TECHNIQUES INCLUDING 2D NMR

The broad lines obtained for solid-state NMR spectra without applying any linenarrowing improvements are due to the different behaviour of nuclear spin interactions in solids compared to liquids. These interactions are averaged to zero or reduced to the isotropic values in liquids by the fast molecular motions, whereas the fixed (and different) orientations (with respect to the external magnetic field Bo) of the local environments of NMR active isotops in the rigid lattice of a solid cause line broadenings. The recorded broad NMR line patterns are superpositions of resonances from randomly oriented individual nuclei due to a random distribution of different orientations, since zeolitic materials usually are microcrystaUine powders. Table 1 summarizes the nuclear spin interactions and their behaviour in liquids versus solids (17).

Line broadening phenomena As already mentioned, the lack of molecular motion in solids gives rise to broad resonances and the received spectral pattern consist of overlapped lines, hiding the valuable analytical information available from the isotropic chemical shifts. In principle, there are three line broadening mechanisms, described in the following (13).

a) Dipolar Interactions Besides the magnetic field causing the Zeeman splitting, there are usually additional magnetic interactions between the magnetic moments of the observed nucleus and those located in the neighbour environment. The strength of these so-called dipolar couplings depends on the magnitude of the magnetic moments of the interacting neighbour nuclei, the distance (decrease with the internuclear distance, 1/r3) and the orientation of the internuclear vector with respect to the external field Bo. This interaction is independent of the applied magnetic field (11, 13). In liquids, fast molecular motion averages the dipolar couplings to zero. However, in a solid no such effect occurs, and broad resonance fines are obtained. The shape of the line is broad and featureless, and the line width (AX)l/2) can be up to many kHz. There are two types of dipolar couplings: the homonuclear dipolar couplings (for example 1H-1H), which means interaction between the spins of like nuclei, and the heteronuclear dipolar couplings (for example 29si-lH), in which the interaction occurs between the nuclear species under observation and spins of different nuclei. Dipolar interactions with protons are usually, very dominating, whereas homonuclear interactions of low natural abundant nuclei, like 29Si and 13C, can usually be neglected due to strong internuclear distance dependence.

b) Chemical Shift Anisotropy (CSA) In addition, the external magnetic field induces magnetic moments due to electron circulation in connection with chemical bonds. The observed chemical shift for a given nucleus depends on the orientation of the molecule and of the chemical bond containing the nucleus relative to the magnetic field B o. In a microcrystalline powder, this chemical bond will have a distribution of orientations relative to the external magnetic field, which leads to

144 a distribution of chemical shifts. The resulting specmma will have the shape of a powder pattern (cf. Figure 1) and this phenomenon is called chemical shift anisotro0y (CSA) (13). The CSA is determined by the symmetry of the electronic charge distribution around the NMR nucleus. The CSA interaction increases linearly with the strength of the external magnetic field B o (11). Table 1 NMR Nuclear Spin Interactions Interaction

Behaviour

Liquid phase

Solid state

Range a) (solid state)

Zeeman interaction

Linear with H o, at high Ho: high population difference

Enery level splitting ~o --"~/" !'Io 50 MHz (same effect in both phases)

Dipol-dipol interaction

depending on ~/ and distance (1/r3), independent on H o

averaged to zero

dominant, short T 2, broad lines

~- 15 kHz

Chemical shift anisotropy (CSA)

linear with H o ("Chemical diagnostic")

isotropic values

anisotropic values

to ~ kHz

Spin-spin coupling (scalar coupling)

independent on Ho

small values

yes, but not dominant

~- 200 Hz

Quadrupolar interaction (I > 1/2)

independent on H o, depending on (3 cos 2 0 - 1)

averaged to zero

dominant 1 - 15 MHz

In addition to those NMR nuclear spin interactions we have to deal with long T1 relaxation times due to the lack of translations and rotations in the solid state. The T1 relaxation times usually control the repetition times of NMR experiments. a) from reference 17.

c) Quadrupolar Interactions Some of the interested nuclei considered in connection with zeolite science, like 27A1 or 170, possess a quadrupolar moment Q (besides the magnetic moment), that means nuclei with a spin quantum number I larger or equal to one. The quadrupolar moment is a result of the non-spherical distribution of nuclear charge (13). Nuclei with a quadrupolar moment interact with the non-spherical, symmetrical electric field gradient. This quadrupolar

145

interaction is determined by the charge distribution of the surrounding electrons and other nuclei and can range up to several MHz, completely dominating the entire spectrum. The quadrupolar powder pattern are mainly affected by the so-called second order quadrupolar interactions which decrease with increasing magnetic field strength (11).

co

l

He

A

b/L I

I

I

1

1

1

Fig. 1. Schematic representation of the 13C NMR absorption of a carbonyl functionality: a) Single crystal with two different orientations b) Polycrystalline sample (contributions from the random distribution of orientations, chemical shift anisotropy, CSA) c) In solution (random motion of the molecules yields the isotropic average chemical shift) (Reproduced by permission of The Royal Society, London).

Experimental techniques to achieve high-resolution solid-state NMR spectra NMR spectra of solids with the quality of high resolution can be achieved when the above mentioned line broadening phenomena are removed or at least considerably reduced. Only in these cases spectra can be obtained which enable, for example, crystaUographically nonequivalent sites of a zeolitic material framework to be resolved as individual resonance lines. High resolution NMR spectra of solids can be received by use of one or more experimental techniques described in the following:

a) Dipolar Decoupling (DD) Heteronuclear dipolar decoupling averages to zero the interactions of the abundant nuclei, like 1H, with the rare spins under observation, as, for example 29Si. Dipolar decoupling is obtained by irradiating the Larrnor frequency of the abundant spin causing the dipolar broadening, while observing the less abundant nucleus to be investigated.During this decoupling, a fast spin flipping of the decoupled nuclei is created, resulting in no influence of the abundant spins on the rare spins.

146

b) Multiple Pulse Sequences (MPS) Homonuclear dipolar decoupling can be achieved by using so-called multiple pulse sequences (MPS). These are carefully tailored short and intense pulse cycles averaging the homonuclear dipolar interactions by reorientation of the nuclear spins.

c) Magic Angle Spinning (MAS) Both, line broadenings caused by dipolar and first order quadrupolar interactions can be removed by magic angle spinning (MAS), discovered by Andrew et al. (18) and Lowe (19). In addition, the anisotropy of the chemical shifts can be reduced to their isotropic values by applying MAS. During magic angle spinning, the sample is rotated quickly about an angle of e = 54044 ' in relation to the axis of the external magnetic field Bo (see Figure 2). All three phenomena (dipolar and first order quadrupolar interactions, CSA) have a dependence on the second-order Legendre polynomial: 3 cos2 e - 1. That means, if e is chosen to be 54044 ' (the so-called magic angle), the expression 3 cos 2 | - 1 becomes equal to zero. In consequence, the nuclear spin interactions discussed are reduced to their values in solution: the dipolar and first order quadrupolar interactions are removed, whereas the chemical shift anisotropy is reduced to their isotropic values. In some way, MAS operates as a substitute for the molecular motion in solids. MAS cannot completely average out the second order quadrupolar interaction, but the resonances are narrowed by a factor of 4 (13).

~ Ho,Z

%,.

~ , ~ / ?\..-- .\.-

I

Fig. 2. Effect of magic angle spinning: By rotation about the magic angle the time averaged value of all binding vectors becomes 54044 ' (Reproduced by permission of Bruker GmbH, Karlsruhe).

d) Dynamic Angle Spinning (DAS) /Double Orientation Rotation (DOR) Quadrupolar nuclei interact not only with the magnetic field in which the sample is placed but also with the electric field gradient. The combination of both effects results in an anisotropy that can not longer be removed by the magic-angle spinning alone (20). A detailed analysis of the averaging process of quadrupolar nuclei shows that second-order quadrupolar interactions depend on a fourth-order Legendre polynomial, described by

147 35 cos 4 0 - 30 cos 2 0 + 3 (20). By introducing only one magic angle, it is never possible to average dipolar interactions, CSA and second-order quadrupolar interactions simultaneously. Pines, Lippmaa and Samoson realized that introduction of two independant angles should average the effects of both tensors (21-23). It should be mentioned that the two magic angles are not unique. There is more than one "magic-angle pair" which can reduce the two terms to zero. In principle, there were two different ways to meet this challenge. One, which is called dynamic angle spinning (DAS), is to rotate the sample sequentially about two different axes, inclined to the magnetic field at angles of 37.38 ~ and 79.19~ the sample is rotated about the first axis, reoriented, rotated about the second axis and so on. The other possibility is double orientation rotation (DOR), in which the sample is spun simultaneously about two axes, the first inclined to the magnetic field at the magic angle and the second at the angle given by zero of the fourth-order Legendre polynomial, that means 30.56 ~ (20) (see Figure 3). To perform DOR in practice is a demanding task from an engineering point of view. The spinning rotors are driven by a flow of gas, which must be delivered to one rotor embedded in another (20). Double-angle rotation will certainly be a major new technique in structure elucidation of zeolitic materials containing nuclei with quadrupolar moments. Figure 4 shows a 27A1 DOR spectrum of hydrated VPI-5 compared to the 27A1 MAS NMR spectrum, telling us that the broad single line pattern corresponding to tetrahedreally coordinated A1 in the MAS spectrum contains much more structural information after recording the DOR spectrum (24).

magneticI field (a) DOR - ~ b r

(b) DAS

~o ~'o 3'o 2'0 lb ~ -1'o-~o 30-;o

Chemicalshift(p.p.m.)

Fig. 4. 27A1 NMR spectra of hydrated VPI-5 Fig. 3. Schematic representation of DOR under conditions of (a) MAS and (b) DOR and DAS NMR (Reproduced by permission (Reproduced by permission of Macmillan of Marcel Dekker, Inc., New York). Magazines Ltd., London).

e) Zero-field NMR Spectroscopy Pines has developed a method called zero-field NMR, in which the sample is placed in a large magnetic field for spin polarization and is then shuttled to a region of low or zero field, where certain field-dependent broadenings disappear (25, 26). Application of zero-field NMR allows the measurement of dipolar couplings, from which distances can be determined.

148 However, zero-field experiments suffer from low sensitivity and can only be applied to materials with reasonably long relaxation times T 1 to allow for the time it makes to shuttle the sample in and out of the field. This field cycling technique measures the evolution of the magnetization at zero-field with the sensitivity of high-field NMR. Zero-field NMR is particularly suited to the investigation of nuclei with quadrupolar moments, since it avoids orientation-dependent broadening.

f) Quadrupole Nutation NMR Spectroscopy Quadrupole nutation NMR is a technique, introduced by Samoson and Lippmaa (27), where in a two-dimensional way the effect of the quadrupole interaction is separated from other line broadening interactions, simply by allowing the magnetization to evolve during an incremented periode tl, under the influence of a strong radio frequency field. As an example, quadrupole nutation NMR of nuclei with half-integer quadrupolar spin in zeolitic materials can distinguish between nuclei of the same chemical element subjected to different quadrupole interactions, the signals of which overlap in conventional spectra. The situation is favourable for half-integer quadrupolar spins since the m=l/2 <-> m= -1/2 transition for these nuclei is broadened by the quadrupole interaction only in second-order perturbation theory. The technique can be usefully applied for the determination of the local environment of A1 in zeolitic catalysts (28). It allows discrimination between species of similar chemical shift but different quadrupolar coupling constants (see Figure 5). The main difficulty in the interpretation is the complex spectrum that results from a nutation experiment since it can consist of many overlapping powder patterns (29). (70 kHz, 60 ppm)

(180 kHz, 0

_ r

4(~0

;

,,

F1 (kHz)

' -400

F2 (ppm)

Fig. 5. 27A1 quadrupole nutation NMR spectrum of 80% NH4-exchanged Y zeolite (Si/AI=2.5). Numbers in parentheses have units of kHz (for F 1) and ppm (for F2), respectively (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam).

149

g) Cross Polarization (CP) The spin-lattice relaxation times T 1 in solids can be very long and the recording time for a simple NMR spectrum of a solid-could be extremely long without any experimental improvement. On the other hand, the transversal relaxation times T 2 are usually short. Since T 1 controls the recycle time between the experiments a method improving the sensitivity (i.e. the signal-to-noise ratio) of the spectra of rare spins (like silicon-29 or carbon-13) has been developed: Cross polarization (CP), which does, however, not influence the resolution of the spectrum. Cross polarization allows transfer of magnetization (or polarization) from an abundant species (usually 1H) to a dilute species, which is under observation. The cross polarization experiment can be described through the following steps: Excitation of the abundant spins by a 90 ~ pulse. Magnetization transfer from the abundant spins to the dilute spins by simultaneously applying "matching" radio frequency fields to both type of spins (spin locking), according to the Hartmann-Hahn condition: ~/1 x B 1 = "t2 x B 2 ( ~/ = magnetogyric ratio). During this period both the abundant and dilute spins are in states with equal fluctuation frequency of the z-magnetization, whereby mutual spin flips of abundant and rare spins are possible. The net magnetization is rapidly transferred from the abundant to the rare spins. 3. Acquisition of the free induction decay of the rare spins by continued irradiation of the 1H field for heteronuclear dipolar decoupling. 4. Repetition of the cycle (see Figure 6). 1. 2.

The benefits are primarily an intensity enhancement of the dilute spin signal by a factor of ~/ abundant/'/ dilute and a reduction of the recycle time between experiments since the ratedetermining relaxation time is now that of the abundant species, rather than that of the rare spins. Usually, the relaxation of the abundant spins are much faster than the dilute spin relaxation (13). The cross polarization experiment may thus be repeated with much shorter intervals, leading to a further increase of the signal-to-noise ratio of the rare spin NMR spectrum within a given period of time. The effectiveness of the cross polarization experiment depends on the strength of the dipolar interaction between the abundant and rare spin systems, i.e. on the distance between the actual nuclei (proportional to r -3, where r is the distance between the abundant and the dilute nuclei) (11). The efficiency of magnetization transfer decreases extremely fast as the distance between the abundant and rare spins increase. One should emphasis, that under normal conditions, the CP experiment does not provide quantitative results. Finally, the cross polarization sequence does not influence the line width.

Combination of techniques to obtain high.resolution solid-state NMR spectra In order to obtain optimum line narrowing and improved sensitivity in a solid-state NMR spectrum of a zeolitic material, the experimental techniques discussed in this chapter may be applied in combination, as, e.g., CP/MAS, DD/MAS, CP/DOR or CRAMPS (Combined rotation and multiple pulse spectroscopy). The rotor synchronization technique provides

150 synchronization of radio frequency pulses to rotor positions for experiments like, for example, CP/MAS, CRAMPS or dynamically adjusted TOSS.

a)

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Fig. 6. a) Pulse sequence used for 1H - 13C cross polarization and b) behaviour of the 1H and 13C spin magnetization during the cross polarization sequence (Reproduced by permission of C.F.C. Press, Guelph). Relevant nuclei for high-resolution solid-state NMR spectroscopy All of the relevant basic nuclei contributing to the framework of zeolitic materials are detectable to NMR investigations by its naturally isotopes: 29Si, 27A1, 31p and 170. Both, the 31p (1=1/2) and 27A1 isotopes are 100% abundant and spectra are easily detected within reasonable time. However, 27A1 has a quadrupole moment which can cause line broadening due to the interaction with the electric field gradient. The 170 isotop has a low natural abundance (0.037%) besides a nuclear quadrupole moment (line broadening!), which in combination makes the registration of 170 NMR spectra without enrichment almost impossible. Interesting investigations of 170 NMR can be done by using enriched zeohtic material.

The 29Si isotope has a natural abundance of 4.7% and no quadrupole moment (1=1/2). The obtained resonance lines for 31p and 29Si are usually narrow, and due to their important role as framework elements (besides 27A1), these nuclei have been widely used in solid-state NMR studies of zeolitic materials for structural investigations.

151 Solid-state 1H NMR of protons, OH groups, adsorbed H20, organic sorbates and probe molecules containing hydrogen in zeolitic materials has been developed as a capable method for getting information about different kinds of hydrogen in terminal or bridging OH groups, varying environments for hydrogen containing probe molecules and acidity investigations. Other nuclei which can substitute isomorphously the usual framework elements in zeolitic materials are observable by solid-state NMR, e.g. liB, 9Be, 73Ge and 69,71Ga. Charge compensating cations, like 7Li, 23Na, 39K, 1-33Cs or 195pt, are suitable for NMR experiments. However, most of those elements possess a quadrupole moment, which limits usually the application. Furthermore, organic compounds used as templates during hydrothermal synthesis or as sorbates in the zeolite framework can be detected by applying 13C CP/MAS NMR spectroscopy. Finally, 129Xe is a very suitable and sensitive isotope for probing the pore architecture of zeolitic materials. The extended Xe electron cloud is easily deformable due to interactions between, e.g. the Xe atoms and the channel wall of a zeolitic framework, and deformation results in a large low-field shift of the Xe resonance. In addition, 129Xe NMR can be used to study metal particles in zeolites, while reduction-oxidation reactions can be monitored (13). Table 2 summarizes the NMR properties of a number of nuclei which have been used in NMR investigations of zeolitic materials (11). Table 2 Properties of NMR active nuclei related to zeolitic materials NMR active nucleus

Spin quantum number

NMR frequency (MHz) a

Natural abundance

Absolute sensitivityb

(%) 1H 7Li 11B 13C 170 23Na 27A1 29Si 31p 51V 69Ga 71Ga 129Xe 133Cs

1/2 3/2 3/2 1/2 5/2 3/2 5/2 1/2 1/2 7/2 3/2 3/2 1/2 7/2

400 155.45 128.34 100.58 54.23 105.81 104.23 79.46 161.92 105.15 96.01 121.98 110.64 52.47

a) at H o = 9.395 Tesla. b) product of relative sensitivity and natural abundance.

99.98 92.58 80.42 1.10 0.04 100 100 4.7 100 99.8 60.4 39.6 26.44 100

1.00 0.27 0.13 1.76-10 -4 1.08-10 -5 9.25- 10-2 0.21 3.69-10 -4 6.63-10 -2 0.38 4.17- 10-2 5.62.10 -2 5.60- 10-3 4.74-10 -2

152

Two-dimensional (2D) solid-state NMR spectroscopy High-resolution solid-state NMR spectroscopy has nowadays become a standard technique in many laboratories, whereas two-dimensional solid-state NMR has not yet reached that status. In liquid-state NMR, the use of two-dimensional techniques provides a lot of information on the connectivities between atoms within molecular structures. Although marV restrictions apply, 2D NMR techniques allow to get increased information in the solid state as well, like Si-O-Si connectivities in zeolite frameworks (30, 31). Fyfe et al. have done pioneering work in connection with the introduction of 2D solidstate NMR spectroscopy with respect to three dimensional connectivities within zeolite lattices (16 and references cited therein). Application of 2D homonuclear COSY (both frequency axes represent 29Si chemical shifts) enabled Fyfe et al. to yield the correct connectivities for the (known) lattice structure of ZSM-39 (16), see Figure 7. As seen in the Figure, a clear connectivity is established between T1 and T 2 as well as for T 2 and T 3, but not between T 1 and T 3. This finding is in line with the known structure of ZSM-39 (space group of the high-temperature form is Fd3). In this study, the used sample of ZSM-39 was enriched to about 85% in 29Si in order to increase the signal-to-noise ratio and enhance the connectivities that result from the 29Si-O-29Si interactions. Fyfe et al. showed that naturalabundance experiments on these systems are indeed feasible, making the technique quite generally applicable (16). Exactly the same results were obtained with spin diffusion experiments performed at ZSM-39. Frequencies can be affected by spin diffusion between sites having different NMR parameters, when, for example, magnetization is transported through a solid by means of mutual spin "flip-flops" that can occur even in the absence of atomic or molecular motion. By monitoring the correlation among frequencies in the different dimensions of a multidimensional NMR experiment, it is possible to learn about the mechanisms and rates of reorientation and diffusion processes in solids (32). The three-dimensional connectivities in a number of zeolitic materials have been investigated by Fyfe and coworkers using the 2D COSY and 2D INADEQUATE (Incredible Natural Abundance Double Quantum Transfer Experiment) sequences (16, 31). In the 2D INADEQUATE, only double quantum coherence signals are observed, that means only signals created by coup 1ed 29S1 " nuclei". The F 2 dommn " is " the normal chermcal shift frequency scale and the F 1 domain represents the double-quantum frequencies of the resonances. Connected signals occur equally on either side of the diagonal of the plot at the same frequencies in F 2 as the corresponding resonances from a simple 1D experiment (16). The COSY experiment is quite easy to carry out, since it does not require detailed knowledge of the coupling constants. However, this experiment was originally designed to indicate homonuclear scalar couplings between abundant nuclei, and its application to the case of dilute nuclei, like 29Si, creates the following difficulty: Most of the 29Si nuclei doesn't show homonuclear scalar coupling, which results in the registration of intense broad signals on the diagonal, hiding cross peaks due to connectivities between spins of small chemical shift differences, like the coupling between T 4 and T 6 in the 2D COSY of ZSM-12 (see Figure 8). Zeolite ZSM-12 is a three-dimensional framework structure in which the asymmetric unit consists of seven crystallographically inequivalent T sites (16, 31).

153 Compared to the 2D COSY experiment, the 2D INADEQUATE experiment has a number of advantages: Those signals due to isolated spins are suppressed (diagonal peaks), which improves the required dynamic range for the connectivities and the detectibility of satellite

Fig. 7. Results of a 29Si COSY experiment on ZSM-39 carried out at 373 K: A) contour plot, B) stacked plot (Reproduced by permission of the Royal Society of Chemistry, Cambridge).

154

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~,o

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ppm

ppm

Fig. 8. Schematic representation of the zeolite ZSM-12 lattice framework and the contour plot of a 29Si COSY experiment on ZSM-12 (Reproduced by permission of the American Chemical Society, Washington).

155 signals. In addition, a better S/N ratio may be reached and any spinning rates may be used, since spinning side bands will not appear in the spectrum. Therefore, it is much easier to observe connectivities between resonances close in frequency which occur close to the large diagonal signals in the COSY experiment (16). Figure 9 shows the 21) INADEQUATE experiment on ZSM-12, including the connectivities between T 4 and T 6, which is clearly resolved here but was ambiguous in the 2D COSY experiment (see Figure 8) due to the close proximity of the cross-peaks to the diagonal (16). However, the main drawback of the INADEQUATE experiment is that a reasonable good estimate of the coupling constant must be available, otherwise the experiment will be very inefficient (31). The direct measurement of the coupling constants in solids is normally not possible due to the large linewidths. However, the 2D spectra recorded by Fyfe et al. allow the measurement of the 29Si-O-29Si scalar couplings in zeolite ZSM-12, ranging from 10-15 Hz (16). In the cases of ZSM-12 and ZSM-22, the results of the natural abundance 29Si/29Si COSY and INADEQUATE 2D NMR experiments are in exact agreement with the lattice structures, the INADEQUATE experiment being particularly successful by detecting all of the connectivities (16).

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ppm

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Fig. 9. Contour plot of a 29Si INADEQUATE experiment on ZSM-12 at 300 K (Reproduced by permission of the American Chemical Society, Washington).

156 One of the most complex zeolite structures known, ZSM-5, has been investigated using 2D solid-state NMR techniques in order to study the three-dimensional bonding connectivities (16). The ZSM-5 structure is difficult to investigate by 2D NMR connectivity experiments since the known structures contain either 24 T sites (monoclinic room temperature form) or 12 T sites (orthorhombic form induced by increasing the temperature or by adding two molecules of p-xylene per unit cell). Therefore, a very large number of 29SiO-29Si connectivities will occur within the 2D plots, and a substantial number of connectivities between signals of similar resonance frequencies will be obscured in the COSY experiments (16). The 2D INADEQUATE experiments on ZSM-5 gave substantially better results than 2D COSY experiments on the same highly siliceous samples. In the case of the orthorhombic form (12 T sites), almost all of the expected connectivities were observed, whereas for the monoclinic form (24 T sites), 38 of the total of 48 connectivities were clearly registrated, allowing the assignment of the spectrum (16). So far, the 2D INADEQUATE experiment turns out to be the method of choice with respect to investigations of three-dimensional bonding connectivities within zeolitic material lattices. This approach is of general interest, since the experiments can be carried out on 29Si natural abundant zeolitic material samples.

Spin coherence transfer from quadrupolar nuclei Interesting solid-state 2D NMR polarization transfer experiments involving quadrupolar nuclei have been reported recently for VPI-5 (16, 33-36). As pointed out earlier, the crosspolarization experiments have been used to transfer magnetization from abundant to rare spin systems. Fyfe et al. (16, 33-35) and Veeman and coworkers (36) introduced crosspolarization experiments involving spin coherence transfer from quadrupolar spin systems to spin-1/2 nuclei, utilizing the very short T1 relaxation times of quadrupolar nuclei to detect spectra of spin-l/2 systems within a reasonable time schedule. In this way, 27A1-O-31p connectivities in aluminophosphates can be followed by NMR. Both 2D heteronuclear correlation experiments using cross-polarization (where the 27A1 polarization is transferred to the 31p spins during spin-locking and the 31p free induction decay is recorded) and 2D TEDOR (Transferred-echo double resonance) experiments have been registered for VPI-5 (33-35) (see Figure 10). Both 2D spectra reveal cross peaks between all three 31p resonances and both the resonances from the tetrahedrally and octahedrally coordinated 27A1 sites are in agreement with the crystal structure of VPI-5. The examples demonstrate the usefulness of TEDOR experiments to observe spin-l/2 nuclei with long T 1 relaxation times which are close to quadrupolar spins, like in zeolitic materials. These experiments (besides TEDOR another dipolar dephasing double resonance experiment was used: REDOR (Rotational echo double resonance)) allow to determine connectivities between quadrupolar and spin1/2 nuclei in zeolitic materials (35). REDOR uses rotational echos from MAS spectra and measures directly dipolar couplings between nuclei, thus determining internuclear distances. In rotational resonance, a multiple of the spinning speed is matched to the chemical shift difference between two nuclei to produce information about the dipolar coupling between the two spins. This technique seems to become the solid-state equivalent of the NOESY

157 (Nuclear Overhauser Enhancement and Exchange Spectroscopy) experiment, since it provides through space distance information (26).

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qt

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Fig. 10. 2D 27A1 to 3lp TEDOR experiment on VPI-5, displaying connectivities between the three 31p resonances and the two resolved 27A1 resonances (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam). 0

3.1

RECENT HIGHLIGHTS ABOUT THE FRAMEWORK CHARACTERIZATION

Zeolites, AIPO4 and SAPO Molecular Sieves

The interested reader will find a number of excellent review papers published during the last 15 years, summarizing the structural information and relationships available through the 29Si NMR data on zeolites and SAPO molecular sieves (1-16). One of the most important results of Lippmaa, Engelhardt and coworkers in the early 1980ies was to establish the relationship between the 29Si chemical shift sensitivity and the degree of condensation of the silicon-oxygen tetrahedra, i.e. the number and type of tetrahedrally coordinated atoms (T-atoms, with T= Si, A1 or other lattice atoms) connected to a given SiO 4 unit. The degree of condensation is symbolized by Si (n A1), with n=0, 1, 2, 3

158 or 4, where n indicates the number of AI atoms sharing oxygens with the SiO4 tetrahedron under consideration. Furthermore, the 29Si chemical shift is influenced by the Si-O-T bond angle and silicon-oxygen bond length, which means, that chemically equivalent but crystaUographically inequivalent Si nuclei may have different chemical shifts (2). Chemical shift ranges for the different SiO4 units are given in Figure 11, and an example of the 29Si MAS NMR spectrum of Y zeolite (Si/AI=2.6) is shown in Figure 12. Generally, the 29Si resonance shifts to lower field by ca. 5 ppm per additional aluminium. Therefore, up to five lines per crystaUographically inequivalent Si site may be observed in the 29Si NMR spectrum. In conclusion, quite a bit of information about the local environment of the SiO4 tetrahedra forming the zeolite lattice can be obtained from the 29Si chemical shift data (11). - -95

A! AI AI O O O AIOSiOAI AIOSiOSi AIOSiOSi O O O AI AI Si

AI O SiOSiOSi O Si

Si O SiOSiOSi O Si

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Fig. 11. The five possible local environments Fig. 12. 29Si M.AS NMR spectrum of of a Si atom in a zeolite framework together Na Y zeolite (Si/A1 ratio=2.6). with the corresponding chemical shift ranges (Reproduced by permission of C.A. Fyfe, Vancouver). One of the most famous examples is the 29Si MAS NMR specmn'a of highly siliceous ZSM-5 (Figure 13), which shows at 295 K 20 well resolved lines (linewidths as narrow as 5 Hz), representing the 24 crystallographically distinct Si sites of the monoclinic form of ZSM5 (37).

159

!

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....I

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o

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silicalite

silicalit

,,.~

-~06

i

'~08

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-~6

i

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_

ppm from TMS Fig. 14. 29Si MAS NMR spectra of silicalite at 191 K, 293 K and 403 K (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam).

160 XRD shows that silicalite (highly siliceous ZSM-5) can exist in the monoclinic and the orthorhombic forms. 29Si MAS NMR detects much more subtle changes in the structure of silicalite which do not necessarily involve symmetry changes detectable by XRD. NMR spectra measured in the range of 191-403 K are sensitive even to small temperature changes and reveal that structural transformations continue over the entire temperature range (see Figure 14). By contrast, the changes in the XRD pattern are slight. This demonstrates the remarkable sensitivity of which NMR can monitor the local environment of atoms. The hightemperature spectrum shown in Figure 14 indicates that above 363 K silicalite contains only 12 nonequvalent sites for silicon in the orthorhombic form. It is interesting to note that the addition of very small amounts of adsorbed organics to dehydrated silicalite induces similar phase transitions and similar changes in the 29Si MAS NMR spectra (15). Finally, the quantitative ratio of tetrahedral Si and A1 in the framework of a zeolite can be directly calculated from the signal intensities (I) according to the following equation:

4 Si _ n = O AI4

[si(n AI) (1)

0.25

nls~c.~)

n=O This method has been used successfully in connection with investigations of dealuminated (ultrastabilized) and realuminated zeolites. One should emphasize, that NMR yields the framework Si/A! ratio since only lattice Si and AI are detected, whereas elemental analysis provides the total sample composition. In addition, equation 1 is independent of the specific structure of the zeolite, but cannot be directly applied to spectra containing overlapping signals from Si (n A1) units of crystallographicaUy non-equivalent Si sites. OH groups (or coordinated water molecules) connected to the outer or inner surface of the porous structure of a zeolite or in lattice defects can be detected by the 1H - 29Si crosspolarization technique. Strong intensity enhancements are observed in the CP spectra for the resonances of (TO)3SiOH or (TO)2Si(OH)2 but not of Si(OT) 4 environments (2). The differences in the 29Si chemical shifts of Si nuclei located in the same chemical environment are mainly due to changes in the bonding geometry of the corresponding Si (n A1) unit. It has been shown, that the chemical shift of a certain Si (n A1) unit is linearly correlated with the average of the four Si-O-T bond angles at the central Si atom and also with the average Si-O bond length. By means of linear regression analysis quantitative relationships can be established and used to estimate mean Si-O-T bond angles from 29Si chemical shifts in zeolites, since a chemical shift change of about 0.6 ppm is to be expected for 1~ change in bond angle, with high-field shifts for larger angles (2). Usually there are two factors in the solid-state 29Si NMR spectra of ze.olites contributing to line broadening: chemical shift anisotropy of the 29Si atoms and heteronuclear dipolar

161 interactions between 29Si and 27A1 or other NMR active nuclei (8, 11). The application of the MAS technique is therefore essential, and usually sufficient, for recording highly resolved 29Si NMR spectra of zeolites (11). Removal of A1 from the lattice and replacing it with Si reduces the line width to about 1 ppm, indicating that A1 is responsible for the line broadening (9). A final example of zeolite framework characterization by using 29Si J-scaled COSY NMR Spectroscopy should be highlighted. This technique scales up the scalar splittings between the cross-peak components, thereby enhancing cross-peak intensities and consequently improving spectral resolution between adjacent diagonal and cross-peaks. The conventional 29Si MAS NMR spectrum of natural abundant, highly siliceous mordenite consists of three signals in the intensity ratio of 2 : 1 : 3 (Figure 15). This may be explained from the known structure of mordenite which contains four distinct crystallographic nonequivalent T-sites in the intensity ratio T1 : T2 : T3 : T4 = 2 : 2 : 1 : 1, with two of the peaks overlapping. T2+T4 T1

i

-110

(b)

I

-112

I

/

-114 -116 ppm from TMS

/

-118

. . . .

F~

F2 Fig. 15. 29Si NMR spectra of highly siliceous mordenite, a) MAS NMR spectrum, b) 2D Jscaled COSY spectrum. Structure of mordenite viewed along the [001] plane (right). The four types of crystallographic sites are indicated (without given relative populations) (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam).

162 Assignment of the peaks on the basis of the correlation between 29Si chemical shifts and the mean Si-O-Si bond angle is not possible in this case. The 2D J-scaled COSY spectrum of highly siliceous mordenite reveals three cross-peaks. On the basis of the known connectivities of the mordenite structure, only two cross-peaks are predicted for the T1 : T4 : T2 + T3 assignment, while the T1 : T3 : T2 + T4 assignment implies that three cross-peaks should be observed. Thus the detection of three cross-peaks in the 2D J-scaled COSY experiment shows that the correct interpretation is T1 9T3 9T2 + T4. Such unambiguous assignment of the spectrum is not possible by one-dimensional NMR or by conventional COSY (15). The main applications of 27AI NMR spectroscopy in connection with zeolitic materials has been (among others) the monitoring of dealumination processes, structure elucidation (for A1PO4 and SAPO molecular sieves) and the detection of extra-framework aluminium. Extra-lattice aluminium is octahedrally coordinated and gives rise to resonance lines at about 0 ppm for zeolites and between -7 and -23 ppm for A1PO4 and SAPO molecular sieves, whereas framework aluminium is tetrahedrally coordinated and resonates between ca. 50 and 65 ppm for zeolites and 29 to 46 ppm for A1PO4 and SAPO molecular sieves. As mentioned before, 27A1 has a nuclear quadrupole moment 0=5/2) which interacts with the electric field gradient caused by the non-spherically symmetric charge distribution around the 27A1 nucleus. Both, line broadening/distortion and chemical shift changes may arise from those quadrupolar interactions resulting in more difficult interpretations of the 27A1 NMR spectra. On the other hand, the high sensitivity of the 27A1 isotope (natural abundance of 100% and generally short relaxation times) allows the determination of very small amounts of aluminium in the samples. Usually, the only aluminium transition recorded in zeolitic material samples is the central +1/2 < - > - 1 / 2 transition, which is only depending on the second order quadrupolar interaction. This interaction decreases with increasing magnetic field strength B o and can partly be reduced by MAS. However, complete removal of the second order quadrupolar interaction can be achieved by applying either DOR or DAS (see chapter 2.). Obeying Loewenstein's rule (forbidden A1-O-A1 linkages), zeolitic materials give rise to quite simple 27A1 NMR spectra, consisting of signals due to only one type of tetrahedral aluminium environment [A1 (OSi)4] besides evtl. octahedrally coordinated aluminium. No relationships between the chemical shifts and the Si, A1 ordering or Si/A1 ratio have been established, whereas relations between the 27A1 chemical shifts and mean Si-O-A1 bond angles exist, with shift values carefully corrected for quadrupolar shift contributions (11). The quantitative use of 27A1 NMR data allows determination of the relative proportions of lattice and extra-lattice A1 in zeolitic samples, providing that all aluminium is detected in the spectra (no signal intensity loss due to quadrupolar interactions, no "NMR invisible Ar'). The distinction between A1 species having the same chemical shifts and strongly overlapping lines but different quadrupolar couplings can be made via the two-dimensional quadrupole nutation NMR technique (13) (see chapter 2.)

163 No doubt, 27A1 MAS NMR is a valuable tool in probing the coordination, location and quantity of AI in zeolitic materials, but less useful than 29Si MAS NMR in detailed studies of the lattice structure (2). However, introduction of DAS and DOR allows much more detailed insight with respect to structural information available by use of 27A1 NMR spectroscopy. Since a number of excellent review papers exist, which summarize the structural information obtained by application of 27A1 NMR on zeolites, I would like to draw the readers attention for further details again to those reviews (1-16) and highlight only few results published recently dealing with framework characterization of zeolites or A1PO4 molecular sieves by using 27A1 NMR spectroscopy.

(a)

(c)

VPI-5

t,

60

I

40

,

!

i

,

,I

20 0 '20 ppm from AI(H20)~

I

-40

Fig. 16. 27A1 NMR spectra of dehydrated and partially rehydrated VPI-5. a) MAS spectrum of dehydrated VPI-5. b) DOR spectrum of dehydrated VPI-5. c) DOR spectrum after 2 days of rehydration, d) DOR spectrum after 23 days of rehydration (24). Structure of VPI-5 (right) (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam). VPI-5 has received great attention during the last years mainly due to the unusual structure and properties of this material with extra-large pores consisting of 18-membered tings. Wu et al. showed that 27A1DOR is capable of resolving discrete framework

164 aluminium sites in VPI-5, permitting quantitative investigation of site-specific adsorbate interactions (typically water interactions) with the framework. Figure 16 shows the 27A1 MAS NMR spectrum of the dehydrated VPI-5 and the DOR spectrum of the same sample (24). Two peaks unresolved in the MAS spectrum, at 33.3 ppm and at 35.9 ppm, are observed in the DOR spectrum. From the 1 : 2 intensity ratio of these two signals the highfield peak is assigned to A1 sites in the double four ring, whereas the low-field signal is assigned to A1 sites in the six-membered tings. During dehydration, 6-coordinated A1 sites are converted to 4-coordinated sites, consistent with the disappearance of the peak at -18.4 ppm. Furthermore, in hydrated VPI-5, 27A1 tetrahedral sites are altered by dehydration to yield different 27A1 tetrahedral environments (15). 31p NMR spectroscopy is considered as the most sensitive technique performing information on the local structure and structural modifications involving the tetrahedrally coordinated framework elements phosphorus and aluminium in A1PO4 molecular sieves. The 31p MAS NMR spectrum of VPI-5 is quite unusual and has been an item of discussion. The structure of VPI-5 consists of two crystallographically unique phosphorus sites, which are located in the 6-membered rings and in the atomic positions that belong to the two adjacent 4-membered tings and are in the atomic ratio of 2:1, respectively (see Figure 16). The 31p MAS NMR spectrum of dehydrated VPI-5 indeed consists of two lines at about-26/-27 and-31/-32 ppm in an area ratio that closely approximates 2:1 (1). However, the 31p MAS NMR spectrum of hydrated or as-synthesized VPI-5 reveals three lines of equal intensity at about -23, -27 and -33 ppm (1). McCusker et al. investigated the structure of hydrated VPI-5 by synchrotron X-ray, and concluded that two water molecules complete an octahedral coordination sphere around the framework aluminium atom between the fused 4-membered rings (38). Furthermore, all the water molecules are located in the 18- ring channels of VPI-5 forming a triple helix structure. According to a large number of investigations, the two low-field signals at-23 and-27 ppm are finally assigned to the P atoms in the 6-membered tings and the remaining high-field signal at -33 ppm represents the P-positions in the double-four tings (1). This assignment has been confirmed by Klinowski using 2D 31p NMR spin-diffusion spectra of hydrated VPI-5 (15), where only 31p_31p dipolar interactions govern the 31p spin diffusion process (see Figure 17). The stability of VPI-5 and the transformation to A1PO4-8, a 14-membered ring A1PO4 molecular sieve, have been extensively studied by XRD and solid-state NMR spectroscopy. Depending on the quality of the sample and the treatment conditions (evacuation, extremely slow heating rate, careful removal of water etc.), the structrue of VPI-5 can be preserved up to at least 400 ~ C, as followed by 31p MAS NMR (1). Under sealed conditions and slightly elevated temperatures (70 to 150 ~ the hydrated VPI-5 undergoes a reversible dehydration/rehydration effect, which results in a merging of the two low-field signals in the 31p MAS NMR spectrum (1) and splitting again after cooling to room temperature. Under more drastic conditions (higher temperatures, faster heating rates and unsealed conditions), VPI-5 transforms to A1PO4-8, monitored by recording of a complete different

165 31p MAS NMR spectrum (see Figure 18). The 31p MAS NMR spectrum of hydrated A1PO4-8 shows three signals at -21, -25 and -30 ppm, with an intensity ration of 1:2:6, which is not incompatible with the five independent crystallographic sites of A1PO4-8, if three of the sites are equivalent with respect to the NMR chemical shift (1).

Fig. 17.2D 31p NMR spin diffusion spectrum of hydrated VPI-5 (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam).

3.2

Mesoporous Materials

A new family of silicate/alurninosilicate mesoporous molecular sieves designated as M41S and kanernite were introduced a few years ago (39-43), and NMR investigations have been done to characterize those materials. MCM-41, one member of the M41S family (see Figure 19), exhibits a hexagonal arrangement of uniform mesopores whose dimensions may be engineered in the range of about 15 to greater than 100 A (39). The 29Si MAS NMR spectra of MCM-41 closely resemble those of amorphous silica, that means the spectra can be separated into three very broad signals at -89/-92 (assigned to Si (2OSi)(2OH) = Q2), -98/-102 (assigned to Si(3OSi)(OH) - Q3) and -108/-111 ppm (assigned to Si(4OSi) = Q4), with the two highfield signals dominating the spectrum (39, 44-50) (see Figure 20). MCM-48, consisting of a cubic arrangement of uniform mesopores, has essentially the same 29Si MAS NMR spectrum as amorphous silica or MCM-41, the only difference being the higher Q3/Q4 ratio (48-50)(see Figure 20).

166

-27.5

42.2 a

"22"611-33.3 ~

A

-30.3 33 1

b

40.9

b

2.9 -24.4 / \-14.4t, -29.8

36.9

c

#

-29.6 d

37.2

d

-24.8

~'

:~o''':2'd"36"-4'6 ppm

"-~6 '~-

al-P MAS NMRspectra

~so ~oo

so

o

ppm

-so -~oo

27-AI MAS NMR spectra

Fig. 18.31 p and 27A1 MAS NMR spectra of a) VPI-5 dried at 60 OC/ovemight, b) VPI-5 evacuated at 54 OC/overnight and calcined at 250 oC/ovemight, c) A1PO4-8 (transformed from VPI-5 by calcination at 400 OC/ovemight) and d) hydrated A1PO4-8. Asterisks denotes spinning side bands (Reproduced by permission of Elsevier Science Publishers B.V., Amsterdam).

167

Fig. 19. Structure of MCM-41 (Reproduced by permission of Verlag Chemie, Weinheim). -110 ppm -102 ppm I I

O/oQ4

Q3/Q4

36

61

0.59

51

47

1.1

56

41

1.4

O/oQ3

=

L

I

I

!

-50

-100

-150

Chemical shift (p.p.m.)

Fig. 20. 29Si MAS NMR spectra of pure siliceous, as-synthesized MCM-41, MCM-48 and MCM-50 (Reproduced by permission of Macmillan Magazines Ltd., London).

168 The relative number of incompletely condensed (Q2, Q3) and fully condensed (Q4) silicon atoms in MCM-41, MCM-48 and MCM-50 (which represents a lamellar structur) can be determined by 29Si MAS NMR, indicating the degree of the formed mesostructures. In other words, the Q3/Q4 ratio measures the extent of silanol condensation, which means lower values for more condensed frameworks (40, 48-50) (see Figure 20). 27A1 MAS NMR spectroscopy has been used to follow the formation of tetrahedrally and octahedrally coordinated aluminium during the synthesis of mesoporous materials, and the main focus has been concentrated on MCM-41. Tetrahedrally coordinated aluminium resonates at about 50 ppm, and these Al-sites are regarded as belonging to the lattice of MCM-41 (44-47). However, the detailed "fine" (or microscopic) structure of these mesoporous materials are not yet elucidated, and the common opinion of the nature of the wall material reflects an amorphous character. The absence of octahedrally coordinated aluminium on MCM-41 samples with decreasing Si/A1 ratios (increasing aluminium content) has been investigated by several authors. Davis and coworkers were able to prepare MCM-41 material containing only tetrahedrally coordinated aluminium with a Si/A1 ratio higher or equal to 29 (45), whereas Corma et al. decreased this ratio to 14 (47), even lower to 8.5 by Schmidt et al. (51) and finally as low as 2 by Clearfield et al. (52). The stability of aluminium in the framework of MCM-41 materials was monitored using 27A1 MAS NMR spectroscopy by Klinowski et al., concluding that the structural A1 in MCM-41 is thermally less stable than, for example, in zeolite Y, because the MCM-41 structure lacks strict crystallographic order at the atomic level and the very small protons cannot satisfy the framework charge balance as efficiently as the sodium cations (53). Corma et al. describe the dealumination of an as-synthesized MCM-41 (with aluminium only in the lattice tetrahedral positions) by direct calcination, leading to decreasing BrCnsted acidity but increasing Lewis acidity due to the formation of extra-framework aluminium as followed by 27A1 MAS NMR (54). A range of MCM-41 materials has been synthesized using different sources of aluminium and characterized in detail by (among others) MAS NMR spectroscopy. 27A1 MAS NMR clearly shows that, when Catapal alumina or sodium aluminate is used, virtually all A1 in the solid is six-coordinated. On the other hand, by using aluminium sulfate, MCM-41 can easily be prepared with all aluminium in four-coordination and a framework Si/Al-ratio as low as 10 (55). Syntheses using monomeric alumina precursers have been shown to yield stable MCM-41 samples, with controlled incorporation of aluminium into the framework being readily achieved as monitored by 27A1 MAS and DOR NMR (56). Japanese researchers characterized kanemite, a highly ordered mesoporous material (FSM-16) prepared from layered polysilicates (41-43): the 29Si MAS NMR spectrum showed a sharp signal at-97 ppm due to Q3 units, indicating a single laffered structure. The spectrum of the silicate-organic complex showed two si~nals due to a QOunit (-99 ppm) and a Q4 unit (-109 ppm). The appearance of the strong Q~ unit clearly indicates the formation of a three-dimensional SiO 2 network from the layered kanemite (41, 42, 57). Mesoporous materials derived from kanemite and MCM-41 samples were synthesized and characterized

169 by Davis and coworkers (58). Both preparations yield mesoporous materials with narrowpore-size distributions and somewhat similar physicochemical properties. However, due to a higher degree of condensation in the silicate walls of the material derived from kanemite (as followed by 29Si MAS NMR spectroscopy), these samples have higher thermal and hydrothermal stability than MCM-41 (58). The synthesis mechanism of the formation of MCM-41 material is still a matter of discussion, since large surfactant molecules are used as templates, and their interactions with the different species during synthesis are quite complex. Therefore, certain efforts have been concentrated on following the template interactions by 13C CP/MAS NMR spectroscopy. The signals from the template (cetyltrimethylammonium bromide/hydroxide=CTMA) located inside MCM-41 were sharper and had markedly different intensity distribution and chemical shifts than those from CFMABr, as reported by Corma et al. (47). Considering the linewidths, the authors suppose that aliphatic chains of CrMA cations are more ordered inside the MCM-41 than in CTMABr and this can be explained by the liquid-crystal arrangement of the template inside MCM-41 and by the high water content of the CTMABr powder (47). Beck and coworkers reinforced the hypothesis that M4IS materials are formed through a mechanism in which aggregates of cationic surfactant molecules in combination with anionic silicate species form a supramolecular structure, whereas microporous materials are formed by molecular organic species, as concluded from their 13C NMR spectra (among others) (59). Furthermore, deuterium NMR was applied for studies related to the micelle formation in connection with the preparation of nanocomposite materials, as outlined by Chmelka et al. (60). The authors concluded that the isotropic 2H NMR signal was consistent with the isotropically mobile spherical micelles in dynamic equilibrium with monomeric surfactant molecules in solution (60). 14N NMR spectra of M41S gels containing CTMA surfactant molecules were recorded by Davis et al. (46) and Anderson et al. (61) in order to collect further information about the liquid-crystal templating mechanism. Their results are consistent with the presence of organic micelles interacting with the silicate species yielding tubular silica encapsulations around the external surface of the micelles. Boron containing MCM-41 molecular sieves were prepared, and l lB NMR spectra confirm the incorporation of boron into the lattice and show the coexistence of several B sites in calcined samples. The stability of such systems is relatively low, and part of the boron can be removed from the lattice by hydrolysis with water vapor at room temperature (62). Substitution of vanadium into the framework of MCM-41 molecular sieves has been achieved, and 51V NMR spectra clearly indicate the presence of two tetrahedral vanadium species with different local environments with isotropic chemical shifts of about -500 ppm (63) (see Figure 21).The absence of an NMR signal with a chemical shift of about -300 ppm indicates that the prepared V-MCM-41 material was free of V20 5 (63). 129Xe NMR spectra were recorded by Ryoo et al. following the formation of A1-MCM-41 applying pH adjustment during synthesis (64). The 129Xe NMR spectrum of A1-MCM-41 prepared with pH adjustment contains a narrow single Lorentzian line appearing at about 71

170 ppm, whereas the line width obtained from A1-MCM-41 samples synthesized without pH adjustment increased due to heterogeneous NMR line broadening (64).

I

-300

I

-.400

I

-500

!

-600

I

-700

~5[ppm]

Fig. 21. 51V NMR spectra of calcined V-MCM-41. a) static NMR and b) MAS NMR (Reproduced by permission of The Royal Society of Chemistry, Cambridge). 3.3

Octahedral Molecular Sieves

Most microporous materials known until recently were zeolites or A1PO4/SAPOs, all of which contain tetrahedraUy coordinated metal atoms. In 1989, a family of microporous titanosilicates (generally denoted ETS) was discovered in which the metal atoms (Ti 4+) are octahedraUy coordinated (65-68). The structure of one prominent member of this family, ETS-10, has been elucidated by Anderson et al. using HREM, electron and X-ray diffraction as well as solid-state NMR spectroscopy (69). This structure comprises corner-sharing SiO 4 tetrahedra and TiO 6 octahedra linked through bridging oxygen atoms. The pore system contains 12-membered rings and displays a considerable degree of disorder. Many ordered variants of ETS-10 exist, some of which are chiral (69). The ETS-10 display characteristics indicating a wide-pore material. The 29Si MAS NMR spectrum of ETS-10 is shown in Figure 22. It shows four lines (one with a shoulder) with chemical shifts of-94.1, -95.8, -96.5 and -103.3 ppm. The intensity ratio of these resonances is 2 9 1 9 1 9 1. By comparing the chemical shifts with other titanosilicate minerals such as zorite and lorenzenite, the three low-field signals can all be assigned to Si (3 Si, 1 Ti) (that is, silicon connected through oxygen bridges to three silicon atoms and one titanium atom) and the resonance at-103.3 ppm can be assigned to Si (4 Si, 0 Ti). These assignments are consistent with the framework connectivity of ETS-10 shown in Figure 23. For the perfect C2/c structure there are eight crystallographic types of Si (3 Si, 1 Ti). From an NMR point of view (short range ordering!) these can be reduced to four distinct

171 types in a 1 91 91 91 ratio (denoted as A, B, C and D in Figure 23). The 29Si MAS NMR spectrum is able to resolve three crystallogaphic types of Si (3 Si, 1 Ti) with two sites overlapping. In conclusion, the structure of ETS-10 is built from sheets and consists of two polymorphs. Polymorph A results in a 12-ring pore system having a zig-zag arrangement with either P41 or P43 symmetry. Polymorph B belongs to the space group C2/c. Consequently, polymorph A has a screw axis and will, therefore, display chiral symmetry

(69).

Si(3Si, 1Ti)

1

Si(4Si, OTi)

l 1

S I 95

I 1 O0 Chemical

shift

,,

I 105

(p.p.m.)

Fig. 22. 29Si MAS NMR spectrum of ETS-10 (Reproduced by permission of Macmillan Magazines Ltd., London). I :

<

.

9

,

.

.

.

.

.

i

Fig. 23. Framework structure of ETS-10 (Reproduced by permission of Macmillan Magazines Ltd., London).

172 The 29Si MAS NMR spectrum of the aluminium containing ETS-10 (so-called ETAS-10) shows at least two additional significant signals at -92 and -90 ppm (compared to the spectrum of ETS-10), indicating that aluminium is part of the lattice structure since no octahedral aluminium was observed in the 27A1 MAS NMR spectrum (66).

3.4

Organic Molecular Sieves/Carbogenic Molecular Sieves (CMS)

Wuest and coworkers demonstrated that the strategy of organic "tectons" can be used to assemble a wide variety of ordered three-dimensional organic molecular sieves and that these assemblies have some of the desirable properties of zeolites and related inorganic materials, including high structural integrity, potentially large void volumes and adjustable microporosity (70). Hypothetical tectons with four tetrahedraUy oriented sticky sites ("adamantane-like" structure with only carbon atoms in all framework positions) are designed to generate diamondoid networks or related three-dimensional lattices. For example, intermolecular hydrogen bonding of the tetrahedrally oriented pyridone rings in a tecton constructed by four pyridone rings at each C-sites in an "adamantane-like" entirely carbon framework structure directs the self-assembly of an interpenetrating diamondoid network that enclathrates guest molecules in large rectangular channels. In crystals of such a clathrate, the average distance between the tetrahedral centers of adjoining tectons is 19.7 ]k, the channels are approximately 4 x 8 tl, in diameter, and 24 % of the total volume is occupied by enclathrated guests (70). Similar networks were prepared and demonstate that the tectons are porous enough to permit exchange and robust enough to resist intact. 1H NMR Spectroscopy and XRD have been applied to monitor complete internal replacements of well defined organic functions in connection with the synthesis of desired modifications, keeping the space group as well as the unit cell dimensions substantially (70). Carbogenic molecular sieves (CMS) are microporous, high-carbon solids. Their preparation has been monitored by Foley (70) using 13C CP/MAS NMR spectroscopy, however, papers dealing with extended NMR studies on CMS systems are still quite rare.

4.

PORE ARCHITECTURE INVESTIGATED BY NMR

One of the main advantages of application of zeolitic or other porous materials is the shape-selectivity of this type of material, which arises due to differential diffusion of molecules with different sizes and shapes in the zeolitic or other porous materials. Therefore, it is very instructive to monitor the pore architecture directly, with a molecule that "observes" the zeolitic type of structure. 129Xe was shown to be a very suitable and sensitive nucleus for this purpose (13). The extended xenon electron cloud is easily deformable (e.g. due to collisions), and deformation results in a large low-field shift of the 129Xe-resonance (13). From an NMR point of view, the 129Xe isotope has a spin 1/2, the natural abundance is 26% and its sensitivity of detection relative to proton is 10"2. Hence, assuming fast exchange, the shift of 129Xe absorbed in zeolites/A1PO 4 molecular sieves can be regarded as the sum of several additive contributions (13, 71-73). Therefore, the chemical shift 8 of xenon adsorbed in a pure zeolite/A1PO 4 is:

173

8 = 80 +

~Xe + 8E + 8S

(2)

where 8 o is the reference (Xe gas at inf'mitely low pressure), ~iXe is a contribution from XeXe collisions, 8 E results from electric field gradients (cations) and ~iS is due to collisions between Xe and cage or channel walls of the zeolite. From equation (2) it is clear that, when extrapolating to vanishing Xe pressure, the xenon chemical shift is determined mainly by the electric field term (which can be neglected for decationized zeolites and those containing only alkali metal cations (73)) and the xenon zeolite/A1PO 4 interaction (13). Indeed, a number of review papers exist, concluding that the chemical shift ~iS of xenon adsorbed on zeolites and extrapolated to zero concentration depends only on the internal void space of the solid (72-75). The smaller the channels or cavities, or the more restricted the diffusion, the greater the ~iS becomes (73). A systematic study of zeolites whose structure is already known demonstrates that the 129Xe chemical shift is linearly related to the pore size of the zeolite (74), and the xenon chemical shift increases with decreasing mean free path for the xenon atoms in the pores (13). A number of different influences, like structure, void space, crystallinity, pore blocking, metal cations (incl. paramagnetic cations), chemisorption of gases etc., have been investigated and reviewed, and I would like to recommend the study of those papers with Ito and Fraissard as the main investigators (72-75). The "129Xe NMR method" has been developed to such a stage where it can be used to study (and predict) the pore architecture of zeolites/A1PO 4 molecular sieves with unknown structures. For example, before the structure of zeolite beta was known, Fraissard et al. were able to determine the approximate form of the internal void volume of this zeolite (76). In conclusion, the application of 129Xe NMR spectroscopy to investigate the pore architecture of zeolites and A1PO 4 type molecular sieves has been very successful. However, extending this method to larger pore containing systems, like the mesoporous materials, has not been demonstrated satisfactorily. Therefore, the search for alternative techniques became evident and certain efforts have been done to study the freezing phenomenon of adsorbed water (or other liquids) confined in high-surface area materials such as silica gel, controlled pore glass and activated charcoal by monitoring the intensity of the 1H NMR signal of water in dependence of the temperature (77). However, these materials consist of non-uniform pore systems, coveting a broad range of irregular and interconnected pores, which lead to complex basic adsorption studies. Consequently, this approach has been successfully extended to mesoporous materials, like MCM-41, by researchers from SINTEF Oslo (78-82). This material shows a well-def'med, uniform hexagonal array of pores, allowing fundamental adsorption studies in a more simple manner. Three samples of purely siliceous MCM-41 with different pore diameters of 45/I, (sample 1), 27 /~ (sample 2) and 18 /~ (sample 3) were studied by Schrnidt et al. using nitrogen adsorption isotherms and proton NMR spectroscopy to monitor the intensity of the liquid water signal when decreasing the temperature (79). The intensity of the liquid water 1H NMR signal drops drastically when the water is froozen, however, the temperature for this transition depends strongly on the pore diameter of the porous material. The nitrogen isotherms varied from a type I-like (sample 3) over a reversible type IV isotherm (sample 2)

174 to a normal type IV isotherm with hysteresis (sample 1). A surprisingly consistency between the nitrogen isotherms and the proton NMR signal intensity versus temperature (IT curves) was observed (see Figure 24). Therefore, it was suggested that 1H NMR spectroscopy could be a valuable technique to characterise the pore architecture of mesoporous materials. In order to investigate this in more detail, the authors attempted to find a mathematical expression describing the proton NMR signal intensity versus temperature and to study the possiblity of using different NMR parameters for the determination of the pore size. For the same "family" of porous MCM-41 materials, a simple linear relationship between spin-lattice relaxation rate (1/T 1) and the inverse pore radius (l/rp) was found by the SINTEF Oslo group (78): 1/T 1 = k m (1/rp)

(3)

where k m is a constant characteristic of the porous material. Combining the well-known Kelvin equation AT = - 2 ~, M To/p AH rp

(4)

(where T O is the normal freezing point, AH the molar heat, T the surface tension, M the molecular weight and p the density of the adsorbate) with eq. 3 suggests a simple linear relation between the spin-lattice relaxation rate (1/T 1) and the lowering of the freezing point (AT) (78). Different MCM-41 materials with similar pore sizes but different concentration of silanol groups (due to the different template removal procedures applied) were used to study the effect of the pore wall composition on the spin-lattice relaxation rate of pore water as a function of temperature (80). The overall spin-lattice relaxation rate was shown to depend strongly on the number of silanol protons and not only on the pore size as suggested in a previous paper (78). A simple linear relation between the spin-lattice relaxation rate (1/T 1) and the inverse pore size (1/rp) can, therefore, only be expected for porous materials with similar chemical compositions. However, the transition temperatures determined by 1H NMR signal intensity measurements versus temperature (see Figure 25) were not sensitive to the chemical composition of the pore walls, making the 1H NMR signal intensity of pore water a more suitable NMR parameter for determining the pore size of mesoporous materials

(80). The same authors derived a mathematical pore size distribution function containing a limited number of adjustable parameters. These parameters were determined exclusively from 1H NMR measurements. Regular mesoporous MCM-41 materials with different pore sizes, ranging from 20 to 30 /~, were synthesized. The 1H NMR technique was used to determine the freezing point of water enclosed in water-saturated samples by recording the 1H NMR signal intensity versus temperature (IT curves). To ensure that the pore system of the MCM-41 materials was not destroyed during the cooling and the following heating procedure applied, the samples were exposed to three consecutive cooling and heating cycles while recording the 1H NMR signal intensity. Within experimental error no changes in the

175

1000

a. }03 E "o .o o o3 -o t~

E

o >

sample

sample

1

sample

2

4~,,,~. 1

45 A

45 A

_>, r

800 c

A

sample 2

27

sample

18A

.a

E

600

400

3

O3 rr

A

18A

sample 3

Z

200

27

0 Q.

0

0

0.2

0.4

0.6

0.8

1

0

P/Po

-20

-40

-60

-80

-1 O0

Temperature [ ~

Fig. 24. Nitrogen adsorption isotherms at 77 K (left) and 1H NMR signal intensities of pore water confined in different siliceous MCM-41 samples (see text) versus temperature (fight). Filled symbols denote adsorption (left) and cooling (right), open symbols denote desorption (left) and heating (right).

v

>pi

O3

Z

LLI

I-Z i

_.I <

Z

i,i

cO

fr Z

-r I

3.8

4.3

4.8

5.3

5.8

IO00/T [K-~] Fig. 25. 1H NMR signal intensity decay of water confined in siliceous MCM-41 (pore diameter of 26/~,) versus temperature.

176 IT curves were observed, indicating no structural change of the mesoporous materials upon freezing of the pore water. By combining nitrogen adsorption and 1H NMR measurements, a simple relation was found between the freezing point depression (AT) and the pore radius

(rp): AT = K f / ( r p - tf) with tf = 3.49 +/-0.36/~

(5)

Kf is a function of properties of the liquid enclosed in the pores (constant). The tf term is tentatively proposed to be identical to the thickness of a surface layer of nonfreezing water, which effectively reduces the actual pore radius from r D to r D - tf. The pore size distribution of amorphous silica determined independently by 1H ~ffMR'and nitrogen adsorption agreed well. However, the application of this method to microporous materials (rp < 10/1,) may be limited. By establishing a correlation between the freezing point depression AT and the pore size r P of mesoporous materials, a new method for the determination of pore size distributions was created (81). The self diffusion coefficient of water enclosed in the pores of the three-dimensional MCM-48, determined by using 1H NMR spin-echo measurements, was found to be significantly larger than that of MCM-41. The high degree of ordering and the three dimensional, interconnected pore system, leads to a high self diffusion coefficient of molecules enclosed in the pores of MCM-48 (82). 5.

IN-SITU NMR STUDIES WITH ZEOLITIC MATERIALS

Studies of the catalytic reactions in the adsorbed phase require conditions of controlled atmosphere, which become especially challenging if the adsorbate is only weakly adsorbed or catalyst-adsorbate samples are air/moisture sensitive and require preparation on a vacuum line, including loading into the MAS rotor. The NMR cells must thus be carefully sealed. On the other hand, to achieve a sufficiently high rotation speed the NMR ceils must be well balanced (83). There are several approaches suiting these requirements for in-situ N M R investigations. Sealed glass ampules containing the catalyst and the adsorbate can be made, fitting precisely into MAS rotors. Highly symmetrical NMR cells are needed, since any asymmetry makes it impossible to obtain high rotation speed and can also lead to explosion of the sample ampule in the rotor due to centrifugal forces. In a second approach glass ampules fit direcly into home-made probes. Extended reviews of in-situ MAS NMR investigations on zeolitic materials can be found in the literature (83, 84). A third approach for investigations of reactions on zeolitic catalysts using in-situ solidstate NMR spectroscopy has been performed by Haw and coworkers applying a special rotor design, abbreviated as CAVERN (cryogenic adsorption vessel enabling rotor nestling) (85, 86). The main advantage of the CAVERN design is that it permits the MAS rotor to be returned to the vacuum line for the adsorption of additional reactants prior to resuming in situ N M R studies. The term CAVERN is now used to designate any device for sealing or unsealing a MAS rotor on a vacuum line. The use of this design was illustrated by

177 demonstrating the sequential adsorption feature of methanol on H-ZSM-5. Although the fast aliquot of methanol exhibited a long induction period in the conversion of methanol to gasoline at 250 ~ none was observed following adsorption of a second aliquot, showing that the products have an influence on the observed kinetics (85). A shallow-bed CAVERN design was developed by the same group allowing the catalysts to be activated under shallow-bed conditions at temperatures up to at least 500 o c prior to adsorption, eliminating the need to handle activated catalysts in a glove box in order to transfer it into the MAS rotor (86). The new device was illustrated by 133Cs MAS NMR spectra of samples of zeolite Cs-ZSM-5 loaded with methanol. The investigation clearly showed that a very homogeneous adsorbate loading was achieved with the shallow-bed CAVERN system (86). Haddix et al. developed an in-situ flow probe which sacrifices the ability to rotate the sample in return for conditions that very closely mimic a bench-top flow reactor (87). A typical experiment with this probe might involve in-situ activation of the catalyst in a flowing gas stream, followed by the establishment of steady-state reaction conditions at elevated temperature through the introduction of one or two reactants into the flow stream

(84).

(a)

Reactants ,~,

(b)

Injectiontube

~

l~~Products ' Rotorcap

]~ --Catalyst I," \

,, ~ Rotor

i

Support I <

>

Injectiontube Bearingjets~~,~~il~ Coil Bearingjets ~'///Fr

~\\\\\\\\\-,~r-

Fig. 26. Design of the MAS rotor (a) and the turbine (b) modified for in-situ MAS NMR investigations of solid-state catalysts (Reproduced by permission of The Royal Society of Chemistry, Cambridge).

178 A new MAS NMR probe for in-situ investigations of hydrocarbon conversion on solid catalysts under continuous-flow conditions has been developed by Hunger et al. (88). This design allows the injection of gaseous educt compounds into the MAS rotor during NMR experiments. Figure 26 shows the in-situ MAS NMR probe built on the basis of a commercial Bruker double-bearing 7 mm MAS probe. A glass tube is placed in the axis of the MAS NMR rotor, and the rotor ejection block on top of the MAS turbine is replaced by a support which fixes the injection tube. The carder gas and educt compounds are injected into the MAS rotor via the injection tube. The gas stream flows inside the rotor from the bottom to the rotor cap and leaves the system through a hole in the rotor cap. After filling the MAS rotor with the catalyst under inert conditions, an axial hole of 2.5 mm diameter is carefully bored into the powder material. This forms a cylindrical catalyst bed at the inner rotor wall. In-situ 13C MAS NMR investigations of alcohol dehydration under continuous-flow conditions were carded out by the same authors using propan-2-ol without isotopic 13C enrichment. Figure 27 shows the 13C MAS NMR spectra of zeolite LaNaY under a certain propan-2-ol flow at 80 and 130 ~ The low temperature spectrum consists of signals at about 24 and 65 ppm originating from propan-2ol and diisopropylether. In the high temperature spectrum, additional signals appear at 19 and in the shift range between 110 and 145 ppm, indicating the formation of propene. Further 13C NMR signals were observed in the shift range between 34 and 42 ppm, indicating the formation of small amounts of branched C 6 alkanes and alkenes (88). The presented design represents a valuable piece of equipment for the in-situ NMR study of reactions on solid catalysts under continuous-flow conditions.

(a) 80oC .

.

.

-,o

.

=

|

200

I

100

h

01 "1

I

130 [ppm]

0

Fig. 27. 13C MAS NMR spectra of dehydrated zeolite LaNaY under a certain propan-2-ol flow (Reproduced by permission of The Royal Society of Chemistry, Cambridge).

179 DIFFUSION OF ADSORBED MOLECULES MONITORED BY NMR

0

Self-diffusion has been defined as the process of molecular migration under conditions of macroscopic equilibrium. For studying the dynamics of small molecules and the intracrystalline mass transfer in the pores of zeolitic materials, a varity of approaches can be chosen. However, the groups of Karger and Pfeifer have done pioneering work with respect to NMR self-diffusion studies in zeolites, especially in connection with the pulsed field gradient (PFG) technique (89). Fast translational motions can be studied by applying this method, in which the actual displacement of a molecule is measured (13). Excellent reviews and papers, describing the molecular migration in zeolitic adsorbate-adsorbent systems, were written by Karger and Pfeifer et al. (3, 89-102). The main parameters determining the transport properties of those systems are the coefficients of intracrystalline and long-range self-diffusion and the molecular intercrystalline exchange rates. The authors showed that these quantities may be determined directly by NMR PFG technique in combination with the NMR tracer desorption technique. The varity of conditions to which molecular migration is subjected in the interior of the different types of zeolitic materials gives rise to characteristic concentration dependences of intracrystalline self-diffusion. The NMR studies yield at least five different patterns of concentration dependence (89). During NMR diffusion studies with ~/2

_I_

)

I-I;

.....

R

___

g

Fig. 28. Schematic representation of the NMR self-diffusion measurements (a-d) and their applications in the spin-echo technique (e-g). Broken lines in (c) and (d) indicate the behaviour with molecular migration. (a) RF pulses, (b) gradient pulses, (c) transverse magnetization M z of different regions, (d) total transverse magnetization M equal to vector sum of (c), (e) RF pulses, (f) gradient pulses and (g) magnetization (Reproduced by permission of John Wiley & Sons, Inc., Chichester).

180 labeled molecules, molecular positions arc simply recorded by the phase of the Larmor precession of the nuclear spins about the external magnetic field. Since the interaction energy between the nuclear spins and the external magnetic field is much less than the thermal energy of the molecules, this way of labelling does not influence the molecular mobility (89). The principle of NMR self-diffusion measurements is most easily understood by considering the pulsed field gradient (PFG) technique (90). In this method, shown schematically in Figure 28, the inhomogeneous field (the "field gradient") is superimposed on the homogeneous field (B) only over two short time intervals. Assuming that the field gradient pulses are of equal magnitude and duration but of opposite sign, the precession of the transverse magnetization M z will depend on the spatial coordinate z during the influence of the field gradient pulse. The first gradient pulse therefore gives rise to a dephasing of the vectors of the transverse magnetization at different positions within the sample, and as a result of this dephasing, the vector sum decays. If the individual species do not change their positions during the time interval between the two gradient pulses, the effect of the first pulse will be exactly counteracted by the second pulse (in the opposite sense) so that the phase of the spins is refocussed and the transverse magnetization vector should be restored to its original value. However, if the molecules (spins) have moved during the time interval between the gradient pulses, the refocussing will be incomplete and the intensity of the transverse magnetization will not be fully restored to its original value. This is indicated by the broken lines in Figure 28. The decrease in the NMR signal becomes larger as the mean square displacement increases (90). A number of papers dealing with this topic have been published since the first review paper appeared in 1987, like the experimental evidence for the self-consistency of the NMR diffusion data obtained by using the 1H- and 19F NMR spectroscopy to investigate the selfdiffusion of CHF2CI in Na X zeolite by varying the intensity of the external magnetic field (92). Conventional 1H NMR signal intensity measurements have been used to monitor macroscopicaUy the kinetics of molecular exchange of deuterium-labeled molecules between the intmcrystalline space of zeolite crystallites and the surrounding atmosphere. After reaching equilibrium, a PFG-experiment was performed within the same sample tube. In this way it became possible to compare results for the intracrystalline migration of adsorbed molecules derived from macroscopic (intensity) and microscopic (PFG) measurements on identical samples. For benzene adsorbed on zeolite Na X the intracrystalline diffusivities resulting from these two techniques were found to be in satisfactory agreement. This result proves that molecular exchange between benzene adsorbed in the intracrystalline space of Na X and the surrounding atmosphere is essentially controlled by intracrystalline mass transfer (93). 129Xe NMR has been successfully used to study the self-diffusion of xenon in zeolites (94). In zeolites Na X and ZSM-5, the self-diffusion coefficients were found to decrease with increasing concentration while for zeolite NaCa A they are essentially constant. The highest diffusivities were observed in zeolite Na X. This is in agreement with the fact that due to the internal pore structure the steric restrictions of molecular propagation in zeolite Na X are smaller than those in Na Ca A and ZSM-5 (94). Mass transfer and chemical reaction in zeolite channels in which the individual molecules cannot pass each other (single-file

181 systems) were studied by Monte Carlo simulations, applying a single jump model for the elementary steps of diffusion (95). The coefficients of intracrystalline self-diffusion of the n-alkanes from propane to nhexane adsorbed in zeolite ZSM-5 are studied by means of the PFG NMR technique over a temperature range from -20 to 380 ~ (96). The diffusivities are found to decrease monotonically with increasing chain lengths. Over the considered temperature range, the diffusivities in ZSM-5 are found to be intermediate between those for Na X and NaCa A zeolite, and the diffusivities of n-alkanes are independent of the Si/A1 ratio of the zeolite lattice (96). Quite a number of PFG NMR measurements have been carried out using 1H NMR spectroscopy with hydrocarbons as probe molecules. Recent progress in the experimental technique of PFG NMR has enabled self-diffusion studies in zeolites using other nuclei than 1H (97). Applying 129Xe and 13C PFG NMR, the temperature dependence of the coefficients of self-diffusion of Xe, CO and CO 2 in zeolites X, NaCa A and ZSM-5 were studied. In all cases the measured diffusivities are found to follow Arrhenius dependence (97). In a second paper dealing with the same zeolites, the authors determined the diffusivities directly from the slope of the echo amplitude vs the field-gradient pulse widths, showing a satisfactory agreement with the corresponding values obtained from an analysis of the NMR tracer desorption curves (98). The process of adsorption of n-hexane in a bed of zeolite Na X was monitored by NMR imaging in combination with PFG NMR (99).The intracrystalline diffusivities were found to depend exclusively on the given sorbate concentration, independent of the time interval elapsed since the onset of the adsorption process. The authors may conclude that adsorbent accommodation during the process of adsorption is not of significant influence on the molecular mobility (99). Monitoring the diffusional barriers on the external surface of zeolite crystaUites was performed by K~rger et al. applying 129Xe PFG NMR spectroscopy (100). The surface permeability of Na X, NaCa A and ZSM-5 was studied. The passage through the external surface of a Na X crystallite was found to be of minor importance, whereas for NaCa A and ZSM-5 zeolites, this process was found to be significantly retarded. The authors conclude that xenon represents a sensitive tool for probing structural distortion in the surface layer of zeolites (100). On the basis of the known results one may conclude that this novel PFG NMR technique can be a routine method for characterization of diffusivities in zeolitic materials. 7.

ACIDITY OF ZEOLITIC MATERIALS

The acidic properties of zeolitic materials are of considerable importance with respect to catalyzed reactions in heterogeneous catalysis. It is vital to know the concentration, strength and accessibility of the BrCnsted and Lewis acid sites and the details of their interaction with adsorbed species (12). For zeolites, for example, 29Si MAS NMR plays a crucial role in the determination of the amount of aluminium which is part of the zeolite lattice as well as the

182 distribution of A1 atoms over distinct crystallographic sites (13). Furthermore, 27A1 MAS and DOR NMR are additional tools to distinguish between framework and non-framework aluminium. While 29Si and 27A1 NMR are suitable for determining the concentration of acid sites, the actual acid strength can be probed either by direct study of the acid protons (Brr acid sites) applying 1H NMR or by using probe molecules (13). Again, a number of review papers exist describing NMR studies on zeolitic material's acidity, and the papers by Pfeifer and coworkers (3, 103) and Klinowski et al. (10, 12 and 104) could be considered as suitable information about this topic of zeolitic materials research. The BrCnsted acidity of zeolitic materials arises from the presence of accessible hydroxyl groups associated with framework aluminium (12). As already mentioned, 1H MAS NMR is an advanced tool for probing the protonic sites in zeolitic materials. However, solid-state 1H NMR is an experimentally difficult technique, particularly in cases where a high concentration of protons cause extreme line broadening due to proton-proton dipolar interactions, which must be removed by multiple quantum decoupling (13). In addition, heteronuclear dipolar interactions of protons with aluminium can complicate the NMR spectra, especially taking into account the generally narrow range of proton chemical shifts. Fortunately for dehydrated zeolitic materials, the proton density is often so low that reasonable 1H MAS NMR spectra can be obtained at high magnetic fields and using fast spinning speeds. In this way, four distinct types of protons have been identified and quantified by their chemical shifts:

o

3. 4.

non-acidic, terminal SiOH groups on the surface of zeolite crystallites and crystal defect sites (1.5 - 2 ppm) A1OH groups at non-framework aluminium (2.6 - 3.6 ppm) acidic, bridging hydroxyl groups SiO(H)A1 (3.6 - 5.6 ppm) ammonium ions (6.5 - 7.5 ppm)

The most important sites are the bridging SiO(H)A1 groups, which represent the catalytically active sites in zeolitic materials. The concentration of the protons in the various sites can be directly determined from the peak intensities. In addition, the deprotonation energy can be measured by chemical shift changes: the chemical shifts increase with "increasing Sanderson electronegativity. Moreover, the 1H chemical shift may be correlated with the proton donor ability of the corresponding site and can thus provide information on the acid strength of the protons (2). Measuring the interactions between surface hydroxyl groups and water by 1H NMR can be used to characterize the strength of BrCnsted acid sites. Variable temperature 1H MAS NMR was used to characterize the structure and dynamics of hydrogen bonded adsorption complexes between various adsorbates and the Brcnsted acid site in H ZSM-5: the Brcnsted proton chemical shift of the active site was found to be extremely sensitive to the amount of type of adsorbate (acetylene, ethylene, CO and benzene) introduced (105). Zscherpel and coworkers performed 13C MAS NMR spectroscopic measurements in order to investigate the interaction between Lewis acid sites in H ZSM-5 and adsorbed CO. A new measure for the "overall" Lewis acidity of zeolites was derived from the 13C MAS NMR spectroscopic data. In addition, the chemical shift of CO adsorbed

183 on Brcnsted acid sites and physisorbed CO was determined to amount to 178 and 186 ppm, respectively (106). Concerning the concentration measurements of hydroxyl groups (Brr acid sites and non-acidic OH groups) NMR spectroscopy has an extremely important advantage compared with IR spectroscopy since the area of a IH MAS NMR signal is directly proportional to the concentration of the hydrogen nuclei contributing to this signal irrespective of their bonding state, so that any compound with a known concentration of hydrogen atoms can be used as a

SAPO-5/1

SAPO-5/2 POH

c

IR

b

iT o.

/~k A '~ SiOH

I 3630

~

1JOH/Cm-1

n 3630

pb 1H MAS NMR

1 b

T

APo.

SiOH

..,

I

6.8

I

I

I

I

3.8 4.8 1.8

I

I

I

3.7 1.1 1.9 0.2 ~iH/[ppm]

Fig. 29. IR and 1H MAS NMR spectra of two different samples of SAPO-5 (Reproduced by permission of Academic Press Ltd., Orlando).

184

reference (3). In Figure 29 the 1H MAS NMR and IR spectra for two differently synthesized samples of SAPO-5 are displayed. While the positions of the various signals in the NMR and IR spectra correspond to each other quite well there are dramatic differences in the relative intensifies. From the IR spectrum of the first sample one would conclude that the concentrations of bridging OH groups (signals indicated by b and c) are approximately equal, and from the IR spectrum of the second sample that the concentration of P-OH groups is about three times larger than that of the bridging OH groups of type b. Therefore, even the relative intensity of an OH stretching vibration band cannot be taken as a measure for the concentration of the respective hydroxyl groups in contrast to the intensity of an NMR signal

(3). On the other hand, there is a surprisingly good correlation between the positions of the various signals in the IR and 1H MAS NMR spectra. With the exception of the cationic OH groups there is a nearly linear interdependence between the wave number vOH of the IR band and the chemical shift 15H of the 1H MAS NMR signal. The straight line approximating the experimental results is given by (3) VOH (cm-1) = 3870 - 67.8. ~i H (ppm)

(6)

Acid sites in zeolitic materials can be sensitively probed by the study of adsorbed molecules like pyridine, ammonia (15N NMR) or trialkylphosphines (31p NMR). There is at present only a handful of publications involving 15N NMR of molecules sorbed on zeolites since enriched adsorbents need to be used to improve the poor signal to noise ratio. Valuable data have been obtained for zeolites X, Y and mordenite, which are summarized in the references (12, 13). A new approach was introduced by Rothwell et al. (107), who explored the utilization of the much more sensitive 31p nucleus as a probe for acidity (13). Trimethylphosphine (TMP) was shown to be an effective probe molecule for studying both Br~nsted and Lewis acid sites in acidic Y-type zeolites. Interaction of the trimethylphosphine with Brcnsted acid sites gives rise to the protonated adduct having 31p MAS NMR chemical shifts in the range -1 to -4 ppm. In a partially oxidized sample the protonated form of TMP oxid exhibits a resonance at 64.6 ppm. TMP also interacts with Lewis acid sites in dealuminated zeolites showing a 31t; resonance at -62 ppm. Depending on the treatment conditions additional resonances were observed in the range of-31 to -58 ppm (108, 109). 8.

CONCLUDING REMARKS

Solid-state NMR spectroscopy has been demonstrated as a well established technique for characterization of zeolites and other porous materials with respect to structure elucidation, pore architecture, catalytic behaviour and mobility properties. The latest progress in the development of NMR techniques, both with respect to software and hardware improvements, has contributed to the present state of the art for NMR within the field of characterization of zeolitic materials. Furthermore, the introduction of NMR imaging (110), two-dimensional quintuple-quantum NMR spectroscopy (111) and transfer of populations in double resonance (TRAPDOR) NMR (112, 113) will extent the horizons of zeolite characterization science. As a final example, the 27A1 ~ 29Si TEDOR experiment directly proves, for the first time, that silicon substitutes for phosphorous atoms in the framework of SAPO-37 (114). The 27A1

185 (31p) and 27A1 (29Si) dipolar-dephasing difference experiments and the 2D 27A1 ~ 31p TEDOR experiment indicate the presence of three types of aluminium environments in SAPO-37: tetrahedral A1 surrounded by symmetrical [4 P] or [4 Si ] environments which give rise to a sharp resonance, tetrahedral A1 surrounded by asymmetrical [3 P, Si ], [2 P, 2 Si ] or [P, 3 Si ] environments, which give rise to a broad resonance, and also extraframework amorphous octahedral A1 (114). I hope that this paper could contribute to highlight recent advances in solid-state NMR spectroscopy related to the characterization of zeolitic materials as well as at the same time presents the latest developments related to new techniques and methods for the zeolite scientists not so familiar with all the details about solid-state NMR spectroscopy.

Acknowledgements Thanks are due to Tordis Whist for technical assistance in connection with the preparation of the figures, and the relevant publishers are gratefully acknowledged for their permission to reproduce the figures as cited in the text. Financial support by The Research Council of Norway (NFR) and SINTEF is gratefully acknowledged. REFERENCES

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83.

84. 85. 86. 87. 88. 89. 90. 91. 92. 93. 94. 95. 96. 97. 98. 99. 100. 101.

M. Springuel-Huet, J. Demarquay, T. Ito and J. Fraissard, Stud. Surf. Sci. Catal., 37 (1988) 183. D.W. Johnson and L. Griffiths, Zeolites, 7 (1987) 484. J. Fraissard, Z. Phys. Chem. (Munich) 152 (1987) 159. R. Benslama, J. Fraissard, A. Albizane, F. Fajula and F. Figueras, Zeolites, 8 (1988) 196. K. Overloop and L. Van Gerven, J. Magn. Reson. Ser. A 101 (1993) 179. D.E. Akporiaye, E.W. Hansen, R. Schmidt and M. StOker, J. Phys. Chem. 98 (1994) 1926. R. Schmidt, M. St6cker, E.W. Hansen, D.E. Akporiaye and O. H. Ellestad, Microporous Mater. 3 (1995)443. E.W. Hansen, R. Schmidt, M. Stticker and D.E. Akporiaye, J. Phys. Chem., 99 (1995) 4148. R. Schmidt, E.W. Hansen, M. St6cker, D.E. Akporiaye and O. H. Ellestad, J. Am. Chem. Soc., 117 (1995) 4049. R. Schmidt, M. Stticker and O.H. Ellestad, Zeolites: A Refined Tool for Designing Catalytic Sites (F,ds.: L. Bonneviot and S. Kaliaguine), Stud. Surf. Sci. Catal., 97 (1995) 149. I.I. Ivanova and E.G. Derouane, Advanced Zeolite Science and Applications (E,ds.: J.C. Jansen, M. Stticker, H.G. Karge and J. Weitkamp), Stud. Surf. Sci. Catal., 85 (1994) 357. J.F. Haw, NMR Techniques in Catalysis (Eds.: A.T. Bell and A. Pines), Marcel Dekker, Inc., New York, 1994, page 139. E.J. Munson, D.B. Ferguson, A.A. Kheir and J.F. Haw, J. Catal., 136 (1992) 504. E.J. Munson, D.K. Murray and J.F. Haw, J. Catal., 141 (1993) 733. G.W. Haddix, J.A. Reimer and A.T. Bell, J. Catal., 106 (1987) 111. M. Hunger and T. Horvath, J.C.S. Chem. Commun. (1995) 1423. J. Karger and H. Pfeifer, Zeolites, 7 (1987) 90. J. K~ger and D.M. Ruthven, Diffusion in Zeolites and other microporous Solids, John Wiley & Sons, New York, 1992. J. Caro, H. Jobic, M. Biilow, J. K~ger and B. Zibrowius, Adv. Catal., 39 (1993) 351. J. K3.rger and R.M. Ruthven, Zeolites, 9 (1989) 267. C. FOrste, J. K~ger and H. Pfeifer, J. Am. Chem. Soc., 112 (1990) 7. W. Heink, J. K3.rger, H. Pfeifer and F. Stallmach, J. Am. Chem. Soc., 112 (1990) 2175. J. K~ger, M. Petzold, H. Pfeifer, S. Ernst and J. Weitkamp, J. Catal., 136 (1992) 283. W. Heink, J. K~ger, H. Pfeifer, K.P. Datema and A.K. Nowak, J.C.S. Faraday Trans., 88 (1992) 3505. J. K~ger, H. Pfeifer, F. StaUmach, N.N. Feoktistova and S.P. Zhdanov, Zeolites, 13 (1993) 50. F. StaUmach, J. K~ger and H. Pfeifer, J. Magn. Reson., A 102 (1993) 270. J. K~ger, G. Seiffert and F. Stallmach, J. Magn. Reson., A 102 (1993) 327. J. Karger, H. Pfeifer, F. Stallmach and H. Spindler, Zeolites, 10 (1990) 288. J. K~ger and G. Fleischer, Trends in Analytical Chemistry 13 (1994) 145.

189 102. 103. 104. 105. 106. 107. 108. 109. 110. 111. 112. 113. 114.

J. K~ger and H. Pfeifer, NMR Techniques in Catalysis (Eds.: A.T. Bell and A. Pines), Marcel Dekker, Inc., New York, 1994, page 69. H. Pfeifer, D. Freude and M. Hunger, Zeolites, 5 (1985) 274. J. Klinowski and M.W. Anderson, Magn. Reson. Chem., 28 (1990) S 68. J.L. White, L.W. Beck and J.F. Haw, J. Am. Chem, Soc., 114 (1992) 6182. D. Zscherpel, E. Brunner, M. Koch and H. Pfeifer, Microporous Mater. 4 (1995) 141. W.P. Rothwell, W.X. Shen and J.H. Lundsford, J. Am. Chem. Soc., 106 (1984) 2452. J.H. Lundsford, P.N. Tutunjian, P.J. Chu, E.B. Yeh and D.J. Zalewski, J. Phys. Chem., 93 (1989) 2590. P.J. Chu, A. de Mallmann and J.H. Lundsford, J. Phys. Chem., 95 (1991) 7362. W. Kolodziejski and J. Klinowski, NMR Techniques in Catalysis (F_.ds.:A.T. Bell and A. Pines), Marcel Dekker, Inc., New York, 1994, page 361. C. Fernandez, J.P. Amoureux, L. Delmotte and H. Kessler, Microporous Mater., in press. C.P. Grey and B.S. Arun Kumar, J. Am. Chem. Soc., 117 (1995) 9071. C.P. Grey and A.J. Vega, J. Am. Chem. Soc., 117 (1995) 8232. C.A. Fyfe, K.C. Wong-Moon, Y. Huang and H. Grondey, Microporous Mater., 5 (1995) 29.

This Page Intentionally Left Blank

H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All rights reserved.

191

Application of surface science techniques in the field of zeolitic materials

Serge Kaliaguine D6partement de g6nie chimique, Universit~ Laval, Ste-Foy, Qu6bec, G1K 7P4, Canada 1.

INTRODUCTION

Among the various surface analysis techniques which are currently available to catalysis chemists, X-ray photoelectron spectroscopy (XPS) is certainly the one having found the widest application in the study of zeolitic materials. Reflecting this significance, this text will mostly dwell upon XPS and its relevance to zeolites, with some mentions of the contributions of such other techniques as Auger Electron Spectroscopy (AES), Ion Scattering Spectroscopy (ISS) and Secondary Ion Mass Spectrometry (SIMS). The topic of XPS investigation of zeolites is specially interesting for two reasons. First the binding energy shifts measured in XPS are reflecting the chemical environment of almost all elements (with the exception of hydrogen and helium). Thus XPS yields chemical information regarding not only the lattice atoms, but also the counter cations and the occluded elements which may be either deliberately inserted in the pores, for example for the preparation of a composite catalyst, or inadvertently produced during the preparation and pretreatment of a zeolite [1]. To some extent XPS may also yield some information about the dispersion and spatial distribution of these occluded materials from the quantitative analysis of XPS peak intensities. However the interpretation of XPS data is seldom straightforward because both BE shifts and peak intensity ratios are not pieces of information which can be univocally associated with one chemical environment and one geometry of deposit respectively. The logical consequence of this is that XPS alone never yields definite proofs but should rather be utilized to verify necessary conditions. It must also, and for the same reason, be used in association with other analytical techniques. With these limitations, the XPS analysis provides nevertheless invaluable and often unique information as we will try to demonstrate below for zeolitic materials. The second aspect which makes the interest of the study of zeolites by XPS is that it allows to understand many unexpected aspects of this spectroscopy, in particular when applied to solids. Zeolites in many respects are materials which may be regarded as model solids and not only because of their regular pore structure. Indeed by varying the Si/A1 ratio, the nature of counter cations, the ion exchange level, the occlusion of extraframework phases, such properties as the density and strength of acidic or basic sites may be varied in a predictable fashion. All these variations are reflected in the XPS spectra yielding data

192 which are not only useful to the analysis of these materials but also to the understanding of the phenomena involved during the XPS analysis itself. In the following text we will try to review this field and to underline the recent progresses associated with these two aspects.

2.

PHOTOELECTRON ANALYSIS

SPECTROSCOPY (XPS) AND OTHER SURFACE

X-ray photoelectron spectroscopy ~ S ) or electron spectroscopy for chemical analysis (ESCA) is a technique whereby a beam of monoenergetic X-ray photons interacts with the atoms of a solid (Figure 1). In this interaction electrons are extracted from their orbitals, provided the energy of the photon is higher than the binding energy of the electron on its particular level. If the collided atom lies too deep below the surface, all emitted photoelectrons will undergo inelastic collisions with other atoms before they can exit from the solid. By contrast if the atom is located within a short distance from the surface, the extracted photoelectrons have a non-zero probability to leave the solid without having experienced a collision.

X-Ray Photon S~

~ ~"~"~"

e-~ Kinetic EnergyAnalyser

-/~'

\ .

~

.

"x x

x

x

...~

,.x

~

,'4: x

..~ l

x

x

x

x

x

Figure 1. Principle of photoelectron spectroscopy. The purpose of XPS analysis is then to detect and count these photoelectrons using a detector which allows to measure the photoelectrons current density as a function of their kinetic energy. Obviously the kinetic energy of the photoelectrons having not been involved

193 in any inelastic collisions bears chemical information on only those atoms located in the near surface region [2-4]. Figure 2 represents the collision energy balance in the case of a conducting sample. As the electron kinetic energy E K is measured with reference to the spectrometer vacuum level, this balance must be expressed as h v = E K + E B * 4),p

(1)

where hv is the incident photo energy, E B the binding energy of the electron on its core level and ~sp the work function of the spectrometer. In the case of a non-conducting sample, this balance must be slightly modified to take into account a steady positive charging of the surface which develops under the steady bombardment conditions of the spectrometer. In this case as shown in Figure 3, the Fermi level of the spectrometer is raised by a value E e corresponding to this charge.

Energy of Moving Electron in Vacuum

IEk

Vacuum Level ,

T,

i

i E]sp Fermi Level ~ / / ~ ~

.... ~

,,

T

Conduction Band

i

i E'B L

Core/~ Levels

Vacuum Level

Fermi Level

of Spectrometer

EB

L

i

,

,

T

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

Figure 2. Photoelectron energy balance - Conductive sample. Therefore:

hv

= E K * E e + 4)=p * E=

(2)

Er must then be determined experimentally for each analysis. On the contrary t~sp which also appears as a correction of the binding energy scale, is a value which depends only on the spectrometer. For a given X-ray source the photon energy hv is known with some precision. The two most common sources use AIK~ (hv = 1486.6 eV) or MgK a (hv = 1253.6 eV) emissions. Thus as long as Er and ~sp are also known, the experimental determination of the electron

194 kinetic energy, E K, yields a determination of the electron binding energy E B. The kinetic energy scale can therefore be converted into a binding energy scale.

Energy of Moving Electron in Vacuum Vacuum Level

i Ek

"-...

Vacuum

Conduction Band ~ / ' / ~ ~ Fermi Level

I

t E]sp '"

.

! E~

Level

Fermi Level of Spectrometer

~/~//~

ValenCeBand

Core I'= Levels ..,.I,

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

Figure 3. Photoelectron energy balance - Non-conductive sample. Figure 4 gives for example the spectrum of the sodium form of zeolite X (NaX) obtained with a magnesium source. It shows main peaks Nals (1072 eV), Na2s (64 eV) and Na2p (31 eV); Ols (532 eV) and O2s (24 eV); Cls (284 eV); Si2s (153 eV) and Si2p (102 eV); A12s (118 eV) and A12p (74 eV). Note that both Sils and Alls are not visible because their binding energies are higher than the photon energy of the Mg source. The spectrum also shows Auger emissions for carbon, oxygen and sodium. This corresponds to electrons emitted following the photoemission and creation of a hole on a core level (1). The rearrangement involves fiUing the photo hole with an electron from an upper layer (2) and emission of another electron from another electronic level (3) in the same layer. As a f'wst approximation the emitted electron has a kinetic energy E K given by: -

-

§

(3)

For example in the KLL Auger emission of sodium levels 1, 2 and 3 are the Nals, Na,2s and Na2p respectively so that the kinetic energy of Auger electrons is E K =- 1 0 7 2 - (64 * 31) = 9 7 7 e V

In the spectrum of Figure 4, this emission appears thus on the binding energy scale at 1254- 977 = 277 eV.

195

140 000 O~s 120 0 0 0 NalS

i 100000 -

>, 8 0 0 0 0 -

~0 000 -

Na (A)

J~

'--

C (A)

I/

a.

ff

N,,s z,a,

O~sKa 0

t

0

,

,

'"

I

200

'

'

'

i

'

'

~

I

'

400 600 Binding Energy leVI

'

'

i

800

'

'

'

I

'

~

'

1000

Figure 4. Example of XPS survey spectrum- NaX zeolite. As the X-ray source was not monochromatized the major XPS peaks show also some satellites due to the K a 3,4 emission of Mg (see Nais, O2s, Si2s and Si2p peaks in Figure 4) and even for the most intense peaks from its KI3 emission (see Oxs peak in Figure 4). As clearly seen in the spectrum of NaX, each major emission is followed by a hump on its high binding energy side thus corresponding to photoelectrons with kinetic energies lower than the main peak. These are the photoelectrons having undergone inelastic collisions in the solid as discussed above. It is also interesting to note that the peak intensities are different for the various emissions. Compare for example the peak heights of Nals, Na,2s and Na2p in Figure 4. This is associated with different probabilities of photoemission for the various levels. It is also apparent that these intensities are depending on the concentration of each atom but not in a simple manner. For example the NaX sample of Figure 4 has the approximate unit cell composition Nass.4Alss.4Silo6.603s4. It is seen that the Ols/Nals intensity ratio is close to 2 whereas the atomic ratio O/Na is close to 4.5. The relationship between XPS intensity ratios and atomic ratios will be discussed below.

2.1 XPS binding energies and atomic chemical environment Equations (1) and (2) describe the energy balance for the photoionization process: A hv > A .

+e"

(4)

E B appears thus as the difference between the energy of the f'mal ionized state and the initial neutral atom A.

196 E~ = E F

E,

-

(5)

Here Eft is referenced to the vacuum level as indicated in Figure 2. There is one difficulty associated with the definition of the final state because after the photoemission the solid undergoes relaxation corresponding to rearrangements both internal to atom A and external to it. If the time scale of the photoemission process is small compared to the relaxation times then the so-called "sudden approximation" applies. According to this approximation the final state in equation (4) involves cation A* before any relaxation. The consequence of this is designated as KOOPMAN's theorem which states that the binding energy on level j, E ~ , is given by: =

(6)

-,j

where e.. is the eigenvalue of the wave function of the j orbital of atom A. (The minus sign ensures that E~ > 0). When the sudden approximation does not hold, E~ must be written J

9

as

E,J --

+

(7)

where Er~ is the relaxation energy. 2.1.1

Initial state effects

2.1.1.1 Chemical shift In XPS one measures core level binding energy differences (AE~) between atoms of the same element in the sample and in some reference compound. This difference is designated as the chemical shift. It follows however from equation (5) that Z~Ee

=

AE

F

-

~E~ ,

(8)

where only the AEI term is related to the chemical state of atom A before photoionization. Thus strictly speaking this difference in binding energy is only reflecting chemical changes in the rather often justified hypothesis: AE.

: - Z~Ew - Z~Ej

(9)

Chemical bonding affects of course primarily e's for the valence orbitals but the core levels are also affected although in a lesser proportion. If atom A is involved in an oxidative bond the attraction from the nucleus is not affected but the repulsion forces exercised on the

197 remaining electrons are decreased. Thus all core levels are lowered with respect to the Fermi level (see Figure 2) and E B increases. Note that the same effect (AF-B > 0) is expected whatever the chemical cause for the decrease in electronic charge of atom A. Thus AF~s may reflect not only a change in the oxidation state of atom A but also any change that affects its charge like a difference in chemical environment, coordination, nature of ligands, and crystallographic position in a lattice. The relationship between E x and the electronic charge qa of atom A is often correctly represented by a simple electrostatic model developed by SIEGBAHN et aL:

Eu~ = E ~ , kq^ * I3 qj j.^ r~

(10)

where E ~ is an energy reference and qj the so-called point charges on neighbouring atoms j. rA- are of course the distances from atom A to j. ~lf atom A is represented as a hollow sphere charged with the valence charge qA then the potential has the same value at each point within this sphere namely qA/rA where r A is the average radius of the valence shell. Thus any change in qA changes this potential by AqA/rA and the model predicts that all core levels will be changed by this amount. With a change in chemical environment for atom A it follows from equation (10) that AE, = k AqA + A V

(11)

V is the summation in the right hand term of equation (10) and is designated as the MAGDELUNG potential. In molecular solids the sum is limited to atoms bonded to A but in ionic solids the summation must be extended to infinity.

2.1.1.2

Spin-orbit splitting

Whenever an orbital does not have spherical symmetry (quantum number 1 ~ 0) any subshell splits into two levels with quantum numbers j = 1 _ Is i- This splitting is therefore increasing with the atomic number on a given subshell (constant n, l) and with a decrease in 1 at constant n. Assuming the two levels have the same photoionization cross-section, the ratio of peak areas in the doublet is given by the ratio of their degeneracies (2j + 1). The values are given in Table 1. Table 1 Parameters for spin-orbit doublets Subshell

1

j

area ratio

s

0

/2

--.-

p d f

1 2 3

1/2, 3/2 3/2, 5/2 5/2, 7/2

1/2 2/3 3/4

198 2.1.2

Final state effects

2.1.2.1 Chemical shift It is indeed surprising that approximation (9) and equation (11) as a rule predict

essentially correctly the observed chemical shifts. Indeed these equations neglect completely the AEF term in equation (8) which following equation (7) should be written as ~E~ = AE j

where AE~ is the difference in relaxation energy for level j. Thus in many cases it seems correct to assume: Z~Ej - 0

but there are cases when this assumption is not justified. For example is has been found [5] that the difficulties in interpretation of the RU3d5/2 lines in Ru Y catalysts [6] were due to the fact that owing to final state effects the RU3d 5R level of R u t 2 was not significantly different from the one of metallic ruthenium in spite of a difference of 4 in oxidation number. The common XPS features discussed below may also be regarded as final state effects, namely the shake up satellites, energy loss bands and multiplet splitting. 2.1.2.2

Shake up satellites

One internal relaxation process is a two electron effect which involves an unpaired valence electron being excited to an upper free level of the valence shell. The corresponding energy is borrowed to the photoelectron so that its kinetic energy is lowered and a satellite peak appears on the high binding energy side of the main peak. Shake up satellites are noticeably high in transition metals compounds with unpaired 3d electrons and in rare earth compounds with unpaired electrons in the 4f shell. The intensity and splitting of these satellites is often of critical diagnostic value. For example Fe(II) in such compounds as FeO, FeMoO4 and Fe(COOH) 2 can be easily identified from its prominent shake up satellite of the Fe2p 3/2 peak [7]. 2.1.2.3 Energy loss peaks The inelastic collision which gives rise to the photoelectrons appearing as a step on the high binding energy side of each main peak (see Figure 4) is obviously one energy loss process which must be considered as an external relaxation event. Another such process is the so-called plasmon loss in which the photoelectron loses energy to a collective oscillation of conduction electrons. There are bulk and surface plasmons with characteristic respective frequencies t ~ and o~s. The plasmon loss peak may be used in several analytical applications. For example carbonaceous materials with high polyaromatic content show a plasmon loss peak of their Cls photoelectrons at 291.2 eV. This peak is clearly resolved from the Cls peak [8].

199

2.1.2.4 Multiplet splitting Whenever the system has unpaired electrons in the valence levels, after ejection of a photoelectron from a core level, the electron remaining on this core level may have its spin parallel to that of the unpaired valence electrons. In that case an exchange interaction can occur yielding a lower energy than for the case of antiparaUel spin.

2.1.3 Auger parameters It may be seen from equation (3) that the chemical shift which affects binding energies will also be reflected in the kinetic energy of Auger electrons. Wagner found that the difference between the kinetic energies of Auger [EK(jkl)] and XPS [EK(i)] emissions was also characteristic of the chemical environment in addition to being independent of reference level and charging effects. Thus he defined the classical Auger parameter as:

(= = EK(]kl ) _ EK(i )

(12)

which was subsequently modified as (='= r * hv = EK(jkl) .Ea(i )

(~3)

The Auger parameter it' is often used in diagrams like the one shown in Figure 5 designated as chemical state plots. In these plots experimental values of E~:(jkl) are plotted as a function of EB(i) so that states at constant values of (x' appear as lines with slope +1. The chemical plot in Figure 5 is for silicon in a series of zeolite. It shows that parameter (x" is only distributed over a short range 1711.3 - 1711.8 eV whereas the Si2a binding energy ranges from 101.3 to 103.5 eV. Another important application of Auger parameters stems from the fact that the extraatomic relaxation energy may be expressed as:

E~

= (1 - ~1) q=

2ro

(14)

where q is the charge, eo the dielectric constant and r o the effective screening distance of the electron. Thus if we assume that the intra-atomic relaxation energy is not affected by the chemical environment, equation (7) yields &EB(i) = - &e(i) + A E ~ ( i )

(15)

Then assuming that changes in initial state orbital energies between the chemical environments is the same for all core orbitals, it may be shown from equations (14) and (15) that:

200 Z~EK(jkl ) = Ae(j) + 3 A E ~ ( j )

(16)

,

where AE~ a (j) is the relaxation energy change associated with the formation of the photohole by XPS, whereas:

a='=

2&E~a(j)

.

(17)

1611

/

/V/o

A > LU v

/

/ ~9

1610

.. ~

1711

tu

1710

~"

UJ

._~ (D

~

1609

.d Y

1608

Z ,,"/ / /OW / 104

/

i//,,

102 103 2P Binding Energy (EV)

E "

~ 1709

a) <

101

Figure 5. Chemical-state plot of silicon [11]. 1. Offretite (Si/A1 = 4.1); 2. Offretite (Si/A1 = 5.0); 3. Offretite (Si/A1 = 5.5); 4. Offretite (Si/A1 = 5.8); 5. H-ZSM-5 (Si/AI = 14.0); 6. H-ZSM-11 (Si/A1 = 10.7); 7. H-mordenite (Si/A1 = 8.5); 8. H-mordenite (Si/A1 = 4.9); 9. NaA; A - NaA; X - NaX; Y - NaY; Z - H-zeolon. Equation (17) indicates that shifts in t~" result solely from the difference in final state extra-atomic relaxation energies between the two chemical environments.

2.1.4 Referencing the binding energy scale As discussed above in relation with equation (2), for each XPS spectrum acquired a value of Ec must be determined before a precise and correct determination of any binding energy is made. Barr has underlined [9,10] that the problem of charging shift Ee must be differentiated from the one of the floating of the Fermi edge of the sample off the Fermi level of the spectrometer. This latter effect may be met either with non conductive samples or with conductors not in good electrical contact with the spectrometer. This corresponds to non-steady state situations and in this case the measurement of accurate binding energies

201 is not possible. It is thus a good laboratory practice to measure systematically the kinetic energy of a characteristic emission (say Si2~, with zeolites) at the beginning and at the end of the spectral acquisition to verify that Ec is not varying with time. Another very difficult problem is associated with the differential charging between a support and a supported phase. This arises when both phases are insulated from one another. It must also be stressed that the use of a flood gun to remove the charging shift is not sufficient to couple the Fermi levels and it is still necessary to reference the binding energy scale when using such a gun. It may some times be possible to use the known value of binding energy for a given element in a known sample. For example a Si2p binding energy of 103.3 eV has often been used in SiO2 and high silica zeolites. This practice must however be used with caution because such effects as metal-support interaction may affect this value so that the binding energies measured with the support may not be valid for the metal-support composite material. Some authors have used the Czs emission of adventitious carbon at a fixed value of 285.0 eV. (Some others prefer 284.4, 284.5 or 284.6 eV without real justification). This practice is not recommended because of the complexity associated with the possible differential charging of carbon impurities. Moreover it is not unusual to observe multicomponent Czs peaks in zeolite spectra [11] and the Czs span in binding energy expands over 8 eV in the literature. We have found that quite consistent data can be reproducibly obtained with nonconductive samples such as zeolites by performing careful gold deposition under vacuum (10 -3 torr) of layers with thickness of the order of 20 A. The binding energy scale may then be referenced to Au4f 7/'2 = 84.0 eV and the other emissions are not overly attenuated. In a recent paper Grttnert et al. [12] have shown that for ion conducting zeolites it is possible to reduce the charging and reach an essentially "charge free" state by heating the sample to a temperature between 570 and 820 K. The sample must be in the form of a very thin layer of zeolite deposited on a metallic sample holder.

2.2 Quantitation of x P s intensifies Obviously quantitative information should also be available from XPS spectra because the flux of photoelectrons emitted is proportional to the number of emitting atoms. The relationship between the peak intensities in spectra similar to the survey spectrum shown in Figure 4, and the surface concentrations of the elements detected is however not simple. The basic reason for this is that the photoelectron flux is attenuated on its path through the solid following a Beer-Lambert law: I - io exp(-d/x)

(18)

where I/Io is the attenuation ratio, d the distance travelled and ~. the mean free path of photoelectrons in the given solid. Thus the attenuation ratio depends not only on the depth z at which a photoelectron is emitted, but also on the angle 0 of the analyzer axis with the surface.

202

(19)

I = Io exp(-z/X sin 0)

Therefore a calculation of the intensity In of collected photoelectrons originating from level n on atom A must involve the integration over z of a differential equation:

d IAn = K DAn oAn CA(Z) exp(-z/XAn sin 0) dz

(20)

In equation (20) K is a constant which depends only on the geometry and other characteristics of the spectrometer provided certain conditions are met. These conditions are not exceedingly restrictive. DAn is the luminosity of the analyzer which is indicated as level dependent here because it depends generally on the kinetic energy of the photoelectrons. The product o~a is the collision cross-section. It depends both on the photon energy and the atomic energy level. Fortunately precise calculations of c ~ have been performed by Scofield for A1K~ and M g I ~ sources. These results extending on all significant energy levels of the elements have been tabulated [13]. CA(z) is the atom A concentration which is depending on depth z except in the trivial case of a homogeneously distributed atom A. The exponential term now involves Z,~ as the photoelectrons mean free path is kinetic energy dependent. Contrary to what is suggested in simple correlations, ~'Aa is alSO dependent on the solid matrix in which the photoelectron is travelling. From equation (20) one may calculate the total flux of photoelectrons exiting the solid at a given angle 0:

IA. = K f; CA(z) oAn exp l_ ;~= zsin 0 /d z

(21)

and the fraction of that flux which originates from a depth smaller than z, which in the case of a uniform solid (C A independent of z) is:

%(z) -

IA, I~ _ IAn

exp

X~ sin O XAn sin O

Figure 6 shows the calculated values for q~(z) at four values of the analysis angle 0. It is seen on these curves that depending on angle 0, the depth of analysis can extend down to three to four times the mean free path. For example at 90~ analysis angle, 63% of the photoelectrons come from the upper layer of depth ~. whereas 5% were generated deeper than 3 ~,. At 0 = 10~ 94.4% of the photoelectrons are generated between 0 and 0.5 ~, and less than 0.3% of them originate from atoms located at a depth exceeding ~..

203 It is thus seen from equation (22) and Figure 6 that even in the simple case of a homogeneously distributed atom A (C A independent of z) the photoelectron escape probability (pc(z) is very strongly dependent on both z and 0.

o.s

__12

o.7

0.9

q~(z)

o

opt(z)

o

o.s

1

q~o(z)

1

qho(z)

90"

4).

o.s

o.8

o.s I

g

"

1).

! '

i

l

l

1

._

~

5.3%

1). "1"-"

ii'~"

=~l~

3:L~I~

3)"I

4)..Io .3%

4).

Z

Z

Figure 6. Photoelectron escape probability %(z). The most common approach to XPS quantitation is the use of an expression derived from equation (20) for the ratio of intensities of detected photoelectron current from two different elements A and B.

(23)

Most of the time this equation is used in the literature with no consideration of the very restrictive hypotheses under which it is derived from equation (20):

204 IAs

-

'=

fz:0 d IAs

(24)

fZo d ,=

Thus assuming that K depends only on the geometry of the spectrometer and the cross sections (rAn are isotropic

I~ _ o~ D~ f : CA(Z) exp(-z/X~ sin O) dz I~

om

(25)

D~ fo Ca(z) exp(-z/X= sin O) dz

Therefore it is mathematically incorrect to derive equation (23) from equation (25) in any other case but

CA(Z) = CAo

Vz

and

CB(Z) = CBo

'V'Z

In this case however this derivation is straightforward noting that

f:

exp

(-

z ,~ sin

0

)dz=XsinO

(26)

Thus equation (23) is only valid for solids in which both elements A and B are uniformly distributed in the volume sensed by the XPS technique, namely over a depth of 2 to 3 times Z, below the sample surface. Due to the highly non-linear variation of the (po(z) probability of escape this condition is very stringent and met in only very restricted cases. In particular with structurally complicated solids such as supported catalysts the use of equation (23) may yield very significant errors. A simple necessary but not sufficient condition for the validity of equation (23) is that the CAo/CBo value calculated from the XPS intensity ratio must be equal to the bulk value

CA/Cs. 3.

XPS.OF ZEOLITES

In the early days of XPS the first applications of the technique to zeolites were dealing with the determination of Si/A1 ratios calculated using equation (23) and their comparison with bulk values [ 14-16]. This of course allowed to detect important compositional gradients in the surface region, a piece of information which is related to the mechanism of zeolite synthesis and which is technically important in order to monitor the concentration of Br0nsted acid sites on the external surface of the zeolite crystals. The measurement of

205 surface (Si/AI) ratio was also applied to detect composition differences among the various particles of a zeolite sample [17]. Figure 7 is showing the results of a study of 60O Na" exchanged ~leaching of Na-ZSM-5 zeolites by refluxing with 0.5 M r~ H* exchanged t HC1 solution [18]. The H + exchanged forms were 50found to be depleted in aluminium in the surface region. A similar depletion in surface aluminium 40content was also reported upon calcination of NH 4 forms of zeolites in air [19]. Analogous studies have been conducted using not only XPS [20,21] but also | 30AES [22], SIMS or FABMS [23] and even electron micropobe with exceptionally large crystals [24]. 2(1In a similar study Corma et al. [25] found that a sample of HY repeatedly dealuminated by deep bed calcination at 550~ showed an A1 rich surface with a two component k12p peak. Thus these authors were framework o ~ , , , , able to discriminate between o lo 2o 3o 4o (E B = 75.0 eV) and extraframework aluminium Su,kS~ (E B = 73.8 eV). Figure 7. Effects of acid leaching on Na-ZSM-5 [18]. The arrows join the Na + and acid leached (H § forms of the same samples. Figure 8 shows the variation of Si2p A12p and 534 Ols binding energies for a series of sodium exchanged zeolites which includes NaA (Si/A1 = 1.0), NaX (Si/AI s3z = 1.25), NaY (Si/A1 = 2.52), NaL (Si/A1 = 3.0), NaO 532 mordenite (Si/A1 = 5.1) and Na-ZSM-5 (Si/A1 - 4 0 ) [26]. These data are in good agreement with those 531 reported by Barr and Lishka [27], Okamoto et al. [28] 104 and more recently by Grtinert et al. [29]. The same trend was also observed for Nals binding energies / ~ o"> 103 although on a more narrow range. Stoch et al. [30] o. published also a set of XPS binding energies of r 102 alumino silicates. There reported trends for Si2p and 101 Ols binding energies agree essentially with the ones 76 displayed on Figure 8 but they show an opposite trend A ..13-'" ...... [2 for the Al2p values which would decrease with an >. 7s f increased Si/A1. As discussed in reference [29] these data are not allowing the fight conclusion because they 74 ~ contain results for both sodium and proton forms of zeolites. In addition the increasing trend is indeed 73 , , , ..... , imposed by the two extreme points, the upper value 2.0 4.0 6.0 40.0 Si/AI Ratio of Zeolites corresponding to mullite (which is not a zeolite) and Figure 8. Binding energy shifts in Na zeolites [26]. t

10-~

e~

206 the lower one to a high silica H-ZSM-5 for which the low A12pbinding energy might correspond to extraframework aluminium. Thus if we exclude this set of data all other literature data on Na-zeolites indicates that all binding energies of lattice atoms and counter ions are shifted in the same direction when the Si/A1 ratio is varied. This is a peculiar behavior if the chemical shift is to be explained by a change in the charges [AqA in equation (11)] because not all charges can increase at the same time. Therefore it is usually expected that if some elements display a positive chemical shift then some other should yield negative binding energy shifts. Barr [27,31] tried to explain this anomaly by introducing the concept of a "chemical mixture" of group clusters SiO2 and N a ~ O 2 which would play the roles of cation-anion units instead of the individual elements. Moreover introducing one cluster say SiO2 into the chemical mixture with the other (NaA102) changes the covalency/ionicity in the M-O bonds. The concept of "chemical mixture" is however not very well defined and its relation to binding energy shifts is not clear. Since the variations in AEB presented in o X zeolites Figure 8 cannot be explained by changes in the /x ZSM-5 zeolites AqA term in equation (11) the question must be 9 Y zeolites formulated in terms of wether the observed ~, 5 3 3 trends are associated to differences in the Magdelung potential AV or in f'mal state effects -= 5 3 2 AER. Both Okamoto et al. [28] and Huang et al. [26] suggested changes in the Magdelung potential as the reason, and the latter authors < 531 based their arguments on the data reported in Figures 9 and 10. Figure 9 gives Al2p, Si2p and 104.0 Ols binding ener~es for alkali exchanged X, Y >, 10,3.5and ZSM-5 zeolites. It is seen that changing c 103.0the electronegativity of the cation affects very --~ 1 0 2 . 5 little the binding energies in ZSM-5 but more significantly the ones in Y and X zeolites. In ~102.0Figure 10 some of these data are reported as a to 1 0 1 . 5 function of the charge calculated using the electronegativity equivalence method and >= electronegativity values listed by Sanderson 533 [32]. It is interesting to note that both Ols and Si2p binding energies vary linearly with the charge thus following equation (11). The two _c 5 3 2 lines corresponding to X and Y zeolites are parallel having thus the same value of k but 0 531 different values of the Magdelung potential. H ti Na b Cs v

(ID e,-

"O t-

122 o.

I

I

i

I

I

I

I

i

I

I

!

,,

lID

W

r m

''1

A

eILl

O)

"O ern

Figure 9. Variations in binding energies of zeolite framework elements with the extraframework cations [26].

207

532.5 532.0

Na .,•Li "

O-531.5

Na o

531.0 530.5 103.0

Rb

Li ~

~

Cs Cs

K

, .., , 0.32 0.34 0.36 0.38 0.40 Negative Charge on Oxygen Atom

. o X Zeolites 9YZeolites

~ co

.. LIo

~ e ~ C s 102.0 101.5

Between X and Y the average AV may thus be estimated as 0.3 eV for Ols and 0.5 eV for Si2p. In reference [29], Griinert et al. reported calculated distributions of Magdelung potentials at O, Si, Al and Na sites for models of NaX, NaY, Na-mordenite and Na-ZSM-5 zeolites. These models were constructed using the "Catalysis" software of Biosym Technologies (San Diego) with lattice energy minimization. The potentials used and their parameters are described in references [33,34]. The models still have arbitrary aspects for example in the population of the cationic sites in NaX and NaY zeolites. Nevertheless the results are reported in Figure 11 which also shows the average Magdelung potential values of each of the X, Y, MOR and ZSM-5 structures.

Ke .,~eLi

"/Na Rb R

Rb/,~

K Cs 9' ' / ! '' ! I 0.02 0.07 0.12 0.17 0.22 Positive Charge on Silicon Atom

Figure 10. Relationship between charge and binding energy [26]. Table 2 Auger parameters of Na-zeolites (eV) [29] zeolite

og(Si)=EB(Si2p) +EK(Si KLL)*

og(O)=EB(Ols) +EK(O I ~ )

og(Na)=EB(Nals) +EK('NaK L L )

NaA NaX NaY Na-mordenite Na-ZSM-5

1711.4 1711.4 1711.6 1711.8 1711.7

1039.4 1038.9 1039.5 1039.1 1039.5

2061.0 2060.8 2061.1 2060.7 2061.1

1460.3 1460.2 1460.3 1460.7/1458.3 1460.9/1458.1

Average

1711.6 __.0.2

1039 _.+0.3

2060.9 _.+0.2

1460.6 _ 0.3/ 1458.2 _* O. 1

.

.

.

.

.

c~'(A1)=EB(AI2p) +EK(A1KLL)* .

,

,

The Si KLL and A1 KLL Auger lines were excited using the bremsstrahlung of the A1 Ka and Mg Ka sources respectively. These results show that the distributions extend over a 4-6 V range with widely different distribution curves. For all four atoms the average values increase in the order X > Y > MOR > ZSM-5 which is the order of the (Si/A1) ratios. It is interesting to note that the differences in average VM values between the NaX and NaY zeolites are of the order of 0.5 V for oxygens and .1 V for silicons. These values are in fair agreement with the AV values estimated from the results in Figure 10 reported above. Moreover Griinert et al. show a convincing similarity between the shapes of the average

208 Magdelung potential as function of A1/Si and the corresponding variations in Ols, Si2p, A12p and Nals binding energies. All this suggests that the trends observed in Figure 8 for the variations in binding energies with the Si/A1 ratio are essentially associated with changes in average Magdelung potential rather than in local charge. This explains the observations that Ols, Si2p, A12p and Nals binding energies all vary in the same direction with the Si/A1 ratio. MOR a)

X

Y

I

!

~o~ o ,o

(~

MOR .SM-5

X

y ~$M-5

T ii

30 20 10

g 0 24

26

28

30

32

-54

34

-52

c)

X~l

-50

-48

VM ,V

VM ,V

MOR ~ (r"'~ZSM'5

50

X

."II

~

Na

y

MOR / ZSM-5

( r~

40

~oI

/J,J-),,' .~ ',, II

/~i 9,', 7i~ 1t-57 II I~,

I

t t l~- /..\ i| i ,~

i ~k/'J

!

i\

,oq ,,,',,//ZI\V/,.".....k -38

-37

-36 VM,V

-35

-34 -11

- 10

-9

-8

-7

-6

VM ,V

Figure 11. Distribution of Magdelung potentials Vu at the atomic of zeolite models [29]. ( e ) NaX, (O) NaY, (-) Na-mordenite, (v) Na-ZSM-5. The possibility that these changes may be associated to final state effects has been ruled out by Grtinert et al. [29] on the basis of a set of measurements of the Auger parameters o( reported in Table 2.

209

Na-A Na-X Na-Y Na-MOR Na-ZSM-5 1395

1390

1385 KE, eV

1380

In Table 2 two values are reported for tx'(A1) of the high silica Na-mordenite and NaZSM-5. These double values arise from the double component A1 KLL Auger lines observed with these two samples as shown in Figure 12. As none of the ct" values varies with the Si/A1 ratio it may be concluded from equation (17) that no change in extra-atomic relaxation energy AE~a is responsible for the variations of binding energies shown in Figure 8. The low kinetic energy Auger AI KLL component line in the Na-mordenite and NaZSM-5 samples have not being assigned. We will come back to this result when we discuss acidic sites in high silica zeolite.

Figure 12. AI KLL Auger lines of Na-zeolites [29]. All this discussion leaves very little doubt that the changes in binding energy' observed in Figure 8 are essentially dominated by changes in the Magdelung potential in other words by changes in the spatial distribution of the charges on their neighbouring atoms. These effects overcome the initial state charges effects. They explain for example why the Si2p line does discriminate Si atoms with different numbers of Al-atoms as second neighbours. The well resolved 29Si MAS NMR signals are indeed reflecting differences in NMR chemical shifts which correspond to different electron densities. They explain also why the Ols lines do not differentiate the oxygens adjacent to the various cations in a mixte counter-cations zeolite lattice. These oxygens have different Lewis base Strengths, as will be shown below, and therefore significantly different electronic charges. The narrow Ols lines observed with a partially exchanged zeolite should therefore not be reflecting uniform oxygen charge within the lattice, but variously compensated Ols binding energies due to induced differences in local values of the Magdelung potential.

4.

ACIDITY IN ZEOLITES

XPS of chemisorbed nitrogen containing basic molecules such as pyridine and ammonia have been employed to monitor the strength of acid sites in H and cationic forms of zeolites. One interesting problem is to follow the changes in concentration of the Br/3nsted and Lewis acidic centers with the temperature of thermal treatment. Two mechanisms have been proposed for the thermal degradation. Dehydroxylation occurs through a process first described by Uytterhoven et al. [35]

210

H

H

I

I

Two Br0nsted acid sites give rise to two Lewis acidic sites, namely the trigonal A1 and Si atoms so that assuming that all A1 is intra-lattice in the initial sample: n~ = B + L

(27)

where n~u, B and L are the numbers of aluminium atoms, Br0nsted and Lewis acid sites per unit cell respectively. However according to Ktihl [36] this process may be followed by framework dealumination which involves the formation of extra-lattice Lewis acid A10§ species:

+

AIO +

When this happens only one Lewis center is formed with the disappearance of two Br0nsted OH's. Then n,~ = B + 2 L

(28)

Kazansky [37] has shown that for zeolites with Si/Al < 5 thermal degradation occurs via the mechanism of Kiihl whereas the process is limited to the Uytterhoven step for higher silica zeolites. 4.1 XPS study of faujasites The early work of Defosse and Canesson [38] had shown that the Nls line of pyridine chemisorbed on NH4Y zeolites calcined at 300 and 400~ displayed a strong peak at 402.4 and a shoulder peak at 400.4 eV. These two peaks were assigned to pyridine respectively chemisorbed on Br0nsted and Lewis acid sites. Figure 13 features the Nls lines of pyridine chemisorbed on a progressively dehydroxylated HY zeolite sample. The precursor was a commercial NH4Y sample with a Si/A1 ratio of 2.33 and 1.2% residual sodium. The dehydroxylated samples were prepared by slowly heating the precursor to temperatures ranging from 300 to 700~ in air for 10 hours in a shallow bed of less than 3 mm in depth [39]. The pyridine desorption temperature was 25~ At a calcination temperature of 300~ two peaks are observed at binding energies identical to the ones reported by Defosse and Canesson. The same peak assignment was therefore adopted. The main peak at 402.4 eV is ascribed to the pyridium ion formed upon adsorption of a pyridine molecule over a protonic OH. The peak at

211 400.4 eV corresponds to pyridine chemisorbed on Lewis acid centers which in this case may be either a residual sodium counter-ion or the Lewis sites X A1 /\

or

/ *Si /\

formed in the Uytterhoven dehy-

droxylation step. Upon heating to 400~ the relative intensity of the two peaks is strongly changed and the Brt~nsted acid sites diminish whereas the Lewis acid sites increase in proportion. From 500 to 700~ one witnesses the appearance of a new Nls peak on the low binding energy side (see also Table 3) which must correspond to the inset of dealumination and formation of AIO+ type of Lewis acid sites. The change in binding energy of this peak (component 1 in Table 3b) with temperature indicates that the nature of the extraframework aluminium oxihydroxide/hydroxide phase which bears these Lewis acid sites is changing with the calcination temperature. The binding energy of this new peak was found to be very close from the main peak observed upon adsorption of pyridine on 7-A1203. This result is in line with the views expressed by Lunsford et al. [40] who concluded that A1203 is formed upon dehydroxylation of HY at 700~ The sites associated with the new Nls peak (component 1 in Table 3b) were found to be weaker acids as pyridine desorbs from them at temperatures lower than from the Lewis acid sites with Nls peak at 402.4 eV.

400

_>, _c

700 392.4

397.4

402.4

407.4

421.4

Eb/eV

Figure 13. Deconvoluted Nls XPS spectra of pyridine chemisorbed on HY zeolites. Sample calcination temperature, in ~ is indicated on the figure [39].

212 Table 3a XPS binding energies and fwmh (eV) for the HY samples of Figure 13 Calcination Temperature ~

(Si/A1) s

(N/AI) s

300 400 500 600 700

3.60 3.18 4.40 4.10 2.48

0.38 0.30 0.30 0.24 0.25

Si2~ 103.3 103.3 103.3 103.3 103.3

(2.4) (2.4) (2.4) (2.3) (2.4)

A]2p 74.8 75.0 74.8 74.8 74.7

(2.5) (2.6) (2.4) (2.4) (2.4)

Table 3 b XPS binding energies and fwmh (eV) for the HY samples of Figure 13 (continued) Calcination Temperature oC 300 400 500 600 700

N~* Oxs 532.3 532.5 532.5 532.3 532.2

(2.6) (2.7) (2.5) (2.5) (2.5)

1

2

3

----399.6 399.3 398.6

400.4 400.2 400.9 400.9 400.4

402.4 402.2 402.5 402.4 402.2

* In this early work the binding energy scale was referenced to Si2p = 103.3 eV ** fwmh = 2.4 eV for all three components It is seen from the data in Table 3 that the (Si/A1) s ratio is higher than the bulk value (2.33). This suggests an aluminium depleted surface region. The low values observed for the ratio (N/A1) s reflect the fact that only part of the Br0nsted acid sites are accessible to pyridine. The pyridine molecule kinetic diameter of 5.9 A does not allow it to enter the sodalite cages with 2.2 A openings. Thus only the acid sites protruding in the supercages can chemisorb pyridine. The number of these molecules is estimated to be 24 per unit cell [41] and since the Y zeolite with a Si/A1 ratio of 2.33 has 57 A1 atoms per unit cell, the maximum N/AI ratio is 0.42. This value is reasonably close to the 0.38 value obtained after calcination at 300~ In a recent paper, Guimon et al. [42] advocate the use of NH 3 as a probe molecule. Interestingly working with commercial NH4Y samples they also find two peaks at 402.7 and 401.2 eV (referencing to Cls = 284.6) when the samples are calcined at 400~ and a third one close to 399 eV when the calcination temperature is 700~ The N/A1 ratio never reaches the value of 1, even at a desorption temperature of 100~ in spite of the fact that ammonia is a stronger base than pyridine.

213 Comparing the high binding energies peaks obtained when adsorbing ammonia and pyridine, it could be concluded that the Br/Snsted acid sites located close to the six oxygen ring of the sodalite cage are as strong acids as the ones in the supercages. 4.2

r

E" i.. v

(D r

8

396.0 400.5 405.0 Binding energy (eV~

High silica zeolites Figure 14 gives the Nls lines obtained with five different samples of H-ZSM-5 zeolites [43]. For their preparation, the sodium precursors were ftrst calcined at 500~ in air for 10 hours. Then the calcined samples were converted to ammonium form by repeated ion exchange with 1 M ammonium nitrate solutions. The protonic form was obtained by air calcination at 500~ for 10 hours and the residual sodium content measured by atomic absorption was less than 0.02%. Sample B was produced by acid leaching sample A with a 0.1 N solution of HC1 at room temperature. Table 4 gives some of the bulk and surface properties of these samples. Sample A shows a 25% difference in (Si/A1) ratio between the bulk and the surface indicating that A1 is not uniformly distributed throughout the zeolite grains. Moreover sample A has a relatively low BET surface area and a low pyridine uptake (at 200~ indicated by its rather low value of (N/A1)B. This suggests the presence of extraframework amorphous and non-acidic AI oxidic phase. Upon acid leaching of sample A the obtained sample B shows increased values of (Si/A1) which indicate extraction of some of the aluminium specially in the surface region. The BET surface area however has not reached the values of samples C, D and E and the pyridine uptake is not significantly increased even though the surface (N/A1) ratio is substantially increased. This suggests only partial extraction of the A1 containing phase upon acid leaching and a preferential extraction in the external surface region of the zeolite crystals.

Figure 14. Deconvoluted Nls XPS spectra of pyridine chemisorbed on H-ZSM-5 zeolites. Sample designation as in Table 4 [43].

214 Table 4 Bulk and surface properties of H-ZSM-5 samples Sample

(Si/A1)B

(Si/A1)s

(N/A1)B

(N/A1)s

So* m2/g

A B C D E

19.9 24.4 39.0 45.9 65.0

14.3 21.4 37.2 47.4 36.6

0.52 0.53 0.85 0.91 0.92

0.37 0.62 0.92 1.07 0.83

362 392 432 422 445

* So: Nitrogen BET surface area Samples C and D are different from sample A. Their (Si/A1)B and (Si/A1)s values are equal within 5% suggesting a uniform spatial distribution of A1 atoms. Their BET surface areas are very close to the known values for ZSM-5 suggesting absence of pore blockage by extraneous material which is confirmed by their high pyridine uptake. The (N/M) ratios both in the bulk and on the surface are close to 1 indicating that every A1 atom is associated with one acidic center (able to adsorb one pyridine molecule). Thus these two samples are specially pure and uniform, and they may be used as model solids for further study. Sample E seems also quite free of extra-lattice phase as its BET surface area is high and its (N/AI) values are both reasonably close to 1. However it (Si/A1)s value is 30% below its bulk value indicating a non-uniform distribution of its A1 lattice atoms.

.O i"r0 0 r 11) u) e~

o

2000c 390 395 400 405 410 Binding energy (eV)

4.2.1 Add strength and Nls binding energies Sample C was used in the study reported in Figure 15 and Table 5. The pyridine desorption temperature was varied from 50 to 400~ These data show that the high binding energy peak corresponds to a site which adsorbs pyridine much more strongly than the other two. The almost constant value observed for (Si/A1)s indicates that the desorption process has not affected the lattice and the constant [N(3)/A1]s ratio indicates that at 400~ pyridine has not desorbed from the sites responsible for the high binding energy component of the Nls signal. The other two sites are obviously weaker acids. Thus once again it is found that the Nls binding energies are in the order of acid strengths. In this respect it is interesting to note that from the Nls binding energies of chemisorbed pyridine it was indeed found that the BrOnsted acid sites are stronger in A1-ZSM-22 and Fe-ZSM-22 [44,45] and weaker in Fe-ZSM-5 and BZSM-5 [46] than in AI-ZSM-5.

Figure 15. Deconvoluted Nxs XPS spectra of pyridine chemisorbed on sample C. Pyridine desorption temperatures, in ~ are indicated on the figure [43].

215 Table 5 Effect of pyridine desorption temperature on some properties of sample C Pyridine desorption T~

XPS

Relative intensity of Nls components %

[N(3)/A1] s

,

(Si/A1)s

(N/A1)s

1

2

3

36.7 33.3 37.2 38.4 39.6

1.10 1.19 0.93 0.56 0.55

19.5 21.0 24.4 9.5 7.7

46.5 48.0 47.6 26.0 29.5

34.0 31.0 28.0 64.5 62.8

50 140 200 300 400

.

.

.

.

.

.

.

.

0.374 0.369 0.260 0.361 0.345

.

4.2.2 Nls peak assignment The assignment of the three component peaks in Nls spectra of H-ZSM-5 samples in Figures 14 and 15 is different from the one discussed in section 4.1 for faujasites. This assignment was made after a careful IR study of pyridine chemisorbed on the same samples. Table 6 reports the relative intensities of the three Nls component peaks for the samples of Figure 14 and Table 4. Table 6 Relative intensities and B/I, ratio for samples A-E. (Calcination temperature 500~ desorption temperature 200~

pyridine

,

Sample

A B C D E

Nls relative intensity %

1

2

3

14.7 26.9 24.4 18.3 26.7

46.4 52.6 47.6 65.4 48.5

38.9 20.5 28.0 16.3 24.8

(B/L)rR

(B/L)sl

(B/L)s2

4.5 2.7 3.7 4.7 4.7

0.64 0.26 0.39 0.19 0.33

5.8 2.7 3.1 4.6 2.7

In Table 6 the Br6nsted to Lewis (B/L) ratio is first estimated from the ratio of absorbances of IR lines at 1545 and 1455 cm -1" (l~L)l R - As eL AL ~.

(29)

The ratio of extinction coefficients I~L/EB was given the value 1.5 as suggested by Rhee Then (B/L) ratios were calculated from the relative intensity ratios of the three component Nls peaks, under two different hypotheses. First (B/L)sl was calculated assuming that peak 2 corresponded to a Lewis acid site and then et al. [47] for high silica zeolites.

216 (B/L)s2 under the assumption that peak 2 was for a BrOnsted acid site. Obviously the first hypothesis is very far from being verified whereas the second hypothesis yields B/L estimates in good agreement with the IR data, specially for samples B, C and D. These three samples are the ones which were found to have surface properties more representative of the bulk in the discussion of Table 4. Thus from these data it may be safely concluded that the three peaks in Figures 14 and 15 are associated to one kind of Lewis acid site (binding energy 398 __. 0.3 eV) and two kinds of Br/3nsted acid sites, one being rather weak (BE = 400.0 _.+0.3 eV) and the other one relatively strong (BE = 401.8 -4-_0.3 eV). Several authors have discussed the existence of two kinds of Br/3nsted acid sites in high silica zeolites. For example Datka and Tuznik [48] measured the desorption of pyridine from H-ZSM-5 at various temperatures while monitoring the changes in intensity of the 3610 cm "1 and the 1545 and 1455 cm "1 bands. They found that a certain amount of pyridinium ion was decomposed after the disappearance of the 1455 cm "1 band and before the reappearance of the 3610 cm 1. They ascribed this amount to pyridine chemisorbed on weak BrSnsted acid sites. From the spectra in Figure 14 it is shown that the component 2 corresponding to weak Br~3nsted acid centers is dominant in all five spectra. It seems thus that XPS is specially sensitive to the presence of these sites compared to bulk techniques such as IR spectroscopy. This suggests that the weak Brt~nsted sites may be surface species. One possibility [49] is that these OH's would be adjacent to a surface silanol:

H

I

0

X /

OH

X/

A1

/X

Si

/

X

Their total concentration would therefore be essentially proportional to the external surface area of the crystals. Similar surface AI atoms may be responsible for the low kinetic energy Auger peaks observed in Figure 12 for high silica zeolites. In this case the sodium cation instead of the proton would be adjacent to the silanol group.

4.2.3 Dehydroxylation process It should be noted that whenever equation (27) applies and dehydroxylation happens solely through the Uytterhoven step, assuming the stoechiometry of pyridine chemisorption is one molecule adsorbed per either BrOnsted or Lewis acid site, then the N/A1 ratio must be equal to one. If however dealumination follows dehydroxylation and equation (28) applies, then the N/A1 is given by:

217 N/L = 1 * B/L 2+B/L

(30)

Table 7 reports some data obtained for the sample D of Table 4 and Figure 14, calcined at increasingly high temperature. Table 7 Effect of dehydroxylation on B/L and N/L ratios of sample D Calcination Temperature

(Si/A1)B

(Si/A1)s

(B/L)IR

(B/L) s

oC 500 675 800 950

(N/AL) s XPS

45.9 45.9 45.9 45.9

47.4 40.1 43.4 41.9

4.71 1.17 0.46 0.49

4.60 0.65 0.46 0.44 ,

1.07 1.04 1.18 0.87

eq. (30)* 0.85 0.62 0.59 0.59

,

* Calculated from equation (30) using the XPS (B/L) s values It is seen from the minor variation of the (Si/Al)s ratio that the migration of hl atoms is rather minor. Since, however, the fB/L)m ratio decreases dehydroxylation is significant. The rather good agreement between (B/L)s and (B/L) m confwms the peak assignment. It is seen that the XPS (N/A1)s average value is equal to 1.03 _+ 0.15 and that the predicted (N/A1)s values calculated by substituting the (B/L) s values in equation (30) are clearly farther from the XPS values. It is thus concluded that the dealumination process of Ktihl [36] leading to equation (28) did not happen in these samples in agreement with the conclusions of Kazansky [37].

5.

BASICITY IN ZEOLITES

In ion exchanged zeolites, the counter cation has electron pair acceptor (EPA) capacity and is therefore a Lewis acid. The lattice plays the role of the conjugated base and has electron pair donor (EPD) properties. Vinek et aL [50] have discussed the relationship between EPA-EPD strength and the catalytic activity of the cation-anion site as well as the mechanism of elimination reactions. They also proposed to measure the EPD strength (or Lewis base strength) of oxides including MgNaX and MgNaY zeolites by their Ols binding energy. As will be shown below this is not a very useful method essentially because it is not sensitive enough. We have recently studied systematically the Lewis basicity of zeolites using pyrrole as a probe molecule. Pyrrole has H-donor properties and adsorbs on basic zeolites forming NH---O bridges. The NH stretching vibration was shown by Barthomeuf [51-53] and others [54] to be a measure of the Lewis base strength of 02. in zeolites.

218 Lavalley has recently compared the suitability of various IR-probe molecules for the surface basicity of oxides [55] and found that pyrrole (PYH) which would decompose to pyrrolate ions (PY') over stronger bases is suitable for zeolites [56]. We have used pyrrole as an XPS probe molecule and monitored the Nls line of this molecule chemisorbed on alkali exchanged X and Y zeolites [26,57]. We also studied this adsorbed species on the same samples by IR [58] and microcalorimetry [59]. A similar study was also performed using chloroform as the probe molecule in IR [60] and XPS [61]. Figure 16 shows the Nls lines of pyrrole chemisorbed on NaX and partially exchanged LiX, KX, RbX and CsX [57]. All samples contain thus Na § counter-ions and it is interesting to note that the peak which dominates the NaX spectrum is present in all of these spectra. These results are also represented in Table 8. Table 8 Nls binding energies for pyrrole chemisorbed on alkali exchanged X zeolites [57] Sample

Cationic composition

Nls binding* energies, eV 1

Relative intensity %

2

3

1

2

3

LiX NaX KX

Li543Na31.1 399.7 Nass.4 399.8 K48_3Na37.1 399.8

400.3 --399.1

401.5 401.5 401.5

37.3 87.0 44.0

50.4 --43.7

12.3 13.0 12.3

RbX CsX-2 CsX-3

Rb37.TNa47.7 399.8 Cs2s.gNa56.6 399.7 Cs59.gNa23.5 399.8

398.7 398.3 398.3

401.6 401.5 401.6

46.3 51.3 45.0

37.1 41.0 51.6

16.6 7.7 3.4

They show that component No 2 decreases regularly in binding energy from Li-X to CsX. A third minor peak at 401.5 eV is believed to belong to secondary adsorption of pyrrole molecules. Our previous IR-study of the same samples had also allowed to detect a common band in the NH stretching vibration region at 3280 cm "l and a second band characteristic of the second cation and ranoino from 3295 cm 1 for LiX to 3175 cm 1 for CsX. A minor component at 3375 cm "1 ascribed to secondary adsorption was also present in all spectra. The results reported in Figure 17 are the measured distributions of differential heats of adsorption of pyrrole on the same solids. Again it is found that the only peak in the distribution for NaX (117 kJ/mol) is present in the curves of KX, RbX and CsX and shifted to 123 kJ/mol in LiX. The second component of these diagrams varies regularly with the cation from 110 kJ/mol on LiX to 146 kJ/mol for CsX. The correlation between the three kinds of data is shown in Figure 18 which shows that the cation characteristic Nls binding energy (component 2) is linearly correlated to the corresponding NH stretching vibration and heat of adsorption of pyrrole.

219

117 1

123

I_iX

NaX

126

0

KX

R L.

Figure 17. Distribution of differential heats of pyrrole adsorption on the samples of Figure 16 [59].

Figure 16. Deconvoluted N~s XPS spectra of pyrrole chemisorbed on a/LiNaX; b/NaX; c/KNaX; d/RbNaX: e/and f./CsNaX [57]. x

'-E 3350 o

o

~Xx~

~'~ o

x~ z

X

"i

o

145 I

to tO

O

E

c~r

,-

9

50 100 150 Differential Heat of Pyrrole Adsorption, Q (kJ/mol)

Binding Energy

A

i

RbX

125 ~

3250-

E O

Ne _q

= 3150 "r" 398 Z

399 4~) Nls Binding Energy (eV)

, 105 401

Figure 18. Relationship between Nls binding energies, NH stretching frequencies and Qmax of chemisorbed pyrrole [59].

220 It seems thus that whenever two counter-ions are present in the same sample the oxygens adjacent to the two cations have different basic strengths and therefore different electron charges. This means that the basic strength is not a collective property of the zeolite lattice but rather a local property. This is in contrast with the Oxs peaks of these zeolites which show almost no increase in the peak width (fwhm) between NaX (2.3 eV) at mixte CsNaX (2.6 eV). This small change does not allow to deconvolute two Oxs component peaks in partially exchanged CsX. Thus the use of the Ols binding energy is less appropriate for the characterization of basic strength. The reason it best seen from Figure 19 which shows the values of Ols, Sizp and A12p binding energies for ion exchanged X and Y zeolites as a function of the Nls binding energy of pyrrole chemisorbed on the oxygen adjacent to the main cation. It is seen from the curves that the Nls binding energy varies from 398.3 eV for CsX to 401.2 eV for LiY, a range of close to 3 eV. The variation ranges for Ols, Si2r, and Al2p for the same samples are respectively 1.4, 1.3 and 1.4 eV. The pyrrole Nls binding energy is therefore a much more sensitive probe of the changes in base strength than the Ols, Si2p and A12p binding energies of the lattice O, Si and A1 atoms. 532.50

O

K Rb,~O .... Cs .-'~

531.50 -

530.50 -

= CS

Ha

.... c, ......

K

Na

101.50

Li

w

F~b

Cs -~176 a~176176176176

10~50

o~176

.......

'U -o

Li

Cs LJ

Na

Rb

74.50

K

_~.....oo'"

o,O o

.~

~176176

<

C

73.50

~

Na

398 Nls Binding Energy (eV)

Figure 19. Changes in binding energies (Nls of pyrrole, 01s, Si2p and A12pof zeolites) depending on the alkali cation. Solid lines: X zeolites; dashed lines: Y zeolites [57].

221 One of the consequences of the existence of oxygens with different basic strengths adjacent to different cations is that in the calculation of charges by the electronegativity equivalence method (see section 3) the various cationic environments must be differentiated. Because cation in the supercages are located near the six oxygen ring, the local composition after pyrrole adsorption is depicted as Si6.nAInO12Mn(C4Hsb0n where M is a monovalent cation and (6-n)/n is the bulk (Si/A1) ratio. This formula was used to calculate atomic charges from Sanderson electronegativities. Figure 20 shows the Nls binding energies of chemisorbed pyrrole (component 2 in Table 8) as a function of the electronic charge on the nitrogen atom. Here again two straight lines are observed for the alkali exchanged X and Y zeolites. These lines have the same slope (-41.6 V) but different ordinates to the origin. From equation (11) this indicates different values of the average Magdelung energy terms in X and Y zeolites. We believe that the charge on nitrogen atom calculated in this manner, is however a good correlating parameter for basicity because as can be seen on Figure 21 the NH stretching frequency represented as a function of this calculated charge yields one single curve for the X and Y zeolites data. A

c X Zeolites 9

401 Z

400

9Y Zeolites

E

Na

>

~ Na

3 400 3300-

KLiNa Rb ~ ~ ~ . ~ C

"~ 3 200 o i

i

i

i

0.18 0.20 0.22 0.24 0.26 Negative Charge on Nitrogen Atom

Figure 20. Relationship between nitrogen electronic charge and Nls binding energy of chemisorbed pyrrole [26].

6.

.o

o X Zeolites 9Y Zeolites

.-s

399 398

-'E 3500

co 3100

"r" Z

O. 8

0.'~0

0.:22. 0.24

s

0.:26

Negative Charge on Nitrogen Atom

Figure 21. Relationship between nitrogen electronic charge and NH stretching frequency of chemisorbed pyrrole [26].

ACTIVE CENTERS IN ZEOLITIC OXIDATION CATALYSTS

The discovery in the early 80's of titanium silicalites [62-64] opened the new application perspective of zeolitic materials as oxidation catalysts. Several reactions of partial oxidation of organic reactants using dilute solutions of hydrogen peroxide could for the first time be performed selectively in very mild conditions. Other elements inserted in the lattice of silicalites have since been shown to have similarly interesting catalytic properties including, vanadium, zirconium, chromium and more recently tin and arsenic [65]. Titanium silicalites with both MFI (TS-1) and MEL (TS-2) structures have however been the object of more attention and they still seem to display unmatched properties. Indeed some of these reactions like the oxyfunctionalization of alkanes [66-69] by H20 2 are not activated by other Ti containing catalysts (with the exception of Ti-A1-Beta [70]). The same situation

222 applies to several other reactions such as aromatics and phenols hydroxylation [62,64b,7174], alcohols oxidation [67,75,76], ketones ammoximation [73,77,78], synthesis of hydrazine [79]. By contrast the epoxidation of olef'ms by H20 2 (and organic peroxides) which is catalyzed by TS-1 and TS-2 [80-83] is also catalyzed by other Ti containing solids including Ti-AI-Beta [84,85], Ti-MCM-41 [86] and mixed TiO2-SiO2 oxides such as the ones described in [87,88]. These exceptional catalytic properties of titanium silicalites have triggered much analytical work aiming at the characterization of the environment of the active titanium site in these lattices. From the very beginning of these efforts, it had been recognized that the Ti2p lines of TS-1 contain a very unusual component doublet with binding energies significantly higher than the one of Ti in TiO 2 [89]. Table 9 shows the Ti2p 3n binding energies of a series of calcined (500~ TS-2 catalysts before and after acid leaching at room temperature with IN HC1 solution followed by another calcination at 500~ [90]. Table 9 Ti2p 3/2 binding energies (eV) and percent peak area of the two component peaks for TS-2 catalysts before and after acid leaching. Acid leached

As prepared Sample

Ti2p 3/2a

Ti2p 3/2b

Ti2p 3/2a

TiO 2 1.6 TS-2 4.2 TS-2 6.4 TS-2 9.1 TS-2 TiSiG c

458.3 (100) --457.1 (71) 457.8 (97) 458.3 (100) ---

--459.8 (100) 459.8 (29) 460.5 (3) --459.8 (100)

--458.0 (19) 458.4 (76) 458.3 (81)

Ti2p 3/2b 460.2 459.9 460.2 460.1

(100) (81) (24) (19)

a/high coordination site; b/low coordination site; c/Ti-Si glass. binding energy scale referenced to Si2p = 103.4 eV. The sample designation indicates the bulk value of the Ti/Ti + Si ratio expressed in %. It is seen that the 1.6 TS-2 sample does not show any line at 458.3 eV, which is the binding energy value in TiO 2. Our peak appears instead at 459.8 eV which is precisely the value obtained for Ti in Titanium Silicon glasses where titanium is known to be in tetrahedral coordination. A similar value was reported by Mukhopadyay and Garofalfiai [91] for tetrahedral Ti in the same kind of glass. The high binding energy Ti2p 3r~ peak was therefore ascribed to tetrahedral titanium in TS-2. It is seen from the data in Table 9 that the percentage area of this low coordination titanium peak increases upon acid leaching. This corresponds to the extraction of extraframework octahedral Ti species. The small upward change in Ti2p 3/2 binding energy observed after leaching was later shown to be associated with the desorption of Na§ ions from the surface. It was also found that pyridine adsorption on TS-2 led to a 0.7- 0.8 eV decrease in the Tip 3/2 binding energies. The Nls line of

223 chemisorbed pyridine was 399.0 eV which according to the discussion in section 4.2.2, should be assigned to pyridine chemisorbed on Lewis acid simms. This result was confirmed by IR analysis and thermodesorption of pyridine which showed the presence of very weak Lewis acid simmsin titanium silicalites [92]. The assignment of the peak with Ti2p 3t2 binding energy close to 460 eV, to tetrahedral titanium was confirmed by the spectra reported in Figures 22A and B [93]. Figure 22A shows the Ti2p photolines of the four TS-2 samples in Table 9, and Figure 22B the XANES spectra for the same hydrated samples. Figure 22B also gives the XANES spectrum for a TiO2-SiO2 glass which is a standard for tetrahedral Ti and for rutile TiO 2 in which Ti is indeed in octahedral coordination. The XANES data confirm the presence of the two coordinations of Ti in TS-2 samples, with tetrahedral species dominating in the sample with lower Ti loading. The interpretation of XANES data is also discussed in [94]. '

1

'

I

~

i

i

'

5

f 8

'

I

'

2

450.5 459.8 469.1 Binding energy (eV) A

'

I ' I'. -1 2

i ' 11 14

Energy (eV) B

Figure 22A. Ti2p photolines for TS-2 samples with 1.6, 4.2, 6.4 and 9.1 Ti / Ti + Si %. Figure 22B. XANES pre-edge features at Ti K edge. Top: tetmhedral Ti in a TiO2-SiO2 glass, from top to bottom 1.6, 4.2, 6.4 and 9.1 TS-2; bottom: octahedral Ti in rutile [93]. In reference [92] it was also shown that the Ols lines of TS-2 samples with a main peak at 532.7 eV for the regular lattice oxygens show a minor component at 529.7 eV whenever the loading is increased and octahedral Ti species appear. Similar values (533.2 and 530.1 eV referenced to Cls = 285.0 eV) were reported by Grohman et al. [95]. These authors also confirmed our Ti2p peak assignment as well as the effect of acid leaching. The capacity to detect tetrahedral Ti species from their Ti2p 3/2 lines was used recently by Bonneviot et al. [96] to establish the substitutional limit for Ti insertion in silicalite-1. The results are shown in Figures 23A and B. Figure 23A shows a series of Ti2p photolines for anatase and TS-1 samples having Ti/Ti + Si of 1.5, 2.4 and 4.6% respectively. Here again the tetrahedral line with Ti2p 3/2 binding energy at 459.8 eV is clearly resolved from the one of octahedral titanium at 457.8 eV. In Figure 23B the fraction of tetrahedral Ti

224 calculated from the XPS surface area ratio is plotted against the bulk Ti/Si ratio. The curve is compared to the one of tetrahedral Ti ratio estimated for the same samples (as well as other TS-1 samples) from the linear fit of Ti-K-edge XANES spectra following the method described in reference [97]. It is seen from Figure 23B that both methods yield a substitutional limit not exceeding 1.5 - 1.7% for Ti/Si. ........

. . -

Anatas

, .

.

~

.

.

.

5 400

i= 1 380~o<

8o

[2.41m

~

38o ~, i-,

-"~" 9 ............... 450 455 460 465 Energy (eV)

470

A

20 [ ' 0

! ' 1, 1

t,

!, I ~ 2 3 Total TVSi (%)

i,

I., 4

,

5

5340

B

Figure 23A. Ti2p photolines for anatase and TS-1 samples with 1.5, 2.4 and 4.6 Ti/Ti+Si %. Figure 23B. Fraction of tetrahedral TI in TS-1 samples estimated from (A) linear fit of XANES at Ti K-edge, (o) Ti2p XPS photolines; (13) unit cell volume [96]. The high binding energy Ti2p 3r2 peak in TS-1 and TS-2 is raising a very fundamental question because the upward shift with respect to Ti in TiO 2 is of the order of 2 eV. In the usual interpretation of XPS data such an increase would be associated with an increase of two of the oxidation number. Clearly here the difference in Ti coordination involves a very significant shift not associated with a difference in electronic charge. Again here a difference in Magdelung potential or a difference in extra-atomic relaxation energy must be playing an important role. In this case the AE~a term may be estimated using the expression given by Moretti [98] for the relaxation energy: E ~ (eV) - 14.4

2

no=

R'(1 * Dr

3)

(31)

Using this formula with an oxygen polarizability (x(O2") = 2.4 A 3 and values of the D parameter reported in reference [98] the calculated AE~a value is lower than 0.1 eV. It seems thus that here again differences in Magdelung potential between the Ti sites in TiO 2 and titanium silicalites are responsible for the significant difference observed in Ti2v binding energy. In reference [88] Stakheev et al. made a systematic XPS-Auger analysis of TiO2-SiO2 mixed oxides with TiO2 content ranging from 3 to 97 wt%. Up to 10% these solids contain essentially tetrahedral titanium and a Ti2p 3/2 binding energy of 460.0 eV is observed whereas this energy decreases down to 458.8 eV at 97% TiO 2. However the binding energy scale

225 was referenced to Cls = 285.0 eV which is obviously to high. Taking Cls = 284.6 eV will bring the 97% sample binding energy to 458.4 eV which is within 0.1 eV of the value adopted in Table 9. With this reference the value for tetrahedral Ti in these solids becomes 459.6 eV which is significantly (by 0.4 - 0.6 eV) lower than the one in Ti-silicalites. Similarly Table 10 reports some XPS data recorded in our laboratory for TAPO-5, TAPO- 11, TAPO- 17 and TAPO-36. Table 10 XPS binding energies for some titanium-aluminophosphates Bulk composition

Sample

XPS surface composition

Binding energies (fwmh) eV ,t,

,

AI2p TAPO-5 TAPO-11 TAPO-17 TAPO-36

A]O.53Po.45Tio.o2 A]o.46Po.52Tio.02 AIo.56Po.41Tio.03 AIo.46Po.47To.07 A]o.45Po.51Tio.04 A10.48P0.48Ti0.04 AIo.58P0.38Tio.04 Alo.49Po.50Tio.01

,,,

,.

P2P

Ti2p

75.3 (2.6) 134.7 (2.4) 458.8 (1.8) 74.8 (2.6) 134.5 (2.5) 458.4 (2.4) 75.5 (2.3) 135.1 (2.4) 458.6 (1.7) 74.9 (2.9) 134.7 (2.9) 459.0 (2.5) .

.

Binding energy referenced to Cls = 284.6 eV It is clear that the Ti2a 3/2 binding energy in these solids is 458.8 _.+0.2 eV which is significantly lower than for tetrahedral Ti in TiO2-SiO2 mixte oxides and than Ti in titanium silicalites. From these few data it appears that these binding energy values are correlated with the activity of the catalyst in oxidation reactions since the TAPO's with the lowest binding energy are not active in epoxidation, whereas the gels with tetrahedral Ti are active in these reactions but not in alkane oxyfunctionalization. Titanium silicalites which are active in both reactions have the highest Ti2p binding energy. This may be in line with the high Magdelung energy which means in this case a higher electron density on the oxygens coordinating the titanium atom. This may be the particular role of the silicalite lattice which by being an insulator would maintain the charge induced by the polarizing action of the Ti ion, on the TiO4 tetrahedra. This charge would be higher on each oxygen for a tetrahedral than for an octahedral environment.

7.

CONCLUSION

The choice of topics dealt with in this text reflects essentially the interests and experience of the author. It encompassed the applications of XPS and Auger spectroscopies to the elements constituting the zeolite lattice, to counter-ions and to probe molecules. This has left aside the very large applications of surface sciences to materials supported or occluded in the zeolitic pore lattice. These materials include highly dispersed metallic panicles, timely spread oxidic phases, entrapped organo-metaUic complexes or metallic clusters. To some extent however the analysis of the supported phase is not specific of the

226 zeolitic support and the problems of applying surface analysis techniques are similar with catalysts supported on more traditional materials such a silica gels or transition aluminas. The surface analysis of such systems have been already reviewed in an encyclopedic manner [16,99]. One of the most difficult aspects of these applications has to do with the very restricted depth of analysis as discussed in section 2.2 in relation with Figure 6. XPS intensity is extremely sensitive to both the size and the spatial distribution of the supported material present in the outermost layer of the zeolite crystal. Therefore even the qualitative analysis of binding energies may not be relevant to the catalytically important species in the rather frequent situation where part of the supported phase is surface segregated. A quick test of this is obtained from the comparison of bulk and surface atomic ratios as was done for example in Tables 4 and 7. In case surface segregation is present, the quantitation of XPS intensity ratios, although difficult, must be performed before the qualitative analysis of binding energies is interpreted. This underlines the interest of a model we developed by extending the well known model which Kerldaof and Moulijn [ 100] obtained by introducing a monodispersed population of particles into the sheet stacking model of Defosse et al. [ 101]. Then a bidispersed population was considered, with the large size mode being surface segregated [102,103]. This model was applied to several zeolite supported catalysts [102,104]. In the present text we have focussed our attention on the elements constitutive of the zeolite lattice and on the characterization of the sites which make zeolites important catalysts. These are the Br~nsted and Lewis acid sites, the Lewis base sites and the oxidoreduction sites exemplified by the isolated tetrahedral titanium atoms in silicalite lattices. From these studies a unified picture of the special features of zeolites which control the binding energies of their constitutive elements emerges. From the discussion in section 3, it is established that the Ols, Si2p and A12p binding energies changes with Si/A1 in Nazeolites are associated with changes in the average Magdelung potential, or with changes in the spatial distribution of charges on the neighbouring atoms in the lattice. Interestingly it seems that the Lewis base sites discussed in section 5 get their basic strength from the local distribution of changes around the alkali counter-ion. It seems that the lattice even at low Si/AI ratio is able to maintain ox3~gen electronic densities which are different in the different cationic environments. It is also the conclusion of section 6 that the spectacular properties of titanium silicalites may be associated with the capacity of the silicalite lattice to maintain high electron densities on the oxygen atoms which surround the tetrahedral titanium atom. Again the local characteristics of the zeolite lattice are emphasized rather than the collective ones and the XPS analysis is sensitive to these different local situations in the same lattice. Another interesting aspect lies in the capacity of nitrogen containing molecules such as basic NH3 and pyridine and acidic (amphotenic) pyrrole, to probe respectively acidic and basic sites in zeolites. This allows among other things to monitor acidic and basic sites in the outer surface of the crystals, a region in which the control of the density and strength of such sites is often critical for catalysts selectivity and deactivation. A unexpected outcome was the discovery discussed in section 4 that the surface region of high silica zeolites bear Br0nsted acid sites of lower strength than the regular sites in the bulk.

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H. Chon, S.I. WOO and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All rights reserved.

231

Computational Approaches in Zeolite Structural Chemistry J. M. N e w s a m

Biosym/MSI, 9685 Scranton Road, San Diego CA 92121 Computer simulation impacts almost all branches of zeolite research. This orientational overview touches applications to zeolite structural characterization and to the prediction of various properties, particularly those associated with sorption, focusing on the issues associated with applying modeling and simulation effectively in these areas. A small number of specific examples are used to illustrate particular points and opportunity areas for progress over the next five years are identified. 1. ROT,F.S OF SIMULATION

Computer simulation impacts almost all branches of zeolite research [1-5]. It is worth repeating this simple statement as it underscores the challenge of attempting to present an instructional overview of a very broad and diverse topic. This particular overview is heavily weighted towards the author's recent work and interests. It is necessarily subjective in its coverage. This subjectivity dictates that m a n y areas and m a n y individual seminal papers are not addressed, referenced or cited. A rather narrow view of the utility of computer modeling methods in zeolite research is not uncommon. It is a relatively new science, perhaps only now emerging solidly as sufficiently mature for widespread and general application. However, atomistic modeling can impact the spectrum of zeolite research from the point of initial phase identification to the interpretation of catalytic performance. The sophistication of modeling methods can advance in consort with the amount that is known, on an atomic scale, about the system (Figure 1) and modeling can play a substantial role in developing this knowledge [5].

232

Figure 1. The modeling 'wedge'. The first part of this article focuses on the 'thin end' of the wedge [6] in Figure 1; the point at which structural details are completely lacking or, at best, incomplete. It is still little appreciated that modeling methods can now provide a tremendous help in this traditionally difficult area. The second part of this article, that briefly considers the simulation of zeolite behavior, emphasizes the middle and thick regions of the wedge. The third, final part considers challenge areas, ones in which we can expect significant and exciting progress over the next 5 years. The emphasis is on simulation at the atomistic and electronic structural level. Computational techniques in general, and simulation in particular, however, play an i m p o r t a n t role t h r o u g h o u t the whole research, development, and production process [5] (Figure 2). Successful translation of an early research result into a product or process with practical business impact requires that a series of hurdles at scale up, economic analysis, process, and engineering levels all be successfully passed.

233

Figure 2. Some Steps Involved in the (Hypothetical) Implementation of a New Process for 2,6-Dialkylnaphthalene Production. While ee are sometimes f r u s t r a t e d by the limits still curtailing our atomistic simulation efforts, it is sobering to consider the complexities of predicting, to an acceptable level, the various economic factors t h a t might impact the viability of a commercial process. This text focuses on applications, with only passing mention of the mechanics of modeling: the descriptions of the intermolecular or interatomic interactions, the methods of q u a n t u m and classical mechanics and the protocols such as energy minimization, Monte Carlo or molecular dynamics t h a t are the basis for computing s t r u c t u r e s and properties. These are discussed in varying degrees of detail in other instructional texts, to which the reader is referred for further background [7-10]. 2. STRUCTURAL CHARACTERIZATION Atomic-level s t r u c t u r a l characterization is not yet a prerequisite for obtaining a composition of m a t t e r p a t e n t on a new microporous solid.

234

However, a knowledge of structure is almost universally desirable. Structural data can (1) confirm and enrich definition of the active components in the system, (2) serve as a basis for assessing and quantifying differences between different preparations or formulations, (3) yield a first-levelunderstanding of catalyst activity, and of structure-activity or structure-selectivity relationships, and (4) be an essential foundation for a rational approach to the improvement of performance or for more detailed investigations. A n improved ability to characterize a zeolite material can substantially strengthen a composition of matter (COM) patent application (see, e.g.,[11]);a strong C O M patent position is based on an ability to differentiate the covered material(s), increasingly requiring quality analytical data, coupled with a definitive interpretation. Improvements in experimentation have contributed to substantial advances in our ability to characterize the atomic structures of even quite complex materials. In parallel, developments in hardware and numerical methods have led to computer modeling approaches becoming a key complement to experiment in the extraction and exploitation of this atomiclevel structural knowledge. Modeling needs to be coupled closely with experiment; simulations m a d e in ignorance of experiment are usually of limited value. A close coupling leads naturally to a powerful role for simulations in the interpretation of analytical data. Modeling in this context does not m e a n molecular graphics, although visualization tools are an invaluable companion of m a n y modeling approaches [2]. Modeling aids in several ways. A computational environment is the preferred means of collating analytical data for a particular sample so that all needed data are available. The usefulness of a piece of analytical information is often governed by the immediacy of its accessibility. Analytical data of diverse types can be simulated based on an appropriate model. Computer modeling techniques are a substantial aid in zeolite structure solutions or refinements, and a means of extracting structural insight from diffraction or other analytical experiments. Sorption results, particle shapes in some cases, diffraction or scattering data, as well as optical, N M R and E X A F S spectra can all be simulated based on an atomic structure and, conversely, analytical data of these various types can be used to guide the development or detailing of an appropriate structural model. One possible implementation of modeling and simulation is to provide a structured framework for developing fundamental insight into the zeolite system of interest. A n R & D program that is founded on a fundamental understanding will, almost certainly, be better positioned than one suggested simply by empiricism. 2.1. S t e p s i n a S t r u ~ ChAT~vterization A zeolite s t r u c t u r a l c h a r a c t e r i z a t i o n effort often begins w i t h s c r u t i n y of a n o n - d e s c r i p t w h i t e powder. If this is a r e s u l t of crystallization from a new

235

compositional domain, the material will almost certainly be of modest crystaUinity and purity. Early compositional and X-ray powder diffraction (XRD) analyses m a y indicate the glimmerings of a possibility for a new and potentially interesting phase. The subsequent combined synthesis - analytical effort then attempts to develop a true fingerprint of the phase, both in terms of X R D profile and other parameters such as chemical composition, sorptive performance, and N M R spectrum. Simultaneously, this developing fingerprint is used as a basis to adjust the synthesis conditions in order to optimize yield of this target phase. As information about the properties of the phase is accumulated, interest in further synthesis optimization m a y grow or diminish. If it emerges truly as an apparently novel material, determination of the framework crystal structure becomes a goal, made significantly more desirable if the properties appear to be propitious (see below). Once a relatively pure (although likely stillimperfect) s~mple is obtained reproducibly, there is a relatively well defined series of analytical and structural questions that we seek to answer. Firstly, definition of the new material, particularly from a composition of matter patent perspective, requires completion of the chemical and structural fingerprinting. Although detailed atomistic structural data are generally not required in this process, they are undoubtedly highly desirable. The subsequent target is then determination of the framework topology. It is interesting that for the more than some 100 zeolite fremework topologies already determined, m a n y of the original structure elucidations had special or unique features. Computational model building approaches that have evolved from the traditional physical model approach are proving valuable, as are new crystallographic developments. In the simulation domain, direct space approaches to structure determination are n o w attractive. With the framework crystal structure determined, subsequent questions to be elucidated include: 1. Definition and quantitation of the micropore architecture, governed to a large degree by the framework structure, 2. Determination of the distribution and configuration of framework cations, 3. Specification of the non-fromework cation configuration, 4. Localization of template molecules, sorbates, or other introduced molecular, semiconductor or metal clusters.

In all of these areas, simulation is already or is beginning to become invaluable.

236 2.2. F r g m e w o r k Crystal Structure Model building: When a single crystal of sufficient size and quality is available, as was the case with, for example, boggsite and cloverite, conventional single crystal X-ray diffraction methods can be used to determine zeolite crystal structures. Most synthetic zeolites, however, cannot by synthesized in particle sizes much larger than some 10~m~ and there are often other sample-related complications. Building on experiences with physical model building and earlier graphical methods, we now have a particularly powerful array of crystallographic modeling techniques to apply to initial framework model construction. Physical model kits do not allow symmetry or unit cell size information to be used as constructional constraints. In a computational crystallographic modeling environment, however, we can impose both chemical and crystallographic constraints [12-16]. As an example, following J. V. Smith's compilations, libraries of 3-dimensional models of polyhedra and cage units have been developed [17]. Following sketching of the appropriate T-atom connectivity, each cluster geometry was optimized by steepest-descent~and conjugate gradient protocols using a simple atom-atom potential function that targets a T-T distance of some 3.05A, with non-bonded repulsions that serve to fully expand the cluster and yield symmetrical arrangements. Once optimized, the cluster point group symmetry is automatically determined and used to check the earlier, manual analyses. Cages or polyhedral units taken from these libraries or other structural motifs such as tetrahedra, molecular fragments, clusters, layers or chains can then be manipulated and combined in 3-dimensions via a graphical interface so as to produce viable crystal structure models. Periodicity and/or s y m m e t r y information can be applied during construction so that as one group is adjusted all related groups are updated appropriately so as to preserve the periodicity and/or symmetry [16]. Alternatively, the 3-dimensional model can be created and the unit ceU then automatically identified by a program that seeks recurrent atom-replications under translation. With the unit cell defined, we can then automatically determine the model space group symmetry by a similar protocol t h a t explores recurrent atom-replications under symmetry operations [13-16]. F u r t h e r structure optimization so as to optimize the match with target distance constraints [18,19] or with more sophisticated interatomic potentials (see, e.g., [20-25]) can then be achieved either in the defined space group symmetry or, omitting symmetry, in a triclinic, P1, description. These tools allow rapid model development and evaluation. For example, starting with a simple T-atom sodalite cage model, the cage connectivity seen in the LTA-framework can be constructed in a few minutes; automatic determination of the unit cell and space group symmetry, completion of the framework by adding the necessary apical oxygen atoms and performing a full constant-pressure geometry optimization cons~_lmes only several minutes

237 more. As one recent example, these methods aided in a detailed evaluation of the nature of planar defects in synthetic FER-fr~mework materials [26]. For the framework that results from fully recurrent application of each of seven distinct planar faulting operations on the parent FER-framework, the unit cells and space group symmetries were determined. Powder diffraction profiles were then simulated for varying degrees of these different types of faults [27]. Comparison against experimental data then allowed the predominant faulting mode to be identified [26]. Even with physical models, once a reasonable geometry has been obtained the crystallographic description must be entered into a computer for further optimization, display and simulation of analytical data. Working exclusively in a crystallographic modeling environment eliminates a tedious data entry step and permits analytical data to be simulated immediately so as to corroborate or fingerprint a model. Perhaps most powerfully, a range of analytical data (such as chemical compositions, geometrical data, diffraction patterns, EXAFS or NMR spectra), all simultaneously computed from the developing model, can be updated interactively as the model is adjusted [28]. By monitoring the match between experimental and simulated data of these types, the model can then be adjusted to best fit with the combined experimental data. It should be emphasized that structural characterization needs to extend beyond definition of a perfect zeolite crystal structure model. Methods for handling certain of the material complexities often present in zeolite samples, such as disorder, local ordering and planar faulting need also to be considered. Non-crystalline phases, such as often used as metal supports or as catalyst binders, present another set of problems. Measured X-ray or neutron scattering data reflect the presence of a distribution of local site environments, requiring t h a t valid atomistic models manifest this heterogeneity. Simulated annealing has already been used to facilitate fitting of the measured scattering patterns [29,30]. Melt-quench molecular dynamics schedules, based on validated interatomic potentials, have been used to develop atomistic models of vitreous silicas and vitreous silica surfaces and to obtain insight into cation ordering phenomena in mixed alkali glasses [31]. Systems in which there occur regions of substantial medium range order, such as the transition aluminas [32] and some microporous carbons, may require hybrid approaches, although modeling will undoubtedly play a substantial role in further atomic-level characterization. Simulated annealing: A structure determination from diffraction data involves the development of an initial model, that is a structure solution step, and then completion and refinement of the model. Why are these steps distinct ? The answer is that today's structure refinement technology can operate effectively only when the starting model is relatively close to the actual model; crystallographic least squares techniques are unable to locate global

238 minima in parameter spaces that are complex, multi-dimensional and which have many local minima, unless the current model is close to this global minimum. A reasonable initial model is required for refinement, hence the need for a separate structure solution step. The structure solution methods t h a t work well in small molecule single crystal work usually cannot, however, be applied directly to powder diffraction data. A successful indexing of the powder diffraction pattern, which can often be done automatically, yields the unit cell dimensions and information on possible space groups. The chemical analysis and sorption data indicate the framework density, or number of tetrahedra per unit cell. The challenge is then to position these tetrahedra within the unit cell such that (1) they fully interconnect in a sensible manner and (2) the necessary analytical data are reproduced. These structural constraints are quantified in an 'energy' expression and simulated annealing [33,34] is employed as the global optimization approach. The first phase of zeolite structure solution requires definition only of the approximate T-atom configuration for which the 'energy' expression is then a sum of terms based on the T-T distances, and the T-T-T angles and average angles observed in known zeolite structures [35,36], and a coordination number term that reflects the requirement of framework 4-connectedness. Additionally, a tendency for T-atoms to reside on special positions is supported by treating all of the T-atoms as being on general positions, but allowing pairs of symmetry related T-atoms to 'merge' when they are separated by less than a defined capture distance. As the total 'energy' expression is a simple sum of terms, other constraints can easily be added. A projection term, for example, provides for a bias in favor of structures that, when viewed in projection down a defined crystallographic direction, appear as 3-connected nets (see, e.g., [37-39]). A pore term can also be used to favor structures that have cylindrical or spherical pores of pre-defined sizes. Given the openness and flexibility of zeolite structures, we have found it advantageous to limit the number of independent variables to as large a degree as possible. The Structure_Solve program [16,36] therefore imposes rigid space group constraints and adjusts the positions only of the unique Tatoms; the positions of the other T-atoms in the coordination shells of the unique T-atoms are generated by the space group symmetry operations. In the simulated annealing procedure, a 'state' is a possible T-atom configuration whose reasonableness is computed by this 'energy' expression. The 'temperature', the amount of thermal 'energy' in the system, determines the relative probabilities of the system adopting two states of different energies. Possible states are considered by the Metropolis importance sampling method [40]. From an initial state of total 'energy' Einitial, the Tatoms coordinates are displaced in a 'jiggle' step; for each T-atom both the direction and, within defined bounds, the displacement extent are chosen at

239 random. The 'energy' of the n e w trial state, Enew, is evaluated and the n e w state is accepted ff Enew < Einitial

(I)

expf " Ene~___~nitial} < rand(0,1)

(2)

or if

where T is the 'temperature', K a constant term and rand(0,1) represents a random number between 0 and 1. At low values of T, the probability of accepting higher energy configurations is small; at high T, however, the probability is high. The 'temperature' is, in fact, initially raised from an arbitrary point in increments or powers until the system becomes 'molten' and it is then slowly reduced in stages. As the system is 'annealed' by this reduction in T, the likelihood of accepting moves t h a t increase the system 'energy' diminishes; KT is in fact a measure of the 'energy' barrier height over which the system can pass at a given value of T; in the limit of low temperature, only energy reducing moves are accepted and the system settles dose to the minimum of its current 'energy' well. The simulated annealing method was initially validated on a test set of 84 known zeolite frameworks [37,39], using only T-T distance, T-T-T angle and average angle, coordination, and merging terms in the zeolite 'energy' function. The unit cell dimensions and space group, and the numbers of crystallographicaUy distinct T-atoms, nunique, and the total number of Tatoms in the unit cell, nT, were t a k e n from those of the representative structure in ZeoFile [39]. A structure type was considered solved if the coordination sequence for one of the final set of structures generated by the simulated annealing matches t h a t of the representative structure of the known material. (The coordination sequences out to a defined shell, k, from the central atoms [41] is defined as the graph of the framework connectivity; for the unit cell volumes typical of most zeolites, if the coordination sequences out to k = 10 for two structures are identical, the topologies are identical with a probability t h a t approaches one). The single first simulated annealing run gave the correct structure in 25 (30%) of the total of 84 cases. Of those structures not solved, those having six or fewer unique T-atoms [39] were selected for further runs, with only the initial random n u m b e r seed being changed between runs. The method was found to reproduce 57 out of these 65 distinct structures. When the crystallographic residual (a quantitative measure of the degree of match between experimental and simulated diffraction data, in the zeolite case powder diffraction data) was added to the 'energy' expression the number of hypothetical but inappropriate structural models was dramatically reduced [36]. The next step which, although seemingly obvious at the time,

240 had far-reaching ramifications, was to test the full generality of the method by using an 'energy' function that included only the degree of match to the target P X D pattern and no geometrical or atom-atom interaction constraints [36]. These trials worked well, setting then the stage for simulated annealing to become a general tool for ab initio structure solution from powder diffraction data for both inorganic and molecular systems [42]. For zeolites, the simulated annealing method has been used to solve the structures of some new materials [35], including the product that results from a framework reconstruction on dehydration of Na6(ZnAsO4)6.8H20 with the sodalite structure at 190~ [43] and the novel aluminosilicate UiO-7 [44]. Analogously to the impact of simulated annealing in structure solution and hypothetical framework structure development, simulation will also be an exciting medium for new approaches to structure completion and refinement. The simulated annealing method can generate large numbers of hypothetical structures in a short time frame. A non-exhaustive series of simulations based on the unit celLsizes and symmetries of 64 known zeolite structures produced over 5000 hypothetical structures [36]. Many of these structures are not 3-dimensional frameworks, instead forming discrete cages, chains or layers. However, for each different set of input data, each derived structure has a unique set of coordination sequences for the inequivalent T-atoms and therefore has a distinct topology. 2.3. A l n m l n u m distributions A l u m i n u m d i s t r i b u t i o n s i n F A U - f r a m e w o r k m a t e r i a l s : Local a|umlnum distributions in zeolites are of substantial interest given their role in governing Brcnsted site acidities and selectivities seen in certain catalytic processes. Except in composition regimes where long-range coherence to the Si-A1 (or Ga, B, Fe etc.) distribution occurs, however, diffraction experiments sample an averaged T-atom site distribution. For structures with crystaUographically inequivalent T-sites diffraction experiments can, in favorable cases, indicate the partitioning of aluminum over the inequivalent sites (see, e.g., [45]), but the aluminum site populations are again the averages of all of the local configurations. Direct information on the relative populations of 29Si nuclei with differing local a l u m i n u m distributions is provided by solid state 29Si NMR. Interpretation of these experimental data can reveal the factors that are at play in governing the aluminum siting. FAU-framework materials are of prime interest in this context, as they are accessible over the full compositional range 1.0 _<Si:A1 < oo, with catalytic and sorptive data available over much of this region. Key questions include "Is the A1 distribution random, or random subject to certain thermodyn~mAc constraints ?", "How labile are the framework A1 atoms ?", and "What information does the A] distribution convey about the mechanics of hydrothermal synthesis ?".

241 Following prior analytical results, early Monte Carlo simulations t h a t employed the powerful simulated annealing optimization technique in an attempt to reproduce experimental Si:nA1 populations, were based on use of the square planar and diamond lattices as simple models of the more complex zeolite framework topologies [46]. To give reasonable agreement with early measurements of the Si-nAl; n=0-4 distributions as a function of Si:A1 ratio, these simple simulations required only a strong, but non-dominant, first neighbor AI-AI repulsion term (Loewenstein's rule ) and a weaker second neighbor A1-A1 repulsion term (Dempsey's rule). Similar simulations have now been performed for the real zeolite framework topology [4]. A model containing several thousand T-sites and with periodic boundary conditions is taken as representative of the bulk crystal structure. The required number of a]~lminum atoms is introduced at random into the all-silica lattice. The Metropolis Monte Carlo method is then used to adjust this initially random arrangement (by attempting, in each step, substitution of a randomly chosen A1 atom onto an adjacent Si site) so that the total system energy, t h a t is the sum of the A1-A1 repulsion terms, is minimized. Simulated annealing is used as the route towards global energy minimization, and the simulations are repeated for different values of the AIA1 repulsion terms. Without the power of the simulated annealing protocol, convergence is not achieved at the lower Si:A1 ratios even for the simpler case of solely Loewenstein constraints [4]. The main result of extensive simulations of A1 placement in the FAUframework topology is that random insertionof A1 into the structure, subject to Loewenstein's rule and to a weaker second neighbor AI-AI repulsion term, does not reproduce the measured Si-nA1 distribution patterns [4]. The details of the aluminum distributions are therefore determined by additional or different factors. This is consistent with Melchior's model of FAU-framework construction from pre-formed 6-ring units [47,48]. The simulation results also highlight the likely limitations of quantum mechanical studies of alumlnum T-site preferences. If the factors controlling the ab~m~num distributions in zeolites X and Y are also at work in other systems, purely energetic arguments will likely have limited direct relevance for application to real materials. A l u m i n u m p l a c e m e n t i n ZSM-5: In ZSM-5 a key question is %Vhich of the 12 crystallographically i n d e p e n d e n t sites accommodate the framework a l u m i n u m content ?" Reasonably accurate determinations of the crystal structures of a series of MFI-framework materials have been reported. At the low ablminum concentrations typical of ZSM-5 it has proved as yet impossible to measure experimentally the distribution of aluminum over the set of 12 crystallographically distinct T-sites in the s t r u c t u r e of orthorhombic s y m m e t r y . Powder X-ray diffraction e x p e r i m e n t s have not yielded information about aluminum placement. Large, quality single crystals are

242 most readily available only for much more siliceous compositions. Close to a pure SiO2 composition the 29Si nmr spectrum can be obtained at exquisite resolution, resolution t h a t allows careful study of the t e m p e r a t u r e dependence of the framework structure and symmetry and the comparable changes that accompany the introduction of particular sorbates. However, as soon as even small ~mounts of aluminum are present in the framework, the resolution is lost and the spectra of aluminosilicate MFI-framework materials afford insufficient definition to allow information about alnmlnum site preference to be extracted; this resolution degradation reflects that a]nminum atoms secozid neighbor to a central 29Si nucleus have a significant effect on its chemical shift [49]. These experimental challenges, combined with the importance of a l u m i n u m site placement, have created a significant opportunity for modeling and simulation approaches. Several groups have attempted to compute the relative site substitution energies for alumim!m into each of the crystaUographically distinct T-sites. In fact, over the last two decades, q u a n t u m mechanical computations on cluster models with this objective serve as a good illustration of the steadily increasing sophistication in simulations made possible by hardware and methodology improvements. A more recent study employs semi-empirical methods applied to calculations on discrete clusters up to some 50 atoms. These types of cluster calculations appear to be converging, perhaps unsteadily, on a picture of quite similar thermodyn~mlc site substitution energies for each of the different T-sites. These calculations yield, subject to some simplifying assumptions, relative T-site alnmlnum substitution energies computed (1) for the thermodynamic equilibrium state, (2) at zero K and (3) for models devoid of non-framework species. Framework zeolites, metastable structures, are produced under kinetic control and if', as indicated by the most recent calculations, the relative T-site substitution energies for the different sites are not grossly disparate, the actual distributions in real materials will be determined by the particular conditions of synthesis. As the molecular-level mechanisms of zeolite synthesis remain obscure, we especially need some experimental indicator of which sites are actually adopted by aluminum in real MFI-framework materials. The unit cell dimensions of quality ZSM-5 materials can be measured by powder X-ray diffraction almost routinely to an accuracy of • 0.005~ or 0.05 ~ (some 0.02%). The character of the changes in the unit cell dimensions that accompany framework T-atom substitution are known to depend on the framework topology and might be presumed to depend on the A1 T-site substitution pattern. Simulation was therefore applied in an attempt to relate changes in the unit cell dimensions, macroscopic observables, to particular patterns of local T-site substitution by aluminum in the MFI-framework [50]. A validated molecular mechanics forcefield of high-quality, eff91_czeo [25], was used as the basis for an exhaustive series of constant-pressure f u l l

243 s t r u c t u r e optimizations of MFI-framework models. A l u m i n u m was substituted onto each of the 12 crystallographically-distinct T-sites in turn, with t h e corresponding anionic framework charge c o m p e n s a t e d by introducing protons at each of the 4 apical oxygen atom positions for each of these 12 T-site choices. Each of these 48 configurations was optimized to a zero-force, energy minimum configuration. Intelligent, but non-exhaustive, searches for possibly deeper local minima were pursued. The m a n n e r in which the observed structural changes scale with varying levels of AI substitution on each site was also evaluated [50]. Comparisons with experimental data available in the literature indicate that the A1 distribution in real materials is, at the least, moderately disordered. This is consistent both with reasonable intuition and with the similar A1 site substitution energies found for the different T-sites in the present and previous cluster simulation results. These exemplary studies illustrate how two different aspects of aluminum placement in zeolite frameworks can be probed by atomistic simulation. Both studies have become viable only quite recently; both studies also illustrate how the combination of calibre analytical observations with suitable simulation can yield invaluable atomic-level insight. 2.4. Micropore arclfitecture For sorptive separations a well-publicized use of zeolites is as molecular sieves, in which the regularly repeated array of micropore apertures limit, on the basis of molecular size, the entry of molecules into the internal micropores. To first order, this phenomena can be rationalized quantitatively based on the relative sizes of the zeolite apertures and those of the sorbate, in its orientation of minimal cross-section. Molecular cross-sections can be m e a s u r e d from scale drawings of the molecules in the appropriate configuration; they can be computed directly as the limiting extents of the molecular van der Waals envelope [14-16]. In a simple case, tabulated molecular dimensions, together with a simple formula for the aperture dimension of AO.O-2.7A, where AO-Q is the separation between oxygen atoms centers across the aperture and 2.7A is twice the effective oxygen radius [37]. For irregular apertures, however, the measurement is better computed from the extremal surfaces of the oxygen atoms defining the r i n g - the Pore_Ruler implements such calculations in an interactive, graphical manner [14-16]. The simple hard-sphere exclusion rule based on r i g i d aperture and molecular sizes overestimates sieving constraints by some 0.3A, even at room temperature. Higher temperatures allow even larger molecules access. For many years this has been attributed to flexibility of the sorbate and, possibly, to a breathing motion of the zeolite pores. Both effects have now been studied explicitly by computation, involving simple geometrical measures, combined with the use of crystal dynamics to probe the effect of temperature on the range of configurations accessible to the guest molecule and host structure.

244 At a simple level, the effective atomic radii are reduced by a scaling factor which measures the extent to which the repulsive component of the potential can be scaled given the thermal energy in the system. The conformations] flexibilitythat is typically a factor in hydrocarbon sorption, can be simulated by molecular dynamics (MD), with the effective cross-sectional area taken as the m i n i m u m such value sampled in the combined M D simulations. Likewise, the framework dynamics can be simulated by periodic M D simulations, termed crystal dynamics [51,52]. These atomistic simulations have the advantage of tracking the manner in which the motion of individual atoms or tetrahedra are coupled - the bonding characteristics of zeolite frameworks are such that the coupling of adjacent tetrahedra can give rise to interesting lattice modes, such as pore-mouth breathing [51]. Nitrogen absorption isotherms, interpreted using the B E T formalism and, particularly, low pressure Ar absorption data provide quite detailed information on pore size distributions down to very small pore sizes, _> ~4A. For microporous crystals, measurements of uptake of molecules of differing sizes and shapes, and product distributions obtained in various test catalytic reactions reveal aspects of the micropore architecture. Direct ways of quantifying pore volumes and pore coordination environments are available and an automatic volume filling algorithm helps quantify the number of molecules of a defined type that can be accommodated within a given micropore volume. The pore volume and its degree of interconnectedness can then be visualized using contour meshes or solid voblme rendering [14-16]. As noted above, known pore architecture information can also be used as a structural constraint in the simulated annealing model development approach [36].

25. Non-frAmework cations The positions of non-framework cations in al,lminosilicate zeolites can control or fine-tune their sorptive and catalytic properties. Measurement, however, requires careful and usually protracted analyses of accurate single crystal or powder diffraction data. In cases for which extensive experimental data are available, statistical mechanics analyses can yield insight into relative site energies [53-55]; earlier analyses have also attempted to quantify the relative importance of short and long-range interactions in controlling site occupancy patterns [56]. Earlier atomistic simulations in this area [57-62] had mixed results. Recent developments in methods and interatomic potentials have allowed non-framework cation positions to be simulated based solely on a knowledge of the framework structure in zeolite systems for which validatory experimental data are available [113]. Illustrative results come from two systems chosen, firstly, because reasonable structural data are available and, secondly, because the S i - A1 distributions are known precisely; they both have Si:A] ratios of unity and hence strict S i - AI alternation. The procedure applied was originally

245

developed for probing the preferred binding sites of molecular sorbates [63] and takes as input a suitable framework model. For zeolite Li-A(BW), this is the unit cell and framework Si, A1 and O coordinates taken from a crystallographic refinement [64,65]. For zeolite A, we again use accurate crystal structure data [66], but reduce the published a = 24.61A supercell to an a = 12.305~k triclinic, P1, subcell by trimming the full supercell contents to the 0 < x < 0.5, 0 _
246 independent fr8mework constituent atoms and mixed Na+-Ca 2+ and Na+-Li + cation systems, However, zeolite 4A is a practical example case. The potentials employed were developed to be simple and readily transferable, r a t h e r t h a n reproducing the full structure to a high degree of accuracy; these results clearly demonstrate their efficacy in this capacity. The structures obtained in these two illustrative zeolite cases are quite sufficient to allow further model refinement against measured powder diffraction data or based on substantially more computationally expensive or more locally appropriate interatomic potentials. The 30 successive packing calculations consumed, respectively for zeolite Li-A(BW) and zeolite 4A, some 3s and 12s on a workstation, with each structure optimization then requiring an additional 2 or 8 min. respectively. In the latter case, in which the local structure provided by the simulations and the sample-averaged structure yielded by diffraction differ, the modeling results also allow the local effects of the site disorder to be explored. These encouraging results set the stage for extensions to still more complex systems and to structures for which less direct experimental data are available. 2.6. F r a m e w o r k Design The simulated annealing method, although initially conceived as an aid to structure solution, can also be used for framework structure design. The term in the 'energy' expression t h a t allows accommodation of one or more cylindrical or spherical pores provides a means of developing s t r u c t u r a l models with pores of a target size or geometry. In principle, a contribution to the 'energy' expression could be made based on w h e t h e r or not rings containing a prescribed number of T-atoms are present. However, for large rings this computation can be time-consuming and therefore inconvenient to perform at each of the many thousands of steps used in the Monte Carlo simulated a n n e a l i n g procedure. Simplistically, successive shells around each T-atom to the shell corresponding to the defined ring size, N, would need to be ssmpled to determine whether closure to form a ring had occurred, a calculation scaling as a power of N. Even with s u b s t a n t i a l l y i m p r o v e d algorithms, this calculation is expensive. The term we use adds a positive contribution to the total cost function for each T-atom that protrudes into the clefined cylindrical or spherical pore volume; clearly this type of constraint could be easily extended to more complicated cases. This simple device, however, n e a t l y allows s t r u c t u r e s to be designed with p a r t i c u l a r pore geometries and, for egRmple, facilitates speculative studies of structures with very large pores.

247 3~OCCLUDED OR SOBBED M

O

~

AND C L I ~ - I ~ S

3.10r~nJc Templates Since the early 1970's much of the richness in zeolite structural chemistry has arisen from the use of organic additives in synthesis [67]. In addition to modifying the gel chemistry, the occlusion of neutral or cationic organic molecules within the crystallized zeolite was the basis for development of the templating concept in which the al~mlnosilicate framework is envisaged to crystallize around the organic molecule or cation. There continues to be active debate as to the importance of this templating phenomena. There are instances, such as ZSM-18, in which there is clearly a close fit between the molecular structure of the organic trisquaternary ~rnmonillm cation and the shape of the pore within which it is housed in the crystallized zeolite. There are, conversely, other zeolites such as ALPO-5 t h a t are accessible using a wide range of different template molecules. Although, as noted further below, we are still a long way from understanding the mechanism of zeolite crystallization at the molecular level, simulation has helped broaden our understanding of the role that organic template molecules might play. Several different authors have used simulation to compute" the strengths of interaction between occluded template molecules in the zeolite and to relate the results with their efficacy as templating molecules [68-71]. The simulation procedure requires, firstly, that the possible locations for the template within the crystallized zeolite structure are correctly identified and, secondly, that there be a reasonable basis for computing the energy of interaction between zeolite and template. Most recent studies have used a hybrid molecular dynamics-Monte Carlo procedure originally developed for specifically this purpose [63]. High t e m p e r a t u r e molecular dynamics is used to generate a sample set of molecular conformations, for each of which docking within the zeolite framework is attempted. The docking procedure tries insertion of the molecule at random within the total zeolite space; configurations of high energy, those that involve molecule-framework clashes, are discarded. For each of the accepted configurations, energy minimization is then used to develop the optimal configuration. By ensuring sufficient statistical sampling, the energy m i n i m u m configuration can be identified from the relative energies of the minimized configurations. As a final step, the entire template+zeolite structure could then be optimized such that the perturbation of the framework geometry by the template is also considered. This procedure and, importantly, the force fields used to describe the zeolite-template interactions in the atom-atom approximation, have been well validated against experiments. Firstly, the locations of a number of templates within zeolites have been determined by single crystal or powder X-ray diffraction. Thus, for example, the templates 1-aminoadamantane and Nm e t h y l q u i n u e l i d i n i u m cation have been used to crystallize the LEV

248 framework and the cation location determined by diffraction. There is excellent agreement between the Monte Carlo docked configurations [72] and the experimental crystal structures [73]. Earlier work has demonstrated, similarly, the location of the tetramethylammonium cation within TMAsodalite and has given insight into the relative populations of TMA + cations in the alpha and beta cages of Zeolite ZK-4. A study of the role of tetraalkylammonium cations in ZSM-5 synthesis promotion has also demonstrated that the tetrabutylammonium cation, at limiting dilution, interacts favorably with the ZSM-5 framework, but once neighboring channel intersections are populated, there are unfavorable clashes between adjacent TBA + cations. Interestingly, optimization of the configuration of the t e t r a p r o p y l a m m o n i u m cation, t h a t preferred for ZSM-5 synthesis, demonstrates that the crystallographica]ly observed configuration is, in part, the result of template-template interactions [74]. The success of these simulations in reproducing the experimental data to a good degree of precision provides further confidence in the approach. These studies focus on the interaction between the template and the crystallized zeolite. The limited experimental data available relating to the molecular basis for zeolite crystallization have also allowed a number of speculative models for zeolite cage formation to be proposed. Harris and Zones have used simulation to develop a relationship between the crystallization rates for both the porosil nonasil and the aluminosilicate chabazite and the host-guest interaction strength for a variety of organic template molecules [68]. The crystallization time is significantly longer for those templates that are computed to interact less favorably with the zeolite framework. This correlation, although still subject to further validation and corroboration, has already been the basis for predicting template performance in terms of crystallization rate [68]. The relationship between interaction energy and template efficacy does not, in itself, yield insight into the mechanism of templating. Earlier molecular dynamics simulations of isolated silicate cages in an aqueous environment have, for example, demonstrated that one role of the template molecule is to act as a "former" in which the dynamics of the cage are constrained such as to maintain an open cage-like geometry and hence presumably facilitate further growth of the zeolite framework [28]. This "forming" concept was a direct and unexpected results of simulations in these systems. It is tempting to consider undertaking simulations that probe, using a sufficiently large simulation cell, the actual interactions between the silicate, aluminosilicate, aqueous, organic, and salt species to probe directly the evolution of the system as a function of the extent of chemical change. Unfortunately, the less computationally expensive simulation protocols that rely on parameterized and simplistic descriptions of the interatomic interactions are poor at tracking bond-making and bond-breaking phenomena. First principles, q u a n t u m mechanical methods, are

249 computational]y demanding and can be applied still to only relatively simple models, generally less than of the order of 200-300 atoms. Further, the time scales of zeolite nucleation and initial growth are many orders of magnitude longer than the femto- or pico-second domain that is tractable today by molecular dynamics.. 3.~ Sorbed Orgauic M o k ~ m ~ For an isolated sorbate within a zeolite, the most natural first question is its preferred site; that is, at which site within the zeolite and at which molecular configuration is the system energy lowest and at which, therefore, the local sorption heat is highest. This is, most simply, a global minimization problem, with the variables being the position of the sorbate and its significant internal degrees of freedom. For simplicity in simulation the zeolite framework and non-framework atom positions are usually assumed fixed, and it is the position and orientation of the potentia]ly-flexible sorbate molecule that is sought. In most zeolites, the energy surface is complex and simplistic optimization methods such as least-squares or conjugate-gradient succeed only in finding local minima immediately adjacent to the initial starting position. Preferred techniques employ molecular dynamics or Monte Carlo methods that can, intrinsically, surmount potential barriers in sampling the energy hypersurface; these techniques, particularly in combination, become essential when sorbate internal degrees of freedom need be considered. The Monte Carlo docking procedure has had reasonable success as a protocol for identifying preferred binding sites [63,71]. The single energy minim~!m configuration will be exclusively adopted only at OK. When thermal agitation is introduced, Boltzmann statistics determine the extent to which a configuration of energy AE higher than this global minimum will be populated as exp(-AE/kT). The structural model evidenced by diffraction represents the temporal average over these configurations, which generally corresponds to the Boltzmann average of the configurations. Monte Carlo methods, using the Metropolis sampling algorithm are a convenient way of sampling the configurational space. The measure by structural probes such as diffraction, and the sorption heat average over all sampled configurations corresponds to the average sorption internal energy for the simulation temperature. Structural studies have been relatively few in number; these are also typically performed at low temperature when sorbates are reasonably well localized. For exRmples, aromatic hydrocarbons in zeolites X, L, Y and good agreement with docking simulations is found. At higher temperatures the effective description of disordered sorbate distributions so as to reproduce the measure diffraction data remains, in general, a challenge [75]. The success of such docking simulations is determined largely by the reliability of the interatomic interaction description. With a sound zeolite

250 structural model and potential set, sorbate locations and sorption heats would be accessible to a good degree of confidence. Larger molecules, with significant degrees of internal freedom present a computational challenge. Consider, for example, the docking of the C16H34 n-alkane into a channel structure such as the CAN-frAmework. Very few attempts at introduction will align the molecule with the channel such that neither end of the molecule clashes with the channel wall; the situation is exacerbated in 2- or 3dimensional pore structures at higher temperatures in which n-alkane configurations other than all trans may be significant. A recent approach to simulating the sorption of longer chain alkanes uses configurational bias Monte Carlo in which the alkane chain is effectively 'grown' within the zeolite, focusing the simulation on more likely configurational domains [76]. The bias is corrected-for subsequently if reliable statistical mechanical data are required. 4. MESO- AND M A ~ U C T U R E Simulation can also be applied to longer length-scale phenomena. Examples include attempts to model the structural and mechanical properties of catalyst pellets, the mesopore structure of particle aggregates and phenomenological studies of crystallization. Here I mention just two examples, studies of crystallite morphology and quantitation of the effect of pore blockages on effective sorption capacities. 4..1 QrystalHte Morphologies Subtle changes to a synthesis composition, such as a low-level addition of divalent cations, can have a drnmatic effect on the morphologies of the zeolite crystallites that result. Small crystals, grown under equilibrium conditions, have habits that are determined by the relative surface energetics, with the most stable faces dominating the crystal habit. The normal from a given position within the crystal interior to a particular face is, in fact, proportional the surface free energy. For cases in which reliable interatomic potentials are available, established surface simulation techniques can be used to predict the relative (internal) surface energies and hence approximate equilibrium morphologies [77]. When crystal growth is governed by kinetic factors, as is almost universally the case in zeolite synthesis, the analysis is more complicated. Additionally, the chemical characteristics of the external zeolite surface are not well-known. Nevertheless, some direct [78] and indirect [79] experimental insight into zeolite external surface structure is becoming available and early simulations of the morphologies predicted for silica sodalite are encouraging [80]. Crystallite morphology prediction and morphology control are important for several kinds of applications. Zeolite membranes can require alignment of many distinct crystaUites, this being facilitated by having uniformly shaped

251 and sized crystallites of similar, high aspect ratios; the success of catalyst particle formulation is, to some degree, also governed by the interaction between the zeolite crystallites and non- or poorly-crystalline binder or filler components. 42 P o r e Planar faults are common in zeolites and related crystalline microporous solids. These can influence the sorptive characteristics in any one of several ways: (i) they can have little influence on the overall accessibility or capacity, but alter the pore architecture, accessibility or ~ i o n a l constraints; (ii) they can reduce the limiting dimensions of pore windows while leaving the total pore volume unaffected; (iii) they can block channels. Pores or pore access can also be blocked by detrital material such as alumina extracted from the framework, coke or sintered metal catalyst particles, immobile organic molecules or non-framework cations in blocking positions. To compute the reduction in effective sorption capacity t h a t accompanies such blockages, Monte Carlo simulations were applied to lattice models [81,82]. For the 1-dimensional channel case, the problem can also be evaluated analytically [81]. There is a precipitous decline in capacity even for relatively low faulting levels in the case of longer crystallites. For example, sorption experiments on polycrystalline gmelinites comprised of crystallites some 0.5 ~tm in length along the c-direction t h a t have an observed factor of some 10 reduction in n-alkane capacity over that expected for the unblocked structure are indicative of a faulting probability of only some 0.04. For the 2- and 3-dimensional cases the channel interconnections allow a substantial fraction of the micropore capacity to be accessible below a percolation t h r e s h o l d in blocking probability. Above this percolation threshold, the accessible micropore volume is limited to the perimeter region of the crystallite [82]. 5. DYNAMICAL BEHAVIOR At the temperatures involved in most sorptive and catalytic processes, the effects of thermal agitation on the local and extended structure can be substantial. Ionic or molecular diffusion can be studied directly by molecular dynamics methods, provided t h a t diffusion is sufficiently rapid for representative motion to be sampled on the classical MD timescale (_< ~ 10 -9 s). The translational diffusion coefficient is related directly to the slope of the linear variation in mean square displacement with time. Although brute force molecular dynamics has been applied quite widely to probing diffusion of lighter molecules [83-85], such as methane in silica]ite (see, e.g., [1,86,87]] for citations to recent literature), it cannot be applied directly to larger molecules, those t h a t do not undergo significant translation on the molecular dynamics

252 timescale [88]. Transition state theory (e.g. [89,90]) and Monte Carlo methods [91] are, however, being applied with encouraging results. There are alternates to probing diffusion by molecular dynamics or transition state theory. For a diffusional path than contains a significant constriction or activation barrier, an additional force along the diffusion direction can be applied to enforce lateral diffusion on the molecule. The 'bias' in quantis the diffusion that results that such an additional force injects can, in principle, be corrected-for so as to yield an estimate of the translational diffusion coefficientin the flee, unperturbed system. Where translational diffusional differences m a y contribute, directly or indirectly to a process outcome, even simpler modeling methods may, on occasion, be insightful. To develop an acidic zeolite-based catalyst that would be selective for production of 2,6-dialkylnaphthalene rather than the 2,7isomer, the research team at Catalytica reasoned that such selectivity might appear in acid zeolites with 1-dimensional channels in which the diffusivity of the 2,7-isomer was significantly curtailed relative to the 2,6-isomer. A n initial and rapid screening of the full set of candidate zeolites compared the molecular dimensions to those of the zeolite channels. For the set of 1dimensional, 12-ring zeolites that remained, a migration path analysis protocol was then used to m a p the interaction energy of the dialkylnaphthalene with the zeolite as a function of its position along the zeolite channel. The lateral and orientational degrees of freedom were optimized at each point along the migration path. Seeking a zeolite for which the energy trough to peak differences differed for the 2,6- and 2,7-isomers, mordenite was indicated to be the best candidate; this indication was substantiated by experiment, the acid mordenite catalyst showing selectivity of some 2.7:1 for production of 2,6-diisopropylnaphthalene versus 2,7 [92,93]. The effect of temperature on the bulk structure can be studied by free energy calculations and by crystal dynamics simulations. Infra-red and R a m a n spectra, and certain inelastic neutron scattering spectra directly reflect aspects of the lattice dynamics. Infra-red spectra can be simulated from the force constant m~trix, based on interatomic potential models [94-97]. The matching of simulated mode frequencies with those measured in R ~ m a n or IR spectra can indeed be used to develop, validate or improve the form and parameterization of the interatomic potential functions [97]. Direct experimental study of the atomic-scale dynamics of zeolite structures is difficult. Despite significant recent progress [94,98], mode assignments of vibrational spectra are complicated. The measured vibrational spectra also provide no direct indication of the manner in which the motions of successive tetrahedra are coupled. Diffraction experiments in principle yield m e a n squared atomic displacements, although measured Debye-Waller factors are usually excessively large on account of a variety of model deficiencies. This is an area to which modeling can contribute physical or mechanistic insight on an atomic scale, while at the s~me time providing

253 data to compare against experiment for validation or corroboration of the modeling results. In applying modeling methods to study zeolite framework dynamics, we have used constant volume crystal dynamics to simulate variations in the aperture dimensions with temperature of the six representative zeolite structure types SOD, RHO, TON, MFI, LTL and *BEA [51]. The framework flexibilities were treated by a crystal mechanics force field using parameters taken from quantitative interpretations of Raman and infrared spectroscopic data [97]. The simulations reveal substantial motion of the framework atoms about their equilibrium positions, with fluctuations in the effective aperture sizes with temperature that depend on the frRmework connectivity. This is consistent with experimental observation. The frequency spectra of the 0 - O distances across the apertures reveal generally well-defined periodicities to the pore window motion. The definition, extent, and period of the motion depend on the framework connectivity. It is most pronounced in the SOD and RHO-frameworks, previously known from experiment to be most susceptible to static framework distortion. Pore-mouth breathing motion is manifested directly in the LTL-fr~mework in the change in cross-sectional area of the 12ring window with time during the dynamics simulations [97]. 6. SORP'rIVE BEHAVIOR 6.1. Thermodynamic parameters Sorption heats at limiting dilution are computed at a defined temperature by averaging the internal energies of a full set of configurations weighted according to their probability of occurrence. The usual procedure is to use the Metropolis Monte Carlo protocol. The sorbate is introduced into the zeolite, and a random displacement and reorientation is used to create a new configuration. This is accepted or rejected based on the energy difference between new and old states, AE, according to the above acceptance criteria (equations (1) and (2)).The procedure is repeated a sufficient number of times that reliable statistical s~mpling of the configurational integral is achieved. Several systems have been examined (see, for exRmple, [3]). Interestingly, the experimental isosteric heats in a given system can often be reproduced quite well using simple models and potentials, although there is an issue with potential transferability. An interesting related study introduced the concept of inverse shape selectivity in molecular sieves [99]. Relative computed adsorption heats for nhexane and 2,2-dimethylbutane in a series of zeolites with 1-dimensional channels were compared with corresponding experimental adsorption data and data for the relative selectivity to production of these two C6 isomers in hydrocracking of n-C16H34. A peak in the relationship between 2,2dimethylbutane:n-hexane selectivity and channel diameter at intermediate pore sizes indicated a channel size domain in which the branched isomer was

254

selectively adsorbed or selectively produced in the hydrocracking reaction. Interestingly, this particular study also provides a nice example of true simulation in lieu of experiment. To complete the data curve, simulations were run for some model channels with dimensions that have not yet been accessed experimentally. Sorption Uptake Isotherms The shape of a zeolite sorption uptake isotherm, a quantitation of the Rmount of a given sorbate taken up as a function of its partial pressure in the gas phase in equilibrium with the zeolite sorbent, depends both on the zeolite sorbate interaction and on the sorbate - sorbate interactions. Simulation of such isotherms for one or more sorbates is accomplished by the Grand Canonical Monte Carlo method. Additional to the molecular reorientation and movement attempts is a particle creation or annihilation, the probability of which scales with the partial pressure [100,101]. This procedure thus simulates the equilibrium between the sorbed phase in the zeolite and an infinite gas / vapor bath. Reasonable reproduction of uptake isotherms for simple gases has been achieved for a small number of systems (e.g. [100,101]), and the molecular simulations have, for example, explained at a molecular level the discontinuity observed in the Ar - VPI-5 isotherm.

6 ~ Improving interatomic potentials Many of the earlier computations of thermodynamic parameters associated with hydrocarbon adsorption into zeolites entailed development of interatomic potentials so as to fit reasonably with one particular set of experimental data. As a result, although the correspondence between simulation and experiment was oi~en reasonable [102], the transferability of the potential set from one zeolite composition to another or from one type of simulation to another was poor. In principle, if the parameterization truly describes the fundamental physics in an approximate way, it should be viable to develop a more generally applicable set of potentials. We have examined whether a simple non-bonded potential can be developed to be (i) transferable from one zeolite to another and (ii) to simulate without parRmeter adjustment isosteric heats at different temperatures and sorption uptake isotherms. The sorption of methane into Na- and K- zeolite X, and Naand K-clinoptilolites was considered. Models for Na-X and K-X were constructed based on the averaged crystallographic results. The non-bonded parameters in a Lennard-Jones potential were iteratively adjusted so as to best reproduce the experimental isosteric heats in Na-X and K-X over a small t e m p e r a t u r e range. Methane-methane interaction parameters were taken from earlier work [89] and a final iteration was made so as to better fit the experimental sorption isotherms in clinoptilolite.This single and simple non-bonded potential parameter set then reproduces to a reasonable degree

255 the different experimental data in these four distinct zeolite structures [103]. A constraint on this process is the iteration time. To ac~mulate each point in the parameter adjustment space entails completing, to within reasonable statistical precision, a full Monte Carlo computation of the internal heats of adsorption in the zeolite X models and a full Grand Canonical Monte Carlo simulation of at least a small number of points on the uptake isotherms. Any theoretical advance that might eliminate or reduce this computational burden to parameter improvement would accelerate the process dr~maticaUy.

7. INTRAZEOLITE CHEMISTRY There is an already impressive literature on the application of various first principles and semi-empirical approaches to aspects of zeolite chemistry (see, e.g., [104-107]). Even a cursory overview of this aspect of zeolite modeling and simulation would be beyond the scope of the present paper. However, two recent development areas are noted. First principles approaches are i m p o ~ t as they avoid many of the pitfalls associated with using p a r a m e t e r i z e d descriptions of the interatomic interactions. Additionally, simulation of chemical reactivity, reactions and reaction kinetics really requires electronic structure calculations [108]. However, such calculations were traditionally limited in applicability to rather simplistic models. Developments in density functional theory are now broadening the scope of what is viable. Car-Parrinello first principles molecular dynamics are now being applied to real zeolite models [109,110], and the combined use of classical and quantum mechanical methods allows quant~lm chemical methods to be applied to cluster models embedded in a simpler description of the zeolite cluster environment [105,111]. 8. SOM31~FIVE YEAR ~ L L ' 5 " ~ T G E Progress on zeolite modeling and simulation over the last decade has been substantial, and the pace of progress can be expected, at a minimum, to be sustained over the next decade. Given t h a t modeling and simulation can impact most dimensions of zeolite research and development, the areas of greatest excitement for this next phase align quite well with opportunity areas for zeolite research in general. 8.1 Cheractzrization The first part of this overview has focused on the many developments that are allowing modeling and simulation to contribute substantially to more rapid and more complete structural characterization of zeolites and related microporous solids. Interpreting various analytical data faithfully in terms of the contributing underlying atomic model, however, remains taxing. It is

256 difficult, even in the case of well defined powder-diffraction data, to develop and optimize the appropriate crystal structure model. However, progress, for example the simulated annealing technology described above, provides the promise of being able to achieve direct data inversion; that is, the derivation of an underlying crystal structure model from a relatively modest amount of analytical data input (to date, chemical composition, micropore volume, and powder diffraction data are being used). As our ability to simulate swiftly and reliably various other types of analytical data improves, it will become feasible to apply similar types of protocols to not only these other types of analytical data, but increasingly to a diversity of analytical data inputs in combinations. It is viable today, for example, to attempt simultaneous refinements of a structure based on powder X-ray diffraction data, a measure of the average, long range ordering, and probes of local structural order such as 29Si NMR, or EXAFS Spectra. Additionally, the rapid evolution of the global computing environment and infrastructure, will make it more straightforward to take advantage of the wealth of information already to hand on a given or related material. The explosion of information available via the Internet has in fact already created the problem of ensuring fight selectivity or effective filtering in the information being accessed.

a2 High

eaing

One of the goals of simulation, and it does remain today largely a goal, is to use computation on a regular basis in advance of experiment. This will become one of the main ways in which simulation contributes practical dollar value to a state-of-the-art R&D program. A small number of the examples cited above illustrate how, in c e ~ areas, simulation is being used to guide and focus an experimental program so that the re!tuber of experimental leads and possibilities to be explored is minimized. As reliability improves, and as access to this simulation technology expands, computational screening of experimental options will become routine. Such screening will combine atomistic simulation techniques and intelligent access to previously compiled information.

a3 True stmca

design

For many of the sorptive and catalytic applications of zeolites, we have an already quite well defined idea of how the zeolite structural attributes influence the physisorptive or catalytic process towards a desirable end result, be it selective uptake of a particular molecule from a mixed feed, or shape selective conversion of a precursor to a particularly desired product. We already have, at least at a qualitative level, the basis for speculating how adjustments to the structure might influence a process outcome. Applying such adjustments iteratively might then allow the design of an optimized structure for the particular application. In the simulated annealing

257 description above, it was clear how definition of a cylindrical or spherical pore, or conceivably other types of pore architecture, can be used as a constraint in the development of crystal structure models. This illustrates one approach to true structure design. As zeolite structural chemistry is reasonably well understood, we will increasingly be able to improve the reliability with which we can develop or predict structures to have fully the desired characteristics. Transition state selectivity in microporous solids is one manifestation of how the relative i m p o ~ c e of different reaction channels can be influenced by structural constraints. In most chemical conversions that involve an activation barrier, subtle changes in the electronic or atomic structure of the transition state can lead to huge differences in reaction kinetics. First principles calculations provide a means of identifying the atomic and electronic structure at the transition state, albeit today for stillquite simple models. These calculations provide the design basis on which to gauge the impact of changes to the active site, or active site environment. Although I a m unaware of any such application to date, even in the molecular domain, it is certainly viable to contemplate using an understanding of the factors that control the barrier height, and the relative degree of constraint in the transition state, relative to reactant and product states, as a basis for designing an optimized active site. If such an active site is a Brcnsted acid function, a supported metal atom or metal cluster, we can also envisage a simulation protocol that develops a three dimensional crystal structure model in which the specified model for the optimal active site is instantiated.

8.4 ~ t i o n phenomena Some of the examples above illustrate how modeling and simulation are contributing significantly to our understanding of the relationship between t e m p l a t e - zeolite structure interaction and efficacy of templating during hydrotherm~l crystallization. Unfortunately, our knowledge of the processes at the molecular level .that contribute to nucleation and crystal growth from a hydrothermal medium is very sparse. The critical length scales of importance, from-8 to -50 ~, place these phenomena outside the bounds that

are accessible to today's analytical techniques. Shorter length scales can be probed by 29Si N M R , E X A F S or other spectroscopies in some cases, and longer length scales can be sampled, albeit not at a molecular level of detail, by high resolution small angle X-ray or neutron scattering. Clearly this is a substantial opportunity area for simulation as the simulation protocols can be validated against the experimental data at both shorter and longer length scales and applied then with reasonable confidence to the intervening lengthscale domain. I believe, based on the nature of progress in the various other areas noted above, that progress here will come not simply from the ability to apply brute force calculations to m u c h larger assemblages of molecules, over m u c h

258

larger time scales. Tl~e many orders of magnitude in time and length scales that separate the molecular dynamics domain from the laboratory chemistry domain in these cases cannot be bridged simply by the anticipated rate of computer hardware and algorithmic developments. Stochastic models are likely to valuable, potentially made more so by parameterizations using high level calculations on relatively simple models. Perhaps the combined use of simulation in conjunction with limited and even rather low resolution analytical data fittingwill be worth pursuing. 8.5 Ties with engineering data In several industrial organizations fundamental zeolite R&D and process development groups are separated, often geographically removed. However, as the practical impact of simulation at the atomic level becomes ever more plain, we can expect a closer alignment [112]. For the simulations approaches mentioned here, this will be manifested in atomistic and process simulations being coupled to a steadily increasing degree. Several of the system parameters provided by simulations at the atomic or electronic structural level, such as molecular diffusivities, thermodynamic parameters, heat capacities and reaction kinetics are raw input to process simulation models. Improvements in accuracy, and new ways to expand discrete molecular simulation data to averages over compositions or configurations are needed. The potential is, however, undoubtedly present for fruitful synergy between experiment and theory, and between chemistry and chemical engineering. a 6 Scrutiny of Subtleties The zeolite chemical literature has many instances of puzzling phenomena that, although often reproducible, are not understood at the molecular l e v e l the sensitivity of many syntheses to subtle changes in the source of starting reagents, the propensity for synthesis of ZSM-5 once it has first been made in a laboratory, the particularly attractive partial oxidation chemistry in TS-1 contrasted with the performance of titanium apparently incorporated into other zeolite frameworks, etc. As with many chemistry domains, it is experimentally taxing to probe the presence and nature of defect sites that are present at low concentration in a disordered fashion throughout a larger matrix, unless such sites have a prominent analytical fingerprint. Simulation, particularly that using first principles methods that don't require the guidance of the simulator in terms of starting model configuration or selection of the description of the interatomic interactions, clearly can yield insight into these types of phenomena. What is important, is that the target phenomena be appropriately quantified, that the simulation approach be suitably structured and unbiased and, as importantly, that the simulation data are interpreted so as to yield practical results.

259 &7 Simulation technology 'on-tap' The proliferation of personal computers, and perhaps even more importantly, the changing face of our global computer infrastructure, driven by the Internet and the World Wide Web (WWW), potentially provide a means of delivering access to the growing power of modeling and simulation to a much broader user comrnul~ty. One can envisage (and we already have in house prototypes of this type of functionality) first using technology on WWW to identify an appropriate strategy to address a particular problem, using combined prior information and simulation, connecting through the net to the appropriate simulation technologies Home Page, and setting up a particular set of calculations t h a t will be performed remotely on the appropriate computer. The results, and potentially even a first interpretation, will then be delivered back to the remote user at his PC or workstation. If efficiently configured such a system could allow immediate access to the latest developments both in simulation technology and in information about the particular system under study. Additionally, such an infrastructure could eliminate many of the chores associated with maintaining the capability and familiarity with the simulation and information technology on one's own computer system. We are still some ways from being able to deliver this type of capability, although the expanding Amount of information on zeolite chemistry available over the Web, when placed in the context of the pace of Internet development, indicates that this type of "on-tap" access to simulation and information technology is not too distant. 9. CONCLUSION This conversational and somewhat subjective overview of computational approaches in zeolite chemistry has illustrated that the field is very diverse, and expanding rapidly. Modeling and simulation at the atomistic or electronic structural level clearly contribute at various levels to practical zeolite research and development programs. Characterization and zeolite physical and chemical property prediction are the most prominent application domains at present. Modeling approaches can be multi-faceted, given t h a t practical advancement of any zeolite technology requires t h a t several different materials and process issues be appropriately addressed. The confidence with which atomistic simulation can be applied will grow steadily as the quantitative reliability of predictions continues to improve and as the library of successful applications expands. Modeling is already being applied successfully, in instances, to zeolite structure determination, analytical data simulation and analysis, to probing molecular interactions and physisorptive phenomena, to computing thermodynamic parameters, and to calculating

260 zeolite electronic structure and chemical reactivity. Further progress in methodology is needed, but steady growth in the practical industrial application of modeling and simulation is anticipated. 10. A C K N O W L ~ M E N T S The Biosym/MSI Catalysis and Sorption Project is supported by a consortium of industrial, academic and government institutions. I also thank the various direct and indirect contributors to the capabilities mentioned in application here, including P. Coulter, C. M. Freeman, A. M. Gorman, A. Ho, C. M. K~ilmel, S. M. Levine, R. J. Lindsay, M. Muir, A. Richards, K. M. Roberts, R. F. Smith, M. Stapleton, M. A. van Daelen, B. Vessal, M. W. Deem, D. H. Gay, M. Katagiri, G. Ricchiardi, S. Veliah, A. K. Cheetham, N. J. Henson, J. Sauer, K.-P. Schr~der, C. R. A. Catlow, A. Rohl, R. G. Bell, R. A. van Santen, T. V. Harris, A. Nakanishi, M. Perrin, J. Pretorius, K.-P. Lillerud, R. Millini, B. H. Toby, S. W. Cart and H. Phala. RI~'~RtENCES 1.

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263 Oo C. J. J. den Ouden, R. A. Jackson, C. R. A. Catlow and M. F. M. Post, J.

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64e 65. 66. 67. 68.

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75.

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V. Alfredsson, T. Ohsuna, O. Terasaki and J.-O. Bovin, Angew. Chemie Int. Edn., 32 (1993). J. M. Newsam, M. M. J. Treacy, D. E. W. Vaughan, K. G. Strohmaier and M. T. Melchior, In: Synthesis of Microporous Materials Volume I. Molecular Sieves, M. L. OcceUi, H. E. Robson (eds.),Van Nostrand Reinhold, N e w York, pp. 454-472, 1992. S. D. Loades, S. W. Cart, D. Gay and A. Rohl, J. Chem. Soc. Chem. Comm., (1994) 1369-1370. J. M. N e w s a m and M. W. Deem, J. Phys. Chem., 99 (1995) 8379-8381. M. W. D e e m and J. M. Newsom, J. Phys. Chem., 99 (1995) 14903-14906. P. Demontis, E. S. Fois, G. B. Suffritiand S. Quartieri, J. Phys. Chem., 94(1990)4329. R. L. June, A. T. Bell and D. N. Theodorou, J. Phys. Chem., 96 (1992) 1051. S. Yashonath and P. Santikary, J. Phys. Chem., 97 (1993) 3849. M. Kawano, B. Vessal and C. R. A. Catlow, J. Chem. Soc. Chem. Commun., 1992 (1992) 879. E. Hernandez, M. Kawano, A. A. Shubin, C. M. Freeman, C. R. A. Catlow, J. M. Thomas and K. I. Zomaraev, In: Proceedings from the Ninth International Zeolite Conference, Vol. I, R. von BaUmoos, J. B. Higgins, M. M. J. Treacy (eds.),Butterworth-Heinemann, Stoneham, MA, pp. 695-702, 1993. L. M. Bull, N. J. Henson, A. K. Cheethom, J. M. N e w s a m and S. J. Heyes, J. Phys. Chem., 97 (1993) 11776-11780. R. Q. Snurr, A. T. Bell and D. N. Theodorou, In: Proceedings from the Ninth International Zeolite Conference, Vol. II, R. von Ballmoos, J. B. Higgins, M. M. J. Treacy (eds.),Butterworth-Heinemann, Stonehom, MA, pp. 71-78, 1993. S. M. Auerbach, N. J. Henson, A. K. Cheetham and H. I. Metiu, J. Phys. Chem., 99 (1995) 10600. S. M. Auerbach and H. I. Metiu, J. Phys. Chem., (1996) submitted. J. A. Horsley, J. D. FeUmann, E. G. Derouane and C. M. Freeman, In: Computer Aided Innovation of N e w Materials II, M. Doyama, J. Kihara, M. Tanaka, R. Yamomoto (eds.),Elsevier, Amsterdam, pp. 985-989, 1993. J. A. Horsley, J. D. Fellmann, E. G. Derouane and C. M. Freeman, J. Catal., 147 (1994) 231-240. K. T. No, D. H. Bae and M. S. Jhon, J. Phys. Chem., 90 (1986) 1772. J. A. Creighton, H. W. Deckman and J. M. Newszm, J. Phys. Chem., 95 (1991) 2099-2101. IC S. Smirnov and D. Bougeard, J. Phys. Chem., 97 (1993) 9434-9440. J. A. Creighton, H. W. Decl~man and J. M. Newsam, J. Phys. Chem., 98 (1994) 448-459. V. A. Maroni, Appl. Spectroscopy, 42 (1988) 487.

265 99. D.S. Santilli,T. V. Harris and S. I. Zones, Microporous Materials, 1 (1993) 329-341. ' 100. G. B. Woods, Z. Panagiotopoulos and S. J. Rowlinson, Molec. Phys., 63 (1988) I. 101. D. M. Razmus and C. K. Hall, AIChE Journal, 37 (1991) 769. 102. A. V. Kiselev and P. Q. Zu, Proc. Nail. Acad. Sci. USSR, Phys. Chem., 241 (1978) 617. 103. S. Veliah, A. Alvarado-Swaisgood, S. M. Levine, A. M. G o m a n , J. M. Newsam and F. yon Trentini, (1996) in preparation. 104. J. Sauer, Chemical Reviews, 89 (1989) 199-255. 105. J. Sauer, In: Zeolites and Related Microporous Materials: State of the Art 1994 (Stud. Surf. Sci. Catal. Vol. 84). Part C, J. Weitkamp, H. G. Karge, H. Pfeifer, W. Htflderich (eds.), Elsevier, Amsterdam, pp. 20392057,1994. 106. B. J. Teppen, D. M. Miller, S. Q. Newton and L. Schiller,(1995) submitted. 107. G. J. Kramer and R. A. van Santen, J. Amer. Chem. Soc., (1996) submitted. 108. E. Wimmer, In: N A T O ASI Series, submitted, 1996. 109. E. Nusterer, P. Bltichland K. Schwarz, Angewandte Chemie, (1996) submitted. 110. R. Shah, J. D. Gale, M. C. Payne and M.-H. Lee, (1996) submitted. 111. C. M. Ktilmel and J. Sauer, (1996) in preparation. 112. R.A. van Santen, Chemical Engineering Science, 50 (1995) 4027-4044. 113. J. M. NewsAm, C. M. Freeman, A. M. Gorman and B. Vessal, (1996) in preparation.

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H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All rights reserved.

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W h a t c a n b e in the c h a n n e l s a n d cavities o f zeolites? Karl Serf * Chemistry Department, University of Hawaii, 96822-2275, U.S.A.

2545 The Mall,

Honolulu, Hawaii

This article discusses the contents of zeolite frameworks rather than the frameworks themselves, to the extent that it is possible to do so. It treats zeolite frameworks as vessels in which, like test tubes, a very broad range of processes:and reactions may occur. It discusses the ions which the framework may need to balance its charge, and the atoms and molecules which it may sorb. It discusses the chemical processes which occur within zeolites, such as ion-exchange and sorption, and others more easily identified as chemical reaction. This article is about the chemistry that occurs in zeolites.

1. Z E O L I T E S AND T H E I R F R A M E W O R K S For the purposes of this article, zeolites are defined as crystalline nanoporous substances whose frameworks are inorganic oxides. This is a narrower definition than the most general, which includes organic and noncrystalline materials, and which remains open with regard to the framework anion: it may perhaps be sulfide or nitride. "Nanoporous" indicates that the dimensions of the pores are of the order of 1 nanometer (1 nm = 10 A), so it is exactly the right word to use in discussing zeolites, whose pores range in size from c a 0.3 to 3 nm.

1.1. Framework types and channel systems There are about a hundred zeolite framework types. 1 Some of these are found in nature, and are classified as minerals. However, most zeolite framework types are not found in nature but are synthetic, with the first having been reported in 1956. 2 More are reported every year due to active research efforts in zeolite synthesis. Each zeolite framework has a system of channels through which ions and guest molecules may enter and leave. The movements of ions and molecules are limited by the diameters of these channels, l, 3 These channels may extend in one dimension, like many

*Acknowledgement is made to the Donors of The Petroleum Research Fund, administered by The American Chemical Society, Grant No. 29025-AC5, for the partial support of this work.

268 parallel non-intersecting tunnels. They may be straight or they may be more or less sinusoidal. If these channels intersect in two dimensions, they form a net of channels, with no access from one such net to another. The most open zeolites have three-dimensional channel systems. For one-dimensional zeolites in particular, tiny concentrations of blockages may make large regions of the zeolite inaccessible. Some zeolite frameworks may be even more complex with, for example, two non-intersecting multi-dimensional channel systems. Other nomenclature is associated with these channels. Narrow points in a channel are called windows or, less definitively, rings. These further limit the movements of ions and molecules. Wider areas between windows and/or at channel intersections are called cavities or cages, and these may be the primary source of the intrazeolitic volume which is available for ions and molecules. This volume can be as large as half of the total volume of a zeolite crystal. 3 Cavities accessible from only one side may be called side-pockets. A zeolite whose channels are of relatively uniform diameter may be viewed as having neither windows nor cages. 1.2. Compositions of zeolite frameworks A given zeolite framework may exist with many different compositions, often continuous within ranges, and is therefore often non-stoichiometric. For example, a given framework type may simply be a pure silicate (a polymorph of silica), or it may be an aluminosilicate with a range of compositions such that the number of silicon atoms is greater than or equal to the number of aluminums, or it may be an aluminophosphate with the ratio of aluminum to phosphorous atoms not free to vary from unity. Other compositions are possible including many atoms in the periodic table such as B, Ga, Ge, Ti, and Zr. Beyond that, compositions with three or more elements included within the zeolite framework (in addition to oxygen) are possible; for example, Si is often substituted into aluminophosphate zeolite frameworks. One does not need to understand chemistry in great detail to know that two substances with different compositions are quite different, so a given zeolite framework type may describe substances with a wide range of properties. As a consequence of its composition, a zeolite framework may be charged or neutral. From the examples above, any silica polymorph and any aluminophosphate would be neutral. An aluminosilicate would carry a negative charge; most zeolites have negative frameworks. In recent years, some zeolites have been synthesized whose frameworks carry a positive charge. 1.3. Zeolite stability is defined with respect to a solvent system Stable zeolites must be defined with respect to a solvent. The solvent from which nearly all zeolites are synthesized, certainly all those found in nature, is water. The bonds of a zeolite must be non-labile in the solvent, or else the zeolite is, at best, extremely unstable. The smallest ions are the least labile, as a rule, so that high charge (3+ or greater), and small atomic number are favored. Within a zeolite composition, those ions which form labile bonds with oxygen atoms of a zeolite framework may move about within that framework, to the extent that channels for such movement are available. They may enter and leave the zeolite crystal, as long as other

269 ions replace them, and therefore cannot be a part of an enduring non-labile zeolite framework. It may be imagined that solvents other than water might be used to synthesize zeolites with anions other than oxides. For example, a sulfur-containing solvent might be used to synthesize sulfide zeolites, or a nitrogen-containing solvent to synthesize nitride zeolites. Both such classes of zeolites would be unstable with respect to reaction with water; accordingly, nearly all reported work involves oxide zeolites and aqueous media. Similarly, the earth's crust is composed nearly entirely of oxides. All zeolite frameworks found in nature are oxides. Nearly all are composed of the three most common elements in the earth's crust: oxygen, silicon and aluminum. 1.4. Zeolite stability with respect to temperature Of course, all zeolites are thermodynamically stable under the conditions of their synthesis. At that time their channels and cavities contain structure (are filled with molecules and ions). However, all desolvated (empty) zeolite frameworks are thermodynamically unstable with respect to decomposition to form more densely packed phases. This includes all neutral zeolite frameworks, and all charged frameworks with just the number of ions (of any kind) needed to balance that charge. It follows that all dehydrated zeolites, for example, will decompose to more densely packed phases when the temperature becomes high enough to allow structural rearrangement to occur. Most zeolites have a wide temperature range, hundreds of degrees Centigrade, between the temperature at which they can be fully desolvated and that at which they begin to decompose noticeably. This stability is a basis for the great utility of zeolites. 1.5. Occluded material within zeolites Zeolite frameworks may contain occluded material. During the course of synthesis, amounts of material which cannot be removed by heating or by washing may have been deposited within zeolitic voids. If an organic molecule or ion is used in the synthesis of the zeolite, it may be removed by burning in oxygen. Otherwise, this "debris," usually a remainder of the synthesis process and therefore composed of some or all of the elements of the zeolite's composition, is usually not present in stoichiometric quantities. Relatively little is known of its structure. If it lies in the main channels, it can interfere with the movement of molecules and ions through the zeolite. It may have ion-exchange capacity. Other occlusions may be introduced as a zeolite is subjected to further chemical treatment. For example, zeolites used as catalysts in oil-cracking or reforming operations may have catalytically active clusters finely dispersed within their structures as occlusions. Upon use, these zeolites may become occluded by carbonaceous deposits, which may be burned off (with oxygen) to reactivate the zeolite.

2. THE CONTENTS OF NEUTRAL ZEOLITE FRAMEWORKS The channels and cavities of zeolites with neutral frameworks may be empty. Certainly no ions are needed to balance the charge of the framework. The most common examples are zeolites of composition SiO 2 (the silica polymorphs) and A1PO4 (the alpo's). These zeolites

270 may exist simply as neutral frameworks containing neither ions nor molecules within their void spaces. This state is readily achieved by mild heating under vacuum. Nearly all zeolite structure models, often seen in laboratories and in the literature, show only the general structure, just the morphology or topology, of the zeolite framework. It is only for these neutral zeolite frameworks that those models can be complete. (Even then, the model is not complete unless all of the elements in the framework are labelled, to indicate, for example, their ordering.) For all other zeolites, much more must be added (the positions of the labile cations, at least) for the model to be considered complete. In general, these models are only convenient starting points for discussions of structure and properties. Zeolites with neutral frameworks may be particularly stable and rather inert. The coordination requirements of all of the cations in the structure are already satisfied by the structure of the zeolite, and there are no sites which are particularly unique or reactive. There are no ions which might have been left in states of particularly low coordination (hence high reactivity) by the removal of guest molecules (generally water). The magnitude of the electric field within these zeolites is relatively low throughout the void volume. However, the size of the cation governs lability, so one would expect a zeolite of composition GeO 2 to be significantly less stable than that of a silica polymorph. The contents of zeolites with neutral frameworks must themselves be neutral. Accordingly, only molecules and/or salts with no excess ions to unbalance charge may be present. Because of the weak electrostatic fields within these zeolites, there is little reason for ionization to occur; in other words, they are poor solid electrolytes. 4 For the same reason, sorption energies are small and these zeolites are relatively easy to empty.

3. THE CONTENTS OF CHARGED ZEOLITE FRAMEWORKS Zeolite frameworks may readily have a net charge. It is an unusual situation where the sum of the charges of the oxide ions equals the corresponding sum for the framework cations. For example, given a silica polymorph, the replacement of a Si4+ ion by A13+ leads to a structure unaltered in its morphology but with a deficiency of one positive charge. Such a zeolite framework would therefore be negatively charged, and the magnitude of this charge, usually given as the framework charge per unit cell, depends upon the degree of such substitution. Although it is possible to synthesize zeolites with positive frameworks, for example by replacing Si4+ with p5+ in a silica polymorph, far less work has been done to explore such systems. None of the zeolites found in nature are of this type. To increase the readability of the remainder of this article, positive frameworks will not be considered further. This will allow us to say simply that it is cations that balance the charge of negative zeolite frameworks.

3.1. Non-framework cations balance framework charges Additional ions, not part of the zeolite framework, are needed to balance the charge of a charged zeolite framework. They must take up some of the void space within zeolites, and may occupy or block windows. The properties of a zeolite are often sensitive to the nature and placement of these extra-framework ions.

271 In a zeolite free of guest atoms or molecules (guests are not formally addressed until Section 5), many or all of these cations must be viewed as being in an extremely unusual chemical situation. During the synthesis of the zeolite, these ions were suitably and reasonably surrounded by neighbors, usually both oxygens of the zeolite framework and solvent (guest) molecules. The treatment which removes the guest molecules usually leaves some or all of the cations in a state of inadequate coordination or association, perhaps extremely so, like ships on mountain tops or fish high on the shore. A useful zeolite framework must be able to withstand this treatment without collapse. Also, cations suitable for a zeolite so to be treated, in their quest for suitable coordination, must not cause the collapse of the zeolite. For example, Ba 2+ ions, if their concentration is sufficiently high, will destroy the structure of zeolite A when the guest water molecules are taken away; 5 Ba-X, in contrast, may be fully dehydrated without loss of structure. 6 Upon removal of guest molecules, suitable cations simply relocate themselves to the best sites available. These are almost always far less satisfactory than the ones they had in the original solvated zeolite crystal. Most unusual coordination geometries are the rule here, with three-coordinate planar being one of the more common. Some cations find themselves not surrounded by coordinating oxygens at all, but up against somewhat of a wall of several oxygens, all on the same side. Many or all cations are left with substantial solid angles in which they lack coordination, and look to relieve this situation, for example by sorption of a wide range of possible guest molecules, or by other chemical reaction.

3.2. Suitable non-framework (exchangeable) cations To exaggerate, the list of cations which can balance the framework charge is as large as the periodic table, and as broad as chemistry will allow. Of course, the most electronegative elements do not form chemically stable cations, e.g. C1+, 0 2+. Of course, cations which interact with framework oxygens in a non-labile manner cannot be considered extraframework, e.g. Al 3+, Si4§ cations with high positive charge, 3+ or higher, are generally non-labile unless they are large. And, of course, cations which are physically too large to fit within the zeolite (these are usually polyatomic cations) cannot be present. Similarly, cations too large to pass through channels and windows may generally not be introduced (see, however, Section 4). Most suitable are the stable 1+ and 2+ cations of the metallic elements, such as Na + which is present from the time of synthesis in many zeolites, and Ca 2§ which can readily be introduced to replace Na + in most zeolites. Thirty or more such familiar ions come easily to mind from an inspection of the periodic table, e.g. Ag + and Zn 2+. The larger of the 3+ cations, such as La 3§ are suitable for some zeolites. Polyatomic cations such as NH4 + and alkylammonium ions (and H30+), and perhaps others like (UO2) 2+, can balance framework charge. One may dream of placing, by resourceful chemical methods, unusual, perhaps previously unknown, cations, e.g. Cd+, Te+ or It , into zeolites. With such thoughts and experiments, zeolites can be used to extend our knowledge of the chemistry of many elements; zeolites may be used as a tool to explore wide ranges of unusual chemistry. 3.2.1. Monopositive cations The cations which balance framework charge may be relatively large with small charges, e.g. 1+. A much larger number of these will be required than of the more highly charged

272 cations. A much larger fraction of the intrazeolitic volume is occupied, and channels may experience a much greater degree of blockage, perhaps being fully blocked. Beyond that, due to cation crowding in the available space, it may be difficult or impossible to prepare a zeolite all of whose cations have large size and small charge. 7 The electric field maxima in the void space of such a zeolite are relatively small, so the forces which can be applied to a guest molecule for the purposes of sorption or catalysis will be weak. A zeolite framework is generally more stable with larger cations of smaller charge. It is more relaxed, less distorted by the local electric fields at the surfaces of the cations. The charge of the zeolite framework is relatively diffuse, amounting to only 1- for the six atoms of a SiA10 4 unit or 2- for an SiTiO 4 unit (where Ti is in the 2+ oxidation state). The former is better balanced locally by many monopositive cations than by fewer cations of higher charge. However, the latter may be better balanced by 2+ cations. It is possible in some cases that a 2+ cation can fit very nicely into a geometric feature of a zeolite, there to be near several negative centers, e.g. SiA104".

3.2.2. Cations of charge 2+ and 3+ Alternatively, these cations may have higher charges, e.g. 2+ or 3+. Fewer of these cations are needed to balance the framework charge, and these cations are smaller. For both reasons, much less void space is needed to accommodate these cations, so the cavities and channels remain more open. Guest molecules have easier access to the zeolite, and more room remains to accept more of them. In addition, because the electric field maxima are larger in a zeolite so exchanged, specifically at the "surface" of these cations, guest molecules are more strongly held. This has consequences for the processes of sorption and catalysis. 3.2.3. Transition-metal cations Transition-metal cations introduce some additional considerations and provide some additional opportunities. These ions, in addition to balancing the charge of the zeolite framework, have coordination requirements which need to be satisfied. Consequently, for example, in order to achieve a more stable coordination geometry, a divalent transitionmetal ion may situate itself in a site different from that which an alkaline-earth ion would select. Specific orbitals, which may be filled, or empty, or which may contain an unpaired electron, may extend into the void space where they may interact with guest molecules, and this may govern processes such as sorption, catalysis, and hydrolysis. A transition-metal cation in an unsatisfactory coordination situation, perhaps in a state of coordinative unsaturation, can achieve additional stability by interacting with a guest molecule. Even by accepting an unsuitable guest into its coordination sphere, even at a long distance, it can achieve a small measure of additional stability. These interactions may be chemisorptive or physisorptive, spanning a broad range of energies. In this way, transition-metal cations may participate in some novel chemical processes involving the guest molecules to which they coordinate. 3.2.4. Hydrogen ions Hydrogen ions may be exchanged into zeolites directly from acidic solution. They may be generated within zeolites by exchange with ammonium ions followed by heating and

273

evacuation to remove ammonia (or alkylamines). They may be inadvertently exchanged into zeolites by washing with pure water, which contains H + ions in low concentration and which may be considered to be a dilute H + ion-exchange solution. They may be introduced into a zeolite as a consequence of simple ion-exchange procedures, so that not only the principle cation of an aqueous ion-exchange solution, but also H +, is introduced to the zeolite. In this regard, the zeolite can be quite selective, sometimes accepting a much greater fraction of H + ions than their mole fraction in the exchange solution. For example, the exchange of Ca2+ from near neutral aqueous solution into zeolite A gives a product with pronounced Br6nsted-acid catalytic activity; to achieve simple Ca 2+ exchange, aqueous Ca(OH)2 should be used. The actual pH of the exchange solution in contact with a zeolite sample is often essential in determining the result of the exchange experiment, that is the composition and structure within the intrazeolitic volume. For example, an increase in pH of less than two units is sufficient to increase the Pb 2+ or Cd 2+ content of zeolite A by about 50% as many hydroxide ions are accepted by the zeolite. This pH may change during ion exchange as hydrogen ions, or alternatively hydroxide ions coordinated to cations, are accepted by the zeolite. Finally, hydrogen ions may appear within zeolites as other cations hydrolyze: water molecules coordinated strongly to transition-metal cations, for example, may dissociate, leaving hydroxyl groups coordinated and generating hydrogen or hydronium ions which locate themselves elsewhere within the structure, or exchange out of the zeolite. The tendency of a transition-metal cation to hydrolyze may be greatly enhanced within a zeolite as it strives to achieve an acceptable coordination geometry, and as the zeolite seeks to have its anionic charge better balanced by more cations of lesser charge. Some dipositive transition-metal cations may hydrolyze twice to become hydrated M(OH)2 molecules, dispatching the smaller hydrogen ions to balance better the local anionic charges of the zeolite framework. Water, although it is the solvent with respect to which all oxide zeolites are defined, greatly complicates the ion-exchange process because of its ability to dissociate. Yet no other solvent is known, with the possible exception NH3(g), which is as successful for ion exchange. 8 Some zeolites are unstable with respect to acid exchange, from whichever of the above sources. Generally this includes zeolite frameworks with high aluminum content. This means that exchange with cations that can hydrolyze can lead to the collapse of the zeolite structure. H + ions bond directly to individual oxygens of the zeolite framework in a dehydrated zeolite, so that these selected oxygens are three-coordinate. When guest molecules are present, they may associate with these H + ions to form hydronium, ammonium, carbenium, oxonium, and other such cations within the zeolite. Oxonium ions are the intermediates in most catalytic processes involving acid zeolites such as H-Y or H-MFI.

274 4. M O L E C U L A R AND IONIC SIEVING The windows and channels of a zeolite allow it to discriminate among guest molecules and cations on the basis of size. This property is so important and easy to understand that "molecular sieve" is taken to be a synonym for "zeolite." However, with this simplicity should come a strong precaution. Firm conclusions made on the basis of a comparison of a zeolite window size with an effective cross-section of a molecule or ion, are often in error. Crystallographic results show readily that many statements found throughout the zeolite literature, saying that an ion, atom, or molecule is too large to enter a zeolite are incorrect. Alternatively, it may be said that one of these species may enter through larger windows, but, incorrectly, that it may not pass through smaller windows to reach other void volumes within the zeolite. Nonetheless, the place of window and channel size in discussions of the sieving properties of zeolites is firmly established. Large alkylammonium ions used to template the growth of some zeolites cannot be removed by ion exchange, clearly because they are too large. Similarly such ions cannot be exchanged into the zeolite from solution. Molecules and ions can readily be too large, not only to pass through windows, but to fit within cavities or channels. Given a mixture, at least a partial separation may be readily achieved if one component presents a much larger cross-section to a zeolite window, e.g. a n n-alkane v s a branched alkane of the same formula.

4.1. Cautionary considerations in the application of size arguments The problem with a strict "fit" or "don't fit" mechanical view of passage is that it is difficult to define the "sizes" of chemical species. Specific sizes of species are generally not discussed in chemistry at all, except with caution and severe disclaimers in specific situations: bonding radii, ionic radii, and van der Waals radii. Each is understood to be only an approximation for the purposes of discussion, and is used carefully. After all, are we not all taught that the electronic wave function of an atom or molecule goes to zero only at infmity? A beginning student of zeolite science often lacks this caution, and reaches conclusions as if rigid parts were being assembled. These "sizes" may be further effected by the flexibility which an ion or molecule may show, either because of its vibrational modes (for a molecule or a molecular ion), or by its interaction with the zeolite framework, which it approaches very closely as it seeks to pass through a window, or pass along a channel. Secondly, the process of passage of an atom, a molecule, or an ion through a window may be viewed as a chemical reaction, and, as such, it may involve a mechanism of several steps. A ring complex involving an ion and one or more molecules may form, and its stability and vibrations may be responsible for an activation energy which will govern the process of passage. For the passage of a cation, one or more solvent molecules, which may readily coordinate to it and perhaps simultaneously hydrogen bond to the zeolite, may participate. The integrity of a zeolite window may itself be compromised. If water is present, one or more of these could actually add to the zeolite framework to give AI-O-H and H-O-Si groups, breaking and reforming the ring as part of the mechanism of passage. Although little is known of this, the failure of ion-exchange into zeolites from all solvents except water 8 and liquid ammonia (unless the window is very large in comparison to the species passing through it) is compatible with such mechanisms.

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4.2. Mechanisms for passage through windows Of course, temperature plays a role in sorption. At higher temperatures, enough energy may be present for a species to pass through a window by overcoming an activation energy of passage. Alternatively, one may think of both the ring and the species at its doorstep as having more pronounced thermal vibrations, and that a momentarily larger window might allow a guest species with a momentarily smaller cross-section to pass through. Simply by the use of Boltzrnann's Equation, one may calculate, from an estimation of the bond energy, the number of windows which are broken at any temperature. Guest species may themselves appreciably lower the energy of such bond breaking (by being present as part of the "activated complex"), and in this way, "allow themselves through," or at least contribute substantially to their own passage. To the degree that such options are available for passage through windows, thermodynamics, rather than kinetics, assumes the dominant role in deciding the outcome. Atoms, ions, and molecules would go to positions of minimum free energy within the zeolite. Some species may not pass through some windows, not because the windows are "too small" (kinetics), but because they prefer not to be on the other side (thermodynamics). For example, TI+ ions, which readily pass through 6-rings to occupy sodalite-unit positions in dehydrated T1-X, do not pass through other 6-rings to occupy double-6-ring cages. 9 Many more stories can be told in this regard. The channel structure of natural mordenite was not foreseen on the basis of sorption experiments because a relatively small number of blockages or dislocations quite adequately blocked the large channels. In an experiment in my laboratory, sodium LTA failed completely to sorb tetrachloroethylene (both fully anhydrous) because of its size (The crystal structure of a single crystal in an atmosphere of ca 100 torr of C2C14 for more than a week was exactly that of fully dehydrated LTA.); we should have honored a simple mechanical calculation of size; apparently no alternative mechanism existed. In contrast, statements that Cs + ions should not be able to enter sodalite cavities in LTA or FAU, have repeatedly been shown crystallographically to be incorrect; such entry of Cs + appears to occur both in the presence and absence of water. It is important to count the number of windows through which passage must occur for a process to reach completion. If only one window is involved, as in passage into a sodalite cavity from the large cavity of LTA or FAU, perhaps an exotic mechanism (such as window breaking) can exist which would accomplish this result within the timespan-of an experiment. Passage through many more windows is needed for bulk sorption into the body of an entire zeolite crystal, and this may appear experimentally to be forbidden for species which are "too large."

4.3. Blockage by exchangeable cations To a degree, exchangeable cations may block windows. Just that they could be exchanged into the zeolite indicates that they are labile, and can continue to move from site to site unless the conditions have changed appreciably, so each "blockage" of a ring by a cation may be considered temporary. At ambient temperatures, where the barriers to migration are easily surmountable, these ions may move easily from site to site, leaving windows temporarily open with sufficient frequency that sorption appears to be unhindered. In addition, the cations and the guests may cooperate in the process of passage; rather than

276 rigidly blocking a window, a cation may interact with a guest, perhaps to form a complex which hinders passage, or perhaps to "escort" it through. Molecular modelling can begin to describe the dynamics of passage of a neutral species through a window which is already occupied by a labile cation. At ambient temperatures, Cs + ions fit nicely at the centers of A14Si40 8 rings (8-rings) in zeolite A. Although these Cs + ions are mobile, when all of the 8-rings are so filled, or even only about 90% according to experiment and percolation theory, small molecules trapped at high pressure (encapsulation, see Section 8) cannot escape from the zeolite. 10 Not only is the number of 8-rings even temporarily vacant too few to allow frequent passage, but also the energy which would allow a Cs + ion to go to some other site within the zeolite, perhaps to trade places with another cation already there, appears to be too high at ambient temperatures. The sorbed and trapped atoms or molecules may contribute to the establishment of this high energy barrier by inhibiting the movements of the cations.

4.4. Hindered diffusion may be incorrectly described as window shrinkage Finally, it is important to consider the larger area of diffusion through zeolites. Very slow diffusion may be incorrectly interpreted as blockage by sieving. Impediments to diffusion, such as crystal damage or blockage of windows or channels by ions or molecules, may lead to the observation that "the windows have shrunk." Of course, this is structurally impossible, except to a small degree, without the destruction of the zeolite framework. In fact, the "shrunken" windows may indeed have become somewhat larger if they have relaxed in the presence of a welcome guest. The classical example of this is zeolite A (LTA) which has been ion-exchanged with K +, left as synthesized with Na + as its exchangeable cation, or largely ion-exchanged with Ca2+. These carry the common names, respectively, of zeolite 3A, zeolite 4A, and zeolite 5A. These names were chosen because they indicate the apparent diameters (very approximately!) of the windows in Angstroms. Sorption characteristics indicate that the windows appear to be larger when fewer smaller cations are present (Ca2+), as compared to the original sodium form of the zeolite, and that they appear to be smaller if larger cations (K +) are in place. The fact of the matter is that these 8-ring windows are all very nearly the same size in all three forms of zeolite A. From a consideration of the structure of zeolite A, and with regard for the general inflexibility of aluminosilicates, it is clear that these rings are :not nearly as flexible as the names 3A, 4A and 5A indicate. The Linde Corporation wished only to describe the sorption properties of its products when it chose these names.

5. SORPTION OF ATOMS AND MOLECULES Neutral species are generally welcome to enter a zeolite through its windows and channels, as long as these avenues of entry are large enough and are not blocked, and as long as sufficient void volume exists within the zeolite. These species may be atoms or neutral molecules. The latter include covalent molecules and salts; the latter especially are free to dissociate within the zeolite upon sorption. A zeolite which is empty of guest molecules and whose exchangeable cations, if any, are relatively few and small, will have the most void

277 volume available for guests. It will also have the greatest number of open windows, those which are not blocked by exchangeable cations at least some fraction of the time. 5.1. Association with and coordination to exchangeable cations The degree of sorption depends upon the forces of interaction between the guest and the zeolite. Usually the strongest forces for sorption involve the exchangeable cations, if any, within the zeolite. These, when they are not well surrounded by framework oxygens, and they often are not (see Section 3.1 .), may be closely approached by guest species, which may be strongly polarized by the electric field of the cation. If the guest is non-polar, the sorption forces are ion-to-induced-dipole in nature. If the guest is polar, the sorption forces are iondipolar, and are much stronger. It is primarily for this reason, for example, that empty cation-containing zeolites are strong desiccants. (In common usage, one would say that activated (cation-containing) zeolites are good drying agents, or that high temperatures (and perhaps vacuum) are needed to activate (fully) a (hydrated) zeolite.) Smaller cations with higher charges will render a zeolite more effective as a sorbing agent for both polar and nonpolar guest species, at least at low loadings. When neutral molecules are sorbed which dissociate to give ions within the zeolite, as can occur easily with salts, acids, and bases, the number of non-framework cations will have increased and anions will have been introduced into the structure. These anions may contribute to the stability of the structure by coordinating to, often bridging between, cations; this, in addition to the availability of suitable sites for additional cations within the zeolite, may be a major driving force for sorption. When the cation of the sorbed molecule is of the same element as cations formerly resident in the zeolite, it is not possible to distinguish between resident and guest cations. The ions of the sorbed molecule simply become participants in a resulting stable arrangement of ions. This often occurs, for example, when ion exchange of transition-metal ions is attempted using halide salts, in solution or as solids. When Cd2+-exchange is attempted from aqueous CdC12 into zeolite A, the product is hydrated Cd9.5C14(OH)3Si12Al12048,11 which may be fully dehydrated without loss of CdC12 or Cd(OH)2. When transition-metal cations are resident within an empty zeolite, they may interact further by a covalent mechanism with a suitable guest. For example, an electron pair from a guest may delocalize into an empty d orbital on the cation. Such interactions are not only stronger, but may be more disruptive to the electronic structure of the guest. They are more demanding in their geometrical and stereochemical requirements, and may play a specific role in further reaction and catalysis. For example, unsaturated homonuclear organic bonds have been found to coordinate sideways to ions such as Ag +, Co 2+, or Mn 2+ within a zeolite. 5.2. Interactions with oxygens of the zeolite framework Rather than with the cations, guest species may choose to interact with the zeolite framework, looking to it as a Lewis base. For example, the heavier dihalogens form linear charge transfer complexes with oxygens of the zeolite framework, and ignore the opportunity of being polarized by coordination to a cation. 12 Sorbed species may interact easily with the zeolite framework by hydrogen bonding, which is enhanced when the zeolite framework is negative. The ammonium cation, and all but the tertalkyl alkylammonium

278 ions, interact with the zeolite not only electrostatically, but also by hydrogen bonding. Those zeolites which lack exchangeable cations, such as polymorphs of SiO 2 or those of composition A1PO4, will be able to interact only in this way with guest species. In general, their interactions will be much weaker, and they will be less effective sorbers. They may sorb without important interaction, simply providing a haven for molecules which do not fit well into a polar phase, such as an aqueous or wet organic phase, which may be in contact with the zeolite.

5.3. Other and multiple interactions Guest molecules may interact favorably with each other upon absorption within a zeolite. Whatever interactions a pure substance may have amongst its atoms or molecules, may be carried into the void space of a zeolite. Polar molecules may continue to orient favorably, hydrogen-bonding molecules may continue that interaction, and formal chemical bonds can reform within the zeolite. Van der Waals liquids may continue to be that upon sorption by a zeolite. An atom or molecule may interact with the zeolite in more than one way. For example, it may associate with a cation while hydrogen bonding to the zeolite framework, or while bonding to other guest atoms or molecules. Each interaction may be perturbed by the other. For example, the polarization of a sorbed molecule by association with an exchangeable cation may enhance its ability to hydrogen bond. Alternatively, the requirements of the stronger interaction may leave the secondary one strained. When such multiple interactions occur, the guest species may be viewed as having selected a "more detailed," perhaps a more oriented, sorption site within the zeolite, and it may be more susceptible to distortion because of the various mechanical and electronic "pulls" which it may be experiencing. The interactions which a guest species has with the zeolite should modify its interactions with other guests. When the guest interacts strongly with the zeolite, it is easy to imagine that the electronic structure of the guest will be effected. When these latter interactions are weak, as amongst the molecules of most gases, the dipoles induced by a primary interaction with the zeolite may allow the interguest interactions to be very much enhanced. A fluctuating dipolar interaction (van der Waals) may become a static dipolar interaction. This is seen in the interactions among Kr atoms, 13 and among Ar atoms, encapsulated within zeolite A. Finally, as a zeolite fills with neutral guest species (atoms or molecules), each successive guest must encounter an at least slightly different sorption environment, and will generally be sorbed with a lesser energy. As a coarse example, water molecules may associate with cations until there are no more such sites, but void volume may remain. Additional water molecules will then be held only by hydrogen bonding, to the zeolite framework and/or to the previous "layer" of water molecules, by interactions enhanced by the additional polarity which the zeolite may have induced upon all of these waters, especially those coordinated to cations. If the exchangeable cations are of several kinds, either chemically or because their sites are not equivalent, a greater range of sorption sites and energies is seen. Often, evacuation of such a zeolite at ambient temperature will remove the more weakly held water molecules, leaving in place those more firmly held.

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5.4. Degree of filling Depending upon the conditions for sorption, there is no requirement that the zeolite fills. The sorption energies may decrease to the point that further sorption is not favorable. Fundamentally, the water content of most zeolites depends upon the humidity; only at high levels may the zeolite be "full." Similarly, and more commonly understood, it depends upon temperature. Usually, however, the concentration of guest species will be greater within the zeolite than it will be extemal to the zeolite, unless there are stronger interactions in the pure phase of the guest substance. So, although one does not see mercury atoms leaving liquid mercury to assemble themselves within a zeolite, some concentration of the vapor, greater than that in the external phase, is to be expected because of the electric fields within the zeolite. 5.5. Retention within the zeolite of aspects of the crystal structure of the sorbate Ultimately, one may expect to recognize aspects of the crystal structure of the (original) pure sorbate within a zeolite after sorption. When the structural units or features of the pure sorbate are too large to enter the zeolite, subunits of these, perhaps individual atoms, may have been sorbed. When this occurs, it may be said that the original substance has reassembled significant aspects of itself, like a ship in a bottle, within the zeolite. At 300 ~ molecules of S 8, which are too large to enter through an 8-ring of zeolite A, have decomposed to form smaller species, perhaps S2, which entered and reassembled to give molecules of octasulfur within the zeolite. 14 In this way, trigonal and tetrahedral oxide and hydroxide clusters of Pb 2+ commonly found in aqueous solution have been found within sodalite cavities of zeolites A15 and X. 16 The same is true for a number of salts, such as CdS, which have been imbibed into or synthesized within zeolites. One may look ahead, in the development of new materials, to the installation of the desirable or functional structural units of a range of substances into zeolites. In all cases, the influence which the zeolite has upon its guest, either by the geometry of its void space or by its electrostatic field and other opportunities for bonding, must have a large impact on the actual geometrical and electronic structure of the guest. These units of structure, which are both similar to and yet different from the original sorbate structure, stabilized and supported in crystallographically regular arrays within zeolites, hold much of the promise of future work in zeolite science.

6. A SINGLE SUBSTANCE MAY REACT WITHIN A Z E O L I T E A single substance may rearrange or decompose. These are considered to be the two unimolecular processes, although the mechanism for each may involve another species (a catalyst). Reversible decomposition (dissociation) is often assisted by the presence of other molecules (a solvent, perhaps). Finally, the molecules of a single substance may oligomerize. All of the above may occur within zeolites. Within zeolites, the reaction rates may be enhanced, the products may be different, and the equilibria may be shifted, perhaps dramatically, especially by interaction of the guest with the labile (exposed) cations which the zeolite may contain. Zeolites with neutral frameworks lack these, and if they are otherwise empty they will generally be at least much less active. (To enhance the activity of

280 these zeolites for both these and other processes, at least some small measure of framework substitution is desirable, so that some cations, perhaps with enhanced individual activity (and selectivity) because of their small concentration, will be present.) Catalytic centers added as occlusions within zeolites (discussed in Section 1.5.) may very much enhance this activity. Finally, a guest atom or molecule may react directly with an exchangeable cation to give product(s), including product cation(s). Chemical reactions involving the zeolite framework are not within the scope of this article, and are not discussed. 6.1. Reaction of molecules with exchangeable cations If the cations are hydrogen ions (see Section 3.2.4.), guest molecules may add to them to give, for example, hydronium, ammonium, oxonium, or carbenium cations. The latter two may rearrange, and then decompose or dissociate to give product(s) which can leave the zeolite. Oxonium ions in particular are central to the most economically important processes of petrochemical industry. In simpler words, hydrogen zeolites are very important catalysts. The reverse of some of the above reactions may be seen in the decomposition by heating, perhaps in oxygen, of polyatomic non-metallic cations which have been ion exchanged into the zeolite, or which were present as templates from the synthesis. These include the ammonium, hydronium, and alkylammonium ions. In all cases, the final products are hydrogen ions. Depending upon the temperature and the zeolite, these may leave with oxide ions of the zeolite framework as water vapor, so that the framework which remains is deficient in oxygens. Molecules capable of ionic dissociation, such as H20 and HCN, sorbed in small to moderate quantities into zeolites containing suitable cations, may be substantially dissociated. This may occur by a hydrolysis or dissociative process at the cation, and will be enhanced by the electric fields within the zeolite. The resulting anion coordinates to one or more exchangeable cations to give a larger cationic cluster, and the newly formed cation may move to another site within the zeolite where it can approach anionic charges such as framework oxygens. An organic disulfide sorbed into a transition-metal form of a zeolite may dissociate to give two thiol radicals. These may coordinate to cations by a mechanism which involves electron sharing or electron transfer, for example to oxidize the cation further and to generate coordinated organic sulfide ions. This was observed with dimethyldisulfide sorbed into Co2+-exchanged zeolite A. 17 Oxides of nitrogen can be decomposed over activated (CuOH)+-exchanged ZSM-5 (MFI) to the elements. 18 6.2. Reaction of metal atoms with exchangeable cations Metal atoms may react with exchangeable cations by a redox process, replacing the original cations within the zeolite by product cations. Alternatively, metal atoms may add to exchangeable cations to form cationic clusters. These clusters may be small (discrete) or very large (continuous). When alkali-metal atoms are involved, either as reagents or products of reaction, a few words of caution regarding synthesis should be noted. Alkali-metal atoms may readily react

281 with a zeolite framework to destroy it, and with the glass vessel which contains the experiment. These effects are avoided or minimized when reaction temperatures are kept to a minimum, the silicon contents of both the zeolite and the glass are minimized, and the size of the alkali metal atom is maximized. As secondary considerations, the time of reaction should be minimized and there should be no excess of alkali metal. 6.2.1. Exchangeable cations and guest atoms of the same element If the atoms and cations are of the same element, then two types of reaction may occur. If the cations are of charge 2+ or higher, they may be reduced by the incoming atoms to give more cations of lower charge. Alternatively, the valence electrons of the incoming atoms may delocalize over the original cations to give cationic clusters. 6.2.1.1. Exchangeable cations may be reduced by guest atoms When cadmium atoms are sorbed into Cd2+-containing zeolites, redox reaction occurs to give cadmium ions in the 1+ oxidation state. These are found both as discrete Cd 1+ cations and (Cd2) 2+ dimers. 19 In addition, neutral atoms may be sorbed without oxidation, in one case to form diatomic molecules. Such reactions are far from general. For them to occur, the 1+ oxidation state must be relatively stable. This reaction is not known for any other element besides cadmium. For example, H-Y exposed to Zn(g) at 420 ~ after being converted quantitatively to Zn-Y, accepted no additional atoms of zinc into its structure. 20 6.2.1.2. Localized cationic clusters Long ago it was observed that the sorption of sodium by anhydrous Na-Y led to a bright red zeolite Y which was shown by ESR to contain clusters of (Na4)3+.21 Not until recently was the structure and location of (Na4) 3+ within the zeolite fully confirmed. 22 One atom of sodium had reacted with three Na + ions to give four equivalent Na § ions, arranged tetrahedraUy within the sodalite unit, with one electron uniformly delocalized about them. Other such clusters have been found, some by crystallographic methods, some by ESR, and some by both. The combination of the two methods yields a relatively complete understanding: the location of the cluster within the zeolite, its interactions with the zeolite, its own physical and electronic structure, and perhaps some information about the rate at which the individual ions interchange within each cluster. These clusters include (N--~2+ a~j , 23,24 (Na5)4+,25,26 (Na6)5+,26,27 (K3)2+,28 (K4)3+,29 (Rb6)5+,30 (Rb6)4+,31 (Cs3)2+, 32 (Cs4)3+, 33 and (Cs4)2+. 34 Not all of the above compositions are fully established, and others have been suggested which are less well established. Some of these clusters are likely to have different shapes in different zeolites, all are sure to be different in their bond lengths and angles in different environments, showing great flexibility. More than one kind of cluster can exist within a zeolite at a time, and this may be related to the extent of sorption.23, 26 For some clusters, it may not be clear whether some peripheral ions should be included in their formulation,30, 31 and some authors have been reluctant to assign specific cluster formulae. 22 For example, the presence of "cations" in double 6-rings very near to tetrahedral clusters in sodalite units, raises serious questions regarding the formulae which should be used.22, 29 However, it is abundantly clear that this type of reaction is quite

282 general and is likely to occur with many variations within any zeolite which has suitable exchangeable cations. These polyatomic Group I cationic clusters can be further categorized. All of the sodium ions in tetrahedral (Na4)3+ are equivalent. The same is true for tetrahedral (K4) 3+. The individual ions in the trigonal clusters (Na3)2+ and (K3)2+ are also equivalent in each cluster. In contrast, the ions in each of the linear polyatomic cations (Cs4) 3+ and (Cs4) 2+ are not equivalent; in each, the two central ions are, by symmetry, not equivalent to the two terminal cesium ions, but all are ions, perhaps of similar charge judging by their approach distances to framework oxygens. Similarly, the three central ions in each of the trigonal cations (Rb6) 5+ and (Rb6) 4+ are not equivalent to the three terminal ions. Finally, the ions at the center of the linear (Na3)2+ 24and the centered-tetrahedral (Na5) 4+ clusters are far from framework oxygens, and so seem by their bonding to be more like atoms than ions; they should be very different from the other ions in their respective clusters. The bonding is different in these categories of clusters. Those in linear (Na3)2+ and centered-tetrahedral (Na5) 4+ are primarily radial, from the central ion (nearly an atom) to the others, with bond distances very near the sum of the corresponding ionic and metallic radii. (This sum is useful for comparison, and even appears to be correct, but has no theoretical basis.) The distances in the remaining clusters are much longer than the corresponding sums, and are much more variable. For those clusters whose ions are all equivalent, the primary bonds are all parallel to the circumference or the surface of the cluster. Three driving forces can be identified for the above reactions, not all of which apply to each situation. First of all, the electrons of the guest atoms have an opportunity to delocalize to a much larger volume. Secondly, the resulting increased number of cations of lesser charge is able to approach more of the anionic framework oxygens, balancing charge better within the zeolite. Finally, especially in the case of the larger cations, the severe repulsions between the cations, some of which must otherwise be very much too close together within the zeolite cavities (cation crowding), may be decreased substantially by using the additional electron(s) to pack some of these cations closer together (by bonding), so that greater intercationic distances become available to the remaining, non-participating cations. No such clusters involving more than one metallic element have been found. Those which have been reported have been recanted. (As occlusions which are not well defined structurally, bimetallic clusters of transition elements are often discussed in catalysis.) For delocalization to occur readily, the energy levels of the partly filled orbitals on the ions involved should be very close. They are, of course, exactly the same when the atoms and ions are of the same element, and this maximizes the opportunity for delocalization. The bonding within these polyatomic cations is weak. One can readily calculate bond orders which are small fractions. There is no suggestion that any of these cations would be stable outside the zeolite, nor that their geometries are as rigid as those of molecules or molecular ions. All of these polyatomic cations conform to the electrostatic requirements of the zeolite framework, somewhat as liquids adopt the shapes of their containers. Perhaps it is reasonable, because the bonding is so weak, not to refer to these as polyatomic cations at all, but rather as electron traps. This more physical description is consistent with the observation that some of these clusters can be prepared (in low concentration) by ),-

283 irradiation. In fact, the cluster (Na4)3+ was discovered in this way, by exposure of (fully dehydrated) Na-Y to gamma rays, not to sodium vapor. 6.2.1.3. Delocalized cationic continua

If the zeolite accepts sufficient atoms, cationic continua may form whose extent is that of the entire zeolite crystal. In these continua, the metal atoms and ions are, for the most part, no longer distinguishable by their bond lengths to each other. From their approach distances to the zeolite framework, nearly all appear to be cations with diminished positive charge. All or most of the valence electrons appear to be delocalized over the entire cationic continum. When Na92-X was exposed to cesium vapor at 450 ~ redox reaction went to completion to give sodium metal, which left the zeolite, and Cs92-X. The reaction did not stop at that point, however. Thirty-six additional atoms of cesium were sorbed per unit cell. It appears from the structure that six Cs + cations did not participate in this latter reaction, but that the remaining 86 Cs + ions had reacted with the 36 Cs atoms to give a cationic continuum of formula (Cs122) 86+ per unit ce11.35,36 This continuum is three dimensional, filling (approximately) all of the supercages. The contents of each supercage engage in multiple contacts through each of the four surrounding 12-rings to the contents of the adjacent four supercages. In addition, an appendix of two cesium ions extends from this continuum into each sodalite cavity. Although an argument can be made for localizing some of the 36 electrons among some of the cesium positions, it is clear that most of them are delocalized among most of the cesium ions. Without such considerations, it may be said that the average charge per cesium ion is +92/128 in this structure, or perhaps +86/122 per cesium ion in the continuum. In either case, the average charge is about +0.7 per cesium cation. A similar reaction was observed when potassium vapor was exposed to Na92-X.37 In this case, the supercages once again filled, more or less, with potassium atoms to give clusters which were in contact to give continua. In addition, another three-dimensional interpenetrating potassium continuum, with no contacts to the first, was seen in the volume of the sodalite units and double 6-rings. It may be said that the zeolite framework and the two potassium continua constitute three interpenetrating three-dimensional continua, all of whose structures can be described in terms of structural units connected tetrahedrally as are the carbon atoms in diamond. It appears that a report of a two-dimensional potassium continuum in zeolite A is incorrect, 38 and that the correct result, involving large localized clusters, appears when a larger unit cell of lower symmetry is used. 22 The properties of these continua have not been explored to date. They may be metallic, and transitions in properties might occur as a function of temperature. They may be viewed as metal allotropes filled with electrons to a level far below their Fermi level. In addition, these "metals" would be very much effected by the electric fields of the zeolite framework. The mean charge on each non-framework cation in the structure can be varied by changing the composition of the zeolite framework, and by varying the extent of sorption.

284 6.2.2. Exchangeable cations and guest atoms not of the same element The electrochemical potential for redox reaction controls the situation where atoms of one element are available to be sorbed by a zeolite containing exchangeable cations of another element. Within the zeolite and even in the absence of water, aqueous reduction potentials are usually capable of deciding whether reaction will occur, with an error due to the difference between the zeolitic environment and aqueous solution of no more than 0.1 (or perhaps 0.2) V. Accordingly there is no question that alkali-metal vapors will reduce transition-metal ions within a zeolite, and that vapors of zinc, mercury, or sulfur will not reduce the cations of the alkali or alkaline-earth metals. By mass action, in an open system where a product can leave, processes which appear to be exceptions to the above, but are not, can be seen. For example, at elevated temperatures, Zn 2+ and Cd 2+ ions can be reduced in a stream of hydrogen gas to their vapors, which are removed. However, for the most part, in equilibrium situations, if the electrochemical potentials are unsuitable for reaction to occur, guest atoms are not absorbed. No important mechanisms to stabilize atoms or clusters of a guest element exist, and the element is generally at a far lower chemical potential in its own phase. (It would be at a far higher chemical potential if it were finely divided and highly dispersed within a zeolite.) Exceptions can be found at high pressures where temperatures are high enough to allow mobility, but the product can persist at ambient pressure only if the mobility of the guest becomes very low. If the cations are reduced to atoms which encounter no serious diffusional impediment within the zeolite structure, they may easily leave the zeolite (migrate to the crystal surface). The entire process is then nothing more than ion exchange. Depending upon the vapor pressure of the leaving element at the temperature of the reaction, it will either deposit in its own phase, coating the surface of the zeolite, or it will leave as the vapor. This ion exchange will have occurred in the (complete) absence of a solvent (such as water). Partly became of the absence of such solvent molecules, which would associate with cations and perhaps impede diffusion, and partly because the free energy changes for these redox reactions are often large, these reactions usually go to completion. Often the resulting atoms do encounter significant obstacles to their departure from the zeolite framework. Such atoms are often generated, for example Pt by reduction of Pt 2+ with hydrogen, to create finely dispersed catalytic occlusions within the zeolite. These occlusions often migrate as atoms or small clusters, especially at elevated temperatures, to form larger particles (sinter) and to leave the zeolite to form large crystaUites on the surface. There is little to keep these occlusions within the zeolite besides the diffusional impediment which the bonding among the metal atoms themselves creates. 6.3. Formation of ion pairs A reaction which was not foreseen in the list of possibilities given in the opening sentence of Section 6. was discovered by Lunsford. Molecules of NO formed the ions NO + and NO" within zeolite y.39 As a result of this, Rabo termed the zeolite "a solid electrolyte" because it was able to generate ion pairs from neutral molecules, not only in the usual way, by dissociation, but also in this unexpected way, by electron transfer. Electric fields within the zeolite had caused an equilibrium to shift, essentially to completion, to generate ions. Of

285 course, the odd NO molecules (radicals) are somewhat prone to react in this way to give even ions.

7. TWO DIFFERENT SUBSTANCES MAY REACT WITHIN A ZEOLITE A binary reaction involving two different reagents may occur within a zeolite. For example, a hydrogen molecule may add to a double bond in an organic molecule to reduce it, or H 2 and CO may combine to produce alcohols or other organic compounds. One substance may be activated by sorption at an active site, for example hydrogen gas can dissociate to atoms on a Group VIII metal cluster, and a second molecule can then react when it reaches that site. In these processes, the zeolite is catalyzing a "fuller" chemical reaction than one of decomposition or rearrangement. One may wish to prepare a zeolite with small particles of an inorganic compound arranged regularly within its cavities. This may be done to prepare a catalyst or a material with desirable solid-state properties. Such a substance might be CdS, a semiconductor whose nanoclusters exhibit non-linear optical behavior. Most such substances may not be simply introduced by sorption because they (1) have negligible vapor pressures at temperatures within the zeolite's range of stability, and (2) are insoluble in a solvent from which sorption might occur. However, it may be possible to synthesize a desired compound, dispersed suitably, within a zeolite by having two reagents react within the zeolite. These may be introduced consecutively as guests from an external phase, each in solution or neat, as a vapor or liquid.

7.1. Two organic substances (catalysis and selectivity) This author is not an expert in this area of zeolite science. Readers are directed to a review40 by Weitcamp and references therein for a presentation of accomplishments to date. The objective in this organic chemistry is not the installation of guest structure within the zeolite, but rather catalysis to give products which leave the zeolite. As such, some very good processes have been developed. This area of science has a great potential for further growth, especially with new large-pore zeolites which have the volume to allow larger reagents and products to participate.

7.2. Two inorganic substances If one wishes to introduce identical particles of a desired size or formula to flU a crystallographic site within the zeolite, the reaction of two reagents within a zeolite is almost uniformly unsuccessful. Diffusional and chemical problems generally do not allow a reaction to go to completion, nor to give only the desired product within the zeolite, nor to give particles of a suitable size. Beyond that, the product may well not be uniformly distributed throughout the volume of the zeolite crystals, and may be at least partly deposited on the surface. The product is generally immobile, and cannot adjust its position from the place where it formed to the most stable site within the zeolite. As a result, a range of geometries may be found for a product synthesized within the zeolite, rather than a single most stable arrangement as is generally found with sorbates, which are very mobile. Such substances

286 are likely to change with time, or to degrade irreversibly upon exposure to reagents such as the components of the atmosphere. The use of metal carbonyls or other such vapors (the method is called "chemical vapor deposition") to introduce neutral species into zeolites encounters the same problems. All of the techniques of physical science, including those of surface chemistry, may be brought to bear to begin to characterize such difficult products. A substance introduced into a zeolite in this way is often not welcome. If it is not needed as is an exchangeable cation to balance charge, nor as a molecule which may coordinate to an exchangeable cation, there may be no thermodynamic reason for it to remain. It is more of an occlusion. It may persist within a zeolite only because the temperature is too low for it to have the mobility to leave easily. Nonetheless, such a product may satisfy the ambitions of the workers who prepared it; the desired properties may have been achieved. The despair of the previous paragraphs of this section may seem inappropriate in the face of success. Simple synthetic sequences are likely to fail to synthesize ideal inorganic nanoclusters within a zeolite. For example, cations of (Cd4Te4) 8+ might be placed within the sodalite cavities of zeolite Y by ion-exchange with Cd 2+ followed by sorption of dimethyl telluride. These clusters might be reduced to a lesser charge or to neutral by the subsequent addition of hydrogen gas. Problems to be expected include uptake of hydroxide during Cd2+-exchange, or other anions by aqueous or other methods, clusters with other formulae and of other sizes in other locations, inadequate retention of tellurium, and loss of CdTe from the zeolite. Reducing the (Cd4S4) 8+ clusters (if successfully prepared) with Cd(g) might work, but could lead to further loss of Te and rearrangement of the clusters, with an ideal product unlikely. If one wished to prepare clusters of Cd4S 4 within zeolite X by ion-exchange with Cd 2+ followed by dehydration and sorption of H2S, followed by further cycles of those three steps, the product would be likely to contain clusters containing the elements Cd, S, O and H, but it would not be easy to guess their compositions, concentrations, and positions within the zeolite. For such a synthesis to be successful, the result of each step must be examined to learn the result. The product of each step may initially not be the one desired. Ion-exchange should occur to give a well-characterized product. Sorption of one and then the second reagent should occur to give desired stoichiometries, and each step should not endanger the good result of a previous step. At the very least, a deep appreciation of intrazeolitic chemistry, rather than just a hope that the reagents will successfully assemble within the zeolite, is needed.

8. ENCAPSULATION It is not particularly difficult to block the pores of a zeolite. Zeolites are generally considered more or less susceptible to this, losing their usefulness when it occurs. The channels of a zeolite filled with cations and/or guests are certainly not open avenues; if the congestion is great enough, or if species have formed which are not mobile, the zeolite may be considered closed. Accordingly, one may readily imagine that various species (atoms, molecules, monatomic and polyatomic ions) may be encapsulated within some or all of the units of the structure of a zeolite.

287 The great ability of a zeolite to encapsulate is demonstrated most clearly with gases which have no important interaction with the zeolite, and so are sorbed only slightly by the zeolite at ambient conditions. These gases can be contained without leakage at densities comparable to those of their condensed phases, corresponding to hundreds or thousands of atmospheres of pressure. To accomplish encapsulation, an empty zeolite containing window-blocking cations is exposed to a guest phase. The temperature must be high enough for these cations to be mobile, allowing the guest species to invade the zeolite, and allowing an equilibrium to be established between the guest molecules in the two phases. A high external pressure may be applied to the system to increase the guest concentrations in both phases. Without change of pressure, the system is allowed to cool to a temperature where the cations are no longer mobile, so that the windows are blocked. At this point, the guest is encapsulated, and the external phase may be removed. Although it is most remarkable that gases can be encapsulated, encapsulation may be accomplished with any substance which can enter the zeolite at the higher temperature. To decapsulate the guest, the temperature may be raised. Decapsulation may also be achieved by allowing polar molecules (e.g. water) to move into the zeolite even at room temperature. By the first method, at least, many cycles of encapsulation/decapsulation may be done without damage to the zeolite. Reversible encapsulation is desired when the guests are to be released at a later time, for example for use. Irreversible encapsulation (entombment) can be accomplished by a process more severe than that described in the preceding paragraph. It is desired when the guests are to be rendered immobile (contained forever) within a zeolite. 8.1. Encapsulation in zeolite LTA Barter and coworkers noted long ago from sorption/desorption studies that some molecules could be trapped in the sodalite units of zeolite LTA. By blocking the 8-rings with large alkali-metal cations, they were able to trap molecules in the large cavity also.41, 42 More recently, this system was studied again with the hope of designing a medium for the storage of hydrogen gas for use as a fuel, to power automobiles for example.43, 44 Hydrogen gas bums very cleanly without polluting the atmosphere. Encapsulation can be accomplished in partially Cs+-exchanged zeolite A. Three Cs + cations per unit cell are sufficient to fill and to block all 8-rings in the structure. In fact, as few as 2.7 Cs + ions is sufficient to prevent the diffusion (percolation) of guest species (even very small ones like H2) at room temperature. At 400 ~ the Cs + ions in LTA are sufficiently mobile that guests (gas molecules for example) may enter the zeolite and come to equilibrium with the surrounding phase. At room temperature, the Cs + ions in LTA are sufficiently immobile that they block the avenues of escape. These then are the conditions and composition for encapsulation in LTA. The encapsulated species may contribute to the apparent immobility of the Cs + ions by blocking their way. The crystal structure of a krypton encapsulate of partially Cs+-exchanged LTA was determined at ambient conditions after treatment at 400 ~ and 635 arm. 13 It showed one Kr atom in the sodalite cavity, less than one Angstrom from its center, and four Kr atoms in the large cavity. The latter were arranged to form a rhombus, nearly a square, with dipoles induced by approaches to framework oxygens and Na + cations alternating. The structure of

288

the zeolite framework and the positions of the exchangeable cations were nearly exactly the same as those of the fully evacuated material, consistent with the weak perturbations to be expected from an inert gas, and showing that the zeolite framework was not strained. The capacity of the zeolite was 55.7 L (STP) of Kr per kg of dry zeolite. This work repeated with argon gas at 660 and 1000 atm showed one atom in the sodalite cavity of each structure as before, with four (a rhombus) and five Ar atoms (a trigonal bipyramid), respectively, in the large cavities. 45 As with the krypton encapsulate, the argon atoms situate themselves in the large cavities so that their dipoles, induced by interaction with the framework and exchangeable cations, are arranged logically. Work on the xenon encapsulate is in progress.

8.2. Encapsulation (entombment) by sealing the surface One may imagine that zeolite crystals containing guests might be coated with an impermeable substance, or sealed in a impermeable phase. Perhaps the surface of each crystallite could be decomposed by chemical or heat treatment to give an impermeable glass, so that the channels end short of the surface. Such methods of entombment have been considered in the packaging of waste substances such as radioactive Kr and Xe for disposal. 8.3. Possible use of encapsulation concepts in chemical reaction If empty zeolite LTA is exposed to cesium gas, and redox reaction with the exchangeable cations of the zeolite can occur, then the product Cs + ions may seal the zeolite crystals. Everything within the zeolite may be encapsulated, including the product atoms, perhaps as neutral clusters. Well, nearly all cations, including Na +, are reduced by Cs(g); only some of the other alkali-metal cations may resist reaction. Depending upon the mobility of the Cs + ions at the reaction temperature, the surfaces of the crystals may be closed initially. If the reaction proceeds, the entire volume of the zeolite may be sealed. The ultimate outcome of the process will depend upon the mobilities of all the atoms and ions in the structure at the reaction temperature, and upon considerations of the stability and lability of any structural subunits which may form. In an attempt to synthesize and capture silver clusters in this way, empty Ag, K-A was exposed to Cs(g) at 250 ~ The structure of the product showed that the potassium ions had all been reduced and were no longer present in the zeolite. The silver ions also had all been reduced to atoms, but most of these were found as hexasilver molecules at the very centers of the large cavities. Each hexasilver molecule was surrounded by fourteen Cs + cations. When this reaction was done with Ag, Ca-A and Rb(g) instead, hexasilver molecules each associated with thirteen Rb + ions were found at the same position. 30 The product calcium atoms had left the structure. Although these experiments were successful at trapping silver clusters within the zeolite structure, it is clear that a zeolite crystal is not sealed at all by this Pompeii rain of large alkali-metal atoms and cations, at least not at 250 ~ because all Ca and K atoms and some Ag atoms in the two structures presented in the previous paragraph had left the zeolite. It was a third reaction, that of Cs(g) with Ag2Ca5-A at 250 ~ which provided profound insight into the mechanism involved. 47 Because the product was Ag2Csl0-A, it could be concluded that cesium atoms were not being sorbed by the zeolite at all, because they would have reduced the Ag + ions also. Apparently the Ca 2+ ions were reduced because only they

289 were mobile within the zeolite, and they must have migrated to the surface there to react with cesium atoms. The Ag + ions, which have a much more covalent interaction with the zeolite framework, must not have been mobile and so could not travel to the surface to be reduced. Therefore Ag + was not reduced by Cs(g), even though the (aqueous) electrochemical potential for this reaction is a vast +3.7 V. This immobility of the Ag + ions is largely responsible for the results of the previous paragraph. The Ag + ions appear to have remained at their cationic sites until a mechanisms was in place for electrons to be transmitted to them from cesium atoms at the surface (perhaps a long line of Cs + ions), at which time the zeolite was very congested and the resulting silver atoms could not all escape.

9. AN EMPTY Z E O L I T E IS A CRYSTALLINE SOLVENT A zeolite may be viewed as a very highly (perhaps ultimately highly) structured solvent. Unlike a conventional solvent, it is solid and, beyond that, crystalline (ultimately regular). As a solid, a zeolite is quite rigid, so it lacks almost completely the great flexibility which a fluid solvent has to surround a guest, to fit to it, and to adjust its approaches to maximize the energies of interaction. The major source of flexibility in a zeolite is its ability to change the positions of some of its cations, which can sometimes move as much as several Angstroms.17,48, 49 In comparison to this, the flexibility of the zeolite framework must be considered negligible. The zeolite, then, offers a small number of already well defined sorption sites to a guest. Unless one or more of these are very suitable, the zeolite cannot be considered an accommodating host. A fluid solvent, in contrast, must be considered very accommodating because of the near continuum of environments which it easily provides. Unlike a fluid solvent, the volume which the guests may occupy in a zeolite exists before solution; in fluids, for the most part, the solvent atoms or molecules must move apart to create this volume. In other ways, a zeolite is like a fluid solvent. A zeolite may host a broad range of guests, and solution (sorption) may occur only slightly or to a great degree. The opportunities for interaction on the extensive inner surface areas of zeolites (extent of sorption) are comparable to those offered by conventional solvents (solubility). Even stronger interaction opportunities, comparable in energy to ordinary chemical bonds, more like those offered by clean surfaces, may be available.

10. AN APPEAL FOR CARE IN THE PREPARATION OF SAMPLES F O R STUDY Much of the work in zeolite science is of limited value because samples were prepared with insufficient thought, and studied, perhaps with very expensive instrtmaents, to give results which make little sense. Most of this work is ultimately recognized as faulty by its authors and is not reported. However, anyone familiar with the zeolite literature knows that different results often appear for samples which are reported as being identical, that is the work is irreproducible. The fault sometimes lies in unappreciated differences in supposedly

290 identical initial zeolite samples. The fault often lies in experimental details which were not recognized to be important, especially to investigators unfamiliar with zeolite chemistry.

10.1. Dehydration Small amounts of water within a zeolite can have a large effect on some experiments, so complete dehydration is often necessary. A zeolite which is to be exposed to alkali-metal vapor, for example, must not be given an opportunity to resorb water from other parts of its vessel after dehydration but before metal vapor sorption. The resulting metal hydroxide or oxide (additional products) could lead to the destruction of the zeolite. To compound the problem, the experimental conditions are often not described in detail in a report in the literature, so the reader can only be suspicious of a link between the reported result and inadequate dehydration. For some work, involving large amounts of powder, this problem may sometimes be of minor importance. Where tiny samples are involved, it is crucial.

10.2. Ion exchange The pH of ion exchange has been shown to be of importance for some ions. The product zeolite may contain H + ions (see section 3.2.4.), or hydroxide (even oxide) anionsl5,16 coordinated to exchangeable cations. Such an assumed simple matter as exchanging Ca2+ into zeolite A from pH 5 or 6 aqueous solution yields a product with approximate composition Ca5.5H-A per 12.25 A unit cell. Such a material is likely to have a very different stability at elevated temperatures, and to interact very differently with some sorbates, such as CO 2, because it is, of course, a Brrnsted acid. For this example, dilute aqueous Ca(OH)2 would yield a simple Ca2+-exchanged product. (Ca2+ is not an unusual ion; this concern is quite general.) Only rarely is the pH of exchange reported, and it often changes during ion exchange due to uptake of OH" or H +. Depending upon the details of an exchange experiment, including its geometry, the product may or may not be able to come to pH equilibrium with the exchange solution. The nature of the product often depends upon these experimental details, and the measurements reported are often sensitive to them in ways which are clear neither to the researchers nor to the readers. Ion-exchange is sometimes reported as having been done "in the usual way," as though, because the process is so easy to perform, the desired product is inevitable. This conceals some valuable experimental details, and the nature and extent of exchange may remain unknown. The purity of the salt, the concentration of the solution, an assurance that the time of exchange and the process (such as, was refreshment of the exchange solution done?) were adequate, and the pH may all be lacking. When cation exchange is performed, the counter ion is often not benign. Halide ions often accompany transition-metal cations into a zeolite structure and continue to coordinate to them. 11 Some anions such as acetate effect the pH, and this may alter the composition of the product zeolite. Washing some zeolites amounts to ion-exchange with dilute acid solution. For a zeolite in its sodium form, some Na + ions may be replaced by H +. Enough of this can lead to the dissolution of a low-silica zeolite. Yet, incorrectly, washing a zeolite is generally considered harmless, and all zeolites are likely to have been washed, at least a little and perhaps extensively, before they reach the hands of the investigator. Sodium zeolite LTA, for example, to deserve its name, should be thoroughly ion exchanged with dilute NaOH, before

291 a final rinse with a small amount of very dilute NaOH. The failure to do so may lead to erroneous results.

11. FINAL COMMENTS Arrangements of atoms which are unfamiliar to chemistry (because they are unstable) may be found in intrazeolitic space (because they are stable within the zeolite). The cationic clusters of the alkali metals (Section 6.2.1.2.) are an example of this. Within the zeolite, such new species may be studied and put to use. Allotropes of elements and compounds (continua), some charged, will continue to be found within zeolites; substances must adopt new structures if they choose to conform to the geometrical requirements of intrazeolitic space, with different structures for different zeolites. In all cases, the properties of these allotropes must be different from those of the native substances in their original crystal structures. To succeed in the above, the techniques of synthetic intrazeolitic chemistry need to be further developed. The consequences of each step of a procedure should be clear to the chemist, so that simple and stoichiometric products are produced. In this regard, zeolite chemistry is a young science with much of a "gold rush" mentality (rush to application) to it, with more fundamental work set aside for much later. The novelty of intrazeolitic crystallography should be noted. The results of a chemical process within a zeolite can be read directly and in detail by determining the crystal structure. There is no problem of growing suitable crystals because they were present from the onset, nor of solving the structure because the crystallographic phases are known sufficiently from a knowledge of the framework structure.

REFERENCES

1.

W.M. Meier and D.H. Olson, Atlas of Zeolite Structure Types, Juris Druck and Verlag AG, Zurich, 1978. a) D.W. Breck, W.G. Eversole, R.M. Milton, T.B. Reed and T.L. Thomas, J. Am. Chem. Soc., 78 (1956) 5963; b) T.B. Reed and D.W. Breck, J. Am. Chem. Soc., 78 (1956) 5972. D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, Inc., New York, 1974. 4. J.A. Rabo, Zeolite Chemistry and Catalysis, American Chemical Society, Washington, D.C., 1976. Y. Kim, V. Subramanian, R.L. Firor and K. Serf, A.C.S. Symposium Series, 135 (1980) 137. Y.H. Yeom, S.B. Jang, Y. Kim, S.H. Song and K. Serf, unpublished work. 7. T.B. Vance, Jr. and K. Serf, J. Phys. Chem., 79 (1975) 2163. 8. K. Ho, H.S. Lee, B.C. Leafio, T. Sun and K. Serf, Zeolites, 15 (1995) 377. 9. Y. Kim, Y.W. Han and K. Serf, unpublished work. 10. a) D. Fraenkel, J. Shabtai, J. Am. Chem. Soc., 99 (1977) 7074; b) D. Fraenkel, J. Chem. Soc., Faraday Trans. 1, 77 (1981) 2029. .

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292 11. 12. 13. 14. 15. 16. 17. 18.

19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40.

41. 42. 43. 44. 45. 46.

L.B. McCusker and K. Serf, J. Am. Chem. Soc., 100 (1978) 5052. K. Seffand D.P. Shoemaker, Acta Crystallogr., 22 (1967) 162. N.H. Heo, K.H. Cho, J.T. Kim and K. Serf, J. Phys. Chem., 98 (1994) 13328. K. Serf, J. Phys. Chem., 76 (1972) 2601. C. Ronay and K. Serf, Zeolites, 13 (1993) 97. G. Nardin, L. Randaccio and E. Zangrando, Zeolites, 15 (1995) 684. V. Subramanian and K. Serf, J. Am. Chem. Soc., 102 (1980) 1881. a) M. Iwamoto, in Future Opportunities in Catalytic and Separation Technology, Eds. M. Misono et al., Elsevier, Amsterdam, 1990, p. 121; b) M. Iwamoto, H. Yahiro, N. Mizuno, W. Zhang, Y. Mine, H. Furukawa and S. Kagawa, J. Phys. Chem., 96 (1992) 9360. S.B. Jang, U.S. Kim, Y. Kim and K. Serf, J. Phys. Chem., 98 (1994) 3796. P.B. Peapples-Montgomery and K. Serf, J. Phys. Chem., 96 (1992) 5962. P.H. Kasai, J. Chem. Phys., 43 (1965) 3322. A.R. Armstrong, P.A. Anderson and P.P. Edwards, J. Chem. Soc., Chem. Commun., (1994) 473. P.A. Anderson, D. Barr and P.P. Edwards, Angew. Chem. Int. Ed. Engl., 30 (1991) 1501. W. Shibata and K. Serf, unpublished work. Y. Kim, Y.W. Han and K. Serf, J. Phys. Chem., 97 (1993) 12663. P.A. Anderson and P.P. Edwards, J. Chem. Soc., Chem. Commun., 14 (1991) 915. J.A. Rabo, C.L. Angell, P.H. Kasai and V. Schomaker, Discuss. Faraday So., 41 (1966) 328. T. Sun and K. Serf, J. Phys. Chem., 98 (1994) 10156. T. Sun and K. Serf, J. Phys. Chem., 97 (1993) 5213. S.H. Song, Y. Kim and K. Serf, J. Phys. Chem., 95 (1991) 9919. S.H. Song, U.S. Kim, Y. Kim and K. Serf, J. Phys. Chem., 96 (1992) 10937. N.H. Heo and K. Serf, Zeolites, 12 (1992) 819. N.H. Heo and K. Serf, J. Am. Chem. Soc., 109 (1987) 7986. A.R. Amastrong, P.A. Anderson, L.J. Woodall and P.P. Edwards, J. Phys. Chem., 98 (1994) 9279. T. Sun, K. Serf, N.H. Heo and V.P. Petranovskii, Science, 259 (1993) 495. T. Sun, K. Serf, N.H. Heo and V.P. Petranovskii, J. Phys. Chem., 98 (1994) 5768. T. Sun, Ph.D. Thesis, University of Hawaii, Honolulu, 1993. T. Sun and K. Serf, J. Phys. Chem., 97 (1993) 10756. C.C. Chao and J.H. Lunsford, J. Am. Chem. Soc., 93 (1971) 71. J. Weitkamp, in Proceedings from the Ninth International Zeolite Conference, Vol. 1, Eds. R. von Ballmoos, J.B. Higgins and M.M.J. Treacy, Butterworth-Heinemann, Stoneham, 1993, p. 13. R.M. Barrer and D.E.W. Vaughan, Surface Science, 14 (1969) 77. R.M. Barrer and D.E.W. Vaughan, Trans. Faraday Soc., 67 (1971) 2129. D. Fraenkel and J. Shabtai, J. Am. Chem. Soc., 99 (1977) 7074. D. Fraenkel, J. Chem. Soc., Faraday Trans. 1, 77 (1981) 2029. N.H. Heo, W.T. Lim and K. Serf, J. Phys. Chem., submitted for publication. M.S. Jeong, Y. Kim and K. Serf, J. Phys. Chem., 97 (1993) 10139.

293 47. Y. Kim, S.H. Song and K. Serf, J. Phys. Chem., 94 (1990) 5959. 48. V. Subramanian, K. Serf and T. Ottersen, J. Am. Chem. Sot., 100 (1978) 2911. 49. S.B. Jang and Y. Kim, Zeolites, 14 (1994) 262.

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H. Chon, S.I. Woo and S.-E. Park (Exlitors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All rights reserved.

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C o n j u g a t e d a n d C o n d u c t i n g N a n o s t r u c t u r e s in Zeolites T. Bein Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA

The inclusion chemistry of conducting polymers and related oligomers in inorganic hosts is reviewed, with emphasis on zeolite molecular sieve hosts. Conducting polymers have been intercalated via various synthetic pathways into layered hosts such as FeOC1, into nanoporous membranes, and into zeolite molecular sieves. The crystalline nature and molecular dimensions of the zeolite cage- and channel-systems make them important hosts for the structuring of matter at the nanometer scale. Significant efforts have been directed at the intrazeolitic polymerization of acetylenes and several heteroaromatics, including pyrrole, thiophene, and aniline. For the latter group, oxidative polymerization is typically used, while polymerization of acetylene proceeds in the acid forms of zeolites. Intrazeolite carbon filaments based on the pyrolysis of intrazeolite polyacrylonitrile have also been prepared. The electronic conductivity of some of the intrazeolitic polymers has been determined, and the critical effects of charge trapping and hopping in more extended systems are emphasized. Encapsulation of conjugated systems in the channels of the mesoporous host MCM-41 leads to conducting filaments at nanoscale dimensions.

1. INTRODUCTION The intensive development of semiconductor circuitry in the past decades has produced an impressive density of electronic components and very high processing speed. However, as photolithography is now being pushed to the limits of resolution, the question arises if we will soon arrive at the point were new paradigms must be developed in order to move much beyond the present powerful capabilities. This situation has inspired scientists to think even smaller: Why not build circuitry up from the bottom, instead of carving it out of large crystals of silicon with the tools of lithography? At the bottom of the size scale, there are atoms and molecules, and the combination of molecules into functional assemblies is precisely what was envisioned. An enormous increase in integration and storage density could be anticipated3 Even if the realization of this concept in its full breadth may only occur in the distant future or found to be impractical, the interface of molecular science and information processing can lead to interesting developments, for example in the areas of chemical sensors or

296 displays, aside from fundamental insights into issues such as charge transfer in molecular systems. In 1974, Aviram and Ratner proposed what was probably one of the first concepts in the new "molecular electronics", and this area has fascinated many scientists since. The idea was to construct a molecular rectifier, by replacing the p-n junction of the semiconductor world with a molecular donor and acceptor, linked by a sigma-bonded separator (Figure 1).2 Theoretical considerations supported the notion of asymmetric barriers to charge transfer. Many other proposals for conductors, rectification, and switching functions 1 were advanced in the meantime, but the experimental demonstration of these concepts proved to be much more difficult. On the other hand, rapid progress in the exploration of entire new families of organic conducting materials, i.e., conducting polymers such as polyacetylene 3 or polyaniline, lend credibility to the notion of charge transport at the molecular level.

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Tetrathiafulvalene (TTF) Acceptor: Tetracyanoquinodimethane (TCNQ)

Figure 1. An example of a rectifier molecule proposed by Aviram and Ratner (after ref. 2). Provided the intriguing concept of molecule-based electronics could be fully realized, we can expect an enormous increase in the integration density and storage capacity of computers (and, eventually, sensors) by many orders of magnitude, compared with present technology. However, several critical issues must be addressed in order to realize the promise of molecular electronics. These include the development of functional units, the simplest of which is the conductor (or wire), access to these units from the macroscopic world, and

297 reliable connections between units. Other issues such as carrier mobility, switching speed, and error rates are also of fundamental importance. New opportunities in structuring the molecular units arose when the power of supramolecular assembly was recognized. For example, Lehn and coworkers immersed conjugated, carotenoid chains in lipid bilayers forming vesicles, and in a series of intriguing experiments demonstrated communication between redox centers spanning the bilayers (Figure 2). 4 This and related work has demonstrated that conjugated systems with appropriate end groups,5,6, 7 and confined aromatic systems 8 can transfer excitations upon photon absorption or after reduction or oxidation. The development of 'organic transistors' with thiophene hexamers demonstrates the potential of molecular materials for electronic applications. 9

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.

Figure 2. Representation of transmembrane electron transfer through a caroviologen described by Lehn (after ref. 4). It appears logical to direct attention initially to the most fundamental of electronic components, the conductor. Thus, the stabilization of molecular conductors at nanometer dimensions is one of the important and achievable goals in the area of molecular electronics. In this context, a major challenge is to achieve charge transfer with low fields (as in metallic wires), and to establish communication with electrically separated nanometer structures. The confinement of conducting polymers in low dimensions has been studied by several groups. Many of the conducting polymer systems reported to date comprise either two-dimensional thin films, or polymer fibrils at much larger than molecular dimensions. For example, polypyrrole, polythiophene and polyaniline have recently been included in layered FeOC1 and V205 (Figure 3). 1~ Other examples include the polymerization of filaments of conducting polymers (ca. 50-1000 nm diameter) in

298 the transverse pores of Nuclepore and Anopore membranes, developed by Martin and coworkers. 11 For example, polypyrrole and poly(3-methylthiophene) fibrils with diameters between 0.03 and 1 ~tm at 10 ~tm length have been synthesized in Nuclepore membranes (Figure 4). 12 In more recent work, fibrils and tubules of conducting polymers, and metal tubules and rods have been studied in similar host membranes. Synthesis of polyaniline in microporus template membranes leads to the growth of a thin skin covering the walls of the host, and higher conductivity (compared with the bulk) that was associated with better order and alignment of the polymer. 13 Template-synthesized polypyrrole also s h o w e d enhanced conductivity. 14 The effect of geometry on the conductivity of polypyrrole and polyaniline tubules shows a crossover from three- to two-dimensional Mott variable range hopping when going from 400 nm to 100 nm tubule diameters. 15 The crossover transition temperature of polypyrrole tubules (3D to 1D) shifts to lower temperature when external pressure is applied36

Figure 3. Intercalation of aniline into FeOC1 (after ref. 10d). Metal microtubules were prepared in a similar manner, with lower internal diameters of 0.8 nm, and the authors could demonstrate that ion transport through such tubules can be switched from positive to negative ions with external potentials applied to the membrane. 17 Gold nanowires (of as little as 10 nm diameter) deposited in nanoporous filtration disks can also act as nanoelectrodes with sensitive cyclic voltammetric detection limits. 18 Finally, the template membrane approach has also been extended to arrays of cadmium chalcogenide phases. Thus, 200 nm-diameter CdSe or graded CdSe/CdTe cylinders were electrodeposited in the pores of Anopore membranes. A Ni-CdSe array was found to be rectifying. 19

Zeolite molecular sieves are very attractive candidates for inclusion chemistry in structurally well-defined, nanometer-size channel structures. The present review focuses on recent studies addressing the encapsulation of molecular conductors in zeolite host channels.

299

Figure 4. Electropolymerization of conducting polymer in a nanoporous membrane.

Properties of Zeolites. Zeolites2~ 23 are crystalline open framework oxide structures (classically aluminosilicates but now encompassing many elemental compositions) with pore sizes between 0.3 and more than 3 nm. The topologies of these systems include 1-D channels, intersecting 2-D channels, and 3-D open frameworks. 24 Ultra-large pore aluminophosphates with 18-ring openings (VPI5), 25 and a gallophosphate (cloverite) 26 with a three-dimensional 20-membered ring channel system and large cages 3 nm across have been introduced. Finally, intriguing mesoporous structures based on the liquid crystal assembly of surfactants and inorganic framework precursors have been discovered,27,28, 29 see Table 1.

300 It is well-known that here are virtually unlimited options to modify the zeolite intrapore surface. In aluminum-containing sieves like FAU or LTA, the pores are typically filled with water from the hydrothermal synthesis procedure and contain alkali metal cations coordinated to oxygen-metal rings with C3v and C2v symmetry. These cations can be exchanged for transition metal ions and the system can be heat-treated to remove water. If the cations are exchanged for NH4+, acid forms are obtained by degassing the ammonium forms at elevated temperature. 3~ Variation of the Si/A1 ratio controls the hydrophilic character of the sieve and the acidity of the acid forms. Zeolites can be stable up to 700oC and higher. Table 1. Representative Zeolite Structure Types. Name

Unit CeU/Comvosition

Window~

Main Ghann~l~/~a

LTA, Linde A FAU, Faujasite MOR, Mordenite

[Na12(A102)12(SiO2)1227 H20]8 Na58[(A102)58(SiO2)134]240 H20 Na8[(A102)8(SiO2)40] 24 H20

MFI, Z S M - 5

Nan[(A102)n(SiO2)96_n] 16 H20

AFI, A1PO4-5 VFI, V P I - 5 MCM-41

Al12P12048R qH20 (R = TPAOH) [Al18P18072] 42 H20 SiO2 and other compositions

.1 *** 8-ring 7.4 *** 12-ring 12-ring/8-rings 6.7x7.0" <-> 2.9x5.7" {5.3x5.6 <-> 10-rings 5.1x5.5} *** 7.3 * 12-ring 12 * 18-ring hexagonal ca. 20-100" channels

a The number of stars (*) indicates the dimensionality of channel connections. The features described above make zeolites attractive candidates as hosts for polymeric conductors. In contrast to glassy matrices, they offer well-defined, crystalline channel structures with dimensions at the molecular level. We note that zeolites have also been used as hosts for a number of other intriguing "nanocomposites", for example in the field of encapsulated quantumsize semiconductor particles such as Se, CdS, CdSe, PbS, and GaP. 31,32,33,34,35 The encapsulation of metals such as Bi, Hg, Sn and Ga in zeolites has been described by Bogolomov. 36 These studies demonstrate the enormous versatility of zeolite host systems for studies and control of structural/electronic relationships.

Properties of representative conducting polymers. Doped conjugated polymers have generated substantial interest in view of possible applications such as lightweight batteries, antistatic equipment, and microelectronics to speculative concepts such as 'molecular electronic' devices.37, 38 These polymers include doped polyacetylene, polyaniline, polypyrrole, and other polyheterocycles (Figure 5). While the conduction mechanism of metals and inorganic semiconductors is well understood and utilized in microelectronics, this is not true to the same

301 extent for the polymeric systems. The interactions in organic polymers are highly anisotropic. 39 While their atoms are covalently linked along the chains, interchain interactions are much weaker, this can cause collective instabilities such as Peierls distortions. Charge transfer between intercalated dopant species and the organic chains can result in substantial local relaxations of the chain geometry. These changes introduce new localized electronic states in the gap.

X

Polyacetylene

k_l J, Polypyrrole

/\1 Polythiophene

Polyaniline Figure 5. Conducting polymers. Important concepts for charge carriers include solitons (Phase boundary defects linking two energetically equivalent configurations and creating midgap states 4~ detected in electronic spectra 42) and polarons 43 (electronic excitations associated with local deformations of the lattice with a corresponding gain in lattice strain energy, and formation of band gap levels close to the band edges). For polymers with nondegenerate ground state, such as poly(3-alkylthiophenes), polypyrrole, and cis-polyacetylene, oxidation (or reduction) leads to the formation of bipolarons (doubly charged spinless species) that are associated with strong local lattice distortions (Figure 6). 44,45 Proposed carrier transport mechanisms include thermally activated hopping between localized electronic

302 states, and quantum mechanical tunneling between 'metallic' domains separated by orientation boundaries or amorphous regions as found in polycrystalline samples. 46 A feature useful in comparing these different transport modes is the temperature dependence of the conductivity. 47

Figure 6. Band structure of polypyrrole as a function of doping level (schematic). (A), neutral polymer; (B) polaron (+), spin = 1/2, 3 new transitions; (C) bipolaron (++), spin = 0, 2 transitions; (D) Heavily doped, bipolaron bands. Polyacetylene (PA) is the simplest conjugated polymer with an extended ~system, which is composed of chains of CH groups 48 in cis- or trans-form. Polyacetylene films become semiconducting (doping level ca. 0.1%) or comparable to a metal (doping level ca. 5%) when doped with oxidizing or reducing agents. The conductivity can be varied over many orders of magnitude upon chemical or electrochemical doping. An interchain hopping mechanism 49 involving solitons has been suggested for semiconducting PA. Kivelson has proposed a mechanism 50 involving mobile neutral solitons and interchain transfer. Experimental evidence for the soliton concept has been challenged because the disorder in typical samples will not allow for motions over noteworthy distances, sl However, an important study 52 has demonstrated that

303 stretched polyacetylene with very low sp 3 defect concentration has much higher conductivity and anisotropy than other materials. This point emphasizes the importance of oriented, low-defect systems if charge transport is principally along the conjugated chains. This discussion illustrates that no complete, consistent theory of transport in PA has yet been developed. Unsaturated heterocycles and aniline have been polymerized either chemically or electrochemically on electrode surfaces. The systems are attractive as they are often significantly more stable than PA under atmospheric conditions. The importance of quinoid vs. aromatic structure becomes apparent if the chemistry and conductivity of polyaniline (PANI) is examined. The initial 'emeraldine salt' product of PANI is believed to have the following composition: 53 {[-(C6H4)-N(H)-(C6H4)-N(H)-][-(C6H4)-N+(H)=(C6H4)=N+(H)-]}x + 2 A-, where A- is an anion. This material can also be formed upon "doping" the corresponding 'emeraldine base' form of polyaniline with aqueous HC1, resulting in a large increase of the number of unpaired spins, 54 probably as diaminobenzene radical cations. 55 The similarity of the electronic properties of protonated phenyl capped octaaniline, poly(p-phenyleneamineimine), and PANI suggested that the dominant transport mechanism is via interchain hopping, with a relatively high density of localized states originating from the proton induced spin unpairing process. Polythiophene (or polythienylene) (PTh) and polypyrrole (PPy) films obtained by electrochemical oxidative polymerization of the respective monomers with anions (e.g.; C104-, HSO4-) present in solution are relatively air-stable and highly conducting. These polymers can also be made by chemical synthesis. The conjugated poly(3-alkylthienylenes) are soluble in usual solvents such as chloroform, toluene, or THF, and can be cast into thin films and subsequently used as semiconducting and metallic polymers. 56 Polypyrrole (PPy) has been studied in the form of thin films deposited on electrode surfaces, 57 by electrochemical oxidative polymerization of pyrrole with anions (e.g.; C104-, HSO4-) present in solution, resulting in relatively air-stable, highly conducting films. Chemical oxidation of pyrrole with Cu(II) or Fe(UI) salts in solution has also been reported.58,59,6~ 61 In polypyrrole (PPy) and other heteroaromatic polymers, the ground state is nondegenerate, that is, the ground state geometry with aromatic structure within rings and single-bonds between rings is more stable than the corresponding quinoid resonance structure. The introduction of a charge on the chain can relax the aromatic to the quinoid structure, because the quinoid structure has a smaller band gap (lower ionization potential and larger electron affinity). The resulting electronic structure of PPy as a function of doping is depicted schematically in Figure 6. At low doping levels, the chains are ionized and produce a radical cation (polaron) which does not contribute significantly to the conductivity.62, 63 However, at higher doping levels the polarons can combine or ionize to form

304 spinless dications (bipolarons) which extend over 4 to 5 rings. The spinless conductivity is then associated with the bipolarons that are assumed to transfer charge via interchain hopping, consistent with the absence of Pauli susceptibility in the highly conducting form of PPy. Composites between polypyrrole and a variety of porous materials such as paper, cloth or wood have been made. Often the respective material was impregnated with an oxidant such as FeC13 64,65 and subsequently contacted with pyrrole vapor or solution. For example, polypyrrole (and polyaniline) have been made in Nafion perfluorosulfonated ionomer membranes by treatment with aqueous ferric chloride and the monomers. 66 2. POLYACETYLENE AND DERIVATIVES IN ZEOLITES Early work regarding the polymerization of acetylene in zeolites was performed by Tsai et al. 67 They reported that the extent of polymerization on Xzeolites was related to the size of the alkali metal cations present in the cages, and that the cations activate the acetylene molecules. The photopolymerization of diacetylenes (3,5-octadiyne-l,8-diol) on various surfaces, including molecular sieves 5A and 13X, was examined. 6s On polar surfaces, the first molecular layers are absorbed through hydrogen bonds, but due to their relative orientation these molecules do not polymerize. However, additional layers can be photopolymerized to form stable polymer films on the surface. The surface acidity of zeolites appears to be an important ingredient for polymerization of acetylene and derivatives. Thus, when diazomethane was used to remove the protonic acidic sites on HZSM-5, no evidence for acetylene polymerization remained, compared to the original acidic form. 69 Transition-metal containing zeolites such as CoY and NiY (but not the Cu, Mn and Zn forms) polymerize acetylene to give trans-polyacetylene with relatively short conjugation length, as indicated by resonance Raman spectroscopy. 70 The polymerization products appeared to be restricted to the zeolite crystal surfaces. The authors also point to the importance of Lewis acidic centers for the polymerization. A whole series of acid forms of zeolites (mordenite, omega, L, Y, beta, ZSM-5, and SAPO-5) were explored as hosts for the polymerization of methylacetylene at room temperature. 71 The monomer was introduced as gas, and yellow to redbrown products were identified as conjugated oligomers of methylacetylene residing in the channels of the zeolite hosts (Figure 7). The acidic form of ZSM-5 was also found to polymerize unsubstituted acetylene, 72 confirming earlier reports (Yin and Peng, ref. 69). Thus, reactions were carried out at temperatures between 298 and 550 K and acetylene pressures between 10 and 1000 torr, and the polymerization rate was found to be proportional to the aluminum content of the zeolite, as expected for acid catalyzed reactions. The oligomers (containing many sp 3 defects) were found to reside in the zeolite host. Hydrogen-bonded precursor species were invoked in the interaction of acetylene, methylacetylene and ethylacetylene with HZSM-5. 73 These precursors

305 are protonated, resulting in colored carbocations which start the oligomerization reactions. Reversible deprotonation of the oligomeric carbocations was also observed. The polymerization of acetylene in different cation forms of mordenites (MOR) was studied with Raman spectroscopy. 74 While the sodium form was inactive, extensive polymerization was observed with the Cs form, particularly at high pressure. The authors confirmed their earlier conclusion (Tsai et al., ref. 67), that the availability and relative distance of the alkali cation coordination sites has a profound effect on polymerization. Pre-treatment of the CsMOR host with CC14 vapor at 573 K led to further enhancement of the acetylene polymerization. This influence was associated with the formation of Lewis acid sites (possibly A1C13 or incorporation of chloride in the zeolite). An extended (resonance) Raman study of these Cs-MOR/acetylene systems suggested that the trans-polymer was formed, having a distribution of conjugation lengths between <6 to about 30 and more. 75 The polymer formed on NaMOR was much shorter and was obtained in much lower yield. In air, oxidative degradation and shortening of the conjugated segments was observed. Acetylene was also polymerized in the channels of A1PO4-5 crystals that were aligned in electric fields. 76

OH

Figure 7. Formation of poly(methylacetylene) in the channels of SAPO-5 (after ref. 71). 3. HETEROAROMATIC CONDUCTING POLYMERS IN ZEOLITES 3.1. Polypyrrole The polymerization of pyrrole in zeolite channels was explored by Bein and coworkers. Pyrrole vapor was equilibrated with copper-exchanged, degassed NaY or Na-mordenite hosts at room temperature, and the formation of dark green to black samples was observed, whose infrared spectra showed the signature of bulk polypyrrole (Figure 8). 77 The absence of detectable polypyrrole on CuNaA zeolite (whose windows are too small for adsorption of pyrrole) and the absent of d.c.

306 conductivity of the CuNaY-polypyrrole samples show that no significant amount of polymer forms on the external zeolite surface. The polymerization reaction is one to two orders of magnitude slower in the zeolite hosts than in homogeneous solution, suggesting diffusional limitations for the pyrrole in the channel system. NaY

+ Cu(NO3) 2 (aq) vac., 620 K

Cu(II)NaY

Pyrrole vapor, ~ 9 5 K

x,~.xp___/ ,

I~ ~~Nr---(/.//

Polypyrrole in Cu(I)NaY

Figure 8. Polymerization of pyrrole in copper-exchanged zeolite Y. Ferric forms of the above zeolites were also found to polymerize pyrrol, 78 while no reaction was observed with the Na or Fe(II) forms. In detailed studies, the electronic and transport properties of polypyrrole in the above hosts were examined.79, 80 The authors found that while the chains (with chain lengths greater than about ten monomers) are fully oxidized by intrazeolite ferric ions and contain bipolaronic charge carriers as well as small concentrations of polarons (about one in 1000 monomers), they do not show significant a.c. conductivity in the frequency range of 100-1000 MHz and 10 GHz. It was argued that this observation as well as the fairly large linewidth of the ESR signal are probably due to trapping of bipolaronic and polaronic charge carriers by the periodic zeolite framework. These observations have important consequences for the design of "molecular wires". The authors suggest that other strategies than low-field conductivity are required to inject charges and possibly transmit information through isolated individual conjugated polymer chains.

307 The polymerization of pyrrole over Cu(II)-exchanged ZSM-5 zeolites was studied with resonance Raman spectroscopy. 81 The authors found that a critical concentration of cupric ions must be exceeded to observe polymerization. Hosts with low Si/A1 ratios gave partially oxidized polypyrrole (having quinoidal and aromatic structures) and pyrrole monomer. The quinoidal structure was associated with the charge carriers. Residual oxygen degraded the polymer. The effect of intrazeolite protons on pyrrole polymerization in faujasite with ferric ions was examined, in order to distinguish the relative influence of acidity and the one-electron oxidant. 82 If water was co-adsorbed with pyrrole, the authors could prepare materials with conductivities varying over a wide range. It is not clear to what extent the conductivity is due to surface-adsorbed polypyrrole, because similar synthetic methods also produced polymer coatings on amorphous aluminosilicates. Instead of cupric or ferric ions, other zeolite-encapsulated oxidants have also been studied for the polymerization of pyrrole. These include small SnO2 particles and oxygen-covered Pd clusters residing in potassium L zeolite. 83 The structure of the clusters was elucidated using x-ray absorption spectroscopy, and ESR and IR data as well as the observation that more monomer than oxidant was present led to the conclusion that the polymerization reaction might proceed in a catalytic fashion, involving air oxidation. The reaction of pyrrole in copper- and nickel-exchanged mordenite was examined with XPS and photoacoustic IR spectroscopy, and it was found that only the copper-exchanged host produced polypyrrole. 84 The authors confirmed the partial oxidation of the polymer, and depth-profiling studies showed that the polymer was distributed throughout the zeolite crystals. Intense IR bands due to ring vibrations typical of oxidized polymer were observed, as well as nitrogen ls XPS spectra indicating the presence of positively changed nitrogen. In a continuation of the above work, the polymerization of pyrrole in copperand proton-exchanged mordenite was examined with EPR and UV-VIS spectroscopy. 85 A decrease of the Cu(II) signal (to about 50%) and the emergence of a radical signal due to pyrrole oligomers or polymers was observed in the EPR spectra. The intensity of the radical signal suggests that the average length of polymer chain associated with a radical species is about 15-20 monomers, while the width of the signal suggested the coexistence of small oligomers. Electronic transitions were observed in the regions 3.6 eV, 2.7-3.0 eV and 1.5-1.9 eV, but assignment of these bands to specific oligomers was uncertain. Microporous hosts related to zeolites, i.e., Cu(II)-exchanged alumina- and chromia-pillared layered a-Sn- and Zr-phosphates, have been studied for the polymerization of pyrrole introduced from the vapor phase, s6 XPS data and optical spectra suggested that more than one type of polymer was formed in the pillared hosts, with mixed neutral/bipolaron states on low-oxidation level polymer. The absence of significant conductance (less than 10-9 S/cm) was associated with short polymeric units. Finally, pyrrole was polymerized in situ in the 0.68 nm channels of the coordination network of [(Me3Sn)3FeIII(CN)6]n .87

308

3.2. Oligo- and Polythiophene The polymerization of thiophene and 3-methylthiophene (3MTh) in the channels of Cu(II) and Fe(III) containing zeolites Y and mordenite has been studied. 88 The intrazeolite Cu(H) and Fe(III) ions were introduced via ionexchange followed by dehydration in oxygen and vacuum (10-5 Torr, 620 K), and serve as oxidants for polymerizationS9, 90 (in the case of the Fe(II)-zeolites, this treatment causes oxidation to Fe(III)). Typical zeolite hosts had unit cell metal contents Na56Y, CulsNa26Y, Na8M, Cu2.4Na3.2M, and Cu8Na80A. The thiophene monomers were introduced into the zeolites either from solutions in water, chloroform, acetonitrile, hexane, or toluene, or via vapor phase adsorption. Vapor phase sorption at 295 K, with 1 Torr of thiophene pressure, resulted in loadings of 25-35 (zeolite Y), 1-2 (mordenite) and 0.2 (LTA) monomer molecules per zeolite unit cell. Spontaneous polymerization was observed after about 30 to 120 min., depending on the zeolite host and experimental conditions, e. g.,

Cu(II)Y + C ~ S

vapor, 295 K ->

CuY/polythiophene.

No reaction was observed with zeolite Cu(II)A, due to size exclusion of the monomer, with the sodium forms (due to the absence of oxidant), or in polar solvents. The latter effect was associated with screening of the intrazeolite metal ions by the polar solvent in the zeolite cages. Strong indications for polymerization within the zeolite pore systems include the following: (i) PTh and P3MTh are formed in the zeolites at rates that are orders of magnitude slower than in solution reactions, showing diffusional limitations in the narrow zeolite channels. (ii) No polymerization was observed with Cu(II)A zeolite, where the small pore opening of 0.4 nm does not allow for monomer diffusion into the zeolite pores and oxidation by intrazeolite oxidants. These diffusion and pore-volume limitations would not have been observed if the polymers had only formed on the crystal surfaces. (iii) Polymerization proceeded only in the presence of oxidant ions that are available at significant levels inside the zeolite crystals. Thus, no detectable polymers were formed in the Na-forms. (iv) Zeolite/polymer pellets showed no significant 'bulk' "conductivity (~ < 10-8 S/cm), which was interpreted as an indication that there was no significant deposition of polymer on the external crystal surfaces, in accordance with scanning electron micrographs. On the other hand, agglomerated P3MTh recovered from a polymer/Cu(II)NaY sample after dissolution of the host in HF showed a conductivity of about 0.01 S/cm. (v) Only minor traces of monomer evolution were detected in pyrolysis mass spectrometry from the zeolite/polymer samples, compared to those containing unreacted monomers (e. g., Na-forms), as expected if polymerization had taken place. Electronic absorption and infrared spectra suggest the presence of polymer/oligomer chains at intermediate oxidation levels compared to materials

309 made electrochemically. For example, the electronic absorption spectrum of P3MTh in Cu(II)NaY shows features at about 2.8 eV, 2 eV, and 1.7 eV. Similar features are observed with the Fe(III)-containing hosts and with PTh-loaded zeolites. The intrazeolite polymers have more complex spectroscopic features than the bulk materials. From the simultaneous observation of interband transitions (ca. 2.8 eV) and the red/near IR absorptions, it was concluded that the polymer chains on average are at intermediate oxidation levels in both zeolite Y and mordenite. A detailed study of the initial steps of thiophene oligomerization in pentasil zeolites such as ZSM-5 and Na-beta was performed by Caspar et al. 91 Chain lengths between 2 and 9 were observed to evolve (for example, terthiophene was converted into the radical cation, the dimer (sexithiophene), and the trimer cations), and an inverse linear relationship between the electronic absorption band energies of polarons/bipolarons and chain length was found (Figure 9). Extrapolation to infinite chain length (polythiophene) suggested that the lowest energy polaron and bipolaron levels of doped polythiophene are close in energy, which could facilitate the transient formation of polarons from bipolarons and play a role in interchain charge hopping processes.

4r9-o, ~ dsorption in ZSM-5

*§ ~ heating in ZSM-5 excess of terthiophene (e.g., 330 K) S

S

S

++

Figure 9. Oligomerization of terthiophene in pentasil zeolites.

310 A range of different thiophene monomers, including thiophene, 3methylthiophene, 2,2'-bithiophene, and terthiophene were introduced into dehydrated proton-, Cu(II)- or Fe(UI)-containing zeolites Y and mordenite from organic solvents or through the vapor-phase. 92 Green/black products are formed within minutes in the large-pore zeolites. The reaction products with thiophene show electronic spectra with a broad absorption between 2.5 and 0.5 eV, similar to bulk polymer, and not the resolved spectra of short oligomers. Conducting polythiophene could be recovered after dissolving the host in dilute HF. 2,2'Bithiophene and terthiophene in acidic Y zeolites gave deeply colored products whose electronic spectra suggest the formation of stable radical cations and dications in the zeolite host. Recent spectroscopic studies of thiophene/zeolite systems confirm the idea of intrazeolite polymerization. It was found that polythiophene formed in coppermordenite but not in nickel mordenite (ref. 84). Polythiophene was partially oxidized in the host. In contrast to polypyrrole (see above), XPS-depth-profiling results suggest that the polythiophene is concentrated in the outermost zeolite channels, possibly due to coordination of the sulfur-containing species to copper(I) ions formed in the zeolite, resulting in limited diffusion in the host. The limited penetration of thiophene/polythiophene was confirmed by a subsequent EPR study (ref. 85). As in similar experiments with pyrrole (see above), the EPR signal of Cu(II) indicated that about 50% of the copper reacted with the thiophene. The length of polymer chain associated with the radical species was found to be about 5-7 monomer units, and the EPR line width pointed to the presence of small oligomers as well. This picture was supported by electronic absorption bands observed at 2.8-3.0 eV (A), 2.3 eV (B) and 1.6-1.9 eV (D, E, F) that were assigned as follows: A and D-F are associated with small oligomers (4-6 monomers long), and B is associated with longer chain polymer. 3.3. Polyaniline and other systems Polyaniline filaments have been synthesized in mordenite and zeolite y.93 Aniline was sorbed from hexane solution into the dehydrated zeolites containing different levels of hydroxyl groups (an alternate route is the exchange of anilinium salt into the host, see ref. 93b). Oxidative polymerization was achieved by immersion of the loaded zeolites into an aqueous solution of peroxydisulfate, as demonstrated by spectroscopic identification and recovery of the encapsulated polymer (Figure 10). As in the case of polythiophenes in zeolite Y and mordenite discussed above, a number of observations established that intrazeolite polymerization had occured, including: (i) much slower polymerization rates were observed with the zeolite host (diffusional limitations), (ii) polymerization was observed only with acidic zeolite forms which have significant proton levels in their pore systems (no polyaniline was formed in the Na-forms), (iii) XPS showed a homogeneous distribution of nitrogen throughout the zeolite crystals, (iv) no significant "bulk" conductivity was measured for the zeolite/polymer samples (c~ < 10-8 S/cm, suggesting the absence of external polymer coatings, while with zeolite

311 samples deliberately covered with a thin film of PANI, c = 10-6 S/cm, and (v), pyrolysis mass spectra show no significant aniline content after polymerization. In both hosts, the encapsulated polyaniline showed spectroscopic features indicative of both emeraldine base and emeraldine salt polymers.

Figure 10. Polymerization of aniline in zeolite Y. Alternative oxidants such as potassium iodate were also explored for the intrazeolite polymerization of aniline in NaY and acidic forms of Y zeolite. 94 With peroxydisulfate, the polymerization proceeded only if a sufficient supply of intrazeolite protons was available. No polymer formed in either NaY or in acid zeolites with neutral iodate solution, but at low pH polyaniline was obtained in all hosts. The open nature of the zeolite host, even when partially filled with polymer, permits the introduction of base (such as ammonia). On admission of ammonia into the emeraldine salt-containing zeolite, the protonated polymer was converted into the neutral emeraldine base form. Encapsulation chemistry similar to that described above (exchange of anilinium, followed by oxidation with peroxydisulfate) was found to produce polyaniline not only in zeolite Y, but also in montmorillonite clay. 95 Spectral features (UV-VIS, IR and EPR) of the products were indicative of emeraldine salt and base formation, respectively. The change in basal spacing of the montmorillonite upon intercalation provided additional evidence for the inclusion polymerization.

312 Layered materials such as zirconium phosphate and zirconium arsenate were also examined as hosts for aniline polymerization, and compared to Y zeolite. 96 Again, the polymerization was effected by oxidation with peroxydisulfate. A free-radical EPR signal was observed at 2.0035-37 for the proton form of the host, and at 2.0047-49 for the sodium form of the host. In both layered hosts but not in the zeolite, saturation was observed. Conductive properties suggested variablerange hopping and tunneling for the layered hosts, but the zeolite-encapsulated material was insulating. It cannot be excluded that external polyaniline phase contributes to the bulk conductivity in the former cases. Instead of chemical oxidative polymerization, electropolymerization can also be considered. A recent study shows that slow but efficient electropolymerization is possible if anilinium-exchanged zeolite Y is subjected to oxidative treatment at the electrode-electrolyte interface. 97 Cyclic voltammetric signatures of the polymerization suggest that it occurs mostly through one dimer (p-aminodiphenylamine) which undergoes oxidative polymerization. Electrochemical polymerization of aniline in zeolite molecular sieves was s t u d i e d . 98 A zeolite-modified electrode showed shape-selectivity for 12molybdophosphoric acid. Furfural on faujasite. The oligomerization of furfural in acidic iron-containing faujasite has been reported. 99 After adsorption of furfural on the zeolite, it turned black; the oligomerization reaction was associated with the zeolite acidity. Some conductivity of the resulting systems (in the range of 10-7 S/cm) was measured. Polyaniline filaments in mesoporous MCM-41. Up to this point, we have discussed the encapsulation of various types of conjugated polymers in the more classical zeolites such as mordenite and zeolite Y. Although these hosts are typically characterized as large pore zeolites, they can only accommodate one or few conjugated chains in one pore. As pointed out above for intrazeolite polypyrrole, significant electronic conductivity (at microwave frequencies) was not observed in zeolite Y and mordenite. The reasons for this observation very likely include charge trapping but also the limited dimensionality of the encapsulated polymer chains. In view of these limitations, we have recently explored the synthesis of conducting structures in molecular sieves with larger pores, i.e., in hexagonal MCM-41. A synthetic protocol for the formation of conducting filaments of polyaniline in the 3 nanometer wide channels of the aluminosilicate MCM-41 was developed (Figure 11). 100 Aniline vapor was allowed to diffuse into the dehydrated channels of the host at room temperature, followed by immersion into an aqueous solution of peroxydisulfate at 273 K. This reaction produced encapsulated polyaniline filaments. Spectroscopic evidence including UV-VIS and infrared data showed that the filaments are in the protonated emeraldine salt form (for example, Raman spectra exhibit modes indicative of the protonated quinone radical cation structure). A single, rather broad (8 G) electron spin resonance line (for an evacuated sample), at g = 2.0032 suggested slightly lower

313 protonation levels than in bulk emeraldine salt, or dipolar intercations with the host channels walls. Gel-permeation chromatography of polymer recovered from the host (with dilute HF) indicated chain lengths of several hundred aniline rings, however the chains were about half as long as those of bulk polyaniline made similarly, suggesting diffusional constraints of the reactants in the host channels. The intrachannel volume after polymerization and thorough evacuation was probed in sorption experiments, and a significant drop in pore volume could be correlated with the encapsulated polyaniline.

Figure 11. Polymerization of aniline in MCM-41 (after ref. 100). Previous studies of bulk emeraldine salt have led to the model of a granular metal where charge hopping in amorphous regions between metallic bundles dominates the macroscopic conductivity.101,102 We ask then how the polymer changes its behavior when it is encapsulated in nanometer channels. The dc conductivity of PANI-MCM was determined to be in the 10-8 S/cm range, similar to the conductivity of unloaded MCM host, and more than seven orders of magnitude lower than for bulk PANI. The dc conductivity of extracted polymer was 10-2 S/cm, a striking difference most likely resulting from the removal of the separating MCM channel walls, and from access to the now exposed polymer. This finding establishes that the polyaniline is located inside the MCM channels and that no percolating conducting paths develop on the exterior of the host. The contactless microwave absorption technique 103 was used to probe conductivity of the encapsulated material. The microwave conductivity of dry

314 PANI-MCM obtained from the perturbation of a rectangular cavity at 2.63 GHz was 0.0014 S/cm, a quarter of the value for evacuated bulk PANI. This significant low-field conductivity demonstrates that conjugated polymers can be encapsulated in nanometer channels and still support mobile charge carriers. In contrast to the experiments with polypyrrole in zeolite Y and mordenite (see above), the channels in the MCM host provide more space and apparently allow some important interchain contact to occur. In an extension of the above study, the stabilization of conducting polyaniline filaments in the 3 nm wide hexagonal channels of the Cu(II) or Fe(III)-containing mesoporous aluminosilicate host MCM-41 was reported. 104 Adsorption of aniline vapor into the dehydrated, metal-containing host produced radical cations or short oligomers that can "lock in" the aniline species in the host channels. If these species are exposed to peroxydisulfate/HC1 under exclusion of air, the conducting emeraldine salt (ES) is formed in the MCM channels. Nitrogen sorption shows the expected decrease in channel volume. Previous exposure to air, however, produces a brown material that can only partially be converted into the emeraldine salt. The spectroscopic properties of the ES-MCM nanocomposites, the chain length of the recovered polymer, and the conductivity are similar to those discussed above for the polyaniline in aluminosilicate MCM without transition metal oxidants.

4. CARBON-BASED CONDUCTING MATERIALS IN NANOMETER CHANNELS Intrazeolite encapsulation of polyacrylonitrile and its pyrolysis products. The conjugated polymers discussed so far are conducting in the bulk when charged carriers are present on their chains. For example, polyaniline is most highly conducting when it is oxidized and protonated (designated as emeraldine salt). In charged zeolite host channels, this could lead to trapping of carriers and drastically reduced conductivity. We have therefore explored another type of c o n d u c t i n g material, i.e., carbonized p o l y m e r s such as p y r o l i z e d polyacrylonitrile, as alternative inclusion systems305,106 Pyrolyzed polyacrylonitrile (PAN) 1~ is a well-known material that can be formed as thin films and fibers. The polymerization of acrylonitrile occurs exothermically in the presence of free radicals or anionic initiators los. Subsequent pyrolysis of the polymer produces a ladder polymer by cyclization through the nitrile pendant group (Figure 12). 109 At increasing pyrolysis temperature a graphite-like structure is formed, which is electrically conductive due to the delocalized electrons31~ Increasing degrees of graphitization at higher pyrolysis temperatures lead to increased conductivity.Ill In related studies, the polymerization of acrylonitrile in montmorillonite 112 and of aminoacetonitrile in layered metal phosphates 113 have been reported, however, these systems contain thin sheets of macroscopic dimensions.

315

radical initiator ~"CN

~--

~X CN

polyacrylonitrile 200 - 300 ~

"~ ~N.

~

~ N ~

graphite-like structure

400- 700 ~ _.~pyrolysis

ladder polymer

Figure 12. Polymerization of acrylonitrile and pyrolysis to graphite-like structures. Vapors of acrylonitrile were adsorbed into the dehydrated forms of different large- and medium-pore zeolites, to saturation levels of 46, 6, and 9 molecules of acrylonitrile per unit cell in dehydrated zeolite NaY, Na-mordenite, and silicalite, respectively. Subsequent reaction with radical initiator (aqueous solution of K2S208 and NaHSO3) produced intrazeolite polyacrylonitrile (no polymer was found in silicalite due to size constraints). The intrazeolite polyacrylonitrile could be recovered after dissolution of the host with dilute aqueous HF, and was very similar to bulk polyacrylonitrile. Gel permeation chromatography revealed a peak molecular weight of 19,000 for polyacrylonitrile recovered from the NaY host, and about 1,000 for the polymer from mordenite. Pyrolysis of the intrazeolite polyacrylonitrile (PAN) was performed under nitrogen, resulting in black encapsulated material that had lost the nitrile groups and hydrogen. The narrow zeolite pores did not permit complete graphitizafion with formation of extended sheet-like structures; instead, intrazeolite pyrolysis reactions at 920 and 970 K (N/C atomic ratios 0.20 and 0.18, respectively) were apparently limited to the ladder structures formed in the bulk at lower temperature, at ca. 800 K. The zeolite host drastically changed the pyrolysis reactions, as shown with thermal analyses under nitrogen. It was observed that in contrast to bulk PAN that loses weight rapidly above 530 K, accompanied by a sharp exotherm at this temperature, the zeolite inclusions exhibit a broad exotherm with much less defined onset of decomposition. The pyrolyzed polyacrylonitrile, recovered from the zeolite hosts, showed electronic dc conductivity of about 10-5 S/cm. Recent studies indicate that the intrazeolite pyrolyzed PAN exhibits measurable microwave conductivity314

316 Conducting carbon wires in the MCM-41 host. The discovery of the MCM-41 family of mesoporous channel systems based on liquid crystal synthesis mechanisms has opened up new opportunities in the design of interesting nanostructured materials. Graphitic carbon is attractive as a candidate for the design of conductive nanostructures because it is stable and exhibits high conductivity. In an extension of the encapsulation of carbon materials in classical zeolites, we have studied the formation of carbon "wires" in the regular, 3-nanometer hexagonal channels of the mesoporous host MCM-41.115 Acrylonitrile was introduced into the host through vapor or solution transfer, and polymerized in the channels with external radical initiators (K2S208 and NaHSO3, at 313 K in aqueous suspension). Gel permeation chromatography of recovered polyacrylonitrile shows a chain length at the order of about 1000 monomers, long enough to span significant parts of the MCM-41 crystals.

Figure 13. Formation of graphite-like filaments in MCM-41 (after ref. 115). Pyrolysis of the intrachannel polyacrylonitrile led to the expected loss of nitrogen, similar to observations in bulk and thin film experiments. As in the case of polyaniline in MCM-41 (see above), nitrogen sorption isotherms showed reduced pore volumes for the inclusion compounds, demonstrating the formation of PAN and carbonized material in the channels of the host. Raman scattering is a sensitive probe for the structure of carbon materials.116,117 Raman spectra of pyrolyzed PAN-MCM show two distinct peaks, the Raman-allowed E2g graphitic peak at ~ 1580 cm -1 (G), and the D band at - 1360 cm -1 (associated with small domain sizes). Empirical correlations between peak intensity ratios (and band widths) and graphitic domain sizes were used to estimate the average domain size in the PAN-MCM samples, and it was found

317 that the carbon phase in the MCM-41 host is consistently more ordered than in bulk samples treated similarly. These observations show that the MCM host plays a key role in ordering the growing graphific sheets, very likely through the parallel alignment of the precursor polymer chains in the channels. Previous conductivity studies of bulk pyrolyzed polyacrylonitrile have established that metallic conduction pathways and localized spins coexist318 An important question relates to the effect of encapsulation on the conductivity of the carbon material. The dc conductivity of pressed pellets of PAN-MCM for all pyrolysis temperatures up to 800oC is in the 10-7 S/cm range (similar to the conductivity of unloaded MCM host under ambient conditions), many orders of magnitude lower than for bulk PAN. We concluded that the carbon material is located inside the insulating MCM-41 channels and that no percolating conducting paths exist on the exterior of the host. The microwave conductivity of dry PAN-MCM increased with pyrolysis temperature. Strikingly, for treatment temperatures of 800 and 1000 oC the conductivity was ten times greater than that of the bulk material treated in a similar way, reaching 10-1 S/cm at a pyrolysis temperature of 1270 K. This surprising result can be explained with the different order of the graphitic material in the two environments: the MCM channels assist in the formation of larger domains of ordered graphite (as shown in Raman spectroscopy), which result in higher conductivity. This study establishes that nanometer-scale carbon filaments with significant low-field conductivity can be generated in an insulating host. The channels in the MCM host appear to provide sufficient space allowing some important transverse delocalization in the graphitic structure. 5. CONCLUSIONS Numerous studies have established the wide scope of inclusion chemistry that is possible with inorganic solids. From layered hosts such as FeOC1 and nanoporous membranes to zeolite molecular sieves, very different dimensions are accessible for the encapsulation of conjugated, potentially conducting materials. Zeolites are distinct hosts because they offer well-defined, crystalline pore systems at molecular dimensions, with sizes in mesoporous systems now reaching even beyond the molecular scale. As demonstrated in this review, many different conjugated polymers have been intercalated into these and other hosts, using synthetic pathways such as inclusion oxidation, acid-catalyzed polymerization, or radical polymerization followed by pyrolysis of the precursor polymer. The intrazeolific polymerization of acetylenes and of several heteroaromatics, including pyrrole, thiophene, and aniline, has been explored to the greatest extent. Furthermore, intrazeolite carbon filaments based on the pyrolysis of intrazeolite polyacrylonitrile have been prepared, and some of the reported structures show significant electronic conductivity. Based on this knowledge, encapsulation of additional polymers and other conducting structures in this family of hosts is anticipated.

318 In view of the potential significance for future "molecular electronics" research, issues including the effect of channel diameter, wall-interactions, and nature of the charge carriers on electronic conductivity, as well as structuring of the host for physical integration into devices deserve future attention. Research in this area is also expected to contribute to an understanding of the relative importance of conductivity along chains vs. interchain hopping in conducting polymers.

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H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V All rights reserved.

323

NEW CATALYTIC APPLICATIONS OF ZEOLITES FOR P E T R O C H E M I C A I ~

C T O CONNOR, E VAN STEEN AND M E DRY

CATALYSIS RESEARCH UNIT, DEPARTMENT OF CHEMICAL ENGINEERING, UNIVERSITY OF CAPE TOWN, RONDEBOSCH, 7700, SOUTH AFRICA

1.

INTRODUCTION

The oil refining industry is presently facing a number of important challenges. Among these are the environmental laws relating to both the quality of the fuels it produces as well as the operation of its refineries. It is also being required to produce more gasoline and, at the same time, more products, such as light alkenes, to serve as feedstocks for the petrochemical industry. The emphasis of the present paper is on recent applications of zeolites for the production of alkenes and aromatics and their conversion to certain petrochemicals. Hence the focus will be on recent developments in the use of zeolites, firstly, in catalytic cracking, which is a key process in producing the classical building blocks of the petrochemical industry, viz. light alkenes and aromatics, and, secondly, on the conversion of these into higher-value products, via: 9 Alkylation of aromatics; 9 Aromatization ofalkanes/alkenes; 9 Skeletal isomerization ofn-butene; 9 Oligomerization ofalkenes; 9 Isomerization of long-chain alkanes.

Figure 1 shows a simplified flowsheet emphasizing the processes to be discussed in this paper. Developments in most of these processes have recently been extensively reviewed [1-7].

324

C=/C, ~

Crude

B'IX

LSR Naphtha V ''~ I Kerosine~ L Diesel -•Reforming] :Lube Oil-----1 C,= GO LVGOI Resid

HVG( HVGO

CCO

iso-Cs

BTX

[;l

"I(c" c") Disti"atet

Alkylation ~Lubricating Oils] ;~HyorocracKing~

Gases I

Tops I NapthaI Jet I Diesel ! Lubes I

Figure 1. Flowsheet emphasizing processes reviewed in this paper. One of the most important properties of zeolites is their ability to carry out shape selective reactions [5]. These can be classified as, firstly, product shape selective reactions in which the only products formed are those which can diffuse out of the pores of the zeolite, secondly, reactant shape selective reactions which occur when some of the molecules in a reactant mixture are too large to diffuse through the catalyst pores, and, thirdly, restricted transition-state selective reactions in which the only reactions which occur are those in which space exists in the pores or cavities to allow the formation of the activated transition state complex~ In some cases where the zeolite is three dimensional the size of the channel intersections will also be a determining factor. This unique catalytic property is related to the pore size of the zeolite and has led to the synthesis of zeolites with a very wide range of pore

sizes. Over the past fif~ years there have been si~ificant developments in the synthesis of zeolites of different pore sizes [6]. Many of these developments have resulted in new applications of zeolite catalysts for petrochemical processes. This is especially so, for example, in the case of catalytic cracking which ideally requires the pore sizes to be tailored to cater for reactions involving a wide range of molecular sizes thus leading to the synthesis of catalysts with macro-, meso- and micropores. Table 1 shows the pore size, dimensionality and structure type of some of the zeolites and molecular sieves which will be discussed in this paper.

325 Table 1 Structural details of some zeolites and molecular sieves Zeolite

Pore Dimension Dimensionality Structure Ring Size [Angstrom] Type [T atoms] ZSM-5 5.3 x 5.6 and 5.1 x 5.5 3-dim MFI 10-membered US-u 7.4 x 7.4 3-dim FAU 12-membered Mordenite 6.5 x 7.0 and 2.6 x 5.7 2-dim MOR ~ 8- and 12-membered Beta 6.5 x 5.6 and 7.5 x 5.7 3-dim BEA 12-membered ZSM-22 5.5 x 4.4 uni-dim TON 10-membered Theta-1 Femerite 4.2 x 5.4 and 3.5 x 4.8 2-dim FER ~10- and 8-membered Rho 3.6 x 3.6 . RHO 8-membered MCM-41 SAPO-11 KL

40 - 65 6.3 x 3.9 7.1 x 7.1

uni-dim uni-dim uni-dim

M41S AEL LTL

depends on synthesis lO-membered 12-membered

* 2 unconnected 3-dim. systems # Intersecting system

Zeolite catalysts are mainly used to promote acid catalysed reactions. The number and strength of acid sites can be controlled by methods such as ion exchange, de/realumination and isomorphous substitution of tetrahedral atoms. Their high thermal stability implies that they are able to be regenerated by the burning off of carbonaceous deposits although in-situ steaming can lead to dealu_mination of the zeolite which may result in a loss of catalytic activity. Finally they are able to act as hosts or carriers of guest atoms which can include transition metal ions or atoms, metal oxide clusters, complexes or chelates, etc. These unique properties enable zeolites to have a wide variety of applications as catalysts.

2.

CATALYTIC CRACKING

2.1

Introduction

The catalytic cracking of oil in modem refineries is mainly aimed at the production of gasoline and diesel and is also one of the main sources of light alkenes and aromatics. The worldwide crude oil processing capacity is about 600M tonne crude per annum which consumes about 300k tonne catalyst per annum There are some 450 catalytic cracking plants worldwide and this capacity is growing at a rate of about 8 000 m3/day [6]. Fhfidized catalytic cracking (FCC) constitutes by far the largest use of zeolite catalysts (some US$ 600M) and, in the US, for example, it is estimated that catalytic cracking accounts for 42% of the estimated US refining catalyst market in 1991 [8]. The impact of zeolites is shown in Figure 2 which shows the changes that have occurred in typical FCC yields as the catalyst has changed over the last fifty years [3].

326

Figure 2. Impact of catalyst type on yields

Developments in the manufacture of more efficient catalysts for FCC have been reviewed by various authors [1,2,3,4,6,9,10,11,46]. Generally the objective of most FCC operations is to maximize the production and the octane number of the gasoline t~action and to minimize the formation of by-products such as LPG, fuel gas, heavy fuel oil and coke. Since the focus of the present paper is on petrochemicals, however, only those developments in the design and formulation of FCC catalysts which are aimed at increasing the production of light alkenes, the key building blocks of the petrochemical industry, will be discussed. In the FCC process, which uses the solid catalyst, ultra-stable zeolite Y (USY), in a fluidized bed reactor, long chain alkanes, polycycloalkanes and alkylaromatics are cracked into alkanes, alkenes and cycloalkanes in the C 1-C 17 range. At the same time aromatics in the C6C12 range are formed by oligomerization, dehydrocyclization and the cracicing of polyalkylaromatics and oligomers. Table 2 shows a typical example of FCC yields. Table 2 Typical yields from an FCC Unit Cut Yield [%] Gas (H2,C1,C~,C2=) 2-4 C3- LPG (65 - 75 % olefinic) 4-6 C4 - LPG (65- 75 % olefinic) 6-11 Gasoline 40-50 LCO 15-20 Bottoms 8-15 Coke 5-6

327

2.2

Role of the catalyst matrix

Modem refineries are required to process more and more heavy feedstocks with a constant increase in the amount of atmospheric resid fed to the FCCU. The consequent increase in the amount of Conradson carbon in the feed reduces the ability to achieve conversion by increasing regenerator temperature and reducing the cat/oil ratio. This problem can be addressed by a proper selection of catalyst in which the zeolite:matrix ratio is carefully tailored to achieve maximum bottoms cracking while not accompanying this with excessive dry gas or coke production. At the same time the matrix can be carefully controlled with respect to factors such as alumina or silica-alumina type, surface area, acidity, and pore size distribution. This will also require that the zeolite part is carefully designed with respect to the nature, content and distribution of extra-framework ahaninham along the zeolite crystals, as well as Si/AI ratio, rare earth content, and unit cell size. The FCC particle consists basically of about 35 wt % zeolite Y [12] incorporated into a matrix usually consisting of silica, which acts as a "glue", and ahtmina, which serves to crack the large molecules. The matrix often includes clay to provide the desired density. One of the roles of the matrix is to increase the resistance of the catalyst to metal poisoning. This will be discussed in Section 2.4. Increasing the matrix content can also increase the hydrothermal stability of the catalyst. Some of the properties which must be developed in the matrix are [13]: Macropores (>IO00A)of low activity to assist in the cracking of the large asphaltenes and to allow deposition/passivation of Ni and V in particular; Mesopores (30-1000A), with higher activity than in the macropores, which are able to cleave side chains from aromatics and napthenic rings and thus produce products boiling in the LCO (light cycle oil) range (220 - 360C); Small pores (<20A), with the highest activity, to crack straight chain alkanes.

Accessibility is a function of the size of the hydrocarbon molecule, the pore size distribution of the catalyst, the plugging of pore mouths as a result of coking, poisoning, contact time of catalyst and oil, etc. The macropores at the entrances to the channel strucu~es should have low activity and surface area but should also be able to act as a guide to transport molecules to the key functional sites in the smaller pores. Improving the accessibility of the feed molecules to the FCC catalyst is a primary area of current research. This improved access would result in better bottoms cracking, better resistance to poisons, improved stripping efficiency in the FCCU and reduced overcracking and secondary reactions. It has also been shown that the relative stripping efficiency can be almost doubled since the more open pore structure not only enhances diffusion into the catalyst particle but transfer out of the particle is faster as well. The improved diffusivity of molecules also results in reduced residence times of the reaction products in the pores and thus overcracking and secondary reactions are reduced. Figure 3 shows how the pore size distribution changes as the accessibility increases [14]. The catalyst with a very open structure and high accessa"oility allows the coke precursors to undergo cracking and thus coke formation is reduced. This implies that reactors with short residence times are more susceptible to coking since there is insufficient time available to bring the precursors into contact with the functional sites due to diffusional limitations.

328

Figure 3. Pore size distribution for catalysts of different accessibility. [ 14]

TONx 104

0

K [ r m "~]

i 10

i

1

20

30

0

i

i

i

i

10

15

20

25

AI/u.c.

AI/u.c.

a.

i

5

b.

Figure 4. Activity for cracking on steam (11) and SiCh([]) deahminated Y zeolites (a: nheptane; b" gas-off) [ 1]

329

Large gasoil molecules cannot penetrate deep into the zeolite pores and only an outershell of the cTystallites will be active for cracldng gasoiL Primary cracking is known to take place on the matrix [15]. Thus all the acid sites measurable by bulk techniques are not necessarily available for primary cracking. This is well illustrated in Figures 4 and 5 [1]. The steam dealuminated zeolite Y possesses mesopores which results in an increase in the ratio of external to internal surface area and thus to increased aceess~ility to acid sites. Thus although the steamed sample has less Bronsted acidity as measured by pyridine TPD [16] and thus shows a lower activity or TON for hexane cracldng, it has a higher activity. This implies that, although the steamed sample has fewer sites, the number of accessa"ole acid sites is greater on the zeolite with mesoporosity. Hence by increasing the ratio of external to internal surface area and generally by increasing the accessibility of molecules to acid sites in a controlled way it should be possible to increase the activity of a given zeolite Y [1]. A recent patent has described the treatment of an ultrastable zeolite Y to produce a catalyst with "primary" pores of about 50A and "secondary" pores of 100-600 A by steaming at about 600~ followed by washing in 0.SN nitric acid at 80~ for 3 h [17].

0.07 Steam

~o eq

0.05

i !i~i i ~

NNNI 0.03

o

E

E

"" I ~ ! ~ SiCI4

ql

Q.

0.01

--1

f

~10

~25

-0.01 ~d/U.r

Figure 5. Mesopore volume for Y zeolites dealuminated by different procedures [ 1].

The diffusion of molecules into the pores of zeolites is a function of their size (Figure 6) and in order to produce catalysts which are capable of selectively cracking the large molecules in crude oil it is necessary to make catalysts with pores large enough to accommodate such molecules. The opening to a pore of zeolite Y is about 7.5A whereas the average diameter of a heavy residue molecule is greater than 20A. The large pore 18 MR

330 molecular sieve VPI-5 theoretically represents a posm'ble candidate for cracking the large molecules. However, when desiring any FCC catalyst hydrothermal stability is of primary concern and it has to be stable when subjected to the high regeneration temperatures such as 720~ in the presence of steam VPI-5 has no acidity and when acidity is introduced by making the SAPO version of this catalyst, it is not thermally stable. Another large pore zeolite which could possa'bly satisfy the above requirement is MCM-41 which can be made with a pore size of between 20 - 100 ~ Recently a series ofMCM-41 zeolites has been prepared to investigate their application as FCC and hydrocracking catalysts [ 18] The Si/A1 ratio of these samples were 6.34, 6.09 and 5.65 and their mean pore sizes were 3.25nm~ 2.15 nm and 3.85 nm respectively. These pores are essentially cylindrical in nature. It was found that their steam stability as determined by meamuSng their surface area and pore volumes after 5 hrs steaming at 788~ was very low and they did not show any promise for resid FCC application. In a subsequent test the MCM-41 sample was used as a 10% additive to Y zeolite in a simple FCC formulation. This sample showed some promise in steam stability and micro - pore retention. In Table 3 some characteristics of the standard cracking catalyst and the catalyst incorporating the wide pore zeolite, MCM, are shown [ 19]. From these results it can be concluded that the catalyst is stabilized in the presence of the ultra-wide pore (UWP) zeolite. The surface area and micro-pore volume atter steaming indicate a bigger part of the Y zeolite is still intact thus leading to a higher activity in the MST test. However there is no indication of an enhanced bottoms conversion capacity [20]. The ultra-wide pore zeolites do not appear to result in an improved performance and, as mentioned above, it would also be necessary to improve the stability of these catalysts drastically in order to enable them to survive the severe regenerator conditions of the FCC unit. De, x 1017 [m21s] 100

10

~

I

e

"/"~/~-~ ~ 1,3-Diisopropy I - ~ X~J~--_j/ benzene benzene -~.v/ /,... /

2-El:hylnaphthalene .,,.

1-Methyl-

(~~~halene /

1-1RhylI-( ~ ~ i naphthalene 1, j "-4.~ " ~

0.1

LO

1,3,5Trimethylbenzene

et

0.01 1,3,5-Triethylbenzene 0.001 0.65

........1~~ (.

I

I

t

!

I

0.7

0.75

0.8

0.85

0.9

Critical Molecular Diameter [nm]

Figure 6. Diffusivity of molecules of various critical molecular diameters.

0.95

331 Table 3 Characteristics of an FCC standard catalyst incorporating unltra-wide pore (UWP) zeolite (MCM) [20] Standard Chemical Composition 35 Y-zeolite wt% d.b. UWP zeolite wt% d.b. Active matdx wt% d.b. 10 Physical analysis Surface area fresh m2.g1 237 Micropore volume ml.g 1 0.110 137 Surface area after Ni deactivation m2.g~ Micropore volume after Ni deactivation ml.g 1 0.053 MST activity at cat:oil = 3.5 63.3 Selectivity at 68 wt% conversion Gasoline wt% 43.7 LCO wt% 17.9 Bottoms wt% 14.1 Coke wt% 2.4 -

MCM 35 10 344 0.174 164 0.073 65 43.8 16.0 16.0 2.6

Another approach to selectively control the cracking process is to crack the heavier resid molecules on an active matrix and to crack the smaller fragments in the Y zeolite. The pre-cracking step can be carried out on a layer of active matrix coating, such as alumina [21]. There has however not been any report to date of a commercial application of such a concept. In order to improve the chances of success for a commercial application the pore size and acidity of the coating will have to be carefully adjusted to perform the pre-cracking. Currently some multi-porous sieves which may have FCC potential are: Boggsite, which is a natural mineral having a 12 membered ring on the outside; SSZ-26 / SSZ-33 / CIT-1, which are potentially very interesting but probably too expensive to prepare [61]; ZSM-50/MCM-22, the latter being an extension of the former which has 12MR pockets so that MCM-22 has interesting 10/12 MRs but only 10 MRs on the outside; 9

NU-87 (SSZ-37) which has 10 MR to the outside and 12 MR intersecting.

Although the concepts of a multi-porous FCC catalyst has much promise, the authors are not aware of any current commercial application. This is mainly due to the cost of these catalysts and the fact that this extra expense is not apparantely justified when the catalysts' performance is compared with the relatively less expensive option of a ZSM-5/Y mixture (cf.

2.5).

332 2.3

Metal contaminants

One of the roles of the matrix is to reduce the effect of metal contamination. Various matrices have been proposed to be suitable for this purpose. Magnesia-alumina has been found to be superior to most other inorganic oxides as a vanadium and nickel metal trap as well as increasing the resistance of the zeolite Y to steam deactivation [4]. Table 4 shows a comparison of the performance of a coated FCC zeolite and a standard FCC catalyst in a micro simulation test both after Ni impregnation and steaming and after cyclic deactivation with vanadium [19]. The coated catalyst shows a better steam stability after Ni impregnation and a much better stab'flit3, after cyclic deactivation with V in the feed. The higher conversion points to a better stability of the zeolite under deactivation conditions. The main advantage of the coated zeolite is the better protection against V attack. In another study it was shown that the zeolite retention, i_e. the zeolite activity after a particular treatment relative to the act'wity of the ~esh catalyst, doubled in the case of a catalyst with increased accessibility after the catalyst had been subjected to both V and Ni poisoning [22]. This is due to the fact that the functional sites which can neutralize these poisons are more accessible to the metal bearing molecules. Table 4 Performance comparison after deactivation with Ni and V of an FCC catalyst and the same coated [19] NonCoated coated RE-Y REoY Ni impregnated 5h 788~ + 1000 ppm Ni Surface area m2.g-1 165 182 Micropore volume ml.g -1 0.048 0.054 Conversion wt% 77.5 77.2 Cat:oil = 3.5 wt/wt Selectivity 68 vwt% conversion Gasoline wt% 46.3 46.5 LCO wt% 17.9 18.1 Bottoms wt% 14.1 13.9 Coke wt% 3.1 3.0 Cyclic deactivation ~ 5000 ppmV Surface area m 2.g-~ 132 170 Micropore volume ml.g -1 0.035 0.051 Conversion wt% 71.8 75.4 Cat:oil = 3.5 Selectivity 68 wt% conversion Gasoline wt% 43.4 44.7 LCO wt% 17.3 17.6 Bottoms wt% 14.7 14.4 Coke wt% 5.9 5.2

333 2.4

Role of additives

2.4.1

ZSM-5 and Beta

Cracking catalysts using combinations of medium and large-pore zeolites in order to maximize the production of products to be used in reformulated gasoline has been reported [23]. In a study of the cracking of n-heptane over MCM-22, ZSM-5 and Beta it was shown that the yield of propene was greatest in the case of MCM-22 and the overall alkane/alkene ratio of the products lay between ZSM-5 (0.94) and Beta (1.17). The addition of ZSM-5 to the FCC catalyst is an important method to increase the amount of light alkenes without increasing coke or dry gas yield. There are now more than 50 commercial units worldwide using ZSM-5 as an additive [24]. The main reason for ZSM-5 being so widely used is that it is very easy for the refiner to add this catalyst when fight alkanes are needed and as soon as addition ceases alkene production stops shortly afterwards. Figure 7 shows that the addition of ZSM-5 is also able to increase both the KON and MON by increasing the iso/normal alkane and alkene ratios and the concentration of gasoline range aromatics [25,26]. At the same time it leads to an increased yield in propene and reduced gasoline yield and an increase in both iso-butene and n-butene (Figures 8 and 9) [25,27]. The addition of ZSM-5 also results in a decrease in the amounts of methyl-pentanes, hexanes and heptanes. The increase in the amount of C5s will result in an increase in the RVP values. Table 5 shows the effect of adding ZSM-5 and also of adding a mesoporous matrix to the KEY FCC catalyst. The effect of adding ZSM-5 is to enhance the cracking of C7 and higher alkenes. This will of course be accompanied by a slight increase in the amount of C5 and C6 alkenes which may crack further to increase light alkene yield [25].

95

82.5

94

, ,,

0.8

9

93.5

0.7 81

~' 92.5

, - ~' ~'&

,,..,

6.

0.65

81 ~

91 90.5

8

0.6

4. ,

~

~

15

80

ZSM-6 Additive ( w t % o n blend)

Figure 7. Effect of ZSM-5 additive on RON and MON [25,26].

~ O'

0.45 5~

10~

15 0.4

ZSM-6 Additive [ w t % o n blend]

Figure 8. Effect of ZSM-5 additive on yields of propane and propene.

334

55 3

==

45

0

5

10

o

15

!

0

5

ZSM-5 Additive [wt% on blend]

10

15

ZSM-5 Additive [wt% in blend]

Figure 9. Effect of ZSM-5 additive on yield of gasoline.

Figure 10. Effect of ZSM-5 additive on yield ofbutene.

Table 5 Different options for the production of light alkenes in FCC [25] Catalyst options Base cat Base cat + Low RE(*) + low RE 3 wt% ZSM-5 mesopore act Reactor temperature, ~ 525 525 525 C2= wt% 2.6 2.6 2.4 LPG wt% 16.4 18.8 15.9 Gasoline wt% 45.0 42.7 46.4

Base cat + high reaction 540 3.3 20.4 44.2

Deep cat cracking 600? 10.5 40.2 22.7

6.3 2.1 5.5 3.7

19.0 6.2 8.3 2.1

Desired Products: C3= wt% iC4= wt% nC4= wt% iC4 wt%

4.2 1.8 4.5 3.5

5.2 2.0 5.0 3.8

Undesired products: C~ wt%

1.4

1.7

1.2

1.8

3.3

1.0 19.0

1.1 21.3

0.6 18.4

1.0 23.6

1.1 50.7

nC4 wt% Total 'Vtet gas" wt% (*) ADZ-50containing catalyst

4.1 2.0 4.5 3.5

335 When ZSM-5 is incorporated as a cracldng catalyst the adsorption of alkyl aromatics, in which the benzene ring leads the alkyl chain, is favoured [28] leading to relatively high yields of benzene. With USY on the other hand the ratio of dealkylation to side chain cracking is an order of magnitude lower than with ZSM-5. It is estimated that with 20% ZSM-5 in the catalyst inventory and using a combination of high ten~erature and, since H-transfer reactions are intrinsically slower than catalytic cracking reactions, short cracking contact time (0.1 - 0.5 sec.) isobutene yield can be 5.4 voL % and combined propene and butene yield can be 35 vol. % [32]. ZSM-5 is particularly selective to making propene relative to C4s with 50 - 60% of the yield shifting to propene. The addition of P to ZSM-5 has greatly enhanced its activity such that only 5-10% ZSM-5 is in the additional catalysts compared to 25% in the original additive catalyst without P. It has been claimed that large pore zeolites such as Beta and ZSM-20 can be incorporated into a cracking catalyst with unusual selectivity for producing compounds boiling in the gasoline range which contribute to high octane. These components are low molecular weight alkenes produced by Beta and aromatics produced by ZSM-20 [33]. The Beta or ZSM-20 in the final catalyst are in the H + form It has also been claimed that when the Beta contains a small amount of gallium or zinc the aromatic content of the gasoline increases [34]. It is worth noting that Beta has a relatively low H transfer activity and this would be conducive to the production of more light alkenes but Beta also has lower gasoil cracking activity due to its problems with accessibility arising from its pore size and structural defects [35]. When Beta is used as an FCC additive in the same way as ZSM-5 much higher amounts of Beta zeolite than ZSM-5 need to be added to see sensible results and, since it is an expensive catalyst, it is not presently economically viable to use it extensively as an additive. 2.4.2

Rare Earth Elements

Cracking catalysts are thermally stable in air at 760~ even when loaded with 3-4% V. In the presence of steam however this stability is greatly reduced and dealumination occurs. Corma et al. [29], however, have shown, by examining the butane/butene ratio in the products obtained during the cracking of a vacuum gas oil at 480~ that, with steam dealuminated zeolites, a sharp decrease in the ratio of H transfer to cracking is observed when the number of A1 atoms per unit cell ~lls below 10. At these conditions the adsorption of alkenes decreases dramatically more that in the case of alkanes implying that the rate of H-transfer reactions will inevitably decrease relative to the rate of cracking. Stability to steaming can be increased however by incorporation of rare earths into the FCC catalyst. The RE impedes the dealumination of the zeolite structure and therefore increases acid site density. Concomitantly a reduction in the RE levels will result in a decrease in the unit cell size and in the acid site density [ 1,30,31] This is accompanied by an increase in the strength of individual sites which will favour the formation of LPG which requires strong sites. A reduced acid site density will also result in a reduced extent of hydrogen transfer which is accompanied by greater isomerization and a reduction in the amount of aromatics formed [30]. This has been explained by the fact that, while cracking is a unimolecular reaction needing one active site, hydrogen transfer, being a bimolecular reaction, needs two close active sites [ 1]. However it should be noted that the higher molecular weight precursors of light alkenes are preferentially saturated and the resulting gasoline range alkanes do not readily crack to light alkenes. A greater concentration in alkenes, which will increase MON, has a greater impact than the effect of reduced aromatics which would decrease the octane number. Generally the RE content in FCC catalysts has been decreasing in recent years in order to allow for an increase in alkene contents in the products by as much as 15% [3]. Low rareearth, high matrix activities also result in high isobutene yields which is also a favourable product if MTBE is being synthesized downstream Engelhard has achieved these effects on

336 their FCC Isoplus series of catalysts in which USY has a with unit cell size <24.29 A and extremely low Na (<0.1%) and rare earth levels. These are commercially proven catalysts which give excellent yields of isobutene and other light alkenes at much lower cost than Beta based catalysts. Currently about 6 units worlwide are using these catalysts in order to obtain high alkene yield [50]. In conclusion it is hnportant to appreciate in the case of catalytic cracldng that modifying reaction conditions can often result in a more si,~nificant increase in alkene production than can be achieved by catalyst modifications. This requires operating the FCC at high severity by increasing the cat/oil ratio which leads to an increase in the yields of C3s and C4s. Raising the reactor temperature, decreasing the feed temperature, and decreasing the regenerator temperature all lead to an increase in alkene production. Overcracking and secondary reactions can also be enhanced by increasing residence times although if the reaction time is too long there exists the possibility for more secondary H-transfer reactions. Obviously excessive overcracking (e.g. conversions > 75%) will lead to decreased gasoline yield and enhanced dry gas yield (Table 5). Lower partial pressures in the FCCU will also increase the formation of C3 and C4 alkenes.

3.

AI~YLATION

3.1

Introduction

In refining processes alkylation of isobutane with propene or butene is important in order to obtain alkylate which has a high octane number and a low vapour pressure. This process is not, however, directly relevant to the focus of attention of this paper and will therefore not be dealt with in any detail It has been well reviewed recently [36]. It is, however, worth noting that recent attempts to develop a zeolite as an alternate to the currently used hydrofluoric or sulphuric acid do not appear to have been successful and it is now assumed that superacid catalysts are the most likely heterogeneous alternatives. For the petrochemical industry the alkylation of aromatics is an important route to the production of alkylaromatic such as ethylbenzene, xylenes, cume, C10 "C18 alkylbenzenes, alkylphenol, alkyl-napthalenes and alkyl-biphenyls which are used in many different processes. Alkylation is an acid catalyzed reaction which traditionally employs aluminium chloride based catalysts or hydrofluoric or sulphuric acid These processes thus require the use of highly corrosiveresistant materials of construction, have a high catalyst consumption and are associated with environmental problems. Consequently there is considerable incentive to replace these catalysts with solid acids such as zeolites. The most important alkylated aromatic compound is ethylbenzene 99 % of which is used after dehydrogenation for styrene production. Several technologies using zeolites are nowadays available for the production of e t h y l b ~ e . The Mobil-Badger process [37] is a vapour phase process at 400 - 450~ 2 - 3 MPa using H-ZSM-5 as a catalyst. The high molar ratio of benzene to ethylene of 5 - 20 ensures an essentially complete ethylene conversion and maximal ethylbenzene selecthhty. The main by-product is diethylbenzene which can be converted into ethylbenzene by disproportionation with benzene. The catalyst life-time is several weeks and it can be regenerated using air. This process is technically proven using a diluted ethylene feed (17.6 voL-% ) using similar ethene:benzene ratios to the pure ethene feed [38,39]. Thus the mass flow through the reactor is slightly increased. It is necessary to eliminate propene from this feedstock because propene alkylates benzene readily and additional distillation facilities are then required. Although the catalyst is much less sensitive to sulphur, the latter was removed from the feedstock. It has been chimed that the liquid phase ethylation of benzene using MCM-22 results in a lower yield ofpoly-substituted ethylbenzenes

337 which would reduce the recycle of these compounds [40]. Processes utilizing catalytic / reaction distillation have been described in literature [41,42]. The formation of ethylbenzene by side-chain alkylation of toluene with methanol using a basic catalyst like alkali-cation exchanged X and Y zeolites has been shown to be feasible [43,44]. Partial oxidation of xylenes yields hnportant monomers for the polymer industry such as phthtalic acid from o-xylene, isophthalic acid from m-xylene and therephthalic acid from pxylene. Of these terephthalic acid is mostly used in industry as a co-monomer like in the production of PET. The demand for p-xylene is hence higher than for the other two xylene isomers. The desired product distribution can be achieved by using product shape selective catalysts like zeolites. Xylenes can be produced either by toluene disproportionation or toluene alkylation using methanol. Toluene disproportionation is an attractive route because it does not require an additional alkylating agent and both the product xylenes and the byproduct benzene are valuable chemicals. In conventional toluene disproportionation processes, the p-xylene content in the fraction of xylenes is ca. 24 %, which corresponds to an equilibrium distribution of the xylene isomers [45]. The Mobil Selective Toluene Disproportionation Process (MSTDP) using large H-ZSM-5 crystals [46,47,48] at 455 470~ 2 - 4 MPa, H 2 to toluene ratio of 3 moFmol, yields a p-xylene content in the fraction of xylenes of 82 - 90 % at 30 % toluene conversion. The use of large crystals is necessary, because aU three xylene isomers are formed inside the pores but due to the orders of magnitude greater diffusivity ofp-xylene relative to the two other isomers the desired product will leave the zeolite crystals and the other two isomers will be converted to p-xylene inside the zeolite's crystal [46, 49]. The high temperature is required because of the high activation energy of the disproportionation reaction and the presence of a noble metal in the catalyst and hydrogen in the feed increases the life-time of the catalyst which operates for up to one year before regeneration becomes necessary [1]. Alkylation of toluene with ethene using shape selective catalysts can yield pethyltoluene which upon dehydrogenation yields p-methylstyrene [51]. Using a modified ZSM-5 catalyst p-ethyltoluene can be produced very selectively (97 %) at high toluene conversion [46]. The polymer from this starting material may possess more interesting properties than polystyrene. Cumene formed by the alkylation of benzene with propene is the major source for the co-production of phenol and acetone. To a minor extent it is also used as the source for amethylstyrene. Originally it was produced using a solid phosphoric acid catalyst at 200 260~ 3 - 4 MPa. In this process 95 wt.-% eumene selectivity could be obtained and the main by-products were diisopropylbenzene and polyaromatic compounds. The reduction in the amount of by-products formed is the main incentive to replace this catalyst and liquid phosporic acid also causes corrosion problems in the downstream apparatus. Cumene can be produced using zeolite Y at ca. 200~ [52]. The once-through selectivity of this process is significantly lower (70-90%) but the by-products viz., polyisopropylbenzenes, can be transalkylated so that an overall process selectivity of 99 % can be obtained. Dow [53] have developed a cumene production unit on the basis of deahlminated Mordenite with aSi/A1 > 40 and preferably-160 [54], and which operates between 130 and 200~ At higher temperatures n-propylbenzene, which cannot be recycled, is formed and at lower temperatures the formation of diisopropylbenzene is favoured [55]. Interesting alternative catalysts for cumene production in the liquid phase are zeolites Beta and ZSM-12 which have shown better stability and higher selectivity in comparison with Y-zeolites [56,57]. Linear alkyl benzene sulfonates, which are important as detergents, are formed by the alkylation of benzene with linear C10 -Cls alkenes and are produced either using anhydrous HF, H2SO, or AICI3 as a catalyst [52]. The corrosive and hazardous nature of these catalysts has led to efforts to replace them with solid acids. Normally, in the alkylation of benzene with

338 linear ct-alkenes a mixture of alkylbenzenes is obtained with the phenylgroup attached to different carbon numbers except for the 1-phenyl isomer which would require a primary carbenium ion as a transition state. This indicates that the double bond isomerisation is much faster than the alkylation step. Ideally, 2-phenylalkenes should be formed because they can be easily converted in the ambient environment. Higher selectivities for this isomer can be obtained using shape-selective catalysts but their major drawback is their relatively rapid deactivation [58,59,60]. Recently, there has been much interest in the shape-selective alkylation of polyaromaties and biphenylic compounds, especially 4,4'-diisopropylbiphenyl ang 2,6dialkylnaphthalene, since upon oxidation they yield monomers for high quality plastics which have interesting applications such as in LCDs [47,62]. For these high technology polymers the narrowest isomer of the dialkylaromatics and dialkylbiphenyls seems to be the one most applied. Thus shape selective acid catalysts can prevent the formation of unwanted isomers and thereby improve the economics of the production of these compounds. Mordenite seems to be the most ideal catalyst for both the selective formation of the dialkylbiphenyl and the dialkylnaphthalene [63,64]. This is supported by molecular modelling studies (Figure 11) which have shown that, in the isopropylation of naphthalene, the 2,6-isomer is the favoured dialkylisomer due to the differences in diffusivity [65].

Figure 11. Model structure of 2,6-DIPN in HM and L.

339 3.2

Mechanism of alkylation

The mechanism of alkylation has recently been extensively reviewed by Venuto [40]. Briefly, alkylation involves an electrophilic addition of a carbenhtm ion which is generated by the alkylating agent. Different alkylating agents, such as alcohols, alkenes, alkyl halides and aromatics, can be used. If alkylaromatics themselves are the alkylating agent yielding the corresponding dialkylaromatic and the aromatic compound it is called disproportionation. Although the formation of the arenium ion from the relatively stable alkylaromatic is less favoured than the formation of the carbenium ions from other alkylating agents like alkenes and alcohols at high reaction temperatures both mechanism might occur. This was also concluded by Mirth and Lercher [67] who showed with IR that in the methylation of toluene at 200~ methanol replaces adsorbed toluene forming methoxonium ions and alkylation takes place by the interaction of this ion with toluene. At temperatures higher than 300~ however, the concentration of these ions becomes very small and therefore the other reaction pathway might prevail [68]. Alkylation reactions can be observed in all processes involving zeolites where aromatics and an alkylating agent are present. Even m-xylene isomerisation, which is classically visualized as a 1,2 methyl shitt [69], proceeds over faujasites partially by a number oftransalkylation steps. This has been shown by using a mixture of hexadeuterated m-xylene (C6H4(CD3)2) and normal m-xylene (C6H4(CH3)2) [70]. The relative importance of the 1,2 methyl shift versus the transalkylation differs over various zeolites [71]. During the xylene isomerisation at 200~ over HY at least 20 % of the reaction occurs via the bimolecular transalkylation. The intermediate complex in the bimolecular reaction needs to be accelerated in a micro-cavity and therefore the importance of the bimolecular reaction is three times less in the isomerization of xylene over H-Mordenite and does not occur over Beta at 2000C. Of interest is the reported absence of the bimolecular reaction over amorphous silica-alumina at 400~ Para/ortho-substitution is strongly favoured in the alkylation of alkylaromatics because of the electron-releasing effect. From a statistical point of view the ratio of ortho to para during alkylation of alkylaromatics should be 2, but, based on resonance considerations, the attack in the para-position is slightly favoured. Although in homogeneous catalyzed systems a high selectivity towards the para-isomer is expected, at slightly elevated temperatures the isomerization yielding the meta-isomer is fast and this decreases the production of the desired para-isomer. Shape-selective zeolites possess the ability to deliver high yields of the paraisomer. The isomerization forming the recta-isomer must, however, be suppressed. It has been shown in the case of toluene alkylation with methanol over H-ZSM-8 between 400 450~ that the shape selective formation of p-xylene is slower than the subsequent isomerization yielding m-xylene [72]. Of interest from a mechanistic point of view, is the observation that toluene disproportionation over Beta at 350 - 400~ yields a p-xylene concentration higher than the equilibrium value thus indicating the primary formation of this isomer in the large pores of this apparently non-shape selective catalyst [73]. The disproportionation of C9 aromatics, however, yielded an excess of o-xylene [73,74] whose concentration approaches the equilibrium value at higher temperatures. This was explained [74] with a biphenyl carbenktm ion intermediate which yields o-xylene as primary product. In the ethylation of toluene the ortho-isomer is the primary product formed over amorphous silica-alumina, whereas the shape-selective zeolite H-ZSM-5 yields the p-isomer with pethyltoluene being essentially absent [75]. Methylation of toluene over wide-pore zeolites like HY also exhibits a product distribution, which, although not thermodynamically limited, cannot be explained in terms of geometrical and diffusion effects [76]. Quantum mechanical calculations seem to indicate that the orbital interaction in the para-position is larger than in the ortho-position [77,78]. This, however, seems to contradict the observations made with amorphous silica-alumina catalysts [75].

340 Shape selective catalysis has been described earlier in this paper [79]. Due to the large difference in the diffusivites of para-, recta- and ortho-xylene in medium pore sized zeolites, the large observed selectivity of the para-isomer has therefore been explained in terms of product shape selectivity. It has been postulated that this product shape selectivity depends on the length of the intracrystalline diffusion pathway and on the tortuosity of the channel system [80, 81, 82]. Hence the selectivity should be governed by the type of the zeolite structure, type of modification and crystal size. If the whole reaction takes place on the internal, shape selective surface of the catalyst, then the observed decrease of the paraselectivity in the toluene methylation with increasing conversion and temperature [83, 84] and decrease in para-xylene selectivity with decreasing crystal size in the toluene disproportionation [85, 86] can be explained. Zeolite crystals possess both an internal, shape selective and an external, non-shape selective surface. Therefore, it was postulated that paraisomers are formed inside the pores and these are converted in a secondary reaction on the external surface of the zeolite [87,88,89,90]. In order to distinguish between the influence of the external and internal surface and to monitor the influence of the diffusion pathway on the selectivity it would be useful to perform the alkylation over zeolites with crystal sizes which differ by at least an order of magnitude and to inertize the external surface of these crystals. The formation of di- and tri-alkyl aromatics and biphenyls l~om their aromatic precursor is a consecutive reaction, in which first the mono-substituted aromatic compound is formed which subsequently reacts to form the di-alkyl compund. This was observed in the shape selective ethylation of biphenyl [91] isopropylation of naphthalene [92] over HMordenite and the isopropylation ofnapthalene over HY [63]. Therefore, if a dialkyl-isomer is the desired product, the reaction conditions (reaction time/residence time, partial pressures and temperature) have to be optimiTed to obtain the the maximum yield of this desired product. With shape selective catalysts the formation of poly-substituted aromatic compounds can be suppressed because they are not able to diffuse out ofthe channels of the zeolites [63].

3.3

Effect of pore size

The alkylation of aromatics over zeolites offers the poss~ility for shape-selective catalysis if the di~sivity of the products in the zeolite pores differ greatly or if certain reaction pathways are blocked due to the geometrical constraint which are put on the transition state of the reaction occuring in the zeolite pore or cavity. The major drawback of the use of zeolites is the possibility of pore blockage which leads to catalyst deactivation. For one-dimensional zeolites in particular, pore blockage will lead immediately to a strong deactivation. This was shown for the transalkylation of C 7 and C 9 alkylaromatics at 400~ in which mordenite showed a high initial conversion and a strong deactivation [73], whereas HZSM-5 was stable for the conversion of the toluene but not active for the conversion of C 9 aromatics due to reactant shape selectivity. Beta showed some deactivation but achieved a steady state conversion which for C 9 aromatics was higher than that obtained with H-ZSM-5. A slight deactivation of Beta was also observed during the isopropylation of toluene and cumene [93]. The zeolites have to be selected according to their pore and cavity dimensions to obtain the desired result. Bellussi et al. [57] studied the propylation of benzene in the liquid phase over H-ZSM-5, H-Beta (Si/AI = 14), HUSY (Si/A1 = 3) and the classical phosphorous impregnated kieselguhr catalyst at 150~ 3 MPa, benzene/propene ratio 7.4 and a space time of 0.06 rain. They observed after one hour on stream, that H-ZSM-5 was hardly active and HY was at that time less active than H-Beta.

341 The choice of the right pore dimensions is very important in the alkylation of polyaromatics and biphenyls. The activity of the medium sized pores of ZSM-5 for the methylation of naphthalene is low in comparison to H-Mordenite and HY. ZSM-5, however, showed high selectivity for 2-methylnaphtalene, whereas Mordenite and did not show shape selective methylation [88,94]. In 1-methylnaphthalene isomerisation at 300~ the selectivity of 2-methylnaphthalene correlates with the Spaciousness Index (SI) [95]. Good isomerization catalysts posses SI between 3 and 20 (HL, H-Beta, HM, EU-1 and ZSM-12). Catalysts with higher value for SI display a low activity and catalysts with a lower SI value showed activity for the disproportionation reaction. Over medium pore-size zeolites, i.e. H-ZSM-5 and HZSM-11, the methylation of 2-methylnaphthalene at 300 - 550~ produced the narrower dimethyl-isomers [96]. The selectivity towards 2,6-dimethylnaphthalene increased with decreasing temperature. Lee et aL [64] described the use of highly siliceous Mordenite for the alkylation of biphenyls to produce selectively 4,4'-diisopropylbiphenyl. Sugi and Toba [47] studied the liquid phase isopropylation of biphenyl over various zeolites at 240~ They observed, that the conversion increased in the order H-ZSM-5 H-Beta > HY> amorphous silica-ahtmina >KE-Y,~HF. Although the production of the 2-phenyl isomer is desirable in the production of LAB, it is much more important to avoid the formation of diphenyl-isomers and branched phenyl isomers. The production of these compounds can be suppressed by using HY as a catalyst [98,60]. The pore size of the zeolites can be modified by introducing cations in the pores or by the formation of coke inside the zeolite crystals, which decreases the diffus'lvity of the products, but also of the reactants, in the zeolite channels. Chen et al observed an increase in p-xylene selectivity with modified and coked H-ZSM-5 [85].

3.4

Effect of crystal size and external surface

As mentioned above, it is di~cu]t to separate the influence of the crystal size and of the external surface on the shape-selectivity of zeolites rigorously. Increasing the crystal size of the zeolite means at the same time reducing the influence of the external surface. For the industrial production it is desirable to operate with small crystal sizes.

342 Using small zeolite crystals (< 0.5~tm) in the methylation of toluene, Chen et al [85] observed at 500~ an equih'bfium mixture of the xylenes (i.e. 23% p-xylene). Increasing the crystal size to 31xm enhanced the para-selectivity to 46%. A further increase in the paraselectivity up to 97% could be obtained by modifying the catalyst with phosphoric acid ending with P-loading of 8.5%. The enhanced para-selectivity was explained by the increase in the diffusional pathway by pore plugging which would favour the outward diffusion of paraxylene. Also for the ethylation of toluene, modification with phosphorous or metal oxides of ZSM-5 was required to obtain high para-selectivities [75]. Kaeding et al [99] stated that the modification with P effectively blocks the external surface. Paparatto et aL [ 100] observed, in the ethylation of toluene and in the isomerisation of m-xylene at low contact times over H-ZSM-5, an excess pf p-ethyltoluene whereas amorphous silica-ahamina yielded an excess of the o-isomer. This indicates that the ethylation of toluene over H-ZSM-5 takes place inside the micro-pores. At higher contact times the product composition reaches the thermodynamic equih'brium distn'bution. With zeolite samples with large primary particles the o-isomer was always absent, whereas small crystals yielded the equilibrium distribution at high contact times. They explained their observations by the primary formation inside the pores of the p-isomer, which can isomerize on the external surface, The conm'bution of the external surface to the total active surface per gram of catalyst becomes larger if the size of the catalysts is reduced. The increase in crystal size also reduced the observed conversion. External acid sites can be eliminated by building an inert iso-structural silica shell around the zeolite by continuing the synthesis in an Al-free synthesis gel. This increases the crystal size and the effective diffusional pathway and is an effective method to enhance the para-selectivity in toluene alkylation but reduces the conversion over the catalyst [ 101]. The modification of zeolites by Chemical Vapour Deposition (CVD) does not only eliminate the external acid sites but also causes pore mouth narrowing. I-h'bino et al. [102] showed that the rate of adsorption of xylenes is decreased by CVD-treatment of H-ZSM-5 with tetramethoxysilane and they ascn"oed their enhanced para-selectivity in the methylation of toluene to pore mouth narrowing. Wang and Ay [103] showed that larger crystals needed less silica on their surface to obtain high para selectivity in toluene ethylation and therefore they regard the role of the active sites on the external surface to be very important. Matsuda et aL [104] studied the disproportionation of 2-methylnaphthalene over HZSM-5 and this zeolite post-treated with (NH4)2SiF 6 to eliminate the external acid sites. They observed that the bulkier isomers were formed over H-ZSM-5, whereas over the treated zeolite only the isomers 2,6 and 2,7-dimethylnaphthalene ws observed. This was explained by a disproportionation reaction in the pores and an isomerization reaction over the external surface. The same was observed in the isopropylation of biphenyl over Mordenite, where modification of the external surface with tn'butyl phosphonate increase the 4,4'diisopropylbiphenyl content in the fraction of dialkylbiphenyls and reduced the deactivation [105]. This was also ascn"oed to the activity of the external surface. Another method to eliminate the external acid sites is the selective poisoning technique in which a stronger base is added to the feed which is too large to enter the pores of the zeolite. It was observed that injection of small amounts of [3-naphthoquinoline during the toluene ethylation over H-ZSM-5 (crystal size 21am) increased the selectivity towards p-ethyltoluene but at the same time decreased the ethene conversion [106]. Regular injections of the base molecule are necessary because of the reversa'ble adsorption and the decomposition of the base under reaction conditions.

343 3.5

Effect of silica to alumina ratio and dealumination

The effect of silica to alumina ratios in the alkylation of aromatics is difficult to study separately because upon changing the Si/A1 ratio in the synthesis gel both the ratio in the zeolite and the crystal size are changed [97], as well as the crystallinity and morphology. If the Si/Al ratio is changed in a post-treatment step by dealumination, which can be done either by acid washing or by steaming, extra framework aluminium species are formed. These species can block pores and thereby modify the diffush~es of the reactants and products in the pores and can form a complex with remaining framework aluminium which may result in a modified acidity of the catalyst. Vinek and Lercher [107] synthesized ZSM-5 with Si/A1 ratios between 20 and 240, but because the pyridine TPD yielded a lower Si/Al ratio the existence of extra-framework aluminium species which were ascrl'bed to be weak acid sites. They obtained a linear correlation between the specific rate of toluene disproportionation and xylene isomerization and the number of strong Bronsted sites, which indicates that the reaction rate is primarily a function of the concentration of acid sites. This was also concluded by Nayak and Riekert [89] and observed for the ethylbenzene disproporfionation [108,109,110]. For toluene dispropordonation a linear relation~ip between the rate constant, assuming the rate to be first order with respect to toluene, and the Si/A1 ratio was obtained [ 111]. This indicates, that the turn-over-number (TON) remains constant and independent of the silica to alumina ratio. Sastre et al. [112] studied the isomerization of m-xylene over Ot~etite and observed monotonical increase in m-xylene conversion upon exchange of the K+-cations. This was ascn'bed to the increase of the concentration of the protons and the increase in accessa'bility of the pores, which resulted in a higher selectivity for the isomerisation reaction at the expense of the disproportionation reaction. Only a slight increase in the p-xylene in the fraction of orthoand para-xylene was observed. Over Beta a maximum activity for the xylene isomersation was observed and this was explained by either a possible existence of a synergistic effect between extra-framework aluminium and the framework Bronsted acid sites or a concentration effect [113]. The alkylation of toluene with methanol is also catalyzed by both strong and weak acid sites [107]. The ideal alkylation catalyst should have a high concentration of weak acid sites and a low concentration of strong Bronsted sites in order to minimize the side-reactions, viz. disproportionation. On the other hand it was observed that for the alkylation of benzene with linear alkenes over zeolite Y the rate increased linearly with the number of t~amework aluminium atoms which means that the turnover number remains constant [98]. However, the turnover number increased with increasing degree of ion-exchange which causes an increases in acid strength. The selectivity towards the desired 2-phenyl-alkane increases with increasing degree of ion-exchange showing that alkylation is a demanding reaction. For the alkylation of polyaromatics and biphenyls Mordenite with high silica to alumina ratios seems to be the preferred catalyst. Lee at al. [64] observed that dealumination of Mordenite by acid washing with 6 N HNO 3 modified the pore structure of Mordenite resulting in an increase in the total pore volume and especially an increase in the volume of pores with a diameter between 20 and 1000 A. In the isopropylation ofbiphenyl an increase in the yield of diisopropylbiphenyl was obtained which might be ascribed to either the enhanced diffusion of the reactants and products via the newly created meso-pores or the decrease in the rate of deactivation during the alkylation reaction.

344 The effect of dealumination of Mordenite by acid washing, leaching with EDTA and steaming has been studied systematically [114]. The selectivity to 2,6-diisopropylnaphthalene in the alkylation ofnapthalene was enhanced by the removal of external sites by leaching with EDTA. On the other hand after deep bed calcination the catalyst with a high external acidity showed a high conversion and a high selectivity. Stezming followed by mild acid washing to remove the extra-framework a~minhnn showed the lowest external activity and the highest selectivity for the formation of 2,6-diisopropylnaphthalene. Coke formation during the alkylation of biphenyl over Mordenite is reduced by using Mordenite with a high silica to aluminium ratio [ 110, 61], but also the nature of the coke is different. Mordenite with a high Si/A1 ratio produces a volatile coke (Td==,vtion= 200 - 340~ which are mainly biphenyl derivates whereas mordenite with a low Si/AI ratio yields hard coke which is burnt off at ca. 500~ The content of 4,4'-diisopropylbiphenyl in the fraction of encapsulated diisopropyldiphenyl isomers in the highly siliceous mordenite is over 80% which indicates the effectiveness of the pore system of Mordenite to produce selectively the desired isomer 4,4'- diisopropylbiphenyl.

4.

AROMATIZATION OF ALKANES/ALKENES

4.1

Introduction

Besides being a key high octane component of gasoline light aromatics are important raw materials for the production of a wide variety of petrochemicals. Benzene ranks third in volume and together with ethylene and propylene accounts for about 75% of the world's petrochemical production. At present catalytic reforming of hydrofined naphtha is the main source of BTX (benzene, toluene and xylene). The standard Pt/Re/A1203/CI catalyst is not very effective for converting C 6 alkanes to benzene, the yield being typically only about 10% as against 60% for methycyclopentane (MCP) and 90% for cyclohexane [58]. When the phasing out of octane-boosting lead from gasoline was started there was considerable interest in the production of additional BTX. To this end several zeolite based processes were developed, e.g., BP/UOP's Cyclar process using refinery C3/C 4 gases as feed, Chevron's Aromax process using C6 to C 8 alkanes as feed. The benzene could also be sold into the growing petrochemical market. However, since the allowable benzene content of gasoline is being lowered to below 1% the interest in these processes waned. To lower the benzene content in reformate it can be alkylated to toluene and xylenes, or it can be extracted and sold into the petrochemical market. The latter option could further depress the need in the short term for new sources of benzene. Only in instances where there is a shortage of aromatics but an ample supply of C 3 to C 8 alkanes and alkenes (as in a Fischer Tropsch complex) may processes such as Cyclar or Aromax be of interest. Nevertheless there is continuing research interest in aromatization using zeolite based catalysts such as Cra/HZSM-5 and Pt/KL.

4.2

Acidic Catalysts

The catalyst of choice remains acidic Ga-HZSM-5. The BP/UOP Cyclar process [115] used this catalyst in the 1000 bpd plant at Grangemouth, Scotland, which operated for about two years and was shut down in December 1991. With butane as feed a typical product spectrum was 65% BTX, 5% hydrogen and 30% fuel gas. UOP's continuous catalyst

345 regeneration process was used. IFffs Aroforming process also uses Ga-HZSM-5 in isothermal tubular reactors which operates on dual cycles [132]. Mitsubishi's Z-Forming process was tested in a 200 bpd unit which was commissioned in September 1991. The success of the HZSM-5 catalyst is no doubt linked to the low coke forming tendency of this particular zeolite. Other acidic zeolites such as HY are initially active but deactivate very rapidly due to coke deposition. Bradley and Kydd [116] investigated the performance of several pillar interlayered clay minerals and although the Ga pillared montmoriUonite was found to be the most effective it had a much lower activity than Cra/HZSM- 5. As is well know gallium addition greatly improves the performance of HZSM-5, eg, HZSM-5 at 550~ has a BTX selectivity of only about 12% while the addition of 5% Ga increases the BTX selectivity up to 70% [117]. The conversion of alkanes or alcohols to aromatics over HZSM-5 involves firstly the formation of alkenes which are then subsequently converted to aromatics, strong acid sites being involved in both steps [118,119]. Alkenes react much faster than alkanes [ 120] and this is in keeping with the deduction that the initial alkane dehydrogenation is a slow step in the overall process. The addition of Ga provides additional routes for dehydrogenation of alkanes, alkenes and naphthenes thus increasing both the overall reaction rate and also the selectivity to aromatics. Dehydrogenation via acid sites involves hydrogen transfer with the formation of low molecular mass alkane such as methane and ethane [120,121] which being inactive represent a loss of feedstock carbon. Dehydrogenation via Ga, however, produces hydrogen gas (which is a valuable by-product in refineries) and so results in a better feedstock carbon utilization. Addition of zinc to HZSM-5 has also been found to be very effective but Ga is preferred because of its higher stability [122]. ZnO is slowly lost through volafflisation at the high operating temperatures. More recently other metals active in dehydrogenation have been investigated as co-catalysts with HZSM-5. Ibm et al [123] claim that when feeding n-pentane to a Ni HZSM-5 catalyst the aromatic yield was equivalent to that obtained with Ga or Zn. They report an aromatic selectivity of 64% with Ni as against 71% for Zn and 66% for Ga. It should be noted, however, that when feeding propane the aromatic selectivity was only 25% which is a poor result. Ono et al [124] found that their Ag-HZSM-5 catalyst produced less methane and ethane than G-a- or Zn-HZSM-5 and concluded that Ag enhances C-H bond cleavage whereas Ga or Zn enhances both C-H and C-C cleavage. With butane and isobutane at 500~ the Ag catalyst gave a higher aromatic selectivity, namely 50 to 60% as against 30% for Ga. It should be noted, however, that here again the reported Ga results appear to be poor. With butene and methanol as feeds aromatic selectivities of 85 and 73% were obtained respectively with the Ag-ZSM5 catalyst. Shpiro et al [125] investigated the effect of adding both Pt and Ga to HZSM-5. They report that Pt promoted Ga reduction and its migration, resttlting in a more stable catalyst with a higher aromatic selectivity.

As gallium plays a key role its effective distn'aution in the zeolite is important. Although it is generally assumed that Ga 3+ is the active form, migration occurs more readily in the reduced Ga "~ state. The addition of Pt promotes Ga reduction by hydrogen spill over [125]. Hamid et al [126] found that the Ga, prepared by ion exchange, was, as one would expect, concentrated on the outer skin of the zeolite particles but with reduction/oxidation cycles the Ga migrated into the interior. They speculated that Ga + migrated as Ga20 vapour. In the regeneration cycle the Ga + is oxidised to the more active Ga 3+ state resulting in an improved performance. Further studies showed that after several reduction/oxidation cycles the performance reached a plateau [ 127]. It was deduced from pyridine infra red studies that the reduction/oxidation cycles resulted in a decreased H + concentration, due to exchange by Ga ions, and an increase in the Lewis acidity due to better Ga dispersion. In another study [117] it was found that H 2 pre-reduction markedly increased the aromatic selectivity of

346 physically mixed Ga20 3 / HZSM-5 but it decreased the aromatic selectivity of samples prepared by incipient wetness impregnation or by ion exchange. It appears therefore that H 2 reductions only improves matters when the Ga is poorly distn'buted in the initial state of the catalyst. If Cra dism~aution is important it could be reasoned that HGa silicate (MFI) would be a good catalysts since the Ga here is atomically dispersed by being incorporated in the ~amework. It has in fact again been reported recently that this zeolite is more effective than Ga/HZSM-5 [128]. Choudhary et al [129] found that the aromatic selectivity of Hgallosilicate increased with the degree of I-I* exchange while it decreased with mcreasing calcination temperature or increasing steam content during calcination. The latter two effects would be due to sintering (ie lower dispersion) of the extra-~amework Ga. Lukvanov, Gnep and Guinet have modelled the kinetics of propene [119] and of propane [120] aromatization over both HZSM-5 and Ga-HZSM-5 obtaining good fits with the experimental results. Propane is converted to propene along two main routes, protolytic craclcing of C-H bonds and dehydrogenation at the Ga sites. Protolytic crack~g of C-C bonds produce methane and ethane. The acid site reactions result in a CH4/H 2 ratio of 2.6 while the Ga sites give a 0.26 ratio which is in line with the observation that Ga HZSM-5 produces more H 2 than H-ZSM5. The propene then oligomerizes to higher alkenes (acid reaction). The oligomers are converted to dienes with both acid and Ga sites contn"outing and the dienes are converted to cyclic alkenes (acid sites). Cyclic alkenes are then converted to cyclic dialkenes and then to aromatics (H transfer at acid sites and de-hydrogenation at Ga sites). It was estimated that the Ga sites contn'bute about 90% to the diene formation and about 50% to the formation of aromatics. The formation of aromatics via H-transfer should result in the production of alkanes but the majority of the latter are again converted to alkenes. The only stable alkanes to emerge are the nonaromatizable methane and ethane. The product spectrum when feeding hexene or octene is very similar to that when feeding propene which is expected i~ as was found, the primary reaction is the craclcing of these higher alkenes to propene and butenes [117]. In general the percentage conversion of the feed and the aromatic selectivity follow the same trend. Likewise C2H4 selectivity follows the BTX selectivity [117,129]. When considering the breakdown of the aromatics it appears that at low temperature (350 ~ xylenes are the dominant aromatics, the benzene being low. As the temperature is raised to 550~ the benzene increases, toluene remains fairly constant and the xylenes decrease [117].

4.3

Platinum on Neutral Zeolites

Although platinum alone or on a variety of neutral supports selectively converts nhexane to benzene most of these catalysts deactivate rapidly due to coke formation. With the neutral zeolite KL as a support, however, much longer on-stream times are feast~ole and within a few years of Bernard's original publication [130] the Aromax process had been developed by Chevron [131]. Table 6 compares the aromatic selectivity obtained with Pt-Ba KL and Pt Re Sn / Al203 - C1 reforming catalysts [58]. Associated with the much higher aromatic selectivity is a lower amount of light gas production. Since the neutrality of the support was an important aspect the influence of doping Pt KL with the alkali series Li to Cs has been investigated by various workers. Hicks and coworkers [133] exchanged BaKL individually with Li to Cs and then added Pt by incipient wetness impregnation. They reported that the activity for aromatic formation increased markedly ~om Li to Cs but that the selectivity only increased slightly. Earlier studies [134]

347 had reported that both the conversion and selectivity increased markedly as Pt/KL was promoted with Li to Cs. Clearly the more basic the catalyst the better the performance. From this point of view it is interesting that promotion with various halogen compounds enhanced performance [135,136] despite the electronegativity of the halogens themselves. Tatsumi et al [ 137] investigated the effects of added KF, KC1, KBr and KI on the performance of Pt/KL. They found that KF and KC1 gave the highest benzene selectivities but that KBr and KI were actually inferior to the unpromoted Pt/KL. Table 6 Alkane Aromatization over Pt on Neutral and on Acidic Supports [58] % Aromatic Selectivity Alkane Feed Pt-BaKL Pt Re Sn/Ai203CI C6 87 25 07 82 45 Cs 80 60

The high selectivity of Pt/KL has been ascribed to the presence of very small Pt particles [138,139] and thus that sintering of these particles is one of the causes of deactivation [140]. It has been shown previously that Pt on zeolites KL, HL, HZSM-5 and silicalite were dispersed by treating with C12 in nitrogen or HC1 in air at 350~ [141]. With standard Pt / A120 3 reforming catalysts the practice of redistn'bution of the Pt (after air regeneration) by treatment with chlorine is well known. In the light of the foregoing it appears probable that the positive effect of halogen pro-treatment [135,136,137] is largely due to its resulting in finely dispersed Pt clusters. Iglesia and Baumgartner [ 142] have pointed out that selective terminal adsorption and dehydrocyclization of hexane to benzene are intrinsic properties of any clean Pt particles and that the role of KL zeolite is that the size of the channels inh~it the formation of coke in these channels thus keeping the Pt clusters there clean. This is in keeping with the opinion expressed previously by Tamm et al [131]. If it were a matter of pore size then one could, however, expect neutral silicalite also to be a satisl~ctory support. It is well known that operating metal catalysts in a hydrogen atmosphere inh~its coke fouling. Hicks et al [143] found that with 0.6% Pt/KBaL the catalyst deactivated due to coke fouling at hydrogen partial pressures below 6 atmospheres. The conversion of heptane increased with increasing hydrogen pressure up to 6 atmospheres. Pt/AI20 3 reforming catalysts commonly also contain Re which improves the catalysts' resistance to coke deposition [131] and this raises the question whether the effect of adding Ke to Pt/KL has been investigated. Pt/KL has been shown to be very sensitive to sulphur poisoning [131,144] and the effect has been ascribed to sulphur accelerating Pt sintering and subsequent blocldng of the zeolite channels rather than normal surface poisoning [144]. The sensitivity to sulphur obviously requires very thorough desulphurization of this feed and from this aspect a feedstock derived from the normal Co or Fe based Fischer Tropseh catalytic process could present an advantage. Fischer Tropsch products, however, contain other non-paraffmic substances such as alkenes, alcohols and carbonyls. The effect of these and other contaminants on the aromatization of hexane over Pt/KL is currently being investigated in the authors' research group. Zeolite supports other than KL have also been investigated. Pt/K Beta because of its higher acidity yielded more isomerized and cracked products than Pt/KL [145]. Ion exchanging with Cs reduced its acidity and improved the aromatic selectivity and Ba improved

348 the dispersion of the Pt which also increased its aromatic selectivity but despite these improvements the Pt Beta catalyst was still inferior to that of Pt/KL. It was, however, less sensitive to sulphur than Pt KL. Ruckenstein et al [146] studied the performance of composite catalysts, consisting of Pt/Ba-K1 with either Pt/beta or Pt/USY. Feeding mixtures of n-hexane, methylcyclopentane and methylcyclohexane the composite catalysts gave higher C7+ aromatics than expected from theindividual catalysts and feeds. An interesting observation was that for all the various individual catalysts (including Pt/Ba-KL) and for the composites n-hexane gave a lower benzene sdecdvity than did methylcyclopentane. This is contrary to the results of others [131]. In normal Pt/AI203-CI naphtha reforming the reaction network is complex because both metal and acidic sites to varying degrees catalyze ring closure, isomerization, dehydrogenation and cracking. Pt apparently is mainly responsible for hydrogenation / dehydrogenation with the acid sites accounting mainly for isomerization [147]. The conversion of n-hexane to benzene apparently goes via methylcyclopentane (MCP) and this is supported by the observation that at low conversions the major product when feeding nhexane is MCP [ 147]. With neutral Pt/KL the reaction network is simpler because the Pt sites mainly account for all the products. A commonly assumed reaction sequence is depicted in Figure 12. 1-6 Ring closure of chemisorbed n-hexane yields cyclohexane while 1-5 closure yields methylcyclopemane. Ring opening of the latter accounts for the two isoalkanes (2MP and 3MP).

Benzene

m

Cyclohexane Hexenes

~~

2MP

T IT MGP

n-Hexane

, P e n t a n e + OH 4

l

Butane

+ OH4

3MP

Figure 12. Reaction sequence for n-hexane conversion over Pt/KL.

It is of interest to compare the observed product concentrations with those predicted by thermodynamics. Table 7 lists several relevant equilibrium ratios. From the values ofthe cyclohexane / n-hexane and the benzene / cyclohexane ratios one would expect that the amoum of cyclohexane emerging from the reactor would be low which indeed it is. At about 40% n-hexane conversion at 450~ a typical exit molar ratio of benzene to cyclohexane is about 300: I, which although much lower than predicted is in line with the known fact that cyclohexane is very rapidly dehydrogenated over Pt at high temperatures. The exit benzene:MCP ratio is about 7 (at 40% n-hexane conversion at 450~ which is also lower than the predicted value. This nevertheless indicates that I-5 ring closure occurs at a reasonable rate compared to I-6 ring closure (the latter being followed by rapid benzene formation). At 350~ and at conversions below 3% the MPC concentration in fact exceeds that of benzene by a factor of 3, which again shows that I-5 ring closure is fairly rapid.

349 Table 7 Equilibrium Ratios of varius mixtures at 1 atm H2 Ratio 600K 700K 800K Cylohexaneln-Hexane 0.017 0.062 0.17 Benzene/Cylcohexane 34 1.8X10 4 2x106 MCPIn-Hexane 0.10 0.56 2.1 Benzene/MCP 5.9 2x103 1.6x105 3MPI2MP 0.51 0.54 0.56 MCP = Methylcyciopentane; MP = methylpentane

The observed 3MP/2MP ratio is about 0.8 which is not very different from the predicted value of about 0.5. The latter ratio does not change much with increasing hexane conversion when, as expected, the benzene level increases. This shows that the near equilibrium state between MCP, 2MP and 3MP persists at different conversion levels indicating that ring opening and closing occurs fairly rapidly. The main cracked products are methane and pentanes which is in line with the known hydrogenolysis activity of Pt. The main alkenes in the product mixtures are trans and cis 2-hexene in that order. While the reaction pathway depicted in figure z is supported by the fact that feeding n-hexane, MCP, 3MP or 2MP individually over Pt KL all result in high and similar aromatic selectivities [131], the actual mechanL~m on a molecular level is still a matter of dispute [139,142].

5.

SKEI~ETAL ISOMERIZATION OF 1-BUTENE

Isobutene is an important petrochemical starting material and best known for its use in the production of MTBE which is added to fuel as an octane-enhancer. It is also used as a monomer for the production of butyl rubber. Furthermore isobutene can be converted into isoprene which is an important monomer for elastomers by the modified Prins reaction with formaldehyde over, for example, H-ZSM-5 at 175 - 4000C [148]. Partial oxidation of isobutene yields methacrolein/methacrylic acid which upon esterification yields alkylacrylates, which are used e.g. for the production of polymers (plexiglass) and in water-soluble paint. Presently, the need for isobutene is covered by its production in the FCC-unit. However, with a strongly increasing demand for this raw material, especially for the m~nufacture of MTBE, alternative routes for the formation of isobutene need to be explored such as the acid catalyzed skeletal isomerization of linear butenes. Thermodynamically the skeletal isomerization of alkenes is favoured at low temperatures and the reciprocal temperature increases with increasing carbon number. The equih~rium concentration of isobutene in the fraction of butenes decreases from ca. 50 % at 200~ to 37% at 500~ [149]. Thus, the conversion of n-butenes into isobutene at these temperatures will be limited by thermodynamic constraints. The skeletal isomerization of the alkenes with more than 4 carbon atoms is a relatively facile reaction step, which is carried out at ca. 290~ over H-Ferrierite [150] or at 340~ over ZSM-5 [151]. This reaction proceeds via the skeletal rearrangement of a carbenium ion yielding a secondary carbenium ion. The singular reaction mechanism indicates that side product formation can be minimized. Even the skeletal isomerization of C 5- and C6-alkanes over Pt-Mordenite, which is thought to proceed

350 via a dehydrogenation step is a relatively facile process [ 151]. This is nowadays an important process for increasing octane numbers [ 153]. Contrary to the isomerization of longer chain alkenes, the formation of iso-butene from n-butene over acid catalysts is a difficult reaction, which proceeds e ~ e r via a mechanism involving oligomerisation, skeletal isomerisation of the oligomers and subsequent cracking or via an energetically unfavourable primary carbenium ion mechanism [ 154]. The first proposed mechanism imnlies the unavoidable formation of C5+-oligomers and C3.-cracked species as by-products m this process. Fluorinated alumina seems to be a promising catalyst for the skeletal isomerization of linear butenes [155,156], but the need to add fluorine to the feed stream together with the associated corrosion and environmental problems might prevent its industrial application [ 157]. A promising alternative to fluorinated alumina are zeolites. A number of zeolites have been studied for their activity and selectivity for n-butene isomerization [156-161]. The conversion of n-butenes over H-ZSM-5 can be observed at 377~ [157], but the iso-butene selectivity for this catalyst is rather low (14 %) and especially the selectivity for C 1-C3 products is quite high. At 500~ high conversions are obtained but the yield is then limited due to thermodynamic constraints [158,160]. The high activity and low selectivity of ZSM-5 has been ascribed to its strong acidity [158]. The acidity of zeolites can be reduced by the incorporation of boron in the zeolite framework [16_2,163] and therefore B-substituted ZSM-5, ZSM-11 and Beta were tested [158,164]. A13§ free boron zeolites are inactive, but these zeolites with low levels of A13+ ions which can be obtained by adding A120 3 binder to the A13+ free boron zeolite have weak acidity and are moderately active at 500 - 600~ and isobutene selectivities of up to 50 % have been reported. At these conditions the observed activity and selectivity of B/A1-ZSM-5, B/A1-ZSM-11 and B/A1-Beta were similar and therefore it was concluded that the pore structure did not play a decisive role in the conversion of n-butene into isobutene [164]. However, A1 which migrates into the pores not only modifies the acidity but also modifies the effective pore diameter. The importance of pore size has been frequently emphasized [150-161]. Figure 13 shows the performance of H-Ferrierite, H-Mordenite and SAPO-11 on the skeletal isomerization of n-butene. The widepore zeolite, mordenite, shows a low conversion and a low selectivity towards isobutene. The selectivity to isobutene obtained with the medium pore size SAPO-11 is higher but, due to the low conversion, a lower isobutene yield is obtained. With H-Ferrierite, a higher selectivity (> 80%) is obtained yielding a composition which is close to thermod~(namic equih'bfium The ~lectivity of zeolite Theta-1, which has narrower pores (5.5 x 4.4 A) than ZSM-5 (5.6 x 5.3 A and 5.5 x 5.1 A) for isobutene at 377 - 379~ was reported to be three times higher but also the conversion over Theta-1 at these conditions was significantly lower. Ferrierite (4.2 x 5.4 A and 3.5 x 4.8 A) showed at these temperatures over 90 % selectivity to isobutene [159]. A comparison of the activity and selectivity of SAPO-5 (ca. 8 A), SAPO-11 (6.7 x 4 A) and SAPO-34 (4.3 A) at 400~ showed that the selectivity to isobutene increased with decreasing pore size [159]. It was further noticed that the conversion and catalyst deactivation decreased with decreasing pore size. The large pore zeolite Mordenite itself is not selective for butene isomerization but Mg-Mordenite has been found to be more selective than H-Mordenite [150]. Zeolites with pores smaller than 4.2 A like Erionite are not useful because of the diffusional constraint of the product isobutene [150, 160]. Studies have shown that Ferderite is an attractive catalyst for n-butene isomerization, because it is both active and selective at relatively low temperatures of 350 - 400~ [150]. Initially small differences in butene conversion and isobutene selectivity for Ferriefite with

351 different Si/AI ratios (Si/A1 = 9 - 43) were observed but after some time on stream the differences were negligible indicating an influence of the acidity on the initial performance of the catalyst but not on its steady-state performance. The selectivity of this zeolite has been explained in terms of shape selectivity and substantiated with computer simulations [160] because the pore structure of the Ferrierite strongly inh~its the diffusion of the intermediate trimethylpentenes and therefore increases the probability of cracking which yields iso-butene. Most laboratory studies has been performed with a highly diluted feed at atmospheric pressure [158,157,159,163,164] because at a butene partial pressure of 1 bar much lower isobutene selectivities [161] and shorter catalyst life-times for medium and large pore zeolites were reported [150]. This is consistent with the postulated tmi-molecular isomerization reaction and multi-molecular oligomerization reaction [158] and the observed first order with respect to the partial pressure of n-butene for the formation of iso-butene and an order larger than one for the formation of by-products assuming low coverage for boron substituted zeolites [164]. For industrial operation, however, it is desirable to operate at higher partial pressure of n-butenes. Ferrierite can operate at higher butene partial pressures with a moderate catalyst life time [ 150,160]. With time on stream the activity of the zeolites for butene conversion decreases and the selectivity for isobutene increases [150,157,159,160,165]. Ferrierite initially produces cracking and oligome"nsation products [165] but with time on stream the rate of formation of these by-products decreases whereas the rate of formation of isobutene first increases before showing a slow decrease. The still active and selective catalyst can contain up to 8 - 10 wt.-% coke which is aromatic in character (H/C = 1) and which reduces the accessible pore volume dramatically [160, 165]. In the case of Theta-1 it was observed by using lower calcination temperatures (325~ instead of 500~ that the isobutene yield increased si~ificantly from 4.6 mol-% to 25.5 tool-% [157]. This has been ascribed to residual template inside the pores [161]. Figure 14 shows the effect of temperature and space velocity on n-butene isomerization over H-Ferrierite. It has been observed, that the isobutene selectivity increases with increasing space velocity and with increasing temperature [150,157]. The usual explanation for an observed increase in selectivity with increasing space velocity is the reduction of secondary conversion of the compound and if the space velocity is high enough its primary formation. Primary formation of isobutene is consistent with the observed different reaction orders for the isomerization and by-product formation [164]. The enhanced selectivity for the coked catalysts can then be explained with a lower diffusivit~ of the reactants into the zeolite and thus a lower butene concentration in the pores, which would favour the reaction with the lower reaction order, i.e. the skeletal isomerization. Assuming the primary formation of isobutene the observed increase in selectivity with temperature would indicate a higher activation energy for its formation in comparison to the oligomerisation. This is mechanistically consistent, because the oligomerisation will proceed via secondary carbenium ions whereas the direct skeletal isomerization of butenes involves an energetically unfavoured primary carbenium ion which might be stabilized by the zeolite structure.

352

40 ) 35 3O

~ F e r r i e r i t e

25 -o 20 SAPO-11

15

"---t

10

H-Mordenite 0

I

I

t

6

12

18

24

Tim 9on Stream [m in]

Figure 13. Yield ofiso-butene from the skeletal isomerization ofn-butene.

100 90

9 9

80

E 0

>=

8

X

70

~

100

400"C, 14 hr-1 425"C. 14 hr-1 400"C, l B h r - 1 425"c, 7 hr-1 Export. (400~, 14hr- 1) qlxxt. (425"C, 14hr-1)

90

7O

~~ _-; so

F ~ y. ( .;2-3~,,. 7"~ - i"/

60 50

o

40

~

4 o l~ /

1~400"C' 18 hr'l

30

m

30 ~-

IX425~:.7",-~

20

20

10

10

O/ 24

48

7"2

Time on Steam [hr]

96

120

0

F''~'~""r-'

)

I

i

t

24

48

72

96

Time on Steam [hr]

Figure 14. Effects of temperature and space velocity pm n-butene isomerization over Ferrierite.

120

353 6.

ALKENE OLIGOMERIZATION

The oligomerization of light alkenes into dimers, trimers, tetramers and higher oligomers represents an important reaction for the production of aromatic-free higher alkenes. Although the use of ZSM-5, with Si/Al ratios of approximately 30 - 40, as an oligomefization catalyst was patented in the early 1980s and has been extensively reviewed [166,167] currently only the Mossgas Refinery in South Africa is using this technology [168]. The process is able to produce, after hydrogenation, mainly low branched alkanes and scarcely any aromatics. The high selectivity to alkenes in the diesel mode of operation is attn~outed to restricted transition shape selectivity which both favours alkene formation and inh~its the formation of typical cyclic coke precursors. These factors together with a high reaction pressure (typically 5MPa) and moderate reaction temperature (200 - 220~ are conducive to the formation of diesel fractions. The typical feed composition is 81.7% alkenes, 15% alkanes, 1.5% aromatics and 1.8% oxygenates and the typical liquid fuel yields, based on alkenes, of 97% and, when operated in distillate mode, yields 78% distillate and 19% gasoline. The product diesel has a high cetane number (about 53). The gasoline has an RON of between 81 and 85 and a MON between 74 and 75. The oxygenates may cause premature catalyst deactivation possa'bly due to stronger and irreversa'ble adsorption on the acid sites [169]. Apart from ZSM-5, there have been reports of the oligomerization activity of other zeolites. As expected the extent of chain branching, usually undesirable in most applications, increases as the pore size increases due to the shape selective nature of the reaction in the zeolite. Oligomerization of higher alkenes represents an important route to the formation of synthetic lube oils [ 180]. After hydrogenation such oils have excellent properties due to their low volat'flity for their viscosity, high thermal and oxidation stability, very low pour point and exceptional low-temperature performance [170]. Such oils however are usually expensive due to the relatively high cost of the olefinic feed. Synthetic lubricant base stocks can be prepared in good yields by oligomefizing long-chain alkenes using catalysts containing large pore zeolites with high Si/Al ratios. Internal alkenes are less reactive than the corresponding alphaalkenes and conversions decrease as the chain-length of the feed alkene increases. In general, however, the zeolites thus far reported are not as good as clay catalysts or the curently used boron trifluoride or aluminittm chloride catalysts. 7.

ISOMERIZATION OF LONG-CHAIN ALKANES

The need for lubes and middle distillate fuels with greater performance, safety, and environmental advantages is increasing. This need has focused attention on highly paraffmic feedstocks due to their high oxidation stability, low volatility for a given viscosity, and high viscosity index (>130). Because highly paraffmic feeds tend to have high wax contents, however, the production of lubes and fuels from these feeds has been limited due to the large loss upon wax removal. An alternative approach is to change the molecular structure of the wax by isomerization, such that low pour point, high performance products can be prepared with a high yield. Since isomerization preserves paraffmicity rather than lowering it, the quality of the feedstocks are maximized. The usefulness of wax isomerization will depend greatly upon its feed flexl"oility, i.e. its ability to produce high yields of dewaxed oils from feeds which vary extensively in boiling range and in chemical composition, particularly wax content [ 171]. Recently anumber of patents have appeared descn~oing the use of zeolites for this isomerization process [ 172]. The catalysts used for dewaxing are usually bifimctional in nature with Pt being the hydrogenation-dehydrogenation component and a large pore typically 12MR zeolite provides the acidic component [1]. ZSM-5 is the catalyst used in Mobil's Distillate Dewaxing (MDDW)

354 or Lube Dewaxing (MLDW) processes. In this process the straight chain, waxy normal or slightly branched alkanes are able to emter the pores where they are selectively cracked and the light products are removed by distillation. Currently more than 70% of the catalytic dewaxing units in operation are based on the Mobil zeolite catalyst and process technology [7]. Table 8 Isomerization of n-octane over Pt catalysts at 1000 psig, 2.8 WHSV, 16 H2/HC, and 30% conv.[177] Catalyst Temperature i-C8sel. 2M-C7/3M-Cr [*c] (wt.%) SiO2-AI2Oz HY ZSM-5 (80 SIO2/AI203) ZSM-5 (650 SIO2/AI203) Na-Beta SAPO- 11

C3+Cs/C4 molar ratio

i-CJn-C4

DM-C6sel. (wt. %)

371 257 260

96.4 96.8 56.6

0.67 0.71 1.54

0.95 0.64 2.1

0.96 3.5 1.1

8.5 12 1.8

343

58.4

0.88

1.2

0.98

5.6

367 331

74.3 94.8

0.70 1.07

0.68 1.0

1.7 0.92

10 2.3

302826242220-

= 9

=. . . . . . . . . . .

- - - o - - R-Silica Alumina

""l,

.~ 1 8 16 .==, o 14=E 1 2 -

/

'1

:~.

1086

Pt-SAPO-11

a ........Pt-ZSM-5

...

. . . . o . . . . -o . . . . -o

o,

,! 2 0 0

2

4

6

8

10

12

Carbon Number

Figure 15. Molar distn"oution of cracked product l~om hexadecane at 1000 psig, 3.1 WHSV, 30 HJI-IC and 94% conversion [177].

14

355 Catalysts containing a hydrogenation component and an intermediate-pore silicoaluminophosphate (SAPO) molecular sieve have recently been found to have a high selectivity for wax isomerization [ 173,174,175]. A new process for dewaxing high alkane lubricating oils, called Isodewaxing [176], is being commercialized by Chevron using Pt/SAPO-11 catalyst. Table 8 shows the hexadecane isomerization selectivities of a number of Pt -loaded catalysts [ 177,178]. SAPO-11 has a low selectivity to dimethyl isomers such that fewer branches are required to obtain a given degree of pour point reduction. Since increased branching reduces the wide-temperature range fluidity of an oil the oil made using SAPO-11 catalyst has a lower sensit'wity of viscosity to temperature. Figure 15 shows that SAPO-11 had a more even distn'bution over the carbon numbers than is commonly associated with intermediate-pore sieves such as ZSM-5 [177]. The cracked also contained fewer isomers with methyl branches separated by less than two carbons than, for example, silica-alumina. Secondary hydrocracking is low and the cracked by-product is all liquid and at the same time hydrocracking of the long chain alkenes is inh~ited. These properties of SAPO-11 for this application appear to be associated with its moderate acid activity and the one-dimensional nature of its pores. Much of the catalysis appears to occur at or near to the sieve external surface. [178]. In a separate study of the relative activities of USY, mordenite, ZSM-5, Beta and SAPO-11, the SAPO-11 was found to be the only catalyst capable of isomerizing normal alkanes in the presence of iso-alkanes without large yield losses due to unwanted cracking [179]. Pt-H mordenite and ferrierite have also been used for this reaction. Recently bifimctional forms of Beta have been found to give better isomerization selectivities relative to hydrocracking and this may represent a superior and economically attractive dewaxing process [3,66]. 8.

ACKNOWLEDGEMENTS

The authors wish to thank all their colleagues both from academic and industrial research groups who kindly contributed much of the source material used in this paper. Their kind assistance and ready response to a request for information is much appreciated.

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363

Synthesis of Intermediates and Fine Chemicals using Molecular Sieve Catalysts Saskia Feast and Johannes A. Lercher University of Twente, Department of Chemical Engineering, P.O. Box 217, 7500 AE Enschede, The Netherlands

Abstract The main principles of using molecular sieve catalysts for intermediates and fine chemical synthesis are reviewed and critically discussed. Emphasis is placed on describing the role of the elementary steps. The role of the catalytic functions (acid-base, redox and host for catalytically active sites) and the role of the pore constraints in activity and selectivity are compared. Examples of successful applications are presented. 1. INTRODUCTION The use of molecular sieves as catalysts or catalyst components for synthesis of intermediates and fine chemical has increased impressively over the last two decades. A large number of reactions has been explored over a growing number of microporous materials. Also the level of understanding of the catalytic chemistry and the structure-activity relationships has greatly improved. Since the first review of Venuto and Landi~ in 1968 [1] and the one of Venuto in 1994 [2] the discovery of medium pore zeolites such as ZSM-5 [3] and of phosphate based molecular sieves [4] had the largest impact on the field. The reasons why they had such an impact, however, were quite different for the two materials. Medium pore materials (especially ZSM-5) have enabled a quantum leap in controlling the selectivity by subtly adjusting the pores size and the tortuosity and so modifying the space available for transition states and/or the diffusivities of reactants and products (i.e., inducing pronounced shape selectivity). At the same time they provide a very robust material that is straightforward to synthesize and withstands severe reaction conditions [5]. Phosphate based molecular sieves such as A1PO4-, SAPO-, MeAPO and MeAPSO, on the other hand, have considerably extended the range of lattice properties and of chemical elements incorporated into the framework [6]. The breadth and depth of these developments are well reflected in a large number of papers, patents and reviews written on the subject during the last two decades (see Fibre 1 and refs.[ 1-2,7,8,11 ]). The growth in the use of molecular sieves as catalysts as compared with macro- and mesoporous oxides was stimulated by several factors: (i) The high concentration of active sites (in comparison with oxides) results in very active catalysts. (ii) The defined pore structure allows to exclude reactants from being converted and/or products to be formed or transported out of the pores due to a too large size. (iii) The active site and the environment of that site can be designed on an atomic level for example by ion exchange [9] or chemical functionalization of the framework [ 10]. (iv) It is possible to tailor the chemical properties of molecular sieves better than those of conventional macro and mesoporous oxides. Most of these advantages relate to the fact that acid/base sites of dense or

364

macroporous oxides, sulfates, etc. depend on the way the bulk is terminated, but that for molecular sieves the entire pore surface and, thus, most acid and base sites are an integral part of the crystal structure. This allows on the one hand an unsurpassed subtlety and reproducibility in the design and modification of the acid/base sites of the molecular sieves. On the other hand it is necessary that the reactant molecules diffuse through the channels (with the pore diameter being of comparable to the size of the molecule) to reach the acid/base sites, where they may experience further severe steric constraints during reaction.

Figure 1. A summary of the number of reviews, articles andpatents published on the synthesis of fine chemicals and intermediates overmicroporous materials during the last 29 years. Thus, the rate and selectivity of catalyzed reactions over molecular sieve catalysts are influenced by factors that are affiliated with the specific interface chemistry (chemi'cal induced selectivity) and the constraints induced by the steric limitations (shape selectivity). This advantage, however, also induces drawbacks that can only be partly overcome by adjustment of the mesoscopic properties of the molecular sieve. The most prominent limitation concerns the size of the molecular sieve channels that does not allow large organic molecules to be converted. Such limitations can be partially overcome by creating a secondary meso/macro pore structure that improves the transport in the microporous materials. The implementaion of such secondary pore structure represents an important option for future improvement of zeolite based catalysts. Another limitation lies in the fact the combination of the presence of larger molecules and of strongly basic or acidic functional groups in the pores causes the desorption of products frequently to be rate limiting or even to be impossible without the help of a co-reactant (adsorption assisted desorption). This is especially important for reactions like condensation, oligomerization and nucleophilic substitution. Finally, the catalytic chemistry inside a zeolite can be seen to occur in a microscopically small tubular reactor in which the active sites are distributed over the whole reactor. It can be intuitively understood that it is difficult to forecome sequential reactions of the same type in such an environment. There is one practical aspect that limits the progress towards a molecular understanding of such processes. The large scale applications of the petroleum industry, permit

365 from an economic point of view to allocate resources for detailed investigations of the catalytic chemistry of a particular reaction. Compared to this the processes in fine chemical industry are usually small scale and allocating comparable resources for catalyst and/or process development is rarely possible. Thus, we have to develop knowledge on the generic reactivity of functional groups of the reacting molecules and their way of interaction with active sites in the molecular sieve catalysts in order to extrapolate knowledge from one process to another one. In this way it should be possible to successfully design new catalysts and/or processes. There is a large number of excellent review articles published on the use of molecular sieves for fine chemical synthesis [ 1,2,7,8,11,12,13,13,14,15,16,17,18,19,20,21,22 ]. Most of these address the problem from the side of the organic conversion and how a molecular sieve changes activity and selectivity for a particular reaction as compared to a macroporous oxide. In the light of what is said above, we have chosen for a more material oriented approach. In first instance the focus will be on outlining the chemical requirements for functional groups in molecular sieves to catalyze a particular reaction and how these groups can be manipulated to optimize activity and selectivity. Then, we will discuss how a change in the pore structure can selectively influence the transport and sorption of reactants and products in the pores and/or the formation of different transition state complexes. Finally, we will provide examples of zeolite catalyzed organic reactions that have been successfully implemented.

2. C H E M I C A L F U N C T I O N A L I T I E S OF M O L E C U L A R SIEVES Molecular sieve catalysts may provide three basic functions. They may act as solid acids and bases, provide sites or be the carrier of sites capable of undergoing valence changes in redox processes and be the host for metalorganic complexes or metal particles offering a unique steric and chemical environment [23]. It will be the purpose of this review to show, how these functionalities can be realized and manipulated within the molecular sieve lattice, at ion exchange positions or by species entrapped in the molecular sieve pores. Our focus will be to highlight the structure - activity relationships characteristic of a generic type'of reaction and to show how the complex interplay between structure and chemical composition can be utilized to obtain a catalyst with highly specialized properties. 2.1. M o l e c u l a r sieves as acids and bases

Molecular sieves consist of a three-dimensional network of metal-oxygen tetrahedra (and to a lesser extent also octahedra) that provides a regularly sized micropore structure in which the acid and base sites are structurally embedded [24,25,26](see Figure 2). Acid sites result from an imbalance between the metal oxygen stoichiometry and the formal charge on the cations. This is seen most clearly in the case ofzeolites which have three-dimensional networks of Si-O tetrahedra. Formally, there is a 4+ charge on the Si cation and a 2- charge on the oxygen anion. As every oxygen belongs to two of such tetrahedra, each of them appears neutral and the resulting lattice does not possess acidic properties. If Si is partially substituted by A1, the formal charge on the metal cation changes from 4+ to 3+. Thus, the tetrahedron which contains the aluminum cation must be negatively charged. This negative charge is balanced by a metal cation or a proton that constitute a Lewis- or a Br6nsted-acid site, respectively [2, 27,28] (see Figure 2). By convention, the bare, negatively charged, tetrahedron must then be seen as the corresponding base. Depending upon the charge on the metal catiow'proton and the oxygen, the acidic or basic properties of the molecular sieves

366

will be dominating and consequently it will be called a solid acid or base [2, 29]. Note that these acid or base properties are not a simple function of the chemical composition, but that also the structure (framework density) has a major impact.

Figure 2. Representation of the three-dimensional network of metaloxygen tetrahedra that provides a regularly sized micropore structure in which the acid and base sites are structurally embedded. The possibilities that emerge when also metal-oxygen tetrahedra with metal cations of formal charges different from 4+ or 3+ are used can be schematically seen in Figure 3. Depending on the combination of the metal cation in the frame work neutral lattices and lattices with cation or anion exchange sites are conceptually conceivable [30]. Up to now only cation exchange molecular sieves have been found and there is considerable doubt, if anion exchanging lattices can be formed because of the instabilities of the metal-oxygen bonds involved in building such a lattice. Intuitively, it can be well understood that only the theoretically highest concentration of the acid (and base) sites can be assessed by comparing the formal charges on the tetmhedra involved in building the molecular sieve framework. The strength of the acid (and base) sites will depend on a variety of factors such as the nature of the lattice cations, the overall chemical composition of the lattice, the crystal structure, etc. [31,32,33,34]. The type of cation incorporated will change the polarizability and the real charge of the lattice oxygens, thus, leading to a wide variety of chemical properties. These relationships between the chemical composition, the crystal structure and the acid/base properties of molecular sieves have been studied extensively and were thoroughly reviewed (see e.g. refs. 6,35,36,37,38,39).

367

0

9

1.4 0/~

/0

O~s1/O~si

i ",,

\

0

~ \

0

~, 0

0

0

Pure-Silica -1

O

M+

O~

Si O~"AI/O

O O

o

O o

Zeolite +1

O

~

+5

o/l

"'' O

O~

/O~p./O

AI

. ~

s~

O O

O O

AIPO4 -n Figure 3. Possibilities of tailoring the zeolite by replacing framework Si4+ or A13+with different metal cations, i.e. ps+, after ref. 7. However, it should be emphasized that the measurement and quantification of the acid/base strength of zeolites is complex and that it is difficult to directly compare the acid/base strength of a solid with that of a liquid. This results from the fact that the stabilization of carbocations and carbanions in a microporous solid differs from that in strongly polar acid and base solutions. For zeolites, it can be stated that the concentration of aluminum in the lattice is directly proportional to the concentration of acid sites and the polarity of the lattice and to a first approximation indirectly proportional to the strength of acid sites [40]. For a given chemical composition of the zeolite, the polarity of the lattice increases with decreasing framework density [41 ].

2.1.2. Acid catalyzed reactions Reactions involving carbon-carbon bond rearrangements The early use and success of molecular sieve catalysis was spurred by the dramatic improvement in activity selectivity for catalytic cracking of vacuum gas oil achieved by using the faujasite based catalysts in comparison to the previously used amorphous SiOJA120 3. These catalysts had a factor of about 103 - 104 higher catalytic activity than the amorphous SiO_-,/AI203catalysts [42]. Paraffin, C4 to C8 isomerization [43] was one of the ftrst successful non-petroleum processing applications using zeolite catalysts. The complexity of tailoring zeolite catalysts, however, is well illustrated by the fact that is only four years back that Shell has developed the first zeolite based process for isomerization ofn-butene to isobutene [44]. Traditionally, industrial isomerization processes involve the use of Br6nsted and Lewis acids, such as H2SO4 and AIC13that are uneconomical to recycle for most of the chemical applications. Replacement of such bulk chemicals with recyclable zeolites is a very attractive

368 option that is only limited by the significantly lower proton (acid site) concentration of molecular sieves in comparison to the above mentioned acids. For example, 1g of H 2 S O 4 contains 0.02 moles of protons whereas 1g of zeolite H'Y, with Si/AI = 5, contains 0.003 moles of protons. This is a rough approximation of the acidic protons available for catalysis, since it assumes 100% dissociation for both samples and that every proton is accessible in the zeolite. Note that lg of H_,SO4 occupies far less volume than the equivalent mass of zeolite, i.e., approx. 0.5 c m 3 compared to 4-6 c m 3. However, due to increasingly stringent environmental policies, the interest in solid acid catalysts remains quite high as can be seen from the number of examples in several reviews [9-22, 45]. The uniformity of the site strength that can be achieved with high silica zeolites such as ZSM5 has been shown by Mirth et al. [46] for m-xylene isomerization using in situ i.r. spectroscopy as means to characterize the concentration of sorbed reactants. The reaction rate was shown to be directly proportional to the concentration of acid sites covered by mxylene indicating that all acid sites of the ZSM-5 zeolite used were able to convert m-xylene to o- and p-xylene with the same activity per proton (see Fig.4). Note at this point that for practical reasons (limitations in the accessibility, diffusional limitations, etc.) not all sites may actually participate in the reaction. The aspects of the shape selectivity that also influence activity in a complex way will be discussed in a later section. It should be emphasized that such uniform behavior of acid sites is usually confined to high silica zeolites or, in general, to molecular sieves with a low density of acid sites. With materials of higher acid site concentrations (such as zeolites Y and X) sites with distinct differences in the acid strength have been observed [47]. These differences were attributed mainly to the existence of neighboring acid sites that produce a local situation not unlike that in the acid H2SO4 also having two protons per molecule with differing acid strength. TOF (molecules/site.s) m-x3'len~r

0.0012

0.0008

0.0004

0

v

0

'

10

20

I

~

30 40 Coverage (%)

!

50

J

t

60

i

70

Figure 4. The reaction rate for the isomerisation of m-xylene to o- and p- xylene is directly proportional to the concentration of acid sites, i.e. the coverage, [46]. The presence of neighboring acid sites, however, may be important when bimolecular reaction steps are involved in the reaction network as illustrated in the following two examples. Over a series of ZSM-5 materials Halik et al. [48] showed that the conversion of

369 ethanol to intermediate size hydrocarbons was a non-linear function of the acid site concentration with a much lower catalytic activity found below a certain concentration of acid sites. This behavior was explained by the necessity of a critical concentration of acid sites being required to maintain reasonable rates of the bimolecular reaction steps that are part of the complex transformation. It should not be interpreted that two protonated species have to react, but rather that a higher concentration of acid sites also results in a higher concentration of reactants in the pores, that favors the bimolecular reactions. Note that Lewis acid sites present in the zeolite may also play a significant role in enhancing the concentration of the reactant in the pores of the zeolite [49]. In a similar way, hydride transfer reactions in alkane/alkene transformations depend in a nonlinear fashion upon the varying concentration of acid sites. Post et al. [50] showed elegantly that the rates of these bimolecular reactions depend upon the square of the concentration of the acid sites, while the rates of the monomolecular reactions (protolytic cracking [51 ]) were linearly dependent on the proton concentration. This suggests that similar effects can also be expected in more complex organic transformations, where less thoroughly developed structure-activity relations exist. The role of Lewis acid sites in such conversions is less understood. Karge et al. [52] showed that La3§ ion exchanged zeolites that do not contain hydroxyl groups are catalytically inactive for ethylbenzene disproportionation suggesting that protons are indispensable for the carbon-carbon bond rearrangement reactions. On the other hand a number of reactions have been reported (the absence of hydroxyl groups is not certain in all those eases) that are well catalyzed by trivalent metal cation exchanged zeolites [53]. The role of the metal cation is in these instances more that of mediating the acid strength and modifying the adsorption strength than being the active site by itself. The skeletal isomerization of tetmhydrodicyclopentadiene into adamantane is an example of a very complex rearrangement that is commercially carried out over strong Lewis acids with a hydride transfer initiator. The reaction can be catalyzed by rare earth (La, Ce, Y, Nd, Yb) exchanged faujasites (Scheme 1) in a Hz/I-IC1atmosphere at 250~ Selectivities to adamantane of up to 50% have been reported, when a metal function, such as Pt, capable of catalyzing hydrogenation is added [54]. Initially acid catalyzed endo- to exo- isomefiz~on of tetmhydro-dicyclopentadiene takes place and then a series of 1,2 alkyl shifts invo'lv~,ag secondary and tertiary carbonium ions leads eventually to adamantane[55]. The possible mechanistic pathways of adamantane formation from tetmhydro-dicyclopentadiene are discussed in detail in ref. [56].

H2/HCI 250 ~ Scheme 1. Skeletal isomerisation of tetrahydrodicyclopentadiene to form adamantane. Zeolites do not only catalyze isomerizations of pure hydrocarbons. Also for molecules bearing a polar functional group, double bond and skeletal rearrangements can be performed without conversion of the functional group. Suitable zeolites should be rather apolar with a low concentration of acid sites, e.g., HZSM5. The interaction of polar functional groups with the pore walls of these rather apolar zeolites are weak [57]and hence polarization and

370 activation of these groups can be minimized. An example for a double bond relocation is the isomerization of2-ethyl propenal into trans-2-methyl-2-butenal over CeBZSM5 [58] (Scheme 2). An example for a skeletal isomerization is the allylic rearrangement of 1,4 diacetoxybutene over ZSM-5 while retaining the functional group intact (see Scheme 3). H3C~ CH2 H2C

CeB-ZSM-5 ~

H3C / C H ' ~ CH3

300 ~

CHO

CHO

Scheme 2. Isomerisation of 2-ethyl propenal to trans-2methyl-2-butenal. o

o

A.

%A.

C

,O..

CH2 Y

CH3 CH.

o

~

ZSM-5 Oc 300

.. "

H2C O

CH3

CH

HC/ll "O~T/CH3 CH2

O

Scheme 3. Skeletal isomerisation of 1,4-diacetoxybutene over ZSM-5.

lsomerization involving heteroatoms Molecular sieves are also well suited as catalysts for isomerization of molecules containing heteroatoms. The weaker strength of the bond between a carbon and a heteroatom compared to a carbon-carbon bond usually allows, and even necessitates, working at relatively low temperatures. Good examples are the isomerization ofhalogenated aromatic molecules such as chlorophenols, chlorothiophene, bromothiophene and iodothiophene over Z.SM-5 zeolites [59]. The optimum reaction temperature for the last three molecules gradually drops from 300~ to 100~ in parallel with the increasingly weaker carbon-halogen bond.

Nucleophilic substitutions In nucleophilic substitutions one can distinguish between two mechanisms, i.e., the two step nucleophilic substitution (SN~)and the one step process (Sin). In the latter route, the highly polar intermediate species or, in the limiting case, the carbocation is stabilized by the catalyst. Direct evidence for the presence of carbonium and carbenium ions in the molecular sieve pores is scarce. Experiments point to such species only in the presence of very strong acid sites provided relatively basic reactant molecules are used [60]. Even in such cases the interpretation of the experimental data does not seem to be unequivocal [61 ]. Most results suggest that the true cation exists only in the transition state resulting in a quite complex reaction coordinate. The course of the reaction is determined by the chemical nature of the leaving and the substituting group, the acid/base properties ofthe molecular sieve, the influence of co-reactants and the availability of space for the reaction to take place. The majority of the nucleophilic substitutions involve the replacement of an-OH group with an-NH,-S,-SH,

371 -OR or another functional group. One of the major problems is that in many cases the resulting product interacts more strongly with the molecular sieve than the reactant. This leads to the situation that many reactions are desorption controlled and need either a reactant to desorb (adsorption assisted desorption) or a gaseous/liquid cocatalyst that also facilitates the desorption of the products without participating in the reaction. Note that for liquid phase reactions the solvent can take over the role of the cocatalyst. Etherifications, conceptionally one of the simplest reactions to catalyze, occur over most zeolites. Molecular sieves, however, have too low acid site density to make them interesting for commercial applications. Usually, resins like amberlyst are used for that purpose [62]. On the other hand etherification is experimentally and theoretically well studied and understood. Using the example of dimethylether formation from methanol one clearly sees, how the reaction conditions influence the reaction mechanism, i.e., whether the reaction proceeds along a Sm or a Sm pathway [63,64,65]. Temperature programmed reaction studies of methanol conversion over HZSM5 suggest that three reaction routes to form dimethylether exist, i.e., via an alkoxonium cation and via t w o alkoxy pathways [65]. At low temperatures the reaction proceeds v/a an Eley-Rideal type mechanism_ In the transition state one methanol molecule forms a methoxonium ion, water leaves the molecule and simultaneously another weakly sorbed methanol binds to the methyl group forming protonated dimethylether (see Scheme 4). The protonated dimethylether donates immediately the proton back to the zeolite and desorbs. M.S. Response (Arb.U.)

V~

nol

,,

~

C H , O H + CH,OH: + ~

D M E + H,O + I-I*

i_~.'. CH~OH + SiOCH3A[ - - ~ DME + SiOHAI CHjOH + SiOCH, ~

!

I

400

500

I

A

600 700 Temperature (K)

....

DME + SiOH

|

800

,

|

900

Scheme 4. Temperature programmed desorption/reaction of methanol on HZSM-5 [65]. As the reaction temperature increases, part of the methanol molecules will be transformed into methoxy groups that replace the proton in bridging (SiOHAI) and terminal (SiOH) hydroxyl groups. These methoxy groups react with weakly associated methanol to form dimethylether under simultaneous restitution of the hydroxyl group. While the methoxy group is covalently bound to the zeolite lattice, its reactivity increases with the acid strength of the hydroxyl group it replaced [65,66,67]. Thus, methoxy groups at bridging hydroxyl

372

groups produce dimethylether at lower temperatures than methoxy groups at terminal hydroxyl groups [67]. Comparison of the chemistry over various zeolites indicates that formation and reactivity of a specific type of methoxy group is connected in a complex way with the polarizability of the lattice and the overall acid/base properties. Methoxy groups at bridging sites are more easily formed and consumed on FAU type materials than on MFI type materials [68]. Recent theoretical calculations by Blazowski and van Santen suggest that indeed the pathway to form DME via the methoxonium ion is energetically favored over the pathway via methoxy groups [69]. The data clearly agree with the observed strong temperature dependence of the reaction mechanism as reported in ref. [65]. Blazowski and van Santen used ab initio calculations to show that the SN_,reaction involves a complex transition state in which four reactions have to proceed in a synchronous manner, i.e., (i) formation of a methoxonium ion by proton donation from the zeolite, (ii) cleavage of water from the methoxonium ion and formation of a methylcarbenium ion, (iii) binding of the methyl carbenium ion to the second methanol molecule to form protonated dimethylether and (iv) donation of the proton back to the zeolite. Note that according to the calculations all must occur in a concerted manner, as the protonated species are only found to be stable in the transition state. If this proves to be true the transition states may be quite difficult to achieve, i.e., transition entropy must be quite low and, hence, also the reaction rates must be low. Stabilization of the methoxonium ion by the catalyst would lead to a less complex transition state and hence, one might expect the intrinsic rates of the reaction to be higher. The initial results of methanol sorption on organic resins and heteropoly acids indicate that such a situation may be attained with these materials [70]. Note that this would make molecular sieves only preferable, if special properties, such as pronounced shape selectivity, would be required. Amination of alcohols follows a mechanistic pathway similar to etherification [71 ]. Due to the basicity of the reactants one might expect that for this reaction, ammonia will be present in the molecular sieve pores in the form of an ammonium ion and the alkyl group of the alcohol would substitute for one of the protons of the ammonium ion. Indeed, if an alcohol is passed over the ammonium form of a zeolite, amines are readily formed [72]. These alkylamines, however, cannot desorb and remain chemisorbed in the zeolite pores under typical reaction conditions (T = 353 ~ The apparent reaction mechanism can be classified a/s S'm, ammonia being stabilized by the molecular sieve in the form of the ammonium ion. In order to obtain a successful reaction, the ammonium ion must protonate the alcohol in the transition state, thus, generating a H20 leaving group. In a simultaneous step the alkyl group must dock onto the lone electron pair of the nitrogen forming an alkylammonium ion. Even under more severe reaction conditions (temperatures higher than 353 ~ the alkylammonium ions are unable to desorb from the acid sites [73,74]. These alkylamines released into the gas phase stem actually from a further nucleophilic substitution in which the alkylgroup of the alkylammonium ion is scavenged by weakly adsorbed ammonia (see Scheme 5) This shows that the type of zeolite might not be as important as the reaction conditions and indeed several zeolites have been claimed to be suitable for amination of alcohols [75,76,77]. The acid strength of such zeolites should be as high as possible in order to assure that all the acid sites are covered by ammonia and amines, thus, preventing the formation of ethers and higher hydrocarbons from the alcohol over free acid sites [78]. The requirement for zeolites of high acid strength for the alkylation of ammonia by alcohols contrasts with the need for weakly acidic zeolites for the addition reactions between alkenes and ammonia [79]. In these reactions the alkene has to be activated by the Bronsted acid site of the zeolite and that is only possible when the acid sites are not fully blocked by

373 ammonium ions. In addition to the weak acid sites of the catalyst high reaction temperatures and high pressures of alkenes are necessary to achieve this. H HAl

H

H

0

+ CH. I~" .N CH.I ~CH 3 ~CH~ O"

0

/ A!\ OO

H CH3 +1 .N~ CH31 CH3

(ii) proton transfer 0

\ / \ / \ / /Si \

O

(i) methyl scavenging

CH

0

\ / \

/Si ~ OO

O-

// S i N O

3

0

/ \ / AI\

O

OO

/ OO

/ si \

o

o

Scheme 5. Proposed mechanism for the removal of methyl amines by scavenging with ammonia, after [72]. A somewhat more involved example is the transformation of oxygen containing heterocycles into nitrogen or sulphur containing heterocycles [80]. Again, not the structure of the molecular sieve has been found to be important (provided there is enough space within in the zeolite pores to accommodate the reactants and products), but rather the acid strength and nature of the acid site. Hatada et al. reported the transformation of y-butyrolactone into 2-pyrrolidinone over a series of metal exchanged Y zeolites (Scheme 6) [81 ]. For alkali metal and alkaline earth metal exchanged FAU a direct dependence of the yield of 2-pyrrolidinone upon the strength of the electrostatic field of the cation was found. This indicates that the strength of the coordination of y-butyrolactone to the metal cation is the most important parameter influencing the catalytic conversion. For transition metal (Co, Ni, Cu and Zn) exchanged zeolite Y a correlation between the cation field strength and the activity was not observed. It is speculated that this is due to the non-spherical nature of transition metal cation orbitals, especially of their partially filled d-orbitals, and thus the simple electrostatic model is no longer applicable [82]. The reaction proceeds via initial polarization of the carbonyl group of y-butyrolactone by the metal cation. In the next step, ammonia binds to the carbon atom of the polarised carbonyl group forming an acid amide, which then rapidly dehydrates under ring closure. The stronger the electrostatic field of the metal cation the stronger the interaction between the carbonyl group and the metal cation leading to a more polarised C=O bond, which is then more reactive towards ammonia. (0.~0

Me-Y

(N-vO

2-pyrolidinone

7-butyrolactone B r o ~ acid catalysed side-reaction

~ A

~

o NH,_

co-hydro xybutryo nit r it e Scheme 6. The transformation of g-butyrolacone into 2pyrrolinone over MeY.

374 In contrast to the previously discussed carbon-carbon bond rearrangements, these results clearly show that Lewis acid sites can also act as catalytically active sites for nucleophilic substitutions. Note that if catalysts without Bronsted acid sites are used (i.e., with zeolites exchanged with monovalent cations) the competitive side reaction leading to o-hydroxybutryonitrile via protonation of the acid amide can be completely suppresscd (Scheme 6). .

.

I)

1I)

.

.

.

.

.

.

.

.

.

.

.

CI

OH

.

.

.

.

.

.

.

.

NH, +

HCI

+

H20

NH2

Scheme 7. The reactions of chlorobenzene with ammonia (I) and phenol with ammonia (1I). . _ _

Nucleophilic substitution of an aromatic ring is difficult to achieve, as the ~-eleetrons will repel the electron density of the incoming molecule and it is difficult for the aromatic ring to accommodate the additional electrons. The substitution becomes more likely when a strongly electron withdrawing group is replaced by a more electron donating one. Examples of this case are the reaction of chlorobenzene with ammonia to form aniline and HC1 or the reaction of phenol with ammonia to give aniline and water (see Scheme 7) [83,84]. The most selective zeolite catalysts are based on mordenite and ZSM-5 and contain copper or cobalt ions [85]. The rate determining step seems to be the release of the electron withdrawing group [76,86]. A similar reaction mechanism seems also to be responsible for the formation of diphenyl from two phenol molecules [87]. The zeolite faciliates nucleophilic substitutionSof aromatics by electron withdrawal from the aromatic ring via coordination on the metal cations

[88].

Addition and elimination reactions of carbonyl compounds The polar nature of the carbonyl group allows for addition ofnucleophiles at the carbon atom. Molecular sieves catalyze these reactions by enhancing the polarity of the carbonyl group through interactions between the Bronsted or Lewis acid sites and the oxygen of the carbonyl group. If the nucleophile retains a proton, water can be easily eliminated and the overall reaction leads to the replacement of the oxygen by another nucleophile (see Scheme 8 and refs. [89,90]). The reactions involve the addition ofH20, ROH, RSH, HCN and HSO3 to the carbonyl group yielding the corresponding hydrates, (semi)acetals, cyanhydrines etc. Acetal and ketal formation from aldehydes, resp. ketones and alcohols occurs over mordenite and other acidic zeolites [91 ] slightly above ambient temperatures in the liquid phase. The reaction is not confined to simple alcohols, diols can also be converted (e.g., cyclohexanone reacts with ethylglycol to 1,4, dioxaspiro(4,5)decane [2]). Note that it is likely that desorption controls the rate of such reactions as the product molecules are larger than the reactants and have, hence, a higher adsorption constant.

375

R.

Null9

R~

R,

,g O H

R

R2 + H20 --H

RI

Scheme 8. General mechanism of nucleophilic addition with subsequent elimination of water. The reaction ofacetonyi acetone to dimethylfuran, catalyzed by HZSM-5, is an example of an intramolecular addition reaction involving two carbonyl groups, followed by a 13elimination of water (Scheme 9). The Bransted acid site of the zeolite protonates one of the carbonyl groups, while the oxygen atom of the second C-O group binds to the positively charged carbon atom of the protonated carbonyl group. The use of the rather hydrophopic zeolite HZSM-5 facilitates the elimination of water after the ring closure reaction. ~___O H3C~ . . .

OH CH,

n +

+ H3C

H3C CH2

"

CH 3

CH 3

H+

OH ~C~ H

_-- o

i IH CH 3

H3C~ C ' - - - C H ~

O ~/

I

+ H20

H3C

acetonylaceto ne

dimethylfuran

Scheme 9. The reaction of acetonyl acetone to give dimethylfiwan over HZSM-5. The products of the ketone or aldehyde conversion with ammonia and amines depends upon the availability of a proton at the nitrogen atom. If such a hydrogen is present, e.g., in the reaction of benzaldehyde with NH3, the addition of ammonia to the carbonyl group is followed by a rapid elimination of water. The so formed benzylidine imine subsequently dehydrogenates to form benzonitrile in the presence of transition metal ions, such as Co, Cr, Cu, Zn or Mn, (Scheme 10) [74]. If the acidic proton at the nitrogen is lacking, e.g., it, ~ e reaction ofdiethylamine with cyclohexanone, the formation ofa C=N bond is prevented during the dehydration step and instead a ring C=C bond is formed. The zeolites used in this case are large pore zeolites such as CaX or HMOR. Note that with these catalysts drying agents have to be added and it appears to be likely that large pore hydrophobic zeolites would be a better choice as catalyst. N

+ NH3

-'~

Scheme 10. Reaction ofbenzaldehyde with ammonia yields benzonitrile.

376 The acidic 10 and 12 membered ring zeolites (H-MOR, ZSM-5, ZSM-11) can also be used to catalyze the condensation ofalkenes with aldehydes to form unsaturated alcohols, acetals etc. (Prins reaction)J92]. Chang et a/.[93] showed that this reaction involves in the initial step the activation of the aldehyde by a Bronsted acid site to generate an electrophilic species. The condensation with, e.g., isobutene leads then to a primary alcohol with a positive charge at the tertiary carbon atom. Elimination of water and addition of further aldehyde molecules may lead to a broad variety of products. Some of these reactions can be effectively blocked by chosing zeolites with the appropriate pore size [94,95].

Reatv'angements of nitrogen containing compounds The most prominent examples of this type of reaction are the Fischer Indole synthesis, the Beckmann rearrangement and the benzylamine rearrangement. For all three reactions rather complex mechanisms have been proposed. On comparing the stmctt~e- activity relationships for these transformations, it becomes clear that generalisations are difficult and that a complex interplay between pore shape and size, the acid strength and the polarity of the zeolite lattice seems to control the activity and selectivity for a given reaction. An example of the Fischer Indole synthesis of substituted indoles involves the initial condensation ofa phenylhydrazine and 3-heptanone to form a phenylhydrazone (see Scheme 11). The phenylhydrazone undergoes (Br~nsted or Lewis) acid catalysed tautomerisation to give the enhydrazine tautomer which further rearranges and then eliminates ammonia to form the indole. Two products are possible, the bulky 2-ethyl-3-propyl-indole (Scheme 11) and the more linear 2-butyl-3-methyl-indole. In the homogeneous phase the selectvity towards one of the two products is controlled by the acid strength of the catalyst. The role of the zeolite in controlling the selectivity in the heterogeneously catalysed process is not unabiguously resolved. Van Bekkum et aL [96] showed that the 'linear' isomer was predominantly produced over most zeolites suggesting at first sight that the constraints in the molecular sieve pores favor the product with the smaller minimum kinetic diameter. However, since HNaX was more selective to the linear isomer than the isostmctural HNaY it must be concluded that the selectivity is not exclusively goverened by classical zeolite shape selectivity.

o

II

+ NHNH,

~

C

~

- H,O - r-

cat

~ - NH 3

"bulky" (III)

_

(i) (II)

Scheme 11.

~

"linear"

(iv) The Fischer Indoie reaction of phenylhydrazine (I) with 3-heptanone (II) giving two indole products.

Similarly, for the vapor phase Beckman rearrangement of, e.g., cyclohexanone oxime into caprolactam (Scheme 12) the zeolite structure was initially thought to be the most decisive factor for selectivity. Small pore zeolite HA (pore size 4fi,) produced caprolactam with only

377 4% selectivity at 14% conversion, whereas medium pore sized HZSM-5 (5.5A) gave 50% selectivity at 100% conversion [97] and large pore HY (7.5A) 89% selectivity at 82% conversion [98]. Recent studies however, suggest that the external silanol groups are the dominant catalytically active sites. Sato et al. [99] observed that the catalytic activity and selectivity to E-caprolactam increased in parallel with the Si/AI ratio of ZSM-5 and were directly proportional to the concentration of weak acid sites on the external surface of H-ZSM5. Interestingly, amorphous silica that also contained a high concentration of such SiOH groups also gave a very high initial conversion, but deactivated rapidly due to coking. Highly crystalline zeolite samples were shown to be more selective and more active indicating that the regularity and/or the low density of such weakly acidic silanols are essential for high lactam selectivity. o

C;

"OH

H

Scheme 12.The Beckmann rearrangement of cyclohexanone oxime into e-caprolactam. Typical examples for benzamine warmngement are the conversion of aniline and 1,3diaminobenzene with ammonia to 2-methylpyridine (Scheme 13) and a-amino-a' -picoline. Although several acidic oxides were found to be active, the best results were obtained with HZSM-5. It was found to be more active than SIOJA1203(48% conversion compared to 29%), but showed similar selectivities (83% over HZSM-5 and 98% over SiO2/Al203) [ 100]. The reaction seems to proceed via the addition of ammonia to a protonated aminobenzene (probably present in the form of an cyclic enamine). After an enamine-amine isomerization, the ring is opened via a reverse aldol-type reaction. Upon addition of the amino group to the imine double bond the ring closes again. After elimination of ammonia from the resulting aminal the final product is obtained [ 101 ]. Note that this potentially provides a new simple route for the production ofaminopyridines replacing the current complicated industrial process [102]. NH~ NH 3 ~

~N]N ~

(I)

CHs

(III)

NH~

NH,_

(If)

HsC

N

(IV)

CH3

Scheme 13. Benamine rearrangements of aaniline (I) and 1,3-diaminobenzene (II) to apicoline (III) and a-amino a'-picoline (IV).

378 Electrophilic substitution on the aromatic ring

In general terms, this type of reaction is characterized by the attack of a species with a positive partial charge, a positively charged species or a radical (i.e, species that are electron deficient) on an aromatic ring, preferably on the carbon atom with the highest negative charge. A broad variety of such electrophilic species has been reported to exist in the pores of molecular sieves (see Fig 5.after ref. [2]). The generation of such species can take place via several pathways from amongst which protonation, hydride abstraction and cleavage of polar groups are the most important ones. A general mechanism can be visualized as depicted in Scheme 14.

RH ArR

o R--

o

\

- - X (X=CI, O C R

ROH, ROR, AK)R

HNO 3, N204, NO 2

Cl 2, Br 2, HBr + 02, 12 + 02

.

D-

|

~

H+

Figure 5. Range of electrophilic agents employed in electrophilic aromatic substitutions over zeolite catalysts, after ref [2]. In the first step, coordinative bonding between the ~-electrons of the aromatic ring and the electrophile frequently occurs. Recent spectroscopic evidence for such an intermediate was reported for the methylation of toluene [ 103]. The aromatic ring should only be weakly held by the zeolite in order not to decrease the availability of the re-electrons. Then, a localized interaction with a carbon atom of the ring preceeds the actual substitution. In the presence of a substitutent on the ring, the carbon atom position at which the interaction with the electrophile occurs will depend upon the inductive effects induced by the ring substituent. For electron donating substituents the preferred carbon atoms to accept the electrophile are those in ortho and para position to the substituent group [ 104]. H

E

E

+

Scheme 14.

H +

General scheme for electrophilic aromatic substitution.

The overall reactivity of the aromatic ring will also depend upon the nature of the substituent. Electron donating properties of the substituents increase the availablity of nelectrons at the aromatic ring, while the electron withdrawal properties reduce it. In that respect alkyl-, hydroxyl-, alkoxy-, or amine groups increase the reactivity, while the presence of halogen or nitro groups will reduce it. The reactivity ofheterocycles also depends upon whether or not the ring is 3z-electron excessive. This results in pyrrole and thiophene being

379 more reactive than benzene, while pyridine is less reactive [92]. The examples that we have chosen to demonstrate the required design of the molecular sieve catalysts and the necessary adjustment of the reaction conditions are alkylation, acylation, nitration and chlorination. Friedel Crafts type alkylations of benzene by alkenes involve the initial formation of a lattice associated carbenium ion, formed by protonation of the sorbed olefin. The chemisorbed alkene is covalently bound to the zeolite in the form of an alkoxy group and the carbenium ion formed exists only in the transition state. As would be expected from conventional Friedel Crafts alkylation, the reaction rate over acidic molecular sieves also increases with the degree of substitution of the aromatic ring (tetramethyl > trimethyl > dimethyl > methyl > unsubstituted benzene). The spatial restrictions induced by the pore size and geometry frequently inhibit the formation of large multisubstituted products (see also the section on shape selectivity). For similar alkylation reactions modified faujasites need lower temperatures to catalyze the reaction with the same rate (under otherwise identical reaction conditions) than amorphous silica-alumina catalysts [ 105]. The difference is explained with the higher site strength and density in the zeolite catalysts. The fact that the original Friedel Crafts catalyst (promoted Lewis acid - AICI3-HC1) is reactive at yet lower temperatures than modified faujasites, suggests that a microporous material with higher acid strength could push the operating temperatures even lower. In general, a suitable catalyst should have high acid site strength and sorb the substituting molecule strongly. A good example of this is the alkylation of benzene with propene for which the reaction rate over divalent cation exchanged Y zeolites was found to decrease in the order Mg--Ca > Sr > Ba in accordance with the decreasing acid strength of the materials [ 106].

IR INTENSITY (2400 cm-I)

I0-

6

2 0

w I

0

.t

t

I

0.000.'3

0.001

0.00 i 5

J,

t

t

0.(}02

0.00"-5

I'OF [molec site s]

F i b r e 6. Reaction rate of methylation of toluene over HZSM-5 is directly proportional to the concentration of chemisorbed methanol [108]. The alkylation of toluene with methanol over HZSM-5 proceeds at low temperatures via a protonated methanol species in the transition state [ 107] and weakly coadsorbed toluene

as classically predicted for Friedel Crafts alkylation. The reaction rate is directly proportional to the concentration of the chemisorbed methanol (in the presence of excess toluene) as shown in Figure 6 [ 108]. Alkylation leads preferentially to ortho- and para- substituted products which rapidly isomerise in the zeolite pores. Specific reaction conditions and tailoring of the catalyst pore structure can be employed so that para- substituted products are preferentially

380 produced [ 109]. The reasons for this selectivity and the methods for optimizing the catalyst performance will be discussed in a later section. The catalysis appears to be completely controlled by the Br/Snsted acid sites with the role of the Lewis acid sites being marginal [ 110]. For the alkylation of the more active phenol both Bronsted and Lewis acid sites are claimed to participate in the catalytic activation [ 111 ]. The Bronsted acid sites activate the alkylating agent by protonation, whereas the Lewis acid sites can activate the alkylating agent and phenol by coordination, and/or phenol by deprotonation. If activation of the alkylating agent and the phenol occurs on the same Lewis acid site, the predominant product will be the ortho- substituted isomer.

OH

OH ~ CH3CO2H

~

O

~ / ~

Scheme 15. The accylation of phenol with acetic acid to yield 2-hydroxyacetophenone.

Acylation is currently carded out industrially with stoichiometric amounts of metal chlorides or mineral acids. Zeolites can replace liquid acids in this two step process consisting ofesterifcation and the Fries rearrangement. Several possible starting compounds for acylation, such as acid halides, carboxylic acids and acid anhydrides exist. The type of acid site (i.e., Br~nsted or Lewis) in the molecular sieve has to be adjusted for the acylating agent. A Lewis acid, such as La3+in the zeolite, will not activate a carboxylic acid to give an acylium ion, but will rather form a carboxylate anion. In contrast, Bronsted acidic hydroxyl groups will readily help to generate an acylium ions. When using an acid chloride as the acylating agent, Lewis acid sites are a better choice than Brrnsted acid sites, since they assist heterolytic dissociation by forming strong bonds to the halogen anion. An example of the need'for Bronsted acid sites is the acylation of phenol with acetic acid to yield 2-hydroxyacetophenone [112] with HZSM-5 as catalyst (Scheme 15). The latter situation is exemplified with the paradirected acylation of toluene with several aliphatic acid chlorides over ZnY [113]. Other examples are the formation of anthraquinone from benzene and phthalic anhydride or from phthalic anhydride alone over NaCe and NaZn exchanged FAU, respectively [ 114]. Acylation ofheterocyclics, such as thiophene, seems to require a lower acid strength of the catalyst which is best met by B-ZSM-5. The reaction ofthiophene with acetic anhydride to 2-acetylthiophene proceeds with 99% selectivity at 24% conversion and the conversion of pyrrole with acetic anhydride to 2-acetylpyrrole with 98% selectivity at 41% conversion [115]. Nitration of aromatic compounds requires very strong acid sites to stabilize the NO2§ cation which is an important intermediate in liquid phase nitration [ 104]. Several nitrating agents such as HNO3, NO_,and N,_O4have been successfully applied using mainly dealuminated mordenites faujasites and elevated pressures [ 116]. The stability of the zeolites is a major problem given the highly acidic reaction medium. A combination of high crystallinity and sufficient extra-zeolite surface area (the presence of extra-lattice material) was found to be beneficial for stabilizing the catalysts.

381 In the halogenation of aromatic molecules the role of the zeolite is to polarize the CI_, or Br2 molecule in order to enable it to attack the aromatic nucleus. The polarization is aided by an alkali or an alkali earth cation [ 117]. In most cases addition of CI,_ to benzene dominates over MFI and FAU type molecular sieves leading to chlorocyclohexane intermediates. A minor portion of the aromatic molecules, however, also reacts directly to chlorobenzenes via electrophilic substitution. Larger pore zeolites usually lead to a higher degree of chlorination which can be explained by the availability of the space in the zeolite pores[ 118].

2.1.2 Reactions catalyzed by basic sites In contrast to the situation found with acid catalyzed reactions, the role of the zeolite is less well defined for base catalyzed reactions. This results from the fact that all "basic" zeolites contain alkali cations that act as (weak) Lewis acid sites. Thus, most of the chemistry described in this chapter involves Lewis acid and base sites. It should be stressed that for all acid/base catalyzed reactions both sites are involved in the reaction sequence. In many of the acid catalyzed reactions the importance of the acid sites dominates so drastically that attention is paid only to the acidic function [ 119]. We speak, therefore, of base catalyzed reactions, if the strength of the base sites is high enough to stabilize anionic or polarized species with a marked negative charge and if these species are part of the catalytic cycle. Interpretation of catalytic results with respect to the role of acid and base sites remains, however, always ambiguous as the stabilizing effect of the metal cation (for zeolites usually an alkali metal cation) is difficult to assess. There is a second problem affiliated with defining the catalytically active site for base catalyzed reactions. Acid sites, irrespective of whether they are of Lewis or Brmasted nature, are always a minority species. The majority of the molecular sieve lattice is comprised of the more electronegative oxygen. Consequently, it is straightforward to characterize the minority species (indeed a large variety of methods have been developed in that respect [ 120], while characterizing the majority species, i.e., base sites, of the catalyst still poses a major problem. Thus, evidence on the location of the base sites in the molecular "sieve channels is ambiguous [ 121 ]. The main question in this respect is whether or not the base sites are localized (e.g., next to the alkali cation) or whether all oxygens of the molecular sieve lattice act as base sites [ 122]. In base catalyzed reactions relatively low rates are achievable compared to acid catalyzed reactions and in many cases minor traces of acidic protons may change the selectivity of a reaction dramatically [ 110]. In order to overcome this problem, catalysts are prepared with a slight excess of the alkali cations. Very strong basic sites have been created by supporting metallic sodium in the zeolite pores [ 123,124]. Recently, the method of using an excess of alkali metal cations has been expanded to load zeolites systematically with alkali metal oxides. This approach results in the zeolites being used more as a support than as base catalyst [125]. The oxidic nanophase particles in the zeolites are created by thermal decomposition of the corresponding alkali metal acetate, nitrate or hydroxide [ 125, 122]. In contrast to the situation found with solid acids, basic mesoporous oxides are excellent catalysts and the use of basic molecular sieves might be advantageous only if shape selectivity is needed for a particular reaction. In general, the action of the basic catalysts can be twofold. On the one hand the high electrostatic field in the pores and the polar lattice of basic molecular sieves facilitates

382 proton abstraction from functional groups of reactant molecules. Depending upon whether this leads to a stabilized carbanion or to a polarized functional group of the reacting molecule, the reaction occurs in a one or a two step process. On the other hand, the functional group is polarised by an electron pair donor/electron pair acceptor interaction with the alkali metal cation. The positive part of the dipole in the polar group and the rest of the molecule may interact with the basic oxygens close to the cation. Hydride transfer, which is frequently part of the catalytic sequence in base catalyzed reactions, is more a consequence of the close vicinity of the sorbed molecules than of being induced by the basic nature of the zeolite. The reactions that will be employed to exemplify these general principals are alcohol dehydrogenation, olefin isomerization, aldol condensation and Meerwein-Ponndorf-Verley reductions. Dehydrogenation of alcohols occurs over base zeolites in the presence and in the absence of oxygen [126]. Dehydration is the prevailing reaction over acid zeolites [ 127]. Higher reaction temperatures are required for dehydrogenation than for dehydration, due to the higher energy of activation for the former reaction [128]. The catalytic activity is related to the concentration and the type of alkali cation, i.e., with increasing size of the alkali cation and increasing level of exchange the rate/selectivity to dehydrogenated products increases [ 126]. Mechanistically, the reaction is thought to proceed via abstraction of a proton from the hydroxyl group of the alcohol by forming an alkoxylate. [3-Hydride abstraction produces hydrogen that desorbs, together with the ketone/aldehyde formed to close the catalytic cycle [129]. Poisoning experiments with pyridine (to block the acid sites) and with phenol (to block the base sites) indeed show that dehydration requires strong acid sites to be catalyzed whilst dehydrogenation requires strong basic sites [130,131]. Conceptually, one might expect that higher concentrations of aluminum in the zeolites and (for a given alkali metal ion) higher concentrations of alkali metal ions would generate a stronger basic zeolite [ 132, 122]. However, the chemical composition of the molecular sieve seems to influence the base strength and, hence, the catalytic activity in a more complex way. Davies et al. [125] reported that Cs exchanged Y type zeolites are an order of magnitude more active than the corresponding X type materials for the catalytic dehydrogenation of isopropanol. In that context it should be emphasized that using the selectivity to dehydration find dehydrogenation of alcohols to characterize the acid and base properties requires the comparison of results at one (arbitary) reference temperature. Since the apparent energies of activation for the two reactions are quite different, it is difficult to judge whether or not changes in the selectivity observed at varying reaction temperatures are induced by changes in the acid/base properties or by the different energies of activation. Basic zeolites are able to catalyze double bond isomerization of olefins [133]. Although this can also be achieved with acidic zeolites, the lower reactivity of basic zeolites towards hydrocarbons (i.e., the complete absence of skeletal isomerization) leads to higher yields [ 134]. A good example for this is the double bond isomerization of 1-octene over potassium loaded NaY. It is claimed that high yields can be achieved in that way and that the impregnation of the zeolite with an excess of alkali cations is important to obtain a good catalyst [ 135]. Aldol condensations are catalyzed by acid and basic zeolites (see Scheme 16). In the base catalyzed route the anionic species is generated by the interaction of the basic site with the hydrogen in a-position to the carbonyl group. The a-carbon atom (bearing a negative partial charge) then forms a new C-C bond with the carbon atom of the carbonyl group of another aldehyde molecule generating a larger 13-hydroxy carbonyl compound. Subsequent

383 dehydration leads to the formation of an a,[3-unsaturated aldehyde. Successful examples include the synthesis of crotonaldehyde from acetaldehyde over SAPO34 [136] and the conversion of acetone into diacetonaldehyde, mesityloxide and subsequent products over various alkali exchanged and alkali oxide loaded large pore zeolites [137]. The pore size of the zeolite influences the product distribution via suppression of the formation of the bulkier products. The condensation of acetone over NaX and NaL type zeolites is an example of this shape selectivity. As outlined above acetone is converted to diacetonalcohol and mesityloxide which may further react to isophorone. The product ratio of mesityloxide to the bulkier isophorone was 0.75 for zeolite X and 1.87 for zeolite L [138,139].

Scheme 16. Side chain alkylation of toluene with methanol over basic zeolites.

A special case of an aldol condensation is the side chain alkylation of alkylaromatics over basic zeolites such as alkali containing faujasites. The reaction requires the complete absence of protons in the zeolite, since these would catalyze ring alkylation with a much higher rate. The most well studied example is the side chain alkylation of toluene with methanol over a variety of alkali containing zeolites. Note that also alkenes can be used as alkylating agents for this reaction, but they require a higher base strength, i.e., the presence of metallic Na [ 137]. The role of the basic zeolite is twofold. It polarizes the methyl group of toluene which leads in the limiting form to a carbanion structure [140,141] and it catalyzes the conversion of methanol to formaldehyde [ 142]. The negatively charged carbon at the toluene carbanion forms a C-C bond with the positively charged carbon atom of chemisorbed formaldehyde forming an intermediate that rapidly eliminates water and ~el'cls styrene (see Scheme 16). The reaction rate seems to be determined by the availability of toluene (which is more readily stabilized in the faujasite pores by the larger alkali cations than methanol) and formaldehyde. Indeed, addition of an extra dehydrogenating function by the addition of ZnO to the zeolite leads to a drastic improvement in the activity [ 143]. The stability of carbanions follows the opposite sequence to that of carbonium ions, i.e., carbanions at primary carbon atoms are more stable than those at secondary or tertiary carbon atoms [144]. Thus, one would expect that it might be possible to convert methane and ethane with methanol. Unfortunately activation and/or proton abstraction from a]kanes seems not to be possible to a significant extent, as attempts to react methanol with methane or ethane have up till now failed. Presumably, one needs to couple such experiments with oxidative dehydrogenation [ 145] in order to achieve feasible conversions. A special case in which a strongly basic catalyst was used to produce 4-methyl thiazole in a simplified reaction sequence (replacing a five step synthesis with a two step synthesis) has been reported recently [ 146]. The catalysts (Cs loaded MFI and BEA) proved to be effective for the conversion of a ketone to an imine, more specifically acetone and rnethylamine into the corresponding imine. In the second step this imine is converted with SO 2 into 4-methyl thiazole (Scheme 17). Using Cs sulfate as the Cs source resulted in the

384 best catalyst and given the acidity and basicity of the reactants, one can speculate that sulfate species may also prevail in the rate determining step.

)=

O

+

~Nk,

CH3NH 9 -

§ SO2

-~

,,

?-

N

\

+

H20

Cszeolite ~ ~ - - ~ S

Scheme 17. Base catalysed conversion of acetone into an imine, which is further reacted to give 4-methyl thiazole.

The Meerwein-Ponndorf-Vedey reaction is conventionally seen as a base catalyzed reduction of a complex aldehyde by a secondary alcohol, e.g., isopropanol. The reaction is catalyzed by alkali metal exchanged zeolites and the product distribution is influenced by the strength of the base sites and/or by spatial constraints in the zeolite pores. An example is the reduction of citronellal (I) with isopropanol (Scheme 18) which gives 86% isopulegol and 14% citronellol at 87% conversion with Li or NaX as catalysts, while with Cs exchanged faujasites 99% citronellol is produced at 77% conversion [ 147]. This change in selectivity is attributed to the steric hindrance induced by the larger Cs § ions,.but the influence of the increasing base strength cannot be ruled out. As with other base catalyzed reactions the role of the catalyst in this example is also twofold, i.e., the basic oxygen helps to abstract a proton from the hydroxyl group of the alcohol, while the metal cation stabilizes the resulting alkoxy species and polarizes the carbonyl group of the aldehyde. If both molecules are adlineated, hydride transfer from the alkoxy group to the polarized aldehyde group takes place inverting the nature of the two reactants. The remaining steps are the reverse reactions of the activation.

Lix

. NaX OH

87%

CsX r92%

~ ]

OH

A<.

isopulegol citronellal citronellol Scheme 18. Selective reduction of citronellal to isopulegol over Li or NaX or to citronellol over CsX.

A recent example, the stereoselective reduction of 4-tert-butylcyclohexanone to cis4-tert-butylcyclohexanol with secondary alcohols over zeolite BEA (95% selectivity at 33% conversion)[ 148] shows however, that not so much the basic character of the molecular

385 sieve, but the presence of Lewis acid sites is an indispensable prerequisite for an active catalyst. As long as metal cations (in the form of extra lattice clusters of aluminum oxide) are present, the zeolite is active and selective. If, by taking special precautions, a HBEA zeolite is produced that is defect free and does not contain Lewis acid sites the zeolite is inactive for the reduction [ 149]. A mechanistically similar example is the Cannizzaro reaction, which is in essence a disproportionation between two aldehydes lacking t~-hydrogen with a parallel addition of water to yield an alcohol and a carboxylic acid [ 144]. Aldol condensation cannot take place since the a-hydrogen is absent. Most likely the formation of aldehydrates takes place in the zeolite pores. NaX and NaY have been reported to be successful catalysts [ 150].

2.2. Molecular sieves as catalysts for oxidation reactions

In contrast to the acid-base chemistry described so far, molecular sieves capable of catalyzing the selective oxidation of organic molecules must contain metal ions in the lattice or at ion exchange sites that are capable of changing their valency and/or are able to strongly coordinate peroxo groups. While these redox properties are readily achievable for metal ions at exchange sites [ 151,152] they are by far more difficult for metal ions coordinated into the molecular sieve lattice. Modifying molecular sieves by ion exchange has been the subject of much research over the past decades [ 153,154,155] whereas the successful in sire synthesis of framework incorporated metal ions is more recent [156,157]. The difficulty of incorporating metal ions into the molecular sieve lattice results from the fact that actually two requirements have to be fulfilled, i.e., (i) the metal cation must have approximately the size of the atom it replaces (Si, A1 or P) and (ii) it must be able to coordinate in a tetrahedral position in the framework. Furthermore, to function as a successful redox catalyst, a change in the valency and/or the coordination of the oxidant must be realized via reversible change of the coordination of the metal cation. Only a limited number of cations have been reported to be incorporated in the framework of zeolite and metal-aluminophosphate molecular sieves. These cations include Co, V, Mn, Cr. Ti [ 158,159] and a short compilation of the structures available (isomorphously substituted molecular sieves) is compiled in Table 1. Generally, it seems that aluminophosphate latti~,es are more easily adaptable for isomorphous substitution, but that the resulting materials have a lower stability than the corresponding zeolite frameworks [ 160]. Both types of molecular sieve catalysts, i.e., those containing exchangeable metal cations and those with metal ions isomorphously substituted into the framework are difficult to tailor with respect to activity and selectivity, and with the exception ofTi silicalite [ 161] none of the catalysts is used in a commercial process. The use of transition metal ions at exchange sites usually leads to preferred total oxidation, since the long residence time of the reactants in the zeolite pores favors this route over selective oxidation. On the other hand, when the transition metal cations are incorporated into the framework they are more labile than their non reducible analogs and are, thus, susceptible to leaching [ 162]. For several molecular sieves (e.g., CoAPO, CrAPO, VAPO) even the assumption that the lattice cations change their oxidation state whilst remaining in the lattice is still under debate [ 163]. Only indirect evidence has been presented so far that indeed the species in the framework and not traces of more active extra lattice species are responsible for the observed catalytic activity [ 163]. The catalytic properties of these materials depend crucially on the local geometry of the active site and the way the reacting molecule can bind to it. Common to all these molecular sieves, however, is the fact

386 that successful catalysts require the transition metal ions to be in an isolated state in the framework [ 164]. Shape selectivity effects are scarce and as the reactions that are dealt with are mostly irreversible and catalytically active sites that are able to interconvert isomer products are usually not present, only reactant selectivity and restricted transition state selectivity may play a role. However, marked progress in molecular sieve synthesis has been made over the last few years and the potential for industrial applications of some of these isomorphously substituted molecular sieves is high. The discussion on the catalytic chemistry will be confined to reactions over such molecular sieves. Titanium containing molecular sieves Titanosilicalite (TS-1)[ 165,166], a highly siliceous MFI type zeolite in which 0.1 to 2.5% of the Si atoms are replaced by Ti, is the most successful example for the use of isomorphously substitited zeolites. As a consequence of the high Si/A1 ratio of TS-1 the material contains only a negligible concentration of strong Br6nsted acid sites. In fact, the presence of acid sites is detrimental to the selectivity of the catalysts, as discussed below. TS-1 has been found to be a selective oxidation catalyst for a wide variety of reactions such as the conversion of alkenes to epoxides [ 167], alcohols to aldehydes [168], alkanes to secondary alcohols and ketones [ 169,170], phenol to hydroquinone and catechol [ 171 ] and amines to hydroxylamines [ 172]. A schematic representation of the chemistry is given in Fig. 7 which is adapted from ref. [ 17].

~

OH

OH 70H +

ArOH

~ NOH

(I) / / a

(1I)

OH

H PhOH

NH 3

O

TS-1 + 30% H20 2

_

/\ RCHCH 2

'CHOHCH2OH

R__o

J

RCHO

Rt/

F i b r e 7. Oxidation reactions catalysed by TS-1 with H202, adapted from ref. 7. The function of the zeolite in these reactions is to activate the oxidizing agent (H202) via the formation of a surface peroxotitanate species, which could exist either in the hydroxylated or the dehydrated state (see Scheme 19). Activation involves weakening or

387 polarization of the peroxidic bond, thereby inducing electrophilic properties in the oxidant. Oxidation occurs by subsequent oxygen transfer from the peroxotitanate to the reactant. As the material is highly hydrophobic, aqueous solutions ofhydroperoxide can be used. The hydrophobicity of the lattice helps the stabilization of relatively high concentrations of the organic reactants and the peroxide in the zeolite pores. The isolation of the Ti atoms in the framework is thought to be necessary to reduce the rate of H202 decomposition which occurs when two neighboring peroxotitianate species react to form H20 and O 2 [ 165]. Hydroxylation of phenol with aqueous H202 to yield a mixture of catechol and hydroquinone is an example of a commercial process employing TS-1 (Fig. 7, products I and II) [ 173]. The reaction is highly sensitive to the presence of extra framework titania that catalyzes the decomposition of H202 and the formation of quinones and coupling products. Therefore, this reaction is also used to probe the fraction of titania that is incorporated into the MFI lattice [ 165]. It is interesting to note that the hydroxylation occurs preferentially in the para position indicating the influence of the pore geometry on the selectivity. Phenol seems to be only very weakly coordinated to the Ti site and this might be one of the reasons why the further oxidation of hydroquinone to quinone is relatively slow. The hydroxylation of other substituted aromatic molecules is possible provided that the n-electron availablity is not significantly lower than that of phenols. For example aromatic molecules with electron withdrawing substituents (e.g., chloro-, bromo- and nitrobenzenes) are unreactive.

O

\

/

OH

/

/Si 0

0

OH I OH O OH

\/

", /

Ti ~

/Si 0

O

O

0

0

\

/

OH

/ si

~

\o /

O--O

OH

Ti

/Si

\/

~o

", /

O

0

(I) (II) Scheme 19. Two possible active peroxotitanate species; (I) hydroxylated or (II) dehydrated.

Epoxidation of olefins is readily catalyzed by TS-1 at reaction temperatures as low as 273 K [ 166]. Similarly to hydroxylation, epoxidation most likely occurs via a heterolytic mechanism that permits the retention of the configuration of the olefin. The relative reactivity depends mainly upon the nucleophilicity of the olefin. The negative effect of the presence of acid sites in TS-1 for the product selectivity in the epoxidation of propene is seen when A1, Fe and Ga containing TS-1 catalysts are compared to pure TS-1 [174]. The selectivi~ changed from 98% epoxide formed over TS-1 to only 6.5% over Ti-Ga-silicalite. The favored product in this latter case was 1-methoxypropan-2-ol formed via acid catalysed addition of methanol to the epoxide. This is in agreement with the observation that the selectivity to the epoxide over TS- 1 can be further increased by poisoning the weakly acidic surface silanol groups with C1-Si-(CH3)3 or CH3COONa [175]. Furthermore Ti-BEA has been employed as catalyst for the epoxidation of 1-octene with H202 [ 176]. The unavoidable presence of Brrnsted acidity in Ti-BEA (it is not possible to synthesize A1 free Ti-BEA) gives rise to unwanted side products via acid catalysed ring opening of the epoxide. Neutralising the acid sites with Li-, Na', K* or Mg2" cations increased the selectivity to the epoxide. Note that the catalytic activity decreased significantly in the presence of potassium and magnesium ions, probably due to spatial constraints or blocking of the Ti sites.

388

The ammoxidation of cyclohexanone to cyclohexanone oxime is catalyzed by TS-1 with 98.2% selectivity to cyclohexanone oxime at 99.9 % conversion [177]. Selective oxidation of the nitrogen of ammonia by hydrogen peroxide is a key step of this reaction. The mechanism is still vividly debated and three possible routes are shown in Scheme 20. Recent evidence [163] seems to support a route via intermediate formation of hydroxylamine [mechanism B]. The high selectivity on the other hand supports the postulate that the reaction proceeds via a concerted reaction step that involves the titanium peroxo species, ammonia and cyclohexanone (mechanism C) [ 177]. The oxidation of secondary and primary alcohols can also be catalyzed over TS-1 with good selectivities at lower conversions [ 178]. The mechanism of the reaction is not clarified, but as for the other oxidations with H202 the reaction seems to involve heterolytic oxygen - oxygen bond cleavage.

Mechanism

A

(>o

-~

NH

~

NOH

TS-1

Mechanism B TS-1 NH 3

H2 O 2

+

(

+

NH 2 OH

~

,-

NH 2 OH

r-

'---NOH

~

NOH

Mechanism C

0

+

\ / Ti

Scheme 20. Possible oxidation mechanisms for the ammoxidation of cyclohexanone to cyclohexanone oxime.

The oxidation of alkanes with H,O~ to alcohols and ketones has received much attention, as it is usually difficult to realize good selectivities at appreciable conversions [179]. Over TS-1 the process is highly selective (up to 90% selectivity based on the consumption of H202 [ 181 ]). At present, it is unresolved whether the reaction proceeds via consecutive one electron steps or via a single two electron step [ 163]. It is remarkable that an apolar substance such as n-hexane can be oxidized in the presence of polar solvents such

389 as acetone and methanol. This clearly demonstrates, the importance of the hydrophobic nature of TS-1 in order to obtain the high selectivities observed for the oxidation reactions studied. Vanadium containing molecular sieves The vanadium silicalites (with MFI and MEL structure) are active oxidation catalyst in gas and liquid phase reactions [ 180]. As for the titanium silicalites, only the framework associated vandium exhibits redox properties [ 181 ]. For example, in the hydroxylation of phenol, silicalite impregnated with vanadium compounds is catalytically inactive [ 182]. The catalytically active vanadium species is speculated to be located in non-tetmhedml positions, most probably chemically bound to the framework. Vanadium bound in that way is not extractable from the lattice [ 183]. A proposed structure of the vanadium site is schematically shown in Scheme 21. Note that the Si-O-V bonds are longer than the Si-O-Ti bonds and that V seems to be more exposed. The redox properties are affiliated with the changes in the oxidation state of vanadium between +IV and +V. Vanadium silicates with Si/V ratios ranging from 40 to 160 have been reported and these high values suggest (in accordance with 51V MAS-NMR measurements) that the V sites are isolated in the lattice.

o

\\

/

Si

si

/ \ ~

o~V---~ \

q

o,,, z_... i

Si-- S i ~ , ~

~

-,4

OH / "

O~i V-O Si

OH OH \/ Si--Si

Scheme 21. Proposed structures of V in a silicalite lattice.

Oxidation employing V-molecular sieves as catalysts tends to be deeper thari with Ti-molecular sieves. A variety of reactions are catalyzed by these materials, the most remarkable examples being oxidative dehydrogenations and hydroxylations. V-silicalites, for example, are highly selective for the oxidative dehydrogenation of propane to propene and methanol to formaldehyde with oxygen or N_,O [ 184] as oxidants. For liquid phase oxidation reactions, using H,_O2as oxidant, titania silicalites and vanadia silicalites (VS-2) are capable of oxidising the secondary carbon atoms of n-parafins. However, only VS-2 will oxidise the primary carbon atoms thus catalysing the formation of primary alcohols and aldehydes [185]. The difference in the oxidising strength of VS-2 compared to TS-2 is further exemplified by the oxidation of aromatics with H20_,. VS-2 is active in the ring oxidation of phenol to o- and p-cresols and in the side chain oxidation to benzyl alcohol and benzaldehyde, whereas TS-2 is only active for the ring oxidation [ 186]. Recently the large pore vanadium containing molecular sieve, V-NCL- 1 with a pore size of 7A, has been shown to be an active catalyst for the oxidation of larger molecules, such as napthalenes, 1,4-napthoquinones and phthalic anhydride (Scheme 22)[ 187]. The as synthesised form ofV-NCL-1 contains atomically dispersed V4+ ions located in framework postions although not neccessarily in tetrahedral coordination. The vandium ions can be oxidised to the pentavalent state by calcination, as evidenced by ESR [ 157], with some

390 vanadium ions leaving the lattice and forming non framework clusters. Reduction returns the vanadium to the tetravalent state. Similarly to VS-2, V-NCL- 1 is able to oxidise primary carbon atoms in alkanes and the side chains of aromatics. The effect of increasing the concentration of V in the zeolite (from SifV 400 to 150) on the catalytic oxidation of cyclohexane to cyclohexanone and cyclohexanol was studied. The selectivity was found to be independent of the vanadium concentration, with the cyclohexanoVcyclohexanone ratio being 0.55. The activity of the catalyst was found to increase with increasing V content. O

O

OH -

H:O~

[V]-NCI-I ~

C +

-.

85%. 31h O 1- a n d 2 - n a p t h o I 85%

1,4-napthoq u inon e 21%

\ /

O

C II O phthal ic anhydride 47%

Scheme 22. Oxidation of napthalene over large pore V-NCL-1.

Other metal substituted molecular sieves Co and Cr have been found to be incorporated into the lattice of aluminiumphosphates in a well dispersed manner [159]. Both elements assume two oxidation states in the lattice depending upon the pretreatment procedures. While it seems certain that during synthesis incorporation can be achieved and that these tetrahedrally coordinated atoms are stable in gas phase reactions, conclusive evidence is lacking that leaching is not an important side reaction in liquid phase studies. Indeed, it seems that for several reactions the highly active complexes that are leached out of the lattice and homogeneously dissolved in the reactant/solvent mixture dominate the catalytic properties. In addition to their strong oxidation potential CoAPO molecular sieves also contain moderately strong acidic sites. Note that the valency of Co suggests that it substitutes for A13§ in the ALPO4-5 structure. In this case, Co present in the 3+ oxidation state restilt~ in a neutral lattice, while Co present in the 2+ oxidation state leads to a metal aluminophosphate with Bmnsted acid sites [ 188]. Since during a red0x cycle acid-base and redox functions could be utilized in a sequential manner_interesting reaction sequences combining acidic steps and redox steps could be envisaged. However, in practice only a few successful examples have been reported. CoAPO-5 has been used to oxidize cyclohexane and n-hexane in the presence of acetic acid to give cyclohexyl acetate and hexyl-2-acetate respectively [ 189]. The active site is regenerated by oxidation of Co(H) to Co(HI) by acetic acid. Another example concems the autooxidation of p-cresol to p-hydroxybenzaldehyde in methanolic sodium hydroxyide solution [ 190]. Similarly CrAPO-5, derived from isomorphous replacement of Al by Cr in A1PO-5, was shown to be an active and selective catalyst for the oxidation of secondary alcohols [ 191 ]. For example, carveol was chemoselectively oxidised by tertiary butyl hydrogen peroxide (TBHP) at the alcohol group (94% selectivity at 62% conversion) rather than at the carbon-carbon double bond. The initial assumption that Cr § coordinated tetrahedrally in the lattice is the active species was later revoked and it seems now that Cr 3+is present as an octahedral species associated with the framework [ 192].

391 2.3 Molecular sieves as host for catalytic functions

Molecular sieves can not only act as catalysts themselves, as previously discussed, but are also able to form the matrix for extra lattice phases which are catalytically active. Most of the applications of these guest species in the molecular sieve concern hydrogenation (metal particles) or oxygenation reactions (metal complexes). If neutral molecular sieve lattices (e.g., the aluminumphosphate VPI-5 [ 193,194]) are used as the matrix, the supported metal particle and/or metal organic complex constitute the only active phase, while bifunctional catalysts are obtained when molecular sieve lattices with acidic functions (e.g, zeoliteY [ 195,196]) serve as matrix. The molecular sieve exerts the function of a crystalline host that surrounds the active species and increases in this way its stability. The regular pore system of the molecular sieve provides an additional advantage, i.e., that only reactant molecules below a defined size can approach the active site. In some cases it can even be realized that the spacial constraints in the pores allow only the interaction of one special functional group of the reactant with the active site resulting in highly selective conversions. Metals in zeolites

Incorporation of metals in zeolites occurs usually v/a ion exchange with the respective metal salt and subsequent reduction. A large body of information exists on the preparation and characterization of mono and bimetallic particles in molecular sieves and the reader is referred to that literature for details [197,198,199]. The knowledge in this field is quite advanced, since (noble) metals form part of a large number of catalysts where a small quantity of the metal needs to be introduced to provide an oxidation (e.g., FCC catalysts) or hydrogenation function (e.g., selectoforming reactions). In most of the applications the function of the (noble) metal is to provide dissociated hydrogen and to keep the catalyst free of coke. These applications will not be discussed and we will refer only to cases where the role of the metal is more involved in the reaction. It is quite challenging to understand in what way the zeolite influences the metal compared to other supports. The electronic changes that could be induced by the pore system are quite subtle and metal particle size effects may overrule these changes [200]. In comp.arison to metal-support interactions on macroporous oxides, the interaction between metal particles and the supporting zeolite matrix seems more pronounced. This may be because the metal particles interact with the zeolite lattice over a substantial fraction of their surface. It has also been suggested that in addition to the intrinsic electronic effects due to the small size of the metal particles in the zeolite cage, a modification of the electronic structure of the metal by the acidic zeolite framework has to be considered [201,202]. Most of the early work concerning metal particles in zeolites focussed on Pt in faujasite (FAU). Benzene hydrogenation activity was shown to be directed by charge transfer effects involving the support, platinum and the reactants. It was suggested that the interaction of the Pt particle with the acid site of the zeolite resulted in an electron deficiency of the Pt atoms compared to Pt on SiO2 [203,204]. Similarly, the rate of benzene hydrogenation over supported Pd catalysts increased in parallel to the increase in the electron deficient character of the Pd cluster in the zeolite [205]. Experiments of Gallezot et al. [201 ] on the adsorption constants of toluene and benzene on various metal catalysts supported this theory. Toluene has a slightly higher basicity than benzene and, thus, the preference to adsorb toluene over benzene will be more pronounced the higher the electron deficiency of the metal is. More recently de Mallmann and Barthomeuf reported a similar experiment using Pt/X-type zeolites suggesting that the electron density at the Pt atom increases [206] with the alkali cation in

392 the sequence Li
o

/CH

~,o .c~ -.CH H

cinnamaldehyde

.=

~./CH

~o .c~ .CH, H 2

-

.CH_,OH -.CH +

x

~

.CH.,OH .CH2 -

CH 2

Scheme 23. Hydrogenation of cinnamaldehyde catalysed by Pt containing zeolite Y.

393 In a more conventional example Pt/ZSM-5 is used to induce reactant (size exclusion) selectivity in the hydrogenation of substituted aromatic molecules. Over Pt/ZSM-5 the rate of styrene conversion is at least 25 times higher than that ofmethylstyrene, while over Pt/Al203 similar rates of hydrogenation were observed for both these compounds. The metal functions can be elegantly combined with the acidic functions of the zeolitic support to obtain a very effective bifunctional catalyst. For example the selective isomerisation followed by dehydrogenation oflimonene to give p-cymene (Scheme 24) can be carried out in one step over a multifunctionalised zeolite [213 ]. With an acidic boron zeolite (SifB= 21) 21% selectivity to p-cymene was obtained at 100% conversion. Addition of 3 wt% Pd increased the selectivity to 70% at the same conversion. Further addition of Ce (1.5 wt% Pd, 3.5 wt% Ce) to the metal loaded zeolite led to 87% selectivity.

1

1

1

Scheme 24. Isomerisation of limonene, followed by dehydrogenation to give p-cymene. Acidic forms of zeolites are well suited as supports for metal functions which are employed for hydrogenation, since they can also withstand the presence of traces of sulfur compounds frequently found in feedstocks of petrochemical industry. It should be noted, however, that hydrogenation is a structure insensitive reaction so it will primarily depend upon the concentration of the accessible metal particles and the adsorption constant of the unsaturated hydrocarbon. This _,nayoffer an explanation as to why Pt catalysts, for example, are still active for hydrogenation, when theft activity for dehydrocyclization or hydrogenolysis (i.e., for structure sensitive reactions) is completely lost (e.g., by poisoning). Recently, noble metals in zeolites have emerged in another context where the influence of the zeolite on the (de)hydrogenation behavior of the noble metal is quite a decisive parameter. Thus, Pt on basic BaKL zeolites will catalyze dehydrocyclization of n-hexane to benzene quite selectively, and such catalysts are used in the Aromax process [214]. Activity for dehydrocyclization follows a reverse trend to that seen for hydrogenation and increases with increasing basicity of the zeolite [215]. The high selectivity for aromatization is attributed to a combined action of the small Pt clusters and the shape selective properties of the L zeolite [216,217]. A recent report of Derouane et al. [218] suggests, however, that the high selectivities for alkane aromatization could also be achieved over MgO supported Pt indicating that the base strenph of the support is more important than the nature of the molecular sieve. This is supported by Mielczarski et al. [219] who reported that the selectivity to aromatization was primarily caused by the absence of acid sites which otherwise catalyzed hydrocracking. Metal organic complexes encaged in zeolites As active metal complexes are mostly too large to enter the molecular sieve pores they have to be synthesized in situ via the so called ship-in-bottle procedure, first described by Romanovsky in 1984 [220] and subsequently adopted by other groups [ 196,221,222,223].

394

In the first step the metal ion is introduced into the zeolite pores by ion exchange or adsorption of a labile metal complex. In the second step the intermediate material is reacted with gaseous complexing ligands, such as 1,2-dicyanobenzene, to form a complex inside the pores that is too large to diffuse out. Alternatively metal complexes can be directly encapsulated inside the zeolite cavities during hydrothermal synthesis, as has been shown for FePc, CoPc, NiPc and CuPc in zeolite X [224]. One of the most studied examples is the mimic of the enzyme cytochrome P-450 in the pores of a faujasite zeolite [196,204,225]. The iron-phthalocyanine complex was encapsulated in the FAU supercage and is used as oxidation catalyst for the conversion of cyclohexane and cyclohexanone to adipic acid, an important intermediate in the nylon production. In this case the two step process using homogeneous catalysts could be replaced by a one step process using a heterogeneous catalyst [ 196]. This allowed better control of the selectivity and inhibited the auto oxidation of the active compound. In order to simulate a catalyst and the reaction conditions which are close to the enzymatic process, the so obtained catalyst was embedded in a polydimethylsiloxane membrane (mimics the phospholipid membrane in the living body) and the membrane was used to limit oxygen availability. With this catalyst alkanes were oxidized at room temperature with rates comparable to those of the enzyme [205]. The catalytic chemistry of these complexes hosted inside the zeolite lattice can be altered by changing the size of the host pores, the acidity/basicity of the host, the redox character of the guest metal (by changing the coordination number [209]) and the ligand electron donating/with&awing properties [204]. An example of the effect of the zeolite lattice is the oxyfunctionalisation of n-alkanes over Fe phthalocyanines in zeolite Y and in VPI-5 [226]. Fe-phthalocyanine in both VPI-5 and zeolite Y catalyzed the oxidation ofalkanes with tertiary butyl hydroperoxide (tBHP) more selectively to ketones than to alcohols. Fephthalocyanine encapsulated in zeolite Y (FePcY) is more active for the oxidation of n-octane with tBHP than the same complex encapsulated in VPI-5, which is in turn was more active than free Fe phthalocyanine. Turnover numbers of 6000, 1500 and 25 were observed for the three types of Fe-phthalocyanines, respectively. Also the shape selectivity, with respect to the position of the oxidation, was higher over FePcY than over FePcVPI-5. VPI-5 has large unidimensional pores of 12.1A compared to the three dimensional 7.4A channel syst&ff of zeolite Y. Molecular modeling studies suggested that FePc is distorted inside zeolite Y, while it does not deform inside VPI-5. The increased activity and selectivity over FePcY is tentatively attributed to a constrained reaction environment and/or the deformed Pc being more reactive. A similar type of activity and selectivity enhancement is found linear salen ligands coordinated to a metal exchanged zeolite, for example Co exchanged FAU. The salen coordinates to Co in a square planar form that is sterically constrained within the zeolite pores. Such salen complexes in faujasites have been shown to act as oxygen carriers mimicking hemoglobin [227], regioselective oxidation catalysts [228] and selective hydrogenation catalysts [229]. An example for subtle control by substitution of the phtallocyanine structure is reported by Partons et al. [193] comparing the activity of FePcY with nitro substituted Fe phthalocyanines in zeolite Y. The electron withdrawing effect of the nitro substituent on the benzene ring was expected to enhance the electrophilic character of the active oxygen species and so to increase the activity. For the oxidation of cyclohexane to cyclohexanone and cyclohexanol with tBHP a 10 fold increase in the turnover frequency (TOF)was found for the nitro substituted complex in zeolite Y in comparison to the unsubstituted [204]. However the nitro substituted Fe phthalocyanines were found to be located at the outer surface of the

395 zeolite crystals and, thus, the increase in the TOF might be attributed to the more easily accessible active site. A recent elegant example of the tailoring the chemical properties of encapsulated metal complexes is the work ofBalka~ et al. who prepared and studied perfluorinated phthalocyanine complexes of Fe, Co, Cu and Ru (Scheme 25)[230] in NaX. Perfluorinating the complexes enhances the stability and catalytic activity of the catalysts in the ox~ctionalisation of light alkanes. The rapid deactivation of the catalysts based on Fe, Co and Cu F16Pc complexes was overcome by using Ru as the metal center. Similar catalysts, i.e.,Co-phthalocyanine (CoPc) encapsulated in zeolite Y, are active catalysts for cyclohexene and 1-hexene epoxidation (Scheme 27)[231 ]. Comparison of the activity of free and encapsulated Co-Pc has shown that the interaction with the zeolite stabilizes the complex. Co-Pc is still active after 24 hrs reaction whereas the free complex in solution is virtually inactive after 15 minutes. F

F F F

F

F

F

F

N I

N

!

F

N

F

N~Ru--N N!

F

N-~ N

"~ F

/

\

F

F F Scheme 25. Perfluorinated phthalocyanine employed by Balkus et al [230] in NaX.

Also other active metal complexes (i.e., several Co"* Schiff base [232] and Mn diimine complexes [233]) have been supported in this way. The high dispersion of the complexes in the cages of the molecular sieves allows to study the redox properties of mononuclear complexes that are unstable in solution [234]. The increased stability of the obtained materials, the easier handling of heterogeneous catalysts and the high yields achieved make these supramolecular systems a very promising candidate for further catalyst development in fine chemical synthesis.

[Ol "~ Co-Pc in zeolite Y

[ ~ o

0

Scheme 26. Epoxidation of cyclohexene and hex-1-ene catalyed by CoPc in zeolite Y.

396

3. PHYSICAL ASPECTS OF MOLECULAR SIEVE C H E M I C A L SYNTHESIS.

CATALYSIS FOR

In the previous section the chemical functionalities of molecular sieves were discussed and how these functionalities can be incorporated into the solids. It was, however, the spatial constraints of molecular sieve pores that initially made zeolites attractive for organic synthesis. It should be noted at this point that discrimination between chemical and structural aspects works well at a conceptual level, but faces quite severe limitations as soon as one tries to separate the contributions of the two effects. This is due to the fact that the chemical properties of a particular molecular sieve are interconnected with its framework density. In general terms, the strength of the acid sites in a molecular sieve with a given chemical composition will increase as the framework density increases and the pore size decreases [235]. Similarly, the polarizability of the lattice seems to be higher for frameworks exhibiting a higher density. Recent theoretical calculations of the group of Mortier [236] suggested that these interconnections might be quite complex and difficult to predict. It is unclear at present, if such properties are determined by localized structural effects, i.e., by local bonding angles and the way the tetrahedra are connected on a microscopic level [237] or if global properties, i.e., the average distance of the framework T atoms dominate [238,239]. We would like to emphasize that the global properties of molecular sieves, often described in terms of hydrophobicity and hydrophilicity [240] or fiamework polarity [241 ] will markedly influence the chemical preference for the sorption of molecules. This may lead to quite different relative concentrations of reactants in the zeolite pores (well demonstrated and discussed for the case of TS-1 in this review [ 163]) and in the intm crystalline void space. The consequence of which is unexpected 'solvent-type' effects. While these possibilities are hardly utilized up to now, one could think of systematically applying this approach for organic synthesis steps in which, for example, a high concentration of reactants is needed around the active site, but for which one wishes to dilute the products in the intra crystalline void to prevent further reaction. In such a case a solvent of different polarity than the reacting substrate should be chosen and the choice of the zeolite should be ma.d~ in accordance to the polarity of the reactant(s). 3.1 Shape selectivity Shape selectivity can be induced by differences in the diffusivities of the reactants and/or the products or by steric constraints of the transition state. A schematic representation of the three types of shape selectivity, i.e., the limitations of the access of some of the reactants to the pore system (reactant selectivity), the limitation of the diffusion of some of the products out of the pores (product selectivity) and constraints in forming certain transition states (transition state selectivity) are given in Fig. 8. Differentiation between the latter two is difficult as the kinetic results may be disguised when the overall rate is influenced by the rates of diffusion. In situ IR and NMR spectroscopy have contributed much to our understanding of these complex phenomena. The aspects of shape selectivity have been extensively discussed and excellent reviews exist [242,243,244]. The examples given here should only illustrate what can be achieved by employing a zeolite and why the pathway of a particular reaction is influenced.

397

Figure 8. Schematic representation of the three types of shape selectivity.

The first example of shape selectivity was reported by Weisz [245] for the dehydration of a n-butanol/iso-butanol mixture over LTA type zeolites. As a consequence of its larger minimum kinetic diameter iso-butanol was excluded from entering the zeolite pores, while n-butanol could easily be dehydrated to butene. This demonstrated for the first time that the catalytically active sites were indeed inside the zeolite pores and that the pores were able to realize a well defined cutoffpoint with respect to the minimum kinetic dia~neter of the reacting molecules. This principle of size exclusion was then frequently used in hydrocarbon processing to remove linear hydrocarbons from a mixture of hydrocarbons (e.g. selectoforming [246]). Note that a complete separation of one group of molecules is not necessary and usually a large difference in the diffusivities of the molecules will suffice. The relatively high apparent energies of activation for configurational diffusion (diffusion through micropores of similar dimensions to the diffusing molecules [247]) often disguises the presence of diffusion control. The high values also indicate that the (relative) rates of transport may change dramatically as the (reaction) temperatures are changed. This should be considered when comparing catalytic data at high temperatures (say for example at 773 K) with diffusion data measured at around ambient temperature. One of the most discussed cases of shape selectivity involving transition state selectivity or product diffusional constraints is the production of p-xylene over chemically modified MFI zeolites [248]. Several processes exist which utilize the shape selectivity of these zeolites, for example the alkylation of toluene with methanol [249], xylene isomerization [250] and selective toluene disproportionation [251 ]. The first two of these examples shall be used to describe in detail the principal possibilities to tailor the reaction pathway by shape selectivity.

398

Toluene alkylation by methanol occurs via methoxonium ions (presumably stable only in the transition state [252]) at low temperatures and via methoxy groups at high temperatures. Initially toluene is alkylated preferentially in o- and p- position of toluene, but all three isomers appear to be primary products (as shown by in situ i.r. spectroscopic measurements [253]). High para selectivity is claimed to be coupled with rapid isomerization of the xylenes. The diffiasion constant of p-xylene is about 103 times higher than that of mxylene [254]. In an idealized model (Scheme 8b) one would, therefore, expect to fred the xylenes in their equilibrium concentrations in the MFI pores. In situ i.r. spectroscopy showed that this is not the case and that preferred sorption of m- and o-xylene and trimethylbenzenes occurs. However, this is not a result of the preferred sorption of any of the reactants or products, as the adsorption constants of the three xylene isomers on HZSM5 are identical [254]. Combining the rate constants for the isomerization of the individual xylene isomers (obtained in separate experiments) and the concentrations of the products in the MFI pores, it could be shown that the selectivity to p-xylene was high, when the rate of internal isomerization was high compared to the overall rate of alkylation. Thus, the results indicate that the secondary isomerization is important for the shape selective production of p-xylene. However, the rates of isomerization are apparently not fast enough with respect to the diffusion to establish equilibrium in the pores. That suggests that the two steps (isomerization and diffusion) are in subtle balance and are at least so close not to allow the assumption of a quasi equilibrium under steady state operation [255].

Figure 9. The minimum kinetic diameter of the transition state for the m ~ p xylene transition is smaller than that for the m--,o xylene transition.

The isomerization ofm-xylene is a good example of transition state selectivity [256]. Irrespective of the temperature and coverage, (a particular sample of) MFI showed a product ratio of p- to o-xylene of 2:l. In the zeolite pores, only m-xylene was found to be sorbed in appreciable quantities. Thus, the reaction rate was concluded not to be influenced by the preferred retention of one of the products. The selectivity must be geverned by the differences in the transition states of the two products. The constant selectivity with varying reaction temperature indicates an identical apparent energy of activation for the formation of p- and o-xylene. Thus, the different selectivities must be caused by differences in the

399 transition entropy. In looking for a possible interpretation of that conclusion we turn to a representation of the transition states in the transformation of m- to o-xylene and p-xylene, respectively. One sees in Fig. 9 that the minimum kinetic diameter of the transition state is smaller for the m--,p xylene transition than for the m~o xylene transition. Given the identical energies of activation and the identical heats of adsorption of all the isomers the larger kinetic diameter of the m ~ o xylene transformation in the transition state is concluded to result in fewer possible configurations in the MFI channels (lower entropy in the transition state). Note that this is expected for many transformations involving shape selectivity and indeed similar effects have been observed for hydrocarbon conversions [257]. Blocking the pore mouth and reducing the diffusivities of the xylenes does not change this overall picture for toluene methylation, but enhances the p- selectivity [258]. As a negative side effect the catalysts deactivate and this has to be balanced with higher reaction temperatures. The higher reaction temperatures are required to open new reaction channels (dealkylation, transalkylation, disproportionation) to drain products from the pores as the longer residence times lead to polymethylated products that are unable to leave the zeolite pores and would eventually block all acid sites [258]. Shape selectivity is not confined to reactions of hydrocarbons in the absence of polar functional groups. MFI type materials have been reported to catalyze the isomerisation of cresols, chlorotoluenes, toluonitriles and toluidines [259]. In the isomerization of aniline derivatives the reaction temperatures have to be relatively mild as under severe reaction temperatures isomerization to methylpyridine would occur [260]. For dimethylanilines it could be shown that only the isomers with the smallest minimum kinetic diameter reacted (reactant selectivity), and that those with a larger kinetic diameter did not form (product selectivity). The isomerization is concluded to occur via a 1,2 methyl shift which is interpreted to indicate transition state selectivity [261 ]. While many of the shape selective processes reported concern small and medium pore zeolites, the synthesis of fine chemicals and intermediates requires larger pore molecular sieves. In this respect shape selective conversions have also been reported for MOR and BEA type catalysts. For example 4,4'-diisopropylbiphenyl, an intermediate for liquid crystals, can be produced from propene and biphenyl in high yields over dealumi0at.ed mordenite [262]. The parent mordenite and other 12 membered ring zeolites such as HY, HL or H-offretite gave poor results. The active mordenite was obtained by severe acid leaching, resulting in a catalyst with a Si/A1 ratio of 1300 and large mesopores. The post synthesis modification converted the mono dimensional channel structure of MOR into a three dimensional structure, in which the micropores are connected by micro and mesopores generated in the leaching procedure. It is concluded that the mesopores are indispensable for efficient mass transport of the bulky molecules and that dealumination reduces coke formation and unselective alkylations, i.e., in the para-meta positions. The recently commercialized process for the synthesis ofcumene and p-diisopropylbenzene from propene and benzene uses the same catalysts [263]. 2-Methylnapthalene (2-MN) and 2,6-dimethylnapthalene (2,6-DMN) can be selectively produced by isomerisation and disproportionation of 1-methylnapthalene (1-MN) or by direct alkylation of naphthalene. The observed reactivity for the isomerisation of 1-MN and 2-MN over HZSM-5, HZSM-11, H-BEA, H-MOR and H-Y led Weitkamp et al. [264] to propose that product shape selectivity dominates, while Fraenkel et al [265] suggest that cavities at the external surface containing strong Brrnsted acid sites are responsible for the selectivity. That idea has been followed up by Derouane proposing nest like structures to be important [266 ]. Intuitively this is intriguing, as the situation resembles the coordination

400 in an enzyme, however, with a much lower flexibility of the inorganic lattice in comparison to the organic macromolecules. While the role of the external and internal acid sites is not completely resolved the combination of silylation and poisoning experiments for xylene isomerization [267] suggest that external acid sites do not markedly contribute to the reaction pathway for such large molecules. A special case of shape selectivity should be mentioned at the end of this section, i.e., the formation of adamantane from 1-hexene in the pores of SAPO-34 [268]. The intra crystalline cavities in the chabazite structure seem to be especially suited to catalyze adamantane formation with high selectivity at temperatures of around 500 K. However, the narrow pore openings do not allow the product to leave the catalyst. High concentrations of adamantane were isolated by dissolving the molecular sieve in HF.

4. CONCLUSION AND OUTLOOK The examples discussed demonstrate that the utilization of molecular sieves as catalysts in free chemical and intermediates synthesis has been advanced dramatically over the last decades. The community has reached a sound level of understanding of the catalytic chemistry and the options to manipulate it. While also the limitations of molecular sieves are now well understood there is still a large number of molecular sieves to be explored. Some trends observed over the last years are noted below. What has to be noted first is that the number of applications for a few molecular sieves is high, however there exist many more structures that currently have no application. The widely employed molecular sieves include various forms of faujasites, mordenites, zeolite BETA, ZSM5, TS-1, zeolite L and to a lesser degree S(Me)APO5. For the other molecular sieves examples of utilization are quite scattered and are mainly confined to comparative studies. This suggests that a move to more catalytic chemistry and less material oriented approaches is required. It has to be critically noted that the quality of the materials used often varies quite substantially and this makes it difficult to derive genuine struc.tureactivity correlations. Another consequence of the rather limited number of molecular sieves that are used, is a high emphasis is being placed on post synthesis modification of molecular sieves. Especially, the introduction of a secondary pore structure (such as achieved for mordenites [263]), the deactivation of outer crystal surface and the adjustment of the acid strength by selected ion exchange procedures are examples for that approach. As an extension of the approach to modify zeolites the incorporation ofmetalorganic complexes and chiral modifiers in molecular sieves finds now increasing use. While the developments are extremely exciting, limitations by the space constraints inside the micropores might prove to be an obstacle for many applications. The rapid pace developments in the area, however, encourages us to expect that industrial exploitations of such catalysts will soon be seen. Especially with (stabilized) molecular sieves having larger cavities such as cloverite, radically new intrapore chemistry can be foreseen. As the fine chemicals that need to be converted are frequently larger than pores of conventional zeolites, the motivation to seek for ultra large pore materials is rather high. Mesoporous materials such as MCM-41 that were recently discovered do offer such large pores, but the inherent advantage of molecular sieves, i.e., the fact that the acid site is an integral part of the catalyst lattice, is lost with these materials. Synthesis of an interesting

401 new material (MCM-36) has been recently reported that contains zeolite layers (from the precursor of MCM-22) and oxide pillars that provide a mesoporous structure. Such combinations allow to combine the local tailoring of acid sites in the zeolite layer and the accessibility of a mesoporous material. Finally, the use of computer modeling is seen to rapidly increase. This growth is likely to continue or accelerate and chemical synthesis strategies should benefit markedly from it. It can be expected that at first especially shape selective application will profit and it will still be largely a visualization technique to understand how a reactant/product molecule adapts and fits into the microporous environment. The challenge on theoretical chemistry will be how to predict reactivity patterns and molecule transformation inside these pores in order to be able to model the chemical behavior. Acknowledgment The authors are indebted to Dr. G. Eder-Mirth for invaluable discussions and editing of the manuscript and Mr. Bert Geerdink for providing graphical illustration. Funding from the Austrian Science Foundation (FWF), the Christian Doppler Laboratories and Dutch Science Organization (NWO) for studies on molecular reactivity in zeolite catalysts is gratefully acknowledged.

402

Table 1. A short compilation o f the zeolites and their isotopical framework structures currently employed. Molecular sieve type

Structure type code [269]

Pore size largest channel (A)

Channel Dimensions

Small pore Linde type A

LTA

4.1

3

Erionite

ERI

3.6 x 5.1

3

Chabazite

CHA

3.8 x 3.8

3

Isotopic framework structures

LZ-215 N-A Gallop hosphate-A Sapo-~,2 Alpha Gallogermanate-A ZK-4 ZK-21 ZK-22 AIPO-17 LZ-220 Linde T CoAPO-44 CoAPO-47 SAPO-34 LZ-218 Linde D Linde R MeAPO-47 MeAPSO-47 Willhendersonite ZK-14 ZYT-6 P s,K)-ZK-5

ZK-5

KFI

3.9

3

Rho

RHO

3.6

3

LZ-214 Pahasaoaite Synthetic _pahapasaite Be-arsenate rho

Medium pore ZSM-5

MFI

5.3 x 5.6

3

ZSM-11

MEL

5.3 x 5.4

3

Ferrierite

FER

4.2 x 5.4

2

ZSM-23

MTT

4.5 x 5.2

1

Yheta-i

TON

4.a x 5.5

1

AIPO.~-I 1 Heulandite

AEL HEU

3.9 x 6.3 3.0 x 7.6

1 3

Silicalite-1 TS-I Boralite D Silicalite-2 TS-2 NU-23 ZSM-35 FU-9 ISI-6 Monoclinic ferrierite Sr-D EU-I 3 ISI-4 KZ-1 ZSM-22 NU-10 ISI-I KZ-2 SAPO-I I Clinootilolite LZ-2 I9

ALPO-31

ATO

6.5

1

~

403

:lable 1. continued.

Large pore Faujasite/X Y

FAU

7.4

Beta Mordenite

BEA MOR

7.6x6.4 6.5x7.0

Offretite

OFF

6.7

Mazzite

MAZ

7.4

Linde Type L

LTL

7.1

ZSM-12

MTW

AIPO4-5

AFI

Hex.aggnal aujas~te

EMT

5.5x5.9

7.3

SAPO-37 Linde X Linde Y Be-p.hosp.hate X n-ptaospnate X LZ-210 ECR-30 ZSM-20 ZSM-3 CSZ-1 Tschemichite Na-D Ca-Q Zeolon Large.po.rt moroenlte Linde-T LZ-217 TMA-0 Omega ZSM'4 LZ-202 Gallosilicate mazzlte Gallosilicate L Perlialite LZ-212 Theta-3 Nu-13 CZH-5 TPZ-12 SAPO-5 SSZ-24 ECR-30 ZSM-20 ZSM-3 CSZ-1

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H. Chon, S.I. Woo and S.-E. Park (Editors) Recent Advances and New Horizons in Zeolite Science and Technology Studies in Surface Science and Catalysis, Vol. 102 9 1996 Elsevier Science B.V. All rights reserved.

413

ZEOLITE-BASED MEMBRANES PREPARATION, PERFORMANCE AND PROSPECTS M.J. den Exter, J.C. Jansen, J.M. van de Graaf, F. Kapteijn, J.A. Moulijn and H. van Bekkum Waterman Institute of Chemical Technology, Delft University of Technology, Julianalaan 136, 2628 BL Delft, The Netherlands

1. I N T R O D U C T I O N Separation processes are widely used in industry. Chemical conversions often run incompletely as dictated by the thermodynamic eqt~bfium or by the wish to obtain high selectivity, which may require relatively low conversion or the application of one of the reactants in excess. Separation methods include distillation, crystallization, centfifugation, extraction, adsorption and membrane techniques. Interest in combining reaction and separation is growing. Thus industrial examples exist of reactive distillation and reactive crystallization. In this respect catalytic membranes [1] and membrane reactors come to the fore and are worldwide studied. Membranes are thin films or layers of material that allow selective passage of one or more components of a gaseous or liquid mixture. Membranes can be classified [2] according to the driving force (concentration or pressure difference) that causes the flow of the permeate through the membrane, or to the material(s) they consist of (organic polymers or inorganic materials). For both type of materials the membrane can be dense or porous. Often combinations of layers are applied in which one thin (< 1 mm) layer is the true separation layer. Examples of dense membranes are palladium foils (selective passage of hydrogen), and films of organic polymers such as polyvinyl alcohol (selective passage of water). Extensively studied is oxygen permeation through dense ceramic membranes (e.g. perovskites). Temperatures > 600 ~ are applied. Here, oxygen splits at the surface and is transported as 02. Porous membranes include porous polymer films (cellulosics, polyamides) as well as amorphous inorganic materials (alumina, silica). For dense and porous membranes somewhat different molecule permeation principles apply; dissolution/diffusion and adsorption/diffusion, respectively. Figure 1 illustrates gas flows through macro-, meso- and microporous membranes and through dense membranes (from top to bottom, respectively). Organic and inorganic membranes have in common that the membrane materials can be selected ranging fi'om hydrophilic to hydrophobic. For dissolution and adsorption the simple rules "like dissolves like" and "like adsorbs like" apply. Porous and especially microporous membranes are able to separate components of a mixture on the size of the molecules. When molecules are simply too bulky to enter the largest pores of a membrane, absolute separation can be achieved.

414

Figure I Mechanisms for permeation of gases through porous and dense gas-separation membranes In case both components of a binary mixture have access to the micro-pores, adsorption strength or diffusion rate is-dominating the separation in the lower and higher temperature region, respectively. Inorganic or ceramic membranes have some disadvantages over polymer membranes (e.g. brittleness and higher cost). Therefore, the use of these membranes is especially directed to applications in which polymer membranes cannot, or only with dit~culty, be applied. In such applications high-temperature (> 200 ~ stability and resistance against extreme conditions (thermal cycles) are required, and it is expected that from these properties the life-time will be longer as regeneration at high temperatures will be possible. As to ceramic membranes [3,4] the focus has been so far in particular on amorphous porous aluminas and silicas. Other inorganics studied include titania, zirconia, non-oxide ceramics (carbides), and microporous carbons. Zeolites as crystalline membrane components can add the advantages or2 (i) strictly defined pore structure and size, (ii) given a particular zeolite structure the possibility of tuning the pore wall character by the Si/A1 ratio and by ion exchange, ('tii) known methods of introducing catalytic entities on the way to catalytic membranes, (iv) several ways of outer surface modification. The pore architecture ofzeolites is unique for each type ofzeolites and can be: - onedimensional (parallel pipes) - twodimensional - threedimensional

415

Figure 2 Zeolite-based membrane configurations It will be clear that in case ofonedimensional zeolites the orientation of the zeolite crystals should be such that the channel direction is perpendicular to the membrane layer configuration. Figure 2 shows in a schematic way several configurations in which zeolites govern - or contribute to membrane permeation. Ad Figure 2.1,2. Zeolite layers can be grown by hydrothermal synthesis onto porous supports (clay, alumina, sintered metal). Especially layers of MFI-type zeolite have been studied [e.g. 5-7]. Such MFI-layers were shown to survive template removal and subsequent thermal cycles up to 350 ~ which is taken as a strong indication for chemical bonding [8] at the support interface. To understand chemical attachment to metals one has to take into consideration that metals - by exposure to air - will be covered with a thin (1-2 nm) oxide film. Sometimes an intermediate mesoporous layer has been applied, e.g. a metakaolin film on clay or on zirconia [5] or metal wool on sintered metal [6]. No adherence exists when a zeolite layer is deposited onto a teflon [9] or a carbon support. Here, binding possibilities between the zeolite layer and the support are lacking. Ad Figure 2.3,4. Another approach is to apply a dense support (e.g. stainless steel) equipped with a regular perforation. The in situ growing of zeolite then aims at a zeolite layer coveting the whole support or at zeolite growth in the openings of the support. For an example see Section 3 of this chapter.

416 Ad Figure 2.5. This system is in a way similar to method 4; the pores of a mesoporous support are filled with zeolite by inside crystallization. Generally several crystallizations will be necessary to achieve essentially complete leak-flee filling. Ad Figure 2. 6. A fundamentally different system is to load a polymer film (e.g. a siloxane) with zeolite crystals. Especially the Twente group studied such composite membranes. The zeolite crystals then just contribute to the permeation by acting as selective reservoirs of components. Hydrophobic (silicalite-1) as well as hydrophilic (zeolite A) zeolites have been studied in such a configuration. Zeolite single crystals may serve as zeolite membrane models. We mention the early elegant work of Hayhurst and Paravar [ 10] on an oriented large silicalite-1 crystal embedded in epoxy resin. In a more recent membrane model study [ 11] a large (twinned) silicalite-1 crystal (100 * 100 * 300 ~tm) was embedded in an epoxy matrix, using an aluminium gasket as a support. Polishing improved the crystal surface exposure. Micropore diffusitivities were measured for benzene, toluene and the xylenes. This work has been extended towards arrays of individual oriented large silicalite-1 crystals in a gas tight matrix (Figure 3).

Figure 3 Schematic view of zeolite crystals, embedded within a mesoporous interlayer Thus Geus et al. studied oriented silicalite-1 crystals in an expoxy matrix on perforated metal with each crystal fully coveting one hole of the metal support [12]. Geus also investigated [13] monolayers of silicalite-1 crystals on macroporous a l u m ~ covered with a thin clay film. Zeolite surface coverages of-- 75% were achieved (Figure 4). The matrix was introduced by a sol-gel technique (silica), by CVD (zirconia), or by deposition of low-melting glazes. To obtain a crackfree thermostabile layer appeared to be difficult. Caro et al. [14] succeeded in embedding silicalite-1 crystals in a metal (Ag or Ni) matrix. The zeolite crystals are brought into a silver-coated glass plate acting as a cathode in a galvanic Ag or Ni bath. The metal matrix is grown electrochemically between the crystals until the top of the crystals is reached. By a slight overgrowth of the crystals by metal additional mechanical stability is obtained.

417 The membrane served to study competitive permeation ofheptane and toluene.

Figure 4 SEM-picture of fluoride-synthesized silicalite prisms, embedded as a monolayer in a thin clay film on a one layer a-alumina support Following the above introductory remarks a discussion will be given on the dynamics of zeolite pores and the access of molecules into them. Then the state of the art of zeolite-based membranes will be overviewed in the sequence: - small pore zeolites - medium pore zeolites - large pore zeolites Section 6 will deal with theory and practise of permeation through zeolite membranes, and finally examples will be given of the use ofzeolites in membrane reactors and catalytic membranes. 2. D Y N A M I C S O F Z E O L I T E P O R E S A N D C O N S E Q U E N C E S F O R ADSORPTION AND PERMEATION The crystallographic pore dimensions of zeolites in the as-synthesized form are generally known with precision [ 15]. Prediction of separation on size of molecules using these zeolite dimensions requires some caution because of the following variables temperature (i) geometric changes due to the activation of the zeolite (ii) (water removal, template calcination) (iii) effects of the cations due to number, size and location phase transitions of the zeolite lattice and adaptation of the (iv) lattice to adsorbates ad (i) Obviously the temperature is an important variable in the adsorption of molecules into zeolites and their diffusion through the pores. Increasing temperature means higher rates of diffusion and lower loading by adsorbates. Moreover, for molecules that are boundary cases due to their size - higher temperatures may give access to the zeolite pores whereas lowering the temperature may establish complete exclusion. ad (ii) Table 1 lists the pore dimensions of eight-membered ring zeolites A and RHO, together with those of the clathrasil DD3R:

418 Table 1

Zeolite type

NaA KA CA RHO DD3R

Pore apertures of eight-membered oxygen ring systems Pore aperture (A) based on O-ring 4.3 4.3 4.3 3.6 3.6

* 4.3 * 4.3 * 4.3 * 3.3 * 4.4

Pore aperture (A) including cation

Pore form

4.1 3.2 4.3 2.3 (dehydr.)

Circular Circular Circular, rigid Circular, flexible Ellipsoidal, rigid

Zeolite RHO changes its pore structure profoundly upon activation. As shown in Figure 5, the neighbouring circular 8-membered tings deform into ellipsoidal shaped apertures that are oriented perpendicular with respect to each other. ~1~

02

01

Figure 5 Structure detail of zeolite Na, Cs-RHO a), the circular pores in the hydrated structure and b), the ellipsoidal pores in the dehydrated structure [ 16] The resulting aperture in projection changes from a diameter of 3.6 A into a reduced opening of 2.3 A in the activated form. The framework deformation is most probably caused by the Na-

419 and Cs-ions that, in the dehydrated form, are coordinating with oxygen from the pore wall. A substantial reduction in the expected amount of n-alkane adsorption based on the pore volume is observed [ 17]. While n-butane as well as n-hexane are 4.3 A in kinetic diameter, which is too large to pass the 2.3 A and the 3.6 A aperture of RHO, they are apparently adsorbed by deformation of the double eight ring from ellipsoidal via circular to a larger pore opening in order to allow the guest molecule to migrate into the pores. ad (iii) As is well-known, the cations in zeolite A can partially block the windows (cf. Table 1). In the parent zeolite NaA the maximum number of (univalent) cations are present and the same holds for KA in which system 45% of the Na-ions have been exchanged for the larger K. In CaA (80% of Na exchanged) the cation number is reduced and sufficient windows are not accompanied by a cation. These eight-membered ring windows are fully available for passage, e.g. of linear organic molecules. ad (iv) Table 2 shows pore dimensions for medium pore zeolites. ZSM-5 (MFI) is known to undergo a phase transition at ~ 80~ from monoclinic to orthorhombic, causing the shape of the channels to change slightly from ellipsoidal to nearly circular. More profound changes are observed upon adsorption of fiat aromatic molecules. Table 2

Pore apertures often-membered oxygen ring zeolites

Zeolite type

Pore dimensions (A)

MFI (straight channels only) ZSM-11 ZSM-22 ZSM-23 ZSM-50 Ferrierite

5.6 5.8 5.4 5.5 5.2 5.7 5.4

* 5.3 (orthorhombic) * 5.2 (monoclinic) * 5.3 * 4.4 * 4.5 * 4.1 * 4.2

pore form Flexible: from circular to ellipsoidal (3.8 * 7.4 A) Ellipsoidal Tear drop Ellipsoidal Ellipsoidal

It is clear that in the case of MFI, the zeolite pore entrances should not be considered as rigid apertures. Instead, zeolite framework topologies can show flexibility. While the O-Si-O angle in the tetrahedral unit is rigid (109 + 1 o), the Si-O-Si angle between the units can vary between 145 and 180 ~ Based on isomorphous substitution of Si by other T-atoms in the framework [18], framework defects [19], cation positions, changes in the water content [16], external forces on the crystalline material [20] and upon adsorption of guest molecules [21] phase transitions can occur that have a dramatic influence in particular cases on the framework atom positions. As shown in Table 3 for the straight channels of MFI, p-xylene, of which the kinetic diameter is actually larger than the pore aperture diameter, can be accomodated by deforming the pore from circular to ellipsoidal. This phenomenon should be taken into account when studying the separation of xylenes with MFI-frameworks in a membrane configuration. It has recently found [22] with single crystal structure analysis, that even molecules as large as naphthalene, with a kinetic diameter of 3.8 * 7.4 ~ can be accomodated in the pores of the MFI-framework, see Table 3.

420 Table 3 Pore dimensions of the straight channel ofMFI (Silicalite-1) containing different aromatic molecules Guest compound

none p-xylene naphthalene

Guest Kin. diameter (A)

5.8 * 3.8 7.4 * 3.8

Host/Guest combination Pore diameter (A) straight channel 5.6 * 5.3 6.2 * 4.8 6.4 * 4.6

Ref.

[ 15] [21] [22]

In particular in the case of naphthalene, the pore apertures deform from a circular to a highly ellipsoidal shape upon adsorption and permeation. Next, this peristaltic behavior of the pore deformation is completed by the partial return of the eUipsoidal into a more circular aperture as the molecule is stabilized between the apertures. This flexibility of the framework of MFI explains why the o-/m-xylenes (3.8 * 6.3 A) can be adsorbed as well. A cascade mode of separation for the xylenes will be required as the separation is typically diffusion controlled. A full comparison of the two discussed zeolite structures and the adsorption properties based on parameters influencing the zeolite pore dynamics are given in Table 4: Table 4 Specifications, parameters of relevance to the deformation of framework, framework dynamics and adsorption properties of zeolite-types MFI and Rt-IO MFI

RHO

>300 hydrophobic none TPA D10R

2 hydrophilic Na, Cs water D8R

specifications: Si/A1 water interaction cations loss upon calcination aperture parameters: cation coordination guest molecule framework dynamics: pore chape change upon adsorption of hydrocarbon adsorption properties: guest molecule

circular to ellipsoidal (mutual perpendicular)

full loading

ellipsoidal to circular (mutual perpendicular)

partial loading

421 Apparently, in both zeolite structures the pore deformation that has to be achieved to adsorb guest molecules is caused by the collisions of these molecules with the pore entrances. In the case of the MFI-type zeolite, p-xylene as well as naphthalene were loaded to the maximum population, which is 8 and 4 molecules/u.c., respectively, in the pores. Thus the attractive forces of the guest molecule with the pore wall are larger than the energy costs of the framework deformation, inducing an ellipsoidal pore and not maintaining the circular pore shape. In the case of zeolite RHO, no full loadings of alkanes were obtained. Thus the guest molecules can just partly distort the pore shape. Here, the counterforces to partly maintain the apertures in the ellipsoidal shape, generated by the coordination of Na § and Cs § to the oxygen atoms in the zeolite wall, are probably larger than in the silicalite-1 framework, resulting in a reduced loading (<50%). As illustrated on two completely different zeolite types, the framework dynamics can drastically change the pore aperture. In conclusion, separations foreseen, must also be based on the possible framework flexibility upon guest molecule adsorption, and not only on crystallographic diameters of as-synthesized or even calcined/activated zeolite structures in some of the frequently studied zeolites.

3. S M A L L P O R E Z E O L I T E S 3.1 General

Small pore (6- and 8-ring) zeolite-based membranes might be used in industrial processes involving hydrogen, in air separation or in separation of linear and branched alkanes. Applying small pore apertures might lead to high separation/selectivity. In the future, the hydrogen demand is expected to increase in petrochemical plants and hydrogen might be used as a clean fuel for transport. Therefore, more attention is focussed on the selective removal of hydrogen from gas streams. The production of hydrogen is roughly divided into three main production methods [23]: - Pressure swing adsorption (PSA) - Cryogenic separation - Steam reforming In PSA-based production, a purity of hydrogen up to 99.9999+% can be obtained by removal of impurities over various adsorption beds. The technique of cryogenic separation is a method often used for the recovery and purification of hydrogen in refineries and petrochemical plants with a purity of 95%. With the aid of this technique, crude ethylene, liquified petroleum gas (LPG) and substitute natural gas (SNG) can be produced as well. Purge gases, reused as fuel until some years ago, are nowadays used as feed for cryogenic purification [23-26]. Steam reforming is applied for the production of hydrogen for refinery hydrotreating and hydrocracking. A purity up to 99.9+% can be reached [23]. The latter produces hydrogen but is not really a separation technology. The expected increase in demand is too large to be accommodated by the refinery industry [27] which could recover hydrogen from offgas streams or expand existing hydrogen-production fa"ctlities. At the long term, solar radiation may be applied for direct or indirect water splitting. Membrane separation, due to the ease of (continuous) operation and low (energy) costs can play its part in the recovery of hydrogen. In Table 5, the advantages and disadvantages of some inorganic membranes for separation of hydrogen are listed.

422 With ceramic membranes (showing Knudsen diffusion) acting on e.g. hydrogen in the presence of a hydrocarbon or of carbon dioxide, the theoretical separation factor amounts to 4-6. For industrial processes, these values are too low [4]. Modification of ceramic membranes and supports by deposition of new materials improves the separation markedly. With controlled modification of thin amorphous silica-layers on ceramics, membranes can be obtained showing separation values upto 150 for Table 5

Comparison of Palladium, Silica-CVD densified, and Zeolite Membranes Palladium

Silica-CVD

Zeolites

Pos.

-Very selective for 1-I2 -Possible catalytic activity -Resistant to acids

-Possibility of densifying -No removal of solvent necessary

-Narrow pore size distribution -Resistant to acids -Thermal stability -Many zeolites to choose from for specific appl.

Neg.

-Expensive -At high temp. migration of Pd particles

-Wide pore size distribution -The structure densities with water

-Difficult gas tight growth on surfaces (pinholes) -Difficult upscaling

hydrogen / butane [4]. Application of such membranes is, however, limited to process temperatures upto 200~ while their chemical resistance needs to be improved. In order to achieve high selectivities with thermostable zeolite-based membranes, zeolites can be choosen with pore apertures matching the kinetic diameters of the molecules to be separated. Moreover, the hydrophobicity of all-silica zeolites provides continuous separation, independently of traces of water in the gas streams applied. In the total spectrum of tectosilica(te)s there is only one all-silica 8-ring system: Deca-dodecasil 3R (DD3R) but several all-silica 6-ring systems (Table 6). 3.2 All-silica s y s t e m s clathrasils The clathrasils form a sub-group within the total group of tectosilica(te)s. In contrast to zeolite frameworks, the clathrasils are marked by a connection of cages through a single window. Guest molecules (templates) are entrapped in the cages during the synthesis. Except for Decadodecasil 3R (DD3R), marked by a connection of the cages through 8-rings, all clathrasils have 6ring pore apertures [28-32]. Due to these small windows, template molecules cannot leave the framework. After calcination, high temperatures will be required to prevent that organic residues remain present inside the structure. In Table 6, the six families of clathrasils with topologically different frameworks are listed. The six-membered ring of Si-O-Si bonds has a pore width of 2.8 A. This makes the group of clathrasils interesting materials for the separation of small gases like hydrogen from a diversity of gas streams.

423 Table 6

Specifications of clathrasils

Clathrasil

Crystal Symmetry

Melanophlogite

cubic

Volume and Number of cages per unit cell* 2[512]

6151262 ]

1.89

160 A3 8151264] 250 h 3

1.86

Dodecasil 3C (D3C)

cubic

Dodecasil 1H (D1H)

hexagonal

70 A 3 161512] 70 A 3 3[512]

Decadodecasil 3R Nonasil

rhombohedral orthorhombic cubic

70 A3 50 As 61435661] 9[512] 35 A 3 70 A 3 8[5464] 81415s] 25 A 3 30 A 3 21466s]

(DD3R)

Silica-sodalite (SOD)

Framework density (g/cm3)

2[435663]

115126s]

1.84

430 A 3 61435126183] 1.76 350 A 3 415s612] 1.94 290 A 3 1.74

The values between brackets denote the various windows in one cage, the value before the bracket denotes the number of the particular cage in one unit cell The ldnetic diameter of hydrogen, calculated fi'om the minimum equilibrium cross-sectional diameter [33] amounts to 2.89 A. Therefore, a high selectivity for hydrogen might be expected. According to Grebner et al. [34], only small gases like helium, hydrogen, neon and ammonia are able to pass a six-membered ring. DD3R is the only 8-membered all-silica structure known. The template used for synthesizing DD3R (1-adarnantanamine) can be removed completely. DD3R's capability of adsorbing small gases is comparable to other zeolites with 8-membered pore apertures like zeolite A [36].Thus, its eUipsoidal pore size of 4.4 x 3.6 A, which matches the kinetic diameters of most small gases closely, makes separations between branched and linear molecules possible. D1H

In Figure 6 a SEM-image is showing the hexagonal-shaped morphology of D1H crystals, formed using 1-adamantanamine as the template. The channels run parallel and perpendicular to the crystal surface. The crystals were obtained using the following molar composition: tetramethoxysilane: 1, 1-adamantanamine: 0.47, ethylenediamine: 4.2 and water: 55. A 30% yield of large crystals can be obtained after heating the synthesis mixture for 20 days at 190~ (Figure 6A). This optimized procedure supplies crystals with the most pronotmced morphology. In order to decrease the long time needed for obtaining a yield of 30%, an ammonia-based synthesis procedure has been developed by the present authors, supplying the same amount of crystals in half the time. In a membrane approach the following molar composition was applied in the presence of a perforated stainless steel support:: aerosi1200: 1, 1-adamantanamine: 0.79, ammonia: 39 and water: 86 (190~ 11 days).

424

Figure 6 A: DIH crystals (crystal size : 2001am), B: stainless steel support (pore size : 60pm-7001am, Stork Veco B.V., The Netherlands), C and D: D1H grown on support, C: upper layer, thickness: 100~rn and D: bottom layer of D1H crystals The obtained layer shown in Figure 6, C and D was synthesized by repeating the synthesis after 14 days for another 6 days in order to obtain a complete coverage of the support. Permeation experiments in a Wicke-Kallenbach cell on the non-calcined layer unfortunately showed the existence of pinholes between crystals (Table 7). Gases were measured at ambient pressure and temperature without a pressure difference over the membrane. At the permeate side, argon was used as a sweep gas. The diffusion of butane reveals the existence ofintercrystalline pores in the membrane layer. As can be seen from the table, the back diffusion of argon through pinholes decreases when stronger adsorbing spedes are used. This can be ascribed to condensation of the gas in the intercrystalline pores, resulting in pore blocking for argon. Study on the growth of D1H on supports showed the difficult reproducability of such membranes. In conclusion, improved growth methods should be developed.

425 Single component permeation on the non-calcined D 1H layer

Table 7

Gas

Permeation ( 10"12mol.m/m~.s.Pa)

Hydrogen Methane Butane

Permeation (10 12 mol.m/m2, s.Pa)

16.0 3.8 0.5

a (Flux H2 / Flux other)

88.5 60.0 47.4

1 4.3 32.1

9counter diffusion of argon Dense silica-based membranes might be an alternative. A disadvantage of such membranes is the sensitiveness for water while their rather low thermostability can lead to crack formation above 200~ DD3R Adsorption details of calcined DD3R crystals are listed in Table 8 [36]. As can be seen from Table 8, a complete separation between linear and branched alkanes can be achieved. Isobutane is excluded from the pore structure of DD3R. The very small adsorption observed is ascribed to extemal surface or intercrystalline sorption. With an increase in carbon number of the adsorbate, the heat of adsorption should increase. Therefore an increase in the adsorbed amount from methane to butane would be expected. This trend can only be seen with methane and ethane, both being in equilibrium with the gas phase within the chosen equilibrium time of one hour. Propane and butane adsorption is markedly lower than expected which is due to the non-eqtfilibfimn situation. Sorption specifications of gases on DD3R at 25~

Table 8

gas

(A) 2

methane

(3.8)

0.70

carbon dioxide

(3.3)

7.68

ethane

(3.8)

4.11

dinitrogen oxide

(3.3)

8.79

propane

(4.3)

0.55 #

ammonia

(2.6)

7.55

butane

(4.3)

1.10#

oxygen

(3.46)

0.57

isobutane

(5.0)

0.15 *

hydrogen

(2.89)

0.003

ethylene

(3.9)

3.73

nitrogen

(3.64)

0.34

propylene

(4.5)

2.55 #

water

(2.65)

1.50 a

1,3-butadiene

(4.2)

9.32 #

wt% (101.3 kPa)

gas

(A) 2

wt% (101.3 kPa)

2: Kin diameter, calculated from the minimum equilibrium cross-sectional diameter [33] "I'~q= 1 hr. extemal/mesoporous sorption, #: Equilibrium not obtained, 1:24 Torr

426

Both molecules are adsorbed, at the temperature applied, at a much lower rate than methane or ethane. The kinetic diameters of propane and butane are larger than that of ethane, due to the curvature of a molecule with three or more carbon atoms, and matches the pore size of the adsorbent more closely, restraining the entering of the pore system. The adsorbed amount of propylene was found to be higher than that of propane, similar as for the C4-adsorbates while the butadiene adsorption was found to be higher than the propylene adsorption. The difference in adsorption behaviour at room temperature is explained in terms of hybridization of the carbon atoms and consequently molecular size. Butadiene contains sp2 hybridized carbon atoms only, in contrast to propylene which contains two spz and one sp3 carbon atom. The cross-section of a methyl group is circular while that of asp z methylene group is more elliptical. Butadiene can therefore probably adapt more easily to the size of the pore aperture than propylene. Besides, the angle of a C-C-C bond changes from 112.5~ to 120~ when a double bond is present, therefore decreasing the curvature of the molecule. In Figure 7, the difference in adsorption rate (the amount adsorbed Q at a certain time divided by the total amount adsorbed derived from the isotherm) between ethylene and propylene is shown. In our group a comparison has been made between the adsorption of some gases (carbon dioxide, ammonia, ethane and ethylene) and water (measured in equilibrium with the vapour pressure at 93.1 kPa and 25~ for the aluminosilicate Na-A [37,38] and the all-silica DD3R.

1.20

- carbon dioxide

o ethylene 8 0

o propylene 0.80

0

E 0

~

O.40

0.00 -

'

0

2

.

'

' 4

'

.

_

,

_

.

.

_

,

.

,

-

' 6

-

-

~

'

' 8

10

~ T i m e (min)

Figure 7 Adsorption rates on DD3R (0.85 atm, 25~

In all cases Na-A, which has a reported pore aperture of 4.1 A, adsorbs larger amounts than DD3K The SiO4 and A104 tetrahedra in zeolite A are linked together to form two types of cavities, one large cavity (diameter ~ 12A) with a 4.3 A pore aperture (a-cage) and a small cavity (13-cage) with a pore aperture of 2.4 A. The diameter of the window without cation (8-membered ring) is aproximately 4.6 A. The presence of sodium ions inside the framework reduces the size of the pore aperture to 4.1 A. The total void

427 volume of Na-A, calculated from the density after drying (1.27 g/cc [38]), amounts to 45 vol% (0.29 ml/g for the or-cage and 0.06 ml/g for the 13-cage respectively). The much higher adsorption of water is understandable considering the presence of the cations of zeolite NaA (Si/A1 = 1) and the fact that water can diffuse into both cages [38]. Ethane, ethylene and carbon dioxide can only diffuse into the large or-cage. For ethane, adsorption values of 4.04 wt% for DD3R were found while Na-A adsorbs 7.4 wt% at similar conditions (700 Torr, 25~ so DD3R adsorbs 54.6% of the amount of ethane adsorbed on zeolite A. The accessible volume of DD3R is 0.48% compared with zeolite A, so based on the void volume, both materials show comparable adsorption of a non-polar molecule. The adsorption of ethylene amounts to 43.5 %, compared to zeolite A (3.65 wt% and 8.4 wt%, respectively). The slightly higher preference of zeolite A for an alkene can be explained in terms of stronger interactions of an unsaturated hydrocarbon with the ionic nature of the zeolite A surface. Calciumexchanged type A zeolites were found to exhibit the same behaviour for propane and propylene

[38]. The adsorption of carbon dioxide on DD3R amounts to 40.5 % of the adsorption on zeolite Na-A (7.62 wt% and 18.8 wt% respectively). It is clear that the preference of zeolite Na-A for adsorption of polarizable gases is larger than that of DD3R. Charged fi'ameworks as well as the presence of cations in the framework structure can induce dipoles within the adsorbate molecule, resulting in stronger interactions with the adsorbent. In gas streams containing small traces of water, it was found that at temperatures up to 100~ a large part of the adsorbing volume of zeolite Na-A is filled with water while DD3R adsorbs water in a very small quantity, leaving much of its pore volume accessible for other gases. The distinction between a very hydrophilic and a hydrophobic surface is dear in terms of the large difference in the adsorbed amounts of water. Up till now, no insitu growth of DD3R on supports or membrane syntheses has been reported. It is to be noted that the synthesis of this clathrasil is marked by a fixed procedure [36], making changes in concentrations or temperature, in order to study the growth on supports, difficult. Zeolite A A-type zeolites show high molecular sieving effects and are widely used as adsorbents in adsorption and separation processes. Industrially, zeolite CaA is used in the UOP Molex and Isosiv processes for the separation of normal and branched alkanes. Therefore, this hydrophilic 8-ring zeolite is a logical candidate for application in zeolite-based membranes. The available literature, however, is scarce. Zeolite A films with a thickness of 35-50 lain have been achieved on alumina cerarrilc filters [39] by repeated hydrothermal synthesis. Thinner A-type layers (7 ~tm) could be grown on polyethylene, glass and poly(tetrafluoroethylene) substrates [40]. A small amount of an unidentified phase co-crystallizes in the latter case, leading to non-pure layers. Jansen et al. [41] have studied the growth of zeolite A on rutile, sapphire and quartz. No permeation data are known on in situ grown layers of zeolite A. A patent [42] describes the preparation of self-supporting membranes, consisting of aggregates of randomly oriented zeolite A crystals. A membrane is made in several steps by first forming a tube or a plate from a mixture of kaolin and zeolte A. The shaped artide, containing additional organic binders to increase its green strength, is is calcined at a temperature of about 700~ At this temperature the organics are removed, kaolin is converted in the easily reacting metakaolin and the article attains enough strength to be handled. The metakaolin is subsequently converted into zeolite A by hydrothermal treatment with an aqueous NaOH solution. The resulting articles consist mainly of aggregated zeolite A crystals. The aggregates are fin-ther processed in order to seal the non

428 zeolitic channels by an additional hydrothermal deposition of zeolite A. It is claimed that a closed membrane can be made in this way. Since the membrane consists of zeolite A, its pore size is dependent on size and number of the exchangeble cations and it was shown that the hydrogen permeability increases dramatically upon exchanging Na to Ca. Pervaporation of water-ethanol on polyvinyl alcohol polymeric membranes filled with (Na, K, Ca)A zeolite has recently been reported [43]. It is noteworthy that, upon increasing the content of KA within the polymer, the flux increases while the separation factor decreases drastically. An explanation on this behaviour might be found in an increase in less selective grain bounderies at higher loadings. The pure polymer is, however, marked by the highest selectivity. At temperatures exceeding 25~ selectivity decreases, probably due to diffusive "holes" within the polymer (see

Figure 8). 200

2

I

E

7 100 ~ 'I

E

.- 70

~

0 F.

O

"= 50 .'7." t-,

30 20

30

40

50

T/~

Figure 8 Effect of temperature on the separation of ethanol-water (80:20) 1. PVA; 2. PVA + KA (11 wt%) [43] As a result, more alcohol permeates through the membrane. From the above mentioned phenomena it can be deducted that zeolite-filled polymeric membranes show permeation behaviour due to two processes: i. adsorption and transport through the zeolite pores and ii. transport along the bounderies in between the zeolite crystals and the polymer matrix. Such two-component systems will probably not take full advantage of the zeolite capabilities. However, in contrast with small molecules like methanol or ethanol, a distict improvement in separation can be seen when applying alcohols with a cross-section larger than the pore aperture of the zeolite. For instance, isopropyl alcohol and ten-butanol are hindered in permeability through the polymer membrane, leading to higher selectivity for water than in case of the unfilled membrane [43]. Combined with the lack of improvement in separation when applying NaX versus the unfilled membrane, it can be stated that the transportation of molecules through the membrane can be partially ascribed to the molecular sieving effect. 4. M E D I U M P O R E Z E O L I T E S

Focus in the field of medium pore (ten-membered oxygen ring) zeolite-based membranes has been almost entirely on MFI-type zeolites. This zeolite and its all-silica form,

429 Silicalite-1, are known to grow relatively easy on various surfaces. Moreover, some principles have been developed by Jansen et al. [8] how to govern the zeolite crystal orientation with respect to the support. Two articles with a reviewing character [7,44] as to MFI-based membranes are recommended to the reader. MFI-based membranes may be categorized as to Al-containing (ZSM-5) membrane layers and M-free (Silicalite-1) layers, or as to the support applied (alumina, metal). The first division will be used in this chapter. ZSM-5 was grown on the outer surface of a porous alumina tube by Matsuda et al. [45]. The tube was first sealed with an inorganic adhesive to avoid crystallization on the inner walls and subsequently vertically positioned in a ZSM-5 synthesis solution. Under stirring conditions at 200~ after 48 hours, a film was formed of ZSM-5 on the outer surface. The synthesis was repeated several times to obtain a 100 ~tm thick and dense layer. Finally, the zeolite composite phase was calcined. To avoid pinholes and cracks, Matsukata et al. [46] optimized the density of the precursor phase by using a dry gel instead of a solution. This was achieved through the use of a slipcasting method. A dry porous alimina plate of 2.2 cm2 was dipped into a gel whereby the support surface was covered with an amorphous aluminosilicate phase. After drying, the sample was exposed to template vapor, triethylamine, ethylenediamine and steam (the Vapourphase Transport Method). However, the zeolite layer (20 lam thick) consisted of a mixture of ZSM-5 and Ferrierite. Nitrogen and oxygen permeation were studied. Gavalas et al. [7] prepared ZSM-5 membranes onto porous (x-alumina disks by in-situ hydrothermal synthesis at 175~ The zeolite layers were formed on the bottom face of disks placed horizontally near the air-liquid interface of clear synthesis solutions. The films grown at the optimized conditions were about 10 ~tm thick and consisted of well-intergrown crystals of about 2 ~m in size. Pure gas permeation measurements of the best preparations yielded hydrogen:isobutane and butane:isobutane ratios of 151 and 18 at room temperature and of 54 and 31 at 185~ respectively. Silicalite-1 membranes have recieved much attention in the last five years. Porous metal supported continuous films of Silicalite-1 have been grown by Sano et al. [47] and by Geus et al. [6] The first group of investigators used a porous stainless steel disk (5 cm diameter) with an average pore diameter of 2 lam, which was positioned on the bottom of an autoclave containing a silicalite-1 synthesis mixture. After 48 hr at 170~ an aggregate of MFI crystals (20-100~m) was formed at the surface. The average thickness of the silicalite-1 layer was 460 ~tm. Upon calcination at 500~ the layer did not disintegrate. Pervaporation experiments on an ethanol/water mixture (5 v/v % EtOH) showed a high separation factor: ot EtOH/H20 = 60. Fluxes amounted to 0.2-0.8 kg m -2 h1 over the temperature range 30-60~ The groups of the present authors, Geus and Bakker [6], prepared continuous layers of MFI on two-layer porous, sintered stainless (AISI 316) steel support disks (diameter 25 mm; thickness 3 mm) provided with a thin (50-150 ~tm) top layer of metal wool (WF 1) which were obtained from KrebsOge (Germany). For the construction of high-temperature membrane modules, similar disks (diameter 20 mm) were enclosed in non-porous stainless steel (AISI 316) tings (cf. Figure 9). Thight contact between both parts was achieved by thermal assemblage (porous support at liquid nitrogen temperature; non-porous disk at 400~ Two stainless steel cylinders , provided with commercial flange connections (LeyboldHeraeus), were subsequently connected to the non-porous part of the disks with a gold alloy.

430

Figure 9 Schematic view of a zeolite layer on top of a porous/non-porous substrate During hydrothermal treatment, the top section of the module (with the smooth top layer) is filled with the synthesis mixture, and the lower section is effectively closed by a Teflon cylinder. Both sides of the module are closed with flange connections using Teflon sealing tings. Especially rather diluted synthesis mixtures at pH 13.5-14.0 were applied. Aerosil 200 (Degussa) was used as the silicon source and the presence of alkali metal ions was kept as low as possible. The TPA to SiO2 ratio was relatively high. After ageing the synthesis mixtures while stirring for 5 hr at ambient conditions, the hydrothermal syntheses were performed at 180~ for 45-50 hr. A formulation leading to a continuous polycrystalline MFI film was (molar ratio): 100 SiO2 : 100 TPA : 50 OH- : 110001-120 Sem pictures of the layer obtained are shown in Figure 10:

Figure 10 a. Sem picture of a continuous MFI layer (top view); b. Sem picture of a cross section of the same two-layer stainless steel supported MFI layer (magnification 600x) As an example, the separation of methane (3.8 A) and n-butane (4.2 A) over the Silicalite-1

431 membrane in a Wicke-Kallenbach setup, is shown. At room temperature, mainly n-butane permeation based on attractive forces is observed (see Figure 11a). At higher temperatures up to 300 ~ (Figure 11b) the methane flux is higher than the butane flux indicating a more diffusion-controUed separation. Silicalite-1 films of 2-7 ~tm in thickness have been prepared by Vroon [48] on a home made alumina support (thickness 2.0 mm; diameter 39 mm; porosity 46% and pore diameter 150 nm) by two subsequent hydrothermal syntheses. The obtained zeolite phase had a low defect level.

0

4

methane

20

ee

1 5 0 "C

2

0

methane

16

*

n-butane

12 .

I

ooo 0

20

40

: 9

9

n-butlme

:::.---,-.... 1 5 0 "C

0

~e'4~e~

o

o 0

350 "C

0

o

0 0

.

60

80

Time (s)

100 Time (rain)

A)

Figure 11

9

B)

a) Transient permeation behaviour of a binary methane/n-butane (50/50) mixture through a stainless steel supported MFI-type membrane at 25~ b) Steady state permeation rates of the 50/50 methane/n-butane mixture as a function of temperature

In particular the layers prepared by two subsequent hydrothermal treatments showed an optimal quality. Flux and separation factors of single gas, methane, n-butane, isobutane and 2,2-dimethylbutane as well as mixtures of methane/n-butane, n-butane/isobutane and nhexane/2,2- dimethylbutane were determined. A continuous Silicalite-1 phase was prepared by Noble et al. [49] on the inner wall of a porous alumina tube by filling this tube with the synthesis mixture and closing both ends with a glazing compound. This was necessary to prevent bypass during the permeation measurements. The synthesis was carried out at 453 K for 12 hours.According to the authors, the crystallization seems to start in the pores of the alumina support, based on continuous evaporation of water through the alumina pores. Next, crystals form a continuous layer on the inner wall of this tube. Calcination of the membrane was carried out by slow heating, over a period of three days up to 738 K, and then this temperature was maintained for 8 hours. Single gas permeances of hydrogen, argon, n-butane, isobutane and sulphur hexafluoride were measured as well as competitive performances of mixtures of hydrogen/isobutane and hydrogen/sulphur hexafluoride at 300 to 737 K. Higher separation factors were obtained with a pressure drop across the membrane than when a sweep gas was used. Altogether, much progress has been made in the preparation of MFI-based membranes and

432 their use in separation. 5. L A R G E P O R E Z E O L I T E - B A S E D

MEMBRANES

(12 membered oxygen ring) 5.1 F e r r i e r i t e Hardly any research has been performed on ferrierite in zeolite membrane configurations. Matsukada et al. [50,51] prepared a ferrierite-based membrane by the frequently used Vapour-phase Transport Method. By using ethylenediamine, triethylamine and steam (under hydrothermal conditions), a porous alumina support, covered with the proper aluminosilicate gel, was transformed into a alumina supported (30 gm thick) ferrierite layer. No permeation with 1,3,5-triisopropylbenzene could be observed, proving the layer to be defect-free. Fluxes of small gases were found in the order of 108-10 .9 mol.m2.sl.Pa "1 and decreased in the order H2>He>CH4>N2>O2>CO2.

5.2 Mordenite Crystallization of mordenite on stainless-steel and polytetrafluoro ethylene plates has been studied by Yamazaki and Tsutsumi [52] from aqueous solutions. Crystallization occurred preferentially in the void spaces of the supports, leading to non-uniform layers with a thickness exceeding 100~m. In another study by Nishiyama et al. [53], the Vapour-phase Transport method was applied on alumina supports. No permeation of 1,3,5-triisopropylbenzene (kinetic diameter 0.85 nm) could be observed through the 10 gm thick membrane. Mordenite has parallel channels with an elliptical pore dimension of 0.65 x 0.7 nm. Pervaporation of benzene-p-xylene (molar ratio 0.86) at 22~ resulted in a separation factor of 164 (total flux 1.19 10"4 mol.mlsl). The theoretical value basecl on the gas-liquid equilibrium amounts to 11.3. Apparently, the mordenite-based membrane shows high selectivity for aromatic hydrocarbons.

5.3 Zeolite NaX Permeation experiments have been performed by Wernick and Osterhuber [54,55] on a 120 ~m single crystal of zeolite NaX (Si/Al=l.29, pore diameter 7.4 A), embedded in an epoxy film. From butane permeation it was derived that as temperature increases, diffusivity increases only slightly. Because the heat of adsorption (12 kcal/mol) is much larger than the activation energy for diffusion (2 kcal/mol), the concentration of butane on the zeolite pore walls dominates permeation in the region of 50-100~ in contrast to the diffusion effect, resulting in a drop in the permeation. Noteworthy is the fact that permeation of butane through the crystal increases in time (1-2 days), while after evacuation the permeation reduces to the initial value and again increases in time. This has been explained in terms of structural transitions occurring in NaX. Initially, partially hydrated sodium cations block pores while upon exposure to butane and gradual drying, sodium cations move from the diffusion pathways, resulting in 103 the original permeation value. Changing temperature resulted in two permeation states, one with a low rate at 100~ and one with a rapid rate at 25~ Gradual cooling of the zeolite crystal (after being heated at 100~ kept the permeation low for 5 hr at 41~ This process was found to be reversible and

433 dependent of temperature and concentration of butane.The rapid state was found to be unselective for C1-C6 hydrocarbons, in contrast with the slow state which exhibited a separation factor of 3.2 (at 41 ~ for isobutane/methane. This transition could, however, not be observed for other crystals of NaX. Defect sites within the structure or slight compositional changes might be responsible for this difficult reproducable effect. 5.4 Zeolite N a Y Zeolite NaY has been grown on several non-porous metal supports from aqueous solutions. Continuous films (1-10 ~m) were grown on Cu, Sn coated Cu and Brass (67.3% Cu/32.7% Zn) [56,59]. No crystallization was, however, observed on Pb, Sn or Cr supports while on Ag supports only discontinuous films could be obtained [56]. Fundamental studies [57,58,60] on Cu supports reveal the influence of pretreatment (e.g. plastic deformation or high temperature treatment) of the supporting material on crystallization. Upon pretreatment for 4 hr at 700~ of Cu, the size of crystals deposited on the plates changes from 2-3 ~m to sizes below SEM resolution [58]. This has been explained in terms of migration of defects within the support from bulk to surface, so as to increase the surface energy. The zeolite nucleation is influenced by the concentration of defects on the surface. It has been shown [60] that crystals of zeolite NaY grow rapidly in cracks or other defects on the Cu surface. This might be due to a more rapid leaching of metal ions from those spots, stimulating gel formation. No data are known on permeation through NaY-based zeolite membranes.

6.

PERMEATION THROUGH SILICALITE-I MEMBRANES: EXAMPLES AND MODELLING

Important for practical implemetation of zeolitic membranes is the acquisition of permeation data of single components and mixtures and the interpretation of these data in the form of macroscopic models. These models describe the permeation flux of components as a function of partial pressure, composition and temperature. Once good models exist separation units can be designed for the separation of multicomponent mixtures. In this paragraph shortly the permeation measurement method is introduced, followed by various examples of permeation through a silicalite-1 membrane on a sintered stainless steel support. This includes unary and binary mixtures as a function of partial pressure, composition and temperature. Finally the present state of modelling permeation through silicalite-1 membranes is reviewed. 6.1. Measurement Globally two methods to measure permeation through a membrane exist. One is a static method the other a continuous method (Figure 12). In the static method one side of the membrane ('permeate side') is connected to an evacuated well-known volume equipped with a sensitive pressure transducer. The other side ('feed side') of the membrane is suddenly exposed to a certain concentration or pressure of the component that is being investigated

434 The pressure at the permeate side is now followed as a function of time. From the rate of the pressure increase the flux can be easily calculated since the volume is known. This method has two major drawbacks: 9 restricted to one-component systems, unless a fast analysis method is available that does not require sample amounts that affect the pressure at the permeate side 9 the conditions are continuously changing at the permeate side (transient operation), so modelling the permeate data should take this into account, for which time dependent models are needed The second method is called the Wicke-Kallenbach (WK) method as these researchers first described this technique for diffusion experiments. Ha, X,Y In this method the membrane is X,u flushed with gas at both sides. At .~.... \ / ._..~ ~---- j I \ the feed side the components that are being investigated are fed, tea~sx~ ta~s/a~ t~rra~se~ while at the permeate side an inert sweep gas is being used that sweeps away the permeated components. The resuking mixture ('the Figure 12. Schematic representation of permeation measurement permeate') is being analysed by methods: (a) static method, (b) continuous method (Wicke- mass spectrometry or gas Kallenbach). chromatography. Also the gas leaving the feed side ('retentate') can be analysed and back-diffusion of the sweep gas can be checked. The pressure difference over the membrane can be zero or not, and can be maintained throughout the experiment. Since it is assumed that the gas flows over the membrane and the permeated components mix well, the flux through the membrane is directly calculated from the gas flow rates and the composition of the permeate and, if back diffusion occurs, of the retentate. For multi component permeation with no back-diffusion the flux is given by eq. (1) Ni =

Yi .FH, (1 - ~-'y;) A

(1)

i

where: iV,. y,F•, A

flux of i through the membrane (mol/s.mz) mol fraction of i in the permeate (-) flowrate of sweep gas helium (tool/s) geometric surface area of membrane (m2)

The WK-method has clearly advantages over the static method since one can use mixtures and one operates under steady state conditions, which makes the modelling of the results easier. Moreover, one can even apply transient conditions by suddenly changing the feed gas flow from inert to a mixture or vice versa and analysing the response of the membrane to this step change. This gives sometimes nice insight in the permeation characteristics of mixtures.

435

6.2. Permeation examples The permeation examples that are given here are based on own work and have been published in different articles [61-66]. The membrane used is of the asymmetric type and consists of a--40 ~tm thick layer of intergrown silicalite-1 crystals on a 3 mm thick layer of highly porous sintered stainless steel. The geometric surface area amounts to 3 cm2. Stainless steel has the advantage of easy mounting in all types of equipment which facilitates practical application compared to ceramic supports. The permeation experiments have been carried out in a WK-type eel with generally at both sides atmospheric pressure, or higher pressures at the feed side. Subatmospheric pressures are obtained by dilution with helium, which is also used as sweep gas. The cell is placed in a GC oven which can operate from 200 K (by cooling with solid CO2) to 700 K. This type of membrane turned out to be very robust, it did not show any deterioration after all kinds of permeation experiments over the temperature range mentioned above and up to pressures of 10 bar. The membrane should be a continuous layer, free of defects. Fortunately, due to the synthesis procedure, this can be easily checked for the silicalite-1 membrane. After synthesis the template molecule, tetrapropyl ammonium hydroxyde, is still present and blocks all the pores. This is removed by thermal decomposition in air ('calcination') at 673 K. During calcination the membrane is placed in the cell and a mixture of Kr in air is used for calcination. A good membrane does not permeate Kr at room temperature and should develop permeability for Kr during the calcination procedure. Ths is illustrated by Figure 13 which shows the permeation development of Kr as a function of time, at a heating rate of 1 K/min. Already around 500 K some permeation is observed, but the large breakthrough is observed above 600 K, until a steady-state permeation level is reached. 6

700

~t~ 4

O0

200

Time [rain] Figure 13. Permeation of l~D~tonduring calci~tion in air/krypton (20%) mixture. Heating rate 1 K/rain.

One-componentpermeation Figure 14 shows the single component permeation of n-alkanes and alkenes as a function of partial pressure around 300 K. Only methane exhibits a nearly linear dependency, the other components show a curvature at increasing pressure levels. Globally, the steadystate flux decreases with increasing chain length, and the alkenes permeate faster than the alkanes. It is noticed, however that ethane permeates slightly faster than methane at low partial pressures.

436

5O

35

,_., 401

,..,30-

Kr S

.,... 0

30

E

E

~ 2o

X

X U_

-

LL

10

o~

20 c

E15.-

20

40

60

80

1O0

Feed partial pressure [kPa] Figure 14. Steady state permeation of n-alkanes and n-alkenes as a function of partial pressure at --300 K.

lo~

5

O: 0

20

I 40

, 60

, 80

, 1O0

Partial feed pressure [kPa]

Figure 15. Steady state permeation of noble at --300 K as a function of partial pressure.

gases

In Figure 15 the permeation of some noble gases are given. Most of them show a linear partial pressure dependency. This is also exhibited by the alkanes/alkenes at higher temperatures [61 ]. This seems more easy to model than the permeation at lower temperatures. Permeation depends on the occupancy of the molecules in the silicalite-1. For weakly adsorbing molecules, like methane this is linear as a function of the partial pressure, but with increasing molecular size and pressure the occupancy is no longer linear, but levels off to a saturation level. Therefore, the permeation of the higher alkanes levels off at lower partial pressures. At high temperatures all molecules have a low occupancy and permeation is linearly dependent on the partial pressure. The fact that the permeation flux decreases with increasing chain length is ascribed to a stronger interaction with the silicalite-1, and, hence, their decreasing mobility (read: diffusivity). The slightly larger flux of ethane at low partial pressure can be explained by a higher occupancy at these conditions that compensates for the lower mobility. The permeation of alkanes as a function of temperature (Figure 16) is quite peculiar. In general the flux increases, passes through a maximum, decreases and finally increases again as a function of the temperature. The temperature of the maximum depends on the component and is not visible in all cases, although it can be inferred to be present at temperatures outside the range investigated. It should be noted that the occupancy of the components is high before the temperature of the maximum and decreases strongly around this temperature, and is low when the flux increases again. The physical interpretation is that at low temperatures the mobilty of the molecules in the nearly saturated silicalite-1 determines the flux. When the concentration in the silicalite-1 decreases strongly, which is determined by the heat of adsorption, the increasing mobility cannot compensate this any more and the flux decreases. At present there exists no good interpretation for the increase at higher temperatures other than an activated di~sion process. The permeation of isobutane is included in Figure 16a and is an illustration of a molecular sieving effect. The kinetic diameter of the molecule amounts to 0.5 nm and approaches that of the pore sizes (0.52-0.55 nm). Compared to that of the nalkanes (0.38 nm) this molecule has difficulties in permeating through or in entering the membrane, resulting in an activated permeation.

437

40

-~

2o

x E

lO

E

~

100

i-butane

300

500

700

Temperature [K]

Figure 16. Steadystate permeationas a function of the temperature for alkanes (feed pressure 1 bar) Partial pressure of methane [kPa] 100 50 .

~

.

80

.

40

~methane

30

' 1 , , .

.

40

20

Partial pressure of ethene [kPa]

0 -o

,,

"-,,.

10 0 0

ethane

2o 40 60 80 100 Partial pressure of ethane [kPa]

00

50

E

-,.<.

,_....,

--~

60 .

-•

8O

i

60

,

40

i

ethane.,

30 20

IT

10

0

i

l ethene

40

E x

20

i

0

20

40

60

80

100

Partial pressure of ethane [kPa]

Figure 17. Steady-statepermeation fluxes as a function of composition for a two-componentsystem at --300 K and 1 bar feed pressure. (a) methane-ethane, (b) ethene-ethane. Solidline one-component,dashed line mixture.

Binary permeation The permeation of two-component mixtures shows interesting features. In Figure 17 the permeation of mixtures of methane/ethane and ethane/ethene are given. The total feed pressure is 1 bar pure hydrocarbon mixture, while the composition varies from 100% of one component to 100% of the other. As is apparent from the one-component permeation methane and ethene permeate faster. In the mixture, however, the ethane permeates faster, while the methane and ethene are hampered in their permeation by the presence of the other component. Ethane itself is even slightly accelerated in the case of ethene. So, the stronger adsorbing component hampers the permeation of less strongly adsorbing components, while the faster molecules are decelerated and the slower ones can be accelerated. For hydrogen/n-butane mixtures this behaviour is nicely and even more extremely exemplified by transient permeation experiments. Here, the membrane is exposed to a stepwise change from helium to 95% hydrogen or 5% n-butane or a mixture of both and the permeation response is followed as a function of time (Figure 18). As single components they both break through after a certain time to their one-component steady-state levels, hydrogen permeating much faster than n-butane. In the experiment with the mixture hydrogen appears firstly, reaches a level of about 10% of the pure component permeation and decreases to a low level.

438

When this level is reached the n-butane appears and reaches its original single component level at those conditions. This results is a n-butane selectivity of more than 300.

Figure 18. Transient permeation of 95% hydrogen and 5% n-butane at 300 K and 1 bar feed pressure as a fuction of time upon a concentration step change. (a) single component breakthrough (He dilution), (b) mixture breakthrough. Permeation of the mixture (50/50) as a function of temperature is given in Figure 19 and compared to a hydrogen/carbon dioxide mixture. Both exhibit similar trends. The stronger adsorbing component permeates the fastest at low temperatures, and hampers the permeation of the other component, depending on its adsorption strength. The flux passes through a maximum and the other component flux increases strongly resulting in a complete reversal in selectivity. The selectivity now goes into the direction of the Kundsen selectivity. A last example of a mixture permeation and shape selectivity is given in Figure 20. Initially 5 kPa of isooctane (2,2,3-trimethylpentane) is passed over the membrane, which results in a low, but still measurable flux. This indicates that the membrane shows some defects of larger pores, since the kinetic diameter of isooctane is 0.60 nm and should not be able to pass

Figure 19. Permeation of 50/50 mixtures at 1 bar feed pressure as a function of temperature. (a) hydrogen and n-butane, (b) hydrogen and carbon dioxide. through the zeolite pores. After 500 s 25 kPa of methane is added which does not affect the isooctane permeation, and methane

439

reaches its single component level at this condition. At 1100 s 25 kPa of nbutane is added, too resuking in a ffl decrease of the methane flux, but not of o.1 '-9 Q,. the isooctane flux These observations O E 0.1 ~t ~ lead to the conclusion that the isooctane E 0.01 t'~ 9 i-octane indeed permeates through defects with X 0.01 _= q~ pores larger than those of the silicalite-1 I.L 0.001 (D 0.001 g 9 and not through the latter, otherwise the rE r flux of methane and n-butane would 12_ " I I I I 0.0001 0.0001 have been negligible. The latter two 30O 6OO 90O 1200 1500 components, however, do pass through Time [s] the silicalite pores as is apparent from Figure 20. Steady-state permeation fluxes as a function of the observed competition effect. time after stepwise concentration increases at--300 K. Addition of 5 kPa isooctane at t--0 s, of 25 kPa methane at t=-500 s, and of 25 kPa n-butane at t= 1100 s. 10

9 9

II

methane

Q.

=,....=

9

9

6.3. Modelling permeation Permeation through a zeolite membrane can be described by a sequence of five reversible steps, depicted in Figure 21. Barrer [67] distinguished: 1. adsorption at the external surface of the zeolitic structure, 2. transport from the external surface into the pores, 3. diffusion in the pores to the other side of the membrane, 4. escape out of the pores to the external surface and 5. desorption from the surface. In addition, a complete model will also take into account the molecular diffusion through the (stagnant) porous stainless steel support. Depending on the conditions the adsorption at the external surface may be significant or not (high temperatures). Steps 1 through 5 are all activated steps, which can be modeled assuming that molecules jump between low-energy sites. Each jump can be correlated to an activation energy and the net flow is calculated from the forward and reverse jumps. Obstructions in the pores can be modeled as occasional intracrystalline energy barriers. The rate determining step in this model is dependent on operating conditions (temperature, partial pressure) and the characteristics of the molecule and the crystalline material. In the following it is assummed that the diffusion in the pores is the rate determining process and that the other steps are that fast that they can be considered to be in quasiequilibrium. The thicker the membrane, the more this assumption is valid. Barrer [67] gives criteria to check this assumption, and in the data shown above this assumption is almost always valid. There are several models to describe Figure 21. Steps in the membrane permeation . process, according to Barrer [67]. mtracrystalline diffusion (step 3) in microporous media. Diffusion in zeolites is extensively

440 described in refs. [68,69]. For the modeling of permeation through zeolitic membranes such a model should take the concentration dependence of zeolitic di~sion into account. Moreover, it should be easy applicable to multi-component systems. Several models will be discussed. As have been seen above adsorption plays an important role in permeation through microporous membranes. So, single and multicomponent adsorption isotherms are required for a successful modelling of the permeation behaviour. An extensive treatment of the recent state of the art of zeolite permeation modelling is given by Van de Graaf et al. [70]. A shortened treatment follows here. Fi c kian diffusion

A general form for the description of diffusion processes is Fick's First Law of Diffusion, eq. (2). (2)

N, = - D r ' a r c ,

Equation 2 implies that the driving force for di~sion is the concentration gradient Vc, in the zeolite. One can argue, however, that the true driving force for diffusion is the chemical potential gradient, and the expression for the flux should be eq. (3). N, = -B, q V ~

(3)

For an ideal gas the chemical potential gradient p; and the partial pressure pi are related in the following way, eq. (4): (4)

~z~ = U ~ + R T l n ( p , )

Combination of eqs. (2)-(4) leads to the definition of the 'corrected' diffusivity, often referred to as the Darken equation [68,69], eq. (5):

D~Ck = D ~ ~ n p~ c~aq

with

D ~ = B, R T

(5)

Three types of diffusion can be distinguished in pores: 1. Bulk or molecular diffusion, dominated by molecule-molecule collisions in the gas phase. This type of diffusion becomes important for relatively large pore diameters or at high system pressures, and should be aplied for the stainless steel support layer. 2. Knudsen diffusion, dominated by molecule-wall collisions. This mechanism is prevailing at low pressures or high temperatures. 3. Surface diffusion, which represents activated transport of adsorbed species along the pore wall.

441

Figure 22. (a) Effect of pore diameter on the order of magnitude of diff~ivities, (b) Effect of pore diameter on the order of magnitude of the activation energyfor diffusion. Adaptedfrom Post [69]. In Figures 22a and b the orders of magnitude of the different di~sion coefficients and the activation energy for diffusion are given as a function of pore size [69]. For diffusion in micropores (< 2 nm in diameter) the diffusivity can vary over several orders of magnitude depending on the size and nature of the diffusing species and the microporous media. Diffusion in micropores is often referred to as configurational diffusion. In the case of surface diffusion, which is likely to occur in micropores, the partial pressure pi is related to the adsorbed phase concentration ci by the adsorption isotherm. Consequently, the choice of the adsorption isotherm influences the diffusion coefficient, according to eq. (5).

Combined gaseous and surface diffusion One way of looking at transport through microporous media is the starting-point that molecules can either retain their gaseous character in the pores or are adsorbed on the surface of the pore wall. In Figure 23 the contribution of the gas phase and the adsorbed phase concentration of a number of gases to the total concentration in silicalite is presented. The contribution of the gas phase concentration becomes more and more important with increasing temperature. The net flow through the pores is a combination of gaseous and surface flow, which are both assumed to be activated processes, eq. (6).

Ny t = N; +N~

(6)

N~ = -~-~ Vp,

(7)

where:

and

442 This type of approach was developed by Wei et al. [72], Ma et al. [73 ], Inoue et e al. [74] for gaseous systems. Yoshida et ; kN al. [75,76] studied the use of combined ~10 3 e")i f .. ~'~,, N~~,'~ methane liquid and surface flow for the description E .....2" of dye permeation through a cellulose O E10 2 membrane. r All these approaches are mainly applied O to single component systems, but hard to ~_~10 ~ r F gas phase extend to multicomponent systems. The O approach developed by Maxwell and I I I O Stefan, however, seems to keep promises o 0 200 400 600 800 for multicomponent mixtures. Temperature [K] The principle of the Maxwell-Stefan Figure 23. Contribution of gas phase (solid line) and diffusion equations is that the force acting adsorbed phase concentration (symbols; A= 1-I2,+= N2, on a species is balanced by the friction that O= CH4, ~=CO2) to the total concentration in silicalite-1 is exerted on that species. The driving (dashed lines) as a function of temperature (p~ = 1 atm). force for diffusion is the chemical potential Adsorption data are taken from [71]. gradient. The Maxwell-Stefan equations were applied to surface diffusion in microporous media by Krishna [77]. During surface diffusion, a molecule experiences friction from other molecules and from the surface, which is included in de model as a pseudo-species, n+l (Dusty-gas model). The balance between force and friction in a multi-component system can thus be written as [77]:

10 4

_ l__vt.t i = RT

V i -- Vj

Vi

Oj D~. s + 0.+, -~sV"+' j=,

,,j

i =1,2, ...n

(8)

Di,n+I

The first term on the fight hand side of the equation denotes the friction between species due to velocity differences, whereas the second term represents friction between a species and the surface. DMSq and D~Si,,+l are the Maxwell-Stefan dif~sivities. The first term on the fight side is often referred to as an exchange term which represents the probability of molecules exchanging places on the surface. Since this exchange is not likely to occur in narrow silicalite1 channels it is commonly neglected. This is called here single file diffusion. The Maxwell-Stefan surface dif~sivity is defined in analogy to the defirfition of the Knudsen diffusivity [77] as: ~/~ = ~D~+a 8,+,

(9)

The chemical potential gradient can be related to a matrix of thermodynamic factors: RTV~i

=

FijVO i

j=a

where:

F o. - 0 , 01np~. c?Oi '

i,j=1,2, ...n

(10)

443 The occupancies can be derived from the adsorption isotherms. For Langmuir adsorption this relation is: O, =

q

c~"t

=

K~p~

l + Y~Kjpj

(11)

Combining equations (8), (9) and (10) and taking:

N~ =pcTOy~

(12)

gives equation (13) and (14) for one or two component systems, respectively.

N~ . .c~~. .Dy~ V01 1-01 N, = -c~~

(13)

DiMs

9 9{(1-O:)-VO, +0~ .VO,} 1-01 -0~_ (single file diffusion)

(14)

On comparing eqs. (13) and (2)+(5), it can be shown that for Langmuir adsorption, the Maxwell-Stefan diffusivity in a one component system is identical to corrected diffusivity in the Darken equation, and predicts also that the Fickian diffusivity will strongly increase with increasing occupancy. In Figure 24 this is demonstrated for the permeation of n-butane at 300 K (from Figure 14) and as a function of the reciprocal temperature (data from Figure 16a). The Maxwell-Stefan diffusivity remains fairly constant as a function of the partial pressure, and thus the occupancy, and exhibits a nice Arrhenius type relationship, whereas the Fickian diffusivity deviates strongly [65]. From eqs. (12) and (13) it follows that if DUSi is independent of concentration, binary fluxes can be predicted on the basis of single component permeation data once the multi-component adsorption isotherm is known. Diffusion through zeolites and consequently, permeation through zeolitic membranes is concentration dependent [61-63,78,79]. It is therefore worthwhile to compare how this concentration dependence is taken into account in different models. Yang [80] proposed a model for diffusion in micropores, in which interaction between molecules on the surface was taken into account by introducing a sticking probability, X. This parameter represents the change of a molecule to move to an occupied site compared to the change of moving to a vacant site on the surface. ~ Is related to the activation energy of diffusion. For a single component system this results in the following relationship for the flux [80]: N i = -c~ satp

D?

(1- O - >L)e~)

V (9~

0 <_~ _<1

(15)

444

5

'E

'

3

4

~

,...,...,

~

2

3

E 2 x i;',

1

1 i/==.~___. ,1[ -I 6 20 40

10 -3

. . . . . . I

I

60

80

Pfeed b u t a n e

~

~ - ,9 :~ "~ = ~ c~

,E

10.4

o.

, . 10-~ = ~:= ~ 10-6 n

DMS

1 o -7

100

1.5

I

2.0

I

I

2.5

3.0

3.5

10 3 T 1 [K 4]

[kPa]

Figure 24. (a) Permeation flux of n-butane (t~) through a silicalite-1 membrane as a function of the feed partial pressure of n-butane (T=300 K, Ptot=100 kPa). Included are the calculated Fickian (Y) (eq.2) and Maxwell-Stefan (v) (eq.13) diffusivities.

In fact 2, is a correction parameter for the Fick diffusion coefficient. This correction has a similar effect on the apparent diffusivity as the correction given in eq. (13). When X is less than 1, the diffusivity increases with occupancy. This correction can also be applied to the MaxwellStefan diffusivity, which results in an even larger effect of concentration on the flux. The concentration dependence of the flux in the Maxwell-Stefan equations depends largely on the adsorption isotherm chosen, since this isotherm determines the thermodynamic factor. For Langmuir adsorption the concentration dependence of the flux increases in the following order using different models: Ni oc

VO i <

1 (1 - (1 - 2i )O i )

Fick [rick+ Yang

1

.VO i < ~ . V 8 (1 - O i ) Maxwell-Stefan

i <

1 (1 - (1 - 2 i )Oi )

1

. ~ V 8 (1 - Oi )

Maxwell-Stefan+ Yang

(

i

16)

In F i g u r e 25 the dependence of the flux on concentration is given, normalized to the flux according to the Fick model, assuming that the diffusion coefficients are constant in all models. The temperature dependence of surface diffusion is contained in the adsorption equilibrium constant and in the diffusivity. For the adsorption constant the following relationship holds: K i = exp, ASi + Q~ "~ R k--/ J

(17)

If the diffusivity is independent of occupancy, its temperature dependence satisfies an Arrhenius-type relation [81,82]:

445

oi =oo "3"

10

"~

8

Z x

6

~

4

v

t

(18)

MaxweiI-Stefan+Y

_

Maxwe

_

..~~~i~+Yang

N

E L_

2

0

Z

I

,

I

I

,

0.00 0 . 2 5 0 . 5 0 0 . 7 5 1.00 Average occupancy

[-]

Figure 25. Normalized flux (N/N'~.~) as a function of occupancy according to ex]. (16). Diffusivity is taken

In Figure 26 the effect of varying the activation energy for diffusion on the temperature dependence of the flux is presented. From this it can be seen that, when using the Maxwell-Stefan description (assuming that intracrystalline transport is rate controlling and adsorption follows a Langmuir isotherm), a maximum in the flux is to be expected when the activation energy for diffusion is smaller than the heat of adsorption. When the activation energy for diffusion is higher than the heat of adsorption, the flux increases with temperature. It does not, however, predict the observed increase as in Figures 16 and 19.

The application of the Maxwell-Stefan theory for diffusion in microporous media to permeation through zeoliticmembranes implies that transport is assumed to occur only via the adsorbed phase (surface diffusion). Upon combination of surface diffusionaccording to the Maxwell-Stefan model (eq. (13)) with activated gas translationaldiffusion (eq. (7)) for a one-component system the temperature dependence of the flux shows a maximum and a minimum for a given set of parameters, Figure 27. At low temperatures surface diffusionisthe most important diffusionmechanism. This type of diffusion is highly dependent on the concentration of adsorbed species in the membrane, which is calculated from the adsorption isotherm. At high temperatures activated gas translational diffusion takes over, causing an increase in the flux at still higher temperatures. From this modelling approach it seems that the combined surface diffusion and activated gas translational diffusion can describe the observed single component permeation behaviour. The interpretation for the latter type of diffusion is not well-crystallized at present. It might have to do with an increasing deformation of the silicalite-1 structure with increasing temperature that causes this apparently activated process. This aspect has not been considered up to now, the silicalite-1 has been considered as a rigid structure. Mukicomponent mixtures are even more difficult to tackle. The Maxwell-Stefan approach with the Yang correction gives the best results up to now [70], but this implies that the permeation, and hence separation behaviour cannot be predicted on the basis of onecomponent data only. Binary data are stil required. The transient permeation behaviour in Figure 18b, however, can be modeled qualitatively well, whereas the Fickian approach is unable to do so [83].

constantforeach model,~.==0.5.

446

25

0.25

Parameter = E a [k J/tool]

20

0.20 ~ E

15

i

O

E

0.15

~

O.lO

E X

0.05

5

"7,

o

X

IT

"7

~' E

E_

E

50

1.0

40

0.8 " ' r

t~

30

0.6 ~-

20

0.4

10

200 400 600 800 1000

.,_:-, L__'7 ......I .k-":

o

0.2 ,~

200 400 600 800 1000

Temperature [K]

Temperature [K]

Figure 26. Temperature dependence of the flux according to the Maxwell-Stefan model for onecomponent. Activation energy for diffusion Ea was varied, heat of adsorption Q was taken to be 25 kJ.mol" Other parameters: AS=-75 J-mol']-K~, csat=l mmol-g-1, D~ "6 , membrane thickness l=50 lxm, 9=1.8.106 g.m-3

Figure 27. Combined gas translational and surface diffusion as a function of temperature. The surface coverageon the feed side is also included. Parameters: AS=-50 J-mol~-K], Csat=l mmol-g"1, Q=25 kJ-mol"~, D~ 6 m2-s~, E'a=15 kJ-mol"x, 1=50~tm, 9=1.8.106g-m3, ESa=15 kJ.mol~, d=-5.5.10 ~~ M=16 g.mol"~.

As has become clear adsorption phenomena play an important, if not, decisive role in this behaviour, and good data and modelling of adsorption are mandatory, too, to serve as the input parameters for the permeation description. This should not be limited to the Langmuir model, but other theories like the IAS (ideal adsorbed solution) and NIAS (non-ideal) should be considered, since they sometines work well for binary systems where the Langrnuir model fails. In addition, molecular modelling studies, that take into account the whole force field of the zeolite and the interaction with and between the molecules, may serve to understand peculiar behaviour that one may observe, like the structured packing of C6 and C7 n-alkanes in silicalite1184,85].

7. Z E O L I T E

MEMBRANES

IN CATALYTIC

CONVERSIONS

Zeolite membranes may play either a passive or an active role in catalytic (organic) conversion reactions and the potential applications of zeolite membrane reactors are quite promising. Both liquid phase and gas phase reactions may advantageously be carried out in a membrane reactor, and transport from the reaction zone is promoted by continuous removal of the permeating molecules. The membrane plays a passive role in case its main function is to:

447 - selectively remove product molecules supply an active component to the reaction zone The membrane plays an active role in case its function is to: catalyse the reaction - function as a carrier for the active catalyst component Selective removal of product molecules is beneficial in equilibrium limited conversion reactions, since the conversion per pass is enhanced and the downstream product purification is simplified. In many processes water or other small molecules are generated during reaction, which should be removed because the reaction is prevented from going by e.g. thermodynamic limitations.In such systems reactors might be applied having a membrane consisting of a narrow pore zeolite. In case water has to be removed, membranes based on one of the forms of the hydrophilic zeolite A seem to be adequate. The advantage of a membrane over in situ drying (batch drying) or continuos drying (column drying) is that a continuous water removal takes place without saturating the adsorbent. Recently [43] Gao et al. applied a zeolite-filled polyvinyl alcohol (PVA) membrane in esterification and acetalization reactions. Zeolites NaA, KA and CaA as well as NaX were loaded into PVA up to 27 wt% and the composites tested in selective water removal during reaction. A pervaporation cell with a membrane area of 22.9 cm2 was connected to a collection system kept at a vacuum of 0.1 mm Hg. A sulfonated resin was used as Br6nsted acid catalyst in the esterification mixture (120 ml). Figure 28 shows the progress of the esterification of salicylic acid and methanol at 60~ The reaction is accelerated considerably as a result of the water removal. 40 -

-

30

.o t== =

20!

10

0

i

i

6

12

I

18

i

'

24

30

t/h Fig 28 Time course of esterification of salicylic acid with methanol 1. blank reation; 2. PVA; 3. PVA+KA (11 wt%); 4. PVA+CaA (11 wt%) On the other hand, organics may be removed from aqueous solutions by applying a high silica membrane of a suitable pore size. An application might be the continuous removal of ethanol by e.g. a Silicalite-1 based membrane from carbohydrate fermentation liquids. The Microorganisms used are deactivated at an alcohol content > 12%. By continuous removal of the alcohol produced its concentration may be kept at a level below 12% enabling a higher final conversion of the carbohydrate feed.

448 Several examples of the use of MFI-based membranes are described in a patent of Haag et al. [ 86]. These authors developed a reactor consisting of an inner porous alumina tube, carrying on its interior wall a 12 lam ZSM-5 membrane. The alumina tube was mounted in an outer steel tube and the inner-tube annulus had a width of about 0.5 cm, both tubes were separately fitted with feed and product lines (see Figure 29).

Figure 29 Set-up W.O. Haag et al. (schematic) [86] This type of reactor was applied in some reactions and the synthesis conditions of the ZSM-5 membrane were adapted to obtain the appropriate Si/AI ratio for the envisaged application. Two applications with the membrane in a "passive" rol folUow: (i) The membrane of the reactor consisted of ZSM-5 having a Si/A1 ratio > 20.000, which is essentially inactive in catalysis. The inner tube of the reactor was loaded with H-ZSM-5 extrudates having a Si/A1 ratio of 70. A feed stream of vapourized cumene was passed over the catalyst at 350~ i bar and a WHSV of 10 h1. Propane and benzene were withdrawn from the annular space which was swept with N2. Under comparable conditions in a conventional reator, the reaction product consisted of disproportionation products of cumene and only traces of propene were formed. (ii) A membrane reator having the same configuration as described before, was equipped with a ZSM-5 membrane having a Si/AI ratio of 700. The inner reactor was loaded with a mixture of 10 g molybdenum oxide and 20 g quartz particles. At 200~ 25 bar and a WHSV of 2 hl, cyclohexane was passed through the inner reactor while 40 ml of oxygen was passed through the outer reactor. The effluent of the inner reactor contained cyclohexanol and cyclohexanone and only little CO2. Compared to a conventional reactor system,where cyclohexane and oxygen are cored to the reactor, the selectivity to the desired product of the membrane reactor system is much higher. In this last example, the role of the membrane is the controlled supply of the reactant oxygen. A recent paper of Casanave et al. [87] describes the use of Silicalite-1 in a-alumina (type 5, Introduction) membrane for selective hydrogen passage in the dehydrogenation of isobutane over a commercial Pt/Snh/-alumina catalyst. Some types of zeolite-based catalytic membrane configurations are schematically depicted in

Figure 30:

449 C . a ~ c z~olitelayer

I

I

suppoxt Catalyston top of zeolitelayer I

I support

Homogeneous membrane ecmtAi~in.

zeolite crystals F--q F"q [~] ~--q ~

r----1 r--I

r----1 ~-1 r--1 f-q F-I ~r--q C Z 3 ~ ~3 v-q r---~F-7 F-q F i g u r e 30 zeolite-based catalytic membrane configurations

In type a., the separating zeolite layer is equipped with catalytic sites (BrOnsted acid sites, Lewis acid sites (cations, special Al-sites), metal clusters, catalytic complexes). In type b., the non-supported side of the zeolite layer serves as a support for catalytic entities, e.g. metal crystallites. In type c., zeolite crystals with catalytic power are embedded in a matrix, e.g. a polymer membrane. The number of examples of zeolite-based catalytic membranes is still low. At the First International Workshop on Catalytic Membranes [88], Lyon-Villeurbarme, 1994, no reports on such membranes were presented. One paper [89] showed the potential of microporous catalytic membranes in poison resistant hydrogenation catalysis. Here, we also mention a composite system developed by Van der Puil et al. [90] in which an MFI (Silicalite-1) layer is completely coveting a platinum-on-fiat support catalyst. The zeolite layer is governing the acces to the platinum and this leads in the competitive hydrogenation of 1-heptene and 3,3-dimethyl-l-butene (see Figure 31 + 3~) to highly selective conversion of 1heptene. 100

12

~

75 0

"8 0 ro

6

50 25

O

0

0

'

'

1

2

3 0

3

time (h)

Figure 31 Competitive hydrogenation of 1-heptene (O) and 3,3-dimethyl-l-butene (A) over TiO2/Pt catalyst at 100~

0

1

2

3

time (h)

Figure 32 Competitive hydrogenation of 1-heptene (O) and 3,3-dimethyl-1-butene (A) over TiO2/Pt/MFI composite at 100~

Without the zeolite layer, the two olefins are converted with the same rate. A type a example is given in the Mobil patent mentioned above [86]. The membrane reactor having the configuration as described before, was equipped with a K-exchanged ZSM-5 membrane having a Si/AI ratio of 220. The membrane was impregnated with chloroplatinic acid to give 0.001 wt% Pt based on total quantity of zeolite of zeolite, and reduced at 500~ in

450 hydrogen to form platinum metal supported on the zeolite covering the inner surface of the membrane tube. The catalyst was applied in the dehydrogenation of isobutane at 560~ and atmospheric pressure. The product from the inner reactor contained isobutene and the product of the annular reactor wa hydrogen. A type c catalytic membrane was developed and tested by Jacobs et al. [91 ]. It consisted of a polydimethylsiloxane polymer matrix loaded with 30 wt% of iron phthalocyanine-containing zeolite Y crystals (see Figure 33). The membrane (thickness 62 ~tm) is in between two liquid streams: cyclohexane and 7 wt% t-butyl hydroperoxide in the membrane and the iron sites inside the zeolite catalyze the oxidation of cyclohexane towards cyclohexanol and cyclohexanone. The oxidation products are distributed over the two phases.

L4 E_o_J \

al~e

4 o- i4oca

;-~ I

ca

ca

~

\~

II

\,.

L,J

Figure 33 Composite catalytic membrane as heterogeneous cytochrome P-450 mimic. matrix: polydimethylsiloxane (PDMS), filler: FePc-loaded zeolite Y (30 wt%) [91 ] Reviews on catalytically active membranes have been written by Zaspalis [92] and Armor [93,94,95]. 8. P R O S P E C T S

The foregoing will have made clear that in the last decade substantial progress has been made in growing gastight zeolite layers onto supports. Especially MFI layers were succesfully grown onto various surfaces and, importantly, with an increasing degree of crystal orientation and a decreasing layer thickness. Experiments in which such MFI membranes were subjected to permeation of single compounds and binary mixtures laid a bails of understanding zeolite membrane action and separation. At low temperatures, adsorption is dominating separation whereas at high temperature diffusion rates come to the fore. Absolute separation on the basis of size is always to be preferred.

451 On the other hand it became clear that each zeolite will require its own and detailed study in combination with the supports selected. Altogether it is expected that the learning curve will be long before arriving at industrially operating zeolitic membranes for separation and catalysis.

REFERENCES

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452 26. 27.

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62. 63. 64. 65. 66. 67.

W.J.W. Bakker, G. Zheng, M. Makkee, F. Kapteijn, J.A. Moulijn, E.R. Geus, H. van Bekkurn, in Precision Process Technology (M.P.C. Weijnen, A.A.H. Drinkenburg, eds.), Kluwer Academic Publishers, Amsterdam, 1993, p. 425 W.J.W. Bakker, F. Kapteijn, J.C. Jansen, H. van Bekkum and J.A. Moulijn, (in Dutch), Procestechnologie 3(12) (1993), 7 W.J.W. Bakker, F. Kapteijn, J. Poppe, J.A. Moulijn, Permeation of a metal-supported Silicalite-1 zeolite membrane, accepted for publication in J. Membrane Sci. F. Kapteijn, W.J.W. Bakker, G. Zheng, J. Poppe, J.A. Moulijn, Chem. Eng. J. 57 (1995), 145 F. Kapteijn, W.J.W. Bakker, G. Zheng and J.A. Moulijn, Microporous Mater. 3(3) (1994), 227 F. Kapteijn, W.J.W. Bakker, J.M. van de Graaf, G. Zheng, J. Poppe and J.A. Moulijn, Cat. Today 25 (1995), 213 R.M. Barrer, Porous crystal membranes, J. Chem. Soc., Faraday Trans. 86(7) (1990), 1123

68. 69. 70. 71. 72. 73. 74. 75. 76. 77. 78. 79. 80. 81. 82. 83. 84. 85. 86. 87. 88. 89. 90.

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454 91. 92. 93. 94. 95.

R.F. Parton, I.F.J. Vankelecom, M . J . A . Casselman, C.P. Bezoukhanova, J.B. Uytterhoeven and P.A. Jacobs, Nature 370 (1994), 541 V.T. Zaspalis, Thesis, University of Twente, 1990 J.N. Armor, Chem. Tech. (1992), 557 J.N. Armor, Appl. Catal. 49 (1989), 1 J.N. Armor, Catal. Today 25 (1995), 199

455

Keyword Index Aromatization 323, 344 Atomastic model 237 Atomic ratios 199 ab initio 240 Acetalization 447 Achiral product 85 Acid catalyst 43 Acid catalyzed reactions 367 Acidity 65 Acid site 53, 191 Acid Type Lewis and Br6nsted 365 Activated complex 274 Adamantane 369 Adsorbed radicals 98 Adsorption 12,22 Adsorption properties 420 Adsorptive capacities 114 Alcohol dehydrogenation 382 Alcohols oxidation 222 Aldol condensation 377, 382 Algorithmic development 257 Alkali L-type zeolites 392 Alkali-Y-type zeolites 392 Alkane oxyflmctionalization 225 Alkylamines 372 Alkylation 336 Allylic rearrangement 370 A1PO4-5 403 A1PO4-11 402 A1PO4-31 402 Amide 374 Amination of alcohols 372 Ammonia 209 Ammoxidation 388 Aniline 374 Annulene 79 Anti-Loewenstein linkage 245 Aromatic guest 83 Aromatics hydroxylation 222

Back-diffusion 434 Basic zeolite 381 Beckmann rearrangement 376 B e e r - ~ b e r t law 201 Benzaldehyde 375, 389 Benzene alkylation 31 Benzene hydrogenation 34 Benzene hydroxylation 33, 34 Benzonitrile 375 Benzylamine rearrangement 376 Benzylidine 375 Bifunctional catalysts 391, 393 Bimetallic clusters 282 Binary permeation 437 Binder 250 Binding energy 192 Biphenyl 341 Blockage 175 Boltzmann's Equation 175 Bonding radii 174 BP/UOP's Cyclar process 344 Bridging OH groups 184 Bronsted acid 204, 290 Bronsted acid sites 182 Brunauer's classification 78 2-Butyl-3-methyl-indole 376 r -Butyrolactone 373

C Cadmium ion

281

456

Cannizzaro reaction 385 Caprolactam 377 Carbanions 383 Carbenium 273 Carbocation 370 Carbogenic molecular sieves 172 Catalyst matrix 327 Catalytic/reaction distillation 337 Catalytic cracking 325 Catechol 387 Cation clusters 281 Cation crowding 282 Cation exchange capacity 50 Cavity 268, 340 Chabazite 402 Channel intersection 324 Channel size 274 Charaterization 141, 197 Charged zeolite framework 270 Charge transfer effects 391 Chemical functionalities 364, 396 Chemical induced selectivity 364 Chemical potential gradient 440 Chemical resistance 422 Chemical shift 196 Chemical shift anisotropy 142, 143 Clinoptilolite Na-, K254 Cloverite 236 Clusters 281 Coke formation 55 Complexation of oxalic acid 63 Conducting polymer 301 Conductivity 66 Conjugated 295 Cross polarization 149 Crystal growth 64 Crystallinity 59 Crystal structure of the sorbate 279 Cyclic coke precursors 353 Cyclic voltammetry 67 Cyclohexanone 374

Cyclohexanone oxime

376, 388

D

Dealkylation 30 Dealumination 217 Decapsulation 287 Deep catalytic cracking 334 Degree of sorption 277 Dehydration/hydrogenation 290 Dehydration 290 Dehydrocyclization 392 Dehydroxylation 209 Deprotonation 380 Dewaxing 353 Diacetonaldehyde 383 2, 6-Dialkylnaphthalene 252, 338 Dianin's Compound 76 Diffusional constraint 251 Diffusivities 396 Dirnethyldisulfide 280 Dirnethylether 371 Dimethy~ 375 Diphenyl-isomers 341 Dipolar interaction 143, 278 Disproportionation 385 Dissociative process 280 Double orientation rotation 146 Dynamic angle spinning 146

Effective cross-section 274 Electrochemical property 66 Electrolyte 270 Electronegativity 206 Electronic wave function 274 Electron pair 267 Electron pair acceptor 217 Electron pair donor 218 Electrophilic substitution 378 Electrostatic field 270 Eley-Rideal type mechanism 371

457 Enamine-amine isomerization 377 Enantiomeric preference 91 Encapsulation 286 Endo-isomerization 369 'End on' adsorption 392 Enzyme cytochrome P-450 394 Epoxidation 387 Epoxidation hexene 33 norbornene 32 olefm 33 terpineol 32 EPR 98 Equilibrium conversion 447 Erionite 402 ESEM 105 ESR 281 Estefification 447 Etherification 371 1-Ethyhaaphthalene 330 Eucapsulation 298 EXAFS 67, 130 Exo-isomerizafion 369

Far-IR 109 FCC 30 FCC catalysts 391 Fermi level 193, 283 Ferrierite 402 H-Ferrierite 350 Fickian diffusion 440 Fine chemicals 363 Fischer indole synthesis 376 Fluid bed reactor 326 Framework defects 419 Framework dynamic 244 Framework polarity 396 Friedel-Crafts alkylation 379 FSM-16 9, 34 A1-FSM-16 15 Funfionalities 365

Gel Chemistry 247 Global optimization approach Graphical method 236 Graphite-link filament 316 Guest atoms 284 Guest moiety 75 Guest molecules 271, 419 Guest species 83

238

H

Halogermtion 381 Hard-sphere exclusion rule 243 Hartmaun-Hahn condition 149 Heterocycles 373 Heterolytic mechanism 387 Hex-l-ene 395 Hexagon 76 Hexyl-2-acetate 390 HMS Ti-HMS 18, 33 V-HMS 20, 33 Host-guest method 88 Hydrocracking 29, 44 Hydrogen bonding 115, 277 Hydrogen ion 272 Hydroisomerization 44 Hydrolysis 280 Hydronium 273 Hydrophilicity 396 Hydrophobicity 396 Hydrothermal alteration 62 Hydrothermal synthesis 240, 415 w-Hydroxybutryonitrile 374 Hydroxylamine 388 Hydroxylation 387 HZSM-5 Ag-HZSM-5 345 Ga-HZSM-5 345

458

Kinetics KL IFP's Aroforrning process 345 Immobile organic molecule 251 Inclusion 75, 89 Indole 376 Infrastructure 256 Initial growth 249 Inorganic adhesive 429 In-situ MAS ~ probe 178 Interatomic interaction 249 Intercrystalline pores 424 Intermolecular interaction 75 Inverse shape selectivity 253 Ion-dipolar 277 Ion-exchange 66, 267, 290, 414 Ionic dissociation 280 Ionic or molecular diffusion 251 Ionic radii 274 Ion-to-induced-dipole 277 IR 106 Isobutane alkylation 30,31 Isobutene 349 Isolated silicate cage 248 Isolated sorbate 249 Isomerization of n-butane 367 Isomorphous substitution 78, 63 Isophthalic acid 337 Isopropanol 384 Isosteric heat 254 Isotopical framework 402 Isotropic nature 75

275

BaKL 393 Pt/KL 348, 392 Knudsen diffusion 422 Kundsen selectivity 438 KOOPMAN's theorem 196

L Langmuir adsorption 443 I . a t ~ i structure 81 Larmor frequency 145 Lattice oxygen mobility 68 Lattice oxygens 366 LCDs 338 LCO (light cycle oil) 327 Leak-free filling 416 Length-scale domain 257 Lennard-Jones potential 254 L E V framework 248 Limonene 393 Linear alkylbenzenes 341 Line broadening phenomena 143 Liquid crystal 3, 35 Liquid phase oxidation 389 Localized cationic clusters 281 Local T-site substitution 242 Loewenstein constraint 241, 245 Long-range coherence 240 Lube dewaxing processes 354

M

J. V. Smith's compilation Jiggle step 238

236

K

Ketal 374 Ketones ammoximation Kinetic diameter 419

222

M41S 1, 29 Macroporous oxides 391 Magdelung potential 197, 207 Magic angle spinning 146 Magnetic property 210 Magnetic susceptibility 211 Magnetization 149 Maleic acid 208

459

Mass spetrometry 112 Maxwell-Stefan diffusivity 443 Mazzite 403 MCM 330 MCM-22 331, 401 MCM-36 401 MCM-41 3, 34 MCM-41 400 AI-MCM-41 18, 34 B-MCM-41 23 Fe-MCM-41 23 Mn-MCM-41 23 V-MCM-41 20, 21, 33 MCM-41 host 316 MCM-48 3, 23 Mn-MCM-48 23 MCM-50 3, 25 Mechanisms for permeation 414 Membrane Dense 413 Large pore zeolite 427 Medium pore zeolite 423 Mesoporous 413 Mieroporous 413 Nanoporous 299 Silicalite-I 428 Zeofite A 422 Zeolite-based catalytic 449 Membrane configuration 444 Mesitylene 49 Mesoporous interlayer 411 Mesoporous materials 1,44 Metal cluster 235 Metal contamination 332 Metal-support interactions 391, 392 Metal wool 415 Methoxonium ion 372 344 Methylcyclopentane 1-Methylnapthalene 330, 399 373 Methyl scavenging 1, 2-Methyl shift 339 4-Methyl thiazole 383 Metropoles sampling algorithm 249 MFI-layers 415 Micelles 3, 12

Microc~orimetry 218 Micropore architecture 235, 243 Microporous carbon 237 Microporous crystal 244 Mitsubishi's Z-Forming 345 Mobil-Badger process 336 Modelling 231 Modelling permeation 439 Modification of ceramic membranes Molecular-based electronics 296 Molecular conformation 247 Molecular diffusivity 258 Molecular sieves i, 35 Molten 239 Monte-Carlo method 233 Mordenite 252, 403, 432 MSU-n 8, 12, 13 Multiple pulse sequences 156 Multiplet splitt~g 198

Nanoclusters 285 Nanocomposites 300 Naphthalenes 389, 420 4-napthoquinones 389 Natural mordenite 275 Neutral framework 269 Ni impregenation 332 Nitration 380 NMR ~A1 150, 163, 17, 27 nB 23,151 ~Be 151 13C 12, 169, 172 l~Cs 177

'~F

190

~H :~W

7

~3Ge XH

170

151 15, 173

4, 169 150

422

460 31p

150

mSi 13, 16, 18,1 50 mV 179 l~Xe 151, 172 NMR self diffusion 179 Non-clashing positions 245 Non-framework A1 235 Non-framework cation 244, 271 Nonostructures 295 Non-stoichiometric 268 Nuclear spin interactions 142, 143 Nucleation 249 Nucleophilic substitution 370, 372

Occluded guest 75 Occluded material 269 Occlusion 247 Octahedral molecular sieves 47,54 Offretite 403 Oil refining 343 Olefm isomerization 382 Oligomerization 29, 30, 34 Oligomerization of light alkenes 353 Omitting symmetry 236 Optically active product 86 Optimal active site 257 Organic templates 247 Organic zeolite 75 Outer surface modification 414 Oxidation aniline 32, 33 cyclododecanol 32 hexane 32 hexene 33 terpineol 32 Oxidation catalysis 50 Oxonium 273 Oxonium ions 280 Oxyfunctionalization 394 Oxygen sites 53

Para/ortho-subsitimtion 339 Para-selectivity 342 Partial oxidation of xylenes 337 Passivation 327 Pd cluster 391 Percolation 287 Permeation measurement 429 Peroxotitanate 386 Perturbation 247 PFG technique 176 Phenol 374 Phenol hydroxylation 222 Phenol tetrahydropyranylation of 30 hydroxylation of 33 Phenylhydrazine 376 Photocatalysis 67 Photocyclization 86 Photooxidation catalyst 63 Phthalocyanines in zeolites 394 Pinholes 424 Polarity 367 Polarizability 376 Polyacetylene 302 Polyaniline 310 Polyatomic cationic cluster 182 Polyhedral unit 236 Polyisopropylbenzenes 337 Polymerization 312 Polypyrrole 305 Poly-substituted ethylbenzenes 336 Polythiophene 310 Pore apertures 418 Pore architecture 244, 251 Pore blockage 250 Pore coordination 244 Pore dimensions 417 Pore size distribution 176, 328 Positive framework 270 Post-treatment 343 Potential transferability 253 Pressure drop 431 Pressure swing adsorption 421

461 Primary cracking 329 Proble molecule 110 Proton transfer 115, 373 Pulsed field gradient techniques Pyridine 209 Pyrrole 217 2-PylTolidinone 373

Quadrupolar interactions 144 Quantitation 201 Quantum mechanical 241, 248 Quasi equilibrium 398 Quinones 387

Raman 123 Raman microspectroscopy 126 Redox process 280 Redox properties 389 Referencing 200 Reflux method 69 Reforming reactions 352 Regioselective oxidation 394 Relaxtion 196 Ring oxidation 389 Rotor synchronization 149

SAPO-34 383 SBA-1 12 SBA-2 12, 15 Secondary pores 329 Second guest 77 Selective oxidation 693, 385 Selectoforming reactions 391 Self-inclusion 75 Shape-selectivity 364, 396 Product selectivity 397

179

Reactant selectivity 397 Ship-in-a-bottle systems 393 Sflicalite 387 Ti-Ga387 Silver cluster 288 Single jump model 181 Skeletal isomerization 349, 369 Sol-gel technique 416 Solid electrolyte 284 Solid state reaction 85 Solvent-type effects 396 Sorption 267,2 77 Spatial constraints 384 Specific interface chemistry 364 Spin coherence transfer 156 Spin diffusion 152 Spinel phases 58 Spin glass material 66 Stability 296 Stacking 76 Stereochemistry 87 Stereoisomer 89 Stereoselective reduction 384 Steric constraints 86 Stochastic model 258 St~-uctural constraint 238, 244 Structure sensitive reactions 393 Supercage 212 Supramolecular 395 Surface diffusion 414, 441 Surface reconstruction 82 Surface silanol gruops 387 Sweep gas 424 Synergistic effect 64 Synthesis of hydrazine 222

Tautomerisation 376 TEM 13, 28 Thermal stability 86 Thin film 75, 413 Thiol radicals 180 Ti-MCM-41 18, 33

462

Time resolved IR 117 Titanosilicalite(TS-1) 386 Toluidines 399 Topology 76 Transition state complex 324, 398 Tributyl phosphonate 342 Trimesic acid 105

Ultra-stable zeolite Y (USY) I.W-Visible 127

V Van der Waals liquid Van der Waals radii VPI-5 394

278 274

W

Wicke-Kallenbach cell 424 Woodward-Hoffmann rule 90 Work function 193

X z

XANES XPS

130, 223 67, 191

Y Y zeolite

202, 209, 214, 326

Zeolite A 271, 276 Zeolite lattice modes Zeolite layer 415

106, 123

326

Zeolite Zeolite Zeolite Zeolite

LTA 287 organic 75 sulfide 269 X Na-, K254 Zn-Y 281 ZSM-5 402 ZSM-22 ZSM-23 ZSM-35 ZSM-50

Pt/ZSM-5 52 52, 402 52 331

393

463 S T U D I E S IN SURFACE SCIENCE A N D CATALYSIS Advisory Editors: B. Delmon, Universite Catholique de Louvain, Louvain-la-Neuve, Belgium J.T. Yates, University of Pittsburgh, Pittsburgh, PA, U.S.A.

Volume 1

Volume 2

Volume 3

Volume 4

Volume 5

Volume 6 Volume 7 Volume 8 Volume 9 Volume 10 Volume 11

Volume 12 Volume 13 Volume 14

Preparation of Catalysts I.Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the First International Symposium, Brussels, October 14-17,1975 edited by B. Delmon, P.A.Jacobs and G. Poncelet The Control of the Reactivity of Solids. A Critical Survey of the Factors that Influence the Reactivity of Solids, with Special Emphasis on the Control of the Chemical Processes in Relation to Practical Applications by V.V. Boldyrev, M. Bulens and B. Delmon Preparation of Catalysts I1. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Second International Symposium, Louvain-la-Neuve, September 4-7, 1978 edited by B. Delmon, P. Grange, P.Jacobs and G. Poncelet Growth and Properties of Metal Clusters. Applications to Catalysis and the Photographic Process. Proceedings ofthe 32nd International Meeting ofthe Societe de Chimie Physique, Villeurbanne, September 24-28, 1979 edited by J. Bourdon Catalysis by Zeolites. Proceedings of an International Symposium, Ecully (Lyon), September 9-11, 1980 edited by B. Imelik, C. Naccache, Y. Ben Taarit, J.C. Vedrine, G. Coudurier and H. Praliaud Catalyst Deactivation. Proceedings of an International Symposium, Antwerp, October 13-15,1980 edited by B. Delmon and G.F. Froment New Horizons in Catalysis. Proceedings of the 7th International Congress on Catalysis, Tokyo, June 30-July4, 1980. Parts A and B edited by T. Seiyama and K. Tanabe Catalysis by Supported Complexes by Yu.l. u B.N. Kuznetsov and V.A. Zakharov Physics of Solid Surfaces. Proceedings of a Symposium, Bechyhe, September 29-October 3,1980 edited by M. Lazni6ka Adsorption at the Gas-Solid and Liquid-Solid Interface. Proceedings of an International Symposium, Aix-en-Provence, September 21-23, 1981 edited by J. Rouquerol and K.S.W. Sing Metal-Support and Metal-Additive Effects in Catalysis. Proceedings of an International Symposium, Ecully (Lyon), September 14-16, 1982 edited by B. Imelik, C. Naccache, G. Coudurier, H. Praliaud, P. Meriaudeau, P.Gallezot, G.A. Martin and J.C. Vedrine Metal Microstructures in Zeolites. Preparation - Properties- Applications. Proceedings of a Workshop, Bremen, September 22-24, 1982 edited by P.A. Jacobs, N.I. Jaeger, P.Jin3 and G. Schulz-Ekloff Adsorption on Metal Surfaces. An Integrated Approach edited by J. Benard Vibrations at Surfaces. Proceedings of the Third International Conference, Asilornar, CA, September 1-4, 1982 edited by C.R. Brundle and H. Morawitz

464 Volume 15 Volume 16

Volume 17 Volume 18 Volume 19 Volume 20 Volume 21 Volume 22 Volume 23 Volume 24 Volume 25 Volume 26 Volume 27 Volume 28 Volume 29 Volume 30 Volume 31

Volume 32 Volume 33 Volume 34

Heterogeneous Catalytic Reactions Involving Molecular Oxygen by G.I. Golodets Preparation of Catalysts III. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Third International Symposium, Louvain-la-Neuve, September 6-9, 1982 edited by G. Poncelet, P. Grange and P.A. Jacobs Spillover of Adsorbed Species. Proceedings of an International Symposium, Lyon-Villeurbanne, September 12-16, 1983 edited by G.M. Pajonk, S.J. Teichner and J.E. Germain Structure and Reactivity of Modified Zeolites. Proceedings of an International Conference, Prague, July 9-13, 1984 edited by P.A. Jacobs, N.I. Jaeger, P.Jin3, V.B. Kazansky and G. Schulz-Ekloff Catalysis on the Energy Scene. Proceedings of the 9th Canadian Symposium on Catalysis, Quebec, P.Q., September 30-October 3, 1984 edited by S. Kaliaguine and A. Mahay Catalysis by Acids and Bases. Proceedings of an International Symposium, Villeurbanne (Lyon), September 25-27, 1984 edited by B. Imelik, C. Naccache, G. Coudurier, Y. Ben Taarit and J.C. Vedrine Adsorption and Catalysis on Oxide Surfaces. Proceedings of a Symposium, Uxbridge, June 28-29, 1984 edited by M. Che and G.C. Bond Unsteady Processes in Catalytic Reactors by Yu.Sh. Matros Physics of Solid Surfaces 1984 edited by J. Koukal Zeolites: Synthesis, Structure, Technology and Application. Proceedings of an International Symposium, Portoro~-Portorose, September 3-8, 1984 edited by B. Dr~aj, S. Ho~evar and S. Pejovnik Catalytic Polymerization of Olefins. Proceedings of the International Symposium on Future Aspects of Olefin Polymerization, Tokyo, July 4-6, 1985 edited by T. Keii and K. Soga Vibrations at Surfaces 1985. Proceedings of the Fourth International Conference, Bowness-on-Windermere, September 15-19, 1985 edited by D.A. King, N.V. Richardson and S. Holloway Catalytic Hydrogenation edited by L. Cerveny New Developments in Zeolite Science and Technology. Proceedings of the 7th International Zeolite Conference, Tokyo, August 17-22, 1986 edited by Y. Murakami, A. lijima and J.W. Ward Metal Clusters in Catalysis edited by B.C. Gates, L. Guczi and H. Kn6zinger Catalysis and Automotive Pollution Control. Proceedings of the First International Symposium, B'russels, September 8-11,1986 edited by A. Crucq and A. Frennet Preparation of Catalysts IV. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fourth International Symposium, Louvain-la-Neuve, September 1-4, 1986 edited by B. Delmon, P.Grange, P.A. Jacobs and G. Poncelet Thin Metal Films and Gas Chemisorption edited by P.Wissmann Synthesis of High-silica Aluminosilicate Zeolites edited by P.A. Jacobs and J.A. Martens Catalyst Deactivation 1987. Proceedings of the 4th International Symposium, Antwerp, September 29-October 1, 1987 edited by B. Delmon and G.F. Froment

465 Volume 35 Volume 36 Volume 37 Volume 38 Volume 39 Volume 40 Volume 41

Volume 42 Volume 43 Volume 44

Volume 45 Volume 46

Volume 47 Volume 48 Volume 49 Volume 50

Volume 51 Volume 52 Volume 53

Keynotes in Energy-Related Catalysis edited by S, Kaliaguine Methane Conversion. Proceedings of a Symposium on the Production of Fuels and Chemicals from Natural Gas, Auckland, April 27-30, 1987 edited by D.M. Bibby, C.D. Chang, R.F. Howe and S. Yurchak Innovation in Zeolite Materials Science. Proceedings of an International Symposium, Nieuwpoort, September 13-17, 1987 edited by P.J. Grobet, W.J. Mortier, E.F.Vansant and G. Schulz-Ekloff Catalysis 1987. Proceedings ofthe 10th North American Meeting ofthe Catalysis Society, San Diego, CA, May 17-22, 1987 edited by J.W. Ward Characterization of Porous Solids. Proceedings of the IUPAC Symposium (COPS I), Bad Soden a. Ts., April 26-29,1987 edited by K.K. Unger, J. Rouquerol, K.S.W. Sing and H. Kral Physics of Solid Surfaces 1987. Proceedings of the Fourth Symposium on Surface Physics, Bechyne Castle, September 7-11, 1987 edited by J. Koukal Heterogeneous Catalysis and Fine Chemicals. Proceedings of an International Symposium, Poitiers, March 15-17, 1988 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, C. Montassier and G. Perot Laboratory Studies of Heterogeneous Catalytic Processes by E.G. Christoffel, revised and edited by Z. Paal Catalytic Processes under Unsteady-State Conditions by Yu. Sh. Matros Successful Design of Catalysts. Future Requirements and Development. Proceedings ofthe Worldwide Catalysis Seminars, July, 1988, on the Occasion of the 30th Anniversary of the Catalysis Society of Japan edited by T. Inui Transition Metal Oxides. Surface Chemistry and Catalysis by H.H. Kung Zeolites as Catalysts, Sorbents and Detergent Builders. Applications and Innovations. Proceedings of an International Symposium, WLirzburg, September 4-8,1988 edited by H.G. Karge and J. Weitkamp Photochemistry on Solid Surfaces edited by M. Anpo and T. Matsuura Structure and Reactivity of Surfaces. Proceedings of a European Conference, Trieste, September 13-16, 1988 edited by C. Morterra, A. Zecchina and G. Costa Zeolites: Facts, Figures, Future. Proceedings of the 8th International Zeolite Conference, Amsterdam, July 10-14, 1989. Parts A and B edited by P.A. Jacobs and R.A. van Santen Hydrotreating Catalysts. Preparation, Characterization and Performance. Proceedings of the Annual International AIChE Meeting, Washington, DC, November 27-December 2, 1988 edited by M.L. Occelli and R.G. Anthony New Solid Acids and Bases. Their Catalytic Properties by K. Tanabe, M. Misono, Y. Ono and H. Hattori Recent Advances in Zeolite Science. Proceedings of the 1989 Meeting of the British Zeolite Association, Cambridge, April 17-19, 1989 edited by J. Klinowsky and P.J. Barrie Catalyst in Petroleum Refining 1989. Proceedings of the First International Conference on Catalysts in Petroleum Refining, Kuwait, March 5-8, 1989 edited by D.L. Trimm, S. Akashah, M. Absi-Halabi and A. Bishara

466 Volume 54

Future Opportunities in Catalytic and Separation Technology edited by M. Misono, Y. Moro-oka and S. Kimura Volume 55 New Developments in Selective Oxidation. Proceedings of an International Symposium, Rimini, Italy, September 18-22, 1989 edited by G. Centi and F.Trifiro Volume 56 Olefin Polymerization Catalysts. Proceedings of the International Symposium on Recent Developments in Olefin Polymerization Catalysts, Tokyo, October 23-25, 1989 edited by T. Keii and K. Soga Volume 57A Spectroscopic Analysis of Heterogeneous Catalysts. Part A: Methods of Surface Analysis edited by J.L.G. Fierro Volume 57B Spectroscopic Analysis of Heterogeneous Catalysts. Part B: Chemisorption of Probe Molecules edited by J.L.G. Fierro Volume 58 Introduction to Zeolite Science and Practice edited by H. van Bekkum, E.M. Flanigen and J.C. Jansen Volume 59 Heterogeneous Catalysis and Fine Chemicals II. Proceedings of the 2nd International Symposium, Poitiers, October 2-6, 1990 edited by M. Guisnet, J. Barrault, C. Bouchoule, D. Duprez, G. Perot, R. Maurel and C. Montassier Volume 60 Chemistry of Microporous Crystals. Proceedings of the International Symposium on Chemistry of Microporous Crystals, Tokyo, June 26-29, 1990 edited by T. Inui, S. Namba and T. Tatsumi Volume 61 Natural Gas Conversion. Proceedings of the Symposium on Natural Gas Conversion, Oslo, August 12-17, 1990 edited by A. Holmen, K.-J. Jens and S. Kolboe Volume 62 Characterization of Porous Solids II. Proceedings of the IUPAC Symposium (COPS II), Alicante, May 6-9, 1990 edited by F. Rodriguez-Reinoso, J. Rouquerol, K.S.W. Sing and K.K. Unger Volume 63 Preparation of Catalysts V. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Fifth International Symposium, Louvain-la-Neuve, September 3-6, 1990 edited by G. Poncelet, P.A. Jacobs, P.Grange and B. Delmon Volume 64 New Trends in CO Activation edited by L. Guczi Volume 65 Catalysis and Adsorption by Zeolites. Proceedings of ZEOCAT 90, Leipzig, August 20-23, 1990 edited by G. (~hlmann, H. Pfeifer and R. Fricke Volume 66 Dioxygen Activation and Homogeneous Catalytic Oxidation. Proceedings of the Fourth International Symposium on Dioxygen Activation and Homogeneous Catalytic Oxidation, Balatonf~red, September 10-14, 1990 edited by L.I. Simandi Volume 67 Structure-Activity and Selectivity Relationships in Heterogeneous Catalysis. Proceedings of the ACS Symposium on Structure-Activity Relationships in Heterogeneous Catalysis, Boston, MA, April 22-27, 1990 edited by R.K. Grasselli and A.W. Sleight Volume 68 Catalyst Deactivation 1991. Proceedings of the Fifth International Symposium, Evanston, IL, June 24-26, 1991 edited by C.H. Bartholomew and J.B. Butt Volume 69 Zeolite Chemistry and Catalysis. Proceedings of an International Symposium, Prague, Czechoslovakia, September 8-13, 1991 edited by P.A. Jacobs, N.I. Jaeger, L. Kubelkova and B. Wichterlova

467 Volume 70 Volume 71 Volume 72

Volume 73 Volume 74 Volume 75 Volume 76 Volume 77

Volume78

Volume79 Volume80 Volume81 Volume82

Volume83 Volume84

Volume85

Poisoning and Promotion in Catalysis based on Surface Science Concepts and Experiments by M. Kiskinova Catalysis and Automotive Pollution Control II. Proceedings of the 2nd International Symposium (CAPoC 2), Brussels, Belgium, September 10-13, 1990 edited by A. Crucq New Developments in Selective Oxidation by Heterogeneous Catalysis. Proceedings of the 3rd European Workshop Meeting on New Developments in Selective Oxidation by Heterogeneous Catalysis, Louvain-la-Neuve, Belgium, April 8-10, 1991 edited by P. Ruiz and B. Delmon Progress in Catalysis. Proceedings of the 12th Canadian Symposium on Catalysis, Banff, Alberta, C3nada, May 25-28, 1992 edited by K.J. Smith and E.C. Sanford Angle-Resolved Photoemission. Theory and Current Applications edited by S.D. Kevan New Frontiers in Catalysis, Parts A-C. Proceedings of the 10th International Congress on Catalysis, Budapest, Hungary, 19-24 July, 1992 edited by L. Guczi, F. Solymosi and R Tetenyi Fluid Catalytic Cracking: Science and Technology edited by J.S. Magee and M.M. Mitchell, Jr. New Aspects of Spillover Effect in Catalysis. For Development of Highly Active Catalysts. Proceedings of the Third International Conference on Spillover, Kyoto, Japan, August 17-20, 1993 edited by T. Inui, K. Fujimoto, T. Uchijima and M. Masai Heterogeneous Catalysis and Fine Chemicals III. Proceedings ofthe 3rd International Symposium, Poitiers, April 5 - 8, 1993 edited by M. Guisnet, J. Barbier, J. Barrault, C. Bouchoule, D. Duprez, G. Perot and C. Montassier Catalysis: An Integrated Approach to Homogeneous, Heterogeneous and Industrial Catalysis edited by J.A. Moulijn, P.W.N.M. van Leeuwen and R.A. van Santen Fundamentals of Adsorption. Proceedings of the Fourth International Conference on Fundamentals of Adsorption, Kyoto, Japan, May 17-22, 1992 edited by M. Suzuki Natural Gas Conversion I1. Proceedings of the Third Natural Gas Conversion Symposium, Sydney, July 4-9, 1993 edited by H.E. Curry-Hyde and R.F. Howe New Developments in Selective Oxidation I1. Proceedings of the Second World Congress and Fourth European Workshop Meeting, Benalmadena, Spain, September 20-24, 1993 edited by V. Cortes Corberan and S. Vic Bellon Zeolites and Microporous Crystals. Proceedings of the International Symposium on Zeolites and Microporous Crystals, Nagoya, Japan, August 22-25, 1993 edited by T. Hattori and T. Yashima Zeolites and Related Microporous Materials: State of the Art 1994. Proceedings ofthe 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by J. Weitkamp, H.G. Karge, H. Pfeifer and W. H61derich Advanced Zeolite Science and Applications edited by J.C. Jansen, M. St6cker, H.G. Karge and J.Weitkamp

468 Volume86 Volume87 Volume88 Volume 89

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Volume 101 Volume 102

Oscillating Heterogeneous Catalytic Systems by M.M. Slin'ko and N.I. Jaeger Characterization of Porous Solids III. Proceedings of the IUPAC Symposium (COPS III), Marseille, France, May 9-12, 1993 edited by J.Rouquerol, F. Rodriguez-Reinoso, K.S.W. Sing and K.K. Unger Catalyst Deactivation 1994. Proceedings of the 6th International Symposium, Ostend, Belgium, October 3-5, 1994 edited by B. Delmon and G.F. Froment Catalyst Design for Tailor-made Polyolefins. Proceedings of the International Symposium on Catalyst Design for Tailor-made Polyolefins, Kanazawa, Japan, March 10-12, 1994 edited by K. Soga and M. Terano Acid-Base Catalysis II. Proceedings of the International Symposium on Acid-Base Catalysis II, Sapporo, Japan, December 2-4, 1993 edited by H. Hattori, M. Misono and Y. Ono Preparation of Catalysts VI. Scientific Bases for the Preparation of Heterogeneous Catalysts. Proceedings of the Sixth International Symposium, Louvain-La-Neuve, September 5-8, 1994 edited by G. Poncelet, J. Martens, B. Delmon, P.A. Jacobs and P. Grange Science and Technology in Catalysis 1994. Proceedings of the Second Tokyo Conference on Advanced Catalytic Science and Technology, Tokyo, August 21-26, 1994 edited by Y. Izumi, H. Arai and M. Iwamoto Characterization and Chemical Modification of the Silica Surface by E.F.Vansant, R Van Der Voort and K.C. Vrancken Catalysis by Microporous Materials. Proceedings of ZFOCAT'95, Szombathely, Hungary, July 9-13, 1995 edited by H.K. Beyer, H.G.Karge, I. Kiricsi and J.B. Nagy Catalysis by Metals and Alloys by V. Ponec and G.C. Bond Catalysis and Automotive Pollution Control III. Proceedings of the Third International Symposium (CAPoC3), Brussels, Belgium, April 20-22, 1994 edited by A. Frennet and J.-M. Bastin Zeolites: A Refined Tool for Designing Catalytic Sites. Proceedings of the International Symposium, Qu@bec, Canada, October 15-20, 1995 edited by L. Bonneviot and S. Kaliaguine Zeolite Science 1994: Recent Progress and Discussions. Supplementary Materials to the 10th International Zeolite Conference, Garmisch-Partenkirchen, Germany, July 17-22, 1994 edited by H.G. Karge and J. Weitkamp Adsorption on New and Modified Inorganic Sorbents edited by A. Dqbrowski and V.A. Tertykh Catalysts in Petroleum Refining and Petrochemicals Industries 1995. Proceedings of the 2nd International Conference on Catalysts in Petroleum Refining and Petrochemical Industries, Kuwait, April 22-26, 1995 edited by M. Absi-Halabi, J. Beshara, H. Qabazard and A. Stanislaus 1lth International Congress on Catalysis - 40th Anniversary. Proceedings ofthe 1lth ICC, Baltimore, MD, USA, June 30-July 5, 1996 edited by J. W. Hightower, W.N. Delgass, E. Iglesia and A.T. Bell Recent Advances and New Horizons in Zeolite Science and Technology edited by H. Chon, S.I. Woo and S. -E. Park