Vol 2 - Structure and Structure Determination 1999.pdf - Supramolecular - ziolek6661

Preface to Volume 2

Once a new natural zeolite is found or a new molecular sieve synthezised,via one

or the other of the methods described in Volume 1 for example, the researchers

face the task of confirming that a novel structure has come into their hands.

However, beyond this basic problem, questions soon arise concerning rather

detailed and subtle structural features.

The classical method of determining crystal structures is X-ray diffraction.

Thus, in Chapter 1 of the present volume, H. van Koningsveld and M. Bennett

provide the reader with information about the enormous progress which has

been made in X-ray structure analysis of zeolites.To a large extent,this is due to

outstanding developments in both experimental techniques and methods of

data evaluation, such as the application of synchrotron radiation and Rietveld

analysis. New methods now enable crystallographers to study very small single

crystals or crystallite powders.This is extremely important with respect to most

of the synthetic micro- and mesoporous materials since the size of primary par-

ticles is usually in the µm range. The authors stress that, in the context of

reliable structure analysis, the determination of the unit cell and space group is

of paramount importance. Modern tools now allow researchers to study subtle

effects on zeolite structures such as those caused by framework distortions,

dealumination, isomorphous substitution or cation and sorbate location.

The study of structures containing light atoms is the particular domain of

neutron scattering, even though this is not its only advantage. The authors of

Chapter 2, A.N.Fitch and H.Jobic demonstrate the way in which neutron scatter-

ing is able to complement structure analysis by X-ray diffraction. In particular,

neutron scattering techniques reveal their strong potential in probing details of

structural arrangements involving hydrogen-containing species (such as water

and hydroxyl groups) as well as determining hydrogen bonds, cation positions,

and the location of adsorbed molecules. Frequently these techniques are suc-

cessfully used for further refinement of X-ray diffraction data.

Chapter 3,written by O. Terasaki ,is devoted to the use of the various kinds of

electron microscopy in the investigation of zeolites and related porous solids.

The author’s contribution focuses on the potential of electron microscopy in

studying crystallite morphologies as well as features of the fine structure, e.g.,

bulk and surface defects; details of the crystal surface (edges and kinks),and,as

such, related to crystal growth; and modification of frameworks. Moreover, the

valuable assistance of electron microscopy in solving new structures is illustrat-

ed by a number of examples.

VIII

Preface to Volume 2

Chapter 4 is contributed by W. Depmeier , and it concerns particular pheno-

mena of the structures of zeolites and related solids which are attracting more

and more interest. Such phenomena are, inter alia, phase transitions as well as

mechanisms of reduction in symmetry and volume as a consequence of tilting,

distortion of the whole framework or framework units, modulations of the

framework, and partial amorphization. These are demonstrated by a variety

of instructive examples, and their importance is pointed out in view of, for

example,catalytic,shape selective and separation properties of zeolite materials.

General problems of zeolite structures are dealt with in Chapter 5 which is

jointly authored by W. M. Meier and C. Baerlocher . It includes basic aspects of

zeolite crystallography such as topology, configuration, and conformation of

framework structures. Similarly, the idea of distinguishing zeolites on the basis

of framework densities is presented. The attempts at classification of zeolite

structure types are critically discussed. The authors then describe the inter-

esting concepts of structural characterization via loop configurations and coor-

dination sequences and also reconsider the long-standing question of whether

zeolite framework structures are predictable.

This volume concludes with Chapter 6,a review devoted to industrial synthe-

sis.Contributed by A. Pfenninger and entitled “Manufacture and Use of Zeolites

for Adsorption Processes”, this chapter provides an extremely useful adjunct to

Volume 1 of this series. Important aspects of industrial synthesis are described

and,simultaneously,the characterization and use of zeolites for separation pro-

cesses are discussed.In these respects,Chapter 6 is something of an introduction

to matters which will be extensively dealt with in Volume 5 (Characterization II)

and Volume 7 (Sorption and Diffusion) of this series.

The originally planned final chapter on the role played by solid state NMR

spectroscopy in the elucidation of structural features of microporous and meso-

porous materials was unfortunately not available at the time of going to press.

However, given the importance of this topic, an appropriate treatment of this

area is intended to appear in Volume 4 (Characterization I).

Thus,Volume 2 presents an extended overview over most of the relevant tech-

niques currently employed for investigations into structural properties of

micro- and mesoporous materials and offers in its last contribution a valuable

addition to the topics treated in Volume 1. From this volume it becomes evident

that the various techniques for structure determination are, to a large extent,

complementary and that evaluation of the experimental data,on the other hand,

is profiting much from recent developments in theory and modeling. It is the

Editors’ hope that Volume 2 of the series “Molecular Sieves – Science and

Technology” will provide the researchers in the field of zeolites and related

materials with the necessary awareness of the great potential in modern

methods for structure analysis.

Hellmut G. Karge

Jens Weitkamp

Contents

H. van Koningsveld and J. M. Bennett:

Zeolite Structure Determination from X-Ray Diffraction . . . . . . . .

A. N. Fitch and H. Jobic:

Structural Information from Neutron Diffraction . . . . . . . . . . . .

O. Terasaki:

Electron Microscopy Studies in Molecular Sieve Science . . . . . . . . .

W. Depmeier:

Structural Distortions and Modulations in Microporous Materials . . . . 113

W. M. Meier and C. Baerlocher:

Zeolite Type Frameworks: Connectivities, Configurations

and Conformations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141

A. Pfenninger:

Manufacture and Use of Zeolites for Adsorption Processes . . . . . . . . 163

Subject Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199

Author Index Vols. 1 and 2 . . . . . . . . . . . . . . . . . . . . . . . . . . 215

Zeolite Structure Determination from X-Ray Diffraction

H. van Koningsveld 1 and J.M.Bennett 2

1 Laboratory of Organic Chemistry and Catalysis, Delft University of Technology,

Julianalaan 136, 2628 BL Delft, The Netherlands; e-mail: havank@cad4sun.tn.tudelft.nl

2 661 Weadley Road, Radnor, PA 19087, USA; e-mail: JMBXrayse@aol.com

1Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

2 Severe Overlap of Reflections in Powder Data . . . . . . . . . . . . . . 3

3 Incorrect Determination of the Space Group . . . . . . . . . . . . . . . 5

4 Effect of Framework Flexibility . . . . . . . . . . . . . . . . . . . . . . . 8

5 Disorder of Non-Framework Species . . . . . . . . . . . . . . . . . . . 16

6 Faulting within the Framework . . . . . . . . . . . . . . . . . . . . . . . 23

7 Isomorphous Replacement of Framework Atoms . . . . . . . . . . . . . 24

8 Crystal Size Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

9 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Introduction

Zeolites and related microporous materials are a class of materials with an ever

widening range of compositions, structures and uses. Since the earliest days of

zeolite science X-ray diffraction has been one of the basic and most useful tools

for characterization.

Initially X-ray diffraction was used to answer simple questions such as:“have

I made a new material?”or:“has the crystallization process gone to completion?”

Now the questions encompass everything that a researcher might want to know

about the structure of a material. Early attempts at determining crystal struc-

tures using X-ray diffraction were often unsuccessful because many of these ear-

ly synthetic materials were available only as powder samples. Fortunately many

of these first synthetic materials had natural counterparts with large single

crystals, and data from these were used to determine the framework structures

Molecular Sieves,Vol. 2

H. van Koningsveld · J.M. Bennett

of their synthetic counterparts. Today, the framework of a new material can be

often determined from powder samples. In addition, single crystal techniques

have improved considerably leading to increased accuracy in the bond angles

and bond distances and to the ability to study crystals of much smaller size.It is

now possible for a single crystal study to reveal details of the structure that show

the interaction of a sorbed material with the framework or movement of cations

within the framework and any ensuing distortions of the framework.Structural

data from powder samples are beginning to reveal similar changes in the crystal

structure with temperature, with sorbed materials and even under catalytic

conditions. Even though the technique of X-ray powder diffraction has im-

proved greatly since the early days of zeolite science, it is still more accurate to

determine the crystal structure of a new material from single crystal data rather

than from powder data.

Many of the advances in the structural information derived for zeolitic mate-

rials are a direct result of major improvements in powder and single crystal

X-ray equipment available, in the development of new structure determination

methods and in the use of new characterization tools including magic angle

spinning NMR, neutron diffraction and electron microscopy, which are de-

scribed in subsequent chapters. Two excellent review papers [1, 2] discuss the

use of X-ray diffraction techniques to study zeolites and the problems en-

countered, and it is recommended that they be used in combination with this

chapter.

The stages in determining the crystal structure of a material have been

described as: (i) obtain a suitable sample, (ii) collect the data, (iii) determine a

trial structure using ab initio methods, and (iv) refine the data.

However, with zeolites it is not as simple as the above infers since subtle

changes in the zeolite framework can influence, to a greater or lesser extent,

both the observed intensities and the symmetry. These subtle changes in the

observed intensities and the symmetry can cause serious problems for crystallo-

graphers performing a zeolite structure analysis.The crystallographic problems

include:

– Severe overlap of reflections in powder data leading to problems with the

techniques used to decompose the peaks into individual reflections

– Incorrect determination of the space group especially when the true symme-

try is masked by pseudo-symmetry

– The effect of framework flexibility on the structure analysis

– Disorder of the non-framework species and its effect on the structure

solution

– Faulting within the framework

– Problems caused by isomorphous replacement of framework atoms

– The effects due to small crystal size and the limits on the crystal size that can

be used

In order to help those in the zeolite community to better appreciate the beauty

of an excellent crystallographic study while learning to evaluate the pitfalls that

are present in an incorrect study,several structures,published in the last decade

and that are examples of the problems listed above, will be reviewed.

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