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| ISBN-13: | 9780854048120 |
|---|---|
| Publisher: | RSC |
| Publication date: | 10/15/2007 |
| Series: | RSC Materials Monographs , #4 |
| Pages: | 444 |
| Product dimensions: | 6.10(w) x 9.30(h) x 1.10(d) |
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Microporous Framework Solids
By Paul A. Wright
The Royal Society of Chemistry
Copyright © 2008 The Royal Society of ChemistryAll rights reserved.
ISBN: 978-0-85404-812-0
CHAPTER 1
Introduction
1.1 Microporous Framework Solids: Definitions
First, a word on definitions. The materials I shall describe in this book will be those with ordered structures that are able to adsorb molecules reversibly and selectively based on differences in their size and shape. For a long time this definition would have applied almost exclusively to aluminosilicate zeolites, crystalline solids with pore sizes up to around 8 Å, and excluded those materials such as porous carbons or ceramics derived from sol-gel preparations that possess microporosity but no regularity of structure. Nowadays the term encompasses families of microporous solids with ever-increasing compositional variety, frameworks with metals in mixed coordination and hybrid metal-organic frameworks. The definition might also include some of the newly discovered class of mesostructured solids, discovered first by researchers at Mobil in the early 1990s. As molecular sieves, it is clear that the pore size of these solids, which extends from 10 Å upwards, enables the adsorption of much larger molecules than is possible for zeolites (Figure 1.1). I have excluded a consideration of layered solids such as pillared clays, which also show porosity, because their order is predominantly in two dimensions, rather than three.
A distinction should also be drawn between framework molecular sieves and the wealth of crystalline solids prepared in the presence of an organic species that becomes incorporated in voids within the structure but cannot be removed, thermally or otherwise, without structural collapse. These are better described as open framework solids. In practice the distinction is blurred and, although only materials that are thermally and hydrothermally robust are likely to be commercially exploited in traditional technologies, open framework solids containing organic species can possess properties that may make them suitable for more specialised use.
1.2 Historical Development of the Subject
There is great current interest in the use of combinatorial methods, or high throughput experimentation (in which a large number of experiments are performed to explore the different effects of many variables), to prepare new solids by optimised routes. I will give an example of this applied to zeolite synthesis in Chapter 5. In a sense, though, the range of geochemical situations that have existed naturally has acted in the same way, and solid state chemists have learnt much from observing and preparing laboratory analogues of natural minerals, including clays, zeolites and other porous silicates and phosphates. Natural zeolites have long been recognised as a class of solids with characteristic properties. The first recorded description of a zeolite mineral was of stilbite, by Cronsted in 1756. Crystals of natural stilbite are shown in Figure 1.2: routes have since been developed for the laboratory synthesis of zeolites with the same framework structure of stilbite, and with a wide variety of compositions. Figure 1.2 also shows a scanning electron micrograph of a synthetic, high silica version of stilbite that has recently been prepared, and a representation of its microporous structure. The property of zeolites to reversibly evolve adsorbed water when heated gives rise to their name (Zeo (Greek, boiling) Lithos (Greek, stone)). Zeolites are a class of 'tectosilicate' minerals that possess tetrahedrally linked, three-dimensional frameworks made up of corner-sharing aluminate and silicate tetrahedra that are sufficiently open to be able to reversibly adsorb molecules.
Under favourable geological conditions significant deposits of useful natural zeolites (such as clinoptililite and mordenite) have resulted and are mined and used in large quantities. More often, and certainly for many of the most important commercial zeolites, mineral forms of the zeolite types only occur in minute quantities. Very small amounts of natural analogues to the widely used zeolites Y, ZSM-5 and Beta have all been found, and are known as faujasite, mutinaite and czernickite, respectively. Such mineralogical occurrences (such as those found in Antarctica) are more than interesting curiosities to the zeolite chemist: they were essential to establishing a structural basis for zeolite science (many zeolite structures have been solved from natural samples) and they point the way to unexpected structural possibilities. Two zeolites reported only as natural minerals are shown in Figure 1.3. The recently discovered structure of tschörtnerite, for example, obtained from a few crystals from the Eifel region of Germany, includes the supercage shown, which has an internal free diameter of 17.3 Å. A synthetic tschörtnerite might be of considerable use in gas separation or in detergency. Boggsite, on the other hand (also shown), possesses an intersecting channel system that would be of more interest in catalysis. Structures such as these are attractive targets for synthesis.
Zeolite mineralogy is certainly interesting and informative, but the development of zeolite chemistry was made possible by their large scale synthesis under laboratory conditions. It was recognised that zeolites could be crystallised hydrothermally from reactive precursor gels under alkaline conditions on timescales of a few days. Early synthetic work, by Richard Barrer, in academia, and industrialists such as Robert Milton, Donald Breck, George Kerr and Edith Flanigen, explored the use of alkali and alkali earth metals to direct the crystallisation of aluminosilicate gels from the 1940s onwards. These pioneering studies gave rise to many of the zeolites that are currently widely used, in particular the zeolites A, X, Y and mordenite. The characteristic zeolitic properties of these materials, such as adsorption and ion exchange, were established, their study receiving impetus with their adoption in gas drying and separation technologies and by the need for replacements for environmentally threatening phosphate ion sequestering agents in detergents. They have since retained their large scale use in these industries, with innovations prompted by new developments and considerations of interest to individual manufacturers (such as the need to prepare new products that are outside existing patent restrictions).
The compositional similarity of zeolites to silica-aluminas in widespread use as solid acid catalysts in catalytic cracking processes suggested that they could also be used in these processes if they were suitably modified. Chemical routes to the incorporation of acid sites were found, and the resulting catalysts were found to be strong solid acids (Chapter 8). In particular, zeolites based on zeolite Y are now widely used on a massive scale in catalytic cracking. Furthermore, the unique pore structure of zeolites was found to give unique and desired product selectivities. Further development in this area was made possible by the discovery by Barrer and Denny that organic cations, particularly alkylammonium ions, could be used to direct the synthesis of new structures in a similar way to alkali and alkaline earth cations. The use of bulky organic cations was found to give zeolites with high silica-to-alumina ratios and consequently high thermal stability. In particular zeolite Beta and the so-called pentasil zeolites ZSM-5 and -11 (pentasil refers to the predominance of five-membered rings in the frameworks) were discovered. These were found to possess unique activities and selectivity for a range of hydrocarbon conversions, in particular in the alkylation and isomerisation of aromatic molecules. The catalytic possibilities were greatly expanded and resulted in further penetration of zeolite catalysts into industrial processes, with increasing replacement of less selective and more corrosive acid catalysts such as supported phosphoric acid or liquid sulfuric acid. The trend continues to the present day, with new zeolite-catalysed hydrocarbon reactions continuing to meet the ever-changing demands of the petrochemical industry (Chapter 8). In parallel, programs of synthesis of novel solids using novel organic structure directing agents (or, more loosely, 'templates') continue today to widen the range of silicate-based solids (Chapters 2 and 5).
By the 1980s most of the aluminosilicate zeolites currently used industrially were known, and the emphasis shifted to the study of these materials using a range of powerful new techniques that came of age at this time. These included, in particular, solid state NMR, X-ray and neutron powder diffraction analysis, high resolution electron microscopy and computational methods. All were ideal for the study of structural details of solids that were rarely available, and never used in industrial applications, other than as microcrystalline powders. All these techniques are applicable to the bulk of the solid - this in turn makes up the (internal) surface, which is accessible to adsorbed molecules. Since the techniques are able to operate under any conditions of gas pressure, they may be used to extract structural details in situ under the operating conditions of ion exchange, adsorption and catalysis. In particular, zeolitic systems have proved ideal for the study, understanding and subsequent improvement of solid acid catalysts.
Already by 1980 it was known that molecular sieve frameworks of compositions other than aluminosilicate could be prepared. Pure silica polymorphs of ZSM-5 and ZSM-11, for example, (patented as silicalite-1 and silicalite-2) were of interest because their internal surfaces were hydrophobic rather than hydrophilic, and offered new possibilities in the separation of organics from aqueous solutions. There was also compelling evidence that elements such as B, Fe, Ga and even P could be introduced, with consequent modification of the acidic properties. Indeed, the announcement by UOP in 1982 of a new series of microporous framework aluminophosphates stimulated a major expansion of the chemical compositions available in the form of molecular sieves. Aluminophosphates possess a chemistry that is sufficiently different from that of zeolites to offer alternative catalytic possibilities, for example in selective oxidation. With a few exceptions, however, the stability of phosphate materials at high temperatures, particularly in the presence of water vapour, is lower than that of silicates, so that their applicability is more likely to be found in fields different from those exploited so successfully by zeolites. A major significance of the aluminophosphates, however, was to open up the possibility of making molecular sieves from an enormously enlarged range of building blocks. Current expression of this is to be found in the great diversity of novel microporous organic-inorganic hybrids described in Section 2.8.
The last decade has seen the increased application of zeolitic solids as catalysts outside the realm of petrochemicals, including the synthesis of fine chemicals and as potential materials for environmentally related catalysis. The development by Enichem of titanosilicate analogues of zeolites as highly efficient catalysts for a range of epoxidations and oxidations is arguably the most significant development in this area, but a wide range of more specialised chemical conversions have also been achieved (Chapter 9). Considerable recent success has also been achieved in the fabrication and use of zeolite membranes in separation technology.
As described above, the field of microporous solids has developed in a series of steps. The discovery of mesoporous silicas, with ordered pores of 20 Å and above, has stimulated a spectacular research effort in synthesis, characterisation and potential applications of these materials, such that the field has grown to a size (in terms of numbers of publications) similar to that of microporous solids. Although lacking the atomic order observed for zeolitic solids, many of the same techniques used to characterise microporous solids are also applicable to them. It is not yet apparent what the most important applications of these solids will be, but it is clear that they have caught the scientific imagination in the same way that microporous solids have, and I include a limited discussion of these materials in the appropriate chapters.
CHAPTER 2Families of Microporous Framework Solids
2.1 Introduction
As described in the first chapter, the continual discovery of novel structure types of microporous and open framework solids and their preparation within a wide compositional range has been one of the most striking features of research in this area. Figure 2.1 illustrates the chronological development of inorganic families of such solids, and emphasises their recent proliferation. The introduction of novel inorganic chemistry is particularly noticeable, and associated with this is the incorporation of framework-forming metal cations that possess five- or six-fold coordination rather than the tetrahedral coordination typical of zeolites and aluminophosphates. Furthermore, novel families of inorganic-organic hybrid solids such as metal phosphonates with inorganic frameworks 'lined' with organic groups and metal-organic frameworks made up of coordination polymers have provided an exciting recent extension. The family of mesoporous solids (of which more than 10 different structure types have so far been identified) can usefully be included in the discussion. In this chapter I will describe the important structural features of these different families.
Considering only frameworks made up entirely of tetrahedral corner-sharing TO species, full details of all the structure types are collected, refereed and published by the Structure Commission of the International Zeolite Association. The most recent publication indicates that around 170 framework types (each of which is given a unique three-letter code) have been unambiguously identified and both hardcopy publications (in particular the so-called 'Atlas of Zeolite Framework Types') and the continuously updated structural summary on the web site (www.iza-structure.org) are indispensable resources for the researcher in this field.
Figure 2.2 illustrates the way in which new types of tetrahedrally connected frameworks have been discovered and their structures solved over the last forty years, and highlights at least two important trends. The first is the decline in the discovery of framework solids formed in the presence of inorganic cations alone as structure directing agents and without the use of organic cations. This decrease is a consequence of the limited choice of such cations that are soluble under alkaline conditions, and the tremendous efforts of exploratory synthesis already made. The major recent growth of structure types has therefore occurred in syntheses that have made use of organic 'templates' and novel framework compositions, often in tandem. This importance of templated syntheses is particularly important in both the high silica zeolites and in metal phosphates (aluminium, gallium, iron phosphates, etc.). In particular the designed synthesis of alkylammonium templates by the groups of Zones, at Chevron, and Corma, at the ITQ in Valencia, has provided a prolific route to the most recent tetrahedrally connected silicate structure types. The acceleration in the discovery of new structure types is therefore largely due to innovative synthetic work, including template design, an increase in the compositional variety and the use of mineralising agents such as fluoride ions (Chapter 5). In addition, improvements in the methods available for the structure solution of these solids (which are typically prepared as microcrystalline powders or as small, weakly diffracting crystals) have contributed to the increase in known tetrahedrally connected structure types. These methods include the remarkable development of methods of structure solution from powder data and also microcrystal diffraction at synchrotron X-ray sources (Chapter 3).
There is currently no comprehensive compilation of structure types that include framework cations with coordination environments other than tetra-hedral. Among the more important of the inorganic-only solids are titanosil-icates such as ETS-4 and -10 (containing octahedrally coordinated titanium), gallium and nickel phosphates, germanates and metal oxide molecular sieves that are described later in this chapter. More recently still, these have been augmented by many inorganic-organic hybrids (including the rapidly expanding field of porous metal-organic framework coordination compounds, or MOFs). These structures are also introduced later in the chapter. Finally, I include a brief description of the structure types of ordered 'mesoporous' solids. Although these are less well denned than truly crystalline solids, enough is now known (from X-ray diffraction, electron microscopy and solid state NMR) to establish their main structural features. These studies, in combination with adsorption measurements, have indicated that several are more accurately described as microporous, with windows less than 2nm connecting larger cages.
Structural diagrams are of great importance in understanding crystalline structures, and indeed the combination of extended frameworks and pore space within these materials makes them particularly attractive for graphical representation. The most useful ways of displaying them are shown in Figure 2.3. These are: line plots showing only the bonds (with or without bridging oxygens); ball-and-stick plots, representing atoms and bonds; ORTEP crystallo-graphic plots, where the atoms are represented by thermal ellipsoids that describe the volume occupied by atoms with a given percent of probability; plots of the coordination polyhedra (tetrahedra, octahedra, etc.); space-filling diagrams. In space-filling diagrams, the oxygen atoms are shown with their van der Waals radii of 1.35 A. In this monograph, most structures are represented either as ball-and-stick models (to emphasise the pore space) or by use of polyhedra, when it is helpful to distinguish different cation coordination geometries within the framework.
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Excerpted from Microporous Framework Solids by Paul A. Wright. Copyright © 2008 The Royal Society of Chemistry. Excerpted by permission of The Royal Society of Chemistry.
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