Silica polymorphs, microporous

The past nearly six decades have seen a chronological progression in molecular sieve materials from the aluminosilicate zeolites to microporous silica polymorphs, microporous aluminophosphate-based polymorphs, metallosilicate and metaHo-phosphate compositions, octahedral-tetrahedral frameworks, mesoporous molecular sieves and most recently hybrid metal organic frameworks (MOFs). A brief discussion of the historical progression is reviewed here. For a more detailed description prior to 2001 the reader is referred to [1]. The robustness of the field is evident from the fact that publications and patents are steadily increasing each year. 

Xenon has been considered as the diffusing species in simulations of microporous frameworks other than faujasite (10-12, 21). Pickett et al. (10) considered the silicalite framework, the all-silica polymorph of ZSM-5. Once again, the framework was assumed to be rigid and a 6-12 Lennard-Jones potential was used to describe the interactions between Xe and zeolite oxygen atoms and interactions between Xe atoms. The potential parameters were slightly different from those used by Yashonath for migration of Xe in NaY zeolite (13). In total, 32 Xe atoms were distributed randomly over 8 unit cells of silicalite at the beginning of the simulations and calculations were made for a run time of 300 ps at temperatures from 77 to 450 K. At 298 K, the diffusion coefficient was calculated to be 1.86 X 10 9 m2/s. This... 

Very small differences in bond strength between different silica polymorphs were found. Since the Extended Huckel Method is too approximate to calculate reliably the small diffferences in energy between low-density material, containing micropores, and high-density material without micropores, work was initiated to study the same problem but now with two rigorous techniques that are currently considered to be state of the art. 

Microporous materials are typified by natural and synthetic zeolites that are crystalline 3D aluminosilicates with open channels or cages. Synthetic and structural concepts of zeolites have to a large extent shaped the development of microporous materials during the past 50 years. For example, the use of organic structure-directing agents in the synthesis of high-silica zeolites and their all-silica polymorphs contributed to... 

The replacement of framework anions (i.e. with chalcogens (e.g. S ) represents a more recent approach for generating microporous materials. The efforts to make microporous chalcogenides began with germanium or tin sulfides. However, germanium or tin sulfides do not form microporous materials similar to all-silica polymorphs of zeolites. It was later found that the incorporation of low-valent cations such as Mn + into the Ge-S composition helped to generate 3D frameworks. 

The history of molecular sieve synthesis encompasses some 40 years and includes the preparation of aluminosilicate zeolites U), phosphorus-substituted aluminosilicates (2 3), and the microporous silica polymorphs (4). More recently, the compositional and... 

Microporous materials with regular pore architectures comprise wonderfully complex structures and compositions. Their fascinating properties, such as ion-exchange, separation, and catalysis, and their roles as hosts in nanocomposite materials, are essentially determined by their unique structural characters, such as the size of the pore window, the accessible void space, the dimensionality of the channel system, and the numbers and sites of cations, etc. Traditionally, the term zeolite refers to a crystalline aluminosilicate or silica polymorph based on comer-sharing TO4 (T = Si and Al) tetrahedra forming a three-dimensional four-connected framework with uniformly sized pores of molecular dimensions. Nowadays, a diverse range of zeolite-related microporous materials with novel open-framework stmctures have been discovered. The framework atoms of microporous materials have expanded to cover most of the elements in the periodic table. For the structural chemistry aspect of our discussions, the second key component of the book, we have a chapter (Chapter 2) to introduce the structural characteristics of zeolites and related microporous materials. 

Crystalline microporous silicas, the porosils, are a family of materials based on [TO4] units with tetrahedral densities below 21 T-atoms per 1000 A, which are synthesized in the presence of teinplating guest molecules. Their silica host frameworks are three-dimensionally four-connected, and in their calcined form, they belong to the large family of silica polymorphs. Porosils with pore openings too small to let the occluded guest molecules out are called clathrasils porosils in which the guests can be removed are called zeosils. 

Nowadays zeolites have to be defined in line with the lUPAC recommendations as a group of solids based on either aluminosilicates up to silica polymorphs, aluminophosphates or metallosilicates or phosphates with a well-defined microporous structure. As zeolite science and application is still a rapidly growing area, the reader may refer to a recently published book "Introduction to Zeolite Science and Practice" [9] for more detailed information. This book represents the latest comprehensive review on the different fields of zeolite chemistry. 

The importance of framework density and molar volume is evident also for large pore, mesoporous silica [33] and for AIPO4 polymorphs [34], Data for the latter are included in Figure 7.19. For mesoporous silica a transition from a regime where cages and pores affects the energetics to one in which the large pores act as inert diluent is reported. A further increase in pore diameter does not appear to increase the enthalpy of the compound [33], The similarity in enthalpy of many different structures shows that the synthesis of metastable microporous framework... 

A number of microporous polymorphs of crystalline silica can now be prepared. One procedure is to attempt the dealumination of a readily available zeolite another approach involves direct synthesis, for example, of ZSM-5, in the form of Silicalite I (31). The considerable amount of recent interest shown in these Al-free zeolites (or porotectosilicates) has been stimulated by the uniformity of their channel structures and their hydrophobic nature. 

Figure 5.6 Enthalpy of transition from quartz vs framework density (FD) for several dense and microporous SiOi polymorphs. Zeolite phases are denoted by their FTC, while minor case codes correspond to the dense phases quartz (q), tridymite (tr), cristobalite (cr), moganite (mo) and coesite (co). The figure suggests the stabilities of silica phases decrease when their densities decrease (except for the high-pressure phase coesite, which is compressed beyond the density of quartz the enthalpy for moganite deviates from the trend, but it was not directly measured and is not as reliable as the rest). The figure also shows pure-silica zeolites are not highly destabilised. Reproduced with permission from P.M. Piccione, C. Laberty, S. Yang, M.A. Camblor, A. Navrotsky and M.E. Davis, /. Phys. Chem. B, 104, 10001. Copyright (2000) American Chemical Society.

The ability of microporous solids to act as high-capacity molecular sieves has long been exploited in a wide range of applications in adsorption and separation. The electrostatic interactions of the traditional cationic forms of aluminosilicates are well suited for the uptake of polar molecules (such as H2O) and are also able to separate oxygen from air. The development of microporous solids with varied chemistry has enabled adsorption and diffusion properties to be finely tuned for particular technologies. Pure silica zeolite polymorphs such as silicalite have particular importance, because they enable separation on the basis of a different range of polarity and on molecular size the absence of aluminium in the framework also prevents the presence of unwanted acidity, so adsorbed hydrocarbons do not undergo any catalytic transformation. 

Camblor MA, CoreU C, Corma A, Dfaz-Cabanas MJ, Nicolopoulos S, Gonzalez-Calbet JM, et al. A new microporous polymorph of silica isomorphous to zeolite MCM-22. Chem Mater 1996 8 2415-7. 

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