Dense silica polymorphs

Note that dense silica polymorphs or silica-rich zeolites (ZSM-5, KZ-2, ZSM-22) are formed at the expense of metastable ZSM-48 for long reactions times in monoamine and diamine bearing systems (see above), indicating that excess of Al in the initial hydrogel does not play a particular role in the formation of these phases. In contrast, a higher Al content in the... 

Generally, force fields available for the study of dense silica polymorphs and zeolites can also be used to obtain reliable structures and vibrational spectra for the frameworks of these materials. In the past it seemed necessary to use molecular mechanics force fields to be able to predict vibrational spectra correctly, but it was shown several years ago that shell model potentials also have this capability. 0 175 Shell model potentials, therefore, seem to be the best-suited force fields for silica polymorphs. These potentials model the more ionic character of the SiO bond, include polarization, allow coordination changes, and contain only a few adjustable parameters. Molecular mechanics potentials have value in molecular dynamics calculations and allow for easier extension with respect to studying the adsorption behavior of organic sorbates in zeolites, whereas the use of shell model potentials is cumbersome. 

The actual values of lattice energies can also be calculated. Comparison of such values for pure silica polymorphs of known zeolite structure types or of aluminophosphates reveal clear trends when normalised to framework cation content and plotted against framework density (Figure 4.2). The framework stability of the observed structures with respect to dense phases such as quartz (Si02) or berlinite (AIPO4) decreases linearly as the framework density... 

The main points of interest of the structures of these polymorphs are (i) the analogies with silica and silicate structures, (ii) the presence of two interpenetrating frameworks in the most dense forms vi and vii (viii), and (iii) the ordering of the protons. Analogies with silica and silicate structures are noted in Table 15.1, namely, ice-iii with a keatite-like structure, ice-vi with two interpenetrating frameworks of the edingtonite type (p. 828), and ice-vii (and viii) with two interpenetrating cristobalite-like frameworks. In these structures, related to those of... 

In the last ten years, at least a dozen polymorphs of pure Si02 have been reported [6], Stishovite, another form of silica obtained at high temperatures and pressures, has, rather than a tetrahedral-based geometry, a rutile (Ti02) structure in which each Si atom is bonded to six O atoms and each O atom bridges three Si atoms [6], Stishovite (found in Meteor Crater, Arizona) is more dense and chemically more inert than normal silica but reverts to amorphous silica upon heating. 

According to the value of df, crystalline silicas were divided by Liebau (10) into pyknosils (df > 21 SiC>2 groups per 1000 A3) and porosils (df < 21 SiC>2 groups per 1000 A3). Pyknosils are defined as polymorphs with frameworks too dense to enclose guest molecules that are larger than helium and neon (10). Pyknosils are nonporous. 

When SAPO-37 was calcined at 1173 K, Si atoms become mobile and detached from the framework to aggregate to form polymorph silica, while Al-P dense phase was formed, corresponding to a new A1 signal which can be distinguished by the a1 MQMAS spectrum. The move of atoms in SAPO-37 is the start of the collapse of the framework. 

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.

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