Silica clathrasils

Figure 2.12 Plot of the area per T-atom vertex SI) versus the average ring size, n, for a variety of zeolites, silica clathrasils and dense silicates. (All zeolites have a silicon aluminium ratio exceeding three, so that the approximate stoichiometry of all these frameworks is Si02). Zeolite and clathrasil frameworks are labelled by the code adopted by the International Zeolite Association [18]). The shaded domain indicates the window of geometrically accessible values of as a function of the ring size. Despite the allowed geometric variability, the value of D is close to 12.2A2 for all these "silicates", regardless of the ring size and consequent intrinsic curvature.

Except for water, silica is the most extensively studied MX2 compound. One of the challenges in studying silica is its complex set of structures. Silica has several common polymorphs under different conditions of temperature [1] and pressure [4], as seen in Figs. 2 and 3. For instance, cristobalite is the crystalline silica polymorph at atmospheric pressure above 1,470°C. It is built on an fee lattice with 24 ions per unit cell. This structure is, in fact, the simplest form of silica. In addition to five polymorphs (quartz, coesite, stishovite, cristobalite, tridymite) that have thermodynamic stability fields, a large and increasing number of metastable polymorphs have been synthesized. These include vitreous silica, clathrasils, and zeolites [2], Except for stishovite, all these structures are based on frameworks of... 

Some frameworks consist only of cages with a maximum ring size of six and have no channels (e.g. the purc-silica clathrasils), but the majority have at least 8-ring channels. These channels can intersect to form 2- and 3-dimensional channel systems, and this can be a critical feature for catalytic or sorption applications. For example, a 1-dimensional channel is much more easily blocked by the formation of coke deposits than is a higher dimensional one where detours are possible. 

Figure 6. Continued. B. AIPO4-I6 or the pure-silica clathrasil octadecasil.

Denote the larger cluster as C. If C and A are now combined, then the structure shown in Figure 6B can be formed. This structure is that of AIPO4-I6 (27) and the pure-silica, clathrasil octadecasil (22). Although the structures shown in Figure 6 can be constructed from the building units described above, it is most likely that controlled combinations of A, B and C will be necessary to form these crystalline materials. 

Clathrasils are host/guest complexes comprised of covalent guest molecules entrapped within cages formed by a silica host framework (1, 2). Like all zeolitic materials, clathrasils have enormous potential as advanced optical and electronic materials whose composite character permits synthetic manipulation of both the molecular structure of the guest species and the extended structure of the host framework (3, 4). Like other zeolites, however, clathrasils also suffer severe handicaps as advanced materials due to a reluctance to form large single crystals and a tendency to form stoichiometrically and structurally defective crystals (5 -10). 

The phase transformation behavior of Py-D3C was far simpler than that reported for tetrahydrofuran/N2- and tetrahydrofuran/Xe-D3C in reference 10. We attribute this difference in part to impurities in the samples employed, samples that contained methanol and ethylenediamine according to 13 C CPMAS NMR spectroscopy. We have observed that use of Si(OCH3)4 as a silica source or ethylenediamine as a catalyst in clathrasil synthesis introduces defects that can alter phase transition temperatures by as much as 30 °C and/or introduce new phase transformations. 

Conversely, in many other cases studied, the stabilization of hydrophobic clathrasils, zeosils and very high silica zeolitic frameworks is induced by neutral guest molecules that only fill the channels and cavities (11, 12). They are thought to form a solid solution on the growing crystals and thereby lower the chemical potential of the framework (13). The energy required to stabilize such a framework mostly derives from weak Van der Waals bonds between the guest molecule and the siliceous framework (12). In that respect, the temptation of ZSM-5 with alcohols (14) or ethers (15) would also have to be interpreted in terms of a pore filling model. 

Synthetic studies on high-silica zeolites showed that silicon end members of structural groups could be prepared. Clearly these are pure silica phases and to date several have been prepared including a synthetic analog of the natural silica phase melanophlogite. These are listed in Table 23. They have sometimes been described as clathrasils, in deference to their strong structural links to clathrates as can be seen from their soap-bubble like frameworks (illustrated in Figure 12). 

Classical SDAs such as alkylamines or alkylammonium cations, which were mentioned in the second part of this article, do not possess special properties which would make them of interest for the construction of materials. Aromatic molecules, in contrast, can be equipped with many different properties. For example, they can act as chromophores, may possess large hyperpolarizabilities (for NLO applications such as SHG, cf the pNA molecule) or stabilize special electronic structures like radicals. However, aromatic molecules are notoriously poor SDAs. One of the few exceptions is pyridine. The hydrothermal treatment of a pyridine - HF - H2O silica solution at 190°C, for instance, results in the formation of large crystals of dodecasil 3C, a clathrasil with small cages. These crystals, which can grow to millimeter size, are acentric at room temperature and exhibit SHG, although the effect is only weak [37]. 

K.R. Franklin and B.M. Lowe, in Hydrothermal Crystallization of Silica Molecular Sieres and Clathrasils from Amine Containing Reaction Mixtures, Stud. Surf. Sci. Catal., 1988, 49, 179-188. 

The porosils have appeared only recently upon the scene [lOj. They include the silica end-products of synthesis of zeolites richer and richer in silica, such as silicalites I and II, and also other species which do not always have a zeolite counterpart. They can be sub-divided into clathrasils in which openings between intracrystalline cavities are too small for molecule migration and zeosils in which these openings are adequate for molecule diffusion. Examples of each sub-division are ... 

Gies. H. Clathrasils and Zeosils Inclusion Compounds with Silica Host Frameworks. In Inclusion Compounds, Atwood. J.L.. Davies. J.E.D.. MacNicol, D.D.. Eds. Oxford University Press Oxford. 1991 Vol. 5. 1-36. 

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. 

The silica frameworks of the clathrasil family are built from cage-like polyhedral structural subunits containing the guest molecules (Fig. 1). The cages are bound by... 

Gies. H. Clathrasils and Zeosils Inclusion Compounds 20. with Silica Host Frameworks. In Inclusion Compounds ... 

Clathrate-type structures also exist with Si02 host lattices, another well known tetrahedrally-based compound, which has several analogous structural polymorphs with H2O. These clathrates are called clathrasils and classified as a subgroup of porous tectosilicates. In addition to their pure silica frameworks, they differ from zeolites by the presence of almost spherical and medium large voids with small apertures limited to six atoms, instead of wide interconnected channels [46]. 

H. Gies, Studies on clathrasils III. Crystal stmcture of melanophlogite, a natural clathrate compound of silica. Z. Kristallogr. 164(3 ), 247-257 (1983)... 

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