Clathrasils

The problem of accessibility in microporous solids is extreme in zero-dimensional zeolite structures such as clathrasils, that is, zeolite-related materials consisting of window-connected cages. The pore openings in these caged structures are restricted to six-membered rings of [Si04] units at most, which corresponds to pore diameters of approximately 0.2 nm [58]. These pores are too small for the removal of templates and, afterward, are impenetrable to typical sorptive molecules for characterization such as N2 and Ar or reactants such as hydrocarbons. Therefore, the intrinsic... 

Clathrasils, which have polyhedral cavities, but with windows that are too small to allow the passage of other molecules, so that enclosed ions or foreign molecules cannot escape. Examples are ultramarines and melanophlogite (Si02)46-8(N2, C02, CH4). 

It has been recognized for some time that SiOj and H2O show a similar tendency to form tetrahedrally linked framework structures (Kamb, 1965). The mineral melanoph-logite is isostructural with cubic gas hydrate I, while the synthetic clathrasil, dodecasil- iC is isostructural with cubic gas hydrate II (Gies et ai, 1981,1982). A section of dodecasil-3C is shown in Fig. 1.41. 

Figurel. l29Xe NMR chemical shifts 6 (filled symbols) and 1/6 (open symbols) as a function of pore diameter. = controlled pore glasses, 75-385A - = ZSM-12 = ALPO-11 A = clathrasil D3C =clathrate hydrates.

Many hydrate cavities have analogs (1) in the clathrasils in which SiC>2 replaces water as a host molecule (Gerke and Gies, 1984) and (2) in the Buckminsterfullerene (covalently bonded carbon cavities) family (Curl and Smalley, 1991). Even with these analogous structures providing estimates of other cavities, for hydrate unit crystals there is the additional restriction that the cavities must be packed to fill space. 

With the exceptions of cavities containing square faces, all hydrate cavities (as well as clathrasil and Buckyball family cavities) follow Euler s theorem (Lyusternik, 1963) for convex polyhedra, stated as (F + V = E + 2). The number of faces (F) plus the vertices (V) equals the edges (E) plus 2. Euler s theorem is easily fulfilled in cavities having exactly 12 pentagonal faces and any number of hexagonal faces except one. 

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. 

Figure 5. Continued. Scanning electron micrographs of (d) MTN-, (e) NUl-type materials, and (f) of a novel clathrasil.

The gas hydrates occur in three types of structures, named type I, type n and type H. The type I has 512 and 51262 polyhedra associated by face-sharing in the ratio 1 3, whereas in type II, 512 and 5,264 polyhedra are combined by face-sharing in the ratio 2 1. These structures have cubic symmetry. The recently discovered type H [779] has hexagonal symmetry and is isostructural with the hexagonal clathrasil-dodecasil 7-H [780]. The host lattice contains 512, 435663 and a larger 51268 void which can accommodate larger organic molecules than the type II cubic structure. 

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). 

The srx-membered ring of Si-O-Si bonds has a pore width of 2.8 A. This makes the group of clathrasils interesting materials for the separation of small gases like hydrogen from a diversity of gas streams. 

Clathrasil Crystal Symmetry Volume and Number Framework density of cages per unit cell (g/cm ) ... 

Up till now, no insitu growth of DD3R on supports or membrane syntheses has been reported. It is to be noted that the synthesis of this clathrasil is marked by a fixed procedure [36], making changes in concentrations or temperature, in order to study the growth on supports, difficult. 

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