Framework hydrate

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

The Langevin model has been employed extensively in the literature for various numerical and physical reasons. For example, the Langevin framework has been used to eliminate explicit representation of water molecules [22], treat droplet surface effects [23, 24], represent hydration shell models in large systems [25, 26, 27], or enhance sampling [28, 29, 30]. See Pastor s comprehensive review [22]. 

The hydrated insular groups may polymerize in various ways by splitting out water this process may be accompanied by the breaking of boron-oxygen bonds within the polyanion framework. 

According to these authors all gas hydrates crystallize in either of two cubic structures (I and II) in which the hydrated molecules are situated in cavities formed by a framework of water molecules linked together by hydrogen bonds. The numbers and sizes of the cavities differ for the two structures, but in both the water molecules are tetrahedrally coordinated as in ordinary ice. Apparently gas hydrates are clathrate compounds. 

As mentioned in the introduction, hydrates may crystallize in either of two frameworks. When discussing ternary systems A+ 13+H20 it is useful to discriminate between systems in which only hydrates of one structure (I or II) are. formed, and those in which hydrates of both structures are formed. 

Fig. 1. The structure of gas hydrates containing a hydrogen-bonded framework of 46 water molecules. Twenty molecules, arranged at the comers of a pentagonal dodecahedron, form a hydrogen-bonded complex about the comers of the unit cube, and another 20 form a similar complex, differently oriented, about the centre of the cube. In addition there are six hydrogen-bonded water molecules, one of which is shown in the bottom face of the cube. In the proposed structure for water additional water molecules, not forming hydrogen bonds, occupy the centres of the dodecahedra, and...

Scheme 4 Representation of equilibria between Ti04 framework species and ri complexes inside TS-1 channels upon dosage of anhydrous H2O2 (left) and between rj and rj complexes upon hydration (right). Adapted from [50] with permission. Copyright (2004) by Wiley-VCH...

We present a molecular theory of hydration that now makes possible a unification of these diverse views of the role of water in protein stabilization. The central element in our development is the potential distribution theorem. We discuss both its physical basis and statistical thermodynamic framework with applications to protein solution thermodynamics and protein folding in mind. To this end, we also derive an extension of the potential distribution theorem, the quasi-chemical theory, and propose its implementation to the hydration of folded and unfolded proteins. Our perspective and current optimism are justified by the understanding we have gained from successful applications of the potential distribution theorem to the hydration of simple solutes. A few examples are given to illustrate this point. 

In Fig. 9.2 we present results of a first-of-a-kind study of the hydration of the first-transition-row metals within the quasichemical framework. The biphasic behavior of the actual hydration free energy is consistent with features inferred experimentally. Removing the ligand field effects reveals the linear decrease [12]. The results shown in Fig. 9.2 are largely outside the purview of extant simulation techniques, but are treated simply in the quasichemical framework developed below. 

We have demonstrated that a combined experimental (27A1 3Q MAS NMR) and theoretical (QM-Pot employing the bare framework model) approach represents a powerful tool for the determination of the local geometry of framework A104 tetrahedra, the prediction of27A1 isotropic chemical shifts in hydrated silicon rich zeolites, and the identification of A1 siting in the framework of silicon-rich zeolites. Experimental evidence is provided for the occupation of at least 10 out of 24 distinguishable framework T sites by A1 atoms in silicon-rich ZSM-5. The conclusion is reached that the A1 distribution over the framework T sites is neither random nor controlled by a simple rule, but depends on the conditions of the zeolite synthesis. 

Diffraction patterns and FTIR spectra of skeletal vibrations of the ZSM-5 and ferrierite zeolites indicated high crystallinity of the analyzed samples. The strong band with a chemical shift of about 55 ppm in the 27Al MAS NMR spectra of hydrated zeolites indicated the presence of more than 97 % Al in the framework in tetrahedral coordination the very low intensity of the peak at 0 ppm indicated less than 3 % rel. of Al in octahedral coordination. 

In dehydrated CoAF the migration of Co2+ ions from Co2 to Co2a sites on heating (Fig. lb) allows the occupancy of the most active a-sites where Co2+coordinates four framework oxygens. The irreversible and progressive decline of catalytic activity during dry-wet-dry cycles is likely to be due to cation hydration and movement from those sites, thus, decreasing their occupancies. 

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