Framework water absorbed

Zeolites can be hydrophilic or hydrophobic due to the different Si/Al ratios within the zeolite framework. Organic molecules rely on H-bonding, electrostatic, and 77-cation interactions for effective zeolite absorption, and these interactions will clearly be influenced by the number of cation sites present. As expected, the more Si present, the more hydrophobic the zeolite and, therefore, the greater the ability of these materials to interact with hydrophobic organic molecules or to exclude hydrophilic molecules, such as water. Zeolites X/Y have a Si/Al content at or close to 1 and are highly hydrophilic absorbants. Pentasil zeolite ZSM-5, which... 

Note that the fractal dimensions discussed here are the fractal dimensions of the excitation transfer paths connecting the hydration centers located on the inner surface of the pores. Due to the low humidity, all of the water molecules absorbed by the materials are bound to these centers. The paths of the excitation transfer span along the fractal pore surface and depict the backbone of clusters formed by the pores on a scale that is larger than the characteristic distance between the hydration centers on the pore surface. Thus the fractal dimension of the paths Dp approximates the real surface fractal dimension in the considered scale interval. For random porous structures, Dp can be also associated with the fractal dimension D, of the porous space Dp = Dr. Therefore, the fractal dimension Dp can be used for porosity calculations in the framework of the fractal models of the porosity. 

Typical UV-Vis spectra of Ti-P and Ox-Ti-P samples are shown in Fig. 2. Isolated Ti(IV) in tetrahedral coordination was reported to exhibit an absorption band at about 212 nm [18a]. The presence of this band has been considered as an evidence for the incorporation of Ti into the framework of various titaniiun silicate molecular sieves such as TS-1, TS-2, TS-48 and Ti-p. The Ox-Ti-P samples had a band at about 220 nm, similar to that of hydrothermally prepared Ti-P [4,10,12]. The relatively high wavelength of this band may be due to the presence of water molecules in the coordination sphere of titanium [18b]. Ti(IV) in an octahedral coordination in a separate phase such as Ti02 is expected to absorb at about 340 nm [18a]. No such band was present in our UV-Vis spectra indicating the absence of segregated Ti02 particles. In conclusion, our UV-Vis data... 

By dehydrating the zeolite, the cations are forced closer to the framework in order to achieve better coordination to the framework oxygen atoms. This means that dehydrated zeolites can often absorb small molecules other than water in their pores. Obviously, the larger the cavities in the zeolite, the larger the molecules it can absorb. 

The open framework structure of the zeolites allows small molecules to be absorbed into their structures. The size and shape of the molecules absorbed depends on the structure of the zeolite, and hence the geometry of its pores. For example, zeolite A readily absorbs water but ethanol is excluded. Figure 7.17 demonstrates the effect of cavity size on the molecules which can be absorbed. 

The structures with D = 3 (frameworks) are usually most stable mechanically, thermally, and chemically. The layered (D = 2) and chain (D = 1) structures, and especially structures with finite anions (D = 0) with similar bond strength and polarity, are more labile many of these may absorb water into the interlayer or interchain space, or hydrolyze to produce amorphous products or even be water-soluble. In addition, structures with D = 1 or 2 usually display strong anisotropy of the conductivity and/or thermal expansion (although some exceptions are possible). Multiple examples with D = 0, 2, and 3 are listed in Sections 7.4—7.6. 

Zeolites are another group of framework silicates similar in structure to the feldspars. Like clays they have the ability to absorb or release water. Zeolites have long been used as molecular sieves, due to their ability to absorb molecules selectively according to molecular size. 

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