Ionization effects, zeolites

For cationic zeolites Richardson (79) has demonstrated that the radical concentration is a function of the electron affinity of the exchangeable cation and the ionization potential of the hydrocarbon, provided the size of the molecule does not prevent entrance into the zeolite. In a study made on mixed cationic zeolites, such as MgCuY, Richardson used the ability of zeolites to form radicals as a measure of the polarizing effect of one metal cation upon another. He subsequently developed a theory for the catalytic activity of these materials based upon this polarizing ability of various cations. It should be pointed out that infrared and ESR evidence indicate that this same polarizing ability is effective in hydrolyzing water to form acidic sites in cationic zeolites (80, 81). 

Fontes tt al. [224,225 addressed the acid—base effects of the zeolites on enzymes in nonaqueous media by looking at how these materials affected the catalytic activity of cross-linked subtilisin microcrystals in supercritical fluids (C02, ethane) and in polar and nonpolar organic solvents (acetonitrile, hexane) at controlled water activity (aw). They were interested in how immobilization of subtilisin on zeolite could affected its ionization state and hence their catalytic performances. Transesterification activity of substilisin supported on NaA zeolite is improved up to 10-fold and 100-fold when performed under low aw values in supercritical-C02 and supercritical-ethane respectively. The increase is also observed when increasing the amount of zeolite due not only to a dehydrating effect but also to a cation exchange process between the surface proton of the enzyme and the sodium ions of the zeolite. The resulting basic form of the enzyme enhances the catalytic activity. In organic solvent the activity was even more enhanced than in sc-hexane, 10-fold and 20-fold for acetonitrile and hexane, respectively, probably due to a difference in the solubility of the acid byproduct. 

The mere exposure of diphenyl-polyenes (DPP) to medium pore acidic ZSM-5 was found to induce spontaneous ionization with radical cation formation and subsequent charge transfer to stabilize electron-hole pair. Diffuse reflectance UV-visible absorption and EPR spectroscopies provide evidence of the sorption process and point out charge separation with ultra stable electron hole pair formation. The tight fit between DPP and zeolite pore size combined with efficient polarizing effect of proton and aluminium electron trapping sites appear to be the most important factors responsible for the stabilization of charge separated state that hinder efficiently the charge recombination. 

Naphthalene, with an ionization potential higher than that of anthracene or perylene, produces a much lower radical concentration in the zeolite (Table I), and appears to have no observable enhancing effect on the formation of anion radicals. Probably only certain sites of high energy are involved in this oxidation, and these sites may not be of the type in which interaction with an adjacent reducing center is possible. 

Giant molecules zeolites de-ionized" water., Temporary hardness and permanent hardness methods of softening water. Heat capacity (specific heat). Van der Waals attraction, boiling point, melting point-dependence on molecular size. Electric dipole moments of molecules—effect on boiling point. Ionic dissocia-... 

The Effect of Water Adsorption on the Spectrum of Ionized Adsorbed Molecules. The adsorption of water molecules on 300°C vacuum treated zeolite with preadsorbed anthraquinone molecules had little effect in the spectrum of anthraquinone. In the case of adsorbed triphenylcarbinol, the adsorption of water caused the disappearance of the bands characteristic for the triphenylcarbonium ion (Figure 2, curves 3 and 11). When the sample was treated in vacuum, the original spectrum of the triphenylcarbonium ion was restored. Analogous behavior was observed (J9) in the case of triphenylcarbinol adsorbed on silica-alumina. 

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