Zeolites charge, point

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. 

The utility of zeolite models in this context will be considered later, but Barrer et al. (4) have already pointed out an important resemblance— i.e., like zeolites, many biological systems contain sites of localized polarity or charge arising from the distribution of the opposing polarity or charge over a macromolecular structure., To this, a second generalization may be added in both zeolites (5) and protein structures (6), water seems to exist in some structured state which lies between those of ice and liquid water. 

Ab initio calculations of [(H03T0T(0H)3]" clusters were also performed by Sauer and Engelhardt (119), whose formulation of the problem was quite similar to that of the previous works. The only distinction in the scheme of computations was that the charge compensation was achieved by means of a crystal field simulated by six point charges q [q = e or e for (Si, Al)- and (Al, Al)2-, respectively]. In addition, the alternative structures with point charges q = e, 2e located at the cation positions in a zeolite were also considered. 

Zeolites are hydrated alcaline or alcaline earth aluminosilicates with the general formula M"+.v/ [(A102).v(Si02)jJ , -, w H20, where x indicates the number of M"+ cations which are necessary to compensate the negative charge of the framework, and w the number of water molecules. From the structural point of view, zeolites exhibit a three dimensional network of corner sharing T04 tetrahedra (T = Al,Si) which delimits interconnected tunnels or cages in which water molecules and M ions are inserted. The elimination of the water molecules keeps intact the framework and the solid becomes porous. 

However, the zeolite framework effect on the reaction is not limited only to a stabilization of charged species. We saw already that a transition state from the cluster approach turns to be an inflection point when the zeolite framework contribution is considered. An effect exists also on transition state. In the case of the shift isomerization transition state, it is found an alternative geometry. Before protonated toluene changes its orientation with respect to the deprotonated Brytasted site, the methyl shift reaction step can be achieved (see Figure 12). 

L. V. C. Rees (Imperial College, London) When zeolites have increasing amounts of polar molecules sorbed in them, the cations show a great increase in mobility when certain low fillings are obtained. Is it not possible that the decrease in the heat of sorption of NH at low isotherm temperatures represents the point where the cations loosen their attachment to the framework This would be an endothermic movement of a positive charge from the negative framework. At 300 °C, the cations would have sufficient thermal energy to mask this effect. 

In the first sum, indices i and denote shells and cations, the second term runs over all point charges in the system, and the third term accounts for interactions of the shells with their cores. The shell model takes into account the polarization of the anions by the crystal field of the solid, which is an important feature. To better reproduce the characteristics of systems with partly covalent bonds, such as zeolites, Eqs. [15] and [16] are supplemented with a term... 

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