Studies on the molecular electrostatic potential inside the microporous material and its relevance to their catalytic activity

Abstract

This chapter deals with the recent studies on the molecular electrostatic potential(MEP) map generation inside the channels and cages of microporous materials. We present the details of our studies in this area preceded by a review of related studies in the literature. Earlier studies reported on microporous zeolitic materials have provided qualitative information to locate the electrophilic and nucleophilic regions. The methodology to derive quantitative information by analyzing the number of MEP points and their actual values are presented. This methodology is exemplified by the results for a metallosilicate, namely TS-I. The substitution of titanium at the 12 crystallographic ‘T’ sites are studied and the reasons for the high activity and selectivity of titanosilicates, in relation to all siliceous form in partial oxidation reactions were brought out. The MEP studies of metal complexes such as metalloporphyrins brought out the existence of regularly repeating patterns of electron density on the periphery of the macro cycle. Molecular fitting of these complexes inside a large pore molecular sieve - VPI-5 is studied. A ‘lock and key’ type fitting for the porphyrins inside the pores of VPI-5 is predicted on the basis of MEP maps. The recent studies on the derivation of MEP by accurate density functional theory method are reported to bring out their potential to predict the reaction mechanisms. In Ga-ZSM-5, the effect of exchanged metal ions, on the electric field inside the pores of the zeolite are studied. The understanding of the activation mode of methane is appreciably aided by the MEP maps. The mechanism of this initial step, as predicted by the MEP calculations is in correspondence with the adsorption and dissociation profile generated from accurate nonlocal density functional theory calculations. The above studies as well as MEP calculations of molecules adsorbed inside cages of siliceous faujasitic zeolite and on clay surfaces, presented here, provide confidence in deriving valid “MEP-activity” correlation. MEP calculations provide clues regarding the favorable adsorption sites for the organic molecule inside the pores of zeolite, which could be used as the starting configuration for more elaborate geometry optimization calculations. The above results are elaborated to conclude the advantages of MEP studies in understanding the catalysis by microporous materials.