Molecular modelling of adsorption and diffusion processes in zeolites in relevance to environment protection

Unlike several other practical heterogeneous catalysts, zeolites are highly crystalline and well characterized materials. The surfaces amenable for th e approach of molecules, the catalytic active sites for adsorption and the space available for their reaction are well-defined. The above facts have l ed to the development of zeolites as the breeding ground for molecular mode lling methods. In turn, the molecular modelling methods have played a cruci al role in improving our understanding of several facets of zeolite catalys is, thus establishing a symbiotic relation. In this work, we bring out the application of molecular modelling methods to understand, interpret and to some extent predict the properties of zeolite based environment-friendly ca talysts. The studies applied to design zeolite catalysts which are prospect ive candidates to replace environmentally hostile catalyst materials such a s mineral acids, chlorides of aluminum, zirconium, iron etc. are presented. Two typical cases, where modelling has been carried out on zeolite catalys ts in order to understand the mechanism of complex organic transformations, are described. In case I, the yields in the conversion of several spirolac tones to enones were rationalized and the role of shape-selectivity in cont rolling the yield are brought out. In case II, the distribution of Na and R E in zeolite-Y and their consequence on the yield of S-N acetals are unders tood. Further, the adsorption and diffusion characteristics of alkylaromati cs in various zeolites are studied by force-field based energy minimization calculations. These studies have brought out the power of molecular modell ing methods for i) the initial screening of zeolite catalysts in shape sele ctive reactions, ii) to identify the energetically favorable and unfavorabl e locations for the molecules insides the pores of zeolites and iii) to ded uce the energy barriers for the diffusion of the molecules. The energetical ly favorable locations of 1,2-dichlorobenzene, its electronic interaction w ith Cl-2 and promoter inside zeolite K-L are studied. The results are usefu l to understand the mechanism of selective formation of 1,2,4-trichlorobenz ene. Additionally, the attempts based on molecular modelling studies toward s the design of zeolite catalysts for pollution control such as deNO(x) and removal of chloroflurohydrocarbons by adsorption over zeolites are describ ed. The mode of activation of CH4 and H2O over Ga-ZSM-5 are brought out. Th e influence of the extra framework cations on the adsorption of CF2Cl2 on C sNaY is revealed. Overall molecular modelling studies throw light on the un derlying chemical forces - their nature and magnitude, which control the be havior of the reacting molecules inside the zeolite pores.