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

Abstract

Unlike several other practical heterogeneous catalysts, zeolites are highly crystalline and well characterized materials. The surfaces amenable for the approach of molecules, the catalytic active sites for adsorption and the space available for their reaction are well-defined. The above facts have led to the development of zeolites as the breeding ground for molecular modelling methods. In turn, the molecular modelling methods have played a crucial role in improving our understanding of several facets of zeolite catalysis, 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 catalysts. The studies applied to design zeolite catalysts which are prospective candidates to replace environmentally hostile catalyst materials such as mineral acids, chlorides of aluminum, zirconium, iron etc. are presented. Two typical cases, where modelling has been carried out on zeolite catalysts in order to understand the mechanism of complex organic transformations, are described. In case I, the yields in the conversion of several spirolactones to enones were rationalized and the role of shape-selectivity in controlling the yield are brought out. In case II, the distribution of Na and RE in zeolite-Y and their consequence on the yield of S-N acetals are understood. Further, the adsorption and diffusion characteristics of alkylaromatics in various zeolites are studied by force-field based energy minimization calculations. These studies have brought out the power of molecular modelling methods for i) the initial screening of zeolite catalysts in shape selective reactions, ii) to identify the energetically favorable and unfavorable locations for the molecules insides the pores of zeolites and iII) to deduce the energy barriers for the diffusion of the molecules. The energetically favorable locations of 1,2-dichlorobenzene, its electronic interaction with C1₂ and promoter inside zeolite K-L are studied. The results are useful to understand the mechanism of selective formation of 1,2,4-trichlorobenzene. Additionally, the attempts based on molecular modelling studies towards the design of zeolite catalysts for pollution control such as deNO_x and removal of chloroflurohydrocarbons by adsorption over zeolites are described. The mode of activation of CH₄ and H₂O over Ga-ZSM-5 are brought out. The influence of the extra framework cations on the adsorption of CF₂C1₂ on CsNaY is revealed. Overall molecular modelling studies throw light on the underlying chemical forces - their nature and magnitude, which control the behavior of the reacting molecules inside the zeolite pores.