NEW APPROACH TO ACID CATALYSIS AND HYDROCARBON ZEOLITE INTERACTIONS

Adsorption of hydrocarbons on zeolites has been the subject of serious concern owing to its important catalytic consequences in petroleum refining, cracking, fuel synthesis and processing. Besides thermodynamic links between zeolites and hydrocarbons during adsorption, diffusion, and cracking, the inclusion of framework and surface parameters, e.g., genus of periodic minimal surface and atomic connectivity, have been postulated in establishing nonspecific type interactions. Keeping this in view, the earlier proposed model that establishes the concept of atomic polarizabilities in zeolite science and acid catalysis (Dixit and Rao, Zeolites, 16, 4, 287-293, 1996) has been extended to examine heats of adsorption of paraffins, naphthenes, olefins and aromatics on the zeolite surfaces. The formalism thus developed has been used to evaluate heats of adsorption and activation energies of hydrocarbon species on the surfaces of faujasites, ZSM-5, mordenite and silicalite by proposing a new relation that reads: L'1H= aM . ~.y + 9r; where aM is the average polarizability, ~and 9r are the constants, and y the average curvature of the zeolite surface. The agreement between theoretical and experimental values of heats of adsorptions of C1- C20paraffins have been found to be very good in the case of ZSM-5 and relates well with their activation energies for cracking. Key words: Zeolites, Heats of adsorption, Activation Energie