Heats of adsorption of hydrocarbons on zeolite surfaces: a mathematical approach

Abstract

The role of electronic polarizabilities in dealing with the phenomenon of molecular adsorption and catalysis by zeolites has been discussed in this work. Making use of the principles of periodic minimal surfaces of crystalline materials and rules of atomic connectivities of zeolite structures, an expression has been derived to evaluate heats of adsorption of hydrocarbons interacting with the surfaces of zeolites that reads ΔH=α_M·β·γ+θ_f,where α_M is the average polarizability of hydrocarbons, β and θ_f are the constants, and γ the average curvature of the zeolite surface. After determining the polarizabilities of hydrocarbons by a quantum mechanical method and establishing the magnitudes of the curvatures of surfaces of zeolites by differential geometrical approach, the formalism has been applied to evaluate heats of adsorption of C₁-C₈ hydrocarbons on the typical surfaces of MOR, FAU, ZSM-5, and silcalite zeolites. The agreement between theoretical and experimental values of heats of adsorptions of paraffins, naphthenes, olefins, and aromatics has been found to be very good and compares fairly well with those obtained from experimental methods as well as derived by other theoretical methods. The applicability of the data so generated has been demonstrated in estimating the driving force of cracking (activation energy, E) following the Evans−Polyani procedure in the case of n-paraffins interacting with ZSM-5 surface. The data so generated has been used to explain negative activation energy for cracking observed for n-hexadecane and higher alkanes (C₁₆ effect).