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

The role of electronic polarizabilities in dealing with the phenomenon of m olecular adsorption and catalysis by zeolites has been discussed in this wo rk. Making use of the principles of periodic minimal surfaces of crystallin e materials and rules of atomic connectivities of zeolite structures, an ex pression has been derived to evaluate heats of adsorption of hydrocarbons i nteracting with the surfaces of zeolites that reads Delta H = alpha(M).beta .gamma + theta(f) where alpha(M) is the average polarizability of hydrocar bons, beta and theta(f) are the constants, and gamma the average curvature of the zeolite surface. After determining the polarizabilities of hydrocarb ons by a quantum mechanical method and establishing the magnitudes of the c urvatures of surfaces of zeolites by differential geometrical approach, the formalism has been applied to evaluate heats of adsorption of C-1-C(8)hydr ocarbons on the typical surfaces of MOR, FAU, ZSM-5, and silcalite zeolites . The agreement between theoretical and experimental values of heats of ads orptions of paraffins, naphthenes, olefins, and aromatics has been found to be very good and compares fairly well with those obtained from experimenta l methods as well as derived by other theoretical methods. The applicabilit y of the data so generated has been demonstrated in estimating the driving force of cracking (activation energy, E) following the Evans-Polyani proced ure 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 fur n-hexadecane and higher alkanes (C-16 effect).