Enthalpy-entropy compensation for n-hexane adsorption on HZSM-5 containing transition metal ions

Abstract

In this work, the values of entropy changes related to n-hexane adsorption onto ion-exchanged ZSM-5 zeolites were calculated from differential heats, obtained from microcalorimetric experiments. The existence of enthalpy-entropy compensation effect, evidenced by the linearity of -Lambda H vs. -Lambda S plots and characteristic for all investigated ZSM-5 zeolites, was found. In the case of ZSM-5 structure, modifying the zeolite structure by ion-exchange gives rise to changes in the heats of adsorption and adsorption entropy in the same manner. The factors that can influence the appearance of entropy-enthalpy compensation were discussed. It was found that compensation effect is governed by ion-induced dipole interaction between highly polarizing cationic centres in zeolite and nonopolar n-hexane molecules, and hence, depends on the size, charge and electron configuration of the cation. It was found also that the compensation temperature is in correlation with the number of zeolites' strong acid centres. Contrary to the adsorption of n-hexane on ZSM-5 zeolites, compensation effect was not found for the adsorption of the same gas on faujasite-type zeolites.