Observation of a compensation relation for n-hexane adsorption in zeolites with different structures: implications for catalytic activity

The Henryʹs equilibrium constant and enthalpy of n-hexane adsorption at low coverage have been determined at 423 K for zeolites of different structural types that have been subjected to different treatments. The Henryʹs constant and enthalpy of adsorption increased as the pore size decreased as MFI > MOR > BEA > FAU. Brønsted acid sites and non-framework Al produced through steaming gave smaller increases, and changes due to alkali or lanthanide ions had little effect. From the enthalpy of adsorption and the Henryʹs equilibrium constant, the entropy of adsorption was calculated. Despite the large differences in structure and composition of the zeolites, there is a linear correlation, or compensation relation, between the entropy and enthalpy of adsorption for all of the zeolites. Combined with previous work in which the same zeolites exhibited a linear kinetic compensation, or Constable relation, for the monomolecular cracking of n-hexane, these results support the proposal that the intrinsic acid strengths of active Brønsted acid sites in zeolites of different structures or silica-to-alumina ratios with Lewis acid sites from nonframework Al are similar.