Heats and entropies of adsorption of saturated hydrocarbons by zeolites X and Y

Abstract

Heats and entropies of adsorption of straight-chain, branched-chain and cyclic saturated hydrocarbons on cation-exchanged forms of zeolites X and Y at various pretreatment temperatures have been determined by gas–solid chromatography. Conclusions as to the mobility of the surface species are made from entropy data by comparison with standard values derived from statistical thermodynamics. The relationship of the heat of adsorption and of the mobility of the hydrocarbon within the zeolite pores to the cationic form and to the Si/Al ratio of the zeolite structure indicates that the cations play an active role in the adsorption of saturated hydrocarbons. The role of cations and structural hydroxy groups as adsorption centres is discussed. A thermodynamic (ΔH–ΔS) compensation effect for adsorption is observed and is discussed in terms of the isoequilibrium temperature and the solvent-like nature of the surfaces.