Sorption in decationated zeolites. I. Gases in hydrogen-chabazite

Abstract

Decationated (hydrogen) chabazites have been prepared and their stability examined. Reversible sorption isotherms have been measured in the crystals at a considerable number of temperatures for Ne, Ar, Xe, H₂, O₂, N₂ and CO₂. The isotherms have been analysed in terms of an isotherm equation based upon an equation of virial form relating concentration and pressure in the sorbed phase. The empirical virial coefficients were functions of temperature and of the particular gas-zeolite system considered. Based upon this isotherm equilibrium constants for the distribution of sorbate between crystals and gas phase, standard heats, entropies and free energies of transfer, and activity coefficients of the sorbed gases were determined. Differential and integral heats of sorption have been derived as functions of amount sorbed for all the guest species. These heats did not vary much with uptake unless the guest molecules had permanent quadrupole moments (N₂, CO₂). The energetic basis of this behaviour is discussed and interpreted. Entropies of the sorbed gases have also been found as functions of amount taken up. Near saturation of the crystals the integral entropy was intermediate between that of the liquid and solid sorbate, in accordance with a fluid mobility limited by the rigid cavity walls and by neighbouring guest molecules.