EFFECT OF CATIONS ON METHANE ADSORPTION BY NAY, MGY, CAY, SRY, AND BAY ZEOLITES

Methane isotherms on NaY, MgY, CaY, SrY, and BaY zeolites are measured between 25 and 70-degrees-C and upto 6760 kPa. All isotherms are of type I. The initial heat of adsorption for divalent cationic forms decreased with decreasing charge density (CaY > MgY > SrY > BaY) except for MgY. Similar anamoly was observed for methane adsorption on MgX (Zhang, S. Y.; Talu, O.; Hayhurst, D. T. J. Phys. Chem. 1991, 95, 1722) and it is attributed to incomplete dehydration (activation) at normal activation temperatures. The cation type significantly affected adsorption properties even at loadings as high as 7.5 molecules/cavity. CaY has the largest capacity per weight, but the methane pore density decreased in order of decreasing cationic size (BaY > SrY > CaY > MgY > NaY) at 25-degrees-C and 5200 kPa fugacity. This unexpected result is attributed to possible differences in molecular packing around the cations at high loadings. The data were satisfactorily correlated by the virial isotherm model.