ADSORPTIVE AND GAS CHROMATOGRAPHIC PROPERTIES OF VARIOUS CATIONIC FORMS OF ZEOLITE X

Abstract

Gas chromatographic retention volumes for O2, N2, CH4, C2H6, C3H8, C4H10, C2H4, and C3H6 have been measured over a range of temperatures between 25 and 400 °C, on columns of the following ion-exchanged forms of zeolite X: Li+, Na+, K+, Mg++, Ca++, Ba++, and Ag+. In addition, for AgX zeolite, full isotherms have been determined for O2, N2, C2H6, and C2H4; and for NaX zeolite, isotherms for C2H6 and C2H4. The gas chromatographic retention volumes agree well with the isotherms in the case of NaX but are significantly low for AgX, presumably as the result of at least a portion of the adsorption process being slow. The heats of adsorption on NaX agree well with literature values for O2, N2, CH4, and C2H6 but are low for C3H8 and C4H10; further indication of slow processes and hence of a limitation of the g.c. technique. The observations can be correlated qualitatively in terms of the interplay of the following effects: (a) cationic field increases with decreasing ionic radius; (b) shielding of smaller cations is greater—at least in the six-membered oxygen-ring sites; (c) divalent cations, while having greater fields, are only half the number of monovalent cations and tend to be found in less exposed sites; and (d) the polarizing power of silver ion, which also occupies all types of exposed sites, is very strong. The most intense interactions were found with AgX; with ethylene a much stronger complex is formed than has been reported for any other silver compound. The significance of molecular quadrupole interactions on these heteroionic surfaces is shown by the much weaker cationic effects observed with O2 (which has negligible quadrupole moment) than with nitrogen.