THE ROLE OF POLARIZATION OF XE BY DI-VALENT AND MONOVALENT CATIONS INXE-129 NMR-STUDIES IN ZEOLITE-A

We consider the role of polarization in the adsorption of Xe in zeolit es of type A by direct comparative analysis of the adsorption isotherm s, distributions of occupancies, and Xe-129 NMR chemical shifts of Xe- n in cages containing CaxNa12-2x ions per alpha cage (x = 0, 1, 2, 3, 5). We find that the qualitative trends in the adsorption isotherms, a nd in the progressions of Xe-n chemical shifts (for n = 0-8 in cages w ith x = 0, 1 Ca2+ ions and for n = 0-5 in cages with x = 2, 3 Ca2+ ion s) upon increasing the level of Ca2+ ion for Na+ ion substitution coul d only be accounted for by including polarization of the Xe atom by th e zeolite framework and its ions. This system, which permits observati on of individual Xe-n peaks and of directly comparable adsorption isot herms in several cage types, provides a good model system for the inte rpretation of the more general case in which only the overall average Xe-129 NMR chemical shift is observed in open network zeolites, arisin g from free exchange of Xe among cavities of variable occupancy and va riable cation distribution. (C) 1997 Elsevier Science B.V.