The role of polarization of Xe by di- and monovalent cations in 129Xe NMR studies in zeolite A

Abstract

We consider the role of polarization in the adsorption of Xe in zeolites of type A by direct comparative analysis of the adsorption isotherms, distributions of occupancies, and 129Xe NMR chemical shifts of Xe n in cages containing Ca x Na 12 −2 x ions per alpha cage ( x = 0, 1, 2, 3, 5). We find that the qualitative trends in the adsorption isotherms, and in the progressions of Xe n chemical shifts (for n = 0–8 in cages with x = 0, 1 Ca 2+ ions and for n = 0–5 in cages with x = 2, 3 Ca 2+ ions) upon increasing the level of Ca 2+ ion for Na + ion substitution could only be accounted for by including polarization of the Xe atom by the zeolite framework and its ions. This system, which permits observation of individual Xe n peaks and of directly comparable adsorption isotherms in several cage types, provides a good model system for the interpretation of the more general case in which only the overall average 129Xe NMR chemical shift is observed in open network zeolites, arising from free exchange of Xe among cavities of variable occupancy and variable cation distribution.