ANALYSIS OF XE-129 CHEMICAL-SHIFTS IN ZEOLITES FROM MOLECULAR-DYNAMICS CALCULATIONS

The qualitative dependence on cavity size of Xe-129 NMR chemical shifts for xenon sorbed in zeolites is usually interpreted in terms of collisions between xenon and cavity walls. Molecular dynamics simulation at infinite dilution is a powerful tool to give insight into the collision effect. Contact time and the number of collisions with oxygen atoms of the framework are derived from MD calculations in pure siliceous zeolites of various cavity size (Y zeolite, silicalite, and mordenite) at different temperatures. Correlation between chemical shifts and the number of binary collisions is found. The occurrence of two signals in mordenite spectra cannot be interpreted by the model of pure siliceous framework. It is assumed a certain number of side pockets are blocked, possibly by cations. The ratio of accessible pockets, derived from NMR spectra, increases with temperature from 0.015% to 12% in Na mordenite. The diffusion coefficient with the blockage of the pockets taken into account has been determined to be 4.3 10(-9) m(2).s(-1) at 300 K.