COMPETITIVE ADSORPTION OF XENON AND ARGON IN ZEOLITE NAA - XE-129 NUCLEAR-MAGNETIC-RESONANCE STUDIES AND GRAND-CANONICAL MONTE-CARLO SIMULATIONS

Investigation of competitive adsorption is carried out using the Xe-Ar mixture in zeolite NaA as a model system. The Xe-n clusters are trapp ed in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for ea ch cluster while the Ar atoms are in fast exchange between the cages a nd also with the gas outside. The Xe-129 nuclear magnetic resonance sp ectra of 12 samples of varying Xe and Ar loadings have been observed a nd analyzed to obtain the Xe-129 chemical shifts and the intensities o f the peaks which are dependent on the average argon and xenon occupan cies. The detailed distributions, f(XenArm), the fractions of cages co ntaining n Xe atoms and m Ar atoms cannot be observed directly in this system,that is, individual peaks for XenArm mixed clusters are not ob served in the NMR spectrum. This information is, however, convoluted i nto the observed Xe-129 chemical shifts for the Xe-n peaks and the dis tributions P-n, the fraction of cages containing n Xe atoms, regardles s of the number of Ar atoms, obtained from their relative intensities. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Ar in a rigid zeolite NaA lattice provide the detailed distributions a nd the average cluster shifts, as well as the distributions P-n. The a greement with experiment is reasonably good for all 12 samples. The ca lculated absolute chemical shifts for the Xe-n peaks in all samples at 300 K range from 75 to 270 ppm and are in good agreement with experim ent. The GCMC results are compared with a strictly statistical model o f a binary mixture, derived from the hypergeometric distribution, in w hich the component atoms are distinguishable but equivalent in competi tion for eight lattice sites per cage under mutual exclusion. The latt er simple model introduced here provides a limiting case for the distr ibutions, with which both the GCMC simulations and the properties of t he actual Xe-Ar system are compared. (C) 1996 American Institute of Ph ysics.