COMPETITIVE ADSORPTION OF XENON AND KRYPTON 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-Kr mixture in zeolite NaA as a model system, The XenKrm clusters are tra pped in the alpha cages of this zeolite for times sufficiently long th at it is possible to observe individual peaks in the nuclear magnetic resonance (NMR) spectrum for the clusters. The Xe-129 nuclear magnetic resonance spectra of several samples of varying Xe and Kr loadings ha ve been observed and analyzed to obtain the Xe-129 chemical shifts and the intensities of the peaks which are dependent on the average krypt on and xenon occupancies. The detailed distributions, f(XenKrm), the f ractions of cages containing n Xe atoms and m Kr atoms can be observed directly in this system from the relative intensities since individua l peaks for XenKrm mixed clusters are observed in the NMR spectrum. Gr and canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Kr in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts. The agreement with experiment is excellent . The calculated absolute chemical shifts for the Xe-n peaks and XenKr peaks at 300 K are in good agreement with experiment. A strictly stat istical model of a binary mixture, derived from the hypergeometric dis tribution, in which the component atoms are distinguishable but equiva lent in competition for eight lattice sites per cage under mutual excl usion provides a limiting case for the distributions, with which the G CMC simulations and the properties of the actual Xe-Kr system may be c ompared, The selectivity coefficients of the Xe-Kr mixture in zeolite NaA is well described by the ideal adsorbed solution model. (C) 1997 A merican Institute of Physics.