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 trapped in the alpha cages of this zeolite for times sufficiently long that 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 have been observed and analyzed to obtain the Xe-129 chemical shifts and the intensities of the peaks which are dependent on the average krypton and xenon occupancies. The detailed distributions, f(XenKrm), the fractions of cages containing n Xe atoms and m Kr atoms can be observed directly in this system from the relative intensities since individual peaks for XenKrm mixed clusters are observed in the NMR spectrum. Grand 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 statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion provides a limiting case for the distributions, with which the GCMC simulations and the properties of the actual Xe-Kr system may be compared, The selectivity coefficients of the Xe-Kr mixture in zeolite NaA is well described by the ideal adsorbed solution model. (C) 1997 American Institute of Physics.