Competitive adsorption of xenon and argon in zeolite NaA. 129Xe nuclear magnetic resonance studies and grand canonical Monte Carlo simulations

Abstract

Investigation of competitive adsorption is carried out using the Xe-Ar mixture in zeolite NaA as a model system. The Xe n clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the Ar atoms are in fast exchange between the cages and also with the gas outside. The 129 Xe nuclear magnetic resonance spectra of 12 samples of varying Xe and Ar loadings have been observed and analyzed to obtain the 129 Xe chemical shifts and the intensities of the peaks which are dependent on the average argon and xenon occupancies. The detailed distributions, f ͑Xe n Ar m ͒, the fractions of cages containing n Xe atoms and m Ar atoms cannot be observed directly in this system, that is, individual peaks for Xe n Ar m mixed clusters are not observed in the NMR spectrum. This information is, however, convoluted into the observed 129 Xe chemical shifts for the Xe n peaks and the distributions P n , the fraction of cages containing n Xe atoms, regardless of the number of Ar atoms, obtained from their relative intensities. Grand canonical Monte Carlo ͑GCMC͒ simulations of mixtures of Xe and Ar in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions P n . The agreement with experiment is reasonably good for all 12 samples. The calculated 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 experiment. The GCMC results are compared with 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. The latter simple model introduced here provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe-Ar system are compared.

The Xe129 nuclear shielding tensor surfaces for Xe interacting with rare gas atoms

“…In Fig. 1 we show a comparison with the previously used Xe-Xe shielding function in our early work, [25][26][27][28][29][30][31][32] obtained by scaling to Xe-Xe from LORG calculations of Ar shielding in Ar-Ar. 20 We see that the scaled Ar-Ar function was not that bad, considering the basis set used for Ar was not as extensive as the 240 basis functions we now use for Xe atom.…”

Hierarchical approach for simulation of binary adsorption in silicalite

“…Given a force field that describes a system well, the simulations allow one to make accurate predictions about real systems of interest. In addition to work on single-component systems, the adsorption of binary mixtures in zeolites has been investigated using atomistic GCMC simu-Ž lations in several studies Karavias and Myers, 1991;Razmus and Hall, 1991;Maddox and Rowlinson, 1993;Dunne and Myers, 1996;Van Tassel et al, 1994b, 1996Dunne et al, 1996;Jameson et al, 1996Jameson et al, , 1997Lachet et al, 1997;Heuchel . et al, 1997;Clark et al, 1998;Macedonia and Maginn, 1999 .…”

Filling of Irregular Channels with Round Cross-Section: Modeling Aspects to Study the Properties of Porous Materials

“…In a more realistic simulation, the spherical particles could be still considered, instead of the ellipsoids, as circumscribed spheres in the ellipsoids, the radius of the sphere being equal to the maximum distance from the center to the surface furthest from the ellipsoid. An example of a real case of a sphere packing in a narrow channel is the adsorption of monatomic molecules of inert gases in zeolites, in which the shape of the neon or argon atoms is well described as a sphere, and the distortions of this form are minimal, see e.g., Reference [ 79 ].…”