Anisotropic Xe Chemical Shifts in Zeolites. The Role of Intra- and Intercrystallite Diffusion

Abstract

We provide observations and interpretations of 129Xe chemical shifts in open zeolite networks in which the Xe is in fast exchange among a large number of cavities and channels, in two zeolite types different from the A types that we have used before, and provide quantitative comparisons of adsorption isotherms and average 129Xe chemical shifts using grand canonical Monte Carlo (GCMC) simulations employing the same methods and shielding functions used previously. We examine the temperature dependence of the Xe chemical shift at a fixed loading in NaY and silicalite and compare the predicted behavior with that of Xe n in the α cages of NaA. Furthermore, we consider the most general case where the chemical shift reported by a Xe atom includes sampling environments in several crystallites and intercrystalline gas during acquisition of the NMR signal.

Molecular dynamics of gases and vapors in nanoporous solids. Final LDRD project report

“…Rhykerd, et al (1991) carried out GCMC of gases in carbon. Razmus and Hall (1991), Jameson et al (1994), and Van Tassel et al (1994a,b) carried out GCMC of gases in zeolite A. Each used varying degrees of accuracy in simulation detail and compared to adsorption and NMR data.…”

129Xenon NMR: Review of recent insights into porous materials

“…The chemical shift is then averaged and may be not truly characteristic of the porosity of the solid studied. The influence of intercrystallite diffusion depends on the size and morphology of the crystallites, the pore size, the crystallite packing, the Xe gas pressure and the temperature [56,57].…”

“…In small pores presenting anisotropic environments (small channels or oblate cavities), in which the xenon atoms undergo anisotropic interactions, the shape of the signals reflects the symmetry of the pores. For example, it is the case for the alumino-phosphate AlPO-11 which presents unidimensional channels with elliptical cross section (4.0×6.5 Å) [60,61], clathrate cages [5], or even tridimensional channel system in ZSM-5 macro-crystals [57,62].…”

Studying porous materials with krypton-83 NMR spectroscopy

“…47,48 Additionally, a nonlinear dependence of υ gas on gas has been demonstrated even for NaX and NaY zeolites. 44 A great deal of progress has been made in the theoretical calculations of 129 Xe chemical shift using grand canonical Monte Carlo simulations 47 -50 by Jameson and coworkers, and similar computational work will likely be helpful in understanding the krypton chemical shift.…”