Grand canonical Monte Carlo simulations of the distribution and chemical shifts of xenon in the cages of zeolite NaA. I. Distribution and129Xe chemical shifts

摘要

The equilibrium distribution of the Xe atoms among the alpha cages of the zeolite NaA have been measured directly by nuclear magnetic resonance (NMR) in ten samples ranging from very low xenon loading up to saturation. These distributions are simulated by a grand canonical Monte Carlo (GCMC) method which reproduces the experimental data quantitatively for all ten samples at 296 K and also at 360 K. The adsorption isotherm of the high loading samples has been determined directly from the chemical shift of the gas in equilibrium with the adsorbed xenon. The data compare favorably with the adsorption isotherms resulting from the simulations. The previously reported129Xe chemical shifts of the individual Xenclusters and their temperature dependences in the range 188ndash;420 K are reproduced quantitatively by the GCMC simulation which makes use of pairwise additiveabinitiointermolecular shielding functions. These cluster shifts and their temperature dependence encode the distribution of configurations for a given Xencluster in an alpha cage. Quantitative agreement with the three experimental measures of the distribution of Xe atoms in NaA (partitioning between the adsorbed phase and the gas phase, distribution of the intrazeolitic atoms among the alpha cages, and the distribution of Xe atoms within an alpha cage containing Xen) as a function of temperature has been achieved for the first time.