¹²⁹Xe NMR chemical shifts in zeolites: Effect of loading studied by Monte Carlo simulations

The NMR chemical shift of ¹²⁹Xe adsorbed in the cavities and channels of zeolites depends on the crystal type (structure and chemistry) and on the loading of Xe. The dependence of the ¹²⁹Xe shift on Xe loading has frequently been interpreted qualitatively in terms of increasing Xe-Xe interactions, but this dependence can be quantitatively and predictively modeled using computer simulations. We show that Monte Carlo simulations of Xe inside Omega, mordenite, AlPO₄-5, L, Y, ZK-4, silicalite, and VPI-5 give Xe-Xe pair correlation functions that are entirely consistent with the shift behavior reported in the literature. The sensitivity of the shift dependence on loading correlates well with the calculated accessible volume; this correlation distinguishes zeolites even with similar unidirectional channels and indicates that an estimate of the available porosity of a zeolite can be made from such data. There is reasonable (but not in all cases quantitative) agreement between the predicted and experimental zeolite shift contribution.