Siting of mixtures in mordenite zeolites: ¹⁹F and ¹²⁹Xe NMR and molecular simulation

¹⁹F and ¹²⁹Xe nuclear magnetic resonance spectroscopy (NMR) investigations were performed to validate predictions from Monte Carlo simulations for molecular siting in mordenite zeolites. For single-component adsorption, CF₄ adsorbs preferentially in the mordenite main channel, while Xe adsorbs about equally in the main channel and the mordenite side pocket. The temperature and pressure dependences of the siting were studied, as well as the influence of zeolite cation exchange on molecular siting. ¹²⁹Xe NMR studies of Xe-CF₄ binary mixtures in Na-mordenite show that the main channel peak decreases with increasing concentration of CF₄, implying that Xe prefers the side pocket while CF₄ prefers the main channel in the binary system. Results show that ¹⁹F NMR of adsorbed CF₄ can be used to characterize different zeolite environments in a manner similar to ¹²⁹Xe NMR but with reduced sensitivity. The NMR results provide a stringent test of molecular simulation of adsorption by providing molecular-level siting that complements macroscopic experimental quantities such as the adsorption isotherm and heat of adsorption. Simultaneously predicting siting and macroscopic quantities is a difficult challenge for simulation. To improve the molecular model, an explicit polarization term was added to the usual Lennard-Jones potential.