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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Surfaces, Coatings and Films
- Materials Chemistry