Adsorption studies of methane, ethane, and argon in the zeolite mordenite:Molecular simulations and experiments

The adsorption of methane, ethane, and argon on sodium mordenite (Na-MOR) a t ambient and cryogenic conditions is investigated experimentally and with grand canonical Monte Carlo (GCMC) simulations. Two different Na-MOR sample s with silicon-to-aluminum (Si/Al) ratios of similar to 5 and similar to 9 are used in the experiments. Simulations are conducted on models with close to the experimentally observed Si/Al ratios and also on a purely siliceous model of MOR. In addition, the impact of varying the zeolite crystal symme try from Cmcm to Pbcn is examined with GCMC. The agreement between the GCMC simulations and previous and current experimental measurements is quite go od at ambient conditions. Differences between the ambient isotherms compute d with the Cmcm and Pbcn structures are slight. However, the two structures exhibit qualitatively different argon adsorption behavior at cryogenic tem peratures (87.3 K). The structure based on symmetry Pbcn shows a much bette r match with experimental isotherms than does the Cmcm structure. Cryogenic adsorption measurements are shown to be sensitive to subtle structural dif ferences in the zeolite lattice. They also provide a rigorous test of force fields used in simulations. It is demonstrated that inclusion of cations a nd framework Al atoms with realistic charge distributions in the simulation s is required to adequately match experimental results.