Force fields for classical atomistic simulations of small gas molecules in silicalite-1 for energy-related gas separations at high temperatures

High temperature (> 573 K) molecular dynamics studies of gas diffusion in microporous zeolites require consideration of the zeolite framework flexibility. Pore windows can expand and contract at high temperatures, affecting phase space and material properties. No studies to date have addressed the application of the condensed-phase optimized molecular potentials for atomistic simulation studies or the consistent valence force field to simulate gas diffusion and adsorption in siliceous MFI (silicalite-1). The current study seeks to validate these intramolecular and intermolecular potentials along with another zeolite-specific force field reported by Nicholas et al. (JACS 113:4792-4800, 1991) for silicalite-1, one of the most extensively investigated zeolites, with respect to diffusion of several gas molecules. The experimental diffusion coefficients of H-2, CO2, CH4, O-2 and N-2 in silicalite-1 obtained using pulse-field gradient-nuclear magnetic resonance and quasi-elastic neutron scattering methods were compared to theoretically derived diffusion coefficients employing these force fields in molecular dynamics simulations. The diffusion coefficients obtained using the three force fields for H-2, CO2, CH4, O-2 and N-2 agreed well with these experimental data. The zeolite-specific force field of Nicholas et al. was employed in grand canonical Monte Carlo simulations to obtain adsorption isotherms of these gases. The adsorption isotherms and isosteric heats of adsorption predicted were also in agreement with the expected range of available experimental and theoretical adsorption data reported in the literature.