Beyond Lattice Models of Activated Transport in Zeolites: High-Temperature Molecular Dynamics of Self-Diffusion and Cooperative Diffusion of Benzene in NaX

Abstract

We employ high-temperature molecular dynamics to investigate self-transport and cooperative transport of benzene in NaX (Si:Al = 1.2). We have refined the benzene-NaX force field for use with our previously developed framework force field for aluminosilicates, which explicitly distinguishes between Si and Al atoms in the frame, and also between oxygen atoms in Si-O-Si and Si-O-Al environments. Energy minimizations and molecular dynamics simulations performed to test the new force field give excellent agreement with experimental data on benzene heats of adsorption, benzene-Na distances, and Na distributions for benzene in NaY (Si:Al = 2.4) and NaX (Si:Al = 1.2). Molecular dynamics simulations are performed over a range of temperatures (600-1500 K) and loadings (infinite dilution to four benzenes per supercage) to evaluate simultaneously the self-diffusivities and cooperative (alternatively Maxwell-Stefan) diffusivities. The simulated diffusivities agree well with pulsed field-gradient NMR and quasi-elastic neutron scattering data. Despite this agreement, we show in the following companion paper that membrane fluxes calculated with our diffusivities overestimate experiments by 1 order of magnitude when support resistance is accounted for in the transport model, and by about 2 orders of magnitude when support resistance is neglected. This discrepancy may arise from the polycrystalline nature of present-day NaX membranes.