A Comparison of Experimental Diffusion Coefficients of Small Gases in Silicalite-1 with Molecular Dynamics Predictions Based On COMPASS and CVFF Forcefields

High temperature (>500°C) molecular dynamics studies of gas diffusion in microporous zeolites require consideration of the zeolite framework flexibility, which can cause expansions and contractions of pore windows, 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 (COMPASS) or the Consistent Valence ForceField (CVFF) to siliceous MFI (silicalite-1). The current study seeks to validate these intramolecular and intermolecular potentials for silicalite-1, one of the most extensively investigated zeolite with reseoct to adsorption and diffusion of guest molecules. The diffusion coefficients of small gases (H₂, CO, CO₂, CH₄, O₂ and N₂) in high silica MFI obtained using Pulse-Field Gradient-Nuclear Magnetic Resonance (PFG-NMR) and Quasi-Elastic Neutron Scattering (QENS) methods are compared to theoretically derived diffusion coefficients employing the available COMPASS and CVFF parameters through molecular dynamics.

The diffusion coefficients obtained using the two forcefields for CO, CO₂, CH₄, O₂ and N₂ were in good agreement with experimental data, while the diffusion coefficients of H₂ were two orders of magnitude greater than the experimental values.