(483e) Temperature and Loading-Dependent Diffusion of Light Hydrocarbons in ZIF-8 As Predicted through Fully Flexible Molecular Simulations

Zeolitic imidazolate framework-8 (ZIF-8), a prototypical member of the ZIF class of metal-organic frameworks (MOFs), has been experimentally shown to exhibit interesting sieving properties with regard to increasing alkane chain length and morphology. The ability to predict diffusion characteristics of these systems using molecular simulations would create opportunities to find similar systems suitable for industrial separations. However, the challenge with simulating diffusion of light key hydrocarbons in ZIF-8 is that diffusion is too slow to observe with standard molecular dynamics (MD).  Previous simulation studies have determined that adsorbate diffusion through cage-type nanoporous materials proceeds by an activated hopping process between low energy states and can be modeled using dynamically corrected transition state theory (dcTST).  Umbrella Sampling (US), a free energy sampling method, has been applied to study small molecule and C1-C4 hydrocarbon diffusion in a fully flexible ZIF-8 model.  Diffusion coefficients of methane, ethane, ethylene, propane, propylene, n-butane, and 1-butene in ZIF-8 are reported over a temperature range of 0 to 150°C and loadings from infinite dilution to saturation.  The results demonstrate how the adsorbate influences the ZIF-8 window during the crossing event.