REDICTING LIGHT HYDROCARBON DIFFUSION THROUGH ZEOLITIC IMIDAZOLATE FRAMEWORKS FOR SEPARATION APPLICATIONS

摘要

The use of nanoporous materials for light paraffin and olefin separations has the potential to disrupt the current industry standard, energy intensive cryogenic distillation. The ability to predict the diffusion characteristics of these nanoporous materials would further the design of either thermodynamic or kinetic separations. We focus on a class of nanoporous materials called zeolitic imidazolate frameworks (ZIFs), a subfamily of metal-organic frameworks (MOFs), for which accurate and efficient predictions of hydrocarbon diffusivities are challenging. The small pore sizes of ZIFs (e.g. ZIF-8), coupled with adsorbates having kinetic diameters equal to or larger than the pore size, produce a wide range of diffusion time scales that cannot be measured directly with equilibrium molecular dynamics simulations. We have computationally measured the hopping rates of 15 different molecules in ZIF-8 via dynamically corrected transition state theory (dcTST). Umbrella sampling, an enhanced molecular dynamics sampling method, along with the one-dimensional weighted histogram analysis method (WHAM) was used to calculate the diffusion free energy barriers. Comparison of the computed diffusivities to extant experimental data shows remarkable agreement within an order of magnitude.