Molecular Simulations of CH4 and CO2 Diffusion in Rigid Nanoporous Amorphous Materials

R Thyagarajan and DS Sholl, JOURNAL OF PHYSICAL CHEMISTRY C, 126, 8530-8538 (2022).

DOI: 10.1021/acs.jpcc.2c01609

Molecular diffusion in nanoporous materials isimportant in determining the rate of equilibration of variousadsorption processes and plays a pivotal role in kinetic separationsand membrane-based separations. Because generating realisticstructures of amorphous nanoporous materials is difficult, far lessis known about diffusion in amorphous nanoporous materials thanin their crystalline counterparts. Here, we present moleculardynamics simulations assessing the room-temperature self- diffusionof CH4and CO2in a wide range of rigid amorphous nanoporousmaterials, including porous carbons, kerogens, polymers of intrinsic microporosity, and hyper-cross-linked polymers. Our results arethe largest collection of molecular diffusivities in amorphous nanoporous materials to date. In each material, the diffusivity increaseswith the adsorbate concentration at low and moderate adsorbate concentrations, reaching a maximum before decreasing due to stericeffects at higher concentrations. The observed diffusivities are much slower than that would be expected based on standarddescriptions of Knudsen diffusivity. We show that the observed diffusivities are not correlated in a simple way with scalar descriptorsof the pore structures such as the pore limiting diameter.

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