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Molecular Design of Microporous Polymer Membranes for the Upgrading of Natural Gas

Liu, Jie, Jiang, Jianwen
Journal of physical chemistry 2019 v.123 no.11 pp. 6607-6615
carbon dioxide, diffusivity, methane, molecular dynamics, natural gas, permeability, polymers, porosity, porous media, solubility, sorption
Microporous polymer membranes (MPMs) are attractive for gas separation due to their high porosity and tuneable functionalization. In this study, two new MPMs (namely PILP-1 and PILP-3) are designed with the contorted linkers as in polymers of intrinsic microporosity (PIMs) and the tetrahedral cores as in benzimidazole-linked polymers. The separation performance of PILP-1 and PILP-3 for a CO₂/CH₄ mixture is investigated by molecular dynamics (MD) simulation under a constant pressure gradient. Unlike conventional MD simulation, our simulation incorporates both polymer flexibility and membrane plasticization during gas permeation. The simulation results are found to agree well with the available experimental data. PILP-1 and PILP-3 are predicted to possess CO₂ permeabilities of ∼10⁴ barrer and CO₂/CH₄ permselectivities of ∼50. Their performance surpasses the Robeson’s upper bound and the permselectivities are 1–2-fold higher than PIM-1. We reveal that the highly permselective separation in PILP-1 and PILP-3 is governed by the solubility difference between CO₂ and CH₄, as the diffusivity difference is small. Due to the strong sorption of CO₂, plasticization is observed in the membranes and the pore sizes are found to increase during gas permeation. A good quantitative relationship exists between the mean pore size and CO₂ permeability. From the bottom-up, this study underpins the important role of gas sorption in determining the separation of the CO₂/CH₄ mixture and it suggests that the newly designed PILP-1 and PILP-3 might be interesting membranes for the upgrading of natural gas.