Abstract
Atomistic simulations were performed to investigate pure- and mixed-gas CO2/CH4 separation properties of a ladder polymer of intrinsic microporosity, PIM-Trip-TB. Despite expected intra-chain rigidity of the polymer, previous experimental reports observed significant loss in CO2/CH4 perm-selectivity under high-pressure mixed-gas conditions. In this work, all-atomistic simulations were applied to accurately predict density, gas uptakes and gas diffusion properties of PIM-Trip-TB. Competitive sorption favoring CO2 over CH4 was apparent in mixed-gas sorption simulations, as previously demonstrated by experimental studies from our group. This effect resulted in enhanced mixed-gas CO2/CH4 solubility selectivity. However, this increase did not translate to increased mixed-gas perm-selectivity because a significant increase in CH4 permeability was observed by co-permeation of CO2 relative to the pure-gas value. Back-calculated diffusion coefficients indicated very low CO2/CH4 diffusion selectivity under mixed-gas conditions, eliminating any gain from competitive sorption. Structural analysis confirmed intact intra-chain rigidity of the polymer; on the other hand, a significant increase in fractional free volume (FFV) and shift to larger pores in the pore size distribution was revealed by our simulations which may be attributed to polymer dilation due a reduction in inter-chain packing.
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