(539a) Metal-Organic Framework/Polymer Mixed-Matrix Membranes for H2/CO2 Separation: A Fully Atomistic Simulation Study

A fully atomistic simulation study is reported to investigate H₂/CO₂ separation in mixed-matrix membranes composed of polybenzimidazole (PBI) and zeolitic imidazolate framework-7 (ZIF-7). PBI is mimicked by the polymer consistent force field (PCFF) and ZIF-7 by a new force filed developed here. The density and glass transition temperature of PBI from simulation and experiment are in good agreement. The simulated lattice constants, bond lengths and angels of ZIF-7 match well with measured data. PBI/ZIF-7 membranes have stronger mechanical strength than PBI as evidenced by the enhanced bulk modulus upon adding ZIF-7. With increasing ZIF-7 loading, the fractional free volume and percentage of large voids (> 3 Å) become larger; the solubilities of H₂ and CO₂ also increase because of stronger affinity with ZIF-7. H₂ diffusion in PBI/ZIF-7 is not retarded by the addition of ZIF-7, in contrast to CO₂ diffusion. Compared to PBI, the permeabilities of H₂ and CO₂ in PBI/ZIF-7 are enhanced, as well as the permselectivity of H₂/CO₂. The performance of PBI/ZIF-7 is found to surpass the Robeson’s upper bound. For the first time, this simulation study provides atomistic insight into the sorption, diffusion and permeation of gas molecules in MOF-filled membranes; and suggests high separation could be achieved in this type of novel membranes.