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 H-2/CO2 separation in mixed-matrix membranes composed of polybenzimidazole (PBI) and zeolitic imidazolateframework-7 (ZIF-7). PBI is mimicked by the polymer-consistent force field and ZIF-7 by a new force field developed here. The predicted density and glass transition temperature of PBI are in fairly good agreement with experimental results. The simulated lattice constants, bond lengths and angles 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 increased ZIF-7 loading, the fractional free volume and percentage of large voids (> 3 angstrom) become larger; the solubilities of H-2 and CO2 increase because of stronger affinity with ZIF-7. H-2 diffusion in PBI membrane is not discernably retarded by adding ZIF-7, in contrast to CO2 diffusion. The simulation results reveal that H-2 and CO2 have higher permeabilities in PBI/ZIF-7 than in PBI, and H-2/CO2 permselectivity is enhanced slightly with increased ZIF-7 loading. For the first time, this simulation study provides atomistic insight into the sorption, diffusion, and permeation of gas molecules in MOF/polymer mixed-matrix membranes.