Metal–Organic Framework/Polymer Mixed-Matrix Membranes for H₂/CO₂ Separation: A Fully Atomistic Simulation Study

Abstract

A fully atomistic simulation study is reported to investigate H 2 /CO 2 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 Å) become larger; the solubilities of H 2 and CO 2 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 CO 2 diffusion. The simulation results reveal that H 2 and CO 2 have higher permeabilities in PBI/ZIF-7 membranes than in PBI, and H 2 /CO 2 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.