Performance and plasticization behavior of polymer-MOF membranes for gas separation at elevated pressures

Mixed matrix membranes (MMMs) based on three distinctively different MOB (MIL-53(AI) (breathing MOP), ZIF-8 (flexible MOP) and Cu3BTC2 (rigid MOP)) dispersed in MatrimidP (R)-PI have been investigated. MOP loading was varied between 0 wt% and 30 wt%. The fabricated MOP-MMMs were characterized for pure and binary gas mixture separations at low and high pressures and their performance in terms of CO2 permeability and CO2/CH4 selectivity was evaluated. The use of a less volatile co-solvent, optimized priming protocol to prepare the MMMs and thermal annealing resulted in a good dispersion of MOP particles in the Matrimid (R)-PI matrix. Incorporation of MOFs resulted in increased density, Tg and improved degradation behavior of the membranes confirming a good compatibility between the polymer and the MOFs. Low pressure gas separation showed moderate enhancement in CO2 permeability and CO2/CH4 selectivity of MOF-MMMs compared to native polymer membranes, but the improvement becomes pronounced at high pressures. At high pressures, the native Matrimid (R)-PI membrane showed typical plasticization behavior, while in MMMs, IMF particles limit the mobility of polymer chains thus suppressing CO2 induced plasticization and maintain large separation factors over a wide pressure range investigated. The respective increase in performance of MMMs is very much dependent on MOP crystal structure and its interactions with CO2 gas molecules. Among the three MOB MMMs, membranes based on Cu3BTC2 showed highest selectivity while ZIF-8 based membranes showed highest permeability. In general it can be concluded that the high CO2 permeability and CO2/CH4 selectivity of MMMs is the combined effect of an increased sorption and diffusion selectivity and reduced plasticization. Overall, this work reveals that MOF-MMMs delay CO2 induced plasticization and show good separation performance even at high pressures, showing their potential to a wide range of newly emerging high pressure energy applications. (C) 2014 Elsevier B.V. All rights reserved.