Crosslinked MOF-polymer to enhance gas separation of mixed matrix membranes

Abstract

Preparation of mixed matrix membranes (MMMs) from rigid polymers like PIM-1 remains challenging because the contorted and rigid polymer chains of PIM-1 severely inhibit self-rearrangement leading to low quality of polymer-filler interaction. MMMs prepared by conventional methods, therefore, often suffer from defects at the polymer-filler interface. In this work, a novel filler having hydroxyl functional groups on the surface (Mg-MOF-74) was used as a filler and mixed with PIM-1 in solution (chloroform). Under optimized solution conditions (65°C and 24h), chemical crosslinking between the hydroxyl groups and the fluoride chain-ends of PIM-1 was facilitated, to completely remove these interfacial defects. The crosslinked MMMs were then characterized by XRD, FTIR, SEM and mechanical tests, while their separation properties were studied for five gases including H2, CO2, CH4, N2 and O2 upon increasing Mg-MOF-74 loading (0–20% wt.). The two main features of MOF-74 (also known as CPO-27), especially Mg-MOF-74, greatly contributing to the membrane gas separation were 1-D hexagonal channels (1.1nm diameter) and an exceptionally high CO2 adsorption capacity under reduced pressures. As the ends of PIM-1 are directly linked with the MOF surface, the intrinsic micropores of PIM-1 are connected to the inner pores of MOF-74 forming an inter-connected micropore network throughout the MMM. Both selectivities and permeabilities of MMM were improved for all the gases, particularly CO2 permeability which increased by 3.2 times from 6500 Barrer for the neat polymer (PIM-1) to 21,000 Barrer for PIM-1 with 20% wt. of MOF-74, meanwhile CO2/CH4 selectivity was improved from 12.3 to 19.1, respectively.