Polymer functionalization to enhance interface quality of mixed matrix membranes for high CO2/CH4 gas separation

Abstract

Hydroxyl-functionalized homo- and co-polyimides 6FDA–(DAM)_x–(HAB)_y (with x : y molar ratio of 1 : 0; 2 : 1; 1 : 1; 1 : 2) and two metal–organic frameworks (MOFs), MIL-53(Al) and NH₂-MIL-53(Al) were synthesized for preparation of mixed matrix membranes (MMMs). The MOF loadings were varied over the range of 10–20 wt% for NH₂-MIL-53(Al) and 10–15 wt% for MIL-53(Al). The incorporation of hydroxyl groups into the polyimide backbone is expected to improve the interfacial interaction between the polymer matrix and fillers, consequently, enhancing gas separation performance of MMMs. A big increase in glass transition temperature (T_g) for MMMs confirmed the polymer chain rigidification, which was caused by a strong interaction between the hydroxyl groups in the copolyimides and the amine groups in NH₂-MIL-53(Al). Additionally, SEM results showed that the hydroxyl groups facilitated the particle dispersion in the MMMs, either was NH₂-MIL-53(Al) or MIL-53 used as filler. Gas separation performances of MMMs were characterized by both CO₂/CH₄ pure gas and binary permeation measurements at 35 °C and 150 psi. The incorporation of NH₂-MIL-53(Al) in the hydroxyl-copolyimides was found to significantly improve the CO₂/CH₄ separation factor while maintaining CO₂ permeability of the MMMs as high as those of the neat corresponding copolyimides, therefore greatly enhancing the MMM separation performance. For example, the MMM prepared from 6FDA–DAM–HAB (1 : 1) copolyimide and 10 wt% NH₂-MIL-53(Al) showed a permeability/selectivity behavior approaching the 2008 Robeson's upper bound making it attractive for practical usage. The significant improvement in CO₂/CH₄ separation factor observed for the MMMs made of the hydroxyl-copolyimides and the amine-functionalized MOFs was due to (i) the enhanced polymer–filler compatibility originating from a mutual interaction between the polymer-functional moieties and the amine-functionalized MOF surface yielding defect-free MMMs and (ii) the high CO₂/CH₄ selective adsorption in the NH₂-MIL-53(Al) framework.