Mixed matrix membranes based on NH2-MIL-53 (Al) and 6FDA-ODA polyimide for CO2 separation: Effect of the processing route on improving MOF-polymer interfacial interaction

Abstract

This work compares the effect of two different fabrication routes on the gas separation performance of mixed matrix membranes (MMM). Flat-sheet symmetric membranes based on 6FDA-ODA as the matrix and 10–20 wt% metal–organic frameworks (MOF) MIL-53 (Al) and NH2-MIL-53 (Al) as fillers were fabricated via two approaches. In approach A, the MOF was incorporated at the early stage of the polyimide synthesis and the 6FDA-ODA polymerization reaction took place in the presence of MOF, while approach B followed the traditional solution mixing technique of MMM preparation. From the samples produced, XRD, N2 adsorption at 77 K, ATR-FTIR, FEG-SEM, TGA-DTG, DMA and density measurements were used to characterize the MOF structure and the membrane properties. Then, single and mixed gas separation performances were evaluated for CO2, CH4 and N2 at 2 and 5.5 bar and 35 °C. As a result of better interfacial interactions between NH2-MIL-53 (Al) and polyimide via hydrogen bonding, the MMM-A series showed excellent dispersion in the polymer matrix compared to MMM-B. At 10 wt% NH2-MIL-53, the membrane prepared by approach A showed 100% improvement in CO2/CH4 mixed gas separation factor (50:50 vol% CO2:CH4 at 35 °C and 5.5 bar) compared to that of neat 6FDA-ODA, which is 27% better than the membrane prepared by approach B. Better interfacial compatibility also improved the thermal and mechanical properties of the MMM, which confirms the efficiency of this approach for the NH2-MIL-53 (Al)/6FDA-ODA system. On the other hand, unfunctionalized MIL-53 (Al) MMM showed no difference between approach A and B in improving the gas separation performance.