Room Temperature Ionic Liquid/ZIF-8 Mixed-Matrix Membranes for Natural Gas Sweetening and Post-Combustion CO2 Capture

We have investigated the effects of zeolite imidazolate framework-8 (ZIF-8) nanoparticles in miscible ionic liquid blend systems for natural gas sweetening and post-combustion CO₂ capture.  The miscible blend systems consists of a polymerizable room temperature ionic liquid (poly(RTIL)) and a “free” room temperature ionic liquids (RTILs). The poly(RTIL) is 1-vinyl-3-butyl imidazolium-bis (trifluoromethylsulfonyl) imidate ([vbim][NTf₂]), while the RTIL is either (i) 1-ethyl-3-methylimidazolium tetrafluoroborate ([emim][BF₄]), (ii) 1-ethyl-3-methylimidazolium  bis (trifluoromethanesulfonyl) imide ([emim][NTf₂]) or (iii) 1-ethyl-3-methylimidazolium tetracyanoborate ([emim][B(CN)₄]). Experimental results show that the free ionic liquids are miscible with poly(RTIL), while ZIF-8 are uniformly dispersed in the MMMs. The presence of ZIF-8 nanoparticles in the MMMs considerably improves gas permeability without much scarifying CO₂/N₂ and CO₂/CH₄ selectivities as compared to their poly(RTIL)/RTIL counterparts. The gas permeability of P[vbim][NTf₂]/[emim][B(CN)₄]/ZIF-8 comprising 25.8 wt% ZIF-8 exhibits a threefold increase in gas permeability. Both the semi-logarithmic addition and Maxwell equations are employed to analyze the transport mechanisms across the newly developed MMMs. The permeability vs. ZIF-8 content of P[vbim][NTf₂]/ZIF-8, P[vbim][NTf₂]/[emim][NTf₂]/ZIF-8 follows exactly the Maxwell prediction, indicating these blend membranes are intrinsically heterogeneous with well dispersed ZIF-8 nanoparticles. However, an interesting phenomenon was observed in the P[vbim][NTf₂]/[emim][B(CN)₄]/ZIF-8 system where the homogeneous poly(RTIL)/RTIL phase turns into a more heterogeneous phase upon the adding of ZIF-8. As a result, a double employment of Maxwell equation is applied to analyze the enhanced gas permeability when a higher ZIF-8 loading is utilized. The P[vbim][NTf₂]/[emim][B(CN)₄]/ZIF-8 system with 25.8 wt% ZIF-8 exhibits impressive performance for post-combustion CO₂/N₂ (50/50 mol%) separation. It has a CO₂ permeability of 906.4 barrers and a CO₂/N₂selectivity of 21 at 35 °C and 3.5 bar.

Keywords: Ionic liquids; Carbon dioxide; Mixed-matrix membrane; Zeolitic imidazolate framework; Maxwell model