Room temperature ionic liquid/ZIF-8 mixed-matrix membranes for natural gas sweetening and post-combustion CO2 capture

Abstract

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 25.8wt% 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 25.8wt% ZIF-8 exhibits impressive performance for post-combustion CO2/N2 (50/50mol%) separation. It has a CO2 permeability of 906.4×3.348×10−19kmolm/(m2spa) (906.4barrer) and a CO2/N2 selectivity of 21 at 35°C and 3.5bar.