M-gallate MOF/6FDA-polyimide mixed-matrix membranes for C2H4/C2H6 separation

Mixed-matrix membranes (MMMs) based on metal-organic frameworks (MOFs) molecular sieves have been extensively studied in gas separation due to the high performance and good compatibility of MOF with polymers. In this work, 6FDA-based polyimides MMMs incorporating M-gallate (M = Ni, Co, Mg) MOF particles were fabricated for the first time and investigated for ethylene/ethane (C2H4/C2H6) separation. By reducing synthesis temperature and introducing agitation, the Ni-gallate particles synthesized in flask (Ni-gallate(F)) with optimized conditions are smaller than Ni-gallate synthesized in autoclave (Ni-gallate(A)). The particle size was reduced from ~7 μm (Ni-gallate(A)) to ~0.8 μm (Ni-gallate(F)), which enables the formation of uniform MMMs. Because of the better compatibility with polyimide and higher adsorption performance of Ni-gallate than Mg-gallate and Co-gallate, the 6FDA-DAM MMM containing Ni-gallate(F) was capable of simultaneously improving C2H4/C2H6 selectivity and C2H4 permeability whereas the MMM with Mg-gallate(F) or Co-gallate(F) failed in performance enhancement. The 6FDA-DAM MMM with Ni-gallate(F) loading of 20% exhibited optimal C2H4/C2H6 separation performance that surpassed the upper-bound of pure polymers. Detailed analysis of sorption and diffusion coefficients suggested that the enhanced transport properties in Ni-gallate(F) MMMs were attributed to the increase of sorption coefficient provided by the higher C2H4 sorption capacity and selectivity, as well as the favorable gas diffusivity via moderately confined pores of the incorporated Ni-gallate(F) fillers.