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.