Asymmetric Polyimide Mixed Matrix Membranes with Porous Materials-Modified Surfaces for CO2/N2 and CO2/CH4 Separations

Membranes with both good permeation and selectivity are highly desired for gas separations. We synthesized a polyimide (PI) asymmetric membrane using the phase-inversion method, and then modified the surface with a mixture of porous fillers and poly(amic acid) (PAA). The porous fillers included the metal organic framework (MOF) of Cu-3(BTC)(2) (copper benzene-1,3,5-tricarboxylate), the zeolite imidazole framework (ZIF) of ZIF-8, and the porous hydrotalcite of MgAl-LDH. A series of asymmetric mixed-matrix membranes (MMMs) were obtained after surface coating and thermal amidation. The MMM structure, CO2, CH4, and N-2 permeance, and the ideal gas selectivity were investigated. With the surface modification, the morphology of the surface separation layers of the asymmetric PUZIF-8, PI/LDH, and PI/Cu-3(BTC)(2) MMMs significantly changed, and the gas separation performance changed accordingly. The PUZIF-8 asymmetric MMM with 5% (w) ZIF-8 doping exhibited both enhanced ideal gas selectivity and permeance; the CO2/N-2 and CO2/CH4 selectivity were as high as 24 and 83, respectively. Thus, this surface modification provides improved MMM gas separation performance.