Supercritical CO<sub>2</sub> treatment of mixed matrix membranes based on polyimides for improvement of their gas transport properties

Mixed matrix membranes (MMMs) were prepared by introduction of metal-organic frameworks ZIF-8 and ZIF-67 in concentration up to 20% w/w in chloroform solution intopolyimides synthesized of diethyltoluenediamine (DETDA) isomers mixture and biphenyl-tetracarboxylic acid dianhydride (BPDA)or hexafluoroisopropylidenediphthalic anhydride (6FDA). The initial polyimides were synthesized by single-stage high-temperature catalytic polycondensation in benzoic acid melt. The studied MMMs weretreated by supercritical CO2 with subsequent decompression. The gas transport and gas selective properties of the initial and exposed to sc-CO2 membranes have been studied. Experimental values of the effective gas permeability and diffusion coefficients of He, H2, O2, N2, CO2, CH4 were obtained, and the effective solubility coefficients of these gases were calculated. It was found that the sc-CO2 treatment of the studied MMMs can significantly increase the level of gas permeability of the membranes, while the effect achieved depends on the gas, the nature of the matrix and the concentration of the introduced particles with the selectivity of gases at the level of the initial ones. It has been established that the treatment effect persists over time with a slight decrease in the permeability of gases, which at the same time remains at a level significantly higher than the initial permeability. The demonstrated effect of improving gas transport properties when treating MMMs based on polyimide matrices 6FDA-DETDA and BPDA-DETDA in sc-CO2 can be used for further application of the proposed modification method in order to increase gas transport through MMMs based on other polymers, including highly permeable ones.