Solubility selectivity-enhanced SIFSIX-3-Ni-containing mixed matrix membranes for improved CO2/CH4 separation efficiency

Abstract The unstable nature of SIFSIX-3-Zn to most organic solvents hinders its application for gas separation albeit its excellent molecular sieving effect and a high affinity towards CO2. Here, we report the enhanced organic solvent resistance of SIFSIX-3-Ni by characterizing their crystallinity and gas adsorption properties after exposure to organic solvents including tetrahydrofuran and N, N-dimethylformaide. SIFSIX-3-Ni microparticles were incorporated into a 6FDA-DAM:DABA (3:2) polyimide (PI) and polysulfone (PSF) matrix to prepare mixed matrix membranes (MMMs) for CO2/CH4 separation. While the PI was an unsuitable matrix for high loading MMMs due to the strong hydrogen bonding between the carboxylic groups of PI and the SiF62− anions of SIFSIX-3-Ni, PSF/SIFSIX-3-Ni MMMs with loading up to 30% were successfully prepared with the aid of the weak interaction between sulfonate groups in PSF and the fluorine atoms of SIFSIX-3-Ni. Interestingly, the CO2 permeability of PSF/SIFSIX-3-Ni MMMs monotonically decreased with increasing CO2/CH4 selectivity as the SIFSIX-3-Ni concentration increased. Also, the PSF/SIFSIX-3-Ni (70/30) showed lower CO2 permeability with higher CO2/CH4 selectivity compared to PSF under an equimolar CO2/CH4 mixed gas condition. Monte Carlo simulations suggest that the strong Coulombic interactions between SIFSIX-3-Ni and CO2 significantly increased the free energy barrier for CO2 over CH4, leading to a reduction in the CO2/CH4 diffusive selectivity of MMMs. Unlike diffusivity, the CO2 solubility of PSF/SIFSIX-3-Ni MMMs increased with increasing SIFSIX-3-Ni contents due to the strong interaction between SiF62− of SIFSIX-3-Ni and CO2, significantly enhancing the CO2/CH4 adsorptive selectivity.