Performance of polysulfone hollow fiber membranes encompassing ZIF-8, SiO2/ZIF-8, and amine-modified SiO2/ZIF-8 nanofillers for CO2/CH4 and CO2/N2 gas separation

Abstract

High tunable properties of metal–organic frameworks (MOFs) allows different modifications to be used for distinct applications. Zeolitic imidazolate frameworks-8 or ZIF-8 are hybrid organic–inorganic porous materials that have shown great potential in membrane-based gas separations. In our study, SiO2/ZIF-8 (S/ZIF-8) nanofiller was synthesized via a facile one-step strategy. Furthermore, the S/ZIF-8 nanofiller was subjected to amine modification using an ammonium hydroxide solution. X-ray photoelectron spectroscopy (XPS) analysis confirms the strong binding of amine and SiO2 on ZIF-8 nanofillers. The synthesized ZIF-8, S/ZIF-8, and amine-modified S/ZIF-8 (A@S/ZIF-8) of 0.5 wt% were incorporated into the polysulfone (PSf) matrix, and the hollow fiber mixed matrix membranes (MMMs) were fabricated using dry–wet phase inversion technique. The inclusion of an amine group in the S/ZIF-8 nanofillers enhances the interaction between the nanofillers and PSf matrices, thus improving the CO2/CH4 and CO2/N2 separation performances. The fabricated membranes were morphologically characterized by Scanning electron microscope (SEM), Brunauer–Emmett–Teller (BET) surface area, and the pore size distribution. It is noteworthy that the BET surface area was enhanced with nanofillers addition and significantly increased gas separation performance. The fabricated asymmetric hollow fiber MMMs were further studied for gas permeation experiments for CO2, N2, and CH4. PSf with A@S/ZIF-8 MMMs showed the best performance as observed from the Robeson upper bound limit, CO2 permeability of 4.25 barrer with the improvements in CO2/N2 and CO2/CH4 selectivities of 89.75 and 61.46% respectively, compared with neat PSf membrane. The enhanced permselectivity of PSf–A@S/ZIF-8 membranes is explained by CO2-facilitated transport, where the high CO2 affinity of the amine group allows the selective transport of CO2 rather than CH4 and N2.