Mixed matrix membranes based on soluble perfluorinated metal-organic cage and polyimide for CO2/CH4 separation

Emerging as a new type of porous materials, metal-organic cages (MOCs) show great potential in adsorption and separation. The high porosity and solubility of MOCs have generated increasing interest in their use as fillers for preparing mixed matrix membranes (MMMs). Herein, we present a series of zirconocene-based MOCs (MOC-1, MOC-1-NH2, and MOC-1-4F), which were used to fabricate MMMs with polyimide 6FDA-DAM. Particularly, the perfluorinated MOC (MOC-1-4F) could form a clear casting solution with 6FDA-DAM in some solvents. The resultant MMMs were analyzed by AFM, SEM, EDX-mapping, XRD, CSM, Raman, TG, and DSC measurements, indicating that MOC-1-4F were homogeneously dispersed in 6FDA-DAM, and the Tg values, mechanical property, CO2 solubility and diffusivity of the membrane were promoted after the introduction of MOC-1-4F. The MOC-1-4F based membranes also outperformed the membranes with MOC-1 or MOC-1-NH2 as filler in CO2/CH4 separation. A low MOC-1-4F loaded MMM (MOC-1-4F-2.17%) exhibited 57% improved CO2 permeability (1228 barrer) in comparison with 6FDA-DAM membrane, and the CO2/CH4 selectivity reached 22, surpassing the 2008 Robeson upper-bound. The plasticization effect was significantly alleviated in MOC-1-4F-2.17%, indicating that the MOC-1-4F-2.17% is potential for CO2/CH4 separation. This work demonstrates that the gas separation performance of MOC-based MMMs could be fine-tuned by the structural tailoring of MOCs.