Efficient CO₂ Separation of Multi-Permselective Mixed Matrix Membranes with a Unique Interfacial Structure Regulated by Mesoporous Nanosheets

A facile strategy to elevate gas separation performances of polymers is to introduce a versatile particle. In this study, the novel F-Ce nanosheets are synthesized, and then F-Ce is functionalized with 1-ethyl-3-methylimidazole thiocyanate (ionic liquids, ILs), obtaining multifunctional f-F-Ce nanosheets by the facile and environment-friendly methods. The multifunctional f-F-Ce nanosheets are incorporated into the Pebax (Pebax 1657) matrix to fabricate mixed matrix membranes (MMMs) for efficient CO2 separation. The f-F-Ce nanosheets play versatile parts in elevating membrane gas separation performance. On the one hand, f-F-Ce tends to arrange horizontally and constructs a unique interfacial structure for cross-layer CO2 transport in MMMs. On the other hand, the abundant mesopores from f-F-Ce construct high-speed CO2 transport channels in MMMs and notably elevate the gas permeability. Moreover, the as-prepared MMMs separate CO2 efficiently due to the comprehensive improvements of diffusivity selectivity, solubility selectivity, and reactivity selectivity. First, the high aspect ratio of f-F-Ce provides the tortuous pathways for gas transport and generates the rigid interface between the Pebax matrix and f-F-Ce nanosheets, increasing the diffusivity selectivity. Second, SCN– groups from ILs show excellent affinity to CO2, enhancing the solubility selectivity. Third, amine groups from ILs with abundant methylimidazole generate reversible reaction with CO2 to elevate reactivity selectivity. Consequently, the f-F-Ce-doped MMMs display excellent CO2 permeability and CO2/CH4 selectivity. In particular, the MMM incorporated with 8 wt % f-F-Ce displays a CO2 permeability of 1823 Barrer and a CO2/CH4 selectivity of 35, overcoming the Robeson upper bound line (2008).