Fabrication of polyimide mixed matrix membranes with asymmetric confined mass transfer channels for improved CO2 separation

Abstract

Narrow-mesoporous SiO2 with ordered helical channels (pore size less than 10 nm) was prepared via a soft template method. Thereafter, the polyimide (PI) mixed matrix membranes (MMMs) incorporated with helical-mesoporous SiO2 were used for CO2 separation. Characterization results show that the formation of asymmetric helical channel structures in narrow-mesoporous SiO2 increased the defect structures and the hydroxyl groups on the pore surface, thus enhancing the affinity between CO2 molecules and SiO2. Due to the stronger CO2 capture ability, the addition of helical-mesoporous SiO2 introduced confined mass transfer channels into the polymer matrix for facilitated transport of CO2, thus significantly enhancing the CO2 separation ability of MMMs. Increasing the loading content of helical-mesoporous SiO2 not only increased the confined channels to facilitate CO2 transport, but also improved the CO2 capture ability of MMMs, thus enhancing their CO2 selectivity. The gas permeability and CO2/N2 ideal selectivity calculated using the Maxwell-Wagner-Sillar model, assuming the helical-mesoporous SiO2 as a collection of parallel helical channels, agreed well with the experimental data of MMMs at low loading content of SiO2. When the loading content exceeded 15 wt%, the SiO2 aggregations caused the formation of non-selective voids in membranes, significantly lowering the CO2/N2 ideal selectivity.