Effective functionalization of porous polymer fillers to enhance the CO₂/N₂ separation performance of mixed-matrix membranes

Prof. Bae’s group at KAIST has developed a series of porous organic polymers (denoted as PBP-x) possessing various functional groups synthesized via Friedel-Crafts alkylation followed by post-synthetic functionalization using amine and sulfonic groups. These functionalized porous polymers were incorporated into an in-house polyimide to fabricate mixed-matrix membranes for CO₂/N₂ separation. Gas permeation testing revealed that a porous filler after being optimized could improve the CO₂ separation performance in a very effective manner. In particular, the addition of 10 wt% PBP-menm (menm=1,2-methylethylenediamine) induced an ultrahigh CO₂ permeability of 2988 Barrers (158% higher than that of a pure polymer membrane), thus realizing excellent performance that surpasses the upper bound limit of polymeric membranes. This study was published in the April 5 issue of the Journal of Membrane Science (Effective functionalization of porous polymer fillers to enhance CO₂/N₂ separation performance of mixed-matrix membranes, J. Membr. Sci., 647 (2022) 120309).

“The membrane-based gas separation process is known to have advantages such as higher energy efficiency, a smaller plant footprint, and lower operating costs over conventional processes such as distillation. Moreover, among various membrane materials, mixed-matrix membranes (MMMs), which integrate the advantages of both polymers and microporous materials, have emerged as a viable option that overcomes the trade-off between permeability and selectivity, as found in conventional membranes,” said first author Yechan Lee, a graduate student in the Department of Chemical and Biomolecular Engineering at KAIST.

“We found that the functionalization of porous filler materials using diethylenetriamine (DETA) and chlorosulfonic acid induced decreases in the CO₂ permeability by 47% and 20%, respectively, with some enhancements in CO₂/N₂ selectivity also observed. However, ultrahigh CO₂ permeability was observed when the polymer was functionalized using a menm solution and incorporated into 6FDA-DAM polyimide. Therefore, we concluded that both the shape of the pores after being functionalized and the degree of interaction between permeating CO₂ molecules and the functional group introduced in the porous filler can play a critical role in the CO₂ separation performance,” said Tae-Hyun Bae, corresponding author and associate professor of chemical and biomolecular engineering at KAIST. “The DETA molecules appended onto the pore surface point toward the center of the pore channel, leading to the formation of a “star-like” pore shape, which is ineffective for the transport of CO₂ molecules. Then, we further hypothesized that pore channels with spherical cross-sections would be highly beneficial for improving the CO₂/N₂ separation performance. Such a channel could be realized by introducing menm into the porous polymer filler materials.” “We successfully fabricated defect-free mixed-matrix membranes consisting of these functionalized porous fillers and optimized both the functionality and pore structure of the porous fillers, thus enhancing CO₂ transport,” Lee said. “We believe that future efforts should be devoted to translating the current dense membrane into an asymmetric hollow fiber configuration for potential applications in the area of industrial gas separation.”
This work was financially supported by the Saudi Aramco-KAIST CO₂ Management Center.