Nitrogen-enriched porous organic polymers for high-performance CO2/N2 separation in mixed-matrix membranes

Abstract

The development of high-performance mixed matrix membranes (MMMs) for CO2/N2 separation requires fillers that simultaneously enhance gas permeability and selectivity without introducing interfacial defects. In this study, we report the synthesis of nitrogen-containing porous organic polymers (POPs) via a one-step Friedel-Crafts polymerization, designed to introduce CO2-philic functional groups directly into the polymer backbone while preserving intrinsic porosity. Structural and gas sorption analyses confirmed that the synthesized POPs exhibit high microporosity, surface area, and nitrogen content, which together enhance CO2 adsorption affinity. When incorporated into Matrimid-based MMMs, the POPs significantly improved CO2 permeability and CO2/N2 selectivity, with the pp-tpta filler (containing the highest nitrogen content) delivering the most pronounced performance enhancement. Further optimization using a high-permeability 6FDA-DAM polyimide matrix yielded a membrane with a CO2 permeability of 1967 Barrer and a selectivity of 33.4 at 20 wt% pp-tpta loading, surpassing the 2008 Robeson Upper Bound. Solubility-diffusivity analyses, supported by both experimental measurements and molecular dynamics simulations, revealed that the pp-tpta filler enhances CO2/N2 separation primarily by increasing CO2 solubility and diffusivity within the membrane matrix. These results underscore the potential of rationally designed, nitrogen-rich POP fillers as effective, scalable materials for advanced gas separation membranes.