Cascade Promotion of Gas Separation Performances in CMS Membranes: MOFs With Functional Groups and Loaded Noble Metals

The rational design of precursor structure serves as a critical determinant for the pore geometry and gas separation performance of carbon molecular sieve (CMS) membranes. Herein, a novel mixed-matrix CMS (MMCMS) membrane was fabricated via a palladium-doped carboxyl-functionalized UiO-66 (Pd/UiO66-COOH)/polyimide (PI) MMM precursor. On one hand, the decomposition of -COOH groups generates abundant micropores, meanwhile the decarboxylation-induced thermally cross-linking enhances the stability of the carbon framework, thus mitigating the collapse of micropores during carbonization and consequently improving H₂ and CO₂ permeability, as well as membrane stability; on the other hand, the synergistic effect of decarboxylation-induced thermally cross-linking and the catalytic graphitization effects of Pd nanoparticles facilitated the formation of more ordered Langmuir domains and narrowed carbon interlayer spacing, thereby enhancing molecular sieving effects. In addition, Pd nanoparticles also contribute to providing abundant H₂ adsorption sites to facilitated transport of H₂ gases. Specifically, the PI/Pd-UiO66-COOH-5-550 MMCMS membrane exhibited superior H₂ permeability of 9134.6 Barrer (PCO2 = 4033.4 Barrer) with H₂/CH₄ selectivity of 118.5 (αCO2/CH4 = 52.3), exceeding the latest Robeson upper bound. Furthermore, the membrane also demonstrated attractive aging resistance, retaining over 90% of its initial H₂ and CO₂ permeability after a 21-day long-term stability test.