Carbon contained mixed matrix membranes based on hydrolyzed polymers of intrinsic microporosity for CO2 separation

Mixed matrix membranes are a promising strategy to overcome the permeability–selectivity trade-off in polymer-based gas separation membranes. In this work, nitrogen-doped biochars derived from chitosan are incorporated at low loadings (1, 2, and 5 wt%) into carboxylated polymers of intrinsic microporosity to fabricate Mixed matrix membranes with enhanced gas separation performance. Gas sorption–diffusion measurements for CO₂, H₂, CH₄, and N₂ reveal a pronounced, gas-dependent effect of carbon incorporation: relative to the neat caPIM membrane, at 5 wt% filler CO₂ solubility increases, CH₄ solubility decreases, and H₂ solubility remains comparable or slightly lower. In parallel, the diffusivities of CO₂, CH₄, and especially H₂ increase, whereas N₂ diffusivity is markedly reduced, leading to higher CO₂ and H₂ permeabilities combined with strong N₂ rejection over the investigated pressure range. The combined effects of selective sorption and polymer–filler diffusion result in outstanding separation performance. The membrane containing 5 wt% nitrogen-doped biochar achieves high ideal selectivities, up to ~ 67 for CO₂/N₂ and in the 102–103 range for H₂/N₂, surpassing Robeson's upper bound for the H₂/N₂ gas pair. These results demonstrate that the biochar acts as a functional, sorption-active phase whose accessible microporosity and surface chemistry cooperatively interact with the caPIM matrix. Overall, nitrogen-doped, bio-derived carbons emerge as intrinsically compatible and selective fillers for high-performance MMMs, offering a sustainable platform for post-combustion CO₂ capture and hydrogen purification from nitrogen-rich gas streams.