Enhanced CO₂/CH₄ separation performance of mixed-matrix membranes through dispersion of sorption-selective MOF nanocrystals

Mixed matrix membranes (MMMs) derived from metal-organic frameworks (MOFs) nanocrystals represent a promising alternative for overcoming the trade-off between permeability and selectivity derived from the pristine polymeric membrane. The selection of MOFs fillers are mainly focused on their diffusion-selectivity property for gas-pairs. However, the improvement of both gas-permeability and selectivity through addition of sorption-selective MOFs are rarely reported. Herein, the incorporation of CO₂-philic KAUST-7 (also referred to as NbOFFIVE-1-Ni) nanocrystals makes the neat 6FDA-Durene polyimide membrane more permeable and more selective, surpassing the state-of-the-art 2008 Robeson upper bound for CO₂/CH₄. Nano-sized (~ 80 nm) KAUST-7 crystals with sharp particle size distribution were first fabricated through a co-solvent synthesis method. The explored gas transportation mechanism indicates that the improvement of CO₂/CH₄ selectivity on MMMs are attributed to the significant raise of the sorption-selectivity rather than the diffusion-selectivity. The favorable interfacial interaction between the imide groups of the 6FDA and the H of the pyrazine in the KAUST-7 enhances plasticization resistance of the neat polymer membrane. The developed MMMs exhibit a promising application in CO₂ capture from natural- and bio-gas.