Zeolite-like MOF nanocrystals incorporated 6FDA-polyimide mixed-matrix membranes for CO₂/CH₄ separation

MOF mixed-matrix membranes (MMMs) are regarded as promising candidates for energy-efficient natural gas purification. This work reports the fabrication of high-performance 6FDA-polyimide MMMs, based on the incorporation of zeolite-like MOF (ZMOF) as fillers, and their associated permeation studies for CO₂/CH₄ separation. To eliminate micron-sized crystals, a facile repeating sedimentation approach was used to harvest nanocrystals from the as-synthesized bulk ZMOF crystalline powder material. This enables the deployment of ZMOF nanocrystals with relatively uniform dimension and morphology in the polymer matrix. Typical 6FDA-polyimides encompassing distinct diamine moieties (6FDA-DAM, 6FDA-DETDA-DABA or PDMC) were explored as polymer matrices to disclose the transport property matching the hosted ZMOF filler. Mixed-gas permeation measurements revealed that the incorporation of the ZMOF filler affords a concurrent enhancement of the CO₂ permeability and the CO₂/CH₄ selectivity for the three tested 6FDA-polyimides. Particularly, the highly permeable 6FDA-DAM showed a considerably enhanced performance for CO₂/CH₄ that transcends the 2008 Robeson upper-bound. Detailed analysis of the sorption data and diffusion coefficients suggest that the enhanced transport property in the ZMOF-based MMM is plausibly attributed to the combination of the higher CO₂ sorption capacity and selectivity, and favorable gas diffusivity via the CO₂-philic framework of ZMOF in moderately confined pores.