Sol–gel asynchronous crystallization of ultra-selective metal–organic framework membranes for gas separation†
Metal–organic framework (MOF) membranes possess superior separation performance, but their inferior processability is an intrinsic drawback. Herein, we report a scalable and versatile sol–gel transformation strategy to synthesize MOF membranes by two steps of precursor-coating for gel dispersion and thermal-treatment for membrane growth, similar to the processing of polymeric membranes. The asynchronous crystallization between the bottom and the top of gels found here is critical for the formation of defect-free MOF membranes. The prepared ZIF-8 membranes with a ligand/zinc ratio of 4 show impressive performance in hydrogen purification and hydrocarbon separation, with H2/CH4, H2/C3H8 and C3H6/C3H8 selectivities of up to 30, 9000 and 200, respectively. Moreover, sol–gel transformation can deposit MOF layers with controllable thickness, down to 130 nm, on both discs and hollow fibers without deterioration in performance in a solvent-/precursor-saving manner.