Preparation of a ZIF-8 membrane induced by ZIF-8-71 hybrid ligand crystal seeds and its gas separation performance

ZIF-8-71 hybrid ligand crystals have been prepared by partially replacing the ligand 2-methylimidazole (HmIM) used in the preparation of ZIF-8 with 4,5-dichloroimidazole (DcIM) and characterized by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR) and scanning electron microscopy (SEM). ZIF-8 membranes were prepared on the inner surface of an α-Al₂O₃ ceramic tube carrier by a secondary crystal growth method using ZIF-8-71 as crystal seeds. The effects of varying the concentration of the crystal seed solution and the flow rate of the membrane growth solution on the microstructure and gas separation performance of the ZIF-8 membranes were investigated. The results show that when the ligand substitution rate was 50%, the particle size distribution of ZIF-8-71 crystals was uniform and they could be coated on the surface of the macroporous carrier to repair the surface defects of the carrier and form a dense seed layer. The thickness of the ZIF-8 film increases with increasing concentration of the ZIF-8-71 seed solution. When the mass fraction of the seed solution was 0.1%, the film was uniform and dense, and the surface was smooth. It was difficult to form a continuous and smooth film when the flow rate of the membrane growth solution was too low or too high. The ZIF-8 film formed at a flow rate of 0.333 mL/min exhibits good compactness and continuity. Under the optimized conditions, the H₂ permeation amount of a ZIF-8 membrane prepared with ZIF-8-71 as crystal seeds was 4.73×10^-6 mol/(Pa·s·m²), and the ideal selectivity coefficients of H₂/CO₂, H₂/N₂ and H₂/CH₄ were 6.32, 8.75 and 12.12, respectively. The gas separation performance of the ZIF-8 membrane prepared with ZIF-8-71 as crystal seeds was better than that of a ZIF-8 membrane prepared with ZIF-8 as crystal seeds.