Formation of ZIF-8 membranes and crystals in a diluted aqueous solution

•Synthesis of ZIF-8 membranes and crystals in the diluted aqueous solution at the same time.•Formation process of ZIF-8 membranes and crystals proposed in the diluted aqueous solution.•Continuous and compact ZIF-8 membrane with an adjustable membrane thickness.

Alumina hollow fiber supported ZIF-8 membranes and crystals have been prepared in diluted aqueous solutions. The concentration of the aqueous solution, synthesis temperature and time were examined in detail. The particle sizes of ZIF-8 crystals increased from about 90 nm to 5.0 μm with the increase of the dilution of the aqueous synthesis solution. Continuous ZIF-8 membranes were obtained by using the diluted synthesis solution with a Zn2+:2-methylimidazolate:H2O molar ratio of 1:70:4952 at 50–70 °C for 24 h, and their thicknesses were about 0.8–1.4 μm. Gas permeation tests indicated the ZIF-8 membranes are continuous and compact. The schematic formation of ZIF-8 membranes and crystals were proposed, and only in the diluted aqueous solution, ZIF-8 membranes and crystals could be prepared at the same time.