Introduction MOF gas separation membranes have attracted much attention due to the large BET surface area, flexible functional groups, adjustable structure, and good thermo-chemical stability, which can significantly improve the separation efficiency and purity of the gas separation process. However, MOF gas separation membranes have unavoidable grain boundaries and are prone to non-selective defects, which greatly destroys the intrinsic separation properties of pure MOF materials, posing a serious of challenges during the preparation and application. MOF glass membranes is an emerging type of gas separation membrane with the advantages of easy processing, no grain boundary defects and permanent porosity, and have become a new access for the preparation of high-performance gas separation membranes. However, the typical MOF glass membranes are normally formed on support layers, and the presence of the supporting layers greatly limits the processability and gas permeability enhancement. In this work, a novel preparation method of self-supported MOF glass membranes is proposed, and a serious of self-supported ZIF-62 glass membranes performing excellent CO2/CH4 gas separation property are obtained.
Methods In the present work, ZIF-62 crystals were prepared according to the previous literature. Zinc hydroxide, zinc acetate dihydrate (purity 99%, manufacturers are Shanghai Aladdin Biochemistry Science and Technology Co., Ltd.), imidazole (purity 99.5%, manufacturers are Anhui Zesheng Science and Technology Co., Ltd.), benzimidazole (purity 96% manufacturers are Shanghai McLean Biochemistry Science and Technology Co., Ltd.), N,N- N,N- dimethylformamide (purity of analytical purity, the manufacturers are Sinopharm Chemical Reagent Co., Ltd.), etc were used. Then, the ZIF-62 crystals were pressed to thin pellets in specific molds, and were heat-treated by a preset heating procedure, so as to prepare the homogeneous and dense self-supported amorphous ZIF-62 glass (agZIF-62) membranes. The difference of the properties between ZIF-62 crystals and agZIF-62 membranes were characterized. After that, self-supported agZIF-62 membranes with different thicknesses were prepared by changing the dosage of ZIF-62 crystals. The influence of thickness on the surface morphology and the CO2/CH4 separation performance of self-supported agZIF-62 membranes were investigated.
Results and discussion It is proved by XRD, DSC and TG curves that the ZIF-62 crystals will be fully melted and transformed from crystalline to amorphous glassy state under the heat treatment condition of holding at 440 ℃ for 20 min. Meanwhile, the chemical bonds and ligands of the periodic crystalline structure of the ZIF-62 crystals will not be changed after heat treatment verified by FTIR and 1H NMR analyses.According to the SEM images, it can be observed that the surface of self-supported agZIF-62 membranes is dense without obvious defects, indicating that the prepared self-supported agZIF-62 membrane qualifies the process requirements of gas separation.Combining with XPS and adsorption isotherms of CO2 and CH4, it is proved that the self-supported agZIF-62 membranes have an excellent CO2/CH4 separation performance. With increasing thickness of self-supported agZIF-62 membranes, the CO2 permeance gradually decreases, while the separation selectivity of CO2/CH4 is improved. What’s more, the self-supported agZIF-62 membrane possesses good performance and long-term stability in CO2/CH4 separation.
Conclusions ZIF-62 crystals with uniform particle size were prepared by liquid-assisted ball milling method, and self-supported ZIF-62 glass membranes were prepared by pressing ZIF-62 crystal powders in special designed molds succeeding with heat treatment.The thickness of the agZIF-62 membranes can be easily regulated to obtain gas separation membranes with excellent gas separation performance. The prepared agZIF-62 membranes also show excellent long-term stability with their performance remaining essentially unchanged during continuous testing for about 48 h, which is potentially advantageous for industrial CO2 separation. The proposed scheme for the preparation of self-supported ZIF-62 glass membranes is highly adjustable. By changing the shape of the mold,self-supported MOF glass membranes with different shape and size can be obtained, giving advantages of the processability of MOF glass membranes, which is of great significance for the application of MOF glass membranes in different fields.