Preparation And Gas Separation Performance Of ZIF-8/Polyimide Mixed Matrix Membranes

As one of the greenhouse gases,carbon dioxide（CO₂）is one of the main factors which can contribute to global warming and climate change.The separation of CO₂ is an important industrial process for greenhouse gas emission reduction,energy and gas purification.Separation technology and effective capture of CO₂ have become a research hotspot in recent years.Compared with the traditional separation technology,membrane separation technology has many outstanding advantages,such as low energy consumption,easy amplification,simple operation,low investment and operation cost,and environmental friendliness.Therefore,membrane separation technology that is regarded as a green and economical alternative method has great application prospects.However,gas separation membrane materials are generally limited by the"trade-off"phenomenon between permeability and selectivity,and the research of organic-inorganic mixed matrix membranes（MMMs）provide new ideas and methods to solve this problem.For the gas separation,the interaction and fusion between inorganic particles and polymer,the properties of separated gases and the preparation method of membranes have important effects on gas separation performance.In this paper,metal-organic framework materials（MOFs）were prepared,and two kinds of ZIF-8/PI MMMs were prepared by in-situ polymerization.ZIF-8/PI MMMs were prepared by direct doping method,and the gas separation selectivity of the ZIF-8/PI MMMs was tested.There are two ways of in situ polymerization.In the part of ZIF-8/PI MMMs prepared by in-situ step by step polymerization,two monomers of ZIF-8 were doped step by step in the prepared polyamic acid solution.Firstly,the ZIF-8/PI MMMs were characterized by FTIR and SEM.The result suggested that ZIF-8 was synthesized successful.Gas separation tests showed that the gas flux increased significantly with the increase of doping amount.When the in-situ step by step doping ZIF-8 was 20 wt.%,the CO₂ flux increased by 4.15 times,and the separation coefficients of H₂/CO₂,CO₂/CH₄ and CO₂/N₂ have little changed.In the part of ZIF-8/PI MMMs prepared by in-situ polymerization,the prepared ZIF-8 was characterized by FTIR,XRD and SEM,and the crystal particles with regular structure and uniform size were obtained.Structural characterization of the membranes showed that the ZIF-8 crystals were damaged and the particles were not obvious during the synthesis of polyimides.However,the gas separation performance of the MMMs showed that when the in-situ doping ZIF-8 was 5 wt.%,the gas flux of CO₂ increased by 5.57 times,and the separation coefficients of H₂/CO₂,CO₂/CH₄ and CO₂/N₂ did not change a lot.In the part of ZIF-8/PI MMMs by direct dispersion method,ZIF-8 was modified by water-wheel phenol（Noria）.The morphology and structure of ZIF-8 crystals were characterized by SEM,XRD and FTIR.The fusion between ZIF-8 and polymer was obviously improved by the modification of Noria.The morphology and structure of the ZIF-8/PI MMMs were characterized by SEM and XRD.The modified ZIF-8 crystals could be well integrated into the polymer,and there were almost no interfacial gaps.The gas separation performance of the MMMs were found that the gas flux of the unmodified ZIF-8/PI MMMs increased significantly with the increase of doping amount.When the doping ZIF-8 was 5 wt.%,the flux of CO₂increased by 7.36 times,and the separation coefficients of H₂/CO₂,CO₂/CH₄ and CO₂/N₂decreased obviously.Because of the better fusion between modified ZIF-8 and PI,the gas flux of ZIF-8/PI MMMs increased with the increase of doping amount.When the doping ZIF-8 was5 wt.%,the flux of CO₂ increased by 7.01 times,and the separation coefficients of H₂/CO₂,CO₂/CH₄ and CO₂/N₂ changed little.Compared with the ZIF-8/PI MMMs with the same doping amount,it can be seen that the gas selectivity of the modified ZIF-8/PI MMMs are significantly higher.