Preparation And Gas Separation Performance Of Hybrid Carbon Molecular Sieve Membrane With Magnetic ZIF-8 Chain Structure

The continuous growth of global CO₂emissions has triggered a series of crises in energy,environment,climate and other aspects.The development of CO₂capture and emission reduction technology is a major demand for China’s economic,industrial and social development.Membrane capture of CO₂demonstrates obvious technical and performance advantages in various CO₂separation and capture technologies,such as low energy consumption,small floor area,good separation effect,environmental friendliness and so on.As a novel high-performance carbon matrix membrane,carbon molecular sieve（CMS）membrane has excellent thermal stability,chemical stability and molecular sieving ability.It has gradually become a research hotspot in the field of gas separation.However,during the pyrolysis of CMS membrane,dead-end and discontinuous pores would be produced,resulting in the tortuous transfer path and membrane aging,so its performance needs to be further improved.However,the imperfect stacking of graphite-like sheets in the pyrolysis process always form a large number of the dead-end and discontinuous pores,resulting in the tortuous transfer path,poor aging resistance and serious damage to the gas separation performance.According to the above problem,this paper combined with the characteristics of carbon membrane,focused on improving the tortuous pore structure of carbon membrane and enhancing anti-aging ability.From the point of view of fine regulation of carbon membrane pore structure,the strategy of magnetic field regulation of carbon membrane microstructure is proposed for the first time.By constructing a vertical through transfer channel in the membrane,the CO₂diffusion rate is increased,and the adsorption of small molecules is avoided to block the pore,so as to improve the aging problem.The magnetic nano materials and hybrid carbon membranes were characterized by means of microscopic test,the magnetic field regulation mechanism and the aging law of carbon membranes were further explored,and the gas separation mechanism of hybrid carbon membranes was put forward,which has theoretical guiding significance for the development and application of carbon membranes.The specific research contents and conclusions are as follows:（1）Carboxylated Fe₃O₄was synthesized by solvothermal method.COO^-on the surface of Fe₃O₄and Zn²⁺were nucleated by electrostatic interaction,and then grew ZIF-8 layers to form magnetic core-shell microspheres（Fe₃O₄@ZIF-8）.Afterwards,Fe₃O₄@ZIF-8 was blended with polyimide to design a precursor membrane by magnetic-field-induced ordered arrangement of magnetic nanoparticles in the membrane,and then the hybrid carbon molecular sieve（HCMS）membranes were obtained by in situ pyrolysis.The SEM,FTIR and EDXS results of the material indicated that the rough ZIF-8 layer was tightly wrapped on the surface of Fe₃O₄,and the coating amount of ZIF-8 in Fe₃O₄@ZIF-8 was calculated using the TAG results.The properties of Fe₃O₄to maintain the ultra-microporous,microporous structure and N active sites of ZIF-8 during carbonization were confirmed by BET,EDXS and CO₂adsorption.The cross-sectional SEM of the hybrid carbon membrane exhibited that Fe₃O₄@ZIF-8 aligned to form a chain-like structure under the induction of magnetic field,and maintains good compatibility with the carbon matrix.The BET results indicate that the hybrid carbon molecular sieve membranes prepared under the induction of vertical magnetic field have high specific surface area,pore volume and porosity.The effects of the pyrolytic carbonization process on the chemical structure and microporous structure of the membrane were investigated,and the effects of Fe₃O₄particle size and ZIF-8 coating times on the gas separation performance of the hybrid carbon membrane during the preparation of Fe₃O₄@ZIF-8 were investigated.The effects of Fe₃O₄@ZIF-8content,magnetic field conditions and CO₂/N₂gas mixture on the gas separation performance and operational stability of the membrane were also discussed.The results exhibited that the hybrid carbon membrane with a vertically oriented loading of 5 wt%Fe₃O₄@ZIF-8 has a CO₂permeability of 5130 Barrer and a CO₂/N₂（CH₄）selectivity of 29（48）.The vertically oriented Fe₃O₄@ZIF-8 nanochain structure in the membrane provided a straight high-speed channel for the diffusion of CO₂,and improved the permeability and selectivity of CO₂.The uniformly distributed N-active sites in the channel enhance the adsorption selectivity of the hybrid carbon membrane.In addition,the hybrid carbon membrane showed excellent aging resistance in 30 d aging performance test,which proved its practical feasibility in flue gas carbon capture and natural gas decarbonization.（2）Fe-ZIF-8 was synthesized by simple precipitation method and chemisorption method,and then MOF-derived Fe-doped magnetic porous carbon（Fe-N/C）was obtained by direct carbonization method.After blending with 6FDA/p-PDA:DABA polyamide acid,the precursor membrane was prepared under magnetic field induction,and the hybrid carbon membrane was obtained by pyrolysis carbonization.BET results exhibited that Fe-N/C was a porous material with high specific surface area.The three-dimensional porous morphology of Fe-N/C was characterized by SEM.The microstructure of the membrane was characterized by chemical properties（ATR-FTIR）,micro morphology（SEM）,thermal properties（TAG）,crystal structure（XRD）and mechanical properties（tensile fracture rate,tensile strength）.ATR-FTIR characterization of carbon membranes revealed the rule of pyrolysis carbonization reaction.TGA test shows that Fe-N/C doped precursor membranes have higher thermal stability.XRD results show that the spacing of Fe-N/C doped hybrid carbon membrane decreases and the pore structure is more compact.The test of mechanical properties confirmed that Fe-N/C nanoparticles were aligned in the direction of the external magnetic field in the membrane.The influence of pyrolysis temperature on the chemical structure and microporous structure of the membrane was studied,and the influence of Fe doping（molar ratio of Fe/Zn+Fe）amount on the gas separation performance of the hybrid carbon membrane during the preparation of Fe-N/C was explored.Simultaneously,the effects of Fe-N/C content,magnetic field conditions（magnetic field direction,magnetic field intensity,action time）and CO₂/N₂mixture on the membrane gas separation performance and operation stability were investigated.The results manifested that the hybrid carbon membrane with vertical orientation loading of 10 wt%Fe-N/C has the best performance,with CO₂permeability of 17690 Barrer,CO₂/N₂selectivity of 29,CO₂/CH₄selectivity of 43.The permeability of CO₂increased by 147%compared with pure carbon membrane,and the selectivity of CO₂/N₂and CO₂/CH₄increased by 65%and 23%,respectively,exceeding the Robertson upper limit revised in 2019.The vertically connected gas transfer channel in the membrane greatly shortened the gas transfer path,enhanced the diffusion rate of CO₂,and improved the permeability and selectivity of CO₂.More importantly,the hybrid carbon membrane can still maintain the CO₂separation performance after the aging performance test,indicating that the membrane has the prospect of industrial application.