Microstructural Engineering And Application Study Of Metal-organic Framework Membranes

Metal organic frameworks（MOFs）,with regular channels and adjustable pore sizes,are promising gas separation membrane materials.Up till now,the number of MOF materials has reached hundreds of thousands and the structure of them also varies a lot.However,the development of MOF membranes still has some challenges.First,flexibility makes MOF membranes lack strict cut-off effect on gas separation.The linker rotation will cause MOF pore sizes widen slightly,thus larger molecules can also pass through the MOF channels.Second,although the pore size of MOF material is tunable,to realize the continuous tuning in sub-nanometer scale is still challenging,so it is difficult to find the suitable MOF membrane to separate some specific gas mixture.Finally,the grain boundary structure is difficult to be optimized,which affecting the gas separation of the membranes.The bad membrane grain boundary structure means a large area of non-selective interfaces between crystals,making the gas separation performance far below expectation.Based on the above problems appearing in the MOF membrane,here,we prepared a series of stiffened ZIF-7_x-8 membranes with the adjustable pore size via the fast current driven synthesis method（FCDS）for CO₂/CH₄separation.Moreover,we prepared the oriented mixed-metal membranes to separate C₃H₆/C₃H₈ and deeply studied the influence of the grain boundary structure and the flexibility on the membrane performance.The specific research contents of this paper are as follows:（1）In order to find a suitable MOF membrane for CO₂/CH₄ separation,we prepared the stiffened ZIF-8＿Cm structure as the parent and in situ introduced different amount of benzimidazole molecules to synthesize a series of stiffened ZIF-7_x-8 membranes with the adjustable pore size via the FCDS.The specific pore size of the mixed-linker membranes was calculated by computional simulation.The experimental results show that ZIF-7₂₂-8membrane has the highest CO₂/CH₄ separation factor with a tenfold increase,up to 25.Moreover,this membrane has the excellent long-term temperature-swing separation performance.The double strategy of―stiffening and narrowing‖developed here is also promising for preparing other types of MOF membranes.（2）In order to understand the effect of the flexibility and the grain boundary structure on membrane performance,we prepared a series of oriented bimetallic Zn_（100-x）Co_x-ZIF membranes for C₃H₆/C₃H₈separation via the FCDS.Here,the Co%content represents the rigidity of the MOF membrane and the OI（orientation index）value represents the quality of the grain boundary structure.The―volcano‖plot and the gradient change curve of the gas separation factor show that neither the rigidity nor grain boundary structure is the exclusive factor affecting the membrane gas separation performance.Further,we got a separation factor contour map which can predict the membrane quality and we found that there is a―seesaw‖behavior between the two factors.From the experimental results,the Zn₈₂Co₁₈-ZIF membrane represents a―sweet spot‖offering the best C₃H₆/C₃H₈ separation factor up to 200.Moreover,this membrane has the long-term operation stability and the pressure resistance capability.In a word,the significance of this paper is to control the structure of the metal organic framework membrane,and to analyze the factors that affecting the gas separation performance of the membrane.