Functionalized MOFs/polyimide Mixed Matrix Membranes Design And Gas Separation Performance Research

In the chemical industry,a large amount of gases,such as H₂,CO₂,CH₄ and other polluting gases are produced by various reaction processes.The existence of these gases will not only affect the quality of chemical products,but also cause waste of energy resources and damage to the ecological environment.Therefore,separating gases from the mixture is required to be solved using energy-efficient technology.Membrane separation technology,which has the advantages of high efficiency and low consumption,small footprint,and low environmental pollution,is widely used in the field of gas separation.Polymer/inorganic filler-based mixed matrix membranes（MMMs）combine the advantages of good separation performance of inorganic membranes and excellent mechanical properties of polymer membranes,and also overcome the trade-off relationships between the permeability and selectivity of polymer membranes.However,the compatibility problem between the inorganic phase and the polymer phase often leads to the existence of defective structures in MMMs,which affects their separation performance.It is necessary to control and optimize the interfacial structure of membrane materials.In this paper,an efficient approach to fine-tune interface interaction force by manipulating metal organic frameworks（MOFs）and polymer functional groups is designed.The concentration of hydrogen bonds in MMMs is regulated by regulating the content of CHO-IM linkers in ZIF-8-90（x）and DABA content in 6FDA-DAM:DABA copolyimide to controll interfacial bonding precisely.The influence of hydrogen bond content on membrane structure and gas separation performance is systematically investigated.In this work,ZIF-8-90（x）fillers with different CHO-IM linker contents are first synthesized as filling phase.A series of 6FDA-DAM:DABA（m:n）copolyimides with different COOH contents are prepared as continuous phase by adjusting the molar ratio of DAM:DABA structural units（3:2 and 1:1）.MMMs are prepared by blending these two materials.The effects of the chemical structures of MOFs and polymers on the physicochemical structure of the membranes are investigated by means of structural characterization.The H₂,N₂,CO₂ and CH₄ gas separation performance of MMMs is systematically investigated at 4 bar and 35℃.The number of hydrogen bonds between MOFs and polymer increases with increasing the proportion of CHO-IM linkers in ZIF-8-90（x）and the content of DABA units in the polymer,which enhances the interfacial adhesion and reduces the interfacial defects,resulting in a decrease in gas permeability of MMMs and a increase in selectivity.The gas permeability of 6FDA-DAM:DABA（3:2）MMMs reaches the maximum when doped with 10 wt.%ZIF-8-90（30）,and the H₂ and CO₂ permeability are 301 Barrer and 187 Barrer,respectively.The ideal selectivities of CO₂/CH₄ and H₂/CH₄ gradually increase and reach the highest in ZIF-90 MMMs due to the number of hydrogen bonds reaches a maximum in ZIF-90 MMMs,the H₂/CH₄ selectivity is as high as 70.7.Compared with the pure membrane,the resistance to CO₂ plasticization of the 10 wt.%ZIF-8-90（30）membrane is slightly enhanced in the CO₂/CH₄ mixture permeation test.In addition,the effect of MOFs particle size on the separation performance of MMMs is negligible.Compared with 6FDA-DAM:DABA（3:2）MMMs,the increase of DABA content in the polymer increases the hydrogen bond strength in MMMs and the compactness of the polymer pore structure,thereby reducing the gas permeability of the membrane material and improving the selectivity.When MMMs are prepared by 6FDA-DAM:DABA（1:1）with higher DABA content,the 10 wt.%ZIF-8-90（30）membrane has the best performance.The H₂/CH₄ separation performance exceeds the Robeson upper bound in 2008,the CO₂/CH₄ separation performance exceeds the Robeson upper bound in 1991.The permeabilities of H₂ and CO₂ are 222Barrer and 128 Barrer,the ideal selectivities of H₂/CH₄ and CO₂/CH₄ are 75.4 and 43.6,respectively,which are 12.1%and 10.7%higher than pure membrane,respectively.Therefore,the separation performance of membrane materials can be precisely regulated by adjusting the ratio of polar linkers in MOFs and the concentration of COOH groups in the polymer,thereby meeting different separation requirements.