Preparation And CO₂-CH₄ Separation Performance Of Mixed Matrix Membranes Based On MOF-808 And Its Modified Materials

The separation of mixtures is an indispensable part of the chemical production process,and traditional separation technologies such as rectification,extraction,and absorption usually have high requirements for equipment and energy consumption,and may cause additional environmental impacts during the production process.In comparison,membrane separation technology shows advantages in energy saving and environmental protection aspects,thus gradually received more and more research attentions.High-performance separation membrane materials are the foundation and core of membrane separation technology.Both the pure polymer membranes and novel porous material membranes have their own problems.Combining novel porous materials with polymers to prepare mixed matrix membranes(MMMs)is one of the effective ways to obtain high-performance membrane materials.The MMMs are expected to combine the advantage of polymer materials in easy-processing with the advantages of porous materials in excellent adsorption and diffusion properties,thus break the limitation in the separation performance of pure polymer membrane and maintain the ability of large-scale preparation.Due to the regular and designable pore structure as well as the diverse and easy-adjustable pore chemical environment,metal-organic frameworks(MOFs)have shown excellent application potential in adsorption separation and membrane separation.In this thesis,the active design of MOFs material(MOF-808)was carried out to control the gas separation performance of the MMMs based on MOF-808.The underlying mechanism was also studied by combining gas separation performance test and membrane microstructure characterization results.The main contents are as follows:?.Nano-sized and micro-sized MOF-808 particles were synthesized by microwave assisted solvothermal method and conventional solvothermal method,respectively.Then,two kinds of MMMs were prepared by compositing the MOF-808samples with polysulfone(PSF),and the CO₂/CH₄ separation properties were measured.The results show that the micro-sized MOF-808 can form a more continuous mass transfer channel in the membrane,so the corresponding MMMs has a greater increase in gas permeability compared with the nano-sized MOF-808 based MMMs.However,the impact of particle agglomeration on membrane performance at higher filler loading is more pronounced for the micro-sized MOF-808,making the CO₂/CH₄ separation factor decreased from the 20 wt%loading.In comparison,the nano-sized MOF-808 based MMMs are easier to maintain their selectivity,which is important to the further reduction of membrane thickness.?.The coordinated formic acid molecules in the structure of MOF-808 is easy to be replaced by other carboxylic acid molecules,providing an efficient opportunity for the modification of the pore chemical environment of MOF-808.In this work,L-histidine(His)was used to replace the formic acid in MOF-808 and engineer the pore structure and chemical environment.The obtained MOF-808-His was then incorporated into the 6FDA-DAM polyimide to examine the effect of His modification on the CO₂/CH₄ separation performance.The results show that His modification can increase the preferential adsorption selectivity of MOF-808 towards CO₂,thereby increasing the local CO₂concentration on the upstream surface of the MOF-808-His/6FDA-DAM MMMs,thus increase the CO₂/CH₄ separation factor of the membrane.?.In order to evaluate the influence of the flexibility of adsorption sites on the mass transfer performance of CO₂ in MOF-808-based MMMs,glycine(Gly),3-aminopropionic acid(Apa),and 4-aminobutyric acid(Aba)were used to modify MOF-808 through the substitution of formic acid.The degree of the freedom of the introduced adsorption sites(amine groups)are determined by the length of the carbon chain.The results show that the greater the flexibility of the amine functional groups,the more facilitated the transport of CO₂ in MMMs,leading to the greater CO₂/CH₄ separation factor.