Molecular Simulation Study Of CO₂ Separation Mechanism By A Class Of Ionic Liquid-based Composite Membrane

Polyimide（PI）is a new type of polymer material that is expected to be used in the CO₂capture process.The latest experimental studies have shown that the mixing of PI and imidazole ionic liquids（ILs）can improve the separation performance of a single polyimide.However,the microscopic mechanism of ILs-PI membrane is not clear,it is impossible to realize the regulation of membrane preparation,and it is more difficult to realize industrial application.Therefore,this paper uses molecular dynamics method to simulate ILs-PI membrane,and analyzes its gas adsorption and diffusion characteristics.Firstly,by molecular dynamics simulation,the mixed system of1-butyl-3-methylimidazole tetrafluoroborate（[BMIM][BF₄]）and water in different proportions is described.Afterwards,the radial distribution function,viscosity,diffusion coefficient,conductivity and other properties are discussed separately.The simulation results show that the diffusion coefficient of anions is greater than that of cations,while the conductivity law is just the opposite.As the temperature increases,the viscosity of the ionic liquid becomes smaller and smaller.The mean square displacement reveals the good absorption of CO₂by the pure system and the mixed system.Then,three kinds of ionic liquids are added with different proportions of CO₂,and the structure of the solution is discussed based on the calculated coordination number.The diffusion coefficient shows that when the CO₂mole fraction exceeds 0.5,the diffusion coefficient of ILs is increased by 2 and 3 times compared with pure ILs.Secondly,molecular simulation technology was used to analyze the structure-property relationship of 1-ethyl-3-methylimidazole bis（trifluoromethylsulfonyl）imide（[EMIM][Tf2N]）with different IL concentrations,1-Ethyl-3-methylimidazole tetrafluoroborate（[EMIM][BF4]）and 1-ethyl-3-methylimidazolium hexafluorophosphate（[EMIM][PF6]）and 4,4-（Hexafluoroisopropylene）diphthalic anhydride（6FDA）and2,4,6-trimethyl-m-phenylenediamine（DAM）,2,5-dimethyl-p-phenylenediamine（DPX）,4,4-oxydiphenylamine（ODA）.The study showed that the CO₂has better diffusion characteristics in the meta-isomer of 6FDA-DAM.As the concentration of the ionic liquid increases,the interaction between the PI chain and IL will weaken,which may be due to the shielding effect of IL.However,the strength of the interaction between different anions and gases shows the following trend:[PF₆]^->[BF₄]^->[Tf₂N]^-.The changes of PI matrix caused by adsorption were studied by radial distribution function and coordination number analysis.The results showed that the CO₂diffusion coefficient decreased with the compatibility of the polyimide structure with CO₂molecules,and increased with the IL concentration increased to 75 wt%.The above research work has analyzed and studied the microscopic mechanism of PI membrane from the molecular level,and provided theoretical guidance and basis for the design and preparation of the membrane.