| Ionic liquid (ILs) membranes have promising prospect of application in CO2separation and purification. One reason is that the evaporation of membrane liquids from the membranes can be prevented due to the negligible vapor-pressure of ILs. On the other hand, ILs membranes can achieve good separation performance for CO2because of the high CO2soluble selectivity of ILs. In this study, new ILs membranes have been developed to improve CO2separation performance and the stability mechanism of supported ionic liquid membranes (SILMs) has been explored to creat well-developed theories of ILs membranes for industrial applications.A new method of adding a certain amount of water into ILs was proposed to improve CO2permeation properties of SILMs by enhancing CO2diffusivity. The effect of water content in1-n-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF4]) on CO2/N2separation performance of polyethersulfone (PES) SILMs was verified theoretically and experimentally. While, the CO2solubility and permeance may decrease at high water content because of the competitive adsorption of water and CO2that is caused by the hydrogen bond interaction between water and [BF4]-. At the cross-membrane pressure difference (ΔP) of0.24MPa, the SILMs own a optimal CO2permeance of13.8GPU and CO2/N2selectivity of60at the water molar fraction of0.10, which are improved by20%than that without water. The research makes a suggestion that SILMs can be applied for gas separation by humidification of gases in practical applications. Gases with a small addition of water impurity can be separated using SILMs directly without drying, and similarly, ILs membrane liquids can be used without drying, either.The effects of ΔP, the structure and pore size of the support membrane, and the affinity of ILs with polymer on the stability of [bmim][BF4] have been investigated in term of experiments and molecular simulation to explore the stability mechanism of SILMs. The results show that the PES SILM, among the PES, PVDF, and N6SILMs, has the best stability owing to the greatest interaction with [bmim][BF4] and the smallest maximum pore size of PES support membrane. A mathematical model based on the solution-diffusion mechanism was established to calculate the structure and dimension changes of the PES SILM. The thickness of the PES SILM can be reduced from97.2μm to59.0μm, and porosity is reduced from64.1%to29.2%when Δp is increased from0.1to0.3MPa. The pore size is decreased from0.18μm to0.12μm. So the deformation of the SILMs under compression is one of the reasons of the membrane liquid loss. However, membrane liquid loss caused by membrane deformation does not lead to the degradation of the selectivity of the PES SILM. On this basis, membrane liquid loss of SILMs can be summarized as three aspects that attributed to different reasons:1) membrane liquid loss from the defects in the SILMs owing to capillary forces;2) membrane liquid loss due to_the deformation of the SILMs under compression;3) SILMs swelling.A new type of mixed membrane,1-n-butyl-3-methylimidazolium hexafluorophosphate ([bmim][PF6])-polyvinylidene fluoride (PVDF) mixed membrane with good performance and high stability has been fabricated with polypropylene (PP) as a support membrane to improve the stability of ILs membranes under high Δp. The [bmim][PF6] and PVDF are found to be partially compatible according to the XRD, DSC, and SEM analysis. The flexibility of the PVDF segments is enhanced and the crystallinity of the PVDF decreases with adding of the [bmim][PF6], which is benefit to the increase of the permeation properties of the membranes. The COt permeation and separation performance is increased with the [bmim][PF6] loading. When the [bmim][PF6] loading increases to80%, the CO2permeability is as high as205.8barter and CO2/N2selectivity is35, which is well above the Robeson’s upper bound trade-off curve. The membrane has good stability that can proceed with the CO2permeance of29.4GPU at0.4MPa, combining with the CO2/N2selectivity as high as35. The CO2permeability and CO2/O2permselectivity of the mixed membrane are predicted based on the Maxwell’s model and compared to experimental results. It is demonstrates that a microphase separation structure may formed in the [bmim][PF6]-PVDF system with the dispersed [bmim][PF6] encapsulating in the PVDF continuous phase.A new amino-functional task-specific ionic liquid (TSIL),1-butyl-3-aminoethyl imidazolium tetrafluoroborate ([NH2ebim][BF4]), has been synthesized to increase the CO2solubility. It is found that up. to0.46moles of CO2can be absorbed per mole of [NH2ebim][BF4] at room temperature and pressure, being close to the theoretical value (0.5mol), and almost completely desorbed by heating to80℃and0.01MPa. The absorption capacity is higher than that of [bmim][BF4] by17times. A small decrease of absorption capacity of the [NH2ebim][BF4] is observed after three cycles. The results show the potential application of the TSIL in CO2separation membrane technology. Forethemore, a new kind of absorption equipment has been designed to improve the test efficiency of CO2solubility, by which the absorption equilibrium time is within60mininites. |
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