The Study of Preparation and Gas Permeation Performance of Supported Ionic Liquid Membranes

Author:Liu Yi Fan

Supervisor:yin jian zhong


Degree Year:2018





Ionic liquids(ILs)exhibit many advantages such as low vapor pressure and wide liquid range.Supported ionic liquid membranes(SILMs),prepared by immobilizing ionic liquids(ILs)into asymmetric mesoporous inorganic membrane,exhibit advantage such as high stability,negligible loss of liquid phase in supports.Therefore,SILMs have attracted many attentions in CO2 capture field.Nevertheless,the high viscosity prevents ILs to imbibe into small pores in supports,making it difficult to prepare high-performance SILMs.In this thesis,the SILMs with high performance were controllably prepared using coating method and supercritical fluid assisted technology(SFAT).The properties of ILs in nanopores were studied using molecular dynamics simulation.The main contents of the dissertation are as follows:(1)Commercial mesoporous inorganic membranes with inevitable defects can not meet the requirements of SILMs preparation.With boehmite sol and nano a-Al2O3 powder,the asymmetric mesoporous inorganic membranes were prepared using hot-coating method,and used as supports.The as-prepared supports were characterized by SEM,permporometry and other characterizations.(2)Based on γ-Al2O3 membrane and ILs with good CO2 solubility such as[BMIM][BF4],[BMIM][Ac],SILMs were prepared using coating method by optimizing the soaking time and temperature to give consideration to both CO2/N2 selectivity(αCO2/N2)and CO2 permeance(Pa,CO2).The gas permeance were measured using pressure-profile method and its precision could reach 0.001 GPU after correcting by temperature fluctuation coefficient kr.The influence of soaking time(0-1800 s)and temperature(18-75℃)on SILMs performance were studied.It was found that changing the ratio of ILs surface tension and viscosity σ/μ by regulating temperature was the key factor to improve theαCO2/N2.Increasing soaking time or temperature led to the increase of liquid membrane thickness and subsequent reduction of gas permeance.Even so,there were still non-negligible small pores,which were difficult to fill.(3)The relationship between SILMs performance and relevant properties of ILs and support were studied.The gas solubility and diffusion coefficient in ILs were correlated to the viscosity and molar volumes.The porosity,tortuosity and other properties of supports were measured by N2 adsorption,He permeation,etc.The CO2 permeability and αCO2/N2 of SILMs could be predicted by combining the properties above.Then the relationship between SILMs performance and ILs properties was discussed.Based on the relationship between SILMs gas permeance and preparation condition,the influence of ILs viscosity,ILs surface tension,soaking time on the effective thickness of liquid membrane was discussed.(4)To improve the filling rate of residual small pores,the SFAT was proposed to prepare SILMs.Using supercritical CO2(scCO2)as solvent,ethanol as co-solvent,[BMIM][BF4],[BMIM][Ac]and[EMIM][Ac]could be dissolved and deposited into the pores by supercritical fluid deposition,then the SILMs were obtained after the CO2 and ethanol were removed by releasing pressure and drying.The influence of preparation conditions on the SILMs performance and ILs loading were investigated,such as preparation time(1-24 h),IL amount(75-400 mg),ethanol amount(0-9 mL),preparation temperature(30-60℃)and pressure(9-21 MPa).Moreover,the scCO2-ethanol-ILs phase behavior and the capillary phase transition in scCO2 were discussed to analyze the mechanism of preparing SILMs using SFAT.(5)The merit and demerit of coating method and SAFT were comparatively analyzed by comprehensive consideration of the Pa,CO2,PCO2 and αCO2/N2 of SILMs.The commercial value and gap with Robeson Upper Bound were discussed.To ensure the separation efficiency of SILMs,it should give consideration to both gas selectivity and permeance.By comparatively analyzing the Pa,CO2 and αCO2/N2 of SILMs,the feature of coating method and SFAT in liquid membrane thickness control and pore filling performance were discussed,as well as their applicability for supports with different pore size and thickness.(6)It is difficult to study the change of structural properties for ILs in nanopores using experimental techniques.Therefore,the molecular dynamics simulation was used to investigate the underlying mechanism of the variation of confined ILs properties and their influence on the SILMs preparation and performance.Different forcefields(EPM2,CLAYFF,etc.)were used to build molecules such as CO2,[BMIM][BF4],[BMIM][Ac]and so on.The slit shaped γ-Al2O3 pores(1.0-3.5 nm)were filled with ILs,and then simulated using NPT ensemble for 2 ns to obtain the stable confined ILs.Then the CO2(12.04 kg/m3)was spontaneously permeated into ILs by 80-200 ns NVT simulation.The influence of pore width,alkyl-chain length on ILs structural properties were studied,such as the density,orientation,radial distribution function and so on.Moreover,the influence of pore width,ILs type on the diffusion coefficient and solubility of CO2 in confined ILs were also studied.