Study Of Synthesis And Performance Of Immobilized Ionic Liquid Used For CO₂/N₂Separation

The immobilization of ionic liquids into poor porous silica support were carriedout with impregnation-evaporation and grafting methods, and the modified particleswere characteristics via FTIR, TG, XPS, DSC and SEM analysis. The effectsregulation of activitied method of SiO2, immobilization method and the structure ofionic liquids on pore structure, CO2adsorption capacity and CO2/N2selectivity wereinvestigated. Furthermore, the thermodynamic study and diffusion coefficicent of CO2adsorption on silica immobilized ionic liquid were discussed in detail. The mainfindings are as follows:(1)According to the FTIR spectroscopy and X-ray photoelectron spectroscopy,the immobilization of ionic liquids into porous silica support were synthesizedsuccessfully through impregnation-evaporation and grafting methods;Thermogravimetric analysis showed that the hydroxyl content of150℃activatedsilica was higher than HCl activated silica. All prepared samples had high thermalstability; DSC analysis indicated the melting points of ionic liquids [Emim]TFSImodified silica decreased with the loading amount of ionic liquids on silica surfacedecrease; The scanning electron micrographs showed that the sample prepared bygrafting method presented more developed porous structure.(2) The activitied method of SiO2, immobilization method and the structure ofionic liquids and feed ratio(IL/SiO2) had the significant effect on CO2adsorptioncapacity and PSD of samples. The Freundlich model fitted the adsorption data well;The samples presented the developed microporous structure and exhibited a higherpore size distribution(PSD) in the0.4～0.8nm range; The HCl activated silicaimmobilized the n-octyl phosphonium ionic liquids prepared with grafting method(QDH-SiO2-P8T-1) showed a better CO2adsorption capacity (4.28wt%) at313K,0.1MPa, and the feed ratio(IL/SiO2) of the sample is0.5:1; The introduced IL phaseon the silica surface showed an improvement in the CO2/N2selectivity, the sampleSHH-SiO2-OMT-1exhibited the best selectivity. The silica immobilized ionic liquidswhich carried with grafting method exhibited the better CO2sorption/desorption cyclecapacity.(3) The isosteric heat of adsorption (H), Gibbs’ free energy change (G) and entropy change (S) of CO2on silica immobilized phosphonium ionic liquids werecalculated. H value showed that the CO2adsorption on the modified silica is aphysical sorption, and the process of adsorption is exothermic. The values of Gindicated a spontaneous sorption process for ILs modified silica. The negative valuesof adsorption entropy implied the decrease in the freedom of CO2molecules.(4)The classical homogeneous surface diffusion model (HSDM) was used toobtain the diffusion coefficient (Ds) of CO2in ILs modified silica. For ionicliquids([P8883][BF4]), the calculated values of Dswere at a level of10-10m2·s-1. Forunmodified and modified silica, the calculated values of Ds were at a level of10-7～10-8m2·s-1. In comparison with bare SiO2, CO2diffusion coefficients for IL modifiedsilica prepared with grafting method were slightly lower. For IL modified silicaprepared with impregnation method, the magnitude of D1, D2were at the level of10-10~10-11m2·s-1and10-7～10-8m2·s-1. For modified silica, the Dswere lower thanbare silica and the D2of sample decreased with the loading amount of ionic liquids onsilica surface increase, which demonstrated more ionic liquids accessed into silicapore with the loading amount increase and impeded CO2diffusion.