Study On The Interaction Mechanism And Absorption Process Optimization Of Gas-phase Toluene And Ionic Liquids

As the main branch of VOCs,aromatic volatile organic compounds have seriously affected human production and life,and their treatment is imminent.Ionic liquids（ILs）are widely used as absorbent because of their good characteristics such as low saturated vapor pressure,high thermal stability and structural designability,as well as the advantages of"zero pollution,zero emission and zero loss"in the absorption and capture process.In this paper,toluene was selected as the representative and ILs was used as the absorbent to study the mechanism of toluene removal and the optimization of absorption process.The specific research contents are as follows:（1）The predictive thermodynamic model COSMO-RS model is applied to the screening of ILs,and the target ILs are screened from 285 ILs including 19anions and 15 cations.Considering the price,viscosity,thermal stability,synthesis conditions,selectivity and other factors of ionic liquids,we select[EMIM][De]as the target ILs.In addition,For toluene,[EMIM][Ca],[EMIM][Pe],[EMIM][TF₂N]and[EMIM][De]have the same selectivity,logarithmic order of magnitude:10000.Select these three ILs as reference ILs.（2）The target ILs was synthesized by two-step synthesis method.It was purified and characterized.The density,viscosity,thermal decomposition temperature and other physical properties of the synthesized ILs were measured.The results showed that the target ILs had been synthesized and had moderate viscosity and density.The thermal decomposition temperature was above215℃,and the thermal decomposition temperature of[EMIM][TF₂N]reached418℃.（3）The absorption experiments of different ILs were carried out.The experimental results showed that among the 13 ILs,the target ILs had the highest toluene absorption saturation capacity,and followed the law:[EMIM][De]>[EMIM][Pe]>[EMIM][Ca]>[EMIM][TF₂N].The accuracy of COSMO-RS model is verified.In addition,experimental studies have proved that low temperature and high pressure are favorable conditions for gas absorption.At the same time,FT-IR and ¹HNMR of the regenerated ionic liquid show that the absorption process is a physical process.It should be noted that after 19 cycles of ionic liquid absorption,the saturation capacity decreased by only 0.2%.The content of toluene detected by GC-MS is 7.2 ppm,which shows the high stability of ILs and reduces the experimental cost,which is conducive to the popularization of industrialization.（4）The thermodynamic behavior of ILs and toluene system was studied.The vapor pressure data at different temperatures and molar compositions show that[EMIM][De]+toluene<[EMIM][Pe]+toluene<[EMIM][Ca]+toluene<[EMIM][TF₂N]+toluene.The average vapor pressure of water is less than the saturated vapor pressure of toluene at the same temperature,and the activity coefficient is greater than 1,indicating that there is a weak interaction between ILs and toluene,and the longer the anionic alkyl chain,the stronger the weak interaction.（5）Using quantum chemical calculation and COSMO-RS model calculation,the phase equilibrium behavior and absorption experimental phenomena of ILs and toluene are explained at the molecular atomic level.There is a hydrogen bonding force between theπbond of the dioxygen atom on the carboxyl group and the toluene methyl group and the hydrogen atom on the adjacent position;There is a weak electrostatic force between alkyl chain and toluene benzene ring.In addition,on the one hand,the growth of alkyl chain shifts theπelectron of carboxyl dioxygen atom to the growth direction of anion chain.This induced effect weakens the polarity of carboxyl group and increases the nonpolarity ability of the whole anion.The corresponding green isosurface is determined by the polarity of anionα,βHydrocarbon atom orientationγ,δHydrocarbon atom expansion;On the other hand,the volume effect of anions is improved.The stacking of the two effects enhances the electrostatic interaction between the whole anion and toluene.Hydrogen bonding force and enhanced electrostatic force are the essential reasons behind the thermodynamic behavior and absorption phenomenon of the system.