Study On Liquid-liquid Equlibrium For Extraction Of P-xylene From N-octane Using Ionic Liquids

N-octane and p-xylene are two important chemical raw materials.P-xylene(PX)is widely used in the fields of polyester resin synthesis,coatings,pharmaceuticals,dyes and so on.N-octane is mainly used as a component for solvent gasoline and industrial gasoline,and also for organic synthesis.These two substances are common components of crude gasoline,so the separation process in the oil industry is very important.However,n-octane and p-xylene have very close boiling points,which creates an azeotrope,resulting in great difficulty in separation.The liquid-liquid extraction technology is the most widely used process in the process of separating aliphatic and aromatic hydrocarbons.Ionic liquid,as a green and excellent solvent,exhibits a good extraction and separation effect under mild conditions.In this work,we mainly do research on liquid-liquid equilibrium for ternary system ?octane(1)+ p-xylene(2)+ IL} and analyze the interaction mechanism between ionic liquids and solutes.The major works accomplished are as follows:(1)The COSMO-RS model was used to predict the extraction capacity of 1-ethyl-3-methylimidazolium([EMIM]?)based ionic liquids(15 species)on separating p-xylene with n-octane at T = 313.15 K.By comparing the calculated results and taking into account technical and economic factors,[EMIM][C2H5SO4],[EMIM][CH3CO2]and[EMIM][CF3CO2]were selected as extractants for this study.(2)The liquid-liquid equilibrium data were determined by experiment for ternary system {n-octane(1)+ p-xylene(2)+ IL(3)} at T= 313.15K and atmospheric pressure.Using the Othmer-Tobias equation to correlate the experimental data,the regression coefficients R2 are all above 0.95,indicating that the data are reliable.The three ionic liquids all showed good extraction effect.The selectivity coefficients follow the order of[EMIM][CF3CO2]>[EMIM][C2H5SO4]>[EMIM][CH3CO2].The partition coefficients are in the following order of[EMIM][CH3CO2]>[EMIM][C2H5SO4]>[EMIM][CF3CO2].In summary,the effect separating p-xylene with n-octane using ionic liquids follows the order of[EMIM][CF3CO2]>[EMIM][C2H5SO4]>[EMIM][CH3CO2].The experimental results are consistent with the results of COSMO-RS prediction,which proves the accuracy of COSMO-RS model prediction.(3)The "Ion pair" is the basis of our research.Quantum chemical calculation was used to obtain the information of ion pairs such as geological structures,electronic properties,orbital properties,topological properties and interaction energies.The interaction mechanism between cation and anion was studied systematically from the microscopic level.The results showed that the structures and properties of ionic liquids are mainly affected by the C-H…O hydrogen bonds between cation and anion,and the hydrogen bonding interactions in the C2 position are stronger.Besides,weaker vander Waals effect and steric effect were showed in ion pairs as well.The order of the interaction energy for the three ion pairs is[EMIM][CH3CO2]>[EMIM][C2H5SO4]>[EMIM][CF3CO2],indicating that[EMIM][CH3CO2]are more prone to aggregation.According to the energy docomposition analysis for interaction energies,it is found that the electrostatic interaction contributes more than 75%to the total attraction,indicating that the hydrogen bonding interaction essentially belongs to electrostatics.The electrons mainly flow from the LP(O)lone pair orbital on the anions to the ?*(C-H)anti-bonding orbital on the cation.The second-order perturbation energy of LP(O)??*(C2-H9)is the strongest.(4)Quantum chemical calculation was used to analyze the interaction mechanism between ionic liquids and solutes.The results show that the interactions between cantion/anions and p-xylene are stronger than that of n-octane.There is no hydrogen bonding interaction,?-? or p-? interaction occurred between cation and solutes.In the case of the same solute,the order of interaction energies between anions and solutes is:[CH3CO2]->[CF3CO2]->[C2H5SO4]-.The hydrogen bonding interactions for ion pair-solute complexes mainly takes place between anions and solutes.The electrons mainly flow from the LP(O)lone pair orbital on the anions to the ?*(C-H)anti-bonding orbital on the cation and solutes.It indicates that the type and size of anions play a dominant role in determining the separation performance of ionic liquids.The effect of the three ionic liquids on the separating p-xylene with n-octane is in the following order:[EMIM][CF3CO2]>[EMIM][C2H5SO4]>[EMIM][CH3CO2],which is consistent with the experimental results.Thereby the experimental phenomena are well explained,and the accuracy of the COSMO-RS model in predicting the trend is verified.