Phase Equilibrium And Simulation Of Dimethyl Carbonate-methanol Azeotrope System Separated By Imidazole Ionic Liquids As Extractants

Dimethyl carbonate（DMC）is a colorless,transparent and volatile liquid with excellent chemical stability and biodegradability.It is widely used in solvents,chemical intermediates,electrolytes and other fields.With the rapid development of the global economy and the improvement of people’s awareness of environmental protection,DMC,as a green solvent and chemical intermediate,its market demand is gradually increasing,especially in the field of new energy,the demand for DMC is expanding.At present,the main industrial production methods of DMC include phosgene method,transesterification method and methanol method.However,DMC and methanol form an azeotrope that is difficult to separate due to the need to use methanol in large quantities.Extractive distillation is an effective method for the separation of azeotropic systems.In this paper,a method of separating dimethyl carbonate and methanol with imidazole ionic liquids as extractants was proposed.The separation effects of four imidazole ionic liquids were studied by combining theory with experiment.The specific research contents and conclusions are as follows:（1）The COSMO-RS model was used to predict and screen ionic liquids for the se paration of methanol-dimethyl carbonate azeotropic system.From the ionic liquids co mposed of 23 cations and 23 anions,1-butyl-3-methylimidazolium hexafluorophosph ate（[BMIM][PF₆]）,1-hexyl-3-methylimidazolium hexafluorophosphate（[HMIM][PF₆]）,1-butyl-3-methylimidazolium bis（trifluoromethanesulfonyl）imide（[BMIM][TF₂N]）and 1-hexyl-3-methylimidazolium bis（trifluoromethanesulfonyl）imide（[HMIM][T F₂N]）were screened.These four ionic liquids were synthesized and characterized.（2）Vapor-liquid equilibrium of methanol-dimethyl carbonate-ionic liquids.The VLE data（T,p,x,y）of the ternary system of methanol（1）+dimethyl carbonate（2）+ILs（3）at different ILs molar concentrations were measured and analyzed by NRTL model at 101.325 k Pa.Experiments show that the addition of four ionic liquids can eliminate the azeotropic point of methanol-dimethyl carbonate system and produce the obvious cross-salt effect.With the increase of ionic liquid concentration,the separation effect is gradually improved.The order of extraction effect is as follows[HMIM][TF₂N]>[BMIM][TF₂N]>[HMIM][PF₆]>[BMIM][PF₆].（3）The 1:1 complex of[BMIM][PF₆],[HMIM][PF₆],[BMIM][TF₂N]and[HMIM][TF₂N]with dimethyl carbonate or methanol were calculated by Gaussian09package at the B3LYP/6-311+G（d）level.The size and nature of the weak interaction were studied by interaction energy and RDG analysis.Studies have shown that the interaction between ionic liquids and methanol,dimethyl carbonate is much greater than the interaction between methanol and dimethyl carbonate.In the interaction between ionic liquids and methanol,dimethyl carbonate,the interaction between ionic liquids and methanol is stronger,and the interaction between ionic liquids and dimethyl carbonate is weaker.For the four ionic liquids,[HMIM][TF₂N]has the best separation effect on methanol-dimethyl carbonate,which is consistent with the experimental results.