The Study Of Recovery Of Imidazolium-based Ionic Liquids By Nanofiltration

Ionic liquis(ILs)have been extensively studied in many fields as a structurally tunable green reagent.In terms of large-scale application of ILs,one of the most prominent obstacles is how to efficiently recycle ILs.Therefore,it has importantly practical significance to develop methods to recover ILs.As an important chemical separation unit,nanofiltration is widely used in the separation and purification fields due to its simple operation and energy saving.In this thesis,a series of imidazole-based ILs were filtrated by nanomembranes,and the effects of molecular size and solution environment on membrane filtration have been studied.What's more,the effect of ion cluster on nanofiltration of long-alkyl-chain ILs aqueous solutions were also investigated.Moreover,solution-diffusion model was rationally improved based on ion cluster,which makes it suitable for aqueous solutions of ILs with different chain lengths.The details are as follows:Firstly,a series of 1-alkyl-3-methylimidazolium-based ILs([Cnmim]X,n represents the number of cationic alkyl side chains,and X represents anion)were filtrated using Dow nanofiltration membranes.The effect of molecular sizes on volumetric membrane flux(Jv)and observed rejection(Ro)was studied,such as the size of the cationic alkyl side chain and the different halide anions.In addition,the effect of solution environment on membrane filtration was investigated,including pH,ionic strength,and macromolecular organic pollutants.The experimental results showed that the molecular size and macromolecular organic pollutants influence the nanofiltration performance by impacting sieving effect,and the pH and ionic strength influence the nanofiltration performance by affecting charge effect.Besides,the solution environment also impacts the physicochemical properties of certain ILs,thereby influencing the nanofiltration mechanism,and thus the nanofiltration performance.Secondly,the effect of ion cluster on concentration of ILs aqueous solution by nanofiltration was investigated.A series of long-alkyl-chain ILs aqueous solutions with different concentrations were filtrated using Dow nanofiltration membranes.The existential state of ion cluster in aqueous solution was studied by experiments and molecular dynamics simulation.The effect of ion cluster on Jv,,Ro,membrane fouling and membrane recycling were investigated.The experimental results showed that ion cluster mainly influences the nanofiltration performance by affecting sieving effect:with the increasing of concentration of ILs,the degree of ion cluster increases,and thus the larger average size of ion cluster,which leads to the decrease of the Jv and the increase of the Ro.In addition,ion cluster alleviates the degree of membrane fouling,It is more difficult for ion cluster to enter membrane pores,because the steric hindrance effect is enhanced,and the probability of the membrane pores being blocked is decreased.What's more,according to the influence of the cluster on the membrane filtration,the filtration performance was enhanced by adjust the molecular size of ion cluster in long-alkyl-chain ILs aqueous solutions.Third,the applicability of the solution-diffusion model to the nanofiltration of ILs aqueous solutions was investigated.A series of long-alkyl-chain ILs aqueous solutions with different concentrations were filtrated using Dow nanofiltration membranes,and their Jv and Ro were measured.Using the solution-diffusion model,Ro was predicted by the experimental value Jv.As a result,it was found that the model is suitable for long-alkyl-chain ILs aqueous solutions with low concentrations,but not for long-alkyl-chain ILs aqueous solutions with high concentrations,and the maximum average relative deviation(ARD)reaches 268%.The solution-diffusion model was reasonably modified based on ion cluster.The molecular size of ion clusters in different concentrations of ILs was quantitatively calculated by molecular dynamics simulation.The relevant parameters in the model were calculated according to the size of ion cluster.The calculation results showed that compared with the unmodified model,the calculated values predicted by the improved model agreed better with the experimental values,and the maximum average relative deviation(ARD)is just 3.4%Fourth,The preliminary economic investigation of nanofiltration combined with distillation to recover ILs was investigated.The costing of nanofiltration combined with vacuum distillation(pre-concentration by nanofiltration and followed by reduced pressure distillation)for recovering ILs aqueous solutions was calculated.The experimental results showed that the costing of the nanofiltration combined with vacuum distillation method is just about 18%of that of simple vacuum distillation.The reason for the reduction in costing is that the gas work is much lower than the energy consumption of heating and evaporation.In addition,a method of two-stage nanofiltration combined with vacuum distillation to purify long-alkyl-chain ILs was proposed:Firstly,the membrane with smaller pore size is used for filtration.As the concentration of ILs increases,the ion clusters are formed and the size gradually becomes larger,and the membrane with a bigger pore size is used at the appropriate moment.Finally,the pre-concentrated ILs was completely purified by reduced pressure distillation.The experimental results show that the costing of the method is further reduced by 13%compared with one-stage nanofiltration.