Facile Separation of Acetic Acid from 1-Ethyl-3-methylimidazolium Acetate Ionic Liquid with the Aid of a Protic Solvent

Abstract

1-Ethyl-3-methylimidazolium acetate (EmimAc), an excellent solvent for cellulosic biomass, is expected to be utilized in chemical conversion, such as in biomass acetylation with acetic anhydride. The corresponding carboxylic acid, acetic acid (AcH), is quantitatively generated as a byproduct and should be separated from EmimAc for recycling. However, the strong interaction between EmimAc and AcH makes their separation difficult under moderate conditions. This study examined the efficacy of protic solvents in distillation and extraction to weaken this interaction through solvation or hydrogen-bonding interactions. The separation efficiency of AcH from EmimAc via distillation increased as the boiling point of the protic solvent increased. Water addition was more effective than the addition of alcohols with boiling points similar to those of water such as 1-propanol and 2-butanol. Furthermore, the favorable effect of water addition on the extraction of AcH was confirmed using common organic solvents, such as diisopropyl ether, diethyl ether, and ethyl acetate. The partition coefficient (α) of AcH between the aqueous and organic phases increased with an increasing dielectric constant of the organic solvent, whereas the α value of EmimAc decreased. Repeated treatments in both distillation and extraction facilitated the complete separation of AcH from EmimAc.