Solvent-free esterification of carboxylic acids with alcohols catalyzed by amphiphilic Brønsted acidic ionic liquids

Five halogen-free amphiphilic Brønsted acidic ionic liquids (ILs) based on 3-(N,N-dimethylalkylammonium)-propanesulphonate quaternary ammonium zwitterions andp-toluene sulphonic acid monohydrate were synthesized. The IL structures were characterized using1H NMR,13C NMR and (ESI)-MS. The IL thermal instability was characterized using TG-DSC. The ILs were used as dual-solvent catalysts for esterification of several carboxylic acids, with different alcohols under mild conditions to study relevant structure-activity relationships. Results showed that the miscibility of the acidic IL, which is affected by the length of the carbon chain in the cation, is an important factor influencing the activity of the catalyst for the esterification reaction. Post-processing steps were simple, and the catalyst could be reused easily.