Significant effects of the anion on the catalytic behaviour of sulfonic acid-functionalized ionic liquids in transesterification reactions – A combined electrochemical/catalytic study

Abstract

This work studies the strong influence of the counterion on the performance of ionic liquids (ILs) with sulfonic acid-functionalized butyl-imidazolic bases as homogeneous acid catalysts. Through a voltammetric analysis, it was shown that the availability of protons from the sulfonic acid groups in ILs with chloride and bisulphate anions is quite different, thus affecting their intrinsic activity as acid catalysts. The study was focused on the chlorides and bisulphates of 1-(4-sulfonic acid)-butyl-3-methylimidazolium ([bsmim]) and 1-(4-sulfonic acid)-butyl-imidazolium ([bsHim]), and the test reaction to evaluate these ILs as catalysts was the transesterification of p-nitrophenyl acetate with methanol. In ILs with counterion bisulphate, protons at the sulfonic acid groups are deeply involved in building the IL´s H-bonded network, so the acceptable catalytic activity for the tested reaction that was verified on these ILs was mostly sustained by the free sulphuric acid that was always present with the ionic pairs. On the contrary, ILs with chloride counterion conform more loosen networks where protons from the sulfonic acid groups are readily available and allow to efficiently catalyse the reaction, independently of the amounts of free hydrochloric acid that may be along with the ionic pair. Remarkably, the same contrast was verified in the catalytic performances of bisulphate and chloride of the non-sulfonated base 1-butyl-imidazolium ([bHim]), where the imidazolic hydrogen acted as an efficient acid catalyst only in [bHim]Cl.