Benzimidazolium-based high temperature ionic liquid-in-oil microemulsion for regioselective nitration reaction

Abstract

Owing to the fascinating applications of ionic liquids (ILs) based non-aqueous microemulsions (MEs) in the field of chemical reactions due to their high thermal stability compared to that of aqueous MEs and requirement of water-free environment, we designed a high-temperature stable cationic IL-in-oil microemulsions using 1-ethyl-3-propylbenzimidazolium bromide ([EPbim][Br]) as a polar phase for the first time. In order to determine the thermodynamic stability of such type of MEs, we measured the distribution of cosurfactant (herein, 1-octanol) between bulk n-decane phase and interface covered by quaternary ammonium head group of cetyltrimethylammonium bromide (CTAB) at elevated temperature. The spontaneous formulation of high-temperature IL-in-oil MEs with organized IL-oil interface was identified by the negative free energy and entropy change at studied temperature range (358–370 K). Interestingly, we observed temperature-independent exothermic behavior at a specific composition, which probably comes from the cation-π interaction between quaternary ammonium head group of CTAB and aromatic moiety of ILs. We performed phase behavior study to demonstrate a wide range of thermally stable of [EPbim][Br]/CTAB/decane MEs. Surprisingly, the droplet size was decreased with progressive addition of IL, which may be due to the difficulty of formation of strong and favourable H-bond between the polar head groups of CTAB and [EPbim][Br] at higher temperature. Furthermore, the aggregation number and molecular interfacial area at higher temperature were calculated, which have direct correlation with the droplet size. Finally, the nitration reaction with three different aromatic compounds was performed in this MEs media at the temperature-independent composition. The reaction was completed with the highest regioselectivity of para isomer product over the ortho one. Moreover, these results clearly highlight an efficient way towards the formulation of high temperature stable MEs with ILs at ambient pressure and open a wide field of potential applications.