Extraction process of sulfur compounds from fuels with protic ionic liquids

Abstract

Sulfur compounds in fuels are hazardous to the environment and present a great challenge. In this study, a series of protic ionic liquids (PILs) was designed, such as N,N-dimethyl-N-cyanoethyl ammonium butyrate ([DMAPN]⁺[Bu]⁻), N,N-dimethyl-N-cyanoethyl ammonium propionate ([DMAPN]⁺[Pr]⁻), N,N-dimethyl-N-cyanoethyl ammonium acetate ([DMAPN]⁺[Ac]⁻), N,N-dimethyl-N-cyanoethyl ammonium formate ([DMAPN]⁺[Fo]⁻), N,N-dimethyl-N-(2-(2-hydroxyethoxy)) ammonium propionate ([DMEE]⁺[Pr]⁻), N,N-dimethyl-N-hydroxyethyl ammonium propionate ([DMEA]⁺[Pr]⁻), N,N-dimethyl-N-hydroxyethyl ammonium acetate ([DMEA]⁺[Ac]⁻), N,N-dimethyl-N-hydroxyethyl ammonium formate ([DMEA]⁺[Fo]⁻), N,N-dimethyl-N-butyl ammonium acetate ([DMBA]⁺[Ac]⁻), N-methyl-N-hydroxyethyl ammonium propionate ([MEA]⁺[Pr]⁻), and N-methyl-N-hydroxyethyl ammonium acetate ([MEA]⁺[Ac]⁻), and their extraction desulfurization capability was evaluated. Through selection, two PILs were chosen to optimize the desulfurization process. The results showed that benzothiophene (BT) and thiophene (T) can be removed using PILs effectively. Both cations and anions have an influence on the extraction efficiencies. However, cations have a greater influence on the desulfurization process. Under optimal conditions, the extraction efficiency can be higher than 80% for one cycle. After 6–8 cycles, the removal efficiency could reach up to 99.5%. It is worth noting that the PILs could be recycled via simple vacuum distillation. After 4 times of recycling, the desulfurization efficiency changed slightly. Finally, a mechanistic study shows that hydrogen bonding interactions between the sulfur atom and active hydrogen of the PILs account for the high desulfurization efficiency. This green process would provide a new route for the extraction desulfurization of fuels.