Green Carboxylic Acid-Based Deep Eutectic Solvents as Solvents for Extractive Desulfurization

The extractive desulfurization of thiophene (TS), dibenzothiophene (DBT), and benzothiophene (BT) in model oil was carried out using carboxylic acid-based deep eutectic solvents (DES). It was found that DES formed by formic acid as hydrogen bond donor and tetrabutylammonium bromide (TBAB) as hydrogen bond acceptor could efficiently separate organosulfur from oils. The influence parameters in this process are discussed, including extraction temperature, reaction time, mass ratio of DES to oil, multistage extraction effect, recycle times, and regeneration of DES. The results showed that the desulfurization selectivity of TBAB/HCOOH followed the order TS < BT approximate to DBT. Under the optimized conditions, the sulfur removal with BT, DBT, and TS were 81.75, 80.47, and 72% in a single stage, respectively, and after three cycles, it rose to 98.32, 98.24, and 97.6%, respectively. The sulfur content in fuels can be achieved to less than 8.4, 8.8, and 12 ppm for BT, DBT, and TS, respectively. In addition, by discussion of the mechanism, it was proven that hydrogen bonding between the sulfide and DES was the main driving force of the extraction desulfurization process.