Natural deep eutectic solvents of γ-valerolactone and formic acid for oxidative desulfurization with superior efficiency

Deep eutectic solvents (DESs) are widely used in oxidative desulfurization (ODS) to produce clean fuel. However, some DESs with good ODS efficiencies are generally consisted of harmful components, and the ODS efficiencies of most DESs with green components need to be further improved. Herein, a DES with two natural compounds of γ-valerolactone (γ-Val) and formic acid (FA) was prepared and used in ODS. The hydrogen bond interaction of DES was characterized by FT-IR, 1H NMR, DSC, and calculated by density functional theory (DFT) computation. The radical quenching experiments, EPR, and kinetic study were carried out to investigate the active radical and apparent activation energy of ODS process. The desulfurization efficiencies of γ-Val and FA (molar ratio 1: 4) DES was capable of reaching 100% of dibenzothiophene (DBT) removal within 100 min at an energy saving way of room temperature (25 °C) with a low O/S molar ratio of 2. When the temperature was raised to 70 °C and the O/S molar ratio was raised to 5, 100% sulfur removal of dibenzothiophene, 4,6-dimethyldibenzothiophene (4,6-DMDBT), and benzothiophene (BT) could be achieved in 4 min, 6 min, and 10 min respectively. DFT calculations revealed that the introduction of γ-Val could reduce the transition state energy barrier of ODS compared with a single FA ODS process. This work provides a novel high ODS efficiency DES with natural compounds, indicating the potential application of green DESs in ODS catalysts.