(21f) Efficient Oxidation Desulfurization of Diesel Fuel Using Amide-Based Ionic Liquids

In the recent years, worldwide environmental regulations towards transportation fuels have been increasingly strict in order to reduce the air pollution and related public health impacts. Many countries mandated stringent legislation to cut diesel fuel S-content down to 10 ppm, which put forth a critical challenge to the refinery industry. In this work, a series of Brønsted acidic ionic liquids having different protonated amide-based cations were synthesized and investigated as solvents and catalysts in the oxidation desulfurization of diesel fuel, with hydrogen peroxide as oxidant. A detailed study was conducted to evaluate their desulfurization performance. Each ionic liquid showed obvious removal of benzothiophene (BT) and dibenzothiophene (DBT) in model oil. Among them, ionic liquids with lactam-based cation exhibited the best performance by completely removing S-components in short time. In order to gain insight into the catalytic mechanism, the detailed structure and conformation of was investigated by both experimental and theoretical methods. The protonated lactam-based cation was proved to exist in a special enol form. The special enol form not only brings a stronger acidity but also allows it to form two interionic hydrogen bonds with anions. These factors were supposed to contribute to high desulfurization performance by improving the formation of peroxides, and a corresponding mechanism was put forward. In the end, an ultra-high S-content Straight-Run Diesel was used to exactly evaluate of its desulfurization performance. The total S-content in the diesel fuel was reduced to only 0.144% from original 2.129% with 93.23% S-removal efficiency.