Catalytic Oxidative-Extractive Deep Desulfurization of Diesel Fuel by N-methyl-2-pyrrolidone-Based Protic Acidic Ionic Liquids (PAILs)

Inexpensive and low-viscosity Protic Acidic Ionic Liquids (PAILs) of N-metheyl-2-pyrrolidone formate, acetate, and propionate abbreviated as [Hnmp][HCOO], [Hnmp][CH3COO] and [Hnmp][CH3CH2COO] respectively, were prepared and characterized by FT-IR and H-NMR. Deep oxidative and extractive desulfurization of a simulated oil and a real diesel containing 1000 ppmw sulfur were studied under the catalysis of the proposed PAILs without additional carboxylic acid and extraction solvent in the system. The desulfurization efficiency in this study was influenced by both amounts of PAILs and concentration of the produced peroxy acids from H2O2. Comparing to [Hnmp][HCOO], [Hnmp][CH3COO] and [Hnmp][CH3CH2COO] had a relatively higher extraction ability however the weaker acidic properties of them resulted in a lower activation of H2O2 to produce peroxy acids and finally led to lower oxidative desulfurization (ODS) efficiency in the same condition. Additionally, the neutralization reaction may not be efficiently gone to the right hand by weaker acids. The effects of the main process variables were studied in detail. The highest desulfurization yield of 97% for BT and 99% for DBT and 4,6-DMDBT, were achieved using [Hnmp][HCOO] in the ODS system at a temperature of 40 °C, H2O2/sulfur molar ratio of 10, VPAIL/Voil of 0.2, and a reaction time of 90 min. The immiscibility of the proposed PAILs in this study with the oil phase caused the easy separation of them after desulfurization. The possible ODS pathway was also proposed.