Brønsted–Lewis Acidic Ionic Liquids and Application in Oxidative Desulfurization of Diesel Fuel

Study on alternative methods to hydrodesulfurization is always an interesting area, because of the ineffectiveness for hydrodesulfurization to remove some cyclic sulfur compounds (S-compounds) and the high cost from harsh operation conditions and expensive catalyst, among which oxidative desulfurization (ODS) using ionic liquids (ILs) as solvent is intensively studied recently. Here, we synthesize a series of Brønsted–Lewis acidic ILs of N-methylpyrrolidonium zinc chloride ([Hnmp]Clₓ/(ZnCl₂)y, x:y from 2:1 to 1:2) and investigate the ODS of both model diesel fuel composed of n-octane and dibenzothiophene and real FCC feedstock where such ILs are used as extractant and catalyst and 30 wt % H₂O₂ is used as oxidant, involving the factors such as IL composition (or x:y), temperature, dose of oxidant (or molar ratio of O/S), dose of IL (or mass ratio of IL/oil), recycling of IL and multistage desulfurization. IL composition has an important effect on sulfur removal (S-removal) efficiency, and [Hnmp]Cl/ZnCl₂ (x:y = 1, the structure nature was characterized with ESI-MS and FT-IR) shows the highest desulfurization capability with good recyclability. With [Hnmp]Cl/ZnCl₂, the S-content in model diesel fuel can be reduced to <1 ppm from 500 ppm with 99.9% S-removal at 75 °C, IL/oil = 1/3 and O/S = 8 after only one stage, while the sulfur removal for real FCC diesel fuel is less than 38% in one stage and can reach 83% after five stages, which might be ascribed to more-complex S-species in real fluidized catalytic cracking (FCC) diesel fuel as indicated by gas chromatography–sulfur chemiluminescence detection (GC-SCD) chromatogram analyses. The sulfur content (S-content) in FCC diesel fuel, however, can be reduced to 5.3 ppm with a total S-removal of 97.6% after five-stage ODS with one more extractive desulfurization with furfural as the extractant. This work shows that such Brønsted–Lewis acidic ILs are potential solvents used in ODS to produce clean fuel oils.