Sulfur compounds in transportation fuels are a pressing issue currently due to the more stringent limits of sulfur content. Sulfur compounds lead to SOx emissions which cause many environmental and health problems. The conventional refinery desulfurization process, hydrodesulfurization (HDS), is conducted at elevated temperatures and pressures, using expensive hydrogen gas and catalysts. This review aims to discuss the merits and drawbacks of the major areas of alternative desulfurization technologies, including biodesulfurization, adsorption, extractive and oxidative desulfurization, with special emphasis on the role of ionic liquids (ILs) as distinctive multi-task fluids. With the rapid development of ILs in the last decades, more effective and reliable desulfurization techniques are foreseeable.

The role of ionic liquids in desulfurization of fuels: A review

Commercially available molybdic compound-catalyzed ultra-deep desulfurization of fuels in ionic liquids

The pyridinium-based ionic liquids (ILs) N-butylpyridinium tetrafluoroborate ([BPy][BF4]), N-hexylpyridinium tetrafluoroborate ([HPy][BF4]), and N-octylpyridinium tetrafluoroborate ([OPy,][BF4]) were found to be effective for the selective removal of aromatic heterocyclic sulfur compounds from diesel at room temperature. The results suggested that the structure and size of the cation greatly affect the extractive performance of ILs. The extractive performance using pyridinium-based ILs followed the order [BPy][BF4] < [HPy][BF4] < [OPy][BF4], and for the IL, the sulfur removal selectivity of sulfur compounds followed the order thiophene (TS) < benzothiophene (BT) < dibenzothiophene (DBT) under the same conditions. The pyridinium-based ILs would not contaminate the diesel due to their insolubility. On the other hand, diesel has a certain solubility in pyridinium-based ILs, varying from 0.49 wt % for [BPy][BF4] to 1.97 wt % for [OPy][BF4]. According to the results, [HPy][BF4] and [OPy][BF4] might be used as promising solvents for the extractive desulfurization of diesel.

Desulfurization of Fuel by Extraction with Pyridinium-Based Ionic Liquids

Abstract Increase in energy demand and consumption has been accompanied by a corresponding increase in sulfur emissions. These pollutants have both health and economic consequences. Furthermore, it significantly reduces the efficiency of advanced emission control systems of diesel engines, thereby indirectly causing more harm to the environment. This resulted in stringent sulfur emission limit down to about 15 ppm or less and in turn served as an incentive for research into alternative sulfur reduction technologies. Although feasible improvements to hydrodesulfurization are currently under investigation, adsorptive, extractive, oxidative and biodesulfurization have also been studied in recent years. Oxidative desulfurization appears to be one of the most promising desulfurization technologies due to its broadness and compatibility with other technologies such as extractive, adsorptive and biodesulfurization. The advent of ionic liquids as extraction solvents has made this even more so. This work, therefore, reviews the different approaches and investigations carried out on oxidative desulfurization while identifying research gaps and giving important recommendations.

Deep oxidative desulfurization of liquid fuels

The demand for clean fuels is increasing throughout the world, with more stringent environmental regulations for transportation fuels including marine fuels, particularly regarding their sulfur content. Moreover, the quality of crude oil and derived petroleum cuts is getting lower while fossil fuels are still in high demand. Heavy oils are characterized by high sulfur content where most sulfur is found in bulky thiophenic structures difficult to remove using conventional high pressure hydrodesulfurization process. However they appeared more reactive in oxidative desulfurization (ODS) process, carried out at mild conditions without hydrogen pressure. This review focuses for the first time on the heavy fuels initially containing more than 0.5 wt.%S and upgraded by the ODS process. Different attractive approaches of the literature towards ODS are reported using homogeneous and heterogeneous catalysis. Recent developments in ODS assisted with ultrasound technology and the use of ionic liquid to enhance ODS efficiency will be fully detailed and discussed to better understand their viability when applied to high sulfur content, high viscosity, and high boiling point feeds.

Wang Jian-long

Papers

Desulfurization of gasoline by extraction with N-alkyl-pyridinium-based ionic liquids