Oxidative desulfurization of liquid fuels using deep eutectic solvents as a catalyst and extractant: A review

For environmental safety and sustainability, sulfur concentration in fuel must be minimized. A Commercial desulfurization method, hydrodesulfurization (HDS), offers fine desulfurization of liquid fuels; however, the major challenge of making the process energy efficient remains intact. To address this, various desulfurization approaches are being explored, such as biodesulfurization, adsorption desulfurization, extraction desulfurization, and oxidative desulfurization. Industrial engineers are finding novel reaction routes, while chemical engineers and chemists are working on preparing catalysts and their modifications to optimize process conditions. Research in oxidative desulfurization (ODS) demonstrates that both photocatalytic and thermal-driven ODS processes exhibit significant potential. Thermally driven extraction and catalytic oxidative desulfurization (ECODS) have gained attention using deep eutectic solvents (DESs) and ionic liquids (ILs) as both catalysts and extractants. However, DES overcomes certain limitations of ILs. In the case of DES, the oxidants (H₂O₂/O₂) oxidize the organic acids in DES to peroxy acid, which in turn oxidizes sulfur compounds of fuel into easily removable sulfones, removed by the same DES (acting as extractant as well). DESs are environmentally benign, possess the capability to work synergistically with additional catalysts such as polyoxometalates (POMs) and metal-free catalysts, can be regenerated using only deionized water, and can be reused multiple times with minimal loss of efficiency. This literature review explores the synergistic, catalytic and extractive potential of DES to overcome the major challenge of energy intensive nature of desulfurization process. Furthermore, various methods are critically analyzed, comparative potential of ionic liquids and DESs in ECODS is discussed, and importance of real system (fuel) studies is emphasized.