The oxidative desulfurization (ODS) of heavy fuel oil (HFO) offers a promising solution for desulfurizing marine fuels under mild conditions, in line with current environmental regulations. While most studies focus on model or light fuels, explaining deactivation through leaching or sulfone adsorption, the deactivation mechanisms of catalysts in HFO remain poorly understood. In this work, Mo-based catalysts supported on alumina were extensively characterized before and after catalytic reactions, and regeneration through air calcination was considered. Techniques such as XRD, Raman spectroscopy, XRF, and TGA, alongside catalytic testing with H₂O₂ as an oxidant, revealed that Mo surface speciation significantly impacted both activity and deactivation. Contrary to well-dispersed polymolybdates, crystalline MoO₃ induced low activity and hindered regeneration. No leaching of the active phase was demonstrated during the reaction. Sulfone adsorption had minimal impact on deactivation, while non-sulphur compounds appeared to be the key contributors. Regeneration outcomes were found to be molybdenum content-dependent: 10Mo/Al recovered its activity, while 20Mo/Al formed inactive phases, like Al₂(MoO₄)₃. Using an organic oxidant (tBHP) during ODS influenced the regeneration, as it prevented Al₂(MoO₄)₃ formation and redispersed crystalline MoO₃, enhancing performance. These findings advance understanding of catalyst deactivation and suggest strategies to extend catalyst life in the ODS of HFO.

This study explores deep eutectic solvents (DESs) as potential alternatives to dimethylformamide (DMF) for extractive desulfurization of gas oil and marine fuel oil. A series of DESs, based on choline chloride (ChCl) and tetrabutylammonium bromide (TBAB), were evaluated using straight-run gas oil (SRGO) and compared to DMF. The influence of an oxidation pre-treatment and water content was also investigated, and the most efficient systems were further applied to marine fuel oil. Firstly, oxidation was found to be essential to enhance sulfur removal by improving the extractability of sulfone compounds. The results show that DES formulation strongly affects desulfurization performance, with TBAB-based DESs achieving efficiencies comparable to DMF (~80% desulfurization rate) and outperforming ChCl-based systems. In ChCl-based DESs, a moderate water content (~10 wt%) improved performance, whereas higher amounts disrupted the hydrogen-bond network, as evidenced by Fourier-transform infrared spectroscopy, leading to a decreased efficiency. When applied to marine fuel oil, similar trends were observed, although lower desulfurization levels were obtained due to the complexity of the feed, and a solvent-to-fuel ratio of 5:1 was required for proper phase separation. Fourier transform mass spectrometry analyses highlighted the persistence of refractory sulfur species, emphasizing the need for tailored solvent design. Overall, DESs represent promising and adaptable alternatives for desulfurization processes.