Metal-Free Catalytic Oxidative Desulfurization of Fuels─A Review

Overdependence of energy consumption around fossil fuels is threatening our environment from different anthropogenic emissions of pollutants. These pollutants (COx, NOx, and SOx) resulting from combustion activities of crude oil and petroleum products are life-threatening. Desulfurization of fuels is an important matter that constantly requires attention from the relevant stakeholders for environmental, economic, and social benefits. Hydrodesulfurization required to lower the sulfur content in the crude oil comes at high operating conditions and cost, especially toward achieving ultradeep desulfurization of <10 or 0 ppm as required for fuel cell catalytic converters. In the recent decades, oxidative desulfurization (ODS) is gaining fast interest among the few alternatives proposed as a complementary approach. This paper reviews the current advances in the metal-free catalytic ODS owing to the effectiveness and promising results in the light of safety, cost, and environmental concerns compared to conventional metal-containing catalytic ODS strategies. Special contributions related to graphitic materials, nitrides, carbides, ionic liquids, deep eutectic solvents, and porous organic polymers with respect to the broad classification of organic, inorganic, and hybrid (organic–inorganic) metal-free catalysts and their mechanism of catalyzing the oxidation process are thoroughly reviewed. The challenges, improvements, merits, and future prospects are discussed in the context of industrial feasibility for strategic plans and further studies.