Deep Aerobic Desulfurization of High-Sulfur-Containing Fuels: Role of Sulfur Inhibition

Aerobic oxidative desulfurization (AODS) of high-sulfur feedstocks is essential for developing efficient, eco-compatible deep-oil refining technologies. However, the influence of the high initial sulfur concentration on AODS efficiency remains poorly understood. This gap is critical, as real feedstocks often contain several thousand parts per million of sulfur. We show that increasing the sulfur content of the model fuel (DBT in dodecane) from 500 to 8000 ppm leads to a pronounced decrease in the oxidation rate: the DBT conversion decreases from 100% to 9% under identical conditions within 60 min at 150 °C, indicating a strong inhibitory effect at high sulfur concentrations. A systematic study was performed on a model fuel containing 8000 ppm of S using a series of Fe, Co, Mn, and Bi molybdate catalysts while varying key process parameters (temperature, catalyst dosage, and air flow rate). To overcome the inhibitory effect, an initiator-assisted strategy was proposed. The addition of tert-butyl hydroperoxide (TBHP) as an initiator increased the DBT conversion from 9% to 33% in 60 min, shortening the induction period and increasing the desulfurization efficiency by generating additional alkyl peroxide radicals. For the first time, complete oxidation of high-sulfur fuel (8000 ppm of S) was carried out using a MnMo catalyst combined with TBHP as an initiator in a two-stage process (150 °C; 0.22 wt % catalyst; air flow rate 6 L/h; stage 1:0.8 wt % TBHP, 120 min; and stage 2:0.4 wt % TBHP, 150 min). The catalyst maintained high activity and structural stability over five cycles. Proposed reaction pathway involving superoxide (O2•–) radical and alkyl peroxide generated from atmospheric O2 and TBHP is discussed. The suggested approach has also been successfully applied to straight-run diesel fuel (10,208 ppm), where the degree of desulfurization has increased from 20 to 45%.