Enhanced oxidative desulfurization of liquid model fuel under microwave irradiation over W2N@C catalyst nanoarchitectonics

Abstract

Oxidative desulfurization (ODS) of liquid fuel was conducted using a series of W-based catalysts like WO3/ZrO2, WO3/AC, and tungsten-nitride@porous carbon (W2N@C), under both microwave (MW) and conventional electric heating to estimate firstly the quantitative contribution of MW in ODS kinetics. At first, because of the favorable effect of MW on ODS, the most stubborn thiophenic compound, thiophene (Th), could be eliminated (97%) within only 30 min at 60 °C under MW irradiation in the presence of the W2N@C catalyst (that showed the highest ODS performance among the studied catalysts). Importantly, the activation energies of Th oxidation over the W2N@C were 22 and 38 kJ⋅mol−1 with microwave and conventional heating, respectively. Moreover, the kinetic constant of Th oxidation with MW was about 3.4 times that of the oxidation with the electric heating at the same temperature (60 °C). The rapid kinetics and low Ea under microwave irradiation might be because of the facile formation of reactive superoxide radicals (⋅O2−) (different from the conventional heating where non-radical mechanism prevails) that could be suggested from the oxidations in the presence of radical scavengers and electron spin resonance experiments. The recyclability of the catalyst, after regeneration from oxidation reactions under MW, was also secured, supporting the possible application of W2N@C in removing of thiophenics (especially under MW) from fuel. Finally, MW heating, compared with conventional electric heating, could be recommended as a highly effective strategy for ODS with rapid kinetics and low Ea because of the ready formation of active radicals.