Ultra-deep desulfurization of diesel by selective oxidation with [C18H37N(CH3)3]4[H2NaPW10O36] catalyst assembled in emulsion droplets

The oxidation of sulfur-containing compounds (benzothiophene, dibenzothiophene, and their derivatives) in diesel was conducted in emulsion oxidative system (water in oil [W/O]) composed of diesel, 30 wt% hydrogen peroxide, and an amphiphilic catalyst [C18H37N(CH3)3]4[H2NaPW10O36] under mild conditions. The amphiphilic catalyst in the W/O emulsion system exhibits very high catalytic activity such that all sulfur-containing compounds in either model or actual diesel can be selectively oxidized into their corresponding sulfones using hydrogen peroxide (O/S ⩽ 3) as an oxidant. The catalytic oxidation reactivity of sulfur-containing compounds was found to be in the following order: benzothiophene < 5-methylbenzothiophene < dibenzothiophene < 4,6-dimethyldibenzothiophene. Although benzothiophene is relatively difficult to oxidize, it can be efficiently oxidized in the emulsion system. The sulfones can be readily separated from diesel by an extractant. The sulfur level of a prehydrotreated diesel can be lowered from 500 to 0.1 ppm after oxidation and then extraction, whereas the sulfur level of a straight-run diesel can be decreased from 6000 to 30 ppm after oxidation and extraction.