New approach plucks more sulfur from diesel fuel

An international industry-academic research team has reported a new approach to removing sulfur from diesel fuel that could improve vehicle performance and reduce the environmental impact of using fossil fuels. Gasoline and diesel contain residual amounts of natural organosulfur compounds that can foul catalytic converters and generate polluting sulfur dioxide. Refineries employ heterogeneous catalysts such as cobalt-doped molybdenum sulfide that use hydrogen at high temperature and pressure to strip out as much sulfur as possible from the fuel. But the most recalcitrant compounds, in particular alkylated dibenzothiophenes, are mostly left untouched. Researchers led by BP’s John W. Shabaker, UCLA’s Kendall N. Houk, and Caltech’s Robert H. Grubbs have devised a noncatalytic solution-based process that employs potassium tert-butoxide and alkylsilanes under mild conditions to create silyl radicals that cleave the C–S bond of the heterocyclic compounds, with the silicon whisking away the sulfur. The researchers show that the method, which they call KOSi, can reduce sulfur in diesel fuel to about 2 ppm, well below the current target of 15 ppm (Nat. Energy 2017, DOI: 10.1038/nenergy.2017.8). The researchers note that their proof-of-concept process will need to be adapted to function as a continuous refinery process, but they envision it could become a “polishing treatment” to supplement current desulfurization processes.