BP, Caltech reduce sulphur from petrol and diesel to 2ppm
Scientists at the California Institute of Technology (Caltech), BP, UCLA, ETH Zürich and China’s Nanjing University have developed a method for reducing levels of sulphur in oil and diesel to ~2ppm, according to a paper published in the journal Nature Energy. It reveals how Caltech graduate student Anton Toutov and colleagues at BP optimised the use of potassium tert-butoxide to remove almost all of the sulphur content in diesel fuel.The next task will be "figuring out how to streamline the process and make it work on an industrial scale", Toutov said.
Sulfur compounds in fuels such as gasoline and diesel create air pollution when the fuel is burned. To address that challenge, large-scale hydrodesulfurization (HDS) at refineries remove the majority of sulfur from fuel down to a government-mandated level. The new technique, however, has the potential to reduce sulfur down to a fraction of that amount, which would further reduce air pollution and extend the lifetime of vehicles’ catalytic converters, which control tailpipe emissions. A paper on their work is published in the journal Nature Energy.

Scientists led by a team at Caltech and BP, and in collaboration with researchers at UCLA, ETH Zürich, and China’s Nanjing University, have developed a new method for potentially removing nearly all sulfur compounds (down to ~2 ppm) from gas and diesel fuel. The method uses Earth-abundant materials (potassium (K), oxygen (O), and silicon (Si)—hence its name, “KOSi”) and operates under mild conditions.

Currently, HDS is performed by treating petroleum with H 2 at high pressures and temperatures (that is, 150–2,250 psi and 400 ˚C) over heterogeneous catalysts such as cobalt-doped molybdenum sulfide supported on alumina … However, certain organosulfur species…are not efficiently removed. Homogeneous strategies employing sophisticated, well-defined transition metal complexes—including those based on platinum, nickel, tungsten, molybdenum, palladium, ruthenium, rhodium, iron, cobalt, and others—have been extensively investigated. While these studies have provided valuable mechanistic insights, several fundamental issues … generally restrict industrial implementation of such methods. … These issues pose a formidable challenge for the development of new HDS methods. Moreover, increasingly strict governmental regulations require limiting the sulfur content in diesel fuel and gasoline (in the US: typically <15 and <30 ppm, respectively) as well as other fuels, rendering the development of new powerful HDS methods a primary global concern.

Herein, we report that the robust KOt Bu/silane-based (that is, KOSi) system is a powerful and effective homogeneous HDS method, which desulfurizes HDS-resistant dibenzothiophenes in good yield and reduces the sulfur content in diesel fuel to remarkably low levels. —Toutov et al.

The new method uses a potassium salt to induce the chemical reactions required to remove sulfur from fuel. Potassium is an abundant element on Earth and cheaper and more environmentally friendly to use than rare metal catalysts that are used for similar reactions.

The discovery that potassium salts can be used to promote key chemical reactions came unexpectedly a couple of years ago. Researchers in the Grubbs laboratory had been testing ways to break carbon-oxygen bonds, which is most efficient when done with a precious metal catalyst such as platinum. Alexey Fedorov of ETH Zürich, who was a postdoctoral fellow in the Grubbs laboratory at the time, ran a control experiment without the metal catalyst and found that the reaction still worked.

After several tests, the researchers confirmed that a potassium salt, called potassium tert-butoxide, was, in fact, driving the reaction. Next, Anton Toutov, a graduate student in the lab of Robert Grubbs, the Victor and Elizabeth Atkins Professor of Chemistry at Caltech, optimized the process and further showed that the reaction produced compounds with carbon-silicon bonds, which normally require metal catalysts to form. Carbon-silicon bonds are found in many products, such as polymers, agricultural chemicals, and semiconductors.

As described in the Nature Energy paper, Toutov and his colleagues in the Grubbs lab have used the potassium salt method to remove sulfur from carbon compounds found in diesel fuel. They partnered with BP to test their method on the company’s refined diesel samples, reducing the sulfur levels down from 8 parts per million (comparable to the highest quality of diesel you can get from a typical gas pump today) to an extremely low 2 parts per million. They also repeated the experiment with diesel spiked with high levels of sulfur and achieved similar results.

The new method could be used as an additional step in the oil refining process to get rid of the last traces of sulfur in fuels.

We were really surprised how well the potassium salt worked. The sulfur is contained in small organic molecules, and this process just rips it right out. The next step is figuring out how to streamline the process and make it work on an industrial scale. —Anton Toutov

Toutov is co-founding a new company, Fuzionaire, to commercialize this technology.

The research was funded by BP. Support for Toutov was also provided by the Resnick Sustainability Institute at Caltech, Dow Chemical Company, Bristol-Myers Squibb, and the Natural Sciences and Engineering Research Council of Canada.

Resources