How to ‘rip out’ all traces of sulfur in gas and diesel

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Sulfur in fuels creates air pollution when it’s burned. Oil refineries can remove a lot of it, but no all.

Simulations show a new method could reduce sulfur to a fraction of that amount. Another potential bonus: it could extend the life of your vehicle’s catalytic converter.

“The next step is figuring out how to streamline the process and make it work on an industrial scale,” says lead author Anton Toutov, a graduate student in the lab of Robert Grubbs, a chemistry professor at the California Institute of Technology.

The new method, described in in the journal Nature Energy, 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.

“We were really surprised how well the potassium salt worked,” says Toutov. “The sulfur is contained in small organic molecules, and this process just rips it right out.”

Potassium surprise

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.

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Next, Toutov 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.

“They left the metal out of the reaction, and it still worked,” says Grubbs. “This was a huge surprise.”

From 8 ppm to 2 ppm

As described in the 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 refinement process to get rid of the last traces of sulfur in fuels. The next step for Toutov, who is co-founding a new company, Fuzionaire, is to commercialize this technology.

“We have a number of ideas in mind on how to do that,” he says, including recycling waste products from other industries for use in the process.

BP, the Resnick Sustainability Institute at Caltech, Dow Chemical Company, Bristol-Myers Squibb, and the Natural Sciences and Engineering Research Council of Canada provided support. Additional coauthors are from UCLA, Nanjing University, and ETH Zürich.

Source: Caltech