Taking the sulfur out of fuel in gentler conditions
When fuel containing sulfur burns, it releases sulfur oxides, which contribute to acid rain and poor air quality. Modern standards such as Euro V and Euro VI therefore limit sulfur in fuel to around 10 parts per million.
The problem with today's approach
Refineries usually remove sulfur with a process called hydrodesulfurisation. It works well for many compounds, but it runs at around 300 to 380 °C under high-pressure hydrogen. Even then, some sulfur molecules are hard to remove because their shape stops them from reaching the catalyst. Dibenzothiophene is a well-known example.
A different route
We study oxidative desulfurisation, which takes another path. Instead of using hydrogen, it uses an oxidant to convert sulfur molecules into a form that is much easier to separate from the fuel. Our catalysts can also be driven by light.
Our catalysts use porous supports with very regular channels, which help sulfur molecules reach the active sites. Some of these supports can be made from halloysite, a natural clay, rather than expensive specialist chemicals.
What we found
In one study, a silver-based catalyst on a support made from halloysite removed 98.66% of dibenzothiophene from a model fuel. The reaction ran under sunlight at 70 °C, using hydrogen peroxide as the oxidant, and the catalyst stayed stable over four cycles.
Why it matters
This could one day complement existing refinery processes by removing the sulfur they leave behind, without extra hydrogen or high pressure. So far, this has been shown on model fuels in the laboratory. Testing on real fuels and at larger scale would be the important next steps.
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