Scientists at ETH Zurich have developed a new catalyst that allows for the easy conversion of natural gas constituents into precursors for the production of fuels or complex chemicals, such as polymers or pharmaceuticals.

“We take a molecule of the natural gas constituent methane (CH4) and replace one of its hydrogen atoms with a bromine atom to form methyl bromide (CH3Br),” explains Javier Pérez-Ramírez, professor of catalysis engineering. It is extremely stable and results in fewer unwanted by-products than other catalysts.

Natural gas can be used as a base material for complex chemical compounds. Image credit: EPA.When methyl bromide is transformed into fuels and chemicals, bromine is released in the form of hydrogen bromide (HBr). “The beauty of our reaction is that it enables the bromine from hydrogen bromide to be embedded back into methyl bromide, using oxygen. In this way, the bromine cycle is closed, and no bromine is lost," says Pérez-Ramírez.

The oxybromination of methane, as the reaction is called, can already be carried out today using reaction accelerators. However, they typically generate large quantities of unwanted products. The ETH catalysis specialists, therefore, looked for a way to improve the reaction’s selectivity. In a multi-step selection process, they investigated a large number of different catalyst materials. Vanadium phosphate proved to be the most suitable.

Vanadium phosphate is a relatively mild oxidizing catalyst, which is exactly what chemists are seeking in the oxybromination of methane. On the one hand, the catalyst is sufficiently strong to allow hydrogen bromide to react with oxygen at the catalyst’s surface. On the other hand, the catalytic action of vanadium phosphate is too weak to oxidize the unwanted methane and the brominated reaction products.

“Our method makes it possible to brominate methane in a single step at atmospheric pressure and at temperatures below 500 degrees Celsius. This makes it an attractive route for industry,” says Vladimir Paunović, a doctoral student in Pérez-Ramírez’s group. Currently, methane is industrially converted into higher-grade chemicals using syngas as an intermediate. However, this method is extremely energy-intensive, as it requires high pressures (up to 30 bar) and high temperatures (up to 1,000 degrees).

Moreover, the new catalyst is exceptionally stable. “Bromine is a halogen. Halogens react very readily with methane, which is desirable in this case, but they also attack the catalyst,” explains Paunović. “Our catalyst resists the corrosive reaction environment, which is essential for its potential industrial application.”