A team of researchers have been able to convert methane to methanol using light and diffuse transition metals such as copper in a process known as photo-oxidation. The response was the best to date for converting methane gas to liquid fuel under ambient temperature and pressure conditions (25 °C and 1 bar respectively).
The term “bar” as a unit of pressure comes from the Greek word for “weight” (baros). One bar is equivalent to 100,000 pascals (100 kPa), which is very close to standard atmospheric pressure at sea level (101,325 Pa).
The results of the study are an important step towards making natural gas available as an energy source for the production of alternative fuels compared to gasoline and diesel. Although natural gas is considered a fossil fuel, converting it to methanol releases less carbon dioxide (CO2) than other liquid fuels in the same category.
In Brazil, methanol plays a key role in the production of biodiesel and in the chemical industry, which uses it to synthesize many products.
In addition, capturing methane from the atmosphere is critical to mitigating the adverse effects of climate change, as the potential of the gas is 25 times greater than that of CO2, for example, to contribute to global warming.
“There is a big debate in the scientific community about the size of the methane reserves on the planet. By some estimates, they could have twice the energy potential of all other fossil fuels combined. When switching to renewable energy, at some point we will have to use all this methane,” said Marcos da Silva, first author of the paper, Agência FAPESP. Silva is a doctoral student in the Department of Physics at the Federal University of San Carlos (UFSCar).
According to Ivo Freitas Teixeira, professor at UFSCar, Silva’s supervisor and last author of the paper, the photocatalyst used in the study was a key innovation. “Our group has made significant innovations by oxidizing methane in one step,” he said. “In the chemical industry, this transformation takes place by producing hydrogen and CO2 in at least two stages and under conditions of very high temperature and pressure. Our success in getting methanol under mild conditions with less energy is a big step forward.”
Teixeira said the results pave the way for future research into using solar energy for this conversion process, potentially further reducing its environmental impact.
Photocatalysts
In the lab, scientists synthesized crystalline carbon nitride in the form of polyheptazinimide (PHI) using base or earth-rich transition metals, especially copper, to produce active visible light photocatalysts.
They then used photocatalysts in methane oxidation reactions with hydrogen peroxide as an initiator. The copper-PHI catalyst generated a large volume of oxygenated liquid products, especially methanol (2900 µmol per gram of material, or µmol g-1 over four hours).
“We found a better catalyst and other conditions needed for a chemical reaction, such as using a lot of water and only a small amount of hydrogen peroxide, which is an oxidizing agent,” Teixeira said. “The next steps include a deeper understanding of the copper active sites in the material and their role in the reaction. We also plan to use oxygen directly to produce hydrogen peroxide in the reaction itself. If successful, this should make the process even safer and more cost effective.”
Another issue that the group will continue to explore concerns copper. “We work with dispersed copper. When we wrote the article, we did not know if we were dealing with isolated atoms or clusters. Now we know that these are clusters,” he explained.
The scientists used pure methane in the study, but in the future they will be extracting the gas from renewable energy sources such as biomass. So far, methane has caused about 30% of global warming since the pre-industrial era, according to the UN. Human methane emissions could be reduced by as much as 45% in the coming decade, avoiding an increase of nearly 0.3°C by 2045.
The strategy of converting methane to liquid fuel using a photocatalyst is new and not commercially available, but its potential in the near future is significant. “We started our study over four years ago. We now have much better results than Prof. Hutchings and his group did in 2017, which motivated our own study,” Teixeira said, referring to the study published in the journal. The science researchers affiliated with universities in the US and the UK, led by Graham Hutchings, Professor at Cardiff University in Wales.