A team of researchers has successfully converted methane into methanol using light and scattered transition metals such as copper in a process known as photo-oxidation. The reaction was the best achieved to date for converting methane gas into liquid fuel at ambient temperature and pressure (25 °C and 1 bar, respectively), according to a study published in the journal Chemical Communications.
The term bar as a pressure unit comes from the Greek word meaning weight (baros). One bar equals 100,000 Pascals (100 kPa), close to the standard atmospheric pressure at sea level (101,325 Pa).
The study’s findings are a crucial step toward making natural gas accessible as a source of energy for the production of alternative fuels to gasoline and diesel. Despite the fact that natural gas is a fossil fuel, its conversion into methanol produces less carbon dioxide (CO2) than other liquid fuels in the same category.
Methanol is vital in biodiesel production and the chemical industry in Brazil, where it is used to synthesize a variety of products.
Furthermore, methane collection from the atmosphere is critical for mitigating the negative consequences of climate change since the gas has 25 times the potential to contribute to global warming as CO2, for example.
“There’s a great debate in the scientific community about the size of the planet’s methane reserves. According to some estimates, they may have double the energy potential of all other fossil fuels combined. In the transition to renewables, we’ll have to tap into all this methane at some point,” Marcos da Silva, first author of the article, told Agência FAPESP. Silva is a Ph.D. candidate in the Physics Department of the Federal University of São Carlos (UFSCar).
The study was supported by FAPESP, the Higher Research Council (CAPES, an agency of the Ministry of Education), and the National Council for Scientific and Technological Development (CNPq, an arm of the Ministry of Science, Technology, and Innovation).
According to Ivo Freitas Teixeira, a professor at UFSCar, Silva’s thesis advisor and the last author of the article, the photocatalyst used in the study was a key innovation. “Our group innovated significantly by oxidizing methane in a single stage,” he said. “In the chemical industry, this conversion occurs via the production of hydrogen and CO2 in at least two stages and under very high temperature and pressure conditions. Our success in obtaining methanol under mild conditions, while also expending less energy, is a major step forward.”
According to Teixeira, the results pave the way for future research into the use of solar energy for this conversion process, potentially reducing its environmental impact still further.
In the laboratory, the scientists synthesized crystalline carbon nitride in the form of polyheptazine imide (PHI), using non-noble or earth-abundant transition metals, especially copper, to produce active visible-light photocatalysts.
They then used the 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 (2,900 micromoles per gram of material, or µmol.g-1 in four hours).
“We discovered the best catalyst and other conditions essential to the chemical reaction, such as using a large amount of water and only a small amount of hydrogen peroxide, which is an oxidizing agent,” Teixeira said. “The next steps include understanding more about the active copper 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 economically viable.”
Another point the group will continue to investigate relates to copper. “We work with dispersed copper. When we wrote the article, we didn’t know whether we were dealing with isolated atoms or clusters. We now know they’re clusters,” he explained.
In the study, the scientists used pure methane, but in the future, they will extract the gas from renewables such as biomass.
According to the United Nations, methane has so far caused about 30% of global warming since the pre-industrial age. Methane emissions from human activity could be reduced by as much as 45% in the decade ahead, avoiding a rise of almost 0.3°C by 2045.
The strategy of converting methane into liquid fuel using a photocatalyst is new and not available commercially, but its potential in the near term is significant. “We began our research over four years ago. We now have far better results than those of Professor Hutchings and his group in 2017, which motivated our own research,” Teixeira said, referring to a study published in the journal Science by researchers affiliated with universities in the United States and the United Kingdom, and led by Graham Hutchings, a professor at Cardiff University in Wales.
“Selective methane photooxidation into methanol under mild conditions promoted by highly dispersed Cu atoms on crystalline carbon nitrides” by Marcos A. R. da Silva, Jéssica C. Gil, Nadezda V. Tarakina, Gelson T. S. T. Silva, José B. G. Filho, Klaus Krambrock, Markus Antonietti, Caue Ribeiro and Ivo F. Teixeira, 31 May 2022, Chemical Communications.
“Aqueous Au-Pd colloids catalyze selective CH4 oxidation to CH3OH with O2 under mild conditions” by Nishtha Agarwal, Simon J. Freakley, Rebecca U. McVicker, Sultan M. Althahban, Nikolaos Dimitratos, Qian He, David J. Morgan, Robert L. Jenkins, David J. Willock, Stuart H. Taylor, Christopher J. Kiely and Graham J. Hutchings, 7 September 2017, Science.
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