Rather than just burning natural gas as a fuel, a mild new method could turn it into a valuable chemical commodity.

A bespoke copper catalyst could slash the energy penalty of one of the most foundational processes in the chemical industry. The catalyst can harness sunlight to selectively oxidize methane, the main component of natural gas, into formaldehyde, a versatile chemical feedstock from which high-value products from polymers to pharmaceuticals can be made.

Despite its natural abundance, methane poses challenges as a chemical feedstock. “It’s difficult to liquefy, has high transportation costs, and cannot be used directly as a chemical raw material,” says Chengyang Feng, a researcher in the sustainable energy advanced catalysis group of Huabin Zhang, who led the research. “Large amounts of methane are directly flared or vented in gas and oil fields, leading to resource wastage and environmental issues,” Zhang adds.

Where methane is still used for chemical production, it is converted first into a more reactive intermediate called syngas: but this energy-intensive conversion requires high temperatures and pressures.

An alternative, mild method of methane conversion could be used to react it with oxygen from the air, in a process powered by sunlight, to create formaldehyde. “Converting methane to formaldehyde transforms the gas into a valuable liquid that is already used widely in the chemical and pharmaceutical industries,” Feng says. “This reduces the difficulty of storage and transportation and improves economic returns.”

The challenge of photo-catalytically reacting oxygen with methane is that it typically generates a mixture of products, including methanol and even carbon dioxide, as well as formaldehyde. The reaction outcome critically depends on the oxygen activation step. Formaldehyde is formed when the oxygen activation generates reactive species called hydroperoxyl radicals.

To promote hydroperoxyl radical generation and formaldehyde production, the team designed a photocatalyst based on a material called a metal-organic framework (MOF). At the nanoscale, these porous crystalline substances consist of metal sites held together with carbon-based organic linkers in a highly regular repeating pattern. By changing the metals and the linkers that the material is made from, different molecular architectures can be accessed.

The researchers created MOFs in which the metal sites were fully bonded to the linkers, forming a chemically inert backbone. “When single copper atoms are then specifically anchored within this framework, these sites become the sole active centers in the catalyst, enabling precise modulation of the reaction pathway,” says Zhang.

Read more at KAUST Discovery.