The ‘holy grail of catalysis’ – converting methane to methanol under ambient conditions using light

An international team of researchers, led by scientists from the University of Manchester, has developed a rapid and economical method of converting methane, or natural gas, into liquid methanol at ambient temperature and pressure. The method takes place under continuous flow over a photocatalytic material using visible light to drive the conversion.

To help observe how the process works and how selective it is, the researchers used neutron scattering on the VISION instrument at Oak Ridge National Laboratory’s Spallation Neutron Source.

The method involves a continuous flow of methane/oxygen saturated water over a novel metal-organic framework (MOF) catalyst. The MOF is porous and contains several components, each of which plays a role in absorbing light, transferring electrons and activating and combining methane and oxygen. The liquid methanol is easily removed from the water. Such a process is widely regarded as “a holy grail of catalysis” and is an area of ​​research supported by the United States Department of Energy. Details of the team’s findings, titled “Direct photo-oxidation of methane to methanol over a mono-iron hydroxyl site,” are published in Nature materials.

Naturally occurring methane is an abundant and valuable fuel used in furnaces, furnaces, boilers, furnaces, automobiles and turbines. However, methane can also be dangerous due to the difficulty of extracting, transporting and storing it.

Methane gas is also harmful to the environment when released or leaked into the atmosphere, where it is a potent greenhouse gas. Major sources of methane in the atmosphere are the production and use of fossil fuels, decaying or burning biomass such as forest fires, agricultural waste products, landfills and melting permafrost.

Excess methane is usually burned or flared to reduce its environmental impact. However, this combustion process produces carbon dioxide, which is itself a greenhouse gas.

The industry has long sought an economical and efficient way to convert methane to methanol, a highly marketable and versatile feedstock used to make a variety of consumer and industrial products. Not only would this help reduce methane emissions, but it would also provide an economic incentive to do so.

Methanol is a more versatile carbon source than methane and is an easily transportable liquid. It can be used to make thousands of products, such as solvents, antifreeze and acrylic plastics; synthetic fabrics and fibers; adhesives, paint and plywood; and chemical agents used in pharmaceuticals and agrochemicals. The conversion of methane into a high-value fuel such as methanol is also becoming more attractive as petroleum reserves dwindle.

Break the tie

A primary challenge in converting methane (CH₄) to methanol (CH₃OH) has been the difficulty of weakening or breaking the carbon-hydrogen (CH) chemical bond to insert an oxygen atom (O) to form a C-OH bond. Conventional methane conversion methods typically involve two stages, steam reforming followed by syngas oxidation, which are energy intensive, costly and inefficient because they require high temperatures and pressures.

The fast and economical methane-to-methanol process developed by the research team uses a multi-component MOF material and visible light to boost conversion. A stream of CH₄ and O₂ saturated water is led through a layer of the MOF grains under the influence of light. The MOF contains several designed components that are placed and held in fixed positions in the porous superstructure. They work together to absorb light to generate electrons which are passed to oxygen and methane in the pores to form methanol.

“To greatly simplify the process, when methane gas is exposed to the functional MOF material containing mono-iron hydroxyl sites, the activated oxygen molecules and energy of the light promote the activation of the CH bond in methane to form methanol,” said Sihai Yang, a chemistry professor in Manchester and corresponding author. “The process is 100% selective — meaning there’s no unwanted by-product — similar to methane monooxygenase, the enzyme in nature for this process.”

The experiments showed that the solid catalyst can be isolated, washed, dried and reused for at least 10 cycles, or about 200 hours of reaction time, without any loss of performance.

The new photocatalytic process is analogous to how plants convert light energy into chemical energy during photosynthesis. Plants absorb sunlight and carbon dioxide through their leaves. A photocatalytic process then converts these elements into sugars, oxygen and water vapour.

“This process has been called the ‘holy grail of catalysis’. Instead of burning methane, it may now be possible to convert the gas directly into methanol, a high-performance chemical that can be used to make biofuels, solvents, pesticides and fuel additives for vehicles,” said Martin Schröder, vice president and dean of science and engineering in Manchester and corresponding author. “This new MOF material may also enable other types of chemical reactions by serving as a kind of test tube in which we can combine different substances to see how they react.”

Using neutrons to visualize the process

“Using neutron scattering to take ‘pictures’ at the VISION instrument initially confirmed the strong interactions between CH₄ and the mono-iron hydroxyl sites in the MOF that weaken the CH bonds,” said Yongqiang Cheng, instrument scientist at the ORNL Neutron Sciences Directorate.

“VISION is a high-throughput neutron vibration spectrometer optimized to provide information about molecular structure, chemical bonding and intermolecular interactions,” said Anibal “Timmy” Ramirez Cuesta, who leads the Chemical Spectroscopy Group at SNS. “Methane molecules produce strong and characteristic neutron scattering signals through their rotation and vibration, which are also sensitive to the local environment. This allows us to unambiguously identify the bond-weakening interactions between CH₄ and the MOF with advanced neutron spectroscopy techniques.”

Fast, economical and reusable

By eliminating the need for high temperatures or pressures and using the energy of sunlight to drive the photo-oxidation process, the new conversion method could significantly reduce equipment and operating costs. The faster speed of the process and the ability to convert methane to methanol without unwanted by-products will facilitate the development of in-line processing that minimizes costs.