Natural gas, consisting mainly of methane, has a relatively low energy density under ambient conditions. Partial oxidation of methane to methanol lifts the energy density and drives the production of numerous chemicals. An international team of researchers, led by scientists at the University of Manchester, has developed a fast and economical method of converting methane, or natural gas, into liquid methanol at ambient temperature and pressure.
Scientists used visible light to drive the conversion under continuous flow over a photocatalytic material. Using neutron scattering at the VISION instrument, they observed the process’s working and how selective it is.
The method involves a continuous flow of methane/oxygen-saturated water over a novel metal-organic framework (MOF) catalyst. Different components in MOF have a role in absorbing light, transferring electrons, and activating and bringing together methane and oxygen. The liquid methanol is easily extracted from the water. This process has commonly been considered “a holy grail of catalysis.”
The difficulty of weakening or breaking the carbon-hydrogen (C-H) chemical link to introduce oxygen (O) atoms to form a C-OH bond has been a major obstacle in the conversion of methane (CH4) to methanol (CH3OH). Steam reforming and syngas oxidation are typically the two phases of conventional methane conversion processes, which need high temperatures and pressures and are energy-intensive, expensive, and ineffective.
The newly developed process is fast and economical. It uses a multicomponent MOF material and visible light to drive the conversion. While exposed to light, a layer of MOF granules is passed through a flow of CH4 and O2-saturated water. The MOF is made up of various designed elements that are fixedly positioned inside the porous superstructure. Together, they absorb light to create electrons, which are then transferred to oxygen and methane inside the pores to create methanol.
Sihai Yang, a professor of chemistry at Manchester and corresponding author, said, “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 from the light promote the activation of the C-H bond in methane to form methanol. The process is 100% selective – meaning there is no undesirable byproduct – comparable with methane monooxygenase, which is the enzyme in nature for this process.”
The investigations showed no performance loss when the solid catalyst is isolated, cleaned, dried, and reused for at least ten cycles, or roughly 200 hours of reaction time.
The novel photocatalytic method is comparable to how plants use photosynthesis to transform light energy into chemical energy. Through their leaves, plants take in carbon dioxide and sunshine. These substances are subsequently changed into sugars, oxygen, and water vapor by a photocatalytic process.
Martin Schröder, vice president and dean of the faculty of science and engineering at Manchester and corresponding author, said, “This process has been termed the ‘holy grail of catalysis.’ Instead of burning methane, it may be possible to convert the gas directly to methanol. This high-value chemical can be used to produce biofuels, solvents, pesticides, and fuel additives for vehicles. This new MOF material may also facilitate other types of chemical reactions by serving as a sort of test tube in which we can combine different substances to see how they react.”
Yongqiang Cheng, instrument scientist at the ORNL Neutron Sciences Directorate, said, “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 C-H bonds.”
Anibal “Timmy” Ramirez Cuesta, who leads the Chemical Spectroscopy Group at SNS, said, “VISION is a high-throughput neutron vibrational spectrometer optimized to provide information about molecular structure, chemical bonding, and intermolecular interactions. Methane molecules produce strong and characteristic neutron scattering signals from their rotation and vibration, which are also sensitive to the local environment. This enables us to reveal the bond-weakening interactions between CH4 and the MOF unambiguously with advanced neutron spectroscopy techniques.”
The new conversion method could substantially lower equipment and operating costs by eliminating the need for high temperatures or pressures and using the energy from sunlight to drive the photooxidation process. The higher speed of the process and its ability to convert methane to methanol with no undesirable byproducts will facilitate the development of in-line processing that minimizes costs.
- An, B., Li, Z., Wang, Z. et al. Direct photo-oxidation of methane to methanol over a mono-iron hydroxyl site. Nat. Mater. (2022). DOI: 10.1038/s41563-022-01279-1