An multinational team of researchers has devised a quick and cost-effective process for turning methane, sometimes known as natural gas, into liquid methanol at room temperature and pressure. Visible light is used to drive the conversion in a continuous flow through a photocatalytic material in this method. Scientists from the University of Manchester led the study.
The researchers employed neutron scattering at the VISION instrument at Oak Ridge National Laboratory's Spallation Neutron Source to assist them understand how the process works and how selective it is.
A continuous flow of methane/oxygen-saturated water is passed across a new metal-organic framework (MOF) catalyst in the technique. The MOF is porous and comprises many components that each have a function in absorbing light, transporting electrons, activating and bringing methane and oxygen together. Methanol liquid is readily extracted from water. Such a process is widely referred to as the "holy grail of catalysis," and it is a focus of study funded by the US Department of Energy. The team's results, titled "Direct photo-oxidation of methane to methanol over a mono-iron hydroxyl site," will be published in the journal Nature Materials today (June 30, 2022).
Natural methane is a plentiful and lucrative fuel that is used in stovetops, ovens, furnaces, water heaters, kilns, cars, and turbines. However, methane is volatile and combustible, and it may also be hazardous owing to the difficulties of collecting, transporting, and storing it.
Methane gas is also a powerful greenhouse gas, which is hazardous to the ecosystem when it is released or escapes into the atmosphere. Fossil fuel production and consumption, rotting or burning biomass, such as forest fires, agricultural waste products, landfills, and melting permafrost, are major producers of atmospheric methane.
Excess methane is typically flared or burnt off to decrease its environmental impact. However, the combustion process still creates carbon dioxide, which is a greenhouse gas.
Industry has long sought a cost-effective method of converting methane into methanol, a highly marketable and versatile feedstock used to manufacture a wide range of consumer and industrial products. This would not only assist to cut methane emissions, but it would also give a financial incentive to do so.
Methanol is a more adaptable carbon source than methane and is a liquid that is easily transported. It may be used to manufacture hundreds of different goods, including solvents, antifreeze, and acrylic polymers, synthetic textiles and fibers, adhesives, paint, and plywood, and chemical agents used in medicines and agrichemicals. As global petroleum stocks deplete, the conversion of methane into a high-value fuel such as methanol is becoming increasingly appealing.
Breaking the bond
The difficulty of weakening or breaking the carbon-hydrogen (C-H) chemical link in order to insert an oxygen (O) atom to create a C-OH bond has been a major barrier in converting methane (CH4) to methanol (CH3OH). Traditional methane conversion technologies usually entail two phases, steam reformation followed by syngas oxidation, which are energy intensive, expensive, and inefficient because to the high temperatures and pressures required.
The study team's rapid and cost-effective methane-to-methanol process is powered by a multicomponent MOF material and visible light. While exposed to light, a flow of CH4 and O2 saturated water is passed through a layer of MOF granules. The MOF is made up of several designed components that are positioned and retained in place within the porous superstructure. They operate together to absorb light and create electrons, which are then transported via the pores to oxygen and methane 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 from the light promote the activation of the C-H bond in methane to form methanol,” said Sihai Yang, a professor of chemistry at Manchester and corresponding author. “The process is 100% selective – meaning there is no undesirable by-product – comparable with methane monooxygenase, which is the enzyme in nature for this process.”
The investigations showed that the solid catalyst may be separated, washed, dried, and reused for at least 10 cycles, or around 200 hours of reaction time, without losing performance.
The novel photocatalytic method is similar to how plants transform light energy into chemical energy during photosynthesis. Plants use their leaves to absorb sunlight and carbon dioxide. These components are subsequently converted into sugars, oxygen, and water vapor via a photocatalytic process.
“This process has been termed the ‘holy grail of catalysis.’ Instead of burning methane, it may now be possible to convert the gas directly to methanol, a high-value chemical that can be used to produce biofuels, solvents, pesticides, and fuel additives for vehicles,” noted Martin Schröder, vice president and dean of faculty of science and engineering at Manchester and corresponding author. “This new MOF material may also be capable of facilitating 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.”
Using neutrons to picture 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 C-H bonds,” stated Yongqiang Cheng, instrument scientist at the ORNL Neutron Sciences Directorate.
“VISION is a high-throughput neutron vibrational spectrometer optimized to provide information about molecular structure, chemical bonding and intermolecular interactions,” explained SNS's Chemical Spectroscopy Group leader Anibal "Timmy" Ramirez Cuesta. “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 unambiguously the bond-weakening interactions between CH4 and the MOF with advanced neutron spectroscopy techniques.”
Fast, economical, and reusable
The novel conversion approach might significantly reduce equipment and operational expenses by removing the need for high temperatures or pressures and instead harnessing the energy from sunshine to drive the photo-oxidation process. The process's increased speed and capacity to convert methane to methanol with no unwanted byproducts would aid in the development of cost-effective in-line processing.
Reference: “Direct photo-oxidation of methane to methanol over a mono-iron hydroxyl site” 30 June 2022, Nature Materials.
DOI: 10.1038/s41563-022-01279-1
Funding and resources were provided by the Royal Society; the University of Manchester; the EPSRC National Service for EPR Spectroscopy at Manchester; the European Research Council under the European Union’s Horizon 2020 research and innovation program; the Diamond Light Source at the Harwell Science and Innovation Campus in Oxfordshire; the U.S. Department of Energy’s Spallation Neutron Source at Oak Ridge National Laboratory and the Advanced Photon Source at Argonne National Laboratory; and the Aichi Synchrotron Radiation Centre in Seto City. Computing resources at ORNL were made available through the VirtuES and ICE-MAN projects funded by ORNL’s Laboratory Directed Research and Development program and Compute and Data Environment for Science.
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