Reaction insights help make sustainable liquid fuels
Methanol, produced from carbon dioxide in the air, can be used to make carbon neutral fuels. But to do this, the mechanism by which methanol is converted to liquid hydrocarbons needs to be better understood so that the catalytic process can be optimized. Now, researchers from ETH Zurich and Paul Scherrer Institute have gained an unprecedented understanding of this complex mechanism using advanced analytical techniques.
As we struggle to combine the impact of emissions with our desire to maintain our energy-hungry lifestyles, using carbon dioxide in the atmosphere to create new fuels is an exciting, carbon neutral alternative. One way to do this is to make methanol from carbon dioxide in the air, using a process called hydrogenation. This methanol can then be converted into hydrocarbons. Although these are then burned, releasing carbon dioxide, this is offset by carbon dioxide which is captured to make the fuel.
Fully developing this sustainable fuel requires a deeper understanding of the mechanism by which methanol is converted into long-chain hydrocarbons in a reaction catalyzed by zeolites, solid materials with unique porous architectures. With this in mind, researchers from ETH Zurich, within the framework of NCCR Catalysis, a Swiss National Research Center for Research, have joined forces with researchers at the Paul Scherrer Institut PSI to reveal the details of this reaction mechanism, whose findings have been published. in the news Nature Catalysis†
“Information is key to developing more selective and stable catalysts,” explains Javier Pérez-Ramírez, professor of Catalysis Engineering at ETH Zurich and director of NCCR Catalysis, who co-led the study. “Before our study, despite many efforts, the key mechanistic aspects of the complex transformation of methanol into hydrocarbons were not well understood.”
The researchers were interested in comparing the process from methanol to hydrocarbon with another process: that of converting methyl chloride into hydrocarbons. Oil refineries often burn large amounts of unwanted methane-rich natural gas. This polluting and wasteful activity results in the typical flares associated with oil refineries. “Converting methyl chloride into hydrocarbons is a kind of bridging technology,” explains Pérez-Ramírez. “Of course we want to get rid of fossil fuels, but in the meantime this would be a way to avoid wasting the huge reserves of valuable methane.”
Volatile gas phase molecules tell the story
The key to understanding complex reaction mechanisms like this is to detect the different species involved, including the intermediates. Traditional techniques look directly at the surface of the catalyst to understand the reaction, but an important part of the story is told by gas phase molecules coming off the catalyst.
“These molecules are often highly reactive and have a very short lifespan, decomposing within milliseconds. This makes identifying them a real challenge, because traditional gas phase analysis methods are simply too slow,” explains Patrick Hemberger, a scientist at the vacuum ultraviolet (VUV) beamline of the Swiss light source SLS, whose advanced analytical techniques would allow the researchers to study the reaction as it took place.
At the VUV beamline, Photoion Photoelectron Coincidence (PEPICO) spectroscopy has recently been established as a powerful analytical tool in catalytic reactions. It combines two different analytical techniques, photoelectron spectroscopy and mass spectrometry, to provide detailed information about the gas phase reaction intermediates, and even allows differentiation between isomers.
“Because we collect two different types of information at the same time, we can quickly identify these volatile species, even in a mixture containing up to a hundred reaction intermediates and products. This gives us an unprecedented insight that is simply not possible with conventional methods,” Hemberger says.
Reaction Paths Revealed
The spectroscopy allowed the researchers to reveal how the carbon-carbon bonds form and the hydrocarbon chain grows by detecting numerous intermediates. For the two processes — methanol to hydrocarbon and methyl chloride to hydrocarbon — the researchers observed different reaction intermediates taking place. From this, they were able to identify two different reaction pathways, one driven by methyl radicals, present in both reactions, and another driven by oxygenated species, so-called ketenes, which only occurred in the reaction of methanol to hydrocarbon.
The researchers were also able to understand an interesting feature of the reactions: After a few days, the catalyst was deactivated and the reaction stopped. This was due to the build-up of an unwanted by-product: coke, which is made from large aromatic hydrocarbons deposited during the reaction.
Using another spectroscopic technique, electron paramagnetic resonance spectroscopy, the researchers saw that the production of methyl chloride into hydrocarbon was much more susceptible to coking than production from methanol. Armed with knowledge of the reaction pathways, the reason for this difference was clear: “The methanol-to-hydrocarbon pathway proceeds along two reaction pathways, while the methyl chloride-to-hydrocarbon pathway can only follow the more reactive methyl radical pathway, which is more prone to coke formation”, explains Gunnar Jeschke, whose team at ETH Zurich conducted the electron paramagnetic resonance spectroscopy studies.
Understanding the mechanism to optimize the process
The insight that this research provides is essential for the future development of liquid fuels in a sustainable way. This may involve finding ways to improve the oxygen-driven pathway, thereby suppressing coke formation.
“We now have a deeper understanding of the reaction mechanism from methanol to hydrocarbons or methyl chloride to hydrocarbons and with this knowledge we can specifically optimize the industrial process to make it more efficient,” Hemberger adds.
Researchers reveal oxygenate-based pathways in syngas conversion over oxide-zeolite bifunctional catalysts
Alessia Cesarini et al, Elucidation of radical and oxygenate driven pathways in zeolite catalyzed conversion of methanol and methyl chloride to hydrocarbons, Nature Catalysis (2022). DOI: 10.1038/s41929-022-00808-0
Quote: Response insights help create sustainable liquid fuels (2022, June 27) retrieved June 27, 2022 from https://phys.org/news/2022-06-reaction-insights-sustainable-liquid-fuels.html
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