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Recyclable reagent and sunlight convert carbon monoxide to methanol

Scientists from the U.S. Department of Energy’s (DOE) Brookhaven National Laboratory and the University of North Carolina Chapel Hill (UNC) have demonstrated the selective conversion of carbon dioxide (CO2) to methanol using a cascade reaction strategy. The two-part process is powered by sunlight, takes place at room temperature and ambient pressure, and uses a recyclable organic reagent similar to a catalyst found in natural photosynthesis.

“Our approach is an important step toward finding an efficient way to convert CO2a powerful greenhouse gas that poses a significant challenge to humanity, into an easily storable and transportable liquid fuel,” said Brookhaven Lab senior chemist Javier Concepcion, lead author of the study.

The research was carried out within the framework of Center for Hybrid Approaches from Solar to Liquid Fuels (CHASE), an energy innovation center based at UNC and funded by the DOE Office of Science. The study is published as a “cover” article in the Journal of the American Chemical Society.

CO conversion at room temperature2 in liquid fuels is a decades-long quest. Such strategies could help achieve carbon-neutral energy cycles, particularly if the conversion is powered by sunlight. Carbon emitted in the form of CO2 By burning single-carbon fuel molecules such as methanol, they could essentially be recycled to make new fuel without adding new carbon to the atmosphere.

Methanol (CH3OH) is a particularly attractive target because it is an easily transportable and storable liquid. In addition to its usefulness as a fuel, methanol serves as a key raw material in the chemical industry to make more complex molecules. Additionally, since methanol contains only one carbon atom, like CO2this avoids the need to create carbon-carbon bonds, which require energy-intensive processes.

However, the key steps involved in the reactions necessary to selectively and efficiently generate solar liquid fuels like methanol remain poorly understood.

“CO conversion2 methanol is very difficult to achieve in a single step. Energy-wise, this is equivalent to climbing a very high mountain,” Concepcion said. “Even though the valley on the other side is at a lower altitude, getting there requires a lot of energy.”

Instead of trying to tackle the challenge in a single “climb,” the Brookhaven/UNC team used a cascade, or multi-step, strategy that works through several easier-to-reach intermediates.

“Imagine climbing several small mountains instead of one big one — and doing it across several valleys,” Concepion said.

The valleys represent the reaction intermediates. But even reaching these valleys can be difficult, because it requires a gradual exchange of electrons and protons between various molecules. To reduce the energy requirements of these exchanges, chemists use molecules called catalysts.

Catalysts allow reaching the next valley through “tunnels” that require less energy than climbing over the mountain,” Concepcion said.

For this study, the team explored reactions using a class of catalysts called dihydrobenzimidazoles. These are organic hydrides – molecules that have two extra electrons and a proton to “donate” to other molecules. They are inexpensive, their properties can be easily manipulated, and previous studies have shown that they can be recycled, a necessary condition for a catalytic process.

These molecules are similar in structure and function to organic cofactors responsible for transporting and delivering energy in the form of electrons and protons during natural photosynthesis.

“Photosynthesis itself is a cascade of many reaction steps that convert atmospheric CO into2, water and light energy into chemical energy in the form of carbohydrates – namely sugars – which can then be metabolized to fuel the activity of living organisms. Our approach of using biomimetic organic hydrides to catalyze methanol as a liquid fuel can therefore be considered an artificial approach to photosynthesis,” said co-lead author Renato Sampaio from UNC.

In study, chemists broke down CO conversion2 in methanol in two stages: photochemical reduction of CO2 to carbon monoxide (CO), followed by sequential hydride transfers of the dihydrobenzimidazoles to convert the CO to methanol.

Their work describes the details of the second stage, as the reaction proceeds through a series of intermediates, including a ruthenium-bonded carbon monoxide (Ru-CO2+), a formyl ruthenium (Ru-CHO+), a ruthenium hydroxymethyl group (Ru-CH2OH+), and finally, light-induced methanol release.

While the first two steps of this scheme are “dark reactions”, the third step which results in the production of free methanol is initiated by the absorption of light by ruthenium hydroxymethyl (Ru-CH2OH+) complex. The proposed mechanism by which this occurs is an excited-state electron transfer between the Ru-CH2OH+ and an organic hydride molecule followed quickly by crushed proton transfer which results in the generation of methanol in solution.

“The one-pot, selective nature of this reaction results in the generation of millimolar (mM) concentrations of methanol – the same concentration range as the starting materials – and avoids the complications that have hampered previous efforts to utilize materials inorganic. catalysts for these reactions,” said Gerald Meyer, UNC co-author and director of CHASE. “This work can therefore be considered an important step in the use of renewable organic hydrides. catalysts looking for decades of catalytic room temperature methanol production from CO2.”

This research was supported by the Center for Hybrid Approaches in Solar Energy to Liquid Fuels (HUNTING), an energy innovation hub funded by the DOE Office of Science.

Brookhaven National Laboratory is supported by the Office of Science of the U.S. Department of Energy. The Office of Science is the largest supporter of basic research in the physical sciences in the United States and strives to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

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