Carbon dioxide (CO2) is a greenhouse gas and it plays a vital role in climate change; it is no wonder Researchers have been searching for solutions to prevent its release into the environment. Although it is a cheap, non-toxic and readily available carbon source, there have been efforts in the past few years to convert carbon dioxide into ‘value-added’ products, or valuable wares.
For example, carbon dioxide allows energy storage by reacting with hydrogen gas - known as the hydrogenation process - in order to transform the mixture into higher energy liquid compounds such as methanol that can be transported easily and used as fuel for cars. In the same way, carbon dioxide hydrogenation in the presence of other chemicals can cause the formation of several value-added products such as formaldehyde, formic acid or formamides that are widely used in industry. These chemicals can also potentially be used for energy storage as, for instance, heating formic acid under certain conditions to allow for the release of hydrogen gas in a controlled and reversible fashion.
The conversion of carbon dioxide into useful products is much complicated because CO2 is the most oxidized state of carbon and by itself a very stable and unreactive molecule. Therefore, the direct reaction between CO2 and hydrogen requires high energy, and as a result, makes the process economically unfavorable. This obstacle can be overcome by using catalysts, which are compounds used in small quantities to speed up chemical reactions.
Most known catalysts are based on precious metals such as ruthenium, rhodium, or iridium for CO2 hydrogenation purposes. As outstanding catalysts, the lack of these precious metals makes it difficult to use them at industrial scales. These metals are also difficult to recycle and potentially toxic for the environment. Other catalysts use inexpensive metals such as cobalt or iron but need a phosphorus-based molecule – known as phosphine - surrounding the metal. Phosphines are not stable around oxygen at all times and sometimes they burn violently in an air atmosphere, which poses another problem for the practical applications.
In order to overcome these problems, the OIST Coordination Chemistry and Catalysis Unit headed by Prof. Julia Khusnutdinova reported in ACS Catalysis innovative and efficient catalysts based on an abundant and inexpensive metal: manganese. Manganese is the third most abundant metal available in Earth’s crust after titanium and iron, and has much lower toxicity than many other metals used in CO2 hydrogenation.
Initially, the Researchers searched for inspiration within the natural world: hydrogenation is a reaction that occurs in various organisms that would not have access to phosphines or precious metals. They examined the structure of specific enzymes – hydrogenases – in order to understand how they could accomplish hydrogenation with the use of simple, Earth-abundant materials. Enzymes utilize a ‘smart’ arrangement where the surrounding organic framework collaborates with a metal atom – like iron – efficiently starting the reaction to facilitate the hydrogenation.
After looking at hydrogenases, we wanted to check if we could make artificial molecules that mimics these enzymes using the same type of common materials, like iron and manganese.
Dr. Abhishek Dubey, the First Author of the study
The key challenge of this research was to create an adequate frame – called a ligand - around the manganese in order to induce the hydrogenation. The Researchers found a simple ligand structure that resembles natural hydrogenase enzymes with a twist from typical phosphine catalysts.
“In most cases, ligands support the metal without directly taking part in a chemical bond activation. In our case, we believe the ligand directly participates in the reaction,” said Dr. Dubey.
The structure of a ligand is strongly linked to its efficiency in ligand design. The new catalyst – the ligand and the manganese together – can achieve over 6,000 turnovers in a hydrogenation reaction and convert over 6,000 times CO2 molecules before decaying. This new ligand, the result of collaboration between an international team including Dr. Robert Fayzullin from Russia and Prof. Carlo Nervi and Mr. Luca Nencini from University of Turin in Italy, is simple to produce and stable in the air.
At present, the catalyst is able to turn carbon dioxide into formic acid, a commonly-used food preservative and tanning agent, and formamide, which has industrial applications. However, the flexibility of this catalyst opens many other possibilities.
Our next goal is to utilize such structurally simple, inexpensive manganese catalysts to target other types of reactions in which CO2 and hydrogen can be converted into useful organic chemicals.
Prof. Julia Khusnutdinova, Head of the Catalysis Unit