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HIGHLIGHTS
• Formic acid is an appealing hydrogen source that can easily be handled and stored.
• A new catalyst for producing formic acid from carbon dioxide and hydrogen was developed that is based on a manganese complex prepared with hemilabile donors that contains pendant arms.
• One of the manganese-based catalysts displays a superior turnover frequency and turnover number compared to most state-of-the-art first-row or precious-metal catalysts.

Finding new strategies for more cost effectively and sustainably producing hydrogen remains a goal of researchers as the current cost of producing and storing this material remains prohibitively high. Hydrogen is used in fuel cells, which are considered an attractive power source particularly for heavy duty vehicles (such as trucks) because the alternative, batteries, is not desirable due to the need to use a large number, adding extra weight that can limit payload capacity and driving range. 

One other benefit of using fuel cells to power trucks is their potential to support the electric grid during periods of downtime. A previous TLT article¹ discussed a theoretical study for how idle trucks powered by fuel cells may be able to support the electric grid in the Canadian province of Alberta. Hydrogen produced either from an onsite electrolyzer situated at a vehicle-to-grid hydrogen fuel station or from market hydrogen is used to enable trucks to generate electricity. The researchers found that at least four fuel cell-powered trucks are needed to enable the electricity generation process to be profitable. 

Finding a cost effective, easy-to-handle source of hydrogen continues to restrict the growth of fuel cells. One potential source of hydrogen is the organic carrier formic acid. Dr. Justin Wedal, postdoctoral researcher at Yale University in New Haven, Conn., says, “Formic acid is produced from several petrochemical pathways. The most common industrial approach is to react methanol with carbon monoxide under basic conditions to form methyl formate, followed by hydrolysis. A second synthetic route is to hydrogenate carbon dioxide. This latter method is not typically used since it is energy intensive, requiring high temperatures and pressures. Metal catalysts can, in principle, lower the energy costs of this process.”

Formic acid is an appealing hydrogen source because it can easily be handled and stored. The catalytic hydrogenate of carbon dioxide has been explored using precious-metal homogeneous catalysts such as iridium, rhodium and palladium. Iridium is one of the metal catalysts that shows promise, but there is concern about this catalyst’s high cost, limited abundance and relative toxicity. 

First-row transition metal catalysts are lower in cost and more readily available. In particular, complexes of iron, manganese and cobalt have been found to exhibit potential in converting carbon dioxide and hydrogen to formic acid. The problem has been that these catalysts are not durable and tend to deactivate after only a short period of time.

One class of iron complexes that has shown potential are those prepared with PNP (phosphorus, nitrogen, phosphorus) pincer ligands. Wedal says, “Transition metal complexes with this type of ligand have been found to activate carbon dioxide and be effective in hydrogenation reactions. Pincer ligands act as chelating agents that bind to three adjacent coplanar sites in a ‘north to south’ (meridional) configuration. They can be tailored by changing substituents for specific applications and are thermally stable making them useful in high temperature processes. Iron PNP complexes have been found to be very active in the hydrogenation of carbon dioxide but only have a limited durability before they lose effectiveness. The best current system is only active for approximately four hours.”

Wedal and his colleagues have now worked with the first-row transition metal manganese, in combination with PNP pincer ligands, to produce more durable catalysts that can effectively generate formic acid by hydrogenating carbon dioxide.

Hemilabile ligands
In developing an alternative manganese catalyst, the researchers took lessons from past work conducted with iron complexes. Wedal says, “We determined the reason for performance issues is the presence of an open coordination site on the iron atom.”

To address this issue, the researchers decided to add a hemilabile ligand to an iron complex. Wedal says, “The purpose of a hemilabile ligand is to bind to the metal center to stabilize key reaction intermediates yet be readily displaced by a substrate at the right time to facilitate a specific reaction. We found that the best hemilabile ligands to use are weak Lewis bases such as ethers and tertiary amines.”

The researchers prepared an iron complex with a hemilabile ether donor linked to a two-carbon bridge and coordinated to the metal atom. The result was a catalyst with a longer operating lifetime but still did not provide desired catalyst performance as measured by turnover frequency and turnover number. 

A series of manganese complexes were prepared with hemilabile donors that contained pendant arms. Evaluation of these complexes was conducted by their use as catalysts in reacting a 1:1 ratio of hydrogen to carbon dioxide at elevated temperature. One of the catalysts demonstrated a turnover frequency and turnover number that exceeds most state-of-the-art first-row or precious-metal catalysts. 

Figure 1 shows an image of one of the compounds used in the manganese study. 


Figure 1. One of the compounds isolated as part of the study to prepare a better catalyst for preparing formic acid from carbon dioxide and hydrogen is shown. Figure courtesy of Yale University.

Wedal says, “The mechanism for how this manganese complex hydrogenates carbon dioxide is relatively straight forward. The reaction follows a circular path where first carbon dioxide inserts into the manganese-hydride bond, followed by displacement of formate by hydrogen gas, and finally deprotonation to close the cycle. The catalyst we have developed has a much longer activity period ranging up to 72 hours and surpasses most of the best current catalysts.”

Future work will involve determining how the manganese complex catalyst can be reactivated. In addition, other substrates will be evaluated to better understand the potential versatility of the catalyst. The outcome from this research and the future potential for this catalysis approach have prompted the researchers to apply for a provisional patent.

Additional information on this work can be found in a recent paper² or by contacting Dr. Nilay Hazari, Chair and John Randolph Huffman Professor of Chemistry at Yale University at nilay.hazari@yale.edu. 

REFERENCES
1. Canter, N. (2024), “Feasibility of fuel cell powered trucks to support electric grid,” TLT 80 (4), pp. 14-15. Available at https://www.stle.org/files/TLTArchives/2024/04_April/Tech_Beat_II.aspx. 
2. Wedal, J., Virtue, K., Bernskoetter, W., Hazari, N., Mercado, B., and Piekut, N. (2026), “Improving productivity and stability for CO2 hydrogenation by using pincer-ligated Mn complexes with hemilabile ligands,” Chem, 12, 102833.