An iridium catalyst for the base-free dehydrogenation of neat formic acid

Increasing strict measures on carbon emissions has highly favored the development of low-carbon technologies-based energy sources. Among the available renewable energy sources, hydrogen has the great potential to reduce carbon emission in both mobile and distributed energy applications. Unfortunately, deploying hydrogen as a clean energy source has been hindered by several challenges among them being hydrogen storage and large-scale utilization.

Recently chemical storage of hydrogen has been developed in the form of formic acid and now considered a promising solution for stationary hydrogen storage due to its stability and less hazardous issues. In particular, the dehydrogenation of formic acid has been given special attention.

Several heterogeneous and homogeneous catalysts for formic acid dehydrogenation have been explored. However, due to their low activity some of these catalysts are not suitable for dehydrogenation of formic acid while more efficient catalysts require base additives and/or diluted conditions hence hampering their practical implementation. Alternatively, iridium-based homogeneous catalysts have been developed for this purpose. Generally, they can efficiently operate in water and have a high potential application as compared to other homogeneous catalysts. On the other hand, base-free conditions for the dehydrogenation of pure and aqueous formic acid are required to enhance the interest of this catalytic process. Unfortunately, this area has not been fully explored.

To this note, researchers at University of Rennes: Shengdong Wang, Haiyun Huang, Dr. Christian Bruneau, Dr. Thierry Roisnel (X-ray Crystallographer) and Dr. Cédric Fischmeister synthesized iridium catalysts for base-free dehydrogenation of pure formic acid. The research team intended to contribute to the world of knowledge about dehydrogenation of aqueous and neat formic acid for use in hydrogen energy generation. Their research work is currently published in the research journal, ChemSusChem.

In brief, the research team synthesized electron-rich iridium(III)Cp*(dipyridylamine) catalysts and cross-examined base-free conditions suitable for aqueous and neat formic acid dehydrogenation. Additional consideration was taken to detect the presence of any carbon monoxide during the catalysis process that would prevent potential application in fuel-cells. Additionally, considering the high stability of the catalysts, its latent behavior and potential for semi-continuous mode application was investigated.

The authors observed that the electron-enriched catalysts were suitable for efficient dehydrogenation of both aqueous and neat formic acid in base-free conditions. This was attributed to high stability of the catalyst under acidic conditions and to hydrogen-bonding assisted formic acid activation. For instance, TOF of 13292 h^-1 was reported at a temperature of 100 ^°C for the dehydrogenation of neat formic acid. Furthermore, the iridium(III) complexes exhibited latent behavior with potential practical applications and the reaction did not generate undesired CO (below 1 ppm).

In summary, the University of Rennes scientists successfully demonstrated dehydrogenation of both aqueous and neat formic acid using iridium complex catalysts which are considered to be among the most efficient catalysts. Altogether, the study will advance exploration and generation of hydrogen energy as an alternative source of renewable energy. Also, it will pave the way for investigation of the catalytic mechanism to unleash their full potential.