A team of researchers at the University of Southern California has developed a robust, reusable iridium catalyst that enables hydrogen gas release from neat formic acid. The catalyst works under mild conditions in the presence of air, is highly selective and affords millions of turnover numbers (TONs).
Although other catalysts exist for both formic acid dehydrogenation and carbon dioxide reduction, solutions to date on hydrogen gas release rely on volatile components that reduce the weight content of stored hydrogen and/or introduce fuel cell poisons; this new catalyst does not. An open-access paper on their work is published in the journal Nature Communications.
Formic acid (HCO2H, FA) is a hydrogen carrier; it can release hydrogen under mild conditions with only CO2 as a by-product. The CO2 can be recycled, in principle, yielding a carbon-neutral fuel cycle.
To date, many efficient heterogeneous and homogeneous catalysts for FA dehydrogenation have been developed. Heterogeneous catalysts have advantages of separability and reusability, while homogeneous catalysts are generally more efficient. The best turnover numbers (TONs) achieved in homogeneous catalysis are (1) >1M, by a catalyst system composed of [RuCl2(benzene)]2, the ligand diphenylphosphinoethane and a FA/Et3N adduct as substrate developed by Boddien et al. and 983,642, by a system composed of an iron pincer complex and LiBF4 developed by Bielinski et al. The highest turnover frequency achieved is 228,000 h−1 by an iridium catalyst developed by Hull et al.
… In heterogeneous catalysis, the highest TOF achieved is 7,256 h−1, by palladium nanoparticles immobilized on carbon nanospheres developed by Zhu et al. Also, homogeneous catalysts generally are more selective, producing less carbon monoxide, a common byproduct of FA dehydrogenation. This is essential, because CO is a fuel cell catalyst poison. Still, no known system is stable and reactive through multiple uses, air and water tolerant, selective against CO formation, and functions in neat FA liquid. Each of these is critical to achieving a usable hydrogen generation system based on FA. Herein we report an iridium-based catalytic system that meets all of these criteria.
… This … is the first known homogeneous system, to the best of our knowledge, to operate in neat FA, thus enabling far greater weight content of H2 release than any other known catalyst for FA dehydrogenation. Moreover, it is the highest turnover system, because, in part, it can be re-used directly with FA substrate that is not rigorously purified or dried.—Celaje et al.
The reaction is operationally simple. Liquid FA is added to the catalyst in a reactor, and heated. Upon completion, the catalyst system remains as a pale-colored precipitate at the bottom of the vessel for re-use. The iridium catalyst delivers very high TONs at low loading with repeated re-use.
The reaction requires base as co-catalyst, but the source of the base is not specific. Furthermore, water does not affect the rate of dehydrogenation significantly.
The catalysts are also air stable. Although dehydrogenation is slower when the catalysts are prepared in air, the system remains active.
The team is currently performing more detailed mechanistic studies, including computational investigation and ligand variation, on the catalyst.
The study was funded by the National Science Foundation and the Hydrocarbon Research Foundation.
Jeff Joseph A. Celaje, Zhiyao Lu, Elyse A. Kedzie, Nicholas J. Terrile, Jonathan N. Lo & Travis J. Williams (2016) “A prolific catalyst for dehydrogenation of neat formic acid” Nature Communications 7, Article number: 11308 doi: 10.1038/ncomms11308