Mechanistic Considerations on Homogeneously Catalyzed Formic Acid Dehydrogenation

The dehydrogenation of formic acid permits the production of hydrogen virtually free of carbon monoxide, which is a key requisite for its use in fuel cells. Moreover, HCOOH is a suitable hydrogen carrier, better in several crucial characteristics than other liquid organic hydrogen carriers, namely, low toxicity, high hydrogen content and recyclability (via CO2 hydrogenation). The most successfully employed catalysts are those based on Ir, Ru, Rh, and Fe metal centers. The catalytic cycles through which homogeneous metal complexes operate show a wide mechanistic diversity. The nature of catalytic cycle depends mainly on the nuclearity of the active species, the type of CO2 formation step (-hydride elimination, hydride abstraction, or outer-sphere mechanism), and whether the rate-limiting step relates to CO2 or H-2 formation. Different outer-sphere interactions have been proposed in the literature in order to explain the success of homogeneous catalyst, which seems to be common ground for most of the catalysts so far reported.