Toward Low-Temperature Heterogeneous Hydrogenation of Carbonyl Compounds: Molecular Systems for Reversible Hydrogen Storage

Hydrogenation of carbonyl compounds to alcohols is an important step in many technological applications, including the emerging molecular systems for reversable hydrogen storage. This class of reactions is experimentally highly challenging as it requires the activation of a very stable C═O bond, which typically occurs under harsh experimental conditions. There is an ongoing discussion on an alternative theoretically predicted reaction pathway for C═O bond hydrogenation, comprising two consecutive reaction steps: keto–enol tautomerization of the carbonyl compound to its enol counterpart followed by hydrogen insertion into the C═C bond of enol. Both reaction steps were theoretically predicted to proceed via significantly lower activation barriers than in case of direct hydrogen insertion into the C═O bond. The major challenge of this predicted mechanism is the low stability of enol species that─if not stabilized─would readily convert to ketone under reactive conditions. In this perspective, we summarize our recent atomistic-level investigations on formation of enol species from a carbonyl compound acetophenone and its stabilization via hydrogen bonding with neighboring carbonyls. In this study, we demonstrate the proof-of-principles experiments confirming the possibility of the low-barrier hydrogenation of carbonyls proceeding via keto-enol tautomerization as the first step. We show that the enol species are formed and stabilized on the surface in form of ketone-enol dimers or ketone-enol-enol trimers via strong hydrogen bonding between the C═O group of the ketone and the OH-group of the enol. The enol species in the ketone-enol dimer can be hydrogenated already at the surface temperature of 240 K, suggesting drastically reduced activation barriers as compared to direct hydrogenation of carbonyl compounds. The stabilization of enol species in form of ketone-enol dimers or ketone-enol-enol trimers turns out to be the crucial reaction step, allowing the enol-containing reaction intermediate to enter the second reaction step─hydrogenation of the olefinic bond.