Low-temperature hydrogenation of butanal proceeding via keto-enol tautomerization

Abstract

Hydrogenation of carbonyl-containing compounds to alcohols is a crucial step in various technological processes, including reversible green hydrogen storage. Activation of the highly stable C=O bond, however, is highly challenging and typically proceeds under harsh reaction conditions. Recent theoretical predictions on low-barrier hydrogenation pathway of carbonyl compounds suggest that this step proceeding over a high activation barrier can be overcome in an alternative low barrier process, which comprises two steps: keto-enol tautomerization followed by hydrogenation of the C=C bond in the newly formed enol. The major challenge of this predicted pathway is the low stability of enol species, which would readily convert to ketone if not intentionally stabilized.In this study, we performed atomistic-level investigation on keto-enol tautomerization and low-temperature hydrogenation of butanal over well-defined Pd(111) model catalyst by combination of surface sensitive infrared reflection absorption spectroscopy (IRAS) and molecular beam techniques. Specifically we show that different types of enol species can be formed on the H-containing Pd(111) surface, whose appearance and abundance strongly dependent on the surface temperature. Importantly, the evolution of the partly hydrogenated product of butanal – C-butoxy species attached to Pd via C atom – was detected already at cryogenic temperatures, which points to a possibility of low temperature hydrogenation. A very clear correlation between the abundance of the enol species and the C-butoxy hydrogenated product was detected: the growing concentration of enol species was found to result in the formation of the C-butoxy product. In contrast, strong decrease in the abundance of enol adsorbates lead to full vanishing of the hydrogenation product C-butoxy. With this, we experimentally show a strong link between the population of the enol species and hydrogenation of butanal at cryogenic temperatures. Finally, we discuss the chemical nature of different enol species, which appear to be stabilized on the surface in different ways via hydrogen bonding, and propose the reaction mechanisms of low-barrier butanal hydrogenation that are consistent with all experimental observations.Obtained atomistic-level insights in low-barrier hydrogenation of carbonyl compounds provide important information on the elementary reaction steps of this potentially highly promising class of reactions for green hydrogen storage. Related phenomena can be employed for controlling keto-enol tautomerization and enol stabilization via mutual lateral interactions on functionalized catalytic surfaces for the broad range of aldehydes and ketones to enable their low-barrier hydrogenation.