Role of Keto–Enol Tautomerization in the Copper-Catalyzed Hydrogenation of Ketones

Hydrogenation of alkoxy intermediates is often rate-limiting in the hydrogenation of carbonyl groups over copper catalysts. Using first-principles microkinetic simulations with coverage-dependent kinetic parameters, we find that an enol hydrogenation pathway, that is, not passing through alkoxy-intermediates, can become dominant, provided the difference in stability between the adsorbed ketone and enol tautomers is below 30 kJmol–1. In this scenario, the surface is covered with spectator alkoxides. When water is added, surface alkoxide species can be protonated by surface water or by hydroxyl groups, and oxygen or hydroxyl hydrogenation becomes rate-limiting. We illustrate this change in mechanism for acetol hydrogenation, where the dominant reaction pathway shifts from enol, to aldehyde, and ultimately to ketone hydrogenation with increasing water:acetol feed ratio.