Catalytic pathways and mechanistic consequences of water during vapor phase hydrogenation of butanal on Ru/SiO2

Kinetic assessments, Fourier-transform infrared spectroscopy, and H/D kinetic and isotopic exchange studies establish the sequence of H-addition steps and reveal how water induces a shift in their kinetic relevance during butanal hydrogenation on Ru/SiO2. Without water, hydrogenation turnovers are slower than the H/D exchange rates at the carbonyl carbon and increase linearly with butanal and H-2 pressures. Thus, hydrogenation occurs via quasi-equilibrated H-addition to the carbonyl carbon, which forms an alkoxy intermediate, before the second, kinetically relevant H-addition to the oxygen. Water vapor does not alter C-H formation rates but promotes the O-H formation rates by stabilizing the transition state through hydrogen bonds, making the first H-addition that forms the C-H bond kinetically relevant, thus decreasing the apparent order to H-2. This study provides the mechanistic context behind the water promotional effects during butanal hydrogenation, as the chemical potential of water increases and hydrogen bonding prevails and mediates catalysis. (C) 2020 Elsevier Inc. All rights reserved.