(616g) Effect of Surface Coverage on the Reduction of Carboxylic Acids on MoO3

Reduction of carboxylic acids to aldehydes is a promising route for upgrading oxygenates into fuels and commodity chemicals. Recently, the selective hydrodeoxygenation of carboxylic acids to aldehydes has been carried out using conventional noble metal catalysts under high hydrogen pressures. Prasomsri et al. demonstrated MoO₃ as a selective hydrodeoxygenation catalyst of aldehydes and alcohols[1]. However, the challenge with using MoO3 for the selective reduction of oxygenates include the harsh conditions required for the reduction of the catalyst surface. In this study, we show the selective HDO of pentanoic acid to pentanal under atmospheric pressure of hydrogen on MoO₃ in the presence of low loadings of Platinum particles. We report the role of hydrogen and hydroxyl groups coverage on MoO₃ surface in the overall energetics of the reaction. Specifically, by combining experimental and first-principles density functional theory calculations, we show that the rate limiting step energetics are highly influenced by surface coverage.

Green et al. studied the hydrodeoxygenation of acetone to propylene on α-MoO₃ (010). Their results indicate that hydrogen dissociation is the rate limiting step in the catalytic cycle (170 kJ/mol)[2]. However, our experimental results reveal that Pt addition can easily accelerate the hydrogen dissociation and oxygen vacancies formation on MoO₃. Fig.1c shows the energetics for the elementary steps involved in the formation of PAL on a clean MoO₃ surface. Following the adsorption of acid on an oxygen defect, a carboxylate intermediate is formed after hydrogen abstraction. The formation of a physiosorbed Pentanal occurs following a hydrogen assisted deoxygenation, which is an endothermic step with an apparent barrier of 96 kJ/mol. In contrast, increasing Hydrogen and hydroxyl groups coverage on the surface alters the overall energetics of the reaction towards lower activation energies.

1. Prasomsri, T. et al. Energy Environ. Sci. 2013.
2. Shetty, M. et al. Phys. Chem. 2017.