(53a) Kinetic Analysis and Reaction Mechanism for Pentanoic Acid Conversion over Promoted Molybdenum Oxide

Selective activation of C-O bonds in biomass-derived molecules while avoiding subsequent hydrogenation of products is crucial for the efficient generation of hydrocarbon fuels and value-added chemicals. Reducible metal oxides have shown great promise for these selective C-O bond activation, often attributed to a reverse Mars-Krevelen type mechanism. Prasomsri et al. have shown that Molybdenum oxide (MoO₃) can convert linear and aromatic aldehydes and alcohols to olefins and aromatic hydrocarbons at low H₂ pressures [1]. The selective conversion of biomass-derived carboxylic acids over these MoO₃ catalysts, however, has not been reported. Herein we investigate the conversion of pentanoic acid over promoted MoO₃, where 0.05% Pt is added to facilitate hydrogen dissociation. H₂ dissociation on MoO₃ surfaces has been suggested as the rate limiting step during selective deoxygenation [2]. Our findings show that the addition of 0.05 wt% Pt on MoO₃ reduces the kinetic barrier by 32 kJ/mol, suggesting that incorporation of Pt could shift the rate limiting step to creation and regeneration of oxygen vacancies (Fig.1a). In this study, we provide additional insights into mechanism of HDO over MoO₃ by using a pulse reactor combined with density functional theory (DFT) calculations, showing that the reaction energetics and selectivities depend on hydrogen coverage. Fig. 1b shows that reduction of the partial pressure of hydrogen by 50% can increase the activation energy by 20 kJ/mol. To decouple highly active sites at the Pt-Mo interface from spillover induced defects on the Mo surface, we also perform this chemistry using a newly developed technique in our lab where active sites are separated by a controlled distance on a conductive carbon nanotube support [3].

[1] Prasomsri, T. et al. Energy Environ. Sci., 2660-2669 (2014); [2] Shetty, M. et al. J. Phys. Chem. C, 17848-17855 (2017); [3] Briggs, N.M. et al. Nat. Commun 9, 3827 (2018).