(214e) Theoretical Investigation of Reaction Pathways of the Deoxygenation of Propanoic Acid to Propane Over Pd(111) Model Surfaces

Hydrotreatment of triglycerides such as vegetable oils and animal fats to green diesel constitutes a promising alternative route for liquid transportation fuel production to the widely studied transesterification of triglycerides with methanol or ethanol. To better understand the reaction mechanism of the deoxygenation of organic acids and ester and to ultimately rationally design novel catalytic materials for the heterogeneous catalytic hydrodeoxygenation of triglycerides from various biomass sources, we studied the reaction mechanism of the deoxygenation of propanoic acid to propane on Pd catalysts. Particularly, first-principles electronic structure calculations based on density functional theory have been employed to examine the catalytic deoxygenation process of propanoic acid to propane over Pd(111) model surfaces. Various hydrodeoxygenation (HDO), decarboxylation (DCX), and decarbonylation (DCN) pathways have been investigated. Transition states for each elementary reaction step have been identified by combining the nudged elastic band method with the dimer method. The most favorable pathway for the HDO consists of the following steps: propanoic acid looses the hydroxyl group to form propanyl (activation barrier: 132 kJ/mol); propanyl is hydrogenated to propanol; propanol looses the hydroxyl group to form propyl (activation barrier: 181 kJ/mol); propyl is hydrogenated to propane. The two dehydroxylation steps, propanoic acid to propanyl and propanol to propyl, are found to be rate limiting under high H2 partial pressure process conditions.