(265a) Sensitivity of Surface Structures Towards Hydrodeoxygenation of Propanoic Acid over Palladium Catalysts

Green diesel, a second generation oxygen-free biomass derived compound with identical calorific value and engine compatibility like fossil diesel, can be produced from triglycerides through catalytic hydrodeoxygenation (HDO), which is an important research area awaiting breakthroughs. Due to regeneration difficulty and short life expectancy of conventional catalysts, development of novel catalysts with appropriate surface architecture is essential for the HDO reactions of biomass conversion.

Two reaction mechanisms - decarbonylation (DCN) and decarboxylation (DCX) have been investigated for the HDO of long-chain carboxylic acid representatives such as propanoic acid. As each metal surface architecture shows distinct d-band congestion and center location (Catalysis Letters 1997, 46, 31-35) with respect to the Fermi level, they exhibit different chemisorption energies and binding modes of adsorbates. In our study, the effect of surface structures on the HDO of propanoic acid has been investigated by density functional theory (DFT) and microkinetic models with comprehensive linear lateral interaction calculations over Pd(100) and Pd(111) surface models at an experimental reaction conditions of 473K. DFT data and Bronsted-Evans-Polanyi (BEP) relationship demonstrated high activity of C-H and O-H bond dissociations on Pd(100) and Pd(111), respectively. Our mechanistic study suggests that decarbonylation of propanoic acid on Pd(100) and Pd(111) via dehydrogenation of α and β –carbons followed by C-OH and C-C bond dissociation is favorable. Although energetics favored DCN over DCX under gas phase reaction conditions, it is still difficult to identify a preferred mechanism without developing a mean-field microkinetic model under realistic reaction conditions. Thus, a mean-field microkinetic model was established based on first-principles data and the steady-state species balances of various adsorbed species.

A coverage dependent microkinetic model results an overall turnover frequency about six order of magnitude higher on Pd(100) relative to Pd(111). Dominance of decarbonylation over decarboxylation has been determined by microkinetic model on Pd(100), however, on Pd(111), decarbonylation and decarboxylation are equally competitive. Moreover, our kinetically preferred reaction pathways have been found similar with our mechanistic study. A sensitivity analysis suggested that the C-OH bond cleavage is controlling the overall reaction rate over the Pd(100) and Pd(111) catalyst surfaces. Furthermore, C-CO₂ bond dissociation to CH₃CH₂ and CO₂ was also found to be a crucial step and is partially rate controlling in the decarboxylation mechanism on Pd(111). Overall, our results imply that the HDO of propanoic acid over Pd catalysts is sensitive to surface structure and Pd(100) is comparatively more active.