Gas-phase, catalytic hydrodeoxygenation of propanoic acid, over supported group VIII noble metals: Metal and support effects

The catalytic, gas-phase hydrodeoxygenation (HDO) of propanoic acid (PAc) over supported group VIII noble metals (M = Pd, Pt, Rh, Ru, Ni) was studied at 1 atm and 200-400 degrees C. The activity and selectivity as a function of the reaction temperature was investigated. The reaction activity based on the TOF follows the order: Pd > Ru > Pt >Rh >Ni. The reaction over Pd, Pt and Rh catalysts proceeds mainly via decarbonylation (DCN) and decarboxylation (DCX) pathways at each reaction temperature. For Ru and Ni catalysts, while decarbonylation and decarboxylation pathways were predominant at lower temperatures (e.g., 200-250 degrees C), at higher temperatures (>300 degrees C) the formation of diethyl ketone was observed. Additionally, the kinetics of Pd over different supports (carbon, SiO2 and TiO2) were examined. The activity based on the TOF decreases in the following order: Pd/O-2 > Pd/TiO2 > Pd/C. The reaction orders in acid and H-2 were found to be approximately 0.5 and zero, respectively, regardless of the support. The apparent activation energies studied in a temperature range of 200-240 degrees C, were 16.7 0.6, 19.3 1.6 and 11.7 +/- 0.7 kcal/mole for Pd/C, Pd/TiO2 and Pd/SiO2 catalysts, respectively. The selectivity for Pd/C and Pd/SiO2 indicated mainly decarbonylation/decarboxylation and hydrogenation reaction pathways. In contrast, Pd/TiO2 at low temperatures (200 degrees C) could generate decarbonylation and esterification products. (C) 2013 Elsevier B.V. All rights reserved.