Theoretical investigation of the decarboxylation and decarbonylation mechanism of propanoic acid over a Ru(0 0 0 1) model surface

Abstract

The hydrodeoxygenation of organic acids is often found to be a rate-controlling process during upgrading of biomass feedstocks into fuels. We developed a microkinetic model based on data obtained from density functional theory calculations for the decarboxylation and decarbonylation mechanisms of propanoic acid (CH3CH2COOH) over a Ru(0001) model surface. The model predicts that the decarbonylation mechanism is two orders of magnitude faster than the decarboxylation mechanism. The most favorable decarbonylation pathway proceeds via removal of the acid –OH group to produce propanoyl (CH3CH2CO) followed by C–CO bond scission of propanoyl to produce CH3CH2 and CO. Finally, CH3CH2 is hydrogenated to CH3CH3. Dehydrogenation reactions that have been observed to be important over Pd catalysts play no role over Ru(0001), and a sensitivity analysis indicates that removal of the acid –OH group is the rate-controlling step in the deoxygenation. Overall, our results suggest that to improve the Ru catalyst performance for the decarbonylation of organic acids, the free site coverage needs to be increased by, for example, adding a catalyst promoter that decreases the hydrogen and CO adsorption strength (without significantly affecting the C–OH bond scission rate), or by raising the reaction temperature and operating at relatively low CO and H2 partial pressures.