Theoretical investigation of the hydrodeoxygenation of methyl propionate over Pd (111) model surfaces

Esters are one of the key components of lipid-rich biomass feedstocks that are potential raw materials for production of green fuels. We present a thorough density functional theory and microkinetic modeling study of the hydrodeoxygenation (HDO) of organic esters over Pd (111) model surfaces. Methyl propionate was chosen as our model molecule since it permits the study of the effect of both alpha- and beta-carbon dehydrogenations on the HDO of esters while still being computationally accessible. An extensive network of elementary reactions was investigated and a microkinetic model was developed at reaction conditions of 473 K, a methyl propionate partial pressure of 0.01 bar, and a hydrogen partial pressure of 0.2 bar to identify the dominant pathway and abundant surface species. Our microkinetic model suggests that decarbonylation pathways of methyl propionate are favored over decarboxylation pathways. We found the most dominant pathway to involve methyl propionate to involve two dehydrogenation steps of both alpha- and beta-carbons to form CH2CHCOOCH3, followed by C-O and C-C cleavages to produce C-2 hydrocarbons and methoxy that eventually get hydrogenated to ethane and methanol (CH3CH2COOCH3 -> CH3CHCOOCH3 -> CH2CHCOOCH3 -> CH2CHCO + OCH3 -> center dot center dot center dot -> CH3CH3 + CO + CH3OH). The most abundant surface intermediates were identified to be H and CO and CH3C. Finally, a sensitivity analysis of our models suggests that the dehydrogenation of the alpha-carbon of methyl propionate, as well as propanoyl-methoxy bond dissociation control the overall rate on Pd (111).