First-principles mechanistic study of ring hydrogenation and deoxygenation reactions of eugenol over Ru(0001) catalysts

Abstract

Catalytic hydrogenation and deoxygenation reactions of ligno-cellulosic (LC) biomass resources are the main valorisation routes of bio-based oxygenates in bio-refineries. Eugenol is a model compound for lignin, which is besides cellulose and hemicellulose a major constituent of wood. Hydrodeoxygenation of eugenol on a Ru(0001) catalyst and in bulk was investigated by plane wave density functional theory (DFT). The adsorption of fragments (benzene, propylene, methanol…) was studied, where several adsorption modes were found. A complex pathway framework, accounting for all elementary steps, was developed and analysed to obtain activation barriers and reaction energies. Additionally, the kinetics of the fragment recombination into methane and water were assessed. To investigate mobility of smaller species, surface diffusion was also modelled explicitly. We found out that homogenously, i.e. without ruthenium, only hydrogenation of allyl functional group proceeded under high hydrogen pressure. On the ruthenium catalyst, however, two predominant cascades were plausible; the hydrogen addition to the substituted benzene ring, followed by oxygen-containing substituent elimination was slightly more favourable in the experiments, while calculations demonstrated that the order of processes can also be reversed. Aromatic ring hydrogenation was found to be endothermic with the rate-determining step (RDS) barriers of approximately 1.0 eV. Methoxyl and hydroxyl were cleaved off differently, depending on aromaticity. With cyclohexane, methoxy, dihydroxy, and hydroxy intermediate moieties were formed, whereas on the aromatic ring (hydro)deoxygenation proceeds via dihydroxy and hydroxyl precursor components. Aryl CO bond cleavage requires 1.8 eV, while for alkyls scission proceeds more readily with activation energies as low as 0.5 eV. Ultimately, completely hydrogenated propyl-cyclohexane is produced. The carbon radical (CHx∗) combination into CH4 is fast enough to avoid coke deposition poisoning, while the formation of H2O is much slower.