A thermochemical ladder to biosynthetic fuels: from quantum chemistry downstairs to the liquid-phase energetics of eugenol hydrodeoxygenation as a model for lignin upgrading

The valorisation of lignin is one of the current challenges for science and industry. Studies on eugenol as a model compound for hydrodeoxygenation considerably facilitate the understanding of the general trends in this complex reaction cascade. The thermochemical properties of the starting material eugenol, the end products (alkylcyclopentanes and alkylcyclohexanes) and the intermediate products of the network, which comprises 18 possible reactions, were evaluated. According to the quantitative thermodynamic analysis, none of the 18 reactions are subject to significant thermodynamic limitations. However, the final distribution of reaction products is determined by acidity, textural properties and active phase valence state at constant experimental conditions. Nevertheless, a quantitative understanding of thermodynamics is indispensable to evaluate the magnitude of the equilibrium constant at a particular temperature and to adjust the type and amount of catalyst, time and temperature and to obtain sufficient yields of biosynthetic fuel compounds. The thermodynamic investigation findings were corroborated through the empirical data. The highest selectivity to 2-methoxy-4-propylphenol (side chain hydrogenation) of 79 % over Ru/HNT with low acidity was observed. In contrast, for the Ru/HNT-t catalyst with enhanced Brønsted and Lewis acidity the major products were propylcyclohexane (42 %, complete hydrodeoxygenation) and 4-propylcyclohexanol (20 %, demethoxylation‑hydrogenation) at 180 °C. An increase in the temperature to 210 °C leads to the quantitative conversion of eugenol and an increase in propylcyclohexane selectivity for both catalysts (Ru/HNT – 38.2 %, Ru/HNT-t – 77.9 %). Other parallel routes are consistent with thermodynamic analysis performed in this work. • Eugenol hydrodeoxygenation as a model process for lignin upgrading studied • Gibbs energies for a general network comprising 18 reactions derived and analysed • Thermodynamic findings supported by experiments over Ru on Al/Si nanotubes