Conversion of Pyrolytic Lignin to Arenes and Cycloalkanes in Consecutive Ethanol and Hydrocarbon Solvents

Abstract

Pyrolytic lignin was first depolymerized in ethanol to produce low-molecular-weight phenolic compounds, followed by complete hydrodeoxygenation in hydrocarbon solvents, achieving a total yield of 21 wt % of arenes and cycloalkanes (arenes/cycloalkanes molar ratio = 1:5) using a 15% Ni/γ-Al₂O₃ catalyst. Ethanol efficiently dissolves and depolymerizes pyrolytic lignin with or without an external hydrogen source. Through etherification and alkylation, ethanol molecules are added to the side chains of the benzene rings, increasing the carbon length of the final products. However, under harsher conditions, ethanol can produce unwanted side products. It can also compete with phenolic intermediates for catalytically active sites, thereby hindering effective hydrodeoxygenation. In contrast, inert hydrocarbon solvents, which lack oxygen-containing functional groups and thus do not adsorb onto the catalyst active sites, were found to be ideal for promoting complete hydrodeoxygenation. Notably, n-hexadecane provided a higher carbon yield compared to hexane and dodecane due to its liquid state at 300 °C and 2 MPa pressure. Catalysts supported on acidic materials, such as Ni/γ-Al₂O₃, demonstrated higher efficacy in promoting depolymerization and suppressing undesirable condensation reactions compared to basic supports like Pt–Ni/MgO. This two-step approach successfully produced oxygen-free products, which significantly reduces separation costs compared to conventional single-step methods. Future research could focus on optimizing mixed solvent systems that combine the beneficial properties of both ethanol and hydrocarbon solvents to balance solubility and selectivity, thereby improving overall process efficiency, reducing catalyst deactivation, and enhancing product selectivity.