Mechanistic insights into selective hydrodeoxygenation of lignin-derived β-O-4 linkage to aromatic hydrocarbons in water

The route for selective hydrodeoxygenation of phenethoxybenzene (PEB, which represents the dominant β-O-4 linkage in lignin) to produce benzene and ethylbenzene is realized by employing a multi-functional Ru/sulfate zirconium (Ru/SZ) catalyst in the aqueous phase. One-pot hydrodeoxygenation of PEB is initially cleaved, forming C₆ phenol and C₈ ethylbenzene via the selective cleavage route of the C_aliphatic-O bond (k₁). While the C₈ ethylbenzene is stable against further hydrogenation under the selected conditions, the C₆ phenol intermediate is hydrogenated to cyclohexanol with a rapid rate (k₂), and the sequential steps of cyclohexanol dehydration (k₃) as well as cyclohexene dehydrogenation (k₄) lead to the target benzene formation. A high temperature (240 °C) together with a low hydrogen pressure (8 bar) is essential for achieving such a specific route, suppressing the side-reactions of aromatic hydrogenation. Compared to Ru/SZ, Pd/SZ catalyzes the high rate for cleavage of ether (k₁), but fails in sequential C₆ phenol hydrogenation (k₂) and cyclohexanol dehydration (k₃), and thus is not able to accomplish the complex cascade reaction. Pt/SZ performs poorly in the primary step of ether cleavage (k₁), and therefore, blocks the following sequential steps. The separate kinetics tests on phenol and cyclohexanol hydrodeoxygenation reveal that benzene is not produced by direct hydrogenolysis, but by the dehydration-dehydrogenation route. The high benzene yield from phenol is probably attributed to few surface adsorbed H species retained on Ru/SZ during phenol hydrodeoxygenation, as evidenced by the model reaction of cyclooctene hydrogenation with the used Ru/SZ in the presence of N₂.