Semi-Batch Hydrotreatment of Lignin-Derived Phenolic Compounds over Raney-Ni with a Continuous Regeneration of the H-Donor Solvent

Lignin can be converted into useful precursors of fuels and fine chemicals by thermochemical conversion followed by catalytic hydrogenation using metal catalysts at severe reaction conditions. Thus, mild hydrogenation would significantly improve the sustainability of lignin valorization. Here, hydrogenation of phenols, alkylphenols, and methoxyphenols was achieved at mild reaction conditions (70 °C and atmospheric pressure) via H-transfer hydrogenation over Raney-Ni catalyst in 2-propanol and 2-butanol solvents. The transfer hydrogenation was feasible at the mild conditions, but the complexity of the reactant greatly decreased or even completely suppressed its reactivity. The position of the functional group (o-, m-, p-position) had a great effect on the reactivity of phenols. Moreover, 2-butanol enhanced the conversion of phenols in comparison with 2-propanol. When comparing classic hydrogenation with H-transfer hydrogenation in presence of external H₂ , it was found that external H₂ not only regenerated H-donor solvent and ensured stable performance but also increased conversion of phenols and alkylphenols. On the other hand, the absence of external H₂ boosted the conversion of methoxy phenols. Finally, phenols extracted from a pyrolysis oil aqueous phase were hydrogenated. The conversion of phenols was greatly affected by competitive adsorption of different compounds present in the reaction mixture. External H₂ promoted hydrogenation of the complex reaction mixture and prevented condensation of the reactive species in contrast to the H-transfer hydrogenation.