Selective Production of 4-Propylphenol from Lignin Oil without Exogenous Hydrogen over a RuNi/NiAl2O4 Catalyst

Recently, the depolymerization of lignocellulosic biomass without exogenous hydrogen sources (named self-hydrogen supplied catalytic fractionation, SCF) was realized under mild conditions, with nearly theoretical yield of lignin phenolic monomers (lignin oil) achieved (Zhou et al., JACS Au, 2023, 3, 1911–1917). Herein, we further developed a new process for the demethoxylation of thus-prepared lignin oil to get 4-propylphenol in the absence of exogenous hydrogen over the RuNi/NiAl2O4 catalyst. In this process, a hint of hydrogen is generated by the splitting of water on the Ni surface, which drives the removal of methoxy groups in lignin oil and produces 4-alkylphenol and methanol, and then the following aqueous phase reforming of methanol supplies more hydrogen and accelerates the entire reaction. Interestingly, water splitting on the Ni surface forms a Ru-NiOx interface, which can improve the C–O bond activation ability and facilitate the removal of methoxy groups. In addition, Ru in Ru-NiOx species is more metallic, which is beneficial for the aqueous phase methanol reforming. Under optimized conditions, 17.0 wt % yield of 4-propylphenol could be acquired in real birch lignin oil. This strategy efficiently produces 4-propylphenol from lignin oil using structural hydrogen in both lignin oil and H2O. The result is a promising and effective approach for lignin oil upgrading.