Investigation on the Catalytic Hydrogenolysis of Lignin over NbOₓ–Ni/ZnO–Al₂O₃

The catalytic hydrogenolysis of lignin was an important route to depolymerize lignin into aromatic monomer products and other high added value chemicals. Herein, the catalytic hydrogenolysis of poplar lignin was carefully studied in the presence of Nbₘ–Niₙ/ZnO–Al₂O₃ catalysts in methanol solvent. The physicochemical properties of Nbₘ–Niₙ/ZnO–Al₂O₃ catalysts were characterized by N₂ adsorption–desorption, X-ray diffraction, H₂-temperature programmed reduction, transmission electron microscopy, high-angle annular dark-field imaging–scanning transmission electron microscopy, and X-ray photon spectroscopy techniques. The structural properties of the raw lignin, oligomers, and monomers were detected by 2D heteronuclear single-quantum coherence NMR, gas chromatography–mass spectroscopy, and gel permeation chromatography. The Nb₂–Ni₁/ZnO–Al₂O₃ (Nb/Ni ratio was 2:1) showed the most attractive lignin depolymerization performance, with an 87.1 wt % bio-oil yield and 22.4 wt % phenolic monomer yield. 2-(2-Methoxyphenoxy)-1-(4-methoxyphenyl)ethanone was employed as model compound to deeply study the mechanism of NbOₓ–Ni/ZnO–Al₂O₃ catalyzed lignin hydrogenolysis. It was found that the introduction of the Nb species was in favor of breaking the β-O-4 bond and avoiding the excessive hydrogenation of aromatic rings.