ZrO<sub>2</sub> Promoted the Depolymerization of Lignin\ninto Phenolic Monomers over 15Ni/SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub>–ZrO<sub>2</sub> Catalyst

Catalytic hydrogenation of lignin is one of the most\neffective\nmethods for valorization of lignin to downstream aromatic compounds.\nHowever, the overhydrogenation of benzene rings during lignin hydrogenation\ncould lower the selectivity of monomer aromatics. In this study, novel\ncomposite catalysts of Ni loaded on Zr-modified SiO<sub>2</sub>–Al<sub>2</sub>O<sub>3</sub> support (Ni/SA-Zr) were prepared by simple wet\nimpregnation methods for the reductive depolymerization of lignin.\nThe as-prepared catalyst exhibited high catalytic activity to achieve\n66.7% yield of lignin oil and 60.5% lignin monomer. Also, the phenolic\nselectivity was up to 80.1% under optimal conditions (catalyst 15Ni/SA-Zr,\n260 °C, 2 MPa H<sub>2</sub>, 4 h). The presence of ZrO<sub>2</sub> not only improved the catalytic activity of the Ni/SA-Zr catalyst\nbut also inhibited the hydrogenation of the benzene ring. The characterization\nof the Ni/SA-Zr catalyst showed that the introduction of ZrO<sub>2</sub> was beneficial to the dispersion of metallic Ni on the surface of\nthe catalyst. In addition, the interaction between the Ni metal and\nthe support was weakened due to the presence of ZrO<sub>2</sub>, which\nwas believed to be the main reason for the high activity of the Ni/SA-Zr\ncatalyst in the lignin depolymerization. The results of X-ray photoelectron\nspectrometry analysis showed that electron-rich Ni species favored\nthe adsorption of hydrogen. However, excess ZrO<sub>2</sub> would\nreduce the activity of the catalyst. The two-dimensional <sup>13</sup>C–<sup>1</sup>H heteronuclear single quantum correlation-nuclear\nmagnetic resonance characterization of lignin samples before and after\nthe depolymerization reaction confirmed that lignin was converted\nto monomeric phenols effectively. Based on the reaction path of the\nlignin model compound 2-phenoxy-1-phenylethanol, a possible reaction\npath of lignin depolymerization was proposed.