ZrO2 Promoted the Depolymerization of Lignin into Phenolic Monomers over 15Ni/SiO2–Al2O3–ZrO2 Catalyst

Catalytic hydrogenation of lignin is one of the most effective methods for valorization of lignin to downstream aromatic compounds. However, the overhydrogenation of benzene rings during lignin hydrogenation could lower the selectivity of monomer aromatics. In this study, novel composite catalysts of Ni loaded on Zr-modified SiO2–Al2O3 support (Ni/SA-Zr) were prepared by simple wet impregnation methods for the reductive depolymerization of lignin. The as-prepared catalyst exhibited high catalytic activity to achieve 66.7% yield of lignin oil and 60.5% lignin monomer. Also, the phenolic selectivity was up to 80.1% under optimal conditions (catalyst 15Ni/SA-Zr, 260 °C, 2 MPa H2, 4 h). The presence of ZrO2 not only improved the catalytic activity of the Ni/SA-Zr catalyst but also inhibited the hydrogenation of the benzene ring. The characterization of the Ni/SA-Zr catalyst showed that the introduction of ZrO2 was beneficial to the dispersion of metallic Ni on the surface of the catalyst. In addition, the interaction between the Ni metal and the support was weakened due to the presence of ZrO2, which was believed to be the main reason for the high activity of the Ni/SA-Zr catalyst in the lignin depolymerization. The results of X-ray photoelectron spectrometry analysis showed that electron-rich Ni species favored the adsorption of hydrogen. However, excess ZrO2 would reduce the activity of the catalyst. The two-dimensional 13C–1H heteronuclear single quantum correlation-nuclear magnetic resonance characterization of lignin samples before and after the depolymerization reaction confirmed that lignin was converted to monomeric phenols effectively. Based on the reaction path of the lignin model compound 2-phenoxy-1-phenylethanol, a possible reaction path of lignin depolymerization was proposed.