Insights into depolymerization pathways and mechanism of alkali lignin over a Ni1.2–ZrO2/WO3/γ-Al2O3 catalyst

This study performed catalytic depolymerization of alkali lignin over Ni-based catalysts. Effects of different promoters (Zr and W), Ni loadings, reaction temperatures, and the addition of formic acid and catalyst on lignin conversion and products distribution were all investigated. The result showed that the highest oil yield (40.1% (mass)) was obtained at 240 °C over Ni1.2/γ-Al2O3 promoted by Zr and W species. Quantitative analysis indicates that Zr and W species prefer to lignin depolymerization while Ni active phase prefer to hydrodeoxygenation and hydrogenation. The interconversion of products derived from lignin depolymerization was determined by gas chromatography-mass spectrometer, which demonstrated that phenolic compounds were dominant products in all lignin derived bio-oils, wherein the proportion of vanillin was highest (65.7%) at 180 °C, while that of alkyl guaiacols increased with the increase of temperature (from 12.45% at 180 °C to 66.67% at 240 °C). Residual lignin obtained after lignin depolymerization was also investigated for detecting differences on functional groups, wherein the disappearing peaks at 1511 cm−1 (stretching of aromatic rings), 1267, 1215 and 1035 cm−1 (vibrations of guaiacyl and syringyl units) were detected by Fourier transform infrared spectrometry. Additionally, the higher O/C ratio measured by elemental analysis also confirmed that alkali lignin was depolymerized effectively under mild conditions.