Evaluating the catalytic performance of various metal oxides in lignin depolymerization: Focus on Bi2O3 for bio-oil production

Catalytic hydrogenolysis of lignin to high value-added chemicals has been a hot topic in recent years. Metal oxide catalysts are prepared by the co-precipitation method. Bi₂O₃ exhibits unique advantages in catalyzing lignin depolymerization for bio-oil production, demonstrating high selectivity in cleaving ether bonds within the lignin structure.The purpose of this study is to compare the performance of bismuth metal with several other metals for the catalytic hydrogenolysis of lignin. Based on the response surface methodology (RSM), the catalytic abilities of several metal oxides for the preparation of bio-oil from lignin under optimal conditions were determined as follows: NiO (77.18 wt %) ≈ Bi₂O₃ (77.04 wt %) > Co₃O₄ (71.12 wt %) > CeO₂ (61.39 wt %); However, comparable performance can be achieved with significantly lower amounts of Bi₂O₃ catalysts. Gas chromatography-mass spectrometry (GC–MS) analysis demonstrated that Bi₂O₃-catalyzed product contained 66.45 wt % yield of phenolic compounds, significantly higher than those catalyzed by other metal oxides. Notably, the combined yield of creosol (G2) and vanillin (G3) reached 21.36 wt %, representing the highest value among all catalysts tested. Scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR) spectra showed that the components in the lignin depolymerization system were uniformly dispersed in the presence of Bi₂O₃ catalyst, which contributed to the cleavage of ether bonds and promoted the generation of a large amount of phenolic compounds. The interactive effects of temperature, time and catalyst dosage on the catalytic hydrogenolysis of lignin and the effect of the addition of the hydrogen donor, formic acid, on the depolymerization were also analyzed. Finally, the recoverability test showed that the catalytic performance of Bi₂O₃ remained active and stable after five consecutive recovery runs.