Efficient depolymerization of alkaline lignin to phenolic monomers over non-precious bimetallic Ni–Fe/CeO₂-Al₂O₃ catalysts

Alkaline lignin is a by-product of the paper manufacturing process that can be processed into valuable aromatic compounds. In this paper, the bimetallic Ni–Fe/CeO₂-Al₂O₃ catalysts with different Ni–Fe ratios were prepared by chemical reduction method to investigate the best Ni–Fe ratio for the depolymerization experiments. The physical and chemical properties of the catalysts were studied by XRD, BET, SEM–EDX, H₂-TPR, NH₃-TPD, and XPS. And the depolymerization products were analyzed by FTIR and GC–MS. The results showed that the highest yield of bio-oil (67.14 wt.%) and the highest quantitative yield of phenolic monomers (8.77 wt.%) of lignin depolymerization products were obtained when the mass ratio of Ni to Fe was 1:1 in the isopropanol reaction system with formic acid as the internal hydrogen donor and the depolymerization reaction at 240 °C for 8 h. Moreover, the catalysts with any Ni–Fe ratios were extremely selective for the orientation of G-type (guaiacol type) phenols (> 98%), indicating that the Ni–Fe/CeO₂-Al₂O₃ catalysts had high ether bonds breaking ability and no hydrogenation of benzene ring, which may be due to the potential synergistic effect of bimetal Ni–Fe and CeO₂-Al₂O₃ support. In addition, comparing the FTIR spectra of lignin before and after the depolymerization reaction, it was found that the depolymerization rate of alkaline lignin was extremely high. Finally, the catalyst cycling experiments were carried, out and it was found that the catalyst still had high stability and depolymerization activity after undergoing seven consecutive cycles of experiments.