Microwave Assisted Depolymerization of Alkaline Lignin over Hydrotalcite-Based CuNiAl Mixed Oxides

In this study, microwave assisted depolymerization of alkaline lignin into bio-oil was investigated in the presence of CuNiAl hydrotalcite based solid base catalysts in methanol. The effects of catalysts, reaction temperature, time, and heating methods were all studied in detail to improve the bio-oil yield. The promotion effect of microwave heating and Cu amount in the CuNiAl based catalysts was obviously observed during degradation of lignin, leading to improved bio-oil yield and lower molecular weight. The highest yield of bio-oil was up to 60.1%, when the depolymerization was conducted at 160 °C for 80 min over CuNiAl based catalyst (metal ratio of 1.5:4.5:2). The monomers and oligomers were identified by GC-MS and MALDI-TOF MS, finding p-hydroxyacetophenone (H2), guaiacol (G1), p-hydroxyacetovanillon (G5), and syringaldehyde (S3) as the main monophenols in bio-oil. The possible chemical structures for oligomers with molecular weight of 288 m/z, 306 m/z, 316 m/z, 330 m/z, 412 m/z, 426 m/z, and 456 m/z were obtained, and plausible depolymerization pathways were proposed in order to facilitate the understanding of possible relationship for the formation of prominent oligomers and monomers. This study confirmed that heterogeneous base catalysts coupled with microwave heating could provide a promising technique to convert lignin to liquid fuels.