Liquefaction of wheat straw alkali lignin under microwave irradiation

The effects of liquefying agent, catalyst, reaction temperature and time on the yield of bio-oil in microwave assisted liquefaction of wheat straw alkali lignin were investigated. And the liquefaction products were characterized by FT-IR, GC-MS and ¹H-NMR. It is found that microwave irradiation greatly shortens liquefaction time with methanol as liquefying agent and ferric sulfate as catalyst, where the yield of bio-oil is up to 55.22% at the relatively low liquefaction temperature of 160 ℃ for 5 min. The structure of residual lignin after liquefaction exhibits very limited variation, indicating that the degraded lignin fragments have lower probability of recondensation, and residual lignin can be reused to improve the utilization rate of lignin. The identified liquefaction products are mainly monophenolic compounds, in which S, G and H type compounds account for 57.72%, 25.28% and 8.98%, respectively. The existence of the signals for β-O-4 and C-C bonds in the ¹H-NMR spectra suggests that the bio-oils include a small amount of dimers and oligomers except for monophenolic compounds.