Investigation on the thermal degradation behavior of enzymatic hydrolysis lignin with or without steam explosion treatment characterized by TG-FTIR and Py-GC/MS

In this study, enzymatic hydrolysis lignin (EHL) extracted from bio-ethanol production residue by enzymatic hydrolysis method was utilized as the raw material, which was subsequently purified to remove cellulose and hemicellulose for gaining the purified lignin (EHL-P) through alkaline solution extraction. Compared with EHL-P, thermal performance and pyrolytic product distribution of EHL-P-SE (which was obtained from EHL-P by steam explosion) were also analyzed by thermogravimetric analyzer coupled with Fourier transform infrared spectroscopy (TG-FTIR) and pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS). Results showed that the degradation process of EHL-P and EHL-P-SE was similarly divided into three stages: a drying stage, a rapid decomposition stage, and a slow carbonization stage. However, their initial weight loss (i.e., ˂ 3 wt% for EHL-P, while ~ 5 wt% for EHL-P-SE before 200 °C), primary weight loss (i.e., ~ 55 wt% for EHL-P, while ~ 48 wt% for EHL-P-SE between 200 and 600 °C), and char yields (i.e., ~ 33 wt% for EHL-P, while ~ 42 wt% for EHL-P-SE at 900 °C) were different. FTIR showed that the evolution of gaseous products changed significantly due to the effect of SE treatment, especially for organic components (i.e., the absorbance of peaks corresponding to carbonyl-containing components such as aldehydes, ketones, and acids decreased, while that related to phenols and aromatics increased). Finally, Py-GC/MS tracked variations of the relative content (%) of phenols (e.g., guaiacol (G), phenol (P), syringol (S), and catechol (C) types) with the increase of degradation temperature (from 300 to 500 °C), and results showed that demethoxylation was more likely to occur at higher degradation temperatures (≥ 400 °C) to realize the transformation from G- and S-type phenols to P- and C-type phenols. Graphical abstract