Toward a further understanding of hydrothermally pretreated holocellulose and isolated pseudo lignin

X-ray photoelectron spectroscopy (XPS) was used to trace pseudo lignin formation during the hydrothermal pretreatment of holocellulose and elaborate the structure of the isolated pseudo lignin. The results showed that XPS could detect the trace amounts of pseudo lignin in the pretreated holocellulose substrate by quantitative determination of carbon (C) and oxygen (O), while fourier transform infrared spectroscopy (FT-IR) and solid-state cross polarisation magic angle spinning carbon-13 nuclear magnetic resonance (CP/MAS ¹³C-NMR) almost had no responses. After extensive pretreatment, C₁ increased from 8.9 to 23.0 %, O/C decreased from 0.68 to 0.54 along with an increase of Klason lignin of the pretreated holocellulose from 1.0 to 6.2 %. As for isolated pseudo lignin, both O₁ and O₂ were involved, whereas only O₁ involved in reference lignin and O₂ in holocellulose sample. In comparison to reference lignin, pseudo lignin was rich in C₁ and less in C₂, suggesting that more cyclic structures and hydroxyl groups were present in the pseudo lignin. The FT-IR and NMR results identified the presence of more aliphatic structures in the pseudo lignin, especially those formed initially; while the subsequent pretreatment could modify the aliphatic toward to aromatic structure.