Aldehydes and diols-induced in-situ modification of lignin for high β-O-4 retention and controllable pyrolysis pathways of lignin

Lignin condensation during pretreatment leads to its high thermal stability, limiting its utilization through pyrolysis. Hence, lignin extraction and modification were realized simultaneously, using different aldehydes and diols as modifiers during organic solvent pretreatment of bamboo leaves. Then, the change in lignin’s fast pyrolysis behavior and the mechanism induced by the modification were explored. The modifiers improved the lignin removal ratio (from 77.03 % to a maximum of 84.67 %), and modified the lignin by forming C_α-alkoxy structures (for diols) and acetals (for aldehydes). This facilitated the decrease of lignin molecular weight to 4760–8590 Da and the significant increase of β-O-4 content to 51.90–68.51 %. As a result, the modification led to lower average activation energy (91.1–110.9 kJ/mol), improved bio-oil yield (maximum of 63.42 wt%), and markedly increased yield of 2-methoxy-4-vinylphenol (to 214 %) and 2,3-dihydrobenzofuran (to 159 %) during lignin pyrolysis. Interestingly, aldehydes-induced modification efficiently changed the pyrolysis pathway, leading to the selective preparation of a specific product (3-hydroxy-4-methoxybenzaldehyde). The results provide a feasible way to control the lignin pyrolysis process and product distribution by modifying the lignin structure during pretreatment.