Interactions between Holocellulose and Lignin during Hydrolysis of Sawdust in Subcritical Water

To clarify the interactions between holocellulose and lignin, the hydrolysis behaviors of sawdust, holocellulose, lignin, and holocellulose–lignin mixture (HLM) in subcritical water were investigated in the present study, where HLM acted as a model of sawdust without covalent linkages between holocellulose and lignin. Holocellulose in sawdust showed negligible effects on lignin at low reaction temperatures, whereas it might promote the hydrolysis of lignin at higher reaction temperatures. In contrast, the hydrolysis of holocellulose in sawdust was inhibited by lignin due to the high bond dissociation energies of covalent linkages between them. The destruction of covalent linkages mainly occurred at the reaction temperatures of 273–315 °C, due to the high activation energy at this temperature range. However, they could barely be destroyed at the reaction temperatures of 192–273 °C. The lignin inhibited the ring scission and glycosidic bond cleavage of holocellulose, resulting in lower yields of total carbohydrates and furan derivates than those of HLM. In addition, it also suppressed the decomposition of holocellulose hydrolysate to levoglucosenone and aromatic-linked chars. The above results could provide the basis for developing models for predicting product distribution during the hydrolysis of biomass.