Elucidation of the interaction effects of cellulose, hemicellulose and lignin during degradative solvent extraction of lignocellulosic biomass

The degradative solvent extraction (DSE) method has been recently developed to deoxygenate and upgrade lignocellulosic biomass wastes into high-quality extracts with a variety of value-added applications. Lignocellulosic biomass is mainly composed of cellulose, hemicellulose and lignin, and the interaction of the three components during the DSE process still remains unclear and needs to be elucidated. Therefore, in this study, the synthetic component mixtures (cellulose/hemicellulose (CH), cellulose/lignin (CL) and hemicellulose/lignin (HL)) were firstly prepared with the model compounds (cellulose, hemicellulose and lignin), and the conversion characteristics of the component mixtures and the superposition of individual component during the DSE process under the same operating conditions were compared to reveal the interaction effect between the three components. The extract yields of cellulose, hemicellulose and lignin were 44.16%, 27.4% and 18.25% respectively, indicating that cellulose contributed mostly to the formation of the extract (the target product) compared with hemicellulose and lignin. Significant interactions were observed for cellulose/lignin and hemicellulose/lignin mixtures, while the interaction effect between cellulose and hemicellulose was relatively small. The interaction intensity between cellulose, hemicellulose and lignin was in the order of HL > CL > CH. The interaction between cellulose/lignin and hemicellulose/lignin promoted the thermal degradation of lignin. Specifically, the interaction between the hemicellulose and lignin mixture reduced the yield of residue by 9.26%. The interaction mechanism between the three components was discussed and revealed based on the detailed characterization of the DSE products. The interaction between the three components could facilitate the production of gaseous products (mainly CO2), indicating that the interaction between the three components significantly promoted the decarboxylation reaction. The interaction between two of the three components all inhibited the aromatization reaction and reduced the aromaticity of the extracts. During the DSE process of cellulose/lignin or hemicellulose/lignin mixtures, the small-molecular free radicals generated by the decomposition of cellulose or hemicellulose could occupy the active sites on lignin fragments, leading to the significant inhibition of the condensation reactions between different lignin fragments. These interactions leaded to the decrease of the extract yields and residue yields during the DSE process of cellulose/lignin or hemicellulose/lignin mixtures.