Structural Insights on Recalcitrance during Hydrothermal Hemicellulose Extraction from Wood

Hydrothermal extraction of hemicelluloses from lignocellulosic biomass for conversion to renewable materials or fuels has captured attention. The extraction is however partial and some lignin is codissolved. Herein, we investigated the role of molecular structure in the recalcitrance. Wood meal of Spruce and Birch were subjected to pressurized hydrothermal extraction at 160 °C for 2 h, which extracted 68–75% of the hemicelluloses. 2D heteronuclear single quantum coherence (HSQC) NMR, HSQC-TOCSY, and 13C NMR were applied for structural studies of both extracts and residues. Subsequent to the known partial hydrolysis of native carbon-2 and carbon-3 acetates in hemicellulose, some acetylation of primary alcohols on hemicelluloses and lignin was observed. Lignin carbohydrate complexes (LCC) were detected in both the extracts and residues. In Spruce extracts, only the phenyl glycoside-type of LCC was detected. Birch extracts contained both the phenyl glycoside and benzyl ether-types. In the hydrothermal wood residues of both species, benzyl ether- and gamma (γ)-ester-LCC were present. Structural changes in lignin included decrease in aryl ether (βO4) content and increases in resinol- (ββ) and phenyl coumaran (β5) contents. On the basis of the overall analysis, the mechanisms and contribution of molecular structure to recalcitrance is discussed.