Abstract
Sulfuric acid lignin (SAL), one of the major biorefinery industry residues from the acid saccharification process on lignocellulosic materials for bio-fuel generation, is becoming a major problem for the sustainable development of the bio-refinery industry. Meanwhile, the recalcitrant and highly complex structure of SAL makes it insoluble in water and other organic solvents, resulting in a low reactivity. This study investigated the effects of hydrothermal reactions on SAL to enhance its solubility and reactivity. A hydrothermal reaction was first conducted on SAL from Cryptomeria japonica to obtain a completely water-soluble lignin. Fourier transform infrared spectroscopy, gel permeation chromatography, gas chromatography, and nuclear magnetic resonance (NMR) analyses revealed that the hydrothermal reaction of the SAL caused depolymerisation, demethylation, and the introduction of phenolic hydroxyls. Furthermore, another experiment was conducted on synthesizing a lignin polymer model compound (DHP) with 13C-labelled on the aromatic ring, followed by hydrothermal reaction and acetylation. Detailed structural information of DHP under hydrothermal reaction was obtained by solid-state cross polarization magic angle spinning NMR analysis. Reactivity of the hydrothermally treated SAL was enhanced by the hydrothermal reaction through the reversed increasing of the phenolic hydroxyl/aliphatic hydroxyl ratio in the condensed lignin units.
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