Biphasic extraction of mechanocatalytically-depolymerized lignin from water-soluble wood and its catalytic downstream processing
Mechanocatalysis is an efficient method for deep depolymerization of lignocellulosic materials. It renders full conversion of lignocellulose into water-soluble products, a mixture of oligosaccharides and lignin oligomers. In an aqueous monophasic system, it is known that further saccharification of these products affords high yields of fermentable sugars in addition to a lignin precipitate. Although much attention has been devoted to upgrading of the carbohydrate fraction, the reactivity of mechanocatalytically-isolated lignin has hitherto been overlooked. In this report, we demonstrate the saccharification of water-soluble lignocellulose in a biphasic mixture of water and 2-methyltetrahydrofuran, to prevent lignin fragments from recondensation, not only with other lignin fragments but also with oligosaccharides and their degradation products. The lignin stream extracted in the biphasic system exhibits a molecular weight much lower than that of the lignin precipitate obtained from monophasic aqueous saccharification, and that of lignin isolated by an Organosolv process with no added acid catalyst. Surprisingly, β-O-4 and phenylcoumaran (β-5 + α-O-4) linkages are quantitatively hydrolyzed during saccharification in the biphasic system. However, the extracted lignin still incorporates resinol bonding motifs (β-β + γ-O-α). Because of the lower molecular weight, low carbohydrate content, and high solubility in organic solvents, the catalytic downstream processing of the extracted lignin leads to higher yields of liquid products (e.g. 2-methoxyphenols and 2,6-dimethoxyphenols) by RANEY® Ni-catalyzed hydrogenolysis (200 °C, 7 MPa H2), compared to reactions performed on the lignin precipitate or Organosolv lignin.