Formation and in situ separation of oligomeric products from complete depolymerization of pubescens using a catalyst-free biphasic system

Abstract

Degradation of cellulose, hemicellulose and lignin in lignocellulosic biomass into fluid-dissolved oligomers is an entry point into biorefinery, and this process greatly improves heat and mass transfer for further processing. In this study, a NaCl–H2O–tetrahydrofuran (THF) biphasic system is successfully applied to the production of oligomers from complete depolymerization of the three main components in pubescens. With the assistance of NaCl, the generated oligosaccharides and lignin-derived oligomers can be separated in situ by dissolution into the aqueous and organic phases, respectively. NaCl is important for the degradation of carbohydrates, while a miscible H2O–THF cosolvent benefits to lignin dissolution. The Cl− ions can break the intermolecular hydrogen bonds of the oligomers and hinder aggregation of the oligomers, thereby obtaining smaller particles with high dispersion in the solution. Acidic products from carbohydrates enhanced by NaCl promote the cleavage of the side-chain linkages in lignin. Acetic acid from the acetyl group on hemicellulose can easily condense with the hydroxyl-terminated lignin unit, occupying the end site and preventing repolymerization of the lignin fragments. Additionally, the inhibition of Cα–OH oxidation by NaCl in lignin side-chain prevents the increase in the oxygen content in the aromatic oligomers, and this active hydroxyl group preserves high potential for further functionalization.