High β-O-4 polymeric lignin and oligomeric phenols from flow-through fractionation of wheat straw using recyclable aqueous formic acid

A biorefinery process characterized with all components valorization based on flow-through fractionation using recyclable aqueous formic acid as solvent is proposed. Within 20 min, wheat straw was efficiently fractionated into cellulose-rich material, hemicellulosic sugars and lignin. Delignification achieved 86%, and the dissolved lignin was further fractionated into water insoluble lignin (WIL) and water soluble lignin (WSL) by dilution using water. WIL accounted for 33-81% of total dissolved lignin depending on treatment severity. GPC and 2D NMR analysis of WIL from 62% formic acid at 130 degrees C revealed non-condensed structure, 81% retention of ether bond, and molar weight of 3016 g/mol. The well retained structure of lignin is crucial for catalytic conversion into aromatic monomers. Characterization of WSL by GPC and Folin-Ciocalteu assay revealed a structure of oligomeric phenols containing 2.8 phenolic hydroxyl groups and five phenylpropane units in each molecule with molar weight of 1123 g/mol on average.