Kinetics of fractionation by SO2–ethanol–water (SEW) treatment: understanding the deconstruction of spruce wood chips

Abstract

The kinetics of SO₂–ethanol–water (SEW) fractionation of spruce were studied using wood meal and chips and compared to those of SO₂–water, acid sulfite (AS) and ethanol–acid sulfite treatment. The SEW lignin removal rate was found to be similar to that of AS at the same free SO₂ concentration, while the lignin sulfonation rate is considerably higher for the acid sulfite systems. No relation between acidity and sulfonation rate was observed putting into question the acid-catalysed nature of this reaction. The observed SEW sulfonation and delignification patterns are consistent with Hägglund's “fast sulfonation–slow hydrolysis” consecutive scheme. The data indicate that during the initial phase hemicelluloses are removed together with lignin as lignocarbohydrate complexes, while cellulose is protected from hydrolytic attack by lignin leading to a lower hydrolysis rate. The SEW hemicellulose dissolution behaviour can be understood by the low tendency of glucomannan to “crystallise” onto cellulose. The understanding of the dissolution pattern of lignin and hemicellulose may help to interpret the enzymatic hydrolysis behaviour of SEW residual solids subjected to different degree of fractionation.