Characteristics of degraded lignins obtained from steam exploded wheat straw

Abstract

Two groups of six lignin preparations degraded during various steam-explosion pre-treatments (L 1a–6a) and released during the sequential alkaline peroxide post-treatment (L 1b–6b) were isolated and characterised. The steam pre-treatment and alkaline peroxide post-treatment, respectively, resulted in a degradation of 11.2–12.4% and 80.6–87.6% of the original lignin, which together removed 92.4–99.4% of the original lignin from wheat straw. L 1b–6b lignin fractions with a sugar content lower than 1.5% were obtained after alkaline peroxide post-treatment of the corresponding steam exploded straw, whereas the lignin fractions L 1a–6a degraded during the steam pre-treatment, contained two to three times higher sugars on the basis of the dry weight of the lignin. The weight-average molecular weight ( M w) of the L 1a–6a lignin preparations ranged between 3460 and 3870 g mol −1 with polydispersity of 1.4–2.0, whereas the M w and M w/ M n of the L 1b–6b lignin fractions released during the alkaline peroxide post-treatment, corresponded to 4270–4640 g mol −1 and 1.8–2.1, respectively. The L 1a–6a lignin preparations were shown to be more condensed and thermally stable than the L 1b–6b lignin fractions. FT-IR and 1H and 13C NMR analyses showed that the steam pre-treatment under the conditions given led to a comprehensive cleavage of β- O-4 ether bonds, condensation reactions, and a certain demethylation of aromatic methoxyl groups in lignin structure. This is particularly true for the L 1a–6a lignin fractions degraded during the steam-explosion pre-treatment.