Isolation of β-O-4-Rich Lignin From Birch in High Yields Enabled by Continuous-Flow Supercritical Water Treatment

The continuous flow supercritical water (scH₂O) treatment of Birch wood (T=372–382 °C; t=0.3–0.7 s; p=260 bar) followed by alkali extraction of lignin allowed for the isolation of lignin and lignin carbohydrate complexes (LCCs) with a high number of β-O-4 moieties in the range 29–57/100 Ar (evaluated by quantitative ¹³C NMR analysis) in yields ranging between 13–19 wt % with respect to the initial wood. A “lightning rod effect” of carbohydrates has been claimed to explain the low degradation of β-O-4 bonds during the process. The structure of the isolated lignin was thoroughly elucidated via comprehensive NMR studies (HSQC, ¹³C and ³¹P). A low degree of condensation (DC)<5 % was found for all the lignin samples, which was only slightly dependent on the reaction severity. The number of aliphatic −OH, phenolic −OH, and −COOH groups was in the range 3.37–5.25, 1.41–2.31 and 0.39–0.73 mmol/g, respectively. The number of −COOH groups increased with increased severity, suggesting that oxidation can occur during the scH₂O treatment. Furthermore, by simply varying the reaction severity, it was possible to tune important lignin properties, like the molar mass and the glass transition temperature (T_g).