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 (Tg).