Comparative study of organosolv lignins from wheat straw

Dewaxed wheat straw was treated with acetic acid–H₂O (65/35, v/v), acetic acid–H₂O (80/20, v/v), acetic acid–H₂O (90/10, v/v), formic acid–acetic acid–H₂O (20/60/20, v/v/v), formic acid–acetic acid–H₂O (30/60/10, v/v/v), methanol–H₂O (60/40, v/v) and ethanol–H₂O (60/40, v/v) using 0.1% HCl as a catalyst at 85 °C for 4 h, in which 78.2, 80.0, 88.2, 89.4, 94.1, 23.5 and 37.4% of the original lignin, and 42.4, 58.7, 70.0, 65.1, 76.5, 14.2 and 22.2% of the original hemicelluloses was released, respectively. Lignins obtained were characterized by their content of hemicelluloses, composition of phenolic acids and aldehydes, molecular weight, thermal stability and by UV, Fourier transform infrared (FT-IR), ¹H and ¹³C nuclear magnetic resonance (NMR) spectroscopy. The results showed that aqueous organic acid was more effective than aqueous organic alcohol for extensive delignification and selective fractionation of cellulose, lignin and hemicelluloses from the straw. In particular, the addition of formic acid gave a significant effect on the dissolution of lignin. All the acid-insoluble lignin fractions contained small amounts of contaminated hemicelluloses as shown by their content of neutral sugars, 0.9–4.3%, and had weight-average molecular weight between 3960 and 4340 g mol⁻¹. An increase in concentration of acetic acid or formic acid in organosolv resulted in an increment in release of guaiacyl units and in lignin condensation. However, the lignin preparations released during the treatment with aqueous organic alcohol without organic acid contained almost equal amounts of non-condensed guaiacyl and syringyl units with fewer p-hydroxyphenyl units. The β-O-4 ether bonds together with β-β, β-5 and 5-5′ carbon–carbon linkages were identified to be present in lignin substructures.