Purification, Structural Characterization, and Modification of Organosolv Wheat Straw Lignin

Biolignin, a wheat straw lignin produced by acetic acid/formic acid/water hydrolysis, was characterized by ³¹P and ¹³C–¹H 2D NMR spectroscopy and by size-exclusion chromatography. Biolignin is a mixture of low molar mass compounds (M_n = 1660 g/mol) made up of S, G, and H units and of coumaric and ferulic acid units. β-5 and β-O-4 interunit linkages are partially acylated in the γ-position by acetate and p-coumarate groups. Deacylated samples with a low content of contaminants were obtained by combining alkaline hydrolysis and solvent extraction. The high phenolic OH content found by ³¹P NMR reflects the presence of condensed aromatic units, such as 5–5 units. Reaction of purified lignin with ethanol and ethane-1,2-diol yielded esterified lignins much more soluble than Biolignin in common organic solvents. During this reaction, the secondary OH of β-O-4 linkages was simultaneously etherified. Phenol hydroxyethylation by 2-chloroethanol yielded samples containing only aliphatic hydroxyl groups.