Effect of steam explosion treatment on chemical composition and characteristic of organosolv fescue lignin

Tall fescue, a grass currently grown as forage, presents a good potential as a feedstock in the developmentof biorefining. In this study, combination of steam explosion and an acetic/formic acid “organosolv”pretreatments are applied on raw fescue to optimize lignin extraction yields and obtain specific products.In a first step, fescue is pretreated by steam explosion under severity factors from 1.4 to 4.2. Thesetreatments allow hemicelluloses solubilization and open the lignocellulosic structure. Steam explodedsolid residues are then treated with an acetic acid (50%)/formic acid (30%)/water (20%) mixture. Extractedlignins are precipitated and characterized by size exclusion chromatography (SEC),31P NMR and HSQCNMR. Results show that acetic and formic acids extracted lignins present G, H and S units with ferulate andp-coumarate. Lignin typical linkages identified are of -O-4 and spirodienone substructures. Combinationof steam explosion and acetic and formic acid pretreatments increases extraction yields from 30% to100%. Moreover, an increase of the steam explosion treatment intensity induces chemical modificationsin lignin structure such -O-4 and spirodienone substructure degradations, increase of free COOH andphenolic OH bonds, decrease of aliphatic OH ferulate and p-coumarate bonds and changes in G/H/S unitsproportions. Steam explosion intensity affects also lignin molecular weights.