Homolytic and Heterolytic Cleavage of β-Ether Linkages in Hardwood Lignin by Steam Explosion

Steam-explosion lignin (SEL) was extracted with ethanol from steam-exploded hardwood (okoume, Aucoumea klaineana Pierre) pretreated at various severities after neutral or acidic impregnation. The SELs were subjected to structural characterization by 2D HSQC NMR, P-31 NMR, and SEC and compared with milled-wood lignin (MWL). A strong decrease in the beta-O-4 content is observed with increasing steam-explosion severity accompanied by a gradual increase in molecular mass. C alpha-oxidized S units (S', Hibbert's ketones) were quantified by NMR and used as a marker of the hydrolytic mechanism; naphthol was used as a carbonium-ion scavenger. It has been observed that mixed reactions of hydrolysis and homolysis are involved, but the SEL is mainly cleaved homolytically, favoring recondensation through radical coupling even at low reaction severity. However, acidic preimpregnation of wood prior to steam explosion enhanced the carbonium-ion pathway.