Steam explosion lignin (SEL) was extracted with ethanol from steam exploded hardwood (okoumé, Aucoumea klaineana Pierre) pretreated at various severities after neutral or acidic impregnation. The SELs were subjected to a structural characterization by 2D HSQC NMR, 31 P NMR, SEC and compared to Milled Wood Lignin (MWL). A strong decrease in the -O-4 content is observed with increasing steam explosion severity accompanied with a gradual increase in molecular mass. C-oxydized S units (S', Hibbert's ketones) were quantified by NMR and used as a marker of the hydrolytic mechanism as well as naphthol 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, favouring recondensation through radical coupling even at low reaction severity. However, acidic pre-impregnation of wood prior to steam explosion enhanced the carbonium ion pathway.

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