Complementary NMR analyses of cellulose-based assembly reveal how multi-scale structure governs saccharification efficiency in steam-exploded wood

Lignocellulosic biomass is a promising alternative to fossil resources to produce biobased products. However, this biomass is highly recalcitrant to enzymatic processes and requires a pretreatment step to overcome it. To understand the role of lignocellulose polymers organization on recalcitrance, characterization at different scales is critical, preferentially by employing non-degradative techniques which maintain sample physical integrity. This requires to combine several techniques thus limiting their application. In this study, structural and morphological modifications caused by steam explosion pretreatment on oak, poplar and spruce wood were enlightened using a combination of advanced ¹³C solid-state NMR and time-domain NMR techniques. Over increasing pretreatment severity, hemicelluloses content was decreased and amorphous cellulose was partially depolymerized, leading to higher cellulose crystallinity: a swelling of the overall polymer matrix was thus observed. A relatively constant water mobility associated to a reduction in pore size were observed, revealing that only a specific pore range of 5-15 nm generated at middle severity pretreatment is favorable to enzyme diffusion and hydrolysis reaction. Interestingly, β-O-4 bonds content and enzymatic hydrolysis yields were highly correlated, confirming the peculiar role of lignin in recalcitrance. Overall, this work demonstrates that structural information based on NMR approaches can reveal biomass species-dependent features explaining the variable saccharification efficiency over steam explosion pretreatment severity.