Multiscale investigation on the chemical and anatomical changes of lignocellulosic biomass for different severities of hydrothermal treatment

International audienceThe chemical changes sustained by lignocellulosic biomass during hydrothermal treatment are reflected at multiple scales. This study proposes to benefit from this multiscale nature in order to provide a global understanding of biomass alterations during hydrothermal treatment. For this purpose, complementary imaging techniques-confocal Raman microscopy and X-ray nanotomography-analysed by image processing and coupled to chemical measurements were used. This unique combination of analyses provided valuable information on topochemical and morphological changes of poplar samples, without the artefacts of sample preparation. At the cell wall level, holocellulose hydrolysis and lignin modifications were observed, which corresponded to anatomical modifications observed at higher scales. Overall, after treatment, samples shrank and had thinner cell walls. When subjected to more severe pre-treatments, cells were disrupted and detached from adjacent cells. Anatomical changes were then used to obtain quantitative indicators of the treatment severity. The effects of treatment at different scales can thus be quantitatively connected in both directions, from micro to macro and from macro to micro. Most biological materials have hierarchically organized structures whose properties are affected by their chemical composition, the interactions between components, and by their spatial distribution and organization. These relationships between properties are observed for lignocellulosic biomass and, particularly, wood. Wood cell walls are composed of layers that differ in their structure, thickness and composition. The main layers are the primary cell wall and the secondary cell wall, which is composed of three layers (S1, S2 and S3). The structure between two adjacent cells is called the middle lamella, but due to the difficulty in distinguishing the primary cell wall from the middle lamella, they are often analysed together as the compound middle lamella 1. In this study, we focus on two of these layers: the secondary cell wall, easier to observe because of its large thickness; and the compound middle lamella, because it ensures bonding between neighbour cells. Even though each layer has distinct characteristics, they are all composed primarily of three polymers: cellulose, hemicellulose and lignin. Cellulose is the main component of wood, accounting for 45-47% and 40-45% of hardwood and softwood weight, respectively 2. It is a polymer of β-D-glucopyranose 1 linked by 1,4-glycosidic bonds 3. Its linear nature and the presence of multiple hydroxyl groups 3 make it likely to form intra-and intermolecular hydrogen bonds. This chemical feature has structural consequences: cellulose is highly organized into microfibrils 4 , forming a crystalline core with a semi-crystalline shell. This structure gives wood its strength. Moreover, the crystalline nature of cellulose also makes it resistant to chemical attack 1. Unlike cellulose, hemicellulose is a type of polymer composed of different pentose and hexose monosaccharides 5. In hardwoods, hemicelluloses constitute 25-40% of wood by weight 2 and are mainly composed of glucuronoxylans, which consist of a xylose unit backbone with 4-O-methylglucoronic acid and O-acety