Mesostructural changes in cellulose within wood cell wall upon hydrothermal treatment at 200 °C

Hydrothermal treatment between 150 °C and 230 °C is widely used in wood processing, from the steam treatment of timber for better dimensional stability and durability to the pretreatment for enzymatic saccharification and chemical pulping. Understanding the ultrastructural changes of wood cell walls through hydrothermal treatments is crucial for controlling and optimizing these hydrothermal treatment-based processes. Here, we studied the ultrastructure of wood cell walls of 24 hardwood species using simultaneous small- and wide-angle X-ray scattering measurements before and after the hydrothermal treatment at 200 °C. Most hardwoods show similar equatorial scattering features, representing the structure in the cross-section of the cell walls. In a water-saturated native state, there is a prominent correlation peak between 0.1 and 0.2 Å⁻¹ and a second peak between 0.2 and 0.4 Å⁻¹. The hydrothermal treatment above 160 ˚C drastically altered the structure at this nanometric scale: the two native correlation peaks disappeared, coincident with a buildup of a correlation peak in the 0.03–0.04 Å⁻¹ range. The hydrothermal treatment likely removed the cell wall matrix component between the microfibrils through autohydrolysis and phase separation, leading to the collapse of microfibrils with each other in the normal wood. In cellulose-rich cell walls, such as the G-layer in tension wood, cellulose microfibrils are already collated in the native state.