Revealing the Molecular Structure Basis for the Recalcitrance of Hardwood and Softwood in Dilute Acid Pretreatment

The intrinsic recalcitrance of lignocellulosic biomass due to the complicated structure of plant cell walls is the main hurdle to efficient deconstruction of biomass but is not fully understood. Thus, this study comparatively characterized main components (e.g. cellulose, hemicellulose, and lignin) of hardwood poplar and softwood Douglas fir in both solubilized fractions and residues which were produced by batch and flowthrough pretreatment under various conditions: 140 °C to 270°C, solid retention time 0 to 70 minutes, acid concentration of 0.05% and 1%, and flowrate of 0 and 25mL/minute. Results based on wet chemistry analysis and NMR revealed very different mechanisms of depolymerization and solubilization of hardwood and softwood under identical pretreatment conditions. Unique molecular structural features that pronounced the specific recalcitrance of hardwood and softwood to dilute acid pretreatment were identified.