In-situ studies of hydrothermal reactions of lignocellulosic biomass using high-pressure differential scanning calorimetry

Highlights: • High-pressure differential scanning calorimetry was applied on whole biomass. • The technique has unique application for study of hydrothermal reactions. • Cellulose and hemicellulose dehydration was observed to be exothermic. • Modelling of exotherm profiles indicates position of hidden lignin exotherm. • Differences in exotherm profiles observed between different biomass species. -- Abstract: Fundamental studies of biomass hydrothermal deconstruction reactions have been carried out under realistic conditions using a novel high-pressure differential scanning calorimetry technique. Exotherms related to cellulose and hemicellulose degradation were identified as separate features, with maxima around 280 °C and 250 °C respectively in the dry state, where curve fitting using a non-isothermal kinetic model was used to postulate the existence of an equivalent lignin exotherm, with a maximum around 240 °C, masked by the carbohydrate features. A downward shift in the hemicellulose exotherm was observed on hydration, of around 25 °C, which may due to the promotion of preceding hydrolytic depolymerisation reactions, which may reduce the kinetic threshold for subsequent dehydration reactions. No corresponding hydration shift was observed for the cellulose exotherm, consistent with the inaccessibility of the crystalline structure of this biomass component. Differences in hydrated degradation exotherm profiles were observed between wheat-straw, Miscanthus and willow biomass species, which in-part corresponded to differences in enzyme digestibility following hydrothermal treatment. The total willow exotherm exhibited lower enthalpy than straw, of 339 J/g and compared to 510 J/g, with both hemicellulose and cellulose exotherm maxima for willow at higher temperatures, at 233 °C and 291 °C, compared to 224 °C and 281 °C for straw, which was consistent with the greater intractability of this woody biomass. The results from the study will be valuable in defining process temperatures and hydration conditions for optimal biomass conversion for downstream thermal and biochemical processing, and also helping to understand phenotypical differences in plant species leading to differing conversion efficiencies.