(548h) Understandings of Thermal Transformation of Cellulose Surface and Crystalline Core By in-Situ Nonlinear Vibrational Spectroscopy

In this study, Total Internal Reflection Sum Frequency Generation Vibrational Spectroscopy (TIR-SFG-VS) combined with non-TIR-SFG-VS enabled detection of the molecular structures of surface layers and the crystalline core of cellulose, which correlates with the biomass recalcitrance. From the SFG spectra in the C-H and O-H regions, it was found for the first time that the surface layers of Iβ cellulose differed in structural and spectroscopic signatures with its crystalline core. This work demonstrates the capacity of TIR and Non-TIR SFG-VS in selectively studying the structures and polymorphs of cellulose. This discovery is significant because it not only challenges the traditional understandings of cellulose materials that neglects the differences between its surface and core region as well as their effects on the cellulose deconstruction reactivity, but also provides a novel spectroscopic tool to measure the spectral signature of the bulk crystalline core and the surface layer of cellulose. The implementation of aqueous pretreatment of cellulose using a heated fluid test bed with dynamic TIR-SFG-VS analysis revealed the structural changes of Avicel and cellulose Iβ caused by thermal effects. The different crystalline structures of Avicel and cellulose Iβ changed during heating and cooling processes. The recrystallization to a new crystalline structure in the process of cooling could strongly affect the recalcitrance of cellulose, leading to new insights on the effectiveness of biomass pretreatment.