High-yield cellulose hydrolysis by HCl vapor: co-crystallization, deuterium accessibility and high-temperature thermal stability

Hydrochloric acid hydrolysis in its gas form was used to produce cellulose nanoparticles from flax shives with a very high yield (> 90%). The efficiency of the transformation was examined by gravimetry, atomic force microscopy and transmission electron microscopy. A novel purpose-built anisotropic line-broadening X-ray diffraction model was used. This XRD method uses a parametric shape in order to address the issue of strongly broadened and overlapping Bragg rays that are characteristic of nano-sized crystallites. This method demonstrated the remarkable stability of the crystallite shapes during hydrolysis and its sensibility was sufficient to detect a minor co-crystallization along the hydrophilic faces. The presence of amorphous material stricto sensu in the form of individual and randomly oriented chains was not necessary to describe the diffractograms accurately. Thermal FTIR with isotopic exchange was also performed using deuterium oxide to characterize the accessibility of the materials between 20 and 260 °C. Further, back-exchange experiments were performed in order to quantify the hysteretic amount of deuterium that was trapped by microstructural reorganization. These experiments showed that hydrolysis cancelled any form of deuterium trapping (water-induced co-crystallization). For the first time, thermal FTIR demonstrated that isotopic labelling of cellulose sources can produce false positives when conducted at room temperature and thermal FTIR can unambiguously distinguish between labelled cellulose groups and free deuterium oxide, which is paramount when measuring the higher accessibility of the nanocelluloses. It was also demonstrated that the high-temperature hydrogen bond reorganization and thermal degradation of the cellulose chains strongly depend on the hydrolysis and on the microstructure of the substrate. Graphic abstract