Subcritical Water: A Method for Green Production of Cellulose Nanocrystals

Abstract

In the present study, an innovative method to produce cellulose nanocrystals is proposed. The conventional method of obtaining these nanocrystals utilizes concentrated solutions of strong acids to promote the hydrolysis of cellulose amorphous regions and hemicelluloses. However, in the conventional method, long duration washing steps and low temperature resistance of the nanocrystals still limit their larger industrialization or some applications that require heat resistance during the transformation (extrusion for exemple) and/or end-uses. Water could also hydrolyze polysaccharides by applying severe reaction conditions. In this context, the increase of temperature and pressure (i.e. subcritical water) allows higher diffusion, activity and ionization of water, partial hydrolysis of cellulose is attended. The cellulose source, the hydrolyzed cellulose and a commercial nanocellulosic material were submitted to different analytical techniques in order to evaluate their morphology (DLS, AFM and optical microscopy) and physicochemical characteristics (XRD, TGA and FTIR). The obtained cellulose nanocrystals presented high crystallinity index, rod-like shape with similar aspect ratio as those known for classic cellulose nanocrystals but also a higher thermal stability even with respect to the original cellulosic source. The exclusive use of water as reagent constitutes a promising process not only for its green characteristics but also for its low corrosion and low cost of reagents.