Subcritical Water: A Method for Green Production of Cellulose Nanocrystals

In the present study, an innovative method to produce cellulose nanocrystals is proposed. The conventional production of nanocrystals uses 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 the nanocrystals low temperature resistance still limit their larger industrialization and some applications in processes or end-uses that require heat resistance, like extrusion. In this context, the use of subcritical water (120 degrees C and 20.3 MPa for 60 min) allows higher diffusion, activity, and ionization of water. With that, partial hydrolysis of cellulose can be attended (with 21.9 wt % NCC yield). The cellulose source, the hydrolyzed cellulose, and a commercial nanocellulose were submitted to different analytical techniques to evaluate their morphology and physicochemical characteristics. The obtained cellulose nanocrystals presented a high crystallinity index (79.0% by XRD), rod-like shape with a similar aspect ratio as those known for dassic cellulose nanocrystals but also a higher thermal stability even when compared with the original cellulosic source (onset around 300 degrees C). The exclusive use of water as a reagent is a promising process not only for its green characteristics but also for its low corrosion, low and cleaner effluent, and low cost of reagents.