Rod-like and spherical cellulose I and II nanocrystals prepared through acidified lithium bromide molten salt hydrate treatment

To achieve large-scale production of cellulose nanocrystals (CNCs), it is crucial to investigate ecologically and environmentally acceptable solvent systems that can effectively hydrolyze cellulose. Lithium bromide molten salt hydrate (LiBr MSH), as a benign cellulose solvent, is thermally and chemically stable, and recyclable with minimal environmental hazard. In this study, microcrystalline cellulose (MCC, cellulose I) was treated with formic acid-acidified LiBr MSH to yield CNCs. Cellulose I and II nanocrystals with tunable morphologies (rod-like and sphere-like) and high yield (up to 71.5%) were isolated. The concentration of LiBr had the most significant influence on the structure of CNCs. With an increased LiBr concentration from 50 to 56%, the CNC allomorph transitioned from cellulose I to a mixture of cellulose I and II and finally to cellulose II. The rod-like CNCs exhibited decreasing lengths from 398.4 nm to 102.9 nm and ultimately transformed into sphere-like nanoparticles. The resulting CNCs showed an overall high crystallinity (up to 65.9%), high thermal stability (T_max up to 334.6 °C), and good suspension stability (no significant settling within 30 days). Therefore, this work provides an environmentally friendly and effective approach to producing CNCs with adjustable size and allomorph.