Integrating solvent-free mechanochemistry and heat curing for the green production of highly charged and highly crystalline phosphorylated cellulose nanocrystals

Abstract

Phosphorylated cellulose nanocrystals (P-CNCs) are important derivatives of cellulose nanocrystals (CNCs), which have attracted much attention in food, cosmetics and composite material. However, P-CNCs reported previously involved water- and energy-intensive processes, namely, aqueous-based pre-phosphorylation containing soaking, drying and heat curing. In this study, we proposed a pre-phosphorylation process consisting of solvent-free ball milling and heat curing for the production of highly charged P-CNCs. Highly transparent P-CNC suspension was obtained by ball milling for 30 min at 650 rpm and a molar ratio of MCC:P₂O₅:Urea = 1:0.5:5, curing at 150 °C for 20 min, followed by washing and high-pressure homogenization. The as-prepared P-CNCs had ultrahigh charge content (4.03 mmol g⁻¹) and high crystallinity (76.3 %). The multiple phosphate structures on P-CNCs, including monophosphate, polyphosphate and cross-linked phosphate groups, were visualized via liquid-state ³¹P NMR analysis. As expected, the energy consumption (0.338 kWh g⁻¹P-CNCs), was significantly lower than that in the aqueous-based phosphorylation process (0.565 kWh g⁻¹). Furthermore, compared to heat soaking-assisted pre-phosphorylation process reported in our previous work, this process was more effective and possessed lower environmental impacts according to the life cycle assessment (LCA) results. Therefore, this efficient process provides an energy-saving and environment-friendly way for the industrial production of P-CNCs.