Rapid, high-yield production of lignin-containing cellulose nanocrystals using recyclable oxalic acid dihydrate

Nanocellulose is of growing interest due to its great potential in many value-added applications and its unique characteristics. However, conventional methods for nanocellulose isolation are challenged by various economic and environmental concerns. With the use of recyclable oxalic acid dihydrate (OAD), this study demonstrated the rapid, high-yield production of lignin-containing cellulose nanocrystals (LCNCs) from thermomechanical pulp (TMP). Serving as the sole solvent and reactant, molten OAD effectively hydrolyzed TMP within 30 min at 110 degrees C, leading to complete removal of hemicellulose (97 %) but retaining most of the cellulose (93 %) and lignin (97 %). This modified TMP can be converted into LCNCs with a high overall yield of >70 % via microfluidization. The LCNCs possessed satisfactory thermal stability and exhibited a nanoscale morphology of highly uniform cellulose nanocrystals (7.0 nm in width, 3.9 nm in height, 200-300 nm in length, and 72 % in crystallinity), randomly implanted by lignin aggregates. The unreacted OAD could also be readily recycled via crystallization from the post-hydrolysis mixture. The recycled OAD showed good performance retention for at least 4 cycles, indicating its suitability for continuous TMP hydrolysis. Further life cycle assessment suggested that the recycling potential could reduce the global warming potential and energy use by 48 % and 47 %, respectively. Overall, it was apparent that recycling molten OAD provided a promising approach to rapidly make clean, high-yield LCNCs with low energy use and improved reagent utilization.