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 °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.

纳米纤维素因其在许多增值应用中的巨大潜力及其独特的特性而受到越来越多的关注。然而，传统的纳米纤维素分离方法受到各种经济和环境问题的挑战。通过使用可回收的二水草酸 (OAD)，这项研究证明了从热机械纸浆 (TMP) 中快速、高产地生产含木质素的纤维素纳米晶体 (LCNC)。作为唯一的溶剂和反应物，熔融 OAD 在 110 °C 下在 30 分钟内有效水解 TMP，导致完全去除半纤维素 (97%) 但保留大部分纤维素 (93%) 和木质素 (97%)。这种经过修饰的 TMP 可以通过微流化转化为 LCNC，总产率 >70%。LCNCs 具有令人满意的热稳定性，并表现出高度均匀的纤维素纳米晶体（宽度为 7.0 nm，高度为 3.9 nm，长度为 200-300 nm，结晶度为 72%）的纳米级形态，由木质素聚集体随机植入。未反应的 OAD 也可以通过从水解后的混合物中结晶而容易地再循环。回收的 OAD 在至少 4 个循环中表现出良好的性能保持率，表明其适用于连续 TMP 水解。进一步的生命周期评估表明，回收潜力可以将全球变暖潜能值和能源使用量分别降低 48% 和 47%。总的来说，很明显，回收熔融 OAD 提供了一种有前途的方法，可以快速制造清洁、高产量、低能耗和提高试剂利用率的 LCNC。