Recyclable and Reusable Maleic Acid for Efficient Production of Cellulose Nanofibrils with Stable Performance

Cellulose nanofibrils (CNFs) have attracted great attention because of their unique mechanical and optical properties for a variety of applications in composites, packaging materials, and electronics. Green, low-cost, and sustainable production of CNFs, however, is still challenging. Herein, an economic system using a type of a recyclable solid organic acid, maleic acid (MA), was developed to prepare carboxylated CNFs from bleached pulp fibers. The physical and chemical properties of the CNF can be tailored by adjusting the acid hydrolysis intensity or combined hydrolysis factor. Because of its low water solubility, MA can be easily recovered with a yield of approximately 90% after hydrolysis reactions through simple crystallization technology. The recyclability experiment of MA showed that the recovery yield of the acid was still 84% in the fourth cycle and the chemical structure of the recycled MA was almost unchanged throughout the recycling cycles. More importantly, the hydrolysis efficiency was not decreased after the recycling process, thereby endowing the resulting CNF with stable performance. Overall, this work provides a green and economically feasible method to recover and reuse MA for sustainable and large-scale production of CNFs.