A Green Cellulose Dissolution System for Producing Tunable Regenerated Nanocellulose Formate

Different from the most used ionic liquids or cosolvents of the cellulose dissolution system, we reported a cellulose dissolution method by coupling LiBr·3H₂O preimpregnation at room temperature (RT) with dissolution of cellulose in formic acid (FA) for controllable production of nanocellulose. This method yields a high solubility (up to 10 wt %) of cellulose, which consequently facilitates the preparation of tunable regenerated nanocellulose formate (RNCF) with versatile applications. The LiBr·3H₂O preimpregnation at RT significantly improved the dissolution efficiency of cotton pulp by increasing cellulose accessibility via largely breaking hydrogen bonds and deconstructing the cellulose crystalline structure. This effect was attributed to the synergistic actions of hydrated Li⁺ coordination with the O of cellulose hydroxyls and the bond of Br^- to H of cellulose hydroxyls, based on the verification of XPS, solid-/liquid-state NMR, and hydrogen-deuterium exchange characterization. The subsequent FA dissolution and regeneration process yielded RNCF with 100% yield, high degree of substitution (DS > 1.2), and tunable degree of polymerization (300-700) of the resultant RNCF. The dissolved CF in FA can serve as a functional building block to reassemble as strong RNCF filaments, films, hydrogels, or aerogels via a versatile regeneration route. The nanosized RNCF is further showcased to stabilize both oil-in-water and high internal phase water-in-oil Pickering emulsions. Therefore, this simple, ambient, sustainable, and cost-effective LiBr·3H₂O preimpregnation approach could largely enhance cellulose accessibility, highly promoting cellulose dissolution and sustainable production of RNCF and boosting the utilization of cellulose-based functional materials.