Rapid dissolution of cellulose in an AlCl3/ZnCl2 aqueous system at room temperature and its versatile adaptability in functional materials

Abstract

This is the first report on the dissolution of cellulose at room temperature (R.T.) by using the AlCl₃/ZnCl₂·4H₂O solvent system. Cellulose with broad degrees of polymerization (DP), even those that reached 4080, can be dissolved rapidly within 120 min at R.T. Minor degradation of cellulose (DP reduced by 30%, based on the original DP) occurred during the dissolution process, which was much lower than that of cellulose dissolved in the traditional 65% ZnCl₂ solvent system at 80 °C (DP reduced by 70%). The dissolution mechanism of the AlCl₃/ZnCl₂·4H₂O solvent system was studied, demonstrating that Al³⁺ ions with a small ionic radius and high charge density first break the hydrogen bonds between cellulose O3H–O5, and then Zn²⁺ ions enter the cellulose chains and break more hydrogen bonds, resulting in the diffusion of cellulose chains to form a clear and transparent cellulose solution even at R.T. The binary inorganic salt solution composed of metal ions with small enough ionic radius, such as AlCl₃/ZnCl₂, LiCl/ZnCl₂, FeCl₃/ZnCl₂, and MnCl₂/ZnCl₂ can dissolve cellulose at R.T. It further proved that the ionic radius is a key factor for efficient dissolution. Importantly, the recovery rate of inorganic salt can reach 95% for each recycling, and still remain 64% after recycling ten times, with the same dissolution efficiency as a fresh solvent, realizing the green and ecofriendly dissolution process of cellulose. Due to the characteristics of inorganic salt solutions, cellulose/AlCl₃/ZnCl₂ solutions were used to prepare cellulose films, and hydrogels with quite high DPs (2976) of regenerated cellulose possessed excellent frost resistance, electrical conductivity, photocatalysis, and antibacterial properties, which can be applied in multifunctional applications, paving a new way for cellulose processing and utilization.