Inorganic molten salt hydrates (IMSHs), such as lithium bromide trihydrate (LBTH, ~61 wt% aqueous solution of LiBr), have drawn increasing attention as cellulose solvents and have been used in biomass processing and conversion (such as hydrolysis of cellulose and saccharification of lignocellulosic biomass, depolymerization and quantitation of lignin, synthesis of high-value products from sugars and biomass, and fabrication of cellulose nanocrystals, films, and aerogels). However, how cellulose is dissolved and regenerated in IMSHs has not been fully understood. This presentation reports our studies on cellulose dissolution and regeneration in the LBTH. It was found that the dissolution of cellulose in LBTH is affected by LiBr concentration, temperature, pH, and cellulose properties (consistency, particle size, and degree of polymerization). It is the ion-dipole coordination between hydrated Li⁺ and cellulose hydroxyl than results in the dissolution of cellulose in the LBTH, which disrupts the intermolecular H-bond of cellulose. ⁷Li NMR verified that the Li⁺ in a concentrated solution (e.g., LiBr●3H₂O) has a stronger interaction with cellulose than that in a dilute solution (e.g., LiBr●5H₂O). Besides, the association of the hydrated lithium cations to cellulose via the coordination reduces overall hydrophobicity of cellulose, which facilitates the hydration, swelling, and dissolution of cellulose. In addition, the polarizable, large-size, and low-electronegativity bromide anion plays a crucial role in the cellulose dissolution in ABTH. It was found that different from that in other cellulose solvent systems, the regeneration of dissolved cellulose takes place in two independent steps in the LBTH. The weak crosslinking through coordination between cellulose hydroxyl group and Li⁺ cation leads to the gelation of the cellulose solution in LBTH during cooling, in which cellulose retains dissolved. Cellulose is not regenerated until the solvent LBTH is exchanged with non-solvent (water) during washing, leading to the formation of porous cellulose network.

Yang Liao

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