Hydration levels in acidic zinc chloride molten salt hydrates for selective fractionation of hemicellulose

Molten salt hydrates (MSH) are efficient, environmentally friendly solvents widely used in polysaccharide dissolution and catalysis. This study thus exquisitely designed a series of acidic ZnCl 2 ·nH 2 O MSH systems with different hydration levels, and investigate their effect toward the selective fractionation of wheat straw components. With the increase in the hydration level of the acidic ZnCl 2 .nH 2 O MSH system, the hemicellulose removal rate gradually decreases, while cellulose retention improves. Among all, the ZnCl 2 ·6H 2 O system, under optimal conditions (5 % sulfuric acid, 100 °C for 20 min), achieved the best performance with 90.34 % cellulose retention, 93.32 % hemicellulose removal, and 34.07 % lignin removal, resulting in 80.55 % enzymatic digestibility of cellulose. The dissolution mechanism of hemicellulose in the acidic ZnCl 2 ·6H 2 O system was then elucidated by the computational simulations, where the results indicate that hemicellulose molecular chains exhibit greater extension and stability within the ZnCl 2 ·6H 2 O system, accompanied by stronger non-bonded interactions and a more uniform distribution of ions and water molecules. These factors enhance the interaction between the solvent and hemicellulose molecules, leading to higher solubilization efficiency. Further calculations revealed that hemicellulose has the strongest interaction energy with the ZnCl 2 ·6H 2 O/H 2 SO 4 system, confirming its superior selectivity for hemicellulose removal.