Fractionation of birch sawdust into carbohydrates and high purity lignin by ZnBr2 molten salt hydrates under mild conditions

The fractionation of lignocellulose is the key step for its efficient utilization. A mild fractionation process was proposed for producing glucan oligomers and high purity lignin using ZnBr 2 molten salt hydrate (ZnBr 2 -MSH). It was found that the cellulose could be hydrolyzed rapidly using ZnBr 2 -MSH with a slight amount of acid (ZnBr 2 -AMSH), giving as high as 88.0 % yield of glucan oligomers together with 92.4 % purity of lignin at 90 °C for 20 min. The long-time molecular dynamics simulation indicated that ZnBr 2 -MSH had more free Br ions to form hydrogen bonds (H-bonds) with H atom in hydroxyl and Zn ions with stronger coordination ability to O atoms in hydroxyl, and hence exhibited that stronger ability to destruct the H-bonds among cellulose bundles. Therefore, ZnBr 2 -MSH could deconstruct cellulose crystal under mild conditions. The lignin obtained by fractionation of ZnBr 2 -AMSH contained more polar functional groups, demonstrating its potential application in a binder for hard carbon (HC) anode in sodium-ion battery, and the resultant electrode contributed to good electrochemical performances and cycling stability. This research work provides a promising approach for efficient fractionation and application of the three components in lignocellulose.