Separation of short-chain glucan oligomers from molten salt hydrate and hydrolysis to glucose

Abstract

Selective production of glucose from the hydrolysis of cellulose is the key step for efficient utilization of lignocellulose biomass. Crystalline cellulose can be dissolved and hydrolyzed into glucose with a high selectivity in molten salt hydrates (MSHs). However, the separation of the formed glucose is challenging due to its high solubility in the MSHs. To address this issue, a stepwise method is introduced, where cellulose is hydrolyzed into short-chain glucan oligomers in the MSH of LiBr. We demonstrate that compared to glucose, the formed glucan oligomers with a degree of polymerization of 4–11 can be efficiently separated from the MSH hydrolysate using an anti-solvent precipitation method. The separated oligomers can be readily converted into glucose under mild conditions and used for other applications. Under optimized conditions, 90.3% of glucan oligomer can be produced from crystalline cellulose and separated from the MSH with the addition of methanol, and the precipitated glucan oligomer can be hydrolyzed into glucose with a yield of 99.7% using dilute sulfuric acid. We show that the precipitation efficiency is influenced by the glucan oligomer chain length, glycoside bond type and concentration. Moreover, the separation of a glucan oligomer from a cotton straw hydrolysate was also investigated in the MSH. 79.2% yield of glucan oligomer was obtained from the hydrolysis of cotton straw at 130 °C for 2 h. With the addition of methanol, the glucan oligomer was precipitated with a selectivity of 60.8%.