Confined synthesis of glucan oligomers from glucose in zeolites†

Glucan oligomer is a unique glucose-derived component with versatile applications as a plant growth elicitor and prebiotic based on its degree of polymerization (DP) and glycosidic bond species. However, in conversional aqueous media, glycosylation can be carried out among 5 free –OH groups on glucose, resulting in a glucan oligomer linked through various uncontrolled glycosidic bonds, which can lead to uncertainty of their application. To address this challenge, the confined synthesis of the glucan oligomer in the micropores of zeolites was developed. Glucose was impregnated into the microporous channel of H-Y with the FAU framework or H-MOR with the MOR framework to restrict its spatial configuration and thereby selectively produce the glucan oligomer with specific glycosidic bonds. The results showed that a 41.7% yield of the glucan oligomer with the DP ranging from 2 to 5 can be produced in H-Y, and 59.2% of the formed glucan oligomer was linked through (1 → 6) glycosidic bonds. When H-MOR was used as a catalyst, a high glucan oligomer yield of 61.7% was achieved with glucose units linked through 87.6% of (1 → 4) linkages. The one-dimensional microporous structure of MOR zeolites is not only beneficial for controlling the glycosidic bond structure, but can also improve the diffusion of the produced glucan oligomer, resulting in facile separation of the glucan oligomer from the zeolites. The H-Y and H-MOR used in glycosylation can be readily regenerated via calcination in air after removing the organic residues, and the catalysts can be recycled more than 5 times without catalytic performance decline.