Deciphering Mechanochemical Depolymerization of Xylan Hemicellulose: Dynamic Structural Evolution Modulates Xylo-oligosaccharide Formation

This study deciphers the dynamic mechanochemical depolymerization of xylan hemicellulose during ball milling and its structure-to-performance linkages. Rapid morphological changes occurred within the first hour of milling. Meanwhile, ball milling-induced cleavage of glycosidic bonds via random scission persisted for up to 7 h. This was evidenced by the linear increase in reducing ends and the exponential decline in molecular weight. Specifically, in the first hour of milling, side chains of hemicellulose were more easily removed, which led to a decrease in the branching degree and an enhancement of hydrogen bonding. Those changes in 1 h ball-milled hemicellulose resulted in the comparable hydrothermal stability to that of the unmilled sample. With the milling time extending to 7 h, the skeleton structure of xylan was broken down and the hydrogen bond intensity was weakened. Hydrothermal conversion revealed that the selectivity for X2–X6 in the xylo-oligosaccharide production was improved from 24 to 39% after ball milling, correlated with reducing ends, arabinan, xylan, and molecular weight. This solvent-free mechanochemistry strategy enhances XOS selectivity, while eliminating chemical inputs, offering a carbon-negative route for biorefinery.