Zeolite Encapsulated Molybdenum Catalysts for Selectively Regulating Glucose Epimerization and Isomerization Routes

Most high-value-added sugars and rare sugars are isomers and epimers of natural carbohydrates. Chemical isomerization or epimerization reactions are typically unselective, leading to complex distributions of products, and are not easy to separate. In this study, an alkaline hydrothermal pretreatment and recrystallization process were used to modulate the zeolite crystal structure and Mo active sites on zeolite (Mo/alk@ZSM) for high-value-added sugar production. New Mo species on zeolite were generated after the recrystallization process. The new species precisely regulate glucose isomerization and epimerization routes by changing interactions between active sites and different carbons on glucose. DFT results indicate that Mo–O–Al sites facilitate epimerization through carbon migration (10.48 kJ/mol), whereas Mo–S sites promote isomerization by electronically polarizing sulfur, which lowers the barrier for hydrogen transfer (23.57 kJ/mol). The catalyst with 85 °C alkaline hydrothermal pretreatments (Mo/alk@ZSM-85) predominantly drives glucose isomerization toward fructose, affording a fructose yield of 32.28 with 74.80% selectivity. Mo/alk@ZSM-65 favors the epimerization pathway, delivering a mannose yield of 30.12% with a selectivity of 95.53%. The etching-recrystallization strategy offers a robust platform for selective conversion of glucose to mannose or fructose, providing a promising approach for high-efficiency biomass valorization.