Efficient epimerization of glucose to mannose by commercial zeolite support molybdenum in ethanol-aqueous media under mild conditions

Mannose has demonstrated high value and versatility in various fields, leading to a growing interest in the catalytic epimerization of glucose to mannose using solid catalysts. Herein, zeolite-supported molybdenum (Mo) catalysts were prepared by impregnation-calcination (Mo/Z) and hydrothermal-calcination methods (Mo/Z-HT), respectively. HR-STEM characterization showed the diameter of Mo active sites (MoO 2 ) in Mo/Z and Mo/Z-HT are 57.83 ± 18.05 and 8.37 ± 1.62 nm, respectively. The hydrothermal-calcination process decreased the spatial scale of Mo active sites. The specific surface area of Mo/Z-HT (363.75 cm 3 /g) is higher than that of Mo/Z (200.46 cm 3 /g). The hydrothermal process promotes the formation of hierarchical structure of ZSM. Mo can be embedded in the hierarchical structure during the hydrothermal process, which improved the stability of catalysts significantly. Catalytic performance shown that Mo/Z-HT achieves a mannose yield of 32.81% with a selectivity of 94.20% in water at 130 °C for 30 min. Mo/Z achieved a mannose yield of 33.13% with a selectivity of 92.50% in ethanol-aqueous at 80 °C after 40 min. The recycling experiment showed that Mo/Z-HT exhibits high stability at the sixth cycle with a 30.76% yield and a selectivity of 94.55%. The different reaction temperatures may be attributed to ethyl glucoside was synthesized in ethanol media, which is more easily selectively epimerized in mild conditions. The protecting groups (-OC 2 H 5 ) at C1 position may decrease the activation energy of epimerization and realize kinetic control of site selectivity.