Synthesis Of Mesoporous And Hollow Manganese Oxides And Research On Their Catalytic Activity

As important functional metal oxides, manganese oxides have been widely used as catalysts, adsorbents, cathode materials for lithium ion batteries and so on.We have successfully synthesized several kinds of manganese oxides in our previous work, such asα-MnO2 nanorods,β-MnO2 nanorods, and Mn3O4 nanorods. And these manganese oxides have been used as catalysts for the oxidation of methylene blue (MB) dye with H2O2 and for the esterification of hexyl acetate. Based on the previous work of our group, in this thesis, manganese oxides with different structures have been fabricated, including mesoporousβ-MnO2 nanofibers and Mn2O3 hollow nanospheres. We demonstrated high catalytic efficiency of the mesoporousβ-MnO2 nanofibers as catalysts in the oxidation of methylene blue (MB) dye with H2O2 and of the Mn2O3 hollow nanospheres as catalysts in the synthesis of hexyl acetate. The relationship between the catalytic properties of the catalysts and their nanostructures has been investigated.Mesoporousβ-MnO2 nanofibers were successfully synthesized based on MCM-41 mesoporous template. Firstly, Mn-MCM-41 was synthesized by impregnating mesoporous MCM-41 template with an aqueous ethanol solution of Mn(NO3)2 and heating it at 450°C for 4 h under air, and then the manganese oxide was obtained through dissolving the MCM-41 template with an aqueous solution of NaOH. The as-prepared manganese oxide was shown to be pure phase ofβ-MnO2 nanofibers with diameters less than 3 nm, through X-ray diffraction, high resolution transmission electron microscopy and N2 adsorption and desorption isotherm characterizations. These orderedβ-MnO2 nanofibers formed a mesoporous structure similar to the MCM-41 template, with a high surface area of 136.5 m2/g. We demonstrated the high catalytic efficiency ofβ-MnO2 nanofibers as catalyst in the oxidation of methylene blue (MB) dye with H2O2. The decoloration rate of 60 mg/L MB aqueous solution reached 97.59 % within 100 min, indicating that the catalyst can degrade high concentration of MB solution efficiently.Mn2O3 hollow nanospheres were prepared and used as catalysts for the synthesis of hexyl acetate. In this thesis, Mn(NO3)2 solution was used as the source of manganese and carbon microspheres were used as templates to fabricate Mn2O3 hollow nanospheres with different characteristics in morphology. After being impregnated for 1-3 days and heated under 450°C for 6 hours in different rate of heat rise, single- and double-walled Mn2O3 hollow nanospheres were obtained respectively. The diameter of the single-walled Mn2O3 hollow nanospheres is about 600 nm, while that of the double-walled Mn2O3 hollow nanospheres is about 300 nm. 89.7 % yield of hexyl acetate was achieved using the double-walled Mn2O3 hollow nanospheres as catalyst under optimum condition: molar ratio of acetic acid to hexyl alcohol, 1.5:1,catalyst dosage, 0.05 g, reaction temperature, 135°C and reaction time, 1.5 h.