Synthesis And Characterization Of New Manganese Oxide Catalysts For Hydrogenation Of Methyl Benzoate

Aromatic aldehydes are important intermediates in fine chemical industries, manufacturing pharmaceuticals, agrochemicals, perfumes and flavors. Various processes have been used for the synthesis of aromatic aldehydes, e. g. partial oxidation of alkyl aromatics, halogenation of alkyl aromatics followed by hydrolysis, or halogenation of aromatic acids followed by hydrolysis, the so-called Rosenmund reduction. However, all these technologies have their own drawbacks, such as low selectivity, low yield and production of a large amount of harmful wastes. The direct hydrogenation of carboxylic acids or carboxylic acid esters to corresponding aldehydes is a more preferable method of synthesizing aldehydes from the point view of green technology. Benzaldehyde was produced via this method with a high yield using chromium modified zirconia catalyst by Mitsubishi Chemicals in 1988. Later on, a large number of metal oxides, including alkali earth oxides, transition metal oxides and rare earth oxides, have been suggested for use as catalysts for the selective hydrogenation of benzoic acid and methyl benzoate. Among these metal oxide catalysts, manganese oxide has shown good activity and selectivity to benzaldehyde at rather low temperature. Recently, new methodologies of material synthesis and new materials appear continuously. It is our duty and obligation to introduce these methodologies and materials into catalysis to improve existing catalysts and technology or to create new catalysts and new technology.In this work, the support effect of manganese oxide catalysts in hydrogenation of methyl benzoate was studied, providing the principles and knowledge for the design of new catalysts. Two types of alumina were synthesized. The mesoporous Mn/Al catalysts were prepared by impregnation and in-situ synthesis method. The Mn/Al and M/Mn/Al hydrotalcite-like compounds were prepared by co-precipitation method and used as precursors to prepare relevant catalysts. The new catalysts were characterized and their catalytic performance in hydrogenation of methyl benzoate was investigated and compared with that of the conventional Mn/γ-Al2O3 catalyst.1. Manganese oxide supported on MgO, γ-Al2O3, SiO2, ZrO2, TiO2 and SiO2-Al2O3 catalysts were prepared. The effect of support on their catalytic behavior for hydrogenation of methyl benzoate to benzaldehyde was studied. The formation of toluene is suppressed on the supported catalysts due to the dilution of oxygen vacancies on the catalyst surface. The benzaldehyde yield of the supported catalysts follows the trend of Mn/γ-Al2O3>Mn/TiO2>Mn/ZrO2>Mn/SiO2-Al2O3>Mn/SiO2>Mn/MgO. XRD measurements show that the Mn nitrate precursor is essentially transformed to highly dispersed MnO2 on the supports after calcination and subsequently to MnO under reaction conditions with an exception of Mn/MgO. TPR and XPS analyses suggest that a strong interaction between manganese oxide and the γ-Al2O3 support plays a positive role in the hydrogenation reaction.2. Two types of mesoporous alumina were synthesized using non-ionic surfactants. The surface areas of meso-Al2O3 and MSU-( were 435.9 and 362.4 m2/g respectively. The XRD and BET results show that the two samples have mesoporous structure. The mesoporous alumina supported manganese oxides catalysts were prepared by impregnation method. With manganese introduced, the mesoporous structure is maintained though the surface area, pore diameter and pore volume are decreased. The catalytic behavior for hydrogenation of methyl benzoate was tested and compared with conventional 10%Mn/(-Al2O3. Mn/MSU-γ catalysts have higher catalytic activity than 10%Mn/(-Al2O3. The conversion of methyl benzoate is 96.5% at 390oC. The activity of Mn/meso-Al2O3 catalyst was poorer than 10%Mn/(-Al2O3 in spite of its higher surface area. The conversion of methyl benzoate is 86.1% at 410oC. 10-30%Mn-MSU-γ catalysts were also prepared by in-situ synthesis method. After calcined at 500oC, the surface areas of catalysts are about 300 m2/g. The conversion of...