Gas-phase Selective Oxidation Of Ethanol Over Metal-doped γ-MnO₂ And Au/M-γ-MnO₂ Catalysts

MnO₂ and Au/MnO₂ catalysts are commonly used in catalytic combustion of organic pollutants due to their high catalytic activity and difficulty in selectivity control for gas-phase oxidation.As a result,the preparation of value-added chemicals by gas-phase selective oxidation over MnO₂ and Au/MnO₂ catalysts remains a great challenge.Selective oxidation of ethanol to acetaldehyde（AC）meets the development needs of the green and sustainable chemical industry,and is considered to be the best choice to replace the ethylene-based high-pollution and high-cost wack oxidation process.However,the applications of MnO₂ and Au/MnO₂ in the gas-phase oxidation of ethanol were focused on catalytic combustion,and the examples on selective ethanol oxidation are scare.In view of the important influence of the structure and composition of MnO₂ on its redox properties,this thesis aims to improve the catalytic performance of MnO₂ and Au/MnO₂ catalysts in the gas-phase selective oxidation of ethanol by adjusting different crystal forms and metal-dopings of MnO₂.The results are as follows:Four types（α,β,γandδ-）of MnO₂ catalysts were initially prepared and evaluated in the gas-phase aerobic oxidation of ethanol and CO.Interestingly,γ-MnO₂ achieved the highest AC yield（～65%）and CO conversion（～95%）at 200 ℃.Then in order to elucidate the structure-performance correlations forγ-MnO₂-catalyzed selective oxidation and complete oxidation,various metal-doped M-γ-MnO₂(M=Cu²⁺,Zn²⁺,Mg²⁺,Co²⁺,Ni²⁺,Ca²⁺,Al³⁺,Fe³⁺,La³⁺)catalysts were prepared by a one-pot hydrothermal method.The results strongly evidenced that Zn-γ-MnO₂(T₉₀=160 ℃)with the highest molar ratio of surface base/acid sites obtained the best activity in the CO oxidation.Whereas Cu-γ-MnO₂with the highest surface reducibility achieved the highest AC yield(～75%at 200 ℃,STY=～2.8 g g^-1_cat h^-1)in the selective ethanol oxidation.This is due to the presence of more Mn²⁺/Mn³⁺defect sites and oxygen vacancies at lower temperatures,which facilitates the low-temperature activation of O₂ and ethanol.Further kinetic studies showed that the concentration of O₂ and ethanol had little influence on the reaction rate,and the isotope effect proved that alcohol hydroxyl cleavage was still the rate-limiting step of the reaction.In order to further increase the activity of M-γ-MnO₂,the Au/M-γ-MnO₂ catalysts were prepared by the colloidal deposition method,which rendered uniform gold particles with average size of～3.0 nm,and significantly improved reducibility compared to the M-γ-MnO₂ supports.The prereduced Au/M-γ-MnO₂catalysts showed higher activity than the preoxidized ones and the corresponding M-γ-MnO₂ supports in the selective ethanol oxidation.Among them,Au/Fe-γ-MnO₂ showed the highest catalytic activity.The Fe doping amounts（3%,10%,15%）have important influence on the catalytic performance of Au/Fe-γ-MnO₂.The catalyst with 10 mol%Fe doping showed the highest catalytic efficiency,achieving～85%acetaldehyde yield and STY～3.2 g g^-1_cat h^-1 at 200 ℃.