| Ester is a pivotal chemical and organic intermediate. Traditionally, it is synthesized by the complicated two-step procedure:first the carboxylic acids or carboxylic acid derivatives are prepared by oxidation of aldehydes or alcohols and sequentially the ester are prepared by esterification. The direct oxidative esterification of aldehydes with alcohols caused much attention due to its short process flow and environmental benefits. Recently, the supported Au-based catalysts have shown a wide application prospect because of its higher performance than that of traditional Pd-based catalysts.In this dissertation, the Au catalysts prepared by different methods were applied for the oxidative esterification of aldehydes. The effects of the supports and the interaction between Au and support were investigated. The main works are shown as follows:(1) The Al2O3, TiO2, ZnO, CeO2 and ZrO2 supported Au catalysts were prepared by deposition precipitation method. The effects of preparation temperature, aging time, initial concentration and reduction step were investigated and the optimum conditions were obtained. These Au catalysts were applied for the oxidative esterification of isobutyraldehyde and the one loaded on self-made CeO2 presented the best performance.(2) The Ce-Zr solid solutions were synthesized using (NH4)2Ce(NO3)6 as precursor, while the mixed oxides were obtained by Ce(NO3)3 precursor. The solid solutions exhibited higher BET surface area, and larger amount of H2 consumption, stronger acidity and basicity than the mixed oxides at the same Ce/Zr mole ratio. Moreover, the Au catalysts supported on the solid solutions exhibited higher catalytic performance than that of catalysts loaded on mixed oxides. Plausible reaction mechanism for oxidative esterification of aldehydes on Ce-Zr solid solution supported Au nanoparticles was proposed. The effects of different substrates and recycling times were also investigated.(3) The Ceo.6Zro.4O2 solid solution was modified by MgO, (NH4)2SO4 or MoO3 respectively through impregnation method and then the corresponding Au catalysts were prepared by deposition precipitation method. The Au particles of these catalysts presented similar size distribution and valent state. All the modified Au/Ceo0.6Zr0.4O2 catalysts demonstrated obvious enhancement in the catalytic activities and the MgO promoted catalyst presented the best performance. According to the correlation of catalytic activity and characteristic data, we speculated that the MgO increased the amount of active oxygen species caused by the enhancement of the interaction between Au and support leading to the improvement of catalytic activity.(4) Three ceria nanoshapes (particles, cubes and rods) were synthesized by hydrothermal method and the CeO2 rods supported Au catalyst presented the best activity for oxidative esterification of MAL (methacrolein). Further, the Ce-M (Zr, La and Fe) solid solution nanorods were prepared by adjusting the concentration of NaOH during the hydrothermal process and their Au catalysts were applied to oxidative esterification of MAL. It is found that the introduction of La decreased the activity and the addition of Zr improved the activity slightly, while the catalyst modified by Fe promoted the performance obviously. The introduction of Fe maybe enhanced the reducibility and basicity of support leading to the improvement of activity according to the H2-TPR and CO2-TPD analysis.(5) The Au/ZnO catalyst was synthesized by colloid deposition method using poly vinyl alcohol (PVA) as protective agent. Three different pre-treatment:heat treatment, hot water washing and water reflux were compared to remove the PVA stabilizer. The catalyst heat treated at 300 ℃ presented the best activity due to the minimum content of PVA. It is found that the distribution and loading content of Au particles was closely related to the pH of the colloid solution. Further, the catalysts with the similar size of Au particles loaded on different supports were successfully prepared. The Au/ZnO presented the best performance, which may be due to the strong basic surface sites and the strong interaction between Au and ZnO.(6) Two series of Au-Pd bimetal catalysts were synthesized by impregnation-reduction method (WIR) and sol-immobilization (SI) method. Au, Pd and Au-Pd particles prepared by WIR method presented big size due to the agglomeration, while the particles prepared by SI method showed uniform distribution and presented the alloy form confirmed by UV-vis and EDS line scan. The addition of Pd improved the conversion of oxidative esterification of MAL, while the selectivity of MMA retained unchanged. |
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