| Benzaldehyde as a high-valuable fine chemical, widely employed in medicines, perfumry, pesticides, dyestuff, etc., shows considerable market prospect and commercial benefit. In recent years, the market demand of high-quality benzaldehyde without chlorine increases greatly in our country. Therefore, the research about the liquid-phase oxidation of toluene to benzaldehyde by hydrogen peroxide has been received considerable attention, because the process produces high quality benzaldehyde at mild reaction conditions. In addition, it is still challenge to simultaneously achieve high conversion of toluene and the high selectivty of benzaldehyde. The development of an effective catalyst is key point in this reaction.In this thesis, we devote to develop an effective solid catalyst for the liquid-phase oxidation of toluene to benzaldehyde by hydrogen peroxide and to deeply understand the function-properties relationship of the catalyst. The vanadium based metal composite oxides were prepared and their catalytic reactivities were investigated in the liquid-phase oxidation of toluene to benzaldehyde. In addition, the structure and properties of these catalysts were characterized by XRD, N2 isothermal adsorption-desorption, H2-TPR etc. The function-properties relationship was discussed.First of all, effects of metal types (Me= Fe, Co, Ni, Mn, Ti, Ce), reaction temperature, Me/V molar ratio, solvents on the reactivity of the V-based catalysts were studied in the reaction. Under the condition of 333 K,3 h,20 mL toluene, hydrogen peroxide/toluene=4:1 (mol ratio),20 mL acetic acid,0.2 g catalyst,34.9% conversion of toluene and 79.4% selectivity of benzaldehyde were obtained on sol-gel prepared Fe-V-0 (Fe/V=1:1, mol ratio) catalyst. Fe-V-0 (Fe/V=1:1, mol ratio) catalyst prepared by sol-gel method had the higher reactivity than any other catalysts.Furthermore, effects of promoters and reaction conditions on the reactivity of Fe-V-0 catalyst were investigated in the reaction. Under the conditon of 333 K,3 h,20 mL toluene, hydrogen peroxide/toluene=4:1 (mol ratio),20 mL acetic acid,0.1 g catalyst,34.2% conversion of toluene and 77.8% selectivity of benzaldehyde as well as 26.6% yield of benzaldehyde were obtained on Co4Fe6V10 catalyst, in which Co was as promoter (n(Co): n(Fe):n(V)=4:6:10). Co4Fe6V10 catalyst showed the highest reactivity among these investigated catalysts.In addition, the structure and properties of V-based composite oxides catalysts were characterized by powder X-ray diffraction (XRD), N2 isothermal adsorption-desorption, H2 temperature-programmed reduction (H2-TPR) technologies. The characterization results indicate, the interaction of metal promoter with vanadium occurred, which significantly affected the specific surface area, redox property and catalytic performance. The structure and crystal phase were determined by relative content of Fe and V in Fe-V-0 catalyst. When Fe/V molar ratio was 1:1, Fe-V-0 catalyst was rutile FeVO4 crystal with high specific surface area and presented the high reactivity than any other catalysts. The structure and properties of Co-Fe-V-0 catalysts varied as a function of Co content. Co4Fe6V10 presented relatively higher reactivity among the investaged catalysts, which was mainly attributed to the interaction of cobalt oxygen species, iron oxygen species and vanadium oxygen species on the catalyst. The capability of electronic transfer could be further enhanced by the presence of Co2+ component in the catalysts. The proper amount of Co added into Co-Fe-V-0 catalsyst favors the liquid-phase oxidation of toluene to benzaldehyde by hydrogen peroxide, due to the improvement of redox properties. |
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