Subject Area物理化学
Pd/WO3–ZrO2 catalysts for direct oxidation of ethylene to acetic acid
王丽霞; 楚文玲; 徐庶亮; 杨维慎; Weishen Yang
Conference NameA Pre-conference of the 14th Intenational Congress on Catalysis
Conference Date2008-7-9
Conference Place中国
Funding Organization大连化学物理研究所
Other AbstractTungstated zirconia materials (WO3-ZrO2) are very attractive, environmentally friendly solid acids. These materials show excellent catalytic activity in the direct oxidation of ethylene to acetic acid if promoted by noble metal Pd. In this work, a series of Pd/WO3–ZrO2 catalysts with different W/Zr ratios, calcination temperatures and Pd precursors were prepared by an impregnation method and investigated by various characterization techniques. Special attention was paid to the oxidation state of tungsten and palladium under different preparation conditions. Our results revealed that tungsten phase composition is the function of W loadings and calcination temperatures. With increasing W/Zr ratios from 0.05 to 0.5 and the calcination temperatures from 873 K to 1273 K, WOx surface species on the catalysts underwent a transformation from monotungstate/polytungstate species to polytungstates/crystalline WO3 coexistence and further to crystalline WO3 particles. At lower W loadings (W/Zr=0.05) and calcination temperature (673 K), isolated monotungstate species predominate on the zirconia surface. These species are difficult to reduce and exhibit poor activity for the ethylene oxidation to acetic acid. With the increase in the W/Zr ratios and calcination temperatures, polytungstate species begin to appear and reach the maximum amount for the sample (W/Zr=0.2) calcined at 1073 K, which exhibits the highest catalytic activity. This suggests that the polytungstate species are the catalytically active sites and play a significantly important role in the oxidation of ethylene to acetic acid. Accordingly, the prevailing presence of crystalline WO3 particles with further increase of W/Zr ratio and calcination temperature inevitably leads to the significant decrease in the catalytic activity. In addition, the results indicate that the selectivity of the catalysts is a strong function of the state of palladium, which is found to be strongly dependent on the Pd precursor as well as the calcination temperatures. Compared with Pd(NH3)2Cl2 and Pd(CH3COO)2 precursor, PdCl2 as the Pd precursor can lead to a strong interaction between the metal and the support. As a result, the formation of higher dispersion Pd2+ species and their stabilization on the support surface appears to be the main reason for the excellent selectivity of the catalyst. Considering the calcination temperatures, it can be found that at the higher calcination temperature (1273 K), the mobility of Pd particles is high enough for agglomeration to take place, leading to predominantly larger metallic Pd clusters, which are active for ethylene combustion to COx. It is proposed that the state of palladium seems to be responsible for the product distributions, in which it was believed that the presence of finely dispersed Pd2+ species appeared to be favorable to obtain a higher selectivity to acetic acid.
Document Type会议论文
Corresponding AuthorWeishen Yang
Recommended Citation
GB/T 7714
王丽霞,楚文玲,徐庶亮,等. Pd/WO3–ZrO2 catalysts for direct oxidation of ethylene to acetic acid[C],2008:60/1.
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