Study in Surface Acidity and Catalytic Behavior of Mesoporous Zirconia-supported Catalysts
|Keywords:||中孔洞氧化鋯;異構化反應;溴化反應;Mesoporous zirconia;Isomerization reaction;Bromination reaction||Issue Date:||2006||Abstract:||
研究中的第二部分，我們利用鎢金屬負載於氧化鋯載體上，經過特定的鍛燒溫度及前處理，使其具有與V-HPO相似的化學官能基，並選用可由UV-vis 偵測的Phenol Red Bromination作為測試反應，溴的來源則來自於NH4Br取代過去的Br2。研究結果發現：以鎢為基礎的仿生酵素在水溶液及近乎中性的反應條件下，所得到的催化效率幾乎和V-HPO在酸性、有機相反應時相當。同時我們也使用了另一種前過渡金屬 “Re” 來取代原先的鎢金屬作為仿生觸媒。利用Re2O7本身獨特的物性及化性，經適溫鍛燒後，使其在大表面積的中孔洞氧化鋯載體上具有高度的分散性，藉此創造出更多的活性位置。此類觸媒的催化效率比V-HPO高出十倍之多，且穩定性高，能經再生後重複使用，是目前芳香族化合物在非均相溴化反應上最佳的觸媒。
Mesoporous zirconia, synthesized from surfactant templating, was employed to disperse alumina and tungstate (AW/m-ZrO2) and then used as a catalyst for the isomerization of n-butane. After calcination at 700 oC, tetragonal phase of zirconia was formed with high surface area. Furthermore, the catalyst AW/m-700 performed exceptionally well, and gave maximum rate and stability at reaction temperature of 250 oC, compared to other tungstated zirconia-based catalyst reported so far. Moderate calcination temperature leads to the presence of more Brönsted acid sites with higher acid strength. In comparison with the normal dense phase zirconia catalyst, AW/m-ZrO2 showed higher activity than the corresponding AW/d-ZrO2. The difference in catalytic activities was examined by various surface characterization techniques, and was attributed to different degree of dispersion of tungstate on the surface of m-ZrO2.
In the second part, we focused on the alumina sulfate supported on mesoporous zirconia (AS/m-ZrO2). Our primary goal was to understand the origin of the catalytic active sites and the roles of these sites in the various stages of reaction. Three kinds of acid sites were determined by pyridine adsorption with XPS (N 1s core-level). The remarkable performance of the Al-promoted catalysts is due to an optimum balance of these acid sites, and the catalytic activity seems to correlate directly with the amount of weak Brönsted acid sites. 27Al MAS-NMR and in-situ DRIFT results elucidate the origin and the role of these acid sites in the various stages of isomerization reaction. EPR experiments explore the existence of O2-; the active species eliminates hydrogen atoms to form n-butene and starts the overall reaction in our experiment.
The promotion effect of Ga on sulfated zirconia catalyst was studied in n-butane isomerization reaction and compared with Al-promoted catalyst. Our NH3-TPD and DRIFT studies reveal that the enhanced acid sites are due to the addition of Al and Ga promoters to mesoporous zirconia. AS/m-ZrO2 promoted by Al seems to bring greater contribution to the acidity of catalyst than that by Ga. Therefore, the improved activity of AS/ZrO2 over S/ZrO2 catalyst is related to the increased acidity created by the Al promoter, while the Ga promotion effect is then through a redox mechanism in the acid catalyzed initiation mechanism.
Finally, highly dispersed biomimetic catalysts tungstated zirconia (WZ) and mesoporous ziconia-supported rhenium oxide (Re/m-ZrO2) were prepared by a simple impregnation method and excellent catalytic performances were obtained in liquid phase bromination of phenol red using hydrogen peroxide and ammonium bromide under neutral condition. It was shown that the activity of phenol red bromination reaction depends directly on the dispersion of WOx or ReOx species over support surfaces.
|Appears in Collections:||化學系|
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