Correlation between acidic properties of nickel catalysts and catalytic activities for ethylene dimerization and butene isomerization
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Catalytic activities of NiO–SiO2 for ethylene dimerization and butene isomerization run parallel when the catalysts are activated by evacuation at elevated temperatures, giving two maxima in activities. The variations in catalytic activities are closely correlated to the acidity of NiO–SiO2 catalysts. Catalytic activities of NiO–TiO2 catalysts modified with H2SO4, H3PO4, H3BO3, and H2SeO4 for ethylene dimerization and butene isomerization were examined. The order of catalytic activities for both reactions was found to be NiO–TiO2/SO42- >> NiO–TiO2/PO43-NiO–TiO2/BO33- > NiO–TiO2/SeO42-> NiO–TiO2, showing clear dependence of catalytic activity upon acid strength. The high catalytic activity of supported nickel sulfate for ethylene dimerization was related to the increase of acidity and acid strength due to the addition of NiSO4. The asymmetric stretching frequency of the S=O bonds for supported NiSO4 catalysts was related to the acidic properties and catalytic activity. That is, the higher the frequency, the larger both the acidity and catalytic activity. For NiSO4/Al2O3–ZrO2 catalyst, the addition of Al2O3 up to 5 mol% enhanced catalytic activity for ethylene dimerzation and strong acidity gradually due to the formation of Al–O–Zr bond. The active sites responsible for ethylene dimerization consist of a low-valent nickel, Ni+, and an acid, as evidenced by the IR spectra of CO adsorbed on NiSO4/γ -Al2O3 and Ni 2p XPS.
KeywordsTiO2 Al2O3 Acidity Catalytic Activity H3BO3
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