Selective hydrogenation of benzene to cyclohexene over Ru–Zn/ZrO2 catalysts prepared by a two-step impregnation method

• Ru–Zn/ZrO2 catalysts are prepared by two-step impregnation.
• Alkali treatment of Ru–Zn/ZrO2 is necessary after reduction.
• Optimal composition of Ru–Zn/ZrO2 is obtained.
• Specific activity of Ru–Zn/ZrO2 catalysts is 4 times that of Ru–Zn catalysts.

Supported Ru–Zn/ZrO2 catalysts are successfully prepared by a two-step impregnation method (Zn deposition–calcination–Ru deposition), and their catalytic performances for selective liquid-phase hydrogenation of benzene are investigated. Physical properties of the catalysts are characterized by N2 adsorption, atomic absorption spectroscopy, X-ray diffraction, temperature-programmed reduction, transmission electron micrographs, dynamic light scattering and X-ray photoelectron spectroscopy. Effects of the Zn content, concentration of alkaline treatment (NaOH) after hydrogen reduction, and hydrogen reduction temperature on the properties of Ru–Zn/ZrO2 catalysts with a fix Ru loading of 10.0 wt% are studied. It is found that the optimal ZnO content is 10.5 wt% in the first-step impregnation (ZnO–ZrO2), and that the alkaline treatment after hydrogen reduction is very important for obtaining the optimal Zn content in the ultimate catalyst. Over the optimal Ru–Zn/ZrO2 catalyst with 10.0 wt% Ru and 2.78 wt% Zn, the selectivity to cyclohexene can reach 80% at the benzene conversion of 50% under the reaction condition (1200 r/min, 150 °C, 5 MPa H2). The specific activity of the catalyst is about 4 times higher than those of Ru–Zn catalysts prepared by coprecipitation. The recycle catalytic performance of the optimal Ru–Zn/ZrO2 catalyst is also explored. There is no significant decrease of catalytic performance after four recycles, indicating good stability of the catalyst prepared by the two-step impregnation method.

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