Selective Hydrogenation of Benzene to Cyclohexene over Ru-Zn Catalysts: Investigations on the Effect of Zn Content and ZrO₂ as the Support and Dispersant

• Main Authors: Haijie Sun, Zhihao Chen, Lingxia Chen, Huiji Li, Zhikun Peng, Zhongyi Liu, Shouchang Liu
• Format: Article
• Language: English
• Published: MDPI AG 2018-11-01
• Series: Catalysts
• Subjects: selective hydrogenation; benzene; cyclohexene; Ru; Zn; ZrO₂
• Online Access: https://www.mdpi.com/2073-4344/8/11/513

m-ZrO₂ (monoclinic phase) supported Ru-Zn catalysts and unsupported Ru-Zn catalysts were synthesized via the impregnation method and co-precipitation method, respectively. The catalytic activity and selectivity were evaluated for selective hydrogenation of benzene towards cyclohexene formation. Catalyst samples before and after catalytic experiments were thoroughly characterized via X-ray diffraction (XRD), X-ray Fluorescence (XRF), transmission electron microscopy (TEM), N₂-sorption, X-ray photoelectron spectroscopy (XPS), H₂-temperature programmed reduction (H₂-TPR), and a contact angle meter. It was found that Zn mainly existed as ZnO, and its content was increased in Ru-Zn/m-ZrO₂ by enhancing the Zn content during the preparation procedure. This results in the amount of formed (Zn(OH)₂)₃(ZnSO₄)(H₂O)₃ increasing and the catalyst becoming more hydrophilic. Therefore, Ru-Zn/m-ZrO₂ with adsorbed benzene would easily move from the oil phase into the aqueous phase, in which the synthesis of cyclohexene took place. The generated cyclohexene then went back into the oil phase, and the further hydrogenation of cyclohexene would be retarded because of the high hydrophilicity of Ru-Zn/m-ZrO₂. Hence, the selectivity towards cyclohexene formation over Ru-Zn/m-ZrO₂ improved by increasing the Zn content. When the theoretical molar ratio of Zn to Ru was 0.60, the highest cyclohexene yield of 60.9% was obtained over Ru-Zn (0.60)/m-ZrO₂. On the other hand, when m-ZrO₂ was utilized as the dispersant (i.e., employed as an additive during the reaction), the catalytic activity and selectivity towards cyclohexene synthesis over the unsupported Ru-Zn catalyst was lower than that achieved over the Ru-Zn catalyst with m-ZrO₂ as the support. This is mainly because the supported catalyst sample demonstrated superior dispersion of Ru, higher content of (Zn(OH)₂)₃(ZnSO₄)(H₂O)₃, and a stronger electronic effect between Ru and ZrO₂. The Ru-Zn(0.60)/m-ZrO₂ was reused 17 times without any regeneration, and no loss of catalytic activity and selectivity towards cyclohexene formation was observed.