Abstract
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.
Highlights
Selective hydrogenation of benzene towards cyclohexene synthesis has been a significant reaction in the field of catalysis research [1,2,3,4,5]
In order to reveal the status of Zn in Ru-Zn/ZrO2 and how it affects the catalytic system, all samples were evaluated for selective hydrogenation of benzene towards cyclohexene formation
ThisHowever, work is itclearly is worth mentioning that the binding energy (BE) of Zr3d5/2 for ZrO2 observed in this work is clearly lower than that lower than that reported in literature (i.e., 182.2 eV) [29], which might be due to the fact that some of reported in literature (i.e., 182.2 eV) [29], which might be due to the fact that some of the electrons the electrons from Ru were transferred to Zr as well
Summary
Selective hydrogenation of benzene towards cyclohexene synthesis has been a significant reaction in the field of catalysis research [1,2,3,4,5]. Other than ZrO2 , zeolite (i.e., SBA-15) [12] and γ-Al2 O3 [13] were reported as the catalyst support for selective hydrogenation of benzene over Ru-based catalysts. It is of great significance to investigate the status of Zn in Ru-Zn/ZrO2 and how it affects the catalytic system, which could provide essential guidance for the development of the supported Ru-based catalysts. In order to reveal the status of Zn in Ru-Zn/ZrO2 and how it affects the catalytic system, all samples were evaluated for selective hydrogenation of benzene towards cyclohexene formation. In comparison to that observed over Ru-Zn/ZrO2 , the support effect of ZrO2 was proposed for the catalytic performance of Ru-Zn on the selective hydrogenation of benzene towards cyclohexene generation
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