The role of La in improving the selectivity to cyclohexene of Ru catalyst for hydrogenation of benzene

Investigation on Mn3O4 Coated Ru Nanoparticles for Partial Hydrogenation of Benzene towards Cyclohexene Production Using ZnSO4, MnSO4 and FeSO4 as Reaction Additives

Mn₃O₄ coated Ru nanoparticles (Ru@Mn₃O₄) were synthesized via a precipitation-reduction-gel method. The prepared catalysts were evaluated for partial hydrogenation of benzene towards cyclohexene generation by applying ZnSO₄, MnSO₄ and FeSO₄ as reaction additives. The fresh and spent catalysts were thoroughly characterized by XRD, X ray fluorescence (XRF), XPS, TEM and N₂-physicalsorption in order to understand the promotion effect of Mn₃O₄ as the modifier as well as ZnSO₄, MnSO₄ and FeSO₄ as reaction additives. It was found that 72.0% of benzene conversion and 79.2% of cyclohexene selectivity was achieved after 25 min of reaction time over Ru@Mn₃O₄ with a molar ratio of Mn/Ru being 0.46. This can be rationalized in terms of the formed (Zn(OH)₂)₃(ZnSO₄)(H₂O)₃ on the Ru surface from the reaction between Mn₃O₄ and the added ZnSO₄. Furthermore, Fe²⁺ and Fe³⁺ compounds could be generated and adsorbed on the surface of Ru@Mn₃O₄ when FeSO₄ is applied as a reaction additive. The most electrons were transferred from Ru to Fe, resulting in that lowest benzene conversion of 1.5% and the highest cyclohexene selectivity of 92.2% after 25 min of catalytic experiment. On the other hand, by utilizing MnSO₄ as an additive, no electrons transfer was observed between Ru and Mn, which lead to the complete hydrogenation of benzene towards cyclohexane within 5 min. In comparison, moderate amount of electrons were transferred from Ru to Zn²⁺ in (Zn(OH)₂)₃(ZnSO₄)(H₂O)₃ when ZnSO₄ is used as a reaction additive, and the highest cyclohexene yield of 57.0% was obtained within 25 min of reaction time.

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

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.

Benzene partial hydrogenation: advances and perspectives

The partial hydrogenation of benzene to cyclohexene is an economically interesting and technically challenging reaction. Over the last four decades, a lot of work has been dedicated to the development of an exploitable process and several approaches have been investigated. However, environmental constraints often represent a limit to their industrial application, making further research in this field necessary. The goal of this review is to highlight the main findings of the different disciplines involved in understanding the governing principles of this reaction from a sustainable chemistry standpoint. Special emphasis is given to ruthenium-catalyzed liquid phase batch hydrogenation of benzene.