Effect of alcohols as additives on the performance of a nano-sized Ru–Zn(2.8%) catalyst for selective hydrogenation of benzene to cyclohexene

In liquid phase hydrogenation of benzene, Ru–Zn(2.8%) catalyst (Ru–Zn catalyst with the Zn mass percentage of 2.8%) exhibited the excellent selectivity to cyclohexene for the nano-size effect and the chemisorbed ZnSO4·3(ZnOH)2⋅7H2O salt formed by the promoter ZnO reacting with ZnSO4. It is found that the cyclohexene yield of Ru–Zn(2.8%) catalyst generally increased with the molecular weight of PEG (Polyethylene glycol) in the presence of ZnSO4. PEG-10000 (Polyethylene glycol with the molecular weight of 10,000) as an additive made Ru–Zn(2.8%) catalyst give the highest cyclohexene yield of 62.3% in presence of ZnSO4 for the following two reasons. Firstly, the addition of PEG-10000 in presence of ZnSO4 could make more Zn2+ chemisorbed on the surface of the catalyst. The Zn2+ ions chemisorbed could be beneficial for stabilizing the formed cyclohexene, accelerating the desorption and hindering the re-adsorption of cyclohexene on the surface of the catalyst. Secondly, the PEG-10000 added might form two molecular hydrogen bonds in eight-numbered rings with the cyclohexene in the liquid phase, which could stabilize the cyclohexene in the liquid phase and hinder the re-adsorption. 2 g of this catalyst with 0.2 g PEG-10000 as an additive pretreated in presence of ZnSO4 gave a selectivity to cyclohexene of 78.6% and the best cyclohexene yield of 64.5% at the benzene conversion of 82.0% at 15 min. Moreover, this catalyst had an excellent stability since ZrO2 and PEG-10000 could effectively inhibit the agglomeration of Ru–Zn(2.8%) catalyst.

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• PEG-10000 made the catalyst give the best cyclohexene yield among all the alcohols.
• More Zn2+ chemisorbed stabilized the formed cyclohexene on catalyst surface.
• The intermolecular hydrogen bonds stabilized the cyclohexene in liquid phase.
• The pretreated catalyst gave the best cyclohexene yield and had a excellent stability.