Effect of (Zn(OH)2)3(ZnSO4)(H2O)5 on the performance of Ru–Zn catalyst for benzene selective hydrogenation to cyclohexene

A series of Ru–Zn catalysts with different Zn contents were prepared by co-precipitation. The catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) – Ar+ sputter, transmission electron micrographs (TEM)–energy dispersion scanning (EDS) and temperature-programmed reduction (TPR). The performances of the catalysts for benzene selective hydrogenation to cyclohexene were investigated in the presence of 0.6 mol/L of ZnSO4. The results showed that the Ru and Zn in Ru–Zn catalyst were in metallic Ru and ZnO respectively and the ZnO was rich on the surface. The ZnO alone could not improve the selectivity to cyclohexene of Ru–Zn catalyst. However, the ZnO on the surface could react with ZnSO4 to form a (Zn(OH)2)3(ZnSO4)(H2O)5 salt. The (Zn(OH)2)3(ZnSO4)(H2O)5 salt chemisorbed played a key role in improving the selectivity to cyclohexene of Ru–Zn catalysts. After Ru–Zn(8.6%) catalyst was pretreated 22 h in ZnSO4 solution at 140 °C and 5 MPa H2, a cyclohexene selectivity of 81.4% at a benzene conversion of 54.0% was achieved at 10 min and a maximum cyclohexene yield of 58.9% was reached. Moreover, the activity was stable above 50% and the cyclohexene selectivity and yield were steadily above 76% and 40% on this catalyst in the first six recycles, respectively.

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► The Zn in Ru–Zn catalyst existed as ZnO that was rich on the catalyst surface.
► The ZnO on the surface could react with ZnSO4 to form a (Zn(OH)2)3(ZnSO4)(H2O)5 salt.
► The salt chemisorbed improved the selectivity to cyclohexene of Ru–Zn catalyst.
► The pretreated Ru–Zn(8.6%) catalyst gave a cyclohexene yield of 58.9% and had a excellent stability.