Enhancing the selective hydrogenation of benzene to cyclohexene over Ru/TiO2 catalyst in the presence of a very small amount of ZnO

Highly efficient and greener hydrogenation of benzene to cyclohexene is of great importance but is challenging. In this work, Ru/TiO2 catalyst was prepared by a simple chemical-reduction method. The catalyst was characterized by transmission electron spectroscopy (TEM), X-ray powder diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), X-ray photoelectron spectroscopy (XPS), and nitrogen adsorption-desorption techniques. It was shown that the Ru nanoparticles with average size of about 2.2 nm were dispersed uniformly on the surface of the TiO2 support. The effect of a very small amount of ZnO in NaOH solution on the selectivity to cyclohexene was investigated under different conditions by using Ru/TiO2 catalyst. It was found that the addition of a small amount of ZnO to the NaOH solution could effectively enhance the selectivity to cyclohexene and that the yield of cyclohexene could reach 41.5%. Control experiments revealed that the main reason for the enhancement of the selectivity to cyclohexene was the presence of ZnO in the form of Na2Zn(OH)(4) in the alkaline solution, which effectively retarded the further hydrogenation of cyclohexene. A recycling experiment showed that the yield of cyclohexene was not obviously decreased after four reuses.