Promoting effect of Zn in high-loading Zn/Ni-SiO₂ catalysts for selective hydrogen evolution from methylcyclohexane

The dehydrogenation of methylcyclohexane to toluene was investigated over high-loading monometallic Ni-SiO₂ and bimetallic Zn/Ni-SiO₂ catalysts. The catalysts were prepared by the impregnation coupled with the advantageous heterophase sol-gel technique. Their performance was tested in a fixed-bed flow reactor at 250-350 °C, 0.1 MPa pressure, equimolar ratio H₂/Ar (24 nL h^-1 in total), and a methylcyclohexane feed rate of 12 mL h^-1. Information regarding the structure of Ni-Zn catalysts was obtained by N₂ and CO adsorption, temperature-programmed reduction, high-resolution transmission electron microscopy, X-ray photoelectron spectroscopy, in situ X-ray diffraction, and in situ X-ray absorption spectroscopy. The results have shown that the addition of zinc leads to the hindrance of Ni reducibility along with weakening the Ni interaction with the silica matrix. This behavior particularly indicated the formation of solid oxide nickel-zinc solutions. The catalytic properties of Zn-modified catalysts in the dehydrogenation of methylcyclohexane appeared significantly superior to their Ni-Cu counterparts. For example, the selectivity of Zn/Ni-SiO₂ catalysts toward toluene formation increased markedly with a decrease in the Ni : Zn mass ratio, achieving 97% at 350 °C over the sample with Ni : Zn = 80 : 20. This is attributed to the promoting geometric and electronic effects arising from the formation of bimetallic Ni-Zn solid solutions. Moreover, a deeper reduction of zinc and a more efficient formation of solid bimetallic solutions are observed after the catalytic tests.