Effect of cobalt complex precursors on reactivity of cationic cobalt catalysts: Cyclohexane dehydrogenation

Abstract

Effect of the cobalt precursors, including Co(bipy)32, [Co(NH3)5Cl]Cl2, [Co(NH3)6]Cl3, and [Co(en)2Cl2] Cl, on reactivity of the cationic Co/SiO2 prepared by strong electrostatic adsorption (SEA) was investigated for the dehydrogenation of cyclohexane as a model reaction. According to the charge density of the cobalt complex, highly dispersed Co2+ species and/or Co3+ oxide can be obtained on the silica surface. The dehydrogenation activity is in the order of Co/SiO2 catalysts prepared by Co(bipy)32 > [Co(NH3)5Cl]Cl2 > [Co(NH3)6]Cl3 > [Co(en)2Cl2] Cl, correlating to the Co2+ content of the final catalysts. The cationic cobalt catalysts are more active than the pre-reduced one. Although metallic cobalt is found to be less active, the activity of cationic cobalt catalyst is enhanced under H2 flow, presumably due to the formation of cobalt hydride intermediate. The inter-conversion of Co2+/cobalt hydride intermediate is readily reversible and regulated by presence of hydrogen.