Dehydrogenation of cyclohexanol on copper-containing catalysts: II. The pathways of the cyclohexanol dehydrogenation reaction to cyclohexanone on copper-active sites in oxidation state Cu0 and Cu+

The mechanism of cyclohexanol and cyclohexanone adsorption and the kinetics and mechanism of cyclohexanol dehydrogenation to cyclohexanone over Cu0 catalysts (on Cu–Mg) and Cu+ catalysts (on Cu–Zn–Al) were studied by IR spectroscopy and kinetic methods. In situ IR spectroscopy data demonstrated that cyclohexanol is adsorbed onto monovalent copper at room temperature, forming molecularly adsorbed cyclohexanol and cyclohexanol alcoholate of Cu+. Cyclohexanol alcoholate species were considered as intermediates for cyclohexanone formation on monovalent copper sites. Unlike in the case of Cu+, the dissociative adsorption of cyclohexanol on Cu0 was observed only at temperatures of 50 °C or higher. Cyclohexanol adsorption on Cu0 was accompanied by formation of a cyclohexanol alcoholate species and a phenolate species. The existence of the two adsorbed species on Cu0 explains the low selectivity of catalysts with this active site. In the case of dehydrogenation on Cu+ copper the kinetics suggests that the reaction proceeds by abstraction of the hydroxyl hydrogen from adsorbed cyclohexanol and formation of the cyclohexanol alcoholate of Cu+. Hydroxyl hydrogen abstraction was considered to be the rate-determining (rds) step of the reaction. For dehydrogenation of cyclohexanol on Cu0, the IR and kinetic data were consistent with a dissociative adsorption of cyclohexanol and formation of cyclohexanol alcoholate of Cu0. The removal of the second nonhydroxyl hydrogen was hypothesized to be the rate-determining step of reaction on Cu0. The proposed kinetic models for dehydrogenation on the catalysts with Cu+ and Cu0 sites give a satisfactory fit to the reaction rate data and provide physically meaningful values for enthalpies and entropies of cyclohexanol and cyclohexanone adsorption.