Selective oxidation of cyclohexane: Ce promotion of nanostructured manganese tungstate

Abstract

Cyclohexane selective oxidation over nanostructured MnWO4 promoted with increasing amounts of Ce (1–5 wt%) has been investigated at mild conditions using molecular oxygen as oxidant. MnWO4 nanorods were found to be an active catalyst for cyclohexane selective oxidation with selectivity to KA oil (cyclohexanol + cyclohexanone) of approximately 85%. The catalytic performance was improved by impregnation with 1 wt% Ce while the textural properties and crystallinity were preserved and Ce was well-dispersed on the surface. XPS analysis of 1%Ce-MnWO4 showed Ce to be present mainly as Ce3+, which is known to promote oxygen adsorption, activation, and mobility. At higher Ce content, the proportion of Ce4+ increased to be the main Ce species and large, heterogeneously-dispersed Ce oxide particles are formed on the catalyst surface. The lower Ce3+ content reduces the promoting effect while the large Ce oxide particles block access to the active sites on the surface of the MnWO4 nanorod. MnWO4 and 1%Ce-MnWO4 nanorods were shown to retain their selective oxidation performance in consecutive reaction runs. Surprisingly, physical mixtures of nanostructured MnWO4 and a CeO2 nanopowder showed enhanced selective oxidation activity compared to MnWO4 alone reaching a plateau at 25–50 wt% CeO2, whereas CeO2 nanopowder itself was found to be inactive at the reaction conditions. Ce promoted MnWO4 shows promise as a catalyst for selective oxidation of cyclohexane and performs at least as well as the most active non-metallic heterogeneous catalysts reported in the literature.