Selective oxidation of cyclohexane: Ce promotion of nanostructured manganese tungstate

• Published in: Applied catalysis. A, General Vol. 568; pp. 95 - 104
• Main Authors: Graça, Inês, Al-Shihri, Saeed, Chadwick, David
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 25-11-2018; Elsevier Science SA
• Subjects: Catalysis; Catalysts; Cerium; Cerium oxides; Cyclohexane; Cyclohexanone; KA oil; Manganese; Manganese compounds; MnWO4; Nanorods; Nanostructure; Oxidation; Oxygen; Selective oxidation; Selectivity; X ray photoelectron spectroscopy; Selective oxidation; Cerium; MnWO4; Cyclohexane; KA oil
• ISSN: 0926-860X; 1873-3875
• DOI: 10.1016/j.apcata.2018.09.025

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•MnWO4 nanorod is an active catalyst for cyclohexane selective oxidation with O2 giving 85% selectivity to KA oil.•Impregnation with 1 wt% Ce forms mainly Ce3+ and promotes catalyst performance.•MnWO4 and 1%CeMnWO4 recycle without loss of performance over 3 reaction cycles.
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.