Gas-phase selective oxidation of cyclohexanol to cyclohexanone over Au/Mg_1-xCu_xCr₂O₄ catalysts: On the role of Cu doping

The industrial production of cyclohexanone from cyclohexanol would benefit from a selective oxidation catalyst. Herein, Cu doping of MgCr₂O₄ supports for gold nanoparticles active in gas-phase oxidation of cyclohexanol was investigated. Mg_1-xCu_xCr₂O₄ exhibited spinel structures (x ≤ 0.25: MgCr₂O₄; x = 1: CuCr₂O₄) onto which 3–4 nm gold nanoparticles could be dispersed. Cu doping led to higher activity. During reaction, surface Cu²⁺ was reduced to Cu⁰, resulting in Au–Cu alloy formation. At low temperature, low-Cu-content catalysts (x ≤ 0.1) showed higher activity than high-Cu-content catalysts, likely because the Au–Cu alloy with highly diluted Cu was more active for the dehydrogenation step of cyclohexanol. However, Au/Mg_0.99Cu_0.01Cr₂O₄ and Au/Mg_0.9Cu_0.1Cr₂O₄ showed lower cyclohexanol conversion at high temperature than samples with high Cu content, because O₂ activation involving Cu becomes rate-limiting. Stable cyclohexanol conversion and cyclohexanone selectivity were 99.1% and 90.2% (space-time yield of 266 g_Ketone g_Au ⁻¹ h⁻¹) for Au/Mg_0.25Cu_0.75Cr₂O₄ at 300 °C.