A robust Au/ZnCr₂O₄ catalyst with highly dispersed gold nanoparticles for gas-phase selective oxidation of cyclohexanol to cyclohexanone

Achieving uniformly dispersed and stable nanoparticles of gold on oxide supports is a challenge in heterogeneous catalysis. Here, we show that zincochromite (ZnCr₂O₄) is a promising support for obtaining high and stable gold dispersion. Despite a low surface area of ZnCr₂O₄, finely dispersed gold (∼3-4 nm particles) could be obtained by a simple deposition-precipitation method, pointing to strong gold-support interactions. Using a combination of XRD, XPS, SEM, TEM, HAADF-STEM, and IR spectroscopy, we confirmed that the calcination temperature of the ZnCr₂O₄ support had a substantial influence on the crystallinity, morphology, and acidic properties of thereof derived Au/ZnCr₂O₄ catalysts. Gold supported on a high-temperature (≥700 °C) calcined ZnCr₂O₄ support displayed the best catalytic performance in gas-phase oxidation of cyclohexanol to cyclohexanone, which is an important intermediate in the chemical industry. When calcined at 800 °C, the material did not show any sign of deactivation in a 90 h stability test, high cyclohexanol conversion (93%) and high cyclohexanone yield (91%) were achieved at 300 °C, with a space-time yield of 250 g_product gAu^-1 h^-1. On the contrary, Au/ZnCr₂O₄ based on supports calcined at lower temperatures (≤600 °C) catalyzed side reactions such as cyclohexanol dehydration to cyclohexene (Lewis acid sites), which further oligomerized to coke deposits that deactivated the catalyst. The CO oxidation trends with respect to calcination temperature were inversed to those in cyclohexanol oxidation, showing that smaller gold particles and the presence of hydroxyls are favorable for CO oxidation to CO₂. DFT calculations provided insight into the (electronic) nature of the strong interactions between Au and ZnCr₂O₄