A Robust Au/ZnCr2O4 Catalyst with Highly Dispersed Gold Nanoparticles for Gas-Phase Selective Oxidation of Cyclohexanol to Cyclohexanone

Abstract

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 gproduct 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.