Efficient base-free direct oxidation of glucose to gluconic acid over TiO2-supported gold clusters

Abstract

The transformation of renewable natural resources is an appealing and sustainable protocol to minimize fossil fuel consumption. Here, a simple incipient wetness protocol is developed to prepare ultrasmall gold clusters, immobilized on TiO₂ (particle size: 1.2–1.7 nm), using anthranilic acid as a stabilizing agent. The Au clusters can be reduced to metallic Au⁰ (Au/TH-150 and Au/TH-200) during 150 and 200 °C annealing in the presence of H₂ gas, while the Au clusters are converted to Au^δ+ species in air at 200 and 500 °C (Au/TA-200 and Au/TA-500), a conclusion supported by XPS and low-temperature (−150 °C) Operando-DRIFTS analysis. Au/TA-200 and Au/TA-500 showed inactivity in the base-free direct oxidation of glucose. For comparison, Au/TH-150 and Au/TH-200 exhibited salient catalytic performance (87–92% conversion and 95–97% selectivity for gluconic acid), revealing that glucose oxidation occurs preferentially on the Au⁰ species. The turnover frequency (TOF) of Au/TH-150 reaches 1908 mol_{reacted glucose} mol_Au⁻¹ h⁻¹, which is much higher than that of commercial Pd–Bi/C under alkaline conditions (TOF: 1298 mol_{reacted glucose} mol_Pd⁻¹ h⁻¹, pH 9.5). The apparent activation energies are 37 (over Au/TH-150) and 47 kJ mol⁻¹ (Au/TH-200), comparable to the unsupported Au colloids, indicating that the oxidation should occur at the Au surface rather than at the perimeter interface between the Au clusters and the supports.