(100g) Selective Oxidation of Ethanol over Zeolite-Supported Gold Catalysts

Selective oxidation of alcohols to aldehydes, acids and esters is required in the production of numerous products in the chemical industry. Current commercial technologies use non-catalytic stoichiometric reactions with oxidants, which are expensive and toxic. It is, therefore, highly desirable for green chemistry and improved efficiency to replace these current technologies with catalytic processes using gas-phase oxygen as the oxidant. In this work, Au/ZSM-5 catalysts were synthesized, characterized and tested for ethanol selective oxidation. Effects of the Au loading, zeolite acidity as well as proximity of Au and zeolite acid sites were studied with XRD, TEM, DRIFTS, and DFT calculations.

The ethanol reaction rate over 1 wt % Au/ZSM-5 (Si/Al=15) was 26 times higher than that over 1 wt % Au/SiO₂. 1 wt% Au/S-1 was found to be more active than Au/SiO₂ by a factor of 3. Furthermore, 1 wt % Au/ZSM-5 exhibited better stability than the SiO₂-supported catalyst. The ethanol reaction rate increased with increasing Al concentration in the zeolite. At the highest tested Al concentration (Si/Al = 15), the ethanol reaction rate over Au/ZSM-5 was 9 times higher than that over Au/S-1 at the same Au loading of 1 wt%. The dependence of the reaction rate on the Au loading for the ZSM-5 support with Si/Al=15 was a volcano curve with a maximum at 0.5 wt % Au.

Physical mixtures of 1 wt% Au/S-1 and blank ZSM-5 (Si/Al = 15) with different ratios were tested to evaluate the influence of proximity of Au and Al atoms. The physical mixtures had reaction rates similar to that of pure 1 wt% Au/S-1, much lower than that of 1 wt% Au/ZSM-5. Therefore, Al zeolite sites do not serve as independent catalytic sites, but they enhance the catalytic activity of Au sites when Au and Al atoms are in proximity.