Selective oxidation of alcohols to aldehydes and acids is important in the production of numerous value-added chemicals. Ethanol is often used a model compound in the development of new selective oxidation processes. Furthermore, ethanol can be produced from biomass and, therefore, is a sustainable feedstock for commodity and specialty chemicals. In this study, a series of zeolite-supported gold catalysts with different Si/Al ratios (Si/Al = 15-140) and Au loadings (0.25-2 wt %) were synthesized and tested for ethanol selective oxidation reaction in vapor phase. Additional supports, fumed SiO2 and silicalite-1 (S-1), were compared to ZSM-5 to evaluate acidity and morphology effects of the support. The Au/ZSM-5 catalysts exhibited high activities at mild reaction conditions. The ethanol reaction rate for 1 wt % Au/ZSM-5 (Si/Al=15) was 26 times higher than that for 1 wt % Au/SiO2 and 6 times higher than for 1 wt % Au/S-1. This rate increase was attributed to Au preferential anchoring on zeolite acid sites and formation of smaller Au particles in the zeolite pores. In addition, Au/ZSM-5 catalysts were also selective to partial oxidation products: acetaldehyde (34-69 mol %) and acetic acid (30-52 mol %). Experimental and computational results demonstrate that Au particles anchor on zeolite framework Al acid sites by replacing H counterions. The dependence of the catalytic activity for the ZSM-5 support with a Si/Al ratio of 15 on the Au loading exhibited a volcano curve with the maximum at 0.5 wt %. This effect was attributed to the formation of larger Au particles on the external surface of the zeolite, as opposed to smaller Au particles on framework Al sites inside the zeolite pores, with increasing Au loading. The dependence of the catalytic activity on the number of zeolite acid sites for the 1 wt % Au loading exhibited the maximum at a Si/Al ratio of 15. The results suggest that zeolite acid sites mostly anchor and disperse Au particles and do not serve as additional catalytic active sites.

Selective oxidation of ethanol over Au/ZSM-5 catalysts

ACS Spring Meeting 2021

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