Steering the Selectivity in Gold–Titanium-Catalyzed Propene Oxidation by Controlling the Surface Acidity
2021
de Boed, Ewoud J.J. | de Rijk, Jan Willem | de Jongh, Petra E. | Donoeva, Baira
Supported nanoparticulate Au/Ti-SiO₂ catalysts are a promising candidate for selective epoxidation of propene with H₂/O₂ mixtures. Here, we demonstrate that by altering the acidity of the surface titanol groups in Au/Ti-SiO₂, the selectivity of these catalysts in propene oxidation can be controlled. That is, Au/Ti-SiO₂ prepared using an alkali base during gold deposition shows basic properties due to the formation of Ti-ONa groups. The catalysts that contained Na⁺ and neutralized acid sites demonstrate high selectivity toward propene oxide. On the contrary, when the acidity of the Ti-OH groups is preserved by using NH₄OH as a base during gold deposition, the catalyst is highly selective toward propanal at a similar propene conversion. This difference in selectivity is explained by the isomerization of initially formed propene oxide into propanal over acidic Ti-OH groups as we demonstrated using stacked bed experiments, where the Ti-support was exposed to propene oxide. When Na⁺ was present, no isomerization was observed, while without Na⁺ present, propene oxide was isomerized to propanal. In short, we demonstrate the crucial role of Na⁺ and acidic Ti-sites in steering the selectivity in gold-catalyzed propene epoxidation.
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