Steering the Selectivity in Gold–Titanium-Catalyzed Propene Oxidation by Controlling the Surface Acidity
- Ewoud J.J. de BoedEwoud J.J. de BoedMaterials Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, Utrecht 3584CG, The NetherlandsMore by Ewoud J.J. de Boed,
- Jan Willem de RijkJan Willem de RijkMaterials Chemistry and Catalysis, Debye Institute for Nanomaterials Science, Utrecht University, Universiteitsweg 99, Utrecht 3584CG, The NetherlandsMore by Jan Willem de Rijk,
- Petra E. de Jongh , and
- Baira Donoeva*
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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