(699b) Role of Oxo-Titanium-Au Interface in Selective Oxidation of Propane to Acetone

Supported Au catalysts have been shown to catalyze selective oxidation of propane below 473 K using a mixture of H₂ and O₂. Interestingly, the primary product changed from propene over Au/TiO₂ to acetone over Au/TS-1 [1]. It was postulated that isolated titanium species in a siliceous TS-1 matrix is responsible for acetone formation. However, these catalysts also differ in their nature and number of Au-titania interface, which has been shown to be important in low temperature oxidation of CO [2]. The objective of this study is to determine the role of oxo-Ti-Au interface in the selective oxidation of propane. This was accomplished by generating the interface in a controlled manner and studying its effect on the reaction.

Two methods were used to form the oxo-Ti-Au interface. One method started with Au particles, formed by the reduction of tetrahydrothiophene gold chloride in the presence of polymethylhydrosilance (PMHS) containing aminopropylsiloxy and diacetylacetonate Ti ligands (Ti-PMHS). The resulting encapsulated Au particles were deposited on SiO₂. The second method started with impregnating Au/TiO₂ with Ti-PMHS followed by aminopropyl-modified PMHS without the Ti ligand (N-PMHS). For both methods, the samples were then subjected to oxidation to remove the hydrocarbon fragments, the completion of which was confirmed by FTIR. This resulted in samples containing oxo-Ti species in close proximity of the Au particles. If the samples were prepared by first using N-PMHS and then followed by a second impregnation with Ti-PMHS, samples with oxo-Ti species farther away from the Au particles were obtained. EXAFS and UV-vis characterization of these samples confirmed that mononuclear oxo-Ti species were present.

These samples were tested for propane oxidation using a mixture containing 5% propane, 5% O₂ and 5% H₂ in He at 458 K. It was found that acetone was formed only on samples containing oxo-Ti species, and a roughly monolayer of silica-dispersed oxo-Ti on Au/TiO₂ nearly completely suppressed the formation of propene, and acetone became the major oxidation product.  Furthermore, a sample with a layer of oxo-Ti-silica adjacent to Au was 3 times more active in the production of acetone than when this layer was one to two-atom-distance away on the average.

The results are consistent with a model of the samples in which silica derived from PMHS forms 3-D patches on the surface of the Au particles, whereas Ti-Si-O layers derived from Ti-PMHS forms a 2-D overlayer. The oxo-Ti species in close proximity of, or likely in contact with, the Au is essential for acetone formation. These results offer an alternative to the literature proposal of propane selective oxidation that involves the formation of hydrogen peroxide as the oxidant for propane. They suggest that acetone is formed at the mononuclear oxo-Ti-Au interface, whereas propene is formed at the multinuclear oxo-Ti-Au interface.

References

[1] J. Bravo-Suarez, K. Bando, J. Lu, T. Fujitani, S.T. Oyama, J. Catal. 255, 114 (2008).  J. Bravo-Suarez, K. Bando, T. Akita, T. Fujitani, T. Fuhrer, S.T. Oyama, S.T., Chem. Comm. 28, 3274 (2008).

[2] I.X. Green, W. Tang, M. Neurock, J.T. Yates Jr., Science, 333, 336 (2011).