Structure and reactivity of single site Ti catalysts for propylene epoxidation

Propylene epoxidation using molecular oxygen and hydrogen mixture on Au-based catalysts has attracted attention because of high propylene oxide selectivity and the use of an inexpensive and environmental friendly oxidant. Single-site titanium on metal oxide supports plays an important role in achieving high reactivity and selectivity in propylene epoxidation. Here we used TiO₂ atomic layer deposition (ALD) to synthesize single-site titanium imbedded in the SiO₂ framework for propylene epoxidation. High temperature calcination was used as post-treatment to control the titania structure and Ti–O coordination number. Using UV–vis spectroscopy and X-ray absorption spectroscopy, we successfully established that under similar propylene conversion the selectivity to propylene oxide (PO) is strongly correlated to the Ti–O coordination number and bond length. Using a cluster model, density functional theory (DFT) calculations indicate that the partial charges of single Ti–SiO₂ sites scale linearly as a function of the coordination number. Also, the predicted Ti–O bond lengths follow the same trend as found in the experiments, providing additional support for the observed experimental activity relationships.