Mechanistic Implications of Aun/Ti-Lattice Proximity for Propylene Epoxidation

Abstract

Previous work on propylene epoxidation using H2/O2 over Au/TS-1 catalysts indirectly supports a “sequential” mechanism: (1) H2O2 formation from H2/O2 on Au and (2) propylene epoxidation using H2O2 on Ti-defect sites. On the contrary, recent kinetic studies suggest a “simultaneous” mechanism which involves attack of adsorbed propylene on H−Au−OOH species. We have employed QM/MM calculations to examine whether the “sequential” mechanism is viable if Au/Ti sites are in proximity inside the TS-1 pores. On the bare Ti-defect site, the calculated ΔEact for Ti−OOH formation (16.8 kcal/mol) and for subsequent propylene epoxidation (20.3 kcal/mol) suggest that epoxidation is viable. However, the Ti-defect site is also the most favorable binding site for small Au clusters. Interestingly, the ΔEact for Ti−OOH formation on Au3/Ti-defect site is 32.1 kcal/mol, suggesting that the “sequential” mechanism is kinetically inhibited due to the proximity between Au clusters and Ti-defect sites. In the “simultaneous” mechanism, propylene is likely to be adsorbed on Au−Ti interface sites (ΔEads ∼ −20.0 kcal/mol) rather than on Ti sites (ΔEads ∼ −10.0 kcal/mol). The predicted adsorption energies are consistent with the reaction order of propylene (0.18 ± 0.04) in the power-law model. We propose that the “simultaneous” mechanism dominates if Ti-defect sites are covered by Au.