(349r) Tuning Electronic Properties of Active Sites for Alkene Epoxidations with H2O2

Atomically dispersed transition metal atoms on catalytically inactive metal oxide supports can activate hydrogen peroxide (H₂O₂) to form intermediates that selectively epoxidize alkenes, including propylene. Despite being reported decades in the past, titanium-silicalite-1 remains the most widely used and benchmark heterogeneous catalyst for this chemistry. Principles to guide the development of more productive and selective dispersed metal catalysts remain elusive. Here, we study the electronic and topological factors that control rates, selectivities, and apparent activation enthalpies for alkene epoxidations with H₂O₂ by examining series of groups 4–6 metals incorporated into the BEA zeolite framework and Ti atoms grafted onto mesoporous supports. We find that electronic structure of the metal active site, assessed quantitatively by spectroscopic and calorimetric methods, correlates strongly with rates, selectivities, and barriers and can be manipulated through choice of the reactive metal and of the supporting oxide.¹

The stabilities of the epoxidation transition state and pertinent reactive intermediates are examined by measured apparent activation enthalpies (∆H^‡_epox) and heats of adsorption of 1,2-epoxyhexane (∆H_ads,epox) onto active sites. Values of (∆H_ads,epox) show W-BEA binds 1,2-epoxyhexane less strongly than Ti- and Nb-BEA (Figure 1c). ∆H^‡_epox values decrease linearly with ∆H_ads,epox, showing that 1-hexene epoxidation exhibits a linear free energy relationship. A similar study on Ti-MO_x reveals Ti atoms supported on SiO₂ (SBA-15) have higher ratesthan Ti supported on other MO_x for 1-hexene epoxidation.

These relationships show that rates, selectivities, and apparent barriers for alkene epoxidations over M-BEA and Ti-MO_x catalysts reflect the electron affinity of dispersed active sites and suggest strategies of fine-tuning these values. Ongoing work will expand on tuning electronic properties of active metal sites by investigating these metal-support interactions. We gratefully acknowledge support from the Army Research Office (W911NF-16-1-0100).

References

1. Ayla, E. Z. et al. ACS Catal. 2021, 11 (1), 139-154.