Tuning Electronic Properties of Active Sites for Alkene Epoxidations with H2O2

Groups 4–5 metals incorporated into zeolite frameworks catalyze epoxidation with H₂O₂, where activation barriers depend on the electronic properties of these metals, such as their Lewis acid strength, as more electrophilic intermediates yield greater epoxidation rates. Group 6 metals are successful homogeneous epoxidation catalysts; however, because they are prone to leaching or restructuring, there is limited understanding for how group 6 metals perform on solid supports.

We have synthesized groups 4–6 metals (Ti, Nb, W) substituted into the zeolite BEA framework through post-synthetic modification to study 1-hexene epoxidation with H₂O₂. Epoxidation selectivities at comparable conditions are greatest on Ti-BEA (93%), followed by Nb- (38%) and W-BEA (20%) and rates are 250- and 60-fold lower on W-BEA and Nb-BEA than Ti-BEA, respectively. These large kinetic differences are not due to differences in the epoxidation mechanism; rather, the disparity in rates and selectivities reflect differences in the electrophilicity of the metal-hydroperoxo and peroxo intermediates. Electrophilicities of these surface species are inferred through measurements of apparent activation enthalpies (∆H^‡) and determination of the relative ligand-to-metal charge transfer energies measured via in situ UV-Vis spectroscopy. The heats of 1,2-epoxyhexane adsorption (∆H_Ads) onto active sites, measured by isothermal titration calorimetry, show W-BEA binds 1,2-epoxyhexane less strongly than Ti- and Nb-BEA. ∆H^‡ values decrease linearly with ∆H_Ads, showing that 1-hexene epoxidation exhibits a linear free energy relationship. These findings suggest rates and selectivities can be simultaneously improved through design principles that increase functional Lewis acidity of active sites. Preliminary results for a similar study reveal Ti catalysts supported on SiO₂ (SBA-15) have higher rates and lower ∆H^‡ than Ti supported on Al₂O₃ for 1-hexene epoxidation. 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).