(741c) Linear Free-Energy Relationships Describe Alkene Epoxidations over Groups 4-6 Lewis Acid Zeolite Catalysts

Epoxidation with hydrogen peroxide is an important chemistry for creating building blocks for polymers used in surfactants, solvents, and advanced materials. Groups 4–5 metals supported on porous silicates catalyze epoxidation with H₂O₂, where turnover rates and selectivities increase exponentially with the functional Lewis acid strength of the active site. Group 6 metals are used in industrial homogeneous epoxidation catalysis; yet, studies suggest these metals are prone to leaching or restructuring on solid supports. Consequently, the literature lacks an understanding for how group 6 metals perform as heterogeneous catalysts.

Here, we synthesize groups 4–6 (Ti, Nb, W) framework-substituted BEA through post-synthetic modification to study 1-hexene epoxidation with H₂O₂. Epoxidation rates (5 mM 1-hexene, 10 mM H₂O₂, 313 K) are 250- and 60-fold lower on W-BEA and Nb-BEA than Ti-BEA, respectively. Epoxide selectivities (defined as epoxidation rates divided by total H₂O₂ consumption rates) at comparable conditions are greatest on Ti-BEA (93%), followed by Nb- (38%) and W-BEA (20%). These large differences in catalysis are not due to differences in the epoxidation mechanism or the coverage of reactive intermediates, which are functionally equivalent for these materials. Rather, the disparity in rates and selectivities reflect differences in the electrophilicity of the reactive metal-hydroperoxo and peroxo intermediates, which are inferred through measurements of apparent activation enthalpies (∆H^‡). 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. Values of ∆H^‡ decrease linearly with ∆H_Ads, showing that alkene epoxidation exhibits a strong linear free energy relationship. Collectively, these findings reveal a rare case in catalysis where both rates and selectivities can be improved through design principles that increase the functional Lewis acid strength of the active site for alkene epoxidation.

We gratefully acknowledge support from the Army Research Office (W911NF-16-1-0100).