Alkene Epoxidations with H2O2 over Groups 4-6 Metal-Substituted BEA Zeolites: Reactive Intermediates, Reaction Pathways, and Linear Free-Energy Relationships

Rates and selectivities for alkene epoxidations depend sensitively on the identity of the active metal center for both heterogeneous and homogeneous catalysts. While group 6 metals (Mo, W) have greater electronegativities and the corresponding molecular complexes have greater rates for epoxidations than group 4 or 5 metals and molecular complexes, these relationships are not established for zeolite catalysts. Here, we combine complementary experimental methods to determine the effects of metal identity on the catalytic epoxidation of 1-hexene with H2O2 for active sites within the BEA framework. Postsynthetic methods were used to incorporate groups 4-6 transition-metal atoms (Ti, Nb, Mo, W) into the framework of zeolite BEA. In situ Raman and UV-vis spectroscopies show that H2O2 activates to form peroxides (M-(eta(2)-O-2)) and hydroperoxides (M-OOH) on all M-BEA but also metal oxos (M=O) on W- and Mo-BEAs, the latter of which leaches rapidly. Changes in turnover rates for epoxidation as functions of reactant concentrations and the conformation of cis-stilbene epoxidation products indicate that epoxide products form by kinetically relevant O-atom transfer from M-OOH or M-(eta(2)-O-2) intermediates to the C=C bond and show two distinct kinetic regimes where H2O2-derived intermediates or adsorbed epoxide molecules prevail on active sites. Ti-BEA catalyzes epoxidations with turnover rates 60 and 250 times greater than Nb-BEA and W-BEA, which reflect apparent activation enthalpies (Delta H double dagger) for both epoxidation and H2O2 decomposition that are lower for Ti-BEA than for Nb- and W-BEAs. Values of Delta H double dagger for epoxidation differ much more between metals than barriers for H2O2 decomposition and give rise to large differences in 1-hexene epoxidation selectivities that range from 93% on Ti-BEA to 20% on W-BEA. Values of Delta H double dagger for both pathways scale linearly with measured enthalpies for adsorption of 1,2-epoxyhexane from the solvent to active sites measured by isothermal titration calorimetry. These correlations confirm that linear free-energy relationships hold for these systems, despite differences in the coordination of active metal atoms to the BEA framework, the identity and number of pendant oxygen species, and the complicating presence of solvent molecules.

Automated summary

Rates and selectivities for alkene epoxidations are sensitive to the identity of the active metal center, with Ti-BEA exhibiting significantly higher catalytic efficiency compared to Nb-BEA and W-BEA. The effects of metal identity on catalytic epoxidation were determined using complementary experimental methods, showing distinct kinetic regimes and differences in activation enthalpies for different metal-BEA catalysts. Linear free-energy relationships hold for these systems, despite differences in metal coordination, oxygen species, and solvent presence.