Micropore-Mediated Interactions Among Surface Intermediates: Role of Oxidant Structure and Pore Size on Alkene Epoxidation in Titanium Silicates

The rational design of materials for desired catalytic transformations and selective separations requires quantitative structure-function relationships to engineer specific interactions that stabilize critical intermediates. Here, we compare n-alkene (C₆ – C₁₀) epoxidation over Ti-substituted zeolite *BEA and SBA-15 with hydrogen peroxide (H₂O₂), t-butyl hydrogen peroxide (TBHP), or cumene hydroperoxide (CHP) as the oxidants and show how the presence of the confining void within zeolites mediates lateral interactions between activated oxidant molecules and adsorbed alkenes during catalysis.

Turnover rates for 1-octene epoxidation are 50 – 1,000-fold greater with H₂O₂ than with TBHP or CHP. These large differences in rates are not due to differences in the mechanism for alkene epoxidation, which are analogous and lead to similar rate expressions. Rather, the differences in rates reflect chemical differences in the electronic structure of the titanium hydro- or alkyl-peroxide intermediates (Ti-OOR; R = H, t-Bu, or cumyl). Larger alkyl groups donate greater electron density to the peroxidic oxygen atoms and lead to lower alkene epoxidation turnover rates, because electrophilic peroxides transfer O-atoms more readily.

Molecular interactions between activated oxidants and adsorbed alkenes depend strongly on size and functionality of the reacting species and surrounding environment. The apparent Gibbs free energy of activation for n-alkene (C₆ – C₁₀) epoxidation with TBHP or CHP relative to 1-hexene (ΔΔG^‡) become increasingly negative with the addition of methylene units within Ti-BEA, which reflect stabilizing interactions among t-Bu or cumyl groups with the aliphatic tail. Within Ti-SBA-15, values of ΔΔG^‡ increase with carbon number, because these mesopores do not significantly mediate interactions among transition states. Collectively, the data and interpretations show that the micropores of *BEA can essentially act as a non-innocent ligand that facilitates the interactions between surface intermediates. This research was supported by the DOE Office of Basic Energy Sciences, under grant DE-SC0020224.