(530b) Influence of Oxidant Chemical Functionality on Alkene Epoxidation over Lewis Acid Zeolites: Intermediate Stabilization through Inner- and Outer-Sphere Interactions
The rates of cyclohexene (C6H10) epoxidation are highly dependent on the combination of oxidant (i.e., H2O2, TBHP, CHP), active site identity (e.g., Ti, Nb), solvent (e.g., CH3CN, CH3OH), and silicate support (e.g., BEA, SBA-15). These differences, however, are not attributed to differences in the general mechanism for epoxidation, which appears to be indistinguishable among catalysts, solvents, supports, and oxidants. For example, within Ti-BEA with CH3CN, rates of C6H10 epoxidation are proportional to the concentration of C6H10 and independent of oxidant concentration at low C6H10-to-oxidant molar ratios (<1 (mol C6H10)(mol oxidant)-1), which suggests that active sites are saturated with Ti-OOR intermediates. Turnover rates become independent of C6H10 concentration and increase linearly with oxidant concentration when the ratio of C6H10-to-oxidant is large (>1 (mol C6H10)(mol oxidant)-1), suggesting that the most abundant surface intermediates are derived from C6H10. These observations are consistent with a mechanism where the oxidant reversibly adsorbs to Ti active sites and is irreversibly activated to form Ti-OOR intermediates, which then react with fluid-phase C6H10 through to form the corresponding epoxide. Despite the similarity in the mechanism for epoxidation, turnover rates using TBHP and CHP as the oxidant are 10- and 20-fold lower than when H2O2 is used. Comparisons of apparent activation enthalpies (ÎHE,Appâ¡) and entropies (ÎSE,Appâ¡) show similar enthalpic stability (41 â 50 kJ mol-1) for the formation of the epoxidation transition states by coordination of C6H10 with the vicinal O-atom of the Ti-OOR intermediate. The large disparity in rates result from differences in ÎSE,Appâ¡ as epoxidations that proceed through Ti-OO-cumyl intermediates lead to the greatest entropic losses (-150 versus -50 J mol-1K-1 for CHP and H2O2, respectively). These inner-sphere interactions between the -RÂ group and the cyclohexyl ring of the C6H10 epoxidation transition state results in the variable entropic losses as the identity of the oxidant is varied. On-going work seeks to further probe inner-sphere interactions between Ti-OOR intermediates and the alkyl group of the transition states while also investigating the role of outer-sphere interactions through changes in the solvent.