Effects of Catalyst Hydrophobicity on Rates and Selectivities of Alkene Epoxidation

Epoxides are essential for the production of various plastics, fragrances, and pharmaceuticals, but conventional oxidants used for the large-scale synthesis of epoxides produce toxic co-products or over-oxidize to form large amounts of carbon dioxide, a greenhouse gas. Highly disperse titanium metal catalysts activate hydrogen peroxide (a green oxidant) for alkene epoxidation, but there is currently not an understanding of how the hydrophilicity (i.e., the silanol density) of such materials affects how quickly and selectively they catalyze epoxidation. Here, a series of titanium zeolite catalysts with varying silanol density but constant pore size and metal content is synthesized via post-synthetic modification of aluminum zeolite samples. The hydrophilicity of these materials is quantitatively determined, and a combination of kinetic, thermodynamic, and spectroscopic measurements are then used to show that the reactivity of these catalysts depends on their hydrophilicity. In particular, activation enthalpies for 1-octene epoxidation decrease with the density of silanols but are accompanied by a concomitant increase in entropy losses. For liquid-phase 1-octene epoxidation in acetonitrile, reaction barriers decrease by 7.7 kJ mol-1 but entropic losses increase by 22.4 J mol-1 K-1 when the catalyst structure is changed from highly-defective to defect-free, which manifests in turnover rates increasing by a factor of 500. Thus, the competing enthalpic and entropic factors must be balanced by carefully selecting the optimal silanol density for a given set of process conditions (e.g., reaction temperature). Together, these findings provide rational design criteria for the design of catalysts to be used in environmentally-friendly epoxide synthesis.Army Research Office #W911NF-18-1-0100Ope