Effects of Solvent–Pore Interaction on Rates and Barriers for Vapor Phase Alkene Epoxidation with Gaseous H<sub>2</sub>O<sub>2</sub> in Ti-BEA Catalysts
Ohsung Kwon, E. Zeynep Ayla, David S. Potts, David W. Flaherty
Abstract
Liquid-like fluids condense spontaneously within micropores of titanosilicates and preferentially stabilize transition states for alkene epoxidations. Consequently, turnover rates increase by an order of magnitude and epoxide selectivities double, despite the absence of a bulk liquid phase. Quantities of acetonitrile (CH 3 CN) condensed within Ti-BEA zeolite pores (CH 3 CN molecules·(unit cell) −1, n CH 3 CN,cell ) are 7 times greater for materials with the highest silanol ((SiOH) x ) densities than for nearly pristine hydrophobic Ti-BEA at conditions relevant for epoxidation catalysis, as determined by in situ infrared spectra and dynamic vapor sorption analysis. Greater (SiOH) x densities and the associated rises in n CH 3 CN,cell confer stability to 1-hexene epoxidation transition states and increase rates and epoxide selectivities by 20-fold and 2-fold, respectively. Turnover rates in hydrophilic Ti-BEA depend more sensitively on the partial pressure of CH 3 CN than rates in hydrophobic Ti-BEA-F, which reflects the stronger dependence of n CH 3 CN,cell on pressure in hydrophilic samples. Apparent activation enthalpies (Δ H ‡ ) and entropies (Δ S ‡ ) increase by 11 kJ·mol –1 and 48 J·mol –1 ·K –1, respectively, as a single value function of n CH 3 CN,cell (0.4–10 molecules·(unit cell) −1 ) on all Ti-BEA materials. Intrapore CH 3 CN molecules coordinate to (SiOH) x and reorganize to accommodate epoxidation transition-state formation, providing excess contributions to Δ H ‡ and Δ S ‡ . These inferences agree with in situ infrared spectroscopy that shows adsorption of 1,2-epoxyhexane disrupts hydrogen bonds with (SiOH) x and displaces CH 3 CN from *BEA pores. Amounts of n CH 3 CN,cell displaced scale with intrapore densities of (SiOH) x and more positive values of Δ H ‡ and Δ S ‡ . These findings show that condensation of unreactive solvents in microporous catalysts influences turnover rates, product selectivities, and kinetic barriers, even without a bulk liquid phase. The origin of these effects demonstrate that this concept applies to broad classes of reactions and requires only that kinetically relevant transition states occupy a different volume than dominant surface intermediates.