Solvent stabilization of alkene epoxidation transition states within Ti-MFI: Interactions near and far from active sites

Abstract

Kinetics of alkene epoxidation in Ti incorporated MFI (Ti-MFI) zeolites that contact liquid methanol (CH3OH) depend strongly on the bulk concentration of water ([H2O]) and the density of silanol groups ((SiOH)x) within the framework, because these species influence excess free energy contributions (Gε) that stabilize transition states for epoxidation and hydrogen peroxide derived reactive intermediates. These effects rely on hydrogen bonds among intrapore molecules that couple catalytic events to the structure of the surrounding solvent molecules. Turnover rates for 1-hexene (C6H12) epoxidation with hydrogen peroxide over Ti-MFI zeolites increase by ten-fold both when either [H2O] or (SiOH)x densities increase. Activation enthalpies and entropies of hydrophilic and hydrophobic Ti-MFI materials are similar at the lowest [H2O] (0.005 M H2O), implying solvation environments near active sites do not depend on defect density in the near-absence of H2O. However, kinetic barriers for these same Ti-MFI materials increase and diverge from one another as non-monotonic functions of [H2O] (0.005–5 M H2O), and these differences reflect stabilizing contributions from discrete solvation shells both adjacent (near) and further (or far) from active sites. The structure of these solvation shells must reorganize to accommodate H2O2 activation and formation of epoxidation transition states. Adsorption enthalpies for the epoxide product become more exothermic with larger values of [H2O] over each Ti-MFI catalyst, and the apparent activation enthalpies generally increase as epoxide adsorption enthalpies become more exothermic. Comparisons to trends previously obtained for Ti-BEA catalysts suggest the negative correlation indicates that excess contributions (Hε, Gε) impact reactive species (Ti-OOH) within the 10-membered ring pores of MFI to a greater extent than 12-membered ring pores due to distinctions among hydrogen bonding interactions among bound reactive species, spectating solvent molecules, and extended surfaces of MFI pores.