Effects of silanol defects and Ti site location within Ti-MWW on alkene epoxidation with aqueous hydrogen peroxide

Abstract

Catalytic reaction kinetics and thermodynamics within zeolites respond to the physical properties of the framework through noncovalent interactions with solvents. Here, we demonstrate that rates of alkene epoxidations depend sensitively upon the topological location of Ti atoms and the density of silanol ((SiOH)x) defects in hydrothermally synthesized Ti-MWW catalysts. Turnover rates for 1-hexene epoxidation do not change systematically with (SiOH)x density unlike previous reports for other zeolite framework. Site-averaged turnover rates on Ti-MWW catalysts with the greatest portion of Ti sites located in sinusoidal channels exceed those on materials with most Ti in supercages by a factor of 40-fold. These differences appear to reflect free energy contributions and compensation between activation enthalpies and entropies, induced by the structural organization of solvent molecules that responds differently to the topological locations of active sites and the presence of defects.