Epoxidation reaction of unsaturated hydrocarbons with H<inf>2</inf>O <inf>2</inf> over defect TS-1 investigated by ONIOM method: Formation of active sites and reaction mechanisms

Abstract

The mechanism of alkene oxidation with hydrogen peroxide over the titanium silicalite-1 (TS-1) defect is investigated using a 65T nanocluster, TiSi 64O97H24 and calculated at the 9T/65T two-layered ONIOM(B3LYP/6-31G(d,p):UFF) level. The intermediate titanium hydroperoxo in the bidentate form, Ti(η2-OOH), occurring through the single-step double proton-transfer mechanism aided by a neighboring silanol group, is proffered as the active species in the oxidation process. It is noted that this species is influenced by the number of water molecules surrounding the active region. The formation of titanium peroxo species, Ti(η2- OO-), consistent with the role of water in hydroperoxo-peroxo interconversion in the TS-1/H2O/H2O2 system, results from the step in which an additional water molecule is introduced into the hydrated Ti(η2-OOH) complex. The step in which oxygen is abstracted during the epoxide formation is determined to be the reaction rate determining step, and is reactive to a number of methyl groups substituted to the active C=C bond of alkene molecules. The evident activation energies for ethylene, propylene, and trans-2-butylene are estimated to be 15.5, 13.6, and 12.2 kcal/mol, respectively. These results agree with the reactivity series of the gas-phase calculations and compare favorably with the known apparent activation energy of 1-hexene of 15.5 ± 1.5 kcal/mol obtained from experiment. © 2007 American Chemical Society.