Effect of the bipodal Ti species on activity and selectivity of propylene epoxidation with H2O2 over TS-1: A theoretical study

In order to probe into the effect of bipodal Ti species on the catalytic performance, the propylene epoxidation with H2O2 over TS-1 catalyst with bipodal Ti site has been studied using an embedded quantum mechanical/ molecular mechanics (QM/MM) approach, and the results shown that the bipodal Ti site has strong Bronsted and Lewis acidity, which not only greatly facilitates the epoxidation reaction but also lower selectivity toward the epoxy products. The excellent epoxidation activity is attributed to the Lewis acidity, in which, H2O2 easily activated to generate active intermediates (i.e., Ti-111-OOH and Ti-112-OOH) that is facile to transfer O atom to oxidize the propylene. The poor selectivity toward PO is also due to the Lewis acidity, which leads to the stronger adsorption ability of PO and excellent ring-opening ability of Ti-OH nucleophile to PO. The introduction of methoxy (CH3O) group is beneficial for the enhancement of the selectivity of PO, although it enhances the activation free energy for epoxidation to generate PO. The identifications about the relationship of the intrinsic selectivity and activity with bipodal Ti site would provide a clue for designing highly-efficient TS-1 catalysts in propylene epoxidation to PO from the theoretical point of view.