Insight into the stereoselectivity of TS-1 in epoxidation of cis/trans-2-hexene: a computational study

Abstract

The mechanism of the stereoselectivity for cis/trans-2-hexene epoxidation in TS-1 zeolite with hydrogen peroxide has been investigated using density functional theory with the ONIOM scheme. A cluster model of 136T with Ti(IV) at the T8 site of TS-1 was adopted to mimic the zeolite catalyst. Four different Ti-hydroperoxo intermediates were constructed and optimized. The stability order is Ti-η²(OOH)–H₂O–(H_β)H₂O > Ti-η²(OOH)–H₂O > Ti-η²(OOH)–(H_β)H₂O > Ti-η²(OOH). The activation energies of cis-2-hexene epoxidation over Ti-η²(OOH), Ti-η²(OOH)–H₂O, Ti-η²(OOH)–(H_β)H₂O, and Ti-η²(OOH)–H₂O–(H_β)H₂O active centers are 38.6, 53.6, 52.1, and 64.0 kJ mol⁻¹, respectively. The corresponding activation energies for trans-2-hexene epoxidation are 60.1, 55.2, 80.5, and 85.8 kJ mol⁻¹, respectively. cis-2-Hexene had a lower activation energy barrier than trans-2-hexene. The Ti-η²(OOH)–H₂O species is likely the real active center; the ratio of the rate constant of the cis-2-hexene to trans-2-hexene epoxidation is 1.7, which agrees nicely with the experimental results. The decisive factor for cis-selectivity depends on the microstructure of the Ti-hydroperoxo intermediate and the confinement effect in TS-1 zeolite. At the transition state, cis-2-hexene is forced to adopt a twisting conformation and embeds suitably in the zeolite cavity under the action of confinement. This leads to increased stability and reduced activation barriers.