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Structure and catalytic activity of a newly proposed titanium species in Ti-YNU-1 zeolite: a density functional theory study

Abstract

Density functional theory was applied to investigate the structure of the framework Ti species in the Ti-YNU-1 zeolite, and to evaluate its catalytic activity for 1-hexene epoxidation with H2O2 as oxidant. The effects of solvent coordination on the sability and reactivity of Ti-hydroperoxo intermediates have been studied. All calculations were performed at the theoretical level of B3LYP/6-31G(d,p) based on cluster models. The results indicate that a defective Ti-hydroxyl species [Ti(OSi)3OH] at the pillar site (the T6 site) between the interlayers in the Ti-YNU-1 zeolite is responsible for the Ti-specific vibrational frequency observed at 930 cm–1, and is in good agreement with the experimental infrared spectra obtained for Ti-rich Ti-YNU-1 zeolite. The stabilities of the possible Ti-hydroperoxo intermediates decrese in the order of Ti-η2(OOH)-H2O > Ti-η2(OOH)-CH3CN > Ti-η2(OOH) as well as Ti-η2(OOH)-H2O > Ti-η1(OOH). The activation barriers of 1-hexene epoxidation over Ti6-η2(OOH), Ti6-η2(OOH)-H2O, and Ti6-η2(OOH)-CH3CN are 33.2, 35.0, and 43.9 kJ mol–1, respetively. The Ti6-η1(OOH) species possesses lower stability and occures structural change t in the transition state for 1-hexene epoxidation, leading to the same transition state structre as that of the bidentate Ti-η2(OOH)-H2O species. For comparison, the resactivity of the perfect Ti(OSi)4 species at the T1 site in the Ti-MWW zeolite were also investigated. It is noted that the transition state of Ti1-η1(OOH) was obtained with the activation barrier of 107.2 kJ mol–1, which is much higher than that of Ti1-η2(OOH)-H2O, revealing that the intramolecular hydrogen bonding in the monodenate five-memvered ring structure does not enhence the stability and reactivity of Ti-hydroperoxo intermediates. The results also indicate that Ti-YNU-1 is more active than Ti-MWW, which is in good agreement with experimental results.

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Publication details

The article was accepted on 07 Aug 2017 and first published on 08 Aug 2017


Article type: Paper
DOI: 10.1039/C7CY01470H
Citation: Catal. Sci. Technol., 2017, Accepted Manuscript
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    Structure and catalytic activity of a newly proposed titanium species in Ti-YNU-1 zeolite: a density functional theory study

    M. Li, Y. Wang, Y. Wu, M. Wang and D. Zhou, Catal. Sci. Technol., 2017, Accepted Manuscript , DOI: 10.1039/C7CY01470H

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