Reactivity of Ti(iv) species hosted in TS-1 towards H2O2–H2O solutions investigated by ab initio cluster and periodic approaches combined with experimental XANES and EXAFS data: a review and new highlights

Abstract

This work is intended to underline how the most-advanced experimental and theoretical physical chemistry tools must be used synergistically to understand the reactivity of Ti-silicalite-1 (TS-1) in an important number of low-temperature oxidation reactions with aqueous H(2)O(2) as the oxidant. Literature results are carefully reviewed and accompanied with new, unpublished highlights of both experimental and computational origin. The first part of this work is devoted to a discussion of the defective nature of the material and to the synergic role played by Si vacancies and Ti insertion in the framework. A summary of the experimental Ti-K-edge EXAFS and XANES literature concerning the activated material in vacuo conditions is then presented and compared to the corresponding Ti geometries obtained by ab initio calculations. From such a comparison, the excellent agreement between experiment and theory is evident. A very good agreement is also obtained for the interaction with water and ammonia. For both H(2)O and NH(3), the agreement is due to the possibility to perform experiments in which the probe molecule is dosed from the gas phase, thus allowing to reach the 1 : 1 (or 1 : 2) ratio between the adsorbing Ti sites and the adsorbed molecule. Then, interaction with hydrogen peroxide is discussed, underlining the problems faced in reaching a common view between experimental and theoretical results, owing to the difficulties both in performing experiments where anhydrous H(2)O(2) is dosed on TS-1, and in taking into account the role played by the aqueous medium in the reactivity of Ti(iv) centres toward H(2)O(2) using ab initio calculations. Only once such difficulties have been overcome, by increasing the complexities of both experiments and ab initio models, is a joint-view finally obtained. Where needed, comparison with other experimental results (X-ray and neutron diffraction, NMR, IR, Raman, UV-vis and resonant Raman) is made.

Propane reacts with O2 and H2 on gold supported TS-1 to form oxygenates with high selectivity

“…The difference in reactivities is likely due to an optimum Ti : Si ratio that allows the formation of a maximum number of effective Ti sites in the zeolite framework. 11 Here, the best Ti : Si ratio of 3 : 100 corresponds well with previously reported maximum contents of Ti in TS-1 zeolite framework positions. 12 Catalytic oxidation with O 2 and H 2 on Au/TiO 2 -SiO 2 likely occurs by in situ formation of H 2 O 2 on Au.…”

Metalloenzyme-like catalyzed isomerizations of sugars by Lewis acid zeolites

“…The structures and energies of intermediates and transition states were determined (SI Appendix, Text S2). Based on previous experimental and theoretical studies, it has been suggested that the open site in Sn-Beta, TS-1, and Ti-Beta is the active site for reactions other than the glucose isomerization (18,19,21,22). The enthalpy profile (Fig.…”

Synchrotron ultrafast techniques for photoactive transition metal complexes

“…This makes XANES a very powerful instrument for direct structural characterization in a wide range of systems, including semiconductors [83,103], superconductors [104], oxides [105][106][107][108][109], catalysts [82,102,[110][111][112][113][114][115][116][117], solutions [118], surfaces and interfaces [119][120][121], and biological [122][123][124][125][126], geological [127][128][129][130] and hybrid organic-inorganic materials [113,[131][132][133]. The relation between geometrical features and XANES response mainly depends on the multiple-scattering processes (figure 5c) of the excited photoelectron after the ionization threshold, which dominate with respect to the single-scattering events due to the lowered photoelectron energy.…”