Ti-Peroxo Species in the TS-1/H₂O₂/H₂O System

Abstract

This work is devoted to the study of the active species present on the TS-1-H 2 O 2 /H 2 O catalytic system and to the elucidation of the mechanisms of formation and interconversion of these species. In particular new experimental data and computational evidence are reported concerning (i) the acidic character of hydroperoxo species, (ii) the role of water in the hydroperoxo-peroxo interconversion, and (iii) the role of water as coreactant in favoring the H 2 O 2 adsorption at Ti sites. It is demonstrated that gentle drying of the TS-1-H 2 O 2 / H 2 O system leads to the transformation of the yellow peroxo groups into noncolored species, still located at Ti centers. The process is reversible and the colored species can be partially restored again by successive rehydration. As the hydroperoxo and peroxo species have different oxidizing ability, these results indicate that the presence of water molecules at hydrophilic Ti sites can correlate with the catalytic activity. As it is well-known, hydroperoxo and peroxo species are unstable and their concentration declines with aging. However, UV-vis spectroscopic results demonstrate that small fractions of peroxo species (characterized by a cream color) persist on the sample even after prolonged aging. On this basis and on EXAFS results, a comprehensive scheme of the formation and mutual conversion of oxidized species is concluded. Spectroscopic results, supported by acidity measurements and titration of active oxygen content, show the following: (i) the O-O species, responsible to the yellow color of the catalyst, is a side-on peroxo complex, probably generated by the reversible rupture of one Ti-O-Si bridge, with the formation of Ti(O 2 H) and H-O-Si groups; (ii) the stability of this peroxo complex is low in the absence of an excess of H 2 O; (iii) a strong enhancement of the acidity of the TS-1/H 2 O 2 /H 2 O system with respect to that of TS-1/H 2 O has been observed, demonstrating a synergic role between Ti(IV) centers and hydrogen peroxide; and (iv) a small fraction of oxidized species has a negligible oxidizing ability and probably does not play any role in the catalysis.

Density Functional Calculations on the Distribution, Acidity, and Catalysis of TiIV and TiIII Ions in MCM‐22 Zeolite

“…All eight T sites are included in the 49-T cluster models and it is ready for direct comparisons of structure and energy among the various T sites. As indicated, the Ti III sites exist in the Ti 3 + -O À form, [2,31,58] and so can be created by designating triplet spin multiplicities (M S = 3) instead of singlet (M S = 1) for the Ti IV cases. Calculations of acidity and catalysis were carried out with 13-T cluster model, which is comparable to those for zeolite surface studies.…”

XPS characterization of a synthetic Ti‐containing MFI zeolite framework: the titanosilicalites, TS‐1

“…The chemical treatments with HCl 0.1 M and H 2 O 2 (5% m/m in water) were based on procedures reported in Refs [8,9], respectively. The TS-1 dissolution in HCl 0.1 M was aimed at dissolving the nontightly bonded titanium (Ti (IV) extra-framework species) while the reaction with H 2 O 2 was sought to better identify the framework titanium reported to act as a catalytic center and to form a yellow-colored peroxide intermediate.…”

“…[9], the same quantities of TS-1 samples as reported above were immersed in 20 ml of H 2 O 2 (5% m/m in water) and stirred for a few minutes at ambient temperature. The filtered yellow residues were dried at 110 • C and then inserted in the spectrometer for analysis.…”

Synthesis and Reaction Chemistry of Nanosize Monosodium Titanate

“…This color change is due to the η 2 -bound protonated hydroperoxo-titanium ligand-to-metal-charge-transfer absorption band at 385 nm. 27,28 Fourier transform infrared spectroscopy (FT-IR) also confirmed the formation of the peroxotitanate species as evidenced by the appearance of an absorption band at 883 cm -1 for the peroxide modified nMST (Figure 4) which is absent in the untreated nMST material. This band is very near the region of 845-875 cm -1 that is reported for the O-O stretching vibration in peroxides.…”