Mechanism of the Decomposition of Aqueous Hydrogen Peroxide over Heterogeneous TiSBA15 and TS-1 Selective Oxidation Catalysts: Insights from Spectroscopic and Density Functional Theory Studies

Abstract

The Ti-based heterogeneous catalysts TiSBA15, Bu cap TiSBA15, TS-1, and [Ti,Al]-MFI were investigated with respect to controlling factors for the competitive decomposition of aqueous H2O2 during selective catalytic oxidations. DRUV–vis spectroscopy revealed that the titanium species in these materials exist mainly in isolated, tetrahedral coordination environments. The observed rates of H2O2 decomposition at 65 °C in acetonitrile decreased in the following order: Bu cap TiSBA15 > TiSBA15 and TS-1 > [Ti,Al]-MFI. The decompositions of H2O2 were also monitored in the presence of inorganic additives and Brønsted acids and bases, in benzene/aqueous biphasic solutions. Significant retardation of the decomposition rates with the KH2PO4 additive was found with TiSBA15, which suggests that the KH2PO4 stabilizer may be useful for optimization of hydrogen peroxide efficiency in catalytic oxidations. DRUV–vis spectroscopy was employed to identify possible catalytically active intermediates, proposed to be Ti(IV)(OOH) species that are produced upon reaction of the Ti-based materials and H2O2. Density Functional Theory (DFT) studies starting from a molecular model, (HO)Ti[OSi(OH)3]3, suggest that three Ti(IV)(OOH) intermediates are in equilibrium, and the formation of Ti–O• and HOO• radical species may be involved in the H2O2 decomposition. In addition, the potential role of KH2PO4 in the H2O2 decomposition process, as a proton acceptor in a [Ti(OO)(HOP(O)(OH)2)] complex, has been investigated.

Switching off H2O2 Decomposition during TS-1 Catalysed Epoxidation via Post-Synthetic Active Site Modification

“…Whilst ICP analysis also revealed a decrease in trace Fe content following etching (Fe content decreased from 326 to 254 ppm), control measurements investigating the ability of Fe-silicalite-1 to decompose H2O2 revealed that the impurity Fe content was not responsible for H2O2 decomposition (ESI Table S3). The inability of this MFI material to catalyse H2O2 decomposition at a similar Fe loading to that found in TS-1 as impurities clearly indicates that Ti is required for decomposition to proceed, in good agreement with previous studies [18]. Accordingly, we re-focused our attention on spectroscopic study of the Ti active sites with more sensitive spectroscopic techniques.…”

“…Although the treatment of various zeolites with NH4HF2 has been found to induce some mesopores formation [17], the impact of this was ruled out given the negligible changes observed in the porosimetry data (Table S1). Similarly, changes to the potential acid/base character of TS-1-an important factor in H2O2 decomposition chemistry [18]-were also ruled out, based on previous observations that NH4HF2 treatment does not change surface acidity (neither Lewis nor Brønsted) to any extent [17]. Fluorine analysis of TS-1B revealed that no residual fluorine remained after treatment (Table S2), and thus changes to the hydrophilicity or polarity of the catalyst based on fluorine are unlikely.…”

Counting Active Sites on Titanium Oxide–Silica Catalysts for Hydrogen Peroxide Activation through In Situ Poisoning with Phenylphosphonic Acid

“…PPA is a known poison for TiO x –SiO 2 catalysts, but has not been used previously for the quantitative determination of active sites 7a. 17…”

Role of pentahedrally coordinated titanium in titanium silicalite-1 in propene epoxidation

“…Tetrahedrally coordinated titanium, which is called framework Ti, is usually considered as the active center for the oxidation reactions. 11 Moreover, anatase TiO 2 can even lead to decomposition of hydrogen peroxide. 10 This 5MR, which was reported by Clerici et al, can oxidize propene to prepare propene oxide.…”