We briefly underline the relevance of TS-1 catalyst for industrial applications in mild oxidation reactions using hydrogen peroxide as oxidant and review the experimental works employed over last two decades for understanding the structure of the Ti centers in the bare TS-1 material. After an animated and controversial debate that has lasted in the literature until 1994, several works (reviewed here in depth) have definitively assessed that Ti atoms occupy framework positions substituting a Si atom and forming tetrahedral [TiO4] units. The literature concerning the interaction of TS-1 with ligand molecules is briefly discussed. There is unanimous consensus that ligand adsorption causes the distortion from the Td-like symmetry of the [TiO4] units. As the TS-1 catalyst works in aqueous solution, particular attention has been devoted to the interaction with water; the same holds for ammonia as it is a reactant in the ammoximation of cyclohexanone to give cyclohexanone oxime. Finally, the interaction of TS-1 with H2O/H2O2 solutions is reviewed in detail. Particular attention is paid to very recent results that have significantly improved knowledge of the catalyst in conditions as close as possible to working conditions. UV-Vis, Raman (under resonance conditions) and X-ray absorption (both in the XANES and EXAFS regions) spectroscopies have been determinant in highlighting the structure of the species formed by adsorption of H2O2 on Ti centers inside the TS-1 framework. The following observations demonstrate a synergic role between Ti(IV) centers and hydrogen peroxide: (i) the O-O species, responsible for the yellow color of the TS-1/H2O/H2O2 system, is a side-on peroxo complex, probably generated by the reversible rupture of one Ti-O-Si bridge, with the formation of Ti(O2H) and H-O-Si groups; (ii) the stability of this peroxo complex is low in the absence of an excess of H2O; (iii) a strong enhancement of the acidity of the TS-1/H2O2/H2O system with respect to that of TS-1/H2O have been observed.
Single site catalyst for partial oxidation reaction: Ts-1 case study
BORDIGA, Silvia;DAMIN, Alessandro Ali;BONINO, Francesca Carla;LAMBERTI, Carlo
2005-01-01
Abstract
We briefly underline the relevance of TS-1 catalyst for industrial applications in mild oxidation reactions using hydrogen peroxide as oxidant and review the experimental works employed over last two decades for understanding the structure of the Ti centers in the bare TS-1 material. After an animated and controversial debate that has lasted in the literature until 1994, several works (reviewed here in depth) have definitively assessed that Ti atoms occupy framework positions substituting a Si atom and forming tetrahedral [TiO4] units. The literature concerning the interaction of TS-1 with ligand molecules is briefly discussed. There is unanimous consensus that ligand adsorption causes the distortion from the Td-like symmetry of the [TiO4] units. As the TS-1 catalyst works in aqueous solution, particular attention has been devoted to the interaction with water; the same holds for ammonia as it is a reactant in the ammoximation of cyclohexanone to give cyclohexanone oxime. Finally, the interaction of TS-1 with H2O/H2O2 solutions is reviewed in detail. Particular attention is paid to very recent results that have significantly improved knowledge of the catalyst in conditions as close as possible to working conditions. UV-Vis, Raman (under resonance conditions) and X-ray absorption (both in the XANES and EXAFS regions) spectroscopies have been determinant in highlighting the structure of the species formed by adsorption of H2O2 on Ti centers inside the TS-1 framework. The following observations demonstrate a synergic role between Ti(IV) centers and hydrogen peroxide: (i) the O-O species, responsible for the yellow color of the TS-1/H2O/H2O2 system, is a side-on peroxo complex, probably generated by the reversible rupture of one Ti-O-Si bridge, with the formation of Ti(O2H) and H-O-Si groups; (ii) the stability of this peroxo complex is low in the absence of an excess of H2O; (iii) a strong enhancement of the acidity of the TS-1/H2O2/H2O system with respect to that of TS-1/H2O have been observed.
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