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Reactive oxo-titanium species in titanosilicate molecular sieves: EPR investigations and structure–activity correlations
- Source :
-
Journal of Catalysis . Jul2003, Vol. 217 Issue 1, p160. 12p. - Publication Year :
- 2003
-
Abstract
- The structure of the reactive oxo-titanium species (hydroperoxo-, peroxo-, and superoxo-titanium) in titanosilicate molecular sieves (TS-1, Ti-<f>β</f>, amorphous Ti–SiO2, and TiMCM-41), generated on interaction with aqueous H2O2 or urea–H2O2 adducts, was investigated by electron paramagnetic resonance (EPR) and diffuse reflectance UV–visible (DRUV–visible) spectroscopies and magnetic susceptibility measurements. Two types of superoxo-titanium species (A and B) were identified in TS-1 and amorphous Ti–SiO2. An additional oxo species, A<f>′</f>, was also detected over Ti-<f>β</f>. TiMCM-41 generated, mainly, the B-type species. DRUV–visible spectroscopy and magnetic susceptibility measurements suggest the coexistence of both the superoxo- and hydroperoxo-titanium species. The A-type oxo-titanium species originate from the framework Ti sites having a tetrapodal (SiO)4Ti structure located inside the pores, while the B-type species originate predominantly from tripodal (SiO)3Ti(OH) structures, also located in the framework, probably at the external surface. The local structural environment of the <f>Ti4+</f> ions, solvent, and temperature influence the concentration of these oxo-titanium species. The concentration of the A-type species decreased in the order: TS-1<f>></f>Ti-<f>β></f>amorphous Ti–SiO2. The B type varied as follows: TiMCM-41<f>></f>amorphous Ti–SiO2<f>></f>Ti-<f>β></f>TS-1. In the oxidation of styrene, the A-type species correlate with styrene oxide formation, while the presence of B-type species lead to nonselective products. Experimental evidence is given for the involvement of the hydroperoxo-titanium species in olefinic epoxidations (styrene to styrene oxide) and the superoxo-titanium in aromatic hydroxylations (phenol to hydroquinone and catechol). [Copyright &y& Elsevier]
- Subjects :
- *SILICATE minerals
*TITANIUM compounds
Subjects
Details
- Language :
- English
- ISSN :
- 00219517
- Volume :
- 217
- Issue :
- 1
- Database :
- Academic Search Index
- Journal :
- Journal of Catalysis
- Publication Type :
- Academic Journal
- Accession number :
- 9853483
- Full Text :
- https://doi.org/10.1016/S0021-9517(03)00060-5