XAFS Study of Ti-Silicalite: Structure of Framework Ti(IV) in the Presence and Absence of Reactive Molecules (H2O, NH3) and Comparison with Ultraviolet-Visible and IR Results

Bordiga, Silvia; Coluccia, S.; Lamberti, Carlo; Marchese, Leonardo; Zecchina, A.; Boscherini, Federico; Buffa, Franco; Genoni, F.; Leofanti, G.

doi:10.1021/j100066a036

Cited by 379 publications

(250 citation statements)

References 3 publications

Supporting

Mentioning

215

Contrasting

Unclassified

Order By: Relevance

“…Diffuse reflectance UV/Vis spectroscopy indicates a red-shift of a multiple band, in thẽ 200-240 nm range in dry samples, [4] and from~200-250 nm in hydrated TS-1. [5] EXAFS experiments confirm that such a redshift occurs along with the formation of penta-and/or hexacoordinated titanium centres. [5,6] Absorption in these ranges, which are characteristic of titanium sites in zeolites, are due to a LMCT mechanism.…”

mentioning

confidence: 73%

See 1 more Smart Citation

Bathochromic Effects in Electronic Excitation Spectra of Hydrated Ti Zeolites: A Theoretical Characterization

2008

View full text Add to dashboard Cite

Titanium-containing zeolites are a class of porous materials having widespread use in industry as catalysts for redox processes in mild conditions. At low titanium concentrations, titanium isomorphously replaces silicon in the tetrahedral sites of the zeolitic framework. Titanium may expand its coordination from four to six, acting as a Lewis acid with respect to bases adsorbed in the channel-and-cage systems typical of zeolites. Herein, we interpret the red-shift (bathochromic effect) of the ligand-to-metal-charge-transfer (LMCT) electronic transitions detected by UV/Vis studies on titanium zeolites upon hydration at the microscopic level by using a DFT-based computational approach which takes into account the full periodicity of the crystalline phase. The relationships among the structures of the zeolitic titanium sites, the adsorbed water molecules and the electronic excitation properties in models of titanium zeolites are discussed. The LMCT bands' profiles, edges and intensities are interlaced with both the location and the coordination of the water molecules at the titanium site.Since the first patent in 1985, [1] titanium silicalite (TS-1) has been widely adopted in partial oxidations of hydrocarbons by using aqueous hydrogen peroxide as oxygen source.[2] In asprepared TS-1, titanium is pentacoordinated, via four framework oxygen atoms and one OH À group.[3] The OH À group is released upon calcination, and, in evacuated (solvent free) zeolites, titanium is coordinated by the four framework oxygen atoms in a tetrahedral geometry. UV/Vis and EXAFS experiments show that water loading causes a reversible expansion of the coordination shell of titanium. Diffuse reflectance UV/Vis spectroscopy indicates a red-shift of a multiple band, in thẽ 200-240 nm range in dry samples, [4] and from~200-250 nm in hydrated TS-1.[5] EXAFS experiments confirm that such a redshift occurs along with the formation of penta-and/or hexacoordinated titanium centres. [5,6] Absorption in these ranges, which are characteristic of titanium sites in zeolites, are due to a LMCT mechanism. [7] While formation and reactivity of oxidizing intermediates in TS-1 have been the subject of many studies, the relationships between the extent of hydration, structure and electronic excitation spectra have received less attention. A description of the electronic excitation in titanium zeolites in non-oxidizing conditions at an atomistic level is, however, a topic of current interest, as it has been recently reported that titanium zeolites might replace the semiconducting TiO 2 layer in photovoltaic cells. [8] Most of the reported data deals with experiments on TS-1, a zeolite with an MFI framework structure [9] and a unit cell content of [Ti x Si 96Àx O 192 ]-too large for extensive ab initio calculations. As a consequence, a number of theoretical studies have been performed on model clusters or by using embedded cluster approaches (see for example, ref.[10]). Smaller titanium zeolites have been studied by ab initio methods by taking into account perio...

show abstract

mentioning

confidence: 73%

“…[5,6] Absorption in these ranges, which are characteristic of titanium sites in zeolites, are due to a LMCT mechanism. [7] While formation and reactivity of oxidizing intermediates in TS-1 have been the subject of many studies, the relationships between the extent of hydration, structure and electronic excitation spectra have received less attention.…”

mentioning

confidence: 99%

Bathochromic Effects in Electronic Excitation Spectra of Hydrated Ti Zeolites: A Theoretical Characterization

2008

View full text Add to dashboard Cite

show abstract

“…2. The pre-edge, small peak(s) before the sharp rise in the spectrum is a result of the excitation of the core electron of Ti to empty bound states and depends on atomic geometry [16]. Titanium foil showed only one strong pre-edge peak at 4,967.9 eV, which is a characteristic of tetrahedral symmetry.…”

Section: Catalyst Characterizationmentioning

confidence: 97%

Characteristics and photocatalytic degradation of methyl orange on Ti-RH-MCM-41 and TiO2/RH-MCM-41

Artkla

Wantala

Srinameb

et al. 2009

Korean J. Chem. Eng.

View full text Add to dashboard Cite

Our purpose was to synthesize, characterize and test photodegradation of methyl orange on two catalysts containing 10 wt% titanium supported on mesoporous MCM-41 synthesized with rice husk silica. The first catalyst was Ti-RH-MCM-41 prepared by adding tetrabutyl orthotitanate (TBOT) in a synthetic gel of RH-MCM-41, and the second catalyst was TiO 2 /RH-MCM-41 prepared by grafting TBOT on the preformed RH-MCM-41. The mesoporous structures were observed on both catalysts and they had surface area of 1,073 and 1,006 m 2 /g. The Ti in Ti-RH-MCM-41 was in the form of Ti(IV) with tetrahedral geometry residing in the mesoporous structure. This form was less active for photodegradation of methyl orange than the other one. The Ti in TiO 2 /RH-MCM-41 was anatase titania with octahedral geometry located outside the mesoporous framework. This form was more an active phase for the photodegradation and the reaction parameters on this catalyst were further investigated. The optimum catalyst weight to methyl orange volume ratio was 5 g/L and the optimum initial concentration of the dye was 2.0 ppm. The degradation rate obeyed pseudo-first order and the adsorption of methyl orange on TiO 2 /RH-MCM-41 obeyed Langmuir isotherm.

show abstract

“…Water, accordingly, may have some influence on the deactivation of Au catalysts. It is likely that water formed during reaction cannot be completely removed from the reactor and partially adsorbs on the catalysts, and results in a more hydroxided surface causing more PO absorption [22,29] and further catalyzing side reactions [29][30][31]. Our recent work [23] showed that the rate of deactivation of Au catalysts could be somewhat suppressed when the Ti-MCM-48 support was trimethylsilylated.…”

Section: Deactivation With Time On Stream: Effect Of Supports Structurementioning

confidence: 99%

Vapor-phase epoxidation of propylene using H2/O2 mixture over gold catalysts supported on non-porous and mesoporous titania-silica: effect of preparation conditions and pretreatments prior to reaction

Qi¹,

Okumura²,

Akita³

et al. 2004

Applied Catalysis A: General

View full text Add to dashboard Cite

XAFS Study of Ti-Silicalite: Structure of Framework Ti(IV) in the Presence and Absence of Reactive Molecules (H2O, NH3) and Comparison with Ultraviolet-Visible and IR Results

Cited by 379 publications

References 3 publications

Bathochromic Effects in Electronic Excitation Spectra of Hydrated Ti Zeolites: A Theoretical Characterization

Bathochromic Effects in Electronic Excitation Spectra of Hydrated Ti Zeolites: A Theoretical Characterization

Characteristics and photocatalytic degradation of methyl orange on Ti-RH-MCM-41 and TiO2/RH-MCM-41

Vapor-phase epoxidation of propylene using H2/O2 mixture over gold catalysts supported on non-porous and mesoporous titania-silica: effect of preparation conditions and pretreatments prior to reaction

Contact Info

Product

Resources

About