XAFS study of Ti-silicalite: structure of framework Ti(IV) in presence and in absence of reactive molecules (H2O, NH3)

Bordiga, Silvia; Boscherini, Federico; Coluccia, S.; Genonic, F.; Lamberti, Carlo; Leofanti, G.; Marchese, Leonardo; Petrini, G.; Vlaic, G.; Zecchina, Adriano

doi:10.1007/bf00824045

Cited by 98 publications

(135 citation statements)

References 25 publications

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“…42,43 This preedge feature is very weak in the spectrum of Cr 2 O 3 (Figure 2b), as it is also for reduced CrO x . 43 12CrAl gives a weaker preedge feature than CrO 3 , while the preedge and near-edge spectral regions in 10V12CrAl and 12Cr10VAl resemble those in Cr 2 O 3 , suggesting the predominant presence of Cr 3+ in these three samples.…”

Section: Resultsmentioning

confidence: 81%

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VO_x/CrO_x/Al₂O₃ Catalysts

2006

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The structure and surface composition of binary oxides consisting of CrO x and VO x dispersed on alumina and their effects on the rate and selectivity of oxidative dehydrogenation (ODH) of propane were examined and compared with those for CrO x and VO x dispersed on alumina. VO x deposition on an equivalent CrO x monolayer on alumina and deposition of CrO x on an equivalent monolayer of VO x deposited on alumina led to CrVO 4 species during thermal treatment with concomitant reduction of Cr 6+ to Cr 3+ .

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Section: Resultsmentioning

confidence: 81%

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VO_x/CrO_x/Al₂O₃ Catalysts

2006

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“…This is consistent with the EXAFS results that the Ti IV coordination numbers can increase from 4 to 5-6 due to NH 3 adsorption. [2,[41][42] In the cases of bare Ti3 IV and Ti3 IV with one and two adsorbed NH 3 molecules, the average Ti À O lengths are approximately 1.745, 1.765, and 1.785 , respectively (Table 3). Accordingly, each NH 3 molecule elongates the Ti À O length by about 0.02 .…”

Section: Resultsmentioning

confidence: 99%

“…This indicates that charge-transfer transitions take place from the NH 3 molecules to the Ti IV atoms, consistent with previous experimental and theoretical studies on TS-1 zeolite. [12,[41][42][43][44] In addition, more charge will be transferred with an increasing number of NH 3 molecules.…”

Section: Resultsmentioning

confidence: 99%

Density Functional Calculations on the Distribution, Acidity, and Catalysis of Ti^IV and Ti^III Ions in MCM‐22 Zeolite

Yang

Zhou

Liu

et al. 2011

Chemistry A European J

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Isolated Ti species in zeolites show unique catalytic activities for a variety of chemical reactions. In this work, density functional calculations were used to explore three current concerns: 1) the distributions of TiIV and TiIII ions in the MCM‐22 zeolite; 2) the Lewis acidity of the TiIV and TiIII sites; and 3) activation of alkane CH bonds by photocatalysis with Ti‐doped zeolites. Neither the TiIV nor TiIII ions are randomly distributed in the MCM‐22 zeolite. The orders of relative stability are very close for the eight TiIV and TiIII sites, and the T3 site is the most probable in both cases. The wavelengths for TiIV–TiIII excitations were calculated to lie in the range λ=246.9–290.2 nm. The Ti3IV site shows Lewis acidity toward NH3 in two different modes, and these two modes can coexist with each other. The calculated TiIV coordination numbers, TiIVO bond elongations, and charge transfers caused by NH3 adsorption are in good agreement with previous results. Similarly, two different NH3 adsorption modes exist for the Ti3III site; the site that exhibits radical transfer from the lattice O to N atoms is preferred due to the higher adsorption energy. This indicates that the Ti3III site does not show Lewis acidity, in contrast to the Ti3IV site. At the Ti3III site, the energy barrier for activating the methane CH bond was calculated to be 33.3 kJ mol−1 and is greatly reduced by replacing the hydrogen atoms with methyl groups. In addition, the reactivity is improved when switching from MCM‐22 to TS‐1 zeolite. The studies on the various Ti species reveal that lattice O atoms rather than TiIII radicals are crucial to the activation of alkane CH bonds. This work provides new insights into and aids understanding of the catalysis by isolated Ti species in zeolites.

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“…The obtained distances are very close to those experimentally found by EXAFS experiments on well-manufactured TS-1 samples. [19][20][21][22]24,25 The authors conclude that, among the 24 different T sites, no clear preferential site was emerging from the energy calculation. Millini et al 27 reached the same conclusion concerning orthorhombic MFI on the basis of local density functional quantum mechanical calculations on pentameric Ti(OSiO 3 H 3 ) 4 cluster.…”

Section: Introductionmentioning

confidence: 99%

Ti Location in the MFI Framework of Ti−Silicalite-1: A Neutron Powder Diffraction Study

Lamberti¹,

Bordiga²,

Zecchina³

et al. 2001

J. Am. Chem. Soc.

193

239

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The first direct evidence that Ti atoms are not equally distributed in the 12 crystallographically independent T sites of the MFI framework is presented on the basis of neutron diffraction data collected at the HRPD instrument of the ISIS pulsed neutron source. We found strong evidence indicating that T6, T7, and T11 are the most populated sites and weak evidence that Ti may be hosted in T10. Ti occupancy can be excluded for sites T1, T2, T4, T5, T9, and T12. The occupancy of the remaining sites is doubtful. Since defective silicalite has been shown to exhibit the same preferential sites (T6, T7, T11, and T10) for Si vacancies, it may be suggested that the incorporation mechanism of the Ti atoms in the MFI framework occurs via the insertion of titanium in the defective sites. This hypothesis implies that titanium has a mineralizing effect on the MFI framework, and it is supported by independent spectroscopic data on both TS-1 and defective silicalite. The results are discussed in comparison with the known substitution mechanisms in the T-sites of MFI-type structures.

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XAFS study of Ti-silicalite: structure of framework Ti(IV) in presence and in absence of reactive molecules (H2O, NH3)

Cited by 98 publications

References 25 publications

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VO_x/CrO_x/Al₂O₃ Catalysts

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VO_x/CrO_x/Al₂O₃ Catalysts

Density Functional Calculations on the Distribution, Acidity, and Catalysis of Ti^IV and Ti^III Ions in MCM‐22 Zeolite

Ti Location in the MFI Framework of Ti−Silicalite-1: A Neutron Powder Diffraction Study

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XAFS study of Ti-silicalite: structure of framework Ti(IV) in presence and in absence of reactive molecules (H2O, NH3)

Cited by 98 publications

References 25 publications

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VOx/CrOx/Al2O3 Catalysts

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VOx/CrOx/Al2O3 Catalysts

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

Ti Location in the MFI Framework of Ti−Silicalite-1: A Neutron Powder Diffraction Study

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Nature, Density, and Catalytic Role of Exposed Species on Dispersed VO_x/CrO_x/Al₂O₃ Catalysts

Nature, Density, and Catalytic Role of Exposed Species on Dispersed VO_x/CrO_x/Al₂O₃ Catalysts

Density Functional Calculations on the Distribution, Acidity, and Catalysis of Ti^IV and Ti^III Ions in MCM‐22 Zeolite