Electronic Spectra of Ti(<scp>IV</scp>) in Zeolites: An Ab Initio Approach

Fois, Ettore; Gamba, Aldo; Tabacchi, Gloria

doi:10.1002/cphc.200500153

Cited by 19 publications

(32 citation statements)

References 30 publications

(33 reference statements)

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“…The electronic excitation spectra were calculated via the time-dependent density functional theory [20] approach (TD-DFT) on a titanium-centred cluster derived from the DFT-optimized crystal, thus obtaining absorption wavelengths in line with experiments. [17] However, similar accuracy was also obtained by calculating the optical conductivity s of periodic models of both titanium zeolites and mesoporous silicas with DFT. [16,21] Herein, the full periodic DFT approach is followed.…”

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“…In contrast to their titanium-containing counterparts, they do not present absorption beyond 180 nm, with the exception of the STF system, for which a very low intensity of absorption is calculated at about 200 nm (Figure 1 a). Such a finding should not be surprising considering that the STF framework is characterized by five-membered rings (5-MRs) as building unit [9,22] and that moderate semiconductor behaviour was observed in a series of zeolites characterized by the presence of 5-MR. [23] Comparison of the calculated ss indicates that the 200-240 nm absorption bands should be related to the presence of [17] is the most symmetric among those studied here. The other model titanium zeolites, characterized by lower symmetry at the titanium centres, show electronic excitation spectra in the same UV region.…”

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confidence: 95%

“…[11,12,[14][15][16] However, to the best of our knowledge, very few modelling studies devoted to electronic transition properties have been published so far. [17][18][19] Recently a mixed approach was presented, where the titanium zeolite structure was optimized with a periodic DFT approach. The electronic excitation spectra were calculated via the time-dependent density functional theory [20] approach (TD-DFT) on a titanium-centred cluster derived from the DFT-optimized crystal, thus obtaining absorption wavelengths in line with experiments.…”

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

2008

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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...

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

2008

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“…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.…”

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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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“…Pseudopotentials and basis set expansion were the same adopted in [56]. Such a computational approach has demonstrated to provide a satisfactory description of diverse systems involving adsorbate-surfaces [108][109][110][111][112][113][114][115][116][117][118][119][120][121][122] or host-guest interactions [123][124][125][126][127][128][129][130][131][132][133][134] including also processes at zeolite interfaces [32,59] and high pressure conditions [97,[135][136][137][138][139][140].…”

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Unravelling the High-Pressure Behaviour of Dye-Zeolite L Hybrid Materials

et al. 2018

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Self-assembly of chromophores nanoconfined in porous materials such as zeolite L has led to technologically relevant host-guest systems exploited in solar energy harvesting, photonics, nanodiagnostics and information technology. The response of these hybrid materials to compression, which would be crucial to enhance their application range, has never been explored to date. By a joint high-pressure in situ synchrotron X-ray powder diffraction and ab initio molecular dynamics approach, herein we unravel the high-pressure behaviour of hybrid composites of zeolite L with fluorenone dye. High-pressure experiments were performed up to 6 GPa using non-penetrating pressure transmitting media to study the effect of dye loading on the structural properties of the materials under compression. Computational modelling provided molecular-level insight on the response to compression of the confined dye assemblies, evidencing a pressure-induced strengthening of the interaction between the fluorenone carbonyl group and zeolite L potassium cations. Our results reveal an impressive stability of the fluorenone-zeolite L composites at GPa pressures. The remarkable resilience of the supramolecular organization of dye molecules hyperconfined in zeolite L channels may open the way to the realization of optical devices able to maintain their functionality under extreme conditions.

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Electronic Spectra of Ti(IV) in Zeolites: An Ab Initio Approach

Cited by 19 publications

References 30 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

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

Unravelling the High-Pressure Behaviour of Dye-Zeolite L Hybrid Materials

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Electronic Spectra of Ti(IV) in Zeolites: An Ab Initio Approach

Cited by 19 publications

References 30 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

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

Unravelling the High-Pressure Behaviour of Dye-Zeolite L Hybrid Materials

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