Structure and Dynamics of the Flexible Triple Helix of Water inside VPI-5 Molecular Sieves

Fois, Ettore; Gamba, Aldo; Tilocca, Antonio

doi:10.1021/jp014276k

Cited by 43 publications

(48 citation statements)

References 29 publications

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“…The chosen system was the LTA structure type, which displays supercages (also called α cages) of diameter 13Å, connected to one another in a cubic symmetry by 8-ring windows of diameter 6Å, and smaller sodalite cages of diameter 7Å (see Figure 1). As opposed to previous simulations of 1D water chains confined in zeolitic materials such as Li-ABW, bikitaite, [25,26] and VPI-5, [27] the medium-size, 3D-connected pores of LTA make it a good candidate for studying the effect of confinement on liquid water condensation in open framework nanoporous materials. Furthermore, the small size of the LTA unit cell (approximately 12Å) makes it a reasonable system for first-principles molecular dynamics simulations.…”

Section: 1 Systems Studiedmentioning

confidence: 89%

Water nanodroplets confined in zeolite pores

et al. 2009

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We provide a comprehensive depiction of the behaviour of a nanodroplet of approximately equal to 20 water molecules confined in the pores of a series of 3D-connected isostructural zeolites with varying acidity, by means of molecular simulations. Both grand canonical Monte Carlo simulations using classical interatomic forcefields and first-principles Car-Parrinello molecular dynamics were used in order to characterise the behaviour of confined water by computing a range of properties, from thermodynamic quantities to electronic properties such as dipole moment, including structural and dynamical information. From the thermodynamic point of view, we have identified the all-silica zeolite as hydrophobic, and the cationic zeolites as hydrophilic; the condensation transition in the first case was demonstrated to be of first order. Furthermore, in-depth analysis of the dynamical and electronic properties of water showed that water in the hydrophobic zeolite behaves as a nanodroplet trying to close its hydrogen-bond network onto itself, with a few short-lived dangling OH groups, while water in hydrophilic zeolites "opens up" to form weak hydrogen bonds with the zeolite oxygen atoms. Finally, the dipole moment of confined water is studied and the contributions of water self-polarisation and the zeolite electric field are discussed.

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Section: 1 Systems Studiedmentioning

confidence: 89%

Water nanodroplets confined in zeolite pores

et al. 2009

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“…refs. [33,34]). Geometry optimizations were performed with a convergence criterium of 1.0 10 À4 a.u.…”

Section: Theory and Methodologymentioning

confidence: 99%

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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“…Mainly because of their impact on catalysis, the location and distribution of extraframework cations in zeolites have been object of many experimental [499][500][501][502][503][504] and computational [505][506][507][508][509][510] studies. In both hydrophilic and hydrophobic frameworks, water behavior has been intensively explored, [217,[511][512][513][514][515][516][517][518] uncovering unexpected supramolecular structures, such as nanohelices (in natrolite), [511] triple helices (in VPI-5), [87,519] cyclic hexamers (in NaY supercages), [520,521] or nanoworms (in hydrophobic silicalite, Figure 5) [522,523] along with the more common nanospheres (in zeolite A) [524] and ice-like nanotubes (in AlPO 4 -5 and SSZ-24). [525] Overall, the message is that water behavior differs significantly in passing from hydrophobic to hydrophilic zeolites ( Figure 31).…”

Section: Supramolecular Organization Of Water and Cations In Confinedmentioning

confidence: 99%

Supramolecular Organization in Confined Nanospaces

Tabacchi

2018

ChemPhysChem

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Empty spaces are abhorred by nature, which immediately rushes in to fill the void. Humans have learnt pretty well how to make ordered empty nanocontainers, and to get useful products out of them. When such an order is imparted to molecules, new properties may appear, often yielding advanced applications. This review illustrates how the organized void space inherently present in various materials: zeolites, clathrates, mesoporous silica/organosilica, and metal organic frameworks (MOF), for example, can be exploited to create confined, organized, and self-assembled supramolecular structures of low dimensionality. Features of the confining matrices relevant to organization are presented with special focus on molecular-level aspects. Selected examples of confined supramolecular assemblies - from small molecules to quantum dots or luminescent species - are aimed to show the complexity and potential of this approach. Natural confinement (minerals) and hyperconfinement (high pressure) provide further opportunities to understand and master the atomistic-level interactions governing supramolecular organization under nanospace restrictions.

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Structure and Dynamics of the Flexible Triple Helix of Water inside VPI-5 Molecular Sieves

Cited by 43 publications

References 29 publications

Water nanodroplets confined in zeolite pores

Water nanodroplets confined in zeolite pores

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

Supramolecular Organization in Confined Nanospaces

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