Aluminum Distribution in Low Si/Al Zeolites:  Dehydrated Na−Clinoptilolite

Ruiz‐Salvador, A. Rabdel; Lewis, Dewi W.; Rubayo-Soneira, J.; Rodrı́guez-Fuentes, G.; Sierra, Leopoldo; Catlow, C. Richard A.

doi:10.1021/jp982211k

Cited by 44 publications

(48 citation statements)

References 48 publications

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“…It was then concluded that In the case of the exchanged samples, the existence of two Gaussians function is also consistent with previous experimental works [22,23] and with computational simulations carried out by A.R. Ruiz-Salvador et al [20,25,26]. In every case, the most occupied sites are M1 and M2 in dehydrated samples, with more accuracy in sites close to these experimentally determined position.…”

Section: Analysis Of the Samples Cis And Ddta Responsesupporting

confidence: 88%

“…So, there must be no steric effect due to the size of the cation hydration shell. 10 As reported for the dry Cli Na − , the coordination number of the sodium cation with the oxygen atoms of the zeolite framework is lower for M1 than for M2 [25], i.e. the de-trapping energy is lower for M1 than for M2 cations.…”

Section: Analysis Of the Samples Cis And Ddta Responsementioning

confidence: 73%

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Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy

et al. 2011

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To cite this version:Gerardo Rodriguez-Fuentees, Sabine Devautour-Vinot, Sekou Diaby, François Henn. Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy. Phys. Chem. Minerals, 2011, 38 (xx) ABSTRACT.Purified natural clinoptilolite from the Tasajeras deposit, Cuba, and some of its metal exchanged forms are studied, at the dehydrated state, by means of Dielectric Relaxation Spectroscopy (DRS) using two different modus operandi: Complex Impedance Spectroscopy and Dielectric Dynamic Thermal Analysis. Data analysis yields the determination of the extra-framework cation (EFC) population into the various possible crystallographic sites of the zeolitic framework as well as of the activation energy characterizing the localized hopping mechanism of EFC. First, it is shown that the DRS responses obtained here match well with the previous reported data which were previously localized EFCs in positions close to M1 and M2 sites when the clinoptilolite is modified to almost homoionic form. From this outcome, it can be concluded that all EFCs are in the same crystallographic situation regarding solvation or, in other terms, that no steric effect can be taken into account to explain cationic selectivity. Second, based on the assumption that the activation energy for EFC hopping is directly connected to the EFC/framework interaction and on simple thermodynamics consideration, we show this interaction does not govern the EFC 2 exchange reaction. So, it is emphasized that EFC/H 2 O interaction is the key factor for cation exchange selectivity.

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Section: Analysis Of the Samples Cis And Ddta Responsesupporting

confidence: 88%

Section: Analysis Of the Samples Cis And Ddta Responsementioning

confidence: 73%

Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy

et al. 2011

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“…In this work we carry out a detailed atomistic simulation study in order to gain a better understanding of the relationship between the nature of the extra-framework cations and the flexibility of the material, taking into account the concomitant impact on molecular sieving properties. Existing approaches for accurately modeling cation location and/or large variations of cell volumes in zeolites consider either poorly deformable frameworks [16][17][18][19], or those having low ratios between the cation number and their potentially accessible sites [20][21][22]. For all these reasons, in the early stage of this work we were unsuccessful in reproducing the crystal structure of several cation-forms of zeolite RHO by applying these methods.…”

Section: Introductionmentioning

confidence: 99%

Understanding Nanopore Window Distortions in the Reversible Molecular Valve Zeolite RHO

et al. 2015

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Molecular valves are nanostructured materials that are becoming popular, due to their potential use in bio-medical applications. However, little is known concerning their performance when dealing with small molecules, which are of interest in energy and environmental areas. It has been observed experimentally that zeolite RHO shows unique pore deformations upon changes in hydration, cation siting, cation type, and/or temperature-pressure conditions. By varying the level of distortion of double 8-rings it is possible to control the adsorption properties, which confers a molecular valve behavior to this material. We have employed interatomic potentials-based simulations to obtain a detailed atomistic view of the structural distortion mechanisms of zeolite RHO, in contrast with the averaged and space group restricted information that can be retrieved from diffraction studies. We have modeled the pure silica zeolite RHO as well as four aluminosilicate structures, containing Li + , Na + , K + , Ca 2+ and Sr 2+ cations. It has been found that the distortions of the three zeolite rings are coupled, although the four-membered rings are rather rigid and both six-and eight-membered rings are largely flexible. A large dependence on the polarizing power of the extra-framework cations and with the loading of water has been found for the minimum aperture of the eight-membered rings that control the nanovalve effect. The energy barriers needed to move the cations across the eightmembered rings are calculated to be very high, which explains the origin of the experimentally observed slow kinetics of the phase transition, as well as the appearance of metastable phases.

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“…[10][11][12] The reason for this difficulty is the paucity of experimental techniques available to discriminate between population of Si and Al or Ti at a given site. The same questions remain unresolved in the case of TS-1.…”

Section: Introductionmentioning

confidence: 99%

The Siting of Ti in TS-1 Is Non-Random. Powder Neutron Diffraction Studies and Theoretical Calculations of TS-1 and FeS-1

et al. 2000

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We present the first direct evidence of non random siting of Ti and Fe in TS-1 and FeS-1, nanoporous metallosilicate selective oxidation catalysts of MFI topology. This was accomplished by using Rietveld analysis of powder neutron diffraction data and exploiting the differences in neutron scattering lengths between Ti or Fe and Si. Previous spectroscopic, X-ray diffraction, and computational approaches have suggested a random substitution of Ti and Fe ions among the 12 crystallographically distinct Si sites in the framework of TS-1 and FeS-1. In contrast, our results indicate that titanium is distributed among only 4 or 5 of the 12 silicon sites with Ti occupying T3, -7, -8, -10, and -12. Of the 2.47 total Ti atoms per unit cell the Ti site occupancies and estimated standard deviation for sample B are as follows: T3 0.30(0.11), T7 0.34(0.14), T8 0.92(0.10), T10 0.41(0.14), and T12 0.50(0.14). In FeS-1 synthesized with 1.5 Fe atoms per unit cell, iron is found only at T8. Several starting models were chosen for initial refinement, and each returned the same specific, nonrandom distribution of Ti in the framework of MFI. We have examined several computational approaches that involve thermodynamic arguments to rationalize the experimental observations, and all have failed to predict the experimentally observed substitution pattern. This suggests that the kinetics of framework formation may play a role in directing the observed metal substitution.

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Aluminum Distribution in Low Si/Al Zeolites: Dehydrated Na−Clinoptilolite

Cited by 44 publications

References 48 publications

Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy

Insights into cation exchange selectivity of a natural clinoptilolite by means of dielectric relaxation spectroscopy

Understanding Nanopore Window Distortions in the Reversible Molecular Valve Zeolite RHO

The Siting of Ti in TS-1 Is Non-Random. Powder Neutron Diffraction Studies and Theoretical Calculations of TS-1 and FeS-1

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