Location Of Cations Research Articles

ChemInform Abstract: A Characterization and Catalytic Study of Y Zeolite Supported Ni/Cu Bimetallics.

A wide range of CuNiNaY andCuNiKY zeolites were prepared by ion exchange. The location of both Ni2+ and Cu2+ cations within the zeolite framework was monitored after various stages of thermal treatment and the reduction process of the transition metal ions in a flowing hydrogen atmosphere was investigated and correlated with reduction time and temperature. The metallic phase generated on reduction was characterized by X-ray diffraction line broadening. The catalytic activity of the reduced zeolites in the hydrogenation of benzene is discussed in the light of these physical characterizations.Keywords: zeolites, supported Ni/Cu bimetals, cation location, crystallite size, catalysis.

Influence of the sodium and calcium non-framework cations on the adsorption of hexane isomers in zeolite BEA

We have performed configurational-bias Monte Carlo simulations to compute pure component adsorption isotherms of n-hexane, 3-methylpentane and 2,2-dimethylbutane in BEA-Polymorphs A and B at 423 K. The effect of the density and nature of influence of non-framework cations was systematically analyzed. Our results show that differences in the type and concentration of the non-framework cations lead to differences in adsorption loading. We found that this behavior is directly related to the preferential adsorption sites of the isomers as well as to the amount and location of the non-framework cations.

Cation Characterization and CO2 Capture in Li+-Exchanged Metal−Organic Frameworks: From First-Principles Modeling to Molecular Simulation

We report a computational study for cation characterization and CO2 capture in Li+-exchanged metal−organic frameworks (Li+-MOFs). Density functional theory is adopted to optimize cation locations and evaluate atomic charges, and molecular simulation is subsequently used to examine the separation of CO2/H2 and CO2/N2 mixtures for pre- and post-combustion CO2 capture. The cations are observed to locate near the carboxylic O-donors of metal clusters. Specifically, H+ ions in dehydrated Li+-MOF form covalent bonds with the O-donors, and H3O+ ions in hydrated Li+-MOF form hydrogen bonds with the O-donors. CO2 is overwhelmingly adsorbed over H2 and N2 in both dehydrated and hydrated Li+-MOFs. Adsorption occurs preferentially near the cations and metal clusters, which possess strong electrostatic potentials, and then in the square channels. At ambient condition, the selectivity is approximately 550 for CO2/H2 mixture and 60 for CO2/N2 mixture, higher than that in nonionic MOFs and other nanoporous adsorbents. Th...

Locating Extra-Framework Cations in Low-Silica Zeolites by a Combinatorial Approach of the Direct Space Method and Rietveld Refinement: Application to Ni2+ and Co2+ Enriched Clinoptilolite

The location of extra-framework cations in low-silica zeolites is determined from in-house X-ray powder diffraction pattern by a successful implementation of a newly developed methodology. The method combines reciprocal and direct space methods plus a cost function that accounts for simultaneous fit of the chemical composition and the X-ray diffractogram. We demonstrated that the iterative combination of relaxation methods (Monte Carlo exploration and lattice energy minimization) helps to improve the structural refinement. The methodology is successfully applied to the study of natural clinoptilolite samples enriched with Ni and Co; in both cases, two different cation sites were found octahedrally coordinated to water molecules. The most populated site is located in the center of the A channel, while the second one is found in the window of the B channel. This result was validated using XANES and EXAFS spectroscopies.

Sorption of nitrogen, oxygen, and argon in Cd (II) exchanged zeolite X: volumetric equilibrium adsorption and grand canonical Monte Carlo study

The adsorption of nitrogen, oxygen and argon has been studied in cadmium (II) cations exchanged zeolite X at 288.2 and 303.2 K. Experimentally measured adsorption isotherms are compared with theoretically calculated data using grand canonical Monte Carlo (GCMC) simulation. Nitrogen showed higher adsorption capacity and selectivity than oxygen and argon in these zeolite samples. The cadmium exchanged zeolite X was showed that increased adsorption capacity for nitrogen, oxygen, and argon with increase in Cd (II)-exchange levels, indicating as charge density increases adsorption capacity also increase. Isosteric heat of adsorption data showed stronger interactions of nitrogen molecules with cadmium cations in zeolite samples. These observations have been explained in terms of higher electrostatic interaction of nitrogen with extra framework zeolite cations. The selectivity of oxygen over argon is explained in terms of its higher interaction with Cd (II)-exchanged zeolites than argon molecules. The selectivity of N2/O2 of cadmium-exchanged zeolite X is better than only sodium containing zeolite-X. Heats of adsorption and adsorption isotherms were also calculated using GCMC simulation algorithm. Simulation studies expectedly show the proximity of nitrogen molecules to the locations of extra framework sodium and cadmium cations.

A DFT periodic study on the interaction between O2and cation exchanged chabazite MCHA (M = H+, Na+ or Cu+): effects in the triplet–singlet energy gap

O(2) adsorption in proton, sodium and copper exchanged chabazite has been studied using periodic and cluster approaches by means of density functional theory. The Grimme's correction has been used to include the dispersion contribution to B3LYP. Two cation locations have been considered: one with the cation at the six-membered ring (MCHA(I)) and the other with the cation at the 8-membered ring (MCHA(IV)). The O(2)-HCHA and O(2)-NaCHA adsorption complexes present a eta(1)-O(2) bent coordination. The adsorption energies, which are due to dispersion, are between -15 and -19 kJ mol(-1), in agreement with the experimental values. On the other hand, the O(2) coordination to CuCHA is through a eta(2)-side-on mode with a square planar coordination around the metal center. This structure favors the Cu d -->pi* O(2) charge transfer which becomes the predominant stabilizing factor. The adsorption of singlet states of O(2) in HCHA and NaCHA, modeled with an ONIOM M12T:48T, is of the same nature as that of the ground state, and only the highest in energy (1)Sigma is significantly more stabilized in MCHA than the triplet state by 14 to 24 kJ mol(-1). The adsorption of singlet O(2) in Cu exchanged zeolites presents a higher electron transfer from Cu(+) to O(2) than that calculated for the triplet species and thus both singlet states are stabilized with respect to the ground state O(2). Generally, singlet oxygen appears more attractive to active zeolite models than those calculated with triplet oxygen, indicating a source of oxidative efficiency for designed structures.

Copper Coordination in Cu-SSZ-13 and Cu-SSZ-16 Investigated by Variable-Temperature XRD

Nitrogen oxides (NOx) are a major atmospheric pollutant produced through the combustion of fossil fuels in internal combustion engines. Copper-exchanged zeolites are promising as selective catalytic reduction catalysts for the direct conversion of NO into N2 and O2, and recent reports have shown the enhanced performance of Cu-CHA catalysts over other zeolite frameworks in the NO decomposition of exhaust gas streams. In the present study, Rietveld refinement of variable-temperature XRD synchrotron data obtained for Cu-SSZ-13 and Cu-SSZ-16 is used to investigate the location of copper cations in the zeolite pores and the effect of temperature on these sites and on framework stability. The XRD patterns show that the thermal stability of SSZ-13 is increased significantly when copper is exchanged into the framework compared with the acid form of the zeolite, H-SSZ-13. Cu-SSZ-13 is also more thermally stable than Cu-SSZ-16. From the refined diffraction patterns, the atomic positions of atoms, copper locations a...

Direct Atomic-Scale Observation of Redox-Induced Cation Dynamics in an Oxide-Supported Monolayer Catalyst: WOx/α-Fe2O3(0001)

For the case of one-third of a monolayer of tungsten grown by atomic layer deposition on a hematite alpha-Fe(2)O(3)(0001) surface, we report direct atomic-scale observations of the structural and chemical changes that occur as this model interfacial system evolves from the as-deposited state to the oxidized state, then to the reduced state, and finally back to the oxidized state. In situ X-ray standing-wave atomic images relative to the hematite lattice show dramatic (but redox-reversible) changes to the W cation locations with respect to the occupied and unoccupied Fe surface sites. These structural changes are concurrent with X-ray photoelectron spectroscopy chemical shift changes, where W is observed to go from the 6+ to 5+ oxidation state. These novel observations of redox-induced cation dynamics in an oxide-supported catalyst are explained by models that account for W incorporation at the interface in Fe sites with various coordination schemes. Our proposed structure models are supported by our X-ray absorption fine structure measurements.

Xenon adsorption in synthetic chabazite zeolites

Xenon adsorption has been studied in different ion-exchanged forms of synthetic chabazite zeolites, K–, Ca–, Li–, Rb– and Cs–CHA, at low pressures and variable temperature. The xenon adsorption capacities vary with the nature of the exchanged cation in a way that is consistent with the known cation locations in the zeolite. Highest uptake was found for Ca–CHA, and lowest for Cs–CHA. The xenon uptakes at room temperature were also consistent with differences in the nitrogen adsorption capacities at 77 K. Enthalpies of adsorption were estimated from the temperature dependence of the adsorption isotherms in Ca–CHA and K–CHA, and found to be very similar. The xenon adsorption was also compared with that of an aluminophosphate (ALPO-CHA) as well as a silicoaluminophosphate (SAPO-34), both having the chabazite-type structure. The high affinity of CHA for xenon is attributed in part to a close match between the size of the xenon atom and the pore dimensions of the CHA structure, the so-called confinement effect.

Crystallography of Chevrel Phases, MMo6T8 (M = Cd, Na, Mn, and Zn, T = S, Se) and Their Cation Mobility

Chevrel phases (CPs), M(x)Mo(6)T(8) (M = metal, T = S, Se) are unique materials, which allow for a fast and reversible insertion of various cations at room temperature. In spite of extensive studies of these materials, the origin of their high ionic mobility remained unclear. In a previous paper we presented for the first time a proper classification of the very complex transport behavior of different cations in the Mo(6)T(8) hosts: (i) apparent immobility of the large M cations such as Pb(2+), Sn(2+), Ag(+) in the ternary phases, MMo(6)T(8); (ii) coupled M+M' diffusion in the quaternary phases, M(x)M'(y)Mo(6)T(8), where both large and small cations can assist; (iii) cation trapping in the Mg-Mo(6)S(8), Cd-Mo(6)S(8), and Na-Mo(6)T(8) systems; (iv) a combination of low- and high-rate diffusion kinetics at the first and last intercalation stages, respectively, for the Cu-Mo(6)S(8), Mn-Mo(6)S(8), and Cd-Mo(6)Se(8) systems, and (v) a fast ionic transport for small cations such as Ni(2+), Zn(2+), and Li(+). It was shown that this behavior could be understood by a relatively simple crystallographic analysis (mapping of all the cation sites and estimations of their potential energy according to the distances of these sites from adjacent anions and cations) of the diffusion routes, which differ for different cations. For this analysis, it was necessary to complete our knowledge about the cation location in the crystal structure for several CPs with known ionic mobility. This article presents the results of a combined Rietveld analysis of powder X-ray and high-resolution neutron diffraction profiles for NaMo(6)T(8), ZnMo(6)T(8), CdMo(6)T(8), and MnMo(6)S(8). All seven compounds can be defined as classic CPs, where cation delocalization from the center of the largest cavity between the Mo(6)T(8) blocks decreases with the length of the ideal chemical bond, M-T. In addition, this work details the effect of the structural parameters on the ionic conductivity in CPs, namely, it shows how subtle changes in cation delocalization may be crucial for diffusion kinetics.

A Conformational Study of 1-Bromo-2-chloroethane in Zeolites

The conformational properties of 1-bromo-2-chloroethane (BCE) in several representative zeolites including silicalite-1, siliceous Y (Si-Y), Na-Y, and zeolite L have been investigated by FT-Raman spectroscopy in combination with thermogravimetric analysis. The results indicate that the conformational and dynamic behavior of BCE inside a zeoltic host is strongly influenced by several factors such as the framework topology, the Si/Al ratio, and the nature and the locations of charge-balancing cations. For siliceous zeolites (silicalite-1 and Si-Y), the anti population of BCE increases significantly relative to pure liquid upon initial loading at room temperature. Interestingly, when lowering temperature, the anti population of BCE in silicalite-1 continues to increase, but the conformational equilibrium of BCE inside Si-Y shifts to the gauche conformer. Zeolite L and Na-Y with extra-framework cations show an almost exclusive preference for the gauche mode of the BCE molecule at room temperature, and the conformational population does not change significantly at low temperatures.

Molecular dynamics study of the silica–water–SDA interactions

In this paper we have applied the molecular dynamics simulations in order to analyse the role of the structure directing tetrapropylammonium ions in the aggregation process that leads to silicalite formation. We address the specific question of how the interactions between silica precursor species and tetrapropylammonium ions/water evolve during the formation of the larger aggregates, that show initial micropore formation from more elementary building blocks. We have followed the dynamics and changes in the position of the tetrapropylammonium ions into the formation of TPA-Si22 complexes. Moreover, the analysis based on the geometries of the systems being studied as well as the radial distribution function allowed us to predict the location of the TPA cations in fully formed nanoslabs. An interesting result is reported that the template cannot be accommodated any more in the newly formed cavities, but is pushed out of the channel like cavities to positions where in a later stage channel cross sections can be formed.

Sulphur-enriched thiacalix[4]arenes in the cone conformation: synthesis, crystal structures and cation binding properties

The synthesis and crystal structure of p-tert-butylthia- and p-tert-butylcalix[4]arene derivatives 1–7 substituted at the narrow rim by diethylthiophosphate ester groups, -PS(OEt)2 are reported. In these compounds the phosphorus atoms are directly attached to the phenolic O atoms of the calix units and the S atoms of the thiophosphate may serve as a two-electron donor to a metal ion. Their crystal structures were solved, which revealed their cone conformation. Their cation-binding properties have been established by liquid-liquid extraction of metal picrates from water into dichloromethane and stability constant determination in acetonitrile using UV-absorption spectrophotometry. Quantitative extraction was achieved for Ag+ (%E = 99) with the tetra-substituted thiacalix[4]arene derivative. The complexes were found to be of 1:1 stoichiometry and the location of the metal cations was shown to be nearby the sulphur atoms by 1H NMR.

Synthesis, 1H-NMR conformational analysis and complexation studies of two di-imidazolyl acetamido p-tert-butylcalix[4]arenes

The synthesis of a new di-imidazolyl-di-methoxy acetamido p-tert-butylcalix[4]arene 4 is reported. 4 has been prepared by reacting the corresponding di-methyl ester di-methoxy derivative with histamine in 1:1 mixture of methanol: toluene. The binding properties of 4 towards alkali, alkaline earth, transition (Zn2+, Co2+) and heavy (Pb2+, Cd2+) metals have been investigated along with the complexes stoichiometries. The 1H-NMR spectra of complexes show the location of cations in receptor 4. Partial cone conformation is observed only with strontium and calcium whereas the cone conformation is detected with most of the cations. Comparison of the complexation results with those obtained for di-imidazolyl acetamido p-tert-butylcalix[4]arene 3 missing the methyl groups is also reported.

Structural study of ammonium metatungstate

Several techniques have been used to study the structure of the Keggin-type polyoxometalate salt ammonium metatungstate (AMT)—(NH 4) 6[H 2W 12O 40]* nH 2O, a potential fuel cell catalyst. The dehydrated salt is comprised of a mixture of crystallites of different unit cells in a centered eutactic cubic configuration, with an average unit cell of a≅12.295. Varied orientations of the Keggin ions in the cubic arrangement create the differences, and orientational variation within each unit cell size represents an energy well. Progressive hydration of each crystallite leads to expansion of the lattice, with the degree of expansion depending on the locations of the water added in relation to the Keggin ion, which is influenced by cation location and hydrogen bonding. The structural hypothesis is supported by electron diffraction of single and multicrystal samples, by powder density measurements, X-ray powder diffraction studies, synchrotron powder X-ray diffraction, and a priori structural modeling studies. Based on the structure, projected active site densities are compared with nanostructured platinum catalysts for fuel cell application.

Phase Diagram of Mg Insertion into Chevrel Phases, MgxMo6T8 (T = S, Se). 3. The Crystal Structure of Triclinic Mg2Mo6Se8

This series of papers is devoted to unique cathode materials for Mg batteries, MgxMo6T8 (T = S, Se, x = 1 and 2) Chevrel phases (CPs). In this part, a combination of neutron and high-resolution synchrotron X-ray diffractions was used to study the crystal structure of Mg2Mo6Se8, which is triclinic at room temperature (space group P1, a = 6.868 A, b = 6.921 A, c = 6.880 A, alpha = 93.00 degrees , beta = 94.40 degrees , gamma = 96.22 degrees ). In contrast to other members of the MgxMo6T8 family, this compound does not follow the classic scheme of successive cation insertion into so-called inner and outer sites: Both the Mg(2+) ions per formula are located in the tetrahedral sites of the outer ring. This surprising cation location, predicted previously for Mg-containing CPs by ab initio calculations, provides the uniform distribution of the cation charge in the triclinic structure, which is similar to that of rhombohedral CPs. A mapping of the cation sites was widely used to demonstrate the variety of cation arrangement in CPs and the factors affecting this arrangement, as well as to clarify the origin of the exceptionally high mobility of the Mg(2+) ions in Mg2Mo6Se8.

Location of Mg Cations in Mordenite Zeolite Studied by IR Spectroscopy and Density Functional Theory Simulations with a CO Adsorption Probe

The location of Mg cations in the channel of mordenite zeolite was studied using a combination of DFT simulations and IR spectroscopy of adsorbed CO. The calculated adsorption energies and frequencies of CO on Mg cations are in good agreement with the results from the IR spectra of adsorbed CO. It is found that the Mg cations can occupy the sites A, C, D and E in mordenite and the distribution of the Mg cations in these sites follows the priority order, site C > site A > site D and site E.

Neutron and Resonant X-ray Diffraction Studies of Zirconolite-2M

AbstractZirconolite (nominally CaZrTi2O7) is a constituent phase of potential waste forms for the safe immobilisation of actinide wastes. Structural studies of such materials provide important information about cation ordering, lattice parameters, and strain effects, and provide input into the modelling of alpha decay damage and the development of future wasteform designs. A suite of zirconolites based on the replacement of Ti with Nb and Fe has been studied using high resolution neutron diffraction and resonant X-ray diffraction to determine the degree of disorder across the available cation sites. Resonant X-ray diffraction is a unique method which allows the location of certain cations to be determined accurately by taking advantage of the change in scattering power close to an absorption edge (e.g., Nb-K and Zr-K). Using standard X-ray diffraction alone this is not possible and there is little scattering difference between Nb and Zr.Raman spectroscopy and measured lattice parameters have shown that the exchange of Ti with Nb and Fe has a non-linear effect on the unit cell dimensions and Raman peak positions while retaining the 2M polytype. Mössbauer spectroscopy has shown that the Fe preferentially fills the Ti split (C2) site. The results from this study provide a more complete picture of the cation order-disorder problem and are generally consistent with the behaviour of lattice parameters across the series.

菱沸石の結晶構造精密化―水分子と交換性陽イオンの結晶学的配置―

Locations of water molecules and exchangeable cations in a hydrated natural chabazite with a composition of Ca1.57Na0.49 [Al3.39Si8.55O24] ⋅12.47H [d2] O were successfully determined by single crystal X-ray diffraction. The structure analyses have revealed the presences of five partially occupied water sites (OW1-OW5) and four partially occupied exchangeable-cation sites (Ca1-Ca4), and have showed that fully coordinated exchangeable cations can adopt sixfold or sevenfold coordination. The distances between water sites and framework oxygen sites and between water sites suggest that the hydrogen bonds formed between water molecules and framework oxygen atoms are very weak, whereas strong hydrogen bonds exist between water molecules.

Cation Dynamics upon Adsorption of Methanol in Na−Y Faujasite Type Zeolites: A Dielectric Relaxation Spectroscopy Investigation

Dielectric relaxation experiments are carried out to address the question of cation location upon adsorption of methanol in a Na-Y faujasite system. They allow us to probe both the evolutions of the number of extraframework sodium ions in the different crystallographic sites and of their de-trapping energy as a function of the adsorbate loading. Redistributions of the cation positions are explained in light of previous molecular dynamic results. It is shown that SI cations remain mainly trapped in their initial sites whatever the methanol loading. At low and intermediate loadings, SII cations migrate within the supercage, due to strong interaction with the adsorbate molecules. They are followed by hopping of SI' cations from the sodalite cage toward the supercage to fill vacant SII sites. At higher loading, the cation motions are hindered due to steric effects induced by absorbates within the supercage. These cation motions are shown to be consistent with the evolution of the de-trapping energy of Na + .