Alpha Cage Research Articles

Zeolite-Type Metal Oxalate Frameworks.

While many metal oxalate salts are known, few are known to form zeolite-type topologies. The construction of zeolite types, especially those with low framework density such as RHO, from linear ligands is generally perceived as less likely, because the 180° metal-ligand-metal geometry deviates too much from the established strategy of using ligands with bent coordination geometry (centered around 145°) to mimic the geometry in natural zeolites. We show the general feasibility of using linear ligands for the synthesis of zeolite types by reporting a family of indium oxalate salts with multiple zeolite topologies, including RHO, GIS, and ABW. Of particular interest is the synthesis of a zeolite RHO net with double 8-rings and large alpha cages, which are highly desirable zeolite features.

Zeolite‐Type Metal Oxalate Frameworks

AbstractWhile many metal oxalate salts are known, few are known to form zeolite‐type topologies. The construction of zeolite types, especially those with low framework density such as RHO, from linear ligands is generally perceived as less likely, because the 180° metal‐ligand‐metal geometry deviates too much from the established strategy of using ligands with bent coordination geometry (centered around 145°) to mimic the geometry in natural zeolites. We show the general feasibility of using linear ligands for the synthesis of zeolite types by reporting a family of indium oxalate salts with multiple zeolite topologies, including RHO, GIS, and ABW. Of particular interest is the synthesis of a zeolite RHO net with double 8‐rings and large alpha cages, which are highly desirable zeolite features.

Structural Evolution of CO2-Filled Pure Silica LTA Zeolite under High-Pressure High-Temperature Conditions

The crystal structure of CO2 filled pure SiO2 LTA zeolite has been studied at high pressures and temperatures using synchrotron based x ray powder diffraction. Its structure consists of 13 CO2 guest molecules, 12 of them accommodated in the large alpha cages and 1 in the beta cages, giving a SiO2:CO2 stoichiometric ratio smaller than 2. The structure remains stable under pressure up to 20 GPa with a slight pressure dependent rhombohedral distortion, indicating that pressure induced amorphization is prevented by the insertion of guest species in this open framework. The ambient-temperature lattice compressibility has been determined. In situ high pressure resistive heating experiments up to 750 K allow us to estimate the thermal expansivity at 5 GPa. Our data confirm that the insertion of CO2 reverses the negative thermal expansion of the empty zeolite structure. No evidence of any chemical reaction was observed. The possibility of synthesizing a silicon carbonate at high temperatures and higher pressures is discussed in terms of the evolution of C-O and Si-O distances between molecular and framework atoms.

Dependence of electron transfer among α cages on the chemical compositions of zeolite LTAs including K clusters

K atoms are loaded in diluted amount into K-form LTA zeolites whose framework compositions are Al(x)Si(24-x)O(48) (6<x<or=12) per alpha cage. The optical and electron spin resonance (ESR) properties of K clusters formed in the alpha cages of the LTAs were investigated to elucidate the influence of the LTA compositions on the transfer of a 4s electron of a loaded K atom among the cages. Optical absorption spectra showed notable absorption at approximately 1 eV originating from cationic K clusters in the alpha cages independent of the x value. The linewidths of the ESR spectra showed a minimum value at x approximately 10. A motional narrowing effect induced by the electron transfer among the alpha cages is the origin of the sharp ESR spectra at x approximately 10. The gradual narrowing of the ESR spectral linewidth that occurred from x=12 to 10 can be explained with the assistance of numerical calculations of the periodic potential for an electron introduced into the LTAs. The potential inside the alpha cage becomes monotonously shallower with decreasing x value. On the other hand, the potential of an eight-membered ring shared with the adjacent alpha cages was insensitive to the x value. The calculation results demonstrate that the electron mobility becomes larger at smaller x values. The discrepancy between the actual ESR results and the calculation results for x<10 might be due to the aggregation of K+ ions to stabilize 4s electrons in the alpha cage in actual systems.

Rotational dynamics of propane in Na-Y zeolite: a molecular dynamics and quasielastic neutron-scattering study.

We report results from molecular dynamics (MD) simulations and quasielastic neutron-scattering (QENS) measurements on the rotational dynamics of propane in Na-Y zeolite at room temperature with a loading of four molecules per alpha cage. Rotational part of the intermediate scattering function F(Q,t) obtained from the MD simulation suggests that rotational motion is faster relative to the translational motion. Various rotational models fitted to the MD data suggest that rotation is isotropic. It is found that the hydrogen atoms lie, on the average, on a sphere of radius 1.88+/-0.05 A, which is also the average distance of the hydrogen atoms from the center of mass of the propane molecule. Results from QENS measurements are in excellent agreement with those obtained from MD, suggesting that the intermolecular potential employed in the MD simulation provides a realistic description of propane motion within faujasite. The rotational diffusion constant D(R) is 1.05+/-0.09 x 10(12) sec(-1) from the QENS data, which may be compared with that obtained from the MD data (0.82+/-0.05 x 10(12) sec(-1)).

Adsorption of xenon and CH4 mixtures in zeolite NaA. 129Xe NMR and grand canonical Monte Carlo simulations

Investigation of competitive adsorption is carried out using the Xe–CH4 mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the CH4 molecules are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of nine samples of varying Xe and CH4 loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average methane and xenon occupancies. The distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of CH4 molecules are obtained directly from the relative intensities of the Xen peaks. From the observed 129Xe chemical shift of each Xen peak can be obtained the average number of CH4 molecules in the same cavity as n Xe atoms. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and CH4 in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all nine samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 80 to 230 ppm and are in good agreement with experiment. We also consider a very simple strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component molecules are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–CH4 system are compared. The ideal adsorbed solution theory gives a first approximation to the selectivity of the adsorption of the Xe and CH4 from a mixture of gases, but starts to fail at high total pressures, especially at low CH4 mole fraction in the bulk.

The role of polarization of Xe by di- and monovalent cations in 129Xe NMR studies in zeolite A

We consider the role of polarization in the adsorption of Xe in zeolites of type A by direct comparative analysis of the adsorption isotherms, distributions of occupancies, and 129Xe NMR chemical shifts of Xe n in cages containing Ca x Na 12 −2 x ions per alpha cage ( x = 0, 1, 2, 3, 5). We find that the qualitative trends in the adsorption isotherms, and in the progressions of Xe n chemical shifts (for n = 0–8 in cages with x = 0, 1 Ca 2+ ions and for n = 0–5 in cages with x = 2, 3 Ca 2+ ions) upon increasing the level of Ca 2+ ion for Na + ion substitution could only be accounted for by including polarization of the Xe atom by the zeolite framework and its ions. This system, which permits observation of individual Xe n peaks and of directly comparable adsorption isotherms in several cage types, provides a good model system for the interpretation of the more general case in which only the overall average 129Xe NMR chemical shift is observed in open network zeolites, arising from free exchange of Xe among cavities of variable occupancy and variable cation distribution.

Distribution and Xe129 NMR chemical shifts of Xen clusters in the alpha cages of zeolite AgA

The distributions and 129Xe NMR chemical shifts of xenon in zeolite AgA have been measured in a series of experiments by Moudrakovski, Ratcliffe, and Ripmeester [Proc. Internat. Zeolite Conference, Quebec, 1995; unpublished]. We carry out grand canonical Monte Carlo (GCMC) simulations of xenon in a rigid zeolite AgA lattice to provide the average Xen cluster shifts, and the distributions Pn for comparison with their experiments. The GCMC results for the distributions, the fraction Pn of the alpha cages containing n Xe atoms, are compared with the experimental distributions in 12 samples and the agreement is excellent. The distributions in NaA and in AgA are very similar, as can be established from the comparison of the dispersion of the distributions, {〈n2〉−〈n〉2}, and both are different from the idealized hypergeometric distribution, in which the component atoms occupy eight lattice sites per cage under mutual exclusion. The calculated chemical shift increments [σ(Xen)−σ(Xen−1)]AgA are in good agreement with experiment. The differences between these and the increments in zeolite NaA,{[σ(Xen)−σ(Xen−1)]AgA−[σ(Xen)−σ(Xen−1)]NaA}, are fairly small and are in good agreement with experiment. The absolute 129Xe chemical shifts of Xen in the alpha cages of AgA are nearly uniformly shifted by about 40 ppm compared to the Xen clusters in NaA. This is attributed to the Fermi contact shifts arising from the Ag0 metal atoms that form the linear Ag32+complexes that are found within the beta cages of AgA.

Competitive adsorption of xenon and argon in zeolite NaA. 129Xe nuclear magnetic resonance studies and grand canonical Monte Carlo simulations

Investigation of competitive adsorption is carried out using the Xe–Ar mixture in zeolite NaA as a model system. The Xen clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the NMR spectrum for each cluster while the Ar atoms are in fast exchange between the cages and also with the gas outside. The 129Xe nuclear magnetic resonance spectra of 12 samples of varying Xe and Ar loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average argon and xenon occupancies. The detailed distributions, f(XenArm), the fractions of cages containing n Xe atoms and m Ar atoms cannot be observed directly in this system, that is, individual peaks for XenArm mixed clusters are not observed in the NMR spectrum. This information is, however, convoluted into the observed 129Xe chemical shifts for the Xen peaks and the distributions Pn, the fraction of cages containing n Xe atoms, regardless of the number of Ar atoms, obtained from their relative intensities. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Ar in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts, as well as the distributions Pn. The agreement with experiment is reasonably good for all 12 samples. The calculated absolute chemical shifts for the Xen peaks in all samples at 300 K range from 75 to 270 ppm and are in good agreement with experiment. The GCMC results are compared with a strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion. The latter simple model introduced here provides a limiting case for the distributions, with which both the GCMC simulations and the properties of the actual Xe–Ar system are compared.

Competitive adsorption of xenon and krypton in zeolite NaA: 129Xe nuclear magnetic resonance studies and grand canonical Monte Carlo simulations

Investigation of competitive adsorption is carried out using the Xe–Kr mixture in zeolite NaA as a model system. The XenKrm clusters are trapped in the alpha cages of this zeolite for times sufficiently long that it is possible to observe individual peaks in the nuclear magnetic resonance (NMR) spectrum for the clusters. The 129Xe nuclear magnetic resonance spectra of several samples of varying Xe and Kr loadings have been observed and analyzed to obtain the 129Xe chemical shifts and the intensities of the peaks which are dependent on the average krypton and xenon occupancies. The detailed distributions, f(XenKrm), the fractions of cages containing n Xe atoms and m Kr atoms can be observed directly in this system from the relative intensities since individual peaks for XenKrm mixed clusters are observed in the NMR spectrum. Grand canonical Monte Carlo (GCMC) simulations of mixtures of Xe and Kr in a rigid zeolite NaA lattice provide the detailed distributions and the average cluster shifts. The agreement with experiment is excellent. The calculated absolute chemical shifts for the Xen peaks and XenKr peaks at 300 K are in good agreement with experiment. A strictly statistical model of a binary mixture, derived from the hypergeometric distribution, in which the component atoms are distinguishable but equivalent in competition for eight lattice sites per cage under mutual exclusion provides a limiting case for the distributions, with which the GCMC simulations and the properties of the actual Xe–Kr system may be compared. The selectivity coefficients of the Xe–Kr mixture in zeolite NaA is well described by the ideal adsorbed solution model.

Xen clusters in the alpha cages of zeolite KA

We have observed the individual signals of the Xen clusters (n=1–5) trapped in the alpha cages of zeolite KA. The 129Xe NMR chemical shift of each cluster in zeolite KA is larger than that of the corresponding Xen cluster in zeolite NaA. The temperature dependence of the chemical shifts of the clusters vary systematically with cluster size as they do in NaA, but the change of the temperature coefficients with n is somewhat more pronounced for Xen in the cages of KA than in NaA. The Xen chemical shifts and their variation with temperature are reproduced by the grand canonical Monte Carlo (GCMC) simulations. GCMC simulations of the distribution of the Xe atoms among the alpha cages in KA provide the fractions of cages containing n Xe atoms which agree reasonably well with the observed equilibrium distributions. The characteristics of Xe distribution and chemical shifts in KA are compared with that in NaA.

Prediction of adsorption of xenon in zeolite NaA with molecular density functional theory

The Tarazona model of molecular density functional theory is used to determine the adsorption isotherm and the density distribution of xenon atoms in zeolite NaA. We consider the alpha cage of NaA to be fully three-dimensional and we introduce a basis set for the density distribution to make the solution computationally tractable. The results of this density functional theory model are compared to previous results of grand canonical Monte Carlo simulations for the same system.

Cage-to-cage migration rates of Xe atoms in zeolite NaA from magnetization transfer experiments and simulations

Xenon trapped in the alpha cages of zeolite NaA exhibits distinct NMR signals for clusters Xe1, Xe2, Xe3,..., up to Xe8. Using multisite magnetization transfer experiments, we have measured the rate constants kmn for the elementary processes that are involved in the cage-to-cage transfer of Xe atoms in the zeolite NaA, that is, for a single Xe atom leaving a cage containing Xen to appear in a neighboring cage containing Xem−1, thereby forming Xem. In a random walk simulation, these rate constants reproduce over a hundred magnetization decay/recovery curves that we have measured in four samples of Xe in zeolite NaA at room temperature, in selective inversion, and complementary experiments for all the significantly populated clusters. The simulations also lead to the correct experimental equilibrium distributions, that is, the fractions of the alpha cages containing Xe1,Xe2,...,Xe8.

Grand canonical Monte Carlo simulations of the distribution and chemical shifts of xenon in the cages of zeolite NaA. I. Distribution and 129Xe chemical shifts

The equilibrium distribution of the Xe atoms among the alpha cages of the zeolite NaA have been measured directly by nuclear magnetic resonance (NMR) in ten samples ranging from very low xenon loading up to saturation. These distributions are simulated by a grand canonical Monte Carlo (GCMC) method which reproduces the experimental data quantitatively for all ten samples at 296 K and also at 360 K. The adsorption isotherm of the high loading samples has been determined directly from the chemical shift of the gas in equilibrium with the adsorbed xenon. The data compare favorably with the adsorption isotherms resulting from the simulations. The previously reported 129Xe chemical shifts of the individual Xen clusters and their temperature dependences in the range 188–420 K are reproduced quantitatively by the GCMC simulation which makes use of pairwise additive ab initio intermolecular shielding functions. These cluster shifts and their temperature dependence encode the distribution of configurations for a given Xen cluster in an alpha cage. Quantitative agreement with the three experimental measures of the distribution of Xe atoms in NaA (partitioning between the adsorbed phase and the gas phase, distribution of the intrazeolitic atoms among the alpha cages, and the distribution of Xe atoms within an alpha cage containing Xen) as a function of temperature has been achieved for the first time.

Grand canonical Monte Carlo simulations of the distribution and chemical shifts of xenon in the cages of zeolite NaA. II. Structure of the adsorbed fluid

The quantitative agreement between the results of a grand canonical Monte Carlo (GCMC) simulation and the various direct experimental measures of the distribution of the Xe atoms between adsorbed phase and gas phase, of intrazeolitic xenon among the alpha cages, and of the distribution of n Xe atoms in a Xen cluster within one alpha cage permit us to consider the structure of the adsorbed fluid in the GCMC simulation as a reasonable description of the actual structure. We provide here the adsorption sites for a single Xe atom in the alpha cage of zeolite NaA, the transition states between these adsorption sites, the one-body distribution functions for the individual clusters Xen inside the alpha cage, the Xe–Xe pair distribution functions for Xe2 through Xe8 at two temperatures, and some of the local minima in the configuration space of the clusters Xe2 through Xe8, i.e., some of the minimum energy configurations of the clusters.

Zeolite adsorption site location and shape shown by simulated isodensity surfaces

The graphics software package Ribbons 1 1 Carson, M. Ribbons models of macromolecules. J. Molec. Graphics 1987, 5, 103-109. is used to display isodensity surfaces of Xe atoms adsorbed in the alpha cage of zeolite NaA. The location, size, and shape of the adsorption sites are highly dependent on the loading and the crystal cation content. When the zeolite has a high number of cations, ellipsoidal sites arrange in a cuboctahedron. When the zeolite has fewer cations, cone-shaped sites arrange in an octahedron at low loading, but at high loading the sites become ellipsoidal and new sites form at cuboctahedral positions. The effect of the nature of the adsorption site on the development of a universal adsorption model is discussed.

Open-system Monte Carlo simulations of Xe in NaA

The grand canonical ensemble Monte Carlo method is used to determine the adsorption isotherm, energy, entropy, occupancy distribution, and density distribution of Xe atoms adsorbed in zeolite NaA, which is modeled as a single alpha cage and as a network of connected cages. The isotherms exhibit multiple plateaus, indicating the existence of certain favored loadings. These plateaus can be predicted by considering constant loading energy and density distributions found previously. A linear regression method of determining the constant loading entropy of Xe atoms is presented. The entropy is a smooth function of loading, which suggests that the structure of the adsorbed Xe is also smoothly varying. The pressures needed to obtain complete loading are quite large and beyond the range of conventional experiment. However, the simulated isotherm and occupancy distribution compare well with the available experimental data. Structural modifications of NaA in the form of cation removal are investigated. A cation-poor model NaA adsorbs fewer Xe at low chemical potential due to its smaller electric field, but has a larger overall adsorption capacity due to its greater accessible volume.

Monte Carlo calculations of Xe arrangement and energetics in the NaA alpha cage

The canonical ensemble Monte Carlo technique is used to calculate density distributions and internal energies of Xe trapped in the alpha cage of zeolite NaA. This cage is modeled as an assembly of ions positioned in accord with crystallographic data for the dehydrated crystal. We explore the effects that cage chemistry, adsorbate loading, and temperature have on the structure and energy of the adsorbed phase. In order to examine the effect of slight changes in the detailed cage structure, we compare current results with those of an earlier study in which a simplified cage structure was used. Intense Xe density maxima are established at discrete adsorption sites, the locations of which change in a discontinuous way as loading increases. The arrangement of these sites depends on the number and position of the charge balancing cations. The sites are arranged at the vertices of a cuboctahedron for the alpha cage of the zeolite NaA, but they make an octahedron for a dealuminated cage (Si/Al = 2·0). When the number of Xe atoms exceeds the number of sites available in the original arrangement, a new polyhedron is formed. The arrangement of the adsorption sites affects the degree of site-site interaction, which must be accounted for in adsorption theories. Trends in the potential energy with loading can be correlated to changes in the Xe density arrangement. The maximum occupancy for which adsorption is energetically favoured in this study is much greater than that found previously for a cage idealized as a truncated cuboctahedron; this observation shows that the isotherm is extremely sensitive even to slight zeolite structural changes.

1 2 9Xe nuclear magnetic resonance studies of xenon in zeolite CaA

The average 129Xe nuclear magnetic resonance (NMR) chemical shift for xenon atoms in alpha cages of zeolite CaA is observed in a single peak dependent on xenon loading (〈n〉=0.5–8.9 Xe atoms/alpha cage) and temperature (240–360 K). The general increase of the shift with increasing average number of xenon atoms per alpha cage is shown to be due largely to the changing distribution of occupancies with increasing 〈n〉, coupled with increasing increments in the chemical shifts of Xen with increasing n. Except at the highest loadings, the results obtained for xenon in CaA are predicted nicely on the basis of δav(T)=(1/〈n〉)Σnnδn(T)Pn (〈n〉,T), where the fractions Pn of alpha cages containing n Xe atoms are imported from the Pn measured in xenon in zeolite NaA. The high loading data in CaA are interpreted in terms of contributions to the average 129Xe chemical shifts associated with xenon atoms in the window positions.

Nuclear magnetic resonance studies of xenon clusters in zeolite NaA

We have observed the equilibrium distribution of Xe atoms trapped in the alpha cages of zeolite NaA at 300 and at 360 K for low to high xenon loadings. The experimental distributions obtained by nuclear magnetic resonance (NMR) spectroscopy differ from two previously proposed statistical distributions. The experimental deviations from these statistical models can be explained by the attractive Xe–Xe interactions which favor clustering at low to medium loading, and the higher energies associated with the overcrowded cage disfavoring clusters of eight Xe atoms at high loadings. The temperature dependence of the 129Xe NMR chemical shift of each cluster has been measured in the range 188–421 K, except that for Xe8, which was determined only up to 300 K. The observed shifts and their temperature dependence are interpreted by using the results of ab initio calculations of the intermolecular shielding function in the 39Ar system as a model for the 129Xe system.