Nonframework Cations Research Articles

Competitive Adsorptive Mechanism of H2/N2 in LTA/FAU Zeolites by Molecular Simulations and Experiments.

For industrial tail gas to be converted into high-purity hydrogen, the H2-N2 mixture needs to be separated efficiently. This work examined the adsorption characteristics and competitive mechanisms of H2 and N2 on LTA- and FAU-type zeolites, at 77 K, 298 K, and 0.1-10 bar by thoroughly analyzing results of adsorption capacity experiments and molecular simulations. In the Grand Canonical Monte Carlo (GCMC) simulations, the force field causing a molecular dipole of H2 and the polarization force field of N2 are first applied. The accuracy of the force field was experimentally verified. The findings indicate that N2 and H2 loading on Ca-FAU (Ca-LTA) are higher than Na-FAU (Na-LTA). On NaX at 77 K, the highest adsorption selectivity (N2/H2) is observed; on NaA at 298 K, it is the opposite. The GCMC data findings demonstrate that H2 and N2 have remarkably similar adsorption sites, with framework oxygen atoms and non-framework cations serving as the main adsorption sites for adsorbate molecules. Furthermore, the rate at which H2 diffuses is higher than that of N2. The study of redistribution charge before and after adsorption demonstrated that N2 has a greater affinity for the framework oxygen atoms than H2. This study provides a molecular theoretical foundation for the adsorption behavior of H2-N2 mixture in zeolites.

Phonon mean free path of silicate glasses: a useful parameter to distinguish between framework and nonframework cations

Phonon mean free path of silicate glasses: a useful parameter to distinguish between framework and nonframework cations

Prediction of water adsorption isotherm on 13X zeolite in different temperatures and investigating the effect of non-framework cations using dynamic simulation

In this article, the adsorption properties of 13X zeolite have been studied using dynamic simulation. The COMPASS force field is used to identify water adsorption isotherm on 13X zeolite. Ewald and Atom-Based group methods were used for the electrostatic and Van der Waals interactions. Li+, Na+, K+, Mg2+and Ca2+ were used as non-framework cations for compensating negative charge. Water adsorption isotherms on the 13X zeolite and different non-framework cations are calculated in the 293.15 – 373.15 K temperature range. Water Adsorption capacity on 13X zeolite for all non-framework cations decreased by increasing the temperature and reducing pressure. Freundlich model was considered for describing water adsorption on 13X zeolite. Isosteric heat of adsorption using the Freundlich model was obtained between the range 44.23 - 83.13 kJ/mol, which has good compliance with the experimental results. The results showed that water has better adsorption on 13X zeolite with Na+ as a non-framework cation compared to the other non-framework cations (Li+, K+, Mg2+and Ca2+), and this type of non-framework cation is a suitable candidate for water adsorption.

AlO1.5/SiO2 substitution effect on the viscosity of alkali silicate melts

The effect of alumina addition on the viscosity of silicate melts is strongly related to the type of non-framework cations; however, the origin of these interactions is not well understood. In this study, we measured the viscosity change in selected alkali disilicate (R2O·2SiO2, R: Na or Li) melts by varying the AlO1.5/SiO2 molar ratio (AlO1.5 = 0–16.8 mol%) within the 1373–1823 K temperature range. We observed that as the molar ratio increased, sodium silicate viscosity increased, whereas lithium silicate viscosity was not affected. Aluminum-27 nuclear magnetic resonance spectra of the quenched glasses indicated that the aluminum cations in both types of alkali silicate melts were mostly present as AlO4 tetrahedra. In contrast to sodium-containing systems, the shear strain on the AlO4 tetrahedron was greater for lithium aluminosilicate glass. Our findings indicated that the degree of shear strain on AlO4 plays an essential role in controlling the viscosity of aluminosilicate systems.

Quadrupolar Interaction with Zeolite and Pressure Swing Adsorption in Portable Medical Oxygen Concentrators for Breathing of Covid-19 and COPD Patients

Pressure swing adsorption (PSA) technology is used in medical oxygen concentrators for selective adsorption of nitrogen under pressure from air to produce oxygen-enriched air for breathing. The functional material in oxygen concentrators is aluminosilicate zeolite like Li-13X, 5A (LTA) with a relatively high Al-content. It selectively adsorbs nitrogen from the air into their molecular sieves due to the strong quadrupolar interaction between the adsorbed nitrogen and the electrical field of the charge balancing non-framework exchangeable cations of aluminosilicate zeolites. In an oxygen concentrator, two adsorbent vessels packed with the zeolite working alternately in a cyclic process are used for a near-continuous production of the oxygen-enriched air.

Pressure-Induced Structural Transitions in Na–Li Silicate Glasses under Compression

The atomic structures of alkali silicate glasses with more than two types of nonframework cations (e.g., Li and Na) at high pressure provide insights into the effects of pressure on the transport p...

Impact of rare earth ion size on the phase evolution of MoO3-containing aluminoborosilicate glass-ceramics

Transition metal and rare earth (RE) elements are important fission products present in used nuclear fuel, which in high concentrations tend to precipitate crystalline phases in vitreous nuclear waste forms. Two phases of particular interest are powellite (CaMoO4) and oxyapatite (Ca2RE8(SiO4)6O2). The glass compositional dependencies controlling crystallization of these phases on cooling from the melt are poorly understood. In the present study, the effect of rare earth identity and modifier cation field strength on powellite and apatite crystallization were studied in a model MoO3-containing alkali/alkaline-earth aluminoborosilicate glass with focus on (1) influence of rare earth cation size (for RE3+: Ce, La, Nd, Sm, Er, Yb) and (2) influence of non-framework cations (RE3+, Mo6+, Na+, Ca2+). Quenched glasses and glass-ceramics (obtained by slow cooling) were characterized by X-ray diffraction (XRD), Raman spectroscopy, X-ray absorption spectroscopy (XAS), and electron probe microanalysis (EPMA). All samples were X-ray amorphous upon quenching, except the Ce-containing composition which crystallized ceria (CeO2), and the sample devoid of any rare earth cations which crystallized powellite. On heat treatment, powellite and oxyapatite crystallized in the majority of the samples, with the former crystallizing in the volume and the latter on the surface. The EPMA results confirmed a small concentration of boron in the oxyapatite crystal structure. RE cations were incorporated in the glass, as well as in powellite, oxyapatite, and in the case of Yb3+, keiviite (Yb2Si2O7). Raman spectroscopy showed that the primary vibration band for molybdate MoO42− in the glasses was strongly affected by the ionic field strength of the modifying cations (alkali, alkaline earth, and RE), suggesting their proximity to the MoO42− ions in the glass, though the MoO bond length and coordination according to XAS suggested little local change.

Mapping Al Distributions in SSZ-13 Zeolites from 23Na Solid-State NMR Spectroscopy and DFT Calculations

Al distributions and locations of corresponding nonframework cations in SSZ-13 zeolites have significant impact on their catalytic and adsorption properties. Herein, we demonstrate the feasibility of elucidating Na+ locations and correlated Al distributions in a series of Na-SSZ-13 zeolites with Si/Al ratios ranging from 4 to 48 by combining 23Na MQ MAS NMR spectroscopy and DFT calculations. As only one crystallographically inequivalent T site exists in CHA cages, the isolated Al and the Al pair (Al–O–Si–O–Si–O–Al) and (Al–O–Si–O–Al) in 6-membered ring (MR) distributions and the corresponding Na+ locations are clearly discriminated. In the high-silica Na-SSZ-13 zeolite, Na+ ions are mainly located at the 6-MR SIIa0 site corresponding to one Al substitution in the CHA cage. However, a small amount of the Al pair in 6-MR is still observable even with an Si/Al ratio of 48, corresponding to two Na+ ions located at 6-MR SIIa1 and 8-MR SIII’a1 sites, respectively. With more Al substitutions in the CHA cage, a p...

Structure and Crystallization of Alkaline-Earth Aluminosilicate Glasses: Prevention of the Alumina-Avoidance Principle.

Aluminosilicate glasses are considered to follow the Al-avoidance principle, which states that Al-O-Al linkages are energetically less favorable, such that, if there is a possibility for Si-O-Al linkages to occur in a glass composition, Al-O-Al linkages are not formed. The current paper shows that breaching of the Al-avoidance principle is essential for understanding the distribution of network-forming AlO4 and SiO4 structural units in alkaline-earth aluminosilicate glasses. The present study proposes a new modified random network (NMRN) model, which accepts Al-O-Al linkages for aluminosilicate glasses. The NMRN model consists of two regions, a network structure region (NS-Region) composed of well-separated homonuclear and heteronuclear framework species and a channel region (C-Region) of nonbridging oxygens (NBOs) and nonframework cations. The NMRN model accounts for the structural changes and devitrification behavior of aluminosilicate glasses. A parent Ca- and Al-rich melilite-based CaO-MgO-Al2O3-SiO2 (CMAS) glass composition was modified by substituting MgO for CaO and SiO2 for Al2O3 to understand variations in the distribution of network-forming structural units in the NS-region and devitrification behavior upon heat treating. The structural features of the glass and glass-ceramics (GCs) were meticulously assessed by advanced characterization techniques including neutron diffraction (ND), powder X-ray diffraction (XRD), 29Si and 27Al magic angle spinning (MAS)-nuclear magnetic resonance (NMR), and in situ Raman spectroscopy. ND revealed the formation of SiO4 and AlO4 tetrahedral units in all the glass compositions. Simulations of chemical glass compositions based on deconvolution of 29Si MAS NMR spectral analysis indicate the preferred formation of Si-O-Al over Si-O-Si and Al-O-Al linkages and the presence of a high concentration of nonbridging oxygens leading to the formation of a separate NS-region containing both SiO4 and AlO4 tetrahedra (Si/Al) (heteronuclear) in addition to the presence of Al[4]-O-Al[4] bonds; this region coexists with a predominantly SiO4-containing (homonuclear) NS-region. In GCs, obtained after heat treatment at 850 °C for 250 h, the formation of crystalline phases, as revealed from Rietveld refinement of XRD data, may be understood on the basis of the distribution of SiO4 and AlO4 structural units in the NS-region. The in situ Raman spectra of the GCs confirmed the formation of a Si/Al structural region, as well as indicating interaction between the Al/Si region and SiO4-rich region at higher temperatures, leading to the formation of additional crystalline phases.

Elastic moduli of XAlSiO4 aluminosilicate glasses: effects of charge-balancing cations

Brillouin spectroscopy is used to investigate the elastic properties of XAlSiO4 aluminosilicate glasses where X=Li, Na, K, Mg0.5, Ca0.5, Sr0.5, Ba0.5, and Zn0.5. The Brillouin frequency shifts obtained in two different scattering geometries allow the calculation of the refractive index, the two sound velocities and Poisson's ratio. Measurements of the mass density give in turn the elastic moduli and the Debye temperature. We find that the elastic properties scale with the atomic density of the glassy network or the charge-balancing cation field strength while they negatively correlate with the glass transition temperature. Further, Poisson's ratio depends on the nature of the non-framework cations in this glass series.

Synthesis and crystal chemistry of microporous titanates Kx(Ti,M)8O16 where M=Sc–Ni

Through salt-flux methods, single crystals of eight novel hollandite-type titanates have been grown with the stoichiometry Kx(Ti8−yMy)8O16 for x=y≈1.5 where M3+=Sc, Ti, V, Cr, Mn, and Fe, and for x/2=y≈0.75 where M2+=Co and Ni. Single crystal and powder X-ray diffraction reveals phase pure crystals with tetragonal space group I4/m. These titanates are microporous oxides with 1-dimensional channels where the non-framework cation, K, resides. In addition to diffraction, spectroscopic methods were used to characterize the chemical composition of both the bulk and surface. X-ray photoelectron spectroscopy affords the oxidation states of the substituted transition metals M on the surface and energy dispersive X-ray spectroscopy the metal ratios, including that of potassium, for an overall accurate description of the chemical composition of both the bulk and surface. Magnetization versus temperature measurements indicate that all phases are Curie paramagnets with effective magnetic moments consistent with the d-electron count given by combined X-ray diffraction and elemental analysis techniques.

Predicting adsorption of n-perfluorohexane (n-C6F14) on BCR-704 zeolite using the first principle force field method

Grand Canonical Monte Carlo (GCMC) simulations with ab initio force field were used to predict the adsorption isotherms of n-C6F14 in BCR-704 faujasite (calcium type) zeolite. The force field parameters were derived using the vapor–liquid equilibrium data for the adsorbate molecules, and ab initio data for the adsorbent (zeolite) with argon as a testing particle. The interactions between the adsorbate molecules and the zeolite were represented by using the Lorentz–Berthelot combining rule. Based on a comparison of ab initio and experimental data obtained for methane absorbed on calcium and sodium type zeolites, it was found that the exposed, non-framework calcium cations (Ca2+) must be partially hydroxylated. As the force field parameters were determined using ab initio data, we were able to estimate the percentage of hydroxylation without interference from the force field parameters by using the experimental adsorption data of argon and nitrogen. The hydroxylation ratio of calcium cations was estimated to be 63%. Based on this model and the combined force field parameters, the adsorption isotherm of n-C6F14 on BCR-704 zeolite was predicted and the results are in reasonable agreement with the experimental data.

Role of Cation–Water Disorder during Cation Exchange in Small-Pore Zeolite Sodium Natrolite

By combining X-ray diffraction with oxygen K-edge absorption spectroscopy we track changes occurring during the K+–Na+ cation exchange of Na-natrolite (Na-NAT) as tightly bonded Na+ cations and H2O molecules convert into a disordered K+–H2O substructure and the unit cell expands by ca. 10% after 50% cation exchange. The coordination of the confined H2O and nonframework cations change from a tetrahedral configuration, similar in ice Ih, with Na+ near the middle of the channels in Na-NAT to two-bonded configuration, similar in bulk water, and K+ located near the walls of the framework in K-NAT. This is related to the enhanced ion-exchange properties of K-NAT, which, in marked contrast to Na-NAT, permits the exchange of K+ by a variety of uni-, di-, and trivalent cations.

Super‐Hydrated Zeolites: Pressure‐Induced Hydration in Natrolites

High-pressure synchrotron X-ray powder diffraction studies of a series of alkali-metal-exchanged natrolites, A16Al16Si24O80·nH2O (A=Li, K, Na, Rb, and Cs and n=14, 16, 22, 24, 32), in the presence of water, reveal structural changes that far exceed what can be achieved by varying temperature and chemical composition. The degree of volume expansion caused by pressure-induced hydration (PIH) is inversely proportional to the non-framework cation radius. The expansion of the unit-cell volume through PIH is as large as 20.6% in Li-natrolite at 1.0 GPa and decreases to 6.7, 3.8, and 0.3% in Na-, K-, and Rb-natrolites, respectively. On the other hand, the onset pressure of PIH appears to increase with non-framework cation radius up to 2.0 GPa in Rb-natrolite. In Cs-natrolite, no PIH is observed but a new phase forms at 0.3 GPa with a 4.8% contracted unit cell and different cation-water configuration in the pores. In K-natrolite, the elliptical channel undergoes a unique overturn upon the formation of super-hydrated natrolite K16Al16Si24O80·32H2O at 1.0 GPa, a species that reverts back above 2.5 GPa as the potassium ions interchange their locations with those of water and migrate from the hinge to the center of the pores. Super-hydrated zeolites are new materials that offer numerous opportunities to expand and modify known chemical and physical properties by reversibly changing the composition and structure using pressure in the presence of water.

The heteropolymolybdate family: structural relations, nomenclature scheme and new species

AbstractType specimens of the molybdoarsenates betpakdalite, natrobetpakdalite and obradovicite and the molybdophosphates mendozavilite, paramendozavilite and melkovite, and similar material from other sources, have been examined in an effort to elucidate the relations among these phases, which we designate as the heteropolymolybdate family of minerals. Using electron microprobe analysis, X-ray powder diffraction and single-crystal X-ray diffraction with crystal structure determination where possible, it was found that natrobetpakdalite, mendozavilite and melkovite are isostructural with betpakdalite and that obradovicite has a closely related structure.The betpakdalite and obradovicite structure types are based on frameworks containing four-member clusters of edge-sharing MoO6 octahedra that link by sharing corners with other clusters, with Fe3+O6 octahedra and with PO4 or AsO4 tetrahedra (T). The structures differ in the linkages through the Fe3+O6 octahedra, which produce different but closely related framework configurations. The structures contain two types of non-framework cation sites, which are designated A and B. In general, there are two or more A sites partially occupied by disordered, generally larger cations that are coordinated to O atoms in the framework and to H2O molecules. The B site is occupied by a smaller cation that is octahedrally coordinated to H2O molecules. The general formula for minerals with either the betpakdalite or the obradovicite structure is: [A2(H2O)nB(H2O)6][Mo8T2Fe3+3O30+7(OH)7–x], where x is the total charge of the cations in the A and B sites (+3 to +7) and n is variable, ideally 17 for arsenates and 15 for phosphates. The ideal total number of A cations is defined as 2 in the general formula, but varies from 1 to 3.8 in analysed samples. Dominant cations at the A site include K, Na and Ca and at the B site Na, Ca, Mg, Cu and Fe. The combinations that have been identified in this study define six new heteropolymolybdate species.A suffix-based nomenclature scheme is established for minerals of the betpakdalite, mendozavilite and obradovicite groups, with the following root names based on the structure types and the T-site cations: betpakdalite (T = As), mendozavilite (T = P) and obradovicite (T = As). Two suffixes of the form -AB, corresponding to the dominant cations in the two different types of non-framework cation sites complete the species name. The historical name melkovite is retained rather than introducing mendozavilite-CaCa.Our investigation of the paramendozavilite type specimen revealed no paramendozavilite, but an apparently closely related new mineral; however, another sample of paramendozavilite analysed had K > Na.

Thermodynamic study of alkali and alkaline-earth cation-exchanged natrolites

A series of synthetic natrolites with different non-framework cations (Li-NAT, Na-NAT, K-NAT, Rb-NAT, Cs-NAT, Ca-NAT, Sr-NAT and Pb-NAT) are investigated by high temperature oxide melt solution calorimetry. The formation enthalpy from the component oxides (ΔHf,ox) becomes less exothermic with increasing ionic potential (Z/r) of the cations. The dehydration behavior of NATs is examined using TG–DSC. The strength of water binding decreases with increasing cation size. Similar to the trend seen in anhydrous zeolites, a linear dependence of ΔHf,ox on Al/(Al+Si) ratio for hydrous zeolites is established.

Growth Mechanism of Microporous Zincophosphate Sodalite Revealed by In Situ Atomic Force Microscopy

Microporous zincophosphate sodalite crystal growth has been studied in situ by atomic force microscopy. This simple model system permits an in depth investigation of some of the axioms governing crystal growth of nanoporous framework solids in general. In particular, this work reveals the importance of considering the growth of a framework material as the growth of a dense phase material where the framework structure, nonframework cations, and hydrogen-bonded water must all be considered. The roles of the different components of the structure, including the role of strict framework ordering, are disentangled, and all of the growth features, both crystal habit and nanoscopic surface structure, are explained according to a simple set of rules. The work describes, for the first time, both ideal growth and growth leading to defect structures on all of the principal facets of the sodalite structure. Also, the discovery of the presence of anisotropic friction on a framework material is described.

Synthesis, structure and characterization of ZrPOF-DEA, a microporous zirconium phosphate framework material

A microporous zirconium phosphate material with the chemical composition |(C4H12N+)6(H3O+)2(H2O)2|[Zr32P48O176F8(OH)16] (ZrPOF-DEA) has been synthesized ionothermally using diethylammonium chloride and oxalic acid to form the deep eutectic solvent. It is closely related to the analagous ethylammonium material ZrPOF-EA, which has already been shown to discriminate very well between CO2 and CH4. The framework structures, with two different 8-ring channels running parallel to the x-axis, one of which intersects a 7-ring channel running parallel to the z-axis, proved to be identical. Only the positions of the non-framework cations and water molecules differ. The structure of ZrPOF-DEA (Pbam, a=20.03757(1) Å, b=37.19099(1) Å, c=6.62488(1) Å) was solved from synchrotron powder diffraction data using the charge-flipping algorithm in Superflip. Difference Fourier analyses were then used to locate the non-framework species in the channels. Rietveld refinement of the final structure converged with RF=0.037 and Rwp=0.138. Although this material, like ZrPOF-EA, is stable up to 410°C, it does not discriminate as well between CO2 and CH4. Thermogravimetric analysis shows that not all of the organic species have been removed at 410°C, and this is presumably the reason for the poorer adsorption results.

Unambiguous Description of the Oxygen Environment in Multicomponent Aluminosilicate Glasses from17O Solid State NMR Computational Spectroscopy

Classical molecular dynamics simulations, density functional theory calculations, and spin-effective Hamiltonians have been used to simulate the 17O MAS and 3QMAS NMR spectra of Ca–Na silicate and aluminosilicate glasses and melts employed as simplified models for basaltic, andesitic, and rhyolitic magmas. The direct comparison of the theoretical NMR spectra of molecular dynamics derived structural models with the experimental counterparts available in the literature has allowed the investigation of the nature of nonframework cation mixing and the extent of intermixing among framework units in Na–Ca aluminosilicate glasses. In particular, in agreement with previous experimental evidence, the results show a nonrandom distribution of the network-modifying Ca and Na in soda-lime glasses with the prevalence of dissimilar Na–Ca pairs around nonbridging oxygens. The oxygen sites are not completely resolved in the MAS spectra of the aluminosilicate glasses. On the contrary, in the 17O 3QMAS spectra the multiple ...

Molecular Simulation of Propane Adsorption in FAU Zeolites

The purpose of this study is to investigate propane adsorption capacity in sodium and calcium form of faujasite (FAU)-type zeolites X and Y by performing Monte Carlo simulations in the grand-canonical ensemble (GCMC). Our results have shown that the nonframework cation number density and the interaction potentials between propane molecules and the nonframework cations are very important factors for influencing propane adsorption in FAU zeolites. For a given ratio of Si/Al, the adsorption of propane in both sodium and calcium FAU increases with the increasing nonframework cation number density, suggesting that an increase in the number of cations produces extra adsorption sites and leads to a higher amount of adsorption. For a given nonframework cation number density, the higher ratio of Si/Al allows the higher adsorption capacity due to the reduced charge density, i.e., the fewer calcium cations, but more sodium cations making the interaction energy more negative and consequently higher affinity between propane and the nonframework cations.