Nonframework Cations Research Articles

Computer Simulation Studies of Zeolite Structure

Abstract This paper describes the use of lattice energy minimization to obtain structural information on zeolites. Examples are given of the use of this technique in the study of non-framework cation distributions, and in energy minimization of complete structures. The zeolites faujasite. zeolite A and silicalite are considered.

Electron beam sensitivity of zeolite L

This paper discusses me of the phenomenological aspects of electron beam damage of zeolites in the elektron microscope, with empjasis on changes in crystallite morphology and composition, Zeolite L is used as an example material. In agreement with previous observations on zeolites, damage rates are found to be most sensitive to the amount of water present in the structure and to the presence of framework aluminum. The presence of H + and NH 4 + cations can significantly increase damage rates. The data suggest a dominating damage mechanism in which AlOSi connectivities are broken after attack by acidic species. This attack can occur following ionization of water by the elektron beam, or by beam-induced migration of H + and NH 4 + non-framework cations.

Synthesis and Structural Characterization of a Hydrated Gallosilicate Zeolite with the Nat-Framework

AbstractGallosilicate NAT-framework zeolites have been synthesized from Na2O:TMA2O:Ga2O3:SiO2:H2O gels and characterized by chemical analysis and powder X-ray diffraction. The structure of a hydrated material (Orthorhombic, Fdd2, a = 18.423(4) Å, b = 18.826(3) Å, c = 6.652(1) Å) was determined using powder neutron diffraction. The framework Si:T ratio, Si-Ga segregation and non-framework cation and water configurations are similar to those in the analogous aluminosilicates. The geometrical effects of framework gallium substitution are consistent with those observed for other zeolite framework topologies.

The structural chemistry of zeolites as viewed by powder neutron diffraction

The catalytic, separations and ion-exchange behavior of the synthetic zeolites is governed largely by the nature of their framework structures, a detailed knowledge of which is then a prerequisite for an in-depth understanding of their function. Despite the complexity of zeolite crystal structures, powder neutron diffraction is now being applied successfully to several facets of zeolite structural chemistry. The present review considers the study of zeolites by powder neutron diffraction and outlines the information that such studies have, to date, provided on zeolite framework geometries and their sensitivity to framework or nonframework cation substitution, the ordering and partitioning of framework cations, the distributions of the nonframework cations, and the locations and structures of sorbed species. The potential impact of ongoing developments in instrumentation and analysis techniques is also addressed.

The structural chemistry of zeolites as viewed by powder neutron diffraction

The catalytic, separations and ion-exchange behavior of the synthetic zeolites is governed largely by the nature of their framework structures, a detailed knowledge of which is then a prerequisite for an in-depth understanding of their function. Despite the complexity of zeolite crystal structures, powder neutron diffraction is now being applied successfully to several facets of zeolite structural chemistry. The present review considers the study of zeolites by powder neutron diffraction and outlines the information that such studies have, to date, provided on zeolite framework geometries and their sensitivity to framework or nonframework cation substitution, the ordering and partitioning of framework cations, the distributions of the nonframework cations, and the locations and structures of sorbed species. The potential impact of ongoing developments in instrumentation and analysis techniques is also addressed.

Rietveld refinement of the crystal structure of the new zeolite mineral gobbinsite

Abstract The crystal structure of the new zeolite mineral gobbinsite, Ca0.6Na2.6K2.2Al6Si10O32 · 12H2O, has been determined in the space group Pmn21 with a = 10.108(1), b = 9.766(1) and c = 10.171(1) Å. The topology of the framework is the same as that of gismondine and can be described as a stacking of 2-dimensional arrays of double crankshaft chains. The structure was refined by the Rietveld technique using X-ray powder data, and converged with R F = 0.126 and R wp = 0.136 (statistically expected R e = 0.121). Fifty soft restrictions on the bond distances and angles of the aluminosilicate framework atoms were included in the least-squares refinement as additional (geometric) observations. This facilitated the location of the non-framework cations and water molecules, and improved the stability of the refinement. As in the closely-related synthetic zeolite Na – P2, the pseudo-tetragonal axis of the gismondine-type framework is parallel to one of the near-equivalent unit cell dimensions (c-axis), not to the unique one as expected. The arrangement of cations within the zeolitic channels suggests an explanation for this distortion of the ideally tetragonal framework.

The tetrahedral framework in glasses and melts ? inferences from molecular orbital calculations and implications for structure, thermodynamics, and physical properties

Results of ab initio molecular orbital (MO) calculations provide a basis for the interpretation of structural and thermodynamic properties of crystals, glasses, and melts containing tetrahedrally coordinated Si, Al, and B. Calculated and experimental tetrahedral atom-oxygen (TO) bond lengths are in good agreement and the observed average SiO and AlO bond lengths remain relatively constant in crystalline, glassy, and molten materials. The TOT framework geometry, which determines the major structural features, is governed largely by the local constraints of the strong TO bonds and its major features are modeled well by ab initio calculations on small clusters. Observed bond lengths for non-framework cations are not always in agreement with calculated values, and reasons for this are discussed in the text. The flexibility of SiOSi, SiOAl, and AlOAl angles is in accord with easy glass formation in silicates and aluminosilicates. The stronger constraints on tetrahedral BOB and BOSi angles, as evidenced by much deeper and steeper calculated potential energy versus angle curves, suggest much greater difficulty in substituting tetrahedral B than Al for Si. This is supported by the pattern of immiscibility in borosilicate glasses, although the occurrence of boron in trigonal coordination is an added complication. The limitations on glass formation in oxysulfide and oxynitride systems may be related to the angular requirements of SiSSi and Si(NH)Si groups. Although the SiO and AlO bonds are the strongest ones in silicates and aluminosilicates, they are perturbed by other cations. Increasing perturbation and weakening of the framework occurs with increasing ability of the other atom to compete with Si or Al for bonding to oxygen, that is, with increasing cation field strength. The perturbation of TOT groups, as evidenced by TO bond lengthening predicted by MO calculations and observed in ordered crystalline aluminosilicates, increases in the series Ca, Mg and K, Na, Li. This perturbation correlates strongly with thermochemical mixing properties of glasses in the systems SiO2-M 1 /+ AlO2 and SiO2-M n+O n/2 (M=Li, Na, K, Rb, Cs, and Mg, Ca, Sr, Ba, Pb), with tendencies toward immiscibility in these systems, and with systematics in vibrational spectra. Trends in physical properties, including viscosity at atmospheric and high pressure, can also be correlated.

Thermochemistry of Charge‐Coupled Substitutions in Silicate Glasses: The Systems Ml/nn+ AlO2‐SiO2 (M = Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, Pb)

Enthalpies of solution of framework aluminosilicate glasses xMl/nn+ AlO2‐(1‐x)SiO2 (M = Cs, Rb, K, Na, Li, Ba, Pb, Sr, Ca, Mg) in molten 2PbO.B2O3 near 973 K were measured. The sequence above represents increasing ability of the nonframework cation M to bond to the bridging oxygen. In that sequence, the enthalpy of the substitution Si4+→+ Al3++ 1/nMn+ in the glass becomes less exothermic, the enthalpy of vitrification increases, and the tendency toward glass‐glass immiscibility becomes more pronounced. These parameters correlate smoothly with the ionic potential, z/r, of the substituting cation.

A thermochemical study of glasses and crystals along the joins silica-calcium aluminate and silica-sodium aluminate

Enthalpies of solution in molten 2PbO · B 2O 3 at 985 K are reported for series of glasses xCa 0.5 AlO 2-(1− x) SiO 2 ( O ≤ x ≤ 0.99) and xNaAlO 2-(1− x) SiO 2 (0 ≤ x ≤ 0.56). The data are compared to values for the corresponding crystalline aluminosilicates and to preliminary data for systems containing KAlO 2 and Mg 0.5AlO 2. The enthalpies of mixing of glasses become more exothermic with increasing basicity of the mono- or divalent oxide. The tendency toward immiscibility on the silica-rich side, indicated by the shape of the heat of mixing curve between x = 0 and x = 0.4, is pronounced in the calcium aluminate system, but not in the sodium aluminate system. The shape of the heat of mixing curve, which is roughly symmetrical about x = 0.5, can be rationalized in terms of glass structure by considering essentially random substitution of Si and Al on a continuous three dimensional tetrahedral framework, with stabilization arising from electrostatic interactions between aluminum and the nonframework cation balancing the destabilizing effects arising from perturbation of the aluminosilicate framework by the nonframework cation. These trends are consistent with the variation of physical properties of aluminosilicate melts.

Some Aspects of Ion-exchange and Catalytic Activity of Faujasite Type Molecular Sieve Zeolites

Natural and synthetic crystalline zeolites are characterized by frameworks originating from linkages of (Si, Al)-tetrahedra, in which structural cages and channels are occupied by water molecules and an appropriate amount of non-framework cations for balancing the negative charge of the available (AlO2)− groups. Most common among these non-framework cations are Na+, Ca2+ and K+ (in both natural and as-synthesized zeolites). Reversible and facile exchange of these cations with others is one of the characteristic properties of zeolites. The extents of such exchanges are guided by factors like crystallographic sites and structural openings involved, ionic hydration energies, hydrated radii of the ions etc. Technologically important ion exchanges in zeolites include Ni2+, RE3+ (e.g. La3+, Ce3+) and NH4+ for sodium.The present paper discusses some aspects of the current knowledge on the phenomenon of ion-exchange in zeolites, the important criteria that are involved, and the modifications in properties that follow complete or partial exchange of sodium in synthetic faujasite-type zeolites. Special emphasis has been given on the development of catalytic activity in these zeolites by virtue of ion exchange and subsequent activation.

Cation location in hydrated NaY zeolites

Sodium cation distribution in a synthetic NaY has been studied by the X-ray powder diffraction technique. The nonframework cation site occupancies are as follows: site S(I), 3.3 Na +; site S(I′), 12 Na +; site S(II), 1.2 Na +, and site S(III), 34.5 Na +. This distribution is different from the previously published ones for natural faujasite dehydrated NaY; the differences are assigned to a thermal treatment which produces a cation migration from the supercage to the smaller cells.

Molecular Sieve Zeolites

A special class of porous hydrous aluminosilicates, some of which occur as minerals in nature and some others can be synthesized in the laboratory, has assumed great importance in the field of chemical technology during the last decade. These are known as molecular sieve zeolites. The structural elements characterizing such materials can conveniently be visualized in terms of conceptual linkages and building units of their aluminosilicate frameworks; they have been reviewed and a structural classification scheme has been proposed. The non-framework cations required for charge compensation can conveniently be exchanged to yield structures with pore dimensions permitting, on dehydration, size-and-shape grading and fractionation of gas or liquid mixtures on molecular scale. The useful properties of the as-synthesized and treated forms, like sorption, sieving, etc have been reviewed in the perspective of their technological applications. Intrinsic catalytic activity of some treated variants has been reviewed in some detail because of their extensive application to the petrochemical industry.

The Faujasite Family of Molecular Sieve Zeolites: Part II—Crystallography and Crystal Chemistry

Research work, up to 1974, on crystallography and crystal chemistry of the faujasite family of zeolites has been reviewed. The nature of the aluminosilicate framework, the intracrystalline zeolitic fluid, compositional range of the different members of this family, and crystallographic siting of non-framework cations and water molecules in the different cavities of the framework wherever it has been possible to do so, have been analysed. Migration of non-framework cations consequent upon dehydration has also been reviewed with the help of schematic diagrams and the way they would lead to activity of the dehydrated forms, so characteristic of this family of zeolites, has been emphasized.