A family of microporous phases with compositions Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-x}(OH){sub x} {center_dot} H{sub 2}O (0 {le} x {le} 0.4) transform to Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-0.5x} perovskites upon heating. In this study, we have measured the enthalpies of formation of the microporous phases and their corresponding perovskites from the constituent oxides and from the elements by drop solution calorimetry in 3Na{sub 2}O {center_dot} 4MoO{sub 3} solvent at 974 K. As Ti/Nb increases, the enthalpies of formation for the microporous phases become less exothermic up to x = {approx}0.2 but then more exothermic thereafter. In contrast, the formation enthalpies for the corresponding perovskites become less exothermic across the series. The energetic disparity between the two series can be attributed to their different mechanisms of ionic substitutions: Nb{sup 5+} + O{sup 2-} {yields} Ti{sup 4+} + OH{sup -} for the microporous phases and Nb{sup 5+} {yields} Ti{sup 4+} + 0.5 V{sub O}** for the perovskites. From the calorimetric data for the two series, the enthalpies of the dehydration reaction, Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-x}(OH){sub x} {center_dot} H{sub 2}O {yields} Na{sub 2}Nb{sub 2-x}Ti{sub x}O{sub 6-0.5X} + H{sub 2}O, have been derived, and their implications for phase stability at the synthesis conditions are discussed.
There is a great need for robust, defect-free, highly selective molecular sieve (zeolite) thin film membranes for light gas molecule separations in hydrogen fuel production from CH{sub 4} or H{sub 2}O sources. In particular, we are interested in (1) separating and isolating H{sub 2} from H{sub 2}O and CH{sub 4}, CO, CO{sub 2}, O{sub 2}, N{sub 2} gases; (2) water management in PEMs and (3) as a replacement for expensive Pt catalysts needed for PEMs. Current hydrogen separation membranes are based on Pd alloys or on chemically and mechanically unstable organic polymer membranes. The use of molecular sieves brings a stable (chemically and mechanically stable) inorganic matrix to the membrane [1-3]. The crystalline frameworks have 'tunable' pores that are capable of size exclusion separations. The frameworks are made of inorganic oxides (e.g., silicates, aluminosilicates, and phosphates) that bring different charge and electrostatic attraction forces to the separation media. The resultant materials have high separation abilities plus inherent thermal stability over 600 C and chemical stability. Furthermore, the crystallographically defined (<1 {angstrom} deviation) pore sizes and shapes allow for size exclusion of very similarly sized molecules. In contrast, organic polymer membranes are successful based on diffusion separations, not size exclusion. We envision the impact of positive results from this project in the near term with hydrocarbon fuels, and long term with biomass fuels. There is a great need for robust, defect-free, highly selective molecular sieve (zeolite) thin film membranes for light gas molecule separations in hydrogen fuel production from CH{sub 4} or H{sub 2}O sources. They contain an inherent chemical, thermal and mechanical stability not found in conventional membrane materials. Our goal is to utilize those zeolitic qualities in membranes for the separation of light gases, and to eventually partner with industry to commercialize the membranes. To date, we have successfully: (1) Demonstrated (through synthesis, characterization and permeation testing) both the ability to synthesize defect-free zeolitic membranes and use them as size selective gas separation membranes; these include aluminosilicates and silicates; (2) Built and operated our in-house light gas permeation unit; we have amended it to enable testing of H{sub 2}S gases, mixed gases and at high temperatures. We are initiating further modification by designing and building an upgraded unit that will allow for temperatures up to 500 C, steady-state vs. pressure driven permeation, and mixed gas resolution through GC/MS analysis; (3) Have shown in preliminary experiments high selectivity for H{sub 2} from binary and industrially-relevant mixed gas streams under low operating pressures of 16 psig; (4) Synthesized membranes on commercially available oxide and composite disks (this is in addition to successes we have in synthesizing zeolitic membranes to tubular supports [9]); and (5) Signed a non-disclosure agreement with industrial partner G. E. Dolbear & Associates, Inc., and have ongoing agreements with Pall Corporation for in-kind support supplies and interest in scale-up for commercialization.
A new family of framework titanosilicates, A{sub 2}TiSi{sub 6}O{sub 15} (A=K, Rb, Cs) (space group Cc), has recently been synthesized using the hydrothermal method. This group of phases can potentially be utilized for storage of radioactive elements, particularly {sup 137}Cs, due to its high stability under electron radiation and chemical leaching. Here, we report the syntheses and structures of two intermediate members in the series: KRbTiSi{sub 6}O{sub 15} and RbCsTiSi{sub 6}O{sub 15}. Rietveld analysis of powder synchrotron X-ray diffraction data reveals that they adopt the same framework topology as the end-members, with no apparent Rb/K or Rb/Cs ordering. To study energetics of the solid solution series, high-temperature drop-solution calorimetry using molten 2PbO {center_dot} B{sub 2}O{sub 3} as the solvent at 975 K has been performed for the end-members and intermediate phases. As the size of the alkali cation increases, the measured enthalpies of formation from the constituent oxides and from the elements ({Delta}H{sub f,el}) become more exothermic, suggesting that this framework structure favors the cation in the sequence Cs{sup +}, Rb{sup +}, and K{sup +}. This trend is consistent with the higher melting temperatures of A{sub 2}TiSi{sub 6}O{sub 15} phases with increase in the alkali cation size.
The synthesis, characterization, and separations capability of defect-free, thin-film zeolite membranes were presented. The one-micron thick sodium-aluminosilicate films of Silicalite-1 and ZSM-5 were synthesized by hydrothermal methods on either disk- or tube-supports. Techniques for growing membranes on both Al2O3 substrates as well as oxide-coated stainless steel substrates were presented. The resulting defect-free zeolite films had high flux rates at room temperature (∼ 10-7 mole/Pa-sec-sq m) and showed selective separations (3-7) between pure gases of H2 and CH4, O2, N2, CO2, CO, SF6. Results from mixed gas studies showed similar flux rates as pure gases with enhanced selectivity (15-50) for H2. The selectivity through both Silicalite-1 and ZSM-5 membranes was compared and contrasted for several gas mixtures. Data comparisons for defect-free and "defect-filled" membranes were also discussed. Under operation, the flow through these membranes quickly reached its maximum value and was stable over long periods of time. Results from experiments at high temperatures, ≤ 300°C, were compared with the data obtained at room temperature. This is an abstract of a paper presented at the 228th ACS National Meeting (Philadelphia, PA, 8/22-26/2004).
Alkylation reactions of benzene with propylene using zeolites were studied for their affinity for cumene production. The current process for the production of cumene involves heating corrosive acid catalysts, cooling, transporting, and distillation. This study focused on the reaction of products in a static one-pot vessel using non-corrosive zeolite catalysts, working towards a more efficient one-step process with a potentially large energy savings. A series of experiments were conducted to find the best reaction conditions yielding the highest production of cumene. The experiments looked at cumene formation amounts in two different reaction vessels that had different physical traits. Different zeolites, temperatures, mixing speeds, and amounts of reactants were also investigated to find their affects on the amount of cumene produced. Quantitative analysis of product mixture was performed by gas chromatography. Mass spectroscopy was also utilized to observe the gas phase components during the alkylation process.
As a participating national lab in the inter-institutional effort to resolve performance issues of the non-elutable ion exchange technology for Cs extraction, they have carried out a series of characterization studies of UOP IONSIV{reg_sign} IE-911 and its component parts. IE-911 is a bound form (zirconium hydroxide-binder) of crystalline silicotitanate (CST) ion exchanger. The crystalline silicotitanate removes Cs from solutions by selective ion exchange. The performance issues of primary concern are: (1) excessive Nb leaching and subsequent precipitation of column-plugging Nb-oxide material, and (2) precipitation of aluminosilicate on IE-911 pellet surfaces, which may be initiated by dissolution of Si from the IE-911, thus creating a supersaturated solution with respect to silica. In this work, they have identified and characterized Si- and Nb-oxide based impurity phases in IE-911, which are the most likely sources of leachable Si and Nb, respectively. Furthermore, they have determined the criteria and mechanism for removal from IE-911 of the Nb-based impurity phase that is responsible for the Nb-oxide column plugging incidents.
The synthesis, structure and some properties of C{sub 2}H{sub 7}N{sub 4}O {center_dot} ZnPO{sub 4} (guanylurea zinc phosphate) are reported. The cationic template was prepared in situ by partial hydrolysis of the neutral 2-cyanoguanidine starting material. The resulting structure contains a new, unprotonated, zincophosphate layer topology as well as unusual N-H-O template-to-template hydrogen bonds which help to stabilize a ''double sandwich'' of templating cations between the inorganic sheets. Crystal data: C{sub 2}H{sub 7}N{sub 4}O {center_dot} ZnPO{sub 4}, M{sub r} = 229.44, monoclinic, P2{sub 1}/c, a = 13.6453 (9) {angstrom}, b = 5.0716 (3) {angstrom}, c = 10.6005 (7) {angstrom}, {beta} = 95.918 (2){sup 0}, V = 729.7 (1) {angstrom}{sup 3}, R(F) = 0.034, wR(F) = 0.034.
Exploratory hydrothermal synthesis in the system Cs2O-SiO2-TiO2 has produced a new polymorph of Cs2TiSi6O15 (SNL-A), whose structure was determined using a combination of experimental and theoretical techniques (29Si and 133Cs NMR, X-ray powder diffraction, and density functional theory). SNL-A crystallizes in the monoclinic space group Cc with unit cell parameters a = 12.998(2) Å, b = 7.5014(3) Å, c = 15.156(3) Å, and β = 105.80(3)°. The SNL-A framework is an unbranched drier single-layer silicate with silicon tetrahedra and titanium octahedra that are linked in 3-, 5-, 6-, 7-, and 8-membered rings in three dimensions. SNL-A is distinctive from a previously reported C2/c polymorph of Cs2TiSi6O15 by orientation of the Si2O52- layers and by different ring geometries. Similarities and differences between the two structures are discussed. Other characterizations of SNL-A include TGA-DTA, Cs/Si/Ti elemental analyses, and SEM/EDS. Furthermore, the chemical and radiation durability of SNL-A was studied in interest of ceramic waste form applications. These studies show that SNL-A is durable in both radioactive and rigorous chemical environments. Finally, calculated cohesive energies of the two Cs2TiSi6O15 polymorphs suggest that the Cc SNL-A phase (synthesized at 200 °C) is energetically more favorable than the C2/c polymorph (synthesized at 1050 °C).
The solution-mediated synthesis and single crystal structure of (CN{sub 3}H{sub 6}){sub 2} {center_dot} Zn(HPO{sub 3}){sub 2} are reported. This phase is built up from a three-dimensional framework of vertex-linked ZnO{sub 4} and HPO{sub 3} building units encapsulating the extra-framework guanidinium cations. The structure is stabilized by template-to-framework hydrogen bonding. The inorganic framework shows a surprising similarity to those of some known zinc phosphates. Crystal data: (CN{sub 3}H{sub 6}){sub 2} {center_dot} Zn(HPO{sub 3}){sub 2}, AI,= 345.50, orthorhombic, space group Fdd2 (No. 43), a = 15.2109 (6) {angstrom}, b = 11.7281 (5) {angstrom}, c = 14.1821 (6) {angstrom}, V = 2530.0 (4){angstrom}{sup 3}, Z = 8, T = 298 (2)K, R(F) = 0.020, wR(F) = 0.025.
The syntheses, crystal structures and some properties of {alpha}- and {beta}-ZnHPO{sub 3}{center_dot}N{sub 4}C{sub 2}H{sub 4} are reported. These two polymorphs are the first organically-templated hydrogen phosphites. They are built up from vertex-sharing HPO{sub 3} pseudo pyramids and ZnO{sub 3}N tetrahedra, where the Zn-N bond represents a direct link between zinc and the neutral 2-cyanoguanidine template. {alpha}-ZnHPO{sub 3}{center_dot}N{sub 4}C{sub 2}H{sub 4} is built up from infinite layers of vertex-sharing ZnO{sub 3}N and HPO{sub 3} groups forming 4-rings and 8-rings. {beta}-ZnHPO{sub 3}{center_dot}N{sub 4}C{sub 2}H{sub 4} has strong one-dimensional character, with the polyhedral building units forming 4-ring ladders. Similarities and differences to related zinc phosphates are discussed. Crystal data: {alpha}-ZnHPO{sub 3}{center_dot}N{sub 4}C{sub 2}H{sub 4}, M{sub r} = 229.44, monoclinic, P2{sub 1}/c, a = 9.7718 (5) {angstrom}, b = 8.2503 (4) {angstrom}, c = 9.2491 (5) {angstrom}, {beta} = 104.146 (1){sup 0}, V = 723.1 (1) {angstrom}{sup 3}, R(F) = 2.33%, wR(F) = 2.52%. {beta}-ZnHPO{sub 3}{center_dot}N{sub 4}C{sub 2}H{sub 4}, M{sub r} = 229.44, monoclinic, C2/c, a = 14.5092 (9) {angstrom}, b = 10.5464 (6) {angstrom}, c = 10.3342 (6) {angstrom}, {beta} = 114.290 (1){sup 0}, V = 1441.4 (3) {angstrom}{sup 3}, R(F) = 3.01%, wR(F) = 3.40%.
The structure of Na{sub 16}Nb{sub 12.8}Ti{sub 3.2}O{sub 44.8}(OH){sub 3.2} {center_dot} 8H{sub 2}O, a member of a new family of Sandia Octahedral Molecular Sieves (SOMS) having a Nb/Na/M{sup IV} (M= Ti, Zr) oxide framework and exchangeable Na and water in open channels, was determined from Synchrotron X-ray data. The SOMS phases are isostructural with variable M{sup IV}:Nb(1:50--1:4) ratios. The SOMS are extremely selective for sorption of divalent cations, particularly Sr{sup 2+}. The ion-exchanged SOMS undergo direct thermal conversion to a perovskite-type phase, indicating this is a promising new method for removal and sequestration of radioactive Sr-90 from mixed nuclear wastes.
A study of zeolite crystallization from sol-gel precursors using the vapor phase transport synthesis method has been performed. Zeolites (ZSM-5, ZSM-48, Zeolite P, and Sodalite) were crystallized by contacting vapor phase organic or organic-water mixtures with dried sodium silicate and dried sodium alumino-silicate gels. For each precursor gel, a ternary phase system of vapor phase organic reactant molecules was explored. The vapor phase reactant mixtures ranged from pure ethylene diamene, triethylamine, or water, to an equimolar mixture of each. In addition, a series of gels with varied physical and chemical properties were crystallized using the same vapor phase solvent mixture for each gel. The precursor gels and the crystalline products were analyzed via Scanning Electron Microscopy, Electron Dispersive Spectroscopy, X-ray mapping, X-ray powder diffraction, nitrogen surface area, Fourier Transform Infrared Spectroscopy, and thermal analyses. The product phase and purity as a function of the solvent mixture, precursor gel structure, and precursor gel chemistry is discussed.
An astonishing variety of inorganic networks templated by organic species have been reported over the last 10 years. A great deal of attention has been paid to the structure-directing role of the organic species, and the structural effect of variously coordinated cations, for example distorted octahedral vanadium and pyramidal tin. Less exploratory work has been carried out on the anionic part of the inorganic network, and most groups reported so far (phosphate, germanate, etc.) invariably adopt tetrahedral coordination. The possibilities of incorporating the pyramidal [HP0{sub 3}]{sup 2{minus}} hydrogen phosphite group into extended structures templated by inorganic, alkaline earth cations was explored a few years ago. In this paper the authors report the synthesis, crystal structure, and some properties of (CN{sub 3}H{sub 6}){sub 4}{center_dot}Zn{sub 3}(SeO{sub 3}){sub 5}, the first organically-templated phase to contain the pyramidal selenite [SeO{sub 3}]{sup 2{minus}} anion.
A number of Hanford tanks have leaked high level radioactive wastes (HLW) into the surrounding unconsolidated sediments. The disequilibrium between atmospheric C0{sub 2} or silica-rich soils and the highly caustic (pH > 13) fluids is a driving force for numerous reactions. Hazardous dissolved components such as {sup 133}Cs, {sup 79}Se, {sup 99}Tc may be adsorbed or sequestered by alteration phases, or released in the vadose zone for further transport by surface water. Additionally, it is likely that precipitation and alteration reactions will change the soil permeability and consequently the fluid flow path in the sediments. In order to ascertain the location and mobility/immobility of the radionuclides from leaked solutions within the vadose zone, the authors are currently studying the chemical reactions between: (1) tank simulant solutions and Hanford soil fill minerals; and (2) tank simulant solutions and C0{sub 2}. The authors are investigating soil-solution reactions at: (1) elevated temperatures (60--200 C) to simulate reactions which occur immediately adjacent a radiogenically heated tank; and (2) ambient temperature (25 C) to simulate reactions which take place further from the tanks. The authors studies show that reactions at elevated temperature result in dissolution of silicate minerals and precipitation of zeolitic phases. At 25 C, silicate dissolution is not significant except where smectite clays are involved. However, at this temperature CO{sub 2} uptake by the solution results in precipitation of Al(OH){sub 3} (bayerite). In these studies, radionuclide analogues (Cs, Se and Re--for Tc) were partially removed from the test solutions both during high-temperature fluid-soil interactions and during room temperature bayerite precipitation. Altered soils would permanently retain a fraction of the Cs but essentially all of the Se and Re would be released once the plume was past and normal groundwater came in contact with the contaminated soil. Bayerite, however, will retain significant amounts of all three radionuclides.
Ongoing hydrothermal Cs-Ti-Si-O-H2O phase investigations has produced several new ternary phases including a novel microporous Cs-silicotitanate molecular sieve, SNL-B with the approximate formula of Cs3TiSi3O9.5 · 3H2O SNL-B is only the second molecular sieve, Cs-silicotitanate phase reported to have been synthesized by hydrothermal methods. Crystallites are very small (0.1 x 2 μm2) with a blade-like morphology. SNL-B is confirmed to be a three-dimensional molecular sieve by a variety of characterization techniques (N2 adsorption, ion exchange, water adsorption/desorption, solid state cross polarization-magic angle spinning nuclear magnetic resonance). SNL-B is able to desorb and adsorb water from its pores while retaining its crystal structure and exchanges Cs cations readily. Additional techniques were used to describe fundamental properties (powder X-ray diffraction, FTIR, 29Si and 133Cs MAS NMR, DTA, SEM/EDS, ion selectivity, and radiation stability). The phase relationships of metastable SNL-B to other hydrothermally synthesized Cs-Ti-Si-O-H2O phases are discussed, particularly its relationship to a Cs-silicotitanate analogue of pharmacosiderite, and a novel condensed phase, a polymorph of Cs2TiSi6O15 (SNL-A). (C) 2000 Elsevier Science B.V. All rights reserved. Ongoing hydrothermal Cs-Ti-Si-O-H2O phase investigations has produced several new ternary phases including a novel microporous Cs-silicotitanate molecular sieve, SNL-B with the approximate formula of Cs3TiSi3O9.5·3H2O. SNL-B is only the second molecular sieve, Cs-silicotitanate phase reported to have been synthesized by hydrothermal methods. Crystallites are very small (0.1×2 μm2) with a blade-like morphology. SNL-B is confirmed to be a three-dimensional molecular sieve by a variety of characterization techniques (N2 adsorption, ion exchange, water adsorption/desorption, solid state cross polarization-magic angle spinning nuclear magnetic resonance). SNL-B is able to desorb and adsorb water from its pores while retaining its crystal structure and exchanges Cs cations readily. Additional techniques were used to describe fundamental properties (powder X-ray diffraction, FTIR, 29Si and 133Cs MAS NMR, DTA, SEM/EDS, ion selectivity, and radiation stability). The phase relationships of metastable SNL-B to other hydrothermally synthesized Cs-Ti-Si-O-H2O phases are discussed, particularly its relationship to a Cs-silicotitanate analogue of pharmacosiderite, and a novel condensed phase, a polymorph of Cs2TiSi6O15 (SNL-A).