Yongjae Lee

Non-framework cation migration and irreversible pressure-induced hydration in a zeolite

Zeolites crystallize in a variety of three-dimensional structures in which oxygen atoms are shared between tetrahedra containing silicon and/or aluminium, thus yielding negatively charged tetrahedral frameworks that enclose cavities and pores of molecular dimensions occupied by charge-balancing metal cations and water molecules. Cation migration in the pores and changes in water content associated with concomitant relaxation of the framework have been observed in numerous variable-temperature studies, whereas the effects of hydrostatic pressure on the structure and properties of zeolites are less well explored. The zeolite sodium aluminosilicate natrolite was recently shown to undergo a volume expansion at pressures above 1.2u2009GPa as a result of reversible pressure-induced hydration; in contrast, a synthetic analogue, potassium gallosilicate natrolite, exhibited irreversible pressure-induced hydration with retention of the high-pressure phase at ambient conditions. Here we report the structure of the high-pressure recovered phase and contrast it with the high-pressure phase of the sodium aluminosilicate natrolite. Our findings show that the irreversible hydration behaviour is associated with a pronounced rearrangement of the non-framework metal ions, thus emphasizing that they can clearly have an important role in mediating the overall properties of zeolites.

Structures and thermodynamics of the mixed alkali alanates

The thermodynamics and structural properties of the hexahydride alanates (M{sub 2}M{sup }AlH{sub 6}) with the elpasolite structure have been investigated. A series of mixed alkali alanates (Na{sub 2}LiAlH{sub 6}, K{sub 2}LiAlH{sub 6}, and K{sub 2}NaAlH{sub 6}) were synthesized and found to reversibly absorb and desorb hydrogen without the need for a catalyst. Pressure-composition isotherms were measured to investigate the thermodynamics of the absorption and desorption reactions with hydrogen. Isotherms for catalyzed (4 mol% TiCl{sub 3}) and uncatalyzed Na{sub 2}LiAlH{sub 6} exhibited an increase in kinetics, but no change in the bulk thermodynamics with the addition of a dopant. A structural analysis using synchrotron x-ray diffraction showed that these compounds favor the Fm3m space group with the smaller ion (M{sup }) occupying an octahedral site. These results demonstrate that appropriate cation substitutions can be used to stabilize or destabilize the material and may provide an avenue to improving the unfavorable thermodynamics of a number of materials with promising gravimetric hydrogen densities.

PRESSURE-INDUCED STABILIZATION OF ORDERED PARANATROLITE: A NEW INSIGHT INTO THE PARANATROLITE CONTROVERSY

Abstract The origin and stability of paranatrolite (approximate formula Na16-xCaxAl16+xSi24-xO80·24H2O), a naturally occurring zeolite with the natrolite topology, has long been debated, with its detailed structure unknown. When taken from an aqueous environment and exposed to the atmosphere, paranatrolite is reported to irreversibly lose water and transform to gonnardite/tetranatrolite, Na16-xCaxAl16+xSi24-xO80·nH2O. Since the latter has a disordered Al/Si distribution over the framework tetrahedral sites, it is believed the same is true for paranatrolite. Natrolite itself (Na16Al16Si24O80·16H2O) has Al/Si ordering, and as recently shown, undergoes a reversible volume expansion (~2.5%) due to pressure-induced hydration (PIH) above 1.2 GPa to a superhydrated phase (Na16Al16Si24O80·32H2O). During this process, an intermediate phase with an even larger volume expansion of ~7.0% has been detected in a narrow pressure range near 1.0 GPa. We report here that this intermediate phase has a unit-cell compatible with the one reported for paranatrolite at ambient conditions with the same 24 water molecules per formula unit and propose that it is paranatrolite with an ordered Al/Si distribution. An unusual watersodium chain is observed in the ordered paranatrolite structure: a sevenfold coordination of sodium cations provided by alternating two water bridges along the expanded elliptical channels. The density of the ordered paranatrolite is lower than those of the 16 and 32 water phases, with its channel openings far more circular than in the low- and high-pressure analogs. The atomistic details of the ordered paranatrolite provide a structural model for the naturally occurring paranatrolite and a complete understanding of this intriguing pressure-volume-hydration mechanism in natrolite, demonstrating the unique role of pressure in controlling the chemistry of microporous materials.

Synthesis and crystal structures of gallium and germanium variants of cancrinite

Abstract A synthetic aluminogermanate and a gallogermanate with the Cancrinite group (CAN) framework topology have been synthesized under hydrothermal conditions and characterized by single crystal synchrotron X-ray diffraction. AlGe-CAN, Na6Cs2Al6Ge6O24xa0·xa0Ge(OH)6, is hexagonal, with the space group P63 and a=12.968(1), c=5.132(1) A, V=747.4(1) A3. The T-sites exhibit complete ordering of Al and Ge atoms, similar to the framework models of aluminosilicate analogues. GaGe-CAN, Na6Cs2Ga6Ge6O24xa0·xa0Ge(OH)6, is hexagonal, apparently with the space group P63mc and a=12.950(2), c=5.117(1) A, V=743.2(2) A3. Although the observed data are consistent with the presence of the c-glide and consequent disordering of Ga and Ge atoms at the T-sites, calculation using a DLS-optimized framework in the space group P63 reveals that the intensities of the hh 2h l reflections with l=2n+1 are less than 0.07% of the strongest (0xa00xa00xa02) reflection, suggesting that P63 is probably the true space group. Resonant diffraction studies performed in the vicinity of the Ga K-edge confirmed the presence of the hh 2h l reflections with l=2n+1 and thus confirmed the ordering of the framework Ga/Ge atoms in GaGe-CAN. Inspection of the framework T–O–T bond angles demonstrates greater relative cell contraction for GaGe-CAN compared to AlGe-CAN and aluminosilicate counterparts. In both the structural models, Ge(OH)6 octahedra are occluded in the 12-ring channels running along the 63-axes. The sodium cations fully occupy the sites above the 6-ring windows in the 12-ring channels. The cesium cations fully occupy the sites in the middle of the cancrinite cages. Subtle differences in the coordination geometries of the extra-framework species are found, perhaps due to the pseudo-symmetry of GaGe-CAN. Thermogravimetry results indicate net weight losses of 3.5% and 3.0% for AlGe-CAN and GaGe-CAN, respectively, which are explainable by the dehydration of the Ge(OH)6 octahedra. In situ synchrotron X-ray powder diffraction demonstrated the formation of GaGe analogue of the nepheline hydrate I type structure at the temperature of complete dehydration.

Synthesis and crystal structures of gallium- and germanium-variants of the fibrous zeolites with the NAT, EDI and THO structure types

Abstract Two synthetic gallosilicates and a gallogermanate with the NAT, EDI and THO framework topologies have been synthesized under hydrothermal conditions and characterized by single crystal synchrotron X-ray diffraction. K-GaSi-NAT, K 8 Ga 8 Si 12 O 40 ·6H 2 O, is tetragonal, space group I 4 2d, with a =13.639(2), c =6.545(1)xa0A. The framework model shows complete disordering of Ga and Si in tetrahedral sites, which is analogous to tetranatrolite but contrasts with the partial ordering in Na-GaSi-NAT. The T-sites of RbNa-GaSi-EDI, Rb 7 NaGa 8 Si 12 O 40 ·3H 2 O, exhibit partial disordering of Ga and Si in space group P 4 2 1 c. This leads to a cell doubling along the chain axis ( c ) with a =9.773(1), c =13.141(3)xa0A, a super cell modification of the Na-exchanged K- F structure. In Rb-GaGe-THO, Rb 20 Ga 20 Ge 20 O 80 ·15H 2 O, a =14.335(3), b =14.198(3), c =13.421(3)xa0A, complete ordering on both tetrahedral and extra-framework sites lowers the symmetry from Pncn to the acentric space group Pn 2 n . An inspection of framework Tue5f8Oue5f8T bond angles, which are related to rotation and distortion of the chains, explains the differences in unit cells between these gallium- and germanium-variants and aluminosilicate analogs. The elliptical 8-ring windows, generated by four crosslinked chains in RbNa-GaSi-EDI and Rb-GaGe-THO, are the sites for (Rb,Na) and Rb cations, respectively, while the helical 8-ring channels formed in K-GaSi-NAT host only water molecules. The T 10 O 20 windows, built by two neighboring chains, provide sites for K or Rb cations in each model. Mechanisms are proposed for cation–framework interactions, which are in turn responsible for the observed framework models.

Structural studies of hydrated germanium X-type zeolite via Rietveld analysis of synchrotron powder X-ray diffraction data

Abstract An aluminogermanate analog of zeolite X has been synthesized by solution reaction, and characterized by Rietveld analysis of synchrotron powder X-ray diffraction data. The hydrated sample has a chemical formula Na 96 Al 96 Ge 96 O 384 · w H 2 O, and the material possesses an ordered arrangement of Al and Ge in the framework. A cell parameter of 25.589(1)xa0A confirms the expected cell expansion over Low Silica X (LSX), although a smaller average framework T–O–T bond angle indicates greater relative cell collapse than for LSX. The distribution of sodium cations in the non-framework sites in NaGeX is significantly different from that in LSX, and appears to deviate from the proposed relationship between Al content and cation siting for hydrated aluminosilicate faujasite-type zeolites.

NOVEL SYNTHESIS AND HIGH PRESSURE BEHAVIOR OF NA0.3COO2X1.3H2O AND RELATED PHASES

We have prepared powder samples of N x CoO 2 .yH 2 O using an alternative synthesis route. Superconductivity was observed in Na 0 . 3 CoO 2 .1.3H 2 O between 4 and 5 K as indicated by the magnetic susceptibility. The bulk compressibilities of Na 0 . 3 CoO 2 .1.3H 2 O, Na 0 . 3 CoO 2 .0.6H 2 O, and Na 0 . 3 CoO 2 were determined using a diamond anvil cell and synchrotron powder diffraction. Chemical changes occurring under pressure when using different pressure-transmitting media are discussed and further transport measurements are advocated.

Pressure-induced hydration in zeolite tetranatrolite

Abstract The tetranatrolite-paranatrolite transformation has remained a key problem in understanding the paragenesis of zeolites in the natrolite family. It is accepted that when paranatrolite, approximate formula Na16-xCaxAl16+xSi24-xO80·24H2O, is removed from an aqueous environment and exposed to the atmosphere, it loses water and transforms to tetranatrolite, Na16-xCaxAl16+xSi24-xO80·nH2O (n ≤ 24). Here we show that this transformation is not only reversible, but that tetranatrolite exhibits two sequential pressure-induced hydrations leading first to paranatrolite and then to a superhydrated tetranatrolite above 0.2 and 3.0 Gpa, respectively. We have previously reported similar behavior for the corresponding system with an ordered Si/Al distribution, i.e., natrolite itself, however the ordered version of paranatrolite exists over a much smaller pressure range. The pressure-induced transformations of natrolite and tetranatrolite thus further supports the supposition that paranatrolite is a distinct mineral species, with a pressure-stability field dependent upon composition.

Understanding negative thermal expansion and ‘trap door’ cation relocations in zeolite rho

In situ time-resolved synchrotron X-ray and neutron npowder diffraction studies indicate that the negative thermal expansion and n‘trap door’ cation relocations observed in zeolite rho result nfrom water-mediated chemical changes that occur during dehydration.

Pressure induced valence and structural phase transition in Ba2PrRu0.8Ir0.2O6

The crystal structure of the ordered double perovskite Ba2PrRu0.8Ir0.2O6 was investigated as a function of pressure at room temperature. High-resolution synchrotron powder diffraction measurements have revealed the occurrence of a first order phase transition at a critical pressure of about 0.5 GPa. Structural refinements indicate that Ba2PrRu0.8Ir0.2O6 transforms from an ambient pressure monoclinic (P 21/n) to a high-pressure tetragonal (P4/mnc) structure. The transition is consistent with a valence change of the Pr ions.