Dehydration-induced water disordering in a synthetic potassium gallosilicate natrolite.

A new potassium gallosilicate zeolite with a natrolite topology (approximate formula K8.2Ga8.2Si11.8O40.11.5H2O) was synthesized under hydrothermal conditions and characterized as a function of temperature using monochromatic synchrotron X-ray powder diffraction and Rietveld analyses. Unlike the previously known tetragonal K8Ga8Si12O40.6H2O phase, the as-synthesized material contains twice the amount of water molecules in an ordered arrangement throughout the channels in an orthorhombic (I212121) symmetry. The ordered configuration of water molecules is stabilized below 300 K, whereas heating above 300 K results in a selective dehydration and subsequent disordering of water molecules in a tetragonal (I2d) symmetry. Above 400 K, the material transforms to a fully dehydrated tetragonal phase with a concomitant volume reduction of ca. 15%. The fully dehydrated material transforms back to its original state when rehydrated over a period of up to 2 weeks. The distribution of potassium cations within the channels remains largely unperturbed during the water rearrangements and their order-disorder transition within the channels.

Experimental and theoretical methods to investigate extraframework species in a layered material of dodecaniobate anions.

The highly charged dodecaniobate Keggin ions [XNb12O40](-16) (X = Si, Ge) and [XNb12O40](-15) (X = P) serve as building blocks of self-assembled, low-dimensional anionic framework materials. In addition to its high charge, the Keggin ion provides optimal binding geometries that render these materials as attractive metal sorbents and ion exchangers. We describe here the synthesis and single-crystal X-ray structure of K(10-x)[Nb2O2][HxGeNb12O40].11H2O (GeNb12-2d; x = approximately 1-1.5), a phase featuring 2D linkage of [GeNb12O40](-16) Keggin ions interlayered with charge-balancing K(+) cations and water molecules. Thermogravimetry, infrared spectroscopy (IR), 1H MAS NMR, and D2O exchange experiments as well as computational studies were used to describe the location and behavior of these interlayer, extraframework species. To model the basicity of the different types of framework oxygen sites appropriately, atomic-centered partial charges were derived from density functional theory (DFT) calculations to model the electrostatic potential. This model enabled the locations and bonding of K(+) cations associated with the framework, as well as K(+) cations bound predominantly to water in the interlayer space, to be accurately computed via Monte Carlo simulation. The poorest agreement between experimental and simulation results was observed for potassium sites that were associated with disordered portions of the framework, namely, the [Nb2O2](6+) bridge between Keggin ions. Finally, through grand canonical Monte Carlo (GCMC) calculations, saturation water loadings consistent with experimental measurements were computed.

Cs[Si(3)O(6)(OH)] and Rb[Si(2)O(4)(OH)]: two novel phyllosilicates.

The crystal structures of two novel phyllosilicates with compositions Cs[Si(3)O(6)(OH)] (caesium hydroxohexaoxotetraotrisilicate) and Rb[Si(2)O(4)(OH)] (rubidium hydroxohexaoxotetraodisilicate) have been characterized by X-ray diffraction. The topology of the caesium phyllosilicate silica sheet consists of interconnected four- and six-membered rings and thus differs from all of the previously reported phyllosilicates. The topology of the rubidium phyllosilicate silica sheet consists of six-membered rings only, in boat conformations, resulting in a corrugated sheet similar to that observed in delta-Na(2)Si(2)O(5). Both of the title compounds exhibit the characteristic sandwich structure of sheet silicates, with the Cs atom ninefold coordinated and the Rb atom eightfold coordinated to the framework O atoms.

An X-ray diffraction and MAS NMR study of the thermal expansion properties of calcined siliceous ferrierite.

Powder and single-crystal X-ray diffraction, combined with MAS NMR measurements, has been used to study the thermal expansion of siliceous zeolite ferrierite as it approaches a second-order displacive phase transition from a low-symmetry (Pnnm) to a high-symmetry (Immm) structure. Below the transition temperature, ferrierite exhibits positive thermal expansivity. However, above the transition temperature a significant change in thermal behavior is seen, and ferrierite becomes a negative thermal expansion material. Accurate variable-temperature single-crystal X-ray diffraction measurements confirm the transition temperature and allow the changes in average atomic position to be followed with temperature. The results from the single-crystal X-ray diffraction study can be correlated with (29)Si MAS NMR chemical shifts for the low-temperature phase. At low temperatures the results show that the positive thermal expansivity is driven by an overall increase in Si[bond]Si distances related to an increase in Si[bond]O[bond]Si bond angles. However, in the high-temperature phase the Si[bond]O[bond]Si angles are approximately invariant with temperature, and the negative thermal expansion in this case is caused by transverse vibrations of the Si[bond]O[bond]Si units.

Synthesis and crystal structure of the first scandium-containing open framework solid.

A novel open framework scandium sulfate phosphate is prepared hydrothermally in the presence of the azamacrocycle cyclen (1,4,7,10-tetraazacyclododecane) in which secondary building units of formula Sc7(S,P)12O48 are linked to give a structure with supercages.