Glover A. Jones

Flexibility of the zeolite RHO framework : effect of dehydration on the crystal structure of the beryllophosphate mineral, pahasapaite

Pahasapaite, a hydrated beryllophosphate mineral (Ca 5.5 Li 3.6 K 1.2 Na 0.2 □ 13.5 )Li 8 Be 24 P 24 O 96 ·38 H 2 O, has a distorted cubic, zeolite RHO-type structure with a = 13.781 A. On dehydration, the structure distorts further to give a cell with a = 13.100(3) A. As with the aluminosilicate RHO framework, Li + is associated with the 6-ring sites and Ca 2+ is sited in the double 8-ring sites, leading to severe distortion of the double 8-rings. The double 8-rings consist of elliptically distorted single 8-rings orthogonal to each other and the degree of their distortion can be described by the Δ/a parameter, where Δ is one-half the difference between the major and minor axes of these elliptical 8-rings and a is the cell edge. This parameter, Δ/a, varies from 0 to 0.178 for the aluminosilicate system, but is 0.201 for the dehydrated pahasapaite mineral. This is clearly the greatest distortion yet observed for this framework. This combination of new framework chemistries (Be/P/O vs. Si/Al/O) and the increased distortion is being examined by theory and experiment with respect to the effect on guest molecules.

Surface mobility of Re2O7 in the system Re7+Pd0γ-Al2O3

Abstract Effect of hydration on the mobility of Re 2 O 7 on γ-Al 2 O 3 in the system Re 7+ Pd 0 γ- Al 2 O 3 was investigated by TPR and XRD techniques. In samples dried at 110 °C, reduction of Re 2 O 7 takes place with T max = 47°C i.e., at a temperature lower by ca. 400 °C than that observed in the absence of metallic palladium. The low-temperature migration of Re 2 O 7 to preexisting Pd 0 crystallites, where reduction takes place, is consistent with the Tamman temperature of Re 2 O 7 . This mechanism is supported by the observed interaction between Pd and Re, manifested by the decreased formation of palladium hydride and by changes in the X-ray diffraction spectra. Dehydroxylation of alumina at 500 °C causes interaction between Re 2 O 7 and Al 2 O 7 to occur, hindering the surface migration. In dehydrated samples most of the rhenium heptoxide is reduced away from Pd crystallites above 400 °C.

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

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

Copolymer modification of nylon-6,6 with 2-methylpentamethylenediamine

Polyamides were prepared in which part or all of the hexamethylenediamine (HMD) in nylon-6,6 was replaced by its isomer, 2-methylpentamethylenediamine (MPMD). The intermediate compositions are considered random copolymers between the two homopolymers (66 and MPMD-6). The crystalline properties of these polymers were studied by differential scanning calorimetry, X-ray diffraction and optical microscopy. It was concluded that the two kinds of units, 66 and MPMD-6, do not co-crystallize. In copolymers containing large fractions of HMD, the MPMD units are excluded from the crystals. With appropriate thermal treatment, MPMD-6 can crystallize with two different unit cells and a variety of spherulitic morphologies. The two crystal forms can be distinguished by their crystallization, melting and optical properties. The crystallization kinetics of the copolymers are slower than those of the homopolymers, which is associated with the slower rate of nucleation formation rather than the transport ability. This is consistent with the dielectric results, which showed that the glass transition temperatures of the different copolymers are similar. It was found that MPMD-6 can be quenched into an amorphous state. The properties of the amorphous portions were further investigated through dielectric relaxations. Although, the incorporation of MPMD tends to shift the dielectric gamma relaxation towards higher temperatures, the total effects of variations on chemical structure were much smaller than those which pertain to the crystalline phase.

Kinetic measurements and XRD investigations in situ of a V2O5-SiO2 catalyst for the vapor-phase oxidation of tetrahydrofuran to γ-butyrolactone

Abstract The kinetics of the oxidation of tetrahydrofuran (THF) to γ-butyrolactone over a V2O5-SiO2 catalyst have been examined. An isothermal, fixed-bed reactor, operating at atmospheric pressure and temperatures between 170 and 215 °C was used to obtain reaction rate data. X-Ray diffraction measurement in situ shows significant proportion of V(IV) on carrying out the oxidation of THF on the catalysts. Reduction in H2 at temperatures above 170 °C gives rise to V4O9, which is similar to the effect when the reduction is effected by THF. The rate data have been correlated by a rate equation developed on the basis of a two-step redox mechanism.

Entrapment and controlled release of xenon in Cd2+-exchanged zeolite rho

Thermal relocation of Cd2+ ions and framework flexibility are used to effect the entrapment and controlled release of xenon from the pores of Cd2+-exchanged zeolite rho.

Encapsulation of hydrogen in cadmium-exchanged zeolite rho; temperature-programmed diffusion studies

Cadmium-exchanged zeolite rho encapsulates 161 µmol g–1 of hydrogen at 100 °C and 620 µmol g–1 at 200°C, (in 2 h and at 1 atm), the highest ever observed for any zeolite at 1 atm; hydrogen-exchanged zeolite rho, however, shows negligible uptake of hydrogen, providing evidence that the cadmium ion is directly responsible for hydrogen encapsulation.

Studies on the effect of temperature on the unit cell parameters of selected polymers

Abstract The dependence of the unit cell parameters of several polymers on temperature from room temperature to the melting point was measured by x-ray diffraction using an automated Rigaku θ-θ diffractometer. The anisotropic thermal expansion was characterized, and in the cases of polytetrafluoroethylene and 66 nylon, additional changes were found below their melting points. These data were combined with the results of pressure-volume-temperature experiments to calculate heats of fusion using the Clapeyron equation.

Flexibility of the zeolite rho framework. Neutron powder structural characterization of Ca-exchanged zeolite rho

Neutron powder diffraction structural characterizations of dehydrated samples of Ca-exchanged zeolite rho show the most pronounced distortions of any zeolite framework yet observed and correspond to a 21% decrease in the lattice volume.