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A series of amorphous silicate materials with the general formula Nax+2yM³⁺xSi1-xO2+y (M³⁺ = Al, Mn, Fe, Y) were studied. Samples were synthesized by a precipitation reaction at room temperature. The results indicate that the ion-exchange capacity (IEC) decreases as follows:  Al > Fe > Mn > Y. Additionally, the IEC increases with increasing aluminum concentration. Structural studies show that the relative amount of octahedrally coordinated aluminum increases with increasing Al content, as does the total amount of AlO4 species increases. The data suggest that the IEC value of these amorphous aluminosilicates is dependent on the tetrahedrally coordinated aluminum. Regeneration of the Al-silicate with acetic acid does not decrease the IEC significantly.

DC745 is a commercially available silicone elastomer consisting of dimethyl, methyl−phenyl, and vinyl−methyl siloxane monomers cross-linked with a peroxide vinyl specific curing agent. It is generally considered to age gracefully and to be resistant to chemical and thermally harsh environments. However, little data exist on the radiation resistance of this commonly used silicone elastomer. We report static ¹H NMR studies of residual dipolar couplings in DC745 solid elastomers subject to exposure to ionizing γ radiation. ¹H spin-echo NMR data shows that with increasing dose the segmental dynamics decrease is consistent with radiatively induced cross-linking. ¹H multiple quantum NMR was used to assess changes in the network structure and observed the presence of a bimodal distribution of residual dipolar couplings, 〈Ωd〉, that were dose dependent. The domain with the lower 〈Ωd〉 has been assigned to the polymer network while the domain with the higher 〈Ωd〉 has been assigned to polymer chains interacting with the inorganic filler surfaces. In samples exposed to radiation, the residual dipolar couplings in both reservoirs were observed to increase, and the populations were observed to be dose dependent. The NMR results are compared to differential scanning calorimetry (DSC) and a two-step solvent swelling technique. The solvent swelling data lend support to the interpretation of the NMR results, and the DSC data show both a decrease in the melt temperature and the heat of fusion with cumulative dose, consistent with radiative cross-linking. In addition, DSC thermograms obtained following a 3 h isothermal soak at −40 °C showed the presence of a second melt feature at Tm ~ −70 °C consistent with a network domain with significantly reduced segmental motion.

Structural speciation of glasses in the systems PbO−B2O3−SiO2, PbO−B2O3−Al2O3−SiO2, and PbO−Al2O3−SiO2 were studied using solid state ²⁹Si, ²⁷Al, ¹¹B, and ²⁰⁷Pb nuclear magnetic resonance (NMR) and Raman spectroscopy. Application of these methods provided insight into the role of Al2O3 incorporation in the lead borosilicate glass networks. The general composition range studied was (PbO)x[(B2O3)1-z(Al2O3)z]y(SiO2)y where x = 0.35, 0.5, and 0.65, y = (1 − x)/2 and z = 0.0, 0.5, and 1.0. Additional insight was obtained via ²⁷Al 2D-3QMAS experiments. The ²⁰⁷Pb spin−echo mapping spectra showed a transition from ionic (Pb²⁺) to covalently bound lead species with increased PbO contents in the borosilicate glasses. The addition of aluminum to the glass network further enhanced the lead species transition, resulting in a higher relative amount of covalent lead bonding in the high PbO content alumino-borosilicate glass. The number of BO4 units present in the ¹¹B MAS NMR decreased with increasing PbO contents for both the borosilicate and the alumino-borosilicate glass systems, with the addition of aluminum further promoting the BO3 species. A deshielding of the ¹¹B chemical shifts and the ²⁷Al chemical shifts with increased lead contents (independent of changes in the quadrupolar coupling) was also observed for both glass systems and was attributed to an increasingly homogeneous glass structure. The ²⁹Si spectra of the borosilicate and alumino-borosilicate glasses showed a downfield shift with increased PbO concentrations representing incorporation of Pb into the silicate and aluminosilicate networks. The Raman spectra were characterized by increased intensities of Si−O−Pb peaks and decreased intensities of Q³ peaks with increased PbO contents and showed no evidence of BO3 or BO4 ring species. Both the NMR and the Raman data point toward systems where lead is increasingly incorporated into the B2O3−SiO2 and the B2O3−SiO2−Al2O3 networks at high PbO concentrations, with the addition of Al2O3 enhancing the trend.

Alkali silicotitanate ternary phases have been studied for a wide variety of applications including catalysis, selective ion exchange, and ceramic nuclear waste form materials. We report here the synthesis and structural characterization of Rb2TiSi6O15 and K2TiSi6O15, structural analogues to SNL-A (Cc-Cs2TiSi6O15). The Rb analogue is the first reported ternary oxide, the rubidium silicotitanate phase. Both the Rb and the K analogues were synthesized nearly pure by hydrothermal treatment of oxide precursor mixtures seeded with SNL-A and could not be synthesized without the addition of the seed. Rietveld refinement gave good structural models in the Cc space group with unit cell parameters:  Rb2TiSi6O15, V = 1357.9 ų, a = 12.736(2) Å, b = 7.3392(3) Å, c = 15.061(3) Å, β = 105.29(2)° and K2TiSi6O15, V = 1318.5 ų, a = 12.570(2) Å, b = 7.2534(3) Å, c = 15.082(3) Å, β = 106.49(3)°. The two isostructural phases were also characterized by thermogravimetry and ²⁹Si NMR. Unit cell parameters, melting temperatures, and ²⁹Si chemical shifts were compared to those of the Cs analogue and related to variation in alkali radius (Cs > Rb > K). The volume, the a-axis, and the b-axis increase linearly with alkali radius, which is directly related to a “relaxation” of the folds of the corrugated silicate layers that lie in the xy plane.

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 Si2O5^2- 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 dynamics and orientations of interlayer terephthalate (TA) anions in mixed-anion hydrotalcite-like compounds [Mg1-x Alx(OH)2](AyB1-y)x/2·nH2O, where A = p-C6D4(COO)2^2- and B = CO3^2-] were investigated by ²H variable-temperature solid-state nuclear magnetic resonance (NMR) spectroscopy. Two dominant 003 d spacings, 7.6 and 14.2 Å, were observed by X-ray diffraction (XRD) in these samples, suggesting that there were at least two interlayer environments. Over the range of layer charges studied (0.37 ≥ x ≥ 0.29), the intensity of the 7.6 Å XRD peak increased with decreasing x, while the intensity of the 14.2 Å peak increased with increasing x. Analysis of ²H NMR line shapes indicated that the motion of some of the TA anions in every sample could be described as rotational diffusion (RD) about the C−COO^- axis at temperatures ranging from 245 to 355 K, with the remainder being essentially immobile. The fraction of TA anions exhibiting rotation (XRD), as estimated by ²H NMR, was positively correlated with the TA content of the interlayer and thus seemed to correspond to the fraction of TA molecules in the 14.2 Å phase. A large difference in XRD was observed between compounds having similar layer charge but different TA mole fractions. At temperatures of ≤330 K, XRD increased with temperature. A secondary minimum in XRD, however, was observed for each of the compounds at temperatures between 330 and 350 K. We conclude that the presence of CO3^2- anions in the interlayer regions, rather than layer charge, is the dominant factor affecting the dynamics and orientation of interlayer TA anions.

Chiral syntheses and separations of both intermediate and final compounds are currently extremely important areas of research, especially for the chemical, pharmaceutical, agrochemical, and biomedical industries. We are investigating the synthesis of new inorganic porous materials for enantiomeric selectivity. We present a study of inorganic zinc phosphate crystalline phases templated with the chiral molecule d-glucosamine hydrochloride. The crystallization of these phases is highly dependent on reaction time, temperature, and stoichiometry. Resultant new phases include a mesoporous phase and a layered phase. Data indicates that the walls of the new 32 Å sized mesoporous phase are crystalline and ordered. Characterization techniques applied include powder X-ray diffraction, TEM, and ³¹P MAS NMR.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

WHEN the butt end of a vibrating tuning-fork is placed on a table, the fork will give out, under certain conditions, in addition to its fundamental, a note which is of frequency one octave higher than the note heard when the fork is held to the ear. This fact has been stated by Ellis (Helmholtz, “The Sensations of Tone” second English edition, p. 54), but apparently no explanation has been put forward.