Michael Mortimer

Aluminosilicates with varying alumina–silica ratios: synthesis via a hybrid sol–gel route and structural characterisation

Aluminosilicates with varying Al2O3:SiO2 molar ratios (3:1, 3:2, 3:3 and 3:4) have been synthesized using a hybrid sol-gel route using boehmite sol as the precursor for alumina and tetraethyl orthosilicate (TEOS) as the precursor for silica. The synthesis of boehmite sol from aluminium nitrate, and its use as the alumina precursor, is cost effective compared to alkoxide precursors. Structural aspects, including bonding and coordination, are studied in detail for samples calcined in the temperature range 400-1400 °C using both NMR and FTIR spectroscopy: the results are correlated with phase formation data (spinel and high temperature phases) obtained from XRD and thermal analysis. FTIR results show a broadening of peaks at 800 °C indicating a disordered distribution of octahedral sites caused by crosslinking between AlO6 octahedral and SiO4 tetrahedral units prior to the formation of mullite. (27)Al MAS NMR spectra are consistent with a progressive decrease in the number of AlO6 polyhedra with increasing temperature corresponding to Al in these units being forced to adopt a tetrahedral coordination due to the increasing presence of similarly coordinated Si species. XRD results confirm the formation of pure mullite at 1250 °C for a 3Al2O3:2SiO2 system. At 1400 °C, phase pure mullite is observed for all compositions except 3Al2O3:SiO2 where α-Al2O3 is the major phase with traces of mullite. The synthesis of aluminosilicates through a hybrid sol-gel route and the detailed insight into structural features gained from spectroscopic and diffraction techniques contributes further to the development of these materials in applications ranging from nanocatalysts to high-temperature ceramics.

Water in poly(ethylene oxide): differential scanning calorimetry and nuclear magnetic resonance experiments

Non-freezable D2O in partially crystalline poly(ethylene oxide) shows residual deuterium quadrupolar splitting. Extrapolation of enthalpies of eutectic melting to zero yields an average hydration number of 0.9.

Ab initio calculation and NMR spin–echo measurement of 19F chemical shielding in the alkali-metal fluorides

Experimental values of 19F chemical shielding in the series of anhydrous alkali-metal fluorides LiF to CsF are reported. An NMR spin–echo technique is used for the measurements and the results are shown to compare favourably with others, in particular those determined by magic-angle spinning (MAS) techniques, reported in the literature. Ab initio methods of calculating 19F chemical shielding in these primarily ionic materials are then discussed in terms of simplified model systems. For all the alkali-metal fluorides, values of absolute 19F chemical shielding are calculated which are significantly less than the free fluoride ion value. In the specific cases of LiF and NaF reasonable agreement between ab initio calculation and experiment is achieved.

Deuterium nuclear magnetic resonance spin echo spectroscopy in molecular crystals

The effects of quadrupole, dipole, chemical shielding and spin–lattice interactions on the properties of the deuterium spin echo response to a 90°–τ–90°90° pulse sequence in powdered molecular crystals are investigated. Procedures are delineted for the selective measurement of these interactions. It is argued that these experiments should contribute to the realisation of the potential of deuterium n.m.r. spectroscopy for studying orientational disorder and motion in molecular solids.

An investigation of the local environments of tin in tin-doped α-Fe2O3

The extended x-ray absorption fine structure (EXAFS) recorded from tin-doped α-Fe2O3, prepared by the mechanical milling of tin dioxide and α-Fe2O3 and by the hydrothermal processing of iron- and tin-containing precipitates, can be interpreted in terms of a model in which tin occupies both substitutional octahedral sites and the interstitial octahedral sites in the corundum-related α-Fe2O3 structure. The EXAFS and 119Sn Mossbauer spectra suggest that structural models derived from x-ray powder diffraction data do not adequately describe the complexity of the local environment of tin in α-Fe2O3. In particular, the EXAFS and 119Sn Mossbauer spectra recorded from materials made by mechanical milling show evidence of more disorder. The 119Sn Mossbauer spectra also indicate that the degree of order in the materials made by both methods is far from perfect and that the microstructural defects are highly sensitive to tin content and the preparative method.

The effect of the NMR spin-echo sequence Py(90°)-τ-Py(180°) in solids containing different dipolar-coupled spin species: applications to KHF2 AND CsHF2

Abstract An experimental investigation of the effect of the NMR sequence P y (90°)-τ- P y (180°) in solids containing different types of dipolar-coupled spin species is reported. The experimental results suggest that the decay of the maximum spin-echo amplitude as a function of pulse spacing depends only on the strength of the homonuclear dipolar interactions. Polycrystalline samples of KHF 2 and CsHF 2 are used as model systems.

Reorientation of D2O in concentrated aqueous solutions of lithium chloride studied by nuclear magnetic relaxation

Deuteron spin–lattice T1 and spin–spin T2 relaxation times have been measured at 10 MHz in aqueous (D2O) solutions of lithium chloride as a function of concentration (R= 3.5 to 6.3 mol D2O per mol LiCl) and temperature (175–320 K). These measurements are analysed using an empirical Cole–Davidson distribution of reorientational correlation times τ2 for D2O. The mean values of τ2 are found to be represented by 2=(1.24 ± 0.27)× 10–13 s exp {(701 ± 21) K/(T–T0)} with T0 values of 137 K (R= 3.7), 129 K (R= 4.8) and 125 K (R= 6.3). At low temperatures, the 2 are similar to the corresponding shear relaxation times s, measured between 149–173 K, while at 298 K, 2, the dielectric relaxation time D and the neutron diffusional correlation time τ0 have similar values. It is, therefore, concluded that the reorientation of the D2O molecules in these solutions is governed by the structural fluctuations associated with the glass transition. T1 measurements at 298 K, in the range R= 3 to 10, are interpreted to show that these solutions have similar structures to the corresponding glasses at 100 K : small clusters of Li+(H2O)4Cl– with excess water incorporated interstitially and having dynamic properties remarkably similar to those of bulk water. Previous T1 measurements for water (D2O) are reanalysed and the 2 shown to be dominated by a thermodynamic singularity at Ts= 228 K.

Dependence of precursor characteristics on low temperature densification of sol-gel aluminium titanate

Abstract Aluminium titanate precursor gels have been prepared by the sol–gel route using monohydroxy aluminium oxide (boehmite) sol and titanium isopropoxide. The influence of calcination temperature of the precursors on phase formation, densification and microstructure development in sol–gel aluminium titanate have been studied. Studies of pre-calcined precursor gels at 600, 800, 1000 and 1200°C show a significant increase in density on raising the calcination temperature from 600 to 1000°C. The treatment at 1000°C favours a phase composition most conducive to low temperature densification of aluminium titanate at 1350°C.

19 F nuclear magnetic resonance spin–lattice relaxation study of polycrystalline lithium trifluoromethanesulfonate, including comments on the influence of cross-correlation

A 19F nuclear magnetic resonance spin–lattice relaxation investigation of polycrystalline lithium trifluoromethanesulfonate, CF3SO3Li, as a function of temperature, and at resonance frequencies of 24.05 and 55.93 MHz, is reported. The 19F spin–lattice relaxation is found to be non-exponential, to varying degrees, over the whole of the temperature range studied, 205–340 K, and this is attributed to the influence of cross-correlation effects. Values of T1(19F) are determined from the initial gradients of appropriate magnetisation recovery plots: this method of measurement is discussed in some detail. The 19F spin–lattice relaxation is attributed to the reorientational motion of the CF3 groups about their threefold symmetry axes and the activation energy for this type of motion is found to be 24.40 ± 0.69 kJ mol–1. The form of the experimental 19F magnetisation recovery at 24.05 MHz and at a temperature where cross-correlation effects are particularly marked, i.e. close to the T1(19F) minimum, is shown to compare favourably with that predicted on the basis of the theoretical treatment of Hilt and Hubbard.

X-Ray absorption and photoelectron spectroscopic investigation of vanadium interaction with NH4+- and Eu3+-exchanged zeolite-Y following calcination and steam treatment

X-Ray absorption near-edge structure, extended X-ray absorption fine structure and X-ray photoelectron spectroscopy have been used to investigate the interaction of vanadium with both NH4+- and Eu3+-exchanged, dealuminated zeolite-Y. In both cases, samples were subjected to either calcination or steam treatment at temperatures in the range 100 to 800 °C. Following deposition of vanadium, in the form of vanadyl(IV) acetylacetonate, there is evidence of a weak interaction between vanadium and the zeolite-Y framework. Under the conditions used in the work, treatment at increasing temperature, either in air or steam, results in the complete oxidation of V(IV) to V(V) at temperatures in the range 300 to 500 °C. At temperatures exceeding 500 °C, V(V) is present in tetrahedrally-distorted vanadium oxide species containing terminal VO bonds, which are highly dispersed but bonded to the zeolite framework. These results show that the migration of vanadium into the zeolite-Y framework is not dependent on the presence of steam, nor is there any detectable influence of the exchanged Eu3+ cations.