Robert S. Maxwell

The interaction of Vanadia with sepiolite. Structural studies by 51V solid-state NMR and raman spectroscopy

Abstract The interaction of vanadium with sepiolite, a layered magnesium silicate used to stabilize cracking catalysts against metal contaminants in crude oils, is investigated. To this end, field-dependent 51 IV wideline, magic-angle-spinning (MAS), and nutation NMR studies, together with X-ray diffraction (XRD) and laser Raman spectroscopy (LRS) results, are reported on sepiolite samples impregnated with a solution of vanadyl napthenate in benzene. These results are discussed in connection with benchmark NMR data of crystallographically well-defined model compounds in the M9O-V 2 O 5 system. When heated near 760°C in the presence of steam, sepiolite decomposes into enstatite (MgSiO 3 ) and silica. If the hydrothermal treatment is repeated in the presence of vanadium, a disordered microcrystalline phase of α-Mg 2 V 2 O 7 forms. Steam treatment at higher temperatures results in the formation of β-Mg 2 V 2 O 7 . In addition, all samples examined contain an amorphous surface phase. Based on 51 V nutation NMR spectroscopy, this amorphous constituent is inferred to have a structural environment similar to β-Mg 2 V 2 O 7 . An MgV 2 O 6 -like, distorted octahedral environment is present only in samples with very high surface coverages and can be suppressed by choosing multiple-step over single-step impregnation.

Thermodynamic Study on Dynamic Water Vapor Sorption in Sylgard-184

The dynamic and equilibrium water vapor sorption properties of Sylgard-184, a commercially available poly(dimethylsiloxane) elastomer (PDMS), were determined via gravimetric analysis from 30 to 70 °C. Described here is a methodology for quantitatively assessing how water vapor diffuses and ad/absorbs into polymeric materials that are traditionally considered hydrophobic. PDMS materials are frequently chosen for their moisture barrier properties; our results, however, demonstrate that moisture is able to penetrate the material over a range of temperatures and humidities. The sorption values measured here ranged from ca. 0.1 to 1.4 cm(3) (STP) H(2)O/g Sylgard. The isotherms exhibited sigmoidal character and were fit to a triple mode sorption model. Asymptotic behavior at low water activities was characterized using a Langmuir type adsorption model, linear behavior was fit to a Henrys law type dependence, and the convex portion at higher activities was fit with good agreement to Parks equation for pooling or clustering. The thermal dependence of these sorption modes was also explored and reported. The dynamics of the sorption process were fit to a Fickian model and effective diffusivities are reported along with corresponding activation energies. The diffusivity values measured here ranged from ca. 0.5 to 3.5 × 10(-5) cm(2)/s depending on the temperature and relative humidity. The concentration dependence of the diffusivity showed a direct correlation with the three modes of uptake obtained from the isotherms. Corrections to the diffusivities were calculated using existing models that take into account adsorption and pooling.

Intermediate-range order in phosphorus-selenium glasses. Constraints from 31P and 77Se NMR spectroscopy

Abstract 31P-77Se spin echo double resonance and in situ high-temperature 77Se nuclear magnetic resonance (NMR) experiments allow the quantification of the ratio of phosphorus-bonded to non-phosphorus-bonded Se atoms in phosphorus-selenium glasses. When interpreted in conjunction with the phosphorus speciations known from 31P NMR, these data allow deductions about the intermediate-range order in these glasses. The results indicate that Se 2 2 , Se=PSe 3 2 , PSe 3 2 and P-P bonded microstructures are mostly randomly linked in these glasses, with a slight preference for the formation of P-Se-P bridges. The experimental selenium speciations strongly disagree with various hypothesized structures based on isolated PSen units, molecular P4Sen clusters, and extended phosphorus-enriched domains with P-Se-P links.

Effects of phase impurities on vanadium-sepiolite interactions

X-Ray diffraction (XRD), laser Raman spectroscopy (LRS), and solid-state 51V nuclear magnetic resonance (NMR) have been used to investigate the interaction of vanadium with a sample of sepiolite in the presence of phase impurities. After thermal and hydrothermal treatments, sepiolite decomposes forming enstatite (MgSiO3). In contrast, in the presence of (⩽ 10%) calcite impurities (as found in naturally occurring samples from Nevada) sepiolite decomposes forming diopside (MgCaSiO6). When the thermal and hydrothermal treatments are repeated in the presence of 5% V, Mg-containing vanadates are not observed, and Ca2V2O7 is the only heat-stable V compound identified. These results suggest that Ca2V2O7 formation is thennodynamically favored over its Mg analog. Formation of this compound is probably responsible for the preservation of cracking activity observed when cracking V-contaminated gas oil with dual-function cracking catalysts (DFCC) containing sepiolite-calcite granules to passivate V impurities.

DEPOLYMERIZATION KINETICS OF GLASS-FORMING CHALCOGENIDE MELTS. COHERENCE TRANSFER NMR STUDIES OF PHOSPHORUS SELENIDE

Abstract The kinetics of the depolymerization of a glass-forming phosphorus selenide melt containing 48 at.% phosphorus have been studied using one- and two-dimensional coherence transfer spectroscopic methods. The depolymerization leads to the creation of molecular P4Se3. Two-dimensional spectroscopy indicates that these units are in slow exchange with the residual polymeric matrix, and corresponding rate constants are extracted from temperature dependent one-dimensional coherence transfer experiments. The results are quantitatively consistent with previous temperature-dependent NMR lineshape studies, yielding an activation energy of 109 kJ/mol

Solid State NMR Measurements for Preliminary Lifetime Assessments in λ-Irradiated and Thermally Aged Siloxane Elastomers

Siloxanes have a wide variety of applications throughout the aerospace industry which take advantage of their exceptional insulating and adhesive properties and general resilience. They also offer a wide range of tailorable engineering properties with changes in composition and filler content. They are, however, subject to degradation in radiatively and thermally harsh environments. We are using solid state nuclear magnetic resonance techniques to investigate changes in network and interfacial structure in siloxane elastomers and their correlations to changes in engineering performance in a series of degraded materials. NMR parameters such as transverse ( T{sub 2}) relaxation times, cross relaxation rates, and residual dipolar coupling constants provide excellent probes of changes crosslink density and motional dynamics of the polymers caused by multi-mechanism degradation. The results of NMR studies on aged siloxanes are being used in conjunction with other mechanical tests to provide insight into component failure and degradation kinetics necessary for preliminary lifetime assessments of these materials as well as into the structure-property relationships of the polymers. NMR and MRI results obtained both from high resolution NMR spectrometers as well as low resolution benchtop NMR screening tools will be presented.