Search

Solid-state NMR and infrared (IR) measurements are combined with computational analysis to examine the structures of major sites formed by grafting C[H.sub.3]Re[O.sub.3] onto amorphous silica-alumina and their relationships with olefin metathesis activity of this material. The results have suggested simple modifications in catalyst preparation, which has led to higher selectivity, less leaching and efficient use of the Re complex.

Hadamard-based pulse sequences are used to determine (super 1)H{(super 13)C}-resolved diffusion coefficients and spin-relaxation time constants for the chemically similar components of an aqueous solution, which exhibit significant spectral overlap in their (super 1)H NMR spectra. The resulting resolved signal decay curves can be used with existing postacquisition analysis techniques to increase significantly the resolution of similar or, in the case of complete spectral resolution, identical decay constants.

A combination of solid-state nuclear magnetic resonance (NMR) spectroscopy, X-ray diffraction, and quantum chemical and empirical force field modeling is used to establish the structure of a novel molecularly ordered two-dimensional (2D) silicate framework in a surfactant-templated mesophase. The intraframework structure of a novel non-3D-crystalline lamellar silicate-C16NMe2Et(super+) surfactant mesophase is determined by the 2D DQ29Si NMR.

The synthesis of new mesoporous silica structures using low molecular weight nonionic ethylene oxide surfactants and poly(alkylene oxide) block copolymers in acid media is described. Results showed that the selected surfactant species not only influence the structure of ordered mesoporous silica, each surfactant type also plays a role in the formation of a specific mesostructured silica phase. It was also observed that the silica formed were highly ordered and thermally stable even in boiling water.

Synchrotron x-ray and neutron powder diffraction studies of the crystal structures of siliceous zeolite ferrierite reveal the presence of a Si-O-Si unit with a bond angle of 170 degrees. Crystal structures predicted by lattice energy minimization method agree with the experimental observations. A bond angle correlation helps assign the well-resolved 29Si magic angle spinning NMR spectrum. This assignment helps determine Si/Al ratios in the multiple T-site system and analyze the complex spectra of various aluminosilicate ferrierite samples.

Inorganic polymer binders (IPBs) are synthesized by the activation of aluminosilicate precursors with an alkaline solution such as sodium hydroxide. This paper studies the relationship between the composition, structure, and solubility of sodium aluminosilicate hydrates (N–A–S–(H)), the primary binding phase in IPBs. It was found that changing the aqueous Si/Al ratio had little effect on N–A–S–(H) Si/Al ratio, but small changes in the aqueous Si/Na ratio led to substantial changes in N–A–S–(H) Si/Al ratio. Early N–A–S–(H) products were found to be X‐ray amorphous, but a rapid transition to the crystalline phase faujasite occurred after several weeks of aging. The transition of the solid amorphous phase to faujasite was accompanied by a rapid drop in aqueous Si(IV) and Al(III) concentrations. Solubility products were determined, temporally, for the N–A–S–(H) before and after the transition to faujasite and represent new contributions to the literature, particularly for the amorphous state. The results presented here provide fundamental insights that are needed for the development of kinetic and thermodynamic models that can establish phase balances and evolutions of IPBs across a range of precursor compositions and synthesis conditions. [ABSTRACT FROM AUTHOR]

A large-sample-volume nuclear magnetic resonance (NMR) spectroscopy probehead has been developed for the detection and characterization of low concentrations of [sup.29]Si species in aqueous solution. The approach described entails the use of a large-diameter radio frequency solenoid coil that permits substantially larger sample volumes to be investigated at moderate magnetic field strengths, compared with conventional NMR probehead configurations. In addition, difficulties presented by long [sup.29]Si [T.sub.1] relaxation times have been circumvented by using the DEFT NMR pulse sequence, which permits more rapid signal averaging. Through a combination of these hardware and methodological improvements, high-resolution [sup.29]Si NMR spectra have been obtained at 4.2 T ([sup.29]Si resonance frequency = 36.8 MHz) for an 800 [micro]M solution of 96% [sup.29]Si-enriched silicic acid, [H.sub.4]Si[O.sub.4] (pH [approximately equals] 8), with a signal-to-noise ratio of 16 and a line width of 31 Hz after 3 h of total measurement time.

The induction of phase transformations from the highly ordered p6mm mesostructure of SBA-15-type mesoporous silicas to mesostructured cellular foams or mesocellular foams (MCF) by the addition of large amounts of organic cosolvents were described. The MCF were made of uniform, large spherical cells interconnected by uniform windows to develop a continuous 3-D pore system. The well-defined MCF pore structure was accurately described by nitrogen sorption measurements.

The species concept in marine phytoplankton is defined based on genomic, morphological, and functional properties. Reports of intraspecific diversity are widespread across major phytoplankton groups but the impacts of this variation on ecological and biogeochemical processes are often overlooked. Intraspecific diversity is well known within coccolithophores, which play an important role in the marine carbon cycle via production of particulate inorganic carbon. In this study, we investigated strain-specific responses to temperature in terms of morphology, carbon production, and carbonate mineralogy using a combination of microscopy, elemental analysis, flow cytometry, and nuclear magnetic resonance. Two strains of the cosmopolitan coccolithophore E. huxleyi isolated from different regions (subtropical, CCMP371; temperate, CCMP3266) were cultured under a range of temperature conditions (10°C, 15°C, and 20°C) using batch cultures and sampled during both exponential and stationary growth. Results for both strains showed that growth rates decreased at lower temperatures while coccosphere size increased. Between 15°C and 20°C, both strains produced similar amounts of total carbon, but differed in allocation of that carbon between particulate inorganic carbon (PIC) and particulate organic carbon (POC), though temperature effects were not detected. Between 10°C and 20°C, temperature effects on daily production of PIC and POC, as well as the cellular quota of POC were detected in CCMP3266. Strain-specific differences in coccolith shedding rates were found during exponential growth. In addition, daily shedding rates were negatively related to temperature in CCMP371 but not in CCMP3266. Despite differences in rates of particulate inorganic carbon production, both strains were found to produce coccoliths composed entirely of pure calcite, as established by solid-state 13C and 43Ca NMR and X-ray diffraction measurements. These results highlight the limitations of the species concept and the need for a trait-based system to better quantify diversity within marine phytoplankton communities. [ABSTRACT FROM AUTHOR]

Layered silicate surfactant mesophases were studied for molecularly ordered inorganic frameworks.