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Alanates

The Experts below are selected from a list of 1839 Experts worldwide ranked by ideXlab platform

Alexandre Alexakis – 1st expert on this subject based on the ideXlab platform

  • improvements and applications of the transition metal free asymmetric allylic alkylation using grignard reagents and magnesium Alanates
    Advanced Synthesis & Catalysis, 2015
    Co-Authors: David Grassi, Alexandre Alexakis

    Abstract:

    Two new N-heterocyclic carbene (NHC) ligands have been synthesized and employed in the transition metal-free asymmetric allylic alkylation (AAA) mediated by Grignard reagents and magnesium Alanates. The employment of these ligands showed high yields and improved regio- and enantioselectivity in the formation of tertiary and quaternary stereocenters. Moreover, the low catalyst loading (up to 0.3 mol%) and high scalability (up to 10 mmol) of this improved methodology provide a convenient access to biologically active compounds and synthetically valuable intermediates.

  • Improvements and Applications of the Transition Metal‐Free Asymmetric Allylic Alkylation using Grignard Reagents and Magnesium Alanates
    Advanced Synthesis & Catalysis, 2015
    Co-Authors: David Grassi, Alexandre Alexakis

    Abstract:

    Two new N-heterocyclic carbene (NHC) ligands have been synthesized and employed in the transition metal-free asymmetric allylic alkylation (AAA) mediated by Grignard reagents and magnesium Alanates. The employment of these ligands showed high yields and improved regio- and enantioselectivity in the formation of tertiary and quaternary stereocenters. Moreover, the low catalyst loading (up to 0.3 mol%) and high scalability (up to 10 mmol) of this improved methodology provide a convenient access to biologically active compounds and synthetically valuable intermediates.

K. J. Gross – 2nd expert on this subject based on the ideXlab platform

  • thermal properties characterization of sodium Alanates
    Journal of Alloys and Compounds, 2005
    Co-Authors: Daniel E. Dedrick, Michael P. Kanouff, B C Replogle, K. J. Gross

    Abstract:

    High energy density hydrogen storage is a critical technology requirement in a hydrogen-based energy infrastructure. Although there are no current storage methods that meet desired energy density goals for vehicular hydrogen storage, complex metal hydride based systems are among the most promising. These materials form compounds with hydrogen under appropriate conditions and release hydrogen by thermal decomposition. The complex hydride, sodium alanate, is particularly useful due to its favorable reversibility. Thermal properties characterization of sodium alanate has been performed at Sandia National Laboratories to gain a detailed understanding of how complex hydrides will behave in a storage system. Thermal properties were investigated using the thermal probe method (ASTM D5334). Custom test hardware was designed and built to accommodate the complex decomposition and recombination of sodium alanate. Thermal conductivity and thermal wall resistance were determined by utilizing analytical and numerical data analysis methods. The thermal conductivity of sodium alanate was found to vary by more than 90% with changes in phase composition and hydrogen gas pressures between 1 and 100 atm. The quality of thermal contact between the alanate and the vessel wall was characterized numerically for various pressures and phase compositions. The contact resistance is high for all states, indicating poor contact between the material and the vessel wall.

  • the effects of titanium precursors on hydriding properties of Alanates
    Journal of Alloys and Compounds, 2003
    Co-Authors: K. J. Gross, E H Majzoub, S Spangler

    Abstract:

    An overview is presented of recent advances in the development of new and improved Alanates for applications and in the fundamental understanding of how Ti-doping enhances hydrogen absorption. Sample materials were produced using approaches based on direct-synthesis and dry Ti-doping methods. It is desirable to introduce Ti through non-reactive processes to avoid the hydrogen capacity loss that occurs through the formation of inactive byproducts (for example Na–halide from the decomposition of Ti–halides and Na–oxides from the decomposition of Ti–alkoxides). We show, for the first time, that Alanates can be Ti-doped using TiH2 or through indirect-doping by pre-reacting TiCl2 with LiH. Both methods result in enhanced kinetics. However, improved rates were achieved only after a prolonged activation period of about a 10 cycles, suggesting that cycling leads to Ti diffusion and substitution into the alanate lattice which provides the mechanism through which Ti-doping enhances kinetics. Thus, the reactive decomposition of Ti–halide and alkoxide precursors in the doping process serves an important but not necessarily required function.

  • catalyzed Alanates for hydrogen storage
    Journal of Alloys and Compounds, 2002
    Co-Authors: K. J. Gross, G J Thomas, Craig M. Jensen

    Abstract:

    The discovery that hydrogen can be reversibly absorbed and desorbed from complex hydrides (the Alanates) by the addition of catalysts has created an entirely new prospect for lightweight hydrogen storage. Unlike the interstitial intermetallic hydrides, these compounds release hydrogen through a series of decomposition/recombination reactions e.g.: NaAlH4⇔1/3Na3AlH6+2/3Al+H2⇔NaH+Al+3/2H2. Initial work resulted in improved catalysts, advanced methods of preparation, and a better understanding of the hydrogen absorption and desorption processes. Recent studies have clarified some of the fundamental material properties, as well as the engineering characteristics of catalyst enhanced sodium alanate. Phase transitions were observed real-time through in situ X-ray powder diffraction. These measurements demonstrate that the decomposition reactions occur through long-range transport of metal species. SEM imaging and EDS analysis verified the segregation of aluminum to the surface of the material during decomposition. The equilibrium thermodynamics of decomposition have now been measured down to room temperature. They show a plateau pressure for the first reaction of 1 bar at 33°C, which suggest that, thermodynamically, this material is ideally suited to on-board hydrogen storage for fuel cell vehicles. Room temperature desorption with slow but measurable kinetics has been recorded for the first time. Studies at temperatures approaching that found in the operation of PEM fuel cells (125–165°C) were performed on a scaled-up test bed. The bed demonstrated surprisingly good kinetics and other positive material properties. However, these studies also pointed to the need to develop new non-alkoxide based catalysts and doping methods to increase the capacity and reduce the level of hydrocarbon impurities found in the desorbed hydrogen. For this reason, new Ti–Cl catalysts and doping processes are being developed which show higher capacities and improved kinetics. An overview of the current state-of-the-art will be presented along with our own studies and the implications for the viability of these materials in on-board hydrogen storage applications.

Maximilian Fichtner – 3rd expert on this subject based on the ideXlab platform

  • M alanate—a material for reversible hydrogen storage?
    , 2020
    Co-Authors: Maximilian Fichtner, Olaf Fuhr, Oliver Kircher

    Abstract:

    Magnesium alanate was synthesized in a metathesis reaction of magnesium chloride and sodium alanate followed by purification. The material obtained was sufficiently pure and it was investigated by X-ray diffraction (XRD) and by thermogravimetry (TG) and mass spectrometry (MS) of the evolved gas, respectively. Thermal analysis showed a decomposition with a release of hydrogen proceeding in two major steps. Measured in vacuum, the peak decomposition temperature of the first step was found to be 163 8C and the residue at 200 8C consisted of MgH and Al which continues to release hydrogen and transforms into an Al Mg /Al mixture at higher temperatures. 2 32 In the first decomposition step 6.6 wt.% of hydrogen was released. To enhance the kinetics of the decomposition, magnesium alanate was doped with a titanium based promoter and ball milled for up to 100 min, resulting in a significantly reduced peak decomposition temperature. First results on determining thermodynamic properties indicate equilibrium desorption pressures in the range of AB 5 compounds. In total, the promising properties determined so far have inspired further investigations on the thermodynamics, kinetics and reabsorption properties of the compound.  2002 Elsevier Science B.V. All rights reserved.

  • wide line solid state nmr characterizations of sodium Alanates
    Journal of Physical Chemistry C, 2009
    Co-Authors: M H W Verkuijlen, Maximilian Fichtner, Jan P M Van Bentum, Wiebke Lohstroh, A P M Kentgens

    Abstract:

    Pure NaAlH4, TiCl3-doped NaAlH4, and pure Na3AlH6 were characterized using 1H, 23Na, and 27Al solid-state NMR. The signal intensities and linewidths of 1H NMR spectra using several spin echo sequences and backprediction of a single pulse experiment were compared to find the optimal experiment to measure wide-line NMR spectra of the Alanates. Second moment calculations using the Van Vleck equations compared with fits of the dipolar coupling line broadening confirm that NaAlH4 has a rigid crystal lattice. On the other hand, for Na3AlH6, a narrowing of the proton and aluminum lineshape was observed, indicating a fast rotational motion of AlH6 clusters at room temperature. A broadening of the 1H and 27Al linewidth was observed upon lowering the temperature. This process is successfully described using thermally activated rotational jumps of AlH6 clusters assuming a fast rotational motion around one single C4 axis and a slower rotation around the other two C4 axes with an activation barrier of Ea = 25 kJ/mol a…

  • Thermal coupling of a high temperature PEM fuel cell with a complex hydride tank
    International Journal of Hydrogen Energy, 2009
    Co-Authors: P. Pfeifer, Clemens Wall, O. Jensen, Horst Hahn, Maximilian Fichtner

    Abstract:

    Abstract Sodium alanate doped with cerium catalyst has been proven to have fast kinetics for hydrogen ab- and de-sorption as well as a high gravimetric storage density around 5 wt%. The kinetics of hydrogen sorption can be improved by preparing the alanate as nanocrystalline material. However, the second decomposition step, i.e. the decomposition of the hexahydride to sodium hydride and aluminium which refers to 1.8 wt% hydrogen is supposed to happen above 110 °C. The discharge of the material is thus limited by the level of heat supplied to the hydride storage tank. Therefore, we evaluated the possibilities of a thermal coupling of a high temperature PEM fuel cell operating at 160–200 °C. The starting temperatures and temperature hold-times before starting fuel cell operation, the heat transfer characteristics of the hydride storage tanks, system temperature, fuel cell electrical power (including efficiency) as well as alanate kinetics were varied by system modelling with gPROMS®. The kinetics of the hydride decomposition was found to have a major influence on the performance of the system. A cumulative output of 0.8 kWh was reached in a test run.