Direct synthesis and NMR characterization of calcium alanate

Brookhaven National Laboratory, New York, New York, United States
Journal of Alloys and Compounds (Impact Factor: 3). 10/2007; 446. DOI: 10.1016/j.jallcom.2006.12.050
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In this work, we present a new synthesis path and characterization results of the alanate, Ca(AlH_4)_2. We have synthesized for the first time, calcium alanate, directly from starting mixtures of AlH_3 and CaH_2 using mechanosynthesis. Ca(AlH_4)_2 has been identified using magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared (FTIR) measurements.

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Available from: Jason Graetz, Jun 23, 2015
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    • "The interest for extensive research on alanates was triggered through the pioneering work of Bogdanovic and Schwickardi [3], which demonstrated fast desorption and reversibility of NaAlH 4 through doping of NaAlH 4 by Ti bearing compounds. Since then research is being conducted on many other complex light weight alkali metal alanates such as LiAlH 4 , Ca(AlH 4 ) 2 , Mg(AlH 4 ) 2 , LiMg(AlH 4 ) 3 and borohydrides like NaBH 4 , LiBH 4 , Zn(BH 4 ) 2 , etc. [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]. Among the various alanates, Mg(AlH 4 ) 2 has drawn significant attention in the recent time due to its high theoretical hydrogen capacity of 9.3 wt%. "
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    ABSTRACT: Magnesium alanate (Mg(AlH4)2) has been synthesized by mechanochemically activated metathesis reaction involving MgCl2 and NaAlH4. Its dehydrogenation kinetics and storage capacity has been studied by using Sievert's type apparatus. We have obtained dehydrogenation capacity of 2.7 wt% H2 from Mg(AlH4)2+2NaCl during the first decomposition step at and 1.1 wt% H2 during second step decomposition at . Efforts were carried out to reduce NaCl content from the product using Soxhlet extraction technique. The Soxhlet extracted product gives the total dehydrogenation capacity of 4.7 wt% H2. To enhance the storage capacity, we have synthesized a complex hydride consisting of mixture: xMg(AlH4)2+yNaAlH4(0<x<1,y⩾1). In the alanate mixture 0.5Mg(AlH4)2+NaAlH4, the dehydriding temperature of NaAlH4 gets lowered by (from to ) with 4 times faster desorption kinetics. The total hydrogen liberated in 180 min from NaAlH4+0.5Mg(AlH4)2(+NaCl) mixture at has been observed to be 3.7 wt% H2.
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    ABSTRACT: 11B NMR spectroscopy has been employed to identify the reaction intermediates and products formed in the amorphous phase during the thermal hydrogen desorption of metal tetrahydroborates (borohydrides) LiBH4, Mg(BH4)2, LiSc(BH4)4, and the mixed Ca(AlH4)2-LiBH4 system. The 11B magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) spectral features of the amorphous intermediate species closely coincide with those of a model compound, closo-borane K2B12H12 that contains the [B12H12]2- anion. The presence of [B12H12]2- in the partially decomposed borohydrides was further confirmed by high-resolution solution 11B and 1H NMR spectra after dissolution of the intermediate desorption powders in water. The formation of the closo-borane structure is observed as a major intermediate species in all of the metal borohydride systems we have examined.
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    ABSTRACT: The reaction of AlMe3 with (dme)2Ca(NPh2)2 in a solvent mixture of toluene and THF yields the addition product [(thf)6Ca] [Me3Al–NPh2]2 (1). In solution the anion of this solvent-separated complex 1 [δ(27Al)=153] partly dismutates into ([AlMe2(NPh2)2]− [δ(27Al)=179] and [AlMe4]− [δ(27Al)=155]. The [Me3Al–NPh2]− anion shows a strong backdonation of negative charge from the nitrogen atom into one phenyl ring whereas the other phenyl group forms hyperconjugative bonds between the p-orbital of the ipso-carbon atom and the σ∗(Al–C) orbital leading to a slight elongation of one of the Al–C bonds.
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