Calcium amidotrihydroborate: A hydrogen storage material

Article abstract of DOI:10.1002/anie.200702240

(Figure Presented) No foaming at the mouth here: A calcium (II) derivative of ammonia-borane, Ca(NH₂BH₃)₂ (see picture; green Ca, red O, blue N, yellow B, gray C, small green H), has thermal properties that are quite different from those of ammonia-borane. Ca(NH ₂BH₃)₂ releases hydrogen over a temperature range of 100 to 170°C without foaming.

Full text of DOI:10.1002/anie.200702240

Angewandte
Chemie
DOI: 10.1002/anie.200702240
Hydrogen Storage
Calcium Amidotrihydroborate: A Hydrogen Storage Material**
Himashinie V. K. Diyabalanage, Roshan P. Shrestha, Troy A. Semelsberger, Brian L. Scott,
Mark E. Bowden, Benjamin L. Davis, and Anthony K. Burrell*
The promise of the hydrogen economy, while great, will only
be realized when many major scientific challenges are over-
come. One of primary challenges is the need to find a safe and
economic method for storing and transporting H2. Conven-
tional storage systems include classical high-pressure tanks,
insulated liquid-hydrogen systems, and storage in hydro-
carbons.[1] Chemical methods, such as the use of metal
hydrides (for example, MgH2),[2] imides (such as LiNH2),[3]
organic frameworks (Zn4O(1,4-benzenedicarboxylate)),[4,5]
alkali-metal tetrahydroboride (such as LiBH4),[6] alanates
(for example, NaAlH4),[7] and chemical hydrides all show
promise. There are several advantages of chemical H2 storage
over compressed H2; an important advantage is that the H2
density available in chemical systems significantly outweighs
even liquid H2.
and the steady release of H2 in the solid state is spread over
much wider temperature and time ranges (see Supporting
Information).
The reaction of NH3BH3 with calcium hydride in THF
leads to calcium amidotrihydroborate as the bis(thf) adduct
(2) in excellent yield (Scheme 1). Compound 2 loses THF
Scheme 1. Synthesis of 1 from calcium hydride and ammonia–borane.
Among the potential candidates for effective chemical H2
storage, ammonia–borane (NH3BH3) has garnered much
interest owing to its ideal combination of low molecular
weight and high H2 storage capacity of 19.6 weight%, which
exceeds the current capacity of gasoline.[8–15] The chemical
nature of the NH3BH3 molecule, with both hydridic and protic
hydrogen atoms in close proximity, provides a unique
environment for the release of H2. Ultimately, NH3BH3 can
be dehydrogenated completely, forming ceramic BN, but only
at temperatures in excess of 5008C.[15–19] Recently, catalytic
hydrogen release from NH3BH3 has also achieved significant
attention.[11,20–23]
once it is removed from solution, giving 1. In most samples of
1, small amounts of THF are still detected, yet the amounts
present are well below stoichiometric ratios. Calcium amido-
trihydroborate is relatively stable in air, with no observable
decomposition over two days in the solid state or in solution.
The boron chemical shift in NH3BH3, 1, and 2 are
effectively identical in the 11B NMR spectrum, with the only
1
À
difference being the B H coupling constants ( JBH = 93.0 Hz
for NH3BH3 and 1JBH = 86.0 Hz for 1). The characteristic
1
differences between 1 and 2 are observed in the H NMR
Herein we report a derivative of NH3BH3, Ca(NH2BH3)2
(1), in which a new covalent bond between the nitrogen atom
of NH3BH3 and calcium has been formed. Compound 1 has
thermal properties that differ significantly from solid
NH3BH3, and undergoes loss of H2 without significant
foaming, which is a common problem with NH3BH3. The H2
loss from 1 is also not as exothermic as in ammonia–borane,
spectrum. Two signals are observed for 1 in [D8]THF, namely
a quartet at d = 1.39 ppm and a broad singlet at d =
À0.64 ppm, corresponding to BH3 and NH2 protons, respec-
tively. On the other hand, the 1H NMR of 2 in C6D6 shows two
multiplets, at d = 3.58 ppm and 1.38 ppm, indicating the
presence of thf ligands, in addition to two broad peaks
corresponding to the BH3 and NH2 protons (at about d =
1.22 ppm and 0.88 ppm, respectively).
In the single-crystal X-ray analysis (Figure 1) of the
crystals obtained from a THF/hexane mixture containing 1,
two thf ligands coordinate to each Ca(NH2BH3)2 molecule,
forming 2. Intermolecular interactions of 2.368 Š are
observed between the CaII ion and hydridic hydrogens of
BH3. This gives rise to an extended structure with a chain-like
arrangement in the solid state structure of 2.
The thermal properties of 1 are of primary importance if it
is to be useful as a H2 storage material. The thermal
gravimetric analysis (TGA) of 1 shows several weight-loss
events (Figure 2). The initial loss, which begins at 708C, is
attributed to THF. This loss was also observed at this
temperature by mass spectrometry (MS). The amount of
THF that remains in 1 varies depending on the surface area of
the sample. Elemental analysis of a typical sample of 1, dried
[*] H. V. K. Diyabalanage, R. P. Shrestha, T. A. Semelsberger, B. L. Scott,
M. E. Bowden, B. L. Davis, Dr. A. K. Burrell
Materials Physics and Applications Division
Los Alamos National Laboratory
Mail Stop J514
Los Alamos NM 87545 (USA)
Fax: (+1)505-665-9905
E-mail: Burrell@lanl.gov
[**] We thank Tom Autrey (PNNL) for very helpful discussions. We
would like to acknowledge the support of the IPHE collaboration
“Combination of Amine Boranes with MgH2 & LiNH2 for High
Capacity Reversible Hydrogen Storage” in the development of this
work and the US Department of Energy, Office of Energy Efficiency
and Renewable Energy for providing funding.
Supporting Information for this article is available on the WWW
under http://www.angewandte.org or from the author.
Angew. Chem. Int. Ed. 2007, 46, 8995 –8997
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
8995
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Communications
Figure 3. Thermal hydrogen-release profile for 1. The colored surface is
fitted to the black data points.
1708C. A small amount of hydrogen release (< 0.3 equiva-
lents of H2) is observed at 908C. However, prolonged heating
at 908C does not result in further release of hydrogen or any
other detectible mass loss (for thermal gravimetric analysis
and differential scanning calorimetry results, see the
Supporting Information). GC experiments indicate that
1.1 equivalents of H2 can be released from 1 at 1208C,
2.4 equivalents at 1508C, and (3.6 Æ 0.2) equivalents at 1708C.
This is a 90% yield at 1708C based upon the four equivalents
Figure 1. X-ray crystal structure of 2, with thermal ellipsoids set at
35% probability, and hydrogen atoms attached to thf carbon atoms
omitted for clarity. A section of the chain (top) showing interactions
involving the hydrogen atoms from the NH2BH3 unit, and (bottom)
the full chain with hydrogen and carbon atoms omitted for clarity.
B yellow, Ca green, O red, N blue, C gray, H small green open circles.
Selected distances(Š): Ca(1)···Ca(1a)=3.147(6), Ca(1)-
B(1)=2.349(12), Ca(1)-B(1a)=2.829(13), Ca(1)-N(1)=2.069(7),
Ca(1)-N(1b)=2.362(7), Ca(1)-O(1)=2.438(4). Symmetry operations:
a: Àx+1, Ày, Àz+1; b: Àx+1, Ày, Àz.
À
À
)
of H2 expected from the combination of four protic ( NH2
À
and four of the six hydridic ( BH3) hydrogen atoms, and
corresponds to a gravimetric concentration of 7.2 weight%
H2. Both the rate and volume of H2 release for 1 varies with
temperature, which is in contrast to solid NH3BH3, where any
temperature above 1208C results in the release of about two
equivalents of H2. This is presumably a consequence of the
endothermic hydrogen release from 1, which was estimated to
have an enthalpy of 3.5 kJmolÀ1 after accounting for the
overlapping THF endotherm in the DSC trace (Supporting
Information). Hydrogen release from NH3BH3 is exothermic
with an enthalpy of À22.5 kJmolÀ1 for the release of the first
equivalent of H2.[17,18]
After about four equivalents of hydrogen are released
from 1 by the facile combination of the boron hydrogen atoms
and ammonia hydrogen atoms, the remaining hydrogen atoms
in the residue ascribed to be Ca(NBH)2 must be lost in an
alternative mechanism at higher temperatures. As the sample
is heated to from 170–2458C, more hydrogen release is
observed by mass spectrometry. Elemental analysis of the
residue that remained after heating 1 to 1708C indicates an
elemental composition of CaN2B2H2C0.2. Elemental analysis
of samples heated to greater than 2458C show steady loss of
nitrogen. Unfortunately, the residue that is formed as 1 is
heated is amorphous and X-ray data cannot be obtained.
Further studies on the high temperature decomposition of the
residue formed after hydrogen release from 1 are underway.
In summary, we have described a straightforward route to
obtain the calcium derivative of ammonia borane, which
possesses different thermal properties that ammonia–borane
and has a lower propensity for borazine release. As a
potential hydrogen storage material, 1 offers some advan-
tages in that the thermal hydrogen release does not suffer
from the induction period present in ammonia–borane.
Figure 2. Thermogravimetric analysis of 1; initial loss of THF begins at
708C. THF loss is also accompanied by a small release of H2 before
the major hydrogen release begins at 1208C.
under a stream of argon, shows less than 10% THF
remaining. Upon heating to 1208C, the onset of the most
significant mass loss is observed. Analysis of the mass loss was
carried out by mass spectrometry, and hydrogen was quanti-
fied below 1708C by calibrated gas chromatography (GC;
Figure 3). The majority of the mass loss is hydrogen gas. Small
amounts of ammonia and borazine can be detected, but these
contribute to less than 0.1% of the weight change below
8996 www.angewandte.org
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8995 –8997
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Angewandte
Chemie
Keywords: ammonia–borane · calcium · gas chromatography ·
hydrogen storage · thermal gravimetric analysis
Experimental Section
.
1: Ammonia–borane (0.20 g, 6.48 mmol) was dissolved in tetrahy-
drofuran (30.0 mL). This mixture was slowly added to a suspension of
calcium hydride (0.14 g, 3.24 mmol) in tetrahydrofuran (20.0 mL) and
allowed to stir overnight. The solvents were removed under vacuum
to obtain the white solid product, Ca(NH2BH3)2 (0.31 g). X-ray
quality crystals of the THF adduct 2 were obtained from a THF/
hexane solution of 2 at À228C. Compound 1 was obtained by vacuum-
drying 2 overnight at room temperature. Yield of 1: 96%; m.p.: 98–
1008C (dec); 1H NMR (300.13 MHz, [D8]THF, 228C, TMS): d = 1.39
(q, 1J(H,B) = 86.0 Hz, 3H, BH3), À0.64 ppm (br.s, 2H, NH2);
11B NMR (96.29 MHz, [D8]THF, 228C): d = À23.62 ppm (q, 1J-
(B,H) = 86.0 Hz, BH3); IR (neat): n˜ = 2977.7 & 2879.6 (N-H), 2196.7
& 2146.2 (B-H), 1533.0, 1459.8, 1260.6, 1168.1, 1041.8, 1010.5, 917.2,
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.
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Crystal Data for 2: C8H26B2CaN2O2, Mr = 244.01, orthorhombic,
a = 7.910(2), b = 17.570(4), c = 4.6700(9) Š, V= 649.0(2) Š3, T=
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,
m(MoKa) = 0.47 mmÀ1, 4461 reflections measured (2qmax = 50.08), and
681 unique (Rint. = 0.0889). The final residuals were R(I>2s; all
data) = 0.0906; 0.1161, and wR(I>2s; all data) = 0.2729; 0.2835, with
residual electron density = 0.484 eŠÀ3. The data were collected on a
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(SHELXL-97). The chain structure was disordered over two posi-
tions, and each atom was refined at one-half normal occupancy. The
hydrogen atom positions were found on the difference map, and
refined with fixed isotropic temperature factors (0.10 Š2). The amine
hydrogen atom positions were restrained. All non-hydrogen atoms
were refined anisotropically. CCDC-646993 contains the supplemen-
tary crystallographic data for this paper. These data can be obtained
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Received: May 21, 2007
Revised: August 21, 2007
Published online: October 26, 2007
Angew. Chem. Int. Ed. 2007, 46, 8995 –8997
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 8997
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