Journal of Alloys and Compounds 446–447 (2007) 264–266
Direct synthesis and NMR characterization of calcium alanate
Houria Kabboura,∗, Channing C. Ahna, Son-Jong Hwangb,
Robert C. Bowman Jr.c, Jason Graetzd
aDivision of Engineering and Applied Science, California Institute of Technology, Pasadena, 91125 CA, USA
bDivision of Chemistry and Chemical Engineering, California Institute of Technology, Pasadena, 91125 CA, USA
cJet Propulsion Laboratory, California Institute of Technology, Pasadena, 91109 CA, USA
dDepartment of Energy Science and Technology, Brookhaven National Laboratory, Upton, 11973 NY, USA
Received 29 October 2006; accepted 13 December 2006
Available online 20 December 2006
In this work, we present a new synthesis path and characterization results of the alanate, Ca(AlH4)2. We have synthesized for the first time,
calcium alanate, directly from starting mixtures of AlH3and CaH2using mechanosynthesis. Ca(AlH4)2has been identified using magic angle
spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared (FTIR) measurements.
© 2007 Elsevier B.V. All rights reserved.
Keywords: Hydrogen storage materials; Mechanochemical synthesis; Nuclear resonances
Complex hydrides have recently shown promise for high
and those that are composed of light elements are of particular
technological interest. Calcium alanate is one such system as it
has a total capacity of 7.8wt.% hydrogen. However, dehydro-
genation takes place over several steps, and only the first two
reactions can be considered for practical applications. These
reactions are listed below with the approximate temperature
Ca(AlH4)2→ CaAlH5+Al + 1.5H2
CaAlH5→ CaH2+Al + 1.5H2
CaH2→ Ca + H2
If we consider the first two reactions (which occur at tech-
nologically viable temperatures), 5.9wt.% hydrogen would be
lowing the reaction (4CaH2+2AlCl3→Ca(AlH4)2+2CaCl2)
[2,3]. In those works, attempts to purify the alanate were unsuc-
E-mail address: email@example.com (H. Kabbour).
cessful. While little interest has been shown in this compound
since the initial work, more recent work has studied alternative
synthesis paths. Wet chemical synthesis under inert processing
conditions has been the recent approach to producing nearly
pure calcium alanate. Typically, a solvent adduct is first formed,
Ca(AlH4)2·x (solvent) (solvent=THF, DEE,...) [4–8], the sol-
vent is then removed by moderate heating under vacuum.
Mechanochemical synthesis  has also been reported using
mixtures of NaAlH4and CaCl2. This approach also results in
the successful synthesis of calcium alanate but NaCl is present
as a by-product.
The structure of the solvent adducts has been determined
using monocrystal X-rays diffraction  while the structure
of the nearly pure calcium alanate, once the solvent has been
the use of DFT calculations to suggest different hypothetical
structures . The most stable of these proposed structures was
found to be similar to the CaB2F8structure (space group Pbca).
The focus of this paper is to present a new simple synthesis
path for calcium alanate by the mechano-reaction of CaH2and
AlH3, and the characterization of the products using IR and
The synthesis of calcium alanate was performed using both the metathesis
reaction (Eq. (4))  or the direct synthesis (Eq. (5)) for the first time using
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved.
H. Kabbour et al. / Journal of Alloys and Compounds 446–447 (2007) 264–266
For the first synthesis path, a mixture of 2.27g of anhydrous CaCl2(Alfa
Aesar, 96%) and 1.518g of LiAlH4(Alfa Aesar, 95%) were used without fur-
ther purification. These compounds were placed in an 80mL steel vessel, with
five 0.5in. diameter steel balls. The vessel was sealed using a rubber gasket
in an Argon glove box. The Argon filled vessel was then placed in a Fritsch-
pulverisette 6 planetary mill. The mixture was subsequently milled at 500rpm
for either a continuous 3h mix, or for a 3h mix during which the system was
allowed to cool for 5min every 15min. We did not notice significant differences
between those two procedures.
For the second path, a mixture of 0.618g of CaH2(Alfa Aesar, 98%) and
0.882g of AlH3was ball-milled in the same set up than previously at different
speeds (from 200 to 500rpm) and different times (from 1 to 10h). We used ?-
AlH3originally synthesized by the Dow Chemical Company in the early 1970s
and provided by Brookhaven National Laboratory.
Solid state FTIR spectra were recorded using a Nicolet 860 Magna
series FTIR spectrometer, under ambient conditions and in air over the range
Solid-state magic angle spinning nuclear magnetic resonance (MAS-NMR)
measurements were performed using a Bruker Avance 500MHz spectrometer
equipped with a Bruker 4mm CPMAS probe. MAS-NMR samples were loaded
in a 4mm ZrO2rotor and sealed with a tightly fitting kel-F cap inside an Argon
glove box in order to avoid any contact with air. The spectral frequencies were
500.23 and 130.25MHz for1H and27Al nuclei, respectively. A typical MAS
spectrum was recorded under MAS spinning rate of 12–14kHz after a short
Use of compressed air was found not to be influential in our experimental setup.
NMR shifts were externally referenced to 1.0M of Al(NO3)3aqueous solution
at 0ppm for27Al nucleus.
3. Results and discussion
Wet chemistry routes can lead to calcium alanate formation.
However, the real formulation of the synthesized compound is
Ca(AlH4)2·x (solvent). Some solvent always remains within the
of the solvent is difficult to achieve without alanate decompo-
sition. A narrow temperature window is generally necessary
to purify the compound without decomposition [4,5], making
forward “dry” method without by product is of great interest
because potential candidates for hydrogen storage such as the
calcium alanate and other complex hydrides are typically syn-
thesized using solvents.
In this work, a new synthesis path for calcium alanate forma-
portions (Eq. (5)). Different parameters have been used for the
mechano-synthesis including milling speed and duration. The
products have been characterized using FTIR and27Al MAS
As a baseline for our spectroscopic results, we have used cal-
cium alanate prepared using the mechano-chemical metathesis
reaction of LiAlH4and CaCl2. The final products are a mix-
ture of the calcium alanate with LiCl.27Al NMR and FTIR
spectra are presented in Figs. 1 and 2, respectively. The shift
in the NMR spectrum at 105.6ppm corresponds to previously
reported values for calcium alanate, and is in good agreement
Fig. 1.27Al NMR spectra of the calcium alanate (105.6ppm) obtained with the
with reported NMR data . LiAlH4and NaAlH4spectra are
also shown in Fig. 1 for comparison. The peak of the lithium
alanate (95.8ppm), used as starting material, is clearly distinct
from the calcium alanate peak, and is not evident in the final
product spectrum. The FTIR spectrum consists mainly of two
wide bands, of which the ∼1800cm−1band corresponds to
the Al–H vibrations. Again, our data are consistent with pre-
viously reported values [5,8]. X-ray diffraction indicated the
presence of LiCl peaks but the quality of the diffractogram
did not allow for further conclusions on a possible alanate
IR data are presented in Fig. 1 for the alanate prepared by
direct synthesis. The position of the different bands compares
well with the metathesis reaction IR data. However, IR data do
not allow us to clearly differentiate calcium alanate peaks from
those of aluminium hydride, which might be among the final
products if the reaction is not complete. Thus, NMR data are
also important in order to distinguish the two phases. These
are presented in Fig. 3 for a selection of syntheses prepared at
Fig. 2. FTIR spectra of the metathesis reaction products (black line) and the
direct synthesis products (dotted line). We can note the similarities. The band
around 1800cm−1corresponds to the Al–H bonds vibrations.
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H. Kabbour et al. / Journal of Alloys and Compounds 446–447 (2007) 264–266
Fig. 3.27Al NMR spectra of the direct synthesis reaction products for different
ball-milling times at 500rpm: 1h (dotted line), 2h (black line) and 3h (grey
Parameters used for direct synthesis and effect on the ratio of the final products
500rpm using various ball-milling time (1, 2 and 3h). In con-
and ∼6ppm, corresponding to the calcium alanate and an alu-
minium hydride, respectively. The latter is clearly identified as
?-AlH3, in agreement with NMR work reported in the litera-
ture . We also note the presence of a weak intensity peak
at around ∼1600ppm corresponding to aluminium metal. This
aluminium metal can either come from the ?-AlH3that is partly
decomposed or from the decomposition during the ball-milling.
Nevertheless, when reasonable times of ball-milling are used,
the ratio of aluminium metal in the final product is negligible.
The relative peak area by integrating the different peaks
that contribute to the spectra can be calculated. In this way,
we can determine the ratio between each species that contains
aluminium. The results for different syntheses are presented
Table 1. With the Dow Chem ?-AlH3, the best synthesis (i.e.
highest ratio for calcium alanate), is obtained when the 500rpm
speed is used for 2h. We note improvement compared to that of
the 1h ball-milled sample at the same speed. We note also that
note a small decrease in the proportion of calcium alanate and,
in parallel, slightly more aluminium metal is observed. If the
time of ball-milling is increased up to 6h at the same speed, the
calcium alanate phase decomposes almost completely.
While we are exploring other parameters in order to improve
the alanate ratio, we note that so far, the use of lower milling
speed and longer ball-milling have not improved the reaction
ratio. Also, very low speeds (200rpm) with up to 10h milling
time led to no reaction.
We have synthesized the calcium alanate, Ca(AlH4)2for the
first time by direct synthesis using CaH2and AlH3ball-milled
together. The compound was characterized by means of NMR
and FTIR. The synthesis using freshly made ?-AlH3can poten-
tially improve the reaction rate and this work is currently in
This work at Caltech was supported by DOE through DE-
Jet Propulsion Laboratory, which is operated by the California
Institute of Technology under contract with the NASA. This
work was partially supported by DOE through Award Number
DE-AI-01-05EE11105. The NMR facility at Caltech was sup-
ported by the National Science Foundation (NSF) under Grant
Number 9724240 and partially supported by the MRSEC Pro-
gram of the NSF under Award Number DMR-0080065. This
work was supported at BNL by the DOE under contract DEA-
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