Mechanochemical preparation and investigation of properties of magnesium, calcium and lithium-magnesium alanates

Article abstract of DOI:10.1016/j.jallcom.2005.06.069

Ball milling of MgCl₂ and CaCl₂ with NaAlH ₄ or LiAlH₄ can be used for the preparation of magnesium, calcium and lithium-magnesium alanates in mixture with NaCl or LiCl. Using wet chemical separation methods, it

Full text of DOI:10.1016/j.jallcom.2005.06.069

Journal of Alloys and Compounds 407 (2006) 78–86
Mechanochemical preparation and investigation of properties of
magnesium, calcium and lithium–magnesium alanates
∗
´
¨
M. Mamatha, B. Bogdanovic , M. Felderhoff, A. Pommerin,
∗∗
W. Schmidt, F. Schuth , C. Weidenthaler
Max-Planck-Institut fu¨r Kohlenforschung, Kaiser-Wilhelm-Platz 1, D-45470 Mu¨lheim an der Ruhr, Germany
Received 22 March 2005; received in revised form 13 June 2005; accepted 13 June 2005
Available online 22 August 2005
Abstract
Ball milling of MgCl2 and CaCl2 with NaAlH4 or LiAlH4 can be used for the preparation of magnesium, calcium and lithium–magnesium
alanates in mixture with NaCl or LiCl. Using wet chemical separation methods, it was possible to obtain these alanates in nearly pure
state. The alanates were characterized by X-ray diffractometry, solid-state 27Al NMR and IR spectroscopy and thermovolumetric (TV)
and differential scanning calorimetry (DSC) measurements. Mg(AlH4)2 dissociates thermally in one step to MgH2, Al and hydrogen; at a
higher temperature, MgH2 and Al transform to Mg–Al alloy and hydrogen. Thermal dissociations of Ca(AlH4)2 and of LiMg(AlH4)3 (in
mixture with NaCl or LiCl) proceeds in several steps, of which the first two can be assigned to the formation of CaH2 and of a MgH2/LiH
mixture, respectively, in addition to Al and H2. Possible intermediates of these two steps are CaAlH5 and LiMgAlH6. Higher temperature
dissociations include formation of MgH2 (LiH) and Ca–Al alloys from CaH2, CaH2 and Al, respectively. Upon ball milling of MgCl2 or
CaCl2 with NaAlH4 or LiAlH4 in the presence of Ti catalysts, only the thermal dissociation products of the expected alanates are obtained.
This indicates that dehydrogenation discharge of earth alkali metal alanates can be catalyzed by Ti. According to DSC measurements, the
thermodynamic stability of Mg(AlH4)2 (ꢀH = 1.7 kJ/mol) is too low for the purpose of reversible hydrogen storage. Determination of ꢀH
values for the second, endothermal step of calcium and lithium–magnesium alanate dissociations gave values of around 31.6 and 13.1 kJ/mol,
respectively.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Alanates; Decomposition; DSC; Hydrogen storage
1. Introduction
Stimulated by this question, in the last years, considerable
research work has been done on preparation and characteriza-
tion of alkaline earth metal alanates, a group of compounds,
which in the past, had attracted little attention. Along these
lines, the highest yet known reversible hydrogen storage
capacity of more than 8 wt.% was reported by Zaluska et al.
[3] by employing lithium beryllium hydrides. A detailed pro-
cedure for the preparation of solvent free magnesium alanate
has been described by Fichtner and Fuhr [4]. In addition, the
compound has been characterized by physical methods such
as, thermal and catalyzed hydrogen release [5]. Fichtner et
al. [6] have performed a quantum chemical calculation of the
Mg(AlH4)2 structure on the density functional theory (DFT)
level. The calculated pattern was found to be congruent with
experimental data. A series of partially as yet unknown Mg,
The reversible dissociation of Ti-doped sodium alanate
(NaAlH4) is presently widely investigated as a potential
hydrogen storage system, which offers high capacity at mod-
erate temperatures [1,2]. With respect to this, the question
arises, if other than sodium-based alanates or other com-
plex hydrides, e.g. boranates can be applied for hydrogen
storage.
∗
∗∗
Corresponding author. Tel.: +49 208 306 2407; fax: +49 208 306 2980.
Co-corresponding author.
E-mail addresses: bogdanovic@mpi-muelheim.mpg.de
(B. Bogdanovic´), schueth@mpi-muelheim.mpg.de (F. Schu¨th).
0925-8388/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2005.06.069

M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
79
Ca and Sr alanates have been obtained by Dymova et al.
[7–10] by application of the ball milling method. Novel bar-
ium and strontium alanates have been discovered and their
structures elucidated by neutron diffraction by Akiba and co-
workers [11,12].
In context of our investigations on alkaline earth metal
alanates, we report here about the mechanochemical prepa-
ration and characterization of magnesium, calcium and
lithium–magnesium alanate via X-ray diffraction (XRD),
infrared spectroscopy (IR), solid-state nuclear magnetic res-
onance (NMR) and thermovolumetric (TV) and differential
scanning calorimetry (DSC) measurements. On the basis of
DSC measurements, an attempt was made to evaluate ther-
modynamic stabilities of the investigated alanates.
comparison with entries from the PDF-2 powder pattern
database.
NMR: The solid-state NMR spectra were measured on a
Bruker Avance 500 WB instrument using a 4 mm MAS
probe. The experimental conditions for 27Al MAS NMR
were 1 s recycle delay, 4000 scans, 13 kHz spinning rate
and 0.6 ␮s π/12 pulse and those for 23Na MAS NMR were
10 s recycle delay, 4000 scans, 12 kHz spinning rate and
0.9 ␮s π/8 pulse. The 27Al NMR spectra without MAS were
recorded with a spin-echo pulse sequence consisting of two
0.6 s pulses with a spacing of 200 s and an appropriate phase
cycling.
IR: A few milligrams of the samples were grinded with dry
KBr under Ar and pressed to pellets. Solid-state infrared
spectra of the alanates were recorded in the range of
400–4000 cm−1 at ambient conditions in air using a Nicolet
Magna-IRTM 750 spectrometer.
2. Experimental
DSC: The DSC measurements were done on a METTLER-
TOLEDO DSC 27HP instrument, using a fully automated
program for evaluation of DSC data. The peak areas of DSC
curves are expressed in mJ units. Before measurements, the
peak areas were calibrated on the basis of averaged heats of
fusion of In, Sn and Bi as standards.
All operations in the described experiments were per-
formed under argon as a protecting atmosphere, using a glove
box or Schlenk techniques.
Starting materials: NaAlH4 (Chemetall) was recrystallized
by addition of pentane to a NaAlH4 tetrahydrofurane (THF)
solution [13]. LiAlH4 (Aldrich) was purified by dissolv-
ing it in diethyl ether, filtering the solution and evapo-
rating the diethyl ether (purity 99% according to TV).
Anhydrous MgCl2, CaCl2 and TiCl3 (99%; Aldrich) were
used without further purification. A sample of colloidal
Ti nanoparticles (Ti*) has been kindly left to us by the
2.1. Magnesium alanate
2.1.1. Mechanochemical preparation of
Mg(AlH4)2 + 2Na(Li)Cl mixtures (Scheme 1)
0.567 g (10.5 mmol) of NaAlH4 or 0.399 g (10.5 mmol) of
LiAlH4 and 0.500 g (5.25 mmol) of MgCl2 were weighted in
the vial of the milling apparatus and the mixture was milled
for 3 h.
¨
group of Prof. Bonnemann [14,15]. Pentane and diethyl
ether were dried over K–Na alloy, toluene over sodium alu-
minum tetraethyl and THF was purified over magnesium
anthracene.
Ball milling: Retsch ball mill 200 MM (30 Hz), 25 ml
milling vial and two steel milling balls of 6.3 g each;
a description of the set-up and measuring of hydro-
gen evolution during ball milling are described in Ref.
[16].
2.1.2. Isolation of Mg(AlH4)2 from the mixture with
NaCl
Each one of the four batches of NaAlH4/MgCl2, 2:1
mixtures was milled for 3 h as described above to yield
altogether ∼8 g of a Mg(AlH4)2 + 2NaCl mixture. Sepa-
ration of Mg(AlH4)2 from NaCl was carried out using
the Soxhlet extraction method, described in Ref. [4]. The
Mg(AlH4)2 + 2NaCl mixture was suspended in 150 ml of
Et2O and extracted for several days using a Soxhlet appa-
ratus, which led to crystallization of the Mg(AlH4)2·Et2O
adduct from the solvent. The solvent was evaporated and
the solid Mg(AlH4)2·Et2O heated to 40 ◦C for 3–4 h in
vacuum. Upon further heating of the solid to 60 ◦C for
6 h in high vacuum, no loss of weight was detected
(yield ∼ 1 g).
XRD: The X-ray powder patterns for qualitative phase anal-
ysis were collected at room temperature on a Stoe STADI
P transmission diffractometer in Debye–Scherrer geome-
˚
try (Cu K␣1: 1.54060 A) with a primary monochromator
(curved germanium (1 1 1)) and a linear position sensitive
detector. For the in situ high-temperature experiments, a
capillary furnace and a 40◦ position sensitive detector were
attached to the diffractometer. Data were collected in the
range between 20 and 90◦ 2θ with a step width of 0.01◦
2θ. For the measurements, the samples were filled into Ø
0.5 mm glass capillaries (for the high-temperature measure-
ments, quartz-glass capillaries were used) in a glove box,
which were sealed to prevent contact with air. During the
in situ experiment, the sample was held under argon. The
sample was heated from room temperature to 470 ◦C in tem-
perature steps of 10 ◦C. The heating rate was 4 ◦C/min.
The measured patterns were evaluated qualitatively by
2.1.3. Hydrogen evolution during ball milling of MgCl2
with NaAlH4 in the presence of Ti* (Scheme 1)
0.567 g (10.5 mmol) of NaAlH4, 0.500 g (5.25 mmol) of
MgCl2 and 0.012 g (2 mol%) of Ti* were milled in the Retsch
mill equipped with a set-up for recording of hydrogen evolu-
tion [16].

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M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
2.2. Calcium alanate
2.3.3. Separation of LiMg(AlH4)3 from the mixture with
LiCl (following Ref. [18])
2.2.1. Mechanochemical preparation of
Ca(AlH4)2 + 2Na(Li)Cl mixtures (Scheme 2)
Ca(AlH4)2 + 2Na(Li)Cl mixtures were prepared in the
same way as the corresponding Mg analogs (Section 2.1),
starting from 0.486 g (9.0 mmol) of NaAlH4 or 0.342 g
(9.0 mmol) of LiAlH4 and 0.500 g (4.5 mmol) of CaCl2.
2.0 g of the LiMg(AlH4)3 + 2LiCl mixture was suspended
in 150 ml of diethyl ether and the suspension stirred for
50 min with a high-speed stirrer (∼2500 rpm). The precip-
itate of LiCl formed was separated by filtration and 200 ml of
toluene were added to the filtrate. During continuous removal
ofthesolventbyheatingonanoilbathat80 ◦C, LiMg(AlH4)3
began to crystallize from the solution. The suspension was
kept for 10–15 min at the same temperature. The plate-like
crystals were separated from the mother liquor by filtration,
washed with pentane and dried in vacuum (10−2 mbar) at
70 ◦C, yielding 0.9 g of LiMg(AlH4)3.
2.2.2. Wet chemical preparation and isolation of
(nearly) pure Ca(AlH4)2
Following Ref. [17], 1.00 g (9.01 mmol) of CaCl2 was sus-
pended in 100 ml of THF and stirred for 1 h with a high-speed
stirrer. The suspension was then added to a solution of 0.97 g
(18.0 mmol) of NaAlH4 in 200 ml THF and the mixture was
stirred for 1 h with a high-speed stirrer. The reaction mixture
was filtered. A portion of the clear filtrate was concentrated in
vacuum to ∼75 ml and thereupon, 200 ml of pentane added
under stirring, resulting in separation of Ca(AlH4)2·4THF
as a fine precipitate. The suspension was cooled in dry ice
for 1 h and then filtrated at low temperature, the precipitate
was washed with pentane and solid-dried for 4 h in vacuum
(yield = 1.3 g).
3. Results and discussion
3.1. Magnesium alanate
Magnesium alanate ether adducts have been formerly
prepared from magnesium halides and lithium or sodium
alanates in diethyl ether or THF [19–21]. Fichtner and Fuhr
succeeded in isolation of neat Mg(AlH4)2 via Soxhlet extrac-
tion followed by heating of the etherate to 90 ◦C in high
vacuum [4]. A mechanochemical preparation of magnesium
alanate–magnesium chloride mixtures by ball milling (Eq.
(1)) has been described by Dymova et al. [10].
2.3. Lithium–magnesium alanate
2.3.1. Mechanochemical preparation of the
LiMg(AlH4)3 + 2LiCl mixture (Scheme 3)
The LiMg(AlH4)3 + 2LiCl mixture was prepared in the
same way as the Mg(AlH4)2 + 2LiCl mixture (Section 2.1),
starting from 0.611 g (16.0 mmol) of LiAlH4 and 0.500 g
(5.25 mmol) of MgCl2.
b.m.
2MgH2 + AlCl3−→ 0.5Mg(AlH4)2 + 1.5MgCl2
(1)
We have found that Mg, Ca and Li–Mg alanate–Na(Li)Cl
mixtures can be prepared by ball milling of sodium or
lithium alanate in conjunction with the corresponding alka-
line earth metal chlorides (Schemes 1–3). In the case of the
Mg(AlH4)2–2NaCl mixture, isolation of the neat Mg(AlH4)2
was carried out according to Ref. [4].
The X-ray powder patterns of thus prepared
Mg(AlH4)2 + 2NaCl or 2LiCl mixtures (Scheme 1) are
shown in Fig. 1(a and b). The full width at half maximum
2.3.2. Ti-catalyzed dissociation of the
LiMg(AlH4)3 + 2LiCl mixture (Scheme 3)
A 3:1 mixture of LiAlH4, MgCl2 (1.10 g) and 2 mol% Ti*
was milled for 3 h. Upon heating of the product to 150 ◦C, no
hydrogen was evolved.
Scheme 1.
Scheme 2.

M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
81
Scheme 3.
(FWHM) of the reflections in the patterns point to particle
sizes of the Mg(AlH4)2 phase of 18 2 nm. Furthermore,
the presence of a high background between 20 and 30◦ 2θ
in the patterns indicates that a part of the material might be
present in X-ray amorphous form. Apart from this and apart
from the presence of diffraction lines of NaCl or LiCl, both
diagrams show reflections, which are in accordance with
those of Mg(AlH4)2 prepared from MgCl2 and NaAlH4
in ether [4,6,22]. Since in both patterns, no diffraction
lines belonging to the starting materials and only those of
Mg(AlH4)2, NaCl and LiCl can be observed, it is reasonable
to assume that the ball milling reaction proceeds until
completion. The XRD pattern of the Mg(AlH4)2 obtained
after separation from NaCl (Fig. 1(c)) is in accordance
with that from Refs. [4,6,23]. Weak absorption bands in the
IR spectra of the compound at 1262 and 2963 cm−1 show
that small amounts of the ether adduct are (still) present.
Mg(AlH4)2 + 2Na(Li)Cl mixtures and the neat compound
were further characterized by solid-state NMR spectroscopy.
All the 27Al NMR spectra show a pronounced doublet
with maxima at 169 and −1 ppm, which can be assigned
to the 27Al resonance signal of Mg(AlH4)2. The 27Al
NMR spectrum of Mg(AlH4)2 (Fig. 2) shows a line shape
characteristic of strong quadrupolar coupling. From the total
width of the line (26 kHz), a quadrupole coupling constant
of about 10 MHz can be deduced. The 23Na MAS NMR
spectrum of the Mg(AlH4)2 + 2NaCl (not shown) exhibits
a strong resonance line of NaCl (7.2 ppm) and a very small
one of NaAlH4 at −9.3 ppm.
The thermal dissociation behavior of Mg(AlH4)2 +
2Na(Li)Cl mixtures and of Mg(AlH4)2 was investigated by
in situ high-temperature X-ray diffraction (HTXRD), TV and
DSC measurements (Section 3.4). As evident from Fig. 3(a),
Fig. 2. Solid-state 27Al NMR spectrum of Mg(AlH4)2.
Fig. 3. The two-step thermal hydrogen evolution curves of: (a)
Mg(AlH4)2 + 2NaCl mixture and (b) neat Mg(AlH4)2 (- - -) temperature in
◦C and curve of hydrogen evolution recorded during ball milling of MgCl2
with NaAlH4 (molar ratio 1:2) in the presence of 2 mol.% of Ti* (c).
Fig. 1. X-ray powder patterns of: (a) Mg(AlH4)2 + 2NaCl; (b) 2LiCl mix-
tures; (c) Mg(AlH4)2 after separation from NaCl; the base diffraction lines
are those from the Refs. [4,6,22], PDF-data base number 47-980.

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M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
during ball milling of MgCl2 with NaAlH4 in the presence
of 2 mol% Ti* [15,16,23] at ambient temperature, hydrogen
evolution takes place (Fig. 3(c)), giving the amount of hydro-
gen expected according to the Scheme 1. The product after
milling displays an X-ray diffraction pattern identical with
that of the Mg(AlH4) + 2NaCl mixture after being heated to
150 ◦C (Fig. 4(b)). This indicates a Ti*-catalyzed dissociation
of the in situ formed Mg(AlH4)2.
3.2. Calcium alanate
Ca(AlH4)2·(THF)x has been earlier prepared from CaH2
and AlBr3 or AlCl3 in dimethyl ether/THF or THF [24–27] or
from CaCl2 and NaAlH4 in THF [17]. As found by Dymova
and co-workers [9], solvent free Ca(AlH4)2, admixed with
CaAlH5 (see below), is formed upon heating or ball milling
of CaH2/AlCl3 mixtures (Eq. (3)) [9]:
b.m.
2CaH2 + AlCl3or−he→ating0.5Ca(AlH4)2(+CaAlH5)
Fig. 4. X-ray powder patterns collected in situ at different temperatures: (a)
Mg(AlH4)2 + 2NaCl; (b) MgH2 + Al + NaCl; (c) Al2Mg3 + Al + NaCl.
+ 1.5CaCl2
(3)
For the thermal dissociation of Ca(AlH4)2, prepared accord-
ing to Eq. (3), on the basis of hydrogen amounts released upon
thermolysis (3:3:2), DTA and XRD, the following sequence
of reactions was proposed:
the thermal dissociation of the Mg(AlH4)2 + 2NaCl mixture
proceeds in two steps, the first step being completed at 150
and the second at 310 ◦C. The amounts of hydrogen liberated
(in wt.%) in the two steps (Scheme 1) are in good agree-
ment with calculated ones (in brackets). However, thermal
dissociation of neat Mg(AlH4)2 at 150 ◦C (Eq. (2), Fig. 3(b))
resulted reproducibly in evolution of only 50–60% of the
expected amount of hydrogen. Probably, a part of hydrogen
together with Et2O is lost during heating the compound to
40 ◦C in vacuum and then to 60 ◦C in high vacuum.
Ca(AlH4)2 → CaAlH5 + Al + 1.5H2
(4)
(5)
(6)
◦
CaAlH5260−−→550 CCaH2 + Al + 1.5H2
◦
CaH2 + Al600−−→700 CCa4Al + H2
The DTA trace for the ball milled CaH2/AlCl3 mixtures was
found to display well-defined exotherms at 130–140 and
330 ◦C [9].
In the present work, Ca(AlH4)2 in mixture with NaCl or
LiCl was prepared by ball milling of CaCl2 with NaAlH4
or LiAlH4 (Scheme 2). Additionally, following Ref. [17], a
nearly pure Ca(AlH4)2 could be obtained from CaCl2 and
NaAlH4 in THF and subsequent desolvation of the THF
adduct at 60 ◦C in high vacuum (Eq. (7)).
150◦C
Mg(AlH4)2 −→ MgH2 + 2Al + 3H2
(a)
310◦C
−→ Mg–Al alloy + H2
(2)
(b)
The HTXRD of the Mg(AlH4)2 + 2NaCl mixture is pre-
sented in Fig. 4. For clearness, only the patterns collected
at r.t., 150 and 370 ◦C are shown. Between 120 and 130 ◦C
Mg(AlH4)2, decomposes and reflections belonging to MgH2
and aluminium appear. In the temperature range of about
250 ◦C, the reflections of MgH2 decrease successively and
newreflectionsintherangebetween34.5and38.9◦ 2θ appear.
The reflections are assigned to an alloy with a composition
very similar to Al2Mg3 [5]. The alloy is stable up to 450 ◦C,
before it starts to melt. The shift of reflections with tempera-
turetolowerdiffractionanglesisduetothethermalexpansion
of the material.
An enhanced kinetics of dehydrogenation of Mg(AlH4)2
promoted by ball milling of the compound with 2 mol%
of TiCl3 has been reported [5]. While the uncatalyzed
Mg(AlH4)2 + 2NaCl mixture (Scheme 1; Fig. 3(a)) disso-
ciates at 150 ◦C to MgH2, Al, NaCl (Fig. 4(b)) and H2,
THF
CaCl2 + 2NaAlH4−→ Ca(AlH4)2 · 4THF + 2NaCl
◦
60−C→/HVCa(AlH4)
(7)
2
A composition of IR spectra of Ca(AlH4)2 before and after
desolvation (Fig. 5(a and b)) shows that complete removal of
the complexed THF could not be achieved. The X-ray pow-
der patterns of Ca(AlH4)2 in mixture with NaCl or LiCl and
of the wet chemically prepared one after desolvation (Eq.
(6)) are shown in Fig. 6(a–c). The FWHM of the reflec-
tions in the patterns assigned to the mechanochemically
prepared Ca(AlH4)2 (Fig. 6(a and b)) indicates small par-
ticle sizes. Furthermore, the high background suggests that
a large part of the material might be present in X-ray amor-
phous form. Apart from this, the X-ray diffraction patterns of

M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
83
Fig. 7. Solid-state 27Al MAS NMR spectrum of Ca(AlH4)2.
products, CaH2 and Al, apart from NaCl, could be identified
by the XRD patterns (Fig. 9(a and b)). The two-step disso-
ciation of Ca(AlH4)2 is in accordance with the findings of
Dymova and co-workers [9] (Eqs. (4) and (5)) and can be
assigned to the formation of the CaAlH5 intermediate (Sec-
tion 3.4).
On the example of the CaCl2 + 2NaAlH4 mixture, it
could be demonstrated that, apparently, also the dissocia-
tion of Ca(AlH4)2 (of 3.1) can be catalyzed by titanium.
As shown in Scheme 2, ball milling of CaCl2, NaAlH4
and 2 mol% of TiCl3 produced an amount of hydrogen
(Fig. 8(c)), approximately equal to that resulting from heating
the Ca(AlH4)2 + 2NaCl mixture to 250 ◦C.
Fig. 5. IR spectra of: (a) Ca(AlH4)2·4THF and (b) Ca(AlH4)2 after removal
of THF in high vacuum/60 ◦C, trace shifted by 40% transmission for clarity.
Ca(AlH4)2 shown in Fig. 6(c) and of the mixtures Fig. 6(a and
b) are showing diffraction lines mainly at 18.8, 25.5, 27.2,
29.9, 31.0 and 32.7◦ 2θ, which can tentatively be assigned
to Ca(AlH4)2. The exact assignment of observed reflec-
tions is speculative. The diffraction lines of Ca(AlH4)2·4THF
Fig. 6(d) differ significantly from those ascribed to solvent-
free Ca(AlH4)2 Fig. 6(c). The 27Al MAS NMR spectrum of
the Ca(AlH4)2 + 2LiCl mixture in Fig. 7 shows a relatively
narrow line at 104.6 ppm. The intense spinning side bands
are mainly caused by the inner satellite transition.
Thermal dissociation of the Ca(AlH4)2 + 2NaCl mixtures
(Scheme 2), as well as that of neat Ca(AlH4)2 (Eq. (8)), was
observed via TV upon heating the sample from r.t. to 250 ◦C
at 4 ◦C/min (Fig. 8(a and b)). As evident from the figure,
the dissociation takes place in two steps with approximately
equal amounts of hydrogen released in both steps. The total
amounts of hydrogen liberated are in good agreement with
calculated ones (given in brackets). The expected thermolysis
250◦C
Ca(AlH4)2 −→ CaH2 + 2Al + 3H2, 5.0(5.5) wt.%
(8)
Fig. 8. Course of thermal dissociations of: (a) Ca(AlH4)2 + 2NaCl mixture
(0.5 g sample); (b) neat Ca(AlH4)2 (0.18 g sample); (c) hydrogen evolution
recorded during ball milling of a CaCl2/2NaAlH4/2 mol% TiCl3 mixture;
(- - -) temperature in ◦C.
Fig. 6. X-ray powder patterns of Ca(AlH4)2 + 2Na(Li)Cl mixtures (a and b),
Ca(AlH4)2 (after removal of THF) (c) and Ca(AlH4)2·4THF (d).

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M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
Fig. 9. X-ray powder patterns of thermal dissociation products of: (a)
Ca(AlH4)2 + 2NaCl and (b) Ca(AlH4)2.
Fig. 11. Thermovolumetric course of the thermal dissociation of the
LiMg(AlH4)3 + 2LiCl mixture; (- - -) temperature in ◦C.
3.3. Lithium–magnesium alanate
Thermolysis of the LiMg(AlH4)3 + 2LiCl mixture
(Fig. 11) proceeds in two steps, the first step being com-
pleted at 100 and the second at 150 ◦C, the molar ratio of
hydrogen released in the two steps being 2:1. According
to Fig. 12(a), the thermolysis products of the mixture are
MgH2, Al and LiCl. On the basis of the thermolysis products,
we assume that the thermolysis of the LiMg(AlH4)3 + 2LiCl
mixture proceeds according to Scheme 3, although diffrac-
tion lines of the expected product LiH could not be identified
in the X-ray powder pattern.
The only source of data on preparation and properties of
lithium–magnesium alanate (LiMg(AlH4)3) goes back to a
work of Bulychev et al. [18], who prepared the compound
from MgCl2, LiAlH4 and NaAlH4 in ether.
As shown in the following, LiMg(AlH4)3 in mixture with
LiCl can be prepared by ball milling of MgCl2 with LiAlH4 in
a molar ratio of 1:3 (Scheme 3). Separation of LiMg(AlH4)3
from the mixture with LiCl was carried out according to the
procedure given in Ref. [18].
The X-ray diffraction lines of LiMg(AlH4)3 in mixture
with LiCl (Fig. 10(a)) and of the neat compound Fig. 10(b) are
inagreementwitheachotherandwiththosefromRef. [18](or
PDF-2 data base no. 34-926). The X-ray patterns (Fig. 10(a
and b)) show that after dissolution of the main crystalline
component LiCl, the powder pattern of the remaining sample
(Fig. 10(b)) shows a very high amorphous background and
reflections with very low intensities. This indicates that after
dissolution of the salt only a very small amount of crystalline
LiMg(AlH4)3 is present.
The 2:1 molar ratio of hydrogen released in the two-
step dissociation of LiMg(AlH4)3 (Fig. 11) reminds of the
two-step thermal (or catalyzed) dissociation of alkali metal
alanates [1,28,29], which takes place according to Eq. (9).
3MAlH4 → M3AlH6 + 2Al + 3H2
→ 3MH + 3Al + 1.5H2, M = Li, Na, K
(9)
It can therefore be assumed that the two-step thermal disso-
ciation of LiMg(AlH4)3 proceeds according to Eqs. (10) and
Fig. 12. X-ray powder pattern of the thermal dissociation products of: (a)
LiMg(AlH4)3 + 2LiCl mixture and (b) MgCl2/3 LiAlH4/2 mol% Ti* mixture
after ball milling.
Fig. 10. X-ray powder patterns of: (a) LiMg(AlH4)3 + 2LiCl mixture and
(b) LiMg(AlH4)3 after separation from LiCl (thin lines, PDF-2 data base no.
34-926).

M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
85
(11), with LiMgAlH6 as an intermediate.
100◦C
LiMg(AlH4)3 −→ LiMgAlH6 + 2Al + 3H2
(10)
◦
LiMgAlH6 + 2Al1−5→0 CMgH2 + (LiH) + 3Al + 1.5H2 (11)
Upon ball milling of MgCl2 with LiAlH4 (molar ratio 1:3) in
the presence of 2 mol% of Ti* (Scheme 3), the X-ray pow-
der pattern of the product (Fig. 12(b)) is in accordance with
that of the LiMg(AlH4)3 + 2LiCl mixture (Fig. 12(a)) heated
to 150 ◦C (except that the amount of amorphous material is
higher than in Fig. 12(a)). This indicates a Ti*-catalyzed dis-
sociation of the in situ formed LiMg(AlH4)3 (Scheme 3).
3.4. Determination of dissociation enthalpies of
Mg(AlH4)2, Ca(AlH4)2 and LiMg(AlH4)3 by means of
DSC
A decisive factor for the application of metal hydrides for
the reversible hydrogen storage is the enthalpy change associ-
ated with hydrogen release or uptake. A dissociation pressure
of a hydride of 1–10 bar at 0–100 ◦C, suitable for many
practical applications, corre−sp1onds to an enthalpy change of
between 15 and 24 kJ molH [2]. DSC measurements were
applied as a suitable and quick method for determination of
dissociation reaction enthalpies [30].
Fig. 13. Charts of DSC measurements on: (a) Mg(AlH4)2 + 2NaCl; (b)
Ca(AlH4)2 + 2NaCl; (c) LiMg(AlH4)3 + 2LiCl.
DSCmeasurementswerecarriedouton1–4 mgsamplesof
mechanochemically prepared Mg(AlH4)2 + 2NaCl mixtures
at heating rates of 2, 5, 10 K/min, which gave identical results
with respect to the heat of reaction. A chart of the DSC mea-
surement of the mixture at 5 K/min is shown in Fig. 13(a). The
DSC curve shows three endothermal peaks with maxima at
137, 273 and 449 ◦C, of which the first two peaks on the basis
of XRD (Fig. 4), can be assigned to the two-step thermal dis-
sociation of Mg(AlH4)2 (Scheme 1). The third (sharp) peak
is due to the melting of the Mg–Al alloy (Ref. [31]; 453 ◦C).
Evaluation of peak areas of three independent DSC measure-
ment (heating rate 2, 5 and 10 K/min) gave enthalpy values of
1.7 kJ/mol for the first decomposition step (endothermic) and
48.8 kJ/mol for the second step (endothermic), respectively.
For the reverse of the second step dissociation, hydrogena-
tion of the Mg3Al2 alloy with formation of MgH2 and Al, an
enthalpy value of 63.4 kJ/mol is reported in literature [32,33],
which in view of different Mg/Al ratios in both cases is a sat-
isfactory agreement.
The DSC chart of the thermal dissociation of the
Ca(AlH4)2 + 2NaCl mixture is given in Fig. 13(b). The DSC
curve shows that the dissociation proceeds in several steps,
of which the first step (127 ◦C), being weakly exothermic,
and the second one (250 ◦C), an endothermic one, can prob-
ably be assigned to dissociation of Ca(AlH4)2 according to
Eqs. (4) and (5). This assignment is supported by the results
of thermovolumetric and XRD measurements (Section 3.2).
Higher temperature endothermic peaks, which are also asso-
ciated with H2 losses, arise possibly from formation of Ca–Al
alloys from CaH2 and Al. Evaluation of peak areas of the first
two peaks of three independent measurements (heating rates
2, 5 and 10 K/min) gave values of −7.4, −7.4, −7.6 kJ/mol
for the first step (exothermic) and 31.1, 31.6 and 32.3 kJ/mol
for the second decomposition step (endothermic) (CaAlH5).
The DSC curve of LiMg(AlH4)3 dissociation (Fig. 13(c))
shows, as in the case of Ca(AlH4)2, a first exothermic
(154 ◦C) and a second endothermic peak (185 ◦C), followed
by two further endothermic peaks. From the peak areas
of two independent measurements (heating rates 5 and
10 K/min), assigned to Eqs. (9) and (10), enthalpy values of
14.9, 15.1 kJ/mol for the first decomposition step (exother-
mic) and 13.1, 13.0 kJ/mol for the second step (endothermic)
are calculated.
4. Summary and conclusion
Ball milling of MgCl2 and CaCl2 with NaAlH4 or LiAlH4
proved to be a simple and efficient method for the preparation
of magnesium, calcium and lithium–magnesium alanates,
although in mixture with NaCl or LiCl (Schemes 1–3). Using
wet chemical separation methods, it was possible to obtain
these alanates in nearly pure state.
The investigated alkaline earth metal alanates were char-
acterized by X-ray diffractometry, solid-state 27Al NMR and
IR spectroscopy and thermovolumetric and DSC measure-
ments. The thermal dissociation products of the alanates
resulting from TV measurements were identified via X-ray
diffractometry.

86
M. Mamatha et al. / Journal of Alloys and Compounds 407 (2006) 78–86
Mg(AlH4)2 (in mixture or neat) dissociates at 150–170 ◦C
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Acknowledgements
This work was partially supported by General Motors Fuel
Cell Activities, in addition to the basic funding by the Max
Planck Gesellschaft. We thank B. Zibrowius for discussion
of NMR measurements.
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