Direct synthesis of unsolvated aluminum hydride involving Lewis and Bronsted acids

Article abstract of DOI:10.1134/S0036023608070048

Unsolvated aluminum hydride has been synthesized by the direct reaction of aluminum bromide or sulfuric acid with an alkali metal tetrahydroaluminate at 90-102°C in pure toluene or in toluene containing 5-10 wt % diethyl ether. The reaction involving aluminum bromide yields a mixture of unsolvated aluminum hydride phases of poor quality. The reaction with sulfuric acid affords a single-phase product as α-AlH₃ at ≤90°C.

Full text of DOI:10.1134/S0036023608070048

ISSN 0036-0236, Russian Journal of Inorganic Chemistry, 2008, Vol. 53, No. 7, pp. 1000–1005. © Pleiades Publishing, Ltd., 2008.
Original Russian Text © B.M. Bulychev, V.N. Verbetskii, P.A. Storozhenko, 2008, published in Zhurnal Neorganicheskoi Khimii, 2008, Vol. 53, No. 7, pp. 1081–1086.
SYNTHESIS AND PROPERTIES
OF INORGANIC COMPOUNDS
“Direct” Synthesis of Unsolvated Aluminum Hydride Involving
Lewis and Bronsted Acids
B. M. Bulychev, V. N. Verbetskii, and P. A. Storozhenko
Faculty of Chemistry, Moscow State University, Leninskie gory, Moscow, 119899 Russia
Received May 21, 2007
Abstract—Unsolvated aluminum hydride has been synthesized by the “direct” reaction of aluminum bromide
or sulfuric acid with an alkali metal tetrahydroaluminate at 90–102°C in pure toluene or in toluene containing
5–10 wt % diethyl ether. The reaction involving aluminum bromide yields a mixture of unsolvated aluminum
hydride phases of poor quality. The reaction with sulfuric acid affords a single-phase product as α-AlH3 at
≤90°ë.
DOI: 10.1134/S0036023608070048
The hydrides of the light nontransition metals
Here, we present two “direct” methods for the syn-
belong to the inorganic compounds richest in bound thesis of unsolvated aluminum hydride, according to
hydrogen.Among these compounds, beryllium and alu- which aluminum bromide or 100% sulfuric acid is
minum hydrides have the highest weight contents and reacted with lithium (sodium) aluminum hydride in a
space densities of hydrogen atoms. Large-scale use of pure arene medium or in an arene containing a small
beryllium hydride is ruled out both because of synthetic amount (<10 wt %) of diethyl ether. By “direct” meth-
difficulties and because of the toxicity of beryllium. ods, we understand methods that do not include the
Aluminum hydride is free of the latter limitation, and synthesis of aluminum hydride etherate (reaction (1))
the main obstacles to its application, including as a dis- as a separate step.
posable source of hydrogen, are synthetic difficulties
and its high cost.
EXPERIMENTAL
The only method to synthesize high-quality alumi-
num hydride known to date is based on the thermo-
All manipulations involved in the purification, pre-
treatment, and storage of the starting chemicals, in
crystallization, and in the processing of the crystalliza-
tion products (washing, drying, and prepackaging)
were done in a dry-argon glovebox or in vacuo using
Schlenk techniques.
chemical cleavage of the Al
O bond in the etherate
AlH3 · xEt2O, which is obtained via the Schlesinger
reaction [1]:
3LiAlH4(sln) + AlCl3 ⋅ xEt2O(sln)
(1)
Et2O/ArH
Benzene and toluene were purified by standard
methods and were distilled from LiAlH4 be fore use.
Diethyl ether and tetrahydrofuran, dried and distilled
from an alkali, were treated with sodium benzophenone
ketyl and were then distilled. Before use, they were
refluxed with LiAlH4 or NaAlH4 and were again dis-
tilled.
4AlH3 ⋅ xEt2O(sln) + 3LiCl↓.
The unsolvated product is crystallized from a
diethyl ether + aromatic hydrocarbon mixed solvent in
the presence of 10–15% LiAlH4 [2]. The quality of alu-
minum hydride (thermal stability, particle size distribu-
tion, and chemical and phase compositions) depends on
the concentration of aluminum hydride etherate in its
solution (it should not exceed 4–4.5 g/l) and on the
purity of all the starting compounds used in reaction (1)
[2, 3]. Furthermore, because this technology is extremely
fire-hazardous and because all of its products are so sen-
sitive to traces of moisture that the reaction should be
conducted in a dry inert atmosphere, it is quite clear why
aluminum hydride is very expensive. Obviously, it is a
challenging problem to develop a cheaper synthetic
method for this product, particularly in the light of its
Since the desolvation mechanism and the stability of
the unsolvated hydride depend crucially on the quality
of the starting chemicals, particularly on the presence
of compounds of transition metals, such as titanium,
iron, and chromium, even if their concentration is
below 10–3 mas. %,all the starting crystalline chemicals
were carefully purified and the apparatus was thor-
oughly cleaned.
Commercial LiAlH4 was recrystallized from a
possible application as a disposable source of hydrogen diethyl ether + toluene mixture (1 : 1, hydride concen-
for mobile power supply units. tration of about 30 g/l) as is described in [3].
1000

“DIRECT” SYNTHESIS OF UNSOLVATED ALUMINUM HYDRIDE
1001
Table 1. Outcomes of the synthesis of unsolvated aluminum hydride from aluminum bromide and lithium (sodium) tetrahydroalu-
minate
Composition of the precipitate washed
with diethyl ether, wt %
Solvent
Al2Br6/LiAlH4
Yield, % Phase composition
Al
H
Benzene
1 : 6
74
8.2
8.3
8.2
8.3
9.1
10
15
13
20
80
γ-AlH3
Toluene
Toluene
o-Xylene
1 : 6
76.3
75
α,α'-AlH3
γ,α'-AlH3
γ,α, α'-AlH3
α,γ-AlH3
1 : 6 (NaAlH4)
1 : 6
80.2
82.2
7 : 6
Benzene
Toluene
Toluene
o-Xylene
Hexane
Heptane
Octane
7 : 6
84.6
85.1
85.0
52
9.3
9.3
9.3
–
84
81
85
0
α,γ-AlH3
α,γ-AlH3
α,γ-AlH3
–
7 : 6 (NaAlH4)
7 : 6
7 : 6
7 : 6
77.8
82.4
8.1
8.6
15
26
α,γ, α'-AlH3
α,γ-AlH3
7 : 6
NaAlH4 was prepared by reacting sodium hydride high-boiling solvents) and was then refluxed at this
temperature for 30–40 min.
with aluminum chloride in THF by a standard proce-
dure [4], but using a high-speed stirrer [5].
X-ray diffraction patterns were obtained on a
Guinier G670 HUBER diffractometer for samples sealed
in vacuum-sealed glass capillaries. Differential thermal
analyses were carried out on a Q1500D thermoanalytical
system at a sample heating rate of 10 K/min. Electron
microscopic examinations were done on a JSM 100C
microscope.
Al2Br6 (pure grade) was sublimed two times from its
melt over an aluminum metal powder. To do this, the
mixture was placed into a Pyrex tube (no more than 5%
by volume) and the tube was evacuated, sealed off,
placed into a tubular furnace by 4/5 of its length, and
heated to 100°ë. The sublimate was condensed in a
colder part of the tube, which was then sealed off. To
sample the product, this part of the tube was unsealed
in a dry inert atmosphere in a glovebox. The product
was completely soluble in toluene, yielding a colorless
solution.
RESULTS AND DISCUSSION
Although homogeneous and homogeneous–hetero-
geneous reactions of alkali metal tetrahydroaluminates
with various Lewis and Brønsted acids in electron-
donor solvents may proceed via different routes, they
ultimately yield solutions or precipitates of solvated
aluminum hydride [6–8]. It is very difficult or even
impossible to convert these products into unsolvated
aluminum hydride. Similar reactions in nonsolvating
(“inert”) solvents have not been carried out as yet, even
though it is known that aluminum bromide, unlike alu-
minum chloride and iodide, is readily soluble in aro-
matic hydrocarbons [9], yielding weakly bonded
molecular π-complexes [10, 11]. In paraffins, Al2Br6 is
much less soluble [9] and does not form any com-
pounds.
Anhydrous sulfuric acid (100%) was prepared by
dissolving oleum (reagent grade) in 96% sulfuric acid
(reagent grade).
Unsolvated aluminum hydride was synthesized by
two procedures:
(1) Isothermal synthesis. A cold solution of alumi-
num bromide or sulfuric acid in ArH or in an ArH +
diethyl ether (5–10 wt %) mixture was added into a 1-l
flask half-filled with a suspension of LiAlH4 (15%
excess over the stoichiometry of reaction (1)) in a sol-
vent and preheated on an oil bath to a preset tempera-
ture (no higher than 102°ë for toluene and xylene). The
solution addition rate was adjusted so that the tempera-
ture of the reaction mixture differed from the preset
temperature by no more than 1°ë.
The interaction between Al2Br6 and lithium or
sodium tetrahydridoaluminate in different arenes above
80°ë occurs in the same way and is independent of
whether the reaction and crystallization are conducted
(2) Polythermal synthesis. Appropriate proportions in the isothermal or polythermal regime, yielding mul-
of all inorganic reactants were combined in a one-com- ticomponent mixtures of unsolvated aluminum hydride
ponent solvent in a 1-l flask. The mixture was heated to phases. The most typical results of this interaction are
the boiling point of the solvent (but not above 100°ë for presented in Table 1. It follows from these data that,
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 7 2008

1002
BULYCHEV et al.
under the assumption that reaction (2) occurs, which is
As the ionization potential of the arene decreases,
similar to the Schlesinger reaction (1), the aluminum the aluminum bromide dimer dissociates increasingly
hydride yield does not exceed 30%.
in solution. Even with mesitylene, aluminum bromide
forms the π-complex AlBr3 · ë6ç2(ëç3)4 [9]. How-
ever, this does not exert any affect on the general char-
acter of the reaction, which was described for other are-
nes, or on the quality of the product. The latter is much
lower than the quality of the hydride obtained by crys-
tallization from diethyl ether–arene media [2, 3]. The
AlH3 samples resulting from reaction (3), in addition to
being multiphase, are ultrafine powders with a mean par-
ticle size of <1.5 µm, and their purity falls to 90–91%
upon two or three washings with diethyl ether. There-
fore, the hydride thus obtained readily undergoes solva-
tion, which causes its partial dissolution in diethyl ether
and an appreciable decrease in the yield of the unsolvated
product. Because of all of these effects, the thermal stabil-
ity of all aluminum hydride samples obtained by the
“direct” method using aluminum bromide is lower than
that of the product synthesized via aluminum hydride
etherate. The thermal decomposition of these samples,
yielding hydrogen, begins at 80–90°ë, and their rapid
decomposition is observed at 110–120°ë (in the polyther-
mal regime at a heating rate of 10 K/min). The samples
obtained in ether–arene media under the same conditions
begin to decompose at 110–120°ë, and their rapid decom-
position occurs at 150–170°ë [3].
The interaction between Al2Br6 and MAlH4 in par-
affins is weak. Even for high-boiling alkanes, the alu-
minum hydride yield does not exceed 26%, irrespective
of the reaction stoichiometry (Table 1). Since a similar
situation was observed in the synthesis of unsolvated
aluminum hydride in diethyl ether + octane mixtures in
the polythermal regime [3], it can be assumed that the
weak nonvalent interactions of the products of reaction
(3), which are Lewis acids, with the π-electron system
of the aromatic hydrocarbon somewhat stabilize the
coordinatively unsaturated monomer AlH3 and favor its
polymerization into an inorganic polymer.
ArH
Al2Br6(sln) + 6MAlH4 s
8AlH3↓ + 6åBr↓. (2)
According to X-ray diffraction data, the precipitate
resulting from the reaction contains no LiBr or NaBr,
although these salts are insoluble in arenes. At the same
time, large amounts of free tetrahydroaluminate are
identified. The elemental analysis of the substance iso-
lated by evaporating the mother solution indicates the
composition MAl2.1–2.3Br7.1–7.4. Therefore, the reaction
yields, besides aluminum hydride, a mixture of bro-
moaluminates consisting of the heptabromo complex
MAl2Br7- and a minor amount of the decabromo com-
plex MAl3Br10. These complexes are well known in alu-
minum halide chemistry [6]. We artificially prepared
these two polyaluminates by melting an alkali metal
bromide with aluminum bromide and established that,
under the above conditions, MAl2Br7 does not react
with the tetrahydroaluminates at all and MAl3Br10
reacts with them to an extent no larger than 3–5%. This
order of reactivities is quite consistent with the stabili-
ties of the complex anions. It is interesting that the same
reaction conducted in an electron-donor solvent (e.g.,
diethyl ether) does not yield an alkali bromide precipi-
tate either, even though it occurs according to chemical
equation (2). Because the intrinsic solubility of the
alkali metal bromides is somewhat higher than is
observed in reaction (2), some authors [12, 13] assume
the formation of soluble hydridobromide complexes
([L · M]+[AlH3Br]–) stable only in solution.
Thus, it follows from the above data that the stoichi-
ometry of the reaction between Al2Br6 and MAlH4 in
arenes should be described by the equation
ArH
7Al2Br6(sln) + 6MAlH4 s
8AlH3↓ + 6MAl2Br7. (3)
Indeed, it is clear from Table 1 that, in this case, the
aluminum hydride yield with respect to the tetrahy-
droaluminate ranges up to 80%. However, this value
was obtained after the by-products were washed out
with diethyl ether and, as a consequence, part of the
desired product was lost due to solvation and subse-
quent dissolution in ether. Therefore, the true aluminum
hydride yield is nearly quantitative. Nevertheless, reac-
tion (3) as a whole is less efficient than the classical
Schlesinger reaction.
Unfortunately, in none of our experiments did we
obtain the pure α-phase of aluminum hydride. Its high-
est percentage in the three-phase mixture of α-, γ-, and
α'-AlH3 (~50 wt %) was attained by performing the
reaction at ~100°ë in high-boiling arenes, such as tolu-
ene and xylene. The reaction carried out in benzene at
the lowest temperature examined for arenes (80°ë)
It was demonstrated in earlier works [2, 3] that
unsolvated aluminum hydride can be crystallized from
ether–arene mixtures only in the presence of a small
amount of LiAlH4. At the same time, aluminum hydride
synthesis in aromatic hydrocarbons can be carried out
using both pure LiAlH4 and pure NaAlH4. This fact can
be viewed as direct evidence that, in all aluminum
hydride technologies using diethyl ether, lithium tet-
rahydroaluminate serves as a desolvating agent favor-
ing the breaking of the Al
O bond in the etherate.
In the absence of ether in the system, there is no agent
to play this role and aluminum hydride, though its qual-
ity is poor, results directly from the reaction between
NaAlH4 and Al2Br6.
As was noted above, solvated aluminum hydride as the
etherate AlH3 · xEt2O can be obtained using both Lewis
yielded the pure γ-phase in some cases. The introduc- acids and Brønsted acids, e.g., sulfuric acid [6–8, 14]. For
tion of diethyl ether (up to 5 wt %) into the solvent (tol- a higher solubility of the starting chemicals, for the
uene) afforded no single-phase aluminum hydride.
sake of safety in conducting the reaction at elevated
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 7 2008

“DIRECT” SYNTHESIS OF UNSOLVATED ALUMINUM HYDRIDE
1003
Table 2. Outcomes of the synthesis of unsolvated aluminum hydride from sulfuric acid and lithium tetrahydroaluminate
LiBH4,
added, wt %
Phase
composition
Excess LiAlH4, wt %
ceff, g/l
tcryst, °C
Yield, %
10
10
15
10
10
10
10
20
10
10
15
–
–
–
–
–
–
–
–
–
–
–
–
10
15
18
24
28
39
10
16
16
14
14
10
90
89
90
90
90
90
92
92
92
96
101
90
73
76
79
82
84
55
64
74
65
72
69
0
α-AlH3
α-AlH3
α-AlH3
α-AlH3
α-AlH3
α,γ-AlH3
α,γ-AlH3
α-AlH3
α,γ-AlH3
α,γ-AlH3
γ,α, α'-AlH3
–
0 (NaAlH4 as a starting
compound)
15 (NaAlH4 as a starting
compound)
–
10
90
0
–
10
10
20
10
10
20
20
50
15
18
14
20
90
96
54
30
22
51
α,γ-AlH3
γ,α, α'-AlH3
γ,α, α'-AlH3
γ,α, α'-AlH3
101
96
temperatures, and considering the data obtained for the noticeably reduces the aluminum hydride yield and
synthesis of unsolvated aluminum hydride using alumi- dramatically spoils the quality of the product. The prod-
num bromide, the aluminum hydride synthesis involving uct that forms in the presence of LiBH4 is always a mix-
100% sulfuric acid was performed in the high-boiling sol-
vent toluene in the presence of 5–10 mas % diethyl ether
in the isothermal reactant combining regime.
ture of phases in which the proportion of α-AlH3, the
most dense and the most stable aluminum hydride
phase, is very small.
In this case, the “direct” method is so different from
“classical” crystallization from ether–arene media [2, 3]
and from the above aluminum hydride synthesis using
aluminum bromide that we can claim that the formation
of the unsolvated product by the reaction
The third distinction of this method is that the for-
mation of the pure α-phase with a crystal size of ~6 µm
is observed only at synthesis temperatures of ≤ 90°ë,
while, in the “classical” and bromide methods, the
probability of the formation of this phase always
increases with an increasing solvent temperature. Further-
more, it was demonstrated by sampling that only the
α-modification crystallizes at this temperature (Fig. 1a)
and a mixture of the γ-, α-, and α'-phases (Fig. 1b)
forms at 93°C and above. Contrary to what was
reported in [3], this mixture persists up to 100–102°ë.
Note also that the quality of the α-phase obtained by
this method, primarily its thermal stability, differs only
slightly from the quality of the samples resulting from
“classical” synthesis and crystallization [3].
Thus, while the first two distinctions of this method
from the other known aluminum hydride synthesis
methods can be attributed to the formal absence of a
distinct step yielding aluminum hydride etherate,
whose solubility in ether determines the effective alu-
minum hydride concentration in the process, and to the
occurrence of the side reaction
ArH/Et2O
2LiAlH4 + H2SO4
2AlH3 + Li2SO4 + 2H2 (4)
takes place via a different mechanism.
The most illustrative results of our experiments are
presented in Table 2. Note that the effective aluminum
hydride concentration achieved in this method (amount
of the resulting AlH3 (g) per total solvent volume (l)) is
higher than that achieved in the syntheses using diethyl
ether. While ceff in the classical synthesis is not higher
than 5 g/l [3], ceff in this method can be raised up to
30 g/l because it is determined not by the solubility of
aluminum hydride etherate, but by the solubility of sul-
furic acid and by the safety regulations at the reactant
combining stage.
The second distinction of this method is that use of
lithium borohydride neither improves the quality of
aluminum hydride nor raises its yield, contrary to what
was claimed in [2, 3]. Conversely, lithium borohydride
ArH/Et2O
2LiÇH4 + H2SO4
Ç2H6 + Li2SO4 + 2H2, (5)
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 7 2008

1004
BULYCHEV et al.
10 µm
10 µm
(‡)
(b)
Fig. 1. (a) α-AlH crystals obtained from sulfuric acid and LiAlH at 90°ë and (b) α, γ-AlH crystals obtained from sulfuric acid
3
4
and LiAlH at 95°ë.
4
which yields diborane, a substance capable of reacting unsolvated aluminum hydride is rather wide and is lim-
with alane to form the rather stable compound ited by only two factors, namely, economic practicabil-
Al(BH4)3 · Et2O [15, 16], the last distinction clearly ity and the quality of the resulting product. For exam-
ple, use of the Lewis acid aluminum bromide is inap-
propriate in terms of both parameters. The technology
involving 100% sulfuric acid is simpler than the con-
ventional technology and affords a product of accept-
able quality, but it suffers from a greater waste of
LiAlH4 per unit weight of the product.
indicates that the AlH3 formation mechanism in this
method is different from that in the “classical” synthe-
sis [2, 3]. We cannot ascertain the cause of the stabili-
zation of the low-density and metastable modifications
of aluminum hydride, which were reported to turn
readily into α-AlH3 upon simple heating [2, 3]. The
only possible, though rather disputable, cause of this
effect is the absence of a lithium halide in the reaction
system. Any unsolvated aluminum hydride sample pre-
pared via an aluminum halide (i.e., by the “classical”
method) contains ≤0.1 mas % lithium and ≤0.05 mas %
chlorine or bromine. The form and location of these
elements in the hydride have not been established; how-
ever, there is good reason to believe that they are incor-
porated in the lattice. The hydride samples obtained
using sulfuric acid contain ꢀ0.1% lithium and no chlo-
rine. It is known from aluminum hydride chemistry that
some alkali metal halides, particularly lithium chloride,
form weakly bonded complexes with aluminum
hydrides, thus changing the solubility of both compo-
nents, and affect the reactivity of the main component
[6, 7, 12, 13, 17]. So, we suppose that the lithium chlo-
ride resulting from the “classical” Schlesinger reaction
(reaction (1)) is not an inert component of the system,
but an active participant processes occurring there,
including the desolvation of the etherate and the phase
transitions of the less dense and thermodynamically
less stable modifications of aluminum hydride into α-
AlH3. However, this hypothesis needs experimental
verification.
ACKNOWLEDGMENTS
This work was supported by INTAS, project
no. 05-1000005-7665: “New Alane: Novel Reversible
Hydrogen Storage Materials Based on the Alloys of Al.”
REFERENCES
1. A. E. Finholt, A. C. Bond, and H. E. Schlesinger, J. Am.
Chem. Soc. 69, 1195 (1947).
2. F. M. Brower, N. E. Matzek, P. F. Reigler, et al., J. Am.
Chem. Soc. 98, 2450 (1976).
3. B. M. Bulychev, V. N. Verbetskii, A. I. Sizov, et al., Russ.
Chem. Bull., Int. Ed. 56, 1305 (2007).
4. A. E. Finholt, G. D. Barbara, G. K. Barbaras, et al., J.
Inorg. Nucl. Chem. 1, 317 (1955).
5. B. M. Bulychev, A. V. Golubeva, and P. A. Storozhenko,
Zh. Neorg. Khim. 43, 1061 (1998) [Russ. J. Inorg.
Chem. 43, 971 (1998)].
6. N. M. Alpatova, V. V. Gavrilenko, Yu. M. Kessler, et al.,
in Complexes of Organometallic, Hydride, and Halide
Compounds of Aluminum (Nauka, Moscow, 1970),
p. 295 [in Russian].
Thus, our experiments demonstrated that the variety
of reactants and reactions suitable for the synthesis of
7. H. Noth and E. Wiberg, Fortschr. Chem. Forsch. 8/3, 321
(1966).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 7 2008

“DIRECT” SYNTHESIS OF UNSOLVATED ALUMINUM HYDRIDE
1005
8. A. F. Zhigach and D. S. Stasinevich, in Hydride Chemis- 13. E. C. Ashby and J. Prather, J. Am. Chem. Soc. 88, 729
try (Khimiya, Moscow, 1969), p. 676 [in Russian].
(1966).
14. E. C.Ashby and N. M.Yoon, J.Am. Chem. Soc. 88, 1464
9. Handbook of Solubility (Akad. Nauk SSSR, Moscow,
(1966).
1962) [in Russian].
15. E. C. Ashby, J. Organomet. Chem. 3, 371 (1965).
16. P. H. Bird and M. G. H. Wallbridge, J. Chem. Soc., 3923
(1965).
10. H. C. Brown and W. J. Wallance, J. Am. Chem. Soc. 75,
6265 (1953).
11. U. C. Sang and H. C. Brown, J. Am. Chem. Soc. 88, 903
(1966).
17. A. P. Savchenkova, S. E. Tokareva, B. M. Bulychev, and
K. N. Semenenko, Koord. Khim. 2, 758 (1976).
12. E. Wiberg, H. Graf, M. Schmidt, and R. Uson, Z. Natur-
forsch., B: Anorg. Chem. Org. Chem. Biochem. Bio- 18. C. W. Heitsch, C. E. Nordman, and R. W. Parry, Inorg.
phys. Biol. 7, 578 (1952).
Chem. 2, 508 (1963).
RUSSIAN JOURNAL OF INORGANIC CHEMISTRY Vol. 53 No. 7 2008