Preparation of ammonia borane in high yield and purity, methanolysis, and regeneration

Article abstract of DOI:10.1021/ic700772a

Ammonia borane (AB) is emerging as a promising solid hydrogen carrier, particularly for power generation in portable devices that employ proton-exchange membrane fuel cells. A preparative-scale synthesis of AB from sodium borohydride and ammonium salts in

Full text of DOI:10.1021/ic700772a

Inorg. Chem. 2007, 46, 7810−7817
Preparation of Ammonia Borane in High Yield and Purity, Methanolysis,
and Regeneration
P. Veeraraghavan Ramachandran* and Pravin D. Gagare
Department of Chemistry, 560 OVal DriVe, Purdue UniVersity,
West Lafayette, Indiana 47907-2084
Received April 23, 2007
Ammonia borane (AB) is emerging as a promising solid hydrogen carrier, particularly for power generation in
portable devices that employ proton-exchange membrane fuel cells. A preparative-scale synthesis of AB from
sodium borohydride and ammonium salts in high yields (g95%) and very high purity (g98%) has been described.
The first systematic study of a transitional metal-catalyzed alcoholysis of AB, comparison of the methanolysis to
the hydrolysis of AB, and regeneration of AB from ammonium tetramethoxyborate also has been described.
Introduction
environmentally benign, and stable material that can be safely
transported without hydrogen loss, which dictates the success
of any chemical for hydrogen storage. AB is also equally
important for synthetic organic chemistry applications. The
literature synthesis of ammonia borane involves the reaction
Ammonia borane (AB) is an excellent source of hydrogen
1
2
(19.4 wt %) for fuel cell applications, and despite its cost,
4
3
is currently being actively pursued. AB is a nontoxic,
5
6
*
To whom correspondence should be addressed. E-mail: chandran@
of lithium or sodium borohydride (SBH) and ammonium
salts, such as ammonium chloride, sulfate, or carbonate. The
low-temperature (-75 °C) isolation of AB is tedious as
purdue.edu.
(
1) (a) Raissi, A. T. Proceedings of the 2002 U.S. DOE Hydrogen Program
ReView; NREL/CP-610-32405; U.S. Department of Energy: Wash-
ington, DC, 2002; http://www.eere.energy.gov/hydrogenandfuelcells/
pdfs/32405b15.pdf. (b) Gutowska, A.; Li, L.; Shin, Y.; Wang, C. M.;
Li, X. S.; Linehan, J. C.; Smith, R. S.; Kay, B. D.; Schmid, B.; Shaw,
W.; Gutowski, M.; Autrey, T. Angew. Chem., Int. Ed. 2005, 44, 3578.
7
reported. A synthetic procedure from diborane and ammonia
in hexane has also been reported. An efficient and economi-
8
cal synthesis and recycling of AB is critical for the realization
of its full potential.
(c) Bluhm, M. E.; Bradley, M. G.; Butterick, R.; Kusari, U.; Sneddon,
L. G. J. Am. Chem. Soc. 2006, 128, 7748. (d) Denney, M. C.; Pons,
V.; Hebden, T. J.; Heinekey, D. M.; Goldberg, K. I. J. Am. Chem.
Soc. 2006, 128, 12048. (e) Baitalow, F.; Baumann, J.; Wolf, G.;
Jaenicke-R o¨ aˆ bler, K.; Leitner, G. Thermochim. Acta 2002, 391, 159.
Herein, we report an optimal procedure for the preparative-
scale synthesis of AB in high yields and purity from sodium
9
borohydride and ammonium salts. The first systematic study
(f) Sit, V.; Geanangel, R. A.; Wendlandt, W. W. Thermochim. Acta
1
987, 113, 379. (g) Cheng, F.; Ma, H.; Li, Y.; Chen, J. Inorg. Chem.
007, 46, 788. (h) Zhang, X.-B.; Han, S.; Yan, J-M.; Chandra, M.;
of the transition metal-catalyzed methanolysis of AB, the
isolation and characterization of the resulting borate salt, and
the recovery of AB has been described. A comparison of
the transition metal-catalyzed methanolysis and hydrolysis
2
Shioyama, H.; Yasuda, K.; Kuriyama, N.; Kobayashi, T.; Xu, Q. J.
Power Sources 2007, 168, 167.
(
2) Borane ammonia complex, 90% Tech. grade, Aldrich catalog price
of $116.50/10 g.
(
3) (a) Stephens, F. H.; Pons, V.; Baker, R. T. Dalton Trans. 2007, 25,
2
613. (b) Mohajeri, N.; T-Raissi, A.; Adebiyi, O. J. Power Sources
(4) (a) Andrews, G. C. Borane-Ammonia. In Encyclopedia of Reagents
for Organic Synthesis; Paquette, L., Ed.; J. Wiley & Sons: New York,
2004. (b) Andrews, G. C.; Crawford, T. C. Tetrahedron Lett. 1980,
21, 693. (c) Andrews, G. C. Tetrahedron Lett. 1980, 21, 697.
(5) Shore, S. G.; Parry, R. W. J. Am. Chem. Soc. 1955, 77, 6084.
(6) Hu, M.G.; Van Paasschen, J. M.; Geanangel, R. A. J. Inorg. Nucl.
Chem. 1977, 39, 2147.
2007, 167, 482. (c) Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid,
B.; Autrey, T. Phys. Chem. Chem. Phys. 2007, 9, 1831. (d) Stephens,
F. H.; Baker, R. T.; Matus, M. H.; G., Daniel J.; Dixon, D. A. Angew.
Chem., Int. Ed. 2007, 46, 746. (e) Baitalow, F.; Wolf, G.; Grolier,
J.-P. E.; Dan, F.; Randzio, S. L. Thermochim. Acta 2006, 445, 121.
(f) Meng, Y.; Zhou, Z.; Duan, C; Wang, B.; Zhong, Q THEOCHEM
2
005, 713, 135. (g) Keaton, R. J.; Blacquiere J. M.; Baker, R. T. J.
(7) Shore, S. G.; Parry, R. W. J. Am. Chem. Soc. 1958, 80, 8.
(8) Mayer, E. Inorg. Chem. 1973, 12, 1954.
Am. Chem. Soc. 2007, 129, 1844. (h) Cheng, F.; Ma, H.; Li, Y.; Chen,
J. Inorg. Chem. 2007, 46, 788. (i) De B., S.; Carewska, M.; Cento,
C.; Gislon, P.; Pasquali, M.; S. S; Prosini, P P. Thermochim. Acta
(9) Parts of this work were presented during (a) the inaugural Purdue
University Energy Center Hydrogen Initiative Symposium, April 5,
2006, and (b) during the symposium entitled, “Role of Catalysts in
Hydrogen Production” sponsored by the FUEL Division of the ACS
during the 232nd ACS National Meeting, San Francisco, CA,
September 11, 2006. (c) For preliminary reports, see: Ramachandran,
P. V. Prepr. SympsACS, DiV. Fuel Chem. 2006, 51 (2), 515.
2006, 441, 184. (j) Dixon, D. A.; Gutowski, M.; Pollack, L.; Windus,
T. L.; De J. W. Prep. Symp., ACS, DiV. Fuel Chem. 2005, 50, 16. (k)
Hu, M. G.; Geanangel, R. A.; Wendlandt, W. W. Thermochim. Acta
1
2
978, 23, 249. (l) D’Ulivo, A.; Onor, M.; Pitzalis, E. Anal. Chem.
004, 76, 6342.
7810 Inorganic Chemistry, Vol. 46, No. 19, 2007
10.1021/ic700772a CCC: $37.00
© 2007 American Chemical Society
Published on Web 08/24/2007

Preparation of Ammonia Borane
of AB has been made. We have recognized and quantified
the liberation of ammonia during the transition metal-
catalyzed hydrolysis. The borate products from the hydrolysis
and the methanolysis of AB have been characterized by
X-ray crystallography.
Experimental Section
General Information. Unless otherwise noted, all manipulations
were carried out under an inert atmosphere in flame-dried glassware,
cooled under nitrogen.10 The B NMR spectra were recorded using
a Varian Gemini-300 spectrometer (64 MHz) with a Nalorac
11
quad probe and were referenced to BF
3
2
‚OEt (0.0 ppm) as an
external standard. Tetrahydrofuran (THF) and methanol were
purchased from Mallinckrodt chemicals. Dry THF was pre-
pared by distillation over sodium benzophenone ketyl. Dry dioxane,
sodium borohydride (SBH), and all of the ammonium salts
were purchased from commercial sources and were used as
received.
Large-Scale Preparation of Ammonia Borane using Sodium
Borohydride and Ammonium Sulfate in THF. Caution:
A significant amount of hydrogen is generated during the syn-
thesis of AB, which is a potential fire hazard. It is recommended
that the reaction be carried out in a well-Ventilated hood and
that the reaction Vessel outlet be led directly into the hood
exhaust.
Figure 1. Experimental setup for hydrogen measurement: (A) thermo-
couple, (B) constant temperature water bath, (C) round-bottom flask, (D)
stop cock, (E) gas burette, (F) leveling flask, (G) stirrer, (H) constant temp
water inlet, (I) water outlet, (J) reflux condenser, and (K) water trap.
Table 1. Synthesis of AB from SBH and Ammonium Salts in THFa
reaction conditions
Sodium borohydride (37.83 g, 1.0 mol) and powdered ammonium
sulfate (132.14 g, 1.0 mol) were added to a 10 L three-neck round-
bottom flask with indentation fitted with an overhead stirrer, a
stopper, and a condenser fitted with a connecting tube. The
connecting tube was vented via an oil bubbler to the hood exhaust.
THF (6 L) was transferred into the flask, and the contents were
ammonium
salt
temp
(°C)
time
(h)
isolated
yield (%)
entry
1
2
3
4
5
6
7
8
9
NH4Cl
NH4Cl
25
40
25
40
25
40
25
40
25
40
25
40
25
40
40
40
40
24
11
4
3
4
10
3
15
5
2
3
5
5
2
5
12
20
45
40
80
85
75
81
85
78
93
96
83
76
93
95
94 (0.3)
90 (1.8)
88 (3)
(NH4)2CO3
(NH4)2CO3
NH4OAc
NH4OAc
NH4F
vigorously stirred at 40 °C. The reaction was monitored by 11
B
NMR spectroscopy. Upon completion (∼2 h), the reaction mixture
was cooled to room temperature (RT) and filtered. The filtrate was
NH4F
concentrated under vacuum to obtain ammonia borane (29.3 g, 96%)
(NH4)2SO4
(NH4)2SO4
NH4NO3b
NH4NO3
in >98% chemical purity, as determined by 11B NMR spectroscopy
10
1
1
1
2
0b
(
64 MHz, -22.1 ppm (q)) and hydride analysis.1
The solvent was recovered and reused for the preparation
of subsequent batches of ammonia borane without further purifica-
tion.
Large-Scale Preparation of Ammonia Borane using Sodium
Borohydride and Ammonium Formate in Dioxane. Caution: A
significant amount of hydrogen is generated during the synthesis
of AB, which is a potential fire hazard. It is recommended that the
reaction be carried out in a well-Ventilated hood and that the
reaction Vessel outlet be led directly to the hood exhaust.
Sodium borohydride (379 g, 10 mol) and ammonium formate
13
14
NH HCO2
4
NH4HCO2
1
1
1
5
6
7
(NH4)2SO4c
d
(NH4)2SO4
(NH4)2SO4e
a
Yields in parentheses show percent impurities determined from the 11
NMR spectrum. The reaction concentration in THF is 0.165 M NaBH4.
B
b
Caution: The reaction of ammonium nitrate with the sodium borohydride
is potentially explosiVe. c The reaction concentration in THF is 0.2 M
NaBH4. d The reaction concentration in THF is 0.4 M NaBH4. The reaction
e
concentration in THF is 0.6 M NaBH4.
(
945 g, 15 mol) were added under a nitrogen atmosphere to a 20
L three-neck round-bottom flask with indentation fitted with an
overhead stirrer, a stopper, and a condenser fitted with a connecting
tube. The connecting tube was vented via an oil bubbler to the
hood exhaust. Anhydrous dioxane (10 L) was transferred into the
flask and the contents were vigorously stirred at 40 °C for ∼2 h
12 h for completion. The reaction mixture was then cooled to RT
and filtered through a Celite bed; the solid residue was washed
with dioxane, and the combined filtrates were concentrated to obtain
294 g (95%) of AB in >98% purity. Elemental analysis: theoretical
B 35.03, H 19.59, N 45.38; found B 35.13, H 19.21, N 45.01%.
The product was stable indefinitely in the solid state but
underwent slow decomposition in THF and dioxane solutions,
precipitating a white solid. Similar behavior is reported in diethyl
ether by Shore and Parry.5
1
1
(
B NMR), until the reaction was complete. Vigorous stirring was
found to be crucial for the rate of the reaction since a similar scale
reaction using a round-bottom flask without indentation required
,7
(
10) (a) Procedures for handling air- and moisture-sensitive compounds
has been described in Brown, H. C.; Kramer, G. W.; Levy, A. B.;
Midland, M. M. Organic Synthesis Via Boranes; Aldrich: Milwaukee,
WI, 2000; Vol. 1, Chapter 9. (b) For the hydride analysis procedure
Hydrolysis of AB. Quantification of Liberated Ammonia. An
aqueous solution of ammonia borane (12 mL, 25 wt %) was placed
in a 2-neck round-bottom flask fitted with septum inlet and a reflux
condenser fitted with a connecting tube. The connecting tube was
(estimation of hydrogen after hydrolysis or alcoholysis), see ref 10a,
pp 241-245.
4
attached to an analytical gas burette filled with CuSO solution via
Inorganic Chemistry, Vol. 46, No. 19, 2007 7811

Ramachandran and Gagare
Table 2. Synthesis of AB: Solvent Effecta
Table 4. Quantity of NH3 Liberated during the Hydrolysis of AB
ammonium
time
(h)
yield (%)
catalyst
(purity, %)b
AB solution
(wt %)
addition
time (min)
NH3
liberated (%)
entry
solvents
salts
entry
MXn
mol %
1
2
3
4
5
6
7
8
9
EtOAc
EtOAc
CH2Cl2
CH2Cl2
DMF
(NH4)2SO4
NH4HCO2
(NH4)2SO4
NH4HCO2
(NH4)2SO4
NH4HCO2
(NH4)2SO4
NH4HCO2
(NH4)2SO4
NH4HCO2
(NH4)2SO4
NH4HCO2
(NH4)2SO4
NH4HCO2
(NH4)2SO4
NH4HCO2
8
9.5
12
15
7
70 (76)
61 (80)
52 (58)
45 (46)
56 (60)
62 (60)
70 (81)
76 (78)
39 (42)
48 (46)
40 (51)
46 (48)
82 (77)
95 (98)
72 (75)
76 (80)
1
2
25
20
15
15
15
15
10
6
RuCl3
RuCl3
RuCl3
RuCl3
RuCl3
RuCl3
RuCl3
RuCl3
RuCl3
NiCl2
0.06
0.06
0.06
0.06
0.2
45
45
45
15
45
15
45
15
45
45
7
6
4
4.4
3
6
1
<1
∼0
6
3
4
DMF
8
5
diglyme
diglyme
CCl4
4
6
0.2
4
7
0.06
0.06
0.06
0.6
10
13
5
8
1
1
1
1
1
1
1
0
1
2
3
4
5
6
CCl4
9
6
20
toluene
toluene
dioxane
dioxane
THF
10
7
10
2
20
25
3
methanol vapor or sublimed borate. A solution of RuCl (0.04 g, 1
mol %) in methanol (4 mL) was syringed, all at once, into the
reaction flask. The hydrogen generated was measured using
THF
a
analytical gas burette. The temperature of the reaction was
The reactions were conducted in 1 M concentrations of the appropriate
b
11
maintained at 25 °C using a constant temperature bath. The
11
B
solvents at 40 °C. On the basis of B NMR spectroscopy.
NMR spectrum (δ 8.7 ppm) revealed the formation of ammonium
tetramethoxyborate. A small amount of borate crystals sublimed
during the reaction and condensed on the inner wall of the
condenser. The wet solid residue was subjected to vacuum (20 Torr)
at RT for 5 min to remove the excess methanol, and the crude
borate was transferred to a sublimation apparatus under nitrogen
and sublimed at 55 °C (oil bath temperature). Yield: 3.1 g (87%).
4
X-ray crystallography revealed the structure of the borate as [NH B-
4 5
(OMe) ] ‚2MeOH.
The aqueous solution from the trap was titrated against standard
.1 N aq HCl. The amount of borate salt sublimed during the
0
reaction was estimated to be 1% (see Results and Discussion).
General Procedure for the Kinetic Study of the Methanolysis
of AB. The transition metal catalyst was placed in a two-neck round-
bottom flask fitted with septum inlet and a reflux condenser fitted
with a connecting tube. The connecting tube was attached to an
Figure 2. ORTEP diagram of {[(NH)4]2[B4O5(OH)4]‚1.4H2O}at 50%
4
analytical gas burette filled with a CuSO solution via a gas bubbler
probability.
containing 100 mL water to trap any methanol vapor or sublimed
borate. A solution of AB (1.45 mL, 2 M) was syringed, all at once,
into the reaction flask. The hydrogen generated was measured using
an analytical gas burette. The temperature of the reaction was
maintained at 25 °C using a constant temperature bath. The time
was recorded for every 5 mL of the hydrogen liberated. The volume
of the hydrogen was plotted against time.
Table 3. Hydrolysis of AB in the Presence of Transition Metal
Catalyst
catalyst
(mol %)
reaction
time (t1/2, min)
entry
catalyst
1
2
3
4
5
6
7
8
RuCl3
RuCl3
RuCl3
RuCl3
CoCl2
CoCl2
Pd/C
0.5
0.2
0.1
0.05
3
<1
2
3
Regeneration of Ammonia Borane from Ammonium Tet-
ramethoxyborate. A suspension of ammonium tetramethoxyborate
10
3
2
12
12
(0.211 g, 0.0013 mol) and ammonium chloride (0.150 g, 0.0027
5
1
5
mol) in THF (3.5 mL) was cooled to 0 °C under a nitrogen
atmosphere. To this, under stirring, a suspension of lithium
aluminum hydride (0.08 g, 0.0016 mol) in THF (3.5 mL) was added,
dropwise, over a period of 1 h at the same temperature. The reaction
mixture was allowed to warm to RT, and the stirring was continued.
The reaction was monitored by 11B NMR spectroscopy. After
completion of the reaction (∼3 h), THF was removed under
vacuum; the solid residue was stirred in diethyl ether (70 mL) at
0 °C for 1.5 h and filtered under nitrogen, and the filtrate was
concentrated under vacuum to obtain ammonia borane. Yield: 0.025
PdCl2
a bubbler containing water (100 mL) (Figure 1). A solution of RuCl
12 mg, 0.06 mol %) in water (0.3 mL) was added dropwise via
3
(
syringe, over a period of 45 min. The temperature of the reaction
was maintained at 25 °C with a constant temperature bath. Upon
completion of the reaction, when hydrogen evolution ceased (∼3
h), the aqueous solution of ammonia from the trap was titrated
3
against standard 0.1 N aq HCl. The amount of NH was estimated
11
to be 7%. The quantity of ammonia liberated depended on the
concentration of AB solution (see Results and Discussion).
Methanolysis of AB and Isolation of Ammonium Tet-
ramethoxyborate. Ammonia borane (0.66 g, 0.0213 mol) was
placed in a two-neck round-bottom flask fitted with septum inlet
and a reflux condenser fitted with connecting tube. The connecting
g (65%) in 98% purity (hydride analysis and B NMR spectros-
copy). 11B NMR (64 MHz, THF): δ -22.1 (q).
Regeneration of Ammonia Borane using Sealed Stainless Steel
Reaction Vessel. A suspension of lithium aluminum hydride (0.16
g, 0.0041 mol) in THF (15 mL) was cooled to -78 °C and was
added, under nitrogen, to a stainless steel reaction vessel (Parr
reactor) containing a mixture of ammonium tetramethoxyborate
(0.422 g, 0.00268 mol) and ammonium chloride (0.3 g, 0.0055 mol),
tube was attached to an analytical gas burette filled with CuSO
solution via a gas bubbler containing 100 mL of water to trap any
4
7812 Inorganic Chemistry, Vol. 46, No. 19, 2007

Preparation of Ammonia Borane
Figure 3. Hydrogen generation by methanolysis of NH3BH3 (2.9 mmol) at 25 °C. Plot of volume of H2 vs reaction time using (a) 0.0625, (b) 0.125, (c)
0
.25, and (d) 0.5 mol % RuCl3.
Figure 4. Hydrogen generation by methanolysis of NH3BH3 (2.9 mmol) at 25 °C. Plot of volume of H2 vs reaction time with (a) 2 mol % NiCl2, (b) 2 mol
CoCl2, (c) 2 mol % Pd/C, (d) 2 mol % Raney Ni, (e) 2 mol % PdCl2, and (f) RhCl3.
%
1
1
and the reaction vessel was sealed immediately. The contents were
stirred at ambient pressure at RT. The reaction was monitored by
removing aliquots and examining them by 11B NMR spectroscopy.
After completion of the reaction (∼8 h), THF was removed under
vacuum, and the solid residue was stirred in diethyl ether (100 mL)
at 0 °C for 1.5 h and was filtered under nitrogen, and the filtrate
was concentrated under vacuum to obtain 0.065 g (81%) of
ammonia borane in 98% purity. 1 B NMR (64 MHz, THF): δ
salts, initially, in THF, with the aid of B NMR spectroscopy
to determine the optimal conditions. The results are sum-
marized in Table 1. Although we obtained excellent yields
(
96%) in THF at 40 °C with ammonium sulfate or am-
monium formate (Scheme 1), a low concentration of the
reaction medium with respect to SBH (e0.165 M) is crucial
for the purity and yield of the product. Higher concentrations
resulted in longer reaction times and trace amounts of
impurities. It was observed that 0.3-3% of impurities were
formed at 0.2-0.6 M concentrations, and the reaction time
also increased to 5-20 h. The B NMR spectrum showed
the impurities as a triplet at -11 ppm (suggesting a tetra-
] species) and a broad peak at -8 ppm.
A 2-8% decrease in the yield of AB was also noted. It is
1
-22.1 (q).
Results and Discussion
1
1
Our repeated attempts to prepare AB from lithium boro-
7
hydride and ammonium chloride in diethyl ether as reported
-
resulted in ∼30% yields. Hence, we examined the preparation
coordinated [>BH
2
from sodium borohydride (SBH) using various ammonium
Inorganic Chemistry, Vol. 46, No. 19, 2007 7813

Ramachandran and Gagare
Figure 5. Methanolysis of AB (first order in [AB]). A plot of concentrations of RuCl3 vs time.
Figure 6. Methanolysis of AB (first order in [AB]) using 2 mol % RhCl3, CoCl2, Raney Ni, and and Pd/C.
noteworthy that the previous report for the preparation of
AB from SBH and (NH CO required 24 h for completion,
as well as purification of AB by sublimation.
and purities of AB was obtained at 40 °C in dioxane (1 M)
using ammonium formate (Scheme 2). Under these optimized
conditions, we carried out the preparation of AB in 10 mol
scale! The solvent was recovered, distilled, and reused in
subsequent AB preparation without loss of yield or purity.
)
4 2
3
6
Scheme 1. Synthesis of Ammonia Borane from SBH and
Ammonium Sulfate in THF.
THF
Scheme 2. Synthesis of Ammonia Borane from SBH and
2
NaBH + (NH ) SO
8 2NH -BH + Na SO + 2H
4
4 2
4
3
3
2
4
2
Ammonium Formate in Dioxane.
4
0 °C
2
h
96%
Dioxane
NaBH + HCO NH
8 NH BH + NaHCO + H
4
2
4
3
5%
3
2
2
4
0 °C, 2 h
The low concentration of the reaction medium was a
9
concern for scaling up the process. Hence, the effect of
solvents on the reaction was examined, restricting ourselves
to ammonium sulfate and formate (Table 2). The best yields
While the hydrolysis11,12 and alcoholysis13,14 of borohy-
drides and other metal hydrides have been examined over
(
11) For the hydrolysis of borohydrides see (a) Brown, H. C.; Brown, C.
(12) For hydrolysis of other metal hydrides, see: (a) Kong, V. C. Y.;
Foulkes, F. R.; Kirk, D. W.; Hinatsu, J. T. Int. J. Hydrogen Energy
1999, 24 (7), 665. (b) Laurent, J. Y.; Gaillard, F.; Capron, P.; Locatelli,
D. Fr. Patent FR 2893606, 2007.
A. J. Am. Chem. Soc. 1962, 84, 1493. (b) Ptichkin, B. B. Plast. Massy
1
1
987, 2, 63. (c) Korobov, I. I.; Mozgina, M. G. Zh. Neorg. Khim.
992, 37, 1465. (d) Amendola, S. C.; Sharp-Goldman, S. L.; Janjua,
M. S.; Spencer, N. C.; Kelly, M. T.; Petillo, P. J.; Binder, M. Int. J.
Hydrogen Energy 2000, 25, 969. (e) Kojima, Y.; Suzuki, K.;
Fukumoto, K.; Sasaki, M.; Yamamoto, T.; Kawai, Y.; Hayashi, H.
Int. J. Hydrogen Energy 2002, 27, 1029. (f) Jeong, S. U.; Kim, R. K.;
Cho, E. A.; Kim, H. J.; Nam, S. W.; Oh, I. H.; Hong, S. H.; Kim, S.
H. J. Power Sources 2005, 144, 129.
(13) For alcoholysis of borohydride, see: (a) Ramachandran, P. V.; Hazra,
D; Raju, B. C.; Reddy M., V.; Bhattacharyya, A. U.S. Pat. Appl.
2005255024, 2005. (b) Davis, R. E.; Gottbrath, J. A. J. Am. Chem.
Soc. 1962, 84, 895.
(14) For methanolysis of other metal hydrides, see: Morita, Y.; Suzuki,
M. Jpn. Patent JP 2003206101, 2003.
7814 Inorganic Chemistry, Vol. 46, No. 19, 2007

Preparation of Ammonia Borane
the past several decades for hydrogen generation, a similar
reaction of alcoholysis of AB has not been studied suf-
ficiently. Unlike borane-THF or borane-dimethyl sulfide,
Table 5. First-Order Rate Constants for the Transition Metal-Catalyzed
Methanolysis of ABa
catalyst
slope
correlation
rate constant (k)
entry catalyst (mol %) (min 1) coefficient (r )
-
2
(m kg
3
-1 -1
s )
15
amine boranes do not react violently with water. AB, with
-
-
3
1a
1
2
3
4
5
6
7
8
9
RuCl3
RuCl3
RuCl3
RuCl3
RuCl3
RhCl3
CoCl2
Pd/C
0.0312
0.0625
0.125
0.250
0.5
2
2
2
2
0.026
0.079
0.174
0.428
1.019
0.504
0.056
0.026
0.070
0.9986
0.9988
0.9989
0.9998
0.9993
0.9996
0.9992
0.9971
0.9982
0.21 × 10
1.27 × 10
5.67 × 10
28.1 × 10
131 × 10
1.62 × 10
0.18 × 10
0.07 × 10
0.27 × 10
a solubility of 23% in methanol, does not liberate hydrogen
readily. A Pd and Raney Ni-catalyzed methanolic cleavage
of borane-amine complexes has been reported.16
3
-3
-3
-
3
-
3
3
3
Scheme 3. Transition Metal-Catalyzed Hydrolysis of AB.
-
-
TM catalyst
NH BH + 4H O
8 NH B(OH) h NH B O
-3
3
3
2
4
4
4
3
3
Raney Ni
RT
(
OH) h NH B O (OH) h [NH ] B O (OH) + 3H
a
4
4
4
5
4
4 2
5
6
4
2
The concentration of AB in methanol was 2 M.
Initially, we examined the hydrolysis of AB (Scheme 3).17
dependent and that such polyborate anions are formed at
higher pH.
We then carried out the alcoholysis of AB with a series
of aliphatic alcohols (see Supporting Information) and
determined that methanol provides the best result. The
screening of several transition metal catalysts for the metha-
1
9
While AB is stable in water,17a we observed the lib-
eration of 3 equiv of hydrogen in the presence of transition
metal catalysts (Table 3). We also observed the liberation
of ammonia at higher concentrations, as was determined
by a change in color of the cupric sulfate solution at the
point of contact in the analytical gas burette. To quantify
the liberated ammonia, the generated hydrogen was
passed through water at RT (Figure 1), which was then
titrated against standard 0.1 N HCl. The quantity of ammonia
liberated depends on the mol % and the rate of addition
of the catalyst (Table 4). For example, it was observed that
nolysis of AB (Table 5, 6) indicated that RuCl
of choice. It was observed that 1 mol % RuCl
3
is the catalyst
liberated 3
3
equiv of hydrogen in ∼1 min. A plot of the volume of
hydrogen liberated versus the reaction time at various mol
%
3
of RuCl and other transition metal catalysts at ambient
conditions are presented in Figures 3 and 4.
The methanolysis of AB, under our experimental condi-
1
5 wt % AB solution produced 6% NH
3
, when 0.2 mol %
, when added,
tions, in the presence of RuCl
Raney Ni was found to be first order in [AB]. However, it
showed zero-order kinetics in the presence of NiCl and
PdCl . The plot of ln([AB]°/[AB]) against time for the first-
3 3 2
, RhCl , CoCl , Pd/C, and
RuCl was added within 15 min, and 3% NH
3
3
slowly, in 45 min. At low catalyst concentrations, the
mode of addition did not show a significant effect on
2
the quantity of NH
solution produced 4.4% NH
was added within 15 min, and 4% NH
5 min. We varied the AB and catalyst concentrations
and found no liberation of NH , detectable by titration, with
wt % AB and 0.06 wt % catalyst under slow addition mode.
3
liberated. Thus, the same 15 wt % AB
, when 0.06 mol % RuCl
, when added within
2
order reaction is shown in the Figures 5 and 6. The values
of the first-order rate constant (Table 5) were determined
from the slopes of the straight lines (integrated rate law for
the first-order reaction, ln([AB]°/[AB]) ) (mcatk/V)t, where
V is reaction volume and mcat is catalyst mass). The slopes
for the zero-order reactions were obtained from the plot of
3
3
3
4
3
6
However, we did not carry out ammonia detection using
electrochemical sensors. Ammonia liberated during AB
hydrolysis can be detrimental to fuel cells, which should be
either curtailed or efficiently trapped.
Depending on the concentrations of AB, the 11B NMR
resonance of the hydrolysis reaction mixture varied between
-16 ppm because of an equilibrium of different borate
species (Scheme 3). At higher concentrations, the borate salt
crystallized from the reaction solution, which showed a
[AB]° - [AB] as a function of time (Figure 7). The rate
constants (k) were calculated (Table 6) using the integrated
rate law for the zero-order reaction ([AB]° - [AB]) )
(m kt)/V.
cat
Scheme 4. RuCl -Catalyzed Methanolysis of AB.
3
8
1
% RuCl3
5
NH BH + 22MeOH RT, 1 min8
3 3
singlet (in H
salt was determined as ammonium tetraborate {[(NH)
(OH) ]‚1.41H O} by X-ray crystallography (Figure 2).
It should be noted that the structure of borate anions are pH
2
O) at δ 12 ppm. The structure of the crystallized
[NH B(OMe) ] ‚2MeOH + 15H
4 4 5 2
4 2
] -
We then focused on the isolation, characterization, and
recycling of the intermediate ammonium tetramethoxybo-
rate formed during the reaction. Because of the solubility
of the borate salt in methanol, we treated AB with stoichio-
metric amounts of methanol in the presence of 1 mol %
[B O
4 5
4
2
1
8
(
15) AB has 26% solubility in water. A 10% water solution of ammonia
borane loses 1.8% hydrogen over 4 days, 4.8% over a month, and
4
5% over 18 months, see ref 1a.
(
16) Couturier, M.; Tucker, J. L.; Andresen, B. M.; Dube, P.; Negri, J. T.
Org. Lett. 2001, 3, 465.
RuCl
ated. A 15% excess is necessary for complete methanolysis
gravimetric density 3.4 wt % hydrogen) (Scheme 4). During
3
and observed that <3 equiv of hydrogen was gener-
(
17) (a) Chandra, M.; Xu, Q. J. Power Sources 2006, 159, 855. (b) Chandra,
M.; Xu, Q. J. Power Sources 2006, 156, 190. (c) Mohajeri, N.;
Adebiyi, O.; Baik, J.; Bokerman, G.; Raissi, A. T. Prep. Symp.sACS,
DiV. Fuel Chem. 2006, 51, 520.
(
the reaction, the crystals of the borate salt condensed on the
inner wall of the condenser. Upon completion of the
(18) (a) Tiess, D. Chem. Tech. 1959, 11, 260. (b) Wilson, J. W. J. Chem.
Soc., Dalton Trans. 1973, 1628.
(19) (a) Bhattacharyya, A.; Hall, D. B. Inorg. Chem. 1992, 31, 3869. (b)
Lin, J.-T.; Tsai, S.-J.; Cheng, S. J. Chin. Chem. Soc. 1999, 46, 779.
(20) Ramachandran, P. V.; Gagare, Pravin D.; Raju, B. C. Patent application
number 60/817,911, filed on June 30, 2006.
Inorganic Chemistry, Vol. 46, No. 19, 2007 7815

Ramachandran and Gagare
Figure 7. Methanolysis of AB (zero order in [AB]) using 2 mol % NiCl2 and PdCl2.
Table 6. Zero-Order Rate Constants for the Transition Metal-Catalyzed
Methanolysis of AB
Table 7. Crystallographic Data of {[(NH)4]2[B4O5(OH)4]‚1.41H2O}
and [NH4B(OMe)4]5‚2MeOH
a
rate
constant (k)
chemical formula
a
b
c
V
Z
fw
H4B4O9‚1.41(H2O)‚2(H4N)
7.1997(8) Å
10.6039(10) Å
7.2604(8) Å
548.15(10) Å3
2
252.75
P21 (No. 4)
260 K
C22H88B5N5O22
22.4122(9) Å
12.0086(3) Å
18,8850 (5) Å
5082.7 (3) Å
4
829.02
Pca21 (No. 29)
150 K
catalyst
slope
correlation
3
-1
2
-1 -1
entry catalyst (mol %) (mol m min ) coefficient (r ) (mol kg
s
)
-
3
3
1
2
NiCl2
PdCl2
2
2
49.72
25.79
0.9999
0.9996
160 × 10
60 × 10
3
-
a
The concentration of AB in methanol was 2 M.
space group
T
λ
0.71073 Å
1.531 g cm
0.064
Mo KR 0.71073 Å
-
3
-3
Dobsd
R (F0)
1.083 g cm
0.051
0.102
2
Rw (F0 )
0.169
which was confirmed by X-ray crystallography to be [NH
4
B-
(
OMe) ‚2MeOH (Table 7). While the B/N ratio is 1:1 in
4 5
]
the ammonium tetramethoxyborate from the methanolysis,
it is 2:1 in the ammonium tetraborate from hydrolysis. In
fact, this should account for a loss of 50 wt % of NH .
3
However, we observed only 1-7% of ammonia liberation,
depending on the concentrations. This may be accounted by
the formation of other ammonium borate species, such as
Figure 8. ORTEP diagram of [NH4B(OMe)4 ]5‚2MeOH at 50% prob-
ability.
NH
solution.
4 4 4 5 6 4 4 3 3 4
B(OH) , NH [B O (OH) ], and NH [B O (OH) ] in
19b
1
1
The recycling of the tetramethoxyborate was then under-
taken to achieve an efficient recyclable hydrogen generation
methanolysis ( B NMR, 8.7 ppm), the crude borate was
subjected to vacuum (20 Torr) at room temperature for a
short period of time. Extensive drying results in the loss of
the borate caused by sublimation. Careful sublimation (∼50-
system from AB. In our initial attempts to reduce NH
OMe) directly to AB using metal hydrides, we obtained
and sodium borohydride
4
B-
(
4
poor (∼10%) yields with LiAlH
4
5
4 °C) of the solid borate provided 87% yield of an
orthorhombic crystalline material, [NH
Figure 8). A unit cell contains four of the asymmetric
pentamer units of ammonium borate with two methanol
molecules of crystallization. The presence of sublimed borate
This is the result of the loss of ammonia from the borate
salt in the presence of LAH. Upon further examination, we
4 4 5
B(OMe) ] ‚2MeOH
(
observed that the addition of NH
was necessary to improve the yield. Thus, the treatment of
the recovered ammonium tetramethoxyborate with LiAlH
and NH Cl in THF for 3 h at RT and atmospheric pressure
4
Cl as a source of ammonia
11
salt in the water trap was confirmed by the B NMR, which
4
11
was quantified by titration (∼1%). The B NMR spectrum
of the titrated solution showed singlet at peak at 19 ppm for
the boric acid. Unlike the hydrolysis, we believe that
methanolysis does not liberate NH
be explained by the structure of the methanolysis product,
4
yielded 65% of AB. We improved the yield further to 81%
by carrying out the reaction in a sealed reactor for 8 h
(Scheme 5). The purity of AB as determined by elemental
analysis and hydrogen estimation is >98%. Thus, we have
3
. This can be probably
7816 Inorganic Chemistry, Vol. 46, No. 19, 2007

Preparation of Ammonia Borane
transition metal-catalyzed hydrolysis and methanolysis of AB
and the isolation and characterization of crystalline am-
monium borates from both of these reactions. The concentra-
tion-dependent ammonia liberation during the hydrolysis can
be detrimental for fuel cell applications and has been
quantified. The ammonia liberation demands low concentra-
tion AB solutions for hydrogen generation via hydrolysis,
which decreases the wt % of hydrogen to ∼1%. While the
recycling of AB from hydrolysis product is a multistep
Figure 9. Ammonia-borane hydrogen cycle.
achieved the first successful hydrogen generation cycle
involving ammonia borane (Figure 9).
3 3 4 3
process (borate f B(OH) f B(OMe) f NaBH f NH -
BH ), an efficient one-pot regeneration of ammonia borane
3
from the methanolysis intermediate, ammonium tetramethox-
yborate, has been achieved.
Scheme 5. Regeneration of AB from Ammonium
Tetramethoxyborate.
THF
Acknowledgment. Financial support from General Atom-
ics/U.S. Army is gratefully acknowledged. We thank Dr.
Karangula Jyothi for some preliminary experiments on the
alcoholysis and Dr. Phil Fanwick of our department for the
X-ray analysis.
NH B(OMe) + NH Cl + LiAlH
8 NH BH + Al
3 3
4
4
4
4
0
°C to RT
-8 h
3
(
OMe) + MeOH + H + LiCl + NH
3 2 3
Conclusion
Supporting Information Available: Experimental details,
spectral data, and X-ray crystallographic data of compounds. This
material is available free of charge via the Internet at http://
pubs.acs.org.
In summary, we have reported a 10 mol scale synthesis
of ammonia borane from sodium borohydride and am-
monium formate in dioxane (1 M) in near quantitative yield
and purity. This synthesis should aid in the progress of
fuel-cell research involving AB. We have also reported the
2
0
IC700772A
Inorganic Chemistry, Vol. 46, No. 19, 2007 7817