Ammonia triborane: A promising new candidate for amineborane-based chemical hydrogen storage

Article abstract of DOI:10.1021/ja064526g

A convenient and safe method for the synthesis of ammonia triborane is reported along with studies of its hydrolytic reactions that demonstrate ammonia triborane is both soluble and stable in water but that upon the addition of acid or an appropriate transition metal catalyst it rapidly releases hydrogen. These studies indicate that ammonia triborane is a promising material for chemical hydrogen storage applications. Copyright

Full text of DOI:10.1021/ja064526g

Published on Web 10/07/2006
Ammonia Triborane: A Promising New Candidate for Amineborane-Based
Chemical Hydrogen Storage
Chang Won Yoon and Larry G. Sneddon*
Department of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323
Received June 26, 2006; E-mail: lsneddon@sas.upenn.edu
Because of their high hydrogen densities, boron based com-
1
pounds, including especially sodium borohydride (NaBH
4
) and
2
3 3
ammonia borane (NH BH ), are now being intensely investigated
as chemical hydrogen storage materials that can release hydrogen
by hydrolytic processes (eqs 1-2).
+
-
2
NaBH (s) + 2H O(l) f Na (aq) + BO (aq) + 4H (g) (1)
4
2
2
+
4
-
2
NH BH (s) + 2H O(l) f NH (aq) + BO (aq) + 3H (g)
3
3
2
2
(2)
Figure 1. Acid-induced hydrogen release from ammonia triborane.
The high hydrogen release capacity that could potentially be
achieved by ammonia triborane, NH , (1) oxidative-hydrolysis
eq 3) should likewise make it an attractive candidate for chemical
3 3 7
B H
(
hydrogen storage. But, owing to the lack of a suitable method for
its efficient synthesis, there have been no previous explorations of
the chemical hydrogen storage properties of this compound. We
NH B H (s) + 6H O(l) f
3
3
7
2
+
-
2
+
NH (aq) + 3BO (aq) + 2H (aq) + 8H (g) (3)
4
2
report here both a convenient and safe method for the synthesis of
ammonia triborane and our initial studies of its hydrolytic hydrogen-
release properties that indicate it is a promising new material for
chemical hydrogen storage applications.
Figure 2. Metal-catalyzed hydrogen release from aqueous (∼0.45 wt %)
ammonia triborane solutions containing (f) RhCl3 (6.9 mol %); (9) 5 wt
%-Rh/Al O (7.0 mol %-Rh); (b) [Rh(COD)(Cl)] (7.2 mol %); (2) 5 wt
2
3
2
%-Rh/Al2O3 (1.4 mol %-Rh); ([) no catalyst.
3
Ammonia triborane was first synthesized by Kodama via the
cleavage reaction of tetraborane, B
LB (L ) tetrahydrofuran or tetrahydropyran) adducts (plus B
that, following displacement of the ethers by reaction with
anhydrous ammonia, then produced NH . Unfortunately,
tetraborane is a volatile unstable compound that is explosive in
air, and, as a result, any large-scale synthesis based on its use is
not feasible.
4
H
10, with ethers to initially form
1 is significantly more soluble in water than NH
% ), with NH B H
3 3 7
3
BH
solutions of at least 33 wt % being attained.
Also, unlike aqueous NaBH , which is stable only in strongly
alkaline solutions,1 aqueous NH
solutions are quite stable
3
(26 wt
2d
H
3 7
2 6
H )
4
b,c
3
B
H
3 7
3 3 7
B H
11
at pH 7.5 in air as evidenced by B NMR studies that showed a
25 wt % solution remained unchanged over 4 days.
Efficient hydrogen release from aqueous solutions of 1 was
obtained upon the addition of either acids or appropriate metal-
catalysts. As shown in Figure 1, quantitative measurements of
hydrogen release using a gas buret showed that when an excess of
aqueous HCl (1 mL of 12.1 M HCl) was added to 12 mg (0.21
mmol) of 1, a near theoretical value of 7.85 equiv (1.65 mmol) of
4
+
-
Binder previously reported that I
in noncoordinating solvents (e.g., CH
14. We have now found that iodine oxidation (with slow
2
oxidation of Me
Cl ) yielded the B
2
4
N B
3 8
H
2
H -dimer,
3 7
6
B H
+
-
warming from -70 to 20 °C) of the air stable Bu
glyme (1,2-dimethoxyethane) (eq 4) allows the efficient preparation
of solutions of the (glyme)B adduct. Direct treatment (eq 5) of
these (glyme)B solutions with anhydrous ammonia then yields
pure NH as a colorless crystalline solid in >80% yields (based
4 3 8
N B H salt in
H
3 7
H
2
was evolved over ∼1 h.
H
3 7
More rapid hydrogen release was achieved using metals to
3
B
3
H
7
catalyze the hydrolysis reaction. Hydrogen release from NaBH
NH BH has previously been attained by metal-catalyzed hydrolysis,
with Ru-catalysts for NaBH
4
and
+
-
+
-
on Bu
prepared5 by iodine oxidation of BH
synthesis of NH is now possible.
4
N B
3
H
8
). Since the Bu
4
N B
H
3 8
salt can itself be readily
, the large-scale, safe
3
3
-
1
and Pt-catalysts2 for NH
BH
4 3 3
4
3
B
3
H
7
exhibiting the highest reactivities. We have now screened a variety
of potential catalysts for ammonia triborane hydrolysis and found
high activities for the rhodium based systems, Rh(0) supported on
+
-
Bu N B H + 0.5I + glyme f
4
3
8
2
alumina, [Rh(COD)(µ-Cl)]
2
(COD ) 1,5-cyclooctadiene), and
(
glyme)B H + Bu NI + 0.5H (4)
3 7 4 2
RhCl (Figure 2). With high catalyst loadings of RhCl
3
3
, ∼7 equiv
of hydrogen were released in only 1.5 min at room temperature.
Although they are unchanged upon initially dissolving in water,
(
glyme)B H + NH f NH B H + glyme
(5)
3
7
3
3
3
7
13992
9
J. AM. CHEM. SOC. 2006, 128, 13992-13993
10.1021/ja064526g CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Figure 5. Rhodium (5 wt %-Rh/Al2O3, 1.0 mol %-Rh) catalyzed H2 release
from a 22.7-wt % aqueous solution of ammonia triborane (1.8 mmol)
yielding 6.1 wt % H2 [wt % ) H2-wt/(NH3B3H7 + H2O + Rh/Al2O3-wts)].
Figure 3. Hydrogen evolution following repeated additions of ammonia
triborane to an aqueous solution containing 5 wt %-Rh/Al2O3.
system. Under these concentrated conditions, 11B NMR studies show
that the hydrolysis reaction yields condensed polyborates, which
-
10
3 4
upon addition of water form mixtures of B(OH) /B(OH) . Thus,
the real utility of this process as a chemical hydrogen storage system
will ultimately depend on the development of new “off-board”
methods to regenerate ammonia triborane from these borates.
Acknowledgment. We thank the U.S. Department of Energy
Center of Excellence for Chemical Hydrogen Storage for support.
We also thank Dr. Goji Kodama for his helpful comments.
Figure 4. Hydrogen release at different temperatures (1, 50; 2, 35; b,
Supporting Information Available: Experimental procedures for
the synthesis of NH B H and the hydrogen release studies. This
3 3 7
material is available free of charge via the Internet at http://pubs.acs.org.
2
0; 9, 0 °C) versus time for 4.9 wt % aqueous solutions of ammonia
triborane catalyzed by 5 wt %-Rh/Al2O3 (1.1 mol %-Rh).
2 3
both the [Rh(COD)(µ-Cl)] and RhCl appear to undergo reduction
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(
2
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9
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