Interaction of lithium hydride and ammonia borane in THF

Article abstract of DOI:10.1039/b812576g

The two-step reaction between LiH and NH₃BH₃ in THF leads to the production of more than 14 wt% of hydrogen at 40 °C. The Royal Society of Chemistry.

Full text of DOI:10.1039/b812576g

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Interaction of lithium hydride and ammonia borane in THFw
ab
c
a
c
abc
Zhitao Xiong, Yong Shen Chua, Guotao Wu, Weiliang Xu, Ping Chen,*
d
d
d
d
Wendy Shaw, Abhi Karkamkar, John Linehan, Tricia Smurthwaite and
d
Thomas Autrey*
Received (in Cambridge, UK) 22nd July 2008, Accepted 22nd August 2008
First published as an Advance Article on the web 29th September 2008
DOI: 10.1039/b812576g
The two-step reaction between LiH and NH BH in THF leads
3
purity of 99.9%. Typically, a suspension was prepared by
blending LiH (67.4 mg, 0.008 mol) and a solution of AB
3
to the production of more than 14 wt% of hydrogen at 40 1C.
(
275.6 mg, 0.008 mol) in THF (30 ml). A commercial Stirred
The demand for high capacity on-board hydrogen storage
systems for hydrogen fuel cell vehicles has stimulated tremen-
dous efforts in materials research and development. Recent
research efforts have focused on materials that are composed
Tank Reactor (STR) combined with a mass spectrometer was
employed to evaluate hydrogen evolution from samples.
Gaseous products were conducted to the mass spectrometer
for analysis. Liquid NMR characterizations were performed
on a Bruker AMX (500 MHz) Fourier transform NMR
1
of hydrogen bound to light elements including borohydrides,
2
3–5
MOFs,
ammonia borane. With a hydrogen capacity of 19.6 wt%,
ammonia borane (NH BH , AB for short) shows promise for
hydrogen storage; however, decomposition gives a volatile side
amide–hydride combination systems,
and
spectrometer. High-field solid NMR was conducted on a
1
6
–9
Varian Unity Inova console operating at 900 MHz
H
frequency. Structural identifications were performed on a
Bruker X-ray diffractometer equipped with an in situ cell.
Fig. 1 shows the time dependence of hydrogen release from
the LiH–AB THF suspension (LiH/AB molar ratio is 1 : 1) at
40, 50 and 55 1C, respectively. Gas evolution from the
suspension is a two-step process; each step has an induction
period which can be effectively shortened through temperature
increase. Hydrogen, the only detectable gaseous product, was
calculated by the ideal equation of state and found to be
ca. 1.0 and 1.8 equiv. for the first and second steps, respec-
tively. Therefore, a total of ca. 2.8 equiv., or 14.3 wt% of
hydrogen based upon added AB and LiH, were evolved at
temperatures as low as 40 1C. Compared with the solid-state
3
3
1
0
product, borazine, that will poison PEM fuel cells. In
addition, the desorption of hydrogen from this chemical
hydride has a sufficient barrier to render the kinetics too slow
6
at temperatures below 80 1C. Our recent effort to modify the
kinetics and thermodynamics of H2 release from AB by
replacing one H with an alkali metal, i.e., alkali amidoboranes
1
1
(
MNH
These amidoboranes release large amount of hydrogen
exp. 10.9 wt%) with no measurable borazine formation,
however, the kinetics for H desorption are still insufficient.
2 3
BH , M = Li and Na) provided some improvement.
(
2
Previous investigations on hydrogen release from complex
hydrides revealed that mass transport or mobility of the
1
2
reacting species is one of the kinetic controlling factors.
The mobility of the species can be enhanced substantially
upon dissolution in a solvent. In the present study we inves-
tigate the reactivity of AB dissolved in tetrahydrofuran (THF)
with a suspension of LiH and observe the formation of
2 3
LiNH BH and enhanced reaction kinetics with an interesting
dependence on AB concentration. More than 2.8 equiv. of
hydrogen can be released in two steps at a temperature as low
as 40 1C.
LiH and AB were purchased from Sigma-Aldrich with
stated purities of 95% and 90%, respectively, and were used
without further purification. THF was a Tedia product with a
a
Dalian Institute of Chemical Physics, Dalian, China 116023.
E-mail: pchen@dicp.ac.cn; Fax: +86 411-84685940;
Tel: +86 411-84379905
Department of Physics, National University of Singapore,
Singapore 117542
Department of Chemistry, National University of Singapore,
Singapore 117542
Pacific Northwest National Laboratory, Richland, WA 99352, USA.
E-mail: tom.autrey@pnl.gov; Fax: +1 509-375-6660
w Electronic supplementary information (ESI) available: Fig. S1–S5.
See DOI: 10.1039/b812576g
Fig. 1 Time dependence of hydrogen release from (a) AB–THF
solution (0.27 M) at 55 1C and from the LiH–AB THF suspension
(AB concentration is 0.27 M at temperatures of (b) 40 1C, (c) 50 1C, (d)
b
c
5
5 1C, respectively. The induction period for the first step ranges from
d
10 minutes to an hour depending on the operation temperatures. The
inset shows the plot of lnk versus 1/T for Steps I and II. The slopes of
the best-fit lines were used to determine activation energies by applying
the Arrhenius equation.
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Chem. Commun., 2008, 5595–5597 | 5595

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dehydrogenation of lithium amidoborane (the intermediate
1
1
product of LiH + AB) the THF-mediated dehydrogenation
takes place at lower temperatures and with increased rates.
The kinetic analyses of the dehydrogenation steps revealed
that the first step follows a first-order rate law, with an
À1
apparent activation energy of 46.6 kJ mol (Fig. 1). Increas-
ing the amounts of AB and LiH resulted in an enhanced
reaction rate (Fig. 2). In the second step, a nearly linear
increase of hydrogen pressure was observed, indicating that
the corresponding reaction followed a zero-order law in
2 3
reactant (LiNH BH , see below) concentration, which is
further supported by the observation of the independence of
the reaction rate with the concentration of reactant (Fig. 2).
Although further in-depth analyses are needed, it indicates
that hydrogen release in the second step may not be directly
from the initial reactant (LiNH BH ). The rate constant
2
3
measured at different temperatures gives apparent activation
À1
energy of 83.2 kJ mol (Fig. 1). Decomposition of AB in
THF alone was monitored at 55 1C for comparison. It is
notable that only negligible amounts of hydrogen was liber-
ated even after 15 h of reaction at the same concentrations.
1
1
Fig. 3 shows the B NMR spectra of the pristine AB–THF
and LiH–AB THF samples collected at different reaction
11
Fig. 3
B NMR spectra of (A) NH BH THF solution reacted at
3
3
intervals. The BH
borazane (CTB, [NH BH ] ), the BH resonance of borazine
2
resonance (at À11.0 ppm) of cyclotri-
55 1C and (B) LiH–NH
indicated reaction times.
3 3
BH THF suspension at 40 1C collected at the
2
2 3
(
(
[NHBH] , at 30.6 ppm) and the BH (À24.3 ppm), BH
3
3
2
À11.3 ppm) and BH (À5.4 ppm) resonances of B-(cyclodi-
position products of AB in THF is B-(cyclodiborazanyl)-
aminoborohydride (BCDB). If this species is formed in com-
1
3
borazanyl)aminoborohydride (BCDB) were detected after
heating the AB–THF solution at 55 1C, respectively. AB
petition with LiNH
to give lithium substitution on the NH
result in the observed downfield chemical shift, i.e., the BH
(À5.4 to À3.8 ppm), the BH (À11.3 to À8.3 ppm) and the
BH
2
BH
3
then a subsequent reaction with LiH
(
BH
of reaction. In contrast, when LiH was introduced to the
AB–THF solution, BH, BH , BH , and BH resonances with
3
at À22.1 ppm) is still the dominant species after 63 hours
2
in the BCDB ring will
2
3
4
2
chemical shifts of À3.8, À8.3, À23.5 and À41.7 ppm, respec-
3
(À24.3 to À23.5 ppm). To confirm this, we added LiH to
tively, were developed in the first induction period. The peak
at À41.7 ppm is close to where LiBH
a solution of AB that was partially decomposed to BCDB and
observed the corresponding chemical shifts (Fig. S1). After
completion of the first dehydrogenation step, the solution
4
is observed but the BH,
BH , and BH peaks at À3.8, À8.3 and À23.5 ppm are
2
3
1
1
interesting new species that may arise from a minor side
reaction(s). From Fig. 3A, we know that one of the decom-
contains no visible solid residue and the dominant B signal
shifts from À22.1 ppm (pristine AB) to À21.9 ppm (Fig. 3B).
7
The Li resonance also appears to shift from À0.1 ppm to
0
.4 ppm (Fig. S2).
Removing the THF solvent at this point yields a polycrys-
talline white solid with a XRD pattern (Fig. S3) identical to
lithium amidoborane, LiNH BH , synthesized by mechanical
2
3
1
2
ball milling of AB and LiH (molar ratio 1 : 1). This
observation supports the chemical transformation as shown
in eqn (1).
NH
3
BH
3
+ LiH - LiNH
2
BH
3
+ H
2
(1)
One of the driving forces for this reaction pathway may
come from an interaction between the Hd+ in NH
LiH to form H
To follow the hydrogen release in the second dehydrogena-
and H in
dÀ
3
2
.
1
1
tion step, the in situ B NMR spectra of LiNH
were monitored at 50 1C, as shown in Fig. 4. The linear
decrease of BH
2 3
BH in THF
3
at À21.9 ppm (Fig. S4) corresponds directly
with the linear increase of hydrogen pressure observed in the
initial experiments shown for the second step in Fig. 1. The
boron species at À23.9 ppm is present through the course of
Fig. 2 Dependence of concentration on hydrogen release from the
LiH–AB (1 : 1 molar ratio) THF suspension in Step I and Step II,
respectively, at 50 1C. & À0.4 M, J À0.53 M, n À0.67 M.
5
596 | Chem. Commun., 2008, 5595–5597
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The THF-mediated reaction of AB in a suspension of LiH
2
yields 2.8 equiv. or 14.3 wt% of H from the mixture of LiH +
AB. The high H capacity and low temperatures for activation
2
suggest that new approaches should be further investigated for
this hydrogen storage complex. Although the solvent contri-
butes extra weight to the system, an appropriate system design
will minimize this side effect. From a scientific point of view,
the overall dehydrogenation is rich in chemistry. Further
investigations are needed to identify intermediates formed in
the induction periods and to understand the dehydrogenation
mechanism.
The authors wish to acknowledge supports from the Dalian
Institute of Chemical Physics, China, the National University
of Singapore, Singapore, and the U.S. DOE CoE in Chemical
Hydrogen Storage. This work was performed as a collabora-
tion established by the IPHE project ‘‘Combination of Amine
Boranes with MgH & LiNH for High Capacity Reversible
1
1
Fig. 4 In situ B NMR spectra of LiNH
0 1C at intervals of 2 minutes.
2 3
BH in THF recorded at
5
2
2
Hydrogen Storage.’’ A portion of the research described in this
paper was performed in the Environmental Molecular
Sciences Laboratory, a national scientific user facility located
at PNNL. PNNL is operated for the DOE by Battelle.
the dehydrogenation; however, the intensity does not appear
to change much with time while that of the LiNH BH species
is clearly decreasing. Although there is no authentic
2
3
1
compound to identify this minor species, a quartet in the H
undecoupled spectra, it is likely the terminal BH3 in a
partially dehydrogenated dimerized LiNH BH , such as
2
3
Notes and references
1
Li [NHBHNHBH ]. It also does not appear to release
4
2
3
1
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2 N. L. Rosi, J. Eckert, M. Eddaoudi, D. T. Vodak, J. Kim, M.
hydrogen. Further investigations are in progress to identify
this species.
O’Keeffe and O. M. Yaghi, Science, 2003, 300, 1127.
A white solid product(s) was precipitated from the solution,
which is amorphous in nature. It is clear from the decrease in
3
4
(a) P. Chen, Z. T. Xiong, J. Z. Luo, J. Y. Lin and K. L. Tan,
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intensity of LiNH
it converts to a THF-insoluble product. The difference in the
weight of the starting material (LiNH BH ) and the weight of
2 3
BH that, as this species releases hydrogen,
2
3
the solid residue corresponds to the weight loss expected for
the quantity of observed hydrogen (ca. 1.8 equiv. H ). High
2
1
field B NMR measurements show that the major B species in
1
5 F. E. Pinkerton, G. P. Meisner, M. S. Meyer, M. P. Balogh and M.
D. Kundrat, J. Phys. Chem. B, 2005, 109, 6.
the solid product has a planar –B(N H) environment in
2
6
(a) G. Wolf, J. Baumann, F. Baitalow and F. Hoffmann,
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Linehan, B. Schmid and T. Autrey, Phys. Chem. Chem. Phys.,
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Taking into consideration the purity of the starting materials
and possible side reaction(s), the full 2.0 equiv. H
from LiNH BH can be represented by eqn (2).
2
released
2
3
2007, 9, 1831; (d) M. Gutowski and T. Autrey, Abstr. Pap. Am.
Chem. Soc., 2004, 227, U1088.
LiNH BH - [LiBNH] + 2H
2
(2)
2
3
7
8
9
M. C. Denney, V. Pons, T. J. Hebden, M. Heinekey and K. L.
Goldberg, J. Am. Chem. Soc., 2006, 128, 12048.
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2
It is interesting to note that there are no BH resonances
observed in either the in situ NMR experiments or the residual
solids, indicating that the 2nd equivalent of hydrogen is
released concurrently with the release of the 1st equivalent
of hydrogen. This observation is important as it suggests the
presence of lithium in the PAB-like products is destabilizing,
resulting in a low barrier pathway for release of the 2nd
equivalent of hydrogen. This is very different from the
25, 2613; (b) R. T. Keaton, J. M. Blacquiere, M. Johanna and
R. T. Baker, J. Am. Chem. Soc., 2007, 129, 1844.
1
0 M. G. Hu, R. A. Geanangel and W. W. Wendlandt, Thermochim.
Acta, 1978, 23, 249.
1
1 Z. T. Xiong, C. K. Yong, G. T. Wu, P. Chen, W. Shaw, A.
Karkamkar, T. Autrey, M. O. Jones, S. R. Johnson, P. P. Edwards
and W. I. F. David, Nat. Mater., 2008, 7, 138.
observations for pristine AB, where several BH
2
species are
observed and there is a higher barrier for release of the 2nd
equivalent of H
products.
12 P. Chen, Z. T. Xiong, L. F. Yang, G. T. Wu and W. F. Luo,
J. Phys. Chem. B, 2006, 110, 14221.
2
from the resulting polyaminoborane (PAB)
1
3 W. Shaw, J. Linehan, N. Szymczak, D. Heldebrant, C. Yonker, D.
Camaioni, R. T. Baker and T. Autrey, Angew. Chem., Int. Ed.,
The overall reaction of LiH and AB in THF can be
expressed by eqn (3).
2
008, 120, 7603.
14 BH with similar chemical shift has been observed in
Ca(NHBHNHBH : J. Spielmann, G. Jansen, H. Bandmann
3
a
3 2
)
NH
3
BH
3
+ LiH - LiNH
2
BH
3
+ H
2
- [LiBNH]+3H
2
(3)
and S. Harder, Angew. Chem., Int. Ed., 2008, 120, 6386.
This journal is ꢀc The Royal Society of Chemistry 2008
Chem. Commun., 2008, 5595–5597 | 5597