Ammonia borane hydrogen release in ionic liquids

Article abstract of DOI:10.1021/ic901560h

The rate and extent of H₂-release from ammonia borane (AB), a promising, high-capacity hydrogen storage material, was found to be enhanced in Ionic-liquid solutions. For example, AB reactions in 1 -butyl-3- methyllmidazolium chloride (bmimCl) (

Full text of DOI:10.1021/ic901560h

Inorg. Chem. 2009, 48, 9883–9889 9883
DOI: 10.1021/ic901560h
Ammonia Borane Hydrogen Release in Ionic Liquids
Daniel W. Himmelberger, Laif R. Alden, Martin E. Bluhm, and Larry G. Sneddon*
Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104-6323
Received August 5, 2009
The rate and extent of H2-release from ammonia borane (AB), a promising, high-capacity hydrogen storage material,
was found to be enhanced in ionic-liquid solutions. For example, AB reactions in 1-butyl-3-methylimidazolium chloride
(bmimCl) (50:50-wt %) exhibited no induction period and released 1.0 H2-equiv in 67 min and 2.2 H2-equiv in 330 min
at 85 °C, whereas comparable solid-state AB reactions at 85 °C had a 180 min induction period and required 360 min
to release ∼0.8 H2-equiv, with the release of only another ∼0.1 H2-equiv at longer times. Significant rate
enhancements for the ionic-liquid mixtures were obtained with only moderate increases in temperature, with, for
example, a 50:50-wt % AB/bmimCl mixture releasing 1.0 H2-equiv in 5 min and 2.2 H2-equiv in only 20 min at 110 °C.
Increasing the AB/bmimCl ratio to 80:20 still gave enhanced H2-release rates compared to the solid-state, and
produced a system that achieved 11.4 materials-weight percent H2-release. Solid-state and solution 11B NMR studies
of AB H2-release reactions in progress support a mechanistic pathway involving: (1) ionic-liquid promoted conversion
of AB into its more reactive ionic diammoniate of diborane (DADB) form, (2) further intermolecular dehydrocoupling
reactions between hydridic B-H hydrogens and protonic N-H hydrogens on DADB and/or AB to form neutral
polyaminoborane polymers, and (3) polyaminoborane dehydrogenation to unsaturated cross-linked polyborazylene
materials.
Introduction
reactions of molecular chemical hydrides. We report here that
ionic liquids provide advantageous media for ammonia bor-
ane dehydrogenation in which both the extent and rate of
hydrogen release are significantly increased. We also present
solid-state and in situ 11B NMR studies of reactions in progress
that provide insight into the intermediates and mechanistic
steps involved in ionic-liquid promoted AB H2-release.
The requirement for efficient and safe methods for hydro-
gen storage is a major hurdle that must be overcome to enable
the use of hydrogen as an alternative energy carrier.1,2 Owing
to its high hydrogen content, ammonia borane (AB) has been
identified as one of the leading candidates for chemical
hydrogen storage, potentially releasing 19.6 wt % H2 accord-
ing to eq 1.3
Experimental Section
Materials. All manipulations were carried out using standard
high-vacuum or inert-atmosphere techniques as described by
Shriver.6 Ammonia borane (Aviabor 97% minimum purity) was
ground into a free-flowing powder using a commercial coffee
grinder. The diammoniate of diborane (DADB) was synthesized
by the literature method.7 The 1-butyl-3-methylimidazolium
iodide (bmimI) was synthesized sonochemically from 1-iodobu-
tane and 1-methyl-imidazole according to literature methods.8
All ionic liquids, including 1-butyl-2,3-dimethylimidazolium
chloride (bmmimCl) (EMD), 1-butyl-3-methylimidazolium tetra-
fluoroborate (bmimBF4), 1-butyl-3-methylimidazolium chloride
(bmimCl), 1-butyl-3-methylimidazolium triflate (bmimOTf), 1-bu-
tyl-3-methylimidazolium hexafluorophosphate (bmimPF6), 1-ethyl-
2,3-dimethylimidazolium ethylsulfate (emmimEtSO4), 1-ethyl-2,3-di-
methylimidazolium triflate (emmimOTf), 1,3-dimethylimidazolium
H3NBH3 f BNþ3H2
ð1Þ
Partial dehydrogenation of ammonia borane can be ther-
mally induced in the solid-state,4,5 but to be useful for hydro-
gen storage, milder conditions and more controllable reactions
still need to be developed. Such reactions could, in principle, be
attained in solution, but practical applications of chemical
hydrogen storage would require a replacement for the volatile
organic solvents that have traditionally been employed for
*To whom correspondence should be addressed. E-mail: lsneddon@
sas.upenn.edu.
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2009 American Chemical Society
Published on Web 09/21/2009
pubs.acs.org/IC

9884 Inorganic Chemistry, Vol. 48, No. 20, 2009
Himmelberger et al.
methylsulfate (mmimMeSO4), and 1-propyl-2,3-dimethylimidazo-
lium triflide (pmmimTf3C) (Aldrich) were dried by toluene azeotropic
distillation to remove any moisture. Tetraethylene glycol dimethyl
ether (Sigma 99%) (tetraglyme) and ethylene glycol dimethyl ether
(Sigma 99%) (glyme) were distilled from sodium under vacuum with
heating.
melted. Data were recorded at 2-5 s intervals depending on the
speed of the reaction. The product residues were extracted with
dry glyme or pyridine and analyzed by 11B NMR.
Results and Discussion
Utilization of waste heat from a PEM fuel cell can provide
for AB H2-release reaction temperatures near 85 °C.5 How-
ever, at 85 °C, H2-release from solid-state AB has been shown
to exhibit an induction period of up to 3 h. After hydrogen
release begins, only the release of ∼0.9 equiv of H2 can be
achieved, rather than the 3 equiv predicted by eq 1, even with
prolonged heating at 85 °C.4,5 As a result, a number of
approaches are now being explored to induce efficient AB
H2-release, including, for example, activation by transition
metal catalysts,10-25 acid catalysts,26 base catalysts,27 and
nano and meso-porous scaffolds.28-30
Physical Measurements. The Toepler pump system used for
hydrogen measurements was similar to that described by Shri-
ver6 and is illustrated in Supporting Information, Figure 1S. The
released gases from the reaction vessel were first passed through
a liquid nitrogen trap before continuing on to the Toepler pump
(700 mL). The released H2 was then pumped into a series of
calibrated volumes with the final pressure of the collected H2 gas
measured ((0.5 mm) with the aid of a U-tube manometer. After
the H2 measurement was completed, the in-line liquid nitrogen
trap was warmed to room temperature, and the amount of any
volatiles that had been trapped was then also measured using the
Toepler pump.
We have previously communicated4 results showing that
AB H2-release is activated in ionic liquids. Ionic liquids are
generally defined as salts that are relatively low viscosity
liquids at temperatures below 100 °C.31-34 Some of the most
common ionic liquids are composed of inorganic anions,
X-, BF4-, PF6-, and nitrogen-containing organic cations,
such as RN,R0N-imidazolium or RN-pyridinium. These salts
have a number of unique properties that make them attractive
substitutes for traditional organic solvents in hydrogen
The automated gas buret was based on a design reported by
Zheng et al.,9 but employed all glass connections with a cold trap
(-78 °C) inserted between the reaction flask and buret to allow
trapping of any volatiles that might have been produced during
the reaction.
While bmimCl is a liquid at 85 °C, it is a solid at room
temperature; therefore, solid-state 11B NMR analyses (at Pacific
Northwest National Laboratories: 240 MHz machine spun at
10 kHz) were used to monitor the products of reactions carried
out in bmimCl. All solid-state 11B chemical shifts were measured
relative to external NaBH4 (-41 ppm) and then referenced to
BF3 O(C2H5)2 (0.0 ppm). The solution 11B NMR (128.4 MHz
3
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Bruker DMX-400) studies in the room temperature ionic-liquid
bmimOTf were carried out by heating reaction mixtures com-
posed of 50 mg of AB (1.6 mmol) or 50 mg of DADB (0.8 mmol)
and 450 mg of ionic liquid at 85 °C in a sealed NMR tube, with
the tube periodically removed from the heating bath to collect
11B NMR spectra of the reaction mixture (recorded at 25 °C).
All solid-state and solution 11B NMR chemical shifts are
referenced to external BF3 O(C2H5)2 (0.0 ppm) with a negative
sign indicating an upfield shift.
3
Procedures for AB H2-release reactions. For the experiments
where the released H2 was measured with the Toepler pump, the
AB (250 mg, 8.1 mmol) was loaded under N2 into ∼100 mL
single neck round-bottom flasks with the ionic liquid (250 mg)
given in Supporting Information, Tables 1S-2S. The flasks were
then evacuated, sealed, and placed in a hot oil bath preheated to
the desired temperature. The flasks were opened at the indicated
times, and the released H2 was quantified using the Toepler
pump system. Post reaction, the flasks were evacuated for 30
min through the cold trap to remove any volatile products from
the reaction residue. The product residues and volatiles in the
cold trap were extracted with dry glyme or pyridine and
analyzed by 11B NMR.
For reactions using the automated gas buret, the AB (150 mg,
4.87 mmol) samples were loaded into ∼100 mL flasks with
calibrated volumes, along with the ionic-liquid (150 mg) or
tetraglyme (0.15 mL) solvents. Under a flow of helium, the flask
was attached to the buret system. The system was evacuated for
30 min for reactions with the ionic-liquid solutions, and for
5 min for tetraglyme solutions. The system was then backfilled
with helium and allowed to equilibrate to atmospheric pressure
for ∼30 min. Once the system pressure equalized, the data
collection program was started, and the flask was immersed in
the preheated oil bath. The data are reported from the point
where the flask was initially plunged into the oil bath, but
H2-release was not observed until the ionic-liquid/AB mixture
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Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9885
Figure 1. H2-release measurements (gas buret) at 85 °C of: (A) 50-wt %
AB (150 mg) in bmimCl (150 mg) and (B) solid-state AB (150 mg).
Figure 3. H2-release measurements (gas buret) of AB (150 mg) in 20.2-wt %
bmimCl (38 mg): (A) 120 °C, (B) 110 °C, (C) 105 °C,(D) 95°C,(E) 85°C, and
(F) 75 °C (The early spike in the data is caused by the initial delay of the buret
to respond to H2-release).
Figure 2. H2-release measurements (gas buret) of 50-wt % AB (150 mg)
in bmimCl (150 mg) at (A) 110 °C, (B) 105 °C, (C) 95 °C, (D) 85 °C, and
(E) 75 °C.
Figure 4. H2-release measurements (Toepler pump) of the reaction of
50-wt % AB (250 mg) at 85 °C in 250 mg of (A) bmmimCl, (B) bmimCl,
(C) emmimEtSO4, (D) bmimBF4, (E) mmimMeSO4, (F) bmimOTf,
(G) emmimOTf, (H) bmimI, (I) bmimPF6, and (J) pmmimTf3C.
storage systems, including: (1) negligible vapor pressures,
(2) stability to elevated temperatures, (3) the ability to dissolve
a wide range of compounds, and (4) a polar reaction medium
that can stabilize ionic transition states and intermediates.
Our earlier reported4 AB H2-release measurements were
periodic values obtained using a Toepler pump, but the new
studies reported herein use an automated gas buret9 that has
provided both more precise and continuous release data for
these reactions. A comparison of the 85 °C H2-release data,
measured with the automated gas buret, obtained from solid-
state AB versus AB dissolved in the 1-butyl-3-methyl-imida-
zolium chloride (bmimCl) ionic liquid (50:50-wt %) is pre-
sented in Figure 1. For the solid-state AB reaction, there was
negligible hydrogen production after 180 min, and only 0.81
equiv of H2 after 360 min. Other samples heated for longer
times (67 h) showed that a total of only 0.9 H2-equiv could
ultimately be obtained from the solid-state AB reactions at
85 °C. In contrast, the AB/bmimCl mixture exhibited no
induction period, with H2-release beginning immediately
after the solution melted, to give release of 1.0 H2-equiv in
67 min and 2.2 H2-equiv in 330 min. The released H2 was
passed through a -78 °C trap before entering the gas buret.
When the reaction was complete, the contents of the trap
were extracted with glyme solvent, but 11B NMR analyses of
the solution showed only trace amounts of borazine.
AB/bmimCl mixture for both the 1st and 2nd H2-equiv were
observed as the temperature was increased with the release of
1.0 H2-equiv in 37 min and 2.2 H2-equiv in 161 min at 95 °C,
1.0 H2-equiv in 9 min and 2.2 H2-equiv in 45 min at 105 °C,
and 1.0 H2-equiv in 5 min and 2.2 H2-equiv in 20 min at
110 °C . The 75 °C reaction did not reach 2 H2-equiv.
The U.S. Department of Energy (DOE) has set a 2015
gravimetric total-system target for H2-storage of 9.0 total-
system-wt %.35,36 The release of 2.2 H2-equiv from a 50:50
AB/bmimCl mixture corresponds to a release of 7.2 mat-wt %
H2 [mat-wt % H2 = H2-wt/(ABþbmimCl-wts)]. For an AB/
ionic-liquid system to attain the DOE total-system targets, an
increase in the mat-wt % by reduction of the weight of the
ionic-liquid component is necessary. As can be seen in
Figure 3, it was found that significantly enhanced H2-release
rates compared to the solid-state could still be obtained when
employing as little as 20.2-wt % bmimCl. Thus, 2.0 H2-equiv
were released from 80:20 AB/bmimCl solutions in only 52
and 157 min at 120 and 110 °C, respectively, with both
solutions then ultimately giving 2.2 H2-equiv at longer times.
The final release observed for these mixtures corresponds to
an 11.4 mat-wt % H2-release.
The AB/bmimCl H2-release plot in Figure 1 also clearly
shows that release appears to occur in at least two steps with
the release rate for the second H2-equiv being significantly
slower than for the first equivalent. Dramatic increases
(Figure 2) in the rate of H2-release of the 50:50-wt %
(35) Satyapal, S. 2007 DOE Hydrogen Program Review; http://www.
hydrogen.energy.gov/pdfs/review07/st_0_satyapal.pdf.
(36) Dillich, S. 2009 DOE Hydrogen Program & Vehicle Technologies
Program; http://www.hydrogen.energy.gov/pdfs/review09/st_0_dillich.pdf. DOE
has recently lowered the 2015 gravimetric total system target toonly 5.5total system
weight %.

9886 Inorganic Chemistry, Vol. 48, No. 20, 2009
Himmelberger et al.
Figure 6. Solution 11B NMR (128.4 MHz) spectra of the residues
(extracted in pyridine) of the 85 °C reaction of (Left) solid-state AB
(250 mg) after H2-release of (a) 0.04 equiv and (b) 0.83 equiv (Right) 50-wt
% AB (250 mg) in bmimCl (250 mg) after H2-release of (c) 0.52 equiv and
(d) 0.95 equiv AB (green circles), DADB (brown stars, BH4-, magenta
triangles BH2þ), PAB (blue diamonds) , BdN (yellow squares).
bmimCl reactions are compared in Figure 6. Consistent with the
observed absence of H2-loss, the spectrum (Figure 6a) of the
residue of the 1 h solid-state AB reaction showed only unreacted
AB (quartet, -22.3 ppm37), whereas the spectrum (Figure 6c)
of the 1 h AB/bmimCl mixture clearly showed mutiple reso-
nances indicating a significant reaction that was consistent with
its measured 0.52 equiv of H2-release. As shown in Figures 6b
and 6d, the spectra of the pyridine soluble residues of the AB
solid-state and AB/bmimCl reactions obtained after the reac-
tions had released 0.83 and 0.95 H2-equiv, respectively, were
similar each showing that the AB resonance had decreased and
the growth of new resonances arising from the diammoniate of
Figure 5. H2-release measurements (Toepler pump) of the reaction of
50-wt % AB (250 mg) in 250 mg of (A) emmimEtSO4, (B) mmimMeSO4,
(C) bmmimCl, and (D) bmimCl at (a) 65 °C and (b) 45 °C.
The 85 °C H2-release data (Toepler pump measurements)
in Supporting Information, Table 1S and Figure 4, show that
AB H2-release is activated in a variety of 50:50-wt % AB/
ionic-liquid mixtures, but that these mixtures exhibit a range
of H2-release extents and rates. The biggest differences were
observed for the release of the second equivalent. The
bmimCl, bmmimCl, bmimBF4, mmimMeSO4, and emmi-
mEtSO4 mixtures all yielded over 2 H2-equiv at reasonably
comparable rates, while the other mixtures showed greatly
decreased release rates beyond the first equivalent. For the
pmmimTf3C mixture, H2-release stopped, as was observed
for the AB solid-state reactions at 85 °C, after only ∼0.9 H2-
equiv. As shown in Supporting Information, Table 2S and
Figure 5, the H2-release rates were significantly decreased
upon lowering the temperature with the mmimMeSO4 and
emmimEtSO4 mixtures being the most active. At 45 °C,
bmimCl and bmmimCl showed little activity.
The bmimCl ionic liquid is a solid at room temperature, but
the 50:50-wt % AB/bmimCl mixtures formed a viscous,
stirrable room temperature liquid. Once H2-release began,
the mixture foamed. As the H2-release neared the loss of
∼1 H2-equiv, the foam began to convert to a white solid. The
entire AB/bmimCl mixture ultimately became solid as the
reaction reached over 2 H2-equiv. Similar behavior was seen
for the other 50:50-wt % AB/ionic-liquid mixtures, but with
some differences in their liquid ranges. On the other hand, the
80:20-wt % AB/bmimCl mixtures formed a moist paste at
room temperature. Upon initial heating, this paste melted,
foam again formed, but then rapidly solidified after the onset of
H2-release. Solid formation was likewise observed in H2-release
reactions of 50:50-wt % AB/tetraglyme systems (discussed
later) to produce a final two-phase liquid/solid mixture.
The 11B NMR spectra of the pyridine soluble products
produced at different stages in the AB solid-state and AB/
diborane, which forms without H2-loss, [(NH3)2BH2]þBH4-
,
(DADB) (-13.3 (overlapped) and -37.6 ppm)7,37 and
branched-chain polyaminoborane polymers (PAB) (-7,
-13.3, and -25.1 ppm).4 As dehydrogenation progressed
past 1 equiv, only a small amount of material was pyridine
soluble; therefore, solid-state NMR was also used to analyze
these materials. Consistent with both the H2-release measure-
ments and the 11B NMR analyses of the pyridine extracts, the
solid-state 11B spectrum (Figure 7) of the reaction of a 50:50-
wt % bmimCl/AB mixture heated at 110 °C also showed the
presence of DADB after 1.0 equiv of H2-release. The solid-
state 11B NMR spectrum of the final product after the release
of 2 H2-equiv showed a broad downfield resonance char-
acteristic of the sp2 boron-nitrogen framework of cross-
linked polyborazylene structures,38-40 indicating that AB
dehydrogenation ultimately produced BdN unsaturated
products. NMR studies of the dehydrogenated products of
AB H2-release promoted by solid-state thermal reactions5,41
have likewise shown the formation of BdN unsaturated final
products after the release of more than 2 H2-equiv.
In situ 11B NMR studies (Figure 8) of AB H2-release at
85 °C in a solution of the room temperature ionic liquid
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Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9887
Figure 7. Solid-state 11B NMR (240 MHz) spectra recorded at 25 °C of
the reaction of 50-wt % AB (150 mg) in bmimCl (150 mg) at 110 °C after
the release of: (a) 1 equiv and (b) 2 equiv.
Figure 9. Solution 11B NMR (128.4 MHz) spectra of the reaction of
10-wt % AB (50 mg) in bmimOTf (450 mg) at 85 °Cfor6h:(a) NMRprobe
at 27 °C and (b) NMR probe at 100 °C.
Figure 10. Solution 11B NMR (128.4 MHz) spectra recorded at 25 °C of
10-wt % borazine (50 mg) in bmimI (450 mg) after (a) initial mixing at
25 °C, (b) 19 h at 85 °C, and (c) the toluene extraction after heating.
sp2 boron-nitrogen frameworks. On the other hand, the in
situ NMR studies of the AB/bmimOTf reactions, as well
as similar NMR studies of the 85 °C H2-release from
AB/mmimMeSO4 and AB/bmimI reactions, showed the
growth of a 16 ppm resonance after the release of 2 H2-equiv.
The 16 ppm resonance in these ionic liquid reactions is thus
shifted almost 14 ppm upfield relative to that normally found
for polyborazylene38-40 or borazine.42 This suggests that if
either of these unsaturated species were formed in these
solutions, the observed chemical shift change could result
from interactions with the ionic-liquid solvent.
Given both their polar compositions and planar aromatic
like structures and properties, a variety of reversible ionic-
liquid interactions would be possible for borazine and poly-
borazylene, including the formation of ionic-liquid hydro-
gen-bonded and/or clathrated43-48 species. Such interactions
Figure 8. Solution 11B NMR (128.4 MHz) spectra recorded at 25 °C of
the reaction of 10-wt % AB (50 mg) in bmimOTf (450 mg) at 85 °C after
the release of (a) 0.0 equiv (0 min), (b) 0.1 equiv (10 min), (c) 0.5 equiv
(30 min), (d) 0.9 equiv (60 min), (e) 1.5 equiv (180 min), and (f) 2.0 equiv
(360 min). (The broad DADB resonance at -13 ppm is obscured by the
AB and PAB resonances).
bmimOTf (10:90 AB/bmimOTf mixture) exhibited features
similar to those observed in the solid-state NMR spectra of
the more concentrated AB/bmimCl reactions. Initially, only
the AB resonance was present, but the appearance, after 10 min,
of the well resolved quintet resonance near -38 ppm indi-
cated significant AB conversion to DADB. After 30 min,
0.5 H2-equiv had been released, but the NMR spectrum
(Figure 8c) indicated that the AB was completely consumed
to produce a mixture of DADB and PAB polymer. Once the
reaction reached the release of 0.9 H2-equiv, the spectrum
(Figure 8d) of the mixture showed a decrease in the DADB
resonance along with a corresponding increase in the PAB
resonances. The spectrum taken after the release of 1.5
H2-equiv (Figure 8e) showed that the DADB had been
almost completely consumed and a new downfield resonance
near 16 ppm had appeared. This 16 ppm resonance continued
(Figure 8f) to grow, and the PAB resonances continued to
decrease as the reaction achieved the release of 2.0 H2-equiv.
The solid-state 11B NMR spectrum, discussed earlier
(Figure 7), of the product of the AB/bmimCl reaction after
the release of 2 H2-equiv showed the broad downfield
resonance near 30 ppm that is characteristic of unsaturated
::
(42) Noth, H.; Wrackmeyer, B. In Nuclear Magnetic Resonance Spectros-
copy of Boron Compounds; Springer-Verlag: New York, 1978; pp 188, 265,
394-395.
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M. J. J. Inclusion Phenom. 1991, 11, 103–114.
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Phenom. 1988, 6, 425–428.
(46) Holbrey, J. D.; Reichert, W. M.; Nieuwenhuyzen, M.; Sheppard, O.;
Hardacre, C.; Rogers, R. D. Chem. Commun. 2003, 476–477.
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2753–2754.

9888 Inorganic Chemistry, Vol. 48, No. 20, 2009
Himmelberger et al.
Figure 11. H2-release measurements (gas buret) of bmimOTf (450 mg)
and 10-wt % (50 mg) of (A) DADB and (B) AB.
Figure 13. Possible pathway for ionic-liquid promoted H2-release from
AB.
reactivity in ionic liquids. These studies showed that the 85 °C
reaction of a pre-synthesized7,37 pure sample of DADB
dissolved in bmimOTf (10-wt % DADB) yielded the same
type of polyaminoborane products, but with faster H2-release
rates, as those found in the AB/bmimOTf reactions.
Figure 12. Solution 11B NMR (128.4 MHz) spectra recorded at 25 °C of
the reaction of 10-wt % DADB (50 mg) in bmimOTf (450 mg) at 85 °C
after the release of(a) 0.0equiv (0min), (b) 0.9 equiv (10min), (c) 1.1equiv
(30 min), (d) 1.3 equiv (60 min), (e) 1.6 equiv (180 min), and (f) 1.9 equiv
(360 min).
-
0:5½ðNH3Þ2BH2þꢀ½BH4 ꢀ f BNþ3H2
ð2Þ
would be expected to decrease as the temperature is increased
and, as shown in the 11B NMR spectra in Figure 9, it was
found that upon recording the NMR spectrum of the final
AB/bmimOTf sample with the NMR probe heated at 100 °C
instead of 27 °C, the resonance at 16 ppm disappeared and
was replaced by a resonance in the more normal 30 ppm
region of borazine. It was likewise found that when glyme
(1:10 glyme) was added to an AB/bmimOTf reaction sample
exhibiting a 16 ppm resonance, this resonance disappeared
and was replaced by a 30 ppm resonance. Additional evi-
dence that borazine could give rise to a shift in this region in
ionic liquid solutions was obtained by recording the spectra
of a pure sample of borazine dissolved in 90 wt % bmimI. The
initial spectrum showed only a broad downfield peak
(Figure 10a), but this resonance then shifted to 16 ppm after
the solution was heated at 85 °C (Figure 10b). That this
interaction was reversible was demonstrated by the fact that
borazine could then be recovered from the bmimI solution by
extraction with toluene (Figure 10c). These results are thus all
consistent with a significant reversible interaction between
borazine and the ionic liquids. Such interactions may play a
key role in retarding the loss of borazine, a likely fuel cell
catalyst poison, during AB H2-release.
The theoretical DADB H2-release reaction in terms of AB
equiv is given by eq 2. The H2-release rates for separate 10-wt %
DADB and AB samples in bmimOTf are compared in
Figure 11, where the faster rate of the DADB reaction is
clearly apparent. While AB/bmimOTf required 96 min to
release 1.0 H2-equiv and 274 min for 1.5 H2-equiv, the DADB
required only 28 min for 1.0 H2-equiv and 94 min for 1.5
H2-equiv. At 400 min, the DADB/bmimOTf reaction had
already released 2.09 H2-equiv, while the AB/bmimCl reac-
tion was still at 1.76 H2-equiv. These results are consistent
with the observation that DADB H2-release is also faster
than that of AB in solid-state reactions.41
As can be seen in the NMR studies in Figure 12, the initial
11B NMR spectrum obtained from a 10-wt % DADB/
bmimOTF sample showed only the broad resonances
þ
expected for the DADB (NH3)2BH2 (-13.3 ppm) and
-
BH4 (-37.6 ppm) components. However, after heating
for only 10 min at 85 °C, most of the DADB had been
converted to PAB. At 30 min, 0.9 H2-equiv had been released
and the 11B NMR spectrum at this point (Figure 12c) showed
that the DADB had been completely consumed. As the
reaction proceeded beyond the release of 1 H2-equiv, a new
resonance grew in that was also at the 16 ppm shift observed
in the AB/bmimOTf reactions (Figure 12d-f).
The fact that DADB is a precursor to the formation of the
polyaminoboranes, rather than just a side reaction, was
demonstrated by H2-release and 11B NMR studies of DADB
The combined solid-state and solution 11B NMR studies of
AB/ionic-liquid and DADB/ionic-liquid H2-release reactions

Article
Inorganic Chemistry, Vol. 48, No. 20, 2009 9889
Figure 14. H2-release measurements (gas buret) of 50-wt % AB (150 mg)
in tetraglyme (150 mg) at (A) 95 °C, (B) 85 °C, and (C) 75 °C.
in progress support a AB dehydrogenation pathway in ionic
liquids (Figure 13) involving the following: (1) ionic-liquid
promoted conversion of AB into its more reactive ionic
DADB form, (2) further intermolecular dehydrocoupling
reactions between hydridic B-H hydrogens and protonic
N-H hydrogens on DADB and/or AB to form polyamino-
borane polymers, and (3) polyaminoborane dehydrogena-
tion to unsaturated cross-linked polyborazylene materials.
The initial formation of DADB has also been proposed as a
key step in thermally induced AB H2-release reactions in the
solid state41,49 and in organic solvents,50 but the highly polar
medium provided by ionic liquids promotes DADB forma-
tion and appears to be the key activating feature of these ionic
liquid reactions.
Figure 15. Solution 11B{1H} NMR (128 MHz) spectra recorded at 80 °C
of the reaction of 10-wt % AB (50 mg) in tetraglyme (450 mg) at 85 °C after
(a) 1.1 equiv (60 min), (b) 1.7 equiv (180 min), and (c) 1.9 equiv (360 min).
Inset shows 1H coupled spectra.
In conclusion, their low solvent volatility, the high extent
of their H2-release, the tunability of both their H2 materials-
weight-percents and release rates, and their product control
that is attained by either trapping or suppressing unwanted
volatile side products continue to make AB/ionic-liquid
based systems attractive candidates for chemical hydrogen
storage applications.
Acknowledgment. We thank the U.S. Department of
Energy for grants from the Center of Excellence for
Chemical Hydrogen Storage and the Division of Basic
Energy Sciences for the support of this research. A portion
of the researchdescribed in this paper was performed in the
Environmental Molecular Sciences Laboratory, a national
scientific user facility sponsored by the Department of
Energy’s Office of Biological and Environmental Research
and located at Pacific Northwest National Laboratory.
We especially thank Dr. Tom Autrey for his generous
invitation to visit PNNL and Dr. Wendy Shaw for her
assistance with the solid-state NMR. We also thank
Mr. Christopher P. Griffin (2007 ACS Project Seed
student) for his assistance in data collection.
The AB H2-release observed in the ionic-liquid solvents
was also compared with that obtained for the conventional
polar organic solvent, tetraglyme. The H2-release data for a
50:50 wt % ratio AB/tetraglyme mixture showed that both
the extent and rate of H2-release were comparable to that of
the AB/bmimCl reactions (Figure 14). However, the 11B
NMR spectra of the AB/tetraglyme reactions showed that,
unlike in the ionic liquids, there was little evidence of PAB
formation, with the major product being instead borazine
-
(30.1 ppm)42 along with smaller amounts of BH4 (-36.8
ppm)42 and μ-aminodiborane (-27.5 ppm)42 (Figure 15).
Thus, ionic-liquid solvents are favored for AB H2-release
since they suppress or retard the formation of these undesired
products.
Supporting Information Available: Diagram of the Toepler
pump apparatus used for some H2-release measurements; tables
of Toepler pump H2-release data for AB/ionic liquid reactions at
different temperatures. This material is available free of charge
via the Internet at http://pubs.acs.org.
(49) Stowe, A. C.; Shaw, W. J.; Linehan, J. C.; Schmid, B.; Autrey, T.
Phys. Chem. Chem. Phys. 2007, 9, 1831–1836, and references therein.
(50) Shaw, W. J.; Linehan, J. C.; Szymczak, N. K.; Helderandt, D. J.;
Yonker, C.; Camaioni, D. M.; Baker, R. T.; Autrey, T. Angew. Chem., Int.
Ed. 2008, 47, 7493–7496.