Amineborane-based chemical hydrogen storage: Enhanced ammonia borane dehydrogenation in ionic liquids

Article abstract of DOI:10.1021/ja062085v

Ionic liquids are shown to provide advantageous media for amineborane-based chemical hydrogen storage systems. Both the extent and rate of hydrogen release from ammonia borane dehydrogenation are significantly increased at 85, 90, and 95 °C when the react

Full text of DOI:10.1021/ja062085v

Published on Web 05/28/2006
Amineborane-Based Chemical Hydrogen Storage: Enhanced Ammonia
Borane Dehydrogenation in Ionic Liquids
Martin E. Bluhm,† Mark G. Bradley,‡ Robert Butterick III,† Upal Kusari,† and Larry G. Sneddon*,†
Departments of Chemistry, UniVersity of PennsylVania, Philadelphia, PennsylVania 19104-6323, and
Widener UniVersity, One UniVersity Place, Chester, PennsylVania 19013-5792
Received March 27, 2006; E-mail: lsneddon@sas.upenn.edu
The requirement for efficient and safe methods for hydrogen
storage is a major hurdle that must be overcome to enable the use
of hydrogen as an alternative energy carrier. Although many
molecular hydride complexes have certain features that might be
attractive for chemical hydrogen storage, the high hydrogen
capacities needed for transportation applications exclude most
compounds. Amineboranes, such as ammonia borane, NH3BH3
(19.6 wt % H2), are thus unique in their potential ability to store
and deliver large amounts of molecular hydrogen through dehy-
drogenation reactions.
Partial dehydrogenation of ammonia borane can be thermally
induced in the solid state,1-4 but to be useful for hydrogen 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 will need
a replacement for the volatile organic solvents that have traditionally
been employed for solution-phase ammonia borane hydrides dehy-
drogenations.1e,5 We report here that ionic liquids provide advanta-
Figure 1. Summary of H2 release at different temperatures (a) from solid
geous media for ammonia borane dehydrogenation in which both
the extent and rate of hydrogen release are significantly increased.
Ionic liquids are salts that are liquid at low temperatures (<100
°C). These salts have unique properties6 that make them attractive
substitutes for organic solvents in hydrogen storage systems,
including (1) negligible vapor pressures, (2) stability to elevated
temperatures, (3) ability to dissolve a wide range of compounds
and gases, (4) weakly coordinating anions and cations that provide
an inert reaction medium which can stabilize polar transition states,
and (5) recycling with little loss of activity.
Comparisons of ammonia borane dehydrogenations in the solid
state versus in 1-butyl-3-methylimidazolium chloride (bmimCl)
solvent are summarized in Figure 1. Reactions in the solid state
were conducted in sealed, evacuated glass vessels that were heated
in a thermostatically controlled oven. The bmimCl/NH3BH3 reac-
tions were carried out by adding equal weights of bmimCl (dried)
and NH3BH3 together in evacuated 100 mL flasks equipped with a
vacuum adapter and then immersing the body of the flasks into an
oil bath at the desired temperature. At the reaction conclusion, the
sealed vessels or flasks were opened to a vacuum line, and the
evolved hydrogen passed through a liquid nitrogen trap in order to
isolate any volatile non-hydrogen products, such as borazine. The
hydrogen was then quantitatively measured in calibrated volumes
using a Toepler pump.
ammonia borane and (b) from equal weight mixtures of bmimCl and
ammonia borane.
evolution after 1 h, 0.8 equiv was obtained after 3 h. Again, as
with the 85 °C sample, prolonged heating at 95 °C (48 h) yielded
a total of only 0.9 equiv of H2.
In contrast to the solid-state reactions, ammonia borane dehy-
drogenations in bmimCl showed no induction period (Figure 1b)
with hydrogen evolution beginning immediately upon placing the
sample in the heated oil bath. Separate samples heated for only 1
h at 85, 90, and 95 °C evolved 0.5, 0.8, and 1.1 equiv of H2, while
samples heated at these temperatures for 3 h produced 0.95, 1.2,
and 1.5 equiv. Heating for 22 h gave a total of 1.2, 1.4, and 1.6
equiv of H2, respectively, which are values significantly greater
than the 0.9 equiv ultimately obtained in the solid-state reactions.
Including the bmimCl weight, the final values correspond to the
evolution of 3.9, 4.5, and 5.4 wt % H2.
The 11B NMR spectra obtained from pyridine (dried) extracts
of the residues of the 85 °C solid-state and bmimCl reactions are
compared in Figure 2. Consistent with the observed absence of H2
loss, the spectrum (Figure 2a) of the residue of the 1 h solid-state
reaction showed only unreacted ammonia borane (-22.4 ppm8),
whereas the 1 h bmimCl sample clearly showed significant reaction.
The spectra of the residues of the 19 h solid-state and 3 h bmimCl
reactions (Figure 2b and d) were quite similar, showing multiple
resonances.
As seen in Figure 1a, for the reactions carried out in the solid
state at 85 °C, there was negligible hydrogen production after 3
h,7 but after 17 h, 0.9 equiv of H2 was produced. Even with
prolonged heating (67 h) at this temperature, no further H2 release
was observed. Similar results were observed at 95 °C, but with a
shorter initial induction period. Thus, while there was no H2
Depending on the conditions, several species have been previ-
ously observed1-5,9 upon heating ammonia borane: borazine
(B3N3H6),1d,e,2,5,9 cycloborazanes1e,5 (e.g., B3N3H12), and polyamino-
borane1-3 (BH2NH2)n. Because they could poison a fuel cell, volatile
products, such as borazine, are undesirable in hydrogen storage
applications, and it is significant that in the bmimCl reactions only
traces of borazine were detected. The diammoniate of diborane,10
† University of Pennsylvania.
‡ Widener University.
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J. AM. CHEM. SOC. 2006, 128, 7748-7749
10.1021/ja062085v CCC: $33.50 © 2006 American Chemical Society

C O M M U N I C A T I O N S
Dehydrogenation to polyaminoborane or cycloborazanes would
release only 1 equiv of hydrogen, but further dehydrogenation
should lead to BdN unsaturation. The weak downfield resonances
in spectra b and e in Figure 2 agree with the calculated shift ranges
(Figure 3, III) for borons in internal BdN or terminal BdNH2 units,
but are somewhat upfield of the value calculated for the terminal
NdBH2 group. In the extended time solid-state reactions, these
resonances did not grow, and even after 67 h, appreciable soluble
saturated polyaminoborane remained (Figure 2c). These resonances
are also not apparent in spectrum 2f, but the more extensive
dehydrogenation found at longer times in bmimCl, while resulting
in the consumption of the polyaminoborane, produced unsaturated
residues that could no longer be extracted (less than ∼20%) into
pyridine.
The role of bmimCl in enhancing the rate and extent of ammonia
borane dehydrogenation has yet to be proven, but it is significant
-
that [(NH3)2BH2+]BH4 has also been reported to form polyami-
noborane upon heating.11 Ionic liquids are known to favor the
formation of polar intermediates and transition states,6 and the
observation that [(NH3)2BH2+]BH4- and/or BH4- are produced in
the bmimCl reaction within the first hour (Figure 2d) suggests that
the activating effect of the ionic liquid may be related to its ability
to induce formation of such ionic species. We are now exploring
this effect in conjunction with the established ability of ionic liquids
to stabilize nanoparticle dehydrogenation catalysts to develop more
active chemical hydrogen storage systems.
Figure 2. 11B NMR spectra (128 MHz) of the residues (extracted in
pyridine) of ammonia borane dehydrogenation: (left) in the solid-state and
+
(right) in bmimCl. Green, ammonia borane; orange, BH4-; purple, BH2
;
blue, polyaminoborane; yellow, BdN. (The signals near -11 ppm may
also include a resonance from pyridine-BH3; the broad humps underlying
the weak spectra in c and f arise from the boron background of the NMR
probe.)
Acknowledgment. We thank the U.S. Department of Energy
Center of Excellence for Chemical Hydrogen Storage and the
Division of Basic Energy Sciences for support.
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