Transition metal catalysed dehydrogenation of amine-borane fuel blends

Article abstract of DOI:10.1039/c0cc03585h

Mixtures containing ammonia-borane and sec-butylamine-borane remain liquid throughout the hydrogen release process that affords tri(N-sec-butyl)borazine and polyborazylene. Concentrated solutions with metal catalysts afford >5 wt% H₂ in 1 h at 80°C and addition of (EMIM)EtSO₄ ionic liquid co-solvent eliminates competing formation of insoluble linear poly(aminoborane) (EMIM = 1-ethyl-3-methyl-imidazolium). The Royal Society of Chemistry.

Full text of DOI:10.1039/c0cc03585h

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Transition metal catalysed dehydrogenation of amine-borane fuel blendswz
Sib Sankar Mal,a Frances H. Stephensb and R. Tom Baker*a
Received 31st August 2010, Accepted 7th January 2011
DOI: 10.1039/c0cc03585h
Mixtures containing ammonia-borane and sec-butylamine-
borane remain liquid throughout the hydrogen release process
that affords tri(N-sec-butyl)borazine and polyborazylene.
Concentrated solutions with metal catalysts afford >5 wt%
H2 in 1 h at 80 1C and addition of (EMIM)EtSO4 ionic liquid
co-solvent eliminates competing formation of insoluble linear
poly(aminoborane) (EMIM = 1-ethyl-3-methyl-imidazolium).
undergo dehydrogenation and thus contribute to overall hydrogen
production from the AB/amine-borane ‘fuel blend’.10
Investigations began with methylamine-borane (MeAB, mp
56 1C, dec. pt. 70 1C) which was attractive due to its low mass
and the low mp (5 1C) of its ultimate dehydrogenation
product, N,N0,N00-trimethylborazine.11,12 Unfortunately,
MeAB proved too volatile and exhibited insufficient thermal
stability, slowly evolving hydrogen even as a solid at 25 1C.
Volatility was also a problem with dimethylamine-borane
(mp 37 1C). While liquid diethylamine-borane (mp ꢀ18 1C)
possesses excellent thermal stability and gives rise to a liquid
dehydrogenation product, it only releases 2.3 wt% hydrogen
and, most importantly, does not readily solubilise AB. In 2006,
Yamamoto et al. reported the synthesis of sec-butylamine-
borane (SBAB),12 which we found to be a stable liquid at
ambient temperatures (mp 2 1C) with a moderate hydrogen
content (5.7 wt%). Moreover, SBAB can solubilise AB at a
1 : 3 molar ratio, giving a liquid mixture potentially capable of
releasing 12.8 wt% H2.
Increasing demands for cleaner energy sources have fueled
intensive efforts towards the development of a hydrogen
economy, a possible long term and sustainable solution.1
While hydrogen is a high energy density fuel, limited
volumetric storage of compressed or even liquid hydrogen
has turned attention to alternative on-board hydrogen storage
solutions.2 Interest in ‘‘chemical hydrides’’ such as N-ethyl-
carbazole derives from the potential advantages associated
with a ‘drop-in’ liquid fuel that would be easy to transport and
would not require costly replacement of existing fuel service
stations.3 For long haul transportation applications, ammonia-
borane (AB) has been widely investigated as a result of its high
gravimetric storage density (19.6 wt%) and low temperature
catalyzed hydrogen release.4,5 Although rapid H2 release has
been demonstrated by metal-catalyzed hydrolysis,6 the recent
focus on catalyzed thermolysis stems from the ease of regeneration7
of the BNHx ‘spent fuel’ (vs. more stable boric acid) and
greatly reduced production of ammonia, a potent fuel cell
catalyst poison.8
In contrast to its tert-butyl and iso-butyl congeners, SBAB
readily loses hydrogen on thermolysis (t1/2 = 12 h at 60 1C) to
afford both aminoborane- and iminoborane cyclic oligomers
as a liquid mixture (Scheme 1; Fig. S1–S4, ESIw). At 85 1C,
230 mg of SBAB is converted in high yield to tri(N-sec-butyl)-
borazine in 30 h (Fig. S5–S8, ESIw).While the aminoborane
intermediates were not isolated, mass spectrometry and multi-
nuclear NMR studies support initial formation of two isomers
of the cyclic dimer, followed by the isomeric trimers.13
Small amounts of the m-aminodiborane compound,
[m-NH(sec-Bu)]B2H5, resulting formally from reaction of
sec-BuNHBH2 with free borane, were also observed.14 This
by-product was not routinely observed in metal-catalysed
reactions (Fig. S9, ESIw) at 60 1C that typically afforded
1–1.3 equiv. of H2 over a 3 h period (Fig. 1).
Previous work demonstrated that controlled hydrogen
release in a vehicle is easily achieved by passing a liquid fuel
(such as basic aqueous sodium borohydride) over
a
heterogeneous catalyst bed.9 As a colorless solid that evolves
hydrogen at its melting point, AB cannot serve as a liquid fuel
without dilution that necessarily reduces its hydrogen
storage capacity. In this report we investigate the use of liquid
N-substituted amine-borane ‘‘solvents’’ that can themselves
While the fastest initial dehydrogenation rates were
obtained with catalysts 115 and 3,16 catalyst 217 was more
effective at releasing the second equivalent of hydrogen
(Fig. 1, Table 1).
a Department of Chemistry and Center for Catalysis Research and
Innovation, University of Ottawa, Ottawa, ON, K1N 6N5 Canada.
E-mail: rbaker@uottawa.ca; Fax: +1 613 562-5613;
Tel: +1 613 562-5698
b Chemistry Division, Los Alamos National Laboratory, Los Alamos,
NM 87545, USA
w This article is part of a ChemComm ‘Hydrogen’ web-based themed
issue.
z Electronic supplementary information (ESI) available: Experimental
details including heterogeneous catalyst synthesis, 11B NMR spectra
and gas burette hydrogen measurements. See DOI: 10.1039/c0cc03585h
Scheme 1 Observed SBAB dehydrogenation products.
c
2922 Chem. Commun., 2011, 47, 2922–2924
This journal is The Royal Society of Chemistry 2011

Fig. 3 Dehydrogenation of SBAB/AB mixture using 1 mol% catalyst
at 80 1C in IL.
borazine and polyborazylene in ether solvents,15 we were
unable to prevent polymer formation by catalyst control in
the SBAB solvent. As earlier studies with Ni N-heterocyclic
carbene catalysts showed drastic variation in AB
dehydrogenation selectivity in different solvents,20 several
co-solvents were investigated with AB/SBAB mixtures. In
other investigations we recently found that insoluble poly-
(aminoborane) formation could be avoided with Ru catalyst
precursor 3 using ionic liquid solvents with moderately
coordinating anions such as (EMIM)[EtSO4] (EMIM = 1-ethyl-
3-methylimidazolium).17 Indeed, while diglyme and sulfolane
co-solvents offered no selectivity improvements, dehydrogenation
of equimolar AB and SBAB in (EMIM)[EtSO4] released
3.4 wt% H2 at 80 1C over 4 h without formation of insoluble
poly(aminoborane) (Fig. 3 and S18, ESIw). Gas burette
measurements confirmed formation of additional hydrogen
(Table S1, ESIw; 1.0 equivalent of H2 released in 20 min and
1.7 equivalents of H2 after a reaction period of 3 h) although
with reduced total storage capacity due to the ionic liquid. While
results have not yet been optimized, a (6 : 18 : 15 : 1) liquid
mixture by weight of AB : SBAB : EMIM : RuCl2(PMe3)4
afforded 3.6 wt% H2 in 18 h at 80 1C.
Fig. 1 H2 release from SBAB using 1 mol% catalyst at 60 1C.
Table 1 Metal-catalysed SBAB dehydrogenation at 60 1C
Catalyst
Time/h
Equivalents H2
[Rh(1,5-cod)(m-Cl)]2(1)
[RuCl2{tBu2PCH2CH2NH2}2] (2)
RuCl2(PMe3)4 (3)
20
23
23
1.03
1.55
1.06
Turning next to SBAB/AB solutions, we noted firstly that
AB dehydrogenation was rapid even at 60 1C (Fig. S9, ESIw)
with complete conversion to borazine and polyborazylene;
an effect that persisted even on dilution with sulfolane
(Fig. S10, ESIw), but not in sulfolane without SBAB
(Fig. S11, ESIw). Moreover, no evidence was obtained for
any mixed dehydrogenation products. Metal-catalysed reactions
at the same temperature afforded a total of 3.5 wt% of H2
after 5 h (Fig. 2) and 4.5 wt% after 23 h (Table S1, ESIw) with
0.8–1.0 equivalents of H2 evolving from the system after a 5 h
initial period. Interestingly, catalyst 3 afforded >5.0 wt% of
H2 in 1 h at 80 1C based on SBAB and AB (Fig. 3) and also
gave 1.0 equivalents of H2 in 10 min (Fig. S17, ESIw). While
titania-supported Ni and Fe heterogeneous catalysts showed
little enhancement for dehydrogenation of SBAB/AB
mixtures, Ni on boron nitride afforded 5.3 wt% in 4 h
(Fig. S19 and S20, ESIw) but only 5.7 wt% in 23 h, suggestive
of product inhibition or catalyst poisoning.18
Current work is focused on optimizing these amine-borane/
IL liquid fuel blends, identifying active, long-lived heterogeneous
catalysts21 and conducting analysis of hydrogen purity and
fuel regeneration studies.
We thank the US Department of Energy, Energy Efficiency
and Renewable Energy office for support of this work through
the Chemical Hydrogen Storage Center of Excellence, the
University of Ottawa’s NMR core facility and Dr William
R. H. Wright for technical assistance.
Notably, all these reactions were accompanied by formation
of a white precipitate that was insoluble in tetrahydrofuran,
indicative of linear poly(aminoborane), (NH2BH2)n.19 While
previous work indicated that catalyst 1 yields primarily
Notes and references
1 L. Schlapbach and A. Zuttel, Nature, 2001, 414, 353–358.
¨
2 S. Satyapal, J. Petrovic, C. Read, G. Thomas and G. Ordaz,
Catal. Today, 2007, 120, 246–256.
3 G. P. Pez, A. R. Scott, A. C. Cooper and H. Cheng. US patent,
7,101,530, 2006; R. H. Crabtree, Energy Environ. Sci., 2008, 1,
134–138; R. Yamaguchi, C. Ikeda, Y. Takahashi and K.-I. Fujita,
J. Am. Chem. Soc., 2009, 131, 8410–8412.
4 F. H. Stephens, V. Pons and R. T. Baker, Dalton Trans., 2007,
2613–2626; T. B. Marder, Angew. Chem., Int. Ed., 2007, 46,
8116–8118; B. Peng and J. Chen, Energy Environ. Sci., 2008, 1,
479–483; C. W. Hamilton, R. T. Baker, A. Staubitz and I. Manners,
Chem. Soc. Rev., 2009, 38, 279–296; A. Staubitz, A. P. M.
Robertson and I. Manners, Chem. Rev., 2010, 110, 4079–4124.
5 S.-K. Kim, W.-S. Han, T.-J. Kim, T.-Y. Kim, S. W. Nam,
M. Mitoraj, Ł. Pieko, A. Michalak, S.-J. Hwang and
Fig. 2 Dehydrogenation of SBAB/AB mixture using 1 mol% catalyst
at 60 1C.
c
This journal is The Royal Society of Chemistry 2011
Chem. Commun., 2011, 47, 2922–2924 2923

S. O. Kang, J. Am. Chem. Soc., 2010, 132, 9954–9960; B. L. Conley
and T. J. Williams, Chem. Commun., 2010, 46, 4815–4817.
6 T. Umegaki, J.-M. Yan, X.-B. Xhang, H. Shioyama, N. Kuriyama
and Q. Xu, Int. J. Hydrogen Energy, 2009, 34, 2303–2311.
7 A. Sutton, K. C. Ott, J. C. Gordon and A. K. Burrell, US patent
application 20100272622, WIPO patent application WO/2010/123551.
8 M. Dresselhaus, et al., Basic Research Needs for Hydrogen Storage,
U.S. DOE, (http://www.sc.doe.gov/bes/reports/files/NHE_rpt.pdf).
9 S. C. Amendola, S. L. Sharp-Goldman, M. S. Jamjua,
N. C. Spencer, M. T. Kelly, P. J. Petillo and M. Binder, Int. J.
Hydrogen Energy, 2000, 25, 969–975.
14 While only one sec-Bu group is observed by 1H and 13C NMR
spectroscopy for m-sec-butylaminodiborane, two 11B resonances
are observed; possibly due to puckering of the four-membered ring.
15 C. A. Jaska, K. Temple, A. J. Lough and I. Manners, J. Am. Chem.
Soc., 2003, 125, 9424–9434; C. A. Jaska and I. Manners, J. Am.
Chem. Soc., 2004, 126, 1334–1335; Y. Chen, J. L. Fulton, J. C.
Linehan and T. Autrey, J. Am. Chem. Soc., 2005, 127, 3254–3255.
16 N. Blaquiere, S. Diallo-Garcia, S. I. Gorelsky, D. A. Black and
K. Fagnou, J. Am. Chem. Soc., 2008, 130, 14034–14035.
17 W. R. H. Wright, E. Berkeley, L. G. Sneddon and R. T. Baker,
Chem. Commun., DOI: 10.1039/C0CC05408A.
10 Y. Kobori, Nippon Oil patent publication JP2005203188.
11 D. T. Haworth and L. F. Hohnstedt, J. Am. Chem. Soc., 1960, 82,
3860–3862; E. Framery and M. Vaultier, Heteroat. Chem., 2000,
11, 218–225; Y. Yamamoto, C. A. Jaska, K. Temple, A. J. Lough
and I. Manners, J. Am. Chem. Soc., 2003, 125, 9424–9434.
12 K. Miyamoto, J. Umeda, Y. Nakatani, T. Yamamoto and
N. Miyaura, J. Organomet. Chem., 2006, 691, 4909–4917.
13 In contrast to AB, attempts to trap (sec-Bu)NHBH2 using
cyclohexene afforded only minor amounts of BCy2NH(sec-Bu).
Cf. V. Pons, R. T. Baker, N. K. Szymczak, D. J. Heldebrant,
J. C. Linehan, M. H. Matus, D. J. Grant and D. A. Dixon,
Chem. Commun., 2008, 6597–6599.
18 R. J. Simonson and M. Trenary, J. Electron Spectrosc. Relat.
Phenom., 1990, 54/55, 717–728.
19 A. Staubitz, A. P. Soto and I. Manners, Angew. Chem., Int. Ed.,
2008, 47, 6212–6215; A. Staubitz, M. E. Sloan, A. P.
M. Robertson, A. Friedrich, S. Schneider, P. J. Gates, J. S. a der
Guenne and I. Manners, J. Am. Chem. Soc., 2010, 132,
13332–13345.
20 R. J. Keaton, J. M. Blacquiere and R. T. Baker, J. Am. Chem. Soc.,
2007, 129, 1844–1845.
21 Preference for a heterogeneous catalyst stems from both the ease of
engineering for controlled hydrogen release and the exclusion of
metals in the fuel regeneration step using hydrazine.
c
2924 Chem. Commun., 2011, 47, 2922–2924
This journal is The Royal Society of Chemistry 2011