Bis-BN cyclohexane: A remarkably kinetically stable chemical hydrogen storage material

Article abstract of DOI:10.1021/ja511766p

A critical component for the successful development of fuel cell applications is hydrogen storage. For back-up power applications, where long storage periods under extreme temperatures are expected, the thermal stability of the storage material is particularly important. Here, we describe the development of an unusually kinetically stable chemical hydrogen storage material with a H₂ storage capacity of 4.7 wt%. The compound, which is the first reported parental BN isostere of cyclohexane featuring two BN units, is thermally stable up to 150 °C both in solution and as a neat material. Yet, it can be activated to rapidly desorb H₂ at room temperature in the presence of a catalyst without releasing other detectable volatile contaminants. We also disclose the isolation and characterization of two cage compounds with S₄ symmetry from the H₂ desorption reactions.

Full text of DOI:10.1021/ja511766p

Communication
pubs.acs.org/JACS
Bis-BN Cyclohexane: A Remarkably Kinetically Stable Chemical
Hydrogen Storage Material
Gang Chen,†,‡ Lev N. Zakharov,‡ Mark E. Bowden,§ Abhijeet J. Karkamkar,§ Sean M. Whittemore,§
Edward B. Garner, III,∥ Tanya C. Mikulas,∥ David A. Dixon,∥ Tom Autrey,§ and Shih-Yuan Liu*,†,‡
†Department of Chemistry, Boston College, Chestnut Hill, Massachusetts 02467-3860, United States
‡Department of Chemistry and Biochemistry, University of Oregon, Eugene, Oregon 97403-1253, United States
§Pacific Northwest National Laboratories, Richland, Washington 99353, United States
∥Department of Chemistry, The University of Alabama, Tuscaloosa, Alabama 35487-0036, United States
S
* Supporting Information
ABSTRACT: A critical component for the successful
development of fuel cell applications is hydrogen storage.
For back-up power applications, where long storage
periods under extreme temperatures are expected, the
thermal stability of the storage material is particularly
important. Here, we describe the development of an
unusually kinetically stable chemical hydrogen storage
material with a H2 storage capacity of 4.7 wt%. The
compound, which is the first reported parental BN isostere
of cyclohexane featuring two BN units, is thermally stable
up to 150 °C both in solution and as a neat material. Yet, it
can be activated to rapidly desorb H2 at room temperature
in the presence of a catalyst without releasing other
detectable volatile contaminants. We also disclose the
isolation and characterization of two cage compounds with
Figure 1. Thermal stability of amine-borane-based chemical hydrogen
storage materials.
S4 symmetry from the H2 desorption reactions.
that need to be considered include storage capacity, cost,
durability/operability, discharging rates, and fuel purity.14 For
back-up power applications, where long storage periods under
ydrogen is currently being investigated as a potential
extreme temperatures are expected, thermal stability of the
storage material is particularly important. On the other hand,
fast and consistent hydrogen desorption without external
energy input is also expected when back-up power is needed
during power outages. A kinetically stable hydrogen storage
material that can readily release hydrogen in the presence of a
catalyst could meet those requirements.
Henergy carrier, with the principal drivers being (1)
petroleum independence, i.e., hydrogen can be produced from
a variety of non-petroleum-based sources, and (2) relatively
high energy conversion efficiencies, i.e., H2 can be efficiently
converted to electrical energy and motive power using fuel
cells.1−4 For example, a well-to-wheels analysis of various
vehicular technologies indicated that proton exchange mem-
brane (PEM, also referred to as polymer electrolyte membrane)
fuel cell electrical vehicles can compare favorably to other
vehicular technologies including battery electric, plug-in hybrid
electric, hybrid electric, and conventional internal combustion
vehicles in terms of CO2 emission and petroleum energy
usage.5,6 In addition to vehicular applications,7−9 potential early
market applications for fuel cell power are being explored.10 In
an analysis by Battelle,11 back-up power (e.g., for tele-
communications towers) has been identified among others as
a potentially attractive market segment for PEM fuel cells.12
The current technology in this area relies on lead-acid batteries
and generators. Compared to those, fuel cells can potentially
offer advantages with regard to run times, maintenance costs,
and durability over a wider range of temperatures.
Compressed hydrogen gas is the current state-of-the-art15 for
vehicular and stationary applications, but it requires high
pressures and/or cryogenic temperatures. To increase the
storage density and mitigate the risks of dealing with
compressed gas, numerous condensed-phase hydrogen storage
approaches including metal hydrides,16 sorbent materials,17 and
chemical hydrogen storage systems18,19 are currently under
investigation. Amine-borane-based chemical hydrogen storage
materials feature relatively high gravimetric storage capacities
and facile H2 desorption kinetics.20 However, these materials
typically are not stable at high temperatures. For instance, a
significant rate of decomposition at temperatures equal to or
below 150 °C is observed for ammonia borane (AB),21−23
various amine-boranes,24 and the cyclic compounds cyclo-
A critical component for the successful development of fuel
Received: November 15, 2014
cell applications is hydrogen storage.13 Important parameters
© XXXX American Chemical Society
A
DOI: 10.1021/ja511766p
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Journal of the American Chemical Society
Communication
triborazane,25,26 1,2-BN-cyclohexane,27 and 3-methyl-1,2-BN-
°C, and the all-carbon cyclohexane melts between 4 and 7 °C.
Cyclotriborazane sublimes at around 157 °C with decom-
position.26 11B and 1H NMR analysis indicated that compound
H does not decompose when kept on the bench for more than
18 months under ambient conditions. No noticeable change
was brought about when H is heated in toluene at 150 °C in a
sealed tube for 24 h. Under the same conditions, more than
50% of 1,2-BN cyclohexane decomposed, and AB, MeAB, and
3-methyl-1,2-BN-cylopentane completely decomposed within 2
h (see Supporting Information). Similarly, it is reported that
cyclotriborazane readily decomposes in solution at temper-
atures above 100 °C.26
cyclopentane28 (Figure 1).
We describe in this Communication the discovery of an
unusually kinetically stable chemical hydrogen storage material
(compound H, Figure 1) with a H2 storage capacity of 4.7 wt%.
Compound H is thermally stable up to 150 °C both in solution
and as a neat material. Yet, it can be activated to rapidly desorb
H2 at room temperature in the presence of a catalyst without
releasing other detectable volatile contaminants.
The Liu laboratory has been focusing on the development of
BN isosteres of classic organic molecules27−31 to expand the
chemical space of existing compounds for potential applications
in biomedical research and materials science.32 In 2011, we
reported the synthesis of 1,2-BN cyclohexane27 and demon-
strated that it readily and cleanly releases 2 equiv of hydrogen
to form a trimer. In order to increase the storage capacity of
1,2-BN cyclohexane, we embarked on the synthesis of a bis-BN
cyclohexane H. As can be seen from Figure 2, the synthesis of
We also performed the thermal stability study of solid H by
thermogravimetric analysis (TGA) accompanied by differential
scanning calorimetry (DSC) experiment (Figure 3). At a
Figure 3. TGA (blue)/DSC (red) data of neat compound H.
heating rate of 1 °C/min, compound H undergoes an
endothermic sublimation process in the range between 150
and 200 °C that is accompanied by total loss of weight. 11B and
1H NMR analysis indicated no decomposition during the
sublimation process.
We then investigated the H2 release of compound H under a
variety of conditions. In view of the high thermal stability
exhibited by H, we surmised that H2 desorption will have to be
mediated by an appropriate catalyst. We recently showed that
simple base metal salts such as FeCl2 and CoCl2 are very
efficient catalyst precursors for the dehydrogenation of 3-
methyl-1,2-BN-cyclopentane.28 However, these catalysts were
ineffective in the dehydrogenation of compound H. When
heated in the presence of CoCl2 (10 mol%), compound H
showed less than 20% conversion at 90 °C in THF for over 1.5
h. Similarly, no noticeable change was detected when FeCl2 (10
mol%) was used under the same conditions (see Supporting
Information). After a brief screening of catalysts, we
determined that Pd/C was suitable in mediating the
dehydrogenation process;34 however, high temperatures (up
to 150 °C) were necessary to achieve high conversion (see
Supporting Information). Gratifyingly, the ruthenium complex
previously used by Fagnou and co-workers35 for the
dehydrogenation of AB proved effective for the dehydrogen-
ation of H. As can be seen from Figure 4, desorption of 2 equiv
of H2 from compound H (i.e., 4.7 wt% H2) can be
accomplished with 5 mol% [Ru] at room temperature in
about 15 min. At 0.5 mol% catalyst loading, the release of 2
equiv of H2 can be achieved in the same time frame at 65 °C.
To probe the purity of the released H2 gas, we performed
residual gas analysis (RGA) of compound H in tetraglyme as
the solvent using 2 mol% [Ru] catalyst at 65 °C. With the
exception of minor tetraglyme background signals, H2 is the
only detectable volatile species (see Supporting Information).
Figure 2. Synthesis of compound H.
compound H can be accomplished in three steps from the
known aminomethylene boronate ester 1.33 Reduction of
compound 1 with LiAlH4 generates the lithium borohydride
intermediate 2 in high yield. Subsequent treatment of 2 with
one equivalent of anhydrous HCl produces the bis-
(trimethylsily)aminomethyl borane 3, which spontaneously
dimerizes to form 1,1,4,4-tetra(trimethylsilyl)-1,4-diazonia-2,5-
diboratacyclohexane 4 in 91% yield from 2. The structure of 4
was unambiguously confirmed by single-crystal X-ray diffraction
analysis, which shows a twist boat conformation as the ground
state in the solid state (Figure 2). Removal of the trimethylsilyl
groups in 4 was achieved with tetrabutylammonium fluoride,
furnishing the target compound H in 69% overall yield from 1.
The identity of compound H was also unambiguously
confirmed by its single-crystal X-ray structure, which shows a
chair conformation that is also adopted by cyclohexane. To the
best of our knowledge, compound H represents the first
reported parental BN isostere of cyclohexane featuring two BN
units.
Compound H is a crystalline solid that sublimes without
decomposition at temperatures higher than 150 °C. As a
comparison, 1,2-BN cyclohexane has a melting point of 75−77
B
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Journal of the American Chemical Society
Communication
consisting of four five- and two six-membered rings, was formed
(Figure 5, eq 2). We were able to obtain single-crystal X-ray
structures for both of the cage compounds, thus unambiguously
confirming their structural assignments (Figure 5). Both cage
compounds 6 and 7 belong to the point group S4, which makes
the nitrogen, boron, and carbon atoms equivalent in each of the
molecules. Both of the cage compounds are air and moisture
stable, and they can be purified by silica gel chromatography.
Compound 7 does not convert to compound 6 when heated in
toluene at 150 °C, with or without Pd/C. Similarly, cage
compound 6 does not convert to 7 when subjected to the
reaction conditions shown in Figure 5, eq 2.39 The lack of
interconversion between 6 and 7 suggests that their formation
is kinetically controlled. G3MP240−44 gas-phase calculations
show that the formation of the six-four cage 6 from H is
exergonic by −56.0 kcal/mol (Figure 5, eq 3), whereas the
analogous reaction to form the six-five cage 7 is exergonic by
−64.3 kcal/mol (Figure 5, eq 4). Thus, the six-five cage 7 is
thermodynamically more stable than the six-four cage isomer 6
by 8 kcal/mol. It is worth noting that the higher temperature
employed in eq 1 (Figure 5), which would seemingly favor the
thermodynamic product, is actually producing the less
thermodynamically stable cage isomer 6.
In conclusion, we have developed the synthesis of compound
H, which is the first parental BN isostere of cyclohexane
featuring two BN units. Bis-BN cyclohexane H is a remarkably
kinetically stable amine-borane, withstanding temperatures up
to 150 °C both as a neat material and in solution. Yet, it can be
activated to release 4.7 wt% of analytically pure H2 at room
temperature in 15 min in the presence of a catalyst. We have
also intercepted and structurally characterized two cage
compounds that might give insight into the possible structures
of polymeric materials that were also formed during the H2
desorption process. The kinetic stability exhibited by
compound H and the ability to readily release its stored H2
content upon exposure to a catalyst would make it a potentially
suitable candidate for stationary fuel cell applications.
Figure 4. Automated volumetric buret measurements of H2 release of
compound H in the presence of the [Ru] catalyst.
In the process of screening various catalytic conditions for
the dehydrogenation of H, we noticed a difference in the
observed 11B NMR spectra when using Pd/C or the [Ru] as the
catalyst. While a doublet at 40.6 ppm (assigned to the BH of
compound 5 (Figure 4, right) was observed in the Pd/C-
catalyzed dehydrogenation, the same signal was not found in
[Ru]-catalyzed system. To further understand the details, we
tried to optimize the reaction conditions to isolate and
characterize the dehydrogenation products from the two
catalytic systems. While some as of yet unidentified polymeric
species36−38 were formed from both of the systems, we were
able to intercept two distinct cage compounds in moderate to
good yields from each of the reactions, respectively. When Pd/
C was selected as the dehydrogenation catalyst, the resulting
cage compound 6, consisting of four six- and two four-
membered rings, was produced as the only non-polymeric
product presumably by the dimerization of two molecules of
the non-isolable intermediate 5 (Figure 5, eq 1). In contrast,
when [Ru] was employed as the catalyst, cage compound 7,
ASSOCIATED CONTENT
■
S
* Supporting Information
Experimental procedures, spectroscopic data, additional
computational details, complete ref 44, and crystallographic
information. This material is available free of charge via the
Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
shihyuan.liu@bc.edu
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
S.-Y.L., T.A., and D.A.D. thank the U.S. Department of Energy
(DE-EE-0005658) for financial support. Pacific Northwest
National Laboratory is operated by Battelle. Funding for the
University of Oregon Chemistry Research and Instrumentation
Services has been furnished in part by the NSF (CHE-
0923589). D.A.D. thanks the Robert Ramsay Chair Fund of
The University of Alabama for support. G.C. thanks Dr.
Senmiao Xu and Gabriel J. Lovinger for helpful discussions.
Figure 5. Isolation and characterization of cage compounds 6 and 7
and predicted energies for their formation.
C
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Communication
(30) Abbey, E. R.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2011,
133, 11508−11511.
REFERENCES
■
(1) Committee on Alternatives and Strategies for Future Hydrogen
Production and Use, National Research Council, National Academy of
Engineering. The Hydrogen Economy: Opportunities, Costs, Barriers, and
R&D Needs; The National Academies Press, Washington, D.C., 2004.
(2) von Helmolt, R.; Eberle, U. J. Power Sources 2007, 165, 833−843.
(3) Eberle, D. U.; von Helmolt, D. R. Energy Environ. Sci. 2010, 3,
689−699.
(4) Satyapal, S.; Petrovic, J.; Read, C.; Thomas, G.; Ordaz, G. Catal.
Today 2007, 120, 246−256.
(5) Well-to-Wheels Greenhouse Gas Emissions and Petroleum Use
for Mid-Size Light-Duty Vehicles. Department of Energy Program
Record No. 13005, revision #1, May 10, 2013; http://www.hydrogen.
energy.gov/pdfs/13005_well_to_wheels_ghg_oil_ldvs.pdf (accessed
Oct 19, 2014, using Firefox 33.0).
̀
(31) Ishibashi, J. S. A.; Marshall, J. L.; Maziere, A.; Lovinger, G. J.; Li,
B.; Zakharov, L. N.; Dargelos, A.; Graciaa, A.; Chrostowska, A.; Liu, S.-
Y. J. Am. Chem. Soc. 2014, 136, 15414−15421.
(32) Campbell, P. G.; Marwitz, A. J. V.; Liu, S.-Y. Angew. Chem., Int.
Ed. 2012, 51, 6074−6092.
(33) Martichonok, V.; Jones, J. B. Bioorg. Med. Chem. 1997, 5, 679−
684.
(34) Jaska, C. A.; Temple, K.; Lough, A. J.; Manners, I. J. Am. Chem.
Soc. 2003, 125, 9424−9434.
(35) Blaquiere, N.; Diallo-Garcia, S.; Gorelsky, S. I.; Black, D. A.;
Fagnou, K. J. Am. Chem. Soc. 2008, 130, 14034−14035.
(36) Denney, M. C.; Pons, V.; Hebden, T. J.; Heinekey, D. M.;
Goldberg, K. I. J. Am. Chem. Soc. 2006, 128, 12048−12049.
(37) Ewing, W. C.; Marchione, A.; Himmelberger, D. W.; Carroll, P.
J.; Sneddon, L. G. J. Am. Chem. Soc. 2011, 133, 17093−17099.
(38) Metters, O. J.; Chapman, A. M.; Robertson, A. P. M.; Woodall,
C. H.; Gates, P. J.; Wass, D. F.; Manners, I. Chem. Commun. 2014, 50,
12146−12149.
(6) Elgowainy, A.; Burnham, A.; Wang, M.; Molburg, J.; Rousseau, A.
SAE Int. J. Fuels Lubr. 2009, 2, 627−644.
(7) Paster, M. D.; Ahluwalia, R. K.; Berry, G.; Elgowainy, A.; Lasher,
S.; McKenney, K.; Gardiner, M. Int. J. Hydrogen Energy 2011, 36,
14534−14551.
(39) Due to the strongly exothermic and exergonic nature of the H2
desorption process to form cage compounds 6 and 7, eqs 1 and 2 in
Figure 5 are likely irreversible.
(40) Curtiss, L. A.; Redfern, P. C.; Raghavachari, K.; Rassolov, V.;
Pople, J. A. J. Chem. Phys. 1999, 110, 4703−4709.
(41) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.
(42) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785−789.
(43) Godbout, N.; Salahub, D. R.; Andzelm, J.; Wimmer, E. Can. J.
Chem. 1992, 70, 560−571.
(44) Frisch, M. J.; et al. Gaussian 09; Gaussian, Inc.: Wallingford, CT,
2009.
(45) (a) Wang, Z.; Lu, G.; Li, H.; Zhao, L. Chinese Sci. Bull. 2010, 55,
239−245. (b) Zhao, L.; Li, H.; Lu, G.; Wang, Z. Dalton Trans. 2010,
39, 4038−4047. (c) Zhao, L.; Lu, G.; Huang, F.; Wang, Z. Dalton
Trans. 2012, 41, 4674−4684.
(8) Contestabile, M.; Offer, G. J.; Slade, R.; Jaeger, F.; Thoennes, M.
Energy Environ. Sci. 2011, 4, 3754−3772.
(9) Eberle, U.; Muller, B.; von Helmolt, R. Energy Environ. Sci. 2012,
̈
5, 8780−8798.
(10) McWhorter, S.; Read, C.; Ordaz, G.; Stetson, N. Curr. Opin.
Solid State Mater. Sci. 2011, 15, 29−38.
(11) Mahadevan, K.; Judd, K.; Stone, H.; Zewatsky, J.; Thomas, A.;
Mahy, H.; Paul, D. Identification and Characterization of Near-Term
Direct Hydrogen Proton Exchange Membrane Fuel Cell Markets.
Prepared for U.S. Department of Energy, DOE Contract No. DE-
FC36-03GO13110; Battelle: Columbus, OH, 2007; http://www1.eere.
energy.gov/hydrogenandfuelcells/pdfs/pemfc_econ_2006_report_
final_0407.pdf (Accessed Oct 19, 2014).
(12) Using Fuel Cells for Backup Power for Telecommunications
Facilities. U.S. Department of Energy, http://www.hydrogen.energy.
gov/permitting/telecommunications.cfm (Accessed Oct 19, 2014).
(13) Eberle, U.; Felderhoff, M.; Schuth, F. Angew. Chem., Int. Ed.
2009, 48, 6608−6630.
(14) Targets for Onboard Hydrogen Storage Systems for Light-Duty
Vehicles. U.S. Department of Energy, September 2009; http://energy.
gov/sites/prod/files/2014/03/f11/targets_onboard_hydro_storage_
explanation.pdf (Accessed Oct 19, 2014).
NOTE ADDED IN PROOF
■
̈
Sterically encumbered derivatives of compound 3 are predicted
to behave like frustrated Lewis pairs.45
(15) Jorgensen, S. W. Curr. Opin. Solid State Mater. Sci. 2011, 15, 39−
43.
(16) Graetz, J. Chem. Soc. Rev. 2009, 38, 73−82.
(17) Zhao, D.; Yuan, D.; Zhou, H.-C. Energy Environ. Sci. 2008, 1,
222−235.
(18) Huang, Z.; Autrey, T. Energy Environ. Sci. 2012, 5, 9257−9268.
(19) Teichmann, D.; Arlt, W.; Wasserscheid, P. Int. J. Hydrogen
Energy 2012, 37, 18118−18132.
(20) Staubitz, A.; Robertson, A. P. M.; Manners, I. Chem. Rev. 2010,
110, 4079−4124.
(21) Wang, P. Dalton Trans. 2012, 41, 4296−4302.
(22) Bowden, M.; Autrey, T. Curr. Opin. Solid State Mater. Sci. 2011,
15, 73−79.
(23) Bowden, M.; Autrey, T.; Brown, I.; Ryan, M. Curr. Appl. Phys.
2008, 8, 498−500.
(24) Mal, S. S.; Stephens, F. H.; Baker, R. T. Chem. Commun. 2011,
47, 2922−2924.
(25) Dahl, G. H.; Schaeffer, R. J. Am. Chem. Soc. 1961, 83, 3032−
3034.
(26) Schellenberg, R.; Kriehme, J.; Wolf, G. Thermochim. Acta 2007,
457, 103−108.
(27) Luo, W.; Zakharov, L. N.; Liu, S.-Y. J. Am. Chem. Soc. 2011, 133,
13006−13009.
(28) Luo, W.; Campbell, P. G.; Zakharov, L. N.; Liu, S.-Y. J. Am.
Chem. Soc. 2011, 133, 19326−19329.
(29) Marwitz, A. J. V.; Matus, M. H.; Zakharov, L. N.; Dixon, D. A.;
Liu, S.-Y. Angew. Chem., Int. Ed. 2009, 48, 973−977.
D
DOI: 10.1021/ja511766p
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