CO2-"Neutral" hydrogen storage based on bicarbonates and formates

Article abstract of DOI:10.1002/anie.201101995

Let the circle be unbroken! One ruthenium catalyst generated in situ facilitates the selective hydrogenation of bicarbonates and carbonates, as well as CO₂ and base, to give formates and also the selective dehydrogenation of formates back to bicarbonates. The two reactions can be coupled, leading to a reversible hydrogen-storage system. dppm=1,2- bis(diphenylphosphino)methane. Copyright

Full text of DOI:10.1002/anie.201101995

DOI: 10.1002/anie.201101995
Hydrogen Storage
CO2-“Neutral” Hydrogen Storage Based on Bicarbonates and
Formates**
Albert Boddien, Felix Gꢀrtner, Christopher Federsel, Peter Sponholz, Dꢁrthe Mellmann,
Ralf Jackstell, Henrik Junge, and Matthias Beller*
Hydrogen is considered to be an attractive candidate as a
chemical “energy vector” for a sustainable energy technol-
ogy.[1] The so-called hydrogen economy is based on the
sustainable production of hydrogen without the release of
stoichiometric amounts of carbon dioxide (CO2).[2] For its
realization, practical solutions for hydrogen storage, distribu-
tion, and usage are also required. The safe and practical
storage of hydrogen in particular constitutes a yet insuffi-
ciently solved issue. Despite progress in the fields of physical
hydrogen storage,[3] metal hydride technologies,[4] and hydro-
gen adsorption,[5] no general solution for hydrogen storage
has yet been developed that meets industrial requirements
(ambient conditions, high power-to-volume ratios).
prerequisites. In this context Himeda et al. were able to
hydrogenate CO2 under basic conditions and dehydrogenate
formic acid under acidic conditions by applying the same
catalysts at different pH.[20]
Ideally, the hydrogen carrier, CO2, should be re-used and
a closed carbon cycle should be achieved. Therefore, trapping
carbon dioxide in the dehydrogenation process is fundamen-
tal. Unfortunately, this has not yet been achieved. Here, we
describe for the first time the design of a reversible hydrogen-
storage cycle based on the redox system bicarbonate/formate
(Scheme 1).
In addition to methanol[6] and methane,[7] formic acid
(FA) and formates have recently gained considerable interest
for hydrogen generation.[8] These chemical compounds are
nontoxic and stable with a hydrogen content of 4.4 wt% (FA)
and 2.35 wt% (for NaHCO2/H2O). As formates are non-
corrosive and nonirritating, they are also easy to handle.
In 2008, parallel to the work of Laurenczy and co-
workers,[9] we demonstrated that hydrogen release from
formic acid/amine adducts proceeds smoothly under mild
conditions in the presence of ruthenium catalysts.[10] Since
then, research groups led by Fukuzumi,[11] Wills,[12] Iglesia,[13]
and others[14] have demonstrated the possibility of using
various other homogeneous or heterogeneous catalysts for
hydrogen release from formic acid. Although most catalysts
are based on noble metals, recently it was shown that even
iron complexes are suitable for this reaction.[15]
After several decades of research, the back reaction of this
hydrogen generation—the catalytic reduction of carbon
dioxide and carbonates—is still a challenging topic; many
reviews have reflected the current research at their respective
time.[16,17] Recently, we reported a ruthenium phosphine
catalyst system[18] and an iron catalyst,[19] which facilitate the
hydrogenation of bicarbonates without the use of additional
CO2. Clearly, the “real” use of formates as a hydrogen-storage
material sets both the release and the uptake of hydrogen as
Scheme 1. Hydrogen uptake and hydrogen release in the bicarbonate/
formate system.
Our concept has the following advantages: Compared to
carbon dioxide, solid bicarbonates are easy to handle and
highly soluble in aqueous media (96 gLÀ1 NaHCO3 at 208C in
H2O). The resulting aqueous bicarbonate solution can be
catalytically converted to a formate solution under much
milder conditions than those required for reactions of
methanol or methane. The nontoxic aqueous solution of
formate is easily stored and transported. Finally, hydrogen
can be released on demand in the presence of a suitable
catalyst. Again, this hydrogen discharge can be performed at
or below room temperature. Most importantly, after full
conversion of the formate, the bicarbonate solution may be
recharged with hydrogen to close the cycle. To the best of our
knowledge, no catalyst system has yet been described that can
facilitate both reaction pathways under basic conditions and
also trap the CO2 formed in the dehydrogenation.
At the start of this work we studied the hydrogenation
activity of a catalyst system comprising [{RuCl2(benzene)}2]
and 1,2-bis(diphenylphosphino)methane (dppm) towards dif-
ferent formates. Here, we examined the hydrogenation of
bicarbonates, carbonates, and carbon dioxide—H2 uptake—
under different conditions (Table 1). Basically, formates can
be obtained by catalytic hydrogenation of a) carbonates and
bicarbonates (often with additional CO2 pressure) or b) CO2
in the presence of inorganic bases. Obviously, method a is the
[*] A. Boddien, F. Gꢀrtner, C. Federsel, P. Sponholz, D. Mellmann,
Dr. R. Jackstell, Dr. H. Junge, Prof. M. Beller
Leibniz-Institut fꢁr Katalyse e.V.
Albert-Einstein-Strasse 29a (Germany)
Fax: (+49)381-1281-5000
E-mail: matthias.beller@catalysis.de
Homepage: http://www.catalysis.de
[**] This work was supported by the BMBF (Spitzenforschung und
Innovation in den neuen Lꢀndern). F.G. thanks the Fonds der
Chemischen Industrie (FCI) for a Kekulꢂ grant. We thank Prof. G.
Laurenczy (EPFL) for fruitful discussions.
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Communications
Table 1: Catalytic hydrogenation of bicarbonates and carbonates as well
as CO2 and base in the presence of [{RuCl2(benzene)}2] and dppm.[a]
analyzed using automatic gas burettes and GC in a setup
described elsewhere.[14a] Besides hydrogen, argon, and to a
minimal extent CO2, no other gas was detected (CO <
1 ppm). Since the formation of hydrogen from formates
requires additional protons, we used an excess of water (11-
fold excess of H2O). In water the liberation of hydrogen
causes a shift of the pH to more basic media. Consequently,
the formed CO2 is captured in the basic solution as
bicarbonate, which precipitates during the reaction. Selected
results of this set of experiments are given in Table 2.
Entry Reagent
Product
pH
[bar][b] Yield [%][c] TON
2 =CO
1
2
3
NaHCO3
KHCO3
NH4HCO3 NH4HCO2
NaHCO3
NaHCO3
NaHCO3
Na2CO3
KHCO3
LiOH
NaOH
KOH
Ca(OH)2
Mg(OH)2
NaHCO2
KHCO2
50/0
50/0
50/0
80/0
80/0
50/30
50/30
50/30
50/30
50/30
50/30
50/30
50/30
35
23
20
96
16
75
45
69
76
38
68
48
39
807
531
461
1108
320
1731
1038
1592
1754
877
1569
1108
900
4[d]
5[e]
6
NaHCO2
NaHCO2
NaHCO2
NaHCO2
KHCO2
LiHCO2
NaHCO2
KHCO2
7
8
9
10
11
12
13
Table 2: Hydrogen generation from formates with the Ru/dppm in situ
catalyst.[a]
Ca(HCO2)2
Mg(HCO2)2
Entry
Formate
VH (3 h)
[mL]
TON (3 h)
Initial
Vol %
CO2
2
TOF [hÀ1
]
[a] Reaction conditions: 5.2 mmol [{RuCl2(benzene)}2], 20.8 mmol dppm
(Ru/P=1:4), 24 mmol reagent, 25 mL distilled H2O, 5 mL THF, 2 h
reaction time, 708C. [b] Pressure at room temperature. [c] Yield based on
1H NMR analysis using THF as an internal standard. [d] 10.4 mmol
catalyst and 4 equiv dppm, 20 h reaction time. [e] 5.0 mmol [{RuCl2-
(benzene)}2], 30.0 mmol dppm (Ru/P=1:6), 20 mmol NaHCO3, 5 mL
H2O, 25 mL THF, RT, 24 h reaction time.
1
2
LiHCO2
490
490
218
14
299
23
2000[c]
2000[c]
889
56
1222
93
2923
2592
377
19
234
126
420
770
33.7
8.3
0.8
0.06
0.9
27.9
34.5
34.2
NaHCO2
NaHCO2
NaHCO2
KHCO2
NH4HCO2
Mg(HCO2)2
Ca(HCO2)2
3[b]
4[d]
5
6
7
8
337
486
1377
1985
most convenient process, since only H2 is needed; however,
higher yields are generally obtained when both CO2 and H2
are added.
[a] Reaction conditions: 5.0 mmol [{RuCl2(benzene)}2], 30 mmol dppm
(Ru/P=1:6), 20 mmol formate, 20 mL DMF, 5 mL H2O, 608C; gas
volumes determined using automatic gas burettes and analyzed by GC;
TON=n(H2)/n(Ru), for the determination of the initial TOF the
conversion was kept below 20%. [b] Reaction temperature 408C, 14 h
reaction time. [c] Full conversion. [d] THF as solvent, 258C.
Several formate salts were obtained in moderate yields
after a reaction time of 2 h at low temperature (708C) in the
presence of 125 ppm of the Ru catalyst. When pure H2 was
used for the hydrogenation of sodium or potassium bicar-
bonate, yields of 35% (TON: 807) and 23% (TON: 531) were
obtained, respectively (Table 1, entries 1 and 2). Similarly,
ammonium bicarbonate was converted in 20% yield and
moderate activity (TON: 461; Table 1, entry 3). However,
when 80 bar of H2 was applied, almost full conversion of
NaHCO3 and an excellent yield (96%) of NaHCO2 were
possible with a TON of 1108 (Table 1, entry 4). Notably,
reasonable activity was observed even at room temperature
(Table 1, entry 5). These results highlight NaHCO3 as the
most suitable candidate for H2 storage since addition of H2 is
sufficient to recover the formate with high yield. Never-
theless, in the presence of additional CO2 the conversion and
activity were higher after 2 h than in the reaction with only
50 bar H2 (yield: 75%, TON: 1731; Table 1, entries 1 and 6).
Interestingly, using sodium carbonate instead of sodium
bicarbonate in the presence of carbon dioxide resulted in a
significant decrease of activity and conversion (Table 1,
entries 6 and 7). The use of carbon dioxide along with
inorganic bases such as calcium, lithium, and magnesium
hydroxide also led to good yields and catalyst activities. The
highest conversions and activities were observed when LiOH
or KOH were used (Table 1, entries 9 and 11). Therefore the
base strength is not the predominant factor. We also observed
significant conversion of CO2 in the presence of Ca(OH)2 and
Mg(OH)2 (Table 1, entries 12 and 13).
To our delight full conversion and selective dehydrogen-
ation (CO < 1 ppm) was observed in all cases. The highest
activity (initial TOF: 2923 hÀ1) was observed when lithium
formate served as the starting material (Table 2, entry 1).
However, in this case the CO2 content of the gas mixture was
relatively high. Notably, comparable activity but significant
lower CO2 content was observed in the case of NaHCO2 and
KHCO2 (Table 2, entries 2 and 5). Interestingly, the reaction
rate drops significantly when the the reaction temperature is
lowered from 608C (TOF: 2592 hÀ1) to 258C (TOF: 19 hÀ1).
However, at low temperatures all CO2 could be collected
(Table 2, entries 2–4). Ammonium formate was converted
with comparable low activity and high CO2 content (Table 2,
entry 6). In the reactions of Mg(HCO2)2 and Ca(HCO2)2
hydrogen was liberated with good activities of 420 hÀ1 and
770 hÀ1, respectively (Table 2, entries 6 and 7). However, CO2
was not trapped effectively during the reaction. Nevertheless,
when NaHCO2 was used, 100% conversion was reached at
408C with a relatively low loss of CO2 (< 1%). A represen-
tative gas evolution curve along with a plot of the H2 content
of the gas mixture is given in Figure 1.
Our investigations indicate that NaHCO2 is a suitable
hydrogen-storage material since only H2 is needed for the
transformation. Both the starting material (sodium bicarbon-
ate) and the product (sodium formate) are nontoxic and
noncorrosive solids and easy to handle. High yields can be
observed for both reactions under mild conditions including
excellent conversion of CO2 to bicarbonate during the release
of hydrogen (Table 1, entry 4 and Table 2, entry 3). Notably,
Next, we investigated the selective dehydrogenation—H2
release—of different formates. We used the same in situ
system comprising the Ru precursor (5.0 mmol) and dppm at
608C. The evolved gases were quantitatively and qualitatively
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96% yield at 708C in water/THF without additional CO2.
Dehydrogenation of sodium formate was achieved with high
conversion (> 90%) under ambient temperature (308C). It
was demonstrated for the first time that the two reactions can
be coupled leading to a closed carbon cycle for hydrogen
storage. In contrast to our previous studies on the dehydro-
genation of formic acid/amine adducts this process is amine-
free.
Experimental Section
All reagents were purchased from a commercial supplier (Aldrich)
and used without further purification. Solvents were degassed and
distilled prior to use. Methods for quantification of gases as well as
determination of conversions and yield were published elsewhere.[14a]
Hydrogen generation from formates: A thermostatically con-
trolled reaction vessel was purged with argon six times to remove any
other gases. The respective formate (20 mmol) was introduced as a
powder together with degassed water (5 mL) and DMF (20 mL).
After addition of dppm (30.0 mmol) the solution was heated to the
desired temperature. After equilibration of the reaction mixture for
15 min [{RuCl2(benzene)}2] (5.0 mmol) was added in a Teflon crucible
and gas evolution started. The evolved gases were collected with an
automatic gas burette. After each reaction a GC sample was taken. At
the end of the reaction period aqueous HCl was slowly added to the
reaction mixture in order to determine the amount of bicarbonate
present (indirectly based on the amount of CO2 formed). CO2
formation was confirmed by a second GC measurement. The
deviation of measured gas volumes for two reactions was typically
1–15%.
Figure 1. Hydrogen evolution curve and plot of H2 content (red dots)
at 408C from NaHCO2/H2O mixture using 10 mmol [{RuCl2-
(benzene)}2]/6 equiv dppm in 20 mL DMF and 5 mL H2O.
in contrast to all previously reported work with formic acid or
formic acid/amine adducts, we have achieved a fully closed
carbon cycle for the first time. This is far from trivial because
of the exigency of CO2 capture for a closed cycle!
Finally, in two different sets of proof-of-principle experi-
ments we demonstrated that the same ruthenium catalyst
system can be used in consecutive hydrogenation/dehydro-
genation sequences (Scheme 2). In the first set of experiments
Hydrogenation of bicarbonates or carbonates and CO2 in the
presence of inorganic base: [{RuCl2(benzene)}2] (2.6 mg, 5.2 mmol)
and dppm (8.0 mg, 21 mmol) were dissolved under argon in a solution
of distilled water (25 mL), 24 mmol bicarbonate, carbonate, or base
and THF (5 mL, to increase solubility of the catalyst). The mixture
was transferred to a stirred (400 rpm) autoclave (100 mL) and
deoxygenated with argon. H2 (in some cases additional CO2) was
introduced to the autoclave at room temperature, and the reaction
mixture was heated at 708C. After 2 h, the autoclave was cooled with
ice water and the pressure was slowly released. All solvents were
removed under reduced pressure, and the formate content in the
Scheme 2. Reversible hydrogen storage with a bicarbonate/formate
system: Catalytic hydrogenation of bicarbonate and hydrogen release
from formate with the same Ru/dppm in situ catalyst. Reaction
conditions: dehydrogenation (DH): 40 mL DMF, 10 mL H2O, 308C,
24–48 h; hydrogenation (H): 20 mL H2O, 10 mL THF, 80 bar H2, 24 h;
sequence I: 71 mmol [{RuCl2(benzene)}2], 0.43 mmol dppm, 39.6 mmol
NaHCO2; sequence II: 10.4 mmol [{RuCl2(benzene)}2], 41.6 mmol
dppm, 23.8 mmol NaHCO3.
1
resulting yellowish white solid was measured by H NMR spectros-
copy with D2O as the solvent and THF as the internal standard with a
relaxation delay (D1) of 20 s. The deviation of determined yields for
two reactions was typically 1–10%.
Proof of principle of hydrogen storage: The formation of sodium
bicarbonate or selective hydrogen generation from formates was
carried out according to the procedures above. After the reactions all
solvents were removed under reduced pressure and the recovered
solid was stored under an inert gas atmosphere. No additional catalyst
was needed for the next hydrogenation or dehydrogenation step.
(sequence I) commercially available sodium formate was
dehydrogenated in the presence of the Ru/dppm catalyst at
room temperature (> 90% conversion). After simple removal
of all solvents, the obtained powder was used in a hydro-
genation reaction under pressure giving back sodium formate
with 80% yield in 20 h. Remarkably, no additional catalyst
was required for the hydrogenation experiment. In a second
set of experiments we started with commercially bicarbonate
(sequence II in Scheme 2). After hydrogenation to sodium
formate with 95% yield, the product was reverted back to
hydrogen and sodium bicarbonate with 80% yield. Again, no
additional catalyst was applied in the second step.
Received: March 21, 2011
Published online: May 23, 2011
Keywords: bicarbonates · homogeneous catalysis · formates ·
.
hydrogen storage · ruthenium
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