Reusable homogeneous catalytic system for hydrogen production from methanol and water

Article abstract of DOI:10.1021/cs500937f

An efficient system catalyzed by a Ru-PNN pincer complex was developed for reforming methanol to H₂ and CO₂ (absorbed by base) under relatively low temperature (around 100 °C), and good yields of H₂ were obtained (～80%). T

Full text of DOI:10.1021/cs500937f

Letter
pubs.acs.org/acscatalysis
Reusable Homogeneous Catalytic System for Hydrogen Production
from Methanol and Water
Peng Hu,† Yael Diskin-Posner,‡ Yehoshoa Ben-David,† and David Milstein*,†
†Department of Organic Chemistry and ‡Department of Chemical Research Support, Weizmann Institute of Science, Rehovot, 76100,
Israel
S
* Supporting Information
ABSTRACT: An efficient system catalyzed by a Ru-PNN
pincer complex was developed for reforming methanol to H2
and CO2 (absorbed by base) under relatively low temperature
(around 100 °C), and good yields of H2 were obtained
(∼80%). The catalyst solution can be reused without isolation
and purification, with no decrease in catalytic activity being
observed for a period of ∼1 month. Decomposition of formic
acid, which is likely to be the last step of the methanol
reforming reaction, was also investigated, and the formic acid
adduct of the catalyst was fully characterized spectroscopically
and by X-ray crystallography.
KEYWORDS: methanol reforming, hydrogen storage, reusable catalyst, ruthenium pincer complex, dehydrogenation
ydrogen is a potentially efficient and clean source of
complexes. Although good turnover numbers were achieved,
1
Henergy, but it is hard to store, being a gas with low the catalysts were not very stable, and the reported methanol
conversions were low.6 Our ongoing study on ruthenium pincer
density under normal pressure at ambient temperature. On the
complexes has resulted in several efficient and “waste-free”
dehydrogenation8,9 and hydrogenation9,10 reactions. We have
recently reported the transformation of primary alcohols to
carboxylic acid salts catalyzed by complex 1 (Figure 1) using
other hand, methanol, a liquid under ambient conditions and
having a high hydrogen storage capacity of 12.6%, is considered
as a very good hydrogen carrier.2−6 Furthermore, methanol can
be produced from renewable resources,2 which makes it even
more attractive for use for hydrogen storage purposes. The
classic process can produce hydrogen and CO2 from a mixture
of methanol and water and is used in the reformed methanol
fuel cell. However, this reforming process is catalyzed by
heterogeneous catalysts and requires a high temperature of
above 200 °C, which may generate CO, poisoning the fuel cell
catalysts. These and some other disadvantages, including the
requirement of high pressure, limit the application of the
system.3−7 For practical purposes, efficient processes catalyzed
by reusable catalysts, without isolation and purification, are
highly desirable for reforming of methanol to hydrogen and
CO2 under relatively low pressure and low temperature.
Figure 1. PNN ruthenium pincer complexes 1−4.
Recently, Beller reported an important homogeneous
ruthenium-catalyzed methanol dehydrogenation reaction at
low temperature (<100 °C) to produce hydrogen and carbon
dioxide. A high turnover frequency up to 4700 h−1 was
reported, and the catalyst remained active for a long time (∼23
days). However, methanol conversion was low (<30% based on
methanol), and the catalytic activity decreased considerably
H2O as the oxygen atom source (eq 1) in basic solvent.11 This
finding led us to explore the possibility to produce formic acid
from methanol, which would then decompose to hydrogen and
carbon dioxide,12,13 as shown in eq 2. Herein, we present the
first example of a reusable homogeneous catalyst system, with
no need for catalyst isolation and purification, to promote the
during the reaction.3,4 In the same year, Grutzmacher presented
̈
another important example of generation of pure hydrogen
from methanol. Catalyzed by 0.5 mol % ruthenium catalyst at
90 °C, conversion of methanol up to 80% was achieved.5
Shortly after these two reports, Beller reported the reforming of
methanol at low temperature catalyzed by iron pincer
Received: July 1, 2014
Published: July 7, 2014
© 2014 American Chemical Society
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Letter
reforming process of methanol, offering hydrogen and CO2
(absorbed by base) as products.
First, a mixture of 0.2 mol % complex 1, 5 mmol MeOH, 10
mmol NaOH, and 0.5 mL H2O was stirred for 24 h at 115 °C
(oil bath temperature), resulting in no observable hydrogen
formation (Table 1, entry 1). However, upon addition of 0.5
after 4 days when only 0.05 mol % catalyst 1 was used (entry
8).
Further optimization revealed that toluene was a better
solvent for the reaction, and 68% yield of hydrogen was
observed after 4 days at the same catalyst loading of 0.05 mol %
(entry 9). Using a mixed solvent of toluene/H2O in 1:1 v/v
ratio resulted in 64% yield of hydrogen after 7 days, even when
a lower catalyst loading of 0.025 mol % was used (entry 10).
When KOH was applied instead of NaOH, hydrogen was
produced in 70% yield after 7 days (entry 11; the average TOF
is 50 h−1)15 and 77% yield after 9 days (entry 12; the average
TOF is 43 h−1). Interestingly, when the organic layer of the
reaction under the conditions of entry 12 was separated after 9
days and reused directly without adding catalyst 1 and toluene,
an 82% yield of hydrogen was collected after 9 days (entry 13;
the average TOF is 45 h−1). Further application of the organic
layer separated from the reaction under conditions of entry 13
resulted in an 80% yield of hydrogen after 9 days (entry 14; the
average TOF is 45 h−1). In all, 0.0024 g of catalyst 1 kept
catalytic activity under the conditions of entry 12 for nearly 1
month without any activity decrease: ∼1.53 g methanol was
fully converted to hydrogen and CO2 and TON of ∼29 000 was
achieved. Thus, the catalyst solution can be stored for extended
periods and does not lose activity during the course of several
heating and cooling (to r.t. or −30 °C) cycles under an inert
atmosphere (see the Supporting Information for details).
On the basis of our former research, alcohols can be
transformed to carboxylic acid salts upon reaction with water
under basic conditions.11 To get further understanding of the
transformation of methanol to H2 and CO2, we investigated the
decomposition reaction of formic acid in water, which is likely
to be the last step of the reforming reaction, using complex 1 as
the catalyst.9 Without stoichiometric base, no gas was produced
at room temperature (Table 2, entry 1), whereas 25% yield of
Table 1. Production of H2 by Reforming Methanol in
a
Water
catalyst
2−
MeOH + 2OH− ⎯⎯⎯⎯⎯⎯→ CO3 + 3H2
cat.
(mol %)
MeOH
(mmol)
H2O
(mL)
solvent
(mL)
t
yield of
H2 (%)
entry
(days)
1
2
1 (0.2)
1 (0.2)
5
5
0.5
0.5
1
2
THF
(0.5)
71
b
3
1 (0.2)
1 (0.2)
2 (0.2)
3 (0.2)
1 (0.2)
1 (0.05)
1 (0.05)
5
5
0.5
0.5
0.5
0.5
0.5
1
THF
(0.5)
1
1
2
1
1
4
4
7
7
9
9
9
c
4
THF
(0.5)
5
6
5
THF
(0.5)
54
3
5
THF
(0.5)
d
7
5
THF
(0.5)
trace
28
68
64
70
77
82
80
8
9
10
10
20
20
20
20
20
THF
(0.5)
1
toluene
(0.5)
10
1
2
toluene
(2)
(0.025)
e
11
1
2
toluene
(2)
(0.025)
e
12
1
2
toluene
(2)
(0.025)
ef
,
13
14
soln
reused
2
eg
,
soln
reused
2
Table 2. Decomposition of Formic Acid Promoted by
a
Catalyst 1
a
Reaction conditions: catalyst, 1 equiv of MeOH, 2 equiv of NaOH,
cat.1
and the specified amounts of water and solvent were stirred at 115 °C
(oil bath temperature, the actual reaction temperature was 90−95 °C
when using THF/H2O as solvent, and 100−105 °C when using
toluene/H2O as solvent). Yields are based on volume of H2 collected
HCOOH ⎯⎯⎯→ CO2 + H2
entry
base (mmol)
H2O (mL)
T (°C)
conv (%)
25
1
2
0.5
0.5
0.5
0.5
0.5
r.t.
b
(with respect to MeOH). Catalytic KOtBu (1.2 equiv to catalyst) was
115
r.t.
c
used without NaOH. Catalytic KOtBu (1.2 equiv to catalyst) and 2
b
3
KOH (5.5)
KOH (5.5)
NEt3 (5.5)
NEt3 (5.5)
d
b
equiv of NEt3 were used instead of NaOH. 60 °C (oil bath
4
115
r.t.
>99
trace
98
e
f
temperature). KOH was used instead of NaOH. Organic layer of
5
g
entry 12 was used. Organic layer of entry 13 was used.
6
r.t.
a
Reaction conditions: 0.09 mol % catalyst 1, KOtBu (1.2 equiv relative
to catalyst), 0.1 mL HCOOH (2.65 mmol), base (as specified in the
Table) and 0.5 mL THF were stirred at r.t. (19−24 °C) or 115 °C (oil
bath temperature) for 24 h. Conversion based on volume of H2
mL of THF, 71% yield of hydrogen was produced (based on
the volume of collected H2 with respect to MeOH) after 2 days
(entry 2). The added THF likely solubilizes the actual catalyst 4
produced in situ (from complex 1 and base). The hydrogen
produced was analyzed by GC and IR, showing no observable
CO or CO2 (the latter being captured as carbonate). Using a
catalytic amount of KOtBu (entry 3) or using NEt3 instead of
NaOH (entry 4) resulted in no hydrogen formation. On the
basis of conditions of entry 2, use of complex 2 resulted in 54%
yield of H2 after 2 days (entry 5), and with complex 3, only 3%
yield was obtained after 24 h (entry 6).14 Decreasing the
temperature to 60 °C dramatically slowed the reaction, and just
a trace amount of hydrogen was produced after 24 h (entry 7).
The efficiency of the reaction was significantly influenced by the
catalyst loading, and only 28% yield of hydrogen was collected
b
collected. KOtBu was not used.
hydrogen was collected at 115 °C after 24 h (entry 2). At room
temperature, no hydrogen was observed when water was
applied, even when stoichiometric KOH (entry 3) and NEt3
(entry 5) were used. When ∼2 equiv of KOH was applied at
115 °C, quantitative hydrogen was produced after 24 h (entry
4). Thus, the formate, which is formed in situ, can undergo
smooth decomposition under the reaction conditions. Interest-
ingly, when ∼2 equiv of NEt3 was used in the absence of water,
a 98% yield of hydrogen was observed at room temperature
after 24 h (entry 6).
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Comparing the results of Tables 1 and 2, it seems that (a)
Decomposition of formic acid/formate is much easier than
formation of formic acid/formate from methanol (Table 2,
entry 2 vs Table 1, entry 3; Table 2, entry 6 vs Table 1, entry
4), and the latter (eq 2) is likely to be rate-determining of the
whole reaction. (b) A stoichiometric strong base is needed to
promote the conversion of methanol to formate. In our former
report on the synthesis of carboxylic acids from alcohols,
addition of the produced carboxylic acid to the actual catalyst 4
resulted in catalyst deactivation, necessitating the use of
stoichiometric base;11 however, it is unlikely to be the major
reason why a strong base is needed in the current case of
methanol, because formic acid can be decomposed in the
absence of base (Table 2, entry 2) or in the presence of a weak
base (Table 2, entry 6), whereas no gas is observed under
similar conditions for reforming of methanol (Table 1, entries 3
and 4). Thus, the process from methanol to formic acid is
favored in highly basic solvent, although the details are still
unclear. (c) Water strongly inhibits the decomposition of
formic acid/formate, probably because addition of water across
catalyst 4 competes with addition of formic acid, as shown in
the following NMR study. This may also be the reason why the
reaction proceeds better in toluene, in which the concentration
of water is lower than in THF.
favored in a high concentration of formic acid and may involve
reaction of complex 5 with another molecule of formic acid.
Adding ∼10 equiv of H2O to the THF-d8 solution of complex 5
rapidly resulted in an orange-brown solution of the hydrido−
hydroxo complex 6 (Figure 2) in quantitative yield, of which
the 1H NMR spectrum contained a doublet at −16.76 ppm (JPH
= 24.6 Hz, Ru-H) and the 31P{1H} NMR spectrum showed a
singlet at 103.27 ppm. As already reported,11 complex 6 can be
obtained by reversible addition of H2O to complex 4 and can
readily lose one molecule of H2O to regenerate complex 4.
On the basis of these results, the equilibria shown in Figure 3
likely exist among complexes 4, 5, and 6 during decomposition
Figure 3. Equilibria among complexes 4, 5, and 6.
of formic acid in water. A higher concentration of water shifts
the equilibrium toward complex 6, inhibiting the generation of
complex 5 and, thus, disfavors the decomposition step.
Crystals of complex 5 suitable for X-ray diffraction were
obtained by slow evaporation of its THF solution. As shown in
Figure 4, a distorted octahedral geometry around the Ru(II)
To gain insight into the mechanism, the addition of formic
acid to complex 4 was studied by NMR spectroscopy. In a J.
Young tube, ∼0.2 mmol of formic acid was added to a 0.6 mL
C6D6 solution of complex 4 (0.02 mmol). The solution color
instantly changed from dark green to deep red, indicative of
formation of the aromatized complex 5 (Figure 2); quantitative
Figure 2. Structures of PNN ruthenium pincer complexes 5 and 6.
Figure 4. X-ray structure of complex 5. Hydrogen atoms (except for
the hydride and the aldehyde hydrogen) are omitted for clarity. (tBu
groups are presented as wireframe for clarity.) Selected bond lengths
[Å] and angles [o]: Ru(1)−C(20) 1.847(2), Ru(1)−N(1) 2.1158(17),
Ru(1)−N(2) 2.0808(16), Ru(1)−P(1) 2.2939(6), Ru(1)−H(1)
1.47(2), Ru(1)−O(2) 2.2097(14); C(20)−Ru(1)−O(2) 101.29(8),
N(1)−Ru(1)−O(2) 81.21(6), N(2)−Ru(1)−O(2) 84.83(6), O(2)−
Ru(1)−P(1) 98.79(4).
1
formation of 5 was indicated by the H NMR spectrum which
showed a doublet at −17.47 ppm (JPH = 23.9 Hz, Ru-H), and
the 31P{1H} NMR spectrum exhibited a singlet at 103.4 ppm.
Because the solubility of complex 5 was poor in C6D6, more
than 1 equiv of formic acid (∼10 equiv relative to the catalyst)
was needed, forming H-bonds with complex 5 and making it
more soluble. However, formic acid slowly decomposed to
hydrogen and CO2 (analyzed by GC) under these conditions,
and a dark red crystalline solid of complex 5 was observed after
2 days. To get clean spectra of complex 5 without the influence
of formic acid, it was generated in THF-d8 instead of C6D6.
Addition of ∼10 equiv (0.2 mmol) of formic acid to a THF-d8
(0.6 mL) solution of complex 4 (0.02 mmol) resulted in a
bright red solution. Formic acid decomposition commenced,
and bubbles were observed. After 4.5 h, no bubbles were
observed, and NMR spectra of the mixture of complex 5 and
center is indicated, with one oxygen atom coordinated (OC(
O)H) cis to two nitrogen atoms and one phosphorus atom of
the bipyridine PNN ligand trans to the hydride ligand.
In conclusion, a reusable, robust catalytic system for the
production of hydrogen from methanol and water under
relatively low temperature was developed. A high conversion of
methanol and good TON were achieved. The outstanding
stability of catalyst 1 holds promise for the development of
even more efficient reusable catalysts for the methanol
reforming procedure.
formic acid were obtained, showing a doublet at −16.04 (JPH
=
24.6 Hz, Ru-H) in the 1H NMR spectrum and a singlet at 105.3
ppm in the 31P{1H} NMR spectrum. Three days later, formic
acid was fully consumed, and clean NMR spectra of complex 5
were obtained (see the Supporting Information for details).
Complex 5 was stable under nitrogen for more than 2 weeks at
ambient temperature. NMR follow-up of formic acid
decomposition suggested that the decomposition reaction was
ASSOCIATED CONTENT
* Supporting Information
Experimental procedures, characterization data, and crystallo-
graphic data for 5 (CIF). This material is available free of
charge via the Internet at http://pubs.acs.org.
■
S
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Letter
2010, 132, 16756−16758. (c) Balaraman, E.; Gunanathan, C.; Zhang,
J.; Shimon, L. J. W.; Milstein, D. Nat. Chem. 2011, 3, 609−614.
(d) Balaraman, E.; Ben-David, Y.; Milstein, D. Angew. Chem., Int. Ed.
2011, 50, 11702−11705. (e) Balaraman, E.; Fogler, E.; Milstein, D.
Chem.Commun. 2012, 48, 1111−1113.
(11) Balaraman, E.; Khaskin, E.; Leitus, G.; Milstein, D. Nat. Chem.
2013, 5, 122−125.
AUTHOR INFORMATION
Corresponding Author
*E-mail: david.milstein@weizmann.ac.il.
Notes
■
The authors declare no competing financial interest.
(12) For decomposition of formic acid to CO2 and hydrogen
catalyzed by ruthenium pincer complexes, see: Filonenko, G. A.;
Putten, R.; Schulpen, E. N.; Hensen, E. J. M.; Pidko, E. A.
ChemCatChem 2014, 6, 1526−1530.
(13) For selected reviews of formic acid decomposition, see:
(a) Johnson, T. C.; Morris, D. J.; Wills, M. Chem. Soc. Rev. 2010,
39, 81−88. (b) Enthaler, S.; Langermann, J.; Schmidt, T. Energy
Environ. Sci. 2010, 3, 1207−1217. (b) Grasemann, M.; Laurenczy, G.
Energy Environ. Sci. 2012, 5, 8171−8181.
(14) For base treatment of complexes 1−3 to obtain the actual
dearomatized catalysts in situ, see refs 8a, 8i, and 10b.
(15) Production of one molecule of hydrogen is counted as one
turnover. See the Supporting Information for details.
ACKNOWLEDGMENTS
■
This research was supported by the European Research Council
under the FP7 framework (ERC No. 246837), by the Israel
Science Foundation (ISF No 1721/13), and by the Bernice and
Peter Cohn Catalysis Research Fund. We thank the Planning
and Budgeting Committee (PBC) of the Council for Higher
Education in Israel for a fellowship to P.H. D.M. is the holder
of the Israel Matz Professorial Chair of Organic Chemistry.
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