Carbon Dioxide to Methanol: The Aqueous Catalytic Way at Room Temperature

Article abstract of DOI:10.1002/chem.201603407

Carbon dioxide may constitute a source of chemicals and fuels if efficient and renewable processes are developed that directly utilize it as feedstock. Two of its reduction products are formic acid and methanol, which have also been proposed as liquid organic chemical carriers in sustainable hydrogen storage. Here we report that both the hydrogenation of carbon dioxide to formic acid and the disproportionation of formic acid into methanol can be realized at ambient temperature and in aqueous, acidic solution, with an iridium catalyst. The formic acid yield is maximized in water without additives, while acidification results in complete (98 %) and selective (96 %) formic acid disproportionation into methanol. These promising features in combination with the low reaction temperatures and the absence of organic solvents and additives are relevant for a sustainable hydrogen/methanol economy.

Full text of DOI:10.1002/chem.201603407

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Accepted Article
Title: Carbon Dioxide to Methanol: the Aqueous Catalytic Way at Room Temperatu-
res
Authors: Gabor Laurenczy
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Chemistry - A European Journal
10.1002/chem.201603407
Carbon Dioxide to Methanol: the Aqueous Catalytic Way at Room Temperature
[
a]
[b]
[c]
[c]
[b]
Katerina Sordakis, Akihiro Tsurusaki, Masayuki Iguchi, Hajime Kawanami, Yuichiro Himeda,*
[
a]
and Gábor Laurenczy*
[
a]
Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL),
Avenue Forel 2, 1015 Lausanne, Switzerland
[
b]
National Institute of Advanced Industrial Science and Technology, Tsukuba Central 5, 1-1-1 Higashi,
Tsukuba, Ibaraki 305-8565, Japan
[
c]
National Institute of Advanced Industrial Science and Technology, 4-2-1 Nigatake, Miyagino,
Sendai, Miyagi, 983-8551, Japan
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Abstract: Carbon dioxide may constitute a source of chemicals and fuels if efficient and renewable
processes are developed that directly utilize it as feedstock. Two of its reduction products are formic
acid and methanol, which have also been proposed as liquid organic chemical carriers in sustainable
hydrogen storage. Here we report that both the hydrogenation of carbon dioxide to formic acid and
the disproportionation of formic acid into methanol can be realized at ambient temperatures and in
aqueous, acidic solution, with an iridium catalyst. The formic acid yield is maximized in water without
additives, while acidification results in complete (98%) and selective (96%) formic acid
disproportionation into methanol. These promising features in combination with the low reaction
temperatures and the absence of organic solvents and additives are relevant for a sustainable
hydrogen/methanol economy.
The catalytic reduction of atmospheric carbon dioxide with “green” hydrogen gas can provide a
pathway for the transformation of a troublesome, yet inexpensive and widely available carbon
[
1,2]
source into value-added chemicals. Among potential products, formic acid’s (FA) significance lies
in its broad range of applications, while methanol (MeOH) serves as an indispensable platform
[
3,4]
molecule in various chemical processes and constitutes the primary fuel of a methanol economy.
[
5]
In addition, both FA and MeOH may be utilized as easy-to-handle H storage media. The group of
2
Sasaki firstly obtained methanol, methane and CO from homogeneous CO hydrogenation with
2
[
6]
Ru (CO) at 240 °C. In recent years, a number of groups has investigated the conversion of CO2
3
12
[
7–14]
derivatives or CO to methanol in the presence of homogeneous catalysts.
Common features of
2
these works were high temperatures (over 100 °C), the employment of organic solvents and, in
certain cases, low selectivities. Alternatively, MeOH can be obtained from a stepwise approach,
comprising CO hydrogenation to formic acid (Equation 1) and disproportionation of the latter into
2
[
15–17]
methanol (Equation 2).
To date, the best results for the FA disproportionation reaction report a
MeOH yield of 50% (with 50% competing dehydrogenation) in THF at 150 °C, in the presence of a
[
16]
ruthenium-phosphine catalyst and a methanesulfonic acid additive.
Himeda et al. have studied the [(Cp*)Ir(dhbp)(OH )][SO ] catalyst (1) (dhbp = 4,4′-dihydroxy-2,2′-
2
4
bipyridine, Cp* = pentamethylcyclopentadienyl) (Figure S4) in bicarbonate hydrogenation to formate
[18]
(
i.e. in basic media) and formic acid dehydrogenation. Herein we report that complex (1) catalyzes
direct CO hydrogenation to formic acid in acidic media without additives, as well as FA
2
disproportionation (98% conversion) into methanol (96% selectivity), notably in aqueous acidified
solution and at ambient temperature (Equation 2).
−
1
H2(aq) + CO2(aq)  HCOOH , ΔG° = −4 kJ mol
(1)
(2)
(aq)
298
−
1
3
HCOOH  CH OH + H O + 2CO , ΔG° = −105 kJ mol
(
l)
3
(l)
2
(l)
2(g)
298
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10.1002/chem.201603407
13
Pressurization of an aqueous 8 mm (millimolal, i.e. mmol per kgH2O) solution of (1) with 20 bar CO
2
and 60 bar H , in the absence of organic solvents and additives, afforded a 0.05 m FA solution after
2
1
.5 h at 60 °C (Figure 1, blue squares). Decreasing the temperature to 25 °C, favored FA formation
due to the exothermic nature of this reaction, yielding a 0.1 m FA solution (Figure 1, grey squares and
Scheme 1a). When the reaction was repeated in the presence of 2.5 m sulfuric acid (H SO ) under
2
4
13
2
0 bar CO and 60 bar H pressure at 70 °C, concomitant production of MeOH occurred (Figure 1,
2
2
13
green circles). Monitoring the reaction by C NMR spectroscopy revealed that formic acid formation
preceded that of methanol, indicative of the latter originating from formic acid disproportionation
and not direct CO hydrogenation (Scheme 1b).
2
1
00
8
6
4
2
0
0
0
0
0
0
10
20
30
40
50
t / h
13
Figure 1. Time course of FA and MeOH formation from CO hydrogenation with (1). P( CO ) = 20 bar,
2
2
P(H ) = 60 bar, n = 15.9 μmol, m = 2.0 g. FA concentrations obtained at 60 °C ( ), at 25 °C ( )
2
cat
H2O
and in 2.5 m H SO at 70 °C ( ). MeOH ( ) concentrations were detected in the presence of H SO at
2
4
2
4
7
0 °C, under these conditions. Dashed curve indicates the observable decrease in FA concentration
due to continuous MeOH formation (for explanation see Figure S1). The trend lines are shown as a
guide and are not a mathematical fit of the data.
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(a)
CO2
HCOOH
d (ppm)
(b)
CO2
HCOOH
CH OH
3
d (ppm)
Scheme 1. Direct CO transformation to FA and MeOH with complex (1). P( CO ) = 20 bar,
13
2
2
13
P(H ) = 60 bar, n = 15.9 μmol, m = 2.0 g. C NMR spectra recorded every 3 h, showing a) the
2
cat
H2O
increase of the FA doublet at 25 °C, b) the increase of the FA doublet and the MeOH quartet in
.5 m H SO at 70 °C.
2
2
4
Even though sulfuric acid favored MeOH production, it was not indispensable; MeOH was also
detected after 20 h at 50 °C or 6 d at 25 °C in the absence of H SO under otherwise identical reaction
2
4
conditions. When an aqueous solution containing 8 mm (1) was left to equilibrate at 20 °C under
13
2
0 bar CO and 60 bar H in the absence of H SO , 0.16 m FA and 0.013 m MeOH were obtained.
2
2
2
4
These results constitute the first demonstration of direct CO transformation to MeOH in a “one-pot”
2
homogeneous reaction, in aqueous solution without additives and at ambient temperature. In
addition, this FA concentration is comparable to the only reported yield for FA synthesis under
[
19]
similar conditions (i.e. in the absence of base).
We subsequently aimed at optimizing the formic acid disproportionation reaction, by directly
13
utilizing FA as substrate. Heating a 5 m aqueous H COOH solution at 50 °C in the presence of 8 mm
catalyst (1) under isochoric conditions (i.e. at constant volume, resulting in a pressure build-up due to
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10.1002/chem.201603407
13
FA dehydrogenation), resulted in the formation of H and CO as the main products (Figure S2).
2
2
13
13
Simultaneously CH OH with a selectivity of 1.2% was also detected in solution by C NMR
3
13
spectroscopy (Table 1, Entry 1). No carbon monoxide ( CO) was observed, indicating that formic acid
decarbonylation did not occur. No further products were detected. Increasing the reaction
temperature obviously favored formic acid dehydrogenation and lowered the methanol selectivity
-
1
-1 [2,15]
(
ΔH°dehyd. = +31.5 kJ mol vs ΔH°
= −35.6 kJ mol )
(Table S1). Methanol formation always
disprop.
started without an activation period and its concentration increased nearly linearly for the first 15 h
at 20 °C (Figure S3).
[
a]
Table 1. Results for FA disproportionation reaction with complex (1) under isochoric conditions
Entry
T (°C)
50
H SO conc. (m)
FA conv. (%)
98 ± 1
t (h)
3.5
8
MeOH sel. (%)
1.2 ± 0.1
10 ± 1
2
4
1
2
3
4
5
6
-
50
0.35
1.25
1.75
2.50
3.70
1.75
2.50
2.50
98 ± 1
50
98 ± 1
30
30
72
90
40
312
72
21 ± 1
50
98 ± 1
29 ± 3
50
98 ± 1
59 ± 1
50
98 ± 1
59 ± 2
[
[
[
b]
c]
d]
7
8
9
50
96 ± 3
3 ± 1
20
60 ± 4
97 ± 2
50
98 ± 1
96 ± 1
13
[
a] 10.0 mmol H COOH, n = 15.9 μmol, m = 2.0 g. FA conversions and MeOH selectivities were
cat
H2O
13
[20]
calculated by quantitative C NMR spectroscopy, unless otherwise stated. The difference between
these values corresponds to FA dehydrogenation (apart from MeOH, H /CO and traces of CH OOCH
2
2
3
were produced). [b] Reaction performed under atmospheric pressure under a N atmosphere.
2
[
c] Pre-pressurization with 100 bar H . [d] Pre-pressurization with 50 bar H , MeOH and FA were
2
2
detected by GC and HPLC, respectively, n = 21.6 mmol, n = 32.2 μmol, m = 4.0 g.
FA
cat
H2O
13
Addition of H SO to an aqueous 5 m H COOH solution containing 8 mm (1), significantly enhanced
2
4
MeOH formation and catalyzed the esterification reaction between FA and MeOH towards methyl
formate (CH OOCH), with the latter, however, being readily reversible (selectivity for CH OOCH was
3
3
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always below 2% under optimized conditions). The best MeOH selectivities were obtained with
complex (1) in the presence of H SO , as demonstrated by screening tests of various catalysts and
2
4
acid additives (Figure S4, Tables S3 and S4). Optimization of the sulfuric acid concentration (Table 1,
entries 2-6) resulted in a MeOH selectivity of 59% for 98% FA conversion, in an aqueous 2.5 m H SO
2
4
1
13
solution at 50 °C (Table 1, entry 5 and Scheme 2). The only products detected by H/ C NMR
spectroscopy, HPLC and GC were methanol, methyl formate, H and CO . Sulfuric acid concentrations
2
2
above 2.5 m did not further promote FA disproportionation (Table S5). Under these reaction
conditions complex (1) provided a cumulative MeOH concentration of 3.2 m after three recyclings
13
(
addition of 10.0 mmol H COOH per cycle) with 99% FA conversion. In a single catalytic cycle,
doubling the initial FA and H SO concentrations (20 m FA and 5 m H SO ) produced a 2 m MeOH
2
4
2
4
solution, corresponding to a selectivity of 60% (Table S6, entry 2).
(a)
CH OH
HCOOH
3
CO2
d (ppm)
(b)
5
4
3
2
1
1.0
0.8
0.6
0.4
0.2
0.0
0
0
20
40
60
t / h
80
100
13
Scheme 2. Time course of a FA disproportionation reaction. a) 100 MHz C NMR spectra recorded
every 1.5 h, showing the decrease of the FA doublet and the increase of the MeOH quartet in the
presence of complex (1) and 2.5 m H SO under isochoric conditions, b) concentrations of
2
4
13
decomposed FA and formed MeOH derived from (a). Experimental conditions: 10.0 mmol H COOH,
ncat = 15.9 μmol, mH2O = 2.0 g, T = 50 °C.
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Monitoring pressure evolution in the presence of H SO revealed decreased gas production with
2
4
increasing solution acidity, in agreement with confined FA dehydrogenation and enhanced
disproportionation (Figure 2). In addition, the CO concentration in the produced gas mixture rose to
2
6
4% in the presence of 2.5 m H SO from 50% in the absence of H SO (originating only from formic
2
4
2
4
acid dehydrogenation). These results corroborated the occurrence of the FA disproportionation
reaction. Control experiments were also performed; heating a 3.6 m aqueous FA/sulfuric acid
mixture (1/0.75) at 70 °C for 48 h in the absence of iridium catalyst resulted in no pressure increase.
8
6
4
2
0
0
0
0
0
without
0
.17 m
1
.25 m
.75 m
0.37 m
0.75 m
1
2
.50 m
0
5
10
15
20
25
30
t / h
Figure 2. Effect of sulfuric acid concentration on pressure increase due to H and CO formation from
2
2
formic acid disproportionation/dehydrogenation reactions in the presence of (1) under isochoric
conditions. Experimental conditions: n = 10.0 mmol, n = 15.9 μmol, m = 2.0 g, T = 50 °C.
FA
cat
H2O
The acidic environment itself was not the only parameter for promoting FA disproportionation; both
acidic media and H pressure resulted in higher MeOH selectivities. When a 5 m FA solution was
2
heated in the presence of 1.75 m H SO under atmospheric pressure (i.e. no pressure build-up),
2
4
methanol selectivity dropped from 29 to 3% (Table 1, entries 4 and 7). Increased methanol
[
16]
selectivities under elevated H /CO pressure were also reported by Cantat and co-workers.
2
2
Therefore, FA disproportionation reactions with a preceding H pressurization step were performed,
2
resulting in an excellent MeOH selectivity of 97% alongside 60% FA conversion at 20 °C (Table 1,
entry 8). Likewise, at 50 °C under 50 bar of initial H gas pressure, practically complete FA conversion
2
(98%) into MeOH was achieved with a methanol selectivity of 96% (Table 1, entry 9). To date, these
are the best values obtained for the formic acid disproportionation reaction and constitute a
significant improvement in terms of methanol selectivity, reaction temperature and type of solvent.
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The “self-induced” formic acid reduction to methanol (Equation 3) with H gas originating from the
2
formic acid dehydrogenation reaction was excluded because it would be unequivocally accompanied
by a “delay” in methanol formation. This delay would be linked to the time necessary to build-up the
required H pressure in order to ensure adequate H concentration in the aqueous solution.
2
2
However, since H SO greatly decreased the FA dehydrogenation/H generation rate, MeOH
2
4
2
formation without an activation period (Scheme 2 and Figure S3) via Equation 3 was ruled out.
Employing isotopically enriched D gas to elucidate the nature of the reactions, resulted in
2
+
+
[21]
inconclusive results due to much faster H /D exchange catalyzed by (1) (Figure S5).
HCOOH(aq) + 2 H2(aq)  CH OH + H O
(3)
3
(aq)
2
(aq)
The decrease in MeOH selectivity under atmospheric pressure can be rationalized through a reaction
mechanism operating via multiple non-classical hydride moieties, dependent on equilibria influenced
by H pressure. A similar phenomenon has already been reported for the iron-catalyzed formic acid
2
dehydrogenation reaction; elevated H pressure hindered H elimination from an FeH(H ) complex
2
2
2
[
22]
and therefore the progression of the catalytic cycle.
The FA-to-MeOH transformation occurring via formaldehyde intermediacy was excluded because the
13
latter was never detected by C NMR spectroscopy or GC/HPLC techniques during our studies. It was
therefore reasonable to assume that the FA-to-MeOH transformation took place in the first
2
coordination sphere of the iridium metal, incorporating a formato moiety, η -H ligand(s) and/or
2
[
23]
terminal hydride(s), with similar iridium polyhydrides already known. Rearrangement of the Cp*
5
3
1
ligand on (1), i.e. ring-slippage from η -C Me to η -C Me and/or η -C Me , could provide the
5
5
5
5
5
5
[
24]
required vacant coordination sites. Throughout FA dehydrogenation reactions in the absence of
1
H SO , a signal for the monohydridic form of (1) at −11.3 ppm, (1’), was present in the H NMR
2
4
[
25]
spectrum. Addition of H SO resulted in the disappearance of (1’), thus explaining the reduced
2
4
catalytic activity for FA dehydrogenation, while multiple new resonances appeared in the hydride
1
region of the H NMR spectrum (Figure S6). Despite several attempts to derive structural information
on these signals from quantification and multiplicity resolution studies, we were limited by fluxional
behavior, low concentration of the dynamically formed catalytic species as well as low catalyst
solubility. In agreement with literature reports, H SO can protonate previously formed hydride
2
4
2
[25]
moieties, yielding species with coordinated η -H ligands, which would then rapidly release H gas.
2
2
2
We reason that the role of H pressure was to stabilize these η -H structures (i.e. shift the equilibria
2
2
towards complexes (II), Figure S7), which were further implicated in FA
reduction/disproportionation.
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In conclusion, we demonstrated that catalyst (1) possesses multiple functionalities that can be
tuned”, depending on the process of interest. First of all, it catalyzes the hydrogenation of CO to
“
2
formic acid in aqueous acidic media without additives, unlike most reported complexes that require
forcing basic conditions. In addition, formic acid can undergo complete disproportionation into
MeOH with unprecedented selectivities of 96% with the same catalyst in the presence of H SO , with
2
4
FA conversions of 98%. The “one-pot” homogeneous production of both FA and MeOH was
demonstrated in a single reaction performed at 20 °C, where an aqueous solution of (1) without
13
additives was pressurized with 20 bar CO and 60 bar H . The catalyst operating in water, endorses
2
2
the “green” character of the system. Conveniently, MeOH-H O solutions do not form an azeotrope
2
[
14]
(in contrast to several MeOH-organic solvent mixtures), facilitating their separation via distillation.
The reactions realized by catalyst (1) are relevant to various fields of research, such as CO2
valorization, sustainable H storage and alternative FA/methanol production.
2
Experimental Section
All experiments were prepared without precluding air, unless otherwise stated. The selectivity for
MeOH was calculated according to Equation 2, as three times the moles of MeOH produced divided
by the amount of FA consumed. Concentrations are given in terms of molality (m), i.e.
mol per kgsolvent. In a typical CO hydrogenation reaction an aqueous solution of (1) was pressurized
2
13
with CO , completed with H to a given pressure, thermostated and then the reaction was
2
2
13
monitored by C NMR spectroscopy. In a typical FA disproportionation reaction an aqueous FA
solution, containing (1) and the acid additive was heated in a sapphire NMR tube, Parr autoclave or
in a Schlenk tube equipped with a condenser under a N atmosphere. The reaction was followed by
2
13
monitoring the pressure increase due to gas evolution as a function of time, and/or by C NMR
spectroscopy, and/or by GC/HPLC techniques (see Supporting Information for details).
Acknowledgements
(K.S.) and (G.L) are grateful to the Swiss National Science Foundation and EPFL for financial support.
Dr. P. Miéville is thanked for support with NMR techniques. (Y.H.) thanks the Japan Science and
Technology Agency (JST), ACT-C for financial support.
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10.1002/chem.201603407
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13
Figure 1. Time course of FA and MeOH formation from CO hydrogenation with (1). P( CO ) = 20 bar,
2
2
P(H ) = 60 bar, n = 15.9 μmol, m = 2.0 g. FA concentrations obtained at 60 °C ( ), at 25 °C ( )
2
cat
H2O
and in 2.5 m H SO at 70 °C ( ). MeOH ( ) concentrations were detected in the presence of H SO at
2
4
2
4
7
0 °C, under these conditions. Dashed curve indicates the observable decrease in FA concentration
due to continuous MeOH formation (for explanation see Figure S1). The trend lines are shown as a
guide and are not a mathematical fit of the data.
13
Scheme 1. Direct CO transformation to FA and MeOH with complex (1). P( CO ) = 20 bar,
2
2
13
P(H ) = 60 bar, n = 15.9 μmol, m = 2.0 g. C NMR spectra recorded every 3 h, showing a) the
2
cat
H2O
increase of the FA doublet at 25 °C, b) the increase of the FA doublet and the MeOH quartet in
.5 m H SO at 70 °C.
2
2
4
13
Scheme 2. Time course of a FA disproportionation reaction. a) 100 MHz C NMR spectra recorded
every 1.5 h, showing the decrease of the FA doublet and the increase of the MeOH quartet in the
presence of complex (1) and 2.5 m H SO under isochoric conditions, b) concentrations of
2
4
13
decomposed FA and formed MeOH derived from (a). Experimental conditions: 10.0 mmol H COOH,
ncat = 15.9 μmol, mH2O = 2.0 g, T = 50 °C.
Figure 2. Effect of sulfuric acid concentration on pressure increase due to H and CO formation from
2
2
formic acid disproportionation/dehydrogenation reactions in the presence of (1) under isochoric
conditions. Experimental conditions: n = 10.0 mmol, n = 15.9 μmol, m = 2.0 g, T = 50 °C.
FA
cat
H2O
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Chemistry - A European Journal
10.1002/chem.201603407
[
a]
Table 1. Results for FA disproportionation reaction with complex (1) under isochoric conditions
Entry
T (°C)
50
H SO conc. (m)
FA conv. (%)
98 ± 1
t (h)
3.5
8
MeOH sel. (%)
1.2 ± 0.1
10 ± 1
2
4
1
2
3
4
5
6
-
50
0.35
1.25
1.75
2.50
3.70
1.75
2.50
2.50
98 ± 1
50
98 ± 1
30
30
72
90
40
312
72
21 ± 1
50
98 ± 1
29 ± 3
50
98 ± 1
59 ± 1
50
98 ± 1
59 ± 2
[
[
[
b]
c]
d]
7
8
9
50
96 ± 3
3 ± 1
20
60 ± 4
97 ± 2
50
98 ± 1
96 ± 1
13
[
a] 10.0 mmol H COOH, n = 15.9 μmol, m = 2.0 g. FA conversions and MeOH selectivities were
cat
H2O
13
[20]
calculated by quantitative C NMR spectroscopy, unless otherwise stated. The difference between
these values corresponds to FA dehydrogenation (apart from MeOH, H /CO and traces of CH OOCH
2
2
3
were produced). [b] Reaction performed under atmospheric pressure under a N atmosphere.
2
[
c] Pre-pressurization with 100 bar H . [d] Pre-pressurization with 50 bar H , MeOH and FA were
2
2
detected by GC and HPLC, respectively, n = 21.6 mmol, n = 32.2 μmol, m = 4.0 g.
FA
cat
H2O
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Chemistry - A European Journal
10.1002/chem.201603407
Text for Table of Contents
An iridium complex was successfully employed in the direct, homogeneous transformation of carbon
dioxide to formic acid and methanol, via formic acid disproportionation, in aqueous solution and at
ambient temperature. Unprecedented methanol selectivities of 96% were obtained for complete
formic acid conversion in the presence of sulfuric acid under optimized conditions. The reactions
reported herein might be beneficial in the fields of CO valorization, sustainable H storage and
2
2
alternative formic acid/methanol production.
Aqueous H SO
2
4
Keywords
carbon dioxide, formic acid, homogeneous, hydrogenation, methanol
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