Catalytic Hydrogenation of CO2to Methanol Using Multinuclear Iridium Complexes in a Gas-Solid Phase Reaction

Article abstract of DOI:10.1021/jacs.0c11927

We report a novel approach toward the catalytic hydrogenation of CO2 to methanol performed in the gas-solid phase using multinuclear iridium complexes at low temperature (30-80 °C). Although homogeneous CO2 hydrogenation in water catalyzed by amide-based iridium catalysts provided only a negligible amount of methanol, the combination of a multinuclear catalyst and gas-solid phase reaction conditions led to the effective production of methanol from CO2. The catalytic activities of the multinuclear catalyst were dependent on the relative configuration of each active species. Conveniently, methanol obtained from the gas phase could be easily isolated from the catalyst without contamination with CO, CH4, or formic acid (FA). The catalyst can be recycled in a batchwise manner via gas release and filling. A final turnover number of 113 was obtained upon reusing the catalyst at 60 °C and 4 MPa of H2/CO2 (3:1). The high reactivity of this system has been attributed to hydride complex formation upon exposure to H2 gas, suppression of the liberation of FA under gas-solid phase reaction conditions, and intramolecular multiple hydride transfer to CO2 by the multinuclear catalyst.

Full text of DOI:10.1021/jacs.0c11927

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Catalytic Hydrogenation of CO2 to Methanol Using Multinuclear
Iridium Complexes in a Gas−Solid Phase Reaction
Ryoichi Kanega, Naoya Onishi,* Shinji Tanaka, Haruo Kishimoto, and Yuichiro Himeda*
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ABSTRACT: We report a novel approach toward the catalytic
hydrogenation of CO2 to methanol performed in the gas−solid phase
using multinuclear iridium complexes at low temperature (30−80
°C). Although homogeneous CO2 hydrogenation in water catalyzed
by amide-based iridium catalysts provided only a negligible amount
of methanol, the combination of a multinuclear catalyst and gas−
solid phase reaction conditions led to the effective production of
methanol from CO2. The catalytic activities of the multinuclear
catalyst were dependent on the relative configuration of each active
species. Conveniently, methanol obtained from the gas phase could
be easily isolated from the catalyst without contamination with CO,
CH4, or formic acid (FA). The catalyst can be recycled in a batchwise
manner via gas release and filling. A final turnover number of 113 was
obtained upon reusing the catalyst at 60 °C and 4 MPa of H2/CO2 (3:1). The high reactivity of this system has been attributed to
hydride complex formation upon exposure to H2 gas, suppression of the liberation of FA under gas−solid phase reaction conditions,
and intramolecular multiple hydride transfer to CO2 by the multinuclear catalyst.
Scheme 1. Pathway for the Hydrogenation of CO2 to
Methanol
INTRODUCTION
■
Methanol, which is produced industrially from syngas, is in
high demand over the world as a promising alternative fuel and
industrially useful bulk chemical.1 Recently, methanol
production from CO2 is highly desirable from the viewpoint
of CO2 utilization and methanol economy.2 There are
numerous reports on heterogeneous catalysts (typically Cu-
based catalysts) used for the thermocatalytic hydrogenation of
CO2 to methanol (HCM).3 These catalysts require harsh
reaction conditions due to the high thermodynamic stability
and low reactivity of CO2, although the entire HCM process is
exergonic (3H2(g) + CO2(g) → CH3OH(l) + H2O(l): ΔG°298
= −8.9 kJ mol−1). Significant obstacles, such as low CO2
conversion and selectivity, still remain for heterogeneous
catalysts due to the equilibrium limitations under the harsh
operating temperatures (>220 °C). Lower reaction temper-
atures will allow a higher theoretical conversion ratio of
methanol from CO2. In the pursuit of a catalyst that can work
at lower temperature, the application of homogeneous
catalysts, which often operate at lower temperatures, in the
HCM has received an increasing amount of attention.4−7
Prakash and co-workers demonstrated efficient HCM using the
Ru-pincer catalyst in triglyme/HNTf2 at 140 °C and 7.5 MPa
with a TON of 9900.19 Wass and co-worker have used the
in ethanol at 12 MPa and 120 °C with a TON of 1938.10 In
addition, the selected reports of HMC using homogeneous
catalysts are provided in Table S1.7−25 The homogeneous
catalytic HCM generally proceeds through stepwise reactions
involving single hydride transfer via formic acid (FA) and its
equivalents (e.g., formate, carbamate, and urea) (Scheme 1). It
is known that the hydrogenation of CO2 to FA in the first step
is an endergonic step, which is strongly limited by
thermodynamic constraints (H2(g) + CO2(g) → HCOOH(l):
ΔG°298 = +31.8 kJ mol−1).26,27 However, homogeneous
catalysts generally lack the ability to promote the reduction
of CO2 to generate formaldehyde and methanol beyond the
single hydride-transfer process to FA or its equivalents. In
addition, the homogeneous catalytic HCM is performed in an
Received: November 14, 2020
Published: January 13, 2021
i
[RuCl2(Ph2PCH2CH2NHMe)2] catalyst in Pr2NH/toluene at
100 °C and 4 MPa to obtain methanol with a TON of 8900.18
Recently, Klankermayer and co-workers reported one-pot
HMC catalyzed by Ru complexes with triphos-type ligands
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organic solvent with additives and at a relatively high
temperature (>100 °C) because most homogeneous catalysts
are moisture-sensitive and exhibit low efficiencies. It is known
that the inherent drawback of homogeneous catalysis is a
difficulty associated with separating the products from the
catalyst. Therefore, a great deal of research effort has been
made to develop alternative routes that circumvent the
inherent barriers of the HCM.4,27
Chart 1. Mononuclear (1), Dinuclear (2), and Trinuclear
(3) Ir Catalysts Used for the Hydrogenation of CO2 to
Methanol
We have developed several efficient iridium catalysts bearing
N,N-bidentate ligands for the hydrogenation of CO2 to
formate under mild reaction conditions in an alkaline aqueous
solution.6,28 Recently, we reported that the homogeneous
HCM via FA formation at 70 °C can be achieved by the acid-
promoted activation of FA in water at elevated pressure (8
MPa), albeit with limited productivity (turnover number
(TON) < 8).29 From these studies, we confirmed the strong
limitations due to the equilibrium between FA and CO2/H2 in
water.30 On the other hand, the production of methanol from
formaldehyde proceeds easily under alkaline conditions.31
These facts imply that the formation of FA as an
intermediate via single hydride transfer from the metal−
hydride complex to CO2 is unfavorable for the HCM. In other
words, the liberation of FA from the intermediate formato
complex via ligand exchange with water as a solvent should be
avoided because the liberated FA readily dehydrogenates to H2
and CO2. Therefore, we envisaged a multinuclear catalyst
capable of concerted multiple hydride transfer to generate
formaldehyde or methanol directly. In contrast to heteroge-
neous catalysts, for which the active species are nonuniformly
distributed among different microenvironments, homogeneous
catalysts that possess well-defined local and tunable structures
may be able to act concertedly and selectively on a reactant by
changing the steric and/or electronic properties of the organic
ligands.32
In addition, we have conceived a gas−solid phase reaction
using complex catalysts to avoid the liberation of FA via ligand
exchange with water. Conveniently, it is unnecessary to
separate the product from the solvent and catalyst in the
gas−solid phase reaction and can avoid the accumulation of
water as a byproduct in the reaction solvent. Furthermore,
water as a solvent for the hydrogenation reaction may be
undesirable because of the poor solubility of H2. However,
only a limited number of examples (mainly reactions with light
olefins) have been reported in regard to gas−solid phase
reactions using complex catalysts.33−37 To date, investigations
on the mechanism in these processes are less well-developed.
Herein, we propose a novel approach for the HCM using
multinuclear iridium complexes in a gas−solid phase reaction
to overcome its inherent obstacles. Multiple hydride transfers
by multinuclear species led to the effective production of
methanol from CO2. In addition, the gas−solid phase reaction
circumvents the liberation of FA from the formato complex.
Therefore, our catalytic system enables the production of
methanol from CO2 under mild reaction conditions.
(Figure S1a and Table S2), while the other catalysts were
obtained as amorphous solids. The standard reaction was
performed by using the iridium catalyst (1: 9.0 μmol; 2: 4.5
μmol; or 3: 3.0 μmol) under 4 MPa of H2/CO2 (3:1) at 60 °C.
Conventional homogeneous catalysis using mononuclear
catalyst 1 (9 μmol) in water without base generated FA, but
the concentration of FA was immediately saturated (entries 1−
3 in Table S4, Figure 1a). After 165 h, a small amount of
methanol (0.3 μmol) was detected, which was probably
formed via the hydrogenation of FA (entry 4 in Table S4).
These results were consistent with those previously reported
with a mononuclear bipyridine-based catalyst.29
In the case of dinuclear catalyst 2-m (4.5 μmol), FA was
immediately produced similar to that of 1, while a trace
amount of methanol (0.05 μmol) was detected after 30 min
and the concentration of methanol gradually increased (entries
6−9 in Table S4, Figure 1a). The rapid generation of methanol
can be attributed to concerted multiple hydride transfers by
two iridium species. The ortho-substituted catalyst (2-o)
showed similar activity to that of 2-m (entry 10 in Table S4),
while methanol production was not detected after 15 h when
using para-substituted catalyst 2-p (entry 11 in Table S4). The
different activities observed among the dinuclear catalysts were
likely dependent on the relative configuration of each iridium
species. These results imply the viability of 2-m for the HCM
via concerted multiple hydride transfer under mild reaction
conditions. However, the efficiency of methanol production
was unsatisfactory even when using 2-m in water because of
the poor productivity of FA from H2/CO2 due to the
equilibrium limitations.
Hydrogenation of CO2 in the Gas−Solid Phase
Reaction. To improve the efficiency of the HCM, we
envisioned an alternate route to circumvent the liberation of
FA into the reaction medium because FA in water is hardly
hydrogenated but easily forms H2 and CO2 by dehydrogen-
ation.30,38 We speculated that the liberation of FA occurs via
ligand exchange with water as a solvent. Thus, we tested an
unconventional gas−solid phase reaction conducted in the
absence of a solvent. Specifically, the complex catalyst was
exposed to 4 MPa of H2/CO2 (3:1) at 60 °C (Figure S2).
After the reaction, the pressurized gases were immediately
released into an aluminum bag containing 2−10 mL of water,
which absorbed the produced methanol, to quantify the
amount of methanol in the gas phase. The residual catalyst
dissolved in an aqueous NaCl solution to suppress the
competing dehydrogenation of FA. Both aqueous samples
were analyzed by using high-performance liquid chromatog-
raphy (HPLC) and gas chromatography (GC) equipped with a
flame ionization detector (FID). Interestingly, methanol was
detected from both samples obtained from the gas phase as
well as the residual catalyst in this system.
RESULTS AND DISCUSSION
■
Homogeneous Hydrogenation of CO2 in Water. We
prepared mononuclear (1), dinuclear (2-o, m, p), and
trinuclear (3) catalysts consisting of picolinamide-based
iridium moieties, which are effective catalysts for the
hydrogenation of CO2 to formate in an aqueous basic solution
and the dehydrogenation of FA in an aqueous acidic solution
(Chart 1).38 Catalyst 2-p can be obtained as a crystalline solid
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Table 1. Hydrogenation of CO2 to Methanol Using Ir
Catalysts in the Gas−Solid Phase Reaction under Stationary
a
Conditions
methanol
b
c
entry
complex/[μmol]
t [h]
[μmol]
TON
1
2
3
4
5
6
1/9
1/9
188
360
1
15
165
15
15
15
15
15
0.4 (0.1)
1.0 (0.2)
0.3 (0.1)
4.6 (2.6)
39.1 (37.0)
0.2 (0)
<0.1
0.1
<0.1
1.0
8.7
<0.1
<0.1
0.65
0.4
1.6
2-m/4.5
2-m/4.5
2-m/4.5
2-o/4.5
2-pc/4.5
2-pa/4.5
2-pac/4.5
3/3
d
e
7
0.1 (0)
8
9
2.6 (2.4)
1.9 (1.8)
4.7 (3.1)
32.1 (29.1)
f
10
11
3/3
165
10.7
a
The reaction was performed under 4 MPa H2/CO2 (3:1) at 60 °C.
The loaded catalyst was an amorphous and powdery solid, except for
2-pc. The total amount of methanol produced from the residual
b
catalyst and gas phase. The parentheses indicate the amount of
methanol obtained from the gas phase. All values were determined
c
from the average of at least two different runs. TON = mol of
d
produced methanol/mol of catalyst. Average of five different runs
e
(see Table S6). A powder was obtained from crushing the crystalline
f
solid in a mortar. An amorphous solid was obtained from an aqueous
solution of the crystalline solid (2-pc) used in entry 7.
solid phase reaction produced a very small amount of methanol
upon prolonging the reaction time (entries 1 and 2 in Table 1).
This implies that intermolecular multiple hydride transfer from
the mononuclear catalysts to CO2 hardly occurs.
Dinuclear catalyst 2-o provided a small amount of methanol
(0.2 μmol) over 15 h, probably due to the relative
configuration of each iridium species being too close (entry
6). Interestingly, in the case of para-substituted catalyst 2-p,
the production of methanol was dependent on the solid state
of the catalyst. Specifically, crushed crystalline 2-pc gave a
negligible amount of methanol after 15 h (entry 7), whereas
amorphous 2-pa provided 2.6 μmol of methanol (entry 8).
Interestingly, amorphous 2-pac obtained from the dissolution of
the less active crystalline catalyst (2-pc) in an aqueous solution
showed reactivated catalytic activity (entry 9). These results
imply that crystalline 2-pc cannot perform efficient intra-
molecular multiple hydride transfer because the iridium species
in 2-pc are too far away from each other (Figure S1b and Table
S3). At this stage, we believe that a suitable configuration of
amorphous 2-pa (or 2-pac) due to π−π stacking led to favorable
intermolecular multiple hydride transfer to produce methanol.
As expected, trinuclear catalyst 3 afforded a similar amount of
methanol to that of 2-m (entries 10 and 11). These results
suggest that the relative configuration of each active species
plays an important role in the production of methanol during
the HCM in the gas−solid phase reaction.
Furthermore, it was observed that the yield of methanol was
significantly improved upon stirring the catalyst by using a
magnetic bar in the reaction vessel, probably due to gas
agitation and catalyst grinding, which exposes the active surface
sites (entries 1−4 in Table 2, Figure 1b). Under these
conditions, the influence of the reaction conditions with 2-m
was examined in detail. Methanol production was proportional
to the amount of catalyst (Figure S5a and Table S8).
Increasing the temperature and pressure led to an exponential
Figure 1. Time course of the production of methanol (red) and FA
(blue) with 1 (9 μmol) and 2-m (4.5 μmol) under 4 MPa of H2/CO2
(3:1) at 60 °C in (a) water (1: squares and dotted line; 2-m: circles
and solid line) and (b) the gas−solid phase (1: squares and dotted
line; 2-m: circles and solid line (stationary conditions), rhombus and
solid line (stirring gases at 500 rpm)).
To our delight, when the reaction was performed with 2-m
as an amorphous powdery solid, the yield of methanol
increased with the reaction time (entries 3−5 in Table 1,
Figure 1b). Furthermore, methanol was exclusively detected
without contamination with CO and CH4 in the gas phase
(Figures S9 and S10). On the other hand, a negligible amount
of FA was observed in the residual catalyst, and formaldehyde
(methanediol) was not detected (Figure S11). It was
confirmed that the reproducibility of this reaction system
was sufficient (Table S6). The methanol produced during the
1
reaction was confirmed by using H NMR spectroscopy in
DMSO-d6 (Figure S3) and the detection of 13CH3OH (m/z
33) generated from 13CO2 in GC−MS (Figure S4). No
methanol was detected in the control experiments lacking
either the catalyst, H2, or CO2 (entries 10−12 in Table S5).
On the other hand, using mononuclear catalyst 1 in the gas−
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Table 2. Hydrogenation of CO2 to Methanol Using 2-m in
a
the Gas−Solid Phase Reaction under Magnetic Stirring
methanol
b
c
entry
T [°C]
P [MPa]
t [h]
[μmol]
TON
1
2
3
4
5
6
7
8
60
60
60
60
60
80
60
70
30
60
70
4
4
4
4
5
4
0.9
0.5
5
15
47
90
336
24
8.9 (8.6)
2.0
4.0
6.7
18.1 (17.9)
30.0 (28.9)
104 (103)
14.1 (14.0)
16.1 (16.0)
20.2 (20.0)
13.4 (13.2)
9.2 (8.2)
23
3.1
3.6
4.5
3.0
2.0
24
768
480
168
336 × 5
9
10
11
d
4
5
(507)
(122)
113
27.1
e
24 × 7
a
The reaction was performed under the desired pressure of H2/CO2
(3:1) by using 4.5 μmol of 2-m as an amorphous powdery solid. All
b
numbers are the average of two runs. The total amount of methanol
produced from the solid and gas phases. The parentheses indicate the
Figure 2. Catalyst recycling experiments using 2-m (4.5 μmol) under
4 MPa of H2/CO2 (3:1) at 60 °C for 336 h in each run. The red bars
indicate the amount of methanol obtained from the gas phase for each
run.
amount of methanol obtained from the gas phase. All values were
c
determined from the average of at least two different runs. TON =
d
mol of produced methanol/mol of catalyst. Recycling experiment: 5
e
cycles, 336 h for each run (see Figure 2). Recycling experiment: 7
Scheme 2. Proposed HCM Mechanism for 2-m in the Gas−
Solid Phase Reaction (Heterogeneous Catalysis: Cycle 1)
and Liquid Phase Reaction (Homogeneous Catalysis: Cycle
2) (X = H or H2O, n = 1 or 2)
cycles, 24 h for each run (see Figure S6).
increase in methanol production (Figure S5b−e). In particular,
the pressure of H2 seems to strongly affect the production of
methanol. It is worth noting that methanol was generated
catalytically, even at low pressure (<1.0 MPa, entries 7 and 8 in
Table 2) and low temperature (30 °C, entry 9 in Table 2).
These results indicate that the catalytic system (i.e., multi-
nuclear catalysts in the gas−solid phase reaction) performs the
HCM under mild reaction conditions.
Conveniently, the methanol produced in the reaction mainly
exists in the gas phase, and its separation from the catalyst is
much easier than that of homogeneous catalysis (i.e., an
aqueous phase reaction). Therefore, the catalyst can be
repeatedly used in a batchwise manner via gas release and
filling with the catalyst exhibiting excellent reusability without
any obvious loss in its activity due to its tolerance to water as a
byproduct (Figure 2 and Figure S6). A recycling experiment
using five cycles afforded 0.507 mmol of methanol with a TON
of 113 under 4 MPa of H2/CO2 (3:1) at 60 °C and 336 h for
each run (entry 10 in Table 2). From the NMR spectra
obtained for catalyst 2-m in D2SO4/D2O before and after the
reaction, apparent degradation was not observed (Figure S12).
Mechanistic Investigation. Our proposed mechanism for
the HCM with dinuclear catalyst 2-m under gas−solid phase
reaction conditions is shown in cycle 1 of Scheme 2. The
following experiments were performed to support the
proposed mechanism.
First, exposing 2-m to H2 led to a rapid color change from
yellow to orange, which is similar to that of the corresponding
hydride complex generated in an aqueous solution of 2-m
under pressurized H2 (Figure S7). The 1H MAS NMR
spectrum shows new peaks at −11.8 and −15.6 ppm, which
may be assigned to its mono- and dihydride complexes,
respectively (Figure S8). The addition of water to the hydride
complex converted to precatalyst 2-m along with the evolution
of H2 in ∼35% yield per iridium species (Figure S13 and Table
S10). The hydride complex seems to be generated at the
interior site as well as the surface, although the species on the
surface undoubtedly act as the most active sites for catalysis
rather than the interior site.37 However, strict identification of
the actual catalytic regime seems to be difficult from these
observations.
Second, the hydride complex generated from 2-m (4.5
μmol) was exposed to 4 MPa of CO2 at 60 °C (Table S11). As
a result, FA (0.4−0.5 μmol) was generated without detection
of methanol and formaldehyde. Furthermore, re-exposure of
the resulting complex to 4 MPa of H2 produced methanol (0.5
μmol) without FA. The slight discrepancy between the amount
of FA and methanol may be due to the decomposition of FA
during the measurement. These results strongly support that
the catalysis is a stepwise reaction, which occurs via a formato
complex.
Third, the effect of the counteranion was examined by using
−
−
−
BF4− and PF6 analogues of 2-m (Table S7). The BF4 and
PF6 analogues provided methanol of 1.6 and 0.9 μmol,
respectively. The activities seem to be correlated to the acidity
of the conjugated acid.39 This result indicates that the
counterion may serve as a proton acceptor.
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Finally, the HMC reaction was performed in suspension of
anhydrous toluene and hexane under standard conditions
(Table S9). When the reaction was performed by using
mononuclear catalyst 1, a negligible amount of methanol was
detected, similar to that under liquid phase and gas−solid
phase conditions. In the case of the dinuclear catalyst (2-m),
the amount of methanol produced in the suspension of toluene
and hexane was higher than that in H2O. The results strongly
support the role of the multinuclear catalysts and the
unsuitability of water as a solvent in this system.
1−3, Table S5). Almost all of the generated formato complexes
(b) remain unchanged, except for intermolecular hydride
transfer with the hydride complexes that exist in a suitable
configuration (step iii*, Scheme S1a). At the end of the
reaction, a negligible amount of FA was detected because the
generated FA will be dehydrogenated to CO2 and H2 during
sampling. The compatibility with efficient catalysts used for
CO2 hydrogenation was investigated (Scheme S2). Several
catalysts provided a small amount of methanol. Similar to that
previously reported,29 homogeneous catalysis (i.e., in water)
using mononuclear catalyst 1 immediately generates FA, whose
concentration was saturated by the equilibrium limitation
between CO2/H2 and FA (Scheme S1b).
The differences observed between the mono- and dinuclear
catalysts were attributed to the intramolecular hydrogenation
of formato moiety B in step iii, which leads to the rapid
generation of methanol upon further hydride transfer. As a
result, only the combination of a multinuclear catalyst and
gas−solid phase reaction conditions exhibits remarkable
performance for the HCM.
In step i, mono- or dihydride complex A may be formed
upon exposure of 2-m to H2. It is known that exposure of the
active metal species to H2 in gas−solid phase reactions
generally leads to its homolytic cleavage.36,40 According to the
results, it was assumed that the heterolytic cleavage of H2 likely
proceeds due to the presence of proton acceptors, such as the
−
counterion (HSO4 ) or coordinated nitrogen atoms, near the
active species.41 In step ii, one of the hydride species in A
reacts with CO2 to generate the formato-hydride complex B. In
step iii, intramolecular hydride transfer from the hydride
species to the formato complex occurs to generate complex C.8
The hydrogenation of FA can be ruled out because FA cannot
be liberated from B for the lack of water. In step iv, further
intra- or intermolecular hydrogenation of the formaldehyde
equivalent in C gives methoxide complex D. Solid-state
1H−13C HETCOR NMR spectroscopy revealed the formation
of a methoxy moiety (Figure 3).42 Finally, liberation of
methanol into the gas phase led to the regeneration of A in
step v.
CONCLUSIONS
■
We have reported the production of methanol via the
hydrogenation of CO2 under mild reaction conditions (30
°C, 5 MPa (TON 2.0) and 0.5 MPa, 70 °C (TON 3.0)) using
multinuclear complexes under gas−solid phase reaction
conditions. The reactivity under mild conditions is due to
(i) facile formation of a hydride complex upon exposure to H2,
(ii) circumvention of the liberation of FA from the formato
complex using unconventional gas−solid phase reaction
conditions, and (iii) multiple intramolecular hydride transfer
to CO2 by the multinuclear catalysts. Apparent differences
were observed with the mononuclear catalyst and homoge-
neous catalysis (liquid phase reaction), in which both provided
a negligible amount of methanol. The combination of
multinuclear catalysts and gas−solid phase reaction conditions
overcame the inherent barriers of the HCM, that is, the
equilibrium limitation between FA and CO2/H2. These studies
provide novel insights into gas−solid phase hydrogenation and
the effect of multinuclear active sites by using a complex
catalyst toward the hydrogenation of CO2. In addition, the
catalytic system can be easily recycled, since the produced
methanol is exclusively released into the gas phase. After five
cycles, 0.507 mmol of methanol with a TON of 113 was
produced under 4 MPa of H2/CO2 (3:1) at 60 °C without any
clear degradation. This catalytic system opens the door to new
possibilities for the practical production of methanol from CO2
at lower reaction temperatures. For advancement of this
system, the surface chemistry of the complex may need to be
established besides development of an efficient catalyst and
detailed analysis of the mechanism.
Figure 3. Solid-state 1H−13C HETCOR spectrum of 2-m after
exposure to 4 MPa of H2/13CO2 (3:1).
On the other hand, in the homogeneous catalysis of 2-m
performed in water, which is a liquid-phase reaction, it was
presumed that the liberation of FA from the formato complex
B via ligand exchange with water preferentially occurs (step vi).
Thus, the liberated FA in water may not be hydrogenated
because of the high catalytic activity of the picolinamide-based
iridium moieties toward the dehydrogenation of FA under
acidic aqueous conditions.43 Consequently, a small amount of
methanol was detected via intramolecular hydride transfer of B
as a minor reaction (step iii).
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/jacs.0c11927.
Figures S1−S27, Tables S1−S11, and Schemes S1 and
S2 (PDF)
Apparent differences were observed in mononuclear catalyst
1 under the gas−solid phase reaction conditions, in which a
negligible amount of methanol and FA were produced (entries
Crystallographic data of 2-m (CIF)
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(8) Wesselbaum, S.; Moha, V.; Meuresch, M.; Brosinski, S.; Thenert,
K. M.; Kothe, J.; Stein, T. V.; Englert, U.; Holscher, M.;
Klankermayer, J.; Leitner, W. Hydrogenation of carbon dioxide to
methanol using a homogeneous ruthenium-triphos catalyst: from
mechanistic investigations to multiphase catalysis. Chem. Sci. 2015, 6,
693−704.
AUTHOR INFORMATION
■
Corresponding Authors
Yuichiro Himeda − Global Zero Emission Research Center,
National Institute of Advanced Industrial Science and
Technology, Tsukuba, Ibaraki 305-8569, Japan;
orcid.org/0000-0002-9869-5554; Email: himeda.y@
aist.go.jp
Naoya Onishi − Global Zero Emission Research Center,
National Institute of Advanced Industrial Science and
Technology, Tsukuba, Ibaraki 305-8569, Japan;
orcid.org/0000-0003-4501-1742; Email: n.onishi@
aist.go.jp
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Authors
Ryoichi Kanega − Research Institute of Energy Conservation,
National Institute of Advanced Industrial Science and
Technology, Tsukuba, Ibaraki 305-8565, Japan;
orcid.org/0000-0003-0790-956X
Shinji Tanaka − Interdisciplinary Research Center for
Catalytic Chemistry, National Institute of Advanced
Industrial Science and Technology, Tsukuba, Ibaraki 305-
8565, Japan; orcid.org/0000-0003-2002-5582
Haruo Kishimoto − Global Zero Emission Research Center,
National Institute of Advanced Industrial Science and
Technology, Tsukuba, Ibaraki 305-8569, Japan
(12) Kothandaraman, J.; Goeppert, A.; Czaun, M.; Olah, G. A.;
Prakash, G. S. Conversion of CO2 from air into methanol using a
polyamine and a homogeneous ruthenium catalyst. J. Am. Chem. Soc.
2016, 138, 778−781.
(13) Schneidewind, J.; Adam, R.; Baumann, W.; Jackstell, R.; Beller,
M. Low-temperature hydrogenation of carbon dioxide to methanol
with a homogeneous cobalt catalyst. Angew. Chem., Int. Ed. 2017, 56,
1890−1893.
(14) Balaraman, E.; Gunanathan, C.; Zhang, J.; Shimon, L. J.;
Milstein, D. Efficient hydrogenation of organic carbonates, carbamates
and formates indicates alternative routes to methanol based on CO2
and CO. Nat. Chem. 2011, 3, 609−614.
(15) Chu, W. Y.; Culakova, Z.; Wang, B. T.; Goldberg, K. I. Acid-
assisted hydrogenation of CO2 to methanol in a homogeneous
catalytic cascade system. ACS Catal. 2019, 9, 9317−9326.
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dioxide-to-methanol single-pot conversion using a C-scorpionate
iron(II) catalyst. Green Chem. 2017, 19, 4811−4815.
Complete contact information is available at:
https://pubs.acs.org/10.1021/jacs.0c11927
Notes
The authors declare no competing financial interest.
(17) Rezayee, N. M.; Huff, C. A.; Sanford, M. S. Tandem amine and
ruthenium-catalyzed hydrogenation of CO2 to methanol. J. Am. Chem.
Soc. 2015, 137, 1028−1031.
ACKNOWLEDGMENTS
■
This research was supported by the Leading Research
Program, Unexplored Challenge 2050 of the New Energy
and Industrial Technology Development Organization
(NEDO) and Grant-in-Aid for Scientific Research
(20K05593) from the Japan Society for the Promotion of
Science (JSPS). Dr. H. Nagashima (Interdisciplinary Research
Center for Catalytic Chemistry, National Institute of Advanced
Industrial Science and Technology) is acknowledged for his
contribution to our solid-state NMR spectrometry study.
(18) Everett, M.; Wass, D. F. Highly productive CO2 hydrogenation
to methanol - a tandem catalytic approach via amide intermediates.
Chem. Commun. 2017, 53, 9502−9504.
(19) Kar, S.; Sen, R.; Kothandaraman, J.; Goeppert, A.; Chowdhury,
R.; Munoz, S. B.; Haiges, R.; Prakash, G. K. S. Mechanistic insights
into ruthenium-pincer-catalyzed amine-assisted homogeneous hydro-
genation of CO2 to methanol. J. Am. Chem. Soc. 2019, 141, 3160−
3170.
(20) Kar, S.; Goeppert, A.; Prakash, G. K. S. Combined CO2 capture
and hydrogenation to methanol: amine immobilization enables easy
recycling of active elements. ChemSusChem 2019, 12, 3172−3177.
(21) Kar, S.; Sen, R.; Goeppert, A.; Prakash, G. K. S. Integrative CO2
capture and hydrogenation to methanol with reusable catalyst and
amine: toward a carbon neutral methanol economy. J. Am. Chem. Soc.
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