Effective hydrogenation of CO2to formate catalyzed by ionic liquid modified acetate-Cu

Article abstract of DOI:10.1039/d0gc03502e

Hydrogenation of CO₂is challenging, especially when non-precious metals are used as catalysts. Herein, we report an effective hydrogenation of CO₂to formate catalyzed by imidazole ionic liquid modified acetate-Cu. The catalytic ability of acetate-Cu was greatly enhanced by ionic liquid modification. The highest turnover number could reach 2005, which is 12.2 times higher than that of unmodified acetate-Cu. Besides, the ionic liquid modified acetate-Cu is easy to be recycled.

Full text of DOI:10.1039/d0gc03502e

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Effective hydrogenation of CO2 to formate
catalyzed by ionic liquid modified acetate-Cu†
Cite this: Green Chem., 2021, 23, 951
Jinling Hu,‡ Jia Liu, ‡ Chenfei Yao, Lei Zhou, Youting Wu,* Zhibing Zhang* and
Xingbang Hu
*
Hydrogenation of CO2 is challenging, especially when non-precious metals are used as catalysts. Herein,
we report an effective hydrogenation of CO2 to formate catalyzed by imidazole ionic liquid modified
acetate-Cu. The catalytic ability of acetate-Cu was greatly enhanced by ionic liquid modification. The
highest turnover number could reach 2005, which is 12.2 times higher than that of unmodified acetate-
Cu. Besides, the ionic liquid modified acetate-Cu is easy to be recycled.
Received 17th October 2020,
Accepted 22nd December 2020
DOI: 10.1039/d0gc03502e
rsc.li/greenchem
metal-catalyzed CO2 hydrogenation involves two steps: the split-
ting of H2 by the metal center to produce a metal hydride com-
Introduction
pound, and the insertion of CO2 into the metal hydride to
produce formate.28–31 Noble metals can efficiently activate H2
and the noble metal based metal-hydride compound is more
stable, both of which are beneficial for the CO2 hydrogenation.
However, the rarity and high cost of noble metals limit their
practical applications. Though the first-row transition metals are
cheaper, Earth abundant and less toxic, their catalytic activity is
much low, such as iron,32–35 cobalt,36–40 nickel and copper.41–44
Copper acetate (Cu(OAc)2), a commercially available cheap
compound, has been reported for its ability to catalyze the hydro-
genation of CO2 to formate with a low TON (165).42 In fact, even
when a ligand with stronger electron donating ability is used, the
TONs of copper based catalysts are still not high, such as a TON
of 500 for the hydrogenation of CO2 to formate catalyzed by tri-
phosphine-ligated copper.43 Bertrand and us developed a cyclic
(alkyl)(amino)carbene copper45 and found that a TON of 1881
can be achieved using cyclic(alkyl)(amino)carbene copper as the
catalyst.44 Though the catalytic ability of copper has been
obviously enhanced, the synthesis of cyclic(alkyl)(amino)carbene
is not easy, which makes this kind of catalyst expensive. Until
now, it has been quite challenging to obtain easily accessed
copper catalysts for the hydrogenation of CO2 to formate.
Ionic liquid (IL) modifications are well-known as a method
to make catalysts recyclable because their solubility can be
adjusted by using different cation–anion combinations.46–49
We earlier reported the structure–reactivity relationships
of acetylacetone-Fe and metalloporphyrin by theoretical
calculations,50,51 which suggested a way to control the reactiv-
ity of organometallic compounds for oxidation reactions.
Herein, for the first time, we found that IL modification can
greatly enhance the catalytic activity of Cu(OAc)2 for the hydro-
genation of CO2 to formate. The highest TON of this simple
copper catalyst is up to 2005.
Carbon dioxide (CO2) is a common greenhouse gas and its
emission is gradually increasing. On the other hand, artificial
CO2 conversion could produce highly valuable chemicals,
which provides
a
chance to reduce CO2 emission.
Hydrogenation of CO2 is becoming an important way to
produce acids, esters, aldehydes, alcohols and even hydro-
carbons. Of them, formic acid is an important chemical raw
material, which can be used as an antiseptic, a pesticide as
well as a material for tanning leather.1 Moreover, formic acid
is widely employed in synthetic chemistry as an acid, reduc-
tant, carbon source and hydrogen storage material.2 Among
these reported methods for the synthesis of formic acid, cata-
lytic hydrogenation of CO2 is an excellent choice because of
the ready availability and low cost of CO2,3,4 and the excellent
atom-economy of CO2 reduction employing hydrogen.
However, the activation of CO2 is difficult due to the thermo-
dynamic stability and kinetic inertness of the CO2 molecule.5
Catalytic reduction of CO2 with hydrogen has been widely
reported using a series of metals since the pioneering work of
Inoue et al.6 Research studies have shown that noble metal cata-
lysts exhibit higher activity for CO2 hydrogenation (such as
Ru,7–18 Rh,19–21 and Ir1,22–27). For example, the TON values of
CO2 hydrogenation in the presence of phosphaamidine based
Rh(I)/Ru(II) complexes, Ru PNP-pincer and Ir(III)-pincer com-
plexes are 4720, 706 500, and 3 500 000, respectively. Usually,
School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210093,
PR China. E-mail: huxb@nju.edu.cn, ytwu@nju.edu.cn, zbzhang@nju.edu.cn;
Fax: +86 2583 336599; Tel: +86 2583 596665
†Electronic supplementary information (ESI) available: Experiment details and
characterization of all compounds. See DOI: 10.1039/d0gc03502e
‡These authors contributed equally.
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420 to 352 (Table 1, entries 12–14). However, alkyl substitution
on the N atom of imidazole is important for enhancing the
Results and discussion
Our research began with the exploration of catalysts containing reactivity. Changing the methyl of [Cu-Imace-C1][Br] by H
different metal centers with acetate or acetylacetonate as results in an obvious decrease of TON from 420 to 194
ligands (Table 1, entries 1–10). Obviously, copper-based cata- (Table 1, entries 12 and 15). Changing the anion is another
lysts are more reactive than others. The TON values of method to control the reactivity of these IL supported catalysts.
Cu(OAc)2 and Cu(acac)2 are at least 3 times higher than those The TON values of catalysts with different anions follow the
of other catalysts investigated here. After modification by ILs order: [Cu-Imace-H][HSO4] > [Cu-Imace-H][Br] > [Cu-Imace-
(Scheme 1), both the catalytic activities of Cu(OAc)2 and H][Cl] > [Cu-Imace-H][NO3] ≫ [Cu-Imace-H][BF4]. A quite
Cu(acac)2 are improved (Table 1, entries 1, 8 and 11–19). In small TON was obtained for [Cu-Imace-H][BF4] because it
particular, for Cu(OAc)2, the influence of IL modifications is almost cannot be dissolved in the solvent.
more remarkable. Further research studies were performed to
The diversity in the reactivity of catalysts with different
synthesize Cu(OAc)2 supported by imidazole ILs with different cations and anions should be induced by the influence of IL
anions and R carbon lengths. Changing the carbon length of modification on the electrical structure of the center metal.
imidazole influences the TON slightly. As the carbon length Copper(II) has a single electron, which provides a chance to
increases from 1 to 6, the TON of the catalysts decreases from observe the influence of a ligand on its electrical structure by
electron paramagnetic resonance (EPR). Based on the results of
EPR (Fig. 1), it is quite clear that IL modification changes the
electrical structure of copper(II) obviously. The electron arrange-
ment around the copper of different catalysts supported by
Table 1 Hydrogenation of CO2 with different catalystsa
[Cu-Imace-R][Br] (R = C1, C3 or C6) is similar to each other
Entry
Catalyst
TONb
(Fig. 1a). However, this is not the case for catalysts supported by
ILs with different anions (Fig. 1b). The paramagnetic resonance
varies greatly when the anion of the catalyst is changed.
To further understand the influence of IL modification on
the electron structures, all the catalysts were fully optimized
1
2
3
4
5
6
Cu(OAc)2·H2O
Zn(OAc)2
Fe(OAc)2
Pd(OAc)2
Co(OAc)2
68
2
2
20
<1
19
AgOAc
7
8
Cu(OAc)2·H2O/PPh3
Cu(acac)2
32
97
9
Co(acac)2
Mn(acac)3
8
4
10
11
12
13
14
15
16
17
18
19
[Cu-ace-mim][Cl]
[Cu-Imace-C1][Br]
[Cu-Imace-C3][Br]
[Cu-Imace-C6][Br]
[Cu-Imace-H][Br]
[Cu-Imace-H][BF4]
[Cu-Imace-H][Cl]
[Cu-Imace-H][NO3]
[Cu-Imace-H][HSO4]
164
420
393
352
194
21
162
122
235
a Reaction conditions: Catalysts (0.02 mmol), DBU (20 mmol), THF
(5.0 mL), CO2 (2 MPa), and H2 (2 MPa), 100 °C, 21 h. b TON = mol
formate/mol catalyst, calculated from the formate 1H NMR integration
relative to an internal standard of isopropanol.
Fig. 1 (a) EPR of catalysts with different cations. (b) EPR of catalysts
with different anions.
Scheme 1 Synthesis of the catalysts.
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Table 2 Some important structural parameters of catalysts
with the UM06-2X/6-311+G*/LANL2DZ method. Both the high
and low spin states were taken into consideration and it was
found that the low spin state catalysts were more stable. It is
interesting to note that one of the anions of the IL modified
catalysts binds with the copper center with a distance ranging
from 1.942 to 2.457 Å (Fig. 2). In agreement with the EPR
results (Fig. 1), the spin densities carried by copper of
[Cu-Imace-R][Br] (R = C1, C3 or C6) are similar to each other,
whereas those of [Cu-Imace-H][X] are quite different (Table 2).
It should be the result of anions coordinating with copper.
According to the frontier orbital theory,52 the gap between the
highest occupied molecular orbital (HOMO) and the lowest
unoccupied molecular orbital (LUMO) is a useful structural
parameter to predict the reactivity of a catalyst. A smaller gap
between the HOMO–LUMO means a more reactive compound.
Theoretical calculation results revealed that the HOMO–LUMO
gaps of all IL modified catalysts are smaller than that of un-
modified Cu(OAc)2 (Table 2). In particular, [Cu-Imace-R][Br]
(R = C1 and C3) has the smallest HOMO–LUMO gaps which
correspond to the most reactive catalysts in the experiments
(Table 1).
QCu
SDCu
Gapc
a
b
Cu(OAc)2
1.247
1.281
1.315
1.327
1.278
1.306
1.473
1.398
1.423
0.637
0.660
0.663
0.664
0.664
0.671
0.726
0.693
0.718
9.815
4.638
4.753
4.849
4.802
6.008
8.350
7.281
7.662
[Cu-Imace-C1][Br]
[Cu-Imace-C3][Br]
[Cu-Imace-C6][Br]
[Cu-Imace-H][Br]
[Cu-Imace-H][Cl]
[Cu-Imace-H][BF4]
[Cu-Imace-H][NO3]
[Cu-Imace-H][HSO4]
a The charge carried by copper. b The spin density carried by copper.
c The HOMO–LUMO gap (in eV) of the catalysts.
The CO2 hydrogenation to formic acid is thermo-
dynamically unfavorable. Usually, organic or inorganic bases
are used to react with formic acid,7–44 which can move the
unfavorable reaction towards the formation of formate. An
organic base can also bind with copper(II),42 thus, changing
the reactivity of these IL modification catalysts (Table 3). The
use of more basic TMG and TBD results in a lower TON
(Table 3, entries 4 and 5). These results indicate that the cata-
lytic activity is not strongly associated with the pKa of the
added base.43 Reactions using LUT or TEA gave quite low
TONs. It may be the result of steric hindrance which makes
the binding between LUT or TEA and the copper center
difficult. Hence, the coordination of the organic base is impor-
tant for the hydrogenation of CO2 catalyzed by IL supported
acetate-Cu.
According to the results listed in Table 1, [Cu-Imace-C1][Br]
is the best choice from the viewpoint of reactivity. This catalyst
was therefore used for further optimized experiments. With
the increase of reaction temperature, the catalytic activity also
improved. However, once the temperature was higher than
120 °C, the TON decreased, which is induced by the decompo-
sition of the catalyst (Table 4, entries 1–3). Through thermo-
gravimetric analysis (Fig. 3), it is found that the catalyst has no
significant mass loss below 200 °C, which indicates that the
catalyst itself is very stable at the reaction temperature. Thus,
the catalyst decomposition may be attributed to the presence
of reduction gas H2 at higher temperature. A higher reaction
pressure and longer reaction time are also beneficial for the
Table 3 Hydrogenation of carbon dioxide with different bases
Entry
Basea
TON
1
2
3
4
5
6
LUT
DBN
TEA
TMG
TBD
DMAP
<1
108
<1
43
164
20
a Base: DBU (1,8-diazabicyclo [5.4.0] undec-7-ene), LUT (2,6-lutidine),
DBN (1,5-diazabicyclo [4.3.0] non-5-ene), TEA (triethylamine), TMG (N,
N,N′,N’-tetramethyl-guanidine), TBD (1,5,7-triazabicyclo [4.4.0] dec-5-
ene), and DMAP (4-dimethylaminopyridine)
Fig. 2 The optimized structures of catalysts calculated by the UM06-
2X/6-311+G*/LANL2DZ method. The shown values are bond lengths in
Å.
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Table 4 Hydrogenation of carbon dioxide under different reaction
conditionsa
Entry
Solvent
T (°C)
P (MPa)
Time (h)
TON
Con.e/%
1
2
3
4
5
6
7
8
THF
THF
THF
CH3CN
CH3OH
DME
Toluene
H2O
THF
THF
80
4
4
4
4
4
4
4
4
2
6
6
6
6
21
21
21
21
21
21
21
21
21
21
48
48
48
86
8.6
42.0
21.0
32.0
<0.1
28.7
10.3
0.2
100
120
100
100
100
100
100
100
100
100
100
100
420
210
320
<1
287
103
2
9
76
7.6
10
11b
12b,c
13b,d
704
2005
1987
1635
70.4
50.1
49.7
40.9
THF
THF
THF
a Reaction conditions: [Cu-Imace-C1][Br] (0.02 mmol) and DBU
(20 mmol). The amount of DBU is 20 mmol. b [Cu-Imace-C1][Br]
(0.005 mmol). c The second run of the catalyst. d The third run of the
catalyst. e Molar ratio of the product and initial DBU determined by 1H
NMR.
Fig. 4 EPR spectrum of the catalyst with DBU.
there is no change in both the oxidation state and coordi-
nation mode of the catalyst during the reaction (Fig. 4).
As for the mechanism of this reaction, previous reports pro-
posed a hydrogenation mechanism that involved a copper
hydride generation and CO2 insertion into the Cu–H
bond.28–31,42,54–58 To verify whether the catalytic system works
according to the above-mentioned mechanism, the reactor con-
taining the catalyst, DBU and THF was first filled with H2 for
2 hours and then H2 was released before it was filled with CO2
for further reaction. Formate was obtained as the final product.
Mixtures of the first step were characterized by EPR (Fig. S11†).
Compared with the reference,59 the hyperfine coupling of the
EPR indicated that a copper hydride was formed in the reaction.
Conclusions
In summary, we have developed a kind of copper catalyst
modified with an ionic liquid, which was efficient at the hydro-
genation of CO2. The catalyst was based on copper acetate,
which is cheap and easy to obtain. The synthesis of the catalyst
is simple and environment friendly. Modification by an imid-
azole ionic liquid can greatly enhance the catalytic reactivity of
copper acetate. A TON up to 2005 can be achieved, which is
obviously higher than these reported data of copper based
catalysts.42–44 The IL modification method should be appli-
cable for other catalysts to enhance their reactivity.
Fig. 3 TGA curves of catalysts.
formation of formate. Different solvents were used in the reac-
tion (Table 4, entries 4–10). The results suggest that protic sol-
vents are not suitable for this reaction. Acetonitrile was earlier
chosen as the solvent in the catalytic system of LCu(MeCN)PF6
and an excellent result was obtained.43 However, acetonitrile
weakened the reactivity of the catalyst in our research. Using
THF as solvent, a TON up to 2005 can be achieved in the pres-
ence of only 0.005 mmol of [Cu-Imace-C1][Br] (Table 4, entry
13). This TON value is far higher than that using Cu(OAc)2
Experimental
General consideration
(TON = 165)53 and it is also higher than that using tripho- All solvents and organic bases were dried and degassed using
sphine-ligated copper (TON = 500)43 and cyclic(alkyl)(amino) standard procedures. Reagents of analytical grade were
carbene copper (TON = 1881) as catalysts.44 In the catalyst re- obtained from commercial suppliers and used without any
cycling experiment, the reactivity of the catalyst decreases purification (commercially available catalysts: Cu(OAc)2·H2O,
about 18% after three cycles (Table 4, entries 11–13). EPR Zn(OAc)2, Fe(OAc)2, Pd(OAc)2, Co(OAc)2, AgOAc, Cu(acac)2, Co
results of the recycled catalyst and the new one indicate that (acac)2,
Mn(acac)3,
2-bromoacetic
acid
and
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N-methylimidazole). NMR spectra were recorded on a Bruker
AV 400 spectrometer. FT-IR spectra were recorded on a Nicolet-
360 infrared spectrometer. H2 (99.999 vt%) and CO2 (99.99 vt%)
were purchased from Nanjing Tianze Gas Center, Nanjing.
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Preparation of catalysts
The IL modified copper acetate ([Cu-Imace-H][X]) and copper
acetylacetonate ([Cu-ace-min][Cl]) were synthesized using a
similar procedure reported by us.60,61 [Cu-Imace-R][Br] was syn-
thesized using the processes presented in Scheme 1. The prepa-
ration of [Cu-Imace-C1][Br] was taken as an example. First, 8.2 g
N-methylimidazole was reacted with 13.9 g 2-bromoacetic acid
in 30 mL acetonitrile under reflux for 48 h. The reaction
mixture was evaporated under reduced pressure at 60 °C, and a
white powder was obtained. Water was used to recrystallize the
powder, and pure [Imace-C1][Br] was obtained. In the next step,
2.21 g [Imace-C1][Br] was reacted with 1.99 g Cu(OAc)2·H2O in
50 mL water at 100 °C. The generated acetic acid was removed
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Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21878141 and 21676134) and Key
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&
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