Efficient synthesis of ethanol by methanol homologation using CO2 at lower temperature

Article abstract of DOI:10.1039/c8gc03320j

CO₂ transformation is an important topic in green chemistry, and methanol homologation using CO₂ is a promising route to produce ethanol. In this work, we studied this reaction using different homogeneous catalytic systems. It was found that a [RuCl₂(CO)₃]₂/Co₄(CO)₁₂ bimetallic catalyst using LiI as a promoter and N-ethyl-2-pyrrolidone (NEP) as the solvent was very effective under mild conditions. The reaction could proceed efficiently at 160 °C, which is much lower than that reported before. The turnover frequency (TOF) of ethanol based on Ru was as high as 7.5 h^-1 and the selectivity of ethanol in total products could reach 65.0 C-mol%, which are obviously higher than those reported in the literature. Ethanol was synthesized through cascade catalysis of a reverse water gas shift (RWGS) reaction and methanol homologation with syngas (CO/H₂). The outstanding performance of the catalytic system originated from the excellent cooperation of the components. The catalyst could be reused at least five times without any obvious decrease of the catalytic performance. The effect of the solvent on this reaction was studied systematically. The mechanism was also discussed based on isotope labeling tests.

Full text of DOI:10.1039/c8gc03320j

Green Chemistry
View Article Online
View Journal
PAPER
Efficient synthesis of ethanol by methanol
homologation using CO at lower temperature†
2
Cite this: DOI: 10.1039/c8gc03320j
a,b
a,b
a
Ying Wang,
Jingjing Zhang, Qingli Qian,
*
a,b
a,b
a
a,b
Bernard Baffour Asare Bediako, Meng Cui, Guanying Yang, Jiang Yan and
a,b
Buxing Han
*
CO
promising route to produce ethanol. In this work, we studied this reaction using different homogeneous
catalytic systems. It was found that a [RuCl (CO) /Co (CO)12 bimetallic catalyst using LiI as a promoter
2 2
transformation is an important topic in green chemistry, and methanol homologation using CO is a
2
3
]
2
4
and N-ethyl-2-pyrrolidone (NEP) as the solvent was very effective under mild conditions. The reaction
could proceed efficiently at 160 °C, which is much lower than that reported before. The turnover fre-
quency (TOF) of ethanol based on Ru was as high as 7.5 h− and the selectivity of ethanol in total products
could reach 65.0 C-mol%, which are obviously higher than those reported in the literature. Ethanol was
synthesized through cascade catalysis of a reverse water gas shift (RWGS) reaction and methanol homolo-
1
Received 23rd October 2018,
Accepted 2nd January 2019
gation with syngas (CO/H ). The outstanding performance of the catalytic system originated from the
2
excellent cooperation of the components. The catalyst could be reused at least five times without any
DOI: 10.1039/c8gc03320j
obvious decrease of the catalytic performance. The effect of the solvent on this reaction was studied sys-
rsc.li/greenchem
tematically. The mechanism was also discussed based on isotope labeling tests.
reported that ethanol could be produced from paraformalde-
Introduction
2 2
hyde, CO and H at 180 °C, where paraformaldehyde was con-
5
Carbon dioxide (CO ) is a notorious greenhouse gas, and it is also verted into methanol before further transformation. Dimethyl
2
a nontoxic, cheap and easily available carbon resource. CO
been widely used as a C1 synthon to produce value added chemi- with CO
cals, such as urea, salicylic acid, carboxylic acids, alcohols, hydro- vity was improved because of the peculiar reaction pathway.
2
has ether could also be used as a substrate to synthesize ethanol
2
and H at 180 °C. In this route, the reaction selecti-
2
6
1
carbons, cyclic carbonates, amides, etc. CO
among the most important issues of CO transformation, where dominantly manufactured from methanol. As a well known
the major achievement focused on the synthesis of C1 products, platform chemical, methanol is a more suitable substrate to
2
hydrogenation is Currently, both paraformaldehyde and dimethyl ether are pre-
2
2
i.e., methanol, CO, methane, and formic acid and its derivatives.
2 2
produce ethanol with CO and H . However, at the present
In many cases, C2+ products, such as ethanol, acetic acid, olefins, stage only one study of ethanol synthesis from methanol and
7
and liquid fuels, are more useful. But it is very difficult to efficien- CO /H has been reported. At 180 °C, the highest TOF of
2
2
−
1
tly synthesize C2+ products from CO
controlled CO hydrogenation with simultaneous C–C bond for- selectivity in the total products was 34.2 C-mol%. In addition,
mation, which is still a challenge in basic science.
Currently, ethanol production via CO hydrogenation gener- higher reaction efficiency at lower temperature, exploration of
2 2
and H because this involves ethanol based on Ru metal was only 1.1 h and the ethanol
2
3
the yield of ethanol based on methanol was 32%. To get
2
ally encounters higher reaction temperature, and low reaction new catalytic systems is highly desirable. Herein, we report the
4
activity and/or selectivity. Synthesis of ethanol using some efficient synthesis of ethanol from methanol, CO2 and H2
2 2 2 3 2 4
substrate to react with CO and H is a feasible way to improve under milder conditions over [RuCl (CO) ] , Co (CO)12 bi-
the catalytic activity and/or selectivity. Recently, it has been metallic catalysts with LiI as a promoter and N-ethyl-2-pyrroli-
done (NEP) as the solvent (Scheme 1).
a
Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Colloid,
Interface and Chemical Thermodynamics, CAS Research/Education Center for
Excellence in Molecular Sciences, Institute of Chemistry, Chinese Academy of
Sciences, Beijing 100190, China. E-mail: qianql@iccas.ac.cn, hanbx@iccas.ac.cn
b
University of Chinese Academy of Sciences, Beijing 100049, China
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
Scheme 1 Synthesis of ethanol via methanol homologation with CO
and H
2
c8gc03320j
2
.
This journal is © The Royal Society of Chemistry 2019
Green Chem.

View Article Online
Paper
Green Chemistry
the subsequent well known Ru–Co catalyzed methanol homo-
Results and discussion
Catalytic system
logation. When single [Ru(CO) Cl ] was used as the catalyst,
3
2 2
ethanol could be generated but the activity and selectivity of
To obtain a better reaction result at lower temperature, we ethanol were much lower (entry 2). In addition, CO was the
tested various catalytic systems on methanol homologation major product, indicating that the Ru catalyst was effective for
using CO
2
, as shown in Table 1.
the RWGS reaction, while no ethanol was formed and the reac-
tion activity was very low when single Co (CO)12 was utilized to
The catalytic system consisting of [Ru(CO)
3
Cl
2
]
2
, Co
4
(CO)12
,
4
LiI, and N-ethyl-2-pyrrolidone (NEP) could efficiently promote catalyze the reaction (entry 3). These results revealed that the
the reaction at 160 °C (entry 1), which is much lower than that Ru complex was the major catalyst and the Co complex acted
7
reported previously. The representative graphs of GC and as a cocatalyst. Obviously, evident synergy existed between the
GC-MS are shown in Fig. S1 and S2.† The TOF of ethanol Ru and Co catalysts (entries 1–3). We combined other
−
1
3 3 4
based on the Ru catalyst was as high as 7.5 h and the selecti- Ru halides, RuCl and RuI , with Co (CO)12, respectively, and
vity of ethanol in total products reached 65.0 C-mol%. Both the results demonstrated that Cl was a better halide ligand
the activity and selectivity of ethanol were much higher than (entries 4 and 5). Further experiments indicated that the co-
7
those in the previous report. Besides CH
4
(13.8 C-mol%), existence of carbonyl and Cl in the Ru precursor gave the best
(CO)12 as a co-
evident CO (21.2 C-mol%) was observed in the product. This catalytic result (entries 1, 6 and 7). Using Co
4
suggested that the reaction occurred via a RWGS reaction and catalyst, we also tried catalysts other than the Ru complex
a
2 2
Table 1 Different catalytic systems for ethanol synthesis from methanol, CO and H
Selectivity (C-mol%)
TOF (h−1)
b
Ethanol
CO
CH
4
Entry
Catalyst precursors
Promoter
c
1
[Ru(CO)
[Ru(CO)
3
Cl
Cl
2
]
]
2
, Co
2
4
(CO)12
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiBr
LiCl
7.5
0.5
0
65.0
5.3
0
62.4
56.4
60.4
4.8
10.9
7.8
4.7
10.4
9.0
1.9
7.3
62.5
46.6
45.0
50.8
46.9
14.7
6.8
4.4
0
0.2
21.2
49.6
5.8
25.9
18.1
24.1
0
13.8
45.2
94.2
11.7
25.5
15.6
95.2
81.5
66.2
77.5
89.6
91.0
97.8
92.7
12.5
27.7
29.4
29.6
31.2
37.8
53.7
26.5
92.3
85.2
11.8
4.9
95.2
53.8
98.6
99.3
14.9
15.9
16.9
19.0
32.8
51.9
100.0
97.3
25.7
2
3
3
2
d
Co
RuCl
RuI , Co
Ru(PPh
Ru(2,2′-bipyridyl)
Mn (CO)10, Co (CO)12
Fe (CO)12, Co (CO)12
FeCl , Co (CO)12
Rh (CO)16, Co (CO)12
RhCl , Co (CO)12
Ir (CO)12, Co (CO)12
IrCl , Co (CO)12
, Co
, CoCl
, CoBr
, CoI
, CoSO
, Ir
, Rh
, Fe
, NiCl
, PdCl
4
(CO)12
, Co
(CO)12
Cl , Co
4
5
3
4
(CO)12
6.1
5.1
5.6
0.1
0.1
<0.1
0.1
0.3
0.2
0.1
0.2
6.4
5.6
4.6
5.3
5.7
1.3
0.5
0.3
0
<0.1
2.3
0.9
<0.1
0.1
<0.1
0.1
5.3
4.1
2.9
1.1
2.9
1.9
0
d
3
4
6
7
3
)
3
2
4
(CO)12
2 4
Cl ·6H O, Co (CO)12
3
2
d
8
2
4
7.5
d
9
3
4
26.1
17.7
0
0
0.4
d
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
2
4
d
d
d
d
6
4
3
4
4
4
3
4
0
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
[Ru(CO)
3
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
Cl
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
]
2
2
(CO)
8
25.1
25.7
25.6
19.6
21.9
47.5
39.5
69.1
7.7
14.6
54.1
79.1
0
31.7
0.1
0
27.9
33.7
32.7
57.2
28.4
25.1
0
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
3
2
2
2
4 2
·7H O
4
(CO)12
(CO)16
(CO)12
d
d
d
d
6
3
2
2
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
, Co
4
4
4
4
4
4
4
4
4
4
4
4
4
4
4
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
(CO)12
34.1
15.9
4.8
14.6
1.4
d
d
d
LiBF
LiOAc
LiOH·H O
2
4
I
2
0.7
NaI
KI
57.1
50.3
50.3
23.9
38.8
23.0
0
d
d
d
d
RbI
CsI
MgI
2
CaI
2
AlI
3
SnI
4
<0.1
0.1
0.4
2.2
2.3
72.1
d
ZnI
2
a
Reaction conditions: 10 μmol Ru catalyst and 60 μmol Co catalyst (based on the metal), 2.5 mmol promoter, 2 mL solvent, 2.6 mmol methanol,
b
2
MPa CO and 6 MPa H (at room temperature), 160 °C, 15 h. TOF denotes moles of ethanol produced per mole of the Ru catalyst per hour in
2 2
c
d
the steady state. Yield of ethanol based on the methanol feedstock was 43.5%. A black precipitate was observed after the reaction.
Green Chem.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
(CO)10, Fe
Paper
(Mn
2
3
(CO)12, FeCl
2
, Rh
6
(CO)16, RhCl
3
, Ir
4
(CO)12, and Table 2 Effect of different solvents on methanol homologation with
a
2 2
CO and H
IrCl ), but all the catalytic results were very poor (entries 8–14).
3
Hence [Ru(CO)
3
Cl
2
]
2
was a suitable catalyst for the reaction.
as the major catalyst, we tested other Co
Products (μmol)
With [Ru(CO) Cl
3
2
]
2
cocatalysts (Co (CO) , CoCl , CoBr , CoI , and CoSO ·7H O), Entry
Solvent
Ethanol
CO
29
CH4
3228
3389
2
8
2
2
2
4
2
where Co
5–19). We also conducted the experiments with cocatalysts
other than the Co complex (Ir (CO) , Rh (CO) , Fe (CO) ,
4
(CO)12 was proved to be the best one (entries 1 and
b
1
2
141
0
1
b
0
4
12
6
16
3
12
b
NiCl
2
, and PdCl
2
), but the catalytic performances were all poor
(CO)12 was an appropriate coca-
3
4
0
0
1407
261
96
17
b
(entries 20–24). Therefore, Co
4
talyst for the reaction.
Based on [Ru(CO)
halides instead of the LiI promoter (LiBr and LiCl), and the
results showed that the promoting effect of the Li halides fol-
lowed the sequence: LiI > LiBr > LiCl (entries 1, 25 and 26). We
b
5
6
7
0
1053
0
776
890
102
101
474
3
2 2 4
Cl ] and Co (CO)12, we used other Li
2072
8
9
1
0
18
0
48
854
6
286
260
also tried other Li salts (LiBF
ethanol yield was very low (entries 27–29). The control test
using iodine (I ) as the promoter demonstrated that the iodide
4 2
, LiOAc, and LiOH·H O), but the
2
anion was necessary for this reaction (entry 30). The peculiar
b
b
b
0
1617
iodide effects in transition metal catalyzed reactions have been
8
systematically discussed elsewhere. On the basis of the iodide 11
0
1268
275
effect, we tested other alkali metal iodides, i.e., NaI, KI, RbI,
and CsI (entries 31–34). It was observed that the sequence of
the promoting effect is as follows: LiI > NaI > KI > RbI > CsI.
1
1
1
1
2
3
4
5
0
0
915
1615
737
627
608
We also tried alkaline metal iodides (MgI
performance was not as good as that of an alkali metal iodide
and they followed the order: MgI > CaI (entries 35 and 36).
After testing some commonly used transition metal iodides
AlI , SnI , and ZnI ), we found that their performances were 16
2 2
and CaI ), but their
1128
289
480
2
2
564
1526
b
(
7
15
153
353
601
2061
1933
723
3
4
2
all very poor (entries 37–39). Consequently, LiI was the best
promoter for the reaction. As discussed above, the eminent
promoting effect of LiI may lie in two aspects: the stronger
1
7
8
1
381
+
Lewis acidity and smaller size of Li , and the better nucleo-
−
philicity of I . In a word, the catalytic system consisting of 19
241
0
872
25
329
176
[
Ru(CO) Cl ] , Co (CO) and LiI was fit for the target reaction.
3 2 2 4 12
2
0
1
2
Solvent effect
2
33
62
14
The role of a solvent in homogeneous transition metal cata-
lyzed reactions is very crucial. Even a small modification of the
solvent structure may cause a large change in the rate and in
2
1071
791
503
9
the pattern of the processes in solution. But the role of the
a
solvent in hydrogenating CO
2
to C2+ products has not been
Reaction conditions: 10 μmol [RuCl
2
(CO) ]
3 2
, 60 μmol Co
4
(CO)12
(
based on the metal), 2.5 mmol LiI, 2 mL solvent, 2.6 mmol methanol,
adequately discussed in the literature. In this report, we con-
ducted systematic investigation of the solvent effect in the
target reaction (Table 2).
b
2
MPa CO
2
and 6 MPa H
2
(at room temperature), 160 °C, 15 h.
A
black precipitate was observed after the reaction.
It is known that dioxane and tetrahydrofuran (THF) are
commonly used solvents in conventional methanol homologa-
1
0
tion with syngas (CO/H
2
). When dioxane was used in this These facts suggested that the RWGS reaction could hardly
work, little ethanol was observed due to decomposition of take place in dioxane or THF, which is necessary for ethanol
dioxane, and methanol was mostly converted to methane formation in the target reaction. When we used diethyl-
(
entry 1). We also tested THF as a reaction solvent, no ethanol methylamine as the solvent, no ethanol was detected and both
was detected and methanol was totally converted to methane catalyst and solvent decomposed significantly (entry 3).
entry 2). In the above two tests, CH I could form easily but Nevertheless, a considerable amount of CO was produced in
(
3
little or no CO was observed. For comparison, when the reac- the reaction, indicating that the amine group could effectively
tion was conducted in THF using 0.5 MPa CO, instead of CO
catalyze the RWGS reaction but not the ethanol formation. The
a considerable amount of ethanol was produced (Fig. S3†). basicity of the amine group may explain its performance in
2
,
This journal is © The Royal Society of Chemistry 2019
Green Chem.

View Article Online
Paper
Green Chemistry
activating acidic CO
2
. We also tested pyridine as the solvent; tronic and steric effects. The ring size of the cyclic amide
the activity of the RWGS reaction was lower than that in di- solvent was also important for the catalytic performance. NMP
ethylmethylamine and the catalyst was also unstable (entry 4). is a five membered ring cyclic amide. When 1-methyl-2-piperi-
We further tried 1-methylpyrrolidine, a cyclic saturated amine, done, a six membered ring cyclic amide, was used as the
as the solvent, and the RWGS reaction was stably and efficien- solvent, both the catalytic activity and selectivity became worse
tly accelerated (entry 5). When we turned to NMP, a cyclic (entry 19). The smaller ring size often possesses a stronger
amide, the catalyst was stable and a remarkable amount of ring tension, which helps to expose the lone pair electrons on
ethanol was generated (entry 6). The coexistence of the amine the N atom as an active site, and accelerate the reaction. The
group and carbonyl group was necessary for ethanol formation relative position of the amine group and carbonyl group also
because no ethanol was observed when cyclopentanone was affected the catalytic performance. When the N-methyl group
used as the solvent and the catalyst was not stable (entry 7). of the 1-methyl-2-piperidone solvent was transferred to the
The carbonyl group helped to dissolve LiI, which is a key com- para position, no ethanol was detected and the catalyst was
ponent for the C–C bond formation step in ethanol synthesis. unstable (entry 20). Similarly, when 1-benzyl-3-pyrrolidinone,
But the cyclopentanone was also unstable and could be readily instead of 1-benzyl-2-pyrrolidinone, was used as the solvent,
hydrogenated to cyclopentanol under reaction conditions. ethanol production was markedly lower (entries 18 and 21). In
When the O atom in NMP was changed to an S atom, no addition, a remarkable amount of a LiI precipitate was
ethanol was generated and the catalyst was unstable; moreover, observed. Therefore, only when the amine group and carbonyl
the RWGS reaction was also inhibited (entry 8). These facts group were located at the ortho position, the target reaction
indicated that there should be some synergy between the could proceed effectively. There is a strong possibility that the
amine and carbonyl groups in the cyclic amide solvent. conjugation between the lone pair electrons of the N atom and
Compared with the carbonyl group, the sulfinyl group is less π electrons of the carbonyl double bond could make the two
polar, which may not be eligible to trigger the synergy during functional groups cooperate well. Moreover, the p–π conju-
the target reaction. The substituent on the amine group was gation between the two groups helps to stabilize the carbonyl
crucial to ethanol formation. When the methyl group on NMP group under reaction conditions. The number of amine
was replaced by a H atom, the RWGS process took place but no groups also influenced the reaction. 1,3-Dimethyl-2-imidazoli-
ethanol was detected in the reaction (entry 9). The cyclic dinone (DMI) was a slightly better solvent than NMP in pro-
nature of the solvent structure was also required for producing moting ethanol synthesis (entries 6 and 22), while it was not
ethanol. When the reaction was carried out in a non-cyclic as good as NEP (entries 14 and 22). The isotope labeling test
1
3
amide like solvent, such as N,N-dimethylformamide (DMF), using CH OH ruled out the possibility that ethanol was
3
N,N-dimethylacetamide (DMA) and 1,1,3,3-tetramethylurea formed from the ethyl group on the NEP molecule (Fig. S4†).
(TMU), no ethanol was formed but the RWGS reaction pro- In a word, NEP was an appropriate solvent for the target reac-
ceeded smoothly (entries 10–12). We further substituted the C tion. In addition, it has low volatility and is widely used as a
atom, with an O atom, near the carbonyl group in the cyclic solvent in different processes such as those in the pharma-
amide, but ethanol generation was totally prohibited (entries 6 ceutical industry.
and 13). In short, the cyclic amide with an alkyl substituent on
an N atom was the elementary structure of a suitable solvent Impact of reaction parameters
for the target reaction. The amine-based group was responsible
for the RWGS reaction, while the alkyl group on the N atom,
the neighboring carbonyl group, and the cyclic structure were
necessary for ethanol production.
Fig. 1 depicts the formation rate and selectivity of ethanol at
different temperatures. The reaction could hardly take place at
120 °C, while obvious ethanol formation was observed at
We further investigated the impact of the substituent of the
N atom on the catalytic performance. Based on the above dis-
cussion, we learned that NMP is an effective solvent for the
target reaction. When the methyl group on the N atom was
replaced by an ethyl group, the activity and selectivity of
ethanol formation were better than those using NMP (entries 6
and 14). The superiority of the ethyl group over the methyl
group has a sound experimental basis under different con-
ditions, as shown in Table S1.† The ethyl group has a stronger
electron donating ability than the methyl group, which could
strengthen the basicity of the N atom and accordingly acceler-
ate the reaction. When the ethyl group was further changed
into larger ones, both ethanol and CO production decreased
Fig. 1 The TOF and selectivity of ethanol at different temperatures.
Reaction conditions: 10 μmol [RuCl (CO) , 60 μmol Co (CO)12 (based
on the metal), 2.5 mmol LiI, 2 mL NEP, 2.6 mmol methanol, 2 MPa CO
(entries 15–18). This phenomenon could be ascribed to the
2
3
]
2
4
steric effect of these substituents. The final impact of the sub-
2
2
stituent on the reaction relied on the balance between the elec- and 6 MPa H (at room temperature), 15 h.
Green Chem.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
Paper
a
Table 3 Effect of reaction parameters on the reaction
Selectivity [C-mol%]
Ethanol CO CH
Ru/Co
Entry (μmol)
LiI
(mmol)
CO
(MPa)
2
/H
2
TOF
−
1
(h
)
4
b
1
2
3
4
5
6
7
8
9
10/60
5/65
10/60
15/55
20/50
60/10
5/30
15/90
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
10/60
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
1
2
3
3.5
4
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2.5
2/6
2/6
2/6
2/6
2/6
2/6
2/6
2/6
2/6
2/6
2/6
2/6
2/6
0/0
0/6
2/0
0.5/1.5
1/3
3/9
1/7
3/5
4/4
5/3
6/2
0.1
2.4
56.4
65.0
62.1
66.2
18.0
59.0
66.6
48.8
61.6
65.1
46.7
39.5
—
96.2
1.4
18.1 25.5
21.2 13.8
23.9 14.0
23.6 10.1
32.4 49.7
21.9 19.1
21.6 11.8
34.7 16.5
25.0 13.5
20.3 14.5
24.3 28.9
22.8 37.7
11.2
7.5
4.8
4.2
0.4
10.3
5.5
4.3
6.6
7.0
5.4
4.6
0
Fig. 2 Arrhenius plots for ln TOF (h− ) versus the inversed temperature 10
1
1
1
1
1
1
1
1
1
1
2
3
4
5
6
7
8
(
from 403 to 433 K) at low conversion. Reaction conditions: 10 μmol
RuCl (CO) , 60 μmol Co (CO)12 (based on the metal), 2.5 mmol LiI,
mL NEP, 2.6 mmol methanol, 2 MPa CO and 6 MPa H (at room
temperature), 6 h.
[
2
3
]
2
4
2
2
2
c
c
—
—
—
—
—
—
0
0
—
—
1.6
4.7
8.7
6.7
7.2
6.8
6.2
5.6
50.6
65.9
60.5
64.9
57.9
53.5
47.8
45.8
15.1 34.3
18.0 16.2
28.4 11.2
18.5 16.5
27.3 14.8
34.0 12.5
35.4 16.9
34.6 19.6
1
30 °C. The yield of ethanol increased rapidly with the elevat- 19
−
1
2
2
2
0
1
2
ing temperature. At 160 °C, the TOF of ethanol reached 7.5 h
and the selectivity of ethanol was as high as 65%. When the
temperature was further increased, the ethanol yield had a minor 23
24
change but the ethanol selectivity decreased sharply due to the
rapid yield of CH . Therefore, the suitable temperature for this
reaction was 160 °C. We calculated the apparent activation (based on the metal), 2.5 mmol LiI, 2 mL NEP, 2.6 mmol methanol, 2
a
4
2 3 2 4
Reaction conditions: 10 μmol [RuCl (CO) ] , 60 μmol Co (CO)12
b
2 2
MPa CO and 6 MPa H (at room temperature), 160 °C, 15 h. No
c
energy from the Arrhenius plot using the data in the temperature
range from 403 K to 433 K, as displayed in Fig. 2. The result indi-
cated that the apparent activation energy of the catalytic system
was 116 kJ mol− , which was much lower than that in ethanol
methanol was added before the reaction. A black precipitate was
observed after the reaction.
1
4d
2
synthesis via CO hydrogenation reported previously. This is
set the total pressure at 8 MPa and changed the ratio of
CO
consistent with the result that the reaction promoted by our cata-
lytic system could proceed at lower temperature.
2 2
/H , and the results showed that 1/3 was a suitable ratio
(
entries 3 and 20–24).
At 160 °C, we studied other reaction parameters (Table 3).
The methanol substrate was necessary for constructing Recyclability
ethanol molecules. When CO hydrogenation was catalyzed by
the optimized catalytic system, little alcohol product was pro-
duced (entry 1). In addition, little methane was observed in
2
Generally speaking, recycling of a homogeneous catalyst is very
difficult. Due to good stability, our catalytic system can be recycled
and reused for at least five cycles, as shown in Fig. 3. The result
showed that the catalytic activity did not change considerably,
and the TON of ethanol reached 555.5 after the five cycles.
CO hydrogenation, which indicated that in the target reaction
2
the methane byproduct was mostly formed from the methanol
substrate (Fig. S1 and S5†). The dosage ratio of the Ru/Co cata-
lysts was crucial to the catalytic performance. We fixed the
total molar amount of the catalysts at 70 μmol and altered the
molar ratio of the Ru/Co catalysts, and found that 10 μmol Ru
and 60 μmol Co gave the best result (entries 2–6). When the
molar ratio of Ru/Co was fixed at 1/6, the appropriate usage of
the catalyst was also 10 μmol Ru and 60 μmol Co (entries 3, 7
and 8). We also tested the usage of the LiI dosage and
observed that 2.5 mmol of LiI was fit for the reaction (entries 3
−
and 9–13). Too much I from LiI would occupy the active sites
of the catalyst, inhibiting the reaction. The gas pressure also
affected the catalytic performance. Without CO and/or H , no
2 2
product was formed, further indicating that ethanol generated
in the reaction was not from NEP (entries 14–16). We fixed the
Fig. 3 The results of the recycling tests. Reaction conditions: 10 μmol
ratio of CO
2
/H
2
at 1/3, and changed the total pressure of the
and 6 MPa H
[RuCl
2
(CO)
3
]
2
and 60 μmol Co
4
(CO)12 (based on the metal), 2.5 mmol LiI,
gases. The results demonstrated that 2 MPa CO
2
2
2
mL NEP, 2.6 mmol methanol, 2 MPa CO
2
and 6 MPa H (at room
2
were suitable for the reaction (entries 3 and 17–19). We also temperature), 160 °C, 15 h.
This journal is © The Royal Society of Chemistry 2019
Green Chem.

View Article Online
Paper
Green Chemistry
1
8
Time course of the reaction
using CH
3
OD and CH
3
OH affirmed that the OH broke away
from CH OH during the reaction, supporting the formation of
3
Fig. 4 illustrates the time course of the reaction. The amount
of methanol decreased continuously during the reaction.
Ethanol emerged at 2 h, and the amount of ethanol grew stea-
dily until 15 h. Then the reaction rate became slower because
the methanol substrate was gradually used up. During the
whole course, the amount of methane increased slowly and
steadily. The content of CO increased rapidly at the beginning
CH
oxidative addition of CH
Step 2). The oxidative addition of CH I to the Co complex has
3
3
I (Fig. S7 and S8†). Secondly, CH Co*I was generated by
3
I to the active Co species (Co*)
(
3
also been reported in other Co catalyzed methanol homologa-
1
2
tion reactions. Thirdly, CO, generated by the RWGS reaction
Step 3), inserted into the CH –Co* bond and resulted in the
(
3
CH
metal–carbon bond is a basic step in organic chemistry.
Fourthly, acetaldehyde was reductively eliminated from
CH COCo*I in the presence of H , during which Co* was
3
COCo* complex (Step 4). The insertion of CO into the
(0–3 h), and then it remained at a constant level with the reac-
1
4
tion continuing. This confirmed that CO generated in situ
rapidly participated in ethanol production.
3
2
Mechanistic discussion
regenerated and HI was formed (Step 5, Fig. S9†).
Acetaldehyde was in situ reduced to ethanol by H catalyzed by
the Ru catalyst (Step 6, Fig. S10†). The tracer experiment using
2
As discussed in the former paragraphs, the target reaction
involved the RWGS reaction to produce CO and subsequent
methanol homologation with CO/H2 (Fig. S6†). The mecha-
1
3
CH
transferred into the methyl group of the CH
cule, which is consistent with the proposed mechanism
Fig. S4†). The tracer experiment using D instead of H
3
OH also verified that the methyl group of methanol was
1
3
1
1
CH OH mole-
3 2
nisms of the Ru catalyzed RWGS reaction and Ru–Co acceler-
1
2
ated methanol homologation have been reported elsewhere.
Based on the mechanistic study in the literature and the
experimental data of our catalytic system, we rendered the
possible mechanism of this work (Scheme 2). Firstly, methanol
reacted with LiI to form CH I and LiOH (Step 1). CH I was
(
2
2
revealed that at most six D atoms could enter one ethanol
molecule, indicating an obvious H–D exchange during the
reaction (Fig. S11†). The detailed study of such exchange reac-
3
3
1
5
tions has been reported elsewhere. Finally, LiOH and HI gen-
erated in situ neutralized spontaneously to form LiI and H
known as the intermediate of methanol homologation and
could form spontaneously from methanol and iodide at elev-
2
O
1
3
(Step 7). After these steps, all the catalytic species were regener-
ated for the next cycle. The active centers of the Ru catalyst in
the RWGS reaction and the Co catalyst in methanol homologa-
ated temperature. The formation of CH I could be acceler-
3
+
7
ated by the Lewis acidic cation (Li ). The tracer experiments
1
1,12
tion with CO/H
2
have been studied elsewhere.
The basic
structure of the Ru and Co active centers in this work should
be similar to those reported in the literature.
Conclusions
In summary, we have studied Ru–Co bimetallic catalysts for
methanol homologation with CO
under mild conditions. The catalytic system consisting of
RuCl (CO) ] /Co (CO) , LiI and NEP shows excellent perform-
2 2
and H to produce ethanol
[
2
3 2
4
12
ance for this reaction. The reaction can be carried out at
60 °C. The turnover frequency (TOF) based on Ru and the
1
−
1
Fig. 4 Time course of the reaction. Reaction conditions: 10 μmol selectivity of ethanol in total products can reach 7.5 h and
RuCl (CO) and 60 μmol Co (CO)12 (based on the metal), 2.5 mmol LiI, 65.0 C-mol%, respectively. In addition, the yield of ethanol
mL NEP, 2.6 mmol methanol, 2 MPa CO
erature), 160 °C.
[
2
3
]
2
4
2
2 2
and 6 MPa H (at room temp-
reaches 43.5%. A detailed study indicates that the reaction pro-
ceeds via the RWGS reaction and subsequent methanol homo-
logation using CO/H . The outstanding activity and selectivity
2
of the catalytic system results from the synergy of the catalytic
components. The solvent is very important for the reaction.
NEP could effectively accelerate the RWGS reaction and metha-
nol homologation, rendering the best catalytic result. The cata-
lyst can be recycled at least five times without any obvious
change of the catalytic performance, and the TON of ethanol
reached 555.5 in five cycles. The transformation of CO into
2
useful chemicals and fuels is one of the major areas in green
chemistry, and this work provides an interesting route to
2
produce ethanol via methanol homologation using CO as a
Scheme 2 Proposed mechanism of the reaction.
C1 feedstock.
Green Chem.
This journal is © The Royal Society of Chemistry 2019

View Article Online
Green Chemistry
Paper
Teflon-lined stainless steel batch reactor equipped with a mag-
netic stirrer. The inner diameter of the reactor was 18 mm. In
a typical experiment, desired amounts of Ru or/and Co or
Experimental
Chemicals
Tris(2,2′-bipyridine)
dichlororuthenium(II)
98+%), iridium
(Co)12, 98+%), iron dodecacarbonyl (Fe
CHO, 99.5%) were provided by Acros CO
Dicobaltoctacarbonyl (Co (CO) ), 1-methyl- pressure at room temperature, respectively. The reactor was
pyrrolidine-2-thione (97%) and diethylmethylamine (98%) placed in an air bath of constant temperature, and the stirrer
were purchased from TCI Shanghai Co., Ltd. was set at 800 rpm. After the reaction, the reactor was cooled
hexahydrate another metal, LiI or another promoter, and methanol and
(
(
Ru(2,2′-bipyridyl) Cl ·6H O,
3 2 2
carbonyl 2 mL NEP or another solvent were added into the reactor. After
Ir
4
3
(CO)12, 99%) and the air in the reactor was replaced with 1 MPa CO
2
three times,
acetaldehyde (CH
Organics.
3
2
and H were charged into the reactor for the desired
2
2
8
Dodecacarbonyltetracobalt (Co (CO) , 98%), hexarhodium in an ice-water bath and the residual gas was released carefully
4
12
hexadecacarbonyl (Rh
anhydrous (RhCl
ium iodide (KI, 99.9%), tin(IV) iodide (SnI , 99.998%), iridium ionization detector (FID) and a DB-5 capillary column using
6
((CO)16
,
98%), rhodium trichloride in a gas bag. The liquid mixture was analyzed by using a gas
3
, 99.9%), sodium iodide (NaI, 99.5%), potass- chromatograph (GC, Agilent 7890 B) equipped with a flame
4
(
III) chloride, anhydrous (IrCl
3
,
99.99%), N,N-dimethyl- toluene as the internal standard. The liquid products were
acetamide (DMA, 99%), lithium chloride (LiCl, 99%), ferrous identified using GC-MS (Agilent-7890B-5977A) and by compar-
chloride (FeCl , 99.5%), magnesium iodide (MgI , 99.996%), ing the retention times with the respective standards in the GC
2
2
calcium iodide (CaI
2
, 99.99%), lithium iodide (LiI, 99.95%), traces. The amounts of the products were calculated from the
rubidium iodide (RbI, 99.8%), cesium iodide (CsI, 99.9%), pal- relevant GC data. The gaseous samples were detected by a GC
ladium chloride (PdCl , 99.9%), ruthenium(III) iodide anhy- (Agilent 4890D) equipped with a TCD detector and a packed
2
drous (RuI
9.5%), manganese carbonyl (Mn
tetrafluoroborate (LiBF4, 98%), lithium hydroxide mono-
hydrate (LiOH·H O, LiOH 56.5%), 1-methyl-2-pyrrolidinone
NMP, 99%), N,N-dimethylformamide (DMF, 99%) and 1,1,3,3- After the reaction, the reactor was cooled down in an ice-water
tetramethylurea (TMU, 99%) were offered by Alfa Aesar China bath and the residual gas was released carefully. The amounts
Co, Ltd. Methanol-D1 (CH OD, 99.5 atom% D) and 1-benzyl-3- of the products were analyzed as discussed above. Then the
3
, Ru 20.5%), cobalt(II) iodide anhydrous (CoI
2
,
column (carbon molecular sieve TDX-01, 3 mm in diameter
9
2
(CO)10, 30.6%), lithium and 1 m in length) using argon as the carrier gas.
Recycling test
2
(
3
pyrrolidinone (98%) were provided by Beijing InnoChem alcohols generated and unreacted methanol were removed
Science & Technology Co., Ltd. Cobalt(II) chloride (CoCl2, under vacuum at 80 °C for 10 h. Then the catalytic system was
9
9%), sulfolane (99%), tetrahydrofuran (THF, 99.5%), iodine used directly for the next cycle.
, 99.8%), 1,4-dioxane (99.5%), 1-methylpyrrolidine (98%),
-methyl-4-piperidone (98%), cyclopentanone (99.5%), zinc
(I
2
1
iodide (ZnI
2
, 98%), lithium acetate (LiOAc, 99%), 2-pyrrolidi- Conflicts of interest
none (98%) and pyridine (99.5%) were obtained from J&K
Scientific Ltd. Lithium bromide (LiBr, 99.5%), tris(triphenyl-
There are no conflicts of interest to declare.
phosphine)ruthenium(II) chloride (Ru(PPh
3 3 2
) Cl , 99%), and
1
,3-dimethyl-2-imidazolidinone (DMI, 98+%) were provided by
Acknowledgements
Adamas Reagent Co., Ltd. Toluene (99.5+%) was purchased
from Beijing Chemical Works. Methanol- O (CH
and 1-methyl-2-piperidone (99%) were offered by Sigma-
Aldrich Co, LLC. Dichlorotricarbonylruthenium dimer
1
8
18
3
OH, 95%)
The authors thank the National Natural Science Foundation of
China (21875262, 21733011, and 21533011), National Key
Research
and
Development
Program
of
China
(
9
[Ru(CO)
8%) and aluminum iodide (AlI
Strem Chemicals, Inc. 1-Ethyl-2-pyrrolidone (NEP, 99%),
3
Cl
2
]
2
, 98%), nickel(II) chloride anhydrous (NiCl
2
,
(
2017YFA0403102), Beijing Municipal Science & Technology
3
, 95%) were purchased from
Commission (Z181100004218004), and the Chinese Academy
of Sciences (QYZDY-SSW-SLH013).
1
1
-benzyl-2-pyrrolidinone (98%), 3-methyl-2-oxazolidone (98%),
-octyl-2-pyrrolidone (98%), 1-cyclohexyl-2-pyrrolidone (99%)
and 1-butyl-2-pyrrolidone (98+%) were obtained from Aladdin.
References
Cobalt(II) bromide (CoBr
Fluorochem Ltd. Cobalt sulfate heptahydrate (CoSO
9.5%) was offered by Amethyst Chemicals. CO (99.99%), H
2
,
98+%) was purchased from
4
·7H O,
2
1 (a) M. Y. He, Y. H. Sun and B. X. Han, Angew. Chem., Int.
Ed., 2013, 52, 9620–9633; (b) Q. Liu, L. P. Wu, R. Jackstell
and M. Beller, Nat. Commun., 2015, 6, 5933; (c) M. Aresta,
A. Dibenedetto and A. Angelini, Chem. Rev., 2014, 114,
9
2
2
(99.99%) and CO (99.99%) were provided by Beijing Analytical
Instrument Company.
1
709–1742; (d) P. G. Jessop, F. Joó and C. C. Tai, Coord.
Catalytic reaction
Chem. Rev., 2004, 248, 2425–2442; (e) J. Klankermayer,
Angew. Chem., Int. Ed., 2016, 55, 7296–7343; (f) H. Zhou
and X. B. Lu, Sci. China: Chem., 2017, 60, 904–911;
The apparatus and procedures were similar to those reported
6
previously. All the reactions were carried out in a 16 mL
This journal is © The Royal Society of Chemistry 2019
Green Chem.

View Article Online
Paper
Green Chemistry
(
g) Q. L. Qian, J. J. Zhang, M. Cui and B. X. Han, Nat.
X. Y. Wang and X. Q. Huang, J. Am. Chem. Soc., 2017, 139,
6827–6830; (e) H. Kusama, K. Okabe, K. Sayama and
H. Arakawa, Energy, 1997, 22, 343–348; (f) K. K. Bando,
K. Soga, K. Kunimori and H. Arakawa, Appl. Catal., A, 1998,
175, 67–81; (g) H. Kurakata, Y. Izumi and K. Aika, Chem.
Commun., 1996, 389–390; (h) T. Inui and T. Yamamoto,
Catal. Today, 1998, 45, 209–214; (i) T. Inui, T. Yamamoto,
M. Inoue, H. Hara, T. Takeguchi and J. B. Kim, Appl. Catal.,
A, 1999, 186, 395–406; ( j) L. S. Davy, D. Nieskens, Y. Ferrari,
R. Liu and J. Kolonko, Catal. Commun., 2011, 14, 111–113;
(k) S. G. Li, H. J. Guo, C. R. Luo, H. R. Zhang, L. Xiong,
X. D. Chen and L. L. Ma, Chem. Lett., 2013, 143, 345–355.
5 J. J. Zhang, Q. L. Qian, M. Cui, C. J. Chen, S. S. Liu and
B. X. Han, Green Chem., 2017, 19, 4396–4401.
6 Q. L. Qian, M. Cui, J. J. Zhang, J. F. Xiang, J. L. Song,
G. Y. Yang and B. X. Han, Green Chem., 2018, 20, 206–213.
7 K. Tominaga, Y. Sasaki, T. Watanabe and M. Saito, Stud.
Surf. Sci. Catal., 1998, 114, 495–498.
8 P. M. Maitlis, A. Haynes, B. R. James, M. Catellani and
G. P. Chiusoli, Dalton Trans., 2004, 3409–3419.
Commun., 2016, 7, 11481; (h) Y. Liu, H. Zhou, J. Z. Guo,
W. M. Ren and X. B. Lu, Angew. Chem., Int. Ed., 2017, 56,
4
862–4866; (i) J. Kothandaraman, A. Goeppert, M. Czaun,
G. A. Olah and G. K. S. Prakash, Green Chem., 2016, 18,
831–5838; ( j) X. Y. Ren, Z. Y. Zheng, L. Zhang, Z. Wang,
C. G. Xia and K. L. Ding, Angew. Chem., Int. Ed., 2017, 56,
10–313; (k) Y. N. Gao, S. Z. Liu, Z. Q. Zhao, H. C. Tao and
Z. Y. Sun, Acta Phys.-Chim. Sin., 2018, 34, 858–872;
l) M. Cui, Q. L. Qian, Z. H. He, Z. F. Zhang, J. Ma,
T. B. Wu, G. Y. Yang and B. X. Han, Chem. Sci., 2016, 7,
200–5205; (m) M. Cui, Q. L. Qian, J. J. Zhang, C. J. Chen
5
3
(
5
and B. X. Han, Green Chem., 2017, 19, 3558–3565.
2
(a) F. L. Liao, X. P. Wu, J. W. Zheng, M. M. J. Li,
A. Kroner, Z. Y. Zeng, X. L. Hong, Y. Z. Yuan, X. Q. Gong
and S. C. E. Tsang, Green Chem., 2017, 19, 270–280;
(
b) S. Kattel, P. J. Ramírez, J. G. Chen, J. A. Rodriguez and
P. Liu, Science, 2017, 355, 1296–1299; (c) S. Wesselbaum,
T. vom Stein, J. Klankermayer and W. Leitner, Angew.
Chem., Int. Ed., 2012, 51, 7499–7502; (d) N. M. Rezayee,
C. A. Huff and M. S. Sanford, J. Am. Chem. Soc., 2015,
9 V. Gutmann, Coord. Chem. Rev., 1976, 18, 225–255.
1
37, 1028–1031; (e) S. Kattel, B. H. Yan, Y. X. Yang, 10 Texaco Inc., US Pat, US4409405-A, 1983.
J. G. Chen and P. Liu, J. Am. Chem. Soc., 2016, 138, 11 (a) K. Tsuchiya, J. D. Huang and K. Tominaga, ACS Catal.,
1
2440–12450.
2013, 3, 2865–2868; (b) W. Li, S. Guo and L. Guo, Catal.
Surv. Asia, 2017, 21, 185–197.
3
4
(a) M. Beller and U. T. Bornscheuer, Angew. Chem., Int. Ed.,
2
1
014, 53, 4527–4528; (b) G. Prieto, ChemSusChem, 2017, 10, 12 M. Röper, H. Loevenich and J. Korff, J. Mol. Catal., 1982,
056–1070. 17, 315–322.
(a) Z. H. He, Q. L. Qian, J. Ma, Q. L. Meng, H. C. Zhou, 13 M. Röper and H. Loevenich, J. Organomet. Chem., 1983,
J. L. Song, Z. M. Liu and B. X. Han, Angew. Chem., Int. Ed., 255, 95–102.
016, 55, 737–741; (b) Q. L. Qian, M. Cui, Z. H. He, 14 (a) G. Braca, G. Sbrana, G. Valentini, G. Andrich and
2
C. Y. Wu, Q. G. Zhu, Z. F. Zhang, J. Ma, G. Y. Yang,
J. J. Zhang and B. X. Han, Chem. Sci., 2015, 6, 5685–5689;
G. Gregorio, J. Am. Chem. Soc., 1978, 100, 6238–6240;
(b) G. Braca, L. Paladini, G. Sbrana, G. Valentini,
G. Andrich and G. Gregorio, Ind. Eng. Chem. Prod. Res. Dev.,
1981, 20, 115–122.
(
c) M. Cui, Q. L. Qian, Z. H. He, Z. F. Zhang, J. Ma,
T. B. Wu, G. Y. Yang and B. X. Han, Chem. Sci., 2016, 7,
5
200–5205; (d) S. X. Bai, Q. Shao, P. T. Wang, Q. G. Dai, 15 T. Junk and W. J. Catallo, Chem. Soc. Rev., 1997, 26, 401–406.
Green Chem.
This journal is © The Royal Society of Chemistry 2019