Efficient synthesis of acetic acid via Rh catalyzed methanol hydrocarboxylation with CO2 and H2 under milder conditions

Article abstract of DOI:10.1039/c7gc01391d

Acetic acid is an important bulk chemical and synthesis of acetic acid via methanol hydrocarboxylation with CO₂ and H₂ is a very promising route. In this work, we studied the reaction over a number of catalytic systems. It was found that Rh₂(CO)₄Cl₂ with 4-methylimidazole (4-MI) as the ligand was very efficient in the presence of LiCl and LiI. Acetic acid began to form at 150°C. The TOF was as high as 26.2 h^-1 and the yield of acetic acid could reach 81.8% at 180°C. The catalytic system had obvious advantages, such as simplicity, high activity and selectivity, milder reaction conditions, and less corrosiveness. The excellent cooperation of CO and Cl^- in Rh₂(CO)₄Cl₂, suitable basicity and aromaticity of the ligand 4-MI, and the hydrogen bonding ability of Cl^- were crucial for the outstanding performance of the catalytic system. The control experiments showed that the reaction did not proceed via the CO pathway.

Full text of DOI:10.1039/c7gc01391d

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DOI: 10.1039/C7GC01391D
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Efficient Synthesis of Acetic Acid via Rh Catalyzed Methanol
Hydrocarboxylation with CO2 and H2 at Milder Condition†
Meng Cui,a,b Qingli Qian,*a Jingjing Zhang,a,b Chunjun Chen,a,b and Buxing Han*a,b
Received 00th January 20xx,
Accepted 00th January 20xx
Acetic acid is an important bulk chemical and synthesis of acetic acid via methanol hydrocarboxylation with CO2 and H2 is a
very promising route. In this work, we studied the reaction over a number of catalytic systems. It was found that
Rh2(CO)4Cl2 with 4-methylimidazole (4-MI) as the ligand was very efficient in the presence of LiCl and LiI. Acetic acid began
to form at 150 C. The TOF was as high as 26.2 h-1 and the yield of acetic acid could reach 81.8% at 180 C. The catalytic
system had obvious advantages, such as simple, high activity and selectivity, milder reaction condition, and less corrosive.
The excellent cooperation of CO and Cl- in the Rh2(CO)4Cl2, suitable basicity and aromaticity of the ligand 4-MI, and the
hydrogen bonding ability of Cl- were crucial for the outstanding performance of the catalytic system. The control
DOI: 10.1039/x0xx00000x
www.rsc.org/
o
o
experiments
showed
that
the
reaction
did
not
proceed
via
CO
pathway.
amount of acetic acid could form via the direct co-conversion of CH4
and CO2, for which severe reaction conditions are required because
of the unfavorable thermodynamics.8 The reduction of CO2 by iron
nanoparticles at 200 oC could generate acetic acid with very low
activity (<0.05 mmol•L-1•h-1) and selectivity (<28.5 mol%).9 Trace
amount of acetic acid was observed in CO2 hydrogenation at 200 oC,
but the product spectrum was broad and CO accounted for 96% of
the total products.10 When the expensive and toxic methyl iodide
reacted with CO2 and H2 by homogeneous catalysis at 240 oC, minor
acetic acid (10.7%) was produced with CO (58.4%) and CH4 (30.9%).
Furthermore, the acetic acid only formed in the presence of two
transition metals (Ru/Co or Ni/Co) and the TOF of acetic acid was
low (<1.4 h-1).11 The progress in this area was very slow in the past.
Currently, the CO2 hydrogenation with simultaneous C-C bond
formation is a grand challenge in CO2 chemistry.12
Recently, we reported a route to produce acetic acid via
methanol hydrocarboxylation with CO2 and H2.13 This route is very
promising because it utilizes cheap and easily available feedstocks
and the efficiency is high. It is a potentially practical way to fix CO2
into value-added chemical on a large scale, although breakthroughs
need to be made before this route is industrially viable. It is obvious
that exploration of efficient and simple catalytic systems,
decreasing reaction temperature, avoiding or reducing the dosage
of corrosive additives are of great importance from both scientific
and practical viewpoints.
Introduction
Carbon dioxide (CO2) is a highly abundant, renewable, easily
available, nontoxic carbon resource.1 Utilization of CO2 as C1
synthon has attracted much attention in recent years, and many
valuable chemicals and fuel have been synthesized, such as alcohols,
hydrocarbons, carboxylic acids, formamides, carbonates, esters,
and so on.2 Carboxylic acids are important and basic chemicals in
industry and human life. Considerable progress has been made in
formic acid synthesis using CO2 and H2 after intensive research for
several decades.3 The carboxylic acids with two or more C atoms
(C2+) are more useful in many cases.4 However, synthesis of C2+
carboxylic acids using CO2 usually uses expensive and/or particular
substrates, such as alkenes, alkynes, arenes, and/or organic halides,
and the metallic reducing agents are generally utilized.5
Transformation of CO2 into C2+ carboxylic acids using cheap and
easily available feedstocks is of great importance, but is challenging.
At present stage, the related progress was seldom reported. In a
pioneering work, carboxylic acids were synthesized using olefins
with CO2 and H2, in which several examples of carboxylic acid (other
than acetic acid) synthesis using corresponding alcohols with CO2
and H2 were discussed.6 It was reported that the reactions followed
the CO pathway under the reaction condition.
Acetic acid is an important bulk chemical that is currently
produced via methanol carbonylation technology using fossil
resource based CO.7 So far, only several preliminary results of acetic
acid synthesis using CO2 were reported.8-11 For example, small
In this work, we found that monometallic catalytic system
consisting of Rh2(CO)4Cl2, 4-methylimidazole (4-MI), LiCl and LiI was
very efficient for acetic acid synthesis from CO2, methanol and H2
(Scheme 1). The catalytic system have some obvious advantages,
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
Scheme 1 Synthesis of acetic acid via methanol hydrocarboxylation with CO2 and H2.
†
Electronic
Supplementary
Information
(ESI)
available.
See
DOI: 10.1039/x0xx00000x
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such as simple, high activity and selectivity, lower reaction stable (entry 2). Thus cocatalyst may be needed to improve the
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-
temperature, and the amount of corrosive LiI used was reduced property of the catalyst. Inspired by theDOroI:le10.o1f03C9l/C7inGCt0h1e39R1Dh
significantly. Detailed study showed that the cooperation of CO and precursor, we screened chlorides of different structures and
Cl- in the Rh2(CO)4Cl2, suitable basicity and aromaticity of the ligand properties. We tried HCl and [Hmim]Cl, respectively, but no acetic
4-MI, and the hydrogen bonding ability of Cl- were crucial for the acid was detected after reaction (entries 14 and 15). We also tested
outstanding performance of the catalytic system. The reaction tetrabutylammonium chloride (TBAC), which may render highly free
mechanism was proposed on the control experiments, which Cl- due to the big steric hindrance of the substituents around the N
indicated that the reaction did not proceed via CO pathway.
atom, but no acetic acid was produced (entry 16). When some
common metallic chlorides (KCl, NaCl) were used as cocatalyst,
respectively, the formation of acetic acid was not observed either
(entries 17 and 18). Remarkable acetic acid was generated and the
catalyst was stable when LiCl was used (entry 1). The superiority of
Results and discussion
Different catalytic systems were tested to synthesize acetic acid
from CO2, methanol and H2, and the results are shown in Table 1.
The product in the reaction solution was mostly acetic acid, and
other products were negligible (Fig. S1 and Fig. S2). Only a small
part of methanol was converted into CH4. The solvent DMI was
stable at reaction condition, and no decomposed product was
detected. At 180 oC, the turnover frequency (TOF) and yield of
acetic acid could reach 26.2 h-1 and 81.8%, respectively (entry 1),
which are much higher than that of the reported Ru-Rh bimetallic
catalyst (10.1 h-1 and 23.2%) at the same temperature.13 Besides
avoiding use of noble Ru compound, the amount of corrosive iodide
salt was only 1/3 of that in the reported catalytic system.13 In brief,
this catalyst is more efficient, less corrosive and could work at
milder condition.
Table 1 Methanol hydrocarboxylation with CO2 and H2 using different catalytic systemsa
Entr
y
Catalyst
precursors
Ligan Cocatal Prom Solve
TOF
Yield
(%)c
d
yst
oter
nt
(h-1)b
1
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
RhI3
RhBr3
RhCl3
Rh6(CO)16
Rh(PPh3)3Cl
[Rh(1,5-
4-MI
4-MI
4-MI
-
LiCl
-
LiCl
LiCl
-
LiCl
LiCl
LiCl
LiCl
LiCl
LiI
LiI
-
LiI
-
LiI
LiI
LiI
LiI
LiI
DMI
DMI
DMI
DMI
DMI
DMI
DMI
DMI
DMI
DMI
26.2
5.5
0
0
0
0.1
0.4
11.2
0
81.8
17.0
0
0
0
0.5
1.1
34.9
0
2d
3d
4d
5d
6d
7d
8
-
4-MI
4-MI
4-MI
4-MI
4-MI
9d
10d
0
0
Effective cooperation between different components of the
catalyst is the key for its outstanding catalytic performance. In the
absence of one or more components, the catalyst behaved poorly
11d
4-MI
4-MI
LiCl
LiCl
LiI
LiI
DMI
DMI
0
0
0
0
COD)Cl]2
RhCl(CO)(PPh
)
or did not work (entries 2-5). The Rh catalyst was the basis of the 12d
3
2
catalytic property and the structure of the Rh precursor played a
decisive role. When RhI3 was used as precursor, trace acetic acid
could be detected and evident black precipitate was observed after
reaction (entry 6). When RhI3 was changed to RhBr3, the catalytic
activity had a minor increase but the catalyst was still unstable
(entry 7). After the reaction using RhCl3 as precursor, the
generation of acetic acid had an obvious increase. Moreover, the
catalyst became stable (entry 8). These indicated that the presence
of Cl- in the Rh precursor is beneficial to enhancing the activity and
stability of the catalyst. When Rh6(CO)16 was used as precursor, no
13d
14
Rh2(OAc)4
Rh2(CO)4Cl2
4-MI
4-MI
LiCl
HCl
LiI
LiI
DMI
DMI
6.8
0
21.2
0
[Hmim]
Cl
TBAC
NaCl
KCl
LiBr
LiI
LiBF4
Li[2-
CBZ]
LiCl
15d
Rh2(CO)4Cl2
4-MI
LiI
DMI
0
0
16d
17d
18d
19d
20d
21d
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
4-MI
4-MI
4-MI
4-MI
4-MI
4-MI
LiI
LiI
LiI
LiI
LiI
LiI
DMI
DMI
DMI
DMI
DMI
DMI
0
0
0
0
0.9
9.5
0
0
0
0
2.7
29.8
acetic acid was produced and the catalyst was unstable (entry 9). To 22d
Rh2(CO)4Cl2
4-MI
LiI
DMI
0
0
our surprise, the carbonyl group (CO), which was invalid for the
reaction in Rh6(CO)16, could cooperated very well with the Cl- in
Rh2(CO)4Cl2 and greatly enhanced the catalytic activity (entry 1).
The cooperation of CO and Cl- in Rh2(CO)4Cl2 was neccessary
because no acetic acid was observed when the Rh precursors
containing Cl- and another ligand were utilized (entries 10 and 11).
Furthermore, when a third ligand was introduced into the Rh
precursor including Cl- and CO, the catalyst did not work either
(entry 12). When we tested Rh2(OAc)4, the Rh precursor used in the
previous work,13 only minor acetic acid was formed and the catalyst
was unstable (entry 13). Hence the Rh2(CO)4Cl2 was a suitable
catalyst for the target reaction. The synergy of Cl- and CO in the Rh
precursor could affect the nature of the active Rh species formed in
situ,14 which may account for the excellent performance of the
Rh2(CO)4Cl2 in the reaction. Although the Rh2(CO)4Cl2 could catalyze
the formation of acetic acid at low content of the corrosive iodide
promoter, the catalytic activity was low and the catalyst was not
23d
24d
25d
26d
27
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
Rh2(CO)4Cl2
4-MI
4-MI
4-MI
4-MI
4-MI
4-MI
LiBr
LiCl
NaI
KI
LiI
LiI
DMI
DMI
DMI
DMI
NMP
DMF
1,4-
Dioxa
ne
H2O
0
0
3.1
2.4
3.2
0
0
0
9.6
7.3
10.1
0
LiCl
LiCl
LiCl
LiCl
28d
LiCl
29d
30d
Rh2(CO)4Cl2
4-MI
LiCl
LiI
0
0
Rh2(CO)4Cl2
4-MI
LiCl
LiI
0
0
aReaction conditions: 30 μmol Rh catalyst (based on the metal), 0.5 mmol 4-MI, 4
mmol cocatalyst, 1 mmol promoter, 2 mL solvent, 12 mmol methanol, 5 MPa CO2
and 5 MPa H2 (at room temperature), 180 oC, 15 h. bTOF denotes moles of acetic
acid produced per mole of Rh catalyst per hour in the steady state. cYield is based
on methanol feedstock (100 × moles of acetic acid product per mole of methanol
feedstock). dBlack precipitate was observed after the reaction.
2 | J. Name., 2012, 00, 1-3
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LiCl may lie in its good solubility in the reaction solvent, the imidazole has two kinds of N atoms, namely, the pyridinic N atom
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stronger Lewis acidity, and proper ion size of the Li+. We also and the pyrrolic N atom.16 The X-ray photDoOelIe: 1c0tr.1o0n39s/pCe7cGtCro0s1c3o91pDy
conducted experiments with other lithium salts (LiBr, LiI, LiBF4), but (XPS) study revealed that the pyridinic N atom coordinated with the
the catalytic results were poor (entries 19-21). It seemed that the Rh catalyst (Fig. S3). The lone pair electrons of the pyrrolic N atom
combination of Li and Cl could give the best results. We further participates in forming conjugated π bond of the heterocycle and
tested lithium 2-chlorobenzoate (Li[2-CBZ]), but no acetic acid was enhances the coordination capability of the pyridinic N atom, which
found after reaction (entry 22). This indicates that LiCl is the best can explain the better performance of the imidazole as the ligand.
cocatalyst for the selected Rh catalyst. Although the usage of the When pyrazole, an isomer of imidazole with much lower
corrosive LiI promoter has been greatly decreased in this work, it is coordination ability, was used as ligand, the catalytic performance
still indispensable for the reaction (entry 3). When other lithium
halides (LiBr, LiCl) were used as promoter, no acetic acid could be
a
Table 2 Effect of different ligands on methanol hydrocarboxylation with CO2 and H2
detected either (entries 23 and 24). We also tried other alkali metal
iodides (NaI, KI), but the catalytic performance was unsatisfactory
(entries 25 and 26). Thus LiI was the best promoter for the catalyst.
The superiority of Li+ may be ascribed to its stronger Lewis acidity
and proper size, which would render better coordination sites in the
reaction. The better performance of I- may be due to its peculiar
nature.15 Firstly, the “soft” iodide ligand binds more firmly to soft
metals (such as Rh) than the other halides or ligands, which is
beneficial to the catalyst stability. Secondly, the size of I- is larger
than other halides and may block nucleophilic attack to Rh, offering
better catalytic selectivity. Thirdly, I- may also participate in
different steps of transition metal catalyzed reaction, such as
oxidative addition, reductive elimination, and substrate activation.
These properties may facilitate the formation of C-C bond in acetic
acid formation. After screening the solvents, we found that 1,3-
dimethyl-2-imidazolidinone (DMI) was the best for the reaction
(entries 27-30). As a weak Lewis base, DMI may not only help to
stabilize the catalyst, but also help to absorb and activate the acidic
CO2.
The ligand is also crucial to the catalytic performance and it
played a distinctive role in this work. Without ligand, the Rh
monometallic catalyst could not catalyze the target reaction and it
was unstable (entry 4 of Table 1). To get further understanding of
the ligand effects, we conducted systematic research, and the
results are given in Table 2. Firstly, we tested the commonly used
PPh3, but no acetic acid was generated and the catalyst was
unstable (entry 1). Then we tried NEt3, but acetic acid could not
form either (entry 2). In addition, the catalyst was not stable. When
we turned to use pyridine, the catalyst was stable and minor acetic
acid was observed after reaction (entry 3). We further tried pyrrole,
a heterocyclic monoamine containing N atom of different nature,
but the catalyst had no activity and was unstable (entry 4). The
reason may be that the Lewis basicity of pyridine is stronger than
that of pyrrole, rendering better coordination capability with the Rh
catalyst. We further studied the performance of piperidine, which
has the same C/N skeleton with pyridine and much stronger basicity,
but it was not effective for the reaction (entry 5). Thus both basicity
and aromaticity of the ligand are important for the performance of
the catalyst.
Ligand
Entry
1b
TOF (h-1)
0
Yield (%)
0
2b
3
0
0
6.2
19.2
4b
5b
0
0
0
0
6
7
0
0
3.3
10.2
8
9.4
0
29.3
0
9b
10
11
12
21.9
3.0
68.3
9.5
10.1
31.5
13
14
6.2
19.3
47.9
15.4
15
16b
17
18
3.4
0
10.6
0
26.2
18.4
81.8
57.5
19
20
18.2
17.1
56.7
53.4
21b
22b
23
5.7
7.0
9.6
17.9
27.9
29.9
We also studied the effect of 2,2’-bipyridine and pyrimidine,
which has two pyridinic N atoms, but the results were poor (entries
6 and 7). When O or S atom coexisted with pyridinic N atom in a
heterocyclic ring, the catalytic performance had little improvement
(entry 8) or was totally inhibited (entry 9). However, when
imidazole was utilized in the reaction, the catalyst had much better
activity, selectivity and stability (entry 10). It is well known that
aReaction conditions: 30 μmol Rh2(CO)4Cl2 (based on the metal), 0.5 mmol ligand,
4 mmol LiCl, 1 mmol LiI, 2 mL DMI, 12 mmol methanol, 5 MPa CO2 and 5 MPa H2
(at room temperature), 180 oC, 15 h. bBlack precipitate was observed after the
reaction.
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was poor (entry 11). We also tested triazole and tetrazole, from 4 MPa to 10 MPa (entries 1-4). But the improvement of
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respectively, which have similar structures with imidazole but lower the catalytic performance became mDinOoIr: 10w.1h03e9n/C7tGhCe01t3o9t1aDl
coordination capacity, the results were not satisfactory (entries 12 pressure was further raised (entries 4 and 5). When the total
and 13). The H atom linked to the pyrrolic N atom also played an pressure was fixed at 10 MPa, the results showed that the
important role. When it was replaced by a methyl group, the ratio of CO2 and H2 affected the efficiency of the catalytic
catalytic performance became poorer evidently (entry 14). One of system, and the best result occurred at 5 MPa CO2 and 5 MPa
the possible reasons may be that the N-H formed hydrogen bond H2 (entries 4, and 6-9). The reaction did not occur without CO2
with Cl-, improving the electronic effect of the ligand.16,17 The Cl- is or H2 (entries 10 and 11). Hence both CO2 and H2 were
much easier to form hydrogen bond with N-H than Br- or I-, which necessary for acetic acid generation and 5 MPa CO2/5 MPa H2
would explain the superiority of LiCl and/or chloric Rh precursor in was suitable for the target reaction. The catalytic efficiency
the reaction.
was enhanced with the elevation of the catalyst dosage, but it
The substituent groups on the C atoms of the imidazole also was less sensitive when the dosage was large enough (entries
markedly affected the catalytic performance. As a whole, the 4, 12 and 13).
electron withdrawing group, such as -Cl and –COOH, greatly
The dosage of 4-MI, LiCl and LiI also remarkably influenced
inhibited the reaction (entries 15 and 16). Because of conjugative the catalytic performance (Figs. 1a-1c). With the increase of
effect, the decrease of electron density on the heterocycle weakens these components, the yield of acetic acid increased quickly.
the coordination capability of the pyridinic N atom, thus hampering However, excess amount of them could suppress the catalytic
the catalytic performance. As for the electron donating group, such performance. Just like 4-MI, the Cl- and I- may also act as ligand,
as alkyl groups or –SH, the electronic effect should be beneficial to and too much of them would occupy the active sites of the
the catalytic properties. The actual influence of such substituents catalyst, inhibiting the reaction. The optimal usage of 4-MI, LiCl
depended on the balance between the electronic and steric effects. and LiI was 0.5 mmol, 4 mmol and 1 mmol, respectively. Fig.
For example, when we used 4-MI, the electronic effect dominated 1d illustrates the effect of reaction temperature. Acetic acid
o
and the catalytic performance was improved significantly (entry 17). began to form at 150 C, and the yield increased rapidly with
However, if the electron donating substituents located next to the the elevating temperature until 180 oC. When the temperature
pyrrolic N, the steric effects dominated and the catalytic activity was further enhanced, the change of catalytic activity became
o
decreased evidently (entries 18-22). There is a strong possibility slow. Thus the appropriate reaction temperature was 180 C.
that particular synergy existed between the two N atoms of Fig. 1e depicts the time course of the reaction. At the
imidazole during the catalytic reaction. The substituents next to the beginning (0-3 h), methyl acetate appeared and increased
pyrrolic N would hinder the synergy, destroying the catalytic because the esterification between methanol and acetic acid
properties. The ligand with more complicated substituent was also generated in situ. After 3 h, obvious molecular acetic acid was
tested, but the catalytic performance was unsatisfactory (entry 23).
In short, the 4-MI could effectively improve the stability and activity
of the Rh catalyst.
The impact of gas pressure and catalyst dosage on the
reaction were studied, and the results are presented in Table 3.
At the fixed ratio of CO2 and H2 (1:1), the yield of acetic acid
increased evidently when the total pressure was increased
Table 3 Effect of reaction parameters on methanol hydrocarboxylation with CO2 and
a
H2
Entry
Rh
(μmol)
30
30
30
30
30
30
30
30
30
30
30
20
40
pCO2
(MPa)
pH2
(MPa)
TOF
(h-1)
0
Yield
(%)
0
1
2
3
4
5
6
7
8
2
3
4
5
6
2
4
6
8
5
0
5
5
2
3
4
5
6
8
6
4
2
0
5
5
5
1.3
4.2
15.8
26.2
27.8
4.5
17.1
18.6
1.9
0
0
12.9
26.8
49.3
81.8
86.7
14.2
53.5
57.9
5.8
0
0
40.2
83.7
9
10b
11b
12
13
Fig. 1 Effect of reaction conditions (a-f) over 30 μmol Rh2(CO)4Cl2 (based on the metal)
in 2 mL DMI with 12 mmol methanol at 5 MPa CO2 and 5 MPa H2: (a) Effect of 4-MI
dosage, 4 mmol LiCl, 1 mmol LiI, 180 oC, 15 h; (b) Effect of LiCl dosage, 0.5 mmol 4-MI, 1
mmol LiI, 180 oC, 15 h; (c) Effect of LiI dosage, 0.5 mmol 4-MI, 4 mmol LiCl, 180 oC, 15 h;
(d) Effect of reaction temperature, 0.5 mmol 4-MI, 4 mmol LiCl, 1 mmol LiI, 15 h; (e)
and (f) Effect of reaction time, 0.5 mmol 4-MI, 4 mmol LiCl, 1 mmol LiI, 180 oC.
aReaction conditions: Rh2(CO)4Cl2 was used as catalyst precursor and its dosage
was based on the metal, 0.5 mmol 4-MI, 4 mmol LiCl, 1 mmol LiI, 2 mL DMI, 12
mmol methanol, 180 oC, 15 h. bBlack precipitate was observed after the reaction.
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detected and it grew continuously with reaction time (3-15 h).
ARTICLE
On the basis of the experimental results, we proposed the
View Article Online
DOI: 10.1039/C7GC01391D
The reaction slowed down when methanol was gradually used possible reaction mechanism as shown in Scheme 2, which is
up (15-18 h). As expected, the methyl acetate generated similar to that of the reported catalyst.13 Firstly, methyl iodide
initially was transformed into acetic acid due to the reverse (CH3I) was generated spontaneously from methanol and LiI
esterification. Because hydrogenation of acetic acid into (Step 1). The second step was the oxidative addition of CH3I to
ethanol at the reaction condition is very difficult, only trace the Rh active species (Rh*) (Step 2). Thirdly, CO2 inserted into
ethanol and/or ethyl acetate were detected after the reaction. CH3Rh*I to form CH3COORh*I (Step 3). Next step was reductive
The consumption of CO2 was consistent with the formation of elimination of acetic acid from the CH3COORh*I with the
acetic acid, and no CO was detected during the reaction (Fig. participation of H2 (Step 4). Finally, the LiOH reacted
1f). We also studied the recyclability of the catalytic system, spontaneously with HI to form LiI and H2O (Step 5). At this time,
and the catalytic activity did not decrease obviously after three all the catalytic species were regenerated for the next cycle.
cycles (Fig. S4). These phenomena suggest that the Rh The 4-MI, with particular structure and strong coordination
monometallic catalyst could effectively promote the methanol capability, was beneficial to the oxidative addition of CH3I
hydrocarboxylation with CO2 and H2 into acetic acid.
(Step 2) and insertion of CO2 (Step 3),7, 18 which are two basic
To understand the mechanism of the reaction, we and key steps of the reaction. With close cooperation with the
conducted the experiment using CO to replace CO2, and other 4-MI, LiCl and LiI, the Rh monometallic catalyst not only
conditions were the same as that given in entry 1 of Table 1, promoted all the catalytic steps smoothly, but also effectively
no acetic acid was formed (Fig. S5), demonstrating that lowered the reaction temperature.
reaction did not proceed via CO route. We also tried a control
experiment without methanol, both acetic acid and CO were
not detected (Fig. S6). Therefore, this work is different from
Conclusions
the well known Rh catalyzed methanol carbonylation with CO.
To get further evidence for this argument, we conducted
tracer experiments using 13CH3OH, CH318OH and CH3OD,
respectively (Figs. S7-S10). When 13CH3OH was used, the
molecular weight of acetic acid formed in the reaction was 61
Daltons (Fig. S7), indicating that the two C atoms in the acetic
acid product were from 13C of 13CH3OH and C of CO2,
respectively. When CH318OH was used to replace methanol,
the molecular weight of acetic acid was still 60 Daltons (Fig. S8),
demonstrating that both O atoms in the product were from
CO2, and the OH group in the methanol was removed in the
reaction. Moreover, as methanol was replaced by CH3OD in
the reaction, the molecular weight of acetic acid generated in
the reaction was also 60 Daltons (Fig. S9), further indicating
that both O atoms in the product were from CO2, and the OH
group in the methanol was removed in the reaction. In
In summary, we have investigated the performance of Rh-
based monometallic catalysts for methanol hydrocarboxylation
with CO2 and H2 to produce acetic acid. It is demonstrated that
the catalytic system consisting of Rh2(CO)4Cl2, 4-MI, LiCl, and
LiI has excellent activity and selectivity for the reaction at
o
milder condition. At 180 C, the TOF and yield of acetic acid
can reach 26.2 h-1 and 81.8%, respectively. In the reaction, CO2
is inserted into the intermediate directly, and the reaction
does not proceed via CO pathway. The catalyst components
cooperate very well in accelerating the target reaction. The
Rh2(CO)4Cl2, 4-MI, and LiI contribute to the formation of acetic
acid, and LiCl greatly promotes the catalytic activity and
stabilizes the catalyst. In addition, Rh2(CO)4Cl2 is unique in that
CO and Cl- cooperatively promote the reaction. Ligand 4-MI is
a very efficient ligand because it has suitable basicity and
aromaticity. This process is greener because it uses CO2 as
feedstock, and in comparison with Ru-Rh bimetallic catalyst,
the catalytic system is simpler, less corrosive and more
efficient at milder condition.
1
addition, H NMR and 13C NMR study showed that CH3 group
of CH3OH was transferred into the acetic acid product in the
reaction (Fig. S10). All the results supported the conclusion
that CO2 took part in the reaction directly, and the reaction did
not follow CO pathway.
Experimental
Chemicals
Tetracarbonyl di-μ-chlorodirhodium(I) (Rh2(CO)4Cl2, 97%) was
purchased from Acros Organics. Rhodium acetate dimer
(Rh2(OAc)4), lithium chloride (LiCl, 98%), and 1,3-dimethyl-2-
imidazolidinone (DMI, 98%) were provided by TCI Shanghai Co.,
Ltd. Hexarhodium hexadecacarbonyl (Rh6(CO)16, 98%),
rhodium(III) chloride (RhCl3, Rh > 38.5%), rhodium(III) bromide
(RhBr3),
rhodium(III)
iodide
(RhI3,
99.9%),
carbonylchlorobis(triphenylphosphine)rhodium(I)(RhCl(CO)(PP
h3)2, Rh 14.9% min), lithium iodide (LiI, 99.95%), lithium
bromide (LiBr, 99%), lithium tetrafluoroborate (LiBF4, 98%),
lithium hydroxide monohydrate (LiOH, 56.5%), sodium iodide
(NaI, 99.5%), sodium chloride (NaCl, 99.99%), potassium iodide
Scheme 2 Proposed mechanism of the reaction over the Rh-based catalytic system.
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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ARTICLE
Journal Name
(KI, 99.9%), potassium chloride (KCl, 99.99%), imidazole (99%), chromatograph (GC, Agilent 7890B) equipped with a flame
View Article Online
2-methylimidazole (97%), 2,4-dimethylimidazole (97%), 2- ionization detector (FID) and an HP-5 cDaOpIi:ll1a0r.y103c9o/lCu7mGnC0u13s9in1Dg
mercaptoimidazole (98%), 4,5-dichloroimidazole (98%), toluene as the internal standard. Identification of the liquid
benzimidazole (99%), pyridine (99%), triphenylphosphine (PPh3, products was done using GC-MS (SHIMADZU-QP2010) as well
99%), triethylamine (99%), piperidine (98%), 2,2'-bipyridine as by comparing the retention times with respective standards
(99%), and 1-methyl-2-pyrrolidinone (NMP, 99%) were in the GC traces. NMR spectra were recorded on a Bruker
obtained from Alfa Aesar China Co, Ltd. 4,5-Dimethylimidazole Avance III HD 400 MHz NMR spectrometer (1H NMR, 400 MHz;
(95%) was obtained from Accela ChemBio Co., Ltd. 13C NMR, 100 MHz). The gas mixture were analyzed using a GC
Tetrabutylammonium
chloride
(TBAC,
98%), (Agilent 4890D) equipped with a TCD detector and a packed
chlorotris(triphenylphosphine)rhodium(I) (Rh(PPh3)3Cl, 98%), column (Carbon molecular sieve TDX-01) using argon as the
chloro(1,5-cyclooctadiene) rhodium(I) dimer ([Rh(1,5-COD)Cl]2, carry gas.
98%), pyrrole (99%), 1,2,4-triazole (99%), and methanol-D1
To test the reusability of the catalytic system, the acetic
(CH3OD, 99.5 atom% D) were purchased from Beijing acid formed in the reaction and the unreacted methanol were
InnoChem Science & Technology Co., Ltd. Pyrazole (99%), removed under vacuum at 85 oC for 5 hrs, and then the
pyrimidine (98%), thiazole (99%), 1-methylimidazole (99%), 4- catalytic system was reused directly for the next run.
methylimidazole (4-MI, 98%), and 2-chlorobenzoic acid (98%)
were provided by J&K Scientific Ltd. Tetrazole (98%), 2-
propylimidazole (98%), imidazole-4-carboxylic acid (98%), and
Acknowledgements
oxazole (97%) were obtained from Adamas Reagent Co., Ltd. 1-
Hexyl-3-methylimidazolium chloride ([Hmim]Cl, purity > 99%)
was purchased from the Centre of Green Chemistry and
Catalysis, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences. Hydrochloric acid (HCl, 37% solution in
water), N,N-dimethylformamide (DMF, 99%), 1,4-dioxane
(99%), and methanol (99.5%) were purchased from Beijing
Chemical Company. Toluene (99.8%) was obtained from Xilong
Chemical Co., Ltd. Methanol-13C (13CH3OH, 99 atom% 13C) and
methanol-18O (CH318OH, 95 atom% 18O) were provided by
Sigma-Aldrich Co. LLC. CO2 (99.99%) and H2 (99.99%) were
supplied by Beijing Analytical Instrument Company. All
chemicals were used as received.
The authors thank the National Natural Science Foundation of
China (21373234, 21533011) and the Chinese Academy of
Sciences (QYZDY-SSW-SLH013).
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DOI: 10.1039/C7GC01391D
Acetic acid can be efficiently synthesized via methanol hydrocarboxylation with CO2 and H2
using Rh monometallic catalyst at milder condition.