Low-Temperature Reverse Water–Gas Shift Process and Transformation of Renewable Carbon Resources to Value-Added Chemicals

Article abstract of DOI:10.1002/cssc.201902404

The use of CO₂ instead of toxic CO in the production of important chemicals has attracted widespread interest, and the reverse water–gas shift reaction (RWGSR) is the key step for this kind of processes. Although the thermodynamic limitations are overcome by the reaction of CO with other compounds, the temperature of most reactions involving RWGSR is usually very high owing to the inertness of CO₂. Herein, it was found that Ru₃(CO)₁₂ could catalyze the RWGSR in the ionic liquid HMimBF₄ without ligand or promoter, and CO could be produced at 80 °C, which was much lower than the temperatures reported to date. Detailed studies showed that the BF₄ ^? in the ionic liquid played a crucial role in the low-temperature RWGSR. Based on the low-temperature RWGSR, three important routes to transform CO₂ into valuable chemicals were developed, including synthesis of xanthone from CO₂ and diaryl ethers, synthesis of phenol and acetic acid from CO₂ and anisole, and production of acetic acid from CO₂ and lignin. The reactions could occur at temperature as low as 80 °C, and low-temperature RWGSR was essential for the reactions under mild conditions. The strategy opens the way to produce value-added chemicals by using CO₂ and H₂ as feedstocks under low temperature.

Full text of DOI:10.1002/cssc.201902404

Accepted Article
Title: Low Temperature Reverse Water-Gas-Shift Process and
Transformation of Renewable Carbon Resources to Value-added
Chemicals at Mild Condition
Authors: Xiaojun Shen, Qinglei Meng, Minghua Dong, Junfeng Xiang,
Shaopeng Li, Huizhen Liu, and Buxing Han
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To be cited as: ChemSusChem 10.1002/cssc.201902404
Link to VoR: http://dx.doi.org/10.1002/cssc.201902404
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10.1002/cssc.201902404
ChemSusChem
COMMUNICATION
Low
Temperature
Reverse
Water-Gas-Shift
Process
and
Transformation of Renewable Carbon Resources to Value-added
Chemicals at Mild Condition
Xiaojun Shen[a,b,c], Qinglei Meng[a,b,c], Minghua Dong[a,b,c], Junfeng Xiang[a,b,c], Shaopeng Li[a,b,c], Huizhen
Liu* [a,b,c], Buxing Han* [a,b,c,d]
Abstract: The use of CO2 instead of toxic CO in the production of
important chemicals has attracted widespread interest and reverse
water-gas-shift reaction (RWGSR) is the key step for this kind of
processes. Although the thermodynamic limitations are overcome by
the reaction of CO with other compounds, the temperature of most
reactions involving RWGSR is usually very high due to the inertness
of CO2. Herein, we found that the Ru3(CO)12 could catalyze the
RWGSR process in ionic liquid HMimBF4 without ligand or promoter
and the CO could be produced at 80 oC, which was much lower than
reaction, RWGSR often needs high temperature over
supported nanometal catalysts and transition metal complex
catalytic system.[2c, 3]
Nowadays, many processes have been reported
involved in RWGSR to produce valuable compounds using
CO2 as the C1 resource. For example, the two-step
mechanism has been proposed for the hydrogenation of CO2
to chemical fuels. The first step is the reduction of CO2 to CO
via the RWGSR and the second step is the hydrogenation of
CO to olefin or higher hydrocarbons via the Fischer–Tropsch
synthesis (FTS).[4] Tominaga and co-workers reported
-
that reported to date. Detailed study showed that BF4 of the ionic
liquid played a crucial role in the low temperature RWGSR. Based on
the low temperature RWGSR, three important routes to transform CO2
into valuable chemicals were developed, including synthesis of
xanthone from CO2 and diaryl ethers, synthesis of phenol and acetic
acid from CO2 and anisole, and production of acetic acid from CO2
and lignin. The reactions could occur at temperature as low as 80 oC,
and low temperature RWGSR was essential for the reactions under
mild condition. The strategy opens the way to produce value-added
chemicals using CO2 and H2 as feedstocks under low temperature.
5]
hydroformylation reaction using CO2 instead of CO.[3c,
Beller et al reported the alkoxycarbonylation reaction using
CO2 as C1 source and the reaction mechanism involved
RWGSR was proposed.[6] Unfortunately, the temperature of
most reactions involving RWGSR is usually higher than 100
oC due to the inertness of CO2, although the thermodynamic
limitations are overcome by the reaction of CO with other
compounds.[2d] The design of highly efficient catalysts to
realize low temperature RWGSR is the key to the production
of important chemicals with CO2 instead of CO under mild
conditions.[3a]
CO2 chemistry has become one of the most important
branches of chemistry because it is an abundant, nontoxic,
and renewable C1 resource.[1] Especially, the use of CO2
instead of toxic CO in the synthesis of important chemicals
has attracted widespread interest and reverse water-gas-
shift reaction (RWGSR) is the key step for these kind of
processes.[2] Based on RWGSR, various valuable chemicals
have been produced using CO2 instead of CO. Due to the
inertness of CO2 and thermodynamic limitation of the
Phenol is widely used to produce valuable compounds
in chemical industry, such as phenolic resin, polyesters,
[7]
caprolactam pharmaceuticals drugs.
The industrial
production of phenol typically involves multi-step cumene
process with low yield (~5%), which is energy consuming and
8]
environmentally unfavourable.[7a,
Xanthone scaffold are
able to interact with different types of drug targets and
attracted interests across a broad spectrum of sciences from
chemistry and biology to medicine,[9] and it has been
synthesized by carbonylation of diaryl ethers using CO as the
carbonyl source.[9] Acetic acid is an important bulk chemical,
which can be used as a material for the synthesis of vinyl
acetate monomer (VAM) and acetic anhydride, and can also
be used as a solvent for the oxidation of xylene to
terephthalic acid.[10] Although acetic acid has been
synthesized by the carbonylation using CO2 as the carbonyl
source, the reaction temperature was above 180 oC.[11]
In this work, we developed a low temperature RWGSR
strategy. The RWGSR could be efficiently catalyzed by
Ru3(CO)12 without ligand or promoter performed in ionic
liquid HMimBF4 under mild conditions. CO begun to form at
[a]
Dr. X. Shen, Dr. Q. Meng, Dr. M. Dong, Prof. Dr. J. Xiang, Dr. S. Li,
Prof. Dr. H. Liu, Prof. Dr. B. Han.
Beijing National Laboratory for Molecular Sciences, CAS Key
Laboratory of Colloid and Interface and Thermodynamics, CAS
Research/Education Center for Excellence in Molecular Sciences,
Institute of Chemistry, Chinese Academy of Sciences, Beijing
100190 (P. R. China).
E-mail: liuhz@iccas.ac.cn (Huizhen Liu); hanbx@iccas.ac.cn
(Buxing Han).
[b]
[c]
Dr. X. Shen, Dr. M. Dong, Dr. S. Li, Prof. Dr. H. Liu, Prof. Dr. B. Han.
School of Chemistry and Chemical Engineering, University of
Chinese Academy of Sciences, Beijing 100049 (P. R. China).
Dr. X. Shen, Dr. Q. Meng, Prof. Dr. J. Xiang, Dr. S. Li, Prof. Dr. H.
Liu, Prof. Dr. B. Han.
Physical Science Laboratory, Huairou National Comprehensive
Science Center, Beijing 101407, (P. R. China).
Prof. Dr. B. Han.
-
80 oC, and the BF4 in the ionic liquid played an important role
[d]
for the low temperature RWGSR. Based on this discovery,
three important cascade reactions were conducted using
CO2 instead of CO by combining the Ru3(CO)12 with other
catalyst in the ionic liquid, including synthesis of xanthone
from diaryl ethers and CO2, synthesis of phenol and acetic
Shanghai Key Laboratory of Green Chemistry and Chemical
Processes, School of Chemistry and Molecular Engineering, East
China Normal University, Shanghai 200062, China.
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201902404
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Table 1. Results for reverse water-gas-shift reaction. a
Products
CO2
Conversion
Temperature
Entry
Catalysts
Solvent
CO2/H2 (MPa)
CO
CH4
(oC)
(%)
(μmol)
(μmol)
1
2
3
4
5
6
7
8
9
10
11
12
13b
14c
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
Ru3(CO)12
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
EMimBF4
BMimBF4
HMimCl
HMimBr
HMimPF6
BPyBF4
Bu4NBF4
n-decane
n-decane
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
3/3
80
100
120
140
160
80
80
80
80
80
78
141
209
250
265
63
69
21
28
0
0
0
0
20
32
0
0
0
0
0
0.29
0.52
0.77
0.92
0.97
0.23
0.25
0.08
0.10
0
80
80
80
80
51
58
49
0
0
0
0
0
0.19
0.21
0.18
0
a Reaction condition: 40 μmol Ru-based catalyst, 3.0 g HMimBF4, 12 h; b LiBF4 (2 mmol) and n-decane (3 mL) were used instead of HMimBF4; c n-decane (3 mL) was
used instead of HMimBF4.
acid from anisole and CO2, and production of acetic acid from
lignin and CO2. The reactions started at about 80 oC, and the
optimized reaction temperature was about 140 C. Detailed
study indicates that low temperature RWGSR was crucial for
the low temperature synthesis of the chemicals.
and 28 μmol respectively (Table 1, entries 8 and 9). The cation
also affected the catalytic performance of the catalytic system. In
EMimBF4 and BMimBF4, which have shorter chains in the cation
than HMimBF4, the amount of CO produced was slightly lower
than that in HMimBF4 (Table 1, entries 1, 6, and 7). The amount
of CO produced in BPyBF4 and Bu4NBF4 was also lower than that
in HMimBF4 (Table 1, entries 11 and 12). However, the yield of
CO in the ionic liquid with BF4- anion was higher than that in the
other ionic liquid. These results indicate that the anion of the ionic
liquids had more significant effect on the reaction than cation, and
o
The catalytic performance of different Ru complexes was
checked in different solvents because Ru complexes have been
reported to be excellent catalysts for the RWGSR, and the results
are shown in Tables 1 and S1. It was found that Ru3(CO)12 and
[RuCl2(CO)3]2 was very active for the RWGSR at 140 oC in
HMimBF4 (Table 1, entry 4 and Table S1, entry 1). The other Ru
complexes checked are inactive (Table S1, entries 2-4).
BF -
liquid. Among the ionic liquids checked, HMimBF4 is the best
solvent for RWGSR. To further confirm the role of BF -
, we studied
4
could greatly promote the low temperature RWGSR the ionic
o
Interestingly, 78 μmol of CO were produced even at 80 C. With
4
the increase of temperature, the amount of CO produced
increased gradually (Table 1, entries 1-5), which is because
RWGSR is an endothermic reaction and high temperature is
the reaction in n-decane with and without LiBF4 (Table 1, entries
13 and 14). 49 μmol of CO was produced in the presence of LiBF4
at 80 oC, while no CO was detected without LiBF4 at this
o
beneficial to conversion of CO2. At 160 C, 32 μmol of methane
temperature. This further indicates that BF -
was essential for the
4
were produced by the hydrogenation of CO (Table 1, entry 5).
We also investigated the effect of the pressure of CO2 and H2
on the RWGSR at 140 C. At a fixed ratio of CO2/H2 (1/1), the
low temperature RWGSR catalyzed by Ru3(CO)12.
Fig. S2 shows the effect of the reaction time on the amount
of CO and methane produced. The amount of CO increased
rapidly in the initial stage of the reaction until 6 h and its amount
reached 196 μmol. Upon prolonging the reaction time to 13 h, the
amount of CO increased slowly and decrease from 13 h. The
methane was gradually increased after 12 h, which was attributed
to the hydrogenation of CO into methane.
To understand the reaction pathway in this catalytic system,
we increased the dosage of Ru3(CO)12 and decreased the dosage
of HMimBF4 to monitor the reaction intermediates. Three new
small peaks were detected in the B-NMR and F-NMR of the
HMimBF4 and Ru3(CO)12 mixture after reaction (Figs. 1a and 1b).
The chemical shift of B moved from -1.18 to -1.25 ppm and the
chemical shift of F changed from -148.41 and -148.46 to -148.20
ppm and -148.35 ppm. Besides, some new signals of the
HMimBF4 and Ru3(CO)12 mixture after reaction appeared in F-
NMR spectra. These results indicate that BF4- could interact with
o
yield of CO rose markedly with the increase of the total pressure,
and 250 μmol of CO could be obtained when the pressure of
CO2/H2 reached 3/3 MPa (Table 1, entry 4 and Table S1, entries
5 and 6). However, the higher pressure of CO2/H2, the larger the
amount of CO produced, and more methane was also produced
at the same time. The effect of total pressure became minor when
the pressure was further increased from 3/3 MPa to 4/4 MPa of
CO2/H2 (Table S1, entry 7). Besides, the ratio of CO2/H2 was also
important for the reaction. When we fixed the total pressure at 6
MPa and altered the ratio of CO2/H2, 3 MPa CO2 and 3 MPa H2
gave the best result (Table S1, entry 4 and Table S1, entry 8-11).
The reaction couldn’t proceed without solvent (Table S1,
entry 14) and ionic liquid promoted this reaction. The structure of
ionic liquids on the catalytic reaction was investigated. The ionic
liquids with BF -
and HMimBr, the amounts of CO generated were only 21 μmol
4
anion led to the higher yield of CO. In HMimCl
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10.1002/cssc.201902404
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COMMUNICATION
Fig. 1 (a) the B-NMR of ionic liquid solution. (black line: pure HMimBF4, blue line: mixture of HMimBF4 and Ru3(CO)12 by mechanical agitation with
HMimBF4/Ru3(CO)12 molar ratio of 50/1, red line: HMimBF4 with Ru3(CO)12 after reaction. Reaction condition: 160 μmol Ru3(CO)12, 8 mmol HMimBF4, 3
MPa CO2, 3 MPa H2, 140 oC, 12 h); (b) the F-NMR of ionic liquid solution, (black line: pure HMimBF4, blue line: mixture of HMimBF4 and Ru3(CO)12
by mechanical agitation with HMimBF4/Ru3(CO)12 molar ratio of 50/1, red line: HMimBF4 with Ru3(CO)12 after reaction. Reaction condition: 160 μmol
Ru3(CO)12, 8 mmol HMimBF4, 3 MPa CO2, 3 MPa H2, 140 oC; (c) the FT-IR spectra of the HMimBF4 solution, black line is purity HMimBF4, blue line is
HMimBF4 with Ru3(CO)12 (the mole ratio of (HMimBF4/Ru3(CO)12) is 50/1), red line is HMimBF4 with Ru3(CO)12 after reaction; Reaction condition: 160
o
μmol Ru3(CO)12, 8 mmol HMimBF4, 3 MPa CO2, 3 MPa H2, 140 C, 12 h; (d) the ESI-MS spectra measured for the HMimBF4 with Ru3(CO)12 after
reaction. Reaction condition: 160 μmol Ru3(CO)12, 8 mmol HMimBF4, 3 MPa CO2, 3 MPa H2, 140 oC, 12 h.
Ru3(CO)12 and promote the RWGSR. Besides, the mixture was
also characterized by FT-IR (Fig. 1c), and the signal of mixture
after reaction was different from that before reaction, indicating
the change of Ru3(CO)12 property. The peak of CO shifted to lower
wavelength and a new peak of C=O appeared, suggesting that
the Ru3(CO)12 was converted to the active Ru species that have
different C≡O environment. The FT-IR of Ru-carbonyl species
(Ru3(CO)12,
[Ru3H(CO)11]-,
[Ru3(CO)11(OCOH)]-,
[Ru3(CO)10(OCOH)]-, [Ru3H2(CO)10(OCOH)]-) was simulated by
DFT calculations (Fig. S3) and the band patterns are similar to the
experimental IR spectrum of HMimBF4 with Ru3(CO)12 after
reaction (Fig. 1a), indicating that these Ru-carbonyl species are
intermediates during the reaction. To further confirm the structure
Scheme 1 Possible mechanism for Ru3(CO)12 catalyzed RWGSR
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10.1002/cssc.201902404
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of these active Ru species, the mixture after reaction were eluted
by methanol and characterized by negative mode ESI-MS. As
shown in Fig. 1d, the main peaks were observed at m/z =630.6,
632.6 and 658.6, respectively, which can be assigned to
[Ru3(CO)10(OCOH)]-,
[Ru3H2(CO)10(OCOH)]-
and
[Ru3(CO)11(OCOH)]- based on the previous report.[12]
Scheme 3. The hydrocarboxylation reaction of anisole as a methyl source,
CO2 and H2 to produce acetic acid. Reaction condition: 1mmol anisole, 40
μmol Ru3(CO)12, 40 μmol Rh2(OAc)4, 3 mmol LiI, 3.0 g HMimBF4, 3 MPa CO2,
3 MPa H2, 140 oC; a conversion of CO2; b Yield of products.
According to the results above and the related knowledge in
5]
the literatures,[3c,
we proposed the possible mechanism of
RWGSR in the catalytic system, which is shown schematically in
Scheme 1. The starting Ru3(CO)12 complex released CO with
ligand exchange with the HMimBF4 to form [Ru3(CO)11(BF4)]- (2).
The [Ru3(CO)11(BF4)]- reacted with hydrogen to generate hydride
complex [Ru3H(CO)11]- (3) and release HBF4. In the next step,
[Ru3H(CO)11]- was attacked by CO2 to obtain a formate complexes
species [Ru3(CO)11(OCOH)]- (4). In the final step, the
[Ru3(CO)11(OCOH)]- (4) reacted with a proton formed in the
previous step to give a carbonyl ligand in Ru3(CO)12 for next
catalytic cycle and water. Then, [Ru3(CO)11(OCOH)]- (4) and
[Ru3H(CO)11]- (3) also performed another catalytic cycle (Scheme
1). In this process, the generated [Ru3(CO)11(OCOH)]- was
dissociated and [Ru3(CO)10(OCOH)]- (5) and CO was released.
Then, the [Ru3(CO)10(OCOH)]- (5) reacted with adsorbed H2 to
form the intermediate [Ru3H2(CO)10(OCOH)]- (6). In subsequent
step, the [Ru3H2(CO)10(OCOH)]- could be degraded into
[Ru3H(CO)11]- (3) and H2O. Finally, the same reaction of
[Ru3H(CO)11]- with CO2 in the last catalytic cycle occurred and the
generated [Ru3(CO)11(OCOH)]- (4) went to the next cycle.
The similar stoichiometric reduction of CO2 to CO with H2 has
been reported with the Ru complex.[3c, 3d] In this reaction cycle, the
BF4- was very important for formation of active Ru species.
Previously,we reported the synthesis of phenylacetate
bycarbonylation of aryl methyl ether with CO using RhCl3 as
catalyst and LiI as promoter.[14] In this work, the carbonylation of
aryl methyl ether was carried out using H2 and CO2 as the
carbonyl source (Scheme 3). Because the RWGSR can produce
CO and H2O, the reaction underwent carbonylation and hydrolysis,
and two important bulk chemicals (phenol and acetic acid) were
o
produced simultaneously. At 80 C, yields of phenol and acetic
acid were 36% and 30%, respectively. The conversion of anisole
was only 38% at this temperature, while the conversion of anisole
increased to 95% at 140 oC, and the yields of phenol and acetic
acid reached 87% and 81%, respectively. Meanwhile the
conversion of CO2 can be up to 4.3%.
It is known that lignin, which is considered as industrial waste,
is abundant of methoxy group (-OCH3).[15] On the basis of results
above, we proposed a strategy to transform -OCH3 in lignin into
acetic acid using CO2 and H2. Because the structure of lignin is
complex and heterogeneous, we studied this process in detail.
Firstly, we used PA-lignin (detail see section S2 in Support
information) from Poplar as methyl source considering the
structural integrity of the lignin and high extraction yield. The
amount of methoxy group in PA-lignin was 17.2% as determined
by the headspace gas chromatographic (HS-GC) method (Fig.
S5).[16] In this work, The reaction was catalyzed by the Ru-Rh
bimetallic catalyst using LiI as promoter in HMimBF4, as shown in
Table 2. The acetic acid was the main product from reaction of
Although CO can be generated by low temperature RWGSR,
the conversion (below 1.0%) of CO2 is very low (Table 1)
Therefore, we explored the cascade sequence of reverse water-
gas shift coupled with a carbonylation or carboxylation reaction to
improve the conversion of CO2. As examples of application of low
temperature RWGSR, we study some interesting reactions at low
temperature using CO2 instead of CO. As discussed above,
xanthone is a very important chemical.[9, 13] It has been reported
that xanthone can be effectively synthesized by the oxidative
double C-H functionalization/carbonylation of diaryl ether with CO
using Pd(OAc)2 as the catalyst.[9] In this work, we tried to use CO2
as carbonyl source to synthesize the xanthone based on RWGSR
by combing Ru3(CO)12 with Pd(OAc)2 (Scheme 2). Fortunately,
our catalytic system could catalyze the reaction of 4-tolyl ether
o
lignin and CO2 and H2 (Fig. S6 and Fig. S7).[17] At 140 C, the
output of acetic acid was as high as 0.16 g/g lignin and yield of
acetic acid was 48.3% based on the content of methoxy group in
the PA-lignin (Table 2, entry 1), which is close to the reported
values using CO and H2O in the literature.[18]
The Ru-Rh bimetallic catalyst is crucial for the synthesis of
acetic acid. Ru3(CO)12 or Rh2(OAc)4 alone was not active (Table
2, entries 2 and 3). Furthermore, the efficiency of the catalytic
systems obtained by combination of Ru3(CO)12 with other Rh
precursors, such as Rh2(CO)4Cl2, RhCl3, RhH(CO)(PPh3)3,
Rh(PPh3)COCl, Rh(PPh3)3Cl2, [Rh(1,5-COD)Cl]2 and Rh(acac)3
were unsatisfactory (Table S2, entries 4-10). Obviously,
synergistic effect existed between the Ru3(CO)12 and Rh2(OAc)4
catalysts in accelerating the hydrocarboxylation reaction (Table 2,
entry 1), in which Ru3(CO)12 catalyzed RWGSR to produce CO
and Rh2(OAc)4 promoted the formation of acetic acid by
hydrocarboxylation reaction.
o
with CO2 and H2 to dimethyl xanthone at 80 C (Fig. S4). After
reaction and purification by column chromatography, the isolated
yield of dimethyl xanthone could reach 71%. In this cascade
reaction, the conversion of CO2 could be up to 3.7%.
In this catalytic system, the promoter is also indispensable.
Without promoter, acetic acid was not formed. LiI was an efficient
promoter in the presence of BF4- (Table 2, entries 1 and 11). The
efficiency of other salts, such as NaI, KI, LiF, LiCl and LiBr was
very low (Table 2, entries 12-16). The superiority of Li+ was
Scheme 2 Oxidative double C-H functionalization/ carbonylation 4-
tolyl ether to dimethyl xanthone. Reaction condition of this work: 1 mmol
4-tolyl ether, 25 μmol Pd(OAc)2, 40 μmol Ru3(CO)12, 2 mmol K2S2O8, 2
mmol LiBF4, 3 ml CF3COOH, 3 MPa H2, 3 MPa CO2, 80 oC, 36 h.
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10.1002/cssc.201902404
ChemSusChem
COMMUNICATION
a
Table 2. Performances of different catalytic systems for producing acetic acid using PA-lignin from poplar, CO2 and H2
.
Conversion of
Output of acetic
acid (g/g)
Yield of acetic
acid (%)
Entry
Catalysts
Promoter
Solvent
CO2
(%)
2.96
0
1
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
-
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
HMimBF4
BMimBF4
EMimBF4
HMimPF6
HMimOAc
HMimCl
0.161
0
48.3
0
2
3
Rh2(OAc)4
0
0
0
4
Ru3(CO)12+ Rh2(CO)4Cl2
Ru3(CO)12+ RhCl3
0.082
0.035
0.053
0.048
0.053
0.035
0.045
0
24.6
10.5
15.9
14.4
15.9
10.5
13.5
0
1.41
0.62
0.90
0.81
0.92
0.63
0.82
0
5
6
Ru3(CO)12+ RhH(CO)(PPh3)3
Ru3(CO)12+ Rh(PPh3)COCl
Ru3(CO)12+ Rh(PPh3)3Cl2
Ru3(CO)12+ [Rh(1,5-COD)Cl]2
Ru3(CO)12+ Rh(acac)3
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
Ru3(CO)12+ Rh2(OAc)4
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
NaI
KI
0.034
0
10.2
0
0.59
0
LiF
LiCl
LiBr
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
LiI
0
0
0
0.043
0.070
0.083
0.052
0
12.9
23.7
24.9
15.6
0
0.84
1.25
1.56
0.98
0
0
0
0
0
0
0
HMimBr
HMimI
0
0
0
0
0
0
Water
0
0
0
DMSO
0
0
0
Cyclohexane
0
0
0
aReaction condition: 200 mg lignin, 40 μmol Ru-based catalyst, 40 μmol Rh-based catalyst, 3 mmol additive, 3.0 g solvent, 3 MPa CO2, 3 MPa H2, 140 oC, 12 h.
18, 21]
reaction, LiI was used to cleave –OCH3 in lignin.[13b,
attributed to the stronger Lewis acidity and proper ion size, which
HMimBF4 plays three roles: solvent for the reaction, the promoter
for ether bond breakage, and low-temperature RWGSR.[14]
Furthermore, since Ru3(CO)12 can catalyze the RWGSR at
80 oC, we studied the effect of the reaction temperature and found
that acetic acid could be produced at 80 oC. With the increase of
temperature, the yield of acetic acid increased gradually from 80
could give appropriate coordination sites during the reaction,
while good performance of I- was due to stronger nucleophilicity,
which was favorable to the formation of C-C bond in the
18-19]
generation of acetic acid.[11a,
Solvents often significantly
influence the efficiency of chemical reactions.[20] As can be known
from Table 2, the reaction did not occur in the ionic liquids without
BF4- (Entries 17-23) at the reaction condition, which is similar to
that in moleculer solvents (Table 2, entries 24-26), further
o
oC to 140 C. The yield of acetic acid reached the maximum at
140 oC, and then decreased from 140 oC to 160 oC (Fig. 2a).
According to CBS-APNO method with Gaussian 09 package,[22]
the standard enthalpy of this hydrocarboxylation reaction is -50.7
kJ/mol (Scheme 4), and this reaction is exothermic, which led to
the decrease of equilibrium conversion at higher temperature.
Besides, the viscosity of HMimBF4 declined at high
temperature and the solubility of hydrogen and carbon dioxide in
-
indicating that BF4 was essential for the reaction. The above
results indicate that Ru3(CO)12/Rh2(OAc)4 bimetallic catalyst in
HMimBF4 using LiI as promoter was most active among the
catalytic systems studied. According to above results, the reaction
of lignin with CO2 and H2 catalyzed by Ru3(CO)12 and Rh2(OAc)4
in HMimBF4 was conducted in the presence of LiI. In this cascade
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10.1002/cssc.201902404
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COMMUNICATION
130.0/6.00–8.00) of the lignin are shown in Fig. S8, and the major
substructures are depicted in Fig. S9 based on recent
publications.[24b, 25] As well known, the cross-peak signals of the
methoxy group appear at about δC/δH 56.5/3.70.[24a] In this work,
it can be seen that the signals of methoxy group in lignin was
disappeared after reaction. Furthermore, the signals of guaiacyl
units (G) were sharply decreased and the catechyl units (C) was
1
formed. Furthermore, the H-NMR spectra of liquid mixture after
reaction showed that the signal of methyl iodide appeared (Fig.
S10).The above NMR results indicate that the aryl methyl ether
structures (G units) is cleaved, and convert into methyl iodide and
phenol structures (C units).[25] To confirm the conclusion from
NMR, we also analyzed the content of methoxy group in lignin
before and after reaction (Table 3 and Fig. S5). As results, 98.2%
methoxy group in lignin was removed after reaction (Fig. S5),
which is consistent with the NMR results.
Fig. 2 (a) Effect of reaction temperature on conversion of lignin, CO2 into
acetic acid. Reaction condition: 200 mg PA-lignin from Poplar, 40 μmol
Ru3(CO)12, 40 μmol Rh2(OAc)4, 3 mmol LiI, 3.0 g HMimBF4, 3 MPa CO2, 3 MPa
H2 (at room temperature), 12 h. (b) Time course of conversion of lignin, CO2
into acetic acid. Reaction conditions: 200 mg PA-lignin from Poplar, 40 μmol
Ru3(CO)12, 3 g HMimBF4, 3 MPa CO2 and 3 MPa H2 (at room temperature), 140
oC.
HMimBF4 increased, which promoted the mass transport during
cascade reaction.[23] So the output of acetic acid increased
We applied this strategy to the production of acetic acid from
variously lignins and the results are given in Table 3. Obviously,
all of lignins could be used as an efficient methyl source for
selective production of acetic acid. No other by-products were
generated except minor methyl iodide and methane. The results
indicate that the catalytic system is versatile for producing acetic
acid from lignin and CO2.
o
o
sharply when the temperature increased from 80 C to 140 C.
We also investigated the influence of the pressure of CO2 and H2
on the yield of acetic acid (see Table S2). The optimized pressure
was 3 MPa CO2/3 MPa H2. The amount of catalyst and promoter
was also optimized and the results are shown in Table S2. These
other reaction condition are consistent with the optimum pressure
of RWGSR discussed above.
In order to further prove that the reaction went through the
RWGSR process, we investigated the yield of CO and acetic acid
over time (Fig. 2b). The CO2, H2 were rapidly converted into CO
and H2O at the initial period of the reaction. The amount of CO
increased quickly in the initial stage of the reaction until 1 h. After
1 h, acetic acid began to generate and the amount of CO
decreased until 7 h. After 7 h, the yield of acetic acid increased
slowly and the amount of CO began to increase again. The
hydrocarboxylation reaction reached equilibrium after about 12 h.
The yield of acetic acid did not increase obviously with time.
These results illustrate that RWGSR is the first step for the
formation of acetic acid. At 140 oC, the yield of acetic acid could
reach 0.16 g/g PA-lignin. In addition, we also carried out the
reaction under 13CO2 atmosphere, the results found that
CH313COOH was generated, which comfirmed the carbanyl group
of acetic acid was from CO2 (Fig. S6 and S7).
On the basis of all the experimental results above and the
related knowledge in the literature,[26] we proposed the possible
mechanism of the hydrocarboxylation reaction of lignin, which is
given in Scheme 5. In the process, LiI firstly cleave the ether bond
of methoxy group in lignin (Ar-OCH3) into CH3I and lithium
-
phenolate of lignin under the catalysis of BF4 . A trace amount of
CH3I was observed in the control experiment, (Fig. S10), which
supports the argument above. The related mechanism of catalytic
cleavage of the ether linkage of methoxy group in lignin have been
discussed in our previous work.[18] Meanwhile, Rh2(OAc)4, CO
generated from RWGSR reacted and formed active Rh species in
situ.[27] Then the process involves the oxidative addition of CH3I to
the active Rh species, insertion of CO and reductive elimination
of acetyl iodide.[18, 26] Finally, the acetyl iodide hydrolyzes into
acetic acid and HI, which further reacted with lithium phenolate of
lignin in situ and generated LiI.
To understand the structural change of the methoxy group in
the lignin during the reaction, the lignin before and after the
reaction was analyzed using the two-dimensional heteronuclear
single quantum coherence nuclear magnetic resonance (2D-
HSQC NMR) (Figs. S8 and S9).[24] The side-chain region (δC/δH
50.0–90.0/2.50–6.00) and aromatic region (δC/δH 100.0–
Scheme 4. The standard enthalpy and Gibbs free energy of
hydrocarboxylation reaction by anisole as lignin model compound, CO2
and H2.
Scheme 5 Possible reaction pathways for hydrocarboxylation reaction
using CO2, H2 and lignin as a methyl source to produce acetic acid.
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10.1002/cssc.201902404
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COMMUNICATION
Table 3. Production of acetic acid by reaction of different lignins with CO2 and H2.a
The amount of
methoxy group
(wt%)
Output of
acetic acid
(g/g)
Yield of
acetic acid
(%)
Conversion of
Entry
Lignins
Catalysts
Promoter
CO2
(%)
1
2
Hardwood-PA-lignin
Softwood-PA-ligninb
Grass- milled wood lignin
(MWL)
Ru3(CO)12+Rh2(OAc)4
Ru3(CO)12+Rh2(OAc)4
LiI
LiI
25.8
17.2
0.27
0.16
53.1
48.2
4.75
2.96
2.27
3
Ru3(CO)12+Rh2(OAc)4
LiI
13.8
0.12
45.5
4
5
6
Organosol lignin
Alkali lignin
Dealkali lignin
Ru3(CO)12+Rh2(OAc)4
Ru3(CO)12+Rh2(OAc)4
Ru3(CO)12+Rh2(OAc)4
LiI
LiI
LiI
22.3
14.2
32.9
0.15
0.11
0.20
35.2
40.6
31.1
2.72
2.10
3.50
a Reaction condition: 200 mg lignin, 40 μmol Ru3(CO)12, 40 μmol Rh2(OAc)4, 3 mmol LiI, 3.0 g HMimBF4, 3 MPa CO2, 3 MPa H2, 140 oC, 12 h.
Chem. Soc. 2015, 137, 10317-10325; d) L. Wu, Q. Liu, R.
Jackstell, M. Beller, Angew. Chem. Int. Ed. 2014, 53, 6310-
6320.
At this time, all the catalytic species were regenerated for the next
catalytic cycle. In this reaction, Ru3(CO)12 is responsible for
catalyzing the RWGSR and Rh2(OAc)4 is responsible for
promoting the carbonylation reaction. We also checked the
activity of Rh2(OAc)4 for RWGSR and it was inactive at 80 oC.
In summary, we found taht RWGSR reaction could be
efficiently catalyzed by Ru3(CO)12-based catalysts without ligand
or promoter in HMimBF4 under mild condition. The CO could be
generated at 80 oC, which was much lower than that reported to
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Acknowledgements
The work was supported financially by National Natural Science
Foundation of China (21871277 and 21703258), Beijing Municipal
Science & Technology Commission (Z1811000042180042) and
Chinese Academy of Sciences (QYZDY-SSW-SLH013).
Keywords: Reverse water-gas-shift reaction (RWGSR) • CO2•
xanthone • phenol • acetic acid • lignin
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Entry for the Table of Contents
COMMUNICATION
Xiaojun Shen, Qinglei Meng, Minghua
Dong, Junfeng Xiang, Shaopeng Li,
Huizhen Liu, Buxing Han
Page No. – Page No.
Low temperature reverse water-gas-
shift process and transformation of
renewable carbon resources to value-
added chemicals at mild condition
Directly utilization of renewable carbon feedstock and CO2 into value-added
chemicals (xanthone, phenol and acetic acid) at mild condition based on the low
temperature reverse water-gas-shift reaction
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