Lignin modified by deep eutectic solvents as green, reusable, and bio-based catalysts for efficient chemical fixation of CO2

Article abstract of DOI:10.1016/j.reactfunctpolym.2020.104502

Lignin is a biodegradable, abundant and renewable natural polymer on earth. In this study, deep eutectic solvents (DESs)-modified lignin heterogeneous catalysts were prepared for the first time via a facile acid-base interaction between choline chloride (

Full text of DOI:10.1016/j.reactfunctpolym.2020.104502

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Lignin modified by deep eutectic solvents as green, reusable, and
bio-based catalysts for efficient chemical fixation of CO2
Xingquan Xiong, Hui Zhang, Shi Lin Lai, Jinbin Gao, Lizhu Gao
PII:
S1381-5148(19)31313-6
DOI:
https://doi.org/10.1016/j.reactfunctpolym.2020.104502
REACT 104502
Reference:
To appear in:
Reactive and Functional Polymers
Received date:
Revised date:
Accepted date:
8 December 2019
7 January 2020
18 January 2020
Please cite this article as: X. Xiong, H. Zhang, S.L. Lai, et al., Lignin modified by
deep eutectic solvents as green, reusable, and bio-based catalysts for efficient chemical
fixation of CO2, Reactive and Functional Polymers (2019), https://doi.org/10.1016/
j.reactfunctpolym.2020.104502
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Lignin modified by deep eutectic solvents as green, reusable,
and bio-based catalysts for efficient chemical fixation of CO2
Xingquan Xiong*, Hui Zhang, Shi Lin Lai, Jinbin Gao, Lizhu Gao
College of Materials Science and Engineering, University of Huaqiao, Xiamen
361021, China.
E-mail address: xxqluli@hqu.edu.cn
Abstract: Lignin is a biodegradable, abundant and renewable natural polymer on
earth. In this study, deep eutectic solvents (DESs)-modified lignin heterogeneous
catalysts were prepared for the first time via a facile acid-base interaction between
choline chloride (ChCl) and p-aminobenzoic acid (PABA) or its derivatives, which
were used to efficiently catalyze the cycloaddition of CO2 and terminal epoxides
under green and mild conditions. Among them, lignin-ChCl-PABA was found to
exhibit the highest catalytic activity for the cycloaddition of epoxides in the presence
of 1.0 MPa of CO2 while retaining high selectivity, which far outperformed those
achieved on the homogeneous EDSs catalytic system. A series of cyclic carbonates
could be obtained in high yields (90-99%), and lignin-ChCl-PABA was stable and
reusable for the cycloaddition reaction (84% yield after 5 recycles). The attaching
DES of ChCl/PABA onto the functional lignin increased the catalytic activity and also
made the catalyst easier to recycle from the reaction mixture. The -OH and -NH2
groups on the surface of lignin-ChCl-PABA played a very important role in the
acceleration of the cycloaddition reaction under solvent free conditions.
Keywords: Lignin; Bio-based catalyst; Deep eutectic solvents; CO2; Cyclic
Carbonates
1. Introduction
Climate change and global warming became major issues for society in the last few
years. CO2 is a major greenhouse gas and the increasing emission of CO2 related to
global warming. Although CO2 is a major cause of global warming, it is a renewable,
abundant, non-toxic, easily available C1 resource to produce valuable chemicals,
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which promotes the carbon cycle in nature [1-4].Chemical conversion and utilization
of CO2 has become an important part of green and sustainable chemistry and has
attracted worldwide attention in recent years [5-10]. Up to now, a series of valuable
chemicals have been synthesized from CO2 [11-15], such as cyclic carbonates, alkynyl
acids, carboxylic acid and its derivatives, carbonyl compounds, formic acid, and
others. Among them, cyclic carbonates are important chemical products with good
biodegradability, solubility and other physical properties. Nowadays, cyclic
carbonates have been widely applied as clean polar solvents, monomers for synthesis
of polymeric materials, chemical intermediates, electrolytes in lithium-ion batteries
with high energy density [16-18].
OH
OH
R
Method I
CO2
O
(1) [O]
O
CO2
O
R
R
O
(2) CO2
R
Method IV
Method II
O
Method III
H2N
OH
NH2
OH
R
Scheme 1. General methods for synthesis of cyclic carbonates.
In general, there are the following methods to synthesize cyclic carbonates: (1)
cyclization of diols with CO2; (2) oxidative carbonylation of olefins; (3) cyclization of
diols with urea; (4) cycloaddition of epoxides with CO2 (Scheme 1). CO2 has many
advantages, such as abundant reserves, low cost, easy availability and environment
friendliness. Therefore, cycloaddition reaction using epoxides as starting material and
CO2 as C1 building block is an important method to synthesize cyclic carbonates [19].
Because of its high atom economy and no by-products, cycloaddition of epoxides
with CO2 meets the requirements of green chemistry and sustainable social
development, so it has been widely studied in chemical synthesis. However, CO2 has
good thermodynamic stability and it is difficult to be activated. Thus, catalysts with
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high catalytic activity are needed to promote the efficient conversion of CO2.
Over the past decades, numerous homogeneous and heterogeneous catalysts have
been used to perform the cycloaddition reaction, such as ionic liquids [20-25] or ionic
polymers [26-28], metal oxides [29, 30], organometallic complexes [31-34], metal
organic frameworks [35-41], porous materials [42-48], etc. Unfortunately, some
heterogeneous catalysts are made from non-renewable sources and composed of
non-biodegradable materials, which will result in environmental and security issues.
In addition, traditional ionic liquids homogeneous catalytic systems still show
shortcomings, such as high cost and low biodegradability, which limit their
applications in real chemical and large scale industrial. Therefore, a highly efficient,
environmental and easily prepared catalytic system is still highly desirable for
chemical fixation of CO2 to cyclic carbonates.
In recent years, DESs have been widely used as a novel class of ILs owing to some
prominent environmentally benign features [49-57]. DESs not only retain the similar
physicochemical properties of ILs, but also have some excellent nature, such as being
easy to prepare from cheap and easily available components, low cost,
environmentally benign and good chemical stability. As far as we knew, only few
articles on the cycloaddition of CO2 with epoxides to produce cyclic carbonates in
DESs catalytic system were found [58-61]. For example, Lü et al. have studied the
cycloaddition of CO2 with epoxides in ChCl-based DESs catalytic system [58]. It was
found that ChCl-based DESs catalytic system exhibited good activity in the
cycloaddition reaction in the absence of any additives under solvent- and metal-free
conditions. In addition, the same group had prepared L-proline/propanedioic
acid/ZnBr2 DESs system to catalyze the cycloaddition reaction of CO2 with epoxides
[59]. The results showed that the DESs system exhibited good activity and selectivity
for the cycloaddition reaction. However, this catalytic system contained toxic metal,
which could cause environmental pollution. Furthermore, in the above homogeneous
catalytic systems, EDSs and ZnBr2 were difficult to separate from the products and
more cumbersome separation procedures were needed. To sum up, an ideal solution is
to use renewable and biodegradable support materials as carriers and load DESs as
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green and efficient catalysts for the chemical conversion of CO2.
It has been reported that hydrogen bonding between functional groups, such as
hydroxyl [62], carboxyl [63], or amino groups [64] and epoxides plays an important
role in the chemical fixation of CO2 into cyclic carbonates, which was helpful for the
further ring opening of epoxide substrates. Therefore, the introducing these functional
groups into the support of heterogeneous catalysts can effectively improve their
catalytic activity towards the cycloaddition reaction. Lignin is one of the most
abundant natural resource on earth. It is amorphous in structure and contains a large
number of phenolic hydroxyl groups, which can be further chemically modified [65,
66]. Because of the biocompatibility, nontoxicity, biodegradability, lignin-based
functional materials have been widely applied in many research areas, such as
bioadsorbents, thermoset polymers, etc [67-72].
In this study, DESs-modified lignin heterogeneous catalysts were prepared for the
first time via a facile acid-base interaction between ChCl and PABA or its derivatives,
which were used to efficiently catalyze the cycloaddition of CO2 and terminal
epoxides under green and mild conditions (Figure 1). Because there are lots of
hydroxyl and amino groups, the obtained lignin-ChCl-PABA was found to act as an
efficient and reusable catalyst in the cycloaddition of terminal epoxides in the
presence of 1.0 MPa of CO2 while retaining high selectivity, which far outperformed
those achieved on the homogeneous EDSs catalytic system[53,54]. A series of cyclic
carbonates could be obtained in high yields (90-99%), and lignin-ChCl-PABA was
stable and reusable for the cycloaddition reaction (84% yield after 5 recycles). The
attaching DES of ChCl/PABA onto the functional lignin increased the catalytic
activity and also made the catalyst easier to recycle from the reaction mixture. The
-OH and -NH2 groups on the surface of lignin-ChCl-PABA played a very important
role in the acceleration of the cycloaddition reaction under solvent free conditions.
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Fig. 1. An illustration for the preparation of DES-modified lignin-ChCl-PABA
catalyst and its application in heterogeneous catalysis of CO2 chemical fixation.
O
O
OH
H
H
H
H
H
H
H
H
O
HO
N
Cl
H
H
O
H
H
O
HO
NaOH+H2O
N2, 80oC, 3h
Lignin
O
Cl
Lignin
DBU, H2O, R. T., 8h
+
OH
O
EPI
H
O
H
H
H
OH
Lignin
O
Lignin-EPI
R
OH
H
O
N
Cl
OH
O
H
O
N
Cl
R
O
O
H
O
O
O
H
H
H
H
H
HO
OH
Cl
H
H
H
O
R
OH
Cl
N
HO
O
N
Cl
O
O
H
Lignin
O
OH
Cl
N
H
O
O
O
N
R
O
Lignin
O
O
H
H
H
O
H
H
O
H
O
Cl
N
R
O
H
Cl
OH
N
OH
Lignin-ChCl
Lignin-DES catalysts
(5) Lignin-PABA
(3) Lignin-PEBA
(2) Lignin-PMBA
(4) Lignin-PNBA
(1) Lignin-BA
OH
OH
OH
H2N
H3
C
OH
O
OH
O2N
O
H3
C
O
O
O
(PABA)
(BA)
(PNBA)
(PMBA)
(PEBA)
Scheme 2. Synthetic route of DESs-modified lignin heterogeneous catalysts through
hydrogen bonds interaction.
2. Experimental
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2.1 Chemicals and reagents
Lignin was purchased from Macklin Reagent Company (dealkaline, product
number: MFCD00147441). Epichlorohydrin, 1,8-diazabicyclo[5.4.0]undec-7-ene
(DBU), NaOH, choline chloride, benzoic acid and its derivatives were obtained from
Aladdin Reagent Company. Dichloromethane, ethyl acetate, and tetrahydrofuran were
obtained from Xiamen Lvyin Reagent Company. All chemicals were obtained
commercially and used as received, without further purification. CO2 (99.99%) was
purchased from Xiamen Kongfente Gas Co., Ltd.
2.2. Instruments
The chemical fixation of CO2 with different epoxides was monitored by TLC
-1
method. FT-IR spectra were registered on a Nicolet Nexus FT-IR instrument at 4 cm
resolution and 32 scans and samples for FT-IR analysis were prepared using the KBr
1
press disc method. H NMR spectra were obtained on a Bruker AVANCE III-500
spectrometer. TGA was run on TG instruments DTG-60H, samples were heated from
25 to 800°C at a heating rate of 20°C min-1 under an Ar atmosphere. SEM images
were obtained using a SU8020 (Hitachi, Japan) scanning electron microscope.
2.3. Preparation of epoxy-functionalized lignin-EPI
Lignin (5.0 g), NaOH (4.3 g, 107 mmol), and distilled water (25.0 mL) were added
into a 3-neck round bottom flask, and then epichlorohydrin (50.0 mL) was add to the
mixture, and then reacted for 3 hours at 80oC under nitrogen condition. The black
solid product could be obtained after simple filtration and washed with water. The
purified product was dried under vacuum at 50oC for 8 hours for further application.
2.4. Synthesis of ChCl-functionalized lignin-ChCl
ChCl (8.0 g) was added into 100.0 mL of aqueous solution containing 4.0 g of
lignin-EPI and the mixture was magnetically stirred at 30°C for 24 hours. Next, DBU
(1.0 g) was added to the dispersion and continued for 6 hours at 30°C. The obtained
lignin-ChCl were washed two times with EtOH and water to remove the excess
choline chloride and DBU, and then dried in vacuo for 12 hours.
2.5. Synthesis of lignin-based catalysts
The obtained product lignin-ChCl was mixed with benzoic acid or its derivative in
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a mass ratio of 1:1 and placed in a 250 mL round bottom flask. After the reaction for
two hours at 80°C, 100.0 mL de-ionized (DI) water was added and then the mixture
was magnetically stirred at 80°C for 24 hours. After simple filtration and washing
with EtOH, the desired lignin-acid catalysts dried at 60oC for further use.
2.6. A general and green procedure for the synthesis of cyclic carbonates
All the chemical fixation of CO2 into cyclic carbonate were carried out in a
Teflon-lined stainless-steel reactor (20 mL) equipped with a pressure gauge and a
magnetic stirrer. In the typical procedure, epoxide (10.0 mmol), lignin-DES catalyst
(100 mg), and the co-catalyst TBABr (1.0 mmol) were placed into the reactor. Then
the reactor was purged 3 times with CO2. Subsequently, the reactor was heated to the
desired temperature and pressure. After completion of the reaction, the reactor was
placed into iced water and then the excess CO2 was released slowly. EtOH (10 mL)
was added into the reaction mixture, and then the lignin-DES catalyst was recovered
by filtration. Subsequently, the obtained catalyst was washed two times with EtOH
and dried under vacuum for the further use.
2.7 Spectroscopic data of the obtained products
4-(Phenoxymethyl)-1,3-dioxolan-2-one (Table 2, entry 1): FT-IR (KBr disc) cm−1:
1788 (C=O), 1167 (C-O), 1052(C-O), 754, 692; 1H NMR (500 MHz, CDCl3) δ: 7.33
(m, 2H), 7.04 (t, J=7.6 Hz, 1H), 6.93 (d, J =7.8 Hz, 2H), 5.05 (m, 1H), 4.63 (t, J=8.3
Hz, 1H), 4.56 (dd, J=5.8, 8.6 Hz, 1H), 4.26 (dd, J=4.3, 10.5 Hz, 1H), 4.17 (dd, J=3.6,
10.5 Hz, 1H).
4-((O-tolyloxy)methyl)-1,3-dioxolan-2-one (Table 2, entry 2): FT-IR (KBr disc)
cm−1: 1789 (C=O), 1163 (C-O), 1049 (C-O), 732, 703; 1H NMR (500 MHz, CDCl3) δ:
7.19 (m, 2H), 6.95 (t, J=7.0 Hz, 1H), 6.80 (d, J =8.6 Hz, 2H), 5.08 (m, 1H), 4.66 (t,
J=8.3 Hz, 1H), 4.60 (dd, J=5.3, 8.4 Hz, 1H), 4.29 (dd, J=3.6, 10.7 Hz, 1H), 4.16 (dd,
J=3.0, 10.6 Hz, 1H), 2.24 (s, 3H).
4-(Hydroxymethyl)-1,3-dioxolan-2-one (Table 2, entry 3): FT-IR (KBr disc) cm−1:
1
3416 (OH), 1770 (C=O), 1170 (C-O), 1048 (C-O), 732, 715; H NMR (500 MHz,
CDCl3) δ: 4.83 (m, 1H), 4.54-4.49 (m, 2H), 3.84-3.73 (m, 2H).
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4-((Allyloxy)methyl)-1,3-dioxolan-2-one (Table 2, entry 4): FT-IR (KBr disc) cm−1:
1
1794 (C=O), 1735 (C=C), 1163 (C-O), 1046 (C-O), 734, 714; H NMR (500 MHz,
CDCl3) δ: 5.86 (m, 1H), 5.28-5.21 (m, 2H), 4.83 (m, 1H), 4.50-4.39 (m, 2H), 4.05 (m,
2H), 3.69-3.61(m, 2H).
4-(Butoxymethyl)-1,3-dioxolan-2-one (Table 2, entry 5): FT-IR (KBr disc) cm−1:
1789 (C=O), 1166 (C-O), 1047 (C-O), 734, 714; 1H NMR (500 MHz, CDCl3) δ: 4.81
(m, 1H), 4.50-4.40 (m, 2H), 3.68-3.61 (m, 2H), 3.51 (m, 2H), 1.56 (m, 2H), 1.37 (m,
2H), 0.92 (t, J=7.39, 3H).
4-(((2-Ethylhexyl)oxy)methyl)-1,3-dioxolan-2-one (Table 2, entry 6): FT-IR (KBr
1
disc) cm−1: 1788 (C=O), 1166 (C-O), 1043 (C-O), 732, 714; H NMR (500 MHz,
CDCl3) δ: 4.81 (m, 1H), 4.49-4.39 (m, 2H), 3.66-3.58 (m, 2H), 3.39 (m, 2H), 1.50 (m,
1H), 1.42-1.19 (m, 8H), 0.92 (m, 6H).
4-(Chloromethyl)-1,3-dioxolan-2-one (Table 2, entry 7): FT-IR (KBr disc) cm−1:
1812 (C=O), 1264, 1162 (C-O), 1073 (C-O), 731, 702; 1H NMR (500 MHz, CDCl3) δ:
4.99 (m, 1H), 4.58 (t, J=8.6 Hz, 1H), 4.39 (m, 1H), 3.81-3.72 (m, 2H).
4,4'-((Ethane-1,2-diylbis(oxy))bis(methylene))bis(1,3-dioxolan-2-one) (Table 2,
entry 8): FT-IR (KBr disc) cm−1: 1784 (C=O), 1170 (C-O), 1048 (C-O), 733, 714; 1H
NMR (500 MHz, CDCl3) δ: 4.85 (m, 2H), 4.53-4.44 (m, 4H), 3.76-3.58 (m, 6H).
4,4'-(1,3-Phenylenebis(ethane-2,1-diyl))bis(1,3-dioxolan-2-one) (Table 2, entry
9): FT-IR (KBr disc) cm−1: 1790 (C=O), 1158 (C-O), 1051 (C-O), 731, 702; 1H NMR
(500 MHz, CDCl3) δ: 7.22 (t, J=8.2 Hz, 2H), 6.59-6.47 (m, 3H), 5.04 (m, 2H),
4.62-4.52 (m, 4H), 4.24 (m, 2H), 4.12 (m, 2H).
4-Phenyl-1,3-dioxolan-2-one (Table 2, entry 10): FT-IR (KBr disc) cm−1: 1789
(C=O), 1163 (C-O), 1065 (C-O), 732, 697; 1H NMR (500 MHz, CDCl3) δ: 7.45-7.37
(m, Ar-H, 5H), 5.69 (t, J=8.4 Hz, 1H), 4.81 (t, J=8.2 Hz, 1H), 4.35 (t, J=8.2 Hz, 1H).
3. Results and Discussion
3.1 Synthesis and characterization of lignin-ChCl-PABA catalyst
The detailed preparation route of the catalyst is shown in Scheme 2. For the
synthesis of epoxy-modified lignin, lignin was reacted with epichlorohydrin to
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produce the lignin-EPI. In the next step, the lignin-ChCl were prepared by reaction of
lignin-EPI with ChCl in the presence of DBU in aqueous solution at room
temperature. The lignin-ChCl was then reacted with PABA or its derivatives to yield
lignin-based heterogeneous catalysts.
The properties of the prepared lignin-ChCl-PABA heterogeneous catalyst were
characterized by FT-IR, SEM, TGA, and elemental analysis. The FT-IR spectra of
original lignin, ChCl, and lignin-ChCl are shown in Figure 2. The FT-IR spectra of
original lignin, PABA, and the lignin-ChCl-PABA catalyst are shown in Figure 3.
The FT-IR spectra of lignin, EPI, and lignin-EPI are shown in Figure S1. In the FT-IR
spectrum of Figure 2a, original lignin sample exhibited a broad peak around 3351
cm−1, attributed to the aliphatic and phenolic hydroxyl groups on the surface of lignin
sample. As shown in Figure S1b, characteristic absorption peaks of epoxy appear at
-1
1265, 905, 852 cm in epichlorohydrin molecule. In the FT-IR spectrum of Figure
S1c, the lignin-ChCl-PABA catalyst exhibited characteristic absorption peaks at 1263,
-1
905, 859 cm , which proved that epoxy groups had been grafted onto the surface of
lignin successfully. As could be seen, characteristic absorption peaks of ChCl
-1
molecule appear at 3259, 2896, 1135 cm (Figure 2b). Compared to the FT-IR
spectrum of original lignin (Figure 2a), some new absorption peaks at 3259, 2896, and
-1
1140 cm appears (Figure 2c), which belong to the stretching vibration of C–H bond
and the bending vibration of C–N bond of the grafted ChCl, respectively. It was an
obvious indication for the presence of ChCl grafted onto the surface of lignin-ChCl.
In the FT-IR spectrum (Figure 3d), characteristic absorption peaks appear at 1663,
-1
1601, 1521, 842, and 771 cm in PABA molecule. After the acid-base interaction
between PABA and ChCl-modified lignin, the characteristic absorption bands of
-1
PABA (1603, 1508, 844, and 775 cm ) appeared in the spectrums of the
lignin-ChCl-PABA catalyst (Figure 3e). These results suggest that PABA had been
grafted onto the surface of lignin-ChCl successfully.
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a
b
2896
2952
c
1135
3259
3259
2896
1140
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 2. FT-IR spectra of (a) lignin, (b) ChCl, and (c) lignin-ChCl.
a
3351
d
842
1521
771
e
1663
1601
844
775
1685
1603
1508
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers (cm-1)
Fig. 3. FT-IR spectra of (a) lignin; (d) PABA; (e) lignin-ChCl-PABA.
The TGA curves of original lignin, lignin-ChCl-PABA, and the recycled
lignin-ChCl-PABA catalyst, are shown in Figure 4. Experiments were performed at
heating rate of 10°C/min in N2 atmosphere up to 800°C. It was clear that the above
three samples showed similar thermal stability. The TGA curves show the first weight
losses around 3.5% at 100°C, which correspond to the loss of the physical adsorbed
water molecules from the samples. At the same time, Figure 4 demonstrates all
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lignin-based products show a two-step degradation profile in the range of 200−300
and 300−500°C. An obvious weight loss between 200 and 300°C is associated with
the breaking of aliphatic chains in the lignin-based samples. While the second broad
mass loss between 300 and 500°C is the breaking of aromatic C−C bonds. Fortunately,
the obtained lignin-ChCl-PABA catalyst is thermally stable up to 200°C, which could
meet the application requirements in heterogeneous catalysis for CO2 chemical
fixation with epoxides. Even after five cycles of recycling, the thermal stability of the
recycled catalyst is still good, and there is no significant difference with the newly
prepared catalyst under 200°C. However, the weight loss of the catalyst after five
times recycling is obvious above 300°C, which may be due to the loss of lignin and
other components in the catalyst during recycling.
100
80
60
a
b
40
c
20
0
100
200
300
400
500
600
700
Temperature (oC)
Fig. 4. TGA of (a) original lignin; (b) lignin-ChCl-PABA; (c) lignin-ChCl-PABA
after five cycles.
The morphology of original lignin and the obtained lignin-ChCl-PABA catalyst
was characterized by SEM (Figure 5). As could be seen, the original lignin exhibited
as irregular plates but the lignin-ChCl-PABA catalyst exhibited layered structure with
rough surfaces in micrometer scale. Lignin-ChCl-PABA showed much looser layered
structure indicating the chemical modification decreased the irregular plates size of
lignin and could improve its dispersibility. The looser layered structure with rough
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surfaces would be helpful to improve the catalytic activity of the catalyst for CO2
chemical fixation with epoxides.
A
B
Fig. 5. The SEM images of original lignin (A) and the lignin-ChCl-PABA catalyst
(B).
3.2 Catalytic performance of DESs-modified lignin catalysts
We then examined the catalytic activity of the different DESs-modified lignin
catalysts by performing the CO2 chemical fixation with glycidyl phenyl ether (Table
1). We used glycidyl phenyl ether as a starting substrate to produce the corresponding
carbonate with CO2 (1.0 MPa) at 110oC in the presence of DESs-modified lignin
catalyst (100 mg). Low yields were observed when CO2 chemical fixation was
performed by using either lignin-ChCl/TBABr, or TBABr as catalyst (<59%, entries 1
and 2, Table 1). However, CO2 chemical fixation proceeded smoothly when using
lignin-ChCl-acid as catalyst and TBABr as co-catalyst. As shown in Table 1, the
moderate to good carbonate yields of 67-87% were obtained by the catalyst
lignin-ChCl-BA, lignin-ChCl-PMBA, lignin-ChCl-PEBA, lignin-ChCl-PNBA, or
lignin-ChCl-PABA (100 mg) after 2 hours by using TBABr as co-catalyst.
Among these heterogeneous catalysts, lignin-ChCl-PABA showed the highest
catalytic activity, which was mainly due to the positive role of PABA supported by
acid-base interaction between ChCl and PABA on the surface of lignin-ChCl-PABA.
Due to the presence of amine groups on lignin-ChCl-PABA catalyst, chemically inert
CO2 molecules can be activated. The detailed reaction mechanism is shown in the
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later mechanism section. As shown in Table 1 (entries 8 and 9), increasing the
amount of lignin-ChCl-PMBA from 50 to 100 mg increased the cycloaddition yield
from 81% to 99%. Obviously, the yield of styrene carbonate rose with increasing
catalyst amount. However, further increase in catalyst amount did not enhance the
yield of styrene carbonate apparently. Under the same experimental conditions, when
the amount of catalyst was increased to 150 mg, the yield still remained 99% (entry
10).
Table 1. Effect of different time, co-catalyst and temperature on the synthesis of
cyclic carbonate.
TBABr
Isolated
Yield
(%)
28
Entry
Catalyst
Cat. (mg)
(mmol) Time (h) Temp (oC)
1
2
None
None
100
100
100
100
100
100
50
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
3
3
3
110
110
110
110
110
110
110
110
110
110
Lignin-ChCl
59
3
Lignin-ChCl-BA
Lignin-ChCl-PMBA
Lignin-ChCl-PEBA
Lignin-ChCl-PNBA
Lignin-ChCl-PABA
Lignin-ChCl-PABA
Lignin-ChCl-PABA
Lignin-ChCl-PABA
72
4
70
5
67
6
75
7
87
8
81
9
100
150
99
10
99
a Reaction conditions: glycidyl phenyl ether (10.0 mmol), CO2 (1.0 MPa).
3.3 Effects of reaction conditions
From the data in Table 1, we can know that the catalyst lignin-ChCl-PABA with
TBABr showed the highest catalytic activity among the tested catalysts. Therefore,
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lignin-ChCl-PABA was further investigated to get the optimized reaction conditions.
In detail, the effects of the reaction temperature and reaction time on the yields were
then discussed, and the obtained results were shown in Figure 6. As shown in Figure
6a, the reaction temperature had a positive impact on the yield of the corresponding
carbonate. The yield of the carbonate product increased with the increasing reaction
temperature in the range of 70–110°C (65%-87%) and levels off upon further
increasing temperature to 120°C (87%). The impact of reaction time under the
condition of 110oC and 1.0 MPa of CO2 could also be obtained in Figure 5b. As
shown in Figure 6b, increasing the reaction time from 1 to 2, 3, 4, and 5 hours
increased the yield of the carbonate product from 77% to 99%. Obviously, glycidyl
phenyl ether was almost completely converted into the desired carbonate within 3
hours.
90
99
99
99
100
95
90
85
80
75
87
87
85
85
80
75
70
65
81
77
87
65
77
70
80
90
100
110
120
1
2
3
4
5
Temperature (oC)
Time (h)
Fig. 6. Effect of reaction parameters on CO2 chemical fixation over the
lignin-ChCl-PABA catalyst: (a) temperature on yield; (b) reaction time on yield.
Reaction conditions: epoxide (10.0 mmol), CO2 (1.0 MPa), TBABr (1.0 mmol),
weight of lignin-ChCl-PABA (100 mg).
3.4 Cycloaddition of CO2 with various epoxides
A promising heterogeneous catalyst not only has high catalytic activity but also has
a broad substrate scope. To demonstrate the practicality and extensiveness of this
lignin-ChCl-PABA catalyst, the cyclization of different terminal epoxides with CO2
was investigated under the optimum reaction conditions. The corresponding results
are summarized in Table 2. As shown in Table 2, all of the mono-substituted terminal
epoxide substrates underwent the transformation smoothly with excellent yields of
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isolated cyclic carbonates (entries 1−10, 90-99% yields). Even for the epoxides with
long alkyl chain, the reaction could also proceed smoothly with excellent yields (91
and 96% yields, entries 5 and 6). The excellent yields are likely due to the presence of
highly electron withdrawing atoms, such as oxygen and chlorine atoms. However, for
cyclohexane oxide, due to its steric hindrance around the epoxide ring, the reaction
activity is not high, resulting in a moderate yield (entry 11, 59% yield). Aside from its
broad substrate scope and excellent catalytic performance, good tolerance towards
various functional groups (such as hydroxyl, alkenyl, halogen, alkoxy, etc.) are
remarkable features of this bio-based lignin-ChCl-PABA catalyst, which exhibiting its
potential applicability in scaled-up production of cyclic carbonates.
Table 2. CO2 fixation into cyclic carbonates catalyzed by lignin-ChCl-PABA.a
O
O
TBABr
O
+
CO2
O
R
Lignin-ChCl-PABA
R
Isolated Yield
(%)
Entry
Substrate
Product
Selectivity (%)
> 99
1
2
99
99
> 99
3
4
5
6
7
> 99
> 99
> 99
> 99
> 99
96
94
96
91
98
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8
9
> 99
>99
94
99
10
11
> 99
> 99
90
59
a
Reaction conditions: epoxide (10.0 mmol), CO2 (1.0 MPa), TBABr (1.0 mmol),
lignin-ChCl-PABA (100 mg).
3.5 Recyclability of the bio-based lignin-ChCl-PABA catalyst
A promising heterogeneous catalyst not only has the outstanding catalytic activity
but also has good reusability. Recycling experiments were conducted in order to
investigate the reusability of lignin-ChCl-PABA in the chemical fixation of CO2 into
cyclic carbonate with glycidyl phenyl ether under 110°C and 1.0 MPa. Obviously, the
catalytic activity of lignin-ChCl-PABA was well retained during the recycling test
after separated from the reaction mixture through simple filtration. As shown in
Figure 7, the catalytic activity of lignin-ChCl-PABA was not decreased after it was
used after five times with the corresponding cyclic carbonate yield around 84%. At
the same time, the selectivity of cycloaddition reaction remained constant after five
cycles, which showed lignin-ChCl-PABA have a quite stable catalytic performance.
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Yield (%)
Selectivity (%)
99
99
99
99
99
99
100
80
60
40
20
0
96
92
90
84
1
2
3
4
5
Runs
Fig. 7. Recyclability and selectivity of lignin-ChCl-PABA. Reaction conditions:
glycidyl phenyl ether (10.0 mmol), lignin-ChCl-PABA (100 mg), TBABr (1.0 mmol),
CO2 pressure (1.0 MPa), reaction time (3 h) and temperature (110 oC).
Chemical stability of heterogeneous catalyst is an important aspect in most
chemical processes. In order to evaluate the stability of the lignin-ChCl-PABA
catalyst after five times of recycling test, reused catalyst has been characterized by
FT-IR and TGA techniques. FT-IR spectra of the fresh lignin-ChCl-PABA catalyst
(Figure 8a) and the reused lignin-ChCl-PABA catalyst after five times recycling are
shown in Figure 8b. As shown in Figure 8b, the characteristic absorption bands of
-1
4-aminobenzoic acid (1603, 1508, 844, and 775 cm ) still appeared in the spectrum of
the reused lignin-ChCl-PABA catalyst, which indicates excellent stability of
lignin-ChCl-PABA after five times recycling. Compared with the fresh
-1
lignin-ChCl-PABA catalyst, the new absorption band at 1805 cm appeared, which
belongs to the bending vibration of C=O bond in cyclic carbonate product
(adsorbed in lignin-ChCl-PABA). The other FT-IR peaks of the reused catalyst were
in good agreement with the fresh catalyst. After five cycles of recycling, the FT-IR
spectrum of the recycled lignin-ChCl-PABA did not change obviously, indicating that
lignin-ChCl-PABA was very stable. Additionally, the thermal stability of the reused
lignin-ChCl-PABA catalyst after five times recycling was as good as that of the fresh
catalyst under 200oC (Figure 4b and 4c). Therefore, it did not affect its practical
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application when used at 110oC.
a
804
774
1686
1603
1508
1030
b
809
756
1700
1508
1603
1034
4000
3500
3000
2500
2000
1500
1000
500
Wavenumbers(cm-1)
Fig. 8. FT-IR spectra of lignin-ChCl-PABA (a) and the reused lignin-ChCl-PABA
after five cycles (b).
3.6 Scale-up procedure for the synthesis of cyclic carbonate
In order to further study the practicability of the bio-based heterogeneous catalyst
lignin-ChCl-PABA, the multigram-scale reaction on the lignin-ChCl-PABA mediated
model cycloaddition of glycidyl phenyl ether with CO2 was also conducted. The
procedure was carried out with lignin-ChCl-PABA in a Teflon-lined stainless-steel
reactor, and the reaction substrates were magnified 2, 4, 6, and 8 times, respectively.
The obtained results showed that the reactions proceeded smoothly without any
extension of reaction time or additional catalyst dosage, to afford an excellent product
yield (80 mmol, product 14.5 g in 93% yield, Table 3). Combining the results of the
scaled-up experiment and the advantages of the lignin-ChCl-PABA catalyst itself,
such as high catalytic activity, simple preparation method, renewable and degradable
performance, it is obvious that lignin-ChCl-PABA is expected to be applied in
industrial production.
Table 3. The results of scaled-up experiment.a
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Entry Scale (mmol)b TBABr (mmol)
Catalyst (g)
0.1
Yield (%)c
99
Product (g)
1.9
1
2
10
20
1
2
0.2
0.4
0.6
0.8
97
94
93
93
3.8
7.3
3
4
5
40
60
80
4
6
8
10.9
14.5
b
a Reaction conditions: 1.0 MPa CO2 initial pressure, temperature 110 °C, 3 h;
c
glycidyl phenyl ether; Isolated yield.
In order to examine the activity and capability of the present catalytic system, we
compared the catalytic activities of lignin-ChCl-PABA in the CO2 chemical fixation
into cyclic carbonate with other reported catalysts (Table 4). As shown in Table 4, the
previously reported synthetic methods suffer from one or more shortcomings, such as
longer reaction time (6 h), higher reaction temperature and pressure (150oC, >1.0
MPa), lower yield of product (< 90%). Furthermore, the homogeneous properties of
the catalytic systems 1-3 in table 4 make them difficult to separate from the product,
limiting their application in industry. It is obvious that our present catalyst
lignin-ChCl-PABA shows promising features in terms of high activity (3 h), excellent
yield (90%), and mild reaction conditions (110oC, 1.0 MPa).
Table 4. Comparison of catalytic activity of lignin-ChCl-PABA with several known
catalysts.
Entry
Catalyst
Condition
Yield (%)
Ref.
73
1
2
EDS (L-pro/PA/ZnBr2)
EDS (ChCl-PEG200)
150oC/5 h/1.2MPa
150oC/5 h/0.8MPa
57
82
74
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3
4
5
(EMIL)2ZnBr4
[HTEA]I
100oC/1.0 h/3.5 MPa
110oC/6 h/2.0 MPa
110oC/3 h/1.0MPa
81
84
90
75
76
Lignin-ChCl-PABA
This work
3.7 Reaction mechanism
Combining the mechanism reported in previous literatures [62-64] and the
structural characteristics of the bio-based lignin-ChCl-PABA catalyst itself, a
plausible mechanism for the CO2 chemical fixation with epoxides over was proposed
(Scheme 3). Initially, the hydrogen bond was formed between the oxygen atom in ring
of the epoxide compound and the –OH group on the surface of lignin-ChCl-PABA,
which was obtained from the ring-opening reaction between epoxide-modified lignin
and choline chloride (Scheme 3). In the next step, the nucleophilic reagent bromine
anion attacked the β-carbon atom with low steric hindrance of epoxide, which had
been activated by hydrogen bond. Because of the presence of the –NH2 group on the
surface of lignin-ChCl-PABA, CO2 could be adsorbed and activated, resulting in easy
insertion of CO2 into the intermediate to form the corresponding carbonate
intermediate. Finally, the desired cyclic carbonate is generated by the simultaneous
intramolecular ring closure and release of bromine anion with regeneration of the
lignin-ChCl-PABA catalyst. Thus, the –OH and –NH2 groups on the surface of
lignin-ChCl-PABA played a very important role in the acceleration of the
cycloaddition reaction under solvent free conditions.
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NH2
O
O
H
H
OH
Cl
N
O
O
O
O
O
Lignin
H
R
O
+
TBABr
R
Br
Br
R
NH2
O
O
O
NH2
Br
O
R
H
H
H
O
Cl
N
H
O
O
O
H
H
Lignin
O
Cl
H
O
O
O
O
Lignin
H
N
O
Br
R
O
C
O
C O
NH2
O
O
H
H
O
Cl
N
O
O
O
Lignin
H
O
H
Scheme 3. A plausible reaction mechanism for CO2 chemical fixation with epoxide
co-catalyzed by lignin-ChCl-PABA and TBABr.
4. Conclusions
In conclusion, we have firstly reported several heterogeneous lignin-acid catalysts
through the acid-base interaction between ChCl on the surface of functional lignin and
benzoic acid derivatives, for CO2 chemical fixation into cyclic carbonates. These
lignin-based catalysts exhibited good catalytic activity for the chemical fixation of
CO2 with epoxides under mild conditions, especially for lignin-ChCl-PABA, with
excellent catalytic activity and high selectivity. The optimal reaction conditions were
110°C, 1.0 MPa CO2 pressure with 1.0 mol% catalyst amount for 3 hours. A series of
cyclic carbonates could be obtained in high yields (90-99%), and lignin-ChCl-PABA
catalyst showed good recyclability with little loss in activity even after five catalytic
runs (84% yield after 5 recycles). The present methodology provides prominent
advantages to previously reported similar catalytic systems for chemical fixation of
CO2 with epoxides including economic viability, easy preparation, and separation,
high catalytic activity, stability, and reusability. The attaching DES of ChCl/PABA
onto the functional lignin has increased the catalytic activity and also has made the
catalyst easier to recycle from the reaction mixture. The -OH and -NH2 groups on the
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surface of lignin-ChCl-PABA played a very important role in the acceleration of the
cycloaddition reaction under solvent free conditions.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of
China (21004024), the Natural Science Foundation of Fujian province (2016J01063),
the Program for New Century Excellent Talents in University of Fujian province
(2012FJ-NCET-ZR03) and the Promotion Program for Young and Middle-aged
Teacher in Science and Technology Research of Huaqiao University (ZQN-YX103).
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