A non-metal Acen-H catalyst for the chemical fixation of CO2 into cyclic carbonates under solvent- and halide-free mild reaction conditions

Article abstract of DOI:10.1016/j.apcata.2020.117646

A metal-free Acen-H catalyst was effectively synthesized by a single step reflux method and successfully tested for the cycloaddition of CO₂ with epoxides to synthesize cyclic carbonates. The Acen-H catalyst exhibited high activity and selectivity for the cycloaddition reaction in the absence of co-catalysts (halides) and solvents at mild reaction conditions of a reaction temperature of 110 °C and a reaction pressure of 1 MPa. Under the optimized reaction conditions, some of the epoxides were successfully converted into corresponding cyclic carbonates with a highest yield of ～98.5%. The composition and structure of the homogeneous catalyst was then systematically evaluated and the reaction kinetics and a plausible reaction mechanism for the cycloaddition of CO₂ with epoxides were proposed. The density functional theory (DFT) calculation provided a corroborated elucidation for the proposed mechanism. The catalytic activity of the Acen-H catalyst was found to be originated from the active hydrogen bond donors (-O-H, =N---H) and imino groups present (-N=) in it, which played a synergistic role in the adsorption and activation of reactants as confirmed by the DFT studies. The structural characteristics of the catalyst was found to be crucial for the cycloaddition of CO₂ with epoxides.

Full text of DOI:10.1016/j.apcata.2020.117646

Journal Pre-proof
A non-metal Acen-H catalyst for the chemical fixation of CO2 into cyclic
carbonates under solvent- and halide-free mild reaction conditions
Zhongxiao Yue (Conceptualization) (Investigation) (Methodology)
(Formal analysis) (Data curation) (Writing - original draft), Manoj
Pudukudy (Writing - review and editing) (Supervision) (Funding
acquisition), Shiyu Chen (Writing - review and editing), Yi Liu
(Writing - review and editing), Wenbo Zhao (Writing - review and
editing), Junya Wang (Writing - review and editing), Shaoyun Shan
(Resources) (Writing - review and editing) (Supervision) (Funding
acquisition) (Project administration), Qingming Jia (Resources)
(Writing - review and editing)
PII:
S0926-860X(20)30239-8
DOI:
https://doi.org/10.1016/j.apcata.2020.117646
APCATA 117646
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
14 March 2020
12 May 2020
18 May 2020
Please cite this article as: Yue Z, Pudukudy M, Chen S, Liu Y, Zhao W, Wang J, Shan S, Jia
Q, A non-metal Acen-H catalyst for the chemical fixation of CO2 into cyclic carbonates under
solvent- and halide-free mild reaction conditions, Applied Catalysis A, General (2020),
doi: https://doi.org/10.1016/j.apcata.2020.117646

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A non-metal Acen-H catalyst for the chemical fixation of CO2 into cyclic
carbonates under solvent- and halide-free mild reaction conditions
Zhongxiao Yue a, Manoj Pudukudy a, b, Shiyu Chen a, Yi Liu a, Wenbo Zhao a, Junya Wang b,
Shaoyun Shan a, *, Qingming Jia a, 1
a Faculty of Chemical Engineering, Kunming University of Science and Technology, Kunming,
Yunnan, 650500, China
b Faculty of Environmental Science and Engineering, Kunming University of Science and
Technology, Kunming, Yunnan, 650500, China
*Email: huagong327@163.com; shansy411@163.com
(1 Deceased author, Date of Death: 25th November 2019)
Graphical Abstract
Highlights

A metal-free Acen-H catalyst was successfully synthesized by a single step reflux
method.
1


The Acen-H catalyst exhibited high catalytic activity for the cycloaddition of CO2 with
epichlorohydrin.


The reactions were perfromed in a solvent- and halide-free reaction condition.
High epoxide converison and product selectivity & yield were achieved at 1.0 MPa
and 110 °C.

A plausible reaction mechanism validated through DFT simulation was proposed.
Abstract
A metal-free Acen-H catalyst was effectively synthesized by a single step reflux method
and successfully tested for the cycloaddition of CO2 with epoxides to synthesize cyclic carbonates.
The Acen-H catalyst exhibited high activity and selectivity for the cycloaddition reaction in the
absence of co-catalysts (halides) and solvents at mild reaction conditions of a reaction temperature
of 110 °C and a reaction pressure of 1 MPa. Under the optimized reaction conditions, most of the
epoxides were successfully converted into corresponding cyclic carbonates with a highest yield of
~98.5%. The composition and structure of the homogeneous catalyst was then systematically
evaluated and the reaction kinetics and a plausible reaction mechanism for the cycloaddition of CO2
with epoxides were proposed. The density functional theory (DFT) calculation provided a
corroborated elucidation for the proposed mechanism. The catalytic activity of the Acen-H catalyst
was found to be originated from the active hydrogen bond donors (-O-H, =N---H) and imino groups
present (-N=) in it, which played a synergistic role in the adsorption and activation of reactants as
confirmed by the DFT studies. The structural characteristics of the catalyst was found to be crucial
for the cycloaddition of CO2 with epoxides.
Keywords: CO2 conversion; Cycloaddition; Cyclic carbonates; Acen-H catalyst; Metal-free
catalysis; Reaction kinetics
1. Introduction
Due to carbon dioxide’s (CO2) impact on global warming, the increased emission of CO2 has
become a major environmental problem, which is recognized by the international community [1].
2

This fact has become a vital driving force for the reduction of greenhouse gas emission for the
development of a sustainable environment. With an emphasis on the carbon oxides-based pollution
and the development of synthetic fuel chemistry, the utilization of CO2 has attracted much attention
in academia and industrial chemists [2]. The environmental impacts of CO2 can be minimized by
its utilization as it can be chemically fixed into highly value-added products and fuels [3]. Among
them, cyclic carbonates [4, 5] and polycarbonates [6, 7] are the successful examples of chemically
fixed CO2.
Currently, there are different ways to synthesize cyclic carbonates from CO2. It includes the
chemical reaction of CO2 with different compounds such as epoxides [4, 5], ethylene glycol and its
derivatives [8, 9], etc. The selection of epoxides as highly active substrates was reported to reduce
the energy demand as it overcame the thermodynamic and kinetic limitations of inert CO2 [4, 10].
Therefore, the synthesis of cyclic carbonates by the ring-opening addition of CO2 with epoxides is
of great importance, which owns the characteristics of simple synthesis and cost-effectiveness
without the formation of by-products. The benefits of this synthesis mainly lie on two aspects: On
the one side, it adheres to the initiation of green chemistry and low carbon economy; on the other
side, it can be widely applied to the production of highly value added chemicals [11, 12]. Cyclic
carbonates has witnessed their valuable applications in industry, because of their unique
characteristics, such as high dipole moment, boiling point, dielectric constant, stability, low toxicity,
and biodegradability [13, 14]. It is widely used not only in the synthesis of fine chemical
intermediates for biomedical precursors and agricultural chemicals [15], but also the synthesis of
novel materials, such as heat recording materials, phenolic resins, thermosetting resins [16],
polycarbonate [7], polyglycerol [17], polyurethane monomers [18], inert non-proton polar solvents
[19], high energy density batteries, capacitor electrolytes [20] and so on. Therefore, the synthesis
of cyclic carbonates is a hot topic in the environmental catalysis, especially, it has attracted much
attention on academia and industry.
The reactions for the synthesis of cyclic carbonates from CO2 and epoxides are mostly focused
on the assistance of metal-based catalysts, which mainly consists of metal organic frameworks [21],
metal porphyrins [22], metal oxides [23], metal ionic liquids [20] and metal Salen or Salophen
complexes [4, 24, 25], etc. The metal-based homogeneous catalysts were reported to have a good
solubility in the reaction mixture and exhibited high efficiency and selectivity in the ring-opening
3

addition of CO2 with epoxides. Some shortcomings of these homogeneous catalysts are that they
would be easily degraded and inactivated in the reaction, and they would be more costly (e.g. metal
based ionic liquids) [26-28]. Thus, exploiting the metal-free Schiff base catalysts is considered to
be of great value in catalysis.
The Schiff base complexes have the advantages of the availability of raw materials, diverse
and adjustable structure, insensitive nature to air and water, multi-functional coordination behavior,
low melting point, relatively low cost and remarkable economic benefits [29-31]. However, in
addition to the use of metal complexes, most of the catalytic systems have used halides as
nucleophilic reagents. In the process of industrialization, there is an obvious defect that
corrosiveness is caused to stainless steel reactor and other related parts [26, 32]. Therefore, it is
imperative to develop highly efficient metal- and halide free Schiff base catalysts.
The catalytic reaction with metal-based catalysts has shown that the Lewis acid provides sites
for the co-ordination of oxygen in the epoxides, and promotes their ring-opening under the
nucleophilic attack by halides [5]. The polarization of epoxides can be generated by hydrogen
bonding to some extent, and the cycloaddition gets promoted. There are many hydrogen bond donor
(HBD) groups in the catalysts such as -OH [33, 34], -NH2, -COOH [35, 36] and so on. These groups
can co-ordinate with the O of epoxide to form hydrogen bondings [26, 37]. Such catalysts can
effectively replace the participation of metal ions, and confirm the concept of greener chemical
production and the strategy of sustainability [38]. Therefore, the synthesis of catalysts for the
activation and ring-opening of the substrates is a key point herein.
Based on the above viewpoints, in this study, a metal-free Schiff base homogeneous catalyst
(Acen-H) was synthesized by a one-step reflux reaction by the condensation of 1,2-diphenyl-1,2-
ethylenediamine with 2,4-pentanedione and their catalytic activity for the potential of cycloaddition
of CO2 with epoxides was explored. It exhibited remarkable catalytic performance for the
carbonation reaction, especially for ring carbonation, under halide and solvent-free conditions at
mild temperatures and CO2 pressures (Scheme 1). Moreover, the effects of ratio of catalyst to
epichlorohydrin (ECH), reaction temperature, time and initial CO2 pressure on the conversion,
selectivity and yield have been investigated. On the other hand, the reaction kinetics and a plausible
reaction mechanism combined with the density functional theory (DFT) were reasonably explained.
4

O
Acen-H catalyst
（ 0.1 mmol）
110 oC, 1 MPa
O
CO
+
O
O
2
R
R
Scheme 1. Schematic representation of the synthesis of cyclic carbonates from epoxides and
CO2.
2. Experimental
2.1. Materials
Salicylaldehyde (AR, 98%), (1R,2R)-(+)-1,2-diphenylethylenediamine (99%), 2,4-
pentanedione (AR, 99%), 3,5-di-tert-butylsalicylaldehyde (98%), epichlorohydrin (ECH) (AR),
propylene oxide (99%), ethylene oxide (99%), glycidol (96%), 1,2-butylene oxide (>99%), styrene
oxide (98%), 1,2-epoxyhexane (96%), isobutylene oxide (>97%), cyclohexene oxide (98%) and
chloroform-d (CDCl3) (D, 99.8%) were purchased from Aladdin Industrial Inc. (Shanghai, China).
CO2 (99.999% purity) was purchased from Chengdu Jingkexin Gases Co., Ltd. and ethanol (AR)
was obtained from Xilong Scientific Co., Ltd. (Sichuan, China). All reagents obtained from the
commercial resources were used without further purification unless otherwise noted.
2.2. Synthesis of homogeneous Acen-H catalyst
A single step method was used to prepare the Acen-H catalyst. A solution of (1R,2R)-(+)-1,2-
diphenylethylenediamine (3.1346 g, 0.0148 mol) in absolute ethanol (80 mL) was taken in a two-
neck round bottom flask (250 mL) and placed in an oil bath with magnetic stirring. A reflux
condenser and a separatory funnel was then placed on the above-mentioned flask. Next, 2,4-
pentanedione (3.07 mL, 0.0295 mol) was dissolved in absolute ethanol (50 mL). The reaction
temperature of the first mixture was increased to 75 °C, then the prepared ethanolic solution of 2,4-
pentanedione was slowly added into the above solution, stirred and heated at 75 °C. The reaction
was performed for 24 h unless it was noted separately. When the reaction was completed, the
reaction mixture was cooled to room temperature, transferred into a one-necked flask (250 mL) and
placed on a rotary evaporator to evaporate the solvents. The formed product was then transferred
into a vacuum oven and dried at 55 °C for 6 h. The product obtained in the form of an amber viscous
solid was named as Acen-H catalyst (VII) (Fig. 5). The schematic diagram of the synthetic route is
5

shown in Scheme 2. Additionally, two Salophen-H ligands (V, VI) (Fig. 5) were also prepared by
the same method [39].
H
O
O
O
O
Ethanol， Reflux
2
+
N
O
N
O
H2N
NH2
75 oC
H
H
(1R,2R)-(+)-1,2-diphenyl-
ethylenediamine
2,4-Pentanedione
Acen-H catalyst
Scheme 2. Schematic representation of the synthesis of metal-free homogeneous Acen-H catalyst.
2.3. Characterization of the prepared samples
Fourier transform infrared spectroscopy (FT-IR) (Bruker Vertex 70 V, Germany) was used to
record the FT-IR spectra of the samples in the wave number range of 4000-500 cm-1. The nuclear
magnetic resonance (1H-NMR, 13C-NMR) spectral analysis was carried out in a Bruker Advance III
HD 600, Bruker BioSpin, Switzerland using CDCl3 as a solvent and TMS as internal standard. The
elemental analysis was performed by using an Elementar Vario EL cube (EA) elemental analyzer
(Elementar, Germany) to determine the elemental contents of C, H and N in the samples. The
thermogravimetric analysis (TGA) was carried out in a thermal analyzer (NETZSCH TG 209F3) to
study the thermal stability of the prepared sample. It was measured from room temperature to 600°C
with a heating rate of 10 °C/min in nitrogen atmosphere.
2.4. Catalytic performance of the prepared samples
The catalytic reactions were carried out in a high-pressure liquid phase reactor (100 mL)
equipped with magnetic stirring and a temperature control. In a typical reaction, 0.1 mmol (0.0377
g) of catalyst and 50 mmol of epoxide were taken in a Teflon lined autoclave. Then, CO2 (99.999%)
gas was introduced into the sealed reactor three times to remove any other gases inside the reactor.
Finally, the required CO2 pressure (0.1 MPa - 3.0 MPa) was set and the reaction mixture was heated
to the desired temperature (60-120 °C) for a specific period of time. Once the reaction was
completed, the product in the reaction vessel was allowed to cool in an ice bath and the cooled
product was collected and detected by FT-IR and NMR analysis. The conversion of epoxide,
product selectivity and yield were calculated using NMR peak integration method [40, 41].
2.5. Density functional theory (DFT) calculation method
6

In order to investigate the reaction mechanism, a density functional theory (DFT) calculation
was performed using Gaussian 09 program. The interactions of the equilibrium structures of the
Acen-H catalyst and the substrates were obtained from the geometry optimization method by
combining the standard B3LYP/6-311+G (d, p) basis set. All bond lengths were expressed in
angstroms (Å) [42, 43].
3. Result and Discussion
3.1. Characterization of the catalysts
（ c）
3202
1025
1618
1452
1492
696
（ b）
（ a）
3352
3374
1720
1622
4000
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Fig. 1. FT-IR spectra of the prepared samples: (a) 2,4-pentanedione, (b) (1R,2R)-(+)-1,2-
diphenylethylenediamine, (c) Acen-H catalyst.
The functional groups on the Acen-H catalyst were studied by FT-IR spectroscopy and the
corresponding FT-IR spectra are shown in Fig. 1. The labels (a) and (b) represent the monomers of
the Acen-H catalyst. The transmission band at 1492 cm-1 could be attributed to the stretching
vibration of the C-N bond [44]. Two transmission bands observed at 1618 cm-1 and 1452 cm-1 in
Fig. 1(c) could be attributed to the stretching vibration of the >C=N and C=C functional groups in
the prepared sample. When a conjugate structure is formed in the sample, the transmission band
was found to be shifted into low frequency by 20 cm-1 [45, 46]. The bands appeared at 3202 cm-1
and 696 cm-1 were attributed to the stretching and deformation vibration of the hydroxyl group
respectively [47]. The band appeared at 1025 cm-1 was due to the stretching vibration of the carbon-
7

oxygen bond in the alcoholic group (C-OH). The band of β-diketone was not observed at 1720 cm-
1 and 1622 cm-1 in Fig. 1(c) [48]. However, a band of hydroxyl group and a carbon-carbon double
bond were observed. In addition, the transmission bands of primary amine (-NH2) were not seen in
the Acen-H catalyst at 3374 cm-1 and 3353 cm-1 whereas the band of carbon-nitrogen double bond
(C=N) appeared [47]. This indicates that a carbonyl group of 2,4-pentanedione and a primary amino
group of (1R,2R)-(+)-1,2-diphenylethylenediamine were dehydrated to form a carbon-nitrogen
double bond (C=N), i.e. secondary ketoimide.
8
9
7
6
4
10
N
4
3
1
5
2
N
O
H
H
O
6,7,8,9,10
3,4,5,6,7
1
3
(c)
5
5
4
3
6
7
2
H2N
NH2
1
1
(b)
2
H
3'
O
O
1'
O
O
2
1'
1
1
2'
1'
(a)
2'
1
2
10
9
8
7
6
5
4
3
2
1
0
Chemical Shift / ppm
Fig. 2. 1H-NMR spectra of the prepared samples: (a) 2,4-pentanedione, (b) (1R,2R)-(+)-1,2-
diphenylethylenediamine, (c) Acen-H catalyst.
In order to study the structural features and changes during the catalyst synthesis, 2,4-
pentanedione, (1R,2R)-(+)-1,2-diphenylethylenediamine and Acen-H catalyst were characterized
by 1H-NMR spectral analysis in CDCl3 (Fig. 2). In Fig. 2(a), the tautomerism of 2,4-pentanedione
composed of a ketonic and an enol configuration was presented. The enol configuration can form
intramolecular hydrogen bond. The active H on the hydroxyl group is unstable, therefore it is not
1
seen clearly in the H-NMR spectra and its chemical shift was about 15.4 ppm [48, 49]. The
chemical shifts at 5.47 and 1.99 ppm were found in Fig. 2(a), which indicated the enol structure of
8

2,4-pentanedione's hydrogen atoms at H2’and H1’positions, respectively. In addition, the chemical
shifts at 3.56 and 2.19 ppm with low peak intensity found in Fig. 2(a) represented the ketonic
structure of 2,4-pentanedione's hydrogen atoms at H2 and H1 positions, respectively [48, 50]. In
1
Fig. 2(b), the H-NMR spectra have resonances at 4.11 and 1.63 ppm, which are assigned to the
hydrogen atoms (H1 and H2) of the (1R,2R)-(+)-1,2-diphenylethylenediamine, respectively. The
1H-NMR signals marked from H3 to H7 in the benzene ring were found to be located in the region
of 7.32-7.22 ppm, as they are in similar chemical environment [51]. In Fig. 2(c), the chemical shifts
at 4.72, 2.17, 2.12 and 1.54 ppm were corresponded to the hydrogen atoms H3, H1, H4 and H5 of
the Acen-H catalyst, respectively [45, 52]. The signal of H2’ in Fig. 2(a) was not positioned at the
same chemical shift value in the Acen-H catalyst (Fig. 2(c)). It was appeared at 4.72 ppm. This shift
could be attributed to the influence of different neighboring chemical environment [45]. The signal
of H1 position at 4.11 ppm in Fig. 2(b) was not detected in Fig. 2(c), which suggests the
condensation of carbonyl group (C=O) and primary amino group (-NH2) had taken place [53, 54].
1
By comparing with the H-NMR signals of (b) and (c), it was observed that the chemical shifts
marked from H6 to H10 in the benzene ring of (1R,2R)-(+)-1,2-diphenylethylenediamine were
observed to be shifted to 7.39-7.19 ppm (Fig. 2(c)).
9
8
7
10
11
12
CDCl
10, 9/11, 8/12
(c)
3
5
6
N
O
N
O
4
3
2
H
H
5
1
7
1
3
4
6
4
3
2
5
(b)
CDCl
6
3
5, 4/6, 3/7
7
1
H2N
NH2
1
2
H
O
O
O
O
1'
(a)
2
2
1
1
3
2'
2'
1'
1'
3'
3'
CDCl
2'
2
3
3
1
200 180 160 140 120 100
80
60
40
20
0
Chemical Shift / ppm
9

Fig. 3. 13C-NMR spectra of the prepared samples: (a) 2,4-pentanedione, (b) (1R,2R)-(+)-1,2-
diphenylethylenediamine, (c) Acen-H catalyst.
On the other hand, the 13C-NMR spectra of the 2,4-pentanedione, (1R,2R)-(+)-1,2-
diphenylethylenediamine and Acen-H catalyst were characterized in CDCl3 (Fig. 3). In Fig. 3(a),
the 13C-NMR spectra have strong resonances at 191.18, 100.40 and 24.75 ppm, which are assigned
to the carbon atoms C2’, C3’ and C1’ in enol configuration of 2,4-pentanedione, respectively [55].
Resonances at 202.16, 58.43 and 30.82 ppm were corresponded to the carbon atoms C2, C3 and C1
of ketonic configuration of 2,4-pentanedione, respectively [55]. In Fig. 3(b), the 13C-NMR spectra
showed singlet resonance signals at 143.33 ppm. It could be attributed to the carbon atom marked
as C2 and the triplet strong resonance signals at 128.26, 127.07 and 126.92 ppm were assigned to
the carbon atoms C5, C4/C6, and C3/C7 in the benzene ring of (1R,2R)-(+)-1,2-diphenyl-
ethylenediamine, respectively [51]. Another strong resonance appearing at 61.88 ppm were
corresponded to carbon atom C1 of (1R,2R)-(+)-1,2-diphenylethylenediamine [51]. In Fig. 3(c), the
appearance of resonance signals at 162.95 ppm attributed to carbon atom C4 of the imine group
moiety (C=N) in the Acen-H catalyst [56]. In addition, the resonance signals of C2 in Fig. 3(a) was
not positioned at the same chemical shift value in the Acen-H catalyst (Fig. 3(c)), which indicated
that the condensation between the carbonyl groups (C=O) of 2,4-pentanedione with the primary
amino groups (-NH2) of (1R,2R)-(+)-1,2-diphenylethylenediamine had taken place [54]. The
appearance of singlet resonance peaks at 143.34 ppm correspond to the carbon atom C7 and the
triplet strong resonance peaks at 128.67, 127.47 and 126.53 ppm were assigned to the carbon atoms
C10, C9/C11 and C8/C12 in benzene ring of Acen-H catalyst, respectively [51]. Other resonance
signals appearing at 61.87, 30.55, 23.23 and 15.33 were assigned to the carbon atoms C6, C3, C1
and C5 of the Acen-H catalyst, respectively [51]. In particular, the chemical shifts of C3, C1 and
C5 in the Acen-H catalyst shifted greatly from the corresponding carbon atoms in Fig. 3(a). Based
on the above characterization results of FT-IR, 1H-NMR and 13C-NMR, the structure of the Acen-
H catalyst is confirmed as shown in Fig.5.
The thermal stability of the Acen-H catalyst was examined by thermogravimetric analysis. The
TGA-DTA curves are shown in Fig. 4. Clearly, the Acen-H catalyst exhibited two stages of weight
loss. There is an initial weight loss about 6.33% in the temperature range of 35-152 °C, this may be
10

due to the loss of water molecules or other gases adsorbed on the sample. However, a sharp drop in
the curve was clearly observed in the range of 154-279 °C, indicating that the catalyst had a major
weight loss (about 99.60%), and its weight loss is attributed to the thermal degradation of Acen-H
catalyst. However, when the temperature exceeds 280 °C, the weight loss of the Acen-H catalyst
reached to zero, which indicates that the Acen-H catalyst was completely decomposed at 280 °C.
Therefore, the Acen-H catalyst can be used in its stable form below this temperature for its catalytic
applications.
step I 6.33%
0
100
80
60
40
20
0
-5
Wt (%)
dTG/dT(%/min) -10
step II 92.36%
-15
-20
-25
-30
0
100
200
300
400
500
600
Temperature (°C)
Fig. 4. TGA/DTG curves of the Acen-H catalyst.
3.2. Catalytic performance for cyclic carbonate synthesis
O
O
O
O
HO
t-Bu
HO
H2N
NH2
t-Bu
I
II
III
IV
N
N
N
N
N
N
t-Bu
OHHO
t-Bu
OHHO
V
OH
HO
t-Bu t-Bu
VI
VII
Fig. 5. Structures of the catalyst monomers (I-IV), Salophen-H ligands (V, VI) and Acen-H
catalyst (VII).
11

In this work, we used epichlorohydrin (ECH) as a model substrate to compare the performance
of the samples shown in Fig. 5 for CO2 cycloaddition to prepare 3-chloropropylene carbonate, as
shown in Table 1. The results show that the no conversion of ECH was observed without any
catalyst (Table 1, entry 1). The precursors such as salicylaldehyde (I), 3,5-di-tert-
butylsalicylaldehyde (II), 2,4-pentanedione (III), (1R,2R)-(+)-1,2-diphenylethylenediamine (IV)
and Salophen-H ligands (V, VI) were found to have no effect in the reaction. However, under the
same conditions, the Acen-H catalyst (VII) exhibited an ECH conversion of 99% (Table 1, entry 8).
It was found that the monomers having hydroxyl, aldehydic, carbonyl or amino group could not
exhibit any catalytic activity. Moreover, even the Salophen-H ligands (V, VI) and the Acen-H
catalyst (VII) contains both -OH and -N=C groups, the Salophen-H ligands (V, VI) does not showed
any catalytic activity, while the Acen-H catalyst (VII) showed high catalytic activity. This could be
explained on the basis of high π delocalization effect in the Salophen-H ligands. Because of the
high delocalization of π electrons, the O in the phenolic group will be more biased towards benzene
ring [57-59] and it will be less susceptible for hydrogen bonding to activate the epoxides [26].
Additionally, the Salophen-H ligand (VI) has large steric hindrance due to t-butyl group
adjacent to the phenolic group, which has an inhibitory effect on the catalytic activity [26].
Therefore, the -OH groups in the Salophen-H ligands (V, VI) were not favorable for hydrogen
bonding with the oxygen of the epoxide. In the absence of metals and co-catalysts or halides, the
tert-butyl groups on the ligands exhibited an inhibition in the catalytic activity for the reaction to
produce cyclic carbonates [26, 60]. However, in the synthesis of polycarbonates, although the ortho
substitution of tert-butyl can inhibit the catalytic activity of Salen and Salophen ligands, it can
increase the solubility of the catalyst in the reaction mixture [26, 61, 62]. Moreover, it could stabilize
the active metal linked to Salen and Salophen species against their decomposition in the reaction
mixture for a better performance for an improved catalytic activity for the selective production of
polycarbonates [60, 62, 63].
There are tautomer of enol and ketone in the Acen-H catalyst, and the enol configuration can
form intramolecular hydrogen bonds [48-50]. The alcoholic group connected to the carbon-carbon
double bond in the olefin has less binding with the π electron cloud and thereby increases the density
of the -OH electron cloud [64]. Therefore, the -OH acts as a hydrogen bond donor to facilitate
bonding with O in the epoxide to form a hydrogen bond and thereby polarizes the epoxide and
12

consequences the ring opening. In addition, the imino group in the Acen-H catalyst has a synergistic
effect with -OH that also acts as a hydrogen bond donor, which enhances its catalytic performance
to catalyze the cycloaddition of CO2 with ECH.
Table 1. The results of ring-opening addition reaction of CO2 and ECH under different catalysts
Entry
Catalyst
Conv.
(%) [c]
Trace
Trace
Trace
Trace
Trace
Trace
Trace
Yield
Select. TON [d]
(%)
0
(%)
1 [a], [b]
2 [a], [b]
3 [a], [b]
4 [a], [b]
5 [a], [b]
6 [a], [b]
7 [a], [b]
blank
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
0
0
0
0
0
0
0
salicylaldehyde (I)
0
3,5-di-tert-butylsalicylaldehyde (II)
2,4-pentanedione (III)
0
0
(1R,2R)-(+)-1,2-diphenylethylenediamine (IV)
N,N'-diphenylidene bis (salicylideneimine) (V)
N, N'-diphenylidene bis (3,5-di-tert-
butylsalicylideneimine) (VI)
0
0
0
8 [a]
N,N'-diphenylidene bis(2-hydroxy-diene
deneimine) (VII)
99.0
98.5
99.4
495.2
[a] Reaction conditions: catalyst (0.1 mmol), ECH (50 mmol), temperature 110 °C, CO2 pressure 1
MPa, time 4 h, 280 rpm magnetic stirring rate, [b] Reaction conditions: catalyst (0.1 mmol), ECH (10
mmol), temperature 120 °C, CO2 pressure 2 MPa, time 12h, 280 rpm magnetic stirring rate, [c]
Determined by 1H-NMR spectroscopy using TMS as an internal standard, [d] TON (turnover number)
= molar amount of initial substrate * conversion / molar amount of catalyst.
The effects of molar ratio of ECH to Acen-H catalyst, reaction temperature, time and initial
CO2 pressure on the reaction efficiency were further investigated. The conversion and turnover
number (TON) of ECH and the yield and selectivity of 3-chloropropylene carbonate under various
reaction conditions are shown in Fig. 6. Firstly, the effect of the molar ratio of ECH to Acen-H
catalyst was investigated, as shown in Fig. 6(a). We had selected mild reaction conditions, that is,
the catalytic reaction was carried out at 80 °C under 1 MPa initial CO2 pressure for a period of 240
min. As expected, when the molar ratio increases, the conversion and TON of ECH and the yield
and selectivity of the product showed a drastic decrease. From this study, we have selected an
ECH/Acen-H catalyst molar ratio of 500 for the next step, i.e. to study the effect of reaction
13

temperature, as shown in Fig. 6(b). With the addition of 0.1 mmol of Acen-H catalyst and an
increase of the reaction temperature from 60 °C to 110 °C, the conversion was increased from 11.5%
to 99.0%, the yield was increased from 10.5% to 98.5%, and the selectivity was increased from
91.3% to 99.4%, respectively, which could indicate that the catalytic reaction is an endothermic
reaction and it plays an important role in the cycloaddition [2]. This might be attributed to the fact
that the cyclic carbonates are thermodynamically more stable than the corresponding
polycarbonates. Moreover, an increase in the reaction temperature up to an optimal level was
observed to be highly beneficial for the enhancement of cyclic carbonate selectivity, as a small
amount of polycarbonate was formed at low reaction temperatures. With an increase in reaction
temperature, the polycarbonate is expected to be undergoing an intra-molecular ring “backbiting”
mechanism, which further leads to the formation of cyclic carbonates [2, 31].
However, when the reaction temperature was increased to 120 °C, the corresponding
conversion (99.2%) remained more or less same as in 110 °C, while the yield and selectivity of the
product was decreased to 97.6% and 96.9%, respectively. This may be due to the formation of other
by-products in the reaction such as polycarbonates, polyethers and ketones, possibly formed by the
isomerization of epichlorohydrin (chloracetone) [65, 66]. Therefore, we have selected 110 °C as the
optimal temperature for the reaction giving high reactant conversion and product yield and
selectivity. From this observation, we can see that increasing the temperature more than 120 °C is
not worth, concerning the conversion, yield and product selectivity of the cycloaddition reaction.
As a consequence, 110 °C was found to be more suitable for the cycloaddition reaction over the
Acen-H catalyst.
The reaction time was further examined using the above ECH to Acen-H catalyst ratio at
110 °C as shown in Fig. 6(c). When the reaction time was increased from 60 minutes to 240 minutes,
the corresponding ECH conversion was increased from 74.6% to 99.0%, the yield increased from
72.1% to 98.5%, and the selectivity increased from 96.6% to 99.4% respectively. This indicates that
the duration of reaction has a positive effect on the cycloaddition, and the optimal reaction time was
240 min. Under the above optimized reaction parameters, the effect of initial CO2 pressure on
catalytic performance was investigated as shown in Fig. 6(d). When the CO2 pressure increased
from 0.1 MPa to 1.0 MPa, the reaction showed a positive effect, and a maximum conversion of
99.0%, a yield of 98.5%, and a selectivity of 99.4% was obtained at 1.0 MPa. As the CO2 pressure
14

increases, the reaction had a negative effect. Similar results were previously reported for other
catalysts [2, 47, 67]. It might be due to difference in the concentration gradient of the reactants in
contact with the catalyst at high CO2 pressures [2]. There are three phase states in the catalytic
system, i.e. the upper CO2 enriched gas phase, the catalyst enriched phase, and the lower ECH
enriched liquid phase. The catalytic reaction is expected to be mainly carrying out at the ECH-
catalyst interface, and there will be a competitive relationship between these three phases. As the
initial CO2 pressure increases upto an optimal level, the CO2 concentration in the liquid phase of
ECH increases and results in a significant increase in the yield of 3-chloropropylene carbonate.
However, when the initial CO2 pressure far exceeds, the conversion of ECH decreases, and the yield
of 3-chloropropylene carbonate also decreases. This might be attributed to the fact that the high
CO2 pressure makes the liquid-phase ECH around the catalyst could be more diluted by the gas-
phase CO2, so that the liquid-phase ECH cannot fully contact with the catalyst. Due to this, the ECH
would not be fully activated by the catalyst and is not conducive to participate in the cycloaddition
reaction [2, 47, 67].
In order to verify the catalytic performance of the Acen-H catalyst, its activity was compared
with some of the previously reported Schiff base catalysts and the results are summarized in Table
2. Most of the reported catalysts were found to be focused on metal complexes (Table 2, entry 1-8)
and used halides (Table 2, entry 1-6, 9) and the catalytic reactions were carried out in solvents (Table
2, entry 1-2). Although the aforesaid catalysts showed good catalytic activity, some of these metal-
based catalysts showed low product yield and selectivity in the reaction. Additionally, the metal-
based catalysts are toxic in nature and are more susceptible to oxidation, sensitive to hydrolysis,
and they would be easily degraded and inactivated in the reaction. Additionally, the use of metal-
based catalysts could lead to the leaching of metal ions in the reaction mixture and the contamination
of the products, while the use of halides or co-catalysts causes the corrosion of the equipments [26-
28]. This is not in line with the concept of green chemistry and economic benefits. In 2019, Wu et
al. [26] reported a homogeneous Salophen-H ligand called N,N'-Phenylenebis(5-tert-
butylsalicylideneimine) for this reaction. Under meta-, halide- and solvent-free reaction condition,
the conversion of ECH and the product yield reached to 84% and 76%, respectively (Table 2, entry
10), at a reaction temperature of 120 °C, a CO2 pressure of 1.0 MPa for a period of 210 minutes.
Our Acen-H catalyst also exhibited high efficiency in the absence of metals, co-catalysts and
15

solvents at 110 °C, a CO2 pressure of 1.0 MPa for a reaction time of 210 minutes. The conversion
was found to be 96.0% and the yield was 95.1%. When the reaction time was increased to 240
minutes, the conversion and yield reached to 99.0% and 98.5%, respectively (Table 2, entry 11).
Therefore, it can be concluded that our Acen-H catalyst shows significant and comparable activity
with metal-based catalysts and can be utilized in the absence of co-catalysts or halides and solvents
under mild reaction conditions. It validates the potential of Acen-H catalyst for the chemical fixation
of CO2 into cyclic carbonates.
(a) 100
100
(b)
90
90
80
80
70
60
50
40
30
20
10
0
Selectivity (%)
Conversion (%)
Yield (%)
70
60
50
40
30
20
10
0
Selectivity (%)
Conversion (%)
Yield (%)
2000 1800 1600 1400 1200 1000 800 600 400 200
ECH/Cat. (mol/mol)
60
70
80
90
100
110
120
Temperature (°C)
(c) 100
100
95
90
85
80
75
(d)
95
90
85
80
75
70
Selectivity (%)
Conversion (%)
Yield (%)
Selectivity (%)
Conversion (%)
Yield (%)
0.0
0.5
1.0
1.5
2.0
2.5
3.0
60
90 120 150 180 210 240 270 300
Pressure (MPa)
Time（ min）
Fig. 6. Effect of reaction parameters for CO2 cycloaddition reaction: (a) effect of ECH/catalyst
molar ratio [Acen-H catalyst: 0.1 mmol, epichlorohydrin (ECH): 20-200 mmol, reaction time 240
min, temperature: 80 °C, initial CO2 pressure: 1 MPa], (b) effect of reaction temperature [Acen-H
catalyst: 0.1 mmol, ECH: 50 mmol, reaction time 240 min, initial CO2 pressure: 1 MPa], (c) effect
of reaction time [Acen-H catalyst: 0.1 mmol, ECH: 50 mmol, temperature: 110 °C, initial CO2
pressure: 1 MPa], (d) effect of initial CO2 pressure [Acen-H catalyst: 0.1 mmol, ECH: 50 mmol,
reaction time 240 min, temperature: 110 °C].
16

Table 2. A comparison of the activity of Schiff base systems for the catalytic ring-opening
addition of CO2 with ECH under different reaction condition.
Entry
Catalyst/ Co-Cat./ Solvent
Reaction conditions
Conv.
(%)
Yield
(%)
96
TON TOF
Ref.
[68]
[40]
[24]
[25]
1
2
3
4
V IV (O)salen (0.01 mol)/ TBAI
(0.01 mol)/ MEK (5.0 mL)
Zn(OH-salC2NH2Am)(0.3 mol%)/
TBAB (10 mg)/ DMF (1.0 mL)
[PS-Zn(II)L] (0.028 mmol)/
TBAB(0.083 mmol)
45 °C, 1.0 MPa, 18 h
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
100 °C, 0.1 MPa, 16
h
n.d.
100
n.d.
96
≥99
99
RT, 0.1 MPa, 8 h
Zn(salphen)-OCH3
100 °C, 2.0 MPa, 4 h
(0.1 mmol)/ N-methylhomo-
piperazine
5
6
7
Zn-CMP(0.1 mol %)/ TBAB
(3.6 mol%)
120 °C, 3 MPa, 1 h
26 °C, 1.0 atm, 24 h
n.d.
100
96.4
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
[69]
[45]
[70]
Bimetallic Al(acen) (2.5 mol%)/
TBAB (2.5 mol%)
Mono-metallic Al(salen) (0.03
mmol)-ISA-1
100 °C, 1.0 MPa,
0.75 h
97(74) 90(68)
240
(181)
n.d.
8
9
Co-Salen-TBB-Py (0.05 g)
Cellulose-based salen complex
(Cell-H2L)(0.57 mmol)/ TBAB(0.28
mmol)
80 °C, 0.5 MPa, 8 h
100 °C, 3.0 MPa, 6 h
100
99
n.d.
>99
n.d.
n.d.
[44]
[71]
n.d.
10
11
N,N′-Phenylenebis(5-tert-
butylsalicylideneimine) (1 mol%)
N,N'-diphenylene bis(2-hydroxy-
diene deneimine) (VII) (0.1 mmol)
120 °C, 1.0 MPa, 3.5
h
84
76
n.d.
n.d.
[26]
110 °C, 1.0 MPa, 4 h
99.0
98.5
495.2 123.8 This
work
Substrate: epichlorohydrin (ECH); TBAI: Tetrabutylammonium iodide; TBAB: Tetrabutylammonium bromide;
MEK: 2-buta-none; DMF: N, N-Dimethylformamide; CMP: conjugated microporous polymers; [PS-Zn(II)L]:
Zinc chloride coordinated polystyrene supported Schiff base ligand [PS-H2L]; n.d. : Means not reported in the
literature.
Based on the above optimized reaction conditions, the catalytic performance of the Acen-H
catalyst was further investigated for the cycloaddition of different epoxides with CO2, and the
17

corresponding results are summarized in Table 3. The experimental data shows that among the nine
epoxides tested, the Acen-H catalyst shows better catalytic activity for four of them than the other
epoxides (Table 3, entry 1-4). Compared with other epoxides, epichlorohydrin (ECH) achieved the
highest yield in 4.0 h (Table 3, entry 1). Obviously, the activity of ECH was superior to other
epoxides. It could be related to the substitution of halide group at the end of epoxide. During the
reaction, the activation of ECH molecule produces some chloride anions (Cl-). These chloride
anions can act as a nucleophile to attack the Cβ position of the epoxide to promote its ring opening
and forms an intermediate compound containing chloride anions, which can also assist in the ring
opening of other epoxide molecules in the reaction mixture [47, 72]. In addition, the chloride of
ECH can act as an electron withdrawing group, thereby reducing the electron cloud density of the
C-O bond of ECH, which further promotes its ring-opening. However, due to the structural
symmetry of ethylene oxide and its uniform electron cloud density around the two C-O bondings
and also due to the absence of any end groups, the probability of the activation of two C-O bonds
of ethylene oxide is high, when it is came in contact with the catalyst [73, 74]. That is, the C-O bond
of ethylene oxide is relatively easy to break in the ring-opening stage of the process compared to
other epoxides, which enhances its cycloaddition reaction. Therefore, the conversion of ethylene
oxide is relatively high (Table 3, entry 3). However, as for cyclohexane oxide, no conversion was
obtained for a period of 30 h of reaction, indicating the incapability of the Acen-H catalyst for
cyclohexane oxide conversion. It could be due to the inner-ring structural effects as well as its high
steric hindrance [75, 76]. Moreover, it requires a higher activation energy to overcome the steric
hindrance to activate the ring opening (Table 3, entry 10). According to the significant variations in
the activity of the prepared catalyst for different epoxides, it can be said that the Acen-H catalyst
catalyzes the cycloaddition of CO2 with terminal epoxides containing a halide or electron-
withdrawing groups as reactants, while with the inner epoxides, it shows very low catalytic activity
at the same experimental conditions. Therefore, Acen-H catalyst is more suitable for catalyzing the
reaction with terminal epoxides containing halide or electron-withdrawing groups as reactants.
Table 3. Performance of the Acen-H catalyst for the ring-opening addition of CO2 with different
epoxides
18

Entry [a]
Substrate
Product
Time Conv. Yield
Select. TON [c]
(%)
(h)
(%) [b]
(%)
O
O
O
1
4.0
99.0
98.5
99.4
495.2
O
Cl
H3C
Cl
O
3.5
6.0
4.0
6.0
47.6
94.6
91.1
99.6
40.9
94.4
89.5
96.3
86.0
99.7
98.3
96.6
238.0
473.2
455.5
498.0
O
O
2
3
4
H3C
O
O
O
O
O
O
O
O
O
4.0
77.9
76.1
97.7
389.5
HO
O
O
HO
O
O
O
3.5
42.7
54.4
42.5
54.2
99.7
99.7
213.4
457.4
O
O
5
6
10.0
O
O
12.0
56.6
56.4
99.6
283.2
O
4.0
16.0
4.0
0
0
0
0
O
7
8
40.4
12.8
34.3
40.1
11.9
32.2
99.2
93.0
94.0
202.2
63.9
171.3
O
O
O
O
O
10.0
O
O
O
20.0
30.0
0
0
0
0
0
0
0
0
9
O
[a] Reaction Conditions: Acen-H catalyst (VII) (0.1 mmol), substrate (50 mmol), temperature
1
110 °C, CO2 pressure 1 MPa, 280 rpm magnetic stirring rate, [b] Determined by H-NMR
spectroscopy using TMS as an internal standard, [c] TON (Turnover number) = molar amount of
initial substrate * conversion / molar amount of catalyst.
3.3. Reaction kinetics and mechanism
The reaction kinetics for the cycloaddition of CO2 with ECH catalyzed byAcen-H catalyst was
further studied. The rate equation for the cycloaddition reaction was expressed as follows:
Rate = k [ECH]a[CAT]b[CO2]c
in which ‘k’ denotes the rate constant, [ECH], [CAT] and [CO2] represents the concentrations
of epoxide, Acen-H catalyst and CO2 respectively, whereas a, b and c denote the orders of the
19

corresponding reactants. Based on the assumption that the formation of cyclic carbonates follows
first order kinetics with respect to CO2 [37, 77] and the concentration of Acen-H catalyst did not
change during the reaction, the above equation can also be written as follows [77]:
Rate = kobs[ECH]a, where kobs = k[CAT]b[CO2]
Rate = -d [ECH]/dt = kobs[ECH]
where kobs represents the pseudo-first order rate constant for the conversion of ECH [78].
By the integration of above equations, the following expression can be obtained.
-In [ECH] = kobst
Additionally, to understand the reaction kinetics in the synthesis of cyclic carbonates from CO2
and epoxides, the parameters were further investigated at different reaction temperatures of 80 °C,
90 °C, 100 °C and 110 °C. The concentration of ECH with reaction time for the ring-opening at
different reaction temperatures is shown in Fig. 7(a). According to the above formula, the -ln [ECH]
was found to be linearly fitted with reaction time. The slope of the fitted line followed pseudo-first-
order kinetics with the rate constant of k at different temperatures. The resulted fitting plots are
shown in Fig. 7(b).
The rate equation, correlation coefficient R2, reaction rate constant kobs and natural logarithm
of ln kobs at different temperatures are listed in the Table 4. It is concluded that the correlation
coefficients R2 was found to be close to 1, which indicates that the reaction rate is linear with the
concentration of ECH and follows pseudo first-order kinetic reaction [52, 77]. Arrhenius equation
was further adopted to calculate the activation energy [52]. The observed rate constant is given by
the expression:
kobs = A exp (-Ea/RT)
In this formula, A: pre-exponential factor (min-1); T: absolute temperature (K); Ea: apparent
activation energy (kJ/mol); R: ideal gas constant (8.314 J∙mol-1K-1).
ln kobs = ln A-Ea/RT
In the temperature range of 80-110 °C, the data was linearly fitted by ln k with 1/T (K-1), and
the fitting equation is given by the expression y = - 4614.9x + 7.7283 (Fig. 7(c)). It was concluded
that the pre-exponential factor and apparent activation energy for the Acen-H catalyst catalyzed
ring-opening addition of CO2 with ECH were found to be A = 2.272 × 103 min-1 and Ea = 38.368
kJ∙mol-1, respectively.
20

The rate equation can be thus written as:
Rate = - d[ECH]/dt = 2.272 × 103 [ECH] exp (-38.368/(RT)).
The activation energy required for the cycloaddition of CO2 with ECH was previously reported
over a series of catalyst systems. For example, Briana et al. [79] reported an activation energy of
63.6 kJ∙mol-1 for the aforesaid cycloaddition reaction over a MIL-101-IP MOF catalyst at 298K and
313K. Wu et al. [80] reported an activation energy of 113.38 kJ∙mol-1 over a Zn-BTC-2MeIm MOF
catalyst for the reaction, performed at 363K-373K. In other work, Samanta et al. [81] reported a bi-
functional graphitic carbon nitride S-CN(UTU)-60 catalyst for the cycloaddition reaction at 333K-
383K, with an activation energy of 55.33 kJ∙mol-1. Wu et al. [82] reported an activation energy of
98.5 kJ∙mol-1 for the reaction over a bifunctional (salen)cobalt(III)-based catalyst. Thus, it is clear
that the activation energy for cycloaddition reaction catalyzed the Acen-H catalyst is quite small
compared to the previously reported metal based catalysts.
100
80 oC
110 oC
100 oC
90 oC
4
(a)
(b)
90 oC
80
60
40
20
0
kobs = 0.0131
kobs = 0.0090
100 oC
110 oC
3
2
1
0
80 oC
kobs = 0.0084
kobs = 0.0044
30
60
90 120 150 180 210 240 270
Reaction time (min)
30
60
90
120 150 180 210 240
Reaction time (min)
-4.2
-4.4
-4.6
-4.8
-5.0
-5.2
-5.4
-5.6
(c)
y
y = - 4614.9x + 7.7283
R2 = 0.9224
0.00260 0.00265 0.00270 0.00275 0.00280 0.00285
1/T (K-1)
x
Fig. 7. The cycloaddition of CO2 catalyzed by Acen-H catalyst at different reaction
temperatures: (a) the curve of ECH residual concentration versus reaction time; (b) -ln [ECH] linear
fitting relationship with reaction time; (c) Arrhenius equation linear fitting to obtain activation
21

energy and the pre-exponential factor. Reaction conditions: Acen-H catalyst (0.05 mmol), ECH (25
mmol), Pressure (CO2) = 1 MPa.
Table 4. Reaction kinetics equation and kinetic parameters at different reaction temperatures
T(°C)
110
100
90
Kinetic equation
y =0.0131x+0.6101
y=0.0090x+0.2820
y=0.0084x-0.0438
y=0.0044x-0.0658
R2
1/T(K-1)
0.0026
0.0027
0.0028
0.0028
kobs (min-1)
ln kobs
0.9585
0.9186
0.9766
0.9576
0.0131
-4.3351
-4.7105
-4.7795
-5.4261
0.0090
0.0084
80
0.0044
O
O
N
N
O
CO2
R
H
H
+
O
O
R
O
Step IV
Step I
O
C
O
O
C
O
R
H
N
N
O
N
O
O
O
H
H
O
R
H
H
O
N
O
H
R
O
O
C
O
O
C
R
Step II
Step III
O
O
C
O
R
O
O
H
O H
N
H
R
O
N
H
O
C
O
Fig. 8. Plausible mechanism for the ring-opening addition reaction of CO2 with epoxide to
synthesize cyclic carbonate over Acen-H catalyst.
A reasonable reaction mechanism is proposed for the catalytic activity. As shown in Fig. 8, the
imino group (-N=) on the Acen-H catalyst provides Lewis basic sites for the adsorption and
activation of CO2 molecule [77, 83]. The -OH group in the Acen-H catalyst acts as a hydrogen bond
donor, which facilitates the epoxide activation by attacking the oxygen in epoxide and forming a
hydrogen bond [40, 84]. The hydrogen bond formation is critical in the epoxide activation. It
polarizes the carbon-oxygen bond in the epoxide and weakens their electron cloud density. Among
them, the Cβ-O bond with less steric hindrance and low extranuclear electron cloud density can be
22

easily opened by the hydrogen bond and an activated intermediate complex would be formed [42,
+
85]. Thereafter, the oxyanion in the activated CO2 attacks the Cβ position of the epoxide (Cβ ) and
a new C-O bond would be formed, which further results in the formation of a carbonic half-ester
intermediate [26]. Moreover, the additional hydrogen-bond (=N---H) formed in between the imino
group (-N=) and H of the hydroxyl group has a synergistic effect in the polarization of C-O bond in
the epoxide [40, 85]. The formed carbonic half-ester intermediate further undergoes a
intramolecular nucleophilic attack followed by an electron transfer [83]. The oxyanion of the
epoxide moiety in the carbonic half ester intermediate further attacks the C atom of the CO2 and the
loop gets closed [86]. As a result, the effects of hydrogen bonding could be weakened and the
formed cyclic carbonate can be desorbed.
The experimental and mechanistic studies of the cycloaddition of CO2 with epoxides show that
the ring-opening of epoxide is considered as a rate control step in the cycloaddition of carbon
dioxide [10]. In addition, because of the high first ionization energy (13.97 eV) and high electron
affinity (38 eV) of CO2, it acts as a weak electron donor and a strong electron acceptor, while the
epoxide acts as an electron donor [87]. Therefore, the interaction of CO2 and epoxides can
effectively enhance the solubility of CO2 in the reaction medium [70].
23

Fig. 9. DFT studies on the intermolecular interaction of the Acen-H catalyst with CO2 and
epichlorohydrin: (a) CO2 and epichlorohydrin, (b) before optimization, (c) after optimization.
In order to validate the reaction mechanism, DFT calculations were performed using Gaussian
09 program with standard B3LYP/6-311+G (d, p) basis set. The interactions of the equilibrium
geometry of Acen-H catalyst with epichlorohydrin and CO2 were considered from the geometric
optimizations method [42, 43] and the obtained results are shown in Fig. 9. The results had shown
that the Cα-O bond in ECH increased from 1.433 Å to 1.444 Å, while the Cβ-O bond increased from
1.433 Å to 1.447 Å, and ∠OCαCβ and ∠OCβCα bond angles changed from 58.4° to 59.7° and 59.5°
respectively. This indicates that the hydrogen bond polarizes the C-O bond of ECH and promotes
its breaking and ring-opening. Among them, Cβ-O bond was easier to open. The hydrogen bond also
increased from 1.647 Å to 1.856 Å, indicating the existence of a polarized complex desorption
24

process. The O-H bond of the hydroxyl functional group in the Acen-H catalyst also increased from
0.963 Å to 0.974 Å during polarization. In addition, the linear CO2 molecule was changed to a 178.3°
curved structure from its 180° geometry. Moreover, the C-N bond was increased to 3.543 Å from
1.877 Å. This indicates that there is a desorption process from the imino group and these
calculations further validated the proposed reaction mechanism.
4. Conclusions
Compared to the commonly reported metal-based Schiff base catalysts containing a metal ion
to provide Lewis active sites, a metal-free Acen-H catalyst was successfully synthesized and used
as an excellent homogenous catalyst in the cycloaddition of CO2 with epichlorohydrin (ECH) to
produce corresponding cyclic carbonates. A high epoxide conversion and cyclic carbonate
selectivity and yield were achieved for several epoxides like epichlorohydrin, propylene oxide and
ethylene oxide over the prepared Acen-H catalyst. The optimization of reaction parameters
indicated that a maximum epoxide conversion of 99.0% with 99.4% selectivity and 98.5% yield
was obtained at a reaction temperature of 110 °C, an initial CO2 pressure of 1.0 MPa and an epoxide
to catalyst ratio of 500 for a period of 240 minutes of reaction. The Acen-H catalyst exhibited higher
catalytic activity compared to the Salophen-H ligands due to the negative effect of delocalized π
electrons in the Salophen-H ligands. The catalytic activity of the Acen-H catalyst is derived from
the active hydrogen bond donors (-O-H, =N---H) and imino groups (-N=), which played a
synergistic role in the adsorption and activation of reactant molecules. These characteristics of
functional groups in the catalyst structure can effectively replace the metal ions and halides for the
activation and ring opening of epoxides. Here, the importance of the structural characteristics of
Acen-H catalyst for the cycloaddition reaction is further illustrated.
Credit authorship contribution statement:
Z. Yue: Conceptualization, Investigation, Methodology, Formal analysis, Data curation,
Writing – Original draft preparation.
M. Pudukudy: Writing – review & editing, Supervision, Funding acquisition.
S. Chen: Writing – review & editing.
Y. Liu: Writing – review & editing.
W. Zhao: Writing – review & editing.
25

J. Wang: Writing – review & editing.
S. Shan: Resources, Writing – review & editing, Supervision, Funding acquisition, Project
administration.
Q. Jia: Resources, Writing – review & editing.
Declaration of Interest Statement:
There are no conflicts of interests to declare.
Conflicts of Interest
There are no conflicts of interests to declare.
Acknowledgements
This study was financially supported by the National Natural Science Foundation of China
(Grant No. 21766016), the Yunnan Ten Thousand Talents Plan Young & Elite Talents Project
(YNWR-QNBJ-2018-198), the Academician Workstation (2019IC002), the Talent Reserve Project
in Yunnan (2015HB014), the Kunming Science and Technology Planing Project (2019-1-G-
25318000003480), and the China Postdoctoral Science Foundation (Grant No. 2019M653845XB).
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