Copper supported on N-heterocyclic carbene-functionalized porous organic polymer for efficient oxidative carbonylation of methanol

Article abstract of DOI:10.1002/aoc.5794

A new heterogeneous catalyst for the oxidative carbonylation of methanol to dimethyl carbonate based on copper coordinated in N-heterocyclic carbene-functionalized porous organic polymer (Cu@PQP-NHC) was presented. The solid catalyst that featured relatively large surface area, hierarchical pore structure, and excellent swelling property, was prepared via a facile copolymerization reaction of tetra-vinylphosphonium salt and bis-vinylimidazolium salt, followed by successful immobilization of CuCl. Accordingly, the resulting Cu@PQP-NHC showed excellent catalytic performance for the oxidative carbonylation of methanol. A 10 mmol/l of Cu usage was sufficient for 9.3percent conversion of methanol with a high TOF number of 57 h^?1. Importantly, the catalyst was easily recovered by simple centrifugation, and could be reused up to 10 consecutive recycles without obvious loss of its initial activity. Also, the solid catalyst showed negligible Cu leaching during the recycling, and 99percent Cu species was still retained after reusing 10 times. The results in this study highlights the advantages of porous organic polymer supported NHC-Cu catalyst as a highly active and stable heterogeneous catalyst, providing a promising route for the synthesis of dimethyl carbonate.

Full text of DOI:10.1002/aoc.5794

Received: 1 February 2020
DOI: 10.1002/aoc.5794
Revised: 2 April 2020
Accepted: 20 April 2020
F U L L P A P E R
Copper supported on N-heterocyclic carbene-functionalized
porous organic polymer for efficient oxidative
carbonylation of methanol
1
,2
1,2
1,2
3
Yizhu Lei
| Meitao Fan
| Guosong Lan
| Guangxing Li
1
School of Chemistry and Materials
Engineering, Liupanshui Normal
University, Liupanshui, Guizhou, 553004,
China
A new heterogeneous catalyst for the oxidative carbonylation of methanol to
dimethyl carbonate based on copper coordinated in N-heterocyclic carbene-
functionalized porous organic polymer (Cu@PQP-NHC) was presented. The
solid catalyst that featured relatively large surface area, hierarchical pore struc-
ture, and excellent swelling property, was prepared via a facile copolymeriza-
tion reaction of tetra-vinylphosphonium salt and bis-vinylimidazolium salt,
followed by successful immobilization of CuCl. Accordingly, the resulting
Cu@PQP-NHC showed excellent catalytic performance for the oxidative car-
bonylation of methanol. A 10 mmol/l of Cu usage was sufficient for 9.3% con-
2
Guizhou Key Laboratory of Coal Clean
Utilization, Liupanshu, Guizhou, 553004,
China
3
Jingchu University of Technology,
Jingmen, Hubei, 448000, China
Correspondence
Yizhu Lei, School of Chemistry and
Materials Engineering, Liupanshui
Normal University, Liupanshui, Guizhou
−1
version of methanol with a high TOF number of 57 h . Importantly, the
catalyst was easily recovered by simple centrifugation, and could be reused up
to 10 consecutive recycles without obvious loss of its initial activity. Also, the
solid catalyst showed negligible Cu leaching during the recycling, and 99% Cu
species was still retained after reusing 10 times. The results in this study high-
lights the advantages of porous organic polymer supported NHC-Cu catalyst as
a highly active and stable heterogeneous catalyst, providing a promising route
for the synthesis of dimethyl carbonate.
5
53004, PR China.
Email: yzleiabc@126.com
Guangxing Li, Jingchu University of
Technology, Jingmen, Hubei 448000,
China.
Email: ligxabc@163.com
Funding information
Academician Workstation of Liupanshui
Normal University, Grant/Award
Number: qiankehepingtairencai
K E Y W O R D S
copper, dimethyl carbonate, N-heterocyclic carbene, oxidative carbonylation, porous organic
polymers
[2019]5604 hao; National Natural Science
Foundation of Guizhou Province, Grant/
Award Numbers: qian ke he ji chu
[2016]1133, qian ke he ji chu [2018]1414,
qiankeheLHzi [2015]7608; Scientific and
Technological Innovation Platform of
Liupanshui, Grant/Award Numbers:
52020-2017-02-02, 52020-2018-03-02
1
| INTRODUCTION
chemical that has been utilized widely as carbonylation
and methylation agent, as well as green solvent in the
[
2,3]
With the growing concerns about sustainable environ-
ment, green and eco-friendly synthesis of organic
chemicals that minimizes the use of toxic substances, has
attracted continuous and increasing attention.
Dimethyl carbonate (DMC) is an important green
synthetic chemistry.
Actually, DMC can also be used
as fuel additive and important building block for the syn-
[
4,5]
thesis of polycarbonate and polyurethane.
In virtue of
[
1,2]
the wide industrial applications, a number of synthetic
[6]
routes of DMC, including phosgenation of methanol,
Appl Organomet Chem. 2020;e5794.
wileyonlinelibrary.com/journal/aoc
© 2020 John Wiley & Sons, Ltd.
1 of 9
https://doi.org/10.1002/aoc.5794

2
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LEI ET AL.
[
7]
variety of organic reactions.[27–29] To facilitate the
product separation and catalyst recovery, a large
oxidative carbonylation of methanol with CO and O
2,
[8]
carboxylation from CO2 and methanol,
urea
have been
[9]
[10]
[30]
methanolysis
and transesterification,
number of supported NHC-M,
such as organic
inorganic silica
[
31,32]
developed. Among these methods, oxidative carbonyla-
tion of methanol is an attractive route because of the low
prices and high activity of CO and O , the good atom
polymer immobilized NHC-M,
[
33]
immobilized NHC-M,
NHC-M,
and POPs immobilized
have also been successfully developed.
[
34–37]
2
economy (87%), and the green byproduct (water).
CuCl was used an active and industrial catalyst for
Among these immobilized catalysts, POPs supported
NHC-M catalysts are particularly appropriate for
catalyzing liquid organic reactions because of their high
surface areas and easy accessibility towards organic
[11]
the oxidative carbonylation of methanol.
However,
the homogeneous CuCl system suffered from some
drawbacks, such as the problems of catalyst recovery
and reuse, equipment corrosion, and catalyst deactiva-
[
37]
substrates.
Recently, a number of POPs supported
NHC-M catalysts have been developed via Friedel-Crafts
[12,13]
[34–37]
tion.
To a certain extent, these drawbacks hamper
reactions.
However, up to date, POPs supported
the large-scale production of DMC via the oxidative
carbonylation route. To address these issues, a number
NHC-M catalysts that prepared by solvothermal and
free radical polymerization method have been seldomly
explored. On the other hand, NHC-M complexes have
displayed high activities in various carbonylation
[13–15]
of immobilized copper complexes
and Cu
[16–18]
nanoparticles
have been developed in the past two
[
38]
decades. Due to these efforts, the problems of catalyst
recovery and deactivation as well as equipment corro-
sion have been alleviated to some extent. However, the
efficiency and reusability of the currently available het-
erogeneous catalysts are still unsatisfactory. Therefore,
it is still a challenge to develop the highly active and
stable heterogeneous catalysts for the oxidative carbon-
ylation of methanol.
reactions,
nevertheless, their applications in the oxi-
dative carbonylation of methanol to DMC have been
rarely reported.
In continuation of our research interest in oxidative
[
13,26,39]
carbonylation reactions,
herein, we designed a
new porous organic polymer supported Cu catalyst
(Cu@PQP-NHC) for the oxidative carbonylation of meth-
anol. The Cu@PQP-NHC catalyst was prepared by a fac-
ile solvothermal and free radical copolymerization of
tetra-vinylphosphonium salt and bis-vinylimidazolium
salt, followed by complexation with CuCl. Characteriza-
tions results suggested that the prepared catalyst featured
relatively large surface areas, hierarchical pore structure,
and excellent swelling property. These properties enabled
the Cu@PQP-NHC catalyst behave as a highly active and
recyclable solid catalyst for the oxidative carbonylation of
methanol to DMC.
Porous organic polymers (POPs), featuring des-
ignable chemical structure and large surface areas, have
recently aroused great interest as attractive materials
[
19,20]
for developing heterogeneous catalysts.
a number of synthetic methods, such as free radical
polymerization, Friedel-Crafts reactions and
Sonogashira coupling, have been developed for the
Nowadays,
[21]
preparation of POPs.
Previous results suggested that
POPs prepared by the solvothermal and free radical
polymerization method have hierarchical pore struc-
tures, flexible frameworks, and excellent swelling
[22–25]
properties.
Consequently, these properties endow
2 | EXPERIMENTAL
2.1 | Materials
the porous catalysts with some characters of quasi-
homogeneous catalysis, and thus, some of these
catalysts showed comparable, or even higher catalytic
[24,25]
activities than their homogeneous analogues.
With
1-Vinylimidazole, 4-bromostyrene, dibromomethane,
phosphorus trichloride, 4-vinylbenzyl chloride (90%),
this background in mind, we recently developed a
phenanthroline-functionalized porous organic polymer
0
tetrahydrofuran, and 2,2 -azobis(2-methylpropionitrile)
supported copper catalyst (Cu@PCP-Phen) via
solvothermal and free radical polymerization
The resulting Cu@PCP-Phen catalyst dis-
a
(AIBN) were purchased from Energy Chemical
(Shanghai) Co. Ltd. Methanol, tetrahydrofuran and
1-vinylimidazole were analytical reagents. CO and O2
[26]
method.
played high catalytic performance for the oxidative car-
bonylation of methanol, with which 6.2% conversion of
were supplied by
vinylphosphonium
tris-(4-vinylphenyl)-phosphonium chloride (QP), was
synthesized via the reaction between 4-vinylbenzyl
a
local company. The tetra-
salt, 4-vinylbenzyl-
−
1
methanol and a high TOF of 38.1 h was obtained at
ꢀ
1
20 C for 2 hr.
[
40]
As a valuable kind of organic ligands, N-heterocyclic
chloride and tris(4-vinylphenyl)-phosphine.
Bis-
carbene-metal (NHC-M) complexes have been exten-
sively utilized as stable and active catalysts for a great
vinylimidazolium salts ([C DVIM]Br) was prepared via
the reaction of 1-vinylimidazole and dibromomethane.
1
[41]

LEI ET AL.
3 of 9
2.2 | Synthesis of PQP-NHC support
and methanol (10 ml) were added into the autoclave.
Then, the reaction mixture was stirred at 80 C for 6 hr
ꢀ
NHC functionalized porous organic polymer precursor
PQP-NHC) was prepared via a solvothermal and free
radical copolymerization of [C DVIM]Br and QP in a
to generate CuCl/NHC in-situ. After cooling to room
temperature, the autoclave was pressurized with CO
(
and O to 30 bar (PCO:PO2 = 2:1). Then, the autoclave
1
2
ꢀ
1
00 ml Teflon autoclave. Generally, the autoclave con-
was heated to 120 C for 2 hr. During the reaction, a
taining [C DVIM]Br (0.85 g, 2.3 mmol), QP (1.15 g,
total pressure of 30 bar was maintained by
1
2
.3 mmol), AIBN (50 mg) and methanol (20 ml) was
supplementing CO and O . The resulting liquid mixture
2
stirred at room temperature for 30 min. Then, the auto-
clave was heated in an oven at 100 C for 24 hr. After the
was analyzed qualitatively by GC.
ꢀ
polymerization reaction, a solid monolith (Figure S1) was
obtained. The resultant solid was washed with ethanol
for three times, dried under vacuum to obtain a pale-
yellow solid. The obtained solid was denoted as PQP-
NHC (2.04 g). Elemental composition of PQP-NHC: C,
2.5 | Characterization
Inductively coupled plasma atomic emission spectroscopy
(ICP, PerkinElmer Optima 8000) was used to analyzed
the content of copper species. Fourier Transform Infrared
Spectroscopy (FT-IR) of the monomers and prepared
samples were examined under ambient conditions by a
Bruker Equinox 55 FT-IR spectrophotometer. Surface
composition and state of Cu species were determined by
X-ray photoelectron spectroscopy (XPS, VG multilab
2000 spectrometer). Elemental analysis of the samples
was collected on an Elementar Vario MICRO Elemental
6
2.63; N, 6.46, H, 4.86%.
2.3 | Preparation of cu@PQP-NHC
t
In a typical procedure, NaO Bu (192 mg, 2 mmol) was
added to a suspension mixture of PQP-NHC (1.0 g) in
DMF (40 ml). After stirring at room temperature for 1 hr
and addition of CuCl (50 mg), the mixture was magneti-
ꢀ
analyzer, the sample was degassed at 90 C before the
ꢀ
13
cally stirred in a pre-heated oil bath at 80 C for 6 hr.
measurements. Solid-state C NMR spectrum was car-
Then, the resulting yellow precipitate, donated as
Cu@PQP-NHC, was isolated by filtration, washed with
ethanol, and then dried under vacuum.
ried out on a Bruker AVANCE III 600 Bruker spectrome-
ter. Specific surface areas of the samples were performed
by a Micrometrics ASAP 2020 surface area analyzer sys-
ꢀ
tem at 77 K, the sample was vacuum dried at 90 C for
12 hr before analysis. The size and morphologies of
2
.4 | Catalytic measurements
obtained samples were observed with SEM and TEM over
the Hitachi S-4800 and FEI Tecnai G2 F30 instruments,
respectively. Thermogravimetric analysis (TGA) of the
sample was carried out with a NETZSCH STA 449 F5
instrument. Quantitative analysis of the reaction mixture
was performance on a gas chromatograph (Scientific™
TRACE™ 1310 apparatus). The chromatographic condi-
tions have been carefully described in our previous
Oxidative carbonylation of methanol was performed in a
high-pressure stainless-steel autoclave. General,
Cu@PQP-NHC (205 mg) and methanol (10 ml) was
added into the autoclave. After pressurizing with CO and
O to 30 bar (PCO:PO2 = 2:1), the autoclave was heated to
the setting reaction temperature for 2 hr. During the
reaction, a total pressure of 30 bar was maintained by
2
[
26]
report.
supplementing CO and O . The resulting reaction mix-
2
ture was separated by centrifugation, the recycling solid
was washed with chloroform. For recycling experiments
of the catalyst, the reaction was scaled-up. Typically,
Cu@PQP-NHC (615 mg) and methanol (30 ml) was
added into the stainless-steel autoclave. After the
catalytic reaction, the solid catalyst was separated by cen-
trifugation, washed with chloroform for three times,
3 | RESULTS AND DISCUSSION
3.1 | Synthesis and characterization of the
catalyst
Copper
supported
on
N-heterocyclic
carbene-
ꢀ
dried at 60 C in vacuum, and then engaged in a new
catalytic run.
functionalized porous organic polymer (Cu@PQP-NHC)
was prepared by a solvothermal copolymerization of QP
Synthesis and catalytic performance of CuCl/NHC
were carried out as follow. Generally, bis-
and [C DVIM]Br monomers, followed by complexation
1
with CuCl (Scheme 1). The tetra-vinylphosphonium salt
[
42,43]
vinylimidazolium salt ([C DVIM]Br, 36.2 mg, 0.1 mmol),
monomer acted as a crosslinking agent,
while the
1
t
CuCl (9.9 mg, 0.1 mmol), NaO Bu (19.2 mg, 0.2 mmol),
bis-vinylimidazolium salt monomer was used as the

4
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LEI ET AL.
SCHEME 1 Synthetic scheme of Cu@PQP-NHC
precursor of NHC ligand. ICP analys is suggested that the
Cu content in Cu@PQP-NHC was 3.10 wt%.
The porous properties of PQP-NHC and Cu@PQP-
NHC were analyzed by nitrogen sorption analysis
these findings also suggested that no obvious morphology
change occurred after immobilizing the copper species.
FT-IR spectra of monomers and the prepared samples
were depicted in Figure 3. FT-IR spectra of PQP-NHC
and Cu@PQP-NHC in Figure 3(c) and (d) showed the
presence of C-H stretching vibrations (2930 and
(
3
Table 1). PQP-NHC had a BET surface area of
23 m ·g and a pore volume of 0.71 cm /g. The immo-
2
−1
3
−
1
bilization of CuCl resulted in an obvious decrease of sur-
face area and pore volume. Nitrogen sorption isotherms
of PQP-NHC and Cu@PQP-NHC in Figure 1 suggested
the samples exhibited similar sorption behavior. At low
2850 cm ) of the saturated methylene (-CH -). Com-
2
pared to the monomers in Figure 3(a) and (b), the C-H
−
1
(>3000 cm ) vibrations of the terminal -C=CH groups
2
were absent in Figure 3(c) and (d). These findings
suggested that the free-radical copolymerization reaction
of the monomers was finished. In addition, PQP-NHC
showed a characteristic peak of imidazolium salts at
relative pressure regions (P/P < 0.01), the steep steps of
0
nitrogen gas uptake indicated the filling of micropore. At
P/P0 = 0.6–1.0 regions, the obvious hysteresis loops
suggested the presence of mesopore. At high relative
−
1
around 1170 cm , and this band was obviously weak-
ened in spectrum of Cu@PQP-NHC. This should be
attributed to the formation NHC-metal complexes.
pressure regions (P/P = 0.9–1.0), the sharp rise of gas
0
uptake curves illustrated a certain amount of macropores.
These findings indicated the hierarchical pore structure
of the prepared samples. Moreover, these results coin-
cided well with pore-size distribution curve of Cu@PQP-
NHC (insert). For solid catalyst, the presence of hierar-
chical pores would be essential to ensure the favorable
catalytic performance because these structures facilitate
13
Solid-state
C NMR spectra of PQP-NHC and
Cu@PQP-NHC in Figure 4 displayed a series of bands at
around 120–152 and 20–60 ppm. The bands at around
120–152 ppm corresponded to aromatic carbons of the
solids, while the bands at around 20–60 ppm should be
[
23–25]
assigned to the polymerized vinyl carbons.
Com-
[35–37]
mass transport of reactants and products.
pared to PQP-NHC, a small band at 188.5 ppm was
The morphology of PQP-NHC and Cu@PQP-NHC
was characterized by SEM and TEM. SEM images in
Figure 2(a) and (b) implied that PQP-NHC and
Cu@PQP-NHC had similar porous morphology which
was composed of small and loosely aggregated particles.
TEM images of PQP-NHC and Cu@PQP-NHC in
Figure 2(c) and (d) further confirmed the porous and
loosely packed morphology of the samples. In addition,
TABLE 1
Textural properties of the prepared catalyst
Pore
Average
volume
pore
radius
(nm)
3
S
BET
(cm /g)
2
a
b
Entry Sample
(m ·g−1)
1
2
PQP-NHC
323
0.71
0.50
6.7
Cu@PQP-NHC 191
4.9
FIGURE 1 Nitrogen adsorption–desorption isotherms of PQP-
NHC and Cu@PQP-NHC, and pore size distribution curve (inset)
of Cu@PQP-NHC calculated by NLDFT method
a
o
Single point adsorption total pore volume of pores at P/P = 0.95.
Adsorption average pore diameter (4 V/A by BET).
b

LEI ET AL.
5 of 9
FIGURE 2 SEM images of (a) PQP-NHC
and (b) Cu@PQP-NHC, TEM images of
(
c) PQP-NHC and (d) Cu@PQP-NHC
+
.[13]
the band at 932.7 eV could be attributed to Cu
As
estimated by measuring the relative proportion of band
+
2+
areas, the molar ratio of Cu /Cu in Cu@PQP-NHC
+
was about 2.0, suggesting the partial oxidation of Cu
species during the immobilization process.
Thermogravimetric analysis of Cu@PQP-NHC was
operated at a heating rate of 10 K·min− and a dynamic
argon atmosphere. As depicted in Figure 6, the initial
1
ꢀ
weight loss that occurred at 25–150 C should be mainly
ascribed to the removal of guest molecules. The rapid
ꢀ
weight losses took place at above 200 C, suggesting that
Cu@PQP-NHC could be stable at least up to a tempera-
ꢀ
ture no less than 200 C.
FIGURE 3 FT-IR spectra of (a) QP, (b) [C1DVIM]Br, (c) PQP-
NHC, and (d) Cu@PQP-NHC
present in Cu@PQP-NHC. This band might be attributed
[37]
to signals of CNHC.
XPS analyses of PQP-NHC and Cu@PQP-NHC were
studied to gain insight into the structural information
and states of Cu species. XPS full spectrum of Cu@PQP-
NHC in Figure 5(a) indicated the presence of P, Cl, C, N,
Br, and Cu elements. Compared to the spectrum of PQP-
NHC, the presence of Cu species and significant decrease
of Br element should be attributed to the formation of
NHC-Cu complexes in Cu@PQP-NHC. Cu2p3/2 spectrum
of Cu@PQP-NHC in Figure 5(b) could be deconvoluted
into two bands, centered at 934.3 and 932.7 eV, respec-
tively. The small band at 934.3 eV combined with the sat-
1
3
FIGURE 4 Solid-state C NMR spectra of (a) PQP-NHC and
(b) Cu@PQP-NHC
2+,[44]
ellite peak (938–947 eV) could assigned to Cu
while

6
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LEI ET AL.
FIGURE 5 XPS spectra of the Cu@PQP-
NHC sample, (a) full spectrum, (b) Cu2p
spectrum
of the NHC ligand led to superior catalytic activity
(
Table 2, entry 3). The catalytic activity of the PQP-NHC
support was also tested under the reaction conditions. No
catalytic activity was observed, suggesting that the Cu
species were the active components of Cu@PQP-NHC
catalyst (Table 2, entry 4).
Next, the effect of reaction temperature on the cata-
lytic activity of the Cu@PQP-NHC catalyst was investi-
ꢀ
gated over the range of 100–140 C. As shown in
ꢀ
Figure 7, with the temperature rising from 100 to 120 C,
the conversions of methanol and TOF numbers gradually
increased. Meanwhile, about 99.3% selectivity of DMC
FIGURE 6 TGA curve of the Cu@PQP-NHC sample
3.2 | Catalytic performance
Catalytic performance of the obtained Cu@PQP-NHC
was then studied for the oxidative carbonylation of meth-
anol with CO and O . The results in Table 2 suggested
2
that Cu@PQP-NHC was an active catalyst for the
ꢀ
carbonylative synthesis of DMC. After reacting at 120 C
for 2 hr, a 10 mmol/l of Cu dosage was sufficient for pro-
moting 9.3% conversion of methanol with a high TOF
number of 57.0 h− (Table 2, entry 1). The TOF number
of Cu@PQP-NHC was about 2.4 times of that of commer-
cial CuCl (Table 2, entry 2). To further investigate the
promotion effect of the NHC ligand, the activity of homo-
geneous CuCl/NHC was tested. Obviously, the presence
1
FIGURE 7 Effect of the reaction temperature. Reaction
conditions: Cu@PQP-NHC (205 mg), methanol (10 ml), CO
(
2
20 bar), O (10 bar), 2 hr, stirring speed (800 rpm)
[a]
TABLE 2
Catalytic results for oxidative carbonylation of methanol
TOF (h−
57.0
23.7
65.9
-
1
)
Entry
Catalyst
[Cu] (mmol/l)
Conversion (%)
Selectivity (%)
1
2
3
Cu@PQP-NHC
CuCl
10
10
10
-
9.3
4.2
10.8
-
99.2
91.3
98.8
-
CuCl/NHC
PQP-NHC
b
4
a
ꢀ
Reaction conditions: catalyst, methanol (10 ml), CO (20 bar), O
PQP-NHC (205 mg).
2
(10 bar), 120 C, 2 hr, stirring speed (800 rpm).
b

LEI ET AL.
7 of 9
was maintained. Further increasing the reaction temper-
ꢀ
ature to above 120 C, a slight decrease in the selectivity
of DMC was observed with methyl formate produced as a
main byproduct, although a higher conversion of metha-
nol was observed.
Then, the stability and recyclability of the Cu@PQP-
NHC catalyst was evaluated. To facilitate the recycling of
the solid catalyst, the catalytic reaction was conducted in
scaled-up quantities. As shown in Figure 8, with three
times of methanol amount, a 9.0% conversion of metha-
nol with 99.4% selectivity of DMC was obtained in the
first catalytic run. After the reaction, the solid catalyst
was separated by centrifugation, washed with chloro-
form, dried under vacuum, and then engaged in a next
catalytic run. The results suggested that Cu@PQP-NHC
exhibited excellent reusability for the oxidative carbonyl-
ation of methanol. After reusing for 10 times, Cu@PQP-
NHC still afforded an 8.7% conversion of methanol and a
FIGURE 8 Recycling of the Cu@PQP-NHC catalyst for the
oxidative carbonylation of methanol. Reaction conditions:
Cu@PQP-NHC (615 mg), methanol (30 ml), CO (20 bar), O2
ꢀ
(
10 bar), 120 C, 2 hr, stirring speed (800 rpm)
−
1
TOF of 53.3 h . Cu leaching in the reaction mixture was
also measured by ICP. Negligible Cu leaching (<0.2 ppm)
was observed for each catalytic cycle. Moreover, ICP
analysis revealed that about 99% Cu species was still
retained in the Cu@PQP-NHC catalyst after reusing
10 times, thus further suggesting the excellent recycling
ability of Cu@PQP-NHC. Additionally, XPS analysis was
used to investigate the state of Cu species in the recycled
Cu@PQP-NHC. The Cu2p3/2 spectrum in Figure 9
suggested that most of the Cu species was remain in +1
+
2+
state, and the molar ratio of Cu /Cu was 2.2. Previous
investigations suggested that the valence of copper spe-
cies has an important influence on catalytic performance,
+
2+
[45–47]
and Cu species was more active than Cu species.
Thus, these results demonstrated that the Cu@PQP-NHC
catalyst is a highly active, stable and recyclable catalyst
for the oxidative carbonylation of methanol.
FIGURE 9 Cu2p3/2 XPS spectrum of the recycling Cu@PQP-
NHC after reusing 10 times
We further compared the catalytic performance of
Cu@PQP-NHC with some other Cu-based catalysts that
TABLE 3
Comparing the catalytic performance of different Cu-based solid catalysts for the oxidative carbonylation of methanol in
liquid phase
1
Entry
Catalysts
Conditions
Conv. (%)
9.3
Sel. (%)
99.2
Run
TOF(h−
57.0
2.32
-
)
Ref.
ꢀ
1
2
Cu@PQP-NHC
CuCl/PS-Phen
120 C, 3 MPa, 2 hr
11
7
This work
[13]
ꢀ
120 C, 3 MPa, 2 hr
15.6
1.76
22.1
3.1
96.8
a
ꢀ
[14]
[15]
[16]
[17]
[18]
[26]
[48]
3
Bpya-CuCl
2
140 C, 2 hr
91.8
3
ꢀ
4
5
6
7
SBA-diamide-CuCl
Cu/NG
120 C, 3 MPa, 5 hr
99.2
4
~5.3
16.9
23.8
17.6
38.1
~27
ꢀ
120 C, 3 MPa, 1.5 hr
97.6
5
ꢀ
Cu@HCS-5
110 C, 3 MPa, 1.5 hr
3.7
90.4
5
ꢀ
Cu/NCNT200
Cu@PCP-Phen
Cu/rGO-CB
110 C, 3 MPa, 1.5 hr
2.44
6.2
97.8
4
ꢀ
8
9
120 C, 3 MPa, 2 hr
99.5
7
ꢀ
120 C, 3 MPa, 1.5 hr
5.6
97.4
5
a
Cu usage (150 mmol/l).

8
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LEI ET AL.
has been reported previously. The results in Table 3 indi-
cated that the prepared Cu@PQP-NHC exhibited much
better catalytic activities and reusability than previously
reported solid Cu catalysts. In term of the physicochemi-
cal properties of the Cu@PQP-NHC, its superior catalytic
performance should be assigned to the relatively large
surface areas, hierarchical pore structure, and excellent
swelling property.
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In summary, a promising approach for the oxidative car-
bonylation of methanol to dimethyl carbonate was devel-
oped using copper coordinated in N-heterocyclic
carbene-functionalized porous organic polymer as a
robust and active catalyst. The new catalyst was synthe-
sized through a simple copolymerization and successive
immobilization method. Physicochemical characteriza-
tions suggested that the prepared catalyst featured rela-
tively large surface areas, hierarchical pore structure, and
excellent swelling property. Under optimal reaction con-
ditions, the solid catalyst displayed high catalytic activity
for the oxidative carbonylation of methanol, which out-
performed commercial CuCl and previously reported
solid Cu catalysts. In addition, the catalyst showed negli-
gible Cu leaching and could be reused at least 10 times
without significant loss of catalytic activity. Thus, this
protocol provided not only an active, robust, and recycla-
ble solid catalyst for the oxidative carbonylation of meth-
anol to dimethyl carbonate, but also paved a way for
developing efficient heterogeneous catalysts for other oxi-
dative carbonylation reactions.
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ACKNOWLEDGMENTS
This work is financially supported by National Natural
Science Foundation of Guizhou Province (qiankehejichu
[
2018]1414 and [2016]1133, qiankeheLHzi [2015]7608
and [2015]7629); the Scientific and Technological Innova-
tion Platform of Liupanshui (52020-2018-03-02 and
[25] Q. Sun, Z. Dai, X. Liu, N. Sheng, F. Deng, X. Meng, F. S. Xiao,
J. Am. Chem. Soc.2015, 137, 5204.
[26] Y. Lei, G. Leng, G. Li, Z. Li, D. Zhu, Appl. Organomet.
5
2020-2017-02-02); the Academician Workstation of
Liupanshui Normal University (qiankehepingtairencai
2019]5604 hao), and Guizhou Key Laboratory of Coal
Chem.2020, 34, e5397.
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We also thank Youming Ni at the Dalian Institute of
Chemical Physics for beneficial discussions.
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ORCID
Yizhu Lei https://orcid.org/0000-0002-9247-6000
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