Ultrasound-assisted synthesis of a stable Co(II) coordination polymer as heterogeneous catalyst for CO2 transformation

Article abstract of DOI:10.1016/j.poly.2020.115016

A stable benzimidazole-containing Co(II) coordination polymer namely [Co(L)_0.5(oba)]n (1) (H₂oba = 4,4′-oxybis(benzoate), L = 1,6-bis(5,6-dimethylbenzimidazolyl) hexane) was successfully synthesized by ultrasonic technique under mild conditions. In especial, the effects of initial reagent concentration, irradiation time and ultrasonic power on the morphology and size of micron scale 1 were discussed in detail. Micron scale 1 appeared exceptional solvent and pH stabilities. Further, as a heterogeneous Lewis catalyst, 1 exhibited a highly activity and recyclability for CO₂ transformation by cycloaddition with epoxide under room temperature.

Full text of DOI:10.1016/j.poly.2020.115016

Polyhedron 196 (2021) 115016
Contents lists available at ScienceDirect
Polyhedron
journal homepage: www.elsevier.com/locate/poly
Ultrasound-assisted synthesis of a stable Co(II) coordination polymer as
heterogeneous catalyst for CO transformation
2
Ce Liu a,b, Lin Liu , Zheng-Bo Han a,
a
⇑
a
College of Chemistry, Liaoning University, Shenyang 110036, PR China
College of Chemical Engineering, North China University of Science and Technology, Tangshan, Hebei 063210, PR China
b
a r t i c l e i n f o
a b s t r a c t
0
Article history:
A stable benzimidazole-containing Co(II) coordination polymer namely [Co(L)0.5(oba)]n (1) (H oba = 4,4 -
2
Received 18 November 2020
Accepted 28 December 2020
Available online 02 January 2021
oxybis(benzoate), L = 1,6-bis(5,6-dimethylbenzimidazolyl) hexane) was successfully synthesized by
ultrasonic technique under mild conditions. In especial, the effects of initial reagent concentration, irra-
diation time and ultrasonic power on the morphology and size of micron scale 1 were discussed in detail.
Micron scale 1 appeared exceptional solvent and pH stabilities. Further, as a heterogeneous Lewis cata-
Keywords:
lyst, 1 exhibited a highly activity and recyclability for CO
2
transformation by cycloaddition with epoxide
Ultrasonic synthesis
Coordination polymer
Catalyst
under room temperature.
Ó 2020 Published by Elsevier Ltd.
CO
2
transformation
Co(II)
1
. Introduction
Coordination polymers (CPs) are a kind of organic inorganic
Ultrasonic irradiation is one of the powerful methods for synthe-
sizing micro- and nano-structure particles with satisfactory sizes
and incomparable properties [17,18]. With ultrasound radiation
on the solution, small bubbles are created that provoke pressure
and temperature alterations. Under these conditions, chemical
reaction can be driven and different structures can be produced
[19]. Compared with the prevalent synthetic methods, the ultra-
sonic method of synthesis material is easy to obtain pure phase
products in short time, and higher yield [20].
hybrid materials composed of metal centers or clusters and organic
ligands which gained widespread attention in the past decades
[
1–3]. CPs have great potential application as catalysts due to their
extremely large surface area, tunable topological structure and
good stability [4,5]. The flexible benzimidazole ligand used
extensively in construction of CPs not only has strong coordination
ability by its unique structure and mutable conformations, but the
presence of the aromatic ring could enhance the stability of the
composite [6].
Hence, we reported the ultrasonic synthesis and characteriza-
tion of a micro-sheets Co(II) CP, namely [Co(L)0.5(oba)]
(L = 1,6-bis(5,6-dimethylbenzimidazolyl) hexane, H
n
(1),
oba = 4,4 -
0
2
Carbon dioxide (CO
2
) as an environment friendly C1 source can
oxybis(benzoate)). The influences of concentration of initial
reagents, sonication time and power on the size and morphology
of 1 had been studied. Moreover, activities of micro-sheets 1 as a
be used to synthesize valuable chemicals with epoxides [7–9].
Several catalysts have been investigated, such as ionic liquids [10],
Schif bases [11], metal centered Salen complex [12] and covalent
organic frameworks [13]. But high temperature, high pressures of
Lewis acid catalyst for CO
2
cycloaddition under mild conditions
were investigated in detail (Scheme 1).
CO
CO
2
and nocuous reactive reagents are usually made transform
uncomfortably. Therefore, an efficient and low energy con-
2
2
. Experimental
.1. Materials and methods
,6-bis(5,6-dimethylbenzimidazolyl) hexane was synthesized
2
sumption catalytic for CO fixation at mild conditions is extremely
desirable of the day. CPs with Lewis acid or base sites are used as cat-
alyst to make the cycloaddition with epoxides more easily, which
are currently receiving wide attention for future application.
Currently, the syntheses of CPs through green and sustainable
technologies have received considerable attention [14–16].
2
1
according to the literature method [21]. Other synthetic starting
reagents were analytical reagent purchased from Sinopharm
Chemical Reagent Co. Ltd. and employed without further purifica-
tion. Powder X-ray diffraction (PXRD) patterns were collected by a
⇑
Corresponding author.
E-mail address: ceshzb@lnu.edu.cn (Z.-B. Han).
Rigaku D/Max-2500 diffractometer with Cu-Ka radiation in the 2h
https://doi.org/10.1016/j.poly.2020.115016
277-5387/Ó 2020 Published by Elsevier Ltd.
0

C. Liu, L. Liu and Zheng-Bo Han
Polyhedron 196 (2021) 115016
to the preset pressure at room temperature and stirring was set at
5
00 rpm. After finishing the reaction, stop the magnetic stirrer and
the reactor was cooled by ice. The micro-sheets 1 were centrifuged
from the reaction solution, and the product yields were identified
by gas chromatograph.
3
. Result and discussion
3.1. Crystal structure of 1
[
Co(L)0.5(oba)]
n
(1) was first reported by Cui [22]. It had 3D
0
framework which consisted of Co(II) as metal center and 4,4 -oxy-
bis(benzoate) and 1,6-bis(5,6-dimethylbenzimidazolyl) hexane as
ligands. In 1, each Co(II) center was four coordinated by three oxy-
gen atoms of two oba anions and one nitrogen atom from the L
ligand, the packing diagram was displayed in Fig. S1. With high
density coordinative unsaturated Co(II) metal centers, 1 could per-
form as Lewis acid catalysts in the context of cycloaddition of
2
ꢀ
epoxides with CO
.2. Morphology
Fig. 1a displayed a Scanning Electron Microscopy (SEM) image
2
to form cyclic carbonates under mind condition.
3
of the precipitate of 1 prepared by the hydrothermal method.
In contrast with the 1 prepared by ultrasonic method, large bulks
of 1 were observed. The size and morphology of the 1 prepared
through the ultrasonic method by changing three conditions (con-
centration of initial reagents, irradiation time and ultrasonic
power) were characterized with SEM (Fig. 1b–f). Initially, two con-
centrations of all reagents (0.01 and 0.005 M) at irradiation time of
Scheme 1. Synthesis of cyclic carbonates by 1.
range of 5–40° at room temperature. Infrared Spectra (IR) were
recorded on an Avatar 360 (Nicolet) instrument in the 500–
000 cm region using the KBr pellet technique. Scanning electron
microscopy (SEM) and Energy dispersion spectrum (EDS) were
observed using a JSM-IT100 system (JEOL Instrument, Tokyo,
Japan), the accelerating voltage of SEM during the measurement
was 20.0 kV. Thermogravimetric analysis (TG) was collected using
a NETZSCH TG 209 thermal analyzer in air at a heating rate of 10 °-
ꢀ
1
4
3
0 min and ultrasonic power of 210 W were performed respec-
tively. Sheets-like 1 were obtained (Fig. 1b and c). When the con-
centration of the initial reagent changed from 0.01 to 0.005 M,
the mean thickness of the sheets decreased from about 490 to
255 nm. It displayed that low initial reagents led to thinner mor-
phology. Hence, concentration of the initial reagent of 0.005 M
was selected for the next experiments.
ꢀ
1
C min
2
in the 30–800 °C range. Ultrasonic cleaning unit KQ-
200DE (Kun Shang Ultrasonic Instrument, Jiangsu, China) was
used for ultrasonic synthesis of 1. Brunauer-Emmett-Teller (BET)
were carried out on a 3H-2000PS2 automated gas sorption ana-
lyzer (Beishide Instruments). Gas chromatography analysis (GC)
were performed by Puxi G5 chromatograph (Beijing General
instrument Co., Ltd.).
Further, the influence of irradiation time on the size and mor-
phology of 1 were monitored by different times (30, 60 and
9
0 min) at constant concentration of 0.005 M and sonication power
of 210 W. (Fig. 1c–e). As shown in Fig. 1c, when the reaction was
performed for 30 min, micron sheets were formed, but their mor-
phologies were not uniform. When the ultrasonic time was
extended to 60 min, thinner (average thickness 160 nm) and uni-
form 1 were prepared (Fig. 1d). While the sonication time
increased to 90 min, too long time increased the thickness of
sheets. The distribution size of 1 became nonuniform and agglom-
erated structure appeared (Fig. 1e). Therefore, the best reaction
time was 60 min.
In addition, the effect of ultrasonic power on the size and mor-
phology of 1 were discussed. The reactions were carried out under
different ultrasonic powers (150 and 210 W). Compared with
Fig. 1f and d, we can see that more aggregated sheets were
obtained by low ultrasonic power (150 W). The results proved that
using higher ultrasonic power was favor to generating thinner
micron sheets than those using lower power. Table S1 showed an
overview of the morphologies and size of 1 compared by the
change of initial reagents concentration, ultrasound time and pow-
ers. Fig. 2(I) showed the physical photographs before and after
ultrasonic reaction.
2
.2. Synthesis of micro-sheets 1
A mixture of 0.1 mmol Co(OAc)
2
ꢁ4H
2
O, 0.1 mmol L, 0.1 mmol
H
2
oba, 100 mg polyethylene glycol (average mol wt 20,000) were
dissolved in 10 m L of H O-EtOH mixed solution (volume ratio of
H2O:EtOH = 4:1) in a beaker with stirring. The mixed solution was
2
V
positioned in the ultrasonic bath, and operated for 30 min at a
power of 210 W. The purple 1 were obtained by centrifugation,
then washed using EtOH three times, and dried in air. These pro-
cesses were also done at concentrations of all reagents of
.005 M, and irradiation time and ultrasonic power at
0 min/210 W, 60 min/210 W, 90 min/210 W and 60 min/150 W
0
3
to research the effects of concentration of initial reagents, sonica-
tion time and power on the size and morphology of 1.
2
.3. Catalytic experiments
Propylene carbonate (PC) synthesis from propylene oxide (PO)
and CO (Scheme S1) was performed in a 25 mL stainless steel
2
high-pressure reactor equipped with a magnetic stirrer. In the typ-
ical operation, PO, TBAB and micro-sheets 1 were transferred into
All experimental results demonstrated that changed the con-
centration of the initial reagent, irradiation time or ultrasonic
power probable were a feasible approach to control the sizes and
morphologies of 1 [23]. Considering that the thinner sheets were
conducive to exposing more active centers, micro-sheets 1 pre-
2
the reactor without any solvent. Then CO pressurized into reactor
2

C. Liu, L. Liu and Zheng-Bo Han
Polyhedron 196 (2021) 115016
Fig. 1. SEM images of 1. 1 prepared via (a) hydrothermal method; (b) 0.01 M, 30 min under 210 W ultrasonic power; (c) 0.005 M, 30 min under 210 W ultrasonic power; (d)
.005 M, 60 min under 210 W ultrasonic power; (e) 0.005 M, 90 min under 210 W ultrasonic power; (f) 0.005 M, 60 min under 150 W ultrasonic power.
0
Fig. 2. (I) Physical photograph before ultrasonic reaction (a) and after ultrasonic reaction (b); (II) Simulated and experimental Powder X-ray diffraction patterns of 1. 1
prepared via (a) 0.01 M, 30 min under 210 W ultrasonic power; (b) 0.005 M, 30 min under 210 W ultrasonic power; (c) 0.005 M, 60 min under 210 W ultrasonic power; (d)
0
.005 M, 90 min under 210 W ultrasonic power; (e) 0.005 M, 60 min under 150 W ultrasonic power.
pared by constant concentration of 0.005 M, ultrasonic time 60 min
and power of 210 W was chosen as catalyst for the catalytic exper-
iments ultimately.
could improve the framework stability, and the formed complexes
usually showed excellent stability.
Infrared spectra (IR) of micro-sheets 1 showed the detailed
information about the characteristic absorption bands of the coor-
2ꢀ
dinated oba and L ligand in 1 (Fig. S4). The strong peaks at 1592
ꢀ1
3.3. PXRD, IR, TG and BET
and 1386 cm were attributed to the asymmetric and symmetric
stretching vibrations of the carboxylic groups. The bands at
ꢀ
1
The PXRD patterns of 1 prepared by the ultrasonic method with
1522 cm
were assigned to v(C@N) stretching vibration of the
– groups
different reaction conditions and the simulated one obtaining by
single crystal X-ray diffraction [22] were compared in Fig. 2(II).
This indicated 1 were successfully synthesized with a good phase
purity. The stabilities of micro-sheets 1 in different pH aqueous
solutions were investigated. The PXRD patterns of micro-sheets 1
indicated that the framework did not collapse after 48 h in the
aqueous solutions pH from 2 to 12 (Fig. S2). Then micro-sheets 1
were immersed in ordinary organic solvents for 48 h. The PXRD
patterns are basically consistent with the synthesized samples
which show that micro-sheets 1 have high solvent stability
benzimidazole rings. The absorption peaks of –CH
2
ꢀ1
appeared at 2931 cm . Thermogravimetric (TG) analysis was car-
ried out to ascertain the thermal stability of micro-sheets 1
(Fig. S5). There was no mass loss before 310 °C, which showed
that 1 had a good thermal stability. After that, the organic ligands
in 1 were decomposed. The N
2
adsorption isotherm plot at 77 K
indicated that micro-sheets 1 was a no-porous structure, which
2
ꢀ1
had the low surface area of 5.98 m
g
, the CO
2
uptake of
2
ꢀ1
micro-sheets 1 reached 6.22 m
g
at 298 K (Figs. S6 and S7).
The PXRD patterns revealed that micro-sheets
retained after activation (Fig. S8).
1 was well
(Fig. S3). This was mainly attributed to that benzimidazole ligands
3

C. Liu, L. Liu and Zheng-Bo Han
Polyhedron 196 (2021) 115016
3
.4. Catalytic properties
of PC increased steadily when the loading of micro-sheets 1 was
increased from 0.24 to 0.48 mol% with a constant TBAB concentra-
tion of 4.8 mol% (Fig. 3a). The saturation point reaching 92.2% at
micro-sheets 1 concentration of 0.48 mol%. Hence the micro-sheets
1 and TBAB was fixed at 0.48 mol% and 4.8 mol% respectively for
The representative catalytic experiments of micro-sheets 1 for
CO
2
cycloaddition with PO were performed at room temperature
pressure for 30 h (Table S2). No significant
(
rt) under 1.5 MPa CO
2
amount of product was produced without catalyst (entry 1).
Entries 2 and 3 showed that reaction with only micro-sheets 1 as
catalyst under the reaction conditions stated above, the yield of
propylene carbonate (PC) was only 11.7%, while only tetrabutylam-
monium bromide (TBAB) as catalyst gave 26.8% PC yield. The yield
was increased with the micro-sheets 1/TBAB mixture as catalyst
which reached to 92.2%, it may be due to synergism between the
active centers of the micro-sheets 1 and TBAB (entry 4). All cat-
alytic systems maintained more than 99% selectivity. Differenti-
further studies. The effect of the CO
at a medium pressure scale (0–1.8 MPa) (Fig. 3b). The yield of PC
increases obviously with an increase of CO pressure from 0 to
0.9 MPa. In low pressure range, with the increase of CO pressure,
the concentration of CO in the liquid phase of the reaction system
increases too, which promotes the reaction. At a CO pressure of
1.5 MPa, the catalyst affords the maximum PC yields, and further
2
loaded pressure was studied
2
2
2
2
increase the CO
yields, a high CO
2
pressure shown little effect on the raise of PC
pressure would retard the interaction between
2
ated from most catalysts for chemical conversion of CO
2
into
epoxide and the active centers on the outer surface of the cata-
lyst [25]. Since the high yields, 1.5 MPa would be appropriate as
reaction condition.Time dependent studies were shown in Fig. 3c.
The yield of PC increased with the increase of reaction time, which
reached the highest at 30 h. After that, the PC yield did not increase
significantly with reaction time attended. So, the optimum reaction
time for the completion was 30 h.
The recyclability of the catalyst was further studied. After the
first cycloaddition reaction, micro-sheets 1 was recovered by cen-
trifugation. The recovered micro-sheets 1 was washed three times
with EtOH and dried at room temperature and used for further run.
cyclic carbonates need harsh conditions of high pressure/tempera-
ture, but micro-sheets 1/TBAB expressed well catalytic activity in
this work, maybe due to the micron scale catalyst brings more
active sites exposure, thus 1 showed its excellent catalytic ability
for CO
The influence of different reaction parameters on the catalyst
effectiveness in the coupling reaction of CO and epoxide were
2
fixation even under mild conditions [24].
2
studied by using micro-sheets 1/TBAB as catalyst (Fig. 3). The effect
of varying catalysts amounts of micro-sheets 1 were investigated
at 1.5 MPa CO pressure at room temperature for 30 h. The yield
2
Fig. 3. Effect of reaction parameters for micro-sheets 1 on the cycloaddition of PO and CO
micro-sheets 1 = 0.48 mol%, TBAB = 4.8 mol%, 30 h, rt); (c) Reaction time (micro-sheets 1 = 0.48 mol%, TBAB = 4.8 mol%, PCO2 = 1.5 MPa, rt); (d) Catalyst reusability (micro-
sheets 1 = 0.48 mol%, TBAB = 4.8 mol%, PCO2 = 1.5 MPa, 30 h, rt).
2
. (a) Catalyst amount (TBAB = 4.8 mol%, PCO2 = 1.5 MPa, 30 h, rt); (b) Pressure
(
4

C. Liu, L. Liu and Zheng-Bo Han
Polyhedron 196 (2021) 115016
2
Fig. 4. Plausible Mechanistic of CO cycloaddition with epoxides by micro-sheets 1/TBAB.
The results showed the activity of micro-sheets 1 in the reuse pro-
cesses without significant decreased, which implied 1 was a high
reusability for applications (Fig. 3d). Micro-sheets 1 recovered after
recycle were analyzed by PXRD, IR, SEM and EDS respectively. The
results from PXRD and IR records indicate that, the recycled cata-
lyst still retained its structural characteristics (Figs. S9 and S10).
The SEM and EDS image shown that after the fourth recycle, the
morphology of the micro-sheets 1 remained well, while C, N, O
and Co elements were detected as expected (Fig. S11). After react-
ing for 12 h, a hot-filtration test was carried out to confirm the
heterogeneous nature of the micro-sheets 1 (Fig. S12). When the
reaction time was continued to 36 h, the yield of PC was only
slightly increased from 38.9% to 52.1%. Even after a long reaction
time, no significant increase in the PC yield was observed, which
indicated that the cycloaddition mediated by the micro-sheets 1
is essentially heterogeneous. All of these results clarified that
(90%), epichlorohydrin gave relatively low conversion (71.5%),
and the butyl glycidyl ether and glycidyl phenyl ether shown least
conversions of 53.6 and 7.4%, respectively. Due to the low response
of electron rich matrix and the large space hindrance of reactants,
the conversion was reduced.
4. Conclusion
In summary, a stable micron scale Co(II) coordination polymer
[Co(L)0.5(oba)] (1) was fleetly synthesized at room temperature
n
by ultrasonic method. The uniform sheets structure could be
achieved by suitable time and power of ultrasonic irradiation.
Without any routine activation, micro-sheets 1 was employed as
a recyclable catalyst for CO conversion to propylene carbonate.
2
It had the advantages of low cost, environmental protection and
energy saving, which had certain potential in the capture and fix-
micro-sheets
transformation.
Based on previous reports [26,27], the potential mechanism for
cycloaddition reaction of CO and epoxide with micro-sheets 1/
1
is
a
stable and efficient catalyst for CO
2
2
ation of CO .
CRediT authorship contribution statement
2
TBAB as catalyst was illustrated in Fig. 4. Considering that micro-
sheets 1 had non-porous structure, the epoxide couldn’t diffuse
in the channels of the CPs, the catalytic reaction taken place on
the surface of the CPs. First, the unsaturated Co(II) atoms on the
surface of micro-sheets 1 acted as Lewis acidic sites attacked on
the O atom of the epoxides. Meanwhile, the N-doped benzimida-
zole ligand from the composition frameworks were also certified
Ce Liu: Investigation, Writing - original draft. Lin Liu: Data
curation, Methodology. Zheng-Bo Han: Conceptualization,
Supervision.
Declaration of Competing Interest
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
as basic sites could effectively activate the carbon-oxygen bond
ꢀ
of CO
2
[28]. Second, the nucleophiles Br from TBAB attacked less
hindered C atoms in the epoxides, thus open the epoxy ring.
Finally, the polarized CO was reacted with the epoxy ring to form
2
Acknowledgements
an intermediate, which transformed into the cyclic carbonate
through the next ring closing step.
In addition, the CO cycloaddition with various epoxides cat-
2
alyzed by micro-sheets 1/TBAB also be carried out (Table S3). It
This work was supported by the National Natural Science Foun-
dation of China (21701076 and 21671090), LiaoNing Revitalization
Talents Program (XLYC1802125), and Liaoning Province Fund
(LZD201903).
is noteworthy that butylene oxide could be efficiently converted
5

C. Liu, L. Liu and Zheng-Bo Han
Polyhedron 196 (2021) 115016
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