High yield room temperature conversion of carbon dioxide into cyclic carbonates catalyzed by mixed metal oxide (CuO-ZnO) nano-flakes/micro-flakes (Cozi-nmf)

Article abstract of DOI:10.1002/aoc.6224

Capturing and converting carbon dioxide (CO₂) into useful organic molecules and polymers is the best way of alleviating excessive release of it from industrial sources to the environment. Cyclic carbonate synthesis by cycloaddition from CO

Full text of DOI:10.1002/aoc.6224

Received: 24 November 2020
DOI: 10.1002/aoc.6224
Revised: 30 January 2021
Accepted: 16 February 2021
F U L L P A P E R
High yield room temperature conversion of carbon dioxide
into cyclic carbonates catalyzed by mixed metal oxide (CuO-
ZnO) nano-flakes/micro-flakes (Cozi-nmf)
Venkateswara Rao Velpuri
| Krishnamurthi Muralidharan
School of Chemistry, University of
Hyderabad, Gachibowli, Hyderabad,
500046, India
Capturing and converting carbon dioxide (CO2) into useful organic molecules
and polymers is the best way of alleviating excessive release of it from indus-
trial sources to the environment. Cyclic carbonate synthesis by cycloaddition
from CO2 and epoxides through a catalytic process is a 100% atom economic
reaction established five decades ago. Despite the availability of many efficient
catalytic systems, there are shortcomings in either tedious preparatory proce-
dure of catalyst, separation problem associated with homogeneous catalysis, or
the requirement of pure CO2. In this work, we report the catalytic system based
on copper and zinc mixed metal oxides (CuO-ZnO) nano-flakes/micro-flakes
(Cozi-nmf) for cycloaddition of CO2 with various epoxides at room tempera-
ture under solvent-free conditions. The novel recyclable heterogeneous catalyst
was prepared via a simple procedure successfully at room temperature by
grinding process. The synthesis of both catalyst and cyclic carbonates described
here is greener and used inexpensive source materials to produce them. The
yields of the synthesized cyclic carbonates were more than 95%, except for
many reactions. The high efficiency of Cozi-nmf as a catalyst is explained
based on the availability of the bare catalyst surface, which promoted the hin-
drance free movement of electrons and adsorption of substrate effectively.
Correspondence
Krishnamurthi Muralidharan, School of
Chemistry, University of Hyderabad,
Gachibowli, Hyderabad 500046, India.
Email: murali@uohyd.ac.in
Funding information
Department of Science and Technology,
Ministry of Science and Technology, India,
Grant/Award Number: PURSE and FIST
programs; University Grants Commission,
Grant/Award Number: BSR Fellowship,
CAS and UPE
K E Y W O R D S
carbon dioxide capture, catalysis, cyclic carbonates, mixed metal oxides
1 | INTRODUCTION
nontoxic renewable carbon resource,[3] the efficient
methods that convert it into valuable chemicals are much
Owing to the industrial revolution, the atmospheric car-
bon dioxide (CO2) level is increasing rapidly, which is the
primary cause of global warming. On the other hand, the
attractive CO2 capture reactions represent an alternative
and safer way of mitigating the excessive release of it from
various industrial sources.[1] Using CO2, it is possible to
prepare many organic compounds by carboxylation reac-
tion and specific transformation reactions into carboxylic
acids, organic carbonates, urea, methanol, and poly-
mers.[2] Because CO2 is an abundant, economic, and
attractive.[4] The production of cyclic carbonates by cyclo-
addition from CO2 and epoxides is a 100% atom economic
reaction established five decades ago.[5] However, because
of the low reactivity and thermal stability of CO2, the
transformation of it into several useful products always
involves a principal step of activating CO2 molecules.[6]
Cyclic carbonates are fascinating compounds finding
a wide range of applications such as electrolyte in
lithium-ion batteries,[7] polar aprotic solvents[8] interme-
diates for fine chemicals, and building blocks in the
Appl Organomet Chem. 2021;35:e6224.
wileyonlinelibrary.com/journal/aoc
© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6224

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VELPURI AND MURALIDHARAN
manufacture of pharmaceuticals.[9] The cyclic carbonates
with different functional groups are used as the precursor
materials for polymeric materials like polycarbonates,
polyurethanes, and polyesters.[10,11] There are many
reports on the development of catalytic systems to acti-
vate CO2. Until now, the efficient catalysts for the cyclo-
addition of CO2 with epoxides are confined to materials
such as ionic liquids.[12] organic metal complexes,[13]
organocatalysts, and quaternary ammonium.[14] Some of
the known catalytic systems for epoxide–CO2 coupling
includes Al (III)@cages,[15] Ti (HPO4)2ꢀH2O,[16] ZnCl2/
Al2O3,[17] succinimide-KI,[18] PCN-222(Co),[19] PIM2,[20]
and some complexes[21] of Zn, Co, Mo, Pd, Cr, Al, Cu,
Mg, and Sn. Much of the catalytic systems reported to the
cyclic carbonate synthesis are homogeneous catalysts.
Further, many of these reactions require high tempera-
tures and high pressures of pure CO2 to induce the reac-
tion with epoxides.
required high temperatures to produce copper oxide-zinc
oxide (CuO-ZnO) nanocomposite.[34] Therefore, we have
designed a simple procedure and synthesized the CuO-
ZnO nano-flakes/micro-flakes (Cozi-nmf) successfully
with the exposed surface and explored their catalytic activ-
ity for CO2 capture reactions. We have synthesized CuO
and ZnO in a 1:1 ratio to attain a high specific surface area.
We have established the novel recyclable heterogeneous
catalysis of Cozi-nmf having exposed surfaces for cycload-
dition of CO2 with various epoxides at room temperature
under solvent-free conditions. The synthesis described
here is greener and used inexpensive source materials to
produce catalyst and cyclic carbonates.
2 | EXPERIMENTAL SECTION
2.1 | Materials
Generally, the pure CO2 is obtained by purifying flue
gases containing large amounts of CO2 released from
power plants and industrial furnaces. These purification
processes are typically time-consuming and involve addi-
tional cost and energy. Despite the good catalytic behavior
manifested by the catalysts mentioned above, their intrin-
sic homogeneous nature precludes re-utilization and limits
their practical application on an industrial scale, especially
those involving tedious preparation procedures.[5] There-
fore, it is essential to develop highly efficient and stable
heterogeneous catalysts for converting industrial CO2 at
atmospheric pressure and the mild temperature.[22]
ZnCl2, CuCl2, and epoxides were purchased from Sigma-
Aldrich. Anhydrous sodium hydroxide was purchased
from Merck, India. Solvents were dried and distilled
using standard procedure and then used for work up and
column chromatography. Thin layer chromatography
was done using commercially available (MERCK) plates
with silica gel 60–120 mess.
2.2 | Instrumentation
The semiconducting oxides such as ZnO, TiO2, CeO2,
ZrO2, and SnO2 having different nanostructures are
appealing materials due to their unique properties. They
find applications in various areas including, catalysis,[23]
photocatalysis,[24] solar cells,[25] and gas sensors.[26] Due to
their exciting surface properties, the metal oxides related
to CuO and ZnO nanostructures like CuO-NiO, CuO-
MgO, CuO-Fe3O4, CuO-TiO2, NiO-ZnO, ZnO-TiO2, and
CuO-ZnO-ZrO2 have been studies elaborately.[27–32] Based
on many reports on mixed metal oxides, we are interested
in developing a versatile catalytic system based on copper
and zinc mixed metal oxides (CuO-ZnO) nano-particles/
micro-particles. These materials have been used in many
chemical, physics, and materials science fields due to their
diversified applications, including catalysis, gas sensors,
solar cells, and optoelectronics devices.[33]
The chemical reactions occur on the catalyst's surface.
To use metal oxides in the form of nano-particles/micro-
particles as a catalyst in any reaction, their active centers
at the surface should be free from any hindrance and
exposed to the interaction with the substrate. Moreover, a
critical scan in the literature revealed that most reactions
through different physical and chemical pathways
Powder X-ray diffraction (PXRD) patterns of Cozi-nmf
was recorded on a Bruker D8 X-ray diffractometer
(λ [Cu-Kα] = 1.54 Å), employing a scan rate of 1ꢁ min−1
for the range of 20ꢁC to 80ꢁC. Field emission-scanning
electron microscopy (FESEM) images of Cozi-nmf and
its energy-dispersive X-ray spectrum (EDAX) were
recorded using Ultra 55 Carl Zeiss instrument) operated
at variable voltages. The Cozi-nmf sample was
suspended in isopropanol and then dispersed on ITO
plates to record FESEM micrographs. TEM images of
Cozi-nmf were acquired by using an FEI Technai G2
20 STEM instrument at an acceleration voltage of 200 kV.
The Cozi-nmf sample for TEM analyses was dispersed in
isopropanol, and one to two drops of dispersion were
dripped onto a carbon-coated copper grid (200 mesh). XPS
spectra were recorded in a Thermo Scientific K Alpha
spectrometer equipped with a micro-focused monochro-
matic X-ray source (Al Kα, spot size ꢂ400 μm) operating
at 70 W. An optimum concentration of Cozi-nmf was
drop-casted onto a quartz substrate for the XPS analysis.
1H NMR and 13C NMR spectra of cyclic carbonates in
CDCl3 were recorded on a 500 or 400 MHz spectrometer
at room temperature. The chemical shifts (δ) are reported

VELPURI AND MURALIDHARAN
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in ppm scale downfield from the internal standard tetra-
methyl silane TMS (δ = 0.0). High-resolution mass spec-
tra (HRMS) were recorded using ESI-TOF techniques.
TGA was recorded on a PerkinElmer–STA 6000. Surface
area analysis was determined from the BET analysis,
which was recorded by the Quantachrome instruments.
Melting points of cyclic carbonates were measured on a
Buchi B-540 apparatus.
evaporated under a vacuum. The reaction mixture was
purified by column chromatography on silica gel with
ethyl acetate and hexane as eluent to get the purified cyclic
carbonates. All compounds were characterized based on
their spectroscopic data (S1–S12; 1H, 13C NMR, and
HRMS) compared to those reported in the literature.
2.5 | Spectral data of cyclic carbonates
2.5.1 | 4-phenyl-1,3-dioxolan-2-one
2.3 | Synthesis of nanocomposite copper
oxide-zinc oxide (CuO-ZnO) nano-flakes/
micro-flakes (Cozi-nmf)
Isolated yield: 93%, white solid, mp: 57ꢁC. 1H NMR
(500 MHz, CDCl3): δ = 4.36 (1H, t, J = 8 Hz), 4.82 (1H, t,
J = 8 Hz), 5.70 (1H, t, J = 8 Hz), 7.35–7.39 (2H, m),
7.43–7.46 (3H, m), and 13C NMR (126 MHz, CDCl3 MHz,
CDCl3): 71.11, 77.93, 125.79, 129.16, 129.64, 135.78,
154.79 HRMS calculated for C9H8O3 [M + Na]+:
187.1571, found: 187.1572.
The grinding process for synthesizing Cozi-nmf is
described as follows: CuCl2 (2 mmol) and ZnCl2 (2 mmol)
were mixed uniformly by grinding in an agate mortar for
5 min until the mixture turned black color. After that
solid NaOH (8 mmol) was added into the mortar, and the
grinding was continued for another 10 min. The resultant
black mass was gathered and washed with water
followed by absolute methanol several times by centrifu-
gation and then dried at 60ꢁC to get light black powder of
Cozi-nmf. The whole grinding process was operated at
room temperature and exposed to the air. We have
extended the reaction to prepare catalyst using 20 mmol
of the starting materials in a single reaction.
2.5.2 | 4-(Phenoxymethyl)-
1,3-dioxolan-2-one
1
Isolated yield: 97%, white solid, mp: 96–97ꢁC. H NMR
(400 MHz, CDCl3): δ = 4.16–4.18 (1H, dd, J = 10 Hz,
J = 7 Hz), 4.24–4.27 (1H, dd, J = 10 Hz, 6 Hz), 4.54–4.57
(1H, q), 4.64 (1H, t, J = 8 Hz), 5.02–5.07 (1H, m), 6.92–6.94
(m, 2H), 7.02–7.05 (1H, m), 7.31–7.34 (2H, m), and 13C
NMR (101 MHz, CDCl3 MHz, CDCl3): δ = 66.25, 66.94,
74.05, 114.65, 122.04, 129.70, 154.56, 157.76. HRMS calcu-
lated for C10H10O4 [M + Na]+: 217.0471, found: 217.0471.
2.4 | Synthesis of cyclic carbonate from
epoxide and CO2
In a typical reaction (Scheme 1), epoxide (1 mmol), t-butyl
ammonium bromide (0.05 mol), and catalyst, Cozi-nmf
(5 mg) were placed in a 50 ml round bottom flask filled
with CO2 gas from a CO2 cylinder. After mixing them, CO2
gas flow was continued purging through the needle dipped
in the reaction mixture. The reactions were continued at rt
for 3–6 h, and CO2 was provided by a gas cylinder (1 atm)
connected to the flask. After completing the reaction, it
was worked up with water and ethyl acetate and dried
using anhydrous Na2SO4. The organic solvent was
2.5.3 | 4-([4-Chlorophenoxy]methyl)-
1,3-dioxolan-2-one
Isolated yield: 95%. White solid, m.p. 101–102ꢁC. 1H
NMR (500 MHz, CDCl3): δ = 4.11–4.15 (1H, dd,
J = 10 Hz, 7 Hz), 4.22–4.26 (1H, dd, J = 10 Hz, 7 Hz),
4.52–4.56 (1H, q), 4.64 (1H, t, J = 8 Hz), 5.02–5.07 (1H,
m), 6.84–6.88 (2H, m), 7.25–7.29 (2H, m), and 13C NMR
(126 MHz, CDCl3): 66.10, 67.27, 73.96, 115.94, 127.03,
129.40, 129.62154.55, 156.36. HRMS calculated for
C10H9ClO4 [M]+: 228.0212, found: 228.0212.
2.5.4 | 4-([4-Bromophenoxy]methyl)-
1,3-dioxolan-2-one
1
Isolated yield: 94%. white solid, mp:103–104ꢁC, H NMR
(500 MHz, CDCl3): δ = 412–4.14 (1H, dd, J = 10 Hz,
7 Hz), 4.22–4.25(1H, dd, J = 10 Hz, 6 Hz), 4.52–4.55
SCHEME 1 Synthesis of cyclic carbonates from terminal
epoxides and CO2 at 1 bar pressure

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VELPURI AND MURALIDHARAN
(1H, q), 4.64 (1H, t, J = 8 Hz), 5.03–5.07 (1H, m),
6.80–6.83 (2H, m), 7.38–7.43(2H, m), and 13C NMR
(126 MHz, CDCl3): 66.11, 67.22, 74.00, 114.32, 116.45,
132.34, 132.55, 154.59, 156.89. HRMS calculated for
C10H9BrO4 [M]+: 271.9565, found: 271.9562, C10H9BrO4
[M + 2]+: 273.9540, found: 273.9540.
J = 10 Hz, 7 Hz), 4.22–4.25 (1H, dd, J = 11 Hz, 6 Hz),
4.52 (1H, q), 4.62 (1H, t, J = 8 Hz), 5.01–5.06 (1H, m),
6.85–6.88 (2H, m), 7.32–7.34 (2H, m), and 13C NMR
(126 MHz, CDCl3): 31.45, 34.15 66.30, 67.06, 74.10,
114.17, 126.47, 144.88, 154.60, 155.55. HRMS calculated
for C14H18O4 [M + NH4] +: 268.1420, found: 268.1420.
2.5.5 | 4-([o-Tolyloxy]methyl)-
1,3-dioxolan-2-one
2.5.9 | 4,40-((1,3-phenylenebis [oxy])bis
(methylene))bis(1,3-dioxolan-2-one)
Isolated yield:97%, white solid, mp: 96ꢁC, 1H NMR
(500 MHz, CDCl3): δ = 2.24 (3H, s), 4.14–4.17 (1H, dd,
J = 13 Hz, 7 Hz),, 4.27–4.30 (1H, dd, J = 14 Hz, 7 Hz),
4.59–4.62 (1H, q), 4.66 (1H, t, J = 8 Hz), 5.06–5.10(1H,
m), 6.78–6.81 (1H, t, J = 5 Hz), 6.93–6.96 (1H, m),
7.17–7.20 (2H, m), and 13C NMR (126 MHz, CDCl3):
16.00, 66.27, 66.99, 74.22, 110.79, 121.67, 126.90, 131.10,
154.83, 155.75. HRMS calculated for C11H12O4
[M + NH4]+: 226.0957, found: 226.0957.
Isolated yield: 90%, colorless gel; 1H NMR (400 MHz,
CDCl3): δ = 4.11–4.14 (2H, dd, J = Hz), 4.23–4.25 (2H, dd,
J = Hz, Hz), 4.51–4.54 (2H, q), 4.63 (2H, t, J = Hz),
5.03–5.07 (2H, m), 6.49 (1H, d, J = Hz), 6.55–6.57 (2H, d,
J = Hz), 7.19–7.23 (1H, t, J = Hz), and 13C NMR
(101 MHz, CDCl3): 66.14, 67.10, 67.15, 74.05, 101.97,
102.06, 107.95, 130.41, 154.81, 154.82, 159.06. HRMS calcu-
lated for C14H14O8 [M + Na] +: 333.0958, found: 333.0958.
2.5.10 | 4-([Benzyloxy]methyl)-
1,3-dioxolan-2-one
2.5.6 | 4-(([1,10-biphenyl]-2-yloxy)
methyl)-1,3-dioxolan-2-one
1
Isolated yield: 93%, colorless liquid, H NMR (400 MHz,
1
Isolated yield: 93%, white solid, mp:105–107ꢁC, H NMR
CDCl3): δ = 3.62–3.65 (1H, dd, J = 11 Hz, 7 Hz),
(500 MHz, CDCl3): δ = 3.60–3.70 (2H, m), 3.96–4.02 (2H,
m), 4.06–4.08 (1H, m), 7.01(1H, d, J = 8 Hz), 7.09 (1H, t,
J = 7 Hz), 7.32–7.36 (3H, m), 7.43 (2H,t, J = 7 Hz), 7.50
(2H, d J = 7 Hz), and 13C NMR (126 MHz, CDCl3):
δ = 63.63, 70.17, 70.49, 113.23, 121.81, 127.14, 128.15,
128.77, 129.38, 130.90, 131.49, 138.36, 155.28. HRMS calcu-
lated for C16H14O4 [M + Na]+: 293.0797, found: 293.0798.
3.71–3.74 (1H, dd, J = 11 Hz, 7 Hz), 4.39–4.42 (1H, q),
4.50 (1H, t, J = 8 Hz), 4.57–4.65(2H, m), 4.82–4.85 (1H,
m), 7.31–7.33(3H, m), 7.36–7.39 (2H, m), and 13C NMR
(101 MHz, CDCl3): 66.32, 68.84, 73.73, 75.00, 127.78,
128.11, 128.60, 137.06, 154.93. HRMS calculated for
C11H12O4 [M + NH4] +: 226.0954, found: 226.0953.
2.5.11 | 4-ethyl-1,3-dioxolan-2-one
2.5.7 | 4-([4-methoxyphenoxy]methyl)
-1,3-dioxolan-2-one
1
Isolated yield: 74%, colorless liquid, H NMR (500 MHz,
CDCl3): δ = 0.98–1.031 (3H, m), 1.71–1.85 (2H, m),
4.05–4.09 (1H, q), 4.50–4.53 (1H, t, J = 8 Hz), 4.62–4.68
(2H, m), and 13C NMR (126 MHz, CDCl3): 19.72, 68.97,
77.96, 155.06. HRMS calculated for C5H8O3 [M + NH4]
+: 134.1539, found: 134.1530.
1
Isolated yield: 96%, white solid, mp: 99–100ꢁC, H NMR
(400 MHz, CDCl3): δ = 3.13 (3H, s), 4.24 (2H, d,
J = 4 Hz), 4.62–4.64 (2H, m), 5.00–5.06 (1H, m),
6.89–6.97 (3H, m), 7.01–7.06 (1H, m), and 13C NMR
(101 MHz, CDCl3): 55.90, 66.42, 69.39, 74.45,
112.57,116.77, 121.04, 123.49, 147.46, 150.43, 154.77.
HRMS calculated for C11H12O5 [M + H] +: 225.0637,
found: 225.0636.
2.5.12 | 4-butyl-1,3-dioxolan-2-one
1
Isolated yield: 68%, colorless liquid, H NMR (500 MHz,
CDCl3): δ = 0.92 (3H, t), 1.34–1.46 (4H, m), 1.64–1.71
(1H, m), 1.77–1.84 (1H, m), 4.04–4.07 (1H, q), 4.52 (1H, t,
J = 8 Hz), 4.66–4.72 (1H, m), and 13C NMR (126 MHz,
CDCl3): 13.84, 22.29, 26.47, 33.61, 68.97, 77.38, 69.42,
155.14. HRMS calculated for C7H12O3 [M + NH4]
+: 167.0684, found: 167.0686.
2.5.8 | 4-((4-[tert-butyl]phenoxy)methyl)-
1,3-dioxolan-2-one
1
Isolated yield: 98%,white solid, mp:98–100ꢁC, H NMR
(500 MHz, CDCl3): δ = 1.31 (9H, s), 4.14–4.17 (1H, dd,

VELPURI AND MURALIDHARAN
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3 | RESULTS AND DISCUSSION
ZnO wurtzite phase (JCPDF No: 036–1451). However, dif-
fraction peaks at 2θ values of 31.6, 34.3, 36.0, 38.5, 47.3,
48.4, 53.5, 56.5, 57.9, and 61.3 (ꢁ) were identified to the lattice
3.1 | Room temperature synthesis of
catalyst and its characterization
−
−
−
planes of [(110), (002), ( 1 11), (111), ( 1 12), ( 2 02),
(020), (021), (202), and (113)], which were matching with
CuO monoclinic phase (JCPDS No: 80–0076). Further,
the existence of three diffraction peaks at 2θ values of Cu
at 31.3, 34.3, and 36.1 (ꢁ) being overlapped completely
with ZnO diffraction peaks with the variation of
0.85–1.14 (ꢁ), which established the formation of ZnO-
CuO composite material.
The analysis of morphological features of Cozi-nmf
were by FESEM images (Figure 2a,b) revealed the for-
mation of flake-like particles of CuO-ZnO composite
materials (Cozi-nmf) with thickness in nanometer's
order while overall size in microns. The elemental map-
ping images by EDS (Figure 2c) demonstrated the pres-
ence of oxygen, zinc, and copper elements throughout
architecture, whereas the atomic ratio was matching
with CuO-ZnO (Cu: Zn: O = 1:1:2) (Figure 2d–f). The
TEM, HRTEM, and SAED analyses of Cozi-nmf are
shown in Figure 3. The crystallinity of material was evi-
dent clearly from the HRTEM image (Figure 3c) The
selected area electron diffraction (SAED) indicated
(Figure 3d) corresponds to the plane (110) at d spacing
value at 2.7 Å of CuO, and the plane (101) at d spacing
value at 2.4 Å ZnO indicated the formation of CuO-
ZnO composite material and the bright spots revealed
the high crystalline nature.
The procedure involved in the synthesis of materials deter-
mines their usability on a large scale. In the present work,
Cozi-nmf was produced at room temperature by the
solvent-free grinding method easily within 10 min. The
required product was purified by water. The X-ray diffrac-
tion pattern gave preliminary information on the formation
of Cozi-nmf. The typical powder XRD pattern (Figure 1)
obtained from the sample of Cozi-nmf exhibited diffraction
peaks at 2θ values of 31.6, 34.3, 36.0, 47.3, 56.5, 62.7, 66.1,
67.7, and 68.8 (ꢁ). The representative (hkl) planes of the
corresponding diffraction peaks (100), (002), (101), (102),
(110), (103), (200), (112), and (201) were identified to the
X-ray photoelectron spectroscopy (XPS) analyses
(Figure 4) of Cozi-nmf disclosed the oxidation state of
elements in the composite. First, the XPS survey spectra
of the sample confirmed the presence of Cu, Zn, and O
atoms in Cozi-nmf. There are no other elements
observed except Si and C from the glass plate. The Zn2p
spectra showed two symmetric peaks, the peak around
1,024.0 eV corresponds to the Zn2p3/2, and the one
FIGURE 1 Powder X- ray diffraction spectrum of Cozi-nmf
FIGURE 2 FE-SEM image of Cozi-
nmf, 200 nm (a), 100 nm (b), EDS
spectrum, and (c), Cu, Zn, and O
elemental mapping (d–f)

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VELPURI AND MURALIDHARAN
FIGURE 3 TEM image of Cozi-nmf at
20 nm magnification (a), high resolution image
of Cozi-nmf (b, d), and SAED pattern (c)
FIGURE 4 XPS spectrum of Cozi-nmf
(a) Zn 2p, (b) Cu 2p, (c) O 1s, and (d) survey
spectrum
around 1,047.1 eV is assigned to Zn 2p1/2, indicating the
oxidation state of Zn (II) ion in the sample. In the Cu2p
spectrum, the peaks around 934.5 and 955.5 eV in all the
samples confirmed the presence of Cu 2p3/2 and Cu
2p1/2, indicating the oxidation state of Cu
(II) (Figure 4b). The O 1s peaks observed around
533.3 eV was assigned to oxygen (Figure 4c).
3.2 | Stability and surface area
The thermal stability of Cozi-nmf was investigated by
thermal gravimetric analysis (TGA). The composite Cozi-
nmf underwent a sharp weight loss of about 6.3% up to
250ꢁC, corresponding to the surface adsorbed residues
(Figure 5). After an initial weight loss, the composite was

VELPURI AND MURALIDHARAN
7 of 11
losing weight of about 4% slowly stable up to 800ꢁC. The
availability of surface freely for the interaction with
the substrate is essential for effective catalysis. Therefore,
the specific surface area of Cozi-nmf was measured
based on single gas adsorption at a constant temperature,
whereas nitrogen gas was used as an adsorbate. The N2
adsorption–desorption isotherm of Cozi-nmf (Figure 6)
was determined from the data points in this pressure
range P/P0 of 0–1.0, and the surface area was calculated
as 45.8 m2 g−1. Envisioning here is that this entire surface
area will be available for catalysis without any hindrance
for the catalytic activity to produce cyclic carbonates.
FIGURE 5 Thermogravimetric analysis of Cozi-nmf
3.3 | Catalytic activity
The metal oxides in the form of nano-/micro-particles are
preferred as catalysts compared to their bulk counterparts
because of increased surface area. However, many chemi-
cal synthesis procedures of nanoparticles use various
long-chain surfactant molecules to stabilize the size of
the particles. These organic surfactant molecules not only
hide the surface but also hinder the electron movement.
Therefore, it is imperative to produce a catalyst whose
surface should be free from any hindrance and exposed
for substrate interaction. Thus, Cozi-nmf is an attractive
material for catalysis. We chose the coupling of phenyl
glycidyl ether (PGE) with CO2 in the presence of Cozi-
nmf as a catalyst to afford phenyl glycidyl carbonate
(PGC) as the model reaction (Scheme 1). First, we have
scanned the reaction conditions, including the reaction
FIGURE 6 N2 adsorption and desorption isotherm of Cozi-nmf
TABLE 1 Solvent optimization
using the model reaction the presence
of Cozi-nmf
Entry
Solvent
CH3OH
toluene
CH3CN
H2O
Time
24 h
24 h
24 h
24 h
24 h
24 h
24 h
6 h
Yield (%)
1
2
3
4
5
6
7
8
15
20
23
No
26
CHCl3
DMSO
DMF
No
No
97
No solvent

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VELPURI AND MURALIDHARAN
temperature, reaction time, and CO2 pressure (1 atm), to
optimize the cyclic carbonates yield. Among the various
reactions tried with solvents like toluene, CH3CN,
CH3OH, CHCl3, DMSO, DMF, and H2O, the reaction
without solvent showed superior activity over other sol-
vents (Table 1) because of the low solubility of CO2 into
conventional solvents.[35]
To test the advantage of having mixed metal oxide
composite as the catalyst, we have performed controlled
reactions of PGE with CO2 under the optimized condition
in the presence of ZnO and CuO nanoparticles, also the
catalyst Cozi-nmf (Table 2). No product was obtained
when the reaction was performed in the absence of Cozi-
nmf. The observed yield of PGC in the presence of ZnO
and CuO was 15% and 35%, whereas a much higher 97%
yield was obtained using Cozi-nmf as the catalyst. The
high surface area of Cozi-nmf that available freely with-
out any capping agent was the reason for better catalytic
activity in this CO2 fixation reaction.
transformed into the corresponding cyclic carbonates
with almost quantitative yields (93–98%) within 3–6 h
under at 1 atm CO2 pressure. Other than phenyl glycidyl
ether, the reaction with three different substrates
(Table 3), styrene oxide (Entry 1, 93%) and 1, 2 epoxy
butane (Entry 11, 74%), and 1, 2 epoxy hexane yielded
the corresponding cyclic carbonate in moderate to good
yields (Entry 12, 68%).
We have compared the catalytic activity of Cozi-nmf
with that of known heterogeneous catalysts used for the
synthesis of cyclic carbonates (Table 4). The comparative
study showed that other reported reactions involved use
of protic compounds, additives, and solvents for epoxide
activation. Moreover, those catalytic systems required
either high pressure,[36] high temperature to activate
CO2, whereas our designed Cozi-nmf having high Lewis
acidic nature of surface might facilitate CO2 insertion at
rt with 1 atm pressure. The simplicity of synthesis makes
Cozi-nmf stand in a better position.
The reaction without solvent at room temperature for
3 h using scanty 5 mg (0.06 mmol) of Cozi-nmf at 1 atm
pressure of CO2 (Table 3, entry 8) gave the best result in
terms of yield (98%) and time. In contrast, with less than
5 mg of Cozi-nmf, the reaction became sluggish with a
low product yield. Using the optimized reaction condi-
tions, we have studied the performance of Cozi-nmf in
the chemical fixation of CO2 over different functional
group substituted epoxides. The epoxides, such as phenyl
glycidyl ether substituted with electron-donating [–CH3,
–OCH3, –C (CH3)3] and electron-withdrawing groups
(–Cl, –Br, –Ph) were used as the substrate in the reaction.
As shown in Table 3 (Entries 1–11), most substrates were
converted efficiently to corresponding cyclic carbonates
with excellent selectivity and high conversion. It was
observed that terminal epoxides with both electron-
donating and electron-withdrawing groups were
3.4 | Recyclability of Cozi-nmf as catalyst
A heterogeneous catalyst needs to be examined for its
ease of separation, recoverability, and reusability. The
reusability of Cozi-nmf was examined in the synthesis of
cyclic carbonate using the illustrative reaction between
CO2 and PGE. After the reaction time, the reaction mix-
ture was centrifuged to separate the solid Cozi-nmf from
the reaction mixture. The catalyst was then thoroughly
washed with distilled water followed by methanol and
dried in an oven at 100ꢁC for 5 h before reuse. As seen in
Figure 7, the catalyst can be efficiently recycled and
reused five times. The results indicated that after five-
time reuse of catalyst, the product's yield decreased from
97% to 90%.
TABLE 2 Synthesis of
phenylglycidyl carbonates with
different catalysts
Entry
Epoxide
Catalyst
CuO
Yield (%)
1
2
3
2
2
2
35
15
97
ZnO
Cozi-nmf

VELPURI AND MURALIDHARAN
9 of 11
TABLE 3 Synthesis of different cyclic carbonates by using Cozi-nmf as catalyst
(3)
(1)
(2)
Yield: 93%
Time: 4 h
Yield: 97%
Time: 3 h
Yield: 95%
Time: 4 h
(4)
(5)
(6)
Yield: 97%
Time: 3 h
Yield: 94%
Time: 5 h
Yield: 93%
Time: 6 h
(7)
(8)
(9)
Yield: 96%
Time: 4 h
Yield: 98%
Time: 3 h
Yield: 90%
Time: 6 h
(10)
(11)
(12)
Yield: 93%
Time: 6 h
Yield: 74%
Time: 6 h
Yield: 68%
Time: 6 h
Note. Reaction conditions: epoxide (1 mmol), Cozi-nmf (5 mg), rt, 1 atm of CO2. Yields refer to those of isolated pure product.

10 of 11
VELPURI AND MURALIDHARAN
TABLE 4 Comparative study on the catalytic activity of Cozi-nmf with numerous catalysts using PGE as substrate
Catalyst
CO2 (bar)
Time (h)
Temperature (ꢁC)
Yield (%)
Ref.
Al (III)@cages
Ti (HPO4)2ꢀH2O
ZnCl2/Al2O3
Succinimide-KI
PCN-222(Co)
PIM2
1
48
6
RT
RT
100
70
54
87
100
94
90
96
97
Ng et al.[15]
1
Chowdhury et al.[16]
Bondarenkoa et al.[17]
Li et al.[18]
Carrasco et al.[19]
Wang et al.[20]
This work
1
4
0.4
1
6
18
8
50
1
130
RT
Cozi-nmf
1
3
(for TEM) University of Hyderabad. Financial and infra-
structure support from the University Grants Commis-
sion, New Delhi, India (through the UPE and CAS
programs) and the Department of Science and Technol-
ogy, Ministry of Science and Technology, India (through
the PURSE and FIST programs), is gratefully
acknowledged.
AUTHOR CONTRIBUTIONS
Venkateswara Rao Velpuri: Conceptualization; formal
analysis; investigation; methodology. Krishnamurthi
Muralidharan: Conceptualization; supervision.
FIGURE 7 Recycling results of the CuO-ZnO catalyst in the
synthesis of cyclic carbonates
4 | CONCLUSION
ORCID
Venkateswara Rao Velpuri https://orcid.org/0000-0002-
2702-338X
Krishnamurthi Muralidharan https://orcid.org/0000-
0003-0529-2447
We have synthesized Cozi-nmf with a high surface area
of around 45.8 m2 g−1 by grinding method within 10 min.
It was used as the catalyst for cycloaddition of CO2 with
various epoxides to produce cyclic carbonates. The
syntheses described here were performed in solvent-free
conditions and used inexpensive and recyclable
surfactant-free heterogeneous catalysts. The yields of the
synthesized products were more than 95% except for sty-
rene and aliphatic carbonates. The high efficiency of
Cozi-nmf as a catalyst is because of the availability of
the bare catalyst surface, which promoted the hindrance
free movement of electrons and adsorption of substrate
effectively. Though many other CO2 capture reactions are
known to proceed in the solvent-free condition, the sim-
plicity in the synthesis of Cozi-nmf and cyclic carbonates
promises large-scale use of the method.
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SUPPORTING INFORMATION
Additional supporting information may be found online
in the Supporting Information section at the end of this
article.
How to cite this article: Velpuri VR,
Muralidharan K. High yield room temperature
conversion of carbon dioxide into cyclic carbonates
catalyzed by mixed metal oxide (CuO-ZnO) nano-
flakes/micro-flakes (Cozi-nmf). Appl Organomet
Chem. 2021;35:e6224. https://doi.org/10.1002/
aoc.6224
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