Electrosynthesis of cyclic carbonates from CO2 and epoxides on a reusable copper nanoparticle cathode

Article abstract of DOI:10.1039/c4ra17287f

A highly stable and reusable Cu NP cathode was prepared by a simple method and used for the electrosynthesis of cyclic carbonates by electroreduction of CO₂ in the presence of epoxides under mild conditions. No added metal catalyst is required and the yields vary from moderate to very good. Furthermore, the activity of the cathode was shown to depend on the size of the Cu NPs.

Full text of DOI:10.1039/c4ra17287f

RSC Advances
View Article Online
View Journal | View Issue
COMMUNICATION
Electrosynthesis of cyclic carbonates from CO2 and
epoxides on a reusable copper nanoparticle
cathode†
Cite this: RSC Adv., 2015, 5, 23189
b
b
b
Laxia Wu,ab Hengpan Yang, Huan Wang and Jiaxing Lu
Received 31st December 2014
Accepted 24th February 2015
*
DOI: 10.1039/c4ra17287f
www.rsc.org/advances
A highly stable and reusable Cu NP cathode was prepared by a simple simple and efficient methodology for the activation of CO2 is a
method and used for the electrosynthesis of cyclic carbonates by demanding challenge for organic chemists.
electroreduction of CO2 in the presence of epoxides under mild
In our former work, electrocatalysis was demonstrated to be
conditions. No added metal catalyst is required and the yields vary an efficient method for the activation of CO2, which could be
from moderate to very good. Furthermore, the activity of the cathode performed under mild conditions with a remarkable effi-
was shown to depend on the size of the Cu NPs.
ciency.17–20 In addition, we employed NHCs as CO2 trans-
formation catalysts with diols, thereby providing a new
procedure to the synthesis of cyclic carbonates.21 In the conti-
nuity of our previous works, we further develop synthesis
methods of cyclic carbonates. In this work, copper nano-
particles (Cu NPs) were successfully prepared by the direct
reduction of copper sulfate with hydrazine hydrate in aqueous
solution. They were compacted into a coin (Fig. S1†) and used as
cathode for the reaction of CO2 with epoxides (Fig. 1). Cyclic
carbonates were obtained in yields ranging from 31% to 86%.
Both the synthesis of Cu NPs and cyclic carbonates were per-
formed under mild conditions, without any added metal
catalyst.
Carbon dioxide is expected to be a non-toxic, inexpensive,
abundant and renewable C1 feedstock.1 Much effort has been
made to develop effective processes for desirable, economically
competitive products from carbon dioxide.2–8 Among them,
cyclic carbonates have commercial importance because of their
extensive applications in chemical and pharmaceutical indus-
tries. They can be used as electrolytes for lithium ion batteries,
as organic solvents, fuel additives, and green reagents.9 In
addition, cyclic carbonates are important raw materials for
polyurethane synthesis, production of urea derivatives, and as
alternatives to phosgene or dimethyl sulfate for methylation
reactions.10 Due to their important roles, much attention has
been attracted to developing synthesis methods. One of the
most promising and atom economical methodologies for the
synthesis of ve-membered cyclic carbonates is the carboxyla-
tion of epoxides with CO2. However, metal complex catalysts are
oen used in typical synthesis procedures, because CO2 is very
stable thermodynamically and quite kinetically inert.11–15
Recently, organic–inorganic hybrid zeolites have also been used
for the synthesis of a variety of cyclic carbonates from epoxides
and CO2 with very good TONs.16 Therefore, the development of a
The synthesis of Cu NPs involved the reduction of CuSO4 by
N2H4$H2O in an aqueous solution. Powder with metallic luster
was achieved aer ltration and drying. They were compacted
into a coin using a tablet press. Firstly, the Cu NPs electrode was
characterized by multiple methods. Fig. 2D displays the X-ray
aSchool of Chemistry and Chemical Engineering, Anqing Normal University, Anqing
246011, China
bShanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, Shanghai
200062, China. E-mail: jxlu@chem.ecnu.edu.cn; Tel: +86-21-62233491
† Electronic supplementary information (ESI) available: Materials, instruments,
experimental details, characterization of Cu nanoparticles and characterization
data for all products. See DOI: 10.1039/c4ra17287f
Fig. 1 Electrosynthesis of cyclic carbonates from CO2 and epoxides
on copper nanoparticles cathode.
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 23189–23192 | 23189

View Article Online
RSC Advances
Communication
Table 1 Electrosynthesis of 2a from 1a and CO2 on Cu NPs (100 nm)
cathodea
Charge
Current density Temperature
Entry Cathode (F molÀ1
)
(mA cmÀ2
)
(ꢁC)
Yieldb (%)
1
2
3
4
5
6
7
8
Cu ake 2.5
Cu NPs 1.0
Cu NPs 1.5
Cu NPs 2.0
Cu NPs 2.5
Cu NPs 3.0
Cu NPs 2.5
Cu NPs 2.5
Cu NPs 2.5
Cu NPs 2.5
Cu NPs 2.5
Cu NPs 2.5
Cu NPs 2.5
Cu NPsc 2.5
Cu NPsd 2.5
Cu NPse 2.5
3.0
3.0
3.0
3.0
3.0
3.0
2.0
4.0
6.0
9.0
3.0
3.0
3.0
3.0
3.0
3.0
25
25
25
25
25
25
25
25
25
25
15
35
50
25
25
25
21
52
66
73
86
85
71
69
59
41
54
60
52
72
89
93
9
10
11
12
13
14
15
16
a
General conditions, 1a concentration: 0.1 M, MeCN: 10 mL, TEAI
concentration: 0.1 M, anode: Mg, cathode diameter: 2 cm, CO2
b
pressure: 1 atm. Mass yield, determined by gas chromatograph (GC).
c
d
e
Cu NPs (300 nm) cathode. Cu NPs (50 nm) cathode. Cu NPs (10
nm) cathode.
To optimize the yield of 2a, the effect of various parameters
on the process such as the charge passed, the current density,
and the temperature was investigated. The results of the elec-
trolyses are summarized in Table 1.
The charge passed during electrolysis strongly inuenced
the yield of product 2a (Table 1, entries 2–6). The yield increased
linearly with the charge from 1.0 to 2.5 F molÀ1. Whereas the
yield did not increase further by increasing the charge to
3.0 F molÀ1 (Table 1, entry 6). The current density also inu-
enced the yield of 2a (Table 1, entries 5 and 7–10). The highest
yield was obtained at 3 mA cmÀ2. Both lower and higher current
Fig. 2 Characterization of Cu NPs. FE-SEM patterns of Cu NPs (100
nm) before compacting (A), after compacting by tablet press (B, E),
after being used for 10 times (C); XRD patterns of Cu NPs electrode
before use (a) and after 10 runs (b) (D). FE-SEM patterns of Cu NPs in
the size range of ꢀ300 nm (F) and 50 nm (G). TEM patterns of Cu NPs
in the size range of 10 nm (H).
diffraction (XRD) patterns of a Cu NPs electrode. Typical densities led to lower yields. At lower current densities, there
diffraction peaks of Cu are observed (Fig. 2D, a), and there is no might not be enough CO2 radical anions produced to react
trace of any other substance such as copper oxides. The SEM efficiently with the epoxide. At higher current densities, the
pattern shows that Cu NPs have a porous structure (Fig. 2A) amount of CO2 radical anions would also be less due to their
resulting from the aggregation of Cu nanograins of the ꢀ100 conversion to CO and oxalate dianion.22
nm size range. Aer being compacted into a coin of 2 cm
The temperature is also a crucial factor for this reaction.
diameter with a tablet press, the size range and porous struc- Normally, the lower the temperature is, the more CO2 could be
tures of Cu NPs electrode (Fig. 2B and E) didn't change. The Cu dissolved in MeCN. As CO2 is the key reagent in this reaction, the
NPs (100 nm) have an average specic surface area of 4.3 m2 gÀ1 concentration of CO2, which in fact depends on the tempera-
according to nitrogen adsorption–desorption isotherms. 2 g Cu ture,23 may affectthe 2ayield. On theotherhand, thetemperature
NPs were compacted into an electrode of 3.14 cm2 geometric will inuence the over potential and reaction rate of the elec-
area, which corresponds to a real surface area of 8.6 m2. trochemical reaction, which may have an impact on the 2a yield
Namely, the real surface area of Cu NPs electrode is 2.7 Â 104 too. Therefore, the effect of the reaction temperature was exam-
times larger than its geometric area.
ined (Table 1, entries 5 and 11–13). Increasing the temperature
Propylene oxide (1a) was chosen as the model compound to from 15 ꢁC to 25 ꢁC favored the reaction (Table 1, entries 6 and
be investigated. A typical galvanostatic electrosynthesis was 11), but a further increase resulted in lower yields (Table 1,
carried out in a mixture of 0.1 M 1a, 0.1 M tetraethylammonium entries 12 and 13). The highest yield was achieved at 25 ꢁC.
iodide (TEAI) in 10 mL acetonitrile (MeCN) using an undivided
The effectiveness of different Cu cathodes was also studied.
glass cell, with a Cu NPs (100 nm) cathode and sacricial Parallel tests were carried out under the same conditions, expect
magnesium (Mg) anode. Correspondingly, propylene carbonate for the cathode. A 21% yield was obtained at the Cu ake cathode
(2a) was obtained (Fig. 1).
as compared to an 86% yield at the Cu NPs (100 nm) cathode
23190 | RSC Adv., 2015, 5, 23189–23192
This journal is © The Royal Society of Chemistry 2015

View Article Online
Communication
RSC Advances
(Table 1, entries 1 and 5). Cu NPs are obviously superior to Cu
akes. To conrm this superiority, Cu NPs with different particle
sizes (300 nm: Fig. 2F, 50 nm: Fig. 2G, 10 nm: Fig. 2H) but similar
crystal form (Fig. S2†) were prepared using multiple methods
(see ESI†) and used for the same reaction. The highest yield was
obtained with the smallest 10 nm particles (Table 1, entry 16)
and the lowest yield with the largest 300 nm particles (entry 14).
The 100 and 50 nm particles gave similar yields (entries 5 and
15). These results show that a compacted Cu NPs electrode is
much more effective than a Cu ake electrode. This agrees with
the reports of other workers showing that smaller particles of Cu
NPs are more active for the electroreduction of CO2.24 In addi-
tion, the electroreduction of CO2 was demonstrated to be the
rst and key step for the synthesis of cyclic carbonates.20 Hence,
the higher effectiveness might be partially attributed to the far
smaller particle size and higher surface area of Cu NPs elec-
trodes. The electrocatalytic activity of Cu ake and Cu NPs
cathode were further compared by cyclic voltammetry.
Fig. 4 Reuse of Cu NPs electrode. Reaction conditions as Table 1
entry 5.
The cyclic voltammograms were recorded in 0.1 M TEAI–
MeCN solution at a sweep rate of 0.2 V sÀ1 (Fig. 3). Compared to
those in N2-saturated solution, a more positive onset potential
and a higher current density were observed in CO2-saturated
solution, which were due to the electroreduction of CO2. In
comparison to the Cu ake cathode, the Cu NPs cathode
exhibited a signicantly higher catalytic activity (larger current
density). This activity increases as the particle size decreases,
the cathode with the smallest Cu NPs (ꢀ10 nm) being almost six
times as active as the Cu ake cathode. This is in agreement
with the electrolysis results: the yield of carbonate increases as
the particle size of the Cu NPs cathode decreases.
2a yield remained around 85% aer 10 runs. To further inves-
tigate the stability of the Cu NPs electrode, it was characterized
by X-ray diffraction before (Fig. 2D, a) and aer (Fig. 2D, b) 10
electrolyses. According to the XRD patterns, its composition did
not change. Moreover, it retained its porous structures and had
the same particle size (Fig. 2C). Therefore, the Cu NPs electrode
is stable and reusable.
Encouraged by the excellent results obtained with propylene
oxide (1a), the reaction was further studied under the condi-
tions of Table 1, entry 5, with the following epoxides: epichlo-
rohydrin (1b), 1-butene oxide (1c), 1-pentene oxide (1d), 2-
The major disadvantage of typical method for the synthesis
of cyclic carbonates involves the utilization of valuable and
hardly recyclable catalysts.11–15 In contrast, no additive catalysts
were needed in our synthesis system. What's more, Cu NPs
electrode has remarkable stability and reusability. It could be
easily cleaned aer each electrolysis and reused. The process
was repeated 10 times with the Cu NPs electrode under the
electrolysis conditions of Table 1, entry 5. As shown in Fig. 4, the
Table 2 Electrosynthesis of cyclic carbonates from CO2 and different
substrates on Cu NPs (100 nm) electrodea
Entry
Substrate
Product
Yieldb (%)
1
2
1a
1b
2a
2b
86
39
3
4
1c
2c
68
54
1d
2d
5
6
7
1e
1f
2e
2f
42
36
31
1g
2g
8
1h
2h
70
a
Substrate concentration: 100 mM, charge: 2.5 F molÀ1, current
density:
3 ,
mA cmÀ2 temperature: 25 ꢁC, MeCN: 10 mL, TEAI
Fig. 3 Cyclic voltammograms recorded at different cathodes in 0.1 M
TEAI–MeCN solution at a sweep rate of 0.2 V sÀ1 at 25 ꢁC.
concentration: 0.1 M, anode: Mg, cathode: Cu NPs, CO2 pressure: 1
atm. b Mass yield, determined by gas chromatograph (GC).
This journal is © The Royal Society of Chemistry 2015
RSC Adv., 2015, 5, 23189–23192 | 23191

View Article Online
RSC Advances
Communication
butene oxide (1e), cyclopentene oxide (1f), cyclohexene oxide
(1g), and styrene oxide (1h). The results are summarized in
Table 2. The low yield of cyclic carbonate 2b (entry 2) might be
attributed to secondary reactions involving the CH2Cl substit-
uent. The yield of cyclic carbonates decreases in the order 2a, 2c,
7 O. Jacquet, C. D. Gomes, M. Ephritikhine and T. Cantat,
ChemCatChem, 2013, 5, 117–120.
8 M. North, R. Pasquale and C. Young, Green Chem., 2010, 12,
1514–1539.
9 P. Tundo and M. Selva, Acc. Chem. Res., 2002, 35, 706–716.
and 2d (entries 1, 3 and 4) as the size of the alkyl substituent of 10 W. N. Haworth and H. Machemer, J. Chem. Soc., 1932, 2270–
the epoxide varies from methyl, ethyl, and propyl. The lowest 2277.
yields of cyclic carbonates were obtained with the disubstituted 11 H. W. Jing, T. Chang, L. L. Jin, M. Wu and W. Y. Qiu, Catal.
epoxides 1e, 1f, and 1g (entries 5–7) probably as a result of the Commun., 2007, 8, 1630–1634.
lesser reactivity of a methine carbon as compared to a methy- 12 C. X. Miao, J. Q. Wang, Y. Wu, Y. Du and L. N. He,
lene carbon (steric effect). The yield of 2h (entry 8) is compa- ChemSusChem, 2008, 1, 236–241.
rable to that of 2c (entry 3) and higher than that of 2d, which 13 J. Melendez, M. North and P. Villuendas, Chem. Commun.,
might be due to the weakening of the benzylic C–O bond of the
oxirane ring of 1h by conjugation to the phenyl ring.
2009, 18, 2577–2579.
14 X. Zhang, Y. B. Jia, X. B. Lu, B. Li, H. Wang and L. C. Sun,
Tetrahedron Lett., 2008, 49, 6589–6592.
In conclusion, an effective and easily prepared compact Cu
NPs electrode has been used in the electrochemical reaction of 15 C. J. Whiteoak, E. Martin, M. M. Belmonate, J. Benet-
CO2 with various epoxides. The corresponding cyclic carbonates
were obtained in moderate to very good yields, without any
Buchholz and A. W. Kleij, Adv. Synth. Catal., 2012, 354,
469–476.
metal-catalyst additives. The Cu NPs electrode has remarkable 16 C. G. Li, L. Xu, P. Wu, H. H. Wu and M. Y. He, Chem.
stability and can be recycled many times.
Commun., 2014, 50, 15764–15767.
17 L. Zhang, D. F. Niu, K. Zhang, G. R. Zhang, Y. W. Luo and
J. X. Lu, Green Chem., 2008, 10, 202–206.
18 H. Wang, L. X. Wu, J. Q. Zhao, R. N. Li, A. J. Zhang,
H. Kajiura, Y. M. Li and J. X. Lu, Greenhouse Gases: Sci.
Technol., 2012, 2, 59–65.
Acknowledgements
Financial support from National Natural Science Foundation of
China (21173085, 21203066, 21373090) is gratefully
acknowledged.
19 L. X. Wu, H. Wang, Y. Xiao, Z. Y. Tu, B. B. Ding and J. X. Lu,
Electrochem. Commun., 2012, 25, 116–118.
20 Y. Xiao, B. L. Chen, H. P. Yang, H. Wang and J. X. Lu,
Electrochem. Commun., 2014, 43, 71–74.
21 L. X. Wu, H. Wang, Z. Y. Tu, B. B. Ding, Y. Xiao and J. X. Lu,
Int. J. Electrochem. Sci., 2012, 7, 11540–11549.
Notes and references
1 T. Sakakura, J. Choi and H. Yasuda, Chem. Rev., 2007, 107,
2365–2387.
2 C. A. Huff and M. S. Sanford, J. Am. Chem. Soc., 2011, 133,
18122–18125.
22 A. Gennaro, A. A. Isse, M. G. Severin, E. Vianello, I. Bhugun
´
and J. M. Saveant, J. Chem. Soc., Faraday Trans., 1996, 92,
3 Z. F. Zhang, S. Q. Hu, J. L. Song, W. J. Li, G. Y. Yang and
B. X. Han, ChemSusChem, 2009, 2, 234–238.
3963–3968.
23 A. Gennaro, A. A. Isse and E. Vianello, J. Electroanal. Chem.,
1990, 289, 203–215.
24 R. Reske, H. Mistry, F. Behafarid, B. R. Cuenya and
P. Strasser, J. Am. Chem. Soc., 2014, 136, 6978–6986.
4 S. G. Liang, H. Z. Liu, T. Jiang, J. L. Song, G. Y. Yang and
B. X. Han, Chem. Commun., 2011, 47, 2131–2133.
5 F. Shi, Y. Q. Deng, T. L. SiMa, J. J. Peng, Y. L. Gu and
B. T. Qiao, Angew. Chem., Int. Ed., 2003, 42, 3257–3260.
6 B. Yu, H. Y. Zhang, Y. F. Zhao, S. Chen, J. L. Xu, C. L. Huang
and Z. M. Liu, Green Chem., 2013, 15, 95–99.
23192 | RSC Adv., 2015, 5, 23189–23192
This journal is © The Royal Society of Chemistry 2015