Biomass-derived Cu/porous carbon for the electrocatalytic synthesis of cyclic carbonates from CO2and diols under mild conditions

Article abstract of DOI:10.1039/d0nj01728k

Biomass-derived Cu/porous carbon (Cu/PC) composites were facilely assembled by using a one-pot hydrothermal approach combined with calcination under a N2 atmosphere, and were used for the electrocatalytic synthesis of cyclic carbonates from CO2 and diols at room temperature and normal pressure without any other catalysts. The results show that the Cu/PC composites with large specific surface areas (SBET > 279 m2 g-1) and high pore volumes (Vp > 0.47 cm3 g-1) possess good catalytic activity for CO2 electrocatalytic fixation. The Cu nanoparticles were uniformly dispersed in the PC, resulting in a remarkable increase in the catalytic activity of the composite compared with those of pure PC and Cu sheets. In addition, the Cu content influenced the electrocatalytic activity. Among the materials used, Cu/PC-III was the best cathode as it had a 46.3percent electrosynthesis yield of propylene carbonate, wide application prospects and was suitable for the synthesis of other o-diols. The yield of 4-propyl-1,3-dioxolan-2-one reached 57.8percent. The prepared composites also displayed satisfactory reusability, and the yield was not reduced even after six cycles of use. These results reveal that the composites could be effectively used for the electrosynthesis of cyclic carbonates from CO2 and diols under mild conditions.

Full text of DOI:10.1039/d0nj01728k

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Biomass-derived Cu/porous carbon for the
electrocatalytic synthesis of cyclic carbonates
Cite this:DOI: 10.1039/d0nj01728k
from CO2 and diols under mild conditions†
Jing-Jie Zhang, Shi-Ming Li, Yi Shi, Qiao-Li Hu, Huan Wang* and Jia-Xing Lu
*
Biomass-derived Cu/porous carbon (Cu/PC) composites were facilely assembled by using a one-pot
hydrothermal approach combined with calcination under a N2 atmosphere, and were used for the
electrocatalytic synthesis of cyclic carbonates from CO2 and diols at room temperature and normal
pressure without any other catalysts. The results show that the Cu/PC composites with large specific
surface areas (SBET 4 279 m2 À1) and high pore volumes (Vp 4 0.47 cm3 À1) possess good catalytic
g g
activity for CO2 electrocatalytic fixation. The Cu nanoparticles were uniformly dispersed in the PC,
resulting in a remarkable increase in the catalytic activity of the composite compared with those of pure
PC and Cu sheets. In addition, the Cu content influenced the electrocatalytic activity. Among the
materials used, Cu/PC-III was the best cathode as it had a 46.3% electrosynthesis yield of propylene
carbonate, wide application prospects and was suitable for the synthesis of other o-diols. The yield of
4-propyl-1,3-dioxolan-2-one reached 57.8%. The prepared composites also displayed satisfactory
reusability, and the yield was not reduced even after six cycles of use. These results reveal that the
composites could be effectively used for the electrosynthesis of cyclic carbonates from CO2 and diols
under mild conditions.
Received 7th April 2020,
Accepted 10th June 2020
DOI: 10.1039/d0nj01728k
rsc.li/njc
Propylene carbonate can also be acquired by direct and indirect
electrosynthesis in MeCN and ionic liquids at room temperature and
Introduction
CO2 is a harmful greenhouse gas but also a cheap, non-toxic C1 normal pressure, but the yield was relatively low (4–21%)11,15,16
resource that is easy to transport and store.1–3 The conversion of CO2 possibly because the cathodes of this electrolysis are flat, and
into useful chemicals has attracted widespread attention.4–7 The use their surface area is fairly small. Hence, the active sites are
of CO2 as a raw material for the synthesis of organic carbonates, relatively few, and the yield is low. Therefore, a cathode with a
especially cyclic carbonates, is an effective approach for its fixation large specific surface area and many active sites is required.
and utilization.8 Five-membered cyclic carbonates are commercially Biomass-derived carbon materials have attracted attention
and widely used as excellent aprotic polar solvents, electrolytes in because they have a large specific surface area, a pore rich
lithium secondary batteries, precursors for polycarbonates and other structure, good charge flow and electrical conductivity, which
polymeric materials, and more recently as useful intermediates in are suitable for the electrochemical field. The commonly used
organic synthesis.9–12 Propylene carbonate can be acquired using biomass includes poplar catkins,17 mangosteen shells,18
conventional organic synthesis, but the reaction needs a complex bamboos19 and xylose.20 Preparing carbon materials from
catalyst, and the conversion levels are low. Du et al.13 used the biomass is cheap and environmentally friendly.21–23 Among
magnesium catalyst to synthesize propylene carbonate with 100% these materials, xylose can be used for the preparation of
selectivity; however, the yield was only 2.8%. Huang et al.14 prepared carbon materials with abundant pore structures using green
a series of zinc oxide catalysts by using alkali salts as modifiers. and facile methods, which are advantageous for electrocatalytic
Among these catalysts, the KI-modified catalyst showed the best reactions. In addition, transition metals are recognized as
catalytic efficiency (the yield of propylene carbonate being 26.0%). powerful materials for CO2 activation.24,25 Among them, copper
has attracted much attention because it is cheap and has high
activity.26 Hence, the xylose-derived carbon material loading Cu
nanoparticle composites may have a high specific surface area
Shanghai Key Laboratory of Green Chemistry and Chemical Processes, School of
Chemistry and Molecular Engineering, East China Normal University, Shanghai,
and greatly increased catalytic sites per unit area, thereby
200062, China. E-mail: hwang@chem.ecnu.edu.cn, jxlu@chem.ecnu.deu.cn
increasing the yield of the reaction. For the first time, we report
the assembly of a biomass-derived Cu/porous carbon (Cu/PC)
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0nj01728k
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Scheme 1 Electrolytic reaction equation.
cathode material from xylose and Cu(NO3)2 for the electro-
synthesis of cyclic carbonates from diols and CO2 at room
temperature and normal pressure (Scheme 1).
Fig. 2 XPS survey spectrum (a), high-resolution spectra of C 1s (b), high-
resolution spectra of N 1s (c) and Cu 2p (d) for the Cu/PC-III composite.
Results and discussion
X-ray diffraction (XRD)
spectrum can be assigned to CQC/C–C, C–O and CQO, respec-
tively (Fig. 2b), indicating that the material contains carboxyl and
hydroxyl carbons, which are beneficial to chelation with Cu to
improve the stability of the composites. As shown in Fig. 2c, the N 1s
spectrum shows two main peaks centred at 398.4 (pyridine N), and
400.5 eV (pyrrolic N), which is obtained from Cu(NO3)2Á3H2O. The
peaks with binding energies of 932.8 and 951.9 eV are assigned to Cu
2p3/2 and Cu 2p1/2 for Cu(0) on the surface, respectively (Fig. 2d);27–29
this result is consistent with the XRD results. Based on the XPS
survey scan and the Cu 2p spectrum, the surface Cu content is low,
indicating that Cu particles may be buried underneath.
The Cu/porous carbon (Cu/PC) composites were facilely assembled
by using a one-pot hydrothermal approach combined with calcina-
tion under a N2 atmosphere (see the Experimental section for
details). The prepared composites were denoted as Cu/PC-x,
where x was I, II, III and IV corresponding to the addition of
0.75, 1, 1.25 and 1.5 g of Cu(NO3)2Á3H2O, respectively. Pure PC
was prepared using the same procedure without the addition of
Cu(NO3)2Á3H2O.
The XRD patterns of PC and Cu/PC composites are shown in
Fig. 1. The XRD pattern of PC reveals one characteristic peak at
2y = 25.31, which was indexed to the amorphous peak of PC. For
Cu/PC composites, the peaks appeared at 2y = 43.21, 50.31 and
74.01, which can be perfectly attributed to the (111), (200) and
(220) planes of Cu(0) according to PDF#04-0836. The absence of
other diffraction peaks adequately demonstrated the high
purity of the composites without the formation of any by-
product.
Thermogravimetric analysis (TGA) and inductively coupled
plasma-atomic emission spectrometry (ICP-AES)
The TGA curves of PC and Cu/PC hybrids are shown in Fig. 3.
The gradual weight loss below 400 1C is due to the removal of
external water molecules and the loss of interstitial water by the
condensation of –OH groups. The further weight loss above
400 1C is ascribed to the complete decomposition of the carbon
skeleton of PC. It can be seen from Fig. 3 that the initial weight
loss temperature of the composite material was lower than that
of PC. This shows that the load of copper decreases the thermal
stability of the material. The possible reason for this was that
the copper nanoparticles wrapped in porous carbon cause
defects in the carbon skeleton, so the weight loss temperature
X-ray photoelectron spectroscopy (XPS)
The surface chemical composition and chemical bonding state
of the Cu/PC composites were further determined via XPS
(Fig. 2). As shown in Fig. 2a, the XPS survey scan for Cu/PC-III
confirms the presence of C, O, N and Cu with binding energies
corresponding to C1s, O1s, N1s and Cu2p, respectively. The split
peaks at 284.7, 285.8 and 288.9 eV in the high-resolution C1s XPS
Fig. 1 XRD patterns of PC and Cu/PC composites.
Fig. 3 TGA curves of PC and Cu/PC composites.
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Table 1 Textural and structural characteristics of the composites
Cu loading (%)
SBET (m2 gÀ1
)
Vp (cm3 gÀ1
b
)
Da (nm) By TGA By ICP
a
c
Sample
PC
Cu/PC-I
Cu/PC-II 382
Cu/PC-III 408
Cu/PC-IV 279
310
332
0.14
0.47
0.56
0.57
0.52
1.85
5.76
5.87
5.67
7.49
12.8
16.8
19.2
23.2
12.3
14.6
19.5
23.2
a
b
SBET: BET surface area. Vp: single point adsorption total pore
volume. Da: adsorption average pore width (4 V AÀ1 by BET).
c
Fig. 5 SEM image of the Cu/PC-III composite.
of the composites decreases. However, the minimum initial
weight loss temperature of the composites was around 350 1C,
indicating that the composite still possesses good thermal
stability. Compared with PC having a 100% weight loss, the
Cu/PC-I, Cu/PC-II, Cu/PC-III and Cu/PC-IV hybrids exhibited a
total weight loss of approximately 87.2%, 83.2%, 80.8% and
76.8%, respectively, under the same condition. Hence, the
contents of Cu in the hybrids are 12.8%, 16.8%, 19.2% and
23.2%, respectively (Table 1). Meanwhile, ICP-AES analysis was
supplemented, showing that the copper contents in the Cu/PC-I,
Cu/PC-II, Cu/PC-III and Cu/PC-IV were 12.3%, 14.6%, 19.5%, and
23.2%, respectively (Table 1). The results of ICP-AES analysis
were basically consistent with the data obtained via TGA.
during their preparation. Furthermore, the porosity of the carbon
material can be clearly observed as shown in Fig. 5.
Nitrogen adsorption–desorption
Nitrogen adsorption–desorption measurements were conducted
to obtain the textural characteristics of the samples (Fig. 6 and
Table 1). As shown in Fig. 6a, the adsorption and desorption
isotherms of the PC increased at low relative pressure and are
relatively flat at high relative pressure, indicating the presence of
micropores in the test range. The composite exhibited a typical
type IV adsorption–desorption isotherm for the Cu/PC compo-
sites. At low relative pressure (P/P0), the isotherm increased,
indicating the presence of some micropores. At high relative
pressure, a significant hysteresis loop was observed, indicating
the presence of meso pores, while large holes were formed by the
accumulation of materials. The hysteresis loop belonged to the
H4 type, and the isotherm had no obvious saturated adsorption
platform, indicating that the pore structure is irregular.17 Fig. 6b
shows that the composites had a wider pore size distribution
than the PC, and microporous mesopores and stacked macro-
pores were observed.30
The total pore volume and the adsorption average pore width
of the Cu/PC were higher than the PC (Vp = 0.14 cm3 gÀ1, Da =
1.85 nm), which facilitated electron transport. Moreover, except
for Cu/PC-IV, the specific surface area of Cu/PC was larger than
that of PC. Among the composites, the specific surface area
initially increased and then decreased, possibly because the
specific surface area increases when small amounts of copper
nanoparticles are uniformly dispersed in the surface or skeleton
of the PC. Cu/PC-III had the maximum specific surface area of
408 m2 gÀ1. With the increase in Cu content, the Cu particles
agglomerated and blocked the pores of the carbon substrate,
Scanning electron microscopy (SEM)
The morphology and structure of the materials were characterised
by SEM (Fig. 4 and 5). As shown in Fig. 4, the Cu nanoparticles are
uniformly dispersed in the PC, and the size of the Cu nanoparticles
gradually increased with an increase in Cu content in the Cu/PC
composites. The average particle sizes were approximately 78, 98,
100 and 145 nm corresponding to the Cu/PC- I, Cu/PC-II, Cu/PC-III
and Cu/PC-IV hybrids, respectively. Fig. 5 shows the SEM images of
Cu/PC-III. Two forms of Cu nanoparticles were obtained from the
SEM images. The first form is embedded on the PC surface
(marked with yellow circles), and the second form is wrapped in
PC (marked with red circles), which support the XPS conjecture.
Hence, the Cu nanoparticles effectively combined with the PC
Fig. 4 SEM images of Cu/PC-I (a), Cu/PC-II (b), Cu/PC-III (c), and Cu/PC-
Fig. 6 N2 adsorption–desorption isotherms (a) and pore size distributions
IV (d).
(b) of PC and the Cu/PC composites.
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thus further reducing the specific surface area of the composite.
This condition is not conducive to catalytic reaction.
In summary, Cu/PC composites with a high specific surface
area and pore volume were successfully prepared. Their char-
acterisation proved the valence and content of copper.
Electrolysis reaction
The obtained Cu/PC composites were used for the electrocatalytic
synthesis of cyclic carbonates from CO2 and diols under mild
conditions. Propylene glycol (1b) was chosen as the model com-
pound to be investigated to obtain propylene carbonate (2b).
As shown in Table 2, the yield of propylene carbonate on the
PC cathode was 15.3% (entry 1). When Cu/PC-I, Cu/PC-II, Cu/
PC-III and Cu/PC-IV were used as the cathodes, the yields were
32.5% (entry 3), 33.9% (entry 4), 38.2% (entry 5) and 36.5%
(entry 6) respectively, which are significantly higher than when
using PC, and the trend of the yield agrees with the electro-
chemical active surface area (ECSA) results (as shown in Fig. S1
and Table S1, ESI†). For the composites, as the Cu content
increased, the yield first increased and then decreased, possibly
because as the Cu content increased, the number of the
electrochemically active site increased, thus increasing the
yield. With further increase in the Cu content, the Cu particles
Scheme 2 The electrolytic reaction mechanism.
agglomerated to reduce the electrochemically active sites (due to With further increase in the current density, the yield dropped,
the ECSA) and the specific surface area (by nitrogen adsorption– because at higher current density, the reaction will generate
desorption) of the composites, thus reducing the yield. Hence, by-products during electrolysis; for example, CO2 may be con-
the yield on the Cu sheet (28.9% entry 2) is lower than those on verted to CO or oxalate dianions,31 thus reducing the yield.
the composites because its specific surface area is small. There- Entries 11, 12, 9 and 13 examine the effect of electric quantity
fore, Cu should be loaded on the carbon material with a large on the yield. When the electric quantity was low, the electro-
specific surface area and good electroconductibility.
generated cyanomethyl anion (Scheme 2 reactions (1)–(4). R3N
The Cu/PC-III cathode possesses a high yield and is used as was detected via GC-MS, as shown in Fig. S2, ESI†) was
a cathode material for further studies. The effect of current insufficient to react with the substrate and CO2. Hence, the
density was investigated (entries 7, 8, 5, 9 and 10). At first, the yield increases as the amount of electric quantity increases.
yield increased with increasing current density. When the When the electric quantity was 0.75 F molÀ1, the yield reached
current density was 12.25 mA cmÀ2, the yield reached 40.0%. the highest (43.0%). If the electric quantity continues to
Table 2 Electrosynthesis of 1,2-propylene carbonate from CO2 and 1,2-propylene glycol under different synthetic conditionsa
Yieldb (%)
Entry
Cathode
j (mA cmÀ2
)
Q (F molÀ1
)
Supporting electrolyte
Propylene carbonate
Dimerization product
1
2
3
4
5
6
7
8
PC
Cu sheet
Cu/PC-I
11.11
11.11
11.11
11.11
11.11
11.11
5.56
1
1
1
1
1
1
1
1
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TBAI
TEAI
TBABr
TEABr
TEACl
15.3
28.9
32.5
33.9
38.2
36.5
21.6
30.5
40.0
29.4
32.3
43.0
35.0
46.3
37.5
40.2
28.9
8.5
9.3
8.1
6.6
10.0
9.2
6.8
8.1
10.5
4.7
5.0
4.8
10.0
8.6
9.3
7.2
5.3
Cu/PC-II
Cu/PC-III
Cu/PC-IV
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
Cu/PC-III
8.33
9
12.25
13.89
12.25
12.25
12.25
12.25
12.25
12.25
12.25
1
1
10
11
12
13
14
15
16
17
0.5
0.75
1.25
0.75
0.75
0.75
0.75
a
General conditions: Two compartment cells, 15 mL MeCN with a 0.2 M supporting electrolyte and 0.15 M 1b, graphite as an anode, CO2 pressure
1 atm, room temperature. The yields of the products were quantified by GC based on 1b (see the ESI1 for more detailed information).
b
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Table 3 Electrosynthesis of cyclic carbonates from carbon dioxide and
increase, the yield declines. According to ref. 32–35, the electro-
generated cyanomethyl anion takes one hydrogen from the diol
molecule to form the corresponding anion, which will capture CO2
in the solution and generate cyclic carbonates eventually (Scheme 2
reactions (5)–(8)). Therefore, when the reaction proceeds to a
certain extent, the intermediate combines with the substrate to
form a dimerisation product of the substrate, rather than binding
to CO2 thereby reducing the yield. The dimerisation product of the
substrate was detected via GC-MS (Fig. S3, ESI†). The yields of the
dimerisation products are shown in Table 2, which are between
4.0% and 11.0%. Entries 12, 14, 15, 16 and 17 examine the effect
of the supporting salt. When the supporting salt was tetraethyl-
ammonium iodide (TEAI), the yield reached 46.3%. Considering
that the space steric effect of TEA+ is smaller than that of TBA+
(tetrabutylammonium), TEA+ is more conducive to stabilising
acetonitrile anions.36 Although 46.3% is not very high, it is much
better than the values of 4%,15 19%,11 and 21%16 previously
reported (Table S2, ESI†).
diolsa
Entry
1
Diols
Cyclic carbonates
Yieldb (%)
28.4
2
3
46.3
47.6
4
5
57.8
Reusability
Cu/PC-III was used to further investigate the stability of the
composite. An electrolysis test was carried out based on the
best conditions (Table 2 entry 14). As shown in Fig. 7, after six
successive cycles, the yield remained stable. The SEM image of
the used material is shown in Fig. S4 (ESI†), which is not much
different from that of fresh Cu/PC-III.
37.9
33.1
6
Substrates expansion
a
General conditions: two department cell, 15 mL MeCN with 0.2 M
TEAI and 0.15 M 1, graphite as the anode, CO2 pressure 1 atm, room
The results of substrate expansion are shown in Table 3 to
investigate the universality of the composites. An electrolysis test
was carried out based on the conditions (Table 2 entry 14) in this
work. Table 3 shows that the yield of 2a, 2b, 2c, 2d, 2e and 2f are
24.8%, 46.3%, 47.6%, 57.8%, 37.9% and 33.5% respectively. Three
types of substrates are selected as shown in Table 3. Based on the
reaction mechanism, the substituent of the diol substrate will have
a certain effect on the yield. In the first case, the substituent is an
electron-donating group, as shown in Scheme 3(a). Moreover, the
acidity of the diol will be enhanced compared with 1a, which is
favourable for the reaction, thus increasing the yield (entries 1, 2, 3
and 4). In the second case, when the substituent is an electron-
withdrawing group as shown in Scheme 3(b), the acidity of the diol
b
temperature. The yield of the products was quantified by GC based on
1 (see the ESI1 for more detailed information).
Scheme 3 Effect of substituents on substrate acidity and basicity.
is weakened compared with 1a; hence, the yield is reduced (entry
5). In the third case when two substituents are present, the steric
hindrance is large, and the yield is low (entry 6). Additionally, the
substituent in entry 5 is an electron-withdrawing group, but the
yield is higher than that of the unsubstituted group (entry 1)
possibly because the electron-withdrawing group stabilises the
negatively charged intermediate. Despite the differences in the
adsorption or push interaction of the substituent group, steric
hindrance and intermediate stability, the composites respond to
these conditions, indicating its good universality.
Conclusion
A biomass-derived Cu-loaded PC (Cu/PC) cathode exhibiting a
high specific surface area and a rich pore structure was facilely
Fig. 7 Stability of the Cu/PC-III composites for electrosynthesis of
1,2-propylene carbonate.
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pluronic F127 (EO106PO70EO106) was added to 30 mL of deio-
nised water and stirred to dissolve. Exactly 3 g of xylose, 0.75 g of
Cu(NO3)2Á3H2O and 1 mL of acetylacetone were successively
added to the above mixed solution with stirring. The mixture
was transferred into a 100 mL high-pressure reactor and held at
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deionised water and dried, followed by calcination in a N2
atmosphere. The specific temperature control conditions are Àas1
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and maintained for 2 h and then raised to 550 1C at 1 1C minÀ1
and maintained for 2 h. Cu sources with varying qualities were
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General electrosynthesis
Galvanostatic electrolysis was conducted in 15 mL of dry MeCN-
TEAI (0.2 M)-1b (0.15 M) saturated with CO2 in a divided cell,
which was separated by a PE-AM homogeneous cationic
membrane. Electrolysis was performed using a graphite anode
and a Cu/PC composite cathode until 1.0 F molÀ1 of charge was
passed. After electrolysis, 0.3 g of anhydrous K2CO3 and 0.3 mL
of MeI were added in the catholyte, and the mixture was stirred
at 55 1C for 5 h. The solution was evaporated, hydrolysed and
extracted with Et2O, and the organic layers were dried over
MgSO4. Yields were determined using GC-2014 (Shimadzu,
Japan).
¨ ¨
22 C. Nitsos, R. Stoklosa, A. Karnaouri, D. Voros, H. Lange,
Conflicts of interest
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