Zinc single atoms on N-doped carbon: An efficient and stable catalyst for CO2 fixation and conversion

Article abstract of DOI:10.1016/S1872-2067(19)63316-4

The cycloaddition of epoxides and carbon dioxide represents a straightforward and atom-efficient method for synthesis of cyclic carbonates and utilization of CO₂. So far, homogeneous metal complexes have been mainly applied for such transformations. Here, we describe the synthesis of novel heterogeneous Zn-based catalysts, which were conveniently prepared by pyrolysis of an active-carbon-supported phenanthroline-ligated Zn(OAc)₂ complex. Detail structural characterizations proved the existence of single zinc sites in the active material. Compared to a Zn-based nanoparticle (Zn-NP) catalyst, the resulting single metal atom catalyst (SAC) displayed improved activity and stability for the cycloaddition of epoxides. By applying the optimal catalyst, a variety of carbonates were successfully obtained in high yields with good functional group tolerance.

Full text of DOI:10.1016/S1872-2067(19)63316-4

Chinese Journal of Catalysis 40 (2019) 1679–1685
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article (Special Issue on Celebrating the 70th Anniversary of Dalian Institute of Chemical Physics, CAS)
Zinc single atoms on N-doped carbon: An efficient and stable catalyst
for CO2 fixation and conversion
Xinjiang Cui a, Xingchao Dai b, Annette-Enrica Surkus a, Kathrin Junge a, Carsten Kreyenschulte a,
Giovanni Agostini a, Nils Rockstroh a, Matthias Beller a,*
a Leibniz-Institut für Katalyse e.V. Albert-Einstein-Str. 29a, 18059 Rostock, Germany
b State Key Laboratory for Oxo Synthesis and Selective Oxidation, Centre for Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese
Academy of Sciences, Lanzhou 730000, Gansu, China
A R T I C L E I N F O
A B S T R A C T
Article history:
The cycloaddition of epoxides and carbon dioxide represents a straightforward and atom-efficient
method for synthesis of cyclic carbonates and utilization of CO2. So far, homogeneous metal
complexes have been mainly applied for such transformations. Here, we describe the synthesis of
novel heterogeneous Zn-based catalysts, which were conveniently prepared by pyrolysis of an ac-
Received 21 January 2019
Accepted 27 January 2019
Published 5 November 2019
tive-carbon-supported
phenanthroline-ligated
Zn(OAc)2
complex.
Detail
structural
characterizations proved the existence of single zinc sites in the active material. Compared to a
Zn-based nanoparticle (Zn-NP) catalyst, the resulting single metal atom catalyst (SAC) displayed
improved activity and stability for the cycloaddition of epoxides. By applying the optimal catalyst, a
variety of carbonates were successfully obtained in high yields with good functional group toler-
ance.
Keywords:
Heterogeneous catalysis
Single atom catalyst
Zinc
Carbon dioxide
Cycloaddition
Carbonate
© 2019, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
been intensely investigated, which proceeds with high atom
efficiency and does not need any stoichiometric amounts of
The development of new procedures for fixation and utiliza-
tion of carbon dioxide (CO2), which is emitted about 36 billion
tons per year (https://www.co2.earth/global-co2-emissions),
continues to attract significant attention from both academic
and industrial researchers [1–12]. In this respect, important
progresses for the valorization of CO2 to energy carriers (me-
thane [13–17]), bulk (methanol [18–22]) and fine chemicals
(formats [7,23–27]) as well as for organic specialties
(N-methylated amines [28–37]) have been reported in recent
years. Apart from all these redox transformations, the synthesis
of cyclic carbonates by cycloaddition of epoxides with CO2 has
reductants [38–40].
The most widely employed catalysts for the formation of cy-
clic carbonates from CO2 are homogeneous metal-based com-
plexes, including mainly salen-type and metalloporphyrin
complexes [41–43]. For example, North et al. [44,45] reported
bimetallic salen complexes which exhibited high activity for the
preparation of cyclic carbonates from both terminal and inter-
nal epoxides. In addition, Kleij and co-workers [46] demon-
strated an easily accessible aluminum complex to be highly
active and productive. Other notable investigations described
homogeneous catalysts based on Zn [47], Cr [48], Co [49,50], Fe
* Corresponding author. Fax: +49(381)1281-51113; E-mail: matthias.beller@catalysis.de
This work was supported by the National Key R&D Program of China (2017YFA0403103), the state of Mecklenburg-Vorpommern and the Bundes-
ministerium für Bildung und Forschung.
DOI: S1872-2067(19)63316-4 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 40, No. 11, November 2019

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Xinjiang Cui et al. / Chinese Journal of Catalysis 40 (2019) 1679–1685
obtained and dried at 60 °C for 3 h by vacuum and was grinded
to a fine solid powder. Then, the grinded powder was pyro-
lyzed at 700 °C for 2 h with a step of 25 °C under Ar. The re-
sulting catalyst was grinded to get the final catalyst as
Zn-SAC@NC-700. As comparison, other samples were prepared
using the similar procedure by varying the pyrolysis tempera-
ture (Zn@N-C-600, Zn@N-C-800, Zn@N-C-1000) and organic
and inorganic supports such as SiC, SiO2, MgO2 and chitosan.
2.2. Preparation of Zn-NP@C catalyst
The support (N-doped carbon) was prepared according to
above procedure in the absence of Zn(OAc)2 and pyrolyzed at
700 °C for 2 h with a step of 25 °C under Ar. The Zn-NP@C cat-
alyst was prepared as the literature [64].
Scheme 1. Known systems and our catalyst for cycloaddition of epox-
ides and CO2.
[51–53], Mg [41], and Nb [54].
Although several of the reported homogeneous catalysts
show impressive activity and work under mild conditions, and
thus allow the formation of sensitive products, the design of
heterogenous catalysts is interesting due to their increased
stability, simple separation and purification of the products as
well as easier integration into continuous flow systems. Com-
pared to molecular-defined complexes, relatively few hetero-
geneous systems, e.g. metal oxides [55] and metal organic
frameworks [56,57], have been explored for the transformation
of epoxides to carbonates. Very recently, metals (Zn, Mg and Ni)
coordinated to mesoporous o-hydroxyazobenzene-based pol-
ymers displayed remarkable activity for the formation of pro-
pylene carbonate [58,59]. However, so far these materials
showed only a narrow substrate scope, and the long reaction
time might limit their potential applications.
Recently, single atom catalysts (SACs) provide new oppor-
tunities to design catalytic materials at the molecular level be-
cause the individual active metal centers can be controlled pre-
cisely by the nature of the neighboring atomic species. Hence,
we thought that such SACs allow for improved activity in car-
bonate synthesis due to metal utilization of up to 100%. En-
couraged by our recent investigations on the preparation and
utilization of N-doped metal nanoparticles and platinum-based
single atom catalyst [60–63], here we describe the preparation,
characterization and catalytic testing of a novel Zn single atom
catalyst (Zn-SAC) supported on N-doped carbon material. The
resulting catalyst allows for efficient and general activation of
functionalized terminal epoxides to give the corresponding
carbonates in high yields.
2.3. Catalyst recycling experiments
With the similar procedure used above, the catalyst was
simply separated by centrifugation, washed with ethyl acetate
and n-hexane, respectively, and used without further reactiva-
tion or purification for the next run.
2.4. Analytic measurements
STEM measurements were performed at 200 kV with an
aberration-corrected JEM-ARM200F (JEOL, Corrector: CEOS).
The microscope was equipped with a JED-2300 (JEOL) ener-
gy-dispersive X-ray-spectrometer (EDXS) and an Enfinium ER
(GATAN) eletron energy loss spectrometer for chemical analy-
sis and spectrum imaging. The samples were deposited without
any pretreatment on a holey carbon supported Cu-grid (mesh
300) and transferred to the microscope. The high-angle annu-
lar dark field (HAADF) and annular bright field (ABF) images
were recorded with a spot size of approximately 0.1 nm, the
collection semi-angles for HAADF and ABF were 70–170 mrad
and 11–22 mrad, respectively.
XPS measurements were performed with
a
VG
ESCALAB220iXL with monochromated Al Kα radiation (E =
1486.6 eV). The electron binding energies (EB) were obtained
without charge compensation. For quantitative analysis the
peaks were deconvoluted with Gaussian-Lorentzian curves,
and the peak areas were divided by a sensitivity factor ob-
tained from the element specific Scofield factor and the trans-
mission function of the spectrometer.
Extended X-ray absorption fine structure (EXAFS) experi-
ments were performed at the Beijing Synchrotron Radiation
Facility (BSRF) in Beijing Institute of High Energy Physics, Chi-
nese Academy of Sciences with storage ring energy of 2.5 GeV
and a beam current between 150 and 250 mA. The Zn K-edge
absorbance of powder catalysts was measured in transmission
geometry at room temperature. EXAFS data analysis was car-
2. Experimental
2.1. Typical preparation of Zn-SAC@N-C-700 catalysts
Zn(OAc)2 (0.5 mmol) and 1,10-phenanthroline (1.5 mmol)
were dissolved in 50 mL ethanol. The reaction mixture was
heated to 60 °C and stirred for 1 h under air atmosphere. Next,
carbon support (VULCAN®XC72R, which is order from PT.
Cabot Indonesia) was used as support and added into the mix-
ture. After further stiring for another 2 h at 60 °C, the reaction
mixture was cooled to room temperature naturally. After
evaporating the ethanol solvent, a black solid material was
ried
out
using
the
IFEFFIT
analysis
program
(http://cars9.uchicago.edu/ifeffit/). Radial distribution func-
tions were obtained by Fourier-transformed k3-weighted χ
function.

Xinjiang Cui et al. / Chinese Journal of Catalysis 40 (2019) 1679–1685
1681
3. Results and discussion
mine the electronic structure and chemical property of this
Zn-SAC, an EXAFS measurement was performed. As shown in
Fig. 1c, the strong peak of R space between 1 and 2 Å is due to a
Zn-N shell which is different from the Zn-O shell. A small peak
in the region of 2–3 Å, which is characteristic for a Zn-Zn shell,
is observed, thus confirming the presence of isolated Zn atoms
and a small portion of Zn nanoparticles.
At the start of this work, we synthesized Zn catalysts sup-
ported on different carriers. The preparation of these catalysts
commenced with the impregnation of different supports (car-
bon, alumina, silica, SiC, etc.) with ethanolic solutions of a phe-
nanthroline-ligated Zn(OAc)2 complex (Zn-phen). The resulting
ZnNx@X-Y (Nx denotes the coordinated nitrogen atoms, while X
and Y represent the support and pyrolysis temperature) cata-
lysts were obtained upon solvent evaporation and subsequent
pyrolysis under inert conditions. Control samples of Zn nano-
particles on carbon (NG) and a Zn-containing catalyst on car-
bon with no N doping were also prepared. A detailed prepara-
tion procedure is described in the Supporting Information.
Preliminary testing of all the different catalysts in the model
reaction of styrene epoxide revealed the best results for the
ZnNx@C-700 system (Table S3). This optimal catalyst was
characterized by high-resolution transmission electron mi-
croscopy (HRTEM) and high-angle annular dark field-scanning
transmission electron microscopy (HAADF-STEM). Surprising-
ly, despite the relatively high metal loading (1.56 wt%), no Zn
nanoparticles or clusters were observed. Instead, as shown in
Fig. 1a, isolated Zn atoms were finely dispersed on the surface
of carbon. N atoms were distributed homogeneously on the
surface of the support rather than embedded in the carbon
support matrix proved by electron energy loss spectroscopy
(EELS) (Fig. 1b). These findings indicate the stabilization of Zn
single atoms by coordination with nitrogen. To further deter-
The contents of Zn and N on ZnNx@C-700 were detected as
1.56 wt% and 2.56 wt% measured by ICP-MS and EA, respec-
tively. The composition of this Zn-SAC on N-doped carbon was
further investigated by X-ray photoelectron spectroscopy
(XPS). As shown in Fig. 1d, three N 1s peaks at 397.9, 400.9, and
402.54 eV are assigned to pyridinic (60%), graphitic (24%),
and oxidic (15%) nitrogen, respectively. Clearly, these N spe-
cies are derived from the pyrolysis of Zn-phen complexe. The
XPS of Zn shows two peaks at a binding energy of 1022.54 and
1045.64 eV (Fig. S6), corresponding to the 2p3/2 and 2p1/2, re-
spectively. Note that the binding energy of Zn 2p3/2 is higher
than the standard of 1022.0 eV for metallic Zn and lower than
that of 1023.0 eV for ZnO [65], which again confirms that main
species in ZnNx@C-700 is Zn-N rather than ZnO or metal Zn0.
The catalytic activity of all the prepared materials was eval-
uated in the cycloaddition of styrene oxide with CO2. To im-
prove the general reactivity, all experiments were performed in
the presence of 2 mol% of tetrabutyl ammonium bromide
(TBAB), which showed little product formation in this bench-
mark reaction. Hence, control experiments in the absence of Zn
catalyst revealed styrene carbonate in 19% yield under neat
Fig. 1. Characterization for ZnNx@C-700. (a) HAADF-STEM, (b) EELS of C and N, (c) Zn K-edge EXAFS, (d) XPS spectra.

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Xinjiang Cui et al. / Chinese Journal of Catalysis 40 (2019) 1679–1685
Fig. 2. Reaction of styrene epoxide with CO2. (a) Time course; (b) Recycling experiments.
condition at 100 °C (Table S3, entry 1). Adding a standard
ZnNx@C catalyst obtained by pyrolysis at 600 °C showed sig-
nificantly improved activity (70% yield; Table S3, entry 2). To
the best of our knowledge, this is the first single atom catalyst
that allows for the cycloaddition of epoxides with CO2. Applying
the ZnNx@C catalyst prepared by pyrolysis at 700 °C exhibited
even better activity with the desired product obtained in 93%
yield (Table S3, entry 3). In comparison, samples pyrolyzed at
higher temperature (800, 900, and 1000 °C) showed lower
yields for this transformation (60%–72%, Table S3, entries
4–6). Zn catalysts immobilized on other supports such as SiO2,
MgO, and SiC yielded much lower yields (29%–55%, Table S3,
entries 7–10). Notably, Zn nanoparticles supported on carbon
gave also lower yield (46%) compared to the single atom cata-
lyst. To compare the reactivity for ZnNx@C-700 and
Zn-NP@N-C appropriately, conversions were measured in de-
tail (Fig. 2a). Under the optimized reaction conditions the Zn
single atom catalyst displayed in general better catalytic per-
formance compared to Zn nanoparticles. We assume that this
higher activity derives from the better dispersion of the zinc
centers on the surface, which activate the epoxide. In addition,
basic nitrogen species on the support are favorable to adsorb
CO2 [66].
Obviously, stability and recyclability of a given catalyst are
most important parameters for any application in industry.
Hence, ZnNx@C-700 was recycled by centrifugation and reused
up to six times (Fig. 2b). To our delight, 90% of 2a was still
achieved at the sixth run. We considered that the loss of the
catalyst after washing operation leads to the slightly decreased
catalytic activity. This is also confirmed by EXAFS that there is
no obvious change of catalyst after reaction, indicating the sta-
bility of the catalysts during the reaction (Fig. S7).
Next, the catalytic activity of the Zn-SAC was studied in the
presence of different CO2 concentrations (Table 1). Although no
carbonate was detected when the cycloaddition was preceded
in air (1 bar), at higher pressure (40 bars) some formation of
2a is observed, which corresponds to approximately 25% con-
version of CO2 in air. This result showed that the Zn-catalytic
system displays certain activity even at very low CO2 concen-
tration which might be interesting for further applications.
Notably, at ambient pressure the desired product was observed
at CO2 concentration of 4000 ppm. In general, at low CO2 con-
centrations, the product yields were increased significantly at
pressured conditions (results in the parentheses in Table 1).
Under gas flow, the yield of 2a was increased to 31% with in-
creasing the content of the CO2 to 25 vol%. Even higher activity
is seen at CO2 contents from 50 vol% to 100 vol%, yielding 2a
in 43% and 55%, respectively. The synthesis of carbonate pro-
ceeded best at 5 bar and 100 vol% CO2 leading to an excellent
yield (Table S4).
Interestingly, the novel active and stable Zn single atom cat-
alyst (ZnNx@C-700) displayed also broad scope with simple
and functionalized epoxides. As shown in Table 2, under the
optimized conditions, various substrates including sensitive
derivatives with chloride, bromide and iodine groups were
converted to the corresponding cyclic carbonates (Table 2,
entries 1–4). Moreover, the cycloaddition of 1e proceeded well
in 95% yield. When oxygen containing substrates were used as
starting materials, 2f and 2g were obtained in 97% and 95%
yields, respectively. Furthermore, when using 100 mmol of 1a
instead of 5 mmol, a 90% yield of 2a was achieved. This result
indicates that the protocol can be scaled up easily.
Short chain aliphatic carbonates have been widely applied
as solvents, electrolytes, additives, and important intermedi-
ates [67]. We then focused on the further investigation of
Table 1
Reaction of styrene epoxide with CO2 under different CO2 concentra-
tiona.
Entry
CO2 concentration (%) Conversion b (%) Yield b (%)
1
2
3
4
5
6
7
0.0400
0.4
1
5
25
0 (2)
1 (10)
2 (17)
8 (30)
32
0 (1)
Trace (9)
2 (15)
7 (29)
31
50
100
45
55
43
55
a Reaction conditions: 5 mmol substrates, TBAB (2 mol%), 20 mg cata-
b
lyst, gas flow, neat condition, 100 °C, 2 h. Detected by GC and the re-
sults in parentheses were obtained at 40 bar.

Xinjiang Cui et al. / Chinese Journal of Catalysis 40 (2019) 1679–1685
1683
Table 2
Table 3
Synthesis of cyclic carbonates from arene containing epoxides and CO2
with ZnNx@C-700 a.
Synthesis of cyclic carbonates from aliphatic epoxides and CO2 with
ZnNx@C-700a.
Entry
1
Substrate
Product
Yield b (%)
93 (90)
Yield b
(%)
Entry
1
Substrate
3a
Product
92
1a
2a
4a
2
3
4
5
93
96
89
91
3b
2
3
4
94
90
98
4b
1b
2b
3c
4c
1c
2c
3d
4d
1d
1e
3e
2d
2e
4e
5
6
95
97
96
6
7
65
97
3f
4f
1f
2f
2g
3g
4g
7
1g
a
Reaction conditions: 5 mmol substrate, TBAB (2 mol%), 20 mg
8
89
93
93
3h
b
catalyst, 5 bar CO2, neat condition, 100 °C, 2 h. Isolated yields. The
result in parentheses was obtained at 100 mmol of 1a.
4h
9
3i
4i
Zn-SAC on the scope of cross transformation of aliphatic epox-
ides to a variety of carbonates with different functional groups.
Indeed, in the case of the cycloaddition of low and medium
chain aliphatic epoxides, higher CO2 pressure (10 bar) was
used to inhibit the evaporation of the low boiling point sub-
strates. For example, propylene oxide, 1-butene oxide, and
1-hexene oxide were converted to the corresponding products
in 92%–96% yields (Table 3, 4a–4d). Interestingly, glycidyl
methyl ether is transformed into the corresponding carbonates
in 91% yield (Table 3, 4e). However, only 67% yield of cyclic
product was achieved in the presence of tert-butyl group (Ta-
ble 3, 4f). Interestingly, the epichlorohydrin was converted to
the corresponding adduct in 97% yield (Table 3, 4g). For the
alkene containing substrates the yields of the corresponding
products were obtained in excellent yields (Table 3, 4h, 4i, and
4j). Moreover, 4k with alkyne group could also be obtained in
88% yield. Cycloaddition of epoxides with steric hindrance
groups proceeded smoothly to the carbonates in good to excel-
lent yields (Table 3, 4l–4n). The electron-withdrawing effect of
Cl probably leads to the low yield for 4m. Finally, silicon con-
taining carbonates (Table 3, 4o–4q), which are useful interme-
diates and are used for the preparation of surfactants, release
coatings and lubricants, are obtained by the cycloaddition of
corresponding epoxides and CO2 in the presence of the Zn-SAC.
10
3j
4j
11
88
3k
4k
O
12
13
90
57
3l
4l
3m
3n
4m
14
15
93
91
4n
3o
4o
4p
16
96
3p
17
97
4q
3q
a
Reaction conditions: 5 mmol substrates, TBBA (2 mol%), 20 mg
catalyst, 5 bar CO2, neat condition, 100 °C, 2 h. b Isolated yields.
atom Zn catalyst with high metal content, which is synthesized
by a simple pyrolysis procedure. The resulting material is used
as a heterogeneous catalyst for the cycloaddition of epoxides to
carbonates and offers high activity with wide substrate scope
4. Conclusions
In conclusion, we describe the first heterogeneous single

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Graphical Abstract
Chin. J. Catal., 2019, 40: 1679–1685 doi: S1872-2067(19)63316-4
Zinc single atoms on N-doped carbon: An efficient and stable catalyst
for CO2 fixation and conversion
Xinjiang Cui, Xingchao Dai, Annette-Enrica Surkus, Kathrin Junge,
Carsten Kreyenschulte, Giovanni Agostini, Nils Rockstroh, Matthias Beller*
Leibniz-Institut für Katalyse e.V., Germany; Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, China
A novel Zn single atom catalyst (Zn-SAC) supported on N-doped carbon
material was prepared. The resulting catalyst allows for efficient and gen-
eral activation of functionalized terminal epoxides to give the corresponding
carbonates in high yields.
Single Atom Catalyst

Xinjiang Cui et al. / Chinese Journal of Catalysis 40 (2019) 1679–1685
1685
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氮掺杂碳上单原子锌: CO2固定和转化的高效稳定催化剂
Xinjiang Cui a, Xingchao Dai b, Annette-Enrica Surkus a, Kathrin Junge a, Carsten Kreyenschulte a,
Giovanni Agostini a, Nils Rockstroh a, Matthias Beller a,*
a莱布尼茨催化研究所, 罗斯托克18059, 德国
b中国科学院兰州化学物理研究所绿色化学与催化中心, 羰基合成与选择氧化国家重点实验室, 甘肃兰州730000, 中国
摘要: 环氧化合物与CO2环加成反应是环状碳酸酯合成和CO2利用的一种直接的原子经济方法, 目前主要采用均相金属络
合物催化此类转化反应. 本文报道了一种新型多相Zn基催化剂, 它可方便地通过热解活性炭负载的邻菲啰啉络合
Zn(OAc)2而得到. 详细的结构表征证实该材料中存在单原子Zn活性位. 与Zn基纳米粒子催化剂相比, 本文制备的单原子
Zn催化剂在环氧化物环加成反应中表现出更高的活性和稳定性. 采用该优化的催化剂成功地以高产率得到了一系列碳酸
酯, 该催化剂具有较好的基团容忍性.
关键词: 多相催化; 单原子催化剂; 锌; 二氧化碳; 环加成; 碳酸酯
收稿日期: 2019-01-21. 接受日期: 2019-01-27. 出版日期: 2019-11-05.
*通讯联系人. 传真: +49-381-128151113; 电子信箱: matthias.beller@catalysis.de
基金来源:国家重点研发计划(2017YFA0403103); 梅克伦堡-西波美拉尼亚州和德国联邦教育和研究部项目资助
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).