Hydroformylation of Olefins by a Rhodium Single-Atom Catalyst with Activity Comparable to RhCl(PPh3)3

Article abstract of DOI:10.1002/anie.201607885

Homogeneous catalysts generally possess superior catalytic performance compared to heterogeneous catalysts. However, the issue of catalyst separation and recycling severely limits their use in practical applications. Single-atom catalysts have the advantages of both homogeneous catalysts, such as “isolated sites”, and heterogeneous catalysts, such as stability and reusability, and thus would be a promising alternative to traditional homogeneous catalysts. In the hydroformylation of olefins, single-atom Rh catalysts supported on ZnO nanowires demonstrate similar efficiency (TON≈40000) compared to that of homogeneous Wilkinson's catalyst (TON≈19000). HAADF-STEM and infrared CO chemisorption experiments identified isolated Rh atoms on the support. XPS and XANES spectra indicate that the electronic state of Rh is almost metallic. The catalysts are about one or two orders of magnitude more active than most reported heterogeneous catalysts and can be reused four times without an obvious decline in activity.

Full text of DOI:10.1002/anie.201607885

Angewandte
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International Edition: DOI: 10.1002/anie.201607885
German Edition: DOI: 10.1002/ange.201607885
Single-Atom Catalysts Hot Paper
Hydroformylation of Olefins by a Rhodium Single-Atom Catalyst with
Activity Comparable to RhCl(PPh3)3
Rui Lang, Tianbo Li, Daiju Matsumura, Shu Miao, Yujing Ren, Yi-Tao Cui, Yuan Tan,
Botao Qiao,* Lin Li, Aiqin Wang, Xiaodong Wang, and Tao Zhang*
Abstract: Homogeneous catalysts generally possess superior
catalytic performance compared to heterogeneous catalysts.
However, the issue of catalyst separation and recycling severely
limits their use in practical applications. Single-atom catalysts
have the advantages of both homogeneous catalysts, such as
and homogeneous catalysts, is an attractive and preferred
option; however, decades of study has proven that this is
extremely difficult and challenging.
[
4]
Single-atom catalysts (SACs), with isolated atoms dis-
persed onto the support surface, are emerging as a new
[
5]
“
isolated sites”, and heterogeneous catalysts, such as stability
frontier in the catalysis field. Single atoms are usually more
reactive than their nanoparticle and nanocluster counterparts
and reusability, and thus would be a promising alternative to
traditional homogeneous catalysts. In the hydroformylation of
olefins, single-atom Rh catalysts supported on ZnO nanowires
demonstrate similar efficiency (TON ꢀ 40000) compared to
that of homogeneous Wilkinsonꢀs catalyst (TON ꢀ 19000).
HAADF-STEM and infrared CO chemisorption experiments
identified isolated Rh atoms on the support. XPS and XANES
spectra indicate that the electronic state of Rh is almost
metallic. The catalysts are about one or two orders of
magnitude more active than most reported heterogeneous
catalysts and can be reused four times without an obvious
decline in activity.
[
6]
owing to their unsaturated coordination environment.
Additionally, the homogeneity of single-atom sites often
[
7]
offers high selectivity towards a specific product. These
features imply that the performance of SACs is similar to that
of homogeneous analogues. On the other hand, while it was
previously thought that these singly dispersed atoms are
thermodynamically unstable, recent studies suggest that they
can actually be tightly bonded to the support by strong
[
8]
[9]
electronic or covalent interactions, and form rather stable
[
10]
catalysts. Therefore, it is believed that SACs could bridge
[
5c]
hetero- and homogeneous catalysis, although this has not
been exclusively demonstrated to date. Herein, we exemplify
development of suitable SACs as a means to heterogenize
a homogeneous catalyst, using hydroformylation of a series of
olefins as a probe reaction.
C
atalysis plays a central role in modern chemical industry. It
has been estimated that more than 90% of chemical processes
[1]
use catalysts. Homogeneous catalysts usually possess supe-
rior activity and/or selectivity compared to heterogeneous
Hydroformylation of olefins is one of the most important
homogeneously catalyzed industrial processes for aldehyde
synthesis, with more than 10 million tons produced globally
on an annual basis. The desired aldehyde products are value-
added fine chemicals that are employed in the synthesis of
pharmaceuticals, agrochemicals, flavorings, and food addi-
[
2]
catalysts. However, the major issue of separating catalyst
from raw materials and products severely limits their use in
[
3]
industrial applications. On the contrary, heterogeneous
catalysts are usually stable and can be readily separated
from a reaction mixture. Therefore, the heterogenization of
homogeneous catalyst, by attachment of the catalyst to an
insoluble support to combine the advantages of both hetero-
[11]
tives.
Nowadays, phosphine-ligand-containing Rh com-
plexes are widely applied in industrial processes because of
their superior catalytic activity. However, the catalyst sepa-
[12]
ration issue remains unsolved. A lot of efforts have been
devoted to heterogenization of homogeneous catalysts but
[
*] Dr. R. Lang, T. Li, Prof. S. Miao, Y. Ren, Y. Tan, Dr. B. Qiao, Dr. L. Li,
Prof. A. Wang, Prof. X. Wang, Prof. T. Zhang
State Key Laboratory of Catalysis, Dalian Institute of Chemical
Physics, Chinese Academy of Sciences
Dalian 116023 (China)
[
13]
with limited success. An alternative method is to develop
heterogeneous catalysts by immobilizing Rh nanoparticles
onto solid supports, but their activities are usually extremely
low compared with the corresponding homogeneous cata-
E-mail: bqiao@dicp.ac.cn
[
13a,14]
taozhang@dicp.ac.cn
lysts.
Herein, we report the development of a Rh SAC
T. Li
supported on ZnO, which demonstrates remarkable activity
in the hydroformylation of a number of olefins; activity is
comparable to that of Wilkinsonꢀs catalyst, a representative
homogeneous catalyst. These findings may pave a new path
toward heterogenization of homogenous catalysts.
University of Chinese Academy of Sciences
Beijing 100049 (China)
Dr. D. Matsumura
Japan Atomic Energy Agency, SPring-8
1-1-1 Kouto, Sayo, Hyogo 679-5148 (Japan)
ZnO nanowires (ZnO-nw; primarily comprising {10-10}
facets) were used as a support material to maximize the
homogeneity of the catalyst, as different crystal faces of the
support may have a distinct effect on the catalytic perfor-
mance. For comparison, three Rh/ZnO-nw catalysts were
synthesized with various Rh weight loadings of 0.3%, 0.03%,
Dr. Y.-T. Cui
Synchrotron Radiation Research Organization
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo, 113-8656 (Japan)
[
15]
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201607885.
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and 0.006%. A detailed synthetic procedure and character-
ization details are provided in the Supporting Information.
The representative scanning electron microscopy (SEM)
images of ZnO-nw are presented in Figures S1A and S1B
(
Supporting Information). The length of ZnO-nw is in the
range of 5–50 mm and the diameter is about 50 nm, which is
similar to the data provided by the supplier. According to
these data and the morphology model of ZnO-nw (Support-
ing Information, Figure S1C), the ratio of {0001} facets is
calculated to be less than one in one thousand, and is
considered negligible. Figure S2 (Supporting Information)
presents the SEM and high-angle annular dark-field scanning
transmission electron microscopy (HAADF-STEM) images
of the 0.3%Rh/ZnO-nw catalyst. The morphology of ZnO
nanowires is retained, and the average diameter of Rh
clusters is measured to be about 0.9 nm. As the Rh weight
loading is decreased to 0.03% and 0.006%, isolated Rh atoms
are distinguishable from the ZnO nanowires in the HAADF-
STEM images (Figure 1; Supporting Information, Figure S3).
A low magnification image (Figure 1A) indicates that Rh
clusters are absent, suggesting that Rh exists primarily as
single atoms in these samples.
Figure 2. DRIFT spectra of CO adsorption at 258C on Rh/ZnO-nw
catalysts with different Rh loadings. All spectra were collected after CO
adsorption (10 min) and He purging (10 min).
0
ꢁ1
linear Rh -CO (2074–2070 cm ) is clearly observed (Sup-
porting Information, Figure S4A). This phenomenon,
together with the presence of bridged-CO species, provide
strong evidence for the existence of Rh clusters, which is in
accordance with HAADF-STEM images (Supporting Infor-
mation, Figure S2). As for 0.03%Rh /ZnO-nw, two obvious
1
ꢁ
1
peaks (2087 and 2014 cm ) are attributed to the symmetric
and asymmetric vibration of gem-dicarbonyl doublet CO on
positively charged Rh atoms, respectively, and another peak
ꢁ
1
0
(
2060 cm ) is the linear CO on the Rh atom. The absence of
bridged-CO suggests that Rh atoms are singly dispersed on
the ZnO support. Previous studies showed that if the position
of IR peaks remains the same under decreased CO coverage,
these peaks could probably be assigned to CO adsorbed on an
[
17]
isolated metal atom. Therefore, lack of a coverage-depen-
0
ꢁ1
dent shift for linear Rh -CO at approximately 2060 cm
further confirms that Rh atoms are primarily isolated from
each other on the support (Supporting Information, Fig-
ure S4B). Similar results are obtained in CO chemisorbed
DRIFTS spectra of 0.006%Rh /ZnO-nw (Figure 2; Support-
1
ing Information, Figure S4C), which is in good agreement
with the HAADF-STEM images.
Figure 1. A,B) HAADF-STEM images of 0.03%Rh /ZnO-nw and
For Rh(CO) gem-dicarbonyl species, the ratio of inte-
1
2
C,D) 0.006%Rh /ZnO-nw with different magnifications. Rh single
1
grated absorbance of asymmetric and symmetric stretches is
atoms are highlighted in orange circles.
[17a]
related to the angle between carbonyl groups.
A quanti-
tative analysis reveals that the angle between carbonyl groups
of 0.3%Rh/ZnO-nw is calculated to be 1208, while those of
0.03%Rh /ZnO-nw and 0.006%Rh /ZnO-nw are about 968
and 948 (Supporting Information, Table S1), suggesting the
geometry of CO adsorption on Rh atoms and clusters are
totally different.
The Rh/ZnO-nw catalysts were further characterized by
in situ diffuse reflectance infrared Fourier transform spec-
troscopy (DRIFTS) upon CO adsorption, as this technique is
reliable for detection of single-site Rh species and Rh
1
1
[16]
aggregates.
As shown in Figure 2, for 0.3%Rh/ZnO-nw,
The chemical state of Rh was further investigated by
X-ray photoelectron spectroscopy (XPS). Considering that
metallic Rh species can be reoxidized easily into the positive
ꢁ
1
peaks at approximately 2102 and 2036 cm are associated
with the symmetric and asymmetric stretches of the positively
[
18a]
charged Rh(CO) gem-dicarbonyl species, and the other two
state by air at room temperature,
we performed in situ
2
ꢁ
1
peaks are attributed to bridged-CO (1860 cm ), and linear-
CO on Rh (2070 cm ). Upon helium purging, the red-shift of
XPS measurements and studied the evolution of Rh 3d and
Zn LMM lines in the spectra (Figure 3; Supporting
0
ꢁ1
2
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ing Information, Figure S6; black and red lines), which
2
ꢁ
provides strong evidence that a portion of the O bonded
2
+
with Zn is removed by H or spillover hydrogen species.
2
Temperature programmed reduction (TPR) patterns (Sup-
[19]
porting Information, Figure S7) and earlier reports have
also suggested the reduction of ZnO in the presence of Rh;
the actual amount of H consumption is about three to four
2
3
+
times higher than the theoretical amount for Rh reduction
Supporting Information, Table S2). As previously presented
(
in STEM images (Figure 1), H reduction at 2008C does not
2
lead to the aggregation of Rh atoms when Rh loadings are
below 0.03%, and we could deduce that Rh atoms would
immediately bond to proximal Zn atoms that lose one or
more O atoms. Consequently, electrons transfer from metallic
Zn to Rh atoms (Zn!Rh) to form near-metallic Rh species.
As a probe reaction, hydroformylation of styrene was
carried out in toluene under a mixture of CO and H gas. The
homogeneous catalyst RhCl3 was initially tested (Table 1,
2
Figure 3. Rh 3d spectra of a,b) 0.3%Rh/ZnO-nw and c,d) 0.03%Rh1/
ZnO-nw catalysts before and after 2008C H reduction, respectively.
2
[
a]
Table 1: Catalytic activity of Rh catalysts.
Information, Figure S5). According to a previous study, the
[
18b]
Zn 2p3/2 peak (binding energy at 1021.8 eV)
was used as an
internal standard for calibration. Notably, a similar result was
obtained using C 1s (284.8 eV) as the internal standard.
The binding energy of Rh 3d5/2 in 0.3%Rh/ZnO-nw
Entry
Catalyst
RhCl
RhCl(PPh3)3
0.006% Rh /ZnO-nw
1
0.03% Rh /ZnO-nw
0.3% Rh/ZnO-nw
l:b
Selectivity [%]
TON
1
2
3
4
5
1
1
1
1
1
83
92
99
99
99
3324
19000
40000
38000
7000
3
(
309.9 eV) is much higher than that of Rh O (308.8 eV),
2
3
indicating that RhCl is the main species before reduction
3
(
Figure 3, line a). After treatment with H at 2008C for 0.5 h
1
2
(
line b), the binding energy shifts to 306.9 eV, suggesting
a metallic or slightly negative charged Rh state, compared
with previous reports. Although the Rh concentration of
[a] Reaction conditions are listed above; 1 (2.5 mmol) was used as
a substrate. Linear:branched (l:b).
[
19]
SAC (0.03%Rh /ZnO-nw) is rather low, the Rh 3d spectra
1
are still distinguishable (lines c,d). Before 2008C reduction,
3
+
+
Rh (309.9 eV) and Rh (308.0 eV) are both present in the
catalyst (line c). A plausible explanation may be that some
Entry 1) as a benchmark for comparison. However, a TON of
only 3324 was achieved, with a ratio of linear to branched (l:b)
of one. Moreover, 17% of the ethylbenzene formed because
of the undesired hydrogenation reaction. The conventional
Wilkinsonꢀs catalyst RhCl(PPh ) gives a TON of 19000, but
about 10% ethylbenzene is also detected (Entry 2).
Comparatively, our SACs (0.03% and 0.006%) give rise to
a TON of about 40000 with the same l:b ratio of one
3
+
Rh precursors are deposited at the O or Zn vacancies of
+
ZnO, and then converted to Rh at room temperature by the
surrounding chemical environment. Similarly, a downshift of
Rh 3d photoemission peaks after 2008C reduction (line d) is
observed, indicating that Rh single atoms are in a metallic
state. The result seems to be different from previous reports,
3
3
[
10b,20]
where the supported single metal atoms (such as Au,
[
7c,21]
[22]
[7a,23]
Pt,
Rh, and Pd
) are generally positively charged.
(Entries 3–4). Accordingly, Rh /ZnO-nw catalysts are about
1
To further confirm the chemical state of the Rh species, we
performed an in situ X-ray adsorption near edge structure
one or two orders of magnitude more active than the majority
of previously reported heterogeneous catalysts (Supporting
Information, Table S3). Additionally, our Rh/ZnO-nw heter-
ogeneous catalysts exhibit 99% chemoselectivity, as no
ethylbenzene hydrogenation side product was detected
(
XANES) measurement. As shown in Figure S5 (Supporting
Information), although the quality of the XANES spectra are
not very good because of the very low Rh loading, absorption
edges of both samples shift slightly to lower energy compared
with Rh foil, suggesting that Rh species exist in an almost
metallic state. The results are consistent with in situ XPS
results. This can probably be explained by the effect of the
strong metal–support interaction (SMSI) between Rh and
(Entries 3–5). As a result, the developed Rh /ZnO-nw SACs
1
are even more effective than traditional homogeneous Rh
catalysts with respect to selectivity. Moreover, complete
conversion of styrene was easily achieved on the
0.006%Rh /ZnO-nw catalyst. As shown in Table S4 (Sup-
1
[
24]
ZnO upon reduction at 2008C,
which results in charge
porting Information), with a metal to substrate molar ratio of
ꢁ5
transfer from partially reduced ZnO to Rh.
2.7 ꢁ 10 (that is, 27 ppm); a 99% yield of the target aldehyde
product was obtained.
ZnO also undergoes partial reduction, and a higher
shoulder at approximately 991 eV after 2008C reduction is
observed in Zn LMM spectra of 0.3%Rh/ZnO-nw (Support-
Hydroformylation substrate scope was explored, as shown
in Table 2. Substitution at the para-position of styrene had
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[
a]
Table 2: Substrate scope of olefins.
In conclusion, we synthesized a Rh SAC supported on
ZnO nanowires for a hydroformylation process with a TON
of about 40000 and 99% selectivity. The utilization of SACs
paves a path toward synthesis of aldehyde fine chemicals. This
work successfully demonstrates that supported hetero-
geneous catalysts with atomically dispersed metal sites can
exhibit activity comparable with homogenous catalysts.
Therefore, rational design and construction of isolated
catalytic sites is significant to the development of new
highly efficient heterogeneous catalysts.
[
a]
Entry
Substrate
Product
Selectivity
%]
TON
[
9
92)
9
40000
(19000)
1
2
3
[
b]
[b]
(
99
26000
9
90)
9
45000
(20000)
[
b]
[b]
Experimental Section
All catalysts were prepared by a facile adsorption method. Typically,
ZnO nanowire (0.2 g) was dispersed into deionized water (15 mL)
and the suspension was stirred at room temperature for 20 min.
(
4
99
86
4000
Subsequently, aqueous RhCl solution (5 mL) with an appropriate
3
ꢁ1
5
6
10000
concentration was added dropwise (1 mLmin ) to the suspension.
After stirring for 3 h and aging for 1 h, the resulting precipitate was
filtered and washed, then dried at 333 K overnight and reduced in 20
9
99)
9
15000
(37000)
[
b]
[b]
(
vol% H /He at 473 K for 0.5 h. The resulting catalysts are denoted
2
[
a] Reaction conditions: 0.8 MPa CO, 0.8 MPa H , 1008C, 0.006%Rh /
2
1
0
.3%Rh/ZnO-nw, 0.03%Rh /ZnO-nw, and 0.006%Rh /ZnO-nw, in
1
1
ZnO-nw catalyst (20 mg), substrate (2.5 mmol), toluene (4 mL) as
solvent, dodecane as internal standard. [b] Homogeneous Wilkinson’s
catalyst (RhCl(PPh ) ) was used. Selectivities and TONs are presented in
agreement with Rh weight loadings of 0.3%, 0.03%, and 0.006%,
respectively. Details about activity tests, and characterization (SEM,
HAADF-STEM, and DRIFTS) are given in the Supporting Infor-
mation.
3
3
parentheses.
a significant electronic effect on reactivity (Entries 1–3). All
substrates exhibited excellent chemoselectivity in the hydro-
formylation reaction, and no hydrogenation product was
observed. Among the substrates tested, 4-Cl-styrene gave the
highest TON (ca. 45000), indicating that Cl-substitution is
well-tolerated under the reaction conditions (Entry 3). These
Acknowledgements
This work was supported by National Natural Science
Foundation of China (21303184, 21573232, 21606222), Stra-
tegic Priority Research Program of the Chinese Academy of
Sciences (XDB17020100), National Key Projects for Funda-
+
results are in accordance with traditional Rh -complex-
mental
Research
and
Development
of
China
catalyzed homogeneous systems, since electron-withdrawing
(2016YFA0202801), the department of science and technol-
[25]
groups are generally considered to promote the reaction.
ogy of Liaoning province (2015020086-101), and dedicated
funds for methanol conversion from DICP. The synchrotron
radiation experiment was performed at the BL14B1 of Japan
Atomic Energy Agency/SPring-8 with the approval of JAEA,
Japan Synchrotron Radiation Research Institute (proposal
no. 2016A3609), and the BL14W1 at the Shanghai Synchro-
tron Radiation Facility, Shanghai Institute of Applied Physics,
China. The authors thank Prof. Shishan Sheng and Dr.
Yongkuan Wu at DICP for their help in performing in situ
XPS measurements.
The control experiments carried out using Wilkinsonꢀs
catalyst (Entries 1 and 3, condition b) show a slightly lower
TON, further demonstrating the superior reactivity of the Rh
SAC. 2-Phenyl-1-propene is also reactive under the reaction
conditions (Entry 4), although a much lower conversion is
obtained, which may be caused by large steric hindrance on
the C=C bond. As shown in Entry 5, the TON with
heptaldehyde is only about 10000, and 14% of the undesired
isomerization products are detected in the reaction residue.
When propylene was used as a substrate, Wilkinsonꢀs catalyst
was more reactive than 0.006%Rh /ZnO-nw (Entry 6). Based
1
Keywords: heterogeneous catalysis · hydroformylation ·
rhodium · single-atom catalysts · styrene
on the above results, we deduce that Rh /ZnO-nw SAC
1
facilitates the hydroformylation reaction of alkenes—
especially those containing an electron-deficient C=C bond.
The Rh /ZnO-nw catalyst can be recycled by centrifuga-
1
tion and reused four times without significant loss of reactivity
and selectivity (Supporting Information, Figure S8). The
amount of Rh leached into the filtrate was below the
detection limit of inductively coupled plasma atomic emission
spectroscopy (ICP-AES), and no product was formed when
the filtrate was used instead of the catalyst. The recycling
experiment confirmed that Rh atoms are strongly bound by
the ZnO support under working conditions, without obvious
leaching or aggregation.
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Received: August 12, 2016
Revised: September 27, 2016
Published online: && &&, &&&&
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

Angewandte
Communications
Chemie
Communications
Single-Atom Catalysts
R. Lang, T. Li, D. Matsumura, S. Miao,
Y. Ren, Y.-T. Cui, Y. Tan, B. Qiao,* L. Li,
A. Wang, X. Wang,
T. Zhang*
&&&& — &&&&
Hydroformylation of Olefins by
a Rhodium Single-Atom Catalyst with
Activity Comparable to RhCl(PPh3)3
Rhodium single-atom catalysts sup-
ported on ZnO nanowires display olefin
hydroformylation efficiency comparable
to that of homogeneous Wilkinson’s
catalyst.
6
www.angewandte.org
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2016, 55, 1 – 6
These are not the final page numbers!