Co3O4Nanoparticles Supported on Mesoporous Carbon for Selective Transfer Hydrogenation of α,β-Unsaturated Aldehydes

Article abstract of DOI:10.1002/anie.201604673

A simple and scalable method for synthesizing Co₃O₄nanoparticles supported on the framework of mesoporous carbon (MC) was developed. Benefiting from an ion-exchange process during the preparation, the cobalt precursor is introduced into a mesostructured polymer framework that results in Co₃O₄nanoparticles (ca. 3 nm) supported on MC (Co₃O₄/MC) with narrow particle size distribution and homogeneous dispersion after simple reduction/pyrolysis and mild oxidation steps. The as-obtained Co₃O₄/MC is a highly efficient catalyst for transfer hydrogenation of α,β-unsaturated aldehydes. Selectivities towards unsaturated alcohols are always higher than 95 % at full conversion. In addition, the Co₃O₄/MC shows high stability under the reaction conditions, it can be recycled at least six times without loss of activity.

Full text of DOI:10.1002/anie.201604673

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201604673
German Edition: DOI: 10.1002/ange.201604673
Supported Catalysts
Co O Nanoparticles Supported on Mesoporous Carbon for Selective
3
4
Transfer Hydrogenation of a,b-Unsaturated Aldehydes
Guang-Hui Wang, Xiaohui Deng, Dong Gu, Kun Chen, Harun Tꢀysꢀz, Bernd Spliethoff, Hans-
Josef Bongard, Claudia Weidenthaler, Wolfgang Schmidt, and Ferdi Schꢀth*
Dedicated to Professor Klaus K. Unger on the occasion of his 80th birthday
Abstract: A simple and scalable method for synthesizing
for product purification. Therefore, design of suitable cata-
lysts and/or catalytic systems that facilitate selective hydro-
genation of C=O bond in the presence of other functionalities
Co O nanoparticles supported on the framework of meso-
3
4
porous carbon (MC) was developed. Benefiting from an ion-
exchange process during the preparation, the cobalt precursor
is introduced into a mesostructured polymer framework that
results in Co O nanoparticles (ca. 3 nm) supported on MC
is highly desirable.
In general, conventional hydrogenation catalysts based on
transition metals (e.g., Pd, Pt, Ru, Rh, Cu, or Ni) show high
3
4
[
4,5]
(
Co O /MC) with narrow particle size distribution and homo-
activity but poor selectivity toward unsaturated alcohols.
To enhance the selectivity for unsaturated alcohols over such
3
4
geneous dispersion after simple reduction/pyrolysis and mild
oxidation steps. The as-obtained Co O /MC is a highly efficient
[6]
[7]
catalysts, additives, stabilizers/ligands, supplemental metal
3
4
[8]
[9]
catalyst for transfer hydrogenation of a,b-unsaturated alde-
hydes. Selectivities towards unsaturated alcohols are always
higher than 95% at full conversion. In addition, the Co O /MC
components, and functional supports were introduced into
catalytic systems or catalysts. In some cases high selectivity
toward unsaturated alcohols and high activity were indeed
achieved using the above approaches. However, owing to the
complexity of reaction mechanisms (e.g., competitive/non-
competitive and dissociative/non-dissociative adsorption, side
reactions), selectivities and activities in these cases can be
affected by a series of factors, including the structure, texture
and composition of the catalysts (e.g., particle size, shape,
molar ratio of different components, crystal structure) and the
reaction conditions (e.g., temperature, pressure and sol-
3
4
shows high stability under the reaction conditions, it can be
recycled at least six times without loss of activity.
T
he production of high-value-added chemicals from biomass
is of major interest since the dependence on petroleum-based
chemicals can be reduced by replacing fossil feedstocks with
[1]
renewable ones. Furan derivatives of furfural (FAL) and 5-
hydroxymethylfurfural (HMF), which can be produced from
hemicellulose and cellulose, respectively, are considered as
promising platform molecules to bridge the gap between
biomass resources and bio-chemicals, since they can be
converted into a variety of high-value-added chemicals and
[10]
vent).
In other words, dramatic decrease in selectivity
and/or activity often occurs, because it is very difficult to
predict and control the influence of these factors precisely,
especially in large scale applications. Therefore, it is necessary
to develop simple methods for scale-up of catalyst synthesis.
Furthermore, the catalysts must be highly active and selective
toward unsaturated alcohols in simple catalytic systems,
which are also controllable and scalable.
[
2]
fuels. Particularly, selective hydrogenation of FAL to
furfuryl alcohol (FOL) and HMF to 2,5-bis-(hydroxymethyl)-
furan (BHMF) has great potential for industrial applications.
Because, FOL and BHMF can be used as precursors in the
synthesis of polymers, resins and adhesives, and as inter-
Compared with hydrogenation reactions involving H2,
transfer hydrogenation is much safer and easier to handle
[1e,f]
mediates for the generation of drugs and crown ethers.
However, owing to the different functionalities of furan-based
because pressure is not an issue and the influence of H
2
[11]
a,b-unsaturated aldehydes (e.g., furan ring, C=C and C=O
pressure on selectivity is not relevant. In transfer hydro-
genation reactions, 2-propanol is often utilized as hydrogen
donor, because it is cheap, environmentally friendly, and easy
[
3]
bonds), selective hydrogenation of only the C=O bond is
very challenging. Many byproducts are often formed by
[
11]
hydrogenolysis of the -CH=O side chain to -CH , or hydro-
to remove. Herein, we report on the transfer hydrogenation
of furan-based a,b-unsaturated aldehydes over spinel-type
cobalt oxide on mesoporous carbon (Co O /MC), where the
3
[
4]
genation of the furan ring and its opening, leading to low
yield of the desired unsaturated alcohols and increasing costs
3
4
selectivity toward unsaturated alcohol is always higher than
5% at full conversion. The synthesis of Co O /MC involves
a hydrothermal process, an ion-exchange, and a pyrolysis/
reduction as well as a mild oxidation step (Figure 1a). In the
first step, a mesostructured polymer gel is synthesized using
9
3
4
[
*] Dr. G.-H. Wang, X. Deng, Dr. D. Gu, Dr. K. Chen, Dr. H. Tꢀysꢀz,
B. Spliethoff, H.-J. Bongard, Dr. C. Weidenthaler, Dr. W. Schmidt,
Prof. Dr. F. Schꢀth
Max-Planck-Institut fꢀr Kohlenforschung
2
,4-dihydroxybenzoic acid and hexamethylenetetramine
(HMT) as polymer precursors, and triblock copolymer
P123) as surfactant under hydrothermal conditions (1308C
for 4 h). Then, cobalt ammine complexes are introduced
45470 Mꢀlheim an der Ruhr (Germany)
E-mail: schueth@mpi-muelheim.mpg.de
(
Supporting information for this article can be found under:
http://dx.doi.org/10.1002/anie.201604673.
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under the diffraction pattern indicate reflections
for the PDF-2 entry 43–1003). The very broad
reflections prove the formation of small Co O
3
4
nanoparticles after the reduction and mild oxida-
tion steps. The Co 2p X-ray photoelectron spec-
trum (XPS) shows peaks with binding energies at
3
+
7
79.6 eV and 794.6 eV corresponding to Co 2p3/2
and 2p , and peaks at 781.1 eV and 796.5 eV
1
/2
2
+
belonging to Co 2p3/2 and 2p1/2 (Figure 1i,
Table S1), which further confirm the formation
of Co O nanoparticles. Nevertheless, the shake-
3
4
2
+
up signal at 785.4 eV belonging to Co is slightly
higher than expected for an ideal Co O surface,
suggesting that Co is enriched in the surface of
3
4
2
+
the Co O4 nanoparticles (Figure 1i, Table S1).
3
Based on AAS analysis (atomic absorption spec-
trometry, PerkinElmer AAnalyst 200), the Co
fraction in Co O /MC is 15 wt%, corresponding
3
4
to 20 wt% Co O . The N sorption isotherm of
3
4
2
Co O /MC (Figure 1j) shows a type-IV curve,
3
4
characteristic for mesoporous materials. The sorp-
tion isotherm does not level out in a plateau at
relative pressures p/p > 0.9, indicating the exis-
0
tence of macropores, fully consistent with the SEM
observations. The surface area, pore volume and
2
À1
3
À1
pore size of Co O /MC are 642 m g , 0.7 cm g
3
4
and 11 nm, respectively.
The synthesis methodology of Co O /MC can
3
4
be extended to synthesize other metal catalysts
supported on MC. Pd/MC (Figure S2) and Ru/MC
Figure 1. a) Synthesis of Co O /MC. b,c) TEM images, d) SEM image and e–
3
4
(Figure S3) were prepared by this method as well,
g) STEM images of Co O /MC; inset in (c) shows the Co O particle size distribu-
3
4
3
4
both of which have narrow particle size distribu-
tion (5 Æ 0.8 nm for Pd; 1 Æ 0.2 nm for Ru) and
homogenous dispersion. Without introducing
tion. h) XRD pattern, i) XPS spectrum, and j) N sorption isotherm of Co O /MC;
2
3
4
inset: the pore size distribution.
2
À1
metals, MC with a surface area of 722 m g ,
3
À1
homogenously into the polymer framework via ion exchange.
Next, the polymer containing the cobalt precursor is pyro-
lyzed at 5008C (10% H2 in argon) to form the carbon,
simultaneously the cobalt precursor is reduced to form metal
nanoparticles supported on the mesopore framework of the
carbon. Finally, Co O nanoparticles are formed within the
a pore volume of 0.7 cm g and a pore size of around 9.5 nm
is generated directly after carbonization of the polymer gel at
8008C in argon (Figure S4). Such carbon material has high
potential for applications as catalyst support or adsorbent.
The catalytic performance of Co O /MC was evaluated
3
4
for transfer hydrogenation of FAL to FOL and HMF to
BHMF. First a blank reaction without catalyst was carried
out; no reaction proceeded (Table S2, entry 1). In the
presence of pure MC, the conversion of FAL was lower
than 4% after reaction for 8 h at 1208C (Table S2, entry 2).
However, when using Co O /MC as catalyst, full conversion
3
4
MC support after mild oxidation at room temperature (1%
O in argon). These synthetic processes can be easily scaled
2
up.
Transmission electron microscopy (TEM) images (Fig-
ure 1b,c) show that the Co O /MC is highly mesoporous. The
3
4
3
4
Co O nanoparticles with a diameter of approximately 3 nm
are finely dispersed on the framework of MC. Many macro-
pores are also observed in the scanning electron microscopy
of FAL into FOL (selectivity > 97%) was achieved after
reaction for 8 h at 1208C (Figure 2a). Also, a 97% selectivity
of BHMFat complete conversion of HMF was realized within
48 h at 1208C (Figure 2b). In both cases, only trace amounts
of byproducts (< 3%) generated from the aldol condensation
of substrates and acetone were observed. Undesired saturated
aldehydes and saturated alcohols were not observed by GC-
MS (detection threshold 0.3%), indicating that Co O /MC is
3
4
(
SEM) images (Figure 1d and Figure S1a in the Supporting
Information), which are caused by the gel structure of the
polymer. Scanning transmission electron microscopy (STEM)
images (Figure 1e–g) further confirm that the Co O nano-
3
4
particles are very homogeneously dispersed on the MC
framework and that they have a narrow particle size
distribution. Most importantly, no bigger Co O particles are
3
4
a truly selective hydrogenation catalyst for the C=O bond.
Extending the reaction time for FAL hydrogenation to 24 h,
the selectivity towards FOL was unchanged (Figure 2a),
further confirming that the C=C bond is unaffected under the
3
4
formed by the synthesis method (Figure 1b,e–g and Fig-
ure S1b). The X-ray diffraction (XRD) pattern (Figure 1h)
shows the typical reflections of the Co O spinel (the lines
reaction conditions. It is unclear whether the surface under
3
4
2
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gives essentially identical results. The rates (calculated
based on the conversion reached over a certain period of
time selected so that conversion over different catalysts were
approximately identical) over Co O -6 nm and Co O -17 nm
3
4
3
4
À3
are 14.0 ꢀ 10
(22% conversion in 4 h) and 4.5 ꢀ
À3
À2 À1
1
0
mmolmCo3O4
h
(15% conversion in 24 h; Table S2,
entries 7,8), respectively. For HMF conversion, the reaction
À3
rates over Co O -6 nm and Co O -17 nm are 4.0 ꢀ 10 (19%
3
4
3
4
À3
À2 À1
conversion in 12 h) and 1.7 ꢀ 10 mmolmCo3O4
h
(11%
conversion in 48 h; Table S2, entries 13, 14), respectively.
Thus, intrinsic activities of the smaller particles seem to be by
about a factor of three higher. Surprisingly, the reaction rates
for FAL and HMF conversions over Co O /MC are 114.5 ꢀ
3
4
À3
À3
1
1
0
0
(64%
mmolmCo3O4
conversion
h
in
2 h)
and
33.9 ꢀ
À2 À1
(64% conversion in 6 h; Table S2,
entries 3,9), respectively, which are much higher than those
over unsupported Co O nanoparticles. This suggests a syner-
3
4
gistic effect between Co O4 and MC, where the porous
3
structure and surface functional groups (Figure S7) of MC
may favor the accumulation of reactants through spatial
[
14]
confinement, and even favor the formation of intermedi-
ates because of the interaction between support and reac-
[
15]
tants, leading to the enhancement of transfer hydrogena-
tion activity over Co O supported on MC. The synergistic
3
4
effect is studied in detail in ongoing work. Notably, compared
with conventional impregnation methods, where the Co O
Figure 2. Catalytic performance in transfer hydrogenation reactions at
208C: a) FAL to FOL and b) HMF to BHMF over Co O /MC, Co O -
3
4
1
3
4
3
4
particle size distributions are generally very broad, the
present synthesis method is simple and generates Co O
4
nanocasting, Co O -6 nm, and Co O -17 nm. Reaction conditions:
3
4
3
4
1
mmol substrate, 10 mL 2-propanol, 50 mg Co O /MC, 25 mg for
3
3
4
Co O -nanocasting, Co O -6 nm, and Co O -17 nm, respectively.
nanoparticles with narrow size distribution (ca. 3 nm) and
homogeneous dispersion, which allows a highly efficient
utilization of Co species on the Co O /MC.
3
4
3
4
3
4
3
4
reaction conditions consists of Co O or somewhat reduced
The effect of temperature on the transfer hydrogenation
reactions was also examined. By increasing the temperature
to 1408C and 1608C, the conversions of FAL reached 100%
within 3 h and 1 h, respectively (Figure S8a). For HMF to
BHMF, the conversions were 100% after reaction for 12 h
and 6 h when increasing the temperature to 1408C and 1608C,
respectively (Figure S8b). Most importantly, the selectivities
towards FOL and BHMF were still higher than 97% in all
cases. These results indicate that increasing temperature to
1608C only enhances the reaction rate without affecting the
selectivity. For FAL conversion, the reaction rates over
Co O /MC were calculated. They increase strongly from
3
4
species. The presence of metallic cobalt, however, is highly
improbable, since comparison experiments using the materi-
als prior to the final oxidation step resulted in a much inferior
activity (Figure S6).
For comparison, mesoporous Co O synthesized by nano-
3
4
[
12]
casting (Co O -nanocasting, Figure S5a,b)
was used as
3
4
catalyst in the same system. The activity of Co O -nanocasting
3
4
was much lower than that of Co O /MC. Conversions of FAL
3
4
and HMF were only 17% and 19% after reaction for 24 h and
8 h, respectively (Figure 2a,b). From the XRD pattern of
Co O -nanocasting one can infer, that its crystalline domain
4
3
4
3
4
sizes are bigger than those of Co O /MC (Figure S5b).
37.5 (64% conversion in 2 h), to 148.2 (63% conversion in
3
4
À1 À1
Furthermore, Co O nanoparticles with different sizes (6 nm
0.5 h) and 401.4 mmolgCo3O4
h
(57% conversion in
3
4
and 17 nm) synthesized by a reported preparation method
0.167 h) by increasing the temperature from 1208C to
1408C, and to 1608C (Table S2, entries 3–5). Also the reaction
rates for HMF conversion over Co O /MC increase from 11.1
[
13]
without any surfactants (Figure S5c–f)
The conversions of FAL over Co O -6 nm and Co O -17 nm
were also tested.
3
4
3
4
3
4
were 46% and 15% within 24 h, indicating that smaller Co O4
(64% conversion in 6 h), to 57.5 (63% conversion in 1 h) and
3
À1 À1
nanoparticles show higher activity (Figure 2a). Similar results
were found for the transfer hydrogenation of HMF, the
Co O -6 nm shows higher activity (Figure 2b). In all cases, the
to 133.3 mmolgCo3O4
h
(73% conversion in 0.5 h), respec-
tively (Table S2, entries 9–11). These values are one or two
orders of magnitude higher than those of Co O -nanocasting,
3
4
3
4
selectivities for FOL and BHMF were always higher than
7% (Table S2, entries 6–8, 12–14), indicating that the
selectivity over Co O is structure insensitive.
Co O -6 nm, and Co O -17 nm (Table S2, entries 6–8, 12–14).
3 4 3 4
Notably, the activity of Co O /MC is comparable to the
3 4
9
activities of catalysts for hydrogenation systems involving H2,
and higher than activities of other transfer hydrogenation
catalysts (Table S3). Moreover, the transfer hydrogenation
system using Co O /MC as catalyst is much more stable and
3
4
The surface area of Co O nanoparticles, assumed to be
3
4
spherical, is used to normalize the reaction rate for FAL
conversion. Basing the normalization on surface areas
determined by nitrogen sorption for both pure samples
3
4
insensitive to factors (e.g., structure/component of catalysts
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and reaction conditions) than the hydrogenation system
from the surface of the used Co O /MC. In this way, the
3 4
involving H (Table S3, entries 5–9).
Co O /MC catalyst was recycled at least six times without
3 4
2
Co O /MC is also broadly applicable for hydrogenation of
considerable change in conversion of FAL (the conversions
were ranging from 28% to 34% after 1 h), and the selectivity
toward FOL was always above 97% (Figure 3b). The TEM
images, XRD patterns and N isotherm of Co O /MC after six
3
4
other a,b-unsaturated aldehydes. As examples, the conver-
sions of cinnamaldehyde and citral were 99% with selectiv-
ities to the corresponding unsaturated alcohols of more than
2
3
4
9
5% (Figure 3a; Table S2, entries 15 and 16). These unsatu-
runs (Figure S9) indicate that the mesoporous structure and
the particle size of Co O were unchanged, confirming the
3
4
high stability of Co O /MC under reaction conditions. In
3
4
addition, a hot-filtration test was carried out to verify the
heterogeneous nature of the reaction. After removal of the
solid catalyst, no reaction proceeded in the filtrate (Fig-
ure S10). Moreover, no Co species was present in the filtrate
(< 0.2 ppm by AAS analysis). These results indicate that the
catalytic effect in this system results from the Co O nano-
3
4
particles and not from leached metal species.
In addition, the transfer hydrogenation process can be
easily scaled up since no high pressure (as in hydrogenation
with H ) is required. A gram-scale experiment was carried out
2
using 1 mL of FAL (ca. 1.16 g) as substrate at 1608C for 4 h.
The conversion of FAL and selectivity for FOL were 100%
and > 97%, respectively (Table S2, entry 17). After isolation
by rotary evaporation at 808C under reduced pressure
(50 mbar), 1.02 g of FOL (Figure 4a) with purity of approx-
imately 95% was obtained (calculation based on NMR data,
Figure 3. a) Catalytic performance in the transfer hydrogenation of
cinnamaldehyde and citral over Co O /MC. Reaction conditions:
3
4
1
mmol substrate, 10 mL 2-propanol, 50 mg Co O /MC, 1208C.
3 4
b) Recycling results for transfer hydrogenation of FAL over Co O /MC.
Reaction conditions: 1 mmol FAL, 10 mL of 2-propanol, 50 mg Co O /
3 4
MC, 1208C for 1 h.
Figure 4. a) Photographs of FAL and the product FOL after transfer
hydrogenation and evaporation. Reaction conditions: 1.16 g FAL,
100 mL 2-propanol, 500 mg Co O /MC, 1608C for 4 h. After isolation
3
4
3
4
by rotary evaporation at 808C under reduced pressure (50 mbar),
.02 g of FOL with purity of around 95% was obtained. b–d) XRD
pattern and TEM images of CMK-5 using the as-obtained FOL as
precursor.
1
rated alcohols are industrially important chemical intermedi-
ates for production of pharmaceuticals, flavors and cosme-
[
10b,c]
tics.
In addition, never were traces of saturated aldehydes
and/or saturated alcohols observed by GC-MS (detection
threshold 0.3%). Therefore, the present catalytic system is
highly promising for use in industrial production of a,b-
unsaturated alcohols.
Figure S11). The as-obtained FOL was used as polymer
precursor directly without further purification to synthesize
ordered mesoporous carbon (CMK-5) via a previously
reported method. The obtained carbon material possesses
the typical highly ordered mesoporous structure (Figure 4b–
d).
[
16]
The reusability of the Co O /MC catalyst was investigated
3
4
using FAL as substrate. For each run, the reaction was carried
out at 1208C for only 1 h. The catalyst after reaction was
filtrated and washed with 2-propanol, followed by drying and
In conclusion, we have developed a simple and scalable
method to synthesize Co O nanoparticles supported on the
treatment under H /Ar at 3008C for 2 h to remove residues
2
3
4
4
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mesopore framework of the MC. Benefiting from the ion-
exchange process, by which cobalt precursor is introduced
homogeneously into the polymer framework, Co O nano-
particles with diameters of around 3 nm are generated which
are finely dispersed on the MC framework. The as-obtained
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[
Co O /MC is an efficient catalyst for transfer hydrogenation
3
4
of furan-based a,b-unsaturated aldehydes. Selectivities
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full conversion. Furthermore, Co O /MC is highly stable
under the reaction conditions and can be recycled several
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3
4
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4
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This work was performed as part of the Clusters of Excellence
“
TMFB” and “RESOLV”, which are funded by the Excel-
lence Initiative by the German federal and state governments
to promote science and research at German universities.
Keywords: aldehydes · hydrogen transfer · mesoporous carbon ·
nanoparticles · supported catalysts
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Received: May 13, 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
Supported Catalysts
A particularly good transfer: Co O
3 4
nanoparticles (ca. 3 nm) dispersed on
mesoporous carbon have been prepared
by a simple and scalable method. The
product shows excellent catalytic perfor-
mance in transfer hydrogenation of furan-
based a,b-unsaturated aldehydes to
unsaturated alcohols with selectivity
higher than 95% at full conversion.
G.-H. Wang, X. Deng, D. Gu, K. Chen,
H. Tꢁysꢁz, B. Spliethoff, H.-J. Bongard,
C. Weidenthaler, W. Schmidt,
F. Schꢁth*
&&&& — &&&&
Co O Nanoparticles Supported on
3
4
Mesoporous Carbon for Selective
Transfer Hydrogenation of a,b-
Unsaturated Aldehydes
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!