Development of N-Doped Carbon-Supported Cobalt/Copper Bimetallic Nanoparticle Catalysts for Aerobic Oxidative Esterifications Based on Polymer Incarceration Methods

Article abstract of DOI:10.1021/acs.orglett.8b02118

Heterogeneous nitrogen-doped carbon-incarcerated cobalt/copper bimetallic nanoparticle (NP) catalysts, prepared from nitrogen-containing polymers, were developed, and an efficient catalytic process for aerobic oxidative esterification was achieved in the presence of a low loading (1 mol %) of catalyst that could be reused and easily reactivated. This protocol enabled diverse conditions for the bimetallic NP formation step to be screened, and significant rate acceleration by inclusion of a copper dopant was discovered. The catalytic activity of the bimetallic Co/Cu catalysts is much higher than that for cobalt catalysts reported to date and is even comparable with noble-metal NP catalysts.

Full text of DOI:10.1021/acs.orglett.8b02118

Letter
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
pubs.acs.org/OrgLett
Development of N‑Doped Carbon-Supported Cobalt/Copper
Bimetallic Nanoparticle Catalysts for Aerobic Oxidative
Esterifications Based on Polymer Incarceration Methods
Tomohiro Yasukawa, Xi Yang, and Shu Kobayashi*
̅
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
*
S Supporting Information
ABSTRACT: Heterogeneous nitrogen-doped carbon-incar-
cerated cobalt/copper bimetallic nanoparticle (NP) catalysts,
prepared from nitrogen-containing polymers, were developed,
and an efficient catalytic process for aerobic oxidative
esterification was achieved in the presence of a low loading
(
1 mol %) of catalyst that could be reused and easily
reactivated. This protocol enabled diverse conditions for the
bimetallic NP formation step to be screened, and significant
rate acceleration by inclusion of a copper dopant was
discovered. The catalytic activity of the bimetallic Co/Cu catalysts is much higher than that for cobalt catalysts reported to
date and is even comparable with noble-metal NP catalysts.
1
1
he development of highly active and stable heterogeneous
catalysts is a crucial subject in modern science. Metal
selective catalysts. NPs are typically synthesized by pyrolysis of
a cobalt complex containing nitrogen atoms in a ligand adsorbed
on carbon black (Scheme 1a). However, it is difficult to control
the structure of NPs using this method because it usually
T
nanoparticle(NP)catalystshavebeenofgreatinterestbecauseof
their unique activity, ease of heterogenization, and robustness.
Metal NP catalysts have been widely studied and applied to
generates a mixture of small NPs, large aggregated NPs, and/or
1
5e,12
transformations of complex organic molecules. However,
single-metal sites.
This is probably because NPs form during
investigations are limited to noble-metal NPs.
the pyrolysis, and the stabilization ability of the ligand is not
Cobalt NP catalysts on conventional supports such as meta2l
oxides and carbon black are useful for bulk chemical synthesis;
however, they are relatively inert with respect to organic
sufficient at very high temperatures (500−1000 °C). Metal−
Scheme 1. Preparation of N-Doped Carbon-Supported Metal
NP Catalysts
3
reactions. A promising strategy to construct active cobalt NP
catalysts is the use of a nitrogen-doped carbon support,
considering that nitrogen atoms influence the properties of the
carbon material and interact with metal species. Nitrogen-doped
carbon-supported cobalt NP catalysts have been studied in the
4
field of electrochemistry and have recently been applied to
5
several organic reactions. Among these, aerobic oxidative
esterification of alcohols or aldehydes is important because
esters are stable and useful synthetic intermediates, and it
generates only water as a coproduct. Noble-metal-based NP
6
,7
catalysts to achieve this transformation have been investigated.
8
Pioneered by Beller in 2013, nitrogen-doped carbon-supported
cobaltNPcatalystswereappliedtothisprocess. However, abase
9
additive was required in most cases and/or a relatively high
catalyst loading was needed (>5.5 mol % under base-free
conditions). Their catalytic activities were not comparable with
noble-metal NP catalysts. Considering formation of bimetallic
10
NPs often tune their activities (and/or selectivities), the
introduction of a second metal to cobalt NPs would be a
promising strategy to improve their activity. However, this has
not been explored for organic transformations.
The structure control of NPs in nitrogen-doped carbon-
supported cobalt catalysts is a key to obtaining highly active and
Received: July 6, 2018
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.8b02118
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
organic frameworks (MOFs) are highly porous crystalline
materials and recognized as relatively stable and rigid templates
Table 1. Aerobic Oxidative Esterification by NCI Catalysts
9f,11,13
for this purpose (Scheme 1b).
Although several pyrolyzed
MOF catalysts have been studied, it remains difficult to flexibly
tune the structure of NPs, especially multimetallic NPs because
atomic-level ordering in multimetallic MOFs remains challeng-
ing. Therefore, a more general method that can be used to
access nitrogen-doped carbon-supported metal NP catalysts that
are more controllable in structure and is applicable to the
preparation of multimetallic NPs is necessary.
a
b
b
entry
Co cat.
t (°C)
2a (%)
3a (%)
1
4
1
2
3
4
5
6
NCI-Co 1
NCI-Co 2
NCI-Co/Cu 3
NCI-Co/Cu 4
NCI-Co/Cu 5
NCI-Cu 6
rt
c
rt
c
50
rt
50
47
24
73
42
79
0
10
12
6
17
6
Our group previously investigated a polymer-incarceration
(
PI) method to immobilize metal NPs by using a polystyrene-
7
1
15
based copolymer with a cross-linking moiety and carbon black.
d
e
7
NCI-Co/Cu 5
92 ± 2
In this method, metal NPs were physically enveloped by the
polymerand stabilized bymultiple interactionsfrom the polymer
a
t is a parameter used for catalyst preparation, shown in Scheme 2.
Determined by GC analysis. The catalyst was prepared without
reduction treatment. The esterification was performed at 70 °C.
The average yield of eight reactions using different batches of the
b
c
d
(
so-called microencapsulation). Further heating then promoted
e
cross-linkingtoaffordaninsolublesolidmaterialthat “lockedup”
metal NPs. We envisioned that the PI method might be used to
construct structurally controlled NP catalysts supported by N-
doped carbon. Thus, nitrogen-containing polymer-encapsulated
metal NPs formed by the chemical reduction might be “cross-
linked” by the pyrolysis to afford a nitrogen-doped carbon-
incarcerated metal NP catalyst (NCI-M). Herein, we report
earth-abundant-metal-based bimetallic NP catalysts for aerobic
oxidative esterification with high activity that is comparable to
noble-metal catalysts. Metal NPs can be effectively stabilized by
the multiple interactions from the polymer, and diverse
conditions for generation of metal NPs, including multimetallic
NPs, can be screened by changing the reduction conditions that
might affect the structure of the NPs (Scheme 1c).
catalyst was shown.
treatment in the preparation of the bimetallic NP catalyst was
again confirmed because the bimetallic catalyst NCI-Co/Cu 4,
prepared without using a reductant, showed moderate activity
(
entry 4).
Theconditionsofcoreductionofmetalsaltscanaffectboththe
structure of bimetallic NPsand the catalytic activity. Ourmethod
could be used to flexibly tune the conditions of NP formation.
Several parameters, such as the reduction temperature, the
reductant, the polymer, the pyrolysis temperature, and the ratio
of metals, were varied, and improved catalytic activity was
observed when the reduction was conducted at 50 °C with
NaBH (NCI-Co/Cu 5, entry 5). These results demonstrate the
16
Poly(4-vinylpyridine) was chosen as a precursor of a nitrogen-
doped carbon and a temporal stabilizer of NPs (Scheme 2).
4
importance of the tunability of the conditions in the NP
formationtoobtainefficientcatalysts. Thecorrespondingcopper
NP catalyst did not give the ester product at all (entry 6). No
productformedinthepresenceofNCI-Cu6, eveninthereaction
started from the aldehyde, which indicated that the active sites of
Scheme 2. Preparation of NCI Catalysts
1
6
NCI-Co/Cu were cobalt species. Finally, a satisfactory high
yield was obtained with good reproducibility when the reaction
was conducted at 70 °C with NCI-Co/Cu 5 (entry 7). No
significant leaching of metals (<0.1%) was detected, and the
heterogeneous nature of catalysts was confirmed by a hot
16
leaching test. We confirmed that copper acetate as an additive
for the reaction with NCI-Co 1 was less effective for the activity
indicating that heterogeneous copper NPs were important to
NaBH as a reductant was added to the mixture of the polymer,
4
metal salts and carbon black to form cobalt NPs stabilized by
multiple interactions from the polymer. A poor solvent was
addedtogenerate aprecipitationof the polymermatrixenclosing
cobalt NPs (microencapsulation). The obtained polymer matrix
was then pyrolyzed at 800 °C to afford the desired catalyst (NCI-
Co 1). As a control, a catalyst (NCI-Co 2) was prepared without
usingareductant. In thiscase, thecobaltcomplexwasenclosedin
the polymer matrix and directly pyrolyzed. Inductively coupled
plasma (ICP) analysis revealed that both catalysts contained
1
6
enhance the activity.
1
6
The comparison of the reaction profiles between the
reactions with the monometallic and bimetallic catalysts revealed
dramatic acceleration of the reaction rate aided by a copper
dopant. According to the initial rate (from 0 to 1 h), the highest
turnover frequency (TOF) of 37.2 h was observed in the
reaction catalyzed by NCI-Co/Cu 5, whereas a TOF of only 3.21
was observed in the case of NCI-Co 1. Such a high TOF has
never been achieved by using reported cobalt catalysts, even in
the presence of a base, and it is even comparable with noble-
6,7
−
1
−
1
h
16
cobalt levels close to the target loading. The catalytic activities
17
were evaluated in the aerobic oxidative esterification of alcohol
1
a in methanol (Table 1). Gratifyingly, the reaction proceeded in
metal NP catalysts. We then took the profiles of the reaction
started from the aldehyde intermediate, which suggested that
both oxidation pathways, an alcohol to an aldehyde and an
aldehyde to an ester, were accelerated by the copper dopant.
Several characterizations of the NCI catalysts were conducted
to elucidate the effect of the reduction treatment. Powder X-ray
diffraction (XRD) patterns of NCI-Co/Cu 4 showed clear, sharp
the presence of 1 mol % of NCI-Co 1 to afford ester 2a in
moderate yield (entry 1), whereas NCI-Co 2 gave almost half the
yield of that with NCI-Co 1 (entry 2). These results indicate that
the reduction treatment during the catalyst preparation
significantly enhanced the catalytic activity. Bimetallic NP
catalysts were also screened, and the catalyst NCI-Co/Cu 3
prepared from Co(II) acetate and Cu(II) acetate improved the
yield dramatically (entry 3). The importance of the reduction
16
1
8
peaks derived from the metallic copper and cobalt (Figure 1a).
However, the peaks of metallic cobalt in NCI-Co/Cu 3 and 5
B
DOI: 10.1021/acs.orglett.8b02118
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Organic Letters
Letter
showed significantly lower activity though no aggregates might
16
form basedon XRDanalysis. Theseobservationsindicated that
a higher ratio of pyrrolic nitrogen might be important for high
catalytic activity.
With the optimized conditions in hand, the substrate scope
was surveyed (Scheme 3). Both electron-donating and -with-
Scheme 3. Substrate Scope*
*
a
b
c
Isolated yield. GC yield. K CO (20 mol %) was used. 48 h.
Figure 1. Characterization of catalysts: XRDand STEM analysisof (a,c)
NCI-Co/Cu 4 and (b,d) NCI-Co/Cu 5, and EDS mapping of (e) NCI-
Co/Cu 5.
2
3
d
e
f
NCI-Co/Cu 5 (2 mol %) was used. At 110 °C. At 120 °C.
drawing group substituted benzyl alcohols were smoothly
converted into the corresponding methyl esters in high yields
under the base-free conditions (2a−g). Whereas substituents at
the m- or p-position were tolerated (2h−i), the presence of a
substituent at o-position decreased the reactivity (2j−k); in one
case, a catalytic amount of base could be used to promote the
reaction (2j). A diol was converted into the corresponding
diester 2l in high yield. Various aryl alcohols, including naphthyl
alcohol, heteroaryl alcohols, and cinnamyl alcohol, were also
suitable substrates, giving the products in good yields under the
base-free conditions (2m−2p). Other alcohol solvents were
examined, andthe correspondingesters wereobtained in goodto
high yields (2q−t). Unfortunately, the reaction with aliphatic
alcohol gave a low yield. We then applied NCI-Co/Cu to the
synthesis of heteroaryl esters from aldehydes (Scheme 4).
Heteroaryl aldehydes are more readily available than the
corresponding alcohols because of many direct synthetic
methods, suchasformylationandtheVilsmeier−Haackreaction.
Various substrates, including pyridine, thiophene, furan, quino-
were significantly suppressed, indicating that the size of
crystalline particles in these catalysts were smaller than those in
16,19
catalyst 4 (Figure 1b).
Scanning transmission electron
microscopy (STEM) analysis revealed the existence of large
aggregates of metal NPs with several size (ca. 30 to ca. 200 nm)
formed during pyrolysis in NCI-Co/Cu 4 (Figure 1c), whereas
relatively small metal NPs (ca. 20 nm) were mainly observed in
NCI-Co/Cu 5 (Figure 1d). These results are consistent with the
XRD patterns, clearly demonstrating that the preformation of
NPs prior to pyrolysis is a key to controlling the size of NPs and
maintaining a small size. A similar tendency was also observed in
16
the monometallic catalysts. Energy dispersive X-ray spectrom-
etry(EDS)mappingrevealedthat, inallcases,bothalloyNPsand
16
16
segregated cobalt or copper NPs were observed (Figure 1e).
The segregation of two metals might occur because of the
20
differences in the reduction potentials between the metals.
To elucidate details of the surface composition, X-ray
2
1
photoelectron spectroscopy (XPS) analysis was conducted.
ThequantitativeanalysisbasedontheCo2p3/2 andCu2p XPS
spectra revealed that a copper-rich surface formed in NCI-Co/
Cu 4, whereas a cobalt-rich surface formed in NCI-Co/Cu 3 and
3/2
Scheme 4. Esterification of Heteroaryl Aldehydes*
5
, althoughthebulkratiooftheloadedmetalsineachcatalystwas
16
almost 1:1 on the basis of ICP analysis. We postulated that the
copper was reduced faster than the cobalt and tended to exist in
the core of the alloy NPs under the current mild chemical
reduction conditions. These results suggest that the reduction
treatment also impacted the structure of the bimetallic NPs and
that a modification of the chemical reduction conditions may be
used to tune the process. However, XPS spectra of N 1s revealed
22
that pyrrolic nitrogen and pyridinic nitrogen were mainly
detected, and that their ratio was almost constant regardless of
the reduction treatment and the deposition of mono- or
bimetallic NPs. A different ratio appeared when the pyrolysis
was conducted at lower temperatures that led to a lower ratio of
pyrrolic nitrogen. The catalysts pyrolyzed at lower temperature
*
a
Isolated yield. 48 h.
C
DOI: 10.1021/acs.orglett.8b02118
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Organic Letters
Letter
line,andindolederivatives,wereconvertedintothedesiredesters
under the base-free conditions.
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NCI catalysts were easily recovered by filtration, and the
16
reusability of the catalyst was examined. A slight loss of activity
was observed after several runs, and the catalyst could be
reactivated by heating at 500 °C. Consequently, the catalyst
could be reused for the tenth run, and around 90% yields were
obtained in every run. We assumed that such a heating treatment
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In conclusion, we have developed nitrogen-doped carbon-
supported cobalt NP catalysts based on a PI strategy. The
chemical reduction of metal salts to form NPs before the
pyrolysis is essential to form a highly active catalyst for aerobic
oxidative esterification, and these conditions can be flexibly
tuned. We discovered for the first time that bimetallic Co/Cu NP
catalystsshowedanorderofmagnitudehigherTOFthanthoseof
cobalt NP catalysts in aerobic esterification reactions, although
copper NPs themselves did not catalyze the reaction. The
esterificationreactionsproceededinthepresenceofonly1mol%
of the catalyst under base-free conditions for most substrates,
including various heteroaryl compounds. Such a remarkable
catalytic performance under relatively mild conditions for a wide
variety of substrates has never been achieved, even with noble-
metal NP catalyst systems. The catalyst was reusable for 10 runs
and easily reactivated. These protocols are suitable for the
construction of more active nitrogen-doped carbon-supported
metal NP catalysts that can be used in various catalysis fields.
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ASSOCIATED CONTENT
Supporting Information
■
Beller, M. J. Am. Chem. Soc. 2013, 135, 10776.
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Experimental procedures, characterization data, and
copies of NMR charts (PDF)
AUTHOR INFORMATION
Corresponding Author
■
(
(
2
*E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp.
ORCID
2
Shu Kobayashi: 0000-0002-8235-4368
Notes
Kiely,C.J.;Hutchings,G.J.Chem.Soc.Rev.2012,41,8099.(e)Ferrando,
R.; Jellinek, J.; Johnston, R. L. Chem. Rev. 2008, 108, 845.
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(
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H.; Pohl, M.-M.; Radnik, J.; Beller, M. Science 2017, 358, 326.
12) (a) Slot, T. K.; Eisenberg, D.; van Noordenne, D.; Jungbacker, P.;
The authors declare no competing financial interest.
(
ACKNOWLEDGMENTS
This work was partially supported by a Grant-in-Aid for Science
Research from the Japan Society for the Promotion of Science
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Chem. - Asian J. 2016, 11, 2594. (c) Cheng, T.; Yu, H.; Peng, F.; Wang,
H.; Zhang, B.; Su, D. Catal. Sci. Technol. 2016, 6, 1007.
■
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13)(a)Shen, K.;Chen, X. D.;Chen, J. Y.;Li, Y. W. ACSCatal. 2016, 6,
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(
JSPS), Global COE Program, The University of Tokyo, MEXT,
5
Japan, and the Japan Science and Technology Agency (JST). We
thank Mr. Noriaki Kuramitsu (The University of Tokyo) for
STEM and EDS analyses.
(
(
(
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