Full text of DOI:10.1002/cctc.201500549

DOI: 10.1002/cctc.201500549
Communications
Hierarchically Structured MnO2-Co/C Nanocomposites:
Highly Efficient and Magnetically Recyclable Catalysts for
the Aerobic Oxidation of Alcohols
Jian Zhi,[a] Sharon Mitchell,[b] Javier PØrez-Ramírez,*[b] and Oliver Reiser*[a]
Dedicated to Manfred Scheer on the occasion of his 60th birthday
Porous MnO2 microspheres distributed around magnetic
carbon-coated cobalt nanoparticles were developed. The per-
formance of these nanoparticles rivals that of Pd-based cata-
lysts for the selective oxidation of alcohols. Excellent recyclabil-
ity was conveniently achieved by magnetic decantation and is
demonstrated in ten consecutive cycles with no apparent ma-
terial or performance losses.
The application of magnetic nanoparticles has rapidly
gained popularity as a convenient and cost-effective platform
to manipulate and recycle catalysts.[12,13] Among those, iron
oxide nanoparticles have been used most widely; however,
their relatively low saturation magnetization (MS,bulk
ꢀ
92 emugÀ1) limits the amount of nonmagnetic active compo-
nent that can be loaded while still permitting the efficient sep-
aration of the resulting core–shell nanocomposites by external
magnetic fields.[14,15]
The oxidation of alcohols to aldehydes or ketones is a widely
used transformation in organic synthesis, and especially, cata-
lytic variants exploiting oxygen as the terminal oxidant are of
eminent importance to substitute established but environmen-
tally problematic methods that necessitate the (over)stoichio-
metric use of heavy metals.[1,2] Significant advances have been
made with the use of ruthenium-, palladium-, and gold-based
catalysts, often supported on carbon or TiO2 carriers to facili-
tate recovery and recyclability.[3] However, these catalysts in-
volve the use of expensive noble metal components. The de-
velopment of inexpensive heterogeneous catalytic systems is
thus highly desirable.[4]
Recently, Stark et al. reported ferromagnetic graphene-
coated cobalt nanoparticles 1 (Figure 1) that can be synthe-
sized by reducing flame-spray pyrolysis on large scale
(>30 ghÀ1).[16,17] The deposition of a thin graphene layer over
the intrinsically pyrophoric metal core enables remarkable ther-
mal stability[18] without any detriment to the magnetization
(MS,bulk =158 emugÀ1). Therefore, 1 seems to be a suitable plat-
form for magnetic hybrid materials if they can be successfully
integrated into catalytic materials.[13] Different strategies, for
example, the direct deposition of metals, the noncovalent and
covalent attachment of molecular metal complexes, or the
growth of dendritic or polymeric metal–ligand assemblies onto
the graphene surface, have been successfully explored.[19] In
each case, the graphene layer on the nanoparticles serves as
an anchor point, but otherwise it does not impact the synthe-
sis of the attached catalytic material. Herein, we report the syn-
thesis of unprecedented hierarchically structured microspheres
composed of MnO2 nanoplatelets, in which Co/C nanoparticles
are homogeneously distributed. The resulting material (denot-
ed 4, Figure 1) functions as a highly efficient and selective cat-
alyst in the aerobic oxidation of aliphatic and aromatic alcohols
at ambient pressure; it outperforms known MnO2-based cata-
lysts[20] and rivals the activity of those based on palladium.[21]
More importantly, owing to homogeneous dispersion of Co/C
nanoparticles, MnO2 species are firmly anchored around the
magnetic component of the catalyst, which ensures efficient
magnetic separation, as established over 10 reaction cycles in
the oxidation of benzyl alcohol.
It is well recognized that the activity of a catalyst is closely
related to its morphology.[5] Targeting more efficient catalyst
utilization, heterogeneous MnOx catalysts with different nano-
structures such as plate-like layers,[6] hollow spheres,[7] octahe-
dral molecular sieves,[8,9] and nanoparticles[10] have been inves-
tigated for the aerobic oxidation of alcohols. Under optimal
conditions, the amount of MnOx applied can be reduced to
10 mol% (0.1 equiv. with respect to the substrate), which pro-
vides high yields of aldehydes and ketones.[10,11] However, the
recovery and reuse of such catalysts is challenging and gener-
ally calls for filtration or centrifugation steps that lead to pro-
gressive material loss, which requires replenishment of the cat-
alyst to preserve the performance.[2,8]
[a] Dr. J. Zhi, Prof. Dr. O. Reiser
Institute for Organic Chemistry
The synthesis of magnetic MnO2 microspheres 4 com-
menced with the in situ growth of MnO2 nanosheets by a mi-
crowave-assisted redox reaction, which was performed by uti-
lizing the graphene layer of 1 as the reducing agent to pro-
duce MnO2 [Scheme S1, Eq. (1) in the Supporting Informa-
tion].[22] Scanning electron microscopy (Figure 1a) revealed
that resulting material 2 consists of large and intergrown
sheetlike structures. The high-angle annular dark field scanning
transmission electron microscopy (HAADF-STEM) image of the
University of Regensburg
Universitätsstrasse 31, 93053 Regensburg (Germany)
E-mail: oliver.reiser@chemie.uni-regensburg.de
[b] Dr. S. Mitchell, Prof. Dr. J. PØrez-Ramírez
Institute for Chemical and Bioengineering
ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zürich (Switzerland)
E-mail: jpr@chem.ethz.ch
Supporting Information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201500549.
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tion and growth of the MnO2
nanoplatelets, which successfully
leads to morphological transfor-
mation into an open intergrown
network in which the cobalt/
carbon nanoparticles are embed-
ded. The diffraction peaks at
2q=38.3, 42.2, and 57.28 in the
XRD pattern are assigned to or-
thorhombic g-MnO2 (JCPDS 14-
644), which confirms the com-
plete phase transition from d-
MnO2.[25] Owing to the protective
graphene layer, no diffraction
peaks belonging to cobalt
oxides are evidenced in 4, which
indicates that the Co nanoparti-
cles are not oxidized during the
synthesis. Examination of a thin
cross section (ꢁ30–50 nm thick)
of a single particle of 4 by
HAADF-STEM (Figure 1g) and
corresponding chemical map-
ping (Figure 1h) of Mn (blue)
and Co (orange) by energy dis-
persive X-ray (EDX) spectroscopy
proves that the redistributed Co/
C nanoparticles are firmly em-
bedded within the matrix of
MnO2
nanoplatelets.[26]
The
metal oxidation states of 4 were
analyzed by X-ray photoelectron
spectroscopy (XPS). The spectra
of the Mn2p region (Figure 2b)
displays two distinct peaks at
binding energies of 642.3 and
653.7 eV, which originate from
the Mn2p3/2 and Mn2p1/2 spin–
orbit peaks, respectively, corre-
Figure 1. a) Synthesis of hierarchically structured MnO2-Co/C catalysts (4). b,c) SEM and HAADF-STEM images of
initial MnO2-Co/C composite 2. d) SEM image of the composite 3 obtained after mechanical milling. e,f) SEM and
TEM images of 4. g) HAADF-STEM image and h) correlative chemical map of Mn (blue) and Co (orange) of a thin
cross section of 4. i) Ar isotherms and j) corresponding NLDFT pore-size distributions of sample 4 before (4, fresh)
and after 5 cycles in the aerobic oxidation of benzyl alcohol (4, used).
thin cross-sectional area reveals that 1 is randomly distributed
among the MnO2 nanosheets (Figure 1b). The X-ray diffraction
(XRD) pattern of 2 (Figure 2a) evidences the presence of the d
phase of MnO2 (JCPDS 42-1317)[22] and of cobalt crystals (reflec-
tions at 2q=44.2 and 51.58). Subsequent mechanical milling
(10 min) to form 3 [68 wt% MnO2 determined by inductively
coupled plasma atomic emission spectroscopy (ICP-AES); Fig-
ure 1c] has two main roles, that is, it compacts the loosely ag-
gregated nanosheets and improves the dispersion of the Co/C
nanoparticles within the MnO2 matrix.
Upon dispersion of 3 in an aqueous solution containing
KMnO4 and MnSO4, MnO2 nanoplatelets quickly form
[Eq. (S2)].[23] After continuous stirring at 608C for 45 min, 3D hi-
erarchical microspheres 4 (66 wt% MnO2 determined by ICP-
AES; Figure 1e,f) are obtained. Both the known instability of d-
MnO2 in acidic solution[24] and the high density of defects gen-
erated by mechanical milling are thought to facilitate nuclea-
!
*
^
Figure 2. a) XRD patterns of 2, 3, and 4. : g-MnO2, : d-MnO2, : Co metal.
b) XPS spectrum of the Mn2p region and c,d,e) STEM image and corre-
sponding EDX maps of 4.
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sponding to Mn4+ [27]
.
Owing to the redistribution of Co/C
so far for such reactions.[11,20,33] For comparison, commercial
MnO2 showed very poor catalytic activity under the same con-
ditions (Table 1, entry 2). With a lower catalyst loading of 4
(0.95 mol%), almost complete conversion of benzaldehyde
could still be obtained after prolonging the reaction time to
10 h (Table 1, entry 3), and a turnover number (TON) of 93 was
achieved. Equally good results were obtained with both elec-
tron-rich- and electron-poor-substituted benzyl alcohols, as
well as with cinnamyl alcohol (Table 1, entries 4–8). Even in the
case of a severely sterically hindered alcohol (Table 1, entry 9)
were a high conversion and yield obtained. Aliphatic, allylic,
and heteroaromatic alcohols also proved to be suitable sub-
strates; however, the reaction time had to be increased to 8 h
(Table 1, entries 10–13).
nanoparticles in 4, Mn (green) and Co (red) are essentially seen
to overlap in the EDX maps of the catalyst acquired by STEM
(Figure 2c–e). Such a homogeneous distribution of the mag-
netic species is beneficial to maintain the magnetization of the
whole microsphere.[28]
To further characterize the porous structure of the final
nanocomposites, 4 was analyzed by argon adsorption at 77 K.
The type IV isotherms evidenced (Figure 1i) display an increas-
ing uptake across the entire range of relative pressures, which
is significantly higher than that displayed by the precursor
nanosheets in 2; this suggests the development of extensive
micro- and/or mesoporosity.[29] This is confirmed by the corre-
sponding nonlocal density functional theory (NLDFT) pore-size
distribution (Figure 1j), which indicates a broad pore-size distri-
bution, with two prominent peaks centered at approximately
d=1.7 and 3 nm. Correspondingly, a BET surface area of
225 m2 gÀ1 is evidenced (Table S1), which is the highest value
reported so far for nanostructured MnO2 materials.[30] The high
porosity originates from the network of interparticle spaces
formed between the MnO2 nanoplatelets (Figure 1d,e),[31]
which offers an excellent balance between accessibility and
surface area of the active phase.[32] As a result, 4 appears to be
promising for applications in catalysis.
The room-temperature magnetization of 4 (Figure S1) was
found to be 78.4 emugÀ1, which is significantly lower than that
of unfunctionalized 1 (158 emugÀ1) as a consequence of the
deposition of substantial amounts of nonmagnetic MnO2.
Nonetheless, complete recovery of the catalyst could still be
achieved within 10 s upon application of an external neodym
magnet (Figure 3a), without any mass loss of MnO2 or cobalt
Thus, 4 (66 wt% MnO2) was investigated as a catalyst for the
aerobic oxidation of alcohols (Table 1). Under optimized reac-
tion conditions (see the Supporting Information) a catalyst
loading of 3.8 mol% was sufficient to achieve the oxidation of
benzyl alcohol to its corresponding aldehyde in high yields
with excellent chemoselectivity (Table 1, entry 1) within 4 h at
808C, which compares favorably to all MnO2 catalysts reported
Table 1. Aerobic oxidation of various alcohols by employing 4 as a
recyclable catalyst.[a]
Entry
R1
R2
t
Conversion[b] Yield[b,c] TON[d]
Figure 3. a) Photo illustrating the magnetic recovery of 4 dispersed in tolu-
ene. b) HAADF-STEM image of 4 after 5 cycles. c) Recyclability test of 4 and
4’ [benzyl alcohol (1 mmol), DMF (0.5 mL), 4 or 4’ (3.8 mol% MnO2), 808C
under bubbling of oxygen, 4 h; total TON: 250 for 4 and 111 for 4’]. d) Ki-
netic profiles for the aerobic oxidation of benzyl alcohol catalyzed by 4
(1 mmol benzyl alcohol, 0.5 mL DMF, and 3.8 mol% MnO2 as catalyst, 808C
under bubbling of oxygen) and commercial Pd/C (10 wt%) [1 mmol benzyl
alcohol, 2 mL of 1.25m aqueous NaOH, 40 mg Pd/C (3.8 mol% Pd), 908C
under bubbling of oxygen]. Yields were determined by GC analysis by using
dodecane as an internal standard.
[h] [%]
[%]
1
C6H5
C6H5
C6H5
4-MeOC6H4
4-ClC6H4
H
H
H
H
H
H
C6H5
H
H
4
4
97
23
94
90
92
95
94
87
85
81
77
73
75
96 (95)
20 (18)
93 (91)
88 (86)
90 (88)
92 (91)
91 (90)
83 (80)
82 (78)
79
25
5
2[e]
3[f]
4
10
4
93
23
23
24
24
22
21
20
19
18
19
5
6
7
8
4
4
4
4
4
8
8
8
4-O2NC6H4
C6H5
C6H5CH=CHÀ
2,6-(CH3)2C6H3
CH3(CH2)2
CH2 =CHCH2
cyclohexanol
furfuryl alcohol
9
10
11
12
13
CH3
–
–
74
70
72
into the product phase, as determined by ICP-AES after each
run (Figure S2). Consistently, the morphology and pore struc-
ture of 4 remained unchanged after 5 cycles (Figures 3b and
1i,j), whereas the constant high activity was evidenced over 10
consecutives runs (Figure 3c). The scanning electron microsco-
py (SEM) micrographs recorded after 10 cycles still showed no
evidence of alteration of the MnO2 hierarchical nanostructures
(Figure S3), and XPS analysis (Figure S4) also confirmed the
–
8
[a] Reaction conditions: Alcohol (1 mmol), DMF (0.5 mL), and 4 catalyst
(5 mg, 3.8 mol% MnO2), 808C, under bubbling of oxygen. [b] Determined
by GC analysis by using dodecane as an internal standard. [c] Values in
brackets are the yields of the isolated products. [d] TON=(mol product)/
(mol MnO2). [e] Commercial MnO2 powder as catalyst (Sigma–Aldrich).
[f] Alcohol (4 mmol), DMF (15 mL), and catalyst
MnO2), 808C, under bubbling of oxygen.
4 (5 mg, 0.95 mol%
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preserved structure of the material, as judged by the un-
changed Mn2p signal.
In contrast to the route described, attempts to directly grow
MnO2 nanoplatelets on 1 (resulting in sample 4’) yielded an in-
ferior catalyst. Aggregated clusters of Co/C nanoparticles were
distributed among the MnO2 nanoplatelet spheres (Figure S6),
which resulted in a low affinity between MnO2 and the mag-
netic cores. Thus, significant leaching of manganese was ob-
served upon application (Figure S2), which led to a sharp de-
crease in activity in consecutive catalytic runs (Figure 3c). Con-
sidering Pd0 as the best catalyst for the title transformation,[20]
we compared the kinetic profiles for the aerobic oxidation of
benzyl alcohol catalyzed by 4 and by Pd0 supported on activat-
ed carbon (Pd/C, 10 wt%). To ensure a fair comparison, the
latter reaction was conducted under the optimal conditions re-
ported, that is, at a reaction temperature of 908C.[34] With the
use of the same catalyst amount (3.8 mol%), the activity of 4
was somewhat lower than that of Pd/C, but a comparable
yield of the product was obtained within 4 h of reaction (Fig-
ure 3d). Considering the cost, inexpensive 4 appears to be
a highly competitive alternative to noble palladium in the oxi-
dation of alcohols, particularly given that considerably lower
amounts of material are required to achieve the same TON for
a given reaction (4 containing 1 mmol MnO2 =0.13 g; Pd/C
containing 1 mmol Pd0 =1.2 g).
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In conclusion, hierarchically structured MnO2-Co/C nanocom-
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for the aerobic oxidations of alcohols. These hybrid systems
offer high catalytic activity, outperform traditional oxidation
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Acknowledgements
This work was supported by the Deutsche Forschungsgemein-
schaft (DFG) (Re 948/8-1, GLOBUCAT) and the EU-ITN Network
Mag(net)icFun (PITN-GA-2012-290248). We thank Turbobeads Llc
for generously providing the magnetic nanobeads. We are grate-
ful to T.C. Keller (ETH Zurich) for Ar adsorption, Prof. R. Witzgall
and Ms. C. Meese (Department of Biology, University of Regens-
burg) for SEM, Prof. Zweck (Department of Physics) for XPS and
TEM measurements, and Maja Günthert and Dr. Fabian Gramm
(ScopeM, ETH Zurich) for assistance with TEM sample preparation
and ChemiSTEM analysis, respectively. The Micromeritics Grant
Program is thanked for the award of the 3Flex.
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Received: May 18, 2015
Published online on August 3, 2015
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