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)
[b] Dr. S. Mitchell, Prof. Dr. J. PØrez-Ramírez
Institute for Chemical and Bioengineering
ETH Zurich
Vladimir-Prelog-Weg 1, 8093 Zürich (Switzerland)
Supporting Information for this article is available on the WWW under
ChemCatChem 2015, 7, 2585 – 2589
2585
ꢀ 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim