A recyclable ferrite-co magnetic nanocatalyst for the oxidation of alcohols to carbonyl compounds

Article abstract of DOI:10.1002/cplu.201200081

Magnetic cleansing: A magnetic Fe₃O₄ Co catalyst was prepared using inexpensive precursors and its catalytic activity was tested for the oxidation of alcohols using tert-butyl hydroperoxide (TBHP) as the oxidant. The corresponding carbonyl compounds were obtained in good to excellent yields (see scheme); furthermore, this magnetic catalyst could be separated by magnetic decantation without significant loss in activity.

Full text of DOI:10.1002/cplu.201200081

DOI: 10.1002/cplu.201200081
A Recyclable Ferrite–Co Magnetic Nanocatalyst for the Oxidation of
Alcohols to Carbonyl Compounds
Manoj B. Gawande,*[a] Anuj Rathi,[b] Isabel D. Nogueira,[c] C. A. A. Ghumman,[d] N. Bundaleski,[d]
O. M. N. D. Teodoro,[d] and Paula S. Branco*[a]
Oxidation of alcohols into carbonyl compounds is one of the
most important and fascinating reactions in organic chemistry
which stems from the fact of it being valuable raw material for
the chemical and pharmaceutical industries. An enormous vari-
ety of oxidizing reagents and catalysts are available for the oxi-
dation of alcohols.[1] Traditional reagents as those derived from
Cr or Mn must be used in stoichiometric amounts and are
often accompanied by the formation of toxic waste products,
which can be deleterious during the work-up procedure, re-
sulting in tricky product isolation, and rendering impossible
the reusability of the catalyst. The use of hypervalent iodine re-
agents such as Dess–Martin periodinane (DMP) presents some
problems, however, advances have been achieved with the de-
velopment of various 2-iodoxybenzoic acid (IBX, a DMP precur-
sor) derivatives which have improved solubility, are non-explo-
sive, and/or are recyclable.[1e]
in handling, and safety of the catalyst. Recently, developmental
work has been carried out on the catalytic aerobic oxidation of
alcohols mediated by several heterogeneous catalyst systems
based on Ru, Mo, Pt, or Pd.[3,4]
Other successful reported procedures involve the nitroxyl
radical TEMPO (2,2,6,6-tetramethylpiperidin-1-yloxyl) employed
as a recyclable oligomeric catalyst (PIPO, polymer immobilized
TEMPO) using sodium hypochlorite as the oxidant and avoid-
ing the use of halogenated hydrocarbon solvents.[5] The main
drawback of this method is the use of a stoichiometric quanti-
ty of oxidant, NaOCl, which generates NaCl as waste. The con-
jugation of TEMPO with noble metals such as Ru or Pd could
circumvent this problem replacing NaOCl with O2 although
a shortcoming is the lack of activity with some alcohols.[5]
Following the first cobalt-catalyzed aerobic oxidation of al-
cohols using cobalt–nitro complexes by Tovrog et al.[6] several
systems for cobalt-catalyzed aerobic alcohol oxidations have
emerged.[7] Although most secondary alcohols are converted
into ketones, primary alcohols are often oxidized to the car-
boxylic acid. There are different strategies for the heterogeni-
zation of redox-active elements in solid matrices.[8] Functional-
ized magnetic nanoparticles (MNPs) are heterogeneous catalyst
supports, which have emerged as viable alternatives to con-
ventional materials because they are robust, inert, inexpensive,
reusable, and recyclable using a simple magnet.[9] A variety of
catalytic systems involving metal-supported MNPs have been
devised and used in synthetic organic reactions such as nano
ferrite-supported Pd,[10] hydroxyapatite-supported Pd,[11] alumi-
na-supported Pd,[12] silver nanoparticles supported on hydrotal-
cites,[13] ruthenium-supported ferrite,[9p] Pd on mesoporous
carbon,[14] gold-supported on metal oxides,[15] and Au–Pd–tita-
nia.[16] Among the numerous methods, free nano-Fe2O3,[9k] and
recyclable SBA-15 TEMPO-supported catalysts are the most
promising because they avoid the use of transition metals.[4b]
With a few exception these protocols include the use of noble
metals, expensive ligands, and involve multistep synthesis for
the catalyst preparation. Therefore, there is an urgent need to
developed inexpensive and benign protocols for oxidation re-
actions. In continuation of our research on the development of
greener protocols,[17] new methods, and nanocatalysis[18] we
have developed a new, simple, and efficient method for func-
tionalization of ferrite MNPs with Co and its application for the
oxidation of alcohols. Fe3O4–Co MNPs were prepared by the
simple wet impregnation method followed by chemical reduc-
tion as reported in the literature[19a,b] (Scheme 1). Characteriza-
tion of the nanocatalyst was achieved through X-ray diffraction
(XRD), inductive coupled plasma atomic emission spectroscopy
(ICP–AES), transmission electron microscopy (TEM), secondary
ion mass spectrometry (SIMS), and X-ray photoelectron spec-
Homogeneous and heterogeneous catalysts combined with
environmentally benign oxidants, such as molecular oxygen,
organic peroxide, and hydrogen peroxide are major challenges
in the oxidation of alcohols.[1d,1g,2] Molecular oxygen and hydro-
gen peroxide are considered to be green oxidants because
they are inexpensive and minimize the chemical waste, having
water as the sole by-product. In line with this, Beller and co-
workers reported selective oxidation of alcohols to aldehydes
and ketones using ruthenium-based catalysts and H2O2 as the
oxidant.[2c]
Notably, the homogeneous catalyst systems show good cat-
alytic activity compared to heterogeneous catalysts[3] but on
an industrial scale the problems related to handling, recovery,
and reuse of the catalyst represent limitations for these pro-
cesses. Instead heterogeneous catalysts have been frequently
used for the oxidation of alcohols owing to easy recovery, ease
[a] Dr. M. B. Gawande, Prof. Dr. P. S. Branco
REQUIMTE, Departamento de Quꢀmica, Faculdade de CiÞncias e Tecnologia
Universidade Nova de Lisboa
2829-516 Caparica (Portugal)
Fax: (+351)21-2948550
E-mail: m.gawande@fct.unl.pt
mbgawande@yahoo.co.in
paula.branco@fct.unl.pt
[b] Dr. A. Rathi
Jubilant Chemsys Ltd.
B-34, Sector-58, Noida-201301, New Delhi (India)
[c] I. D. Nogueira
Instituto de CiÞncia e Engenharia de Materiais e Superfꢀcies
IST, Lisbon (Portugal)
[d] C. A. A. Ghumman, Dr. N. Bundaleski, Prof. Dr. O. M. N. D. Teodoro
Centre for Physics and Technological Research (CeFITec)
FCT, UNL, Lisbon (Portugal)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cplu.201200081.
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Scheme 1. Synthesis of Ferrite (Fe3O4) and Fe3O4–Co MNPs.
troscopy (XPS). The weight percentage of Co was determined
to be 8.12% by ICP–AES analysis (ESI).
Figure 2. TEM of Fe3O4–Co MNPs at 100 nm.
Characterization of the catalyst by SIMS reveals the presence
of Co over the surface of ferrite. The positive and negative
mass spectra of intact and sputtered surfaces are shown in
Figure 1. Distinct ions of ferrite are observed at m/z 56 [Fe]+,
72 [FeO]+, and 73 [FeOH]+. The characteristic ions of cobalt
oxide (CoO) were observed at m/z 59, 75, 76, and 77, corre-
sponding to Co+, CoO+, CoOH+, and CoOH2+, respectively. In
negative ion mode the cobalt oxide ions, CoHÀ, CoOÀ, CoOHÀ,
CoOH2À, CoO2À, CoO2HÀ, CoO2H2À, CoO3À, and CoO3HÀ were
observed at their respective m/z as shown in Figure 1b.
Figure 3. Survey spectrum of Co–Fe3O4 powder supported on In plate. The
spectrum was taken in FAT 44 mode with energy step of 0.5 eV and acquisi-
tion time of 0.5 s.
by arrows. Besides the main O, Fe, and Co lines we also ob-
serve a C 1s line which is an expected impurity.
The surface composition of the powder was determined
from the characteristic XPS peak intensities of Co, Fe, O, and C
which are Co 2p3/2, Fe 2p3/2, O 1s, and C 1s, respectively. To im-
prove the sensitivity, the lines were recorded with long acquisi-
tion times. As for the sensitivity factors, we use recommended
values for the MgKa line.[19c] Oxygen appears to be the most
abound element in the powder (49%) followed by carbon
(38%), cobalt (8%), and iron (5%). The presence of carbon cor-
responds to surface contamination. The main contribution of
the carbon peak, which should correspond to adventitious
carbon, is at 285.5 eV instead of the more common value
285.0 eV, thus indicating some charging effects. The character-
istic peaks of cobalt (Co 2p) and iron (Fe 2p3/2) are presented in
the Figure 4 and Figure 5, respectively. The spectra were taken
in fixed analyzer transmission mode with the pass energy of
22 eV, the energy step was 0.1 eV, and the acquisition time
window was 12 s.
Figure 1. Positive (a) and negative (b) TOF-SIMS spectra from Fe3O4–Co ana-
lyzed with an upgraded VG Ionex, equipped with a Ga+ liquid metal ion
gun.
The TEM image of ferrite-Co, Figure 2, shows that magnetic
nanoparticles are obtained in the nano size range with uni-
formly sized particles presenting a somewhat spherical mor-
phology with an average diameter range from 10 to 30 nm.
The survey XPS spectrum of the sample taken in fixed ana-
lyzer transmission mode with the pass energy of 44 eV (FAT 44)
is given in Figure 3. The most intensive XPS lines are marked
The main Co 2p3/2 and Co 2p1/2 peaks shown in Figure 4 are
at 780.0 eV and 795.6 eV, respectively. In addition, both peaks
are ‘accompanied’ by broad shake-up satellites meaning that
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sented in Figure 4 is very similar to that of Co3O4 (110) surfa-
ce,[19g,j] suggesting that the ratio between the amounts of Co2+
and Co3+ is about 1:2. However, being aware of the charging
problems owing to which the peaks are slightly shifted to-
wards higher binding energies, the relative amount of Co3+ is
probably overestimated.
The characteristic photoelectron peak of iron Fe 2p3/2 is pre-
sented in Figure 5. By using the pseudo-Voigt GL (30) as the
peak model and the constraint that the width of each contri-
bution is constant, the peak can be fitted to two contributions.
The main one at 710.8 eV and relative intensity of about 75%
perfectly matches Fe3O4,[19k,l] The second contribution at
713.1 eV and relative intensity of 25% is most probably due to
the sample charging.
Figure 4. Co 2p XPS line taken with the energy step of 0.1 eV and acquisition
time window of 12 seconds.
The synthesized and well characterized Fe3O4–Co MNPs were
explored for the oxidation of alcohols. The oxidation of 1-phe-
nylethanol (1) to acetophenone (2a) was used as a model re-
action and the results are collected in Table 1. Under catalyst-
free and oxidant-free conditions no reaction was observed
Table 1. Optimization of reaction conditions.[a]
Figure 5. Fe 2p3/2 XPS line taken with the energy step of 0.1 eV and acquisi-
Entry
Catalysts
Oxidant
Time [h]
Yield [%][b]
tion time window of 12 seconds.
1
2
3
4
5
Fe3O4–Co
Fe3O4–Co
Fe3O4–Co
Fe3O4
–
48
24
6
6
24
NR
20
92
trace
NR
H2O2 (30%)
TBHP (70%)
TBHP (70%)
TBHP (70%)
cobalt is in the paramagnetic state. The position of the Co 2p
multiplet, low intensity of the satellites, and their shift with re-
spect to the main peaks of about 8.5 eV clearly indicate that
cobalt is present in the sample as Co3O4. Indeed, Co3O4 spinel
surface is characterized by sharp Co 2p peaks at 779.8 eV and
795.7 eV with the weak and broad satellite structure located
about 9 eV higher in binding energy with respect to the main
peaks.[19d–f] The satellite structure is usually used to differentiate
between CoO and Co3O4 oxide phases, because the former
have satellite peaks having about 26% of the main peak inten-
sity.[19g,h] Therefore, the CoO phase was not registered. Because
of the same reason we may also exclude the presence of
Co(OH)2 for which the satellite peak intensity should be about
33% of that of the main peak.[19h,i] However, one should have
in mind that Co3O4 spinel has a unit cell consisting of
56 atoms: 32 O2À anions, 16 Co3+ cations occupying octahedral
sites and 8 Co2+ cations occupying tetrahedral sites.[19j] Accord-
ing to the study of Chuang et al.[19d] the Co 2p structure in the
photoelectron spectrum can be separated into two contribu-
tions attributed to Co2+ and Co3+. The contributions of Co+2
cations are shifted towards higher binding energies with re-
spect to the latter. Besides, satellites are present only for the
ionization state +2. Because the crystalline structure should
not be formed on our sample, the shape of the Co 2p peak
most probably corresponds to the mixture of oxides in which
cobalt is present as both Co2+ and Co3+. The spectrum pre-
–
[a] Reaction conditions: 1-phenylethanol (1 mmol), oxidant (1.2 mmol),
catalyst (100 mg; 13.7 mmol% of Co), 808C. [b] Yield of isolated product.
NR=no reaction.
(Table 1, entries 1 and 5). Using H2O2 (30%) as the oxidant the
reaction proceeds sluggish and only 20% yield was obtained
(Table 1, entry 2). Using TBHP (tert-butyl hydroperoxide; 70%)
as the oxidant an excellent yield of 2a was obtained (92%) in
short reaction time (Table 1, entry 3). However, cobalt-free
Fe3O4 gave only a trace amount of 2a (Table 1, entry 4).
Other alcohol substrates were subject to the oxidation reac-
tion with the well characterized Fe3O4–Co MNPs (Table 2) with
excellent results. The reaction under solvent-free conditions
proved to be of large applicability to a wide variety of structur-
ally and electronically diverse primary and secondary alcohols.
The reaction was also successfully carried out for benzyl alco-
hols containing electron-releasing or electron-withdrawing
groups and the corresponding ketones 2 were obtained in
high yields (Table 2).
This oxidation protocol tolerates a variety of functional
groups such as methoxy, chloro, bromo, methyl, amino, and
nitro groups. Aliphatic secondary alcohol such as cyclohexanol
reacted well under these conditions and cyclohexanone was
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Table 2. Ferrite-Co catalyzed oxidation of aldehydes and ketones.[a]
Entry Alcohol
Product
Time [h] Yield [%][b] Entry Alcohol
Product
Time [h] Yield [%][b]
1
2a
5
6
92
89
10
11
2j
5
5
83
82
2
2b
2k
3
4
2c
6
6
85
88
12
13
2l
5
5
84
93
2d
2m
5
2e
5
92
14
2n
5
88
6
7
8
9
2 f
2g
2h
2i
5
5
5
5
94
91
90
86
15
16
17
18
2o
2p
2q
2r
5
5
5
6
94
82
79
85
[a] Reaction conditions: alcohol (1 mmol), oxidant (1.2 mmol), catalyst (100 mg; 13.7 mmol% of Co), 808C. [b] Yield of isolated product.
obtained in 85% yield. The functional group at the ortho or
para positions of the benzyl alcohol did not have much influ-
ence on the reaction. The present protocol offers a good cata-
lytic system for the oxidation of benzoin, with no cleavage of
carbon–carbon bonds, which is generally observed with other
conventional methods (Table 2, entry 15).[20]
To understand the efficiency of our protocol with respect to
others for the oxidation of diphenylmethanol to benzophe-
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to a magnet and used for up to
seven reaction cycles. We believe
that this magnetic nanocatalyst
is a viable alternative for existing
protocols.
Table 3. Comparison of reported procedures for the oxidation of diphenylmethanol to benzophenone (2m).
Entry
Reaction conditions
Yield [%][a]
E Factor
Mass intensity
1
2
3
4
5
resin-dispersion of Pd; K2CO3, H2O, Reflux, O2, 20 h[8e]
SBA-15 Pd NPs, K2CO3,toluene, 808C, O2/air, 20 h[21]
IBX, Bu4NHSO4, water, oxone, nitromethane, 708C, 8 h[23]
NAP–Mg–Pd(0), K2CO3, toluene, RT, 10 h[22]
85
99
88
99
93
35.10
32.61
31.77
19.96
0.98
40.76
34.77
33.27
21.11
2.40
Fe3O4–Co, TBHP (70 %), 808C, 5 h (this study)
Experimental Section
[a] Yield of isolated product.
All commercial reagents were used
as received unless otherwise men-
tioned. For analytical and prepara-
tive thin-layer chromatography, Merck, 0.2 mm and 0.5 mm Kiesel-
gel GF 254 percoated were used, respectively. The spots were vi-
none, the results compared in Table 3 clearly indicate that the
present conditions are the most efficient, clean, and compara-
tively more sustainable (Table 3, entry 5). However, other proto-
cols have their own advantages. The protocol reported by
Karimi et al.[21] and Kantam and co-workers[22] (Table 3, entry 2
and 4, respectively) presents comparable yields, but not the re-
lated E Factor (cost and environmental impact of the process)
and mass intensity. Also, the main drawbacks of these proto-
cols are the use of toluene as solvent and Pd metal, which is
quite expensive. For specific reaction conditions see the corre-
sponding literature and the Supporting Information
for calculations.
1
sualized using UV light. H NMR spectra were recorded on a Bruker
400 spectrometer, 5 mm at 400 Hz. 1H shifts are reported relative
to internal tetramethylsilane.
Preparation of Ferrites/Fe3O4
The FeCl3.6H2O (5.4 g) and urea (3.6 g) were dissolved in distilled
water (200 mL) at 85 to 908C for 2 h. The solution turned to
brown. To the resultant reaction mixture was cooled to RT before
To prove that the reaction is heterogeneous, a stan-
dard leaching experiment was conducted by the hot
filtration method. The model reaction of oxidation of
1-phenylethanol proceeded for 20 minutes in the
presence of the Fe3O4–Co at 808C. The hot filtered re-
action mixture was then stirred without catalyst for
12 hours. Notably, no formation of the corresponding
product was observed, even after 12 hours, indicating
that no homogeneous catalyst was involved.
Inductively coupled plasma atomic emission spec-
tra (ICP–AES) analysis of the filtrate (hot) revealed the
absence of Fe and Co species in the filtrate. The sta-
bility of Fe3O4–Co was tested for the oxidation of 1a
to 2a by recycling and reuse of the catalyst seven
times without any significant loss of the catalytic ac-
tivity. After completion of the reaction, and between
each cycle the nanocatalyst was easily separated by
simple decantation using an external magnet,
washed with ethyl acetate, dried under vacuum, and
subjected to the next cycle. Notably, after the 7th
cycle negligible loss in the yield was observed
(Figure 6). The Co content Fe3O4–Co catalyst was ob-
served to be 8.12% before the reaction and 8.03%
after the reaction and so indicated negligible Co
leaching.
In summary, we have designed a robust magneti-
cally separable and inexpensive nanocatalyst for oxi-
dation of alcohols to corresponding ketones that ren-
ders the present protocol highly cost effective and
environment friendly. This magnetic nanocatalyst can
be easily prepared by the impregnation method. In
addition, the nanocatalyst can be efficiently recov-
Figure 6. Reusability of Fe3O4–Co MNPs (Inset catalyst separated by magnet); Reaction
conditions: 1-phenylethanol (1 mmol), oxidant (1.2 mmol), catalyst (100 mg), 808C (for
ered from the reaction medium by decantation of
the reactions mixture by simply exposing the mixture details please see the Supporting Information).
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General procedure for the oxidation reaction
A mixture of alcohol (1 mmol), 70% aqueous tert-butyl hydroperox-
ide solution (TBHP) (1.2 mmol), and 100 mg of Fe3O4–Co MNPs
(13.7 mmol% Co), was stirred at 808C, for an appropriate time.
After completion of the reaction, ethyl acetate (5 mL) was added
and the catalyst removed by simple decantation. The organic sol-
vent was evaporated under vacuum and the crude product puri-
fied by column chromatography on silica gel using n-hexane and
ethyl acetate as eluent (90:10).
Acknowledgements
This study was supported by Fundażo para a CiÞncia e a Tecnolo-
gia through grant no. PEst-C/EQB/LA0006/2011. M.B.G. also
thanks the PRAXIS program for a research fellowship SFRH/BPD/
64934/2009.
Keywords: heterogeneous catalysis · magnetic nanoparticles ·
oxidation reactions · recyclable · sustainable protocol
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Received: April 9, 2012
Revised: July 18, 2012
Published online on && &&, 0000
ChemPlusChem 2012, 00, 1 – 7
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chempluschem.org
&
7
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These are not the final page numbers!

COMMUNICATIONS
M. B. Gawande,* A. Rathi, I. D. Nogueira,
C. A. A. Ghumman, N. Bundaleski,
O. M. N. D. Teodoro, P. S. Branco*
&& – &&
A Recyclable Ferrite–Co Magnetic
Nanocatalyst for the Oxidation of
Alcohols to Carbonyl Compounds
Magnetic cleansing: A magnetic Fe3O4–
Co catalyst was prepared using inexpen-
sive precursors and its catalytic activity
was tested for the oxidation of alcohols
using tert-butyl hydroperoxide (TBHP) as
the oxidant. The corresponding carbon-
yl compounds were obtained in good
to excellent yields (see scheme); further-
more, this magnetic catalyst could be
separated by magnetic decantation
without significant loss in activity.
&8
&
www.chempluschem.org
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2012, 00, 1 – 7
ÝÝ
These are not the final page numbers!