Magnetically retrievable MFe2O4 spinel (M = Mn, Co, Cu, Ni, Zn) catalysts for oxidation of benzylic alcohols to carbonyls

Article abstract of DOI:10.1039/c3ra45327h

The catalytic activity of the MFe₂O₄ spinel (M = Mn, Co, Cu, Ni, Zn) was investigated for the oxidation of benzylic alcohols to respective carbonyls using tert-butyl hydroperoxide (TBHP) as an oxidant. The combination of CoFe₂O₄/TBHP in a dimethyl sulfoxide (DMSO) catalytic system was found to be most efficient for this catalytic conversion. A CoFe₂O₄ catalyst is magnetically separable and could be reused with no considerable loss in catalytic activity as proved for 5 consecutive cycles.

Full text of DOI:10.1039/c3ra45327h

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Magnetically retrievable MFe2O4 spinel (M ¼ Mn,
Co, Cu, Ni, Zn) catalysts for oxidation of benzylic
alcohols to carbonyls†
Cite this: RSC Adv., 2014, 4, 6597
a
a
b
b
Received 24th September 2013
Accepted 19th December 2013
*
Anand S. Burange, Sandip R. Kale, Radek Zboril, Manoj B. Gawande
a
*
and Radha V. Jayaram
DOI: 10.1039/c3ra45327h
www.rsc.org/advances
The catalytic activity of the MFe2O4 spinel (M ¼ Mn, Co, Cu, Ni, Zn) was Co(OAc)2/NHPI,14 HNO3/Br2,15 FeBr3,16 PDC/Adogen464,17
investigated for the oxidation of benzylic alcohols to respective Zr(OtBu)4,18 hydrotalcite-supported Pd(OAc)2/pyridine,19 silica-
carbonyls using tert-butyl hydroperoxide (TBHP) as an oxidant. The supported Ru complex,20 Mo–V–O oxide,21 and several other
combination of CoFe2O4/TBHP in a dimethyl sulfoxide (DMSO) cataly- important catalysts and reagents.22–24 Most of these protocols
tic system was found to be most efficient for this catalytic conversion. suffered from various drawbacks, such as the use of non-benign
A CoFe2O4 catalyst is magnetically separable and could be reused with chemicals, non-recoverable catalysts, less efficient and expensive
no considerable loss in catalytic activity as proved for 5 consecutive catalytic systems. Thus, the use of a heterogeneous catalyst
cycles.
system in the liquid phase represents an attractive approach for
the liquid phase oxidation reactions. In recent scenario much
attention has been paid in the application of spinel and
perovskite type metal oxides as catalysts for various organic
transformations.25–28 These catalysts are found to be thermody-
namically stable at high temperatures and can be employed for a
wide range of temperatures in various organic reactions such as
reduction, oxidation etc.29–35
The synthesis, structure and properties of various spinels are
well documented in a literature and they were successfully
employed in numerous applications.36–42 Nano size and
magnetic properties of some spinel ferrites made them more
imperative in the eld of science and technology.43 Very
recently, spinel ferrites successfully synthesized by template
method and its catalytic activity was tested for oxidation of
benzyl alcohols under benign conditions. The high selectivity
and excellent conversion was observed on cobalt ferrite with
H2O2 as a green oxidizing agent (Scheme 1).44
Oxidation of alcohols to carbonyl compounds is a fundamental
organic transformation for the synthesis of a large variety of
versatile intermediates in synthetic organic chemistry.1 Tradi-
tionally this transformation is performed with a stoichiometric
amount of oxidants such as permanganate and dichromate, but
these reagents are toxic and produce large amounts of
hazardous waste causing serious environmental problems.2–4
Noble metals such as palladium and gold supported on metal
oxides have also been shown to have promising catalytic
activity,5–7 but they are expensive. Because of the industrial
importance of oxidation reaction and development of green
synthetic processes, the use of less-hazardous oxidant is an
important challenge in this arena.
Over the past several years, the stoichiometric reagents for
oxidation reaction are gradually replaced by various catalytic
systems, including homogeneous and heterogeneous catalysts.
This includes RuCl3/(diacetoxyiodo)benzene,8 RuCl2(PPh3)3/
TEMPO,9 Pd(OAc)2,10 NHPI/vanadium cocatalysts,11 palladium(II)
bathophenanthroline,12 CuCl2/tetrabutylammonium bromide,13
In continuation of our research activities on nanomaterials,
benign protocols and magnetically recyclable catalysts,45–48
herein we report oxidation of benzylic alcohols using TBHP as a
aDepartment of Chemistry, Institute of Chemical Technology (Autonomous), N. Parekh
Marg, Matunga, Mumbai 400 019, India. E-mail: rv.jayaram@ictmumbai.edu.in; Fax:
+91(22)241456144
bRegional Centre of Advanced Technologies and Materials, Faculty of Science,
ˇ
˚
Department of Physical Chemistry, Palacky University, Slechtitelu 11, 783 71,
Olomouc, Czech Republic. E-mail: mbgawande@yahoo.co.in; manoj.gawande@upol.cz
† Electronic supplementary information (ESI) available: Experimental details,
characterization data and 1H NMR, 13C NMR and GCMS spectra of all prepared
compounds. See DOI: 10.1039/c3ra45327h
Scheme 1 Oxidation of benzhydrol to benzophenone over MFe2O4
spinel (M ¼ Mn, Co, Cu, Ni, Zn).
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clean oxidant over CoFe2O4 as a potential catalyst in DMSO
solvent. In addition, the catalyst system was effective for a
synthesis of wide range of substrates under mild reaction
conditions. In recent years, oxidation chemistry is well investi-
gated by our group.49,50
The catalysts MFe2O4 (M ¼ Mn, Co, Cu, Ni, Zn) were
prepared according to previously reported technique.51 To
investigate the catalytic activities of prepared catalysts, oxida-
tion of benzhydrol was performed over 10 mol% of catalyst
using TBHP as an oxidant in acetonitrile. The activity of
prepared catalyst was studied at different reaction conditions
with the key variables including catalyst loading, solvent, and
temperature. Notably, CoFe2O4, MnFe2O4 and ZnFe2O4 showed
excellent conversions than NiFe2O4 and CuFe2O4 catalysts
(Table 1).
Fig. 1 XRD of CoFe2O4.
Table
3 Effect of temperature and solvent on oxidation of
Further, CoFe2O4, MnFe2O4 and ZnFe2O4 employed for the
time optimization study (Table 2). Among chosen metal ferrites
for time optimizations, CoFe2O4 catalyst showed 100% conver-
sion in 12 h, hence chosen for further studies (Table 2, entry 2).
XRD of prepared catalyst proves crystalline nature of mate-
rial and conrmed formation of spinel crystal lattice (Fig. 1).
The effects of reaction temperature and solvent type were as
also investigated for the oxidation of benzhydrol using TBHP as
an oxidant. In acetonitrile solvent, maximum activity of catalyst
benzhydrola
Entry
Temperature (ꢀC)
Solvent
Conversionb (%)
Time (h)
1
2
3
4
5
6
7
8
9
60
70
80
90
90
90
90
90
90
ACN
ACN
ACN
100
100
100
100
100
32
62
100
00
14.5
12
9.5
8
ACNc
DMSO
DMF
EtOHc
5
22
22
3.5
5
ꢀ
was observed at reux temperature of 90 C (Table 3, entry 4).
c
CCl4
Non-polar solvent CCl4 showed 100% conversion with 100%
selectivity in 3.5 h (Table 3, entry 8), while polar protic solvent
EtOH showed only 62% conversion with 80% selectivity in 22 h
(Table 3, entry 7).
The decreased selectivity observed in ethanol would be related
to intermolecular dehydration reaction between benzhydrol and
DMSOd
a
Reaction conditions: benzhydrol (1 mmol), 70% TBHP (3 mmol),
b
CoFe2O4 (10 mol%). Conversion and selectivity is determined by GC
and GC-MS analyses. c Reux condition. d Without catalyst.
ethanol. The formation of ether was conrmed by GC-MS. Other
polar aprotic solvents, DMSO showed better results next to CCl4
with 100% conversion in 5 h (Table 3, entry 5). Keeping in mind
catalytic data and also environmental issues and green approach,
DMSO was selected for further studies.
Table 1 Comparison of catalytic activities of MFe2O4 (M = Mn, Co, Cu,
Ni, Zn)a
Entry
Catalyst
Conversionb (%)
The concentration of the catalyst is another variable
having a strong inuence on the % conversion of desired
carbonyls. The concentration of CoFe2O4 catalyst was opti-
mized by employing the reaction with 5, 10, 15 and 20 mol%
of catalyst.
1
2
3
4
5
MnFe2O4
CoFe2O4
NiFe2O4
CuFe2O4
ZnFe2O4
100
100
58
98
100
It was observed that 10 mol% of catalyst provided excellent
conversion (100%) and selectivity (100%) of corresponding
a
Reaction conditions: benzhydrol (1 mmol), 70% TBHP (3 mmol),
catalyst: MFe2O4 (10 mol%), temperature (70 ꢀC), time (22 h), solvent
(DMSO). Conversion and selectivity is determined by GC and GC-MS
b
analysis.
Table 4 An effect of amount of CoFe2O4 catalyst on oxidation of
benzhydrola
Table 2 Time optimization study for MFe2O4 (M ¼ Mn, Co, Zn)a
Catalyst Loading
(mol%)
Initial rate Entry
(M minꢁ1
Conversionb (%)
Time (h)
Entry
Catalyst
Conversionb (%)
Time (h)
)
1
2
3
4
5
10
15
20
100
100
100
100
8.5
5
4.5
4.5
1
2
3
MnFe2O4
CoFe2O4
ZnFe2O4
100
100
100
15
12
16
0.14
0.24
0.18
a
a
Reaction conditions: benzhydrol (1 mmol), 70% TBHP (3 mmol),
Reaction conditions: benzhydrol (1 mmol), 70% TBHP (3 mmol),
b
b
catalyst (10 mol%), temperature (70 ꢀC), solvent DMSO. Conversion solvent DMSO. Conversion and selectivity is determined by GC and
and selectivity is determined by GC and GC-MS analysis.
GC-MS analysis.
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Table 5 Application of reaction protocol to various substratea
Entry
Reactant
Product
Time (h)
5.5
Conversionb (%)
100
Selectivityb (%)
100
1
2
3
4
5
100
100
100
100
100
100
5.5
4.5
5
6
4
5
100
100
00
00
7
8
6.5
5
100
80
60
80
9
5.5
6.5
100
100
52
98
10
11
12
13
6
100
100
78
100
100
100
6.5
6
14
6
86
90
a
b
Reaction conditions: benzyl alcohol (1 mmol), 70% TBHP (3 mmol), CoFe2O4 (10 mol%), temperature (90 ꢀC), solvent: DMSO. Conversion and
selectivity determined by GC and GC-MS analysis.
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Table 6 Recyclability study
product (Table 4, entry 2), while further increase in the catalyst
loading had no signicant effect on reaction time required. To
nd out the effect of oxidant, reaction was carried out in the
presence of hydrogen peroxide under optimized conditions.
However no product formation was observed because of
decomposition of hydrogen peroxide over ferrite catalyst.52
In order to explore the general applicability and efficacy of
TBHP/CoFe2O4 catalytic system, oxidation of various benzylic
alcohols were performed under the optimized conditions.
Secondary benzylic alcohols such as 1-phenylethanol, 2-
hydroxy-1-phenyl ethanol, 4-chloro-1-phenylethanol, bromo-
benzhydrol showed 100% conversion with 100% selectivity
(Table 5, entries 1–4), while primary benzylic alcohols viz.;
benzyl alcohol, 4-methyl benzyl alcohol, 4-methoxy benzyl
alcohol showed lower selectivity, because of over oxidation to
corresponding carboxylic acid (Table 5, entries 7–9). Primary
benzylic alcohols like 4-nitro benzylic alcohol, 2-nitro benzylic
alcohol, 2-chloro benzylic alcohol, 4-bromo benzylic alcohol
showed 100% selectivity, while m-phenoxy benzylic alcohol
98% selectivity. When the same catalytic protocol was applied
to benzoin and mandelic acid, it was observed that both
produced benzoic acid exclusively with 100% conversion
(Table 5, entries 5–6).
Run
1
2
3
4
5
% Conversion
100
100
100
100
100
Conclusions
In conclusion, we have developed an efficient and clean
protocol for the oxidation of benzylic alcohols using TBHP as an
oxidant over magnetically recyclable CoFe2O4 catalyst. The
prepared catalyst is found to be useful for the selective
conversion of benzylic alcohols to carbonyls with a high
conversion. The developed methodology was effectively appli-
cable for various benzylic alcohols excluding primary benzylic
alcohols with electron donating substituents.
Acknowledgements
ASB is greatly thankful to UGC for providing Junior Research
Fellowship (Green Tech-JRF). SRK is thankful to CSIR for
providing SRF. The authors gratefully acknowledge the support
by the Operational Program Research and Development for
Innovations – European Regional Development Fund (project
CZ.1.05/2.1.00/03.0058) and by the Operational Program
Education for competitiveness – European Social Fund (project
CZ.1.07/2.3.00/30.0041) of the Ministry of Education, Youth and
Sports of the Czech Republic.
General procedure for the oxidation
reaction
To a clean dry 10 mL round-bottom ask containing 1.0 mmol
benzhydrol, 3.0 mmol equivalent of 70% TBHP was added. This
was followed by the addition 10 mol% of MFe2O4 catalyst and
2 mL solvent. The mixture was stirred at 90 ꢀC. The progress of
the reaction was monitored by TLC and GC. Aer completion of
the reaction, the reaction mixture was cooled to room temper-
ature and the catalyst was separated by ltration method. The
products were identied by GC-MS.
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