Natural hydroxyapatite-supported MnO2: a green heterogeneous catalyst for selective aerobic oxidation of alkylarenes and alcohols

Article abstract of DOI:10.1002/aoc.3505

Natural hydroxyapatite-supported MnO₂ (MnO₂@NHAp) was easily prepared in situ from reduction of potassium permanganate with natural hydroxyapatite derived from cow bones in water at room temperature, and its structure was characterized using flame atomic absorption spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and energy dispersive spectroscopy. The catalytic activity of the synthesized catalyst was investigated for the aerobic oxidation of alkylarenes and alcohols. MnO₂@NHAp shows excellent catalytic performance for the oxidation of alkylarenes and alcohols to their corresponding carbonyl compounds without using any other oxidizing agent. This catalyst can be readily recycled and reused for several runs without any significant loss of efficiency. Copyright

Full text of DOI:10.1002/aoc.3505

Full paper
Received: 27 January 2016
Revised: 3 April 2016
Accepted: 9 April 2016
Published online in Wiley Online Library
(wileyonlinelibrary.com) DOI 10.1002/aoc.3505
Natural hydroxyapatite-supported MnO2: a
green heterogeneous catalyst for selective
aerobic oxidation of alkylarenes and alcohols
Ahmad Shaabani*, Hadi Afaridoun and Shabnam Shaabani
Natural hydroxyapatite-supported MnO2 (MnO2@NHAp) was easily prepared in situ from reduction of potassium permanganate
with natural hydroxyapatite derived from cow bones in water at room temperature, and its structure was characterized using
flame atomic absorption spectroscopy, X-ray diffraction, thermogravimetric analysis, scanning electron microscopy and energy
dispersive spectroscopy. The catalytic activity of the synthesized catalyst was investigated for the aerobic oxidation of alkylarenes
and alcohols. MnO2@NHAp shows excellent catalytic performance for the oxidation of alkylarenes and alcohols to their
corresponding carbonyl compounds without using any other oxidizing agent. This catalyst can be readily recycled and reused
for several runs without any significant loss of efficiency. Copyright © 2016 John Wiley & Sons, Ltd.
Keywords: natural hydroxyapatite; aerobic oxidation; alcohol; alkylarene; manganese dioxide
As a part of our ongoing research programme on oxidation
reactions of alkylarenes and alcohols and using bio-supported
Introduction
Hydroxyapatite (HAp) as a calcium phosphate-based material is a
major component of animal bones. The composition of bone is
approximately 25–75% HAp by weight and 35–65% HAp by
volume.[1] Recently, synthetic HAp has found various applications
in biomaterials,[2] adsorbents,[3] ion exchangers,[4] drug delivery
agents[5] and catalyst supports.[6] The results of recent studies have
shown that synthetic HAp-supported metals or metal oxides such
heterogeneous catalytic systems such as cellulose, chitosan, wool
and silk or synthetic supports such as graphene oxide,[35–42] here
we report a novel heterogeneous catalytic system using MnO2 as
a green and cheap transition metal oxide and natural hydroxyapa-
tite (NHAp) as a support for aerobic oxidation of alkylarenes and
alcohols to their corresponding carbonyl compounds without using
any other oxidizing agent.
as Pd,[7] Au,[8–11] Ag,[12] Ru[13–15] and V2O5 exhibit outstanding
[16]
catalytic activities in aerobic oxidation transformations. Because of
its potential applications, various synthetic methods have been
reported for the synthesis of HAp.[17–21]
Since the direction of science and technology is shifting to an
emphasis more on environmentally friendly resources and reusable
catalytic processes, natural biomaterials such as HAp are attractive
candidates for use as solid supports for catalytic applications.
However, to the best of our knowledge, the use of cow bone as
an attractive candidate for solid-supported oxidation reactions
has not been explored until now.
The catalytic aerobic oxidation of alkylarenes and alcohols plays
an important role in green organic transformations in both labora-
tory and industrial synthetic chemistry. From both environmental
and industrial points of view, there is an important demand for
greener and more atom-efficient approaches which employ
aerobic catalytic oxidation processes with minimum amount of
waste.[22–26]
Experimental
General
All reagents were obtained from Aldrich or Merck and used with-
out further purification. Mn(IV) determination was carried out
using a flame atomic absorption spectroscopy (FAAS) instru-
ment (Shimadzu model AA-680) with a Mn hollow cathode lamp
at 240.7 nm, using an air–acetylene flame. Thermogravimetric
analysis (TGA) was carried out using an STA 1500 instrument at
a heating rate of 10°C minÀ1 in air. Products were analysed using
a Varian 3900 GC. X-ray diffraction (XRD) patterns were recorded
using a STOE STADI P with scintillation detector, secondary
monochromator and Cu Kα radiation (λ = 0.1540 nm). Scanning
electron microscopy (SEM) observations were carried out with
MnO2 is one of the currently used reagents for the oxidation of
organic compounds. Its activity and selectivity depend on its
synthesis procedure and morphology. MnO2 is important from
the viewpoint of its accessibility and economics, but has a low
surface area and activity. In recent reports these problems have
been overcome with the use of natural or synthetic supports with
high area-to-volume ratio.[27–34]
*
Correspondence to: Ahmad Shaabani, Faculty of Chemistry, Shahid Beheshti
University, GC, PO Box 19396-4716, Tehran, Iran. E-mail: a-shaabani@sbu.ac.ir
Faculty of Chemistry, Shahid Beheshti University, GC, PO Box 19396-4716, Tehran,
Iran
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.

Shaabani A., Afaridoun H. and Shaabani S.
a Philips XL-30 ESEM. All samples were sputtered with gold
before observation.
Oxidation of Alkylarenes in presence of MnO2@NHAp
In a general reaction, an alkylarene (1.00 mmol), K2CO3
(0.50 mmol), MnO2@NHAp (0.05 g, 8.50 mol%) and p-xylene
(5 ml) were added to a two-necked flask. The mixture was
heated under air blowing at 100°C for the required time. TLC
was used to investigate the progress of the reaction. Upon com-
pletion of the reaction, analysis of the crude product was done
using GC.
Preparation of NHAp
Cow bones were boiled in water for 12 h. Then, they were
soaked in 5% (w/v) NaOH aqueous solution under reflux condi-
tions for 5 h. Finally they were washed with distilled water,
dried and broken into smaller pieces and were ground with a
mill (Fig. 1).
Oxidation of alcohols in presence of MnO2@NHAp
In a general reaction, an alcohol (1.00 mmol) was added to a
two-necked flask containing MnO2@NHAp (0.05 g, 8.50 mol%),
K2CO3 (0.50 mmol) and p-xylene (5 ml) and stirred under air
blowing at 80°C for the required time. The progress of the reac-
tion was followed using TLC. Upon completion, the reaction
mixture was filtered and the crude product analysed using GC.
The yields were determined from the integrals of the GC
analysis.
Preparation of MnO2@NHAp
NHAp (2.00 g) was added to 250 ml of KMnO4 solution (0.02 M)
over 2 h at room temperature. The mixture was irradiated with
ultrasound for 20 min and stirred at room temperature for 72
h. Finally, the resulting brown powders was filtered and washed
with H2O (3 × 10 ml) and EtOH (3 × 10 ml) and it was dried at
100°C (Fig. 1(c)).[39]
Results and discussion
NHAp is a natural, green, cheap and accessible support, the sur-
face of which possesses P-OH groups that act as sorption sites
for MnO2.[43] Therefore, it is not only able to adsorb MnO2, but
also the catalytic activity of MnO2 is increased in comparison
with active MnO2 and the leaching of catalyst into the reaction
mixture is decreased considerably. The MnO2 content of the
synthesized catalyst was determined as 15% using FAAS. It is
important to note that MnO2@NHAp was prepared using
KMnO4 as the precursor of MnO2 in the absence of any reducing
agent.
The TGA curves of NHAp and MnO2@NHAp were obtained un-
der air atmosphere (Fig. 2). In the TGA curves of NHAp and
MnO2@NHAp, two stages can be observed. The first stage oc-
curs in the low-temperature range up to 300 and 220°C for
NHAp and MnO2@NHAp, respectively. In this stage, the mass
slowly decreases by 6 and 2% for NHAp and MnO2@NHAp, re-
spectively; this is related to the removal of adsorbed water on
the surface of the catalyst and loss of lattice water. In the
second stage, in the range between 300 and 220°C to 1000°C
Figure 1. Images of (a) cow bone, (b) prepared NHAp and (c) MnO2@NHAp.
Figure 2. TGA curves of NHAp and MnO2@NHAp.
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Appl. Organometal. Chem. (2016)

MnO2@NHAp as catalyst for oxidation of alkylarenes and alcohols
Table 1. Optimization of reaction conditions for aerobic oxidation of
arenesa
Entry
Catalyst
Amount of catalyst (mol%) Solvent Yield (%)b
1
Active MnO2
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
0.01 g (10.00)
0.03 g (5.00)
0.04 g (7.00)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
n-Hexane
Toluene
H2O
20
55
75
95
60
20
40
75
0
2
3
4
Figure 3. XRD pattern of MnO2@NHAp.
5c
6d
7
8
9e
10e MnO2@NHAp
11e MnO2@NHAp
12e MnO2@NHAp
13e MnO2@NHAp
MeOH
0
EtOH
0
EtOAc
0
CH3CN
25
0
14
NHAp
p-Xylene
aReaction conditions: indane (1.00 mmol), solvent (5 ml), K2CO3
(0.50 mmol), air oxidant, 100°C, 8 h.
bYield determined by GC analysis with internal standard.
cKOH (0.5 mmol) was added as a base.
dIn the absence of base.
eReflux.
Table 2. Aerobic oxidation of various alkylarenes to corresponding
ketonesa
Entry
Alkylarene
Product
Time (h)Yield (%)b
1
2
3
Diphenylmethane
Ethylbenzene
Benzophenone
Acetophenone
1-Phenylbutan-
1-one
8
8
8
80
90
85
Butylbenzene
Figure 4. (a, b) SEM images and (c) EDS analysis of MnO2@NHAp.
4
5
Octylbenzene
1-Phenyloctan-
1-one
12
8
70
80
1,2,3,4-
3,4-
for NHAp and MnO2@NHAp, respectively, a large weight loss is
observed which corresponds to the decomposition of organic
compounds associated with the NHAp. Finally, the loading
amount of MnO2 in MnO2@NHAp is estimated to be 15% (w/
w) which corroborates the amount of MnO2 in MnO2@NHAp de-
termined using FAAS.
The MnO2@NHAp catalyst was also characterized using powder
XRD (Fig. 3). There are five characteristic peaks for NHAp at 2θ
values of 25.76°, 31.88°, 33.91°, 39.77° and 46.70° and four for
MnO2 at 2θ values of 28.88°, 35.46°, 41.97° and 49.39°.[44]
Figures 4(a) and (b) show SEM images of MnO2@NHAp. The
SEM analysis of the catalyst shows the morphology of
MnO2@NHAp which is composed of particles with irregular
shapes and different sizes. The chemical composition of the
MnO2@NHAp catalyst was investigated using energy dispersive
spectroscopy (EDS; Fig. 4(c)). The EDS spectrum exhibits peaks
for P, Ca, Mn and O, which are the major constituents of NHAp
and MnO2.
Tetrahydro-
naphthalene
2,3-Dihydro-1
H-indene
Dihydronaphthalen-
1(2H)-one
6
2,3-Dihydro-1
H-inden-1-one
9H-Fluoren-9-one
9H-Xanthen-9-one
Isochroman-1-one
8
95
7
8
9
9H-Fluorene
9H-Xanthene
Isochromane
8
8
8
90
85
80
aReaction conditions: alkylarene (1.00 mmol), MnO2@NHAp (0.05 g),
p-xylene (5 ml), K2CO3 (0.50 mmol), air oxidant, 100°C.
bYield determined by GC analysis.
The catalytic activity of MnO2@NHAp was investigated in the
aerobic oxidation of alkylarenes and alcohols. Optimization studies
of the effects of solvents, bases and amount of catalyst were
performed in order to find the best reaction conditions for the
Appl. Organometal. Chem. (2016)
Copyright © 2016 John Wiley & Sons, Ltd.
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Shaabani A., Afaridoun H. and Shaabani S.
Table 3. Optimization of reaction conditions for aerobic oxidation of
Table 4. Aerobic oxidation of various alcohols to corresponding alde-
alcoholsa
hydes and ketonesa
Entry
Alcohol
Product
Time Yield
(h) (%)b
Entry
Catalyst
Amount of catalyst (mol%) Solvent Yield (%)b
1
2
3
4
1-Phenylethan-1-ol
Phenylmethanol
9H-Fluoren-9-ol
Acetophenone
Benzaldehyde
9H-Fluoren-9-one
Benzil
2
2
2
1
90
99
95
99
1
Active MnO2
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
MnO2@NHAp
0.01 g (10.00)
0.02 g (3.00)
0.03 g (5.00)
0.04 g (7.00)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g (8.50)
0.05 g
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
n-Hexane
Toluene
H2O
25
40
60
85
97
90
80
70
10
30
80
0
2
3
2-Hydroxy-1,2-
4
diphenylethan-1-one
1-Phenylpropan-1-ol
Diphenylmethanol
5
5
6
7
Propiophenone
Benzophenone
2
3
3
85
97
90
6
7
8c
9d
10
11
12
2,3-Dihydro-1H-inden-1-ol 2,3-Dihydro-1H-inden-1-
one
8
9
(E)-3-Phenylprop-2-en-1-ol Cinnamaldehyde
(4-Fluorophenyl)methanol 4-Fluorobenzaldehyde
3
3
3
3
3
3
3
70
80
80
85
90
80
85
10 (4-Bromophenyl)methanol 4-Bromobenzaldehyde
11 (4-Chlorophenyl)methanol 4-Chlorobenzaldehyde
12 (4-Nitrophenyl)methanol 4-Nitrobenzaldehyde
13e MnO2@NHAp
14e MnO2@NHAp
15e MnO2@NHAp
16e MnO2@NHAp
MeOH
0
EtOH
0
13 p-Tolylmethanol
14 (4-Methoxyphenyl)
methanol
4-Methylbenzaldehyde
4-
EtOAc
0
CH3CN
40
5
Methoxybenzaldehyde
Hexanal
17
NHAp
p-Xylene
15 Hexan-1-ol
6
6
6
6
70
73
75
77
aReaction conditions: diphenylmethanol (1.00 mmol), solvent (5 ml),
K2CO3 (0.50 mmol), air oxidant, 80°C, 3 h.
bYield determined by GC analysis.
cKOH (0.5 mmol) was added as a base.
dIn the absence of base.
16 Cyclohexanol
17 Cyclooctanol
18 Octan-2-ol
Cyclohexanone
Cyclooctanone
Octan-2-one
aReaction conditions: alcohol (1.00 mmol), MnO2@NHAp (0.05 g), K2CO3
(0.50 mmol), p-xylene (5 ml), air oxidant, 80°C.
bYield determined by GC analysis.
eReflux.
aerobic oxidation of alkylarenes to their corresponding carbonyl
compounds. As indicated in Table 1, with NHAp the reaction does
not proceed, and with active MnO2 the yield is 20% (entries 1 and
12). The best results are obtained using MnO2@NHAp with optimal
MnO2 loading of 8.50 mol% in the presence of 0.50 mmol of K2CO3
as a base in p-xylene as a solvent (entry 4).
As indicated in Table 2, the reactions of active alkylarenes such as
1-phenylethane, indane and fluorene proceed in short times and
give high yields in this catalytic system.
In the next step, optimization studies for aerobic oxidation of
alcohols were conducted. As indicated in Table 3, the optimal
MnO2 loading is 8.50 mol% in the presence of 0.50 mmol of
K2CO3 as a base in p-xylene as a solvent.
Figure 5. Reusability of MnO2@NHAp catalyst in oxidation of
diphenylmethanol.
As evident from Table 4, under optimum reaction conditions, the
efficiency of the catalyst for the aerobic oxidation of benzylic
alcohols is higher than for primary alcohols; for the aerobic oxida-
tion of primary benzylic alcohols with electron-withdrawing substi-
tutions is higher than for alcohols with electron-releasing
substituents; and for the aerobic oxidation of secondary aliphatic
alcohols is higher than for primary aliphatic alcohols.
The recyclability of catalyst for the model reaction of alcohols
was investigated. For this purpose, dried recovered catalyst was
reused for five repeat runs without a significant decrease in its
activity and selectivity (Fig. 5). The MnO2 content of the catalyst
before and after reaction was determined as 15 and 14.6%, respec-
tively, using FAAS. Also, the concentration of Mn that had leached
out into the solution during the reaction was 3 ppm.
Conclusions
MnO2@NHAp as a new bio-supported heterogeneous catalytic
system was easily prepared from in situ reduction of potassium
permanganate in the presence of NHAp. Combination of MnO2
as a green and inexpensive catalyst with NHAp increased its cat-
alytic activity with higher molar ratio of substrate to MnO2. Ad-
vantages of this heterogeneous catalytic system are high
activity, yield and selectivity, easy synthetic procedure and easy
separation from reaction mixture, excellent recyclability and the
use of air as a green oxidant without the need for any additional
oxidation agents.
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MnO2@NHAp as catalyst for oxidation of alkylarenes and alcohols
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Acknowledgments
We gratefully acknowledge the financial support of the Iran
National Elites Foundation (INEF) and the Research Council of
Shahid Beheshti University.
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