Natural hydroxyapatite supported cobalt tetrasulfophthalocyanine: A green, renewable and biomaterial-based heterogeneous catalyst for selective aerobic oxidation of alkyl arenes and alcohols

Article abstract of DOI:10.1039/c6ra24294d

Natural hydroxyapatite supported cobalt tetrasulfophthalocyanine (CoTSPc-NHAp) as a green, renewable and biomaterial-based heterogeneous catalyst that mediates oxidation of various secondary benzylic C-H bonds, secondary benzylic and primary and secondary aliphatic alcohols to their corresponding aldehydes and ketones in high yields and selectivity without any additional oxidizing reagent is reported. The structure of the synthesized catalyst was characterized by inductively coupled plasma-optical emission spectrometry (ICP-OES), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and the energy dispersive spectroscopy (EDS) analysis.

Full text of DOI:10.1039/c6ra24294d

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Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
Natural Hydroxyapatite Supported Cobalt Tetrasulfophthalocyanine: A
Green, Renewable and Biomaterial-Based Heterogeneous Catalyst for
Selective Aerobic Oxidation of Alkyl Arenes and Alcohols
*
a
a
a
a
Ahmad Shaabani, Hadi Afaridoun, Shabnam Shaabani and Mina Keramati nejad
Natural hydroxyapatite supported cobalt tetrasulfophthalocyanine catalyst promotes aerobic oxidation of alkyl arenes and alcohols.
Herein, we report a green, renewable and biomaterial-based heterogeneous catalytic system that mediates oxidation of various
secondary benzylic C–H bond, secondary benzylic alcohols, primary and secondary aliphatic alcohols to their corresponding
aldehydes and ketones in high yields and selectivity without any additional oxidizing reagent.The structure of the synthesized
catalyst was characterized by inductively coupled plasma-optical emission spectrometry (ICP-OES), Fourier transform infrared (FT-
IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), and the energy dispersive
spectroscopy (EDS) analysis.
1
. Introduction
Carbonyl compounds such as aldehydes and ketones are important functional groups in industrial and laboratory
1
fine-chemical production and are often prepared by oxidation of their corresponding alcohols or hydrocarbons. In
2
3
4-6
7
conventional, various hazardous oxidizing agents such as metal oxides, halides, peroxides or ozone were
examined in some of the oxidation processes which were not so environmental friendly. From the green chemistry
approach, there is an important demand to develop oxidation procedures such as using green oxidants and
heterogeneous nano-biosupported and recoverable catalysts under mild conditions which is particularly suitable for
8
, 9
industrial units.
Synthetic hydroxyapatite (HAp), Ca10(PO
4 6
) (OH)2, as a biocompatible compound is used for replacement and
rehabilitation of bone materials. Recently, nanotechnology has helped to synthesis hydroxyapatite nanostructures
10
that have very similar properties to natural hydroxyapatite. Due to its extensive properties such as good chemical
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1
1
12
and thermal stability, bioactivity, and noninflammatory behavior and its potential applications such as matrix for the purification,
13
14-19
bioceramics, chemical sensors, and catalysts support
hydrothermal synthesis, sol–gel route and so on have been reported for the synthesis of HAp.
efficient support in heterogeneous catalysts exhibited a significant performance for oxidation transformations.
various synthetic methods such as chemical precipitation, solid-state reaction,
20-24
The synthetic HAp as a suitable and
2
5-29
From the environmental
and economic viewpoint, natural hydroxyapatite (NHAp) as a renewable biomaterial could be an efficient replacement for the synthetic
HAp. Such a strategy represents "green chemistry" in that it is efficient using a natural resource (bone) which is often considered a waste
product.
Metallophthalocyanines (MPcs) based catalysts, due to the high catalytic activity, easy preparation in large scale, low costs, low toxicity,
3
0
and relatively high chemical and thermal stability have attracted considerable attention. However, many homogeneous MPcs catalysts
often have a limited lifetime of activity because of the formation of inactive aggregates and bridged µ-oxo dimers in solution, which block
3
1-33
the access of the reagents to the catalytically active sites and diminishes the catalytic efficacy.
Therefore, the immobilization of these
catalysts to solid supports is an interesting approach to overcome such drawbacks, where the macrocycle can be isolated and its catalytic
3
4
35
36
site can be protected. Accordingly, the methods about anchoring of MPcs onto various supports like carbon fibers, silica, zeolites,
3
7
38
resins and clays via covalent bond or axial ligation of the metal center were investigated. However, there is still a need to develop new
efficient heterogeneous catalysts in more simple and efficient way.
3
5,
In recent years, the design of catalysts focusses on the development of new catalytic materials based on biomimetic and green principles.
3
9-45
43, 44
Due to this fact, MPcs as biomimetic catalysts have been widely employed in several oxidation reactions.
In view of both eco-
friendly and green chemistry approach using air as a green and low-cost oxidant and using natural and biocompatible support for
heterogeneous catalysts are important. Therefore, in a continuation of our efforts to synthesis of bio-supported heterogeneous catalytic
4
6-50
51
52-56
systems,
metallophthalocyanines and aerobic oxidation of alkyl arenes and alcohols,
herein, we describe the selectively aerobic
oxidation of alkyl arenes and alcohols to their corresponding carbonyl compounds using natural hydroxyapatite supported cobalt
tetrasulfophthalocyanine catalyst (CoTSPc-NHAp) under heterogeneous conditions without using any oxidizing reagents.
2
. Experimental
General
All experiments were performed in compliance with the relevant laws and institutional guidelines. All reagents were purchased from
Aldrich or Merck and used without further purification. A 1200W, 40 MHz Spectro Vista-Pro ICP-OES (Varian, Australia) with a concentric
nebulizer and a cyclonic model spray chamber was employed for the determination of Co. The auxiliary gas (Ar) flow rate, carrier gas (Ar)
-
1
flow rate and plasma gas (Ar) flow rate were 1.5, 1.5, and 15 Lmin , respectively. The wavelength used was Co(II) 228.6 nm. The solution
-1
uptake rate was 1.4 mLmin and the delay time was 15s. Co determination was also carried out on a flame atomic absorption spectroscopy
FAAS) (Shimadzu model AA-680 atomic absorption spectrometer) using an air-acetylene flame. Thermogravimetric analysis (TGA) was
(
−1
carried out using STA 1500 instrument at a heating rate of 10 °C min in air. Products were analyzed using a Varian 3900 GC. X-ray
diffraction (XRD) pattern of product was recorded on a STOE STADI P with scintillation detector, secondary monochromator using Cu Kα
radiation (λ = 0.1540 nm). Scanning electron microscopy (SEM) observations were carried out on an electron microscopy Philips XL-30
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ESEM. All samples were sputtered with gold before observation. Melting points were measured on an Electrothermal 9200 apparatus. IR
spectra were recorded on Bomem MB-Series FT-IR spectrophotometer.
a
2
.1 Preparation of NHAp
Cow bones were boiled in water for 12 h to remove the traces of meat. Then, they were soaked in 5% (w/v) NaOH aqueous solution under
5
7
reflux conditions for 5 h to remove proteins, lipids, oils and other organic impurities. Finally, they were washed with distilled water, dried,
broken in smaller pieces and ground with mill (Fig. 1).
b
c
a
Fig. 1. Images of bone (a), prepared NHAp (b) and CoTSPc-NHAp (c).
2
.2 Preparation of CoTSPc
5
1
CoPc was synthesized according to our previously reported method. Chlorosulfonicacid (5.00 mmol) was added dropwise to the well-
o
stirred mixture of CoPc (1.00 mmol) in trichlorobenzene (30.00 ml), during one hour and then it was stirred for 2 hour at 170 C over reflux
conditions. After the mixture had cooled, it was added slowly to 500 ml distilled water, then, the mixture was filtered and the obtained
o
solution was treated with saturated NaHCO
3
solution. Finally, the resulting mixture was dried at 70 C under vacuum conditions.
2
.3 Preparation of CoTSPc-NHAp catalyst
CoTSPc-NHAp catalyst was prepared using a chemical immobilization method. To a 50 ml of CoTSPc solution (0.5% w/v), NHAp (2.00 g) was
slowly added under vigorous stirring conditions over 1 h at room temperature. Then, the mixture was irradiated with ultrasound for 20
minutes. After stirring at room temperature for 48 h, the resulting bluish powders were filtered and washed three times with distilled
water and ethanol, and dried overnight at 70 °C (Fig 1c). The content of cobalt in catalyst was determined 0.4 % (w/w) by ICP-OES
and FAAS analysis.
2
.4 General procedure for the oxidation of alkyl arenes
An alkyl arene (1.00 mmol), N-hydroxyphthalimide (0.02 g, 0.15 mmol), CoTSPc-NHAp (0.05 g, 0.34 mol%), and p-xylene (10 ml)
as a solvent were added to a two-necked round-bottom flask. The mixture was heated under air blowing at 100 °C for the
indicated times in Table 2. The thin layer chromatography (TLC) method was used to investigate the progress of the reaction.
Upon completion of reaction, analysis of the crude product has been done by gas chromatography (GC) method.
2.5 General procedure for the oxidation of alcohols
An alcohol (1.00 mmol) was added to a two-necked flask containing CoTSPc-NHAp (0.05 g, 0.34 mol%), KOH (0.50 mmol), and p-
xylene (5 ml) and stirred under air blowing at 80 °C for the indicated times in Table 4. The progress of the reaction was followed
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by TLC. Upon completion, the reaction mixture was filtered and the crude product analyzed by GC method. The conversions
were determined from the integrals of the GC analysis.
3
. Results and discussion
According to thermal gravimetric analysis (TGA) under air flow (heating rate 10 °C/min), the TGA curves of NHAp and catalyst
showed small weight loss (5 7 wt %) before 130 °C, due to the loss of lattice water and removal of adsorbed water on the
∼
surface of NHAp and catalyst. The TGA curve of the CoTSPc-NHAp showed that the sample lost a large mass from 270 to 580 °C,
possibly due to decomposition of CoTSPc.
100
9
9
8
8
7
7
6
5
0
5
0
5
0
5
0
200
400
600
800
Temperature (°C)
Fig. 2. TGA curves of NHAp and CoTSPc-NHAp
.
In addition to TGA, XRD pattern of NHAp and CoTSPc-NHAp was also employed to investigate the structure of the catalyst (Fig.
). The crystallinity of the NHAp was confirmed by strong peaks at 2θ values of 25.77°, 31.88°, 33.91°, 46.72°, 49.46° and 53.14°.
The pattern of CoTSPc-NHAp is similar to NHAp indicating that the NHAp remains intact at the end of the procedure without loss
3
5
8
in the crystallinity.
CoTSPc-NHAp
NHAp
10
20
30
40
50
60
70
80
2
θ(degree)
Fig. 3 . XRD pattern of NHAp and CoTSPc-NHAp.
FT-IR has been used to analyze the presence of functional groups in the synthesized catalyst. In Fig. 4, spectra of NHAp and
CoTSPc-NHAp have been shown after background subtraction for each spectrum. The absorption bands at 960-1115, 559-602
4
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−
1
3-
and 467cm are attributed to PO ions. The broad band at 3518 cm and a small band at about 1666 cm are due to absorbed
−1
−1
4
1
−
water. The shoulder at 3672 cm and the absorption band at 694 cm corresponds to the stretching vibration and vibrational
−1
mode of hydroxyl group. Existence of carbonate ion could be confirmed through absorption bands at 1658, 1747, 2063 and 2129
−
1
cm .
CoTSPc-NHAp
NHAp
3
900 3400 2900 2400 1900 1400 900 400
-1
Wavenumbers (cm )
Fig. 4. Infrared spectra of NHAp and CoTSPc-NHAp
As shown in Fig. 5(a, b), the SEM analysis has been used to study the structure and morphology of the CoTSPc-NHAp. The SEM
analysis of the catalyst showed that the morphology of the CoTSPc-NHAp which is composed of particles with irregular shapes
and different size. The chemical composition of the CoTSPc-NHAp was investigated using energy dispersive spectroscopy (EDS)
analysis in Fig. 5c. The EDS analysis illustrated the presence of P, Ca, O, C, Co and S elements in the catalyst.
a
b
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c
Fig. 5. SEM images (a, b) and EDS analysis of CoTSPc-NHAP (c).
Various interactions such as π-π stacking, van der Waals and electrostatic contribution could be imagine between metal and supports.
According to the nature of tetrasulfophthalocyanine as a ligand and natural hydroxyapatite as a support maybe π-π stacking interactions of
5
9
ligand (TSPc) and electrostatic contribution of cobalt with NHAp are effective interactions in this heterogeneous catalyst.
Catalytic activity of CoTSPc-NHAp was investigated in aerobic oxidation of alkyl arenes and alcohols. The effects of solvents, bases and the
amount of catalysts were screened in order to find the best reaction conditions. The aerobic oxidation of indane as a model reaction was
2 3
carried out using different solvents: H O, MeOH, EtOH, EtOAc, n-hexane, CH CN, toluene, DMF and p-xylene. As indicated in Table 1, p-
xylene was found to be the most suitable solvent. However, the reaction was not carried out in polar and protic solvents (entries 8-11) or
were found to be very slow with low yields after 8 h (entries 12 and 13). These results may be explained, due to insolubility or less solubility
of substrate in polar and aprotic solvents and lower boiling point and reaction temperature of nonpolar solvent such as n-hexane. The
strong base, KOH, was not appropriate in aerobic oxidation of alkyl arenes. As indicated in Table 1, the optimum conditions of reaction
were obtained using 0.34 mol% CoTSPc in the presence of 0.15 mmol NHPI as an initial reagent in p-xylene as a solvent.
O
0
.34 mol% CoTSPc-NHAp , NHPI
Ar
R
Ar
R
p-xylene, air, 100 °C
Scheme 1. Oxidation of alkyl arenes using CoTSPc-NHAp.
a
Table 1. Optimization of the reaction conditions for aerobic oxidation of arenes.
Amount of catalyst
mol%)
Reagent/base
(mmol)
b
Entry
Catalyst
Solvent
Yield (%)
(
1
2
3
4
5
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
p-Xylene
p-Xylene
p-Xylene
p-Xylene
p-Xylene
KOH (0.50)
NHPI (0.05)
NHPI (0.10)
NHPI (0.15)
-
10
60
85
95
5
6
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c
6
7
8
9
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
NHAp
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g
n-Hexane
NHPI (0.15)
NHPI (0.15)
NHPI (0.15)
NHPI (0.15)
NHPI (0.15)
NHPI (0.15)
NHPI (0.15)
NHPI (0.15)
NHPI (0.15)
0
80
0
Toluene
2
H O
c
MeOH
0
c
1
0
1
2
3
4
EtOH
0
c
1
1
1
1
EtOAc
DMF
0
5
c
CH
3
CN
5
p-Xylene
10
a
b
Reaction conditions: Indane (1.00 mmol), solvent (10 ml), NHPI (0.15 mmol), air oxidant, 100 °C, 8 h. Yields determined
c
by GC analysis. Reaction conditions: Indane (1.00 mmol), solvent (10 ml), NHPI (0.15 mmol), air oxidant, reflux, 8 h.
a
Table 2. Aerobic oxidation of various alkyl arenes to corresponding ketones.
b
Entry
Alkyl arene
Product
Time (h)
Yield (%)
1
2
3
4
5
6
7
8
8
85
80
8
75
12
8
65
80
8
95
8
85
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8
9
8
8
90
80
a
Reaction conditions: alkyl arene (1.00 mmol), CoTSPc-NHAp (0.05 g), solvent (10 ml), NHPI (0.15
mmol), air oxidant, 100 °C. b Yields determined by GC analysis.
As indicated in Table 2, various alkyl arenes are converted into the corresponding carbonyl compounds in high yields when the reactions
are carried out in p-xylene at 100 for 8 or more hours. This is a highly useful reaction for the preparation of ketones, which are
℃
important intermediates in the synthesis of many organic compounds. Although the oxidation of C–H groups is thermodynamically
favorable, it also has large activation barriers that translate into their characteristic inert nature against most reagents. To test the site
selectivity of this catalytic system, molecules with different secondary sites were oxidized (Table 2, entries 3, 4, 5). As indicated in Table 2
60, 61
this catalytic system mediates the benzylic oxidation preferentially over other oxidized products via benzylic C-H abstraction.
In the next step, optimization studies for aerobic oxidation of alcohols was investigated. As indicated in Table 3, the optimal
o
CoTSPc loading is 0.34 mol% in the presence of 0.50 mmol KOH in p-xylene as a solvent at 80 C.
Scheme 1. Oxidation of alcohols using CoTSPc-NHAp.
a
Table 3. Optimization of the reaction conditions for aerobic oxidation of alcohols.
Amount of catalyst
b
Entry
Catalyst
Solvent
Base (mmol)
Yield (%)
(
mol%)
1
2
3
4
5
6
7
8
9
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
CoTSPc-NHAp
0.03 g (0.20)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
0.05 g (0.34)
p-Xylene
p-Xylene
p-Xylene
n-Hexane
Toluene
KOH (0.50)
90
98
5
KOH (0.50)
2 3
K CO (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
KOH (0.50)
0
90
0
2
H O
c
MeOH
0
c
EtOH
0
c
EtOAc
0
c
1
1
1
1
1
0
1
2
3
4
CH
3
CN
10
0
c
CH
2
Cl
2
c
THF
0
DMF
10
30
AcOH
8
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1
1
5
6
CoTSPc-NHAp
NHAp
0.05 g (0.34)
0.05 g
p-Xylene
p-Xylene
-
5
5
KOH (0.50)
a
o
Reaction conditions: diphenylmethanol (1.00 mmol), solvent (5 ml), air oxidant, 80 C, 4 h. Yields determined by
b
c
GC analysis. Reaction conditions: diphenylmethanol (1.00 mmol), solvent (5 ml), air oxidant, reflux, 4 h.
a
Table 4. Aerobic oxidation of various alcohols to corresponding aldehydes and ketones.
b
Entry
Alcohol
Product
Time (h)
Yield (%)
10
11
12
13
14
15
16
17
4
90
4
4
3
5
4
5
6
95
90
99
85
98
98
70
1
8
9
6
6
80
85
1
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2
0
1
6
5
5
5
8
8
8
8
75
85
70
70
70
78
75
80
2
2
2
3
2
24
25
26
27
a
Reaction conditions: Alcohol (1.00 mmol), CoTSPc-NHAp (0.05 g), KOH (0.50 mmol), p-
o
xylene (5 ml), air oxidant, 80 C. Yields determined by GC analysis.
b
As indicated in Table 4, aerobic oxidation of various alcohols such as primary and secondary benzylic, cinnamyl and primary and
secondary aliphatic alcohols were investigated. According to the yields and reaction times, secondary benzylic alcohols were
more active than others in this catalytic system. Primary benzylic alcohols with electron-withdrawing substitution were better
than primary benzylic alcohols with electron-donor substitution. Cinnamyl alcohol showed relatively good activity. Aliphatic
alcohols needed more time for oxidation and secondary aliphatic alcohols were more active than primary aliphatic alcohols.
The mechanism of this reaction probably proceeds through the radical proton abstraction from alkyl arenes by NHPI and for the
alcohols through the removal of a hydrogen atom from the carbon attached to the hydroxyl group of alcohols by
4
9
tetrasulfophthalocyaninesuperoxocobalt(III) as shown in Scheme 3.
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Scheme 2. Plausible mechanism of the aerobic oxidation of alkyl arenes and alcohols.
The efficiency of the synthesized catalyst in comparison with other reported catalysts is given in Table 5. The results showed that
in the case of CoTSPc-NHAp, the reaction conversion was increased with higher molar ratio of substrate to CoTSPc and air was
used as an oxidant.
Table 5. Comparison of literature catalysts and this work for oxidation of benzyl alcohol.
Temperature
-1 a
TOF (h )
Entry
Catalyst
Oxidant
Solvent
o
Time (h)
Yield (%)
Ref.
(
C)
metal complex of
Ir(III)
1
2
---
Toluene
110
80
48
4
89
88
45
44
62
63
Co(II)–SBC
---
H O
2
3
4
5
CoPc
O
O
2
2
Xylene
140
65
0.75
3.50
4
94
76
95
6.3
40
64
CoTSPc-PANI
CH
3
CN
20.6
69.8
b
This
work
CoTSPc-NHAp
Air
p-Xylene
80
a
−1 b
Turn over frequency (TOF): moles of substrate converted per mol of Co in the catalyst h . TON of catalyst = 279.4.
Catalyst recyclability is an important feature from a cost point of view. Thus, the recyclability of the CoTSPc-NHAp catalyst was investigated
in the oxidation reaction of diphenylmethanol and diphenylmethane under optimized conditions. For this purpose, dried recovered catalyst
was reused for five repetitive times without significantly decrease in its activity and selectivity. The cobalt content of the catalyst before
and after reaction was determined as 0.40 and 0.38% (w/w), respectively, using FAAS. The concentration of Co that had leached out into
the solution during the reaction was 1 ppm.
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Fig. 6. Reusability of the catalyst for oxidation of diphenylmethanol and diphenylmethane by CoTSPc-NHAp.
Conclusions
In conclusion, natural hydroxyapatite as a renewable biomaterial was used as a support for cobalt tetrasulfophthalocyanine and the
catalytic behavior of synthesized CoTSPc-NHAp was investigated in selective aerobic oxidation of various secondary benzylic C–H bond,
secondary benzylic alcohols, primary and secondary aliphatic alcohols. In this system, secondary benzylic alcohols were more active than
secondary and primary aliphatic alcohols. Advantages of this heterogeneous catalytic system are high activity, yield and selectivity, easy
synthetic procedure and separation from mixture, excellent recyclability and using air as green oxidant without using any additional
oxidizing reagents.
Acknowledgements
We gratefully acknowledge financial support of the Iran National Elites Foundation (INEF) and the Research Council of Shahid
Beheshti University.
Notes and references
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Natural
Tetrasulfophthalocyanine:
Hydroxyapatite
Supported
Green, Renewable
Cobalt
A
and
Biomaterial-Based Heterogeneous Catalyst for Selective Aerobic
Oxidation of Alkyl Arenes and Alcohols
*
Ahmad Shaabani, Hadi Afaridoun, Shabnam Shaabani and Mina Keramati nejad
Natural hydroxyapatite supported cobalt tetrasulfophthalocyanine catalyst promotes aerobic oxidation of alkyl arenes
and alcohols. Here, we report a green renewable and biomaterial-based heterogeneous catalyst system that mediates
oxidation of various secondary benzylic C–H bond, secondary benzylic alcohols, aliphatic primary and secondary
alcohols to their aldehyde and ketones in high yields and selectivity without any additional oxidizing reagent.The
structure of synthesized catalyst was characterized by inductively coupled plasma-optical emission spectrometry (ICP-
OES), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron
microscopy (SEM), and the energy dispersive spectroscopy (EDS) analysis.