Aqueous heterogeneous oxygenation of hydrocarbons and sulfides catalyzed by recoverable magnetite nanoparticles coated with copper(ii) phthalocyanine

Article abstract of DOI:10.1039/c2gc35837a

In this work, a novel magnetically recoverable phthalocyanine catalyst was prepared by immobilization of the Cu(ii) phthalocyanine-tetrasulfonic acid tetrasodium complex (CuPcS) on the silica coated magnetic nanoparticles (Fe ₃O₄@SiO₂, SMNP) via the amine functionality (ASMNP). The epoxidation of olefins and the oxidation of saturated hydrocarbons to the related ketones and also sulfides to the sulfones by an aqueous solution of tetra-n-butylammonium peroxomonosulfate (n-Bu₄NHSO₅, TBAOX) were efficiently enhanced with excellent selectivity under the influence of the catalytic activity of the magnetically separable catalyst. Sulfoxides could also be selectively produced in the mixture of water-ethanol, which makes the title methodology a good alternative for both sulfoxide and sulfone production. The separation and recycling of the catalyst and the reduced form of the oxidant were simple, effective and economical in this clean oxidation method. The FT-IR and leaching experiments after seven successive cycles showed that the catalyst was most strongly anchored to the magnetite nanoparticles.

Full text of DOI:10.1039/c2gc35837a

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PAPER
Aqueous heterogeneous oxygenation of hydrocarbons and sulfides catalyzed
by recoverable magnetite nanoparticles coated with copper(II)
phthalocyanine†
Abdolreza Rezaeifard,* Maasoumeh Jafarpour,* Atena Naeimi and Reza Haddad
Received 30th May 2012, Accepted 19th September 2012
DOI: 10.1039/c2gc35837a
In this work, a novel magnetically recoverable phthalocyanine catalyst was prepared by immobilization of
the Cu(II) phthalocyanine-tetrasulfonic acid tetrasodium complex (CuPcS) on the silica coated magnetic
nanoparticles (Fe3O4@SiO2, SMNP) via the amine functionality (ASMNP). The epoxidation of olefins
and the oxidation of saturated hydrocarbons to the related ketones and also sulfides to the sulfones by an
aqueous solution of tetra-n-butylammonium peroxomonosulfate (n-Bu4NHSO5, TBAOX) were efficiently
enhanced with excellent selectivity under the influence of the catalytic activity of the magnetically
separable catalyst. Sulfoxides could also be selectively produced in the mixture of water–ethanol, which
makes the title methodology a good alternative for both sulfoxide and sulfone production. The separation
and recycling of the catalyst and the reduced form of the oxidant were simple, effective and economical
in this clean oxidation method. The FT-IR and leaching experiments after seven successive cycles showed
that the catalyst was most strongly anchored to the magnetite nanoparticles.
phthalocyanine complexes onto different supports to obtain het-
Introduction
erogeneous catalysts.4 Recently, nanoparticles have emerged as
The macrocyclic structure of metallophthalocyanines (MPcs)
resembles that of porphyrin complexes widely used by nature in
the active sites of enzymes responsible for biological oxidation.1
In contrast to the porphyrin complexes, phthalocyanine counter-
parts are cheap and readily accessible on an industrial scale: their
worldwide production is about 80 000 t/year, making them very
attractive as potential industrial catalysts.2 However, the low
solubility of MPcs is perhaps the most significant limitation in
their application as catalysts. By suitable substitution on the aro-
matic ring, a variety of phthalocyanine derivatives have been pre-
pared in an attempt to improve their solubility in different
solvents, particularly in aqueous media.3 Nevertheless, the sepa-
ration of water-soluble catalysts from the aqueous phase is a time
consuming process, which requires making them out of recycled
materials, and also the tendency of water-soluble phthalocya-
nines to aggregate in aqueous solution, even at low concen-
tration, has reduced considerably the catalytic activities. This
problem can be overcome by dispersion and fixation of the
efficient alternatives for the immobilization of homogeneous cat-
alysts. Nevertheless, difficulties in recovering the nanometer-
sized particles from the reaction mixture, resulting from the
effective dispersion in solution by forming stable suspensions,
severely limits their wide applications.5 There is currently
intense interest in the use of magnetic nanoparticles (such as
Fe3O4) for a wide range of biomedical and technological appli-
cations.6 They have been coated with metal catalysts or conju-
gated with enzymes, to combine the separating power of the
magnetic properties with the catalytic activity of the metal
surface or enzyme conjugate.6
In continuation of our ongoing research on the development
of novel catalytic oxidation methods using tetra-n-butyl-
ammonium peroxomonosulfate (n-Bu4NHSO5, TBAOX),7 we
very recently explored an innovative heterogeneous strategy for
the aqueous oxidation of olefins, hydrocarbons, alcohols and
sulfides, catalyzed by dispersed Fe(III) and Mn(III) porphyrins
and copper(II) phthalocyanine as the hydrophobic nanocatalysts
in the absence of any organic co-solvents, surfactants and hydro-
philic auxiliaries.8 To benefit the easy and complete recovery of
the catalyst in the aqueous system we decided to immobilize the
Cu(II) phthalocyanine-tetrasulfonic acid tetrasodium complex
(CuPcS) on the nano-sized Fe3O4 as a magnetic nanoparticle
(MNP) coated with a silica layer (SMNP). The epoxidation of
olefins and the oxygenation of saturated hydrocarbons to
ketones, and also selective oxidation of sulfides to both sulfoxide
and sulfone products, was enhanced in the presence of CuPcS@
Catalysis Research Laboratory, Department of Chemistry, Faculty of
Science, University of Birjand, Birjand, 97179-414, Iran.
E-mail: rrezaeifard@gmail.com, rrezaeifard@birjand.ac.ir,
mjafarpour@birjand.ac.ir; Fax: +98 561 2502515;
Tel: +98 561 2502516
†Electronic supplementary information (ESI) available: Optimization
results for the oxidation of indane and thioanisole, and the TEM images
of CuPcS@ASMNP in aqueous solutions of other oxidants used. See
DOI: 10.1039/c2gc35837a
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Scheme 1 Aqueous oxidation of hydrocarbons and sulfides using
TBAOX catalyzed by CuPcS@ASMNP.
ASMNP as a novel magnetically recoverable nanocatalyst using
an aqueous solution of TBAOX in the absence of surfactants,
organic co-solvents, and co-catalysts (Scheme 1). The catalyst
and reduced form of oxidant (n-Bu4NHSO4) were easily recov-
ered and reused in the procedure.
Fig. 1 FT-IR spectra of MNP (A), SMNP (B), ASMNP (C) and
CuPcS@ASMNP (D).
The FT-IR spectra of MNP, SMNP, ASMNP and the CuPcS@
ASMNP are given in Fig. 1. The presence of a magnetite core
in the prepared nanomaterials was confirmed by the obser-
vation of two bands at around 430–600 cm−1 (Fig. 1A).10 The
broad peak at about 900–1200 cm−1, assigned to the Si–O–Si
and Si–OH stretching vibrations, showed the silica coating of
magnetite nanoparticles (Fig. 1B).11 The anchoring of amino-
propyl groups on SMNP was confirmed by stretching vibrations
that appeared at about 2800–3000 and 3446 cm−1 (Fig. 1C).6i,9b
Other bands were derived from the deformation and stretching
vibrations of the phthalocyanine complex electrostatically
anchored to the magnetic support via amine functionality
(Fig. 1D).
The crystalline structure of MNP, with a cubic structure, was
identified with XRD (Fig. 2A). The same set of characteristic
peaks was observed for CuPcS@ASMNP (Fig. 2B), indicating
the stability of the crystalline phase of the nanoparticles during
the subsequent surface modification.12 The size of the particles
were estimated to be 7.70 and 9.40 nm for MNP and CuPcS@
ASMNP respectively, using the Debye–Scherrer equation
according to the (311) reflection.
Results and discussion
Preparation and characterization of CuPcS@ASMNP
We used magnetic nanoparticles (MNP), of approximately
10 nm diameter, which were prepared by the co-precipitation
method.9 The particles were subsequently coated with a dense
silica layer, using tetraethoxysilane (TEOS) as the silica source
and ammonia solution. The silica layered system (SMNP) then
offers binding sites (Si–OH units) for the heterogenization of
the molecular catalysts. For grafting on the surface, we used
(3-aminopropyl) triethoxysilane (APS) as the chelating ligand
(ASMNP).9a,b It was reacted with CuPcS in water to yield the
corresponding magnetically recoverable catalyst [CuPcS@
ASMNP] (Scheme 2).
Fig. 3A shows the transmission electron microscopy (TEM)
image of CuPcS@ASMNP, which further confirmed an assem-
bly of the catalyst and its size being less than 8.8 nm (including
the SiO2 shell and Pc complex). The SEM image of the sup-
ported catalyst showed the uniformity and spherical morphology
of the nanoparticles (Fig. 3B). The aggregation gives rise to an
increase in the size of the observed nanoparticles, with an
average diameter of ca. 80 nm.11
The thermal behavior of CuPcS@ASMNP (Fig. 4) showed
the degradation of CuPcS@ASMNP at 250 °C, which exhibits
the high thermostability of catalyst. The organic parts decom-
posed completely at 520 °C. According to the TGA, the amount
of CuPcS is evaluated to be 6.50 wt%. Therefore each gram of
Scheme 2 Preparation of CuPcS@ASMNP.
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Fig. 2 XRD patterns of MNP (A) and CuPcS@ASMNP (B).
Fig. 4 TGA curves of CuPcS@ASMNP.
Table 1 Screening of solvents and the quantity of the catalyst in the
oxidation of 1-methyl cyclohexenea
Catalyst
(mol%)
Time
(min)
1-Methyl cyclohexane
oxide%b
Entry Solvent
1
2
3
4
CH2Cl2
CHCl3
Toluene
n-Hexane
0.50
0.50
0.50
0.50
90
90
90
90
10
12
5
—
5
6
7
8
Ethylacetate 0.50
90
90
90
600
90
90
90
80 (90)
—
51
39
30
18
42
80
Fig. 3 TEM (A) and SEM (B) images of CuPcS@ASMNP.
Ethanol
Methanol
H2O
H2O
H2O
H2O
H2O
0.50
0.50
0.00
0.20
0.30
0.40
0.50
9
heterogeneous catalyst contains 66.00 μmol g−1 of Cu or CuPcS
complex. This result is in agreement with that of ICP-AES
(6.65 wt%, 67.50 μmol g−1).
10
11
12
13
85 (100)c
87–92 (100)
H2O
1.00 to 5.00 80 (90)
It should be noted that, the black Fe3O4 nanoparticles can be
easily oxidized to yellow Fe2O3 nanoparticles when they are
exposed directly to air and their magnetism decreases signifi-
cantly. However, after six months of storage, when exposed to
air at room temperature, the color and the magnetic properties of
CuPcS@ASMNP remained almost unchanged. The coating of
Fe3O4 with a SiO2 layer containing CuPcS attached to surface
shielded the nanomagnetite surface from oxygen and improved
greatly its stability.13
a Reaction conditions: 0.50 mmol 1-methyl cyclohexene, 1.50 mmol
TBAOX, 1.00 ml solvent, X mmol CuPcS@ASMNP as catalyst. The
reactions were run at 70 °C except for low bp solvents which were
performed under reflux conditions. b GC yield. c 89% isolated yield.
hydrocarbons. Virtually no catalytic activity was observed in
commonly used organic solvents (Table 1, entries 1–7). The
examination of six solvents, such as dichloromethane, chloro-
form, toluene, n-hexane, ethylacetate, methanol, ethanol and
water, showed that protic solvent (in particularly water), pro-
moted the yield of the epoxide product. The possibility of using
water as a standard “green” reaction medium is a remarkable
advantage, especially due to the very low solubility of the
organic compounds in water, providing easy separation of the
products. Our results also showed that the yields were crucially
affected by the catalyst loading. The desired reaction rate and
conversion were achieved by using 0.50 mol% of the immobi-
lized catalyst (Table 1, entries 9–13). However, to furnish
favored yields of the oxidation product, higher temperatures up
to 70 °C should be used (Fig. 5), as established in a previous
report using hydrophobic copper(II) phthalocyanine.8d Under
these optimum conditions 1-methylcyclohexene converted com-
pletely to the related epoxide within 90 min.
Aqueous oxidation of olefins and saturated hydrocarbons
Preliminary experiments were applied to the oxidation of
1-methylcyclohexene (0.10 mmol) with TBAOX (0.30 mmol) in
neat water under air, which did not proceed in the absence of cat-
alyst at room temperature as well as at 70 °C. When a small
amount of CuPcS (5.00 ×10−4 mmol) was added to the reaction
mixture the conversion rate was enhanced significantly, and
1-methylcyclohexene oxide was produced in 94 and 100% yield
within 4 and 1.5 h at 25 and 70 °C, respectively. Nevertheless,
the recovery of this water-soluble catalyst from aqueous solution
was a time consuming process, actually making it out of recycled
material. Thus, we decided to immobilize CuPcS on the mag-
netic nanoparticles (CuPcS@ASMNP) and evaluate its catalytic
potential in the heterogeneous aqueous oxidation of
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Fig. 5 The effect of temperature on the aqueous oxidation of 1-methyl-
cyclohexene using TBAOX catalyzed by CuPcS@ASMNP. The reac-
tions were run under air for 90 min and the molar ratio of olefin–
TBAOX–catalyst was 200 : 600 : 1.
Fig. 7 TEM image of CuPcS@ASMNP stirred in an aqueous solution
of TBAOX at 70 °C.
activity of the title catalyst in the aqueous solution of other oxi-
dants is probably caused by the low dispersity of the catalyst
under these conditions, as can be seen in the TEM images
(Fig. S1 in ESI†). To confirm the effect of the dispersity degree
on the catalytic activity of CuPcS@SMNP, oxidation of
1-methylcyclohexene under different conditions was evaluated
using an aqueous solution of TBAOX at 50 °C. With magnetic
stirring, a 40% epoxide yield was observed within 90 min. In the
second experiment, an aqueous mixture of the catalyst was
initially sonicated for 10 min. After that, TBAOX was added and
then the reaction mixture was stirred magnetically, which led to a
75% epoxide yield in the same amount of time. When ultrasoni-
cation was used as the stirring method, a significant increase in
the epoxide yield was observed (100%). However, the thorough
mixing of reactants and the production of hot spots should also
be taken into account when enhancing the reaction rate by ultra-
sonic irradiation.15
As explored in previous studies, the addition of additives (10
times vs. catalyst), [such as imidazole (84%, 80 min), pyridine
(76%, 60 min), and sodium dodecyl sulfate under CMC con-
ditions (0.008 mol L−1) (80%, 60 min)], the use of standard buf-
fered solutions (76, 72, 83, 75, and 78%, at pH = 2, 4, 7, 9, 13
within 80 min), and also performing the reaction in an inert
atmosphere (Ar, 83 and 98% after 80 and 90 min) did not
improve the yield of epoxidation product.
Having proved the activity and stability of CuPcS@ASMNP
in the model reaction, the scope of the protocol was sub-
sequently extended to a range of olefins, as shown in Table 2.
Excellent conversion and selectivity to the epoxide were gener-
ally obtained within 90 min (entries 1–10, Table 2). The system
led to complete conversion of cyclooctene, norbornene and
cyclohexenes with the formation of the corresponding epoxides
as the sole products (entries 1–4, Table 2). A notable feature of
this oxidation system is that a moderate epoxide yield is obtained
in the oxidation of electron-poor 2-cyclohexene-1-one (40%,
entry 5, Table 2), confirming the superiority of the supported
catalyst compared to the free one (19 and 15% epoxide yield for
Fig. 6 The oxidation of 1-methyl cyclohexene catalyzed by CuPcS@
ASMNP with different oxidants. The reactions were run under air
for 90 min and the molar ratio of olefin–TBAOX–catalyst was
200 : 600 : 1.
To evaluate the oxidizing potential of other common oxidants,
1-methyl cyclohexene was subjected to the oxidation protocol
using O2, H2O2, t-butylhydroperoxide, PhIO, and Oxone® under
the catalytic influence of CuPcS@ASMNP (Fig. 6). Only trace
amounts of the oxidation products were observed. Nevertheless,
the addition of an equimolar amount of n-Bu4NHSO4
(TBAHSO4) to Oxone® improved the oxidation rate and a 40%
yield of the epoxide was obtained within 90 min. When the reac-
tion was carried out using TBAOX, full conversion of olefin was
observed in the same amount of time.
The good dispersity of CuPcS@SMNP in the aqueous solu-
tion of TBAOX (Fig. 7) seems to be the most important factor
that affects the efficiency. TBAOX, as a quaternary salt, acts as a
stabilizer which improves the dispersibility of the catalyst, as
described for the enhanced catalytic activity of unsupported
pencil-like hydrophobic CuPc.8d,14 Accordingly, the lower
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Table 2 Aqueous oxygenation of hydrocarbons using TBAOX
catalyzed by CuPcS@ASMNPa
CuPc and CuPcS, respectively). The chemoselectivity of the pro-
cedure was impressive. While, primary alcohols containing C–C
double bonds (allylic and homoallylic alcohols) were oxidized
completely to the corresponding epoxides under the influence of
this catalytic system (entries 6 and 7, Table 2), secondary
α, β-unsaturated alcohols were converted to the related ketones
as the sole product (entry 8, Table 2).8c,d
Yield
%
b
Entry Substrate
1
Productc
1H NMR δ(ppm)
96
2.9d
2.9d
However, the oxidation of styrenes gave exclusively the
related carbonyl compounds, indicating that the epoxides
undergo ring-opening reactions (entries 9 and 10, Table 2). It
seems that benzaldehyde and acetophenone are favorably formed
at high temperatures in the oxidation of styrene and α-methyl-
styrene, respectively, because the high temperature will supply
enough energy to break the CvC bond.16 These results are
further supported by the oxidation of styrene oxide as a substrate
under the same conditions. Benzaldehyde was formed as a sole
product in 100% yield (GC) within 90 min. It is worth mention-
ing that, when free CuPc was used under the same conditions,
68 and 100% epoxide selectivity was observed for the oxidation
of styrene and α-methylstyrene, respectively.8d Thus, it seems
that the attachment of CuPc to the magnetic solid increased the
rate of ring-opening of the epoxide. It should be noted that in
all the above mentioned reactions, no phenylacetaldehyde
product was observed in the oxidation of either styrene or
styrene oxide.
2
92
3
4
5
93
89
40
3.1d
2.9d
2.8, 3.4d
6
95
2.8d
7
8
92
80
2.5d
2–2.4, 6, 7.1
When a saturated benzylic C–H bond, such as indane, was
subjected to the oxidation protocol using CuPcS@ASMNP at
70 °C, no oxidation product was observed (Table S1 in ESI†).
However, an increase in the catalyst loading (up to 1 mol%) and
in the temperature (up to 80 °C) enhanced remarkably the con-
version rate and a yield of 90% indanone was secured,
accompanied by a small percentage of the related alcohol (6%)
within 2.5 h (entry 11, Table 2). Under these conditions, other
benzylic hydrocarbons can be functionalized in moderate/good
yields (58–85%) with the formation of the corresponding
ketones as the main products (entries 12–13, Table 2). The selec-
tive oxidation of inactive cyclooctane to the cyclooctanone in
12% yield is a promising result for the title oxidation system
(entry 14, Table 2).
9
90
97
7.5–7.9, 10
10
2.6, 7.3–7.6, 7.9
11
12
90
85
2.7, 3.1, 7.4–7.7
2.6, 7.3–7.6, 7.9
Aqueous oxidation of sulfides
13
14
58
12
2.1, 2.6–2.9,
7.2–7.5, 8
Encouraged by the promising results obtained in the oxidation of
hydrocarbons, the oxidation activity of this clean catalytic
system towards organic sulfides was evaluated. Knowing that the
oxidant has the ability to trigger sulfide oxidation, we performed
the blank experiment using 1.00 mmol thioanisole (0.10 g) and
3.00 mmol of TBAOX (1.00 g) in neat water at 25 °C under air,
which afforded a mixture of the related sulfoxide (31%) and
sulfone (39%) after 1 h, according to GC analysis. Nevertheless,
the addition of a catalytic amount of CuPcS@ASMNP
(0.50 mol%) to the aqueous mixture led to complete conversion
of thioanisole within 1 h, with the formation of the correspond-
ing sulfone as the sole product under the same conditions (pro-
cedure A). Our attempts to control the reaction for the selective
and high yielding production of sulfoxide in neat water were
unsuccessful (Table S2 in ESI†). Nevertheless, the addition of
ethanol to the aqueous mixture (water–ethanol: 1 : 2 v/v) limited
1.2–1.9, 2.4
a The reactions were run under air for 90 min at 70 °C for olefins and
150 min at 80 °C for saturated hydrocarbons. The molar ratio of
substrate–TBAOX–catalyst was 200 : 600 : 1 for olefins (entries 1–10)
and 100 : 300 : 1 for saturated hydrocarbons (entries 11–14). b Yield of
isolated products. c The products were identified by 1H NMR,8a,d,17 or
by comparison with the retention times of authentic samples of GC
analysis. The selectivity of the products were >99%, except for entries
11–13, which were 94, 93 and 93 respectively. d δ (ppm) of C–H oxiran
ring.
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significantly the overoxidation process and PhSOMe was
obtained in excellent yield and selectivity (>98%) using one
equivalent of TBAOX at room temperature (procedure B) (Fig. 8
and also Table S2 in ESI†).
Next, different sulfides were subjected to the reaction systems
(methods A and B ) in the presence of CuPcS@ASMNP and
the results are listed in Table 3. All substrates could be smoothly
converted to sulfoxides and sulfones with high/excellent yields,
and excellent selectivities were obtained under mild conditions
(Table 3). A salient feature of the present oxidation system is its
excellent selectivity in both procedures. Sulfoxides and sulfones
could be nearly stoichiometrically produced and only a very
small percentages of by-product (the related sulfoxide and
sulfone) was detected, which makes the title method a good
alternative for sulfoxide and sulfone production. Also sulfides
possessing a benzylic C–H bond (entry 3, Table 3), carbon–
carbon double bond (entries 4–6, Table 3), and hydroxyl group
(entry 8, Table 3) were cleanly transformed into the correspond-
ing sulfoxides and sulfones in excellent yield without the for-
mation of any epoxide, alcohol and carbonyl by-products.8c
Recovery of the catalyst
Recovery of CuPcS@ASMNP catalyst was easy and efficient.
When the magnetic stirring was stopped, the catalyst absorbed
onto the magnetic stirring bar. The catalyst was recovered by
decantation of the reaction mixture in the presence of an external
magnet. It was then washed with ethylacetate or ethanol as safe
solvents, dried under vacuum, and used directly for the next
round of reaction without further purification. The ease of recov-
ery, combined with the intrinsic stability of the silica-protected
Fe3O4 nanoparticle component, allows the catalyst to be recov-
ered efficiently over at least seven times in the oxidation of
different kinds of substrates used in this study (Fig. 9). Compari-
son of the IR spectra (Fig. 10) and TEM images of the used
CuPcS@ASMNP (Fig. 11) with fresh catalyst showed that the
Fig. 8 The effect of ethanol on the oxidation of thioanisole using an
aqueous solution of TBAOX catalyzed by CuPcS@ASMNP. The reac-
tions were run under air at r.t. for 60 min and the molar ratio of olefin–
TBAOX–catalyst was 200 : 200 : 1.
Table 3 Oxidation of various sulfides by TBAOX catalyzed by CuPcS@ASMNP at room temperature in water
Entry
Sulfide
Method Aa
Method Bb
Sulfoxide product
Sulfone product
Yieldc (selectivity)d [1H NMR (δppm)]
1
2
3
97 (100) [3, 7.6–7.9]
90 (>98) [7.5–7.9]
70 (90) [4.35, 6.9–8]
92 (100) [2.7, 7.4–7.6]
63 (90) [7.4–7.6]
80 (100) [4.3, 7.5–7.7, 8.1]
4
5
95 (100) [5.8–6.9, 7.5–7.9]
85 (96) [3.8, 4.9–6, 7.3–7.9]
70 (100) [5.8–6.7, 7.4–7.6]
76 (100) [3.53, 5.2–5.6, 7.4–7.6]
6
7
8
89 (100) [6.1–6.6]
96 (96) [5.9–6.4]
90 (100) [3.2, 1.8, 1.5, 0.96]
90 (94) [3, 3.7, 4.7, 7.8]
82 (100) [0.94, 1.45, 1.7, 2.9]
95 (93) [2.7, 3.7, 4.6, 7.5]
a The reactions were run under air at room temperature in water after 60 min with a molar ratio of 200 : 600 : 1 for sulfide–TBAOX–catalyst. b The
reactions were run under air at room temperature in the mixture of water–ethanol (1 : 2) after 90 min with molar ratio of 200 : 200 : 1 for sulfide–
TBAOX–catalyst. c The yield of isolated products. The products were identified by comparison with authentic samples.18 d The remainders are the
corresponding sulfoxides and sulfones by-products.
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Fig. 9 Recycling of the catalytic system for the oxidation of thio-
anisole, 1-methylcyclohexene and indane in water according to the pro-
cedures mentioned in Tables 2 and 3.
Fig. 11 TEM image of CuPcS@ASMNP after 7 reuses.
synthesized using an anionic CuPcS complex anchored electro-
statically on functionalized silica coated magnetic nanoparticles.
The epoxidation of olefins and the oxidation of saturated hydro-
carbons to the related ketones with excellent selectivity using an
aqueous solution of TBAOX were efficiently enhanced in the
presence of a low catalyst loading of the title nanocatalyst. More-
over, sulfides converted selectively to the sulfones in neat water
and to the sulfoxides in a water–ethanol mixture using this
heterogeneous oxidation system. No surfactants, additives, toxic
reagents or solvents, and by-products were involved and no
laborious purification was necessary. These conditions, along
with the use of water as a standard “green” solvent, as well as
the easy and safe work-up procedure, reusability of catalyst, and
lack of by-products, makes the title methodology a cost effective,
environmentally benign system that possesses high generality,
which makes it suitable for industrial goals.
Fig. 10 FT-IR spectra of fresh CuPcS@ASMNP (A) and after 4 (B)
and 7 (C) reuses.
structure and morphology of the catalyst remained almost intact
after seven recoveries. Furthermore, the recovered heterogeneous
catalyst was found to contain 6.60% wt% of Cu, which indicates
less than 1.00% leaching during the reaction cycles (compared
with 6.65 wt% according to ICP analysis) confirming that the
complex was firmly anchored to the SMNP support. Moreover,
the by-product of the oxidant (TBAHSO4) was separated by lyo-
philizing the aqueous phase (>90%), and was reused in the prep-
aration of TBAOX. Therefore, the title methodology is cost
effective and environmentally benign due to the use of water as
a reaction medium, reuse of the catalyst and oxidant, easy iso-
lation of the hydrophobic organic products and lack of a need for
surfactants, nitrogen donors, toxic reagents or solvents. These
advantages of this high yielding oxidation method offered ready
scalability. For example the use of a semi scale-up procedure
(10.00 mmol) for the epoxidation of 1-methylcyclohexene in the
presence of CuPcS@ASMNP led to the isolation of the related
epoxide in 91% yield.
Experimental section
General remarks
Conversion and yields of the products were accomplished by
GC-FID on a Shimadzu GC-16A instrument using a 25-m
CBP1-S25 (0.32 mm ID, 0.5 μm coating) capillary column. IR
spectra were recorded on a Perkin Elmer 780 instrument. UV-Vis
spectra were recorded on a 160 Shimadzu spectrophotometer.
NMR spectra were recorded in CDCl3 solutions with a Brucker
Avance DPX FT-NMR 250 MHz instrument. The residual
CHCl3 in conventional 99.8 at% CDCl3 gives a signal at δ =
7.26 ppm, which was used for calibration of the chemical shift scale.
Mass spectra were recorded on a Shimadzu GC-MS-QP5050A.
SEM micrographs were obtained by SEM instrumentation (SEM,
XL-30 FEG SEM, Philips, at 20 kV), TEM images were obtained
by TEM instrumentation (Philips CM 10). X-ray diffraction (XRD,
D8, Advance, Bruker, AXS), TGA (Shimadzu 50) and ICP (Varian,
visa-pro) were employed for characterization of the heterogeneous
catalyst. The Cu(II) phthalocyanine-tetrasulfonic acid tetrasodium
complex (CuPcS) was purchased from Fluka chemical company.
Conclusion
In summary, a novel magnetically recoverable nanocatalyst
(<10.00 nm) with high thermostability (>250 °C) was
3392 | Green Chem., 2012, 14, 3386–3394
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The freshly prepared n-Bu4NHSO5,19 was a much stronger oxidant
than commercially available samples.
2.00 mL) to the reaction flask. The organic phase was separated
and dried with sodium sulfate. The solvent was removed under
reduced pressure to give the corresponding compound (Table 2)
and the separated catalyst was washed with EtOH, H2O, and
used directly for a subsequent round of reaction without further
purification. The purity of the products, analyzed by GC, was
found to be more than 99% without any chromatographic
purification.
Large-scale synthesis of nanomagnetite particles (Fe3O4)
A solution of FeCl2 (5.40 g) and FeCl3 (2.00 g) in aqueous
hydrochloric acid (2.00 M, 25.00 mL) at room temperature, was
sonicated until the salts dissolved completely. Aqueous ammonia
(25%, 40.00 mL) was added slowly over 20 min to the mixture
under an Ar atmosphere at room temperature followed by stirring
about 30 min with mechanical stirrer. The Fe3O4 nanoparticles
were separated by an external magnet and washed three times
with deionized water and ethanol. The final product was
obtained after drying under vacuum. Also it could be re-
suspended in ethanol and stored in the refrigerator for future use.9b
General procedure for the oxygenation of saturated
hydrocarbons
To a mixture of saturated hydrocarbon (1.00 mmol), CuPcS@
ASMNP (0.14 g, 0.01 mmol), in bidistilled water at 80 °C
was added freshly prepared TBAOX (3.00 mmol, 1.00 g). After
completion of the reaction, the catalyst was separated by an
external magnet followed by the addition of ethyl acetate (3 ×
2.00 mL) to the reaction flask. The organic phase was separated
and dried with sodium sulfate. The solvent was removed under
reduced pressure to give the corresponding compound (Table 2)
and the separated catalyst was washed with EtOH, H2O, and
used directly for a subsequent round of reaction without further
purification. The purity of cyclooctanone was determined to be
more than 98% without any chromatographic purification,
according to GC analysis. For other carbonyl products, further
purification was performed by silica chromatography eluted with
n-hexane–ethyl acetate (10 : 2).
Synthesis of silica-coated magnetite nanoparticles (SMNP)
The synthesized Fe3O4 was suspended in 35.00 mL ethanol and
6.00 mL deionized water and sonicated for 15 min. 1.50 mL of
tetraethyl orthosilicate (TEOS) was added slowly to the mixture,
which was then sonicated for 10 min. Aqueous ammonia (10%,
1.40 mL) was then added slowly over 10 min under mechanical
stirrer. The mixture was heated at 40 °C for 12 h. The iron oxide
nanoparticles with a thin layer of silica (Fe3O4@SiO2, SMNP)
were separated by an external magnet and washed three times
with ethanol and were then dried under vacuum.9b
Silanation of silica-coated magnetite nanoparticles (ASMNP)
General procedure for the oxidation of sulfides to sulfones
10.00 g of dry SMNP powder was mixed with 200.00 mL of
toluene to produce a homogeneously mixed solution, followed
by sonication of the mixture for 30 min. Then 2.50 mL of
(3-aminopropyl) triethoxysilane (APS) was added under mech-
anical stirring and the mixture was slowly heated to 105 °C and
kept at this temperature for 20 h. The final sample (ASMNP)
was separated by an external magnet and washed three times
with methanol or ethanol and dried under vacuum. The concen-
trated product was stored in a refrigerator for use later.9b
To a mixture of sulfide (1.00 mmol), CuPcS@ASMNP (0.07 g,
5.00 × 10−3 mmol), in bidistilled water at 25 °C was added
freshly prepared TBAOX (3.00 mmol, 1.00 g). After completion
of the reactions, the catalyst was separated by an external magnet
followed by the addition of ethyl acetate (3 × 2.00 ml) to the
reaction flask. The organic phase was separated and dried with
sodium sulfate. The solvent was removed under reduced pressure
to give the corresponding compounds (Table 3, method A) and
the separated catalyst was washed with EtOH, H2O, and used
directly for a subsequent round of reaction without further purifi-
cation. The purity of products was more than 90% without any
chromatographic purification according to GC analysis. Further
purification was performed by silica chromatography eluted with
n-hexane–ethyl acetate (10 : 1).
Synthesis of magnetite nanoparticle-supported CuPcS
(CuPcS@ASMNP)
4.00 g of dry ASMNP powder was mixed with 200.00 mL water
to produce a homogeneously mixed solution, followed by soni-
cation of the mixture for 40 min. 0.20 g CuPcS was added under
mechanical stirring and the mixture was heated to 90 °C for 7 h.
The unsupported phthalocyanines were removed through Soxhlet
extraction with water. The concentrated product stored in a
refrigerator to use.
General procedure for oxidation of sulfides to sulfoxides
To a mixture of sulfide (1.00 mmol), CuPcS@ASMNP (0.07 g,
5.00 × 10−3 mmol), in the mixture of water–ethanol (2.00 ml,
1 : 2), at 25 °C was added freshly prepared TBAOX (2.00 mmol,
0.70 g). After completion of the reactions, the catalyst was separ-
ated by an external magnet followed by removing the ethanol
under reduced pressure. After the addition of ethyl acetate (3 ×
2.00 mL) to the reaction flask, the organic phase was separated
and dried with sodium sulfate. The solvent was removed under
reduced pressure to give the corresponding compounds (Table 3,
method B) and the separated catalyst washed with EtOH, H2O,
and used directly for a subsequent round of reaction without
General procedure for the epoxidation of olefins
To a mixture of olefin (1.00 mmol) and CuPcS@ASMNP
(0.07 g, 5.00 × 10−3 mmol) in bidistilled water (2.00 mL) at
70 °C was added freshly prepared TBAOX (3.00 mmol, 1.00 g).
After completion of the reaction, the catalyst was separated by an
external magnet followed by the addition of ethyl acetate (3 ×
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without any chromatographic purification according to GC
analysis. Further purification was performed by silica chromato-
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Recycling procedure
After completion of the oxidation reaction, the catalyst was sep-
arated by an external magnet followed by washing with EtOH
and H2O. The catalyst was then used directly for next round of
reaction without further purification. The aqueous phase was lyo-
philized to obtain the reduced form of the oxidant (TBAHSO4)
which can be reused in the preparation of TBAOX.
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Acknowledgements
Support for this work by the Research Council of the University
of Birjand is highly appreciated.
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This journal is © The Royal Society of Chemistry 2012