Nanomagnet-supported partially brominated manganese-porphyrin as a promising catalyst for the selective heterogeneous oxidation of hydrocarbons and sulfides in water

Article abstract of DOI:10.1002/ejic.201200753

A novel, magnetically recoverable porphyrin catalyst has been prepared by the coordinative anchoring of the [Mn(Br₂TPP)OAc] complex on silica-coated magnetic nanoparticles (Fe₃O₄@SiO ₂, SMNP) through an amine functionality. The heterogeneous catalyst was characterized by powder X-ray diffraction, transmision electron microscopy, scanning electron microscopy, and FTIR spectroscopy. The catalyst loading on the magnetic support was determined by thermogravimetric (TG) analysis and inductively coupled plasma atomic emission spectroscopy. The catalyst nanoparticles were estimated to have a diameter of less than 10 nm. The TG curve demonstrated that the nanocatalyst was thermally stable up to almost 350 °C, exhibiting high thermostability. The epoxidation of olefins and the oxidation of saturated hydrocarbons to the corresponding ketones as well as sulfides to sulfones in an aqueous solution of tetra-n-butylammonium peroxomonosulfate (n-Bu₄NHSO₅, TBAOX) were efficiently enhanced with excellent selectivity under the influence of the magnetically separable catalyst. Sulfoxides could also be selectively produced in a mixture of water and ethanol, which makes the title methodology a good alternative for both sulfoxide and sulfone production. The separation and recycling of the catalyst and the byproduct of the oxidant were simple, effective, and economic in this clean oxidation method. FTIR and leaching experiments after seven successive cycles showed that the catalyst was very strongly anchored to the SMNP support. Copyright

Full text of DOI:10.1002/ejic.201200753

Job/Unit: I20753
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Date: 27-09-12 10:10:35
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FULL PAPER
DOI: 10.1002/ejic.201200753
Nanomagnet-Supported Partially Brominated Manganese–Porphyrin as a
Promising Catalyst for the Selective Heterogeneous Oxidation of
Hydrocarbons and Sulfides in Water
[a]
[a]
[a]
[a]
Abdolreza Rezaeifard,* Maasoumeh Jafarpour,* Parisa Farshid, and Atena Naeimi
Keywords: Nanostructures / Nanoparticles / Magnetic properties / Porphyrinoids / Green chemistry / Oxygenation / Sulfur
oxides / Heterogeneous catalysis
A novel, magnetically recoverable porphyrin catalyst has
been prepared by the coordinative anchoring of the
rated hydrocarbons to the corresponding ketones as well as
sulfides to sulfones in an aqueous solution of tetra-n-butyl-
[Mn(Br
2 4 5
TPP)OAc] complex on silica-coated magnetic nano- ammonium peroxomonosulfate (n-Bu NHSO , TBAOX) were
particles (Fe
3
O
4
@SiO
2
, SMNP) through an amine function-
efficiently enhanced with excellent selectivity under the in-
fluence of the magnetically separable catalyst. Sulfoxides
could also be selectively produced in a mixture of water and
ethanol, which makes the title methodology a good alterna-
tive for both sulfoxide and sulfone production. The separa-
tion and recycling of the catalyst and the byproduct of the
oxidant were simple, effective, and economic in this clean
oxidation method. FTIR and leaching experiments after
seven successive cycles showed that the catalyst was very
strongly anchored to the SMNP support.
ality. The heterogeneous catalyst was characterized by pow-
der X-ray diffraction, transmision electron microscopy, scan-
ning electron microscopy, and FTIR spectroscopy. The cata-
lyst loading on the magnetic support was determined by
thermogravimetric (TG) analysis and inductively coupled
plasma atomic emission spectroscopy. The catalyst nanopar-
ticles were estimated to have a diameter of less than 10 nm.
The TG curve demonstrated that the nanocatalyst was ther-
mally stable up to almost 350 °C, exhibiting high thermosta-
bility. The epoxidation of olefins and the oxidation of satu-
Introduction
compared with homogeneous systems.[4] To overcome this
limitation, the size of the support particles should be kept
as small as possible providing nanoparticles with a high ex-
ternal surface area. Consequently, a high loading of catalyt-
ically active sites can be obtained and diffusion will no
longer limit the kinetics. Accordingly, nanoparticles have re-
cently emerged as efficient alternatives for the immobiliza-
Problems often encountered in oxidation reactions cata-
lyzed by metal–organic complexes are the oxidative degra-
dation of ligands, difficulties in separating the products, and
contamination by residual catalyst.[ In the age of “green
chemistry”, heterogeneous reactions have become an im-
portant objective as these processes are used in industry
and help to minimize the problems of industrial disposal
and waste treatment.[ For practical applications of hetero-
geneous systems, the lifetime of a catalyst and its recovery
and reusability are very important factors. Considerable ef-
fort has been devoted to the heterogenization of porphyrin
complexes as often-used biorelevant catalysts within dif-
ferent organic and inorganic supports to enhance their
chemical stability and allow catalyst recovery by simple fil-
1]
[
5]
tion of homogeneous catalysts. Nevertheless, difficulties in
recovering the nanometer-sized particles from the reaction
mixture as a result of their effective dispersion in solution
through the formation of stable suspensions has severely
2]
[
6]
limited their widespread applications. There is currently
intense interest in the use of magnetic nanoparticles (such
as Fe O ) for a wide range of biomedical and technological
3
4
applications. These nanoparticles are coated with metal cat-
alysts or conjugated with enzymes to combine the separat-
ing power of the magnetic properties with the catalytic ac-
[
3]
tration. However, due to the diffusion of substrates and
products through the pores of the support materials, a sub-
stantial decrease in the reaction rate is frequently observed
[
7]
tivity of the metal surface or enzyme conjugate. For exam-
ple, horseradish peroxidase (HRP)-entrapped magnetic
nanoparticles have been used for biocatalysis and biosepar-
[
a] Catalysis Research Laboratory, Department of Chemistry,
Faculty of Science, University of Birjand,
Birjand 97179-414, Iran
[
8]
ation. These dual-functional nanoparticles are composed
of two parts: Their cores provide a magnetic function and
their shells allow catalysis.
Fax: +98-561-2502515
E-mail: rrezaeifard@gmail.com
rrezaeifard@birjand.ac.ir
mjafarpour@birjand.ac.ir
Furthermore, a recoverable catalyst requires a sustain-
able reaction media for application in scale-up procedures
for practical purposes. Discarding harmful organic solvents
Homepage: www.birjand.ac.ir
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201200753.
Eur. J. Inorg. Chem. 0000, 0–0
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A. Rezaeifard, M. Jafarpour, P. Farshid, A. Naeimi
FULL PAPER
is the major problem in the chemical industry, accounting oxidant (nBu NHSO ) were easily recovered and reused in
4
4
for around 80% of their waste. Thus, a new challenge is to the procedure.
make innovative “clean” methods by using nontoxic sol-
vents, in particular, aqueous media.[ Moreover, it has been
found that reactions in water can facilitate access to dif-
ferent reactivity and selectivity patterns compared with
those observed in common organic solvents due to its
unique physical and chemical properties.[ In this regard,
the use of water as a reaction solvent has attracted great
attention in the recent past and has become an active area
of research in “green” chemistry.[
9]
10]
9]
Following the initial promising report on olefin epoxid-
ation and C–H bond hydroxylation with tetra-n-butylam-
monium peroxomonosulfate (nBu NHSO , TBAOX) cata-
4
Ͼ5
lyzed by [Mn(TPP)OAc] in dichloromethane,[ various or-
11]
Scheme 1. Aqueous oxygenation of hydrocarbons and sulfides cata-
lyzed by magnetically recoverable [Mn(Br TPP)OAc@SMNP].
2
ganic substrates have been successfully subjected to oxi-
dation with this versatile oxidant.[
12]
Nevertheless, sup-
ported porphyrins have virtually shown no catalytic activity
in this oxidation system to date. For example, in the pres-
ence of robust sulfonated manganese and iron porphyrins
supported on poly(viny1pyridinium) polymers, the slope of
the conversion of cyclooctene in the epoxidation reaction is
the same as that of the noncatalyzed reaction after 2 h of
reaction at room temperature.[ Very recently we devel-
Results and Discussion
Preparation and Characterization of [Mn(Br TPP)
OAc@SMNP]
2
13]
We used magnetic nanoparticles (MNP) with a diameter
oped a novel heterogeneous strategy for the oxidation of of approximately 10 nm, which were prepared by the co-
[
15]
olefins, hydrocarbons, alcohols, and sulfides in aqueous precipitation method.
The particles were subsequently
III
III
solution by using Fe – and Mn –porphyrins and cop- coated with a dense silica layer by using tetraethoxysilane
per(II)–phthalocyanine as water-insoluble catalysts in the (TEOS) as the silica source and ammonia solution as the
[
14]
absence of any organic co-solvents and surfactants.
To hydrolyzing agent. The silica-layered system (SMNP) offers
benefit the easy and complete recovery of the catalyst in binding sites (Si–OH units) for the heterogenization of the
this new aqueous oxidation system we decided to immobi- molecular catalysts. For grafting on to the surface we used
lize the porphyrin catalyst on nanosized Fe O as magnetic (3-aminopropyl)triethoxysilane (APS) as chelating li-
3
4
[
15b]
nanoparticles (MNP) coated with a silica layer (SMNP). gand.
The modified SMNP was then treated with
The epoxidation of olefins and oxygenation of saturated hy- [Mn(Br TPP)OAc] in ethanol to yield the corresponding
2
drocarbons to ketones as well as the selective oxidation of magnetically
sulfides to both sulfoxides and sulfones were enhanced by OAc@SMNP] (Scheme 2).
using [Mn(Br TPP)OAc@SMNP] as a novel, magnetically The FTIR spectra of MNP, SMNP, ASMNP, and
recoverable
catalyst
[Mn(Br TPP)-
2
2
recoverable nanocatalyst in an aqueous solution of TBAOX [Mn(Br TPP)OAc@SMNP] (as KBr pellets) are presented
2
in the absence of surfactants, organic co-solvents, and co- in Figure 1. The presence of the magnetite core in the pre-
catalysts (Scheme 1). The catalyst and reduced form of the pared nanomaterials was confirmed by observation of two
2
Scheme 2. Preparation of SMNP, ASMNP, and [Mn(Br TPP)OAc@SMNP].
2
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Nanomagnet-Supported Manganese–Porphyrin as Catalyst
bands at around 430–600 cm (Figure 1, A).[7f] The broad
–
1
–
1
peak at about 900–1200 cm assigned to Si–O–Si and Si–
OH stretching vibrations show the silica coating of the mag-
netite nanoparticles (Figure 1, B). The anchoring of ami-
nopropyl groups on SMNP was confirmed by stretching
–
1
vibrations at about 2800–3000 and 3465 cm (Figure 1,
C).[
15]
In the FTIR spectrum of [Mn(Br TPP)
2
–
1
OAc@SMNP] (Figure 1, D) the band at 1045.7 cm was
assigned to the deformation vibration of the pyrrole frag-
ments of the porphyrin molecule. The stretching vibration
band of the phenyl rings in the meso positions appears at
–
1 [16]
1
602.7 cm .
Figure 2. XRD patterns of A) MNP and B) [Mn(Br
OAc@SMNP].
2
TPP)-
Figure 3A shows the transmission electron microscopy
(
TEM) image of [Mn(Br TPP)OAc@SMNP] in water,
2
which further confirms the assembly of the catalyst and a
size of less than 10 nm (including the SiO shell and por-
2
phyrin complex). The SEM image of the supported por-
phyrin catalyst shows the uniformity and spherical mor-
phology of the nanoparticles with an average size of 15 nm
(Figure 3B). Nevertheless, aggregation leads to an increase
in the size of observed nanoparticles, as can be seen in some
[
7e]
parts of the SEM image.
Figure 1. FTIR spectra of A) MNP, B) SMNP, C) ASMNP, and
D) [Mn(Br TPP)OAc@SMNP].
2
The crystalline structure of the MNP was determined to
have a cubic structure by XRD (Figure 2, A). The same set
of characteristic peaks was observed for [Mn(Br TPP)-
2
OAc@SMNP] (Figure 2, B), which indicates the stability of
the crystalline phase of the nanoparticles during the subse-
quent surface modification.[ The particles were estimated
to be 8.09 and 8.95 nm in diameter for MNP and
17]
[
Mn(Br TPP)OAc@SMNP], respectively, determined by
2
applying the Debye–Scherrer equation to the (311) reflec-
tions.
Figure 3. A) TEM and B) SEM images of [Mn(Br
OAc@SMNP].
2
TPP)-
3
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A. Rezaeifard, M. Jafarpour, P. Farshid, A. Naeimi
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The thermal behavior of [Mn(Br TPP)OAc@SMNP] organic/inorganic nanomaterial in heterogeneous catalysis
2
(Figure 4) shows a weight loss at about 132 °C related to reactions that have virtually no catalytic activity in com-
desorption of water molecules from the catalyst surface. monly used organic solvents with metalloporphyrin systems
The thermal gravimetric (TG) curve demonstrates the de- (Table 1, entries 1–7). The epoxidation of 1-methylcyclohex-
gradation of [Mn(Br TPP)OAc@SMNP] at 350 °C, which ene (1 mmol) as a model olefin with 2 equiv. of TBAOX
2
indicates the high thermostability of the catalyst. The or- (0.7 g) in six solvents, namely dichloromethane, chloroform,
ganic parts decomposed completely at 511 °C. The final methanol, ethanol, and water, showed that protic solvents,
weight loss at 700 °C can be related to the formation of and especially water, significantly enhanced the yield of the
metal oxides. From the TGA, the amount of [Mn(Br TPP)- epoxide product. The possibility of using water as a stan-
2
OAc] was evaluated to be 4.20% (w/w), which is in agree- dard “green” reaction medium is a remarkable advantage
ment with the value determined by inductively coupled especially because the very low solubility of organic com-
plasma atomic emission spectroscopy (ICP-AES; 4.60%). pounds in water allows easy separation of the products.
Therefore each gram of heterogeneous catalyst contains
–
1
Table 1. Screening of solvents and catalyst loading in the oxidation
of 1-methylcyclohexene with TBAOX in aqueous solution.
4
3 μmolg of Mn or the [Mn(Br TPP)OAc] complex.
2
[
a]
Entry Solvent
Catalyst
Time
[min]
1-Methylcyclohex-
ane oxide [%]
[
b]
[mol-%]
1
2
3
4
5
6
7
8
9
1
1
1
1
CH
CHCl
toluene
n-hexane
ethyl acetate
ethanol
2
Cl
3
2
1
1
1
1
1
1
1
0
45
45
120
120
120
45
45
600
45
45
45
40 (45)
40 (45)
15
17
1
2
1
83
50
30
25
53
70
methanol
H
2
H
2
H
2
H
2
H
2
H
2
O
O
O
O
O
O
0.2
0.3
0.4
0.5
1 to 5
0
1
2
3
[
c]
89 (100)
87–92 (100)
Figure 4. TGA diagram of [Mn(Br
2
TPP)OAc@SMNP].
[a] Reaction conditions: 0.5 mmol 1-methylcyclohexene, 1 mmol
TBAOX, 1 mL solvent, x mmol [Mn(Br TPP)OAc@SMNP] as cat-
Note that the black Fe O nanoparticles can be easily alyst. The reactions were performed at 70 °C except for those in
2
3
4
low b.p. solvents which were performed at reflux. [b] GC yield. [c]
oxidized to yellow Fe O when they are exposed directly to
2
3
95% isolated yield.
air and their magnetism decreases significantly. However,
after storage for six months, on exposure to air at room
temperature, the color and magnetic properties of the
Our results also showed that the yields were crucially af-
fected by catalyst loading (Table 1, entries 8–13). Blank ex-
periments proceeded sluggishly and gave a very low yield
of 1-methylcyclohexane oxide after a long period of time
[
Mn(Br TPP)OAc@SMNP] remained almost unchanged.
2
The coating of Fe O with a layer of SiO and [Mn(Br -
3
4
2
2
TPP)OAc] attached to the surface shields the nanomagnet-
ite surface from oxygen and greatly improves its stability.
(Table 1, entry 8). The desired conversion rate was achieved
[
18]
when 0.50 mol-% of [Mn(Br TPP)OAc@SMNP] was used
2
(Table 1, entry 12). This catalyst loading is less than that
used in reactions carried out in the presence of free por-
phyrin catalysts.
However, to obtain higher yields of oxidation products a
Aqueous Oxidation of Olefins and Saturated Hydrocarbons
[
14a,14b]
As a part of our ongoing research on the aqueous oxi-
dation activity of different hydrophobic iron– and manga- higher temperature up to 70 °C should be used (Figure 5)
nese–porphyrins using TBAOX, we explored the lower as established in previous reports using free catalysts.[
14]
epoxidation activity of [Mn(Br TPP)OAc] compared with Under these optimum conditions, 1-methylcyclohexene was
2
[
Mn(TPP)OAc], [Mn(TDCPP)OAc], [Mn(Br TPP)OAc], converted completely within 45 min and 95% of the corre-
3
and [Mn(Br TPP)OAc] as unsupported catalysts. For exam- sponding epoxide was secured after a simple extraction
4
ple, the epoxidation of 1-methylcyclohexene with 2 equiv. from the aqueous phase (Table 1, entry 12). Note that the
TBAOX catalyzed by [Mn(Br TPP)OAc] (1 mol-%) at 70 °C application of these conditions to the epoxidation of 1-
2
gave a 67% yield of epoxide compared with the yields of methylcyclohexene in the presence of 0.5 mol-% of free
Ͼ97% obtained with the other porphyrin complexes men- [Mn(Br TPP)OAc] gave only a 54% yield of epoxide under
2
tioned above under the same conditions. However, the im- the same conditions.
mobilization of [Mn(Br TPP)OAc] on SMNP enhanced its
To evaluate the oxidizing potential of other common oxi-
2
catalytic efficiency and full conversion of the olefinn was dants 1-methylcyclohexene was subjected to the oxidation
observed in water under heterogeneous conditions. This protocol with H O , tert-butyl hydroperoxide, PhIO,
2
®
2
promising result encouraged us to evaluate the catalytic po- NaIO , and Oxone as oxidants under the catalytic influ-
4
tential of [Mn(Br TPP)OAc@SMNP] as a novel magnetic ence of [Mn(Br TPP)OAc@SMNP] (Figure 6). Only trace
2
2
4
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Nanomagnet-Supported Manganese–Porphyrin as Catalyst
Having proved the activity and stability of [Mn(Br TPP)-
2
OAc@SMNP] in the model reaction, the scope of the pro-
tocol was subsequently extended to a range of olefins, as
shown in Table 2. Excellent conversions and selectivities to
epoxide were generally obtained within 45 min (entries 1–
10). Complete conversion of cyclooctene, norbornene, and
cyclohexenes was achieved with the formation of the corre-
sponding epoxides as the sole products (entries 1–4). A no-
table feature of the title oxidation system is the desired ep-
oxide yield obtained in the oxidation of electron-poor 2-
cyclohexen-1-one (entry 5, 60%), which confirms the supe-
riority of the supported catalyst over the free catalyst
(
29%). The chemoselectivity of the procedure was notable.
Whereas the primary alcohols containing C=C double
Figure 5. Effect of temperature on the epoxidation of 1-methylcy- bonds (allylic and homoallylic alcohols) were oxidized com-
2
clohexene by TBAOX catalyzed by [Mn(Br TPP)OAc@SMNP] in
aqueous solution. The reactions were performed in air for 45 min
and the molar ratio of olefin/TBAOX/catalyst was 200:400:1.
pletely to the corresponding epoxides (entries 6 and 7), the
secondary α,β-unsaturated alcohol was converted into the
corresponding ketone as the sole product (entry 8).[
14b]
In
addition, styrenes were oxidized to give exclusively the re-
lated carbonyl compounds, which indicates that the epox-
ides undergo ring-opening reactions (entries 9 and 10). It
would seem that benzaldehyde and acetophenone are favor-
ably formed at a high temperature in the oxidation of styr-
ene and α-methylstyrene, respectively, because sufficient en-
amounts of the oxidation products were observed, which
indicates the inability of the title catalyst to activate these
oxidants under these conditions. However, addition of an
®
equimolar amount of nBu NHSO (TBAHSO ) to Oxone ,
4
4
4
enhanced the oxidation rate and 45% yield of the corre-
sponding epoxide was obtained. Although the formation of
a new oxidant (TBAOX) under these conditions may be re-
sponsible for the increase in rate, the fact that quaternary
[19]
ergy is supplied to break the C=C bond. Note that when
free [Mn(Br TPP)OAc] was used under the same condi-
2
tions, yields of 20 and 47% of the epoxide products were
observed along with benzaldehyde and acetophenone in the
oxidation of styrene and α-methylstyrene, respectively.
Thus, it would seem that the attachment of Mn–porphyrin
to a magnetic solid increases the rate of ring-opening of
epoxides. These results are further supported by the oxi-
dation of styrene oxide as substrate under the same condi-
tions. Benzaldehyde was formed as the sole product in
yields of 76 and 100% (GC) within 45 min in the presence
of free and supported Mn-porphyrin, respectively. Note
that in all the above reactions, no phenylacetaldehyde prod-
uct was observed in the oxidation of either styrene and styr-
ene oxide.
TBAHSO acts as a stabilizer improving the dispersibility
4
of the catalyst should also be taken into account.[14d] When
the reaction was carried out with TBAOX, full conversion
of the olefin was observed.
When a saturated benzylic C–H bond such as indane was
subjected to the oxidation protocol with [Mn(Br TPP)-
2
OAc@SMNP] at 70 °C, no oxidation product was observed
(
see Table S1 in the Supporting Information). However, an
increase in the catalyst loading up to 1 mol-% together with
equiv. of TBAOX enhanced remarkably the conversion
rate and 71% indanone was secured along with a small per-
Figure 6. Oxidation of 1-methylcyclohexene with different oxidants centage of the related alcohol (6%) within 3 h (Table 2, en-
3
2
catalyzed by [Mn(Br TPP)OAc@SMNP]. The reactions were per-
formed in air at 70 °C for 45 min and the molar ratio of olefin/
oxidant/catalyst was 200:400:1.
try 11). Other benzylic hydrocarbons were oxidized under
these conditions with the formation of the corresponding
ketones as the main products in good-to-high yields (43 and
As found in previous studies, the addition of additives 61%; entries 12 and 13). The selective oxidation of inactive
additive/catalyst = 10:1) such as imidazole (80%/40 min) cyclooctane to cyclooctanone in 15% yield is a promising
(
and pyridine (69%/40 min) to the reaction mixture, the use result for the title oxidation system (entry 14).
of micellar solutions of sodium dodecyl sulfate at the CMC These results demonstrate well the significant catalytic
0.008 mol/L; 60%/40 min), and standard buffered solu- activity of the title magnetic nanocatalyst under the oxi-
tions (70, 69, 75, 80, and 61% at pH = 2, 4, 7, 9, 13 within dation conditions. The good dispersity of [Mn(Br TPP)
(
2
40 min) did not improve the yield of 1-methylcyclohexane OAc@SMNP] in aqueous solution (Figure 3, A) seems to
oxide.
be the most important factor for its catalytic efficiency. Ac-
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A. Rezaeifard, M. Jafarpour, P. Farshid, A. Naeimi
Table 2. Aqueous oxygenation of hydrocarbons with TBAOX cata- cordingly, the lower catalytic activity of the free catalyst
FULL PAPER
lyzed by [Mn(Br
2
TPP)OAc@SMNP].[
a]
compared with that of the supported catalyst can be greatly
affected by the high aggregation tendency of free
[
Mn(Br TPP)OAc] in aqueous media, as can be seen in the
2
TEM image (Figure 7). To confirm the effect of the degree
of dispersity on the catalytic activities of free and immobi-
lized catalysts, the oxidation of 1-methylcyclohexene under
different conditions was evaluated in aqueous solutions of
TBAOX at 50 °C. With magnetic stirring, yields of 35 and
59% of epoxide were observed within 45 min with free and
supported catalysts, respectively. In a second experiment,
aqueous solutions of each catalyst were initially sonicated
for 10 min. After that TBAOX was added and then the re-
action mixtures were stirred magnetically, which led to
yields of 42 and 83% of epoxide in the presence of free and
immobilized catalysts, respectively, for the same reaction
time. When ultrasound was used as the stirring method, a
significant increase in the epoxide yield was observed (75
and 100% yields with free and immobilized catalyst, respec-
tively). However, thorough mixing of the reactants and the
production of hot spots by ultrasonic irradiation that en-
hance the reaction rate should also be taken into ac-
[
21]
count.
2
Figure 7. TEM image of free [Mn(Br TPP)OAc] in water.
Aqueous Oxidation of Sulfides Catalyzed by [Mn(Br TPP)-
2
OAc@SMNP]
The promising results obtained in the oxidation of hydro-
carbons encouraged us to evaluate the oxidation activity of
this clean catalytic system towards organic sulfides. Know-
ing that the oxidant has the ability to trigger sulfide oxi-
dation, we performed a blank experiment with 1 mmol
thioanisole (0.1 g) and 3.0 mmol of TBAOX (1.0 g) in neat
water at 25 °C in air, which afforded a mixture of the related
sulfoxide (31%) and sulfone (39%) after 1 h according to
[
14c]
GC analysis.
However, the addition of a catalytic
[
a] The reactions were performed in air at 70 °C for 45 and 180 min
amount of [Mn(Br TPP)OAc@SMNP] (0.5 mol-%) to the
aqueous mixture led to complete conversion of thioanisole
2
for olefins and saturated hydrocarbons, respectively. The molar ra-
tio of substrate/TBAOX/catalyst was 200:400:1 for olefins (en-
tries 1–10) and 100:300:1 for saturated hydrocarbons (entries 11– within 45 min with the formation of the corresponding sulf-
1
4). [b] Yield of isolated products. [c] The products were identified
one as the sole product under the same conditions
Method A, Table 3). Note that when the catalyst was re-
placed by the same quantity of unsupported [Mn(Br TPP)-
1
[14,20]
by H NMR spectroscopy
retention times of authentic samples in GC analysis. The selectivity
of the reactions was Ͼ99% except for entries 11–13, which
were 94, 93, and 93% respectively. [d] δ [ppm] of the C–H oxirane OAc], the sulfone selectivity dropped to 77%. Our attempts
or by comparison with the
(
2
ring.
to control the selectivity of the reaction to give high yields
6
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Nanomagnet-Supported Manganese–Porphyrin as Catalyst
of sulfoxide in neat water were unsuccessful (see Table S2
in the Supporting Information). However, the addition of
ethanol to the aqueous mixture (water/ethanol. 1:2, v/v) re-
stricted significantly the rate of overoxidation and PhSOMe
was obtained in excellent yield and selectivity (Ͼ98%) with
1
equiv. of TBAOX at room temperature (Method B,
Table 3, Figure 8).
Table 3. Oxidation of various sulfides to sulfoxides and sulfones
with TBAOX catalyzed by [Mn(Br
solution.
2
TPP)OAc@SMNP] in aqueous
Figure 8. Effect of ethanol on the oxidation of thioanisole with
TBAOX in aqueous solution catalyzed by [Mn(Br TPP)-
2
OAc@SMNP]. The reactions were performed in air at room tem-
perature for 1 h with a molar ratio of sulfide/TBAOX/catalyst of
200:200:1.
the corresponding sulfoxides and sulfones in excellent yields
without the formation of any epoxide, alcohol, and carb-
[
14c]
onyl byproducts.
It should be noted that when 1-(all-
yloxy)benzene and benzyl alcohol as parent compounds of
entries 4 and 7, respectively, were subjected to oxidation,
the former remained completely intact and the latter was
oxidized partially to benzaldehyde (12%/1 h) in the pro-
cedure used for the oxidation of sulfides.
Catalyst Recovery
Recovery of the [Mn(Br TPP)OAc@SMNP] catalyst was
2
easy and efficient. When the magnetic stirring was stopped,
the catalyst adsorbed onto the magnetic stirring bar. It was
recovered by decantation of the reaction mixture in the
presence of an external magnet. After washing with ethyl
acetate or ethanol as safe solvents and drying under high
[
a] Yield of isolated products. The products were identified by com-
[22]
parison with authentic samples. [b] The byproducts were the cor-
responding sulfoxides and sulfones. [c] The reactions were per-
formed in air at room temperature in water for 45 min with a molar vacuum, it was used directly in the next reaction without
ratio of sulfide/TBAOX/catalyst of 200:600:1. [d] The reactions
further purification. The ease of recovery, combined with
were performed in air at room temperature in a mixture of water/
ethanol (1:2) for 1 h with a molar ratio of sulfide/TBAOX/catalyst
of 200:200:1.
the intrinsic stability of the silica-protected Fe O nanopar-
ticle component, allows the catalyst to be recovered ef-
ficiently at least seven times in the oxidation of different
3
4
Next, different sulfides were subjected to the two reac- substrates (Figure 9). Note that when the supported catalyst
tion systems (methods A and B) in the presence of was replaced by the free catalyst, similar results were ob-
[
Mn(Br TPP)OAc@SMNP] (Table 3). All the substrates tained for the oxidation of thioanisole (96%) and 1-methyl-
2
were readily converted into sulfoxides and sulfones in high- cyclohexne (80%) after seven cycles. However, in the oxi-
to-excellent yields and excellent selectivities were obtained dation of the less active indane, a remarkable loss of activity
under mild conditions (Table 3). A salient feature of the was observed after five runs (15%). These results once
present oxidation system is its excellent selectivity in both again confirm the significant improvement in the catalytic
procedures. Sulfoxides and sulfones could be obtained properties of [Mn(Br TPP)OAc] after immobilization on
2
nearly stoichiometrically and very small percentages of by- the magnetic nanoparticles. A comparison of the FTIR
product (the corresponding sulfoxide and sulfone) were de- spectra of fresh and used [Mn(Br TPP)OAc@SMNP]
2
tected, which makes the methods good alternatives for sulf- shows that the structure and morphology of the catalyst
oxide and sulfone production. Also, sulfides with a benzylic remains almost intact after seven cycles (Figure 10).
C–H bond (entry 3), C=C double bond (entries 4 and 5), Furthermore, the recovered heterogeneous catalyst was
and hydroxy group (entry 7) were cleanly transformed into found to contain 4.54 wt.-% Mn, which indicates that less
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A. Rezaeifard, M. Jafarpour, P. Farshid, A. Naeimi
FULL PAPER
than 1.5% leaching occurs during the reaction cycles (com- 1-methylcyclohexne and norbornene in the presence of
pare with 4.60 wt.-% according to ICP analysis) and con- [Mn(Br TPP)OAc@SMNP] led to the isolation of the cor-
2
firms that the complex is firmly anchored to the SMNP responding epoxides in yields of 91 and 94%, respectively.
support. Moreover, the byproduct of the oxidant
(
TBAHSO ) was separated by lyophilizing of the aqueous
4
phase (Ͼ90%) and reused in the preparation of TBAOX. Conclusions
Therefore the title methodology is cost effective and envi-
A novel organic–inorganic nanohybrid (Ͻ10 nm) with
high thermostability (Ͼ350 °C) has been synthesized by co-
ronmentally benign because of the use of water as the reac-
tion media, the reuse of the catalyst and oxidant, the easy
isolation of hydrophobic organic products, and also the ab-
sence of surfactants, nitrogen donors, toxic reagents, or sol-
vents. These advantages for this high-yielding oxidation
method offer ready scalability. For example, the use of a
semi-scale-up procedure (25.0 mmol) for the epoxidation of
ordinative anchoring of the [Mn(Br TPP)OAc] complex on
2
silica-coated magnetic nanoparticles through an amine
functionality to create an active, magnetically separable,
oxidation catalyst. The epoxidation of olefins and oxidation
of saturated hydrocarbons to the corresponding ketones
with TBAOX in aqueous solution were efficiently enhanced
with excellent selectivity by using a low catalyst loading.
Moreover, sulfides were converted selectively into sulfones
in neat water and to sulfoxides in water/ethanol mixture by
using this heterogeneous oxidation system. No surfactants,
additives, toxic reagents, solvents, or byproducts were in-
volved and no laborious purifications were necessary. Our
results have demonstrated well that the efficiency, selectiv-
ity, and oxidative stability of the porphyrin catalyst is
greatly improved by anchoring to magnetite nanoparticles.
The use of water as a standard “green” solvent as well as
the easy and safe work-up procedure and reusability of the
catalyst and oxidant byproduct render this catalytic system
cost effective and environmentally benign with high gener-
ality, and thus the title methodology is suitable for indus-
trial purposes.
Experimental Section
General: Conversion and product yields were determined by FID-
GC with a Shimadzu GC-16A instrument equipped with a 25 m
CBP1–S25 (0.32 mm i.d., 0.5 μm coating) capillary column. IR
spectra were recorded with a Perkin–Elmer 780 spectrometer. UV/
Vis spectra were recorded with a 160 Shimadzu spectrophotometer.
Figure 9. Recycling of the catalytic system in the oxidation of
thioanisole, 1-methylcyclohexene, and indane in water according to
the procedures reported in Tables 2 and 3.
NMR spectra were recorded in CDCl
FT-NMR 400 MHz spectrometer. The residual CHCl
.26 ppm) in conventional 99.8.% CDCl was used for calibration
3
with a Bruker Avance DPX
3
(δ =
7
3
of the chemical shifts. Mass spectra were recorded with a Shimadzu
GC–MS QP5050A apparatus. SEM micrographs were obtained
with a Philips XL-30 FEG SEM instrument at 20 kV. TEM images
were obtained with a Philips CM 10 instrument. X-ray diffraction
(
(
XRD, D8, Advance, Bruker, axs), TGA (Shimadzu 50), and ICP
Varian, visa-pro) were employed for characterization of the hetero-
geneous catalyst. The free-base porphyrin, Br
2 2
TPPH , and its man-
[
23]
ganese complex were synthesized as reported in the literature.
[24]
The freshly prepared nBu
4
NHSO
5
was a much stronger oxidant
thanϾ commercially available samples.
Large-Scale Synthesis of Nanomagnetite Particles (Fe
3 4
O ): A solu-
tion of FeCl (5.4 g) and FeCl (2.0 g) in aqueous hydrochloride
2
3
acid (2.0 m, 25.0 mL) at room temperature was sonicated until the
salts had dissolved completely. Aqueous ammonia (25%, 40.0 mL)
was added slowly over 20 min to the mixture under Ar at room
temperature followed by stirring for about 30 min with a mechani-
3 4
cal stirrer. The Fe O nanoparticles were separated with an external
Figure 10. FTIR spectra of A) fresh [Mn(Br
2
TPP)OAc@SMNP]
magnet and washed three times with deionized water and ethanol.
The final product was obtained after drying under vacuum. This
and B) [Mn(Br TPP)OAc@SMNP] after being used seven times.
2
8
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Nanomagnet-Supported Manganese–Porphyrin as Catalyst
could be resuspended in ethanol and stored in a refrigerator prior
then ethyl acetate (3ϫ 2.0 mL) was added to the reaction flask.
The organic phase was separated and dried with sodium sulfate.
The solvent was removed under reduced pressure to give the corre-
sponding compound (Method A, Table 3) and the separated cata-
[
15b]
to use.
Synthesis of Silica-Coated Magnetite Nanoparticles (SMNP): The
synthesized Fe was suspended in ethanol (35.0 mL) and deion-
ized water (6 mL) and sonicated for 15 min. Tetraethyl orthosilicate
TEOS, 1.5 mL) was added slowly to the mixture and sonicated for
0 min. Then aqueous ammonia (10%, 1.4 mL) was added slowly
over 10 min under mechanical stirring. The mixture was heated at
0 °C for 12 h. The silica-coated iron oxide nanoparticles
Fe @SiO ) were separated with an external magnet and washed
three times with ethanol and dried under vacuum.
3
O
4
2
lyst was washed with EtOH and H O, and finally used directly in
the next reaction without further purification. The purity of prod-
ucts was more than 92% without any chromatographic purification
according to GC analysis. Further purification was performed by
silica chromatography eluting with n-hexane/ethyl acetate (10:1).
(
1
4
(
3
O
4
2
General Procedure for the Oxidation of Sulfides to Sulfoxides:
Freshly prepared TBAOX (2.0 mmol, 0.7 g) was added to a mixture
[15b]
of sulfide (1 mmol) and [Mn(Br
0
2
with an external magnet. The ethanol was then removed under re-
duced pressure. After the addition of ethyl acetate (3ϫ 2.0 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 (Method B, Table 3) and the
2
TPP)OAc@SMNP] (0.1 g,
.005 mmol) in a mixture of water and ethanol (2.0 mL, 1:2) at
5 °C. After completion of the reaction, the catalyst was separated
Silanation of Silica-Coated Magnetite Nanoparticles (ASMNP):
Dry SMNP powder (10.0 g) was mixed with toluene (200.0 mL) to
produce a homogeneously mixed solution. This mixture was soni-
cated for 30 min. Then (3-aminopropyl)triethoxysilane (APS,
2.5 mL) was added under mechanical stirring and the mixture was
slowly heated to 105 °C and kept at this temperature for 20 h. The
final sample was separated with an external magnet and washed
three times with methanol or ethanol and dried under vacuum. The
concentrated product was stored in a refrigerator prior to use.[
separated catalyst was washed with EtOH and H
used directly in the next reaction without further purification. The
Catalyst purity of products was more than 90% without any chromato-
TPP)OAc@SMNP]: Dry ASMNP powder (4.0 g) was graphic purification according to GC analysis. Further purification
2
2
O, and finally
15b]
Synthesis
Mn(Br
of
Magnetite-Nanoparticle-Supported
[
mixed with ethanol (200.0 mL) to produce a homogenously mixed
solution. This mixture was sonicated for 30 min. The
was performed by silica chromatography eluting with n-hexane/
ethyl acetate (10:1).
[
2
Mn(Br TPP)OAc] complex (0.3 g) was added under mechanical
Recycling Procedure: After completion of the oxidation reaction,
the catalyst was separated with an external magnet and washed
stirring and the mixture was heated at 80 °C for 8 h. The free Mn–
porphyrin was removed by Soxhlet extraction with ethanol. The
concentrated product was stored in refrigerator prior to use.
with EtOH and H
2
O. The catalyst was then used directly in the
next reaction without further purification. The aqueous phase was
General Procedure for the Epoxidation of Olefins: Freshly prepared
TBAOX (2 mmol, 0.7 g) was added to a mixture of olefin
lyophilized to obtain the reduced form of the oxidant (TBAHSO
which can be reused for the preparation of TBAOX.
4
),
(
2
1.0 mmol) and [Mn(Br TPP)OAc@SMNP] (0.1 g, 0.005 mmol) in
Supporting Information (see footnote on the first page of this arti-
cle): Results for the optimization of the oxidation of indane and
thioanisole.
bidistilled water (2.0 mL) at 70 °C was added. After completion of
the reaction, the catalyst was separated with an external magnet
and ethyl acetate (3ϫ 2.0 mL) was then added to the reaction flask.
The organic phase was separated and dried with sodium sulfate.
The solvent was removed under reduced pressure to give the corre- Acknowledgments
sponding compound (Table 2) and the separated catalyst was
washed with EtOH and H
2
O, and finally used directly in the next
Support for this work by the Research Council of the University
of Birjand is highly appreciated.
reaction without further purification. The purity of the products
was analyzed by GC to be Ͼ99% without any chromatographic
purification.
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A. Rezaeifard, M. Jafarpour, P. Farshid, A. Naeimi
FULL PAPER
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Published Online: ᭿
10
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Date: 27-09-12 10:10:35
Pages: 11
Nanomagnet-Supported Manganese–Porphyrin as Catalyst
Magnetite-Porphyrin Nanocatalyst
The synthesis and catalytic performance of
a novel magnetically separable Mn–por-
phyrin nanocatalyst in the selective oxi-
dation of olefins, saturated hydrocarbons,
and sulfides in aqueous media are de-
scribed.
A. Rezaeifard,* M. Jafarpour,* P. Farshid,
A. Naeimi ....................................... 1–11
Nanomagnet-Supported Partially Bromin-
ated Manganese–Porphyrin as a Promising
Catalyst for the Selective Heterogeneous
Oxidation of Hydrocarbons and Sulfides in
Water
Keywords: Nanostructures / Nanoparticles /
Magnetic properties
/ Porphyrinoids /
Green chemistry / Oxygenation / Sulfur ox-
ides / Heterogeneous catalysis
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