Immobilized metalloporphyrins on 3-aminopropyl-functionalized silica support as heterogeneous catalysts for selective oxidation of primary and secondary alcohols

Article abstract of DOI:10.1007/s00706-011-0684-2

Iron(III), manganese(III), and cobalt(II) complexes of meso-tetrakis(p- chlorophenyl)porphyrin (Fe(TCl-PP)X, Mn(TClPP)X, and Co(TClPP)X, X = Cl or OAc) were immobilized onto 3-aminopropyl-functionalized silica (SF-3-APTS). SF-3-APTS acts as both axial bas

Full text of DOI:10.1007/s00706-011-0684-2

Monatsh Chem (2012) 143:1031–1038
DOI 10.1007/s00706-011-0684-2
ORIGINAL PAPER
Immobilized metalloporphyrins on 3-aminopropyl-functionalized
silica support as heterogeneous catalysts for selective oxidation
of primary and secondary alcohols
Rahmatollah Rahimi
Mohammad G. Dekamin
•
Seyyedeh Zahra Ghoreishi
•
Received: 25 May 2011 / Accepted: 2 November 2011 / Published online: 3 January 2012
Ó Springer-Verlag 2011
Abstract Iron(III), manganese(III), and cobalt(II) com-
plexes of meso-tetrakis(p-chlorophenyl)porphyrin (Fe(TCl-
PP)X, Mn(TClPP)X, and Co(TClPP)X, X = Cl or OAc)
were immobilized onto 3-aminopropyl-functionalized silica
Introduction
Oxidation of alcohols to the corresponding carbonyl com-
pounds is a key reaction on both laboratory and industrial
scales because of the importance of carbonyl compounds as
chemical intermediates for the synthesis of various per-
fumes, drugs, and pharmaceuticals [1, 2]. Recent attempts
have been directed to develop more efficient and inex-
pensive, cleaner, and reusable catalytic systems for the
oxidation reactions [3–5]. Along this line, metalloporphy-
rins have been used as biomimetic models for the
cytochrome P450 enzyme under homogeneous conditions
[6]. These compounds play an important role in the oxi-
dation of alcohols [7–9], sulfides [10, 11], epoxidation of
alkenes [12, 13], and hydroxylation of alkanes [14–16].
Indeed, the synthesis of metalloporphyrins is challenging,
often with low yield and tedious workup procedures. Fur-
thermore, molecule aggregation caused by p–p stacking
interactions or biomolecular self-oxidation reaction path-
ways lead to deactivation of metalloporphyrin species.
However, immobilization of metalloporphyrins on solid
supports not only enables the reusability of these catalysts
but also reduces undesirable effects of the homogeneous
conditions. Therefore, the supported metalloporphyrins
will be more cost-effective [17]. In the past few decades,
different kinds of supports such as silica [18–20], clay [21],
zeolites [22, 23], MCM-41 [24, 25], and organic polymers
(
SF-3-APTS). SF-3-APTS acts as both axial base and support
for immobilization of these metalloporphyrins. The obtained
heterogeneous catalysts were characterized by Fourier trans-
form infrared (FT-IR), UV–Vis, and inductively coupled
plasma (ICP) spectroscopy, scanning electron microscopy
(
SEM), energy-dispersive X-ray (EDX), and thermogravi-
metric analysis (TGA) techniques. Their catalytic activity
as biomimetic catalysts was investigated for the selective
oxidation of primary and secondary benzylic alcohols to
the corresponding carbonyl compounds with t-butylhydro-
peroxide as oxidant. SF-3-APTS–Fe(TClPP)Cl demonstrated
higher catalytic activity than SF-3-APTS–Mn(TClPP)Cl
and SF-3-APTS–Co(TClPP)OAc. The presence of electron-
withdrawing substituents on benzylic alcohols enhances the
rateofcatalyticoxidation.SF-3-APTS–Fe(TClPP)Clcouldbe
reused at least four times without significant loss of its cata-
lytic activity.
Keywords Metalloporphyrins Á
Biomimetic alcohol oxidation Á
3
-Aminopropyl-functionalized silica Á
Heterogeneous catalysis Á t-Butylhydroproxide
[
26] have been used for immobilization of metallopor-
phyrins. Amongst these different supports, silica gel is
believed to be one of the most attractive inorganic supports
due to its nontoxicity, high stability, good accessibility,
low cost, and good surface attraction toward functional
groups of organic compounds. In this work, we have
studied the biomimetic enzymatic activity of the iron(III),
manganese(III), and cobalt(II) complexes of meso-tetrakis-
R. Rahimi (&) Á S. Z. Ghoreishi Á M. G. Dekamin (&)
Department of Chemistry, Iran University of Science
and Technology, Narmak, Tehran 16846-13114, Iran
e-mail: rahimi_rah@iust.ac.ir
M. G. Dekamin
e-mail: mdekamin@iust.ac.ir
123

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032
R. Rahimi et al.
Scheme 1
(
p-chlorophenyl)porphyrin (Fe(TClPP)X, Mn(TClPP)X, and
The results indicated that there is approximately 1 mmol of
3-APTS groups per gram of silica.
Co(TClPP)X, X = Cl or OAc) immobilized on silica gel
modified with 3-aminopropyltriethoxysilane (3-APTS) groups
for oxidation of alcohols. Herein, we report these hybrid
materials, SF-3-APTS–Fe(TClPP)Cl (1b), SF-3-APTS–
Mn(TClPP)Cl (1c), and SF-3-APTS–Co(TClPP)OAc (1d), as
suitable catalysts for the selective oxidation of primary and
secondary benzylic alcohols with t-butylhydroperoxide
In the next stage, metalloporphyrins and modified silica
gel (SF-3-APTS) (1a) were mixed in toluene at 80 °C for
48 h. Finally, after filtration and washing the hybrid
materials, the heterogeneous catalysts were obtained and
characterized by FT-IR and solid-state UV–Vis spectro-
scopy along with thermogravimetric analysis (TGA),
scanning electron microscopy (SEM), and energy-disper-
sive X-ray (EDX) techniques.
(TBHP) as oxidant (Scheme 1).
FT-IR spectra of silica gel and 1a–1d are presented in
Fig. 1. In the case of silica gel (Fig. 1a), a very broad band
Results and discussion
-
is observed at 3,700–3,200 cm due to the O–H stretching
1
The preparation of silica metalloporphyrin heterogeneous
catalysts is shown in Scheme 2. In the first stage, the silica gel
surface was modified with 3-APTS groups. Functionalization
of silica gel with amine groups was confirmed by Fourier
transform infrared (FT-IR) spectra (Fig. 1a). The amount of
vibration of the hydroxyl groups. Another band at
-
1,649 cm is attributed to the H–O–H bending vibration
1
of the adsorbed water. Furthermore, a strong band between
-
1
-1
1,200 and 1,000 cm and a band at 804 cm are assigned
to the Si–O–Si stretching vibrations [27]. After function-
alization of the silica gel with 3-APTS groups, the intensity
3
-APTS groups on the silica gel support was estimated
-
1
by CHN elemental analysis (C: 5.75%, H: 1.47%, N: 1.58%).
of the broad band of silica gel at 3,450 cm decreased
Scheme 2
1
23

Immobilized metalloporphyrins on 3-aminopropyl-functionalized silica
1033
which confirmed the modification of silica gel with 3-ami-
nopropyl groups. Additionally, new bands appear at2,925 and
hand, immobilization of metalloporphyrins on the silica sup-
port was confirmed by the appearance of new bands at
-1
-1
2
,854 cm duetoCH stretchingvibrationsofpropylgroups.
2
1,603 cm (Fig. 1c–e). These bands are associated with the
The NH stretching vibrations are also overlapped with the
2
stretching vibrations of C=N and C=C in the pyridyl and
phenyl rings in porphyrins [28].
-1
O–H vibration band at 3,450 cm (Fig. 1b). On the other
The morphology of 1a–1d determined by SEM is shown in
Fig. 2. Immobilization of metalloporphyrins on the silica
support was confirmed by these images. Furthermore, no
considerable changes in the shape of the silica particles were
observed after immobilization of the metalloporphyrins.
The presence of metalloporphyrins on the silica support
was also confirmed by means of solid-state UV–Vis spec-
troscopy (Fig. 3). No absorption band is observed in the
UV–Vis spectrum of silica gel (Fig. 3a). However, the Q
and Soret bands appear after immobilization of metallo-
porphyrins on the silica support. Furthermore, a red shift in
Q and Soret bands is observed after immobilization of the
metalloporphyrins comparing with the pure metallopor-
phyrins. This shift is assigned to the symmetry changes of
the metalloporphyrin after its metal ion coordinates to the
NH group of 1a.
2
The quantitative analysis of metalloporphyrin loading in
the heterogeneous catalysts 1b–1d was estimated using
inductively coupled plasma (ICP) spectroscopy. The obtained
amounts were as follows: 4.9% Fe(TClPP)Cl in 1b, 4.1%
Mn(TClPP)Cl in 1c, and 5.2% Co(TClPP)OAc in 1d. Fur-
thermore, the results of EDX analysis were consistent with the
aforementioned loadings.
Fig. 1 FT-IR spectra of silica (a), 1a (b), 1b (c), 1c (d), 1d (e), and
recovered catalyst 1b (f)
Fig. 2 Scanning electron
micrographs of 1a (a), 1b (b),
1
c (c), and 1d (d)
123

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034
R. Rahimi et al.
Oxidation of benzhydrol by TBHP in the presence
of heterogeneous catalysts 1b–1d
Benzhydrol (2a) was used as the model substrate for oxi-
dation by TBHP in the presence of heterogeneous catalysts
1b–1d. In a systematic study, the effect of different factors
such as catalyst loading, temperature, kind of active metal
ion in the catalysts, and reaction time were investigated.
Effect of catalyst loading of 1b, reaction temperature,
and oxidant
Table 1 shows the influence of catalyst loading of 1b on the
yield of the model reaction. As the data in Table 1 show, only
a trace amount (about 7%) of the oxidation product of ben-
zophenone (3a) was detected at 60 °C after 10 h in the
absenceofcatalyst1b (entry1). However,theyieldofreaction
was improved with 4.4 mol% catalyst loading of 1b (50 mg
per 1 mmol of the substrate) to the reaction mixture under
similar conditions. By decreasing the catalyst loading to
Fig. 3 UV–Vis spectra of 1a (a), 1c (b), 1d (c), and 1b (d)
2.2 mol%, only 35% of the desired product was obtained. On
the other hand, higher yields of 82 and 84% were obtained by
the use of 8.8 and 17.6 mol% catalyst loadings, respectively
(entries 4, 5). The best result in terms of calculated turnover
number (TON) and yield was obtained with a catalyst loading
of 8.8 mol% (entry 4). Furthermore, the progress of oxidation
of 2a was very slow at room temperature (entry 6). Therefore,
thetemperature isanimportant factorfor this catalytic system.
The results of TGA of the heterogeneous catalysts
1
b–1d are shown in Fig. 4. The weight loss in samples of
b–1d occurs in three steps: below 100 °C, 200–420 °C, and
1
above 420 °C. The first step (T \ 100 °C) can be assigned to
the evaporation of volatile solvents and water molecules
which are physically adsorbed in the samples. The secondstep
can be related to decomposition of metalloporphyrins and
organic groups. The weight loss above 420 °C can be attrib-
uted to the further oxidation of the remaining organic
compounds and the loss of silanol groups. These results
indicate that the hybrids of silica metalloporphyrins 1b–1d
show relatively high thermal stability.
Ontheother hand, H
of benzhydrol under similar reaction conditions (cf. entries 4,
7). It is well known that H can produce hydroxyl radicals
O affordedlower yieldsintheoxidation
2 2
O
2 2
(OHÁ) which are more reactive than t-butoxyl (t-BuOÁ) radi-
cals and may destroy the porphyrin moiety in the catalyst
structure [29].
Fig. 4 TGA curves of 1c (a),
1
b (b), and 1d (c)
1
23

Immobilized metalloporphyrins on 3-aminopropyl-functionalized silica
1035
Table 1 Effect of catalyst loading on oxidation of benzhydrol (2a) in
the presence of catalyst 1b
Table 3 Effect of reaction time on oxidation of benzhydrol (2a) by
TBHP in the presence of catalyst 1b
a
Yield /%
b -1
TOF /h
OH
O
Entry
Reaction time/h
TON
TBHP, 1b
1
2
3
4
5
6
1
22
45
62
82
85
87
2.5
5.1
7.1
9.3
9.7
9.9
2.50
1.28
0.88
0.93
0.69
0.41
CH CN
4
3
8
2a
3a
10
14
24
a
Yield /%
b
Entry
Catalyst loading/mol%
T/°C
TON
1
2
3
4
5
6
No catalyst
2.2
60
60
60
60
60
25
60
7
–
Conditions: benzhydrol (1 mmol), TBHP (1 mmol), 8.8 mol% cata-
3
lyst 1a, 10 cm CH CN, 60 °C
3
35
66
82
84
32
49
15.9
15
a
Yields obtained by GC
4.4
b
Turnover frequency: mol of product per mol of catalyst per hour
8.8
9.3
4.8
3.6
5.7
17.6
8.8
c
7
8.8
3
Conditions: benzhydrol (1 mmol), TBHP (1 mmol), 10 cm CH
3
CN,
1
0 h
a
Yields obtained by GC
b
c
Turnover number: mol of product per mol of catalyst
was used as oxidant
2 2
H O
Effect of metal ion
Catalytic oxidation of benzhydrol by TBHP in the presence
of silica supported metalloporphyrins 1b–1d was also
studied. Comparison of the catalytic activity of these het-
erogeneous catalysts shows that iron porphyrin 1b
demonstrates higher catalytic activity than manganese and
cobalt porphyrins (1c and 1d, respectively) in the oxidation
of 2a under similar conditions (Table 2). Therefore, 1b was
selected as the catalyst of choice for further catalytic
studies in the benzhydrol oxidation by TBHP.
Fig. 5 Changes of product yield with reaction time. Reaction
conditions: benzhydrol (1 mmol), TBHP (1 mmol), and 8.8%mol
3
b, 10 cm CH
1
3
CN, 60 °C
yield was obtained after 10 h (82%) in terms of both calcu-
lated turnover frequency (TOF) and TON values. Only slight
improvements in conversion of 2a were experienced after
14 h (84%) or even 24 h (86%). On the other hand, lower
yields were observed after reaction times less than 10 h.
Effect of reaction time
Oxidation of different benzylic alcohols by 1b
The influence of reaction time on the yield of product 3a
was also investigated in the oxidation of benzhydrol by
TBHP. As the results show in Table 3 and Fig. 5, the best
Primary benzylic alcohols were also subjected to oxidation
under the optimized reaction conditions (8.8 mol% 1b,
CH CN). In general, primary alcohols are more reactive
3
toward oxidation than secondary alcohols. Therefore, they
require milder reaction conditions including lower tem-
perature and shorter reaction time. Furthermore, the
corresponding aldehydes or carboxylic acids can also be
obtained. Therefore, controlled oxidation from an alcohol
to an aldehyde, avoiding overoxidation to the carboxylic
acid, is very important [30]. In our hands, primary benzylic
alcohols 2b–2e were selectively oxidized at room temper-
ature and in shorter reaction time than with secondary
benzhydrol (Table 3, entries 1–4) to afford high to quan-
titative yields of the corresponding aldehydes 3b–3e.
Table 2 Oxidation of benzhydrol (2a) by TBHP in the presence of
silica-supported metalloporphyrin catalysts 1b–1d
a
Yield /%
Entry
Catalyst
TON
1
2
3
SF-3-APTS–Fe(TClPP) (1b)
SF-3-APTS–Mn(TClPP) (1c)
SF-3-APTS–Co(TClPP) (1d)
82
78
51
9.3
8.9
5.8
Conditions: benzhydrol (1 mmol), TBHP (1 mmol), 8.8 mol% cata-
3
lyst 1b–1d, 10 cm CH
3
CN, 10 h, 60 °C
Yields obtained by GC
a
123

1
036
R. Rahimi et al.
Benzyl alcohol (2b) and 4-nitrobenzyl alcohol (2c) affor-
ded quantitative yields of the corresponding aldehydes 3b
and 3c, respectively. On the other hand, 4-chlorobenzyl
alcohol (2d) and 4-methylbenzyl alcohol (2e) produced 88
and 82% of the desired products 3d and 3e, respectively.
In the case of 4-methylbenzyl alcohol, complete conversion
of the alcohol was observed. However, about 18% of
considered by interaction of two intermediates IV to give
rise to the product 3 and alcohol 2 (Scheme 3, bottom).
Furthermore, the corresponding hydroperoxide of the used
alcohols was not detected in the reaction product, most
probably due to fast decomposition of any peroxide by the
catalyst 1b.
4
-carboxybenzyl alcohol was detected as a by-product of
Recyclability of the catalyst 1b
the reaction. This observation can be attributed to the
competitive oxidation of both benzylic positions in this
substrate (Table 4).
The stability of the heterogeneous catalyst 1b was deter-
mined by examining its reuse in consecutive oxidation
reactions. After each reaction, the catalyst was completely
washed in a Soxhlet extractor with the organic solvents
acetonitrile, acetone, and methanol and then dried before
using in subsequent runs. As the results in Table 5 show,
the catalyst could be reused for four consecutive runs
without significant loss of its catalytic activity. Further-
more, comparison of the FT-IR spectra before and after the
reaction confirmed that no structure change of the catalyst
occurred during the reaction (Fig. 1f).
Proposed mechanism for oxidation of benzylic alcohols
by TBHP in the presence of 1b
The following mechanism for oxidation of benzylic alco-
hols 2a–2e by TBHP in the presence of 1b could be
proposed according to literature and substituent effects
(
Scheme 3). When the ‘‘singlet oxygen donor’’ oxidants
such as hydrogen peroxide and organic peroxides are used
in oxidation catalyzed by metalloporphyrins, two general
routes can be expected [31]: (i) the homolytic cleavage of
the peroxidic O–O bond in the intermediate I which pro-
duces t-BuOÁ as active species (pathway A); (ii) the
heterolytic cleavage of O–O bond in the intermediate
I which produces high valent oxoiron(IV) species (inter-
mediate III, pathway B). Subsequent hydrogen abstraction
from the benzylic carbon of alcohol 2, probably in a rate-
determining step, gives the corresponding radical IV in
both pathways. Hence, the difference in the reactivity of
primary benzylic alcohols and benzhydrol can be inter-
preted by considering more steric effects in benzhydrol.
Finally, hydride transfer from the intermediate IV to
intermediate II affords product 3 and water as a by-product
Experimental
Materials
All chemicals and solvents were purchased from Aldrich
and Merck with maximum purity degree and used as
received. t-Butylhydroperoxide (TBHP, 70% aqueous
solution) was also obtained from Merck and used without
further purification. Toluene and acetonitrile were dried
with calcium hydride before using in reactions. Silica gel
(Merck, 60/70–230) was activated at 250 °C under vacuum
for 2 h. meso-Tetrakis(p-chlorophenyl)porphyrin was syn-
thesized in our laboratory according to Adler’s method
[
32]. An alternative route for the last step may be
[
33]. Metallation of this complex of porphyrin was per-
Table 4 Oxidation of benzylic alcohols with TBHP catalyzed by 1b
Conditions: substrate (1 mmol), TBHP (1 mmol), 8.8 mol% catalyst
formed according to the literature [34].
(
3
1
3
a, 10 cm CH CN, 3 h, 25 °C)
Instrumentation
O
2
, 3
R
H
b
c
d
e
H
NO
Cl
OH
FT-IR spectra were obtained using KBr pellets on a Shi-
-1
madzu 8400S spectrophotometer in the 4,000–500 cm
R
R
2
range. Electronic spectra (UV–Vis) were recorded on a
double beam UV–Vis Shimadzu 2550 spectrophotometer.
TGA was performed by using a Netzsch TG-209 F1
thermogravimetric analyzer in the 25–850 °C temperature
CH
3
a
Yield /%
Entry
Product
Time/h
TON
-
1
range with a heating rate of 10 °C min
under N2
1
2
3
4
a
3b
3c
3d
3e
3
3
3
3
[99
[99
88
11.4
11.4
10
atmosphere. Scanning electron micrographs of samples
were imaged by a Philips XL30 SEM system. A Philips
XL30 EDX system was used for determination of metal-
loporphyrin contents on the silica support. Elemental
analysis (CHN) measurements were carried out in a Per-
kin–Elmer 2400 instrument. The yield of oxidation
b
82 (18 )
9.3
Yields obtained by GC
b
4
-Carboxybenzyl alcohol was detected as by-product of the reaction
1
23

Immobilized metalloporphyrins on 3-aminopropyl-functionalized silica
1037
FeIII
t-BuOOH
Pathway (A)
Pathway (B)
Homolytic cleavage
Heterolytic cleavage
H
H
OH
R
OH
O
OH
R
t-BuOH
Fe IV
t-BuO.
FeV
2
2
II
t-BuOH
III
OH
OH
Ot-Bu
OH
Ot-Bu
O
OH
O
R
FeIV
R
+
FeIV
FeIII
FeIII
+
IV
II
I
II
I
IV
O
R
H O
2
3
OH
OH
O
H
R
R
R
IV
3
2
Scheme 3
Table 5 Results of recovery of catalyst 1b in oxidation of
benzhydrol
at 80 °C for 24 h. The product was filtered and exhaus-
tively washed in a Soxhlet extractor with absolute ethanol
for 24 h, then dried at 80 °C under vaccum for 24 h. The
obtained solid 1a was characterized by FT-IR and CHN
elemental analysis [20].
a
Yield /%
Run
TON
1
2
3
4
82
80
75
74
9.3
9.0
8.5
8.4
Immobilization of metalloporphyrins on modified
silica 1a
Conditions: benzhydrol (1 mmol), TBHP (1 mmol), 8.8 mol% cata-
3
lyst 1a, 10 cm CH
3
In a 250-cm flask, a mixture of 0.3 g metalloporphyrin and
3
3 g SF-3-APTS was suspended in 80 cm toluene. Then,
3
CN, 10 h, 60 °C
a
Yields obtained by GC
the mixture was vigorously stirred by magnetic stirring at
80 °C for 48 h. After cooling to room temperature, the
solid was filtered and exhaustively washed in a Soxhlet
extractor with dichloromethane (14 h), methanol (12 h),
and ether (10 h) to remove any non-supported metallo-
porphyrin. The obtained solid was dried in vaccum at
reactions was calculated using a Shimadzu GC-2010 gas
chromatograph equipped with a flame ionization detector
(
FID) detector and capillary column (30 m).
Modification of silica gel surface with 3-APTS groups
25 °C for 12 h. The presence of metalloporphyrin on the
The following procedure was used for preparing of
silica support was confirmed by solid state UV–Vis and
FT-IR spectroscopy and SEM [35]. Furthermore, the
amount of immobilized metalloporphyrins on silica was
determined by ICP and EDX analysis.
3
-aminopropyl-functionalized silica 1a: A mixture of 5 g
3
3
silica gel, 5 cm 3-APTS, and 75 cm toluene was degas-
sed and sealed under nitrogen. The suspension was refluxed
123

1
038
R. Rahimi et al.
General procedure for oxidation of benzhydrol
catalyzed by silica-supported metalloporphyrins 1b–1d
10. Moghaddam M, Tangestaninejad S, Mirkhani V, Mohammad-
poor-Baltork I, Abbasi-Larki AA (2008) Appl Catal A Gen
3
49:177
1
1. Rahimi R, Azhdari AR, Azizpoor-Fard M, Mirmohammadsadegh
B, Khavasi HR (2009) Catal Commun 11:232
A mixture of benzhydrol (2a, 1 mmol), TBHP (1 mmol),
3
and 100 mg heterogeneous catalyst 1b–1d in 10 cm
12. Vinhado FS, Martins PR, Masson AP, Abreu DG, Vidoto EA,
Nascimento OR, Imamoto YK (2002) J Mol Catal A Chem
3
CH CN was stirred in a 50-cm two-necked flask equipped
3
1
88:141
with a septum at 60 °C. The progress of the reaction was
monitored by TLC. The heterogeneous catalysts were
separated from the reaction mixture by filtration. The fil-
trate was analyzed by GC after 10 h. The yield of the
reaction was calculated from the calibration curves using
benzophenone (3a) and n-octane as authentic sample and
external standard, respectively.
1
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Catalyst reusability
1
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The reusability of the heterogeneous catalysts 1b–1d was
investigated by using them in sequential oxidation reac-
tions of benzhydrol as model substrate. The used catalyst
was completely washed in a Soxhlet extractor with acetone,
methanol, and acetonitrile and dried before reusing in
another oxidation reaction.
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Acknowledgments We are grateful for the financial support from
The Research Council of Iran University of Science and Technology
2
2
2
(
IUST), Iran.
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