Preparation of catalysts based on iron(III) porphyrins heterogenized on silica obtained by the sol-gel process for hydroxylation and epoxidation reactions

Article abstract of DOI:10.1590/S0103-50532010000700020

Solid catalysts have been prepared by chemical interaction of iron(III) porphyrins with the surface of the pores of a silica matrix obtained by the sol-gel method. The presence of the complexes in the silica matrix and the morphology of the obtained parti

Full text of DOI:10.1590/S0103-50532010000700020

J. Braz. Chem. Soc., Vol. 21, No. 7, 1329-1340, 2010.
Printed in Brazil - ©2010 Sociedade Brasileira de Química
0103 - 5053 $6.00+0.00
Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica
obtained by the Sol-Gel Process for Hydroxylation and Epoxidation Reactions
Kelly A. D. F. Castro,a Matilte Halma,a Guilherme S. Machado,a Gustavo P. Ricci,b
Geani M. Ucoski,a Katia J. Ciuffib and Shirley Nakagaki*,a
aUniversidade Federal do Paraná, Departamento de Química - Laboratório de Química
Bioinorgânica e Catálise, CP 19081, 81531-990 Curitiba-PR, Brazil
bUniversidade de Franca, Departamento de Química, CP 82, 14404-600, Franca-SP, Brazil
Catalisadores sólidos foram preparados pela interação de ferroporfirinas com sílica obtida pelo
processo sol-gel. A presença da ferroporfirina na matriz de sílica e a morfologia das partículas
obtidas foram estudadas por espectroscopia eletrônica na região do ultravioleta e visível,
difratometria de raios X de pó, espectroscopia vibracional na região do infravermelho, microscopia
eletrônica de transmissão, ressonância paramagnética eletrônica e análises termogravimétricas.
A atividade catalítica dos sólidos obtidos foi investigada frente a oxidação dos substratos (Z)-
cicloocteno, cicloexeno e cicloexano por iodosilbenzeno em meio de diclorometano e acetonitrila.
Os resultados catalíticos obtidos foram comparáveis àqueles observados quando as ferroporfirinas
em solução (catálise homogênea) foram utilizadas.
Solid catalysts have been prepared by chemical interaction of iron(III) porphyrins with
the surface of the pores of a silica matrix obtained by the sol-gel method. The presence of the
complexes in the silica matrix and the morphology of the obtained particles were studied by
UV-Vis spectroscopy, powder X-ray diffractometry, infrared spectroscopy, transmission electron
microscopy, electron paramagnetic resonance and thermogravimetric analysis. The catalytic
activity of the immobilized iron(III) porphyrins in the oxidation of (Z)-cyclooctene, cyclohexene
and cyclohexane was evaluated in dichloromethane/acetonitrile 1:1 solvent mixture (v/v) using
iodosylbenzene as oxidant. Results were compared with those achieved with the homogeneous
counterparts.
Keywords: iron porphyrin, silica, sol-gel, oxidation, cytochrome P-450 mimetic systems
Introduction
double hydroxides,13,14 tubular and fibrous matrices,15
silica matrix obtained by the sol-gel process, among
others.16-23 This versatile class of molecules is efficient and
selective for the heterogeneous catalytic hydroxylation and
epoxidation of a wide variety of organic substrates.
Catalyst immobilization enables site-isolation of the
metal center and is one of the approaches to reducing
metalloporphyrin degradation because it may prevent
molecular aggregation or bimolecular self-destruction
reactions,20 which all lead to deactivation of the catalytically
active metalloporphyrin species.19,21 Indeed, catalyst
stability is essential for recycling procedures and recovery
of valuable metalloporphyrins.23,24
Porphyrinsandrelatedmacrocyclesprovideanextremely
versatile synthetic base for a variety of applications
in the area of new materials.1-3 Metalloporphyrins are
important examples of macrocyclic complexes4 that have
demonstrated catalytic ability in a wide variety of reactions
over the last 30 years. Such reactions include oxidative
processes, especially in homogeneous media.
More recently, porous metalloporphyrin networks
have proven to be potentially applicable as efficient
heterogeneous catalysts5 after immobilization in organic
amorphous polymers, amorphous inorganic matrices, or
crystalline inorganic materials such as silica,6,7 zeolites,8,9
clay from the smectite group (montmorillonite),10-12 layered
To obtain an ideal heterogeneous catalytic system
consisting of porphyrin-matrix, the porphyrin active site
must be readily accessible for both charge transfer and mass
transport. Therefore, a study on the best possible approach
*e-mail: shirley@quimica.ufpr.br

1330
Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica
J. Braz. Chem. Soc.
to building heterogeneous catalysts based on porphyrins
is crucial to obtain economically viable applications for
these systems.
Thisworkreportstheimmobilizationofthreestructurally
different iron porphyrins, namely: [Fe(TMPyP)]Cl5],
[5,10,15,20-tetrakis(4-N-methylpyridyl)porphyrin iron(III)
chloride]; [Fe(TDFPP)]Cl], [5,10,15,20-tetrakis(2,6-
difluorophenyl)porphyrin iron(III) chloride] and
[Fe(TDCPP)]Cl, [5,10,15,20-tetrakis(2,6-dichlorophenyl)
porphyrin iron(III) chloride] (Figure 1) in the silica
structure obtained by the hydrolytic sol-gel process.25 After
characterization, the catalytic activity of the obtained solid
compounds in oxidation reactions was investigated.
Although various works in the literature have reported
the heterogenization of metalloporphyrin catalysts, we have
verified that much has to be done in terms of optimization
and matrix preparation. The investigation of factors such
as particle shape and size, as well as active site distribution
on the matrices, may furnish catalysts with potentially
applicable features in a wide variety of fields in both the
scientific and industrial areas.
Stöber et al.25 have described a method for the
preparation of monodispersed spherical silica particles
of nanometric dimensions through tetraethylorthosilicate
(TEOS) hydrolysis in ethanol, employing basic conditions
and ambient temperature. This method is widely applied
nowadays because its simplicity enables the production of
spherical particles of different sizes.21 Thus, this can be an
alternative process in the search for more efficient matrices
for catalyst immobilization.
Our research team18,19 and others20,21 have observed
that the addition of porphyrins to silica matrices obtained
by the sol-gel methodology can affect particle porosity
and morphology. As already reported,21 the presence of
porphyrins can contribute to the polymerization of the
silica network.
In the sol-gel methodology, control of the preparation
conditions can easily alter the structure and properties of the
obtained materials. The use of an acid catalyst can also be
an attractive alternative approach in the preparation of the
heterogeneous iron porphyrin catalysts. In the case of silica,
the utilization of acidic catalysts leads to transparent gels
of low porosity, whereas basic catalysts furnish translucent,
highly porous gels.21
Experimental
Materials
All chemicals used in this study were purchased from
Aldrich, Sigma or Merck and were of analytical grade. In
particular, tetraethylorthosilicate (TEOS) was acquired
from Aldrich; ethanol (EtOH), dichloromethane (dcm),
dimethylformamide (dmf), methanol (MeOH), isopropyl
alcohol (PriOH), and tetrahydrofuran (thf) were obtained
from Merck. Iodosylbenzene (PhIO) was synthesized by
hydrolysis of iodosylbenzenediacetate,26 and the obtained
solid was carefully dried under reduced pressure and kept at
5 °C. PhIO purity was periodically controlled by iodometric
titration.27 Deionized water was used in all experimental
procedures.
Po r p h y r i n s : T h e f r e e b a s e p o r p hy r i n s
5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin
([H2(TMPyP)]4+), 5,10,15,20-tetrakis(2,6-fluorophenyl)
porphyrin ([H2(TDFPP)])] and 5,10,15,20-tetrakis(2,6-
dichlorophenyl)porphyrin([H2(TDCPP)])weresynthesized,
purified, and characterized following the previously
described methodology.28-30
Figure 1. Schematic representation of the iron(III) porphyrins employed in this work.

Vol. 21, No. 7, 2010
Castro et al.
1331
Iron(III) porphyrins: Iron(III) porphyrins (FePors)
were obtained by metallation of the free base with
ferrous chloride tetrahydrate in dmf following the
method described by Adler et al.31,32 The FePors were
purified by column chromatography on silica or
alumina. The products were characterized by UV-Visible
and EPR spectroscopy, and the data were consistent
with the expected compound after the metallation
reaction. [Fe(TMPyP)] (deionized water): 390 nm
(e = 13×103 L mol-1 cm-1), [Fe(TDFPP)] (dichlorometane)
412 nm (e = 45×103 L mol-1 m-1), [Fe(TDCPP)]
(dichlorometane) 412 nm (e = 47×103 L mol-1 cm-1), which
were denoted FePor 1, FePor 2, and FePor 3, respectively.
The positive charges and the chlorine counter ion will be
omitted in the text for simplification purposes.
70 °C for complete dryness. The final materials, gelatinized
after about 7 days, consisted in transparent brown-emerald
green stable gels similar to the FePor-SGB solids. The wet
gels were also extensively washed with MeOH, EtOH, H2O
and dcm by Soxhlet procedure, followed by drying for 24 h
at 70 °C. In the case of the acid sol-gel process, UV-Vis
spectroscopy detected no FePor in the solvent recovered at
the end of the washing process.A control sample containing
no added FePor was also synthesized in the same conditions
described above.
Catalytic oxidation reaction
Catalytic oxidation reactions were carried out in a 2 mL
thermostatic glass reactor equipped with a magnetic stirrer
placed inside a dark chamber. Alkene and alkane reactions
were accomplished using FePor-SGB or FePor-SGA as
catalysts (about 0.01 g), and (Z)-cyclooctene, cyclohexene
or cyclohexane as substrates. The alkenes had been
previously purified on an alumina column. In a standard
experiment, the solid catalyst and iodosylbenzene (FePor/
PhIO molar ratio = 1:50) were suspended in 0.400 mL of
the solvent (dichloromethane/acetonitrile 1:1 mixture v/v)
inside a 2 mL vial and degassed with argon for 15 min. The
substrate (FePor/substrate molar ratio = 1:5000) was added,
and the oxidation reaction was carried out for 1 to 24 h,
under magnetic stirring. Sodium sulfite was added at the end
of the reaction, to eliminate the excess of iodosylbenzene.
The soluble reaction products were separated from the solid
catalyst by centrifugation and transferred to a volumetric
flask. Then, the solid catalyst was washed several times
with dichloromethane and acetonitrile, in order to extract
any remaining product. The solution containing the final
reaction solution plus the solvents from the washings of the
solid catalyst was analyzed by gas chromatography. Product
yields were quantified on the basis of PhIO, using n-octanol
(acetonitrile solution, 1.0×10-2 mol L-1) of high purity
degree (99.9 %) as internal standard. Control reactions were
carried out using the same procedure in the case of (a) the
substrate alone, (b) substrate + PhIO, and (c) substrate +
PhIO + silica (without FePor). The FePor in solution was
also tested as a catalyst (homogeneous catalysis), and the
experimental procedure in this case was similar to that used
for the heterogeneous catalysis.
Immobilization of 1, 2, and 3 in silica prepared by the sol-
gel process (FePor-SGB and FePor-SGA solids)
Basic catalysis
The FePor-SGB catalysts were prepared by the Stöber
methodology,25 through hydrolysis of TEOS in ethanol/
methanol, employing NH4OH as catalyst, 1×10-5 mol of
the desired FePor, and the following molar ratios: H2O/Si
(TEOS) = 50, NH3/EtOH = 10, and H2O/NH3 = 200. An
alternative methodology utilizing isopropyl alcohol (PriOH)
instead of ethanol (NH3/PriOH = 10) in the presence of
7×10-6 mol of FePor 1, 2 or 3 was also employed. The
reaction mixture was kept at 40 °C; the solution was
stirred for 30 min and kept at about 60 °C for complete
dryness. The final materials, gelatinized after about 14
days, consisted in transparent brown-emerald green stable
gels. The wet gels were extensively washed with MeOH,
EtOH, H2O and dcm by Soxhlet procedure, followed by
drying at 70 °C for 24 h. The FePor loading in the silica
was quantified by UV-Vis spectroscopy, by measuring
the amount of non-immobilized FePor recovered during
the washing of the solid. A control sample containing no
FePor was also synthesized in the same way as described
above, furnishing a transparent colorless gel that was also
washed with MeOH, EtOH, H2O and dcm by the same
Soxhlet procedure, and then dried in an oven at 70 °C for
24 h.
After the first use, the solid catalyst was thoroughly
washed and dried for reuse in another reaction, under the
same experimental conditions employed in its first use.
Acid catalysis
The FePor-SGA catalyst was prepared by the sol-gel
process through hydrolysis of TEOS in ethanol/methanol,
employing HCl as catalyst, 1×10-5 mol of the desired
FePor and the following molar ratios: H2O/Si (TEOS) = 4,
thf/HCl = 308, and H2O/HCl = 92.21 The reaction mixture
was stirred for 1 h at room temperature, and kept at about
Characterization and apparatus
Electronic spectra were recorded on an HP 8452A
DiodeArray UV-Vis spectrophotometer, in the 200-800 nm

1332
Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica
J. Braz. Chem. Soc.
range. Spectra of the solid samples were recorded in a
0.1 cm path length quartz cell (Hellma) in nujol mull.
FTIR spectra were registered on a Biorad 3500 GX
spectrophotometer in the 400 to 4000 cm-1 range, using KBr
pellets. KBr was crushed with a small amount of the solid
samples, and the spectra were collected with a resolution
of 4 cm-1 and accumulation of 32 scans.
hydroxyl or silanolate anions attack Si directly. Hydrolysis
of alkylsilicates followed by condensation in appropriate pH
furnishes high purity silica particles.25 The choice between
acid or basic catalysis for silica synthesis influences the
nature of the prepared catalyst. For this reason, we decided
to study the role played by pH in the catalytic activity of
the materials prepared herein.
For the X-ray diffraction (XRD) measurements, self-
oriented films were placed on neutral glass sample holders.
The measurements were performed in the reflection mode
using a Shimadzu XRD-6000 diffractometer operating at
40 kV and 40 mA (Cu-Ka radiation, l = 1.5418 Å) with a
dwell time of 1q/min.
Electron paramagnetic resonance (EPR) measurements
of the powder materials were carried out on an EPR
BRUKER ESP 300E spectrometer (standard concavity:
4102-SP, frequency X band 9.5 GHz), at 293 or 77 K,
using liquid N2.
The control silica sample, containing no FePor, is white,
whereas the solids obtained after FePor immobilization are
of a light brown-green color that gradually deepens with
increasing FePor concentration.
After the solids were dried, FePor loading in the silica
was determined by measuring the difference between
the absorbance of the complex in the solution prepared
before the immobilization process and the absorbance
of the solution and combined washings from the Soxhlet
procedure obtained after FePor immobilization (Table 1).
Table 1. Immobilization of FePors in silicas obtained by the sol-gel
process
Thermogravimetric (TGA) measurements were
performed on a Thermal Analyst TA Instrument SDT 600
Simultaneous DTA-TGA-DSC, in nitrogen, with a heating
rate of 20 ºC min-1, from 25 to 1200 ºC.
Solid
Percentage of
immobilization (%)a
Loading (concentration of
FePor in the silica)
Transmission electron microscopy (TEM) was
accomplished on an 80 kV JEM 1200EXII Electron
Microscope. A drop of a suspension of the catalyst powder
was deposited on a 300 mesh copper grid (formvar/carbon).
Products from the catalytic oxidation reactions were
identified using a Shimadzu GC-14B gas chromatograph
(flame ionization detector) equipped with a DB-WAX
capillary column, 30 m (J & W Scientific).
Specific surface areas were determined by applying the
BET method33 to the corresponding nitrogen adsorption
isotherms, obtained by using a physical adsorption analyzer
Micrometrics ASAP 2020. The samples were previously
degassed by treatment at 180-200 ºC until the system
pressure reached 10 μmHg. The nitrogen adsorption data
were obtained using 0.2 g of the sample.
(mol g-1)
8.5 × 10-6
2.2 × 10-5
9.0 × 10-6
2.3 × 10-5
8.4 × 10-6
1.8 × 10-5
% (m/m)
0.83
2.1
FePor 1-SGA
FePor 1-SGB
FePor 2-SGA
FePor 2-SGB
FePor 3-SGA
FePor 3-SGB
100
98.0
99.6
95.4
100
0.75
1.9
0.81
1.7
96.7
a Iron porphyrin immobilized in the silica considering the initial mass used.
Table 1 reveals high FePor immobilization (> 95%) in
the silica support, and FePor immobilization is superior
in the materials prepared by acid catalysis. The structure
of the final silica materials explains the FePor percentage
of immobilization. An open silica network arises during
acid catalysis, which provides a larger number of available
sites and facilitates FePor entrapment. In contrast,
materials achieved by means of basic catalysis consist in
interconnected silica spherical particles, thereby leading to
lower FePor percentage of immobilization.
Acid catalysis and low H2O/Si ratios furnish weakly
branched polymeric sols and gels, which results in highly
microporous xerogels with a very fine texture. Basic
catalysis and high H2O/Si ratios, on the other hand, produce
colloidal particles that give rise to meso- or macroporous
xerogels composed by translucent gels of high porosity.21,34
The mechanism through which FePor immobilization
occurs in neutral medium is not completely understood.
Interactions between the π-conjugated electron cloud
Results and Discussion
The FePor-SGA and FePor-SGB solids were prepared
through hydrolysis and condensation of the alkoxysilane
(TEOS) in alcohol and water, using ammonia or HCl as
catalysts in the presence of one of the FePors (1, 2 or 3).
The hydrolysis and polycondensation reactions initiate at
numerous sites as soon as TEOS and H2O are mixed. Under
normal conditions, both hydrolysis and condensation may
occur by acid or base-catalyzed bimolecular nucleophilic
substitution reactions. The acid-catalyzed mechanisms
proceed via rapid deprotonation of the -OR or -OH
substituents bound to Si,21 whereas under basic conditions

Vol. 21, No. 7, 2010
Castro et al.
1333
of the macrocyclic rings and the structure of the highly
hydroxylated silica are possible. Furthermore, the FePor
can insert into the three-dimensional network of the silica
resulting from Si-O-Si links. This leads to agglomeration
of the molecules, thereby forming larger particles and
agglomerates, or a colloidal sol (Figure 2).
In the case of FePor 1, in addition to the possible
interactions mentioned above, electrostatic interactions
between FePor 1 and the silica surface could also take
place, giving rise to FePor immobilization on the surface
of the support.
for the control silica SGB and FePor-SGB shows that the
silica particles are more spherical in the presence of FePors.
However, particle morphology is irregular in the cases of
the FePor 2-SGB and FePor 3-SGB hybrid materials, but
spherical for FePor 1-SGB.
The positive character of the charged FePor 1 may favor
agglomeration of the FePor 1-SGB particles, which would
account for the production of beads only in the presence of
this FePor, under the experimental conditions of this work.
Wang et al.23 observed that silica prepared by the
Stöber25 methodology tends to absorb cationic porphyrins
strongly. This is because this methodology can produce
negatively charged spherical particles depending on the pH
conditions employed during silica preparation. The silanol
groups present on the material achieved at high pH can
dissociate and produce negative charges, thus facilitating
FePor adsorption. This is an excellent approach for the
preparation of materials consisting of spheres.23
The solids obtained by immobilization of FePors1, 2 and
3 in the silica achieved by the sol-gel process (acid and basic
catalysis) as well as the control silica (without FePor) were
analyzed by XRD (Figure not shown). The XRD patterns
of all prepared solids revealed only a halo in the region of
20 and 30º for 2q, which means that they are amorphous.
The presence of FePor in the hybrid FePor-SG solids
was confirmed by EPR analysis. Figure 4 depicts the EPR
spectra of SGA (a), FePor 1-SGA (b), FePor 1-SGB (c),
FePor 2-SGA (d), FePor 2-SGB (e), FePor 3-SGA (f),
and FePor 3-SGB (g) at 77 K. The control silicas SGA
(Figure 4a) and SGB (not shown) display EPR-silent
spectra. The absence of EPR signals indicates that these
solids are free of contaminating paramagnetic species that
Figure 2. Schematic representation of the FePor-silica solid formed by
the sol-gel process.
Figure 3 presents the transmission electron micrographs
of the FePor-SG catalysts and the control materials
(containing no FePor). As observed previously by us,19
addition of porphyrins to silica matrices obtained by the
sol-gel methodology can affect particle morphology and
agglomeration. Comparison of the micrographs obtained
Figure 3. Transmission electron microscopy (TEM). (a) FePor 1-SGA; (b) FePor 1-SGB; (c) FePor 2-SGA; (d) FePor 2-SGB; (e) FePor 3-SGA; (f) FePor
3-SGB; (g) SGA-blank; (h) SGB-blank.

1334
Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica
J. Braz. Chem. Soc.
could have been inserted into the silica during the synthetic
process. All the prepared FePor-SG solids display EPR
signals at g = 6.0 (1000-1200 gauss), characteristic of
high-spin iron(III) in axial symmetry, 24,35-37 and a small
signal at g = 4.3, typical of high-spin iron(III) in rhombic
symmetry.37,38 The latter EPR signal suggests that, upon
immobilization, some or all FePor molecules may undergo
a rhombic distortion because of their interaction with the
structure of the support. Nevertheless, the axial symmetry
signal at g = 6.0 is still present in high intensity.
of the FePor-SG materials. For instance, the characteristic
Si-O stretching band shifts to lower wavenumber after
FePor immobilization. This shift is more pronounced for
the FePor 1-SGA solid, indicating that this FePor interacts
with the silica siloxane groups, and that FePor 1 might be
located inside the matrix pores.39
The presence of the typical EPR iron(III) signal in
the silica solids confirms that during the immobilization
process no significant demetallation of the FePors took
place. Demetallation could have been caused by binding
of the metal ion to the coordinating groups in the solid
derivative, in the presence of the hydroxyl groups.37
Figure 5. FTIR spectra (a) FePor 1-SGA; (b) FePor 1-SGB; (c) FePor
2-SGA; (d) FePor 2-SGB; (e) FePor 3-SGA; (f) FePor 3-SGB.
The prepared solids were also analyzed by UV-Vis
spectroscopy in nujol mull (Figure 6). The spectra of all
FePor-SG solids displayed a typical peak enlargement40
and presented the Soret band characteristic of FePors in
the region of 400 nm, thereby confirming the presence of
FePor 1, 2, or 3 in the prepared silicas. However, the Soret
band of all the FePor-SG materials was shifted to lower
energy compared with the spectrum of the parent FePor
in solution (spectrum collected in Nujol oil emulsion -
figure not shown). This demonstrates that no demetallation
(characterized by a blue shift of the Soret band typical of the
presence of a significant amount of free-base porphyrin)18 or
significant metal ion exchange between the metallocomplex
and the support occurs during the immobilization process.
On the other hand, the red-shifted Soret band obtained in
the case of the immobilized FePors gives evidence that
FePor immobilization in the solid causes no important
steric constraints that would substantially modify the
metalloporphyrin structure in the supported catalyst.36 In
fact, the EPR spectrum (Figure 4) displays signals both
at g = 6.0 and g = 4.3, thus confirming that only some
distortion of the FePor takes place after the immobilization
process. The UV-Vis spectrum of the solid does not allow
for a more detailed analysis of the visible region. For
metalloporphyrins in solution, this region often exhibits
bands that aid understanding of the axial coordination of
Figure 4. EPR spectra of solid samples obtained at 77K. (a) SGA; (b)
FePor 1-SGA; (c) FePor 1-SGB; (d) FePor 2-SGA; (e) FePor 2-SGB; (f)
FePor 3-SGA; (g) FePor 3-SGB.
Figure 5 shows the FTIR spectra of the solids FePor-SG
prepared in this work (Figure 5a-5e).
The main characteristic of the FTIR spectrum of pure
silica is an intense, wide band in the region of 3420 cm-1,
due to the typical axial deformation of superficial SiO-H
groups. Also, there is a large band at 1630 cm-1, ascribed
to water molecules adsorbed/absorbed in the silica. The
band in the region of 1100 cm-1 is characteristic of the
stretching of the 180-degree angle of the Si-O-Si groups of
the four tetrahedral SiO2. The band at 960 cm-1 is assigned
to Si-OH vibration, and the band at 798 cm-1 is associated
with the distorted structure of SiO4.36 The FTIR bands of
the FePors are not observed, probably because of the low
concentration of the adsorbate compared with the support
(Table 1). Nevertheless, some differences are noted when
the spectrum of the control solid is compared with those

Vol. 21, No. 7, 2010
Castro et al.
1335
the metal and help determine whether dimeric species are
formed during the immobilization process.41
600 ºC, attributed to FePor decomposition, followed by
condensation and collapse of the inorganic structure,37
resulting in SiO2. FePor loss occurs approximately between
400 and 600 ºC, and the residual groups are lost from 200
to 650 ºC. There is no exothermic effect. Similar results
were obtained for all the FePor-SG solids.
The surface area and porosity of the materials,
determined from nitrogen adsorption, are shown in Table 2.
Comparison of the materials obtained by acid or basic
catalysis reveals that the SGB materials have the largest
surface areas. Indeed, low surface area values are found
for materials obtained by acid catalysis.
Table 2. Physicochemical properties of FePor-SGA and FePor-SGB
materials
Sample
S
BET (m2 cm-1)a
Vp (cm3 g-1)a
Dp (Å)a
98.0
64.5
24.0
--
FePor 1-SGB
FePor 3-SGB
FePor 2-SGA
FePor 3-SGA
266
255
35
0.65
0.41
0.02
--
Figure 6. UV-Vis spectra (a) FePor 1-SGA; (b) FePor 1-SGB; (c) FePor
2-SGA; (d) FePor 2-SGB; (e) FePor 3-SGA; (f) FePor 3-SGB.
0.01
FePor dimeric species (e.g., m-oxo dimer species) are
often observed when FePors are obtained in alkaline media,
preferably in the presence of water traces. In this case,
the FePor solution usually acquires a green color, and so
does the solid obtained by immobilization of the dimeric
species. The UV-Vis spectrum of the dimeric species in
solution typically presents two bands, namely one in the
570-580 nm region (more intense) and another between
610-620 nm (shoulder). Furthermore, the Soret band shifts
to lower energy, compared with the corresponding band
for the monomeric species.41 Due to the difficult analysis
of the visible region of the spectrum in the case of FePor
immobilization in solid supports like silica, the presence of
some dimeric species in the support cannot be discarded,
mainly in the silica solids obtained by the base-catalyzed
sol-gel process. However, if this dimerization process
took place, it occurred in minor proportion, since the EPR
spectra (Figure 4) indicate the presence of monomeric
iron(III) porphyrin species. The μ-oxo dimeric species
of FePors are EPR silent, due to the antiferromagnetic
coupling of the iron ions via μ-oxo bridge.41
From these results, we can assume that the FePor
molecules were really introduced into the silica gels.21
Thermogravimetric and differential thermal analyses
(TGA and DTA) (figure not shown) provide information
on the thermal stability of the solids obtained after FePor
immobilization. Such analyses give evidence of two
events. Firstly, there is a mass loss of around 20%, which
is associated with an endothermic transition centered
at 150 °C and corresponds to loss of adsorbed H2O
molecules. There is another mass loss between 200 and
aBET surface area (SBET), average pore volume (Vp) and average pore
diameter (Dp) of the FePor-SGA/SGB samples.
As already mentioned, the pH employed in the
preparationofthesilicasupportinfluencesthecharacteristics
of the obtained material. A highly branched material is
achieved by acid catalysis (SGA), which usually contains
a large number of micropores. If the FePor is located on
the surface of SGA, it hinders nitrogen gas adsorption
by the FePor-SGA catalyst, thereby making this type of
analysis impossible.
Catalytic oxidation reactions
ThecatalyticactivitiesofFePors1,2and3(homogeneous
catalysis) and those of the corresponding supported
catalysts (heterogeneous catalysis) in the oxidation of
three substrates, namely two alkenes ((Z)-cyclooctene
and cyclohexene) and one alkane (cyclohexane), were
investigated (Figure 7). The results are displayed in Tables
3 to 5. The catalytic yields depicted in these tables can be
attributed to all the solids resulting from immobilization
of the FePs in the silicas (SGA or SGB), since control
reactions carried out with the pure matrices (containing no
FePor) did not furnish any oxidation products.
Since the first report of Groves et al.,42 iodosylbenzene
(PhIO) has been frequently employed as oxygen transfer
agent in oxidation reactions using metalloporphyrins.
Those authors demonstrated that this reactant is relatively
inert in the absence of the catalyst and reacts with FePors
to produce the intermediate active species oxoiron(IV)

1336
Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica
J. Braz. Chem. Soc.
Table 3. (Z)-cyclooctene oxidation by PhIO catalyzed by FePors in
homogeneous and heterogeneous mediaa
Catalystb
Run
Epoxide yield (%)
1 hc
60
42
27
65
86
60
68
63
65
5
24 hd
66
28
44
79
97
69
83
75
83
-
FePor 1
1
2
FePor 1-SGA
FePor 1-SGB
FePor 2
3
4
FePor 2-SGA
FePor 2-SGB
FePor 3
5
6
7
FePor 3-SGA
FePor 3-SGB
SGA + PhIO (control)
SGB + PhIO (control)
PhIO (control)
8
Figure 7. Schematic representation of the catalytic oxidation reactions
using FePor-SG as heterogeneous catalysts.
9
10
11
12
porphyrin cation radical complex (O=FeIVporphyrin•+).
Furthermore, the polymeric nature of this oxidant is efficient
to suppress metal-mediated free radical propagation
reactions often observed in the case of cytochrome P-450
mimetic systems.43
A UV-Vis spectrum of the supernatant reaction solution
was recorded after all the catalytic reactions. The typical
Soret band of FePors was not detected in any of the cases,
proving that no catalyst leaching form the support occurred
in any of the reaction conditions investigated in this study.
In other words, the catalysis is truly heterogeneous.
5
-
5
-
aThe yield of cyclooctene oxide was calculated on the basis of the amount
of PhIO used in the reaction. The results represent an average of at least
duplicate reactions. FePor/PhIO/(Z)-cyclooctene molar ratio = 1:50:5000.
bThe piece of glass obtained from the sol-gel process was hardly grounded,
and the resulting powder was used as catalyst. Reaction time: c1 and d24 h.
density on the porphyrin ring, thereby stabilizing the
catalyst against oxidative degradation.45 Furthermore, the
relative solubility observed for the three FePors in the
reaction solvent (dichloromethane/acetonitrile mixture) is
FePor 3 > FePor 2 >> FePor 1, and this relative solubility
can also explain the catalytic yields depicted in Table 3.
All the immobilized FePors display catalytic activity
for (Z)-cyclooctene oxidation, thus confirming that the
immobilization process and the experimental conditions did
not destroy or transform the FePors into inactive species
(Table 3).
In reactions performed with the solid obtained by the
sol-gel process (heterogeneous catalysis, Table 3, runs 2,
3, 5, 6, 8 and 9) the overall results obtained at one hour
of reaction are generally lower than those obtained at the
same reaction time with the corresponding homogeneous
catalysts. It is frequently reported that the catalytic activity
of metalloporphyrins is reduced upon immobilization,
because the access of both the substrate and the oxidant to
the active site is limited by the structure of the support.46
This seems to be the case with the heterogeneous catalysts
investigated here, except for the FePor 3-SGA solid
(Table 3, run 5), which furnished higher yields after FePor
immobilization in SGA. Table 3 shows that better catalytic
results are achieved at 24 h. This suggests that access of the
reactants to the metallic catalytic center is facilitated after
longer contact time. FePor 1-SGA is an exception (Table 3,
run 2), since longer reaction time actually decreased
(Z)-Cyclooctene
It is well established that the oxidation of (Z)-
cyclooctene by metalloporphyrin/PhIO systems produces
epoxide as the sole oxidation product, with no traces of
allylic alcohol or ketone.44 For this reason, this alkene is
frequently employed as a diagnostic substrate in biomimetic
catalytic systems involving metalloporphyrins. In this work,
we used this substrate to investigate the efficiency and
stability of the immobilized FePors as catalysts for alkene
oxidation by PhIO. This study also provided information
about the accessibility of the substrate and the oxidant to
the iron(III) sites in the immobilized catalyst.
Table 3 shows the results from (Z)-cyclooctene
oxidation with different catalysts. The reactions were
accomplished in both homogeneous and heterogeneous
environments.
The homogeneous FePors 2 and 3 furnish high epoxide
yields (Table 3, runs 4 and 7). FePor 1 is not completely
solubilized in the reaction solvent, accounting for the
lower yields (run 1). Both FePors 2 and 3 are expected
to be highly efficient catalysts for oxidation reactions.
This is because the electronegative substituents on their
phenyl rings contribute to a reduction of the electronic

Vol. 21, No. 7, 2010
Castro et al.
1337
Table 4. Cyclohexane oxidation by PhIO catalyzed by FePors in
product yield. This might be related to the FePor structure,
which is not robust enough to support long reaction times.
Thus, some FePor destruction may have occurred.
homogeneous and heterogeneous mediaa
Catalystb
Run
Alcohol yield
(%)c
Ketone yield
(%)d
Generally, the solids prepared by acid catalysis (FePor-
SGA) led to better catalytic yields (Table 3, runs 2 and
5) than those synthesized by the base-catalyzed sol-gel
route (FePor-SGB) (Table 3, runs 3 and 6). These results
agree with the characterization data obtained for the solids
by EPR and UV-Vis spectroscopy, suggesting that some
dimeric FePor species (e.g., m-oxo dimer species) could
be present together with the monomer in SGB, prepared
by the base-catalyzed sol-gel process. The dimerization of
FePor 1 and FePor 2 can take place because both porphyrin
ligands do not have bulky substituents like FePor 3, which
contains chlorine atoms that protect the porphyrin ring from
dimerization. In fact, some dimeric and other aggregated
species have been well characterized for FePors 147-49 and
2. Moreover, dimeric FePor 1 presents low catalytic yield
compared with the monomeric species.50
FePor 1
1
2
6
7
1
-
FePor 1-SGA
FePor 1-SGB
FePor 2
3
2
1
1
-
4
55
4
FePor 2-SGA
FePor 2-SGB
FePor 3
5
6
3
14
2
20
7
-
7
37
2
FePor 3-SGA
FePor 3-SGB
SGA + PhIO (control)
SGB + PhIO (control)
PhIO (control)
8
9
7
10
11
12
trace
trace
trace
-
-
aThe yields of the reactions were calculated on the basis of the amount
of PhIO used in the reaction. The results represent an average of at least
duplicate reactions. FePor/PhIO/cyclohexane molar ratio = 1:50:5000.
bThe piece of glass obtained from the sol-gel process was hardly
grounded, and the resulting powder was used as catalyst. cCyclohexanol
and dCyclohexanone.
Cyclohexane
In general, the oxidation of cyclohexane with
iodosylbenzene in the presence of metalloporphyrins
commonly yields cyclohexanol and cyclohexanone as
major products, so this kind of catalyst is frequently
considered cytochrome P-450 biomimetic models. High
selectivity for the alcohol product is also observed.Alcohol
formation occurs via proton abstraction by the active
intermediate species oxoiron(IV) porphyrin π-cation radical
complex, responsible for the hydroxylation reaction.51
Cyclohexane is a very useful substrate to investigate the
efficiency of FePors as catalysts for alkane hydroxylation
by iodosylbenzene. The activation of inert C-H bonds
in alkanes calls for more drastic conditions than those
necessary for alkene functionalization, thus allowing for
a better differentiation between the performances of a
metalloporphyrin catalyst in solution and immobilized in
a solid support.10,42,48,52,53
of the catalytic intermediate active species oxoiron(IV)
porphyrin π-cation radical complex is directly related to
the electronic nature of the porphyrin ligand.54 Probably,
the electron-deficiency of the porphyrin ligands containing
chlorine and fluorine substituents is very similar, but any
difference in their oxidizing capacity is more evident when
the substrate is hard to oxidize, as in the case of cyclohexane
and other alkanes.55
In line with the results from (Z)-cyclooctene oxidation
(Table 3), immobilization of all three FePors rendered
solids with low catalytic capacity for selective cyclohexane
hydroxylation as well as lower alcohol yield, compared
with the homogeneous counterparts. For the less reactive
cyclohexane, access to the catalytic centre could be even
more critical than in the case of highly reactive substrates
like (Z)-cyclooctene.38
Table 4 shows that FePors 1, 2 and 3 were catalytically
active for cyclohexane oxidation (homogeneous catalysis).
However, we had expected that both FePors 2 and 3 would
present higher efficiency than FePor 1 (Table 4, run 1), since
the former contain robust, electron-deficient porphyrin
structures. In fact, high product yields (59%) and high
selectivity for cyclohexanol were observed for FePor 2
(Table 4, run 4), but not so high for FePor 3 (Table 4, run
7, 37%). The relative solubility and electron-deficiency
caused by the electron-withdrawing substituents on the
porphyrin ring of both FePors 2 and 3 could explain these
results. Nam et al. observed that the oxidizing power
In contrast with homogeneous media, reactions 6, 8 and
9 (Table 4) reveal large ketone production. This behavior is
a consequence of the proposed reaction mechanism.51 For
homogeneouscatalyticsystemsusingFePors, Lindsay-Smith
etal.56 observedthat,dependingontheporphyrinstructureand
catalysis conditions, ketone was formed from further alcohol
oxidation.As for the heterogeneous catalysis investigated in
the present work, the pores of the silica structure can delay
diffusion of the alcohol into the solution, and the presence
of this product near the active metal centre can result in its
further oxidation, in a competitive process with cyclohexane.
This situation is more dramatic when the structure of the

1338
Preparation of Catalysts based on Iron(III) Porphyrins Heterogenized on Silica
J. Braz. Chem. Soc.
Table 5. Cyclohexene oxidation by PhIO catalyzed by FePors in
porphyrin ring contains bulkier substituents, as in the case
of FePor 3 (Table 4, run 2). In fact, we recently observed
the same catalytic behavior when metalloporphyrins
immobilized in LDH (layered double hydroxide)55 and
silica19 were employed as catalysts for alkane oxidation.
Catalyticexperimentsatdifferentreactiontimescorroborated
ketone production from alcohol re-oxidation.
homogeneous and heterogeneous mediaa
Catalyst
Run
Epoxide yield Ketone + Alcohol
(%)b
36
16
26
13
34
57
35
21
72
6
(%)c
FePor 1
1
2
40
FePor 1-SGA
FePor 1-SGB
FePor 2
>100
>100
36
3
4
Cyclohexene
FePor 2-SGA
FePor 2-SGB
FePor 3
5
22
6
>100
42
The products generated from cyclohexene oxidation
mediated by PhIO/FePor systems result from a competition
between the C=C and allylic C-H groups on the alkene
for the electrophilic active species oxoiron(IV) porphyrin
π-cation radical complex57,58 formed in the reaction
between FePor and iodosylbenzene. Oxidation of these
groups should lead to cyclohexene oxide and/or allylic
alcohol (1-cyclohexen-3-ol) and ketone (1-cyclohexen-3-
one), respectively.59,60 Homogeneous PhIO/FePor systems
furnish allylic products in minor yields compared with the
epoxide, and the efficiency and selectivity of the catalytic
reaction toward the latter compound is controlled by the
reaction conditions (solvent, temperature, inert atmosphere
and reactants molar ratio),61 the structure of the porphyrin
ring, and the presence of axial ligands to the iron center.35,36
In a preliminary examination (Table 5), the use
of the three investigated FePors in homogeneous and
heterogeneous systems for cyclohexene oxidation gave
epoxide and significant yields of allylic oxidation products
(Table 5). However, the control reactions indicate that the
silica (SGA and SGB) support alone largely contributes to
the total product yields (Table 5, runs 10 and 11), and the
reaction performed with PhIO and substrate only (Table 5,
run 12) also leads to significant amounts of allylic products.
Therefore, the results concerning the final percentage
of product yields obtained after deduction of the yields
achieved in the control reactions should provide a more
realistic picture of the catalytic results obtained with both
homogeneous and heterogeneous systems.
The product distributions and yields achieved in the
oxidation reactions catalyzed by the FePors are consistent
with the presence of dioxygen in the reaction medium,
especially in the case of the heterogeneous systems.
Controlling the presence of dioxygen is a problem inherent
to heterogeneous catalysis since this gas might be present
in the voids of the small pores and cavities of the silica,
making it difficult to ensure its absence. The presence of
dioxygen in the support was confirmed by the high yields
of allylic products obtained when only the SGA or SGB
support (with no immobilized FePor) was used as catalyst
(Table 5, runs 10 and 11).
7
FePor 3-SGA
FePor 3-SGB
SGA + PhIO (control)
SGB + PhIO (control)
PhIO (control)
8
>100
>100
>100
>100
25
9
10
11
12
6
-
aReaction time: 1 h. FePor/PhIO/cyclohexene molar ratio = 1:50:5000.
The yield of the reaction was calculated on basis of the amount of PhIO
employed used in the reaction. The results represent an average of at
least duplicate reactions. All the catalyst solids used were macerated.
bEpoxide =cyclohexene oxide.cKetone +Alcohol = 2-cyclohexen-1-one +
2-cyclohexen-1-ol.
In fact, we recently reported that the oxidation of
cyclohexene in the presence of immobilized FePor and air,
under magnetic stirring, in catalytic reaction conditions
similar to those employed here, resulted in the preferable
conversion of cyclohexene to allylic products.35 This result
was consistent with a free-radical autooxidation mechanism
mediated by the FePor in solution or immobilized
into ZnnAl-LDH.62 Most of the allylic products from
cyclohexene conversion are produced when the substrate
and solvent are stirred magnetically in air, for the same
time period and under the same temperature conditions as
the catalytic reaction.
In spite of the fact that the allylic products yields were
superior to 100% because of the uncontrolled presence of
oxygen from air, the catalyst FePor 2-SGB (Table 5, run 9)
furnished high epoxide yield, showing that this system can
be very efficient for cyclohexene oxidation.
The recyclability of the solids resulting from
immobilization of the three FePors was investigated in
the oxidation of cyclohexene. It was done in the same
experimental conditions employed during the first use
of each catalyst, after the solids had been recovered and
washed for removal of traces of products (or iodobenzene
and iodosylbenzene) that could have remained inside the
silica structure. UV-Vis analysis of the solvents used in the
washing process proved that there was no FePor leaching
from the support. The catalyst solids were reused in two
further catalytic reactions and, in spite of the high quantity

Vol. 21, No. 7, 2010
Castro et al.
1339
of allylic products, the epoxide yields were similar to those
achieved during the first use of the catalyst.
it led to higher yields for (Z)-cyclooctene and cyclohexene
oxidation compared with the corresponding homogeneous
system. Nevertheless, it must be borne in mind that FePor
heterogenization is advantageous since it enables catalyst
recycling, thereby resulting in high total turnovers.
Surprisingly, despite the low amount of FePor in
the silica, there is a non-linear dependence of catalytic
efficiency on surface area and loading. Additionally, there
is no direct relationship between catalytic efficiency and
surface area or FePor loading on the support for any of the
studied substrates. Surface area depends solely on the silica
preparation method (acid or basic catalysis).
Based on the catalytic results and characterization of
the solids, we argue that catalyst efficiency is more related
to FePor dispersion on the support than to the final surface
area of the catalyst. This dispersion can be affected by such
parameters as FePor interaction with the support, which in
turn is governed by the pH used during silica preparation.
Strong interactions between the catalyst and the support
may lead to better FePor dispersion in the matrix.
Acknowledgments
The authors are grateful to Conselho Nacional de
Desenvolvimento Científico e Tecnológico (CNPq),
Coordenação de Aperfeiçoamento de Pessoal de Nível
Superior (CAPES), Fundação Araucária, Fundação
da Universidade Federal do Paraná (FUNPAR) and
Universidade Federal do Paraná (UFPR) for financial
support. They also thank the Centro de Microscopia
Eletrônica da UFPR for the TEM analyses.
Finally, it is noteworthy that the catalysts prepared here
have lower FePor loadings compared with other systems
reported in the literature. Nonetheless, our catalysts led to
similar (Z)-cyclooctene conversion results. For example,
Battioni et al.63 reported that the FePor-supported silica
system catalyzed the epoxidation of cyclooctene by PhIO
with high epoxide yields (> 90%), by employing materials
with high surface area (ca. 700 m2 g-1) and catalyst loading
of 150 mg g-1 support. The catalytic yields are similar to
those achieved with the SGA-FePor or SGB-FePor catalysts
prepared by us, but it may be considered that the catalyst
prepared by Battioni et al.63 has an iron porphyrin content
of 15 g g-1 (15%), which is 10 times higher than the content
in our SG-FePor catalysts (Table 1, 0.8 to 2.1 g g-1).
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