Catalytic activity of anionic iron(III) porphyrins immobilized on grafted disordered silica obtained from acidic leached chrysotile

Article abstract of DOI:10.1016/j.molcata.2005.08.019

Hydrated disordered silica obtained by leaching chrysotile with hydrochloric acid was grafted with 3-APTS and reacted with aqueous iron porphyrins solutions of [Fe(TDFSPP) and Fe(TCFSPP)]. The obtained materials were characterized by powder X-ray diffraction (PXRD), UV-vis, FTIR and electron paramagnetic resonance (EPR) spectroscopies and investigated as catalysts in oxidation reaction of cyclohexane using iodosylbenzene as oxidant. The catalytic activities obtained in heterogeneous media for Fe(TDFSPP) was superior to the results obtained in homogeneous conditions but the opposite effect was observed for the Fe(TCFSPP), indicating that instead of the structural similarity of both iron porphyrins (second generation porphyrins), the immobilization way produced different catalysts. The best catalytic activity of the Fe(TDFSPP)/Si-3-APTS (65%) compared to Fe(TCFSPP)/Si-3-APTS (33%) can be explained by the easy access of the oxidant and the substrate to the catalytic sites in the former. A schematic representation for the immobilization and a mechanism for the oxidation reaction have been presented.

Full text of DOI:10.1016/j.molcata.2005.08.019

Journal of Molecular Catalysis A: Chemical 243 (2006) 44–51
Catalytic activity of anionic iron(III) porphyrins immobilized on grafted
disordered silica obtained from acidic leached chrysotile
Matilte Halma, Alesandro Bail, Fernando Wypych, Shirley Nakagaki∗
Universidade Federal do Parana´, Departamento de Qu´ımica, CP 19081, 81531-990 Curitiba, PR, Brazil
Received 23 May 2005; received in revised form 10 August 2005; accepted 10 August 2005
Available online 19 September 2005
Abstract
Hydrated disordered silica obtained by leaching chrysotile with hydrochloric acid was grafted with 3-APTS and reacted with aqueous iron por-
phyrins solutions of [Fe(TDFSPP) and Fe(TCFSPP)]. The obtained materials were characterized by powder X-ray diffraction (PXRD), UV–vis,
FTIR and electron paramagnetic resonance (EPR) spectroscopies and investigated as catalysts in oxidation reaction of cyclohexane using iodosyl-
benzene as oxidant. The catalytic activities obtained in heterogeneous media for Fe(TDFSPP) was superior to the results obtained in homogeneous
conditions but the opposite effect was observed for the Fe(TCFSPP), indicating that instead of the structural similarity of both iron porphyrins
(second generation porphyrins), the immobilization way produced different catalysts. The best catalytic activity of the Fe(TDFSPP)/Si-3-APTS
(65%) compared to Fe(TCFSPP)/Si-3-APTS (33%) can be explained by the easy access of the oxidant and the substrate to the catalytic sites in the
former. A schematic representation for the immobilization and a mechanism for the oxidation reaction have been presented.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Iron porphyrin; Supported catalysts; Leached chrysotile; Grafting; Oxidation; Catalysis
1. Introduction
associated with an easily recyclable solid, which can be reused
[19].
Metalloporphyrins are important examples of macrocyclic
complexes and have attracted much interest in the study of
various oxygenation reactions of hydrocarbons under mild con-
ditions [1–4]. This class of compounds is used in solution or
following immobilization in organic amorphous polymers and
crystalline inorganic materials, such as silica [5–7], zeolites
[8,9], montmorillonite clay [10,11] and others [12–16]. The
use of metalloporphyrins substituted with electron-withdrawing
groups (the so-called second generation porphyrins [17]) and
their immobilization has resulted in efficient and selective cat-
alysts for oxidation reactions since the support can impose
shape selectivity and promote a special environment, favoring
the approach of the substrate to the active catalytic species
[5–7,12,13,18]. In addition, the immobilization may prevent
molecular aggregation or bimolecular self-destruction reactions,
which leads to deactivation of catalytically active metallopor-
phyrin species. The immobilization of metalloporphyrins is also
Chrysotile, one of the alternative source of highly hydrox-
ylated silica, is classified in the kaolin/serpentine mineral 1:1
group with tri-octahedral site occupancy [20–22]. This solid dis-
plays substitution of aluminum atoms, which form a compact
brucite-like layer [20]. The mismatch of the brucite-like octahe-
dral sheet with the silica tetrahedral sheet causes the curvature
of the layers, which can roll into tight tubes, the characteristic
chrysotile fibers [21]. A drastic leaching of concentrated acid
on chrysotile fibers transform this natural polymer by remov-
ing the brucite-like layer, resulting in an excellent source of
hydrated disordered silica [23], with physico-chemical charac-
teristics similar to silica gel.
Silica gel is an example of inorganic solid that present silox-
anes groups (Si O Si) in the bulk and a high population of
silanols groups (Si OH) at the surfaces [24], being the sur-
face groups available for grafting reactions. The silane coupling
agents (general formula (RꢀO)3SiR)) covalently bonded to inor-
ganic supports, are one interesting alternative to immobilize
catalysts, including metalloporphyrins [16,23].
To test the performance of the new silica, we report in this
paper the immobilization of a second generation iron porphy-
∗
Corresponding author. Tel.: +55 41 33613180; fax: +55 41 33613186.
E-mail address: shirley@quimica.ufpr.br (S. Nakagaki).
1381-1169/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2005.08.019

M. Halma et al. / Journal of Molecular Catalysis A: Chemical 243 (2006) 44–51
45
The chrysotile sample with a fiber length below 2.0 mm
˜
(SAMA7ML) was supplied by SAMA-Minerac¸ao de Amianto
´
Ltda, mined in Uruac¸u, state of Goias, Brazil. The white solid
of disordered silica was obtained by treating chrysotile with
hydrochloric acid, as previously described [21,22].
(3-Aminopropyl)triethoxysilane-3-APTS),
NH2(CH2)3
Si(CH2CH2O)3 (Aldrich), toluene (Synth) and ethanol
(Nuclear) were all reagent grade. Deionized water was used in
all experimental procedures.
Fig. 1. Schematic representation of iron(III) porphyrins.
2.2.1. Porphyrins
rin, 5,10,15,20-tetrakis(2-fluor-6-chloro-3-sulfonatophenyl)po-
rphyrinate] iron(III) [Fe(TCFSPP)] and the already used
also second generation iron porphyrin 5,10,15,20-tetrakis-
(2,6-difluor-3-sulfonatophenyl) porphyrinate] iron(III), (Fe-
(TDFSPP) (Fig. 1) at the surface of the grafted disordered
hydrated silica derived from the chrysotile structure.
The solid materials were investigated in the oxidation reac-
tion of cyclohexane using iodosylbenzene as oxidant and the
results compared to the immobilization and catalytic behavior
of the iron porphyrins systems previously reported [19].
Free base porphyrins Na4[H2(TDFSPP)] and Na4[H2
(TCFSPP)] were synthesized, purified and characterized follow-
ing the methodology previously described [25–28].
2.2.2. Iron(III) porphyrins
Iron(III) porphyrins (FePor) were obtained by metallation
of the free ligand with ferrous chloride tetrahydrate in DMF
following the method described by Adler et al. [29,30]. The
iron porphyrins were purified by column chromatography on
exchange resin (Sephadex), using deionized water as eluent.
The products were characterized by UV–visible and EPR
spectrometry and the data were consistent with the expected
compound after the metallation reaction [Fe(TDFSPP)Cl]4−
(deionized water): 390 nm (ε = 37 × 103 L mol−1 cm−1),
2. Experimental
2.1. Physical measurements
504 nm
(ε = 3 × 103 L mol−1 cm−1);
[Fe(TCFSPP)Cl]4−
(deionized water): 390 nm (ε = 15 × 103 L mol−1 cm−1). The
negative charges and the chlorine counter ion will be omitted in
the text for simplification purposes [31].
UV–vis spectra of the solids obtained after the immobi-
lization process were recorded from CCl4 suspension and the
porphyrins and metalloporphyrins were recorded in deionized
water solutions, using a Hewlett Packard-8452 A diode array
spectrophotometer. FTIR spectra were recorded on a Biorad
2.2.3. Oxidant
3500 GX spectrophotometer in the range of 400–4000 cm−1
,
Iodosylbenzene (PhIO) was prepared as previously described
[19,31]. It was obtained through the hydrolysis of iodosylben-
zenediacetate following the methods described by Saltzmann
and Sharefkin [32,33]. The purity was measured by iodometric
assay.
using KBr pellets. KBr was crushed with a small amount of the
solids and the spectra were collected with a resolution of 4 cm−1
andaccumulationof32scans. FortheX-raydiffractionmeasure-
ments, self-oriented films were placed on neutral glass sample
holders. The measurements were performed in reflection mode
using a Shimadzu diffractometer XRD-6000 operating at 40 kV
2.2.4. Catalyst preparation
˚
and 40 mA (Cu K␣ radiation, λ = 1.5418 A) with a dwell time
Silica derived from chrysotile (700 mg) was activated at
100 ◦C for approximately 8 h. The organo-functionalization
reaction consisted in refluxing (at 110 ◦C) the suspension of
silica (under mechanical stirring) in toluene (150 mL), under
inert argon atmosphere. After starting the reflux, 4.5 mL of
(3-aminopropyl)triethoxysilane (3-APTS) were added dropwise
(1.0 mL/h) to the suspension and the reaction maintained for
10 h. After cooling, the product was centrifuged and washed
with toluene, ethanol and a large volume of deionized water and
dried at 70 ◦C for 24 h (Si-3-APTS).
of 1◦/min. Electron paramagnetic resonance (EPR) measure-
ments were performed with a Bruker ESP 300E spectrometer
at X-band (approximately 9.5 GHz) at 293 or 77 K, using liquid
N2. Products from catalytic oxidation reactions were identified
using a Shimadzu CG-14B gas chromatograph (flame ionization
detector) with a DB-WAX capillary column (J&W Scientific).
Elemental analysis was made using a EUROVECTOR equip-
ment, model EA 3000 CHNS.
2.2. Materials
The iron porphyrin [Fe(TDFSPP)] immobilization was con-
ducted in a typical experiment, by suspending Si-3-APTS
(500 mg) in 20 mL of deionized water, under magnetic stir-
ring. Drops of a 1% (w/w) hydrochloric acid were added to
the solution until pH 5–6 and the stirring was maintained for
15 min. Following, 1.2 × 10−5 mol of iron(III) porphyrin was
dissolved in 5 mL of deionized water (1.5 × 10−3 mol/L), which
was added dropwise to the grafted silica acidic suspension.
The reaction was conducted under reflux conditions for 5 h at
All solvents and reagents were of commercial grade (Merck
and Aldrich) unless otherwise stated. Authentic samples (alco-
hol and ketone) were purchased at their highest commercial
purity (Aldrich) and used as-received. All substrates were stored
at 5 ◦C and purged with argon just before use. After use, all the
reagents were discarded in an appropriate container for later
treatment for reuse when it was possible or for final disposal.

46
M. Halma et al. / Journal of Molecular Catalysis A: Chemical 243 (2006) 44–51
100–110 ◦C following by a rest of 15 h at 5 ◦C. Finally, the
green solid obtained was recovered by centrifugation (10 min at
3000 rpm), washed thoroughly with deionized water and dried at
70 ◦C for 24 h. The iron porphyrin Fe(TCFSPP) immobilization,
product purification and characterization were done following
the same procedure. The amount of iron porphyrin retained
within the Si-3-APTS support (solids Fe(TCFSPP)/Si-3-APTS
and Fe(TDFSPP)/Si-3-APTS), after the end of the immobiliza-
tion process, was determined by the UV–visible spectroscopy
analyses of the amount of iron porphyrins in the reaction solu-
tions and all extracts obtained in the washing processes. The
content of 2.3 × 10−5 and 1.3 × 10−5 mol of FePor/g functional-
ized silica for Fe(TDFSPP) and Fe(TCFSPP), respectively, were
determined. The immobilization processes were also carried out
for both iron porphyrins in the absence of hydrochloric acid and
similar percentage of iron porphyrin retention in the silica sup-
port were observed. When raw silica derived from chrysotile was
used as support to immobilize the iron porphyrin, no significant
retention of the metallocomplexes was observed as only a white
solid was obtained and no traces of iron porphyrin Soret band
were detected by UV–vis analyses. The molar absortivities (ε)
of the iron porphyrins Soret band were determined by analyzing
a solution of Fe(TDFSPP) or Fe(TCFSPP) in deionized water
using the UV–vis measurement of successive dilutions.
Fig. 2. FTIR spectra of hydrated disordered silica (a), Si-3-APTS (b),
Fe(TDFSPP)/Si-3-APTS (c) and Fe(TCFSPP)/Si-3-APTS (d).
Mg OH bond on the inorganic structural layer [21,23]. The
broad bands at 3447 and 1640 cm−1 are attributed to stretching
modes of hydrogen-bonded silanol groups and trapped water
molecule on the structure. The broad band at 1088 cm−1 can be
attributed to the Si O Si symmetric stretching vibrations [23].
The spectrum relating to the precursor silica shows the band
at 955 cm−1, which is attributed to the surface Si OH, being
the band at 463 cm−1, associated to deformation of Si O. The
result for modified silica (Fig. 2b) was characterized by the
reduction of the intensity of the band at 955 cm−1 and presence
of small bands in the region of 2800–3000 cm−1, attributed to
typical CH2 stretching modes, both characterizing the effective
reaction of the coupling agent to the surface of hydrated silica
(Fig. 2a).
Fig. 2 presents also the spectra of Fe(TDFSPP)/Si-3-APTS
(c) and Fe(TCFSPP)/Si-3-APTS (d). The bands attributed to the
ironporphyrin are absent, due to the low concentration of the
complexes in the matrix [19].
The silica derived from chrysotile present almost 50% of the
silicon atoms bonded to hydroxide group (SiO1.75(OH)0.5) [22].
According to the chemical analysis (Table 1), after the graft-
ing reaction, almost 25% of the reactive hydroxide groups were
reacted to 3-APTS, where the silicon atoms have 1.7 OH and
0.3 ethoxyde group (1 bonding to the surface). After the addi-
tion of the acid and immobilization of the iron porphyrin, partial
2.2.5. Catalytic oxidation reaction
Catalytic oxidation reactions were carried out in a 2 mL ther-
mostatic glass reactor equipped with a magnetic stirrer inside of
a dark chamber [10]. In a standard experiment within the reac-
tor, solid catalyst and iodosylbenzene (in different FePor:PhIO
molar ratios—1:5, 1:50, 1:100 and 1:500) were suspended in
0.350 mL of solvent (dichloromethane–acetonitrile 1:1 mixture,
v/v) and degassed with argon during 10 min. The substrate
(cyclohexane, FePor:substrate molar ratio = 1:1000) was added
and the oxidation reaction was carried out during 1–24 h, under
magnetic stirring. To eliminate the excess of iodosylbenzene,
sodium sulfite was added and the products of the reaction were
separated from the solid catalyst by exhaustive washing and
centrifugation of the solid with an acetonitrile–dichloromethane
mixture. The extracted solution was analyzed by capillary gas
chromatography and the amount of the products was deter-
mined by using the internal standard method. No products were
detected when the catalyst used was grafted silica without any
iron porphyrins or in solutions set up without catalyst. After
the first catalytic run for each catalyst solid, the supported cata-
lysts were recovered from the solution reaction by filtration and
washed thoroughly in Soxhlet extractor with different solvents
and finally dried to be reused.
Table 1
Percentages of hydrogen (H), carbon (C) and nitrogen (N), C/N ratio observed
and calculated on the samples
3. Results and discussion
The infrared spectroscopy was used as a tool to provide infor-
mation from the surface silanol group region of the hydrated
silica (Fig. 2).
The main characteristic of the spectrum of hydrated dis-
ordered silica (Fig. 2a) is the complete absence of a band
in 3686 and 3644 cm−1 attributed to typical inner surface
Sample
Found
C (%)
Calculated
C/N
N (%)
H (%)
C/N
Silica (Si)
Si-3-APTS
Fe(TCFSPP)/Si-3-APTS
0.10
6.80
5.04
0
2.21
1.70
1.48
2.06
1.67
0
3.08
2.97
0
3.09
2.72

M. Halma et al. / Journal of Molecular Catalysis A: Chemical 243 (2006) 44–51
47
Fig. 3. Powder X-ray diffraction patterns of hydrated disordered silica (a), Si-3-
APTS (b) and Fe(TCFSPP)/Si-3-APTS (c) and raw chrysotile (d). C, chrysotile;
T, talc.
Fig. 4. EPR spectra of hydrated disordered silica (a), Si-3-APTS (b),
Fe(TCFSPP)/Si-3-APTS (c) and Fe(TCFSPP) (d).
hydrolysis occur and the amount of immobilized iron porphyrin
is about 0.2%. The same behavior was already described for the
immobilization of metalloporphyrins in kaolinite grafted with
3-APTS [16].
cess. On the other hand, coordinative interaction by the terminal
amino groups and iron from the complexes is also possible since
substantial iron porphyrin immobilization was also observed in
the absence of hydrochloric acid. In both cases, the distortion
of the porphyrin ring is expected, characterized by the g = 4.3
signal [16].
The presence of the iron(III) porphyrins in the solid FePor/Si-
3-APTS was also confirmed by UV–vis spectra (in CCl4 suspen-
sion; Fig. 5). The measurements suggest that no demetallation
(characterized by a blueshift of the Soret band that is associated
Fig. 3 shows the powder X-ray patterns of hydrated disor-
dered silica (a), Si-3-APTS (b), Fe(TCFSPP)/Si-3-APTS (c) (the
result of this analysis for the Fe(TDFSPP)/Si-3-APTS was the
same) and raw chrysotile (d). In all the cases, it was observed
the typical diffraction pattern of amorphous samples, with only
two sharp peaks of low intensity near 12◦ and 24◦ (2θ) denoted
by “C”, attributed to main diffraction peaks of the residual
chrysotile. Chrysotile shows a small contamination of talc (T),
removed during the acid leaching process in the form of a cloudy
suspension. It can also be seen that the functionalization and
immobilization process do not change the crystallinity of silica.
EPR spectrum of the starting Fe(TCFSPP) shows a signal
with g = 6.0, typical of iron(III)porphyrin high spin 5/2 complex
(Fig. 4d) [8]. Hydrated disordered silica (Fig. 4a) and Si-3-APTS
(Fig. 4b), shows a very small contamination of high spin Fe(III)
in rhombic symmetry (g = 4.3) and low spin Fe(III) in g = 1.95.
Fig. 4c shows the presence of the typical Fe(III) EPR signal in
the solid Fe(TCFSPP)/Si-3-APTS (g = 6.0 to the Fe(III)), con-
firming the immobilization and excluding the demetallation in
the process [10,11].
The EPR results to the Fe(TDFSPP)/Si-3-APTS were similar
to the EPR results obtained to the solid Fe(TCFSPP)/Si-3-APTS,
showing the same peaks (not shown). The FePor/Si-3-APTS
spectra show that some distortion from the planarity is possible
for the porphyrin ring, suggesting that the iron porphyrins are
immobilizedatthesurfaceofthegraftedsilica. Itisproposedthat
electrostaticinteractionofthepositivechargedprotonatedamino
terminal groups provided from the funcionalization groups and
the sulfonated negative charged porphyrin ring. In this case,
the acid media would be necessary to the immobilization pro-
Fig. 5. UV–vis spectra of Fe(TCFSPP)/Si-3-APTS (a) and methanol solution
of Fe(TCFSPP) (b).

48
M. Halma et al. / Journal of Molecular Catalysis A: Chemical 243 (2006) 44–51
Fig. 6. Schematic representation of the support preparation, surface modification and immobilization of the iron porphyrin in acid media.
with a significant amount of free base porphyrin [12–14]) or sig-
nificant exchange of the Fe(III) ion with the supports occurred
during the preparation process. The Soret peaks at 412 nm for
Fe(TDFSPP)/Si-3-APTS (Fig. 5a) and for Fe(TCFSPP)/Si-3-
APTS (not shown) were red-shifted when compared to those
of iron porphyrins in solution (methanol water), 388 nm for
Fe(TCFSPP) (Fig. 5b) and 386 nm for Fe(TDFSPP) (not shown).
A similar behavior was observed previously [19], when metal-
loporphyrins were immobilized in different inorganic supports
[10,12–14,34]. This behavior was attributed to steric constraints
caused by the support, which modified the iron porphyrin struc-
ture substantially in these supported catalysts [34].
3.1. Catalytic properties of the supported iron(III)
porphyrins
The catalytic activity of the both iron porphyrin sup-
ported catalysts, (Fe(TDFSPP)/Si-3-APTS and Fe(TCFSPP)/Si-
3-APTS), was investigated on the oxidation of weakly reactive
alkanes, such as cyclohexane. It is a very useful substrate and
frequently used for the Fe(III) porphyrin-catalyzed hydroxyla-
tion by iodosylbenzene (PhIO) [6,10,35,36] and was used as test
reaction to compare the catalyst activity of the iron porphyrins.
The results are presented in Table 2 (Fe(TDFSPP)/Si-3-APTS)
and Table 3 (Fe(TCFSPP)/Si-3-APTS). When using 1.4 mmol
of substrate in the solvent mixture CH3CN:CH2Cl2 (1:1, v/v,
ratio), the supported catalysts do not release their iron porphyrin
and different proportions of PhIO relative to the iron porphyrin
supported catalyst led to selective formation of alcohol (based
Fig. 6 shows schematically the possible process of the support
preparation by leaching the brucite like sheet from the chrysotile
structure, surface modification by grafting and immobilization
of the porphyrin, in acid media.
Table 2
Hydroxylation of cyclohexane by PhIO catalyzed by iron(III) porphyrin, Fe(TDFSPP) (homogeneous catalysis) and supported iron(III) porphyrin, Fe(TDFSPP)/Si-
3-APTS (heterogeneous catalysis)a
Catalyst
Run
1
FePor:PhIOb
1:50
Time (h)
1
Alcohol yield (%)c,d
13
Yield/time (%/h)e
Fe(TDFSPP)f
Fe(TDFSPP)/Si-3-APTS
2
3
4
5
6
7
8
9
10
1:50
1:50
1:50
1:50
1:100
1:100
1:100
1:100
1:500
1
3
6
24
1
3
6
24
1
36
57
63
61
25
48
50
48
8
7.0
2.0
7.0
0.3
1st Reutilization
11
12
1:50
1:100
24
24
22
9
0.5
0
Si-3-APTS
13
1
<1
a
Typical conditions: purged argon, catalyst:oxidant:cyclohexane molar ratio = 1 mmol:10 mmol:1000 mmol; solvent mixture dichloromethane/acetonitrile (1:1,
v/v) (350 ␮L) at room temperature. When high Fepor:PhIO molar ratio was used (>1:100), proportional increase of the solvent and substrate was used. Homogeneous
catalyses were made under identical conditions as heterogeneous catalyses.
b
Iron porphyrin:iodosylbenzene = FePor:PhIO molar ratio (mol:mol).
Yields based on starting PhIO obtained after 1, 3, 6, 12 or 24 h of reaction. It was assumed that two mols of PhIO are necessary for ketone formation. Trace yields
c
of cyclohexanone gave results smaller than 5% in all reactions.
d
Total yields of cyclohexanol.
e
Ratio % yield product/time of the reaction. E.g. to run 2 and 3: 57% − 36% = 21% / 3 h = 7.0% / h.
f
Homogeneous catalysis performed in dichloromethane/acetonitrile solvent mixture (1:1, v/v).

M. Halma et al. / Journal of Molecular Catalysis A: Chemical 243 (2006) 44–51
49
Table 3
Hydroxylation of cyclohexane by PhIO catalyzed by iron(III) porphyrin, Fe(TCFSPP), (homogeneous catalysis) and supported iron(III) porphyrin, Fe(TCFSPP)/Si-
3-APTS (heterogeneous catalysis)a
Catalysts
Run
1
FePor:PhIOb
1:50
Time (h)
1
Alcohol yield (%)c,d
48
Yield/time (%/h)e
Fe(TCFSPP)f
Fe(TCFSPP)/Si-3-APTS
2
3
4
5
6
7
8
9
10
1:50
1:50
1:50
1:50
1:100
1:100
1:100
1:100
1:500
1
3
6
24
1
3
6
24
1
15
25
30
33
6
16
20
26
1
3.0
0.8
3.0
0.8
0.5
1st Reutilization
11
12
1:50
1:100
24
24
8
8
0.2
0.3
Si-3-APTS
13
1
<1
a
Typical conditions: purged argon, catalyst:oxidant:cyclohexane molar ratio = 1 mmol:10 mmol:1000 mmol; solvent mixture dichloromethane/acetonitrile (1:1,
v/v) (350 ␮L) at room temperature. When high Fepor:PhIO molar ratio was used (>1:100) proportional increase of the solvent and substrate was used. Homogeneous
catalyses were made under identical conditions as heterogeneous catalyses.
b
Iron porphyrin:iodosylbenzene = FePor:PhIO molar ratio (mol:mol).
Yields based on starting PhIO obtained after 1, 3, 6, 12 or 24 h of reaction. It was assumed that two mols of PhIO are necessary for ketone formation. Trace yields
c
of cyclohexanone gave results smaller than 5% in all reactions.
d
Total yields of cyclohexanol.
e
Ratio % yield product/time of the reaction. E.g. to run 2 and 3: 25% − 15% = 10% / 3 h = 3.0% / h.
f
Homogeneous catalysis performed in dichloromethane/acetonitrile solvent mixture (1:1, v/v).
on PhIO) within 1, 3, 6, 12 and 24 h (the results obtained for
12 h reaction was omitted in Tables 2 and 3 because they were
similar to the results for 24 h). The consumption of PhIO in
all reactions was monitored by the presence of iodobenzene
(PhI).
For Fe(TDFSPP), the heterogeneous catalytic results pre-
sented superior yield of alcohol than the homogeneous catal-
ysis (run 1, Table 2) and the opposite behavior was observed
for Fe(TCFSPP) (run 1, Table 3). In homogeneous catalysis,
one ortho-chlorine substituent in each meso-phenyl porphyrin
groups in the Fe(TCFSPP) can avoid molecular interactions,
which can deactivate (by destruction of the iron porphyrin
or dimerization) and/or avoid the catalytic active species for-
mation [8,24,37]. So, higher yields of oxidation reaction are
expected to this iron porphyrin in comparison to Fe(TDFSPP).
On the contrary, after immobilization, the best catalytic results
observed for Fe(TDFSPP) could be due to an easy access
of PhIO and substrate to the iron site, based on the small
size of the two ortho-fluorine substituent in comparison to
ortho-chlorine substituent from Fe(TCFSPP) [24]. Besides, the
immobilization process avoids any molecular interaction pos-
sible in homogeneous catalysis, mainly when low percent-
age of iron porphyrin immobilized in the Si-3-APTS solid
is used.
For both catalysts, the increase of the alcohol yield was
accompanied by a modest increase of ketone, (yields below
5% in all of the runs) suggesting a good selectivity to alco-
hol, for both immobilized iron porphyrins for all conditions
(Tables 2 and 3). Besides, the best results were observed when
the iron porphyrin–iodosylbenzene molar ratio was 1:50 and
worst for 1:500, suggesting that the high amount of PhIO could
be blocking the access of the reactants to the catalytic site and/or
destruction of the catalyst. This phenomenon is more dramatic to
the Fe(TCFSPP), which have a structure with bulk chlorine atom
substituents (Fig. 1). This result also suggests that the access of
For the catalyst Fe(TDFSPP)/Si-3-APTS (Table 2) in the best
condition, 63% of conversion to the alcohol was observed (run
4). This catalyst presented better oxy-functionalization results
(run 2) than the iron(III) porphyrins in solution in the same
amount of oxidant (run 1), indicating that the immobilization
favors the catalytic activity of these metalloporphyrins. The low
solubility of the iron porphyrins in a CH2Cl2:CH3CN (1:1, v/v,
ratio) solvent mixture in homogeneous catalysis certainly is an
important factor responsible for the low yields of the iron por-
phyrin itself. The possibility of the molecular interaction of the
Fe(TDFSPP) species in solution could be another reason for
the low catalytic activity observed in the homogeneous cataly-
sis because it can be accompanied by ␮-oxo complex formation
[17,35,36]. The catalyst oxidative degradation in solution is fre-
quently responsible for the low yield in catalytic reaction using
metalloporphyrins [17] but not to second generation iron por-
phyrins and in FePor/oxidant molar ratio low like the used in
run 1 (1:50).
Table 3 presents the results of the catalytic activity of
Fe(TCFSPP) (run 1) and Fe(TCFSPP)/Si-3-APTS. It can be
observedthatthebestyieldforimmobilizedsystemwasobtained
in run 5 (33% of alcohol). In opposite behavior to Fe(TDFSPP)
catalyst, Fe(TCFSPP) present substantially better yield in homo-
geneous catalysis (run 1) when compared with this iron por-
phyrin immobilized, under same reaction condition (run 2).

50
M. Halma et al. / Journal of Molecular Catalysis A: Chemical 243 (2006) 44–51
the reactants to the catalytic site is an important factor to the
catalytic activity when the iron porphyrins are immobilized in
Si-3-APTS.
On the other hand, instead of the fact that the so-called sec-
ond generation iron porphyrin are able to resist to some extent
to the oxidative destruction, because of the steric and elec-
tronic structure [15–17,19], some destruction of the catalyst can
be occurring in high molar ratio. This destruction can explain
the observed drastic yields reduction in the proportions 1:500
FePor/oxidant (runs 10, Tables 2 and 3).
Acknowledgments
The authors are grateful to Conselho Nacional de Desen-
volvimento Cient´ıfico e Tecnolo´gico (CNPq), Coordenac¸a˜o
de Aperfeic¸oamento de Pessoal de N´ıvel Superior (CAPES),
Fundac¸a˜o Arauca´ria, Fundac¸a˜o da Universidade Federal do
Parana´ (FUNPAR) and Universidade Federal do Parana´ (UFPR)
for the financial support. They gratefully acknowledge M.Sc.
Geraldo R. Friedermann and Dr. Antonio S. Mangrich for the
EPR facilities and analyses.
In general, it was observed that the yield of the conver-
sion of cyclohexane to alcohol increases with the increase of
the reaction time from 1 to 24 h for both the iron porphyrins
(runs 2–5, Tables 2 and 3). The same behavior was observed
in the high FePor:PhIO molar ratio conditions 1:100 (runs 6–9,
Tables 2 and 3). In general, the yield of alcohol became constant
after 6 h. The observation that in high molar ratio, increase the
alcohol yield (1:100 at 6 h or more time reaction) suggests that
the access to the iron is an effective factor to the catalytic perfor-
mance of the ironporphyrin immobilized. Normally, in this high
oxidant molar ratio, a decrease of the yield could be expected
with the increase of the reaction time, as a consequence of the
some iron porphyrin destruction.
It is noteworthy that reactions using the Si-3-APTS support
itself without iron porphyrin gave very low hydroxylation yields
(under 1% of alcohol), indicating that the catalytic effect in the
hydroxylation of the cyclohexane can be attributed to the pres-
ence of the adsorbed iron porphyrin.
The results of the catalyst reuse in a second reaction showed
lower yields than the first reaction, suggesting that the catalysts
can be detached from the surface after the first use by simple
filtration and washing process in Sohxlet extractor.
They also thank Prof. Kestur Gundappa Satyanarayana for
reading the manuscript, critical comments and helpful sugges-
tions including English language.
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