Anionic iron(III) porphyrins immobilized on zinc hydroxide chloride as catalysts for heterogeneous oxidation reactions

Article abstract of DOI:10.1016/j.apcata.2011.10.046

This work describes the immobilization of an anionic iron(III) porphyrin (FePor) family on zinc hydroxide chloride (ZHC), a layered hydroxide salt prepared by reacting an aqueous zinc chloride solution with an ammonium hydroxide solution. The FePor immobilization was performed at room temperature under magnetic stirring, under air atmosphere, of each complex ethanol solution and the ZHC solid support suspension. The materials obtained were characterized by X-ray powder diffraction (XRPD), ultraviolet-visible spectroscopy (UV-vis) (solid samples), Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR). The catalytic activity of the solids was investigated in cyclooctene, cyclohexane and n-heptane heterogeneous catalytic oxidation reactions with iodosylbenzene as the oxygen donor. The solid catalyst's reutilization capacity was also investigated and the heterogeneous character of the catalytic process was confirmed. The compounds and the catalytic activity of FePor-ZHC were compared with the synthesis and catalytic activity of the same FePor immobilized on zinc hydroxide nitrate (ZHN). Though the matrixes are similar, the results obtained were exactly the opposite when the selectivity was analyzed.

Full text of DOI:10.1016/j.apcata.2011.10.046

Applied Catalysis A: General 413–414 (2012) 94–102
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Anionic iron(III) porphyrins immobilized on zinc hydroxide chloride as catalysts
for heterogeneous oxidation reactions
Guilherme Sippel Machadoa,b, Fernando Wypychb, Shirley Nakagakia,∗
a Universidade Federal do Paraná, Departamento de Química – Laboratório de Bioinorgânica e Catálise – CP 19081, CEP 81531-980, Curitiba, Paraná, Brazil
b Universidade Federal do Paraná, Departamento de Química – Centro de Pesquisa em Química Aplicada (CEPESQ) – CP 19081, CEP 81531-980, Curitiba, Paraná, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 July 2011
Received in revised form 17 October 2011
Accepted 28 October 2011
Available online 4 November 2011
This work describes the immobilization of an anionic iron(III) porphyrin (FePor) family on zinc hydroxide
chloride (ZHC), a layered hydroxide salt prepared by reacting an aqueous zinc chloride solution with an
ammonium hydroxide solution. The FePor immobilization was performed at room temperature under
magnetic stirring, under air atmosphere, of each complex ethanol solution and the ZHC solid support
suspension. The materials obtained were characterized by X-ray powder diffraction (XRPD), ultraviolet-
visible spectroscopy (UV–vis) (solid samples), Fourier transform infrared spectroscopy (FTIR) and electron
paramagnetic resonance (EPR). The catalytic activity of the solids was investigated in cyclooctene, cyclo-
hexane and n-heptane heterogeneous catalytic oxidation reactions with iodosylbenzene as the oxygen
donor. The solid catalyst’s reutilization capacity was also investigated and the heterogeneous character
of the catalytic process was confirmed. The compounds and the catalytic activity of FePor-ZHC were
compared with the synthesis and catalytic activity of the same FePor immobilized on zinc hydroxide
nitrate (ZHN). Though the matrixes are similar, the results obtained were exactly the opposite when the
selectivity was analyzed.
Keywords:
Porphyrin
Zinc hydroxide chloride
Heterogeneous catalyst
Oxidation
© 2011 Elsevier B.V. All rights reserved.
1. Introduction
The strategy to obtain and use a solid catalyst for heterogeneous
catalysis, where the catalytic species are immobilized, can hinder
Inspired by biological systems that have catalytic activity, many
different compounds have been synthesized and investigated as
oxidation catalysts [1–3]. In this context, synthetic metallopor-
phyrins are investigated as mimics of cytochrome P-450 [2]. In
live organisms the cytochrome P-450 performs oxidation reactions
with high selectivity and efficiency [1]. Many different investi-
gations involving metalloporphyrin catalytic activity have been
performed, initially in homogeneous media (catalyst and sub-
strate in the same solvent phase) [4–6]. Under these conditions,
deactivation of the catalysts has been detected, for example, by
dimerization or by destructive auto-oxidation. In the former pro-
cess, two porphyrin-ring metal ␮-oxo bridges are established and
in the latter, active catalytic species approach other porphyrin
molecule, deactivating both [7]. One of the most significant con-
tributions to obtain efficient, selective and reusable catalysts using
metalloporphyrins, seeking possible technological application, was
the immobilization of porphyrinic compounds on solids such as clay
minerals [7,8], silica [9], and other inorganic-supports [10].
undesirable approximations between activated and non-activated
catalytic species (this species can lead to a deactivating process
caused by secondary reactions between porphyrinic rings [7]) and
also can create materials that can be reused in several reaction
cycles.
In this context, the present work reports the study of a family
of anionic iron(III) porphyrins (FePor) (Fig. 1), which were immobi-
lized on zinc hydroxide chloride (ZHC), a non-exchangeable layered
hydroxide salt [11]. Layered hydroxide salts have been studied for
a great number of applications [11–13], such as intercalation reac-
tions [14,15], catalyst support [12], oxide precursors [16,17], and
others [11]. This class of compounds consists of modified brucite-
like layers [12], where the Mg2+ metallic center, surrounded by
hydroxyl groups, is replaced by another M2+ metallic center, such
as Zn2+, Co2+ or Cu2+ [12,13]. Partial substitution of hydroxyl groups
by other anions or water molecules creates a positive charge at the
layers, which needs to be compensated by the presence of inter-
layer anions. In the case of nitrate anions, the compound is an
anionic exchanger and in the case of chloride, as these anions are
grafted directly to the layers, the compound is neutral [11]. Specif-
ically, zinc hydroxide chloride (ZHC) is represented by the formula
Zn5(OH)8Cl2·H2O [17,18] and the basic structural unit contains a
vacancy in a quarter of the octahedral zinc sites coordinated to
∗
Corresponding author. Tel.: +55 41 33613180; fax: +55 41 33613186.
E-mail address: shirleyn@ufpr.br (S. Nakagaki).
0926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.10.046

G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
95
Fig. 1. Structure of the iron porphyrins employed in this study: [Fe(TDFSPP)Na4]+ = [tetrasodium – 5, 10, 15, 20 – tetrakis (2,6-difluoro-3-sulfonatophenyl) porphyrinate
iron(III)], [Fe(TCFSPP)Na4]+ = [tetrasodium – 5, 10, 15, 20 – tetrakis (2-chloro-6-fluoro-3-sulfonatophenyl) porphyrinate iron(III)], and [Fe(TDCSPP)Na4]+ = [tetrasodium – 5,
10, 15, 20 – tetrakis (2,6-dichloro-3-sulfonatophenyl) porphyrinate iron(III)].
hydroxyl groups, while the upper and lower sides of this vacancy
contain tetrahedral zinc sites, originating a positive layer charge
[11]. In the tetrahedral zinc site, three vertices are occupied by
hydroxyl groups from the octahedral sheet, and in the fourth posi-
tion the metal is coordinated to chloride ions, building a neutral
layered structure. In the ZHC structure, water molecules are also
present between the layers [18], generating a solid with basal dis-
these three FePor will be represented in this work by [Fe(TDFSPP)]
– FeDF, [Fe(TCFSPP)] – FeCF, and [Fe(TDCSPP)] – FeDC, respec-
tively, so hereafter no mention of the FePor porphyrin charges
will be made, to avoid repetition. The Soret bands of the FePors
obtained after the metal insertion reaction were the following:
FeDF (ethanol) 390 nm (ε = 28 × 103 L mol−1 cm−1), FeCF (ethanol)
412 nm (ε = 73 × 103 L mol−1 cm−1), and FeDC (ethanol) 390 nm
(ε = 74 × 103 L mol−1 cm−1).
˚
tance of approximately 7.8 A (JCPDS card: 07-0155) [17].
The solids obtained after the FePor immobilization on ZHC were
investigated as oxidation catalysts for cyclooctene, cyclohexane
and n-heptane using iodosylbenzene as the oxygen donor.
2.3. Preparation of the solid FePor-ZHC catalysts
A ZnCl2 solution prepared in distilled water (0.73 mol L−1
)
2. Experimental
was kept under magnetic stirring and heated to approximately
50–60 ◦C. Drops of ammonia aqueous solution (28%) were added
to the solution and immediately a white cloudy solid precipitate
was observed. The total volume addition of 3 mL of NH4OH drop by
drop occurred in the time of 1 h. During the base addition, the pH
was verified each 5 min to keep it constant at 7 in order to avoid
precipitation of other insoluble zinc hydroxides [12]. The suspen-
sion containing the solid ZHC was centrifuged and the supernatant
separated. The solid was washed 5 times with distilled water and
dried in an oven at 50 ◦C for 48 h. The reaction’s final yield was 38%
in relation to the initial amount of zinc.
2.1. Reagents
All chemicals used in this study were purchased from Aldrich,
Sigma, or Merck, and were of analytical grade. Iodosylbenzene
(PhIO) was synthesized by hydrolysis of iodosylbenzenediacetate
[19]. The solid was carefully dried under reduced pressure and kept
at 5 ◦C; its purity was periodically controlled by iodometric titration
[20].
2.2. Porphyrins and metalloporphyrins
Although ZHC has a generally neutral structure, the anionic
iron porphyrins [Fe(TDFSPP)], [Fe(TCFSPP)] and [Fe(TDCSPP)] were
chosen for immobilization on it because they can interact eas-
ily with the residual positive charges at the layered crystal edges
of the support. The other reason for choosing the anionic com-
plexes is for behavior comparison purposes, since these same FePor
were already immobilized on the anionic exchanger zinc hydroxide
nitrate (ZHN) [12].
The system used for immobilization consisted of magnetic stir-
ring at room temperature in ethanol solution. This technique does
not lead to the collapse of the hydroxide salt structure, as happens
when magnetic stirring and reflux conditions were used [12]. For
The anionic free base porphyrins Na4[H2(TDFSPP)],
Na4[H2(TCFSPP)], and Na4[H2(TDCSPP)] and their correspond-
ing iron(III) complexes ([Fe(TDFSPP)Na4]+ = [tetrasodium
– 5,
10, 15, 20 – tetrakis (2,6-difluoro-3-sulfonatophenyl) porphyri-
nate iron(III)]; ([Fe(TCFSPP)Na4]+ = [tetrasodium – 5, 10, 15, 20
–
tetrakis (2-chloro-6-fluoro-3-sulfonatophenyl) porphyrinate
iron(III)] and ([Fe(TDCSPP)Na4]+ = [tetrasodium – 5, 10, 15, 20 –
tetrakis (2,6-dichloro-3-sulfonatophenyl) porphyrinate iron(III)])
were synthesized, purified, and characterized following a previ-
ously described method [21,22]. For the sake of simplification,

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G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
Table 1
the reaction, 10 mg of each FePor was dissolved in 50 mL of ethanol
and kept under magnetic stirring. Then 500 mg of ZHC was added to
the solution and the mixture was stirred for 5 h. In the sequence, the
suspensions were centrifuged and the supernatant placed in vol-
umetric flasks for posterior analysis by UV–vis spectroscopy. Each
solid was washed with a minimum of 5 portions of ethanol, and the
used ethanol was also added to volumetric flasks for metallopor-
phyrin quantification. The solids obtained were dried under air for
48 h.
Degree of FePor immobilization onto the ZHC support.
FePor
Supported
FePor
Immobilization
degreea (%)
Loadingb
(mol g−1
)
[Fe(TDFSPP)]
[Fe(TCFSPP)]
[Fe(TDCSPP)]
FeDF-ZHC
FeCF-ZHC
FeDC-ZHC
23
35
30
5.1 × 10−6
5.3 × 10−6
4.3 × 10−6
a
Amount of FePor (%) immobilized to the ZHN relative to the initial amount of
FePor used in the immobilization process.
b
FePor-ZHC (mol g−1).
After use, all reagents were discarded in an appropriate con-
tainer for later treatment and reuse, or for final disposal.
Electron paramagnetic resonance (EPR) measurements were
performed with a Bruker ESP 300E spectrometer operating in the
X-band (approximately 9.5 GHz), at 298 or −196 ◦C, using liquid N2.
All the products from the catalytic oxidation reactions were
identified using an Agilent 6850 gas chromatograph (flame ioniza-
tion detector) equipped with a 30 m long DB-WAX capillary column
with 0.25 mm internal diameter (J&W Scientific). The oven temper-
ature program used for determination of the oxidation products
from cyclooctene and cyclohexane started at 100 ◦C. Then the tem-
perature was increased to 150 ◦C at 10 ◦C min−1, followed by further
temperature rise to 200 ◦C at 50 ◦C min−1, which was maintained
for 1 min. For determination of the products originated from n-
heptane, the temperature program started at 70 ◦C, followed by
a temperature elevation to 100 ◦C at 5 ◦C min−1, maintained for
2.4. Heterogeneous catalytic oxidation of cyclooctene,
cyclohexane, and n-heptane by PhIO using FePor-ZHC solid as
catalyst
The solids obtained by immobilization of the anionic FePor onto
the synthetic ZHC matrix were used as catalysts in the oxida-
tion reactions. These reactions were carried out in a thermostatic
glass vessel (2 mL) equipped with a magnetic stirrer bar [7,8,23].
The catalyst (20 mg) and PhIO (0.5 mg) were suspended in solvent
(0.300 mL of acetonitrile), and the substrate (cyclooctene, cyclo-
hexane or n-heptane) was then added to the reaction mixture,
resulting in a constant compound/oxidant/substrate molar ratio
of 1:20:2000. The oxidation reaction was allowed to proceed for
1 h, under magnetic stirring. Sodium sulfite was added to the reac-
tion mixture to eliminate excess PhIO and to quench the reaction
after the experimental time was over. The reaction products were
separated from the FePor-ZHC catalyst by centrifugation and trans-
ferred to a volumetric flask. Next, the FePor-ZHC solid employed
in the reaction was washed several times with methanol and ace-
tonitrile, in order to extract any reaction product that might have
been retained in the catalyst. The solution containing the final
reaction products and the solvents from the washings of the FePor-
ZHC was analyzed by gas chromatography. Product yields were
quantified on the basis of PhIO, and high-purity n-octanol (99.9%)
(acetonitrile solution, 1.0 × 10−2 mol L−1) was employed as internal
standard. Control reactions were carried out using the same pro-
cedure in the case of (a) the substrate, (b) substrate + PhIO, and (c)
substrate + PhIO + ZHC (without FePor).
The corresponding FePor in homogeneous solution were also
investigated as catalysts (homogeneous catalysis) using similar
reagent molar ratio. The experimental procedure in this case was
similar to that used for the heterogeneous catalysis.
Catalyst reuse tests were also performed for all solids with
all substrates. At the end of each reaction, the catalytic solid
was separated and extensively washed with water, methanol and
acetonitrile in sequence. All the solvents used in the washing pro-
cedure were analyzed by UV–vis spectroscopy to detect possible
lixiviation of the catalyst from the support.
1 min, and a further temperature increase to 200 ◦C at 20 ◦C min−1
kept for 1 min.
,
3. Results and discussion
3.1. Characterization of FePor-ZHC catalysts
The immobilization process of FePor on the ZHC support occurs
probably by electrostatic interaction between the anionic FePor and
the positively charged surface edges of ZHC. In comparison to ZHN,
the grafted chloride ion in the solid cannot be exchanged as can the
nitrate, so the immobilization rates on ZHC are lower, even when
the same FePor is immobilized and similar conditions are used.
The degree of FePor immobilization on the support ZHC, deter-
mined by UV–vis spectroscopy analysis, and the codes for the
synthesized solid catalysts are presented in Table 1.
On the ZHN support, the immobilization degrees were almost
100% [12]. This observation indicates differences between the two
matrixes (ZHN and ZHC). Although they have similar structures,
they generated distinct immobilization degrees and a possible
hypothesis for the immobilization of the porphyrinic complexes
in ZHC is shown in Fig. 2.
The basal distance of the layered solid ZHC is approximately
˚
7.8 A [17] and the sulphonate metalloporphyrin medium size is
˚
close to 15 A [24]. As chloride is covalently bonded to the layer
[11], an exchange reaction between this anion and the FePor was
not expected, not even at the surface of the layer’s crystals. In the
present case, the possible location of the porphyrin anions is on
the residual positive charges at the layered crystal edges, which
can explain the lower immobilization degrees. In the proposed
model, only the interaction by two sulphonate groups is proposed
since the interaction with the four negative charges would gener-
ate an unstable conformation of the porphyrin ring [12]. The outer
sulphonate groups are probably compensated by Na+/NH4+ or even
H+.
2.5. Characterization of the FePor-ZHC catalysts
For the X-ray powder diffraction (XRPD) measurements, self-
oriented films were placed on neutral glass sample holders. XRPD
patterns were obtained in reflection mode using a Shimadzu
XRD-6000 diffractometer operating at 40 kV, 40 mA, using CuK␣
◦
˚
radiation (ꢀ = 1.5418 A) and a dwell time of 2 /min.
FTIR spectra were recorded with a Bomem MB spectrophotome-
ter in the range of 400–4000 cm−1, 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 and accumulation of 32 scans.
Ultraviolet–visible (UV–vis) spectra were registered in the
200–800 nm range with a Varian Cary 100 Bio Spectrophotome-
ter. Analyses were accomplished with a 1 cm path length cell or
with a Teflon® support for solid samples.
Fig. 3 shows the XRPD analysis for the synthesized solids. Fig. 3a
presents the characteristic pattern of a ZHC solid [17] with a basal
˚
˚
distance of 7.78 A, very close to the reported value of 7.8 A [11].
Fig. 3b–d shows, respectively, the XRPD patterns of FeDF-ZHC,
FeCF-ZHC and FeDC-ZHC. It can be observed that the XRPD patterns
are virtually the same and no shift of basal peaks was detected,

G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
97
Fig. 2. Schematic representation of the FePor immobilized on the solid support ZHC.
suggesting the absence of any porphyrin intercalation between
the ZHC layers [12,14]. This finding reinforces the hypothesis of
anionic FePor immobilization on the support layered crystal edges.
The change of the basal peak’s intensity and the overall improve-
ment of the ZHC crystal quality can be explained by the crystal’s
ripening due to their extensive contact with the solvent during the
immobilization process [8].
is obtained as a by-product of the ZHC synthesis and was not com-
pletely removed by washing.
Fig. 4b–d shows, respectively, the FTIR analysis for the immo-
bilized solids FeDF-ZHC, FeCF-ZHC and FeDC-ZHC. Very small
differences between these analyses in comparison to pure ZHC
were observed, although these differences are linked to the disap-
pearance or decrease in intensity of bands related to NH4Cl. This can
occur as a result of the immobilization process and new material
washing procedures. The band groups relative to ZHC are practically
the same in the immobilized solids.
For comparison, Fig. 4e presents the analysis for pure
[Fe(TDCSPP)], with a characteristic sulphonate porphyrin spec-
trum, with the band related to the SO3 group at 1100 cm−1 and
the aromatic C C at 1600 cm−1 [27]. The analyses for [Fe(TDFSPP)]
and [Fe(TCFSPP)] (data not show) are very similar to that reported
in Fig. 4e. The bands attributed to FePor were not observed in the
immobilized solids, due to the high intensity of support bands and
the low amount of porphyrinic complex immobilized [8,23]. Finally,
Fig. 4 depicts the FTIR analysis for the synthesized solids.
Fig. 4a shows the pure ZHC spectra, with a characteristic view for
these compounds [18,25], the broad band in 3400–3600 cm−1 is
attributed to vibration of water molecules [11] and the defined
peaks at 3500 and 3454 cm−1 refer to the stretching of the O
H
groups from the layer [25]. The band at 1622 cm−1 occurs due to
interlayer water angular vibration [12]. The band group at 1045, 906
and 725 cm−1 refers to O H group deformation [25] and the bands
at 576, 532 and 466 cm−1 are attributed to the vibrational modes
of Zn O [12]. A Zn Cl bound vibration could not be detected, since
these vibrations are observed only below 400 cm−1 [18]. Bands at
3200, 1394 and 1250 cm−1 are due to NH4Cl [26], as this compound
Fig. 4. FTIR spectra of ZHC (a), FeDF-ZHC (b), FeCF-ZHC (c) and FeDC-ZHC (d) and
[Fe(TDCSPP)] (e).
Fig. 3. XRPD patterns of the synthesized ZHC (a), FeDF-ZHC (b), FeCF-ZHC (c) and
FeDC-ZHC.

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G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
Fig. 5. Solid UV–vis spectra of ZHC (a), FeDF-ZHC (b), FeCF-ZHC (c) and FeDC-ZHC
Fig. 6. EPR spectra of ZHC (a), FeDF-ZHC (b), FeCF-ZHC (c) and FeDC-ZHC (d),
(d).
[Fe(TDFSPP)] (e), [Fe(TCFSPP)] (f) and [Fe(TDCSPP)] (g).
for the supported solids, a tiny band appears at 1510 cm−1, mainly
in FeCF-ZHC and FeDC-ZHC, attributed to carbonate ions adsorbed
during the immobilization process, due to the solution’s contact
with atmospheric carbon dioxide [11].
In order to demonstrate the presence of FePor on the support
ZHC, the solids FeDF-ZHC, FeCF-ZHC and FeDC-ZHC were analyzed
by UV–vis spectroscopy (Fig. 5b, c and e, respectively). Soret bands
were observed at 416, 416 and 434 nm for FeDF-ZHC, FeCF-ZHC and
FeDC-ZHC, respectively, and Q bands were observed in the region
of 500–600 nm, for each metalloporphyrin.
The Soret band of the immobilized porphyrins, when com-
pared to FePor in ethanol solution, is shifted to the lower spectral
energy region [23]. Similar behavior has been reported in other
works involving metalloporphyrin immobilization [8,12] and is
attributed to interaction between the porphyrinic complex and the
solid surface, creating steric limitations to the immobilized metal-
loporphyrin [28], generating the dislocation to red spectral region
[12,29].
symmetry at g = 5.8 and at g// = 2.0 are observed. This is a com-
⊥
mon behavior for metalloporphyrin in solid state [22,32], as is also
a signal with smaller intensity at g = 4.3. In comparison, the relative
signal intensities for Fe(III) in axial and rhombic symmetries have
small change after the immobilization process. The immobilization
process normally increases the rhombic character of porphyrinic
complexes [23,33], consequently increasing the signal intensity at
g = 4.3. When the immobilization occurs of a relative uniform and
flat surface, as observed for layered double hydroxide [26], the
FePor is submitted to a small distortion, but when the immobi-
lization surface is corrugated, as in ZHN [12], the FePor rhombic
distortion increases considerably.
The EPR analysis provides more evidence that the metallopor-
phyrin’s immobilization occurred on the edges of the ZHC layers.
The distortion in their structure is small, the opposite of what would
occur if the immobilization took place at surface tetrahedra [12].
Finally, for the immobilized solids, the free radical present in initial
ZHC has smaller intensity, due probably to the increase of crys-
tallinity by the ripening process.
Fig. 5a shows the UV–vis spectroscopic analysis of the pure
ZHC matrix, where the absence of any band attests that the bands
present in the other analyses are due to the immobilized FePor.
EPR analysis was used for solid characterization and also to show
the presence of FePor in the ZHC matrix. Fig. 6a shows the analy-
sis for pure ZHC, where an intense signal is observed at g = 1.95,
attributed to possible oxygen atoms vacancies in the crystalline
solid structure. This behavior is common in zinc oxide [30,31]. For
ZHC, some tiny signals near 2800–3300 G region are also present.
Signals in that region are observed in materials submitted to elec-
tron irradiated ZnO and are assigned to zinc atoms vacancies in the
structure [31]. In the case of zinc layered hydroxide salts, according
to our knowledge no article describes EPR analysis of pure ZHC, and
we suppose that during the solid synthesis, defects are formed in
the lattice structure, originating as oxygen atoms so zinc atoms
vacancies. In the analysis of ZHN [12], although a radical signal
was not detected, signals with minor intensity were also observed,
showing this is a typical behavior for the class of zinc hydroxide
salt compounds.
3.2. Investigation of FePor-ZHC solids as catalysts in
heterogeneous catalytic oxidation reactions
The first substrate used to investigate the oxidation catalytic
activity of the synthesized solids was cyclooctene, a substrate
commonly employed for catalytic activity diagnosis [8,23] and fre-
quently cited in the literature [2,24,34,35], in homogeneous and
heterogeneous catalysis. In reactions where metalloporphyrins are
employed as catalysts for this substrate, only cyclooctenoxide is
obtained as a final product. This behavior is attributed to the high
stability of intermediate radicals formed during the oxidation reac-
tion [5], where the probably active catalytic species ferryl porphyrin
␲-cation radical FeIV(O)P•+ shows high reactivity for the double
bond of cyclic alkenes, normally generating high catalytic results
[4,6,8].
The results obtained for cyclooctene oxidation catalysis with the
solids obtained by immobilization of FePor on ZHC can be seem in
Table 2. Yields greater than 90% for cyclooctenoxide production
were observed when the catalysts containing the supported por-
phyrinic complexes (Runs 1, 3 and 5) were used; these results are in
agreement to other catalysts based on metalloporphyrins [9]. For all
synthesized solids, improved reaction yields were observed when
compared with homogeneous catalysis (Runs 7–9). This observa-
tion can be attributed to different factors, e.g. absence of catalytic
The EPR spectra for FePor immobilized solids (Fig. 6b–d, respec-
tively, FeDF-ZHC, FeCF-ZHC and FeDC-ZHC) showed characteristic
signals for Fe(III) (S = 5/2) high spin in axial symmetry at g = 5.8
[22,32] and in rhombic symmetry at g = 4.3 [12], confirming again
the presence of FePor in the synthesized solids. Fig. 6e–g shows,
respectively, the EPR analysis of pure [Fe(TDFSPP)], [Fe(TCFSPP)]
and [Fe(TDCSPP)], where signals of high spin Fe(III) in axial

G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
99
Table 2
immobilized on synthetic tubular kaolinite [7] 28% of cyclohexanol
was obtained and when it was immobilized on a synthetic alumi-
nosilicate [26], 25% of cyclohexanol was produced. Instead of it,
in the case of ZHC support, despite of the yield results, it shows
an advantage in comparison to the other supports, it is a support
with a easiest synthesized route and easier manipulation until the
obtation of the solid catalyst with FePor. By contrast, when com-
pared to the same FePor immobilized on a layered double hydroxide
[12], only 8% of cyclohexanol is obtained, showing that the catalyst
reported in the present work have a better performance.
In general, when FePors are used as oxidation catalysts for
cyclohexane, in both homogeneous and heterogeneous reac-
tions, selectivity for cyclohexanol production has been observed
[4,7,9,26,36]. This is particularly clear for the catalysts obtained in
FePor immobilization on the ZHC support. This behavior can be
attributed to the place where FePor immobilization occurs, since
FePor interact with the layered crystal edges on a relative uni-
form surface, presenting low distortion in the porphyrinic ring,
as observed by EPR. When the same FePor were immobilized on
ZHN support [12], an unusual selectivity for cyclohexanone was
observed, where the FePor probably interact with the corrugated
surface tetrahedra of the layered crystals, generating an increase
in the rhombic character of the immobilized FePor, modifying the
catalytic reaction mechanism [12].
In catalytic studies of the reaction of intermediate species,
it has been noted that the active catalytic species of oxidation
reactions using FePor is the mentioned ferryl porphyrin ␲-cation
radical FeIV(O)P•+ [1–6,12]. Fig. 7 illustrates the reaction mecha-
nism. After the active catalytic species formation by interaction
between the FePor with PhIO, a hydrogen atom from the C H bond
of the substrate was abstracted and originated a new intermediate
species [FeIV OH + R•] (R = Substrate) [35], named as “solvent cage”.
The production of cyclohexanol and/or cyclohexanone is directly
dependent on “cage” stability. If the OH group linked to the iron
is rapidly directed to the radical (R• species) formed in the sub-
strate, cyclohexanol is produced. However, if the OH and radical
recombination does not occur rapidly, the radicalar species gener-
ated can escape from the “cage”, originating other products such as
cyclohexanone [12,35,40,41].
In the case of FePor immobilized on ZHN, it is possible that
the place of complex immobilization hampers maintenance of the
[FeIV OH + R•] species and the recombination process generating
radical escape and causing cyclohexanone production [12]. The ZHC
structure showed the same surface tetrahedra as ZHN [11], so it can
be supposed that the immobilized FePor on this support also can
generate ketone production. However, this was not observed. As
described previously, the immobilization does not occur on tetra-
hedra’s tops, but rather on layered crystal edges, contributing to
“solvent cage” control, preferentially generating alcohol as the oxi-
dation product. This fact is very common in systems where the
support surface is more uniform, as in the case of layered dou-
ble hydroxides [23,38], synthetic aluminosilicates [26] or silica [9].
When these solids were used in immobilization, the selectivity of
the synthesized catalysts was directed to cyclohexanol production.
The small amount of cyclohexanone obtained in heterogeneous
reactions was possibly generated from a new oxidation of cyclo-
hexanol previously formed [26,41].
Cyclooctenoxide yields achieved in the oxidation of cyclooctene by PhIO catalyzed
by FePor and FePor-ZHC.a
Catalyst
Run
Cyclooctenoxideb yield (%)
FeDF-ZHC
1st reuse
FeCF-ZHC
1st reuse
FeDC-ZHC
1st reuse
[Fe(TDFSPP)]
[Fe(TCFSPP)]
[Fe(TDCSPP)]
PhIO only, no catalyst
ZHC without immobilized FePor
1
2
3
4
5
6
7
8
9
98
92
91
87
95
93
85
70
78
10
11
10
11
a
Reaction conditions: reactant molar ratio 1:20:2000 (FePor/PhIO/substrate), at
room temperature under argon and 1 h of reaction. Homogeneous catalysis were
performed under similar conditions to those employed for heterogeneous catalysis
(similar reagent molar ratio), using iron porphyrin 1 mg, PhIO 3 mg and about 200 ␮L
of substrate.
b
Yield based on starting PhIO.
solubility factors [7,26] and more catalytic resistance to oxidative
degradation.
Another great advantage of heterogeneous systems is avoidance
of undesirable approximations between active catalytic species
that hinder the product’s formation [3,36] and also the possibility
of catalyst reuse [7,12,35].
The capacity to reuse the catalyst solids was also investigated
in heterogeneous reactions (Runs 2, 4 and 6), where low losses
of catalytic activity for the three solids were observed. During the
first use of the catalyst, slight leaching of immobilized FePor was
observed by UV–vis monitoring of the reaction solution, where
approximately 10% of the immobilized complex was lost, contribut-
ing to the decrease of the catalytic yield. In the reuse reactions,
progressive leaching of the metalloporphyrin from ZHC was not
observed. This fact suggests that around 10% of FePor, which left
the solid support after the catalyst’s first use, was weakly bonded
to the ZHC solid. Indeed, in the second and third reuse reactions for
FeDF-ZHC, similar catalytic yields of cyclooctenoxide to the first
reuse were obtained (data not inserted in Table 2), confirming the
stability of the catalyst on the support after the first use. The control
reactions, using only iodosylbenzene (Run 10) and pure ZHC (Run
11) showed very low catalytic yields, confirming that the observed
catalytic activity can be attributed to the immobilized FePor.
The synthesized solid was also investigated as a catalyst in the
oxidation of cyclohexane, a cyclic saturated hydrocarbon. This class
of compounds showed less reactive oxidation reactions than the
unsaturated substrates, such as cyclooctene [7,24,37]. Cyclohexane
is often used by research groups to investigate oxidation reactions
[4,12,24,35,37,38]. This substrate is also suitable to investigate the
selectivity of the obtained catalysts for a determined product. In the
case of cyclohexane oxidation, cyclohexanol and cyclohexanone are
obtained as products [7]. Another advantage of using cyclohexane
as a substrate for oxidation reaction investigation is the industrial
importance of this compound as a precursor for producing many
polymers [39].
Table 3 shows the results obtained for cyclohexane oxidation
with the prepared catalysts. The catalysts employed in heteroge-
neous catalysis (Runs 12, 14 and 16) presented very close results
for the three immobilized FePor. These results were slightly bet-
ter than those of homogenous catalysis with the same FePor (Runs
18–20) in relation to the total yield (the final result was very close to
homogeneous catalysis only for [Fe(TDFSPP)]), although the selec-
tivity for cyclohexanol production increased considerably for all the
catalysts synthesized. These cyclohexane oxidation yield results are
lower than the other systems where the same metalloporphyrins
are immobilized [7,26]. For example, when the [Fe(TDFSPP)] was
The heterogeneous reactions were investigated also in the sol-
vent mixture acetonitrile:dichloromethane (1:1, volume:volume).
In these reactions the same behavior for the catalysts was observed:
the selectivity for cyclohexanol was maintained, although there
was a decrease in the alcohol yield (cyclohexanol: 15, 20 and 17%,
respectively for FeDF-ZHC, FeCF-ZHC and FeDC-ZHC), and cyclo-
hexanone yield remained practically constant. Consequently, the
solvent system using only acetonitrile was the most effective to
evaluate the performance of the synthesized catalysts.

100
G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
Table 3
Oxidation of cyclohexane by PhIO catalyzed by FePor and FePor-ZHC.a
Catalyst
Run
Alcohol yieldb (%)
Ketone yieldb (%)
Total yield (%)
Alcohol/ketone ratioc
FeDF-ZHC
1st reuse
FeCF-ZHC
1st reuse
FeDC-ZHC
1st reuse
[Fe(TDFSPP)]
[Fe(TCFSPP)]
[Fe(TDCSPP)]
PhIO only, no catalyst
ZHC without immobilized FePor
12
13
14
15
16
17
18
19
20
21
22
18
8
2
2
3
2
2
2
6
5
4
20
10
25
13
25
17
21
17
20
1
9.0
4.0
7.3
5.5
11.5
7.5
2.7
2.8
4.5
1
22
11
23
15
16
14
18
<1
2
<1
1
3
2
a
Reaction conditions: reactant molar ratio 1:20:2000 (FePor/PhIO/substrate), at room temperature under argon and 1 h of reaction. Homogeneous catalysis were performed
under similar conditions to those employed for heterogeneous catalysis (similar reagent molar ratio), using iron porphyrin 1 mg, PhIO 3 mg and about 150 ␮L of substrate.
b
Yield based on starting PhIO (it was assumed that 2 mol of PhIO was used for ketone formation).
Selectivity for alcohol formation in relation to ketone formation.
c
The results of the first heterogeneous reuse of the solid cata-
lysts (Runs 13, 15 and 17), like those observed in the reaction using
cyclooctene, presented a decrease in the reaction yields, mainly the
alcohol yield for the three catalysts. This can also be attributed to
the porphyrinic complex leaching from the support during the first
use of the solids, and also to deactivation of catalyst molecules due
to the more drastic reaction medium generated in alkane oxidations
[7,26], although the selectivity for cyclohexanol was maintained in
the reuse reactions despite the decrease in that value too. In the case
of first reuse of the catalysts based on FePor-ZHN [12], a significant
decrease in comparison to the first use was observed, probably due
to the complex leaching or catalyst deactivation. In other systems,
as the [Fe(TDFSPP)] immobilized on synthetic aluminosilicate [26],
only a small deactivation was observed, showing that these reuse
capacity is specific to each support.
As observed for cyclooctene oxidation, the second and third
reuse reactions using FeDF-ZHC catalyst for cyclohexane oxidation
showed maintenance of the yields observed in first reuse, around
7% of cyclohexanol and only 1% of cyclohexanone in all cases (data
not shown), indicating the catalyst’s stability in reuse reactions.
In order to verify the real heterogeneous character of the cat-
alytic reaction and the contribution of the roughly 10% of FePor
leached from the support during the first use for any homogeneous
catalysis character, the solid catalyst was removed from the reac-
tion media after 1 h of reaction and the reaction was carried out
for 1 and 2 h more. The monitoring of the reaction solution did
not show any significant increase of the reaction yields. The same
experiment was performed at different reaction times and the same
results were observed, confirming that the yield observed in fact
can be attributed only to the presence of the solid catalyst in the
reaction solution.
The control reactions with only PhIO (Run 21) and pure ZHC
(Run 22) showed very low yields, confirming once more that the
observed catalytic activity is due to the immobilized FePor.
The linear alkane n-heptane was also investigated as a substrate
in this work. Linear alkanes have shown higher resistance to oxi-
dation than cyclic alkanes [7,39], so when this class of substrates
is used, a great expectation is created in relation to the possible
terminal positions of oxo-functionalization [8]. Very few examples
are described in the specialized literature regarding efficient cata-
lysts for linear alkane position 1 or position 2 selective oxidation
[39,42]. The linear alkane C H bond dissociation energy decreasing
by 104, 95.3 and 91 kcal mol−1, respectively, for primary, secondary
and tertiary carbons, this fact justifies in part because the majority
of previous works on linear alkane oxidation have shown bet-
ter results for products at positions 2 and 3 of the carbon chain
[8,26,36,43], despite the contribution of the statistic factors [7]. In
this sense, the development of an effective method for not activat-
ing alkane C H bond oxidation is very important.
Table 4 presents the results obtained for n-heptane oxidation
with the synthesized catalysts. In the reactions using immobilized
FePor (Runs 23, 25 and 27), there was high selectivity for alco-
hol production, as observed in the cyclohexane oxidation catalysis.
The use of FePor catalysts in linear alkane oxidation commonly
produces alcohols [7,39]. In comparison with the same FePor immo-
bilized on ZHN [12], the selectivity was the same. The n-heptane
linear structure contributes to “solvent cage” maintenance, and
indeed with the support surface irregularities in the case of ZHN,
Fig. 7. Cyclohexane oxidation mechanism with FePor as catalyst [12,35].

G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
101
Table 4
Oxidation of n-heptane by PhIO catalyzed by FePor and FePor-ZHC.a
Catalyst
Run
n-Heptane oxidation products yieldb (%)
1-olc
2-olc
3-olc
4-olc
Total ol
2-oned
3-oned
4-oned
Total one
ol/onee
FeDF-ZHC
1st reuse
FeCF-ZHC
1st reuse
FeDC-ZHC
1st reuse
[Fe(TDFSPP)]
[Fe(TCFSPP)]
[Fe(TDCSPP)]
PhIO only, no catalyst
ZHC without immobilized FePor
23
24
25
26
27
28
29
30
31
32
33
4
1
2
–
2
–
–
–
–
–
–
10
7
6
4
6
11
8
7
6
8
5
–
–
–
–
–
4
2
3
2
3
2
–
–
–
–
–
29
18
18
12
19
10
17
13
15
–
-
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
3
3
2
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
3
3
2
–
–
29
18
18
12
19
10
3
17
13
15
–
5.7
4.3
7.5
–
–
–
–
a
Reaction conditions: reactant molar ratio 1:20:2000 (FePor/PhIO/substrate), at room temperature under argon and 1 h of reaction. Homogeneous catalysis were performed
under similar conditions to those employed for heterogeneous catalysis (similar reagent molar ratio), using iron porphyrin 1 mg, PhIO 3 mg and about 130 ␮L of substrate.
b
Yield based on starting PhIO.
Heptanol.
Heptanone.
c
d
e
Selectivity for alcohol formation in relation to ketone formation.
Table 5
Comparative results for n-heptane oxidation with immobilized FePor on ZHC and ZHN in first catalyst utilization.
FePor
ZHC
ZHN
ola total
oneb total
ol/onec
ola total
oneb total
ol/onec
[Fe(TDFSPP)]
[Fe(TCFSPP)]
[Fe(TDCSPP)]
29
18
19
–
–
–
29
18
19
22
13
25
2
3
3
11
4.3
8.3
a
Heptanol.
Heptanone.
b
c
Selectivity for alcohol formation in relation to ketone formation.
the mechanism was directed to cyclohexanol production by the
traditional route [12]. For ZHC, the immobilized FePor on the crys-
tal’s edges contributed once again to alcohol production via the
traditional mechanism [40]. In relation to catalytic activity, the
immobilized catalysts on the two supports showed similar results
for total reaction yields, although the FePor immobilized on ZHC
was more selective than the ZHN support (Table 5).
The FeDF-ZHC presented the best catalytic result for n-heptane
oxidation, while for cyclohexane oxidation reaction it showed a
slightly lower result than for the others. This can be linked to the
FePor immobilization mode on the support [7]. The fluoride groups,
smaller than chloride atoms of the other two FePors, can allow more
effective interaction between the metalloporphyrin and the solid
surface, generating an increase in catalytic activity and also 4% yield
of 1-heptanol, which is a difficult oxidation product to obtain [39].
We stress that the comments about FeDF-ZHC catalytic activity are
hypotheses to explain the catalyst behavior.
ketone at position 3 of the carbonic chain. This can be assigned
to more restricted substrate access to the active catalytic species in
the metallic center, generated by the bulk and anionic substitutions
(sulphonates) on porphyrin aromatic rings [26,36]. With the FePor
immobilization on the solid surface, the arrangement leads to a
decrease in active catalytic center restriction, exposing the metal-
lic center and altering the previous selectivity for alcohol only at
position 2 to alcohol selectivity at all the positions of the carbon
chain.
Finally, for n-heptane, in the control reactions without the cata-
lysts presence and only with the oxidant PhIO (Run 32) and the pure
ZHC support (Run 33), insignificant product formation occurred.
Heptaldehyde formation was also not detected in any of the exper-
iments.
4. Conclusions
The catalysts reuse reactions (Runs 24, 26 and 28) showed, as
observed for cyclohexane, a lower yield in relation to that observed
in the first use of the solid. The leaching of the immobilized species
also contributed to the smaller yields. The selectivity continued to
favor alcohol production and ketone formation was not observed.
As in the other two substrates, the second and third reuse reac-
tions for FeDF-ZHC catalyst showed small reduction of total alcohol
yield (17 and 17%, respectively), indicating the absence of com-
plex leaching from the support after the first use. In ZHN system
[12], as already described for cyclohexane oxidation, in the first
reuse, considerable decreases were observed, indicating a common
behavior to the class of compounds containing metalloporphyrins
immobilized on layered hydroxide salts.
A family of anionic iron(III) porphyrins (FePor) were immobi-
lized on zinc hydroxide chloride (ZHC), previously obtained by
reaction between a ZnCl2 and NH4OH solution. The synthesized
materials were characterized by X-ray powder diffraction, infrared
and UV–vis spectroscopy and electron paramagnetic resonance,
with this last analysis evidencing that the FePor was immobilized
on ZHC.
The solids obtained were investigated as catalysts for het-
erogeneous oxidation reactions of cyclooctene, cyclohexane and
n-heptane. Very good results were observed for cyclooctenoxide
production, including in reuse reactions. For cyclohexane and n-
heptane, considerable oxidation results were observed. Despite
the great selectivity for alcohol production in these two cases,
the reuse reactions showed a higher decrease in comparison to
the cyclooctene reuse reaction, possibly attributed to active phase
leaching from the support or some catalytic deactivation during the
oxidation of these two more inert alkanes.
In comparison to homogeneous catalysis (Runs 29–31), the
supported catalysts presented better catalytic efficiency and excel-
lent selectivity for alcohol production. The homogeneous reactions
showed selectivity for alcohol production at position 2 and for

102
G.S. Machado et al. / Applied Catalysis A: General 413–414 (2012) 94–102
The catalytic yields were compared to the results obtained in
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the same FePor immobilization on zinc hydroxide nitrate (ZHN)
[12]. For this solid, an unusual selectivity for the cyclohexane oxi-
dation toward cyclohexanone was observed. This selectivity can
be attributed to the metalloporphyrin immobilization mode on
ZHN, which modifies the reaction mechanism by a radicalar route.
ZHC and ZHN structures are very similar, although the FePor were
immobilized in different modes on ZHC, generating alcohol pro-
duction in cyclohexane oxidation catalysis.
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Acknowledgements
We thank the National Council for Scientific and Technological
Development (CNPq), the Office to Improve University Research
(CAPES), the Araucária Foundation, the Paraná Federal University
Foundation (FUNPAR) and Paraná Federal University (UFPR) for
financial support. G.S. Machado thanks CAPES for a doctoral grant.
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