Efficient cyclohexane oxidation with hydrogen peroxide catalysed by a C-scorpionate iron(II) complex immobilized on desilicated MOR zeolite

Article abstract of DOI:10.1016/j.apcata.2013.05.022

The hydrotris(pyrazol-1-yl)methane iron(II) complex [FeCl ₂{η³-HC(pz)₃}] (Fe, pz = pyrazol-1-yl) immobilized on commercial (MOR) or desilicated (MOR-D) zeolite, catalyses the oxidation of cyclohexane with hydrogen peroxide to cyclohexanol and cyclohexanone, under mild conditions. MOR-D/Fe (desilicated zeolite supported [FeCl₂{η³-HC(pz)₃}] complex) provides an outstanding catalytic activity (TON up to 2.90 x 10³) with the concomitant overall yield of 38%, and can be easy recovered and reused. The MOR or MOR-D supported hydrotris(pyrazol-1-yl)methane iron(II) complex (MOR/Fe and MOR-D/Fe, respectively) was characterized by X-ray powder diffraction, ICP-AES, and TEM studies as well as by IR spectroscopy and N₂ adsorption at -196 C. The catalytic operational conditions (e.g., reaction time, type and amount of oxidant, presence of acid and type of solvent) were optimized.

Full text of DOI:10.1016/j.apcata.2013.05.022

Accepted Manuscript
Title: EFFICIENT CYCLOHEXANE OXIDATION WITH
HYDROGEN PEROXIDE CATALYSED BY A
C-SCORPIONATE IRON(II) COMPLEX IMMOBILIZED
ON DESILICATED MOR ZEOLITE
Author: L.M.D.R.S. Martins A. Martins E.C.B.A. Alegria
A.P. Carvalho A.J.L. Pombeiro
PII:
S0926-860X(13)00303-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2013.05.022
APCATA 14234
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
27-2-2013
15-5-2013
18-5-2013
Please cite this article as: L.M.D.R.S. Martins, A. Martins, E.C.B.A. Alegria,
A.P. Carvalho, A.J.L. Pombeiro, EFFICIENT CYCLOHEXANE OXIDATION
WITH HYDROGEN PEROXIDE CATALYSED BY A C-SCORPIONATE IRON(II)
COMPLEX IMMOBILIZED ON DESILICATED MOR ZEOLITE, Applied Catalysis
A, General (2013), http://dx.doi.org/10.1016/j.apcata.2013.05.022
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*Graphical Abstract (for review)
Page 1 of 35

*Highlights (for review)
-
-
-
The C-scorpionate Fe(II) complex is suitably supported on desilicated MOR
zeolite
Generation of mesoporosity in commercial MOR improves the anchorage of Fe
complex
The heterogeneous Fe system is notably efficient for the oxidation of
cyclohexane
-
The catalytic activity of the Fe complex is highly improved by heterogenization
Page 2 of 35

*Manuscript
EFFICIENT CYCLOHEXANE OXIDATION WITH HYDROGEN PEROXIDE
CATALYSED BY A C-SCORPIONATE IRON(II) COMPLEX IMMOBILIZED ON
DESILICATED MOR ZEOLITE
a,b,
a,c
a,b
d
L.M.D.R.S. Martins *, A. Martins , E.C.B.A. Alegria , A.P. Carvalho , A.J.L.
b
Pombeiro
a
Área Departamental de Engenharia Química, Instituto Superior de Engenharia de Lisboa,
Rua Conselheiro Emídio Navarro, 1, 1959-007 Lisboa, Portugal. e-mail:
lmartins@deq.isel.ipl.pt, Telephone: +351 218317163, Fax: +351 218317267.
b
CQE, Complexo I, Instituto Superior Técnico, Technical University of Lisbon, Av. Rovisco
Pais, 1049-001 Lisboa, Portugal
c
CIEQB, Instituto Superior de Engenharia de Lisboa, Rua Conselheiro Emídio Navarro, 1
1
959-007 Lisboa, Portugal.
d
Departamento de Química e Bioquímica and CQB, Faculdade de Ciências, Universidade
de Lisboa, Campo Grande Ed. C8, 1749-016 Lisboa, Portugal
Abstract
3
The hydrotris(pyrazol-1-yl)methane iron(II) complex [FeCl { -HC(pz) }] (Fe, pz =
2
3
pyrazol-1-yl) immobilized on commercial (MOR) or desilicated (MOR-D) zeolite,
1
Page 3 of 35

catalyses the oxidation of cyclohexane with hydrogen peroxide to cyclohexanol and
cyclohexanone, under mild conditions. MOR-D/Fe (desilicated zeolite supported
3
[
FeCl { -HC(pz) }] complex) provides an outstanding catalytic activity (TON up to 2.90
2
3
3
x 10 ) with the concomitant overall yield of 38%, and can be easy recovered and reused.
The MOR or MOR-D supported hydrotris(pyrazol-1-yl)methane iron(II) complex (MOR-
Fe and MOR-D/Fe, respectively) were characterized by X-ray powder diffraction, ICP-
AES, and TEM studies as well as by IR spectroscopy and N2 adsorption at
-196 ºC. The catalytic operational conditions (e.g., reaction time, type and amount of
oxidant, presence of acid and type of solvent) were optimized.
Keywords: C-scorpionates, iron(II), cyclohexane oxidation, supported catalysis, desilicated
MOR zeolite
1
. Introduction
The current interest in tris(pyrazolyl)methanes e.g., HC(pz) (Tpm, pz = pyrazol-1-yl)
3
arises from their diverse applications [1]. Of particular synthetic interest is the use of
transition metal complexes bearing such ligands in oxidation catalysis, namely toward
organic synthesis. In fact, some of us have demonstrated [2] that several tris(pyrazol-1-
yl)methane complexes can act, in homogeneous media, as catalysts or catalyst precursors
for relevant alkane oxidation reactions, namely peroxidative oxygenations (to give alcohols
and ketones) and carboxylations (to produce carboxylic acids), a field that remains rather
2
Page 4 of 35

unexplored due to the difficulty of the activation of the C-H and C-C bonds in such
unreactive compounds that prevents their broad usage for the direct syntheses of added
value chemical products [3]. An example with industrial significance concerns the
oxidation of cyclohexane to cyclohexanol and cyclohexanone that are important reagents
for the production of adipic acid and caprolactam used for the manufacture of nylon. The
industrial process uses a homogeneous cobalt species as catalyst and dioxygen as oxidant,
at a considerably high temperature (150 ºC), forming the oxidation products in a low yield
to achieve a good selectivity [3g-i] and so the need to search for more active systems under
milder reaction conditions has been recognized [3g-i,4].
The immobilization of a catalyst or a catalyst precursor complex on a solid support is a
common and suitable procedure to combining the advantages of homogeneous and
heterogeneous catalyses. In fact, the heterogeneization of homogeneous catalysts has
become an important strategy to obtain supported catalysts that retain the homogeneous
catalytic sites, with the advantages of easy separation and recycling [5,6]. The mechanical
stability and high porosity of zeolites could be advantageous during catalysts recycling and
reuse procedures. Nevertheless, the purely microporous character of zeolites can be a
drawback for processing large molecules of nanometer scale sizes, since it imposes
diffusion limitations due to the restricted access of those molecules to the porous structure.
This is the main reason why zeolites are not widely used as supports for large active
species, when compared with mesoporous solids. In fact, due to the microporous nature, the
immobilization is typically confined to the external surface resulting in limiting uptake and
protection of the guest molecule [7]. In this sense, the use of zeolites with hierarchical
porosity, combining micro and mesoporosity, can efficiently combine the properties
3
Page 5 of 35

inherent to zeolites with the presence of mesopores. The desilication, a post-synthesis [8]
controlled extraction of silicon from the zeolite framework in alkaline medium, results on
the generation of mesoporosity in zeolite crystals, and also on the enlargement of the
existent micropores which can be translated in a certain hierarchical porous structure. In the
last years, the volume of publications concerning the characterization and potential
applications of desilicated zeolites is increasing, especially focusing MFI zeolite [9,10] but
also other structures like BEA [11], FER [12] and MOR [9,13,14]. Although the first
explored applications are in the domain of heterogeneous catalysis, recent publications have
been showing more potentialities for desilicated zeolites as adsorbents [15] or enzyme
carriers [16]. However, the application of hierarchical zeolites as hosts for transition metal
complexes is scarcely reported: Guo and Kim [17] reported the immobilization of a salen Ti
complex on MFI with mesoporosity and achieved high activity and stability in the
epoxidation of (-)--pinene with H O ; Jin et al. [18] described the effect of the
2
2
immobilization of palladium acetate in thin ionic liquid layers on mesopore walls of
hierarchical MFI zeolite and found improved catalytic activity and high stability for the
Suzuki reaction.
Mordenite (MOR) is a member of the large pore zeolite family. It consists of a
1
2-membered ring (MR) system of pore channels of 0.67 × 0.70 nm interconnected by 8-
MR pores of 0.34 × 0.48 nm. Since this zeolite has excellent thermal and mechanic stability
it is one of the most used zeolites in industrial applications, especially as heterogeneous
catalysts in processes such as alkylation [19] and short-chain paraffin hydroisomerization
4
Page 6 of 35

[20]. The desilication conditions for this structure were already studied and optimized by
some of us [9].
In the present study, we aim to explore the ability of desilicated MOR zeolite (MOR-D) as
a support for the immobilization of tris(pyrazol-1-yl)methane-type scorpionate complexes
and evaluate the catalytic potential of the heterogeneized species for the peroxidative
oxidation of cyclohexane to the cyclohexanol and cyclohexanone mixture under mild
conditions, namely concerning the possibility of recycling.
2
. Experimental
2
.1. Catalyst and support preparation
The hydrotris(pyrazol-1-yl)methane [HC(pz) , Tpm, pz = pyrazol-1-yl] [21], as well as the
3
3
iron complex [FeCl { -HC(pz) }] (Fe, Figure 1) [2f], were prepared according to
2
3
published procedures, (the complex) under an atmosphere of dinitrogen (Air Liquid
Portugal) using standard vacuum and inert-gas flow techniques. The solvents were purified
by standard procedures and freshly distilled immediately prior to use.
The parent material used as support was a mordenite structure (MOR) in ammonium form,
supplied by Zeolyst (CBV21A; lot 2200-83), that according with the technical report has a
SiO /Al O = 20. The alkaline treatment was performed using NaOH (Merk p.a.) solution
2
2
3
as desilicating agent, following the procedure previously published [9]. Briefly, the zeolite
was stirred with 0.5 M NaOH solution using a proportion of 330 mg/10 mL solution. After
5
Page 7 of 35

the treatment, the zeolite suspension was filtered off, washed thoroughly up to pH 7 and
then dried overnight at 90 ºC.
3
2
.2. Immobilization of the iron complex [FeCl { -HC(pz) }]
2
3
For the immobilization of Fe in MOR and MOR-D the wet-impregnation method was used
following the procedure previously reported by some of us [1h]. The iron complex (10 mg)
was dissolved in methanol (5 mL, Lab-Scan). The solution was added to the zeolite sample
(350 mg) and the mixture was refluxed at 80 ºC for 5 h. The supported complexes
(MOR/Fe and MOR-D/Fe, respectively for Fe heterogenised on commercial and
desilicated zeolite) were separated by vacuum filtration using a G4 porous plaque, washed
several times with ethanol (Riedel-de-Haën) and dried, firstly overnight at ambient
temperature and then at 50 ºC for about 4 h.
2
.3. Physicochemical characterization
The structural characterization of the supports (MOR and MOR-D) and complex loaded
samples (MOR/Fe and MOR-D/Fe) was undertaken using X-ray powder difractograms
(
XRD) that were obtained at room temperature in a Philips PX-1730, using a CuKα
radiation as incident beam. Difractograms were obtained by continuous scanning from 5º to
0º 2θ, with a step size of 0.02º 2θ and a time step of 0.4 s.
4
6
Page 8 of 35

-1
Far infrared spectra (400-200 cm ) were recorded on a Vertex 70 spectrophotometer, in
CsI pellets.
The metal contents present on the immobilized complexes were analysed by ICP using a
ICP-AES Horiba Jobin-Yvon model Ultima equipament at Laboratório de Análises, IST
Lisbon, Portugal.
Transmission electron microscopy (TEM) of parent and desilicated support was carried out
in a Hitachi, model H-8100 microscope, operating at 200 kV. The images were captured
from the thin edges of the particles supported on a carbon grid.
Textural characterization of the supports and complex-loaded samples was made by N2
adsorption isotherms at -196 ºC in an automatic apparatus Micromeritics ASAP 2010.
Before the isotherms acquisition the samples (≈ 50 mg) were outgassed for 2 h at 150 ºC,
-2
under vacuum better than 10 Pa.
2
.4. Typical procedure for the catalytic oxidation of cyclohexane and products analysis
The peroxidative oxidation reactions were carried out in Schlenk tubes and under
dinitrogen atmosphere as follows: 0.43-6.6 mol of MOR/Fe and MOR-D/Fe catalysts and
0
.08-1.6 mmol of pyrazine carboxylic acid (Hpca) (Aldrich) or of nitric acid (Fluka) (i.e.
catalyst:acid molar ratio of 1:100 to 1:2500) were added to 3.00 mL of NCMe (Riedel-de-
Haën) with vigorous stirring, whereafter 5.00 mmol of cyclohexane (Aldrich) (0.54 mL)
and 10.00 mmol of H O in a 30 % H O solution (Fluka) (1.02 mL) were added and the
2
2
2
reaction solution was stirred for 10 h, at room temperature and normal pressure. For the
7
Page 9 of 35

products analysis, 90 L of cycloheptanone (Aldrich, internal standard), 10.00 mL of
diethyl ether (Riedel-de-Haën, to extract the substrate and the organic products from the
reaction mixture) were added. The obtained mixture was stirred during 10 min and then a
sample (1 L) was taken from the organic phase and analysed by gas chromatography (GC)
by the internal standard method. Subsequently, an excess of solid triphenylphosphine was
added to the final organic phase (to reduce the cyclohexyl hydroperoxide, if formed, to the
corresponding alcohol, and hydrogen peroxide to water), and the mixture was analysed
again to estimate the amount of cyclohexyl hydroperoxide, following a method developed
by Shul’pin [22]. For determination of oxygenate concentrations only data obtained after
treatment of the reaction sample with PPh were used. Authentic samples of all oxygenated
3
products were used to attribute the peaks in chromatograms. Blank tests indicate that no
oxidation takes place in the absence of the metal catalyst or the oxidant. Eventual leaching
of the supported complex was also tested.
GC measurements were carried out using a FISONS Instruments GC 8000 series gas
chromatograph with a FID detector and a capillary column (DB-WAX, column length: 30
m; internal diameter: 0.32 mm). The temperature of injection was 240 ºC. The initial
temperature was maintained at 100 ºC for 1 min, then raised 10 ºC/min to 180 ºC. Helium
was used as the carrier gas. GC-MS analyses were performed using a Perkin Elmer Clarus
6
00 C instrument. The ionization voltage was 70 eV. Gas chromatography was conducted
in the temperature-programming mode, using a SGE BPX5 column (30 m × 0.25 mm ×
.25 µm). Reaction products were identified by comparison of their retention times with
known reference compounds, and by comparing their mass spectra to fragmentation
0
8
Page 10 of 35

patterns obtained from the NIST spectral library stored in the computer software of the
mass spectrometer.
3
. Results and discussion
3
The hydrotris(pyrazol-1-yl)methane iron(II) complex [FeCl { -HC(pz) }] (Fe, Figure 1)
2
3
was immobilized in parent (MOR) and desilicated MOR (MOR-D) zeolite supports, using
the wet-impregnation method, by the same procedure previously reported by some of us
[
1h].
-
Figure 1 -
3
.1. Physicochemical characterisation
The powder XRD patterns of parent (MOR) and desilicated (MOR-D) samples reveal the
same long-range crystal ordering, since no important changes in peaks position are
registered, as previously discussed [9]. However, in MOR-D the peaks intensity is affected
by the alkaline treatment, leading to a decrease of about 25 % of the crystallinity, when
compared with the parent zeolite. Upon complex immobilization no significant losses of
crystallinity are observed when compared with MOR and MOR-D.
TEM micrographs for MOR and MOR-D samples are displayed in Figure 2. For the parent
MOR zeolite (Fig. 2a) the images show the presence of solid dense crystals. Upon
desilication (Fig. 2b) the image presents corroded areas at the crystal external surface as
9
Page 11 of 35

well as lighter zones due to the perforation of the crystals as a consequence of the alkaline
treatment with relatively high concentrated NaOH solution (0.5 M).
-
Figure 2 -
The metal contents present on the immobilized complexes were quantified by ICP in
MOR/Fe and MOR-D/Fe samples and are displayed on Table 1. The loading level of iron
immobilization was shown to be 0.28% (wt/wt) of Fe on the MOR-D support,
corresponding to a catalyst loading of 18.1 mmol per gram of MOR-D, whereas MOR/Fe
was shown to be 0.40% (wt/wt) of Fe on the MOR support, corresponding to a catalyst
loading of 25.8 mmol per gram of MOR. The higher content of Fe on the parent sample
indicates that the interaction of the metal complex with the zeolite is affected by the amount
of internal/external surface groups. Indeed, the alkaline treatment performed on zeolites
leads to some lost of surface groups, that is, acid sites, along with a decrease in
crystallinity, especially when more severe experimental conditions of temperature, time or
base concentration are applied, as it was the case of the conditions used to prepare MOR-D.
Some of us have already reported [13] the decrease on the acid sites density of desilicated
MOR zeolite with the increase of the duration of the treatment, using 0.2 M NaOH
solution. Hence, it is predictable that the use of a higher base concentration (0.5 M) leads to
a significant loss of acid sites, and decreases the number of possible interactions between
the iron complex and the zeolite surface group, leading to a lower amount of Fe
immobilized on the support.
1
0
Page 12 of 35

The presence of Fe loaded on MOR or MOR-D is also detected at the MOR/Fe or MOR-
-1
D/Fe infrared spectra in the 400-200 cm range. In fact, both spectra display, compared to
-1
MOR (Fig. 3e) or MOR-D (Fig. 3b) spectra, new bands in the 255 - 235 cm range (Fig. 3d
or Fig. 3a, respectively for MOR/Fe or MOR-D/Fe) assigned to (Fe-Cl) of Fe (Fig. 3c),
which attest the presence of the Fe complex immobilized on the zeolitic supports.
-
Figure 3 -
In Figure 4 the N adsorption-desorption isotherm for MOR, MOR-D and MOR-D/Fe are
2
displayed. The curve of the parent structure belongs to type I isotherm in BDDT
classification [23], which is in accordance with the microporous nature of MOR zeolite. At
relative pressure near 1 a small upwards deviation of the plateau is noticed, pointing out
some mesoporosity or external surface, resulting of the aggregation of the small crystallites,
typical of commercial zeolites. Upon desilication treatment MOR-D sample exhibits a mix
type I+IV isotherm revealing the microporosity of the sample along with a considerable
development of mesoporosity, denoted by the increase of the N uptake at relative pressures
2
near unity. After complex immobilization there is a significant reduction of the adsorption
capacity of the samples indicating that most probably the complex molecule blocks the
entrance of the porous channels.
-
Figure 4 -
1
1
Page 13 of 35

In order to obtain a quantitative evaluation of porosity evolution the adsorption data were
analysed applying the BET equation, to compute the apparent surface area (ABET), and the
s method (taking as reference the isotherm determined on nonporous silica [24]) to obtain
the micropore volume (Vmicro). These textural parameters along with the external surface
area are displayed in Table 1. The desilication conditions used to prepare MOR-D promoted
an accentuated development of mesoporosity along with a small decrease of the volume
correspondent to micropores. This evolution reveals that the mesoporosity results mainly
from the corrosion of the external surface of the crystals, that is, intercrystalline
mesoporosity. On the other hand, the decrease in microporous volume also indicates the
occurrence of intracristalline mesoporosity. The effect of the complex immobilization on
the zeolitic supports is clearly shown as a considerable decrease on all the textural
parameters is observed.
Table 1 - Iron content, apparent surface area (A BBB EEE TTT ), micro (Vmicro) and total pore volume
(
Vtotal), and external area (A ) of the samples
.
ext
Sample
%Fe
ABET
Vmicro
Vtotal
Vmeso
Aext
2
3
3
3
2
(
m /g)
(cm /g)
(cm /g) (cm /g) (m /g)
MOR
-
502
43
0.20
0.01
0.17
0.01
0.25
0.05
0.38
0.10
0.05
0.04
0.21
0.09
24
22
MOR/Fe
MOR-D
MOR-D/Fe
0.40
-
500
45
125
25
0.28
1
2
Page 14 of 35

Considering the predicted dimensions of the iron complex (maximum width ~0.8 nm, by
3
comparison with related  -HC(pz) Cu complexes) [25] we must consider that the decrease
3
of the micropore volume accessible to N is most likely the result of the blockage of the
2
pore channels characteristic of MOR structure (0.67 x 0.70 nm). In the case of sample
MOR-D/Fe there is also an important decrease of the mesopore volume, compared to the
value presented by the same sample before the immobilization of Fe complex (see Table 1)
pointing out that at least a fraction of the complex must be located in the intercrystaline
mesoporosity created upon desilication. In fact, the mesopore size distribution of the MOR-
D reveals the presence of a large fraction of pores with width around 12.5 nm, what is
compatible with the predicted dimensions of the complex.
3
.2. Catalytic oxidation of cyclohexane
We found that MOR-D/Fe exhibits a remarkable catalytic activity (turnover numbers up to
3
ca. 2.90 x 10 , Table 2, entry 8) under mild conditions for the peroxidative oxidation of
cyclohexane to the cyclohexanol and cyclohexanone mixture, in a single-pot process
(Scheme 1). The catalytic system is based on the above hydrotris(pyrazol-1-yl)methane
3
Fe(II) complex [FeCl { -HC(pz) }] (Fe) supported on desilicated MOR zeolite (MOR-D),
2
3
hydrogen peroxide (30% aqueous solution) as the oxidizing agent, and acetonitrile as the
solvent, in a slightly acidic medium (typically with the presence of pyrazinecarboxylic acid,
Hpca), at room temperature and 10 h typical reaction time.
1
3
Page 15 of 35

Scheme 1
In order to determine the concentrations of all cyclohexane oxidation products the samples
of reaction solutions were analysed twice, that is, before and after treatment with an excess
of solid triphenylphosphine (in order to reduce the formed cyclohexyl hydroperoxide to the
corresponding alcohol, and hydrogen peroxide to water), following a method developed by
Shul’pin [22] and used previously by some of us [2a-d,2f,26]. By applying this method in
the present work, we demonstrate that the oxidation of cyclohexane affords predominantly
the cyclohexyl hydroperoxide CyOOH as a primary product (Scheme 1) which decomposes
to form cyclohexanol and cyclohexanone (a significant increase of the alcohol amount, by
ca. 40%, with a concomitant decrease of that of the ketone when the final reaction solution,
prior to GC analysis, is treated with an excess of PPh was observed).
3
The achieved TON is remarkably high, much above that found for MOR/Fe, Fe [2f] (91
and 232, Table 2, entries 9 and 10, respectively) or for other mononuclear iron complex
catalysts [2b,27,28] in homogeneous medium, in spite of the mild conditions, in aqueous
acetonitrile at a low temperature with a green oxidant, in the presence of a weak acid.
The yield (up to 37.5% Table 2, entry 8) is also much higher than those observed for
MOR/Fe (1.8%, Table 2, entry 9), Fe (up to 13%) [2f], the related Li[FeCl {SO C(pz) }]
2
3
3
1
4
Page 16 of 35

(up to 24%) [2f], [Fe{HOCH C(pz) } ] [Fe OCl ](Cl) ∙4H O (pz = pyrazolyl; up to 14%)
2 3 2 2 2 6 2 2
[
2b], [Fe(gma)(PBu )] (gmaH = glyoxal-bis(2-mercaptoanil); up to 8%) [27a], the Co(III)-
3
2
Fe(III) Schiff base complex [Co Fe OSae ]·4DMF·H O (H Sae = salicylidene-2-
4
2
8
2
2
ethanolamine) (up to 26%) [29] and the hexanuclear iron(III) p-nitrobenzoate complex
Fe O (OH)(p-NO C H COO) (dmf) ] (20%) [30]. It is lower than those for Fe(III)-Cr(III)
[
6
3
2
6
4
11
4
hydroxides [31] (up to 45%) but the latter system requires the use of a strong acid (HNO3)
and leads to a much lower TON value of 30. The combination of such a high yield and
TON is rare and indicates an exceptional activity of the catalytic system based on MOR-
D/Fe. Moreover, the oxidation of cyclohexane with hydrogen peroxide in the presence of
decamethylosmocene, [Os(Me C ) ] and pyridine, also an exceptionally active system [32],
5
5 2
4
needs 24 h to exhibit higher TON values (up to ca. 5.1  10 ) and the yields remain low (up
4
4
to ca. 6%). For [Os (CO) ] TON values up to 6.0  10 (i.e. 2.0  10 per Os [22b]) are
3
12
obtained at 60 ºC, whereas in the present work the reactions were performed at room
temperature.
A high selectivity towards the formation of cyclohexanol and cyclohexanone is exhibited
by our systems, since no traces of by-products were detected by GC-MS analysis of the
final reaction mixtures. No alkane oxidation products were obtained in the absence of Fe
(Table 2, entries 1 and 2) or hydrogen peroxide.
a
Table 2 - Oxidation of cyclohexane with H O (selected data).
2
2
Entry
Catalyst
n(H O )/n(catalyst) n(acid)/n(catalyst)
Time
Total
Total Yield
2
2
1
5
Page 17 of 35

-3
-3
b
c
x 10
x 10
h
TON
(%)
1
2
MOR
20
20
20
20
20
20
20
20
20
20
40
20
20
20
20
20
0
0
0
0
0
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
1
2
0.1
10
10
6
-
-
0.0
0.0
0.8
0.5
4.3
0.0
0.0
MOR-D
d
3
Fe [2f]
161
25
334
-
4
5
MOR/Fe
10
10
10
10
10
10
6
MOR-D/Fe
MOR
e
6
7
e
MOR-D
-
3
8
MOR-D/Fe
MOR/Fe
2.9 x 10
91
37.5
1.8
9
d, f
1
1
0
1
Fe [2f]
232
2.3
d, f
Fe [2f]
6
522
2.6
1
1
1
1
1
1
1
1
2
2
3
4
5
6
7
8
9
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
3
575
7.4
3
3
6
1.9 x 10
1.7 x 10
438
24.0
21.8
5.6
12
17
24
10
10
10
10
10
10
10
272
3.5
3.8
7.5
15
30
20
20
20
410
5.3
672
9.9
3
3
3
1.8 x 10
2.9 x 10
1.4 x 10
447
22.9
37.2
18.2
5.7
0
f
2
2
2
1
2
3
f
f
609
7.8
1
6
Page 18 of 35

g
h
i
2
2
4
MOR-D/Fe
MOR-D/Fe
MOR-D/Fe
20
20
20
1
1
1
10
10
10
478
407
371
6.1
5.2
3.1
5
2
6
a
Reaction conditions (unless stated otherwise): acetonitrile (3.0 mL), cyclohexane (5.0 mmol), r.t., under
3
4
dinitrogen; 0.43-6.6 mol of Fe, catalyst:Hpca (molar ratio 1:0 to 1:2.5 x 10 ), H O (1:1 to 3 x 10 :1 molar
2
2
ratio of oxidant to Fe catalyst). Percentage of yield, TON determined by GC analysis (upon treatment with
b
PPh3). Total (cyclohexanol + cyclohexanone) turnover number (moles of product per mol of supported
c
catalyst). Molar yield (%) based on substrate, i.e. moles of products (cyclohexanol and cyclohexanone) per
d
2f
1
00 mol of cyclohexane. The reaction time leading to the highest yields and TONs for the homogeneous
e
f
g
system. Reaction in the presence of Hpca or HNO . Reaction in the presence of HNO . Reaction with t-
3
3
h
i
nd
BuOOH as oxidant. Reaction in water. 2 run of catalyst used in entry 21.
The high catalytic activity of MOR-D/Fe must be related with the textural parameters of
the MOR-D support since after immobilization the Fe complex must be located in the
intercrystalline mesopores then the accessibility of the reactant molecules is strongly
improved leading to higher catalytic activity.
For the purpose of optimization of the catalytic system, the effects of various reaction
parameters were examined. Hence, the dependences of the yields of products and the TONs
on the oxidant-to-catalyst molar ratio, acid-to-catalyst molar ratio, reaction time and type of
acid, oxidant and solvent were investigated for the most active catalyst MOR-D/Fe.
3
.2.1. Effect of the reaction time
1
7
Page 19 of 35

The effect of the reaction time on the TON for MOR-D/Fe in the cyclohexane oxidation is
depicted on Figure 5. The overall TON (and yield) increases during the first 10 h
whereafter a drop is observed, conceivably due to over oxidation. In fact, some products of
overoxidation such as 1,4-cyclohexanedione and 1,2-epoxycyclohexane were detected by
GC-MS for the conditions of entry 16 in Table 2.
-
Figure 5 -
3
.2.2. Effect of the amount and type of acid
The presence of Hpca (pyrazinecarboxylic acid) is essential for these reactions to proceed
significantly (Table 2, compare entries 5 and 8, Figure 6). The addition of Hpca (up to a
Hpca:catalyst molar ratio of 1000:1) leads to a remarkable yield growth for both alcohol
and ketone (Figure 6). The co-catalytic effect of Hpca in vanadium-catalysed oxidations
was discovered by Shul’pin et al. [4f,4h]. Later, a high promoting effect of that
heteroaromatic acid on the peroxidative oxidation of alkanes catalyzed by homogeneous
iron species [4a,27,33], on the oxyfunctionalization of cyclohexane with dioxygen
catalyzed by scorpionate vanadium complexes [2e], as well as by pyrazole-rhenium
complexes supported on modified silica gel [1h], was also recognized [34]. The role of
Hpca, via coordination of pca (the conjugate base) to iron, conceivably consists in assisting
proton-transfer from ligated H O to an oxo- or a hydroxo-ligand, thus promoting the
2
2


formation of hydroperoxyl (HOO ) and hydroxyl (HO ) radicals, as it was proposed
1
8
Page 20 of 35

[
4a,4d,4f,4h,22b,33,35] for vanadium catalyst-Hpca-H O systems and supported by
2 2
theoretical calculations [36].
The activity of MOR-D/Fe is also promoted by addition of a mineral acid such as HNO3
(
Table 2, entries 21 - 23; Figure 6), as recognized by us for other metal catalysts
2a,2d,2e,2g,26b-d,37], but in a much lower extent.
[
-
Figure 6 -
3
.2.3. Effect of the amount and type of oxidant
The effect of the peroxide-to-catalyst molar ratio was investigated and is depicted in Figure
4
7
. The increase of the peroxide amount up to n(H O )/n(MOR-D/Fe) molar ratio of 2x10
2 2
leads to the maximum products yield of 37.5%. Further increase of the oxidant amount does
not result in a higher product yield, on account of overoxidation at higher H O amounts
2
2
(1,4-cyclohexanedione was detected by GC-MS for the conditions of entry 20 in Table 2).
-
Figure 7 -
Hydrogen peroxide behaves as a more effective oxidizing agent than tert-butyl
hydroperoxide (70%) (Table 2, entries 8 and 24, respectively), conceivably due to the bulky
character of the t-butyl group in the latter (resulting in a higher difficulty to reach the
supported iron complex).
1
9
Page 21 of 35

3
.2.4. Effect of the type of solvent
Acetonitrile was chosen as the typical solvent for our systems due to its high resistance to
oxidizing agents and also in view of the good solubility of the substrate. It has also been
used [1h,2] in other cases as the most appropriate solvent for cyclohexane oxidations,
3
namely with [FeCl { -HC(pz) }] in homogeneous media [2f]. The attempt to replace
2
3
acetonitrile by a greener solvent, such as water, results in a drastic decrease of activity
from 37.5% to 5.2%, Table 2, entries 8 and 25, respectively).
(
3
.2.5. Catalyst recycling
In order to investigate the recyclability of the MOR-D/Fe system, on completion of a stage,
the products were analysed as usually and the supported catalyst was recovered by filtration
from the reaction mixture, washed several times with acetonitrile and dried overnight at 50
º C. The subsequent cycle was initiated upon addition of new standard portions of all other
reagents; MOR-D/Fe exhibited 54% of its initial activity (Table 2, entry 26), while
maintaining the selectivity. This recycling possibility was not possible [2f] for Fe for the
peroxidative oxidation of cyclohexane.
Moreover, the amount of Fe on MOR-D/Fe after the first catalytic run was quantified. A
decrease of about half of the initial content was determined (0.28% to 0.12% of supported
Fe on MOR-D), which indicates the occurrence of lixiviation phenomena during the
2
0
Page 22 of 35

catalytic reaction or catalyst recovery and that the type of interaction between Fe and the
support is weak (probably of the electrostatic type) [38].
Surprisingly, only traces of cyclohexanol and cyclohexanone were found in a run
performed with the filtered solution from a first run (Table 2, entry 22) taken to dryness, to
which new standard portions of all other reagents were added. This suggests the formation
of new inactive species from Fe upon the release from the zeolitic support.
4
. Conclusions
This study has contributed towards the development of the still unexplored application of
transition metal complexes supported on desilicated zeolites as catalyst and/or catalyst
precursor for the oxidation of cyclohexane to the cyclohexanol and cyclohexanone mixture,
under mild conditions and with an environmentally friendly oxidant (H O ).
2
2
The generation of mesoporosity in commercial MOR by desilication revealed to be a good
3
strategy to improve the immobilization of the [FeCl { -HC(pz) }] complex on the zeolitic
2
3
support and appears to be crucial for its catalytic activity. In fact, the heterogeneous MOR-
D/Fe system is notably more efficient (much higher TON and yield values and lower
loading of oxidant) than the homogeneous C-scorpionate iron(II) complex.
The heterogeneous system also offered practical advantages of recycling (by the facile
separation from products), allowing successive cycles, although with a considerable loss of
st
nd
activity, mainly due to lixiviation (54 % from the 1 to the 2 cycle), an aspect which
deserves to be further addressed.
2
1
Page 23 of 35

Acknowledgments
This work has been partially supported by the Fundação para a Ciência e a Tecnologia
(FCT), Portugal, and projects PTDC/QUI-QUI/102150/2008, PTDC/EQU-EQU/122025-
2
010 and PEst-OE/QUI/UI0100/2011, PEst-OE/QUI/UI0612/2011 (CQB) and PEst-
OE/EQB/UI0702/2011 (CIEQB).
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