Fe-TUD-1 for the preferential rupture of the substituted CC bond of methylcyclopentane (MCP)

Article abstract of DOI:10.1016/j.catcom.2012.07.009

Fe-TUD-1 catalysts with Si/Fe ratios of 85, 65 and 45 were prepared via hydrothermal one-pot synthesis. The samples were characterized by XRD, N ₂ physisorption, UV-vis and XPS spectroscopies, which show the incorporation of Fe^{3 +} in

Full text of DOI:10.1016/j.catcom.2012.07.009

Catalysis Communications 27 (2012) 141–147
Contents lists available at SciVerse ScienceDirect
Catalysis Communications
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Short Communication
Fe-TUD-1 for the preferential rupture of the substituted C\C bond of
methylcyclopentane (MCP)
c
a
Saliha Haddoum a,b, Ioana Fechete a, , Bertrand Donnio , Francois Garin ,
⁎
Doina Lutic d, Chems Eddine Chitour b
a
Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse, UMR 7515 CNRS, Université de Strasbourg, 25 rue Becquerel, 67087, Strasbourg Cedex 2, France
Département Génie de l'Environnement, Ecole Nationale Polytechnique, 10 Av. H. Badi, BP 182, El Harrach, Alger, Algérie
Institut de Physique et Chimie des Matériaux de Strasbourg, UMR 7504 CNRS, 23 rue du Loess BP 43, F-67034, Strasbourg Cedex 2, France
b
c
d
Department of Chemistry, Al. I. Cuza University of Iasi, 11 Carol I Bd. 700506, Iasi, Romania
a r t i c l e i n f o
a b s t r a c t
Article history:
Fe-TUD-1 catalysts with Si/Fe ratios of 85, 65 and 45 were prepared via hydrothermal one-pot synthesis. The
samples were characterized by XRD, N2 physisorption, UV–vis and XPS spectroscopies, which show the incor-
poration of Fe3+ in tetrahedral coordination. The ability of these catalysts was tested by examining the
conversion of the reaction of methylcyclopentane (MCP) with hydrogen at atmospheric pressure and tem-
peratures between 200 and 400 °C. The active sites, tetrahedrally coordinated Fe and isolated atomic Fe
sites, were responsible for the endocyclic C\C bond rupture between substituted secondary-tertiary carbon
atoms, while the small clusters serve as active sites for the successive C\C bond rupture.
© 2012 Elsevier B.V. All rights reserved.
Received 26 March 2012
Received in revised form 30 June 2012
Accepted 10 July 2012
Available online 20 July 2012
Keywords:
Methylcyclopentane
Fe-catalysts
n-Hexane
Ring opening
1. Introduction
of the environmental and economic advantages, good catalytic per-
formance and the competitiveness with the noble metals. In this con-
With increasingly stringent environmental and economic regula-
tions, the foremost challenge facing in all industries is to find greener
processes with better atom economy [1,2]. In particular, the petro-
chemical industry has placed a strong research emphasis on the ring
opening of naphthenic molecules over noble metal catalysts [3–6] to
improve the cetane number of diesel fuels and to minimize harmful
emissions. Diesel fuels can be improved by selective ring opening of
naphthenic compounds following hydrogenation of aromatics com-
pounds [3]. Taking into account the fact that each dearomatization re-
action followed by a hydrogenation step leads to a cyclopentane, the
understanding of MCP chemistry is very important. In this case, al-
though MCP is not a component of diesel fuel, the ring opening of
MCP must first be understood because it serves as a model molecule
and provides the basis for understanding the behavior of all other
molecules. The ring opening of MCP is one of the most-used model re-
actions for exploring the structural sensitivity of hydrocarbon conver-
sion catalyzed by noble metals. Despite the effectiveness of supported
noble metals, which efficiently catalyze the conversion of MCP with
atom economy, the high cost and limited availability of precious
metals restrict their wide application. A practical solution is to find
a clean and efficient heterogeneous catalyst to replace noble metal
catalysts. It is cheaper to employ base-metal oxide catalysts because
text, mesoporous materials appear to be good candidates for the
conversion of hydrocarbons because of a large internal surface area,
high thermal stability and surface properties that create new oppor-
tunities for heterogeneous catalysis [7]. In the family of mesoporous
materials, TUD-1 (Technische Universiteit Delft) [8] is an interesting
material because of its 3D, sponge-like pore structure, which favors
mass-transfer kinetics and diffusion of reactant and product mole-
cules and possesses high thermal/hydrothermal stability. Although
the discovery of mesoporous materials has brought about the begin-
ning of a new age in the field of material synthesis, these materials
are known to be of limited use as catalysts because the catalytic func-
tionalities are missing. These mesoporous materials can be function-
alized by depositing heteroatoms onto the silica framework [9–11].
A promising approach for the creation of active sites is that of
substituting heteroatoms for Si by one-pot synthesis, which creates
active sites that enhance the activities of mesoporous solids and
allow catalysis [9–11]. However, it is noteworthy that few reports
are available on the catalytic properties of mesoporous materials for
the conversion of MCP [11,12]. Therefore, in this investigation, an at-
tempt has been made to synthesize and characterize high-quality
Fe-TUD-1 and to test its ability to catalytically convert MCP. The
goal is to understand the basic surface chemistry of iron in a reductive
environment greatly lags behind that of noble metals. Iron is of high
interest due to its wide range of applications in catalysis [13–16].
However, most studies of redox mesoporous catalysts published
⁎ Corresponding author. Tel.: +33 3 68852756; fax: +33 368852761.
E-mail address: ifechete@unistra.fr (I. Fechete).
1566-7367/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.catcom.2012.07.009

142
S. Haddoum et al. / Catalysis Communications 27 (2012) 141–147
thus far dealt with oxidative environments. Despite the successful de-
velopment of catalytically active Fe-containing mesoporous mate-
rials, little is known about their behavior in reductive environment
and the influence of isomorphous substitution on their properties.
The present paper reports on the effect of isomorphous substitution
on the catalytic properties of TUD-1 silicate molecular sieves. It is
known [17] that the isomorphous substitution of Fe3+ in the silicate
framework leads to acid catalysts by creating Si\O\Fe bonds and
being the genesis of Bronsted acidity. To obtain a gradation of the
acidity of the Fe-TUD-1 samples, a challenging controlled-synthesis
method was used to substitute iron for silicon atoms to obtain several
Si/Fe ratios. We used several physicochemical techniques to investi-
gate the location of Fe ions, to identify the environmental coordina-
tion of Fe ions, and to understand the nature of the active sites in
the samples.
spectrometer equipped with an AlKα X-ray source (1486.6 eV). The ele-
ment binding energies were referenced to the C (1 s) line situated at
284.6 eV.
2.4. Catalytic tests
The reaction of MCP was performed using a pulse method under
hydrogen at atmospheric pressure. The reaction conditions were as
follows: the range of temperatures studied was 200-400 °C; the
mass of catalyst was 200 mg; and the volume of MCP injected was
5 μl. Before the catalytic tests with H2, all catalysts were reduced in
an H2 atmosphere for 4 h at 500 °C. The gas flow used for treatments
and catalytic tests was fixed at 40 cm3 min−1. It should be noted that
the preliminary experiments were performed at 400 °C to determine
the rate of surface catalyst deactivation. In this case, we considered
the mean value of pulses 2–4. The experiments were repeated several
times to confirm the reproducibility of the data obtained. The identi-
fication of products was performed in a gas chromatograph (GS-2010
Plus, Shimadzu) with helium as the carrier gas using a capillary
column and a flame ionization detector (FID).
2. Experimental section
2.1. Reagents and materials
The synthesis of the mesoporous catalysts was performed using the
reagents without pretreatment. The tetraethylorthosilicate (TEOS, 98%)
and triethanolamine (TEA, 97%) were supplied by Acros, and the
tetraethyl ammonium hydroxide (TEAOH, 35%) was supplied by
Aldrich. The iron source was FeCl3 purchased from Aldrich. For
the catalytic experiments, the methylcyclopentane (MCP) was supplied
by Aldrich (purity>99%). Hydrogen (Air Liquide, Grade I), helium
(Air Liquide, Grade U) and air (Air Liquide) were used after purifi-
cation by a gas-cleaning filter system (Chrompack).
The performance of the catalysts was measured in terms of con-
version and selectivity.
The conversion of MCP is defined as:
n
P
ði=nÞCi
i¼1
:
ðmol%Þ
100
Conversion ¼
n
P
Cn0
þ
ði=nÞCi
i¼1
The selectivity is defined as:
2.2. Catalyst synthesis
Ci
Si ¼
ðmol%Þ
n
P
The mesoporous Fe-TUD-1 catalysts with different Si/Fe ratios were
synthesized hydrothermally from a gel with the following molar compo-
sition: 1 SiO2: x Fe: 0.5 TEAOH: 1 TEA: 11 H2O, with x=0.0118; 0.0154;
0.0222. The synthesis procedure was adapted from the literature [14].
The gel was prepared as follows: the triethanolamine was added to dis-
tilled water, which was then added to a solution containing a mixture of
tetraethylorthosilicate and the metal source; the resulting mixture was
blended using a magnetic stirrer for 45 min and was then added to the
tetraethyl ammonium hydroxide. The resulting gel was aged for 24 h
at room temperature, dried at 90 °C for 24 h, transferred to a stainless-
steel Teflon-lined autoclave and heated under static conditions. After
8 h at 180 °C, the mesoporous solids were calcined at 600 °C for 10 h.
Ci
i¼1
where Ci is the mole percent of product with i carbon atoms and Cn0 is
the mole percent of MCP (n=6) in the feed.
3. Results and discussion
3.1. Characterization of the structure of Fe-TUD-1 samples
3.1.1. Chemical analysis
Mesoporous Fe-TUD-1 catalysts with different Si/Fe ratios were
prepared by hydrothermal one-pot synthesis. Table 1 lists the Si/Fe ra-
tios in the wet gel and in the calcined samples. The Fe content in the
calcined samples is almost the same as the Fe content in the wet gel.
This correlation indicates that most of the Fe was incorporated into
the mesoporous TUD-1 by hydrothermal one-pot synthesis at 180 °C.
2.3. Physicochemical characterizations
The chemical analysis of the solid samples was performed by the
Service Central d'Analyse of CNRS-Solaize, France. X-Ray diffraction
(XRD) measurements were performed on a Bruker D8 powder diffrac-
tometer (40 kV and 50 mA) with Cu Kα radiation (λ=0.15418 nm).
The diffractograms were collected in the 2θ range of 0.5–6° and 10–
90°. Transmission electron microscopy (TEM) images were obtained on a
TopCon 2100 FCs microscope operated at 200 kV. Nitrogen physisorption
measurements were performed at −196 °C using a Tristar Micromeritics
automated apparatus. The samples were pretreated at 250 °C in vacuum
for 24 h before nitrogen adsorption. The specific surface area was calcu-
lated by the BET (Brauner–Emmet–Teller) method. The total pore volume
was determined from amount adsorbed at a relative pressure of approx-
imately 0.99, and the pore diameter distribution was calculated via anal-
ysis of the adsorption branch of the isotherm using the BJH (Barrett–
Joyner–Halenda) model equation with a cylindrical-pore approximation.
Diffuse reflectance spectra in the UV–vis region were collected on a
PerkinElmer spectrophotometer equipped with a diffuse reflectance ac-
cessory and using BaSO4 as a reference. The spectra were recorded be-
tween 200 and 800 cm−1. XPS signals were collected on Multilab 2000
3.1.2. X-ray diffraction (XRD)
The XRD measurements were performed to obtain information
about changes in the structure of the TUD-1 mesoporous host system
after isomorphous substitution via one-pot synthesis. The XRD pat-
terns at low angles for the calcined and reduced Fe-TUD-1 samples
with different Si/Fe ratios are shown in Fig. 1, which clearly shows a
single diffraction peak at low angles of 0.7–0.8°, indicating that the
Fe-TUD-1 catalysts are amorphous mesostructured materials [18]. It
must be noted that a significant change was not observed between
the Fe-TUD-1 samples after metal loading and after calcination and
reduction treatment. Although the intensity of the peak decreases
slightly with metal loading, the mesoporous structure of the Fe-TUD-1
samples was preserved. The Si/Fe=85 and 45 samples, with the Fe con-
tent increasing, were compared, but a shift of the 2θ peaks to lower an-
gles was nearly imperceptible. These XRD patterns coincide well with
data reported in the literature [19]. To identify any non-framework Fe

S. Haddoum et al. / Catalysis Communications 27 (2012) 141–147
143
Table 1
Physicochemical characteristics of calcined Fe-TUD-1 with different Si/Fe ratios.
Fe-TUD-1
Si/Fe=85
Si/Fe=65
Si/Fe=45
Si/Fe molar ratio gel
calcined solida
85
88
93
White
White
640
646
0.93
65
67
76
White
White
612
617
0.93
45
45
54
White
White
570
561
0.91
Si/Fe = 85 calc.
Si/Fe = 85 red.
Si/Fe = 65 calc.
Si/Fe = 65 red.
Si/Fe = 45 calc.
Si/Fe = 45 red.
XPSb
Color calcined solid
reduced solid
BET (m2/g)c calcined solid
Reduced solid
Pore volume (cm3/g)
calcined solid
Reduced solid
Pore diameter (nm)
calcined solid
0.93
6.6
0.93
6.6
0.90
6.6
0
10
20
30
40
50
60
70
80
90
2 Theta/degree
Reduced solid
6.6
6.6
6.6
Crystallinity of iron (DRX)
Metal coordination (UV–vis) Tetrahedral Tetrahedral Tetrahedral/octahedral
Active site (XPS, UV–vis)
Amorphous Amorphous Amorphous
Fig. 2. Wide-angle XRD patterns of the Fe-TUD-1 samples with Si/Fe ratios of 85, 65 and 45.
Fe3+
Fe3+
Fe3+
a
Atomic ratios measured by chemical analysis.
Atomic ratios of the calcined samples.
BET-specific surface area.
3.1.4. N2-physisorption
b
c
In addition to XRD analysis, N2-physisorption is a method used to
characterize mesoporous materials. Consistent with the above
XRD analysis, all samples have similar N2-sorption isotherms.
The N2-physisorption isotherms of calcined/reduced Fe-TUD-1 samples
(Fig. 4) were all type-IV isotherms, according to the BDDT classification
[20], and showed a hysteresis loop characteristic of wormhole type dis-
ordered mesoporous materials [21]. All samples showed steep increases
in the volume of adsorbed nitrogen at relative pressures of P/P0=0.6–0.8
due to the onset of capillary condensation within mesopores. The surface
area was calculated following the BET method, and the pore volume and
diameter were calculated from the desorption branch using the BJH
method. The results are presented in Table 1. The surface area decreased
gradually with increased the metal loading, from 640 m2/g for Si/Fe=85
to 612 m2/g and 570 m2/g for Si/Fe=65 and Si/Fe=45, respectively.
The incorporation of more metal decreases the pore volume from
0.93 cm3/g for Si/Fe=85 to 0.91 cm3/g for Si/Fe=45. Meanwhile, all
oxide phases, XRD patterns for all samples were recorded in the
wide-angle region (Fig. 2). The diffractograms do not show any patterns
in this region. A diffraction peak corresponding to the crystalline phase of
Fe was not detected. It is deduced that the added Fe may have been high-
ly dispersed on the TUD-1 samples. These results were confirmed by the
color of the calcined and reduced samples, which were completely
white, indicating that iron was part of the framework of the mesoporous
solid and that no iron oxides are formed. It must also be noted that the
absence of a diffraction peak can be attributed to the low content of Fe
when the particles are small and thus might not be visible in the powder
XRD pattern due to the detection limit of the XRD apparatus. A broad
peak centered at 23° (2-theta) originates from the amorphous nature
of pore walls of the TUD-1 support was observed in all samples.
(a)
3.1.3. Transmission electron microscopy (TEM)
The highly dispersed Fe species in the silica framework of TUD-1
and the absence of the crystalline phase of Fe were also confirmed
by the TEM analysis. A typical TEM image of the calcined and reduced
Fe-TUD-1 sample with a Si/Fe ratio of 45 is shown in Fig. 3. The im-
ages show a fully disordered sponge-like or wormhole mesoporous
structure, and with a three-dimensional regular pore structure.
These TEM images of the Fe-TUD-1 samples with a Si/Fe ratio of 45
showed a structure comparable to that of Si-TUD-1 mesoporous ma-
terials [8].
(b)
Si/Fe = 85 calc.
Si/Fe = 85 red.
Si/Fe = 65 calc.
Si/Fe = 65 red.
Si/Fe = 45 calc.
Si/Fe = 45 red.
5.0
3.0
4.0
2.0
0.5
1.0
2 Theta/degree
Fig. 3. TEM micrographs of the calcined (a) and reduced (b) Fe-TUD-1 sample with Si/Fe
Fig. 1. Low-angle XRD patterns of the Fe-TUD-1 samples with Si/Fe ratios of 85, 65 and 45.
ratio of 45.

144
S. Haddoum et al. / Catalysis Communications 27 (2012) 141–147
coordination (Fe3+O6), was absent in the samples with Si/Fe ratios of
85 and 65, indicating that these samples were free of the ferric oxide
species. Compared with the samples with Si/Fe ratios of 85 and 65, a
red-shift is observed as the iron content increased for the sample with
a Si/Fe ratio of 45. Nevertheless, the Fe-TUD-1 sample with a Si/Fe ratio
of 45 shows a broad absorption band in the UV region between 260 and
400 nm. The UV–Vis spectra of this sample shows three absorption
bands at 255 nm, 290 nm and 320 nm. The band at 255 nm is typical
of dπ-pπ charge transfer between the Fe and O atoms in the framework
of TUD-1 and demonstrated the formation of Fe\O\Si bonds and the
presence of tetrahedrally coordinated iron atoms. The d-d transition
band at 290 nm indicates the presence of highly dispersed, isolated, oc-
tahedrally coordinated Fe entities, which are the prevalent species [24].
The absorption band at 320 nm observed for the Fe-TUD-1 with a Si/Fe
ratio of 45 stresses that the Fe-TUD-1 samples contain oligomers of low
nuclearity 2d-FeOx species in low amounts. In Fe-TUD-1 with Si/Fe
ratio of 45, some Fe\O\Fe clusters seem to coexist with isolated Fe
sites of Si\O\Fe bonds. However, it is worth noting that the charge
transfer transition bands between 300 and 400 nm are attributed to oc-
tahedral Fe3+ in small oligomeric FexOy clusters [22,24]. No absorption
band was observed between 400–600 nm, revealing the absence of
bulk iron oxide. The UV–vis spectra in the wavelength range of 200 to
800 nm for the samples reduced for 4 h under H2 are also displayed in
Fig. 5. It was observed that the reductive treatment does not cause a no-
ticeable change in the mode of iron-ion stabilization and that the charge
transfer bands are not essentially shifted. This observation is in good
agreement with the high stability of isolated iron atoms in a reductive
environment, which suggests that no partial or total breaking of the
Si\O\Fe framework linkages take place. Our results agree with those al-
ready reported [25], which show that the isolated tetrahedral sites incor-
porated into the framework is not reduced up to 500 °C.
Si/Fe = 85 calc.
Si/Fe = 85 red.
Si/Fe = 65 calc.
Si/Fe = 65 red.
Si/Fe = 45 calc.
Si/Fe = 45 red.
0
0.2
0.4
0.6
0.8
1
Relative Pressure (p/po)
Fig. 4. N2 physisorption isotherms (−196 °C) for Fe-TUD-1samples with Si/Fe ratios of
85, 65 and 45.
Fe-TUD-1 samples exhibit a pore diameter of 6.6 nm, suggesting that
pore shrinkage does not occur with increasing Fe content and that pore
diameter is virtually independent of Fe loading.
3.1.5. DR UV–vis spectroscopy
For the sake of gaining insight into the Fe-TUD-1 structure and
clarifying the nature/coordination environment of the active sites,
UV–vis spectroscopy was employed in this study. The UV–vis spectra
of calcined and reduced Fe-TUD-1 samples with different Fe contents
are shown in Fig. 5. The UV–vis spectra of Fe-TUD-1 with Si/Fe ratios
of 85 and 65 mainly show one absorption band between 200 and
280 nm with a maximum centered at ca. 235 nm. This maxima
value should be ascribed to Laporte-allowed ligand-to-metal charge
transfer that involves isolated tetrahedrally coordinated Fe3+. This
means a t1 →t2 and t1 →e transition with Fe3+ in an isolated tetrahe-
dral geometry (Fe3+O4) [22,23]. For low iron content, Si/Fe ratios of
85 and 65, the Fe\O\Si bond of the isolated tetrahedral iron ions
in the silica framework of TUD-1 is dominant, indicating that Fe was
successfully incorporated into the silica TUD-1 network. Moreover,
the concentration of tetrahedral species increases with the crystallo-
graphic incorporation of Fe in TUD-1, as observed by the increased in-
tensity of the absorption band for the sample with a Si/Fe ratio of 65.
It is known that the ligand-to-metal charge transfer transition for iso-
lated Fe3+ sites gives rise to bands below 300 nm and becomes
red-shifted with increasing numbers of coordinating oxygen ligands
[22], reflecting the fact that our samples with Si/Fe ratios of 85 and
65 contained only the isolated sites. Absorption at higher wave-
lengths is essentially absent for the samples with Si/Fe ratios of 85
and 65, indicating that these samples contain predominantly isolated
tetrahedral framework Fe3+ species and that the formation of the
Si-O-Fe bond was not prevented from forming in our experimental
conditions. A characteristic band above 320 nm, typical of octahedral
3.1.6. XPS‐spectroscopy
Fig. 6 illustrates the parts of the XPS spectra associated with the Fe
2p1/2 and Fe 2p3/2 binding energy regions for calcined and reduced
Fe-TUD-1 samples, with Si/Fe ratios of 85, 65 and 45. All the spectra
are centered at 724.1 eV and 710.8 eV for Fe 2p1/2 and Fe 2p3/2, re-
spectively, which are typical values for Fe3+ [26]. The spin orbit sep-
aration between the peaks is 13.4 eV and is identical for all the
samples. The Fe 2p3/2 peaks (Fe 2p3/2 has a degeneracy of four states)
are narrower and stronger than those of Fe 2p1/2, which only has two
states, because of the spin-orbit (j–j) coupling. The intensity of the Fe
2p peaks increases with the Fe content. It is clearly seen from this re-
sult that the surface iron species of the Fe-loaded samples mainly
existed in the form of Fe3+ species. As shown in Fig. 6, the XPS signal
is unaffected by the reduction treatment, suggesting that the isolated
Fe ions in the TUD-1 situated in tetrahedral sites are stable under
reductive conditions up to 500 °C. The Fe in Fe-TUD-1 cannot be
255
235
290
320
724.1
Fe 2p
710.8
Si/Fe = 85 calc.
Si/Fe = 85 red.
Si/Fe = 65 calc.
Si/Fe = 65 red.
Si/Fe = 45 calc.
Si/Fe = 45 red.
Si/Fe = 85 calc.
Si/Fe = 85 red.
Si/Fe = 65 calc.
Si/Fe = 65 red.
Si/Fe = 45 calc.
Si/Fe = 45 red.
0
100 200 300 400 500 600 700 800
705
710
715
720
725
730
Wavelength (nm)
Binding energy (eV)
Fig. 5. UV–vis spectra of Fe-TUD-1 samples with Si/Fe ratios of 85, 65 and 45.
Fig. 6. Fe 2p XPS spectra of Fe-TUD-1 samples with Si/Fe ratios of 85, 65 and 45.

S. Haddoum et al. / Catalysis Communications 27 (2012) 141–147
145
reduced to Fe0 up to 500 °C, or even to Fe2+ by one electron transfer.
Table 2
Effect of the Si/Fe ratio and temperature on the conversion of MCP.
However, was observed [27] that isolated Fe atoms in Fe-ZSM-5 can-
not be reduced to Fe0, even up to 800 °C. This means that the
Si\O\Fe framework containing the isolated Fe atoms is stable in
the reductive environment. When introduced in the reductive envi-
ronment at a high temperature, the presence of Fe3+ in the catalysts
can be due to the nonmobility of Fe ions which are located in
Si\O\Fe framework. It must be noted that for the Fe-TUD-1sample
with Si/Fe=45, the UV–vis results stressed the presence of octahe-
dral coordination ions situated on their surface, but the XPS analysis
did not detect Fe2+. Fe3+ is present in the Fe-TUD-1 with a Si/Fe
ratio of 45, but the nature of this oxidation state is unclear. One
may expect the presence of Fe2+ or Fe0, but it is not detected in the
XPS spectra. Thus, the amount of these species either is below the de-
tection limit or is absent. The calculated Si/Fe ratio from integral inten-
sities of Si 2p and Fe 2p photoelectron spectra are summarized in Table 1.
A slight but systematic increase in the Si/Fe ratio was observed compared
with the chemical analysis. The spectrum of O 1 s from the XPS spectra of
each sample is composed of a single peak centered at 532.4 eV. Other ox-
ygen peaks attributable to iron oxide (at approximately 530 eV) and to
adsorbed –OH groups (at approximately 531.1 eV) are very low (Fig. 7).
Fe-TUD-1
T
Conversion Ring opening Cracking
Ring enlargement
(°C) (%)
selectivity,
selectivity, selectivity,
n-H (%)
C1 (%)
Bz (%)
Si/Fe=85
200
250
300
350
400
0.32
82
78
72
64
55
18
22
28
36
45
0
0
0
0
0
0.53
0.79
1.09
1.47
Si/Fe=65
Si/Fe=45
200
250
300
350
400
0.67
0.98
1.89
3.41
4.87
78
72
67
61
52
22
28
33
39
48
0
0
0
0
0
200
250
300
350
400
1.58
3.02
4.56
9.46
31.08
64
51
39
23
14
36
49
61
73
70
0
0
0
0
16
3.2.1. Conversion of MCP
3.2. Catalytic activity in the conversion of MCP
All catalysts displayed an activity in the conversion of MCP. In-
creasing the iron content of TUD-1 significantly enhanced the MCP
conversion, suggesting that iron can function as the active sites in
this reaction. For all catalysts, the activity increased with tempera-
ture. If evaluated at the same reaction temperature, the order of con-
version (catalytic performance) of the samples was as follows:
Fe-TUD-1, Si/Fe=45>Fe-TUD-1, Si/Fe=65>Fe-TUD-1, Si/Fe=85
(Table 2). This order is in good agreement with the sequence of the
density of active sites of these catalysts.
Isolated tetrahedral Fe3+ entities appear to be selective sites for
n-H, while the small aggregates serve as active sites for the C1 crack-
ing products, as previously proposed for Fe/KIT-6 prepared by a
solid-solid method [12]. For completeness, the reactivity of isolated
tetrahedrally/octahedrally coordinated Fe cations and small aggre-
gates in the Fe-TUD-1 catalysts were evaluated in the conversion of
MCP. The Fe-TUD-1 catalysts with different Si/Fe ratios were investi-
gated for the first time, in this study, with respect to MCP conversion
as a function of reaction temperature and metal loading. The effect of
the temperature on the activity was investigated in the temperature
range of 200–400 °C for Si/Fe ratios between 85 and 45. Before the cata-
lytic tests, all samples were reduced for 4 h in hydrogen at 500 °C.
The potential products in the conversion of MCP are n-hexane (n-H),
2-methylpentane (2-MP) and 3-methylpentane (3-MP), which are
formed from a ring-opening reaction of MCP. Cyclohexane (Ch) and ben-
zene (Bz) can be formed via ring-enlargement reactions, while methane
(C1), ethane (C2), propane (C3), butane (C4)/i-butane (i-C4) and pen-
tane (C5)/i-pentane (i-C5) can be formed from cracking reactions.
Table 2 compares the catalytic properties of Fe-TUD-1 catalysts with
different Si/Fe ratios as a function of reaction temperature for the con-
version of MCP. It must be noted that olefins were observed for the
Fe-TUD-1 with an Si/Fe of 45 over the entire range of studied tempera-
tures, but the olefins were present in small concentrations (b 0.02 %).
3.2.2. Ring-opening selectivity
As shown in Table 2, n-H was the only species among the
ring-opening products that was formed irrespective of the reaction
temperature or the Fe loading. The ring-opening reaction was domi-
nant in the samples with Si/Fe ratios of 85 and 65, irrespective of
the reaction temperature. These results suggest that the highly isolat-
ed, tetrahedrally coordinated Fe entities on the samples with Si/Fe ra-
tios of 85 and 65 were responsible for the rupture of endocyclic C\C
bonds in MCP between the substituted secondary-tertiary carbon
atoms. The results were very interesting because a different behavior
was observed when compared with noble metal catalysts. For exam-
ple, Pt/Al2O3 catalysts can open the MCP ring by breaking the substituted
or the unsubstituted C\C bonds in MCP [28,29] to form 2-MP, 3-MP and
n-H. Ir/Al2O3 catalysts exhibit the tendency to break the endocyclic C-C
bonds in MCP between unsubstituted secondary-secondary carbon
atoms, which only form 2-MP and 3-MP [28–30]. However, it must be
noted the ability of Ni/Al2O3 catalysts for the conversion of methyl-
cyclopentane [31]. A recent work [32] reported the selective formation
of n-H on noble metal catalysts, but the experimental conditions were
different. The conditions in that work were 10–50 torr under H2. Logical-
ly, the 1–6 cyclization is favored under these conditions, as already
reported [33].
For high Fe loading on the sample with a Si/Fe ratio of 45, the
ring-opening reaction with the selective formation of n-H prevailed
at the low temperature of 200 °C. Only the highly isolated, tetrahe-
drally coordinated Fe entities are favorable sites for the endocyclic
rupture of C\C bonds at the substituted position. For the other reac-
tion temperature, the cracking C1 product was dominant. Taking into
account the UV–vis results, isolated tetrahedrally/octahedrally coordi-
nated Fe species and small Fe clusters seem to coexist in this sample.
This result is consistent with the suggestion that the tetrahedrally coor-
dinated and atomically isolated sites are responsible for the rupture of
endocyclic C\C bonds between substituted secondary-tertiary carbon
532.4
O 1s
Si/Fe = 85 calc.
Si/Fe = 85 red.
Si/Fe = 65 calc.
Si/Fe = 65 red.
Si/Fe = 45 calc.
Si/Fe = 45 red.
515
520
525
530
535
540
Binding enrgy (eV)
Fig. 7. O1s XPS spectra of Fe-TUD-1 samples with Si/Fe ratios of 85, 65 and 45.

146
S. Haddoum et al. / Catalysis Communications 27 (2012) 141–147
Scheme 1.
atoms. It must be noted that the ring-opening reaction is dominant but
not exclusive, suggesting that only a part of the Fe ions are involved in
the ring-opening reaction and that the mononuclear species of Fe formed
in these cases can be responsible for the successive rupture of C\C bonds
(which will be discussed in the next section). A decrease in the n-H selec-
tivity with the temperature is a common feature observed in all samples.
However, these results were significantly better than what was observed
for W mesoporous catalysts [11], for which no products derived from
ring-opening reactions were observed under the same conditions or for
Fe/KIT-6, Mo/KIT-6 and FeMo/KIT-6 prepared by solid-solid methods in
which the cracking products prevailed [12].
Now, we need to explain the results obtained from the MCP
ring-opening reaction and the high selectivity to n-hexane formation on
the Fe-TUD-1 catalysts. This reaction, also called “methylcyclopentane
hydrogenolysis”, was extensively studied in the 1970s and was exclusive-
ly performed on noble metals, mainly on alumina-supported platinum
catalysts with various mean metallic particle sizes. As a consequence of
such studies, it was noted that in the skeletal rearrangements of hydrocar-
bons, two types of isomerization mechanisms occur: the Bond Shift (BS)
and the Cyclic Mechanism (CM). To distinguish between them, the 13C
Tracer Technique was used (Scheme 1) [29]. It was noticed that the CM
was strongly favored on small Pt aggregates with diametersb2 nm and
that three types of hydrogenolysis take place: a nonselective one, occur-
ring on highly dispersed catalysts and corresponding to an equal chance
of breaking any C\C bond of the ring; a selective one, allowing only the
rupture of bisecondary C\C bonds and a “partially selective” mechanism
competing with the selective one on catalysts of low dispersion. Coming
back to our results in which we have only n-H formed, we have to assume
the reaction of a metallocyclobutane intermediate with the exocyclic
methyl, as already suggested [34]. After the metallocyclobutane exocyclic
intermediate is created, the C\C bond rupture of the ring leads to the
formation of a carbene-olefin, which gives a π-adsorbed vinyl group
via a 1,2 hydride shift, and n-hexane is formed. In addition, using deute-
rium exchange reactions on Fe films, it was observed that the vinylic
hydrogen is very mobile and is very easily exchanged [35]. Such observa-
tions on Fe catalysts reinforce the proposed mechanism, which may take
place on the Fe-TUD-1, as noticed on the Scheme 2. Such a mechanism
can explain the predominant n-H formation, the aromatization and the
extensive cracking reactions when the conversion or the temperature is
increased.
3.2.3. Cracking selectivity
Regardless of the Fe loading in the TUD-1 and the reaction tem-
perature, the only major product formed was determined to be C1, in-
dicating that a deep cracking process occurred with the Fe-TUD-1
catalysts. The high selectivity toward C1 is attributed, most likely, to
the hindered desorption of products that are formed, followed by
the successive C\C bond rupture on the Fe active sites present on
the TUD-1. Compared with the samples with Si/Fe ratios of 85 and
65, impressive values of selectivity toward cracking reactions were
observed for the sample with a Si/Fe ratio of 45 in the temperature
range of 250–400 °C, which suggests that octahedral Fe entities/
small clusters and high temperatures increase the C1 selectivity. The
color of this sample with a Si/Fe ratio of 45 was gray, while the
color of the samples with Si/Fe ratios of 65 and 85 remained white
after the catalytic tests. It must be recalled that the color of all re-
duced samples was white (Table 1). In other words, a slight deactiva-
tion was observed on the sample with a Si/Fe ratio of 45. A plausible
explanation could be the agglomeration of small clusters. It seems
that these small clusters are susceptible to easily forming the iron car-
bide, taking into account the fact that in the Fe\O\Fe clusters, the
binding energy of O\Fe is small compared with that of C\Fe. When
the Fe is isolated and tetrahedrally coordinated in the Si\O\Fe
framework, the binding energy is strong compared with C\Fe and
the Fe carbide cannot be formed, which is explained in this case by
the white color after catalytic tests. Noticeably, for the sample with
a Si/Fe ratio of 45 (the point of the framework partial damage) and
a high temperature, the convertibility between Fe clusters and Fe car-
bide was observed. These latter moieties are responsible for the
cracking reaction.
3.2.4. Ring-enlargement selectivity
Under our conditions, benzene was only detected for the sample
with an Si/Fe ratio of 45. Enlargement of the MCP ring was only
observed at temperatures of 400 °C. This result can be attributed to
thermodynamic phenomena because aromatization is favored at
Scheme 2.

S. Haddoum et al. / Catalysis Communications 27 (2012) 141–147
147
high temperatures. Keeping in the mind that ring-enlargement prod-
uct formation was hindered at low temperatures and at low Fe con-
tents, this result represents an advantage of Fe catalysts due to their
ability to hinder aromatization. This also holds true for the samples
with high Fe loadings but at low temperatures.
IDECAT network of excellence and the scientific program TASSILI for
their support.
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Acknowledgments
Financial support by the CNRS France is gratefully acknowledged by
Ioana Fechete. We are pleased to acknowledge the REALISE network, the