Study of Ir/WO3/ZrO2-SiO2 ring-opening catalysts: Part II. Reaction network, kinetic studies and structure-activity correlation

Article abstract of DOI:10.1016/j.jcat.2007.11.016

The present paper is the second part of a systematic study of the influence of W and Ir loading on the activity of Ir/WO₃/ZrO₂-SiO₂ catalysts for the ring-opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of Si-stabilized tungstated zirconias, WO_x/ZrO₂-SiO₂, containing up to 3.5 atom W/nm², was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by impregnation of these solids. Characterization of the metal dispersion and catalyst acidity was described in a previous article. The objective of the present study was to determine the best metal/acid balance for optimal performance of Ir/WO_x/ZrO₂-SiO₂ catalysts in the ring-opening reaction of MCH. Monofunctional (acid WO_x/ZrO₂-SiO₂ or metal Ir/ZrO₂-SiO₂) and bifunctional (Ir/WO₃/ZrO₂-SiO₂) catalysts were examined. Based on the analysis of the yields and products distributions, a reaction network was proposed, and kinetic data (e.g., activation energies, initial rates) were calculated. Correlations between characterization results obtained earlier (e.g., acidity, dispersion) and catalytic performance are also reported. The monofunctional acid catalysts WO_x/ZrO₂-SiO₂ showed a low selectivity for ring opening. The ring-contraction activity developed for W surface density above a threshold value of 1 atom W/nm². This was attributed to the appearance and the development of a relatively strong Broensted acidity monitored by infrared measurements. MCH ring contraction and C5 naphthene ring opening occur according to a classic acid mechanism. For low conversions, the monofunctional metal catalysts Ir/ZrO₂-SiO₂ exhibited significant selectivity for ring opening that decreased with increasing conversion. Because of the lack of ring-contraction products, the observed activity was attributed to the direct ring opening of the MCH. Ring opening and cracking occur according to a dicarbene mechanism. The study of MCH conversion on Ir/WO_x/ZrO₂-SiO₂ catalysts indicated that MCH ring contraction to alkylcyclopentanes occurs before ring opening. The best yields for ring opening were obtained with the 1.2% Ir/WO_x/ZrO₂ (1.5 atom of W/nm²). Further increases in W surface density led to a decrease in the indirect ring-opening yield, attributed to a decrease in Ir dispersion. For bifunctional metal/acid catalysts, analysis of the mechanism is less straightforward. The activation energy for C6 ring contraction and indirect C6 ring opening is a function of the metal/acid ratio. For high ratios, indirect ring opening occurs essentially over metallic sites. A decrease in the metal/acid ratio enhances the contribution of acid mechanism.

Full text of DOI:10.1016/j.jcat.2007.11.016

Journal of Catalysis 254 (2008) 49–63
www.elsevier.com/locate/jcat
Study of Ir/WO /ZrO –SiO ring-opening catalysts: Part II.
3
2
2
Reaction network, kinetic studies and structure–activity correlation
∗
Sébastien Lecarpentier, Jacob van Gestel, Karine Thomas, Jean-Pierre Gilson, Marwan Houalla
Laboratoire Catalyse et Spectrochimie, ENSICAEN-Université de Caen-CNRS, 6 Bd. Maréchal Juin, 14050 Caen cedex, France
Received 19 July 2007; revised 23 November 2007; accepted 25 November 2007
Available online 8 January 2008
Abstract
The present paper is the second part of a systematic study of the influence of W and Ir loading on the activity of Ir/WO /ZrO –SiO catalysts for
3
2
2
the ring-opening reaction of naphthenic molecules using methylcyclohexane (MCH) as a model compound. A series of Si-stabilized tungstated
2
zirconias, WOx/ZrO –SiO , containing up to 3.5 atom W/nm , was prepared. Ir-based catalysts containing up to 1.2 wt% were obtained by
2
2
impregnation of these solids. Characterization of the metal dispersion and catalyst acidity was described in a previous article. The objective of the
present study was to determine the best metal/acid balance for optimal performance of Ir/WOx/ZrO –SiO catalysts in the ring-opening reaction of
2
2
MCH. Monofunctional (acid WOx/ZrO –SiO or metal Ir/ZrO –SiO ) and bifunctional (Ir/WO /ZrO –SiO ) catalysts were examined. Based on
2
2
2
2
3
2
2
the analysis of the yields and products distributions, a reaction network was proposed, and kinetic data (e.g., activation energies, initial rates) were
calculated. Correlations between characterization results obtained earlier (e.g., acidity, dispersion) and catalytic performance are also reported.
The monofunctional acid catalysts WOx/ZrO –SiO showed a low selectivity for ring opening. The ring-contraction activity developed for W
2
2
2
surface density above a threshold value of 1 atom W/nm . This was attributed to the appearance and the development of a relatively strong
Brönsted acidity monitored by infrared measurements. MCH ring contraction and C5 naphthene ring opening occur according to a classic acid
mechanism. For low conversions, the monofunctional metal catalysts Ir/ZrO –SiO exhibited significant selectivity for ring opening that decreased
2
2
with increasing conversion. Because of the lack of ring-contraction products, the observed activity was attributed to the direct ring opening of the
MCH. Ring opening and cracking occur according to a dicarbene mechanism. The study of MCH conversion on Ir/WOx/ZrO –SiO catalysts
2
2
indicated that MCH ring contraction to alkylcyclopentanes occurs before ring opening. The best yields for ring opening were obtained with the
2
1
.2% Ir/WOx/ZrO (1.5 atom of W/nm ). Further increases in W surface density led to a decrease in the indirect ring-opening yield, attributed
2
to a decrease in Ir dispersion. For bifunctional metal/acid catalysts, analysis of the mechanism is less straightforward. The activation energy for
C6 ring contraction and indirect C6 ring opening is a function of the metal/acid ratio. For high ratios, indirect ring opening occurs essentially over
metallic sites. A decrease in the metal/acid ratio enhances the contribution of acid mechanism.
©
2007 Elsevier Inc. All rights reserved.
Keywords: Tungstated zirconia; Iridium; Methylcyclohexane conversion; Selective ring opening
1
. Introduction
high cycle oils [HCOs]) and coke, a significant fraction of light
cycle oil (LCO), the boiling range of which is similar to that
of diesel. To date, LCO is added to heating and heavy fuels
to adjust their viscosity [1]. Because of the increasing demand
for diesel, a more interesting application would be to blend the
LCO fraction in the diesel pool. However, the LCO characteris-
tics are very different from diesel specifications, especially in
Europe. LCO contains from 0.2 to 0.7% of sulfur and from
To meet an increased demand for transportation fuels, the
petroleum industry has to maximize the valorization of vari-
ous feedstocks originating from distillation or treatment units.
Specifically, the fluid catalytic cracking (FCC) process pro-
duces, in addition to gasoline, light gases, heavy products (e.g.,
4
2
8 to 69% of polyaromatics yielding a cetane index of 22–
5 [1], compared with the European norms of an upper limit
*
Corresponding author. Fax: +33 2 31 56 73 50.
E-mail address: marwan.houalla@ensicaen.fr (M. Houalla).
URL: http://www-lcs.ensicaen.fr.
of 50 ppm of sulfur (10 ppm in January 2009), 11% of poly-
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.11.016

50
S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
aromatics, and a minimum cetane index of 51. Thus, the LCO
fraction must be subjected to important transformations. Sul-
fur and nitrogen content can be reduced by conventional hy-
drotreatments and, if necessary, by other desulfurization tech-
niques [2]. The polycyclic aromatic fraction can be lowered by
hydrogenation, which also improves the cetane index. But this
increase is not sufficient, and a ring opening of naphthenic com-
pounds appears to be required [3–5]. Such ring opening must
preferably be highly selective toward the formation of linear or
mono-branched paraffins instead of multibranched ones, which
exhibit a lower cetane index.
reportedly decreases the Brönsted acidity of the solids, leading
to a decrease in cracking products. Similarly, the addition of an
acid function to metals active for hydrogenolysis of C5 rings
enhances C6 ring opening by promoting C6 to C5 ring contrac-
tion [3,20,21].
In general, a detailed examination of the results for metal,
acid-catalyzed, and bifunctional ring-opening highlights the
importance of judicious control of the metal to acid ratio in a
given catalyst. Brönsted acid sites must be sufficiently strong
and abundant to obtain good performance for ring-opening ac-
tivity while limiting cracking.
In principle, naphthene ring opening can be achieved by
metal or acid catalysis. Previous studies of catalysis by metals
focused on Ir-, Pt-, Ru-, and Rh-based solids. The hydrogenol-
ysis ability of these solids increases in the order Pt < Rh <
Ir < Ru [6,7], whereas the ring-opening ability depends on a
number of factors, such as particle size [8,9], the nature of the
support [10], and the naphthenic compound [3,11]. Thus, for
MCH conversion, large metal particles of alumina-supported
Ir/Al2O3, Ru/Al2O3 [7] and Rh/Al2O3 [10] catalysts exhib-
ited higher intrinsic activity, whereas the opposite results were
reported for Rh/SiO2 [10]. McVicker et al [3] examined the in-
fluence of the degree of substitution of the ring and the position
of the substituting group. They noted that for Ir/Al2O3, the yield
for methylcyclohexane ring opening is significantly higher than
that for 1,2,4-trimethylcyclohexane. They also showed that the
ring-opening yield for various C7-alkylcyclopentanes (ECP,
The present work is a systematic study of the influence of
the acidic and metallic functions on ring-opening activity. The
selected catalytic system was Ir supported on tungstated zir-
conia. Zirconia appeared to be an attractive choice because of
the possibility of modulating its acidity by controlled tungsten
deposition [22,23]. Furthermore, the mesoporous nature of the
support reduces the risk of diffusion limitations and enhanced
cracking previously encountered in the case of some zeolites.
Ir was selected because of its reportedly high C5 ring-opening
activity [24]. Characterization of the metal dispersion and the
catalyst acidity was described in a previous article [25]. Acid-
ity and Ir dispersion were monitored by low-temperature (77 K)
and ambient-temperature CO adsorption, respectively, followed
by infrared spectroscopy.
The present paper describes the catalytic activity for the con-
version of methylcyclohexane (MCH). The selection of MCH
as a model molecule for the evaluation of C6 ring-opening ac-
tivity was motivated by its relative simplicity, which limits the
number of possible reaction products and facilitates the estab-
lishment of a reaction network, and subsequent kinetic model-
ing enabling structure-catalytic performance relationships [26].
Based on the proposed reaction network, kinetic parameters are
calculated, and earlier characterization results (acidity; Ir dis-
persion) are correlated with the catalytic performance.
1
,1-DMCP, 1,2-DMCP, 1,3-DMCP) is directly proportional to
the number of unsubstituted C–C bonds present in a given iso-
mer. Except for Ru-based catalysts, which exhibited essentially
extensive cracking activity, Ni-, Pt-, Rh-, and Ir-based sys-
tems showed greater ring-opening activity for C5 rings than for
C6 rings. Consistent with this finding, the results [3] demon-
strated that the ease of converting two-ring naphthenes contain-
ing combinations of five- and six-membered rings over 0.9%
Ir/Al2O3 is directly related to the relative number of saturated
five-membered rings in the molecule.
2. Experimental
The studies of acid-catalyzed ring opening have been per-
formed primarily on zeolites. Kubicka et al. [12] showed that
decalin ring opening is preceded by ring contraction. The ac-
tivity is reportedly dependent on the number and strength of
Brönsted acid sites and on the porosity of the support. Thus,
the best ring-opening results were obtained with zeolites ex-
hibiting medium acidity (Hβ-25, Hβ-75, and HY-12). The low
activity measured with more acidic zeolites, such as morden-
ite, was attributed to coke formation. Similarly, Santikunaporn
et al. [13] showed that intermediate acidity is a key factor in
the ring-opening selectivity of decalin and tetralin molecules.
With respect to the influence of porosity of the solids, previ-
ous studies have indicated that large-pore zeolites inhibit the
dealkylation of alkylnaphthenes by limiting the steric hindrance
2.1. Materials
Zirconia doped with 1.2 wt% silica was prepared by drop-
wise addition of a 10 M ammonium hydroxide solution to
a 0.2 M solution of zirconyl chloride octahydrate (ZrOCl2,
FLUKA) up to a pH of 9.9. Preliminary experiments indicated
that substantial stabilization of the surface of the tetragonal
zirconia support can be achieved through the incorporation of
1.2 wt% Si [25]. Thus, subsequent studies of the influence of W
deposition were conducted with this solid. The silicon was in-
troduced before precipitation using the prerequisite amount of
SiCl4. The precipitated material was heated at 363 K for 48 h
and then left to stand at room temperature for 96 h in the so-
lution. The final product was recovered by vacuum filtration.
It was then redispersed in deionized water to remove residual
chlorine and filtered. This operation was repeated until no traces
[
14,15]. In general, the use of large-pores zeolites favors the
ring-opening reaction by limiting deactivation due to coke de-
posits [12,16,17].
A survey of the literature also shows that acid- or metal-
catalyzed ring-opening reactions are enhanced by the use of
bifunctional catalysts [13,18,19]. Doping zeolites with metals
−
of Cl could be detected with an AgNO3 solution. The washed
product was dried at 393 K for 16 h and calcined in air at 1023 K
for 3 h.

S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
51
The W phase (0–12 wt% W) was deposited by impregna-
tion of the support using an ammonium metatungstate solution,
followed by calcination at 1023 K for 3 h. The Ir (0–1.2 wt%)
was added by impregnation with an iridium chloride solution,
followed by calcination at 723 K with a 20% O2 + 80% N2
flow of 25 ml/min. The catalyst was reduced in situ in flowing
H2 at 623 K at atmospheric pressure before use. The tungstated
zirconias were designated ZSiWx, where x is the W surface
2
density expressed as at. W/nm . The series of Ir deposited on
tungstated zirconia was designated yIr/ZSiWx, where y is the
nominal weight percentage of Ir in the catalysts.
2
.2. Methylcyclohexane ring opening
Scheme 1. Possible products formed on conversion of MCH.
Methylcyclohexane (MCH) ring opening was carried out at
a total pressure of 5 MPa and a temperature range of 523–
98 K in a continuous-flow fixed-bed microreactor containing
.25–1 g of catalyst diluted in silicon carbide. The partial pres-
sures of hydrogen and MCH were set at 4.8 and 0.2 MPa,
respectively.
3.1.1.1. Reaction products Scheme 1 shows the products that
could be obtained by ring opening of methylcyclohexane. To
simplify the figure, the cracking products, containing <7 atoms
of carbon, were not included. Two types of ring opening are
noted:
5
0
MCH was delivered through a liquid chromatography pump
(
Gilson 302). The product gas mixture was collected at at-
– Direct ring opening (broken arrows) by hydrogenolysis of
the C–C endocyclic bond of MCH by the metallic function
– Indirect ring opening (continuous arrows) by ring contrac-
tion of the MCH to alkylcyclopentane on the acidic func-
tion of the catalysts followed by hydrogenolysis of an en-
docyclic C–C bond of the alkylcyclopentane.
mospheric pressure and analyzed online with a Varian 3400 gas
chromatograph equipped with a flame ionization detector and
a Chrompack CP-SIL 5 CB WCOT fused silica capillary col-
umn.
Before testing, each catalyst was activated in situ under at-
mospheric pressure at 673 K with flowing dry air for 2 h, cooled
to 623 K under flowing dry He and reduced at 623 K in a dry
hydrogen flow for 1 h. The MCH reactant was dried over acti-
vated 3A zeolite. Hydrogen (Air Liquide, Grade I) and helium
As can be seen from Scheme 1, more linear C H hydro-
7
16
carbons are obtained by direct ring opening. Note that for the
same number of carbon atoms, linear chain hydrocarbons ex-
hibit higher cetane index than their branched counterparts. In
the present article, the products are grouped as follows:
(
Air Liquide, Grade U) gases were further purified from water
and oxygen contaminants with 3A zeolite and BTS (FLUKA)
traps.
The MCH conversion was calculated from the equation
– Cracking (products with fewer than 7 carbon atoms).
ꢀ
ꢁ
–
–
Ring contraction (C7H14 isomers): ethylcyclopentane
ECP); dimethylcyclopentane (1,1-DMCP, 1,2-DMCP,
,3-DMCP).
f = 100 (Area(total) − AreaMCH)/Area(total) ,
(
1
where Area(total) designates the total peak areas of the chro-
matogram and AreaMCH, the peak area for MCH. The weight
Ring opening (C7H16 isomers): n-heptane (Hp); methyl-
yield was expressed by Ri = 100{Areaproduct i/Area
}. The
(
total)
hexanes (2-MHx, 3-MHx); dimethylpentanes (2,3-DMP,
weight selectivity of a group of products was evaluated by
3
2
,3-DMP, 2,4-DMP, 2,2-DMP), 3-ethylpentane (3EP);
,2,3-trimethylbutane (2,2,3-TMB).
ꢀ
Si = 100{Ri/f }. The weight selectivity S of a product i within
ꢀ
a group of products j was calculated from the relation S =
i
1
00{Ri/Rj }. For the molar selectivity of a product within the
cracking group, the peak area for a given compound was cor-
rected for the differences of molecular weight.
3
.1.1.2. Time-on-stream experiments In a given test, MCH
conversion was measured at four different temperatures (523,
5
2
28, 573, and 598 K) and three contact times (10, 13, and
0 g h/mol) for each temperature. To monitor the eventual de-
3
3
3
. Results
activation of the catalyst, conversion in standard conditions
.1. Reaction network
(
572 K, W/F = 20 hg/mol) was measured periodically. No
significant deactivation was detected for the duration of the
experiment (96 h). In standard conditions, the yields for ring
opening, ring contraction, and cracking were 40, 18, and 23%,
respectively.
.1.1. Methodology
The following section describes the methodology adopted
in the present study with respect to the experimental protocol,
product classification, and combined use of contact time and
reaction temperature to achieve an extended variation of con-
version. A bifunctional catalyst 1.2Ir/ZSiW2.5 was used for the
purpose of illustration.
3.1.1.3. Variation of MCH conversion by combined changes in
contact time and reaction temperature In a typical reaction

52
S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
test, the variation of MCH conversion is obtained by changing
the contact time at a given temperature or by varying the reac-
tion temperature at a given contact time. These two approaches
were combined in the present study to obtain a wider range
of variation of conversion. Although this is not a very rigor-
ous treatment of the results, it has the advantage of giving a
clearer picture of the evolution of various products with MCH
conversion. An example of this approach is shown in Fig. 1 for
the 1.2Ir/ZSiW2.5 catalyst. The ring-contraction products are
primary in nature, whereas those of ring opening and crack-
ing are secondary in nature. Alkylcyclopentanes obtained by
ring contraction are subsequently hydrogenolyzed to form ring-
opening products. The decrease in the ring-opening selectivity
for high MCH conversions is associated with the cracking of
ring-opening products.
3.1.2. Monofunctional acid catalysts (ZSiWx): influence of W
surface density
3.1.2.1. Activity Fig. 2a shows, for a series of ZSiWx cata-
lysts (with x = 0; 0.6; 0.9; 1.5; 2.5; 3.4), the variation of MCH
conversion as a function of W surface density for a contact
time of 20 g h/mol at a reaction temperature of 573 K. The
results indicate no significant MCH conversion for W surface
2
densities ꢀ0.9 at/nm . Further increases in W surface density
lead to a significant increase in conversion. Fig. 2b reports for
active catalysts, the yields of various types of products as a
function of contact time. For all contact times, the yields for
ring-contraction largely exceed those measured for ring open-
ing.
3
.1.2.2. Selectivity Fig. 3a depicts, for the ZSiWx series, the
selectivity for the three types of products: ring contraction, ring
opening, and cracking as a function of MCH conversion at
5
48 K. The results clearly show that for a given conversion,
essentially the same selectivities are obtained for the three ac-
tive catalysts. This suggests that similar types of active sites
are present in these solids (e.g., Brönsted acid sites of similar
strengths). Note that the selectivity for ring contraction tends
toward 100% with decreasing conversion, whereas that for ring
opening and cracking tends toward 0%. This indicates that the
ring contraction reaction precedes the ring-opening and crack-
ing reactions. In the absence of a metallic function, the mecha-
nism for ring opening is purely acidic, by β-scission. The crack-
ing products consisted primarily of C3 and C4, characteristic of
acid-catalyzed cracking. Furthermore, minor amounts of olefins
and aromatics were detected, which agrees with this mecha-
nism. Clearly, active monofunctional acid catalysts (ZSiWx)
were not selective for ring opening.
Fig. 3b shows the selectivities for C H isomers within the
7
14
ring contraction products obtained with the ZSiWx series as a
function of MCH conversion. The extrapolation to zero of the
ring-contraction yield indicates that 1,3-DMCP is the main iso-
mer, followed by 1,2-DMCP and ECP in comparable amounts
Fig. 1. Selectivity of ring contraction, ring opening, and cracking obtained with
the catalyst 1.2Ir/ZSiW2.5 as a function of MCH conversion for temperatures
between 548 and 598 K and for contact times of 1–20 g h/mol.
Fig. 2. (a) MCH conversion at 573 K for a contact time of 20 g h/mol as a function of tungsten surface density. (b) Yields for ring contraction (full symbols) and
ring opening (open symbols) obtained with ZSiW1.5 and ZSiW2.5 at 573 K as a function of contact time.

S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
53
Fig. 3. Selectivities obtained with ZSiWx catalysts (x = 1.5, 2.5, 3.4) as a function of MCH conversion at 548 K and for contact times between 10 and 20 g h/mol.
a) Selectivities for ring contraction, ring opening, and cracking. (b) Selectivities of C7H isomers within the ring-contraction group of products.
(
14
Fig. 4. (a) MCH conversion obtained with yIr/ZSiW0 catalysts as a function of Ir content at 573 K and for a contact time of 14 g h/mol. (b) Yields for ring
opening (open symbols) and cracking (full symbols) obtained with the yIr/ZSiW0 catalysts (y = 0.3; 0.6; 1.2 wt% Ir) as a function of contact time and at a reaction
temperature of 573 K.
and 1,1-DMCP as a minor product. The low ECP selectivity at
low conversion is indicative of the presence of strong Brönsted
acid sites [3].
ucts were detected. Note that catalysts containing different Ir
loadings exhibited similar selectivity for a given conversion.
This reflects the fact that only metal-catalyzed reactions are in-
volved. For low conversions, the selectivities for ring opening
and cracking were around 80% and 20%, respectively. The ring-
opening products were primarily 2 and 3 methylhexanes, char-
acteristic of a selective breaking of a nonsubstituted carbon–
carbon bond of MCH on Ir (Scheme 1). The ability of Ir to
open C6 rings has been reported by McVicker et al. [3]. The se-
lectivity for cracking (20%) observed at low MCH conversions
suggests that some of the cracking products are primary in na-
ture.
3
.1.3. Monofunctional metallic catalysts yIr/ZW0: influence
of Ir content
.1.3.1. Activity Fig. 4a shows the variation of MCH conver-
3
sion as a function of Ir content for the yIr/ZSiW0 catalysts at a
given contact time of 14 g h/mol and a reaction temperature of
5
73 K. A steady increase in conversion with increasing Ir con-
tent can be seen. Fig. 4b shows the yield in ring-opening and
cracking products for the catalyst yIr/ZSiW0 as a function of
contact time and at a reaction temperature of 573 K. No ring-
contraction products were detected. This is consistent with the
absence of an acid function. Direct ring opening is the predom-
inant reaction observed.
The cracking products consisted primarily of C1 and C6
compounds in comparable amounts (Fig. 5b). In principle, this
can be attributed to dealkylation of MCH. However, the amount
of cyclohexane detected was much lower than that would be
expected from such a process. A more likely explanation is
to attribute C and C formation to a “deep” hydrogenoly-
3
.1.3.2. Selectivity Fig. 5a reports the selectivities for ring
opening and cracking for yIr/ZSiW0 catalysts (y = 0.3; 0.6;
.2 wt% Ir) as a function of MCH conversion. In these solids,
1
6
1
sis. Consistent with this interpretation is the fact that the main
C6 products observed were 2 and 3 methylpentanes, which
could be formed by hydrogenolysis of ring-opening products
the zirconia support is not sufficiently acidic to isomerize MCH;
thus, only direct ring opening of the C6 ring and cracking prod-

54
S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
Fig. 5. (a) Selectivity for ring opening (full symbols) and cracking (open symbols) obtained with the yIr/ZSiW0 catalysts (y = 0.3; 0.6; 1.2 wt% Ir) as a function of
MCH conversion. (b) Selectivity within the cracking products obtained with the catalysts 1.2Ir/ZSiW0 and 0.6Ir/ZSiW0 as a function of cracking yield.
Fig. 6. (a) MCH conversion as function of tungsten surface density for 1.2Ir/ZSiWx catalysts for a contact time of 20 g h/mol. (b) Ring-opening yield obtained with
the series 1.2Ir/ZSiWx as a function of contact time at a reaction temperature of 573 K.
2
and 3 methylhexanes. This second breaking of the C–C bond
may occur without desorption of methylhexane intermediates
MHx), which may explain why cracking appears partially as a
This can be tentatively explained by a decreased dispersion of
the Ir phase on W addition [25].
(
Fig. 6b shows the variation of ring-opening yield for 1.2Ir/
ZSiWx catalysts as a function of contact time for a reaction
temperature of 573 K. For the sake of clarity, ring contraction
and cracking yields were not included. Note that for the same
contact time (20 g h/mol), the evolution of ring-opening yield
was similar to that observed for the MCH conversion.
primary reaction. The ring-opening selectivity decreased with
increasing MCH conversion, which can be attributed to deep
hydrogenolysis.
3
.1.4. Bifunctional catalysts (1.2Ir/ZWx): influence of W
surface density
.1.4.1. Activity Fig. 6a shows the variation of MCH con-
3
.1.4.2. Selectivity Fig. 7 shows the ring-opening selectivity
3
for 1.2Ir/ZSiWx catalysts. Two different patterns for the evo-
version for 1.2Ir/ZSiWx catalysts as a function of W surface
density for a reaction temperature of 573 K and a contact time
of 20 g h/mol. Two different zones can be distinguished. For
low W surface densities (i.e., catalysts with an acidity not suf-
ficient for MCH ring contraction), MCH conversion was rel-
lution of the selectivity with MCH conversion can be distin-
guished. For low W surface densities (x ꢀ 0.9 at. W/nm ), the
2
ring-opening selectivity, which was very high for low conver-
sions, decreased with increasing MCH conversion. For higher
W surface densities, the selectively increased with increasing
conversion up to a maximum reached for MCH conversion be-
tween 65 and 75%, followed by a decrease for higher conver-
sions.
2
atively low. For W surface densities ꢁ1.5 at. W/nm (i.e.,
catalysts sufficiently acidic to promote MCH ring contraction),
MCH conversion decreased with increasing W surface density.
It should be noted that the 1.2Ir/ZSiW0.9 catalyst exhibited a
lower activity than that measured for the 1.2Ir/ZSiW0 solid.
As noted earlier, catalysts with low W surface densities
(1.2Ir/ZSiW0 and 1.2Ir/ZSiW0.9) exhibited weak acidity and

S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
55
Fig. 9. Selectivity for cracking obtained with the 1.2Ir/ZSiWx catalysts as a
function of MCH conversion.
Fig. 7. Ring-opening selectivity obtained with the 1.2Ir/ZSiWx catalysts as a
function of MCH conversion.
were noted (Fig. 8). The increase in ring contraction selectiv-
ity can be qualitatively explained by a decrease in ring-opening
activity due to the observed decrease in Ir dispersion with in-
creasing W surface density [25].
Fig. 9 shows the cracking selectivity for the same catalysts.
At low conversions, the 1.2Ir/ZSiW0 and 1.2Ir/ZSiW0.9 cata-
lysts exhibited around 20% cracking, indicating a behavior typ-
ical of monofunctional metallic catalysts. (Cracking products
are in part primary in nature.) For higher W surface densities,
the selectivity for cracking at low conversions was negligible
reflecting the secondary nature of cracking compounds. The ob-
served increase in selectivity with increasing conversion can be
attributed to the cracking of ring-opening products.
Fig. 10 shows the distribution within the group of cracking
products for 1.2Ir/ZSiWx catalysts (x = 0 and 3.4) as a func-
tion of cracking weight yield. Analysis of the results shows
that C1 and C6 cracking products characteristic of deep hy-
drogenolysis were essentially obtained with the monofunctional
metallic catalyst 1.2Ir/ZSiW0. In contrast, cracking products
for the 1.2Ir/ZSiW3.4 catalyst consisted primarily of C3 and
C4 compounds typically found in acid-cracking. The catalyst
Fig. 8. Selectivity for ring contraction obtained with the 1.2Ir/ZSiWx catalysts
as a function of MCH conversion.
thus negligible ring contraction activity. Hence, ring-opening
products are typical of those observed for direct opening of the
C6 ring. The behavior of the remaining catalysts (presence of
a maximum) appears to characterize indirect ring opening. The
best ring-opening selectivity was observed for 1.2Ir/ZSiW1.5
1
.2Ir/ZSiW1.5 exhibited an intermediate behavior. These re-
sults suggest that the addition of W decreases the hydrogenol-
ysis ability of Ir (by decreasing the number of exposed metal
centers) and increases the cracking of ring-opening products
over Brönsted acid sites of the support.
(
ca. 90% selectivity for 75% conversion) [26]. Further increases
in W surface density led to a significant decrease in the maxi-
mum selectivity for ring opening.
3.1.5. Bifunctional catalysts (yIr/ZW1.5): influence of Ir
content
Fig. 8 shows the ring contraction selectivity for 1.2Ir/ZSiWx
as a function of MCH conversion. The results show that for
low conversions, catalysts active for ring contraction exhibited
a selectivity of ca. 100%. This indicates that ring contraction
of MCH to alkylcyclopentane was the first step in ring open-
ing. Similar results have been reported by Schultz et al. [20]
for the conversion of cyclohexane. For a given conversion, the
ring contraction selectivity increased with increasing W surface
density. Thus, the observed decrease in ring-opening selectiv-
ity (Fig. 7) around 75% conversion with increasing W surface
density can be associated with an increase in ring contraction
selectivity, because no major changes in cracking selectivity
In the previous section, analysis of the influence of W
surface density in the case of bifunctional catalysts 1.2Ir/
ZSiWx showed superior performance in ring-opening activ-
ity for x = 1.5. Catalysts with such a W surface density and
different Ir contents were examined. As shown in Fig. 11a,
no significant improvement in activity of the ZSiW1.5 solid
occurred on addition of 0.3% Ir. This is consistent with the cat-
alytic performance for toluene hydrogenation reported earlier
[25]. Further increases in Ir content bring about a sharp in-
crease in MCH conversion. Fig. 11b shows the weight yields for
ring opening and ring contraction obtained with yIr/ZSiW1.5

56
S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
Fig. 10. Molar selectivity for C1, C6, C3 and C4 products as a function of cracking yield for 1.2Ir/ZSiW0 (a) and 1.2Ir/ZSiW3.4 (b) catalysts.
Fig. 11. (a) MCH conversion obtained with the yIr/ZSiW1.5 catalysts as a function of Ir content at 573 K for a contact time of 14 g h/mol. (b) Yields for ring
opening (open symbols) and ring contraction (full symbols) obtained with the catalyst yIr/ZSiW1.5 as a function of contact time at a reaction temperature of 573 K.
catalysts. The results show that the ring-opening yield sharply
increased with increasing Ir content from 0.3 to 1.2 wt%, reach-
ing ca. 80% for a contact time 14 g h/mol and at a reaction
temperature of 573 K.
3
.1.6. Reaction network and kinetic modeling
In agreement with the literature [3], the results obtained
for the solid 1.2Ir/ZSiW0 illustrate the ability of Ir to open
a 6-carbon atom cycle. Thus, a direct path for ring opening
(
(
rdo) must be considered in the proposed reaction network
Scheme 2). The evolution of the ring-opening selectivity in-
dicates different behavior for catalysts containing sufficient W
content to induce C6 to C5 ring contraction. It has been shown
that for these solids, the ring contraction reaction precedes that
of ring opening. Thus, the observed ring opening is that of
alkylcyclopentanes formed on acid-catalyzed ring contraction
of MCH. Such indirect path for ring opening is illustrated in
Scheme 2. Reaction network for the conversion of MCH over monofunctional
WO /ZrO –SiO ; Ir/ZrO –SiO ) and bifunctional (Ir/WO /ZrO –SiO ) cat-
alysts.
(
3
2
2
2
2
3
2
2
reported in [12] by the cracking step of ring contraction prod-
ucts.
Scheme 2 by two consecutive reactions: ring contraction (rrcont
)
For kinetic modeling (see supplementary material), all of the
reactions shown in Scheme 2 are considered to be first order.
Initial rates are calculated for a reaction temperature of 523 K.
followed by ring opening (rio). In the proposed reaction net-
work, two possibilities for cracking are considered: cracking
of the ring-opening products and direct cracking of C5 rings.
Direct cracking from MCH by dealkylation has not been con-
sidered, because cyclohexane yield has been, consistently, very
low. Note that the proposed reaction network differs from that
◦
For the optimization of initial rates (r ) and activation energies
(Ea), the experimental data are treated as a function of the con-
tact time and the reaction temperature. The reaction network

S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
57
Fig. 12. Comparison between experimental weight fractions obtained for the 1.2Ir/ZSiW2.5 catalyst and those calculated using the proposed model. (a) Reaction
temperature of 548 K. (b) Reaction temperature of 573 K.
Table 1
Initial rates at 523 K and activation energies expressed respectively in mol/
(h kg) and kJ/mol obtained for ZSiWx catalysts
Catalyst
rrcont
Ercont
r
io
E
r
E
crack2
io
crack2
ZSiW1.5
ZSiW2.5
ZSiW3.4
9.4
25.5
33.6
38
38
38
1.2
1.5
1.4
50
50
50
2.3
4.1
7.9
59
59
59
has been used to model the performances obtained with var-
ious catalysts. Fig. 12 shows, for the catalyst 1.2Ir/ZSiW2.5,
the predicted (continuous lines) and experimental weight frac-
tions of various groups of products and of unconverted MCH as
a function of contact time for reaction temperatures of 548 and
5
73 K. A very good agreement was obtained between the calcu-
lated weight fractions and experimental results. Similar results
were obtained for other solids. In the subsequent sections, we
report on the use of this model to investigate the relationships
between the kinetic parameters and characterization results.
Fig. 13. Initial rates for ring contraction, indirect ring opening and cracking for
the ZSiWx catalysts as a function of W surface density.
3
3
.2. Kinetic studies
.2.1. Monofunctional acid catalysts ZSiWx
the abundance of these acid sites increases with no apparent in-
crease in their strengths [25].
Table 1 reports the initial rates and activation energies cal-
culated for the solids ZSiW1.5, ZSiW2.5, and ZSiW3.4. As
noted earlier (Fig. 3a), for these solids, ring-opening com-
pounds clearly appear as secondary products. The direct ring-
Fig. 13 shows the evolution of the initial rates calculated for
the ZSiWx as a function of W surface density. No significant
◦
◦
ring contraction activity (r
) was detected for W surface
opening path (r ) was thus deleted from the reaction network.
rcont
do
2
densities ꢀ0.4 at. W/nm . The activity appeared to develop
Furthermore, fitting of the data gives a zero initial rate for the
2
◦
for a W surface density around 0.7 at. W/nm and increased
reaction of cracking of ring-opening products (rcrack1). This can
steadily with a W surface density. Similar behavior was noted
be attributed to low yields for ring-opening products. The acti-
vation energies for ring contraction (38 kJ/mol), indirect ring
opening (50 kJ/mol), and cracking (59 kJ/mol) appear to be
little affected by W surface density. This suggests a similar na-
ture of the active sites (i.e., changes in the abundance of various
active sites and not in their intrinsic activity with W surface
density). W deposition on zirconia leads to the development
of strong and relatively strong acid sites that were not present
◦
for the cracking reaction (r
). The initial rate for indirect
crack2
◦
ring opening (r ) was consistently low and did not exhibit a
io
clear evolution with W surface density.
3.2.2. Monofunctional metal catalysts yIr/ZSiW0
Table 2 reports the calculated initial rates and activation en-
ergies for the yIr/ZSiW0 catalysts. No ring contraction prod-
ucts were detected for these catalysts. Thus, the indirect ring-
2
in the original support. For W surface densities >0.9 at./nm ,

58
S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
◦
opening (r ) pathway via ring contraction of MCH was not
3.2.3. Bifunctional catalysts
io
considered in the modeling of the results. The reaction network
3.2.3.1. 1.2Ir/ZSiWx catalysts: influence of W surface density
Table 3 reports the initial rates and activation energies calcu-
lated for the 1.2Ir/ZSiWx catalysts. As noted previously for
the ZSiWx series, no ring contraction products were detected
◦
comprises two consecutive reactions: direct ring opening (r )
od
followed by cracking. Within the range of Ir loadings examined
in the present study, the Ir content had no significant affect on
the activation energy calculated for direct ring opening of MCH
2
for W surface densities ꢀ0.4 at. W/nm . The activation en-
(
about 138 kJ/mol). For the three catalysts, the activation en-
ergy, Ercont, obtained for the solid 1.2Ir/ZSiW0.9 (117 kJ/mol)
was similar to that reported by McVicker et al. for MCH ring
contraction over bifunctional catalysts (117–134 kJ/mol) [27].
◦
ergy for the cracking reaction (rcrack1) was ca. 59 kJ/mol, and
the activation energy was set at this value for optimization of
other kinetic parameters.
2
For W surface densities ꢁ0.9 at. W/nm , the activation en-
Fig. 14 shows the initial rates as a function of Ir content.
A minimum Ir content (0.25%) appears to be required for the
ergy for ring contraction decreased with increasing W surface
density. This can be explained by an increased contribution of
the acid mechanism (38 kJ/mol obtained for the ZSiWx se-
ries). This evolution appears to reflect the reported increased
abundance of Brönsted acid sites and the decreased dispersion
of the metallic phase with increasing W surface density [25].
The observed decrease in the activation energy for indirect ring
opening is consistent with this hypothesis. In contrast, no sig-
nificant changes in the activation energy for direct ring opening
(Edo) with W surface density can be seen.
◦
development of direct ring-opening activity (r ). For higher
do
loadings, a steady increase of the initial rate with Ir content can
be seen. Similar behavior has been observed for CO adsorption
◦
[
25]. The cracking rate (r
) increased sharply with addi-
crack1
tion of Ir, followed by a more progressive increase at higher Ir
content.
Table 2
Initial rates at 523 K and activation energies expressed respectively in mol/
Fig. 15 shows the initial rates calculated from the experi-
mental data for the 1.2Ir/ZSiWx catalysts as a function of W
surface density. As noted for the ZSiWx solids, a minimum of
W surface density appears to be required for the development of
(h kg) and kJ/mol obtained for yIr/ZSiW0 catalysts
Catalyst
r
do
E
r
E
crack1
do
crack1
0.3Ir/ZSiW0
0.6Ir/ZSiW0
1.2Ir/ZSiW0
0.3
1.1
3.5
138
138
138
10.1
12.6
19.1
59
59
59
◦
ring contraction activity (r ). Above this threshold, the ring
cont
Fig. 14. Variation of the initial rates for direct ring opening and cracking as a
Fig. 15. Initial rates calculated from experimental data for 1.2Ir/ZSiWx cata-
function of Ir content for the yIr/ZSiW0 catalysts (y = 0.3, 0.6, and 1.2 wt% Ir).
lysts as a function of W density.
Table 3
Initial rates at 523 K and activation energies expressed respectively in mol/(h kg) and kJ/mol obtained for the 1.2Ir/ZSiWx catalysts
Catalyst
rrcont
Ercont
r
do
E
r
io
E
r
E
r
E
crack2
do
io
crack1
crack1
crack2
1
1
1
1
1
1
.2Ir/ZSiW0
3.5
3.7
3.0
2.8
0.2
138
138
121
151
155
19.1
16
26.2
–
–
–
59
55
59
–
–
–
.2Ir/ZSiW0.4
.2Ir/ZSiW0.9
.2Ir/ZSiW1.5
.2Ir/ZSiW2.5
.2Ir/ZSiW3.4
0.2
15.2
35.1
55.4
121
117
92
28.8
126
117
63
107.9
36.1
23.5
8.0
3.8
6.0
100
92
92
80
42

S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
59
Table 4
Initial rates at 523 K and activation energies expressed respectively mol/(h kg) and kJ/mol obtained for the yIr/ZSiW1.5 catalysts
%
0
Ir
rrcont
Ercont
r
E
r
E
r
E
r
E
crack2
do
do
io
io
crack1
crack1
crack2
9.4
4.1
9.1
38
63
88
1.2
8.4
26.4
51
63
75
2.3
3.3
2.6
8.0
59
25
42
0.3
0.6
1.2
0.1
0.8
2.8
146
113
151
–
–
–
–
15.2
117
151
117
100
contraction rate increased linearly with increasing W surface
density. The decrease in the initial rate for direct ring opening
◦
(
r ) is consistent with the observed decrease in Ir dispersion.
do
2
Note that for W surface densities ꢁ0.9 at. W/nm , the indi-
rect ring-opening rate was significantly higher than the direct
ring opening. This confirms that C5 rings can be more read-
ily opened than C6 rings. For W surface densities >1.5 at.
2
W/nm , the rate for indirect ring opening decreased signifi-
cantly with W. This agrees with the observed decrease in the
dispersion of the Ir phase reported earlier [25]. For low W sur-
2
face densities (ꢀ0.9 at. W/nm ), the high initial rate for crack-
ing reflects the behavior observed for monofunctional metal
catalysts yIr/ZSiW0.
3
.2.3.2. yIr/ZSiW1.5: Influence of Ir content Table 4 re-
ports the initial rates and activation energies calculated for the
yIr/ZSiW1.5 catalysts. The activation energy for the ring-con-
traction reaction (Ercont) increased with Ir content. This may be
attributed to an evolution of the solids from monofunctional
acid for ZSiW1.5 to bifunctional for Ir-rich catalysts. This
interpretation is consistent with the activation energy (117–
Fig. 16. Initial rates for ring contraction, direct and indirect ring opening, and
cracking as a function of Ir content for yIr/ZSiW1.5 catalysts.
4
. Discussion
1
34 kJ/mol) reported by McVicker et al. [27] for MCH ring
4
.1. Monofunctional acid and metal catalysts: correlation
contraction in the case of a bifunctional catalyst. The acti-
vation energy for direct ring opening varied within a limited
range (134 ± 21 kJ/mol) with Ir content. The calculated val-
ues were close to those observed with monofunctional metallic
catalysts yIr/ZSiW0 (138 kJ/mol). A parallel evolution of the
activation energy for ring contraction (Ercont) and that of in-
direct ring opening (Eio) can be seen. Note that there was
no clear pattern for the variation in the activation energy for
cracking of alkylnaphthenes (Ecrack2) with varying Ir con-
structure–catalytic performance
4
.1.1. Monofunctional acid catalysts ZSiWx: correlation with
acidity measurements
As noted earlier, a minimum W surface density (ca. 0.9 at.
2
W/nm ) is required for the development of MCH ring-con-
traction activity. For higher values, the ring-contraction rate
increased linearly with increasing W surface density. The lack
of variation of the activation energy for ring contraction in the
range of W surface densities investigated is indicative of the
presence of Brönsted acid sites of similar strengths. Fig. 17
shows the initial ring contraction rate (r
tent.
◦
Fig. 16 reports the initial rates for ring contraction (r
direct ring opening (r ), indirect ring opening (r ), and crack-
ing (r ) as a function of Ir content. The initial rate for ring
),
rcont
◦
◦
◦
) calculated for a
do
io
rcont
◦
reaction temperature of 523 K as a function of the areas of
infrared bands at 2167 and 2175 cm attributed to the inter-
do
◦
−1
contraction (r
) appeared to increase with Ir content. How-
rcont
ever, the rate calculated for the Ir-free solid ZSiW1.5 was higher
action of CO with relatively strong and strong Brönsted acid
sites, respectively [25,28]. There is a linear relationship be-
tween the abundance of these acid sites and the rate for MCH
ring contraction to alkylcyclopentanes. No such correlation has
been observed with Lewis acid sites. This is consistent with the
hypothesis that active sites are associated with strong and rel-
atively strong Brönsted acid sites of tungstated zirconia. Thus,
the ring contraction mechanism is purely acidic [16,29,30], that
is, protonation of methylcyclohexane forming carbenium ion
and molecular hydrogen, followed by ring contraction via pro-
tonated cyclopropane intermediate (Scheme 3). Hydride trans-
than that determined for the solid with the lowest Ir content
◦
(
0.3Ir/ZSiW1.5). The rate for indirect ring opening (r ) in-
io
creased significantly with Ir content. The rate of direct ring
◦
opening (r ) also increased with Ir content; however, it re-
do
mained ca. two orders of magnitude lower than that for in-
◦
◦
direct ring opening (r ). The initial rate for cracking (r )
io
do
was low for low Ir content, then increased steadily with in-
creasing Ir loadings. A significant rate for cracking of ring-
◦
opening products (r
) was measured only for high Ir con-
crack1
tent.

60
S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
Fig. 17. Initial ring-contraction rate rrcont at 523 K for ZSiWx catalysts as a
function of the area of the infrared bands at 2167 and 2175 cm characteristic
of CO vibration.
Scheme 3. One possible acid mechanism for ring contraction: (a) carbeniu-
mion formation; (b) ring contraction via protonated cyclopropane intermediate;
adapted from [12,14].
−
1
Scheme 4. Dicarbene mechanism for ring opening [31].
Scheme 5. Selective ring opening of methylcyclopentane via a dicarbene mech-
anism.
Fig. 18. Initial rate for MCH direct ring opening (r ) over yIr/ZSiW0 catalysts
do
as a function of the infrared band area for CO adsorbed at saturation on Ir.
a direct relationship between hydrogenation rate and Ir con-
tent [25]. An activation energy of 138 kJ/mol was calculated
for the hydrogenolysis of a 6-carbon atom ring on Ir. The low
amount of n-heptane observed points to a selective ring open-
ing by cleavage of unsubstituted C–C bonds, consistent with a
dicarbene mechanism (Scheme 4). This has been previously re-
ported for methylcyclopentane ring opening [31] (Scheme 5)
and by McVicker for methylcyclohexane ring opening on 0.9%
fer from another MCH molecule yields DMP or ECP. The rate
for indirect ring opening remains low with an activation energy
of ca. 50 kJ/mol. This activation energy can thus be considered
characteristic of an acid-catalyzed mechanism. The cracking
products were essentially C3 and C4 compounds, again con-
sistent with this mechanism.
Ir/Al O catalyst [3]. A fraction of MCH is converted by hy-
2
3
drogenolysis without desorption yielding cracking compounds
as primary products.
4
.1.2. Monofunctional metallic catalysts yIr/ZSiW0:
correlation with Ir dispersion
Fig. 18 shows, for the yIr/ZSiW0 catalysts, the initial rate
◦
for direct ring opening (r ) as a function of the area of the
4.2. Bifunctional catalysts: correlation structure–catalytic
performance
od
infrared band for CO adsorbed on Ir at saturation. A direct
correlation between these two parameters can be seen, indi-
cating that direct ring opening occurs on metallic Ir sites by
hydrogenolysis. The threshold observed for the development of
ring-opening activity and for the detection of the infrared bands
of CO adsorbed on Ir suggest that part of the Ir is not accessi-
ble to the reactants and the IR probe molecule. This appears to
be in contradiction with toluene hydrogenation results showing
4.2.1. Ir/ZSiWx catalysts: influence of W surface density and
correlation with Brönsted acidity and Ir dispersion
◦
Fig. 19 shows the initial ring-contraction rate (r
) cal-
rcont
culated for a reaction temperature of 523 K as a function of
the relatively strong and strong Brönsted acid sites (areas of
−1
the νCO vibration infrared bands at 2167 and 2175 cm , at-

S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
61
◦
of the initial rate for direct ring opening r exhibited similar
do
behavior; however, the correlation was less straightforward be-
cause of the low rates measured and thus the high uncertainty
associated with the results.
◦
The rate for C5 ring opening, r , at 523 K decreased for
io
2
W surface density ꢁ1.5 at. W/nm (Fig. 15) and appears to be
directly related to Ir dispersion. The rate of direct ring opening,
◦
r , also decreased with increasing W surface density; however,
do
in this case, the activation energy (Edo) was not significantly
affected by the W surface density (Table 3). This is a further
indication that two different mechanisms are in play for ring-
opening reactions.
4
.2.2. yIr/ZSiW1.5 catalysts: influence of Ir loading and
Fig. 19. Initial ring-contraction rate rrcont at 523 K for the 1.2Ir/ZSiWx cata-
correlation with Ir dispersion
lysts as a function of the areas of the infrared CO vibration bands at 2167 and
For monofunctional metal catalysts, the introduction of a
small amount of Brönsted acid sites of sufficient strength on
W deposition drastically changed the reaction network. The
preferred pathway became ring contraction, followed by ring
opening. The activation energy for ring contraction increased
progressively with increasing Ir content from a value that could
be considered typical for an acid mechanism (38 kJ/mol)
to that representative of a bifunctional metal/acid system
−
1
2175 cm
.
−
1
(
117 kJ mol ) [27]. A similar behavior was observed for the
activation energy of indirect opening of a 6-carbon atom ring
evolution from 50 to 117 kJ/mol). This indicates a similar
(
change of mechanism from acid to bifunctional with increas-
ing Ir content. Taking into consideration that little variation
in Ir dispersion was observed for this series, such a change
in mechanism can be explained by a decrease in the distance
of the metal-acid center with increasing Ir content at constant
dispersion, leading to the participation of more Brönsted acid
sites in a bifunctional mechanism. We discuss this hypothesis
later.
Fig. 20. Initial rates for direct and indirect ring opening as a function of the Ir
dispersion obtained for 1.2Ir/ZSiWx catalysts.
Analysis of the results for direct ring opening shows that
within the range of Ir loadings considered, the activation energy
for direct ring opening does not change with Ir content. This
can be taken as an indication that direct ring opening of MCH
occurs by the same mechanism operating for monofunctional
metal and bifunctional metal/acid systems. Fig. 21 represents
tributed to the interaction of these sites with CO [25,28]). The
results indicate a direct relationship between the abundance of
these acid sites and the rate for MCH ring contraction to alkyl-
cyclopentanes. Note that higher initial ring-contraction rates
were observed for these solids compared with the correspond-
ing ZSiWx catalysts. This indicates the participation of a bi-
functional mechanism for ring contraction on Ir deposition. The
increase in W surface density for a given Ir content led to an in-
crease in the number of Brönsted acid sites and a decrease in
the number of metallic centers due to the observed decrease
in Ir dispersion. Consequently, the surface metal/acid ratio de-
creased with increasing W surface density. This results in a
greater contribution of the acid mechanism for ring contrac-
◦
◦
the initial rates for direct (r ) and indirect ring opening (r )
do
io
versus the rates for toluene hydrogenation measured for the
yIr/ZSiW1.5 catalysts. Both initial rates appear to be directly
correlated with the rate for toluene hydrogenation, suggesting
that direct and indirect ring-opening reactions are correlated
with the abundance of accessible metal sites. The correlation
observed between the initial rate for direct or indirect ring open-
ing and that for toluene hydrogenation illustrates the essential
role of the metallic function for ring-opening reaction.
tion, reflected in a decrease of activation energy E
to 80 kJ mol (Table 3). The same behavior was observed for
from 121
cont
−1
4
.3. MCH ring-opening mechanism
the activation energy of indirect ring opening (Eio). Fig. 20 re-
◦
◦
ports the initial rates for direct r and indirect r ring-opening
do
io
reactions for 1.2Ir/ZSiWx as a function of Ir dispersion. A di-
rect correlation can be seen between the initial rate for indirect
ring opening and Ir dispersion, indicating that metal sites are
involved in the reaction of indirect ring opening. The evolution
Our results indicate that direct ring opening occurs on metal
sites by a dicarbene mechanism as proposed in the literature
[3,31]. The close similarity in the calculated values of the acti-
vation energy over solids of the yIr/ZSiW0, yIr/ZSiW1.5, and

62
S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
and ring-opening products will be formed in parallel reac-
tions (dimethylcyclopentene or ring opening). Thus, ring-
opening products will be detected as primary compounds.
This is in apparent contradiction with the observed selectiv-
ities. Only when the desorption rate of dimethylcyclopen-
tene is much higher than that of ring opening may ring-
opening products appear as secondary species. In that case,
the proposed mechanism may account for the observed re-
sults and cannot be excluded.
–
Ring opening by two parallel mechanisms, a dicarbene
mechanism over the metallic sites and a β-scission mecha-
nism over the acid sites. The activation energy measured
in the case of an acid mechanism for ring opening of
MCP over Ir-free acid catalysts was 50.2 kJ/mol, whereas
that of MCH ring opening over metallic Ir sites was ca.
1
38 kJ/mol. Assuming the same activation energy for C5
and C6 ring opening, the two mechanisms (β-scission
over acid sites and dicarbene mechanism over metal sites)
lead to a weighted contribution of each activation en-
ergy. For low Ir dispersion and high W surface density
Fig. 21. Correlation between initial rates of direct and indirect ring opening
versus toluene hydrogenation rate for yIr/ZSiW1.5 catalysts.
(
1.2Ir/ZSiW3.4), the calculated activation energies reflect
1
.2Ir/ZSiWx series suggests that the reaction is not very sen-
a predominant acid mechanism. In contrast, for relatively
high Ir dispersion and low W surface density (1.2Ir/ZSiW
1
tervention of two parallel mechanisms is consistent with
the observed evolution of selectivity.
sitive to the acidity of the support or to Ir particle size; only
the number of accessible metallic sites appears to determine the
reaction rate.
With respect to the mechanism for indirect ring opening, the
situation is more complex. The parallel evolution of the activa-
tion energies for ring opening and ring contraction cannot be
explained by one type of mechanism (metal or acid). Several
hypotheses may be considered:
.5), metallic mechanism predominates. The proposed in-
5
. Conclusion
The present work studied the catalytic performances of
monofunctional acid (ZSiWx) or metal (yIr/ZrW0) and bi-
functional (1.2Ir/ZWx and yIr/ZW1.5) catalysts for the ring-
opening reaction of MCH. Kinetic analysis based on our pro-
posed reaction network allowed the determination of initial
rates and activation energies for various reactions. From the
comparison of the results with previous characterization of the
metallic and acid functions, we can draw the following conclu-
sions:
–
An acid mechanism (Scheme 3). This hypothesis can be
discarded, because it fails to account for the observed in-
crease in the initial rate for ring opening with increasing Ir
content for the yIr/ZSiW1.5 solids.
A dicarbene mechanism (Scheme 4) over the metal sites as
proposed for the direct ring opening of MCH. This mech-
anism is consistent with the observed relation between the
reaction rate and the number of accessible metal sites. For
catalysts with sufficient acidity to ring-contract MCH, ring
opening becomes a reaction consecutive to ring contraction
–
– Monofunctional acid catalysts (ZSiWx) display low selec-
tivity for ring opening. The isomerization activity develops
for W surface density above a threshold value of 1 atom
(
indirect ring opening). The isomer thus formed can read-
2
sorb over the metal, in line with the dicarbene mechanism;
however, for indirect ring opening, a variation in the acti-
vation energy for the solids yIr/ZSiW1.5 and 1.2Ir/ZSiWx
was observed, whereas no such variations were noted for
direct ring opening. Thus, the results cannot be fully ex-
plained by a dicarbene mechanism.
A bifunctional mechanism for both indirect ring opening
and ring contraction. One such mechanism proposed by
Walter [32] consists of hydride abstraction by an acid site
W/nm . This is attributed to the appearance and the devel-
opment of a relatively strong Brönsted acidity monitored
by infrared measurements. Over these acid solids, contrac-
tion and indirect opening of C5 rings occur according to a
classic acid mechanism by β-scission described previously.
– Monofunctional metal catalysts (yIr/ZrW0) display high
selectivity for ring opening that decreases with increasing
conversion. Because of the lack of isomerization products,
the observed activity is attributed to the direct MCH ring
opening. Over these catalysts, ring opening and cracking
occur according to a dicarbene mechanism.
–
(
proton) with hydrogen formation, ring contraction by the
protonated cyclopropane intermediate, and ring opening by
β-scission, without desorption of dimethylcyclopentane or
ethylcyclopentane intermediates. The metal hydrogenates
the olefin thus formed and regenerates Brönsted acidity.
Without desorption of the intermediate, ring contraction
– The study of MCH conversion on bifunctional (1.2Ir/ZWx
and yIr/ZW1.5) catalysts indicates that MCH isomerization
to alkylcyclopentanes occurs before ring opening. The best
yields for ring opening are obtained with the 1.2Ir/ZW1.5

S. Lecarpentier et al. / Journal of Catalysis 254 (2008) 49–63
63
catalyst. Further increases in W surface density lead to a
decrease in the indirect ring-opening rate, attributed to a
decrease in Ir dispersion. For bifunctional metal/acid cat-
alysts, analysis of the mechanism is less straightforward.
The activation energy for ring contraction and indirect C5
ring opening is a function of the metal/acid ratio. For high
ratios, indirect ring opening occurs essentially over metal-
lic sites. A decrease in the metal/acid ratio enhances the
contribution of acid mechanism.
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This detailed study (Part I [25] and II) prepares the ground
for a better understanding of bifunctional catalysts. This could
lead to even more effective Ir-based catalysts for such mole-
cules as decalin, tetralin, and alkyl-naphthalenes, which are
more representative of a deeply hydrotreated LCO produced in
the FCC unit of an oil refinery.
[
[
[
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Acknowledgments
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Murzin, J. Catal. 227 (2004) 313.
The work of S.L. was supported by a PhD thesis grant
from the French Ministry of Education. The authors thank Drs.
C. Collet, W. Vermeiren, and J.-P. Dath (Total S.A.) for fruitful
discussions.
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Supplementary material
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[
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The online version of this article contains the development
of the kinetic equations.
Please visit DOI: 10.1016/j.jcat.2007.11.016.
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