Influence of the support on the selective ring opening of methylcyclohexane and decalin catalyzed by Rh-Pd catalysts

Article abstract of DOI:10.1016/j.molcata.2014.12.011

Rh-Pd catalysts supported on Al₂O₃, SiO₂ and SiO₂-Al₂O₃ (SIRAL 40) were studied for methylcyclohexane (MCH) and decalin ring opening reactions. It was found that the Rh/Pd atomic ratios were similar to the theoretically expected one and the metal dispersion values varied between 30 and 50%. On bimetallic catalysts supported on Al₂O₃ and SiO₂, Pd and Rh were present mainly in the form of monometallic particles, whereas a heterogeneous distribution constituted of large Pd particles and small bimetallic ones were observed on SIRAL 40. Total and Bronsted acidities followed the order: SIRAL 40 >> Al₂O₃ > SiO₂. Al₂O₃ supported catalysts were the most suitable for MCH ring opening, while the opening of decalin was favored by using SiO₂-Al₂O₃ (SIRAL 40). The catalysts supported on SiO₂ produced mainly dehydrogenated compounds. The higher total and Bronsted acidities of SIRAL 40 series had a great impact on the opening of bicyclic naphthenes, whereas the metal function of the catalyst and the hydrogenolytic activity of Rh had a major role during MCH reaction. Using bimetallic catalysts in the MCH reaction decreased the formation of cracking and dehydrogenated products, and the selective formation of RO products could be optimized by tuning the working temperature.

Full text of DOI:10.1016/j.molcata.2014.12.011

Journal of Molecular Catalysis A: Chemical 398 (2015) 203–214
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Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Influence of the support on the selective ring opening of
methylcyclohexane and decalin catalyzed by Rh–Pd catalysts
Silvana A. D’Ippolitoa, Catherine Especelb, Laurence Vivierb, Stéphane Pronierb,
Florence Epronb, Carlos L. Piecka,∗
a Instituto de Investigaciones en Catálisis y Petroquímica (INCAPE) (FIQ-UNL, CONICET), Santiago del Estero 2654, 3000 Santa Fe, Argentina
b Université de Poitiers, CNRS UMR 7285, Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP), 4 rue Michel Brunet, TSA 51106, 86073 Poitiers
Cedex 9, France
a r t i c l e i n f o
a b s t r a c t
Article history:
Rh–Pd catalysts supported on Al2O3, SiO2 and SiO2–Al2O3 (SIRAL 40) were studied for methylcyclohexane
(MCH) and decalin ring opening reactions. It was found that the Rh/Pd atomic ratios were similar to the
theoretically expected one and the metal dispersion values varied between 30 and 50%. On bimetallic cat-
alysts supported on Al2O3 and SiO2, Pd and Rh were present mainly in the form of monometallic particles,
whereas a heterogeneous distribution constituted of large Pd particles and small bimetallic ones were
observed on SIRAL 40. Total and Bronsted acidities followed the order: SIRAL 40 » Al2O3 > SiO2. Al2O3 sup-
ported catalysts were the most suitable for MCH ring opening, while the opening of decalin was favored
by using SiO2–Al2O3 (SIRAL 40). The catalysts supported on SiO2 produced mainly dehydrogenated com-
pounds. The higher total and Bronsted acidities of SIRAL 40 series had a great impact on the opening of
bicyclic naphthenes, whereas the metal function of the catalyst and the hydrogenolytic activity of Rh had
a major role during MCH reaction. Using bimetallic catalysts in the MCH reaction decreased the formation
of cracking and dehydrogenated products, and the selective formation of RO products could be optimized
by tuning the working temperature.
Received 14 September 2014
Received in revised form 4 December 2014
Accepted 5 December 2014
Available online 16 December 2014
Keywords:
Rh–Pd
Bifunctional catalysts
Ring opening
Decalin
Methylcyclohexane
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
tronic properties and the presence of acid sites can significantly
change the chemistry of the process [6]. The zeolites with larger
LCO (Light cycle oil) is a waste stream from the bottom of the
fluid catalytic cracking (FCC) unit that is used for heating and is
added to heavy fuel to adjust its viscosity [1]. Due to the increasing
diesel demand, an interesting application would be to combine the
LCO fraction to the diesel pool. However, the LCO characteristics
are very different from the diesel fuel specifications Therefore, the
LCO fraction must be subjected to major changes. The content of
sulfur and nitrogen can be reduced by conventional hydrotreating
[2] and the polycyclic aromatic by hydrogenation to improve the
cetane index (CI). However, this improvement is not sufficient and
it is necessary to open the naphthenic rings [3,4]. The CI increases
considerably when decalin is converted to linear or monobranched
paraffins. The selective ring opening (SRO) of naphthenes is a very
complex chemistry, largely depending on the operating conditions
and characteristics of the catalyst system [5].
pore size, such as HY, are considered as among the most suitable
supports for this purpose. In general, the use of zeolites of large
pore size favors the ring opening reaction, limiting the deactiva-
tion due to coke deposits [7–9]. Also the size of the crystal [10],
the number and distribution of the strength of Bronsted acid sites
[10,11] are important parameters for obtaining ring opening (RO)
products. Santana et al. reported in a study devoted to ring opening
of decalin and tetralin that Pt/HY is more effective than HY without
metallic promoters [12]. Pt in Pt/USY bifunctional catalysts signifi-
cantly increases the rate of isomerization and thus the formation of
ring opening products compared to monofunctional USY catalysts.
It has been shown that the addition of Pt to acidic materials reduces
the strength of Bronsted acid sites and significantly improves the
isomerization and ring opening of decalin [13]. The use of zeolites
can lead to non-selective cracking (undesirable side reactions) and
pore constraints issue steric impediments [7,12,14,15].
The support has a strong influence on the SRO reaction because
the interaction with the metallic particles can change their elec-
The opening reaction of C6 naphthenes is enhanced by the use
of bifunctional catalysts [16–19]. This can be explained consider-
ing that on the acid function of the catalyst takes place contraction
of C6 ring to C5 one, which is easier to open on the metal func-
tion [4,18,19]. Kustov et al. [20] studied Pt, Rh, Ru and Ni catalysts
∗
Corresponding author. Tel.: +54 342 4533858; fax: +54 342 4531068.
E-mail address: pieck@fiq.unl.edu.a (C.L. Pieck).
http://dx.doi.org/10.1016/j.molcata.2014.12.011
1381-1169/© 2014 Elsevier B.V. All rights reserved.

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S.A. D’Ippolito et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 203–214
supported on alumina and silica for ring opening (RO) of cyclohex-
ane in conditions close to the industrial ones. It was observed that
Rh catalysts have higher activity and inhibit the dehydrogenation
reaction as compared to the Pt catalysts. Other authors [13,21] also
proposed and studied Rh-based catalysts for the reaction of SRO due
to their large hydrogenolytic capacity. Metals such as Pt, Pd, Ir, Ru
and Rh supported on Al2O3 were observed as active and selective
for RO of methylcyclopentane to C6 paraffins [22,23].
(ICP–OES) after digestion of the sample in an acid solution and
dilution. Chlorine contents of catalysts in their final state, i.e., after
activation, were measured with the Charpentier–Volhard method,
in which the sample was dissolved in concentrated sulfuric acid
and Cl ions precipitated with silver nitrate solution. Excess silver
solution was back titrated with a thiocyanate solution to give the
amount of chlorine on the original sample.
It was reported that the hydrogenolytic activity follows the
order: Pt < Rh < Ir < Ru [24,25]. Over bifunctional catalysts, charac-
terized by the presence of acidic sites and a hydro-dehydrogenating
function, the reactions occur via carbenium ions, i.e., ␤-scission and
isomerization. Do et al. [5] pointed out that decalin ring opening
is catalyzed both by the acid function through ␤-scission and by
the metal via dicarbene mechanism leading to highly isomerized
products.
The research has begun with noble metal catalysts supported
on mesoporous silica-based materials [7,26]. In this type of bifunc-
tional catalysts the acid function allows the contraction of the
ring, and then the metal sites more easily produce hydrogenolysis
of C C bonds. The silica–alumina mixed phases contain not only
silica–alumina, but also pure silica and aluminum groups which
exhibit strong Bronsted acid sites with strengths comparable to
that of zeolites [27]. The concentration of Bronsted acid sites can
be adjusted by varying the silica content [28]. This support is pre-
sented as an alternative to zeolites that commonly lead to excessive
cracking activity.
In previous works [29,30], we studied the influence of the
SiO2/Al2O3 ratio and the Rh–Pd content on the ring opening of
methylcyclohexane and decalin using silica–alumina as support.
In line with this research, this work studies the influence of the
support (Al2O3, SiO2 and Al2O3–SiO2) on the properties of Rh and
Pd monometallic and Rh–Pd bimetallic catalysts. The objective is to
optimize the metal/acid functions for favoring the selective opening
of the rings of methylcyclohexane and decalin.
2.3. Temperature programmed desorption of pyridine
This test was used for measuring the amount and strength
of the acid sites. Samples of 200 mg were impregnated with an
excess of pyridine. The samples were then rinsed and the excess
of physisorbed pyridine was eliminated by heating the sample in a
nitrogen stream at 110 ◦C for 1 h. Then the temperature was raised
at a rate of 10 ◦C min−1 to a final value of 700 ◦C. To measure the
amount of desorbed pyridine, the reactor exhaust was connected
to a flame ionization detector. The error associated to the peak
position and areas has been determined to be of about 7% [31].
2.4. Isomerization of 3,3-dimethyl-1-butene (33DM1B)
The equipment used was described previously [29]. The catalyst
(50 mg) was pretreated in situ, by reduction with H2 (60 cm3 min-1
,
450 oC, 1 h). The sample was then cooled in N2 (30 cm3 min-1) to
the reaction temperature adjusted in order to have small conver-
sion values to avoid secondary reactions. Then the feed from the
saturator was injected. The reagent partial pressure and flow rate
were 20.9 kPa and 15.2 mmol h-1, respectively. The error associated
to the test of 33DM1B isomerization was determined by calculat-
ing the variance of the conversion in a set of seven experiments
(variance = 6.5%).
2.5. H2 chemisorption
2. Experimental
This technique was used in order to estimate the metallic acces-
sibility of the Pd, Rh and Rh–Pd particles on the surface of the
2.1. Catalysts preparation
catalyst. The sample (100 mg) was reduced at 500 oC (10 oC min-1
,
␥-Al2O3 (Cyanamid Ketjen CK-300, pore volume = 0.5 cm3 g−1
Sg = 180 m2 g−1), volume = 0.31 cm3 g−1
SiO2 (SIL, pore
Sg = 130 m2 g−1
,
,
H2 30 cm3 min-1) for 1 h. Then argon (30 cm3 min-1) was made to
flow over the sample for 2 h at 500 oC in order to eliminate adsorbed
hydrogen. Finally the sample was cooled down to 70 oC in argon
and calibrated pulses of H2 were injected into the reactor (HC1).
These pulses were sent until the sample was saturated. After flush-
ing the system with argon during 30 min, a second set of pulses
was injected (HC2). The difference HC1–HC2 allows one to estimate
the metallic accessibility considering the stoichiometry between a
hydrogen atom and a Pd or Rh surface atom (H/Pd and H/Rh) equal
to 1.
)
and SiO2–Al2O3 provided by Sasol (SIRAL
40, 60.7 and 39.3 wt% of Al2O3 and SiO2, respectively; pore
volume = 0.9 cm3 g−1 Sg = 514 m2 g−1
were used as support.
,
)
Previously, they were calcined at 450 ◦C for 4 h (10 ◦C min−1, air,
60 cm3 min−1). Rh and Pd were added by a common coimpreg-
nation method. An aqueous solution of HCl (0.2 mol L−1) was
added to the support and the system was left unstirred at room
temperature for 1 h. Then an aqueous solution of RhCl3 and/or
PdCl2 (Sigma–Aldrich) was added in order to have a total metal
charge of 1 wt%. For the bimetallic catalysts, the Rh/Pd atomic
ratio was 0.5, 1 and 2. The slurry was gently stirred for 1 h at room
temperature and then it was put in a thermostated bath at 70 ◦C
until a dry solid was obtained. Drying was completed in a stove
at 120 ◦C overnight. Finally, the samples were calcined in flowing
air (60 cm3 min−1) at 300 ◦C for 4 h and reduced under flowing
H2 (60 cm3 min−1, 500 ◦C, 4 h). The monometallic catalysts were
named Pd1 or Rh1/support while the bimetallic catalysts were
named Rx/support, Rx corresponding to the Rh/Pd atomic ratio.
2.6. Transmission electron microscopy (TEM) measurements
TEM measurements were performed on a JEOL 2100 electron
microscope operating at 200 kV with a LaB6 source and equipped
with a Gatan ultra scan camera. The powder was ultrasonically
dispersed in ethanol, and the suspension was deposited on an alu-
minum grid coated with a porous carbon film. Average particle sizes
were determined by measuring at least 100 particles for each sam-
ple analyzed, from at least five different micrographs. The particle
size distribution was obtained from TEM pictures calculating the
surface average particle diameter from d = ꢀnidi3/ꢀnidi2. Micro-
analysis of Pd and Rh was carried out by energy dispersive X-ray
spectroscopy (EDX) in the nanoprobe mode.
2.2. Measurement of the Pd and Rh contents
The composition of the metal function was determined
by inductively coupled plasma-optical emission spectroscopy

S.A. D’Ippolito et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 203–214
205
2.7. Temperature programmed reduction (TPR)
The tests were performed in an Ohkura TP2002 apparatus
equipped with a thermal conductivity detector. At the beginning
of each TPR test, the catalyst samples were pretreated in situ by
heating in air at 400 ◦C for 1 h. Then they were heated from 0 to
700 ◦C at 10 ◦C min−1 in a gas stream of 5.0% hydrogen in argon
(molar base).
2.8. Fourier transform infrared (FTIR) spectroscopy study of
adsorbed CO
Transmission FTIR spectra were collected in the single-beam
mode, with a resolution of 2 cm−1, using a Nicolet Nexus FTIR
spectrometer. Catalyst samples were prepared as self-supported
wafers with a diameter of 12 mm and a “thickness” of approxi-
mately 20 mg cm−2. Prior to CO adsorption experiments, all the
catalysts were reduced in situ for 1 h under H2 at 500 ◦C, then
outgassed at this temperature before to be cooled to room tem-
perature. Reference spectra of the clean surfaces were collected at
room temperature. CO was introduced to the cell by small doses up
to saturation, and was then purged to remove any weakly bonded
CO. Spectra were collected until a steady state was reached. Differ-
ence spectra between the samples and the corresponding reference
are shown in this paper.
Fig. 1. Temperature programmed desorption of pyridine of the supports.
2.12. Ring opening of decalin
The reaction conditions were: stainless steel autoclave reac-
tor, temperature = 350 ◦C, hydrogen pressure = 3 MPa, stirring
rate = 1360 rpm, volume of decalin = 25 cm3, catalyst loading = 1 g.
The decalin had 37.5% cis isomer and a trans/cis ratio of 1.63. The
experiments conditions and the equipment were described pre-
viously, also it was checked that diffusional limitations to mass
transfer are negligible [30].
2.9. Cyclopentane hydrogenolysis
3. Results and discussion
The equipment used was described previously [29]. Before the
reaction, the catalysts were reduced for 1 h at 500 ◦C in hydrogen
(60 cm3 min−1). Then they were cooled down in hydrogen to the
reaction temperature (300 ◦C). The other conditions were: cata-
3.1. Metal content
lyst mass = 150 mg, pressure = 0.1 MPa, H2 flow rate = 36 cm3 min−1
,
Table 1 shows the Rh/Pd atomic ratio as well as the weight
percentage of Rh and Pd as determined by ICP analysis and the the-
oretical values for all prepared mono and bimetallic catalysts. The
R1/SIRAL 40 presents the highest difference between the deposited
and theoretical total metal content (25%). This difference is lower
for the SiO2 supported catalysts. However, whatever the catalyst,
the difference between the deposited and theoretical Rh/Pd atomic
ratio is at most of 6%.
cyclopentane (CP) flow rate = 0.483 cm3 h−1. The error in the con-
version value in the CP hydrogenolysis test is about 8%.
2.10. Cyclohexane dehydrogenation
The equipment used was described previously [29]. Reaction
conditions were: catalyst mass = 50 mg, temperature = 350 ◦C, pres-
sure = 0.1 MPa, H2 flow rate = 36 cm3 min−1, cyclohexane (CH) flow
rate = 1.61 cm3 h−1. Before the reaction the catalysts were reduced
in hydrogen (60 cm3 min−1, 500 ◦C, 1 h). The average error of the
measurements was less than 3% [31].
3.2. Evaluation of the acidity by TPD of pyridine and
isomerization of 33DM1B
Fig. 1 shows the profiles of pyridine temperature programmed
desorption (TPD) determined on the three supports. The SIRAL 40
support presents a peak at 503 ◦C, and the alumina a peak with a
maximum at 225 ◦C and a shoulder extending from 270 to 500 ◦C.
Finally the silica shows two small peaks at 2 0 2 and 387 ◦C. The
desorption temperature is associated with the strength of the acid
sites and the area under the TPD curve indicates the total acidity.
SIRAL 40 presents a high concentration of strong acid sites; Al2O3
has weak to moderate acidity, as SiO2, but in larger quantities. As
indicated at the bottom of Table 2, the total acidity, referred to
SiO2, is equal to 5.2 and 14.8 for Al2O3 and SIRAL 40, respectively.
The addition of Pd and Rh increases the total acidity on the three
supports (Table 2). This could be due to the presence of Cl− incor-
porated by the metal precursors or by the HCl used as competitor to
produce a homogeneous metal distribution on the support [32]. In a
previous work, we found that Al2O3–Cl has about three times more
acidity than Al2O3 [33]. As it can be seen in Table 2, the chlorine
content is in correlation with the total acidity. The electronic inter-
action of the metal particles with the support could be responsible
for the changes in acidity. Some authors reported the same behav-
2.11. Ring opening of methylcyclohexane (MCH)
MCH hydrogenolysis was performed in gas phase in a fixed-bed
continuous reactor described previously [29]. Reaction conditions
were: catalyst mass = 1.5 g, total pressure = 3.9 MPa, molar ratio
H2/MCH = 8, WHSV = 2 h−1 and temperature = 250–450 ◦C. Previ-
ously, the catalysts were reduced in situ at the reaction pressure
at 500 ◦C for 1 h using 60 cm3 min−1 of H2. The conversion was var-
ied by changing the reaction temperature, in 25 ◦C steps. The initial
reaction temperature was chosen according to the metal loading
and the support of each catalyst. After reaching a conversion near
100%, the temperature was not increased more. Four measure-
ments were performed at each temperature; the values given in the
results correspond to the average of these four measurements. The
absence of intra and extra granular diffusion limitations in these
experimental conditions were checked [29]. The error associated
with the data was at maximum 5%.

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Table 1
Percentage (wt%) of Rh and Pd and Rh/Pd atomic ratio determined by ICP analysis.
Catalyst
Rha
(wt%)
Rhb
(wt%)
Pda (wt%)
Pdb (wt%)
Pd + Rhb
(wt%)
Rh/Pdb
Pd1/Al2O3
R0.5/Al2O3
R1/Al2O3
R2/Al2O3
Rh1/Al2O3
Pd1/SiO2
R0.5/SiO2
R1/SiO2
R2/SiO2
–
–
1.00
0.67
0.51
0.34
–
1.00
0.67
0.51
0.34
–
1.00
0.67
0.51
0.34
–
0.83
0.61
0.43
0.28
–
0.87
0.59
0.46
0.29
–
0.78
0.54
0.39
0.28
–
0.83
0.92
0.86
0.84
0.81
0.87
0.88
0.93
0.84
0.91
0.78
0.80
0.75
0.80
0.79
–
0.33
0.49
0.66
1.00
–
0.33
0.49
0.66
1.00
–
0.31
0.43
0.56
0.81
–
0.29
0.47
0.55
0.91
–
0.53
1.03
2.07
–
–
0.51
1.06
1.96
–
Rh1/SiO2
Pd1/SIRAL40
R0.5/SIRAL40
R1/SIRAL40
R2/SIRAL40
Rh1/SIRAL40
–
0.33
0.49
0.66
1.00
0.26
0.36
0.52
0.79
0.5
0.95
1.92
–
a
Theoretical.
Determined by ICP.
b
ior pointing to the importance of the redistribution of the acid sites
[13,34–41].
The conversion values for the isomerization reaction of 33DM1B
were taken as a measure of the concentration of Brønsted acid sites.
Kemball and co-workers [42,43] demonstrated that the Lewis acid
sites are not involved in this reaction. Only two isomers are formed
(2,3-dimethyl-2-butene and 2,3-dimethyl-1-butene) at low reac-
tion temperature (<300 ◦C) [44]. The SiO2 series was not evaluated
in 33DM1B isomerization due to the low acidity of the support. The
activities of Al2O3 and SIRAL 40 for the 33DM1B isomerization were
evaluated as a function of temperature between 100 and 300 ◦C. The
activation energy is of 63.5 and 35.6 kJ mol-1 on Al2O3 and SIRAL 40,
respectively, indicating that SIRAL 40 is by far the support present-
ing the highest concentration of Bronsted acid sites. Consequently,
for comparing the effect of the presence of the metal on both sup-
ports, the reaction was performed at 100 ◦C for the SIRAL 40 series
and at 250 ◦C for the Al2O3 one. Fig. 2 shows the conversion extrap-
olated at zero reaction time for the monometallic and R1 bimetallic
catalysts supported on Al2O3 and for the complete SIRAL 40 series.
It must be noticed that the differences between the catalysts of
each series are very difficult to infer because the experimental
error is 6.5%. It is important to point out that the Bronsted acidity
depends strongly on the support and is affected to a lesser degree
Fig. 2. Initial conversion values for the 33DM1B isomerization of the catalysts sup-
ported on Al2O3 and SIRAL 40.
Table 2
Pyridine TPD area (relative to SiO2), chlorine content (wt%), chemisorption values (H/M) and metal particle sizes (d).
Catalyst
Pyridine
Chlorine
H/M
(%)
db
(nm)
TPDa
(wt%)
Pd1/Al2O3
R0.5/Al2O3
R1/Al2O3
R2/Al2O3
Rh1/Al2O3
Pd1/SiO2
R0.5/SiO2
R1/SiO2
R2/SiO2
13.5
18.6
16.4
14.1
13.1
4.9
4.9
4.7
4.3
4.9
40.5
48.1
50.2
56.9
44.4
0.9
16
39
42
45
43
32
33
31
33
30
37
50
46
34
36
5.4
2.3
2.1
1.9
1.9
2.9
2.7
2.9
2.6
2.8
2.5
1.8
1.9
2.6
2.3
0.88
0.92
0.94
0.89
0.7
0.69
0.72
0.73
0.68
1.4
Rh1/SiO2
Pd1/SIRAL40
R0.5/SIRAL40
R1/SIRAL40
R2/SIRAL40
Rh1/SIRAL40
1.41
1.44
1.39
1.37
a
Pyridine TPD area relative to SiO2:SiO2 = 1.00, Al2O3 = 5.2; SIRAL 40 = 14.8.
Deduced from H2 chemisorption.
b

S.A. D’Ippolito et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 203–214
207
Fig. 3. Representative TEM images and particle size distribution for SIRAL 40 supported catalysts.

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S.A. D’Ippolito et al. / Journal of Molecular Catalysis A: Chemical 398 (2015) 203–214
by the nature of the metal, especially on the most acidic support
(SIRAL 40).
ficult to obtain an homogeneous particle size distribution on SIRAL
40 than on Al2O3.
3.4. TPR
3.3. Hydrogen chemisorption and TEM analysis
The degree of metal-metal and metal-support interactions were
evaluated by temperature programmed reduction (TPR). Fig. 4
shows the TPR profiles of the three series of catalysts. In all cases,
the hydrogen consumption indicates that the metals are completely
reduced at temperatures below 500 ◦C. Monometallic Pd and Rh
catalysts supported on Al2O3 show a reduction peak with a maxi-
mum at 75 ◦C and 143 ◦C, respectively. When supported on alumina,
Pd is reduced at a lower temperature compared to the other sup-
ports. It was shown by TEM that Pd particles supported on alumina
are mostly agglomerated with a mean size of 5.8 nm determined
from H2 chemisorption. Then, it can be inferred that the presence
of Pd agglomerates, with low metal-support interactions favor the
reduction of Pd oxides. On the Pd-containing catalysts, whatever
the support, no reduction peak was observed at room temperature
which would be assigned to a reduction of large Pd-oxide species,
according to the lirerature [45].
Bimetallic catalysts supported on Al2O3 have a single reduc-
tion peak between 1 2 2 and 133 ◦C, suggesting a simultaneous
reduction of Pd and Rh oxides. The increase in Rh content shifts
the reduction of the metal phase to higher temperatures. On the
bimetallic R2/Al2O3 catalyst, TEM demonstrated that Pd is either as
monometallic well dispersed nanoparticles with diameter around
1 nm or in bimetallic particles. Then, compared to the Pd1/Al2O3
catalyst, the shift of the reduction peak to higher temperatures may
be attributed to both the reduction of smaller Pd monometallic par-
ticles in strong interaction with the support and the co-reduction
of Pd and Rh.
Table 2 shows the hydrogen chemisorptions values (H/(Rh + Pd)
in %) of the catalysts. Monometallic catalysts, i.e., Pd1 and Rh1
samples, exhibit quite comparable H/M values for a given support,
except in the case of Rh1/Al2O3 which displays a greater disper-
sion than Pd1/Al2O3 (43% compared to 16%). In the case of the
bimetallic catalysts supported on SIRAL 40, the metal dispersion
decreases when increasing the Rh content. An opposite behavior is
found for the Al2O3 series while SiO2 supported catalysts show no
appreciable changes. The monometallic Rh or bimetallic catalysts
supported on SiO2 present lower dispersion than their counterparts
supported on SIRAL 40 and Al2O3. This may be due to the absence
of anion exchange capacity of the SiO2 support, leading therefore
to less metal-support interaction and favoring thus the migration
of species and the development of large crystals.
For the sake of simplicity, only TEM pictures of catalysts sup-
ported on SIRAL 40 are presented (Fig. 3). In agreement with the
values of hydrogen chemisorption TEM experiments show that
metal particles are well dispersed on the Al2O3 and SIRAL 40, except
on Pd1/Al2O3. The particles have a mean diameter between 2.1 and
3.0 nm, not very different from the mean diameters determined
from H2 chemisorption results (Table 2). The R2/SIRAL40 sample
shows some large particles around 10 nm, which were analyzed
by EDX as corresponding to monometallic Pd particles. On this
sample, bimetallic particles were also identified with a majority
of particles enriched in Rh, with Rh/Pd atomic ratio between 2.66
and 3.30. Similar results were reported by Hensen et al. [28] who
found the formation of very small well-dispersed PdRh alloy par-
ticles and few bigger particles which might correspond to some
isolated single metal aggregates (Pd preferentially located on the
particle surfaces). On the R2/Al2O3 sample, most of the particles
are monometallic, with small Pd particles about 1 nm and larger Rh
particles of 2 nm, also some bimetallic particles with Rh/Pd atomic
ratio around 1.6 were analyzed, i.e., with more Pd than the nominal
value. In conclusion, TEM and EDX analysis show that is more dif-
Pd1 and Rh1 catalysts supported on SiO2 show a reduction peak
at 1 0 0 and 132 ◦C, respectively, and bimetallic catalysts exhibit a
similar behavior to those supported on Al2O3.
In the case of monometallic catalysts supported on SIRAL 40,
the reduction ends at temperatures higher than 250 ◦C, which
can be ascribed to the presence of very small metal particles in
strong interaction with the support. In addition, for the R0.5 and
R1 bimetallic catalysts, the peaks clearly appear to be composed of
Fig. 4. Temperature programmed reduction of the catalysts.

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209
Fig. 6. Cyclopentane conversion/cyclohexane conversion ratio obtained in the
cyclopentane hydrogenolysis and cyclohexane dehydrogenation.
adsorbed on Rh0. The two other bands may be attributed to gem-
dicarbonyl Rh+(CO)2 species in agreement with Smith et al. [51],
who demonstrated that the vibrational frequencies of Rh+(CO)2
supported on alumina appears in the region 2105–2088 cm-1 for
the symmetric and 2040–2016 cm-1 for the antisymmetric vibra-
tions. Consequently, the bands at 2107 2 cm-1and 2047 5 cm-1
observed on the Rh1 catalysts are attributed to the vibrations of the
gem-dicarbonyl Rh+(CO)2 species with symmetric and antisym-
metric modes, respectively, [52–54]. The presence of Rh+ species
is generally explained by Rh oxidation in presence of CO involving
hydroxyl groups of the support [55].
Fig. 5. CO–FTIRspectrum of the Pd1 and Rh1 monometalliccatalysts and R1 bimetal-
lic catalysts supported on Al2O3 and SIRAL 40.
R1/Al2O3 and R1/SIRAL 40 bimetallic catalysts show similar
FTIR–CO absorption bands with a preponderance of the bands
corresponding to CO adsorbed on rhodium at 2105, 2042 and
at minimum 2 peaks, one around 110 ◦C, attributable to the reduc-
tion of monometallic palladium particles, and another one at 175 ◦C
due to the reduction of oxides Rh particles. In agreement with the
TEM and EDX analysis, it seems that it is more difficult to obtain
a homogeneous particle size distribution on the SIRAL 40 support
than on Al2O3.
It is important to point out that the TPR trace of the monometal-
lic Rh catalysts supported on Al2O3 and SiO2 show only one
reduction peak. This implies that all the rhodium species immo-
bilized on the respective support material possess the same redox
properties. Meanwhile, the broad reduction peak and the shoulder
found in the Rh1/SIRAL40 catalyst indicate the presence of rhodium
species with different redox properties.
1870 cm-1. Both bimetallic catalysts have a band at 1968 cm-1
,
which may be attributed to bridged CO species adsorbed on Pd. Due
to the complexity of the spectra obtained on mono and bimetallic
catalysts, it is not possible to clearly conclude on particular inter-
actions between Pd and Rh in the bimetallic catalysts from the
CO–FTIR spectra.
3.6. Dehydrogenation of cyclohexane and hydrogenolysis of
cyclopentane
The reaction of cyclohexane (CH) dehydrogenation to benzene
can be used as an indirect method to characterize the metal sites on
supports of mild to moderate acidity such as alumina or silica; the
reaction rate is proportional to the active surface metal atoms. It
is considered insensible to the structure of the catalyst, i.e., it does
not need a special ensemble of atoms [56]. Inversely, cyclopen-
tane (CP) hydrogenolysis is a structure–sensitive reaction, which
requires a number of metal atoms with a given configuration [22].
CP conversion/CH conversion ratio (noted CP/CH) represents the
ratio between a demanding reaction and a non-demanding one. It
can be seen in Fig. 6 that the CP/CH ratio increases with the Rh
content in the three catalyst series, which means that, with the
increase in Rh loading, the hydrogenolytic behavior dominates over
the dehydrogenating ones. This is an important data because during
naphthenic ring opening reaction the dehydrogenation to aromat-
ics must be decreased while hydrogenolysis must be favored.
3.5. CO–FTIR results
The CO–FTIR spectra of the monometallic and R1 bimetallic
catalysts supported on Al2O3 and SIRAL 40 are show in Fig. 5.
Metallic palladium forms carbonyls that are generally character-
ized by bands at wavenumbers below 2100 cm−1 [46–48], with, in
general, one band around 2100 cm−1 attributed to linearly bonded
CO (L band), and broad and convoluted bands below 1995 cm−1
assigned to bridged CO (B band). The band corresponding to bridged
CO species may be decomposed into three main types of species:
B1, close to 1970 cm−1, and B2, around 1955 cm−1, corresponding
to di-coordinated bridged CO species on two types of Pd planes
and a band below 1830 cm−1 generally assigned to triply bonded
CO species [49]. The spectra of CO adsorbed on Pd1/SIRAL40 and
Pd1/Al2O3 present peaks attributable to linear and doubly bridged
CO.
3.7. MCH ring opening
The monometallic catalysts of Rh1/Al2O3 and Rh1/SIRAL 40
show three bands centered at 2107 2 cm-1, 2047 5 cm-1 and
1870 3 cm-1. According to Yang and Garland [50], the broad bands
in the 1800–1900 cm-1 range may be ascribed to bridged CO species
Fig. 7 shows the conversion values of MCH as a function of
the reaction temperature for the three series supported on Al2O3,
SiO2 and SIRAL 40. As expected, for all the catalysts the conversion
increases with the reaction temperature. It was reported that crack-

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Fig. 7. MCH conversion as a function of the temperature reaction for the three series of catalysts.
ing of MCH is an irreversible endothermic reaction that is favored
at high temperature [10,57,58]. Previous experiments have shown
that the deactivation occurring during the MCH reaction was neg-
ligible. This was checked by the following procedure. After having
reached the highest reaction temperature and the maximum con-
version, the temperature was decreased by steps of 25 ◦C to the
lowest one. The MCH conversions as well as the products distribu-
tion obtained were similar.
Monometallic catalysts supported on SIRAL 40 show higher con-
version values at a given temperature than the ones of the Al2O3
and SiO2 series. In the three series, the Rh1 catalysts display a
higher activity than the Pd1 ones, in agreement with the higher
hydrogenolytic activity of Rh, as previously demonstrated. Bimetal-
lic catalysts have an intermediate activity between those of the
Rh1 and Pd1 catalysts, except for the R0.5/SIRAL40 catalyst which
exhibits an activity slightly lower than the Pd1/SIRAL40 sample.
On the SiO2 supported catalysts, higher temperatures are needed
to reach the conversion range of the other supported catalysts. The
lower activity of the SiO2 supported samples is probably due to
their low acidity as reported in Table 2.
The reaction products of MCH are classified in: (i) cracking prod-
ucts, i.e., C1–C6 resulting from either deep hydrogenolysis or acidic
cracking; (ii) isomerization products, i.e., saturated cyclic alkanes
containing the same number of carbon atoms as MCH but with
C5 ring; (iii) ring opening (RO) products, i.e., C7 alkanes resulting
from the opening of MCH and its isomers; and (iv) dehydrogenation
products, i.e., toluene.
The selectivities were calculated by dividing the yield to each
kind of products by the amount of MCH converted, as previously
reported [59]. Fig. 8 shows the selectivity to ring opening prod-
ucts as function of the conversion for each catalyst series. The ring
opening selectivity decreases with increasing MCH conversion due
to deep hydrogenolysis and cracking [17]. For each series, Rh1 cat-
alyst is more selective to RO than the corresponding Pd1 one. The
bimetallic catalysts show the following order of selectivity to RO
products, according to the nature of the support: Al2O3 > SIRAL
40 > SiO2. At low conversions, the selectivity to RO increases with
the Rh content on Al2O3 and SIRAL 40 series, probably due to
the increase of the hydrogenolysis activity, as previously high-
lighted by the CP/CH ratio. The bimetallic catalysts supported on
SiO2 show an opposite trend, with slight variations as function
of the metal composition. At high conversions (close to 80%), the
R0.5/Al2O3, R0.5/SiO2 and R2/SIRAL40 catalysts exhibit finally the
highest selectivity to RO among all bimetallic catalysts of each
series. Fig. 9 shows the selectivity to cracking, isomerization, ring
opening and dehydrogenation products as a function of the con-
version obtained with these three catalysts. It can be seen that the
selectivities to isomerization and ring opening decrease as the con-
version increases, due to cracking of these products on R0.5/Al2O3
and R2/S40 and due to dehydrogenation on R0.5/SiO2. Finally, the
R2/SIRAL40 and R0.5/Al2O3 catalysts display a good combination
of metal and acid functions, allowing the preferential ring opening
of MCH by a bifunctional mechanism, involving a contraction step
of the C6 ring to alkylcyclopentanes with a subsequent opening to
paraffins. It is obvious that the selectivity is affected by the sup-
port acidity and by the composition of the metallic phase. Catalysts
with low total and Bronsted acidity, such those supported on SiO2,
produce mainly dehydrogenated compounds and low amounts of
isomers and cracking products. On the other hand, catalysts with
high total and Bronsted acidity, such those supported on SIRAL 40,
produce more cracking products and higher amounts of isomers.
R0.5/Al2O3 catalyst has selectivity to dehydrogenated products
higher than that of R2/SIRAL40 due to its higher content Pd, which
favors more dehydrogenation than hydrogenolysis. However, by
considering the metal function only, the CP/CH ratio was previously
observed to be higher on R0.5/Al2O3 catalyst than on R2/SIRAL40
(Fig. 6).Then, less dehydrogenated products would be expected on
R0.5/Al2O3 because hydrogenolysis reaction might prevail on the
metallic sites. The negligible formation of dehydrogenated prod-
ucts on R2/SIRAL40 catalyst could be due to the high acidity that
favors the MCH ring contraction and the subsequent ring opening
of the as-formed C5 ring.
Among the cracking products, the major part is constituted of C1
and C6 products, mainly produced by hydrogenolysis on the metal-
lic function. It can be seen in Fig. 10 that the C1 and C6 products
increase linearly with the yield to cracking products for the three
series of catalysts. An acid-catalyzed cracking would have yield C3
and C4 products.
From the MCH reaction, it can be concluded that, on all sup-
ports, it is possible to reduce both the formation of cracking and
dehydrogenated products, which are undesirable products, during
MCH ring opening reaction by using an appropriate formulation of
Pd–Rh bimetallic catalyst. In the bimetallic catalysts (results not
shown), Pd contributes to limit the formation of cracking products

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211
Fig. 8. Selectivity to ring opening products as a function of the MCH conversion for the three series of catalysts.
(negligible cracking reactions are observed on Pd1 samples, what-
ever the support) and Rh contributes to limit the dehydrogenation
of MCH to toluene.
decane isomers, as well as cracking of RO and dehydrogenation of
decalin.
The decalin reaction products were classified according to the
same criterion used in a previous work [62]. The products of
reaction are lumped into: cracking products (C1–C9 products);
ring opening (RO) C10 products; ring contraction (RC) products;
and dehydrogenated products (naphthalene and heavy dehydro-
genated products). The results of the decalin reaction are presented
in Fig. 11, in terms of percentages of cis and trans decalin and
the selectivity to cracking, RC, RO and dehydrogenated products
obtained after 6 h reaction time at 350 ◦C.
On acid catalysts, trans/cis decalin ratios were observed to
increase in the course of the reaction, which could be due to a
higher reactivity of the cis isomer [12,63] and also to the catalytic
cis/trans isomerization reaction [64]. On Al2O3 and SIRAL 40 series,
3.8. Decalin ring opening
On bifunctional catalysts the hydroconversion of decalin
occurs through two parallel reactions [60,61]: ring contraction
on Bronsted acid sites yielding skeletal isomers with five-
membered naphthenic rings (ring contraction products) followed
by hydrogenolysis of these isomers on metal sites to C10 one-ring
naphthenes (ring opening products), as well as direct hydrogenoly-
sis on metal sites yielding ring opening which is less favored in the
case of six-membered naphthenic rings. Also a second ring opening
step can occur from C10 one-ring naphthenes leading to open chain
Fig. 9. Selectivity to cracking, isomerization, ring opening and dehydrogenated products as a function of the MCH conversion for the R0.5/Al2O3, R0.5/SiO2 and R2/SIRAL40
catalysts.

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Fig. 10. C1 and C6 products as a function of selectivity to cracking obtained during MCH reaction for the three series of catalysts.
Fig. 11. Trans and cis-decalin and the selectivity to cracking, ring contraction, ring opening and dehydrogenated products obtained after 6 h reaction time. Values between
brackets are trans/cis-decalin ratio.
the trans/cis ratio decalin roughly increases as a function of the
conversion (initially the trans/cis ratio is of 1.63). These results are
in agreement with those reported by Lai and Song [65] and Moraes
et al. [66]. The Rh1 and bimetallic catalysts supported on SiO2 show
higher amounts of cis-decalin than their counterparts on Al2O3 or
SIRAL 40. The lowest acidity of these samples forces the reaction
to proceed on the metal function where both isomers have the
same reactivity. Finally, the SIRAL 40 series have a higher trans/cis
decalin ratio than the corresponding catalysts of Al2O3 and SiO2
series. It is known that cis-decalin is more selectively converted to
RO products than trans-decalin [12]. In general, a lower amount
of cis-decalin obtained at the end of the reaction corresponds to

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213
higher yields in RO products. The stereoisomerization of cis-decalin
to trans-decalin occurs on the metal sites via a hydrogenation-
dehydrogenation mechanism. Besides, the acid sites promote the
stereoisomerization. The isomerization is an essential step in the
ring opening of decalin and occurs on Brønsted acid sites. In the
absence of Bronsted acid sites, no isomerization occurs nor ring
opening [7,8]. This is evidenced by the results of Fig. 11, where
the catalysts supported on Al2O3 and SiO2 with the lowest total
and Brønsted acidities show the lowest conversion, i.e., a high per-
centage of decalin in the final reaction medium, and a very low
selectivity to RC and RO. Al2O3 and SiO2 supported samples produce
mainly dehydrogenated compounds (naphthalene and others). On
the contrary, SIRAL 40 series, which is the most active, exhibits
the best selectivity to RO products. On this support, the activity is
enhanced by the increase in the Rh content, highlighting the impor-
tance of the balance between the hydrogenolytic function (Rh) and
the acid sites for selective ring opening. Moreover, R2/S40 cata-
lyst having the highest metal-metal interaction and the highest
acidity of the SIRAL 40 series displays a slightly better selectiv-
ity to RO. In the Al2O3 and SiO2-supported catalysts series, the
increase in Rh content produces a slight increase of the conver-
sion, which is associated with the increase in cracking products in
the SiO2 series.
stability of one-ring compounds compared to that of two-ring com-
pounds. Moreover, for the decalin and MCH ring opening reactions,
a first step consisting of ring contraction is necessary to facilitate
the hydrogenolysis reaction.
4. Conclusions
Hydrogen chemisorption showed that the metal dispersion of
the catalysts varied between 30 and 50%. Monometallic Pd/Al2O3
catalyst formed agglomerated nanoparticles with low metal-
support interactions. Bimetallic particles were seemingly difficult
to obtain on the alumina support. The same trend was observed
on silica. The bimetallic catalysts supported on SIRAL 40 presented
large particles of Pd and small bimetallic particles. The CP conver-
sion/CH conversion ratio (parameter of demanding/no demanding
metal catalyzed reaction) increased with the Rh content. Moreover
in most cases, the CP/CH ratio was higher on the Al2O3 supported
catalysts than in the SiO2 and SIRAL 40 series.
The SIRAL series 40 had the highest concentration of strong
acid sites, whereas the concentration of weak and moderate acid
sites was higher on alumina than on silica. The following order of
total and Brønsted acidities was found for the three series: SIRAL
40 » Al2O3 > SiO2. For the three series, the incorporation of metals
increased the acidity of the support and the strength of the acid
sites decreased by increasing the Rh content.
The selectivity to ring contraction products is higher for the
SIRAL 40 series than for the SiO2 and Al2O3 ones. This trend clearly
shows the importance of the acidity on the overall performance
of the catalyst, the RC compounds being furthermore more easily
transformed into RO products [4].
The best support for the ring opening of MCH was Al2O3,
whereas SIRAL 40 had the best performances for SRO of decalin,
SiO2 support being inappropriate in both reactions. The addition
of Rh to Pd resulted in an increase in MCH conversion and in the
formation of cracking products, while decreasing the isomerization
and dehydrogenation products.
3.9. Comparison between MCH and decalin reactions
Comparison of MCH and decalin conversion is not applicable
because the decalin reaction was performed in batch reactor and
the MCH one in continuous flow reactor. However, selectivities val-
ues observed at the same reaction temperature could be useful to
evaluate the catalysts.
At equal values of metal dispersion and metal content, the cat-
alysts with higher amount of total and Bronsted acid sites (i.e.,
supported on SIRAL 40) showed higher selectivity to RO products
during SRO of decalin.
As already seen, the SIRAL 40 supported catalysts notably exhibit
better performances in the decalin reaction (higher conversion and
higher selectivity to ring opening products) than those supported
on Al2O3 or SiO2 (Fig. 11). Moreover, there is no major variation in
the RO selectivity between the SIRAL 40 supported catalysts. Con-
sequently, the acidity of these catalysts plays a more important role
than the metal function in decalin RO as compared to the conversion
of MCH. The metal function of the catalyst plays a more relevant role
in the opening of naphthenes of one ring such as MCH, compared
to the one of two rings. Indeed, the rhodium content produces a
marked increase in the selectivity to cracking products in the MCH
reaction (Fig. 9), whereas this effect remains negligible in decalin
ring opening.
In the MCH reaction, the monometallic Pd/SiO2 catalyst pro-
duces very little amounts of RC products by isomerization, due
to a low acidity. Higher amounts of isomers are obtained on Pd
supported on Al2O3 and SIRAL 40, due to the higher acidity of
these samples combined with the low hydrogenolysis activity of
Pd, which limits the conversion of the formed five-membered
rings. The incorporation of Rh leads to catalysts with a higher
hydrogenolysis activity, and, as a consequence the RC products
are transformed. Furthermore, during the decalin RO reaction, only
SIRAL 40 supported catalysts produce significant amounts of RC
products, highlighting that strong acid sites are required to produce
ring contraction of double rings.
Acknowledgement
The authors gratefully acknowledge the ECOS-MINCyT program
n◦ A12E02 (2013-2015) which allowed the collaboration between
the two research teams.
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