S.A. D’Ippolito et al. / Applied Catalysis A: General 469 (2014) 532–540
533
large-sized pores (USY, beta, mordenites) as ring opening catalysts
2.2. Measurement of the Pd and Rh contents
are more selective to RO products as compared to middle pore size
ones; consequently, this parameter is very important in RO catal-
ysis [11]. Zeolites of larger pore size, such as HY, are considered
as one of the most appropriate supports for ring opening catalysts
The composition of the metal function was determined by
inductively coupled plasma-optical emission spectroscopy (ICP-
OES) after digestion in an acid solution and dilution.
[
12,13]. The zeolite crystal size [14] and the number and strength
distribution of the acid sites [11,15] are important parameters for
ring opening activity and selectivity. Kubicka et al. [15,16] found an
important influence of acidity in the SRO of bicyclic naphthenes, the
presence of Brønsted sites is required for ring opening and isomer-
ization. Santikunaporn et al. [17] studied the contraction and ring
opening of decalin and tetralin and found that the Pt/HY catalysts
are more effective than HY catalysts without metal promoter. The
addition of Pt to USY zeolites was also found to greatly increase the
rate of isomerization and therefore the formation of ring opening
products. For these bifunctional metal–acid catalysts, the formation
of ring opening products increases with the proximity between the
Pt and the acid sites and also with the increase of the metal/acid
ratio [18].
Amorphous silica–alumina (ASA) may replace advantageously
zeolites, which often lead to excessive cracking activity, as car-
riers for bifunctional metal–acid catalysts. ASA supports contain
not only silica–alumina mixed phases, but also pure silica and
aluminum clusters [19]. They present strong Brønsted acid sites;
with strength comparable to that of zeolites but in much lower
concentration, which can be adjusted by varying the silica con-
tent [20]. It has been widely reported that the SiO2 increases
acidity of silica–alumina support improving catalyst performance
particularly for deep hydrodesulfuration (HDS) of diesel fuel [21].
Nassreddine et al. [22,23] demonstrated that ASA supports provide
a good ring-opening and contraction selectivity to iridium for
tetralin hydroconversion in the presence of sulfur. It was shown
that the most active and selective Ir/ASA catalyst for tetralin con-
version was the one with 40% of silica in the support, which presents
the highest Brønsted acidity.
2.3. 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 room
◦
◦
−1
temperature to 700 C at 10 C min in a gas stream of 5.0% hydro-
gen in argon (molar base).
2.4. X-ray diffraction
The analysis was performed with a Shimadzu XD-D1 diffrac-
tometer. Diffraction patterns were recorded using Cu K␣ radiation
◦
◦
−1
,
filtered with Ni in the 10–60 range at a scan rate of 2 min
operating at 30 kV and 40 mA.
2.5. H chemisorption
2
This technique was used in order to estimate the metallic acces-
sibility of the Pd–Rh bimetallic particles on the surface of the
◦
−1
◦
−1
catalyst. The sample (100 mg) was reduced at 500 C (10 C min ,
3
−1
3
H 30 cm min ) for 1 h. Then argon (30 cm min ) was made to
2
◦
flow over the sample for 2 h at 500 C in order to eliminate adsorbed
◦
hydrogen. Finally the sample was cooled down to 70 C 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 surface Pd or Rh atom (H/Pd and H/Rh) equal
to 1.
In this work, the influence of the Brønsted acidity of the support
on the selective ring opening of two-ring naphthenic compounds
was studied. Mono and bimetallic Pd–Rh catalysts supported on
SiO –Al O with different SiO2 contents (SiO = 5, 20 and 40 wt%)
2.6. Temperature-programmed desorption of pyridine
2
2
3
2
and different atomic Rh/Pd ratios were used. Decalin was taken as
model molecule.
The amount and strength of the acid sites of the catalysts were
assessed by means of temperature programmed desorption of pyri-
dine. An amount of 200 mg of the catalyst to be tested were first
immersed in a closed vial containing pure pyridine (Merck, 99.9%)
for 4 h. Then the vial was open and excess pyridine was allowed
to evaporate in a ventilated hood at room conditions until the sur-
face of the particles was dried. The sample was then loaded into a
quartz tube microreactor and supported over a quartz wool plug.
2
. Experimental
2.1. Catalysts preparation
Three commercial SiO –Al O supports provided by Sasol
2
2
3
−
1
(
SIRAL 5, SIRAL 20 and SIRAL 40) were used as support. Previously,
A constant flow of nitrogen (40 mL min ) was made to flow over
the sample. A first step of desorption of weakly adsorbed pyridine
◦
◦
−1
3
−1
they were calcined at 450 C for 4 h (10 C min , air, 60 cm min ).
Rh and/or Pd were added by a common impregnation method. An
aqueous solution of HCl (0.2 mol L ) was added to the support and
the system was left unstirred at room temperature for 1 h. Then an
aqueous solution of RhCl and/or PdCl (Sigma–Aldrich) was added
◦
and stabilization was performed by heating the sample at 110 C
−1
◦
−1
for 1 h. Then the temperature was raised at a rate of 10 C min to
a final value of 700 C. The reactor outlet was directly connected to
◦
3
2
a flame ionization detector.
in order to have a 1 wt% of total metal charge. For the bimetal-
lic catalyst, the Rh/Pd atomic ratio was x = 0.5, 1 and 2. The slurry
was gently stirred for 1 h at room temperature and then it was
2.7. Isomerization of 3,3-dimethyl-1-butene (33DM1B)
◦
put in a thermostated bath at 70 C until a dry solid was obtained.
The reaction was performed in a microreactor of U shape
(length = 20 cm, diameter = 0.6 cm). The feed was generated by
passing a nitrogen stream through a saturator contactor containing
◦
The drying was completed in a stove at 120 C overnight. Finally,
3
−1
◦
the samples were calcined in flowing air (60 cm min ) at 300 C
3
−1
◦
◦
for 4 h and reduced under flowing H2 (60 cm min , 500 C, 4 h).
The monometallic catalysts are named Pd1/Sy or Rh1/Sy, while the
bimetallic are named Rx/Sy, where Sy is the support (SIRAL) and y
the liquid reagent and immersed in an ice bath at 0 C. The catalyst
3
−1
,
(50 mg) was pretreated in situ by reduction with H (60 cm min
2
◦
3
−1
450 C, 1 h). The sample was then cooled in N2 (30 cm min ) to
the reaction temperature, which was changed in order to have small
conversion values to avoid secondary reactions. It was fixed at 100,
is the weight percentage of SiO . In the case of the bimetallic cata-
2
lysts, R corresponds to the Rh/Pd atomic ratio and x is the value of
this ratio.
◦
150 and 200 C for SIRAL 40, 20 and 5, respectively. Then the feed
D'Ippolito, Silvana A.
