Ir/Pt-HZSM5 for n-pentane isomerization: Effect of iridium loading on the properties and catalytic activity

Article abstract of DOI:10.1016/j.jcat.2012.07.012

The effects of iridium loading on the properties of Ir/Pt-HZSM5 and n-pentane isomerization were studied. XRD, IR, and NMR results indicated that increasing iridium loading did not much change the properties of catalysts, but eliminated the perturbed silanol groups at 3700 and 3520 cm^-1, whereas IR and ESR spectroscopy confirmed that increasing iridium loading continuously decreased the permanent Lewis and Bronsted acid sites and inhibited the formation of protonic acid sites induced by hydrogen. At low iridium loading (0-0.3 wt%), cracking process proceed through dimerization-cracking step, whereas high iridium loading (0.5-2.0 wt%) reduces the contribution of dimerization-cracking step and promotes the contribution of hydrogenolysis. The excessive amount of iridium loading, with the presence of a low amount of active protonic acid sites and hydrogen gas, accelerated the hydrogenolysis process. The activity of Ir/Pt-HZSM5 was marginal in the absence of hydrogen, showing the dependence of activity on promotive effect of hydrogen.

Full text of DOI:10.1016/j.jcat.2012.07.012

Journal of Catalysis 294 (2012) 128–135
Contents lists available at SciVerse ScienceDirect
Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat
Ir/Pt-HZSM5 for n-pentane isomerization: Effect of iridium loading on the
properties and catalytic activity
a
a
b,c,
⇑
H.D. Setiabudi , A.A. Jalil , S. Triwahyono
a
Institute of Hydrogen Economy, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
Ibnu Sina Institute for Fundamental Science Studies, Faculty of Science, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
Nanotechnology Research Alliance, Universiti Teknologi Malaysia, 81310 UTM Johor Bahru, Johor, Malaysia
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
The effects of iridium loading on the properties of Ir/Pt-HZSM5 and n-pentane isomerization were stud-
Received 26 May 2012
Revised 11 July 2012
Accepted 12 July 2012
Available online 11 August 2012
ied. XRD, IR, and NMR results indicated that increasing iridium loading did not much change the proper-
ties of catalysts, but eliminated the perturbed silanol groups at 3700 and 3520 cm , whereas IR and ESR
ꢀ1
spectroscopy confirmed that increasing iridium loading continuously decreased the permanent Lewis and
Brønsted acid sites and inhibited the formation of protonic acid sites induced by hydrogen. At low iridium
loading (0–0.3 wt%), cracking process proceed through dimerization-cracking step, whereas high iridium
loading (0.5–2.0 wt%) reduces the contribution of dimerization-cracking step and promotes the contribu-
tion of hydrogenolysis. The excessive amount of iridium loading, with the presence of a low amount of
active protonic acid sites and hydrogen gas, accelerated the hydrogenolysis process. The activity of
Ir/Pt-HZSM5 was marginal in the absence of hydrogen, showing the dependence of activity on promotive
effect of hydrogen.
Keywords:
Ir/Pt-HZSM5
Protonic acid sites
Dimerization
Hydrogenolysis
n-Pentane isomerization
Ó 2012 Elsevier Inc. All rights reserved.
1
. Introduction
Bifunctional heterogeneous catalysts, consisting of a noble me-
zation process. Yang and Woo [14] reported that bimetallic
Pt–Ir/NaY maintained better activity for the n-heptane reforming
reaction than the Pt/NaY catalyst, due to a decrease in the forma-
tion of coke. Additionally, Aboul-Gheit et al. [15] reported that
the combination of platinum and iridium in the HZSM-5 zeolite
produces a more active catalyst compared to the monometallic
Pt/HZSM5 catalyst and thus enhanced the production of iso-
hexane. Recently, we also have studied the effect of iridium on
Pt-HZSM5 for the isomerization of n-pentane [6]. We found that
the presence of 0.1 wt% iridium increased the number of strong
Lewis acid sites and protonic acid sites via hydrogen spillover phe-
nomenon and led to increase the selectivity of iso-pentane. In addi-
tion, the bonding of iridium oxide to perturbed silanol groups
inhibited the formation of hydroxyl groups from molecular hydro-
tal supported on microporous and mesoporous materials, have
been widely studied because of their efficiency for the isomeriza-
tion process and for the synthesis of high octane numbers [1–3].
It has been reported that the isomerization process over bifunc-
tional heterogeneous catalysts was influenced by the hydrogen
spillover phenomenon [4]. The promotive effect of hydrogen has
been interpreted by the generation of protonic acid sites, in which
the hydrogen migrates or spills over from noble metal sites onto
the acidic oxide support, during the reaction. However, this hydro-
gen spillover effect has only been observed for a limited class of
catalysts, including zeolite supported metal catalysts [1,5,6] and
zirconia based acid catalysts [7–12], with different mechanisms
and rate formation of protonic acid sites [13]. Therefore, the
development of new catalysts with a better hydrogen spillover
phenomenon and higher activity is necessary for the isomerization
process.
ꢀ1
gen at 3380, 3600 and 3680 cm , where those hydroxyl groups
may participate in the enhancement of the cracking reaction.
In the present report, the amount of iridium loaded on
Pt-HZSM5 was studied, in order to elucidate the effects and limita-
tions of iridium loading on the properties and activity of the
Pt-HZSM5 catalyst. XRD, BET, and FTIR measurements were em-
ployed to observe the structural properties of the catalyst, and
the adsorption of 2,6-lutidine was performed to identify the acidic
nature of the catalyst. In addition, the hydrogen spillover phenom-
ena of the catalysts were studied by hydrogen adsorption ESR and
IR spectroscopy. In the catalytic activity studies, hydrogen and
nitrogen were used as carrier gases in order to examine the promo-
tive effect of hydrogen.
In the recent years, the interest in bimetallic heterogeneous cat-
alysts has been increasing, since the presence of a second metal can
influence catalytic properties, improving their activity, stability,
and selectivity. In particular, bimetallic catalysts composed by irid-
ium and platinum were found to be active and stable for isomeri-
⇑
Corresponding author. Fax: +60 7 5536080.
E-mail address: sugeng@utm.my (S. Triwahyono).
0
021-9517/$ - see front matter Ó 2012 Elsevier Inc. All rights reserved.
http://dx.doi.org/10.1016/j.jcat.2012.07.012

H.D. Setiabudi et al. / Journal of Catalysis 294 (2012) 128–135
129
2
. Experimental
2.3. Isomerization of n-pentane
2
.1. Catalyst preparation
The isomerization of n-pentane was performed in a microcata-
lytic pulse reactor according to the method described in the litera-
tures [19]. Initially, 0.2 g of catalyst was treated in an oxygen
stream (FOxygen = 100 ml/min) for 1 h followed by hydrogen stream
(FHydrogen = 100 ml/min) for 3 h at 673 K and cooled down to 548 K
Commercial HZSM5 (Zeolyst International) with Si/Al atomic
ratio of 23 was used as a catalyst support. Pt-HZSM5 and
Ir/Pt-HZSM5 were prepared according to the method described in
previous report [6]. In brief, Pt-HZSM5 was prepared by incipient
wetness impregnation of HZSM5 with the requisite quantity of
in a hydrogen stream. A dose of n-pentane (43 lmol) was passed
over the activated catalyst, and the products were trapped at
77 K before flash-evaporation into an online 6090 N Agilent gas
chromatograph equipped with HP-5 Capillary Column and FID
detector. The intervals between each dose were kept constant at
20 min. Since all the catalysts reached steady state condition with-
in 7 pulses (140 min), results at 7 pulses were used.
The conversion of n-pentane (Xn-pentane) was determined by
P
H
2 6
PtCl ꢁH
2
O (Merck) in aqueous solution to obtain 0.1 wt% Pt in
the finished catalyst. After Pt impregnation, the product was dried
at 383 K overnight and calcined at 823 K for 3 h in air. Subse-
quently, the prepared Pt-HZSM5 catalyst was impregnated with
aqueous solution of IrCl
3 2
ꢁ3H O (Merck) to obtained bimetallic Ir/
Pt-HZSM5 catalyst, followed by drying at 383 K overnight and cal-
cination at 823 K for 3 h in air. The content of iridium was adjusted
to 0.1, 0.3, 0.5, 1.0, and 2.0 wt%. The prepared Ir/Pt-HZSM5 catalyst
denoted as xIr/Pt-HZSM5, where x is the weight percentage of
iridium.
A
i
ꢀ An-pentane
Xn-pentane
where A
¼
P
ꢂ 100
ð1Þ
A
i
i
and An-pentane are corrected chromatographic area for par-
ticular compound and for residual n-pentane. The selectivity (S
i
)
and yield (Y ) to particular product was calculated according to
Eqs. (2) and (3), respectively.
i
2.2. Catalyst characterization
The crystalline structure of the catalyst was determined by X-
Ai
S
i
¼ P
ꢂ 100
ð2Þ
ð3Þ
ray diffraction (XRD) recorded on a powder diffractometer (Bruker
Advanced D8, 40 kV, 40 mA) using a Cu K radiation source in the
A
i
ꢀ An-pentane
a
range of 2h = 2–50°. Percentage crystallinity was calculated using
the average peak intensity at 2h = 7.9° and 8.9°, in which Pt-HZSM5
was used as a reference.
X
n-pentane ꢂ S
i
Y
i
¼
1
00
2
The BET analysis of the catalyst was determined by N adsorp-
tion–desorption isotherm using a Quantachrome Autosorb-1
instrument. The catalyst was outgassed at 573 K for 3 h before
3. Results and discussion
being subjected to N
2
adsorption.
3.1. Structural properties of catalysts
MAS NMR spectra were obtained using a Bruker Avance
4
00 MHz spectrometer. 29Si MAS NMR spectra were recorded at a
Table 1 shows the XRD crystallinity and BET surface area of Pt-
HZSM5 and Ir/Pt-HZSM5 catalysts with different iridium loading.
Similar with our previous assignment [6], the introduction of
0.1 wt% iridium on Pt-HZSM5 slightly increased the percentage
crystallinity of Pt-HZSM5 due to the elimination of the distorted
aluminium sites caused by the interaction of iridium with per-
turbed silanol groups, which stabilizing the crystalline structure
of Pt-HZSM5 and leading to a more ordered framework structure.
An increase in the iridium content from 0.1 to 0.3 wt% did not alter
the percentage crystallinity of the catalyst. However, further in-
creases in the iridium content from 0.5 to 2.0 wt% resulted in a
slight decrease in the percentage crystallinity, which may be due
to the presence of excess amounts of iridium on the surface of
Pt-HZSM5 or partially collapse of Pt-HZSM5 structure. Based on
the XPS results in our previous assignment, the state of iridium
frequency of 79.4 MHz using a 4
cycle delay of 60 s and spinning rate of 7 kHz using a 4 mm zirconia
sample rotor. Al MAS NMR spectra were obtained at 104.2 MHz
using pulse length of 1.9
delay of 2 s.
Fourier Transform Infra Red (FTIR) measurements were carried
out using Perkin–Elmer Spectrum GX FT-IR Spectrometer. The cat-
alyst was prepared as a self-supported wafer and activated under
hydrogen flow at 673 K for 3 h, followed by outgassing at 673 K
for 3 h [10]. In the 2,6-lutidine adsorption process, the activated
catalyst was exposed to 2 Torr of 2,6-lutidine at room temperature
for 30 min, followed by outgassing at room temperature, 373 and
ls radio frequency pulses, a re-
2
7
ls, spin rate of 7 kHz, and relaxation time
4
73 K for 30 min, respectively. The formation of protonic acid sites
from molecular hydrogen was observed according to the method
described in the literature [12]. The 2,6-lutidine pre-adsorbed cat-
alyst was exposed to 100 Torr of hydrogen at room temperature.
The catalyst was then heated stepwise from room temperature to
4
+
on Pt-HZSM5 is Ir , possibly as IrO [6]. Thus, the excess amount
2
of Ir species on the external surface of catalysts is most probably
in the form of metal oxide (IrO ). For BET surface area analysis,
2
4
73 K in 50 K increments. All spectra were recorded at room tem-
the presence of 0.1 wt% iridium in Pt-HZSM5 increased the BET
2
ꢀ1
perature. In order to compare the surface coverage of the adsorbed
species between different wafer thicknesses, all spectra were nor-
malized using the overtone and combination vibrations of the MFI
surface area from 444 to 463 m g due to the increase (approxi-
mately 13.3%) in the crystallinity of Pt-HZSM5. However, the BET
surface area of 0.1Ir/Pt-HZSM5 and 0.3Ir/Pt-HZSM5 catalysts are
similar, indicating the 0.1–0.3 wt% iridium loading did not change
the textural properties of the catalysts. Moreover, further increases
in iridium loading, up to 2.0 wt%, led to a decrease in the BET sur-
face area of the catalysts may be due to the blockage of a portion of
the micropores with iridium oxide or partially collapse of
Pt-HZSM5 structure. The change in the BET surface area of the
catalyst, caused by the different amounts of iridium loading, was
observed for an Ir/Beta catalyst, reported by Nea ßt u et al. [20]. They
reported that increasing the amount of iridium loading (1.0–
ꢀ1
between 2100 and 1550 cm after activation [16].
A JEOL JES-FA100 ESR Spectrometer was used to observe the
formation of electron holes or unpaired electrons in vacuo heating
and to observe the interaction of the electron holes or unpaired
electrons with electrons formed from molecular hydrogen at room
temperature to 473 K. The catalyst was outgassed at 673 K for 3 h
followed by the introduction of 50 Torr of hydrogen at room tem-
perature. The catalyst was then heated stepwise from room tem-
perature to 473 K in 50 K increments [17,18].

1
30
H.D. Setiabudi et al. / Journal of Catalysis 294 (2012) 128–135
Table 1
Structural properties of Pt-HZSM5 and iridium-modified Pt-HZSM5.
Pt-HZSM5, a dominant signal at approximately ꢀ112.8 ppm is
assigned to (ꢃSiO) Si while the shoulder signal between ꢀ103
and ꢀ108 ppm corresponded to (ꢃSiO) Si in the HZSM5 frame-
4
Catalysts
Iridium
XRD
BET surface
area (m /g)
3
2
content (wt%)
crystallinity (%)
work [23]. Moreover, a small shoulder signal in the range of
ꢀ120 to ꢀ135 ppm may be attributed to the cystallographically
inequivalent sites in the Pt-HZSM5 catalyst [24]. The introduction
of iridium on Pt-HZSM5 developed a new shoulder in the range be-
tween ꢀ90 and ꢀ103 ppm and eliminated the shoulder in the
range between ꢀ120 and ꢀ135 ppm, indicating the formation of
Pt-HZSM-5
0
100.0
113.3
113.5
112.3
102.6
100.6
444
463
460
443
419
407
0.1Ir/Pt-HZSM5
0.3Ir/Pt-HZSM5
0.5Ir/Pt-HZSM5
1.0Ir/Pt-HZSM5
2.0Ir/Pt-HZSM5
0.1
0.3
0.5
1.0
2.0
2
(ꢃSiO) Si sites by eliminating the cystallographically inequivalent
sites. As we previously observed [6], the elimination of the cystall-
ographically inequivalent sites in Pt-HZSM5 may be related to the
elimination of the distorted aluminium sites caused by the interac-
tion of iridium with perturbed silanol groups of Pt-HZSM5. As the
iridium loading was increased up to 2.0 wt%, no major changes
were observed in the intensity of both signals, indicating that nei-
ther silicon nor aluminium strongly interacted with the iridium
oxide. A similar trend was observed for molybdenum loaded on
HMCM-22 zeolite reported by Ma et al. [25], in which no signifi-
5
.0 wt%) led to a decrease in the surface area of the Ir/Beta catalysts
may be due to a blockage of the pores, caused by the deposition of
iridium in large aggregates. A similar phenomenon was observed
for a cobalt–iridium impregnated zirconium-doped mesoporous
silica catalyst, in which the textural properties of the catalysts
showed a moderate decrease as the iridium loading is increased,
indicating the blockage of the mesopores with iridium species
[
21]. In contrast, Qun et al. [22] found that the specific surface area
29
cant changes were observed in the Si MAS NMR spectra of all
and total pore volume of all iridium-modified Fe-USY catalysts
were similar, which indicated that the addition of iridium (0.1–
Mo/HMCM-22 catalysts. All samples were prepared by impregna-
tion of the HMCM-22 zeolite with an aqueous solution of ammo-
nium heptamolybdate. Ma et al. suggested that the loading of
molybdenum (2–10 wt%) does not destroy or change the silicate
framework of the Mo/HMCM-22 catalysts.
2
.0 wt%) into Fe-USY did not change the textural properties of
Fe-USY, due to the well-controlled deposition of iridium onto
Fe-USY.
27Al and 29Si MAS NMR offer a strong and effective tool for char-
acterizing the structure of zeolite. In general, species with different
structures or different environments will have a different chemical
Fig. 2 shows the IR spectra of the activated Pt-HZSM5 and irid-
ium-modified Pt-HZSM5 catalysts in the region of hydroxyl groups.
There were five bands observed in this region. The narrow band at
shift in their Al and Si MAS NMR spectra. Fig. 1A shows the 27Al
MAS NMR spectra of Pt-HZSM5 and Ir/Pt-HZSM5 catalysts with dif-
ferent iridium loading. Pt-HZSM5 catalyst shows three signals at
2
7
29
ꢀ1
3
740 cm is assigned to the non-acidic terminal silanol groups
(
Si–OH) located on the external surface of the zeolite, while a
ꢀ1
shoulder band at approximately 3665 cm is assigned to the ex-
tra-framework Al–OH species originating from limited hydrother-
6
4, 55, and 0 ppm corresponding to the distorted tetrahedral sites,
tetrahedral aluminium in the framework structure, and octahe-
drally coordinated extra-framework aluminium, respectively [23].
The introduction of iridium eliminated the peak at 64 ppm and
slightly increased the intensity of the peak at 0 ppm. This result
suggests that the presence of iridium could stabilize the MFI
framework structure by eliminating the distorted aluminium sites
in Pt-HZSM5, as previously observed [6]. As the iridium loading
was increased from 0.1 to 2.0 wt%, there was no significant differ-
ence observed in the intensity of the signals at 55 and 0 ppm,
indicating that increasing iridium loading (0.1–2.0 wt%) has little
effect on the chemical environment of the aluminium atom.
mal degradation during the calcination process [26]. A band at
ꢀ1
3
610 cm is assigned to the Brønsted OH band associated with
bridging hydroxyl groups (Si(OH)Al) located inside the zeolite
ꢀ1
structure, whereas the weak band at 3700 cm and broad band
ꢀ1
at 3520 cm
may be associated with the perturbation of OH
groups through their surroundings by lattice defects or extra-
lattice oxygen [27]. The introduction of iridium (0.1–2.0 wt%) on
Pt-HZSM5 did not change the intensity of the bands at 3740,
ꢀ1
3
665 and 3610 cm , indicating neither non-acidic terminal silanol
groups, nor acidic bridging hydroxyl groups (Si(OH)Al) interacted
with the iridium. However, the presence of 0.1 wt% iridium in Pt-
2
9
The Si MAS NMR spectra of Pt-HZSM5 and Ir/Pt-HZSM5 cata-
lysts with different iridium loading are shown in Fig. 1B. For
ꢀ1
HZSM5 broadened the band at 3740 cm to higher wavenumber
Fig. 1. (A) 27Al MAS NMR (B) 29Si MAS NMR spectra of (a) Pt-HZSM5; (b) 0.1Ir/Pt-
HZSM5; (c) 0.3Ir/Pt-HZSM5; (d) 0.5Ir/Pt-HZSM5; (e) 1.0Ir/Pt-HZSM5; and (f) 2.0Ir/
Pt-HZSM5.
Fig. 2. IR spectra of activated (a) Pt-HZSM5; (b) 0.1Ir/Pt-HZSM5; (c) 0.3Ir/Pt-
HZSM5; (d) 0.5Ir/Pt-HZSM5; (e) 1.0Ir/Pt-HZSM5; and (f) 2.0Ir/Pt-HZSM5 in the
ꢀ
1
region of 3800–3200 cm
.

H.D. Setiabudi et al. / Journal of Catalysis 294 (2012) 128–135
131
ꢀ
1
at 3745 cm and slightly decreased the intensity of the bands at
3
lutidine at the outgassing temperature of 473 K and below. All
the catalysts showed the absorbance bands at 1700, 1680, 1650
ꢀ1
700 and 3520 cm . As we previously reported [6], these results
may be related to the formation of non-acidic geminal silanol
groups at 3745 cm by the interaction of iridium with perturbed
silanol groups at 3700 and 3520 cm . The formation of non-acidic
ꢀ1
and 1640 cm , which are associated with the 2,6-lutidinium cat-
ions adsorbed on Brønsted acid sites. The absorbance bands at
ꢀ1
ꢀ1
ꢀ1
1605, 1585, 1490, 1460, 1410, and 1385 cm are assigned to the
ꢀ
1
geminal silanol groups at 3745 cm is in agreement with the for-
mation of (ꢃSiO) Si in the NMR results. The interaction of iridium
with perturbed silanol groups of Pt-HZSM5 most probably as
ꢃSiO) Si(OH) (IrO ). In contrast, as the iridium loading was
increased up to 2.0 wt%, the bands corresponding to perturbed
silanol groups at 3700 and 3520 cm were eliminated after the
addition of 0.3 wt% and no further changes were observed with
increasing amount of iridium loading (0.5–2.0 wt%). The elimina-
tion of the bands at 3700 and 3520 cm may be related to the
interaction of iridium with perturbed silanol groups of Pt-HZSM5.
This result indicated that all perturbed silanol groups of Pt-HZSM5
were possibly having a full interaction with iridium, when the
amount of iridium loading achieved 0.3 wt%. The change of IR spec-
tra with increasing metal loading was also observed in nickel
loaded on a PtHY catalyst, as reported by Aziz et al. [17]. They re-
ported that the addition of 0.1 wt% nickel eliminated the broad
Lewis acid sites [29]. The changes in the acid sites are more clearly
illustrated in Fig. 3B, in which the absorbance of the IR bands at
2
ꢀ1
ꢀ1
Brønsted acid sites (1640 cm ) and Lewis acid sites (1585 cm
)
(
2
2
2
were plotted as a function of iridium loading. The presence of
0.1 wt% iridium in Pt-HZSM5 slightly increased the intensity of
the bands associated with the Brønsted and Lewis acid sites. Based
on our previous assignment [6], the generation of acid sites may be
due to the interaction of iridium with perturbed silanol groups,
ꢀ1
ꢀ1
probably as (ꢃSiO)
2 2 2
Si(OH) (IrO ). However, a slight decrease in
the Brønsted and Lewis acid sites was observed with increasing
iridium content from 0.1 to 2.0 wt%, which may be related to the
presence of bulk iridium oxide on the external surface of the
catalyst which may hinder the accessibility of acidic sites to
the 2,6-lutidine probe molecule.
Partial coverage of the acid sites with iridium was also observed
with the Ir/HZSM5 catalyst, as reported by Aboul-Gheit et al. [30].
ꢀ
1
bands in the range of 3500–3720 cm and partially decreased
the band assigned to the terminal silanol groups at 3740 cm
The NH
3
-TPD results showed that the addition of 0.35 wt% iridium
H from 98.4 to 87.5 J/g,
ꢀ1
.
to HZSM5 slightly decreased the values of D
Additionally, as the nickel loading was increased up to 3.0 wt%,
the intensity of the band corresponding to terminal silanol groups
at 3740 cm decreased markedly and a new band assigned to
indicating a small decrease in acid sites. The authors suggested
that a slight decrease in the acid sites may be caused by the cover-
age of a portion of the acid sites with iridium. A similar phenome-
non was observed with an Ir–Mo/HZSM5 catalyst in which the
addition of 0.5 wt% iridium caused a decrease in all species of acid
sites, from 553 K up to the end of the TPD run at 1073 K [31]. These
authors also reported that a decrease in all species of acid sites was
due to the coverage of a portion of the acid sites with iridium
species.
ꢀ1
(
–(OH)Ni), located on the surface of the zeolite framework, ap-
ꢀ1
peared at 3700 cm
.
3.2. Nature of acidity
The type of acid sites in all catalysts was qualitatively probed by
,6-lutidine adsorption using IR spectroscopy. 2,6-lutidine (pK
.4), which demonstrated more basic character than the pyridine
= 8.8), is used to study the acidic nature of catalysts, particu-
2
7
b
=
3.3. Hydrogen molecule-originated protonic acid sites
(
pK
b
larly in the observation of the relatively weak Brønsted acid sites
and the acidic centers of Lewis acid sites [28]. Fig. 3A shows the
IR spectra of 2,6-lutidine adsorbed on activated Pt-HZSM5 and
iridium-modified Pt-HZSM5 catalysts in the region of 1750–1350
cm . The spectra were recorded after the adsorption of 2,6-
lutidine at room temperature, followed by removal of 2,6-lutidine
at 473 K. Since the catalysts were outgassed at 473 K, the acid sites
under consideration are only strong acid sites that can retain 2,6-
Fig. 4A shows the changes in the IR spectra, when the 2,6-luti-
dine pre-adsorbed catalysts were heated in hydrogen at 473 K. As
the catalysts were heated in hydrogen, the intensity of the bands
ꢀ1
at 1605, 1585, 1490, 1460, 1410 and 1385 cm , corresponding
to the Lewis acid sites, decreased with a concomitant increase in
ꢀ1
0.40
0.40
0.30
0.20
0.10
0.00
A
0.5
B
1
640
A
0.5
385
B
0
0
.30
.20
2
.0
1
650
(f)
1
Brönsted
1460
1
410
(1640 cm-1
)
(e)
1
6051490
585
1.6
Brönsted
(1640 cm )
1680
-1
1
(d)
1
700
1.2
0.8
0.4
0.0
(c)
(
f)
(
e)
0.10
0.00
(
(
b)
a)
Lewis
1585 cm )
(
(
d)
c)
-1
(
Lewis
1585 cm )
(
b)
a)
-1
1750 1650 1550 1450 1350
0
0.5
1
1.5
2
(
(
Wavenumber [cm-1]
Iridium Loading [%]
1
750 1650 1550 1450 1350
0
0.5
1
1.5 2
Fig. 4. (A) Spectral changes when 2,6-lutidine pre-adsorbed samples (dotted lines)
were heated in the presence of hydrogen at 473 K (solid lines). (a) Pt-HZSM5;(b)
-
1
Wavenumber [cm ]
Iridium Loading [%]
0
.1Ir/Pt-HZSM5; (c) 0.3Ir/Pt-HZSM5; (d) 0.5Ir/Pt-HZSM5; (e) 1.0Ir/Pt-HZSM5; (f)
2.0Ir/Pt-HZSM5. (B) The changes in the peak intensity of the IR bands for Brønsted
and Lewis acid sites when the catalysts were heated in hydrogen at 473 K. [L1585
and [B1640 BG represent the peak intensity of the Lewis and Brønsted acid sites
before hydrogen adsorption. [L1585 H2 and [B1640 H2 represent the peak intensity of
the Lewis and Brønsted acid sites in the presence of hydrogen at 473 K.
Fig. 3. (A) IR spectra of 2,6-lutidine adsorbed on activated (a) Pt-HZSM5; (b) 0.1Ir/
Pt-HZSM5; (c) 0.3Ir/Pt-HZSM5; (d) 0.5Ir/Pt-HZSM5; (e) 1.0Ir/Pt-HZSM5; and (f)
.0Ir/Pt-HZSM5 catalysts at room temperature followed by removal of 2,6-lutidine
at 473 K. (B) Variations in the absorbance of the IR bands for Brønsted and Lewis
acid sites after removal of 2,6-lutidine at 473 K.
]
BG
2
]
]
]

1
32
H.D. Setiabudi et al. / Journal of Catalysis 294 (2012) 128–135
the intensity of the bands at 1700, 1680, 1650 and 1640 cmꢀ1,
which are attributed to the peaks of 2,6-lutidine on Brønsted acid
sites. This result demonstrates the formation of protonic acid sites,
in the presence of hydrogen, with a simultaneous decrease in the
number of Lewis acid sites, by switching the acidic character from
Lewis acid sites to protonic acid sites. The effect of hydrogen was
essentially similar for all catalysts, indicating that the concept of
OH groups. Although both catalysts exhibited a similar phenome-
non, 0.1Ir/Pt-HZSM5 showed largest changes in the intensity of
the signal, over that of 2.0Ir/Pt-HZSM5, indicating that the forma-
tion of electrons via the hydrogen spillover mechanism is much
easier for 0.1Ir/Pt-HZSM5, compared to that of 2.0Ir/Pt-HZSM5
catalyst.
The formation of electrons for Pt-HZSM5 and iridium-modified
Pt-HZSM5 catalysts is more clearly illustrated in Fig. 6, in which
the variation in the relative intensity of the ESR signal at g = 1.99
for all catalysts was plotted as a function of the heating tempera-
ture. As the heating temperature was increased, the variation in
the relative intensity of the ESR signal at g = 1.99 decreased, due
to the formation of electrons via the hydrogen spillover mecha-
nism. Even though all catalysts exhibited a similar phenomenon,
it is obvious that the generation of electrons is higher for 0.1Ir/
Pt-HZSM5 catalysts, followed by 0.3, 0.5, 0, 1.0, and 2.0 wt%. The
results of the ESR study are consistent with the FTIR study of
hydrogen adsorption on 2,6-lutidine pre-adsorbed catalysts, in
which the introduction of 0.1 wt% iridium increased the formation
of protonic acid sites and electrons, whereas an increase in iridium
loading decreased the formation of protonic acid sites and
electrons.
‘
‘molecular hydrogen-originated protonic acid sites’’ is applicable
to Pt-HZSM5 and all Ir loaded on Pt-HZSM5 catalysts. It should
be noted that the increase in the intensity of protonic acid sites
is not similar to the decrease in the intensity of Lewis acid sites, be-
cause of the different extinction coefficients for the protonic and
Lewis acid sites, as well as the ability of 2,6-lutidine to easily pro-
duce protonated species and its weaker affinity for Lewis acid sites.
The changes in the acid sites are more clearly illustrated in
Fig. 4B, in which the absorbance of the IR bands at Brønsted acid
ꢀ1
ꢀ1
sites (1640 cm ) and Lewis acid sites (1585 cm ) were plotted
as a function of iridium loading. The introduction of 0.1 wt% irid-
ium on Pt-HZSM5 slightly increased the formation of protonic acid
sites originating from molecular hydrogen, whereas increasing
amount of iridium loading (up to 2.0 wt%) slightly decreased the
formation of protonic acid sites from molecular hydrogen. These
results may be due to the changes in the number of strong Lewis
acid sites, as evidenced by IR spectra of 2,6-lutidine adsorption.
Although some differences were observed, the effect of hydrogen
is essentially the same as that observed for Pt/HZSM5 [6], Zn/
The effect of hydrogen on the ESR signal is similar to the results
observed for Pt/HY [17] and MoO –ZrO [18] catalysts, in which
3 2
heating the catalysts, in presence of hydrogen, partially eliminated
the ESR signal assigned to the trapped electrons, or unpaired elec-
trons, that have localized on metal cations (electron-deficient me-
tal cations), representing the formation of electrons via hydrogen
spillover phenomenon. However, for Ni/HY catalysts [17], heating
the catalyst in the presence of hydrogen did not significantly
change the ESR signal at g = 1.9866, indicating a low ability of
the Ni/HY catalyst to form electrons via the hydrogen spillover
phenomenon.
3 3 2
HZSM5 [5], Pt/MoO [32], and MoO –ZrO [12] catalysts. In con-
trast, no protonic acid sites were formed for a Ni/HY catalyst due
to the inability of nickel to facilitate the formation of protonic acid
sites from molecular hydrogen [17].
Fig. 5 shows the ESR signals of (A) 0.1Ir/Pt-HZSM5 and (B) 2.0Ir/
Pt-HZSM5 catalysts, when the catalysts were outgassed at 673 K
for 3 h, followed by heating in the presence of hydrogen at differ-
ent temperatures. The ESR signal at g = 1.99 is assigned to the
trapped electrons, or unpaired electrons, that have localized on
metal cations (electron-deficient metal cations) [33]. As the cata-
lysts were outgassed at 673 K for 3 h, the intensity of the ESR signal
at g = 1.99 increased. This may be due to the desorption of hydro-
xyl groups from the surface of the catalysts, which subsequently
leave the electron-deficient metal cations. The introduction of gas-
eous hydrogen, followed by heating, resulted in the formation of
electrons and protonic acid sites where the electrons were trapped
in the electron-deficient metal cations, resulting the reduction of
the ESR signal at g = 1.99. Moreover, the protonic acid sites were
stabilized near the surface oxygen atoms as hydrogen-bonded
3.4. Isomerization of n-pentane
The catalytic activities of Pt-HZSM5 and Ir/Pt-HZSM5 catalysts
with different amounts of iridium loading were evaluated with
respect to n-pentane isomerization at 548 K in a microcatalytic
pulse reactor, in which hydrogen or nitrogen was used as a carrier
gas. The outlet consisted of cracking products, iso-pentane, residual
n-pentane, and higher hydrocarbons. No cyclo-alkane product was
observed, indicating that dehydrocyclization was not taking place
on Ir/Pt-HZSM5. In fact, these types of catalysts are active in the
ring-opening of cyclo-alkanes [34].
A
B
g =1.99
2
00
200
1.00
g =1.99
(
(
(
f)
(f)
e)
d)
(e)
0
0
0
0
0
.80
.60
.40
.20
.00
(
(
d)
c)
(
(
c)
b)
(b)
(
a)
(
a)
3
10 315 320 325 330 335
310 315 320 325 330 335
223
323
423
523
Field [mT]
Field [mT]
Temperature [K]
Fig. 5. ESR signals of (A) 0.1Ir/Pt-HZSM5 and (B) 2.0Ir/Pt-HZSM5. (a) Before
outgassing; (b) after outgassing at 673 K and heated in the presence of 50 Torr
hydrogen at (c) 323 K, (d) 373 K, (e) 423 K, and (f) 473 K.
Fig. 6. Relative intensity of the ESR signal at g = 1.99 as a function of heating
temperature. (s) Pt-HZSM5; (ꢄ) 0.1Ir/Pt-HZSM5; (j) 0.3Ir/Pt-HZSM5; (N) 0.5Ir/Pt-
HZSM5; (h) 1.0Ir/Pt-HZSM5; (D) 2.0Ir/Pt-HZSM5.

H.D. Setiabudi et al. / Journal of Catalysis 294 (2012) 128–135
133
Fig. 7 shows the effect of iridium loading on the conversion of n-
pentane, yield of iso-pentane and yield of cracking products in the
presence of hydrogen or nitrogen. In the presence of nitrogen gas,
all catalysts were not active in n-pentane isomerization. In fact, the
yield of iso-pentane and cracking products did not typically exceed
5
%, indicating that the presence of permanent strong Brønsted and
Lewis acid sites do not directly affect the catalytic isomerization of
n-pentane. This result also confirmed that thermal cracking does
not take place on these catalysts. In the presence of hydrogen
gas, the addition of 0.1 wt% iridium to Pt-HZSM5 has increased
the yield of iso-pentane and decreased the yield of cracking prod-
ucts. As we previously observed [6], the enhancement in the yield
of iso-pentane with the addition of 0.1 wt% iridium may be caused
by the interaction of iridium with lattice defects or extra-lattice
oxygen resulting in an increased in the number of strong Lewis
and protonic acid sites from molecular hydrogen, whereas the sup-
pression of cracking products may be caused by the inhibition of
the formation of hydrogen bonding interactions from molecular
hydrogen at 3680, 3600, and 3380 cm . However, increasing irid-
ium loading up to 0.3 wt% has reduced the conversion of n-pen-
tane, while the ratio of iso- to cracking products was almost
similar. The further loading of the iridium reduced continuously
the yield of iso-pentane, but increased markedly the yield of crack-
ing products. Additionally, the conversion of n-pentane and selec-
tivity for cracking products exceeded than 98% on 2.0 wt% iridium
loading on Pt-HZSM5.
The distribution of cracking products and higher hydrocarbons
are shown in Fig. 8. At low iridium loading (0–0.3 wt%), cracking
products consisted of methane, ethane, and propane. Additionally,
small amount of iso-hexane and n-hexane were observed as higher
hydrocarbons. However, only methane and ethane were observed
for high iridium loading (0.5–2.0 wt%). The large amount of meth-
ane and ethane, without higher hydrocarbons, for high iridium
loading indicates that the cracking process proceeds via a hydrog-
enolysis process, whereas the small amount of iso-hexane and n-
Fig. 8. Effect of Iridium loading on the distribution of (A) cracking products and (B)
higher hydrocarbons in the presence of hydrogen.
ꢀ
1
Lewis acid sites. Lowering the acidity led to a reduction in the
activity of isomerization and an excessive amount of iridium pro-
ceeded intensively the hydrogenolysis of n-pentane, in the pres-
ence of hydrogen, in the gas phase.
Scheme 1 shows the plausible reaction mechanisms of n-pen-
tane isomerization over Pt-HZSM5 and Ir/Pt-HZSM5 catalysts with
different amounts of iridium loading. Isomerization process oc-
curred for all amounts of iridium loading (0–2.0 wt%) while the
cracking process route proceeds differently with amounts of irid-
ium loading. Isomerization process proceeds by acid catalyzed
reaction mechanism in which the isomerization was induced by
the interaction between n-pentane and protonic acid site to form
n-pentyl carbenium ion. Then, the carbenium ion was isomerized
to mono- or multi-branched pentyl carbenium ion. The product
of iso-pentane was produced by the interaction between branched
pentyl carbenium ion and hydride ion followed by desorption form
the surface of catalyst. The presence of protonic acid site and hy-
dride ion was significant in n-pentane isomerization, and the for-
mation of iso-pentane products increased by the concentration of
protonic acid sites. At low iridium loading (0–0.3 wt%), cracking
process proceeds via a dimerization-cracking reaction route in
1 3
hexane with the presence of cracking products (C –C ) for low irid-
ium loading (0–0.3 wt%) indicated the possibility of the dimeriza-
tion-cracking route. The reduction in iso-pentane product
followed by an increase in n-pentane conversion for 0.5–2.0 wt%
iridium loading may be related to the increase in the perturbed sil-
anol groups-covered by iridium, which lowered both Brønsted and
5
which the process was initiated with the dimerization of n-C to
form n-C10, followed by the formation of cracking products and
higher hydrocarbons. However, for higher iridium loading (0.5–
Conversion
Yield of iC
5
Yield of C
1
-C
4
1
00
100
80
60
40
20
0
100
80
60
40
20
0
2
.0 wt%), the hydrogenolysis of n-pentane took place intensively
A
B
C
and the possibility of a dimerization-cracking route can be ex-
cluded due to the absence of higher hydrocarbon products. Most
H
2
8
6
4
2
0
0
0
0
0
2
probably, hydrogenolysis occurs on the surface of IrO with hydro-
gen assisting in its gas phase and/or by contributing small amount
of protonic acid sites.
H2
H2
N2
N2
N2
0
0.5
1
1.5
2
0
0.5
1
1.5
2
0
0.5
1
1.5
2
Iridium
Loading [%]
Iridium
Loading [%]
Iridium
Loading [%]
Fig. 7. Effect of Iridium loading on the (A) conversion of n-pentane, (B) yield of iso-
pentane, and (C) yield of cracking products, in the presence of hydrogen (black
symbol) and nitrogen (white symbol).
Scheme 1. Plausible reaction mechanisms for n-pentane isomerization over Pt-
HZSM5 and Ir/Pt-HZSM5 catalysts with different amounts of iridium loading.

1
34
H.D. Setiabudi et al. / Journal of Catalysis 294 (2012) 128–135
A similar phenomenon was observed for the conversion of
heptane over Pt/H-Beta zeolite, as reported by Blomsma et al.
35]. Pt/H-Beta catalysts with different Pt loading (0.05–1.50 wt%)
ure shows the ratios of yield of iso-pentane to elimination of Lewis
acid sites (Yield/ L) and to the formation of protonic acid sites
(Yield/ B). The Yield/ L changes more extensively to that of
Yield/ B, indicating that the yield of iso-pentane is directly corre-
lated with the changes in the amount of formed Brønsted acid
sites.
D
[
D
D
D
were prepared by incipient wetness impregnation. They found that
low Pt loading (below 0.1 wt%) caused the cracking reaction to
proceed via dimerization-cracking reaction. However, as Pt loading
increased (up to 1.5 wt%), the hydrogenolysis route became domi-
nant, due to the excessive amount of metal sites. In addition, it
has been reported that the hydrogenolysis of n-alkanes usually
takes place mainly on noble metals of Group VII [36–38] and their
hydrogenolysis ability increase in the order of Pt < Pd < Ir < Rh < Ru
4
. Conclusion
Introduction of iridium (0.1–2.0 wt%) on Pt-HZSM5 did not
ꢀ1
interact with the non-acidic terminal silanol groups (3740 cm ),
[
38]. Yang and Woo [14] studied the n-heptane reforming reaction
ꢀ1
extraframework Al–OH species (3665 cm ) or acidic bridging
over Pt–Ir/NaY catalyst in which Ir is present as agglomerated IrO
2
ꢀ1
hydroxyl groups (3610 cm ), but interacted with perturbed sila-
2+
on the exterior surface while Pt is present as Pt in the supercage
ꢀ1
nol groups of Pt-HZSM5 (3700 and 3520 cm ). This interaction
4
+
and Pt in sodalite cage after calcination of Pt–Ir/NaY at 500 °C.
eliminated the distorted tetrahedral sites caused by the presence
of platinum. The presence of 0.1 wt% iridium slightly increased
the number of Lewis and Brønsted acid sites and increased the
number of protonic acid sites via hydrogen spillover phenomenon
due to the interaction between iridium and perturbed silanol
The catalytic activity study showed that an increase in the Ir con-
tent led to increase C –C products due to the high activity of Ir
1 6
for the hydrogenolysis reaction.
The promotive effects of hydrogen on hydrocarbon isomeriza-
tion or cracking reactions have been reported by several research
groups. Fujimoto et al. [1] studied the hydroconversion of n-pen-
tane over hybrid catalysts under hydrogen and nitrogen atmo-
spheres. In the absence of hydrogen, the conversion of n-pentane
was dramatically reduced and an oligomerization reaction became
dominant. They suggested that the spillover hydrogen play an
important role in n-alkane hydroconversion. In addition, Shishido
and Hattori [39] reported the promotive effect of hydrogen on
ꢀ1
groups at 3700 and 3520 cm . Moreover, the interaction between
iridium and perturbed silanol groups resulted in the enhancement
of iso-pentane selectivity.
However, an increase in iridium loading (0.3–2.0 wt%) did not
significantly change the structural properties of catalysts, but de-
creased continuously the permanent Lewis and Brønsted acid sites
and inhibited the changes of Lewis and Brønsted acid sites induced
by hydrogen in the gaseous phase. In addition, the introduction of
0.3 wt% iridium totally eliminated the peaks corresponding to the
2ꢀ
the catalytic activity of Pt/SO4 –ZrO
2
for cumene cracking reac-
tions. They found that the presence of hydrogen enhanced the
ꢀ1
perturbed silanol groups at 3700 and 3520 cm . Further increases
2
4
ꢀ
activity of Pt/SO –ZrO
2
by forming the protonic acid sites gener-
in iridium loading formed bulk iridium oxide on external surfaces.
ated from molecules of hydrogen. However, no promotive effect
This led to increases in C
pentane. The presence of small number of higher hydrocarbon and
–C cracking products for <0.3 wt% Ir loaded on Pt-HZSM5, indi-
cated the possibility of the dimerization-cracking route. In particu-
lar, large amount of protonic acid sites enhanced the
isomerization of n-pentane while a small amount of protonic acid
1 2
and C products via hydrogenolysis of n-
2ꢀ
2 2
of hydrogen was observed on the SO4 –ZrO and Pt/ZrO catalysts.
Fig. 9 shows the effect of iridium loading on the yield of iso-pen-
tane, elimination of Lewis acid sites and the formation of protonic
acid sites, induced by hydrogen in gas phase. The introduction of
C
1
3
a
0
.1 wt% iridium to Pt-HZSM5 slightly increased the elimination of
,
Lewis acid sites and increased the formation of protonic acid sites
from molecular hydrogen. Additionally, 0.1 wt% iridium loading
slightly increased the yield of iso-pentane from 51.89% to 57.22%.
However, increasing iridium loading slightly decreased the elimi-
nation of Lewis acid sites and the formation of protonic acid sites
and successively decreased the yield of iso-pentane. The inset fig-
sites, with the presence of hydrogen in gas phase on the surface of
IrO2, may enhance the hydrogenolysis of n-pentane. In general, the
selectivity and yield of iso-pentane decreased with iridium loading,
while the conversion of n-pentane increased, due to the increase in
cracking products.
The catalytic activity studies confirmed that n-pentane
isomerization over Ir/Pt-HZSM5 catalysts strongly depends on the
promotive effect of hydrogen as a carrier gas. In fact, the yield of
iso-pentane and cracking product did not typically exceed 5% for
isomerization under nitrogen gas.
Acknowledgments
This work was supported by the Research University Grant,
Universiti Teknologi Malaysia No. 00H95. Our gratitude also goes
to Universiti Malaysia Pahang for the award of Skim Fellowship
Universiti Malaysia (Herma Dina Setiabudi) and the Hitachi Schol-
arship Foundation for the Gas Chromatograph Instruments Grant.
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