Catalytic performance of nanostructured Pt/ZSM-5 catalysts synthesized by extended Charnell's method in hydroisomerization of n-pentane

Article abstract of DOI:10.1016/j.poly.2019.01.056

Na-ZSM-5 zeolite, synthesized by extended Charnell's method was used to prepare Pt/ZSM-5 catalyst for n-pentane hydroisomerization. The influence of ion exchange, alkaline treatment and method of platinum incorporation on the Na-ZSM-5 zeolite was investigated. It was found that the presence of Trien affected the physicochemical properties of the synthesized catalyst. The synthesized catalysts showed high performance so that the sample prepared in rotating mode at 24 h and Pt loaded via impregnation showed conversion and selectivity about 75 and 97%, respectively. Also, a maximum yield for this catalyst was achieved at 250 °C.

Full text of DOI:10.1016/j.poly.2019.01.056

Accepted Manuscript
Catalytic performance of nanostructured Pt/ZSM-5 catalysts synthesized by ex-
tended Charnell’s method in hydroisomerization of n-pentane
N. Charchi Aghdam, M. Ejtemaei, S.M. Sharafi, A.A. Babaluo, A. Tavakoli, B.
Bayati
PII:
S0277-5387(19)30078-6
https://doi.org/10.1016/j.poly.2019.01.056
POLY 13729
DOI:
Reference:
To appear in:
Polyhedron
Received Date:
Revised Date:
Accepted Date:
6 September 2018
13 January 2019
23 January 2019
Please cite this article as: N. Charchi Aghdam, M. Ejtemaei, S.M. Sharafi, A.A. Babaluo, A. Tavakoli, B. Bayati,
Catalytic performance of nanostructured Pt/ZSM-5 catalysts synthesized by extended Charnell’s method in
hydroisomerization of n-pentane, Polyhedron (2019), doi: https://doi.org/10.1016/j.poly.2019.01.056
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Catalytic performance of nanostructured Pt/ZSM-5 catalysts
synthesized by extended Charnell’s method in hydroisomerization
of n-pentane
N. Charchi Aghdam a,b, M. Ejtemaei , S. M. Sharafi a,b, A. A. Babaluo a,b,, A. Tavakoli
a,b
a,b, B. Bayatib,c
a
Chemical Engineering Department, Sahand University of Technology, P.O. Box 51335/1996, Tabriz, Islamic
Republic of Iran
b
Nanostructure Materials Research Center (NMRC), Sahand University of Technology, P.O. Box 51335/1996,
Tabriz, Islamic Republic of Iran
c
Department of Chemical Engineering, Ilam University, Ilam, Islamic Republic of Iran

Corresponding author at: Chemical Engineering Department, Sahand University of Technology, P.O. Box
5
1335/1996, Tabriz, Islamic Republic of Iran. Tel.: +98 4133459081; fax: +98 411 3444355
E-mail address: a.babaluo@sut.ac.ir (A. A. Babaluo)
1

ABSTRACT
Na-ZSM-5 zeolite, synthesized by extended Charnell’s method was used to prepare
Pt/ZSM-5 catalyst for n-pentane hydroisomerization. The influence of ion exchange, alkaline
treatment and method of platinum incorporation on the Na-ZSM-5 zeolite was investigated. It
was found that the presence of Trien affected the physicochemical properties of the
synthesized catalyst. The synthesized catalysts showed high performance so that the sample
prepared in rotating mode at 24 h and Pt loaded via impregnation showed conversion and
selectivity about 75 and 97 %, respectively. Also, a maximum yield for this catalyst was
achieved at 250 ℃.
Keywords: ZSM-5, Extended Charnell’s method, Trien, Nanocatalyst, Hydroisomerization
2

1
. Introduction
Nowadays isomerization of light linear alkanes to iso-alkanes is the best way to
produce environment-friendly gasoline with favored combustion properties and higher
octane number [1,2]. The current industrial catalyst which has been mostly used in recent
years is Pt/Mordenite catalyst. However, the two-dimensional structure of Mordenite makes
it vulnerable in the case of coke formation as one of the main problems of isomerization
reactions [3,4]. Hereupon a great effort has been conducted to use the ZSM-5 zeolite as a
catalyst in the isomerization of normal Paraffin. ZSM-5 has widespread use in catalytic
processes due to unique properties such as intrinsic acidity, uniform pore size, high thermal
stability, and the unique capability to stabilize metal species [5,6]. The special pore size of
ZSM-5 prevents coke formation in its channels. Moreover, due to the very low rate of
carbonaceous deposits, it has a long catalytic life. Also, the three-dimensional structure of
the ZSM-5 zeolite active sites makes them easy to access [7,8].
Catalytic activity and acidic properties of ZSM-5 zeolite mainly depend on the Si/Al
ratio and an actual number of Al atoms [3]. It was reported that in order to achieve high
catalyst activity in the isomerization of n-pentane, Si/Al molar ratio must be low between
1
0–50 [9]. However, it is difficult to synthesize the ZSM-5 samples with low Si/Al ratios
(<25), high purity and without the presence of other phases [10]. With increasing the Al
content in zeolite structure, nucleation increases and zeolite with small particles is produced
and consequently, control of zeolite properties becomes more difficult. The reason is that the
3
+
nucleation rate increases with increasing the Al concentration in the reactant solution and it
results in small crystals. To overcome this problem Charnell developed a method to
synthesize large crystals of Al-rich ZSM-5 by adding chelating agents. Therefore, the
crystals grow to be large [11]. However, in Charnell’s method, triethanolamine plays both a
structure-directing role and a chelating role. Yokomori and Idaka improved Charnell’s
method and called this synthesis method the "extended" Charnell’s method or the "chelate
addition" method. In their method the chelating reagent and the templating reagent are
different. The addition of Trien (Triethylenetetramine) is important in this synthesis. Trien
3
+
acts as a chelation reagent of Al and also suppresses the formation of nuclei containing
3
+
Al leading to bigger crystals and better control of properties [12]. Consequently, a zeolite
with well-defined structure could be synthesized at low Si/Al ratio.
3

Notwithstanding chelating agents play an important role in the synthesis of zeolites;
few works report the development of chelation agent-based methods in the synthesis of
ZSM-5 zeolite. The synthesis parameters, such as temperature, time and mode of stirring
have been reported to influence the physicochemical and catalytic properties of zeolite
catalysts [5]. Karimi et al investigated some effective parameters on ZSM-5 synthesized with
chelate addition method [13]. But to the best of our knowledge, there are no investigations
reported in the open literature on the catalytic properties and performance of ZSM-5 zeolites
prepared by Charnell and extended Charnell methods and possibly that how zeolites with
high Al content may be affected by acid and alkaline treatments for catalytic applications.
In the present study, structural changes occurring in ZSM-5 synthesized by extended
Charnell’s method at different levels of converting the Na-ZSM-5 zeolite to Pt/ZSM-5
catalyst like ion exchange by acid treatment, alkaline treatment, and Pt incorporation were
investigated. The synthesized samples were characterized using X-ray diffraction (XRD),
Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM),
energy dispersive X-ray (EDX) and BET specific surface area techniques. Also, activity and
product selectivity of the synthesized Pt/ZSM-5 catalysts were evaluated in n-pentane
hydroisomerization.
2
. Experimental
.1. Catalyst synthesis and characterization
2
2
.1.1. Preparation of nano-crystalline Na-ZSM-5 and H-ZSM-5 zeolites
Nanostructure Na-ZSM-5 zeolite was synthesized following the procedure reported
earlier [13]. Briefly, solution A was prepared by adding Tetrapropylammonium hydroxide
and deionized water to Tetraethyl orthosilicate. Solution B was prepared from aluminum
isopropoxide and deionized water. Triethylenetetramine (Trien) and a mixture of NaOH and
deionized water were added to solution A. Then solution B was added to the mixture A under
continuous stirring. After completing the addition, the mixture was stirred for 2 h to obtain a
homogeneous mixture. The mixture was transferred to a stainless-steel autoclave including
Teflon holder and sealed autoclave. Crystallization was carried out at 175 ℃ for a given time
To evaluate the effect of autoclave rotation during crystallization, the crystallization was
studied in both stationary and rotating mode. When the synthesis was completed, the
.
4

autoclave was quenched and the products were filtered, washed with distilled water and dried
at 100 ℃.
The Na-type ZSM-5 (Na-ZSM-5) samples were obtained by calcination of the as-
synthesized samples at 550 ℃ to remove the template. The calcined Na-ZSM-5 samples were
converted to proton form by ion exchange with 1 mol/L NH Cl solution at room temperature
4
for 48 h. The solids were then separated by centrifugation and washed with deionized water
to remove chloride ions, dried at 110 ℃ and calcined at 540 ℃ for 4 h to obtain H-ZSM-5.
The notation used along the manuscript was ZS for the zeolites synthesized in stationary
mode and ZR for the zeolites synthesized in a rotating mode. Also, IX suffix was used for
samples converted to H-form by ion exchange.
2
.1.2. Alkaline treatment
.00 g of calcined Na-ZSM-5 was added to 100 ml of 0.1 mol/L NaOH aqueous
2
solution at 70 ℃ stirred for 8 h. The zeolite suspension was then cooled down immediately
using an ice-water bath, filtered, washed with deionized water to remove remaining NaOH
until neutral pH and dried at 120 ℃ for 12 h [14]. The prepared samples were denoted by AT-
ZS. The alkaline-treated sample was converted into the H-form by the conventional cation-
exchange procedure described in Section 2.1.1. The obtained sample in this step was labeled
as AT-ZS-72-IX.
2
.1.3. Synthesis of Pt/ZSM-5 nanocatalyst
Platinum incorporation was carried out by using two different techniques: impregnation
and ion-exchange methods. The nominal platinum concentration was 1 % g/g Pt. In the
impregnation method, 1.00 g of the H-ZSM-5 sample was impregnated with a minimum
amount of H PtCl solution required to wet the solid and stirred for 16 h at ambient
2
6
temperature. Then, the solvent was removed by evaporation at 100 ℃ for 12 h. Ion exchange
was performed by adding 1.00 g of the H-ZSM-5 sample in 0.0025 mol/L solution of
H PtCl . The mixture was kept under agitation at ambient temperature for 16 h, followed by
2
6
filtration, washing with deionized water and drying at 100 ℃. Finally, the samples prepared
℃
under different conditions were carefully calcined at a heating rate of 0.5 min to 450 ℃ in
flowing air and maintaining this temperature for 2 h. The Pt/ZSM-5 catalysts synthesized by
impregnation and ion exchange methods were labeled as PtM and PtX respectively.
2
.1.4. Characterization
5

X-ray powder diffraction was performed using the Siemens-D5000 diffractometer with
Cu-Kα radiation (30 kV, 40 mA) with a 2θ range of 5-50°. Scherrer equation can be used to
calculate the size of crystals and is as follows:
0
.9휆
퐷 =
(1)
훽cos 휃
Where D is the crystallite size (nm), λ is the radiation wavelength (0.15406 nm), θ is
the diffraction peak angle and β is the corrected half-width at half-maximum intensity
FWHM) [9]. The average crystal size was determined from the average of crystal size
(
between 2θ=23–25° and 2θ=7–8°. The Fourier transform infrared spectra were recorded on
Unicam 4000 FT-IR spectrometer using KBr pellets. The surface morphology of samples
was characterized by Quanta tax-QX2 scanning electron microscopy (SEM) equipped with an
energy dispersive X-ray analysis (EDX) attachment which could provide the catalyst surface
elemental composition information. The surface area of the samples was determined using
Quantachrome3000 physical adsorption series with a BET method. The catalysts were
outgassed at 200 ℃ prior to the surface area measurements.
2
.2. Catalytic tests for n-pentane isomerization
The isomerization of n-pentane was carried out in a stainless steel fixed-bed
continuous flow reactor (i.d. = 4.5 mm). As can be seen in Fig. 1, a hot air furnace placed
around the reactor provides the reaction temperature. The mixture of hydrogen and n-pentane
was fed into the reactor simultaneously. The liquid n-pentane was injected by a syringe pump
and the gas flow rates were controlled by electronic mass flow controllers. Prior to the
reaction the catalysts were pelletized, crushed and sieved in the mesh size of 20–40. For each
test 0.5 g of catalysts were loaded to the reactor and pre-treated in drying air flow (20
mL/min) at 400 ℃ for 2 h to remove the impurity absorbed on the surface of the catalyst.
Then the metallic phase was reduced by a hydrogen flow (20 mL/min) at 450 ℃ for 2 h.
Catalytic tests were performed at atmospheric pressure and the n-pentane conversion and
selectivity of i-pentane as a function of time on stream over Pt/ H-ZSM-5 catalysts was
investigated at 250 ℃ with weight hour space velocity (WHSV) of 0.3, and a H /n-C molar
2
5
ratio of 3. Also, the effect of temperature on the isomerization of n-pentane over ZS-24-PtM
catalysts was examined at the same reaction conditions. Reaction products were analyzed on-
line using a Teif-Gostar GC-2550 TG gas chromatograph equipped with a flame ionization
detector, capillary column (30 m, DB-1 column) and automatic valve for continuous analysis.
6

The reactor effluent stream was passed through a heated line to reach the automatic valve for
analysis. Repeated experiments confirmed that the data obtained were closely reproducible.
The conversion and selectivity of the products were calculated based on the carbon number of
the components as defined by formulas given below
푐표푛푠푢푚푒푑 푛­푝푒푛푡푎푛푒
푐표푛푣푒푟푠푖표푛 (%) =
(2)
(3)
푖푛푙푒푡 푛­푝푒푛푡푎푛푒
푖­푝푒푛푡푎푛푒
푠푒푙푒푐푡푖푣푖푡푦(%) =
× 100
푐표푛푠푢푚푒푑 푛­푝푒푛푡푎푛푒
(Fig. 1)
3
3
3
. Results and discussion
.1. Characterization results
.1.1. Influence of ion exchange in different autoclave modes
To investigate the effect of different autoclave modes on ion exchange, ZS-72 and ZR-
2 samples were synthesized at static and rotatory modes respectively. The XRD patterns of
7
these samples are shown in Fig. 2. It is evident that both samples have a typical MFI structure
with high phase purity, but ZS-72 sample has more crystallinity. The physicochemical
properties of all the samples at different steps are summarized in Table. 1. The average
crystallite size obtained from the Scherrer equation showed a slight decrease in ZR-72 sample
compared to ZS-72 sample.
(Fig. 2)
Comparing the Si/Al molar ratio of two samples revealed that this ratio decreased
dramatically from 92 in ZS-72 to 25.5 in ZR-72 implying that rotation of autoclave during the
crystallization time facilitated the entrance of aluminum atoms in zeolite structure (Table. 1).
It is well-known that by entering the aluminum in the structure of zeolite, the structure is
dissipated and consequently crystallinity is reduced. Also, as can be seen in Table 1, it led to
a decrease in the size of zeolite crystals. These results were also confirmed by SEM
micrographs in Fig. 3. So that, ZS-72 sample had a well-defined cubic structure with twinned
morphology while in the case of ZR-72 many small and large amorphous particles could be
seen. In addition, in the static mode, larger particles were formed which made the control of
7

their properties easier and led to twinned morphology with high crystallinity. Meanwhile,
these larger particles caused to lower specific surface area of ZS-72 sample compared to
other as shown in Table. 1.
(Table. 1)
Fig. 4 shows the FTIR spectra of the synthesized samples. The presence of the ZSM-5
-1
zeolite in both samples is indicated by the infrared band at 545 cm , which has been assigned
to the five-membered ring of the pentasil zeolite structure. The bands near 1080 (internal
asymmetric stretch), 790 (external symmetric stretch), 540 (double ring vibration) and 450
-1
cm (T-O bend) in both samples are the signature of ZSM-5 phase FTIR [10]. These patterns
also confirmed the higher crystallinity of ZS-72 compared to that of ZR-72 sample due to its
higher pick intensity.
(
Fig. 3)
Fig. 4)
(
In catalytic applications, Na-form zeolites are usually converted to H-form using ion
exchanging of the zeolite in acidic solutions to enhance their acidic properties. In general, this
treatment leads to extraction of Al from the framework and increase in the Si/Al ratio,
consequently. The created extra-framework aluminum species can act as Lewis acid centers
and benefit certain catalytic applications especially in the isomerization of n-pentane.
Contrary to published results in the literature, as can be seen in Table. 1, ion exchange of ZS-
7
2 and ZR-72 samples led to a decrement in the Si/Al molar ratio. The reason for such a
behavior was probably due to the presence of Trien and its effect on Al atoms in the structure
3
+
of the zeolite. Trien acts as a chelation reagent of Al as a kind of mineralizer and affected
3
+
the nuclei containing Al [12]. Meanwhile, Al is more difficult to extract. This fact can be
explained by the negative charge associated with Al tetrahedra in the zeolite framework,
hindering its extraction of Al through hydrolysis of Si-O-Al bonds by negatively charged
3
+
hydroxyl groups [6]. Consequently, because of the strong position of Al , acid treatment
leads to framework Si extraction, referred to desilication in the structure of zeolite alongside
dealumination.
(Fig. 5)
8

As is evident in Fig. 5 removal of Si from zeolite structure influenced its morphology
and crystallinity. These changes were not very significant in ZS-72-IX sample but in the case
of ZR-72-IX sample, it was more striking. This behavior could be attributed to the lower
Si/Al ratio and consequently higher Al content of ZR-72. The higher amount of Al makes
zeolite structure unstable. The small particles existed in ZR-72 (Fig. 3b), due to the formation
of amorphous silica species in the hydrothermal synthesis, disappeared after ion exchange
and the surface of the particles also became rough. The disappearance of small particles
during the desilication further confirmed that these particles should be amorphous silica. This
also is based on the results revealed by the XRD analysis. XRD pattern of ZR-72-IX and ZS-
7
2-IX samples showed that desilication during the ion exchanging might lead to a decrement
in crystallinity. Additionally, some impurity peaks that were present in ZS-72 and ZR-72, had
disappeared in ZS-72-IX and ZR-72-IX probably due to the removal of Si from the structure
of the zeolite. The average crystallite size of both samples after ion exchange are listed in
Table. 1. It was observed that the average crystallite size decreased after ion exchange in both
samples indicating that treatment with acid not only increased the acid strength but also
improved the diffusion properties.
The results illustrated that the rotation of autoclave during crystallization decreased
the Si/Al ratio which consequently increased the acidity. On the other hand, it decreased the
crystallinity of the zeolite structure. In order to overcome this problem and maintain the
desired crystallinity in the low Si/Al ratios, the ZR-24 sample was synthesized in the rotary
mode but for short crystallization time. Since by increasing the crystallization time, more Al
entered the structure, the low Si/Al ratio was balanced by high crystallinity. As it was
expected ZR-24 sample was
a typical twin hexahedron with perfect crystalline surfaces and highly ordered
structures (Fig. 6a-b) with high Al content simultaneously (Si/Al ~ 31). Also because of the
higher crystallinity of this sample compared to ZR-72, it had a higher specific surface area
(Table. 1). SEM images of ZR-24 after ion exchange are shown in Figure 6c. Ion exchange
affected this sample to some extent, but the relatively complete crystalline structure of the
zeolite was preserved.
(Fig. 6)
Also, the higher specific surface area of this sample in comparison with two other
samples can be attributed to smaller particle size and a well-defined crystalline structure
9

(Table. 1). It can be said that with increasing the Si/Al ratio due to the lower content of Al,
the effect of ion exchange on the structure of zeolite decreased. Also, comparing the ZS-72-
IX, ZR-72-IX, and ZR-24-IX samples showed that the higher the original Si/Al ratio of the
zeolite was, the greater the number of lost Si species. Moreover, the higher decline in Si/Al
ratio illustrated that Si species were easier to remove from high Si content ZSM-5 zeolites
which is in agreement with previous works over ZSM-5 [15].
3
.1.2. Effect of alkali treatment
ZS-72 sample was used to investigate the effect of alkaline treatment on the ZSM-5
zeolite synthesized in the presence of Trien. The XRD patterns of alkali-treated ZSM-5
zeolites are presented in Fig. 2. The intensity of most of the peaks was drastically decreased,
which can be explained with a decrement in crystallinity by removal of Si from the
framework. MFI major peaks in FTIR patent of treated zeolite showed that notwithstanding
the severe changes in the structure, intrinsic crystalline structure of ZSM-5 remained but with
-1
lower crystallinity. This was shown by the presence of the infrared band at 550 cm , assigned
to the five-membered ring of the pentasil zeolite structure (Fig. 4). As shown in Fig. 7a-b,
alkaline treatment of the Na-ZSM-5 zeolite leads to the destruction of zeolite structure.
Several large pores and holes appeared on the zeolite's surface. A part of the framework had
also collapsed. Si/Al ratio of the sample was decreased drastically from 92 to 5.2 indicating
that deep desilication occurred and it destroyed the zeolite framework. Average crystallite
size of sample decreased from 53.6 to 45.18 after treatment. In general, the application of the
alkaline treatment is discussed as a promising alternative to increasing porosity, [16] but
contrary to what was expected as it is reported in Table. 1, specific surface area of AT-ZS
sample showed drastic decrement. This behavior can be attributed to the structure collapse of
AT-ZS. This result illustrates that crystallinity has an important role in the BET surface area
of zeolite [5]. The high surface area of zeolites is primarily caused by a network of uniform
micropores that provides shape selectivity opportunities [16]. It is worth noting that drastic
changes in zeolite structure may be due to severe condition applied in this work. Treatment in
the alkaline medium for a longer time and at higher concentration results in a higher degree
of Si removal and consequently a decrement in crystallinity of zeolite. The decrease in the
BET surface area after alkaline treatment also reported by Groen et al. They reported that
BET surface area of FER zeolite decreased because of longer time and a higher concentration
of NaOH led to a higher degree of Si removal. Also, they reported that a certain decrease of
1
0

the characteristic reflections was observed in all the zeolite examined but the main structure
of zeolite preserved [17].
Fig. 7c gives SEM image of AT-ZS sample after ion exchanging which denoted as AT-
ZS-IX. It can be seen that ion exchange led to the disappearance of edges and the structure of
the sample became spongy. In addition, the particles became small upon the ion exchange.
Likewise, the average crystallite size of the sample decreased after ion exchanging from
4
5.18 to 35.57.
(Fig. 7)
3
.1.3. Effect of platinum incorporation method
The method used in the incorporation of metal has a strong effect on metal particle
size and structure [9]. Various techniques are available to introduce metals into zeolites, the
most widely used being impregnation and ion exchange [18-20]. XRD patterns of the ZS-72-
PtM and ZS-72-PtX samples are shown in Fig. 2. The platinum should be appeared at 2θ
=
39.76 and, 46.24; however, a noise accompanied at these positions in addition to the low
platinum concentration; therefore, no lattice fringes were observed in the figure from most of
the nanoparticles, presumably because of their small sizes and random orientations. Also,
these patterns showed that PtM had higher peak intensities and consequently higher
crystallinity. It can be said that incorporation of Pt by impregnation method has a lower effect
on the crystalline structure of zeolite compared to ion exchange method. SEM micrographs of
both samples are shown in Figure 8. In the case of PtM sample, some bright spots were seen
attributed to Pt particles. EDS analysis confirmed that these luminous spots were Pt particles
which were not present in PtX sample. It was reported that in the impregnation, the relatively
large metal particles were usually obtained while ion exchange resulted in a high initial
dispersion and smaller particles which agree well with our results [18]. From obtained results,
it can be concluded that higher dispersion of Pt was reached by ion exchange method whereas
in impregnation method Pt particles remained large as they are explicit in SEM images.
Babůrek et al also reported that Pt dispersion affected by Pt loading method and ion exchange
method led to high Pt dispersion [21].
(Fig. 8)
1
1

3
.1.4. Catalytic isomerization of n-pentane
The n-pentane hydroisomerization experiments were carried out to compare the
performance of synthesized catalysts. The conversion of n-pentane and selectivity toward i-
pentane over these catalysts are shown in Fig. 9 and 10 respectively. Obviously, the decrease
of conversion and selectivity during the reaction time was virtually negligible over all the
catalysts. Also, it can be seen that the conversions of n-pentane and selectivity of i-pentane
over the different catalysts showed the great difference. The higher conversion of n-pentane
was obtained over ZR-24-PtM catalyst. The reason for such a behavior is higher intrinsic
acidity and higher crystallinity simultaneously. Acid strength, crystal/particle size and their
morphology influence acidic zeolite activity and product selectivity [14,22]. It is generally
acknowledged that the density of acid sites and the acid strength decreases with the
increasing Si/Al molar ratios for ZSM-5 zeolites. On the other hand, with increasing Al atoms
in structure, phase purity and crystallinity of ZSM-5 decreases. Comparing the performance
of ZR-24-PtM and ZS-72-PtM catalysts showed that the first catalyst had higher conversion
due to the lower Si/Al ratio and higher acidity. Also, smaller crystals offered shorter diffusion
paths for the reactants and products and possessed a higher number of external active sites
[22]. Comparing the performance of ZS-72-PtX and ZR-72-PtX shows that rotation during
the crystallization time had a desirable effect on the catalytic performance of ZSM-5 catalyst.
The better performance of the ZR-72-PtX catalyst could be explained in terms of the higher
acidity and a more suitable specific surface area of this system. In fact, as mentioned above
because of lower Si/Al ratio of ZR-72 and also higher BET specific surface area, the sample
prepared in rotating mode showed higher conversion and selectivity compared to ZS-72
sample prepared in static mode. The less selectivity to i-pentane in the ZR-PtX catalyst can
also be related to this higher strength of acid sites, which led to an increase in cracking
products.
The significant changes in the acidity and the average crystallite size of ZSM-5 caused
by alkali treatment may influence the catalytic activity. Thus, the hydroisomerization
performance of alkaline treated Pt/ZSM-5 catalyst was examined, and the results are
presented in Fig. 9 and 10. It is clearly observed that the alkaline treatment in the HZSM-5
zeolite catalyst synthesized in the presence of Trien enhances the n-pentane conversion
remarkably. As can be seen in Table. 1 and mentioned above, the average crystallite size of
ZS-72 sample reduced from 53.6 nm to 45.18 after alkali treatment. Smaller crystallite size
suggested shorter diffusion paths, better performance of catalyst and higher conversion. Also,
1
2

alkali treatment caused severe desilication and consequently drastic decline in the Si/Al molar
ratio of ZS-72 sample from 92 to 5.5. The enhanced activity can be attributed to extra-
framework Al created during the alkaline treatment and desilication. Song et al whose
alkaline treatment method was used in this work reported that the acidity of ZSM-5 was
increased after alkaline treatment and their results showed that extra-framework Al formed in
the alkali treatment and new stronger acid sites originated from treated ZSM-5 [14].
Moreover, some other papers reported the formation of strength acid sites with NaOH
treatment [23]. Though there was a drastic change in the structure of ZSM-5, the increasing
conversion with simultaneous high selectivity clearly showed that alkaline treatment was an
effective method to improve isomer yield for ZSM-5 zeolite synthesized in the presence of
Trien.
Comparing the activity results obtained from ZS-PtM and ZS-PtX samples showed
that the conversion of n-pentane and selectivity of i-pentane for the catalysts prepared by
impregnation method was greater than for the ion-exchanged counterparts. Based on the
literature it is clearly observed that the preparation technique including ion-exchange and
impregnation methods hardly affects the state of platinum in ZSM-5 [24]. These spectacular
changes in conversion and selectivity are evident from Fig. 9 and 10. The Pt impregnated
catalyst showed 22 times higher conversion compared to Pt ion exchanged one which has
increased from 2.55 % to 50 % after 1 hour and there is a 37.5 % increase in i-pentane
selectivity. Additionally, comparing the other results showed that all the catalysts in which Pt
introduced by the impregnation method, showed higher conversion and selectivity compared
to the one prepared by ion exchanging method. However, the selectivity for i-pentane over all
the catalysts prepared by this method was identical and about 100 %.
Pt is assumed to play the dominant role in the activation of hydrogen which helps in
the (de) hydrogenation reaction steps. This is important on the one hand in the proton transfer
and formation and reactions of carbenium ions, and in suppressed deactivation of the
catalysts, on the other hand. Also, according to Babůrek et al, higher dispersion of Pt
enhanced the cracking reactions compared to the isomerization reaction and consequently
selectivity was decreased over Pt ion-exchanged sample [21]. A similar effect was reported
by Rodr´ıguez-Reinoso et al. They reported high activity of small Pt clusters in the C-C
bonds splitting. The dispersion effect is probably of more complicated nature, as also the Pt-
H distances should be considered, which differs from large Pt particles located partially on
1
3

the surface of the zeolite to small ones embedded predominantly inside the zeolitic cavities
25].
[
The catalysts, in terms of activity, can be listed as follow: ZR-24-PtM, AT-ZS-72-
PtM, ZS-PtM, ZR-72-PtX, and ZS-72-PtX. The ZR-24-PtM sample due to higher acidity and
crystallinity and also a method of Pt introduction exhibited a far superior activity. The effect
of reaction temperature over the most active catalyst was investigated and the results are
shown in Fig. 11. It can be seen that the conversion of n-pentane increased with temperature
while the increasing of reaction temperature resulted in a substantial decrease of the
selectivity to i-pentane. This behavior can be explained in terms of the exothermicity of the
reaction. Since the reaction is equilibrium-limited, low temperature favors high
concentrations of branched isomers and, furthermore, cracking products decrease with low
reaction temperatures [26,27]. The highest yield of i-pentane over ZR-24-PtM was obtained
at 250 ℃.
(Fig. 9)
(
Fig. 10)
Fig. 11)
(
3
.2. Our results compared with literature
As already mentioned, in recent years ZSM-5 zeolite has attracted more attention due
to its unique characteristics for use as an acidic function in the bifunctional catalyst in the
isomerization of linear alkanes. It is noteworthy that a strict comparison of our results with
the results reported in the literature cannot be made, because of the difference in the amount
of catalyst, weight percent of platinum in the catalyst, reactor configuration and reaction
conditions. However, we have attempted to compare our data with comparable results from
literature and results are summarized in Table. 2. It can be seen that most of the reported
catalysts were active at 250 ℃ and higher temperatures and almost no data are reported below
this temperature. In general, zeolite catalysts are active at higher temperatures, which is not
suitable from the process and economic point of view. Whiles ZR-72-PtM synthesized in the
present study, has the i-pentane yield of about 60 %. In fact, it can be claimed that it was a
first ZSM-5 based catalyst which is highly active in n-pentane isomerization at 220 ℃. Also,
it is clear that the synthesized catalyst showed better performance compared to the others at
higher temperatures. However, it was the first study on the catalytic use of ZSM-5
1
4

synthesized in presence of Trien by extended Charnell’s method and its performance can be
improved with further investigations. Research Octane Number (RON) of product stream is
also calculated and reported in Table. 2. As seen, the RON of products is in a suitable range.
It is important to mention that in the industrial isomerization the reactor outlet stream enters
the adsorption column. These adsorption columns separate branched molecules from
unconverted linear molecules that are transferred to the catalyst section in a recycle step
which leads to higher research octane number.
4
. Conclusion
Presence of Trien led to decrease in Si/Al ratio of HZSM-5 obtained from ion
exchange of Na-ZSM-5 zeolites synthesized by extended Charnell’s method. Influence of ion
exchange on the structure of zeolite affected by the mode of autoclave stirring and synthesis
time as rotation of autoclave during the crystallization time facilities the entrance of Al atoms
in zeolite structure and consequently led to lower Si/Al ratio. The new Pt/ZSM-5
nanocatalysts exhibited promising catalytic activity for the production of i-pentane from
hydroisomerization of n-pentane. High n-pentane conversion and i-pentane selectivity of the
synthesized Pt/ZSM-5 nanocatalyst showed that extended Charnell’s method is a very useful
method for preparation of zeolite for the catalytic application. Although, alkaline treatment
causes a severe desilication and change in zeolite structure it significantly improved catalytic
performance of Pt/ZSM-5 which can be attributed to the smaller crystallite size and lower
Si/Al ratio and the higher acidity of the prepared sample. Impregnation method resulted in
very high n-pentane conversion and i-pentane selectivity. By contrary, ion exchange method
led to lower conversion and selectivity which could be related to higher dispersion of Pt. In
this case, higher dispersion of Pt enhanced the cracking reactions and decreased selectivity
and n-pentane conversion. Among the tested catalysts ZR-24-PtM showed the highest
conversion of 75 % and selectivity of 97.6 % at 250 ℃ where a short crystallization time (24
h) in the rotary mode was applied and the introduction of Pt was done by impregnation
method.
Acknowledgments
The Authors wish to thank Sahand University of Technology (SUT) for the financial
support of this work. Also, thank co-workers and technical staff in the department of
chemical engineering and nanostructure materials research center (NMRC) of SUT for their
help during various stages of this work.
1
5

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1
8

Table. 1. Average crystallite size, specific surface area and Si/Al molar ratio of different samples.
Average
BET specific
Sample
ZS-72
crystallite size surface area
Si/Al
92
53.6
51.08
26.48
44.0
33.5
-
236.05
ZR-72
336.07
25.5
31
ZR-24
342.5
ZS-72-IX
ZR-72-IX
ZR-24-IX
AT-ZS
-
42
-
20
-
52.53
-
24
45.18
35.57
5. 5
3.2
AT-ZS-IX
1
9

Table. 2. Comparison of the results obtained in this study with the literature on n-pentane
isomerization in the presence of Pt/ZSM-5 catalyst
º
T( C)
200
220
250
300
Ref.
X1
%
-
S2 Y3 RON
X
%
-
S
%
-
Y
RON
X
%
10
-
S
%
60
-
Y
RON
X
S
Y
RON
%
-
%
-
%
-
%
6
%
40
%
30
%
12
[27]
[19]
[28]
[25]
[29]
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
22.1* 96.4* 21.3*
-
10
-
93 9.3
-
-
-
-
-
77.2 93.9 72.5
80
40
75
66
63
50
50.4
20
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
-
50
95 47.5
75
56.25
60
-
[8]
-
-
-
-
-
-
68 98.6 67
91.6
-
This
20 100 20
68
60 98.8 59.28
80
75 97.6 73.2 84.8 71.2* 89* 63.36*
84*
work
1
Conversion
Selectivity
Yield
2
3
*
º
2
80 C
2
0

Fig.1. Process flow diagram of experimental set-up for n-pentane isomerization
2
1

0
51
33
13
0
11
0
0
20
3
Fig. 2. XRD patterns of the samples: (a) ZR-72, (b) ZR-72-IX, (c) ZS-72-PtX, (d) ZS-72-PtM, (e) ZS-
2, (f) ZS-72-IX, (g) AT-ZS-72 and (h) AT-ZS-72-IX.
7
2
2

Fig. 3. SEM micrographs of Na-ZSM-5 samples. (a) ZS-72 (b) ZR-72
2
3

(e)
(d)
(c)
(b)
(
4
000
3400
2800
2200
1600
1000
400
Wave number (cm-1)
Fig. 4. FTIR spectra of ZSM-5 samples: (a) ZR-72, (b) ZS-72, (c) ZS-72-IX, (d) AT-ZS-72, (e) AT-
ZS-72-IX
2
4

Fig. 5. SEM micrographs of ion-exchanged samples (a) ZS-72-IX and (b) ZR-72-IX.
2
5

Fig. 6. SEM micrographs of (a-b) ZR-24 and (c) ZR-24-IX samples.
2
6

Fig. 7. SEM micrographs of alkaline treated and ion-exchanged alkaline treated ZSM-5 samples (a-b)
AT-ZS-72 and (c) AT-ZS-72-IX.
2
7

Fig. 8. SEM micrographs of Pt incorporated ZSM-5 samples (a-b) ZS-72-PtM and (c-d) ZS-72-PtX.
2
8

1
00
ZS-72-PtM
ZR-72-PtX
ZS-72-PtX
AT-ZS-72-PtM
90
80
70
60
50
40
30
20
10
0
ZR-24-PtM
0
10
20
30
40
50
60
TOS (min)
Fig. 9. Conversion of n-pentane as a function of TOS over various Pt/ZSM-5 catalysts
2
9