Effect of hydrohalogenation of PtRe/H-ZSM-5 for cyclohexene conversion

Article abstract of DOI:10.1016/S1872-2067(11)60372-0

Extended use of supported Pt catalysts causes thermal migration of Pt particles to form large agglomerates, thus decreasing the catalytic activity. The combination of Pt with Re protects Pt against migration. In addition, Cl^- injection into the reactor assists Pt particles redispersion to prolong catalyst life. In this work, a PtRe/H-ZSM-5 catalyst was treated with either HCl or HF to investigate their role in activating or deactivating the catalyst. The conversion exceeded 90% in the whole temperature range with the PtRe/H-ZSM-5(HCl) catalyst, and its activity for the direct isomerization of cyclohexene to methylcyclopentenes (MCPEs) was the lowest but the activity for the hydrogenation of the MCPEs to methylcyclopentane was the highest. The reactivities of MCPEs and cyclohexadienes on the catalysts were similar because both are dehydrogenation reactions. Benzene production was significantly higher on the hydrochlorinated catalyst than on the other catalysts, and its hydrocracking activity was the lowest, which is a good characteristic for processing catalysts where cracking is undesired.

Full text of DOI:10.1016/S1872-2067(11)60372-0

CHINESE JOURNAL OF CATALYSIS
Volume 33, Issue 4, 2012
Online English edition of the Chinese language journal
Cite this article as: Chin. J. Catal., 2012, 33: 697–705.
ARTICLE
Effect of Hydrohalogenation of PtRe/H-ZSM-5 for
Cyclohexene Conversion
Sameh M. K. ABOUL-FOTOUH1,*, Noha A. K. ABOUL-GHEIT2
1Chemistry Department, Faculty of Education, Ain-Shams University, Cairo 11757, Egypt
2Process Development Department, Egyptian Petroleum Research Institute, Cairo 11787, Egypt
Abstract: Extended use of supported Pt catalysts causes thermal migration of Pt particles to form large agglomerates, thus decreasing the
catalytic activity. The combination of Pt with Re protects Pt against migration. In addition, Clꢀ injection into the reactor assists Pt particles
redispersion to prolong catalyst life. In this work, a PtRe/H-ZSM-5 catalyst was treated with either HCl or HF to investigate their role in
activating or deactivating the catalyst. The conversion exceeded 90% in the whole temperature range with the PtRe/H-ZSM-5(HCl) catalyst,
and its activity for the direct isomerization of cyclohexene to methylcyclopentenes (MCPEs) was the lowest but the activity for the hy-
drogenation of the MCPEs to methylcyclopentane was the highest. The reactivities of MCPEs and cyclohexadienes on the catalysts were
similar because both are dehydrogenation reactions. Benzene production was significantly higher on the hydrochlorinated catalyst than on
the other catalysts, and its hydrocracking activity was the lowest, which is a good characteristic for processing catalysts where cracking is
undesired.
Key words: platinum, rhenium, cyclohexene, hydroconversion, hydrofluorination, hydrochlorination, H-ZSM-5 zeolite
Cyclohexene (CHE) conversion on bifunctional catalysts
goes through a number of reactions that each requires a dif-
ferent active site type. Hence, CHE is frequently selected as a
probe molecule for characterizing catalyst preparation [1–4].
The experimental conditions, temperature and presence or
absence of hydrogen and the pressure of hydrogen, have im-
portant effects on the catalytic behavior. In the absence of
hydrogen, disproportionation [5,6] and dehydrogenation [7,8]
reactions take place. The former reaction gives benzene and
cyclohexane (CHA) and the reaction is hydrogen transfer.
When the reaction is carried out in the presence of metal
catalysts, CHE behaves as both hydrogen donor and acceptor.
If the metal is supported on Al2O3 [9], the acid sites are weakly
Lewis acid and not strong enough to promote carbonium ion
formation, and hence isomerization and cracking reactions do
not occur (monofunctional catalyst). The use of the H-form of
zeolites, which possess strongly acidic Brönsted and Lewis
sites, can promote CHE isomerization to a mixture of me-
thylcyclopentenes (MCPEs) and cracking to lower molecular
mass hydrocarbons and the reaction scheme is more compli-
cated (bifunctional catalyst). However, the treatment of the
cation-exchanged zeolites with aqueous HCl or HF acid leads
to loss of cations and partial dealumination of the zeolite
framework.
Isomerization of CHE over very pure alumina with and
without Cl– ions has been studied by Ozimek et al. [10,11].
Introducing Cl– ions into aluminate aluminas in different ways
causes Brönsted acid sites to appear, and both skeletal isom-
erzation and total conversions were much increased [12].
CHE hydrogenation and dehydrogenation are much faster
than benzene hydrogenation and CHA dehydrogenation on
noble metal catalysts. Aboul-Gheit and coworkers have studied
aromatic hydrogenation [13], hydroconversion of n-paraffins
in light naphtha [14], cyclohexane dehydrogenation [15] and
hydroisomerization, hydrocracking and dehydrocyclization of
n-paraffins [14,16] using Pt/Al2O3 catalysts promoted with
other metals (Rh, Ir, Re or U) and Cl– or F–. Rhenium is a
promoter for platinum in c atalysts for petroleum naphtha
hydroisomerization and reforming used for upgrading octane
numbers. Although Pt-Re catalysts have received enormous
attention since the discovery of the Re promotion effect by
Kluksdahl [17], monometallic Re catalysts have not received
Received 31 October 2011. Accepted 13 February 2012.
*Corresponding author. Tel: +202-227-57840; Fax: +202-225-81243; E-mail: saboulfotouh@hotmail.com
Copyright © 2012, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.
DOI: 10.1016/S1872-2067(11)60372-0

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
much consideration. Researches on Pt promoted by Re were
1.1.4 PtRe/H-ZSM-5(HF) catalyst
mainly concerned with catalyst protection to extend its lifetime.
However, these catalysts are also conventionally treated with
injected light chlorohydrocarbons to generate HCl, which
improves catalyst acidity and Pt dispersion. Since Kluksdahl
[17] showed that Re addition to platinum is required to give
superior petroleum naphtha reforming catalysts, the most
commercially successful bimetallic catalysts used in refineries
contain Pt-Re combinations [18–20].
CHE hydrogenation and dehydrogenation have been inves-
tigated using Ni, Pt, Rh, and Pd catalysts [6,8,21–28]. CHE
hydroconversion over catalysts containing 0.35 wt% of the
single metals Rh, Pt, Ir, Re, and their combinations on ꢀ-Al2O3
was carried out at temperatures of 50–250 oC [9]. The effect of
doping with 3.0 wt% of Cl– or F– was examined. The bimetallic
combinations gave more active catalysts than the monometallic
catalysts. In the present work, a Pt-Re loaded H-ZSM-5 cata-
lyst was treated with 3.0% HCl or HF to examine their effects
on CHE hydroconversion in a flow reactor operated in pres-
ence of hydrogen at 50–400 oC.
The PtRe/H-ZSM-5 catalyst prepared in section 1.1.2 was
doped with HF solution sufficient to give 3.0% HF. It was dried,
calcined, and then reduced as above.
All the catalysts basically retained the crystallinity of the
original H-ZSM-5 zeolite, which was shown by X-ray diffrac-
tion (XRD) analysis. The main diffraction peak in all forms
appeared at 2ꢀ of 23.0o. However, a very slight increase in the
diffraction peak at 2ꢀ of 10.884o for the PtRe/H-ZSM-5(HCl)
and PtRe/H-ZSM-5(HF) catalysts indicated a slight bulk
dealumination of the H-ZSM-5 zeolite.
1.2 Hydroconversion reactor and product analysis
The reactor was a silica glass tube containing 0.2 g catalyst
heated in an insulated wider silica tube jacket thermostated to
1 oC. Hydrogen gas was used as a carrier and simultaneously
as a reactant at a flow rate of 20 ml/min in all runs. The feed
(CHE) was introduced into the reactor using a H2 flow passed
o
into a closed jar thermostated at a fixed temperature of 28 C
1 Experimental
whereby the quantity of cyclohexene fed to the catalyst was
8.33 ꢁ 10–3 mol/h, i.e., the H2/CHE molar ratio was 6.4 in all
runs. The amount of CHE condensed between saturator and
reactor was considered in calculating the actual mass of CHE
flowing over the catalyst. The reaction temperatures were
1.1 Catalyst preparation
1.1.1 H-ZSM-5 catalyst
o
o
o
50–400 C, at 25 C decrements starting from 400 C down-
wards. The gaseous reaction effluent from the reactor was
injected into a Perkin-Elmer Autosystem XL gas chromato-
graph with a 15 m capillary column of Carbowax 20M bonded
in fused silica and analyzed using a FID detector and a Tur-
bochrom Navigator program. The GC column separated the
product effluent into hydrocracked components, MCPEs, me-
thylcyclopentane (MCPA), CHE, CHA, cyclohexadienes
(CHDEs) (1,3- and 1,4-), and aromatics (benzene, toluene, and
xylenes). The examination after a time on-stream of 180 min at
400 oC did not show any decrease in the activity of the catalyst.
Moreover, examination of the catalysts for carbon deposition
using temperature-programmed oxidation (TPO) in a differen-
tial scanning calorimeter (DSC) did not show any CO2 peak.
This catalyst was prepared by exchanging Na-ZSM-5 zeolite
with 0.6 mol/g NH4NO3 solution followed by drying at 110 oC
overnight and calcination at 550 oC for 4 h. The Na-zeolite was
provided by Süd Chemie, München, Germany. The catalyst had
a Si/Al molar ratio of 23.8, specific surface area of 440 m2/g,
and pore volume of 0.16 cm3/g. The H-form had a sodium
content < 0.002%.
1.1.2 PtRe/H-ZSM-5 catalyst
The PtRe/H-ZSM-5 catalyst was prepared by two successive
impregnations: first with an aqueous solution of hexachloro-
platinic acid followed by a second impregnation with ammo-
nium perrhenate to give 0.35 wt% Pt + 0.35 wt% Re in the
finished catalyst. The catalyst was dried and calcined as in
section 1.1.1 after each impregnation. It was reduced in a hy-
drogen flow in the catalytic reactor at 500 oC for 8 h before the
reaction.
1.3 Catalyst characterization
The temperature-programmed desorption (TPD) of pre-
sorbed ammonia on the acid sites of the zeolite supports was
carried out in a DSC using nitrogen as a purge gas according to
the procedure previously reported [29,30]. The NH3 desorption
enthalpy values from the higher temperature endothermic peak
was used to correlate the acid site density of the three
PtRe/H-ZSM-5 catalysts. The peak temperature was used to
correlate the strength of these acid sites.
1.1.3 PtRe/H-ZSM-5(HCl) catalyst
The PtRe/H-ZSM-5 catalyst prepared in section 1.1.2 was
doped with a solution containing the required quantity of HCl
acid to give 3.0% HCl. The catalyst was then dried, calcined,
and reduced as above.
The dispersion of PtRe in the catalysts (metal fraction ex-

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
posed) was determined by hydrogen chemisorption using a
2.1 Effect of hydrochlorination or hydrofluorination on
pulse technique similar to that used by Freel [31]. A known
mass of a calcined catalyst was heated in the chemisorption
furnace at 500 oC for 3 h in a flow of 50 ml/min of ultra-pure
hydrogen. The flow was then replaced by oxygen-free nitrogen
gas at 30 ml/min for 2 h at 500 oC (degassing). The furnace was
shut off and the catalyst was cooled to room temperature. Hy-
drogen was then pulsed into the nitrogen carrier till saturation,
i.e., until the appearance of hydrogen peaks corresponding to
complete non-adsorbed pulses. The hydrogen uptake was cal-
culated as hydrogen atoms adsorbed per total Pt-Re atoms on
the basis of 1:1 stoichiometry.
the PtRe/H-ZSM-5 catalyst
With 3.0% HCl doping, the acid site density (Table 1) was
enhanced (ꢁHd = 96.0 J/g) over that of the untreated catalyst
(ꢁHd = 90.9 J/g). Hydrofluorination gave a lower density of
acid sites (ꢁHd = 94.2 J/g) than hydrochlorination. However,
the effect of hydrofluorination was higher than hydrochlorina-
tion for enhancing the strength of the acid sites, with the am-
monia desorption peak temperature being in the order of
PtRe/H-ZSM-5(HF)
ZSM-5.
>
PtRe/H-ZSM-5(HCl)
>
PtRe/H-
The X-ray diffraction (XRD) patterns of the catalysts were
carried out using a Phillips X, Pert Diffractometer PW 1390 at
40 kV and 30 mA with Ni Filter and Cu Kꢁ radiation. The XRD
measurements were carried out up to 2ꢀ of 60o. The XRD
patterns obtained (not given here) for the catalysts showed
fairly similar intensities for the diffraction peaks. The main
crystalline peak appeared in all samples at 2ꢀ of 23.0o. The HCl
and HF doped samples showed slightly higher relative intensity
of the lowest 2ꢀ peak, indicating slight dealumination of the
zeolite support.
Treating PtRe/H-ZSM-5 with the acids affected Pt-Re dis-
persion (Table 1). With the HCl treatment, the metal dispersion
was increased over that using the HF treatment. The dispersion
order was PtRe/H-ZSM-5(HCl) > PtRe/H-ZSM-5(HF) >
PtRe/H-ZSM-5.
The comparison of the properties of the Re/H-ZSM-5 [32]
and PtRe/H-ZSM-5 catalysts (Table 1) before and after HCl or
HF treatments indicated that dispersion of either mono-metal
(Re) or bimetal (Pt-Re) in the H-ZSM-5 support was increased
after halogenation. Also, the acid site number (ꢁHd) and
strength (NH3 peak temperature) were increased by this
treatment. Hydrofluorination was significantly more effective
than hydrochlorination when the catalyst contained Re alone.
In case of the PtRe/H-ZSM-5 catalyst, the HF-treated case was
less significantly affected than the HCl-treated version for both
metals dispersion and acidic properties. Since HF is a stronger
acid than HCl, it can be assumed that the combination of Pt and
Re is not as good for metal dispersion and acidity than when Re
was loaded alone in the zeolite. Pt when present with Re seems
to have played a role in diminishing the spreading of the metal
on the surface of the zeolite as well as blocking of the acid sites.
This can be attributed to that the deposition of fluoro-silicon
and fluoro-aluminum debris was more than that of
chloro-aluminum debris.
2 Results and discussion
Light organic chlorohydrocarbons are normally injected into
industrial reactors used in the light naphtha hydroisomerization
process. This procedure is applied for catalyst chlorination with
commercial Pt/Al2O3 catalysts since these chlorohydrocarbons
produce HCl and small hydrocarbons at higher temperatures.
The HCl formed assists the redistribution of the platinum
crystallites and inhibits their growth into agglomerates, besides
enhancing the acidity of the catalyst. This treatment differs
from that normally applied in preparing chlorinated Pt/zeolite
catalysts where the support is first halogenated before im-
pregnating or exchanging with a Pt precursor. The use of the
hydrofluorination of noble metal-containing catalysts may be
an alternative for HCl doping because the Al–F bond is
stronger than the Al–Cl bond and the requirement for the con-
tinuous injection of a fluorohydrocarbon into the reactor is
therefore unnecessary.
The metal mainly used in light naphtha hydroisomerization
and heavy naphtha reforming catalysts is platinum. However,
another metal such as Ir, Re, Pd, Ti or Sn is normally combined
with Pt in the catalyst, mainly to improve the catalyst lifetime
by restricting Pt agglomeration, sulphur poisoning and deac-
tivation by carbon deposition. It is important to know the be-
havior of the promoter metal towards the presence or absence
of HCl or HF when the metal is supported on H-ZSM-5 zeolite.
In the present work, the doping of PtRe/H-ZSM-5 catalysts
with HCl or HF was investigated with the hydroconversion of
CHE.
In our recent work [33], pore volume distribution curves for
catalysts containing 2% Pt/H-ZSM-5 before and after doping
Table 1 NH3-TPD, and Re and Pt-Re dispersion by H2 chemisorption in
the Re/H-ZSM-5 and Pt-Re/H-ZSM-5 catalyst
Acid site strength distribution
Peak temperatureb Metal disper-
Catalyst
ꢁHa/(J/g)
(oC)
sion (%)
56.8
Re/H-ZSM-5c
90.9
99.6
105.3
90.9
96.0
94.2
380.1
392.3
415.7
376.1
390.3
395.9
Re/H-ZSM-5(HCl)c
Re/H-ZSM-5(HF)c
PtRe/H-ZSM-5
59.8
63.8
58.8
63.5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
60.8
aProportional to acid site number.
bProportional to acid site strength.
cReference 32.

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
with 4% HF for the alkylation of toluene with methanol were
carbons increased continuously with temperature.
shown. In the catalyst treated with 4% HF, the large pores in the
volume distribution curve (in the range of 2.7–5.5 nm) had
lower values than those of the untreated catalyst, while the
smaller pores (0.3–2.7 nm) has higher values. This was attrib-
uted to the filling of most of the large pores by the debris
formed from the leaching of Al and Si at 4% HF concentration.
Figure 1(b) exhibited a distinct variation where the MCPEs
comprises ~90% of the product between 50 and 175 oC, beyond
which these cycloolefins decline to < 20% at 400 oC. Another
major product was MCPA, the hydrogenation product of
MCPEs, which reached a maximum of ~50% at 300 oC beyond
o
which it declined to ~20% at 400 C. All other intermediates
were generally less.
2.2 Effect of HCl or HF doping of PtRe/H-ZSM-5 catalyst
on the conversion of cyclohexene
Figure 2 compares the CHE hydroconversion activities of
the PtRe-containing catalysts as a function of reaction tem-
perature. The highest activity was given by the HCl treated
catalyst in the whole reaction temperature range of 50–400 oC.
The lowest activity was given by the untreated catalyst at low
reaction temperatures. However, at 200–400 oC, the HF treated
catalyst became the least active.
The distribution of the CHE hydroconversion products using
the PtRe/H-ZSM-5, PtRe/H-ZSM-5(HCl) and PtRe/
H-ZSM-5(HF) catalysts are shown in Fig. 1. The general fea-
tures are a continuous decrease of CHE with reaction tem-
perature and an increase of CHA at 50–200 oC. At intermediate
temperatures, MCPEs, and MCPA passed through maxima at
2.3 Hydrogenation of CHE to CHA
o
300–325 C, beyond which they decreased. At higher tem-
peratures, CHE dehydrogenation to aromatics via CHDE for-
mation and hydrocracking to lower molecular mass hydro-
CHE is an unsaturated hydrocarbon molecule, and its initial
adsorption is largely through the most energetically favorable
donor ꢂ and acceptor ꢂ* orbitals with the appropriate metal
surface orbitals. On an atomically flat surface, CHE can form a
chemisorption state in which the C6-ring is oriented in a plane
parallel to the surface plane. The first reaction of chemisorbed
CHE, taking place at low reaction temperatures, is its exo-
thermic hydrogenation to CHA. Thus, the hydroconversion
activity of CHE (Fig. 2) can be attributed mainly to its hydro-
genation to CHA (Fig. 3) throughout the low temperature range
Cracked product
(a)
100
80
60
40
20
MCPA
CHA
MCPEs
CHE
CHDEs
Aromatics
o
(50–200 C). The activities of the catalysts are in the order of
PtRe/H-ZSM-5(HCl)
H-ZSM-5.
>
PtRe/H-ZSM-5(HF)
>
PtRe/
0
100
(b)
The hydrohalogenation treatment of the PtRe/H-ZSM-5
catalyst has greatly enhanced the reactivity for CHE hydro-
genation. The HCl treatment had more effect than the HF
treatment. Figure 3 shows that PtRe/H-ZSM-5 gave a maxi-
mum CHA production of 46.3% at 150 oC, while
PtRe/H-ZSM-5(HF) gave a maximum of 60.0% at 125 oC, and
80
60
40
20
Cracked product
MCPA
CHA
MCPEs
CHE
CHDEs
Aromatics
100
80
0
100
(c)
Cracked product
MCPA
MCPEs
CHDEs
CHA
80
60
40
20
0
CHE
Aromatics
60
40
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
20
PtRe/H-ZSM-5(HF)
50
100
150
200
250
300
350
400
0
50
Reaction temperature (oC)
100
150
Reaction temperature (oC)
Fig. 2. Hydroconversion of cyclohexene using different catalysts.
200
250
300
350
400
Fig. 1. Distribution of cyclohexene hydroconversion product with
PtRe/H-ZSM-5 (a), PtRe/H-ZSM-5(HCl) (b), and PtRe/H-ZSM-5(HF)
(c).

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
layer and decreases the basic Lewis sites. It also induces a
100
80
60
40
20
0
stronger “inductive effect” on the neighboring hydroxyl groups
on the zeolite surface, weakening the O–H bond, and rendering
the proton more acidic. Le Van Mao et al. [37] has doped
H-ZSM-5 zeolite with a low concentration of F– species and
found that its acidity was enhanced by forming new Brönsted
acid sites, in addition to the strengthening of some acid sites of
the parent zeolite.
Several authors have shown that acid sites in zeolites act as
hydrogenation sites, particularly at higher temperatures. Sano
et al. [38] reported that H-ZSM-5 zeolite becomes an active
hydrogenating catalyst and it converts 95% of ethylene to
ethane at a temperature of 535 oC. Because no metal impurity
in the zeolite was detected, Sano et al. concluded that the hy-
drogenation took place on the acidic sites of the zeolite. Jacobs
and co-workers [39] also concluded that the Brönsted acid sites
of zeolites act as active hydrogenation sites at higher tem-
peratures. Senger et al. [40] have provided a deeper insight into
this type of catalytic hydrogenation that proceeds without the
participation of a transition metal.
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
50
100
150
200
250
300
350
400
Reaction temperature (oC)
Fig. 3. Cyclohexane production using different catalysts.
PtRe/H-ZSM-5(HCl) exhibited the highest activity with ~90%
o
o
in the temperature range 50–175 C. Beyond 175 C, with a
further increase of temperature, the hydrogenation activity
declined since the reaction equilibrium was reversed from
exothermic hydrogenation towards endothermic dehydrogena-
tion. The enhancement of the hydrogenation activity with the
HF treatment can be attributed to: (1) A better dispersion of
PtRe bimetal in the support from 58.8% to 60.8% (Table 1); (2)
A decrease in the basic Lewis sites present originally in the
H-ZSM-5 zeolite via neutralization with HF; (3) A stronger
“inductive effect” on the neighboring hydroxyl groups due to
the influence of fluoride ions adsorbed on the surface of the
H-ZSM-5 support [12].
The higher activity of the HCl-treated zeolite compared with
the HF-treated one can be attributed to a physical effect rather
than a chemical effect. The higher strength of HF than HCl
allows it to attack Si in the zeolite, causing it to form debris of
fluoro-silicon species that deposit in the channels of the zeolite
that causes diffusion restriction that decreases the ability of
CHA to diffuse and react.
2.4 Isomerization of CHE to MCPEs and MCPA
On the other hand, HCl-treatment of PtRe/H-ZSM-5
changed the behavior of CHE hydrogenation (Fig. 3) such that
CHA in the product significantly exceeded those obtained with
the other two catalysts, particularly in the lower temperature
region. This implied a significant activity enhancement of the
PtRe bimetal on the surface of the PtRe/H-ZSM-5(HCl) cata-
lyst. Previous studies by Aboul-Gheit [34,35] showed that
benzene hydrogenation was completely inactive on a 0.6%
Re/Al2O3 catalyst under high hydrogen pressure and tempera-
Figure 4 depicts the yield of methylcyclopentenes (MCPEs)
as a function of reaction temperature. With the fluorinated and
untreated catalysts, these olefinic isomers increased with
temperature to reach a maximum, beyond which they decrease
with a further increase of temperature. However, with the HCl
treated catalyst, MCPEs increased slowly with temperature up
to 400 oC. The rates of formation of these isomers were in the
o
60
tures of 100–140 C. This is evidence that Re by itself at a
concentration of ~0.6% cannot act as a hydrogenation com-
ponent.
PtRe/H-ZSM-5
50
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
40
Recently, Aboul-Gheit et al. [36] doped unloaded H-ZSM-5
zeolite with HCl or HF and found that the acid site number,
strength and hydrogenation activity on the H-ZSM-5(HCl) and
H-ZSM-5(HF) catalysts were increased, and H-ZSM-5(HCl)
exhibited a significantly higher hydrogenation activity than
H-ZSM-5(HF). This was attributed to a larger increase in the
acid site number in the former catalyst although the acid site
strength was more enhanced in the HF-treated catalyst. In the
present study, the loading of H-ZSM-5 zeolite with PtRe re-
sulted in the same response towards HCl and HF treatments.
Arena et al. [12] showed that the Cl– ion adsorbed on the sur-
face of Al2O3 changes the electronic properties of the outer
30
20
10
0
50
100
150
200
250
300
350
400
Reaction temperature (oC)
Fig. 4. MCPEs yield as a function of reaction temperature using differ-
ent catalysts.

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
where I refers to the initial selectivity. Tjandra and Zaera [42]
60
50
40
30
20
10
0
and Jolly et al. [43] have agreed with Dwyer et al., which
indicates that the maximum of the isomerization from the
six-membered ring to the five-membered one is not a critical
catalyst specification. The reaction sequence in Eq. (1) shows
that a molecule of CHE rearranges itself on an acid site to
produce an equilibrium mixture of MCPEs, which are then
hydrogenated on a PtRe site to produce a MCPA molecule.
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
PtRe (+H2)
Acid site
cyclohexene
methycyclopentenes
methylcyclopentane (1)
Figure 5 shows that the hydrogenation activity of the
PtRe/H-ZSM-5(HCl) catalyst greatly exceeds that of the
HF-treated and untreated catalysts. The maximum conversion
50
100
150
200
250
300
350
400
Reaction temperature (oC)
o
of MCPA was 53.6% at 300 C on the PtRe/H-ZSM-5(HCl)
Fig. 5. MCPA conversion as a function of reaction temperature using
different catalysts.
catalyst as compared with 11.9% and 17.3% on
o
PtRe/H-ZSM-5(HF) and PtRe/H-ZSM-5 at 300 and 250 C,
respectively. These results are in agreement with the results of
the hydrogenation of CHE to CHA (Fig. 3), and both orders
were the same as the order of acid site number and strength as
well as the PtRe dispersion (Table 1). The effect of Cl– and F–
ions in the catalysts in decreasing the basic Lewis sites, a
stronger inductive effect on the neighboring O–H bond, and
rendering the proton more acidic should also contribute to
these results.
order of PtRe/H-ZSM-5
>
PtRe/H-ZSM-5(HF)
>
PtRe/H-ZSM-5(HCl). This order is the reverse of the order of
the hydrogenation activities (Fig. 3). In other words, the yield
of these MCPEs was higher with a less active hydrogenating
catalyst and vice versa. Because these MCPEs are unsaturated
isomers of CHE, they can be saturated to MCPA under the
same reaction conditions. Hence, the more active hydrogenat-
ing catalysts will produce lower yields of MCPEs (Fig. 4).
MCPEs products from the untreated and fluorinated catalysts
were relatively high and reached their maxima (38.3% and
44.0%, respectively) at 250 and 300 oC, respectively.
In the hydrogenation of MCPEs to MCPA (Fig. 5), the three
unsaturated isomers (MCPEs) of CHE are saturated giving
CHA isomer. The overall isomerization activities of the cata-
lysts equal the sum of unsaturated and saturated C5-ring con-
taining compounds, i.e., MCPEs + MCPA (Fig. 6). Dwyer et al.
[41] defined the relative rates of isomerization (monomolecular
reaction) and bimolecular hydrogen transfer as follows:
Isomerization/Hydrogen transfer =
2.5 Dehydrogenation of CHE to CHDEs
CHE first slowly dehydrogenates to a mixture of 1,3- and
1,4-CHDEs, then further dehydrogenates at higher rates with
o
increasing temperature from 275 to 400 C, since dehydroge-
nation is endothermic (Fig. 7, Scheme 1). Land et al. [44]
assumed that CHE at low temperatures is bonded to a noble
metal substrate as di-ꢃ bonded CHE. This may undergo con-
version analogous to olefin metathesis to form the
bis-(alkylidene) and 1,6-hexadialkylidene. Furthermore,
CHDEs dehydrogenate to benzene, which is the most stable
dehydrogenation product.
I(MCPEs + MCPA)/I(CHA + MCPA)
3
PtRe/H-ZSM-5
60
50
40
30
20
10
0
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
2
1
0
50
100
150
Reaction temperature (oC)
Fig. 7. Cyclohexadienes production using the PtRe/H-ZSM-5 catalysts.
200
250
300
350
400
50
100
150
200
250
300
350
400
Reaction temperature (oC)
Fig. 6. Total isomerization products using different catalysts.

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
30
25
20
15
10
5
C1-C6 fragments
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
CH3
CH3
CH3
CH3
Methylcyclopentenes
0
50
100
150
200
250
300
350
400
Reaction temperature (oC)
Cyclohexadienes
+
Fig. 8. Total dehydrogenation products using the PtRe/H-ZSM-5
catalysts.
CH3
CH3
CH3
diffusion restriction in case of the HF treated PtRe/H-ZSM-5,
due to fluoro-aluminium debris deposition in the zeolite
channels.
Acidic function
Scheme 1. Proposed scheme for cyclohexene hydroconversion.
2.7 Benzene production
Figure 7 shows the sum of 1,3- and 1,4-CHDEs in the
product as a function of reaction temperature. With the un-
treated PtRe/H-ZSM-5 and PtRe/H-ZSM-5(HF) catalysts, the
maximum CHDE amounts were 2.1%, and 2.7% at 300 and
Benzene is the most stable dehydrogenation product (final
product), and hence a more active dehydrogenating catalyst
will give a higher benzene production. Benzene production
was low using the untreated and HF-treated catalysts (Fig. 9),
and it was highest on the PtRe/H-ZSM-5(HCl) catalyst, it was
~2.0 at 50–150 oC and then increased continually beyond 150
oC to reach 15.3% at 400 oC. However, on the PtRe/H-ZSM-5
and PtRe/H-ZSM-5(HF) catalysts, benzene production in-
o
375 C, respectively. With the PtRe/H-ZSM-5(HCl) catalyst,
the CHDEs increased gradually with temperature up to 0.23%
at 400 oC.
2.6 Further dehydrogenation
o
creased slowly with temperature up to 400 C. Benzene pro-
Dehydrogenation intermediates 1,3- and 1,4-CHDEs dehy-
drogenate further on Pt (111) to benzene. Su et al. [45] and Tsai
et al. [46] found that CHE is converted to benzene on all Pt
planes. Gland et al. [47] observed that CHE dehydrogenation
on Pt (111) occur at some intermediate state before chemi-
sorbed benzene is generated. CHDEs with different strengths
of adsorption on the catalysts play a significant role in pro-
ducing the ultimate products. More strongly adsorbed CHDEs
retard aromatic formation. Peck and Koel [48] have assumed
that the chemisorption of CHDEs on Pt (111) is irreversible and
the chemisorbed monolayer dehydrogenates to form benzene
upon heating. In the present work, the CHDEs reached maxi-
mum values of 0.23%–2.66% in the product (Fig. 7), which
means that the CHDEs do not all remain chemisorbed on the
metal surface till completely dehydrogenated to benzene. Fig-
ure 8 shows that the PtRe/H-ZSM-5(HCl) catalyst gave sig-
nificantly more dehydrogenation products than the untreated
and HF treated catalysts, particularly at the higher temperatures.
At 400 oC, the dehydrogenation products of CHE amounted to
21.46% using the chlorinated PtRe/H-ZSM-5 catalyst, while
with the fluorinated catalyst, the total dehydrogenation prod-
ucts amounted to 9.78%. This can be attributed to the higher
duction on the catalysts was in the order of
PtRe/H-ZSM-5(HCl) > PtRe/H-ZSM-5 ꢄ PtRe/H-ZSM-5(HF).
This is the order of acidic site strength distribution and PtRe
dispersion in the catalysts.
2.8 Alkylated products
Benzene production was much higher on the
16
12
8
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
4
0
50
100
150
200
250
300
350
400
Reaction temperature (oC)
Fig. 9. Benzene yield using different catalysts.

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
70
PtRe/H-ZSM-5
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
6
5
4
3
2
1
60
50
40
30
20
10
0
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
50
100
150
200
250
300
350
400
Reaction temperature (oC)
50
100
150
200
250
300
350
400
Reaction temperature (oC)
Fig. 12. Hydrocracked products using the PtRe/H-ZSM-5 catalysts.
Fig. 10. Toluene in the product using the PtRe/H-ZSM-5 catalysts.
cyclohexene. The catalysts were in the order of PtRe/H-ZSM-5
> PtRe/H-ZSM-5 (HF) > PtRe/H-ZSM-5(HCl) (Fig. 12). In
contrast, unloaded H-ZSM-5 gave the following order of hy-
drocracking activities of H-ZSM-5(HF) > H-ZSM-5(HCl) >
H-ZSM-5 [36], which is the same order of the acid site strength.
This correlation showed that the bimetal combination, PtRe,
which is a conventional metal combination in commercial
industrial hydroisomerization catalysts, has a significant in-
fluence on inhibiting the hydrocracking activity of the catalysts.
However, in our previous work [49] using a monometallic
catalyst (Pt/H-ZSM-5), the same effect of hydrochlorination
and hydrofluorination on the hydrocracking of cyclohexene
PtRe/H-ZSM-5(HCl) catalyst (Fig. 9) than on the two other
catalysts. However, toluene and xylenes (Figs. 10 and 11,
respectively) were produced in lower quantities on this hy-
drochlorinated catalyst, indicating that more of the benzene
was produced during the alkylation (by methyl groups pro-
duced through cyclohexene hydrogenolysis).
The production of toluene and xylenes by alkylation is
normally dependent on catalyst acidity. Nevertheless, these
alkylaromatics were produced in larger amounts on the unhy-
drohalogenated catalyst, PtRe/H-ZSM-5. The catalysts treated
with HCl or HF possessed lower alkylation activities, although
their acidities were higher. This can be attributed to the absence
of chloro- and/or fluoro-alumino species deposited in the pores
of the PtRe/H-ZSM-5 catalyst.
was observed as Pt/H-ZSM-5
> Pt/H-ZSM-5(HF) >
Pt/H-ZSM-5(HCl). This indicates that the addition of Re to Pt
in H-ZSM-5 zeolite results in a significant alteration of the
hydrocracking activity. It is to be noted that the inhibition of
CHE hydrocracking by hydrohalogenation with HF or HCl
(particularly HCl) is a favorable effect since this reaction
causes a loss of the valuable hydrocarbon feedstock.
2.9 Hydrocracking reactions
Traditionally, the cracking of a probe compound (cumene) is
used to compare the strength of the acid sites in catalysts.
However, in the present work, it is found that the bimetal-
lic/zeolite catalyst (PtRe/H-ZSM-5) that was not treated with
either HCl or HF gave the highest hydrocracking activity for
3 Conclusions
The hydroconversion of CHE was excellent and was highest
in the whole temperature range with PtRe/H-ZSM-5(HCl). At
lower temperatures, ~90% of its activity was for the hydro-
genation of CHE to CHA. The activity was the lowest on the
untreated catalyst due to its lowest PtRe dispersion, and lowest
number and strength of acid sites. Dehydrogenation activity
was also the highest on the hydrochlorinated catalyst. The
lowest hydrocracking activity was attained with the hydro-
chlorinated catalyst, while the highest was attained with the
untreated catalyst. This study exhibited the advantage of the
HCl treated catalyst since hydrocracking is a loss of the hy-
drocarbon reactant.
PtRe/H-ZSM-5
PtRe/H-ZSM-5(HCl)
PtRe/H-ZSM-5(HF)
4
3
2
1
0
50
100
150
200
250
300
350
400
References
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1
Bartok M, Molnar A. Stereochemistry of Heterogeneous Metal
Fig. 11. Xylenes in the product using the PtRe/H-ZSM-5 catalysts.

Sameh M. K. ABOUL-FOTOUH et al. / Chinese Journal of Catalysis, 2012, 33: 697–705
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