Temperature effects in benzene alkylation with ethane into ethylbenzene over a PtH-MFI bifunctional catalyst

Article abstract of DOI:10.1016/j.apcata.2013.01.012

Benzene alkylation with ethane into ethylbenzene (EB) over a PtH-MFI bifunctional catalyst was thoroughly studied at six different temperatures between 290 and 490 °C. Our work demonstrates that temperature affects in a different way the thermodynamic equilibrium of two major reaction steps: (i) ethane dehydrogenation into ethene over Pt sites and (ii) benzene alkylation with ethene over acid sites. This, in turn, leads to significant differences in the product distribution and catalyst performance at different temperatures. It is shown that with increasing temperature ethane dehydrogenation is highly accelerated, while the EB formation rate increases in much lesser extent. As a consequence, high concentration of ethene is observed at high temperatures (450-490 °C), while the maximum concentrations of EB are very similar at all temperatures. Ethene, which is formed in excess at high temperatures, is converted via oligomerization and cracking steps into higher alkenes that alkylate benzene (or aromatic products). These alkylation reactions are followed by cyclization and dehydrogenation steps leading to formation of polyaromatics (coke precursors) and subsequent catalyst deactivation. On the other hand, at lower temperatures (290-410 °C) ethene reacts preferably with benzene forming EB. Hence, side reactions are suppressed and high EB selectivity and catalyst stability are observed at these temperatures. Based on the analysis of the catalyst activity, selectivity and stability, it is concluded that 370-410 °C can be considered as the optimum temperature range for the direct benzene alkylation with ethane into EB. Further improvement of the process may be achieved by selective removal of hydrogen from the reactor and by optimization of the ethane/benzene ratio in the feed.

Full text of DOI:10.1016/j.apcata.2013.01.012

Applied Catalysis A: General 454 (2013) 137–144
Contents lists available at SciVerse ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Temperature effects in benzene alkylation with ethane into
ethylbenzene over a PtH-MFI bifunctional catalyst
1
2
∗
Kae S. Wong , Tanya Vazhnova, Sean P. Rigby , Dmitry B. Lukyanov
Catalysis and Reaction Engineering Group, Department of Chemical Engineering, University of Bath, Claverton Down, Bath BA2 7AY, UK
a r t i c l e i n f o
a b s t r a c t
Article history:
Benzene alkylation with ethane into ethylbenzene (EB) over a PtH-MFI bifunctional catalyst was thor-
oughly studied at six different temperatures between 290 and 490 C. Our work demonstrates that
temperature affects in a different way the thermodynamic equilibrium of two major reaction steps:
Received 27 November 2012
Accepted 4 January 2013
Available online xxx
◦
(i) ethane dehydrogenation into ethene over Pt sites and (ii) benzene alkylation with ethene over acid
sites. This, in turn, leads to significant differences in the product distribution and catalyst performance at
different temperatures. It is shown that with increasing temperature ethane dehydrogenation is highly
accelerated, while the EB formation rate increases in much lesser extent. As a consequence, high con-
Keywords:
Alkylation
Dehydrogenation
Bifunctional catalysis
Zeolite
◦
centration of ethene is observed at high temperatures (450–490 C), while the maximum concentrations
of EB are very similar at all temperatures. Ethene, which is formed in excess at high temperatures, is
converted via oligomerization and cracking steps into higher alkenes that alkylate benzene (or aromatic
products). These alkylation reactions are followed by cyclization and dehydrogenation steps leading to
formation of polyaromatics (coke precursors) and subsequent catalyst deactivation. On the other hand,
◦
at lower temperatures (290–410 C) ethene reacts preferably with benzene forming EB. Hence, side reac-
tions are suppressed and high EB selectivity and catalyst stability are observed at these temperatures.
◦
Based on the analysis of the catalyst activity, selectivity and stability, it is concluded that 370–410 C can
be considered as the optimum temperature range for the direct benzene alkylation with ethane into EB.
Further improvement of the process may be achieved by selective removal of hydrogen from the reactor
and by optimization of the ethane/benzene ratio in the feed.
©
2013 Elsevier B.V. All rights reserved.
1
. Introduction
EB was first produced on the commercial scale in Germany
and the United States in the 1930s. Its manufacturing increased
during the World War II due to the high demand for synthetic
polystyrene, i.e. butadiene rubber [4]. In the 1950s, roughly 40%
of the world’s EB capacity was produced in Friedel-Crafts liquid-
Ethylbenzene (EB) is the primary feedstock for the synthesis
of styrene that is subsequently polymerized into polystyrene [1].
It was reported that the world annual production capacity of EB
was around 29.2 million metric tons in 2010, with an average
annual growth rate of approximately 3% from 2010 to 2015 [2].
In the chemical industry, over 90% of EB is used in the synthesis of
styrene, while the remaining ∼10% of EB is used in manufacturing
of acetophenone, cellulose acetate, diethylbenzenes (DEBs), ethyl-
anthraquinone, ethylbenzene sulfonic acids, and ␣-methylbenzyl
alcohol. A mixture of EB and xylenes is used as a solvent in the
paint industry and for producing pesticides in agriculture [3].
phase alkylation with aluminum chloride-(AlCl ) based catalysts
3
[3]. In the 1960s, in attempt to reduce corrosion problems asso-
ciated with the use of the AlCl3 catalysts, supported catalysts
such as BF /Al O and SPA (supported phosphoric acid) were
3
2
3
considered [5,6]. These catalysts however were not used on an
industrial scale, as they did not provide solutions of the corrosion
problems [6]. A significant improvement in the EB synthesis was
achieved in the 1980s, when a new gas-phase alkylation process
using zeolite-based catalysts was commercialized, overcoming the
main drawbacks (environmental and corrosion problems) of the
AlCl3 and SPA technologies [1,3,5–7]. Since that time zeolite-based
processeshave been licensedby a numberof companies, e.g., Mobil-
Badger, Lummus-UOP, Unocal, CDTech, etc. (see refs. 3, 5 and 6 for
more details).
∗ Corresponding author. Tel.: +44 1225 383329; fax: +44 1225 385713.
E-mail addresses: KaeShin.Wong@UGent.be (K.S. Wong),
T.Vazhnova@bath.ac.uk (T. Vazhnova), Sean.Rigby@nottingham.ac.uk (S.P. Rigby),
D.B.Lukyanov@bath.ac.uk (D.B. Lukyanov).
1
Present address: Laboratory for Chemical Technology, Ghent University, Tech-
The current industrial synthesis of EB is based on the reaction
of benzene alkylation with ethene over acidic zeolites [3,5,6]. This
reaction is accompanied by several side reactions: about 5–15% of
nologiepark 914, B-9052 Zwijnaarde, Belgium.
Present address: Department of Chemical and Environmental Engineering, Uni-
versity of Nottingham, University Park, Nottingham, NG7 2RD, UK.
2
0
926-860X/$ – see front matter © 2013 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.apcata.2013.01.012

1
38
K.S. Wong et al. / Applied Catalysis A: General 454 (2013) 137–144
◦
EB react further with ethene forming DEBs and about 10–20% of
DEBs are alkylated into triethylbenzenes (TEBs). A stoichiometric
excess of benzene in the alkylation loop minimizes the alkylation
of TEBs with ethene to polyethylated benzenes. The DEBs and TEBs
are recovered and react with benzene in the transalkylation reactor
producing EB [3,5,6,8]. As a result, the overall EB process is highly
selective (≥98%).
one temperature only (370 C) [12,14,15]. Hence, it is not clear if the
chosen temperature is in fact the optimum reaction temperature.
Indeed, no experimental or theoretical data is available to conclude
if, for example, a 2-fold decrease or increase in the ethane conver-
sion to ethene (lower or higher temperatures, respectively) would
have a beneficial or detrimental effect of the catalyst performance.
The enthalpies of reactions (2) and (3) indicate that the tempera-
ture should have very different effects on these two major steps
of the direct benzene alkylation with ethane reaction, as the first
step (ethane dehydrogenation, Eq. (2)) is highly endothermic, while
the second step (benzene alkylation, Eq. (3)) is highly exothermic.
The interplay of these two reaction steps at different temperatures,
which is currently unknown, is of obvious interest from both the
practical and theoretical viewpoints.
The present work was undertaken in order to establish the
effects of temperature on the reaction pathways of the direct
benzene alkylation with ethane into EB over a Pt-containing H-
MFI catalyst. The obtained experimental results have been used
together with the thermodynamic analysis to understand the inter-
play between different reaction steps at different temperatures and
The ethene feed used in the conventional EB synthesis is
produced from steam cracking of hydrocarbons such as ethane,
liquefied petroleum gas (LPG), naphtha, gas condensate, and gas
oil. These highly endothermic cracking processes are very energy
intensive, and the energy input is generated by direct burning of
fossil fuel, leading to large CO2 emissions [9,10]. Hence, the new
catalytic reaction pathway (Eq. (1)), which substitutes ethene with
ethane, as the alkylating agent, in the production of EB would lead
to commercial and environmental benefits [7,11,12]. In addition to
the elimination of ethene as the feedstock, the direct benzene alkyl-
ation with ethane into EB offers an attractive economical reaction
path to convert ethane, one of the most abundant hydrocarbons,
into more valuable aromatics [13].
C H
2
5
∆H 2
98
= +22.6 kJ/mol
+
C H
+ H2
r
2
6
(1)
The direct and highly selective benzene alkylation with ethane
into EB at 370 C has been recently reported [12,14]. The Pt-
containing H-MFI catalysts with moderate and low acidity (Si/Al
ratios of 36 and 140) demonstrated [12] stable catalyst performance
up to 49 hours with EB selectivity in the aromatic products above
◦
to ascertain the optimum temperature for the EB synthesis via this
novel environmentally benign reaction.
2. Experimental
9
0 mol.%. The PtH-MFI catalyst with higher acidity (Si/Al = 15) was
2.1. Catalyst preparation and characterization
less stable [14]. This difference was explained in the subsequent
work [15] by the different rates of coke formation and by the dif-
ferent location of coke species in the PtH-MFI catalysts of different
acidity. The experimental results, obtained with all PtH-MFI cat-
alysts [12,14], indicated that the EB formation proceeds via two
reaction steps: (i) ethane dehydrogenation into ethene and hydro-
gen over Pt sites (Eq. (2)), and (ii) benzene alkylation with ethene
The H-MFI zeolite with the Si/Al ratio of 36 (Zeolyst) was used
as a parent material in this study. High crystallinity of the zeo-
lite and the absence of other phases were confirmed by X-ray
diffraction analysis. The preparation of the Pt-containing zeolite
catalyst (1 wt.% Pt), defined as PtH-MFI, was carried out using
incipient wetness impregnation, as described previously [12,14].
This method was chosen because it is one of the most widely
used catalyst preparation procedures in industry. Additionally, the
impregnated catalyst, which is not washed, is expected to produce
similar designed and actual metal loadings [18]. After impregna-
tion, the catalyst was dried slowly at room temperature for 48 h
◦
into EB over acid sites (Eq. (3)). At 370 C, the initial step of a series
of alkene oligomerization and cracking reactions [16], that is the
ethene dimerization step, proceeds with a low rate due to the inher-
ently low concentration of ethene in the reactor at this temperature
[
12,14]. As a consequence, many side reactions, including coke for-
◦
mation, are suppressed at 370 C, and the catalysts demonstrate
high EB selectivity and stability. It is worth noting that similar (high)
selectivity of the Pt-containing H-MFI catalysts was also reported
◦
and then calcined in a thin layer in a muffle furnace at 530 C for
◦
4
h (heating rate was 1 C/min). The Pt dispersion was 12% and
was determined on a purpose-built adsorption system (Johnson
for direct alkylation of toluene with ethane into ethyltoluene at
◦
Matthey) from the uptake of strongly chemisorbed CO at 25 C and
◦
3
50 C [17].
∆H 2 = +136.4 kJ/mol
98
C H
C H + H
2 4
2
2
6
r
(2)
C H
2
5
298
∆H
r
= –113.8 kJ/mol
+
C H
2 4
(3)
Clearly, the temperature effects should lead to significant
changes in the catalyst activity and may result in essential changes
in the EB selectivity and catalyst stability. In the initial study of the
benzene alkylation with ethane reaction [12], the reaction temper-
assuming CO/Pt adsorption ratio of 1. For kinetic studies, the cata-
lyst powder samples were pressed into disks, crashed, and sieved
to obtain particle sizes in the range of 250–500 ␮m.
◦
ature of 370 C was chosen as at this temperature the conversion of
2.2. Kinetic studies
ethane into ethene (see Eq. (2)) is ∼0.5% (due to thermodynam-
ics). It was expected that such low conversion would limit the
maximum possible concentration of ethene and slow down the
ethene dimerization steps that lead to many other side reactions.
The kinetic experiments [12,14] have proven the validity of this
approach. However, the work reported up to date was carried out at
Benzene alkylation with ethane was investigated at atmospheric
pressure in a continuous flow reactor at six temperatures: 290, 330,
370, 410, 450 and 490 C. At all temperatures the maximum tem-
◦
perature gradient within the catalyst layer was found to be below
◦
2 C. The feed was comprised of ethane (90 mol.%) and benzene

K.S. Wong et al. / Applied Catalysis A: General 454 (2013) 137–144
139
(
10 mol.%), and the reaction mixture was analyzed by on-line GC
8
6
4
2
0
(
a)
as detailed in previous reports [12,14]. Prior to the kinetic experi-
ments, the catalyst samples were activated at 530 C under flowing
air (30 ml/min) for 4 h (heating rate was 1 C/min). Then the reac-
tor was cooled down to 200 C and the catalyst sample was purged
with N (50 ml/min) for 1 h before switching to H flow. Following
the established procedures [12,14], the reduction of the catalyst
was carried out in a flow of H (60 ml/min) at 500 C for 1 h (heat-
ing rate was 5 C/min), before cooling the sample to the reaction
◦
◦
◦
2
2
◦
2
◦
temperature.
Two types of kinetic experiments were performed: (i) experi-
ments at different values of contact time (ꢀ) and (ii) experiments
at different time on stream (TOS). Contact time was defined as
WHSV , where WHSV is the total weight hour space velocity of
ethane and benzene as shown in the equation below:
0.0
0.2
0.4
0.6
0.8
−
1
(
b) 25
2
1
1
0
5
0
5
0
total mass flowrate (gfeed/h)
WHSV =
mass of catalyst (gcat
)
Different contact times were achieved by varing catalyst
loadings (50–500 mg) and mass flow rates of the reactants
0.36–9.38 g/h). TOS experiments were carried out up to 48 h on
stream.
The product selectivities are characterized in this work in three
ways: (i) by the selectivity in aromatic products, (ii) by the selectiv-
ity in all carbon-containing products, and (iii) by the selectivity in all
products (including hydrogen). All selectivities are based on molar
(
0
.0
0.2
0.4
0.6
0.8
-1
WHSV (h gcat/gfeed
)
product concentrations (C ), as illustrated below for the selectivity
i
in aromatic products.
Fig. 1. Effect of contact time on the conversions of ethane (a) and benzene (b) at
◦
Ci
(ꢀ) 290, (ꢀ) 330 (ꢁ) 370, (ꢂ) 410, (᭹) 450 and (ꢃ) 490 C. The values of conversions
Seli,AR = ꢀ
× 100%
were determined at TOS of 20–22 h.
Ci,AR
conversion was only 40% with the same temperature rise (from 12.3
to 17%). Similar effect is observed when the temperature increases
3
. Results and discussion
◦
from 450 to 490 C (Fig. 1). This observation indicates that at tem-
◦
3.1. Catalyst activity and stability at different temperatures
peratures above ∼370 C, the rate of the ethane dehydrogenation
grows steadily, while the benzene alkylation step becomes the rate
limiting step in formation of EB.
Fig. 1 shows ethane and benzene conversions at different con-
◦
tact times in the temperature range between 290 and 490 C. At
The assessment of catalyst stability was first carried out within
a small range of benzene conversions (11–15%) to eliminate the
conversion effect (benzene conversions were lower for 290 and
◦
4
90 C, the conversions of ethane and benzene reach their highest
levels (7.3 and 20%, respectively). It can be seen that the ethane con-
version increases rapidly and achieves 5.3% at the contact time of
◦
330 C due to essentially lower catalytic activity at these temper-
0
.06 h gcat/gfeed. After that, ethane conversion rises slower, from 5.3
atures). Stable ethane and benzene conversions were achieved for
◦
to 7.3%, between the contact times of 0.06 and 0.65 h gcat/gfeed. Sim-
ilarly, a steep increase in benzene conversion (to 11.8%) is observed
up to the contact time of 0.06 h gcat/gfeed. At low contact times
at least 40 hours on stream at 290, 330, 370 and 410 C (Fig. 2).
◦
Lower catalyst stability was observed at 490 C, where ethane and
benzene conversions drop from 6.2 to 5.5% and from 15.8 to 13%,
respectively. Slight decrease in ethane and benzene conversions
(
below 0.06 h gcat/gfeed), ethane and benzene conversions increase
◦
more rapidly at high temperatures (Fig. 1). This indicates that the
rates of the forward ethane dehydrogenation reaction (Eq. (2)) and
benzene alkylation with ethene reaction (Eq. (3)) are dominating
at low contact times, and are increasing with the growing temper-
ature. At 290 C, the catalyst activity is very low, as evident from
the low conversions of ethane (<0.3%) and benzene (<2.5%) at this
temperature (Fig. 1).
(by 0.4 and 0.7%, respectively) was also observed at 450 C during
45 hours on stream. The observed lower stability of the catalyst
◦
at 450 and 490 C appears to be caused mainly by the tempera-
ture effect rather than the contact time effect. Indeed, as can be
◦
◦
◦
seen from Fig. 3, stable catalyst performance is observed at 370 C
at the contact times of 0.13 and 0.32 h gcat/gfeed, while at 490 C,
both the ethane and benzene conversions decrease at these con-
tact times. A steeper decline in ethane and benzene conversions is
observed at higher contact time (0.32 h gcat/gfeed), showing that the
◦
At low temperatures (290–370 C), the effect of temperature on
the conversions of both the ethane and benzene is strong (Fig. 1). For
example, an increase in temperature from 330 to 370 C doubles the
◦
◦
deactivation at 490 C increases slightly with the feed conversion.
ethane conversion, i.e. from 0.9 to 1.8% (␶ = ∼0.65 h gcat/gfeed), and
the benzene conversion rises ∼1.8 times, i.e. from 7 to 12.3%. This
suggests that the first reaction step, i.e. ethane dehydrogenation
The higher conversion of ethane, observed at high temperatures
(Fig. 1), leads to the enhanced formation of ethene, as shown in
◦
Fig. 4. Although the ethene concentration at 490 C is 7 times higher
◦
to ethene (Eq. (2)), is the rate limiting step at temperatures below
than that at 370 C, comparable EB concentrations are obtained at
◦
3
70 C. At higher temperatures, the temperature effect on ethane
both temperatures. This indicates that the enhanced formation of
ethene at high temperatures does not promote the alkylation of
benzene with ethene into EB (Eq. (3)). As a result, the benzene
conversion into EB does not change with increasing temperature
even though more ethene is available for the transformation of
conversion is essentially more pronounced than the effect on ben-
zene conversion, as seen in Fig. 1. For instance, about 4% of ethane is
◦
converted at 450 C, which is more than double the ethane conver-
◦
sion obtained at 370 C. At the same time, the increase in benzene

1
40
K.S. Wong et al. / Applied Catalysis A: General 454 (2013) 137–144
8
6
4
2
(a)
(
a)
5
4
3
2
1
0
0
.0
0.2
0.4
0.6
0.8
0
0
0
10
20
30
40
50
(
b)
1
1
0
0
0
0
.5
2
1
1
(
b)
.2
.9
.6
.3
.0
5
0
5
0
0
.0
0.2
0.4
cat feed
0.6
0.8
-
1
WHSV (h g /g
)
0
10
20
30
40
50
Time on stream (h)
Fig. 4. Effect of contact time on the concentrations of ethene (a) and EB (b) at (ꢁ)
◦
3
70 and (ꢃ) 490 C.
Fig. 2. Effect of time on stream on the conversions of ethane (a) and benzene (b) at
◦
◦
(
(
ꢀ) 290 C, ꢀ = 0.67 h gcat/gfeed; (ꢀ) 330 and (ꢁ) 370 C, ꢀ = 0.32 h gcat/gfeed; (ꢂ) 410,
◦
᭹) 450, and (ꢃ) 490 C, ꢀ = 0.13 h gcat/gfeed.
◦
benzene into EB at 490 C. The high concentration of alkenes in the
reaction mixture would result in a lower catalyst stability, which
is observed in Figs. 2 and 3, as alkenes are known as important
contributors to the coke formation over zeolite catalysts [19–21].
The ethene molecules, which are in excess at high temperatures,
are converted via rapid oligomerization and cracking into higher
alkenes [16] that alkylate benzene (or aromatic products). These
alkylation reactions are followed by cyclization and dehydrogena-
tion steps leading to formation of polyaromatics (coke precursors)
[21–23] and subsequent catalyst deactivation. In contrast, at lower
(
a) 10
8
6
4
2
◦
temperatures (290–410 C) ethene reacts preferably with benzene
forming EB. As a result, side reactions, including coke formation,
are suppressed and high EB selectivity and catalyst stability are
observed at these temperatures.
0
0
10
20
30
40
50
Based on this consideration, it is clear that a change in
ethane/benzene ratio in the feed should affect the catalyst selec-
tivity and stability. Indeed, a decrease in this ratio would favor
ethene reaction with benzene in EB at the expense of formation
of side products, including polyaromatics and coke. In our opinion,
this effect, which is outside of the scope of this study, should be
(
b) 25
2
1
1
0
5
0
5
0
◦
more pronounced at temperatures above 370 C and could lead to
enhanced selectivity and stability at high temperatures.
The discussion presented above suggests that the rates of coke
formation should be different at low and high temperatures. To ver-
ify this suggestion, we analyzed the amount of coke on the PtH-MFI
◦
catalyst samples after 44 h on stream at 370 and 490 C (at contact
time of 0.32 h gcat/gfeed in both experiments). The thermogravi-
metric analysis, which was carried out following the procedure
0
10
20
30
40
50
described earlier [15], revealed that the coke content was 1.9 and
Time on stream (h)
◦
4
.2 wt% on the samples that worked at 370 and 490 C, respec-
tively. This data is in very good agreement with our interpretation
of the lower catalyst stability observed at 490 C in comparison with
lower temperatures.
Fig. 3. Effect of time on stream on the conversions of ethane (a) and benzene (b) at
◦
◦
◦
3
70 C and ꢀ of (ꢁ) 0.13 and (ꢄ) 0.32 h gcat/gfeed, and at 490 C and ꢀ of (ꢃ) 0.13 and
(ꢅ) 0.32 h gcat/gfeed.

K.S. Wong et al. / Applied Catalysis A: General 454 (2013) 137–144
141
Table 1
Table 2
a
Effect of temperature on the thermodynamic equilibrium constants of the reactions
of ethane dehydrogenation (KD) and benzene alkylation with ethene (KA).
Aromatic product selectivities during benzene alkylation with ethane over the PtH-
ZSM-5 catalyst at 330, 370, 410 and 490 C.
◦
◦
−1
◦
T ( C)
KD (bar)
C2H6 = C2H4 + H2
KA (bar )C6H6 + C2H4 = EB
Temperature ( C)
330
0.71
370
0.91
410
2.38
490
5.81
Ethane conversion (%)
Benzene conversion (%)
Selectivity (mol.%)
Toluene
Ethylbenzene
Xylenes
Propylbenzenes
Ethyltouene
Diethylbenzenes
Triethylbenzenes
Total
6.60 × 10−
7
1331
304
83.4
26.7
9.71
3.93
5.14
4.71
13.7
14.2
2
3
3
4
4
4
90
30
70
10
50
90
−6
−5
−4
−4
3
4.90 × 10
2.80 × 10
1.40 × 10
5.68 × 10
0.61
95.93
0
1.20
0.12
1.56
0.58
0.34
93.25
0
3.96
0.31
1.62
0.52
0.80
86.41
0.14
7.35
0.94
2.54
1.82
1.63
81.47
0.65
10.98
1.41
2.04
1.73
100
2.01 × 10−
a
The calculations were performed using heat capacities cp(T) at constant pressure.
100
100
100
3
.2. Consideration of thermodynamic equilibrium
The idea of coupling the reaction of ethane dehydrogenation into
exothermic reaction (3) leads to the equilibrium conversion of ben-
zene into EB reaching a plateau around 400 C and then decreasing
at higher temperatures (Fig. 5).
ethene and hydrogen (Eq. (2)) on metal sites with the reaction of
benzene alkylation by ethene (Eq. (3)) at Brønsted acid sites (BAS)
using bifunctional zeolite catalyst was discussed earlier [12,14]. The
feasibility of such coupling (see Eq. (1)) was successfully demon-
strated in this previous work.
◦
Thus, based on the thermodynamic analysis, it is clear that the
experimental results presented in Section 3.1 are affected consid-
erably by the equilibrium conversions in reactions (2) and (3). The
linear increase of the equilibrium benzene and ethane conversions
In the present study, in order to understand the observed
inhibited formation of EB at high temperature (see Section 3.1), we
conducted the analysis of the equilibrium conversions of ethane
and benzene in reactions (2) and (3) at different temperatures with
the feed comprised of ethane (90 mol.%) and benzene (10 mol.%).
This was done in two steps. First, the equilibrium constants of the
two reactions were calculated using thermodynamic data (the cal-
culations were performed using heat capacities cp(T) at constant
pressure). The obtained equilibrium constants (Table 1) were then
used to determine the equilibrium conversions of ethane and ben-
zene. To calculate these equilibrium conversions, we have assumed
that reactions (2) and (3) take place simultaneously and there
are no side reactions involved. The results of our calculations are
presented in Fig. 5 (solid lines) and show that the equilibrium con-
version of ethane in the reaction of ethane dehydrogenation (Eq.
◦
at temperatures lower than 410 C (Fig. 5) can explain similar rise
of the benzene and ethane conversions that is observed between
◦
3
30 and 370 C (see Fig. 1). The enhanced ethane conversion at
higher temperatures, as shown in Fig. 1, is a result of the exponen-
tial rise of the equilibrium ethane conversion at these temperatures
(Fig. 5). Additionally, the restricted EB formation at high temper-
atures, revealed by Fig. 4, is limited by the equilibrium benzene
conversion. Hence, the excess ethene formed at high temperatures
does not react with benzene to form EB, but instead, it serves as a
source for secondary reactions, which are responsible for the rise
of the ethane and benzene conversions above the equilibrium val-
ues, as discussed in the following section. Importantly, some of the
secondary reactions lead to formation of polyalkylaromatis that are
normally considered as coke precursors [13,21,22].
(
2)) increases exponentially with temperature. This result can be
expected as ethane dehydrogenation is an endothermic reaction
see Eq. (2)) and its equilibrium constant increases with increasing
3.3. Product selectivities and reaction pathway
(
temperature (Table 1). On the other hand, as shown in Table 1, the
equilibrium constant of benzene alkylation by ethene is decreasing,
in agreement with the exothermic nature of this reaction (see Eq.
Fig. 5 shows that the conversion of ethane is rising with tem-
◦
perature approaching the equilibrium conversion at 370 C and
then surpassing it at higher temperatures. Similarly, the conver-
(
3)), and its equilibrium is shifting to the left with increasing tem-
◦
sion of benzene rises sharply at temperatures higher than 290 C,
perature. Such an interplay between endothermic reaction (2) and
◦
reaches the equilibrium level at around 370 C and continues to
rise above the equilibrium conversion with growing temperature.
These observations indicate clearly that different side reactions are
taking place at higher temperatures, and this conclusion is sup-
ported by the data presented in Fig. 6.
At all temperatures, the EB selectivity in aromatic products
approaches 100 mol.% at very low benzene conversions, and then
decreases slowly with increasing benzene conversion (Fig. 6a). Two
regions can be clearly observed here. The first corresponds to ben-
zene conversions up to around 8–10%. In this region, the drop in
the EB selectivity is associated with the rise in selectivities to such
secondary products as isopropylbenzene (iPB) and DEBs (Fig. 6b
and c). With the rise in temperature, the selectivity to iPB increases
2
1
1
0
5
0
5
(
Fig. 6b), and that leads to more pronounced drop in EB selectivity,
0
2
which still remains very high (above or around 90%, depending on
temperature) at benzene conversions lower than 8–10% (Fig. 6a).
In this region, other secondary or tertiary products are formed in
very small quantities (Fig. 6d and e). At benzene conversions higher
than 10–11% (the second region), another drop in the EB selectiv-
ity is observed (Fig. 6a) that is due mainly to the formation of such
tertiary products as TEBs and toluene (see Table 2, Fig. 6d and e).
Additionally, heavy aromatic hydrocarbons (C10+), including naph-
00
300
400
500
Temperature (ºC)
Fig. 5. Conversions of ethane and benzene at different temperatures: thermody-
namic equilibrium conversions of ethane (×) and benzene (+) in reactions (2) and
(
3) occurring simultaneously; experimental total conversions of ethane (᭹) and ben-
zene (ꢃ); experimental conversions of ethane into ethene and EB (ꢀ) and benzene
into EB (ꢅ) according to reactions (2) and (3). The thermodynamic equilibrium con-
versions were calculated with two assumptions: (i) reactions (2) and (3) take place
simultaneously and (ii) there are no side reactions in the catalytic system.
◦
thalene, were observed at 490 C indicating that the consecutive

1
42
K.S. Wong et al. / Applied Catalysis A: General 454 (2013) 137–144
(d)
(
a) 100
6
5
4
3
2
1
0
9
8
7
6
0
0
0
0
0
5
5
5
10
10
10
15
15
15
20
20
20
25
25
25
0
5
10
15
20
25
(
b) 15
(e)
6
5
4
3
2
1
0
1
0
5
0
0
0
5
10
15
20
25
Benzene conversion (%)
6
5
4
3
2
1
0
(
c)
0
Benzene conversion (%)
Fig. 6. Effect of benzene conversion on the selectivities to aromatic products: (a) EB, (b) iPB, (c) DEBs, (d) TEBs and (e) toluene. Temperature: (ꢆ) 290, (♦) 330, (ꢄ) 370, (ꢂ)
◦
4
10, (᭹) 450 and (ꢃ) 490 C.
alkylation and condensation reactions take place at this tempera-
ture.
to the secondary and tertiary products have been discussed in detail
previously [14].
Based on the data in Fig. 6 one can conclude that the forma-
tion of aromatic products follows similar pathways at high and
low temperatures. EB is the primary product of benzene transfor-
mation as it is observed at very low benzene conversion. iPB and
DEBs, which are observed at benzene conversions lower than 1.5%,
are the secondary products that are formed in benzene alkylation
with propene and further alkylation of EB with ethene, respectively.
Toluene, ethyltoluene and TEBs are the tertiary products that are
formed only at benzene conversions higher than ∼10%. Toluene
is likely to be produced in the hydrogenolysis of EB, while ethyl-
toluene and TEBs are formed in alkylation of toluene and DEBs,
respectively, with ethene. Note that all possible reactions leading
Table 3 shows that at similar benzene conversions (11–12%), a
dramatic difference in ethene and EB selectivities in all carbon con-
taining products was observed in the temperature range between
◦
370 and 490 C. This observation is expected, as we have already
◦
discussed the data on formation of ethene and EB at 370 and 490 C
(Fig. 4). Table 3 shows that the excess of ethene, at high temper-
ature, promotes oligomerization and cracking reactions of alkenes
that are responsible for the increase in the selectivities to propene
and butenes. The product distribution shown in Table 3 indicates
that the rates of different reaction steps are affected by temper-
ature differently. The rate of ethane dehydrogenation is highly
accelerated at high temperature and, hence, promotes the steps

K.S. Wong et al. / Applied Catalysis A: General 454 (2013) 137–144
143
Table 3
60
(
a)
Selectivities of all carbon containing products of benzene alkylation with ethane
over the PtH-ZSM-5 catalyst in the temperature range of 370–490 C.
◦
5
4
3
2
0
0
0
0
◦
Temperature ( C)
370
1.57
410
2.13
450
3.30
490
5.25
Ethane conversion (%)
Benzene conversion (%)
Selectivity (mol.%)
Methane
Ethene
Propane
Propene
Butanes
Butenes
Toluene
11.1
12.0
11.6
11.0
0.39
0.31
0.39
0.39
14.27
2.47
0.11
0.18
0
0.82
75.33
0.09
2.05
0.39
1.86
1.97
0.07
39.49
1.36
0.27
0.13
0.04
0.34
51.50
0.06
3.94
0.37
1.47
0.40
0.33
64.01
0.49
0.34
0.07
0.11
0.14
29.93
0.06
3.01
0.20
0.87
0.06
0.33
77.08
0.18
0.57
0.07
0.21
0.12
18.98
0.06
1.61
0.08
0.40
0.06
0.19
10
0
0.0
0.5
1.0
1.5
2.0
Ethylbenzene
Xylenes
6
5
0
(
b)
Propylbenzenes
Ethyltoluenes
Diethylbenzenes
Triethylbenzenes
Other aromatics
0
40
3
0
of alkene oligomerization and cracking. The rates of the secondary
and tertiary reactions of aromatics are also accelerated consider-
ably at high temperatures, as reflected by the enhanced formation
of the secondary and tertiary aromatic products at these temper-
atures (Fig. 6). On the other hand, the rate of benzene alkylation
with ethene into EB is improved marginally at high temperature.
20
1
0
0
0
2
4
6
8
Ethane conversion (%)
As a result, comparable EB concentrations are observed at 370 and
◦
4
90 C, as shown in Fig. 4.
Fig. 7. Effect of ethane conversion on the selectivities to (ꢂ) hydrogen, (᭹) ethene
◦
and (ꢃ) EB at (a) 370 and (b) 490 C.
The optimum temperature of the direct benzene alkylation with
ethane into EB and hydrogen should be relatively low in order
to prevent the excess formation of ethene, which does not trans-
form into the desired product. Based on the thermodynamic as
well as product distribution analyses, it appears that the opti-
mum temperature for the benzene alkylation with ethane is likely
ethane conversion higher than 6%. This indicates clearly that the
formation of EB is not the sole reaction consuming ethene at high
temperatures. The major other reaction of ethene is its dimeriza-
tion that is followed by the oligomerization and cracking reactions
of alkenes. The alkenes formed in these reactions alkylate benzene
and EB producing heavy aromatics and coke [13,21,22] leading to
catalyst deactivation as shown in Figs. 2 and 3.
◦
to be in the region between 370–410 C. In this temperature
range, excellent catalyst stability is achieved with high EB selec-
tivity at benzene conversions close to the equilibrium values (see
Fig. 5).
◦
At 370 C, the selectivities to ethene and hydrogen are very close
to 50 mol.% at very low ethane conversion (Fig. 7a). The equimo-
lar amounts of ethene and hydrogen are observed once ethane
started to react, indicating that the hydrogen and ethene are the
primary products. Hydrogen, which is not consumed in the sub-
sequent reaction steps, maintains 50 mol.% selectivity at different
conversions of ethane (Fig. 7a). The drop of ethene selectivity with
increasing ethane conversion corresponds to the rise of EB selec-
tivity, thus demonstrating that EB is formed from the consecutive
alkylation of benzene with ethene. Additionally, the selectivity to
EB drops to zero as ethane conversion approaches zero, indicating
further that EB is the secondary product of ethane transforma-
tion.
4. Conclusions
Detailed kinetic study of the benzene alkylation with ethane
into EB over a PtH-MFI bifunctional catalyst was carried out in the
temperature range between 290 and 490 C, during which the sta-
bility, activity and EB selectivity was investigated at six different
temperatures.
◦
It has been concluded that similar reaction pathways of benzene
alkylation with ethane into EB operate at all temperatures studied
in this work. The EB formation proceeds via two consecutive reac-
tion steps: (i) ethane dehydrogenation into ethene and hydrogen on
Pt sites, and (ii) benzene alkylation with ethene into EB over BAS.
In addition to benzene alkylation, ethene is also involved in the
dimerization reaction that is followed by oligomerization, cracking
and further alkylation reactions producing various side products.
This work demonstrates that temperature has very different
effects on the thermodynamic equilibrium of ethane dehydro-
genation and benzene alkylation reactions and, as a result, on
the product distribution and catalyst performance. Ethane dehy-
drogenation is highly accelerated, while the EB formation rate
increases in much lesser extent with increasing temperature. As
a consequence, high concentration of ethene is observed at high
temperatures, while the maximum concentrations of EB are very
similar at all temperatures. Ethene, which is formed in excess at
high temperatures, is converted via oligomerization and cracking
steps in higher alkenes that alkylate benzene (or aromatic prod-
ucts). These alkylation reactions are followed by cyclization and
Fig. 7b depicts the product selectivities as a function of ethane
◦
◦
conversion at 490 C. Similar to 370 C, ethene and hydrogen selec-
◦
tivities can be extrapolated to 50 mol.% at 490 C when small
amount of ethane is converted. The selectivity of hydrogen remains
at 50 mol.% up to 6% of ethane conversion and starts to increase
slightly to 53 mol.% at higher ethane conversion. This rise of hydro-
gen selectivity is likely to be associated with secondary reactions
producing hydrogen. Aromatization, which may produce hydrogen
and is more favorable at higher ethane conversion, could be one of
such reactions. The dramatic drop in ethene selectivity at ethane
conversion higher than 6% supports the suggestion that aroma-
tization may take place at high ethane conversions. In contrast
◦
to 370 C, when ethene is converted mainly into EB via benzene
◦
alkylation with ethene, the selectivity of EB at 490 C remains
nearly constant with the significant drop in ethene selectivity at

1
44
K.S. Wong et al. / Applied Catalysis A: General 454 (2013) 137–144
dehydrogenation steps leading to formation of polyaromatics (coke
precursors) and subsequent catalyst deactivation. On the other
hand, at lower temperatures (290–410 C) ethene reacts preferably
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