A kinetic study of benzene alkylation with ethane into ethylbenzene over bifunctional PtH-MFI catalyst

Article abstract of DOI:10.1016/j.jcat.2008.05.020

Benzene alkylation with ethane into ethylbenzene (EB) was studied at 370 °C over a PtH-MFI bifunctional catalyst of high acidity (Si/Al = 15). Highly selective and stable catalyst operation was observed at benzene conversions up to 10-12%, which are close to the thermodynamic equilibrium conversion value of 13.5%. Kinetic analysis of the experimental data reveals that EB is formed via two consecutive reaction steps: (i) ethane dehydrogenation into ethene over Pt sites and (ii) benzene alkylation with ethene over acid sites. These reactions dominate at low contact times and benzene conversions up to 10-12%. At higher conversions (i.e. higher contact times) other reactions start to compete with the EB formation steps leading to a steep drop in the EB selectivity and to a decrease in the catalyst stability. The analysis of these side reactions results in a better understanding of the requirements for the selective and stable benzene alkylation with ethane into EB over bifunctional zeolite catalysts.

Full text of DOI:10.1016/j.jcat.2008.05.020

Journal of Catalysis 257 (2008) 382–389
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Journal of Catalysis
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A kinetic study of benzene alkylation with ethane into ethylbenzene over
bifunctional PtH-MFI catalyst
∗
Dmitry B. Lukyanov , Tanya Vazhnova
Catalysis and Reaction Engineering Group, Department of Chemical Engineering, University of Bath, 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) was studied at 370 C over a PtH-MFI bifunctional
catalyst of high acidity (Si/Al = 15). Highly selective and stable catalyst operation was observed at
benzene conversions up to 10–12%, which are close to the thermodynamic equilibrium conversion value
of 13.5%. Kinetic analysis of the experimental data reveals that EB is formed via two consecutive reaction
steps: (i) ethane dehydrogenation into ethene over Pt sites and (ii) benzene alkylation with ethene over
acid sites. These reactions dominate at low contact times and benzene conversions up to 10-12%. At
higher conversions (i.e. higher contact times) other reactions start to compete with the EB formation
steps leading to a steep drop in the EB selectivity and to a decrease in the catalyst stability. The analysis
of these side reactions results in a better understanding of the requirements for the selective and stable
benzene alkylation with ethane into EB over bifunctional zeolite catalysts.
Received 17 November 2007
Revised 12 May 2008
Accepted 16 May 2008
Available online 16 June 2008
Keywords:
Ethane
Ethylbenzene
Dehydrogenation
Alkylation
Bifunctional catalysis
Zeolites
©
2008 Elsevier Inc. All rights reserved.
1
. Introduction
products of direct alkylation, i.e. EB and propylbenzenes (PB). The
studies of benzene alkylation with ethane over bifunctional zeo-
lite catalysts [5–7] were performed at 450–600 C and showed the
◦
Ethylbenzene (EB) is
a
key intermediate in production of
polystyrene and is annually produced in the amount of 23 million
metric tons [1]. The current technologies of EB production [1,2]
are based on the zeolite catalyzed benzene alkylation with ethene,
which is produced in the steam reforming of ethane and naphtha
complexity of the reaction pathways at these temperatures. It was
suggested [6,7] that EB was produced over acid sites via benzene
alkylation with ethene, which was formed initially in ethane dehy-
drogenation over Pt sites. However, no attempt was made in these
studies [5–7] to investigate experimentally the reaction pathways
that led to the observed low EB selectivities.
[3]. Both latter processes are very energy intensive, consume fos-
sil fuels and generate a greenhouse gas, CO2 [3]. The replacement
of ethene with ethane, as the alkylating agent in the synthesis of
EB, would allow elimination of the ethene production step and, as
a consequence, would lead to the commercial and environmental
benefits in the EB production. Obviously, the new process, benzene
alkylation with ethane into EB, would be commercially viable only
if high EB selectivity and catalyst stability are achieved.
The possibility of the direct benzene alkylation with ethane into
EB was first established by Olah et al. [4]. With fluoroantimonic
acid (HF-SbF5) as a catalyst, the EB yield was 1 mol% and the selec-
tivity to EB in the aromatic products was around 76 mol% [4]. Later
on, several research groups investigated benzene alkylation with
ethane [5–7] and propane [5,7–11] over a number of Pt- and Ga-
containing zeolite catalysts, which are free from the corrosion and
environmental problems associated with the superacidic catalytic
systems. However, the zeolite-based catalysts produced mixtures of
various alkyl and dialkylbenzenes with very low selectivities to the
In our recent paper [12] we have identified three groups of
side reactions, namely, (i) ethane hydrogenolysis, (ii) reactions of
alkenes and (iii) transformation of EB, which were likely respon-
sible for the low EB selectivities previously reported for benzene
alkylation with ethane over bifunctional zeolite catalysts. We then
analyzed the thermodynamic and kinetic limitations associated
with the target and side reactions. This analysis suggested that
alkene reactions could be suppressed at lower temperatures and
that EB formation steps and the steps of its transformation could
be decoupled by using catalysts of low or moderate acidity [12].
This suggestion was then verified by the experiments on benzene
◦
alkylation with ethane at 370 C over two PtH-MFI catalysts with
the Si/Al ratios of 36 and 140. Both catalysts displayed a remark-
ably stable performance (during 45–49 h on stream) with EB selec-
tivity in the aromatic products in the range between 92–95 mol%
and the highest EB yield (benzene based) of 10.7 mol% [12].
Thus, our recent communication [12] has demonstrated for the
first time the feasibility of the highly selective and stable alkylation
of benzene with ethane into EB over bifunctional zeolite catalysts
and suggested a number of reaction steps that may be involved
Corresponding author. Fax: +44 1225 385713.
E-mail addresses: d.b.lukyanov@bath.ac.uk (D.B. Lukyanov),
*
t.vazhnova@bath.ac.uk (T. Vazhnova).
0
021-9517/$ – see front matter © 2008 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2008.05.020

D.B. Lukyanov, T. Vazhnova / Journal of Catalysis 257 (2008) 382–389
383
in this complex reaction. The aim of the present study was to get
further insight into the reaction pathways of benzene alkylation
with ethane into EB over Pt-containing MFI catalysts. To achieve
this goal, we conducted a detailed kinetic study of this reaction
catalyst sample was purged with N2 (50 ml/min) for 1 h before
switching to the flowing H2 (60 ml/min). The catalyst sample was
◦
◦
then heated (5 C/min) to 500 C and kept at this temperature for
◦
1 h before cooling the sample to the reaction temperature (370 C).
◦
−1
−1
at 370 C over a PtH-MFI zeolite catalyst with a Si/Al ratio of 15.
Contact time (τ) was defined as WHSV , where WHSV (h )
This catalyst was chosen since it was of interest to verify if sim-
ilar, highly selective and stable performance can be attained with
a higher acidity catalyst when compared to the catalysts of lower
acidity that were used previously [12].
is the total weight hour space velocity of ethane and benzene. Dif-
ferent levels of conversions of benzene and ethane were obtained
by performing experiments at different values of contact time that
was changed in the range between 0.0018 and 0.65 h (variation of
−1
WHSV was in the range between 557 and 1.54 h , respectively).
To achieve such a variation in contact time, the experiments were
performed with different catalyst loadings (18.5–1000 mg) and dif-
ferent mass flow rates of the reactants (1.54–10.3 g/h). Two quartz
reactors with the inner diameters of 3 and 8 mm were used in the
experiments with the low (18.5, 40 and 125 mg) and high (500
and 1000 mg) catalyst loadings, respectively. The absence of the
mass transfer limitations, which could be due to the changes in
the flow rates, were confirmed by the same results obtained at
the same contact times using different catalyst loadings (40 and
2. Experimental
2.1. Catalyst preparation and characterization
A sample of H-MFI zeolite with a Si/Al ratio of 15 (ZEOLYST)
was used in this work as a parent material. High crystallinity of
the zeolite and the absence of other phases were confirmed by
X-ray diffraction analysis. The size of the zeolite crystallites was
in the range between 0.4 and 0.7 μm, as determined by scan-
ning electron microscopy. The number of acid sites in the zeolite
was determined by FTIR spectroscopy using pyridine as a probe
molecule. FTIR spectra of the self-supported catalyst discs were
125 mg) and flow rates (1.54 and 4.81 g/h). Time on stream (TOS)
studies revealed stable catalyst performance during 30 h of the re-
action at all contact times used in this work (see Section 3.3), and
the analysis of the reaction pathways was performed on the basis
of the experimental data obtained at TOS between 20 and 24 h.
−1
collected at a resolution of 2 cm
using a Bruker Equinox 55
FTIR spectrometer and a purpose-built IR cell that allowed high-
temperature treatment of samples in situ [13]. Pyridine adsorption
◦
was carried out at 150 C, after activation of the samples under
vacuum (10
−5
◦
mbar) at 400 C overnight. These experiments re-
3. Results and discussion
vealed that the number of all acid sites in the H-MFI zeolite was
−
1
5
35 μmol g , and that the numbers of the Brønsted and Lewis
This section starts with consideration of the effect of contact
time on ethane and benzene conversions and product selectivi-
ties, and then proceeds to the detailed analysis of the reaction
pathways of benzene alkylation with ethane into EB over the PtH-
MFI catalyst. This leads to the identification of the sequence of the
main reaction steps, which are used later on in the discussion of
the TOS experiments at the end of this section.
−1
acid sites were 448 and 87 μmol g , respectively.
The Pt-containing zeolite catalyst (1 wt% Pt), defined as PtH-
MFI, was prepared by incipient wetness impregnation of the H-MFI
zeolite with an aqueous solution of tetraammineplatinum(II) ni-
trate, Pt(NH3)4(NO3)2. After impregnation the catalyst was dried
slowly at room temperature (∼48 h), and then calcined (in a
◦
thin layer) in a muffle furnace at 530 C for 4 h (heating rate
◦
was 1 C/min). For kinetic studies, the catalyst powder samples
were pressed into disks, crashed, and sieved to obtain catalyst
particle sizes in the range of 250–500 μm. The Pt dispersion in
the PtH-MFI catalyst was 12%. It was determined on a purpose-
built adsorption system (Johnson Matthey) from the uptake of
3
.1. Effect of contact time on feed conversion and product selectivities
Fig. 1 shows that the conversions of benzene and ethane in-
◦
crease with growing contact time and reach their highest levels
21.5 and 5.8%, respectively) at the contact time of 0.65 h. It can be
strongly chemisorbed CO at 25 C and assuming CO/Pt adsorption
(
ratio of 1. Prior to CO adsorption experiments the catalyst sam-
◦
seen that the increase in benzene conversion is quite steep up to
ples (0.5 g) were first oxidized at 530 C for 4 h in flowing air
◦
the level of about 10% (τ = 0.04 h) and then slows down steadily
(30 ml/min), and then reduced at 500 C for 1 h in a flow of pure
◦
at higher contact time values (Fig. 1). At the temperature of 370 C,
H2 (30 ml/min). Based on the assumption of the uniform Pt distri-
bution (as spherical particles), the diameter of the Pt particles was
calculated to be around 10 nm.
the reaction between benzene and ethane into ethylbenzene and
hydrogen
2
.2. Kinetic studies
Alkylation of benzene with ethane was studied at atmospheric
◦
pressure in a continuous flow reactor at 370 C with the feed
comprised of ethane (90 mol%) and benzene (10 mol%). The re-
action mixture was analyzed by on-line GC using Varian CP-3800
Gas Chromatograph, which was equipped with a molecular sieve
13X packed column and a thermal conductivity detector (TCD) for
analysis of H2, and a 25 m long PLOT Al2O3/KCl capillary column
with a flame ionization detector (FID) for analysis of hydrocarbons
(argon was used as a carrier gas in both columns). Identification
of the retention times for different hydrocarbons was done using
reference hydrocarbon mixtures. TCD was calibrated using gas mix-
tures with different H2 concentrations. Prior to the kinetic experi-
◦
ments, the catalyst samples were heated (1 C/min) in the reactor
Fig. 1. Effect of contact time on benzene and ethane conversions over the PtH-MFI
catalyst at 370 C. Ethane to benzene molar ratio in the feed was 9:1, and the ex-
perimental data were obtained at TOS between 20 and 24 h.
◦
◦
under flowing air (30 ml/min) to 530 C and kept at this tempera-
ture for 4 h. Then the temperature was reduced to 200 C and the
◦

384
D.B. Lukyanov, T. Vazhnova / Journal of Catalysis 257 (2008) 382–389
This conclusion is fully supported by Fig. 2, which displays the
selectivities to the aromatic products as functions of benzene con-
version. Two distinct regions can be easily identified in Fig. 2.
The first region corresponds to benzene conversions up to about
10% where EB selectivity in the aromatic products is very high
(
(
above 90 mol%) and decreases slowly with increasing conversion
Fig. 2A). From Fig. 2 it follows that this decrease is mainly as-
sociated with the rising selectivities to PB and diethylbenzenes
DEB). In the second region, at benzene conversions above 10%,
(
a steep drop in EB selectivity is observed (Fig. 2A) indicating
clearly that formation of other aromatic products from benzene
is taking place. Figs. 2C and 2D reveal that these aromatic prod-
ucts include toluene, xylenes, triethylbenzenes (TEB), ethyltoluenes
(ETT) and other aromatics, which include different alkylbenzenes,
naphthalene and its derivatives. Based on these observations and
taking into account the experimental data shown in Fig. 1, we can
conclude that at low contact times (up to 0.04 h) the conversions
of benzene and ethane are rising steeply mainly due to their trans-
formation into EB and hydrogen, while at higher contact times the
slow increase in the feed conversion is associated with formation
of other products, which can be formed via reactions involving EB
as discussed in Section 3.2.
3
.2. Reaction pathways of benzene alkylation with ethane over PtH-MFI
catalyst
To get better understanding of the reaction pathways of ben-
zene alkylation with ethane, we decided to analyze formation of
the reaction products at low and high contact times separately,
since it appears that different reactions are taking place in these
two regions.
3.2.1. Reaction steps at low contact times
In the region of low contact times the following products
were observed: hydrogen, ethene, EB, diethylbenzenes, propene,
propane, and propylbenzenes. The ratio of iso-propylbenzene (IPB)
to n-propylbenzene (NPB) was in the range between 35 and 45.
Fig. 3 shows the concentrations of these products as functions of
contact time and demonstrates that hydrogen, ethene and EB are
the major reaction products (note the difference in the scales be-
tween Figs. 3A and 3B). Analysis of the concentration curves in
Fig. 3 allows us to conclude that hydrogen and ethene are the ini-
tial reaction products, while DEB, propene, propane and PB are the
products of the secondary reaction steps. The situation is more
complicated in the case of the EB formation steps, since Fig. 3A
shows that EB is a secondary product formed via benzene alky-
lation with ethene (ethene concentration clearly passes through a
maximum) but does not allow us to exclude the possibility of EB
formation directly via benzene alkylation with ethane. This possi-
bility was noted by Smirnov et al. [7] who proposed that ethane
dehydrogenation and ethene protonation into carbenium ion may
occur over Pt-acidic centers that allow the direct alkylation of ben-
zene with ethane into EB (Eq. (1)).
Fig. 2. Effect of benzene conversion on the selectivities in the aromatic products
formed during benzene alkylation with ethane over the PtH-MFI catalyst at 370 C:
◦
(
(
A) EB; (B) DEB and PB; (C) toluene, xylenes and TEB; (D) ETT and other aromatics
C10 –C12 alkylbenzenes, naphthalene and its derivatives).
In order to clarify the mechanism of EB formation, we con-
sidered the selectivities to hydrogen, ethene and EB as functions
of ethane conversion. These selectivities are shown in Fig. 4 and
demonstrate clearly that EB is the secondary reaction product and
that it is not formed via direct alkylation of benzene with ethane
over the PtH-MFI catalyst used in this work. Our selectivity data
(
1)
is strongly limited by thermodynamics, and the equilibrium con-
version of benzene into EB is around 13.5%, as calculated on the
basis of thermodynamic data [14] for the feed composition used
in this work. Since the maximum benzene conversion (21.5%) ob-
served in this work (Fig. 1) is essentially higher than the equilib-
rium benzene conversion into EB, one can conclude that benzene
is transformed not only into EB but in other products as well.
(
Fig. 4) verify experimentally that EB formation proceeds via clas-
sical bifunctional mechanism [15] that involves two distinct active
sites catalyzing two distinct reaction steps. First, ethane dehydro-
genation into ethene and hydrogen (Eq. (2)) is taking place over
Pt sites and this reaction is then followed by benzene alkylation
with ethene into EB (Eq. (3)) over Brønsted acid sites (BAS) via for-
mation of ethyl carbenium ions. It is worth noting that the above

D.B. Lukyanov, T. Vazhnova / Journal of Catalysis 257 (2008) 382–389
385
[12]. Therefore, ethene concentration is very low at any point in
the reactor and the whole process of EB formation is driven by
benzene alkylation with ethene into EB (Eq. (3)) that withdraws
ethene molecules from the reaction mixture and pushes ethane
dehydrogenation (Eq. (2)) forward. The inherently low ethene con-
centration in the reactor hinders significantly the formation of all
side reaction products (DEB, propene, propane and PB) that are
observed at low contact times (see Fig. 3B). Indeed, it slows down
the formation of diethylbenzenes that are formed via EB alkylation
with ethene over BAS [1,2] as shown below.
(4)
Also, the low ethene concentration leads to a significant decrease
in the rates of the acid catalyzed bimolecular ethene dimeriza-
tion steps (Eq. (5)). In our case, these steps are the initial steps in
the sequence of alkene oligomerization and cracking reactions that
produce different alkenes [16], as illustrated below by the reactions
(6)–(8) for the formation of hexenes, pentenes and propene.
C2H4 + C2H4 ꢀ C4H8
C2H4 + C4H8 ꢀ C6H12
C6H12 ꢀ C3H6 + C3H6
C H + C H ꢀ C H
(5)
(6)
(7)
(8)
Fig. 3. Effect of contact time (in the region of low conversions) on the concentra-
2
4
3
6
5
10
tions of (A) hydrogen, ethene and EB, and (B) propene, propane, PB and DEB formed
◦
during benzene alkylation with ethane over the PtH-MFI catalyst at 370 C (ethane
According to the literature [17–19], alkene distribution resulting
to benzene molar ratio in the feed was 9:1, and TOS was between 20 and 24 h).
At contact time of 0.04 h, conversions of ethane and benzene were 1.51 and 10.3%,
respectively.
from the oligomerization and cracking reactions is a strong func-
tion of alkene partial pressure and is shifted to small alkenes at
low alkene concentrations. Therefore, not only the rates of alkene
reactions were suppressed under reaction conditions of this study,
but the formation of the higher molecular weight alkenes was hin-
dered as well. As a consequence, only ethene and propene were
observed in the reaction mixture at low contact times. Formation
of propene has led to formation of propane via platinum catalyzed
hydrogenation reaction (9) and to formation of PB via acid cat-
alyzed benzene alkylation reaction (10) as shown below.
C3H6 + H2 ꢀ C3H8
(9)
(10)
Quite obviously, the observed low concentrations of propane
and propylbenzenes in the reaction mixture (Fig. 3B) can be ex-
plained by very low propene concentration that slowed down the
rates of the reactions (9) and (10).
Fig. 4. Selectivities to hydrogen, ethene and EB as functions of ethane conver-
sion over the PtH-MFI catalyst at 370 C. Ethane conversion of 1.5% corresponds
to benzene conversion of 10.3%. The sum of hydrogen, ethene and EB selectivities
is 99.9 mol% at ethane conversion of 0.35% (isopropylbenzene and propene make
together the remaining 0.1 mol%).
◦
3.2.2. Reaction steps at high contact times
Fig. 5 shows the distribution of the products of benzene alky-
lation with ethane in the whole range of contact times used in
this study. As contact time increases from 0.04 to 0.65 h, the new
products are observed in the reaction mixture: methane (Fig. 5A);
toluene and xylenes (Fig. 5C); ethyltoluenes, triethylbenzenes and
other aromatics (Fig. 5D), which include different alkylbenzenes,
naphthalene and its derivatives. Simultaneously with formation of
these products a significant increase is observed for propane con-
centration (Fig. 5C), while the concentrations of EB, DEB and PB
pass through the maximums (Figs. 5A and 5B). Analysis of the ex-
perimental data in Fig. 5 allows us to suggest a number of reaction
steps that are responsible for the changes in the product concen-
trations at high contact times.
conclusion agrees well with the low Pt dispersion in our catalyst
(12%). Indeed, it is clear that most of the Pt species are present
in the catalyst as large particles (∼10 nm). Such Pt particles can
be located only on the outer surface of the zeolite crystallites and,
therefore, are spatially separated from BAS, which are located in-
side the channel system of the PtH-MFI catalyst.
C2H6 ꢀ C2H4 + H2
(2)
(3)
◦
The kinetic study reported in this paper was carried out at 370 C.
At this temperature the equilibrium ethane dehydrogenation con-
version into ethene (for pure ethane as feed) is around 0.55%
Formation of methane is clearly associated with formation of
toluene via hydrogenolysis (hydrocracking) of EB over Pt sites of
the catalyst

386
D.B. Lukyanov, T. Vazhnova / Journal of Catalysis 257 (2008) 382–389
and (15)). It is known (see, for example, the paper by Cortright et
al. [20] and references therein) that hydrogenolysis reactions re-
quire several adjacent Pt sites. According to the data on the Pt
dispersion (12%), our catalyst contains large Pt particles and, there-
fore, is likely to catalyze hydrogenolysis reactions (in particular, at
high contact times that lead to higher ethane conversions and, as
a consequence, to higher concentrations of H2 in the reaction mix-
ture).
(
12)
13)
(
C2H6 + H2 → CH4 + CH4
C3H8 + H2 → C2H6 + CH4
(14)
(15)
The experimental data of this work does not allow estimation
of the relative contributions of the reactions (12)–(15) to methane
formation, but it is worth noting that hydrogenolysis of toluene
(
reaction (12)) can by itself explain that methane is formed in
higher amounts than toluene (Figs. 5A and 5C). Hydrogenolysis of
PB (Eq. (13)) is likely to be responsible for the observed maxi-
mum in PB concentration (Fig. 5B), while EB hydrogenolysis (Eq.
(
(
11)) appears to be the main reaction that limits EB concentration
Fig. 5A). Another reaction, which can contribute essentially to EB
transformation is the acid catalyzed EB alkylation with ethene into
DEB (Eq. (4)). Indeed, Fig. 5D shows that this reaction is obviously
followed by DEB alkylation with ethene into TEB as shown below.
(16)
The latter reaction leads to the relatively high concentrations of
TEB (Fig. 5D) and therefore requires essential amounts of EB, as
a starting compound, for its production. It is very likely that TEB
formation is responsible for the observed maximum in DEB con-
centration (Fig. 5B).
Formation of xylenes can be explained by EB hydroisomeriza-
tion reaction
(17)
that proceeds via a sequence of reaction steps that occur over Pt
and acid sites, as discussed by Moreau et al. [21] in their study of
EB transformation over Pt containing mordenite catalysts.
Formation of ethyltoluenes is likely to occur via toluene alkyla-
tion with ethene over BAS (Eq. (18)) and DEB hydrogenolysis over
Pt sites (Eq. (19)) as shown below.
Fig. 5. Effect of contact time on the concentrations of the reaction products formed
during benzene alkylation with ethane over the PtH-MFI catalyst at 370 C: (A) hy-
◦
drogen, methane, EB and ethene; (B) DEB, PB and propene; (C) toluene, xylenes and
propane; (D) ETT, TEB and other aromatics (C10 –C12 alkylbenzenes, naphthalene and
its derivatives). Ethane to benzene molar ratio in the feed was 9:1, and the experi-
mental data were obtained at TOS between 20 and 24 h.
(18)
(19)
(11)
However, taking into consideration that methane is formed in es-
sentially higher amounts than toluene (Figs. 5A and 5C), we can
conclude that other hydrogenolysis reactions are taking place. The
most likely reactions are shown below (reactions (12), (13), (14)
Alkylation of toluene with ethene (Eq. (18)) appears to be more
important for ETT formation due to much higher toluene con-
centration in comparison with the concentration of DEB (Figs. 5B
and 5C).

D.B. Lukyanov, T. Vazhnova / Journal of Catalysis 257 (2008) 382–389
387
Finally, let us consider formation of other aromatic products
Fig. 5D) that include C10–C12 alkyl and dialkylbenzenes, naphtha-
molecules (including naphthalene and its derivatives) that were
observed in this study.
(
lene and its derivatives. The term “C10–C12 alkyl and dialkylben-
zenes” is used in this paper to define aromatic molecules that con-
tain 10–12 carbon atoms, i.e. benzene molecules substituted with
relatively short alkyl groups. Most of the dialkylbenzenes were rep-
resented by ethylpropylbenzenes, which are clearly the products of
the acid catalyzed EB alkylation with propene and PB alkylation
with ethene
From Figs. 1 and 2D it follows that at contact time of 0.32 h
−1
(WHSV = 3.1 h ) C10–C12 alkylbenzenes and naphthalenic com-
pounds constituted about 12 mol% of all aromatic products formed
over the PtH-MFI catalyst used in this work. Interestingly, under
the same reaction conditions [12], formation of these aromatic
compounds was not observed over the Pt-containing MFI catalysts
with the Si/Al ratio of 36 and 140. It is likely that this difference
is due to the lower acidity of the latter catalysts when compared
to the PtH-MFI catalyst (Si/Al = 15) used in this work. Indeed, as
shown above, C10–C12 alkylbenzenes and naphthalenic compounds
are formed via a long sequence of the reaction steps, some of
which are catalyzed by the Pt sites (e.g. dehydrogenation steps)
while the other proceed over BAS (e.g. alkylation steps, alkene
oligomerization and cracking steps). As a consequence, formation
of these aromatic compounds over the PtH-MFI catalysts of lower
acidity is likely to occur at higher contact times, as compared to
the highly acidic PtH-MFI catalyst used in this work.
(20)
(21)
Formation of C10–C12 alkylbenzenes at high contact time can be
explained by benzene alkylation with butenes, pentenes and hex-
enes that are the products of alkene oligomerization and cracking
reactions (Eqs. (5), (6) and (8)). Once formed, C10–C12 alkylben-
zenes can be transformed into naphthalene and its derivatives,
which were also observed at high contact times. Based on the
chemistry of the aromatization (dehydrocyclization) reactions es-
tablished for the acid zeolite catalysts [19,22–24] and taking into
consideration the presence of Pt sites in our MFI catalyst, we sug-
gest that C10–C12 alkylbenzenes undergo dehydrogenation over Pt
sites (Eq. (22)) that is followed by cyclization and isomerization
steps over BAS (Eq. (23)) and, finally, by the dehydrogenation steps
catalyzed by Pt sites (Eq. (24)), as illustrated below for the trans-
formation of pentylbenzene into methyl-naphthalene.
3
.3. Time on stream experiments
In order to investigate the stability of the PtH-MFI catalyst at
different levels of the feed conversions, we performed three TOS
experiments at different contact times. Fig. 6 shows the results
of these experiments and reveals stable performance of the PtH-
MFI catalyst during 45–46 h on stream at benzene conversions
of ∼8 and ∼12%. For initial benzene conversion of 8%, no essen-
tial changes were observed in the product distribution and, as a
consequence, the selectivity to EB in the aromatic products (see
Fig. 7, the data corresponding to τ = 0.024 h) was very stable
during 45 h. For initial benzene conversion of 12% (τ = 0.081 h),
the EB selectivity was nearly constant during 28 h and then in-
creased slightly (from ∼ 84 to ∼87%) during the following 18 h
of the experiment (Fig. 7). Simultaneously with this increase a
(22)
(23)
(24)
With purely acidic zeolite catalysts, formation of the hydrogen-
deficient hydrocarbons, which are the products of the reactions
(22) and (24), proceeds entirely via bimolecular hydrogen trans-
fer (HT) steps with the involvement of carbenium ions formed by
alkenes on BAS [19,22–24]. Thus, it is possible that the HT steps
operate in parallel with the dehydrogenation steps in the case of
bifunctional catalysts that contain both the metal and acid sites.
The relative importance of these two routes in the dehydrocycliza-
tion reactions depends on the zeolite structure, number of metal
and acid sites in the catalysts, and alkene concentration in the re-
action mixture [16,25,26]. As discussed above, under the reaction
conditions used in this work, the concentration of alkenes, which
are required for the HT reactions, is very low at any point in the
reactor. Therefore, it is likely that the dehydrogenation reactions
Fig. 6. Effect of time on stream on (A) benzene and (B) ethane conversions over
PtH-MFI catalyst at contact times of (Q) 0.024, (2) 0.081 and (") 0.32 h at 370 C.
◦
(22) and (24) dominate in the formation of the hydrogen-deficient
Ethane to benzene molar ratio in the feed was 9:1.

388
D.B. Lukyanov, T. Vazhnova / Journal of Catalysis 257 (2008) 382–389
Table 1
Comparison of the selectivities to aromatic products formed over the steady oper-
ating and partially deactivated PtH-MFI catalyst at benzene conversion of 17.5%
Catalyst
Steady operatinga
Partially deactivated
Contact time (h)
Time on stream (h)
Benzene conversion (%)
Aromatics selectivity (mol%)
Toluene
0.21
20–24
17.5
0.32
46
17.4
13.5
60.5
1.3
13.8
60.3
1.1
Ethylbenzene
Xylenes
Propylbenzenes
Ethyltoluenes
Diethylbenzenes
Triethylbenzenes
Other aromatics
0.8
4.8
1.7
8.4
9.0
0.9
4.1
1.8
7.8
Fig. 7. Effect of time on stream on EB selectivity in aromatic products produced
10.2
during benzene alkylation with ethane over PtH-MFI catalyst at contact times of
◦
a
The data for the steady operating PtH-MFI catalyst at benzene conversion of
(
Q) 0.024, (2) 0.081 and (") 0.32 h at 370 C. Ethane to benzene molar ratio in the
feed was 9:1.
17.5% were obtained by interpolation of the experimental data shown in Figs. 1
and 2.
slight decrease was observed in the selectivities to toluene, ETT,
TEB, C10–C12 alkylbenzenes and naphthalenic compounds, which
can be considered (according to Fig. 2) as the end aromatic prod-
ucts of the reaction over the PtH-MFI catalyst used in this study.
These observations suggest that very slow coke formation is taking
place in the PtH-MFI catalyst when benzene alkylation with ethane
is carried out at the contact time of 0.081 h. The level of the cat-
alyst deactivation is very low under these conditions, as evident
from the fact that no noticeable changes have been observed in
the conversions of benzene and ethane during 46 h of the reaction
and that the deactivation simply leads to a decrease in “effective”
contact time due to the decrease in the number of active sites
available for the reaction. Further studies are in progress to verify
this suggestion and get better understanding of the coke forma-
tion and its effect on the Pt and acid sites present in the PtH-MFI
catalyst.
4
. Conclusions
(Fig. 6).
At the initial benzene conversion of ∼20% the PtH-MFI catalyst
This study demonstrates for the first time that highly selec-
demonstrates stable performance during the first 28 h on stream
(
tive and stable benzene alkylation with ethane into EB can be
achieved (Figs. 2, 6 and 7) with the highly acidic PtH-MFI cat-
Figs. 6 and 7, the data corresponding to τ = 0.32 h), but then
a decrease in the conversions of benzene and ethane is observed.
This decrease is possibly associated with a higher coking rate un-
der these reaction conditions. In our view, that can be related to
the enhanced formation (at high contact time) of C10–C12 alkylben-
zenes and naphthalenic compounds, which are usually considered
as coke precursors in reactions over zeolite catalysts [23,27–30].
Indeed, the concentration of these compounds (Fig. 5D) correlates
inversely with the stability of the catalyst performance in three
TOS experiments that were carried out at different contact times
◦
alyst at 370 C at benzene conversions up to 10–12%, which are
close to the equilibrium benzene conversion of 13.5%. Analysis of
the product selectivities (Fig. 4) confirms the suggestion made ear-
lier [6,7,12] that EB is formed via the sequence of two reaction
steps: (i) dehydrogenation of ethane into ethene over Pt sites and
(
ii) benzene alkylation with ethene over acid sites. These two re-
actions control transformation of both the ethane and benzene at
low contact times up to benzene conversions of ∼10% due to the
low rates of many side reactions. Some of these reactions (such
as ethane and EB hydrogenolysis) are slow because of the rel-
atively low hydrogenolysis activity of the PtH-MFI catalyst used
in this work, while the other (such as reactions of alkenes) are
(
Figs. 6 and 7). This observation agrees well with our previous re-
sults on very stable operation of two PtH-MFI catalysts of lower
acidity [12] when formation of C10–C12 alkylbenzenes and naph-
thalenic compounds was not observed at all.
The deactivation of the PtH-MFI catalyst, which was used in
this work, was observed at the contact time of 0.32 h between 28
and 46 h on stream. During this time ethane conversion decreased
from 5.3 to 4.5%, while benzene conversion declined from 20 to
7.4% (Fig. 6). The decrease in the feed conversion was accompa-
nied by an increase in the EB selectivity in the aromatic products
Fig. 7) that changed from ∼50 to ∼60 mol% between 28 and 46 h
◦
suppressed by the inherently low ethene concentration at 370 C.
With further increase in benzene conversion (due to the increase
in contact time) the selectivity of benzene transformation into EB
drops steeply and is about 40 mol% at benzene conversion of 21.5%
1
(
Fig. 2A). It is shown that this loss of the EB selectivity is asso-
ciated with a number of side reactions that take place at high
contact times. The main side reactions include EB hydrogenolysis
into toluene and methane, EB alkylation with ethene into DEB and
TEB, formation of C10–C12 alkylbenzenes and naphthalenic com-
pounds. The latter compounds are likely to be responsible for slow
deactivation of the PtH-MFI catalyst that was observed in this work
at benzene conversion of ∼20% after 28–30 h on stream.
(
on stream. Thus, the deactivation of the catalyst active sites has
led to the gain in the EB selectivity of ∼10 mol%. Consideration
of the data in Fig. 2A shows that practically the same change in
the EB selectivity is observed when benzene conversion is chang-
ing between 17.5 and 20% due to the increase in contact time. In
both cases (catalyst deactivation and variation of contact time),
the increase in the EB selectivity is associated with decreasing
selectivities to toluene, TEB, ETT, and C10–C12 alkylbenzenes and
naphthalenic compounds. Table 1 compares the aromatic product
selectivities observed over the steady working and partially deac-
tivated PtH-MFI catalyst at benzene conversion of 17.5%. The com-
parison reveals that catalyst deactivation and variation of contact
time have very similar effects on the distribution of the aromatic
products. This suggests that there is no preferential deactivation
of the Pt or acid sites in the PtH-MFI catalyst used in this work,
Analysis of the reaction pathways allows us to conclude that
four requirements should be met in order to achieve highly se-
lective and stable benzene alkylation with ethane into EB over
bifunctional zeolite catalysts. These requirements include: (i) the
low hydrogenolysis activity of the catalyst; (ii) the low concentra-
tion of alkenes in the reactor; (iii) the decoupling of the steps of
EB formation and the steps of its further transformation; and (iv)
the elimination (prevention) of the steps of formation of C10–C12
alkylbenzenes and naphthalenic compounds.

D.B. Lukyanov, T. Vazhnova / Journal of Catalysis 257 (2008) 382–389
389
Acknowledgments
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We thank Dr. Paul Millington (Johnson Matthey) for analysis of
the Pt dispersion and valuable comments, and Dr. Sean Rigby (Uni-
versity of Bath) and Dr. Stan Golunski (Johnson Matthey) for help-
ful discussions. This work was supported by the Engineering and
Physical Sciences Research Council of the UK (grant EP/C532554).
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