Alkylation of toluene with ethanol to para-ethyltoluene over MFI zeolites: Comparative study and kinetic modeling

Article abstract of DOI:10.1016/j.cattod.2014.08.019

The production of para-ethyltoluene (p-ET) from the alkylation of toluene with ethanol was investigated over three MFI zeolites with varying SiO₂/Al₂O₃ ratio (80, 280, and 2000). The ethylation reaction was conducted in a batch fluidized-bed reactor at a temperature range of 300-400 °C, reaction times of 5-20 s and molar feed ratio of toluene to ethanol at 1:1. Toluene conversion increased with temperature over all the MFI zeolites except for MFI-80, which showed a maximum conversion of 29% at 300°C. The product distribution exhibited ethyltoluenes as major product with a maximum yield of 26% over MFI-80. At 400°C, constant toluene conversion of 14% and 100% ethanol conversion, para-selectivity to p-ET was 100% over MFI-2000 compared with 27% and 48% over MFI-80 and MFI-280, respectively. The high para-selectivity over MFI-2000 is attributed to the combined effects of higher SiO₂/Al₂O₃ ratio, very weak acid sites and larger crystal size (longer diffusion length). The experimental data were analyzed for each MFI zeolite and suitable reaction mechanism for toluene ethylation was proposed based on the Langmuir-Hinshelwood model. The activation energy for the formation of p-ET over MFI-280 and MFI-2000 is 30 kJ/mol and 65 kJ/mol, while the heat of adsorption of ethanol is 19 kJ/mol and 29 kJ/mol, respectively.

Full text of DOI:10.1016/j.cattod.2014.08.019

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Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Alkylation of toluene with ethanol to para-ethyltoluene over MFI
zeolites: Comparative study and kinetic modeling
Babatunde A. Ogunbadejo, Mogahid S. Osman, Palani Arudra, Abdullah M. Aitani,
Sulaiman S. Al-Khattaf∗
Center of Research Excellence in Petroleum Refining & Petrochemicals, King Fahd University of Petroleum & Minerals, Dhahran 31261, Saudi Arabia
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 4 June 2014
Received in revised form 10 August 2014
Accepted 18 August 2014
Available online xxx
The production of para-ethyltoluene (p-ET) from the alkylation of toluene with ethanol was investi-
gated over three MFI zeolites with varying SiO2/Al2O3 ratio (80, 280, and 2000). The ethylation reaction
was conducted in a batch fluidized-bed reactor at a temperature range of 300–400 ◦C, reaction times of
5–20 s and molar feed ratio of toluene to ethanol at 1:1. Toluene conversion increased with tempera-
ture over all the MFI zeolites except for MFI-80, which showed a maximum conversion of 29% at 300 ◦C.
The product distribution exhibited ethyltoluenes as major product with a maximum yield of 26% over
MFI-80. At 400 ◦C, constant toluene conversion of 14% and 100% ethanol conversion, para-selectivity to
p-ET was 100% over MFI-2000 compared with 27% and 48% over MFI-80 and MFI-280, respectively. The
high para-selectivity over MFI-2000 is attributed to the combined effects of higher SiO2/Al2O3 ratio, very
weak acid sites and larger crystal size (longer diffusion length). The experimental data were analyzed
for each MFI zeolite and suitable reaction mechanism for toluene ethylation was proposed based on
the Langmuir–Hinshelwood model. The activation energy for the formation of p-ET over MFI-280 and
MFI-2000 is 30 kJ/mol and 65 kJ/mol, while the heat of adsorption of ethanol is 19 kJ/mol and 29 kJ/mol,
respectively.
Keywords:
MFI
Crystal size
Para-selectivity
Para-ethyltoluene
Toluene ethylation
Ethanol
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
and B-, Fe-, Ga-isomorphously substituted zeolites of MFI structure
with Si/metal ratios between 50 and 64 [10]. They reported that
The alkylation of toluene with ethanol produces ethyltoluenes
which find applications in the petrochemical and chemical indus-
tries [1–5]. The ethylation of toluene with ethanol produces a wide
range of hydrocarbon products which include C1–C4 gases (mostly
obtained from dehydration and disproportionation reactions of
ethanol and toluene, respectively), ethylbenzene, xylenes, and mix-
tures of ethyltoluene isomers [4–6]. In addition, other products
such as styrene and methylstyrene could also be obtained as a result
of side-chain alkylation, a condition favored by basic zeolites. The
composition of the products may vary depending on the structural
and morphological properties of the zeolites used [7–9].
an increase in MFI crystal size results in a reduction in the active
sites available on the external surface thereby inhibiting the isom-
erization of p-ET formed in the pores to other isomers. This is in
conformity with the reaction mechanism proposed by Paparatto
et al. using MFI (SiO2/Al2O3 = 25, 58, 63) and MEL (SiO2/Al2O3 = 40,
80). They concluded that p-ET is initially formed in the zeolite chan-
nels but undergoes isomerization if there are available acid sites
ˇ
on the external surface [8]. Wichterlova and Cejka [11] studied
para-selectivity from the view point of the diffusion coefficient
and coke deposition. It was observed that coke deposition did
not influence para-selectivity, however, surface silylation with
tetraethylorthosilicate (TEOS) could enhance p-ET due to pore nar-
rowing.
Another important factor that could enhance para-selectivity
is the external surface area. A correlation has been established
between the crystal size and external surface area of MFI with
SiO2/Al2O3 ratios of 22 to 600 [11–14]. An increase in the crystal
size translates to a reduction in the external surface which enhances
the para-selectivity by impeding isomerization into meta and ortho
isomers. Further improvement of para-selectivity over MFI can be
Several researchers have investigated various parameters to
enhance para-selectivity and catalyst activity. Parikh et al. studied
the effects of crystal size and acidity on para-selectivity over A1-MFI
∗
Corresponding author. Tel.: +966 13 860 2029; fax: +966 13 860 4509.
E-mail addresses: ogunbadejo.babatunde@yahoo.com (B.A. Ogunbadejo),
mogahid@kfupm.edu.sa (M.S. Osman), arupalya@kfupm.edu.sa (P. Arudra),
maitani@kfupm.edu.sa (A.M. Aitani), skhattaf@kfupm.edu.sa (S.S. Al-Khattaf).
http://dx.doi.org/10.1016/j.cattod.2014.08.019
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: B.A. Ogunbadejo, et al., Alkylation of toluene with ethanol to para-ethyltoluene over MFI zeolites:
Comparative study and kinetic modeling, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.019

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in standing air at 550 ◦C for 5 h (ramping rate of 3 ◦C min−1), in
Nomenclature
order to get the H-form. MFI with a SiO2/Al2O3 of 2000 (MFI-2000)
was prepared using hydrothermal techniques, in a typical synthe-
sis of this sample 4.26 g tetrapropylammonium bromide, 0.7407 g
ammonium fluoride and hydrated aluminium nitrate 0.0750 was
dissolved in 72 ml of water and stirred well for 15 min. 12 g silica
(381276 Aldrich) was added and stirred well until homogenized.
The obtained gel was autoclaved and kept at 200 ◦C for 2 days. The
molar composition of gel was 1 SiO2: 0.08 (TPA) Br: 0.10 NH4F:
0.0005 Al2O3: 20 H2O. The solid product obtained was washed
with water and dried at 80 ◦C overnight. The template was removed
by calcination in air at 750 ◦C for 5 h. These zeolites are hereafter
referred as MFI-80, MFI-280 and MFI-2000, where the number rep-
resents the nominal SiO2/Al2O3 ratio. Silicalite-1 was synthesized
by the same procedure as MFI-2000 but without addition of alu-
minium source i.e. aluminium nitrate.
Ci
concentration of specie i in the riser simulator
(mol/m3)
CL
confidence limit
Ei
apparent activation energy of the ith reaction
(kJ/mol)
Ki
ki
adsorption equilibrium constant of component i
apparent rate constant for the ith reaction (m3/kg of
catalyst .s)
koi
pre-exponential factor for ith reaction after re-
parameterization (m3/kg of catalyst s)
molecular weight of specie i
universal gas constant (kJ/kmol K)
reaction time (s)
MWi
R
t
T
reaction temperature (K)
Toluene and ethanol were obtained from Sigma-Aldrich and no
further attempt was made to purify the chemicals.
To
V
Wc
Whc
average temperature of the experiment (K)
volume of the riser (45 cm3)
mass of the catalyst (0.81 g)
total mass of the hydrocarbon injected the riser
(0.162 g)
2.2. Catalyst characterization
ꢀSa0ds,i entropy for adsorption for componenet i
ꢀHa0ds,i enthalpy for adsorption for componenet i
The MFI zeolites were characterized using several techniques.
The amounts of SiO2 and Al2O3 the catalysts were determined
by atomic absorption spectrometer (Perkin-Elmer AAS Analyst
100). Textural properties were characterized by N2 adsorption-
desorption measurements at 77 K, using Quantachrome Autosorb
1-C adsorption analyzer. Samples were outgassed at 220 ◦C
under vacuum (10−5 Torr) for 3 h before N2 physisorption. The
Brunauer–Emmett–Teller (BET) specific surface areas were deter-
mined from the dsorption data in the relative pressure (P/P0) range
from 0.06–0.3, assuming 0.164 nm2 for the cross-section of the N2
molecule. Contribution of micropore and mesopores was derived
from the t-plot method according to Lippens and de Boer [21].
High-angle X-ray diffraction patterns were recorded on a Rigaku
Miniflex II XRD powder diffraction system using CuK␣ radia-
Greek letters
ϕ
˛
apparent deactivation function
catalyst deactivation constant (time on stream
model)
achieved by impregnation of the zeolite channels with metal or
non-metal oxides [15–18]. Modification of MFI (SiO2/Al2O3 = 50)
with different elements (B, P, Mg, Si, La, or Cd) showed an increase
in para-selectivity to p-ET due to the decreased concentration of
strong acid sites and the sorption capacity of the zeolite [16–19].
The heat of adsorption for toluene and ethanol was reported
at 56 kJ/mol and 35 kJ/mol, respectively, with surface activa-
tion energy of 62 kJ/mol [16]. Parikh reported the kinetics of
the ethylation reaction using a monolith reactor on which MFI
(SiO2/Al2O3 = 24) was wash-coated [20]. The proposed rate expres-
sion indicated that p-ET is the primary product of the alkylation
reaction. o-ET was not considered due to negligible quantities while
the net rate of m-ET formation was a result of the total rate of
toluene consumption to form p-ET and the subsequent isomeri-
zation rate of p-ET to m-ET.
Most of the zeolites that showed high para-selectivity for
toluene ethylation were subjected to post-synthesis steps either by
modification of zeolite channels or deactivation of external surface.
Consequently, the aim of this paper is to present aspects related
to the development of a para-selective MFI-zeolite for toluene
ethylation to p-ET without the need to modify either the zeolite
pore channels or external surface. The paper addresses the effects
of SiO2/Al2O3 ratio, crystal size and reaction temperature on p-
ET formation. It also focuses on development of a kinetic model
accounting for all reaction steps i.e. adsorption, surface reaction
and desorption.
˚
tion (ꢁK␣1 = 1.54051 A, 30 kV and 15 mA). The XRD patterns were
recorded in the static scanning mode from 1.2–60◦ (2␪) at a detec-
tor angular speed of 2◦/min and step size of 0.02◦. The crystal
sizes of the MFI zeolites were measured using a scanning elec-
tron microscopy (SEM) by Nova NanoSEM FEI with an accelerating
voltage of 30 kV.
In order to assess the acid sites on the MFI zeolites and their
properties, NH3 temperature-programmed desorption (TPD) and
FTIR spectroscopy of adsorbed pyridine were used. NH3-TPD was
carried out using Quantachrome Autosorb 1-C/TCD to determine
total acid sites on the catalysts. Samples were pretreated at 450 ◦C
in a stream of helium (25 ml min−1) for 2 h. This was followed by
the uptake of ammonia (5 vol. % in helium) at 100 ◦C for 30 min. The
samples were then subjected to flow of helium for 2 h at 120 ◦C so
as to remove loosely bound ammonia (i.e. physisorbed ammonia).
After that, the samples were heated from 100–700 ◦C at a rate of
10 ◦C/min in a flow of helium (25 ml min−1) while monitoring the
evolved ammonia using TCD.
Infrared spectroscopy of adsorbed pyridine was used to iden-
tify the nature of available acid sites (i.e. Brønsted and/or Lewis
acid sites). The measurements were conducted using a Fourier
transform infrared with Nicolet FTIR spectrometer (Magna 500
model). The samples, as self-supporting wafers (ca. 60 mg in weight
and 20 mm in diameter) were obtained by compressing a uniform
layer of the powdered samples. The wafer was then inserted into
an infrared vacuum cell equipped with KBr windows (Makuhari
Rikagaku Garasu Inc., Japan), and preheated under vacuum (ca.
10−3 torr) at 450 ◦C for 2 h. The adsorption temperature of pyridine
was 150 ◦C. The IR cell was then cooled down to ambient temper-
ature and placed in an IR beam compartment while under vacuum
2. Experimental
2.1. Materials
Two of the MFI zeolites used in this work were procured from
Zeolyst; MFI-80 (CBV8014, NH4-form) and MFI-280 (CBV28014,
NH4-form). Prior to catalyst testing, the zeolites were calcined
Please cite this article in press as: B.A. Ogunbadejo, et al., Alkylation of toluene with ethanol to para-ethyltoluene over MFI zeolites:
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and transmission spectra were recorded. The bands and absorption
2 ␮m [25]. For this crystal size range, one can expect that the value
of the effectiveness factor should be close to one. Considering this
fact, the effect of diffusion has not been incorporated in the kinet-
ics analysis. Marin and co-workers reported the kinetic modeling
of ethylbenzene dealkylation over Pt promoted MFI by neglecting
the contribution of the diffusion resistance [26].
coefficients used are as follows: pyridine (PyH+) band at 1545 cm−1
ε = 0.078 cm ␮mol−1; pyridine (PyL) bands at 1461 and 1454 cm−1
ε = 0.165 cm ␮mol−1 [22,23].
,
,
2.3. Catalytic test
Langmuir–Hinshelwood mechanism with surface reaction con-
trol was proposed as descriptor of the kinetic of the reaction
of toluene with ethanol. The proposed model considers only the
adsorption of ethanol and ethyltoluenes on the catalyst sites, the
adsorption of toluene and other products are negligible as com-
pared to ethanol and ethyltoluenes. Ethanol has strong tendency to
adsorb on catalysts site thereby transforming into surface ethoxy
groups which further interacts with lightly adsorbed toluene to
form ethyltoluenes [8]. According to the proposed reaction scheme,
the rate of toluene disappearance and the rate of ethyltoluenes
formation can be written as:
The ethylation of toluene with ethanol was conducted in a
batch fluidized-bed reactor operated under atmospheric pressure.
The reactor was designed to simulate an industrial scale reactor
operated in batch mode with an impeller to aid gas mixing. The
comprehensive description of the riser simulator can be found else-
where [24].
The experiments for toluene ethylation with ethanol were per-
formed using 0.81 g of catalyst with 1:1 molar ratio of toluene
and ethanol (equivalent to 67:33 wt%) at different reaction times
of 5, 10, 15 and 20 s and temperatures between 300 and 400 ◦C.
Before each experimental run, the catalyst was activated for 15 min
at 610 ◦C in a continuous flow of air after which the temperature
was brought down to the desired reaction level. Once the reactor
gets to the reaction temperature, the impeller was started with a
speed of 5500 rpm and the feedstock was injected into the reac-
tor via a pre-loaded syringe. At the end of the reaction, the port
valve automatically opened ensuring termination of the reaction
and the product was sent to an online gas chromatograph through
the vacuum box. Each run was repeated to ensure the results can be
reproduced with standard deviation in the range 2%. The response
factor of each product was taken into account in analyzing the
product distribution. The conversion, product yield, selectivity and
para-selectivity were determined using the following relations:
Rate of disappearance of toluene (To)
ꢀ
ꢁ
V
dCTo
k1 KEtOH KEtTol CEtOH CEtTol
−
=
ϕ
(1)
2
WC dt
1 + KEtOHCEtOH + KEtTol CEtTol
(
)
Rate of ethyltoluenes formation (EtTol):
ꢀ
ꢁ
V
dCEtTol
k1 KEtOH KEtTol CEtOH CEtTol
1 + KEtOHCEtOH + KEtTol CEtTol
=
ϕ
(2)
2
WC dt
(
)
where Ci is molar concentration of each species in the system, V is
the volume of the reactor, Wc is the mass of the catalyst, t is time in
seconds, ϕ is the apparent deactivation function, k1 is the apparent
rate coefficient and Ki is the adsorption coefficient of each species
on the catalyst.
toluene in feed − toluene in product
The reaction rate constant was represented with the tempera-
ture dependence using the following form of Arrhenius equation:
% toluene conversion =
× 100
toluene in feed
ꢂ
ꢂ
ꢃꢃ
−E
1
T
1
T0
wt% of product i
wt% of toluene in feed
k = k0 exp
−
(3)
% yieldi =
× 100%
R
where ki0 is pre-exponential factor at the average temperature T0
and E is apparent activation energy of the reaction.
The adsorption equilibrium constants with the temperature
dependence can be expressed according to the following thermo-
dynamic relations [27,28]:
wt% of ethyltoluenes ET in product
(
)
% selectivity =
× 100
wt% of aromatics in product
wt% of p-ET in product
wt% of ET in product
% para-selectivity =
× 100
ꢄ
ꢅ
ꢀSa0ds,i
R
ꢀHads,i
0
ꢂ
ꢃ
1
T
1
To
Ki = exp
−
−
(4)
R
2.4. Kinetic modeling
where ꢀHa0ds,i is the change of enthalpy and ꢀSa0ds,i is the change
of entropy of the adsorption.
Phenomenological based kinetic model demonstrating the
alkylation of toluene with ethanol is developed based on Scheme 1
and the observed data obtained from the fluidized-bed reactor
at different reaction temperatures and varying reaction times.
Isothermal reactor condition was assumed given that the amount
of the reacting species is relatively small and the contribution of
heat of reaction can be considered negligible.
Within the experimental conditions, the alkylation reactions
were considered to be free from internal and external mass trans-
fer limitations. This assumption is reasonable given the fact that the
MFI sample used in this study has small crystallite sizes of 0.5 ␮m to
The effects of catalyst deactivation has been taken into account
by considering a time on stream deactivation function as described
in Eq. (5) [29].
ϕ = exp −˛t
(5)
(
)
where, ˛ is a constant and t is the time the catalyst is exposed to
reaction.
A nonlinear regression algorithm (MATLAB, ODE 45-4th order
Runge–Kutta method and least-square curve fitting “lsqcurvefit”
routine) was used to solve the model equations and to obtain the
CH3
CH3
CH3
CH3
CH3
k1
H2O
+
Ethanol
+
CH3
CH3
Scheme 1. Reaction network of toluene alkylation with ethanol.
Please cite this article in press as: B.A. Ogunbadejo, et al., Alkylation of toluene with ethanol to para-ethyltoluene over MFI zeolites:
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Table 1
Physico-chemical properties of zeolite samples.
SiO2/Al2O3 ratioa
N2 sorption
V (cm3/g)c,d
Zeolite sample
e
dspacing (Å)b
SBET (m2/g)
Smeso (m2 g−1
)
MFI-80
86
3.861
3.856
3.833
3.824
425
400
367
363
0.28 (0.19)
0.23 (0.21)
0.19 (0.18)
0.19 (0.18)
68
33
5.0
3.0
MFI-280
MFI-2000
Silicalite-1
295
2130
n.d.
a
AAS analysis.
Evaluated by XRD (hkl = 501).
Total pore volume.
Number in parenthesis corresponds to micropore volume calculated using the t-plot.
Smeso includes the mesoporous and external surface area, n.d. = not detected.
b
c
d
e
kinetic parameters. The optimization criteria for the model evalu-
ation are that all the rate constants and the activation energies had
to be positive and consistent with physical principles. The details
of the regression analysis are described in Waziri et al. [30].
3. Results and discussion
3.1. Catalyst characterization
Table 1 shows the SiO2/Al2O3 ratio for the MFI zeolites obtained
by AAS analysis. The results show that MFI-80, MFI-280 and MFI-
280 have SiO2/Al2O3 ratio of 86, 295 and 2130, respectively. The
nitrogen adsorption isotherms results of the MFI zeolites and
silicalite-1 are presented in Fig. 1. The textural parameters calcu-
lated from the nitrogen sorption isotherms are compiled in Table 1.
The surface area of silicalite-1 is 363 m2/g, which is lower than that
of other MFI zeolites. The incorporation Al atoms in the framework
increased the pore volume as well as surface area of the mate-
rial [31]. The total pore volume of MFI-2000 was similar to that
of silicalite-1 due to the very low concentration of Al present in
MFI-2000.
Fig. 2. XRD patterns of MFI-80, MFI-280, MFI-2000 and silicalite-1.
The XRD patterns of catalyst samples are presented in Fig. 2.
The patterns of all samples exhibit XRD reflections that conform
with the characteristic of MFI structure in the ranges between 8–9◦
and 22–25◦ [32,33]. The intensity of the peaks in the range 22–25◦
slightly decreased with an increase in SiO2/Al2O3 ratio showing
a slight decrease in crystallinity [34].The values of lattice spacing
d5 0 1 determined by an XRD peak at (hkl) of 5 0 1 reflection are
presented in Table 1. The values matched with standard MFI in the
database [32].
The SEM micrographs of the three MFI zeolites are shown in
Fig. 3. MFI-80 has a crystal size of about 0.5–1 ␮m and MFI-280 has
2–3 ␮m size, whereas MFI-2000 has a large crystal size of about
30–35 ␮m. The formation of uniform crystallite for MFI-2000 was
also observed.
TPD profiles of desorbed ammonia for the three MFI are pre-
sented in Fig. 4. Two peaks appear in ammonia TPD profile for
zeolites as reported by Kadata and Niwa [35]. They assigned the
peaks appeared at a higher temperature and those appeared at a
lower temperature to the ammonia desorbed from the acid sites of
+
zeolites and ammonia molecules adsorbed either on NH4 species
formed on Brønsted acid sites or on Na+ cations, respectively. In the
present study, the numbers of acid sites were estimated from the
areas of the peaks at a higher temperature and listed in Table 2. The
number of acid sites increased with a decrease in SiO2/Al2O3 ratio
of MFI zeolites, which was expected. Pyridine adsorption by FT-IR
spectroscopy was carried out to evaluate the strength and types
of acid sites. Table 2 presents the amount of Lewis and Brønsted
acid sites in the MFI zeolites. For MFI-80 and MFI-280, the pres-
ence of both types of acid sites was observed, whereas in the case of
MFI-2000 and silicalite-1 these sites were very weak and were not
detected by FT-IR spectra recorded after the adsorption of pyridine
and subsequent evacuation at 150 ◦C. The Lewis acid sites observed
by IR of adsorbed pyridine are not the Na+ cations, but coordina-
tively unsaturated Al3+ cations formed by dehydroxylation of the
OH groups bridging to Al and Si atoms (Brønsted acid sites).
Fig. 1. N2-adsorption–desorption isotherms of MFI-80, MFI-280, MFI-2000 and
silicalite-1.
Please cite this article in press as: B.A. Ogunbadejo, et al., Alkylation of toluene with ethanol to para-ethyltoluene over MFI zeolites:
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Fig. 3. SEM images of MFI-80, MFI-280, and MFI-2000.
3.2. Catalytic activity
undergoes isomerization to meta and ortho isomers at the external
surface, subject to availability of Brønsted acid sites [8].
Table 3 presents the catalytic performance of MFI zeolites for
toluene ethylation with ethanol at different temperatures. The
results comprise toluene conversion, product yields, selectivity to
ethyltoluenes, p-ET, m-ET, and o-ET. In toluene ethylation over
MFI-2000, it is evident that the major product at all temperatures
studied was p-ET with trace amount of EB, xylene and benzene
indicating a negligible degree of other side reactions such as dispro-
portionation and dealkylation. This is attributed to the low acidity
of the catalyst due to its low Al content [36]. The selective forma-
tion of p-ET over MFI-2000 may be attributed to the combined effect
of acid sites, absence of external acid sites and crystal size. p-ET is
formed within the zeolite pores being smaller in size than other iso-
mers and its ease of diffusing over large crystal size in the range of
35–40 ␮m. The meta- and ortho-isomers are bulkier than p-ET and
steric hindrance within the zeolitic pore could be expected. p-ET
The formation of m-ET and o-ET isomers over MFI-280 was
noticed at the reaction conditions studied. The product distribu-
tion over MFI-280 showed a higher quantity of disproportionation
products than over MFI-2000. The production of diethylbenzenes
(DEBs) over MFI-280 can be attributed to the availability of more
Brønsted acid sites (Table 2). Over MFI-80, the product distribu-
tion became wider indicating significant presence of side reactions
such as toluene disproportionation and isomerization yielding
benzene, ethylbenzene, xylenes, DEBs and trimethylbenzenes
(TMBs).
Ethanol conversion was much higher compared to toluene con-
version at all temperatures although the same molar amount of
ethanol and toluene was injected as feed. The main reason for
higher ethanol conversion is attributed to the alkylation and dehy-
dration reactions which consume the additional ethanol molecules.
Table 2
Acid sites characteristics and pyridine sorption data for zeolite samples.
a
NH3-TPD (m mol g−1
)
Pyridine FT-IR (m mol g−1
B sites
)
Zeolite sample
TA
L.T. < 300 ◦
C
H.T. 300–550 ◦
C
TA
L sites
MFI-80
0.16
0.07
0.01
n.d.
0.09
0.05
0.004
n.d.
0.07
0.02
0.006
n.d.
0.14
0.02
n.d.
n.d.
0.08
0.01
n.d.
n.d.
0.06
0.01
n.d.
n.d.
MFI-280
MFI-2000
Silicalite-1
a
Absorptivity ratio ε1455/ε1545 = 1.33 was used to calculate the acidity, TA = total acidity; B = Brønsted acidity; L = Lewis acidity; nd = not detectable L.T. = low temperature;
H.T. = high temperature; n.d. = not detected.
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Fig. 4. TPD ammonia profiles of MFI-80, MFI-280 and MFI-2000.
The water molecules formed in alkylation of toluene by ethanol are
adsorbed on Lewis acid sites to form protonic acid sites, and on the
strong protonic acid sites to form hydronium ions which are weaker
acid than the bridged OH groups. Therefore, water molecules might
change the type of acid sites (Lewis acid to protonic acid) as well as
weaken the protonic acid sites.
3.2.1. Effect of temperature
As shown in Fig. 5, there is an incremental increase in toluene
conversion with the increase in temperature from 300 to 400 ◦C at
20 s reaction time for MFI-280 and MFI-2000 except that for MFI-
80 which showed a reduction in toluene conversion from ∼29%
to ∼22%. This drop in toluene conversion may be attributed to
Table 3
Catalytic performance of MFI-80, MFI-280 and MFI-2000 in toluene ethylation with ethanol at 20 s reaction time and 1:1 toluene:ethanol molar ratio.
Catalyst
MFI-80
MFI-280
MFI-2000
350
Reaction temp. (◦C)
300
350
400
300
350
400
300
1.5
400
14.2
Toluene conversion (%)
Product yield (%)
p-ET
m-ET
o-ET
29.0
29.0
22.2
18.6
22.3
23.8
8.0
7.3
16.0
2.8
6.3
14.2
3.2
3.6
8.2
2.1
9.4
4.5
-
11.2
11.1
0.3
8.5
13.0
1.0
1.5
-
-
7.7
-
-
13.5
-
-
Total-ET
Gases
Benzene
Ethylbenzene
Xylenes
DEBs + TMBs
ET selectivity (%)
ET composition (%)
p-ET
m-ET
o-ET
26.1
3.9
0.2
0.9
0.9
23.7
4.5
0.8
1.5
2.0
13.9
7.4
2.5
1.8
3.36
13.1
2.5
0
0.3
0.1
0.6
92.9
22.6
2.3
0
0.6
0.5
22.5
2.1
0.2
0.9
1.1
1.5
10.8
0
0.03
0
0
7.7
8.8
0.04
0.1
0.2
0
13.5
7.9
0.1
0.3
0.3
0
0.9
90.0
0.9
82.0
0.7
64.7
0.4
93.8
0.3
90.0
98.0
95.8
95.0
27.8
61.4
10.8
26.4
59.9
13.7
25.9
59.3
14.8
67.6
32.4
–
49.6
49.0
1.4
37.8
57.7
4.5
100
–
–
100
–
–
100
–
–
DEBs = diethylbenzenes, TMBs = trimethylbenzenes.
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7
Fig. 5. Effect of temperature on toluene conversion over MFI-80 (ꢀ), MFI-280 (ꢁ)
and MFI-2000 (ꢂ) at reaction time of 20 s.
coke formation which is accelerated by strong acid sites of MFI-
80. Comparing MFI-80 and MFI-280 at 400 ◦C, the amount of C1–C4
gases for MFI-80 is 3 times more than that of MFI-280. Since these
gases are produced from the decomposition of ethanol, it indicates
that there is less amount of ethanol available for alkylating toluene
to ethyltoluenes over MFI-80 as shown in Fig. 6(a) and Table 3.
The formation of secondary alkylation products from toluene dis-
proportionation or transalkylation with ET i.e. benzene, xylenes,
EB, TMB and DEB over MFI-80 and MFI-280, means that some of
the gases were consumed as methyl and ethyl groups. At 400 ◦C,
the yield of ethyltoluenes over MFI-80 was 13.9% compared with
22.5% over MFI-280. A similar trend was also observed by Bhan-
darkar and Bhatia [17] over MFI (SiO2/Al2O3 = 50) who attributed
it to the decomposition of the alkylating agent and the possibility
of reversible reaction within 300–400 ◦C. However, the increase in
the yield of gases with the increase in reaction temperature over
MFI-80, contrary to the behavior of MFI-280 and MFI-2000, may be
attributed to the high amount of Brønsted acid sites in MFI-80 [37].
The yield of gases over MFI-2000 was the highest compared
with MFI-80 and MFI-280. This may be attributed to the presence of
weak acid sites in MF-2000 which catalyze dehydration of ethanol
but not so effective for isomerization of ET’s (high p-ET selectivity)
and disproportionation and transalkylation of toluene (low yields
of benzene, xylene, EB, DEBs, TMBs). However, ass the reaction tem-
perature increased from 300 to 400 ꢀC, the amount of C1–C4 gases
produced over MFI-2000 dropped from 10.8% to 7.9%. This may be
due to the partial consumption of these gases as shown in the prod-
uct distribution in Table 3. Over MFI-2000, there was an increase
in toluene conversion with the increase in temperature at different
reaction times; however, the rapid increase was at a temperature
of 400 ◦C. As the reaction time increased from 5 to 20 s, toluene
conversion over MFI-2000 increased drastically especially at 400 ◦C
as shown in Fig. 6(b). Maximum toluene conversion of 14% was
obtained at 400 ◦C and a reaction time of 20 s.
Fig. 6. Effect of reaction time on toluene conversion over (a) MFI-80 and (b) MFI-
2000 at 300 ◦C (᭹), 350 ◦C (ꢀ) and 400 ◦C (ꢂ).
3.2.2. Effect of SiO2/Al2O3 ratio
The influence of SiO2/Al2O3 ratio on the selectivity to p-ET
over MFI zeolites is presented in Fig. 7 at 400 ◦C and constant
toluene conversion of 14%. The para-selectivity to p-ET over MFI-
2000 was 100% compared with 27% and 48% over MFI-80 and
MFI-280, respectively. A similar trend was observed by Al-Khattaf
and co-workers who studied the effect of SiO2/Al2O3 ratio in
Fig. 7. para-Ethyltoluene selectivity over MFI-80, MFI-280 and MFI-2000 at 14%
toluene conversion.
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Table 4
8
Estimated kinetic parameters for toluene ethylation over MFI-280 and MFI-2000.
Parameter
MFI-280
MFI-2000
k01 (m3/kgcat s)
E1
0.32 0.01
30.2 8.2
19.1 6.2
32.7 9.5
0.26 0.01
65.2 9.3
29.1 5.1
21.1 4.2
−ꢀHEtOH
−ꢀHEtTol
disproportionation study over MFI [40]. They found that high con-
tent of p-DEB was noticed over large crystals than smaller ones
due to the absence of diffusion limitation. Comparing the product
distribution obtained over MFI zeolites, both MFI-80 and MFI-280
yielded appreciable amount of meta and ortho isomers due to their
small crystal sizes (short diffusion length) meaning high active sites
could be obtained on the external crystal surface. The absence of
isomerization over MFI-2000 with higher crystal size gives strength
to this assertion.
The effect of crystal size on toluene ethylation showed that
the shape selectivity over MFI-zeolites falls under configurational
diffusion-controlled selectivity. Chen et al. [41] explained that the
differences between transition-state shape selectivity and configu-
rational diffusion selectivity are the factors on which each of them
depends, for configurational diffusion selectivity; it is affected by
channel diameter and crystal size [42].
Fig. 8. Effect of temperature on ethyltoluenes selectivity over MFI-80 (ꢂ), MFI-280
(ꢁ) and MFI-2000 (ꢀ) at reaction time of 20 s.
diethylbenzenes synthesis over MFI and reported the same phe-
nomenon for para-diethylbenzene selectivity [38].
It seems that irrespective of the reaction and product obtained
over MFI zeolite, an optimum SiO2/Al2O3 ratio generally favors
para-selectivity [19]. Bhandarkar and Bhatia [17] obtained 56%
para-selectivity at a toluene to ethanol ratio of 4:1 over MFI with
SiO2/Al2O3 of 50, whereas Paparatto et al. [9] reported 57% para-
selectivity with similar feed ratio using MFI with SiO2/Al2O3 of 25.
The selectivities of the equilibrium mixture obtained over MFI-80
(p-ET 26.4%, m-ET 59.9%, o-ET 13.7%) were in agreement with those
reported by Lee et al. over MFI with SiO2/Al2O3 of 90 (p-ET 33.1%,
m-ET 50.2%, o-ET 16.7%), keeping in mind that the conditions were
slightly different [8].
For MFI-80 and MFI-2000, which have high and low concen-
tration of acid sites, respectively, variation in temperature did not
show any appreciable alteration to their para-selectivity. To eluci-
date the effect of SiO2/Al2O3 ratio, silicalite-1 (SiO2/Al2O3 = ∞) was
tested for toluene ethylation. It was confirmed that neither activity
nor selectivity was observed. At 300 ◦C and 20 s, toluene conversion
was less than 0.4% and yield of ethyltoluenes was 0.2%. From the
patterns observed over the three MFI zeolites, it is concluded that
optimum SiO2/Al2O3 ratio and large crystal size are necessary for
maximum para-selectivity to p-ET. However, low SiO2/Al2O3 ratio
is required for high catalyst activity in the ethylation of toluene.
Fig. 8 presents the selectivity to ETs over the three MFI zeolites at
different temperatures (300, 350, and 400 ◦C). The selectivity to ETs
was observed to decrease with the increase in temperature for all
zeolites associated with an increase in xylene, benzene and EB. The
reduction in the selectivity to ET was most significant over MFI-80
in which the selectivity decreased from 90.0% to 64.7%. As shown
in Fig. 8 and Table 3, the selectivity to ET over MFI-280 and MFI-
2000 decreased by about 3% due to the formation of xylene and
EB. The results obtained in our study is unique because the toluene
to ethanol ratio used was 1:1 in contrast to what is reported in
the literature, where high para-selectivity can only be achieved by
using higher toluene to ethanol ratio.
3.3. Kinetic model evaluation
Kinetic modeling of the toluene ethylation reaction has been
developed based on Scheme 1 and the equations presented in Sec-
tion 2.4. Langmuir–Hinshelwood model equations (Eq. (1) and (2))
incorporating the Arrhenius relation, the temperature dependence
form of the equilibrium adsorption constants and deactivation
function were simultaneously solved using a nonlinear regression
analysis. The estimated kinetic parameters and their 95% confi-
dence limits are shown in Table 4. The apparent activation energy
(E) obtained from the regression analysis for toluene ethylation
over MFI-2000 is more than double (65 kJ/mol) that of toluene ethy-
lation over MFI-280 (30 kJ/mol). This may be attributed to both
acidity and diffusion effects. The apparent rate constant for the
MFI-2000 is smaller than that of MFI-280 due to the difference in
external surface area. This has led to a lower activation energy for
MFI-280 compared withMFI-2000 [43]. Furthermore, the acidity of
3.2.3. Effect of crystal size
The high para-selectivity observed over MFI-2000 could also
be attributed to its large crystal size of 30–35 ␮m [39]. Arsenova-
Hartel et al. proposed an inverse relationship between the rate
of isomerization and crystal size of the catalyst in their EB
Fig. 9. Comparison between the experimental data (symbols) and predicted values
(dotted lines) for the concentration of toluene (ꢂ) and ethyltoluenes (ꢀ) at 400 ◦C.
Please cite this article in press as: B.A. Ogunbadejo, et al., Alkylation of toluene with ethanol to para-ethyltoluene over MFI zeolites:
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Acknowledgments
9
The authors are grateful to King Abdulaziz City for Science &
Technology (KACST) for financial support of this research through
Project No. AR-34-22. The authors also acknowledge the support
from the Ministry of Higher Education, Saudi Arabia in establish-
ing the Center of Research Excellence in Petroleum Refining &
Petrochemicals at King Fahd University of Petroleum & Minerals
(KFUPM). We are also grateful to Mr. Mariano Gica for the technical
assistance provided during the experimental work.
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Please cite this article in press as: B.A. Ogunbadejo, et al., Alkylation of toluene with ethanol to para-ethyltoluene over MFI zeolites:
Comparative study and kinetic modeling, Catal. Today (2014), http://dx.doi.org/10.1016/j.cattod.2014.08.019