Analysis of decisive structural parameters of zeolites for alkylation of benzene with ethylene

Article abstract of DOI:10.1016/j.apcata.2019.117379

The effects of the zeolite structure and the Br?nsted and Lewis sites on the activity and selectivity in alkylation of benzene with ethylene to ethylbenzene were investigated using three series of zeolites with different structures, Si/Al ratios and conte

Full text of DOI:10.1016/j.apcata.2019.117379

Journal Pre-proof
Analysis of decisive structural parameters of zeolites for alkylation of
benzene with ethylene
Somayeh F. Rastegar, Galina Sadovska, Radim Pilar, Jaroslava
Moravkova, Dalibor Kaucky, Libor Brabec, Jana Pastvova, Petr
Sazama
PII:
S0926-860X(19)30534-4
DOI:
https://doi.org/10.1016/j.apcata.2019.117379
APCATA 117379
Reference:
To appear in:
Applied Catalysis A, General
Received Date:
Revised Date:
Accepted Date:
4 September 2019
11 December 2019
14 December 2019
Please cite this article as: Rastegar SF, Sadovska G, Pilar R, Moravkova J, Kaucky D, Brabec
L, Pastvova J, Sazama P, Analysis of decisive structural parameters of zeolites for alkylation
of benzene with ethylene, Applied Catalysis A, General (2019),
doi: https://doi.org/10.1016/j.apcata.2019.117379
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Analysis of decisive structural parameters of zeolites for alkylation of benzene with
ethylene
Somayeh F. Rastegar, Galina Sadovska, Radim Pilar, Jaroslava Moravkova, Dalibor Kaucky,
*
Libor Brabec, Jana Pastvova, Petr Sazama petr.sazama@jh-inst.cas.cz
J. Heyrovsky Institute of Physical Chemistry, Academy of Sciences of the Czech Republic,
Dolejskova 3, 182 23 Prague, Czech Republic,
*Corrresponding author
Graphical abstract
1

Highlights




The catalytic activity is controlled by the concentration of Brønsted acid sites.
Channels sterically constrained for heavy by-products are decisive for selectivity.
Lewis sites do not affect catalytic activity or selectivity.
*
Superior activity and selectivity are obtained over Al-rich H- BEA zeolites.
Abstract:
The effects of the zeolite structure and the Brønsted and Lewis sites on the activity and
selectivity in alkylation of benzene with ethylene to ethylbenzene were investigated using three
series of zeolites with different structures, Si/Al ratios and contents of Lewis sites. The 12-
*
membered three-dimensional channels of beta zeolite ( BEA) provide higher selectivity to
ethylbenzene compared to faujasite zeolite (FAU) with cavities in which the di-alkylated and
heavier by-products are formed. The activity is controlled by the concentration of Brønsted
acidic sites, while Lewis sites do not contribute to the activity or affect the selectivity. The
*
highest activity and selectivity are obtained using Al-rich H- BEA (Si/Al 4.2) with a high density
of Brønsted groups in a well-defined channel structure sterically constrained for heavy by-
products. It is shown that a three-dimensional channel structure with Brønsted acid sites in a
shape selective environment is a key parameter controlling the activity and selectivity.
Keywords: Alkylation of benzene; Ethylbenzene; Beta zeolite (*BEA); Organotemplate-free Al-
rich Beta; Mordenite (MOR); Faujasite (USY)
2

1
.
Introduction
Due to the substantial increase in ethylbenzene (EB) production, an important petrochemical
intermediate in styrene production, considerable effort has been exerted to find an efficient
catalytic process which can conventionally produce EB [1]. EB is mostly synthesized by benzene
(
B) ethylation over acidic catalysts. Traditional processes were based on the use of strong
mineral and Lewis acids, such as H SO , BF , and HF. The emphasis on cleaner and safer
technologies without the problematic handling, corrosion and waste disposal of these catalysts
2] has resulted in the development of faujasite (FAU, USY) zeolite alkylation catalysts effective
2
4
3
[
in EB production in the liquid phase [3]. In the early 1990s, a patent by Chevron showed that, in
*
the liquid-phase, the alkylation/transalkylation catalysis can be performed on a beta ( BEA)
zeolite [4]. Shortly after this, it was shown that MCM-22 zeolite can also be used as an active
catalyst in benzene alkylation [5]. Although the catalytic activity of MCM-22 is comparable
*
(
with a lower deactivation rate) to USY, is still approximately 2.4 times less than BEA zeolite
5]. The strength and concentration of Brønsted acidic sites in the zeolites and the transport of
[
the products from the inner channels of the zeolites are assumed to be responsible for the activity
of zeolites in the alkylation reaction [1, 2, 4, 6-12]. The latter is predominantly governed by the
size of the zeolite crystals and the kinetic diameters of the reactants and products [13] relative to
the diameter of the zeolite channels (see Figure 1). It has been demonstrated that the shape
selectivity of zeolite in alkylation/transalkylation is related to the geometry and dimensions of
the internal channels [14]. For example, medium-sized pore ZSM-5 zeolites with 10-ring channel
openings (pore diameter of about 0.55 nm) have been shown to be a suitable catalyst for the
*
synthesis of para-dialkyl benzene isomers. However, the 12-ring channels in BEA zeolite do not
exhibit para-selectivity but are advantageously used in the synthesis of monoalkyl benzenes, like
3

EB and isopropylbenzene. As the kinetic diameters of the alkylbenzene molecules are close to
*
the diameter of the inner channels of BEA zeolite, the complex reaction mechanism and thus
product composition are controlled by shape-selective effects [15]. In the absence of a product
shape-selectivity effect, the monoalkylation of benzene is followed by the formation of other
polyalkylates, since the benzene aromatic ring is activated in the presence of an alkyl group.
Therefore, in order to obtain high EB selectivity, it is advisable to operate the process at a high
benzene/ethylene ratio. On the other hand, the unfavourable polyalkylates produced in the
alkylation process can be transalkylated back in the presence of benzene.
This work studies the relationships between the concentration and type of acid centres
(Brønsted/Lewis) and their concentration on the catalytic activity in the alkylation of benzene to
*
EB and selectivity to dialkylated products and by-products in the liquid phase on BEA zeolites,
*
comparing the shape selectivity of BEA zeolites with that of a traditional USY zeolite catalyst.
*
The study exploited the progress in the synthesis of Al-rich BEA zeolite [16-20], which enabled
a significant increase in the concentration of Al in the zeolite framework, for the preparation of
catalysts with a high concentration of Brønsted acid sites. The study analyses the effects of high
*
concentrations of Brønsted and Lewis acid sites in Al-rich BEA zeolites on the catalytic activity
and selectivity.
2
.
Experimental
2
.1
Preparation of zeolites
The faujasite zeolite (CBV 712, Si/Alꢀ=ꢀ6) denoted as FAU/6, mordenite zeolite (CBV10ꢀA,
*
Si/Al 5.8) denoted as MOR/5.8 and high-silica BEA zeolites (CP814B, Si/Al 12.5 and CP814C,
Si/Al 18) denoted as BEA/12.5 and BEA/18, respectively, were kindly provided by Zeolyst
4

*
International. Al-rich BEA zeolites with Si/Al ratios 4.2 and 4.8, denoted as BEA/4.2 and
BEA/4.8, respectively, were obtained by hydrothermal syntheses performed using an
organotemplate-free procedure at 140 °C. BEA/4.8 was synthesised using synthesis gel with a
molar composition of 1 SiO
sources of alumina and silica, respectively. BEA/4.2 was synthesised from synthesis gel with a
molar composition 1 SiO : 0.13 Al : 0.4 NaOH : 7.5 H O prepared using Al(OH) and Bindzil as
2
: 0.08 Al : 0.4 NaOH: 8.5 H
2
O. NaAlO
2
and Aerosil were used as
2
2
3
sources of alumina and silica, respectively. Calcined beta zeolite (Si/Al 10, Zeolyst International)
was added in the amount of 10 wt.% to the suspension of the aluminosilicate gel to act as the
*
seeds in the both synthesis procedures. Al-rich BEA zeolites with Si/Al ratios 4.5 and 5, denoted
as BEA/4.5 and BEA/5, respectively, were provided by Euro Support Manufacturing Czechia.
All the zeolites were ion-exchanged three times with 100 ml 0.5 M NH
4
NO solution for 12 h,
3
filtered and dried at 70 °C overnight. The obtained ion-exchanged zeolites were typically
calcined at 520 °C for 4 h to obtain the protonic form. Table 1 lists the zeolites used in this study.
2
.2
Characterization of zeolites
X-ray diffraction analysis (XRD) was used to analyse the phase composition of powdered
zeolites using a MiniFlex 600 (Rigaku, Japan) diffractometer equipped with a
vertical goniometer (with goniometer accuracy of ± 0.02°) in the 2θ range of 10° – 50°.
A copper anode (U = 40 kV, I = 15 mA) was used as a source of X-ray radiation. The textural
properties of powders were analysed by adsorption isotherms of nitrogen at 77 K performed with
a Micrometrics 3FLEX volumetric adsorption unit. All the samples were outgassed, before the
adsorption experiment, at 400 °C for 24 h to purify the surface of the samples. Fitting the
Broekhoff–de Boer t-plot was used to determine the volume of the micropores and the external
5

surface area. HR-TEM images were obtained by JEOL JEM-2100Plus high-resolution
transmission electron microscope (HR-TEM) operated at 200 kV (LaB6 cathode, point resolution
1
.9 Å). Dispersion of zeolite powder was cast on the lacey carbon copper grid and dried at 120
C before inserting into the microscope. HR-SEM micrographs were obtained using a Hitachi S
800-I field emission scanning electron microscope. The FTIR spectra of adsorbed d
°
4
3
-
acetonitrile were used to analyse the concentrations of the Brønsted and Lewis acid sites. The
spectra were recorded at RT on a Nicolet Nexus 670 FTIR spectrometer operating at resolution
−
1
of 1 cm by collecting 256 scans for each spectrum. The zeolites were evacuated at 450 °C for
80 min prior the adsorption of d -acetonitrile (10 mbar of CD CN at room temperature for 15
min with subsequent evacuation at room temperature for 15 min). The characteristic absorption
bands of the C≡N vibrations and the extinction coefficients obtained in our previous study (ε
1
3
3
B
=
−1
−1
2
.05 cm μmol and ε = 3.60 cm μmol ) [21] were used to determine the concentrations of the
L
acid sites. Competitive adsorption of benzene and EB was analysed on two Al-rich and Si-rich
*BEA zeolites. Benzene was adsorbed (18 Torr for 15 min) and subsequently EB was adsorbed
(3 Torr for 15 min) at RT. Then the samples were evacuated for 30 min at RT. The procedure
leads to the adsorption of benzene/EB at Brønsted sites and the absence of adsorbed molecules
on silanols. Comparison of the intensity of the vibrations in the benzene rings and the vibrations
of the ethyl groups provided information on the benzene/EB competitive adsorption on Si-rich
*
and Al-rich BEA zeolites.
2
.3 Analysis of catalytic properties
Alkylation of benzene with ethylene was carried out in a 500 ml stirred batch reactor at a
temperature of 180 °C with n-heptane as GC internal standard. To determine the activation
6

energy, the activity was also measured for selected samples in the 150 to 200 °C temperature
range. The liquid mixture was washed twice with N to remove O and then 0.5 g of catalyst was
2
2
added to the reactor. The catalysts were activated in the dryer at 220 °C/16 h (air) before adding
to the reaction mixture. Ethylene was added to the reactor at 180 °C and the pressure was kept
constant at 40 bar. The consumed ethylene was continuously replaced in the reactor to maintain a
constant pressure. To suppress the formation of unwanted by-products, excess benzene is used in
all the experiments (the benzene/ethylene molar ratio was kept at ~ 5). Samples for analysis were
taken at 15 min intervals. Analysis of the products was carried out by a Perkin-Elmer Clarus 580
gas chromatograph equipped with a capillary CP-Sil PONA CB (100 m x 0.25 mm x 0.5 µm)
column and a flame ionization detector (FID). The catalytic activity of each zeolite was defined
in terms of conversion of benzene (X
B
), the yield of EB (YEB) and selectivity of each product as
follows:
푁ꢀ
푁퐻ꢁ푡ꢂ0)
× 푁퐻 ꢁ푡) ꢃ × 1ꢄꢄ
ꢁ푡)
%푋퐵 = (1 −
푁ꢀ ꢁ푡ꢂ0)
ꢆ 퐸퐵 ꢁꢇ) − ꢆ퐸퐵 ꢁꢇꢈꢉ) ꢆꢊꢁꢇꢈꢉ)
%
푌
= ꢅ
×
ꢋ × 1ꢄꢄ
퐸퐵ꢁ퐷퐸퐵)
ꢆ퐵 ꢁꢇꢈꢉ)
ꢆꢊ ꢁꢇ)
푁
푁퐻ꢁ푡ꢂ0)
ꢌꢀ ꢁ푡)
%푆퐸퐵 ꢁ표ꢇℎ푒푟 푝푟표푑푢푐ꢇ푠) = (− ꢁ푁
× ꢃ × 1ꢄꢄ
ꢍ푁ꢀ ꢁ푡ꢂ0
)
푁퐻 ꢁ푡)
ꢀ
ꢁ푡)
Where the N stands for a number of moles of reactants or products, YEB corresponds to the yield
of EB (or DEB and heavier by-products) and S is the selectivity of EB, DEB, and other by-
products. We used the appropriate kinetic tests to confirm that the reaction is not affected by
mass transfer limitations. To analyse the possible effects of external diffusion, the catalytic test
was performed on a very active catalyst BEA/5 at different stirring rates from 25 to 250 rpm
using a U-type stirrer designed for intensive mixing of catalytic reactors. Practically identical
conversions were achieved at a stirring rate of 100 - 250 rpm. On the basis of these results, the
7

activity was analysed for all the catalysts in this study using a stirring rate of 150 rpm. To
*
determine the effect of intra-crystalline diffusion on the activity of BEA zeolites, we analysed
the conversion of benzene and selectivity for EB, DEB and higher products for two Al-rich
BEA/4.2 and BEA/4.5 zeolites of very similar compositions but with different crystal sizes of 0.4
-
0.5 μm and 0.1 - 0.2 μm, respectively. The same attained conversions of benzene clearly
indicate the absence of the effect of intra-crystalline diffusion on the observed activities. Since
BEA/4.2 is the sample with the largest crystals and the highest concentrations of active sites of
all the beta zeolite samples used in the study, we can exclude the effect of intra-crystalline
*
diffusion on the activity of the other BEA samples with smaller crystal size a lower
concentrations of active sites.
3
.
Results and discussion
3
.1
Structure of the zeolite catalysts
*
Table 1 lists a series of BEA zeolites with molar Al/Si ratios from 4.2 to 18 and Al-rich MOR
and USY zeolites with a similar molar ratio Al/Si ~ 6 used for analysis of the effects of the Si/Al
ratio and zeolite channel structure on the alkylation of benzene with ethylene, respectively. The
XRD patterns and electron microscopic images of the zeolites are depicted in Figures 2 and 3.
The characteristic patterns and intensities of the XRD lines, the microporous volumes ~ 0.15 –
3
-1
*
3
-1
0
.2 cm .g for BEA and MOR zeolites and ~ 0.3 cm .g for USY, and the electron scanning
*
images indicate a well-developed crystalline structure for all the parent zeolites. Al-rich BEA
*
zeolites consist of well-defined crystals in the form of a truncated octahedron, Si-rich BEA
consist of aggregates of small intergrown crystals and USY shows crystals with cubic
morphology. The zeolite crystal size for all the samples is in the range from 0.1 to 1 μm. A shift
8

*
in the intense XRD reflections of the Al-rich BEA zeolites ~ 0.4° 2θ to lower values with
respect to the reflections of the Si-rich BEA/12 and BEA/18 samples reflects the higher
concentration of Al in the zeolite framework. A detailed study of the distribution of Al atoms in
*
the framework of Al-rich BEA zeolites and concentration and nature of related acidic sites was
reported previously [22]. A detailed structural analysis of the MOR and USY samples has been
2
7
*
described elsewhere [20, 23]. The previous Al MAS NMR studies on the BEA and MOR
zeolite samples showed that aluminium is tetrahedrally coordinated in the framework without a
*
significant signal for non-lattice aluminium. Si/AlFr values for the BEA and MOR zeolites
2
9
obtained from the Si MAS NMR spectra are similar to those obtained from chemical analysis
[
20, 22].
3
.2 Analysis of activity and selectivity in benzene alkylation with ethylene
3
.2.1 Effect of the zeolite structure
To understand the role of the channel structures in the alkylation of benzene with ethylene,
*
conversion of benzene as a dependence on the reaction time over the Al-rich BEA, MOR and
*
USY zeolites was analysed and plotted in Figure 4. Note that FAU, MOR, and BEA zeolites
exhibit very similar framework composition and particle size and the catalytic activity was
measured with the same pre-treatment and under identical reaction conditions. The figure shows
that the conversion of benzene over the BEA/5 is significantly higher than for FAU/6 and
MOR/5.8. The results indicate that the geometry and dimensionality of the channel system of the
zeolites play a decisive role in their activity. The interaction of the reactant molecules in the
confined reaction space with Brønsted acid sites and with the inner walls of the zeolite channels
are factors controlling the reaction rate, where a more confined reaction space in the pore
9

structures and the higher acid strength and concentration of acid sites yield a higher reaction rate
[
24-26]. The Brønsted OH groups corresponding to Al-Si-Al sequences in the framework of
*
FAU zeolites exhibit lower acid strengths compared to Al-rich BEA due to mutual interactions
of close OH groups [20, 22, 27, 28]. The greater reactant confinement and the stronger van der
*
Waals interactions in the interior of the BEA zeolitic channels compared to the FAU zeolite [29,
3
0] with relatively large inner cavities with a diameter of 1.2 nm together with the presence of a
*
high concentration of acid sites in Al-rich BEA zeolite enable the alkylation reaction at ca 2.5
times higher rates compared to the FAU/6 zeolite. The very low activity of the MOR zeolite is
associated with the diffusion-restricted accessibility of the acid sites in the 8-ring for C
6
molecules and mono-dimensional 12-ring channels in MOR lowering the efficiency of the
catalytic process by mass transfer effects [23, 31]. Partial dealumination or desilication of Al-
rich mordenite, which opens the side 8-ring channel pockets and re-organises the channel
structure from mono-dimensional to three-dimensional, is necessary for improving the MOR
activity in the alkylation reaction [32, 33].
Figure 4B shows a comparison of the selectivity of the FAU/6 and BEA/5 samples in EB,
DEB, and heavier by-products at similar benzene conversions. FAU/6 zeolite produces more
DEB and much heavier by-products (C ≥ 9) due to the lack of shape-selectivity in the supercages
and three dimensional channel system with 0.74 x 0.74 nm openings accessible to large DEB
*
isomers. High selectivity of the BEA zeolite is related to the well-defined sterically constrained
environment of 12-membered three-dimensional channels (0.66 nm × 0.67 nm, 0.56 nm × 0.56
nm) which are readily accessible for benzene and EB, in which both the DEB isomers and
heavier by-products barely diffuse out of the zeolite pores. Figure 1 illustrates the diameters of
1
0

*
the pore openings of the BEA and FAU zeolites and the kinetic diameters of benzene, EB, and
three DEB isomers.
3
.2.2 Effect of the Si/Al ratio
*
Figure 5A depicts the effect of the aluminium content in the BEA zeolites on the conversion in
the alkylation of benzene with ethylene. Consistent with the reports of Bellussi et al. [6], Yilmas
et al. [19] and De Baerdemaek et al. [34], the conversion increases with an increasing amount of
*
framework Al in the BEA zeolites. The conversion of benzene gradually increases with a
decrease in the Si/Al ratios up to the maximum content of Al achievable by the zeolite synthesis,
i.e. up to 4.2 in the Al-rich BEA/4.2 sample. A significant increase in the activity of Al-rich
*BEA zeolites (Si/Al 4 – 5) compared to Si-rich zeolites (Si/Al ≥ 10) was previously also
observed in the hydroamination of aromatics [20], isomerization of alkanes [28, 35] and cracking
and hydrocracking of aromatics and alkanes [20, 34, 36]. Both analysis of ammonia desorption
1
and H MAS NMR and FTIR spectroscopy investigations [22, 28, 34, 37] of the concentration
*
and acid strength of Brønsted sites in Al-rich BEA zeolites showed that the concentration of
Brønsted sites is approximately proportional to the increased Al content in the framework
without significant changes in their acid strength which is similar to the Brønsted sites in Si-rich
*
-1
BEA zeolites. The comparable apparent activation energies of ~ 70 kJ mol for Al-rich BEA/5
and Si-rich Al-rich BEA/12.5 and a higher pre-exponential factor for Al-rich BEA/5 compared to
Si-rich Al-rich BEA/12.5 were obtained from the Arrhenius plots for the benzene alkylation
reaction (Figure 5B). This is consistent with the higher concentration of the Brønsted active sites
in the BEA/5 sample without marked change of their acidic properties. The previous studies on
*
the structure of Al-rich BEA zeolites showed that a predominant part of the Al atoms in the
1
1

zeolite framework is located in Al-Si-Al sequences [22, 34, 38-40]; however in contrast, e.g., to
the faujasite structure, these Al-Si-Al sequences form the zeolite wall separating two
*
neighbouring channels. Al atoms of the Al-Si-Al sequence in Al-rich BEA thus face into two
channels providing two non-interacting Brønsted OH groups located in different channels [22].
The deprotonation energies of these non-interacting Brønsted sites corresponding to Al-Si-Al in
*
Al-rich are comparable with those for Al-Sin≥2-Al sequences in Si-rich BEA zeolites, indicating
a similar strength of the acid sites [20]. The high Al concentration in the framework Al-rich
*BEA zeolites and the corresponding Brønsted centres with sufficient acid strength thus allow a
*
significant increase in the activity compared to the hitherto most active Si-rich BEA zeolites.
Recycling the BEA/5 catalyst three times showed no loss in conversion of benzene or selectivity
for EB.
*
Figure 5C compares the selectivity of the Al-rich and Si-rich BEA zeolites. The
*
selectivity for EB is slightly higher for the Al-rich BEA samples producing less DEB and
heavier by-products compared to the Si-rich BEA/12.5 zeolite for similar conversion of benzene.
Yilmaz et al. associated high selectivity towards the monoalkylated products in alkylation of
benzene with the polarity of the zeolite framework increasing with the content of the aluminium
*
[
19]. High polarity of Al-rich BEA zeolite could favour adsorption of benzene over that of EB,
which decreases subsequent alkylation reactions due to the higher rate of desorption of the EB
product. We employed infrared spectroscopy to determine whether the greater polarity of Al-rich
zeolites could, in fact, result in greater coverage of the acid sites with benzene. We adsorbed a
*
mixture of benzene and EB on Si- and Al-rich BEA zeolites and observed the intensity of the
absorption bands of the benzene ring and the ethyl group under conditions where benzene and
EB are adsorbed predominantly on Brønsted sites (Figure 6). The spectra show higher relative
1
2

*
intensity of the adsorption bands corresponding to the benzene ring on the Al-rich BEA zeolite
sample at the same intensity of the bands of the ethyl group on both zeolites. They clearly show
slightly greater relative benzene coverage, which could be responsible for the greater selectivity
*
for ethyl benzene during the alkylation reaction on Al-rich BEA zeolites. Direct evidence could
be obtained by monitoring the adsorbed entities under the reaction conditions; however, analysis
of the adsorbed molecules at high pressures and temperatures in the liquid phase is not
experimentally feasible.
We could not exclude that the textural properties also contribute to the higher selectivity
*
*
obtained over Al-rich BEA zeolites. Al-rich BEA consists of well-developed crystals (Figure 3,
Table 1) with a much smaller external surface area, possibly implying high shape-selectivity
effects of the defined reaction environment of the regular inner pores compared to small
*
crystallites of Si-rich BEA zeolites with a large external surface (Table 1). We analysed the
selectivity for EB, DEB and higher products at a conversion of ~15% for two Al-rich BEA/4.2
and BEA/4.5 zeolites of very similar compositions but with different crystal sizes of 0.4 - 0.5 μm
and 0.1 - 0.2 μm, respectively. Figure 7 shows practically identical selectivity for EB for both
samples. The larger and better developed crystals of BEA/4.2 did not significantly alter the
*
reaction selectivity. It seems that the selectivity of Al-rich BEA zeolites is not significantly
affected by the particle morphology and the external surface. This is consistent with the fact that,
for most reactions acid catalysed by zeolites, the van der Waals interactions of the reacting
molecule in the confined interior of the zeolitic channels and the presence of the acid sites in the
environment of the channels are essential [35]. However, it cannot be excluded that the increase
in the external surface area and the reduction of diffusional path lengths do not contribute to the
1
3

changes in the reaction selectivity when the crystal size is reduced below 100 nm for Si-rich
*BEA zeolites.
*
3
.2.3 Effect of the concentration of Lewis and Brønsted acid sites in BEA zeolites
*
Compared to zeolites with other structural topologies, the BEA zeolite exhibits a greater
tendency both to reversible and non-reversible changes in the coordination of the T atoms,
especially aluminium, and to easier creation of the Lewis sites either in the form of the perturbed
framework Al-defective sites or extra-framework Al species [20, 41]. Lewis sites play the role of
active centres in many acid-catalysed reactions, affect the specific activity of the Brønsted sites,
or influence the reaction mechanism and the selectivity of the reaction [42, 43]. The presence of
Lewis sites in the close vicinity of Brønsted acid sites in zeolite catalysts was shown to increase
the participation of hydrogen transfer reactions in the complex process of transformation of
methanol to hydrocarbons (MTH) [44, 45] and in cracking of alkenes [46]. The pairs of Brønsted
and Lewis acid sites in zeolite catalysts provided much higher reaction rates for alkane cracking
[
47-49]. In previous studies [1, 6, 7, 50], the activity in alkylation of benzene by ethylene over
*BEA zeolites was associated with the content of aluminium in the zeolite without analysing the
nature of the acid sites. To assess the role of Brønsted and Levis acid centres in the benzene
*
alkylation reaction, a series of BEA zeolite samples was prepared by calcining BEA/4.8 at
temperatures from 450 to 600 °C. The concentration and nature of acidic active sites in the series
*
of BEA/4.8 zeolites calcined at the different temperatures and in the BEA zeolites with different
Si/Al ratios were determined by d
3
-acetonitrile adsorption monitored by FTIR spectroscopy.
*
The FTIR spectra of OH groups for the Al-rich and Si-rich BEA zeolites are shown in
Figure 8A. The spectra in the region of the OH groups exhibit absorption bands characteristic of
1
4

−
1
the bridging OH groups with maxima at 3610 – 3614 cm and bands reflecting the presence of
−
1
the silanol groups at 3745 cm . The increase in the intensity of the absorption band of the
bridging OH groups for the Al-rich compared to the Si-rich zeolites is approximately
proportional to the increase in the aluminium concentration in the zeolite. However, the
−
1
absorption at 3610 – 3614 cm corresponds to only non-interacting bridging OH groups, while
hydrogen-bonded silanols and Brønsted hydroxyls are associated with a broad background signal
-1
with a maximum at about 3500 cm [51]. The high intensity of the absorption bands for the
terminal Si-OH groups for the Si-rich zeolites is given by the large external surface of the small
-1
crystal particles. No absorption bands were found in the 3650 – 3600 cm region, indicating the
absence of Al–OH species for all the zeolite samples. Adsorption of d -acetonitrile resulted in
the appearance of absorption bands at 2290 and 2325 cm corresponding to the ν(C≡N)
stretching mode of d -acetonitrile adsorbed on Brønsted and Lewis sites, respectively (Figure
B). The intensity of both the absorption bands obviously decreases with increasing Si/Al ratio.
3
−
1
3
8
The results of quantitative analysis of the acid sites using the integral intensities of the absorption
bands are summarised in table 2. They show that the concentrations of the acid sites correspond
to the aluminium contents in all the zeolites with predominant Brønsted sites and significant
*
concentrations of Lewis sites typical of BEA zeolites [52, 53].
The spectra of adsorbed acetonitrile on BEA/4.8 calcined at temperatures from 450 to
6
00 °C in figure 9A show a gradual decrease in the intensity of the absorption bands
-1
characteristic of the C≡N group interacting with the Brønsted sites at 2290 cm and an increase
-1
in the intensities of the bands of the Lewis-coordinated acetonitrile at 2325 cm with increasing
calcination temperature. The dehydroxylation of the Brønsted groups occurred progressively
with increasing calcination temperatures and led to the formation of the Lewis sites from 0.5
1
5

-1
-1
mmol.g for 450 °C to 1.28 mmol.g for 600 °C (Table 2 and Figure 9B). Proportionally, the
-1
concentration of Brønsted sites gradually decreased from 1.71 mmol.g for 450 °C to 1.07
-1
mmol.g for 600 °C. The Lewis electron acceptor sites are probably formed by step-wise
weakening of the bonds in Si–OH–Al Brønsted groups and progressive dehydroxylation with
increasing temperatures [54].
The alkylation activity of the series of BEA/4.8 zeolites calcined at different temperatures
is depicted in figure 10. The figure clearly shows that the obtained conversions of benzene
gradually decrease with increasing calcination temperature. The decrease in the concentration of
Brønsted OH groups by their partial transformation to Lewis centres results in a decrease in the
activity. The dependence of the benzene conversion on the concentration of Brønsted sites for
the series of BEA/4.8 zeolites and the Si-rich BEA/12.5 and BEA/18 zeolite samples is shown in
Figure 11A. The data show that the conversion is a function of the concentration of Brønsted
sites and the activity increases with the increasing concentration of Brønsted sites regardless of
the absence or presence of large numbers of Lewis sites. We also plotted the concentrations of
Lewis sites and the conversion of benzene. The plots of the conversion of benzene against the
concentrations of the Lewis sites did not show any relationship (Figure 11B). The dependence
*
clearly shows that the activity of the BEA zeolite is controlled by the concentration of Brønsted
sites and that the Lewis sites do not significantly contribute to the activity.
The effect of the Lewis sites on the selectivity of alkylation was analysed using the series
of BEA/4.8 samples providing essentially different ratios between the concentrations of Brønsted
and Lewis sites, e.g. ratios of C
B
/C 3.4 and 0.8 for samples calcined at 450 and 600 °C,
L
respectively. The selectivity for EB, DEB and heavier by-products for the samples with the
different concentrations of Lewis sites examined at benzene conversion of ~ 15% is shown in
1
6

Figure 11C and Table 3. The reaction selectivity for EB is about 95% for all the samples with
about 5% DEB and only about 0.5% other by-products. The results clearly show that the Lewis
centres do not affect the selectivity of the reaction, at least under the described reaction
conditions.
It can be summarised that the benzene conversion increases with an increase in the
concentration of acid Brønsted sites regardless of the presence of Lewis sites and the greatest
activity with preserved selectivity is observed for samples containing the greatest concentration
of the Brønsted sites.
Conclusions
Analysis of the activity and selectivity with respect to the zeolitic structure and concentration and
nature of the acid sites revealed that:
i)
The selectivity for EB is controlled by shape-selectivity effects where a three-
*
dimensional channel structure with 12-membered ring openings of BEA zeolite
provides high yields of EB without excessive formation of DEB isomers and heavier
by-products.
ii)
The concentration of Brønsted acid sites controls the activity of the reaction, while
*
even a very high density of acid sites in BEA zeolites does not negatively influence
the selectivity of the reaction. Superior activity and selectivity are obtained over Al-
*
rich BEA zeolites where the Brønsted OH groups associated with high concentration
*
of Al in the zeolite framework Al-rich BEA zeolite represent acidic sites with
strength and specific activity comparable or greater compared to Si-rich zeolites.
1
7

iii)
Even a very high concentration of the Lewis sites does not contribute to the activity
or influence the selectivity of the reaction. The Lewis sites are not involved in the
alkylation reaction and play the role of spectators.
It can be concluded that the high concentration of Brønsted OH groups in the shape-selective
environment of the 3D channel system associated with a high concentration of Al in the zeolite
framework is the decisive factor for the high activity and selectivity in the alkylation reaction.
Author Contribution Statement
Dr. Faal Rastegar Seyyedeh Somayeh, Postdoctoral researcher, J. Heyrovsky Institute of Physical Chemistry of the
Academy of Sciences of the Czech Republic, Dolejskova 3, Prague 8, Czech Republic, tel.: +(420) 26605 3555, fax:
+(420) 28658 2307, e-mail somayeh.rastegar@jh-inst.cas.cz: Kinetic analysis, manuscript preparation.
Dr. Galina Sadovska, Scientist, J. Heyrovsky Institute of Physical Chemistry of the Academy of Sciences of the
Czech Republic, Dolejskova 3, Prague 8, Czech Republic, tel.: +(420) 26605 3555, fax: +(420) 28658 2307, e-mail
galina.sadovska@jh-inst.cas.cz. Structural analysis.
Dr. Radim Pilar, Postdoctoral researcher, J. Heyrovsky Institute of Physical Chemistry of the Academy of Sciences of
the Czech Republic, Dolejskova 3, Prague 8, Czech Republic, tel.: +(420) 26605 3555, fax: +(420) 28658 2307, e-
mail radim.pilar@jh-inst.cas.cz: Synthesis of petr.sazama@jh-inst.cas.cz zeolites.
Dr. Jaroslava Moravkova, Scientist, J. Heyrovsky Institute of Physical Chemistry of the Academy of Sciences of the
Czech Republic, Dolejskova 3, Prague 8, Czech Republic, fax: +(420) 28658 2307, e-mail jaroslava.moravkova@jh-
inst.cas.cz. Microscopic analysis and analytical methods.
Dr. Dalib Kaucky, Senior researcher, J. Heyrovsky Institute of Physical Chemistry of the Academy of Sciences of the
Czech Republic, Dolejskova 3, Prague 8, Czech Republic, tel.: +(420) 26605 3775, fax: +(420) 28658 2307, e-mail
dalibor.kaucky@jh-inst.cas.cz: Kinetic analysis.
Dr. Libor Brabec, Senior researcher, J. Heyrovsky Institute of Physical Chemistry of the Academy of Sciences of the
Czech Republic, Dolejskova 3, Prague 8, Czech Republic, tel.: +(420) 26605 3775, fax: +(420) 28658 2307, e-mail
libor.brabec@jh-inst.cas.cz: HR-SEM analysis.
Dr. Jana Pastvova, Postdoctoral researcher, J. Heyrovsky Institute of Physical Chemistry of the Academy of
Sciences of the Czech Republic, Dolejskova 3, Prague 8, Czech Republic, tel.: +(420) 26605 3555, fax: +(420)
2
8658 2307, e-mail jana.pastvova@jh-inst.cas.cz: FTIR analysis.
Dr. Petr Sazama, Senior scientist, J. Heyrovsky Institute of Physical Chemistry of the Academy of Sciences of the
Czech Republic, Dolejskova 3, Prague 8, Czech Republic, tel.: +(420) 26605 3325, fax: +(420) 28658 2307, e-mail
petr.sazama@jh-inst.cas.cz: Research management, manuscript preparation.
Acknowledgments
1
8

This work was supported by the Grant Agency of the Czech Republic under Project No. 18-
0303S and the Technology Agency of the Czech Republic under Project No. TH03020184. The
authors acknowledge the assistance provided by the Research Infrastructure Pro-NanoEnviCz
Project No. CZ.02.1.01/0.0/0.0/16_013/0001821), supported by the Ministry of Education,
Youth and Sports of the Czech Republic.
2
(
1
9

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2
5

Figure 1. The channel structure and diameters of the openings of the 12-membered rings in
*
the structure of a) FAU and b) BEA zeolites, c) the reaction scheme for alkylation of benzene
to EB and DEB, and d) the kinetic diameters of the benzene, EB and DEB isomers.
The kinetic diameters of the DEB isomers were calculated based on the work of Lilov et al.
[
13]. The data are in Å.
2
6

*
Figure 2. X-ray diffraction patterns of the BEA, FAU and MOR zeolites.
2
7

Figure 3. SEM and HR-TEM micrographs for zeolites BEA/4.2 (A), BEA/4.5 (B), BEA/4.8
C), BEA/5 (D), BEA/12 (E), BEA/18 (F), and USY/6 (G).
(
2
8

Figure 4. Effect of the zeolitic structure on alkylation of benzene with ethylene over Al-
rich
*BEA, MOR and FAU with similar framework compositions. A) Conversion of benzene
*
as a function of time. B) The comparison of selectivity of Al-rich BEA with FAU zeolite
for EB, DEB and heavier by-products (C ≥ 9) at similar conversions of benzene ~ 15%.
2
9