Transformation of ethylbenzene-m-xylene feed over MCM-22 zeolites with different acidities

Article abstract of DOI:10.1016/j.apcata.2014.02.004

Transformation of ethylbenzene (EB, 22 wt%)-m-xylene (78 wt%) mixture was carried out over zeolite MCM-22 catalysts with different acidities. The modification of the number and strength of the acid sites was attained by: (i) dealumination by steaming and successive acid treatment and (ii) isomorphous substitution of framework Al for boron ([Al,B]MCM-22). The effect of platinum introduction was also investigated. The strong decrease in the Br?nsted acidity, concomitant to the dealumination procedure, appears unfavorable since it leads to a drastic diminish of the degree of EB conversion, not compensated by satisfactory level of m-xylene isomerization. [Al,B]MCM-22 is a modification with much better performance, most probably because it contains both strong, Al-connected acid sites but in lower amount than in the parent sample, and weaker boron-generated sites and practically no any Lewis sites. Parent zeolite [Al]MCM-22 as well as B-substitution possess promising properties for ethylbenzene-m-xylene mixture transformation catalyst with adequate degree of EB conversion and extent of p-xylene approach to equilibrium as well as low xylene loss.

Full text of DOI:10.1016/j.apcata.2014.02.004

Applied Catalysis A: General 476 (2014) 19–25
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Transformation of ethylbenzene-m-xylene feed over MCM-22 zeolites
with different acidities
a,∗
a
a
b
Magdolna R. Mihályi , Márton Kollár , Szilvia Klébert , Vesselina Mavrodinova
a
Institute of Materials and Environmental Chemistry, Research Centre for Natural Sciences, Hungarian Academy of Sciences, Pusztaszeri út 59-67, Budapest
025, Hungary
Institute of Organic Chemistry with Centre of Phytochemistry, Bulgarian Academy of Sciences, Acad. G. Bonchev str., bl. 9, 1113 Sofia, Bulgaria
1
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Transformation of ethylbenzene (EB, 22 wt%)-m-xylene (78 wt%) mixture was carried out over zeolite
MCM-22 catalysts with different acidities. The modification of the number and strength of the acid sites
was attained by: (i) dealumination by steaming and successive acid treatment and (ii) isomorphous
substitution of framework Al for boron ([Al,B]MCM-22). The effect of platinum introduction was also
investigated. The strong decrease in the Brønsted acidity, concomitant to the dealumination procedure,
appears unfavorable since it leads to a drastic diminish of the degree of EB conversion, not compensated
by satisfactory level of m-xylene isomerization. [Al,B]MCM-22 is a modification with much better per-
formance, most probably because it contains both strong, Al-connected acid sites but in lower amount
than in the parent sample, and weaker boron-generated sites and practically no any Lewis sites.
Parent zeolite [Al]MCM-22 as well as B-substitution possess promising properties for ethylbenzene-
m-xylene mixture transformation catalyst with adequate degree of EB conversion and extent of p-xylene
approach to equilibrium as well as low xylene loss.
Received 2 December 2013
Received in revised form 29 January 2014
Accepted 4 February 2014
Available online 12 February 2014
Keywords:
Dealuminated and B-substituted zeolite
MCM-22
Pt/H-MCM-22
Ethylbenzene + m-xylene mixture
transformation
©
2014 Elsevier B.V. All rights reserved.
1
. Introduction
first time in the conversion of ethylbenzene-m-xylene mixed feed
9]. The results reveal the promising properties of this catalyst for
[
The improvement of the processes connected to the manu-
the selective elimination of EB from C aromatics feed at admissible
8
facturing of xylenes from C8 aromatics cut containing o-xylene,
m-xylene and 20% to 40% ethylbenzene (EB), produced in catalytic
reforming of naphtha and naphtha pyrolysis is an important task
for researchers. The transformation of this mixed feed on acidic
catalysts should be mainly directed to isomerization of the xylenes
ingredients up to the equilibrium value and to EB dealkylation and
disproportionation with a minimum loss of xylenes [1–3]. The rate
of these reactions is not only influenced by the strength and the
number of the acidic centers, but also by the pore size of the zeolites
xylene loss, compared to that of the large-pore BEA and medium-
pore ZSM-5 zeolites.
Several types of zeolites, with different structures and acidity
characteristics, bifunctional or in their H-form, have been exam-
ined in the latter process [4–8] and thoroughly reviewed by Degnan
et al. [10] but not the zeolite MCM-22. This zeolite has a layer
structure and, as such, is a member of the MWW family [11]. In
a template directed synthesis, first a lamellar precursor (MCM-
22(P)) is prepared. Each layer includes a two-dimensional 10-MR
(10 membered-ring) sinusoidal channel system and a hexagonal-
array of cups, having 12-MR openings on the external surface of
the layer. In the precursor material both the intralayer channels
and the interlayer space are filled with the organic template. By
removing the template, the MCM-22 material is obtained. During
this thermal decomposition, the adjacent parallel layers of the pre-
cursor are linked together through oxygen bridges and supercages
are formed from the cups facing each other.
[
4–8]. Two types of zeolites are applied for the industrial processing
of the C8 aromatic feed. In the process using mordenite-based,
large-pore zeolite as bifunctional catalyst, EB is mainly converted
into xylenes, whereas EB dealkylation and disproportionation are
the main reaction routes over medium-pore pentasil-based cata-
lysts.
In our previous contribution the catalytic performance of zeolite
[
Al]MCM-22 with intermediate pore structure was studied for the
MCM-22 has found fast industrial application for production of
EB through liquid-phase alkylation of benzene with ethylene at low
temperature. It was shown that the acid sites located in the 12-MR
surface cups are responsible for the high selectivity in this reaction.
∗ Corresponding author. Tel.: +36 1 438 1100; fax: +36 1 325 7554.
E-mail address: mihalyi.magdolna@ttk.mta.hu (M.R. Mihályi).
http://dx.doi.org/10.1016/j.apcata.2014.02.004
926-860X/© 2014 Elsevier B.V. All rights reserved.
0

2
0
M.R. Mihályi et al. / Applied Catalysis A: General 476 (2014) 19–25
Table 1
Aluminum content, ion-exchange capacity and textural properties of the samples.
IECa,b
mmol/g)
Surface area
(m /g)
a,c
Micropore
volume (cm /g)
Sample
Dealumination method
Aluminum content
2
a,d
3
(
Steaming
Acid refluxa
Si/Al (mol/mol)
ꢀAla (mmol/g)
[
Al]MCM-22
–
–
–
16.5
22.3
25.6
37.9
45.4
32.2
1.08
1.12
0.68
1.09
0.69
0.46
0.96
0.72
0.63
0.43
0.36
0.75
578
528
526
534
538
465
0.18
0.16
0.16
0.15
0.15
0.14
D350
D350/Ox
D500
623 K, 3 h
623 K, 3 h
773 K, 3 h
773 K, 3 h
–
Oxalic 24 h
–
Oxalic 24 h
–
D500/Ox
[Al,B]MCM-22
a
◦
All data were related to 1 g of sample calcined at 1000 C.
The ion exchange capacity (IEC) of the sample, obtained as the amount of ammonia, evolved from the NH4+-form sample in the 453–923 K temperature range during the
b
TPAE run.
c
Calculated from the B point of the adsorption isotherm.
Calculated from the ˛s plots.
d
The reverse reaction over the same catalyst is only scarcely studied
12] while that of the conversion of C8 mixed isomerization feed
has not been investigated yet, although MCM-22 has a structure
similar to those of the two commercial zeolite catalysts applied in
the industrial process.
of the acidic hydroxyls bridged between the framework Si and Al
atoms, a ratio of about 0.5 was found [20] designating that in the
boron-containing catalyst half of the acid sites are Al-connected
Brønsted sites. This ratio corresponds to the relative aluminum
content of the samples determined by AAS (Table 1).
[
The particular structure of the zeolite MCM-22 holds out inter-
esting opportunities for modification by different methods like
dealumination [13–16], change in the Si/Al ratio [17,18], etc. In
the present work modified MCM-22 zeolites with varying com-
position of the building lattice elements and diverse number and
strength of the acid sites were used for the selective transformation
of EB in presence of m-xylene at high temperature and atmospheric
pressure. Two methods for modification of this zeolite thoroughly
studied by us before [19,20], were applied for moderating the acid-
ity of MCM-22: (i) dealumination by steaming and successive acid
treatment and (ii) isomorphous substitution of framework Al for
boron. Pt-loaded MCM-22 was also tested in this reaction in order
to study the effect of introduction of hydrogenation agent.
0.5 wt% of Pt was loaded by incipient wetness impregnation
method on the zeolite sample in the form of tetraammine-
platinum(II) hydroxide hydrate precursor. The Pt-complex was
decomposed in an oven at 673 K for 1 h in air. Before catalytic run
it was reduced in H2 at 673 K for 1 h in order to obtain bifunctional
Pt/H-zeolite sample.
2.2. Catalysts characterization
The structure and crystallinity of the samples were verified by
a Philips PW 1810 powder diffractometer. The particle size and
the morphology were examined using SEM (Hitachi). Adsorption
isotherms were determined by N2 sorption at 77 K using a Quanta-
chrome NOVA Automated Gas Sorption instrument. The specific
surface area was calculated from the N adsorption capacity by the
multiple BET method. The micropore volumes were estimated from
the alpha-s plots.
2
2
. Experimental
2.1. Catalysts preparation
IR spectra were recorded by a Nicolet FT-IR spectrometer Impact
4
00 using the wafer transmission technique. The wafers of the cata-
The [Al]MCM-22 precursor for both modifications was prepared
lysts were activated in high vacuum at 673 K for 1 h then contacted
with pyridine (Py) at 473 K and 5.7 mbar Py pressure for 30 min,
cooled down to 373 K in Py atmosphere and degassed then at
increasing temperatures. The spectra were obtained at room tem-
by hydrothermal synthesis applying rotating industrial autoclave
3
(
∼1 m ) according to the procedure described in Refs. [20,22].
The crystallization temperature and time were 418 K and 10 days,
respectively. After washing with distilled water and drying at 343 K,
the preparation was calcined in air (programmed heating, 1.5 K/min
up to 823 K for 3 h) in order to remove the template. Twofold ion
−1
perature by collecting 32 scans at a resolution of 2 cm . Spectra
2
were normalized to wafer thickness of 5 mg/cm .
The NH4+-ion exchange capacity (IEC) of the samples were char-
acterized by TPAE (Temperature-Programmed Ammonia Evolution
exchange with 1 M NH NO3 solution was applied for the prepara-
4
tion of its ammonium form. The H-form of the sample with Si/Al
molar ratio of 16 was obtained by in situ decomposition of the
[
21]) and presented in Table 1. About 300 mg of samples were
NH4+-form in N at 823 K and is indicated as MCM-22 (Table 1). The
heated from 453 K to 873 K at a rate of 10 K/min in a 20-ml/min flow
of dry nitrogen. From the effluent NH3 was absorbed in distilled
water. The pH of the absorbing solution was kept between pH = 5.5
and 6.0 by automatically titrating the absorbed NH with 0.1 M HCl
solution. The ammonia, evolved from the sample between 423 and
2
procedure for dealumination [19] was carried out by steam treat-
◦
◦
ment at 623 K (350 C) and 773 K (500 C), (samples designated as
D350 and D500) followed by 0.5 M oxalic (samples D350/Ox and
D500/Ox) acid reflux at 373 K (Table 1).
3
8
23 K during TPAE run, was taken as equivalent with the IEC of the
In case of [Al,B]MCM-22 zeolite, sodium tetraborate hydrate
borax) was used as a source of boron [20]. The template was
removed in air at 853 K from both [Al]- and [Al,B]-MCM-22 mate-
sample and the Brønsted acid site concentration of the deammoni-
ated sample.
(
rials, then the calcined samples were ion exchanged with NH Cl
4
solution at room temperature. The Al content was determined by
atomic adsorption spectroscopy (AAS) after digestion of the sam-
ples with hydrofluoric acid. The boron concentration was measured
by induced coupled plasma (ICP) emission after sample extrac-
tion with 1 M HCl solution at 373 K. By calculating the integrated
2.3. Catalytic studies
The test reaction has been carried out in a fixed-bed flow
reactor at atmospheric pressure, reaction temperatures of 723 K
and 683 K and various contact times. N2 carrier gas was passed
through a saturator filled with a mixture of ethylbenzene and
m-xylene (weight ratio 22%:78%) and equilibrated at 293.2 K so
−1
absorbance of the bands at 3620 cm
in the FT-IR spectra of
[
Al,B]MCM-22 and [Al]MCM-22, due to the stretching vibration

M.R. Mihályi et al. / Applied Catalysis A: General 476 (2014) 19–25
21
Fig. 1. XRD pattern of the parent, dealuminated and boron-containing MCM-22
samples.
Fig. 2. N2 adsorption desorption isotherms of the parent, dealuminated and boron-
containing MCM-22 samples.
that almost equal partial pressure of 0.9 kPa to be attained for
both components of the mixture (boiling points 409.2 K and 412 K,
respectively). Adjusting of this ratio has been made regularly by
controlling its value in the range of ± 1% with the bypass. On-line
analysis of the reaction products has been performed using HP-GC
with 25 m FFAP capillary column.
the steamed (D500, D350) and subsequently acid-treated samples,
it can be concluded that upon acid reflux the micropore volume of
the samples practically does not change. Results suggested that the
lower N2 adsorption capacity of the dealuminated zeolites is not
due to the loss of crystallinity considering the even higher inten-
sity of the XRD reflections of the dealuminated samples. Rather the
presence of EFAl entities inside the pores, causing some adsorption
limitations even for the small N2 molecule, could be the reason for
this effect as was concluded in [20].
3
. Results and discussion
3.1. Physicochemical characteristics of the samples
3.2. Acidity
The X-ray diffraction patterns (Fig. 1) show that between 5 and
5 2ꢁ the intensity of the reflections for the samples steamed at
73 K and 623 K (D500 and D350) and their acid-treated varieties
1
7
(
3.2.1. Ion exchange capacity
The IEC of the zeolites, determined as the amount of evolved
ammonia, is usually equivalent to the concentration of Brønsted
acid sites in the generated H-forms [21]. The difference between the
total aluminum content (1.08 mmol/g) and the IEC (0.96 mmol/g)
of the initial [Al]MCM-22 material indicates that about 11% of Al is
in extra-framework positions still in the parent calcined sample.
The amount of aluminum in the steamed samples D350 and
D500 remains similar to that of the parent material but their
ion-exchange capacity decreases significantly by 25% and 55%,
respectively. The results indicate that substantial portion of the Al
atoms has been removed from tetrahedral positions, the higher the
more rigorous the hydrothermal treatment.
The subsequent acid leaching results in a similar decrease, by
about 40% of the total Al content, for the acid-treated materials.
The results suggest that both extra-framework and tetrahedrally
coordinated aluminum atoms have been expelled from the zeolite
lattice upon the acid reflux (Table 1).
D500/Ox and D350/Ox) is higher than that of the parent MCM-22
material. The enhanced intensity of these reflections also detected
by Wu et al. [23] in case of dealuminated MCM-22 materials can
be due, according to Camblor et al. [24] to the removal of Al from
lattice positions. The authors found out that the XRD pattern of
ITQ-1, the pure silica analog of MCM-22, was better resolved than
those of [Al]MCM-22 and indicated an improved crystallinity. No
diffraction peaks due to another crystalline phase were observed
for our steamed and steamed/acid-leached materials.
According to the XRD data presented in Ref. [20] the boron
modification, [Al,B]MCM-22, is also well crystalline and phase-pure
MCM-22 materials. The calculations based on the shift of the reflec-
tions in the XRD patterns used for determining the lattice a and c
parameters showed that both these values are lower in case of the
B-containing material. This result is an evidence, like in the case of
[
B]ZSM-5 [25] for boron incorporation in the MCM-22 framework
which leads to significant lattice contraction.
On Fig. 3A the amount of NH3 evolved upon the TPAE measure-
ment is plotted as a function of temperature. The derivative of the
curves, i.e., the plots of desorption rate vs. temperature are also
shown (Fig. 3B). The NH+4-form of the [Al]MCM-22 zeolite released
ammonia in a broad temperature interval ranging from 523 K to
873 K (Fig. 3A) with a maximum at about 660 K (Fig. 3B). This maxi-
mum corresponds to NH3 evolved from the stronger, Al-connected
Brønsted acid sites which concentration is about twice higher in
the initial, not modified with boron catalyst. Replacement of half of
the lattice Al atoms by B leads to reduction of this type of sites. The
The specific surface area as well as the pore volume of
[
Al,B]MCM-22 determined by N2 adsorption are lower than that of
the B-free material (Table 1) and correspond to the ones reported
in the literature. The smaller total pore volume of [Al,B]MCM-22
than that of [Al]MCM-22 is explained by the smaller diameter of
boron atoms substituting, in tetrahedral framework positions, the
larger aluminum or silicon T-atoms.
On Fig. 2 the N2 adsorption and desorption isotherms of the
parent, dealuminated and B-containing catalysts are presented. As
the data show, the decrease in the micropore volume for the mildly
ammonia desorption from the NH -[Al,B]MCM-22 material exhibit
4
steam-treated samples (D350 and D500) determined by N adsorp-
tion is in the range of 11–17% (Table 1). Comparing the isotherm of
two maxima, the first one at lower temperature (503 K, Fig. 3B). The
low-temperature peak is due to the presence of framework boron
2

2
2
M.R. Mihályi et al. / Applied Catalysis A: General 476 (2014) 19–25
Fig. 4. FT-IR spectra of the MCM-22 samples in the ␯(OH) region after evacuation at
6
0
73 K for 1 h (A) and subsequent Py adsorption at 473 K and degassing at 473 K for
.5 h (B).
Fig. 3. Temperature-programmed ammonia evolution curves (A) and their deriva-
tive plots (B) for NH4-[Al]MCM-22 and NH4-[Al,B]MCM-22.
bands at 3700 cm 1 and at 3527 cm in case of the acid-leached
−
−1
samples.
generating weaker Brønsted acid sites. This result suggests that the
modification with B in [Al,B]MCM-22 possesses substantial amount
of weaker Brønsted acid sites on the expense of the stronger ones.
Calculations of the number of strong acid sites in the partially B-
substituted sample shows Al content of 0.46 mmol/g, whereas the
IEC is 0.75 mmol/g. This is an additional indication that this prepa-
ration also contains a substantial amount of weak acid sites due
to the isomorphous substitution of Al by B. The effect is quite well
visualized in the TPAE curves in Fig. 3.
−1
The band at 3727 cm is also registered by Meloni at al. [26] in
MCM-22 materials with higher Si/Al ratio and corresponds to the
“hydroxyl nests” detected on steamed [27], acid-leached [28] or
steam and acid-treated [23] zeolites of the MCM-22 type. In our case
the intensity of the band at 3727 cm is the highest for the more
−1
severely steam-treated D500/Ox. The FT-IR study combined with
27
the Al-MAS NMR examinations made by us [19], has led to the
conclusion that the effect of the dealumination of MCM-22 zeolite
on the Al atoms distribution consists in: (i) neutralization of part
of the bridging OH groups by cationic extra-lattice Al species and
(ii) partial transformation of the remaining tetrahedral framework
Al to tri-coordinated lattice aluminum. These Al rearrangements
would suggest substantial change in the zeolite activity.
3.2.2. FT-IR studies
In order to obtain information about the type of the acid sites in
the parent, steamed and subsequently acid-treated materials, FT-IR
spectra before and after Py adsorption were made (Fig. 4). The spec-
tra in the hydroxyl stretching region show (Fig. 4A) that steaming
Upon pyridine adsorption the bridged hydroxyl band at
−
1
3620 cm
disappeared and, indicating the formation of pyri-
+
(
D350 and D500) results in a strong reduction of the band attributed
dinium ions (PyH ), bands of the Py-ring vibrations appeared at
−1
−1
−1
to bridged OH groups (Al(OH)Si) at 3620 cm . Simultaneously, an
increase in the intensity of the external Si–OH groups (3745 cm
is observed. The following acid treatment step leads to an increased
1545(1635) cm . The further bands at 1454(1622) cm are gener-
ally assigned to vibrations of Py, bound coordinately to strong Lewis
acid sites, i.e., to cationic extra-framework aluminum (EFAl) species
(often visualized as AlO ). By calculating the integrated absorbance
of the bands at 3620 cm
−
1
)
−
1
+
concentration of the internal silanols (3727 cm ) on the expense
of the external Si–OH (3745 cm−1) and to the appearance of new
−1
and that of the pyridinium bands at

M.R. Mihályi et al. / Applied Catalysis A: General 476 (2014) 19–25
23
Fig. 5. Ethylbenzene conversion (A) and xylene loss (B) as a function of TOS for the dealuminated catalysts at 723 K and different contact times in the transformation of
ethylbenzene-m-xylene mixture (22:78 m/m%).
−
1
1
546 cm in the FT-IR spectra of [Al,B]MCM-22 and [Al]MCM-22
the introduction of boron in the zeolite lattice lead to reduction of
the total acidity of the parent material. Generation of additional,
weakly acidic Brønsted sites on the expense of the stronger ones is
observed on the boron containing preparation.
ratio of about 0.5 was found [20] designating that in the boron-
containing catalyst half of the Brønsted acid sites are Al-connected.
This ratio corresponds to the relative aluminum content of the sam-
ples determined by AAS (Table 1). Comparing the intensity of the
band at 1454 cm in the FT-IR spectra of the samples (Fig. 4B) it can
be concluded that boron-containing sample possesses the lowest
amount of Lewis acid sites. These results proved that introduction
of boron into MCM-22 lattice reduce both the Brønsted and Lewis
acidity (Fig. 4).
−1
3.2.3. Catalytic activity
The effect of the acidity modification on the activity of the dealu-
minated and B-substituted series of catalysts for the mixed feed
conversion is presented in Figs. 5 and 6, respectively. The reduction
in the total number of Brønsted acid sites in both modifications lead
to substantial decrease in the ethylbenzene conversion (5A and 6).
A vast enhancement in the contact time for the dealuminated and
The results from the acidity determinations show that dealu-
mination, together with the consecutive acid treatment, as well as
Fig. 6. Ethylbenzene conversion (A) and xylene loss (B) as a function of TOS for the B-containing catalysts at 723 K and different contact times in the transformation of
ethylbenzene-m-xylene mixture (22:78 m/m%).

2
4
M.R. Mihályi et al. / Applied Catalysis A: General 476 (2014) 19–25
Table 2
Products distribution at the 5th min from the start of the reaction of ethylbenzene-m-xylene feed (22:78 m/m%) transformation at close ethylbenzene conversion attained
at different contact times.
Contact times (h)
Catalysts
1.1
0.96
0.41
0.41
0.29
0.41
D500a
D500(Ox)a
[Al,B]MCM-22a
D350a
[Al]MCM-22a
Pt/[Al]MCM-22b
Products (wt%)
Ethane
Benzene
Toluene
Ethylbenzene
Xylenes
Trimethylbenzenes
Ethyltoluenes
EB conversion (%)
Xylene loss (%)
Degree of p-xylene approach to
1.93
5.25
3.53
14.14
74.67
0.27
0.20
37.1
3.5
1.46
4.41
4.3
14.47
74.38
0.63
0.34
34.5
4.4
1.89
4.76
3.09
13.97
75.98
0.26
0.04
36.2
2.6
1.77
4.33
4.05
13.63
75.62
0.59
0.06
35.7
3.9
2.15
4.89
4.62
13.09
74.66
0.44
0.14
39.4
4.6
2.79
5.53
5.4
11.79
75.26
0.09
0.12
45.9
3.6
74.5
75.0
81.0
65.7
76.9
82.0
equilibrium (%) (Eq. = 25.5%)
c
(
%) Isomerisation
32.4
1.4
35.0
1.1
40.4
1.05
32.2
1.0
36.9
1.03
53.1
1.01
p:o-Xylene ratio (eq. = 0.96)
a
Treact. = 723 K.
Treact. = 683 K.
b
c
(
%) Isomerisation = ((p-xylene + o-xylene) in product/m-xylene in feed) × 100.
oxalic-acid treated samples is needed for to reach close activity
toward ethylbenzene conversion to that of the initial [Al]MCM-22
xylenes have proceeded in accordance to Silva et al. [5] and Benazzi
et al. [4], respectively. Considering the data in Table 2, it can be
inferred that the lowest xylene loss registered on the B-containing
catalyst results from the limitation of the above side reactions.
Replacement of the strong Al-connected acid sites by weaker ones
is most probably the reason for the favorable restriction of alkyl
transfer over [Al,B]MCM-22.
(Fig. 5A). As was concluded in [19], the generation of EFAl enti-
ties by the steaming procedure not only reduces the number of
Al-connected proton sites but also causes, most probably, blocking
of the zeolite micropores and leads to significant diminish in the
adsorption capacity and catalytic activity.
It may be inferred from the catalytic data presented above and
in accordance to the FT-IR that the Lewis acid sites created upon
steam treatment provoke, with the exception of D350/ox, larger
m-xylene loss (Fig. 5B) and faster deactivation (Fig. 5A) without
inducing favorable effect on the xylene yield. The partial extraction
of these sites by oxalic acid does not restore the active Brønsted
sites. It also does not eliminate completely the harmful effect on
the xylene isomers yield but only slightly reduce it (Fig. 5).
In Fig. 6 comparison of the catalytic performance of the parent
and boron-containing catalyst by equalizing the EB conversion with
varying the contact time is made. In accordance to the FT-IR acidity
determinations, the data presented above prove that the reduction
of the number of strong Brønsted acid sites and the lack of Lewis
acidity (Fig. 4B) together with the presence of weaker sites (Fig. 3)
are obviously the reasons for the improved catalytic performance
of [Al,B]MCM-22. It consists in lower xylene loss and much higher
catalytic stability, positive effects studied in a wider range of con-
tact times and EB conversion degrees for two reaction temperatures
Analogous effect of selective improvement in xylene isomeriza-
tion and significant reduction in xylene loss by effective elimination
of the strong acid sites of zeolite ZSM-5 is observed by Bauer et al.
[28]. The authors also introduce hydroisomerization function by
loading Pt to the modified support. The ability of platinum for con-
verting EB to xylenes is well known [29] and zeolites impregnated
with this noble metal are widely studied in reactions of mixed EB-
m-xylene feeds transformation [4,5,7,30]. As it was expected, the
introduction of Pt into our [Al]MCM-22 leaded to higher net gain
of products of EB conversion through dealkylation and transalky-
lation as well as isomerization to the aimed xylenes (Table 2) as
the above authors have observed for ZSM-5, BEA and mordenite
catalysts. Pt-modified [Al]MCM-22 possesses highest isomeriza-
tion activity and degree of p-xylene approach to equilibrium at
◦
40 C lower reaction temperature. In accordance with the reduced
formation of bulky transalkylation products, the main cause for
(Fig. 7).
Thus, it may be concluded that the introduction of boron in lat-
tice positions is more effective way of modification, compared to
dealumination, as an approach to get milder acidity, appropriate for
the conversion of EB-m-xylene mixed feed. The absence of Lewis
acid sites (Fig. 4B), the generation of weaker Brønsted acid sites
and the reduction of the micropore volume are, most possibly, the
main favorable factors responsible for the limitation of the detri-
mental side reactions of transalkylation and coking in which xylene
products participate intensely.
The distribution of the reaction products on the parent and mod-
ified catalysts at close EB conversion is presented in Table 2. The
selectivity to side products formed in transethylation and xylene
disproportionation is higher over the dealuminated samples com-
pared to the parent material. The much higher than one molar ratio
between toluene and trimethylbenzene (TMB) for all catalysts is an
indication that not only direct xylene disproportionation to those
two products takes place, but also secondary reactions of toluene
formation by transmethylation of benzene or ethylbenzene with
Fig. 7. Ethylbenzene conversion (filled symbols) and xylene loss (empty symbols)
in the transformation of ethylbenzene-m-xylene mixture (22:78 m/m%) at 683 and
723 K over [Al]MCM-22 and [Al,B]MCM-22 as a function of contact times.

M.R. Mihályi et al. / Applied Catalysis A: General 476 (2014) 19–25
25
coke accumulation, this catalyst shows very high stability, same
as that of [Al,B]MCM-22 presented in Fig. 6.
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Al]MCM-22 and the presence of hydrogenation sites provide ade-
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quate degree of EB conversion (about 40%) and p-xylene approach
to equilibrium (above 70%) at low xylene loss (less than 5%) for the
studied interval of reaction conditions.
[
[
Al,B]MCM-22 possesses better catalytic performance than the
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Steam dealumination with or without consecutive acid treat-
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Acknowledgment
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Bulgarian Inter-Academic Exchange Agreement (project no. 9) is
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