Effects of NaOH solution treatment on the catalytic performances of MCM-49 in liquid alkylation of benzene with ethylene

Article abstract of DOI:10.1016/j.apcata.2010.05.029

MCM-49 zeolite was alkali-treated with NaOH solutions under mild and severe conditions. The samples before and after alkali-treatment were both used as catalysts in liquid alkylation reaction of benzene with ethylene. XRD, XRF, TPD, TPO, IR, SEM, ^29<

Full text of DOI:10.1016/j.apcata.2010.05.029

Applied Catalysis A: General 383 (2010) 102–111
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Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Effects of NaOH solution treatment on the catalytic performances of MCM-49 in
liquid alkylation of benzene with ethylene
Kefeng Liua,b, Sujuan Xiea, Guoliang Xua, Yuning Lia, Shenglin Liua, Longya Xua,∗
a State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences,Dalian 116023, PR China
b Graduate University of Chinese Academy of Science, Beijing 100049, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
MCM-49 zeolite was alkali-treated with NaOH solutions under mild and severe conditions. The samples
before and after alkali-treatment were both used as catalysts in liquid alkylation reaction of benzene with
ethylene. XRD, XRF, TPD, TPO, IR, SEM, 29Si NMR, N2 adsorption and desorption techniques were employed
to characterize the samples. The mildly alkali-treated samples exhibited higher catalytic stability than
the parent HMCM-49, as the amorphous particles on the MCM-49 crystal surface were cleaned and the
diffusion of the reactant and product was enhanced. The severe alkali-treatment condition resulted in
the selective extraction of silicon atoms and the introduction of extra mesopores. The catalytic stability
of the catalysts that had been severely alkali-treated was not as good as that of mildly treated ones, but
was still better than that of the parent HMCM-49 catalyst if the alkali-treatment conditions were not too
severe.
Received 20 November 2009
Received in revised form 10 May 2010
Accepted 20 May 2010
Available online 27 May 2010
Keywords:
MCM-49
Alkali-treatment
Benzene
Ethylene
© 2010 Elsevier B.V. All rights reserved.
Liquid alkylation
1. Introduction
USY, Beta and MCM-22 have been used as catalysts in the liquid-
phase alkylation processes [1,3,4,6]. Although MCM-22 catalyst
Ethylbenzene is an important raw material in the petrochemical
industry [1]. At present, most ethylbenzene is used as feedstock for
the production of styrene [2,3], an important material for plastic
and rubber production. Nowadays, more than 90% of ethylbenzene
is produced by the alkylation of benzene with ethylene catalyzed
by acidic substances. The conventional catalyst for this process is
AlCl3, which still accounts for 24% of the worldwide ethylbenzene
production in 2009. As utilization of this catalyst involves prob-
lems of separation, handling, safety and corrosion [4], immense
efforts have been devoted to the development of new catalysts for
ethylbenzene production.
Compared with AlCl3, zeolites possess advantages in easy sep-
aration, low corrosion, high surface area as well as uniform
microporous structure coupled to shape selectivity, and thus they
are better acidic catalysts when applied in petrochemical indus-
try [5]. When zeolites are used as catalyst, there are mainly two
kinds of processes: vapor-phase and liquid-phase processes. Com-
pared with the vapor-phase process, liquid-phase process exhibits
obvious advantages such as better thermal control, higher selectiv-
ity to target products and longer catalyst life [4]. Zeolites such as
is somewhat low in catalytic activity, it exhibits higher selectiv-
ity for ethylbenzene. Interestingly, it was reported that MCM-49,
which possesses a structure nearly identical to that of the calcined
MCM-22 [7–9], showed even higher ethylbenzene selectivity than
MCM-22 [10].
Although MCM-49 exhibits good catalytic performances in some
petrochemical processes, the small sizes of its apertures and cavity
channel greatly limit the diffusion of reactants and products with
large molecules [5,11]. An effective way to reduce diffusion limi-
tation is to introduce mesopores into zeolites by post-treatment.
Steam treatment and acid leaching are useful methods to produce
irregular mesopores by removing aluminium from zeolites, but at
the same time the acid properties of such samples are seriously
affected [12–14]. Different from the methods above, works of Groen
et al. [15,16], Tao et al. [11,17] and our group [18] indicate that
alkali-treatment is also an effective way to introduce mesopores
into most zeolites.
Additionally, many zeolites, especially those abundantly
synthesized and post-treated in factories, contain amorphous
silico-aluminate. And these particles may hinder the diffusion of
molecules into/out of pores when zeolites are used as catalysts or
adsorbents. Subjecting zeolites to a caustic solution can remove
these amorphous materials, which can result in the improvement
of diffusion and catalytic properties [19–22].
∗
Corresponding author at: Dalian Institute of Chemical Physics, Chinese Academy
of Sciences, 804 Group, #457 Zhongshan Road, Dalian 116023, Liaoning, PR China.
Tel.: +86 411 84693292; fax: +86 411 84693292.
In this study, hierarchically structured MCM-49 samples were
prepared through a controlled silicon extraction by severe alkali-
E-mail address: lyxu@dicp.ac.cn (L. Xu).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.05.029

K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
103
Table 1
Alkali-treatment conditions and chemical compositions of MCM-49 before and after alkali-treatment.
a
Catalysts
cNaOH (mol/l)
Temperature (◦C)
Time (min)
–
Si/Al2 (mol ratio)
HMCM-49
–
–
75
75
75
75
75
25
75
25
25
25
25
20.4
20.1
17.0
15.0
13.5
11.9
20.7
20.4
20.8
20.9
21.0
21.8
0.1AT–75C–120M
0.2AT–75C–120M
0.5AT–75C–120M
0.65AT–75C–120M
0.8AT–75C–120M
0.1AT–25C–30M
0.1AT–75C–30M
0.025AT–25C–3M
0.025AT–25C–15M
0.025AT–25C–30M
0.025AT–25C–60M
0.10
0.20
0.50
0.65
0.80
0.10
0.10
0.025
0.025
0.025
0.025
120
120
120
120
120
30
30
3
15
30
60
a
Measured by XRF.
treatment, and the MCM-49 zeolite catalysts with high crystallinity
were prepared by mild alkali-treatment. The parent and alkali-
treated HMCM-49 samples were both applied for the liquid-phase
alkylation reaction of benzene with ethylene. The catalytic per-
formances of the catalysts were well correlated to their porous
structures and acidic properties.
NH3 temperature programmed desorption (NH3-TPD) was
used for the measurement of acid distribution. In a typical run,
0.1400 g of sample was loaded in a U-shaped quartz micro-reactor
(i.d. = 4 mm) connected to an on-line gas chromatograph (Shimadzu
GC-8A) equipped with a TCD detector. Before the NH3-TPD mea-
surement, the catalyst was pretreated at 600 ◦C for 1 h in flowing
He (25 ml/min), then cooled down to 150 ◦C and saturated with
NH3. After a flat baseline was obtained, TPD process was carried
out from 150 to 600 ◦C at a heating rate of 18.8 ◦C/min.
2. Experimental
Infrared (IR) measurements were performed on a Bruker 70 IR
spectrometer. For the framework IR detection, samples were mixed
with KBr (msample:mKBr = 1:100), and the mixtures were pressed
into wafers with a diameter of 13 mm. The spectrum of a KBr wafer
of the same size was used as background. For the pyridine adsorp-
tion IR (Py-IR), the sample was first pressed into a self-supported
wafer and then the wafer was pretreated at 500 ◦C for 2 h in vacuum
(10−2 Pa). After cooling down to the room temperature, a spectrum
was recorded as background. Subsequently, the wafer was exposed
to the pyridine vapor for 20 min at 0 ◦C, and then outgassed at
150 ◦C for 30 min. IR spectra were collected at room temperature.
The Brønsted/Lewis acid (B/L) molar ratio was calculated according
to the determination methods reported in Ref. [23].
Temperature program oxidations (TPO) of used catalysts were
carried out in a U-shaped quartz micro-reactor (i.d. = 4 mm) con-
nected to an on-line mass spectrometer (MS). In a typical run,
0.0800 g of used sample was loaded in the reactor and heated from
50 to 850 ◦C at a heating rate of 10 ◦C/min with the flowing of 10%
O2/Ar (50 ml/min).
2.1. Catalyst preparation
The calcined MCM-49 zeolite (Si/Al2 molar ratio = 20.4) was pro-
vided by Tieling catalyst factory of China. The as-received MCM-49
was treated with NaOH solutions (m (zeolite):V (solution) = 1:10
(g/ml)) with certain concentrations at two different temperatures
(25 and 75 ◦C) under vigorously stirring for various times. Then the
mixture was cooled down by ice water and the solid was recovered
by filtrating, washing and drying. Subsequently, the as-received
MCM-49 or the treated samples were ammonium exchanged in
0.8N NH4NO3 solution at 75 ◦C for three times, and the final H-
form zeolite was obtained by calcination of the NH4-form zeolite
at 540 ◦C for 4 h. Before evaluation, all the samples were pelletized,
crushed and sieved to 20–40 mesh particles.
The alkali-treatment conditions and the chemical compositions
of the prepared samples are listed in Table 1. The sample without
alkali-treatment was named as HMCM-49, while the alkali-treated
samples were named as xAT–yC–zM, where x, y and z represent
NaOH concentration (N), treatment temperature (◦C) and time
(min), respectively. For example, 0.1AT–75C–120M is an MCM-49
sample which was alkali-treated in 0.1N NaOH solutions at 75 ◦C
for 120 min.
2.3. Catalyst evaluation
The liquid alkylation of benzene with ethylene was carried out in
a stainless-steel fixed bed reactor. In a typical run, 2 g of catalyst was
located in the center of the reactor and pretreated at 500 ◦C for 1 h
in nitrogen. After the reactor was cooled down to 220 ◦C, benzene
was pumped into the reactor to fully fill the bed before ethylene
was introduced. The reaction conditions, which ensured that the
reaction was performed in liquid-phase, were as follows: 3.5 MPa
of pressure, 220 ◦C of temperature, 0.5 and 1.5 h−1 of weight hourly
space velocity (WHSV) of ethylene, 12/1 of benzene/ethylene molar
ratio. The product was analyzed by Varian 3800 gas chromatograph
with a flame ionization detector (FID).
2.2. Characterizations
X-ray diffraction (XRD) patterns of samples were obtained on
an X Pert Pro X-ray diffractometer operated at 40 kV and 40 mA
using Cu K␣ radiation. The relative crystallinity of the samples was
calculated according to the sum of the peak intensities at 2ꢀ of 10.0◦,
14.3◦, 16.0◦, 22.7◦ and 26.0◦.
The chemical composition of MCM-49 samples was analyzed on
a Philips Magix 601X X-ray fluorescence (XRF) spectrometer.
The morphology of the samples was investigated by scanning
electron microscopy (SEM) on an FEI Quanta-200F field emission
scanning electron microscope.
3. Results
29Si NMR experiments were carried out on a Bruker DRX-400
spectrometer using 4 mm ZrO2 rotors. The spectra were recorded
at 79.5 MHz with the magic angle-spinning rate of 4 kHz.
N2 adsorption and desorption measurements were carried out at
−196 ◦C on an ASAP 2010 instrument. Prior to analysis, the samples
were pretreated at 350 ◦C in the vacuum of 10−3 Pa for 5 h.
3.1. Severe alkali-treatment of MCM-49 zeolite
In the following experiments, MCM-49 is severely alkali-treated
in 0.1–0.8N NaOH solution for 120 min at 75 ◦C. From the chemical
compositions of the parent and treated samples listed in Table 1, it

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K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
can be observed that the Si/Al2 molar ratio of 0.1AT–75C–120M
is similar to that of HMCM-49, while it decreases from 20.4
to 11.9 with increasing the NaOH solution concentration from
0.2 to 0.8N. The results suggest that Si is more easily extracted
from framework than Al, which is consistent with the results in
Refs. [11,24].
Table 2 presents the textural properties of the samples. With
the increase of NaOH solution concentration from 0.1 to 0.65N,
the mesopore volume of the samples increases from 0.29 to
0.40 cm3/g and the extra surface area (Sext) increases from 79.2 to
174.2 m2/g. In contrast, the microporous area decreases from 409.1
to 241.2 m2/g.
XRD patterns and relative crystallinity of the samples are shown
in Fig. 1(A). The diffraction pattern of HMCM-49 is in agree-
ment with that in Ref. [8]. Compared with HMCM-49 (relative
crystallinity: 100%), the relative crystallinity of 0.1AT–75C–120M
increases to 134%, which may be attributed to the purification of
zeolite by cleaning the amorphous particles [20]. With increasing
the NaOH solution concentration from 0.2 to 0.65N, the relative
crystallinity of samples decreases from 86 to 40%. Especially, as
the NaOH solution concentration is 0.8N, only 9% of relative crys-
tallinity is left.
29Si NMR spectra are employed to investigate the chemical
environment of the framework silicon atoms in MCM-49. The
deconvoluted spectra show that there are at least five resonance
peaks that can be attributed to different chemical environments
of T sites (where T is tetrahedral site in the proposed hexagonal
structure of MCM-49) in Q4 region (Fig. 2). Some of these peaks
are overlapped due to the similar chemical shifts [25,26]. The res-
onance peak at about −106 ppm is usually attributed to Si (0Al) on
T2 site, although some contribution from Si (1Al) site cannot be
excluded. The peak at −121(−116) ppm (marked as T1+ (T6+)) is
mainly ascribed to Si on T1 (T6) site, and it may also contain some
contribution from silicon atoms on T4 and T5 sites [27,28]. It is clear
that some of the peak intensity decreases upon alkali-treatment
severity (through increasing NaOH solution concentration). In par-
ticular, the peaks at −121 (T1+) and −114.5 ppm (T3) shrink more
obviously than the others (T2, T7 and T8), along with the increase
of peak area at about −98 ppm of Q3 (Fig. 2(B)). The deconvolu-
tion quantification data of T1+ and Q3+ (some contribution of Q2
possibly involved) for several samples are calculated and listed in
Table 3.
In Fig. 3, the framework IR spectra of the samples are depicted.
The vibrations at about 1080, 800 and 450 cm−1 are assigned to
the “internal” structure vibrations of the SiO4 or AlO4 tetrahedra.
The absorption bands between 540 and 630 cm−1 are related to
the presence of double rings in the framework [29,30]. From the
deconvoluted spectra in Fig. 3(B), we can see that the peak inten-
sity of blocks (550 cm−1) and 5MR chains (1250 cm−1) decreases
with the increase of NaOH solution concentration, especially when
it is higher than 0.2N, indicating the partial damage of the 5MR
structures after the severe alkali-treatment.
Fig. 4 displays the SEM images of the parent and alkali-treated
samples. The sample HMCM-49 (Fig. 4(a)) exhibits morphology of
5–30 ␮m aggregates constituted of thin crystal sheets 30–40 nm of
thick. Apertures between the sheets of HMCM-49 are stacked with
very small particles (Fig. 4(a)) which are absent in the as-received
NaMCM-49. This might be due to the instability of MWW type zeo-
lite while it is calcined in air [29,31,32]. After being alkali-treated
with 0.1N NaOH solution, the sheets of sample are maintained
and the small particles between the sheets disappear (Fig. 4(b)).
The outer surface of the crystal sheet starts to dissolve when the
NaOH solution concentration increases to 0.2N, as is marked out
in Fig. 4(c). When the concentration amounts to 0.5N, most sheets
on the surface of the aggregates are destroyed. When the concen-
tration is increased to 0.65N, even the aggregates collapse and only
Fig. 1. XRD patterns of samples before and after alkali-treatment. (A) a:
HMCM-49, b: 0.1AT–75C–120M, c: 0.2AT–75C–120M, d: 0.5AT–75C–120M,
e: 0.65AT–75C–120M and f: 0.8AT–75C–120M; (B) a: HMCM-49, b:
0.025AT–25C–30M, c: 0.1AT–25C–30M and d: 0.1AT–75C–30M; (C) a: HMCM-
49, b: 0.025AT–25C–3M, c: 0.025AT–25C–15M, d: 0.025AT–25C–30M and e:
0.025AT–25C–60M.
cracked sheets are left as shown in Fig. 4(e), due to the quite serious
alkali-treatment [33].
The acidity of HMCM-49 samples before and after alkali-
treatment was characterized by the NH3-TPD method (Fig. 5). The
quantitative results shown in Fig. 5 and Table 3 were obtained
according to Gauss curve fitting by peak deconvolution method. The
experimental curves are satisfactorily fitted by at least three peaks

K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
105
Table 2
Texture properties of samples before and after alkali-treatment.
Catalysts
SBET (m2/g)
Smicro (m2/g)
Sext (m2/g)
Vmicro (cm3/g)
Vmeso (cm3/g)
HMCM-49
488.3
458.4
469.1
461.3
415.4
448.0
437.2
489.0
442.7
431.9
402.3
409.1
368.1
354.2
301.4
241.2
372.2
378.1
407.6
385.2
378.6
361.0
79.2
90.3
114.9
159.9
174.2
75.8
59.2
81.4
57.5
53.3
0.20
0.18
0.17
0.15
0.11
0.18
0.19
0.20
0.19
0.19
0.18
0.29
0.29
0.30
0.38
0.40
0.26
0.19
0.30
0.24
0.21
0.19
0.1AT–75C–120M
0.2AT–75C–120M
0.5AT–75C–120M
0.65AT–75C–120M
0.1AT–25C–30M
0.1AT–75C–30M
0.025AT–25C–3M
0.025AT–25C–15M
0.025AT–25C–30M
0.025AT–25C–60M
41.3
Table 3
Acid properties and deconvoluted Si NMR results of samples before and after alkali-treatment.
Catalysts
B/L
Acid quantity (mmol/g)
Weak
T1+ (%)
Q3+(%)
Medium
Strong
HMCM-49
1.97
1.87
1.67
1.46
1.32
0.16
0.19
0.26
0.32
0.35
0.19
0.19
0.15
0.13
0.10
0.44
0.49
0.40
0.35
0.29
11.52
11.49
8.66
4.17
1.33
8.28
8.25
8.95
9.60
11.22
0.1AT–75C–120M
0.2AT–75C–120M
0.5AT–75C–120M
0.65AT–75C–120M
Fig. 3. (A) Framework IR spectra of samples before and after alkali-treatment:
a: HMCM-49, b: 0.025AT–25C–3M, c: 0.025AT–25C–15M, d: 0.025AT–25C–30M,
e: 0.2AT–75C–120M, f: 0.5AT–75C–120M and g: 0.65AT–75C–120M. (B) Decon-
voluted Framework IR spectra of a: HMCM-49, e: 0.2AT–75C–120M and g:
0.65AT–75C–120M.
Fig. 2. (A) 29Si NMR spectra of MCM-49 zeolite catalysts before and after
alkali-treatment: a: HMCM-49, b: 0.025AT–25C–3M, c: 0.025AT–25C–15M, d:
0.025AT–25C–30M, e: 0.1AT–75C–120M, f: 0.2AT–75C–120M, g: 0.5AT–75C–120M
and h: 0.65AT–75C–120M. (B) Deconvoluted 29Si NMR spectra of a: HMCM-49, f:
0.2AT–75C–120M and h: 0.65AT–75C–120M. T1+ (T6+): peak at −121(−116) ppm
mainly included Si on T1 (T6) site, may also involve Si on T4 or T5 site. Q3+: peak at
about −98 ppm is mainly contributed by Q3 type Si, Q2 type Si may be also partly
included (Qn = Si(OSi)n(OH)(4−n)).
that represent three types of acid sites with different strengths
(Fig. 5A(e)). These three peaks centered at about 246, 317 and
430 ◦C are denoted as weak, medium and strong acid sites, respec-
tively. For the samples alkali-treated under severe conditions, each

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K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
Fig. 4. SEM of MCM-49 zeolite catalysts before and after alkali-treatment. (a): HMCM-49, (b) 0.1AT–75C–120M, (c) 0.2AT–75C–120M, (d) 0.5AT–75C–120M, (e)
0.65AT–75C–120M, (f) 0.025AT–25C–15M, (g) 0.025AT–25C–30M and (h) 0.025AT–25C–60M.
total acid amount is all around 0.80 mmol/g. However, the medium
and strong acid amounts as well as the B/L ratio decrease obvi-
ously when the NaOH solution concentration increases from 0.1
to 0.65N (Table 3), which means that the medium and strong
acid sites and the Brønsted acid sites are more sensitive to caus-
tic solution than are the weak acid sites and the Lewis acid
sites.
3.2. Mild alkali-treatment
The results in Section 3.1 indicate that mesopores can be pro-
duced in the MCM-49 zeolite when the NaOH concentration of
alkali-treatment is higher than 0.2N. However, the crystallinity of
MCM-49 zeolite cannot be preserved, which is dissimilar to the
situation in the case of ZSM-5 [18,34]. Further investigations are

K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
107
MCM-49 samples alkali-treated under the mild conditions possess
high relative crystallinity (∼130%, Fig. 1(B and C)) and similar Si/Al2
molar ratios to that of the parent sample (∼20, Table 1). In addition,
the total acid amount and the B/L ratio of these samples show little
difference (∼0.8 mmol/g and ∼1.9, respectively). Furthermore, the
distribution of their weak, medium and strong acid amount (not be
listed here) shows little change compared with that of HMCM-49.
As shown in Table 2, the microporous properties of MCM-49
zeolites are not obviously affected by the alkali-treatment with
NaOH solution of 0.025N at 25 ◦C. Even if the alkali-treatment time
is prolonged to 60 min, 90% of microporous area and of volume
are preserved. However, the mesoporous related properties are
affected intensively. When the treatment time is increased from
0 (HMCM-49) to 60 min, the Sext decreases from 79.2 to 41.3 m2/g
and the Vmeso decreases from 0.30 to 0.19 cm3/g. This means that
the mesopores formed by the aggregation of small particles [1,35]
are reduced by these mild alkali-treatment.
As shown in Fig. 1(C), when MCM-49 is mildly alkali-treated
in an NaOH solution of 0.025N for different times, the samples
show high and similar relative crystallinity (∼130%). The 29Si NMR
spectra show that the intensity distributions of the five peaks
of 0.025AT–25C–3M, 0.025AT–25C–15M and 0.025AT–25C–30M
are quite similar (Figs. 2(A)(b–d)), and the FT-IR spectra (Figs. 3
(A)(b–d)) also exhibit similar framework vibrations, indicating that
the zeolite frameworks are slightly affected. In addition, as shown in
Fig. 4(f–h), similar to 0.1AT–75C–120M, small particles disappear
when the samples are treated in NaOH solution of 0.025N. Clear
crystal sheets and the aggregates can be observed in the samples
of 0.025AT–25C–15M, 0.025AT–25C–30M and 0.025AT–25C–60M,
which is different from the case for the samples treated in NaOH
solution of ≥0.2N (Fig. 4(c–e)).
3.3. Evaluation of the MCM-49 catalysts
The parent and alkali-treated HMCM-49 samples were both
used as catalysts for the liquid alkylation of benzene with ethy-
lene. Ethylene conversion changes with the time on stream (TOS),
while the product distributions have little difference. As shown in
Table 4, the ethylation selectivity is over 99.0% for all the samples,
and the selectivity to toluene is lower than 0.1%.
The evolutions of ethylene conversion with the TOS over the
MCM-49 catalysts are shown in Fig. 6. For the parent HMCM-49
and the alkali-treated samples under severe conditions (Fig. 6(A)),
their initial ethylene conversions are all at ∼99.9%, indicating that
the MCM-49 catalysts are very active for this reaction. However,
the catalytic stability of the parent HMCM-49 is not very good,
ethylene conversion decreases from 99.9% at 1 h of TOS to 91.0%
at 27 h. The catalytic stability is improved over the alkali-treated
MCM-49 zeolite catalysts except for the severely treated sample of
0.65AT–75C–120M. At TOS of 27 h, the ethylene conversions over
the xAT–75C–120M samples where x = 0.1, 0.2 and 0.5 are 97.6%,
94.5% and 93.8%, respectively.
For the samples alkali-treated with 0.1N NaOH solution, the
milder the conditions are (by reducing the treatment temperature
and treatment time), the better the corresponding catalytic stability
is (Fig. 6(A) iconograph). When the concentration of NaOH solution
is 0.025N, there is almost no difference for the performances of the
catalysts alkaki-treated for different times at the ethylene WHSV
of 0.5 h−1 (ethylene conversion is always higher than 99.0% within
27 h of TOS). In order to differentiate the catalytic performances, we
enhanced the ethylene WHSV to 1.5 h−1. As shown in Fig. 6(B), the
catalyst, even alkali-treated for only 3 min at 25 ◦C in NaOH solution
of 0.025N shows higher ethylene conversion and better catalytic
stability than HMCM-49. When the treatment time was between 3
and 60 min, the samples show similar initial ethylene conversion,
but their catalytic stability varies with the treatment time. Among
Fig. 5. NH3-TPD spectra of MCM-49 zeolites catalysts before and after alkali-
treatment. (A) a: HMCM-49, b: 0.1AT–75C–120M, c: 0.2AT–75C–120M,
d: 0.5AT–75C–120M and e: 0.65AT–75C–120M. (B) a: HMCM-49, b:
0.025AT–25C–30M, c: 0.1AT–25C–30M and d: 0.1AT–75C–30M. (C) a: HMCM-
49, b: 0.025AT–25C–3M, c: 0.025AT–25C–15M, d: 0.025AT–25C–30M and e:
0.025AT–25C–60M.
carried out under mild alkali-treatment conditions, during which
the crystallinity of the samples can be well preserved or even
increased, as shown in Table 1 and Fig. 1.
The influences of the NaOH solution concentration and treat-
ment temperature on mild alkali-treated MCM-49 zeolite are
firstly investigated by preparing the samples of 0.025AT–25C–30M,
0.1AT–25C–30M and 0.1AT–75C–30M. The effect of alkali-
treatment time is also studied by immersing MCM-49 zeolite in
the 0.025N NaOH solutions at 25 ◦C for 3, 15, 30 and 60 min. All the

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K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
Table 4
Reaction performance over the MCM-49 catalysts via alkali-treatment conditions (a) HMCM-49, (b) 0.65AT–75C–120M, (c) 0.5AT–75C–120M, (d) 0.2AT–75C–120M, (e)
0.1AT–75C–30M, (f) 0.025AT–25C–15M and (g) 0.025AT–25C–30M.
Catalysts
ax
bx
cx
dx
ex
ay
ey
fy
gy
Ethylene conversion (mol%)
96.9
82.9
98.4
99.7
99.9
76.0
89.7
92.1
91.4
Product selectivity (mol%)
Ethyl benzene
m-Diethyl benzene
p-Diethyl benzene
95.1
1.4
1.2
1.6
99.3
96.2
1.0
0.9
1.5
99.6
0.1
0.03
0.25
0.02
95.7
1.5
1.2
1.1
99.5
96.1
1.3
1.1
1.1
99.6
96.2
1.5
0.9
0.9
99.5
97.2
0.8
0.7
1.1
99.8
96.4
1.1
0.9
1.3
99.7
96.6
1.1
0.8
1.2
99.7
0.1
0.02
0.17
0.01
97.4
0.7
0.6
0.9
99.6
0.1
0
o-Diethyl benzene
ꢀ
(Ethylation)
Methylbenzene
C9 aromatics
C10+ aromatics
Paraffins
0.07
0.07
0.08
0.08
0.04
0.03
0.05
0.56
0.02
0.07
0.33
0.03
0.06
0.23
0.03
0.03
0.37
0.02
0.01
0.11
0.04
0.01
0.23
0.03
0.28
0.02
Reaction conditions: 3.5 MPa, 220 ◦C, 9 h of TOS, B/E (mol ratio) 12. x: 0.5 h−1 of ethylene WHSV and y: 1.5 h−1 of ethylene WHSV.
all these samples investigated, 0.025AT–25C–15M shows the best
catalytic performances.
for 0.65AT–75C–120M). In addition, the intensity of peaks of the
severely alkali-treated samples decrease compared with that on
HMCM-49.
The TPO technique was employed to investigate the nature of
coke deposited on the used samples. As shown in Fig. 7, three peaks
centered at about 345, 440 and 620 ◦C can be seen in the TPO curve
of HMCM-49. The TPO curves of the mildly alkali-treated samples
(x = 0.025 and 0.1N) are very close to that of HMCM-49 (Fig. 7(B
and C)), indicating that the total coking amount and carbon species
generated in the alkylation reaction are very similar. The TPO peaks
shift to lower-temperature positions for the severely alkali-treated
samples (cNaOH ≥ 0.2N), especially for the peak at ∼620 (to 550 ◦C
4. Discussion
According to the results above, the structure evolutions of
the MCM-49 catalysts during the alkali-treatment process can be
depicted as in Scheme 1. It is well known that HMCM-49 is com-
posed of crystal sheets with “pockets” (hemi-supercages) on the
external surfaces (Scheme 1(A)) [7]. The amorphous particles gen-
erated by calcinations may scatter between the sheets and partly
fill in the pockets if they are small enough, which results in the
hindered diffusion for substance into/out of the “pockets”. The
aggregation of these small particles produces inter-particle meso-
pores, which contributes a lot to the mesopore volume and extra
surface area [35].
For the MCM-49 samples alkali-treated under the severe condi-
tions, the fact that the relative crystallinity of 0.1AT–75C–120M is
higher than that of HMCM-49 (Fig. 1(A)) is mainly attributed to
the purification of zeolite [20,22]. Bonilla et al. investigated the
effect of alkali-treatment conditions on ferrierite zeolite and found
that too severe extraction of Si atoms would lead to collapse of the
zeolite framework [36]. Bao and co-workers also showed that the
relative crystallinity of ZSM-5 would decrease with the increase
of alkali-treatment severity [37,38]. However, unlike ZSM-5, the
relative crystallinity of MCM-49 (Si/Al2 molar ratio = 20.4) cannot
be preserved when the concentration of NaOH solution is above
0.2N, and it is only 40% when the concentration of NaOH solution
is 0.65N, which indicates that the framework of MCM-49 zeolite is
less stable than that of ZSM-5 [18].
When the NaOH solution concentration is higher than 0.2N, the
external surface area and mesopore volume of the sample obtained
increase along with the loss of micropores (Table 2, Scheme 1(C)).
The formation of mesopores can be ascribed to the extraction of Si
from framework, during which a little amount of Al is also eluted
[39,40]. Bao and co-workers and Groen et al. investigated the alkali-
treated MFI type zeolite by N2 adsorption and reported that the
extraction of Si usually started from the boundaries and defects
[37,41]. On the alkali-treated ZSM-5, Ogura et al. found that the
dissolution of Si started from the surface of zeolite crystals [42].
In our experiments, the morphology of MCM-49 changes with the
severe alkali-treatment conditions. For the severely alkali-treated
MCM-49 samples (Fig. 4(c–e)), the edges of crystal sheets, espe-
cially sheets on the surface of aggregates, are dissolved. According
to the Ref. [37], the boundaries and defects are the places where
the crystallization is poor, and they are easily dissolved in the alkali
solution.
Fig. 6. Ethylene conversion as a function of reaction time over the MCM-49 zeolite
catalysts. (A) 3.5 MPa, 220 ◦C, B/E (mol ratio) = 12, ethylene WHSV = 0.5 h−1: a:
HMCM-49, b: 0.1AT–75C–120M, c: 0.2AT–75C–120M, d: 0.5AT–75C–120M,
e: 0.65AT–75C–120M and f: 0.8AT–75C–120M. (B) 3.5 MPa, 220 ◦C, B/E
: a: HMCM-49, b: 0.1AT–25C–30M,
c: 0.025AT–25C–3M, d: 0.025AT–25C–15M, e: 0.025AT–25C–30M and f:
0.025AT–25C–60M.
(mole ratio) = 12, ethylene WHSV = 1.5 h−1

K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
109
Scheme 1. Imagine of alkali-treatment process: (A) HMCM-49, (B) MCM-49 sample alkali-treated under mild conditions and (C) MCM-49 sample alkali-treated under severe
conditions.
Camblor et al. reported that, when the layered precursor
was transformed into the 3D framework upon calcination, most
connectivity defects exist at T1–O–T1 and T2–O–T3 sites [26].
Corma et al. have succeeded in preparation of ITQ-2 with MCM-
22(p) as a precursor [28], during which the potential linkages of
T1–O–T1 were hindered. This indicates that T1–O–T1 linkages are
more fragile than the others, and that the Si on T1 site is eas-
ily extracted with the damage of 5MR including T1 site. Fig. 2
shows that the intensities of peaks at −121 and −114.5 ppm, which
are attributed to Si on T1+ and T3 sites, respectively, obviously
decreases compared with that of peaks of T2, T7 and T8. The
peak area percent of T1+ decreases from 11.52% (HMCM-49) to
only 1.33% (0.65AT–75C–120M), indicating that Si atoms on T1
sites are more sensitive to alkali-treatment. Simultaneously, the
percent of Q3+ site increases from 8.28% (HMCM-49) to 11.22%
(0.65AT–75C–120M). Thus we can conclude that Si atoms located
on T1 and T3 sites are easier to be extracted than those located
on other sites (T2, T7 and T8) when MCM-49 is exposed to alkali-
treatment. The other Si atoms which are adjacent to T1 and T3 sites
would exist in the form of (OSi)3SiOH, which results in the increase
of peak area centered at −98 ppm. As shown in the framework IR
spectra (Fig. 3), when the NaOH solution concentration is higher
than 0.2N, with the increase of NaOH solution concentration, the
peaks between 540 and 630 cm−1 and at 1250 cm−1 shrink more
obviously than other peaks. Especially when the NaOH solution
concentration is 0.65N, the area of the peak centered at 550 cm−1
of 0.65AT–75C–120M shrinks to only 20% compared with that of
HMCM-49, and the area of the peak at 1250 cm−1 also reduces
by 20%. The similarity of its IR spectrum to that of ITQ-2 men-
tioned in Ref. [28] indicates the dissociation of T1–O–T1 linkage in
0.65AT–75C–120M. Combining with the 29Si NMR results, we can
propose that the extraction of Si most likely starts from the T1 and
T3 sites, along with the break of blocks and damage of five member
rings involving Si on T1.
The results of NH3-TPD reveal that the total acid amount of sam-
ples alkali-treated under different conditions has a little change.
The reason that 0.1AT–75C–120M possesses more strong acid sites
(0.49 mmol/g) than HMCM-49 (0.44 mmol/g) can be ascribed to
its higher Al content resulting from desilication. Strong acid sites
of samples decrease when the concentration of NaOH is higher
than 0.2N. For the sample 0.65AT–75C–120M, only 66% of strong
acid sites are left compared with HMCM-49, which might be due
to the collapse of zeolite framework as a result of the extraction
of too much Si. Similar results are also observed on the alkali-
treated ZSM-5 sample [18]. The alkali-treatment condition also has
some effect on the B/L ratio of the MCM-49 samples. The B/L ratio
decreases from 1.87 to 1.32 when the concentration of NaOH solu-
tion is enlarged from 0.1 to 0.65N (Table 3). The decrease of the
Brønsted acid amount with the increase of alkali-treatment sever-
ity may partly account for the decrease of catalytic stability, as the
Brønsted acid site of catalyst is generally considered as the active
center in the reaction of ethylene alkylation with benzene [1,6].
In addition, extra mesopores are introduced (Scheme 1(C)) with
the decrement of micropores (Table 2), and the catalytic stability
increases at a certain degree (Fig. 6(A)) when the NaOH solution
concentration is below 0.5N. This indicates that the mesopores can
improve the catalytic stability by means of improving the diffu-
sion of reactants and products [6]. The lower catalytic stability of
0.65AT–75C–120M compared with HMCM-49, is related with its
low Brønsted acid amount and poor relative crystallinity, although
it has a lot of mesopores.
Although the mesopore of the MCM-49 zeolite does not increase
during mild alkali-treatment (cNaOH = 0.025N), its total acid amount
and its B/L ratio are both preserved (Fig. 5). Simultaneously, the
samples alkali-treated under such conditions have limited differ-
ences in reaction performance and they all show better catalytic
stability than the HMCM-49. As shown in Fig. 4, amorphous par-
ticles and crystal splinters are cleared by mild treatment with

110
K. Liu et al. / Applied Catalysis A: General 383 (2010) 102–111
small silica particles would not aggregate, but would tightly scatter
on the surface [43,44], and some of the “pockets” would be par-
tially or totally covered (marked in Scheme 1(B)). This results in
the bad diffusion conditions for bigger molecular species. To some
extent, this reason can account for the worse catalytic stability of
0.025AT–25C–60M being worse than that of 0.025AT–25C–15M or
0.025AT–25C–30M.
5. Conclusions
MCM-49 zeolite was alkali-treated in NaOH solution under vari-
ous conditions. After the suitable alkali-treatment (in 0.025N NaOH
solution at 25 ◦C for 15–30 min), not only are the amorphous par-
ticles and crystal splinters within the MCM-49 zeolites cleared,
but also their framework and acid properties are preserved. For
the severely alkali-treated samples, mesopores were introduced by
the selective extraction of silicon on T1 and T3 sites in the MWW
framework, covering the decrease of micropore amount as well as
the damage of MCM-49 structure. In the liquid alkylation reac-
tion of benzene with ethylene, the samples alkali-treated under
mild conditions show better stability for ethylene conversion than
the parent HMCM-49, and this becomes much outstanding under
the high ethylene WHSV. Furthermore, the samples that were
alkali-treated under severe conditions also exhibit better catalytic
stability than the parent HMCM-49 unless the alkali-treatment con-
dition is too severe.
Acknowledgement
We acknowledge the National Basic Research Program of China
(Nos. 2009CB623501 and 2005CB221403) for financial support.
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