Size-controlled synthesis of MCM-49 zeolites and their application in liquid-phase alkylation of benzene with ethylene

Article abstract of DOI:10.1039/c4ra15470c

Size-controlled synthesis of MCM-49 zeolites was achieved via topology reconstruction from NaY zeolites with different sizes. SEM images showed that the sizes of the reconstructed H-MCM-49 zeolites were controlled by those of the parent NaY zeolites. Smal

Full text of DOI:10.1039/c4ra15470c

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Size-controlled synthesis of MCM-49 zeolites and
their application in liquid-phase alkylation of
benzene with ethylene†
Cite this: RSC Adv., 2015, 5, 13420
*
Yanchun Shi, Enhui Xing, Wenhua Xie, Fengmei Zhang, Xuhong Mu and Xingtian Shu
Size-controlled synthesis of MCM-49 zeolites was achieved via topology reconstruction from NaY zeolites
with different sizes. SEM images showed that the sizes of the reconstructed H-MCM-49 zeolites were
controlled by those of the parent NaY zeolites. Smaller NaY zeolites improved the diffusion of reactants
on the parent NaY zeolites during the topology reconstruction, the final H-type zeolites, the relative
crystallinity, the BET surface areas, and the number of Brønsted and total acid sites. Moreover, a
substantial improvement in both ethylene conversion and ethylbenzene selectivity was observed for the
H-type catalyst originating from the smaller NaY zeolite (300 nm) in liquid-phase alkylation of benzene
with ethylene. This alkylation performance may provide a new strategy for enhancing both catalytic
activity and product selectivity, and our method could be applicable to many diffusion-limited or
external surface reactions.
Received 29th November 2014
Accepted 13th January 2015
DOI: 10.1039/c4ra15470c
www.rsc.org/advances
sinusoidal 10 MR pores (0.41 ꢀ 0.54 nm). Some reports19,20
1. Introduction
have claimed that active centers of MWW zeolites are mainly
located in the 12 MR cups on the outer surface so that they are
more easily accessed during liquid-phase alkylation of benzene
with ethylene. Therefore, the 12 MR cups on the outer surface
do not restrict the diffusion of reactant toward the active
centers. Cheng et al.10 demonstrated that the acid sites on the
external surface were the main active centers based on the
following comparison tests. 2,4,6-Trimethyl-pyridine (0.62 ꢀ
0.56 nm)-doped and undoped H-MCM-22 were used as catalysts
in the liquid-phase alkylation of benzene with ethylene at 220 ꢁC
with a benzene/ethylene molar ratio of 3.5. Compared with the
high ethylene conversion (95.6%) over undoped H-MCM-22,
2,4,6-trimethyl-pyridine-doped H-MCM-22 was almost inactive,
giving only 1.4% ethylene conversion. The results indicated that
2,4,6-trimethyl-pyridine is mainly adsorbed on the external
surface of H-MCM-22 (12 MR cups) catalysts, and thus the active
centers for liquid-phase alkylation of benzene with ethylene are
mainly located in 12 MR cups.
To increase the number and accessibility of active centers,
equal to improving the alkylation performance, much effort has
been devoted to synthesizing MWW zeolites, including preparing
small grain sizes,21 pillaring,22,23 swelling,24 interlayer-expan-
sion,25,26 delaminating,27,28 and introducing mesopores with basic
solution.29–31 In these methods, it is very important to prepare
smaller MWW zeolites to tailor catalytic properties, including
high surface activity, short diffusion path lengths, and low
carbon deposition. Although most research has focused on
materials and conditions to synthesize smaller MWW zeolites via
conventional hydrothermal treatment, little progress has been
made. Hexamethyleneimine (HMI) is an effective structure-
Ethylbenzene (EB) is an important material for plastic and
rubber production. Global demand grew by about 4.1% annu-
ally in the period 2009–2014 and is forecast to grow by 3.3% per
year over the next decade.1,2 Compared with the conventional
AlCl3 process, the zeolite-based vapor-phase (ZSM-5) and liquid-
phase (USY, Beta, MCM-22) EB processes show easy separation,
low corrosion, high activity and shape selectivity.3–8 Because of
the low reaction temperature, there are obvious advantages to
the liquid-phase process such as high selectivity and long
catalyst life. USY zeolite is seldom used because of its relatively
inferior EB selectivity and quick deactivation. Owing to their
superior activity and selectivity, Beta and MCM-22 zeolites are
the most widely used catalysts in commercial liquid-phase
alkylation of benzene with ethylene.
Despite their low catalytic activity, MWW zeolites have
attracted much attention because of their superior EB selectivity
at a lower benzene/ethylene molar ratio with respect to energy
input.9–11 MWW zeolites,12–18 including PSH-3, ERB-1, SSZ-25,
MCM-22, MCM-49, MCM-56, MCM-36, ITQ-1, ITQ-2 and UZM-
8, have a MWW structure: 12 MR “cups” (0.71 ꢀ 0.71 ꢀ 0.91
nm) on the external surface, supercages dened by 12 MR
(0.71 nm external diameter ꢀ 1.82 nm height) through 10 MR
opening windows (0.41 ꢀ 0.54 nm), and two-dimensional
State Key Laboratory of Catalytic Materials and Reaction Engineering, Research
Institute of Petroleum Processing, Sinopec, Xueyuan Road 18, Beijing 100083,
China. E-mail: zhangfm.ripp@sinopec.com; Tel: +86 010 82368698
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ra15470c
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directing agent for synthesizing MWW zeolites; therefore, it is Co., Ltd) were mixed in a conical ask with NaOH (96 wt%,
difficult to avoid the aggregation of MWW layers, and to achieve Beijing Chemical Works), HMI (98 wt%, TCI), solid silica gel
size-controlled synthesis of MWW zeolites. Typically, MWW (90 wt% SiO2, Qingdao Haiyang Chemical Co., Ltd.), and
zeolite crystals are sheet-like discs or aggregates of cross-linked deionized water. The molar ratios of the batch composition
discs 2–5 mm in diameter and 10–40 nm in thickness.32,33 were: SiO2/Al2O3 ¼ 25, NaOH/SiO2 ¼ 0.18, HMI/SiO2 ¼ 0.3 (300
Although the use of cationic polymers could decrease the size of nm: HMI/SiO2 ¼ 0.2), H2O/SiO2 ¼ 15. The order of addition was
MWW zeolites,21 this method still requires further consideration deionized water, NaOH, silica gel, NaY zeolite and HMI with
because of its economic and environmental impact. Nanosized continuous stirring. Topology reconstruction was carried out in
MCM-22 catalysts demonstrated higher methane conversion, a Teon-lined autoclave at 145 ꢁC and 30 rpm. As a comparison
higher benzene yield, higher catalyst stability and lower carbon for sample 1, MCM-49 zeolite was synthesized in the 1 m3
deposition during methane dehydro-aromatization compared demonstration unit at Hunan Jianchang Petrochemical
with conventional micro-sized MCM-22 catalysts.21
Company (Sinopec) via temperature-controlled phase transfer
Recently, we investigated the potential of topology recon- hydrothermal synthesis.35 The molar ratios of the batch
struction (direct zeolite-to-zeolite transformation) and succeeded composition were: SiO2/Al2O3 ¼ 25, NaOH/SiO2 ¼ 0.18, HMI/
in synthesizing MWW zeolites with FAU parent zeolites,34 which SiO2 ¼ 0.3, aniline/SiO2 ¼ 0.2, H2O/SiO2 ¼ 15. The synthesis was
provided us with a novel strategy to synthesize smaller MWW carried out at 145 ꢁC for 72 h with a stirring speed of 15 Hz. The
zeolites and improve their catalytic performance. According to products were recovered by ltration, washed with deionized
ꢁ
our previous research, MWW zeolites were reconstructed gradu- water until they reached pH 7 and dried in an oven at 100 C
ally from the exterior to the interior of FAU zeolites, which was overnight to remove physically adsorbed water molecules.
clearly demonstrated by the core (FAU)–shell (MWW) co-existing
H-type zeolites. H-type zeolites were prepared by two repe-
zeolites as intermediates. No extra-framework Al was observed in titions of liquid-phase ion exchange with NH4NO3 solution at
ꢁ
ꢁ
topology reconstruction, and there was almost no detection of 90 C for 2 h. The products were ltered and dried at 100 C
27Al resonances in the mother liquor. The FAU structure was overnight, and calcined at 550 C for 6 h, to obtain the corre-
ꢁ
reconstructed into the MWW structure without complete disap- sponding H-type zeolites with a Na2O content of less than 0.05
pearance of crystalline phase, which led us to consider whether wt%. The mass ratio composition of the MCM-49
the sizes of target MWW zeolites could be controlled by the zeolites : NH4NO3 : deionized water was 1 : 1 : 20. According
original sizes of the parent FAU zeolites via topology recon- to the sizes of the parent NaY zeolites (1000, 500, and 300 nm),
struction. Through this strategy, it may be easy to synthesize the products were named HYM, HYM500 and HYM300,
smaller MWW zeolites and improve their alkylation perfor- respectively.
mance. In this work, we intentionally select NaY zeolites with
Catalyst preparation. The NH4-type samples (70 wt%) and
different sizes as parent zeolites for the size-controlled synthesis Al2O3 (30 wt%) were mixed and extruded. Al2O3, which showed
of MWW zeolites. We also examine the effects of the size of the almost no activity for the liquid-phase alkylation of benzene
parent NaY zeolites on the topology reconstruction and the with ethylene, was used as binder to increase the mechanical
liquid-phase alkylation of benzene with ethylene over the strength of the catalysts. The extruded catalysts were crushed
reconstructed MWW catalysts. Our method may be suitable for with dimensions of 16–20 mesh to collect particles that were
the size-controlled synthesis of target MWW zeolites.
then subjected to calcination at 550 ꢁC for 6 h to obtain the
corresponding H-type catalysts.
2. Experiments
2.1 Synthesis of MCM-49 and preparation of H-type catalysts
2.2 Characterization
Topology reconstruction from FAU to MWW structure. All
Intermediate phase characterization. X-ray diffraction (XRD)
materials were used as purchased. Commercial NaY zeolites patterns of samples were collected on an X'pert X-ray diffrac-
(Table 1; average sizes: 1000, 500, and 300 nm, Sinopec Catalyst tometer (PANalytical Corporation, Netherlands) with ltered Cu
Table 1 Topology reconstruction from NaY to MCM-49 zeolites
Parent zeolites
Conditions of topology reconstructiona
Products
b
Samples SiO2/Al2O3 Na2O (wt%) Size (nm) SiO2/Al2O3 HMI/SiO2 NaOH/SiO2 H2O/SiO2 Time (h) SiO2/Al2O3 XRD
R.C.c (%)
MCM-49 100
1
2
3
4
5
—
—
—
25
25
25
25
25
0.3
0.3
0.3
0.2
0.3
0.18
0.18
0.18
0.18
0.18
15
15
15
15
15
72
88
72
72
72
23
21
21
21
21
5.0
5.0
5.0
5.0
10.5
10.1
9.8
1000
500
300
300
MCM-49 100
MCM-49 102
MCM-49 103
9.8
ZSM-35
—
a
All crystallization temperatures, 145 ꢁC. b SiO2/Al2O3 of products by XRF analysis. c Relative crystallinity (%): sample 1 synthesized by conventional
hydrothermal method dened as R.C. ¼ 100%.
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Ka radiation at a tube current of 40 mA and a voltage of 40 kV. (8 mL) was placed in the center of the reactor and it was
The scanning range of 2q was 5–35ꢁ. The relative crystallinity of nitrogen-purged. Benzene was pumped into the reactor to ll
the samples was calculated according to the sum of the peak the bed fully under reaction pressure, and then the temperature
intensities at 2q of 14.3ꢁ, 22.7ꢁ, 23.7ꢁ and 26.0ꢁ. 29Si MAS NMR was raised to 200 ꢁC before ethylene was introduced. The
experiments were performed on a Bruker AVANCE III 500WB alkylation conditions were as follows: pressure of 3.5 MPa, a
spectrometer at a resonance frequency of 99.3 MHz using a 7 weight hourly space velocity (WHSV) of benzene of 3 hꢃ1, a
mm double-resonance magic-angle spinning (MAS) probe. The benzene/ethylene molar ratio of 12/1, and temperatures of 200
ꢁ
MAS speed was 5 kHz in all experiments, and a typical p/6 pulse to 260 C. The reaction products were not collected until every
length of 1.8 ms was used for 29Si resonance. The chemical shi temperature point lasted at least for 15 h, three times a day,
of 29Si was referenced to tetramethylsilane (TMS). 27Al MAS which ensured that the reaction was completely steady. The
NMR experiments were performed on a Bruker AVANCE III reaction products were analyzed by an Agilent 6890 gas chro-
600WB spectrometer at a resonance frequency of 156.4 MHz matograph (GC) equipped with a ame ionization detector and
using a 4 mm double-resonance MAS probe at a sample- a capillary column. The alkylation of benzene with ethylene is a
spinning rate of 13 kHz. The chemical shi of 27Al was refer- consecutive reaction, and the reaction products consisted of
enced to 1 M aqueous Al(NO3)3. 27Al MAS NMR spectra were ethylbenzene (EB), para-diethylbenzene (p-DEB), ortho-diethyl-
recorded by the small-ip angle technique using a pulse length benzene (o-DEB), meta-diethylbenzene (m-DEB), trie-
of 0.4 ms (<p/15) and a recycle delay of 1 s.
thylbenzenes (TEB), and other by-products (diphenylethane).
Characterization of H-MCM-49 zeolites. The crystal The ethylene conversion, EB selectivity, DEB selectivity and DEB
morphology was measured on a Quanta (FEI) scanning electron distribution of the selectivity were calculated based on the
microscope (SEM). The elemental analyses of the solids were following formulas.
performed on a MagiX (Philips) X-ray uorescence (XRF) spec-
trometer. Nitrogen adsorption–desorption isotherms were
recorded on a Micromeritics ASAP 2010 instrument. The
samples were out gassed under vacuum at 90 ꢁC for 1 h and at
H ¼ xEB/MEB + 2xDEB/MDEB + 3xTEB/MTEB
+ xdiphenylethane/Mdiphenylethane
xDEB ¼ xp-DEB + xo-DEB + xm-DEB
350 ꢁC for 15 h. The total surface area was obtained by using the
BET equation using the relative pressure range of 0.05–0.16 in
the nitrogen adsorption isotherm as the range of linearity
(molecular cross-sectional area for N2 of 0.162 nm2). The
micropore volume was calculated by the t-plot method.
Acidity characterization. The acidity of H-type zeolites was
measured by temperature programmed desorption (TPD) and
Fourier transform infrared (FTIR) spectrometry, using NH3 and
pyridine as probe molecules, respectively. TPD of ammonia (NH3-
TPD) was carried out in an Autochem II 2920 unit equipped with a
thermal conductivity detector. The samples (ꢂ200 mg, 20–40
mesh) were pretreated at 600 ꢁC in a ow of He (25 cm3 minꢃ1) for
60 min. Aerwards, pure NH3 (25 cm3 minꢃ1) was adsorbed at
room temperature for 30 min followed by He purging at 100 ꢁC for
90 min. Desorption of ammonia was monitored in the range of
where M is the molar mass (g molꢃ1), x is the mass percentage
concentration (wt%) from GC analysis, and H is the molar
number percentage concentration (mol%) of ethyl.
(1) Ethylene conversion (%): Cethylene
¼
H/(H + xethylene/Methylene) ꢀ 100
(2) EB selectivity (%): SEB
¼
xEB/(xEB + xDEB + xTEB + xdiphenylethane) ꢀ 100
(3) DEB selectivity (%): SDEB
¼
xDEB/(xEB + xDEB + xTEB + xdiphenylethane) ꢀ 100
(4) DEB distribution of selectivity:
m(p-DEB)/m(DEB) ¼ xp-DEB/(xp-DEB + xm-DEB + xo-DEB) ꢀ 100
m(m-DEB)/m(DEB) ¼ xm-DEB/(xp-DEB + xm-DEB + xo-DEB) ꢀ 100
m(o-DEB)/m(DEB) ¼ xo-DEB/(xp-DEB + xm-DEB + xo-DEB) ꢀ 100
100–600 C using a heating rate of 10 Cminꢃ1. Py-FTIR spectra
were obtained on a FTS3O00 FT-IR spectrometer with 64 scans and
a resolution of 4 cmꢃ1. Self-supporting thin wafers were pressed
and placed in a quartz IR cell with CaF2 windows, which was used
in pyridine adsorption studies. Prior to the measurements, each
ꢁ
ꢁ
3. Results and discussions
3.1 Synthesis of MCM-49 zeolites via topology
reconstruction
sample was dehydrated under vacuum (10ꢃ3 Pa) at 350 ꢁC for 1 h, NaY parent zeolites (SiO2/Al2O3 ¼ 5.0) with different sizes
and then cooled to 50 ꢁC for pyridine adsorption. The IR spectra of (average sizes: 1000, 500, and 300 nm) were used to provide all
the samples before pyridine adsorption were recorded at different Al and some Si in the following experiments. Supplementary Si
temperatures (200 and 350 ꢁC), and aer adsorbing pyridine for 10 was provided by solid silica gel to meet the mass balance for the
s, the samples were purged under vacuum (10ꢃ3 Pa) and then synthesis of high-silica MCM-49 zeolites. The chemical
heated at a rate of 10 ꢁC minꢃ1. Next, the IR spectra of pyridine on compositions of the parent and target samples are listed in
samples were recorded at different temperatures (200 and 350 ꢁC). Table 1. First, the SiO2/Al2O3 molar ratios of MCM-49 zeolites,
All the spectra presented in this work are difference spectra.
which were obtained by the topology reconstruction of NaY
zeolites with different sizes, were lower than those obtained by
initial feeding of 25. All MCM-49 zeolites had high relative
crystallinity compared with MCM-49 synthesized via the
2.3 Liquid-phase alkylation of benzene with ethylene
The liquid-phase alkylation of benzene with ethylene was conventional hydrothermal method (relative crystallinity:
carried out in a stainless-steel xed bed reactor. The catalyst 100%). Second, it is worth noting that it took longer for NaY
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(1000 nm) to complete the whole reconstruction process XRD during the topology reconstruction. The amorphous phase
(Sample 2, 88 h in Table 1 and Fig. 1a) than NaY (500 nm, was associated with the solid silica gel used to provide extra
Sample 3, 72 h in Table 1 and Fig. 1b) under identical condi- SiO2. The amount of solid silica gel accounted for about 75 wt%
tions. Additionally, despite the crystallization time being the of the total feeding, explaining the large amount of amorphous
same, less HMI (HMI/SiO2 ¼ 0.2) was needed for 300 nm NaY to phase. For example, from 48 to 72 h in Fig. 1a, the amorphous
accomplish topology reconstruction than that needed for 500 phase decreased and nally disappeared at 88 h; therefore, the
nm NaY (HMI/SiO2 ¼ 0.3), which also showed that the pure FER amorphous SiO2 was gradually consumed during the topology
zeolite was formed thermodynamically with HMI/SiO2 of 0.3 reconstruction and completely consumed at the end of the
(Table 1, sample 5).
topology reconstruction. Second, all MCM-49 zeolites were
XRD patterns of the as-made samples during the topology formed via the topology reconstruction from NaY zeolites with
reconstruction are shown in Fig. 1. The patterns of topology different sizes, and XRD patterns showed that MCM-22P with
reconstruction from FAU (1000 and 500 nm) to MWW (MCM-49, 2D structure did not form. During the topology reconstruction,
SiO2/Al2O3 > 20) were similar; the diffraction peaks of NaY there were no typical diffraction peaks of MCM-22P, such as the
decreased at 48 h, and peaks corresponding to MCM-49 typical (002) diffraction peak located at 2q of 6.6ꢁ.33 MCM-49
emerged and increased with increasing crystallization time. with a 3D structure was reconstructed from NaY zeolites with
Clearly, the FAU-MWW zeolites were the intermediates during a 3D structure. In other words, the topology reconstruction
the topology reconstruction. Furthermore, at 48 h, there were proceeded via partial destruction without complete destruction
stronger diffraction peaks for MCM-49 in Fig. 1b for the 500 nm of the 3D connections. This is the only plausible way that the
parent NaY zeolite than those in Fig. 1a for the 1000 nm parent size-controlled synthesis of MCM-49 zeolites could be achieved
NaY zeolite. There are two points that should be emphasized. efficiently from parent zeolites with different sizes.
First, there was a large amount of amorphous phase observed by
The 29Si/27Al MAS NMR spectra of samples obtained via the
topology reconstruction from NaY (300 nm) at various crystal-
lization time have been explained in detail in our previous
paper.32 Similarly, the 29Si/27Al MAS NMR spectra were
employed to investigate the chemical environments of the
framework Si/Al in topology reconstruction processes from two
other parent NaY zeolites (1000 and 500 nm). It appears that
there were similarities in the topology reconstruction of the
three kinds of parent zeolites. Fig. 2a and b shows that for NaY
there were the typical resonances of framework Si that were
assigned as follows:36 ꢃ88 ppm Si(3Al), ꢃ95 ppm Si(2Al), ꢃ100
ppm Si(1Al), and ꢃ105 ppm Si(0Al). The resonances of the FAU
framework Si were decreased by the destruction of the FAU
structure at 48 h, and with further crystallization time, the
resonances of the MCM-49 zeolite at around ꢃ100 and ꢃ105 to
ꢃ119 ppm (ref. 13) became clear from 48 to 88 h. Eventually, the
MWW framework was fully formed with the complete disap-
pearance of the FAU framework Si. The results clearly revealed
the FAU destruction and MWW growth during the topology
reconstruction, in which the hydrothermal destruction of the
FAU structure should be the rate-determining step for the whole
topology reconstruction. Additionally, there are also some
differences. Fig. 2a shows the transformation of the framework
Si from NaY (1000 nm) to MCM-49 zeolite, and at crystallization
time from 48 to 72 h, there were no resonances between ꢃ113
and ꢃ119 ppm. Fig. 2b shows two strong resonances at ꢃ115
and ꢃ119 ppm at 48 h for the transformation from NaY (500
nm) to MCM-49 zeolite that were mainly ascribed to Si(0Al). This
indicates that NaY (500 nm) was easier to reconstruct into
MCM-49 than NaY (1000 nm), which was in accordance with the
XRD results.
Furthermore, the transformations of framework Al in the
topology reconstruction are shown in Fig. 2c and d. With the
increase of the crystallization time, the 27Al MAS NMR reso-
nances at 50, 56 and 60 ppm (MWW) gradually increased with
the decrease of the resonance at 62 ppm (FAU), which showed
clearly that the framework Al of the FAU structure was gradually
Fig. 1 XRD patterns of as-synthesized samples obtained from the
topology reconstruction of FAU zeolites of different sizes (NaY: a, 1000
nm; b, 500 nm) at various crystallization time.
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Fig. 2 29Si/27Al MAS NMR spectra of as-synthesized samples obtained from the topology reconstruction of FAU zeolites of different sizes (a: 29Si
NMR-NaY 1000 nm; b: 29Si NMR-NaY 500 nm; c: 27Al NMR-NaY 1000 nm; d: 27Al NMR-NaY 500 nm) at various crystallization time.
transformed to the framework Al of the MWW structure by Si Table 1, which indicated that the MWW structure was perfectly
insertion. Moreover, Al transformation for NaY (500 nm) was preserved during the ion-exchange process.
faster than that of NaY (1000 nm) from 48 to 72 h. During the
Our initial aim was to achieve size-controlled synthesis of
entire process, there was no extra-framework Al formation, MWW zeolites and to tailor their alkylation performance. Fig. 4
which demonstrated direct transformation from the FAU to the shows SEM images of parent zeolites with different average
MWW structure.
sizes (1000, 500 and 300 nm) and the increase in the volume of
The XRD and NMR results suggest that the smaller sizes of the corresponding products. The sizes of HYM were larger than
the starting NaY zeolites presented better diffusion, which 1000 nm. The sizes of HYM500 were close to 1000 nm. The sizes
helped the other reactants (HMI, NaOH and silica source) to of HYM300 were smaller than 1000 nm. Obviously, the sizes of
diffuse to achieve the topology reconstruction. This could mean
that the reconstruction of the smaller parent zeolites to the
target zeolites is easier. The direct transformation from NaY to
MCM-49 zeolites, dened as topology reconstruction, indicated
that size control of the target MWW zeolites was possible.
Through this method, size-controlled synthesis of MCM-49
zeolite can be easily achieved, and it was critical to tailor the
catalytic performance of MWW zeolites in the liquid-phase
alkylation of benzene with ethylene.
3.2 Physicochemical properties of H-type zeolites
XRD patterns of H-type zeolites are presented in Fig. 3. The
similarity in the XRD patterns between the as-synthesized
samples and H-type zeolites indicated that the reconstructed
MCM-49 zeolites were mostly retained aer NH4+ ion-exchange
and calcination. Furthermore, the relative crystallinity was
higher for all H-type products (HYM: 100%; HYM: 105%;
HYM300: 108%) than the corresponding Na-MCM-49 zeolites in Fig. 3 XRD patterns of H-type zeolites.
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Fig. 4 SEM images of NaY and corresponding H-type zeolites.
the products can be controlled by the size of the parent zeolite two desorption peaks at around 215 and 400 ꢁC, corresponding
via the transformation from FAU to MWW structure, even to the weak and strong acid sites, respectively. HYM300 con-
though the volume increases. Therefore, these results demon- tained the highest number of acid sites, as characterized by
strate a new route for achieving our initial objective, and for the NH3-TPD and Py-FTIR (Table 3 and Fig. S1 and S2 in ESI†). The
size control of the target product via topology reconstruction. order of the number of acid sites was HYM300 > HYM500 >
Generally speaking, smaller target zeolites may offer a larger HYM. For Brønsted acid sites the results were 620 mmol gꢃ1
number of active centers on the exterior surface and better (HYM300) > 501 mmol gꢃ1 (HYM500) > 365 mmol gꢃ1 (HYM) at
accessibility to the active centers because the diffusion is less 200 ꢁC; and 544 mmol gꢃ1 (HYM300) > 447 mmol gꢃ1 (HYM500) >
limited, which is benecial for liquid-phase alkylation of 324 mmol gꢃ1 (HYM) at 350 C. The total number of Brønsted
benzene with ethylene. and Lewis acid sites exhibited similar variations at 200 and 350
ꢁ
Table 2 shows no obvious changes for SiO2/Al2O3 ratios in 21 ꢁC, although there were some uctuations in the number of
of the H-type zeolites, as well as in Na-type zeolites. The texture Lewis acid sites in all products. These results also demonstrated
properties of H-type zeolites were characterized by the nitrogen that smaller zeolites possessed a higher number of Brønsted
adsorption–desorption method. The data showed both the acid sites, which may offer a higher number of active centers in
surface areas and total pore volumes increased slightly as the the liquid-phase alkylation of benzene with ethylene.
sizes of the parent zeolites decreased. The BET surface areas
Generally, the number of total acid sites was strongly
were 465 m2 gꢃ1 for HYM < 489 m2 gꢃ1 for HYM500 < 496 m2 gꢃ1 dependent on the sizes of the probe molecules. With pyridine as
for HYM300. The micro surface areas were 406 m2 gꢃ1 for HYM the probe molecule, HYM300 showed the highest number of
< 408 m2 gꢃ1 for HYM500 < 410 m2 gꢃ1 for HYM300. The acid sites of all the products, and HYM showed the lowest
external surface areas were 59 m2 gꢃ1 for HYM < 81 m2 gꢃ1 for number. However, the NH3-TPD results showed only slight
HYM500 < 86 m2 gꢃ1 for HYM300. The total volumes were 0.44 differences (Fig. 5). This may be because HYM300 possessed the
cm3 gꢃ1 for HYM < 0.51 cm3 gꢃ1 for HYM500 < 0.54 cm3 gꢃ1 for highest surface area and pore volume. Furthermore, the pyri-
HYM300. Generally, the micro surface areas and micro volumes dine molecules have a kinetic diameter of 0.58 nm, which
showed no substantial differences. The effect of the parent NaY results in a higher mass transfer restriction within the micro-
zeolite size was greater on the external surface areas and pore channels than NH3 molecules (kinetic diameter 0.26 nm).
external pore volumes, which contributed to the increase in the The XRF results conrmed that the reconstructed MCM-49
BET surface areas and the total pore volumes, although it did zeolites had almost the same SiO2/Al2O3 ratios of 21, which
not affect the micropore data. HYM300 possessed the largest indicated that they may have same number of acid sites;
surface area and pore volume, and HYM had the smallest however the number of acid sites determined by both NH3-TPD
surface area and pore volume in Table 2, demonstrating the and Py-FTIR increased as the H-type zeolites size decreased,
effect of the parent zeolite size on the texture properties of the which is usually observed in nanosized zeolite catalysts. We
nal H-type zeolites. This can be explained by the decreased propose that the increased number of acid sites increases the
crystal size of MWW zeolites, which contributes little to the number of active centers and improves the accessibility of active
increase in the micropore properties, although it does centers for alkylation. All results indicate that HYM300 may
contribute to the external pore properties.
possess superior accessibility of active centers, which is very
There was a similar trend in the acidity of H-type zeolites important for zeolites to act as catalysts in the liquid-phase
obtained by topology reconstruction. Fig. 5 presents the NH3- alkylation of benzene with ethylene.
TPD results. For all reconstructed H-type zeolites, there were
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Table 2 Texture properties of H-type zeolites
Samples
SiO2/Al2O3
SBET (m2 gꢃ1
)
Smicro (m2 gꢃ1
)
Sext (m2 gꢃ1
)
Vmicro (cm3 gꢃ1
)
Vtotal (cm3 gꢃ1
)
HYM
HYM500
HYM300
21
21
21
465
489
496
406
408
410
59
81
86
0.20
0.19
0.19
0.44
0.51
0.54
250 ꢁC, and 99.3% at the nal temperature of 260 ꢁC. It is
expected that the ethylene conversion would increase with the
temperature, and that the EB selectivity would decrease.
The HYM300 catalyst that originated from the 300 nm NaY
zeolite showed the best ethylene conversion and EB selectivity
of all the H-type catalysts, whereas the catalyst from the largest
zeolite (1000 nm) exhibited the poorest performance. Fig. 6a
and b shows that the order of ethylene conversion in catalysts
was HYM300 > HYM500 > HYM, which was the same as the
order of EB selectivity. These results strongly suggest that the
small HYM300 catalyst possessed the highest ethylene conver-
sion because of their higher BET surface areas, smaller sizes,
and larger number of active centers. Therefore, a larger number
of active centers were exposed to the reactants and there were
more opportunities for the reagents to access the active centers
(12 MR cups) on the outer surface. The 12 MR cups on the outer
surface did not restrict the diffusion of the reactants toward the
active centers to improve ethylene conversion and EB selectivity.
Moreover, the increase in acid sites, as determined by Py-FTIR,
improved the accessibility of the acid sites to pyridine. There-
fore, as the size of the H-type catalysts decreases, the accessi-
bility of the active centers to benzene molecules is improved
and this enhances the ethylene conversion and EB selectivity.
Furthermore, there is almost no trade-off between ethylene
conversion and EB selectivity in the consecutive reaction as the
size of the H-type catalysts decreases. This is mainly attributed
to the better diffusion of the reactant in smaller H-type zeolites.
In addition, because of the same ethylene feeding, the increase
in activity of smaller catalysts means that more EB is produced,
decreasing the amount of ethylene available to react with EB to
produce DEB, TEB and other by-products. Therefore, the results
highlight an important strategy for designing alkylation cata-
lysts, which demonstrates that an excellent catalyst, with
smaller particles, higher relative crystallinity, higher BET
surface areas and a larger number of acid sites offers more
opportunities for benzene to access active centers, and can help
Fig. 5 NH3-TPD curves of H-type zeolites.
3.3 Liquid-phase alkylation of benzene and ethylene over H-
type catalysts
Ethylene conversion and EB selectivity. The alkylation of
benzene with ethylene is a consecutive reaction with EB as the
target product. Therefore, the trade-off between ethylene
conversion and EB selectivity must be carefully considered;
usually, improved ethylene conversion is accompanied by
decreased EB selectivity. The alkylation performance of cata-
lysts synthesized through topology reconstruction from NaY
zeolites is shown in Fig. 6a and b. For the HYM catalyst,
ethylene conversion was improved from 89.5% to 98.8% by
increasing the temperature from 200 to 260 ꢁC, although the EB
selectivity decreased from 95.4% to 94.5%. Similarly, in the
HYM500 catalyst, increasing the temperature from 200 to
260 ꢁC increased ethylene conversion from 95.4% to 99.4% and
decreased EB selectivity from 95.7% to 94.9%. Although EB
selectivity slightly decreased from 95.8% to 95.4%, HYM300
showed a steady ethylene conversion of 98.4% from 200 to
Table 3 Acid properties of H-type zeolites
200 ꢁ
C
350 ꢁ
C
Lewis acid
Brønsted acid
Lewis acid
Brønsted acid
Samples
(mmol gꢃ1
)
(mmol gꢃ1
)
(mmol gꢃ1
)
(mmol gꢃ1
)
HYM
HYM500
HYM300
342
271
343
365
501
620
263
212
276
324
447
544
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Fig. 6 Ethylene conversion (%, a), ethylbenzene selectivity (%, b), DEB selectivity (%, c), and ꢃD1EB distribution (%, d) of H-type catalysts. Liquid-
phase alkylation conditions: catalyst, 8 mL; T ¼ 200 to 260 ꢁC; p ¼ 3.5 MPa; benzene WHSV ¼ 3.0 hꢃ1; benzene/ethylene molar ratio ¼ 12.0.
decrease the trade-off between high ethylene conversion and
high EB selectivity.
Liquid-phase alkylation of benzene with ethylene over the
conventional H-MCM-49 and HYM300 catalysts. First, we
measured the properties of the conventional H-MCM-49 cata-
lyst, the relative crystallinity of which was set as 100%. By using
the conventional hydrothermal method, the SiO2/Al2O3 of H-
MCM-49 was determined to be 23 by XRF analysis. The BET,
micro and external surface areas were 48, 410, and 75 m2 gꢃ1
respectively. The micro and total pore volumes were 0.19 and
0.56 cm3 gꢃ1, respectively. The micro surface area and pore
volume of conventional H-MCM-49 were similar to those of the
zeolites reconstructed from NaY. Additionally, the number of
acid sites in conventional H-MCM-49 zeolite was as follows: 402
mmol gꢃ1 for Brønsted acid sites and 189 mmol gꢃ1 for Lewis
acid sites at 200 ꢁC; 375 mmol gꢃ1 for Brønsted acid sites and 168
DEB selectivity and DEB distributions. Fig. 6c and d shows
DEB selectivity and DEB distributions over H-type catalysts in
the liquid-phase alkylation of benzene with ethylene. The DEB
selectivity for all H-type catalysts increased with the tempera-
ture and the size of the zeolite (Fig. 6c). The smaller the MCM-49
zeolite, the smaller amount of DEB was produced and the better
the EB selectivity. At low temperatures, all H-type catalysts
showed a high degree of selectivity for o-DEB (Fig. 6d), and the
amount of o-DEB decreased when the temperature increased
from 200 to 260 ꢁC. In contrast, a low degree of selectivity for m-
DEB was achieved. p-DEB has the smallest molecular diameter
of the three DEB isomers, whereas space-lling models show
that o-DEB has the most favorable conguration for the active
centers of H-type catalysts. As the temperature increased, the
amount of m-DEB produced by the isomerization of o-DEB
increased, whereas the selectivity of p-DEB was almost
unchanged. Additionally, the proportion DEB isomers moved
toward the thermodynamic equilibrium values and the quality
ratios of DEB isomers (m-DEB : p-DEB : o-DEB) were main-
tained as 64 : 30 : 6.10 Generally, generating less DEBs allows
better EB selectivity to be achieved. Therefore, the HYM300
catalyst reconstructed from NaY (300 nm) showed the lowest
DEB selectivity and the highest EB selectivity accompanying the
highest ethylene conversion.
mmol gꢃ1 for Lewis acid sites at 350 C. The texture and acidic
ꢁ
properties of conventional H-MCM-49 were close to those of
HYM500, and lower than those of HYM300. Table 4 gives the
detailed alkylation performance of conventional H-MCM-49
from 200 to 260 ꢁC. The HYM300 catalyst showed higher
ethylene conversion and EB selectivity than the conventional H-
MCM-49 catalyst at low reaction temperatures. The increase in
ethylene conversion for HYM300 was different from that for the
conventional H-MCM-49 catalyst, and unlike the conventional
H-MCM-49 catalyst, HYM300 exhibited small changes in
ethylene conversion as the temperature changed, which indi-
cates that the active centers in HYM300 were very accessible.
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Table 4 Ethylene conversion and EB selectivity over conventional H-MCM-49 catalysta
Temperature (ꢁC)
200
210
220
230
240
250
260
Ethylene conversion (%)
EB selectivity (%)
96.58
95.42
97.21
95.26
97.94
95.10
98.25
95.14
98.50
95.10
98.61
94.80
99.23
94.95
a
Liquid-phase alkylation conditions: catalyst, 8 mL, T ¼ 200 to 260 ꢁC, p ¼ 3.5 MPa, benzene WHSVꢃ1 ¼ 3.0 hꢃ1, benzene/ethylene molar ratio ¼
12.0.
The results also conrm that an excellent catalyst that is smaller of size on the topology reconstruction, texture properties and
with high relative crystallinity, high BET surface area and a acidity of H-type zeolites were observed. Smaller parent NaY
larger number of active centers, can produce a signicant zeolites improved the diffusion of reactants on the parent
breakthrough in activity without loss of selectivity. Thus, the zeolites during the topology reconstruction, and also the rela-
topology reconstruction of zeolites can achieve the size- tive crystallinity, BET surface areas, external pore volumes and
controlled synthesis of target zeolites, and also prevent the number of acid sites determined by Py-FTIR. This indicates that
trade-off between ethylene conversion and EB selectivity the smaller MCM-49 possessed a larger number of active centers
because of the better accessibility of active centers in the (12 MR cups) on the outer surface and allowed better accessi-
HYM300 catalyst.
bility to active centers for benzene. The most promising result
According to our results, we can summarize the effects of the was the simultaneous improvement of both the ethylene
NaY zeolite size on the topology reconstruction, chemical and conversion and EB selectivity as the size of the zeolites
physical properties, size, and acidity of H-type zeolites. Smaller decreased, because the diffusion of the reactant over smaller H-
parent NaY zeolites improved the diffusion of the reactants on type catalysts improved. In particular, HYM300 showed the
the NaY zeolites during the topology reconstruction, and also highest ethylene conversion and EB selectivity. This demon-
the relative crystallinity, the BET surface areas owing to the strates that smaller MWW zeolites and higher geometrical
increase in external surface areas, and the number of acid sites. regularities can ensure excellent catalytic performance in the
Most importantly, ethylene conversion and EB selectivity were liquid-phase alkylation of benzene with ethylene. This alkyl-
improved by decreasing the size of parent NaY zeolites because ation performance may indicate a new strategy for enhancing
of the better accessibility of the active centers on small H-type both catalytic activity and selectivity, and our method should be
catalysts. In particular, HYM300 showed the highest value for applicable to many diffusion-limited or surface reactions.
both the ethylene conversion and EB selectivity. We successfully
controlled the sizes of the target zeolites to improve the acces-
sibility of active centers through topology reconstruction from
Acknowledgements
smaller parent zeolites.
Thus, we show that size-controlled synthesis of MWW
zeolites via topology reconstruction of parent NaY zeolites with
different sizes is a potential method to improve both ethylene
conversion and EB selectivity. In the topology reconstruction,
we emphasize that size-controlled synthesis of MWW zeolites
This work was supported by the National Basic Research
Program of China (973 Program, no. 2012CB224805). Special
thanks to the Department of Analysis in Research Institute of
Petroleum Processing Sinopec.
can be achieved for smaller zeolite catalysts, which possess a
larger number of active centers and allow better access for
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