Shape selectivity of beta and MCM-49 zeolites in liquid-phase alkylation of benzene with ethylene

Article abstract of DOI:10.1016/j.molcata.2016.03.039

NH₃, pyridine and 2,4,6-trimethyl-pyridine were selected as the base probes to characterize the different pore acidity of H-beta and H-MCM-49 zeolites, revealing the essence in their catalytic performance from the view of shape selectivity in d

Full text of DOI:10.1016/j.molcata.2016.03.039

Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Shape selectivity of beta and MCM-49 zeolites in liquid-phase
alkylation of benzene with ethylene
∗
Yanchun Shi, Enhui Xing, Wenhua Xie, Fengmei Zhang , Xuhong Mu, Xingtian Shu
State Key Laboratory of Catalytic Materials and Reaction Engineering, Research Institute of Petroleum Processing, Sinopec, Beijing 100083, China
a r t i c l e i n f o
a b s t r a c t
Article history:
NH , pyridine and 2,4,6-trimethyl-pyridine were selected as the base probes to characterize the differ-
ent pore acidity of H-beta and H-MCM-49 zeolites, revealing the essence in their catalytic performance
3
Received 3 December 2015
Received in revised form 24 March 2016
Accepted 25 March 2016
from the view of shape selectivity in different pore systems. Based on the size of base probes, reactants,
product, transition states and pore channels, the difference in catalytic performance over H-beta and
H-MCM-49 was investigated. Easy diffusion of reactants into and long diffusion distance for ethylben-
zene escaping out of confined space within beta are the main reason for superior activity and poorer
ethylbenzene (EB) selectivity. As for H-MCM-49, acid sites located in 10 MR channels contributed little
to the formation of EB due to transition state selectivity; Acid sites located in 12 MR cups on the outer
surface with little restriction on the diffusion of reactant/product toward/from the active centers were
the major active centers for EB formation, which could be detected by 2,4,6-trimethyl-pyridine; There
was diffusion limitation on reactant/product in 12 MR supercages through 10 MR windows to influence
activity/selectivity. The enhancement in activity and selectivity in liquid-phase alkylation of benzene
with ethylene over MWW zeolites should be mainly focused on transforming confined 12 MR supercages
into 12 MR cups with improved openness.
Available online 26 March 2016
Keywords:
Shape selectivity
Beta
MCM-49
Liquid-phase alkylation
Ethylbenzene
©
2016 Published by Elsevier B.V.
1
. Introduction
smart fitting between dimensions of transition states and confined
spaces within zeolites. Shape-selective catalysis of zeolite has been
extensively proven and industrialized in petro-refining and petro-
chemical processes [1–8].
Zeolites have been widely utilized in the refining and petro-
chemical processes with superior selectivity, environmental
benign and easy reproducibility in catalytic reactions, based upon
their framework structure with ordered micropores and adjustable
acidic property dispersed within regular pores [1,2]. Because of
their confined space around active centers and restricted access to
and escape from the internal surface, zeolites facilitate only certain
reactions between molecules having specific dimensions, so-called
the concept of “shape-selective catalysis” by Weisz, and there
are well known as reactant selectivity, transition state selectiv-
ity and product selectivity according to sizes of reactant/transition
state/product, respectively [3–5]. Based upon mass transport lim-
itation (reactant/product), molecules that are suitable to zeolitic
pores could diffuse freely within the inner pores and access/escape
to/from the active centers, which contributes to improve activ-
ity and selectivity over zeolite catalysts. Additionally transition
state control of reactions could be significantly enhanced due to
Catalytic performances of zeolite-based catalysts are influ-
enced but not limited by their morphology, textual properties
and chemical compositions, especially determined by the match
between zeolite pores/spaces and the sizes of reactant/transition-
state/product [9–12]. As well known, the liquid-phase alkylation of
benzene with ethylene is a well-established process in both aca-
demic research and the petrochemical industry, in which *BEA
and MWW zeolites have been proven to be superior catalysts
because of easy separation, low corrosion, valuable activity, high
selectivity and long catalytic life [13–21]. Generally speaking, *BEA
zeolites show better activity of ethylene conversion; on the con-
trary, MWW zeolites exhibit higher selectivity of ethylbenzene (EB)
especially at lower molar ratio of benzene/ethylene. *BEA zeolite
has a three-dimensional pore system consisting of 12-membered
ring (MR) channels (0.66 nm × 0.67 nm
&
0.56 nm × 0.56 nm),
while MWW zeolites have three complex pore systems: 12
MR cups (half supercages 0.71 nm × 0.71 nm × 0.91 nm) on the
external surface, the interlayer12 MR supercages (0.71 nm out-
side diameter × 1.82 nm height) through10 MR opening windows
(0.41 nm × 0.54 nm & 0.41 nm × 0.59 nm), and the intralayer two-
∗ Corresponding author at: Research Institute of Petroleum Processing Sinopec,
Xueyuan Road 18, Beijing, China.
E-mail address: zhangfm.ripp@sinopec.com (F. Zhang).
http://dx.doi.org/10.1016/j.molcata.2016.03.039
1
381-1169/© 2016 Published by Elsevier B.V.

Y. Shi et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
87
dimensional sinusoidal 10 MR pores (0.41 nm × 0.54 nm) [22–25].
As solid acidic catalysts, acid sites dispersed in different pore
system of *BEA and MWW zeolites would derive the issue on acces-
sibility of active centers, diffusion lengths for reactant/product and
reaction spaces for transition state, determining the apparent cat-
alytic performances in liquid-phase alkylation of benzene with
ethylene.
Based upon reactant selectivity, the acidity of H-type zeolites are
usually determined by base probes with different molecule sizes,
such as NH3 (kinetic diameter 0.26 nm) in TPD characterization
and pyridine (kinetic diameter 0.58 nm) in FTIR experiments. Both
NH3 and pyridine are effective to access acid sites located in micro
pores, however they can offer little information about acid sites in
different pore systems and on the internal/external surface, espe-
cially for MWW zeolites with three different pore systems. It has
been well reported that 2,4,6-trimethyl-pyridine (kinetic diameter
filtrated, washed with deionized water until pH = 7, and dried at
◦
◦
100 C overnight. Samples were calcined at 550 C in ambient air
for 6 h in a muffle furnace to remove organics and decompose NH4+
to form H-type zeolites.
2.1.3. H-type catalysts
The as-synthesized beta and NH4-MCM-49 zeolites were mixed
with Al2O3 (30 wt.%) and extruded. Al2O3 showed almost no activ-
ity for the liquid-phase alkylation of benzene with ethylene and was
used as binder to increase the mechanical strength of catalysts. The
extruded catalysts were then crushed, and the −16/+20 mesh frac-
◦
tion was collected and subjected to calcination at 550 C for 6 h to
obtain corresponding H-type catalysts.
0.74 nm) was used to detect the acid sites in 12 MR cups on the
outer surface of MWW zeolites, because 2,4,6-trimethyl-pyridine
fit snugly into the 12 MR cups but not penetrate 10 MR pores and
windows [20,26–28]. Furthermore, there is little reaction in the
2
.2. Characterization of beta and MCM-49 zeolites
X-ray diffraction (XRD) patterns of samples were collected on
2
,4,6-trimethyl-pyridine-doped runs confirming that the ethyla-
a D/MAX-III X-ray diffractometer (Rigaku Corporation, Japan) with
filtered Cu K ␣ radiation at a tube current of 35 mA and a voltage
of 35 kV. The scanning range of 2ꢀ was 5–35 . The crystal morphol-
ogy was measured on a FEI Quanta scanning electron microscope
tion occurred almost exclusively in the 12 MR cups on the outer
surface of MWW zeolites.
◦
As common recognized, H-type zeolites show different acidity
with different base probes due to reactant selectivity via gas-
solid heterogonous reaction, which may have a similar tendency in
liquid-solid heterogonous reaction. The relationship between guest
molecules (base probes, reactants, transitional states and products)
and the sizes of zeolite pore systems has always been interesting
and challenging topic for zeolite-catalyzed reaction, which needs
comprehensive consideration on distribution and accessibility of
acid sites in different pore systems to tailor their catalytic per-
(SEM). The elemental analyses of the solids were performed on an
X-ray fluorescence (XRF) spectrometer MagiX (Philips). Nitrogen
adsorption-desorption isotherms were recorded on a Micromerit-
ics ASAP 2010 instrument. The samples were first out-gassed under
vacuum at 363 K for 1 h and at 623 K for 15 h. The total surface area
was obtained by application of the BET equation using the relative
pressure range of 0.05–0.16 in the nitrogen adsorption isotherm
as range of linearity (using a molecular cross-sectional area for N2
of 0.162 nm ). The micropore volume was calculated by the t-plot
method.
formances. Therefore NH , pyridine and 2,4,6-trimethyl-pyridine
3
2
were selected as the base probes to characterize accessibility of
the different pore acid sites over H-beta and H-MCM-49 zeolites in
this paper. Focusing on the perspective of shape-selective catalysis,
we anticipate to reveal potential essence of different liquid-phase
alkylation performances over *BEA and MWW zeolites, and propose
corresponding strategies to enhance ethylene conversion without
loss of EB selectivity over *BEA and MWW zeolites respectively.
The acidity of H-type zeolites was measured by temperature
programmed desorption (TPD) and Fourier transform infrared
spectrometer (FTIR),using ammonia(NH3) and pyridine/2,4,6-
trimethyl-pyridine as probe molecules, respectively. NH -TPDwas
3
carried out on an Autochem II 2920 unit equipped with a ther-
mal conductivity detector. For FTIR spectroscopy of adsorbed
probe molecules (pyridine and 2,4,6-trimethyl-pyridine), all sam-
ples were pressed into self-supporting wafers and measured in
transmission mode in an FTS3O00 FTIR spectrometer by 64 scans
with a resolution of 4 cm . Prior to the measurements, each sam-
ple was dehydrated under vacuum 10 Pa at 350 C for1 h, and
then cooled to 50 C for pyridine adsorption. The IR spectra of the
samples before pyridine adsorption were recorded at different tem-
peratures (200 and 350 C), and after adsorbing pyridine for 10 s,
the samples were purged under vacuum 10 Pa to higher tem-
perature at a heating rate of 10 C min . Then the IR spectra of
pyridine on samples were recorded at different temperatures (200
and 350 C), while IR spectra of pyridine was collected at 200 C.
All the spectra given in this work were differential spectra. The
number of Brønsted and Lewis acid sites was calculated according
to adsorbed pyridine IR peak area, and the number of acid sites
by 2,4,6-trimethyl-pyridine was calculated based upon adsorbed
2
2
2
. Experimental
.1. Preparation of beta, MCM-49 zeolites and H-type catalysts
−1
−
3
◦
.1.1. Synthesis of MCM-49 zeolites
◦
Beta zeolite was provided by Research Institute of Petroleum
Processing, Sinopec. All materials such as solid silica gel, NaAlO2,
NaOH, hexamethyleneimine (HMI), aniline (AN) and deionized
water were used as purchased. Detailed conditions of synthesis are
provided in Table 1. The hydrothermal synthesis of MCM-49 was
◦
−3
◦
−1
◦
carried out for 72 h at 145 C in a stirred vessel (15 Hz) [29,30]. The
◦
◦
products were filtrated, washed with deionized water until pH = 7,
◦
◦
and dried at 100 C overnight. Samples were calcined at 550 C in
ambient air for 6 h in a muffle furnace to remove organics.
2
,4,6-trimethyl-pyridine IR peak height.
2
.1.2. H-type zeolites
29
Si/27Al MAS NMR experiments were performed on a Bruker
Usually Na O content of H-type catalysts for liquid-phase alky-
2
AVANCE III 500WB/600WB spectrometer at a resonance frequency
of 99.3/156.4 MHz using a 7/4 mm double-resonance MAS probe
with a recycle delay of 4/1 s respectively. The magic-angle spin-
ning rate was 5/13 kHz in all experiments, and a typical length of
lation of benzene with ethylene are required below 0.05 wt.%.
H-beta (Na O <0.01 wt.%) was obtained via direct calcination of
2
as-synthesized beta without further ion-exchange. NH -MCM-49
4
was first prepared by twice liquid-phase ion-exchange of Na-MCM-
29
27
◦
1.8/0.4 ␮s was adopted for Si/ Al resonance respectively. The
4
9 with NH NO solution at 90 C for 2 h to reduce Na O content
4
3
2
chemical shift of 29Si/ Al was referenced to tetramethylsilane
27
below 0.05 wt.%. The mass ratio composition of MWW zeolites,
NH NO , and deionized water was: 1:1:20. The products were
(
TMS)/1 M aqueous Al (NO3) respectively.
3
4
3

8
8
Y. Shi et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
Table 1
Synthetic condition of beta and MCM-49.
◦
Samples
Si source
Feeding composition (mol/mol)
Temperature ( C)
Time (h)
SiO2/Al2O3
NaOH/SiO2
H2O/SiO2
TEAOH/SiO2
HMI/SiO2
AN/SiO2
Beta
MCM-49
Si-Al gel
Solid silica gel
25
25
–
0.18
7
15
0.1
–
–
0.1
–
0.2
120/145
145
12/36
72
2.3. Liquid-phase alkylation of benzene with ethylene
The liquid-phase alkylation of benzene with ethylene was car-
3
. Results and discussion
ried out in a stainless-steel fixed bed reactor. The catalyst (8 mL)
was placed in the center of the reactor and purged with nitro-
gen. Benzene was pumped into the reactor to fill the bed fully
3.1. Characterization of beta and MCM-49 zeolites
under reaction pressure, and then the temperature was raised to
Both beta and MCM-49 zeolites were intentionally synthesized
with the feeding SiO /Al O molar ratio at 25 to diminish the
◦
2
00 C before ethylene was introduced. The alkylation conditions
2
2
3
were as follows: pressure of 3.5 MPa, weight hourly space velocity
effects of framework compositions. Tetraethylammonium hydrox-
ide (TEAOH) was used as the structure-directing agent (SDA) to
synthesize beta zeolite. Temperature-controlled phase transfer
hydrothermal synthesis of MCM-49 was achieved with HMI as a
SDA coupled with aniline as structure-promoting agent [29,30].
XRD patterns of as-synthesized/calcined beta and MCM-49 zeolites
are shown in Fig. 1, which agreed well with the published data with-
out any diffraction peaks of other competing phases [23,36]. As can
be seen from the SEM images shown in Fig. 2, beta showed much
smaller particles with diameters at about 50 nm, while MCM-49
was of rose-like shape at about 2–5 ␮m and 20–40 nm in thickness.
−
1
(
1
WHSV) of benzene of 3 h , the molar ratio of benzene/ethylene
◦
2/1, and 200–260 C of temperature. At each temperature the
reaction was given at least 15 h to reach a steady state condition
before samples were collected for analysis. The reaction prod-
ucts were analyzed by an Agilent 6890 gas chromatograph (GC)
equipped with a flame ionization detector and a capillary column.
It is well known that the alkylation of benzene with ethylene is
a consecutive reaction, and the reaction products were made up
of EB, para-diethylbenzene (p-DEB), ortho-diethylbenzene (o-DEB),
meta-diethylbenzene (m-DEB), triethylbenzenes (TEB), and other
by-products (diphenylethane) in our work. The ethylene conver-
sion, EB selectivity, DEB selectivity, DEB selectivity distribution and
ethylation selectivity were calculated based upon formulas as fol-
lows:
29
27
Fig.
3
presents the
Si/ Al MAS NMR spectra of as-
synthesized/calcined beta and MCM-49 zeolites. The resonances
a
as-synthesized
calcined
H = xEB/MEB + 2xDEB/MDEB + 3xTEB /MTEB + xdiphenylethane
/Mdiphenylethane;
xDEB = xp-DEB + xo-DEB + xm-DEB;
M: molar mass (g/mol);
x: mass percentage concentration (wt.%) from GC analysis;
H: molar number percentage concentration (mol%) of ethyl;
Ethylene conversion (%) : Cethylene = H/(H + xethylene/Methylene
)
×
100%
(1)
5
10
15
20
25
30
35
o
2
Theta ( )
EB selectivity (%) : SEB = xEB/(xEB + xDEB + xTEB + xdiphenylethane
100%
)
×
(2)
b
as-synthesized
calcined
DEB selectivity (%) : SDEB =xDEB/(xEB+xDEB+xTEB + xdiphenylethane
)
×
100% (3)
TEB selectivity(%) : STEB = xTEB/(xEB + xDEB + xTEB + xdiphenylethane
)
×
100%
(4)
5
10
15
20
25
30
35
o
Ethylation selectivity (%) : Sethyl = (xEB/MEB + 2xDEB/MDEB
3xTEB/MTEB)/H × 100%
2 Theta ( )
+
(5)
Fig. 1. XRD patterns of as-synthesized/calcined beta (a) and MCM-49 (b).

Y. Shi et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
89
Table 2
Textural properties of H-type zeolites.
2
2
2
3
3
Samples
Na2O wt%
SiO2/Al2O3
SBET (m /g)
Smicro (m /g)
Sext (m /g)
Vmicro (cm /g)
Vtotal (cm /g)
H-beta
H-MCM-49
<0.01
0.02
25
23
571
485
471
410
100
75
0.22
0.19
0.62
0.56
SBET were calculated by BET method. Micropore volume and surface area were calculated by t-plot method. Sexternal = SBET − Smicro.
of MCM-49 were similar to those presented in previous publi-
cations with six resonances at about −100, −105, −110, −110,
of MCM-49 windows on its assessment of some acid sites may
show less acid sites detected by pyridine with larger molecule size.
Because of 12 MR pore system, H-beta has larger space for pyri-
dine to access acid sites than H-MCM-49 with 12 MR and 10 MR
pore systems. An excellent summary of Brønsted and Lewis acid
sites concentration of MWW zeolites was reported by Gil et al. [37].
There are some differences in the concentration of acid sites. Main
reason lied in the absorption coefficients in different group.
−
115 and −119 ppm, which were mainly associated with Si sites
of MWW zeolites as follows: −100 ppm ∼ Si (1Al) and −105 ppm
to −119 ppm ∼ Si (0Al) [31–35]. Usually the resonances at about
−
110, −110 and −115 overlapped. While for beta, the resonance at
about −103 ppm was ascribed to Si (1Al), and −105 to −119 ppm
2
7
to Si (0Al) [36]. As for Al MAS NMR spectra of MCM-49 were both
comprised of at least two tetrahedral Al resonances centered at
According to published results, 2,4,6-trimethyl-pyridineis a
bulky base to distinguish acid sites on the inner pores and
outer surface areas, however it failed in characterizing inter-
nal/external acidity of H-beta with 12 MR pore system [20,26–28].
Fig. 5 shows spectra of H-beta and H-MCM-49 before/after 2,4,6-
trimethyl-pyridine adsorption. 2,4,6-Trimethyl-pyridine adsorbed
on Brønsted and Lewis acid sites gave the band at about ∼1650
∼50 and ∼56 ppm. Upon calcination, the shoulder at ∼49 ppm was
further reduced while the resonance at ∼60 ppm also appeared as
a shoulder and was more visible. A sharp peak at ∼0 ppm due to
octahedral aluminum can be clearly observed, indicating that cal-
cination caused dealumination to some extent [23]. The framework
Al resonance at ∼55 ppm of as-synthesized beta was divided into
two peaks located at ∼56 and ∼54 ppm after calcination, causing
the formation of extra-framework Al with a sharp resonance at
−
1
and ∼1633 cm
[38]. What’s more, there were obvious physi-
∼
0 ppm and a broad resonance at ∼5 ppm.
The compositions, BET surface areas and pore volumes of H-
type zeolites are shown in Table 2. The SiO /Al O ratio of H-beta
2
2
3
was identical to that of feeding composition at 25, however the H-
MCM-49 showed a slightly lower SiO /Al O ratio at 23 than the
2
2
3
feeding composition at 25. H-beta with 12 MR channels showed
larger surface areas (both micro and external) and pore volumes
than H-MCM-49 with 12 MR and 10 MR pore systems. It is reason-
able to conclude that larger pore systems surely lead to the increase
in adsorption capability of N2 (kinetic diameter 0.36 nm), which
may be less restriction and larger space for reactant/transition
state/product than H-MCM-49.
3.2. Acidity of H-beta and H-MCM-49 zeolites
H-MCM-49 had lower SiO /Al O ratio at 23 than H-beta
2
2
3
(
SiO /Al O = 25), indicating that H-MCM-49 may exhibit a higher
2 2 3
Al content and acidic density. However, not all of these acid sites (or
active centers) could be accessed by base probes and more impor-
tantly cannot be accessed by reactants during catalytic reactions.
In this section, base probes with different molecule sizes were
selected to detect the accessibility of acid sites in different pore
system of these two zeolites. NH3 was selected based upon little
diffusion restriction into *BEA and MWW pore systems. Pyridine
with identical kinetic diameter as benzene (0.58 nm) was selected
to reveal reactant selectivity. 2,4,6-Trimethyl-pyridine with size
larger than benzene and smaller than the sum of benzene and ethy-
lene (0.39 nm), has similar size to the transition state of benzene
ethylation to indicate the transition state selectivity.
Fig. 4 shows NH -TPD curves of H-beta and H-MCM-49 zeolites.
3
Obviously, H-MCM-49 had more weak and strong acid sites due to
its slightly lower SiO /Al O ratio. While Py-FTIR analysis (Table 3)
2
2
3
−
1
shows reverse results of total acid sites: H-beta (423 ␮mol g of
Brønsted acid sites and 271 ␮mol g of Lewis acid sites at 200 C;
96 ␮mol g of Brønsted acid sites and 227 ␮mol g of Lewis acid
sites at 350 C) showed a little higher acidic concentration than H-
MCM-49 (402 ␮mol g of Brønsted acid sites and 189 ␮mol g of
Lewis acid sites at 200 C; 375 ␮mol g of Brønsted acid sites
and 168 ␮mol g of Lewis acid sites at 350 C). NH molecule has
−
1
◦
−
◦
1
−1
3
−
1
−1
◦
−1
−1
◦
3
smaller kinetic diameter to present little diffusion restriction into
inner channels of H-beta and H-MCM-49 zeolites. The restriction
Fig. 2. SEM images H-beta (a) and H-MCM-49 (b).

9
0
Y. Shi et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
a
as-synthesized beta
b
H-beta
H-MCM-49
as-synthesized MCM-49
-90
-100
-110
-120
-130
-140
-90
-100
-110
-120
-130
-140
Chemical shift
Chemical shift
56
H-beta
H-MCM-49
c
as-synthsized beta
as-synthesized MCM-49
d
61
49
54
120
100
80
60
40
20
0
-20
120 100
80
60
40
20
0
-20
-40
Chemical shift
Chemical shift
Fig. 3. 29Si/27Al MAS NMR spectra of as-synthesized and H-type zeolites.
Table 3
Acid properties of H-type zeolites.
◦
◦
200
C
350
C
Lewis acid (␮mol g 1
−
)
BrÖnsted acid (␮mol g
−1
)
B/L
Lewis acid (␮mol g
−1
)
BrÖnsted acid (␮mol g
−1
)
B/L
H-beta
H-MCM-49
271
189
423
402
1.50
2.13
227
168
396
375
1.74
2.23
Table 4
the sizes of reactants, product, base probes and zeolitic channels.
Samples
Kinetic diameter (nm)
Ethylene Benzene
EB
Transitional state
intermediate
NH3
Pyridine
2,4,6-Trimethyl-pyridine
0.39
0.58
0.58
0.58 < d < 0.97
0.26
0.58
0.74
H-beta
√
√
√
√
√
√
√
√
1
0
2 MR channels (0.66 nm × 0.67 nm &
.56 nm × 0.56 nm)
√
√
√
√
√
√
√
H-MCM-49
2 MR cups
0.71 nm × 0.71 nm × 0.91 nm)
2 MR supercages via 10 MR windows
√
√
√
√
√
√
√
√
√
√
√
√
1
(
1
(
0.41 nm × 0.54 nm &
0
1
0
.41 nm × 0.59 nm)
√
√
√
√
√
0 MR channels (0.41 nm × 0.54 nm &
.41 nm × 0.59 nm)
The sizes of pore and channel are referenced from [40].

Y. Shi et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
91
H-beta
H-MCM-49
a
0
.2 abs.u
Hydroxyl (-OH)
,4,6-trimethylpyridine
2
100
200
300
400
500
600
o
Temperature ( C)
3800 3600 3400 3200 3000 1700 1600 1500 1400
-1
Wavenumber (cm )
Fig. 4. NH3-TPD curves of H-beta and H-MCM-49.
b
0.2 abs.u
cally adsorbed 2,4,6-trimethyl-pyridine with bands at ∼1485 and
Hydroxyl (-OH)
,4,6-trimethylpyridine
−
1
∼
1395 cm
effects of zeolitic channels on guest molecules [39]. However
,4,6-trimethyl-pyridine cannot penetrate the inner channels
of H-MCM-49 (10 MR channels and 12 MR supercages with
on the inner surface of H-beta due to activation
2
2
1
2
0 MR windows) [20,26–28]. Meanwhile physically adsorbed
,4,6-trimethyl-pyridine could be easily desorbed under treat-
ing conditions of FTIR, therefore there were no physically
adsorbed bands of 2,4,6-trimethyl-pyridine detected over H-MCM-
−
1
4
9. According to the height of adsorption band (∼1633 cm ),
the amount of 2,4,6-trimethyl-pyridinewas calculated to give the
−
1
accessibility of active centers, 34.9 ␮mol g
5
for H-MCM-49 and
for H-beta. Clearly H-beta shows more acid sites
accessed by 2,4,6-trimethyl-pyridine by about 51% than H-MCM-
9. In the case of H-beta’s three-dimensional 12 MR pore systems,
an easy penetration of 2,4,6-trimethyl-pyridine into its pores led
−1
2.7 ␮mol g
3
800 3600 3400 3200 3000 1700 1600 1500 1400
4
-1
Wavenumber (cm )
−
1
to complete disappearance of the OH acidic band at ∼3610 cm
observed. However, for H-MCM-49, the slight decrease in the inten-
Fig. 5. 2,4,6-Trimethylpyridine spectra of H-beta (a) and H-MCM-49 (b).
−1
sity of the acidic OH band at ∼3610 cm indicates that only a small
part of acid sites could be assessed by 2,4,6-trimethyl-pyridine.
According to previous results, 2,4,6-trimethyl-pyridine could only
detect acid sites residing in 12 MR cups on the outer surface of H-
MCM-49.From the view of reactant selectivity, the more acid sites
accessed by 2,4,6-trimethyl- pyridine, the better catalytic activity
in liquid-phase alkylation of benzene with ethylene.
MCM-49 is considered as a promising catalyst for liquid-alkylation
of benzene with ethylene due to its better EB selectivity, which
may lead to less energy consumption at lower benzene/ethylene
ratio. Better EB selectivity means that less DEBs and TEBs were
produced over H-MCM-49 than H-beta (Fig. 6c and d). The distri-
butions of DEBs over these two catalysts are shown in Fig. 6e. The
high degree of selectivity over H-MCM-49 for o-DEB is interest-
ing. The kinetically favorable o-DEB was firstly generated and then
transformed into the more thermodynamically favorable m-DEB.
For H-beta, with the rise of temperature, thermodynamic equi-
librium distributions of three DEB isomers could be approached
m(m-DEB):m(p-DEB):m(o-DEB) = 64:30:6 [20]. However, for the
thermodynamic equilibrium distributions over H-MCM-49 can-
3
.3. Alkylation performances over H-beta and H-MCM-49
catalysts
Usually the ethylene conversion is almost 100% to ensure no
ethylene to enter trans-alkylation reactor in industrial unit. If there
was unconverted ethylene, fast deactivation will happen on ethy-
lation and trans-alkylation catalysts. Also with benzene/ethylene
ratio at 12, which is similar to that of single ethylation stage with
multi-ethylene feeding points, is enough to guarantee the solution
of ethylene in the benzene. The catalytic alkylation of benzene over
H-beta and H-MCM-49 catalysts is shown in Fig. 6. Clearly, H-beta
presented superior activity with ethylene conversion at 100% in
the temperature range from 200 to 260 C, but a slight decrease
in EB selectivity. As for H-MCM-49, there was a stepwise increase
of ethylene conversion from 96.6% at 200 C to 99.2% at 260 C,
which indicated diffusion limits may be one of the key factors to
impact the activity. Similar to H-beta, a slight decrease in EB selec-
◦
not be reached even at 260 C, which was also evidence of lower
isomerization activity over H-MCM-49. With ethylation selectiv-
ity considered, H-MCM-49 showed no poorer results than H-beta
(Fig. 6f).
The difference in alkylation performances depends on sizes
of reactant/transition-state/product and zeolite pore systems
based upon shape selectivity (Table 4). Benzene (0.58 nm) could
penetrate the 12 MR channels of H-beta (0.66 nm × 0.67 nm &
0.56 nm × 0.56 nm), and more importantly the12 MR pore chan-
nels of H-beta are large enough to accommodate the formation
of transition state. Therefore, liquid-alkylation of benzene with
ethylene over H-beta is recognized as the inner channels reaction
with better accessibility of active centers, which was strongly sup-
ported by the strong adsorption of 2,4,6-trimethyl-pyridine and
◦
◦
◦
◦
tivity was also observed over H-MCM-49 from 95.4% at 200 C to
9
poorer EB selectivity. Regardless of lower ethylene conversion, H-
◦
4.9% at 260 C. On the whole, H-beta exhibits better activity but

9
2
Y. Shi et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
9
6
1
00.0
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
a
b
9
95
9
94
9
93
9
9
92
9
H-beta
H-MCM-49
H-beta
91
H-MCM-49
9
9
90
200
210
220
230
240
250
260
200
210
220
230
240
250
260
o
o
Temperature ( C)
Temperature ( C)
8
7
6
5
4
3
0.35
H-beta
c
H-beta
d
H-MCM-49
H-MCM-49
0.30
0.25
0.20
0.15
0.10
0.05
200
210
220
230
240
250
260
200
210
220
230
240
250
260
o
o
Temperature ( C)
Temperature ( C)
7
0
1
00.0
9.9
9.8
99.7
9.6
9.5
9.4
99.3
9.2
9.1
9.0
e
f
9
60
9
50
H-beta: m-DEB
H-beta: p-DEB
H-beta: o-DEB
9
40
9
30
H-MCM-49: m-DEB
H-MCM-49: p-DEB
H-MCM-49: o-DEB
9
20
H-beta
H-MCM-49
9
10
9
0
9
2
00 210 220 230 240 250 260 270 280 290 300 310 320
200
210
220
230
240
250
260
o
o
Temperature ( C)
Temperature ( C)
◦
Fig. 6. Liquid-phase alkylation of benzene with ethylene over H-beta and H-MCM-49 (Liquid-phase alkylation conditions: 8 mL catalysts, T = 200–260 C, p = 3.5 MPa, benzene
−
1
−1
WHSV = 3.0 h , benzene/ethylene molar ratio = 12.0).
the complete disappearance of the OH acidic band at 3610 cm−1
.
the target product. Usually for a consecutive reaction, the decrease
of intermediate product selectivity is usually accompanied by the
increase of conversion [41,42]. For H-beta, longer diffusion dis-
tance out of confined space in inner channels surely would drive
ethylation forward to the formation of DEBs and TEBs, which may
be the real reason for the lower EB selectivity. Therefore as for
H-beta, enhancement in EB selectivity should be given the prior-
ity rather than activity through synthesizing nano or hierarchical
beta to decease the diffusion length out of confined space in inner
channels.
Meanwhile for reactions proceeded in the inner channels of zeo-
lites, guest molecules including reactants and base probes could
be activated by zeolitic channels due to the negative charge of
zeolitic framework. This hypothesis is also proven by strong phys-
ically adsorbed 2,4,6-trimethyl-pyridine in the pores of H-beta,
which is more difficult to be desorbed than H-MCM-49. In such
a way, easy diffusion into confined space in beta channels, suf-
ficient accommodation for the transition states and activation of
guest molecules by zeolitic channels are the key to superior activity
over H-beta, however superior activity is not the only consideration
especially for a consecutive reaction with intermediate product as
While for H-MCM-49, things become more complicated due to
three different pore systems. Although there are abundant acid sites

Y. Shi et al. / Journal of Molecular Catalysis A: Chemical 418 (2016) 86–94
93
inside the 10 MR channels, they seem to contribute little to EB for-
mation, which has been proven in many publications [20,26–28].
Both benzene (0.58 nm) and ethylene can penetrate the 10 MR
channels. Also it has been extensively proven that 10 MR channels
are large enough to accommodate benzene, ethylbenzene even p-
DEB [20]. However there are insufficient spaces to accommodate
the transition state during formation of EB, i.e., transition state
selectivity. Also some reports have claimed that active centers use-
ful for EB synthesis were mainly located in 12 MR cups on the
external surface of MWW zeolites, which does not restrict the dif-
fusion of reactant/transition state/product toward/from the active
centers [20,26–28]. 2,4,6-Trimethyl-pyridine can only be adsorbed
on the acid sites located in the 12 MR cups on the outer surface of
H-MCM-49 with little restriction on the reactants and EB, indicat-
ing less chances to be further ethylated to form DEBs and TEBs.
Different from confined spaces in the channels of beta catalyst,
without loss of EB selectivity over MWW zeolites, much attention
should be put into promoting the diffusion through 10 MR windows
governing 12 MR supercages. It has been proven that 3D struc-
ture of MWW zeolite could be changed to corresponding lamellar
precursor via a reversible rearrangement in presence of HMI or
piperidine, which returned reversibly to the 3D MWW structure by
further calcination [46,47]. We have reported to tailor the aggrega-
tion of MWW layers, crystals and particles to improve accessibility
of active centers (one supercage is divided into two 12 MR cups) via
synthesis of MCM-56, directing-gel method, zeolite–zeolite trans-
formation and post-synthesis to improve catalytic performances
in liquid-phase alkylation of benzene with ethylene [45,48–52]. In
future research, the degree of 12 MR supercages splitting into 12
MR cups should be assessed and adjusted.
4. Conclusion
1
2 MR cups on the external surface of MWW catalysts could be
viewed as open space for catalysis i.e. surface pocket catalysis.
Poor activation of guest molecules by open space of 12 MR cups
of MWW zeolite than inner channels of beta zeolite may lead to
inferior activity. However open space of 12 MR cups accounts for
higher EB selectivity over MWW catalysts because of easy escaping
from acid sites to avoid further alkylation. As for 12 MR supercages
with 10 MR windows, there are few reports on its influences on
activity and selectivity in liquid-phase alkylation of benzene with
ethylene. Benzene and ethylene could penetrate 12 MR supercages
through 10 MR windows, and 12 MR supercages are large enough
to form the transition state of EB. The key lies in the poorer diffu-
sion of benzene into of 12 MR supercages through 10 MR windows,
Based upon the shape selectivity, comprehensive research were
conducted on acidity characterization and catalytic performance
in liquid-phase alkylation of benzene with ethylene over *BEA
and MWW zeolites. According to reactant selectivity, H-beta and
H-MCM-49 were characterized by base probes with different
molecule sizes. NH , pyridine and 2,4,6-trimethyl-pyridine could
3
penetrate H-beta channels to access the acid sites. Although both
NH3 and pyridine could detect the acid sites on internal and exter-
nal surface of H-MCM-49, 2,4,6-trimethyl-pyridine, which could
dominantly be adsorbed on the acid sites located in 12 MR cups
on the outer surface of H-MCM-49, gave less number of acid sites
than H-beta. Based on the sizes of base probes/reactants/transition
states/products/pore channels, the difference in catalytic perfor-
mance over H-beta and H-MCM-49 was summarized. Easy diffusion
into confined spaces and activation of reactant molecules by zeolitic
channels accounts for superior activity, however longer diffusion
distance for EB escaping results in poor EB selectivity. Therefore
as for H-beta, enhancement in EB selectivity should be given the
priority rather than activity through synthesizing nano or hierar-
chical beta to decease the diffusion length out of confined space in
inner channels. As for H-MCM-49, acid sites located in 10 MR chan-
nels contributed little to the formation of EB due to transition state
selectivity. Acid sites located in 12 MR cups on the external surface
of MWW zeolites with little restriction on the diffusion of reac-
tant/product toward/from the active centers were the major active
centers for EB formation. However the acid sites on the outer sur-
face of H-MCM-49 only accounted for a small part of total acid sites,
which could be determined by 2,4,6-trimethyl-pyridine. Therefore,
to enhance activity without loss of EB selectivity over MWW zeo-
lites, much attention should be put into promoting the diffusion
through 10 MR windows governing 12 MR supercages, i.e. trans-
forming confined space (12 MR supecages) into open space (12 MR
cups).
which led to a stepwise increase of ethylene conversion from 200 to
◦
2
60 C. There has been several reports confirmed our proposal that
2 MR supercages could catalyze alkylation of benzene even with
1
long chain alkenes [43,44]. The other issue is selectivity. Although
the kinetic diameters of EB is similar to benzene at about 0.58 nm,
EB molecules show poorer diffusion capability than benzene due
to the ethyl group, which usually caused the formation of DEBs,
TEBs and heavier aromatics resulting in poorer EB selectivity and
deactivation. The 12 MR supercages also contribute to the fast deac-
tivation of catalysts especially in alkylation of longer chain alkenes
[
44]. The reason also lies in the poor diffusion of products out of 12
MR supercages through 10 MR windows. With more supercages, Y
zeolite suffered a much faster deactivation and poorer selectivity
regardless of 12 MR windows [20].
In our recent paper, it has been proven that liquid-phase alky-
lation of benzene with ethylene over MWW zeolites is an inner
diffusion-limited reaction. With the decrease of MWW crystal sizes,
the ethylene conversion and EB selectivity could be simultaneously
increased [45]. Extreme condition of inner diffusion is considered
as the external surface reaction, strongly supporting previous pro-
posal that liquid-phase alkylation of benzene with ethylene over
MWW zeolites is the external surface reaction [20,28,38]. Conven-
tionally active centers useful for EB synthesis are thought to locate
in 12 MR cups on the external surface of MWW zeolites, which
do not restrict the diffusion of reactants/products toward/from the
active centers [20]. That is to say, most of acid sites residing in
the inner 10 MR channels are not utilized during the liquid-phase
alkylation of benzene with ethylene, and cannot be utilized because
of transition state selectivity. As concluded in this paper, the key
to increase the alkylation performances of MWW zeolites is to
increase the diffusion of benzene/EB into/out of 12 MR supercages
through 10 MR windows, equal to increase the activity/selectivity
of this reaction. According to reports, the acid sites on the external
surface of MWW zeolites account for less than 15% of total acid sites,
that is, only a small portion is utilized in the liquid-phase alkylation
of benzene with ethylene [20,26–28], which also leads to poorer
activity over MWW zeolite than beta. Therefore, to enhance activity
Acknowledgments
This work was supported by the National Basic Research Pro-
gram of China (973 Program, No. 2012CB224805). Special thanks
to the Department of Analysis in Research Institute of Petroleum
Processing Sinopec.
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