Design of micro-mesoporous zeolite catalysts for alkylation

Article abstract of DOI:10.1039/c6ra08099e

Hierarchical zeolite Y was hydrothermally synthesized using ionic liquid 1-methyl-3-[3′-(trimethoxysilyl)propyl]imidazolium chloride, which was added into the conventional synthesis composition for crystalline microporous aluminosilicate as a mesopore-directing agent. An extremely short diffusion path length and abundant exposed acid sites in the hierarchical zeolite Y made it suitable for the catalytic transformation of bulky molecules. Then the catalytic performance of hierarchical zeolites Y was investigated using the alkylation of o-xylene with styrene. The ionic liquid directed hierarchical zeolite Y exhibited a remarkably high catalytic activity and long catalytic lifetime for the production of 1-phenyl-1-xylylethane via alkylation reaction, as compared with that of solely microporous zeolite Y. The distinct catalytic performances of the novel hierarchical zeolite Y could be attributed to the enhancement of medium strong acid amount and its micro-mesoporous structural properties which are favorable for the fast diffusion of products.

Full text of DOI:10.1039/c6ra08099e

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PAPER
Design of micro–mesoporous zeolite catalysts for
alkylation
Cite this: RSC Adv., 2016, 6, 50630
a
a
a
b
a
a
Xiaoqin Fu, Xiaoli Sheng, Yuming Zhou,* Zhiwei Fu, Shuo Zhao, Xiaohai Bu
a
and Chao Zhang
0
Hierarchical zeolite Y was hydrothermally synthesized using ionic liquid 1-methyl-3-[3 -(trimethoxysilyl)
propyl]imidazolium chloride, which was added into the conventional synthesis composition for
crystalline microporous aluminosilicate as a mesopore-directing agent. An extremely short diffusion path
length and abundant exposed acid sites in the hierarchical zeolite Y made it suitable for the catalytic
transformation of bulky molecules. Then the catalytic performance of hierarchical zeolites Y was
investigated using the alkylation of o-xylene with styrene. The ionic liquid directed hierarchical zeolite Y
exhibited a remarkably high catalytic activity and long catalytic lifetime for the production of 1-phenyl-1-
xylylethane via alkylation reaction, as compared with that of solely microporous zeolite Y. The distinct
catalytic performances of the novel hierarchical zeolite Y could be attributed to the enhancement of
medium strong acid amount and its micro–mesoporous structural properties which are favorable for the
fast diffusion of products.
Received 29th March 2016
Accepted 13th May 2016
DOI: 10.1039/c6ra08099e
www.rsc.org/advances
5
6
sulfated zirconia/titania, modied mesoporous mordenite
have been adopted as catalysts, all of which have improved the
1
. Introduction
The alkylation of o-xylene with styrene to give 1-phenyl-1- catalytic activity. But they still cannot satisfy the requirements
arylethane is an industrially important reaction. Phenylxylyl- of the practical applications due to their operation loss, low
ethane (PXE) is widely used as a high-energy fuel for turbojets, mechanical strength, rapid catalytic deactivation, and high
rockets, jets, missile engines; as an important solvent for mass transfer resistance.
pressure-sensitive record materials, plasticizer, heating
Nanoporous materials such as zeolites, metal–organic
medium, electric-insulating oil; and as a high-stability lubri- frameworks (MOFs), porous solids are widely used as hetero-
1
cant. One of the main problems in this reaction is the forma- geneous catalysts in petrochemical and ne chemical
7–10
tion of styrene oligomers generated as byproducts, especially synthesis.
Among the nanoporous materials used in indus-
when homogeneous Brønsted or Lewis acid catalysts are used. trial scale, zeolite Y is the most widely employed aluminosili-
Traditionally, various homogeneous acid catalysts (such as cate, which is a highly versatile faujasite molecular sieve with
H SO , BF , HF, AlCl or FeCl ) have been used in this reaction. 0.74 nm diameter pores and a three-dimensional pore struc-
2
4
3
3
3
However, these homogeneous acid catalysts possess some ture, has been found widespread applications in catalysis due to
inherent shortcomings of high corrosiveness, poor shape its catalytic activity, structural stability, strong acid strength,
11–13
selectivity, environmental non-friendliness, troublesome and excellent shape selectivity.
However, these relatively
product recovery and purication, and impossibility of catalyst narrower micropores put limits on the application of zeolite Y in
recycling. Thus many efforts have been made to develop envi- the conversion and separation of bulky molecules, because the
ronmentally friendly catalytic technologies for the alkylation sole presence of micropores oen associates with slow mass
reaction. Recently, heterogeneous catalysts in liquid phase transfers into and out of the catalytic active sites inside the
1
4,15
reactions have received overwhelming attention in response to zeolite frameworks.
To overcome this disadvantage, the
16,17
their advantages such as high catalytic activity and selectivity, introduction of mesopores into zeolites Y is the key.
A varied
ease of separation, no corrosion or disposal of effluent prob- assortment of top-down and bottom-up approaches are avail-
lems, reusability, etc. For PXE synthesis, solid acids such as able to synthesize hierarchically structured zeolites. Meso-
2
3
4
heteropolyacid salts, cation-exchange resins, silica-alumina,
porous zeolites Y are hitherto successfully synthesized via a top-
down approach through dealumination or sequential
18
a
dealumination-steaming treatment. But the post-treatment
School of Chemistry and Chemical Engineering, Southeast University, Jiangsu
Optoelectronic Functional Materials and Engineering Laboratory, Nanjing 211189, process is too difficult to control, which may result in an arbi-
P. R. China. E-mail: ymzhou@seu.edu.cn; Fax: +86 25 52090617; Tel: +86 25 trary mesoporous network or even the collapse of zeolites
5
2090617
structure. A one-step bottom-up approach using so or hard
b
Nuclear and radiation safety center, MEP, Beijing 100082, P. R. China
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2 2
template has been considered as one of the most promising aluminate (NaAlO ), aluminum sulfate octadecahydrate (Al (-
methods to prepare hierarchical zeolites with tailored meso- SO ) $18H O, 99%), sodium hydroxide (NaOH, AR, 96%) were of
4
3
2
19
porosity. As turn out to be the state-of-art in the design of analytical grade, and all of them were purchased from Aladdin
various hierarchical zeolites by different templates, it provides Chemistry Co. Ltd (Shanghai, China). Deionized water was used
many inspirations for the development of new and improved in all experiments. All chemicals were used as received without
template for mesoporous zeolite synthesis.
further purication.
Ionic liquids, which are molten salts at room temperature,
have been attracting considerable attention in recent time due
to their superior properties of negligible vapor pressures, non-
2
.2 Synthesis of IL
IL had been synthesized using 21.75 g of 1-methylimidazole and
2.66 g of 3-chloropropyl trimethoxysilane (with the molar ratio
20,21

ammability and good thermal stability.
Their special
5
structures of the organic cation and inorganic anion make them
acted as multiple roles of cosolvent, cosurfactant as well as salt
of 1 : 1) in a solvent-free process by an ultrasonic/microwave
synergistic method. Firstly, the two hybrid materials were
poured into a reaction vessel, and placed into a microwave oven
equipped with ultrasound. The ultrasonic power was set at 300
W with a cycle of working 1.1 S and intermitting 1.1 S. The
22
in the system. Ionic liquid can form extended hydrogen-bonds
systems in the liquid state, which is the basis of molecular
23
recognition and self-assembly process. And the negligible
vapor pressures and ease with which ionic liquids may be
recycled make such hydrothermal synthetic routes both eco-
friendly and simple to implement. Hence, they can induce
mixture was irradiated at 320 W, reaction temperature was
ꢀ
120 C and irradiation continued for a total of 2 min. Then the
product was directly dried under continuous evacuation
24
micro or mesoporous materials. Zheng et al. synthesized
ꢀ
(1 mmHg) at 100 C for 12 h without any purication. Aer
hollow silica spheres using a mixed template of a long-chain
cooling to room temperature, the product was obtained as
yellowish viscous liquid, which was stored under dry condi-
tions. Lastly, IL was characterized by H and C NMR spec-
ionic
liquid
1-dodecyl-3-methylimidazolium
bromide
([C12mim]Br) and an anionic surfactant of sodium dodecyl
1
13
sulfate (SDS). They had investigated the phase behavior and the
vesicle formation of SDS/[C12mim]Br in aqueous solution to
illuminate the formation mechanism of hollow silica spheres.
In fact, the ionic liquid 1-hexadecyl-3-methylimidazolium
chloride ([C mim]Cl) had been described to present superior
1
troscopy. H NMR (CDCl ): d ¼ 10.412 (s, 1H, –N–CH–N–), 7.591
3
(s, 1H, –N–CH]CH–N–), 7.366 (s, 1H, –N–CH]CH–N–), 4.25 (t,
2H, –N–CH –), 4.0 (s, 3H, –N–CH ), 1.9 (quint, –CH –CH –CH –
2
3
2
2
2
1
3
)
(
, 3.433 (t, 9H, –Si–OCH
CDCl
3
2
), 0.5 (t, 2H, –Si–CH –). C NMR
1
6
3
): d ¼ 138.01 (–N–CH–N–), 122.43 (–N–CH]CH–N–),
templating properties in the hydrothermal synthesis of a two-
121.73 (–N–CH]CH–N–), 36.498 (–N–CH
2
–), 51.652 (–N–CH
3
),
).
dimensional hexagonal p6mm mesopore structure (MCM-41
24.069 (–CH –CH –CH –), 5.885 (Si–CH –), 50.625 (–Si–OCH
2
2
2
2
3
25
type), as compared with commonly used ammonium based
surfactants (i.e. cetyltrimethylammonium bromide (CTAB) or
cetyltrimethylammonium chloride (CTAC)). Furthermore, ionic
liquids have very recently allowed for the synthesis of meso-
1
In addition, the H NMR spectroscopy was used to characterize
the trace amounts of remaining 1-methylimidazol and 3-chlor-
opropyl trimethoxysilane, respectively. The maximum content
of remaining 1-methylimidazol is 1.95%, and the content of
remaining 3-chloropropyl trimethoxysilane is 2.70%. Thus, the
yield of IL is 95.35%.
26
porous zeolites by a facile bottom-up approach.
The present work was aimed to conduct framework geometry
analysis of hierarchical zeolite Y, to investigate the changes of
pore system and acidity responsible for increased catalytic
2.3 Synthesis of hierarchical zeolite Y
activity for PXE synthesis. A bifunctional ionic liquid 1-methyl-3-
0
[
3 -(trimethoxysilyl)propyl] imidazolium chloride, denoted as IL, In a typical synthesis experiment, the mixed solution involved
which is a highly commercialized product, was used as a meso- 15.379 g of Na
porous template for synthesizing hierarchical zeolite Y. Addi- addition of precursor into the solution (the precursor was ob-
tionally, great efforts targeted at the expansion of the SiO /Al O
tained by mixing 0.07 g of NaOH, 2.20 g of Na SiO , 0.16 g of
2 3
SiO , 52 mL of deionized water, followed by the
2
2
3
2
3
ratio had been made, since it is well known that the properties of NaAlO and 7.4 mL of deionized water, then the mixture was
2
ꢀ
ꢀ
the aluminum-containing materials and the number of acid sites stirred at 50 C for 1 h and aged at 50 C for 5 h), aer which the
ꢀ
can be tuned by changing the SiO
/Al O
2 2 3
molar ratio during mixture was stirred at 50 C for 1.5 h, during which 6.46 g of IL
synthesis. The hierarchical micro–mesoporous zeolite Y, was slowly added dropwise. Aer the mixture was further stirred
combining the advantages of mesoporous characteristics and the for another 1.5 h, 0.18 g of NaOH, 3.49 g of Al (SO ) $18H O and
2
4 3
2
2
microporous crystalline structure of zeolite, is expected to be 1.17 g of NaAlO were added under vigorous magnetic stirring.
a suitable catalyst for catalytic reactions involving bulky mole- The resultant aluminosilicate gel had a molar composition of
cules. Then a series of studies have revealed the unprecedented 1.0 Na O/0.93 SiO /0.088 Al O /21.86 H O/0.15 IL. The resulting
2
2
2
3
2
catalytic activity of the hierarchical zeolites Y.
synthesis mixture was further vigorously stirred for another 12
h, a homogeneous milk white solution was observed. Then the
resultant aluminosilicate gel was transferred into a Teon-lined
2. Experimental section
ꢀ
autoclave and heated in an oven at 100 C for 24 h (referred to as
2.1 Materials
the autoclaving treatment time). The crystallization products
ꢀ
1-Methylimidazole (99%), 3-chloropropyl trimethoxysilane were collected by ltration with deionized water, dried at 100 C
(98%), sodium metasilicate (Na
2
SiO
3
, SiO
2
44–47%), sodium in air. Then calcination was carried out with a heating rate of 3
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ꢀ
ꢁ1
ꢀ
C min , and kept at the target temperature 600 C for 6 h to electrostatic potential (MEP) were determined at the same level
remove the template.
of theory B3LYP/6-31G* with the optimized structure.
Similarly, a series of hierarchical zeolites Y were synthesized
by varying the aluminum contents with x ¼ 11, 10.5, 9.5, and 8.5 2.6 Catalytic reactions
(
x: the molar ratio of SiO to Al in the resultant alumino-
2
2 3
O
To decrease the Na content of zeolite Y and thus increase its
acidity, H was used for ion-exchange of Na in zeolite NaY. The
H-form samples of zeolite Y were obtained by three times of ion
silicate gel). And the nal products were denoted as NaY-x. In
addition, conventional microporous zeolite NaY was synthe-
sized with SiO
the addition of IL and denoted as “NaY”.
+
+
2
/Al
2
O
3
¼ 10.5 by the same procedure except for
ꢀ
exchange with 1 M NH
4
Cl solution at 90 C for 4 h, and then
+
samples were converted to the H form via calcination in air at
50 C for 6 h. They were denoted as HY-x or HY.
ꢀ
5
The alkylation reactions were carried out in a continuously
2
.4 Characterizations
ꢀ
stirred oil batch reactor at 120 C for 3 h under reux condi-
tions. 45 g of o-xylene, 5.89 g of styrene (molar ratio of o-xylene
to styrene is 7.5 : 1) and 1.00 g of catalyst were added in a 100
mL three-neck ask equipped with a condenser. Firstly, one
third of o-xylene were added to the round-bottom ask, followed
by 1.00 g of catalyst, then the mixture of 5.89 g of styrene and the
remaining o-xylene was dropwise added into the ask for 2 h.
Lastly, the nal reaction mixture remained for another 1 h. Aer
the reaction, unreacted o-xylene was distilled out under atmo-
spheric pressure, and the obtained liquid was denoted by crude
product which was analyzed with GC-9890A gas chromatograph
equipped with OV-1 capillary column and a ame ionization
detector (FID). The yield of PXE was dened as follows:
1
13
H NMR and C NMR spectra were performed on an Avance II
3
200 spectrometer (Bruker), using CDCl as a solvent. Powder X-
ray diffraction (XRD) patterns were recorded on a Bruker D8
Advance diffractometer (Germany) system equipped with Cu-Ka
radiation (operation at 40 kV, 30 mA). And the XRD patterns
ꢀ
were collected in the range of 3–50 in 2q/q scanning mode with
ꢀ
ꢀ
ꢁ1
a 0.02 step and scanning speed of 10 min . The nitrogen
adsorption and desorption isotherms were measured with an
ASAP-2020 (Micromeritics USA) at 77 K. The samples were out
gassed for 6 h at 350 C before the measurements. The specic
surface area (SBET) was determined from the linear part of the
Brunauer–Emmett–Teller (BET) equation (P/P
The pore size distribution was calculated using the Barrett–
Joyner–Halenda (BJH) method from the desorption branch of
ꢀ
0
¼ 0.05–0.25).
actual product weight
Yield of PXE ð%Þ ¼
ꢂ 100%
theoretical product weight
2
the N isotherm. The total pore volume (Vtotal) was estimated
from the amount of nitrogen adsorbed at a relative pressure (P/
P0) of ca. 0.995. Transmission electron microscopy (TEM)
experiments were conducted on a JEM-1230 microscope oper-
ated at 100 kV. The samples for the TEM measurements were
dispersed in ethanol ultrasonically and were supported on a Cu
grid. The morphologies and microstructures of as-synthesized
zeolites were characterized by obtaining scanning electron
microscope (SEM) images using a Hitachi S-3400N SEM at an
acceleration voltage of 20 kV. Energy dispersion X-ray analysis
Actual product weight ¼ crude product weight
ꢂ PXE (chromatography)%
3
. Results and discussion
3
.1 Effect of the IL template
kV. Samples were xed using a conductive carbon double-sided The X-ray diffraction (XRD) patterns of the conventional zeolite
sticky belt. Surface acidity was measured by NH -TPD in a TP- NaY and the mesoporous zeolite NaY-10.5 templated by the IL
000 apparatus (made in China) at atmospheric pressure. The are shown in Fig. 1. The diffractions of the two samples are
(EDX) was conducted on a JEM-1230 microscope operated at 100
3
5
ꢀ
sample (150 mg) was preheated at 500 C for 1 h, and then consistent with the typical diffraction patterns of zeolite Y in the
ꢀ
ꢀ
cooled down to room temperature under a owing He stream. high 2q region of 3–50 , in which well-resolved peaks at ¼ 5.97 ,
ꢀ ꢀ ꢀ ꢀ
At this temperature, sufficient pulses of NH
adsorption saturation. TPD was carried out from 100 to 600 C (331), (533), (642), (555) reections, respectively. The only
3
were injected until 15.25 , 23.29 , 26.48 , 30.90 are corresponded to the (111),
ꢀ
15
ꢀ
ꢁ1
with a heating rate of 10 C min and with helium (30 mL difference between mesoporous zeolite NaY-10.5 and conven-
min ) as the carrier gas.
ꢁ
1
tional zeolite NaY indicated by XRD is a signicant increase in
the intensity of characteristic peaks as the addition of IL, which
indicates its higher crystallinity. The positive contribution of
template IL to the crystal growth in the crystallization process is
2.5 Calculations
Density functional theory (DFT) was used as a very useful tool reasonably concluded.
for understanding molecular properties and for describing the Fig. 2a shows N adsorption–desorption isotherm of hierar-
behavior of atoms in molecules. The optimized structure of IL chical zeolite NaY-10.5. The isotherms exhibit a type IV hyster-
2
27
29
was performed at the Becke3–Lee–Yang–Parr (B3LYP) level esis loop, as dened by IUPAC. The sample NaY-10.5 displays
theory in combination with the 6-31+G(d,p) basis set with a steep uptake in the low relative pressure region (P/P < 0.1), as
0
28
Gaussian 03 program. This basis set provided accurate a result of micropore lling. A sharp inection at relatively high
geometry and electronic properties for a wide range of organic pressure (0.4 < P/P < 0.8) suggests the capillary condensation of
0
compounds. Besides, frontier molecular orbitals and molecular
N
2
in mesopores, indicating the presence of mesoporous
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Fig. 3 SEM images of as-synthesized products: (a) conventional
zeolite NaY and (b) hierarchical zeolite NaY-10.5; TEM images of as-
synthesized samples: (c) NaY, and (d) NaY-10.5 at different
magnifications.
Fig. 1 Powder X-ray diffraction patterns of the hierarchical zeolite
NaY-10.5 and conventional zeolite NaY in the high 2q region.
observation of the morphology of the samples. Only the
ordered microporous channels are presented in the TEM
image of conventional NaY (Fig. 3c). Whereas, TEM images
3
0
structures in the zeolite NaY-10.5. The curve continues rising
above P/P condensation in the
¼ 0.8, which is attributed to N
0
2
2
7
void volume between the particles. The mesopore size distri-
butions can be estimated from the adsorption branch of the
isotherms using the Barrett–Joyner–Halenda (BJH) model. The
sample NaY-10.5 exhibits a narrow pore size distribution of
mesopore diameters centered at 3.89 nm (Fig. 2b). These results
conrm the generation of mesoporosity in zeolite NaY-10.5
prepared by the IL. However, the conventional zeolite NaY
exhibits an extremely narrow micropore size distribution of
about 0.74 nm with Horvath–Kawazoe equation, which does not
contain any mesopores.
(
Fig. 3d) give direct evidence for the presence of intracrystal-
line mesostructure in the hierarchical zeolite NaY-10.5. These
1
5
results are consistent with those of other literature. The
observation of disordered wormlike mesostructure conrms
the presence of mesopores (the mesoporosity in the crystal is
marked by orange lines). The mesoporous structure are well
present in the nearest neighboring frameworks, combining
2
with the corresponding N adsorption–desorption isotherms,
indicating that the formation of hierarchical zeolite Y frame-
work. Moreover, the diameters of wormlike mesostructures
vary through the mesoporous channels and the mesoporous
channels are not straight. It may be attributed to the disor-
dered aggregation of IL, reecting that the IL micelles are
Fig. 3 shows the SEM and TEM images of conventional
zeolite NaY and hierarchical zeolite NaY-10.5. The typical
octahedral structure is observed in the sample NaY (Fig. 3a),
while hierarchical NaY-10.5 exhibits
a “cauliower-like”
3
1
mesoporous templates. The phenomena suggest that intra-
crystalline mesoporosity could be formed through the
sequential anchoring and templating mechanism of IL. In the
morphology (Fig. 3b) formed by primary octahedral structure.
The TEM images recorded from samples NaY and NaY-10.5 are
shown in Fig. 3c and d, which provide us with a direct
Fig. 2
2
N adsorption–desorption isotherms of hierarchical zeolite NaY-10.5 and conventional zeolite NaY; (a), the corresponding pore size
distribution calculated by BJH model based on the desorption curves (b).
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case of hierarchical zeolite NaY-10.5, the mesopore walls 3.2 Effect of SiO /Al O ratio
2
2 3
possess large external surface areas where the reactions are
catalyzed. The unique pore system can provide a short diffu-
sion route for the guest molecules, and reduce secondary
reactions, which are important for catalytic performance
involving bulky molecules.
The plausible formation of hierarchically micro–meso-
porous zeolite Y is illustrated in Scheme 1. Under the strong
basic condition of the typical Y synthesis system, alkoxy-
Fig. 4 shows the XRD patterns of the samples synthesized with
the initial gel composition of 1.0 Na
IL (crystallized at 100 C for 24 h) by varying the aluminum
2
O/0.93 SiO
2
/21.86 H
2
O/0.15
ꢀ
contents with x ¼ 11, 10.5, 9.5, and 8.5 (x: the molar ratio of SiO
2
to Al ). Highly crystallized zeolites Y are obtained in the case
2 3
O
of suitable aluminum contents (x ¼ 9.5, 8.5, and 10.5).
Decreasing the aluminum content to a SiO /Al O molar ratio of
2
2 3
11 (Fig. 4d), the XRD intensity is very weak, reecting a lower
functionalized (CH O)–Si– bonds can quickly hydrolyze to
3
crystallinity. Zeolites are crystalline aluminosilicate composed
of silicon–oxygen and aluminum–oxygen tetrahedra. The
negative charge of the aluminum–oxygen tetrahedra is
HO–Si– and form rm –Si–O–Si–, –Si–O–Al– bonds anchoring in
32
the zeolite framework. Moreover, the electrostatic interaction
between the positively charged imidazole head group and
negatively charged aluminosilicate gels, and micelles formed
through self-assembly, act as an effective structure-directing
agent for the zeolite, promoting the formation of meso-
porosity within the zeolite Y crystals. Therefore, mesoporous
channels are constructed with crystalline microporous zeolite Y
by calcination to eliminate the IL.
33
neutralized by cations. The incorporation of Al atoms into the
framework can contribute one negative charge unit. The nega-
tively charged units will preferentially attract cations, including
+
+
34
H or Na into zeolite framework to play a stabilizing role.
However, the incorporation of Si atoms cannot produce
a negative charge. Thus –Si–O–Al– bonds are more easily formed
Scheme 1 Schematic diagram represents the synthesis of hierarchical zeolite Y using IL as a template.
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2
ꢁ1
exhibits a biggest BET surface area of 713 m g . In the case, 90
2
ꢁ1
m g is from the external surface and the mesoporous volume
3
ꢁ1
is increased to 0.16 cm
g . Therefore, a synthesis system
consisting of IL and the aluminum content in proper propor-
tions can lay a foundation for directly synthesizing hierarchi-
cally micro–mesoporous zeolite Y is reasonably concluded.
The SEM image (Fig. 6) shows that the products are synthe-
sized with different molar ratio of SiO
1 (Fig. 6a) displays chaotic block structure. With decreasing
SiO /Al O molar ratio, the morphology changes from a small
2 2 3
/Al O . The sample NaY-
1
2
2 3
block structure, to a “cauliower-like” structure (Fig. 6b), with
diameters of around 0.85 mm. Further increasing the aluminum
content maintains the cauliower-like nature, with a small
increase in size to 0.95 mm with a SiO
2 2 3
/Al O molar ratio of 9.5
(Fig. 6c), but with a much greater increase to 2.4 mm for a ratio
of 8.5 (Fig. 6d). Lowenstein's rule describes morphology varia-
tion where silicon species can link either silicon or aluminum
whereas aluminum species can only connect with silicon
Fig. 4 XRD patterns of these samples induced by various molar ratio
of SiO /Al O .
2 2 3
36
species. It suggests that the small block structure fabricating
the cauliower-like is relative to the low SiO /Al O ratio, in
2
2 3
than –Si–O–Si– bonds in an Al-rich synthesis system. It suggests
that high SiO /Al ratio is harmful for the crystallization
process. In addition, the other difference indicated by XRD is
a gradual increase in the diffraction peak widths as the SiO
Al molar ratio increased, which indicates that the size of the
agreement with the earlier reports that an Al-rich system could
2
2
O
3
37
accelerate the nucleation and growth of zeolites.
TEM images at high magnication reveal the presence of
disordered mesopores in those samples (the mesoporosity in the
crystal is marked by orange lines) (Fig. 7). The mesoporous
channels are present in the neighboring microporous crystalline
zeolite frameworks. The wormlike mesoporous structures are not
straight, and the width is differed, which may be attributed to the
aggregation of the template IL. Unlike other templates, this IL
template can be homogeneously dispersed in aluminosilicate gel
and well incorporated within the microporous zeolite frame-
works, which resulted in the formation of hierarchical zeolite Y.
It can be attributed to the templating and anchoring mechanism
2
/
2 3
O
35
crystalline domain decreased. The decrease in crystal sizes can
be conrmed by electron microscopic investigations.
N adsorption/desorption isotherms of samples are shown in
2
Fig. 5a, which exhibits type IV hysteresis loop characterizing the
presence of mesopores. In a sharp contrast, the adsorption of
NaY-9.5 in the medium-pressure region of 0.4–0.7 is the highest,
showing the volume of mesopores is largest than the others.
The pore size distributions shown in Fig. 5b give clear evidence
that the sample NaY-9.5 has a large pore size distribution at
15
of the bifunctional template IL, avoiding phase separation
3.89 nm. From the physicochemical properties summarized in
between the IL and aluminosilicate gel. The alkoxy-
Table 1, the proper molar ratio of SiO /Al O is 9.5, which
2
2 3
functionalized (CH O)–Si– bonds can quickly hydrolyze to
3
Fig. 5
2 2 2 3
N adsorption–desorption isotherms of the samples induced by different molar ratio of SiO /Al O (a), the corresponding pore size
distribution calculated by BJH model based on the desorption curves (b).
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Table 1 The specific surface area and pore structure parameters of the samples
a
2
ꢁ1
b
2
ꢁ1
3
ꢁ1
3
ꢁ1
c
d
Samples
S
BET (m g
)
S
ext (m g
)
V
micro (cm g
)
V
total (cm g
)
Pore size (nm)
2 2 3
SiO /Al O
NaY-11
NaY-10.5
NaY-9.5
NaY-8.5
NaY
479
690
713
632
595
24
76
90
57
11
0.11
0.28
0.26
0.24
0.29
0.16
0.39
0.42
0.32
0.31
3.89
3.89
3.89
3.89
0.74
5
4.6
4.0
3.8
4.3
a
b
c
d
BET method. t-plot method. BJH model applied to the desorption branch of the isotherms. EDX method.
ꢀ
desorption peaks at 300–400 and 500–600 C, corresponding to
38
medium and strong acid sites, respectively. When the SiO
2
/
Al ratio is 9.5, the peak for medium strong acid sites is
2 3
O
increased, whereas, the peak for strong acid sites is decreased.
Decreasing the aluminum content to 10.5, hierarchical zeolite
ꢀ
HY-10.5 (curve c) shows only one desorption peak at 300–500 C,
demonstrating that zeolite obtained in the present investigation
has only medium acidity. Both for sample HY-11 (curve d) and
sample HY (curve e), the temperature of the peaks maxima shi
toward lower temperature, which can be ascribed to the
6
decrease of the strength of acid site. For acid-catalyzed alkyl-
ation reaction, the weaker acidity may mean relatively lower
catalytic activity.
Fig. 6 SEM of the samples induced by different SiO
a) 11, (b) 10.5, (c) 9.5, and (d) 8.5.
2 2 3
/Al O
molar ratio 3.3 Theoretical study
(
The addition of IL into conventional synthesis compositions for
zeolite Y led to the introduction of mesopores. A strong inter-
action between the alkyloxy-functionalized IL and aluminosili-
cate gel presents the fact that IL is easily formed mesopore
structuring agent in alkaline media. Density functional theory
(DFT) calculations were applied to understand molecular
properties of IL. The optimized structure of the template IL is
shown in Fig. 9.
The reactive ability of the chemical species is considered to
be closely related to the frontier molecular orbitals, thus the
lowest unoccupied molecular orbital (LUMO) of the IL is
calculated at the B3LYP/6-31G* level. Fig. 10 shows the distri-
butions and energy levels of LUMO orbitals (ELUMO). According
to the frontier molecular orbital theory, ELUMO represents the
electron affinity and characterizes the ability of the molecule to
accept electrons. The lower value of ELUMO suggests the
molecule accepts electrons more probable. It is clearly shown
that the LUMO is mainly distributed on the imidazole ring,
which indicates that the region around the positively charged
imidazole ring is the preferred active sites for a nucleophilic
attack.
Fig. 7 TEM images of samples (a) NaY-11, (b) NaY-9.5, and (c) NaY-8.5
the mesoporosity in the crystal is marked by orange lines). The
pictures on the right of each image are the local enlargements.
39
(
HO–Si– and form rm –Si–O–Si–, –Si–O–Al– bonds incorporating
in the resultant aluminosilicate frameworks, while organic tail of
32
IL directs the mesoporous channels. Therefore, wormlike
mesoporous channels are constructed with microporous crys-
talline zeolite Y by calcination to eliminate the IL.
Molecular electrostatic potential (MEP) map is related to the
local electronic charge density. The MEP of the alkyloxy-
functionalized IL was calculated at the B3LYP/6-31G* opti-
mized geometry (Fig. 11). MEP map provides an approach for
identication of template attractive forces. Generation of MEP
map for this study provides justication for molecular
phenomena and is ideal for predicting a molecule's ability for
The acidity of aluminum-containing zeolites is seriously
inuenced by the local environment of Al atoms. The acid
properties of the zeolites Y have been measured by the NH3-
TPD, and the results are showing in Fig. 8. All samples had been
+
transformed into H form by ion exchange before measure-
28
electrophilic attack and nucleophilic reactions. The negative
ment. Apparently, hierarchical zeolite HY-8.5 (curve a) has two
(orange) regions of MEP are related to electrophilic reactivity
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Fig. 8 NH -TPD profiles of samples (a) HY-8.5, (b) HY-9.5, (c) HY-10.5, (d) HY-11 and (e) conventional HY.
Fig.
9 Optimized structure of the ionic liquid (white spheres,
hydrogen; gray spheres, carbon; blue sphere, nitrogen; red spheres,
oxygen; dark cyan sphere, silicon; green sphere, chlorine).
Fig. 11 Molecular electrostatic potential map of the IL. Orange regions
of the map are the most electron-rich regions of the molecule, and
yellow regions are electron poor. Order of increasing electron density
is yellow < orange < red.
and the positive (yellow) ones to nucleophilic reactivity.
As easily can be seen that the positive regions are mainly
over the imidazole ring, it is indicated that a nucleophile will
preferentially attack the positively charged imidazole ring.
The result is in agreement with the aforementioned frontier
orbitals analysis. Here, the anionic aluminosilicate gel that
may play a nucleophile role can interact with the cationic IL
by electrostatic interactions. In addition, the presence of
Fig. 10 The LUMO orbital density distribution for the ionic liquid
3 3
(CH O) Si– in the IL can hydrolyze to –Si–OH, and yield –Si–O–
(
white spheres, hydrogen; gray spheres, carbon; blue sphere, nitrogen;
red spheres oxygen; dark cyan sphere, silicon; green sphere, chlorine). Al– and –Si–O–Si– covalent linkages to be integrated into the
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Table 2 Activity of various catalysts in alkylation of o-xylene with
a
styrene
b
c
Catalyst
PXE yield (%)
PXE selectivity (%)
Conventional HY
HY-11
HY-10.5
HY-9.5
HY-8.5
12.67
67.77
82.01
91.73
71.37
14.49
67.9
82.93
93.76
73.57
a
Reaction conditions: o-xylene : styrene ¼ 7.5 : 1, reaction temperature
ꢀ
¼
120 C, reaction time ¼ 3.0 h, catalyst loading ¼ 20% (w/w of styrene).
b
c
Isolated yield based on the amount of styrene. Including all
products.
32
resultant zeolite framework, while the imidazole ring and alkyl
tail promotes the formation of mesoporosity within zeolite Y
crystals.
Fig. 12 Catalytic stability of the hierarchical HY-9.5 and conventional
HY catalysts in the alkylation of o-xylene with styrene.
3.4 Catalytic activity
To investigate the effect of the facile diffusion and acidity in the
hierarchical zeolites Y on catalytic performance, the alkylation
of o-xylene and styrene was chosen as a probe reaction, which
involves bulky molecules and can be catalyzed by medium
strong acid sites. The catalytic activity of different catalysts is
showed in Table 2. The detailed reaction scheme is shown in
Scheme 2: reaction (1) is the PXE formation reaction, whereas
reaction (2) and (3) display the formation of styrene oligomers
and more substitutes, respectively.
As listed in Table 2, all the hierarchical zeolites HY-x show
high catalytic activity and selectivity compared with the
conventional zeolite HY as expected. The high catalytic activity
of hierarchical zeolites HY-x can be attributed to a facile diffu-
sion due to its large mesoporous diameter and the relatively
high specic area of the external surfaces compared with
conventional HY. It is worth of noting that the sample HY-9.5
shows highest catalytic performances, indicating the unique
pore system and acidity play essential roles in the generation of
40
PXE during the alkylation process.
An important advantage of heterogeneous catalysis over the
homogeneous counterpart is the possibility of reusing the
41
catalyst by simple ltration, without loss of activity. The
catalytic reusability of the hierarchical HY-9.5 and conventional
HY catalysts was evaluated by carrying out the reaction with
used catalyst under the optimized conditions (Fig. 12). Aer
each run, the catalyst was recovered by ltration, followed by
ꢀ
drying, and then calcination at 550 C for 1 h before reuse. It is
noteworthy that the generation of PXE decreased with the
number of cycles for both hierarchical zeolite HY-9.5 and
conventional zeolite HY catalysts, but conventional zeolite HY
loses catalytic activity completely in four recycling experiments.
In contrast, the deactivation of hierarchical zeolite HY-9.5
catalyst is much slow and its PXE yield still maintains 84.2%
conversion even aer four recycling experiments. The
outstanding durability of the hierarchical HY-9.5 catalyst may
be attributed to its maximum medium strong acid amount and
the formation of mesopores, as acid sites located at the meso-
pore walls or external surface of the hierarchical zeolite can
catalyze reactions involving bulky molecules that cannot enter
the microporous framework.
4. Conclusions
In this work, zeolite Y with intracrystalline mesoporosity,
prepared by using ionic liquid as a mesopore director, is an
extraordinarily selective catalyst for the alkylation of o-xylene
with styrene. In addition to affording high catalytic activity, the
hierarchical HY-9.5 catalyst can be used for more than
four times without any signicant loss of activity. This
remarkable catalytic performance of hierarchical HY-9.5
Scheme 2 Reaction scheme of alkylation of o-xylene with styrene
over a heterogeneous catalyst.
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2
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(
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the Central Universities (3207046409, 3207046302 and
207045421) and a project funded by the Priority Academic
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