M. Liu et al.
technological applications [1–3]. However, it has been
repeatedly shown that purely microporous network often
imposes limitations on mass transfer rate that hinders
zeolites catalytic reaction involving large molecules [1–5].
To minimize or even eliminate these limitations, a number
of strategies had been proposed by researchers, such as
synthesis of nanosized zeolite crystals to shorten the
microporous diffusion path and ordered mesoporous silica
materials. Nevertheless, the application of these materials
was limited because of difficulties in separating zeolite
nanocrystals from reaction mixture and intrinsic weakness
in terms of hydrothermal stability and acidity [4–10].
Recently, hierarchical zeolites have stimulated much
research because they can not only enhance diffusion rate
but also retain stability and strong acidity, like conven-
tional microporous zeolites [10, 11]. Generally, a second
pore system in zeolite crystals can be introduced via two
ways. One is the top-down method that zeolites are syn-
thesized firstly and mesopores are then obtained by
leaching the framework atoms selectively (e.g. desilication
and dealumination) [12, 13]. This method can hardly
control the mesoporsity of zeolites and may lead to partial
destruction of zeolite structures [14]. The other is the
bottom-up way in which the second templates (e.g. carbon
and organic polymers) are incorporated into the zeolite
synthesis gel [15, 16]. It offers the opportunity to generate
controllable mesoporous system based on the molecular
size, the crystallization temperature or the amount of the
mesoporous additives [4, 7, 9, 10].
and acted as a high efficient catalyst for carbonylation of
dimethyl ether [22]. Nevertheless, the resultant zeolites did
not have larger external surface areas because of the larger
assemble size of nanosheets. Therefore, its catalytic perfor-
mances of reactions involving macromolecules should be
investigated further.
Herein, Hi-MOR nanosheets were synthesized success-
fully in 2 days by self-assembly between subnanocrystals
and industrial products monovalent cationic surfactant (i.e.
CTAB) and polyethylene glycol (PEG). The competition
between self-assembly of CTAB and zeolites crystallization
was restrained due to the combination of CTAB and PEG by
electrostatic attraction. The interplay between CTAB/PEG
and the charge balancing effects of the inorganic cations may
lead to the formation of sheet morphology [23]. Hierarchical
pore systems provided a large quantity of Brønsted acid sites.
For the toluene disproportionation, Hi-MOR showed a
higher activity, xylene selectivity and a longer catalytic life
than conventional MOR.
2 Experimental
5.5 g of NaCl and 11.00 g of Al2(SO4)3Á18H2O, were
dissolved in 40.00 ml of deionized water, followed by
addition of 3.60 g of H2SO4 (98 wt%, Sinopharm Chem.
Reagent Co., AR), the result transparent solution was
labeled A. Then, 111.00 g of Na2SiO3Á9H2O was added
into solution A dropwise under continuous stirring condi-
tion to obtain a gel. After stirring for 10 min at room
temperature, 12.00 ml NH3ÁH2O (22–25 wt%, Sinopharm
Chem. Reagent Co., AR) and 4.50 g of mordenite seed (Si/
Al = 20) were introduced into the gel. The gel mixture was
left for 12 h at 303 K for aging, 4.75 g of hexadecyl tri-
methyl ammonium bromide and 4.75 g of polyethylene
glycol (PEG10000) were then added. After stirring for 3 h
at room temperature, the gel mixture was transferred into a
stainless-steel autoclave lined with polytetrafluoroethylene
(PTFE) to crystallize at 448 K for 48 h under autogeneous
pressure. Finally, being filtered at room temperature and
dried at about 363 K overnight, the obtained crystalline
products were calcined at 823 K for 4 h to move templates.
For comparison, conventional mordenite (C-MOR) zeolite
with particle size of 5–7 lm was synthesized under the
similar procedures except for the absence of CTAB and
PEG and used as seeds.
Mordenite (MOR) is a zeolite with two different channels
lacking of interconnectivity, i.e., two porous sizes of
˚
6.5 9 7.0 and 2.6 9 5.7 A, but the latter is so small that
most molecules cannot enter, limiting diffusion efficiency as
catalysts or absorbents. To overcome these disadvantages,
Hi-MOR had been prepared by both top-down and bottom-
up methods [17–22]. Li et al. prepared mesoporous mor-
denite by three methods (i.e. leaching, soft templating and
hard templating) and found that amphiphilic organosilane
and carbon as second template could not generate real
mesoporous mordenite [18]. MOR nanorods and nanosheets
were synthesized by using designed multivalent cationic
surfactants [20]. However, when monovalent cationic sur-
factant or hexadecyl trimethyl ammonium bromide (CTAB)
was selected as capping agent, MOR-bulk was obtained. In
addition, the complex synthesis routine of multivalent sur-
factants would make the Hi-MOR zeolites costly. Ryoo et al.
synthesized Hi-MOR zeolites in the presence of organosi-
lane surfactant by adding bulk crystal seed way [21]. Owing
to the low decomposition temperature of organosilane,
4 days were still needed to crystallization though the bulk
zeolite seeds could accelerate the crystallization process.
More recently, it is reported that H-mordenite nanosheets
assemblies were prepared without using any template agent
3 Results and Discussion
Figure 1a and Fig. S1 (Supporting Information) show the
X-ray diffraction (XRD) patterns and FT-IR spectrum of
Hi-MOR and conventional MOR, respectively. It
123
Liu, Min
