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413
complicated multisteps were usually involved for the dispersion
2. Experimental
and compatibility of the hard template in the synthesis solu-
tion because of the incompatibility between hard substrates and
precursor species [25]. In addition, most of generating mesopores
in zeolites by the hard template strategy show the characteristic of
isolated secondary porosity that are unsuitable for the diffusion of
large molecules.
Another method that is distinguished from hard templates
is soft templating route. In this approach, surfactant molecules
are self-organized into a supramolecular micelle that can be
used to synthesize hierarchical zeolites. The micelle is called
a soft template duo to the characteristic of flexibility of sur-
factant micelle during the zeolite synthesis processes. The
soft-templating approach has the higher versatility in tailoring
the mesoporous structure as compared with the hard-templates
mentioned above through the molecular manipulation of func-
tional groups and geometrical packing parameter of the surfactant
2.1. Synthesis of organic surfactant
The gemini-type tetraquaternary ammonium surfactant is
synthesized according to the literature procedures [36]. In a
typical synthesis, first 0.01 mol of 1-Bromohexadecane (98%, J&K)
ꢀ
ꢀ
and 0.1 mol of N,N,N ,N -tetramethyl-1,6-hexanediamine (99%,
J&K) were dissolved in 50 mL toluene/acetonitrile mixture with
v/v = 1:1; then the mixture was refluxed with stirred for 8 h at 65 C;
after cooling to room temperature and solvent evaporation, the pre-
cipitated product was [C16H33–N (CH ) –C H –N(CH ) ][Br ].
◦
+
−
3
2
6
12
3 2
The precipitate was collected by filtration and washed with diethyl
◦
ether, and dried in a vacuum oven at 50 C for 8 h. Second, 0.005 mol
+
−
ꢀ
[C16H33–N (CH ) –C H –N(CH ) ][Br ]and 0.0025 mol ␣,␣ -
3
2
6
12
3 2
dichloro-p-xylene (98%, J&K) were dissolved in 25 mL of
chloroform, which was refluxed with stirred for 12 h at 62 C.
◦
[
26–28]. However, it is difficult for the surfactants to mod-
After cooling to room temperature and solvent evaporation, the
precipitate was filtered and washed with diethyl ether, and dried
ulate the zeolite crystal growth into a mesoporous structure
due to the strong phase-separation tendency between the
employed ordinary organic surfactants and the rapidly growing
zeolite crystals. Thus a rationally designed surfactant molecule
with functional groups is the key to control the interac-
tions between these templates and zeolitic species during
crystallization. In this respect, substantial efforts have been
made by different groups [29–32]. Among them, the typi-
cal ones include cationic or silylated polymer and amphiphilic
organosilane. For example, Choi et al. synthesized microporous
crystalline molecular sieves with a mesoporous skeleton by using
amphiphilic organosilane molecules as a meso-structuring agent
in 2006 [10]. Wang et al. used mesoscale cationic polymers
such as polymers polydiallyldimethylammonium chloride (PDAD-
MAC) and the dimethyldiallyl ammonium chloride acrylamide
copolymer (PDDAM) to prepare the hierarchical mesoporous zeo-
lites [33]. Wang and Pinnavaia prepared MSU-MFI with small
intracrystal mesopores (average pore size 2.0–3.0 nm) using a
silane-functionalized polyethylenimine as the mesoporogen [11].
Srivastava et al. synthesized nanocrystalline MFI-zeolites with
intracrystal mesopores by using alkyltriethoxysilane [31]. Among
these studies, an advanced preparation method was proposed
by Ryoo’s group [34–36] for the preparation of single-unit-cell
nanosheets of zeolite and mesoporous molecular sieves by using
specially designed surfactant. In 2011, Na and co-workers [36]
reported on the synthesis of mesoporous molecular sieves pos-
sessing crystalline microporous walls with zeolitelike frameworks
using special designed surfactant. In this process, the surfac-
tant played a dual-functional role as both microposity-directing
agent and mesoposity-generating template. In a following research,
a hierarchical mesopore–micropore Beta zeolite was prepared
using various surfactants that can function as a hierarchical
structure-directing agent [37]. In our previous work, we prepared a
mesoporous ZSM-5 type molecular sieve through a dual-functional
surfactant [38].
◦
in a vacuum oven at 50 C for 8 h to obtain the final product, the
+
+
+
[C16H33–N (CH ) –C H –N (CH ) –CH –(p–C H )–CH –N (CH )
3
2
6
12
3
2
2
6
4
2
3
+
−
−
–C H –N (CH ) –C16H33][Br ] [Cl ] surfactant; it is desig-
2
6
12
3
2
2
2
nated as Ph(C–N–C –N–C ) .
6
16 2
2.2. Synthesis of the hierarchically structured Beta zeolite
In a typical synthesis, a homogeneously mixed solution was
first prepared from tetraethylorthosilicate (TEOS, 98%, J&K),
Ph(C–N–C –N–C16)2 surfactant, NaAlO2 (44.7 wt% Na O, 52 wt%
6
2
Al O , J&K), NaOH, ethanol and H O with a molar ratio of 3.58
2
3
2
Na O:16 SiO :0.4 Al O :0.8 Ph(C–N–C –N–C16)2 surfactant:128
2
2
2
3
6
EtOH:1143 H O. The resultant gel mixture was aged under mag-
2
◦
netic stirring at 65 C for 12 h. The final gel obtained was transferred
into 50 mL Teflon-lined stainless steel autoclave and heated at
◦
150 C for 5 days. After crystallization, the product was collected
◦
by filtration, dried in air, and calcined at 580 C for 6 h to remove
the template. The resulting hierarchically structured Beta zeolite
sample is denoted by Beta-H. For comparison, conventional Beta
sample was prepared in this study following the literature reports
[39], which is denoted by Beta-C.
2.3. Characterization
Powder X-ray diffraction (XRD) patterns were recorded on a
Rigaku Rotaflex Diffractometer using CuK␣ radiation (ꢀ = 1. 5418 A˚ ).
Scanning electron microscopy (SEM) images were recorded on a
JSM-6330F electron microscope at an acceleration voltage of 10 kV.
Transmission electron microscopy (TEM) images were obtained
with a JEM-2100HR electron microscope operated at 200 kV. The
FTIR spectra of samples in the form of KBr pellets were recorded
at room temperature on a Bruker Vector 33-IR spectrometer with
−
1
a resolution of 1 cm . The thermal behavior of a washed, as-
synthesized sample was followed on a Perkin-Elmer Pyris 6 TGA
◦
In the present work, extending the synthesis strategy, a meso-
porous/microporous hierarchically structured Beta zeolite was
synthesized by employing a single gemini-type tetraquaternary
ammonium surfactant, which could generate micropores and
mesopores simultaneously. The mesopores were disordered and
the zeolite frameworks were highly crystalline, and this mate-
rial exhibited the high steam stability. The catalytic activity for
the esterification reaction of benzyl alcohol with hexanoic involv-
ing large organic molecules was tested as a probe reaction using
the prepared hierarchically structured Beta zeolites. Results indi-
cated that the hierarchically structured Beta exhibited an improved
catalytic performance compared with conventional Beta zeolites
featuring only micropores.
analyzer with a heating rate of 5 C/min under a nitrogen flow of
30 mL/min. N2 adsorption–desorption isotherms were measured
with a Micromeritics ASAP 2020 system at 77 K. The samples were
◦
outgassed for 12 h at 150 C before the measurements. The spe-
cific surface areas of materials were calculated from the adsorption
branch of the isotherm using the Brunauer–Emmett–Teller (BET)
equation, and the pore-size distribution was analyzed by using
the Barrett–Joyner–Halenda (BJH) method. 27Al MAS NMR spec-
tra were recorded on a Bruker Avance AV 400 spectrometer at
104.26 MHz. A saturated aqueous aluminum sulfate solution with a
chemical shift 0.0 ppm was used as an external reference. NH –TPD
3
was performed on a homemade apparatus; 50 mg powder sam-
ple was placed in a quartz tubular reactor and heated at a rate of