Effect of synthesis conditions on the structural and catalytic properties of hierarchically structured ZSM-5 zeolites

Article abstract of DOI:10.1039/c4ra00124a

Hierarchically structured ZSM-5 zeolites were synthesized by employing only one dual-porogenic surfactant, and the hydrothermal crystallization of hierarchical structured zeolites were investigated under rotating and static synthesis conditions. X-Ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption isotherms, ²⁷Al magic-angle spinning nuclear magnetic resonance (²⁷Al MAS NMR) and thermogravimetric analysis (TGA) were used to characterize the structural and textural features of the resultant zeolite products. By tuning the synthesis conditions from rotating to static, the morphology of the as-obtained hierarchical structure ZSM-5 zeolites changed from silk-like to nanoparticles with a disordered arrangement. Furthermore, the obtained zeolites under static conditions showed a higher BET surface area and lower total pore volume than those of zeolites synthesized under rotating conditions. In addition, quantum chemical calculation results show that the inner ammonium groups may have greater contribution in directing the zeolite structure than tailed ammonium groups. More importantly, the hierarchical structured ZSM-5 zeolites synthesized under static conditions exhibited a higher benzyl alcohol conversion rate and lower ester selectivity compared to those of the samples prepared under rotating conditions. Consideration of the effects of synthesis conditions may be useful for modulating the textural and catalytic properties of hierarchical zeolites.

Full text of DOI:10.1039/c4ra00124a

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Effect of synthesis conditions on the structural and
catalytic properties of hierarchically structured
ZSM-5 zeolites
Cite this: RSC Adv., 2014, 4, 13831
Baoyu Liu, Liming Zheng, Zhihong Zhu, Kai Zhang, Hongxia Xi* and Yu Qian
Hierarchically structured ZSM-5 zeolites were synthesized by employing only one dual-porogenic
surfactant, and the hydrothermal crystallization of hierarchical structured zeolites were investigated
under rotating and static synthesis conditions. X-Ray diffraction (XRD), Fourier-transform infrared
spectroscopy (FTIR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N
2
27
27
adsorption–desorption isotherms, Al magic-angle spinning nuclear magnetic resonance ( Al MAS
NMR) and thermogravimetric analysis (TGA) were used to characterize the structural and textural
features of the resultant zeolite products. By tuning the synthesis conditions from rotating to static, the
morphology of the as-obtained hierarchical structure ZSM-5 zeolites changed from silk-like to
nanoparticles with a disordered arrangement. Furthermore, the obtained zeolites under static conditions
showed a higher BET surface area and lower total pore volume than those of zeolites synthesized under
rotating conditions. In addition, quantum chemical calculation results show that the inner ammonium
groups may have greater contribution in directing the zeolite structure than tailed ammonium groups.
More importantly, the hierarchical structured ZSM-5 zeolites synthesized under static conditions
exhibited a higher benzyl alcohol conversion rate and lower ester selectivity compared to those of the
samples prepared under rotating conditions. Consideration of the effects of synthesis conditions may be
useful for modulating the textural and catalytic properties of hierarchical zeolites.
Received 7th January 2014
Accepted 3rd March 2014
DOI: 10.1039/c4ra00124a
www.rsc.org/advances
sites can increase the possibility of secondary reactions, which
1
. Introduction
can cause polymerization of by-products or reaction interme-
6
Zeolites are a family of crystalline aluminosilicate materials diates and result in the formation of cokes. Coke can cover
with a regular arrangement of uniform micropores and have active sites within the microporous channels and leads to
shown great (or potential) applications in many modern catalyst deactivation, decreasing catalytic lifetime in industrial
industrial processes related to catalysis, adsorption, and sepa- applications.
ration, especially in the eld of petrochemical industry due to
the fact that these materials possess high surface area, adjust- aluminosilicate materials with larger adjustable pore sizes,
To resolve the diffusive limitations, mesoporous silica and
1,2
7
8
able pore size, and high thermal and chemical stability. So far, such as MCM-41 and SBA-15 were successively developed in
00 structure types of zeolite have been crystallographically the 1990s. These materials have the ability to overcome the pore
classied. Conventional zeolite crystals are normally labeled as size constraints of microporous zeolites and allow the diffusion
2
‘
molecular sieves’ based on their small pore sizes. This of lager molecules. Unfortunately, these ordered mesoporous
molecular sieving’ effect has enabled the development of materials show low hydrothermal stability and acidity
‘
molecular size- or shape-selective applications. However, the compared to zeolites crystals, which can be attributed to the fact
sole presence of so many micropores oen imposes diffusion that the mesopore walls in ordinary mesoporous silica are built
limitations. Bulky molecules with sizes beyond the micropore with amorphous frameworks. The amorphous nature is a crit-
dimensions are excluded from the internal zeolite surface. ical factor that prevents their framework acidity from rising to a
Moreover, even if the smaller molecules could enter the zeolitic level that would satisfy use in practical applications, especially
micropores, the catalytic performance can be seriously limited in the petrochemical processes. Thus, it is highly desirable to
by the diffusion of reactants or products in the microporous fabricate a hierarchically structured zeolites that possess an
3–5
domains. Slow mass transport to and away from the activated open mesoporous structure in addition to microporosity
inherent in zeolites. This material can integrate the merits of
mesopores and the microporous crystalline structure of
zeolites. Mesopores in such crystalline materials are suitable for
School of Chemistry and Chemical Engineering, South China University of Technology,
Guangzhou, Guangdong 510640, P.R. China. E-mail: cehxxi@scut.edu.cn; Fax: +86
catalyzing reactions of bulky molecular species that cannot
0
20 87113735; Tel: +86 020 87113501
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ꢀ
diffuse into the microporous region, while the micropores could with diethyl ether, and dried in a vacuum oven at 50 C for 8 h.
+
provide a size- or shape-selective catalytic process for guest Second, 0.005 mol of the [C H –N (CH ) –C H –N(CH ) ]-
2
2
45
3 2
6
12
3 2
ꢁ
molecules. The hierarchically mesoporous-microporous struc- [Br ] and 0.0025 mol 1,6-dibromohexane (98%, J&K) were dis-
ture enables zeolites to have maximum structural functions in a solved in 40 mL of acetonitrile, then the mixture was reuxed
9
ꢀ
limited space.
with stirring for 12 h at 75 C, aer cooling to room temperature
In the past few decades, there were numerous reports and solvent evaporation, the precipitated product was collected
focusing on synthesizing mesoporous aluminosilicate materials by ltration and washed with diethyl ether, and dried in a
1
0–17
ꢀ
with zeolite-like frameworks.
included are the alkaline or acid leaching of bulk zeolite to [C22
Among them, typically vacuum oven at 50 C for 8 h to obtain the nal product, the
+
+
+
H
45–N (CH
3 2 6 3 2 6 3 2 6 12
) –C H12–N (CH ) –C H12–N (CH ) –C H –
fabrication of zeolites N (CH ) –C22H45][Br ] surfactant, which is hereby desig-
1
8–22
+
ꢁ
generate intracrystalline mesopores,
in the form of small nanoparticles in order to form mesoporous nated as C22–N –C22. The purity of the nal solid product was
channels by intercrystalline void space, adding the ‘hard’ analyzed by H-NMR measurements in a CD OD solution
3
2
4
4
14
1
3
templates as a mesopore structure-directing-agent (SDA) to with a Bruker Avance Digital 400 spectrometer.
23–25
prepare zeolites containing mesopores,
and incorporating
the ‘so’ templates into the zeolite framework as a mesopore-
2
.2. Synthesis of the hierarchically structured ZSM-5 zeolites
15,26,27
generating agent to synthesize the mesoporous zeolites.
In this respect, a great breakthrough has been made by
In a typical synthesis, a homogeneously mixed solution was rst
prepared from fumed Silica (99%, J&K), C –N –C surfactant,
NaAlO
H
N
aged under magnetic stirring at 65 C for 12 h. The nal gel
obtained was transferred into 50 mL Teon-lined stainless steel
autoclave and heated at 150 C under rotating and static
conditions for 5 days respectively. Aer crystallization, the
products were collected by ltration, dried in air, and calcined
at 580 C for 6 h to remove the template. The resulting hierar-
28,29
22
4
22
Ryoo's research group.
A series of gemini-type multi-
2
(44.7 wt% Na
2
O, 52 wt% Al
2
O
3
, J&K), NaOH, ethanol and
ammonium surfactants were used as structure directing agents
for hierarchically structured zeolites with crystalline micropo-
2
O with a molar ratio of 5.43Na
2
O–24SiO –0.6Al –1.2C22–
2
2
O
3
4
–C22–192ethanol–1706H
2
O. The resultant gel mixture was
30–33
rous frameworks.
These surfactants are composed of
ꢀ
hydrophilic quaternary ammonium groups and hydrophobic
alkyl chain moieties. The ammonium head groups caused the
silica precursor to form the microporous crystalline zeolitic
framework while the self-assembled surfactant tails directed the
zeolite crystal morphology into a mesostructure. These surfac-
tants played a dual structure-directing functional role as both a
zeolite-structure-directing agent and mesoporosity-generating
template in the absence of other SDAs. It is quite attractive to
fabricate hierarchical zeolites using such a dual-functional
surfactant approach.
ꢀ
ꢀ
chically structured ZSM-5 zeolites samples are denoted by ZSM-
5
a conventional ZSM-5 sample was purchased from Nankai
University catalyst Co., Ltd, which is denoted by ZSM-5-C.
-R (rotating) and ZSM-5-S (static) respectively. For comparison,
Herein, extending their work, we report the results obtained
by employing a bifunctional tetra-quaternary ammonium type
2.3. Characterization
surfactant as SDA to synthesize hierarchically structured ZSM-5 Powder X-ray diffraction (XRD) patterns were recorded on a
zeolites under static and rotating conditions. This study was Rigaku Rotaex Diffractometer using CuKa radiation (l ¼
˚
undertaken to investigate the effect of synthesis conditions for 1.5418 A). N adsorption–desorption isotherms were measured
2
zeolite structure generation. The zeolite structure, porosity, with a Micromeritics ASAP 2020 system at 77 K. The samples
ꢀ
morphology and catalytic performance were analyzed by were outgassed for 12 h at 150 C before the measurements. The
2
complementary combination of X-ray diffraction, N adsorp- specic surface areas of the materials were calculated from the
tion, electron microscopy and probe reaction. Besides, we study adsorption branch of the isotherm using the Brunauer–
the molecular properties of the surfactant by means of the Emmett–Teller (BET) equation, and the pore-size distribution
density functional theory (DFT) method. We report these results was analyzed by using the Barrett–Joyner–Halenda (BJH)
because this information may provide valuable guidance for method. Scanning Electron Microscopy (SEM) images were
practical industrial applications.
recorded with a Carl Zeiss, ZEISS Ultra 55 at a low landing
energy (5.0 kV). Samples were mounted using a conductive
carbon double-sided sticky belt, and a thin coating of gold
sputter was deposited onto the samples in order to reduce the
effects of charging. Transmission Electron Microscopy (TEM)
2. Experimental
2.1. Synthesis of organic surfactant
The tetra-quaternary ammonium surfactant is synthesized images were recorded with a JEOL JEM-2100 electron micro-
29
according to the literature procedures. In a typical synthesis, scope operated at 200 kV and equipped with a Gatan 832 CCD
rst 0.01 mol of 1-bromodocosane (98%, J&K) and 0.1 mol of camera. The FTIR spectra of samples in the form of KBr pellets

0
0
N,N,N ,N -tetramethyl-1,6-hexanediamine (99%, J&K) were dis- were recorded at room temperature on a Bruker Vector 33-IR
ꢁ
1
solved in 50 mL toluene–acetonitrile mixture with v/v ¼ 1 : 1, spectrometer with a resolution of 1 cm . The thermal behav-
ꢀ
then the mixture was reuxed with stirring for 8 h at 65 C, aer iour of a washed, as-synthesized sample was observed on a
ꢀ
cooling to room temperature and solvent evaporation, the Perkin Elmer Pyris 6 TGA analyzer with a heating rate of 5 C
+
ꢁ1
ꢁ1 27
precipitated product was [C22
[
H45–N (CH
3
)
2
–C
6
H
12–N(CH
3
)
2
]- min under a nitrogen ow of 30 mL min
.
Al MAS NMR
ꢁ
Br ]. The precipitate was collected by ltration and washed spectra were recorded on a Bruker Avance AV 400 spectrometer
13832 | RSC Adv., 2014, 4, 13831–13838
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at 104.26 MHz. A saturated aqueous aluminium sulfate solution
with a chemical shi 0.0 ppm was used as an external reference.
2.4. Calculations
DFT methods have become very popular recently due to accu-
racies that are similar to other methods while requiring less
time and smaller investments from a computational point of
34
view. In this work, a hybrid functional B3LYP level at the
-31G* basis set was adopted to calculate molecular structure
and electronic density of the cationic C22–N –C22 surfactant.
This basis set provided accurate geometry and electronic
6
4
35,36
properties for a wide range of organic compounds.
All the
calculations were performed using the Gaussian 03 program
package. Besides, molecular electrostatic potential (MEP) and
frontier molecular orbitals were determined at the same level of
theory B3LYP/6-31G* with the optimized structure.
Fig. 2 IR spectra of samples: (a) ZSM-5-R, (b) ZSM-5-S and (c) ZSM-5-C.
conditions are shown in Fig. 1. The XRD patterns conrm that
2.5. Catalytic reaction
Before the catalytic tests, all the samples were ion-exchanged all samples are highly crystallized ZSM-5 zeolite, exhibiting the
ꢀ
29
three times with 1 M NH NO solution at 90 C and then con- characteristics of a crystalline MFI reported by Ryoo et al.
4
3
+
verted to the H form via calcination in owing nitrogen (120 Comparing the as-synthesized and calcined ZSM-5-R with ZSM-
ꢁ1
ꢀ
mL min ) at 550 C for 6 h. Here, the esterication reaction of 5-S samples reveals that the diffraction patterns of ZSM-5-R
benzyl alcohol and hexanoic acid was chosen as a model reac- have a more broader peaks than those of ZSM-5-S samples
tion involving large molecules in order to compare the catalytic respectively. However, the XRD patterns of ZSM-5-R and ZSM-5-S
ability of ZSM-5-R, ZSM-5-S and ZSM-5-C. In a typical reaction, a samples are well matched to reections of ZSM-5-C sample, and
reaction mixture containing toluene (12 mL), benzyl alcohol (15 no other crystalline phase was detected. In addition, upon
mmol), hexanoic acid (18 mmol) and catalyst (0.2 g) was heated removal of the surfactant by calcinations at high temperature, the
ꢀ
under stirring at 110 C for 6 h in a three-necked ask with a intensity of Bragg reection of the calcined ZSM-5-R and ZSM-5-S
reux condenser under N atmosphere. Aer the reaction, the increases to some extent compared with the as-synthesized
2
mixture was centrifuged and analyzed using a GC-MS (QP2010) ZSM-5-R and ZSM-5-S respectively, suggesting that the calcination
and Waters HPLC system (UV detection wavelength was set at process may promote siloxane cross-linking and thereby some-
37
257 nm).
what improve the ordering of the framework.
The FT-IR test was carried out to further identify the crystal
ꢁ
1
phase (Fig. 2), all of the samples show a broad band at 451 cm
3
. Results and discussion
ꢁ
1
in the region of 400–500 cm , which corresponds to a Si–O
3
8
3.1. Characterization of the hierarchically structured ZSM-5
bending mode observed in all amorphous SiO2 materials.
ꢁ
1
zeolites
However, other another absorptions at 545, 553 and 551 cm
were detected in the spectrum of ZSM-5-C, ZSM-5-R and ZSM-5-S
respectively, which can be generally attributed to the ve-
The X-ray diffraction patterns of hierarchical structure ZSM-5
zeolite samples synthesized under rotating and static
3
9
membered rings of T–O–T (T ¼ Si or Al) in MFI-type zeolites,
indicating that the ZSM-5-R/ZSM-5-S samples contain the
primary units of zeolite MFI. In addition, a band is observed at
about 622 cm only in the ZSM-5-C sample, which is evidence
ꢁ1
40
for a higher extent of zeolitisation.
Fig. 3 shows the SEM and TEM images of ZSM-5-R and ZSM-
-S samples. The ZSM-5-S sample, shown in Fig. 3a, is
5
comprised of nanoparticles with a randomly organized struc-
ture, while ZSM-5-R sample exhibits a silk-like morphology
(Fig. 3b). No physically isolated crystals or particles of the two
phases were detected throughout the entire ZSM-5-R/ZSM-5-S
samples. SEM only shows the surface morphology of the hier-
archically structured ZSM-5 zeolites. A close examination on
each sample was conducted by TEM observation (results shown
in Fig. 3c–f). Fig. 3c shows the TEM image of ZSM-5-R, which
exhibits aggregation of short piece-like morphology, and
these individual short pieces were assembled irregularly.
Fig. 1 Wide-angle XRD patterns of (a) as-synthesized ZSM-5-R, (b)
ꢀ
calcined ZSM-5-R at 580 C, (c) as-synthesized ZSM-5-S, (d) calcined
ꢀ
ZSM-5-S at 580 C and (e) conventional ZSM-5.
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Fig.
4
Nitrogen adsorption–desorption isotherms and pore size
distributions of ZSM-5-S, ZSM-5-R and ZSM-5-C.
0.6), the ZSM-5-R and ZSM-5-S samples show different hyster-
esis loops, indicating they have different mesoporous and/or
macroporous structures. For the ZSM-5-R samples, the hyster-
esis loop has the general type A form, indicating that the
ZSM-5-R samples have well-dened cylindrical-like pore chan-
41
nels. For the ZSM-5-S samples, the hysteresis loop shows the
general type B form, which shows that the material has slit-
shaped pores, and the distribution of pore size and/or shape is
41
not well dened.
Correspondingly, the Barrett–Joyner–
Halenda (BJH) model was used to estimate the mesopore size
distributions of ZSM-5-R, ZSM-5-S and ZSM-5-C samples. The
results exhibit mesopores in the range of 2.3–31 nm, centered at
7.5 and 5.6 nm for ZSM-5-R and ZSM-5-S respectively. In
contrast, the ZSM-5-C sample does not contain any mesopores.
Fig. 3 SEM images of as-synthesized products: (a) ZSM-5-S and (b) A more detailed analysis (Table 1) reveals that the ZSM-5-S
ZSM-5-R; TEM images of as-synthesized samples: (c) and (d) for samples have higher BET surface areas and external surface
ZSM-5-R, (e) and (f) for ZSM-5-S at different magnifications.
areas compared with ZSM-5-R samples. However, the total pore
volumes and microporous volumes of ZSM-5-S are smaller than
those of ZSM-5-R. These different textural properties of the
Furthermore, the lattice fringe can be clearly seen in the high- ZSM-5-R and ZSM-5-S samples may be due to the different
resolution TEM (HR-TEM) image of the ZSM-5-R sample synthesis conditions of the zeolites.
2
7
(Fig. 3d). ZSM-5-S sample shown in Fig. 3e and f, however,
Al MAS NMR technique is a well-established tool to deter-
indicates a fully crystalline structure with mesopores pene- mine the coordinations of the aluminum species present in the
42
trating through the micropore nanoparticle frameworks. zeolite samples, and the chemical activity of aluminosilicate
Amorphous silica domains were not detected in electron materials is generally correlated to the presence of intra-frame-
43
microscopic images for either of the ZSM-5-R and ZSM-5-S work aluminium atoms incorporated into the silica framework.
samples. These results indicated that the morphology of nal The coordination state of Al in the ZSM-5-R and ZSM-5-S was
2
7
samples can be affected through rotating and static synthesis checked by Al MAS NMR spectroscopy (Fig. 5). It is clear that
conditions. both ZSM-5-R and ZSM-5-S exhibit a single resonance peak at a
N adsorption–desorption isotherms were used to reveal the chemical shi of 60.22 and 60.13 respectively, which can be
2
porosity features of parents ZSM-5-R and ZSM-5-S samples. assigned to the Al species with a tetrahedrally coordinated
Fig. 4 shows that the synthesized ZSM-5-R and ZSM-5-S samples framework. Compared with the tetrahedral Al signal, the absence
exhibit type-IV N
2
isotherms with a hysteresis loop corre- of a peak at 0 ppm in both ZSM-5-R and ZSM-5-S samples
sponding to capillary condensation at high relative pressure, suggests that there is no non-framework octahedrally coordi-
suggesting a relatively large mesopore size. Comparatively, no nated Al, which demonstrates that Al is entirely incorporated in
obvious N
2 0
condensation can be observed at P/P > 0.1 for ZSM- the framework of the sample. However, in the ZSM-5-R sample,
5-C zeolite, indicating a traditional microporous material the NMR peak corresponding to the intra-framework Al was
without mesopores. At relatively low pressures (P/P0 < 0.02 slightly broad compared with that of ZSM-5-S sample, which
corresponding to the micropore lling), all the samples have could be attributed to the different conditions employed during
similar isotherms, indicating that they have similar micropo- zeolite synthesis, resulting in slight differences in the environ-
rous structures. In the higher relative pressure region (P/P
0
>
ments around Al in the zeolite frameworks.
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Table 1 Textual properties of the selected samples
b
c
c
c
d
c
S
BET
S
ext
S
mic
V
tol
V
ext
V
mic
a
2
ꢁ1
2
ꢁ1
2
ꢁ1
3
ꢁ1
3
ꢁ1
3
ꢁ1
Sample
Si/Al
[m g
]
[m g
]
[m g
]
[cm g
]
[cm g
]
[cm g ]
ZSM-5-C
ZSM-5-S
ZSM-5-R
20
17
19
347
630
518
132
439
355
215
191
163
0.25
1.18
1.32
0.09
1.03
1.19
0.16
0.15
0.13
a
b
c
Si/Al ratio was obtained by ICP/AES analysis.
SBET is the specic BET surface area obtained from N adsorption. Micropore and external surface
2
d
areas and micropore volumes by t-plot method. Total pore volumes is obtained at P/P
0
¼ 0.95.
incorporated in the hierarchical structure. It is important to
note that the dual functional surfactant can interact strongly
16
with the zeolite framework, so it is removed in later stages.
However, compared with ZSM-5-R, the amount of the weight
loss for ZSM-5-S was lower in all temperature ranges. This result
indicates that the ZSM-5-R may have a relatively larger pore
volume than that of ZSM-5-S, which is consistent with the
2
results of textural analysis using N adsorption.
3
.2. Computational study
The surfactant used in the present work was composed of four
quaternary ammoniums connected with –C 12– alkyl spacers
and two hydrophobic –C22 45 alkyl tails. Such a cationic
6
H
H
amphiphilic surfactant could easily disperse into the synthesis
solution and assemble cooperatively with silicate anions to
generate hierarchical structure ZSM-5 zeolite, in which the
multi-ammonium head group acted as an effective structure-
directing agent for the microporous zeolite, while the hydro-
phobic interaction between the long-chain tails induced the
formation of a mesoscale micellar structure. Herein, quantum
chemical calculations were performed in order to investigate
the molecular structure and electronic density of the cationic
27
Fig. 5
Al MAS NMR spectra of (a) ZSM-5-R and (b) ZSM-5-S.
C
4
22–N –C22 surfactant. Molecular electrostatic potential (MEP)
is related to the electronic density and is a very useful tool in
understanding the sites for electrophilic attack and nucleo-
44
philic reactions. To the predict reactive sites for nucleophilic
attack for the title surfactant, MEP was calculated based on
optimized geometry (Fig. 7a). MEP image (Fig. 7b) shows that
the positive (blue/cyan) regions are mainly over the ammonium
groups, especially on the inner ammonium groups when
+
compared to the two tailed ammonium groups [N (CH
3
)
2
Fig. 6 Thermogravimetric (TG) curves of as-synthesized ZSM-5-R and
ZSM-5-S.
bonded directly to the C22H45 tail], indicating that the preferred
sites for nucleophilic attack that is relevant to anionic alumi-
nosilicate species are the inner ammonium groups, followed by
45
the tailed ammonium groups.
The frontier orbital of a chemical species is very important in
ZSM-5-R and ZSM-5-S samples using the C22–N –C22 template in dening its reactivity. Fig. 7c shows the electric/orbital density
4
Thermogravimetric analysis (TGA) of the as-synthesized
46
the presence of nitrogen is presented in Fig. 6. Both the ZSM-5-R distributions of LUMO (the lowest unoccupied molecular
and ZSM-5-S exhibit four distinct regions of weight loss. The orbital) for the surfactant. The LUMO represents the ability of
ꢀ

rst weight loss below 140 C is attributed to desorption of the molecule to accept electrons. The lower value of LUMO
physically adsorbed water in the porous material. Very signi- indicates the molecule accepts electrons more probable. As can
cant weight losses were recorded with three steps in the be seen from Fig. 7c, the LUMO is located in the inner two
ꢀ
ꢀ
ꢀ
temperature ranges of 140–310 C, 310–400 C and 400–700 C ammonium groups, indicating that the inner ammonium
respectively, which could be assigned reasonably to the groups may have more contribution as the zeolite-structure-
complete cracking and combustion of the surfactant directing function than two tailed ammonium groups. A similar
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Fig. 7 (a) Optimized structure (b) molecular electrostatic potential map and (c) molecular orbital surface and energy level of the ‘bifunctional’
cationic surfactant (atom legend: white ¼ H; gray ¼ C; blue ¼ N).
47
30
conclusion was drawn by Jung et al. and Park et al. who average pore diameter of ZSM-5-S is less than that of ZSM-5-R,
conrmed that the gemini-type surfactants containing only the indicating that ZSM-5-R has better diffusion property than that
tailed ammoniums were unable to generate any zeolite frame- of ZSM-5-S. However, the esterication reaction is mainly cata-
works at all.
lyzed by Brønsted acids located at the surface of the mesopore
48
walls. While ZSM-5-S has higher external surface area than
that of ZSM-5-R, which suggests that ZSM-5-S can supply more
active sites for the catalytic reaction than those of ZSM-5-R.
From this point of view, the external surface area played a
decisive factor. Thus, ZSM-5-S exhibited a higher conversion of
benzyl alcohol than that of ZSM-5-R. The ester selectivity of
ZSM-5-C, ZSM-5-R and ZSM-5-S was 91.4%, 64.9% and 53.6%,
respectively. The ZSM-5-C exhibited the highest ester selectivity
due to the sole presence of micropores inhibiting the dehy-
drative condensation between two alcohol molecules. The
average pore diameter may be responsible for the difference of
ester selectivity between ZSM-5-S and ZSM-5-R. The large
average pore diameter could increase the diffusion freedom of
moleculars, resulting in low selectivity. Apparently, the hierar-
chically structured ZSM-5 zeolites exhibit a remarkable benet
for the model esterication reaction of benzyl alcohol and
hexanoic acid. Furthermore, the hierarchically structured
zeolites can be extended to many other catalytic reactions
involving large molecules, where diffusion constraints and/or
adsorption of reactant molecules onto the acid sites are the
main concern.
3
.3. Catalytic activity
The liquid phase esterication reaction of benzyl alcohol and
hexanoic acid (Fig. 8) was employed as a probe reaction to
examine the catalytic performances of ZSM-5-R and ZSM-5-S in
a diffusion-constrained reaction. It can be seen from Fig. 8,
compared with ZSM-5-C, ZSM-5-R and ZSM-5-S exhibit higher
benzyl alcohol conversions. Because the esterication reaction
involves large molecular species, it may take place primarily at
the surface of mesopore walls. Thus, the low conversion of ZSM-
49
5
-C could be attributed to a diffusion limitations due to its small
pore diameter. The high catalytic activities of ZSM-5-R and ZSM-
-S indicate that interconnected micropore-to-mesopore and
5
mesopore-to-mesopore frameworks can lead to easier access to
the active sites for reactants and faster diffusion of products
during the reaction.
On the other hand, the ZSM-5-S exhibited a higher benzyl
alcohol conversion rate of 69% than that of ZSM-5-R (51%). We
attribute the difference between ZSM-5-S and ZSM-5-R to their
differences in external surfaces and average pore diameters. The
4. Conclusion
A tetra-quaternary ammonium type surfactant was employed as
SDA to synthesize hierarchically structured ZSM-5 zeolites
under rotating and static conditions in this study. By tuning the
synthesis conditions (rotating and static), the morphology and
porosity of the resultant zeolites were changed. The resulted
samples exhibited piece-like morphology and nanoparticle
frameworks under rotating and static conditions, respectively.
Besides that, the ZSM-5-S sample showed higher BET surface
areas and external surface areas than those of ZSM-5-R samples.
Nevertheless, the total pore volumes and microporous volumes
of ZSM-5-S were smaller compared to ZSM-5-R. In addition,
quantum chemical calculations were used to gain more infor-
mation about the molecular structure of the dual-functional
tetra-quaternary ammonium type surfactant. Moreover, the
hierarchical structure ZSM-5-R and ZSM-S zeolites showed
Fig. 8 Catalytic activity of ZSM-5-S, ZSM-5-R and ZSM-5-C in the
esterification reaction of benzyl alcohol and hexanoic acid.
13836 | RSC Adv., 2014, 4, 13831–13838
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different catalytic activities in case of esterication reaction of 14 L. Tosheva and V. P. Valtchev, Chem. Mater., 2005, 17, 2494–
benzyl alcohol and hexanoic acid. The ZSM-5-S exhibited a 2513.
higher benzyl alcohol conversion rate and lower ester selectivity 15 M. Choi, H. S. Cho, R. Srivastava, C. Venkatesan, D.-H. Choi
than those of ZSM-5-R. The present study indicates that the and R. Ryoo, Nat. Mater., 2006, 5, 718–723.
different synthesis conditions (rotating and static) can change 16 B. Y. Liu, Y. Z. Tan, Y. Q. Ren, C. Li, H. X. Xi and Y. Qian,
the textural and catalytic properties of hierarchical structure J. Mater. Chem., 2012, 22, 18631–18638.
ZSM-5 zeolites guided by C22–N –C22 templates. Further work 17 B. Y. Liu, C. Li, Y. Q. Ren, Y. Z. Tan, H. X. Xi and Y. Qian,
would be necessary for a detailed investigation concerning the Chem. Eng. J., 2012, 210, 96–102.
effects of synthesis conditions on the structural and catalytic 18 J. C. Groen, J. A. Moulijn and J. P ´e rez-Ram ´ı rez, J. Mater.
properties for targeted catalytic reactions. In a word, the effect Chem., 2006, 16, 2121–2131.
of synthesis conditions (rotating and static) provided here 19 J. C. Groen, W. Zhu, S. Brouwer, S. J. Huynink, F. Kapteijn,
4
would be helpful for investigating the hydrothermal crystalli-
zation of other zeolites under various synthesis conditions in
J. A. Moulijn and J. P ´e rez-Ram ´ı rez, J. Am. Chem. Soc., 2007,
129, 355–360.
the future. Further research is currently under way in our 20 A. Corma, V. Fornes, J. Martinez-Triguero and S. B. Pergher,
laboratory.
J. Catal., 1999, 186, 57–63.
2
1 J. C. Groen, L. A. A. Peffer, J. A. Moulijn and J. P ´e rez-Ram ´ı rez,
Microporous Mesoporous Mater., 2004, 69, 29–34.
2 J. C. Groen, L. A. A. Peffer, J. A. Moulijn and J. P ´e rez-Ram ´ı rez,
Chem.–Eur. J., 2005, 11, 4983–4994.
Acknowledgements
2
This work was supported by the National Natural Science
Foundation of China (no. 20936001 and 21176084), the 23 S. S. Kim, J. Shah and T. J. Pinnavaia, Chem. Mater., 2003, 15,
National High Technology Research and Development Program 1664–1668.
of China (no. 2013AA065005), SRFDP (no. 20130172110012) and 24 Y. Tao, H. Kanoh and K. Kaneko, J. Phys. Chem. B, 2003, 107,
Guangdong Natural Science Foundation (S2011 030001366) are
gratefully acknowledged.
10974–10976.
25 W. Fan, M. A. Snyder, S. Kumar, P.-S. Lee, W. C. Yoo,
A. V. McCormick, R. L. Penn, A. Stein and M. Tsapatsis,
Nat. Mater., 2008, 7, 984–991.
6 F.-S. Xiao, L. F. Wang, C. Y. Yin, K. F. Lin, Y. Di, J. X. Li,
R. R. Xu, D. S. Su, R. Schlgl, T. Yokoi and T. Tatsumi,
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