Competition balance between mesoporous self-assembly and crystallization of zeolite: A key to the formation of mesoporous zeolite

Article abstract of DOI:10.1016/j.jallcom.2012.12.111

A hierarchical meso-/microporous aluminosilicate has been synthesized through kinetic control over the competition balance between mesoporous self-assembly and microporous zeolite crystallization by using hexadecyl trimethyl ammonium bromide (CTAB) and tetrabutylammonium hydroxide (TBAOH) as meso- and micro-porogens, respectively. A very small pH value range of 11.10-11.30, which can be well tuned by added ethanol volume fraction and/or NaOH addition, was found to be suitable for the formation of mesoporous zeolite without phase separation. Balanced inorganic species adsorptions onto the meso- and microtemplates, and the subsequent electrostatic interaction between such adsorption-formed meso- and micro-colloids are discussed and proposed to be the two key underlined mechanisms in the successful synthesis. The material shows perfect crystallization of zeolite frameworks and relatively high surface area and meso-/micropore volumes. The prepared mesoporous zeolite showed much higher catalytic activity in the reaction between lauric acid and ethanol than those when using both amorphous mesoporous materials and conventional ZSM-5 zeolite as catalysts.

Full text of DOI:10.1016/j.jallcom.2012.12.111

Journal of Alloys and Compounds 556 (2013) 71–78
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Competition balance between mesoporous self-assembly and crystallization
of zeolite: A key to the formation of mesoporous zeolite
Hua Li a,1, Huazhong Wu b, Jian-lin Shi c,
⇑
a Department of Inorganic Materials, College of Chemistry Chemical Engineering and Materials Science, Soochow University, 199 Renai Road, Suzhou 215123, Jiangsu Province,
PR China
b Department of Chemistry and Chemical Engineering, MinJiang University, Fuzhou 350108, Fujian Province, PR China
c State Key Lab of High Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, 1295 DingXi Road, Shanghai 200050,
PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A hierarchical meso-/microporous aluminosilicate has been synthesized through kinetic control over the
competition balance between mesoporous self-assembly and microporous zeolite crystallization by using
hexadecyl trimethyl ammonium bromide (CTAB) and tetrabutylammonium hydroxide (TBAOH) as meso-
and micro-porogens, respectively. A very small pH value range of 11.10–11.30, which can be well tuned
by added ethanol volume fraction and/or NaOH addition, was found to be suitable for the formation of
mesoporous zeolite without phase separation. Balanced inorganic species adsorptions onto the meso-
and microtemplates, and the subsequent electrostatic interaction between such adsorption-formed
meso- and micro-colloids are discussed and proposed to be the two key underlined mechanisms in the
successful synthesis. The material shows perfect crystallization of zeolite frameworks and relatively high
surface area and meso-/micropore volumes. The prepared mesoporous zeolite showed much higher cat-
alytic activity in the reaction between lauric acid and ethanol than those when using both amorphous
mesoporous materials and conventional ZSM-5 zeolite as catalysts.
Received 9 November 2012
Received in revised form 20 December 2012
Accepted 20 December 2012
Available online 29 December 2012
Keywords:
Hierarchical porous structure
Mesoporous materials
Zeolites
Kinetics
Competition balance
Heterogeneous catalysis
Ó 2012 Elsevier B.V. All rights reserved.
1. Introduction
synthesized mesoporous aluminosilicates [7,8], typically the
steaming-assisted crystallization methods; (3) the assembly of
A series of crystalline microporous aluminosilicates, e.g., ZSM-5
type zeolites, are widely used in catalysis, separation and environ-
mental fields [1,2] due to their high hydrothermal stability and
strong solid acidity. The catalytic activity of zeolite, however, is
inevitably subject to the diffusion limitations of bulky reactants/
products in their micropore systems (typically smaller than
1.2 nm), especially in the reactions involving macromolecules
[3,4]. To solve this problem, the introduction of nanosized pore
network into the zeolites may help. Thus, the synthesis of hierar-
chically porous zeolites has attracted extensive attentions among
material and catalysis researchers.
To date, a number of promising strategies have been developed
for synthesizing hierarchically porous zeolites, most of which were
mesoporous zeolites. These approaches can be summarized briefly
as follows: (1) dealumination or sequential desilication–dealumi-
nation [5,6] which involves post-etching of intact zeolites to gen-
erate mesoporosity; (2) crystallization of the framework of pre-
nanosized zeolite crystallites [9], in which the mesopores were
formed among the crystallites. Generally, in later two routes, tet-
rapropylammonium (TPAOH) or tetrabutylammonium hydroxide
(TBAOH) was commonly used as micropore structure-directing
agent (SDA) for ZSM-5 zeolites while various hard or soft tem-
plates were used as mesoporogens. By using hard templates, such
as various carbon materials and mono-disperse polystyrene (PS)
spheres, complicated multi-steps were usually involved due to
the incompatibility between hard substrates and precursor species
[10], and in some cases isolated secondary porosity would form,
which was inapplicable for the diffusion of large molecules [11].
In recent years, some kinds of soft templating methods
[10,12,13] including cationic or silylated polymer and amphiphilic
organosilane have been successfully employed as templates in gen-
erating hierarchical structures without phase separation between
the mesophase and crystalline zeolite. For example, stable single-
unit-cell nanosheets of zeolite MFI [14] were firstly fabricated with
a specially designed di-quaternary ammonium-type surfactant
(C22-6-6), in which diammonium head group acted as an effective
structure-directing agent for the MFI zeolite and the hydrophobic
interaction between the long-chain tails induced the formation of
mesoscale micellar structure. These routes, unfortunately, required
⇑
Corresponding author.
E-mail addresses: lihua123@suda.edu.cn (H. Li), jlshi@sunm.shcnc.ac.cn (J.-l.
Shi).
1
Tel./fax: +86 512 65880089.
0925-8388/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.jallcom.2012.12.111

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H. Li et al. / Journal of Alloys and Compounds 556 (2013) 71–78
uniquely designed templates, which were synthesized by very com-
plex processes and unavailable on market up to date.
amorphous zeolite seeds further assemble with each other to form
mesoporous zeolite precursor by using CTAB as mesotemplate and
ethanol as additive. And the final hierarchical porous zeolite was
obtained through a steam-assisted crystallization (SAC) method.
However, the synthesis was found very sensitive to experimental
parameters, which was similar to most of literature reports. It is
important to find the underlined key factors affecting the success-
ful synthesis of mesoporous zeolites. In the present work, we re-
port our findings of the key factor governing the competition
between mesoporous self-assembly and crystallization of zeolite
through adjusting the amount of used ethanol and pH value, and
the balanced inorganic species adsorption competition onto the
meso- and microtemplates and electrostatic interaction between
meso- and microtemplate-based colloids were proposed to be
the underlined mechanisms responsible for the formation of mes-
oporous zeolite.
In contrast to the special template above mentioned, it would
be facile to prepare hierarchical zeolites by using ordinary surfac-
tants as the mesoporogens, such as hexadecyl trimethyl ammo-
nium bromide (CTAB) [15,16]. However, synthetic strategy using
ordinary surfactant template still remains a great challenge in syn-
thesizing mesoporous zeolites: either phase separation between
meso- and micro-phases or non-crystallization of the framework
was almost unavoidable. The failure in the synthesis of mesopor-
ous zeolite using conventional soft templates has been mainly
attributed to the weak binding between these soft templates and
silicate species [17–19]. In addition to this, the size and property
of zeolite seeds [11,20,21] were also found to be an important fac-
tor. For example, last year, our group reported that the aging time
of zeolite seeds could strongly influence the synthesis of hierarchi-
cal porous zeolite in a hydrothermal route in which zeolite seed
size was found to be a key factor responsible for the mesoporous
zeolite by matching CTAB-assisted mesostructure formation and
zeolite crystallization [11]. In such a report, ethanol was added
to the reaction mixture to stabilize those zeolite seeds. In a word,
as the crystallization of zeolites is kinetically a very slow process
compared with the rapid formation of the mesoporous phase at
low temperature while a rapid crystallization happens once it
reached a relatively high temperature (usually 150 °C for ZSM-5),
therefore, a mismatch between kinetics and thermodynamics will
easily result in the failure in the fabrication of mesoporous zeolites.
Apart from the synthetic factors mentioned above, the phase
separation may possibly happen from the beginning of synthesis.
From the kinetic point of view, there are two factors which need
to be addressed. First one is the adsorption competition: mesotem-
plates (for example, CTAB micelles) and microtemplates (for exam-
ple, TPAOH) would compete with each other to adsorb silicate
species (namely, adsorption competition), which would result in
phase separation once two kinds of porogens were mixed together.
If mesotemplates are more attractive to silicate species than micro-
templates, the silicate species previously adsorbed on the micro-
templates would tend to be desorbed from them and transfers
onto mesotemplates. As a result, mesoporous phase with non-crys-
talline framework would be achieved. In contrast, the crystalline
microporous zeolites without intracrystal mesoporous structure
could be obtained. If the conditions were in between the above
two extreme cases, either a mixture of zeolite crystal and meso-
phase, or well-crystallized mesoporous zeolite would be obtained,
which was decided by the competition balance between these two
adsorption competitions. The second one is colloid interaction. The
interaction between two kinds of organic/inorganic composites,
i.e., meso-colloids of silicate species adsorbed onto mesotemplates,
(i.e., CTAB micelle/inorganic composite colloid) and micro-colloids
of silicate species adsorbed onto microtemplates (i.e., TBAOH/inor-
ganic composite colloid), respectively, would decide whether the
phase separation would happen or not in the final product. Two or-
ganic/inorganic composite colloids may have different surface
charges due to different pH value or zeta potential when silicate
species were adsorbed onto different porogens. The phase separa-
tion would not happen under the condition that the electrostatic
repulsion between two kinds of composite colloids was so low as
not to separate them with each other. As the pH value, i.e., Zeta po-
tential plays a decisive role in modulating the electrostatic repul-
sion/attraction, therefore, it is highly valuable to explore the
relationship between the formation of mesoporous zeolite and
pH value.
2. Experimental
2.1. Materials synthesis
Sample Hp-E was prepared according to the procedure which was described
elsewhere (Hp-ZSM) [16], however, with lots of improvements. Briefly, three inde-
pendent steps were involved in the synthesis of Hp-E: first, the synthesis of amor-
phous zeolite seeds in aqueous solution, second, mesoscale self-assembly among
zeolite seeds as inorganic source and CTAB as mesotemplate using ethanol and dis-
tilled water as mixed solvent, and finally, the steam-assisted crystallization by
employing the dried meso-/micro-amorphous materials as precursor. Hp-Na was
prepared similar to Hp-E but with NaOH (0.9 ml, 0.5 mol/L aqueous solution) being
added to adjust pH value. The factors influencing the final structure, including the
amount of ethanol and pH value, were investigated. Herein, according to the above
synthetic conditions, the products were named respectively as Hp-E(X) and Hp-
Na(X), here E and Na are the abbreviations of ethanol and NaOH, respectively, while
X refers to the volume (ml) of ethanol in the mixing solution.
The synthesis of wet gel was basically the same as in the previous report and
the whole stirring process was carried out at 35 °C. The zeolite seeds solvent was
statically aged at room temperature for 24 h. Therein, the molar ratios of the dried
gel mixture were Al2O3:SiO2:Na2O:TBAOH:CTAB = 0.017:1:0.017:0.153:0.099 for
Hp-E(X) and Al2O3:SiO2:Na2O:TBAOH:CTAB = 0.017:1:0.025:0.153:0.099 for Hp-
Na(X), respectively. During all reactions, the total volume was kept at 21 ml by
using EtOH and H2O as mixed solvents and the amount of TEOS was fixed at
0.66 mol/L. All of the gels were dried at 50 °C. The dried gel was steaming-thermally
treated at 155 °C for 40 h. Afterwords, the products were washed repeatedly with
distilled water, dried at 100 °C and then calcined at 550 °C for 6 h to remove the or-
ganic agents. The effects of pH value and ethanol were the focuses of this study.
The synthetic process of ZSM-5 was synthesized using conventional hydrother-
mal route, in which the mixed sol of precursors was stirred to form a gel, then aged
at ambient temperature and finally, hydrothermally theated at 155 °C for 72 h.
2.2. Characterization
Powder XRD patterns were recorded by using a Rigaku D/Max 2200PC diffrac-
tometer with Cu Ka
radiation (40 kV and 40 mA) with a scanning rate of 0.6 minÀ1
for small-angle testing and 10° minÀ1 for large-angle testing. The N2 sorption iso-
therms were measured using Micromeritics ASAP 2020 porosimeters at 77 K. The
mesoporous specific surface area, pore-size distribution, and pore volume were cal-
culated using the Brunauner–Emmett–Teller (BET) and Barrett–Joyner–Halenda
(BJH) methods, respectively. The micropore specific surface area and volume were
calculated by the t-plot method. FE-SEM (field-emission-scanning electron micros-
copy) analysis was performed on a Hitachi S4800 electron microscope. TEM images
were obtained on a JEOL-2010F electron microscope operated at 200 kV.
2.3. Catalytic reaction
The catalytic reactions were carried out in a three-necked round bottom flask
equipped with a reflux condenser (351 2 K), a thermometer and a sampling sys-
tem. The whole system was kept in an oil bath, which was placed upon a magnetic
stirrer. A certain amount of lauric acid and ethanol to form totally 5 ml mixture
with the molar ratio between lauric acid and ethanol fixed at 1:4. Then, 0.1 g cata-
lyst was added into the mixture. The yield of ethyl laurate was determined period-
ically by using GC–MS (Agilent, 6890/5973 N).
Recently, we reported a simple route to synthesize the hierar-
chical porous zeolite [16]. Our process started from the synthesis
of zeolite seeds solution by using TBAOH as microtemplate, and
then a mesoscale self-assembly process was applied to allow the

H. Li et al. / Journal of Alloys and Compounds 556 (2013) 71–78
73
Ethanol is miscible with water. When ethanol was added into
the reactive solution, the solubility of surfactant CTAB would in-
crease as the dielectric constant decreased in the whole solution.
This change enhanced electrostatic repulsion among surfactant
ions within micelles and weakened their hydrophobicity, which
would lead to decreased stability of surfactant micelles and in-
creased CMC value (critical micelle concentration) as well as de-
creased micellar size [22]. Specially, micelles could not form
under an excessive amount of ethanol in the mixing solution
(Hp-E(17)). A similar phenomenon was also found in the mesopor-
ous zeolite synthesis by hydrothermal route [11].
3. Results and discussion
As discussed above, the phase separation could happen from the
beginning of reaction, depending on different experimental param-
eters. In our experiments, pH value was primarily explored since it
would affect the aggregation of mesotemplates, zeta potential in
the mixed system and the electrostatic interaction among the tem-
plates and inorganic species.
3.1. Effect of ethanol volume on pH value in the reaction system
Fig. 2a shows an apparently increased 2-theta value in SAXRD at
the increased ethanol adding amount, implying that the micelle
size and the corresponding mesopore size in the self-assembled
products became smaller.
The relationship between the pH value and the ethanol adding
amount under the presence (Hp-Na) and absence (Hp-E) of NaOH,
and also the phase diagram for the mesoporous zeolite formation,
were given in Fig. 1. The pH values of every sample can be found
according to their ethanol volumes indicated in the sample names.
It can be seen that the pH value increases with the increase of eth-
anol amount for both Hp-E(X) and Hp-Na(X). The pH values for all
Hp-Na(X)-type samples are higher than those of Hp-E(X) at the
same amount of ethanol added due to the addition of NaOH. The
shaded area was found in this study to be a zone in which meso-
porous zeolite could be obtained. Apart from this area, as we found
in this report, only phase-separated mixture of zeolite crystals and
amorphous mesophase, or amorphous mesoporous materials could
be observed. The pH value for the formation of microporous zeo-
lites was also indicated at 11.9 in the figure.
The effect of pH value on mesoporosity can be essentially attrib-
uted to the concentration of counter ions (OHÀ) to CTAB micelles.
Counter ions, OHÀ, can compress the double-layer of micelles
and also reduce electrostatic repulsion among those surfactant
ions in a micelle by compensating the positive charges of CTAB
when entering the core of CTAB micelles, which will enhance the
stability of the CTAB micelles. Therefore, when NaOH was added
into the reaction system, well ordered mesoporous structure can
be achieved even at the highest content of ethanol (Fig. 2c, Hp-
Na(17)) due to the enhanced stability of CTAB micelles. Apart from
this, the ratio of Vmicro/Vtotal (Table 1) was found decreased with the
increasing amount of ethanol, which was similar to the literature
report [11] and the authors contributed it to the hindering of eth-
anol to the crystallization of zeolite during hydrothermally treat-
ment process. In our experiment, however, most of ethanol had
been evaporated out before steam-treatment (crystallization),
therefore, ethanol in fact played a different role in the formation
of mesoporous zeolites in the present case. That the pore size be-
came smaller at increased ethanol amount implys the number in-
creases of the CTAB micelles and the corresponding mesopores
under the same content of CTAB, leading to the increased meso-
pore volume and decreased ratio of Vmicro/Vtotal, accordingly.
On the other hand, from Fig. 2a and b, one can know that a certain
amount of ethanol addition do not affect the formation of meso-
phase but over high amount of ethanol would lead to the destruction
of ordered mesophase, and meanwhile, even at very high pH value
by NaOH addition, mesostructure can still be retained.
Table 1 lists the textual properties of the successfully synthe-
sized mesoporous zeolites in the shaded area in Fig. 1. The peak
pore size decreased with the increasing ethanol amount from
2.54 nm for Hp-E(10) to 1.95 nm for Hp-E(13) of Hp-E series and
from 2.23 nm for Hp-Na(0) to 1.65 nm for Hp-Na(7) of Hp-Na series,
respectively. The relative percentages of microporous volume (Vmic
/
Vtotal) were calculated and are also listed for comparison (Table 1).
Similar to the literature repot in which the amount of ethanol was
found to decrease microporous volume percentage in hydrothermal
route [11], the present research also demonstrated that ethanol had
influenced the microporous properties significantly: Vmic/Vtotal de-
creased apparently at the increased amount of ethanol after NaOH
was added into reaction mixture (Hp-Na series).
3.2. Factors affecting the formations of mesophase and zeolite
crystallization
ZSM-5 zeolite usually form at high pH value (here, pH = 11.9), as
reported in a large number of literatures [23,24]. At lower pH val-
ues, zeolite cannot be obtained, such as sample Hp-E(0) of
pH = 10.7, which was found to be amorphous mesoporous silica
with ordered pore structure (MCM-41). With the increase of pH va-
lue, crystallized zeolite phase gradually formed in products, such
as Hp-E(4). In our experiments, the minimum pH value for the zeo-
lite crystallization was found to be 10.80.
The pH value and the amount of ethanol greatly affected the
formation of mesophase as illustrated in Fig. 1 and in more detail
in Fig. 2.
In an ethanol/water mixed solution under the presence of both
micro- and mesotemplates (TBAOH and CTAB in the present case),
pH value would increase significantly with the increase of ethanol
amount and the addition of NaOH (Fig. 1), which affected the inter-
actions among various species in the mixture and finally resulted
in thoroughly different products.
3.3. Mesoporous zeolite formation as a result of adsorption
competition and electrostatic interaction balances
3.3.1. Adsorption competition balance for the co-existence of meso-
and micro-colloids
Under the co-existence of CTAB and TBAOH templates, an
adsorption competition balance to the inorganic species between
CTAB and TBAOH must be achieved for the final formation of mes-
oporous zeolite, i.e., inorganic species should not be preferentially
Fig. 1. Relationship between pH value and EtOH content with (Hp-Na(X)) and
without (Hp-E(X)) the addition of NaOH under the presence of CTAB and TBAOH as
the meso- and microtemplates, respectively.

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H. Li et al. / Journal of Alloys and Compounds 556 (2013) 71–78
Table 1
Textual properties of mesoporous zeolites synthesized in the shaded area in Fig. 1a.
SBET (m2 gÀ1
)
Sexternal/meso (m2 gÀ1
)
Smicro (m2 gÀ1
)
Vtotal (cm3 gÀ1
)
Vmicro (cm3 gÀ1
)
Dv (d,nm)
Vmic/Vtotal
pH Value
Hp-E(10)
Hp-E(12)
Hp-E(13)
Hp-Na(0)
Hp-Na(4)
Hp-Na(7)
445
364
374
418
370
410
330
198
185
242
219
242
115
166
189
176
151
168
0.365
0.200
0.228
0.220
0.246
0.355
0.057
0.086
0.097
0.090
0.078
0.087
2.54
2.35
1.82
2.23
1.98
1.65
0.16
0.43
0.42
0.43
0.32
0.25
11.10
11.25
11.30
11.12
11.24
11.30
a
SBET (surface area) by BET method; Smicro (micropore surface areas) and Sexternal (external surface areas) and Vmicro (micropore volumes) by t-plot method; Vtotal (total pore
volumes of P/P0 = 1) was calculated by BJH method. Dv was the peak pore size according to pore size distribution. The pH value was measured in the reaction system.
Fig. 2. Small-angle XRD (a and c) and wide-angle XRD (b and d), patterns of Hp-E(X) (a and b), and Hp-Na(X) (c and d).
adsorbed to only one kind of templates (such as CTAB) but not to
the other (such as TBAOH), or vice versa. This means that meso-
and micro-colloids, formed by silicate species adsorbing onto mes-
otemplates (CTAB in the present study) and microtemplates
(TBAOH) should be co-existent in the reaction systems. For exam-
ple, according to Fig. 1, at relatively high pH value, i.e., at the in-
creased [OHÀ], the double-layer of CTAB micelles was compressed
and meanwhile, the stability and the charge density of the CTAB mi-
celles were enhanced. Comparatively, the electric charging of
TBAOH molecules was almost unchanged. Therefore, the inorganic
species preferentially and mostly adsorb onto CTAB micelles when
the pH value is excessively high, e.g., higher than 11.3 as illustrated
in Scheme 1. As a result, only mesoporous structure without micro-
porosity (or zeolite crystallization) can be generated. As a typical
example, Hp-Na(10) was found to be amorphous mesoporous
structure, because Hp-Na(10) had excessively high NaOH content.
Therefore, it is very necessary to properly reduce pH value in pri-
mary reaction system. In our experiment, the maximum pH value
for the successful formation of mesoporous zeolite was 11.30.
3.3.2. Balanced electrostatic interaction for the subsequent formation
of mesoporous zeolite
As discussed above, in a slightly lowered pH value, inorganic
species can be adsorbed on both meso- and microtemplates in
comparable extents and an organic/inorganic composite colloidal
solution can be obtained. Under this circumstance, keeping a bal-
anced electrostatic interaction between meso-colloids and micro-
colloids should be a next essential key for binding the two kinds
of colloids together for the formation of mesoporous zeolite phase
without the phase separation between the mesoporous phase and
zeolite crystal phase. Such an electrostatic interaction between two
template colloids was also affected by pH value, i.e., by the surface
charge densities of two kinds of composites. In this experiment, the
pH values of all reactions solution are higher than 10.5, and in this

H. Li et al. / Journal of Alloys and Compounds 556 (2013) 71–78
75
Scheme 1. The adsorption competition, electrostatic repulsion and mesoporous self-assembly from zeolites seeds under various pH values. SEM image is that of a typical
product, sample Hp-E(7) showing the phase separation.
range, report [25] shows that the absolute zeta potential value of
silicate species would decrease as pH value increases, implying
the weakened electrostatic repulsion resulted from the surface
charge between two kinds of composite colloids. When pH value
was in a range of 10.8–11.1 in our experiment, the electrostatic
repulsion between meso-colloid and micro-colloid is still high en-
ough to compel each other resulting in phase separation (such as
Hp-E(4) and Hp-E(7)). Once the pH value was higher than 11.1,
as indicated in Fig. 1, the electrostatic interaction (as well as abso-
lute zeta potential value) is inferred to have decreased substan-
tially, leading to favorable affinity and/or integration between
these two kinds of colloids for further free energy minimizing in
the mixture. As a result, a mesoporous zeolite precursor without
any phase separation was obtained during the following steam-as-
sisted crystallization process. All above experimental and literature
results means that there may exists a pH range where the inor-
ganic species were adsorbed onto both meso- and microtemplates
in a comparable way and the two kinds of composite colloids can
be integrated together with each other under substantially mini-
mized electrostatic repulsion, forming desired integral mesoporous
zeolite phase, instead of amorphous mesoporous phase alone or
phase-separated mesophase and zeolite crystals. In the present
study, such a pH range was found be very narrowly in 11.10–11.30.
In summary, pH value is the most important key to the successful
synthesis of mesoporous zeolite. Only in a very narrow pH range of
11.10–11.30, achieved by tuning the ethanol adding amount and/or
NaOH addition, mesoporous zeolite was found to be able to obtain.
Higher or lower pH value would lead to either phase separation (a
mixture of zeolite crystals and amorphous silica), or formation of
amorphous phase alone, or even disordered amorphous silica, due
to the preferential adsorption of inorganic species only onto one
kind of templates and/or over high electrostatic repulsion between
the composite colloids in solvent of high dielectric constant.
4. Microstructure and catalytic properties
Sample Hp-Na(0) was used as an example for further character-
izing its meso- and micro-structure and catalytic performance. This
sample was chosen due to its typical meso-/microporous structure,
as well as the fact that no previous investigation has been made on
the catalytic property of synthesized mesoporous zeolite using
CTAB as mesoporogen but without using ethanol [11,16].
Representative TEM images of a synthesized mesoporous zeo-
lite, sample Hp-Na(0), are given in Fig. 3 to reveal the micro/
meso-structure. Fig. 4 is a corresponding SEM image.
A higher magnification image at the particle rim shown in
Fig. 3b reveals the three-dimensional sponge or worm-like meso-
porous structure of the synthesized mesoporous zeolite. The HR-
TEM image (Fig. 3c) taken from the square area marked in Fig. 3b
demonstrates clear single crystalline lattice fringes co-existing
with the meso-structure, which is confirmed by the corresponding
electron diffraction pattern. A d-placing of 0.98 nm measured from

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H. Li et al. / Journal of Alloys and Compounds 556 (2013) 71–78
the HR-TEM images is in agreement with XRD results (at 2h = 8.98°,
to (020) lattice plane). The co-existence of a meso-structure and
zeolite lattice should be attributed to properly controlled pH value
in the synthesis.
The catalytic application of synthesized mesoporous zeolite in
biodiesel fabrication was explored (see Scheme 2). Fig. 6 shows
the yield vs. time plots of ethyl laurate, in which Hp-Na(0) demon-
strates distinctively higher activity than those of ZSM-5, Al-MCM-
41 in the first 8 h. Typically the yield of ethyl laurate when using
Hp-Na(0) as the catalyst is about 2.3 and 3.4 times of those using
ZSM-5 and Al-MCM-41 as catalysts in 16 h of reactions under the
same condition. This result confirmed that the reaction was con-
trolled both by a diffusion step in the pore network and by an acid
catalytic step. The diffusion limitation in the micropore channel of
ZSM-5 or the weak acidity in the mesoporous Al-MCM-41 have led
to significantly lower catalytic efficiencies in this reaction. In the
other word, the extensive mesoporosity and the high acidic inten-
sity of Hp-Na(0) ensures its high production yield of ethyl laurate
due to its much more efficient diffusion of large molecule reac-
The mesoporous structure of Hp-Na(0) was measured by
recording the nitrogen sorption isotherm (Fig. 5), which exhibits
a typical type-IV behavior of mesoporous structure. A strong up-
take of N2 as a result of capillary condensation can be observed
in a relative pressure (P/P0) range of 0.15–0.45 which indicates that
the material has a typical mesoporous structure with mesopore
size about 2.2 nm from the pore size distribution plot (inset in
Fig. 5). A high Brunauer–Emmet–Teller (BET) surface area of
418 m2 gÀ1, a total pore volume of 0.22 cm3 gÀ1, and the corre-
sponding microporous volume, the meso- and micropore surface
areas can be found in Table 1.
Fig. 3. (a) TEM image of a representative particle of Hp-Na(0), (b) high magnification image at its rim, (c) HR-TEM image taken from the square frame in (b) and the inset is
the corresponding electron diffraction pattern.

H. Li et al. / Journal of Alloys and Compounds 556 (2013) 71–78
77
Fig. 6. Ethyl laurate yield in the esterification of lauric acid with ethanol using
different kinds of catalysts.
Fig. 4. Representative SEM image of hierarchical zeolite, sample Hp-Na(0).
static repulsion between the composite meso- and micro-colloids.
The obtained mesoporous zeolite showed greately enhanced cata-
lytic activity in the esterification of lauric acid with ethanol for
ethyl laurate production.
Acknowledgements
The authors thank the financial support from National Natural
Science Foundation of China (No. 20633090 and 20703055), Natu-
ral science fund for colleges and universities in Jiangsu Province
(No. 09KJD430010) and in Fujian Province of China (No. JB10131)
and also thanks the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
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