Hierarchical mesoporous zeolites: Direct self-assembly synthesis in a conventional surfactant solution by kinetic control over the zeolite seed formation

Article abstract of DOI:10.1002/chem.201101401

By kinetic control over the zeolite seed formation, we report the direct fabrication of hierarchical mesoporous zeolites using hexadecyl trimethyl ammonium bromide (CTAB) as the soft template in a conventional solution route. Nanometer-sized, subnanocrystal-type zeolite seeds with a high degree of polymerization are essential to prevent the formation of a separate amorphous mesoporous phase and the phase separation between the mesophase and zeolite crystals in the presence of CTAB and a certain amount of ethanol. The mechanisms for the formation of hierarchically porous zeolites in the solution process, including the effect of mother liquid aging, formation of subnanocrystal zeolite seeds and their self-assembly effect with CTAB, and the role of ethanol are proposed and discussed in detail. The prepared mesoporous ZSM-5 zeolite showed much higher catalytic activity than conventional counterparts for aldol condensations involving large molecules, especially in the synthesis of vesidryl.

Full text of DOI:10.1002/chem.201101401

DOI: 10.1002/chem.201101401
Hierarchical Mesoporous Zeolites: Direct Self-Assembly Synthesis in a
Conventional Surfactant Solution by Kinetic Control over the Zeolite Seed
Formation
Yan Zhu,[a] Zile Hua,[a] Jian Zhou,[a] Lijun Wang,[a] Jinjin Zhao,[b] Yun Gong,[a]
Wei Wu,[a] Meiling Ruan,[a] and Jianlin Shi*[a]
Abstract: By kinetic control over the
zeolite seed formation, we report the
direct fabrication of hierarchical meso-
porous zeolites using hexadecyl tri-
methyl ammonium bromide (CTAB) as
the soft template in a conventional so-
lution route. Nanometer-sized, subna-
nocrystal-type zeolite seeds with a high
degree of polymerization are essential
to prevent the formation of a separate
amorphous mesoporous phase and the
phase separation between the meso-
phase and zeolite crystals in the pres-
ence of CTAB and a certain amount of
ethanol. The mechanisms for the for-
mation of hierarchically porous zeolites
in the solution process, including the
effect of mother liquid aging, formation
of subnanocrystal zeolite seeds and
their self-assembly effect with CTAB,
and the role of ethanol are proposed
and discussed in detail. The prepared
mesoporous ZSM-5 zeolite showed
much higher catalytic activity than con-
ventional counterparts for aldol con-
densations involving large molecules,
especially in the synthesis of vesidryl.
Keywords: aldol condensations
·
heterogeneous catalysis · kinetics ·
surface chemistry
zeolites
· surfactants ·
Introduction
offer a higher para selectivity for para-xylene over meta-
xylene.[4,7] On the other hand, the adjustable acidity pro-
motes the formation of carbocations, which are reactive and
can accelerate reactions like isomerization. However, ZSM-
5 zeolites fail to catalyze larger molecules or reactions in-
volving macromolecules due to their diffusion limitations in
the micropore channels, and carbon deposition in the pore
system during the reaction would consequently lead to
severe catalyst deactivation.[8] To solve the diffusion prob-
lem, aluminum-substituted mesoporous silicates have been
synthesized by using hexadecyl trimethyl ammonium bro-
mide (CTAB) as a soft template since 1992.[9] Although the
ordered mesopores (around 2 nm in diameter) could easily
be used in diffusion restricted reactions, the poor hydrother-
mal stability and weak acidity, resulting from the noncrystal-
line frameworks, greatly limited practical applications.[10] Al-
ternatively, the diffusion limitation of reagents through zeo-
lite micropores can be solved when nanozeolites (particles
no bigger than 100 nm) are employed. Within the past few
decades, nanozeolites have been successfully synthesized by
the hydrothermal treatment of colloid solutions at relatively
low temperatures with or without the addition of a growth
inhibitor or nucleation promoter.[11] However, zeolite crys-
tals smaller than 100 nm might be thermodynamically unsta-
ble due to high surface energy and vast amounts of surface
defects. Meanwhile, nanocrystals are difficult to handle and
have low yields during synthesis, in which the majority of
the building units are left unused in the mother liquid. Con-
sequently, the synthesis of hierarchical and microsized zeo-
lites, which combines the acidic activity and shape selectivity
of micropores and the free diffusion properties of meso-
As important heterogeneous catalysts, ZSM-5 type zeolites
are widely used in petrochemistry and the fine chemical in-
dustry for a series of reactions involving cracking,[1] alkyla-
tion,[2] acylation,[3] isomerization,[4] aromatization,[5] and
aldol condensation.[6] The penetrating and intersecting mi-
cropore networks, composed of straight channels along the
b axis (0.53 nmꢀ0.56 nm) and zigzag channels along the a
axis (0.51 nmꢀ0.55 nm), endow the catalyst high shape and
size selectivity towards reactant and/or product molecules.
The enhanced selectivity of ZSM-5 zeolites was evidenced
in the separation of xylene isomers, especially for highly
pure p-xylene. In toluene disproportionation reactions or
the alkylation of toluene with methanol, ZSM-5 catalysts
[a] Dr. Y. Zhu, Dr. Z. Hua, Dr. J. Zhou, Dr. L. Wang, Dr. Y. Gong,
W. Wu, M. Ruan, Prof. J. Shi
State Key Laboratory of High Performance Ceramics and
Superfine Microstructures
Shanghai Institute of Ceramics, Chinese Academy of Science
No.1295 Ding-xi Road
Shanghai 200050 (P.R. China)
Fax : (+86)21-5241-3122
E-mail: jlshi@mail.sic.ac.cn
[b] Dr. J. Zhao
School of Material Science and Engineering
Shijiazhuang Tiedao University
17 Northeast, Second Inner Ring
Shijiazhuang, Hebei (P.R. China)
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201101401.
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FULL PAPER
pores, has attracted great attention among researchers in
chemical and material sciences in the last few years.
enlargement of charge densities, the hierarchical structures
listed above could be created without phase separation be-
tween the mesophase and crystalline zeolite. At the same
time, unfortunately, the synthesis of these unique templates
is very complex, leading to a high cost for the synthesized
mesoporous zeolite materials.
To date, the synthesis of hierarchical mesoporous zeolites
falls into three categories: 1) the selective etching of presyn-
thesized zeolites to create mesopores in zeolite crystals,[12]
including selective acid etching for framework dealumina-
tion and alkali etching for framework desilication; 2) crys-
tallization of the framework of presynthesized mesoporous
aluminosilicates, keeping the mesopores unaffected during
zeolite formation, which includes traditional hydrothermal
treatment and steam-assisted crystallization methods;[13] and
3) the assembly of nanosized zeolite crystallites, in which
the mesopores are formed among the crystallites. Generally,
in the framework crystallization approach, mesoporogens
and microporogens are used simultaneously in one system.
Tetrapropylammonium (TPAOH) is a commonly used mi-
cropore structure-directing agent (SDA) for ZSM-5 zeolites.
A number of hard or soft templates have been used as mes-
oporogens. Compared with soft templates, most of the hard
templates serve as scaffolds, either through endo- or exo-
templating,[14] such as carbon particles,[6a,15] carbon aero-
gel,[16] and 3DOM carbon.[17] Also monodisperse polystyrene
(PS) spheres[18] and poly(methylmethacrylate) (PMMA)
nanospheres[19] have been used as hard templates for gener-
ating mesopores in zeolites with unique morphologies. Nev-
ertheless, most of the mesoporous materials with isolated
secondary porosity so far are unsuitable for the diffusion of
large molecules, because of the rigidity of the hard tem-
plates. Combined with the high cost and complexity in the
fabrication of hard templates, soft templates may offer a
more promising alternative approach for the preparation of
hierarchical mesoporous zeolites.
Soft templates can be distinguished from hard templates
by diverse interactions with zeolite precursors, such as cova-
lent bonds, van der Waals forces, hydrogen bonds, and elec-
trostatic forces. They can be classified as organosilanes, cat-
ionic polymers, silylated polymers, general surfactants, and
other agents with unique molecular structures. Organosilane
soft templates, which were proposed by Ryoo et al., prevent
phase separation during crystallization by using specially de-
signed bifunctional coupling agents.[6b] On the other hand,
Xiao et al. synthesized hierarchical zeolite beta and ZSM-5
by using the cationic polymers polydiallyldimethylammoni-
um chloride (PDADMAC) and the dimethyldiallyl ammoni-
um chloride acrylamide copolymer (PDDAM) as mesoporo-
gens, and the mesopore size could be tuned by the amount
of polymers used.[20] Also, Park et al. prepared MSU-MFI,
with intracrystal mesopores of 2.2 and 5.2 nm, using silylated
polypropylene oxide diamine and polyethylenimine poly-
mers as templates.[21] Furthermore, stable single-unit-cell
nanosheets of zeolite MFI were successfully fabricated with
the specially designed and synthesized bifunctionalized
Using ordinary surfactants, such as CTAB, to prepare hi-
erarchical zeolites has been of great interest since the dis-
covery of MCM-41. Several years ago, Pinnavaia et al. syn-
thesized steam-stable aluminosilicate mesostructures using
zeolite precursors. MSU-S (MFI) and MSU-S (BEA) type
mesostructures were prepared by assembling the respective
zeolite seeds with CTAB under hydrothermal conditions.[22]
Nevertheless, diffraction peaks were not present in the
XRD patterns, indicating a noncrystalline framework, al-
though infrared (IR) spectroscopy with a band in the 550–
600 cmÀ1 region confirmed the presence of five-membered
ring subunits. Then in 2006, Bagshaw et al. synthesized an
intimate composite material, composed of highly ordered
mesoporous MSU-S (BEA) and a well-crystallized zeolite
Beta over microscale domains, which displayed Beta-type
XRD diffraction pattern.[23] However, this material was still
a mixture of ordered mesophase and zeolite crystals with in-
ferior catalytic activity and stability, and phase separation
could not be avoided when using traditional surfactants in
reactions with the zeolite precursors. To solve this problem,
Zhu et al. proposed a new strategy to prepare hierarchical
zeolites, which used CTAB coupled with tert-butyl alcohol
(TBA) and 1,3,5-trimethylbenzene (TMB) as a cosolvent
and additive, respectively, enhancing the stability of the sur-
factant micelles.[24] This coincided with the strong interac-
tions required between zeolite precursors and templates.
All of the above reports focused on designing and/or
choosing different types of surfactants, yet little attention
has been paid to modulation of the zeolite building blocks,
or zeolite seeds, during the production of hierarchical meso-
porous zeolites. The crystallization process of zeolites, which
can be distinguished in a chronological sequence as the in-
duction period, nucleation, and crystal growth, is a heteroge-
neous process with constituents typical of different polymer-
ization degrees, even in the same period, such as induc-
tion.[11c,25] Due to the important effect of the zeolite precur-
sor in the induction period on the nucleation, great efforts
have been made to characterize the primary units, oligo-
mers, and nanoparticles.[26] 29Si NMR spectroscopy combined
with other techniques clearly quantified the distribution of
the Qn bands in the zeolite precursor at an early stage, and
the latest results by Petry et al. proved the presence of olig-
omers and their evolution into aggregated nanoparticles,
which demonstrated the connectivity in nanoparticles is
about 0.3 units above oligomers.[26b] At the same time, tech-
niques such as XRD, SEM, and TEM could be used to char-
acterize crystal growth.[25a,d] However, the in situ detection
of nucleation at relatively high temperatures is still challeng-
ing. By aging the zeolite precursor of silicalite-1 at 908C,
Kirschhock et al. and Bals et al. proposed the presence of
nanoslabs and tablets, the structures of which are still un-
agent C22H45-NACHTUNGTRENNUNG(CH3)2-C6H12-NAHCTUGNTNERN(NGU CH3)2-C6H13 (C22-6-6), which
possesses two quaternary ammonium groups spaced by a C6
alkyl linkage and a long chain alkyl group (C22) on the
end.[6c] Based on the strong interactions between the zeolite
precursor and mesoporogen, either by chemical bonding or
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J. Shi et al.
clear, and prepared ordered mesoporous materials by alter-
ing the reaction conditions to be unfavorable for zeolite
growth.[25e,27] Hence, the bond making and cleavage of Si-O-
Si and subsequent rearrangement of the subunits into small
nuclei centers with high regularity are still obscure and fur-
ther work is necessary.
ment at 1508C. Finally, the zeolites with mesopores pene-
trating through the micropore system were formed. This
strategy differs from the soft templating approach proposed
by Ryoo et al,[6b] in which specially designed bifunctional
surfactants were employed but zeolite seed formation was
not necessary, and differs from the early reports,[22] in which
zeolite subunits were used directly as the building blocks
but a crystallized zeolite phase was not obtained as evi-
denced by XRD. Moreover, the present work is also differ-
ent form our steam-assisted crystallization approach recently
reported, in which phase separation was prevented by avoid-
ing the liquid phase during the framework crystalliza-
tion.[13b,c] The present approach is based on the principle
that the formation of discrete mesophases, separate from
zeolite crystals, can be effectively prevented by generating a
large amount of subnanocrystals with relatively high poly-
merization degree combined with the use of ethanol.
In addition, the catalytic conversions involving large or-
ganic molecules were tested using the prepared mesoporous
zeolites in the aldol condensation between 2-hydroxyaceto-
phenone and benzaldehyde and between 2’,4’-dimethoxyace-
tophenone and 4-methoxybenzaldehyde. Results indicate
that the presence of mesoporosity in the zeolite particles
can lead to enhanced catalytic activities for aldol condensa-
tion reactions.
Although there is no clear definition and demarcation
due to the uniqueness and complexity of different reaction
systems, zeolite precursors can be approximately classified
into primary units of either oligomers (composed of differ-
ent subunits) and nanoparticles or subnanocrystals and
nanocrystals, according to the polymerization degree or
framework regularity. Therefore, most of the above zeolite
seeds before interaction with mesoporogen should be oligo-
mers and nanoparticles, and few of them contain subnanoc-
rystals or nanocrystals. Nevertheless, the crystallization of
zeolites is a rather slow process compared with the rapid
formation of the mesoporous phase even at low temperatur-
es.[25a–d] Based on the kinetic priority for the formation of
the mesophase and the small energy differences in the ther-
modynamics between microporous zeolite frameworks and
ordered mesophases,[25c] constituents with low polymeri-
zation degrees would redissolve, or rearrangement could
occur under the directing effect of mesotemplates. Hence,
zeolite nucleation and/or growth can be largely interrupted
accordingly. Finally, an amorphous silicate mesophase re-
sults, or a mixture of zeolite and mesophase is obtained for
prolonged hydrothermal treatment processes, as demonstrat-
ed by Pinnavaia et al.[22] In the end, this paradox results in
separate mesostructures and zeolite crystals with meso-
porosity apart from the microporous zeolite lattice.
Fortunately, the constituents of the zeolite precursor,
which is kinetically controlled, can be tuned by heat treat-
ments for different time periods. Therefore, to diminish a
mismatch between kinetics and thermodynamics in the fab-
rication of mesoporous zeolites, further efforts are needed
in the design of experiments, from the perspective of kinetic
control, to synthesize mesoporous zeolites without phase
separation in solution.
Herein, the kinetic control for the zeolite seed formation
was first attempted in the direct fabrication of hierarchically
mesoporous ZSM-5 zeolites by controlling the zeolite seed
formation and using CTAB as a soft template in our experi-
ments, in which the key was to produce a large number of
subnanocrystal-type zeolite seeds in a heating process. They
were then allowed to interact with the surfactant for meso-
structure assembly. These unique subnanocrystal-type zeo-
lite seeds were fabricated by aging a water solution of tet-
raethylorthosilicate (TEOS), aluminum isopropoxide (Al-
Results and Discussion
Synthesis of hierarchical ZSM-5 zeolites with mesoporous
structures: ZSM-5 zeolites with different hierarchical porous
structures were prepared with different amounts of CTAB
(C) and ethanol (E) and designated as HMZ-xC-yE, in
which x and y represent the molar ratio of CTAB to H2O
times 10À3 and of ethanol to H2O, respectively. For compari-
son, ZSM-5 zeolites without using CTAB were synthesized,
while keeping the other conditions unchanged, and labeled
as HMZ-0C-yE. The yield of the zeolites without CTAB
was very low, as described in detail in the Experimental Sec-
tion, about 10% compared with yields higher than 85% for
samples assembled with CTAB. This means that CTAB
plays an important role in the mesostructure assembly pro-
cess with subnanocrystals. Table 1 lists the textual properties
of HMZ-xC-yE. The relative percentages of microporous
volume (Vmic/Vtot) and the external surface area (Sext/SBET
)
were calculated and are listed for comparison (Table 1)
along with the data for conventional ZSM-5 zeolite, which
has a small external surface area of about 43 m2 gÀ1 because
of the absence of mesostructures. Small differences in the
total BET surface area between HMZ-0.28C-0E and HMZ-
0C-0E were found, but the external surface area increased
from 103 m2 gÀ1 (HMZ-0C-0E) to 172 m2 gÀ1 (HMZ-0.28C-
0E), as clearly indicated by the Sext/SBET ratios. For the
sample HMZ-0C-0E, the jump at P/P0 >0.9 in the nitrogen
sorption isotherms belongs to the intercrystal porosity due
to the aggregation of small zeolite crystals. However, for the
sample HMZ-0.28C-0E, the slow rise at P/P0 <0.4 contrib-
ACHTUNGTRENNUNG(iPrO)3) and tetrapropylammonium (TPAOH) in sealed
Schott Duran bottles at 1008C for two to three days. Such
zeolite seed solutions are believed to contain mostly subna-
nocrystals, which have much higher polymerization degrees
than the corresponding oligomers and nanoparticles before
aging, and tend to crystallize upon subsequent treatment.
This nearly clear solution was then mixed with CTAB in an
ethanol/water solution with subsequent hydrothermal treat-
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Hierarchical Mesoporous Zeolites
FULL PAPER
Table 1. Textual properties of hierarchical ZSM-5 zeolites.[a]
Figure 1 shows SEM images
of HMZ with different amounts
of CTAB. Sample HMZ-0.14C-
0.035E (Figure 1A) was com-
posed of ellipsoidal particles
with a rough surface and a par-
ticle size of about 150 nm.
Compared with HMZ-0.14C-
SBET
[m2 gÀ1
Sext
[m2 gÀ1
Smic
[m2 gÀ1
Vtot
[cm3 gÀ1
Vext
[cm3 gÀ1
Vmic
[cm3 gÀ1
]
Vmic/Vtot Sext/SBET
G
]
E
]
G
]
N
]
G
]
R
HMZ-0.28C-0E
357
379
416
431
440
412
450
384
367
349
172
185
229
272
289
309
240
178
133
103
225
241
43
185
194
187
159
151
103
210
206
234
246
173
161
327
0.200
0.213
0.257
0.273
0.289
0.256
0.294
0.265
0.244
0.232
0.263
0.270
0.192
0.116
0.125
0.172
0.201
0.221
0.21
0.197
0.171
0.136
0.118
0.176
0.162
–
0.084
0.088
0.085
0.072
0.068
0.046
0.097
0.094
0.108
0.114
0.083
0.108
–
0.42
0.41
0.33
0.26
0.24
0.18
0.33
0.35
0.44
0.49
0.32
0.40
–
0.48
0.49
0.55
0.63
0.66
0.75
0.53
0.46
0.36
0.29
0.67
0.60
0.12
HMZ-0.28C-0.012E
HMZ-0.28C-0.024E[b]
HMZ-0.28C-0.035E
HMZ-0.28C-0.047E
HMZ-0.56C-0.035E
HMZ-0.21C-0.035E[b]
HMZ-0.14C-0.035E
HMZ-0C-0.035E
0.035E,
synthesized with
amount of CTAB, shows
HMZ-0.28C-0.035E,
a
higher
a
HMZ-0C-0E
HMZ-0.28C-0.035E (3d HT) 398
HMZ-0.28C-0.035E (12h ST) 402
larger particle size and a more
regular, cubic morphology simi-
lar to conventional ZSM-5 zeo-
lites (Figure 1). An enlarged
SEM image in Figure 1D shows
the typical rough surface of the
mesoporous zeolite. All of the
particles in these samples show
rough surfaces and mesoporous
structures in the high resolution
conventional ZSM-5
370
[a] SBET is the BET surface area; Smic and Vmic are values calculated from t-plot analyses; Vtot is the single-point
adsorption total pore volume at P/P0 =0.984; Sext =SBETÀSmic and Vext =VtotÀVmic. In our experiments, the ex-
ternal surface areas and external pore volumes were the calculated results for the secondary porosity except
the intrinsic micropores. Sext and Vext are effective parameters to evaluate diffusion limitations in zeolites,
which are negligible for the conventional ZSM-5 zeolite. The samples labeled HMZ-0.28C-0.035E (3d HT)
and HMZ-0.28C-0.035E (12h ST) are the treated HMZ-0.28C-0.035E samples after aging for 3 days at 1008C
for the hydrothermal stability test and after a steam treatment for 12 h at 7008C, respectively. The textual
properties for conventional ZSM-5 zeolite are listed for comparison. [b] Values of sample HMZ-0.28C-0.024E
and HMZ-0.21C-0.035E, were given by Micromeritics ASAP 2020, while others were calculated by Micromer-
itics Tristar 3000.
utes evenly to the external surface area compared with the
contribution of intercrystal pores. Adding ethanol into the
mixture, the BET surface area increases from 357 m2 gÀ1
(HMZ-0.28C-0E) to 431 m2 gÀ1 (HMZ-0.28C-0.035E), and
the corresponding external surface area increases to
272 m2 gÀ1, and the micropore volume remains almost un-
changed. Even for the zeolite preparation without CTAB,
the BET surface area increased slowly at increased ethanol
amounts. This is because a certain amount of ethanol hin-
ders the overgrowth of zeolite crystals and slows down the
crystallization process,[11b] which favors the self-assembly of
zeolite subnanocrystals or nanocrystals under the direction
by micro- and/or mesopore directing agents. However, high
amounts of ethanol (sample HMZ-0.28C-0.047E) prevent
the formation of CTAB micelles and subsequent self assem-
bly with the zeolite seeds, resulting in phase separation be-
tween the mesophase and zeolite crystals. Consequently, to
synthesize zeolites with mesopores penetrating through the
micropore framework, the molar ratio of ethanol to H2O
needs to be smaller than 0.035, under these conditions.
Furthermore, different amounts of CTAB were tested and
the results are listed in Table 1. A clear relationship between
the BET surface area and the amount of CTAB was not
found. With increasing CTAB amounts, the BET surface
area first increases from 384 (HMZ-0.14C-0.035E) to
450 m2 gÀ1 for HMZ-0.21C-0.035E then decreases slightly
for samples HMZ-0.28C-0.035E and HMZ-0.56C-0.035E.
The mesopore and micropore surface areas change accord-
ingly. This means the meso- and microporosity can be tuned,
to a certain extent, by changing the using amount of the sur-
factant and ethanol in the preparation of hierarchical zeo-
lites.
Figure 1. SEM images of hierarchical zeolites: A) HMZ-0.14C-0.035E;
B) HMZ-0.21C-0.035E; C) and D) HMZ-0.28C-0.035E; E) TEM image
and F) HR-TEM image and SAED patterns (inset) of HMZ-0.28C-
0.035E.
field emission scanning electron microscopic (FE-SEM)
images demonstrating the presence of mesoporosity in the
zeolite particles. Therefore, by choosing the right type of
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J. Shi et al.
zeolite seed precursor, hierarchical ZSM-5 with regular mi-
croporosity and a co-existing mesoporosity of about 2.5 nm
can be synthesized.
For HMZ-0.28C-0.035E, the total BET surface area was
431 m2 gÀ1, and the external surface area and pore volume
reach 272 m2 gÀ1 and 0.201 cm3 gÀ1, respectively, with micro-
porosity comparable to other samples. Therefore, further
characterization and catalytic reactions were made using
this sample. Figure 3a shows the X-ray diffraction pattern of
The nitrogen adsorption and desorption isotherms (Fig-
ure 2a) of some samples give more detailed information on
the pore structures. The sharp increase at P/P0 <0.05 in all
of the samples indicates the presence of micropores, but the
gradual increase in the P/P0 range of 0.05–0.3 is observed in
most of the samples except HMZ-0C-0E. Even for HMZ-
0.28C-0E, without ethanol, there is a slow uptake before
the appearance of a distinct plateau. Combined with pore
size distribution in Figure 2b, the results for HMZ-0.28C-
0.035E and HMZ-0.28C-0.024E clearly indicate the pres-
ence of mesopores of about 2–3 nm co-existing with micro-
pores. Only microporosity is present in HMZ-0C-0E
(Figure 2); however, mesopores of about 2.5 nm are formed
in HMZ-0.28C-0E, to which CTAB was added. Mesoporosi-
ty becomes more and more apparent with increased ethanol
content from sample HMZ-0.28C-0.024E to HMZ-0.28C-
0.035E. All of the above results demonstrate that the forma-
tion of mesoporous zeolites is a result of self-assembly be-
tween the subnanocrystal zeolite seeds and the mesostruc-
ture directing agent, CTAB, with synergetic assistance by a
certain amount of ethanol.
Figure 3. a) Wide-angle XRD patterns of HMZ-0.28C-0.035E and b) the
corresponding sample after 3 days of hydrothermal treatment, labeled as
HMZ-0.28C-0.035E (3d HT), to test its hydrothermal stability.
HMZ-0.28C-0.035E, which exhibits typical and sharp dif-
fraction peaks for the ZSM-5 lattice structure. Moreover,
the high-resolution TEM (HR-TEM) image of the sample
and the selected area electron diffraction (SAED) patterns
(Figure 1F) prove that this domain is a single crystal with
mesopores (about 3 nm) penetrating through the micropore
framework. Even for small particles with irregular shape,
TEM images (Figure S1, Supporting Information) show the
crystal structure.
The hydrothermal stability of HMZ-0.28C-0.035E was
tested by subjecting the calcined sample (0.2 g) in water
(50 mL) to 1008C for three days under vigorously stirring.
This sample is labeled as HMZ-0.28C-0.035E (3d HT). A
further test for the steam stability of HMZ-0.28C-0.035E
was carried out by exposing the calcined sample (0.2 g) to a
flow of N2 (50 mLminÀ1) saturated with about 40% water
vapor at 7008C for 12 h. This sample is designated as HMZ-
0.28C-0.035E (12h ST). The samples showed a BET surface
area loss of only around 10% (Table 1), which, compared
with the severe destruction of ordered mesoporous materials
MCM-41(Al) (Table S1, Supporting Information), proves
the as prepared hierarchical ZSM-5 zeolites to be enough
stable for applications in catalytic reactions. Furthermore,
SEM images (Figure S2, Supporting Information) and X-ray
diffraction patterns (Figure 3b) also demonstrate that this
material possesses high hydrothermal and steam stability.
Figure 2. a) Nitrogen adsorption and desorption isotherms of HMZ-xC-
yE and b) the corresponding pore size distribution calculated by the BJH
method using the desorption branch of the isotherms. The isotherms in a)
are shifted by 0, 80, 120 and 160 cm3 gÀ1 , respectively, along the y axis to
avoid overlapping. The isotherms in b) are shifted by À0.01, 0.01, 0.03
and 0.05 cm3 gÀ1 respectively, along the y axis.
The catalytic properties of hierarchical ZSM-5 zeolites and
other counterparts: Chalconoids and flavanoids have attract-
ed increasing attention in pharmaceutical fields due to their
antitumor, anti-inflammatory and antifungal properties.
They are commonly catalyzed by acids or bases under ho-
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Hierarchical Mesoporous Zeolites
FULL PAPER
mogeneous conditions using the Claisen–Schmidt condensa-
tion and subsequent isomerization. However, the difficulties
in the separation and recovery of the catalysts makes heter-
ogeneous catalysis by solid acids or bases a much more at-
tractive and environmental friendly alternative. Thus, in our
experiments, H-ZSM-5 zeolites (HMZ-0.28C-0.035E and
conventional ZSM-5), and ordered mesoporous materials
(MCM-41) with the same Si/Al ratio (50:1) were used as the
catalysts for aldol reactions between 2-hydroxyacetophe-
none and benzaldehyde and between 2’, 4’-dimethoxyaceto-
phenone and 4-methoxybenzaldehyde without any solvents.
For the reaction between 2-hydroxyacetophenone and ben-
zaldehyde, conventional ZSM-5 zeolite showed a moderate
conversion rate (19.1%), significantly lower than for HMZ-
0.28C-0.035E (41.6%, Table 2). In spite of the relatively
was from the combined effect of framework acidity and
mesoporosity.
Mechanism for the formation of mesoporous zeolites
through kinetic control of zeolite seed formation: To test
the influence of the zeolite precursor on the synthesis of hi-
erarchical mesoporous zeolites, the precursor was aged at
1008C for different time periods. The samples are designat-
ed as HMZ-nd, in which n represents the number of days
the zeolite precursor was aged at 1008C. All of the other
conditions were the same as for sample HMZ-0.28C-
0.035E, which was aged for 2 days at 1008C. According to
the experimental results, four interaction models between
CTAB and the zeolite precursor at different stages are pro-
posed and illustrated in Scheme 1–4 . These are as follows.
Table 2. Catalytic data of aldol condensations using hierarchical zeolites
and their counterparts.[a]
Conventional
ZSM-5 (Si/Al=50)
Hierarchical
ZSM-5 (HMZ-0.28C-0.035E)
MCM-41
ACHTUNGTRENUN(NG Si/Al=50)
Scheme 1. The primary units are [SiO4] and/or [AlO4] tetrahedrons. Self-
assembly synthesis of ordered mesoporous materials under structure di-
rection by CTAB.
19.1 (61/39/0)[b]
41.6 (67/33/0)[b]
9 (58/42/0)[b]
À
1) For zeolite precursors only with primary units of [Si O]
tetrahedrons, ordered mesoporous materials such as
MCM-41 can be prepared by a standard self-assembly
synthesis under hydrothermal conditions.
2) When the zeolite precursor is mainly composed of sili-
cate oligomers and nanoparticles of low polymerization
degree, the self-assembly of TPA+ around these low-
polymerized species can be destructed by strong interac-
tions between the highly hydrophilic species and the hy-
drophilic ends of the CTAB molecules. This is a result of
the competition between self-assembly (between the
mesoporogens and the oligomers and/or nanoparticles)
and crystallization of these species under the direction of
the microporogens. Therefore, either mesophase or large
zeolites or a mixture of the two are formed, depending
on the experimental parameters. For example, Kirsch-
hock et al. and Bals et al. prepared ordered mesoporous
materials using much higher polymerized species, such as
nanoslabs or tablets, by lowering either the hydrothermal
treatment temperature or the pH value of the reaction
system to avoid the nucleation of the zeolite.[25e,27] More
generalized rules are: 1) oligomers larger in size (by sev-
eral nanometers, see below) than the primary units may
not be as well structure-directed by the mesopore struc-
ture directing agent (surfactant) for self-assembly as the
primary units, which always leads to the formation of the
mesophase with less ordered mesostructure and 2) the
polymerized species and nanoparticles propagate into
nuclei centers and quickly grow into large zeolite crystals
by hydrothermal treatment at elevated temperatures.
<2.0
15.1
6
[a] Catalytic properties were compared on the basis of the same weight
of the catalyst (see the Experimental Section for details). The BET sur-
face areas were 431 m2 gÀ1 for HMZ-0.28C-0.035E, 370 m2 gÀ1 for the con-
ventional ZSM-5, and 987 m2 gÀ1 for MCM-41(Al). [b] These data indi-
cate percentage selectivity of flavanone/chalcone/others, and the other
data indicate the reactant conversion [%].
small molecular size of the reactants and products in the cat-
alytic synthesis of flavanone and 2’-hydroxychalcone, the dif-
ference in the catalytic activities between HMZ-0.28C-
0.035E and the conventional ZSM-5 zeolite was significant.
Furthermore, HMZ-0.28C-0.035E exhibits a much more en-
hanced catalytic activity in the synthesis of vesidryl (2’, 4, 4’-
trimethoxychalone) compared with the conventional ZSM-5
zeolite (Table 2), which shows less than 2% conversion.
Therefore, the presence of mesoporosity in HMZ-0.28C-
0.035E can greatly enhance the catalytic activity in the reac-
tions, especially involving large organic molecules. More-
over, the MCM-41(Al) catalyst without diffusion limitations
displayed very low catalytic activities in aldol condensations.
This can be attributed to the lack of strong acid sites be-
cause of the short-range atomic order in MCM-41 compared
with highly crystalline zeolites. Therefore, the enhanced cat-
alytic activity by hierarchical zeolites in aldol condensations
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J. Shi et al.
for 2–3 days is key for the
creation
of
hierarchical
porous zeolite in a conven-
tional solution process. All
of the oligomers and/or
nanoparticles or other low-
polymerized species, which
are responsible for the for-
mation of amorphous meso-
phases, except for pure zeo-
lite
crystals
(HMZ-0d,
Scheme 2) and for mesopo-
rous zeolite particles (HMZ-
1d, Scheme 3A), could be
eliminated and were con-
verted to subnanocrystals in
the mother liquid by aging
Scheme 2. Oligomers and/or nanoparticles leading to amorphous and not well-ordered mesoporous phases or a
mixture of the mesophase and pure zeolite crystals. Silicate oligomers are aggregated subunits such as three-
membered rings, four-membered rings or five-membered rings with relatively low degrees of polymerization.
Nanoparticles are aggregated oligomers with connectivity above oligomers. The final products depend on the
competition between mesoporogens (CTAB) and microporogens (TPAOH). TEM (top) and SEM (bottom)
images are for the HMZ-0d sample prepared from this type of zeolite seed.
Therefore, the most common results (Scheme 2) are the
formation of large zeolite crystals accompanied by con-
siderable amounts of amorphous mesoporous silica. A
SEM image of sample HMZ-0d, without having been
aged at 1008C (Scheme 2) shows the presence of micro-
meter-sized zeolites with smooth surfaces surrounded by
a large amount of disordered mesophase (TEM image,
Scheme 2). Small-angle XRD patterns (Figure S3, Sup-
porting Information) showed a small, broadened diffrac-
tion peak around 28 (2q), which demonstrates the par-
tially but much less ordered mesoporous structure com-
pared with traditional MCM-41, which has three distinct
peaks. This result of phase separation does not differ
from earlier reports by Pinnavaia et al.[22]
3) In the literature, subnanocrystals (ca. 2–10 nm) are re-
garded as species with the typical, regular atomic ar-
rangement of the MFI structure and are easy to crystal-
Scheme 3. Subnanocrystals in the self-assembly synthesis of hierarchical
mesoporous zeolites (HMZ) resulting in a low ordered mesoporous net-
work penetrating through the zeolite microporous framework. Subnanoc-
rystals are of much higher polymerization degree and atomic arrange-
ment ordering than oligomers and/or nanoparticles and are obtained by
properly aging the seed solution at relative high temperatures. A) SEM
and B) TEM images of the structure of HMZ-1d for zeolite precursors
aged for 1 day at 1008C. The circled area (A) shows the mesophase. C)
The enlarged image of the dotted square in B which shows the crystalline
structure. D) The SEM and E) TEM images for HMZ-3d (the zeolite
precursor aged at 1008C for 3 days), and F) is a HR-TEM image of the
zeolite crystals in E.
lize. However, the crystal structure could not be detected
by XRD. In the present study, these subnanocrystals
were formed in mother liquids by aging at 1008C with
the concentrations tunable by varying the heating time.
However, different from sample HMZ-0d without aging,
the aged zeolite seeds should have experienced some
kind of structure alteration into a more ordered atomic
arrangement, and the subsequent interaction between
the aged seeds and CTAB at this stage led to the forma-
tion of mesoporous zeolites. The SEM image of the
HMZ-1d sample (Scheme 3A) shows the zeolite parti-
cles with mesoporous structure, together with a few mes-
ophase particles (circled), and the corresponding TEM
images (Scheme 3B and C) clearly show the zeolite lat-
tice structure. Zeolite precursors aged at 1008C for 2–3
days formed hierarchical mesoporous zeolites without
detectable isolated mesophase particles. The SEM and
TEM images (Scheme 3D–F) indicate that the particles
from sample HMZ-3d are regularly shaped, similar to
the HMZ-0.28C-0.035E sample aged for 2 days (Fig-
ure 1C–F). Therefore, aging zeolite precursors at 1008C
for enough time. As a result, the strong interaction be-
tween subnanocrystals and CTAB micelles, similar to the
case between the oligomers and CTAB, led to the crea-
tion of less ordered mesostructures among the zeolite
subnanocrystals, and the subsequent crystallization of the
subnanocrystal framework resulted in the regularly
shaped hierarchical zeolites with penetrating mesoporous
network in the particles.
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Hierarchical Mesoporous Zeolites
FULL PAPER
4) In the case of sufficient zeolite seed crystallization, the
mother liquids contained mainly small zeolite nanocrys-
tals tens or hundreds of nanometers in size. The presence
of such nanocrystals of about 100 nm was verified in
TEM images (Figure S4, Supporting Information) after
aging the zeolite precursor at 1008C for 4 days. However,
due to the size mismatch between the CTAB micelles
and the zeolite precursor at this stage, only aggregates of
the nanocrystals were obtained (HMZ-4d, SEM image,
Scheme 4).
Figure 4. Particle size distributions of zeolite seeds measured using dy-
namic light scattering (DLS) without or after aging at 1008C for different
time periods: a) 0 h, b) 24 h, c) 72 h, and d) 96 h.
over-aging (4 days, for example) led to decreased numbers
of subnanocrystals.
The presence of a size peak at about 400 nm before aging
was attributed to the aggregation of the oligomers and/or
nanoparticles, which is similar to the ordered liquid phases
(OLP) proposed by Kirschhock et al.[25e] The peak shifts to
about 130 nm after 24 h of aging (Figure 4b), implying the
de-aggregation and transformation of some oligomers and/
or nanoparticles into subnanocrystals, corresponding to the
sharpening of the size distribution peak for subnanocrystals.
Aging for 3 and 4 days led to much larger particles about
800 nm in size, which is believed to be the aggregates of
nanocrystals of about 100 nm (TEM image, Figure S4, Sup-
porting Information).
Scheme 4. Nanocrystals and their aggregates of about 100 nm.
The above results demonstrate that a kinetic control over
the zeolite seed formation is vitally important for the prepa-
ration of mesoporous zeolites. In the present case, the for-
mation of zeolite subnanocrystals of several nanometers in
size by aging the mother liquids at 1008C for 2 or 3 days is
key. The presence of the surfactant, CTAB, played an impor-
tant role in mesostructure formation; however, well-ordered
mesostructures could not be created in cooperative self-as-
sembly with subnanocrystals.
Conclusion
An essential piece of evidence to further support the
above conclusion would be to characterize the structures
and sizes of the silicate oligomers and/or nanoparticles, sub-
nanocrystals, and nanocrystals. Unfortunately, however, the
structures of most zeolite seeds cannot be directly detected
by TEM or SEM characterization due to the quick structure
destruction of the seeds under high-power electron beams.
Hence, dynamic light scattering (DLS) was employed to
roughly measure the sizes of the seeds during the aging pro-
cess of the mother liquid (Figure 4). In all the cases, seeds of
around 10 nm are present, which could be ascribed to the
presence of silicate oligomers and/or nanoparticles and/or
subnanocrystals. It is worth noting that the real seed size is
remarkably smaller, especially for small size particles, than
those detected by DLS, which gives the hydrated particle
size. The aging did not alter the seed size significantly; how-
ever, structure change, such as polymerization and atomic
arrangement, must have occurred within the seeds during
aging, leading to significantly different material structures,
which, unfortunately, could not be directly detected in the
present study. Aging for 2 or 3 days resulted in a significant-
ly sharpened size distribution of subnanocrystals; however,
In summary, by kinetic control of zeolite seed formation in-
stead of by changing surfactants, CTAB can be used directly
for the synthesis of hierarchical mesoporous ZSM-5 zeolites
through the self-assembly with subnanocrystal-type zeolite
seeds in the presence of a certain amount of ethanol. The
resultant materials showed a typical cubic, regularly-shaped
morphology with mesopores penetrating through the micro-
pore system. Catalytic results showed significantly higher ac-
tivities for the prepared mesoporous zeolites than for
common zeolites and amorphous mesoporous silica for reac-
tions involving bulky molecules in aldol condensations. This
is due to the 2.5 nm mesoporous structure and crystalline
nature of the prepared catalysts.
A detailed investigation showed that the dominant pres-
ence of subnanocrystals typical of a high degree of polymer-
ization and several nanometer in size, in a nearly transpar-
ent zeolite seed solution, was the prerequisite for the direct
self-assembly synthesis of the mesoporous zeolite when
using CTAB as the mesoporogen and ethanol as a self-as-
sembly modulator. Subnanocrystals can be prepared by
aging the mother liquid at 1008C for 2–3 days, but the direct
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J. Shi et al.
droxyacetophenone with benzaldehyde was performed by heating a mix-
ture of 2-hydroxyacetophenone (1.36 g, 10 mmol), benzaldehyde (1.59 g,
15 mmol), and the catalyst (150 mg) at 1508C for 12 h. In the synthesis of
vesidryl, 2’,4’-dimethoxyacetophenone (1.80 g, 10 mmol), 4-methoxyben-
zaldehyde (1.36 g, 10 mmol), and the catalyst (70 mg) were mixed, and
the reaction was conducted at 1408C for 24 h. After the reaction, the
samples were diluted and analyzed by GC-MS (Agilent, 6890/5973N).
The conversion rate and selectivity were given by drawing a standard
curve to determine the concentration of corresponding substances.
assembly of the silicate oligomer seeds without aging with
CTAB led to a phase separated mixture of amorphous mes-
oporous silica and pure zeolite crystals. Over-aging the seed
solution resulted in nanocrystals of about one 100 nm in size
and large aggregates of the zeolite. This study presented the
first example for the preparation of mesoporous zeolites in
an ordinary solution route using conventional surfactants.
Experimental Section
Acknowledgements
Synthesis of hierarchical mesoporous ZSM-5 zeolites (HMZ): First, Al-
ACHTUNGTRENNUNG(iPrO)3 (0.3066 g, 1.50 mol) and TEOS (15.624 g, 0.075 mol) were mixed
The authors thank the National Natural Science Foundation of China
(Nos. 20633090, 20703055, 50872140, and 50702072) for financial support.
with deionized water (25.92 g) at room temperature. After stirring for
2 h, TPAOH (aqueous, 25%, 21.963 g, 0.027 mol) was added dropwise
into the resultant emulsion, which became a clear solution after stirring
at 408C for 2–3 h. Finally, the clear solution was transferred to an oil
bath at 1008C and aged for 2 days under continuous stirring. The molar
ˇ
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Received: May 8, 2011
Revised: August 8, 2011
Published online: November 14, 2011
Chem. Eur. J. 2011, 17, 14618 – 14627
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
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