Well-organized zeolite nanocrystal aggregates with interconnected hierarchically micro-meso-macropore systems showing enhanced catalytic performance

Article abstract of DOI:10.1002/chem.201101594

Preparation and characterization of well-organized zeolitic nanocrystal aggregates with an interconnected hierarchically micro-meso-macro porous system are described. Amorphous nanoparticles in bimodal aluminosilicates were directly transformed into highly crystalline nanosized zeolites, as well as acting as scaffold template. All pores on three length scales incorporated in one solid body are interconnected with each other. These zeolitic nanocrystal aggregates with hierarchically micro-meso-macroporous structure were thoroughly characterized. TEM images and ²⁹Si NMR spectra showed that the amorphous phase of the initial material had been completely replaced by nanocrystals to give a micro-meso-macroporous crystalline zeolitic structure. Catalytic testing demonstrated their superiority due to the highly active sites and the presence of interconnected micro-meso-macroporosity in the cracking of bulky 1,3,5-triisopropylbenzene (TIPB) compared to traditional zeolite catalysts. This synthesis strategy was extended to prepare various zeolitic nanocrystal aggregates (ZSM-5, Beta, TS-1, etc.) with well-organized hierarchical micro-meso-macroporous structures.

Full text of DOI:10.1002/chem.201101594

FULL PAPER
DOI: 10.1002/chem.201101594
Well-Organized Zeolite Nanocrystal Aggregates with Interconnected
Hierarchically Micro–Meso–Macropore Systems Showing Enhanced
Catalytic Performance
Xiao-Yu Yang ,*[a, b, e] Ge Tian,[b] Li-Hua Chen,[b] Yu Li,[a, b] Joanna C. Rooke,[b]
Ying-Xu Wei,[c] Zhong-Min Liu,*[c] Zhao Deng,[a] Gustaaf Van Tendeloo,[d] and
Bao-Lian Su*[a, b]
Abstract: Preparation and characteri-
zation of well-organized zeolitic nano-
crystal aggregates with an interconnect-
ed hierarchically micro–meso–macro
porous system are described. Amor-
phous nanoparticles in bimodal alumi-
nosilicates were directly transformed
into highly crystalline nanosized zeo-
lites, as well as acting as scaffold tem-
plate. All pores on three length scales
incorporated in one solid body are in-
terconnected with each other. These
zeolitic nanocrystal aggregates with hi-
erarchically micro–meso–macroporous
structure were thoroughly character-
ized. TEM images and 29Si NMR spec-
tra showed that the amorphous phase
of the initial material had been com-
pletely replaced by nanocrystals to give
a micro–meso–macroporous crystalline
zeolitic structure. Catalytic testing
demonstrated their superiority due to
the highly active sites and the presence
of interconnected micro–meso–macro-
porosity in the cracking of bulky 1,3,5-
triisopropylbenzene (TIPB) compared
to traditional zeolite catalysts. This syn-
thesis strategy was extended to prepare
various zeolitic nanocrystal aggregates
(ZSM-5, Beta, TS-1, etc.) with well-or-
ganized hierarchical micro–meso–mac-
roporous structures.
Keywords: crystal growth · hetero-
geneous catalysis · mesoporous ma-
terials · microporous materials ·
zeolites
Introduction
with controlled shape and porosity are preferred to mini-
mize diffusion limitations. Recently, various successful at-
tempts to synthesize mesoporous or macroporous zeolites
with bimodal porosity and framework acidity have been re-
ported.[4–15] These materials hold great promise, particularly
in catalysis and separation processes in which optimization
of the diffusion and confinement regimes is required.
Whereas micro- and/or mesopores provide size and shape
selectivity for the guest molecules, and thus enhance the
host–guest interactions, the presence of macropores can con-
siderably favor diffusion of the guest molecules and thus
their access to the active sites.[1,16–36] This is particularly im-
portant for the diffusion of large molecules or in viscous sys-
tems. However, these conventional as-synthesized zeolite
catalysts are usually extruded with inorganic binders to
make them suitable for practical applications. Several at-
tempts have already been made in this field;[37,38] for exam-
ple, 3D macroporous monolithic silicalite-1 by using poly-
mer spheres,[39] and hierarchically structured monolithic zeo-
lites by using a carbon template.[40–42] In these cases, the or-
ganic scaffold templates must be removed by calcination. It
would therefore be interesting to develop a more facile and
organic scaffold free route to self-supporting well-organized
zeolite nanocrystal aggregates with interconnected hierarchi-
cally micro–meso–macroporous systems. Ideally, such mate-
rials should have both well-defined macropores and inter-
connected mesopores within walls which have been con-
structed from tunable microporous units. More importantly,
Zeolites have excellent thermal, hydrothermal, and chemical
stability and have been successfully employed in vapor-
phase chemistry.[1] However their use is much less frequent
in organic reactions, as the size of the molecules can exceed
the size of the small zeolite pores.[1–3] Thus, zeolite materials
[a] Dr. X.-Y. Yang , Dr. Y. Li, Dr. Z. Deng, Prof. Dr. B.-L. Su
State Key Laboratory of Advanced Technology for
Material Synthesis and Processing
Wuhan University of Technology
Luoshi Road 122, Wuhan 430070 (P.R. China)
[b] Dr. X.-Y. Yang , Dr. G. Tian, Dr. L.-H. Chen, Dr. Y. Li,
Dr. J. C. Rooke, Prof. Dr. B.-L. Su
Laboratory of Inorganic Materials Chemistry (CMI)
University of Namur (FUNDP), 61, rue de Bruxelles,
5000 Namur (Belgium)
Fax : (+32)81-725414
E-mail: xyyang@fundp.ac.be
liuzm@dicp.ac.cn
bao-lian.su@fundp.ac.be
[c] Dr. Y.-X. Wei, Prof. Dr. Z.-M. Liu
Dalian National Laboratory for Clean Energy
Dalian Institute of Chemical Physics, Chinese Academy of Sciences
[d] Prof. Dr. G. Van Tendeloo
EMAT, University of Antwerp, Groenenborgerlaan 171
B-2020 Antwerpen (Belgium)
[e] Dr. X.-Y. Yang
Chargꢀ de recherchꢀ
FNRS (Fonds National de la Recherche Scientifique)
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on all length scales, the larger pores should be connected to
the smaller pores, to result in a greater range of potential
applications, such as the fluidized catalytic cracking process.
Here we describe the synthesis and characterization of zeo-
lite nanocrystal aggregates with interconnected hierarchical-
ly micro–meso–macroporous systems prepared by a quasi-
solid-state crystallization process. These hierarchically
porous materials are constructed from zeolite nanocrystals
and exhibit high catalytic activity. The general synthesis pro-
cedure is illustrated in Figure 1. A meso–macroporous alu-
minosilicate,[36] which could be easily synthesized without
any external template, was mixed with a structure-directing
agent, such as tetrapropylammonium (TPA+), to aid zeolite
formation. A supplementary silica source was added to facil-
itate transformation of the amorphous phase of the meso–
macroporous aluminosilicate into a crystalline micro–meso–
macroporous aluminosilicate with zeolite ZSM-5 architec-
ture. The initial meso–macroporous aluminosilicate acted as
silica and alumina source, as well as scaffold template.
During the transformation, the macroporosity was not af-
fected and thus entirely transferred to the final materials
constructed from nanocrystals. The resultant material has
pores on three length scales: a well-defined macroporous
structure with highly interconnected mesopores, which de-
veloped during the growth of the microporous zeolite nano-
crystals on the basis of the mesoporosity of the precursor
materials, and micropores which could be tailored by con-
trolling the zeolite type. Most importantly, the resultant ma-
terials not only have stable catalytically active sites due to
the microporous zeolites, but also bulky molecules can
easily diffuse into the macropores and access the channels
of the mesopores and make contact with these active sites.
These multimodal porous materials constructed from zeolite
nanocrystals are highly desired in the design of hierarchical-
ly porous zeolites. The key to the design was to find a suita-
ble crystallization process that not only transformed the
amorphous phase into zeolite crystals but also protected the
meso–macroporous structure. The structure of the meso–
macroporous aluminosilicate would be easily destroyed
during conventional hydrothermal reactions owing to its
amorphous framework. The question then arises whether an
atypical hydrothermal process can be used for the synthesis,
that is, a method that minimizes the damage to the mesopo-
rous and macroporous structure caused by hydrothermal
synthesis. Some successful methods were demonstrated by
Kanno et al., Campos et al., Coppens et al., and Kaliaguine
et al.,[24,25,43–50] who used glycerol as a medium to directly
synthesize zeolites, to recrystallize mesoporous silica to zeo-
lite, to embed zeolite nanocrystals into a well-connected
amorphous matrix, and to coat the walls of mesostructured
materials with zeolite nanoclusters, respectively. However,
the generation of crystalline micro–meso–macroporous alu-
minosilicates constructed entirely from zeolite nanocrystals
with interconnectivity on three length scales still remains a
great challenge, since the structures of the mesoporous alu-
minosilicates would still be easily destroyed during reactions
even when using the glycerol system. In spite of the lack of
entirely crystalline nanosized zeolites with direct intercon-
nectivity of meso- or macroporosity, reflected by TEM
images, this is still an interesting idea, as it can provide a
new and mild synthesis route to introduce zeolite structures
into mesostructured bodies, and thus minimize the damage
caused by hydrothermal synthesis owing the recrystallization
process of the multiporous structure. The crystallization pro-
cess was performed under quasi-solid-state conditions by
using mixture of dried gel and glycerol, as opposed to fully
hydrothermal conditions with water. As the glycerol be-
comes more fluidlike it acts as a flux which improves Brow-
nian motion, aiding the diffusion of solid particles, a rate-de-
termining step in solid-state chemistry. This meant that the
structure-directing agent could still strongly interact with
growing crystal domains, through the formation of covalent
bonds with other SiO2 and Al2O3 sources, yet by fine-tuning
the synthetic system, that is, the synthetic gel, medium, tem-
perature, and so on, the mesoporous and macroporous struc-
Figure 1. Schematic representation of the synthesis of hierarchically
micro–meso–macroporous aluminosilicates constructed from zeolite
nanocrystals by a quasi-solid-state crystallization process. Top: The initial
meso–macroporous aluminosilicates constructed from amorphous parti-
cles (inset). Middle: The quasi-solid-state crystallization process from an
amorphous framework to a crystalline framework by means of a struc-
ture-directing agent (TPA+) and glycerol. Bottom: Micro–meso–macro-
porous aluminosilicates constructed from zeolite nanocrystals (inset).
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Micro–Meso–Macroporous Zeolite Nanocrystal Aggregates
FULL PAPER
tures could be preserved. This method uses glycerol as it
would not attack the amorphous aluminosilicate network to
the same degree that H2O would at high vapor pressures,
yet as it is heated it becomes less viscous and thus plays the
role of water. The quasi-solid-state crystallization process in
glycerol system was thus employed to transform the nano-
sized amorphous particles into crystalline zeolites, and
thereby govern the mesoporosity within the meso–macropo-
rous aluminosilicate to yield an unprecedented well-organ-
ized zeolite nanocrystal aggregate with interconnected hier-
archically micro–meso–macroporous system. Another ad-
vantage of this chemical crystallization process is that it ena-
bles aggregation or condensation of zeolite nanocrystals
with each other, which is a critical factor to avoid collapse
of the walls of the macropores. Moreover, the smaller meso-
pores of the initial meso–macro-porous aluminosilicates
appear to be governed by nanoparticle aggregation, which is
in contrast to the mesoporous materials used by the above-
mentioned groups. While the amorphous walls and original
mesopores gradually disappeared with increasing reaction
time, larger mesopores were generated. The microporous
zeolite crystals possibly grew by using the amorphous phase
as a source to create a new mesostructure. The key to this
synthesis is that the macroporous structure can easily be
maintained during the transformation, owing to relatively
thick walls (ca. 2–5 mm).
Results and Discussion
Studies by SEM and TEM (Figures 2 and 3) revealed that a
well-organized hierarchically micro–meso–macroporous ar-
chitecture was formed at 1308C through post-crystallization
synthesis over various reaction periods (initial, 1 d, and 2 d,
Figure 3. TEM investigation of the formation of micro–meso–macropo-
rous aluminosilicate. A–C) correspond to MMM(1) with A) bright-field
and B) dark-field images taken on the same area and C) a higher-mag-
nification view of A). D–F) correspond to MMM(2) with D) bright-field
and E) dark-field images taken on the same area and F) a higher-magnifi-
cation image of D). G–I) HRTEM images of MMM(2). The insets in C),
F), and G) are the corresponding selected-area electron diffraction pat-
terns taken from numerous particles. Scale bars: A,B,D,E) 1 mm,
C,F) 500 nm, G) 50 nm, H,I) 10 nm.
Figure 2. SEM investigation of the formation of micro–meso–macropo-
rous aluminosilicate. A–C) MMM(0), D–F) MMM(1), G–I) MMM(2),
corresponding proposed schematic drawing (Figure 1) of initial precursor
(C) and final product (I) and high-magnification image of the same parti-
cles (inset of C). Scale bars: A,D,G) 20 mm, B,E,H) 1 mm, C,F,I) 100 nm.
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B.-L. Su, X.-Y. Yang, Z.-M. Liu et al.
designated MMM(0), MMM(1), and MMM(2), respective-
ly). The precursor material exhibits a macroscopic network
with relatively homogenous and straight channel-shaped
macropores of 2–5 mm in diameter. The macrochannels are
arranged parallel to each other (Figure 2A and B). The
walls around the macrochannels are composed of very ho-
mogenous particles of about 150 nm in size (Figure 2B and
C and inset), which result in interparticle porosity. These
particles are themselves formed by aggregation of smaller
nanoparticles (Figure 2C and inset) and thus have accessible
smaller mesovoids, similar to previously reported meso–
macroporous materials.[36] As the reaction time proceeded
(1 d), material MMM(1) retained its meso–macroporous
structure (Figure 2D–F) and zeolite MFI nanoparticles
began to form on the macropore walls (Figure 2F, Fig-
ure 3A). The dark-field TEM image (Figure 3B) was ac-
quired on the same region as the bright-field TEM image
(Figure 3A). The bright spots in the image correspond to
MFI nanocrystals (Figure 3B). Figure 3C shows the wall
separating two macrochannels. The crystalline zone and the
remaining amorphous phase are easily distinguishable. The
nanocrystals that developed around the macropore walls
gradually embedded themselves within the continuous amor-
phous inorganic matrix and thus formed a new crystalline
framework while preserving the meso- and macroporous
scaffold (Figure 3A–C). These zeolite MFI particles, located
within the framework of the meso–macroporous aluminosili-
cate precursor (Figure 2G–I, Figure 3D) became more crys-
talline after 2 d at 1308C. Notably, the bright-field image
(Figure 3D) showed that the amorphous phase of the initial
material had been completely replaced by the nanocrystals
and thus suggests that this process aided the crystallization
of the amorphous framework leading to a micro–meso–mac-
roporous aluminosilicate structure. Crystallization or the
transformation of amorphous aluminosilicate phase into
well-crystallized zeolite phase did not affect the meso–mac-
roporous framework of the precursor (Figure 2G and H).
The parallel macrochannel structure is well preserved. The
dark-field image (Figure 3E) further indicated that the
meso–macroporous framework was mainly crystalline, owing
to the presence of bright spots corresponding to the MFI
nanocrystals fully occupying the matrix. Figure 3F shows an
image of the wall separating two macrochannels, which is
clearly composed of an assembly of nanocrystals. In addi-
tion, higher-magnification TEM images (Figure 3F and G)
and circular streaking in the electron diffraction pattern
(inset of Figure 3G) indicated that the micro–meso–macro-
porous aluminosilicates were constructed from randomly
oriented zeolite nanocrystals. Furthermore, high-resolution
(HR) TEM studies (Figure 3G–I, H and I) confirm that the
sample has very uniform nanocrystals with high crystallinity
observed by SEM (Figure 2G). The ZSM-5 nanoparticles in
the macroporous wall of MMM(3) show a high degree of
crystallinity, and the lattice fringes of 0.892 (Figure 3H) and
0.996 nm (Figure 3I) correspond to the (210) and (200) crys-
tal planes of the tetragonal structure of ZSM-5 (JCPDS card
no. 044-0002), respectively. Electron microscopy investiga-
tion revealed that the amorphous domain gradually disap-
peared with increasing reaction time, and subsequently
larger mesopores and voids, compared to the initial meso-
pores, were generated. Owing to the effect of the structure-
directing agent TPA+, the microporous zeolite crystals were
able to grow by using the amorphous phase as a source of
aluminum and silicon atoms to create new mesostructure.
Interestingly, the entirely zeolite architecture exhibits a uni-
form zeolite crystal size of about 150 nm (Figure 2I and Fig-
ure 3D, F–I), and this results in relatively uniform meso-
pores or mesovoids, which lead to improved catalytic activi-
ty. The crystal size of 150 nm is approximately the same size
as that of the amorphous aggregate of nanoparticles found
in the precursor material (ca. 150–200 nm, see Figure 2C).
Moreover, TEM studies (Figure 3C and F) reveal that the
nanocrystals of the surface of the macroporous channels did
not grow excessively with increasing reaction time, while the
crystal size remained around 150 nm. This phenomenon is
unique, and quite different from other routes reported pre-
viously.[24–25,43–50] As the size of the microporous zeolite crys-
tal formed is similar to that of the starting amorphous aggre-
gate of nanoparticles, this could potentially indicate that
each aggregate of nanoparticles was transformed into a zeo-
lite crystal, possibly due to good accessibility for the struc-
ture-directing agent and supplementary silica source of the
unique mesostructure formed by aggregation of the nano-
particle precursor and the relatively mild glycerol system.
This is probably a critical factor in the formation of uniform
crystals and maintenance of the mesostructure. The macro-
porous structure was also unaffected due to the relatively
thick macroporous wall in the meso–macroporous alumino-
silicate and relatively mild reaction system (Figures 2G–I
and 3B–F). This indicates that the overall meso–macropo-
rous structure within the initial material was maintained
during the transformation (Figures 2 and 3). Furthermore,
even after calcination at 5508C for 5 h, the nanocrystals in
macroporous walls do not obviously aggregate with each
other, and thus the mesopores or mesovoids are not affected
(Figure 3). This means that the hierarchical materials have
highly thermal stability, which is mostly important for recy-
cling. In summary these observations clearly suggest that a
hierarchically micro–meso–macroporous material which has
well-defined macropores and interconnecting mesopores
within the macropore walls was constructed from zeolite
nanocrystals with tunable micropores.
Figure 4 shows XRD patterns of micro–meso–macropo-
rous aluminosilicates MMM(0), MMM(1), and MMM(2)
synthesized at 1308C. Notably, the peaks in the XRD pat-
terns of samples MMM(1) and MMM(2) are characteristic
of ZSM-5 crystal symmetry (JCPDS card no. 044-0002), and
the enhanced wide-angle XRD pattern suggested that the
degree of crystallinity gradually increased and the amor-
phous phase of the precursor gradually disappeared with in-
creasing reaction time (Figure 4). These results are in good
agreement with the electron microscopy investigation.
Figure 5 shows N2 adsorption isotherms of MMM(0),
MMM(1), and MMM(2) calcined at 5508C. The isotherm
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Micro–Meso–Macroporous Zeolite Nanocrystal Aggregates
FULL PAPER
Figure 4. Wide-angle XRD patterns of A) MMM(0), B) MMM(1), and
C) MMM(2).
Figure 6. 27Al MAS NMR spectra of A) calcined MMM(0) and B) cal-
cined MMM(2).
correspondingly reduced. This indicates that the amorphous
aluminum atoms were gradually incorporated into the
micro–meso–macroporous framework in tetrahedral posi-
tions during the post-crystallization process. The transforma-
tion of Al atoms located in octahedral positions can be rea-
sonably attributed to complete condensation of the amor-
phous domain during the zeolite crystallization process,
which is further evidenced by 29Si MAS NMR spectroscopy
(Figure 7). Notably, the spectrum of the final material shows
a highly intense resonance at d=ꢀ112 ppm and a shoulder
peak at d=ꢀ102 ppm indicating that the micro–meso–mac-
roporous material consists primarily of cross-linked Q4 silica
units [d=ꢀ112 ppm, Si
ACHTUNGTRENNUNG
Si(OSi)3(OH) and/or Si(OSi)3ACHTNGUTERNNU(G OAl)] as deduced from a very
G
E
Figure 5. Top: N2 isotherms of calcined samples. A) MMM(0),
high Q4/Q3 ratio of 5. No Q2 units were observed. In con-
trast, the initial material has typical peaks corresponding to
B) MMM(1), and C) MMM(2). Isotherms
B and C are offset by
100 cm3 gꢀ1 and 600 cm3 gꢀ1, respectively, along the vertical axis for clarity.
Bottom: Pore size distributions of MMM(2) (macropore size distribution
curve obtained by mercury porosimetry, and the micropore and meso-
pore size distribution curves obtained by nitrogen adsorption–desorption
isotherms).
ACTHNUTRGNEUNG
Q2, Q3, and Q4 silica species, with a Q4/(Q3+Q2) ratio of 1,
revealing the presence of large amounts of amorphous alu-
minosilicate in the meso–macroporous framework. Com-
plete condensation of the framework would result in more
aluminum atoms on octahedral sites becoming tetrahedrally
coordinated. This complete condensation may be attributed
to the post-crystallization process, which could efficiently
crystallize amorphous aluminosilicate to a crystalline zeolite
phase.
The acidity of the samples (initial, 1 d, 2 d) was character-
ized by temperature-programmed desorption of ammonia
(Figure 8). For comparison, commercial zeolite ZSM-5 and
MMM(0) with similar Si/Al ratio (75–85) were also studied.
All three samples show a large desorption peak at 1508C,
corresponding to desorption of physisorbed NH3. Another
desorption peak of ammonia, albeit small but evident on
MMM(2), was located at about 5608C, and indicates strong
acid sites on the product, similar to commercial zeolite
ZSM-5 (Td =5208C). In contrast, MMM(0) with a similar Si/
changed progressively from type IV to type I with increasing
reaction time, and this suggests that the microporosity of the
material gradually increased. The final material MMM(2)
exhibits a type I isotherm with hysteresis at higher P/P0 indi-
cating clearly the presence of interparticle mesoporosity
(Figure 5). The changes in isotherm shape and form, as well
as the observation of hysteresis suggesting generation of mi-
croporosity, are in good agreement with TEM and XRD re-
sults. This was further confirmed by a 27Al MAS NMR in-
vestigation (Figure 6). The sharp, symmetrical signal cen-
tered at about d=53 ppm, which corresponds to tetrahedral
aluminum, was gradually enhanced with increasing reaction
time. Simultaneously, the signal centered at about d=
0 ppm, corresponding to an octahedral environment, was
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B.-L. Su, X.-Y. Yang, Z.-M. Liu et al.
Figure 7. 29Si MAS NMR spectra of A) calcined MMM(0) and B) cal-
cined MMM(2).
Al ratio to MMM(2) and amorphous framework displayed a
desorption peak at 4508C, indicating weak acid sites. These
results indicate that micro–meso–macroporous material
MMM(2) has strong acidity, even stronger than that of com-
mercial ZSM-5 crystals with similar Si/Al ratio, owing to full
crystallization of the framework.
A micro–meso–macroporous structure with strong acidity
has great advantages in catalytic reactions involving large
organic molecules, as diffusion constraints and/or adsorption
of reactant molecules onto the strong acid sites are the main
concern. Catalytic activities for the cracking of the large-
molecule hydrocarbon 1,3,5-triisopropylbenzene (TIPB)
over various catalysts are compared and summarized in
Table 1. The kinetic molecular dimension of TIPB is much
larger than the entrance dimensions of MFI zeolites, and
such molecules cannot penetrate into the internal channels
of zeolite ZSM-5. Cracking reactions can thus be realized
only at the external surface of ZSM-5 crystals. With a con-
tact time of 24 ms, HZSM-5, a commercial product with Si/
Al ratio around 75, is much less active (23.1%) owing to its
relatively small pore size with respect to the large diameter
of the reactant molecules. Al-MCM-41 and MCM-41 show
no activity. In contrast, calcined MMM(2) with the same
contact time yields the highest activity (88.6%). These ob-
servations further confirm that MMM(2) has strong acidity
and high catalytic activity due to its greater porosity. Owing
Figure 8. Temperature-programmed curves for the desorption of ammo-
nia for A) MMM(0), B) ZSM-5, and C) MMM(2).
to the large size of the TIPB molecule, the cracking reaction
on ZSM-5 zeolite crystals is independent of the contact
time. The increase in contact time augments the diffusion
effect in reaction systems. The advantages of micro–meso–
macroporous zeolite architectures for reactions involving
large organic molecules are evidenced. The presence of
large meso–macroporosity favors the access of TIPB mole-
cules to the active sites. The sharp increase in cracking activ-
ity clearly illustrates the superiority of MMM(2) as catalyst
in TIPB cracking due to the increased diffusion effect. From
Table 1, it is clear that the increase in contact time favors
deep cracking of the TIPB reactant, as well as the IPB and
DIPB intermediates formed during the reaction, to form
propylene and benzene. No secondary reaction products are
formed, since the cracking reaction was realized only at the
external surface of the ZSM-5 catalyst. The MMM(2) cata-
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Micro–Meso–Macroporous Zeolite Nanocrystal Aggregates
FULL PAPER
Table 1. Catalytic activity for cracking of 1,3,5-triisopropylbenzene and the structural parameters obtained
through BET measurements for various samples.[a]
general approach for the syn-
thesis of hierarchically micro–
Conv. Contact time BET surface area Si/Al
[%] [ms]
[m2 gꢀ1
Product distribution [%]
meso–macroporous
with zeolitic architecture.
materials
G
]
P1
P2
3.30
3.01
P3
P4
P5
P6
–
MMM(2)
ZSM-5
28.59 12
44.46 18
88.63 24
17.25 12
23.26 18
23.97 24
562
80
25.29
25.91
7.59 63.8
6.57 62.86
–
–
1.64
37.14 13.39
4.73
4.44 12.42 27.87
Conclusion
302
75
40.75 16.98 25.51 16.75
–
–
–
–
–
–
49.22 25.92 18.57
6.32
5.59
24.48 26.41 16.85
The current micro–meso–mac-
roporous materials are the first
Al-MCM-41
MCM-41
–
–
24
24
996
1075
82
1
–
–
examples
of
hierarchically
[a] Catalytic reactions were performed by pulse injection, and the data presented in this table are average
values of five injections. Conditions for each run: catalyst (50 mg), pulse-injected reactant (0.4 mL), flow rate
53.7 mLminꢀ1, temperature 3508C. The products are propylene (P1), benzene (P2), isopropylbenzene (P3), dii-
sopropylbenzene (P4), C4–C6 hydrocarbons (P5), and C7–C10 aromatics (P6). Samples were calcined at 5508C
for 5 h.
micro–meso–macroporous alu-
minosilicates and titanosilicates
with zeolitic architecture and
strong acidity or interesting oxi-
dation sites, obtained through a
lyst not only facilitates the production of propylene and
benzene, with increasing yield on increasing contact time,
but also formation of secondary products such as C4–C6 hy-
drocarbons and C7–C10 aromatics, which stem from the cycli-
zation and oligomerization of propylene during secondary
reaction, can be observed in the mesopores and/or internal
channels of ZSM-5 zeolite. This experiment clearly illus-
trates the benefits of hierarchically micro–meso–macropo-
rous zeolite architectures, as in certain reaction systems a
cascade of reactions can be carried out simultaneously by
using different functionalities of catalyst. MMM(2) is thus
an excellent candidate for a catalyst in industrial cracking of
petroleum, particularly for petroleum residues, for which
high reaction temperatures, large pore sizes, and cascade-
type reactions are required to obtain products with high
added value. Furthermore, this hierarchically micro–meso–
macroporous aluminosilicate with zeolitic architecture, con-
structed from zeolite nanocrystals, may have important tech-
nological implications for many other catalytic reactions in-
volving large molecules, whereas purely mesoporous or mac-
roporous materials with amorphous frameworks lack the re-
quired acidity.
Figure 9. A) SEM image (inset: high magnification image), B) wide-angle
XRD pattern, C) N2 isotherm, and D) 27Al MAS NMR spectrum of
MMMACHTNUTRGNEUNG(Beta). E, F) SEM images and wide-angle XRD patterns (inset of
Synthesis of micro–meso–macroporous materials with
zeolitic architecture by the post-crystallization process in
glycerol system is not only limited to ZSM-5 zeolite. It
could be successfully extended to the synthesis of micro–
F) of MMM
F) 100 nm.
ACHTUNGTREN(NUNG TS-1). Scale bars: A) 100 nm, inset: 100 nm, E) 1 mm,
meso–macroporous zeolite beta (MMM
ACHTUNGTREN(NUGN Beta), Figure 9A–
52]
D) and zeolite TS-1 (MMM
N
.
SEM and XRD studies reveal that zeolite beta and zeolite
TS-1 architectures with meso–macroporous structure are
successfully synthesized in the glycerol system (Figure 9),
similar to the case of hierarchically micro–meso–macropo-
rous aluminosilicate with zeolite ZSM-5 architecture.
post-crystallization process. They represent two significant
advances in the search for new catalysts and candidates for
catalyst supports. Firstly, these materials show a well-defined
macroporous structure, which has a highly interconnected
mesoporous system, in addition to the mesopores that devel-
oped during the growth of the microporous zeolite nano-
crystals. Secondly, and more importantly, the zeolite type
and its micropore size can be altered by a simple and clean
method that could yield new insight into how the synthesis
of these hierarchically micro–meso–macroporous materials
can be controlled. Catalytic testing demonstrates their supe-
riority in the cracking of 1,3,5-triisopropylbenzene (TIPB)
MMMACHTUNGTRENNUNG(Beta) shows a type I isotherm (Figure 9C) with hys-
teresis at higher P/P0 due to the presence of microporosity
and interparticle mesoporosity. The 27Al MAS NMR spec-
trum of MMMACHTUNGTRENNUNG(Beta) (Figure 9D) shows a very strong peak
around d=55 ppm, corresponding to tetrahedral Al sites in
crystalline zeolite. It is believed that the strategy developed
here provides a unique, effective, low-cost, and potentially
Chem. Eur. J. 2011, 17, 14987 – 14995
ꢁ 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.chemeurj.org
14993

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.
Acknowledgements
This work was realized in the frame of a Belgian Federal Government
(Belspo PAI-IAP) project, INANOMAT, P6/17, and a Belgium–Viet
Nam bilateral cooperation project, (BL/13V11). X.-Y.Y. thanks FNRS
(Fonds National de la Recherche Scientifique in Belgium) for a “Chargꢀ
de Recherche” position. B.L.S. acknowledges the Chinese Central Gov-
ernment for an “Expert of the State” position in the frame of “Thousand
Talents Program” and the Chinese Ministry of Education for a “Chang-
jiang Chair Visiting Scholar” position at Wuhan University of Technolo-
gy. We thank Prof. Jing Li and Mrs. Li-Na Wang of the Fourth Military
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