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10536 Chem. Commun., 2011, 47, 10536–10547 This journal is c The Royal Society of Chemistry 2011
Cite this: Chem. Commun., 2011, 47, 10536–10547
Recent advances in hierarchically structured zeolites: synthesis and
material performances
Zi Le Hua, Jian Zhou and Jian Lin Shi*
Received 14th January 2011, Accepted 10th June 2011
DOI: 10.1039/c1cc10261c
Hierarchically structured zeolites (HSZs) have attracted increasing attention in the last few years,
thanks to their unique hierarchical porous structures combining micro- and mesoporosity and
superior material performances, especially in the bulky molecules-involved catalysis and
adsorption applications. In this Feature Article, the recent advances in the HSZs synthetic
methodologies and material performances in catalysis are overviewed. Further, some perspectives
for the future development of HSZs are discussed.
1. Introduction
Zeolites are an important class of crystalline porous materials
with cage- or channel-like structures. Due to their well-defined
micropore sizes for molecular shape selectivity, large specific
surface areas, intrinsic acidity, and high (hydro)thermal
and chemical stability, they have shown great (or potential)
applications in many modern industrial processes related to
catalysis, adsorption, and separation. However, the small sizes
(typically o1.2 nm) of the inherent intracrystalline micropore
channels or windows of pure zeolites are becoming a more
and more significant obstacle for their applications in bulky
molecules-involved processes. The diffusion limitation of
relatively large molecules in the micropore channels would
result in a poor access of reactants to the zeolite active sites,
and more severely, the frequent blocking of the diffusion path
and the fast deactivation of the zeolites.
To solve the diffusion problems of guest species in zeolites,
ordered mesoporous materials (OMMs) with adjustable larger
pore sizes (2–30 nm), such as M41s,1 SBA-n,2 and MSU-x,3
have been successively invented since the 1990s. However,
until now, their practical applications are still far from the
extensive success, especially in the petrochemical processes,
owing to their amorphous frameworks and the resultant low
stability and lack of (strong) acidity. Around 2001, some
progress in the templating synthesis of highly stable and acidic
mesostructured materials was made through the incorporation
of zeolite secondary building units (SBU) in the mesoporous
frameworks, whose precursors could be the aged zeolite
State Key Lab of High Performance Ceramics and SuperfineMicrostructure, Shanghai Institute of Ceramics, Chinese Academy ofSciences, 1295 Dingxi Road, Shanghai 200050, P. R. China.E-mail: jlshi@mail.sic.ac.cn; Fax: +86-21-52413122
Zi Le Hua
Zile Hua obtained his MSdegree from South ChinaUniversity of Technology in1999 and his PhD degreefrom Shanghai Institute ofCeramics, Chinese Academyof Sciences (SICCAS) in2002. Then he joined the StateKey Lab of High PerformanceCeramics and SuperfineMicrostructure at SICCAS.In 2004, he worked atUniversity of Manchester asa visiting scholar on thesolvothermal synthesis ofyttria-stabilized zirconia. He
is now an associate professor at SICCAS. His current researchinterests include hierarchical porous materials, mesoporous thinfilms and environmental catalysis.
Jian Zhou
Jian Zhou obtained his BSdegree from China Universityof Geosciences in 2006, andthen continued his postgraduatestudy under the supervision ofProf. Jianlin Shi in the StateKey Lab of High PerformanceCeramics and SuperfineMicrostructure at SICCAS.His main research topic is thepreparation and catalyticapplications of hierarchicalmicro-/mesoporous materials.
ChemComm Dynamic Article Links
www.rsc.org/chemcomm FEATURE ARTICLE
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This journal is c The Royal Society of Chemistry 2011 Chem. Commun., 2011, 47, 10536–10547 10537
synthesis sols4,5 or the destructive products of pre-synthesized
zeolites.6 Unfortunately, no typical X-ray diffraction peaks
assigned to the specific zeolite structures appeared in their
wide-angle XRD patterns, and strictly speaking, these synthe-
sized products were still amorphous materials. Thus, a novel
class of hierarchically structured zeolites (HSZs) or so-called
mesoporous zeolites is highly desired, which should have
additional intra- or inter-crystalline mesoporosity in addition
to the inherent microporosity of zeolites. These HSZs would
integrate the characters of mesoporosity and the microporous
crystalline structure of zeolites, viz., improved mass transport-
ability of guest species through the interconnected micropore-to-
mesopore and mesopore-to-mesopore networks and zeolitic
stability and activity. When ordinary mesoporous surfactant
templates and small molecule organic ammonium agents for
microporous zeolites were directly used for the preparation of
HSZs, earlier attempts achieved little success, because there
was no expected synergistic or consecutive structure-directing
effect occurring between those two size-scale templates. Actually,
the obtained products were either microporous zeolite crystals
or amorphous mesoporous materials or phase-separated
mixtures of them, rather than the desired single-crystal(-like)
HSZs.7 In the following years, many new kinds of organic or
inorganic mesoporogens, classified as ‘‘soft’’ or ‘‘hard’’
templates, including some specially designed and/or synthesized
bifunctional agents, have been introduced in this templating
synthesis of HSZs. Among them, the typical ones include
amphiphilic organosilanes,8 polymers,9–11 carbon materials,12,13
and nanosized organic/inorganic particles.11,14,15 Specifically,
single-crystal-like HSZs microspheres, evidenced by the
continuous and well-crystallized lattice structure coexisting
with the mesoporous network in the HRTEM image, were
synthesized in our laboratory.11 In this report, poly(methyl
methacrylate) (PMMA) nanospheres were found to be a dual-
functional template for the generation of mesoporosity and
spherical HSZs particles, and an average mesopore size of 13 nm
was obtained, thanks to the great volume shrinkage of PMMA
nanospheres during the heat treatment. Very interestingly,
by a twice hard-templating approach, HSZs with ordered
mesoporosity were reported for the first time by Tsapatsis
et al. in 2008.15 In this report, three-dimensionally ordered
mesoporous (3DOm) colloidal crystals composed of mono-
disperse silica nanoparticles (10–40 nm) were used as templates
for replicating mesoporous carbons at first, then zeolite
precursors were introduced into the resultant carbon replicas
with large enough mesopore sizes, and a subsequent steam-
assisted crystallization (SAC) treatment led to the formation
of uniform and isolatable nanocrystals and single-crystal
zeolites with ordered imprinted mesoporosity (HSZs).
Alternatively, different from above mentioned ‘‘bottom-up’’
synthesis for HSZs, starting from the pre-synthesized micro-
porous zeolites, a post-demetallation process by steaming or
acid/base selective leaching is another effective strategy to
fabricate HSZs with additional intracrystalline mesoporosity.16,17
In fact, by this post-synthesis demetallation process, HSZs or
so-called modified zeolites have been extensively studied for a
long time and some of them have been actually used in the
petrochemical industry, such as the well-known ultrastable Y
(USY) catalysts.18 Better catalytic performances (activity,
selectivity, durability, and recyclability) were achieved compared
with their parent zeolites. Possible modifications induced by
the post-synthesis demetallation treatment were not limited
only to micro/mesoporous structures, but also to material
compositions and active site distributions, all of which would
cooperatively contribute to the improvement of material
performances.
In a word, whether by template synthesis or by the post-
synthetic demetallation process, a common target for HSZs
preparation is that the resultant products should be the single-
crystal(-like) materials with well-controlled and penetrating
mesoporous structures, and maximally preserved native char-
acters of pure zeolites (microporosity, acidity, and stability).
In the past decade, significant progress has been made in the
development of HSZs synthetic methodologies and material
performances, such as catalytic properties, and a number of
excellent reviews about HSZs have also been published in the
last few years.16,17,19–22 Interestingly, a very recent review,
which was given by Chal et al., outlined HSZs’ industrial
perspectives, however, no detailed information on the catalytic
performance of HSZs was addressed.22 Here, in this Feature
Article, combined with our new results on HSZs synthesis
with the ordinary cationic or copolymer surfactants as the
mesoporogens, the most recent advances in this rapidly growing
research field will be reviewed, mainly focusing on the
publications in the last three or four years. To concentrate
our reviewing and also limited by the article length, mainly
the direct synthesis of HSZs by using soft templates and
post-synthesis demetallation/desilication will be concerned in
this article.
2. Templating synthesis of HSZs
2.1 HSZs synthesized with specially designed templates
Direct synthesis of HSZs with multi-length scale templates is a
straightforward idea originated from the successful preparation
of OMMs via supramolecular micelles. However, as mentioned
Jian Lin Shi
Jianlin Shi obtained his PhDdegree in 1989 at the ShanghaiInstitute of Ceramics, ChineseAcademy of Sciences (SIC-CAS). His main researchfields include processingscience of advanced ceramics,sintering theory, inorganicnano-structured materials andmesoporous materials. Morethan 260 papers have beenpublished and these publica-tions have been cited by otherscientists for more than 3400times with an H-index of 32.Professor Jianlin Shi is now
the director of the State Key Lab of High Performance Cera-mics and Superfine Microstructure. He has been awarded ofShanghai Natural Science Prize (the first grade) and alsoreceived several other awards.
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10538 Chem. Commun., 2011, 47, 10536–10547 This journal is c The Royal Society of Chemistry 2011
above, these preliminary attempts were unsuccessful due to the
strong phase-separation tendency between the employed meso-
scale templates and the rapidly growing zeolite crystals. To get
the single-crystal(-like) HSZs with this direct synthesis method,
mesoscale templates and/or synthesis processing should be well
designed to control the interactions between these templates and
zeolitic species during crystallization. In this respect, a great
progress has been made by Ryoo et al. in 2006.8 Through the
specially designed and synthesized amphiphilic organosilanes,
[3-(trimethoxysilyl)propyl]hexadecyldimethylammonium chloride
([(CH3O)3SiC3H6N9CH3]2C16H33]Cl, TPHAC), and their struc-
tural analogues, the covalent linkages between the growing crystal
domains and mesostructure directing agents were formed, which
were stable in the basic synthesis solutions and effectively
suppressed the exclusion of mesoscale templates from the growing
zeolite crystal phases. Consequently, mesoporous zeolites built
with randomly oriented zeolite nanocrystals were obtained.
In recent years, using these amphiphilic organosilanes as
mesoscale templates, this methodology has been applied
to HSZs synthesis with different framework structures and
compositions, such as (Me)-ZSM-5,23,24 LTA zeolite,8,25
TS-1,26 microporous aluminophosphate,27,28 and even basic
sodalite.29 Their mesopore diameters could be finely tuned
depending on the chain lengths of the organosilanes and/or the
hydrothermal synthesis parameters (temperature, time, etc.).
Moreover, as reported in LTA HSZs synthesis, the mesopore
diameters could even be expanded up to 24 nm when the
excessive organosilane surfactants or the auxiliary pore-
expanding agents, such as EO20PO70EO20 triblock copolymers,
were introduced.25 NH3-TPD or CO2-TPD characterization
results confirmed that the synthesized HSZs possessed the
similar acidic or basic properties to those of corresponding
purely microporous zeolites, whereas the amorphous mesoporous
or SBU-containing counterparts just showed much weaker
acidity or basicity. Superior catalytic performances had been
achieved when the synthesized HSZs were applied in a series of
small and/or bulky molecules involved reactions, such as the
transformation of methanol to olefin/gasoline,8,30 organic
synthesis of jasminaldehyde and vesidryl,8 benzene hydroxylation
to phenol,24 base-catalyzed Knoevenagel condensation and
Claisen–Schmidt condensation,29 etc. Moreover, the additional
micro-/mesopore networks in HSZs not only enhanced the
catalyst activity and product selectivity, but also significantly
improved the catalyst stability and recyclability. In a following
research, the surface Al species on the mesopore walls of the
synthesized HSZs were selectively removed by the bulky tartaric
acid, while the internal Al in the micropores was mostly
preserved.31 As a result, for the bulky molecules involved
reactions, the catalytic activities of the dealuminated HSZs were
lost almost completely, though the as-synthesized HSZs were
more active than the purely microporous zeolites and amorphous
OMMs counterparts. However, for small molecular reactions, the
dealuminated HSZs still exhibited high catalytic activities as the
parent HSZs. Based on these facts, it has been concluded that
bulky molecules involved reactions were catalyzed by the active Al
sites located at the surface of the mesopore walls. The number of
Al sites on the mesopore walls was also determined by the
chemisorption of 2,6-di-tert-butylpyridine, which was correlated
with the high catalytic activities of the synthesized HSZs.
Very recently, a bifunctional diquaternary ammonium-type
surfactant (as shown in Fig. 1) was designed by the same
group, in which the diammonium head group acts as an
effective structure-directing agent for the MFI zeolite, while
the hydrophobic interaction between the long-chain tails
induced the formation of a mesoscale micellar structure.32
The resultant materials were mesostructured, which were
composed of three-dimensionally intergrown multilamellar
or even unilamellar zeolite nanosheets depending on Na+
concentrations and pH values of the synthesis mixtures.33
More interestingly, through pillaring post-treatment, a widely
used process for the pore-generation or pore-expansion upon
the layered materials, the template-containing MFI multilamellar
nanosheets could be transformed into the pillared zeolite
nanosheets, as shown in Fig. 2.34 An almost ideal micro-/
mesoporous HSZ was successfully prepared, which possesses
long-range structural ordering in both micro- and mesopore
regimes. Compared with those of purely microporous zeolites,
greatly enhanced catalytic activities and increased catalyst
lifetimes were achieved in the typical reactions of bulky
molecules-involved cracking of branched polyethylene, protection
of benzaldehyde with pentaerythritol, condensation of
2-hydroxyacetophenone with benzaldehyde, or methanol-
to-gasoline conversions, due to their significantly enhanced
external surface areas, and consequently the increased number of
easily accessible surface acid sites of the zeolite nanosheets.32
Also, in a very recent report, surfactant-modified MFI-zeolite
nanosheets were used as highly efficient anion-exchangers for the
removal of nitrate ions from the aqueous effluents.35
On the other hand, in Xiao’s recent report, a cationic
polymer, polydiallyldimethylammonium chloride (PDADMAC),
was used as the mesoscale template.36 The electrostatic inter-
action between cationic polymer templates and negatively
charged beta nanocrystals (30–80 nm) induced the formation
of micrometre-sized (2–20 mm) and mechanically stable
HSZs microspheres. In addition to a relatively large mesopore
volume, similar pore characters and more importantly, similar
catalytic performances were obtained between the synthesized
HSZs microspheres and the corresponding zeolite nano-
crystals. When large-scale industrial applications were considered,
easier catalyst separation, whether in material preparation or
in catalyst recycling via a filtration route, would be the major
advantage of HSZs microspheres compared with zeolite
nanocrystals. Similarly, micrometre-sized HSZs particles
composed of nanosized zeolite crystals could also be prepared
with Pluronic triblock copolymer F12737 or even without the
additional mesoscale templates.38,39
As one of the most important oxidation catalysts, TS-1
HSZs could also be synthesized using the amphiphilic organo-
silanes as the soft templates.26 However, although the resultant
Fig. 1 The three-dimensional molecular structure of ‘bifunctional’
cationic surfactant (C22-6-6). Reproduced with permission from ref. 32
Copyright 2009, Macmillan Publishers Limited.
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products possessed the expected interconnecting micro-/
mesopore systems and better performances in the catalytic
oxidations of bulky substrates in the hydrophobic media,
lower activity or even complete ineffectiveness in phenol
hydroxylation was observed when aqueous H2O2 was used
as the oxidant. Different from the hydrophobicity of pure TS-1
zeolite, the presence of a large number of surface defects
and consequently the hydrophilicity of these synthesized TS-1
HSZs should be the main reason.
Fortunately, an interesting result was recently reported by
Li et al.40 Using nanosized carbon black as the mesoporous
hard template, TS-1 HSZ was prepared via the SAC process,
whose particles were composed of many nanosized zeolite
crystals. Its mesoporosity was confirmed with variable-
temperature 129Xe NMR spectroscopy and the Ti species has
almost the same coordination environment as those in purely
microporous TS-1 zeolite. Importantly, in the following model
reaction of phenol hydroxylation, TS-1 HSZs showed similar
product selectivity but significantly higher catalytic activity
(phenol conversion up to 33% in 4 hours, a maximum based
on a phenol-to-H2O2 ratio of 3) than TS-1 zeolite (19.3%
under identical reaction conditions). Similarly, enhanced
catalytic activity for phenol hydroxylation was also obtained
for the alkaline-treated TS-1 catalyst.41 The penetrable
mesopores were thought to be the main reason for the
enhanced activity of TS-1 HSZs. However, both of them did
not give any catalytic properties about bulky molecules
involved reactions.
2.2 HSZs synthesized with conventional soft templates
It is well-known that the ordinary surfactant templates for
OMMs synthesis, e.g. cetyltrimethylammonium bromide
(CTAB) and triblock copolymer P123, are not the suitable
agents for HSZs synthesis, because of their limited charge
densities and consequently weak interactions with zeolitic
species. However, our recent results showed that under the
controlled synthesis conditions, HSZs could also be synthe-
sized with these ordinary mesoscale templates, especially via a
steam-assisted crystallization (SAC) process.42–45 As an extension
of our previous work,46 when CTAB was used as the
mesoporous template, HSZs hollow spheres with an average
mesopore size of around 3.0 nm were synthesized through
carefully controlling the synthetic procedure.42 More specifically,
the key parameters for the successful synthesis of HSZs by
this process have been identified to be the pre-hydrolysis
temperature of sol precursors and the following hydrothermal
treatment. TEM images gave the direct evidence for the
coexistence of the mesoporous structure and the zeolite crystal
lattice structure.
The SAC process is also called vapor-phase transport (VPT)
or dry gel conversion (DGC), which is one of the effective
routes to synthesize conventional zeolites, e.g. Silicalite-1,
ZSM-5 and beta zeolite.47,48 Unlike in the conventional
hydrothermal synthesis where zeolite crystallization occurs in
a large-scale solid–liquid system, in the SAC process, dried gel
precursors contact only with water and/or microporous
template steam. Since mass transport during this framework
crystallization transformation is greatly suppressed due to the
absence of a continuous liquid phase, this crystallization
process is considered to be nearly an in situ course. Therefore,
if the precursors possess mesoporous structures and cautious
control is made, phase separation tendency between the zeolite
precursors and the template molecules within the dried gel
precursors can be inhibited during the SAC process, and a
hierarchical micro/mesoporous structure could be achieved by
this strategy. A few years ago, using dried zeolitic species/
CTAB hybrids49 or AlMCM-41/TPAOH as precursors,50 the
Fig. 2 High-resolution SEM (A) and TEM (B) images of the ordered
multilamellar MFI zeolite nanosheets synthesized with C22-6-6Br2,
in which the interlayer spacing has been supported with silica
pillars (pointed by the white arrows in parts (A) and (B)). Reproduced
with permission from ref. 34. Copyright 2010, American Chemical
Society.
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10540 Chem. Commun., 2011, 47, 10536–10547 This journal is c The Royal Society of Chemistry 2011
DGC route was usually adopted for the synthesis of mesoporous
zeolites (or composites). The resultant products exhibited
higher adsorption capacities than microporous zeolite and
amorphous mesoporous counterparts. Recently, a new type
of hierarchically structured composites of ZSM-5 nanocrystals
embedded in a well-connected mesoporous matrix was
also synthesized by the DGC method,51 in which TPAOH
simultaneously worked as the microporous template and the
mesoporous scaffolding agent.51,52 Though the resultant
TUD-C or TUD-M materials do not have a fully crystalline
structure, they showed an improved hydrothermal stability.
More importantly, by varying the crystallization conditions,
the zeolite crystal size and the mesopore size can be tuned
systematically.
Based on the SAC process, very recently, c-TUD-1, one
kind of HSZs with zeolitic mesoporous frameworks,43,44 has
been prepared by our group by employing Al-doped TUD-1
mesoporous silica as precursors.53,54 In this synthesis, triethanol-
amine (TEA) and its polymer derivative poly-TEA work not
only as the mesoscale templates, but also as the scaffolds to
keep the intactness of the mesostructures during the SAC
process.44,55 As shown in Fig. 3, the obtained HSZs particles
showed the penetrable mesoporosity and, more importantly,
the single crystalline features as revealed by the HRTEM
image and the electron diffraction pattern on a large area
(Fig. 3C). As a result, a greatly improved hydrothermal
stability was observed with the resultant HSZs, which showed
only B10% loss of specific surface areas even after 200 h
hydrothermal treatment. As a comparison, an B40% specific
surface area loss was found after 50 h hydrothermal treatment
for the corresponding amorphous mesoporous precursors.43
Applicability of such a SAC approach has also been verified
in the preparation of TS-1 HSZ, which exhibits high catalytic
activity and a strongly prolonged lifetime in the selective
oxidation of 2,3,6-trimethylphenol (TMP) to trimethyl-p-
benzoquinone (TMBQ).45 This excellent catalytic performance
is closely related to its hierarchical micro/meso-structures, and
high stability against mesostructure collapse and Ti leaching
during the catalytic applications. Further, this process could
be extended with other conventional surfactant templates, such as
F127, P123, and Brij series, which are highly commercialized
low-cost products and now widely used for the synthesis of
mesoporous materials.56 Similar to using TEA agent, the
resultant HSZs displayed the disordered but well-defined
mesoporosity. Also, the synthesized materials demonstrated high
catalytic activity and remarkably improved anti-deactivation
performance in the cracking of TIPB and also esterification
reactions.56 In a word, the key to the successful synthesis of
HSZs by the SAC process using these conventional soft
templates is believed to be the suppression of phase separation
between the mesoporous templates and the silica species
during the crystallization transformation, where extensive
liquid phase should not be present.
Based on the similar considerations as the above mentioned
SAC process to suppress the mobility of silicates and/or
templates during crystallization, direct hydrothermal synthesis
of BEA HSZs is also feasible when one type of cyclic
diammonium (CD) was used as a structure-directing agent.57
The highly crystallized BEA HSZs product was composed of
Fig. 3 (A) TEM image of a representative c-TUD-1 particle (scale
bar = 0.2 mm) (inset: SEM image of a c-TUD-1 particle (scale bar =
500 nm)); (B) higher magnification TEM image of the rim of c-TUD-1
(scale bar = 50 nm); (C) HRTEM image of the square frame in
(B) (scale bar = 20 nm) (inset: the corresponding SAED). Reproduced
with permission from ref. 43. Copyright 2009, Wiley-VCH Verlag
GmbH & Co. KGaA, Weinheim.
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interconnected zeolite nanocrystals of 10–15 nm in diameter.
More interestingly, the synthesized HSZs inherited the
morphology and meso- and/or macroporous characters of
the precursor gels or diatomaceous earth sources (pseudo-
morphic crystallization). A possible explanation lies in the
strong interactions between CD and silicates and the rigidity
of the CD molecule, which prevented phase separation during
hydrothermal treatments. Due to the enhanced molecular
diffusion through mesopores in the HSZs catalyst, in a model
reaction of the isopropylation of naphthalene, the BEA HSZ
still retained 74% of its initial catalytic activity after 5 h of
employment, while the conventional zeolite lost its catalytic
activity completely.
3. HSZs synthesis by a demetallation route
HSZs synthesis via a demetallation route starts from a pre-
synthesized zeolite precursor. Through selective leaching of
framework atoms (Al3+, Si4+, Ti4+, etc.), intracrystalline
mesopores can be introduced within the zeolite precursor
particles. Possible leaching processes include steam or acid
treatment for dealumination,58 base treatment for desilication,16
and H2O2 treatment under microwave irradiation for
detitanation.59 Among them, the best known way to create
mesopores in zeolite is dealumination. Although this treatment
improves the catalytic performance of zeolites, recent 3D-TEM60
and PFG-NMR61 analyses indicated that the mesopores
introduced by dealumination do not yield an interconnected
pore network and therefore contribute little to molecular
diffusion. Desilication is now considered to be a more effective
strategy than dealumination for introducing mesopores in
zeolites. Up to date, besides NaOH solution, generally used
alkaline treating solutions include inorganic (LiOH, KOH,
Na2CO3) and organic (TMAOH, TPAOH, and TBAOH)
compounds. Some multi-constituent leaching agents or multistep
treating processes were also adopted recently. For example, a
two-step route comprising treatment in sodium aluminate
followed by acid extraction had been successfully developed
to introduce secondary mesoporosity in ZSM-5.62 Compared
with ordinary desilication by NaOH solution, this two-step
route led to equivalent mesopore surface areas. However, a
higher solid yield is attained due to the one order of magnitude
lower dissolution rate of silicon. Consequently, the resultant
HSZs have much smaller mesopores (5 nm) and the Si/Al ratio
is very similar to that of the starting precursor.
3.1 HSZs synthesis by desilication: effect of micro-/
mesoporous templates and leaching conditions
To modulate the desilication process for HSZs synthesis,
partial detemplation (microporous structure directing templates)
of zeolites followed by desilication in alkaline medium has
been attempted.63 This process is based on the fact that the
template-containing zeolite is virtually inert to Si leaching
upon treatment in aqueous NaOH solutions. Depending on
the pre-desilication calcination temperature, the microporous
template was partially removed and these template-free portions
would be susceptible to the subsequent alkaline leaching.
Thus, under typical desilication conditions, mesopore surface
areas of the resultant HSZs could be controlled in the range of
20–230 m2 g�1. In the catalytic pyrolysis of low-density
polyethylene, the progressively enhanced activity was obtained
with the increasing amount of mesopores. More interestingly,
even for ZSM-5 zeolite with the template being completely
removed, bulkier quaternary ammonium cations (TPA+ or
TBA+ ions) could also work as efficient pore-growth
moderators, which were introduced by impregnating the parent
zeolites with the corresponding salts prior to NaOH treatment,
or in the form of hydroxide compounds as one part of alkaline
sources.64 The quaternary ammonium cation moderates the
pore growth by buffering the OH� attack and prevents the
over-fast Si extraction (leaching rate and leaching degree).
This unique feature leads to the resultant HSZs with smaller
mesopores and better preserved microporosity. Compared
with parent zeolites and solely NaOH-treated samples,
faster molecular diffusion and higher catalytic performances
in the benzene alkylation with ethylene were observed in
(NaOH + TPAOH)-treated zeolite. Optimal ZSM-5 HSZ
was obtained upon the desilication process with the mixed
(NaOH + TPAOH) alkaline solution.
Surfactant CTAB, a commonly used template for the
preparation of M41s materials, was found to be helpful for
the pseudomorphic transformation of pre-synthesized zeolite
Y to HSZs, though the detailed mechanism is not clear yet.65
TEM images of parent and alkaline-treated zeolites indicated
that the resultant product maintained the typical crystalline
morphology of the precursor zeolite Y and possessed an
interconnected network of micro- and mesopores. Another
different feature from classical desilication processes is that the
presence of CTAB ensured no preferential silica extraction
under the reported treating conditions and the Si/Al molar
ratios before and after alkaline leaching kept almost constant.
However, the N2 adsorption–desorption isotherm and
NH3-TPD results showed obvious decreases in microporosity
and the strength and intensity of the strong acid sites in the
synthesized HSZs.
Although attempts to modify the porous characteristics of
ferrierite by dealumination were unsuccessful, desilication,
with aqueous NaOH solutions or with a sequential treatment
in aqueous NaAlO2, HCl, and NaOH solutions, has been
proven to be feasible under relatively harsher conditions than
that for other frameworks (MFI, MTW, and MOR) with a
similar Si/Al ratio.66,67 As shown in Fig. 4A, under optimal
conditions (0.2 M NaOH solution, 353 K, 9 h), the N2 uptake
of sample AT-16 at lower relative pressures is similar to that of
the parent (P) material, which evidences the preservation of
microporosity. The increased uptake at middle-to-high relative
pressures and a hysteresis loop present in the mesopore region
indicate the appearance of mesoporosity in the alkaline-treated
ferrierite samples. Further, as shown in Fig. 4B, the NH3-TPD
results confirm the similarity of acid strength and acid
intensity between sample P and sample AT-16. However, for
the over-treated sample AT-11 (0.5 M NaOH solution, 353 K,
3 h), serious damage of the intrinsic microporous crystallinity
led to the decrease of the strong acid intensity. In the following
model reaction, the pyrolysis of low-density polyethylene
(LDPE), the introduction of mesoporosity in the optimally
alkaline-treated sample AT-16 promoted the LDPE degradation
with respect to the parent zeolite, and what appeared to be
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more interesting was the possibility of tuning the product
distribution, though this required further verification.
3.2 HSZs synthesis by the combined dealumination and
desilication
For the post-synthetic desilication process, in some cases, a
sequential treatment of first acid-leaching dealumination and
then alkaline-leaching desilication became necessary, since
there exists an optimal Si/Al molar ratio in the range of
25–50 for the desilication process16 and some zeolite precursors
have lower Si/Al ratios. As reported by van Bokhoven et al.,68
the synthesized mordenite HSZs showed near 100% conversion in
3 h in the alkylation of benzene with benzyl alcohol, but the
conversions with other catalysts (conventional mordenite,
mordenite HSZs synthesized with amphiphilic organosilanes
or carbon templates) were less than 3%. However, in some
other cases, the multi-step treating sequence should be
optimized, i.e. first alkaline-leaching desilication (introduction
of mesoporosity) and then acid washing (surface acidity
modification), as reported in ZSM-5 HSZs synthesis for
shape-selective xylene isomerization.69 In addition to the
development of secondary mesoporosity, the alkaline-leaching
desilication treatment also induced substantial aluminium
redistribution, increasing the density of Lewis acid sites
located at the external surface and the concentration of
Brønsted centers at the pore mouths. As a consequence,
for just one-step alkaline-treated zeolite, although a higher
o-xylene conversion was obtained compared to the purely
microporous counterpart, the selectivity to p-xylene decreased
and fast deactivation occurred. A subsequent mild HCl washing
became necessary, which could selectively eliminate the
deactivated sites and lead to an approximately twofold increase
in p-xylene yield and a reduced deactivation rate. From these
results, it is clear that for the HSZs preparation with a
post-synthetic desilication process, the detailed processing
parameters (treating sequence, alkaline/acid sources, compositions,
temperature, etc.) should be carefully selected, which heavily
depends on the zeolite precursor frameworks, active site
characters and the specific requirements of the catalytic
process under investigation.
4. HSZs performances and characterizations
4.1 Molecular adsorption and diffusion characteristics in
HSZs
With the development of HSZs synthesis methodology, attention
should also be paid to their structural characters and perfor-
mances in the practical applications. Actually, although
in the reported (model) reactions the improved material
performances of HSZs have often been attributed to the
enhanced diffusion capability of reactants and/or products in
the highly developed micro-/mesopore systems, little direct
evidence of the diffusion characteristics had ever been given.
To answer these questions, some newly published results could
be most helpful.
In Groen’s research, the taper element oscillating micro-
balance (TEOM) technique has been adopted to get an
accurate assessment of the diffusion characteristics of large
and gradient-free ZSM-5 zeolite and the corresponding
ZSM-5 HSZs obtained from post-synthetic desilication.70
Neopentane was used as a probe molecule to exclude the effect
of coke formation during tests. Since the intrinsic diffusivity of
neopentane in the micropores should be the same in both
samples, they concluded that the characteristic diffusion path
length in HSZs is dramatically reduced due to the presence of
an accessible interconnected network of micro/mesopores.
Consequently, compared with that of purely microporous
zeolite, a two orders of magnitude faster diffusion of neopentane
in the HSZs was found. This was the first time that the
enhanced diffusion capability of HSZs was evidenced directly.
Similar conclusions could be found in other reports about
the diffusivity characterization of toluene in template-synthesized
mordenite HSZs68 and of cumene in NaOH-treated ZSM-5
Fig. 4 (A) N2 isotherms of the parent (P) and alkaline-treated
ferrierite samples (AT-11 and -16). Inset: adsorption BJH pore size
distribution. (B) NH3-TPD profiles of the parent and alkaline-treated
ferrierite samples. Reproduced with permission from ref. 66.
Copyright 2009, Elsevier Inc.
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HSZs.71 For LTA HSZs synthesized with amphiphilic
organosilanes, xenon uptake and 120Xe NMR measurement
at 297 K revealed that due to the highly developed mesoporous
networks, the xenon diffusion in LTA HSZs could occur
200 times more rapidly than in purely microporous zeolite.25
In Xu’s research, more complete characterization of the diffusion
mechanism, the adsorption dynamics and the diffusion
thermodynamic properties of the probe cumene molecule were
studied.71 Results showed that during the adsorption process
on ZSM-5 HSZs, it covered three steps of diffusion, phase
transition, and re-arrangement of cumene molecules. The
introduction of mesopores in the zeolites decreased the diffusion–
adsorption activation energy by a factor of 4.6, which implied
an enhanced diffusion rate and a decreased adsorption energy
barrier for cumene molecules in the HSZs. Consequently,
although the amount of strong Brønsted acid sites decreased
in HSZs, the conversion of cumene was still two times higher
than that on the purely microporous ZSM-5 catalyst.
4.2 Effect of surface acidity and mesoporosity of HSZs on
material performances
To evaluate the possible differences in type, concentration,
strength of acidic sites and their locations between purely
microporous zeolites and HSZs (carbon template synthesis),
pyridine, d3-acetonitrile and 2,6-tert-butyl pyridine were used
as probe molecules for FT-IR characterization.72 Results
showed that no substantial difference could be observed when
smaller sized pyridine and d3-acetonitrile were used. In
contrast, for larger 2,6-di-tert-butyl pyridine molecules, due
to the steric limitation effect, high concentrations of Brønsted
sites at the mesopore surface of HSZs were identified. More-
over, through model reactions of toluene disproportionation,
toluene alkylation with isopropyl alcohol, and p-xylene
alkylation with isopropyl alcohol, the impact of the additional
mesopores on material performances was clarified. For the
diffusion-controlled reactions (toluene disproportionation or
p-xylene alkylation with isopropyl alcohol), reactant conversion
increased with increasing mesopore volume. Comparatively,
reactant conversion would not increase significantly with
increasing mesopore volume for the product-desorption-
controlled reactions (toluene alkylation with isopropyl alcohol).
On the other hand, when para-selectivity was concerned in
products, the presence of mesopores might show a negative
effect, since the shortened diffusion length is detrimental for
their selectivity.
Very recently, the positive impact of mesoporosity on MFI
zeolite catalyst lifetime has been confirmed for practical
methanol-to-hydrocarbon (MTH) reactions through control
experiments.30,73 As shown in Fig. 5, a roughly linear correlation
between the mesoporosity and the catalytic lifetime was
illustrated.30 For HSZs, higher external surface area means
longer catalyst lifetime, which is not sensitive to the adopted
synthesis methodology, whether by post-synthetic desilication
or by hard-template synthesis with nanocarbon materials or
by soft-template synthesis with amphiphilic organosilanes.
Since the coke deposition in the micropores is highly
detrimental to the catalytic activity and stability, it has
been evidenced that the enlarged external surface area and
shortened diffusion path length would facilitate the mass
transfer of coke precursors from the micropores to the external
surfaces and consequently prevent the catalysts from quick
deactivation. Similar phenomena were observed in aromatization
and isomerization of 1-hexene over alkali-treated HZSM-5
zeolite.74 As a simplified consideration, Fig. 6 gives a scheme
of the reduced channel blockage of the alkali-treated HZSM-5
zeolite by the micro-/mesoporosity. In HSZs, due to the
interconnection between micropores and mesopores, the
diffusion path in micropores is greatly shortened. As a result,
the aromatization and isomerization products or even coke
Fig. 5 Correlation between catalyst lifetime (t1/2) in the MTH
reaction and external surface areas of zeolite (ZST-12) and hierarchical
zeolites (CTZ-16, BTZ-13, OSD-2 and OSD-5). Reproduced with
permission from ref. 30. Copyright 2009, Elsevier Inc.
Fig. 6 Scheme of the reduced channel blockage of the alkali-treated
HZSM-5 zeolite with micro-mesopore porosity. Reproduced with
permission from ref. 74. Copyright 2009, Elsevier B. V.
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precursors formed in the micropores can diffuse out of the
pores on time, while coke deposition tends to occur in the
mesopores of HSZs. Thus, the improved catalytic stabilities
in aromatization and isomerization can be achieved due to
the reduced coke formation/diffusion path blockage in the
micropore network.
Using silylated polymers as mesoporogens, Pinnavaia et al.
reported MSU-MFI HSZs with adjustable mesopore size.75
Very recently, these HSZs have been evaluated as the gas-oil
cracking catalysts.76 Results showed that compared with the
purely microporous ZSM-5 counterpart, MSU-MFI HSZs
possessed not only higher catalytic activity and longer lifetime,
but more importantly, the prior selectivity for desired
products. Since ZSM-5 is currently used as a commercial fluid
catalytic cracking (FCC) additive to boost octane ratings and
liquefied petroleum gases yields, the application of MSU-MFI
HSZs is expected.
4.3 HSZs as catalytic supports
In conventional zeolites, the small sized micropore network
can be seldom employed to load active guest species due to the
resulting pore narrowing or even blockage of the micropore
channels by the guests. The HSZs, possessing an inter-
connected micropore–mesopore network, are not only the
active catalysts by themselves, but also the ideal supports for
the other functional species, like the commonly used (porous)
carbon, OMMs, and g-Al2O3, etc. Compared with amorphous
OMMs and purely microporous zeolite supports, positive
effects of HSZs lie in the enhanced mass transport capability
of reactants/intermediates/products, high dispersion of guest
species within the additional mesopores, inhibition of possible
sintering of active metals, and synergistic effects between
active guests and the acidic supports.77–80 Typical examples
could be found in the recent reports by Prin’s77 and Xiao’s
groups.79 These two groups, independently and almost simul-
taneously, demonstrated the improved catalytic performance
of noble metals incorporated HSZs (ZSM-5 or b-zeolite)for the hydrodesulfurization (HDS) of bulky 4,6-dimethyl-
dibenzothiophene (4,6-DMDBT), though their selected HSZs
varied in the synthesis methodology and the resultant micro-
and mesoporous structures. In addition to the favored fast
diffusion of bulky 4,6-DMDBT molecules in larger mesopore
sizes of HSZs, more importantly, the strong acidity of HSZs
made great and also key contributions to the improved
catalytic performance. First, the strong acidity within HSZs
is favorable for demethylation and scission of a C–C bond
connecting the two rings of a 4,6-DMDBT molecule, which
may transform the refractory component into more reactive
species and thus accelerate HDS. Second, since partial electron
transfer can occur from the supported metal particles to acidic
sites of HSZs, the resultant electron-deficient metal particles
have, no doubt, higher sulfur resistance and better hydrogenation
properties.
4.4 Quantitative indexing for the micro-/mesoporosity of
HSZs
Although a number of methodologies of HSZs syntheses
have been developed and their catalytic performances been
investigated extensively, reports on quantitative indexing for
the micro-/mesoporosity are still rare. Very recently, two kinds
of indexes have been proposed for the quantitative assessment
of the micro-/mesoporosity of HSZs. One is an accessibility
index (ACI)81 and the other is the hierarchy factor (HF).64
ACI is derived from infrared spectroscopy of probe molecules
over HSZs or other porous solids. For ZSM-5 zeolite, the
selected probe molecules could be pyridine (0.57 nm),
2,6-lutidine (0.67 nm), and 2,4,6-collidine (0.74 nm). Since
probe molecules could be adsorbed on specific surface sites
determined by their physical sizes and chemical properties,
ACI would reflect the (relative) amount of the accessible
specific surface sites (e.g., the accessible number of Brønsted
acid sites among the total ones). As shown in Fig. 7, it gives the
ACI of substituted alkylpyridines as a function of the meso-
porous surface areas of ZSM-5 zeolite parent (P) and three
ZSM-5 HSZs derivatives (H1/H2/H3) synthesized from a
post-synthetic desilication treatment. Apparently, with the
increase of material mesopore surface area or so-called the
extent of mesoporosity, ACI of the largest probe molecule
(ACI(Coll)), 2,4,6-collidine, increases from 0.06 for sample P,
to 0.11, 0.15 and 0.38 for sample H1, H2, and H3, respectively.
For bulky molecules involved reactions, higher ACI value
means higher accessibility of surface Brønsted acid sites and
consequently possible superior material performances.
On the other hand, based on the hypothesis that the
catalytically functional centers are located in the micropores
(active sites) and the guest accessibility is contributed by
the auxiliary mesopores, HF is defined as the product of the
relative micropore volume (Vmicro/Vtotal) and the relative
mesopore surface area (Smeso/SBET).
Its effectiveness has been confirmed by the above alkylation
reaction of benzene with alkaline-treated ZSM-5 under the
presence of pore-growth moderator organic ammonium cations,
in which a perfect linear dependence of ethylbenzene (EB)
productivity on sample’s HF was obtained (Fig. 8).64 The
maximization of HF should be the target for the newly
structured HSZs design and the novel synthesis methodology
Fig. 7 Accessibility index (ACI) of pyridine (Py), lutidine (Lu), and
collidine (Coll) versus the mesopore surface area of the ZSM-5 zeolites.
Reproduced with permission from ref. 81. Copyright 2009, Elsevier Inc.
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development, which reflects a favorable balance between the
enhanced mass transport contributed by auxiliary mesoporous
structures and the native microporosity/acidity of zeolites in
HSZs for an overall catalytic performance. Generally, Fig. 9
shows the hierarchy factor plotted in a contour plot as
functions of the relative mesoporous surface area and relative
microporous volume from the available all four textural
parameters in the published references. As expected, the
enhanced catalytic performances can be found clearly along
with the increased HF value from zeolite nanocrystals, to
desilicated and some template-synthesized HSZs. Comparatively,
purely microporous zeolites or OMMs, suffering from the
absence of wide enough and effective fast diffusion pathways
or of intrinsic strong acidity, showed much decreased HF values
and consequently lower catalytic activity or performance than
HSZs. Nevertheless, it should be noted that since the HF index
is still based on a simplified hypothesis and only several
specific examples were verified, further work is necessary to
confirm its generality and to upgrade it or re-build a
new general index for the characterization of the structural
features and material performances of HSZs and their essential
correlations.
5. Summary and outlook
To diminish the diffusion limitations present in purely micro-
porous zeolites during applications, hierarchically structured
zeolites (HSZs) have attracted ever-increasing attention in
recent years. Up to now, either through direct synthesis or a
post-synthetic demetallation (desilication) process, various
kinds of HSZs of multiple framework structures and compositions
have been prepared, all of which possess auxiliary inter- or
intracrystalline mesoporous structures in addition to the
native microporosities. As a consequence, improved material
performances, i.e. high (hydro)thermal stability, superior
activity, and increased catalytic performance and lifetime,
have been achieved in above summarized model and/or practical
catalysis applications.
Among those two synthetic methodologies, direct synthesis
is actually a ‘‘bottom-up’’ pathway and always associated with
soft or hard templates as mesoporogens and/or mesoporous
structure directing agents. The resultant products could
possess the well-controlled mesoporous characters (mesopore
sizes, size distributions, mesopore surface areas, and mesopore
volumes, etc.). However, because of the strong exclusion effect
by the growing zeolitic species, HSZs would be the kinetically
controlled intermediate phases. Thus, sol (or dry gel) preparation
and following crystallization parameters should be carefully
tuned to get the uniform and single-crystal(-like) HSZs
products, especially when the ordinary mesoscale templates,
such as CTAB, TEA, and F127, were used as in our recent
reports.42–45,56 The key to the direct ‘‘bottom-up’’ HSZs
synthesis by using mesoscale templates is to prevent the phase
separation between the templates and the growing zeolitic
species, e.g. by covalent bonding between the mesoporogens
and the silicon precursors through specially designed templates
such as amphiphilic organosilanes,8 or by excluding the presence
of a continuous liquid phase during crystallization when those
ordinary mesoscale templates were used. Importantly, the
synthesized HSZs particles with these ordinary templates
showed a single crystalline feature as evidenced by TEM and
electron diffraction characterizations and the mesopores are
Fig. 8 Correlation between ethylbenzene (EB) productivity and the
hierarchy factor (HF) (P and QAx for the parent and alkaline-treated
zeolites, respectively). Reproduced with permission from ref. 64.
Copyright 2009, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
Fig. 9 The hierarchy factor (HF) plotted in a contour plot as a
function of the relative mesoporous surface area and the relative
microporous volume of different zeolite types prepared by different
methods. Reproduced with permission from ref. 64. Copyright 2009,
Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
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10546 Chem. Commun., 2011, 47, 10536–10547 This journal is c The Royal Society of Chemistry 2011
the penetrating intracrystal ones. Such a structure feature
ensures the high performance and stability of the resultant
materials. On the other hand, for the ‘‘top-down’’ post-
synthetic desilication process, the pre-synthesized zeolite
precursors could endow the resultant HSZs with a high degree
of microporous crystallization as in purely microporous
zeolites. Although it is well accepted now that the post-
synthetic desilication process is superior compared with that
of dealumination, the mesoporous characters of the resultant
HSZs are still less-controlled and the material compositions
and surface states may be changed during this selective
leaching process. Sequentially multistep treatments and/or
some additional moderating agents are required to get HSZs
materials with balanced performances.
In general, whether by direct synthesis or by the post-
synthetic desilication process, a suitable balance between
microporosity and mesoporosity in the resultant HSZs is very
important. In this respect, the hierarchy factor (HF) is a
simplified and effective index in evaluating the HSZs properties
or designing newly structured HSZs, in which both the native
functions of micropores and the enhanced diffusion capability
of reactants/products in mesopores are considered. Further
research of HSZs should include not only the new processes/
methodologies to realize the simultaneous and accurate
controlling over the compositions and structures on micro-
and mesoporous scales, catalytic performance such as activity,
product selectivity, coke formation and lifetime, but also a
general index to characterize and to correlate the structural
features and material performances of HSZs.
In addition to the further investigation on HSZs themselves,
the synthesis of HSZs-based composites will be another focus
in future. The loading of heterogeneous active species such as
metallic nanoparticles on zeolites, or the introduction of active
guests in the pore channels of conventional mesoporous silica,
has been proved to be effective in enhancing the material
performances of zeolites or conventional mesoporous materials.
One can expect that the loading of active species in the
hierarchical pore channels of HSZs should be equally effective
as in conventional mesoporous materials. Little work has
been reported in this direction but the results have been very
encouraging.77–80 Such a composite combines the advantages
of zeolite crystals of native catalytic activity, mesoporous
structures benefiting the fast diffusion of reactants/products,
and the additional functions of introduced guests and their
possible synergistic effect with the HSZs supports.
The authors thank the NSFC (No. 20633090, 20703055,
50872140) for financial support.
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