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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10544 Chem. Commun., 2011, 47, 10536–10547 This journal is c The Royal Society of Chemistry 2011

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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