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Hierarchical zeolites: materials withimproved accessibility and enhancedcatalytic activity
D. P. Serrano,1, 2 J. Aguado3 and J. M. Escola3
DOI: 10.1039/9781849732772-00253
1 Introduction
Conventional zeolites are crystalline aluminosilicates formed by a network of
SiO4 and AlO4 tetrahedra linked by shared oxygen atoms, which are usually
obtained in the form of crystals/particles of micrometer size (W1mm).1 The
presence of aluminium atoms into their framework confers them acidic
properties (both Bro nsted and Lewis) although they can also incorporate
other heteroatoms such as Ti, V, Sn, etc., allowing their application ascatalyst in many different types of chemical reactions.2 However, their major
feature is likely the occurrence of micropores of molecular dimensions
(generally 0.40. 75 nm), which turn them into molecular sieves. This unique
property has been also called shape selectivity and allows the zeolites to
discriminate among different reactants, products or even transition states
according to their shape and size. This phenomenon has led to remarkable
selectivities exhibited by zeolites in a large number of reactions. This is the
case of toluene disproportionation, wherein enhanced yields and selectivities
towards the p-xylene isomer have been attained over the ZSM-5 zeolite dueto its improved diffusion through the channel systems with regard to the
m/o- isomers.3 In fact, many of the current successful applications of zeolites
are based on their molecular sieves character.
However, zeolites fail when dealing with bulky substrates, usually
exhibiting low activities, often even below the values obtained with
amorphous materials such as silica-alumina. This result is to be expected as
the bulky substrates cannot access the active sites located inside the zeolite
micropores due to small size of the latter. Thus, only those sites situated
over the external surface of the catalyst particles and crystals, or close to the
micropore openings, are accessible for large molecules, and these represent
usually a low share (o 5%) of the total content of active sites. A clear-cut
example of this circumstance can be found in the catalytic cracking of
polypropylene over ZSM-5 zeolite,4 wherein just 11% of conversion
was observed at 3751C, despite the high acid strength of this zeolite, while a
99% conversion was obtained over the mild acid strength Al-MCM-41
mesoporous material. On the other hand, even if the substrate can enter into
the zeolite micropores, the diffusion rate is usually too slow bringing about
the appearance of intracrystalline mass transfer constraints, which limit
meaningfully the performance of the zeolite catalyst.5
1Department of Chemical and Energy Technology, ESCET, Universidad Rey Juan Carlos,c/ Tulipan s/n, 28933, Mostoles, Madrid, Spain2IMDEA Energy Institute, c/Tulipan s/n, 28933, Mostoles, Madrid, Spain3Department of Chemical and Environmental Technology, ESCET, Universidad Rey Juan
Carlos, c/ Tulipan s/n, 28933, Mostoles, Madrid, Spain
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Seemingly, the increasing need of processing bulky compounds put the
zeolites into a difficult situation, specially when ordered mesoporous
materials such as MCM-416 and SBA-157 were discovered in the early and
late 90s, respectively. These materials possess pore sizes whose dimension
may be tailored within the 1.5 30.0 nm range by a suitable choice of
synthesis conditions (mostly type of surfactant, temperature and aging
time). However, the huge number of publications devoted to these materialssoon realized their main limitations, mainly derived from the amorphous
nature of their walls. Accordingly, ordered mesoporous aluminosilicates do
not possess the high hydrothermal stability and strong acidity of zeolites.
Therefore, zeolites are still the preferred choice in numerous industrial
applications.
An alternative solution to deal with bulky substrates has been the syn-
thesis and application of nanozeolites (crystal size below 100 nm) since they
show a high share of external surface area whose active sites are accessible
to large molecules.
8
However, nanozeolites are usually produced in lowyields since they are very difficult to separate from the synthesis medium.9,10
Recently, several new strategies have appeared in order to improve the
properties of zeolites when processing bulky substrates. Delaminated
zeolites,11 large pore zeolites12,13 and hierarchical zeolites14,15 are examples
of these new strategies. Delaminated zeolites, such as ITQ-2, result from the
delamination of a layered zeolite precursor (e.g. MCM-22) giving rise to
thin zeolite sheets (B2.5 nm thick) having a huge external surface area
(Z700m2 g1).11 However, this method of synthesis is inherently bound to
the occurrence of a zeolite layered precursor. The preparation of large pore
zeolites, having channels formed by rings with more than 12 members, is
another strategy that has been pursued in the late years. One of the main
achievements in this line has been the discovery of ITQ-33, a silicogerma-
nate having circular pores of 18 member-rings interconnected by 10
member-ring channels and a crystallographic pore diameter of 1.22 nm.13
This means a substantial enhancement in pore size considering that
a conventional zeolite Y shows roughly 0.75 nm of pore dimension.
Nevertheless, the main disadvantage of this approach is the need of
employing Ge in the synthesis, as well as the addition of special structure
directing agents (SDA) which are usually expensive reagents.One of the most successful strategies for improving accessibility is likely
the case of hierarchical zeolites.14,15 These zeolites are characterized by the
presence of a bimodal pore size distribution, formed by both micropores
and mesopores. The microporous structure is the one inherent to the
classical zeolite topology while the secondary mesoporous structure can be
generated by a variety of specific synthetic procedures. The presence of a
secondary porosity in hierarchical zeolites, usually in the mesopore range, is
responsible for the improved mass transfer properties of these materials,
which have proved to be advantageous in numerous reactions. On the otherhand, the surface area associated to this secondary porosity implies the
presence of active sites that are not sterically hindered for interacting with
bulky molecules. In some cases the mesopore surface area is confused with
the external surface area, likely due to the application of the t-plot method
for the joint calculation of both parameters. Although the nature and
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performance of the active sites located in both mesopore and external
surface area are probably quite similar, they are different concepts. External
surface refers to the surface area located in the outer part of the zeolite
particles, whereas mesopore surface corresponds with the surface area in the
mesopore walls. Therefore, the occurrence of high external surface area
does not mean the presence of mesopores in the material. Thus, nanozeolites
show high external surface area but many times mesopores are absent. Inthis regard, hierarchical zeolites cannot be regarded truly as nanozeolites
(although they usually consist of zeolitic nanounits as building blocks).
From a catalytic viewpoint, most part of the active sites in hierarchical
zeolites are placed inside both micropores and mesopores, the latter having
improved accessibility. Henceforth, the main preparation strategies of
hierarchical zeolites as well as their applications in different reactions are
commented.
2 Methods of preparation of hierarchical zeolitesHierarchical zeolites can be prepared by different procedures which show
distinct features. Although all of them lead to materials with bimodal pore
size distributions, the features and contribution of the generated secondary
mesoporosity depends heavily on the chosen procedure. Some of these
synthesis strategies present common aspects which allow them to be clas-
sified as follows:
Dealumination
Desilication
Hard templating by carbon materials
Hard templating by polymers
Incorporation of organosilanes
Other methods
2.1 Dealumination
Dealumination methods represent the most classical alternative for de-
veloping mesoporosity in zeolites. They comprise steaming at elevated
temperatures and acid leaching.16 Steaming at high temperature of zeolite Y
has been traditionally performed for its application as catalyst in fluidcatalytic cracking reactors (FCC) since it creates mesopores by removal of a
certain amount of the aluminium from the zeolite framework. This treat-
ment causes an enhancement of the hydrothermal stability of the zeolite as
well as an improvement of the diffusion of bulky molecules inside the zeolite
pores. The steaming treatment consists of first performing an ion exchange
of an alkali metal containing zeolite Y with an ammonium salt solution in
order to reduce the alkali content to 1025% of its starting value, followed
by washing to remove the excess salt. Subsequently, the zeolite is heated to
2006001C to enable the migration of sodium ions towards easilyexchangeable sites. Then, a second ion exchange is performed with an
ammonium salt solution to remove completely the sodium. Finally the
zeolite is heated under a steam atmosphere at a temperature between
6008001C.17 The non framework aluminium resulting from the steaming
treatment can be removed from the zeolite by subsequent washing with
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dilute acids. In addition, as a result of the steaming treatment, some
shrinkage of the zeolite framework of around 0.020.03 nm takes place.
Acid leaching also allows mesopores to be created by a quite similar
mechanism, i.e. through the extraction of some aluminium from the
framework positions. A typical procedure is the treatment of the zeolite in
an slurry with a solution of ethylenediaminetetraacetic acid (EDTA) under
reflux for 18 h.18
Afterwards, the zeolite is heated with an inert gas at 8001C.However, this method has the drawback of removing preferentially the
aluminium present over the external surface leading to non uniform
distributions within the crystals.19 A related method which employs a strong
acid treatment has been reported recently by Van Oers et al.20 They attained
hierarchical Beta from nanoparticles of this zeolite using a procedure
consisting in the ageing at 1401C of a zeolite Beta nanoparticle solution,
followed by stepwise cooling or quenching. Then, this mixture is strongly
acidified with concentrated HCl and subjected to a second hydrothermal
treatment at 1501
C for 72 h. The obtained mesopore size depended on theconditions employed in the cooling step, giving rise to roughly 10 nm pores
with slow cooling and 6.0 nm pores with quenching.
Nevertheless, recent experimental evidences have been found that ques-
tion markedly the goodness of the mesoporous structure created in zeolite Y
by steaming. 3-D TEM technique has allowed the nature of the obtained
mesopores to be studied, indicating that their size encompass a broad range
(2 50 nm).21 But the most important finding has been that the mesopores
consist chiefly in cavities connected by micropores instead of a network of
mesopore channels connecting the outer surface and the internal micro-
pores. This is nicely illustrated in Fig. 1, which shows the 2-D and 3-D TEM
micrographs of a USY zeolite obtained by steaming of a NH4Y sample.
This finding has important consequences since this mesopore structure is
not expected to cause significant enhanced diffusion rates inside the zeolite
crystals. Likewise, other works have been devoted specifically to elucidate
this fact by measuring intracrystalline diffusion rates in steamed USY
Fig. 1 TEM micrographs of USY zeolite: a) 2D-TEM image of a crystal, b) 3D TEMreconstruction of a crystal. (Reprinted with permission from ref. 21, Copyright Wiley-VCHVerlag GmbH & Co KGaA, 2001)
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zeolites using the PFG NMR technique.22 Two probe molecules were used:
n-octane and 1,3,5 triisopropylbenzene, with molecular diameters smaller
and larger than the zeolite micropores, respectively. The main conclusion of
this work was that the intracrystalline diffusion inside zeolite USY is
practically not affected by the presence of the mesopores generated by
steaming, due to the absence of an interconnected network structure. This
result indicates that the classical procedure of steaming is not reallyappropriate for the preparation of a network of mesopores, highlighting the
importance of developing new methods of synthesis of hierarchical zeolites.
2.2 Desilication
The technique of desilication is based on the treatment of zeolites with a
base (usually sodium hydroxide) under relatively mild conditions
(To363 K). Mesopores are formed due to the preferential removal of silica
from the framework. This is a remarkable difference from the acid leachingtreatment wherein aluminium atoms are selectively extracted. Ogura et al.23
carried out the desilication of ZSM-5 zeolite with 0.2 M NaOH aqueous
solution at 353 K. After the treatment, the ZSM-5 sample retains its crys-
tallinity and the majority of their microporous structure, whereas it contains
additional mesopores. The morphology of the zeolite is meaningfully
altered by the alkaline treatment leading to the appearance of voids and
grooves over the zeolite surface. This is clearly appreciated in Fig. 2 which
shows the SEM micrograph of both the raw sample and the alkaline treated
ZSM-5 zeolite. These authors also observed that the acidity of the zeolite
was little affected after the alkaline treatment according to ammonia TPD
measurements.24 Moreover, it was concluded that the size of the pores
formed by the treatment with NaOH is around 1.8 nm, being indeed
supermicropores.25
A key parameter of desilication by alkaline treatment is the Si/Al atomic
ratio of the zeolite.26 Thus, a Si/Al atomic ratio in the range 20 50 has been
found as optimal in the case of ZSM-5 zeolite giving rise to intracrystalline
mesopores with a size around 10 nm. In contrast, for lower Si/Al atomic
ratios, the high aluminium concentration in the framework prevents the
removal of silicon leading to almost no mesopore formation. On the con-trary, if the Si/Al is very high (Si/Al W 200), too much silicon is extracted
creating large pores. This can be appreciated in Fig. 3, illustrating the
evolution of the porous structure of the material with the alkali treatment
for different Si/Al atomic ratios.
On the other hand, the presence of extraframework aluminium proved to
inhibit silicon extraction due to realumination during the alkaline treat-
ment. However, this problem could be solved by previous acid washing in
order to eliminate the extraframework aluminium species.27 An interesting
fact that highlights the importance in this method of the Si/Al atomic ratiois the existence of Al concentration gradients in large ZSM-5 crystals. Thus,
the external zeolite surface is often rich in Al species, while the concen-
tration of aluminium in the inner part of the crystals is much lower. In this
case, the application of the desilication procedure inevitably brings about
the generation of hollow zeolite architectures.28 Therefore, special synthetic
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methods which lead to more uniform Al concentration into the zeolites are
required if the desilication procedure is to be applied.
The alkali treatment can be applied using also organic bases (TPAOH and
TBAOH) instead of NaOH, which causes a higher extent of the aluminumleaching.29 In this way, the mesopore volume can be optimized by incorpor-
ating in the alkaline treatment both NaOH and quaternary ammonium cat-
ions,30 such as TPA or TBA . These cations act as pore growth moderators,
being necessary for the success of the method that they do not enter the zeolite
micropores (TMA does not work). The addition of these cations together
with the alkaline solution allows the mesopore surface area to be increased
without reducing the micropore volume (the ratio between these two
parameters has been called hierarchy factor). In addition, the mesopore size
decreased from 10.0 nm to 4.5 nm on augmenting the TPA
concentration.Based on FTIR measurements using CO as probe molecule, it has been
concluded that the acid strength of the Bro nsted sites of the zeolitic
materials did not change significantly after desilication.31 However, the
appearance of new strong Lewis acid sites have been observed assigned to
dislodged Al species. The enhanced accessibility of the Bro nsted acid sites
Fig. 2 SEM micrographs of the original (a) and alkaline treated ZSM-5 (b). (Reprinted withpermission from ref. 23, Copyright Japan Chemical Society)
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in the desilicated zeolites has been proved by FTIR using substituted
alkylpyridines such as 2-6-lutidine (size of 0.67 nm) and 2,4,6-collidine (size
of 0.74 nm).32 Collidine is too bulky to enter the MFI zeolite micropores
while lutidine can probe certain amount of the zeolite sites (below 50%).
This fact allowed an accessibility index (ACI) to be established as the ratio
of the Bro nsted acid sites detected with the probe molecule with regard to
the total number of Bro nsted sites, measured using a fully accessible probe
molecule such as pyridine (size of 0.57 nm). This accessibility index showed
a remarkable increase with the mesopore surface area reaching maximum
values of 0.4 and 1 with collidine and lutidine, respectively, for a mesopore
surface area of 277 m2 g 1.
The desilication procedure has been extended to other zeolites differentfrom ZSM-5, such as mordenite33 and ZSM-12.34 Its main advantage is that
the procedure to be applied is quite simple and does not involve the use of
expensive reagents. However, one inherent limitation of the method is the
need of having a Si/Al atomic ratio within a given range. Thereby, its
applicability is lower than other reported procedures that do not show this
bound. In addition, desilication involves a partial destruction of the zeolite
structure what should be carefully controlled in order to avoid the
appearance of amorphous material or the generation of extraframework
aluminium species.
2.3 Hard templating by carbon materials
The outburst in the synthesis of mesoporous/hierarchical zeolites has been
largely marked by the work of Jacobsen and col.35 It was preceded by the
development of the confined synthesis method for the preparation of
Fig. 3 Evolution of the porous structure of ZSM-5 zeolite during desilication by alkalitreatment as a function of the Si/Al ratio. (Reprinted with permission from ref. 27, CopyrightWiley-VCH Verlag GmbH & Co KGaA, 2005)
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nanozeolites, performed also by this research group.36 The latter consists of
the synthesis of nanozeolites inside a mesoporous inert carbon matrix.
Subsequently, the carbon is burnt off releasing the nanozeolites with a
crystal size of around 20 nm that could be tailored by controlling the pore
size of the used inert matrix. The preparation of mesoporous zeolites could
be carried out by modifying slightly this method. Thus, if an excess of
zeolite gel is used in the synthesis, the zeolite grows over and around theparticles of the carbon matrix forming large zeolite crystals which en-
capsulates the inert carbon matrix. Once the zeolite crystal is formed, the
carbon matrix is eliminated by combustion in air giving rise to large zeolite
crystals containing mesopores roughly of the size of the carbon matrix
particles (see Fig. 4).
Seemingly, the dimension of the mesopores can be easily controlled by
adjusting the size of the starting carbon particles. In this sense, in the original
work of Jacobsen, carbon black pearls 2000, with a mean size of 12 nm, were
employed leading to a mesopore volume of 1.01 cm
3
g
1
and a pore sizewithin the range 5 50 nm. The dimensions of the zeolite particles so obtained
were relatively large (0.3 1.2 mm), being formed by agglomerates of smaller
crystals. However, these nanocrystals show their zeolite planes aligned
spanning throughout the entire agglomerate which is indicative of them being
organized as a single zeolite crystal. In addition, electron diffraction patterns
also indicate that the apparent agglomerates are actually single zeolite crys-
tals. Other carbon sources have been also used as matrixes such as multiwall
carbon nanotubes (MWNT) which led to narrower mesopore distributions
than with carbon black pearls.37 In addition, the use of carbon fibres also
allowed cylindrical mesopores to be obtained with low tortuosity.38
This method was further applied successfully to the synthesis of a rela-
tively large number of hierarchical zeolites, such as MFI,39 MEL,40 MTW,41
BEA and CHA.42 However, one of the limitations of the method is the
availability of adequate carbon templates to obtain the desired mesoporous
structures. Accordingly, a modification of the carbon template method has
been proposed also by Jacobsen et al.43 which allows at least partially this
drawback to be overcome. They devised a procedure consisting of using a
Fig. 4 Scheme of the synthesis of mesoporous zeolites by carbon templating according to thework of Jacobsen. (Reprinted with permission from ref. 35, Copyright American ChemicalSociety, 2000)
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carbohydrate (sucrose) instead of a carbon template. The method comprises
the incipient wet impregnation of silica gel with a sucrose solution, followed
by its thermal decomposition under inert atmosphere (Ar). Thus, a carbon-
silica composite is formed. Subsequently, a base and the corresponding
zeolite template are added and the mixture is left crystallizing. Afterwards,
the carbon template is eliminated by combustion in air and a hierarchical
zeolite is formed. The resulting mesoporous structure can be tailored tosome extent by controlling the concentration of the sucrose solution. Fig. 5
shows TEM micrographs of one of the mesoporous zeolites so obtained.
It should be remarked that hierarchical zeolites obtained by the carbon
templated method of Jacobsen et al. contain interconnected micropores
and mesopores, so enhanced mass transfer rates are to be expected, unlike
it occurred with the hierarchical USY zeolites obtained by steaming. This
fact has been proved by 3D-TEM of a hierarchical zeolite synthesized
using carbon nanotubes as hard-templates. The 3D images showed that
the mesopores formed defined channels spanning throughout the zeolitecrystal.44
Carbon aerogels monoliths have been also used as templates for the
generation of mesoporosity in zeolites.45 In this case, the mechanism of
formation of mesopores is slightly different from those shown above
according to the Jacobsen method. Carbon aerogels were obtained from
resorcinol-formaldehyde gels after drying with CO2 under supercritical
conditions and pyrolysis under nitrogen atmosphere at 1323 K. The
resulting carbon aerogel possessed mesopores with 23 nm size. The for-
mation of the hierarchical zeolites takes place by the crystallization of the
ZSM-5 zeolite inside the mesopores of the carbon aerogel. The latter is
Fig. 5 TEM micrographs of hierarchical zeolites obtained using concentrated sucrosesolutions as precursors of the carbon template. (Reprinted with permission from ref. 43,Copyright American Chemical Society, 2007)
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subsequently removed by burning off under oxygen leaving available the
space corresponding to the carbon aerogel pore walls. The obtained
material is a ZSM-5 zeolite with a mesopore size of 11 nm. This method
presents the added advantage that the mesoporous zeolite has been
synthesized as a monolith.
Ordered mesoporous carbons, prepared by nanocasting, have been also
used successfully as templates for the synthesis of hierarchical zeolites.46,47
This is the case of CMK-3, an ordered mesoporous carbon attained by
nanoreplication of pure silica SBA-15. The hierarchical zeolites obtained
employing CMK-3 as template present mainly supermicropores or small
mesopores with a size around 2 nm. These are mesopores smaller than those
reported previously with other carbon sources. The textural properties of
the hierarchical zeolites can be tuned by changing the type of CMK-3
carbon used. It should be pointed out that the size of the mesopores in
CMK-3 carbon can be tailored using SBA-15 templates with different
mesopore size. A modification of this method consists of impregnatingdirectly the composite SBA-15/carbon or MCM-41/carbon with TPAOH
and left the mixture crystallizing hydrothermally under steam.47 After cal-
cination, a mesoporous ZSM-5 is formed. In this case, the size of the
mesopores is around 3.5 nm and 10 15 nm depending on whether MCM-
41 or SBA-15 are used as raw templates, respectively.
In summary, carbon-templating offer many possibilities for synthesizing
hierarchical zeolites due to the large availability of carbon materials that
can be employed. In contrast, the main disadvantage of these carbon-based
strategy is related to the need of burning the carbon template, which
represents a great amount of material being destroyed and having, in many
cases, a high cost. Moreover, the carbon combustion may generate high
temperatures that can damage the zeolite structure.
2.4 Hard templating by polymers
Closely related with the use of carbon based templates, is the application of
polymers as hard templates for the synthesis of hierarchical zeolites. In this
regard, the work of Xiao et al.48 deserves special mention. These authors
have employed mesoscale cationic polymers, like polydiallyldimethyl-
ammonium chloride, for the preparation of hierarchical Beta zeoliteby a one-step hydrothermal route. The size of the mesopores obtained
varies in the range 5 40 nm, which is in agreement with the molecular
dimension of the cationic polymer used. According to the authors, the
method can be extended to the synthesis of other zeolites different from Beta.
Polystyrene spheres have also been employed for obtaining hierarchical
zeolites with a microporous/macroporous pore structure.49 Likewise, the
use of nanospheres of poly(methyl methacrylate) (PMMA) has allowed
mesoporous microspheres of zeolite ZSM-5 to be prepared by a mechanism
involving self assembly of the aluminosilicate source, the PMMAnanospheres and TPAOH under basic conditions.50 Subsequently, hydro-
thermal crystallization takes place and the hierarchical zeolite is obtained
after calcination. Mesopores with about 13 nm diameter are achieved
with this method despite that the PMMA nanosphere size is far higher
(around 80 nm). Mesoporous zeolite A was also obtained using as templates
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aerogels formed by sol-gel polymerization of resorcinol-formaldehyde.
The hierarchical zeolite A so obtained showed mesopores with a size of
roughly 15 nm. This method has the advantage that the nanoscale structure
of the polymer aerogel template can be tuned just by changing the resorcinol -
catalyst ratio, allowing some modification and control of the mesopore
size.51
2.5 Incorporation of organosilanes
One alternative initially attempted to obtain hierarchical zeolites was based
on putting together in the synthesis gel the structure-directing agent of the
zeolite and that of an ordered mesoporous solid in order to obtain a hybrid
material formed by an array of ordered mesoporous with crystalline zeolitic
walls. However, this approach has failed to get the desired product as it
usually led to a physical mixture of segregated phases.52 Pinnavia et al.53
succeeded in solving this problem by a two-step synthesis strategy involving
the previous formation of a precursor solution containing the zeolite seedsthat subsequently were assembled into the mesostructure by aggregation
around surfactant micelles. However, the materials so obtained lack of
zeolitic crystalline features, being wide-angle X-ray amorphous, which
makes difficult to establish their real crystalline character.
In order to avoid the phase separation of the mesoporous material
and the zeolite, Choi et al.54 designed a specific amphiphilic organosilane
((3-trimethoxysilyl) propyl/hexadecyldimethylammonium chloride) of
formula [(CH3O)3SiC3H6N(CH3)2C16H33]Cl. This amphiphilic organosi-
lane is a surfactant molecule that contains a hydrophobic alkyl chain, a
quaternary ammonium group (zeolite structure-directing agent) and a
hydrolysable trimethoxysilyl moiety. The latter interacts strongly with the
growing zeolite crystals due to the formation of covalent bonds with the silyl
moiety avoiding the separation of the phases. The silanization constitutes a
crucial step for the success of the method. The amount of amphiphilic
organosilane used is low (4% mol), being first added to a typical syn-
thesis composition of ZSM-5 zeolite, containing tetrapropylammonium
hydroxide (TPAOH). The evolution of the crystallinity of the products
obtained using this method with the synthesis time is shown in Fig. 6,
which depicts the low and wide-angle XRD patterns. At short synthesistimes (3 h), only a mesoporous phase with amorphous walls is obtained.
However, after 12 h, the appearance of zeolite MFI begins to be appreciated
in the high angle XRD pattern, and finally, after 2 days, the presence of a
mesoporous structure with zeolitic pore walls is appreciated.
The authors suggested a solution-based mechanism to explain the for-
mation of the mesoporous zeolite, involving the dissolution of the starting
mesophase, followed by the crystallization of the mesoporous zeolite crys-
tals from the dissolved species. The mesopore diameter can be tailored
within the range 2 20 nm by changing the length of the alkyl chain of theorganosilane or by modifying the hydrothermal conditions of the synthesis.
The mesoporosity created by this procedure is claimed to be rather uniform.
On the other hand, the mesopore size can be widened up to 24 nm by using
triblock copolymers (EO20PO70EO20) as pore expanding agents.55 This
method for the synthesis of hierarchical zeolites has allowed the preparation
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of MFI, LTA54,55 and sodalite56 materials. Regarding the acidity of the
samples, and according to infrared mass spectroscopy/temperature
programmed desorption measurements, hierarchical ZSM-5 synthesized by
this method contains Bro nsted acid sites of similar strength as those of
conventional ZSM-5 but in lower number.57
Different approachs have been also developed by other authors to syn-thesize hierarchical zeolites by perturbing the crystallization mechanism
through the addition of organosilanes. Thus, our research group5861
envisaged a procedure for the preparation of hierarchical zeolites by using
seed silanization agents. The method is based on the fact that during the early
stages of crystallization of MFI zeolites from clear synthesis solutions, the
precursors are nanounits with a particle size of 25 nm.6264 If the
agglomeration of these nanocrystals into bigger entities is partially hindered
by the presence of bulky organic substituents anchored over their external
surface, the obtained zeolite shows, after calcination, the occurrence of
mesopores whose dimension correspond to the voids occupied by the bulky
substituents during aggregation. This strategy comprises the following stages:
a) Precrystallization of the zeolite synthesis gel to promote the formation
of ZSM-5 protozeolitic nanounits, using tetrapropylammonium hydroxide
(TPAOH) as structure directing agent.
Fig. 6 XRD patterns of mesoporous MFI zeolite generated using amphiphilic organosilanealong different synthesis times. (Reprinted with permission from ref. 54, Copyright McMillan
Publishers Ltd., 2006)
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b) Functionalization of the zeolite seeds by anchoring organosilanes on
their external surface. These organosilanes form a surface passivating layer
that avoids or reduces meaningfully the nanounit aggregation.
c) Crystallization to complete the zeolitization of the functionalized
protozeolitic units. This stage can be performed hydrothermally under
standard zeolite crystallization temperatures (e.g. 1701C).
d) Calcination to remove the silanization and the structure directingagents rendering accessible both micropores and mesopores in the hier-
archical zeolite.
As it can be appreciated in Fig 7, the hierarchical ZSM-5 so obtained is
made up by 200 400 nm aggregates formed by ultrasmall ZSM-5 units
whose size varies within the 5 10 nm range depending on the synthesis
conditions. Interestingly, the lattice fringes share their orientation among
the different nanounits which indicates that they are not really independent
nanocrystals but a significant degree of intergrowth exists between them.
This is an important aspect as it implies that the actual size of the crystallinedomains is quite larger than the size of the nanounits, which is expected to
improve the stability of these materials compared to nanozeolites formed
solely by independent nanocrystals.
On the other hand,27Al-MAS NMR analyses point out that most of
the aluminium atoms are incorporated inside the framework of the
as-synthesized zeolites. Additionally, 1D and 2D NMR analyses provided
evidences about the location of the TPA and silanization agent (pheny-
laminopropyltrimethoxysilane, PHAPTMS) moieties bearing out that they
Fig. 7 TEM images of hierarchical ZSM-5 obtained by crystallization of silanized seeds.(Reprinted with permission from ref. 8, Copyright Royal Society of Chemistry, 2008)
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are located inside the zeolite nanounits and grafted over their external
surface, respectively. This can be appreciated clearly in the schematic
diagram shown in Fig. 8.
The effects induced by the seed silanization treatment heavily depend
on the chosen synthesis variables, in particular upon the nature and
concentration of the seed silanization agent and on the precrystallization
stage. Different bulky organosilanes have been used as seed silanization
agents: octadecyltrimethoxysilane, isobutyltriethoxysilane, 3-aminopropyl-
trimethoxysilane and phenylaminopropyltrimethoxysilane.8 The former
(octadecyltrimethoxysilane) was only anchored onto the protozeolitic units
in a reduced extent (1.3 wt %), being not successful in developing meso-
porosity. In contrast, both isobutyltriethoxysilane and 3-aminopropyl-
trimethoxysilane led to materials having mesopores with a size of 8.0 nm
and an external surface area of around 200 m2 g1. But the most interesting
result has been achieved with phenylaminopropyltrimethoxysilane, leading
to materials having 2.0 3.0 nm mesopores, their walls being formed by the
smallest ZSM-5 nanounits (5 10 nm). Regardless of the organosilane used,
the crystallinity of all the samples has been proved by both XRD patterns,which indicate the absence of amorphous material, and FTIR, that shows
the appearance of the 550 cm 1 band, typical of the asymmetric stretching
mode of five membered rings present in ZSM-5 zeolite.65
The concentration of the seed silanization agent is also an important
variable. Thus, an optimum of 12 mol%, referred to the total silica content,
appears to exist with phenylaminopropyltrimethoxysilane as seed silaniza-
tion agent.61 On the other hand, the precrystallization step for the
formation of the nanozeolite precursors containing TPA occluded is
completely necessary, since in its absence the synthesis of an amorphousmaterial took place. In addition, the temperature used in the pre-
crystallization step influenced deeply the BET surface areas of the samples:
698m2 g 1 at 901C and 785m2 g1 at 401C. The latter value is specially
remarkable since it represents the highest reported BET surface area for a
hierarchical ZSM-5 zeolite.61
Fig. 8 Location of TPA and PHAPTMS moieties in hierarchical ZSM-5 obtained from sila-nized seeds. (Reprinted with permission from ref. 61, Copyright American Chemical Society, 2009)
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The seed silanization approach can be considered as a general route for
synthesizing hierarchical zeolites since it is not bound to just ZSM-5 zeolite.
Hierarchical Beta,59 mordenite66 and TS-160 zeolites have been prepared by
the seed silanization procedure. In the case of hierarchical Beta, BET
surface areas as high as 857 m2 g 1 have been reported using phenylami-
nopropyltrimethoxysilane as seed silanization agent. A great proportion of
supermicropores (1.7 1.8 nm) were formed in addition to mesopores inhierarchical Beta zeolite.
Another method based on the use of organosilanes has been devised by
Pinnavaia et al.67 In this case, the procedure consists of employing a silane
functionalized polymer, as shown in the schematic diagram depicted in
Fig. 9. During the nucleation of the zeolitic entities, a silylated polymer
(e.g. silylated polyethyleneimine oxide) is grafted over their external surface
hindering the nanozeolite aggregation but not avoiding the zeolite crystal
formation. This leads to the generation of an intracrystalline polymer
network inside the zeolite structure. Finally, the polymer is removed bycalcination forming a zeolite with intracrystalline mesopores. Fig. 10 shows
the N2 adsorption isotherm at 77 K of hierarchical zeolite (MSU-MFI),
obtained using this approach, compared with a reference ZSM-5 sample
wherein the differences in the porosity between both samples are evident (see
the steep adsorption of MSU-MFI in the p/p0 range of 0.15 0.6). The
polymers employed had molecular weights within the range 600 25000
leading to mesopore sizes around 2.0 and 3.0 nm, respectively. Silylation has
Fig. 9 Mechanism of formation of a mesoporous zeolite using a silylated polymer. (Reprintedwith permission from ref. 67, Copyright Wiley-VCH Verlag GmbH & Co KGaA, 2006)
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probed to be essential for the success of the procedure since the use of a
non-silylated polyethyleneimine was not effective. In addition, the degree of
silylation is also important for the generation of mesopores. The procedure
is not limited to ZSM-5 zeolite but it has also been applied to the synthesis
of hierarchical zeolite Y.
These organosilanes based methods have the advantage of the possibility
of tailoring the mesopore size by choosing the adequate synthesis
conditions. This is particularly true for the seed silanization and theamphiphilic organosilane strategies. In addition, although organosilanes
may be expensive reagents, the employed amounts are relatively small and
their removal by calcination takes place at temperatures (300 5001C) low
enough for not damaging the structure of the hierarchical zeolite.
2.6 Other methods
A variety of other methods, that cannot be classified within any of the
previous categories, has been reported for the preparation of hierarchical
zeolites. Just as interesting examples, three of them are described in thissection. Zhang et al.68 employed bacteria (Bacillus subtilis) as templates in
the synthesis of microporous/macroporous MFI hierarchical zeolites. The
zeolite nanounits grow occupying the void spaces of the bacterial template,
so the formation of a microporous/macroporous framework occurs after
calcination. The final products are fibres formed by 0.5 mm channels and
Fig. 10 N2 adsorption isotherm at 77 K of a hierarchical ZSM-5 (MSU-MFI), prepared usinga silylated polymer, and of a conventional ZSM-5. (Reprinted with permission from ref. 67,
Copyright Wiley-VCH Verlag GmbH & Co KGaA, 2006)
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100 nm thick walls of silicalite. Other curious method to prepare
hierarchical zeolites is the one based on the usage of the leaves of the plant
Equisetum arvense as template.69 The presence of silica in the plant pro-
motes zeolite crystallization leading to a microporous/mesoporous zeolite
(roughly 0.79 cm3 g1 of intracrystalline mesoporosity). On the other hand,
inorganic compounds have also been used as templates for the preparation
of hierarchical zeolites. In this regard, nanosized CaCO3 can act as templatefor the preparation of hierarchical silicalite-1. After the synthesis, the car-
bonate can be easily dissolved by acid treatment, developing the secondary
porosity (pore size within 50100 nm). In this procedure, the presence of
hydroxyl groups over the surface of the nanosized CaCO3 is essential to
interact with the silanol groups of the silica leading to the encapsulation of
the salt inside the zeolite crystal. Thus, if the hydroxyl groups are protected
by fatty acids leading to hydrophobic CaCO3, the synthesis is unsuccessful
yielding conventional zeolites instead of the hierarchical ones.70
3 Singular features of hierarchical zeolites
The presence of a bimodal micro/mesoporous structure provides hier-
archical zeolites with an improved accessibility to the active sites, which in
many cases influences positively on their catalytic activity compared to
conventional zeolites with micrometer crystal sizes. Hierarchical zeolites
possess a collection of singular properties that are commented henceforth
according to the following order:
Improved surface area.
Increase in mass transfer rates.
Resistance to deactivation.
High dispersion of active phases.
3.1 Improved surface area
For conventional zeolites, having crystal sizes in the micrometer range, the
proportion of external surface area is usually negligible, i.e. the BET surface
area corresponds almost completely with the surface area associated to the
micropores. However, in the case of hierarchical zeolites the presence of
mesoporosity implies that a great part of the surface area is related to thelatter, while a reduction is usually observed in the micropore surface area
compared with the standard zeolites. This is an essential aspect since the
mesopore surface area is not sterically hindered, as it occurs with the surface
area of the micropores, being capable of adsorbing and interacting with
bulky compounds.
Other interesting fact is that in most cases hierarchical zeolites have been
reported to present enhanced BET surface area, which also occurs for
nanozeolites. This fact can be interpreted as a result of the strongly
restricted adsorption that takes place within the zeolite micropores, whereasadsorption on the external/mesopore surface area is not so limited. Thus, in
the case of nanocrystalline ZSM-5, a good correlation has been found
between the enhancement of the BET surface area and the reduction in
the size of the nanocrystals.9 A similar trend is expected to be also valid
for hierarchical zeolites, with an increase of the BET surface area as the
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contribution of the secondary mesoporosity is more pronounced. Accord-
ingly, the BET surface area can be used as a parameter for comparison
among the different afore commented synthesis procedures of hierarchical
zeolites.
Fig. 11 illustrates the ranges, as well as the average values, of BET
surface areas reported for hierarchical ZSM-5 zeolite synthesized with the
most important methods previously discussed. It can be observed that thestrongest effect on the BET surface area is obtained with the seed silani-
zation strategy61 reaching values close to 800 m2 g1. Moreover, this
method allows the BET surface area to be adjusted in a wide range by
changing the synthesis conditions. Significant enhancements in the BET
surface area of ZSM-5 are also obtained with the use of silane-containing
polymers,67 desilication29 and amphiphilic organosilanes,54 although with
clearly lower values (up to around 600 m2 g1). This is a remarkable
achievement as the BET surface area of a conventional ZSM-5 is about
400m
2
g
1
. In contrast, lower BET surface areas are exhibited by thehierarchical ZSM-5 samples obtained using different types of carbon
materials as hard-templates. Thus, the use of carbon blacks71 just gives rise
at most to a BET surface area of 418m2 g 1, very close to the value
corresponding to a conventional ZSM-5. These results indicate that the
1 2 3 4 5 6 7 80
100
200
300
400
500
600
700
800
900
1000
Carbonnanotubes
Carbon
aerogels
Carbon
blacks
Mesostructured
carbons
Amphiphilic
organosilane
Desilication
Silane
polymer
Seed
silanization
BETsur
facearea(m2g-1)
Method of preparation
Fig. 11 Ranges of BET surface area values reported for hierarchical ZSM-5 samples preparedaccording to different synthesis methods (seed silanization,8,58,61 silane polymer,79
desilication,26,30 amphiphilic organosilanes,76,77 mesostructured carbons,46,47 carbonblacks,42,70 carbon aerogels45 and carbon nanotubes44).
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degree of modification of the zeolite textural properties depends strongly on
the type of method employed for the generation of the hierarchical porosity.
Those methods based on the incorporation of organosilanes into the
synthesis gel appear to cause a stronger and more effective modification of
the textural properties of the zeolite in terms of surface area than the route
using carbon templates.
The improvement of the BET surface area, as well as the presence ofa high share of non-microporous surface area, opens the possibility for
the functionalization of hierarchical zeolites with different agents.
Thus, hierarchical ZSM-5 has been successfully functionalized with
3-aminopropylsilane, reaching the incorporation of 25% more organic
groups than in the case of using an ordered mesoporous SBA-15 support.
The organic-functionalized hierarchical zeolites show the added advantages
of their high hydrothermal stability and reusability. This has been proved in
the Pd-catalyzed Sonogashira cross coupling reaction72 of terminal alkynes
with chlorobenzenes under the presence of Na2CO3. In this reaction,the crystallinity of the hierarchical zeolite made possible their reuse since
conversion just decreased slightly (from 96 to 91%) after 5 reaction cycles.
In contrast, for SBA-15 the conversion values dropped from 84 to 38% due
to the lower hydrothermal stability of this material compared to the
mesoporous zeolite.
3.2 Increase in mass transfer rates
The existence of an interconnected network of mesopores and micropores is
expected to favour the intracrystalline mass transfer phenomena in hier-
archical zeolites. The mesopores allows a faster diffusion of the reacting
molecules towards the active sites leading to enhanced kinetics. In the same
way, the products may diffuse faster from the active sites reducing the extent
of secondary reactions which alters the obtained selectivity. This fact has
been nicely shown by Christensen et al.73 in the catalytic alkylation of
benzene with ethene. Fig. 12.a) shows the activity so obtained (TOF values)
versus the temperature, while Fig 12.b) illustrates the selectivity towards
ethylbenzene versus benzene conversion. These results indicate clearly
that the activity of the mesoporous zeolite is always higher than that of
the conventional ZSM-5 in the whole range of temperatures studied(583 643 K). Fig 12.b) evidences that selectivities towards ethylbenzene
attained over the hierarchical ZSM-5 are higher than the values obtained
over the conventional ZSM-5 catalyst. The occurrence of mass transport
constraints when using a conventional ZSM-5 zeolite with regard to the
hierarchical zeolite was concluded from the lower values of the activation
energies obtained with the former (59 vs 77 kJ/mol, respectively). Con-
sequently, this enhanced mass transport rates drives to higher benzene
conversions for the hierarchical zeolite as well as to larger selectivities
towards ethylbenzene. The latter effect has been ascribed to the shorterdiffusion path present in the hierarchical zeolite which suppresses the
secondary ethylation reaction.
Further analyses confirmed that the mass transport rates of both benzene
and ethylbenzene are diffusion limited over the conventional zeolite
since the value of the Thiele modulus was higher than 0.1, while over the
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hierarchical zeolite was much lower. In addition, experiments of diffusion
of isobutane point out that the effective diffusivity is three times higherover the hierarchical zeolite.74 These results have been obtained using
hierarchical zeolites prepared according to the carbon templating route.
Similar conclusions have been drawn by Groen et al.75 with hierarchical
zeolites synthesized using the desilication method. Thus, a two-order of
magnitude improvement has been denoted in the diffusion of neopentane
inside desilicated ZSM-5, due to the shorter diffusion path length and the
presence of an accessible network of mesopores.
3.3 Resistance to deactivationBesides the above described improvement of the intraparticle mass trans-
port, hierarchical zeolites show another key feature when compared with
conventional ones: increased catalyst lifetime due to their high resistance to
deactivation. The large size of the mesopores hinders the occurrence of
micropore blocking phenomena by coke, typically found in many catalytic
Fig. 12 Benzene alkylation with ethane over conventional and mesoporous ZSM-5 catalysts:a) TOF; b) selectivity towards ethylbenzene. (Reprinted with permission from ref. 73,Copyright American Chemical Society, 2003)
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reactions when using conventional zeolites. In this regard, coke precursors
can diffuse out of the hierarchical zeolite easily due the presence of
mesopores, which avoids or delays their transformation into coke deposits,
slowing down the zeolite deactivation. This fact has been clearly shown by
Ryoo et al.76 for a hierarchical MFI zeolite, synthesized using an amphipilic
organosilane, in three different reactions: isomerisation of 1,2,4
trimethylbenzene, cumene cracking and esterification of benzyl alcohol withhexanoic acid. Fig. 13 illustrates the deactivation curves obtained in these
three reactions wherein the superb performance of the hierarchical
MFI zeolite over conventional MFI zeolite and ordered mesoporous
Al-MCM-41 can be appreciated.
In the isomerisation of 1,2,4-trimethylbenzene (Fig. 13.a), the conversion
obtained over conventional MFI catalyst drops quickly from 30 to 8% after
30 min, while for Al-MCM-41 falls from 13 to 5%. In contrast, the con-
version over hierarchical MFI slowly decreases from 25 to 17% after much
longer reaction times (180 min). In cumene cracking (Fig. 13.b), the con-version obtained over Al-MCM-41 is below 10% along the whole reaction
time (140 min). Hierarchical MFI and conventional MFI initially show al-
most complete conversion (B 95%) for this reaction but the evolution of
the activity with the time shows rather different trends between them.
Thus, hierarchical MFI keeps its high conversion (W 90%) while for the
conventional MFI conversion drops to around 50% after 140 min.
Likewise, as it can be observed in Fig. 13.c), the benzyl alcohol conversion
over conventional MFI and Al-MCM-41 drops after 5 reaction cycles from
22% and 81%, respectively, to almost zero. In contrast, over hierarchical
MFI only a slight decrease from 90 to 80% in conversion is observed after 5
reaction cycles.
The slow deactivation undergone by hierarchical zeolites has been mainly
ascribed to the presence of mesopores that favour a fast diffusion of the
coke precursors out of the zeolite, avoiding their accumulation inside the
micropores which would cause pore blocking phenomena. Likewise, the
difference in performance between the ordered mesoporous Al-MCM-41
materials and the hierarchical ZSM-5 zeolite (both of them possess
mesopores) has been assigned to the stronger acidity of the hierarchical
ZSM-5 as well as to the higher concentration of Al sites in the Al-MCM-41.The latter proposal takes into account that the Al sites are relatively close in
Al-MCM-41, which can promote the formation of polymeric coke
precursor species.
Another example of resistance to deactivation over hierarchical MFI has
been denoted in the methanol to hydrocarbon reaction (MTH).77 The
lifetime of the catalyst has been observed to increase more than three
times over the hierarchical ZSM-5 with regard to conventional ZSM-5.
This effect has been also related to the deposition of coke on the mesopores
of the hierarchical zeolite, while for the conventional sample it takes placepreferentially inside the zeolite micropores having a stronger deactivating
effect by pore blockage. This is nicely shown in Figs 14.a and 14.b, re-
spectively, wherein the coke content as well as the hydrocarbon production
obtained over conventional (ZST-12) and mesoporous ZSM-5 (OSD-5) are
shown.
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Fig.
13
Deactivationperformanceofhierarchical
MFIindifferentreactions:a)1
,2,4,
TMBisomerisation,
b)cumenecrackingandc)benzylalc
oholesterificationwith
hexanoicacid
.(Reprintedwithpermissionfromref.76
,CopyrightRoyalSocietyo
fChemistry,
2006)
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It can be observed that the hydrocarbon production over the con-
ventional ZSM-5 catalyst drops to practically 0% after a time on stream of
40 h while over OSD-5 it is still around 50% after 130 h, indicating the
longer lifetime of the mesoporous ZSM-5. In addition, both Figs 14.a and
14.b provides information of the coke content, distinguishing between in-ternal coke (inside the micropores) and external coke (mainly within the
mesopores). It is clear that the internal coke is formed before and in a far
greater amount over the microporous ZSM-5 than over hierarchical ZSM-5,
wherein the coke is mostly external (W80%). The internal coke possesses a
stronger deactivation effect, which explains the fast decrease observed in the
conversion with the conventional ZSM-5.
3.4 High dispersion of active phases
The presence of mesoporosity in hierarchical zeolites offers great oppor-tunities for the preparation of bifunctional catalysts through the in-
corporation of other active phases, such as metals and metal oxides, in close
contact and with an improved interaction with the zeolite support. The
presence of mesopores is expected to lead towards better dispersions of the
active phase. This has been shown by Christensen et al.78 when impreg-
nating both mesoporous and conventional zeolites with Pt (B2 wt %), PtSn
alloy (B1 wt %) and a b- MoC2 carbide (B10 wt %). For the conventional
zeolites the metals are deposited mainly over the outer surface, frequently as
large particles according to EDS scan measurements associated to the TEMimages. In contrast, the same technique indicates that for the mesoporous
zeolite the metal nanocrystals are evenly placed within the mesopores.
Moreover, in this case, large metal particles are not observed. This result is
not only ascribed to the higher BET surface area and pore volume of the
hierarchical zeolite but to the occurrence of a much higher amount of lattice
Fig. 14 Evolution along the time on stream of the coke and the hydrocarbon production in themethanol conversion: a) microporous ZSM-5 (ZST-12) and b) mesoporous ZSM-5 (OSD-5).(Reprinted with permission from ref. 77, Copyright Elsevier, 2010)
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defect sites than in the case of conventional zeolites. These defect sites are
assumed to act as preferred deposition points for the active phase, leading to
improved dispersions. Additional examples of the benefits derived from
using hierarchical zeolites as supports of different active phases are de-
scribed in the next section dealing with the catalytic applications of these
materials.
4 Catalytic applications
The remarkable and singular properties shown by hierarchical zeolites have
brought about the potential catalytic applications of these materials in
numerous reactions, specially those wherein steric or diffusion limitations
are encountered. The next paragraphs review the literature works dealing
with the application of hierarchical zeolites in a variety of reactions, which
have been classified into three groups (oil refining and petrochemical re-
actions, fine chemicals reactions and environmental catalysis).
4.1 Oil refining and petrochemical reactions
A potentially interesting application of hierarchical zeolites is in petroleum
refining processes since conventional zeolite catalysts cannot refine about
20% of a petroleum barrel due to the steric hindrances posed by bulky
molecules. The application of hierarchical zeolites could diminish this
amount increasing meaningfully the profitability of the refining. In addition,
it is expected that the share of gasoline and light alkenes might also be
enhanced by the application of hierarchical zeolite catalysts.
This approach has been tested by Pinnavaia et al.79 that studied the
cracking of gas-oil over a mesoporous zeolite (MSU-MFI) and compared it
with a conventional ZSM-5. These authors observed increased conversions
over mesoporous MSU-MFI, accompanied by higher yields of gaseous
products (LPG), gasoline and light cycle oil (LCO) and lower amounts of
coke. In addition, much more light olefins were also detected.
Another petroleum-related application of hierarchical zeolites is the
desulfuration of gasoline and diesel fuels, which is a very important process
in order to comply with the target of reducing their sulphur content below
10 ppm.80 Thereby, 4,6, dimethyldibenzothiophene, a highly refractorysulphur-containing organic compound, has been hydrodesulfurized (HDS)
over Pt, Pd and Pt-Pd mesoporous ZSM-5 zeolite (total metal content of 0.5
wt %) leading to higher sulphur removal efficiency than metal/microporous
zeolite or metal/g-Al2O3. Fig. 15 shows the HDS conversion attained over
the different Pd containing catalysts: mesoporous Na(90%)/H (10%)-
ZSM-5 (MNZ-5), conventional Na-ZSM-5 (NZ-5) and g-Al2O3. It can be
appreciated the remarkable performance of Pd/mesoporous Na/H-ZSM-5
reaching 86% conversion with only 3% for conventional Pd/Na-ZSM-5 and
21% for Pd/g-Al2O3. This result has been ascribed to a proper combinationof acidity and mesoporosity in the hierarchical zeolite. In line with this
result, Pd/mesoporous Beta hydrodesulfurized 4,6 dimethyldibenzothio-
phene at 2501C under 62 bar of hydrogen better than Pd/Al-MCM-41 (51 vs
35%) due to the higher acidity of the zeolite.81 Additionally, Pd/mesopor-
ous Beta has shown to be more active in the hydrogenation, at 2501C and 40
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bar of hydrogen, of the bulky aromatic pyrene than Pd/conventional Beta,
Pd/Al-MCM-41 and Pd/g-Al2O3. The difference in performance with the
Pd/conventional Beta lies in its larger mesopore volume, whereas regarding
the other two catalysts this result has been also related to its greater
acidity.82
Other reactions of interest are the aromatization and isomerisation of
1-hexene wherein hierarchical zeolites, obtained by desilication, showed
enhanced stability. Thus, after a time on stream of 14 h at 3501C, the
hierarchical zeolite prepared by desilication with 0.5 M NaOH shows
selectivity towards aromatics of 19.1% while the selectivity values obtained
with the conventional ZSM-5 drop to 5.1%.83 Likewise, in butene aroma-
tization at 3501C it has been found that after a time on stream of 34 h, the
conversion over hierarchical ZSM-5 remains rather stable at 99% while the
conversion over conventional HZSM-5 drops to 93%.84 This performance
has been ascribed to a lower deposition of coke inside the micropores,reducing the extent of micropore blocking.
Hierarchical Mo/HZSM-5 also shows enhanced selectivity to aromatics
due to a larger tolerance to coke in the catalytic dehydroaromatization
of methane.85 Hence, after 720 min of reaction at 1003 K, the methane
conversion was 11% with hierarchical Mo/HZSM-5 while the conversion
over conventional Mo/HZSM-5 dropped to 3.9%. In addition, the select-
ivity towards benzene was 68% over the hierarchical zeolite instead of 37%
over the conventional catalyst.
In the case of 1-butene isomerisation at 701C, an enhanced activity hasbeen found over H3PW12O40 supported on mesoporous silicalite-1 com-
pared with H3PW12O40 supported on a conventional silicalite-1.71 These
differences are specially remarkable since the initial conversion was 72% for
the former and less than 1% for the latter. Alkylation of benzene with
ethene over microporous ZSM-5 has also been improved over mesoporous
Fig. 15 HDS conversion of 4,6, dimethyldibenzothiophene obtained over different Pd-containingcatalysts. (Reprinted with permission from ref. 80, Copyright Wiley-VCH Verlag GmbH & CoKGaA, 2008)
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ZSM-5, as it was previously commented in point 3.b).73 Mesoporous
mordenite33 have also been tested for the same reaction at 438 K. This
catalyst brings about a 5-6 fold increased production of ethylbenzene
compared to conventional mordenite. The explanation to this behaviour has
been the presence of mesopores which reduces the deactivation of the
catalyst.
On the other hand, confocal fluorescence microscopy86
has been used tostudy the oligomerization of 4-methoxystyrene. The bimodal porous
structure of the mesoporous zeolite gives rise to the preferential formation
of dimeric carbocations due to the shortening of the micropore diffusion
path, precluding the appearance of higher oligomers. This result is clearly
bound up to the above indicated resistance to deactivation shown by
hierarchical zeolites.
4.2 Fine chemistry reactions
Hierarchical ZSM-5 has been tested in various reactions involving bulkymolecules with potential application in Fine Chemistry, like the protection
of benzaldehyde with pentaerythritol, condensation of benzaldehyde with
2-hydroxyacetophenone or the esterification of benzyl alcohol with
hexanoic acid.87 High activities were observed for these three reactions over
hierarchical ZSM-5. In addition, by means of experiments of dealumination
of the mesopore walls with tartaric acid, it was concluded that bulky mol-
ecules react at the Al sites placed over the mesopore walls.
Hierarchical ZSM-5 has been evaluated in the Friedel-Crafts acylation of
anisole at 1201C with either acetyl chloride or acetic anhydride as acylating
agent and compared with nanocrystalline HZSM-5 and Al-MCM-41.88
Superior conversions were achieved over the hierarchical ZSM-5 due to the
right combination of improved accessibility provided by the mesopores and
the high acidity and crystallinity of the zeolite.
Mesoporous sodalite has been also successfully applied in base catalyzed
reactions such as Knoevenagel condensation, Claisen-Schmidt conden-
sation and acetonyl acetone cyclization.56 This material shows enhanced
activity with regards to CsNaX and KAl-MCM-41 catalyst. Thus, in
Knoevenagel condensation of 4-isopropylbenzaldehyde with ethylcyanoa-
cetate at 353 K, the conversion obtained with K-mesoporous sodalite was78%, while KAl-MCM-41 and CsNaX gave 45 and 36% conversion values,
respectively. This result has been related to the basic sites present in the
mesopores of the hierarchical zeolite.
Pd exchanged mesoporous sodalite and NaA zeolite have been also
applied for different aryl coupling reactions (Suzuki, Heck and Sonoga-
shira) involving bulky substrates.89 These hierarchical catalysts have been
reported to show high activity and reusability avoiding the usual problem of
Pd leaching and agglomeration provided that the reactions are carried out
under air. In this regard, the mesoporous zeolite stabilizes the Pd2
specieseliminating the formation of unwanted agglomerates. Mesoporous MFI
zeolite show much higher activity (98%) than conventional ZSM-5 (3.9%),
Al-MCM-41 (25%) and ZSM-5 seed assembled mesoporous materials
(SAM, 64%) in the synthesis of jasminaldehyde (all of them were prepared
with Si/Al=20). In addition, the selectivity obtained with the mesoporous
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zeolite was outstanding (98%), since the other materials presented values
below 80%. These results have been ascribed to the highly mesoporous
structure and strong acidity of the hierarchical MFI.54 These same authors
also tested the synthesis of vesidryl with the same catalysts obtaining again
superb results with the mesoporous MFI zeolite.
Other interesting application of hierarchical zeolites as catalysts is in
epoxidation reactions using Ti-containing hierarchical zeolites. Thus, in theepoxidation of cyclohexene39 with hydrogen peroxide, a yield of products
ten times higher has been detected over mesoporous TS-1 compared to
conventional TS-1. Likewise, hierarchical TS-1, obtained through the seed
silanization route, has been employed as catalyst in the epoxidation of
1-octene60 at 1001C with a bulky oxidant (TBHP, hindered to enter the
zeolite micropores), leading to much higher conversion (42% vs 5% of
conventional TS-1) with 100% epoxide selectivity and TBHP efficiency
higher than 90%. This result is specially relevant as it has opened the
possibility of using organic hydroperoxides as oxidants, instead of solelyhydrogen peroxide, in combination with TS-1 zeolite.
4.3 Environmental catalysis
Hierarchical zeolites have been also investigated in a number of reactions
within the field of environmental catalysis. Among them, the decomposition
of NO over hierarchical Cu-ZSM-11 and Cu-ZSM-5 deserves special
mention.90 In this case, the improved accessibility causes an enhanced
activity of the hierarchical zeolites because of the formation of dimeric and
oligomeric Cu species within the mesopores instead of the preferential
formation of monomeric Cu species over conventional Cu-ZSM-11 and
Cu-ZSM-5. In addition, hierarchical Cu-ZSM-11 was two-fold more active
than mesoporous Cu-ZSM-5 due to the occurrence of solely straight
microporous channels, wherein the active sites are preferentially located.
Likewise, desilicated Fe-ZSM-5 samples have shown enhanced N2O
decomposition activity.91 This has been assigned to the fully exchange of
iron in desilicated ZSM-5 samples due to its enhanced accessibility without
formation of iron oxides. In contrast, for large zeolite crystals, iron
exchange is diffusion controlled and leads to the deposition of inactive iron
species formed by hydrolysis.One less successful application of hierarchical zeolites has been the
catalytic pyrolysis of lignocellulose92 from beech wood at 5001C. Hier-
archical Beta zeolite yields less liquid bio-oil and more coke and char than
Al-MCM-41, leading to increased production of aromatics and PAH due to
the stronger acidity of their acid sites.
On the other hand, hierarchical zeolites have shown to be remarkable
catalysts for the cracking of polyolefins. The latter are bulky substrates
wherein an easy access to the acid sites leads towards higher activities. Thus,
a bulky polyolefin such as polypropylene has been cracked at 3601C overhierarchical Beta and ZSM-5 zeolites, obtained both by a seed silanization
procedure, being compared to the conventional catalysts.58 The improved
accessibility of these hierarchical catalysts for bulky polypropylene
molecules caused a four-fold greater conversion than conventional zeolites.
A similar result has been obtained in the catalytic cracking of low density
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polyethylene at 3401C using a plastic/catalyst mass ratio=100 with differ-ent hierarchical ZSM-5 samples prepared also by a seed silanization
method. In this case, while a TOF value of just 0.018 s1 is achieved over
conventional ZSM-5, the hierarchical ZSM-5 samples leads towards TOF
values far higher, within the 0.3840.901 range, depending on the combin-
ation of mesoporosity/acidity of the hierarchical sample.61
5 Concluding remarks
The field of synthesis and applications of hierarchical zeolites has been
extremely fruitful in the last decade. A wide range of synthesis strategies
have been developed that successfully allowed these zeolites to be prepared
with bimodal microporous/mesoporous structure. In addition, the number
of potential applications is growing every year, mainly as catalysts in a large
variety of reactions. In this respect, a bright future can be envisaged for
hierarchical zeolites, specially in transformations dealing with bulky sub-strates or suffering of strong deactivation by pore blockage.
The development of hierarchical zeolites has obliged to change the con-
ventional vision of zeolites as just pure microporous materials with shape
selectivity properties. At present, the occurrence of mesoporosity is an
added feature to a zeolitic material which markedly improves its properties:
enhanced textural properties, faster intraparticle transport, reduction of
steric and diffusion constraints, improved dispersion of active phases, better
resistance to deactivation and higher catalytic performance in many re-
actions. Interestingly, these improvements in many cases do not imply ne-cessarily a decrease in the selectivity exhibited by the zeolite.
The availability of a large variety of synthesis methods, which for sure
will be optimized and enlarged in the future, makes possible to tailor and
define the contribution and features of the mesoporosity present in hier-
archical zeolites. Consequently, the research in the field of hierarchical
zeolites is expected to grow, specially when dealing with the following goals:
Better control of the mesopore size and distribution.
Optimization of the ratio mesopore/micropore surface to achieve the
best catalyst performance.
Deeper characterization of the nature and strength of the acid sites
present in the mesopores.
Increase in the number of catalytic applications, with a strong focus on
reactions requiring bifunctional catalysts, prepared by incorporation of
different active phases to hierarchical zeolites.
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