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Hierarchical Zeolites A Proven Strategy to Combine Shape Selectivity with Efficient Mass Transport.

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Highlights
Zeolites
Hierarchical Zeolites: A Proven Strategy to Combine
Shape Selectivity with Efficient Mass Transport**
Martin Hartmann*
Keywords:
carbon · ethylbenzene · mesoporous materials ·
zeolites
Although a variety of chemical reactions of industrial interest are catalyzed
by zeolites or zeolite-analogue materials, zeolite-based catalysts have almost
exclusively found application in refinery
and petrochemical processes where the
shape-selective properties of the microporous zeolites are exploited.[1] One of
the reasons that zeolites have not yet
found a wider range of industrial applications is the sole presence of micropores which imposes diffusion limitations on the reaction rate. Mass transport to and from the active sites located
within the micropores is slow (even
compared to Knudsen diffusion) and
limits the performance of industrial
catalysts. To overcome this limitation,
there has been a long-standing drive to
either minimize the size of the zeolite
crystals or to increase the pore size of
zeolites. The latter strategy has led to
the discovery of various large-pore zeolites and zeolite analogues (i.e. VPI-5,[2]
UTD-1,[3] and more recently ECR-34,[4]
SSZ-53, and SSZ-59[5]) and also to the
discovery of mesoporous molecular
sieves.[6] However, the use of these novel
materials in industrial applications is
rather limited. Another possibility is to
decrease the size of the zeolite crystals
and several synthesis schemes have been
reported which allow the preparation of
very small (< 50 nm) zeolite crystals.[7]
However, none of these attempts has
produced an easy means of controlling
the crystal size. Moreover, separation of
the small zeolite crystals from a reaction
mixture by filtration is difficult owing to
the colloidal properties of these materials.
Zeolites with hierarchical pore architecture (that is, zeolites containing
both micro- and mesopores) have been
found to present a solution to this
problem. The effect of the presence of
mesopores is already used in a number
of industrial processes that make use of
zeolite catalysts, such as, the cracking of
heavy oil fraction over zeolite Y, the
isomerization of the C5/C6 cut of the
light naphtha fraction to increase the
octane number, and cumene production
over dealuminated mordenite.[8]
To prepare zeolites with hierarchical
pore structure four approaches can be
followed: 1) Small zeolite crystals are
supported, for example, on latex
spheres, carbon fibers, or surfactants,
the support is then removed by calcination.[9] 2) The walls of mesoporous silicates (e.g. MCM-41 or SBA-15) are
recrystallized or zeolite precursors are
deposited on the walls of mesoporous
supports.[10] 3) A widely applied method
to generate mesopores involves steaming and acid-leaching treatments of
zeolite crystals.[8] These methods generate mesopores by extraction of aluminum from the zeolite lattice. However,
the mesopores formed during steaming
are predominantly cavities in the zeolite
crystals rather than cylindrical pores
connecting the external surface with
the interior of the crystal.[8] 4) Mesopores are templated with carbon during
zeolite synthesis. This method was recently developed by researchers from
Haldor Topsoe.[11–14] A carbon source,
for example, carbon black, carbon nanotubes, or nanofibers,[15] is impregnated
with a zeolite precursor solution after
which the material is subject to a hydrothermal treatment to grow the zeolite
crystals. In a subsequent calcination
step, the carbon and the template are
burned away resulting in intracrystalline
mesopores in the zeolite (Figure 1).
Proper choice of the carbon source and
the synthesis conditions allows tuning of
size, shape, and connectivity of the
mesopores in the system. Furthermore,
the mesoporosity and composition of
[*] Priv.-Doz. Dr. M. Hartmann
Fachbereich Chemie, Technische Chemie
Technische Universit)t Kaiserslautern
Postfach 3049, 67653 Kaiserslautern
(Germany)
Fax: (+ 49) 631-205-4193
E-mail: hartmann@chemie.uni-kl.de
[**] The author’s work in this area has been
funded by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen
Industrie.
5880
Figure 1. Growth of zeolite crystals around carbon particles. Nucleation of the zeolite occurs between the carbon particles; the crystal grows continues within the pore system of the carbon
template.
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
DOI: 10.1002/anie.200460644
Angew. Chem. Int. Ed. 2004, 43, 5880 –5882
Angewandte
Chemie
the material can be varied independently.[11–14] In this context, the recent publication from the group of Christensen is
worth mentioning.[16] They report the
performance of zeolites with a hierarchical pore structure in an industrially
relevant reaction, namely the ethylation
of benzene to ethylbenzene, and compare the results to a conventional microporous HZSM-5 catalyst. The worldwide demand for ethylbenzene, which is
the raw material for styrene production,
is roughly 22 B 106 t per year and is
almost exclusively produced by this
reaction. The alkylation of benzene with
ethylene over the HZSM-5 catalyst
constitutes the heart of the Mobil–
Badger process, which was first brought
on stream in 1980. This high-temperature vapor-phase alkylation process
offers various advantages over liquidphase processes (by Monsanto or UOP)
based on AlCl3 or BF3 catalysts. Compared to these Friedel–Craft catalysts,
zeolites have the clear advantages of
being noncorrosive and harmless to the
environment. With the HZSM-5 catalyst
an excellent selectivity for ethylbenzene
of more than 98 % is obtained at a
benzene conversion level of about 20 %.
The uniform channels (dimensions
(dp) = 0.51 B 0.55 nm) of zeolite ZSM-5
permit the entrance of the feed molecules as well as departure of the product
molecules ethylbenzene and diethylbenzene isomers, while higher alkylated
products are restricted from leaving.
Any of the higher alkylated products
formed within the ZSM-5 channels are
forced to undergo transalkylation or
dealkylation to facilitate their diffusion
back to the bulk. Steric hindrance of the
necessary transition states is also believed to inhibit the formation of polyalkylated products. Although these are
the clear advantages of using a shapeselective microporous catalyst, the problems of catalyst deactivation by coke
formation and low reaction rates owing
to mass transport limitations are major
obstacles to overcome.
Christensen et al. showed convincingly that the zeolite catalyst with the
hierarchical pore architecture is significantly more active than the conventional zeolite catalyst under the reaction
conditions employed (TR = 583 to
643 K; pR = 0.25 MPa), which are sufficiently close to the conditions employed
Angew. Chem. Int. Ed. 2004, 43, 5880 –5882
in the industrial process (TR 700 K;
pR = 2 to 5 MPa).[16] Moreover, the selectivity to ethylbenzene increases by 5
to 10 % depending on the benzene
conversion. The increased activity is
ascribed to the improved mass transport
in the mesoporous zeolites which is
indicated by an increase of the apparent
activation energy from 59 to 77 kJ mol 1.
Attributing the higher activity to improved mass transport is further substantiated by the differences in selectivities observed for the two catalysts. The
higher selectivity to ethylbenzene is
explained by Christensen et al. in the
following way: Whenever an ethylbenzene molecule is formed, it can either be
transported into the product stream or
undergo further alkylation. However, in
a mesoporous zeolite the diffusion path
is significantly shorter than in the conventional zeolite and further alkylation
is suppressed.
The hierarchical catalyst used in this
study was prepared by impregnating
carbon black pearls (particle diameter = 12 nm) to incipient wetness with a
clear zeolite synthesis gel. After a hydrothermal treatment at 180 8C for 72 h
in a stainless steel autoclave, the product
was collected by filtration, washed, and
the carbon black was removed by controlled combustion in air at 550 8C for
8 h. The mesoporous zeolite obtained is
characterized by a significantly higher
specific pore volume (0.59 versus
0.10 cm3 g 1) and the presence of mesopores with a diameter of approximately
12 nm.[16] The size of the mesopores is
predefined by the primary carbon particles which are encapsulated by the
growing zeolite crystal and are later
removed by the controlled combustion
in air. The shape, size and tortuosity of
the mesopores can be controlled by the
carbon source used. To produce large
zeolite crystals (with a hierarchical pore
architecture) rather than nanosized zeolite crystals, it is essential that an excess
of sufficiently concentrated gel is used
and that the carbon matrix has pores of a
size that allows crystal growth to proceed through the pore system. It is still
not fully understood which synthesis
parameters promote the formation of
large mesoporous crystals over nanosized crystals located in the carbon
pores. However, it seems reasonable to
assume that the ration of the nucleation
www.angewandte.org
rate relative to the growth rate is of
primary importance.[11]
Hierarchical zeolites have also been
shown to be superior catalysts in the
epoxidation of oct-1-ene and cyclohexene[12, 17] as well as in the isomerization
of n-hexane and n-heptane.[18] In both
cases, the activity as well as the selectivity for the desired products is higher
than with the conventional zeolite catalyst.
In conclusion, the primary advantages of these new mesoporous zeolites
compared with conventional micronsized zeolite crystals are related to their
higher specific external surface area
causing 1) higher reaction rates for diffusion-limited reactions, 2) better transport properties resulting in improved
selectivities for the target molecules,
3) better contact between active components in bifunctional catalysts, 4) slower
deactivation caused by blocking of the
pore mouth, and 5) easier burn-off of
coke deposits. Compared with nanocrystal zeolites, the advantages of hierarchical zeolites are related to their
larger crystal size, that is, easier processing owing to noncolloidal properties and
higher stability in high-temperature
processes including catalyst regeneration. While the effect of mesoporosity
on the catalytic performance of zeolite
catalysts has been known for some time
and is already employed in commercial
catalysts, the major achievement in the
work by Christiansen et al. is that the
mesopores can be produced in a more
controlled manner. Moreover, the direct
connection of the mesopores to the
external surface represents a significant
improvement compared to the (predominantly internal) mesopores generated
by steaming or acid leaching. However,
different areas can be identified where
further research and development are
required, that is, tailoring the properties
of the carbon source with respect to
particle size and morphology and the
use of different zeolites. The effect of a
mesoporous secondary pore system on
mass transport is expected to be much
larger for zeolites with one-dimensional
pore systems. In selecting materials used
in industrial processes, mordenite or
zeolite L might be prime targets. Moreover, synthesis strategies for hierarchical zeolites need to be developed for
2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5881
Highlights
zeolites that grow from viscous gels or
require seeds to crystallize.
Published Online: October 8, 2004
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Angew. Chem. Int. Ed. 2004, 43, 5880 –5882
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