Mesoporous materials with zeolite framework: Remarkable effect of the hierarchical structure for retardation of catalyst deactivation

Article abstract of DOI:10.1039/b612116k

Hierarchical MFI zeolite was synthesized following a synthesis route using organic-inorganic hybrid surfactants; the resultant zeolite with mesoporous/microporous hierarchical structure exhibited remarkably high resistance to deactivation in catalytic activity of various reactions such as isomerization of 1,2,4-trimethylbenzene, cumene cracking, and esterification of benzyl alcohol with hexanoic acid, as compared with conventional MFI and mesoporous aluminosilicate MCM-41. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b612116k

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Mesoporous materials with zeolite framework: remarkable effect of the
hierarchical structure for retardation of catalyst deactivation{
Rajendra Srivastava, Minkee Choi and Ryong Ryoo*
Received (in Cambridge, UK) 22nd August 2006, Accepted 19th September 2006
First published as an Advance Article on the web 29th September 2006
DOI: 10.1039/b612116k
hierarchical MFI zeolite exhibited remarkably high resistance to
deactivation in catalytic activity of various reactions, such as gas
phase isomerization of 1,2,4-trimethylbenzene and cumene crack-
ing, and liquid phase esterification of benzyl alcohol with hexanoic
acid. Improvement in catalytic activity using mesoporous materials
composed of zeolite frameworks was anticipated10 and recently
reported.9,11 However, to the best of our knowledge, such
retardation of catalyst deactivation for zeolite with mesoporous/
microporous hierarchical structures is reported here for the first
time.
Hierarchical MFI zeolite was synthesized following a synthesis
route using organic–inorganic hybrid surfactants; the resultant
zeolite with mesoporous/microporous hierarchical structure
exhibited remarkably high resistance to deactivation in catalytic
activity of various reactions such as isomerization of 1,2,4-
trimethylbenzene, cumene cracking, and esterification of benzyl
alcohol with hexanoic acid, as compared with conventional
MFI and mesoporous aluminosilicate MCM-41.
Zeolites are crystalline microporous aluminosilicates, widely used
as catalysts in a variety of chemical reactions in the petrochemical
industry.1 The success of zeolite catalysis is attributed to the
presence of strong acid sites on the frameworks with well-defined
microporous structures that are responsible for the shape-
The hierarchical MFI zeolite was prepared according to a
procedure in which 3-[(trimethoxysilyl)propyl]hexadecyl-dimethyl-
ammonium
chloride
([(CH3O)3SiC3H6N(CH3)2C16H33]Cl,
TPHAC) was added into an initial synthesis composition for
ordinary MFI zeolite. Tetraethyl orthosilicate (TEOS) was used as
the silica source. Tetrapropylammonium bromide (TPABr) was
used as the MFI structure director. The initial gel composition was
1.0 Al2O3/4.0 TPABr/10.0 Na2O/41.8 SiO2/2.2 TPHAC/7200 H2O.
Conventional hydrothermal reaction was employed for the
crystallization during 2 d at 170 uC. Inductively coupled plasma
analysis of the zeolite product gave Si/Al = 20. For comparative
purposes, other aluminosilicate catalysts such as conventional MFI
zeolite and mesoporous MCM-41 (Al-MCM-41) with Si/Al = 20
were prepared according to the reported procedures.9 Scanning
electron micrographs (SEM) (Fig. 1) and N2 adsorption isotherms
˚
selectivity (pore diameter , 15 A). For instance, zeolite-based
cracking processes correspond to an added value of 40 billion US
dollars/year.2 However, in catalytic reaction of bulky molecules,
the sole presence of micropores inhibits facile mass transfer to and
from the active sites, often limiting the catalytic applications. The
last decade has witnessed the development of ordered mesoporous
aluminosilicate materials such as M41S and SBA.3 However,
despite their large pores, the mesoporous materials with a
noncrystalline framework were low in acidity and hydrothermal
stability, thus limiting their practical applications.
In recent years, various strategies have been developed in order
to enhance the pore accessibility of zeolites: synthesis of zeolite
nanoparticles,4 synthesis of ordered mesoporous materials using
zeolite seeds,5 and generation of mesopores inside zeolite particles
through steaming6 or other chemical etching routes.7 Mesoporous
zeolites have also been successfully synthesized by using nanosized
carbon templates.8 Very recently, we developed a new synthesis
strategy for the direct, hydrothermal crystallization of zeolites
having a mesoporous/microporous hierarchical structure.9 The
mesopores were disordered, but the pore diameters could be
uniformly tailored. The zeolite frameworks were highly crystalline
and exhibited strong acidity. The hierarchical zeolites showed
remarkably enhanced catalytic activity for bulky molecular
transformation.9
In the present communication, we report our finding of another
favourable effect of such hierarchical zeolite structures. That is, the
Centre for Functional Nanomaterials, Department of Chemistry, Korea
Advanced Research Institute of Sciences and Technology, Daejon,
305-701, Republic of Korea. E-mail: rryoo@kaist.ac.kr;
Fax: 82-42-869-8130; Tel: 82-42-869-2830
{ Electronic supplementary information (ESI) available: TEM image of
hierarchical MFI, XRD profiles for hierarchical MFI and MFI,
experimental procedure for the catalytic activity measurements. See
DOI: 10.1039/b612116k
Fig. 1 SEM images of hierarchical MFI (a and c) and MFI (b and d).
Chem. Commun., 2006, 4489–4491 | 4489
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and TeMB in hierarchical zeolite. In the hierarchical zeolite
catalyst, it is reasonable that the diffusion path would be
significantly shorter than that in the conventional zeolite catalyst.
The low selectivity of xylenes (3%) can be attributed to the short
diffusion path, which would prevent further disproportionation
reactions. If only the disproportionation reaction took place, the
molar ratio between xylenes and TeMB should be one. However,
in the case of conventional MFI, a surplus of xylenes was
observed. This result indicates that TeMB could be trapped in the
micropores of MFI as a precursor to cokes. Indeed, the formation
of cokes was identified by the black colour of the catalyst
recovered after reaction.
The three catalyst samples exhibited remarkably different
behaviours for deactivation. Slow deactivation was observed for
hierarchical MFI, whereas conventional MFI was deactivated very
rapidly, and Al-MCM-41 was about in the middle. The stability of
the hierarchical MFI against deactivation, as compared with
conventional MFI, may be attributed to the facile diffusion of
reactants and products through mesopores. It is well known that
catalyst deactivation in zeolites can occur due to coke formation at
the pore mouths, which is produced via secondary reactions. The
large surface area of the mesoporous structure would allow facile
molecular diffusion into and out of the micropores in the zeolite
frameworks. The micropore blockage would occur very slowly
because of the large number of pore mouths at the surface of the
mesopore walls. Similar slow deactivation has also been reported
with nanocrystalline zeolites that have much higher external
surface area than bulk zeolites.13
Fig. 2 N2 adsorption/desorption isotherms of MFI and hierarchical
MFI materials (inset shows their pore size distributions).
(Fig. 2) were obtained and analyzed in the same way as in our
previous work.9 The N2 adsorption isotherms and SEM images of
the hierarchical zeolite prepared in the present work are similar to
those reported in our previous work, except for the slight variation
of mesopore diameters depending on details of synthesis conditions
(Table 1).
Cumene cracking was also tested with the three catalyst samples
mentioned above. This reaction is a well-established test reaction
for hydrocarbon cracking activity that requires strong acidity.14
Benzene and propylene were found to be the major products in the
present investigation (ESI). C4–C6 hydrocarbons, ethylbenzene,
toluene and dipropylbenzene were minor products. The C4–C6
hydrocarbons can be attributed to the transformation of propylene
produced in the course of cumene cracking. Alkylation of cumene
with propylene leads to dipropylbenzene.11 The catalytic activity of
Al-MCM-41 was found to be very low (cumene conversion =
14%), due to its low acidity, whereas both conventional and
hierarchical zeolites exhibited high activities (cumene conversion =
97 and 94% respectively). The two MFI zeolite samples showed
similar activities during the initial reaction period (Fig. 3(b)). With
prolonged reaction time on stream, the two zeolite samples showed
a remarkable difference in their deactivation behaviour. That is,
catalytic activity of the conventional MFI decreased very rapidly
with time whereas no significant deactivation was detected in the
case of hierarchical MFI zeolite. Al-MCM-41 catalyst showed low
initial activity, and further, the catalyst was rapidly deactivated
with time on stream.
Gas phase isomerization and disproportionation reaction of
1,2,4-trimethylbenzene [1,2,4-TMB]12 was performed in order to
investigate the catalytic activity of these catalyst samples. Details of
the experimental proceedures are given in the ESI.{ Isomerization
of 1,2,4-TMB led to the simultaneous formation of 1,2,3-TMB and
1,3,5-TMB. The disproportionation produced all isomers of xylene
and tetramethylbenzene [TeMB] (Fig. 3(a), ESI). During the initial
reaction period, conventional MFI exhibited slightly higher
activity than the hierarchical MFI zeolite. The 1,2,4-TMB
conversion was 30 and 25% for MFI and hierarchical MFI,
respectively. Al-MCM-41 exhibited somewhat lower activity
(1,2,4-TMB conversion = 11%). The selectivity for TMB and
TeMB was higher in the case of hierarchical MFI (96%) and Al-
MCM-41(95%) than for conventional MFI (74%). The selectivity
difference can be ascribed to the improved mass transport of TMB
Table 1 Textural properties of various catalysts used in the present
study
Interestingly, the hierarchical zeolite also indicated very slow
deactivation in the liquid phase esterification reaction between
benzyl alcohol and hexanoic acid (Fig. 3(c)), compared with the
other catalysts. In this reaction, the ester was produced as a major
product while dibenzyl ether was obtained as a by-product due to
a dehydrative condensation between two alcohol molecules.
During the first run, the hierarchical MFI, conventional MFI
and Al-MCM-41 samples gave 92, 23 and 83% conversion of
benzyl alcohol, respectively. The ester selectivity was 75.2, 98.2
and 67.0%, respectively. Notably, both the conventional MFI and
aSBET
bVtotal
bVmeso
Catalysts
(m2 g21
)
(ml g21
)
(ml g21
)
cd (nm)
MFI
Hierarchical MFI
Al-MCM-41
a
350
590
930
0.22
0.50
0.69
0.08
0.38
0.61
—
5.2
2.6
SBET is surface area calculated using the Brunauer–Emmett–Teller
equation. Vtotal is total pore volume and Vmeso is mesopore
b
c
volume. d is mesopore diameter calculated using the BJH method
with adsorption branch.
4490 | Chem. Commun., 2006, 4489–4491
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Fig. 3 Deactivation behaviour of hierarchical MFI, MFI and Al-MCM-41: (a) 1,2,4-TMB isomerization, (b) cumene cracking, and (c) esterification of
benzyl alcohol and hexanoic acid.
Al-MCM-41 catalysts lost catalytic activity completely in five
recycling experiments. However, the hierarchical MFI still
exhibited 78% benzyl alcohol conversion even after five recycling
experiments.
flow processes, because of the cost-down effect due to less frequent
interruption for catalyst regeneration or replacement. Further
studies are in progress in our laboratory.
This work is supported by the Ministry of Science and
Technology through the Creative Research Initiative Program.
The facile diffusion through mesopores can explain the data in
Fig. 3(a) and 3(b), where the deactivation of hierarchical MFI was
comparable to that of Al-MCM-41 and much slower than that of
conventional MFI. However, it is noteworthy that hierarchical
MFI is much more stable than Al-MCM-41, in the case of
esterification of benzyl alcohol. The esterification reaction involves
large molecular species, hence takes place mainly at the surface of
mesopore walls (compared with the small molecular reactions
shown in Fig. 3(a) and Fig. 3(b)). We attribute the difference
between hierarchical MFI and Al-MCM-41 in Fig. 3(c) to their
differences in acid strength and Al concentration at the mesopore
walls. It is reasonable that all Al sites in Al-MCM-41 are
concentrated exclusively at the mesopore-wall surface because of
the post-synthesis incorporation.9 Despite a similar Al content, the
hierarchical MFI zeolite can have Al locations inside the mesopore
walls (zeolite framework) as well as at the wall surface. That is, Al
sites are much more highly concentrated in Al-MCM-41. The
short distance between adjacent Al sites in the Al-MCM-41 surface
seems to be favourable for the formation of polymeric coke species
that can deactivate the catalyst.
Notes and references
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In summary, the MFI zeolite with mesoporous/microporous
hierarchical structure exhibited very slow deactivation in the three
cases of catalytic applications shown above, as compared with
MFI without mesoporosity. Further work would be necessary for
the generalization of the slow deactivation phenomena into other
catalytic reactions. Nevertheless, through the present reactions, it is
proposed that the generalization would be highly probable. It is
reasonable that facile diffusion through mesopores can improve
access of reactant molecules to the strong acid sites and also
minimize the diffusion length of coke precursors out of the
micropore. This effect can lead to a significant change in the
product selectivity, increase the catalytic activity, and increase
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