Catalytic Benzene Alkylation over Mesoporous Zeolite Single Crystals: Improving Activity and Selectivity with a New Family of Porous Materials

Article abstract of DOI:10.1021/ja037063c

Mesoporous zeolite single-crystal catalysts are shown to be both more active and more selective than conventional zeolite catalysts in the alkylation of benzene with ethene. The superior catalytic properties are ascribed to improved mass transport in the

Full text of DOI:10.1021/ja037063c

Published on Web 10/09/2003
Catalytic Benzene Alkylation over Mesoporous Zeolite Single Crystals:
Improving Activity and Selectivity with a New Family of Porous Materials
†
†
†
,†,‡
Christina Hviid Christensen, Kim Johannsen, Iver Schmidt, and Claus Hviid Christensen*
Haldor Topsøe A/S, NymølleVej 55, DK-2800 Lyngby, Denmark, and Department of Chemistry, Building 206,
Technical UniVersity of Denmark, DK-2800 Lyngby, Denmark
Received July 4, 2003; E-mail: chc@kemi.dtu.dk
Zeolites and zeotype-materials have been found to catalyze a
wide variety of chemical reactions. In industry, zeolite catalysts
Table 1. Zeolite Catalyst Acidity Expressed as Si/Al
1
27Al NMR
catalyst
NH
3
IR
are primarily used for catalytic fuel-upgrading in refineries and also
for the production of various petrochemicals. When zeolite catalysts
were first introduced in oil-cracking to replace the amorphous
silica-alumina catalysts, it was estimated that the higher gasoline
yield resulting from the zeolite catalyst corresponded to an added
conventional
mesoporous
71
116
70
110
70
120
Table 2. Selected Properties of Zeolite Catalysts
pore volumea
mL/g
mesopore sizea
nm
2
SEM
µm
value of 40 billion U.S. dollars/year. Much of the success of zeolite
catalysis can be attributed to the presence of micropores (diameter
catalyst
XRPD
conventional
mesoporous
MFI
MFI
2.0
2.1
0.10
0.59
none
12
<
20 Å) with well-defined structures that are responsible for the
shape-selectivity exhibited by such catalysts. However, in many
cases, the sole presence of micropores can also be a major limitation
because mass transport to and from the active sites located within
the micropores is slow, much slower than, for example, Knudsen
diffusion, which is often limiting the performance of industrial
catalysts. To overcome this limitation, there has been a long-
standing drive either to minimize the crystal size of zeolite catalysts
or to increase the pore size of available zeolites.4,5 Here, we show
that a new family of zeolite catalysts, the mesoporous zeolite single
a
Mesopores were quantified by the BJH method. The conventional
zeolite contains only micropores, whereas the mesoporous zeolite has both
micropores and mesopores.
3
6
crystals, exhibit significantly improved catalytic activities and
selectivities as compared to conventional zeolite catalysts, in the
alkylation of benzene with ethene. This is a major industrial process
responsible for production of almost all ethylbenzene which in turn
is the raw material for styrene manufacture. The mesoporous zeolite
single crystals combine in each individual zeolite crystal, the
crystallographic, intracrystalline micropore system typical of zeolites
with a noncrystallographic, intracrystalline mesopore system. This
is shown to dramatically improve mass transport in zeolite crystals.
7
6
Comparable samples of conventional and mesoporous zeolite
powders were prepared according to published procedures. The
resulting powders were pressed into catalyst tablets which were
crushed and sieved to a particle size of 150-300 µm. Next, 150
mg of each catalyst was tested in an isothermal plug-flow reactor
(4 mm inner diameter) at temperatures between 583 and 643 K
and pressures of 2.5 and 5 bar. In the feed, the molar ratio of
benzene to ethene was 5.1:1, and this limits the maximum
conversion of benzene to ca. 20%. Flow rates corresponding to a
weight-hourly space velocity (WHSV) (mass of feed/mass of
catalyst/time) of 225 h were used, and the exit gas was analyzed
on-line by GC.
Figure 1. SEM images of the conventional and mesoporous zeolite catalysts
used for alkylation of benzene with ethene.
-
1
Table 2 shows the results from scanning electron microscopy
(SEM), X-ray powder diffraction (XRPD), and N2 adsorption/
desorption (at 77 K) studies.
In Table 1, the acidities of the two catalysts as determined by
8
9
27
ammonia desorption, IR of adsorbed pyridine, and Al MAS
8
NMR are compared.
Both catalysts are seen to contain exclusively zeolite ZSM-5
It is seen that an excellent agreement between the different
experimental techniques for determining acidity exists for the zeolite
catalysts. The conventional zeolite catalyst has almost twice as many
acidic sites as the mesoporous zeolite catalyst.
(with MFI structure). Moreover, the average crystal size as
determined from SEM is almost the same, but the mesoporous
zeolite crystals are slightly larger than the conventional zeolite
crystals as shown in Figure 1. The typical coffin-shape of MFI
crystals is observed for both samples, and the very different surface
morphologies are easily seen.
†
Haldor Topsøe A/S.
Technical University of Denmark.
‡
13370
9
J. AM. CHEM. SOC. 2003, 125, 13370-13371
10.1021/ja037063c CCC: $25.00 © 2003 American Chemical Society

C O MMU N I C A T I O N S
Figure 3. Selectivity to ethylbenzene for mesoporous zeolite and conven-
tional zeolite catalyst obtained at 583-643 K and 2.5 bar.
Figure 2. Arrhenius plot illustrating the activity difference between
conventional and mesoporous zeolite catalysts. The activities are expressed
as turn-over frequencies using the Si/Al ratios in Table 1 as a measure of
the number of active sites.
experiments, the major byproduct was dialkylated benzene, and less
than 0.5% was converted into polyalkylated products that were not
further analyzed.
The higher selectivities to ethylbenzene observed with meso-
porous zeolite catalyst can be understood in simple terms. Whenever
a benzene molecule is ethylated, it can either be transported into
the product stream or undergo further ethylation. In the mesoporous
zeolite catalyst, the diffusion path is significantly shorter than that
in the conventional zeolite catalyst, and further ethylation is
suppressed. Hence, it is possible with the mesoporous zeolite
catalysts to obtain both a higher activity and a higher selectivity to
the desired product. Recently, several other successful attempts to
Thus, the major difference in this comparison between the
conventional and the mesoporous zeolite catalysts is the presence
of noncrystallographic intracrystalline mesopores resulting in a very
high porosity and a large average pore size for the mesoporous
zeolite. The mesoporous zeolites are members of a new family of
materials characterized by a high porosity attributed to a network
of mesopores introduced into each individual crystal. Recently,
similar materials have been reported for other compounds.10
However, the mesoporous zeolite single crystals are unique in the
sense that they contain interconnected micropores and mesopores
inside each individual single crystal. In Figure 2, the importance
of such a bimodal pore size distribution in the catalytic alkylation
of benzene with ethene is clearly shown. The activity measurements
are conducted at conditions that are comparable to those used in
industrial processes but generally at somewhat lower conversions.
Therefore, the effect of the enhanced mass transport with the
mesoporous zeolite catalyst could be even larger than shown here.
First of all, it is seen that the mesoporous zeolite catalyst is
significantly more active than the conventional zeolite catalyst.
Furthermore, it appears reasonable to conclude that this activity
difference can be ascribed to improved mass transport in the
mesoporous zeolite because the presence of a diffusional limitation
is manifested by a lower apparent activation energy. For the
conventional and mesoporous zeolite catalysts, we find apparent
overcome the transport limitation with zeolite catalysts in various
processes have been reported,1
3,14
and there has also been much
focus on establishing new synthetic routes to mesostructured zeolite
catalysts.1
5,16
Most industrial zeolite catalysts are operated at
conditions where the reaction rate is limited by diffusion rates. This
suggests that use of mesoporus zeolite single crystals can lead to
improvements in existing processes or allow entirely new applica-
tions of zeolite catalysts.
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