Catalytic cracking of large molecules over hierarchical zeolites

Article abstract of DOI:10.1039/b600547k

A hierarchical zeolite catalyst was synthesized by transforming the skeletons of a bimodal pore silica gel into a zeolite through a steam-assisted conversion method, and shows high catalytic activity and a long catalyst lifetime for catalytic cracking of large molecules. The Royal Society of Chemistry 2006.

Full text of DOI:10.1039/b600547k

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Catalytic cracking of large molecules over hierarchical zeolites
a
Qian Lei, Tianbo Zhao,* Fengyan Li, Lingling Zhang and Yue Wang
a
b
b
b
Received (in Cambridge, UK) 16th January 2006, Accepted 6th March 2006
First published as an Advance Article on the web 16th March 2006
DOI: 10.1039/b600547k
A hierarchical zeolite catalyst was synthesized by transforming
the skeletons of a bimodal pore silica gel into a zeolite through
a steam-assisted conversion method, and shows high catalytic
activity and a long catalyst lifetime for catalytic cracking of
large molecules.
423 K for 24 h. The precursor solution was prepared by mixing
aluminium isopropoxide, sodium hydroxide, distilled water and
tetrapropylammonium hydroxide (TPAOH) following the molar
composition: 1 Al O : 1 Na O : 1266.7 H O : 6 TPAOH.
2
3
2
2
Thereafter, the samples were dried at 373 K for more than 24 h
and calcined at 873 K for 6 h. Finally, the protonated form of the
Nowadays, hierarchically structured zeolites are considered to
have potential impact in heterogeneous catalysis, adsorption and
separation, so hierarchical zeolites have become the subject of
sample was obtained by ion-exchange with NH
4
Cl, followed by
calcination at 823 K for 6 h.
The scanning electron microscopy (SEM) images (Fig. 1) clearly
show the morphology of the original macroporous silica and the
hierarchical zeolite catalyst. The morphology of the macroporous
silica gel shows the ‘interconnected structure’ (Fig. 1a), where both
the silica skeletons and the macropores (about 3 to 8 mm in
diameter) are continuous. The interconnected macroporous
morphology is formed when transitional structures of spinodal
1,2
intense research for such applications.
As compared with
conventional zeolites, these hierarchical zeolites could overcome
the limitations of the pore size, possessing the capability of
cracking large molecules. The general synthetic approach to
hierarchical zeolites previously relied on an endotemplating or
3
exotemplating route, such as molecular, supramolecular, colloidal
4
crystal template, latex spheres beads, carbon fiber, mesoporous
5
15
decomposition are frozen-in by the sol–gel transition of silica. It
6
7
8
silica spheres, ceramic foams, and wood cell. However, they
suffered from restricted mechanical stability and controlled
morphology. Recently, Nakanishi et al. reported the preparation
of a novel template with macropores and mesopores based on
phase separation, in the presence of organic polymer, in a silicon
alkoxide sol–gel process. Meanwhile, macroporous silica had been
applied in high-performance liquid chromatography (HPLC)
columns, which showed superior separation properties in analysis
is clear that the morphology of the hierarchical zeolite catalyst
(Fig. 1b) is quite similar to that of the macroporous silica gel.
Fig. 2 shows the X-ray diffraction (XRD) patterns of the
original macroporous silica gel, the hierarchical zeolite catalyst and
microporous ZSM-5. The hierarchical zeolite catalyst showed a
pattern typical of ZSM-5 as well as the amorphous silica,
9,10
compared with conventional packed columns. Takahashi
11,12
et al. have reported the preparation of silica–alumina catalysts
with continuous macropores and mesopores. They showed higher
catalytic activity than commercial silica–alumina catalysts in
11
12
cracking of cumene and dehydration of 2-propanol. The
results clearly show that macropores with interconnected structure
work effectively as pathways for rapid diffusion and improve the
12
reaction kinetics.
In this study, we synthesized novel catalysts with hierarchically
structured zeolites by transforming the skeletons of a bimodal pore
silica gel into a zeolite through a steam-assisted conversion method
1
3
(
SAC). Firstly, the macroporous silica used in this experiment
14
was
prepared,
following
published
procedures.
Tetramethoxysilane (TMOS) was hydrolyzed under acidic condi-
tions in the presence of various additives such as water soluble
polymers that induced a phase separation during the polymeriza-
tion stage. Then, the macroporous silica gel was immersed in the
precursor solution and subjected to steam-assisted conversion at
a
The Institute for Chemical Physics, Beijing Institute of Technology,
Beijing, 100081, P.R. China. E-mail: zhaotb@bit.edu.cn;
Fax: 86 10 6871 9687; Tel: 86 10 8168 2929
Department of Applied Chemistry, Beijing Institute of Petrochemical
b
Technology, Beijing, 102617, P.R. China.
E-mail: lifengyan@bipt.edu.cn; Fax: 86 10 6871 9687;
Tel: 86 10 8692 1705
Fig. 1 SEM images of the macroporous silica gel (a) and the hierarchical
zeolite catalyst (b).
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Fig. 2 XRD patterns of the macroporous silica gel (a), the hierarchical
zeolite catalyst (b) and microporous ZSM-5 (c).
indicating that the skeletons of the macroporous silica gel were
incompletely converted into the MFI zeolite structure. Actually,
when this hierarchically structured zeolite monolith is used as a
catalyst, not all the skeletons need to be completely converted into
a zeolite structure. By reducing the alkalinity of the precursor
solution and the steam treatment time, most of the interconnected
skeletons were preserved as shown in Fig. 1b, and some silica
skeletons dissolved and were subsequently transformed into zeolite
crystals. Therefore, we consider that the macroporous silica gel
provides both a template and a silica source.
Fig. 3 Nitrogen adsorption–desorption isotherms and pore size distribu-
tions of the macroporous silica gel (a) and the hierarchical zeolite catalyst
As shown in the inset in Fig. 3, the hierarchical zeolite catalyst
exhibits a monolithic shape of 10 mm in length and 5 mm in
diameter. Additionally, the retained mechanical strength of the
hierarchical zeolite catalyst was adequate for practical applications.
The adsorption–desorption isotherm of the macroporous silica
(b). The inset is a photograph of the macroporous silica gel and the
hierarchical zeolite catalyst.
+
(NH
the hierarchical zeolite catalyst.
-TPD) analysis gave acidity values of 0.266 H mmol/g for
3
gel is a type-IV isotherm and shows little increase until P ,
0
The SAC requires an appropriate amount of water in the
bottom of the autoclave to maintain the saturated vapor pressure,
and a higher amount of steam greatly enhances the rate of
16
indicating the presence of both mesopores and macropores. But
the adsorption–desorption isotherm of the hierarchical zeolite
catalyst is a type II isotherm as shown in Fig. 3, which is usually
considered to be indicative of adsorption in the macroporous
material. It is interesting to note that for the hierarchical zeolite
catalyst the hysteresis loops turn to type H3, indicating the
18
crystallization. Thoma et al. estimated that the role of the water
in the VPT solvent mixture was likely rehydration of surface
siloxane groups to create hydroxyl groups that could act as
adsorption sites for the structure-directing agents (SDA), and to
facilitate bond cleavage required for molecular rearrangement to
construct the crystalline cage of the zeolite. Serrano and van
1
7
presence of extra mesoporosity. The corresponding pore size
distribution of the macroporous silica shows a narrow peak
centered at 6.6 nm, while the hierarchical zeolite catalyst shows a
broad pore-size distribution range from 10 nm to 30 nm. The BET
1
9
Grieken took the direct conversion from gel to zeolite as evidence
for the solid–solid transformation in their recent review article.
However, the direct conversion to zeolite in the steaming step is a
complicated process. The mechanisms of crystallization in SAC are
still open questions.
2
specific surface area of the macroporous silica gel (492 m /g) is
2
higher than that of the hierarchical zeolite catalyst (253 m /g). The
micropore surface area and micropore volume were calculated by
the t-plot method. The macroporous silica gel shows negligible
micropores, while the hierarchical zeolite catalyst has a reasonable
The catalytic cracking performances of the hierarchical zeolite
catalyst and the microporous HZSM-5 were tested in a fixed-type
reactor for catalytic cracking of 1,3,5-triisopropylbenzene (TIPB)
at 623 K. The feed rate of TIPB was 0.07 ml/min and the catalyst
3
micropore volume (0.0233 cm /g) and micropore surface area
2
51.7 cm /g). Ammonia temperature-programmed desorption
(
Table 1 Initial catalytic activity of the microporous ZSM-5 and the hierarchical zeolite catalyst for the cracking of 1,3,5-triisopropylbenzene
a
Selectivity (%)
Catalyst
Conversion (%)
Benzene
IPB
MIPB
PIPB
others
Microporous ZSM-5
Hierarchical zeolite catalyst
14.3
97.8
—
13.1
—
33.9
66.8
22.7
—
5.1
33.2
25.2
a
IPB, MIPB and PIPB denote isopropylbenzene, m-diisopropylbenzene and p-diisopropylbenzene respectively.
1
770 | Chem. Commun., 2006, 1769–1771
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zeolite structure. The continuous macropores provide pathways
for transportation of larger molecules and the microporous
HZSM-5 provides acidic sites for catalytic activity. These results
suggest that this hierarchical zeolite catalyst may lead to
improvements in the catalytic cracking of large molecules.
Notes and references
1
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Fig. 4 TIPB cracking activity of the hierarchical zeolite catalyst.
volume was 3.9 g. The microporous ZSM-5 afforded an initially
low TIPB conversion of 14.3%, as shown in Table 1. The
microporous ZSM-5 has 10-ring pores with diameter no larger
than 0.56 nm, while TIPB has a dynamic diameter of 0.94 nm and
cannot diffuse through the inner pores of ZSM-5. Therefore,
catalytic reaction can only occur on the external surface of ZSM-5
1
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1
2
1
2
2
0
crystals. However, the hierarchical zeolite catalyst showed an
initially high catalytic activity corresponding to 98% conversion.
After 3 h TIPB conversion was still 85% . Fig. 4 shows that
isopropylbenzene and m-diisopropylbenzene were the major
products. The yield of m- and p-diisopropylbenzene increased
with prolongation of reaction time, while the yield of isopropyl-
benzene and benzene decreased. It is interesting that the conversion
and product distributions are basically stable when the reaction
time is longer than 1.5 h. The remarkable catalytic reactivity of
large molecules is attributed to the presence of the hierarchical
1
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1
1
1
4
1
2
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Chem. Commun., 2006, 1769–1771 | 1771