Received: December 4, 2014 | Accepted: December 17, 2014 | Web Released: January 14, 2015
CL-141120
Synthesis of a Silicalite-1-coated Titanium Silicalite-1 (TS-1) Zeolite
and Its Catalytic Activity in Liquid-phase Oxidation
Yusuke Sugiura,* Yuichiro Hirota, Yoshiaki Uchida, and Norikazu Nishiyama1
1
2
1,3
1
Division of Chemical Engineering, Graduate School of Engineering Science, Osaka University,
1
-3 Machikaneyama, Toyonaka, Osaka 560-8531
Department of Chemical Engineering, Graduate School of Science and Engineering, Tokyo Institute of Technology,
-12-1 Ookayama, Meguro-ku, Tokyo 152-8550
Japan Science and Technology Agency, PRESTO, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012
2
2
3
(
E-mail: sugiura.0126@cheng.es.osaka-u.ac.jp)
Silicalite-1/titanium silicalite-1 (TS-1) composites were
synthesized by growing silicalite-1 on the surface of TS-1.
Oxidation of 1-hexene was studied using both the silicalite-
silicalite-1/TS-1 composite. If the silicalite-1 layer were poly-
crystalline, the mechanical strength of the composite would
not be sufficient for use as a fluidized catalyst. In addition,
the diffusion resistance would be large if the polycrystalline
1
/TS-1 composite and uncoated TS-1. The silicalite-1/TS-1
1
4,15
composite showed a higher selectivity toward 1,2-epoxyhexane
than uncoated TS-1 because sequential reactions on the external
surface of TS-1 were inhibited. The catalytic activity of the
silicalite-1/TS-1 composite was almost the same as that of TS-1,
suggesting that the silicalite-1 layer is very thin and that the
diffusion resistance is negligibly small.
silicalite-1 layer is thick. From our previous work,
when
the concentration of the silica source and the organic structure
directing agent (OSDA) in the coating solution is high, silicalite-
1 crystals are formed not only on the surface of the core zeolite
catalysts but also in the solution. The homogeneous nucleation
of silicalite-1 crystals in the solution is a problem because it is
not easy to separate silicalite-1/zeolite composites from the
silicalite-1 crystals formed in the solution. Thus, in this study,
we have first examined the effect of the composition of the
coating solutions to obtain a single-crystal-like silicalite-1/TS-1
composite. Second, the effect of the coating of the single-crystal-
like silicalite-1/TS-1 composite on the product selectivity in the
oxidation of 1-hexene has been examined.
The catalytic oxidation of olefins to epoxides is of great
importance to the chemical industry, because epoxides are
valuable intermediates for a wide variety of bulk chemicals,
polymers, and fine chemicals. In olefin epoxidation processes,
titanium silicalite-1 (TS-1) shows a high selectivity to epoxy
compounds in the liquid-phase oxidation of a wide range of
hydrocarbons, including alkenes, alkanes, alcohols, and aromat-
TS-1 zeolites were synthesized according to the procedures
1
6
reported by Tatsumi’s group. The TS-1 crystals were used as
the core zeolite for the silicalite-1 coating. The precursor
solution for the coating consisted of fumed silica, tetrapropy-
lammonium hydroxide (TPAOH), ethanol, and deionized water,
and the molar ratios are shown in Table 1. Approximately, 0.3 g
of the TS-1 crystals was immersed in 17 g of the precursor
solution. The crystallization was carried out under hydrothermal
conditions at 453 K for 24 h in a closed Teflon-lined stainless
steel vessel with agitation. The products were rinsed repeatedly
using deionized water, dried overnight at 363 K, and then
calcined in air at 823 K for 10 h.
1
8
ics. The high selectivity to epoxy compounds in oxidation
over TS-1 arises from pore size of the structure of MFI-type
9
zeolite. On the other hand, on the external surface, the reactions
take place without spatial restriction based on the zeolite
structures. Therefore, the selectivity toward aimed products is
decreased due to the reaction over the active sites on the external
structure.
One of the solutions to this problem is to coat zeolite
catalysts with inactive materials such as silica.1
013
However,
when the mouth of the pore is reduced in size by the deposition
of silica, the activity of catalysts decrease. Another way of
coating with inert materials is by zeolite overgrowth. Our group
has proposed a silicalite-1 (Al-free MFI zeolite) coating on
zeolite catalyst particles.1 Silicalite-1 has the same crystalline
structure (MFI) as ZSM-5. An epitaxial growth of silicalite-1 on
the external surface of ZSM-5 crystals (MFI structure) resulted
in a direct connection of zeolite pores between core and shell
zeolites. Therefore, the diffusion resistance for desired products
does not increase much. Therefore, not only high selectivity but
also high conversion (catalytic activity) was attained.
The uncoated TS-1 and silicalite-1/TS-1 composites were
characterized by XRD, SEM, N2 adsorptiondesorption mea-
surements, and UVvis and FT-IR spectroscopy. The XRD
patterns of the uncoated TS-1 and silicalite-1/TS-1 composites
are shown in Figure 1. They show a XRD pattern typical of the
MFI structure, confirming its high purity and crystallinity by
comparing to the XRD pattern of the MFI structure provided by
Structure Commission of the International Zeolite Association
4,15
9
(IZA).
In this study, we have coated TS-1 crystals with silicalite-1.
The silicalite-1/TS-1 composite was used for the oxidation of 1-
hexene. The purpose of this study was to increase the selectivity
of 1,2-epoxyhexane by inhibiting subsequent reactions on the
external surface of TS-1 crystals. As described above, the
morphology of the composite is important for the formation of
a defect-free silicalite-1 layer. Epitaxial growth of silicalite-1 on
the TS-1 crystal may be necessary to obtain a single-crystal-like
Table 1. Molar ratio of precursor solutions for silicalite-1 coating
Fumed silica
mol
TPAOH
/mol
Ethanol
/mol
H2O
/mol
Sample
/
a
b
c
d
1.5
1.0
1.0
0.5
0.12
0.12
0.06
0.06
8
8
8
8
120
120
120
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