Silica-alumina-supported molybdena catalyst for propene metathesis

Article abstract of DOI:10.1246/cl.2000.66

Without reductive pretreatment, 10 wt% MoO₃/SiO₂-Al₂O₃ (JRC-SAH-I) exhibits high catalytic activity for propene metathesis. The Mo ions on SiO₂-Al₂O₃ may exist as dispersed polyanions, and they can be reduced easily in contact with propene even at room temperature. The reduced Mo ions formed in the initial step of the reaction are efficient for metathesis.

Full text of DOI:10.1246/cl.2000.66

66
Chemistry Letters 2000
Silica-Alumina-Supported Molybdena Catalyst for Propene Metathesis
Hirofumi Aritani,* Osamu Fukuda, Takashi Yamamoto,† Tsunehiro Tanaka,† and Seiichiro Imamura
Faculty of Engineering and Design, Kyoto Institute of Technology, Sakyo-ku, Kyoto 606-0962
†Department of Molecular Engineering, Graduate School of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-8501
(Received September 24, 1999; CL-990820)
343 K for 6 h to form a paste. The paste was then dried
overnight and calcined at 773 K for 6 h. The metathesis reac-
tion of propene was carried out in a conventional circulating
reaction system. Each catalyst sample (0.5 g) was placed in the
reaction vessel (total reaction volume of 871 cm3) and treated
in vacuo at 773 K for 1 h before the reaction. In the case of
prereduced sample, it was treated with hydrogen (80 Torr) at
773 K for 1 h, and then evacuated at 773 K for 1 h. After the
pretreatment, reactant gas mixture consisting of propene (10
Torr) and helium (90 Torr) was introduced in the reaction ves-
sel. The reaction gas was periodically analyzed by using on-
line gas chromatograph (Shimadzu GC-8A) equipped with VZ-
7 column (3 m). Since deuterated propene reactant was not
used, only the products of 'productive metathesis'14,17,18 such as
Without reductive pretreatment, 10 wt% MoO3/SiO2-
Al2O3 (JRC-SAH-1) exhibits high catalytic activity for propene
metathesis. The Mo ions on SiO2-Al2O3 may exist as dispersed
polyanions, and they can be reduced easily in contact with
propene even at room temperature. The reduced Mo ions formed
in the initial step of the reaction are efficient for metathesis.
Molybdenum-containing heterogeneous catalysts for olefin
metathesis have been of great interest1 since Banks et al.2
reported the first example. At the same time, the active Mo
species for metathesis have been characterized. SiO2-support-
ed molybdenum is a typical catalyst exhibiting high metathesis
reactivity as reported by Kazansky et al.3,4 They proposed that
the active species for propene metathesis are tetrahedrally coor-
dinated Mo4+-monoxo formed by prereduction with CO under
photo-irradiation.5,6 For MoO3/Al2O3, Grünert et al.7,8 also
concluded that Mo4+ species formed by H2 prereduction is
active for metathesis. Whereas, Zhang et al.9 reported that
Mo5+ dioxo-species on SiO2, which is formed by photo-reduc-
tion with CO, is effective for metathesis. Many workers have
concluded that Mo4+ acts as to the active species, however,
several workers have shown that Mo5+ and/or its dimer species
show the activity for metathesis. The detailed conclusion about
the valence of Mo is still unclear, however, the reaction mecha-
nism of metathesis has been provided. It is accepted generally
that the reduced Mo ions can easily form Mo-alkylidene
species by contact with olefins, and then, the metathesis reac-
tion proceeds via metallacyclobutane intermediate.10-12 The
reduced Mo ions are thus necessary to form the alkylidene
intermediates. In addition, it is also accepted that extent of
reduction give a decrease of the activity. For instance, Tanaka
et al.13,14 proposed that MoO3/β-TiO2 with x=2.3-2.9 is effec-
tive for propene metathesis. Segawa et al. reported that
reduced MoO3/TiO2 in x=1.9 shows the maximum activity for
propene metathesis.15,16 As described above, appropriate pre-
reduction needs to give an active Mo species before the
metathesis reaction. Nevertheless, we found that MoO3/SiO2-
Al2O3 catalysts exhibit the catalytic activity for propene
metathesis even at room temperature without reductive pre-
treatment. The detail is shown in this report.
could be analyzed, and 'degenerate-metathesis' reactivity could
not be evaluated. Other products such as ethane, propane, or
hexenes were not detected. In order to examine whether the
reaction takes place catalytically or not, twice reaction was car-
ried out successively. In this successive reaction, the catalyst
was evacuated without any other treatment at the reaction tem-
perature between the first and second reactions. UV-Vis spec-
tra were recorded with a Perkin-Elmer Lambda19 spectrometer
in a diffuse reflectance mode at room temperature.
Table 1 shows the metathesis reactivity of the supported
molybdena catalysts at 473 K. Prereduced MoO3/Al2O3 and
MoO3/SiO2 show higher conversion than unreduced ones. On
the other hand, prereduced MoO3/SiO2-Al2O3 catalysts such as
HY, SAH-1, and SAL-2 show lower activity than unreduced
ones. It is noted that MoO3/SAH-1 and MoO3/SAL-2 exhibit a
much higher activity for propene metathesis than other cata-
lysts. These results suggest that the silica-alumina support give
a different effect on the formation of active molybdena species
from other supports. The reactivity of MoO3/SAH-1 with vari-
ous molybdena contents at 293 and 473 K is examined. The
result is summarized in Table 2. In the both reaction tempera-
tures, 10 wt% MoO3/SAH-1 possesses the maximum activity.
The result of the 2nd reaction is also shown in the table. For
Supported molybdenum catalysts were prepared by
impregnation of each metal oxide support with an aqueous
solution of (NH4)6Mo7O24·4H2O (Nacalai Tesque). The oxide
supports used in this study were SiO2 (Aerosil), γ-Al2O3
(Nacalai Tesque; calcined at 873 K), TiO2 (Wako; anatase-
type) HY-zeolite (JRC-Z-Y5.6; containing 20.4 wt% of Al2O3,
deionized to remove sodium ions by using HNO3 solution and
calcined at 873 K) and SiO2-Al2O3 containing 28.6 wt% of
Al2O3 (JRC-SAH-1; designated as SAH-1) and 13.8 wt% of
Al2O3 (JRC-SAL-2, designated as SAL-2). The impregnating
solution was stilled at room temperature and then evaporated at
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
67
containing reduced ions act as the active center for metathesis.
It was reported that the extent of the reduction of loaded
molybdena on SiO2-Al2O3 shows a maximum when Al2O3 con-
tent is 25 wt%.22 Thereby, the molybdena on SAH-1 can be
reduced easier than that on other supports. It is concluded that
MoO3/SAH-1 can be reduced easily to form active species for
metathesis even by contacting with propene. The reductive
pretreatment is unnecessary to form Mo-alkylidene species as
an intermediate of metathesis.
This work is supported by a Grant-in-Aid (No. 11750675)
from the Ministry of Education, Science, Sports, and Culture.
References and Notes
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all MoO3/SAH-1 samples at 473 K, the conversion of propene
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MoO3/SAH-1 containing 695 µmol/g of MoO3, 806 µmol/g of
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