Selective synthesis of methylal from methanol on a new crystalline SbRe2O6 catalyst

Article abstract of DOI:10.1246/cl.2000.674

A novel SbRe₂O₆ catalyst was active for the selective methanol oxidation to methylal (CH₂(OCH₃)₂). Other known Sb-Re oxides, Sb₄Re₂O₁₃ and SbOReO₄·2H₂O produced almost no methylal. The bulk and surface of the SbRe₂O₆ catalyst was characterized by XRD, Raman, XPS and SEM. The high performance of SbRe₂O₆ may be attributed to the Re-oxide octahedra connecting with Sb-O chains.

Full text of DOI:10.1246/cl.2000.674

674
Chemistry Letters 2000
Selective Synthesis of Methylal from Methanol on a New Crystalline SbRe2O6 Catalyst
Youzhu Yuan, Haichao Liu, Hideo Imoto, Takafumi Shido, and Yasuhiro Iwasawa*
Department of Chemistry, Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
(Received March 6, 2000; CL-000220)
A novel SbRe2O6 catalyst was active for the selective
thesis from methanol, where three methanol molecules are incor-
porated into a methylal molecule. In this paper we wish to report
the first selective synthesis of methylal by the catalytic methanol
oxidation on a novel SbRe2O6 catalyst.
Three Sb-Re-O compounds, SbOReO4⋅2H2O, SbRe2O6 and
Sb4Re2O13 were prepared according to the procedures reported
elsewhere.10-14 For comparison, Sb2O3- and SiO2-supported
Re2O7 catalysts were also prepared by an incipient wetness
impregnation method. Catalytic performances were examined in
a conventional fixed-bed flow reactor at 1 atm (He/O2/MeOH =
86.3/9.7/4.0 (mol%)).
The activities of various catalysts for methanol conversion
and the selectivity toward methylal formation are shown in Table
1. Sb oxides such as Sb2O3, Sb2O4 and Sb2O5, a mechanical
mixture of Re2O7 and Sb2O3 (Re2O7 + Sb2O3), and Re oxides
supported on Sb2O3 and SiO2 (Re2O7/Sb2O3 and Re2O7/SiO2,
respectively) showed no or negligible activities for the methylal
formation at 573 K. Sb4Re2O13 and SbOReO4⋅2H2O produced
almost no methylal either. Only SbRe2O6 among the crystalline
Sb-Re oxides synthesized thus far was active for the selective
methanol oxidation to form methylal. The methylal selectivity
was as high as 92.5% at a conversion of 6.5% (Table 1).
Recently, a high selectivity of 76.2% was reported on 2 mol%
Mo supported on MCM-41, but it was achieved at a very low
conversion of 0.7% at 543 K.7 Further, the Mo/MCM-41 catalyst
was rapidly deactivated due to a significant leaching of Mo
species from the channels of MCM-41.7 On the other hand, no
deactivation of the SbRe2O6 catalyst occurred after the 6 h reac-
methanol oxidation to methylal (CH2(OCH3)2). Other known
Sb-Re oxides, Sb4Re2O13 and SbOReO4⋅2H2O produced almost
no methylal. The bulk and surface of the SbRe2O6 catalyst was
characterized by XRD, Raman, XPS and SEM. The high per-
formance of SbRe2O6 may be attributed to the Re-oxide octahe-
dra connecting with Sb-O chains.
Numerous efforts have been made with development of
selective oxidation catalysts for methanol conversion to oxy-
genates such as formaldehyde, methyl formate and
dimethoxymethane (methylal) from both industrial and academ-
ic points of view. The oxidation of methanol to formaldehyde
has been extensively studied and commercialized on silver and
ferric molybdate catalysts.1 It has also been demonstrated that
methyl formate can be produced with high yields by the catalyt-
ic oxidation of methanol on V-Ti oxides,2 Sn-Mo oxides3 and
Bi-based mixed oxides.4 However, methylal, which is used as a
gasoline additive, a solvent in perfume industry, and key inter-
mediates for preparing high concentration formaldehyde and
also in organic synthesis, has not successfully been produced by
the catalytic oxidation of methanol. The catalytic methylal syn-
thesis from methanol has been reported on V/TiO2,1 V-Mo-O,5
PMoH-5.75/SiO2,6 Mo/MCM-41,7 a MoO3(100) plane,8 and
electrocatalysts,9 but the selectivities to methylal on those cata-
lysts were very low. Hence, discovery of a new selective oxida-
tion catalyst is the key issue to realize the direct methylal syn-
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
675
tion at 573 K. The methylal formation from methanol was also
reported on PMoH-5.75/SiO2 and V/TiO2.1,6 The selectivities
toward methylal on these catalysts were as low as 40–56%, even
the activities were higher than SbRe2O6.
A series of pulsed reactions of methanol in the absence of O2
indicated that lattice oxygen atoms of the SbRe2O6 catalyst con-
tributed to the selective catalysis. To examine the property of the
lattice oxygen atoms, we investigated temperature-programmed
desorption (TPD) of oxygen for the SbRe2O6 catalyst. The O2-
TPD experiments demonstrated that there existed two types of
lattice oxygen species responsible for the selective methanol oxi-
dation to methylal. The lattice oxygen atoms exhausted in the
methylal formation were regenerated by O2 during the catalytic
methanol oxidation.
Recently, we demonstrated that SbOReO4⋅2H2O, SbRe2O6
and Sb4Re2O13 were more or less active for the selective oxida-
tion of isobutane and isobutylene to produce methacrolein.13,14
This is entirely different from the present results that only the
SbRe2O6 compound showed the catalytic performance of methy-
lal synthesis. The methylal formation from methanol may pro-
ceed on dual sites with redox and acidic properties.
Figure 1 shows the catalytic methanol oxidation on SbRe2O6
under GHSV = 10000 ml⋅h-1⋅g-cat and He/O2/MeOH =
-1
86.3/9.7/4.0 (mol%) as a function of reaction temperature. The
100% conversion of methanol corresponds to a reaction rate of
16.4 × 10-3 mol⋅h-1⋅g-cat-1. The reaction rate and the methylal
selectivity increased with increasing temperature up to 573 K,
where the selectivity to methylal reached maximum of 92.5%.
Dimethyl ether was a main by-product whose formation
decreased with temperature. An increase in the partial pressure
of methanol resulted in a decrease in the reaction conversion,
while the selectivity remained unchanged. The reaction rate was
independent of O2 concentration in the range of 1–10 mol%. The
conversion of methanol on SbRe2O6 at 673 K increased to 86%,
keeping a high selectivity of 85%. However, at this temperature
SbRe2O6 partially decomposed to Sb2O4 and Re2O7 as proved by
X-ray photoelectron spectroscopy (XPS), mircro-confocal laser
Raman spectroscopy (Raman), X-ray diffraction (XRD), and
scanning electron microscopy (SEM).
In conclusion, the novel compound SbRe2O6 shows high
performance for the selective catalytic oxidation of methanol to
methylal. The selective oxidation is structure-specific. The high
performance of SbRe2O6 may be ascribed to the Re-oxide species
stabilized by the specific connection with Sb oxides at the crystal
surface.
This work has been supported by Core Research for
Evolutional Science and Technology (CREST) of Japan
Science and Technology Corporation (JST).
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