V-MCM-41 for selective oxidation of propane to propene and acrolein

Article abstract of DOI:10.1246/cl.2001.194

The V-MCM-41 with lower V content catalyzes the oxidation of propane to acrolein with a yield of 3.3% and selectivity of 20%, while that with higher V content mainly produces propene.

Full text of DOI:10.1246/cl.2001.194

194
Chemistry Letters 2001
V-MCM-41 for Selective Oxidation of Propane to Propene and Acrolein
Qinghong Zhang, Ye Wang,* Yoshihiko Ohishi, Tetsuya Shishido, and Katsuomi Takehira*
Department of Applied Chemistry, Faculty of Engineering, Hiroshima University,
Kagamiyama, Higashi-hiroshima 739-8527
(Received November 16, 2000; CL-001038 )
The V-MCM-41 with lower V content catalyzes the oxida-
tion of propane to acrolein with a yield of 3.3% and selectivity
of 20%, while that with higher V content mainly produces
propene.
V-containing mesoporous materials such as V-MCM-41
have received much attention since they can provide high con-
centration of isolated vanadium species, which are desirable for
many partial oxidation reactions. Although many publications
have contributed to the syntheses and characterizations of V-
MCM-41,1–3 the reports on the use of it as catalyst are still
scarce and mainly limited to liquid phase oxidation reactions
using H2O2 or tert-butyl hydroperoxide.4,5 On the other hand,
vanadium-substituted zeolites such as V-silicates6 and VAPO-
57 have shown good performances in the oxidative dehydro-
genation of lower alkanes. Recently, we found that V-MCM-41
showed unique catalytic properties in partial oxidation of lower
alkanes such as C3H8. Not only alkene but also oxygenate
could be obtained with moderate selectivity depending on Si/V
ratio. This communication is the first report on the partial oxi-
dation of C3H8 to C3H6 and acrolein using V-MCM-41.
V-MCM-41 was prepared by hydrothermal synthesis at
120 °C for 96 h. Sodium silicate, vanadyl oxalate and cetyl-
trimethylammonium bromide were used as silicon, vanadium
sources and surfactant template, respectively. The resultant
solid after hydrothermal synthesis was recovered by filtration,
washed thoroughly with deionized water, dried in vacuum at
40 °C for 24 h, and finally calcined at 550 °C for 6 h. V con-
tent in each sample was determined by ICP spectrometry. ICP
measurements also showed that nearly no Na+ remained in the
sample. Catalytic reactions were performed using a U-typed
fixed-bed flow reactor. Standard reaction conditions were as
follows: T = 400–550 °C, P(C3H8) = 12.2 kPa, P(O2) = 6.1 kPa,
W = 0.2 g, F = 50 mL/min.
Figure 1 shows the XRD patterns of V-MCM-41 samples
with different Si/V ratios. The peaks (100), (110) and (200)
indexed to hexagonal regularity of MCM-41 were clearly
observed for all samples except the one with the highest V con-
tent. The peak of (210) was not clearly observed for the V-
MCM-41 and similar phenomenon has been reported by Kevan
and coworkers.2 Table 1 shows the results obtained from N2
adsorption measurements. BET surface area was ca. 1000 m2
g–1 for MCM-41 and the samples with low V content and the
value decreased for the samples with high V content. Pore size
distribution determined by DH method showed a sharp peak at
2.3–2.7 nm depending on Si/V ratio.
gave a band at 956 cm–1 probably ascribed to framework
Si–O–V stretching. These features resemble those observed for
V-silicate zeolite,8 suggesting that vanadium was mainly incor-
porated into the framework of MCM-41.
Table 2 shows the catalytic results for C3H8 oxidation at
550 °C. No reaction occurred without catalyst under the reac-
tion conditions used here. C3H8 was also converted over MCM-
41. The incorporation of V remarkably increased C3H8 conver-
sion and the selectivity for partial oxidation products. C3H6 was
formed with high selectivity over the catalysts with high V con-
tent and the C3H6 yield was comparable with that obtained over
V-containing zeolites. It should be noted that not only C3H6 but
UV–vis spectroscopic measurements exhibited a main peak
at 275 nm for all samples, indicating that vanadium was in
tetrahedral coordination environment. H2-TPR showed one
reduction peak at 550–580 °C, and Laser-Raman spectroscopy
Copyright © 2001 The Chemical Society of Japan

Chemistry Letters 2001
195
acrolein was also obtained over V-MCM-41. The selectivity
and yield of acrolein were the highest over the sample with
Si/V ratio of 120. Further increase in V content decreased
selectivity to acrolein and increased that to C3H6. C3H8 conver-
sion and C3H6 selectivity decreased over the catalyst with Si/V
ratio of 23, probably due to the decrease in BET surface area of
this sample.
The oxidation of C3H8 to acrolein has been reported over
complex metal oxide catalysts containing Bi, Mo, Ni and many
other components and a good catalyst generally gives yield of
ca. 3%.9 No report has shown the production of acrolein over
V-containing silica or zeolite catalysts. Thus, V-MCM-41 is
very unique for the selective oxidation of C3H8.
To investigate the influence of acidity on the catalytic reac-
tion, NH3-TPD was performed and the results are shown in
Figure 2. For MCM-41 and the samples with low V content, a
shoulder at 700–730 K corresponding to acid sites with medium
strength was observed along with the main peak at 490–500 K
ascribed to weak acid sites. The medium acid sites disappeared
for the sample with high vanadium content. Such change in
acidity corresponds well with the shift of partial oxidation prod-
uct from acrolein to C3H6 with the increase in V content. Thus,
it is likely that C3H6 or allylic intermediate produced on the
catalysts with low V content may interact strongly with the
medium acid sites and be further oxidized into acrolein. On the
other hand, the desorption of C3H6 must be facile over the cata-
lysts without such acid sites. The elucidation of the nature of
acid sites and their influences on adsorption and conversion of
C3H6 is underway.
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