Photooxidation of methane to methanol by molecular oxygen on water-preadsorbed porous TiO2-based catalysts

Article abstract of DOI:10.1246/cl.2000.314

At the temperature below 350 K, methanol was formed from the photooxidation of CH₄ by molecular oxygen on water preadsorbed porous TiO₂-based catalysts, Mo-containing porous TiO₂ catalysts exhibit higher catalytic activity than pure TiO₂.

Full text of DOI:10.1246/cl.2000.314

314
Chemistry Letters 2000
Photooxidation of Methane to Methanol by Molecular Oxygen
On Water-preadsorbed Porous TiO2-based Catalysts
Xihui Chen and Shuben Li*
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of
Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China
(Received October 15, 1999; CL-990880)
At the temperature below 350 K, methanol was formed
from the photooxidation of CH4 by molecular oxygen on water
preadsorbed porous TiO2-based catalysts, Mo-containing
porous TiO2 catalysts exhibit higher catalytic activity than pure
TiO2.
Although the photocatalytic directly conversion of methane
into oxygen-containing organics over solid oxide semiconduc-
tors has been studied for many years, few attempts to convert
directly methane with high conversion and high selectivity have
been successful.1,2 On selective photooxidation of methane
using semiconductor oxides as catalysts, the photooxidation of
methane only produced CO2 over TiO2 (P25) at room tempera-
ture and atmospheric pressure under UV irradiation1; The selec-
tive photooxidation of methane over anatase TiO2 produced a
small amount of HCHO(0.5 µmol h-1) at 493 K under UV irra-
diation2; the selective photooxidation of methane over
MoO3/SiO2 and ZnO produced a small amount of CH3OH at
473-530 K under UV irradiation, the yield of CH3OH was 0.7
and 0.4 µmol h-1 respectively.3,4 In these reports, the yield of
CH3OH is very low. So it is necessary to find out new photo-
catalysts and reaction routs by which CH3OH can be directly
synthesized from CH4 and O2 under mild conditions. It is inter-
esting to study the photooxidation of CH4 by O2 using water
preadsorbed porous oxide semiconductors catalysts. Now, the
study on the photooxidation of CH4 on water preadsorbed
porous oxide semiconductors has not been reported. Here, we
report the photooxidation of CH4 by O2 on water preadsorbed
made home porous TiO2-based catalysts at the temperature
below 350 K and atmospheric pressure.
The porous TiO2-based catalysts were prepared by simple
sol-gel hydrolysis of titanium butyloxide (Ti(OC4H9)4) - 2-
propanol solution (alkoxide : alcohol molar ratio 1:32). The
determined amount of H2O and (NH4)6Mo7O24 aqueous solu-
tion were added to alkoxide alcohol solution respectively to
form sol under vigorous stirring at room temperature, then 100
mL of acetic acid aqueous solution(1 mol/L) was added into the
sol under stirring to produce white precipitate. After having
been aged for 3 days at room temperature, the gel was washed
with a large amount of distilled water to remove the alcohol and
dried at room temperature, then calcined at 723 K for 8 h.
Chemical analysis using ICP atomic emission spectroscopy the
Mo content in the porous TiO2 samples were 0.23 wt%. The
crystal form of the porous TiO2 was determined as anatase by
XRD (Figure 1). The textural properties of the porous TiO2-
based samples were listed in Table 1. Commercial TiO2 (rutile
mass component 36%, TiO2(C)) was used as a reference catalyst.
The photocatalytic reactions were carried out in a quartz
glass cell with volume of 50 mL. Prior to the photoreactions,
the catalysts were evacuated at 473 K for 2 h. Slurry of catalyst
was made by adding a small amount of water to the catalyst in
the cell, the slurry was coated on the inter-surface of the cell
and dried. After the catalyst in the cell adsorbed water, the cell
was cleaned by passing pre-mixed reaction stock (CH4: O2: He
molar ratio 9:1:10) and stock gas was introduced into the cell at
room temperature and atmospheric pressure. UV irradiation
was carried out with a high-pressure mercury lamp (250 W)
with a cooling water jacket. Chemical actinometry using potas-
sium ferrioxalate (K3Fe(C2O4)3) revealed that the number of
photons irradiated into the cell was 7.2 × 10-9 einstein s-1 for
250-500 nm. After photoreaction, the CH3OH and HCHO were
collected by ice-trap (273 K). The products were determined
and analyzed by ion trap detector and gas chromatography.
The reaction results were summarized in Table 2. Only
COx was detected when dehydrated TiO2 was used as catalyst.
It is consistent with the result reported by M. Grätzel et al.1
Methanol was detected besides COx when water-preadsorbed
TiO2 was used as catalyst. According to the reaction results,
Copyright © 2000 The Chemical Society of Japan

Chemistry Letters 2000
315
the Mo-containing TiO2 catalyst exhibits higher photocatalytic
activities than pure TiO2 catalysts. The XRD patterns of the
catalysts show in Figure 1. In Figure 1, the XRD pattern of the
Mo-containing TiO2 catalyst is similar with that of TiO2, but is
completely different with that of MoO3. It shows that MoO3
crystal is not formed in the Mo-containing catalyst. In Figure
2, the UV-vis diffuse reflection spectra of the Mo-containing
TiO2 sample is different with those of TiO2 and MoO3, suggest-
ing the molybdenum oxide species dispersed on the TiO2. In
Figure 2, a significant shift to the longer wavelength in the
absorption band is also observed in the Mo-containing TiO2
sample as compared to that of pure TiO2, suggesting the molyb-
denum oxide to sensitize the TiO2. These may be responsible
for the higher catalytic activity of the Mo-containing TiO2 cata-
lyst. On the other hand, the photooxidation rate of CH3OH over
molybdemun-oxide containing TiO2 reduces slightly as com-
pared to that over TiO2.5 It might be one of cause that the yield
of CH3OH increased slightly when the Mo-containing TiO2 was
used as catalyst. Table 2 also shows the conversion of CH4 and
the yield of CH3OH increase with temperature. It may be one of
cause that desorption of products from catalyst surface can be
promoted at higher temperature. The systematic investigation
on the reaction is underway and will be reported later.
References and Notes
1
2
3
4
5
K. R. Thampi, J. Kiwi, and M. Grätzel, Catal. Lett., 1, 109
(1988).
K. Wada, K. Yoshida, T. Takatani, and Y. Watanabe, Appl.
Catal. A: General, 99, 21 (1993).
K. Wada, K. Yoshida, Y. Watanabe, and T. Suzuki, J.
Chem. Soc., Chem. Commun., 1991, 726.
K. Wada, K. Yoshida, and Y. Watanabe, J. Chem. Soc.,
Faraday Trans., 91, 1647 (1995).
Y. C. Liu, G. L. Griffin, S. S. Chan, and I. E. Wachs, J.
Catal., 94, 108 (1985).