Photoinduced non-oxidative coupling of methane over silica-alumina and
alumina around room temperature
Yuko Kato,a Hisao Yoshida*a and Tadashi Hattorib
a Department of Applied Chemistry, Graduate School of Engineering, Nagoya University, Nagoya 464-8603, Japan.
E-mail: yoshida@apchem.nagoya-u.ac.jp
b Research Center for Advanced Waste and Emission Management, Nagoya University, Nagoya 464-8603, Japan
Received (in Cambridge, UK) 2nd September 1998, Accepted 18th September 1998
Around room temperature, photoinduced coupling of meth-
ane proceeds without any oxidant molecules on silica-
alumina and alumina evacuated at 1073 K; many coupling
products in the gaseous phase are obtained from silica-
alumina, while most of products on alumina are obtained
only through thermal desorption.
specific surface area was 679 m2 g21. The silica-alumina
samples, SiO2–Al2O3(L) and SiO2–Al2O3(H), employed were
reference catalysts of the Catalysis Society of Japan, JRC-SAL-
2 and JRC-SAH-1, respectively.15,16 The alumina contents were
13.75 and 28.61 mass% and the specific surface areas were 560
and 511 m2 g21, respectively.15,16 The alumina sample was also
the JRC sample, JRC-ALO-4 (surface area; 174 m2 g21).15,16
The reaction tests were carried out in a closed quartz reaction
vessel (82 cm3). The sample (1.0 g) was spread on the flat
bottom of vessel (19.6 cm2), and was treated with 60 Torr (1
Torr = 133 Pa) O2 for 1 h at 1073 K, followed by evacuation for
1 h at 1073 K. Methane (99.95%) was purified by a vacuum
evaporation before use and introduced into the reactor. The
initial pressure of methane (100 mmol) in the reactor was 21
Torr and no oxidant molecules were introduced. The sample
was irradiated with a 250 W Xe lamp for 18 h. Under
photoirradiation, the temperature of the sample bed was
measured to be ca. 310 K. Products in the gaseous phase were
collected with a liquid-N2 trap and analysed by GC. Then
adsorbed products were thermally desorbed by heating (573 K,
15 min), collected, and analysed by GC.
Table 1 shows the product yields in photoinduced non-
oxidative coupling of methane, in the absence of gaseous
oxidants, over silica, silica-alumina and alumina. Note that in all
cases no oxygenates (MeOH, HCHO, CO2, CO) were de-
tected.
For the empty reactor (run 1), only a trace amount of C2H6
was formed upon photoirradiation. On the silica sample (run 2),
a small amount of C2H6 and C3H8 were obtained in the gaseous
phase, and a trace amount of C2H4 and C3H6 were observed as
the thermally desorbed products.
The oxidative coupling of methane is an expedient reaction to
convert natural gas into useful chemicals. However, it is very
difficult to obtain the coupling products in high yield, because
oxidation of the coupling products to COx proceeds more
selectively than the coupling reaction. If the oxidant molecules
are removed to avoid complete oxidation, the reaction requires
very high temperature1 and has no practical use. Photoinduced
reactions are one of the most available reactions taking place at
low temperature where complete oxidation could be minimized.
Recently, it was reported that photo-induced coupling of
methane proceeded at 373–473 K in the presence of oxygen
2
over TiO2 which is the most widely used photocatalyst.
However the selectivity of COx was very high and the yield of
coupling products was only ca. 0.5%. Use of N2O as oxidant
improved the selectivity of coupling products on MgO, but the
yield was < 0.2%.3,4 The possibility of the photoinduced non-
oxidative coupling, where no oxidant molecules are employed,
was suggested by using transition metal oxides, such as
V/SiO2,5 TiO26 and Mo/SiO2.7 However, the highest yield was
only ca. 0.007% in the gaseous phase and < 0.4% even after
forced desorption by heating or by admission of water vapor.7
Here, we describe that the coupling products are obtained in
yields as high as 5% without formation of CO and CO2 in the
non-oxidative coupling of methane over silica-alumina and
alumina under photoirradiation. Silica-alumina, which is a
member of silica-based materials recently attracting a great deal
of attention as a new photocatalyst family,8–13 was found to be
photoactive; it exhibited a characteristic phosphorescence
emission spectrum.9 The photoreactivity of silica-alumina
toward gaseous molecules, however, has not been investigated,
and the present report is the first in this area.
Over the silica-alumina samples (runs 3 and 4), the
conversions were obviously much higher than that over silica.
In the gaseous phase, a large amount of C2–C4 alkanes was
obtained while smaller amounts of C2–C6 alkanes and alkenes
were desorbed upon heating. Among the thermally desorbed
products, alkenes were the major products. In the dark (in an
electric furnace, run 6) at 473 K, no products were detected,
clearly indicating that photoirradiation is necessary for the
above reaction. On SiO2–Al2O3(L), the total yield reached
The silica sample was prepared from Si(OEt)4 by the sol–gel
method followed by calcination in dry air at 773 K14 and its
Table 1 Results of photoinduced non-oxidative coupling of methanea
Yield of gaseous phase product (C%)b Yield of thermal desorption product at 573 K (C%)b
Total
Run
Sample
C2H6
C3H8
C4H10
Total
C2H4
C2H6
C3H6
C3H8
C4H8
C4H10 C5,6
Total
(C%)b
1c
2
3
4
5
—
SiO2
tr.
0.08
0
0
0
0.14
0.03
0
tr.
—
tr.
0.42
0.29
0.33
0
—
0
0.01
0.02
2.64
0
—
tr.
0.27
0.24
0.03
0
—
0
0.02
0.01
1.18
0
—
0
0.20
0.12
0
—
0
tr.
tr.
0.49
0
—
0
0.45
0.22
0.16
0
—
tr.
1.37
0.90
4.83
0
tr.
0.01
0.85
0.27
0.02
0
0.09
4.53
2.12
0.50
0
0.09
5.90
3.02
5.33
0
SiO2–Al2O3(L) 3.54
SiO2–Al2O3(H) 1.82
Al2O3
SiO2–Al2O3(L)
Al2O3
0.48
0
0
6d
7d
a
0
0
0
0
0
0
0
0
b
0
0
0
0
0
0
c
d
Reaction temperature = ca. 310 K, reaction time = 18 h, CH4 = 100 mmol. Based on the initial amount of CH4. A blank test. Reaction at 473 K
without UV-irradiation. tr. = trace.
Chem. Commun., 1998, 2389–2390
2389