Enhancement effects of methanol on the reactivity for methane partial
oxidation in the gas phase reaction of CH4–O2–NO2
Yonghong Teng,*a Yoichi Yamaguchi,b Tetsuya Takemoto,a Lianxin Dai,a Kenji Tabataa and Eiji Suzukia
a Research Institute of Innovative Technology for the Earth (RITE), 9-2, Kizugawadai, Kizu-cho, Soraku-gun, Kyoto,
619-0292. Japan. E-mail: teng@rite.or.jp
b Kansai Research Institute (KRI), Kyoto Research Park, Science Center Bldg. 17, Chudoji-Minami-machi,
Shimogyo-ku, Kyoyo, 600-8813 Japan
Received (in Cambridge, UK) 6th December 1999, Accepted 31st January 2000
The partial oxidation of methane in the gas phase reaction of
CH4–O2–NO2 was enhanced with the addition of a small
amount of methanol; the selectivity of methanol at the same
level of CH4 conversion was enhanced in the presence of
methanol which showed this effect exclusively in the
presence of NO2.
the decrease of reaction temperature is derived from the
presence of both CH3OH and NO2 in the reaction mixture. The
enhancement effects of CH3OH are clearly seen on an ON–OFF
experiment for the supply of methanol (Fig. 2). An increase of
reaction temperature after the addition of methanol was
observed but amounted to < 3 K in the center of the reactor.
On varying the methanol concentration, it was found that
0.016% CH3OH was the lower limit for which CH4 conversion
was enhanced.
The selectivity of CH3OH for the reaction in the presence of
methanol is higher than that without methanol in the lower
conversion region (Table 1). The selectivity of CH2O in the
system CH4–O2–NO2–CH3OH had somewhat smaller values in
the same region. The selectivity of CH3OH in the presence of
methanol exceeded that in the absence of CH3OH for the
regions of CH4 conversion shown in Table 1.
Methanol is a desirable product in the partial oxidation of
methane. Recently, we found high yield methanol formation at
808 K under atmospheric pressure in the gas phase reaction of
CH4–O2 upon the addition of a small amount of NOx
(x = 1,2).1,2 The transition barrier energy of hydrogen atom
abstraction from CH4 by NO2 was lower than that by O2.3,4 This
brought about the enhancement effect of NO2 on the reactivity
of methane in CH4–O2–NO2. Since successive oxidation of the
produced methanol has been reported at high temperatures for
the CH4–O2 gas phase reaction,5,6 the successive oxidation of
methanol could also occur in the CH4–O2–NOx gas phase
reaction. The initial hydrogen atom abstraction is the first step
of successive oxidation of methanol that probably initiates
radical chain reactions to enhance methane partial oxidation,
since Burch and co-workers5 reported that the addition of C2H6
lowered the reaction temperature by ca. 50 K at the same
conversion of CH4 for the gas phase reaction of CH4–O2. On the
other hand, it was found in our previous study that the
selectivity of the produced methanol was scarcely affected by a
change of space velocity at a given temperature.1,2,4 This
indicates that methanol could be a fairly stabilized product in
the gas phase. In addition, gaseous additives have been
examined under high pressure so far,7–9 but an enhancement
effect of methanol was not found. Therefore it is important to
clarify the behavior of methanol in the gas phase reaction of
CH4–O2–NO2. In this study, a small amount of methanol was
added to the CH4–O2–NO2 system to investigate its effects on
the partial oxidation of methanol in the gas phase.
The reaction was carried out with using a test gas (CH4: 28%,
O2: 14%, NO2: 0.5%, He: 57.3 or 57.5%, CH3OH: 0 or 0.16%).
A quartz tube (7 mm i.d.) was used as a reactor and the length
of heated reaction zone was 200 mm. The temperature was
measured with a thermocouple attached on the outside of the
reactor. Products were analyzed with two gas chromatographs.
The carbon balance before and after the reaction exceeded 98%.
Each reaction was performed for 30 min under each of the
conditions and then the products were analyzed. All experi-
mental data shown here were shown to be reproducible.
Conversion and selectivities were calculated as follows (unit is
mol): CH4 conversion = (initial CH4 2 final CH4)/initial CH4;
selectivity of a product = product/reacted CH4; selectivity of
methanol = total methanol/reacted CH4.
Fig. 1 CH4 conversion vs. reaction temperature. Reaction gas: CH4(28%)–
O2(14%)–NO2(0.5%)–He(57.3%)–CH3OH(0.16%) (:), CH4(28%)–
O2(14%)–NO2(0.5%)–He(57.5%) (5), CH4(28%)–O2(14%)–He(57.8%)–
CH3OH(0.16%) («), CH4(28%)–O2(14%)–He(58%) (2).
Fig. 1 shows CH4 conversion in each different reaction gas
mixture as a function of reaction temperature. The temperature
at the level of 10% CH4 conversion is lowered by ca. 30 K (827
K) with addition of methanol (0.16%) to CH4–O2–NO2. CH4
conversion was scarcely observed at 827 K for both CH4–O2
and CH4–O2–CH3OH systems. These differences indicate that
Fig. 2 The enhancement effect of CH3OH on CH4 conversion in CH4–O2–
NO2. 0.16% CH3OH was fed into the reaction gas consisting of CH4(28%)–
O2(14%)–NO2(0.5%)–He(57.5%).
DOI: 10.1039/a909587j
Chem. Commun., 2000, 371–372
This journal is © The Royal Society of Chemistry 2000
371
Teng, Yonghong
