Direct synthesis of acetylene from methane by direct current pulse discharge
Shigeru Kado,* Yasushi Sekine and Kaoru Fujimoto
Department of Applied Chemistry, School of Engineering, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo
113-8656, Japan. E-mail: tt97208@mail.ecc.u-tokyo.ac.jp
Received (in Cambridge, UK) 25th September 1999, Accepted 1st November 1999
In non-catalytic direct conversion of methane to acetylene by
using direct current pulse discharge under conditions of
ambient temperature and atmospheric pressure, the selectiv-
ity of acetylene from methane was > 95% at methane
conversion ranging from 16 to 52%; coexisting oxygen was
very effective in removing deposited carbon and stabilized
the state of discharge.
defined as follows; selectivity (%) = yield of the product
(Carbon mmol)/sum of all the products (Carbon mmol) 3
100.
Upon dc pulse discharge, methane was activated readily to
form acetylene with a selectivity of > 90% in the absence of
catalyst. Fig. 2 shows methane conversion, yield of C
2
hydrocarbons and selectivity of acetylene, ethylene and ethane
vs. supplied power. While methane conversion increased up to
52% by the increasing power supply, acetylene selectivity was
very stable at ca. 95%. Applying a discharge for methane
activation thus led to the selective formation of acetylene with
Methane, which is a major constituent of natural gas, is so stable
that high reaction temperatures such as 1273 K or higher are
1
required for its pyrolysis to ethylene or acetylene. Although
high reaction temperature is favorable for high conversion,
higher temperature promotes the consecutive decomposition of
the products to carbon.1 Recently, direct conversion of
methane to higher hydrocarbons using plasma technology has
been studied to improve the selectivity and yield of the desired
products.3 For example, in microwave plasma reaction,
acetylene was produced with a selectivity of > 90% from
2
high C yield and stable selectivity, not observed in conven-
tional homogenous gas phase reactions. Other hydrocarbon
products such as prop-1-yne and buta-1,3-diyne were at < 1%.
Some carbon deposition occurred on both electrodes and the
reactor wall during the reaction and eventually resulted in
unstable discharge and sometimes in cessation.
,2
–8
6
methane at as low a reaction pressure as 4.5 kPa. The plasma
catalytic conversion of methane by using dc corona discharge
was also found to produce acetylene with high selectivity and
yield under atmospheric pressure in the temperature range
3
43–773 K. The nature of the catalyst surface in contact with the
plasma was very important for homogeneous activation of
methane and NaY zeolite gave the highest yields of C
hydrocarbons. The highest yield of C hydrocarbons (32%) was
obtained in a hydrogen-containing plasma at a flow rate of 10
2
2
3
21 7
cm min . In this study, dc pulse discharge was applied to
non-catalytic direct conversion of methane at ambient tem-
perature and under atmospheric pressure to prepare C
carbons selectively, with high methane conversion.
2
hydro-
A flow type reaction apparatus which was composed of a
Pyrex glass tube of 4.0 mm internal diameter was used as the
reactor. Reactant gas which was premixed at a given ratio was
fed at a constant flow rate. Stainless steel electrodes of 2 mm
diameter were inserted from each end of the reactor as shown in
Fig. 1 and the distance between the electrodes was 1.5 mm. A dc
pulse discharge was initiated by supplying a negative high
voltage with a dc power generator. The wave signal was
observed by a digital oscilloscope and the pulse duration was ca.
Fig. 2 Effect of supplied power on conversion and selectivity. Reaction
conditions: pure methane, 10 cm3 min
21
(NTP) flow rate, ambient
temperature, 0.1 MPa, 1.5 mm electrode distance, (-) methane conv., (5)
yield, (2) acetylene selectivity, (Ω) ethylene selectivity, (8) ethane
selectivity.
C
2
In order to prevent carbon deposition during discharge, O
and Ar were added to give a resultant feed gas composition
2
3
4 2
CH –O –Ar = 5+1+4. Reaction results under a flow of 10 cm
1
0 ms. All the reactions were conducted at atmospheric pressure
and ambient temperature without any catalyst and all products
were analyzed by gas chromatography. Product selectivity was
21
min at normal temperature and pressure (NTP) and 25 W
power supply are given in Table 1. Carbon monoxide was
produced as well as carbon dioxide (selectivity < 1% to CO
in addition to C compounds. Comparing results with those of
the reaction with pure methane, there was little difference in C
2
),
2
2
yield, which indicates that the increased methane conversion
was essentially due to the carbon monoxide formed. Addition-
ally, the composition of C
In the reaction using pure methane, the C
the amount of acetylene in the C products was 94.4%, while the
reaction in the presence of O gave 91% acetylene selectivity in
terms of non-CO/CO carbon products with yields of C
2
compounds was not much affected.
2
yield was 40.6% and
2
2
2
2
compounds and CO of 38 and 20%, respectively. These results
suggest that the precursor of the deposited carbon is converted
to carbon monoxide by reaction with oxygen. Also the reaction
in the presence of O
The effect of total flow rate (from 3 to 225 cm min in the
presence of O ) with a supplied power of ca. 25 W is also shown
in Table 1. Methane and oxygen conversion was remarkably
2
led to stable discharge.
Fig. 1 Schematic diagram of the reactor. (a) 2.0 mm diameter stainless steel
electrode, (b) 4.6 mm internal diameter quartz tube, (c) thermocouple, (d)
negative high voltage, (e) ground, (f) direction of the flow gas, (g) direct
current high voltage power supply, (h) digital oscilloscope.
3
21
2
Chem. Commun., 1999, 2485–2486
This journal is © The Royal Society of Chemistry 1999
2485
Kado, Shigeru
