Nonthermal plasma assisted co-processing of CH4 and N 2O for methanol production

Article abstract of DOI:10.1039/c3ra45103h

Oxidation of CH₄ with N₂O into value added products like methanol, formaldehyde etc., was attempted in a nonthermal plasma reactor operated under ambient conditions. Typical results highlight the potential of plasma during the direct decomposition of N₂O into N₂ and nascent oxygen that converts CH₄ into methanol, formaldehyde, carbon dioxide and carbon monoxide with desired methanol selectivity ～28%. It was observed that the high CH₄/N₂O ratios favor the formation of methanol.

Full text of DOI:10.1039/c3ra45103h

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Nonthermal plasma assisted co-processing of CH4
and N2O for methanol production†
Cite this: RSC Adv., 2014, 4, 4034
*
Shaik Mahammadunnisa, P. Manoj Kumar Reddy and Challapalli Subrahmanyam
Received 13th September 2013
Accepted 9th December 2013
DOI: 10.1039/c3ra45103h
www.rsc.org/advances
Oxidation of CH4 with N2O into value added products like methanol, warming.4,7 In this direction, various attempts have been made
formaldehyde etc., was attempted in a nonthermal plasma reactor to convert methane into methanol.8–10 It has been rst demon-
operated under ambient conditions. Typical results highlight the strated by Lunsford and co-workers that molybdenum sup-
potential of plasma during the direct decomposition of N2O into N2 ported on silica was an excellent catalyst for partial oxidation of
and nascent oxygen that converts CH4 into methanol, formaldehyde, CH4 with N2O.11,12 However, the selectivity to methanol is not
carbon dioxide and carbon monoxide with desired methanol selec- high. However, conventional thermal methods for MPOM typi-
tivity ꢀ28%. It was observed that the high CH4/N2O ratios favor the cally require extreme reaction conditions such as high
formation of methanol.
temperature (up to 1000 ꢁC) and pressure (>10 atm), which limit
the industrial applications of the catalytic partial oxidation
process.13,14 As methane is a very stable compound due to high
C–H bond strength and perfect symmetry, a methane cracking
process is necessary for its partial oxidation to methanol
(MPOM).12
Alternatively, decomposition of CH4 by using nonthermal
plasmas (NTP) under ambient conditions is one possible new
route for the utilization of methane derived from natural gas or
other sources for conversion to more valuable products (H2,
CH3OH and HCHO) and offers unique advantages of inducing
gas phase reactions by electron collisions, ease of operation and
production of energetic electrons that may activate even a stable
molecule like CH4.15–17 In a dielectric barrier discharge (DBD),
the distribution of micro discharges in the discharge volume
may be uniform, making DBDs more suitable for practical
applications. In the present study, NTP assisted co-processing
of nitrous oxide (N2O) and methane (CH4) was attempted.
Inuence of various conditions like specic input energy (SIE)
and mole ratio of the feed gases was studied.
In order to understand the inuence of feed ratio of the
reacting gases on the performance of the plasma reactor (shown
in Fig. S1†), mole ratio of CH4 and N2O were varied between
5 : 1, 1 : 1 and 1 : 5 (diluted in argon) at a constant gas ow rate
of 60 mL minÀ1 that corresponds to the gas residence time of
27.2 s with discharge gap of 3.5 mm. The energy dissipated
during the one period of voltage is calculated from the V–Q
Lissajous diagram taken at different applied voltages and at
xed discharge gap of 3.5 mm and 50 Hz frequency shown in
Fig. S2† (shown for feed ratio of CH4/N2O-5 : 1 from 12–22 kV).
The specic energy, measured in kJ LÀ1 of feed gas, was varied
Natural gas is a mixture of several hydrocarbons (at least 95%)
and non-hydrocarbon gases such as nitrogen (up to 5%), carbon
dioxide and hydrogen sulde. Methane (CH4) is the principal
component (between 70 and 90%) of most natural gas
reserves.1–3 For decades, methane has been the feed stock for
chemical industry, especially for the production of hydrogen
and related energy applications. However, this makes methane
an important source for the production of valuable chemicals
and fuels, such as hydrogen gas, higher hydrocarbons, syngas (a
mixture of CO and H2), methanol (CH3OH) and formaldehyde
(CH2O). Development of efficient natural gas conversion tech-
nologies is therefore urgent and essential for a sustainable
feedstock for the chemical industry and for protecting our
environment. In conventional steam methane reforming reac-
tion and CO2 reforming, partial oxidation of methane to syngas
will be carried out that may be converted into a variety of
products in a set of secondary processes.4–6 However, partial
oxidation of CH4 directly to methanol or formaldehyde offers
interesting opportunities and may lead to the production of
liquid fuels and chemicals. In addition, CH4 and N2O are long-
lived in the atmosphere and are major contributors for global
Energy and Environmental Research Lab, Department of Chemistry, Indian Institute of
Technology (IIT) Hyderabad, Andhra Pradesh, 502205, India. E-mail: csubbu@iith.ac.
in; Fax: +91 40 23016032; Tel: +91 40 23016050
† Electronic supplementary information (ESI) available: A detailed description of
the NTP-DBD reactor and the energy dissipated during the one period of voltage is
calculated from the V–Q Lissajous diagram has been explained. See DOI:
10.1039/c3ra45103h
4034 | RSC Adv., 2014, 4, 4034–4036
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Table 1 Conversion and selectivity (in moles per L Â 10À2) at 6 kJ LÀ1
for various mole ratios
5 : 1
1 : 1
1 : 5
CH4
N2O
2.23
2.45
0.57
0.26
0.39
0.18
2.00
1.43
4.68
4.95
0.98
0.44
1.33
0.66
3.34
3.43
2.00
5.57
0.15
0.11
0.71
0.66
0.66
1.65
SCH OH
3
SHCHO
SCO
SCO
2
SH
2
Scarbon
Fig. 1 Conversion of CH4 and N2O during the partial oxidation of CH4
to CH3OH in a DBD reactor, flow rate – 60 mL minÀ1 and discharge
gap – 3.5 mm.
the highest selectivity was achieved for CH4/N2O-5 : 1 (18, 46%
respectively for HCHO and H2) that decreased to 13, 36% and 5,
17% for 1 : 1 and 1 : 5 feed ratios. Fig. 2 also conrms the best
selectivity to deep/total oxidation (selectivity to CO and CO2) at
CH4/N2O-1 : 5, which may be due to the formation of highest
amount of oxygen by N2O decomposition. The selectivity of CO
was 16% for CH4/N2O of 5 : 1 that increased to 29 and 36% on
changing the feed ratio to 1 : 1 and 1 : 5. Under the same
experimental conditions, CO2 selectivity was 7.5, 14.5 and
33.5%, respectively. Conversion and selectivity (in moles per
L Â 10À2) at 6 kJ LÀ1 for various mole ratios expressed in molar
basis as shown in Table 1.
Fig. 3 presents the carbon mass balance (to the desired
products listed above) at 6 kJ LÀ1. Among the all feeds studied,
CH4/N2O-1 : 5 showed best selectivity (up to ꢀ81%), whereas,
1 : 1, 5 : 1 showed slightly lower values of 75 and 70%. In
addition to the main products (CH3OH, HCHO, H2, CO and
CO2), formation of other products such as ethane, propane,
propene, etc. were identied by GCMS, however they were not
quantied. Concerning the mechanism of plasma activation of
CH4 and N2O, many reports deal with the radical processes.22
The reaction may be initiated by the formation of meta stable
argon that reacts with N2O via energy transfer reactions to
by changing the applied voltage. As seen from Fig. 1, at any
given SIE, the conversions of CH4 decreased on increasing the
feed ratio, whereas N2O increased with the increasing CH4/N2O
feed ratio. At 6 kJ LÀ1, CH4 conversion was 10, 35 and 45%,
whereas N2O conversion was 55, 37 and 25%, respectively for
feed ratio variation 5 : 1, 1 : 1 and 1 : 5 mole ratios.
In general, it is believed that the generation of the energetic
electrons is the initial step in plasma initiated reactions. These
electrons may collide and dissociate the reactant gas. Hence,
the increasing conversion at higher SIE may be due to the
presence of more number of energetic electrons.18–21 The
selectivity to ve main by-products, i.e. CH3OH, HCHO, H2, CO
and CO2 for different CH4/N2O molar ratios at a SIE 6 kJ LÀ1 are
shown in Fig. 2 that presented the selectivity to CH3OH, HCHO
and H2 decreases, while the selectivity of CO and CO2 increased
on increasing CH4/N2O molar ratio.
Above tendencies are reasonable, as at CH4/N2O-5 : 1 ratio,
more CH4 molecules are available in the discharge region that
may be converted into hydrogen and oxygenated hydrocarbons.
The selectivity to methanol was highest (28%) for CH4/N2O-5 : 1
that decreased to 23 and 13% respectively for 1 : 1 and 1 : 5
ratios. A similar trend was observed for HCHO and H2, where
Fig. 2 Selectivities of H2, CH3OH, HCHO, CO and CO2 during the Fig. 3 Carbon balance during the partial oxidation of CH4 to CH3OH
partial oxidation of CH4 to CH3OH in a DBD reactor at SIE – 6 kJ LÀ1
flow rate – 60 mL minÀ1 and discharge gap – 3.5 mm.
,
in a DBD reactor at SIE – 6 kJ LÀ1, flow rate – 60 mL minÀ1 and
discharge gap – 3.5 mm.
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produce atomic oxygen,23 whereas, electron collision reaction of
N2O and CH4 may form other products like CH3, CH2, CH, C2H,
OH and C2 radicals. Reaction of atomic oxygen with methane
may form methanol. For 5 : 1 mole ratio of CH4 and N2O, the
conversion increased with increasing input power. At 4.5 W
input power and discharge length of 15 cm, the rate of
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4036 | RSC Adv., 2014, 4, 4034–4036
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