The partial oxidation of methane to methanol with nitrite and nitrate melts

Article abstract of DOI:10.1515/znb-1998-0705

The effect of reduced oxygen species on the partial oxidation of methane to methanol was examined with nitrite melts. The experimental results support the suggestion that the formation of methanol or C₂ compounds depends on different reduced oxygen species, as observed in our previous work using nitrate melts. It has been suggested that the partial oxidation of methane proceeds to CH₃OH or C₂ compounds via parallel pathways. This suggestion was verified by increasing the oxygen concentration to carry out the partial oxidation of methane in 25 mol% NaNO₃ - 75 mol% KNO₃ melts. A methanol selectivity of 8.2% and a methanol yield of 0.43% were observed with CH₄/O2 = 15/1 at 575 °C, whereas with CH₄/O₂ = 7/1 methanol selectivity and yield increased to 23.7% and 1.1%, respectively. The results further confirm the contribution of the superoxide ion O₂^- on methanol formation.

Full text of DOI:10.1515/znb-1998-0705

The Partial Oxidation of Methane to Methanol with Nitrite and Nitrate Melts
Bor-Jih Lee, Shigeo Kitsukawa, Hidemoto Nakagawa, Shukuji Asakura*, Kenzo Fukuda
Department of Materials Science and Chemical Engineering, Faculty of Engineering, Yokohama
National University, 79-5 Tokiwadai, Hodogaya-ku Yokohama, 240-8501 Japan
Z. Naturforsch. 53 b, 679-682 (1998); received February 26, 1998
Superoxide Ion, Nitrite, Nitrate, Partial Oxidation, Methanol
The effect of reduced oxygen species on the partial oxidation of methane to methanol was
examined with nitrite melts. The experimental results support the suggestion that the formation
of methanol or C2 compounds depends on different reduced oxygen species, as observed in our
previous work using nitrate melts. It has been suggested that the partial oxidation of methane
proceeds to CH3OH or C2 compounds via parallel pathways. This suggestion was verified by
increasing the oxygen concentration to carry out the partial oxidation of methane in 25 mol%
NaNC>3 - 75 mol% KNO3 melts. A methanol selectivity of 8.2% and a methanol yield of
0
.43% were observed with CH4/O2 = 15/1 at 575 °C, whereas with CH4/O2 = 7/1 methanol
selectivity and yield increased to 23.7% and 1.1%, respectively. The results further confirm the
contribution of the superoxide ion O2- on methanol formation.
Introduction
O2- + V2O2 — 0
2
2
-
(6)
(7)
The potential of molten salts as media for chem-
ical reactions is well known [1, 2]. The use of
molten carbonates [3, 4], hydroxides [5] and chlo-
rides [6]as reaction media for the oxidative coupling
of methane to hydrocarbons has been investigated.
In these studies, the authors suggested that reduced
oxygen species played an important role in the for-
mation of C2 compounds [3-5]. Thermodynamic
and electrochemical studies in molten barium hy-
droxide [5, 7] and alkali carbonates [4, 8, 9] have
shown that peroxide ion was the stable form of re-
duced oxygen species in basic media, and may in-
crease the selectivity of formation of C2compounds
0 22- + 0 2 ^ 202~
Our results indicated that there are two reduced oxy-
gen anions (0 2~ and 022-) in nitrate melts, 0 2_ be-
ing the dominant species the concentration of which
is much larger than that of 022~. It was also shown
that the variation of the amount of 0 2 _ followed
the trend of the methanol selectivity. Furthermore, a
comparison of the results with those of other studies
4, 5] which suggested that 0 22- plays an important
role in the C2 compound formation indicated that
2 _ should lead to methanol formation [10] while
22~ should lead to C2 compound formation [4, 5],
and nitrate ions were considered to promote the ox-
[
0
0
[4, 5].
idation of O2- and 022- to C>2~ according to eqs
4) and (5).
In our previous work [10], we used (Na, K)NC>3
(
melts as reaction media for the partial oxidation
of methane to methanol, and tried to estimate the
amounts of reduced oxygen species by thermo-
chemical calculation according to eqs (1) - (7).
In order to further confirm the role of 0 2 ~ in
methanol formation, in the present work we used
molten (Na, K)N02 as medium to perform the par-
tial oxidation of methane to methanol. Theoretical
NO3- ^ n o 2~ + V2O2
(1) calculations indicate that the major reduced oxygen
species in nitrite melts is O22-, whereas 0 2 ~ is the
2
2
N02~ ^ O2- + N2 + 3/20 2
N02~ ^ O2- + 2NO + V2O2
(2)
major active oxygen species in nitrate melts. The
(3)
analysis of eqs. (1) - (7) suggests that the increase
o 2~ + n o 3 ^ n o 2~ + 0 22“
22“ + 2NO3 ^ 2N02- + 202~
(4) of oxygen pressure should increase the amount of
O2- . Also, we tried to increase the amount of O2-
in the (Na, IQNO3 system by increasing the oxygen
concentration in the feed in order to improve the
selectivity and the yield of methanol.
0
(5)
*
0
Reprint requests to Prof. S. Asakura.
932-0776/98/0700-0679 $ 06.00 (c) 1998 Verlag der Zeitschrift für Naturforschung. All rights reserved.
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B.-J. Lee et al. ■The Partial Oxidation of Methane to Methanol with Nitrite and Nitrate Melts
Table I. Experimental results of the partial oxidation of methane in the 25 mol % NaNC>2 - 75 mol% KNO: melts.
Temp.
°C)
Conv.
(mol%)
Selectivity (mol%)
CO
NO
(•10~4mol) (• 10~4mol)
N^
O22“
(• 10_4mol)
o2~
(
CH,OH
C 0 2
(• 10~4mol)
5
5
5
6
25
50
75
00
2.7(1.8) 0.15(14.1) 73.9(24.2) 25.9(61.7) 1.25 (0.10) 6.19 (0.13) 113.2(0.91) 15.9 (50.8)
4.9(3.6) 0.08(13.1) 25.1(31.5) 74.8(55.4) 1.72 (0.19) 9.00 (0.27) 187.1(1.07) 26.5 (74.6)
8.5(5.2) 0.05 (8.2)
10.3(7.7) 0.02 (2.4)
0.5(38.6) 99.5(53.2) 2.92 (0.64) 16.3 (0.46) 237.2(1.86) 34.0 (93.0)
0.2(45.0) 99.8(52.6) 3.93 (1.52) 25.9 (0.59) 352.5(2.26) 51.0(134.0)
Reaction conditions: 1 atm, 525 - 600 °C, ChL/CWHe = 15:1:4, 50 ml/min. The selectivity of C2 compounds was
below 0.1% in each run above 575 °C. () The data obtained with (Na, KJNO3 [10]. The amounts of NO and N2 are
measured values.
Experimental
The experimental device and analysis methods were
the same as previously described [8]. Experiments were
carried out under atmospheric pressure and at tempera-
tures in the range of 525 - 600 °C. NaN02, K N 02, N aN 03,
and KNO3, used were of reagent grade. 100 g of the salts
of the chosen ratio (Na/K = 25 mol%/75 mol%) were
dried at 150 °C for 12 h in a vacuum and melted under
helium atmosphere, then heated up to the reaction tem-
perature. A reactant gas mixture of CHjAIWHe = 15/1/4,
5
00
525
550
575
600
625
1
1/1/3 or 7/1/2 was bubbled through the molten phase
TOC)
at a constant flow rate of 50 ml/min. All products were
analyzed by conventional GC techniques with thermal
conductivity and flame ionization detectors. The conver-
sion of C H 4 , the selectivity and the yields of products
were calculated from the GC analysis, while CO2 was
estimated from the carbon balance between products and
reacted methane. NO* products emitted by the thermal
decomposition of molten nitrates were quantified by the
Saltzman absorptiometry method [11],
Fig. 1. The variation of the amounts of reduced oxygen
species with temperature in nitrate melts.
Results and Discussion
The experimental results obtained in 25 mol%
NaNO, - 75 mol% K N 02 melts at 525 - 600°C
are given in Table I together with the amounts of
reduced oxygen species. The data of our previous
work are also shown for comparison. The amounts
of reduced oxygen species in molten nitrites were
calculated according to eqs (1) - (7) using thermo-
chemical data as in previous work [10]. Methane
T(°C)
Fig. 2. The variation of the amounts of reduced oxygen
species with temperature in nitrite melts.
conversion is larger than that obtained with ni- (Fig. 1), but in nitrite melts (Fig. 2) the amount
trate melts, but the methanol selectivity is very low. of O22- is larger. This may account for the low
Methane was almost completely oxidized to CO2 methanol selectivity obtained in nitrite melts, where
and H2O above 575 °C. The variation of the amounts C>22~ competes with O2- to react with methane thus
of O2- and O22- in nitrate and nitrite melts is shown reducing the reaction of O2- with methane. O22" is
in Fig. 1 and 2, respectively. The amount of Oz~
apparently too active and promotes methane oxida-
is much larger than that of O22“ in nitrate melts tion to CO2 and H2O in nitrate melts.
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B.-J. Lee et al. • The Partial Oxidation of Methane to Methanol with Nitrite and Nitrate Melts
681
Table II. The results of the partial oxidation of methane to methanol in the 25 mol % N aN 02 - 75 mol% K N 02 melts
under various oxygen cenrations.
CH4/0 2 Temp.
°C)
Conv.
(mol%)
CH3OH select.
(mol%)
yield
(mol%)
NO
(• 10- 4mol)
n 2
( 10~4mol)
O72-
(• 10~4mol)
o2-
(
(• 10_4mol)
1
5:1
525
550
1.8
3.6
5.2
7.7
1.4
2.9
4.8
7.6
1.1
2.6
4.6
7.7
14.1
13.1
8.2
0.25
0.47
0.43
0.18
0.29
0.54
0.67
0.21
0.33
0.69
1.10
0.32
0.10
0.19
0.64
1.52
0.07
0.14
0.36
1.40
0.05
0.11
0.26
1.23
0.13
0.27
0.46
0.59
0.09
0.21
0.31
0.50
0.06
0.18
0.25
0.44
0.91
1.07
1.86
2.26
0.62
0.82
1.59
1.89
0.35
0.52
1.06
1.78
50.8
74.6
93.0
134.0
62.1
87.8
125.2
142.8
72.4
100.3
145.6
154.8
[
10]
5
6
75
00
2.4
1
1:1
525
21.0
18.7
13.9
2.8
29.7
26.5
23.7
4.2
5
5
6
50
75
00
7
:1
525
5
5
6
50
75
00
In our present and previous work [10] trace fied. The present results suggest that superoxide
amounts of C2H2 and C2H4 were observed above 0 2 _ brings about methanol formation and peroxide
5
75 °C. This suggests that the formation of methanol
0 2 2~ brings about C2 compound formation. Based
or C2 compounds may follow parallel reaction on this suggestion, a simplified reaction route for
routes requiring different reduced oxygen species the partial oxidation of methane with nitrate melts
for their formation. The suggestion is consistent has been proposed as follows:
with the proposal made by Walsh et al. [12, 13]
°
2'J-r C H 3Ox ------► C H 3OH
that the partial oxidation of methane to CH3OH or
C2 compounds proceeds via parallel pathways.
Formaldehyde has not been found in nitrite and
nitrate melts. CO selectivity increases as the tem-
perature increases from 525 °C to 600 °C, whereas
the methanol selectivity is greatly decreased. This
suggests that CO is a secondary product of the fur-
ther oxidation of methanol.
CO, C 02, H20
C,H
2
4
In order to verify this assumption, we tried to
increase the amount of O2- in the (Na, IQNO3
system by increasing the oxygen concentration as
suggested by eqs (1) - (5). We performed the ex-
The partial oxidation of methane involves sev- periments with CH4/O2 = 11/1 and 7/1 in the melts
eral elementary reactions, and its reaction pathways of 25 mol% NaNÜ3 - 75 mol% KNO3, and com-
have been discussed by many authors [12 - 18]. Al- pared the results to our previous results obtained
though it is difficult to make a direct comparison of with CH4/Ü2 = 15/1 [10]. The results are given in
the reaction pathways suggested for different reac- Table II. As expected, methanol selectivity depends
tion conditions, a simple classification is possible:
CH4 ^ CH3 C2H4 -»> COx for ox- gas. A methanol selectivity of 8.2% and a methanol
idative coupling, and CH4 —>CH3- — >■ CH3OO —» yield of 0.43% were obtained with CH4/02 =15/1
CH3OOH CH3O -> CH3OH -> CO* for the at 575 °C, whereas with CH4/02 = 7/1 methanol
significantly on the oxygen concentration in the feed
C2H6
direct conversion to methanol. The initiating step selectivity and yield increased to 23.7% and 1.1%,
is common for both pathways and most likely best respectively. The amounts of NO and N2 decreased
represented by:
as the oxygen concentration increased, indicating
that the thermal decomposition of NO3- was sup-
pressed. The amounts of NO and N2 at 600 °Cwere
c h 4 + o 2 -> c h 3- + H 02
The methyl and hydroperoxy radicals then continue much larger than those below 575 °C, indicating that
the reaction to form C2 compounds or methanol. Al- the thermal decomposition of NO3~ is violent above
though O2 makes a major contribution in the above 600 °C, and it apparently influenced the methanol
reactions, the oxygen species has not been clari- selectivity. The effect of the oxygen concentration
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82
B.-J. Lee et al. ■The Partial Oxidation of Methane to Methanol with Nitrite and Nitrate Melts
the ratio of CH4/O2 increases, but at 600 °C this
variation disappears due to complete oxidation of
methane, indicating that the role of reduced oxygen
species is much less significant at high temperature.
Studies [19 - 23] of the partial oxidation of
methane to methanol in homogeneous and het-
erogeneous systems showed that methane conver-
sion increases and methanol selectivity decreases
as the oxygen concentration is increased. A maxi-
mum methanol yield is obtained with medium oxy-
gen concentrations. The present results indicate that
CH4/O2
methane conversion decreases as the oxygen con-
centration is increased, while methanol selectivity
and yield increase as the oxygen concentration is
increased. This seems to be a very special feature
Fig. 3. The influence of oxygen cocentration on methanol
selectivity. □: 525 °C, O: 550 °C; o: 575 °C; A: 600 °C.
on methanol selectivity is shown in Fig. 3. Methanol for molten salt reaction systems, in which oxygen
selectivity greatly decreases at 525 - 575 °C as participates in the ion equilibria.
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