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Energy & Fuels 2002, 16, 864-870
Synthesis of Oxygenates and Higher Hydrocarbons Directly from Methane and Carbon Dioxide Using Dielectric-Barrier Discharges: Product Distribution Yang Li,†,‡ Chang-Jun Liu,*,†,‡ Baldur Eliasson,‡,§ and Yu Wang†,‡ State Key Laboratory of C1 Chemical Technology, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, China, ABB Plasma Greenhouse Gas Chemistry Laboratory, Tianjin University, Tianjin 300072, China, and Energy and Global Change, ABB Corporate Research Ltd., Baden, CH5405, Switzerland Received November 26, 2001
In this work, a direct conversion of methane in the presence of carbon dioxide using dielectricbarrier discharge plasmas has been conducted. The product includes syngas (H2 and CO), gaseous hydrocarbons (C2 to C5), liquid hydrocarbons (C5 to C11+), and oxygenates. The liquid hydrocarbons are highly branched, representing a high octane number, while the oxygenates mainly consist of series of alcohols and acids. A detailed analysis of product distribution has been performed under variable feed conditions with different reactor configurations. At the high CH4/CO2 feed ratio, the wider discharge gap (1.8 mm) is more favored for the formation of methanol and ethanol. For the production of acetic acid, the narrower discharge gap (1.1 mm) is better, especially, with the existence of after-glow zones. Conditions favored for the production of acetic acid are also good for the production of liquid fuels.
Introduction Methane is the principal constituent of natural gas. It is very promising to utilize methane as raw material for production of highly valuable chemicals and clean fuels. There have already been a lot of reviews focusing on the coupling of methane or the partial oxidation of methane.1,2 A major difficulty for such direct methane conversion is to activate the stable C-H bonds in methane molecules using conventional catalysis. The methane conversion in the presence of oxidants, like oxygen, is thermodynamically favored for the production of COx. Catalytic conversion of methane to more useful chemicals and fuels remains a challenge for the 21st century.3 It is very important for us to improve the conventional catalysts, and at the same time, to exploit other potential techniques for methane conversion. Regarding the difficulty in the activation of methane conventionally, gas discharge plasmas have been investigated extensively these years for methane conversion to C2 hydrocarbons,4-12 to oxygenates,13-17 to liquid * Author to whom correspondence should be addressed at P.O. Box 796666, Tianjin University, Tianjin 300072, P. R. China. Fax: +8622-27890078. E-mail:
[email protected]. † State Key Laboratory of C 1 Chemical Technology, School of Chemical Engineering and Technology, Tianjin University. ‡ ABB Plasma Greenhouse Gas Chemistry Laboratory, Tianjin University. § Energy and Global Change, ABB Corporate Research Ltd. (1) Fox, J. M. Catal. Rev. Sci. Eng. 1993, 35 (2), 169-212. (2) Guczi, L.; Santen, R. A. V.; Sarma, K. V. Catal. Rev. Sci. Eng. 1996, 38 (2), 249-296. (3) Lunsford, J. H. Catal. Today 2000, 63, 165-174. (4) Suib, S. L.; Zerger, R. P. J. Catal. 1993, 139, 383-391. (5) Liu, C.; Mallinson, R. G.; Lobban, L. L. Appl. Catal. A 1999, 178, 17-27. (6) Liu, C.; Lobban, L. L.; Mallinson, R. G. J. Catal. 1998, 179, 326334.
fuels,18,19 and to syngas.18-25 The plasmas operated at low pressure, like microwave discharge, were applied for methane conversion at the very beginning. Then the atmospheric pressure plasmas, like corona discharge,5-10,12 gliding arc,21,22 plasma torch,25 and dielectric-barrier discharge,13,14,16-20,26,27 were developed. One of the advantages in plasma methane conversion is that reactions can be operated at low gas temperature (as low as room temperature), while the methane conversion is significantly high (even more than 60% for some cases). Such plasma methane conversions mostly employ a co-reactant or a dilution gas to avoid the possible carbon deposit. Oxygen,5,6,13,15-17 carbon (7) Liu, C.; Mallinson, R. G.; Lobban, L. L. Appl. Catal. A 1997, 164, 21-31. (8) Yao, S. L.; Nakayama, A.; Suzuki, E. AIChE J. 2001, 47 (2), 413418. (9) Yao, S. L.; Ouyang, F.; Nakayama, A.; Suzuki, E.; Okumoto, M.; Mizuno, A. Energy Fuels 2000, 14, 910-914. (10) Kado, S.; Sekine, Y.; Fujimoto, K. Chem. Commun. 1999, 24852486. (11) Oumghar, A.; Legrand, J. C.; Diamy, A. M.; Turillon, N. Plasma Chem. Plasma Process. 1995, 15, 87-107. (12) Zhu, A.; Gong, W.; Zhang, X.; Zhang, B. Sci. China B (in Chinese) 2000, 30 (2), 167-173. (13) Okumoto, M.; Rajanikanth, B. S.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 1998, 34 (5), 940-944. (14) Matsumoto, H.; Tanabe, S.; Okitsu, K.; Hayashi, Y.; Suib, S. L. J. Phys. Chem. A 2001, 105 (21), 5304-5308. (15) Yao, S. L.; Ouyang, F.; Nakayama, A.; Suzuki, E. Trans. Mater. Res. Soc. Jpn. 2000, 25 (1), 373-376. (16) Bugaev, S. P.; Kozyrev, A. V.; Kuvshinov, V. A.; Sochugov, N. S.; Khryapov, P. A. Plasma Chem. Plasma Process. 1998, 18 (2), 247262. (17) Larkin, D. W.; Caldwell, T. A.; Lobban, L. L.; Mallinson, R. G. Energy Fuels 1998, 12, 740-744. (18) Eliasson, B.; Liu, C.; Kogelschatz, U. Ind. Eng. Chem. Res. 2000, 39 (5), 1221-1227. (19) Liu, C. J.; Xue, B.; Eliasson, B.; He, F.; Li, Y.; Xu, G. H. Plasma Chem. Plasma Processing 2001, 21 (3), 301-310.
10.1021/ef0102770 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/04/2002
Synthesis of Oxygenates and Higher Hydrocarbons
dioxide,5,9,18-20,24,27 nitrogen monoxide,14 hydrogen,5,6 or helium20,28 has been used as the co-reactant or dilution gas. It is very promising to use carbon dioxide as the co-reactant for plasma methane conversion since carbon dioxide can provide extra carbon atoms for methane conversion, while carbon dioxide also serves as a better oxidant, compared to oxygen or air. The co-feed of carbon dioxide will also increase the methane conversion and the yield of objective products.18,19 However, the introduction of carbon dioxide into the feed will lead to a complex product. In addition to syngas, gaseous hydrocarbons, liquid fuels, and oxygenates have been produced in plasma methane conversion with the co-feed of carbon dioxide.18-20 To clarify the product distribution in such plasma methane conversion is very important since it will determine the feasibility or potential application of plasma methane conversion technologies. It is also very important for us to control the plasma reactions to get the desired product. The development of a production technology of highly valuable oxygenates such as acids or alcohols, directly from plasma methane conversion, will probably be more economically desired. However, due to the poor understanding on plasma chemistry, the fundamental investigation is still very necessary because the reaction mechanism and kinetics remain unclear. The previous investigation has discussed a co-generation of syngas and higher hydrocarbons under the conditions of DBD plasmas.18-20 The objective of the present investigation is to investigate the product distributions, especially the oxygenates, from plasma methane conversion in the presence of carbon dioxide using dielectric-barrier discharges. Experimental Section The reactors we used in this investigation were similar to that reported previously, but smaller in size.19 Figure 1, a and b, shows the schematic configurations of the reactors used in this work. Three reactors were used in these experimental investigations. For all three reactors, the high voltage electrode was an aluminum foil attached to the inner surface of the quartz tube. And a stainless steel tube around the quartz tube served as the grounded electrode. The width of discharge gap was equal to the distance between the steel tube and the quartz tube. As shown in Figure 1a, the outer diameter of the quartz tube (dielectric) was 8.2 mm, the gap for the discharge was 1.8 mm and the length of discharge zone was 250 mm in Reactor 1; The outer diameter of quartz tube in Reactor 2 was 9.6 mm with a gap width of 1.1 mm and a length of 250 mm in discharge zone. The flow rates of feed were about 60 mL/ min and 40 mL/min in Reactor 1 and Reactor 2, respectively. The residence times of feed were about 14 s for both reactors. (20) Li, Y.; Xu, G. H.; Liu, C. J.; Eliasson, B.; Xue, B. Z. Energy Fuels 2001, 15 (2), 299-303. (21) Czernichowski, A. Private communications. (22) Mutaf-Yardimci, O.; Saveliev, A. V.; Fridman, A. A.; Kennedy, L. A. Int. J. Hydrogen Energy 1998, 23 (12), 1109-1111. (23) Kado, S.; Urasaki, K.; Sekine, Y.; Fujimoto, K. Chem. Commun. 2001, 415-416. (24) Gesser, H. D.; Hunter, N. R.; Probawono, D. Plasma Chem. Plasma Process. 1998, 18 (2), 241-245. (25) Bromberg, L.; Cohn, D. R.; Rabinovich, A. Energy Fuels 1998, 12, 11-18. (26) Chang, M.; Huang, C. J. Adv. Oxid. Technol. 1999, 4 (3), 333338. (27) Liu, C.-J.; Li, Y.; Zhang, Y.-P.; Wang, Y.; Zou, J.; Eliasson, B.; Xue, B. Chem. Lett. 2001, 1304-1305. (28) Okumoto, M.; Su, Z.; Katsura, S.; Mizuno, A. IEEE Trans. Ind. Appl. 1999, 35 (5), 1205-1210.
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Figure 1. Configurations of the DBD reactors. On the basis of the configuration of Reactor 2, the high voltage electrode in Reactor 3 was divided into five parts with equal length, illustrated in Figure 1b. A distance of 12 mm between every part served as an after-glow zone in order to investigate its effect on reactions. The flow rate of feed was fixed at 40 mL/min in this reactor. A high voltage AC generator supplied a voltage from 0 kV to 30 kV with a nearly sinusoidal waveform at a frequency of 25 kHz. The voltage and current measurements were conducted using a digital oscilloscope (Tektronix 2440) with a high voltage probe (Tektronix P6015 A) and a pulse current transformer (Pearson Electronics 411). The discharge power was detected with a power-meter (Keithley 2000). The feed gases were introduced into the reactors via mass flow controllers. The gaseous products were analyzed with an online gas chromatograph (HP5890, TCD and FID) and mass selective detector (HP5971) equipped with a HP-PLOT Q column (30 m × 0.53 mm × 40 µm). The liquid products were collected in a trap cooled by a mixture of ice and water. The liquid hydrocarbons were analyzed using the same GC with a CBP-1 column (50 m × 0.25 mm × 0.1 µm). Oxygenates, including water, alcohols, acids, and others, in the condenate were analyzed with the HP-PLOT Q column. The temperature of all the reactions was adjusted at about 65 °C with circulating oil. All the experiments were operated at atmospheric pressure. The residence time in the discharge zone and the discharge power was fixed at 14 s and 100 W, respectively, in this work.
Results and Discussion The influence of the CH4/CO2 feed ratio on conversions of methane and carbon dioxide was investigated first. As shown in Figure 2 and Figure 3, the trends of conversion variation of methane and carbon dioxide were similar for all three reactors. The conversions of methane decreased with the increasing methane in the feed. The conversions of carbon dioxide increased with
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Figure 2. Conversions of methane in three reactors.
Figure 3. Conversions of carbon dioxide in three reactors.
the increasing methane in the feed. The conversions of methane and carbon dioxide in Reactor 2 were close to those obtained in Reactor 3. The conversions in Reactor 2 and 3 were much higher than those obtained in Reactor 1. Therefore the effect of discharge gap on conversions of feed gases was much more significant than the effect of after-glow zones was. The less the width of discharge gap was, the higher conversions of methane and carbon dioxide were. The effect of gap width could be explained through the mean electron energy in discharge zone. The narrower discharge gap leads to a generation of higher energetic electrons. The electrons with higher mean energy could dissociate more methane or carbon dioxide. The average energy of the electrons generated can be easily adjusted by changing the product of gas density and gap width in DBD plasmas, which is a great advantage of the DBD plasmas over many other discharges.29 With the decreasing product of gas density and gap width, the mean electron energy increased. It suggested that conversions of feed gases could be improved by raising the mean electron energy. The existence of after-glow zones improved the conversions little under the experimental conditions. Due to the high stability of methane and carbon dioxide molecules at ambient condition, it is very difficult to decompose them in the after-glow zones. The effect of the after-glow zones on conversions was very small but (29) Eliasson, B.; Kogelschatz, U. IEEE Trans. Plasma Sci. 1991, 19 (2), 309-323.
Li et al.
its effect on the selectivity of products was significant as discussed below. The selectivities of the main products, including gaseous hydrocarbons (GHC), methanol and ethanol (Alcohol), series of acids (Acid), and carbon monoxide (CO) are shown in Figure 4a-e. Liquid hydrocarbons, trace amount of other oxygenates, and plasma polymers were not illustrated in these figures. The concentrations of methane in the feed were variable from 82.8 vol% to 34.0 vol% in Figure 4a-e. Gaseous hydrocarbons and syngas (H2 and CO) were the main products under the experimental condition employed. Their selectivities were changeable by adjusting the component of feed gases. There were optimum compositions of the feed to make the selectivities of alcohol and acid maximum. Gaseous products mainly consisted of light hydrocarbons (from C2 to C5) and syngas. Table 1, Table 2, and Table 3 show the selectivities of gaseous hydrocarbons produced in three reactors. For all three reactors, the selectivities of paraffin were much higher than that of olefin in the gaseous products. These results were significantly different from the results using corona discharges.5-8 In comparing Table 2 to Table 3, selectivities of most hydrocarbons were higher with the existence of after-glow zones in Reactor 3 when methane was the primary component in the feed. It suggested that the radicals already generated in the discharge zone could continue to combine into products in afterglow zones. On the other hand, with the decreasing methane concentrations in the feed, all the selectivities of gaseous hydrocarbons decreased in three reactors. The selectivities of hydrocarbons sharply decreased with the increasing carbon atom numbers in the molecules for all three cases. A simple power function was found, which is between the carbon atom numbers in molecules and the total selectivity of gaseous hydrocarbons containing the same carbon atom number. It can be described as
Sn ) A‚nb
(1)
where Sn stands for the selectivity of hydrocarbon with n carbon atoms; n is the number of carbon atoms in one hydrocarbon molecule, n should be less than 5 or equal to 5 here; and A and b were empirical coefficients derived from the experimental data. Table 4 shows the empirical values of A and b in the equations derived from the experimental data. According to eq 1, the value of Sn was deeply influenced by the value of b, especially when n > 3. The value of b was negativesthat means the selectivities of hydrocarbons produced decreased with the increasing number of carbon atoms in hydrocarbon molecules. The selectivity of hydrocarbons containing more carbon atoms would increase with a higher value of b according to these equations. The value of A was in the range from 0 to 1 in all the equations. A was an empirical coefficient related to the equilibrium concentration of methane. Regarding the exponent, b was a more important influencing factor on the selectivity than A was. It was considered that there was a correlation between the values of A and b and the mechanism of carbon chain propagation in hydrocarbons. However, further investigation is required for the theoretical explanation of A and b.
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Figure 4. Selectivity of products on the basis of carbon atoms. Table 1. Selectivities of Gaseous Hydrocarbons in Reactor 1a CH4 in the feed/vol%
C2H4
C2H2
C2H6
C3H6
82.8 75.1 67.4 50.4 34.0
1.2 1.0 1.0 0.6 0.2
1.6 1.6 1.4 0.8 0.2
21.0 19.6 19.4 14.6 7.8
0.6 0.3 0.6 0.3 0.0
a
selectivity based on carbon atoms/% C3H8 i-C4H10 C4H10 10.5 9.9 9.3 6.6 2.7
2.8 2.4 2.4 1.2 0.4
3.2 2.8 2.8 1.6 0.8
n-C5H12
i-C5H12
C5H12
1.0 1.0 1.0 0.5 0.0
2.5 2.0 2.0 1.0 0.5
0.5 0.5 1.0 0.5 0.0
Discharge power of 100 W; gap width of 1.8 mm; flow rate of 60 mL/min.
As discussed above, the decomposition of methane and carbon dioxide mainly took place in the discharge zone and hardly in the after-glow zones, while the combination of radicals could happen in the after-glow zones. Free radical processes were widely accepted as the main mechanism in nonthermal plasma reactions by many researchers.4,6,8,11,15,16 Therefore the mechanism of producing the gaseous hydrocarbons was hypothesized as below:
1. Free radical reactions were the main reactions; 2. The reactions took place in the discharge channels and in the bulk of gases; 3. The initiation of free radicals occurred in the discharge channels. The main reactions were described as below. The initiation of radicals in the discharge channels could be expressed below.
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Table 2. Selectivities of Gaseous Hydrocarbons in Reactor 2a CH4 in the feed/vol%
C2H4
C2H2
C2H6
C3H6
82.8 75.1 67.4 50.4 34.0
1.0 0.6 0.6 0.4 0.2
1.2 0.8 0.6 0.4 0.2
17.4 14.6 14.0 11.8 7.8
0.6 0.3 0.3 0.3 0.0
a
selectivity based on carbon atoms/% C3H8 i-C4H10 C4H10 9.9 8.1 7.5 5.7 3.3
2.8 2.0 2.0 1.2 0.4
n-C5H12
i-C5H12
C5H12
1.0 1.0 0.5 0.5 0.5
2.5 2.0 1.5 1.0 0.0
1.0 0.5 0.5 0.5 0.0
n-C5H12
i-C5H12
C5H12
1.0 1.0 1.0 0.5 0.0
2.5 2.0 2.0 1.0 0.5
0.5 1.0 0.5 0.5 0.0
3.2 2.8 2.4 1.6 0.8
Discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min. Table 3. Selectivities of Gaseous Hydrocarbons in Reactor 3a
CH4 in the feed/vol%
C2H4
C2H2
C2H6
C3H6
82.8 75.1 67.4 50.4 34.0
1.0 0.8 0.6 0.4 0.2
1.4 1.0 0.8 0.4 0.2
18.0 16.2 15.4 11.6 7.0
0.6 0.3 0.3 0.0 0.0
a
selectivity based on carbon atoms/% C3H8 i-C4H10 C4H10 10.5 9.6 8.1 5.7 2.4
2.8 2.8 2.4 1.6 0.4
3.6 2.4 1.6 1.2 0.8
Discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min; with after-glow discharge zones.
CH4 + e f •CH3 + H• + e′
(2)
C2H6 + e f •CH3 + •CH3 + e′
(3)
C2H6 + e f •C2H5 + H• + e′
(4)
C3H8 + e f •C3H7 + H• + e′
(5)
Table 4. Empirical Coefficients in Equations of Gaseous Hydrocarbon Selectivities and Number of Carbon Atomsa CH4 in the feed/v% coefficients Reactor 1 Reactor 2
where e′ represents the electrons with less energy. The reactions of propagating the length of carbon chain, which took place in the bulk gases, could be expressed as:
Reactor 3
A b A b A b
82.8
75.1
67.4
50.4
34.0
0.952 0.901 0.852 0.808 0.736 -3.000 -3.017 -2.991 -3.328 -4.082 0.626 0.524 0.528 0.556 0.738 -2.679 -2.704 -2.798 -3.117 -4.038 0.671 0.607 0.590 0.566 0.569 -2.697 -2.732 -2.828 -3.158 -3.895
a Selectivity of GHC ) moles of GHC produced/(moles of CH 4 converted + moles of CO2 converted) × 100 mol %.
CH3• + CH3• f C2H6
(6)
CH3• + C2H5• f C3H8
(7)
CH4 in the feed/vol%
82.8
75.1
67.4
50.4
34.0
CH3• + C3H7• f C4H10
(8)
Reactor 1a Reactor 2b Reactor 3c
2.6 3.2 3.4
2.1 2.4 2.6
2.0 1.9 2.0
1.1 1.5 1.2
0.7 0.8 0.8
C2H5• + C2H5• f C4H10
(9)
Furthermore, radicals would rearrange to more stable structure with more branches in the discharge zone, especially when the hydrocarbons contained more than five carbon atoms. It suggested that the branched hydrocarbons should be easily produced compared to the straight chains. The other component in the gaseous product was syngas. As shown in Figure 4a-e, the selectivity of CO increased proportionally with the increasing CO2 concentration in the feed. The ratio of H2/CO is an important parameter if the syngas is used as the feed for further synthesis. As shown in Table 5, the ratio of H2/ CO was changeable with different components of feed gases. The reactor with the narrower discharge gap (Reactor 2 and Reactor 3) was very good for the production of syngas with a high ratio of H2/CO, especially when feed gases with a high concentration of methane were used. Comparing the case of Reactor 2 to that of Reactor 3, the ratio of H2/CO was higher in the existence of after-glow than that without after-glow zones when methane was the principle composition in feed gases. The effect of after-glow zones was much more significant on the production of hydrogen than on the production of carbon monoxide. When the concentration of carbon dioxide increased in the feed gases, the ratio
Table 5. Ratio of H2/CO in Three Reactors
a Reactor 1; discharge power of 100 W; gap width of 1.8 mm; flow rate of 60 mL/min. b Reactor 2; discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min. c Reactor 3; discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min.
of H2/CO was lower in Reactor 3 than that in Reactor 2. It was considered that the formation of oxygenates from carbon dioxide took a great amount of hydrogen atoms in the after-glow, which reduced the amount of hydrogen. The results of oxygenates will provide us a further proof as discussed below. It was sure that liquid hydrocarbons were from the propagation of gaseous hydrocarbons. Therefore the distribution of liquid hydrocarbons should be similar to the case of gaseous ones. Figure 5 shows the distribution of hydrocarbons distinguished by the number of carbon atoms. Almost 50 wt% of liquid hydrocarbon products contained 7 and 8 carbon atoms. The concentrations from C5 to C6 were relatively low because some of them could not be completely condensed at the temperature of the mixture of ice and water. The concentration of C11+ stands for the hydrocarbons with 11 carbon atoms and more. These liquid hydrocarbons were just in the range of gasoline. There were over 130 components in the mixture. Most of the hydrocarbons were highly branched, which represents a high octane number. Regarding the number of branches, Table 6 shows the distribution of liquid hydrocarbons. More than half of
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Energy & Fuels, Vol. 16, No. 4, 2002 869
Figure 5. Distribution of liquid hydrocarbons with the number of carbon atoms. Table 6. Distribution of Liquid Hydrocarbons with the Number of Branch Groupsa components
concentrations/ wt%
number o components
direct chain with 1 branch with 2 branches with 3 branches with 4 branches and more
2.97 18.50 39.10 30.25 9.18
8 27 50 30 14
a Most branch groups were methyl, several of them were ethyl, and a few of them were propyl; results from the mixture of hydrocarbons produced in three reactors.
Table 7. Concentrations of Hydrocarbons (C5 to C11+) in the Condensate CH4 in the feed/vol%
50.4
34.0
concentrations in the condensate/wt% Reactor 1a 21.9 6.6 1.7 0.5 Reactor 2b 27.9 15.5 3.9 3.3 Reactor 3c 29.8 16.2 3.6 3.2
82.8
75.1
67.4
0.5 1.1 0.4
a Reactor 1; discharge power of 100 W; gap width of 1.8 mm; flow rate of 60 mL/min. b Reactor 2; discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min. c Reactor 3; discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min.
hydrocarbons were branched with 2 and 3 radical groups. The distribution of products was distinct from the results of Fischer-Tropsch synthesis. Benzene and other aromatics were not detected in the liquid products. The concentration of olefins was about 5.5 wt%. Table 7 also shows the concentrations of hydrocarbons in the condensate for three reactors. The concentration of methane in the feed was an important factor for the total amount of hydrocarbons produced. More methane in the feed will produce more hydrocarbons. As to the influence of gap width of discharge, concentrations of hydrocarbons were higher with the gap of 1.1 mm than those with the gap of 1.8 mm under the experimental conditions. Figure 6a-c shows the selectivity distributions of oxygenates including formic acid (A-C1), acetic acid (A-C2), propanoic acid (A-C3), and butanoic acid and isobutanoic acid (A-C4), methanol (MeOH), and ethanol (EtOH). The selectivity of oxygenates reached maximum with methane of 67.4 vol% in the feed in all the reactors.
Figure 6. Distributions of alcohols and acids with the number of carbon atoms.
Acetic acid and ethanol were two major components in all cases. The selectivity of ethanol was higher than that of methanol. However, the concentration of alcohols containing more than two carbon atoms rapidly decreased in the condensate. There was a similar trend in the generation of the series of acids in this investigation. Acetic acid was far more than any other acids. Bugaev16 et al. reported that the most products are formic acid with the feed of methane and oxygen using DBD plasmas. This difference in product distributions between these two results suggested that carbon dioxide probably provided one of carbon atoms in the generation of acetic acids in this investigation. Table 8 shows the quantity of other oxygenates not discussed above. It mainly included formaldehyde, acetone and some other ketones with a group of hydroxyl.
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Table 8. Other Oxygenates in the Condensate CH4 in the feed/vol%
50.4
34.0
concentrations in the condensate/wt% Reactor 1a 11.2 11.1 5.9 0.9 Reactor 2b 7.4 8.5 3.7 1.6 Reactor 3c 7.4 6.4 5.1 3.4
82.8
75.1
67.4
0.3 1.5 2.0
a
Reactor 1; discharge power of 100 W; gap width of 1.8 mm; flow rate of 60 mL/min. b Reactor 2; discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min. c Reactor 3; discharge power of 100 W; gap width of 1.1 mm; flow rate of 40 mL/min.
The composition of feed gases was an important factor to influence the product distribution in this work. Obviously, more methane in the feed will produce more hydrocarbons. As shown in Table 7, hydrocarbons decreased sharply with the decreasing methane concentration in the feed. Distinguished from the case of hydrocarbons, the selectivities of alcohols and acids were controlled by both methane and carbon dioxide in the feed. Therefore there existed an optimal composition of feed gases to attain the maximum alcohols or acids, respectively. The length of carbon chain in acids increased though the reaction of free radicals derived from methane. So the distribution of acids was similar to the case of hydrocarbons. For carbon dioxide involved in the generation of acids, the maximum concentration of acids was not reached when methane concentration was highest. Regarding the effect of discharge gap on the product distribution, more methanol and ethanol were produced with the discharge gap width of 1.8 mm in Reactor 1, compared to the results with the gap of 1.1 mm in Reactor 2 and Reactor 3. It is hydroxyl that is the common group in all three molecules. On the contrary, the concentrations of hydrocarbons and acids were larger with the gap of 1.1 mm than those with the gap of 1.8 mm. This provided us a clue to control the selectivity of component by adjusting the gap width of discharge. The effect of after-glow zones was the key to understanding the reaction mechanism and controlling the selectivity of products. The amount of liquid hydrocarbons was larger with the existence of after-glow zones than that without these zones when methane was predominant in the feed. However, with increasing carbon dioxide in the feed, much more CxHy radicals would combine with the groups containing oxygen so that more oxygenates were generated. The combination of radicals could take place in the after-glow zones although the dissociation of methane or carbon dioxide
could not happen in these zones, which was proven by the results of methane and carbon dioxide conversions. An interesting result was the generation of acetic acid directly from methane and carbon dioxide. This reaction could be expressed as:
CH4 + CO2 f CH3COOH ∆G298K ) 71.17 kJ/mol (10) The reaction of producing acetic acid from methane and carbon dioxide is a perfect atomic economy reaction. Unfortunately, this reaction is not feasible thermodynamically. Under the condition of nonequilibrium dielectric-barrier discharges, this reaction becomes true. Especially, a higher CO2/CH4 feed ratio is more favored for this reaction. Further research is in progress to clarify the mechanism of plasma reactions. Conclusions Distribution of products generated from methane and carbon dioxide was investigated in this work. The influences of the composition of feed gas, the width of discharge gap, and the after-glow zones were studied. For the effect of the composition of feed gas, the amounts of gaseous hydrocarbons and liquid hydrocarbons increased with the increasing methane concentration in the feed. The selectivity of CO was almost proportional to CO2 concentration in the feed. There existed an optimal composition of feed to attain the maximum amount of oxygenates. As for the influence of the discharge gap width, conversions of CH4 and CO2 were higher with the gap of 1.1 mm than those with the gap of 1.8 mm. The same discharge gap also favors the formation of liquid hydrocarbons and acids. An atomic economic reaction of acetic acid synthesis directly from methane and carbon dioxide has been thereby achieved effectively. On the other hand, much higher concentrations of methanol and ethanol were obtained with the discharge gap of 1.8 mm. Moreover, the liquid hydrocarbons produced in this work were highly branched with about 5.5 wt% of olefins. Acknowledgment. Support from ABB Corporate Research Ltd., Switzerland, and the Key Foundation of the Tianjin City Committee of Science and Technology is very appreciated. The assistance from Dr. Tao Jiang, Ms. Yue-ping Zhang, and Prof. Guo-liang Fan in Tianjin University is also appreciated. EF0102770
