Methane conversion via microwave plasma initiated by a metal initiator**

Studies in Surface Science and Catalysis J.J. Spivey, E. Iglesia and T.H. Fleisch (Editors) 9 2001 Elsevier Science B.V. All rights reserved.

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Methane conversion via microwave plasma initiated by a metal initiator** XU Yunpeng, TIAN Zhijian*, XU Zhusheng, ZHU Aimin and LIN Liwu Dalian Institute of Chemical Physics, the Chinese Academy of Sciences, Dalian 116023, China, Usually, the conversion of methane in a continuous microwave plasma was conducted under low pressures. In this study, the atmospheric pressure microwave discharge plasma for methane conversion was investigated, and the main useful products were acetylene, ethylene, ethane and hydrogen. 88% acetylene selectivity and 6% ethylene selectivity at 76% methane conversion could be obtained with methane and hydrogen mixtures as the feed gas. A selfdesigned metal initiator was utilized to maintain the continuous microwave discharges under atmospheric pressure and ambient temperature. The effects of diluting gas were investigated. Hydrogen was found to be able to suppress the formation of coke, while the presence of argon favored the production of coke. 1. INTRODUCTION Declining crude oil reserves and the great resources of natural gas lead to a large number of researches on methane processing for valuable chemical products [1]. Methane is the main component of natural gas, and it is thermodynamically one of the most stable hydrocarbons, so that selective conversion of methane to more useful organic chemicals is difficult [2]. A number of promising methods for natural gas conversion are under extensive development, one of them is the use of plasma technology for selective activation of methane [2-12]. For example, Kado et al. presented a process of non-catalytic direct conversion of methane to acetylene by using direct current pulse discharge under ambient temperature and pressure. The selectivity of acetylene from methane was >95% at methane conversions ranging from 16 to 52% [7]. Liu et al. converted methane to acetylene by using dc corona discharge with high selectivity and yield under atmospheric pressure in the temperature range of 343-773K, and catalysts were utilized in their study. A high yield of C2 was obtained in a hydrogencontaining plasma at a flow rate of 10cm3/min [8]. Other researchers have investigated microwave plasma methods for the conversion of methane to higher hydrocarbons at low pressure [4-6, 9-12], and the main products included acetylene, ethylene and ethane. Onoe et

""Supported by Youth Science Foundation of Laser Technology of China (No.98-11) "To whom correspondence should be addressed

76 al. used a microwave plasma reaction to produce acetylene from methane with a selectivity of >90% at a low reaction pressure of 4.5KPa [4]. Wan et al. also reported that pulse high-power microwave radiation was used to convert methane to acetylene, ethylene and ethane over active carbon under a pressure of 100KPa, in their studies, pulse microwave discharge in catalyst bed could be observed [2]. In the present paper, we report on the conversion of methane via a continuous microwave discharge plasma under atmospheric pressure, which was carried out in a home-made reactor. During these experiments, a continuous microwave was used as the power source, and stable continuous microwave discharges could be observed. The main useful products of the experiments were acetylene, ethylene, ethane and hydrogen. The variations in selectivity and conversion with flow rates as well as gas dilutions were also explored.

2. EXPERIMENTAL The methane used in our experiments was obtained directly from natural gas (98.5% methane, 1.3% ethane and 0.2% nitrogen). Hydrogen (purity of 99.99%) and argon (purity of 99.99%) were used as diluting gases for the experiments. Flow rates of the feed gases were controlled by bubble flow meters. Microwave radiation was supplied by a variable continuous microwave generator with work frequency of 2450 MHz and power of 10 KW. The reaction cavity was a modified section of the wave-guide where a quartz tubular reactor (30cm long and 2.5 cm in diameter) was inserted, and its longitudinal axis was perpendicular to the length-wide side of the rectangular wave-guide. A self-designed metal initiator of microwave discharge plasma was fixed in the center of the reaction zone. The feed gas flew into the reactor through the metal initiator. In the beginning of the experiment, the feed gas was fed into the reactor. After the air in the reactor was replaced by the feed gas completely, the reaction zone was subjected to continuous microwave irradiation with a power of 600 W. By varying the position of the plunger, the microwave cavity could be made to resonate at the working frequency, and the discharge was initiated. A spherical discharge plasma could be observed near the metal initiator, and the diameter of which was about 6 mm. Methane alone, mixtures of methane and argon or mixtures of methane and hydrogen were used as feed gases during the experiments. The tail gases were analyzed by a GC-950 chromatograph with a flame ion detector (FID) and carbon molecular sieve columns. The molar percentages of methane (C1), acetylene (C2), ethylene (C3) and ethane (C4) in the tail gases were determined by the external standard method. Trace amounts of higher hydrocarbons except C2 hydrocarbons were found in our products. We could infer that the following four reactions were the main routes of methane conversion in our experiments: a CH4 ~ a/2 C2H 6 + a/2 H2 b CH4 ~ b/2 C2H4 + b H2 c CH 4 ~ c/2 C2H2 + 3c/2 H 2

(1) (2) (3)

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d

CH 4 ~

d C + 2d

H2

(4)

a, b, c and d were respectively the molar percentages of methane conversion through the four reactions mentioned above. The molar percentage of methane in the feed gas was known to be C~~ Basing on mass balance, we could describe molar percents of methane, acetylene, ethylene and ethane in the tail gas (C1, C2, C3 and C4) by a, b, c, d and C1~ according to the following four equations: (1-a-b-c-d)C~~ + 3b/2 + 5c/2 +2d)C~ ~ + 100 - C~~ =C~/100 (C~~ a/2)/((a + 3b/2 + 5c/2 +2d)C~~ + 100 - C~~ = C2/100 (CI ~b/2)/((a + 3b/2 + 5c/2 +2d)C~~ + 100 - Ct ~ = C3/100 (C~~ c/2)/((a + 3b/2 + 5c/2 +2d)C~~ + 100 - C~~ = Ca/100

(5) (6) (7) (8)

From these four equations we could calculate the values of a, b, c and d, and the conversion of methane as well as the selectivities to acetylene, ethylene, ethane and coke could be obtained. 3. RESULTS AND DISCUSSIONS 3.1. Plasma under atmospheric pressure The data of Tables 1-3 show that methane can be effectively converted to higher hydrocarbons, such as acetylene, ethylene and ethane by the atmospheric pressure microwave discharge plasma. Usually, microwave plasma reactions of pure methane and its mixtures were conducted under low pressures [5] because stable continuous plasmas of those gas were difficult to maintain under atmospheric pressure. Accordingly, the vacuum technique had to be employed to control the plasma of pure methane or its mixtures in those studies. In the present study, a metal microwave plasma initiator designed by us was utilized for the experiments, which could initiate easily continuous discharge plasmas under ambient temperature and pressure with continuous microwave irradiation. Thus, stable continuous microwave discharges could be produced when pure methane or its mixtures was used as the reagent gas, which usually were difficult to achieve under atmospheric pressure in a microwave field. Furthermore, a reactor designed by ourselves could make the plasma so produced to locate just at the center of the tubular reactor, while the wall of the reactor could not contact with the discharge plasma zone (the diameter of the discharge plasma was only 6mm, but the diameter of the reactor was about 24mm), so that the reactor would not be damaged by the high temperature produced by continuous microwave discharges. However, large amounts of coke produced in the experiment would stick on the wall of the tubular reactor and absorb the microwave to yield a high temperature, and this would exterminate the discharge plasma and damage the reactor. So suppressing the coke formation was important for obtaining stable continuous microwave discharge. A diluting gas could have certain effect on coke formation, and this effect will be discussed in Section 3.2. of the following text.

78 From Tables 1-3 we can see that the main product with or without diluting gas was acetylene, and the selectivity towards coke varied with different diluting gases fed into the reactor. The conversion of methane attained was between 27-79%, which increased with the total flow rate of the feed gas so long as the microwave power input was kept constant at 600 W during the experiments. The total selectivities towards acetylene, ethylene and ethane could reach as high as almost 100% under appropriate flow rate when certain amounts of hydrogen was used as the diluting gas. Table 1. Results of methane reactions without diluting gases via microwave discharge plasma T0tal flow Conv. of . . . . . . . . . . Molar Sele.(%) . . . . . . rate STW CH4 (%) Acetylene Ethylene Ethane Coke .... (ml/min) ...... 100 ........ 66 ;74.4 ~,.5 0.5 20.6 300 42 77.2 3.7 1.6 17.5 400 35 83.2 4.2 2.3 10.3 "Reaction conditions: ambient temperature, a~nospheric pressure, n~icrowave power of 600W_ a Standard temperature and pressure Table 2. Results of the reaction of a methane and argon mixture via microwave discharge plasma Total flow ...... Moiar 'Conv. of' Molar Sel'e.(~ ............. rate STW ratio of CH4 (%) _ Coke ...... (ml/min) CH4/Ar Acetylene Ethylene Ethane 37 0.85 79 71.0 128 0.9 47 75.3 235 1.0 37 71.2 Reaction conditions- ambient temperature, a~nospheri'c a Standard temperature and pressure

2.0 0 2.1 1.3 2.4 2.1 pressure, microwave power

27.O 21.3 24.3 of 600W(-

Table 3. Results of the reaction of a mixture of methane and hydrogen via microwave discharge plasma Total fl0w Molar Conv. of . . . . . . Molar Sele.(%) . . . . . . . . . . . . rate STP a ratio of CH4 (%) . . . . . . . . . (ml/min) CHJH2 Acetylene Ethylene Ethane coke 37 ....... 0.6 . . . . . . . . . 76 88.0 87 1.1 49 95.0 182 1.0 39 95.0 361 1.1 27 93.0 Reaction conditions: ambient temperature, atmospheric a Standard temperature and pressure

610 0 6.0 3.9 1.1 0 3.0 2.0 0 4.0 3.0 0 pressure, microwave power of 600W~-

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3.2. Effects of diluting gases Table 2 shows that the presence of argon led to more coke formation as compared with the data in Table 1. However, from Table 3 we can see that the presence of hydrogen can suppress the formation of coke and the selectivity towards coke approached almost zero. Under such a condition, we did not observe any coke formation on the wall of the reactor or the metal initiator. This would stabilize the continuous microwave discharge during the experiments. Previously, Suib et al. have reported that during microwave plasma reaction, H-atom abstraction is the major initial reaction of methane, while hydrocarbon radicals are produced at the same time [5]. CH4 --->CH 3 + H C H 3 -"-> CH 2 + H CH2 --->CH + H CH --->C + H

(9) (10) (11) (12)

Some steps of this process are reversible [2]: CH 2 + H --->CH3 CH + H ~ CH2 C + H --->CH

(13) (14) (15)

And recombination of the radicals is responsible for product formation [5]: 2 CH 3 ---->C2H6 2 CH 2 --->C2H4 2 CH --->C2H2

(16) (17) (18)

When hydrogen is fed into the reactor, H atoms are produced in the discharge plasma [2]: H2 ~ 2 H

(19)

This reaction results in an increasing amount of H atoms, which are advantageous to reactions (13), (14) and (15). On the other hand, reaction (15) can eliminate the coke produced by a deep dehydrogenation of methane. Thus, the presence of hydrogen could reduce the formation of the coke in the plasma reaction of methane, enhance the selectivities of higher hydrocarbons and stabilize the continuous microwave discharge. The effect of argon on methane conversion was different from that of hydrogen, because the former is a good "discharge conductor" [1] and can easily produce discharge under microwave irradiation. It was reported that argon could enhance the conversion of methane [1 ]. In our work, however, argon might have enhanced deep dehydrogenation of methane, thus resulted in more coke formation.

80 4. CONCLUSION A self-designed metal initiator could initiate stable continuous microwave discharge plasma under ambient temperature and atmospheric pressure. Methane could be effectively converted to acetylene, ethylene, ethane and hydrogen by this approach. It was also found that the presence of hydrogen could prevent coke formation, while the presence of argon led to a converse result. REFERENCES 1. Y:Y. Tanashev, V.I. Fedoseev, Y.I. Aristov, V.V. Pushkarev, L.B. Avdeeva, V.I. Zaikovskii and V.N. Parmon, Catal. Today, 42 (1998) 333-336. 2. M.S. Ioffe, S.D. Pollington, and J.K.S. Wan, J.Catal., 151 (1995) 349-355. 3. G. Bond, R.B. Moyes and D.A. Whan, Catal. Today, 17 (1993) 427. 4. K. Onoe, A. Fujie, T. Yamaguchi and Y. Hatano, Fuel, 76 Iss 3 (1997) 281-282. 5. S. L. Suib and R. P. Zerger, J.Catal., 139 (1993) 383-391. 6. J.H. Huang, and S.L. Suib, Res. Chem. Intermed., 20 (1994) 133. 7. S. Kado, Y. Sekine and K. Fujimoto, Chem. Commun., 24 (1999) 2485-2486. 8. C. Liu, R. Mallinson and L. Lobban, J. Catal., 179 (1998) 326. 9. R.L.McCarthy, J. Phys. Chem., 22 (1954) 1360. 10. Y. Kawahara, 3'. Phys. Chem., 73 (1969) 1648. 11. R. Mach, H. Drost, J. Rutkowsky, and U. Timm, in "ISPC-7, Eindhoven, 1985,"p.531. 12. S.L. Suib, R.P. Zerger, and Z. Zhang, "Proceedings, Symposium on Natural Gas Upgrading II", p. 344. Div. Petroleum Chem., ACS, Washington DC, 1992.