YOSHIOKAWAHARA
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Decomposition of Hydrocarbons in a Microwave Discharge. I.
Methane. Effect of Power by Yoshio Kawahara Radiochemistry Research Laboratory, The Dow Chemical Company, Midland, Michigan 68660
(Received August 86, 1988)
The decomposition of methane in the microwave discharge was investigated in a fast-flow system at 10 Torr using microwaves at 2450 MHz with incident power ranging from 200 to 500 W. The disappearanceof methane was studied using the reciprocal of linear flow rate as the basis of the residence time, the reaction followed first-order kinetics at a given incident power, and the rate of disappearance of methane was found to be an exponential function of the incident power. The principal products are acetylene and hydrogen at a high conversion of methane. When the conversion is low, ethane and ethylene are also obtained in significant quantities. Trace amounts of numerous other hydrocarbons were detected. The product distribution at a given conversion of methane is independent of the incident power as long as the reaction pressure is unchanged. The yields of ethane and ethylene reach maximum values of about 8 and 9%, respectively, while the yield of acetylene keeps on increasing with residence time to exceed 40% based on methane. The product distribution indicates distinct differences between the microwave discharge reaction of methane and those in electrode discharges.
Introduction Methane subjected to electric discharges undergoes transformation, producing various hydrocarbons and hydrogen. Early studies suggest that the different types of discharges may cause a variety of product distributions.1-4 The decomposition of methane in a microwave discharge has been reported only briefly thus far. McCarthy reported the formation of acetylene from methane in the microwave discharge at pressures as high as 0.5 atm.6 He observed the formation of ethane and ethylene only when the efffuent of the microwave discharge was collected on the wall cooled with liquid nitrogen. Vastola and Wightman6 and Wightman and Johnston’ have obtained polymer films with small amounts of low-molecular-weight gaseous hydrocarbons. Very little information is available to date concerning the kinetics and mechanism of the decomposition of methane in various types of electric discharges. As far as the microwave discharges are concerned, such information is almost nonexistent. Recently, Borisova and Eremin studied the kinetics and mechanism of methane transformation in the glow discharge between metallic electrodes.*~g They assumed a first-order dependence of the input power on the rate of reaction and observed the variation in the rate and composition of the products with changes in pressure and current. Preliminary experimental results obtained by the author suggested that t,he microwave reaction of methane may be of a distinctly different nature from those reactions in electric discharges between electrodes. In the present study, methane was decomposed in the microwave discharge a t 2450 MHz with various incident power levels and residence The Journal of Physical Chemistry
times to elucidate the effect of those factors on the reaction.
Experimental Section The microwave discharge reactions were carried out in a Vycor tube (13 mm o.d., 10.6 mm i.d.) which constituted a part of a fast-flow system. Methane gas flow was metered with a capillary flowmeter calibrated against a wet test meter (Precision Scientific Co.). The pressure of the gas in the flow system was monitored by a fused quartz Bourdon gauge (Texas Instruments). A gas flow of finely controlled pressure and flow rate was obtained by adjusting a number of finemetering needle valves (Nupro and Whitey) located both upstream and downstream from the discharge tube. The power source for the discharge was a Raytheon microwave power generator (Model PGM-100) delivering a fixed frequency, continuous wave at 2450 MHz. The output from this generator was introduced into a guillotine-type vane attenuator and then guided (1) G. Glocker and 8. C. Lind, “The Electrochemistry of Gases and other Dielectrics,” John Wiley & Sons, Inc., New York, N. Y.,1939. (2) C. L. Thomas, G. Egloff, and J. C. Morrell, Chem. Rev., 28, 1 (1941). (3) V. N. Kondrat’ev, “Chemical Kinetics of Gas Reactions,” Pergamon Press Ltd., London, 1964. (4) H.Wiener and M. Burton, J . Amer. Chem. Soc., 75, 5815 (1953). (5) R. L. McCarthy, J. Chem. Phys., 22, 1360 (1954). (6) F. J. Vastola and J. P. Wightman, J . A p p l . Chem., 14, 69 (1964). (7) J. P. Wightman and N. J. Johnston, Preprints, 153rd National Meeting of the American Chemical Society, Division of Fuel Chemistry, Miami, Fla., April 1967,Vol. 11, No. 2,p 473. (8) E. N. Borisova and E. N. Eremin, Zh. Pis. Khim., 36, 1261 (1962); Russ. J. Phys. Chem., 36, 667 (1962). (9) E. N. Borisova and E. N. Eremin, Zh. Piz. Khim., 41, 137 (1967); R U S S . J. Phys. Chem., 41, 69 (1967).
DECOMPOSITION O F HYDROCARBONS 1N A MICROWAVE DISCHARGE through a Raytheon power monitor and tuner unit. A monitor meter combined with a detector unit (Type 1N21B) indicates both incident and reflected powers. The incident microwave power was then coupled to the gas flowing in a discharge tube through a tapered matching section. As soon as the discharge was initiated, the pressure of the reaction zone (adjusted to be 10.0 Torr prior to the discharge) increased somewhat due to the composition and temperature changes caused by the reaction, reaching a steady value within a few seconds. The pressure became as high as 12 Torr when the conversion of methane was 50%. After the discharge reaction, the product gas mixture was collected in a sample tube located downstream of the discharge and was analyzed using a CEC mass spectrometer (Model 21-103). The discharge was usually terminated after a sample was taken, although it could have been maintained as long as the microwave excitation was continued. From the per cent composition of the discharge reaction products, the initial amount of methane introduced into the discharge was determined, based on hydrogen balance. The corresponding value based on carbon balance was also calculated in order to check the validity of the value by hydrogen balance. These two values were generally in good agreement, with relative difference being less than 5%. The difference became significant when the reaction was carried out to a conversion higher than about 80% because of a considerable polymer or soot formation. The yields of individual products were calculated as relative amounts compared to the initial amount of methane based on hydrogen balance. Residence time used in kinetic treatment of the data in this study was calculated using the microwave resonator thickness, which is exactly 10 mm, the cross-sectional area of the Vycor discharge tube (0.882 cm2), and the precisely controlled flow rate of methane. The size of visible glow discharge appears to increase as the increasing incident power. However, the measurement of the exact size of the discharge zone was practically impossible due to the diffused nature of the discharge. The residence time used in the present study is therefore not to be confused with the residence time in the discharge. The residence time, thus defined, ranged in the present study from 0.796 to 4.16 msec, corresponding to a linear flow rate from 1260 to 241 cm/sec and a mass flow rate from 2.35 to 0.449 mol/hr. Methane used in the present study was obtained from The Rfatheson Co., Inc. (CP grade containing about 0.2% ethane as a sole impurity detected by mass spectrometry) and was used without further purification.
Results and Discussion Products of the Reaction. The major products of the
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r
01 0
i
I
I
I
I
1
2
3
4
J
Residence Time ( m sec)
Figure 1. Relative yields of major reaction products as a function of residence time; incident microwave power 300 W.
reaction of methane in a microwave discharge a t 10 Torr were acetylene, ethylene, ethane, and hydrogen. The minor products included hydrocarbons such as C3H4*,propylene, propane, diacetylene, vinyl acetylene, C4H6*, C4H8*, C5H6*, CSH8*, pentane, triacetylene, benzene, and toluene. (The structure identification of the asterisked products was not performed.) The deposition of carbon and soot on the inner wall of the discharge tube downstream of the microwave resonator cavity became significant when the conversion of methane exceeded about 80%. Figure 1 demonstrates a typical variation of the product distribution against residence time. As the residence time increases, all of the major hydrocarbon products, ie., ethane, ethylene, and acetylene, increase initially. The rates of formation of ethylene and acetylene are seemingly almost identical during the initial stage of the reaction, After reaching a maximum value of about 8% of the theoretical yield [(C2&)/ (CH4)o = 4%] at about 1.4-msec residence time, the yield of ethane gradually decreases, whereas that of ethylene goes through a peak value of about 9% of the theoretical yield [(C2H4)/(CHJo= 4.5%] at about 2.4 msec of residence time. A sharp increase in acetylene yield is observed until the value for (C2H2)/(CH4)oexceeds 10%. Although the rate of acetylene formation after that period tends to decrease with residence time, the yield reaches well above 40% of the theoretical value [(GH2)/(CH4), = 20%] in the reactions with incident powers of 400 or 500 W. The existence of the formation and successive decomposition of ethane in electrical discharge reactions Volume 78, Number 6 June 1069
YOSHIOKAWAHARA
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of the product distributions due to the residence time change may suggest ethane being the precursor for the formation of both ethylene and acetylene. Further study to elucidate the reaction mechanisms is currently underway. Rate of the Disappearance of Methane. The disappearance of methane in the microwave discharge appears to follow first-order kinetics rather precisely. Figure 2 shows the plots of logarithm of [(CHJ/(CHd)o] against residence time. The first-order rate expression for the disappearance of methane may be written as
0.8
0.7 0.6 0.5
0.4 0.3
0,2
0.1 500 watts
0
1
\ 2
3
4
where (CH,) is the concentration of methane at time t, and kl denotes the over-all rate coefficient for the first-order disappearance of methane. The rate coefficients corresponding to the reactions a t various incident microwave powers obtained from Figure 2 are listed in Table I.
Xesidence Time (m seo)
Figure 2. First-order plots for methane disappearance in the microwave discharge; In [ (CH~)/(CH~)O] us. residence time.
of methane has been a point of argument and has often been neglected in the proposed mechanisms. Wiener and Burton4 rejected the formation of ethane in their high-intensity electrical discharge studies because of the absence of ethane in a significant amount in their reaction product mixture. Burton and Mageeloproposed a mechanism of a discharge reaction of methane involving a successive excitation process by low energy electrons, completely ignoring the formation of ethane and ethylene. On the contrary, Borisova and Eremins showed that, in their low pressure glow discharge reactions of methane between steel electrodes, the main product was ethane. The yield of ethane passed through a certain maximum value with increases of power or residence time. This maximum value reached about 23% based on the initial concentration of methane. The second major product in their reaction was acetylene, and its yield also passed through a maximum value which never exceeded 16% based on the initial concentration of methane. Ethylene production in the above study never became important. In view of the observations described above, it should be noted that the microwave discharge reaction of methane produces a distinctly different product distribution from those observed in other types of electrode discharges. Whether or not ethane and ethylene are formed and decomposed successively in the discharge of methane and are the precursors in the formation of acetylene is not the conclusion that may be drawn from the present limited experimental results. However, the variation The Journal of Physical Chemistry
Table I : First-Order Rate Coefficient for the Disappearance of Methane a t 10.0 Torr Incident power, W
200
300 400
500
ki msec - 1
0.180 0.368 0.710 1.403
It is evident from Table I that the first-order rate coefficient, kl, is a function of the incident power, w. When the logarithm of kl was plotted against incident power, w, a linear relationship was obtained as shown in Figure 3. This empirical relationship may be expressed as kl = KeAw (2) where A is the slope and K is equal to exp(intercept in Figure 3). Using this relationship, the rate eq 1 for the disappearance of methane after integration becomes -In [(CHJ/(CHJo] = KeAu(t - to)
(3)
where to is the residence time at zero conversion. The empirical equation obtained for the linear relationship in Figure 3 is written as ICl = exp(6.87 X 10-aw
- 3.10)
(4) Therefore, in rate expression 3, K = 0.045 msec-' W-l. and A = 6.87 X (10) M. Burton and J. L. Magee, J . Chem. Phye., 23, 2196 (1966).
DECOMPOSITION OF HYDROCARBONS IN A ~IICEOWAVE DISCHARGE
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w
0
200
0 0
300’W
400 W
0
500 W
5.0
0 1.0
E
0.5
I
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0.1
t
I
0
100
200
300
400
500
Incident Miorowwe Power (watts)
Figure 3. The first-order rate coefficients us. incident microwave powers.
The effect of electric power on the rate of an electrical discharge reaction has not been well understood to date. Wiener and B ~ r t o n in , ~ their investigation of methane decomposition a t atmospheric pressure between brass and copper electrodes, fitted their data to the following purely empirical rate equation
where I is the current, V is the voltage, and IC is a constant. Borisova and Eremin8 correlated their data in the low pressure glow discharge reactions with “specific energy” which is defined to be the product of power, U (watts), and the reciprocal of flow rate, (l./hr). Their rate expressions assuming a first-order dependency of power seemingly fit their experimental data. However, they have presented no quantitative explanation on the fact that the variation of current resulted in changing the over-all rate as well as the product composition. Because the microwave transmitted into this resonator cavity is considered to be mostly confined within this cavity, the oscillating electric field induced by the
CH4 Conversion
I
1 0
50
(%I
Figure 4. Relative concentration of acetylene produced us. the conversion of methane.
microwave is presumably restricted to the same area. Therefore, the residence time in the present study is actually the time spent by a particle traveling through the electric field, and it is merely a relative time scale to interpret the rate of conversion under given gressure and power level. The product distribution in the present microwave discharge reaction is found to be a function of the conversion of methane and independent of incident power used to achieve the particular conversion. This fact, demonstrated in Figure 4 showing the relationship between the yield of acetylene and the conversion of methane, clearly indicates that the increase in incident microwave power merely accelerates the rate of reaction exponentially. It does not alter the course of the chemical processes. In the electrodeless discharge, the effect of incident power on the chemical reaction may appear differently from that in the discharges between electrodes. The variation of power is suspected to cause, in the electrodeless discharge (1) a change in the size of the plasma, thus altering the size of the reaction zone, and (2) a change in the degree of excitation brought about by the change in the electron densities and/or the electron energies in the plasma. Further studies are in progress to investigate whether or not the above factors account for the exponential dependence of the methane decomposition rate on the incident power.
Vohme 73, Number 6
June 1069
