17866
J. Phys. Chem. 1996, 100, 17866-17872
Effect of Dielectric Constant, Cavities in Series, and Cavities in Parallel on the Product Distribution of the Oligomerization of Methane via Microwave Plasmas Carolina Maru´ n,† Steven L. Suib,*,†,‡ Mery Dery,§ Jeffrey B. Harrison,§ and Mahmoud Kablaoui§ U-60, Department of Chemistry and Institute of Material Science, UniVersity of Connecticut, Storrs, Connecticut 06269-3060, Department of Chemical Engineering, UniVersity of Connecticut, Storrs, Connecticut 06269-3060, and Texaco Research Center, Texaco, Inc., P.O. Box 509, Beacon, New York 12508. ReceiVed: April 1, 1996; In Final Form: July 8, 1996X
The use of microwave-induced plasmas as a method to oligomerize methane to higher hydrocarbons has been studied. The pressure range used was 10-20 Torr and the applied power was 60 W. The microwave power is coupled to the plasma by means of either an Evenson or a Beenakker cavity, the Beenakker being the most effective. We explored the effect of the presence of a dielectric material on the product distribution for this reaction. The values of the dielectric constants for these materials varied from 2.6 for Pb(Ac)2 to 10 000 for MnO2 relative to the vacuum. No direct correlation was found, but in some cases the selectivities toward C6s to C8s were enhanced. TiO2 and Li2CO3 increased the selectivities toward C6s. SnO2 was the best for selectivities to C7s and C8s. When a coating of Si/SiC on the reactor walls was present in the plasma zone, the selectivities toward C6s and C7s increased with respect to both materials (Si and SiC) by themselves. We also studied the effect of cavities in series and cavities in parallel on the oligomerization of methane with and without dielectric material in between the cavities. When methane and iodine are activated separately and then recombined, it seems that the oligomerization of methane is enhanced toward higher hydrocarbons. We found that when a dielectric material is placed in between and when the distance between the two cavities in series is the largest, the oligomerization of methane toward high molecular weight hydrocarbons is maximized.
I. Introduction The conversion of methane (main component of natural gas, which is abundant in nature) to more valuable and useful chemicals and fuels has received considerable attention in the last two decades. A process in which methane could be directly converted to a specific product would be of great significance and could have great economic advantages. The methane molecule is thermodynamically stable with respect to its components.1 The reactions to make other hydrocarbons (all of which are less stable than methane around 1000 °C) have unfavorable free energies of reaction and are strongly limited by equilibrium. The use of natural gas as an oil substitute could be possible if methane could be converted to petrochemical species or gasoline components. For this purpose, the methane molecule must be functionalized, which means that a very strong C-H bond (104 kcal/mol) needs to be selectively broken. Many processes have been developed for this activation. These include coupling of methane with oxygen over alkali-metal catalysts,2,3 coupling of methane with chlorine,4,5 electrochemical activation of methane, photocatalytic oxidation of methane, partial oxidation of methane over redox catalysts, and catalytic reactions involving transition metal complexes,2,6-15 etc. Most recent efforts have focused on the development of direct methane conversion by means of oxidative coupling into ethane and ethylene,2,16 partial oxidation to methanol at high pressures,17,18 electric discharges,19 etc. * Author to whom correspondence should be addressed. Phone (203) 4862797. Fax (203) 4862981. † Department of Chemical Engineering, University of Connecticut. ‡ Department of Chemistry and Institute of Material Science, University of Connecticut. § Texaco Research Center. X Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)00955-0 CCC: $12.00
The requirements of energy to reach temperatures above 1000 °C are minimized by means of microwave plasmas in which there is an abundance of free radicals and metastable excited species. It has been demonstrated that a plasma process activates C-H bonds, and many reactions that are difficult to catalyze under thermal conditions proceed easily in a highfrequency plasma reactor. Several applications of microwaveinduced catalysis in chemical reaction including the synthesis of higher aliphatic and aromatic hydrocarbons from methane have been studied,20 as well as pulsed microwave catalytic decomposition of olefinic hydrocarbons,21 production of acetylene by microwave catalytic reaction of carbon and water,22 and a microwave-assisted cracking of benzene to produce acetylene.23 Methane has been converted to acetylene, ethylene, and ethane over carbon at atmospheric pressures using pulsed highpower radio frequency or microwave irradiation.24 Dimerization25,26 and conversion of methane to higher hydrocarbons via microwave heating has been studied in the last few years,27,28 as well as the partial oxidation of methane to methanol through microwave plasmas.29 In this paper, we examined the effect of the presence of dielectric materials, type of cavity (Beenakker or Evenson), cavities in series or in parallel, and expansion and compression effects, as well as the presence of a radical initiator (I2, in this case), on the product distribution of methane oligomerization via microwave plasmas. II. Experimental Section A. Plasmas Reactors. The oligomerization of methane was carried out in a 3/8 in. quartz reactor, which was placed inside the cavity (Beenakker or Evenson). The microwave power was generated from a magnetron. For the Beenakker cavity we used an Opthos Microwave Generator Model MPG 4M and for the © 1996 American Chemical Society
Oligomerization of Methane via Microwave Plasmas
J. Phys. Chem., Vol. 100, No. 45, 1996 17867
SCHEME 1. Plasma Reactor Apparatus
TABLE 1: Information about Dielectric Materials material
source
dielectric constant30 a
T (K)
V (Hz)
MnO2 TiO2 SnO2 Si CaO BaSO4 SiC Li2CO3 SiO2 Pb(Ac)2
Diamond Shamrock Chemicals Fisher Scientific Mallinkrodt Chemical Works Atlantic Equipment Co. Baker and Adanson Merck & Co. Aldrich Mallinkrodt Chemical Works Silica gel, Davisil Aldrich Baker Chemical Co.
10000 170 14 12.1 11.8 11.4 9.72 4.9 4.6 2.6
298 300 298 4.2 283 288 298 291 298 288
104 104-106 104-1010 107-109 2 × 106 108 IR 2 × 105 9.4 × 1010 106
a
Referred to vacuum.
Evenson cavity a Raytheon Microwave Generator was connected. Both generators provided up to 100 W of power at a fixed frequency 2.54 GHz. Meters were used to measure the forward and reflected power. B. Dielectric Materials. The plasma reactions were run under a continuous methane flow as shown in Scheme 1. HP grade methane obtained from Matheson was used. The dielectric materials were purchased from different companies (see Table 1 for more information), and they were used as received. In Table 1 we show the values of the dielectric constant for each material in reference to a vacuum and the temperature and frequency at which they were measured.30 Approximately 0.5 g of the dielectric material was placed inside the reactor which was plugged at one end with glass wool. A pressure transducer MKS Model PDR-C-1C was used to measure the pressure in the plasma reactor. An electronic flow meter (J&W Scientific Gas Flowmeter) was utilized to measure the flow rate of methane, and a rotameter (Matheson R7630 Series) coupled with fine metering valves was used to control the flow rates. The conditions we used for this set of experiments were power/power reflected, 60 W/10 W; pressure range, 10-20 Torr; mass flow rate of methane (at room temperature and 1 atm), 3.5 mL/min. C. Product Analysis. We collected the products and the unreacted reactant by using a liquid nitrogen trap. After
collection time elapsed, the system was isolated by closing valves 20 and 10. Then, the line was allowed to warm to room temperature. Whenever necessary, the system was pressurized with UHP helium (purchased from Matheson) using a needle valve (18) and a stopcock (17), in order to bring the sample to the gas-sampling valve. The products were analyzed by connecting the four-port valve (23) and the sampling loop (24) between the He tank and HP5890 Series II chromatograph which was equipped with a thermal conductivity detector and a mass detector. The two detectors are connected parallel to the analytical column. The GC also has a sampling valve and another six-port valve for column switching. Three analytical columns were used for the separation. A precolumn (HP Porapack Q 1/8 in. × 6 ft) was used to separate the permanent gases from the rest of the sample. The permanent gases are further separated individually on a molecular sieve column (HP Molsieve 25 m × 0.53 mm × 50 µm), while the rest of the sample mixture is back-flushed from the precolumn and passed through a split to a Poraplot Q column (HP Poraplot Q 25 m × 0.32 mm). The splitting ratio is typically 30:1. The permanent gases, after passing through the Molsieve column, reenter the precolumn and then enter the split and the Poraplot Q column. D. CVD Studies. To determine if variation in how the dielectric material was placed in the reactor had any effect on the product distribution, chemical vapor deposition (CVD) was used to deposit Si/SiC film on the reactor wall. Methyltrichlorosilane (MTS) was put in a bubbler. Hydrogen at a flow rate of 23 mL/min was passed through the bubbler. The MTS carried by hydrogen entered a furnace at 850 °C for 1 h where the CVD took place. E. Experiments with Iodine. For the study of the effect of cavities in parallel, iodine was used as a dielectric material. The iodine was vaporized by working at a pressure lower than the vaporization pressure of the material. In the case of expansion and compression effects, the runs where made for a 2 in. expansion and 1/8 in. compression for the quartz reactor. The power for these two cavities was set at 60 W. Experiments with cavities in series with and without dielectric materials in between were performed. In this case, the dielectric material
17868 J. Phys. Chem., Vol. 100, No. 45, 1996
Figure 1. Effect of dielectric constant on conversion.
used was SnO2 since it was the best for the enhancement of selectivities toward C7s and C8s (vide infra). The different distances we tried were 0.75, 1.25, and 4.5 in. The power set for the cavities in series was again 60 W. III. Results A. Dielectric Studies. The reactions were carried out in a straight 3/8 in. quartz reactor. The plasma was ignited with a Tesla coil. The area of the plasma was maximized with a tuning stub, and in the same way the reflected power was minimized. The plasma always fills the full cross section of the reactor. Under the experimental conditions that were used, the plasma is blue colored. For the Beenakker cavity, the plasma zone has a length of about 5 mm, and for the Evenson cavity the length was usually about 10 mm. The plasma is easier to ignite with the Beenakker than with the Evenson cavity. For both cavities, coke deposition on the walls of the quartz tube is observed. The coke is deposited nonuniformily along the wall of the plasma reactor. In comparison to the Beenakker cavity, coke formation is greater with the Evenson cavity, which makes it more difficult to maintain the plasma which will eventually extinguish. When some of the dielectric materials (CaO, MnO2, TiO2, Si, and BaSO4) were placed in the reactor, coke formation on the walls of the reactor was minimized. The conditions chosen for these studies were limited by the stability of the plasma with the Evenson cavity, since over a pressure of 20 Torr the plasma cannot be maintained. The total conversion of methane was estimated by a carbon balance, and the selectivities were not reported for a specific compound.31 Instead, we added all the compounds multiplied by the number of carbons present in that molecule, and we report selectivities (Cis) as the selectivities toward the sum of the compounds with i number of carbon atoms. The compounds analyzed using the gas chromatograph were methane, carbon dioxide, carbon monoxide; C2s, ethylene,
Maru´n et al. acetylene, and ethane; C3s, propene, propane, 1,2-propadiene, propyne; C4s, 2-methylpropane, 2-butene, 1-buten-3-yne, 1,2butadiene, butadiyne, 1-butyne, 2-butyne; C5s, 3-penten-1-yne; C6s, benzene; C7s, toluene, C8s, ethylbenzene, ethynylbenzene, and ethenylbenzene. For the dielectric effect on product distribution in a methane plasma induced via microwaves, we used the Evenson cavity and conditions that were previously mentioned. The range of conversion obtained using all the dielectric materials was between 0.7 and 0.9 as can be seen in Figures 1 and 4. Only in the case of expansion, compression, and cavities in parallel (Figure 7) does the conversion seem to decrease in comparison with the other cases. In general, selectivities toward C2s to C4s did not change significantly with the variables we studied (type of cavity, dielectric constant, cavities in series or in parallel, etc.) at the conditions we used (Figures 2, 5, and 8). In Figures 2 and 3 we present the selectivities for reactors containing different dielectric materials. Ethylene, acetylene, and ethane (C2s) are the major products of this reaction (∼70%), being followed by C4s (9-13%) and C3s (8-12%). There is no direct correlation between the value of the dielectric constant and the product distribution at the conditions we used. There are some cases where an increase in the selectivities toward some of the products was observed, as for example: MnO2 showed an increase in selectivity toward C7s, TiO2 and Li2CO3 were the best for enhancing the selectivities toward C6s and C7s, and SnO2 was the best for C7s and C8s (Figure 3). Note that carbon dioxide was present as a product for this reaction being in larger quantities for MnO2 (4.3%), Pb(Ac)2 (3.2%), and BaSO4 (3.1%). B. CVD Coatings. Direct placement of the dielectric material in the reactor and a coating of Si/SiC deposited by CVD were examined to determine if these variables had any effects on the product distribution. There was a small increase in the selectivities toward C6s and C7s in the case of coating compared to the materials placed in the reactors by themselves (Figure 3). C. Effect of Cavity Type and Number. In Figures 4-6 we show the effect of cavity type (Evenson or Beenakker) with and without SnO2 as a dielectric material. The effects of cavities in series with different distances in between and with or without SnO2 are also shown. The Beenakker cavity is more effective than the Evenson cavity (Figure 4). The selectivities toward C5s and C6s were larger for the former (Figure 6). Using the Evenson cavity, with SnO2 placed in the reactor, selectivities toward C7s and
Figure 2. Effect of dielectric constant on product distribution (selectivities from C2s to C4s are shown.)
Oligomerization of Methane via Microwave Plasmas
J. Phys. Chem., Vol. 100, No. 45, 1996 17869
Figure 3. Effect of dielectric constant on product distribution (selectivities from C5s to C8s and CO2 are shown.)
might recombine better in an expansion process (Figure 7). Cavities in parallel in which methane and iodine are activated before they recombine favor the oligomerization of methane, enhanced conversion, and selectivities toward C5s to C8s (Figures 8 and 9).
IV. Discussion Figure 4. Effects of type of cavity, dielectric material and cavities in series on conversion. B ) Beenakker; E ) Evenson; Eser ) Evenson in series; the number in the parentheses refers to the distance in between the cavities; the dielectric material used was SnO2.
C8s were enhanced. When the Beenakker cavity was used, only selectivities toward C8s were increased (Figure 6). A combination of two Evenson cavities in series is more effective with regard to conversion and selectivities than a single Evenson cavity and as effective as a Beenakker cavity by itself (Figure 4). Selectivities toward C6s to C8s are enhanced when two cavities in series are used (Figure 6). When two Evenson cavities in series were used with a dielectric material in between, the selectivities toward C6s to C8s were enhanced significantly and maximized when the separation between the cavities is the largest (Figure 6). Traces of C9s were detected in the latter configuration. Compression and expansion effects do not seem to favor the oligomerization of methane, contrary to the idea that radicals
A. Efficiencies of the Cavities and General Mechanistic Ideas. The electromagnetic field is stronger in a TM010 Beenakker cavity in comparison to a less efficient resonant cavity, such as an Evenson cavity, at a comparable microwave energy input. This is why the plasma is easier to maintain and ignite in a Beenakker cavity. The coke that is formed on the walls of the reactor will further induce thermal coke formation from methane. When heat is not removed fast enough from the reactor, then the reactor begins to absorb microwave energy. As a consequence, less microwave energy can penetrate the reactor wall and eventually the plasma will extinguish. The larger tube diameter of the Evenson cavity leads to power dissipation of thermal energy in comparison to the Beenakker cavity. A vortex reactor has been utilized in plasma reactions which is more efficient at minimizing coke formation.21 A plasma is an electrically conducting gas, which contains ions, electrons, and ground and excited state species. It is known that the collisions of electrons and molecules generate large
Figure 5. Effects of type of cavity, dielectric material, and cavities in series on product distribution. B ) Beenakker; E ) Evenson; Eser ) Evenson in series; the number in the parentheses refers to the distance in between the cavities; the dielectric material used was SnO2. (Selectivities from C2s to C4s are shown.)
17870 J. Phys. Chem., Vol. 100, No. 45, 1996
Maru´n et al.
Figure 6. Effects of type of cavity, dielectric material, and cavities in series on product distribution. B ) Beenakker; E ) Evenson; Eser ) Evenson in series; the number in the parentheses refers to the distance in between the cavities; the dielectric material used was SnO2. (Selectivities from C5s to C8s and CO2 are shown.)
is presented below:
CH4 f C2H6 f C2H4 f C2H2
Figure 7. Effects of compression, expansion, and cavities in parallel on conversion. Iodine (I2) was used as a dielectric material.
amounts of radicals and metastable excited species, which are the key ingredients in plasma reactions. Radicals and reactive species, such as CH, CH2, CH3, and H (from CH4) are generated in the plasma. Radical recombination is perhaps responsible for the product formation:
2CH3 f C2H6
(1)
2CH2 f C2H4
(2)
2CH f C2H2
(3)
Another viable mechanism for ethylene and acetylene formation
(4)
The mechanism by which C4s to C8s are formed could be very complicated, and many different pathways could be involve in the production of these compounds. In order to determine the kinetics and mechanisms by which these reactions occur, several experiments need to be performed. One might expect that having a dielectric material present in the plasma zone will have an effect on the activities and product distribution for the oligomerization of methane toward higher olefins via microwave plasmas. When a dielectric material is placed in an electric or electromagnetic field, the material becomes polarized and stores electric energy through polarization. The level and mechanism of polarization available to materials depend on the state and composition of the material and the frequency of the applied electric field.32 The resulting polarization lags behind the changes of the electric field and causes dielectric heating of the material.33 The ability of a molecule or bulk substance to absorb microwave radiation is quantified by the dielectric property of the material. This direct absorption can lead to localized introduction of energy to a specific site or region.34 Many solids absorb microwaves and can be heated very rapidly to high temperatures. This opens up the possibility of using the coupling to microwaves of one component in a mixture
Figure 8. Effects of compression, expansion, and cavities in parallel on product distribution. Iodine (I2) was used as a dielectric material. (Selectivities from C2s to C4s are shown.)
Oligomerization of Methane via Microwave Plasmas
J. Phys. Chem., Vol. 100, No. 45, 1996 17871
Figure 9. Effects of compression, expansion, and cavities in parallel on product distribution. Iodine (I2) was used as a dielectric material. (Selectivities from C5s to C8s and CO2 are shown.)
to drive a chemical reaction with a second component which may be transparent to microwaves.35 The desired chemical selectivity in the products is typically achieved by appropriate microwave pulsing as well as changing the material used as a sensitizer.20-24,36-41 B. Dielectric Materials. Dielectric properties are usually strongly dependent on temperature and frequency. For example, in the case of water, data published in the literature show that there was a change of 1 order of magnitude in the dielectric constant value as the frequency and temperature were changed by 5 and 2 orders of magnitude, respectively.42 The dielectric constant values used were measured at different frequencies, but if there was any effect on the product distribution and activity due to the presence of the dielectric material in the plasma zone, it would have been noticeable within the range of dielectric values of the materials that we used (4 orders of magnitude). The presence of dielectric materials in microwave plasmas had a very small effect on the product distribution, and no direct correlation between the dielectric constant value and the selectivity toward higher hydrocarbons (Figures 1-3) existed. It is possible that the way the dielectric material was placed in the reactor was not the most effective one. A homogeneous dielectric media where the plasma could be maintained would be the best choice, since the ionizing media will have more contact with the dielectric material. In our case only a small region of the total area of the dielectric material was in contact with the plasma zone. However, one still might expect considerable effect on activity, selectivity, and coke formation.43,44 The presence of carbon dioxide as a product for this reaction can be explained by considering that dielectric materials, such as MnO2, can release oxygen. In other cases where oxygen was not a component present in the material, the formation of CO2 could come from sputtering of the reactor walls (quartz). C. CVD Coatings. The increase of selectivities of C6s and C7s in the case of the coating might be due to better contact of the plasma with the dielectric material (Figure 3). This effect may not be significant in practical applications due to the large amount of coke formed during the reaction. Similar large amounts of coke have been observed when catalysts have been placed directly in cavities.27,28 D. Effect of Various Cavities and Configurations. It is not surprising that the Beenakker cavity is more effective than the Evenson cavity (Figure 4). The Evenson cavity is a larger cavity than the Beenakker cavity. The large volume of the cavity decreases the electromagnetic field density in the cavity
and the power density. Having more coke formation with the Evenson cavity also decreases the efficiency for the same power applied for both cavities. It was surprising that the conversion of methane was not enhanced significantly by using two cavities in series (Figure 4). The selectivities toward C6s to C8s increased by using two cavities in series, because the feed for the second plasma region was not only methane but other higher hydrocarbons formed during the first reaction zone (Figure 6). Therefore, these hydrocarbons will be ionized, and larger radical chains will be formed, which will recombine to lead to higher oligomers. It is possible that having two cavities in series too close with a dielectric material in between might affect the efficiency of both cavities (Figure 4). This may explain why the selectivities toward C6s to C8s were enhanced and maximized with the presence of a dielectric material when the separation between the cavities was the largest (Figure 6). As we increased the distance between the cavities, the residence time of the products formed during the first plasma zone was increased (Figure 6). Therefore, it might be possible that the products formed during this stage changed as we changed the position of the dielectric material. Results suggest that as we increase the distance between the cavities the oligomerization of methane is optimized and that oligomerization was enhanced even more when the dielectric material was present. The magnitude of the compression and expansion effects that we used did not affect the product distribution (Figures 8 and 9). It is possible that the reaction occurs mainly in the plasma zone, although other evidence suggests that the free radicals survive long after passing the plasma zone.45 In addition, the data for cavities in series also suggest that the residence time of the radicals and recombined species needs to be long in order to maximize oligomerization (Figure 6). It was more effective to activate methane and the radical initiator separately (I2 in this case) than to mix them together and activate the mixture (Figure 7). It is possible that having an activated radical initiator as a reactant will enhance the lifetime of the radicals coming from the methane plasma or will create different initial species that will recombine in different ways. V. Conclusions It has previously been shown that plasmas can be used to activate C-H bonds in methane molecules. The major products observed in these earlier reactions are C2H2, C2H4, and C2H6. We studied the effect of type of cavity, presence of a dielectric
17872 J. Phys. Chem., Vol. 100, No. 45, 1996 material, and cavities in series and in parallel on the product distribution of the oligomerization of methane via microwave plasmas. In order to favor the oligomerization of methane to higher hydrocarbons, different dielectric materials were placed in the reactor in contact with the plasma zone. However, for the oligomerization of methane via microwave plasmas we did not observe a significant effect on the product distributions and activities as has been reported before in the literature for microwave-heated systems.20-24,36-41 In some cases (MnO2, TiO2, Li2CO3, and SnO2) the selectivities toward C6s to C8s were enhanced, but no direct correlation between the dielectric constant and the product distribution was found. How the dielectric material is placed in the plasma zone seems to affect the selectivities toward higher hydrocarbons. Cavities in series with a large separation and with a dielectric material in between optimize the oligomerization of methane toward C6s to C8s with traces of C9s formed during reaction. Compression and expansion effects do not influence the product distribution. Finally, cavities in parallel may favor the oligomerization of methane and lead to enhanced conversion. Acknowledgment. We thank the National Science Foundation, EPRI under Grant CTS-9413394 of the joint NSF/EPRI Initiative on Microwave-Induced Reactions, and Texaco, Inc. for providing support for this work.
Maru´n et al. (20) Tse, M. Y.; Depew, M. C.; Wan, J. K. S. Res. Chem. Intermed. 1990, 13, 221-236. (21) Cameron, K. L.; Depew, M. C.; Wan, J. K. S. Res. Chem. Intermed. 1991, 16, 57-70. (22) Bamwenda, G.; Moore, E.; Wan, J. K. S. Res. Chem. Intermed. 1992, 17, 243-262. (23) Bamwenda, G.; Depew, M. C.; Wan, J. K. S. Res. Chem. Intermed. 1993, 19, 553-564. (24) Ioffe, M. S.; Pollington, S. D.; Wan, J. K. S. J. Catal. 1995, 151, 349-355. (25) Huang, J.; Suib, S. L. J. Phys. Chem. 1993, 97 (37), 9403-9407. (26) Huang, J.; Suib, S. L. Res. Chem. Intermed. 1994, 20 (1), 133139. (27) Suib, S. L.; Zerger, R. P.; Zhang, Z. Proceedings, Symposium on Natural Gas Upgrading II; American Chemical Society: Washington, DC, 1992; Petroleum Chem., p 344. (28) Suib, S. L.; Zerger, R. P. J. Catal. 1993, 139, 383-391. (29) Huang, J.; Badani, M. V.; Suib, S. L.; Harrinson, J. B.; Kablauoi, M. J. Phys. Chem. 1994, 98 (1), 206-210. (30) CRC Handbook of Chemistry and Physics, 73rd ed.; Lide, D. R., Ed.; CRC Press, Inc.: Boca Raton, FL, 1992-1993. (31) The conversion is calculated as follows:
XCH4 ) (CT - CCH4exit)/CT where CT is the sum of the concentration of each compound (Ci) multiplied by the number of carbons present in the molecule i (ni), i.e.:
CT )
i i
The selectivities toward compounds with number of carbons i (Si) was calculated by
References and Notes (1) Billand, F. G.; Gueret, C. P.; Baronnet, F.; Weill J. Ind. Eng. Res. Chem. 1992, 31, 2748-2753. (2) Amenomiya, T.; Birss, V. I.; Goledzinowski, M.; Galuszka, J.; Sanger, A. R. Catal. ReV.-Sci. Eng. 1990, 32 (3), 163-227. (3) Keller, G. E.; Bhasin, M. M. J. Catal. 1982, 73, 9-19. (4) Benson, S. F. U.S. Patent 4199533, 1980. (5) Senkan, S. M. Chem. Eng. Prog. 1987, 22, 58-61. (6) Shilov, A. E. The ActiVation of Saturated Hydrocarbon by Transition Metal Complexes; D. Reidel: Dordrecht, The Netherlands, 1984. (7) Pitchai, R.; Klier, K. Catal. ReV.-Sci. Eng. 1986, 28, 13-88. (8) Wada, K.; Yoshida, K.; Watanabe, Y.; Suzuki, T. J. Chem. Soc., Chem. Commun. 1991, 726-717. (9) Otsuka, K.; Konatsu, T. J. Chem. Soc., Chem Commun. 1987, 388. (10) Hutchings, G. J.; Scurrell, M. S.; Woodhouse, J. R. Chem. Soc. ReV. 1989, 18, 251-283. (11) Ogura, K.; Kataoka, M. J. Mol. Catal. 1988, 43, 371-379. (12) Sayyel, B. A.; Stair, P. C. J. Phys. Chem. 1990, 94, 409. (13) ActiVation and Functionalization of Alkanes; Hill, C. L., Ed.; Wiley: New York, 1989. (14) Ryabov, A. D. Chem. ReV. 1990, 90, 403-424. (15) Methane ConVersion by OxidatiVe Process; Wolf, E. E., Ed.; Van Nostrand Reinhold: New York, 1992. (16) Lee, J. S.; Oyama, S. T. Catal. ReV. Sci. Eng. 1988, 30, 249-280. (17) Edward, J. H.; Fostes, N. R. Fuel. Sci. Tech. Int. 1986, 4, 365. (18) Vedeneev, V. J.; Goldenberg, M. Y.; Gorban, N. I.; Teitelboim, M. A. Kinet. Catal. 1988, 29, 8-14. (19) Shepelev, S. S.; Gesser, H. D.; Hunter, N. R. Plasma Chem. Plasma Process. 1993, 13 (3), 479-488.
∑n C
∑n C )/(C
Si ) (
i i
T
- CCH4exit)
(32) Adu, B.; Otten, L.; Afenya, E.; Groenevelt, P. J. MicrowaVe Power Electromagn. Energy 1995, 30 (2), 90-96. (33) Jacob, J.; Chia, L. H. L.; Boey, F. Y. C. J. Mater. Sci. 1995, 30, 5321-5327. (34) Krieger-Brockett, B.; Mingos, D. M. P.; Wan, J. K. S. Proc. MicrowaVes-Induced React. Workshop. 1993, 4-1-4-4. (35) Mingos, D. M. P.; Baghurst, D. R. Chem. Soc. ReV. 1991, 20, 1-47. (36) Dinesen, T. R. J.; Tse, M. Y.; Depew, M. C.; Wan, J. K. S. Res. Chem. Intermed. 1991, 15, 113-127. (37) Cameron, K. L.; Depew, M. C.; Wan, J. K. S. Res. Chem. Intermed. 1991, 16, 57-70. (38) Depew, M. C.; Lem, S.; Wan, J. K. S. Res. Chem. Intermed. 1991, 16, 213-223. (39) Wan, J. K. S. Res. Chem. Intermed. 1991, 16, 147. (40) Wan, J. K. S. Ioffe, M. S. Res. Chem. Intermed. 1994, 20 (1), 115132. (41) Pollington, S. D.; Ioffe, M. S.; Westergard, M.; Wan, J. K. S. Res. Chem. Intermed. 1995, 21 (21), 59-68. (42) Reid, W. Res. Chem. Intermed. 1994, 20, 97-114. (43) Majetich, G.; Hicks, R. Proc. MicrowaVes-Induced React. Workshop 1993, A6-1-A6-19. (44) Mingos, D. M. P. Proc. MicrowaVes-Induced React. Workshop 1993, A8-1-A8-11. (45) McCarty, R. J. J. Chem. Phys. 1954, 22, 1360.
JP960955X
