Origin of Carbon Oxides during the Oxidative Coupling of Methane

Jul 15, 1994 - Chunlei Shi, Michael P. Rosynek, and Jack H. Lunsford' ... the ratio of the rate constant for COX formation from C2H4 to that from CH4...
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J . Phys. Chem. 1994, 98, 8371-8376

8371

Origin of Carbon Oxides during the Oxidative Coupling of Methane Chunlei Shi, Michael P. Rosynek, and Jack H. Lunsford' Department of Chemistry, Texas A & M University, College Station, Texas 77843 Received: February 22, 1994; In Final Form: May 24, 1994"

Carbon oxides (CO and CO,), which are generated as side products during the oxidative coupling of methane, can potentially be derived from CH4, C2H4, or C2H6. The source of COXwas determined by adding W 2 H 6 or 13C2H4to the reactants and analyzing the isotopic composition of the COX. In order to eliminate the extent of reaction as a factor, a parameter, R, has been defined as the ratio of the percentage of 13Cin the COXproducts to that in the feed gas. If the amount of COXderived from C2H6 is small, it can be shown that R is approximately the ratio of the rate constant for COXformation from C2H4 to that from CH4. Values of R have been obtained for several oxide catalysts by introducing measured amounts of 13C2H4into a CH4/02 reaction mixture and then determining the percentage of 13Cin the COXreaction product. Additional experiments, involving 13C2H6 addition to the reaction mixture, have established that the primary coupling product, C2H6, is selectively converted into C2H4; i.e., virtually no direct conversion of C2H6 into COXoccurs during the methane coupling reaction. For selective oxidative coupling catalysts, the experimentally determined values of R at 700 "C were 2.8 for Sr/LazO, and Ba/MgO and 4.7 for Li+/MgO. The value of R increased with increasing reaction temperature, along with the C2 selectivity and yield, indicating that the direct conversion of CH4 into COXis less important a t higher temperatures. At 850 OC, for example, the value of R for Ba/MgO was 6.2. It has been noted previously that, over a Li+/MgO catalyst, lower reaction temperatures favor the formation of COX from CH4. This occurs because of decreased CH4 conversion and, thus, lower partial pressures of C2H4, not because of a particularly small value of R . At 650 "C, in fact, the value of R for Li+/MgO was 3.4. Over a wide range of operating conditions, the rate constants for COXformation from C2H4 were several times as large as those for COXformation from CH4. This observation is consistent with the moderate C2 yields that have been achieved during the oxidative coupling of CH4, although large values of R may also indicate that CH4 is converted into C2H6 very selectively.

Introduction

In recent years, the oxidative coupling of methane has been extensively studied as a possible new method for more efficient use of abundant natural gas reserves. Considerable progress has been made in elucidating many aspects of the mechanism of the reaction, including the formation and reaction of methyl radicals, the effect of carbon oxides on the kinetic behavior of the catalysts, and the relative rates of the elementary reaction steps. However, the mechanism for the formation of COX(CO and CO2) has not been thoroughly investigated. The origin of carbon oxide products can be described by the simplified mechanistic scheme shown in Figure 1. In principle, carbon oxides may be formed either heterogeneously or homogeneously from any of the three principal hydrocarbons (viz., methane, ethane, or ethylene) that are present during the coupling reaction. Although the homogeneous oxidation of methane yields mainly carbon monoxide,carbon dioxide is the major nonselective product produced heterogeneously over most catalysts because good coupling catalysts generally are also effective for the conversion of CO to COz. Part of the difficulty in identifying the source(s) of carbon oxides is due to the fact that individual reaction rates of pure hydrocarbons over a given catalyst may be different from those observed for a mixture of two or three of the same hydrocarbons, particularly under oxygen-limiting conditions.' This is because each of the three hydrocarbons in Figure 1 may compete for the same set of active centers. Consequently, results obtained with individual hydrocarbons cannot reflect thesituation when all three hydrocarbons are present simultaneously. Current knowledge of the mechanism involved in generating carbon oxides is much more limited than that for the formation

* To whom correspondence should be addressed.

Abstract published in Advance ACS Abstracts, July 15, 1994.

0022-3654/94/2098-8371$04.50/0

Figure 1. Reaction scheme showing possible pathways for the formation of COXand other products.

of ethane via the methyl radical coupling reaction. Lin et al.2 proposed that an important source of carbon oxides is the homogeneous oxidation of methyl radicals through the formation and reaction of methyl peroxy radicals, CH3O2.. The latter are produced by the reaction of methyl radicals with gas phase molecular oxygen:

CH,'

+ 0,

CH302'

i-

(1)

Carbon oxides are then produced by subsequent reactions of CH302*in the gas phase:

-

CH302' 2CH3Q,'

-

C H 2 0 + OH'

CH,O

CH,O, CH,OH

+ CH,OH + 0,

-

CO, CO,

(2) (3) (4)

Nelson et u I . , ~on the other hand, proposed a mechanism involving surface reactions of CH3O2*, because their results were inconsistent with a mechanism consisting of purely gas phase reactions of CH3O2.. They speculated that CH3O2' radicals, once formed, 0 1994 American Chemical Society

8372 The Journal of Physical Chemisfry. Vol. 98, No. 34, 1994 would undergo an oxidation reaction on the catalyst surface to produce COX, particularly at lower temperatures. At higher temperatures, theequilibrium reaction 1 shifts totheleft,and the reaction of CHIOz' radicals becomes much less important. A kinetic model developed in our laboratory'also indicated that the gas phase reaction involving methyl peroxy radicals does not contribute significantly to the formation of C02. Another pathway for the formation of carbon oxides during the catalytic oxidative coupling of methane is the heterogeneous oxidation of methyl radicals on the surfaces of the metal oxide catalysts. With the assumption of a reasonable concentration of methyl radicals in the catalyst bed, a given methyl radical will collide with a surface approximately IO' times before it couples with another methyl radical. Obviously, if there is a high probabilitythatacollision with thesurfaceresultsin the formation of carbon oxides, the reaction of methyl radicals with a surface becomes a significant source of these undesirable products. Recently, Tong and Lunsford' investigated the manner in which methyl radicals react with metal oxide surfaces and found that the radicals react mainly by a reductive addition process, such as

-

CH,' -I M"" -I 02- M"+(OCHJ

(5)

to form surface methoxide ions. On transition metal oxides, the methoxide ions are readily oxidized to carboxylates and, ultimately, to C02. With a modified form of the MIESR system, thereactivestickingwefficients, ~(theprohabilitythatacollision with a surface results in a reaction), for the reaction of methyl radicals with several metal oxides weredetermined. It was found that the values of u for ZnO, MgO, and Li+/MgO are IO-', IO-', and IO-', respectively. Thus, only in the case of ZnO is it likely that an appreciable amount of CO, would be formed by the reaction of CHI' radicals with the oxide. In general, oxide catalysts that contain metal ions having more than one possible oxidation state react much more readily with CHI' radicals than do those containing only a single cationic oxidation state. One other possible source of carbon oxide formation is the secondary oxidation of the CZ products (ethane and ethylene) that are formedduring theoxidativecoupling reaction. Although these may be present in relatively low concentrations compared to that of methane, they are more reactive than methane under oxidativecouplingconditions. As discussed by Nelson and Cant.6 secondaryoxidation of Czproductscan occur bothon thesurface and in the gas phase. On the surface, ethane and ethylene may compete with methane for the active surface centers and produce CO. in a purely heterogeneous process. In the gas phase, ethane and ethylene can react with a multitude of gas phase radicals, such as CH3, OH, H02, efc., to produce intermediates which may, in turn, be oxidized to CO,. The most definitive experiments on the origin of COXhave been carried out using IT-labeled hydrocarbons. Ekstrom et al? were the first to use this method to demonstrate that the oxidationsofethane and ethylene were responsible for significant carbonoxide formation overa Smz03catalyst. By adding I3CzH6 and I3CzH4 to a CH,/Oz reaction mixture and observing the amount of "C in the COX products, they showed that the percentage of I3C in the COX products greatly exceeded the percentage of I3C in the feed gas. They reported, for example, that, over a SmZO3catalyst at 700 OC, the introduction of ca. 4% of 13CzH4into the 10%Oz/CHo feed resulted in COXproducts that contained 22% 13C. Inaddition,on the basisoftheobservation that slightly more 13COxwas present when W Z H 4was added to the feed than when the same amount of I3CzH6was added, they concluded that COXis formed primarily via secondary oxidation of the Cz products and that C2H4 is oxidized to CO, more readily than is CZH6. Nelson and Cant6carried out similar studies over Li+/MgO catalysts. From data obtained at T > 740 "C, they concluded that CZoxidation is responsible for the formation of

Shi et al.

(GCIMSI Loop selection valve

Gas storage

Vac. Pump

Buffer

Cirarlating pump

Flow Mode Figure 2. Diagram of the reactor system.

up to 80% of the CO,. However, at T < 700 OC, more than 90% of COXis derived from methane. Although the studies of Smz03by Ekstrom et a/? and of Li*/ MgO byNelsonandCant6aresignificant.a number ofimportant variables, such as the type of catalyst, the methane-to-oxygen reactant ratio, thenatureoftheoxidant,and theextentofmethane conversion, were not systematically explored. In order to gain a better understanding of the origin of carbon oxides in the oxidative coupling of methane, experiments using W-labeled ethane and ethylene as additives to the feed gas were carried out in our laboratory to extend the work of these two groups. The results obtained in this study significantlyaffect our interpretation concerning the origin of CO, during the oxidative coupling of CH4 at T < 700 OC over a Li+/MgO catalyst. Experimental Section The Li+/MgOcatalystused in theseexperiments was prepared by adding MgO (Aldrich, 98%) to an aqueous solution of LizCOl. The resulting material, which contained 4.1 wt % Li, was dried in air at 140 "C and then calcined in air at 750 OC for 16 h. TheZmol%Ba/MgOcatalysts waspreparedusingBa(N03)z (Matheson, ACS certified) and MgO (Fischer, ACS certified). An appropriateamount ofBa(NO,)z solutionwasaddeddropwise to a suspension of MgO in deionized water. The resulting slurry was stirred overnight at room temperature, evaporated todryness, and crushed into 2 N 2 mesh granules. The catalyst particles were then calcined in air at 800 OC for 1 h. The 1 wt % Sr/Laz03 catalyst, which was obtained from Ammo, was treated in flowing helium at 850 "C for 1 h before the reagent mixture was introduced. 13C2H6,13CzH4,and 1TH4wereobtainedfrom MSD Isotope and Isotec. The catalytic reaction system used in this study was capable of being switched between a flow mode and a recirculating mode, as shown in Figure 2. When the system was switched to the flow mode, it operated asaconventional flow reactor, with thereagent gas mixture passing through thecatalyst bed where thereactions took place. Analysis of the feed gas was achieved by placing two 3-way switching valves before and after the reactor, enabling the feed gas to bypass the reactor for analysis, while helium was simultaneously flowed through the catalyst bed. The effluent gas mixture was passed through a 3-way switching valve where it couldbedirectedeither throughthegaschromatographsample loop for analysis or to vent.

Origin of COXduring Oxidative Coupling of Methane The circulating mode of the reaction system was used primarily for the isotopic labeling experiments. In this mode, an isotopically labeled gas was first admitted into an isolated and previously evacuated introduction volume, with the pressure inside thevolume being measured by a pressure transducer. A Cole-Parmer Model 7530-40 gas circulating pump was then used to mix this gas with a previously prepared reagent gas mixture. To ensure complete mixing, gas mixtures were circulated several times before being diverted through the catalyst bed and the reaction was initiated in the circulating mode. The circulation rate was adjusted within the range 0-400 mL/min using two switching valves and a needle valve. Sampling of gases in the circulating system was achieved by opening a valve to introduce the mixture into a previously evacuated sample loop. Conversions were kept low (CH4 conversions [C2H4], (iii) [l2COX]>> [13COx], and (iv) the conversion is small so that the concentration of COXcan be approximated by the product of the initial rate of reaction and

The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8373

TABLE 1: Effect of the Amount of lJCzH4 Addition on the Value of R I3C2H4added, 13CzH4/ % of COX catalvst Torr 12C2Hd fromC2 R 4.1 wt % Li+/MgO' 0.07 0.50 1.7 6.9

~~~

1 wt % Sr/La203b

0.20 0.47 0.88 1.74 1.01 2.45 3.59

0.7 1 0.73 0.87 1.53 0.64 1.90 2.85

2.5 4.2 5.9 8.5 36.6 42.7 45.6

5.5 5.2 4.6 4.6 4.3 4.2 4.2

200 mg; T = 700 O C ; P(CH4) = 200 Torr, P(02) = 10 Torr, balance helium; CH4 conv. = 3-5%. 5 mg; T = 800 "C; P(CH4) = 200 Torr, P(02) = 10 Torr, balance helium; CH4 conv. = 9-10%.

the time of reaction. Thus,

An additional assumption is that the major pathway for ethane oxidation is uia ethylene, which means that kd is small compared to kb, k,, and k,. It is generally observed that over these oxidative coupling catalysts the selectivity for the conversion of C& to CzH4 is large.6 When the added I3C-labeled hydrocarbon is I3CzH4, a value of R = 0 would indicate that none of the COXcame from CzH4, while a value of R > 1 would indicate that COXwas formed faster from C2H4 than from CH4 and C2H6 combined. However, when the added I3C-labeled hydrocarbon is 13C2H6,a value of R = 0 would indicate that all of the COXcame from CH4, and a value of R > 1 would indicate that COX was formed from Cz hydrocarbons faster than from CH4. Since the natural abundance of 13C in ethylene is ca. 2.2%,the amount of 13CzH4added to the system must be larger than this value in order to obtain reliable measurements of R. It is evident from the results shown in Table 1 that when the amount of 13C2H4 added is 20.9 Torr and I3C2H4/l2CzH4 2 0.9, R becomes independent of the initial pressure of l3CzH4, while the percentage of COXfrom CZoxidation continues to increase. Here, I3CzH4/ 'ZCzH4 refers to the ratio of labeled-to-unlabeled ethylene in the product. It should be noted that the percentage of COXfrom Cz oxidation reported in Table 1 for Li+/MgO is several times smaller than that for Sr/Laz03, while the R values for these two catalysts are approximately the same. This is because the data for Sr/ La203 wereobtainedat much higher conversionlevels, uiz., 9-lo%, as compared to 3-5% for the Li+/MgO. At higher conversions, the Cz concentration in the system is higher and, as a result, more COXis formed from C2. The R value, however, does not change significantly because it is the ratio of two rate constants. Sample calculations for the entries in the tables are given in the appendix. Influence of Surface Carbonate Pool. Over the Li+/MgO catalyst, we observed that the percentage of "CO from l3C2 oxidation was always higher than that of W O z while, over other catalysts, the percentages of I3COand W O zfrom I T z oxidation were approximately the same. This can be explained by the fact that, in the temperature range studied, an exchangeable pool of carbonate species exists on the Li+/MgO surface, due to its basic nature. The I3CO2exchanges with this pool and releases I2C02 into the gas phase. Although the Ba/MgO is also highly basic,

8374 The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 TABLE 2 Effect of Methane Conversion on Value of R over 1 wt 7% Sr/La203 Catalyst' circulation time, min CH4 conv., 5% C2 select., 9% % of COXfrom C2 R 3 6 9 12 25 57

2.8 3.7 4.4 4.9 7.3 11.0

43.2 41.4 39.7 39.0 37.8 35.4

11.2 18.4 19.4 20.6 25.9 34.6

3.2 3.2 3.2 3.2 3.3 3.3

Shi et al.

TABLE 3 Dependence of R on Reaction Temperature over 4.1 wt 5% Li+/MgO Catalyst' temp., CH4, W2H4 added, 13C2H4/ 5% of 13C % of COX OC conv.,% Torr 12C2H4 infeed fromC2 R 650 615 100 725 150 (I

5 mg; T = 725 OC; P(CH4) = 200 Torr; CH4/O2 = lO/l; I3C2H4 added = 3.5 Torr.

the reaction temperature range employed with this catalyst was considerably higher than that used for Li+/MgO (vide infra). Consequently, the steady-state surface carbonate pool, if any, was too small to effect significant exchange with gas phase 13C02. The size of the exchangeable pool on the Li+/MgO surface was determined by passing a known mixture of 12C02/13C02 over the catalyst and monitoring the 13C content of the C02 as a function of circulating time. The results showed that the exchange reached its equilibrium within a very short time over this catalyst. On the basis of these experiments, the size of the exchangeable carbonate pool on a 220 mg sample of 4.1 wt % Li+/MgO catalyst surface was determined to be 0.5-1 .O mg in the temperature range 620-720 OC. This value corresponds to only 3% of that expected if all the Li in the catalyst were present as Li2CO3. The effect of the exchangeable carbonate pool was taken into account in calculating the percentage of COXfrom C2 and the values of R. An equilibrium distribution coefficient, K , was obtained at a particular temperature by circulating a mixture of 13CO2/12CO2,similar in comparison to that of an actual catalytic methane oxidative reaction, over a catalyst that initially contained only I2C carbonate ions. The distribution function is the ratio of the exchangeable I3CO2to the final W02in the gas phase. For example, a mixture of T O 2 and 12C02 (0.46and 2.51 Torr, respectively) was circulated over 200 mg of 4.1 wt % Li+/MgO at 650 O C until equilibrium was reached. Analysis of the gas phase showed that the equilibrium mixture consisted of 2.68 Torr I2CO2and 0.30Torr I3CO2. Thus, over this catalyst K = 0.53. Subsequently, a typical oxidative coupling experiment was carried out over a fresh 4.1 wt % Li+/MgO catalyst, with W2H6added to the reagents. The products contained 0.24 Torr 13C02and 2.43 Torr WOz. It was desirable to calculate, using the value of K , a pressure of 13C02that had been corrected for the loss of W02into the catalyst as carbonate ions. In this particular case, the corrected W02pressure was 0.37 Torr (i.e., 0.24 + 0.24 K ) . The corrected values for W02formation from I T 2 were within flO%ofthosefor W O . Theobservation that thel3Ccomposition of CO and C02 was essentially the same is consistent with the view that CO is a primary reaction product and is oxidized further to C02. It is highly unlikely, for example, that C02 is derived from the heterogeneous oxidation of CHI and that C O arises from the homogeneous oxidation of C2H4. At higher temperatures and over less basic catalysts, the amount of W02exchanged was small compared to the total amount of C02 in the system. The corrections amounted to only a few percent of the 13C02;therefore they generally were omitted. Effect of CH4 Conversion on R. The effect of methane conversion on the value of R was investigated over a Sr/La203 catalyst at 725 OC in therecirculating batch reactor. Themethane conversion was increased by increasing the circulating time. The results, reported in Table 2, again show that the value of R is essentially invariant with methane conversion. This is to be expected because R is the ratio of two rate constants. The value of R reported here (ca. 3.3) is less than the value of 4.2 reported in Table 1 for Sr/La203 because of the difference in reaction temperature (vide infra).

1.9 2.5 5.5 1.8 14.7

1.27 1.02 1.42 1.19 1.31

29.9 18.0 4.2 1.5 0.3

1.4 1.2 1.4 1.4 1.5

6.0 5.8 8.6 10.9 28.3

3.4 3.1 4.2 4.0 3.9

200 mg; P(CH4) = Torr, P(02) = 100 Torr, balance helium.

TABLE 4 Effect of C&/O2 Ratio on Origin of C02 over 4.1 wt % Li+/MgO. CH4/02 CH4 C2 select., I3C2H4 I3C2H4/ W of COX ratio conv., % % added, Torr 12C2H4 from C2 R 2 5 10 a

8.8 7.9 6.7

56.8 70.2 81.6

1.43 1.96 2.11

1.5 1.3 1.2

12.2 19.4 22.5

4.0 4.5 4.1

200 mg; T = 700 OC; P(CH4) = 200 Torr.

Effect of Reaction Temperature on R. One of the most important influences on the origin of COXduring CH4 coupling is that of reaction temperature. As discussed earlier, Nelson and Cant6 reported that, over Li+/MgO catalysts, the principal origin of COXat lower temperatures ( T < 680 "C) was the direct oxidation of CH4, while at higher temperatures ( T > 740 "C) oxidation of CZhydrocarbons became the major source of COX. We carried out similar experiments over Li+/MgO in the temperature range 650-750 O C using a methane-to-oxygen ratio of 2,and the results are reported in Table 3. As shown in eq 8, the value of R is the ratio of the rate constants for COXformation from C2H4 relative to that from CH4. The results indicate that, throughout the temperature range studied, the rate constant for COXformation from C2 was always several times larger than that from CH4 and, moreover, increased slightly with increasing temperature, changing from 3.4 a t 650 OC to 3.9 a t 750 OC. Of course, since R is a ratio of rate constants, its value does not indicate the actual reaction rates, which will depend on the partial pressures of the hydrocarbons. At 700 OC,for example, with an initial CHI partial pressure of 200 Torr and with 1.42 Torr of added 13C2H4,ca. 8.6% of the COXwas derived from CzH4. From 650 to 750 OC, the percentage of COXderived from C2 oxidation increased almost 5-fold (from 6% to 28%), which is in agreement with the results of Nelson and Cant.6 This increase in R with increasing temperature was even more pronounced with Ba/MgO and Sr/La203, as will be discussed below. Effect of CH4/02 Ratio and Nature of Oxidant on R. Since the active site responsible for methane oxidation is believed to be associated with a surface oxygen species, the methane-to-oxygen reactant ratio is also an important quantity because it changes the availability of oxygen. In addition, the extent of gas phase reaction is also strongly influenced by the availability of 0 2 . The effect of the methane-to-oxygen ratio on the origin of CO2 was investigated over Li+/MgO, by maintaining a CH4 partial pressure of 200 Torr and varying the pressure of 0 2 . Since R is a ratio of rate constants and is, therefore, a function of temperature, the experiments were carried out a t a fixed temperature of 700 O C . It is evident from the results in Table 4 that R increases with increasing methane-to-oxygen ratio. This may indicate that nonselective oxygen-containing sites, possibly 03-ions, formed on the catalyst surface via

0,+ 0-a 0,-

(9)

oxidize CH3' radicals, or that heterogeneous conversion of CH3O2*,formed via reaction 1,occurs. As the methane-to-oxygen ratio increases, this direct pathway for COXformation from CH4 becomes less significant. An alternative explanation is that at high methane-to-oxygen ratios less C02 is produced and, hence,

Origin of COXduring Oxidative Coupling of Methane

The Journal of Physical Chemistry, Vol. 98, No. 34, 1994 8375

TABLE 5 Effect of Oxidant on Origin of COXover 4.1 wt % Li+/MgO. temp., CHI C2 select., I3C2H4 % of COX oxidant OC cow.,% % added,Torr fromC2 R 4.4 47.2 5.8 NzOb 650 4.8 93.1 02'

615 650 700

8.1 4.4 7.2

89.8 11.5 87.3

4.8 3.1 3.5

69.1 18.4 26.2

6.4 3.9 4.5

250 mg; circulating time = 12 min. P(CH4) = 150 Torr, P(N20) = 600 Torr. CP(CH4)= 150 Torr, P(O2) = 15 Torr, balance helium.

TABLE 6

Origin of COXover Various Catalysts' temp., CHI C2 select., I3C2H4 % of COX OC conv.,% % added,Torr fromC2 R

catalyst Li+/MgOb 100 Ba/MgOc 700

750 800 850

Sr/LaZOf

100

750 800 800

6.7 0.7 2.4 4.3 9.7 2.6 6.1 9.4 10.5

70.1 31.2 55.5 71.1 14.7 33.3 43.1 50.8 3.8

2.2 4.7 4.2 4.4 4.9 4.1 3.5 3.9 1.1

22.5 15.6 26.4 41.2 90.8 16.1 24.9 40.6 7.8

4.1 2.8 4.3 5.1 6.2 2.8 3.4 4.0 7.1

CeO# P(CH4) = 200 Torr; CH4/02 = 10;balance helium. 200 mg. 5 mg. 100 mg, diluted to 1 cm3 with quartz chips.

more of the I3C2H4reacts, resulting in larger R valuesU8Increased gas-phase oxidation of C2H4, on the other hand, would have an effect on R that is opposite to that observed here. In effect, high methane-to-oxygen ratios can be achieved by using N 2 0 as the oxidant, since very little molecular 0 2 is then available to react, for example, according to reaction 6. Comparative experiments were performed with 0 2 and N 2 0 as oxidants, using WzH4 as the labeled hydiocarbon additive. The results in Table 5 show that, at temperatures below 700 OC, the value of R using N20 is much larger than that when 0 2 is used. Additionally, at approximately the same level of CHI conversion, but with N 2 0 as the oxidant, a higher C2 selectivity was attained, which may also be related to the inhibition of reaction 6.9 It should be noted that the increase in R observed with increasing methane-to-oxygen ratio and with the use of N2O as the oxidant may be caused more by a decrease in the rate of CH4 conversion than by an increase in that of C2H4. Comparison of the Origin of COXover Various Catalysts. In addition to the case for Li+/MgO, the origin of COX was investigated over three other metal oxide catalysts, viz., Ba/ MgO, Sr/Laz03, and Ce02, and the results are reported in Table 6. A significant difference in behavior was observed between Ba/MgO and Sr/LazO3 catalysts at 800 "C, in that the ratio R for the Ba/MgO catalyst was significantly larger. This may be due to the existence of a nonselective center on the surface of the Sr/La203 catalyst which reacts with CH3' or CH4 to form COX. Over the Ba/MgO catalysts, such a center is either absent or of low concentration. It is also consistent with the fact that the C2 selectivity with Sr/LazO3 is lower than with Ba/MgO. It is evident from the results shown in Table 6 that the R value for Li+/MgO at 700 OC is the largest among the three catalysts studied. With both Ba/MgO and Sr/La203, the value of R increases significantly with increasing temperature. At >700 OC, the homogeneous oxidation of C2H6 becomes important, even in a packed bed.1° Since C2H4 is more readily oxidized than is C2H6 in the gas phase, the increasing R values at T > 700 OC may be attributed to an increasing occurrence of homogeneous C2H4 oxidation. Although one might expect that with increasing R values the C2 yield over a given catalyst would decrease correspondingly, the relationship between R and the Cz yield is a direct one. In principle, it should be possible to minimize the nonselective homogeneous reactions by using catalysts that function well at

TABLE 7: Behavior of Ethane as Labeled Reactant over 4.1 wt % Li+/MgOa temp., CH4 conv., I3C2Hs % of I3C % of COX OC % added,Torr in feed fromC9 R ~~

650 700 750

3.1 9.5 14.8

0.44 0.55 0.63

1.0 1.6 1.7 200 mg; P(CH4) = 200 Torr, P(O2) = 20 Torr, balance helium. 0.74 0.82 0.91

9.1 21.1 40.9

T < 700 "C, such as Li+/MgO. However, none of the currently known catalysts that operate at lower temperatures give superior C2 yields. From the large values of R obtained over the Li+/ MgO catalyst at T I 700 OC (Table 3), it appears that heterogeneous reactions also convert CzH4 to COX. The exceptionally large value of R (7.7) observed with Ce02, which is a complete oxidation catalyst, must be interpreted with care. It is a reflection of the fact that nearly all of the added 13C2H4is oxidized to COX,rather than an indication of the relative rate constants for COXformation. Under these conditions, the average rates were no longer meaningful, since the oxygen conversion was 100%. In such cases, the value of R only indicates the extent of I3C enrichment in the COXproducts. Under the experimental conditions employed, only ca. 7.8%of the COXcame from C2 oxidation. Use of *3C& Additive. Experiments involving 13CzH4addition are more straightforward to interpret because 13C2H4is the only source of WO,. Interpretation of results from I3C2H6addition are complicated by the fact that the resulting I3CO, can arise from both I3C2H6and I3C2H4, since oxidative dehydrogenation of W2H6 occurs simultaneously with the methane coupling reaction. Nevertheless, one set of data was obtained using 13Clabeled ethane, and the results are summarized in Table 7 . When I3C2H6was used as the labeling compound, the percentage of COXfrom C2 oxidation at 700 OC was similar to that reported in Table 6, but the value of R was significantly smaller than that observed with W2H4 as the additive. The latter is due to the fact that only a fraction of the added l3C2H6was oxidized to W2H4 and then to WO,. This smaller value of R observed with the T 2 H 6additive is also consistent with the view that subsequent reaction of the C& formed as a primary product of theoxidative coupling of CHI, is primarily via oxidative dehydrogenation to C2H4, rather than direct oxidation to COX.

Conclusion The results of an investigation on the origin of carbon oxides formed during the oxidative coupling of methane over various catalysts showed that, under most conditions, the rate constant for COXformation from subsequent C2 oxidation is several times larger than that of COXformation directly from CH4. This is true even over Li+/MgO catalysts at reaction temperatures