11524
J. Phys. Chem. 1993,97, 11524-1 1529
Transient Isotropic Labeling Using 1 6 0 2 / 1 8 0 2 over Alkali-Metal-PromotedMolybdate Catalysts in Oxidative Coupling of Methane Sharon A. Driscoll and Umit S . Ozkan* Department of Chemical Engineering, The Ohio State University, Columbus, Ohio 43210 Received: June 21. 1993; In Final Form: August 16, 1993"
The methane coupling reaction was investigated over MnMo04 and alkali-metal (Li, N a , K)-promoted MnMo04 a t 700 OC using isotopically labeled oxygen under both methane-free and oxygen/methane cofeed reaction conditions. The addition of K to MnMoO4 has previously been found to increase the selectivity to C1 hydrocarbons under methane coupling conditions.' Results of the isotopic labeling studies performed under steady-state conditions indicated that the interaction of the catalyst surfaces with gas-phase oxygen and the mobility of subsurface lattice oxygen may play a significant role in determining the selectivity to Ca hydrocarbons.
Introduction The study of various metal oxides and alkali-metal-promoted metal oxide catalysts has received much interest in recent years after the earlier reports of ethylene synthesis through oxidative coupling of methane2 and of achieving a high selectivity over a Li/MgO catalyst under methane and oxygen cofeed condition^.^ Several oxide catalysts containing promoter ions have been studied to determine the effect of the promoter ion on catalytic activity and selectivity.&16 Alkali-metal-promoters have been found to behave differently in different systems, affecting both the activity and selectivity of the promoted catalysts compared to the pure oxide. Isotopic labeling of the feed gas has also been used recently in order to investigate the role of lattice oxygen and the residence time of carbon species for various oxidative coupling catalysts. One isotopic labeling technique involves a switch from an unlabeled oxygen or methane source to a labeled one under steady-state conditions.17-19 Information is obtained by following the transient changes in the amounts of unlabeled and labeled reactants and products by mass spectrometry. Steady-state conditions are maintained using this method, so the total amount of any one product will not change due to the isotopic switch. The relative intensity of the signals corresponding to the molecular weights of the unlabeled and labeled compounds can thus be used to obtain their isotopic concentration in the gas phase. Our work has recently focused on the use of alkali-metal promoters for a simple molybdate catalyst, MnMo04, for the oxidative coupling reaction. Previous studies using molybdates dealt with unpromoted forms of these catalysts. A study ofvarious unpromoted molybdates (Na, Li, K, Mg, Ba, Mn, Co, Fe, Cu, Zn, and Ni) by Kiwi et a1.20321 showed that with the exception of NiMoO4, the molybdate catalysts were stable for long periods of time under the reaction conditions required for oxidative coupling. At a methane conversion level of about 60%, the selectivity of molybdate catalysts to C2 hydrocarbons ranged from 9.8% to 16.6%. Mn and K molybdates were found to be the least selective catalysts. Another molybdate, PbMoO,, was studied by Baerns et a1.22with 19% selectivity to C2 hydrocarbons and an 11.4% selectivity to formaldehyde at 1% conversion. In our studies, we have found that the addition of alkali-metal promoters to MnMoOl catalyst changes the product distribution in favor of partial oxidative coupling products, with the addition of potassium producing the most significant increase in C2 selectivity.' In this paper we have focused on experiments using an isotopic transient technique with labeled oxygen to help explain To whom correspondence should be addressed. Phone: 614-292-6623. FAX: 614-292-3769. E-mail:
[email protected]. Abstract published in Advance ACS Abstracts, October 1, 1993. @
0022-3654/93/2097- 11524$04.00/0
the behavior of the promoted catalysts compared to pure MnMo04 in the methane coupling reaction.
Experimental Section Simple molybdate catalysts were prepared by a precipitation reaction as outlined previously.23 Alkali-metal-promoted catalysts containing lithium, sodium, or potassium were prepared through wet impregnation of the molybdate with the alkali-metal carbonate followed by drying in an oven overnight to drive off the water. The pure manganese molybdate and the alkali-metal-promoted catalystswerecalcinedinoxygenfor 4 hat 800 OC. Thesecatalysts were characterized through a number of techniques, including BET surface area measurements using krypton (Micromeritics 2100E Accusorb), X-ray diffraction (Scintag PAD V diffractometer with Cu K a radiation), X-ray photoelectron spectroscopy (Physical Electronics/Perkin-Elmer, Model 550), laser Raman spectroscopy (Spex 1403 and Triplemate 1877),and temperatureprogrammed studies. A quartz fixed-bed reactor with 9-mm 0.d. and 5-mm i.d. was used for the catalytic reaction experiments. The diameter was reduced to 2 mm at the end of the catalyst bed to allow rapid exiting of the gas stream. The isothermal portion of the quartz tube was determined to be 20 mm. The catalyst bed length ranged from 8 to 11 mm, with a quartz wool plug inserted to hold the bed in place. The total surface area of the catalyst was kept constant at 0.1 m2. Blank studies using an empty reactor or a reactor filled with quartz chips revealed minor conversion under reaction conditions, mainly to ethane and formaldehyde. The reaction feed gas consisted of methane, oxygen, and nitrogen or helium (ratio = 2:1:3) with a total flow rate of 9.3 cm3 (STP)/ min. The feed gas composition was maintained using mass flow controllers (Tylan), and the reaction gas composition was continuously monitored during the isotopic experiments by a quadrupole mass spectrometer ( H P 5989A M S engine). The isotopic experiments utilized a four-port Valco valve in the gas flow stream, as shown in Figure 1, to switch the oxygen feed gas source from l602/He (Matheson) to an I802/He (Icon, 99 atom %pure 1 8 0 ) . These oxygen-exchange experiments were performed in both the presence and absence of methane in the feed gas. The overall flow rate remained the same for both methane and methane-free experiments through the substitution of additional helium. The product stream composition was determined using an automated HP 5890 gas chromatograph. CH4, C02, CO, N2, 02, HCHO, and Cz hydrocarbons were separated and analyzed by a thermal conductivity detector using an Haysep T column connected to a molecular sieve 5A column through an isolation 0 1993 American Chemical Society
Transient Isotopic Labeling
The Journal of Physical Chemistry, Vol. 97, No. 44, 1993 11525 1 .o
0.8 Nitrogen
Thermoconlroller
0.6
Methane
0
Li/MnMo04
0
Na/MnMo04
0.4
TO Vent
0,Melium Helium
Figure 1. Schematicof the experimentalsystem used in isotopic labeling
studies. valve. Hydrocarbons were also separated and analyzed through a second Haysep T column connected to a flame ionization detector. The isotopic labeling studies included experiments to measure the residence time of an inert gas in the system, also known as the gas-phase holdup. The gas-phase holdup was found by switching from argon to oxygen and following the argon decay. To determine if there is a significant homogeneous mechanism for "mixing" of gas-phase 1 6 0 2 and 1 8 0 2 to form cross-labeled oxygen, 160180, a "blank" experiment was performed in which the isotopic switch from '602/He to I802/He was made over quartz wool under the same conditions used for examining the catalysts. The mass spectrometer signals corresponding to the unlabeled and labeled species for each compound were normalized to produce isotopic concentrations so that the sum of all possible combinations was unity. For example with oxygen this would correspond to the signals for m / e = 32 (1602), m / e = 34 (160180), and m / e = 36 (1802). In addition, the normalized transients for the experiments in which methane was included in the feed were then multiplied by the corresponding flow rates to produce the isotopic yield. The oxygen readily available for exchange from the catalyst surface can be obtained from the normalized transient oxygen exchange curves. This is achieved by integrating the total 1 6 0 content, which is then corrected for gas-phase holdup and for bulk diffusion and multiplied by the oxygen flow rate. The bulk contribution is defined as the continued offset in the l60 content of the exiting stream after "pseudo-steady-state" is reached, and it refers to the continued replenishment of surface/subsurface oxygen by diffusion from the catalyst lattice.
0.0
0.4
0.8
0.6
1.0
Time (min)
1 .o
0.8
0.6 --b
160180
0.4
0.2
0.0 0.0
0.2
0.4
3
0.8
0.6
Time (min)
Figure 2. Normalized transients for argon decay and "blank" oxygen
exchange. 6
0
1
2
3
4
5
Time (min)
Time (min)
ReSults
Catalyst Characterization. As has been previously reported,' the addition of the alkali-metal-promoter ions to manganese molybdate resulted in a decrease in the catalyst surface area. N o change was found between fresh and spent catalysts with respect to the binding energies of Mn and Mo through X-ray photoelectron spectroscopy. Sodium and potassium were present in detectable quantities on both fresh and spent catalysts, but the low sensitivity of the technique to lithium prevented its detection. Laser Raman spectra revealed no major changes in the molybdate structure of the promoted catalysts compared to pure MnMo04. The effect of the promoter on the X-ray diffraction patterns compared to the pure MnMo04 revealed mainly changes in the relative intensities of the pattern, with no new phases observed. The X-ray diffraction patterns of the spent catalyst samples did not show any changes compared to those of the fresh catalysts. Oxygen-Exchange Transients. The gas-phase holdup time in the system and the homogeneous reaction contribution to the oxygen exchange are shown in Figure 2. The gas-phase holdup time was found to be the same for each catalyst, as shown by the overlap of the data. The residence time for oxygen in the absence of a catalyst is shown to be the same as that of the inert gas, with
0.2
1.o
0.8
0.8
0.6
0.6 0.4
0.2 00
Time (min)
.
.
.
.
.
.
0
1
2
3
4
5
Time (min)
Figure 3. Normalized oxygen isotope concentrations without methane.
the remaining signals from l a 0 2 and l60'*0 representing the impurity level of the feed gas oxygen. The oxygen-exchange transients in the absence of methane are plotted out to 5 min in Figure 3. It is seen that almost no crosslabeled oxgyen ('80160) is formed over pure MnMo04 catalyst, and only small quantities are formed over Li- and Na-promoted
11526 The Journal of Physical Chemistry, Vol. 97, No. 44, 1993
Driscoll and Ozkan
TABLE I: '60 Flux into the Cas Phase in the Absence of Methane for Unpromoted and Alkali-Promoted Manganese Molybdate Catalysts pseudo-steady-state total 1 6 0 incorporation total surface/subsurface 1 6 0 flux at t = 1 min l60flux at t = 14 min into the gas phase l6O incorporation into the gas phase over 14 min (atomslmz) catalyst (atoms/mZ min) (atoms/mZ min) over 14 min (atoms/mZ) 5.1 x 1019 3.5 x 1019 MnMo04 5.6 X 1V0 1.1 x lozo Li/MnMoO, 7.2 x 1019 5.5 x 1019 8.3 X lozo 1.3 X lom 10.9 x 10'9 5.9 x 1019 Na/MnMoO4 10.8 X lozo 3.1 X 1020 23.2 x 1019 5.1 x 1019 K/MnMo04 12.9 X 1Po 6.3 X lom a Corrected for pseudo-steady-statecontribution. catalysts. However, the I 6 0 2 signal for these catalysts does not decrease to the concentration seen for the blank run but maintains a steady value greater than the impurity level. Over the K-promoted catalyst, however, the formation of the cross-labeled oxygen is very pronounced, and while the decay of the l 6 0 2 signal is slow, it falls below that of the other catalysts by the time a pseudo-steady-state is reached. The rates of oxygen exchange over the four catalysts are summarized in Table I. The flux of l 6 0 (atoms/m2 min) from the catalyst surface was greatest for the K-promoted catalyst, with a rate 4.5 times that of pure MnMo04 catalyst 1 min after the isotopic switch. But the sharpest decrease in the flux of ' 6 0 was also over the K/MnMoO4 catalyst, and by the 14th minute, the flux of I6O from the K-promoted catalyst decreased to a pseudo-steady-state value less than values for the Li- and Napromoted catalysts. The pure MnMo04 catalyst continued to show the lowest flux of 1 6 0 throughout the experiment. The total amount of l60incorporated into the gas phase, and the amount of surface/subsurface oxygen exchanged over the 14min time period were also calculated and listed in Table I. The K-promoted catalyst was able to incorporate the greatest amount of 1 6 0 into the gas phase, both overall and in terms of surface/ subsurface oxygen, while the lowest totals were again found for the pure MnMo04 catalyst. Transients of Reaction Product Stream. Under the conditions used for the transient isotopic labeling studies, the percent conversion of methane over the catalysts was 3.6, 4.2, 7.9, and 9.6 for pure MnMo04and the K-, Li-, and Na-promoted MnMoO, catalysts, respectively. The quantified products included carbon oxides, ethane, ethylene, and formaldehyde, with the calculated production of water based on the relative yields of the other products. Under the experimental conditions used in these runs, the K-promoted molybdate was again the most selective catalyst for CZ formation, exhibiting the same trend that was observed previously under different experimental conditions.' Similarly, the unpromoted MnMo04 had the lowest selectivity for CZ production. Formaldehyde was detected during the product gas analysis, but during the 15 min the transients were followed, essentially all of the oxygen incororated into HCHO was found to be 1 6 0 . We found the changes in the main ion fragment for formaldehyde ( m l e = 29) to be insignificant, and there was no production of the labeled ion fragment ( m / e = 31). Therefore formaldehyde isotope transients are not presented. The oxygen transients obtained under reaction conditions are shown in Figure 4. Again, very little 1 6 0 1 8 0 is formed over the MnMoO, catalyst, with only small amounts formed over the Liand Na-promoted catalysts. However, the K-promoted catalyst is seen to exhibit the same behavior as was seen under methanefree conditions, with cross-labeled oxygen constituting a significant portion of the total oxygen at the exit, The offset in the 1 6 0 2 signal under reaction conditions decreased from that observed in the oxygen exchange experiments in the absence of methane for all catalysts except K/MnMoOd. The carbon dioxide transient curves are presented in Figure 5 . All four catalysts show a continued contribution of lattice oxygen in the formation of carbon dioxide. The Na- and Lipromoted catalysts produce similar quantities of carbon dioxide,
0.8
0.6 0.4 0.2
0.0 0 Time
1
2
(mm)
3
4
5
Time (min)
1 .o
1 .o
0.8
0.8
0.6
I
Na/MnMo04
0.6-/!
0.4 0.2
0.2
0.0
0.0 0
1
2
-
3
4
0
5
1
2
Time (min)
I
1602
3
4
5
Time (min)
....A....
1618 0
--o--
0
lao 2
1
Figure 4. Normalized oxygen isotope concentrations during reaction.
0
5
10
1s
0
5
I
0
5
10
10
15
Time (min)
Time (min)
15
h
0
Time (min)
1
5
10
15
Time (min)
c160180
--0--
c184
Figure 5. Carbon dioxide isotope yields during reaction.
and rapidly approach a point where 50% of the COz formed is cross-labeled, C 1 6 0 1 8 0 . This is followed by a slow decline of C160180in favor of C1*02formation. The K-promoted catalyst also quickly reaches approximately 50% C160180 formation, but even though the amount of carbon dioxide produced is lower than either the Li- or Na-promoted catalyst, the K-promoted catalyst
The Journal of Physical Chemistry, Vol. 97, No. 44, 1993 11527
Transient Isotopic Labeling
B E
!$ i!d
0
5
10 Time (min)
15
0
5
10
15
Time (min) Time (min)
Time (min)
Time (min)
Figure 6. Carbon monoxide isotope yields during reaction.
exhibits a much faster rise in C1802 production. The pure MnMo04 catalyst produced the least amount of carbon dioxide and continued to use mainly I 6 0 in its formation, with very little C1802 formed even after 15 min of reaction time. The transient curves for carbon monoxide isotopes are presented in Figure 6. The K/MnMo04 catalyst shows the fastest decline in unlabeled C O formation, with the l6O incorporation rate approaching zero around 5 min. This is especially noteworthy, since thiscatalyst has thelowest COformationrateofthecatalysts studied. The second fastest decline in unlabeled CO formation rate is observed over Na/MnMo04 catalyst. The pure MnMo04 and Li/MnMoO4 show a much slower decline in the amount of 1 6 0 incorporation into CO. The transient curves for water isotopes are presented in Figure 7 . As seen from the plots, both the Li- and Na-promoted catalysts formed only minor amounts of H2180, even though they were the largest producers ofwater. As with the other products, the signal for labeled water rose much more rapidly over the K/MnMoO4 catalyst. The formation of labeled water over MnMo04 catalyst was significantly higher than those observed over the Li- and Na-promoted catalysts, however about 70%of all water formed over MnMo04 16 min after the switch was still unlabeled. The fluxes of 1 6 0 into the gas phase in the form of molecular oxygen or as part of the reaction products after 1 and 14 min past the isotopic switch are compared in Table I1 for all four catalysts. The most significant decline in l6Ocontent of the product stream occurred over the K/MnMo04 catalyst, with minimal flux decreases for the other catalysts. The total amount of l6O incorporation is also presented in Table 11, the order of decrease being Na > Li > K > MnMo04. The percent l 6 0 content of each individual component in the product stream is given in Tables I11 and IV for the 1st and 14th minutes, respectively. Also included are the total percentages of l 6 0 in the product stream with and without molecular oxygen. The widest distribution of values was seen in the 1 6 0 contents of CO and COS, with the largest value for MnMo04 and the smallest value for the K/MnMoO4 in each case. The Li/MnMoOd catalyst shows the highest percentage of 1 6 0 incorporation into theoxygen containing reaction products, followed by similar values for Na-promoted and unpromoted MnMoO4 catalysts. When molecular oxygen is included, however, the percent of unlabeled oxygen in the product stream is highest
Time (min)
Time (min)
Time (min)
Figure 7. Water isotope yields during reaction.
for the K/MnMo04 catalyst, and all of the promoted catalysts are seen to incorporate significantly greater amounts of unlabeled oxygen than pure MnMo04. The percentages at 14 min after the isotopic switch show the significant decrease in unlabeled oxygen incorporation for the K/MnMo04 catalyst. It continues to incorporate the largest amount of l 6 0 into molecular oxygen but has the smallest 1 6 0 content in the products.
Discussion The studies performed over promoted manganese molybdate catalysts have shown significant changes in catalytic behavior due to the presence of the promoter. The preliminary results suggest that the pronounced differences observed in selectivity and activity may be related to the effect of the promoter cations on the reactivity of the lattice oxygen and the type of adsorbed oxygen. The isotopic exchange results revealed a much different behavior for the potassium-promoted catalyst as compared to the pure MnMo04 or the Na- or Li-promoted MnMo04, especially in the formation of the cross-labeled oxygen, 1 6 0 l 8 0 . As with the "blank" reaction, only a small amount of cross-labeled oxygen was formed over the unpromoted MnMoO,, or the Na- and Lipromoted MnMo04 catalysts, even during the gas-phase holdup time whensignificant quantitiesof both I 6 0 2 and l 8 0 2 were present in the reactor. There were, however, significant amounts of crosslabeled oxygen formed over,K/MnMo04. The l 6 0 content of molecular oxygen for K/MnMo04 remained higher than the other three catalysts, in both methane-free oxygen-exchange experiments, and steady-state reaction experiments when methane was present in the feed. Our oxygen-exchange results for unpromoted MnMo04 are in goad agreement with oxygen-exchangeresults previously obtained by Klissurskiunder static conditions.24-25 Thelackof cross-labeled oxygen formation suggests two main possibilities: either there is no significant exchange taking place, or the mechanism involves one oxygen molecule from the gas-phase exchanging with two oxygen atoms from thelattice. Thelatter mechanism was reported by Klissurski and co-workers as the main mechanism of oxygen exchange in MnMo04 and, in our case, is supported by the large offset in the m / e = 32 signal. Molybdates have also been found to show a high rate of oxygen self-diffusion, with little difference
11528 The Journal of Physical Chemistry, Vol. 97, No. 44, 1993
Driscoll and Ozkan
TABLE Ik '60 Flux into Product Stream under Reaction Conditions for Unpromoted and Alkali-Metal-Promoted Manganese Molybdate Catalysts pseudo-steady-state total 1 6 0 incorporation total "surface/subsurfacc" 1 6 0 flux at t = 1 min l 6 0 flux at f = 14 min into the gas phase l 6 0 incorporation into the gas (atoms/m2 min) over 14 min (atoms/m2) phase over 14 min (atoms/m')@ catalyst (atoms/m2 min) 8.1 x 1019 MnMoO4 10.1 x 1019 11.5 X lozo 0.58 X 1020 17.6 x 1019 Li/MnMoO4 20.9 x 1019 24.4 X 1020 0.49 X 1020 Na/MnMo04 25.6 x 1019 18.6 x 1019 26.1 X lozo 0.83 X lom K/MnMo04 27.2 x 1019 6.5 x 1019 15.0 X 1020 6.16 X 1020 Corrected for pseudo-steady-statecontribution. TABLE II1: Percentage of '60 Incorporated into Products Stream at the 1st Minute total products + catalyst 0 2 C02 CO H20 HCHO products oxygen MnMoO4 3.2 86 86 82 100 85 12 91 25 Li/MnMoOd 3.7 74 81 98 100 Na/MnMoO4 8.4 62 56 100 100 87 31 K/MnMo04 27.3 51 37 80 100 71 33 TABLE I V Percentage of '60 Incorporated into Products Stream at the 14th Minute total products + catalyst 0 2 C02 CO H20 HCHO products oxygen 71 100 75 10 1.7 77 74 MnMoO4 97 100 80 21 Li/MnMoO, 2.2 38 61 99 100 74 22 Na/MnMo04 2.0 26 20 K/MnMoO, 6.1 12 4.5 17 100 20 8 between the surface oxygen and oxygen in the catalyst bulk. This could explain why no cross-labeled oxygen was observed over MnMo04 even after 14 min, as the amount of oxygen exchanged represents only a small fraction of the lattice oxygen content. One possible explanation for the observed differences between K/MnMoO4 and the other three catalysts studied for oxygen exchange would be a difference in the surface structure due to the addition of the promoter ion, possibly resulting in a surface/ subsurface that is not replenished as rapidly through oxygen diffusion from the catalyst bulk. Alternatively, there could be a change in the mechanism of oxygen exchange predominating on the catalyst surface. The addition of the promoter ion K and, to a much lesser extent, Na and Li may provide additional sites at which dissociative adsorption can occur, allowing the formation and subsequent desorption of 1 6 0 1 8 0 species. As K/MnMo04 catalyst proved to be the most selective catalyst for C2 hydrocarbons, the sites associated with the formation of cross-labeled oxygen may also be involved in the coupling reaction. There was no observable gas-phase exchange between labeled and unlabeled oxygen, as shown by the blank oxygen-exchange experiment. However, with the large amounts of cross-labeled carbon dioxide (Cl6O180)produced, it is worthwhile to consider the possibility of secondary and gas-phase exchange between oxygen and carbon dioxide. The molecular oxygen source after the gas-phase holdup time was I8O2. Thus, to be observable, gas-phase exchange would have to occur between Is02 and C1602 or C160180,in each case resulting in the formation of 160180. As there was no significant formation or change in the amount of 160180 for the unpromoted MnMo04 or the Li- and Napromoted MnMoO4, it is unlikely that there was any significant gas-phase exchange between oxygen and carbon dioxide. Secondary oxygen exchange of the oxygenated products H 2 0 , CO, and C02 with the metal oxide surface is also possible. Such secondary exchange has been investigated in other systems for COZand H2026-28and has been found to occur even when there is no molecular oxygen exchange on thecatalyst surface. It would be difficult, in our case, to rule out this possibility completely without further experimentation to examine these variables.
However, our results provide clear evidence as to the involvement of lattice oxygen, especially for water formation over the Li- and Na-promoted catalysts. As we observed no significant change in the labeled oxygen concentration for the Li- or Na-promoted catalysts, an increase in lattice oxygen mobility would still be required to explain our observed results. Since these catalysts exhibited the highest activity, the lack of isotopically labeled water is significant and seems to indicate that the increase in the methane activation may be due to an increase in the reactivity of lattice oxygen. The addition of the promoter ions Li and N a may have increased the mobility of the lattice oxygen, bringing about this effect. Alternatively, the promoter ion may somehow affect the regeneration of methane activation sites, acting to speed up the process. As formaldehyde was also unlabeled, adsorbed oxygen would seem to be responsible mainly for nonselective oxidation for the Li- and Na-promoted catalysts and the unpromoted MnMoO,. Also, regeneration of the methane activation sites appears to occur predominantly from lattice oxygen rather than adsorbed oxygen for these catalysts. The K-promoted catalyst as compared to the Na-promoted catalyst is seen to be more selective due to the lack of COX formation more than an increase in CZformation. Although the catalyst bed lengths in this study were different for each catalyst, this trend was also observed in our previous study using equal surface area and bed length.' In the previous study, the catalysts were also compared at equal conversion levels, with a 41% selectivity to Cz hydrocarbons over the K-promoted catalyst compared to lo%, 3%, and 1% for Na- and Li-promoted and unpromoted MnMo04 catalysts, respectively. The large contrast in behavior of the K-promoted catalyst with the others studied may indicate the presence of different promoter effects for the different promoter ions. The differences may be associated with the formation of a different type of site for the K-promoted catalyst. This new site appears to affect the surface interaction with gas-phase oxygen and is likely to be responsible for an easier path for dissociative adsorption of gas-phase oxygen, leading to the continued formation of 160180, even under reaction conditions, and the rapid incorporation of l80into each of the oxygenated products once the surface/subsurface oxygen is depleted. Although similar sites may be formed to a lesser extent on the Lior Na-promoted catalyst, the addition of Li or N a appears to have a greater effect on the lattice oxygen behavior. It is possible that the smaller size of Li and N a allows them to become incorporated into the crystal structure at lattice defect sites and hence provides a faster diffusion path for lattice oxygen. The study of pure MnMo04 catalyst and alkali-metal-promoted MnMo04 through 1 6 0 2 / 1 8 0 ~ experiments has provided some important information about differences in oxygen utilization by the catalysts. These results will be combined with information from 12CH4/1TH4 and temperature programmed studies tocome up with a clearer picture of the catalysts which explains the observed selectivity and activity changes under methanecoupling reaction conditions.
Acknowledgment. The financial support provided by the National Science Foundation through Grant CTS-8912247 is gratefully acknowledged.
Transient Isotopic Labeling
References and Notes (1) Driscoll, S. A.; Zhang, L.; Ozkan, U. S. CatalyticSelective Oxidation; Oyama, S. T.; Hightower, J. W., Eds.; American Chemical Society: Washington, D.C.; ACS Symp. Ser. 1993, 523, 340. (2) Keller, G. E.; Bhasin, M. M. J . Catal. 1982, 73, 9. (3) Ito, T.; Lunsford, J. H. Narure 1985, 314, 721. (4) Aganval, S. K.; Migone, R. A.; Marcelin, G. Appl. Catal. 1989,53, 71. (5) Bartsch, S.; Hofmann, H. New Developments in Selective Oxidation; Centi, G., Trifiro, F., Eds.; Elsevier: Amsterdam, 1990; p 353. (6) Gaffney, A. M.; Jones, C. A.; Leonard, J. J.;Sofranko, J. A. J. Catal. 1988, 114,422.
( 7 ) Baronetti, G . T.; Lazzari, E.; Garcia, E. Y.; Castro, A. A.; Fierro, J. L.; Scelza,0. A. React. Kiner. Catal. Lerr. 1989, 39, 175. (8) Korf, S. J.; Roos, J. A.; Derksen, J. W. H. C.; Vreeman, J. A., Van Ommen, J. G.; Ross, J. R. H. Appl. Catal. 1990, 59, 291. (9) Lo, M.-Y.; Agarwal, S. K.; Marcelin, G. J. Catal. 1988, 112, 168. (10) Matsuura, I.; Utsumi, Y.; Naki, M.; Doi, T. Chem. Lerr. 1986,1981. (1 1) Miro, E. E.; Santamaria, J. M.; Wolf, E. E. J. Caral. 1990,124,465. (12) Otsuka, K.; Liu, Q.;Morikawa, A. Inorg. Chim. Acta 1986, 118, 1 - 3
LLJ,
(13) Otsuka, K.; Liu, Q.; Hatano, M . ; Morikawa, A. Chem. Letr. 1986, 467. (14) Jones, C. A.; Leonard, J. J.; Sofranco, J. A. J . Catal. 1987,103,311. (15) Iwamatsu, E.; Moriyama, N.; Takasaki, T.; Aika, K. Methane
Conuersion; Bibby, D. M., Chang, C. D., Howe, R. F., Yurchak, S., Eds.;
The Journal of Physical Chemistry, Vol. 97, No. 44, 1993 11529 Elsevier: Amsterdam, 1988; p 373. (16) Burch, R.; Squire, G. D.; Tsang, S. C. Appl. Catal. 1988, 43, 105. (17) Peil, K. P.;Goodwin, J. G., Jr.; Marcelin, G. J. J . Phys. Chem. 1989, 93, 5977. (18) Peil, K. P.; Goodwin, J. G. Jr.; Marcelin, G. J. J. Catal. 1991, 131,
143. (19) Mirodatos, C.; Holmen, A.; Mariscal, R.; Martin, G. A. Catal. Today 1990. 6. 601. (20) Kiwi, J.;Thampi, K. R.; Gratzel, M. J. Chem. Soc., Chem. Commun. 1990, 1690. (21) Kiwi, J.;Thampi, K. R.;Gratzel, M.;Albers, P.; Seibold, K.J. Phys. Chem. 1992, 96, 1344. (22) Carreiro, J. A. S.P.; Follmer, G.; Lehmann, L.; Baems, M. Proceeding ~
~~
of the 9th International Congress on Catalysis;Calgary, Phillips, M. J., Teman, M., Eds.; Chemical Institute of Canada: Ottowa, 1988; Vol. 2, p 891. (23) Ozkan, U. S.; Gill, R. C.; Smith, M. R. J. Catal. 1989, 116, 171. (24) Klissurski, D. G. Rev.Port. Quim. 1976, 18, 29. (25) Klissurski, D. G.; Ross, R. A.; Griffith, T. J. Reu. Port. Quim. 1977, 19, 7. Or: Klissurski, D. G. J . Catal. 1979, 59, 448. (26) Perl, J. B. J . Phys. Chem. 1975, 79, 1582. (27) Koranne, M. M.; Goodwin, J. G., Jr.; Marcelin, G. New Frontiers in CatalYSis; Proc. 10th Int. COW Cat& Budapest; Guczi, L., et al., Eds.; Elsevier: Amsterdam, 1993; p 219. (28) Matsuura, I.; Kirishiki, M. Chem. Lett. 1986, 1441.
