J . Phys. Chem. 1989, 93, 2896-2898
2896
TABLE III: Calculated Average D i d e Moments0 for 9.Y-BiantbryI ref 7 this work
solvent benzene
method
en - e,)
(/.t2)’12
b
dioxane
b
0.146 0.412 0.0775 0.4615
9.6 5.7 15.7 6.4
c C
/.tmd
(r2)’12
6.5 3.9 11.0 4.5
7.1 7.8
/.tmd
4.9 5.5
“In debye, near 22 OC. bThe factor t”/(e0 - e,) in eq 1 is determined from the estimated molecular reorientation time and eq 3 (Debye relaxation) as in ref 12. CThefactor e”/(~,, - e-) is determined from the ratio of dispersion to absorption, ( U T * ) - ’ . dCalculated using the equilibrium constants for LE s CT from ref 1 1.
-
-
the slower C T L E step.) The observed relaxation time of 10 ps in this case would be that for the slower C T LE step. If both pathways are involved the situation is more complicated. The fast relaxation affects the calculation of the dipole moment. Visser et al.? in the absence of the microwave phase data, assumed the relaxation to follow the simple Debye form with an appropriate long relaxation time. Their calculation is equivalent to the use of eq 3 to give t”/(co - em) = 0.146. Now that the relaxation time is known to be considerably shorter, that calculation is not correct. It is reasonable to use the new relaxation time in eq 3 to give a value of t”/(cO - L) = 0.412 as in the third column of Table 111. Using this method, the calculated dipole moment is smaller by the square root of the ratio of the values. The new values (which represent the average over the LE G C T equilibrium) are given in Table 111. In the present work, the amplitudes of the observed signals were also compared to those for a reference compound. The dipole moments so determined are given in Table 111. With both solvents the average dipole moment is smaller than that given previ~usly.~
The reason for the difference is not clear but must involve the method of absolute calibration. The dipole moment of the C T form can be calculated if the equilibrium constant is known. Values are given by Kang et al.” which correspond to 64 or 68% CT for benzene and dioxane, respectively.21 On this basis, the moment of the C T form is 7.1 or 4.9 D in benzene and 7.8 or 5.5 D in dioxane. The two values for each solvent represent the two absolute calibrations. These values are considerably smaller than that for transfer of a full electronic charge but it is not clear whether transfer of a full charge should necessarily occur for the C T state in these low polarity solvents.s,6 The values of the dipole moments given here depend on the absolute calibration and assumption of Debye relaxation, as mentioned above, and the values for the equilibrium constants. Conclusion The measurements of both components of the microwave signal associated with the excitation of BA are unusual in that a relaxation time of about 10 ps is indicated. An internal charge CT’ or C T LE CT’, which rearrangement, either C T reverses the direction of dipole moment is suggested to account for the small observed relaxation time. The rate of this process increases by only a moderate amount as the temperature is raised.
-
- -
Acknowledgment. The research described herein was supported by the Office of Basic Energy Sciences of the Department of Energy. We thank Dr. V. Nagarajan for kindly providing the sample of 9,9’-bianthryl. (21) The mechanism of the relaxation between the two CT forms is not known in detail so the ratio of the forward and reverse rates cannot be used to determine the equilibrium constant. A relaxation time of 10 ps coupled with a forward rate of 2 ps would imply a larger value than that reported.
Secondary Reactions of Methyl Radkals with Lanthanide Oxides: Their Role in the Selectfve OxMation of Methane Youdong Tong, Michael P. Rosynek, and Jack H. Lunsford* Department of Chemistry, Texas A&M University, College Station, Texas 77843 (Received: December 13, 1988)
Reactions between methyl radicals and certain members of the lanthanide oxide series have been studied by using a matrix isolation electron spin resonance technique. Methyl radicals react extensively with the lanthanide-metal oxides Ce02,Pr@l], and Tb4O7,all of which have multiple cationic oxidation states. By contrast, the oxides La203,Nd2O3, SmZO3, Eu203,and Yb203 react with CH; radicals to only a small extent. Consistent with this observation,the former group of oxides is ineffective in the generation of CH3’ radicals which emanate into the gas phase. The reaction of CeO, with CH3’ radicals is strongly inhibited by the addition of Na2C03,and consequently radical production is enhanced. This effect is manifested in the catalytic properties of Ce02, which is a complete oxidation catalyst in its pure form but becomes a good catalyst for the oxidative coupling of methane following the addition of Na2C0,.
Introduction There is now convincing evidence that the oxidative dimerization of CH, to C2H6,and subsequently C2H4,occurs over several oxide catalysts by a heterogeneous-homogeneous mechanism.’S2 That is, methyl radicals are formed on the surface of the oxide and then emanate into the gas phase where they either couple to yield the desired product, C2H6, or enter into chain branching reactions that ultimately result in complete oxidation. In a typical catalyst
particle, or catalyst bed, each methyl radical would have an opportunity to collide with the metal oxide surface many times before reacting with another radical, and if these subsequent collisions result in a reaction, products other than C2Hs would be formed. ~,~ Except for two specific observations from our l a b o r a t ~ r y ,the reactions between CH3’ radicals and metal oxides have not been previously reported. In this study we show that very large differences occur in the reactivity of methyl radicals with certain lanthanide oxides and that the reactivity of an oxide can be
(1) Campbell, K. D.; Morales, E.; Lunsford, J. H. J . Am. Chem. Soc. 1987,
109, 7900.
( 2 ) Campbell, K. D.; Lunsford, J. H. J . Phys. Chem. 1988, 92, 5792.
0022-3654/89/2093-2896$01.50/0
( 3 ) Campbell, K. D.; Zhang, H.; Lunsford, J. H. J . Phys. Chem. 1988,92, 750.
0 1989 American Chemical Society
The Journal of Physical Chemistry, Vol. 93, No. 8, 1989 2897
Letters
CH3' GENERATOR
Figure 1. Apparatus for studying the reactivity of CH3' radicals with
metal oxides.
Experimental Section The matrix isolation electron spin resonance (MIESR) system used previously to study the generation of radicals was modified, as shown in Figure 1, in order to determine the reactivities of lanthanide oxides with methyl radical^.^ The oxide (scavenger) under investigation was placed between the radical generator and the leak into the low-pressure region of the apparatus. In these studies, Sm203was used as the CH3' generator because it provided a stable source of radicals after 15 h in a flow reactor. The temperatures at the generator and scavenger were 650 and 470 OC, respectively. In blank experiments, it was demonstrated that the scavenger did not function as a significant radical generator at the lower temperature. For example, with La203 in the position of the scavenger, but with no Sm203present, the rate of radical formation at 470 OC was only 4% of that obtained with SmzO3 present as the radical generator. The distance between the generator and the scavenger was ca. 2.3 cm, and the pressure in this region was ca. 1.5 Torr. Under these conditions, the loss of radicals from recombination was about 30%during the time that the gas flowed from the generator to the scavenger. In a typical experiment, the two oxides were heated in flowing O2 for 1 h, with the generator at 550 OC and the scavenger at 380 O C . After stopping the O2 flow and evacuating the gas for 10 min, a gas mixture having the following compositions (at STP) was allowed to flow over the oxides: 3.8 mL mi& Ar, 1.1 mL min-' CH4, 0.025 mL min-' 02.The oxides were heated to the reaction temperatures and maintained under these conditions for 1 h before radical collection commenced. Reaction efficiencies (RE) of the oxides are reported relative to quartz chips by using the equation
where [CH3*lQand [CH,'], are the radical amounts determined by ESR using quartz chips and the oxide of interest, respectively, as the scavenger. Quartz chips have previously been shown to be almost inert for both generating and scavenging methyl radicals. The oxides La203 (99.99%), C e 0 2 (99.9%), Pr6011(99.9%), Nd203 (99.9%), S m 2 0 3 (99.9%), Eu2O3 (99.99%), and Yb2O3 (99.9%) were obtained from Aldrich, and Tb407 (99.9%) was obtained from Alfa. In addition, Aldrich Na2C03 (gold label) was used. The sodium-modified cerium oxides, Na+/Ce02, were prepared by evaporating to dryness slurries containing appropriate amounts of Ce02 and aqueous solutions of Na2C03. The materials were calcined in static air at 700 OC for 10 h. Catalytic measurements were carried out in a fused quartz reactor having an inside diameter of 10 mm containing 0.5 g of catalyst as 20-40 mesh chips. The catalyst was pretreated for 3 h at 780 OC in flowing O2before exposure to the reactants. The data reported here were obtained after the catalysts had been on stream for times greater than 15 h.
Results and Discussion The results shown in Figure 2 provide a comparison of net radical formation rates and the reaction efficiencies for selected members of the lanthanide oxide series. The data in Figure 2a (4) Martir, W.; Lunsford, J.
H.J .
Am. Chem. SOC.1981, 103,
.C Pr Nd Sm Eu
Tb
Yb
La Ce Pr Nd Sm Eu
Tb
Yb
La
moderated by adding sodium carbonate.
3728.
100 r?
2E
80
0 I Y
at i
60
E
40
'0
E
W C
B
20
8
d
0
Figure 2. Comparison of (a) CH3' formation rate and (b) rate of reaction with selected members of the lanthanide oxide series.
r 100
/
0.0
2.0
4.0
6.0
1.0
10.0
c
.-
a .
0
E LL
20
s 0
a
-. 0.0
a 0
2:O
4:O
610
810
10.0
No Content, wt% Figure 3. Effect of NazCOpaddition to CeO, on (a) the catalytic oxidation of CH4and (b) the production of CH3' radicals and their reaction with the catalyst: 0, CH4 conversion; 0 , O2conversion; A,combined C2 and C4 selectivity; 0, relative formation rate of CH3' radicals; e, reaction efficiency of CH,' radicals with the catalysts. The catalytic reaction was carried out at 770 OC, 1 atm, and a flow rate of 34 mL min-'. The partial pressures of CH4and O2were 180 and 67 Torr, respectively. The CH3' formation rates and the reaction efficiencies were determined with the catalysts at 760 and 470 "C, respectively. The gas pressures were 1 and 1.5 Torr for the two cases.
have been reported previously, where it was noted that the oxides which exhibit multiple cationic oxidation states (Ce02, Pr6OlI, and Tb407) are not effective in generating gas-phase CH3' radi c a l ~ .It~is now clear from the results in Figure 2b that these three oxides also react very efficiently with the CH,' radicals. Thus,
2898
The Journal of Physical Chemistry, Vol. 93, No. 8. 1989
even if radicals were formed on these oxides, they would not survive long enough to exit the catalyst bed in a MIESR experiment. The nature of the surface species that is formed when these oxides react with CH,' is not known; however, it appears likely that methoxide ions would be produced by the reaction
+
M(n+1)+02- CH3'
-
Mn+(OCH3)-
(2)
Infrared and ESR evidence suggests that methyl radicals react with supported MOO, by an analogous r e a ~ t i o n . ~Methoxide ions are an intermediate in the formation of formaldehyde during the oxidation of methanol. One might expect that formaldehyde would be a product of the methane oxidation reaction, but at the temperatures employed, complete oxidation would undoubtedly occur. The remaining lanthanide oxides studied (LazO3, Nd2O3, Sm203,Eu203, and Yb2O3) are relatively poor radical scavengers; therefore, the rates of radical formation (Figure 2a) reflect the true activities of these materials. That is, ytterbium oxide is intrinsically a poor radical former. It was pointed out previously that the relative activities of these oxides parallel their basicities., Although cerium oxide is a nonselective catalyst for the partial oxidation of CH, to C2H4 and C2H6(C2 compounds), it can be made into a selective catalyst, as shown in Figure 3a, by the addition of sodium carbonate. With pure Ce02, all of the O2was consumed, and 20%of the CH4 was converted completely to COP The addition of 10 wt % Na+ resulted in an increase in the C2 and C4 selectivity from 0 to 59%. The origin of the greatly improved C2 and C4 selectivity upon addition of Na2C03can be understood when one considers the results in Figure 3b. The effect of Na2C03is clearly to decrease the rate of reaction of C e 0 2 with CH,' radicals, thus providing an opportunity for the radicals to escape into the gas phase where they may couple. The addition of sodium to Pr6OI1has a similar positive effect on the selective ~
(5) Liu, H.-F.; Liu, R.-S.; Liew, K. Y.; Johnson, R. E.; Lunsford, J. H J . A m Chem SOC.1984, 106,4117.
Letters conversion of CH, to higher hydrocarbons, in part because the alkali-metal ion (or its carbonate) inhibits the reaction of methyl radicals with the oxide.6 Na2C03 partially covers the C e 0 2 surface, as indicated by X-ray photoelectron spectra of the catalysts. Moreover, Na2C03, or more likely Na202,7is about 70% as active for the generation of CH,' radicals as is the 5 wt % Na+/Ce02 catalyst when compared on the basis of unit surface area. Perhaps an even more significant observation was that 4 wt % Na+/Yb203 is twice as effective in generating CH3' radicals, per unit surface area, as is the 5 wt % Na+/Ce02 catalyst. It is evident, therefore, that, in addition to inhibiting the reaction of CH,' with Ce02, the Na2C03surface phase may also serve as a source of CH< radicals. A more detailed account of the catalytic properties of the Na+/Ce02 system will be published elsewhere. In summary, these results show that secondary reactions of methyl radicals with metal oxide surfaces are a significant factor in determining the flux of radicals that emanate into the gas phase. Members of the lanthanide oxide series that have multiple cationic oxidation states are particularly reactive with respect to methyl radicals. It is likely that the Ln3+/Ln4+multivalency in CeO,, Pr601,, and Tb407 facilitates the electron transfer needed to generate methoxide species, according to reaction 2. These radical-oxide reactions are strongly inhibited by the presence of Na2C03on the surface, and by such a modification a catalyst that results only in the complete oxidation of CH4 can be transformed into one that promotes the oxidative dimerization reaction.
Acknowledgment. We acknowledge financial support of this work by the Division of Chemical Sciences, Office of Basic Energy Sciences, U S . Department of Energy. (6) Gaffney, A. M.; Jones, C. A.; Leonard, J. J.; Sofranko, J. A. P r e p . Am. Chem. Soc., Diu.Pet. Chem. 1988, 33,445-452. ( 7 ) Otsuka, K.; Said, A. A,; Kiyotaka, J.; Komatsu, T. Chem. Lett. 1987, 71.
