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The Catalytic Conversion of Methane to Higher Hydrocarbons
A. Holmen et al. (Editors), Nuturul Gas Conversion 1991 Elsevier Science Publishers B.V., Amsterdam
3
THE CATALYTIC CONVERSION OF METHANE TO HIGHER HYDROCARBONS J.H. Lunsford Department of Chemistry, Texas A&M University, College Station, Texas 77843 SUMMARY The oxidative coupling of methane over metal oxide catalysts is an example of a heterogeneoushomogeneous reaction in which surface-generated CH,. radicals initiate gas-phase reactions. Thcse chain-branching gas-phase reactions consume additional CH, and may result in the kinetic isotope effects that have been observed. Moreover, they are responsible, in part, for the conversion of C2.H6 to q H 4 and for the formation of COX. Secondary reactions of CH,. radicals with the metal oxide also may contribute to the formation of CO,. INTRODUCTION The oxidative coupling of methane to form ethane and ethylene, and to a limited extent higher hydrocarbons, CH,
+
0,
cat. -*
q H 6 , q H 4 , higher hydrocarbons, H,O, CO, CO,, H,
650"900°C
(1)
is now a well established reaction that has potential for the improved conversion of natural gas to more useful chemicals and fuels. Numerous catalysts have been shown to be moderately effective for the coupling reaction; however, combined conversions and selectivities are only marginally adequate for commercialization.
Moreover, under the severe conditions at which the catalysts
operate, their limited lifetime is an additional problem. These catalytic systems provide an excellent example of the coupling between heterogeneous and homogeneous (gas phase) reactions. The free radical chemistry that prevails involves both well known chain reactions in the gas phase and little known radical reactions at the catalytic surface. Several aspects of the complex reaction mechanism have been recently reviewed by Lunsford (l), and some of these will be more fully developed in this paper. At the outset one should recognize that the oxidative coupling of methane is a relatively new process, and consequently there does not exist a definitive mechanism that will quantitatively conform to all of the experimental data, CATALYSTS Before discussing the mechanistic details of oxidative coupling a brief summary of the classes of catalysts will be given. A much more detailed description of the catalysts is provided in a recent review by Hutchings et a/. (Z),wherein approximately 100 catalysts are listed which have some degree
4
of effectiveness for the oxidative coupling reaction. Additional catalysts are being reported each month. With such a large number of catalysts being capable of promoting the coupling reaction one might ask whether all classes of catalysts would be effective, and the answer is no. In general group VIII metals, acidic metal oxides (e.g. acidic zeolites) and many unmodified transition metal oxides
(e.g. Cr203, Fe,O,, NiO) are not selective for methane oxidation. A possible exception to this generalization is PdO, which is capable of providing 35% selectivity to ethane at 40O0C, albeit at a conversion of only 0.2% (3).
The more effective catalysts may be broadly categorized as described in Table 1, where the references are representative rather than complete. Another arbitrary means of classification would be to divide the catalysts between those that operate in the cofeed mode and those that operate in a cyclic mode; ie., CH, and 0, are alternately passed over the catalyst. An example of the latter from Table 1 is NaMn04/Mg0. Catalysts which operate in the cyclic mode clearly must have oxygen storage capacity. Table 1. Classification of Catalysts for the Oxidative Dimerization of Methane General Type
Examples
Probable Active Center
Ref.
Gr IA ion in Gr IIA oxide
Li'/MgO, Na+/CaO
[M'O-] center
4s
Certain lanthanide oxides
La203, Sm203
0,- or
Gr I A oxide/carbonate on basic oxide
Na2C03/Mg07 Na,COJCeO,
0:-
Gr IIA oxide on basic oxide
BaO/CaO
0,2- (?)
10
Modified transition metal oxide
NaMnOflgO (Na2C0&&jMn08)
0;-
11
Monophasic oxide
BaPbO,, LiNiO, LiCa2Bi30,C16
Group
M
0;
0,2- (?)
P-
' 0
tr
67
12-14 t
(?)
IA ions are present in many of the catalysts that are most effective for the coupling
reaction. The role of these ions has been the subject of several investigations (15-18), and it appears that, depending on the host oxide, they serve (i) to create active centers of the type [M'O], form active surface peroxides (particularly with Na')
(ii) to
and (iii) to inhibit secondary reactions of
5
intermediates and products with the host oxide. These functions will be discussed in more detail in subsequent sections. MECHANISM A Kinetic Model
Since the coupling reaction is carried out at temperatures where the purely homogeneous oxidation of CH, could occur, it is first necessary to consider whether this reaction, in fact, could account for results that have been attributed to a heterogeneous process. Most investigators have carried out "blank" reactions, sometimes in reactors packed with an inert material and occasionally in reactors that contain a large free volume. Based on "blank reactor" studies, Yates and Zlotin (19) concluded that the purely homogeneous oxidation of CH, contributed significantly to the conversion over Li+/MgO and that the catalyst was mainly responsible for the conversion of C O to CO,. Unfortunately, the reactor system of Yates and Zlotin had a large free volume that was heated. We have subsequently shown that such a volume is conducive to the homogeneous reaction, but if the volume is packed with an inert material the CH, conversion is much less at comparable residence times (20). Lane and Wolf (21) have shown that the partial pressure of the reagents, or more correctly the dilution ratio given by the partial pressure of methane and oxygen divided by the total pressure, affects the extent of the homogeneous reaction, with higher ratios resulting in a greater homogeneous contribution. There is now a considerable body of evidence which confirms that ethane is the major primary product of the coupling reaction and that ethylene is a secondary product (2,22,23). Even so, there may exist a minor direct pathway for the formation of ethylene (2,24). Both qualitative and quantitative results obtained with a matrix isolation-electron spin resonance (MIESR) technique confirm that over many catalysts the coupling of methyl radicals to form ethane occurs mainly in the gas phase (25-27). This seems to be the case even on catalysts containing transition metal ions, such as LiNiO, and NaMn04/Mg0 (28). What has not been appreciated in the past is the fact that these surface-generated CH,. radicals initiate chain branching reactions in the gas phase which may result in the conversion of additional CH,.
That is, more CH, may be consumed in the gas phase than is
consumed on the surface. A kinetic model has been developed in our laboratory in which CH,. radicals produced a t the
surface are allowed to react in the gas phase through well established elementary reactions (29). The model consists of 156 reactions, one of which is the coupling of CH,. radicals to form G H @ The initial generation of CH,. radicals is via the catalytic surface, or in the purely homogeneous case is by the reaction CH4
+
02
4
(333.
+
HOy
(2)
The model also includes an alternate step for the removal of H 2 0 2 H202
+
H20
+
1/20,
(3)
6
which adequately accounts for the effect of a solid surface in decreasing the purely homogeneous conversion of CH,. This particular wall effect is very important because the competing homogeneous decomposition of H202 H,O,
+
M
20H.
+
M
(4) produces OH. radicals which are major chain carriers in the oxidation reaction. Some of the other +
important reactions in this model are: CH3CH3. CH4 CH4 CH4
+ +
+
+ +
0,
+
CH30.
02
+
CH2O
0
CH3*
+
OH* H*
+
+
CH,*
CHy
+
+
+
0
(5)
OH.
(6)
OH*
+
(7)
HzO
(8)
H2
(9) through which additional CH, is consumed, and CH3* radicals are formed in the gas phase. The +
model accurately describes the purely homogeneous reaction of CH4 when the effects of the reactor wall and other surfaces that might be present are included. Moreover, as shown in Table 2 the model is reasonably effective in predicting the conversions and selectivities obtained over a Li+/MgO catalyst. The catalyst replaces reaction 2 as the major source of CH3. radicals, and under these conditions reaction 3 becomes unimportant. From the model the number of CH, molecules reacted
on the surface and in the gas phase through chain branching reactions may be compared with those reacted on the surface. This number, which is defined as the chain length, is a function of the 0, partial pressure, and under typical reaction conditions it may exceed 2. Table 2. Comparison of Experimental and Calculated Results for Methane Conversion over Li+,MgO
Selectivities, % qH6
CzH4 CO,
co
KIE Chain length
Experimentala
Calculated
12.2 (8.8)b 13.7 (8.9)
10.1 (7.9) 17.4 (10.6)
30.1 (33.0) 18.7 (12.4) 50.5 (50.9) 0.7 (3.6)
35.0 (49.3) 7.4 (6.6) 57.5 (43.9) 0.0 (0.0)
1.39
1.28 2.7
a 0.5 g Li+/MgO, 7WC, 25% CH,, 12.5% O,, FR = 80 mLmin”.
Numbers in parentheses obtained from the oxidation of CD,.
Heretofore we have emphasized the importance of the gas phase equilibrium
I
CH,.
+
0,
* CH3Oy
(10) as a possible explanation for the greater C,selectivities that are obtained at higher temperatures (5).
The equilibrium constant for this reaction decreases with increasing temperature, thus shifting the reaction to the left and making more CH,. radicals available for the coupling reaction. These forward and reverse reactions are included in our mechanism, but if they are omitted the results are essentially unchanged. Thus, it does not appear that reaction 10 is affecting the C, selectivity. Instead, the model indicates that the positive effect of temperature on C, selectivity is achieved by the greater production rate of CH,. radicals via the surface reaction and reactions 7-9. The heterogeneous-homogeneous kinetic model may be used to interpret the kinetic isotope effects (KIE) for CH, conversion over several catalysts that have recently been reported (23,30-32). The KIE in its simplest form is defined as the ratio of the conversion rate of CH, to that of CD,.
A KIE also may be determined from the ratio of the amount of CH, to the amount of CD, in the ethane fraction, which is composed of q H 6 , CH,CD, and C,D,. A summary of the KIE's obtained at several different reaction conditions is given in Table 3. Table 3. Kinetic Isotope Effects in the Partial Oxidation of Methanea Catalyst
ConditionsMethod
KIE
Li+/MgO
750°C, 10% methaneb, 7.2% oxygen; methane conv.
1.5
Li+/MgO
68O"C, 20% methane, 10% oxygen; methane conv.
1.59
31
Li+/MgO
750°C, 45% CH,, 45% CD,, 10% 0,; H and D distribution in ethane
1.43
31
Li+/MgO
715"C, 15% CH,, 10% CD,, 12% 0,; H and D distribution in ethane
1.73 k 0.15
23
Li+/MgO
70O0C, 25% methane, 12.5% oxygen; methane conv.
1.39 zk 0.05
This study
65O0C,2.7% methane, 1.3% oxygen; methane conv.
1.3
32
Sm203
a
zk
Ref. 0.03
30
KIE = k,RD Total pressures are 760 torr.
A KIE of cu. 1.5 for methane conversion over a Li+/MgO catalyst led Cant et af. (30) to conclude that the rate limiting step in the catalytic cycle
0,
+
CH3.
20% 02-
CH,
+
+
4
OH,
-+.
CH3* 0”
[ ]
+
+
CH3*
+ M qH6 + + [ ] + H2O 4
(11)
M
(12) (13)
1/20, -20,
(14)
is reaction 11, which involves the breaking of a C-H bond. Earlier, Ito et al. (4) had proposed that this step was fast, and that either reaction 13 or 14 must be rate limiting. With the realization that methane also may be reacted via reactions 7-9 and that these reactions may contribute to the KIE, we have obtained KIE values over a Li+/MgO catalyst at different methane-to-oxygen ratios. The results are depicted in Fig. 1 (29). If the gas phase reactions give rise to the KIE and reaction 11
is not rate limiting, one would expect that the KIE effect would approach unity at a large methaneto-oxygen ratio, which indeed was observed. Although most of the data reported in Table 3 were obtained at moderate methane-to-oxygen ratios, Nelson el al. (31) carried out an experiment at a ratio of 9. From their FTXR data it is possible to determine the isotopic composition of their ethane, and from this conclude that the KIE was 1.4. The reason for the discrepancy between their KIE and that indicated in Fig. 1 is not evident.
1.8
I
I
I
I
I
L?/M go 1.6
-
T=700°C P(CH4)=190(torr)
1.4
1.2 1.o
0.8
-
0
I
I
I
I
I
2
4
6
8
10
12
Methane to Oxygen Ratio Figure 1. Influence of CH4:02 ratio on the kinetic isotope effect, determined from the conversion of CH, and CD,: 4, experimental result; A,calculated from the rcaction model.
9
When the theoretical KIEs of 1.8 and 2.1 at 700°C (33)were introduced, respectively, in all of the C-H(D) and 0-H(D) bond breaking reactions of the kinetic model, but not in the formation of methyl radicals at the surface, the resulting overall K E indicated by the dashed line in Fig. 1 was obtained. The calculated KIE's reflect the general trend exhibited by the experimental values, which supports the hypothesis that gas phase reactions are largely responsible for the KIE. Our current view of the surface reaction is that 0; is a transient active center because the hole at nearly every (electron) is in rapid motion among the oxygen ions. Moreover, CH, reacts with Oscollision (34). Therefore the rate of CH,. radical formation via surface reactions is given by CH3* OL
r.[cH41[o;l
(15)
and
+ 052- *
+
[L~+o-I~ [ L ~ + O ~ - ] ~0, (16) Here r is the lifetime of the 0; center on the surface, and [ L i + 0 l b is the concentration of a bulk center that has been characterized by ESR spectroscopy (35). If reaction 14 were a t equilibrium, then the concentration of [O;]would be proportional to [O,].'
In the kinetic model the production
of CH3- radicals is assumed to be first order with respect to CH4 and zero order with respect to 0,. Essentially the same results would have been obtained assuming a zero order surface reaction with respect to CH, since the conversion of CH, was small. Among the reactions 11-14it appears likely that reaction 13 is the slowest. Cant et nl. (30)have argued that reaction 13 is not rate limiting because the addition of H20 or D 2 0 had no discernible effect on the KIE. But if the KIE were a result of gas phase reactions, one would expect this result. The observation that there is no KIE at high CH4:02 ratios (Fig. 1) is more problematic if reaction 13 is indeed rate limiting, as this reaction involves the breaking of an 0-H bond. It may be, however, that the rate offormation of water is limited by the removal of oxygen from the lattice and not by the breaking of an 0-Hbond. Therefore, the usual kinetic isotope effect would not be expected.
- Radicals with the Catalysts. Secondaxv Reactions of CH,. One feature of the kinetic model that is lacking is the incorporation of reactions between the catalyst and radical intermediates. Generally, such rate data do not exist; however, we are beginning to obtain both qualitative and quantitative data on the reactions of CH3- radicals with metal oxides that are of interest in the coupling reaction (7,36). Based upon a reasonable concentration of CH,. radicals in a catalyst bed (26) it may be shown that a given radical will collide with a surface approximately
1 6 times before it reacts with another CH,.
radical. Obviously, if there is a high
probability that a collision with the surface results in an undesirable reaction, the selectivity for
C,
formation would be small. The relative reaction rates between various members of the lanthanide oxide series and CH,. radicals are very different, and those oxides that react significantly with the
10
radicals are nonselective catalysts (9). The metal ions in the most reactive oxides have multiple accessible oxidation states (e.g. Pr6011), and presumably the CH,. radicals react via a reductive addition, forming the reduced form of the metal ion and a surface methoxide species. These secondary reactions may be minimized by covering the reactive oxide with a relatively inert compound such as Na2C03. Using a modified form of the MIESR system the reactive sticking coefficient, u, (the probability that a collision with a surface results in a reaction) and the activation energy for the reaction of CH,. radicals with ZnO and MgO were determined. These oxides were selected because the former
is among the most reactive of the oxides that we have studied and the latter is relatively inactive.
The results are summarized in Table 4. Even on ZnO the reactive sticking coefficient is surprisingly small. Nevertheless, it is sufficiently large to account for the nonselective behavior of Z n O in the oxidation of CH4 (37). By contrast, the reactive sticking coefficient of CH,. is two orders of magnitude less on MgO than on ZnO at 482°C and at 720°C. Table 4. Sticking Coefficients and Activation Energies ~
oa
Oxides
E, (KcaVmol)
ZnO
1.8 10”
2.6
MgO
1.2
lo-’
5.7
a
T = 755 K
The Oriein of COX Perhaps the most thoroughly studied, and yet most controversial aspect of the oxidative coupling reaction is the origin of the COXproducts. Since selectivity is an important factor in any economic evaluation of the process, it is necessary to determine the manner in which the nonselective products are formed. The origin of COXmay be best discussed using the relatively simple mechanistic scheme kl
cH4
k2
- GH6- GH4
9
p
4
cox
p
5
11
which shows that COX,in principle, may be produced by each of the three hydrocarbons. Part of the difficulty in establishing the primary source of COXresults from the fact that the reaction rates of pure hydrocarbons over a given catalyst are different from those obtained with a mixture of two or three hydrocarbons (38,39). This effect is particularly evident in oxygen-limiting reactions. Moreover, factors such as the CH,:O,
ratio, the total pressure of the reactants, the CH, conversion, the
temperature of the reaction, the free volume in the reactor and the type of catalyst play a role in determining the major source of CO,. The magnitudes of these potential variables are often quite different among the various studies that have been reported, therefore it is difficult to compare results. Moreover, conclusions that have been based on data obtained at a particular set of conditions cannot be generalized (4,38-43). For example, Ito et al. (4) studied the formation of COX over Li'MgO
at 620°C, with an initial CH,:O, ratio of 4.4 and at a CH, conversion of 1.6%, and
concluded that most of the CO, was derived from step 3; ie., the direct catalytic oxidation of CH,. By contrast, over the same catalyst operating at 720°C and with a CH4:02 ratio of ca. 20, Roos et al. (38) obtained data which allowed them to conclude that the COXproducts were formed solely from CzH4. The most definitive results have been obtained recently by adding 13%H6 and 13%H4 to a CHdO, mixture and observing the amount of I3C in the COXproducts. Ekstrom et al. (39) have
used this method to show that the percentage of 13C in the COX products greatly exceeded the percentage of 13C in the feed gas. Over a Sm203 catalyst operating at 700°C with 10% 0, in CH, they reported that with 19% 13C present as l 3 q H 4 in the feed there was 86% I3C in the CO,. There was slightly more l3COXpresent with l3%H, than with an equivalent amount of 13%H,. The authors concluded that COXis largely formed via the % products and that %H4 is oxidized somewhat faster than GH, to CO,.
Using a similar method Nelson and Cant (42) have studied the oxidation
of CH, over Li+/MgO at several different temperatures. In agreement with Ito et al. (4) they found
C, products accounted for less than 10% of the COX,but at temperatures greater than 740"C, C, oxidation was responsible for the formation of 3040% of the
that at T < 700°C the oxidation of
co,.
It is of interest to know whether the q H 6 and the %H4 are converted to COXin the gas phasc
or on the surface of the catalyst. The extent of the surface and gas phase reactions will depend on the CH4:0, ratio, etc. Under the oxygen-limiting conditions employed by Roos el al. (38) it was concluded that at 720°C the reactions were mainly heterogeneous, but at 800°C there could have been a considerable homogeneous component. Geerts et al. (41) also studied the source of COXover a Li+/MgO catalyst at 800°C and concluded that an appreciable part of the reaction sequence takes place in the gas phase. The results of the model (Table 2) indicate that COXcould be formed via gas phase reactions at 700°C. Here we report the COXas CO, because over Li+/MgO CO is extensively converted to CO, (40). Even at much greater CH4:02 ratios (e.g lo), the model indicates the formation of COXin the gas phase.
12
CONCLUSIONS Recent experimental and modeling studies on the catalytic oxidation of methane over metal oxide catalysts have provided insight into the complex mechanisms through which this reaction occurs. The catalyst is the origin of CH,. radicals that enter into chain-branchinggas-phase reactions. These gasphase reactions account for much of the chemistry that is observed, including kinetic isotope effects and the formation, in part, of products such as G H 4 and CO, The partial pressure of 0, influences the extent of the homogeneous component; the role of CH,Or in equilibrium with CH,. and 0, appears to be a less significant factor in determining selectivity than was previously thought. Secondary reactions of gas phase CH3*radicals with the metal oxide may provide an alternate route to CO,
The similar catalytic behavior observed over many different metal oxide catalysts may be
attributed to the dominant effects of these homogeneous reactions. ACKNOWLEDGMENTS The author is indebted to Mr. Chunlei Shih for his part in developing the reaction model and for carrying out the KIE experiments. Mr. Youdong Tong performed the MIESR experiments. The research in our laboratory was supported by the National Science Foundation under Grant CHE-
8617436. REFERENCES
1. 2. 3. 4. 5. 6. 7. 8. 9. 10.
11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22.
J.H. Lunsford, Catal. Today, 6 (1990)235. G.L. Hutchings, M.S. Scurrell and J.R. Woodhouse, Chem. Soc. Rev., 18 (1989)251. K.R. Thampi, J. Kiwi and M. Gratzel, Catal. Lett., 4 (1990)49. T. Ito, J.-X. Wang, C.-H. Lin and J.H. Lunsford, J. Am. Chem. Soc., 107 (1985)5062. C.-H. Lin, J.-X. Wang and J.H. Lunsford, J. Catal., 111 (1988)302. K. Otsuka, K. Jinno and A. Morikawa, J. Catal., 100 (1986)353. K.D. Campbell, H. Zhang and J.H. Lunsford, J. Phys. Chem., 92 (1988)750. E. Iwamatsu, T. Moryama, N. Takasaki and K. Aka, J. Chem. Soc., Chem. Commun., (1986)19. Y. Tong, M.P. Rosynek and J.H. Lunsford, J. Phys. Chem., 93 (1989)28%. S.J. Korf, J.A. Roos, J.W.H.C. Derksen, J.A. Vreeman, J.G. van Ommen and J.R.H. Ross; in Natural Gas Conversion Symposium, Oslo, 1990. J.A. Sofranko, J.J. Leonard, C.A. Jones, A.M. Gaffney and H.P. Withers, Catal. Today, 3 (1988) 127. K.C.C. Kharas and J.H. Lunsford, J. Am. Chem. Soc., 111 (1989)2336. M. Hatano and K. Otsuka, Inorg. Chim. Acta, 146 (1988)243. J.M. Thomas, W. Ueda, J. Williams and K.D.M. Harris, Faraday Discuss. Chem. Soc., 87 (1989) 33. J.H. Lunsford, C.-H. Lin, J.-X. Wang and K.D. Campbell, in: M.M.J. Treacy, J.M. Thomas and J.M. White (Eds.), Microstructure and Properties of Catalysts, Materials Research Society, Pittsburgh, 1988,pp. 305-314. C.-H. Lin, T. Ito, J.-X Wang and J.H. Lunsford, J. Am. Chem. Soc., 109 (1987)4808. A.M. Gaffney, C.A. Jones, J.J. Leonard and J.A. Sofranko, J. Catal., 114 (1988)422. Y. Tong, M.P. Rosynek and J.H. Lunsford, J. Catal., in press. D.J.C. Yates and N.E. Zlotin, J. Catal., 111 (1988)317. M. Hatano, P.G.Hinson, K.S. Vines and J.H. Lunsford, J. Catal., 124 (1990)557. G.S. Lane and EE. Wolf, J. Catal., 113 (1988)144. P.F. Nelson, C.A. Lukey and N.W. Cant, J. Phys. Chem., 92 (1988)6176.
13
23. C.A. Mims, R.B. Hall, KD. Rose and G.R. Myers, Catal. Lett., 2 (1989) 361. 24. G.L. Hutchings, J.R. Woodhouse and M.S.Scurrell, J. Chem. Soc., Faraday Trans. 1,85 (1989) 2507. 25. D.J. DriscoII, W.Martir, J.-X. Wang and J.H. Lunsford, J. Am. Chem. Soc., 107 (1985) 58. 26. KD. Campbell, E. Morales and J.H. Lunsford, J. Am. Chem. Soc., 109 (1987) 7900. 27. KD. Campbell and J.H. Lunsford, J. Phys. Chem., 92 (1988) 5792. 28. Y. Tong and J.H. Lunsford, J. Chem. Soc., Chem. Commun., (1990) 792. 29. C. Shih, M.P.Rosynek and J.H. Lunsford, unpublished results. 30. N.W. Cant, C.A. Lukey, P.F. Nelson and R.J. Tyler, J. Chem. Soc., Chem. Commun., (1988) 766. 31. P.F. Nelson, C.A. Lukey and N.W. Cant, J. Catal., 120 (1989) 216. 32. K Otsuka, M. Inaida and Y. Wada, Chem. Lett., (1989) 1531. 33. L. Melander, Isotope Effects on Reaction Rates, Ronald Press, New York, 1960. 34. D.K. Bohme and F.C. Fehsenfeld, Can. J. Chem., 47 (1%9) 2717. 35. J.-X. Wang and J.H. Lunsford, J. Phys. Chem., 90 (1986)5883. 36. Y. Tong and J.H. Lunsford, unpublished results. 37. H.-S. Zhang, J.-X Wang, D.J. Driscoll and J.H. Lunsford, J. Catal., 112 (1988) 366. 38. J.A. Roos, S.J. Korf, R.H.J. Veehof, J.G. van Ommen and J.R.H. Ross, Appl. Catal., 52 (1989) 147. 39. A. Ekstrom, J.A. Lapsmicz and I. Campbell, Appl. Catal., 56 (1989) L29. 40. G.A. Martin, A. Bates, V. Ducarme and C. Mirodatos, Appl. Catal., 47 (1989) 287. 41. J.W.M.H. Geerts, J.M.N. van Kasteren and K van der Wiele, Catal. Today,4 (1989) 453. 42. P.F. Nelson and N.W. Cant, J. Phys. Chem., 94 (1990) 3756. 43. J.S. Lee and S.T.Oyama, React. Kinet. Catal. Lett., 41 (1990) 257.
3
THE CATALYTIC CONVERSION OF METHANE TO HIGHER HYDROCARBONS J.H. Lunsford Department of Chemistry, Texas A&M University, College Station, Texas 77843 SUMMARY The oxidative coupling of methane over metal oxide catalysts is an example of a heterogeneoushomogeneous reaction in which surface-generated CH,. radicals initiate gas-phase reactions. Thcse chain-branching gas-phase reactions consume additional CH, and may result in the kinetic isotope effects that have been observed. Moreover, they are responsible, in part, for the conversion of C2.H6 to q H 4 and for the formation of COX. Secondary reactions of CH,. radicals with the metal oxide also may contribute to the formation of CO,. INTRODUCTION The oxidative coupling of methane to form ethane and ethylene, and to a limited extent higher hydrocarbons, CH,
+
0,
cat. -*
q H 6 , q H 4 , higher hydrocarbons, H,O, CO, CO,, H,
650"900°C
(1)
is now a well established reaction that has potential for the improved conversion of natural gas to more useful chemicals and fuels. Numerous catalysts have been shown to be moderately effective for the coupling reaction; however, combined conversions and selectivities are only marginally adequate for commercialization.
Moreover, under the severe conditions at which the catalysts
operate, their limited lifetime is an additional problem. These catalytic systems provide an excellent example of the coupling between heterogeneous and homogeneous (gas phase) reactions. The free radical chemistry that prevails involves both well known chain reactions in the gas phase and little known radical reactions at the catalytic surface. Several aspects of the complex reaction mechanism have been recently reviewed by Lunsford (l), and some of these will be more fully developed in this paper. At the outset one should recognize that the oxidative coupling of methane is a relatively new process, and consequently there does not exist a definitive mechanism that will quantitatively conform to all of the experimental data, CATALYSTS Before discussing the mechanistic details of oxidative coupling a brief summary of the classes of catalysts will be given. A much more detailed description of the catalysts is provided in a recent review by Hutchings et a/. (Z),wherein approximately 100 catalysts are listed which have some degree
4
of effectiveness for the oxidative coupling reaction. Additional catalysts are being reported each month. With such a large number of catalysts being capable of promoting the coupling reaction one might ask whether all classes of catalysts would be effective, and the answer is no. In general group VIII metals, acidic metal oxides (e.g. acidic zeolites) and many unmodified transition metal oxides
(e.g. Cr203, Fe,O,, NiO) are not selective for methane oxidation. A possible exception to this generalization is PdO, which is capable of providing 35% selectivity to ethane at 40O0C, albeit at a conversion of only 0.2% (3).
The more effective catalysts may be broadly categorized as described in Table 1, where the references are representative rather than complete. Another arbitrary means of classification would be to divide the catalysts between those that operate in the cofeed mode and those that operate in a cyclic mode; ie., CH, and 0, are alternately passed over the catalyst. An example of the latter from Table 1 is NaMn04/Mg0. Catalysts which operate in the cyclic mode clearly must have oxygen storage capacity. Table 1. Classification of Catalysts for the Oxidative Dimerization of Methane General Type
Examples
Probable Active Center
Ref.
Gr IA ion in Gr IIA oxide
Li'/MgO, Na+/CaO
[M'O-] center
4s
Certain lanthanide oxides
La203, Sm203
0,- or
Gr I A oxide/carbonate on basic oxide
Na2C03/Mg07 Na,COJCeO,
0:-
Gr IIA oxide on basic oxide
BaO/CaO
0,2- (?)
10
Modified transition metal oxide
NaMnOflgO (Na2C0&&jMn08)
0;-
11
Monophasic oxide
BaPbO,, LiNiO, LiCa2Bi30,C16
Group
M
0;
0,2- (?)
P-
' 0
tr
67
12-14 t
(?)
IA ions are present in many of the catalysts that are most effective for the coupling
reaction. The role of these ions has been the subject of several investigations (15-18), and it appears that, depending on the host oxide, they serve (i) to create active centers of the type [M'O], form active surface peroxides (particularly with Na')
(ii) to
and (iii) to inhibit secondary reactions of
5
intermediates and products with the host oxide. These functions will be discussed in more detail in subsequent sections. MECHANISM A Kinetic Model
Since the coupling reaction is carried out at temperatures where the purely homogeneous oxidation of CH, could occur, it is first necessary to consider whether this reaction, in fact, could account for results that have been attributed to a heterogeneous process. Most investigators have carried out "blank" reactions, sometimes in reactors packed with an inert material and occasionally in reactors that contain a large free volume. Based on "blank reactor" studies, Yates and Zlotin (19) concluded that the purely homogeneous oxidation of CH, contributed significantly to the conversion over Li+/MgO and that the catalyst was mainly responsible for the conversion of C O to CO,. Unfortunately, the reactor system of Yates and Zlotin had a large free volume that was heated. We have subsequently shown that such a volume is conducive to the homogeneous reaction, but if the volume is packed with an inert material the CH, conversion is much less at comparable residence times (20). Lane and Wolf (21) have shown that the partial pressure of the reagents, or more correctly the dilution ratio given by the partial pressure of methane and oxygen divided by the total pressure, affects the extent of the homogeneous reaction, with higher ratios resulting in a greater homogeneous contribution. There is now a considerable body of evidence which confirms that ethane is the major primary product of the coupling reaction and that ethylene is a secondary product (2,22,23). Even so, there may exist a minor direct pathway for the formation of ethylene (2,24). Both qualitative and quantitative results obtained with a matrix isolation-electron spin resonance (MIESR) technique confirm that over many catalysts the coupling of methyl radicals to form ethane occurs mainly in the gas phase (25-27). This seems to be the case even on catalysts containing transition metal ions, such as LiNiO, and NaMn04/Mg0 (28). What has not been appreciated in the past is the fact that these surface-generated CH,. radicals initiate chain branching reactions in the gas phase which may result in the conversion of additional CH,.
That is, more CH, may be consumed in the gas phase than is
consumed on the surface. A kinetic model has been developed in our laboratory in which CH,. radicals produced a t the
surface are allowed to react in the gas phase through well established elementary reactions (29). The model consists of 156 reactions, one of which is the coupling of CH,. radicals to form G H @ The initial generation of CH,. radicals is via the catalytic surface, or in the purely homogeneous case is by the reaction CH4
+
02
4
(333.
+
HOy
(2)
The model also includes an alternate step for the removal of H 2 0 2 H202
+
H20
+
1/20,
(3)
6
which adequately accounts for the effect of a solid surface in decreasing the purely homogeneous conversion of CH,. This particular wall effect is very important because the competing homogeneous decomposition of H202 H,O,
+
M
20H.
+
M
(4) produces OH. radicals which are major chain carriers in the oxidation reaction. Some of the other +
important reactions in this model are: CH3CH3. CH4 CH4 CH4
+ +
+
+ +
0,
+
CH30.
02
+
CH2O
0
CH3*
+
OH* H*
+
+
CH,*
CHy
+
+
+
0
(5)
OH.
(6)
OH*
+
(7)
HzO
(8)
H2
(9) through which additional CH, is consumed, and CH3* radicals are formed in the gas phase. The +
model accurately describes the purely homogeneous reaction of CH4 when the effects of the reactor wall and other surfaces that might be present are included. Moreover, as shown in Table 2 the model is reasonably effective in predicting the conversions and selectivities obtained over a Li+/MgO catalyst. The catalyst replaces reaction 2 as the major source of CH3. radicals, and under these conditions reaction 3 becomes unimportant. From the model the number of CH, molecules reacted
on the surface and in the gas phase through chain branching reactions may be compared with those reacted on the surface. This number, which is defined as the chain length, is a function of the 0, partial pressure, and under typical reaction conditions it may exceed 2. Table 2. Comparison of Experimental and Calculated Results for Methane Conversion over Li+,MgO
Selectivities, % qH6
CzH4 CO,
co
KIE Chain length
Experimentala
Calculated
12.2 (8.8)b 13.7 (8.9)
10.1 (7.9) 17.4 (10.6)
30.1 (33.0) 18.7 (12.4) 50.5 (50.9) 0.7 (3.6)
35.0 (49.3) 7.4 (6.6) 57.5 (43.9) 0.0 (0.0)
1.39
1.28 2.7
a 0.5 g Li+/MgO, 7WC, 25% CH,, 12.5% O,, FR = 80 mLmin”.
Numbers in parentheses obtained from the oxidation of CD,.
Heretofore we have emphasized the importance of the gas phase equilibrium
I
CH,.
+
0,
* CH3Oy
(10) as a possible explanation for the greater C,selectivities that are obtained at higher temperatures (5).
The equilibrium constant for this reaction decreases with increasing temperature, thus shifting the reaction to the left and making more CH,. radicals available for the coupling reaction. These forward and reverse reactions are included in our mechanism, but if they are omitted the results are essentially unchanged. Thus, it does not appear that reaction 10 is affecting the C, selectivity. Instead, the model indicates that the positive effect of temperature on C, selectivity is achieved by the greater production rate of CH,. radicals via the surface reaction and reactions 7-9. The heterogeneous-homogeneous kinetic model may be used to interpret the kinetic isotope effects (KIE) for CH, conversion over several catalysts that have recently been reported (23,30-32). The KIE in its simplest form is defined as the ratio of the conversion rate of CH, to that of CD,.
A KIE also may be determined from the ratio of the amount of CH, to the amount of CD, in the ethane fraction, which is composed of q H 6 , CH,CD, and C,D,. A summary of the KIE's obtained at several different reaction conditions is given in Table 3. Table 3. Kinetic Isotope Effects in the Partial Oxidation of Methanea Catalyst
ConditionsMethod
KIE
Li+/MgO
750°C, 10% methaneb, 7.2% oxygen; methane conv.
1.5
Li+/MgO
68O"C, 20% methane, 10% oxygen; methane conv.
1.59
31
Li+/MgO
750°C, 45% CH,, 45% CD,, 10% 0,; H and D distribution in ethane
1.43
31
Li+/MgO
715"C, 15% CH,, 10% CD,, 12% 0,; H and D distribution in ethane
1.73 k 0.15
23
Li+/MgO
70O0C, 25% methane, 12.5% oxygen; methane conv.
1.39 zk 0.05
This study
65O0C,2.7% methane, 1.3% oxygen; methane conv.
1.3
32
Sm203
a
zk
Ref. 0.03
30
KIE = k,RD Total pressures are 760 torr.
A KIE of cu. 1.5 for methane conversion over a Li+/MgO catalyst led Cant et af. (30) to conclude that the rate limiting step in the catalytic cycle
0,
+
CH3.
20% 02-
CH,
+
+
4
OH,
-+.
CH3* 0”
[ ]
+
+
CH3*
+ M qH6 + + [ ] + H2O 4
(11)
M
(12) (13)
1/20, -20,
(14)
is reaction 11, which involves the breaking of a C-H bond. Earlier, Ito et al. (4) had proposed that this step was fast, and that either reaction 13 or 14 must be rate limiting. With the realization that methane also may be reacted via reactions 7-9 and that these reactions may contribute to the KIE, we have obtained KIE values over a Li+/MgO catalyst at different methane-to-oxygen ratios. The results are depicted in Fig. 1 (29). If the gas phase reactions give rise to the KIE and reaction 11
is not rate limiting, one would expect that the KIE effect would approach unity at a large methaneto-oxygen ratio, which indeed was observed. Although most of the data reported in Table 3 were obtained at moderate methane-to-oxygen ratios, Nelson el al. (31) carried out an experiment at a ratio of 9. From their FTXR data it is possible to determine the isotopic composition of their ethane, and from this conclude that the KIE was 1.4. The reason for the discrepancy between their KIE and that indicated in Fig. 1 is not evident.
1.8
I
I
I
I
I
L?/M go 1.6
-
T=700°C P(CH4)=190(torr)
1.4
1.2 1.o
0.8
-
0
I
I
I
I
I
2
4
6
8
10
12
Methane to Oxygen Ratio Figure 1. Influence of CH4:02 ratio on the kinetic isotope effect, determined from the conversion of CH, and CD,: 4, experimental result; A,calculated from the rcaction model.
9
When the theoretical KIEs of 1.8 and 2.1 at 700°C (33)were introduced, respectively, in all of the C-H(D) and 0-H(D) bond breaking reactions of the kinetic model, but not in the formation of methyl radicals at the surface, the resulting overall K E indicated by the dashed line in Fig. 1 was obtained. The calculated KIE's reflect the general trend exhibited by the experimental values, which supports the hypothesis that gas phase reactions are largely responsible for the KIE. Our current view of the surface reaction is that 0; is a transient active center because the hole at nearly every (electron) is in rapid motion among the oxygen ions. Moreover, CH, reacts with Oscollision (34). Therefore the rate of CH,. radical formation via surface reactions is given by CH3* OL
r.[cH41[o;l
(15)
and
+ 052- *
+
[L~+o-I~ [ L ~ + O ~ - ] ~0, (16) Here r is the lifetime of the 0; center on the surface, and [ L i + 0 l b is the concentration of a bulk center that has been characterized by ESR spectroscopy (35). If reaction 14 were a t equilibrium, then the concentration of [O;]would be proportional to [O,].'
In the kinetic model the production
of CH3- radicals is assumed to be first order with respect to CH4 and zero order with respect to 0,. Essentially the same results would have been obtained assuming a zero order surface reaction with respect to CH, since the conversion of CH, was small. Among the reactions 11-14it appears likely that reaction 13 is the slowest. Cant et nl. (30)have argued that reaction 13 is not rate limiting because the addition of H20 or D 2 0 had no discernible effect on the KIE. But if the KIE were a result of gas phase reactions, one would expect this result. The observation that there is no KIE at high CH4:02 ratios (Fig. 1) is more problematic if reaction 13 is indeed rate limiting, as this reaction involves the breaking of an 0-H bond. It may be, however, that the rate offormation of water is limited by the removal of oxygen from the lattice and not by the breaking of an 0-Hbond. Therefore, the usual kinetic isotope effect would not be expected.
- Radicals with the Catalysts. Secondaxv Reactions of CH,. One feature of the kinetic model that is lacking is the incorporation of reactions between the catalyst and radical intermediates. Generally, such rate data do not exist; however, we are beginning to obtain both qualitative and quantitative data on the reactions of CH3- radicals with metal oxides that are of interest in the coupling reaction (7,36). Based upon a reasonable concentration of CH,. radicals in a catalyst bed (26) it may be shown that a given radical will collide with a surface approximately
1 6 times before it reacts with another CH,.
radical. Obviously, if there is a high
probability that a collision with the surface results in an undesirable reaction, the selectivity for
C,
formation would be small. The relative reaction rates between various members of the lanthanide oxide series and CH,. radicals are very different, and those oxides that react significantly with the
10
radicals are nonselective catalysts (9). The metal ions in the most reactive oxides have multiple accessible oxidation states (e.g. Pr6011), and presumably the CH,. radicals react via a reductive addition, forming the reduced form of the metal ion and a surface methoxide species. These secondary reactions may be minimized by covering the reactive oxide with a relatively inert compound such as Na2C03. Using a modified form of the MIESR system the reactive sticking coefficient, u, (the probability that a collision with a surface results in a reaction) and the activation energy for the reaction of CH,. radicals with ZnO and MgO were determined. These oxides were selected because the former
is among the most reactive of the oxides that we have studied and the latter is relatively inactive.
The results are summarized in Table 4. Even on ZnO the reactive sticking coefficient is surprisingly small. Nevertheless, it is sufficiently large to account for the nonselective behavior of Z n O in the oxidation of CH4 (37). By contrast, the reactive sticking coefficient of CH,. is two orders of magnitude less on MgO than on ZnO at 482°C and at 720°C. Table 4. Sticking Coefficients and Activation Energies ~
oa
Oxides
E, (KcaVmol)
ZnO
1.8 10”
2.6
MgO
1.2
lo-’
5.7
a
T = 755 K
The Oriein of COX Perhaps the most thoroughly studied, and yet most controversial aspect of the oxidative coupling reaction is the origin of the COXproducts. Since selectivity is an important factor in any economic evaluation of the process, it is necessary to determine the manner in which the nonselective products are formed. The origin of COXmay be best discussed using the relatively simple mechanistic scheme kl
cH4
k2
- GH6- GH4
9
p
4
cox
p
5
11
which shows that COX,in principle, may be produced by each of the three hydrocarbons. Part of the difficulty in establishing the primary source of COXresults from the fact that the reaction rates of pure hydrocarbons over a given catalyst are different from those obtained with a mixture of two or three hydrocarbons (38,39). This effect is particularly evident in oxygen-limiting reactions. Moreover, factors such as the CH,:O,
ratio, the total pressure of the reactants, the CH, conversion, the
temperature of the reaction, the free volume in the reactor and the type of catalyst play a role in determining the major source of CO,. The magnitudes of these potential variables are often quite different among the various studies that have been reported, therefore it is difficult to compare results. Moreover, conclusions that have been based on data obtained at a particular set of conditions cannot be generalized (4,38-43). For example, Ito et al. (4) studied the formation of COX over Li'MgO
at 620°C, with an initial CH,:O, ratio of 4.4 and at a CH, conversion of 1.6%, and
concluded that most of the CO, was derived from step 3; ie., the direct catalytic oxidation of CH,. By contrast, over the same catalyst operating at 720°C and with a CH4:02 ratio of ca. 20, Roos et al. (38) obtained data which allowed them to conclude that the COXproducts were formed solely from CzH4. The most definitive results have been obtained recently by adding 13%H6 and 13%H4 to a CHdO, mixture and observing the amount of I3C in the COXproducts. Ekstrom et al. (39) have
used this method to show that the percentage of 13C in the COX products greatly exceeded the percentage of 13C in the feed gas. Over a Sm203 catalyst operating at 700°C with 10% 0, in CH, they reported that with 19% 13C present as l 3 q H 4 in the feed there was 86% I3C in the CO,. There was slightly more l3COXpresent with l3%H, than with an equivalent amount of 13%H,. The authors concluded that COXis largely formed via the % products and that %H4 is oxidized somewhat faster than GH, to CO,.
Using a similar method Nelson and Cant (42) have studied the oxidation
of CH, over Li+/MgO at several different temperatures. In agreement with Ito et al. (4) they found
C, products accounted for less than 10% of the COX,but at temperatures greater than 740"C, C, oxidation was responsible for the formation of 3040% of the
that at T < 700°C the oxidation of
co,.
It is of interest to know whether the q H 6 and the %H4 are converted to COXin the gas phasc
or on the surface of the catalyst. The extent of the surface and gas phase reactions will depend on the CH4:0, ratio, etc. Under the oxygen-limiting conditions employed by Roos el al. (38) it was concluded that at 720°C the reactions were mainly heterogeneous, but at 800°C there could have been a considerable homogeneous component. Geerts et al. (41) also studied the source of COXover a Li+/MgO catalyst at 800°C and concluded that an appreciable part of the reaction sequence takes place in the gas phase. The results of the model (Table 2) indicate that COXcould be formed via gas phase reactions at 700°C. Here we report the COXas CO, because over Li+/MgO CO is extensively converted to CO, (40). Even at much greater CH4:02 ratios (e.g lo), the model indicates the formation of COXin the gas phase.
12
CONCLUSIONS Recent experimental and modeling studies on the catalytic oxidation of methane over metal oxide catalysts have provided insight into the complex mechanisms through which this reaction occurs. The catalyst is the origin of CH,. radicals that enter into chain-branchinggas-phase reactions. These gasphase reactions account for much of the chemistry that is observed, including kinetic isotope effects and the formation, in part, of products such as G H 4 and CO, The partial pressure of 0, influences the extent of the homogeneous component; the role of CH,Or in equilibrium with CH,. and 0, appears to be a less significant factor in determining selectivity than was previously thought. Secondary reactions of gas phase CH3*radicals with the metal oxide may provide an alternate route to CO,
The similar catalytic behavior observed over many different metal oxide catalysts may be
attributed to the dominant effects of these homogeneous reactions. ACKNOWLEDGMENTS The author is indebted to Mr. Chunlei Shih for his part in developing the reaction model and for carrying out the KIE experiments. Mr. Youdong Tong performed the MIESR experiments. The research in our laboratory was supported by the National Science Foundation under Grant CHE-
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