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Ind. Eng. Chem. Res. 1997, 36, 4028-4032
Quantum Chemical Study of the Catalytic Oxidative Coupling of Methane† Isik Onal* Department of Chemical Engineering, Middle East Technical University, Ankara, Turkey 06531
Selim Senkan* Department of Chemical Engineering, University of California, Los Angeles, California 90095
Oxidative coupling of methane reaction pathways on MgO and lithium-modified MgO were theoretically studied using the semiempirical MNDO-PM3 molecular orbital method. The surface of the MgO catalyst was modeled by a Mg9O9 molecular cluster containing structural defects such as edges and corners. Lithium-promoted magnesia was simulated by isomorphic substitution of Mg2+ by Li+; the excess negative charge of the cluster was compensated by a proton connected to a neighboring O2- site. Heterolytic adsorption of methane was found to be directly related to the coordination number of both the lattice oxygen and the metal sites. Energetically the most favorable site pair was Mg3c-O3c with a neighboring Li4c site present. Various sequential oxygen and methane adsorption pathways were explored resulting in CH3OH formation with lower energy barriers for the Li-modified MgO cluster as compared to unmodified MgO. Introduction Methane is the most abundant component of natural gas, usually containing over 90 mol % of the hydrocarbon fraction, and represents a possible raw material for the synthesis of more valuable products such as ethylene. However, methane’s high molecular stability compared to other aliphatics makes its use difficult, and no significant amount of ethylene is produced commercially from methane today. Extensive experimental studies have been conducted on the oxidative coupling of methane since the works of Keller and Bhasin (1982) and of Hinsen and Baerns (1984). Work by Ito et al. (1985) showed that lithiumdoped magnesium oxide has high activity for converting methane to C2+ compounds in the presence of O2; however, the volatility of lithium hampered the potential utility of these catalysts. Several authors such as Lee and Oyama (1988), Hutchings et al. (1989), Dubois and Cameron (1990), and Amenomiya et al. (1990) published review articles on oxidative coupling of methane. In recent years, several theoretical studies involving ab initio and semiempirical methods have been conducted to better elucidate the mechanism of the oxidative coupling of methane (OCM) process. In particular, the adsorption of the methane molecule on pure and modified oxide surfaces was studied. Ito et al. (1991) examined chemisorption of methane on MgO clusters by means of ab initio molecular orbital methods. In this study it was shown that methane heterolytically dissociates on the nearest pair of three-coordinated surface magnesium and oxygen atoms which were the most active sites. Zhanpeisov et al. (1990, 1995) performed quantum chemical MINDO/3 calculations for adsorption of methane on pure and modified MgO, CaO, and ZnO cluster surfaces. These studies were limited to calcula* To whom correspondence should be addressed. † Contributed from Computational Chemistry and Its Industrial Applications, American Institute of Chemical Engineers Annual Meeting, November 14, 1996. S0888-5885(96)00726-9 CCC: $14.00
tions of heat of formation for adsorbed complexes, and they did not involve reaction coordinate computations. The most favorable active sites were identified to be lowcoordinated adjacent-pair Mg-O sites for the cases of both unmodified and Li-modified MgO. In this paper the energetics of the OCM process on MgO and Li-doped MgO catalysts using the semiempirical MNDO-PM3 formalism is explored (Stewart, 1989, 1991). Reaction mechanisms for the formation of C2 hydrocarbons and nonselective products were identified through computations of relative energy barriers and heats of adsorption along hypothetical reaction coordinate steps and compared with experimental data as well as some ab initio calculations reported in the literature. The objectives of our studies were to theoretically estimate trends in reaction energetics and to elucidate mechanisms, as well as to evaluate the feasibility of using semiempirical quantum chemistry as a rapid screening tool in catalyst discovery. The determination of accurate activation energies was not the purpose of this study due to the current limitations of semiempirical quantum chemical methods. Surface Model and Calculation Method Chemical activity of magnesium oxide in catalytic reactions is often connected with low-coordinated magnesium and oxygen ions of various surface irregularities such as edges, corners, and faces. The MgO catalyst was modeled as a Mg9O9 molecular cluster exhibiting such surface structural defects. This cluster size is significantly larger than the molecular sizes of CH4, O2, and OCM products, so that the chemisorption properties of the surfaces should be minimally altered. Early in the program, the effects of cluster size on the energetics of the OCM reaction was also explored. From these studies we determined that larger clusters did not significantly change the energetics of corner or edge atoms. The Mg9O9 cluster was assumed to exhibit the cubic rock salt structure of MgO with Mg-O bond length of 2.106 A (Ito et al., 1991). Molecular clusters of Li-doped MgO were then simulated by the isomorphic © 1997 American Chemical Society
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4029
Figure 1. The Mg9O9 cluster and optimized structure of CH4 during different structural changes of the dissociative adsorption system along the reaction coordinate where H-O3c distance is (a) 4 A, (b) 0.98 A, and (c) 0.94 A (optimized final structure).
substitution of Mg by Li, and by the connection of a proton to the adjoining O atom to compensate for the differences in valencies. The ionic radii of Li and Mg ions are close to each other (0.68 and 0.82 A, respectively), and substitution of Mg by Li which has the smaller radius is favorable. As a general computational procedure, the energies of the reactants, intermediates, and the catalysts were calculated by fixing the crystal structure of the clusters and by optimizing the remaining structural parameters of the system. That is, deformations of the cubic crystal structure were ignored with the underlying assumption that the effect of the coordination of the atoms that form the adsorption site is more important than that of the cluster relaxation as also concluded by Poveda et al. (1996) in theoretical work on carbon chemisorption on a nickel cluster. In general it is known that physical oxide surfaces are relaxed (have a lattice parameter different from that of the bulk) and rumpled (have different surface planes for anions and cations). For MgO, however, LEED studies and shell-model calculations agree in indicating a small (less than 5%) contraction of the surface lattice and rumpling of approximately the same magnitude (Fowler and Tole, 1988). The experimental data on the structures of oxides sufficient to examine relaxation in the surface layers is very sparse, and there is no such detailed data under reaction conditions, with reactive intermediates adsorbed. Results and Discussion In Figure 1, the sketch of the Mg9O9 cluster used in the simulations and the geometry of the CH4 molecule during its various stages of adsorption are presented. These calculations were made by considering the H3CH-O3c interatomic distance as the reaction coordinate, where O3c is the corner atom with a coordination number of 3. As can be seen from these figures, as the CH4 molecule approaches the cluster, the CH3 group also strongly interacts with the Mg3c atom, resulting in the stretching on the C-H bond distance, finally leading to the dissociative adsorption of CH4. These events can be represented by the following elementary reactions:
CH4 (g) + [ ]O3c ) [H3CH]O3c [H3CH]O3c + [ ]Mg3c ) [H]O3c + [CH3]Mg3c Similar behavior was observed when CH4 approaches
Figure 2. Relative energy change for CH4 adsorption on O3c, O4c, and O5c sites of MgO.
the oxygen atom at the edge of the cluster (O4c). That is, the CH3 group still was preferentially adsorbed on the Mg3c site as opposed to the edge, Mg4c site, and is shown below:
CH4 (g) + [ ]O4c ) [H3CH]O4c [H3CH]O4c + [ ]Mg3c ) [H]O4c + [CH3]Mg3c In Figure 2, the relative energies of the CH4 + Mg9O9 complex are plotted as a function of the H-O interatomic distance as the CH4 molecule approached the O3c, O4c, and O5c sites. The relative energy is defined to be the difference between the total enthalpy of formation of the CH4 + Mg9O9 complex at any H-O interatomic distance and the sum of the enthalpies of formation of the free Mg9O9 cluster and the approaching CH4 molecule. As evident from Figure 2, the adsorption of CH4 on MgO is, as expected, an exothermic process. The calculated heat of adsorption was about 35 and 18.4 kcal/mol for sorptions on O3c and O4c sites, respectively. This indicates a preferential adsorption on the site with the lower coordination number. This result is not surprising because O3c has a greater fraction of its dangling orbitals exposed than the O4c atoms. Calculations performed for the CH4 molecule approaching the O5c site indicate that thermodynamically it is not a favorable process as shown in Figure 2. As evident from Figure 2 also, the dissociative adsorption of CH4 exhibits an activation energy barrier height of about 31.8 kcal/mol on to the O3c site and about 44.7 kcal/mol to the O4c site. The relative magnitudes of these energy barriers are in accord with the heats of adsorption through Evans-Polanyi considerations and again suggest the preferential adsorption of methane on the O3c site. Adsorption to the O4c site also exhibited an earlier transition state. In Figure 3 the relative energy of the CH4 + Mg9O9 complex is plotted as a function of the C-H interatomic distance to illustrate the dissociation and migration of the CH4 molecule upon adsorption. The changes in the charge distributions on Mg, O, H, and the CH3 group are similarly presented in Figure 4. As evident from this figure, the H and CH3 groups become increasingly charged upon adsorption, indicating a heterolytic dis-
4030 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 Table 1. Relative Energy Barriers and Heats of Chemisorption for Methane Adsorption on Li-Doped and Undoped MgO Cluster active site pair
energy barrier (kcal/mol)
heat of adsorption (kcal/mol)
Mg3c-O3c Li3c-O3c Li3c-O3c
31.8 32.4 26.5
44.7 -1.8 0.9
Mg3c-O5c O5c-O3c
All-Li Surface very high 68.8
59.5
Figure 3. Energy change for CH4 adsorption on O sites with different coordination.
Figure 5. Effect of the neighboring Li4c site on adsorption energetics of CH4 on the Mg3c-O4c pair.
Figure 4. Changes of charge on the Mg, H, and O3c atoms and the methyl group.
sociative process. It is also interesting to note that the charge density on Mg decreased and that on O increased slightly upon the adsorption of CH4. These results, energetically, electrostatically, and mechanistically are in excellent agreement with ab initio calculations and experimental observations (Ito et al., 1991). For the case of Li-modified Mg9O9, the cluster was modeled by substituting one or more Mg atoms, i.e., Mg3c, by lithium. Chemisorption energetics similarly were calculated by considering the H-Oxc interatomic distance as the reaction coordinate where x denotes the coordination number. The relative energy levels determined for the case of H-O3c are compared for the Lisubstituted and unsubstituted MgO cluster in Table 1. As in the case of the Mg9O9 cluster, the dissociative adsorption process on Li3cMg8O9H leads to the chemisorption of CH3 on the Li3c site and H on the O3c site which is consistent with the experimental studies of Ito et al. (1985). However, although the energy barrier for CH4 chemisorption on the Li3c-O3c pair site is approximately the same as on Mg3c-O3c at about 32.4 kcal/ mol, the heat of adsorption was very slightly endothermic at -1.8 kcal/mol. This is a significant effect induced
by lithium and clearly suggests that the CH3 radicals should be desorbed more easily from the Li-doped MgO catalyst and form C2 products under the OCM conditions. These results do not change significantly when surface Mg atoms are all substituted by lithium atoms as also shown in Table 1. However, when O5c is used as an active site, there is a substantial effect of an allLi surface on the energy barrier of the dissociation process compared with an all-Mg surface as given in Table 1. While CH4 dissociation does not take place on the O5c site of the Mg9O9 cluster (infinitely high energy barrier), it requires a barrier height of 68.8 kcal/mol for the all-Li surface case with a heat of formation of 59.5 kcal/mol (see Table 1). More interestingly the active site pair is now O3c-O5c which is another indication that the presence of the Li atom on the surface statistically increases the type of possible active sites for CH4 dissociation as also pointed out by Zhanpeisov et al. (1995). However, their conclusion was made only on the basis of a positive heat of formation result. Another case providing further evidence to the synergistic interplay between Mg and Li atoms is when calculations are performed using H-O3c as the reaction coordinate for a Li4cMg8O9H cluster where Li4c is a neighbor to the active Mg3c site. As observed in Figure 5, there is a slight decrease in the activation energy of CH4 dissociation on the Mg3c-O3c pair site. However, this effect of neighboring Li is very pronounced for the case of Mg3c-O4c as active sites as illustrated in Figure 6. In this case, the energy barrier decreases from 44.7 to 15.9 kcal/mol, indicating a kinetically favorable CH4 dissociation site. The heat of formation is also lower (30.8 vs 18.4 kcal/mol) whereby the release of the methyl radical to the gas phase becomes easier. These conclu-
Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997 4031
Figure 6. Effect of the neighboring Li4c site on adsorption energetics of CH4 on the Mg3c-O3c pair.
Figure 7. Adsorption of O2 on the Li3c and Mg3c sites with the methyl radical already attached.
sions certainly support widely obtained experimental data concerning the promotional effect of Li on MgObased catalysts. A simulation of the OCM reaction involving the sequential sorption of O2 and CH4 with interchangeable order was also investigated over the MgO and Limodified MgO clusters. In the first study, methane was initially adsorbed by the active sites Li3c-O3c and Mg3cO3c of the respective clusters as described previously, and then oxygen was adsorbed by bringing O2 to the active metal sites Mg3c and Li3c. The energetics of these adsorption processes are presented in Figure 7. Oxygen dissociatively adsorbs on the Li3c site with a high heat of adsorption of 55 kcal/mol and with almost no energy barrier. The methyl radical is simultaneously displaced and forms a CH3O complex with a total charge of -0.096. When another CH4 molecule was brought in to react with it, CH3OH was released to the gas phase and a CH3O-Li3c complex formed on the surface. The energetics of this last step is favorable with an energy barrier height of 23.9 kcal/mol and a heat of adsorption of 62.3 kcal/mol as shown in Figure 8. The final optimized structure is depicted in Figure 9. For the case
Figure 8. Reaction of the CH4 molecule with the CH3O-O complex attached to the Li3c site.
Figure 9. The final optimized structure of the CH4 molecule reacting with the CH3O-O surface complex formed on the Limodified MgO cluster.
of the Mg3c site, however, due to the high strength of the Mg3c-CH3 bond molecular oxygen weakly adsorbs on this site without significant dissociation. The contrast between the two adsorption processes may explain the experimentally observed low methane conversion of unmodified MgO even to nonselective combustion products such as CO and CO2 (Dubois and Cameron, 1990). As for the Li-modified case it is very probable under OCM conditions that CH3OH will be converted to combustion products of CO and CO2; therefore, the O2 adsorption process described above is likely to be among the possible routes to lower C2+ selectivity. This point was confirmed by further calculations involving oxygen adsorption, but this time O2 was adsorbed first by bringing it to the O3c site and then CH4 was adsorbed on the same site by the same dissociation mechanism given above. The final outcome is approximately the same; the Li-modified cluster produces CH3OH with a much lower activation barrier as compared to unmodified MgO. These results involving oxygen adsorption may have a direct relation to the recent experimental evidence concerning the nonselective pathways of weakly adsorbed oxygen on MgO-based catalysts as reported by Mallens et al. (1996) and Buyevskaya et al. (1994).
4032 Ind. Eng. Chem. Res., Vol. 36, No. 10, 1997
The computational study presented above confirms the important role surface lattice oxygen with low coordination plays in OCM catalysis by providing a mechanism for heterolytic dissociation of methane leading to methyl radicals in the gas phase which would then combine to form higher molecular hydrocarbons. In summary, semiempirical quantum chemistry, as applied to the OCM process, allows the prediction of reaction energetics and mechanism in complete agreement with the experimental data. Since semiempirical methods have a distinct computational speed advantage over more rigorous ab initio wavefunction and density functional methods, semiempirical quantum chemistry represents a powerful screening tool in catalysis. Semiempirical methods can be used to establish the relative reactivity trends of a large number of clusters with modest computational resources, thereby steering the experiments toward most promising directions in catalyst discovery and development. Acknowledgment This research was funded, in part, by the National Science Foundation, the U.S. Environmental Protection Agency, the UCLA Center for Clean Technology, and the UCLA Chemical Engineering Department. Literature Cited Amenomiya, Y.; Birss,V. I.; Goledzinowski, M.; Galuszka, J.; Sanger, A. R. Conversion of Methane by Oxidative Coupling. Catal. Rev. Sci. Eng. 1990, 32, 163. Buyevskaya, O. V.; Rothaemel, M.; Zanthoff, H. W.; Baerns, M. Transient Studies on the Role of Oxygen Activation in the Oxidative Coupling of Methane over Sm2O3, Sm2O3/MgO, and MgO Catalytic Surfaces. J. Catal. 1994, 150, 71. Dubois, J. L.; Cameron, C. J. Common Features of Oxidative Coupling of Methane Cofeed Catalysts. Appl. Catal. 1990, 67, 49. Fowler, P. W.; Tole, P. Effects of Coordination Number on Surface Anions: An ab Initio Study of LiF and MgO. Appl. Catal. 1988, 457. Hinsen, W.; Bytyn, W.; Baerns, M. Oxidative Dehydrogenation and Coupling of Methane. In Proceedings, 8th International Congress on Catalysis, Berlin; Verlag Chemie: Frankfurt, 1984; Vol. 3, p 581.
Hutchings, G. J.; Scurrell, M. S.; Woodhouse, J. R. Oxidative Coupling of Methane Using Oxide Catalysts. Chem. Soc. Rev. 1989, 18, 251. Ito, T.; Pang, X.; Lin, C. H.; Lunsford, J. H. Oxidative Dimerization of Methane over a Lithium-Promoted Magnesium Oxide Catalyst. J. Am. Chem. Soc. 1985, 107, 5062. Ito, T.; Tashiro, T.; Kawasaki, M.; Watanabe, T.; Toi, K.; Kobayashi, H. Adsorption of Methane on Magnesium Oxide Studied by Temperature-Programmed Desorption and ab Initio Molecular Orbital Methods. J. Phys. Chem. 1991, 95, 4476. Keller, G. E.; Bhasin, M. M. Synthesis of Ethylene via Oxidative Coupling of Methane. J. Catal. 1982, 73, 9. Lee, J. S.; Oyama, S. T. Oxidative Coupling of Methane to Higher Hydrocarbons. Catal. Rev. Sci. Eng. 1988, 32, 249. Mallens, E. P. J.; Hoebink, J. H. B. J.; Marin, G. B. An Investigation of the Oxygen Pathways in the Oxidative Coupling of Methane over MgO-Based Catalysts. J. Catal. 1996, 160, 222. Poveda, F. M.; Sierraalta, A.; Villaveces, J. L.; Ruette, F. Modelling Carbon Chemisorption on a Nickel Catalyst. J. Mol. Catal. A: Chem. 1996, 106, 109. Stewart, J. J. P. Special Issue-MOPAC-A Semiempirical Molecular Orbital Program. J. Comput.-Aided Mol. Des. 1989, 4, 1. Stewart, J. J. P. Optimization of Parameters for Semiempirical Methods. J. Comput. Chem. 1991, 12, 320. Zhanpeisov, N. U.; Baerns, M. Cluster Quantum Chemical Study of the Chemisorption of Methane on a Lithium-Promoted Magnesium Oxide Doped by Zinc Oxide. J. Mol. Catal. A: Chem. 1995, 99, 139. Zhanpeisov, N. U.; Pelmenchikov, A. G.; Zhidomirov, G. M. Cluster Quantum Chemical Study of Interaction of Molecules With a Magnesium Oxide Surface. Kinet. Catal. 1990, 31, 563. Zhanpeisov, N. U.; Staemmler, V.; Baerns, M. A. Quantum Chemical MINDO/3 Study of Methane and Oxygen Interactions with a Pure and a Modified Calcium Oxide Surface. J. Mol. Catal. A: Chem. 1995, 101, 51.
Received for review November 8, 1996 Revised manuscript received May 2, 1997 Accepted June 16, 1997X IE960726X
X Abstract published in Advance ACS Abstracts, September 1, 1997.
