Industrial & Engineering Chemistry Research, 36, 4028-4032 (1997).

Quantum Chemical Study of the Catalytic Oxidative Coupling of Methane

Isik Onal^# and Selim Senkan^*

Department of Chemical Engineering, University of California, Los Angeles, California 90095

* Tel: 310-206-4106 Fax: 310-206-4107 E-Mail: senkan@seas.ucla.edu

^# Permanent address, Middle East Technical University, Department of Chemical Engineering, Ankara, Turkey.

ABSTRACT

Oxidative coupling of methane reaction pathways on MgO and Lithium-modified MgO were theoretically studied using the semi-empirical MNDO-PM3 molecular orbital method. The surface of MgO catalyst was modelled 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 O^2- 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 Mg_3c-O_3c with a neighboring Li_4c site present. Various sequential oxygen and methane adsorption pathways were explored resulting in CH₃OH formation with lower energy barriers for Li-modified MgO cluster as compared to unmodified MgO.

INTRODUCTION

Methane is the most abundant component of natural gas, usually containing over 90 mole % 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 lithium-doped 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 OCM process. In particular, the adsorption of methane molecule on pure and modified oxide surfaces were 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 calculations of heat of formation for adsorbed complexes, and they did not involve reaction coordinate computations. The most favorable active sites were identified to be low-coordinated 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 semi-empirical MNDO-PM3 formalism is explored (Stewart 1989 and 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 substitution of Mg by Li, and by connecting 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 (.68 A and .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, as well as the geometry of the CH₄ molecule during its various stages of adsorption are presented. These calculations were made by considering H₃CH-O_3c interatomic distance as the reaction coordinate, where O_3c 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 Mg_3c 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 the oxygen atom at the edge of the cluster (O_4c). That is, the CH₃ group still was preferentially adsorbed on the Mg_3c site as opposed to the edge, Mg_4c site, and 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 O_3c, O_4c and O_5c 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 kcal/mole and 18.4 kcal/mole for sorptions on O_3c and O_4c sites, respectively. This indicates a preferential adsorption on the site with the lower coordination number. This result is not surprising because O_3c have a greater fraction of its dangling orbitals exposed than the O_4c atoms. Calculations performed for CH₄ molecule approaching O_5c 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/mole on to the O_3c site and about 44.7 kcal/mole to the O_4c 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 O_3c site. Adsorption to the O_4c 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 dissociative 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. Mg_3c, by Lithium. Chemisorption energetics similarly were calculated by considering H-O_xc interatomic distance as the reaction coordinate where x denotes the coordination number. The relative energy levels determined for the case of H-O_3c are compared for Li-substituted and unsubstituted MgO cluster in Table 1. As in the case of the Mg₉O₉ cluster, the dissociative adsorption process on Li_3cMg₈O₉H leads to the chemisorption of CH₃ on Li_3c site and H on the O_3c site which is consistent with the experimental studies of Ito et al.(1985). However, although the energy barrier for CH₄ chemisorption on Li_3c-O_3c pair site is approximately the same as on Mg_3c-O_3c at about 32.4 kcal/mole, the heat of adsorption was very slightly endothermic at -1.8 kcal/mole. 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 O_5c is used as an active site there is a substantial effect of an all Li 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 O_5c site of Mg₉O₉ cluster (infinitely high energy barrier) it requires a barrier height of 68.8 Kcal/mole for the all Li surface case with a heat of formation of 59.5 Kcal/mole (see Table 1). More interestingly the active site pair is now O_3c-O_5c which is another indication that the presence of Li atom on the surface statistically increases the type of possible active sites for CH₄ dissociation as also pointed out by Zhanpeisov et al.(1992). However, their conclusion was made only on the basis of 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-O_3c as reaction coordinate for a Li_4cMg₈O₉H cluster where Li_4c is a neighbor to the active Mg_3c site. As observed in Figure 5, there is a slight decrease in the activation energy of CH₄ dissociation on Mg_3c-O_3c pair site. However, this effect of neighboring Li is very pronounced for the case of Mg_3c-O_4c as active sites as illustrated in Figure 6. In this case , the energy barrier decreases from 44.7 to 15.9 Kcal/mole indicating a kinetically favorable CH₄ dissociation site. The heat of formation is also lower (30.8 vs 18.4 Kcal/mole) whereby the release of methyl radical to the gas phase becomes easier. These conclusions certainly support widely obtained experimental data concerning the promotional effect of Li on MgO-based 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 Li-modified MgO clusters. In the first study, methane was initially adsorbed by the active sites Li_3c-O_3c and Mg_3c-O_3c of the respective clusters as described previously and then oxygen was adsorbed by bringing O₂ to the active metal sites Mg_3c and Li_3c. The energetics of these adsorption processes are presented in Figure 7. Oxygen dissociatively adsorbs on Li_3c site with a high heat of adsorption of 55 Kcal/mole and with almost no energy barrier. The methyl radical is simultaneously displaced and forms 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-Li_3c complex formed on the surface. The energetics of this last step is favorable with an energy barrier height of 23.9 Kcal/mole and a heat of adsorption of 62.3 Kcal/mole as shown in Figure 8. The final optimized structure is depicted in Figure 9. For the case of Mg_3c site, however, due to the high strength of Mg_3c-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 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 O_3c site and then CH₄ was adsorbed on the same site by the same dissociation mechanism given above. The final outcome is approximately the same ; 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).

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 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 represent 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 steer the experiments towards most promising directions in catalyst discovery and development.

ACKNOWLEDGEMENT

This research was funded, in part, by the National Science Foundation, the US Environmental Protection Agency, the UCLA Center for Clean Technology and the UCLA Chemical Engineering Department.

LITERATURE CITED

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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/mole)	Heat of Adsorption (kcal/mole)
Mg3c - O3c	31.8	44.7
Li3c - O3c	32.4	-1.8
Li3c - O3c (All Li surface)	26.5	0.9
Mg3c - O5c	Very High	--
O5c - O3c	68.8	59.5

Figures

Figure 1. The Mg₉O₉ cluster and optimized structure of CH₄ during different structural changes of dissociative adsorption system along the reaction coordinate where H-O_3c distance is : (a) 4 A; (b) 0.98 A; and (c) 0.94 A (optimized final structure).

Figure 2. Relative energy change for CH₄ adsorption on O_3c, ,O_4c and O_5c sites of MgO.

Figure 3. Energy change for CH₄ adsorption on O sites with different coordination.

Figure 4. Changes of charge on the Mg, H, and O_3c atoms and the methyl group.

Figure 5. Effect of neighboring Li_4c site on adsorption energetics of CH₄ on Mg_3c - O_4c pair.

Figure 6. Effect of neighboring Li_4c site on adsorption energetics of CH₄ on Mg_3c - O_3c pair.

Figure 7. Adsorption of O₂ on Li_3c and Mg_3c sites with methyl radical already attached.

Figure 8. Reaction of CH₄ molecule with CH₃O-O complex attached to Li_3c site.

Figure 9. The final optimized structure of CH₄ molecule reacting with the CH₃O-O surface complex formed on Li-modified MgO cluster.