Quantum Chemical Study of the Catalytic Oxidative Coupling of Methane

Isik Onal§ and Selim M. Senkan

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

§ Permanent address, Middle East Technical University, Department of Chemical Engineering, Ankara, Turkiye.

ABSTRACT

Oxidative coupling of methane reaction pathways on MgO and Lithium-modified MgO was 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 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 activation energies for Li-modified MgO cluster as compared to unmodified MgO.





Figure 1. The Mg9O9 cluster used in the simulations and the optimized final structure of dissociatively adsorbed CH4 molecule:
	    (a) Ball and stick representation
            (b) Wire frame model representation


Figure 2. Evidence for the synergistic interplay between Mg and Li atoms. Calculations were performed using (H3C)H-O3 interatomic distance as the reaction coordinate.

Here is a recent paper from our group on this subject.