Cluster quantum-chemical study of the chemisorption of methane on a lithium-promoted magnesium oxide doped by zinc oxide

The reaction of a methane molecule with a lithium-doped magnesium oxide catalyst ( Li MgO) containing small amounts of Zn²⁺ cations (Zn/Li/MgO) was theoretically studied using a modified MINDO/3 method and applying a supermolecular approach. The surface of magnesium oxide (MgO) was modelled by a Mg₃₂O₃₂ four-layer molecular cluster containing all types of structural defects i.e., low-coordinated magnesium and oxygen ions (Mg²⁺_LC and O^2-_LC) of various faces, edges, corners etc. Molecular clusters of lithium-promoted magnesia ( Li MgO) were simulated by isomorphic substitution of Mg²⁺_LC by Li⁺_LC; the excess negative charge of the cluster was compensated by a proton connected to an O^2-_3C site. For Zn-doped Li MgO or MgO an isomorphic substitution of Mg²⁺_LC by Zn²⁺_LC was assumed. The calculations indicate that for Zn/Li/MgO or Zn MgO the substitution of a threefold coordinated magnesium cation by zinc is more favourable by energetics than for four- and five-fold coordinated Mg cations. The computational results are used to interpret the experimentally observed increase of C₂₊ hydrocarbons selectivity in the oxidative coupling of methane when doping a NaOH-promoted CaO catalyst with minor amounts of Zn²⁺ cations.