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 oxid e catalyst (Li/MgO) containing small amounts of Zn2+ cations (Zn/Li/Mg O) was theoretically studied using a modified MINDO/3 method and apply ing a supermolecular approach. The surface of magnesium oxide (MgO) wa s modelled by a Mg32O32 four-layer molecular cluster containing all ty pes of structural defects i.e., low-coordinated magnesium and oxygen i ons (Mg-LC(2+) and O-LC(2-)) of various faces, edges, corners etc. Mol ecular clusters of lithium-promoted magnesia (Li/MgO) were simulated b y isomorphic substitution of Mg-LC(2+) by Li-LC(+); the excess negativ e charge of the cluster was compensated by a proton connected to an O- 3C(2-) site. For Zn-doped Li/MgO or MgO an isomorphic substitution of Mg-LC(2+) by Zn-LC(2+) was assumed. The calculations indicate that for Zn/Li/MgO or Zn/MgO the substitution of a threefold coordinated magne sium cation by zinc is more favourable by energetics than for four- an d five-fold coordinated Mg cations. The computational results are used to interpret the experimentally observed increase of C-2+ hydrocarbon s selectivity in the oxidative coupling of methane when doping a NaOH- promoted CaO catalyst with minor amounts of Zn2+ cations.