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

✍ Scribed by N.U. Zhanpeisov; M. Baerns

• Publisher: Elsevier Science
• Year: 1995
• Tongue: English
• Weight: 411 KB
• Volume: 99
• Category: Article
• ISSN: 1381-1169
• DOI: 10.1016/1381-1169(95)00030-5

✦ Synopsis

The reaction of a methane molecule with a lithium-doped magnesium oxide catalyst (Li/MgO) containing small amounts of Znzt cations (Zn/Li/MgO) was theoretically studied using a modified MIND0/3 method and applying a supermolecular approach. The surface of magnesium oxide (MgO) was modelled by a MgX20X2 four-layer molecular cluster containing all types of structural defects i.e., low-coordinated magnesium and oxygen ions (Mgtg and 0;; ) of various faces, edges, comers etc. Molecular clusters of lithium-promoted magnesia (Li/MgO) were simulated by isomorphic substitution of Mg:: by Li&; the excess negative charge of the cluster was compensated by a proton connected to an 0:; site. For Zn-doped Li/MgO or MgO an isomorphic substitution of MgtG by Zntg 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.