An MO study on Na-Mninteraction in Na-W-Mn/SiO2 catalyst

The performance of Na - W - Mn/SiO2 catalyst for oxidative coupling of meth ane shows that manganese loaded alone is very active and results in deep ox idation of hydrocarbons to form CO,. Adding sodium will strongly restrain t he activity of manganese containing catalysts and give high C-2 selectivity . Study on the electronic structure of the catalyst and the nature of molec ular orbital of possible metal sites reveals the mechanism of the Na-Mn int eractions. Sodium in the catalyst will give out free electrons,. and mangan ese in the bulk phase produce a virtual band at about - 3 eV. Over the cata lyst surface, manganese exists as dispersed tetrahedral [MnO4] or oxide clu sters. The energy level of the LUMO in the Mn-O sites, dispersed centers, o r clusters, is rather low ( - 7 eV similar to - 5 eV). The low LUMO level o f the Mn-O sites leads to strong ability to oxide hydrocarbons. When sodium is added, the free electron produced will occupy the virtual band produced by bulk manganese or take the LUMO of the surface Mn-O sites. As a consequ ence, adding sodium will not significantly increase the basicity and the ca talysts will not be poisoned due to subsequent adsorbing CO2 However, the M n-O sites will lose their strong oxidation activity after accepting electro ns from sodium. The Na-Mn interaction plays a key role in the high C-2 sele ctivity of the Na - W - Mn/SiO2 catalyst.