关键词: 钠, 钨, 锰, 二氧化硅, 催化剂, 协同效应, 分子轨道理论, 氧化, 偶联, 甲烷

The performance of Na-W-Mn/SiO~2 catalyst for oxidative coupling of methane shows that manganese alone is very active and results in deep oxidation of hydrocarbons to form CO~x. Adding sodium will strongly restrain the activity of manganese containing catalysts and give high C~2 selectivity. Study on the electronic structure of the catalyst and the nature of molecular orbital of possible metal sites reveals the mechanism of the Na-MN interaction. Sodium in the catalyst will give out gree electrons, and manganese in the bulk phase produce a virtual band at about - 3eV. Over the catalyst surface, manganese emists as dispersed tetrabedral [MnO~4] or oxide clusters. The energy level of the LUMO in the Mn-0 sites, dispersed centers, or clusters, is rather low (-7eV ～-5eV). The low LUMO level of the Mn-0 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-0 sites. As a consequence, adding sodium will not significantly increase the basicity and the catalyst will not be poisoned due to subsequent adsorbing CO~2. However, the Mn-0 sites will lose their strong oxidation activity after accepting electrons from sodium. The Ma- Mn interaction plays a key role in the high C~2 selectivity of the Na-W- Mn/SiO~2catalyst.

Key words: SODIUM, TUNGSTEN, MANGANESE, SILICON DIOXIDE, CATALYST, SYNERGISTIC EFFECT, MOLECULAR ORBITAL THEORY, OXIDATION, COUPLED, METHANE