Ti-MWW分子筛钛氧活性中心与溶剂分子吸附作用的理论研究

采用密度泛函理论方法,研究了钛硅分子筛Ti-MWW与H₂O₂所形成的钛氧活性中间体的结构,以及溶剂分子吸附对其几何结构和电子结构的影响。结果表明,骨架Ti与H₂O₂作用生成两种钛氧活性中间体,即五元环的Ti-η¹-OOH和三元环的Ti-η²-OOH,骨架Ti中心可再吸附一个溶剂分子,形成六配位络合物结构,不同溶剂分子的吸附能力为H₂O > CH₃OH > CH₃CN。两种钛氧活性中心与溶剂分子的吸附作用略有不同,表现为Ti-η¹-OOH > Ti-η²-OOH,而Ti的落位也对吸附效应有很大影响, T1位点的Ti中心与溶剂分子的吸附作用明显强于T3位点的Ti中心。溶剂分子吸附还将影响活性氧的亲电性和催化活性。计算结果表明在Ti-η²-OOH活性中心吸附CH₃CN可降低氯丙烯环氧化反应的活化能。

Theoretical Study of the Solvent Adsorption on Ti-Peroxo Active Center in Ti-MWW Zeolite

Density functional theory is used to study the structures of the Ti-peroxide species in a Ti-MWW/H₂O₂ system and the influence of the coordination with solvent molecules on the geometries and electronic features. The result indicated that the framework Ti can interact with H₂O₂ to form two kinds of Ti-peroxide species, the five-membered ring Ti-η¹-OOH and the three-membered ring Ti-η²-OOH. The framework Ti center can further interact with solvent molecules of H₂O, CH₃OH, and CH₃CN to form 6-coordinated complexes. The adsorption energies decreased as H₂O > CH₃OH > CH₃CN. Both Ti-peroxide species display different adsorption capacity as Ti-η¹-OOH > Ti-η²-OOH. The location of Ti also has a distinct effect on the adsorption. The central Ti at T1 prefers to adsorb solvent molecule over the Ti at T3. In addition, solvent adsorption also influences the electrophilicity of the active oxygen atom. The calculated results indicate that on the Ti-η²-OOH active center with adsorbed CH₃CN, the activation energy of the epoxidation of chloropropene has been reduced.