Incorporation of transition-metal complexes in functionalized mesoporous silica and their activity toward the oxidation of aromatic amines

The highly ordered mesoporous material H_ISiO₂ was prepared at room temperature and low pH utilizing a high concentration of nonionic surfactant to achieve a hexagonal ordered phase with a pore size of ∼3.5 nm. The grafted amino ligand was covalently bonded to the internal pore surface of H_ISiO₂ through a silanation procedure. Thereby, immobilized transition-metal-aquo complexes such as Mn^II-aquo (I), Cu^II-aquo (II), Co^II-aquo (III), and Zn^II-aquo (IV) were coordinated to the supported wall without impregnation on the surface. Diffuse reflectance spectroscopy (DRS) and electron paramagnetic resonance (EPR) studies observed that a proportion of the Mn^II complex was oxidized to a higher oxidation state, particularly Mn^IV. The kinetics and mechanism of redox reactions between o-aminophenol, o-phenylenediamine, and p-pheneylenediamine and the incorporated transition-metal-aquo-propylamine complexes have been investigated. The oxidation products of the amines have been monitored by UV-vis spectroscopy. The reaction follows first-order kinetics, and the rate constant of the oxidation of amines decreases in the following order: Mn^IV/Mn^II → Cu^II → Co^II → Zn^II. This trend is attributed to the reduction potential of the metal ions in the reaction medium. The most obvious feature of the oxidation reaction of amines with complexes III and IV is that there is a well-defined induction time, whose rate depends on the reactivity and the initial concentration of these amines, prior to a rapid growth in the production of the oxidation product of amines. The experimental results indicate that the outer-sphere mechanism is probably followed in this redox system. Extensive studies of the transition-metal complexes on H_ISiO₂ have been conducted before and after the redox reaction by a wide variety of characterization techniques which include powder X-ray diffraction, DRS, the Brunauer-Emmett-Teller method for nitrogen adsorption and surface area measurements, NMR, EPR, and IR.