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

The highly ordered mesoporous material HISiO2 was prepared at room temperat ure and low pH utilizing a high concentration of nonionic surfactant to ach ieve a hexagonal ordered phase with a pore size of similar to3.5 nm. The gr afted amino ligand was covalently bonded to the internal pore surface of HI SiO2 through a silanation procedure. Thereby, immobilized transition-metal- aquo complexes such as Mn-II-aquo (I), Cu-II-aquo (II), Co-II-aquo (III), a nd Zn-II-aquo (IV) were coordinated to the supported wall without impregnat ion on the surface. Diffuse reflectance spectroscopy (DRS) and electron par amagnetic resonance (EPR) studies observed that a proportion of the Mn-II c omplex was oxidized to a higher oxidation state, particularly Mn-IV. The ki netics and mechanism of redox reactions between o-aminophenol, o-phenylened iamine, 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 fi rst-order kinetics, and the rate constant of the oxidation of amines decrea ses in the following order: Mn-IV/Mn-II --> Cu-II --> Con --> Zn-II. This t rend is attributed to the reduction potential of the metal ions in the reac tion medium. The most obvious feature of the oxidation reaction of amines w ith complexes III and IV is that there is a well-defined induction time, wh ose rate depends on the reactivity and the initial concentration of these a mines, prior to a rapid growth in the production of the oxidation product o f amines. The experimental results indicate that the outer-sphere mechanism is probably followed in this redox system. Extensive studies of the transi tion-metal complexes on HISiO2 have been conducted before and after the red ox reaction by a wide variety of characterization techniques which include powder X-ray diffraction, DRS, the Bnxnauer-Emmett-Teller method for nitrog en adsorption and surface area measurements, NMR, EPR, and IR.