Incorporation of Transition-Metal Complexes in Functionalized Mesoporous Silica and Their Activity toward the Oxidation of Aromatic Amines

Abstract

The highly ordered mesoporous material H I SiO 2 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 I SiO 2 through a silanation procedure. Thereby, immobilized transition-metal-aquo complexes such as Mn IIaquo (I), Cu IIaquo (II), Co IIaquo (III), and Zn IIaquo (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 f Cu II f Co II f 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 I SiO 2 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.

Cyclam‐Functionalized Silica‐Modified Electrodes for Selective Determination of Cu(II)

“…Only a very small decrease in the intensity of diffraction peaks was observed. This can be explained by partial structure collapse of the ordered phases [36] or due to contrast matching between the inorganic framework and the organic ligands [37]. As expected, the grafting process resulted in a decrease in both the specific surface area, pore volume and pore diameter due to the space occupied by cyclam moieties in the porous structures.…”

Heterogeneous kinetic studies of the hydrogen peroxide decomposition with some transition metal‐heterocyclic complexes

“…This means that the energy necessary for the formation of the active species (intermediate) at the beginning of the reaction was smaller than that necessary for the H 2 O 2 decomposition. The H # value of activation is in the range of the chemical reaction control [29,38]. The G # value of activation was found to be in the 81-101 kJ mol −1 range.…”

“…However, in the case of Ni(II) complexes the following order was found: (Table III). It is clear that the greater the stability constant of the complex, the higher is the rate constant of the reaction (Tables I and III) [38].…”

Nanoporous Silica-Supported Nanometric Palladium: Synthesis, Characterization, and Catalytic Deep Oxidation of Benzene

“…However, this method has some difficulties in preventing the active phases from agglomerating outside the pores of the supports. Postsynthesis grafting procedures involve modifying the silica host with some types of organosiloxane containing organic functional groups, such as thiol (14), amine (15), EDTA (16), etc. In most cases, the organic moieties are planted onto the silica wall by refluxing the dry toluene or hexane containing both pre-synthesized silica and organosiloxane.…”