The photoacoustic spectroscopy (PAS), Electron paramagnetic resonance (EPR) and thermogravimetric analysis (TGA) techniques have been used to characterize the redox properties of Mn(III) ions in MnAPO-5 molecular sieves of various manganese contents.
It was showed that a part of Mn(II) ions occluded in MnAPO-5 was oxidized to Mn(III) ions during the calcination and oxidized Mn(III) ions were easily reduced to Mn(II) by reducing agents.
When the adsorption of cyclohexene and other adsorbates was performed on the calcined MnAPO-5, the corresponding organic radicals were formed with the simultaneous reduction of Mn(III) ions to Mn(II). The Mn(III) ions probably function as electron-acceptor sites. Therefore, MnAPO-5 molecular sieves are expected as a potential oxidative catalyst by the redox characteristic of mangan ions.
The CoAPO-5 and CoAPO-11 samples were hydrothermally synthesized using triethylamine (TEA) and diisopropylamine (DiiPA) as template agents, respectively. PAS study of CoAPO-5 molecular sieve revealed that the amount of cobalt that can be incorporated into the framework is limited at about 0.5 mol%.
EPR methods were used to elucidate the radical formation over CoAPO-5 and CoAPO-11 molecular sieves. When olefins such as cyclohexene, cyclopentene, 1,4-cyclohexadiene and tetramethylethylene (TME) were adsorbed on the calcined samples, the corresponding allyl or cation type radicals were formed interacting with framework cobalt ions.
The CoAPO-11 sample with a small and elliptical pore gave less resolved spectra of cycloolefin radicals due to restriction in intramolecular motion while CoAPO-5 with a relatively large and spherical pore produced the isotropic EPR spectra. In the case of TME cation radical CoAPO-11 sample produced only monomer radicals whereas formation of dimer radicals was observed for CoAPO-5 sample.
When 2,3-dimethyl-1-butene and 3,3-dimethyl-1-butene compounds were adsorbed on CoAPOs samples, both compounds were isom...