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 isomerized to more stable TME cation radicals over the samples.
Large novel anionic aluminophosphate-based layered compounds were synthesized using $Al(OH)_3$ as an aluminum source and cyclohexylamine as an organic template. The characterization studies by means of XRD, SEM, thermal analysis, MAS NMR, FT-IR and elemental analysis revealed that cobalt ions could be substituted for Al in the tetrahedral framework and protonated cyclohexylamine cations were intercalated as a double layer form between macroanionic AlPO sheets. The removal of cyclohexylamine by the calcination process resulted in a collapse of layered structure to amorphous phase.
The empirical formula of AlPO layered compound was generally 3.4 $C_6H_{12}NH_2 [Al_3H_3P_4O_{16}]$, showing an unusual molar ratio of an Al : P=0.75 and higher content of organic amines compared with that of AlPO molecular sieves.
Large cobalt-rich CoAPO-44 molecular sieves were crystallized in either higher temperature or longer time than the crystallization of cobalt-substituted AlPO layered compound.
