Sol-gel synthesis of NiFe2O4 with PVA matrices and their catalytic activities for one-step hydroxylation of benzene into phenol

Nickel ferrite (NiFe2O4) was synthesized using citric acid (CA) as a chelating agent and varying amounts of polyvinyl alcohol (PVA) as powder binder via a combustion process. The influence of PVA/NiFe2O4 blend composition on the phase, crystal structure, and morphology has been investigated by X-ray diffraction (XRD), scanning electron microscopy, and Fourier transform infrared spectroscopy, respectively. The probable assignments of the thermal degradation products of PVA/NiFe2O4 were studied by complementary thermogravimetric analysis and differential thermal analysis. The PVA/NiFe2O4 crystals preferentially oriented along the (311) plane as revealed by XRD, owing to diffusion of alpha-Fe2O3 particles into the lattice matrix where the rate of diffusion increased significantly with the PVA concentration increase from 0.1 to 0.3 mol%. A vibrational doublet at 1,644 and 1,609 cm(-1) for the a parts per thousand Fe-O-COOH complex was emitted in the spectra of PVA-impregnated NiFe2O4 to assure the attachment of Fe(III) to the chelating agents CA/PVA. Thermal kinetic consideration based on Coats-Redfern and Horowitz-Metzger equations at subsequent decomposition steps of CA/PVA/NiFe2O4 illustrated that the values of activation free energy Delta G* increase significantly, indicating the non-spontaneous behavior. The one-step direct hydroxylation of benzene toward phenol has been extensively investigated using hydrogen peroxide as an oxidant.