Synthesis of magnetically recyclable spinel ferrite (MFe2O4, M = Zn, Co, Mn) nanocrystals engineered by sol gel-hydrothermal technology: High catalytic performances for nitroarenes reduction

Abstract

Highly stable and magnetically recoverable MFe2O4 (M=Zn, Co, Mn) spinel ferrite nanoparticles; synthesized using sol gel-hydrothermal technology via utilizing polyvinyl alcohol surfactant, were proposed as heterogeneous catalysts for the reduction of nitroarenes. The morphological characteristics, structural exploration, surface, optical, vibrational and magnetic properties were performed using powder X-ray diffraction, high-resolution transmission electron microscopy, energy dispersive X-ray, N2 sorptiometry, diffused UV–vis reflectance spectroscopy, FTIR, point of zero charge, vibrating sample magnetometer (VSM) and thermokinetic analysis via TGA technique. The results showed that MnFe2O4 exhibited the best performance in the reduction of 4-nitrophenol (4-NP), 2,4,6 tri-nitrophenol (2,4,6-NP) and 4-nitroaniline (4-NA) and revealed 100% conversion into the corresponding amino derivatives in 270sec with rate constant equal 0.01061s−1, 0.01134s−1 and 0.01355s−1, respectively. The superiority of the catalytic reduction of MnFe2O4 was due to increasing the pore radius and pore volume (6.75nm, 0.227cm3/g) values compared to other nanoferrites. The synthesized nanoferrites indicate independence of the activity on crystallite sizes due to the insignificant margin of change (from 6 to10nm). Conversely, decreasing the activity of ZnFe2O4 was due to increasing the Zn2+ ions size that induces an increase in lattice parameter values and thus increases the long-range electron transfer between Fe2+–Fe3+ ions. The MnFe2O4 catalyst that presented the highest saturation magnetization (135emu/g) indicated the highest reduction potential for 4-NA comparatively in the presence of NaBH4 and the reduction reaction followed pseudo first-order kinetics. Increasing the reduction performance of 4-NA compared to other nitroaromatics on MnFe2O4 was explained based on the formed intermediates, their reactivities, hydrophobicity and to point of zero charges. It has been explored that the reduction efficacy of 4-NP was enhanced on MnFe2O4 with the addition of ammonium oxalate and benzoquinone where it was inferior following the addition of tertiary-butyl alcohol. More important correlations concerning the effect of octahedral sites, residual organics, heat of activation (Ea) and activation free energy (ΔG*) values; determined while the removal of citrate/PVA moieties, on nitroarenes reduction rates were well scrutinized and discussed.