ABSTRACT
In this study, metal-doped niobates and perovskites were obtained by a solid-state reaction. The solids were evaluated in the esterification of glycerol in the presence of acetic acid to produce valuable esters of glycerol. The structural features of the solids indicated the ZnNb2O6, Pb2.8Nb2O7.8 and CuNb2O6 columbite main phases and La2MnFeO6 double-perovskite. Density functional theory (DFT) studies of Pb2.8Nb2O7.8 clearly confirmed the existence of a robust orthorhombic structure and its electronic properties were correlated with the Nb and Pb interactions. The morphological and elemental analyses also indicated that not all surface elements, as well as morphology, were crucial for catalytic properties. All solids were active and selective toward triacetin formation upon glycerol esterification with acetic acid. The catalytic performance depends mainly on the availability of the surface and its structural stability, as well as defects formation. Recyclability studies indicated that the La2MnFeO6 double-perovskite was an efficient catalyst, achieving glycerol conversion of 68% and triacetin selectivity of 25% up to 4 cycles of use in the reaction. The structural defects near the Mn4+/Mn3+ surface sites resulted in the diffusion of anions and an increased concentration of oxygen vacancies contributed to the stable performance of the solid in glycerol ester production.
ABSTRACT
Iron niobates, pure and substituted with copper (Fe1-xCuxNbO4 with x = 0-0.15), were prepared by the solid-state method and characterized by X-ray diffraction, Raman spectroscopy, and magnetic measurements. The results of the structural characterizations revealed the high solubility of Cu ions in the structure and better structural stability compared to the pure sample. The analysis of the magnetic properties showed that the antiferromagnetic-ferromagnetic transition was caused by the insertion of Cu2+ ions into the FeNbO4 structure. The pure FeNbO4 structure presented an antiferromagnetic ordering state, with a Néel temperature of approximately 36.81K. The increase in substitution promoted a change in the magnetic ordering, with the state passing to a weak ferromagnetic order with a transition temperature (Tc) higher than the ambient temperature. The origin of the ferromagnetic ordering could be attributed to the increase in super-exchange interactions between Fe/Cu ions in the Cu2+-O-Fe3+ chains and the formation of bound magnetic polarons in the oxygen vacancies.
