Investigation of the preparation and catalytic performance of alkali ferrites M2FeO2 and MFeO2 (M=Li, Na, K) as heterogeneous basic catalysts for transesterification of vegetable oils

This work aims to prepare ecofriendly alkali ferrites M2FeO2 and MFeO2 (M=Li, Na, K) as heterogeneous basic catalysts for vegetable oils transesterification. The catalysts were obtained by conventional precipitation, wet impregnation, and calcination of impregnated iron oxides under O2, CO2, N2, and static atmospheres. The influence of the preparation parameters, including alkali to iron oxides molar ratio, calcination temperature, time, and atmosphere, on catalyst performance was evaluated. The catalysts were characterized using TGA, SEM-EDX, FT-IR, XPS, XRD, and titration. The results showed that potassium-based catalysts had better catalytic activities than lithium and sodium ferrite catalysts. The ethyl ester content of the biodiesels was 99.80 % and 95.05 % for K/FeII and K/FeIII, respectively. Optimum preparation conditions for catalysts were achieved at an activation temperature of 600 °C, a calcination time of 2 h, and impregnation ratios of 1/1 and 1/2 for K/FeII and K/FeIII, respectively. Increasing the alkali-to-iron oxide molar ratio to 2/1 and the calcination temperature to 800 °C led to a significant rise in the K/Fe ratio (i.e., ≥ 3), which induced a marked decrease in catalytic activity for both catalysts. Calcination under N2 improved catalyst stability, enabling reuse for 5 and 3 cycles with K/FeII and K/FeIII, respectively. These values are higher than those obtained for catalysts calcined under static, O2, and CO2 atmospheres. Catalysts calcined in O2 exhibited the highest surface oxygen density, with O/(C1s+K+Fe) ratios around 1.35, whereas values close to 1.25 were observed under N2 and static atmospheres, conditions that favoring reusability. The superior recyclability of K/FeII compared with K/FeIII likely results from its apparent ferromagnetic behavior and the high intrinsic activity of the K2FeO2 phase, supported by contributions from KFeO2, K2O, and K2CO3. Potassium ferrites are promising heterogeneous catalysts for the transesterification of vegetable oils.