Low-temperature conversion of base precursor KNO₃ on core–shell structured Fe₃O₄@C: Fabrication of magnetically responsive solid strong bases

Solid strong bases can catalyze a variety of organic reactions under mild conditions. Their preparation consists of loading base precursors (e.g. KNO₃) on porous supports followed by activating to convert base precursors to basic sites (e.g. K₂O). However, high temperatures are usually required for activation, which consumes lots of energy and damages the structure of supports. Moreover, filtration or centrifugation is mainly adopted for the recovery of solid base catalysts, which is tedious and limits their applications to some extent. In this paper, we constructed multifunctional core–shell structured Fe₃O₄@C microspheres as the supports owing to the reducibility of carbon shell and the magnetism of Fe₃O₄ core, thus creating magnetically responsive solid strong bases under mild conditions. Strong basic sites can be formed at a low temperature of 400 °C due to the redox reaction between the carbon shell and the base precursor, yielding the solid base K-400@Fe₃O₄@C. Such a temperature is obviously lower than the temperature for the decomposition of KNO₃ on the frequently used supports SiO₂ (750 °C) and Al₂O₃ (500−700 °C). The resultant solid base K-400@Fe₃O₄@C exhibits excellent catalytic activity in transesterification reaction, and the yield of target product dimethyl carbonate is higher than some benchmarks like MgO, CsX, and Na₂O/Al₂O₃. In addition, thanks to the superparamagnetism, the solid base catalysts can be recovered within 10 s with the assistance of external magnetic fields.