Ionic liquid-controlled synthesis of Fe3O4 nanoparticles (NPs) for the preparation of magnetic solid acids

The use of magnetic solid acid can effectively solve the problem of waste acid recovery in chemical reaction process, thus protecting the environment. Based on the co-precipitation method, Fe3O4 NPs were prepared by adding an ionic liquid, [BMIM]BF4, and then coated by Stöber method to obtain Fe3O4@SiO2. Finally, a nano-magnetic solid acid Fe3O4@SiO2-SO3H is obtained through sulfonation. The structure and morphology of Fe3O4 NPs, Fe3O4@SiO2 and Fe3O4@SiO2-SO3H were characterized by TEM, EDS, FTIR, XRD, and XPS, and the magnetic properties, acid density and catalytic performance were also measured. It is found that the Fe3O4 NPs obtained with 10 % (v/v) [BMIM]BF4 exhibit the most optimal particle sizes and the best dispersibility for the solid acid preparation. The corresponding Fe3O4@SiO2-SO3H solid acid is composed of NPs with sizes of 30–50 nm, and its magnetization and acid density are measured to be 42.33 emu·g−1 and 2.73 mmol·g−1. The cellulose hydrolysis with 1.5 g/g Fe3O4@SiO2-SO3H solid acid at 140 °C and the water content of 1 wt% can give the highest glucose yield up to 54.89 %. Fe3O4@SiO2-SO3H can be easily separated and recycled from the reaction residue by an external magnet.