Sulfonated porous polystyrene microspheres as heterogeneous catalyst for preparation of biodiesel were prepared by a facile two-step synthesis process in this work. Firstly, monodisperse porous cross-linked polystyrene microspheres were prepared by reflux-precipitation polymerization, in which different ratio of styrene (St) and divinylbenzene (DVB) were used as monomer and cross-linker, respectively. Then the obtained polystyrene microspheres were sulfonated by chlorosulfonic acid. The structure and composition of the related polystyrene microspheres were characterized by FT-IR, X-ray photoelectron spectroscopy, elemental analysis, thermogravimetric analysis, Brunauer-Emmett-Teller (BET) surface area analysis, transmission electron microscope, particle size and zeta potential analysis. In order to adjust the sulfonated degree and acid density, the reaction parameters such as solvent dosage, swelling time, amount of chlorinated sulfonic acid, sulfonated temperature and sulfonated time were investigated carefully. The optimum conditions for the sulfonated reaction are as follows:0.5 g of cross-linked polystyrene microspheres was swollen with 5 mL of CCl₄, and then sulfonated with 0.3 mL of chlorosulfonic acid at 50℃ for 75 min. In addition, the effect of crosslinking degree on the specific surface area and acid density of the sulfonated polystyrene microspheres were also studied. The as-prepared sulfonated polystyrene microspheres exhibited high acid density (2.611 mmol·g^-1), good thermal stability (up to about 200℃) and appropriate specific surface area. The sulfonated porous polystyrene microspheres as catalyst were applied in the esterification reaction of oleic acid and methanol. Our experimental results showed that the functional particles possessed very high catalytic activity, which was much higher than polystyrene ion exchange resin (Ameberlyst-15) and close to concentrated sulfuric acid. The catalytic activity still maintained 92% of the initial activity after 3 runs of recycling use, and the shedding of sulfonic acid groups was almost negligible. This paper demonstrates a simple, green and controllable strategy to develop sulfonated porous polystyrene microspheres with high catalytic activity and good durability for preparation of biodiesel.

Key words: reflux-precipitation polymerization, polystyrene microsphere, sulfonated reaction, biodiesel