Magnetic solid base catalyst (K3PO4/Fe3O4) was prepared using the incipient impregnation method. In a typical procedure, 3.8 g
potassium phosphate trihydrate (K3PO4_3H2O, purchased from Aladdin) was firstly dissolved in specific volume of deionized water,followed by adding 4.0 g magnetic nanoparticle (Fe3O4, 99.5% metal basis, 20 nm, purchased from Aladdin) with ultrasonic treatment for 3 h.