Preparation of a magnetic and recyclable superparamagnetic silica support with a boronic acid group for immobilizing Pd catalysts and its applications in Suzuki reactions

Abstract

Palladium is one of the best metal catalysts for Suzuki cross-coupling reaction to synthesize unsymmetrical biaryl compounds. However, homogeneous palladium (Pd) is limited in an industrial scale due to the high cost, separation, removal, and recovery issues. In this paper, a novel, high activity magnetic nanoparticles (Fe₃O₄@SiO₂-APBA-Pd) catalyst was prepared by a simple, cost-effective procedure. The as-prepared functional nanoparticles (Fe₃O₄@SiO₂-APBA) with boric acid group immobilized Pd through adding Pd(OAc)₂ to Fe₃O₄@SiO₂-APBA in absolute ethanol and maintaining for a certain time under a nitrogen atmosphere. The as-prepared catalyst was characterized by FT-IR, SEM, EDX, TEM, ICP-MS, XPS, and XRD. The results showed that the Pd (0.2–0.6 nm) was successfully anchored on the magnetic silica material with boric acid group. The amount of Pd was 0.800 mmol g⁻¹. This magnetic nanostructure (8–15 nm) is especially beneficial as a nanocatalyst because each nanoparticle can catalyze a reaction in a certain time without steric restriction, which could effectively improve the reaction efficiency. The current nanoparticles with the Pd catalyst could be used as a novel, green, and efficient heterogeneous catalyst for Suzuki reactions. This catalyst showed promising catalytic activity and excellent yields toward 14 kinds of Suzuki coupling reactions under mild reaction conditions, which was similar to homogeneous Pd and many reported heterogeneous Pd catalysts. In addition, the turnover number (TON) and turnover frequency (TOF) for the Suzuki reaction were high. TOF and TON were 9048 h⁻¹ and 20 250 for the Suzuki reaction of bromobenzene and phenylboronic acid. Furthermore, the nanoparticles could be easily separated by a magnet, and could be used repeatedly seven times without any significant loss in activity.