Supported polymer magnets with high catalytic performance in the green reduction of nitroaromatic compounds

Abstract

Magnetic Fe₃O₄ nanoparticles (MNPs) were prepared by a simple co-precipitation method using molar ratios of Fe²⁺ : Fe³⁺ = 1 : 2 in ammonia solution, and subsequently were modified with tetraethyl orthosilicate (TEOS) via a modified Stöber method and also with 3-aminopropyl triethylenesilane (APTES) via a silanization reaction. Then, a layer of TiO₂ shell was coated directly onto the silica coated magnetite core–shell nanoparticles (Fe₃O₄@SiO₂) followed by dispersion of a Ag nanoparticle layer on the surface of the TiO₂ shell. Also, the amino-silane coated magnetite nanoparticles (Fe₃O₄-APTES) were successfully coated with polyethylene glycol (PEG) via the formation of covalent bonds between –NH₂ and –COOH to afford well-defined polymer-coated magnetic nanoparticles. Several transition metal nanoparticles (such as Cu, Ag, Co, Ni, Pb, Zn and Mn) were then loaded on the surface of Fe₃O₄-APTES–PEG. We exhibit herein the synthesis and modification of magnetic nanoparticles as solid phase catalysts and their use in the reduction of nitroaromatics (nitrophenols, nitroanilines) in the presence of an excess amount of sodium borohydride (NaBH₄). The kinetic activity of the catalysts used in the reduction of nitroaromatic compounds was studied using UV-visible spectrophotometer. The reduction reaction followed first-order kinetics. Moreover, these magnetic core–shell nanocomposites showed convenient magnetic separability as well as quite good stability after five recycles.