Preparation And Properties Of Magnetic Core-shell Structure Nano Catalysts

As a kind of important organic intermediates, the synthesis of benzimidazoles has been a hot research at home and abroad. But most of the reported methods exhibited some problems, such as harsh reaction conditions and difficulties in the separation of the used catalysts. In order to improve the separation efficiency of catalysts, a series of magnetic core-shell structure nano catalysts, with magnetic Fe₃O₄ as the core and chitosan or silica as the shell, were prepared and applied in the green synthesis of benzimidazoles in this work.Firstly, Fe₃O₄@Chitosan@ZnO magnetic core-shell nano catalyst was prapared by in situ chemical precipitation method and applied in the synthesis of 2-substituted benzimidazoles. The catalysts was fabricated with magnetic Fe₃O₄ as core, natural compound chitosan as inner shell and ZnO as outer active component, and possessed a large specific surface area. Moreover, the catalyst had strong superparamagnetic and could be easily separated from the mixture by an external magnetic field. The yield kept over 90 % in seven runs. Thus a mild condition, environment friendly method for synthesis of 2-benzimidazoles was successfully established.Then, Fe₃O₄@Chitosan@（NH₄）₆Mo₇O₂₄ magnetic core-shell nano catalyst was prepared. In this catalyst, the active（NH₄）₆Mo₇O₂₄ was highly dispersed on the surface of chitosan, resulting its large specific surface area and good catalytic activity and selectivity in the synthesis of 2-substituted benzimidazoles. In addition, the catalyst could be quickly separated from the reaction solution by an external magnetic field and recycled seven times without significantly reduced of activity which showed good stability.Finally, on the basis of previous work, two kinds of magnetic nano catalysts Fe₃O₄@nSiO₂@mSiO₂@（NH₄）₆Mo₇O₂₄ and Fe₃O₄@nSiO₂@mSiO₂@（NH₄）₆Mo₇O₂₄@mSiO₂ were prepared. Taking the synthesis of 2-phenyl benzimidazole from o-phenylenediamine and benzaldehyde as a model reaction, their activities and stabilities were comparatively studied.