Aerobic selective oxidation of benzyl alcohols to benzaldehyde catalyzed by bidentate Schiff base dioxomolybdenum(VI) complex immobilized on CPS microspheres

The Schiff base reaction between aldehyde (AL) group-modified crosslinked polystyrene (CPS) microspheres, AL–CPS microspheres, and glycine (GL) was conducted, resulting in the microspheres ALGL–CPS, on which bidentate Schiff base ligand ALGL were chemically anchored. Subsequently, the coordination reaction between the ligand ALGL of ALGL–CPS microspheres and molybdenyl acetylacetonate (MoO2(acac)2) was carried out, obtaining a new immobilized dioxomolybdenum(VI) complex with bidentate Schiff base-type, CPS-[MoO2(ALGL)2] microspheres, namely a new heterogeneous dioxomolybdenum(VI) complex catalyst was prepared. The microspheres CPS-[MoO2(ALGL)2] were fully characterized by many means such as FTIR, UV/Vis absorption spectrum and AAS. They were used in the oxidation reaction of benzyl alcohol by molecular oxygen. The experimental results show that through the coordination reaction between ALGL–CPS microspheres and molybdenyl acetylacetonate (MoO2(acac)2), the immobilized Schiff base dioxomolybdenum(VI) complex can be smoothly prepared. In the oxidation reaction of benzyl alcohol with molecular oxygen as oxidant, the heterogeneous dioxomolybdenum(VI) complex catalyst, CPS-[MoO2(ALGL)2], has very high catalytic activity. Under the mild conditions such as at ordinary pressure of dioxygen and at a lower temperature of 90 °C, benzyl alcohol can be transformed to benzaldehyde with a conversion of 63% in 10 h. It is namely the yield of benzaldehyde because benzaldehyde is the single oxidation product. It is obvious that CPS-[MoO2(ALGL)2] has excellent catalytic selectivity, and it specially catalyzes the oxidation reaction of benzyl alcohol to benzaldehyde. It was found that the solvent polarity effects on the catalyst activity greatly. The weaker the polarity of the solvent, the higher the activity of the catalyst CPS-[VO(SAAM)2] is and the higher the yield of benzaldehyde is.

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► We prepared new bidentate Schiff base ligand-anchored polymeric microspheres.
► A novel immobilized dioxomolybdenum(VI) complex catalyst was prepared.
► The heterogeneous catalyst was used in the oxidation of benzyl alcohol by molecular oxygen.
► The catalyst had very high catalytic activity and excellent catalytic selectivity.
► A catalytic reaction mechanism was proposed.