A synergistic catalyst based on a multivalence monodisperse-porous microspheres with oxygen vacancies for benzyl alcohol oxidation

Abstract

A heterogeneous catalyst for selective oxidation of benzyl alcohol (BzOH) was synthesized by immobilization of Pd nanoparticles (Pd NPs, 3 nm) on monodisperse-porous manganese oxide (MnOx, 4 μm) microspheres obtained by “staged shape templating protocol”. MnOx microspheres containing Mn5O8 (97 %) and MnO2 (78 %) phases were evaluated as supports. The catalysts carrying immobilized Pd NPs were also synthesized, using monodisperse-porous SiO2 and TiO2 microspheres with similar size and porous properties (i.e. Pd@SiO2 and Pd@TiO2). Pd NP decorated-Mn5O8 (Pd@Mn5O8) microspheres provided the highest catalytic activity with BzOH conversion of 92.2 % and benzaldehyde (BzCHO) selectivity of 96.3 %. Higher catalytic activity of Pd@Mn5O8 was explained by mixed valence nature of Mn5O8 microspheres assisting the formation of abundant oxygen vacancies. Mn (II), Mn(III) and Mn(IV) oxidation states were shown by deconvoluted x-ray photoelectron spectra. A synergistic catalytic activity was reported for the first time, by combination of Pd NPs with Mn5O8 microspheres. Pd@Mn5O8 microspheres could be reused up to 5 times without remarkable loss in the activity and selectivity towards BzCHO. A methodology for comparison of catalytic activity and synergistic interaction between active site and support was also exemplified by choosing a nanoparticle type and a monodisperse-porous microsphere type.