Fabrication of Ag/Mn3O4 nano-architectures for the one-step selective oxidation of 3-picoline to niacin: a key to vitamin B3 production

We have synthesized Ag nanoparticles (with sizes of 2–5 nm) supported on 1D Mn3O4 spinel nanorods by a surfactant-assisted single-step preparation method using a room-temperature synthesis protocol for the first time. The unique 1D rod-like morphology increases the amount of adsorption sites of the support, leading to less sintering and higher dispersion of Ag nanoparticles, resulting in significantly enhanced catalytic activity for the industrially important selective oxidation of 3-picoline to niacin. The catalyst was thoroughly characterized by XRD, XPS, SEM, TEM, Raman, FTIR and UV-visible spectroscopy. XRD revealed the formation of Ag nanoparticles supported on hausmannite-type tetragonal Mn3O4. Electron microscopy studies confirmed the formation of the 1D structure of the catalyst. The generation of this 1D nanostructure has been revealed in detail and correlated with its superior catalytic activity. Various reaction parameters like temperature, substrate : H2O2 molar ratio, reaction time, etc. were studied. A niacin selectivity of 97%, with a 3-picoline conversion of 55%, was achieved over this catalyst at 70 °C. It was noticed that, when halide ions (Br−) were present in the synthesis gel mixture, only then were the morphology and activity of the catalyst proven to be superior. The thorough dispersion of Ag nanoparticles on Mn3O4 nanorods and the synergy displayed between Ag nanoparticles and Mn3O4 are the detrimental factors for its high activity. The catalyst was reused 5 times and thereby displayed its true heterogeneous character.