Ag₂O nanoparticles/MnCO₃, –MnO₂ or –Mn₂O₃/highly reduced graphene oxide composites as an efficient and recyclable oxidation catalyst

Silver oxide nanoparticles doped manganese (IV) oxide along with varying percentages of highly reduced graphene oxide (HRG) [Ag₂O(1%)–MnO₂/(X%)HRG] nanocomposites were fabricated through a simple co-precipitation method followed by calcination at 400 °C. The as-prepared nanocomposite upon calcination at 300 °C and 500 °C temperatures, yields the manganese carbonate (MnCO₃) and manganese (III) oxide (Mn₂O₃) composites i.e. Ag₂O(1%)–MnCO₃/(X%)HRG and Ag₂O(1%)–Mn₂O₃/(X%)HRG, correspondingly. The structural composition of the prepared nanocomposites has confirmed by several characterization techniques. The nanocomposites have successfully utilized as a catalyst for liquid-phase oxidation of aromatic alcohols in presence of O₂ as a green oxidant under alkali-free conditions. In addition, a comparative study was performed to assess the activity of the manganese carbonates and manganese oxides for aerial oxidation of benzyl alcohol into benzaldehyde as a model reaction. Effects of various parameters have thoroughly examined in detail and the Ag₂O(1%)–MnO₂/(5%)HRG catalyst exhibited the highest activity in the aerial oxidation of benzyl alcohol to benzaldehyde with a 100% conversion and >99% selectivity in a remarkably short reaction time (35 min) than the undoped precursor i.e. Ag₂O(1%)–MnO₂. The presence of HRG dopant greatly enhanced the catalytic performance of Ag₂O–MnO₂ nanocatalysts could be attributed to the presence of carbon vacancies and topological defects as well as oxygen carrying functionalities on the HRG surface and increase in the surface area. The as-prepared catalyst could be efficiently recycled and reused up to five times without a discernible drop in its catalytic performance and the product selectivity remained unchanged. The prepared catalyst i.e. Ag₂O(1%)–MnO₂/(5%)HRG was employed as oxidation catalyst for a series of various substituted benzylic and aliphatic alcohols into their respective aldehydes and yielded complete conversion with excellent product selectivity with no further oxidation to acids.