Abstract
Background: There are various conventional ways for the synthesis of aromatic ketones but suffer from disadvantages like the use of toxic reagents, stoichiometric reagents producing huge amount of byproducts, thus causing hazard to the environment, whereas homogeneous protocols or the noble metal which support catalytic processes are not economically viable. On this ground, an effort was made to develop new green catalytic protocol to overcome these environmental concerns. The catalytic activity of manganese dioxides prepared by different recipes was investigated for the oxidation of allylic and benzylic alcohols.
Methods: MnO2 B sample was prepared by calcinations of manganese nitrate while sample ANMnO2 was prepared by the reduction of potassium permanganate using triethanolamine as a reductant. All the samples were well characterized by XRD, SEM, EDX and TEM techniques. All the prepared catalyst samples along with commercial MnO2 were tested for the oxidation of alcohols using TBHP as a clean oxidant, where % conversion and % selectivity were determined by Gas Chromatography. The products were further confirmed by the GC-MS and NMR techniques. Results: Of all the oxides, nano amorphous manganese dioxide exhibited significant catalytic activity and selectivity for the corresponding carbonyls. Change from bulk to nano structure enhanced the catalytic activity because of its higher surface area and change in Mn3+/ Mn4+ ratio. The nano amorphous MnO2 (ANMnO2)/TBHP in acetonitrile solvent catalytic system was found to be most efficient with substrate compatibility. In mechanistic investigations, it was observed that from the bulk to the nano structure, Mn3+ species content in an oxide increased which may play a crucial role in the activity. It was also confirmed by hydrogen peroxide decomposition studies. The catalyst ANMnO2 was found to be reusable for five consecutive cycles with no significant loss in catalytic activity. Conclusion: In conclusion, change in the catalyst’s preparation recipe not only alters the particle size but also affects the ratio of Mn3+ to Mn4+ species on the surface as well as on the bulk and thereby catalytic activity. On comparison of initial rate m-2g of all the catalyst samples for the said reaction, there was no significant difference observed which clearly proved the role of Mn3+ in catalytic activity.Keywords: Manganese dioxide, amorphous catalysis, nanomaterials, oxidation, nanocatalysis.
Graphical Abstract
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