MnO2 nanostructures as sustainable catalysts for selectivity tuning and syntheses of amine coupling products with bio-derived glycerol

Abstract

This work reports crystalline MnO2 nanomaterials as a sustainable catalyst, for economical synthesis of N-substituted formamides and N-methyl amine products in the presence of air:O2 mixture (9:1) using bio-derived glycerol. This process provides flexible selectivity tuning between N-formamides and N-methyl amines, based on MnO2 crystal structure and glycerol concentration. The as-prepared (α, β, γ & δ) MnO2 nanostructures have been characterized in detail. This process was tested further with γ-MnO2 nanorods, which delivers yields in the range of 43-90% for a diverse substrate library of 16 amine substrates for N-formamide products and β-MnO2 for synthesis of N-methyl amines from secondary amines. The varied crystalline MnO2 nanostructures and their surface Mn (III)/Mn (IV) species ratios have been shown to steer the selectivity of the reaction differently, where α- and γ-MnO2 forming N-formamide with higher selectivity, while β- and δ-MnO2 leans more towards formation of N-methyl amine and N-formamide equally. The reusability of MnO2 catalysts up to 7 runs has been shown. The gram scale synthesis of formamides also shows yields of up to 75% using γ-MnO2 at pre-determined parameters.