MnO2 polymorphs for catalytic carboxylation of 1-butanamine by CO2

Abstract

N,N′-dibutylurea is an important fine chemical/intermediate with a wide range of applications. Here, the synthesis of N,N′-dibutylurea through catalytic carbonylation of 1-butanamine by CO2 over MnO2 heterogeneous catalysts was systematically investigated. Four polymorphs of MnO2, including α-MnO2, β-MnO2, δ-MnO2 and γ-MnO2 with various morphological shapes were synthesized by hydrothermal method through altering the synthesis conditions (raw materials or parameters). Significant impact of the lattice structure of the MnO2 oxides on the catalytic activity was demonstrated. Among them, δ-MnO2 possessed the most favorable catalytic activity with N,N′-dibutylurea yield as high as 85.78 % at 160 °C for 24 h in CO2, while it is only 76.70 % for the α-MnO2, although they showed the similar specific surface area of around 10 m2 g−1. To understand the different catalytic behaviors among the different structured MnO2, the catalysts were characterized by X-ray photoelectron spectroscopy and temperature-programmed desorption of CO2. It was discovered that the excellent performance of δ-MnO2 was ascribed to its high surface basicity from its high concentration of lattice oxygen on surface of MnO2, while the large amount of surface unsaturated coordination manganese also positively affected the catalytic performance of γ-MnO2. It suggests enhancement of surface basicity and defect concentration is critical for the development of superior MnO2-based catalysts for the green synthesis of N,N′-dibutylurea from catalytic carbonylation of 1-butanamine by CO2.