Efficient Cu-ZnO/TiO2 Catalyst for Direct N-Methylation of N-Methylaniline with CO2 and H2

Abstract

The preparation of methyl-substituted amines by direct N-methylation of amines with CO₂/H₂ is an important reaction for CO₂ utilization and organic synthesis. However, the design and construction of effective and stable non-noble-metal catalysts for this reaction remain challenging. Here, a cheap non-noble-Cu-based catalyst was prepared and applied in the direct N-methylation reaction of N-methylaniline with CO₂ and H₂. 5%Cu-ZnO(3.75%)/TiO₂ showed 98.8% yield with an initial TOF of 7.7 h^–1 after 24 h under 453 K, 1.5 MPa CO₂, and 4.5 MPa H₂, which was superior to 5%Cu/TiO₂, 5%Cu/ZnO, and reported Cu-based catalysts. Furthermore, 5%Cu-ZnO(3.75%)/TiO₂ can be reused at least four times and applied to the direct N-methylation of various amines with excellent selectivity. The various characterizations and experimental results showed that Cu⁰ was the active site, and 5%Cu-ZnO(3.75%)/TiO₂ exhibited the largest surface Cu⁰ amount, thereby contributing to superior catalytic performance. The reaction mechanism of the direct N-methylation reaction of N-methylaniline with CO₂ and H₂ was proposed based on spectroscopic studies, kinetic studies, and control experiments. The reaction proceeded with the HCHO and N-methylformanilide intermediates, and the formation of HCHO from H₂ and CO₂ was the rate-determining step.