Selective N-methylation of amines, nitro compounds and nitriles with CO2/H2 by Cu-ZnO/TiO2 catalyst

Abstract

N-methylation of amines, nitro compounds and nitriles with CO₂ and H₂ offers a promising strategy for CO₂ utilization. However, developing highly efficient and stable Cu-based catalysts for selective mono-N-methylation of aniline remains challenging. Herein, we report the successful synthesis of a Cu-ZnO/TiO₂ catalyst, with the optimized 5 %Cu-7.5 %ZnO/TiO₂ catalyst achieving a 96.7 % yield of N-methylaniline (MA) and an initial turnover frequency (TOF) of 7.4 h⁻¹ after 6 h under 453 K, 2 MPa CO₂ and 4 MPa H₂. This performance surpasses all reported Cu-based catalysts and rivals noble metal-based systems. The catalyst demonstrated reusability at least four cycles and versatility in the N-methylation of various amines, nitro compounds, and nitriles. Kinetic studies revealed reaction orders of 0 with respect to aniline concentration and CO₂ pressure, and 1 with respect to H₂ pressure. Based on kinetics and extensive characterization, we propose a reaction mechanism over 5 %Cu-7.5 %ZnO/TiO₂, identifying Cu nanoparticles as the active sites. The process proceeds via N-phenylcarbamic acid and N-phenylformamide intermediates, with the hydrogenation of N-phenylcarbamic acid as the rate-determining step.