One-Pot Synthesis of Secondary Amines from Nitroarenes and Aldehydes on Supported Copper Catalysts in a Flow Reactor: The Effect of the Support

The effect of the support on the properties of copper catalysts supported on γ-Al 2 O 3 , SiO 2 , and TiO 2 –SiO 2 with a ~5 wt % Cu content was studied in the one-pot synthesis of N -heptyl- p -toluidine from p -nitrotoluene and n -heptanal. The catalysts were characterized by elemental analysis, X-ray diffraction analysis, transmission electron microscopy, temperature-programmed reduction, and low-temperature nitrogen adsorption. The reaction was carried out in a flow reactor with the use of molecular hydrogen as a reducing agent. It was established that the nature of the support exerts a profound effect on the yield of the target secondary amine; in this case, 5%Cu/Al 2 O 3 was found the most active catalyst. A combination of high catalyst activity in the hydrogenation of a nitro group to an amino group with the presence of acid sites, which facilitate imine formation as a result of the interaction of n -heptanal with p -toluidine, on the catalyst surface is necessary for reaching the greatest yield of N -heptyl- p -toluidine. The study of reaction mechanism on the 5%Cu/Al 2 O 3 catalyst showed that p -nitrotoluene inhibits the hydrogenation of n -heptanal, and aldehyde hydrogenation into alcohol begins only after the conversion of the major portion of p -nitrotoluene as a result of the selective adsorption of the nitroarene under the conditions of the simultaneous presence of p -nitrotoluene and n -heptanal in the reaction mixture.