Reaction pathways over copper and cerium oxide catalysts for direct synthesis of imines from amines under aerobic conditions


​Copper(II) oxide, cerium oxide, and several copper(II) oxide supported on cerium oxide catalysts are explored in the aerobic oxidative homocoupling of benzylamine to form N-benzylidenebenzylamine in DMSO at 110 °C. Although both CuO and CeO2 alone are shown to catalyze the reaction, CuOCeO2 catalysts are shown to most efficiently catalyze the reaction, providing higher rates (per g Cu) due to the presence of both copper and ceria species in the reactor. Catalysts with lower copper loadings and increased ceria content reduce the product yield, as ceria domains can catalyze the decomposition of the desired product as well. The amine conversion occurs with a significant induction period, associated with the putative formation of an initial benzylimine intermediate in the case of catalysis with CeO2 alone or from slow copper solubilization in cases where supported or unsupported copper catalysts are used, followed by rapid conversion to the N-benzylidenebenzylamine product. Copper leaching studies clearly demonstrate that catalysis using copper-containing catalysts is primarily associated with turnover by soluble copper species. A series of experiments targeted at elucidating the reaction pathway suggests that copper oxide domains promote the coupling of the initial intermediate, benzylimine, with benzylamine to produce the N-benzylidenebenzylamine product (path A), with a maximum production rate of 888 μmol/m2 h (13.3 mmol/gCu h) over the pure CuO catalyst or 22.9 μmol/m2 h (26.1 mmol/gCu h) over the CuOCeO2 catalyst. In contrast, cerium oxide domains are suggested to primarily convert the benzylimine to benzaldehyde, followed by condensation of the benzaldehyde with benzylamine in a rapid step to yield the N-benzylidenebenzylamine product (path B), with a maximum amine production rate of 2.74 μmol/m2 h. Both ceria and copper(II) oxide domains promote the initial benzylimine formation at comparable rates. Although the CuOCeO2 catalyst leaches about 11% of the copper during the reaction, and these soluble copper species are largely responsible for the catalytic turnover, the recovered solid can be recycled until the copper is depleted, catalyzing the reaction with an identical rate per g Cu in a second cycle, after calcination. The copper–ceria family of catalysts offers an alternative, potentially lower cost composition for the target oxidative homocoupling reaction than previously studied precious metal catalysts, although copper leaching is a distinct drawback to the catalyst composition.