Sustainable diamine synthesis via continuous-flow reductive amination over a stable Re1Ox–Ni interface

Abstract

Bis(2-morpholinoethyl) ether (DMDEE) is a value-added diamine widely used in polyurethane synthesis and drug intermediate production. Conventional DMDEE manufacturing processes are based on the use of halogens or corrosive acids, posing significant environmental and operational challenges due to poor atom economy and hazardous byproducts. Herein, we report a halogen- and acid-free, continuous-flow approach for the direct synthesis of DMDEE via reductive amination of diethylene glycol (DEG) with morpholine over stable Re-modified Ni/Al₂O₃ catalysts. Systematic investigation of Re loadings revealed that atomically dispersed Re₁O_x species significantly enhance both activity and selectivity by stabilising Ni dispersion and promoting alcohol dehydrogenation. The optimised Ni–1Re/Al₂O₃ catalyst achieved full DEG conversion and 77% selectivity to DMDEE under mild conditions (473 K, 2 MPa), maintaining performance for over 1200 hours on stream without deactivation and with minimal metal leaching. In-depth characterisation using PXRD, HAADF-STEM, in situ XPS, XAFS, CO-DRIFTS, and H₂-TPR demonstrated that the formation of Re₁O_x–Ni interfacial ensembles not only modulates the redox behaviour of Ni but also enables enhanced H₂ adsorption and reaction pathway steering. This work establishes a robust structure–performance relationship for Re–Ni catalytic systems and offers a scalable and sustainable route for the industrial production of commercial-grade diamines via atom-economical reductive amination of alcohols.