Silver cluster-promoted heterogeneous copper catalyst for N-alkylation of amines with alcohols

Abstract

A series of metal(M)-loaded Al₂O₃ catalysts (M/Al₂O₃) and bimetallic catalysts, Cu_xAg_1−x/Al₂O₃ with different compositions, Cu_0.95Ag_0.05/MOx with different supports (MOx), and Cu_0.95M′_0.05/Al₂O₃ with different promoter (M′), were prepared by an impregnation method, followed by H₂-reduction at 600 °C. For the N-alkylation of amines with alcohols, Al₂O₃-supported copper–silver bimetallic catalysts with a Cu/Ag molar ratio of 95/5 (Cu_0.95Ag_0.05/Al₂O₃) was found to be the most effective heterogeneous catalyst. The alkylation of anilines and aliphatic amines with various alcohols (benzyl and aliphatic alcohols) was achieved with a small amount of the catalyst (1 mol%). Mechanistic studies show that the reaction proceeds through a hydrogen-borrowing mechanism initiated by alcohol dehydrogenation as the rate-limiting step. Structural studies indicate that small Ag nanoclusters are supported on Cu nanoparticles possibly through Ag–O–Cu bonds at the silver–copper boundary. This bimetallic structure can be crucial to an effective promotion of the alcohol dehydrogenation and hydride transfer to the imines.