Gas-phase oxidative coupling of alcohols and amines over bimetallic solid catalysts

The formation of amide bonds is estimated to be the most common bond forming reaction that occurs in biological systems and pharmaceutical production. Many synthetic polymers such as nylon and Kevlar contain amide bonds which are crucial to their structural strength and stability. Therefore, understanding the kinetics of amide formation would benefit many chemical industries and biological sciences. Amide synthesis remains largely unexplored in heterogeneous catalysis, which in turn could hinder efforts to move from batch-wise to continuous processes in the pharmaceutical and chemicals industries. Bimetallic solid catalysts catalyze selective oxidative alcohol-alcohol and alcohol-amine coupling; in particular, those with group 10 and 11 transition metals (M; e.g., silver or palladium) as minor components in gold or copper hosts. A series of bimetallic M-Au and M-Cu catalysts (i.e., nanoporous sponges and silica-supported alloy nanoparticles) were synthesized and their physical properties characterized using X-ray diffraction, N₂ physisorption, scanning and transmission electron microscopies, and bulk elemental analyses. Gas-phase oxidative coupling reactions performed in packed bed reactors (1-10 kPa methanol, 0.0.02-0.15 kPa dimethylamine, 0.5-10 kPa O₂, 348-473 K) have been successful in demonstrating methanol self-coupling reported by others previously, and in methanol coupling with dimethylamine to form dimethylformamide (DMF). Measurements of apparent reaction orders, activation energies, and reaction pathways suggest a highly covered surface, and similar kinetics are observed regardless of the identity of the metal alloy. Per dilute-metal-atom (e.g., Pd in PdAu) DMF site-time-yields (398 K) vary by orders of magnitude with the bulk Pd:Au ratio, suggesting more reactive site ensembles form as metal domains responsible for oxygen dissociation (i.e., Pd islands) are increasingly isolated. At temperatures above 423 K, unexpected oligomeric products including tetramethyl urea and methyl dimethyl carbamate form, with the identity of the high temperature products dictated by the identity of the dilute metal present in the coinage metal host. Computational studies based on Density Functional Theory (DFT) help rationalize experimental observations on reaction mechanism, energetics, and coverage. Ongoing research will expand to leverage in situ and operando transmission infrared spectroscopy to identify surface species and likely reaction intermediates.