Selective vapor-phase formation of dimethylformamide via oxidative coupling of methanol and dimethylamine over bimetallic catalysts

Abstract

The gas-phase oxidative coupling of methanol and dimethylamine (DMA) was investigated using bimetallic gold-based catalysts in a packed bed reactor. The selective coupling reaction yields dimethylformamide (DMF), a useful solvent for many chemical industries. Characterization of AgAu/SiO₂ and PdAu/SiO₂ catalysts reveals changes to the crystal and electronic structures of the metals indicative of predominantly alloyed nanoparticles. These alloy phases catalyze the oxidative coupling reaction of methanol and DMA to proceed with high selectivity towards DMF at low temperatures, whereas monometallic Ag or Pd catalysts are unselective and monometallic Au catalysts are unreactive. For PdAu/SiO₂ catalysts, increasing dilution of Pd in Au during synthesis results in increased gravimetric reaction rates (by 25 times) and rates per mol Pd (by 663 times), until both gravimetric and per Pd rates decrease as Pd is increasingly diluted in Au. These results suggest there is an optimum surface Pd ensemble size for oxidative coupling reactions, and that isolated Pd atoms are likely unreactive for this chemistry. DMF selectivity increases from monometallic Pd to alloyed Pd (from 45% to >75%), consistent with increasing isolation of oxygen rich domains capable of successive C–H cleavages leading to total oxidation products. Kinetic measurements show low reaction orders for the methanol (0–0.2), dimethylamine (0.1–0.5), and oxygen (0.1–0.2) reactants, alluding to high surface coverages. Co-feeding H₂O increased the rate of DMF formation, consistent with the increasingly basic surface adsorbates (i.e., hydroxyls) that are most reactive for kinetically relevant oxidative bond cleavages (e.g., C–H cleavage). Finally, DFT calculations and microkinetic modelling reveal surfaces of Pd(111) to be covered by surface oxygen and dehydrogenated DMA, the presence of which affects the reaction kinetics.