(246d) Dimethylformamide Synthesis Via Oxidative Coupling of Methanol and Dimethylamine over Alkali Promoted PdAu/SiO2 Catalysts

Oxidative coupling of methanol and dimethylamine to form dimethylformamide serves as a model reaction to study bimetallic catalysts under oxidative conditions. Per-metal-atom dimethylformamide site time yields and selectivities are higher over dilute metal-in-gold catalysts relative to their monometallic analogues. Oxygen dissociation is facile over the dopant metal, the resultant oxygen adatoms spillover onto gold and catalyze selective partial oxidation. We previously reported that colloidally synthesized PdAu nanoparticles (~15 nm) dispersed on SiO₂ catalyze this reaction, that the dimethylformamide selectivity decreased with increasing O₂/methanol ratio, and that all apparent reaction orders were <0.5. DFT calculations indicate a high surface coverage of dehydrogenated dimethylamine; thus the dimethylformamide rate is relatively insensitive to reactant concentrations. Co-feeding H₂O resulted in an 1000% increase in dimethylformamide rate, potentially due to either increased mobility of surface hydroxyls relative to oxygen adatoms, or to reduced barriers for C-H cleavage when hydroxyls act as H acceptors. Small (<5 nm) PdAu particles dispersed on SiO₂ were synthesized by strong electrostatic adsorption (SEA). These catalysts had higher per-metal dimethylformamide rates and increased selectivities relative to ~15 nm nanoparticles. The measured reaction orders were distinct; O₂ order increased (to 0.1 from 0.15), methanol order decreased (from 0 to -0.3), and co-fed H₂O had no impact. A series of 1:5 Pd:Au catalysts were synthesized using NH₄OH, NaOH, KOH, and CsOH to control pH during SEA resulting in alkali-promoted PdAu/SiO₂. The dimethylformamide rate and selectivity both increased in the order Cs > K > Na > NH₄. Literature precedent suggests that alkali ions disrupt the high coverage of reactants allowing for increased rates. Major conclusions from this work are i) oxidative coupling rates, selectivities, and kinetics depend on nanoparticle size, ii) the identity of the base should be considered when performing SEA, and iii) alkali promotion increases oxidative coupling rates over bimetallic catalysts.