N-Doped Sub-3 nm Co Nanoparticles as Highly Efficient and Durable Aerobic Oxidative Coupling Catalysts

A nano-coating associated with sulfuric acid leaching protocol was developed to prepare N-doped sub-3 nm Co-based nanoparticle catalyst (Co-N/C) using melamine-formaldehyde resin as the N-containing precursor, active carbon as the support, and Co(NO3 )2 as the Co-containing precursor. By thermal treatment under nitrogen atmosphere at 800 °C and leached with sulfuric acid solution, a stable and highly dispersive Co-N coordination structure was uniformly dispersed on the formed Co-N/C catalyst with a Co loading of 0.47 wt % and Co nanoparticle size of 2.55 nm. The Co-N/C catalyst was characterized with XRD, XPS, Raman, SEM, TEM, ICP, and elemental analysis. The Co-N/C catalyst showed extremely high catalytic efficiency with a TON of 257 for the aerobic oxidative coupling of aldehydes with methanol to directly synthesize methyl esters with molecular oxygen as the final oxidant. The Co-N/C catalyst also showed broad substrate range and stable recyclability. After recycling for 7 times, no obvious deactivation was detected. It was confirmed that the sub-3 nm Co-N coordination structure formed between metallic Co nanoparticles and pyridinic nitrogen doping into graphitic layers functions as the active site to activate molecular oxygen for the β-H elimination from generated hemiacetal intermediates to produce methyl esters. The nano-coating associated with acid leaching protocol provides a novel strategy to prepare highly efficient non-precious metal-based catalysts.