On the nature of the 'heterogeneous' catalyst: Nickel-on-charcoal

Results from aromatic aminations and Kumada couplings, together with spectroscopic analyses (TEM, EDX, ICP-AES, React-IR), reveal that catalysis using nickel-on-charcoal (Ni/C) is most likely of a homogeneous rather than heterogeneous nature. In the course of a reaction with Ni/C, nickel bleed from the support was calculated to be as high as 78%. However, the existence of an equilibrium for this homogeneous species between nickel located inside vs outside the pore system of charcoal strongly favors the former, thus leaving only traces of metal detectable in solution. This accounts for virtually complete recovery of nickel on the charcoal following filtration of a reaction mixture and allows for recycling of the catalyst. TEM and EDX data were used to explain different reactivity profiles of Ni/C, which depended upon the method of reduction used to convert Ni(II)/C to Ni(0) as well as the level of nickel loading on the support.