How addition of a nickel cyclohexyl-salen complex impacts a one-pot synthesis of nickel/hierarchically porous carbon monolith catalyst

Synthesis of hierarchically porous carbons incorporating metal nanoparticles can be carried out by a “one-pot” process in which the metal precursor is mixed with polymer precursors and structure directing agents, that undergoes a sol-gel self-assembly polymerization and then pyrolyzed under an inert atmosphere. Such syntheses are potentially cheaper and could make carbon catalysts more compatible with organic solvents. Previous work had shown that addition of metal salts interfered with the self-assembly process. To prevent this interference, and increase thermal stability, a nickel cyclohexyl salen complex was used as metal precursor. Dynamic light scattering (DLS) of the precursor solutions shows the presence of micelles in the absence of added complex. Addition of the complex is observed to give large agglomerates and a reduction in micelle concentration. SEM of the carbons formed shows that the nickel particles have a wide distribution of particle sizes and a heterogenous spatial distribution. Gas adsorption showed only modest changes resulting from nickel complex addition. XRD confirms the formation of nickel nanoparticles. The heterogenous distribution of nanoparticles observed in SEM is consistent with the large agglomerates observed in DLS being nickel cyclohexyl-salen complex which is not fully dispersed in the polymer and therefore gives heterogenous dispersion in the resulting carbon. The metal dispersion and stability of the carbon were improved over that observed with other salen complexes. The resulting catalysts were found to be catalytically active for the reduction of p -nitrophenol by sodium borohydride with catalytic activity consistent with the particle size distributions.