Magnetite catalyst shell

For the reduction process of magnetite a core-and-shell mechanism was proposed and tested in detail (8). Thereafter, a catalyst particle is continuously converted into a porous shell of metallic iron topotactically formed on a nonre-duced core by a propagating reaction front. It was, however, found that this simple concept has to be modified and that the reduction process is, in fact, rather complex in the presence of the promoters (9). In particular, it was concluded that the resulting ammonia-iron consists of Fe particles with a preferred (111) texture determined by the preferential migration of iron ions separated from each other by particles of the structural promoter. 

Another magnetically recoverable catalytic silica microreactors 156 were prepared and tested in the ATH of ketones in aqueous medium [135]. Magnetite nanoparticles were coated with shells synthesised via co-polymerisation of Si(OEt)4 and the Ru-TsDPEN complex 157 functionalised with a trimethoxysilane group. The transport of the hydrophobic reactants from the water medium to an entrapped catalyst in a sol-gel matrix was ensured by the surfactant CTAC. Under these conditions, ring-substituted acetophenones were almost quantitatively reduced with HC02Na giving the corresponding alcohols in over 90 % ees within 24 h (Fig. 49). 

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