Hydrogenation of 3-nitroacetophenone over rhodium/silica catalysts: effect of metal dispersion and catalyst support

The effect of physical properties of rhodium/silica catalysts on the hydrogenation of 3-nitroacetophenone was investigated using a series of catalysts with different particle sizes, pore sizes and dispersions. The hydrogenation reaction was carried out in the liquid phase at different temperatures (313–343 K) and hydrogen pressures (2–5 barg). The hydrogenation of 3-nitroacetophenone was found to be a consecutive reaction with 3-aminoacetophenone, 1-(3-aminophenyl) ethanol, and 1-(3-aminocyclohexyl) ethanol the main products; traces of 3-aminoaniline were also detected. The reaction was zero order with respect to the concentration of 3-nitroacetophenone and half order with respect to hydrogen. The rate of formation and disappearance of intermediates increased with increasing temperature and hydrogen pressure. The rate of ring hydrogenation to form 1-(3-aminocyclohexyl) ethanol shows a marked increase at high temperatures. The catalytic activity varies with both the pore size of silica support and the rhodium dispersion with no significant influence of catalyst particle size. The turnover frequency increases with decreasing metal dispersion (or with increasing average metal crystallite size) revealing an antipathetic particle size effect. This behaviour can be related to changes in the electronic and structural make up of small metal crystallites but suggests that the reaction takes place on the plane face surface of the FCC close-pack structure of rhodium in which the number of face surface atoms increases at the expense of the edge and corner sites as the metal crystallite size increases