Hydrogenation of 4-nitroacetophenone over Rh/silica

• Kinetics of hydrogenation of 4-nitroacetophenone and 4-aminoacetophenone determined.
• Mechanisms of consecutive, A → B → C → D reactions outlined.
• Turnover frequency increases with increasing average metal crystallite size.
• High yields (>94%) to intermediates B and C achieved.

The hydrogenation of 4-nitroacetophenone (4-NAP) and 4-aminoacetophenone (4-AAP) was examined over rhodium/silica catalysts. The reactions were carried out using isopropanol as the solvent under a range of temperatures (303–333 K) and pressures (1–5 barg). An activation energy of 50 ± 4 kJ mol−1 was determined for 4-NAP hydrogenation and 48 ± 2 kJ mol−1 for 4-AAP hydrogenation. Orders of reaction were obtained for 4-NAP (zero order) and hydrogen (first order) and a kinetic isotope effect of 3.0 was observed for 4-NAP hydrogenation and ∼1.4 for 4-AAP hydrogenation when deuterium was used. Under specific conditions high yields (∼94%) to 4-aminoacetophenone and 1-(4-aminophenyl) ethanol could be obtained from 4-NAP hydrogenation. An antipathetic metal crystallite particle size effect was observed for both reactants.

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