The hydrogenation of para-toluidine over rhodium/silica: The effect of metal particle size and support texture

The hydrogenation of para-toluidine was studied over a series of rhodium/silica catalysts. The reaction exhibits an antipathetic particle size effect, suggesting that plane face surface atoms, such as C39 sites, are the active site for ring hydrogenation. The reaction is zero-order in p-toluidine and first-order in hydrogen, with the mechanism of formation of the cis and trans isomers likely through surface immine and enamine intermediates. The activation energy for the trans   isomer (62±4 kJmol−1) was systematically greater than that of the cis   isomer (51±6 kJmol−1). The support was also shown to play a part, adsorbing trans-4-MCYA in the early stages of the reaction, resulting in a varying cis:trans ratio. The effect of pore size was also studied; a dramatic decrease in activity was observed at pore diameters <64 Å.