Steric effects in the selective hydrogenation of cinnamaldehyde to cinnamyl alcohol using an Ir/C catalyst

The liquid phase selective hydrogenation of cinnamaldehyde to cinnamyl alcohol has been carried out over a graphite-supported iridium catalyst. The effect of reaction parameters such as temperature, pressure, concentration of reactant, the effect of addition of product to the feed and pre-reduction of the catalyst were studied. In situ pre-reduction of the catalyst with hydrogen had a very significant enhancing effect on the conversion of cinnamaldehyde and selectivity of the catalyst to cinnamyl alcohol. Kinetic analysis of the pre-reduced catalyst showed that the reaction is zero order with respect to cinnamaldehyde and first order with respect to hydrogen. The reaction follows an Arrhenius behaviour with an activation energy of 37 kJ mol(-1). Detailed analysis of the reaction showed that hydrogenation of the C=C double bond to give hydrocinnamaldehyde predominantly occurred at low conversions of cinnamaldehyde (<5%). In addition, it was found that the C=C double bond in cinnamyl alcohol could not be hydrogenated to give phenyl propanol, whereas the less thermodynamically favoured hydrogenation of the C=O bond in hydrocinnamaldehyde could readily proceed. It was also found that the addition of cinnamyl alcohol to the feed greatly decreased the rate of hydrogenation of the C=C bond in cinnamaldehyde. These results indicated that the cinnamyl alcohol adsorbs on the surface of the catalyst during the reaction and creates a steric effect that inhibits the hydrogenation of the C=C bond of cinnamaldehyde. (C) 2004 Elsevier B.V. All rights reserved.