Hydrogenation of unsaturated hydrocarbons—40 years on: Hydrogenation of 1,3-pentadiene over Pd/alumina

The hydrogenation of 1,4-pentadiene, E-1,3-pentadiene, and a Z/E-1,3-pentadiene mix have been studied over a Pd/alumina catalyst in the liquid phase at 313 K. 1,4-Pentadiene was found to react in a manner similar to 1-pentene. However, E-1,3-pentadiene caused rapid deactivation of the catalyst. When a Z/E-1,3-pentadiene mixture was hydrogenated no such deactivation was observed and the initial rate of E-1,3-pentadiene hydrogenation was reduced. It is proposed that the adsorbed Z-1,3-pentadiene disrupts π-stacking of the E-isomer. The pentene isomer selectivity indicated that 3,4-addition was favoured, which indicated that the internal double bond was more reactive than the terminal double bond. The reaction of 1,4-pentadiene in competition with 2-pentyne resulted in an increased rate of reaction for both reactants. Whereas the reaction of Z/E-1,3-pentadiene in competition with 2-pentyne or 1-pentyne resulted in a decreased rate of reaction for both reactants. In this case the alkene isomer selectivity could be modelled by simple averaging of the individual selectivities.