C-5 alkene hydrogenation: Effect of competitive reactions on activity and selectivity

The hydrogenation of 1-pentene, cis-2-pentene and trans-2-pentene was studied over a 1% Pd/alumina catalyst. The rate of hydrogenation was cis-2-pentene > 1-pentene > trans-2-pentene. As the system moved towards thermodynamic equilibrium in pentene isomers the rate constant changed to that of the trans-isomer in a competitive environment. In competitive hydrogenation between the alkenes the hydrogenation rate of 1-pentene and trans-2-pentene are enhanced while that of cis-2-pentene is decreased. In a competitive system between alkene and alkyne, the rate of 1-pentene hydrogenation is increased 20-fold, even after all the pentyne is reacted. Enhanced hydrogen transfer through a modified hydrocarbonaceous deposit is believed to be the cause. The results also support the concept of separate sites for alkyne and alkene in a competitive environment.