Catalyst performance changes induced by palladium phase transformation in the hydrogenation of benzonitrile

The influence of hydrogen pressure on the performance of a gamma-alumina-supported palladium catalyst was studied for the multiphase selective hydrogenation of benzonitrile to benzylamine and byproducts. Semi-batch experiments of benzonitrile hydrogenation in 2-propanol were performed with hydrogen pressures between 2.5 and 30 bar, at a constant temperature of 80 degrees C. The intrinsic property of palladium to absorb hydrogen into its lattice structure has a strong influence on activity and selectivity. The transformation to stable palladium beta-hydride above a threshold hydrogen pressure of 10 bar induces a persistent change in turnover frequency and byproduct selectivity. The turnover frequency increases from 0.32 s(-1) to a maximum of 0.75 s(-1) at this threshold pressure and decreases to 0.25 s(-1) with increasing hydrogen pressure. The palladium beta-hydride phase suppresses the hydrogenolysis to toluene changing the selectivity from 6.5% to 2.0% and increasing the selectivity of the condensation to dibenzylamine from 1.6% to 2.7%, attributed to modified electronic interactions between catalyst and substrates. The selectivity to the desired product benzylamine is always high and increases with hydrogen pressure from 92.7% to 95.3%. The palladium catalyst state is mainly determined by the activation or operational hydrogen pressure, whichever was the highest, if the activation pressure was above 10 bar. (C) 2010 Elsevier Inc. All rights reserved.