Pd catalysed hexyne hydrogenation modified by Bi and by Pb

Two Pd/Al2O3 catalysts of different loadings and dispersions were modified by the addition of various amounts of Bi and studied in the hydrogenation of 1-hexyne and 2-hexyne and in the consecutive reactions of the products formed. Catalyst behaviour was compared with a commercial Lindlar catalyst and Pb-free Pd/CaCO3. Results are consistent with a preference of Bi to occupy step and edge sites while leaving the terraces and extended facets relatively unaffected. Results show that while Bi had little effect on the rate of the 1-hexyne hydrogenation, the rate of subsequent reactions of the 1-hexene formed were suppressed. In this context, Bi was a more effective modifier than Pb. This situation was reversed when reactions were performed using 2-hexyne. Results are discussed in terms of the reaction mechanism and key intermediates in the process and the roles of bismuth, relative to lead, in creating an appropriate ensemble of surface Pd atoms to permit control of selectivity.