Catalytic performance and elution of Pd in the Heck reaction over zeolite-supported Pd cluster catalyst

The Heck reaction was carried out in N,N-dimethylacetamide (DMAc) solvent with Pd-supported zeolites as catalysts. Particular attention was paid to the influence of pretreatment conditions and the kind of zeolites on the catalytic activity and the elution of Pd. Pd2+ ion-exchanged zeolites exhibited relatively high activity in the reaction. However, a considerable amount of Pd was dissolved in the solvent. Deposition of agglomerated Pd0 was observed after the reaction, suggesting the dissolved Pd2+ species was reduced with DMAc during reaction. Pd0-loaded zeolites were prepared by the reduction of Pd oxide/zeolites with H2. Among Pd0-loaded catalysts, Pd0/H–Y exhibited high activity. The dissolution of Pd was significantly suppressed over Pd0/H–Y, as indicated by ICP analysis of the solution. The recycled use of Pd0/H–Y was possible through oxidation and successive reduction with H2. The growing process of Pd clusters in H–Y was followed by energy-dispersive XAFS (partly Quick XAFS) during temperature-programmed reduction in diluted H2. It was found that stable Pd13 clusters that interacted with Brønsted acid sites were generated in the pore of H–Y. The Pd13 clusters were ascribed to the active and insoluble species in the Heck reaction.