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.