Stabilization of Palladium Catalysts for the Heck Reaction by Support Functionalization and Solvent Selection

Pd nanoparticles were synthesized on nonfunctionalized and functionalized SBA-15 grafted with thiol or amine groups. The resulting materials were used as catalysts to study the influence of these functional groups for a similar Pd particle size of approximately 2 nm on the Heck reaction that uses iodide substrates. The nonfunctionalized catalysts lost their activity quickly because of Pd leaching and particle growth. However, if a mixed solvent that consisted of toluene and DMF was used and thiol groups were grafted on SBA-15, we were able to reduce the Pd leaching and particle growth to thus obtain a more stable catalyst in which an important role of the ligands was to recapture Pd. However, heterogeneity tests, such as a hot filtration test and poisoning experiments, gave a strong indication that Pd still leached from the support and that homogeneous Pd species were mainly responsible for the activity. Pd on S-functionalized SBA-15 was able to catalyze the Heck reaction using different substrates to achieve good activity and selectivity in most cases.