(257f) Colloidal Nanoparticle Catalysis

Gold nanoparticles (AuNPs) have attracted enormous attention as of late due to their unique catalytic activities. Colloidal AuNPs provide benefits of selectivity, greater surface area per mass of catalyst compared to supported catalysts, activity under mild conditions and potential for chiral catalysis. We have demonstrated that colloidal AuNPs have higher catalytic activity than the supported counterparts on a per metal basis. The caveat is that colloidal AuNPs require surface ligand functionalization in order to prevent aggregation and loss of activity. This results in surface passivation and significant reduction in catalytic activity. Colloidal catalytic activity is strongly dependent on ligand packing and conformation on the AuNP surface. High molecular weight polymer ligands provide increased available surface area owing to decreased surface coverage due to steric hindrance. This leads to increased catalytic activity, while low molecular weight ligands lead to complete AuNP surface passivation. A major challenge for colloidal AuNP catalysts is the catalyst recovery and reuse. To alleviate this, we have developed pH responsive ligands that enable recursive recovery and reuse of colloidal AuNP catalysts by altering the pH to selectively precipitate or phase transfer catalyst into organic solvents. This was achieved by employing poly(acrylic acid) as benchmark polymer ligand where a specific number of acid groups can be grafted to with thiol moieties for surface binding. In this case there is a fine balance between providing adequate nanoparticle colloidal stability, recoverability, and maintaining catalytic activity.