(31c) pH Sensitive Colloidal Gold Nanoparticle Catalysts for Enhanced Recovery and Reuse

Gold nanoparticles (AuNPs) have attracted enormous attention due to their unique catalytic activities. Colloidal AuNPs provides the benefit of selectivity, greater surface area per mass of catalyst compared to supported catalysts, catalyzes reactions under mild conditions and are very effective for chiral catalysis. Recovery of colloidal catalysts involve tedious methods like pH or temperature induced aggregation, solvent extraction, etc which renders these catalysts uneconomical. For colloidal catalysts, surface functionalization with ligands is a prerequisite to prevent aggregation but surface passivation with ligands causes significant reduction in catalytic activity. On the other hand, functionalization of colloidal AuNPs with stimuli responsive ligands provides enhanced recovery and reuse of catalysts. In this work, colloidal AuNPs have been functionalized with pH responsive thiolated-polyacrylic acid (PAA-SH) to impart pH triggered recoverability in the gold nanoparticles (AuNP-SPAA). Two pH triggered recovery schemes were investigated: a) pH triggered aggregation-separation-redispersion and b) pH triggered phase transfer into chloroform by a phase transfer agent. In AuNP-SPAA with low thiol content, loss of activity was ~100% within 5 cycles of reuse when the reuse scheme was (a). On the other hand catalytic activity was maintained when a (b) reuse scheme was employed. In AuNP-SPAA with higher thiol content, loss in reaction rate was observed due to increased surface passivation but with much greater recoverability for scheme (a) and (b) recovery. Small angle neutron scattering was used to characterize the polymer conformation and ligand shell solvation to corroborate our observations.