Reversible ligand exchange of triphenylphosphine with thiols on gold nanoparticles for Janus modification and catalyst recycling

Separation and regeneration of ultrafine catalysts in reaction system still remain challenging. In this study, we demonstrate a simple route to recycle gold nanoparticle (Au NP) catalysts by alternating surface wettability. Hydrophilic Au NPs protected with 3-mercapto-1,2-propanediol (MPD) were applied to catalyze the reduction of 4-nitrophenol (4-NP) using sodium borohydride as reductant, and were then separated from the suspension by interfacial ligand exchange with a toluene solution containing triphenylphosphine (TPP), led to amphiphilic Janus Au NPs with both MPD and TPP ligands at the toluene-water interface. The catalyst was regenerated by a reversible ligand exchange with a MPD aqueous solution. Experiments show that interfacial ligand exchange between phosphines and thiols on gold surface is reversible, although the forward and backward rates are different. As a result, the Au NP catalyst capped with MPD exhibited an excellent catalytic activity with an apparent rate constant (k) of 2.79 min⁻¹, and the performance could be maintained for more than 10 regeneration cycles. The facile conversion between hydrophilic and amphiphilic Janus Au NPs provides a new pathway to explore monophasic and multiphasic reactions with convenient catalyst separation and regeneration.