Trace thioether inserted polyamine patches on a support mediate uniform gold nanoclusters as ultrahigh active catalysts

Abstract

Ultrasmall and uniform gold nanoclusters (AuNCs), either freestanding or supported, are usually indirectly available unless mediated with abundant strong ligands, but strong ligands usually suppress the catalytic activity towards certain chemical reactions. The dilemma is that a weaker ligand/mediator allows better catalytic activity but tends to mediate larger gold nanoparticles (AuNPs) and vice versa. Here we show a strategy to overcome this issue in the case of supported AuNCs. When trace amounts of moderately strong ligands of thioethers are introduced into sphere-like polyamines patched on a support, uniform and highly catalytically active AuNCs are directly available. Experimentally, in the presence of a porogen, suspension polymerization of 4-vinylbenzyl chloride with divinylbenzene affords mesoporous beads. The exposed benzyl chloride groups on the beads are replaced by a-few-thioether-modified branched polyethylenimine (PEI) molecules. The modified PEI molecules appear as patches (diameter of 1.86 nm) on the support. Uniform AuNCs (e.g. 1.6 ± 0.1 nm) are obtained from the cooperation of several factors: PEI sequesters and in situ reduces the chloroauric acid species; the trace thioethers enhance nucleation of Au atoms; the patch shapes the AuNCs. At an optimal thioether level of 1 mol% (based on amino units), AuNCs show the best catalytic performance, with a turnover frequency (TOF) as high as 354 h⁻¹ available. Analysis suggests that only 1.1% of the dosed thioether groups have a chance to come into contact with the AuNCs. As a result, an ultralow dosage level of 0.12 ppm for AuNCs in a reaction system conducts 98% reductive conversion of 4-nitrophenol within 17 min, and the catalyst is facilely recyclable.