Rapid microwave heating and fast quenching for the highly efficient production of long-term stable supported Ag nanoclusters

Given the exciting potential of metallic clusters in a variety of fields, the development of novel preparation methods to accurately controlling the cluster size has become a research priority. Specifically, for catalytic applications, the synthesis and deployment of metallic nanoclusters on a proper substrate is perhaps the main bottleneck. Here, we have adopted an alternative reactor that uses simultaneous ice cooling and microwave heating (unlike water ice is a low microwave absorber) for the synthesis of Ag nanoclusters directly over a support with ordered mesopores (SBA-15). The reactor design exploits the selectivity of microwave heating, assuring a rapid localized nucleation followed by a nearly instantaneous quenching that largely avoids the aggregation of nascent clusters as well as Ostwald ripening mechanisms. We have compared this new synthesis approach with some previously reported methods for the production of supported silver nanoclusters: conventional batch reactor and also a continuous flow microreactor. The resulting Ag clusters were initially analyzed in terms of size distribution, textural properties and catalytic activity in the reduction of 4-nitrophenol. Finally, encouraged by the good results obtained, these nanoclusters were also employed in the production of different cyclic organic compounds, building blocks for pharmaceutical and photochemical applications. The nanoclusters displayed a high catalytic activity, lowering the metal loading required to achieve high yield and selectivity. Furthermore, the stabilization of the clusters over the mesoporous substrate allowed their reuse in several reaction cycles. In fact, the method produced exceptionally stable Ag clusters, whose catalytic properties were preserved even after one year of storage.