Silver Cluster Catalysts for Green Organic Synthesis

Abstract

Development of platinum-group-metal (PGM)-free catalysts for green chemical synthesis is an important topic in synthetic catalysis, because PGM will soon be in short supply in the near future. Our group has paid attention to the catalytic functions of Ag clusters. Here, we summarize our recent work on size- and support-specific catalysis of Ag clusters for green organic synthesis. Ag clusters supported on Al₂O₃ act as effective heterogeneous catalysts for (1) oxidant-free dehydrogenation of alcohols to carbonyl compounds, (2) coupling of alcohols with amines to form amides and H₂, (3) N-alkylation of anilines with alcohols, (4) C–C cross-coupling reaction of alcohols, (5) selective hydrogenation of nitroaromatics, and (6) direct synthesis of N-substituted anilines from nitroaromatics and alcohols. To establish a catalyst design concept, effects of Ag particle size and acid-base character of support oxides are investigated. The structure-activity relationships for all reactions show similar tendencies; metallic Ag clusters with smaller size and acid-base bifunctional nature of the support oxide are preferable. We propose that cooperation between coordinatively unsaturated Ag sites of Ag clusters and acid-base pair sites at the metal-support interface is a key concept for the design of Ag cluster catalysts for the above reactions.