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 acidbase pair sites at the metal-support interface is a key concept for the design of Ag cluster catalysts for the above reactions.

On the origin of the changes in color of Ag/Al2O3 catalysts during storage

“…The smaller Ag-Ag distance compared to that of bulk silver likely results from the small size of the particles. Similar contraction of the metal-metal distance compared to bulk has been observed in several cases of supported Ag samples after reduction [17,18,80]. It can be mentioned that in the case of Au nanoparticles, a direct correlation was established between the size of the Au 0 particles and the extent of the contraction of the Au-Au distances [81].…”

“…Silver-based catalysts, such as Ag/Al2O3, have been broadly studied in reactions such as selective reduction of nitrogen oxides by hydrocarbons [1][2][3][4][5][6][7][8][9][10][11][12][13][14], ethylene epoxidation [15,16] and reactions of green organic synthesis [17]. The most used method to prepare and activate this type of catalyst is very simple; it consists in impregnating the alumina support with an aqueous solution of silver nitrate, followed by a drying step and a calcination treatment in air at around 600 °C.…”

Ag-Based Catalysts in Heterogeneous Selective Oxidation of Alcohols: A Review

“…The TPR data showed that the phase of alumina support affected the interaction with Ag + species, and a weaker interaction led to a facilitated reduction of Ag + species into Ag n δ+ clusters that according to authors' opinion are more active in dehydrogenation in comparison with Ag + and Ag 0 species. Shimizu and Satsuma studied oxidant-free dehydrogenation of alcohol over small silver clusters supported on Al 2 O 3 [53]. The authors emphasize an important role of basic and acid sites of the support in the cooperative dehydrogenation of alcohols into carbonyl compounds ( Figure 2).…”

“…Proposed mechanism for oxidant-free dehydrogenation of alcohol. Reprinted with permission from reference[53]. Copyright 2011 Japan Petroleum Institute.…”

Supported Cobalt Catalysts for Acceptorless Alcohol Dehydrogenation

“…Noble metal-based catalysts, i.e. Pt, [18] Pd, [19] Ru, [20,21] Re, [22] Au, [23][24][25] and Ag, [26][27][28][29][30] have been shown to be active towards acceptor-less alcohol dehydrogenation. Due to their higher abundancy and lower costs, non-noble metal catalysts have also been investigated, namely Cu, [31][32][33] Ni, [34,35] and Co [36] which proved to be also active.…”