Background: Polyoxometalates are an ever more essential class of environmentally benign catalysts. Among them, Keggin heteropolyacids have been highlighted because they are catalysts with attractive redox and acid properties, which may be easily tunable through structural changes. In particular, the total or partial exchanges of their protons by large cations make heteropolyacid insoluble salts with high surface area, potentially useful in a plethora of acid or redox catalyzed reactions. The purpose of this mini review is to highlight some recent advances achieved in the total or partially exchanged heteropolyacid salt-catalyzed routes for the production of chemicals and fuels.
Methods: We undertook a structured search of papers that were published in high-impact journals, which confirm the quality of retrieved papers. The main characteristics of screened papers were described, with emphasis on the aspects that included structural properties, synthesis methods and catalytic activity of metal exchanged heteropolyacids.
Results: Forty-three papers were included in the references of this mini-review. In almost papers cited discuss the surface and catalytic properties of heteropolyacids. Among the papers, the majority (32) are related to the class of polyoxometalates, in which the Keggin heteropolyacid and its salts were included. Eleven papers describe the physical and chemical properties of metal exchanged Keggin salts as well as the synthesis processes. Twenty-seven papers outline the catalytic activity of heteropolyacid salts in different reactions such as oxidation, Friedel-Crafts acylation, transesterification, hydrolysis and so on. In most of the papers, the effects of Brønsted and Lewis acidities on activity and selectivity of heteropolyacid- based catalysts are discussed. The reactivity of heteropolytungstate and heteropolymolybdate metal transition salts is a central theme of most of the papers (32).
Conclusion: The findings of this mini-review confirm that understanding and modulating the physical and chemical properties of heteropolyacid salts comprise a key aspect for obtaining highly active and selective heterogeneous catalysts. Moreover, solid salts of metal exchanged heteropolyacid are attractive options to the solid-supported traditional catalysts.
Keywords: Keggin heteropolyacids, metal-exchanged heteropoly salts, lacunary salts, solid catalysts, hydrolysis, POMs.