Lacunar Keggin Heteropolyacid Salts: Soluble, Solid and Solid-Supported Catalysts

Polyoxometalates are an important class of environmentally friendly catalysts. The Keggin heteropolyacids containing tungsten or molybdenum addenda atoms have received great attention because they have attractive acid and redox properties, which may be easily tunable through changes in their composition and structure. The replacement of their protons by large radium cations makes insoluble the Keggin heteropolyacids and increases their surface area. On the other hand, the removal of tungsten or molybdenum atoms from the Keggin heteropolyanion generates vacancies, which can activate different organic substrates. Therefore, solid lacunar heteropolyacid salts are potentially active catalysts and selective in a plethora of reactions. These catalysts are used either in homo- or heterogeneous conditions. In this review, we wish would highlight recent advances achieved in the chemistry of lacunar Keggin heteropolyacids. We describe their use as solid, soluble or solid-supported catalysts in reactions of oxidation of alcohols and olefins, oxidative desulfurization and acid-catalyzed reactions such as acetalization, esterification, and transesterification.