Keggin Heteropolyacid Salt Catalysts in Oxidation Reactions: A Review
Márcio José da Silva, Alana Alves Rodrigues, Neide Paloma Gonçalves Lopes
Abstract
Keggin heteropolyacids (HPAs) are metal–oxygen clusters with strong Bronsted acidity. The conversion of HPAs to metal salts can result in Lewis acidity, improving their performance in oxidation reactions. In this review, the main routes for the synthesis of Keggin-type heteropolyacids salts, as well their use as catalysts in oxidation processes of a plethora of substrates, such as monoterpenes, olefins, aldehydes, terpene alcohols, and aromatics, are described. Green reactants such as hydrogen peroxide and molecular oxygen are used as oxidants. These reactions are of interest to several industries because they can be used to produce drugs, additives, fragrances, and fine chemicals. The high efficiency of Keggin HPA with green oxidants contributes to a reduction in the environmental impact of these processes, as preconize the principles of green chemistry. Moreover, Keggin HPAs can be converted to bifunctional catalysts by the modification of their structure, total or partial replacement of their protons with Lewis acid metal cations, or the introduction of these cations into the Keggin anion structure, replacing the addenda atoms (i.e., W and Mo). Aspects linked to the synthesis and characterization of these catalysts are discussed herein, with emphasis on infrared spectroscopy and powder XRD patterns. The most recent advances achieved in the development of catalytic oxidation systems based on Keggin HPA salts are also addressed.