Citation: da Silva, M.J.; Rodrigues, A.A.; Lopes, N.P.G. Keggin Heteropolyacid Salt Catalysts in Oxidation Reactions: A Review. Inorganics 2023, 11, 162. https://doi.org/10.3390/ inorganics11040162 Academic Editor: Kirill Grzhegorzhevskii Received: 12 February 2023 Revised: 25 March 2023 Accepted: 27 March 2023 Published: 11 April 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). inorganics Review Keggin Heteropolyacid Salt Catalysts in Oxidation Reactions: A Review Marcio Jose da Silva * , Alana Alves Rodrigues and Neide Paloma Gonçalves Lopes Chemistry Department, Federal University of Viçosa, Viçosa 36590-000, Brazil * Correspondence: silvamj2003@ufv.br 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. Keywords: Keggin heteropolyacids; solid heteropoly salts; monoterpenes; aromatic aldehydes; olefins; terpene alcohols 1. Introduction The development of bifunctional catalysts has attracted attention for economic and environmental reasons [1]. Many bifunctional catalysts have either Lewis or Brønsted acidity functionality, which can promote either acidic or oxidative transformations [2]. Another possible use of the term “bifunctional” is concerning catalysts that can promote hydrogenation and dehydrogenation [3]. Catalysts that can promote reactions for which the main steps require Lewis or Brønsted acid sites are still scarce. Although it is expected that these two types of active sites catalyze different elementary steps within an overall reaction, it is also possible that they participate in the same step. There are various types of bifunctional catalysts, such as bimetallic catalysts [4]. Nor- mally, these catalysts comprise noble metals as nanoparticles, solid-supported catalysts, and metal oxides [5]. In general, these catalysts have active hydrogenation or oxidation sites [6]. Perovskite oxides are bifunctional catalysts with the general formula ABO 3 or A 2 BO 4 , where A sites normally represent rare-earth, alkaline-earth, or other large metal cations, while the B sites are transition metal cations. These compounds have been demonstrated to be a class of effective bifunctional catalysts due to their plentiful stoichiometries and crystal structures, as well as multimetal active centers [7]. Bifunctional catalysts frequently combine active metal sites for hydrogenation/dehydrogenation and active acidic sites for protonation/deprotonation steps [8]. Salts containing different counterions can also be used as bifunctional catalysts [9,10]. Keggin heteropolyacids are compounds belonging to the polyoxometalate class that can also be used as bifunctional catalysts [11,12]. Their strong acidity and their structural Inorganics 2023, 11, 162. https://doi.org/10.3390/inorganics11040162 https://www.mdpi.com/journal/inorganics