Pickering emulsions are surfactant-free dispersions of two immiscible fluids that are kinetically stabilized by colloidal particles. For ecological reasons, these systems have undergone a resurgence of interest to mitigate the use of synthetic surfactants and solvents. Moreover, the use of colloidal particles as stabilizers provides emulsions with original properties compared to surfactant-stabilized emulsions, microemulsions, and micellar systems. Despite these specific advantages, the application of Pickering emulsions to catalysis has been rarely explored. This Minireview describes very recent examples of hybrid and composite amphiphilic materials for the design of interfacial catalysts in Pickering emulsions with special emphasis on their assets and challenges for industrially relevant biphasic reactions in fine chemistry, biofuel upgrading, and depollution.

Polyoxometalates (POMs) are nanometric metal-oxide anions with unique chemical and physical properties. During last decade, significant efforts have been made to give POMs surface activity and self-assembly properties that are essential for catalysis applications and for producing organic-inorganic hybrid materials. In this work, POMs based surfactants are produced spontaneously through non-covalent interactions in water by mixing non-ionic surfactants with POM. The most common POMs of tungstosilicate and tungstophosphate, have indeed an unexpected strong tendency to adsorb on polar and neutral interfaces. Micelles in water and water/air interfaces were investigated by SAXS and ion flotation showing the POM anions adsorbed at the micelle surface and on monolayers of non-ionic surfactants. This general property of POM provides a unique opportunity for deeper understanding of many medicinal effects of POMs, i.e. their antiviral and antitumor activities that involves their specific adsorption on biological surfaces.

Decyl-, dodecyl-, and tetradecyltrimethylammonium cations were combined with the catalytic polyoxometalate [PW(12)O(40)](3-) anion to give spherical and monodisperse nanoparticles that are able to stabilize emulsions in the presence of water and an aromatic solvent. This triphasic liquid/solid/liquid system, based on a catalytic surfactant, is particularly efficient as a reaction medium for epoxidation reactions that involve hydrogen peroxide. The reactions proceed at competitive rates with straightforward separation of the phases by centrifugation. Such catalytic "Pickering" emulsions combine the advantages of heterogeneous catalysis and biphasic catalysis without the drawbacks (e.g., catalyst leaching or separation time).