Recent advances in interfacial liquid–liquid oxidation in emulsions and microemulsions with surface-active molecular and nanoparticulate catalysts

Abstract

This review presents recent advances in catalytic oxidations performed in emulsified biphasic systems using primary oxidants (O₂, H₂O_2, and t-BuOOH) mediated by surface-active molecular or nanoparticulate catalysts. Emphasis is placed on the activation of these sustainable oxidants by redox catalysts compatible with aqueous biphasic environments. A distinctive feature of this review is its colloid-focused perspective, examining how the formulation and composition of surface-active catalyst/oil/water systems drives emulsion morphology, droplet size, demixing kinetics, and ultimately the ease of product recovery and catalyst recycling. Particular attention is given to the design of redox-active amphiphiles, which enhance interfacial coverage and facilitate (nano)emulsification. Three classes of systems are considered: (i) systems using separate phase-transfer agents (PTAs) and redox catalysts; (ii) integrated systems involving catalytic surfactants or catalytic ionic liquids (CILs) that combine interfacial and redox functions within a single species; and (iii) amphiphilic nanoparticles that adsorb at the oil–water interface, serving as both emulsion stabilizers (the so-called Pickering emulsions) and oxidation catalysts. A dedicated section addresses the challenge of chemically generated singlet oxygen (¹O₂) in organic synthesis. Due to its short lifetime in water (≈4 μs), ¹O₂ must be generated in aqueous nanodroplets of a microemulsion system to ensure its diffusion into the organic phase before deactivation occurs. Recent innovations in Pickering interfacial catalysis (PIC) highlight that particle-based systems are becoming increasingly elaborated, both for controlling the physicochemical properties of the emulsion and for enhancing catalytic performance. However, several challenges still remain to be overcome before they can be scaled up for industrial applications.