Catalysis at the Solid–Liquid–Liquid Interface of Water–Oil Pickering Emulsions: A Tutorial Review

The development of reactive-separation processes, in which products are separated from the reaction media (i.e., reactants and catalysts) in a single reaction unit, is of great interest in industry as energy-intensive separation processes can be obviated. One promising alternative to accomplish this process intensification is the utilization of biphasic emulsion systems, where products and reactants can be separated based on differences in solubility. Additionally, when solid nanoparticles with amphiphilic character and catalytic activity are employed in biphasic systems, it is possible to simultaneously stabilize the liquid–liquid interface and catalyze chemical reactions. This new technology leads to (a) increased interfacial area, (b) enhanced mass transfer of molecules between the two phases, (c) simplified reaction/separation processes by using a recoverable solid catalyst instead of surfactants, and (d) effective separation of products from the reaction mixture by differences in their water/oil solubility and thus avoiding distillation that leads to product decomposition. To this end, the present tutorial review explores the fundamentals of Pickering interfacial catalysis (PIC) and its application in biomass upgrading (upgrading of sugars and pyrolysis oil), biogas to liquid products via Fischer–Tropsch synthesis, and biodiesel production in the context of the United Nations sustainable development goals. We conclude this contribution with an outlook of the current bottlenecks in the commercial exploitation of the technology and the possible future directions in which PIC can be employed to facilitate the energy transition and the C-circular economy.