Abstract
Multiphase reactions combining gas and liquid phases and a solid catalyst are widespread in the chemical industry. The reactions are typically affected by the low gas solubility in liquids and poor mass transfer from the gas phase to the liquid, especially for fast reactions, leading to much lower activity than the intrinsic catalytic activity. In practice, high pressure, temperature, and cosolvents are required to increase the gas solubility and boost the reaction rate. Gas-liquid-solid (G-L-S) microreactors based on particle-stabilized (Pickering) foams rather than conventional surfactant-stabilized foams can increase the contact between the gas and liquid phases, together with surface-active catalytic particles, and dramatically accelerate G-L-S reactions. Unlike surfactants, surface-active catalytic particles can be recycled and reused and reduce coalescence, Ostwald ripening, and aggregation by adsorbing selectively at the G-L interface, promoting stability. In this Account, we present first a taxonomy of microstructured G-L-(S) interfaces to build G-L-S microreactors (catalytic membrane contactors, microdroplets, micromarbles, microbubbles, and particle-stabilized bubbles/foams). Within this taxonomy, we provide a critical appraisal of surface-active catalytic particles to engineer particle-stabilized aqueous and oil foams. We address the fundamental thermodynamics and dynamics aspects of particle adsorption at the G-L interface and examine the foaming stabilization mechanisms. We further enumerate the possible interactions between particles and G-L interfaces and elucidate how the interfacial self-assembly of surface-active particles can discourage foam destabilization mechanisms. We also discuss strategies for the synthesis of surface-active particles, including surface modification of preformed hydrophilic particles, synthesis of organic-inorganic hybrids, coprecipitation, and bottom-up synthesis, including methods for depositing catalytic centers. Various types of particles capable of stabilizing foams are identified including silica particles modified with hydrophobic and hydrophilic chains, silica particles functionalized with oleophobic and oleophilic chains, biphenyl-bridged organosilica particles, and surface-active polymers. Finally, we highlight recent advances from our group, including catalytic oxidation, hydrogenation, and tandem reactions, facilitated by tailor-designed surface-active particles in aqueous/nonaqueous foam. The relationship between the structure, properties, and foaming performance of surface-active particles, along with their catalytic efficiency within foams, is elucidated. It is our hope that this Account will inspire innovative designs of surface-active particles with tailored properties for the advancement of industrially relevant multiphase reactions. Looking ahead, developing data-driven computational tools would be highly beneficial, allowing the in silico design of particles with tailored foaming, foam stability, and local G-L miscibility for defined G-L systems, thus precluding trial-and-error approaches. Parameters such as the three-phase contact angle of particles, the line tension, and the optimal particle size and shape to ensure gas regeneration could be modeled and implemented.