Now, Yilin Wang, Bing Liu and co-workers developed a type of emulsion, where the droplets are covered by a densely packed colloidal SiO2 ring monolayer (pictured). Besides enhancing the stability, the SiO2 rings are hollow so that the liquid–liquid interface of the droplets remains largely accessible, enabling efficient diffusion and operation of the catalysts. First, fluorescent rhodamine B was used to visualize the enhanced diffusion through the interface of water-in-cyclohexane ring-stabilized Pickering emulsion (RPE) compared to conventional nanosphere-stabilized PEs (SPE). Having this beneficial feature confirmed the researchers set out to use it for enhancing catalytic reactions. Lipase was added to the water phase of toluene-in water RPE. The enzyme accumulated at the interface and catalysed the hydrolysis of oil-soluble 4-nitrophenyl palmitate to water-soluble p-nitrophenol, which quickly diffused to the water-phase. This set-up outperformed other conventional emulsion systems in terms of reaction conversion and specific activity of lipase. Then, the authors applied the RPE for the haemoglobin-catalysed oxidation of pyrogallol that not only takes place at the interface but also in in the water phase. RPE again showed significantly faster conversion compared to other PE systems, which was attributed to the faster diffusion of the products from the aqueous phase to the organic phase. Finally, a Pd NP-loaded RPE was generated, and it was shown that it performed better than the corresponding SPE system for the oxidation of benzyl alcohol in batch and continuous flow interfacial catalysis due to higher accumulation of Pd NPs at the interface.
Taken together, the researchers have successfully addressed the problem of low interface accessibility of particle-stabilized Pickering emulsions and have convincingly demonstrated the advantages of the developed ring-stabilized system for catalytic applications.
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