Pickering emulsions (PEs), emulsions stabilized by solid emulsifiers, are already of great importance for the food, pharmaceutical and biomedical industry. More recently, PEs are also being increasingly used as advanced...
Pickering emulsions (PEs), emulsions stabilized by solid particles, have shown to be a versatile tool for biphasic catalysis. Here, we report a droplet microfluidic approach for flow PE (FPE) catalysis, further expanding the possibilities for PE catalysis beyond standard batch PE reactions. This microreactor allowed for the inline analysis of the catalytic process with in situ Raman spectroscopy, as demonstrated for the acid‐catalyzed deacetalization of benzaldehyde dimethyl acetal to form benzaldehyde. Furthermore, the use of the FPE system showed a nine fold improvement in yield compared to the simple biphasic flow system (FBS), highlighting the advantage of emulsification. Finally, FPE allowed an antagonistic set of reactions, the deacetalization–Knoevenagel condensation, which proved less efficient in FBS due to rapid acid‐base quenching. The droplet microfluidic system thus offers a versatile new extension of PE catalysis.
Ta ndemcatalysis combines multiple conversion steps, catalysts, and reagents in one reaction medium, offering the potential to reduce waste and time. In this study,P ickeringe mulsionsemulsions stabilized by solidp articles-are used as easy-topreparea nd bioinspired, compartmentalized reaction media for tandemc atalysis. Making use of simple and inexpensive acid and base catalysts, the strategy of compartmentalization of two noncompatible catalysts in both phases of the emulsion is demonstrated by using the deacetalization-Knoevenagel condensation reaction of benzaldehyde dimethyl acetal as a probe reaction. In contrast to simple biphasic systems, which do not allow for tandem catalysis and show instantaneous quenching of the acid and base catalysts, the Pickering emulsions show efficient antagonistic tandemc atalysis and give the desired product in high yield, as ar esult of an increased interfacial area and suppressed mutual destruction of the acid and base catalysts.Biomimicry,t hat is, science inspired by biological entitiesa nd processes, has served catalysis well, for instanceb ym imicking enzyme active sites for the development of new atom-efficient conversionsa nd the design of new biomimetic catalysts. [1] However,l ess attention has been given to bio-inspiredr eactor and process design, emulating the efficiency with which living cells are capable of performing multiple sequential and parallel reactions simultaneously. [2] In this study,w ea im to emulaten ature's strategy of compartmentalization to efficiently perform coupled, one-potr eactions and, in particular,t oa llow antagonistic orthogonal tandem catalytic reactions. [3,4] Orthogonal tandemc atalysis has been defined by Lohr et al. as ao ne-pot reactioni nw hich sequentialc atalytic processes occur through two or more functionally distinct, and preferably non-interfering catalytic cycles. [4] The major challenge of operating tandemr eactions for non-interfering catalysts is that the optimal process param-eters and kinetic regimes for each are typicallyq uite different. Furthermore, noninterference cannot always be avoided and catalystnon-compatibility in fact offers another main challenge for efficient tandem catalysis, for example when combining antagonistic catalysts such as an acid andabase. In that case, the two catalysts need to be kept physically separate, but still be accessible fort he substrates. Various approaches have been taken towards the design of bifunctional acid-base catalysts, often relying on the spatial separation of the reactive entities on polymeric or oxidic support materials, [5,6] for example, in the form of yolk-shellm aterials, [7] metal-organic frameworks (MOFs), [8][9][10][11] shell cross-linked micelles, [12,13] or star polymers. [14] An alternative strategy is to use bio-inspired reaction media and process options in which compartmentalization can be reversibly induced to physically separate soluble antagonistic catalysts. Herein, we reporto ns uch ac ompartmentalization approachi nt he form of aP ickering emulsion ...
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