Tandem Catalysis with Antagonistic Catalysts Compartmentalized in the Dispersed and Continuous Phases of a Pickering Emulsion

Abstract

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 ...

Recent developments in catalysis with Pickering Emulsions

“…Bruijnincx et al showed that the solid particles in PEs cannot only serve as stabilizing components but also aid in compartmentalizing antagonistic catalysts. 92 They formulated w/o PEs containing both homogeneous acid and base catalysts. The acid catalyst was confined within the aqueous phase while the organic acid was dissolved within the oil phase (Table 1, entry 5).…”

Bifunctional Janus Silica Spheres for Pickering Interfacial Tandem Catalysis

“…Recently, such solid particle‐stabilized emulsions, so‐called Pickering emulsions (PEs) have been receiving increased attention for their application in biphasic catalysis [17–19] . The solid particles at the boundary layer in PEs serve to compartmentalize and to protect the dispersed phase from the continuous phase, for example, allowing multiple reactions to be mediated consecutively [20–22] . The catalysts can then be dissolved in both phases, or the solid emulsifier itself can be endowed with catalytic properties.…”

Continuous Flow Pickering Emulsion Catalysis in Droplet Microfluidics Studied with In Situ Raman Microscopy

“…This microreactor was used to demonstrate the advantage of Pickering stabilized droplets over non‐stabilized droplets for biphasic flow catalysis, using the acid–base catalyzed deacetalization–Knoevenagel condensation of benzaldehyde dimethyl acetal as probe reaction. We previously showed PEs to positively influence this reaction in batch [15] . In this microreactor, the progress of the acid‐catalyzed deacetalization reaction, could be followed with in situ Raman spectroscopy, at various temperatures with both stabilized and non‐stabilized droplets.…”

“…[9] Previous research has mainly focused on the formation and stabilityo fP Es [10][11][12] to replace surfactant-stabilizede mulsions for pharmaceutical and cosmetic applications. [13] More recently, the use of batch PEs as reaction media for catalysis [14][15][16] has drawnattention,a na pproach that comes with considerable potentialb ut also with experimental challenges. Batch PEs are preparedb yv igorous stirring and as ar esult show broad droplet size distributions, making it difficult to measure kinetics, for example.…”

“…We previously showed PEs to positively influencet his reaction in batch. [15] In this microreactor, the progress of the acid-catalyzed deacetalization reaction, could be followed with in situ Ramans pectroscopy, at various temperatures with both stabilizeda nd non-stabilized droplets. Secondly, ap ositive effect of Pickering stabilization of am icrofluidic flow system is shown for the deacetalization-Knoevenagel tandem reaction, now using ex situ GC analysis.…”