Pickering Emulsion-Derived Liquid–Solid Hybrid Catalyst for Bridging Homogeneous and Heterogeneous Catalysis

Abstract

We describe a novel method to prepare a liquid-solid hybrid catalyst via interfacial growth of a porous silica crust around Pickering emulsion droplets, which allowed us to overcome the current limitations of both homogeneous and heterogeneous catalysts. The inner micron-scaled liquid (for example, ionic liquids) pool of the resultant catalyst can host free homogeneous molecular catalysts or enzymes to create a true homogeneous catalysis environment. The porous silica crust of the hybrid catalyst has excellent stability, which makes it amenable to packing directly in fixed-bed reactors for continuous flow catalysis. As a proof of concept, the enzymatic kinetic resolution of racemic alcohols, Cr III (salen) complex-catalyzed asymmetric ring opening of epoxides and Pd-catalyzed Tsuji-Trost allylic substitution reactions were used to verify the generality and versatility of our strategy for bridging homogeneous and heterogeneous catalysis. The hybrid catalyst-based continuous flow system exhibited a 1.6~16-fold enhancement in activity relative to homogeneous counterparts even at more than 1500 h, and the afforded enantioselectivities were completely equal to those obtained in the homogeneous counterpart systems. Interestingly, the catalytic efficiency can be tuned through rational engineering of the porous crust and the dimensions of the liquid pool, resulting in features of an innovatively designed catalyst. This contribution provides a new method to design efficient catalysts that can bridge the conceptual and technical gaps between homogeneous and heterogeneous catalysis.

Catalysis in Pickering emulsions

“…Various biocatalytic reactions were studied in ref. 46 and 47. The use of IL as the dispersed phase is crucial as it confines the biocatalyst and allows the exchange of molecular reactants with the continuous organic phase.…”

“…47 ILs can even in some cases enhance the selectivity of a reaction. 43,45,46,48 Tao et al carried out the hydroformylation of long chain alkenes in an IL/o Pickering emulsion system. 48 The overall catalytic performance was superior to that of a w/o system.…”

“…86 Sometimes the presence of the particle stabilizer at the liquid-liquid interface is not sufficient to bring the desired robustness for a specific application. 46,55,67 Cross-linking of interfacial particles forming an inorganic shell around the droplets may be required, e.g. for flow applications to avoid leakage of the internal liquid 46 or to assemble droplets containing opposing reagents in the same vessel for multi-step cascade reactions.…”

Recent Advances in Continuous‐Flow Enantioselective Catalysis

“…Through the immobilization strategy mentioned above, although these amazing catalytic designs provide excellent conversion, enantioselectivity, and recyclability, owing to their own limitations, they are unable to reproduce an ideal homogeneous microenvironment. Recently, Yang and co‐workers elucidated the design and implemented the method of a liquid–solid hybrid catalyst made by interfacial growth of a porous silica crust around Pickering emulsion droplets . The catalyst or enzyme was trapped in a crack‐free but permeable capsule to provide a homogeneous catalysis environment.…”

Enzyme–Metal Hybrid Catalysts for Chemoenzymatic Reactions

“…Enzymes are powerful tools for asymmetrically synthesizing high‐value organic compounds under mild reaction conditions. They are widely applied in the manufacture of substantial chiral pharmaceuticals, agrochemicals, and functional materials . Unfortunately, compared to the unlimited demand of synthesizing new chiral structures, the types of enzymatic reactions as well as the substrate scope are quite limit.…”