The use of Pickering emulsions has recently received increased attention in catalyzed multiphase reactions. Here, the ultrafiltration of Pickering emulsions is studied for product separation and to retain the catalyst in the reactor. To find the optimum between a high specific surface area for high reaction rates and a suited drop size distribution for high permeate fluxes, the preparation method of Pickering emulsion was investigated. It was found that the stability of the emulsion during filtration does not only depend on the solid particle content, but also on the drop size distribution.

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Unmodified natural clay particles, halloysite nanotubes (HNTs; l = 800 nm, d outer = 50 nm, d inner = 15 nm), are used to stabilize oil-in-water Pickering emulsions (PEs). The hydrophilic HNTs are laterally attached to the oil/water interface and accumulate in bundles because of capillary forces. The nanotubes cover the oil droplets and prevent them from coalescing. The influence of HNT concentration on the oil droplet size and stability of the emulsions is investigated. An increase in the HNT concentration results in a nonmonotonous decrease in the droplet size, which can be attributed to changes in the packing parameter. The high mechanical stability of PEs allows for the separation of the oil droplets from the continuous phase by membrane filtration, and the emulsion phase could be highly concentrated up to around 90 vol % oil phase fraction. This study facilitates an accurate and more efficient application of HNT-stabilized PEs, e.g., for recyclable or continuous liquid/liquid reaction systems.

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