Stimulus-responsive Pickering emulsion droplet-based microreactors for green biphasic catalysis.

Pickering emulsions (PEs) are constantly emerging as promising platforms in biphasic catalysis. However, the existing nanoparticle-stabilized PE systems suffer from several drawbacks, such as reactant diffusion, particle leaching, incompatible reaction environments, tedious workups and poor catalyst recovery. The use of nanogel particles in PE catalysis could be highly advantageous in streamlining processes. Herein, we report a general approach to designing a PE droplet-based reactor by using a combination of egg white nanogel particles (EWNGs, 0.01 wt.%) and the newly synthesized switchable surfactant 3-(10-carboxydecyl)-1-methyl-1H-imidazolium bromide (ImDec-COOH) at a very low concentration of 0.50 wt.%. This versatile approach helps EWNGs in PEs overcome existing limitations, such as reactant diffusion and incompatible reaction environments, while stimulus-responsive surfactants increase the pH of PE systems. Thus, nanogel particles stabilize the PE system, and imidazolium-based surfactants effectively facilitate organic transformation through biphasic catalysis. As a proof-of-concept, the as-developed microreactor was employed to achieve a one-pot synthesis of benzothiazoles and benzoxazole in high yield, in situ product separation and excellent catalyst recovery.