Process Intensification of 2,2′-(4-Nitrophenyl) Dipyrromethane Synthesis with a SO3H-Functionalized Ionic Liquid Catalyst in Pickering-Emulsion-Based Packed-Bed Microreactors

A stable water-in-oil Pickering emulsion was fabricated with SO3H-functionalized ionic liquid and surface-modified silica nanoparticles and used for 2,2′-(4-nitrophenyl) dipyrromethane synthesis in a packed-bed microreactor, exhibiting high reaction activity and product selectivity. The compartmentalized water droplets of the Pickering emulsion had an excellent ability to confine the ionic liquid against loss under continuous-flow conditions, and the excellent durability of the catalytic system without a significant decrease in the reaction efficiency and selectivity was achieved. Compared with the reaction performance of a liquid–liquid slug-flow microreactor and batch reactor, the Pickering-emulsion-based catalytic system showed a higher specific interfacial area between the catalytic and reactant phases, benefiting the synthesis of 2,2′-(4-nitrophenyl) dipyrromethane and resulting in a higher yield (90%). This work indicated that an increase in the contact of reactants with catalytic aqueous solution in a Pickering-emulsion-based packed-bed microreactor can greatly enhance the synthetic process of dipyrromethane, giving an excellent yield of products and a short reaction time. It was revealed that Pickering-emulsion-based packed-bed microreactors with the use of ionic liquids as catalysts for interfacial catalysis have great application potential in the process of intensification of organic synthesis.