Hydrophilic silica nanoparticles (100 nm in length and of 20 nm diameter) and larger hollow Halloysite nanotubes (HNTs; 800 nm in length with an outer diameter of 50 nm and an inner diameter of 15 nm) are used to stabilize an oil-in-water emulsion. These particle-stabilized Pickering emulsions (PEs) are used for the hydroformylation of a long-chain olefin (1-dodecene). Rhodium (Rh) and the water-soluble ligand sulfonated 4,5-bis(diphenylphosphino)-9,9-dimethylxanthene are used as catalyst. The emulsions are prepared by sonication and Ultra-Turrax in a specially designed vessel to protect the catalyst from oxygenation and to control the temperature of each sample during the preparation process. The Rh catalyst shows interfacial active behavior and strongly influences the mean droplet size of the emulsions, stability, wettability, and energy of detachment. Further, the Rh catalyst stabilizes an emulsion even in the absence of particles. In a mixture of Rh catalyst and particles, both attach at the interface if the droplet size is in a magnitude of micrometers. These PEs show a monotonous droplet decrease with increasing particle concentration. It is shown that hydroformylation is possible in all emulsions stabilized by the Rh catalyst, silica nanoparticles, or HNTs. However, the conversions in the emulsions are different. The highest conversion is observed in silica-stabilized emulsions with above 40 wt % after a reaction time of only 5 h. Further, high selectivity for aldehyde was observed for all emulsions. A model for the behavior of the emulsions in the reactor is postulated. Interestingly, the emulsions stabilized by the Rh catalyst and silica nanoparticles are destroyed after the reaction, but the HNTs-stabilized PEs remained stable.
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