Physico-Chemical Properties of Pickering Emulsions Stabilized by Different Nanoparticles for Hydroformylation of Long-Chain Olefins

Abstract

Solid particle-stabilized Pickering emulsions can be used as alternative reaction systems, for example, for the homogeneously catalyzed hydroformylation reaction. This study addresses the understanding of the physicochemical behavior of Pickering emulsions in terms of the hydroformylation in a recycable process. In the first part (see chapter 4), fumed silica with different hydrophobicities were used to stabilize the emulsions. Hence, the droplets with W/O Pickering emulsions exhibit a size that is a function of particle concentration and energy input during the preparation. Furthermore, adsorption of interface impurities on the particles is observed, resulting in an increase of the interfacial tension. In addition, the Pickering emulsions are highly stable in a batch reactor. Hence, the hydroformylation reaction in Pickering emulsions was optimized and a complete recycling cycle with a membrane filtration was successfully demonstrated. In chapter 5, hydrophilic particles with different particle shapes, so-called Halloysite nanotubes and fumed silica, which stabilize an O/W Pickering emulsion were used due to higher conversions. The larger Halloysite nanotubes initially exhibit an isotropic interface orientation that converts to a radial configuration by increasing the particle concentration. It was possible to modify the Halloysite nanotubes, but the change in wettability was not strongly pronounced. Furthermore, emulsions stabilized by pristine Halloysite nanotubes or by silica show a dependency on the particle concentration, hence, in the case of Halloysite nanotubes, the droplet size does not decrease monotonically. The addition of the interface-active Rh-catalyst leads to a droplet size in the order of nanometers, resulting in droplets without adherent particles. The increase in the droplet size to the micrometer scale leads to an adherence of the particles. Thus, a corresponding model of Pickering emulsions is postulated in a batch reactor, with intermediate emulsion stability promoting the reaction. The last chapter (see chapter 6) investigates the interaction between positively charged particles and the negatively charged rhodium (Rh-) catalyst in terms of emulsion structure and hydroformylation. The positively charged polystyrene particles used stabilize a W/O emulsion while the modified positively charged Halloysite nanotubes stabilize an O/W emulsion. It is shown that the Rh-catalyst adsorbs at the particle surface, which does not change the emulsion type. Further, in the case of polystyrene-stabilized Pickering emulsions, the particle density at the interface is also not affected by the adsorption of the Rh-catalyst. However, the diffusion behavior of the polystyrene particles at the interface is influenced by the adsorption of the Rh-catalyst on the particle surface. In general, it is demonstrated that the positive surface charge for both particle types leads to a higher conversion and selectivity in comparison to their negatively charged analogous.