Pickering Emulsions as Compartmentalized Reaction Media for Catalysis

Abstract

The work in this PhD Thesis focused on the separation of incompatible reagents and catalysts in Pickering Emulsions (PEs) as compartmentalized reaction media, inspired by nature’s strategy of compartmentalization. For instance, a living cell contains all kind of physically separated compartments in which multiple parallel and consecutive reactions can take place without interfering with each other. Here, we use PEs to mimic this strategy, emulsions stabilized by solid micro- or nanoparticles rather than surfactants as is the case for classical emulsions. The benefit of using PEs over classical emulsions, is the higher stability of the former, due to the strong adsorption of the particles at the liquid-liquid interface. Two general types of PEs can be distinguished, the water-in-oil (w/o) and the oil-in-water (o/w) type of emulsion. Which type of emulsion is obtained, depends on the wettability of the particles, and therefore on the contact angle of the particles with the liquid phases. Hydrophobic stabilizing particles are more easily wetted by oil than by water, resulting in a contact angle > 90° with water, giving w/o PEs. The opposite is the case for hydrophilic particles. All research described in this PhD Thesis was performed in w/o PEs stabilized by (partially) hydrophobic silica particles. In Chapter 2 the antagonistic deacetalization-Knoevenagel condensation reaction of benzaldehyde dimethyl acetal was performed. This acid-base catalyzed reaction could not be executed in a normal biphasic system due to quenching of the catalysts. In a PE, the catalysts were compartmentalized, resulting in tandem catalytic activity. In Chapter 3 the stability of alkylphenol formulated PEs and their performance for 5-HMF extraction was studied. In Chapter 4 the performance of these alkylphenol formulated PEs was tested for the antagonistic dehydration-Knoevenagel condensation reaction of fructose, via 5-HMF to 5-HMFDEM using homogeneous catalysts. In Chapter 5 a heterogeneous approach was taken for this tandem catalytic reaction. The solid stabilizing particles were modified to have mono- or bicatalytic properties while still maintaining their stabilizing properties. This was the first time that acid-base Janus particles were synthesized. In Chapter 6, a microfluidic approach was used to study the performance of PEs. Due to the transparant tubing, the acid catalyzed deacetalization of benzaldehyde dimethyl acetal could be followed using in-situ Raman spectroscopy. Again PEs outperformed normal biphasic systems in the antagonistic deacetalization-Knoevenagel condensation reaction.