Pickering emulsion stabilizers, such as silica, have gained recent interest in the areas of underground oil recovery and heterogeneous catalyst supports due to their ability to undergo functionalization and tune the surface chemistry [1-4]. Yet, little knowledge is available showing how changing particle and emulsion properties can be used to influence reaction selectivity. In this contribution, Pd nanoparticles are supported on spherical silica nanoparticles with varying degrees of hydrophobic functional groups. A combination of detailed emulsion characterization is coupled with phase specific chemical reactions operating under mass transfer limitations to quantify both the location of functionalized silica nanoparticles at an oil/water interface, as well as the role of diffusion across the interface for enhanced reaction control.
A combination of diffusion NMR, acoustic spectroscopy, and optical microscopy is used to quantify the influence of various surface functional groups on the interfacial area stabilized by silica nanoparticle emulsions. This information is coupled with chemical reactions of olefins and small oxygenates with exclusive solubility in either the oil or water phase to measure the number of catalytic particles in each phase, as well as the rate of diffusion across the oil/water interface. Finally, this information is used to prepare two novel biphasic systems for enhancing reaction selectivity under mass transfer limitations. The first system utilizes blends of functionalized silica to create a gradient in catalytic particles along the oil/water interface enabling for enhanced control of chemical reactions in biphasic systems. The second system uses silica functionalized with a temperature responsive polymer allowing for phase transfer between the oil and water by controlling the temperature. By controlling which phase the catalyst is present the reaction can be turned on or off and the products can be easily recovered by phase transferring catalyst particles.
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Shi, D., Faria Albanese, J. A., Pham, T. N., & Resasco, D. E. (2014). Enhanced Activity and Selectivity of Fischer-Tropsch Synthesis Catalysts in Water/Oil Emulsions. ACS Catalysis.