Water is a desirable solvent to use in chemical transformations from both a green chemistry and economic perspective. Because water is polar, protic, and able to form hydrogen bonds, it has been reported to alter the reaction pathway on metal and oxide surfaces in a complex manner. In addition to being employed as a monophasic solvent, water has also been incorporated with immiscible oils to form water/oil Pickering emulsions stabilized by solid catalyst particles. One of the advantages of such emulsions is the potential for improving catalytic activity for reactions involving both water- and oil-soluble species. It is also possible to design catalysts such that reactions in either the aqueous or organic phase can be selectively promoted. Here, we describe how surface functionalization with organic monolayers can be used to control reaction selectivity at the water-oil interface. For example, surface modification of Pd/Al2O3 was used to promote desired product formation during vanillin hydrodeoxygenation (HDO) in a water/decalin emulsion environment. Organophosphonic acids (PAs) were applied as modifiers to control the surface hydrophobicity, thereby determining the continuous and dispersed emulsion phases. The PAs also introduced Brønsted acid sites that enhanced the rate of vanillin HDO. Catalyst performance greatly depended on the surface hydrophobicity, which influenced the selectivity to different reaction pathways and emulsion structures. We will also describe how amphiphilic Janus particles created by asymmetric functionalization with organic monolayers can result in remarkable changes in phase selectivity during hydrogenation of various substrates in oil-water mixtures.

Controlling catalysis at oil-water interfaces with functionalized nanoparticles

ACS Fall 2020 Virtual Meeting

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