(530c) Controlling Selectivity in Complex Fluids with Catalytic Janus Particles

A powerful method for combining phase selectivity, emulsion stability, and recyclability is a heterogeneous catalyst with two distinct chemical surfaces and a catalytic metal selectively deposited on one surface.¹ Known as Janus particles, these surface-asymmetric particles have attracted widespread attention over the last decade with applications including solid surfactants, self-propelled motors, and self-assembly.² As catalytic Janus particles marry recyclability, emulsion stability, and catalytic phase selectively, their utility is especially apparent for selective catalytic treatment of complex mixtures without the need for separations. The amphiphilic particles preferentially assemble at the oil/water interface, providing effective emulsion stability and greatly increasing the liquid-liquid interfacial area. Furthermore, the asymmetric functionalization of Janus particle limits the particle rotation at the interface.³ Thus, a catalyst selectively loaded onto the hydrophobic moiety should only be exposed to the oil phase and catalyze transformations of oil soluble species. The solid particles can then be easily separated. Catalytic Janus particles have great potential for selectively degradation of organic pollutants while avoiding catalytic degradation of the valuable water-soluble chemical additives also present in the complex fluid.

Materials and Methods

Silica nanoparticles are asymmetrically functionalized using the water/wax technique.⁴ Briefly, a suspension of 500 nm silica particles are partially hydrophobized, paraffin wax is added, the solution is heated to above the wax melting point, and vigorous stirring is applied to create an emulsion. The solution is then cooled, causing the wax to solidify and trapping the particles in the solid wax. The silica/solid wax “colloidosomes” are redispersed and the exposed silica surface is rendered hydrophobic. The wax is dissolved to realize Janus particles with two chemically distinct hemispheres: bare hydrophilic silica and silane-coated hydrophobic silica. Catalytic Pd nanoparticles can then be either loaded unselectively across the whole particle (termed Pd Janus) or selectively loaded onto the hydrophobic moiety (Janus Pd).

Results and Discussion

The catalytic activity and selectivity of the synthesized catalysts is probed using the hydrogenation of benzaldehyde in a biphasic environment. The reaction is particularly interesting as a probe because benzaldehyde is oil soluble, the intermediate benzyl alcohol is predominantly water soluble, while the final product toluene is very oil soluble. Samples taken at time intervals during the reaction allow a time profile for each species to be constructed. For both Janus Pd and Pd Janus, the benzaldehyde is rapidly consumed to form benzyl alcohol at comparable rates, suggesting similar loadings of Pd on the hydrophobic hemisphere for both catalysts. The benzyl alcohol migrates to the water phase where over the Pd Janus catalyst it is rapidly consumed to form toluene, which migrates back to the oil phase. However, over the Janus Pd catalyst (no Pd exposed to the water phase), the benzyl alcohol profile remained relatively constant and toluene production is much slower. These results are consistent with the hypothesis that the Janus Pd particle is selectively catalyzing oil-soluble species and the water-soluble species (benzyl alcohol) are being “marooned” in the water phase and not undergoing any further reactions.

Significance

This work shows how rational catalyst design can be used to effectively control selectively in complex oil/water emulsions. Through the study, fundamental insights into mass transfer between phases, emulsion stability, and solvent effect on catalytic performance are elucidated.

References

1. Faria, J., Ruiz, M. and Resasco, D. Syn. & Cat., 352(14-15), pp.2359-2364. (2010)
2. Poggi, E. and Gohy, J. Col. & Poly. Sci. 295(11), pp.2083-2108. (2017)
3. Binks, B. and Fletcher, P. Langmuir, 17(16), 4708-4710. (2001)
4. Perro, A., Meunier, F., Schmitt, V. and Ravaine, S. & Sur. 332(1), pp.57-62. (2009)