A Pickering Emulsion/Organic-biphasic System For Aqueous Catalytic Hydrogenation

With the recent advances in nanoscience and nanotechno logy, more and more plenteous nanoparticle catalysts that feature high accessibility of active sites and high surface area are explored for a variety of chemical transformations, and their popularity has been gained unprecedentedly in the liquid-phase synthesis. The practical applications of these active catalysts in the liquid-phase synthesis, however, are implicated by notorious difficulty in separation and recycling. In some cases, using water as reaction medium can simplify the organic product separation and facilitate catalyst recycling. Albeit these merits and appearing potentials, the organic/aqueous biphasic systems (OABS) lead to an extremely high mass transfer resistance especially for highly hydrophobic substrate. Thus, catalytic reactivity in water media commonly exposed an intrinsic shortage. Therefore, a conceptually novel methodology has been explored for recycling nanoparticle catalysts for aqueous reaction becomes an important and desirable theme.A conceptually novel methodology has been explored for recycling nanoparticle catalysts, which is based on transforming a conventional organic/aqueous biphasic system to a Pickering emulsion/organic-biphasic system (PEOBS). We used a mixture of hydrophobic and relatively hydrophilic silane to modify the silica microsphere via covalent linkage to obtain the modified silica microsphere. It is well documented that a mixture of water and organics can form so-called Pickering emulsion in the presence of interfacially active particles. Noble metal nanoparticles were loaded on the material, resulting in an interfacially active solid catalyst. The obtained catalysts’structures were estimated by N2 sorption, X-ray Photoelectron Spectroscopy (XPS) and transmission electron microscopy (TEM). Abundant hydrogenation reactions in aqueous phase were employed to evaluate catalytic performances. It is found that the Pickering emulsion/organic-biphasic system exhibited remarkable superiority (4 times higher) than the conventional organic/aqueous biphasic system. Furthermore, recycling test indicated that the Pickering emulsion/organic-biphasic system showed a excellent stability without loss of catalytic efficiency. In all cases the Pickering emulsion/organic-biphasic system enables the organic product to be directly separated through a simple liquid-liquid phase separation and the nanoparticle catalyst to be in situ recycle without need for "isolation". The merits of Pickering emulsion/organic-biphasic system such as facile catalyst recycling, and high catalysis efficiency are intensively demonstrated though aqueous hydrogenations of unsaturated C=C, C=N, C=O and alkynes.Finally, by investigating the activation energy and desorption capacity of the Pickering emulsion/organic-biphasic systems, we will have an insight into the fundamental explanation of the reasons for the high activity and recycling nanoparticle catalysts of the Pickering emulsion/organic-biphasic systems.