The interfacially active TiO 2 nanoparticles were prepared via a simple post-grafting method. Schematic illustration of the pH-switched Pickering emulsion strategy. The organic phase in the upper layer can be isolated by simple phase separation, while the solid catalysts residing in water phase will be directly used for the next reaction cycle after tuning the pH by the addition of base. After reaction, the emulsions were broken completely by adding a small amount acid in the system. Because of the increased interface area, the catalytic efficiency could be improved. At the beginning of reaction, an oil-in-water (O/W) Pickering emulsion system was formed using amino-functionalized TiO 2 catalysts as stabilizer ( Scheme 1b). As illustrated in Scheme 1a, the surface wettability of the catalyst can be switched by adding acid or base. Herein, in this work, we further investigated the pH responsive Pickering emulsion system for in situ catalyst recycling using interficially active TiO 2 catalyst nanoparticles as emulsifier. ![]() However, there are little reports for its application in Pickering emulsion catalytic system, especially in pH-responsive system. It has been widely used in photocatalytic degradation of organics in water and water splitting to generate hydrogen. Titanium dioxide (TiO 2) is a traditional support in catalysis and also a potential photocatalyst due to its low cost, safety, resistance to acid and base and highly photocatalysis activity. On the other hand, for pH-switched Pickering emulsion systems, silica or organic polymers are often used as the solid stabilizers, more diverse of useful solid emulsifier are expected to be developed. Such a new strategy might provide new opportunities for separating and recycling solid catalysts, however, still less studied as far as we know. Because of the large interface area in the Pickering emulsions, the interfacial reaction efficiency could be improved by several orders of magnitude compared to bulk systems. The surface hydrophilic/hydrophobic properties of the silica emulsifier can be easily altered by tuning pH values, leading to the reversible circulation process of emulsification/demulsification and achieving the recovery and reuse of the solid catalyst. In this system, an interfacially active silica nanoparticle prepared with a pH-sensitive (MeO) 3-SiCH 2CH 2CH 2(NHCH 2CH 2) 2NH 2 group on the surface was utilized. Recently, our group developed a novel pH-switched Pickering emulsion catalytic system for multi-phase reactions. Therefore, exploring more efficient methods for achieving the easy recovery and separation of nanoparticle catalysts are still highly desirable. Magnetic field assisted isolation is one of the relatively effective ways to combat these problems, but external magnetic fields are required. ![]() Moreover, these methods often involve the consumption of extra energy in particularly when the particle size is very small. Though traditional filtration or centrifugation methods can be used, these operations will inevitably lead to the loss of catalyst during transferring the reaction solution from one vessel to another. However, a serious obstacle for practical utilization of nano-catalysts is the recycling and separation of the catalysts after chemical reaction, especially in multi-phasic systems. Interfacially active TiO 2 nanoparticles pH-responsive Pickering emulsion Catalyst recyclingĪlong with the development of nanotechnology, nano-catalysts play an increasingly important role in the field of catalysis due to their high efficiency and unique properties compared with the corresponding bulk catalysts.
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