Fibrous intrinsically zeolitic pickering emulsifier

Abstract

The intrinsic amphiphilicity that enables grafting-free zeolite Pickering emulsification has been reported previously by us. However, only one type of anisotropic form, i.e., layer morphology, is reported. In this study, we successfully synthesized another anisotropic form, i.e., fibrous zeolitic TON, with high crystallinity and dispersion. Furthermore, we defined the anisotropic index to evaluate the anisotropy of the zeolite particles. The high anisotropy (anisotropic index = 17.7) and dispersion result in an amphiphilicity that facilitates the stabilization of the emulsion by the bare zeolite. To our surprise, the emulsion exhibits a very stable state in a basic environment, in contrast with the previous emulsion that is stable at low and medium pH (the Brønsted acid site will be quenched under alkaline conditions). In addition to the anisotropy and dispersion, the zeta potential of fibrous particles plays important roles regardless of the counterpart cations present in the zeolite (H⁺, Na⁺, or a mixture). This feature indicates their applicability in catalytic reactions under alkaline conditions. Finally, Pd nanoparticles are further deposited on zeolites, leading to a catalyst for the bromobenzene/water biphasic Suzuki–Miyaura C–C coupling. Pd/TON acts as not only an emulsifier but also a catalyst in the reaction, making Pd/TON a Pickering interfacial catalyst (PIC). The advantages of PICs are: (1) almost 100% product yield is achieved within 20 min; and (2) easy recovery of the product just by filtration or centrifugation, which makes the reaction system environmentally friendly.