Tuning the Interfacial Activity of Mesoporous Silicas for Biphasic Interface Catalysis Reactions

Interface-active particle materials that are able to assemble at the oil/water interface so as to stabilize droplets, are gaining unprecedented interest due to the intriguing applications in catalysis and materials synthesis, etc. In contrast to these potential applications, this kind of materials are still limited and cannot meet some particular demands of practical utilizations such as rationally designed interfacial activity and high stability against concentrated salts. In this contribution, interface-active mesoporous silica nanospheres (MSS@C_xZ_y) are synthesized through simultaneous incorporation of extremely hydrophilic zwitterionic moiety and hydrophobic octyl moiety in the shell. The textural properties of these materials are characterized by transmission electron microscopy (TEM), powder X-ray diffraction (XRD), and nitrogen sorption. The successful decoration of these functionalities in the shell is confirmed by Fourier transform infrared spectra (FT-IR), ¹³C nuclear cross-polar magnetic resonance (¹³C CP/MAS NMR), and ²⁹Si nuclear cross-polar magnetic resonance (²⁹Si CP/MAS NMR). The prepared mesoporous silicas exhibit tunable interfacial activity, so that oil-in-water (O/W) and water-in-oil (W/O) Pickering emulsions can be easily obtained by varying the molar fraction of these two functionalities. The MSS@C_xZ_y-stabilized Pickering emulsions exhibit high stability to coalescence even at 6.0 M NaCl and have relatively low surface coverage of droplets due to electrostatic repulsion, which is normally difficult to obtain for conventional particles. Interestingly, such interface-active mesoporous silicas can also carry polyoxometalate that is hosted in the nanopore to assemble at the oil/water interface and thus efficiently promotes biphasic epoxidation reactions without any external stirring, exemplifying an innovative application of theses developed mesoporous silicas.