Preparation And Application Of Interfacially Active Silica Microspheres

Water is the most ideal green solvent because of its nontoxicity, noncombustibility, and simple separation of organic products. However, most catalytic reactions in water proceed sluggishly because of low solubility, which leads to incomputibility of a reaction system.Catalytic reactions in water proceed more slowly when the catalyst is insoluble solid, which leds to the reaction system contains oil, water and solid phases. Furthermore, for liquid phase reactions, solid catalysts are usually separated from products through filtration or centrifugation. These separation methods are widely used in both small and large scale synthesis, but bothersome especially when the catalyst particle sizes are in a submicrometer-to-micrometer range because of catalyst loss, blocking of filters, high time and energy-consumption, and risk of air oxidation during the course of catalyst transport. To solve the two problems, we carry out the following work:We synthesize modified silica microsphere via covalent linkage between the silica microsphere and the mixture of hydrophobic (MeO)3Si(CH2)7CH3and relatively hydrophilic, pH-sensitive (MeO)3SiCH2CH2CH2(NHCH2CH2)2NH2. The obtained modified silica microsphere was characterized with elemental analysis. The results show that SiO2surface chemistry can be designed through changing the molar fractions of these two organosilanes with opposite properties. When triamine silane/mixture silanes=4%(molar ratio), we can get the functional silica microsphere, which is interfacially active for producing emulsion and pH-responsive. The obtained functional silica microsphere was characterized with zeta potential and water contact angle. The results show that hydrophilicity/hydrophobicity of material was switched by regulating pH=3-4to pH=7-8, which leads to an emulsion inversion. This functional silica microsphere-stabilized emulsion system features the toleration of a variety of oils, high internal oil phase, flexible formulation, rapid inversion and good ability to resolve oil. After10cycles Pickering emulsion was still kept and the neat organic phase could be well resolved.Noble metal Pd nanoparticles were loaded on the functional silica microsphere, resulting in a solid catalyst. The hydrogenation of olefin in the organic/aqueous biphasic system was used as a model reaction to evaluate the catalysis efficiency. It is found that the solid catalyst-stabilized o/w emulsion system has much higher catalytic efficiency than other biphasic systems. At the end of reaction, the emulsion is inverted from o/w to w/o after raising the pH. Based on Pickering emulsion inversion, we have successfully developed a conceptually novel, built-in method to in situ separate and recycle submicrometer-sized solid catalysts by tuning the pH. This method is time/energy-saving for the catalyst separation and recycling. Its high effectiveness is highlighted by36reaction cycles and negligible catalyst loss.