Fabracation And Performance Of Mesoprous SiO₂ Microspheres Through A Pickering Emulsion Template Strategy

Mesoporous silica microspheres are widely used in chemical industry because of their excellent physical and chemical stability,low density,and easy separation from the reaction system.More desirable is that these mesoporous silica microspheres that possess multicompartmentalized interior structures,not only due to their unique structure can significantly benefit mass transportrelevant processes,but also due to their ability to spatially control local physical and chemical processes occurring in their interiors.Therefore,the preparation of silica microspheres and the tuning of their interior structure have been received extensive attention.There are many methods for the fabrication of hollow micron-sized spheres,including droplet microfluidics,spray drying,and templating method.Unfortunately,these methods are barely able to produce silica microspheres with complex interior structures.Therefore,it is of great significance to develop a fast and efficient method for large-scale preparation of mesoporous silica microspheres with controlled interior structures.In this dissertation,we propose a novel emulsion confinement strategy,in which each nanoparticle-stabilized water droplet serves as a geometrical template for growing an outer shell through an interfacial sol-gel process,and the surfactants located in the water droplets can be utilized to construct interior compartments through a surfactant assembly-directed sol-gel process.Such a protocol is shown to enable tuning of the sol-gel process occurring at the oil/water interface and within the droplet-confined space,independently.The applications of these microspheres in CO₂ adsorption and enzymatic catalysis were investigated.The main results are as follows:A series of multi-compartmentalized mesoporous silica microspheres were prepared by combinaton a solid particle-stabilized Pickering emulsion with the self-assembly of molecular surfactants in the droplet confined space.Each nanoparticle-stabilized water droplet serves as a geometrical template for growing an outer shell through an interfacial sol-gel process.The surfactants located in the water droplets are utilized to construct interior compartments through a surfactant assembly-directed sol-gel process.By simply varying the synthesis conditions,such as the amounts of catalyst,surfactant,and silica precursor,a series of unprecedented mesoporous microspheres were obtained.At the same time,through control experiments and the theoretical basis of selfassembly in the droplet confined space,the mechanism for the evolution of interior structure within the microspheres was proposed.Interestingly,such a protocol was proven to be flexible and extendable by varying the surfactants and other synthesis conditions.These microspheres exhibit excellent permeability to extraneous molecules due to their porous shell and large interior void space.The obtained microspheres can be packed directly in fixedbed reactors,and exhibit significantly enhanced performances in catalysis and CO₂ capture due to the large void spaces and spatially separated nanocompartments within the microsphere.The above multi-compartmentalized mesoporous silica microspheres show excellent performance in CO₂ adsorption applications.However,this adsorbent needs to be obtained through a post-impregnation process,and the cycle stability of the adsorbent is limited.To this end,we extend the above synthesis process to a system using tetraethylenepentamine as a catalyst,methyl orthosilicate and 1,2-bis?triethoxysilyl?ethane as a mixed silica precursors.It is worth mentioning,tetraethylenepentamine?TEPA?here is not only served as the base to catalyze the hydrolysis and condensation of the silica precursors,but also the adsorbent of CO₂.This protocol is fairly simple and time and energy-saving since the whole process mainly involves a one-step synthesis without need removing the surfactant cetyltrimethylammonium chloride?CTAC?.At the same time,the retained CTAC help TEPA dispersing homogeneously in the microspheres.The interior structure and size of the microspheres can be tuned by simply changing the synthesis parameters.The effects of the size,structure and composition of the solid-liquid hybrid mesoporous microspheres on the CO₂ adsorption capacity and adsorption kinetics were also systematically investigated.To improve the water stability of the sorbents,its surface was modified by hydrophobic octyltrimethoxysilane.It was found that the moisture stability of the microspheres was significantly improved after hydrophobic modification,and at the same time,CO₂ capture performance was not affected.This simple surface modification method can significantly improve the moisture stability of the material.This study opens up a horizon in design of solid-liquid hybrid materials for further innovative applications.This study breaks through the limitations of using conventional surfactant based emulsions or Pickering emulsions alone for preparation of microspheres,reveals the mechanism of combining Pickering emulsions with molecular surfactant assembly inside the droplet to prepare microspheres with complex structures,and achieves the goal of regulation of the shell and interior structure of microspheres,independently.This synthesis strategy provides a new platform for the synthesis of other multi-compartment structural materials.The revealed formation mechanism and the droplet confinement effect provide a general principle for the preparation of microspheres with complex interior structures.