Synthesis of hierarchically macro/mesoporous silica beads
AffiliationChemical and Biomolecular Engineering
Document TypePhD thesis
Access StatusThis item is embargoed and will be available on 2021-01-21.
© 2018 Dr Stephan Burger
Hierarchically porous materials have potential applications in energy storage and conversion, catalysis, separation and bioengineering. The hierarchically porous structure provides the material with both large quantities of active sites and enhanced mass transport throughout the entire material. These properties are critical for the material's performance since they will influence the capacity and kinetics of the involved reactions or processes. Even though hierarchically porous materials show significant potential, many hierarchical supports contain disordered porous networks or altered catalytic active sites, which influence their reactivity and hamper their performance in comparison to purely meso- or microporous materials. Therefore, to reach their potential industrial application, it is of critical importance to develop improved synthesis methods to obtain hierarchically porous structures with controllability on all the length scales. In addition to improved control over the hierarchically porous structure, it is also critical to develop scalable synthesis strategies, which provides control over the shape and size of these materials. The target applications of the materials synthesized in this thesis are macromolecular applications, such are biocatalysis or the adsorption or separation of larger macromolecules such as proteins. Due to the size of macromolecules, mesoporous materials have an advantage over microporous materials since the biomolecules can enter the larger mesopores, in contrast to the size exclusion effects encountered with the smaller micropores. Therefore, the focus of this thesis was on hierarchically macro/mesoporous materials. One of the strategies used to synthesize hierarchically macro/mesoporous materials are the co-micelle/emulsion templating (co-MET) method, which was the basis of the strategies used in this thesis. With the above-mentioned challenges in mind, the overall objective of this thesis was to gain a deeper understanding of the co-micelle/emulsion templating synthesis method and to develop improved strategies to allow control over the mesopores, macropores and the bead sizes. This objective was overarched by the ultimate aim to develop hierarchically porous materials which are suitable for macromolecular applications. A sedimentation polymerisation method in combination with the co-MET method was used to obtain silica beads with a hierarchically macro/mesoporous structure. Briefly, the co-MET method is a dual templating approach in which a high internal phase emulsion (HIPE) serves as a template for the formation of macropores while surfactant micelles serve as a template for mesopores. This hierarchical structure was captured by silica condensation and acrylamide polymerisation reactions and after the removal of the templates, a hierarchical porous structure was obtained. Multiple interactions exist between the multiple components and structures, which can influence the obtained material properties in various ways. Several synthesis parameters were investigated to determine their effects on the macropore and mesopore sizes as well as the bead mechanical strengths. In addition to the knowledge gained regarding the effect of the various parameters on the material properties, the need to develop a strategy to synthesise co-MET silica beads without the reliance on the acrylamide polymerisation was identified. Acrylamide was removed from the reaction mixture and a new circulation polymerisation synthesis strategy was developed and used to synthesise hierarchically macro/mesoporous silica beads. Precursor droplets were formed by means of a 3D printed coaxial flow device and the circulation of the precursor droplets provided enough time for the silica condensation reaction to form beads, which could maintain their spherical shape during additional ageing and calcination. The removal of the acrylamide from the reaction mixture resulted in a much cleaner mesoporous structure. Furthermore, various parameters were investigated, and this new synthesis method allowed the independent control of the bead sizes, macropore sizes and mesopore sizes while maintaining a well defined and highly interconnected macroporous structure. The stability of these beads was tested in aqueous environments and the results indicated that the beads are suitable for applications at lower temperatures, which is typically used for biomolecular applications. Additionally, the adsorption kinetics of macromolecules onto the hierarchically porous beads were investigated. In conclusion, a new and improved synthesis strategy was developed to synthesize hierarchically macro/mesoporous silica beads by means of the co-MET method. Additionally, a deeper understanding of the synthesis parameters was gained, allowing better control over the bead sizes, macropore sizes and interconnectivity as well as the mesopore sizes. These improvements allow hierarchically macro/mesoporous silica beads to take a step closer to their industrial application.
Keywordssilica beads; hierachically macro/mesoporous, co-micelle/emulsion templating; sedimentation polymerisation; circulation polymerisation
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