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Scalable self-assembly of mesoporous silica microparticles via spray drying
thesisposted on 22.02.2017, 00:34 by Waldron, Kathryn Elizabeth
Porous materials have been shown in nature to be extremely useful for the intended applications. These naturally porous materials have inspired a vast array of synthetic porous materials, especially with the pore sizes in the nanoscale. Of particular interest are the mesoporous materials which have pores in the range of 2 – 50 nm. Thus the pores are the optimum sizes for a wide range of applications being large enough to adsorb/host the larger molecules (bio-molecules) yet small enough to entrap them. Mesoporous silica is of particular interest as its characteristic high surface areas, controllable mesostructures and pore size, coupled with high chemical and thermal stability and availability for surface functionalization could be ideal for a range of applications including adsorption, catalysis, or as drug carriers and biosensors due to their biocompatibility and low cytotoxicity. Traditionally mesoporous silica is synthesised through a “wet chemistry” or solvent evaporation method whereby the mesostructures form in solution and are recovered as the precipitate. However, conventional solution or solvent-evaporation precipitation procedures to synthesize mesoporous materials do present several drawbacks. Firstly, most of them are still conducted in time-consuming and costly batch operations that are not amenable to scalable fabrication. More importantly, most mesoporous silica materials produced via hydrothermal methods are fine powders with non-uniform and/or small sizes, which often have to be post shaped to uniform and large microparticles before use in real applications. Evaporation methods, on the other hand, enable high precision in either particle diameter or thickness of film to produce the particle sizes that are better suited for practical applications, such as in adsorption, separation and catalysis, and packed columns where dynamic streams with high pressure, temperature and fluid flow are often involved. Therefore, fast and scalable fabrication of uniform mesoporous silica microparticles with large geometrical sizes (tens of micrometers) and mesopore size (> 2 – 50 nm) is preferred as they can provide large surface area with accessible pore volumes, without inducing excessive fluid pressure drop across the bed. An alternative method for the synthesis of mesoporous silicas is evaporation-induced self-assembly (EISA) which is the result of competition between condensation of silica and self-assembly of micelles. The EISA process facilitates the control of the final structure though chemical parameters such as the composition of the precursor solution, pH, and aging time, and through processing parameters such as the evaporation rate (partial vapour pressures, temperature, convection), and droplet diameter. Spray drying offers a fast and scalable method of producing mesoporous silica, yet the mechanism behind the self-assembly and the effect of many parameters has yet to be fully elucidated. There are also drawbacks associated with spray drying that are not present in more traditional methods that must be overcome in order for it to be useful in a wide range of applications. These gaps in the knowledge with regards to effects of parameters (drying temperature, initial solute content, precursor composition, solvent, hydrolysis time, post-synthesis treatment) and possible mechanisms are explored in this thesis. We have found that the mesostructure is dependent on the composition of the precursor and for SBA-15 the inlet temperature, while the particle size and shape can be controlled through the initial solute content, inlet temperature, solvent, and hydrolysis time. We propose that the mechanism of assembly is the result of a “wet-pocket” formation with the mesostructure not fully set in the initial spray drying phase, instead it continues to self-organise during secondary drying. The drawbacks, such as the skin that is the result of spray drying which reduces the efficacy of the product in applications, have been overcome through the use of post-synthesis hydrothermal treatments and the addition of co-templates.