Enzyme Immobilization in Mesoporous Silica
Abstract: Catalysts are widely used in many industries for production of chemicals, pharmaceuticals, fuels and energy. There is a need for more sustainable production processes, with reduced energy and raw material consumption together with a reduction in waste and toxic by-products. Conventional synthetic catalysts often demand harsh chemical conditions and multi-step processes. Biocatalysts, i.e., enzymes, have high substrate specificity and good catalytic efficiency under mild reaction conditions and are therefore alternatives to synthetic catalysts. Immobilization on a solid support is a method to overcome limitations, such as low long-term stability and poor reusability of the enzymes. Mesoporous silica is a promising material for this purpose. The large surface area and the well-ordered and uniform pores make them suitable as hosts for enzymes. This thesis describes the immobilization of four types of hydrolytic enzymes into mesoporous silica with varying pore size, particle size and morphology. The pH during the immobilization was also varied in order to find the most optimal conditions for high loading, proper stability and good catalytic activity. Mesoporous silica with hexagonally ordered and narrow pore size distribution was synthesized (SBA-15). Three pore sizes, 5, 6 and 9 nm, were obtained by varying the hydrothermal temperature. The particles were rod-shaped, about 2 µm in length, and somewhat agglomerated. Three types of silica particles with varying size and morphology but with the same pore size (9 nm) were also synthesized. Two of them were obtained by modifying the synthesis conditions of conventional SBA-15; one type being broad and about 1 µm long, the other more rod-shaped and about 300 nm long. By decreasing the stirring time during the condensation, particle agglomeration could be avoided. The third particle type was synthesized by forming a composite material with silica and polystyrene clusters inside the droplets of an oil-in-water emulsion (HMM). Spherical, 40 nm small particles with non-ordered pores and a wide pore size distribution were obtained. The materials have been characterized with electron microscopy, nitrogen sorption and small angle X-ray scattering. Trypsin from bovine pancreas (BPT) was immobilized yielding a rapid and large loading. The optimal pore size was found to be 6 nm. A larger pore size gave less protection against autolysis and a smaller pore diameter turned out to be too narrow. Mucor miehei lipase (MML) was immobilized at a lower rate and also gave lower loading, probably due to a larger molecular size and an unfavorable surface charge. The 9 nm pores provided the largest loading and the highest activity. The immobilized lipase was more active compared to free lipase, most likely because of interfacial activation at the silica surface. The highest lipase activity was found in the medium sized particles; however, the loading did not differ significantly between the particles with varying sizes. Hindered substrate diffusion in the large particles and a very wide pore size distribution in the small particles can explain the difference in activity. Lipase should be immobilized in its most active state, which was found to be at pH 8. Immobilization of feruloyl esterases (FAE) resulted in a desired altered enzymatic activity towards transesterification compared to the use of free enzyme in solution. The immobilization was rapid and the stability and reusability was good.
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