Immobilisation of catalysts for applications in organic reactions

The use of catalysts generally requires the immobilisation of the catalyst at specific locations, a requirement that can pose a significant challenge. The use of mesoporous silicate materials as supports offers some valuable advantages and can provide a more stable environment for catalysts. In this instance, this project was focused on the preparation, characterisation and optimization of immobilized catalysts (enzyme and metal based) for applications in organic reactions. The goal is to obtain an optimal catalytic performance combined with long-term operational stability. In order to achieve this goal, a detailed understanding of the solid supports, the catalysts, and of the immobilization process is essential. Cytochrome c and Candida antartica lipase B (CALB) were immobilised on to the mesoporous silicate SBA-15 and a porous spherical silicate material (PPS) in order to determine the influence of differences in the properties of supports such as pore volume, surface area and morphology on factors such as loading, activity and stability of enzymes. The catalytic activity of cytochrome c was similar on both supports, while the loading and catalytic activity of CALB on SBA-15 was higher when compared to PPS. These results indicate that the amount of adsorbed enzyme and activity are a reflection of the different surface areas, pore shapes and pore volumes of the two materials. Leaching of the enzyme from the support can result in substantial losses in activity, particularly on reuse. To overcome this disadvantage, it is necessary to utilise surface modification of the silica support to specifically bind the immobilised enzyme. In order to explore this effect, covalent attachment of Nickel/Cobalt-cyclam onto MCF were examined. The amount of cyclam modified onto the surface of MCF increased as the amounts of starting materials were increased. Ni/Co-cyclam modified materials were tested for their ability to immobilise His6-alanine racemase (HT-AlaR). The catalytic activity of immobilized HT-AlaR was comparable to the free enzyme and also maintained its activity after 5 cycles. After successfully immobilising cytochrome c, lipase CALB and HT-AlaR on silicate materials by physical adsorption and covalent methods, the next step was to utilize these methods to immobilize glucose oxidase. Numerous experiments were performed to immobilise glucose oxidase. The aim was to prepare highly active and stable biocatalysts which have capability to work in extreme reaction conditions. After examined immobilisation of GOx using physical adsorption, covalent bonding and cross linking methods. GOx immobilized by cross linking methods offers many advantages such as high activity and operational stability over other methods. The synthesis and characterisation of the manganese (Mn) complex 4,7-dimethyl-1,4,7-triazacyclonone (DMTACN) immobilised on mesoporous silica was performed. FT-IR, Elemental analysis and X-Ray photoelectron spectra indicated the presence of catalytic species in an active conformation. Under the reaction condition employed in this work, manganese complexes of 1,4,7-trimethyl-1,4,7-triazacyclonone (TMTACN) produced the maximum yield of 90% in 1 hr reaction time. The immobilised catalyst, 4,7-dimethyl-1,4,7-triazacyclonone (MCF-DMTACN) on the other hand showed the highest yield (56%) in the presence of a co-catalyst. A series of reaction conditions were examined to determine the best conditions to combine immobilised GOx and the Mn complex. The aim was to catalyse the epoxidation of styrene by the immobilised Mn complex with in-situ generation of hydrogen peroxide by immobilised GOx as tandem system. On combining the catalysts the epoxidation yield was very low for the Mn complex in the presence of immobilised enzyme. The reason for the low yield may be due to deactivation of Mn complex. The use of powdered forms of mesoporous silicate as support for catalysts in industrial application requires high back pressures in flow reactors, due to the small particle sizes (10 to 25 μm). To overcome this limitation, immobilisation of catalysts (glucose oxidase and manganese complex) on to functionalised MPS monoliths was performed. Glucose oxidase immobilised in the monolith microreactor showed low activity, while the Mn-functionalised monolith recorded a 75% yield of epoxide at a flow rate of 0.2 ml/min. This 75% yield was not reported before under similar reaction condition. These results have the potential to selectively immobilize different metal-based catalysts on monoliths for such reactions.