Kee, S.P.; (2007) Microreactor engineering studies for asymmetric chalcone epoxidation. Doctoral thesis , University of London.

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Abstract

Advances in the field of microreaction technology offer the opportunity to combine the benefits of continuous processing with the flexibility and versatility desired in the pharmaceutical and fine chemicals industry. Microreactor devices, also offer their own unique advantages over traditional continuous processing, such as improved heat and mass transfer, safer handling of exothermic reactions and easy containment of explosive and toxic materials. A reaction system can be quickly scaled-up to production levels by increasing the number of units operating in parallel, allowing significant savings in time and R&D costs. Most studies of microreactor systems to date focus on the development and performance of individual microdevices. However, a top down approach is preferred, with the focus on the requirements of the process and a suitable device design derived to meet those requirements. This work aims to demonstrate the suitability of the poly-L-leucine catalysed asymmetric epoxidation of chalcone reaction for continuous processing as well as the process and choices of designing and scaling a microchemical system. A suitable continuous reaction protocol was established for this reaction system, achieving a conversion of 88.4 % and enantioselectivity of 88.8 %. Mixing was found to be critical due to the low diffusivity ( 10"u) of the polymeric catalyst. Design criteria were established and a microstructured reactor with a footprint of 110 mm x 85 mm and production rate of - 0.5 g/day was designed for the system. An external scale-out structure was selected. The staggered herringbone mixer was selected for enhancing the mixing in the microstructured reactor. A method for characterizing the mixing performance in the staggered herringbone mixer based on stretching computations using particle tracking methods was developed, which allowed the required mixer length to be derived directly. Mixer lengths of 40 mm were provided for both deprotonation and epoxidation mixers. The effects of varying operating temperature, residence time and reactant concentrations on reaction performance in the fabricated microstructured reactor were investigated. The base case condition (13.47 g/1 PLL, 0.132 mol/1 H202, 0.0802 mol/1 chalcone, 0.22 mol/1 DBU) was found to be optimal, achieving a conversion of 86.7 % and enantioselectivity of 87.6 %. Several unexpected phenomena such as bubble clogging and increased viscosity due to the polymeric catalyst were encountered. A scaled-out system was designed and experiments carried out. Flow maldistribution, attributed to fabrication errors and bubble clogging, resulted in poor reaction performance (conversion -31.4 % and enantioselectivity 82.7 %) due to unequal residence times and imperfect mixing ratios of reactants. The commercial potential of the research was evaluated. Micro and macro level analysis of the market and industry were favourable and a suitable commercialisation route was suggested.

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