Microfluidic plasma reactor for organic synthesis

The reactive species produced by atmospheric pressure plasma (APP) are useful in many applications including disinfection, pretreatment, catalysis, detection and chemical synthesis. Most highly reactive species produced by plasma, such as ·OH, 1O2 and 𝑂2 ∙ −, are highly reactive and short-lived; therefore, in-situ generation is essential to transfer plasma products to the liquid phase or use them at the gas-liquid interface efficiently. In this thesis, a macro-scale plasma reactor was first studied to identify the design features required for the final design to be implemented. The preliminary device was an adapted fluidic diverter valve coupled with a DBD plasma generation site to generate a bubbly flow for the treatment of dyes. This preliminary design led to the development of a novel flow-focusing microfluidic plasma reactor capable of transferring plasma reactive species into various liquid solutions via microbubbles for treating solutions and organic synthesis. The integration of microbubbles in microfluidic devices for improved mass transfer has been successfully demonstrated by several research groups. However, transfer of short-lived plasma species for enhancement of organic synthesis has not been demonstrated before. Therefore, the focus of this thesis is to investigate chemical reactions at the interface of plasma microbubbles.

Firstly, this novel microfluidic device that generates DBD plasma in the vicinity of the gas-liquid interface and disperses the reactive species generated using microbubbles of ca. 200 μm in diameter has been developed and tested. As the bubble size affects the mass transfer performance of the device, the effect of operating parameters and plasma discharge on generated bubbles size has been studied. The mass transfer performance of the device was evaluated by transferring the reactive species generated to an aqueous solution containing dye and measuring the percentage degradation of the dye. Monodisperse microbubbles were generated under all examined conditions but for gas flow rate exceeding a critical value, a secondary break-up event occurred after bubble formation leading to multiple monodisperse bubble populations. The generated microbubble size increased by up to ~ 8% when the device was operated with the gas plasma in the dispersed phase compared to the case without the plasma due to thermal expansion of the feed gas. At the optimal operating conditions, initial dye concentration was reduced significantly by ~60% in a single pass within a short residence time of 5-10 s.

The microfluidic reactor was then applied to complex organic synthesis. Novel organic synthesis routes that circumvent the need for a catalyst and reduce unwanted by-products are highly sought by industry. Reactive species produced by an APP can be used as a potential oxygen donor to carry out this epoxidation reaction without waste stream and catalysts, but mass transfer limitations result in low product yields. This microfluidic reactor in this study was used to facilitate trans-stilbene epoxidation via a chemical reaction at the interface of microbubbles. The effect of initial trans-stilbene concentration, oxygen content in the feed gas mixture and reaction time on the epoxide formation was studied to optimise the chemical reaction. The optimum operating conditions were found to be short bubble-liquid contact times (~2 s) with frequent exposure to freshly generated microbubbles containing reactive species by continuous liquid recirculation, and under these conditions, the overall epoxide yield was ~94% with an overall epoxide selectivity of 10:1. This study demonstrates a strategy for maximizing the use of short-lived reactive species over long-lived reactive species where the desired reaction path requires the former and the reaction scheme involves multiple parallel reactions.