Automated flow synthesis and click-chemistry of quinones

The ever-growing global energy consumption is a challenge to be achieved by promising renewable energy sources such as solar and wind. The intermittent nature of such renewable energy sources creates a large-scale energy storage problem. Redox flow batteries are considered one of the most promising technologies for low-cost large-scale energy storage. Employing electrolytes with dissolved redox active organic molecules as active material in such batteries offers a potential route to low cost and low environmental impact energy storage. However, one of the significant challenges for redox flow battery technology is not only to develop redox-active organic molecules that are easily synthesized, and that follow a sustainable and fast synthesis approach. But also, for these redox-active molecules to provide a battery performance on par with or better than their more conventional and more expensive inorganic counterparts. For practical applications, the development of redox-active species with high redox potential is very limited and needs further investigation. Therefore, it is crucial to have materials readily obtained from bio-based raw materials to achieve a green technology in its complete lifecycle analysis.

The main focus of the thesis is accelerated synthesis and optimization of green redox-active materials. Therefore, the first part of the thesis focuses on the development of the framework for continuous synthesis, reaction monitoring, and optimization of reaction conditions to increase product yield or conversion.

The continuous synthesis approach offered by flow chemistry is adapted to the chemical synthesis setup with a flow reactor, which is discussed in detail, along with the organic reaction chosen to develop a setup capable of autonomous reaction optimization based on an IR absorbance feedback signal. Characterization techniques such as NMR and IR spectroscopy are also discussed for the selected reaction to understand the chemistry. The correlation of IR absorbance with the relative concentration of reaction components for reaction monitoring is presented in this thesis.

The second part of the thesis is devoted to the exploration of click-chemistry routes for modification of quinone and naphthoquinone cores. Finding conditions where substituents may be introduced in few reaction steps, without much heat input, and carried out in aqueous reaction media is a potential route to low-cost and sustainable synthetic processing of active materials for flow batteries. Molecules from the quinone family have been shown to be promising candidates for use in an organic redox flow battery.

One of the exploratory works employs an electrosynthesis approach via electro-oxidation followed by a chemical step. Although this approach is seen in literature but usually for a different application, this investigation has provided interesting insights into an attempt to form phenazine molecules from the cross-coupling reaction of quinone with aromatic diamines. During this study, electropolymerization of aromatic diamines was encountered and formation of various electropolymerization products. The other exploratory work involved modification of the main precursor naphthoquinone molecule through the concept of clickchemistry and understanding its challenges concerning water solubility, pH, reactant conversion and purification. A widespread click-chemistry synthesis strategy, Michael addition reaction, i.e. thiol-Michael and aza-Michael reaction for different naphthoquinones, are explored and presented in this thesis. For naphthoquinone derivative synthesis (with MPSNa), lawsone-thiol derivative seems to be promising over menadione-thiol derivative due to its single reversible redox wave nature seen over a potential range of -1 V to 0V. In general, it was consistently observed that a second Michael addition step was challenging to be driven by a simple oxidation technique of bubbling air through an open reaction flask. Unlike, thiol-Michael addition reaction, signs of aza-Michael addition reaction were not seen in acidic pH instead were seen in an alkaline pH and in “water only” solvent reaction system. It was interesting to find that Lawsone underwent a Michael addition reaction in the presence of thiol as the nucleophile but underwent a substitution reaction in the presence of primary amine as the nucleophile.