Electrosynthesis as an environmentally friendly production method: A screening methodology towards upscaling
Organic electrosynthesis is a field within electrochemistry that concerns the synthesis of organic products using the electron as a redox agent instead of chemical reductants or oxidants. It offers several important advantages to conventional synthetic methods, such as mild process conditions as reactions can be carried out at ambient temperature and pressure, higher selectivity due to precise control of the reaction by control of the electrode potential, ability to produce unstable or hazardous reagents in situ and less generation of pollutants and waste streams. It is a versatile and inherently environmentally friendly technique, and is labeled as a “clean” and “green” process. Its advantages make it an interesting alternative for industrial applications. However, despite its potential, its implementation is limited. A major drawback is the lack of the crucial combined knowledge on organic synthesis, electrochemistry and engineering aspects. This work aims to bridge the gap between the engineering aspects and the fundamental electrochemical mechanistic aspects of a reaction by developing an innovative screening approach towards industrial implementation.
In a first part, the electrochemical pathway of an industrial relevant reaction is studied: the aldol reaction of acetone to diacetone alcohol. The aldol reaction is a useful and common reaction for C-C bond formation, however, the conventional process exhibits drawbacks to which electrosynthesis offers an answer. A thorough electrochemical investigation is performed to screen this electrochemical reaction. Key parameters are identified and their influence on the reaction is quantified. The obtained information is subsequently used to develop a modular continuous flow electrochemical microreactor setup, which can be combined with analytical techniques to acquire a versatile research platform. The setup is used to investigate the capabilities of performing the electrosynthetic case study in flow. Electrochemical microreactors offer additional advantages such as a lower energy input and less auxiliaries while maintaining the same throughput and product yield as a batch setup.
The influence of reactor parameters such as inter-electrode distance and electrolysis configuration are investigated and described.
In a second part, a more mechanistic approach is applied. Sufficient knowledge of the reaction mechanism is essential to tune the electrocatalyst in order to increase the activity and to improve the viability of the electrochemical pathway. To unravel the underlying electrode reaction mechanisms, electron paramagnetic resonance (EPR) is a well-suited technique to acquire information on the chemical nature and electronic structure of the different intermediates. Even though EPR spectroelectrochemical experiments (i.e. in situ EPR studies performed during electrochemical reactions) were first reported in 1958, it is presently vastly underexploited in electrocatalytic research. On the one hand this can be attributed to the fact that EPR equipment is not widely available and requires specific knowledge, on the other hand there is a considerable lack or even absence of commercially available equipment for such combined experiments. To overcome this problem, a simple, inexpensive and versatile platform consisting of an innovative electrode construction is developed, specifically optimized for electrocatalytic research. The setup is validated on a case study: the reductive intramolecular cyclisation of allyl 2-bromobenzyl ether to 4-methylisochromane. Intermediate products are detected and identified, confirming the proposed mechanism.
Combining these two parts, a methodology comprising the different aspects of an in-depth investigation of an organic electrochemical pathway is developed. Necessary setups were constructed to provide versatile research platforms where available equipment was insufficient or non-existing.