Scalable Microreactor Concept for the Continuous Kolbe Electrolysis of Carboxylic Acids Using Aqueous Electrolyte

Abstract

The Kolbe electrolysis is a promising reaction to combine the usage of electrons as reagents and the application of renewable generated carboxylic acids as raw materials producing value added chemicals. Within this study, the electrolysis was conducted in a specially developed concept electrochemical microreactor and draws the particular attention to continuous operation and reuse of the aqueous electrolyte as well as of the dissolved unreacted feedstock. The electrolysis was conducted in alkaline aqueous solution using n‐octanoic acid as model substance. Platinized titanium as anode material in an undivided cell setup was shown to give Kolbe selectivity above 90 %. During the technically relevant conditions of current densities up to 0.6 A cm −2 and overall electrolysis times of up to 3 h, a high electrode stability was observed. Finally, a proof‐of‐concept continuous operation and the numbering up potential of the ECMR could be demonstrated.

Closing the Gap: Towards a Fully Continuous and Self‐Regulated Kolbe Electrosynthesis

“…[27] Also Baumgarten et al observed a negative impact while re-saturating carboxylate during a Kolbe electrolysis. [14] Adding neutralized (via addition of KOH) carboxylate instead of the acid itself, however leads to a further increased carbonate concentration with every resaturation -enhancing negative effects on the Kolbe electrolysis. This problem is avoided by re-dosing the acid itself.…”

“…Thus, the products of electrochemical transformations (e. g., hydrogenations, hydrodeoxygenations, decarboxylations) of polar biogenic compounds are often significantly less polar than their precursors, leading to a reduced solubility and therefore the option to achieve a separation of the insoluble product phase (liquid or solid) from the electrolyte phase. [8,[12][13][14] Kolbe reactions are generally performed at slightly acidic pH values. [15] This pH range can be understood as a compromise between the necessary acidic reaction conditions to avoid the Hofer-Moest-reaction [16] (see Scheme 1) on the one hand and the required deprotonation of the carboxylic group for a sufficiently high carboxylate concentration.…”

“…This is comparable to the latest publications. [14] As high temperatures are known to decrease radical stability, it is recommended to perform Kolbe coupling at moderate temperatures. [15] Depending on the reactor's electrical resistance, the required high currents cause joule heating and are likely to change the temperature.…”

“…Today, many micro-flow reactors are presented to be easily scaled up. [14,20,21] The perspective given is the addition of an arbitrary number of compartments in a "numbering-up" approach, which is rather a "scale-out" than a "scale-up". [21,22] In principle, numbering-up can be used to enable large production quantities.…”

“…Using aqueous electrolyte solutions also offers an elegant product separation opportunity. Thus, the products of electrochemical transformations (e. g., hydrogenations, hydrodeoxygenations, decarboxylations) of polar biogenic compounds are often significantly less polar than their precursors, leading to a reduced solubility and therefore the option to achieve a separation of the insoluble product phase (liquid or solid) from the electrolyte phase [8,12–14] . Kolbe reactions are generally performed at slightly acidic pH values [15] .…”