The Dilemma of Supporting Electrolytes for Electroorganic Synthesis: A Case Study on Kolbe Electrolysis

Abstract

Remarkably, coulombic efficiency (CE, about 50 % at 1 Farad equivalent), and product composition resulting from aqueous Kolbe electrolysis are independent of reactor temperature and initial pH value. Although numerous studies on Kolbe electrolysis are available, the interrelations of different reaction parameters (e.g., acid concentration, pH, and especially electrolytic conductivity) are not addressed. A systematic analysis based on cyclic voltammetry reveals that solely the electrolytic conductivity impacts the current-voltage behavior. When using supporting electrolytes, not only their concentration, but also the type is decisive. We show that higher concentrations of KNO3 result in reduced CE and thus in significant increase in electric energy demand per converted molecule, whereas Na2 SO4 allows improved space-time yields. Pros and cons of adding supporting electrolytes are discussed in a final cost assessment.

Elektrifizierung der organischen Synthese

“…[62] Theo utcome of aK olbe electrolysis depends mainly on the reaction conditions.C urrent density,s ubstrate concentration, and pH value are major factors. [63] Thef ormation of ac arboxylate layer on the anode seems to be crucial for the reactions,while solvent gets desorbed. High current densities (> 250 mA cm À2 )a re recommended to favor this layer formation, and as uitable pH value ensures carboxylate formation.…”

Elektrobioraffinerien: Synergien zwischen elektrochemischen und mikrobiologischen Stoffumwandlungen nutzbar machen

“…[62] Bemerkenswerterweise wurde die Muconsäure durch Fermentation gewonnen und kçnnte auch als Baustein füra ndere wichtige Monomere,z .B.A dipinsäure,w irken. [67,68] Diese erfolgt entweder durch Erzeugung von C-C-Bindungen zwischen aliphatischen organischen Säuren oder durch Kreuzkupplung mit anderen Reaktionspartnern wie Phenolen oder Thiofuranen. B. die Gewinnung von Furfuralen [65] und Va nillin ein.…”

Deterioration of Aqueous n‐Octanoate Electrolysis with Electrolytic Conductivity Collapse Caused by the Formation of n‐Octanoic Acid/n‐Octanoate Agglomerates

“…[22] Whereas both, C 8 and C 5 are mid-chain n-carboxylic acids/ n-carboxylates, they differ significantly in their physical chemical properties. The results are compared to the recent observations that Na 2 SO 4 acts as inert supporting electrolyte, improving spacetime-yields of C 5 (i. e. aqueous solutions containing n-valeric acid and n-valerate) electrolysis without notably affecting the CE.…”

“…[20] This study focusses on the electrochemical oxidation of saturated n-carboxylates in aqueous reaction media as typically derived from microbial conversion processes. [22] Consequently, high current densities can already be achieved at anode potentials slightly above the potential at which carboxylate oxidation starts (> 2.4 V vs. NHE respectively 2.2 vs. Ag/AgCl (sat. Furthermore, a higher educt concentration, which is easily achievable for soluble n-carboxylic acids and their respective n-carboxylates, results in an improved electrolytic conductivity and hence mass transfer limitations are minimized.…”

“…[1,20,21] For ncarboxylic acids, which are completely soluble in water, the above mentioned favorable conditions for dimer formation, i. e. neutral to slightly acidic pH and high educt concentrations, can be achieved without difficulty. [7,22] However, more hydrophobic n-carboxylic acids possessing a higher carbon chain length are only slightly soluble in water, e. g. n-heptanoic acid (0.24 g in 100 g water at 20 8C) or n-octanoic acid (0.08 g in 100 g water at 20 8C). [22] Consequently, high current densities can already be achieved at anode potentials slightly above the potential at which carboxylate oxidation starts (> 2.4 V vs. NHE respectively 2.2 vs. Ag/AgCl (sat.…”