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.

Scheme 1

Mechanistic reaction scheme of Kolbe electrolysis.

Figure 1

Flow chart of single‐pass Kolbe electrolysis in water.

Figure 2

Conversion (left axis) and applied faradaic equivalents (right axis) (a), selectivity for Kolbe products tetradecane and heptane/1‐heptene (b) and anodic FE for all main Kolbe products (tetradecane, heptane, 1‐heptene), oxygen evolution reaction (OER, O₂) and cathodic FE based on the hydrogen evolution reaction (HER, H₂) (c) in relation to different current densities. In all electrolysis experiments, the residence time was set to 2.55 s (15 mL min⁻¹ per cell). The concentration of n‐octanoic acid was 1.0 m and potassium hydroxide (1.5 m) was used as electrolyte. Electrolysis was performed in single‐pass mode with two cells in parallel using two different setups, namely platinum‐coated stainless steel as anode and nickel‐coated stainless steel as cathode (SS‐Pt||Ni‐SS) and platinum‐coated titanium as anode and platinum‐coated stainless steel as cathode (Ti‐Pt||Pt‐SS). Error bars result from standard deviation of sample measurements.

Figure 3

Flow rate variation at constant current densities of 0.28 A cm⁻² (a–c) and 0.48 A cm⁻² (d–f). Electrolysis was performed in single‐pass mode with two cells in parallel using two different setups, namely platinum‐coated stainless steel as anode and nickel‐coated stainless steel as cathode (SS‐Pt||Ni‐SS) and platinum‐coated titanium as anode and platinum‐coated stainless steel as cathode (Ti‐Pt||Pt‐SS). Concentration of n‐octanoic acid was 1.0 m and potassium hydroxide (1.5 m) was used as electrolyte. Graphs show conversion (left axis) and applied faradaic equivalents (right axis) (a,d), selectivity for Kolbe products tetradecane and heptane/1‐heptene (b,e) and anodic FE for all main Kolbe products (tetradecane, heptane, 1‐heptene), oxygen evolution reaction (OER, O₂) and cathodic FE based on the hydrogen evolution reaction (HER, H₂) (c,f). Error bars result from standard deviation of sample measurements.

Figure 4

a) Selectivity and FE for main Kolbe products tetradecane, heptane and 1‐heptene and conversion depending on different system pressures. b) Selectivity for non‐Kolbe products (heptanal, ester, heptene (non‐Kolbe)) relative to increased system pressure. Error bars result from standard deviation of sample measurements.

Figure 5

Productivity (green bars, right scale) as well as conversion, selectivity and FE for Kolbe products tetradecane, heptane, 1‐heptene (left scale) in relation to the number of electrochemical cells operated in parallel. Error bars result from standard deviation of sample measurements.