(384g) Upcycling of Organic Acid Salt Waste By Sustainability Applications of Kolbe Electrolysis System

Alkaline agents such as sodium hydroxide are widely used as neutralizers and catalysts in various industries, including the food industry. And about 40% of it is waste in the form of organic acid salts. However, incineration in waste treatment is unsuitable because of damage and blockage by salt of combustion to boilers¹⁾, and wastewater treatment, such as activated sludge treatment, is also highly loaded due to the high concentrations of organic matter. Thus, there is currently no suitable treatment method for this problem.

We focused on Kolbe electrolysis as a new treatment method. This reaction has previously been reported to proceed only with water-soluble short-chain organic acids²⁾. In contrast, we considered that organic acid salts would dissociate and are soluble in water, making them applicable to longer chains. Then, Kolbe electrolysis was successfully used to convert organic acid salts with 8 carbon chain length into dimerized long-chain hydrocarbons and regenerate them as alkali, showing the potential of a new complete circulation system for organic acid salts³⁾.

In this study, to improve the practicality of this system, its application to longer-chain organic acid salts generated in large quantities by food industry, the balance between input and generated energy, and the possibility of replacing the electrode material, Pt, with a cheaper material were investigated.

Experimental method

Fatty acid sodium salts with 8, 10, and 12 carbon chain lengths(C8, C10, and C12) were used as raw materials, in aqueous solutions of 0.12 or 0.24 mol/dm³. The basic electrode material was Pt, with stainless steel (SUS), Ti, and Ni as cathodes, and Cu, Ag, Au, and carbon as anodes. In the electrolysis experiments, 100 cm³ of raw material solution was kept at 50℃ with sufficient stirring and a voltage of 10 V was applied. The electric charge, which is an indicator of reaction progress, was kept constant. In the analysis, reactants were extracted with hexane, its concentration was measured by GC-FID, and the product yield was calculated.

Results and Discussions

First, to investigate the effect of carbon chain length on Kolbe electrolysis, the experiments were conducted using fatty acid salts with C8, 10, and 12. In this case, Pt mesh electrode was used, and the electric charge was set at 1000 C. In the case of all carbon chain lengths, the formation of dimerized hydrocarbons, mono alcohol, and mono aldehyde was observed as products of Kolbe electrolysis. Therefore, it is considered that the same mechanism as reported for water-soluble carboxylic acids proceeds when applied to long-chain fatty acid salts. The total yield of products tended to decrease as the carbon chain length increased. However, the current efficiency remained constant, suggesting that this was due to the increase in the yields of alcohol and aldehyde requiring more charge.

To examine the feasibility of this electrolysis system, the energy input was determined from the experimental conditions, and the low heating value was determined from the product volume. The low heating value increases with the carbon chain length of the feedstock, from 9.3 kJ/mol for hydrocarbons produced in the case of feedstock C8 to 14.5 kJ/mol in the case of C12. It was confirmed that the generated energy was at least 5 MJ/kg higher than the input energy for all carbon chain lengths. From the above, although there is a negative effect of lower total yield with increasing carbon chain length, there is a significant contribution from the positive effect of greater generated energy.

Next, an alternative electrode material to the expensive Pt electrode was investigated. In this system, Kolbe electrolysis occurs at the anode and hydrogen generation at the cathode. Since the cathode material is not directly involved in Kolbe electrolysis, it is considered appropriate as long as it is conductive and inexpensive. For the anode material, a material with a low ionization tendency is considered appropriate to avoid metal elution. In the electrolysis experiment, fatty acid salts with C8 were used as the raw material, and an electric charge was set at 500 C. In examining the cathode material, each electrode yielded a Kolbe product equivalent to Pt. Therefore, the expensive Pt (32 US$/g) is considered to be substitutable for the least expensive SUS (0.0034 US$/g). In examining the anode material, in the case of Au, Ag, and Cu, the target reaction did not proceed because the electrode itself was oxidized and current could not flow. On the other hand, with carbon electrode, the reaction proceeded, and the amount produced was about half that of Pt, but the dimer selectivity was as high as 85%. This indicates that carbon electrode promotes dimerization reactions that require fewer electrons for generation. Therefore, it is considered that the least expensive carbon (0.0023 US$/g) can be substituted for carbon.

1)T. Kawamoto et al., JP2016-79289A

2)B. Zhang et al., Nature., 606, 313 (2022)

3)K. Hiromori et al., JAOCS., 99, 103 (2022)