Resource Recycling Japanese Researchers Develop Breakthrough Method for Fossil Fuel-Free Acetylene Production

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Acetylene, a key industrial chemical, is traditionally produced using fossil fuels, contributing to a high carbon footprint. Japanese researchers have now developed a groundbreaking electrochemical method that converts CO₂ into acetylene with a 92 % efficiency, bypassing the need for fossil fuels and enhancing carbon recycling.

Since its discovery, in 1836, acetylene has emerged as an essential chemical compound in industry, widely used as a chemical building block and fuel. It has applications in the raw material for resins, such as vinyl chloride, welding gas, and illumination. Recent developments aimed at reducing the dependence on petroleum feedstocks have shown that acetylene is a promising platform molecule for producing various base chemicals. Additionally, polyacetylene, a crucial semiconducting material, is made from acetylene. Currently, acetylene is mainly produced through two methods: the calcium carbide process and cracking. The calcium carbide process involves reacting coal-derived coke and quicklime to form calcium carbide (CaC₂), which is then reacted with water to produce acetylene. In cracking, the thermal decomposition of fossil fuels is used to make acetylene. Both methods depend heavily on fossil fuels, contributing significantly to the carbon footprint. Therefore, developing a new sustainable method that does not rely on fossil fuels is essential.

In a recent study, a team of Japanese researchers proposed an innovative electrochemical process using high-temperature molten salts. This method produces acetylene indirectly by the formation of CaC₂ from carbon dioxide (CO₂). In this process, CaC₂ is obtained through electrodeposition and is subsequently reacted with water to yield acetylene. However, the current efficiency of this process is less than 10 % in most conditions.

Addressing this issue, in a new study, the team, led by Assistant Professor Yuta Suzuki (first author) from Harris Science Research Institute, Doshisha University, and Professor Takuya Goto (corresponding author) from the Department of Science of Environment and Mathematical Modeling, Graduate School of Science and Engineering, Doshisha University, and including Dr. Tomohiro Isogai from the Technology and Innovation Center at Daikin Industries Ltd., developed a more efficient process with selective CaC₂ formation. Dr. Suzuki explains, “To increase the current efficiency of acetylene formation, it is necessary to selectively produce CaC₂ from CO₂, which depends on controlling interfacial phenomena between the electrode and the molten salt electrolyte. One approach is to separate the reactions of carbon electrodeposition from CO₂ and the Ca(II) ions reduction on the CO₂-derived solid carbon”.

In this method, acetylene is produced through a two-step electrolysis process. First, an electrolytic cell containing iron as the working electrode is immersed in a molten salt mixture (NaCl–KCl–CaCl₂) at 823 K under a CO₂ atmosphere. This results in the electrodeposition of carbon, from the CO₂ onto the iron electrode, forming a carbon electrode. In the second step, this electrodeposited carbon is selectively converted to CaC₂ under an inert Argon atmosphere, which can then be used to synthesize acetylene. This second electrolysis can be carried out either using the carbon electrode formed in the first step as is or the carbon residue after hydrolysis. In this study, the researchers utilized the carbon electrode from the first step.

This method achieves acetylene synthesis with a high current efficiency of 92 %. Moreover, unlike conventional methods using fossil fuels, the CaC₂ produced in this process does not contain sulfur and phosphorous in the product gas and contributes to carbon recycling by realizing highly efficient CaC₂ production.

The researchers also investigated the mechanism of the formation of CaC₂ and found that it was induced by the electrochemical reaction of Ca(II) ions at the carbon’s basal plane, rather than through interlayer reactions, representing a unique interfacial phenomenon. Highlighting the potential of this method, Prof. Goto remarks, “This method can lead to the realization of an acetylene-based chemical industry with acetylene made from CO₂. This process will help build a new industry based on resource recycling without using fossil fuels.”

Original Article: High-efficient acetylene synthesis by selective electrochemical formation of CO2-derived CaC2; Chemical Engineering Journal; DOI:10.1016/j.cej.2024.153013

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