A new way to make oxalic acid from CO₂ and alkali formates: Using the active carbonite intermediate

Conversion of CO₂ to valuable chemicals such as polymers via the electrochemical reduction of CO₂ to formate followed by the formate to oxalate coupling reaction (FOCR) is an interesting concept to replace fossil feedstocks with renewable ones. Yet, the activation of CO₂ is challenging and energy-intensive and today the production of one oxalate molecule first requires the reduction of two CO₂ molecules. Recently we confirmed the crucial role of the reactive carbonite intermediate in the FOCR. Due to its high reactivity, this intermediate might also be a strong enough nucleophile to react with CO₂ directly. If this is the case, we can form oxalate directly from CO₂ and formate and avoid the need for double electrochemical CO₂ reduction in oxalate production. In this work, we successfully established the conversion of CO₂ (with a theoretical yield of 52%) to oxalate (via the reaction with carbonite), as well as to formate and carbonate. The direct reaction of the reactive carbonite intermediate with CO₂ was the dominant pathway for CO₂ incorporation in oxalate. For enhancing the CO₂ incorporation in oxalate, we found a reaction temperature of 200°C, stoichiometric amounts of the base, and the presence of CO₂ in the supercritical state most suitable. The residence time is strongly depending on the reactor type but should be kept to a minimum to avoid carbonate formation. The presence of high amounts of hydride and supercritical CO₂ appeared to also cause the formation of carbonates as a side-product. The carbonate formation increased with higher temperatures and longer reaction times, which suggests a consecutive decomposition of oxalate formed in the reaction.