Synthesis of Oxalate from CO₂ and Cesium Carbonate Supported Over Porous Carbon

抄録

Oxalic acid is an attractive chemical platform potentially available from CO₂ due to its established applications and chemical characteristics enabling it to serve as a mediator in hydrometallurgy including iron-making. However, a method for synthesizing oxalic acid from CO₂ has yet to be established. In the present work, the formation of oxalate scaffold during heating of cesium carbonate (Cs₂CO₃) in the presence of CO₂ and H₂ as reactants was experimentally investigated with a particular focus on the influence of supporting Cs₂CO₃ over porous materials. Among the support materials examined, activated carbon (AC) had a notable effect in improving the reaction rate and yield of total carboxylates (formate and oxalate) during experiments with an autoclave. An important problem was the dominant presence of formate, the intermediate between carbonate and oxalate, accounting for over 90% of the carboxylates. Changing the reaction conditions, including temperature, reaction time, partial pressure of gas components, and amount of loaded Cs₂CO₃, did not alter the situation. Alternatively, re-heating of the formate-rich salts over AC under CO and CO₂ enhanced the oxalate fraction while maintaining the total carboxylates yield. Benefiting from the employment of support material, the two-step conversion was carried out using a gas-flow type reactor with a packed bed of Cs₂CO₃ supported over AC. In this reaction system, because water, acting as a promoter, was absent, the total carboxylates yield was lower than that in the autoclave, while the oxalate fraction was higher, being 71.8% with a yield of 43.2% on a Cs₂CO₃-carbon basis.