Direct use of low-concentration CO2 in the synthesis of dialkyl carbonates, carbamate acid esters, and urea derivatives

The CO₂ in thermal power plant and factory exhaust gases and the air must be addressed because all have low-concentration CO₂. However, applying low-concentration CO₂ to developed reactions that use high-purity CO₂, energy- and cost-intensive pretreatments, such as CO₂ purification, concentration, and compression, are required. Thus, a technology that directly converts low-concentration CO₂ into useful chemicals without these pretreatment processes is attractive for energy saving and cost reduction. Such technology has been achieved in the fields of methane and methanol synthesis via the hydrogen reduction of CO₂. Recently, the concept of directly using low-concentration CO₂ has been applied to the synthesis of high value-added chemicals without hydrogen reduction. The synthesis of useful chemicals from CO₂ without hydrogen reduction is a technology that will be put into practical use without waiting for the widespread use of green hydrogen. Thus, this review introduces the technology for the direct synthesis of useful chemicals, such as dialkyl carbonates (DRCs), carbamic acid esters (CAEs), and urea derivatives from low-concentration CO₂ contained in exhaust gas or air, without hydrogen reduction. DRCs, CAEs, and urea derivatives are synthesized via the non-hydrogen reduction route of CO₂. They are expected to contribute to long-term, large-volume CO₂ fixation because they are raw materials of functional polymers, which have long-material-life and large market potential. Details on how to synthesize these compounds directly from low-concentration CO₂ are presented, focusing on the efforts to devise CO₂ capture processes, reactants, and catalysts.