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

Abstract

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.

Keywords

• Low-concentration CO2
• Direct utilization
• Dialkyl carbonate
• Carbamate acid ester
• Urea derivative