Experimental Study of High-Pressure CO2 Absorption by Amino Acid-Based Deep Eutectic Solvents

Carbon dioxide (CO2) is a major greenhouse gas and an undesirable impurity in natural gas due to its corrosive effects, hydrate formation, and reduced calorific value. Its efficient removal is therefore vital for both environmental protection and industrial operations. Deep eutectic solvents (DESs) have recently emerged as sustainable alternatives to conventional amine-based solvents owing to their biodegradability, low vapor pressure, and simple synthesis. In this study, six amino acid–based DESs were synthesized and evaluated for CO2 absorption using a high-pressure system. The effects of operating parameters─including pressure, molar ratio, amino acid molecular weight and side chain length, and water content were systematically examined and compared with conventional amine solvents used in natural gas sweetening. To prove the formation of DES, several appearance parameters, viscosity, and 1H NMR experiments of the prepared solutions were investigated. The results revealed that DESs composed of l-proline and monoethanolamine (MEA) showed the most favorable absorption capacity and stability. The l-proline–MEA (1:2) DES achieved a maximum loading value of 1.26 mol CO2/mol DES after 6 h at 20 bar and 35 °C with a concentration of 2.5 M. These findings demonstrate the potential of amino acid–based DESs as efficient and stable solvents for CO2 capture in industrial gas purification.