Unraveling the synergic effect of H2O in CO2 capture by aminoalcohols

Abstract

Removal of CO2 from biogas and landfill gas by aminoalcohols is a widely used method. The absorption mechanism at molecular level was analyzed by DFT. The synergic effect of H2O in CO2 capture was studied in aminoalcohols solution including monoethanolamine (MEA), diethanolamine (DEA), and n-methyldiethanolamine (MDEA). The interaction between MDEA and CO2 was found to be very weak, with binding energy of −9.1 kJ mol−1. Meanwhile, the binding energy of the most stable conformer of MDEA–H2O was obtained at −35.0 kJ mol−1. However, both CO2 and H2O interact with MDEA to give a positive Gibbs free energy of formation, indicating a thermodynamically unfavored reaction. In aqueous solution, the hydrogen bonds between H2CO3 and MDEA makes the adsorption much more energetically favorable (binding energy = −69.9 kJ mol−1). The synergic effect of H2O in CO2 capture by MDEA was calculated to be −85.5 kJ mol−1, and the synergic effect make the CO2 capture to be thermodynamically favored (Gibbs free energy of formation = −56.3 kJ mol−1). This is the same conclusion for MEA and DEA. The formation of the OH‧‧‧O and OH‧‧‧N hydrogen bonds are the driving forces during CO2 removal. According to the changes of the Gibbs free energy of formation upon different temperatures, CO2 can be desorbed from the aminoalcohols at 394 K, 411 K, and 415 K in MDEA, DEA, and DEA, respectively. Moreover, the nature of the hydrogen bond was further analyzed by using electron densities and Laplacian densities at the bond critical points, and the values fall in the range of a hydrogen bond. The energy decomposition analysis was further used to analyze the essence of interactions. This study may provide theoretical basis for the selection of new adsorbents in the future.