Computational Studies Of Solvent Effect And Structure Of Amines On CO₂ Absorption Process In Amine Solutions

Since the industrial revolution,environmental problems such as the greenhouse effect caused by excessive emissions of carbon dioxide?CO₂?have prompted countries around the world to formulate a number of emission reduction strategies to reduce atmospheric CO₂concentration.Capturing CO₂ from the flue gas of power plant by post-combustion CO₂ capture?PCC?technology can effectively restrain CO₂ emission.The chemical absorption method with amine solution as the capturing agent is considered to be the most promising application for post-combustion CO₂ capture due to its advantages of high reactivity,fast reaction speed,high absorption capacity and recyclable absorption.However,the high energy consumption in the regeneration stage limits its large-scale application in power plants.The chemical process of capturing CO₂ in amine solution directly affects the capture performance of absorbents.Therefore,understanding the mechanism of capturing CO₂ in amine and the factors affecting its chemical reaction process is helpful to better design and optimize PCC technology.In this study,the influence of solvent effect on the chemical process of CO₂ absorption by reference MEA aqueous solution and the influence of CO₂ absorption mechanism and diamine structure characteristics?amine group distance and amine group combination?on CO₂ absorption by diamine aqueous solution were investigated.The main research contents and results are as follows:Various computational methods including PM3-PDDG/MM,B3LYP/6-31+G?d,p?/MM and AIMD were employed to investigate the zwitterion formation from the reaction of MEA with CO₂ in water,monoethanolamine,propylamine,methanol and chloroform solvent by explicitly considering the solvent effect.We found,the solvents with higher hydrogen bond capacity facilitate the zwitterion formation.Parameters determining the kinetics of zwitterion formation,are more relevant to the hydrogen bond capacity than to dielectric constant of the solvents.The CO₂ absorption mechanism in diamine aqueous solution was studied by quantum chemical calculation,and the influence of diamine structure?diamine group distance and amine group combination?on the CO₂ absorption process of new diamine was evaluated.We found,the main products are mono-carbamate and di-carbamate in diamine solutions containing tertiary amino group and diamine solutions containing primary/secondary amino group,respectively.The type of amine group and the length of carbon chain have little influence on the activation free energy of intermediate formed by the reaction of amine group with CO₂.In diamine aqueous solutions,the intermolecular proton transfer is barrierless.The energy barrier of intramolecular proton transfer reaction decreases with the increase of carbon chain,and the reaction rate constant increases with the increase of carbon chain.Through the comparison of the reaction energy barrier between proton diamine and free diamine with CO₂ and the comparison of reaction enthalpy between the formation of intermolecular and intramolecular proton products,it was found that the intramolecular proton transfer products is more conducive to the absorption of CO₂ by diamine solution and the reduction of regeneration energy consumption.