Mechanism And Mass Transfer Of CO₂ Absorption Into Aqueous Solutions Of Amino-functionalized Ionic Liquids

Greenhouse gas effect and global warming has caused worldwide attention.As one of the main greenhouse gases,the major anthropogenic sources of carbon dioxide emission are the burning of fossil fuels and the development and utilization of land.Flue gas from power plant has the characteristics of large volume,complex composition and relatively low partial pressure of CO2.Therefore,it has requirements for industrial CO2 absorbent with large absorption capacity,fast absorption rate,good CO2 selectivity and no secondary pollution.Traditional organic amine absorbents are difficult to meet all of the above requirements,so the development of new highly efficient CO2 chemical absorbents is an urge need for current carbon capture and storage.Ionic liquids are widely used in many fields due to their outstanding physicochemical properties and regulatable structures.They also have good performance on CO2 absorption,but significantly increased viscosity of pure ionic liquids after CO2 absorption limits their further development.Hence,combining with water and organic amine absorbents is an effective way to reduce the significantly high viscosity of ionic liquids after absorption and maintain their great absorption performance,therefore promoting the ionic liquids absorption of CO2 towards industrialization.To solve the problem of large viscosity of ionic liquids,the ionic liquids are dissolved in aqueous solution for CO2 absorption,and a series of amino acid anion functionalized ionic liquids are synthesized.We compared absorption performance of ionic liquids with five cations and found that the effect of cations could be ignored since they do not participate in the chemical reaction of CO2 absorption,and tetramethylammonium hydroxide was selected as cation of ionic liquid for subsequent studies.Different amino acid anions were also screened according to the number of amino groups.It was found that the complex anions were not suitable for aqueous solution absorption.Finally,five amino acids including glycine,alanine,serine,lysine and arginine were selected as anions for subsequent studies.The two important properties of the absorption capacity and absorption rate of this series of ionic liquids were investigated.Among them,the aqueous solution of those monoamino ionic liquids have close CO2 absorption loadings to each other.The absorption capacities of[N1111[Gly],[N1111][Ala]and[N1111][Ser]at 30 ℃ are 0.611,0.685 and 0.580 mol CO2·(mol IL)-1,respectively.The absorption loading of ionic liquid aqueous solution containing two or more amino groups was significantly improved,and the absorption capacities of[N1111][[Lys]and[N1111][Arg]aqueous solutions under the same conditions were 1.197 and 1.234 mol CO2·(mol IL)-1,respectively.As the absorption temperature increases,the absorption capacity of the ionic liquid aqueous solution decreases gradually,and the absorption rate first increases and then becomes steady.Reaction mechanism and mass transfer reaction kinetics were also explored.For the aqueous solution of monoamino anion functionalized ionic liquid,the reaction mechanism was summarized as following:amino group firstly reacts with CO2 to form carbamate during absorption,then the carbamate is partially hydrolyzed to HCO3-with presence of solvent and CO2.For anions containing more than one amino group,specific treatment is required.One of the amino groups in[N1111][[Lys]preferentially reacts with CO2,and then another amino group is reacted and formed a carbamate which is partially hydrolyzed to bicarbonate in the second stage of the reaction.The reaction of[N1111][Arg]aqueous solution and CO2 is similar to[N1111][Lys].The amino group near the carbonyl group preferentially reacts with CO2,and the presence of the carbamate intermediate is observed,followed by reaction between another amino group and CO2.The structures of the ionic liquids were constructed and optimized in Gaussian16 software,and the Mulliken charges of N on the amino group were calculated.The influence of electronegativity of N atom on the absorption performance was also discussed.Based on the reaction mechanism obtained,the mass transfer reaction kinetics of the aqueous solution of ionic liquid and CO2 were investigated.The viscosity and density of the absorbents under different CO2 loadings at different temperatures were measured,and the related kinetic parameters such as reaction rate constant were calculated.And according to the Arrhenius equation fitting curve,the correlation was high and the fitting result was ideal.Lastly,we combined ionic liquid with N-methyldiethanolamine/piperazine,screened the best formula with optimal ratio,investigated the CO2 absorption and desorption performance of the absorbent under the optimal ratio,and explored the reaction mechanism,to provide theoretic data for push the application of ionic liquid for CO2 absorption towards industrialization.