Mass Transfer-reaction Mechanisms And Kinetics Of CO₂ Capture By Amino Functionalized Ionic Liquid Organic Solvents

Global warming caused by greenhouse gases will lead to a series of serious environmental problems.Carbon dioxide（CO₂）as the most important greenhouse gas,its control and emission reduction are of great significance for mitigating the climate change.As the largest coal country in the world,it is very necessary for China to reduce CO₂ emissions in the utilization of fossil resources to achieve the goal of carbon neutralization.Carbon capture,utilization and storage（CCUS）technology is the only technical choice to realize the low-carbon utilization of fossil energy on a large scale.Considering the large total emissions and low CO₂ partial pressure of power plant flue gas,the chemical absorption is the most effective CO₂ capture technology to popularize the carbon emission reduction of flue gas of coal-fired power plants.Various chemical absorbents have their own advantages in capturing CO₂,but they also have some defects.For example,organic amine and ionic liquid aqueous solutions face the problem of high regeneration energy consumption,while pure ionic liquids and amine non-aqueous solutions face the problem of high viscosity.In view of the shortcomings of traditional absorbents,developing novel efficient and low-consumption CO₂ absorbent has become the research focus in this field.This work creatively proposed an absorption system composed of amino functionalized ionic liquids（AFILs）and organic solvents at targets of high absorption and desorption performance,low regeneration energy consumption,low viscosity and low corrosivity.Novel AFILs with efficient performance were synthesized,and then suitable organic solvents were selected to construct an ideal composite absorption system.Based on the investigation of absorption and desorption performance,the regeneration energy consumption and corrosion ability of the solution were evaluated.In addition,the mechanisms and kinetics of mass transfer reaction were further explored.Firstly,tetraethylene pentamine（TEPA）containing multiple amino groups was selected as the cation,and 5-amino-4-imidazolecarboxamide（AICA）containing amino and azozole structure was selected as the anion to synthesize tetraethylene pentamine 5-amino-4-imidazolecarboxamide salt（[TEPAH][AICA]）,which was dissolved in the mixed solution of N-propanol（NPA）and ethylene glycol（EG）for CO₂ capture.The CO₂ absorption loading of[TEPAH][AICA]/NPA/EG solution could reach 1.78 mol·mol^-1,which was 3.6 times that of traditional ME A solution,and the viscosities of the solution before and after absorption were 3.84 mPa·s and 9.57 mPa·s,respectively.The solution could be desorbed by heating under atmospheric pressure.The regeneration efficiency could reach 95.2%,and the regeneration energy consumption was 2.30 GJ·t^-1 CO₂,which was 39.5%lower than that of the traditional MEA aqueous solution.Tafel curve of the solution was measured by the electrochemical method,and then the corrosion ability to 20#carbon steel was investigated.The corrosion rates of the fresh and saturated solutions were 4.51 ×10^-3 mm·a^-1 and 6.71×10^-3 mm·a^-1,which were much lower than that of MEA and other ionic liquid aqueous solutions.Secondly,the products evolution of[TEPAH][AICA]/NPA/EG solution during the absorption and desorption process was analyzed by 13C NMR and quantum chemical calculation,and then the reaction mechanism was clarified.The amide group in[AICA]did not participate in the reaction,while the other amino groups could absorb CO₂ to form the carbamates,which followed the zwitterion mechanism.The tertiary nitrogen anion could directly absorb CO₂ to form carbamate without the zwitterion formation and deprotonation process.Some carbamates further reacted with NPA and EG to convert into alkyl carbonates.The physicochemical parameters such as density,viscosity,Henry coefficient and diffusion coefficient of the solution were also measured for the further kinetic study.Based on the reaction mechanism,the mass transfer reaction kinetic model of CO₂ capture was established according to the classical mass transfer model and two-film theory,and a series of kinetic parameters were consequently obtained.The total reaction rate constant and enhancement factor were 18245.90 s^-1 and 80.17,respectively.The relationship between the second-order reaction rate constant and temperature could be described by Arrhenius equation,which was expressed as:k2=7.2075×10⁸ exp（-3093.5/T）,and its reaction activation energy was 25.72 kJ·mol^-1.Although[TEPAH][AICA]/NPA/EG solution had efficient performance,energy saving and low corrosion advantages,[TEPAH][AICA]still had some defects to be improved from the perspective of industrial application and promotion.The material cost of AICA was high and the synthesis conditions of ionic liquid were harsh.In addition,the amino groups in[AICA]-did not react with CO₂,but increased the viscosity and corrosivity.Therefore,the anion was replaced by 2-methylimidazolium（2-MI）with good performance and low price.The synthetic method of tetraethylene pentamine 2-methylimidazolium salt（[TEPAH][2-MI]）was relatively simple,and its organic solution could achieve equivalent performance and energy-saving advantages.The absorption loading of the optimized solution was 1.72 mol·mol^-1,the regeneration efficiency was up to 98.8%,and the regeneration energy was 2.98 GJ·t^-1 CO₂,which was 21.6%lower than that of MEA solution.Due to the elimination of the unreacted amino groups,the viscosity and corrosion ability of the solution were reduced.The viscosities of the fresh and saturated solutions were 3.66 mPa·s and 7.65 mPa·s,and the corrosion rates decreased to 2.43 ×10^-3 mm·a^-1 and 2.53 ×10^-3 mm·a^-1,which was conducive to its storage and transportation during the CO₂ capture industrial process.Finally,the reaction mechanism of CO₂ capture in the[TEPAH][2-MI]/NPA/EG solution was analyzed by 13C NMR and quantum chemical calculation.All amino groups and nitrogen anion could absorb CO₂.The physicochemical parameters of the solution were also measured and the mass transfer reaction kinetic was explored.The solution showed equivalent reaction ability of CO₂ absorption as that of[TEPAH][AICA]/NPA/EG solution,of which the total reaction rate constant and enhancement factor were 13411.66 s^-1 and 67.77,respectively.The relationship between the second-order reaction rate constant and temperature could be expressed as k2=5.6192×10⁸ exp（-3111.6/T）,and the reaction activation energy was 25.87 kJ·mol^-1.Compared with the traditional absorbents,[TEPAH][2-MI]/NPA/EG solution had significant advantages and application potential.