The Design And Regulation Of Functionalized Ionic Liquids For Low-concentration SO₂ Capture In Flue Gas

“Lucid waters and lush mountains are invaluable assets.”The research and practicefocusing on environmental protection have been fruitful in China.Meanwhile,both the aim of“peak carbon dioxide emissions”,“carbon neutrality”,and the people’s pursuit on higher quality life put forward higher requirements for gaseous pollution processing.Sulfur dioxide（SO₂）is regarded as a fearful pollutant threatening both environmental safety and human health.On one hand,acid rain,resulting from the excessive emissions SO₂ in flue gas,is still existed in the southern region of China.On the other hand,due to the inherent disadvantages,such as secondary pollution and non-reusable defect,the development of traditional SO₂ abatement technologies is restricted.Ionic liquids（ILs）are considered as good substitutes for the absorption of acidic gas on account of their unique advantages（negligible vapor pressures,splendid thermal stabilities,highly designable property and so on）,and that providing a new thought to solve the problem of desulfurization.This paper aimed at the absorption of low-concentration SO₂ in flue gas（about 2000 ppm）.The quantum chemical calculation and spectroscopic analysis methods（such as nuclear magnetic resonance）were used for designing and tuning the structure of ionic liquids.Additionally,in-depth investigation was carried out on the structure-performance relationship.New types of functionalized ionic liquids with outstanding absorption performance and environmentally friendly advantage were designed to capture low-concentration SO₂.The main contents are as follows:Firstly,a strategy that predicted SO₂ absorption/desorption behaviors quantitativelywas reported.The equilibrium constant was used as a bridge to associate the absorption enthalpy and absorption capacity,in which the absorption enthalpy could be obtained by density functional theory calculations.After that,some efforts have been made to verify the feasibility and optimize model.The optimized model predicted absorption capacity accurately.This model was applied to predict the absorption/desorption performance of thirteen different imidazolium ionic liquids,in which the optimal ionic liquid exhibited high available absorption capacity（0.68 mol SO₂/mol ILs）,good stability,outstanding SO₂/CO₂ selectivity,and splendid reversibility（5 cycles）.Secondly,in order to solve the separation problem of low-concentration SO₂（about2000 ppm）and CO₂（about 15%）in flue gas and the difficulties preparation in traditional ionic liquids,several simple-prepared protic ionic liquids（PILs）were designed and synthesized.A strategy of cationic regulation and synergistic effect was developed to regulate the selectivity of 2000 ppm SO₂/15%CO₂.Furthermore,the density functional theory calculations and spectroscopic analysis methods were used to indicate the significant influence of the cation on selectivity.The designed[THEEDH][Tetz]showed the best selectivity of 120.4 under the atmosphere of 2000ppm of SO₂ and 15%CO₂,outstanding absorption capacity（0.54 mol SO₂/mol IL）,and excellent reversibility（9 cycles）.Thirdly,the unsatisfactory thermal stability of the simple-prepared protic ionic liquids limits their applications in industry seriously.Therefore,a series of facile-prepared chelate-based ionic liquids were designed and developed by one-step method,which can capture 2000 ppm SO₂ effectively.On the basis of the anion-tunable strategy,the idea of fine-tuned strategy on cation was proposed.Subsequently,the density functional theory calculations were used to explore the influence of anions,ligands,and metal ions.The ideal ionic liquid,[Na（PEG400）][Tetz]showed splendid available absorption capacity of SO₂（0.57 mol SO₂/mol IL）,which was equivalent to that of traditional ionic liquids.Meanwhile,the superiorities of facile preparation and low-cost made it a good potential for industrial application.Fourthly,the increasingly stringent regulation on emission of SO₂ making the"zero emissions"a general trend.This study proposed the“clean-up emissions”of SO₂ by ionic liquids.The anion of ionic liquids was designed by prediction model to meet the standard in advance.After that,the influence of the type and chain length of nonfunctional cations on absorption enthalpy and absorption behavior was studied by theoretical calculation based on density function theory.The ideal ILs,[P₄₄₄₂][Bentriz],exhibited the ability of clean-up emissions（the SO₂ concentration of tail gas less than15 ppm）,good thermal stability（T_d>350 ^oC）,and excellent reversibility（6 cycles）.This study provided a novel method for tuning the property of ionic liquids by nonfunctional cation.