Design Of Functional Deep Eutectic Solvents And Their Applications In Flue Gas Desulfurization And Decarbonization

Coal is an important fossil energy and the main energy source in China.However,the conversion and utilization of coal produce a large amount of acidic gases,such as sulfur dioxide（SO₂）and carbon dioxide（CO₂）,causing great harm to the environment and human health.It is of great practical significance to capture acid gases from flue gas before it was discharged into the atmosphere.Deep eutectic solvents（DESs）are recognized as an ionic liquid（IL）analogue,with properties of low volatility,adjustable structure and high solubility to acidic gases,as well as simper synthesis process and lower cost compared with ILs,which show great advantages in the field of acid gas capture.However,the types of DESs that have been reported are very limited,and the available DESs suffer from the problem of low absorption capacity,high viscosity,difficult regeneration and lack of research on low concentration gas capture.The purpose of this study is to design and synthesize functional DESs that can efficiently capture low concentration SO₂ and CO₂,and explore law of absorption/desorption and the interaction mechanism.The main conclusions are as follows:1.Enlightened by the charge transfer interaction between halogen anion and SO₂,series of guanidine-based DESs were designed and synthesized,and the law of their absorption and desorption of low concentration SO₂ and interaction mechanism were investigated.The experimental results show that the guanidine-based DESs have an excellent absorption capacity for low concentration SO₂.The mass absorption capacity of[TMG][Tetz]:Im（1:2）DES was up to 0.4 g SO₂/g DES（2 mol SO₂/mol DES）for 2000 ppm SO₂,which is at an optimal level among functionalized DESs reported in the literature.Moreover,the saturated[TMG][Tetz]:Im（1:2）DES could be regenerated at 80℃.After 5 absorption-desorption cycles,its absorption capacity maintained unchanged,indicating that it has good stability and reusability.The curve of the relationship between partial pressure and absorption capacity showed that the DES absorbed SO₂ by physical and chemical interactions.The results of color rendering and spectral characterization showed that the charge transfer interaction mainly occurred between[TMG]⁺and SO₂,and the chemical interaction sites are imidazole and tetrazole anions.2.There is a sharp increase in viscosity of amino functionalized DESs after absorption of CO₂,due to the formation of a strong and dense hydrogen bond network between the products-NHCOO^-and-NH₃⁺.As for the problem,a series of low viscosity aromatic amine DESs with a new chemical reaction route were designed and synthesized.The experimental results show that 4-aminopyridine（AP）:ethylene glycol（EG）DES has a good ability to absorb CO₂.EG has a significant activation effect on AP,which can greatly improve the CO₂ absorption capacity of AP.The absorption capacity of AP:EG（1:3）DES can reach 0.6 mol CO₂/mol DES（0.1 g CO₂/g DES）,and the saturated DES can achieve rapid regeneration within 20mins at 80℃.The regeneration performance is better than most similar absorbers.At the same time,the viscosity of the absorbent before and after absorption was less than70 m Pa·s.The spectral analysis results show that proton transfer occured between-NH₂ and-N=,and the reaction products are-N-COO^-and-N⁺H=,accompanied by charge transfer interaction.The curve of the relationship between partial pressure and absorption capacity showed that the DES captured CO₂through physicochemical absorption especially physical absorption.Partial pressure had a significant effect on the absorption capacity of AP:EG（1:3）DES.3.Aiming at solving the problems of low absorption capacity of DESs for low concentration CO₂,a series of environmentally friendly choline DESs with high absorption capacity for the capture of low concentration CO₂ was synthesized.The law of absorption and desorption of low concentration CO₂and the interaction mechanism were investigated.The results show that the choline DESs have excellent absorption ability for low concentration CO₂.At30℃,1 atm and 0.15 atm,the mass absorption capacity of[Ch][2-OP]:NMP（1:1）DES was 0.12 CO₂/g DES and 0.10 g CO₂/g DES,respectively.The curve of the relationship between the partial pressure and the absorption capacity showed that[Ch][2-OP]:NMP（1:1）DES absorbed CO₂ mainly through chemical reaction,the partial pressure had little effect on the absorption capacity.Even at 50℃,the absorption capacity of[Ch][2-OP]:NMP（1:1）DES for 0.15 atm CO₂ only dropped to 0.81 g CO₂/g DES,which proved that the[Ch][2-OP]:NMP（1:1）DES has excellent absorption ability for high temperature and low concentration CO₂,and is suitable for the decarbonization demand of high temperature flue gas.Moreover,the saturated DES can be regenerated at 80℃,100 m L/min nitrogen purge.After 5absorption-desorption cycles,its absorption capacity maintained unchanged,indicating that it has good stability and reusability.The mechanism study showed that the absorption site of the DES is located on the[2-OP]^-,which has two kinds of tautomerism,enol and keto.So,pyridine nitrogen and O^-are both effective absorption sites.¹³C NMR results showed that there are both carbamic acid and carbonate in the product as well as a small amount of HCO₃^-/CO₃^2-.In addition,the synergistic effect between O^-and pyridine N on[2-OP]^-endow[Ch][2-OP]:NMP（1:2）DES with high absorption capacity and good desorption performance.