Preparation Of Functionalized Ionic Liquids And Their Absorption Performances For SO₂ And CO₂

Sulfur dioxide and carbon dioxide in coal-fired flue gas are the main sources of air pollution and greenhouse effects.Recovery of SO2 and CO2 from coal-fired flue gas is of great significance to environmental protection and sustainable economic development.Ionic liquids(ILs)have become the most promising candidates of desulfurizer and decarbonizer for coal-fired flue gas due to their low vapor pressure,wide liquid temperature range and adjustable properties.In this paper,four novel ILs were designed and developed to study the absorption performances of SO2 and CO2,respecticely.Aiming at the low concentration of SO2 in coal-fired flue gas and poor SO2 absorption ability of conventional ILs,a series of functional diacation ionic liquids(DILs)containing ether oxygen group and azole group were designed and synthesized.The results show that the maximum available absorption capacity of imidazole-based DIL is 0.267 g SO2·g-1 at 40℃ and 0.2 vol%SO2,which is the highest level known at present.It is found that the high absorption ability of imidazole-based DIL is due to the multi-site chemical absorption of SO2 by anions and the physical absorption of SO2 by ether groups in the dication.The synergistic effect of these two absorption modes makes the absorbents have appropriate absorption enthalpy,which improves the desorption performance of SO2 while maintaining high absorption capacity.In order to solve the problems of high cost of ILs and the sharp increase in viscosity of organic amine after absorbing SO2,a binary system composed of methylated polyethylenimine(mEIP)and 1-methyl-3-ethylimidazoline tetrazolium([Emim][Tetz])was developed for the absorption of low-concentration SO2.The study of absorption mechanism shows that the binary absorbent can significantly improve the absorption rate of SO2,the viscosity of the absorption system and the absorption capacity of SO2.The absorption of 0.2 vol%SO2 could reach equilibrium within 120 min,and the total absorption capacity is up to 0.253 g SO2·g-1.It is found that the high SO2 absorption capacity of the binary system is mainly due to the synergistic chemical interaction between mEIP and[Emim][Tetz].In view of the low CO2 absorption capacity of conventional ILs and the sharp increase in viscosity of functionalized ILs after CO2 absorption,a new low viscosity and low melting solvent composed of 1,8-diazabicycloundecano-7-ene(DBU)and imidazole(Im)was prepared for CO2 capture.The results show that DBU-Im(1:2)can maintain a lower viscosity as well as a higher CO2 saturated absorption capacity.The CO2 absorption capacity of DBU-Im(1:2)is as high as 0.22 g CO2·g-1 at 40℃ and 1.0 atm,while the viscosity of the absorption system is only 478 cP.The study of absorption mechanism shows that the efficient absorption of CO2 by DBU-Im is mainly attributed to the Lewis acid-base interaction between DBU and CO2.In order to reduce the cost and desorption energy consumption of functionalized ILs,a new type of deep eutectic solvent composed of tetramethylguanidine(TMG),glycerol and its amino derivatives(Gly&Devs)was prepared for CO2 absorption.It is found that the maximum CO2 absorption of TMG-Gly&Devs is 0.196 g CO2·g-1 at 40℃ and 1.0 atm.The study of absorption mechanism shows that TMG,as a proton acceptor,captures the proton hydrogen in Gly&Devs to generate[TMGH]+,which results in Gly&Devs carrying negative charge attack on CO2 to form guanidinium carbonate.