Study On Absorption And Catalytic Conversion Of SO₂ By Deep Eutectic Solvents

Sulfur dioxide(SO2)produced by coal combustion is extremely harmful.However,SO2 is still an important sulfur resource in industrial applications.Therefore,it is necessary to develop SO2 capture and utilization strategy.In the ways of SO2 utilization,the cycloaddition reaction of SO2 has attracted our attention due to its 100%atomic economy.At present,deep eutectic solvents(DESs)are widely used in the field of gas absorption.This article aims to find a DES and apply it to the absorption and in-situ conversion of SO2,so that DES can be regenerated while obtaining high value-added chemicals.(1)Two conventional DESs,1-ethyl-3-methylimidazole bromide([Emim]Br)-polyethylene glycol(PEG200)and choline chloride-ethylene glycol(EG)DES,were synthesized.Two internal salt functional DESs,L-carnitine(Lcar)-EG and betaine(Bet)-EG DES,were synthesized.The absorption performance of DESs on SO2 and the catalytic effect on reaction of SO2 and propylene oxide to form 1,2-propanediol sulfite were studied.In[Emim]BrPEG200(1:1)DES,the absorption of 2 vol%SO2 under 40℃ can reach to 0.56 mol S02/mol DES and the absorption of pure SO2 reached to 4.06 mol SO2/mol DES under the same absorption conditions.After reacting at 12O℃ for 5 h,the conversion of pure SO2 was as high as 89.6%.However,[Emim]BrPEG200 DES still has low absorption of low-concentration SO2.Thus,it cannot efficiently capture low-concentration SO2 in flue gas.While,the internal salt functional DESs can capture low-concentration SO2 through chemical action.At 40℃,L-car-EG(1:3)DES can absorb 2 vol%SO2 up to 0.85 mol SO2/mol DES and the absorption of pure SO2 reached to 2.66 mol SO2/mol DES.Reacted at 80℃ for 5 h,in the presence of potassium iodide,the conversion of pure SO2 can only reach to 41.5%,which proves that the catalytic effect of the reaction between SO2 and propylene oxide in L-car-EG(1:3)DES is not ideal;(2)Aiming at the fact that conventional DESs cannot efficiently capture low-concentration SO2 and internal salt functional DESs cannot efficiently convert SO2,a series of polyamine functional DESs based on tetraethylenepentamine halogen salts([TEPA]X,X=Cl/Br/I)have been synthesized.The absorption performance of DESs on SO2 and the catalytic effect on the reaction of SO2 and propylene oxide to form 1,2-propanediol sulfite were studied.Among them,[TEPA]L0.4Br-EG(1:5)DES synthesized by introducing lactic acid(L)can absorb pure SO2 up to 5.34 mol SO2/mol DES at 40℃.And reacted at 80℃ for 5 h,the conversion of pure SO2 was as high as 93.7%.[TEPA]L0.4Br-EG(1:5)DES can absorb 2 vol%SO2 up to 2.32 mol SO2/mol DES.After reacting at 80℃ for 5 hours,the conversion of 2 vol%SO2 has also reached 85.3%.In the acidic environment provided by lactic acid,in addition to the reaction of SO2 and propylene oxide,the system also undergoes a transesterification reaction which forms ethylene glycol sulfite.With the increase of the amount of lactic acid,reacted in[TEPA]LBr-EG(1:5)DES at 80℃ for 5 h,the yield of 1,2-propanediol sulfite reached to 42.4%,and the yield of ethylene glycol sulfite reached to 55.1%.The conversion of pure SO2 was as high as 97.5%which indicated that conversion of almost all SO2 has been achieved.Although the introduction of lactic acid reduced the absorption of 2 vol%SO2 and the selectivity of the reaction of SO2 and propylene oxide,it greatly increased the conversion of SO2.Finally,the SO2 absorption mechanism in[TEPA]LBr-EG DES and its catalytic pathway for the reaction of SO2 and propylene oxide are proposed.