Synthesis And DFT Calculation Of The Desulfurization Of Functionalized Ionic Liquids

As environmentally friendly desulfurizer, Room temperature ionic liquids（ILs）are gaining increasing interest for overcoming many disadvantages of traditionaldesulfurization. The studied desulfurization ionic liquids have low vapor pressure,good stability and preferable absorbing SO₂capacity. However, they also have highviscosity and exorbitant price, which limit industrial application. The question ofselecting low-cost and superior performance of ILs is becoming more urgent. It is alsoneeded to explore the application form. Quantum chemistry methods provide thetheory basis for design and development of new desulfurization ionic liuids, reducingblindness of screening ILs by the experimentally tried methods.In this paper, three kinds of ionic liquids are selected to investigate the behaviorsof SO₂absorption and desorption.They are Brominated1-propyl-3-methyl sulfonated C3MIM Br, Brominated1-butyl-3-methyl sulfonated BMIM Br and Brominated1-pentyl-3-methyl sulfonated C₅MIM Br.This kind of ILs is inexpensive and easilysynthesized. Study results indicate that three kinds of ILs have good ability to absorbSO₂. On conditon of25℃and101.3KPa, it spends inside50min reaching absorptionequilibrium. Balance absorptive amounts are respectively1.44，1.61，1.79molSO₂/mol ILs. With increasing of the length of the side chain alkyl, the concentration ofSO₂and the flow, the balance absorptive amounts grow. With increasing oftemperature, the balance absorptive amounts reduce. Equilibrium time reduces withincreasing of the concentration of SO₂, the flow and temperature,it grows with the sidechain alkyl increased. ILs regenerate by heating.Balance desorption rate is affected bytemperature and time.ILs can recycle after desorption. Compare the infrared spectrumof ILs before absorption with the one of ILs after absorption, it can be found that theregenerated ILs have not recovered completely.The absorbing SO₂capacity of silica gel improves after being loaded by C₅MIM Br. The internal pores of silica gel are changed and the specific surface areasreduce. ILs, meanwhile, increase the absorbing SO₂capacity. C₅MIM Br/SiO₂has thelargest balance absorptive amounts（0.21gSO₂/g C₅MIM Br/SiO₂）, when quality ratioof ILs and SiO₂is0.5:1. Analysing the infrared spectrum, it finds that both ILs and silica gel have interaction with SO₂.Calculations are performed with Gaussian03. The stucture optimization andfrequency analysis are carried out at the B3LYP/6-31+G （d） level of DFT theory,conclusions follows: The anion mainly interacts with cation around the cation, with aweak interaciton with alkyl side chain. The interaction energy between anion andcation is larger than normal hydrogen-bonding energy. It becomes larger when anioninteracts with H that is between two N atoms. The molecule of SO₂is easy to interactwith hydrogen atom in the imidazole ring. The interaction energy between SO₂andcaion is in the range of hydrogen-bonding energy, consequently, hydrogen bonds playan important role at the interaction between SO₂and caion. Because of the molecule ofSO₂, the amount of hydrogen bonds between anion and cation reduces. The interactionbecomes weak. The interaction energy between SO₂and ion pairs is also larger thannormal hydrogen-bonding energy. Anion plays an important part in the interactionbetween ILs and SO₂.