Design And Performance Study Of Ionic Liquids For Sulfer Dioxide Absorption

SO₂is mainly produced by the burning of oil or coal in petroleum refineries and thermal powerplants.It has received more and more attention in recent years because of its huge threat to the ecological environment and human health.In the past few decades,people have developed a variety of flue gas desulfurization（FGD）methods（wet,dry,and semi-dry）.Although about 90%of SO₂is removed by the limestone-gypsum process based on an equimolar reaction,it is still necessary to treat mass-produced low-quality gypsum and wastewater.Based on this,we need to develop new absorbents to efficiently and greenly absorb SO₂gas.As a new type of green solvent,ionic liquid has the characteristics of structural tunability,negligible vapor pressure,low melting point,and excellent thermal stability.It has been widely used in gas absorption,battery materials,extraction,new functional materials,catalysis,organic synthesis and other fields.In recent years,a large number of researchers have discovered that functionalized ionic liquids have a greater ability to capture SO₂than conventional ionic liquids.In this paper,a series of functionalized ionic liquids have been designed and synthesized for the application in efficient absorption of SO₂.The main research contents include the following three aspects:（1）Several functionalized ionic liquids have been designed and synthesized.Their structureswere characterized by nuclear magnetic resonance spectroscopy（NMR）and infrared spectroscopy（FT-IR）.Their density,viscosity and other related physical properties were measured,as a result,they were found to have excellent thermal stability and low temperature properties.The density（ρ）and the logarithm of the density（lnρ）show a good linear relationship with the temperature（T）,the viscosity（η）and the temperature（T）conform to the formulaη＝η0×exp(^D_T-T0),and the logarithm of viscosity（lnη）and the temperature（T）conforms to the Arrhenius formula lnη=lnA+E_vis/（RT）.（2）Six kinds（three groups）of quaternary phosphonium ionic liquids(tributyl ethyl phosphonium2-hydroxypyridine[P₄₄₄₂][PyO],tributyl ethyl phosphonium phenol[P₄₄₄₂][PhO],tributyl ethyl phosphonium 2-picolinic acid[P₄₄₄₂][PyCOO],tributyl ethyl phosphonium benzoic acid[P₄₄₄₂][PhCOO],tributyl ethyl phosphonium 2-pyridine sulfonic acid[P₄₄₄₂][PySO₃],and tributyl ethyl phosphonium benzene sulfonic acid[P₄₄₄₂][Ph SO₃])were used for the absorption of SO₂,and the effects（SO₂partial pressure,temperature,and water）on the performance of ionic liquids for SO₂absorption were studied.The mechanism of interaction between this kind of functionalized ionic liquids and SO₂was studied through absorption experiments,spectroscopic analysis（FT-IR and NMR）and DFT calculation.The results show that under the same absorption conditions,the ionic liquids with pyridyl group anion has a higher absorption capacity than the corresponding ionic liquids without pyridyl group due to the N···SO₂chemical interaction between SO₂and N on the pyridyl group.And the strength of the interaction is obviously related to the alkalinity of the ionic liquids anion.There are two chemical sites（N and O）on the pyridyl anion,and they act simultaneously.（3）Several functionalized ionic liquids(tetradecyl trihexyl phosphonium 2-hydroxypyridine[P₆₆₆₁₄][2-PyO],tetradecyltrihexylphosphonium3-hydroxypyridine[P₆₆₆₁₄][3-PyO],tetradecyltrihexylphosphonium 4-hydroxypyridine[P₆₆₆₁₄][4-PyO],tributyl ethyl phosphonium3-hydroxypyridine[P₄₄₄₂][3-PyO],tributyl ethyl phosphonium 4-hydroxy-pyridine[P₄₄₄₂][4-PyO],tetramethylguanidine 2-hydroxy-pyridine[TMG][2-PyO],tetramethylguanidine 3-hydroxy-pyridine[TMG][3-PyO])with different anions and cations were synthesized and applied to SO₂absorption.The effects of partial pressure of SO₂and temperature on the SO₂absorption behavior of these ionic liquids were investigated,and the reaction mechanism was studied by FT-IR and NMR analysis and quantum chemistry calculation.The results show that:1)aprotic cations weaken the interaction between cations and anions,thus better releasing the activity of the sites on the anions.2)the position of N on the anions has a great influence on the charge distribution of the anions,leading to adjustability of SO₂absorption capacity.