Theoretical Study On The Structural Properties Of Ionic Liquids And Their Applications In Dissolution Of Lignin

Ionic liquids as novel solvents have been applied to processes,such as catalysis,gas absorption,electrolytes and biomass treatment.However,the structural properties of ionic liquids and the mechanism of lignin dissolution in ionic liquids lack in-depth exploration.In this work,the effect of co-solvent on the structure of ionic liquids is studied by computational chemistry in combination with experiments,and the mechanism of lignin dissolution in imidazolium-based ionic liquids is particularly studied by DFT calculations.The main research is summarized as following:(1)Excess infrared(Excess IR)and DFT calculations were applied to study the hydrogen bonding interactions between dimethyl sulfoxide(DMSO)and N-butyl-pyridinium dicyanamide([Bpy][DCA]).The results show that DMSO forms H-bonds with[Bpy]+ and[DCA]-simultaneously and H-bond between DMSO and[Bpy]+ is stronger than that between[DCA]-and DMSO.H-bonds between ionic cations and anions are gradually weakened and the whole hydrogen bonding interactions in the solutions are enhanced by DMSO.Positive and negative peaks at fixed positions in excess IR of[DCA]-show the presence of stable interactions of Cring-H…N?C,Calkyl-H…N?C and CDMSO-H…N?C.When the molar fraction of DMSO is in the range from 0.3 to 0.4,hydrogen bonding interaction reaches the maximum than that in other solutions.The sequence of the interactions in the binary solutions is:2[Bpy]+-2[DCA]--DMSO>[Bpy]+-[DCA]--DMSO>[Bpy]+-[DCA]->DMSO-[Bpy]+>DMSO-[DCA]-.(2)The interactions between anions([OAc]-,[Cl]-,[MeSO4]-,[PF6]-),cations([Mmim]+,[Emim]+,[Bmim]+,[Hmim]+),16 kinds of ion pairs and lignin model compound guaiacyl glycerol-?-guaiacyl ether(GG)is studied by DFT calculations to reveal the dissolution mechanism of lignin in ionic liquids.The results show that one single anion forms stronger H-bond with GG than cation does.The optimal position for anion-GG interactions is the a-OH of GG and anions follow[OAc]->[Cl]->[MeSO4]->[PF6]-.The optimal position for cation-GG interactions is the,?-OH of GG and cations follow[Mmim]+>[Emim]+>[Bmim]+>[Hmim]+.It seems that the alkyl chain lengths do not have obvious effects on cation-GG interactions,but the types of anions have significant effects on anion-GG interactions.Further calculations at the B3LYP-D3/6-311+g**level confirm that anions form H-bonds with C2-H of cations anda-OH of GG,and ?-? stacking effect exist between pyridinium ring and benzene ring of GG.The stronger H-bonds are dominated by electrostatic interaction,and the weaker H-bonds and?-? stacking effect are dominated by van der Waals interaction.H-bonds and ?-?interaction are essential for the dissolution of lignin in ionic liquids.