Piperazinium-and Guanidinium-Based Ionic Liquids: Synthesis, Property And Application

Ionic liquids (ILs) as novel "green" materials have attracted much attention. In this dissertation, a total of 57 kinds of piperazinium-and guanidinium-based ILs are synthesized. Among them,14 kinds of room-temperature ILs are selected for further investigations of their properties and applications. They are 1-methylpiperazinium lactate ([C1pi][Lac]),1-ethylpiperazinium lactate ([C2pi][Lac]),1,1,3,3-tetramethylguanidinium lactate ([TMG][Lac]),1,4-dimethylpiperazinium dilactate ([CiC1pi][Lac]2), 1-ethyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C2C]C14pi][NTf2]),1-propyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C3C1C14pi][NTf2]), 1-allyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([(C2=C1)C1C14pi][NTf2]),1-butyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C4C1C14pi][NTf2]),1-pentyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C5C1C14pi][NTf2]),1-hexyl-1,4-dimethylpiperazinium bis(trifluoromethylsulfonyl)imide ([C6C1C14pi][NTf2]),2-ethyl-1,1,3,3-tetramethylguanidinium ethylsulfate ([TMG(C2)][C2OSO3]),2-ethyl-1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide ([TMG(C2)][NTf2]),2,2-diethyl-1,1,3,3-tetramethylguanidinium ethylsulfate ([TMG(C2)2][C2OSO3]), and 2,2-diethyl-1,1,3,3-tetramethylguanidinium bis(trifluoromethylsulfonyl)imide ([TMG(C2)2][NTf2]).Conformational isomerism of [C2C1Ci4pi][NTf2] is discussed. The inversion of the -CH3 group bonded to the ternary nitrogen atom on the piperazinium ring in the [C2C]C14pi]+ cation results in the cis-trans isomerism with the equatorial or axial N-CH3 group. The'H NMR spectra show the preponderance of the cis-isomer to the trans-one with a fluctuating mole ratio around 65:35 when the IL remains subcooled under ambient conditions. When the IL becomes solid or one polar solvent is added, only the cis-isomers are observed. Quantum chemistry calculations reveal that the difference of energy between two isomers is as small as 1.26 kJ·mol-1, and the energy barrier for the transformation of two isomers is 26.37 kJ·mol-1, leading to the frequent and facile conversion between the isomers under ambient conditions. The conformational isomerism significantly influences the thermal transition behaviours of the IL and the excess molar volumes of the IL mixtures.Properties and interactions of some liquid-liquid binary systems containing guanidinium-based IL are investigated. The IL-IL mixtures with the same cation or anion of two IL components are [TMG(C2)][C2OSO3]+[TMG(C2)][NTf2] ([TMG(C2)][C2OSO3];f[NTf2]1-x), [TMG(C2)][C2OSO3]+[TMG(C2)2][C2OSO3] ([rMG(C2)MTMG(C2)2]1-x[C2OSO3]), [TMG(C2)2][NTf2]+[TMG(C2)][NTf2] ([TMG(C2)2]x[TMG(C2)]1-x[NTf2]), and [TMG(C2)2][C2OSO3]+[TMG(C2)2][NTf2] ([TMG(C2)2][C2OSO3]x[NTf2]1-x). The binary mixtures with one component of molecular solvent are [TMG(C2)][C2OS03]+ethanol or 1-propanol and [TMG(C2)][NTf2]+ethanol or 1-propanol. The 1H NMR spectra and density data of the IL-IL mixtures are determined. The optimized structures of the ILs and the binary mixtures are obtained by quantum chemistry calculation. These IL-IL binary mixtures show relatively small excess molar volumes, which indicates the interactions among ions are not significantly changed after mixing of ILs, and the mixing process seems to be nearly ideal. Meanwhile, the measured densities and viscosities of the IL-alcohol mixtures give much larger values of excess/deviation properties, and the IL-alcohol systems with strong H-bonding interactions (spans within 2 A) between IL and alcohol show more significant deviation from the ideal mixtures relative to the IL-IL systems. The alcohol molecules with the smaller molecular sizes can get into the free spaces among ions, resulting in the volume compression of mixtures. Therefore, significant differences of volumetric properties and interactions between IL-IL mixtures and IL-molecular solvent mixtures can be presented.Desulfurization performance of [Cipi][Lac], [C2pi][Lac], [TMG][Lac] and [C1Cipi][Lac]2 are investigated by extracting sulfur compounds from hydrocarbon-based fuels. Several factors such as extraction time, temperature, IL/fuel mole ratio, and IL or fuel species are evaluated. At 30? and with the IL/fuel mole ratio of 1:1, [TMG] [Lac] has the highest sulfur removal ratio and selectivity of thiophene against toluene after extraction of 30 min. More than 50% of the sulfur compounds are extracted from 93# gasoline to [TMG][Lac] through one extraction stage, and the selectivity of thiophene against toluene turns out to be 13.5. These lactate ILs are cheap, facile to synthesize, and low toxic. They are promising materials for fuel desulfurization.SO2 absorption abilities of [TMG(C2)][C2OSO3}, [TMG(C2)2][C2OSO3], [TMG(C2)][NTf2] and [TMG(C2)2][NTf2] are investigated. At 20? and 100 kPa of SO2, they can absorb 3.15,3.93,1.93 and 2.25 mol SO2/(mol IL), respectively. When the partial pressure of SO2 decreases to 10 kPa (containing 90 kPa of N2), the corresponding capacities decrease to 0.71,1.08,0.17 and 0.24 mol SO2/(mol IL), respectively. At 40? and with 10 kPa of SO2 (containing 90 kPa of N2), [TMG(C2)][C2OSO3] and [TMG(C2)2][C2OSO3] absorb 0.36 and 0.55 mol SO2/(mol IL), respectively. The'H NMR and FT-IR characterization results indicate that no chemical process is involved, and the absorption process is physically driven. On the basis of the quantum chemistry calculation, the interactions between ILs and SO2 molecules are discussed and the interactions between the IL anion and SO2 probably play the predominant role. Furthermore, the ILs show good regeneration abilities through 10 cycles of absorption/desorption. As a result, the guanidinium-based ILs exhibit great potential in SO2 capture.