Piperazinium-Based Ionic Liquids with Lactate Anion for Extractive Desulfurization of Fuels

Three kinds of piperazinium-based room-temperature ionic liquids (RTILs), namely, N-methylpiperazinium lactate ([C(1)pi][Lac]), N-ethylpiperazinium lactate ([C(2)pi][Lac]), and N,N'-dimethylpiperazinium dilactate ([C(1)C(1)pi][Lac](2)), have been synthesized by the direct reaction of N-alkyl-substituted piperazines and lactate acid. Together with 1,1,3,3-tetramethylguanidinium lactate ([TMG][Lac]), they are employed as new extractants for removing aromatic sulfur compounds, thiophene (TS), benzothiophene (BT), dibenzothiophene (DBT), and 4-methyldibenzothiophene (4-MDBT), from various hydrocarbon fuels. The effects of the temperature, extraction time, and amount of ionic liquid (IL) on the sulfur removal are investigated systematically. The mutual solubility measurements show that the ILs are dissolved in n-heptane with the mass fraction less than 0.01 at 30 degrees C. The solubility values of 93 gasoline in the ILs are observed with the following sequence: [C(1)C(1)pi][Lac](2) (0.007 in mass fraction) < [C(1)pi][Lac] (0.014 in mass fraction) < [TMG][Lac] (0.017 in mass fraction) < [C(2)pi][Lac] (0.070 in mass fraction), and the sulfur distribution coefficient follows the order: [TMG][Lac] (1.08 in mass fraction) > [C(2)pi][Lac] (0.98 in mass fraction) > [C(1)pi][Lac] (0.78 in mass fraction) > [C(1)C(1)pi][Lac](2) (0.53 in mass fraction) for 93 gasoline. Selectivity between TS and toluene is observed higher than 4 with the following sequence: [TMG][Lac] (13.19 in mass fraction) > [C(1)pi][Lac] (10.59 in mass fraction) > [C(2)pi][Lac] (7.12 in mass fraction) > [C(1)C(1)pi][Lac](2) (4.94 in mass fraction), revealing that these ILs are more preferable to extract TS than toluene from hydrocarbon fuels. The used as can be recycled without a significant decrease of desulfurization activity after extraction 5 times. These fundamental results hopefully provide useful information for future commercialization and practical desulfurization.