Extractive Deep Desulfurization of Liquid Fuels Using Lewis-Based Ionic Liquids
A new class of green solvents, known as ionic liquids (ILs), has recently been the subject of intensive research on the extractive desulfurization of liquid fuels because of the limitation of traditional hydrodesulfurization method. In present work, eleven Lewis acid ionic liquids were synthesized and employed as promising extractants for deep desulfurization of the liquid fuel containing dibenzothiophene (DBT) to test the desulfurization efficiency. [Bmim]Cl/FeCl3 was the most promising ionic liquid and performed the best among studied ionic liquids under the same operating conditions. It can remove dibenzothiophene from the model liquid fuel in the single-stage extraction process with the maximum desulfurization efficiency of 75.6%. It was also found that [Bmim]Cl/FeCl3 may be reused without regeneration with considerable extraction efficiency of 47.3%. Huge saving on energy can be achieved if we make use of this ionic liquids behavior in process design, instead of regenerating ionic liquids after every time of extraction.
Due to environmental awareness and economic competition, fuel oil standards are more stringent. Sulfur present in transportation fuel leads to sulfur oxide (SOx) emissions into the atmosphere and causes many environmental problems. It also inhibits the performance of vehicles pollution control equipment. Hence, much recent interest in the deep desulfurization [1, 2] of light oil feedstock is directed. Inconsiderable attention has been given to deep desulfurization of gasoline and liquid fuels due to strict environment regulations on the sulfur limit of fuels . The maximum sulfur content will be limited to 10–50 ppm in 2016, as compared to today’s permitted value of 500 ppm in most western countries .
In India, the present norms were decided by the Central Pollution Control Board (CPCB); the current value of total sulfur in liquid fuels is limited to 350 ppm which has to be lowered down into possible extent . As a result the deep desulfurization of liquid fuels has fascinated increased attentunity worldwide. In the petroleum industry, low sulfur fuels are often obtained from hydrocracking processes .
In petroleum and hydrocarbon industries, various solvents such as ethers, amines, alcohols, and other volatile organic compounds have been used for the processes like extraction, absorption, azeotropic distillation, and so forth . These solvents have their own limitations in terms of environmental issue, recycle ability, and so forth, which can be overcome by the use of ionic liquids as green solvent due to their very low vapour pressure and wide range of application with unique physical and chemical properties . Among these, deep extractive desulfurization is an attractive technology, as it can be carried out at ambient temperature, pressure, and without hydrogen as an catalyst. A good extractant much have the following attributes : good extractive ability for sulfur compounds, free of contamination to the fuels, nontoxicity, environmental benignity, and stability for repetitive use. Ionic liquids have been studied for many possible applications for green chemical processes [10–16].
Ionic liquids (ILs) are usually consisted by various combinations of heterocyclic organic cations, and anions show liquid at room temperature with unique properties such as nonvolatility, nonflammability, and a wide temperature range for the liquid phase [17–21].
In present work, eleven Lewis acid ionic liquids, [Bmim]Cl/FeCl3 (Bmim: 1-butyl-3-methylimidazolium chloride), [Omim]Cl/FeCl3 (Omim: 1-Octyl-3-methylimidazolium Chloride), T8Cl/FeCl3 (T8Cl: Tri-octyl methyl ammonium chloride), T4Cl/FeCl3 (T4Cl: Tri-butyl methyl chloride), D10Cl/FeCl3 (D10Cl: didecyl dimethyl ammonium chloride), and [Emim]Cl/AlCl3 (Emim: 1-ethyl-3-methylimidazolium), have been synthesized for desulfurization of liquid fuels. The extent of sulfur removal from the model fuel by the ILs has been used to decide the best ILs. Further the reusability of the IL has been investigated.
2.1. Materials and Reagents
n-dodecane (AR grade), dibenzothiophene (DBT) (98%), ethyl acetate (99.5%), [Bmim]Cl (98%), [Omim]Cl (99%), [Emim]Cl (98%), anhydrous FeCl3 (99%), anhydrous ZnCl2 min (99%), anhydrous SnCl2 (99%), anhydrous MnCl2 (99%), and anhydrous CoCl2 (99%) are of Acros (USA) available commercially and used as received without further treatment.
2.2. Preparation of Ionic Liquids
[Bmim]Cl/FeCl3 ionic liquid was synthesized as mentioned in the literature . [Omim]Cl/FeCl3, [Bmim]Cl/AlCl3, [Bmim]Cl/ZnCl2, [Bmim]Cl/SnCl2, [Bmim]Cl/MnCl2, [Bmim]Cl/CoCl2, and [Emim]Cl/AlCl3 were prepared by adding the desired amount of chloride to the imidazolium salt. The reaction was carried out with stirring for 2 hours. The synthesis of T8Cl/FeCl3, T4Cl/FeCl3, and D10Cl/FeCl3 was followed by a similar procedure .
2.3. Preparation of Model Liquid Fuel
A model liquid fuel containing 500 ppm sulfur dibenzothiophene (DBT) (98%) was prepared by dissolving DBT in n-dodecane.
2.4. Extractive Desulfurization of the Model Liquid Fuel
The desulfurization experiments were carried out in a 100 mL two necked flask by mixing of model liquid fuel (10 mL) and specific amount of ionic liquid with the fixed mass ratio between model liquid fuel to ionic liquid as 5/1 at 30°C in a water bath for 30 minutes with vigorous stirring.
On completion of the reaction, the upper phase (model liquid fuel) was withdrawn and analyzed by Inductively Coupled Plasma-Atomic Emission Spectrometer (ICP-AES) (Arcos, M/s. Spectro, Germany.), Indian Institute of Technology, Mumbai (M.S.), India, to determine the concentration of sulfur in the model liquid fuels. Then % removal of sulfur can be calculated by the following equation :
% sulfur removal = [DBT] initial – [DBT] final/[DBT] initial × 100.
3. Results and Discussion
3.1. Desulfurization of the Model Liquid Fuel (DBT in n-dodecane) with Various Ionic Liquids
The experiments were carried out to test the feasibility of various ionic liquid for desulfurization of liquid fuels. The results of experiments on the selective extraction of using Lewis acid ionic liquids are given in Table 1.
The sulfur removal efficiency of [Bmim]Cl/FeCl3, [Omim]Cl/FeCl3, T8Cl/FeCl3, T4Cl/FeCl3, D10Cl/FeCl3, [Bmim]Cl/AlCl3, [Emim]Cl/AlCl3, [Bmim]Cl/ZnCl2, [Bmim]Cl/SnCl2, [Bmim]Cl/MnCl2, and [Bmim]Cl/CoCl2 for model liquid fuel is shown in Table 1. The ionic liquids based on FeCl3 showed the best sulfur removal efficiency among other ionic liquids. This experiment clearly demonstrates that the extraction ability of [Bmim]Cl/FeCl3 is suitable in comparison with the other ionic liquids. It was also found that the extraction process went on quickly, and it could reach extraction equilibrium in little time. (Say 30 minutes). The amount of DBT extracted can be increased with an increased molar ratios of FeCl3/[Bmim]Cl which can be recognized to the increased Lewis acidity of the resulting IL at higher molar ratios of FeCl3/[Bmim]Cl . However the DBT was broadly extracted from the model fuel at the [Bmim]Cl/FeCl3 at higher molar ratios. In general FeCl3 alone exhibited lower extraction ability than the Fe-containing ILs, signifying the important role of [Bmim]Cl . The IL with one or two acidic hydrogen atoms on the imidazolium ring exhibited much lower extraction ability than the ILs containing two or three alkyl groups. The highest DBT extraction was obtained with [Bmim]Cl/FeCl3 that existed as liquids at room temperature. These results may imply that the ability of Fe-based ILs to remove DBT is also affected by the fluidity of the IL. Similar results were reported  among the three ionic liquids; [Bmim]Cl/AlCl3 with desulfurization of 45% represents better extraction ability than the other [Emim]Cl; two different chloroaluminate melts suggest a certain influence of the ionic liquids cation as for [Bmim]+ cation containing Lewis acidic ionic liquids, the desulfurization efficiency of different anions expressed different extraction ability. [Bmim]Cl ionic liquid showed higher extractive ability than the other ionic liquids . It was also found that [Bmim]Cl/FeCl3 is less viscous than other ionic liquids. However, [Bmim]Cl/FeCl3 is considered as a promising extractive agent in extractive deep desulfurization for the removal of dibenzothiophene (DBT).
3.2. Ionic Liquid Reusability without Regeneration
In order to examine the reusability of the ionic liquid, the spent [Bmim]Cl/FeCl3 was studied. Table 2 shows the desulfurization efficiency of [Bmim]Cl/FeCl3 which was reused for three times without regeneration. It was observed that the spent ionic liquid was able to extract dibenzothiophene from model liquid fuel even without regeneration, however, at a lower efficiency of 47.3% from 75.6% with fresh ionic liquid.
Eleven Lewis-acid-based ionic liquids were screened to investigate the desulfurization efficiency. FeCl3-based ILs can be used as effective extractant for removing DBT from liquid fuel. [Bmim]Cl/FeCl3 was found to be the best ionic liquid as a kind of novel extractant for desulfurization of liquid fuel, which exhibits a better extractive performance for dibenzothiophene. The sulfur removal of dibenzothiophene containing model liquid fuel was 75.6% with single-stage extraction process at 30°C in 30 minutes. Lewis acid ionic liquid with long carbon chains can obtain 70.2% with simple extraction. The used ionic liquid was able to extract DBT from model liquid fuel even without regeneration. Furthermore, Fe-based IL system shows considerable promise for providing a technology to meet future needs for low sulfur fuel (less than 50 ppm), that is, clean fuels.
The authors gratefully acknowledge, for the financial support, the Council of Scientific Research (CSIR) Grant no. (22(0492)/09/EMR-II), Government of India, India (principal investigator: Dr. K. L. Wasewar).
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