VIETNAM JOURNAL OF CHEMISTRY VOL. 51(2) 151-155
INFLUENCE OF SOME FACTORS ON DEEP DESULFURIZATION EXTRACTION CAPACITY OF IONIC LIQUIDS
Bni Thi Le Thuy
Hanoi University of Mining and Geology - HUMG Received 6 August 2012 Abstract
Some ionic liquids with different structures were used as solvents for extraction of sulfur compounds in model oils. Influence of some factors such as the structures of ionic liquid anions and cations, the stmcture of sullur confounds on the deep desulfinization extraction capacity were investigated. The extraction process was performed at room ten^eratures and under normal pressures; no high hydrogen pressure was needed.
Keywords: Extraction, desulfurization, ionic liquids.
1. INTRODUCTION
Sulfur compounds in petroleum lead to equipment corrosion and SOx emissions that cause air pollution and acid rain. In recent years, regulations on sulfur limits of fuel are increasingly strict [1]. In Vietnam, sulfur limits of fuel are still high compared to those of developed countries.
However, for protecting community health, reaching environmental standards and integrating, the sulfur content in fuel in Vietnam should also be consistent with international regulations for sulfur. Therefore, the investigation on deep desulfurization of fuel is necessary for developing refinery in the near future in Vietnam.
The conventional desulfunzation method in industry is catalytic hydrodesulflinzation (HDS).
However, it is difficult for that method to meet the demand of deep desulfiirization because benzothiophenes and dibenzothiophenes (DBTs) are highly resistant to hydrogenation and require the use of more severe conditions including more active catalysts and higher consumption of hydrogen, which bring about a number of problems, such as high investment and high operating costs [2]. Therefore, extensive research has been carried out during the past decades in industrial and academic research laboratories to deeply desulfiirize fijels [3,4].
In recent years, some sulfur compound removing processes operated under moderate conditions without requiring H2 have been extensively investigated. Desulfurization using biocatalysts to convert sulfur to sulfate form was reported [5]. Complex containing nickel and
platinium was used effectively for alkyldesufurization of dibenzothiophene [6].
Oxidesufurization using hydroperoxide and formic acid was investigated by Zhao et al [7].
Ionic liquids exhibit a new group of solvents and catalysts. Their properties can be adjusted to be suitable for particular applications by changing or modification ofthe ions.
During the last decades ionic liquids (ILs) were introduced as a new class of liquid material [8 - 11].
ILs are defined as salts melting below 100°C. They have very low vapour pressure and thus a loss by evaporisation is impossible. Therefore, they are intensively discussed as environmentally friendly and less hazardous solvents or catalysts in industry [12-15]. In addition, ionic liquids have high compatibility, low melting point so that they are suitable as solvents for extraction. Extraction process is based on the sulfur compounds being more soluble in ionic liquids than in hydrocarbons.
Some desulfurization investigations show that they can be used as extraction solvents [16-18] or in combination with oxidizing agents [19-25]. The advantages of desulfurization using ionic liquid are the performance under normal conditions, easy phase separation, non-volatile solvents, high reusability, particularly adjustable extraction ability by changing the structure of ionic liquid cations and anions. Promising results encourage further research on this subject.
2. EXPERIMENTAL
1 -Methylimidazole, 1 -chlorobutane, 1 -chloro-
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ctane, thiophene, benzothiophene, and dibenzothiophene were purchased from Merck and used without further punfication. Sodium acetate, sulfunc acid, tri butyl phosphate were purchased from Guangdong Guanghua, China and used without further purification.
2.1. Preparation of ionic liquids l-Methyl-3-ankylimidazolium chloride (ICnMIMlCI)
Equimolar amounts of 1-methylimidazol and 1- chloroctane was stirred for about 100 h at 70*'C. The ionic liquid was washed three times with ethyl acetate to remove unreacted material and then dried at 110°C.
l-Methyl-3-alkyl imidazolium hydrosulfate ([C„MIMl|[HS04l)
An equimolar amount of sulphuric acid was added dropwise to a solution of [CnMIM]Cl] in dichloromethane. The mixture was stirred for 48 h at room temperature and slowly distilled at 80°C until no HCl released (tested with damp litmus paper), and finally dried at 120°C.
l-methyl-3-alkyl imidazolium acetate ([CnMIMHICHjCOOl)
Ionic liquid l-methyl-3-alkyl imidazolium acetate was prepared by adding equal amounts (mole) of [CnMIM]CI] and sodium acetate to methyl alcohol, and then the mixture was agitated at room temperature for 48 h. Then the resulting precipitate was filtered off, and the solvent was removed by evaporation to leave a brown liquid.
l-methyl-3-butyl imidazolium dibutylphosphate ([BMIM][DBP])
Ionic liquid [BMIM][DBP] were prepared by reacting equimolar quantities of 1-ethylimidazole and the tributylphosphate at 120°C for 24 h with a yield of 98%. The resulting yellowish viscous liquid was cooled to room temperature and then washed several times with diethyl ether followed by evaporation for 12 h to remove all volatile residues.
A'-butyl-pyridinium hydrosulfate ((BMIM]1[HS04])
Solution of [BMIMjCl] in dichloromethane was added into a round bottomed flask. Then an equimolar amount of sulphuric acid was added dropwise to. The mixture was stirred for 48 h at room temperature and slowly distilled at 80°C until no HCl released (tested with damp litmus paper), and finally dried at 120°C.
2.2. The desulfurization procedure
Desulfiirization of the model oils were carried
Hui inline Thuy out as follows: thiophene, benzothiophene, dibenzothiophene were dissolved in n-heptane. The sulfur concentration of the model oils is 498 mg/L (498 ppm).
All the desulfurization experiments were conducted in a 100 mL glass flask. The model oil and ionic liquids were added into the flask at room temperature. The volume ratio of ILs to the model oil was 1:1. The biphasic mixture was then stirred for 30-40 min. The sulfur content of stratified mixture was quantitative analyzed by a TCVN 3172- 2008 (ASTM D 4294) in Trung Tam Ky Thuat tieu chuan do luong 1, Tong cue Tieu chuan do luong (Quality Asurance and Testing Center 1, Directorate for Standards Metrology and Quality).
2.3. Regeneration of ionic liquids
Used ionic liquids were regenerated by re- extracting with ethyl acetate for 3 times. The volume ratio of ILs to the ethyl acetate was 1:1. The regenerated ionic liquids were used as solvent for sulfur extraction.
3. RESULTS AND DISCUSSION
3.1. Some physical properties of synthesised ionic liquids
Some physical properties of synthesized ionic liquids such as molar mass, density, viscosity and melting point are given m Tab. I
3.2. Desulfurization of the model oil with different ILs
The synthesized ionic liquids were used as solvents to extract dibenzothiophene in model oils.
The desulfiirization results of these ionis liquids are shown in Tab. 2. When the mass ratio of ILs to the model oil is 1:1, [OMIM][CH3COO] has the best extracting ability to remove dibenzothiophene from the model oil among the six ionic liquid at the same conditions.
The sulfur extraction properties of ionic liquids are related to their structural organization. It should be noted that aromatic compounds interact via CH (imidazolium)-!! bonds (aromatic) [26]. Further the ionic liquid stabilizes through C2-X hydrogen bonds (X = anion) [27]. The molecular structures ofthe lonpair [BMIM] and [BF4"] are stabilized by hydrogen bonds between the P atom in the anion and various H atoms of the cation [27]. It was expected that this chemistry is similar to that involved in the
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stability of ionpair [BMIM] and [CHjCOO].
Influence of some factors on deep...
Table 1: Physical properties of synthesised ionic liquids Property
Moi mass (g/mol) Density (g/cm^) Viscosity
(cP) Melting
point CO
[OMIM][CHjC00]
255
1.05 545.1
-13
[0MIM][HS04]
292
1.29 520.7 25
[BMIM] [DBF]
348.1
1.37 284.49
6-10
[BMIMKHSOJ 236
1.27 164 (at84°C)
28
[BMIM][CHjCOO]
198
1.02 440
<-20
Monte Carlo simulations [28] reported that the highest probability of finding an anion around the cation was m the neighborhood of the C2 carbon atom. The C2 carbon atom is the center ofthe largest positive charge on the cation, so this is where the anion localizes. Decreasing the strength of this hydrogen bond wdth the anion increases the aromatic (thiophene and derivatives) extraction capacity of thelL.
The imidazolium cation gives greater selectivity as compared to the six-membered cation families of pyridinium because of its similar structure to thiophene and denvatives.
Increasing the alkyl chain length ofthe 1-alkyl side chain of cation leads to an increase in the extraction capacity (Tab. 2) for the ionic liquids, since the increased van derWaals volume leads to a decrease in the strength of C2-X hydrogen bond of the imidazoliumcation strength with the anion. This is consistent with the simulation work carried out by Hanke et al. [29] and also reported for polar systems byJorketal. [30].
The imidazolium cation gives greater selectivity as compared to the six-membered cation families of pyndinium because of its similar structure to thiophene and derivatives.
The results from Tab. 2 show that if ionic liquids have the same cation ([BMIM]) then the extraction capacity of ionic liquids follows the order of anions CH3COO" > DBF" > HSO4'. This is consistent with the COSMO-RS based predictions [31]. The distribution of charge in anion plays an important role for extraction ability. The large distribution of negative charge gives low solvent capacity.
Table 2: Desulfurization results of ionic liquids
1 2 3 4 5 6
ILs
[OMIM]
fCHjCOO]
[OMIM]
[CHjCOO]
[OMIM]
[CHjCOO]
[OMIM]
[HSO,]
[OMIM]
[HSO.l [OMIM]
[HSO4]
Sulfur compound
dibenzothiophene benzothiophene
thiophene dibenzothiophene
benzothiophene thiophene
Sulfur removal
r\IV.
71 60.9 53.7 58 52 50.7
KN
2.5 1.56 1.16 1.35 0.91 0.52
3.3. Influence of sulfur compound structure on desulfurization
The aromatic sulphur compounds including thiophenes, benzothiophenes, and their alkylated derivatives are generally more difficult to convert over hydroprocessing catalysts. Therefore, the aromatic sulphur compounds present the most difficult challenges to the HDS processes [32].
In this section, two ionic liquids [0MIM][CH3C00] and [0MIM][HS04] that gave the highest desulfurization yields were chosen to extract thiophene, benzothiophene, and dibenzothiophene in model oils. The results are given in Tab. 3.
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Table 3: hifluence of sulfur compound structure on desulfurization
1 2 3 4 5 6
Ionic liquids [BM1M][CH]C00]
[OMIM][CHjCOO]
[BMIM][DBP]
[BMIM][HSO,]
[0MIM][IIS04]
[BPy][HS04]
Sulfur removal 70.00 71.08 61.80 57.97 58.97 45.8
KN
2.4 2.5 1.67 1.34 1.43 0.84 For the ILs the sulfur removal selectivity of sulfur compounds followed the order thiophene
< benzothiophene < dibenzothiophene under the same conditions. These results indicate that extraction is favored for those aromatic heterocyclic sulfur compounds having higher density aromatic p- electrons. A possible p-p interaction between aromatic sulfur compounds and the imidazolium rings of ILs was suggested as extraction mechanism [32J.
The results show that desulfurization yields of 71%, 60,9% and 53,7% obtained when using [OMIMJCHjCOO ionic liquid as extraction solvent.
3.4. Reusable ability of ionic liquid
After re-extracting with ethyl acetat, ionic liquid [BMIM][CHjCOO] was used to remove sulfur from oil. The results in table 4 show that the sulfur exfraction yield of regenerated ionic liquid is slighfly lower than that of fresh ionic liquid. A small decrease in sulfur exfraction yield of regenerated ion liquid exhibits recylmg the potential of ionic liquids.
Ethyl acetate can be recovered by simple distillation.
Table 4: Sulfur extraction yield of fresh and regenerated ionic liquids
Sulfur removal
7 ) / %
Fresh [BMIM][CHjCOO]
70
Regenerated [BMIM][CH,COO]
66
4. CONCLUSIONS
Some ionic liquids with different structures were used as solvents for exfraction of thiophene.
Bui Thi Le Thuy benzothiophene and dibenzothiophene in model oil.
For the removal of thiophene and its derivatives it was found that imidazolium cation because of its similar sfrucfrire with thiophene gave greater selectivity as compared to the six-membered cation families of pyridinium. Increasing the alky! chain length ofthe 1-aIkyl side chain of cation leads to an increase in the exfraction capacity for the ionic liquid. [OMIM][CH3COO] has the best cxfracting ability to remove dibenzothiophene from the model oil among the six ionic liquid at the same conditions.
For investigated ionic liquids the sulfir removal selectivity of sulfur compounds followed the order thiophene < benzothiophene < dibenzothiophene under the same conditions.
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Corresponding author: B u i T h i L e T h u y
H a n o i University of Mining and Geology - H U M G D o n g N g a c , T u Liem, Hanoi
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